i united states department of the interior...

$: I United States Department of the Interior SURVEYdggs.alaska.gov/webpubs/usgs/of/text/of72-0255.pdf · LL . I" ; United States Department of the Interior \ -r GEOLOGlCAL SURVEY$
,-, LL .

)". . ' . , * -.-~ I" ; United States Department of the Interior

\ - r

GEOLOGlCAL SURVEY

SURFICIAL GEOLOGY OF THE JUNEAU URBAN AREA AND V I C I N I l ' Y , ALASKA,

WITH 131PHASIS OX EARTI-IQUIZKE AND OTHER GEOLOGIC HAZARDS

Open- f i l e report

1972

This report is preliminary and has not been edited or reviewed for conformity with U. S. Geological Survey standards or nomenclature

UNITED STATES DEPARTMENT OF THE I N T E R I O R

GEOLOGICAL SURVEY

S U R F I C I A L GEOLOGY OF THE JUNEAU URBAN AREA AND V I C I N I T Y , ALASKA

WITH EMPHASIS ON EARTHQUAKE AND OTHER GEOLOGIC nAZARDS

BY

Robert D. M i l l e r

Open-f i le r e p o r t

1972

This r epor t i s p re l imina ry and has not been e d i t e d o r reviewed f o r conformity with U . S. Geological Survey standards o r nomenclature

CONTENTS

Page Introduction------------------------------------------------------- 1 Acknowledgmunts---------------------------------------------------- 3 Glossf i ry- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4 Faults------------------------------------------------------------- 7 Earthquakes-------------------------------------------------------- 10

Probability--------------------------------------------------- 11 Tsunamis----------------------------------------------------------- 18 Summary eva lua t ion of relative p r o b a b i l i t y o f occurrence o f

c e r t a i n hazardous geologic events-------------------------------- 20 Geomoqhology------------------------------------------------------ 2 0 Geology and engineer ing interpretations---------------------------- 2 2

Bedrock---------------**-------------------------------------- 2 3 S u r f i c i a l deposits-------------------------------------------- 25

Manmade fill--------------------------------------------- 25 bIine dulnps (nld) - - - - - -- -- - - -- - - - - - - - - - - - - - - - - - - - - - -- 26 Undifferent iated fill C m f ) ----- - ---- 2 7 Waste dwnp c~~)------------------------------------- 29

Muskeg----------------------------------------------+---- 3 0 Peat (Qmk)------------------------------------------ 3 1

Mass-wasting deposits-------------------------------------- 33 Colluvium ( Q C ) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 34 Talus (Qta)----------------------------------------- 36 Debris-flow depos i t s (~fl)-------------------------- 3 8 Rockslide-avalanche d e p o s i t s (Qra)------------------ 4 3 Undifferentiated l a n d s l i d e s (Qs1)------------------- 4 6 Colluvial(?) diamicton (Qud)------------------------ 49

Glac ia l deposits----------------------------------------- 5 1 Pitted outwash deposits (Qop)----------------------- 5 1 Moraine (Qm)---------------------------------------- 5 2 Younger outnash d e p o s i t s (Qoy)---------------------- 55 Late glacial-outwash d e p o s i t s (Qo1)----------------- 5 8 Older g l a c i a l ( ? ) a l luvium (Qpm)--------------------- 60 Older t i ll (Qpt)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 1

Alluvial deposits---------------------------------------- 62 Modern a l luv ium (Qal)------------------------------- 6 2 Fan depos i t s (Qf)--*-------------------------------- 6 3 Terrace depos i t s (~t)------------------------------- 6 6 Rubble depos i t s (Qar)------------------------------- 6 6

D e l t a i c deposits----------------------------------------- 67 Younger delra d e p o s i t s (Qdyl------------------------ 6 7 Older d e l t a depos i t s (Qdo)-------------------------- 7 0

Beach deposits--------------------------------------------- 7 3 Modem beach depos i t s (Qby)------------------------- 74 Spi t depos i t s (Qb)---------------------------------- 75 Young r a i s e d beach d e p o s i t s (Qrb)------------------- 76 Older r a i s e d beach d e p o s i t s , t h i n and cont inuous,

(Qbe)---------------------*----------------------- 7 7 Older r a i s e d beach d e p o s i t s , t h i c k and l o c a l , (Qbo)- 79

Geology and engineer ing in te rpre ta t ions- -Cont inued S u r f i c i a l deposits--Continued Page

Marine deposits------------------------------------------ 8 O I n t e r t i d a l d e p o s i t s (Qts)--------------------------- 8 0 Emergcnt i n t e r t i d a l dcpos i t s (Qe)------------------- 82

G l n c i o m a r i n e deposits------------------------------------ 83 Glaciomarine d e p o s i t s , f i r s t phase (Qmb)------------ 9 3 Glaciomarine d e p o s i t s , second phase (Qms)----------- 96 Glaciomarine d e p o s i t s , t h i r d phase (we)------------ 9 7 Und i f f e ren t i a t ed glaciornarine d e p o s i t s (Qmu)-------- 100

Selected references---------------*-*------------------------------ 101

ILLUSTRATIONS

Page Figure 1. Location o f s e l e c t e d major f a u l t s i n sou theas t e rn Alaska

t h a t might cause ear thquakes t h a t could a f f e c t t h e Juneau area------------------------------------------- 8

2 . Location o f major f a u l t s i n t h e Juneau area------------- 9

3. Location o f e p i c e n t e r s and approximate magnitude o f ear thquakes i n sou theas t e rn Alaska and ad jacen t regions----------------------------------------------- 1 3

5. Seismic p r o b a b i l i t y map f o r most of Alaska-------------- 19

6 . Sketch showing r e l a t i o n s h i p s o f f o l i a t i o n and l aye r ing and j o i n t s e t s t o each o t h e r i n t h e mountain s lope above t h e Gast ineau Avenue-Franklin S t r e e t a r e a , Juneau, Alaska---------------------------------------- 4 1

7. Fence diagram showing i n t e r p r e t a t i o n of the depth t o , and conf igura t ion o f , geologic u n i t s beneath t h e Mendenhall val ley area, based p r i m a r i l y on se i smic data-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -*- - - - - - [ i n p o c k e t l d

8. Diagrammatic ske tches showing t h e r e l a t i o n s h i p s between f ans , d e l t a s , and fan deltas-------------------------- 6 4

9. Cross s e c t i o n s A-A' and B - B ' , through p a r t s o f Juneau, Alaska, and C - C ' , through t h e no r the rn p a r t o f Douglas, Alaska, and bor ing logs o f d r i l l ho l e s shown on t h e c r o s s sections------------------------------------- [ i n pocket] ./

10. Diagrammatic ske tches showing i n t e r p r e t a t i o n s of manner o f glaciomarine depos i t i on and accumulation o f r e l a t e d depos i t s dur ing t h e t h r e e d e p o s i t i o n a l phases o f t h e glaciomarine d e p o s i t s i n t h e Juneau, Alaska, area------------------------------------------------- 86

4' / P l a t e 1. Geologic map (sheets I and 11)-------------------------- [ s epa ra t e ] J

Overlay 1. Map overlay showing p o t e n t i a l l a n d s l i d e hazards ,

0 Juneau urban a r e a , Alaska ( shee t s I and 11)--------- [separate]

2. Map overlay showing foundat ion cond i t i ons , Juneau urban a r e a , Alaska ( shee t s I and TI)---------------- [ separa te ]

. .

TABLES

Page 1. P a r t i a l l i s t of earthquakes r epo r t ed f e l t , o r poss ib ly f e l t ,

i n t h e Juneau r eg ion of A l a s k a from 1847 through 1969----[in p o c k e t ] J

2 . Abridged blodified lrlcrcalli i n t e n s i t y scale--------------------- 12

3. Relative p r o b a b i l i t y of occurrence of earthquake and s e l e c t e d llazardous geo log ic events------------------------------------ 2 1

4 . Average pcrccntage o f p a r t i c l e s passing 14-inch sc reen from se lec ted geologic units-------------------------------------- so /

5. F o s s i l s c o l l e c t e d fram massive glaciomarine d e p o s i t s i n the Juneau, Alaska, v i c i n i t y by R. D. Miller--------------------- 88

6 . Molluscan f o s s i l s c o l l e c t e d from glaciomarine d e p o s i t s i n 1958 i n t h e Juneau, Alaska, v i c i n i t y by Don J . Miller-------- 9 0

7 . Foraminifera c o l l e c t e d from glaciomarine depos i t s i n 1958 i n t h e Juneau, Alaska, v i c i n i t y by Don J . Miller------------- 9 2

Tabular Tex t - -Sur f i c i a l geology of the Juneau urban a r e a and v i c i n i t y , Alaska------------------ [in pocket] /

iii

SURFICIAL GEOLOGY OF THE J U S E A U URB.4N AREA AND VICINITY, ALASKA WITH EMPHASIS ON EARTHQUAKE AND OTHER GEOLOGIC HAZARDS

BY

Robert D . M i l l e r

INTRODUCTION

Th i s r e p o r t r e s u l t s from a geologic s tudy of s u r f i c i a l d e p o s i t s i n and near t h e Juneau urban a r e a . The i n v e s t i g a t i o n i s p a r t of an ear thquake- hazards s tudy i n Alaskan coastal conmunities by t h e U.S. Geological Sur- vey. The p r i n c i p a l o b j e c t i v e s of t h e p r o j e c t a r e t o i n v e s t i g a t e and eva lua t e t h e p o t e n t i a l geologic e f f e c t s of ear thquakes and o t h e r c a t a s t r o p h i c geologic events i n t h e Juneau a rea .

The accompanying geologic map was compiled on p a r t s of three U.S. Geo- l o g i c a l Survey quadrangles: Juneau A-2, Juneau B - 2 , and Juneau B-3. Subsurface geology shotin on t h e c r o s s s e c t i o n s o f c e r t a i n a r e a s o f Juneau and Douglas i s i n t e r p r e t e d from reco rds o f d r i l l e d t e s t h o l e s . The topography used i n cons t ruc t ing t h e c r o s s s e c t i o n s was obta ined from topographic maps prepared by t h e engineer ing f i r m of Wyller, Van Doren, and Hazard, a t a s c a l e of 1 inch = 400 f e e t , f o r t h e communities o f Juneau and Douglas. The geologic c h a r a c t e r of each u n i t shown on the map and c r o s s s e c t i o n s i s b r i e f l y descr ibed i n t h e t a b l e s and more ex tens ive ly d iscussed i n t h e t e x t of t h e r e p o r t .

• In order t o a i d p lanners , engineers , developers , and o t h e r s concerned wi th land u s e , two t r a n s p a r e n t ove r l ays a r e provided t o be used i n con- j unc t ion wi th t h e geologic map. One ove r l ay d e p i c t s areas known o r be- l i e v e d t o be s u s c e p t i b l e t o t h e e f f e c t s o f l a n d s l i d e s . The o t h e r shows an i n t e r p r e t a t i o n of t h e r e l a t i v e s u i t a b i l i t y of geologic d e p o s i t s f o r foundat ions , judged p r i n c i p a l l y from t h e expected behavior o f t h o s e de- p o s i t s dur ing a severe ear thquake. These over lays r ep re sen t t h e bes t i n t e r p r e t a t i o n s I can make of t h e probable ground response of t h e v a r i - ous geologic depos i t s . The i n t e r p r e t a t i o n s a r e based on l a b o r a t o r y t e s t s a s wel l a s on many f i e l d observa t ions of such cond i t i ons as ground s t a b i l i t y , d e n s i t y , t h i ckness , and s a t u r a t i o n . I wish t o emphasize t h a t t h e s e over lays a r e only genera l gu ide l ines f o r f u t u r e urban planning and a r e not intended t o t a k e t h e p l ace of d e t a i l e d geologic i n v e s t i g a t i o n s of s p e c i f i c s i t e s .

The map and t a b l e s accompanying t h i s r e p o r t show t h e d i s t r i b u t i o n of t h e geologic formations, some of t h e i r phys i ca l p r o p e r t i e s , u ses , and prob- a b l e r e a c t i o n s t o a severe earthquake. However, it is no t t h e purpose of t h i s s tudy t o p r e d i c t i n d e t a i l how an earthquake would affect any one p lace . The u n p r e d i c t a b i l i t y of magnitude, a c c e l e r a t i o n , d i r e c t i o n , pe r iod o f s e i smic energy, and t h e d i s t a n c e of an ear thquake e p i c e n t e r from Juneau makes an unqua l i f i ed p r e d i c t i o n of ground behavior impossible . The i n t e r p r e t a t i o n s shown i n t h e accompanying t a b l e should be regarded

as i n d i c a t i o n s of how c e r t a i n geologic d e p o s i t s w i l l behave du r ing an ear thquake. ?.!any o f t h e i n t e r p r e t a t i o n s a r e based on t h e behavior o f s i m i l a r d e p o s i t s during ear thquakes elsewhere, and o t h e r s a r e based on l abo ra to ry t e s t s of t h e phys i ca l p r o p e r t i e s of c e r t a i n d e p o s i t s ,

This r e p o r t inc ludes a l s o a t abu la r t e x t t h a t b r i e f l y summarizes t h e p e r t i n e n t c h a r a c t e r i s t i c s of each geologic formation. Both t h e t e x t and t h e t a b l e s desc r ibe f i r s t t h e geology of each formation and then in - t e r p r e t t h e probable behavior o f each formation under t h e in f luence o f a severe earthquake, o r as a consequence of man's u se of t h e formation.

Juneau i s i n southeaster11 Alaska on t h e n o r t h e a s t e r n s i d e of Gast ineau Channel, a f i o r d t h a t s e p a r a t e s Douglas I s l and from t h e mainland. Gas- t i n e a u Channel j o i n s Stephens Passage t o t h e sou theas t ; i t t e rmina te s about 8 mi les northwest from Juneau n e a r t h e mouth of t h e Mendenhdl River v a l l e y where it merges wi th Fritz Cove. West Juneau i s on Douglas I s l and nea r t h e west end of t h e b r idge from Juneau t h a t c ros ses Gast ineau Channel. The town of Douglas i s on Douglas I s l and about 2 miles south- e a s t from West Juneau,

The Juneau a r e a , a s used i n thi.5 r e p o r t , c o n s i s t s o f t h a t p a r t of t h e Grea t e r Juneau Borough t ha t a d j o i n s Gast ineau Channel, F r i t z Cove, Auke Bay, Lena Cove, and Tee Harbor, and inc ludes t h e smal l i s l a n d s i n t h e waterways. Geologic mapping was g e n e r a l l y r e s t r i c t e d t o shore and v a l l e y a r e a s , and along mountainsides t o a l t i t u d e s l e s s t han 700 f e e t above s e a l e v e l .

The coas t mountains on t h e mainland r i s e sha rp ly from t i d e w a t e r t o an a l t i t u d e of more than 3,500 f e e t a t Mount Juneau, l e s s t han 1 mi le from t h e c i t y of Juneau. About 7 o r 8 mi les eastward from t idewa te r l i e s the Juneau I c e F i e l d , which covers much of t h e high p a r t of t h e moun- t a i n s . Only one g l a c i e r , t h e Mendenhall G lac i e r , extends i n t o t h e mapped area; it l i e s at t h e head of t h e Mendenhall River v a l l e y , a v a l l e y about 4 mi les long and from about 1 t o 2 mi l e s wide.

In a d d i t i o n t o t h e Mendenhall River v a l l e y , t h e v a l l e y s of Salmon, Lemon, and F i s h C r e e k s e i t h e r a r e o r probably w i l l be used f o r urban development. O f these, :he v a l l e y s of Salmon and Lemon Creeks a l r eady con ta in homes o r o t h e r s t r u c t u r e s . F ish Creek v a l l e y probably w i l l be developed a f t e r t h e proposed road t o Cropley Lake and t h e planned s k i development a r e r e a l i t i e s ,

Low-lying shore lands provide most of t h e d e s i r a b l e homesite p r o p e r t i e s i n t h e mapped area because o f t h e s t eepness of t h e mountains. Much of t h e land along t h e mairland shore northward from Juneau t o Tee Harbor is developed a s r e s i d e n t i a l p roper ty . Urban development on t h e e a s t coas t o f Douglas I s l and has been slow and sporadic ; consequent ly, t h e r e a r e a r e a s t h e r e t h a t remain undeveloped. Lack of l a r g e water s u p p l i e s i s one of t h e causes f o r t h e slow urban growth on Douglas I s l and .

ACKNOWLEDGblENTS

F'lny o rgan iza t ions and i n d i v i d u a l s con t r ibu ted va luab le information, advice , and unpublished d a t a on va r ious a spec t s of t h e Juneau a r e a . Mr. F e l i x Toner, o f t h e engineering firm o f Toner and Nord l ing , gave per- missian t o d r i l l on h i s propcr ty and both h i s firm and t h a t of Il 'yller, Van Doren, Kil lewich, and Hazard drew on t h e i r knowledge of t h e a r e a and made t h e i r f i l e s a v a i l a b l e t o me t o provide engineering and des ign data f o r t h r Juneau a r e a . b I r . George Davidson, C i t y of Juneau, provided a ladge-sca le base map of Juneau. PIIrs. blargaret F r i t s c h , Juneau Borough Planning Departmefit, made d a t a f rom t h e i r f i l e s a v a i l a b l e t o me. Elrs. Susan H. L. Barrow, Cura tor , Alaska H i s t o r i c a l L ibrary , provided photographs t h a t helped reveal t h e l o c a t i o n and form of geologic and topo- g raph ic f e a t u r e s now obscured by bu i ld ings and manmade f i l l , and made a v a i l a b l e o l d newspaper accounts quoted i n t h i s r e p o r t . Spec ia l acknowl- edgment i s made of t h e a s s i s t a n c e provided by Ray D . M i l l e r and Robert J . Munson, Juneau D i s t r i c t , Alaska Department of Highways, and by Keith Hart, Juneau, who volunteered observa t ions and made s p e c i a l e f f o r t s t o provide samples of geologic d e p o s i t s t h a t were no t a c c e s s i b l e t o me.

C. C. Fenn, U.S. Army Corps of Engineers, helped coord ina te the t e s t - d r i l l i n g program undertaken a s p a r t of t h i s s tudy by R . S. Vel ikanje , Geologis t , U.S. Army Corps o f Engineers. L . E , Jack and K , J. Metca l f , U.S. Fores t Serv ice , provided copies o f unpublished r e p o r t s on t h e g l a c i e r and vege ta t ion i n t h e Mendenhall v a l l e y . In add i t i on , F. R. Stevens and D . N. Swanston, I n s t i t u t e of Northern Fores t ry , U.S. Fores t Serv ice , d i scussed s o i l s and d e b r i s f lows with m e bo th i n t h e o f f i c e and i n t h e f i e l d .

L. A. Yehle a s s i s t e d f o r s e v e r a l weeks i n the f i e l d , A Geological Survey mobile l abo ra to ry under t h e d i r e c t i o n of R. A. Farrow and E . E. McGregor was used f o r about 1 month t o provide s e l e c t e d s o i l t e s t s , and f o r geophysical t r a v e r s e s us ing se i smic and r e s i s t i v i t y techniques . Members of t h e Geological Survey 's o f f i c e i n Juneau provided l o g i s t i c support and made a v a i l a b l e unpublished wel l r eco rds f o r c r i t i c a l a r e a s ; i n p a r t i c u l a r I want to-acknowledge t h e a s s i s t a n c e of V. K. Berwick, C . H. Clark, C. W. Boning, J. A . McConaghy, R. L . Cartmill, L. J. Whist ler , and J e s s i e Skrzynski , D . A. Brew and A. B. Ford, U.S. Geological Survey, provided d a t a c o l l e c t e d i n connection wi th t h e i r s t u d i e s o f t h e bedrock geology of t h e Juneau reg ion , which was most he lp fu l t o me.

GLOSSARY

T x h n i c a l terms t h a t a r e used ex tens ive ly i n t h i s r e p o r t a r e de f ined he re f o r r e a d e r s who may not be f a m i l i a r wi th them.

Craep. The slow, g e n e r a l l y impercept ib le , downslope movement of e a r t h ma te r i a l .

Dim;cton. A nonsorted OT poorly s o r t e d sediment t h a t con- s is ts o f pa r t i c l e s l a rge r than sand i n a matrix o f sand, s i l t , and c l ay - s i ze pa r t i c l e s . The term i s noncommittal as t o how t h e depos i t was formed.

Dip. The angle which a bed, l a y e r , d ike , f a u l t , f i s s u r e , o r s i m i l a r p l ana r geologic feature forms wi th an imagi- '

nary h o r i z o n t a l s u r f a c e when measured a t r i g h t angle t o t o t h e s t r i k e .

Fault. A f r a c t u r e o r f r a c t u r e zone along which t h e r e has been s t r u c t u r a l displacement of t h e two s i d e s r e l a t i v e t o one another p a r a l l e l t o t h e f r a c t u r e .

FoZiation. Banding o r laminat ion of c r y s t a l l i n e rock t h a t r e s u l t e d from seg rega t ion of minera ls dur ing metamorphism o r from l ame l l a r flow.

Footing. Manmade support ing p o r t i o n s of a s t r u c t u r e , p laced on t h e foundat ion.

Foundation. Natural o r a r t i f i c i a l l y emplaced e a r t h mate- r i a l on which manmade s t r u c t u r e s axe placed.

HoZoeene. The most r e c e n t epoch i n geologic t ime; it in - c ludes t h e p re sen t . Used in te rchangeably wi th t h e term Recent. As used i n t h i s r e p o r t t h e Holocene Epoch con- sists of approximately t h e l a s t 10,000 yea r s of geologic t <me.

HypsithemaZ. The prolonged i n t e r v a l of mild c l ima te i n t h e Holocene Epoch, which s t a r t e d about 8,000 y e a r s ago and may have ended as l a t e as 3,500 years ago i n south- e a s t e r n Alaska.

InfiZtration, The slow e n t r y of water from t h e ground s u r f a c e i n t o s u ; f i c i a l d e p o s i t s o r i n t o bedrock.

Intensity. Refers t o t h e s e v e r i t y of ground motion (shaking a t a s p e c i f i c l o c a t i o n dur ing an ear thquake and is based on t h e sensa t ions of people and on v i s i b l e e f f e c t s on n a t u r a l and manmade o b j e c t s . The most widely used i n t e n s i t y s c a l e i n t h e United Stares i s t h e Modified Merca l l i i n t e n s i t y scale, (See t a b l e 2.)

Joint. A f r a c t u r e i n bedrock a lang which t h e r e has been no movement para l le l t o t h e f r a c t u r e . Movement a t r i g h t angle t o a f r a c t u r e , however, may t a k e p l a c e and produce an open j o i n t . . Av

J0Zr.t set. h grcup o f j o i n t s p a r a l l e l i n s t r i k e and d i p with each o the r .

Nagnitude. Refers t o t h e total energy r e l e a s e d a t t h e source of an ear thquake. I t i s based on se i smic r eco rds o f an earthquake a s recorded on seismographs. Unlike in - t e n s i t y , t h e r e i s only one magnitude a s soc i a t ed wi th one ear thquake. The s c a l e i s exponent ial i n c h a r a c t e r , and where appl ied t o shal low earthquakes, an i n c r e a s e o f 1 u n i t i n magnitude s i gn i f i e s approximately a 32-fold in- c r e a s e i n seismic energy r e l e a s e d .

d m t . The he ight t o which sea l e v e l formerly extended as con t r a s t ed wi th p re sen t s e a l e v e l . I t i s used t o i n d i c a t e t h e approxirnatc amount o f r e l a t i v e r i s e of t h e land s u r f a c e above p re sen t s e a l e v e l .

Muskeg. Organic-r ich d e p o s i t s c o n s i s t i n g o f pea t and o t h e r decaying vege ta t ion ; commonly found i n swamps and bogs.

Neoglaciation. An episode of r e l a t i v e l y cool climataq;,that followed t h e Hypsithermal, and which extended from about 3,500 yea r s ago i n sou theas t e rn Alaska t o t h e present,

PZeistocene. An epoch of geologic t ime cha rac t e r i zed by . .*

worldwide cool ing and by major g l a c i a t i o n s ; also c a l l e d the ' ' g l ac i a l epoch" o r Ice Age. The P le i s tocene Epoch denotes t he time from about 2 m i l l i o n t o 10,000 years ago.

Seismic! seiche. Waves set up i n a body of water by t h e passage of se i smic waves from an ear thquake, or by t i l t i n g o f a w a t e r - f i l l e d bas in .

S t r i k e . The compass d i r e c t i o n of a l i n e formed by t h e i n t e r s e c t i o n of a bed, bedding su r f ace , f r a c t u r e , f a u l t , f o l i a t i o n , o r o t h e r e s s e n t i a l l y planar f e a t u r e with a *

- . h o r i z o n t a l p lane .

Talus. Accumulated heap o f rock fragments der ived from and ly ing a t t h e base of a c l i f f o r very s teep s lope . The term a p p l i e s t o t h e body of rock fragments as a u n i t . The heap u s u a l l y has a form determined by g r a v i t y and t h e angle o f rest of the m a t e r i a l (Varnes, 1958).

Tsunami. A sea wave, otherwise known as a seismic sea wave, generated by sudden large-scale ver t ica l d i s - placement of t he ocean bottom as a result of submarine earthquakes or vo lcan ic a c t i o n . Tsunamis i n t h e open ocean a re long and low, and have speeds o f 425-600 mi l e s pe r hour . As t hey enter shallow coas ta l wate rs they can g r e a t l y increase i n wave he igh t and a l s o in he igh t of runup onto land.

FAULTS

F a u l t s r eco rd ground movement t h a t occurred i n the pas t , and i n d i c a t e t h e p o s s i b i l i t y of renewed movement i n t h e f u t u r e . Consequently, one of t h e obj e z t i v e s o f t h e f i c l d niLlpping was t o determine i f r ecen t movement on any f au l t s could be e s t a b l i s h e d i n t h e Juncau a r e a . P a r t i c u l a r a t t e n t i o n was given t o a search f o r gcomorphic f e a t u r e s t h a t might in- d i c a t e f a u l t displacement of t h e r e l a t i v e l y young d e p o s i t s of P l e i s t o - cene o r Holocene age, Broad f l a t t i s h s u r f a c e s on s u r f i c i a l d e p o s i t s and t e r r a c e s \<ere examined on t h e ground f o r evidence o f v e r t i c a l o r horizon- t a l movement, and a e r i a l p h o t o ~ r a p h s were inspec ted f o r l i n e a r f e a t u r e s i n t h e surficial m a t e r i a l s , which might i n d i c a t e f a u l t t r a c e s . In addi - t i o n , exposures of glaciomarine d e p o s i t s were examined f o r any i n t e r n a l evidence o f f a u l t movement. I have concluded t h a t i n t h e a r e a mapped no r e c e n t f a u l t a c t i v i t y has d i sp l aced s u r f a c e d e p o s i t s of P l e i s tocene o r IIolocene age. Khether t h e r e has been f a u l t movement beneath t h e waters o f Gast ineau Channel during t h a t t ime, however, i s n o t known. Thus, a l l t h e f a u l t s shown on t h e geologic map ( p l , 1, sheets I and 11) probably a r e of pxe-Pleis tocene age.

F a u l t s have been recognized i n sou theas t e rn Alaska and i n and nea r t h e Juneau a r e a f o r many years ( f i g . 1 ) . The f a u l t s shown on f i g u r e 2 have been p l o t t e d from e a r l i e r work by o t h e r s . F . E . and C. W, Wright (1908, p . 21-22; p l , I V ) proposed t h a t t h e f i o r d marked by Lynn Canal and i t s southern ex tens ion , Chatham S t r a i t , e x i s t e d because o f a major f a u l t ( s ee f i g . 1, t h i s r e p o r t ) . Th i s f i o r d , which i s only 3-6 miles wide along most of i t s o l e n g t h i s more than 250 mi les long and t r e n d s a t an angle o f about 30 t o t h e s t r i k e of t h e bedrock. Buddington and Chapin (1929, p. 291) also considered t h e Lynn Canal-Chatham S t r a i t f i o r d t o be t h e r e s u l t o f e ros ion along a f a u l t zone. S t . Amand (1954, p , 1350; 1 9 5 7 , p . 1 3 5 7 , f i g . 7) c o n c l u d e d t h a t t h e L y n n C a n a l s e g m e n t i s p a r t o f . t h e Denali f a u l t and pos tu l a t ed it t o be a s t r i k e - s l i p f a u l t wi th r i g h t - l a t e r a l movement. Th i s means, i n e f f e c t , t h a t t h e p a r t s of t h e e a r t h on d i f f e r e n t s i d e s of t h e f a u l t moved i n oppos i te d i r e c t i o n s and b a s i c a l l y i n a h o r i z o n t a l p lane , with t h e western s i d e of t h e f a u l t moving no r th - westward r e l a t i v e t o t h e e a s t e r n s i d e . He a l s o suggested t h a t t h e near - l y s t r a i g h t alignment of Cowee and Boulder Creeks (northwest of t h e mapped area) alor: wi th Windfall and Montana Creeks and Gast ineau Channel, may mark an important f a u l t branch of t h e Denali f a u l t . Twenhofel and Sainsbury (1958, f i g . 2) show an i n f e r r e d f a u l t t r end ing southeastward from t h e main Lynn Canal-Chatham Strait f a u l t through Berners Bay as t h e Gast ineau Channel f a u l t (?) ( f i g . 2 , t h i s r e p o r t ) . The t r a c e of t h i s i n f e r r e d f a u l t i s be l ieved t o be r ep re sen ted by deformed rocks i n t h e saddle i n t h e r i d g e no r theas t o f t h e Juneau a i r p o r t (C. L. Sainsbury, o r a l comrnun., 1966). Subject t o cont roversy , the e x i s t e n c e of a f a u l t a long Gast ineau Channel i s given support by se i smic d a t a t h a t show a V-shaped r a t h e r t han U-shaped v a l l e y a t depth (Gene Rusnak, w r i t t e n commun., 1967). Twenhofel and Sainsbury (1958, f i g . 2) a l s o show an in- f e r r e d f a u l t t r end ing northwest-southeast t h a t approximately b i s e c t s Douglas I s l and and fol lows F i sh Creek on t h e no r the rn p a r t o f t h e i s l a n d ( f i g . 2). Barker (1957) mapped shear zones along Windfall and Montana

Creeks, showcd a f a u l t along Peterson Creek on t h e western p a r t of D o u ~ l n s Is lancl , and showcd p o s s i b l e f a u l t s a t t h e base of and p a r a l l e l to =luke ?Tountnin, no r th of Auke Bay, and from Lena Cove t o near Point h u i s a on Auke Bay ( f i g . 2 t h i s r e p o r t ) . Brew and o t h e r s (1966, f i g . 5 - 5 ) a l s o show :I major f a u l t o r l ineament t r end ing northwest along Gas- t i : lcLiu Chmncl and blontana Creek. The s t e e p l y d ipping beds a t Sunny P o i n t a l s o may be r e l a t e d t o a f a u l t o r f a u l t s a long t h e channel . In a d J i t i o n , a water wel l d r i l l e d j u s t no r theas t o f t he bedrock r i d g e t h a t forns Sunny Poin t d i d no t p e n e t r a t e bedrock u n t i l it reached t h e depth of 1q5 f e e t (Andrct~ IIaskins, Alaska D r i l l i n g Corp., Juneau, Alaska, o r a l Cow.mm., 1965). This sugges ts t o me t h a t t h e bedrock a t t h e Sunny Point pro:nontory may be a bedrock remnant contained between f a u l t s . Spencer (1?d6, p l . IV) f i r s t mapped t h e Silverbow f a u l t a long Snowslide Gulch, ad jacen t t o Gold Creek, which t r e n d s Gastineau Channel. The f a u l t loca- t i 2 a shown on t h e geologic map, p l a t e l , shee t I , of t h i s r e p o r t , was p :~v ided by A . B. Ford and D. A . B r e w , U.S. Geological Survey. Although f j r ? ~ and o t h e r s (1966, p. 167) cons ider t h a t p o s t - T e r t i a r y movements ~ l l c n g e x i s t i n g f a u l t s i n southeas te rn Alaska helped t o develop t h e IY rssen t -day conf iguxat ion of t h e land, ev iden t ly no such movements have o c c u r r e d nea r Juneau. The n e a r e s t known example of h i s t o r i c f a u l t move- n:cr:t occurred about 100 miles west of Juneau i n 1958 (Tocher and M i l l e r , 1959; D . J . M i l l e r , 1960; Page, 1969) along t h e Fairweather f a u l t ( f i g . 1).

EARTHQUAKES

A l t h o u g h movement has no t occurred a long f a u l t s i n o r near Juneau i n historic t ime, t h e a r e a has been r epea t ed ly shaken by ear thquakes from m r e d i s t a n t ep i cen te r s . Furthermore, t h e type and d i s t r i b u t i o n of some or' t h e geologic d e p o s i t s a t Juneau l eads me t o b e l i e v e t ha t i n p r e h i s t o r i c ti=..e, t h e a r e a was s e i s m i c a l l y ac t ive . The geologic evidence i n support of t h i s conclusion includes t h e rocksl ide-avalanche d e p o s i t s and some of the o t h e r l a n d s l i d e d e p o s i t s . The rocksl ide-avalanches along lower Gold C r z e k and on Douglas Is land and the l a n d s l i d e s i n Lemon and Salmon Creeks %:J along Nugget Creek r e f l e c t c a t a s t r o p h i c events o f t h e f a i r l y r e c e n t s t Other condi t ions could have t r i g g e r e d t h e s e s l i d e s , bu t r e c u r r i n g s t l s m i c a c t i v i t y seems t o be t h e most l i k e l y cause. Because of Juneau ' s 2_7:3tion a t t h e l i s e of s t e e p mountain s lopes , t h e r e is an ever -present -; - - s . i from f a l l i n g and s l i d i n g rock. This r i s k i s g r e a t l y increased dur - .- - A*. - a severe earthquake, and hence, r o c k f a l l s should be considered i n l::z-range planning f o r urban development.

X l z h o u g h damage t o bu i ld ings from se ismic shak ing i s not s o l e l y a geo- I t . z i c - e f f e c t , t h e pe r iod and amplitude of ground motion depend on geo- l:;ic f a c t o r s . I t i s vene ra l ly known t h a t v i b r a t i o n i s more i n t e n s e i n ti:isk unconsol idated m a t e r i a l s than i n bedrock. Th i s r e p o r t i n par-t . - z - s a l s s e s t h e probable response of t h e var ious geologic m a t e r i a l s - t o t f i k

+ + a*-. - - - s e t s o f a hypo the t i ca l nearby severe ear thquake.

!te-spaper and s c i e n t i f i c accounts record t h e occurrence o f ear thquakes - - -: v a r i o u s i n t e n s i t i e s s i n c e about 1900. Table 1 i s a compilat ion of

earthquakes since 1847 t h a t va r ious sources r epo r t ed as f e l t i n Juneau o r t h a t poss ib ly should h,lve been f e l t i n t h e Juneau a r e a . Table 2 i s t h e blodified filcrctilli i n t c ~ l s i t y sca le , whicR c l a s s i f i e s t h e s e v e r i t y of ear thquakes by a nurncrical r a t i n g t h a t i s based on ground behavior , human r e a c t i o n s , and damage. On September 10 , 1899, miners working underground on Douglas I s l a n d a1:J i n t h e Silverbow Basin rushed t o t h e su r f ace a f t e r t h e s t ronges t ear thquake t o occur t h a t day a t Yakutat, Alaska (The Alaska bliner, Sep t . 16 , 1899). Tnrr and ? [ a r t i n (1912, p . 48) r e f e r t o r e p o r t s t h a t f r a c t u r e s o r furrol is occurred i n " incoherent sand f l a t s i n t h e Lynn

, and (p. 82) t o eyewitness accounts t h a t r e p o r t e d Canal reg ion * * * " ground waves, d i s h e s be ing broken, and boulders r o l l i n g down t h e moun- t a i n s i d e s from t h e shocks i n t h e Berners Bay a r e a . The d i r e c t i o n of motion and s e v e r i t y of t h e quake a t Berners Bay is suggested by a person who s t a t e d t h a t " I t seemed t o come from t h e northwest . If walking nor th- west one s taggered forlsard, and if walking no r theas t one s taggered s i d e - wise." (Tarr and Mart in , 1912, p. 8 2 ) . Another eyewitness r epo r t ed tha t i cebe rgs f i l l e d t h e nearby waterways of Taku I n l e t and Stephens Passage, as well a s Gast ineau Channel, from t h e s h a t t e r i n g of Taku Glac i e r . Buildings along t h e Juneau water f ront shook and swayed and windows rat- t l e d dur ing an ear thquake i n 1909 (Daily Alaska Dispatch, Feb. 16, 1909). A l a r g e ear thquake t h a t occurred February 12, 1934, on t h e nor thern p a r t of Admiralty I s l and ( f i g . 3) had an i n t e n s i t y of 5 a t Juneau; it caused t h e rock dump a t t h e A - J mill t o s e t t l e i n p l aces and t i pped over a s t a c k e r . An earthquake shook Juneau on September 2 3 , 1934, and caused f u r n i t u r e t o move (Davis and Echols, 1962, no. 224). Another ear thquake shook bu i ld ings i n Juneau on September 27, 1947 (Davis and Echols, 1962, no. 246). As a r e s u l t of t h e ear thquake along t h e Fairweather f a u l t near I cy Poin t on J u l y 10 , 1958, Juneau shook and merchandise f e l l t o t h e f l o o r , minor r o c k s l i d e s occu~red i n t h e h ighland p a r t of town, and people f l e d t o t h e s t r e e t s a s t h e 12-s tory Mendenhall apartment b u i l d i n g and p r i v a t e homes swayed (Daily Alaska Empire, J u l y 10, 1958). A s a r e s u l t o f t h e g r e a t Alaska earthquake on March 27, 1964, a f l o a t p lane on t h e water i n t h e harbor a t Douglas f l i p p e d over "as a r e s u l t o f un- u sua l t i d a l ac t ion" (Juneau-Daily Alaska Empire, Mar, 29, 1964). Von Hake and Cloud (1966, p. 54) s t a t e t h a t t h e 1964 ear thquake was f e l t p r i n c i p a l l y i n t h e northwest p a r t of Juneau, e s p e c i a l l y i n t h e Gast ineau Channel a r e a , bu t with " the heav ie s t r o l l i n g shocks appa ren t ly concen- t r a t e d i n t h e Mendenhall va l ley ."

P r o b a b i l i t y

The h i s t ~ r i c a l record i n d i c a t e s t h a t ear thquakes s t r o n g enough t o a f f e c t Juneau most l i k e l y would occur along t h e Fairweather-Queen C h a r l o t t e I s l a n d s f a u l t , and t h a t t h e i r d i r e c t e f f e c t s on Juneau would depend on t h e e p i c e n t s a l d i s t a n c e , magnitude, and f o c a l depth and on t h e acce l e ra - t i o n , amplitude of t h e ground waves, and du ra t ion of shaking a t Juneau. A long du ra t ion of shaking even from an ear thquake of f a i r l y low magnitude can result i n g r e a t e r damage than one might p r e d i c t from the magnitude alone.

Table 2 . - -Ah~vXged / t !o~l i f i sd MercaZZi intensity s c a l ~ d

I Detected on ly by s ens i t i ve ins t ruments

I1 Felt by a f e w p e r s o n s a t r e s t , e s p e c i a l l y on upper f l o o r s ; delicate suspended o b j e c t s may swing

111 F e l t no t i ceab ly indoors , bu t no t always recognized a s a quake; s t and ing a u t o s rock s l i g h t l y , v i b r a t i o n l i k e pas s ing t r u c k

I V F e l t indoors by manv, outdoors by a few; a t n i g h t some ,

awaken; d i s h e s , ~ . ~ i n d o w s , doors d i s t u r b e d ; motar cars rock no t i ceab ly

V F e l t by most people; some b r e a k a g e of d i shes , windows, and p l a s t e r ; d i s turbance of t a l l o b j c c t s

V L F e l t by a l l ; many f r i gh t ened and run outdoors ; falling p l a s t e r and chinineys; damage smal l

V I I Everybody runs outdoors; damage t o b u i l d i n g s v a r i e s , depending on q u a l i t y of c o n s t r u c t i o n ; no t i ced by d r i v e r s of cars

V I I I Panel w a l l s thrown ou t o f f rames; f a l l of w a l l s , monuments, chimneys; sand and mud e jec ted; d r i v e r s of a u t o s d i s t u r b e d

IX Buildings s h i f t e d o f f foundat ions , cracked, thrown out of plumb; ground cracked; underground p ipes broken

X Most masonry and frame s t r u c t u r e s des t royed; ground cracked; rails ben t ; l a n d s l i d e s

XI New s t r u c t u r e s remain s t and ing ; b r idges des t royed; f i s s u r e s i n ground; p ipes broken; l a n d s l i d e s ; r a i l s bent

X I 1 Damage total; waves seen on ground s u r f a c e ; l i n e s of s i g h t and l e v e l d i s t o r t e d ; o b j e c t s thrown up i n t o a i r

L

A b r i d g e d from Wood and Newnann (1931)

P r e d i c t i o n of t h e s i z e and l o c a t i o n of f u t u r e ear thquakes and t h e i r e f f e c t s i s d i f f i c u l t and tenuous. F igure 3 shows t h e l o c a t i o n o f epi- c e n t e r s and :~pproxirnnte magnitude o f most of t h e recorded ear thquakes t h a t occurred between 1899 and 1969 i n , o r ad jacent t o , sou theas t e rn A l a s k a . Circ les of 50- and 100-n i le r a d i i g i v e some i d e a of t h e h i s t o r i - c a l occurrcnccs o f e a r t h q u a k e s i n l oca t ion and magnitude wi th in a d i s - t ance t h a t could s t r o n g l y a f f e c t t h e Juneau a rea . Earthquakes beyond t h e 100-mile c i r c l e probably ~ o u l d not s e r i o u s l y a f f e c t Juneau. I t i s i n t e r e s t i n g t o no te t h a t on ly two recorded ear thquakes o r i g i n a t e d with- i n 50 mi lcs of Ju:leau s i n c e a t l e a s t t h e t u r n of t h e century . The ep i - c e n t e r o f one o f these earthqucikes, which occurred on January 20, 1964, l i e s about 25 miles no r theas t o f Juneau. Although t h i s ear thquake was not f e l t at Juneau, it was reported by the In te rna t iona l Seisrno1ogica.J- Centre at Edinburgh (1967). A s noted on f i g u r e 3 , t h e optimum accuracy of t h e e p i c e n t r a l l o c a t i o n s i s wi th in 10-15 mi l e s , bu t some o f t h e epi- c e n t e r s of t h e e a r l y 1 4 0 0 ' s may be misplaced a s much as 50 miles o r more; n e v e r t h e l e s s , t h i s 1964 ear thquake apparent ly o r i g i n a t e d c l o s e r t o Juneau than any o the r k n o ~ n a t present.

An at tempt t o p r e d i c t s t a t i s t i c a l l y t h e p r o b a b i l i t y , o r r ecu r rence i n - t e r v a l s , o f ear thquakes t h a t might a f f e c t t h e Juneau a r e a o r t h e i r po- t e n t i a l magnitudes can be based only on h i s t o r i c a l r eco rds , and a t Juneau t h o s e records extend back only t o n e a r t h e t u r n o f t h e century . A r eco rd o f no l e s s t han seve ra l c e n t u r i e s i s regarded by Lomnitz (1967) a s nec- e s s a r y f o r computer p r e d i c t i o n s of earthquake recur rence i n t e r v a l s . In an at tempt t o provide a r e sea rch guide f o r des ign and insurance u s e s , Milne (1967) compiled a l l of the recorded ear thquakes i n Canada. One r e - s u l t of t h e s tudy was a computer p r i n t - o u t based on r eco rds from 1898 through 1960 t h a t provided t h e d a t a f o r compilat ion of a s t r a i n - r e l e a s e map designed p r i n c i p a l l y f o r western Canada, bu t inc luding c o a s t a l Alaska (Milne, 1967, f i g . 11, p . 809). On t h i s map energy r e l e a s e is shown by contours numbered from 0 t o 6 . Because each contour i s based on t h e maximum s t r a i n t h a t has been r e l e a s e d i n t h e h i s t o r i c a l p a s t , t h e contours can be used t o suggest t he p o t e n t i a l s i ze of f u t u r e ear thquakes. Juneau l i e s between t h e 0 and 1 contours on Milne 's s t r a i n - r e l e a s e map. Accord- ing t o Milne (1967, t a b l e 11,' p. 805) t h e 0 contour r e q u i r e s a magnitude 3 . 7 earthquake once every 100 yea r s t o r e l e a s e t h e accumulated s t r a i n , and t h e 1 contour i n d i c a t e s r h a t a magnitude 5 ear thquake every L O O yea r s i s needed t o t o t a l l y r e l e a s e t h e s t r a i n . Earthquakes of t h e s e magnitudes a r e gene ra l ly not considered t o be d e s t r u c t i v e i n a r e a s of well-designed and p rope r ly b u i l t s t r u c t u r e s . These magnitudes a r e t h e o r e t i c a l l y t h e 2 most severe earthquake a n t i c i p a t e d pe r 10,000 km per 100 yea r s , based on t h e h i s t o r i c a l d a t a a v a i l a b l e t o blilne. I t should be poin ted o u t , however, t h a t t h e h ighes t s t r a i n - r e l e a s e contour p l o t t e d , t h e 6 contour , i s only about 80 mi les from Juneau where it extends n o r t h from a p o i n t south of I cy Point (shown on f i g . 1). This s t r a i n - r e l e a s e contour i n d i - c a t e s a magnitude 7 .7 ear thquake a s t h e t h e o r e t i c a l maximum; more even t s o f lesser magnitude would be needed t o r e l e a s e t h e accumulated s t r a in . For example, 3.1 events o f magnitude 7 o r 100 even t s o f magnitude S would be needed t o r e l e a s e t h e s t r a i n per 100 years . The Icy Poin t a r e a was near t h e e p i c e n t e r of t h e 1958 Fairweather f a u l t ear thquake t h a t s o d rama t i ca l ly a f f e c t e d L i t u y a Bay ( D . J . M i l l e r , 1960); t h i s ear thquake had a magnitude of about 8 (see f i g . 3 ) . '

Some workers be l i eve t h a t t h e se i smic a c t i v i t y i n t h i s p a r t o f south- e a s t e r n Alsska has moved westward, away from t h e Lynn Canal-Chatham S t r a i t f a u l t dur ing P le i s tocene and ~ o l o c e n e ' t imes . Lemke and Yehle (19723) sum~ilarize t h e rcgiorrril t e c t o n i c s concerning southeas te rn Alaska i n more detail t h a n t h i s paper permi ts . B r i e f l y , t h e t e c t o n i c t h ink ing r e l a t i v e t o t h e Juneau a r e a i s r e f l e c t e d i n t h e fol lowing s ta tements . Grantz (1966, p . 5 2 , 76) sugges ts t h a t t h e main se i smic a c t i v i t y i n sou theas t e rn Alaska now occurs along t h e Fairweather-Queen C h a r l o t t e I s l ands f a u l t , and t h e r e l a t e d Chugach-St. E l i a s f a u l t ( f i g . 1 ) . R ich te r and Matson (1971, p . 15333 cons ider t h a t "Movement, and e s p e c i a l l y l a t e r - a l moverntznt, along t h a t p a r t of t h e Denali f a u l t southeas t of t h e Tot- schunda f a u l t system and t h e remainder of t h e f a u l t s i n t h e Denal i system extending i n t o Canada and sout'heast Alaska may have been n e g l i g i b l e s i n c e middle P le i s tocene time." Figure 3 shows t h e concent ra t ion o f se i smic a c t i v i t y west and northwest o f Juneau, near t h e coast and i n t h e areas t r a v e r s e d by t h e Fairweather and Chugach-St. E l i a s f a u l t s , shown on f i g u r e 1.

The se i smic s t a t u s of t h e sou theas t p a r t o f t h e Denal i f a u l t system, however, remains unresolved. Although t h e Lynn Canal-Chatham S t r a i t f a u l t has shown no d e t e c t a b l e se i smic a c t i v i t y dur ing t h e p a s t 60 yea r s , according t o Tobin and Sykes (1968, p. 38391, o r dur ing r ecen t micro- ear thquake s t u d i e s by Rogers (1972, p. 2261, i n t e r p r e t a t i o n s d i f f e r as t o t h e meaning o f t h i s lack of a c t i v i t y . R ich te r and Matson (1971, p . 1534) be l i eve t h a t t h e Totschunda f a u l t system shows Holocene r i g h t - l a t e ra l displacement; this, coupled with lack of evidence o f such rnove- ment sou theas t of t h e Denali-Totschunda junc t ion , sugges ts t o them t h a t t h e Totschunda f a u l t system may extend t o t h e Fairweather f a u l t , and se i smic a c t i v i t y may be bypassing t h e sou theas t e rn p a r t o f t h e Denali f a u l t , i nc lud ing t h e Lynn Canal-Chatham S t r a i t f a u l t . Evaluat ing t h e absence of se i smic a c t i v i t y , Tobin and Sykes (1968, p. 3840) suggest t h a t prolonged q u i e t per iods over par t of an o therwise a c t i v e ear thquake b e l t might be a guide t o t h e accumulation of s t r a in which might be re- l ea sed suddenly a s an ear thquake. Boucher and F i t c h (1969, p . 66481, summarizing t h e i r microearthquake s e i s m i c i t y s t u d i e s along t h e Denal i f a u l t system, s t a t e t h a t " * * * t h e r e s u l t s of t h i s s tudy i n d i c a t e t h a t t h e Deanli f a u l t i s a c t i v e i n some sense along i t s e n t i r e l e n g t h e a s t of Mount bIcKinley, t h e westernmost p o i n t v i s i t e d i n t h i s survey, and it should probably no t be dismissed a s a r e l i c f a u l t o f no cu r r en t t e c t o n i c importance." On t h e b a s i s of t h e s e s ta tements , I f e e l t h a t t h e Lynn Canal-Chatham S t r a i t f a u l t cannot be ignored merely because of t h e lack of h i s t o r i c a l ear thquakes having e p i c e n t e r s r e l a t e d t o it.

Attempts have been made by o t h e r s t o zone p o r t i o n s of t h e e a r t h ' s s u r - f a c e according t o t h e probable maximum magnitudes of ear thquakes t h a t might a f f e c t t h e va r ious a r e a s . The purpose of such zoning i s t o a id i n development of design c r i t e r i a and insurance r a t e s , and i s not t o a i d i n p r e d i c t i o n of t h e specif ic s i z e o r frequency of earthquakes. A se is - mic zone map ( f i g . 4 ) from t h e 1970 e d i t i o n of t h e Uniform Building Code ( I n t e r n a t l . Conf. Building O f f i c i a l s , 1970) p l a c e s Juneau i n zone 2 , a zone where the l a r g e s t expec tab le ear thquakes would have magnitudes of

between 4 . 5 and 6 and where moderate damage t o manmade s t r u c t u r e s is p o s s i b l e . On t h e ot l ler hand, f i g u r e 5 of t h i s r e p o r t i s a se i smic prob- a b i l i t y zone map c u r r e n t l y used by t h e U.S. Army Corps of Engineers f o r des ign requirements (Alaska D i s t r i c t , w r i t t e n commun., June 13 , 1968). On t h i s map Juneau l i e s lc i thin zone 3, which c o n s i s t s of t h e arca i n which an ear thquake g r e a t e r rhnn magnitude 6 . 0 might occur and where major daniage t o manmade s t r u c t u r e s might occur . Thus, t h i s map p l aces Juneau i n a cons iderably h igher ca tegory of r i s k than e i t h e r Mi lne ' s (1967) s t r a i n - r e l e a s e map o r t h e se i smic zone map i n t h e Uniform Build- i ng Code. The h igher r i s k assignment by t he Corps of Engineers seems reasonable t o me, u n t i l t h e se i smic a c t i v i t y , o r lack of i t , i n t h e Lynn Canal arca i s b e t t e r understood. The hazard eva lua t ions t h a t a r e d iscussed f o r t h e va r ious geologic d e p o s i t s a r e based upon t h e assumption t h a t a magnitude 6 o r s t ronge r ear thquake could occur i n t h e Juneau a r e a . A magnitude event with an e p i c e n t e r wi th in 10 mi l e s of Juneau could cause more shaking and r e s u l t i n g damage than a much s t r o n g e r ear thquake 100 mi l e s away. Thus, i n t h e t a b u l a r t e x t t h e column t i t l e d "Probable ground rcsponse t o a severe earthquake" d i s c u s s e s t h e probabZe r e a c t i o n t o a seve re ear thquake on t h e premise t h a t , if such an ear thquake occu r s , p a r t o r a l l o f t h e responses probably w i l l occur .

TSUNAMI S

The p o s s i b i l i t y t h a t t h e Juneau area i s s u s c e p t i b l e t o tsunamis, o r se i smic sea waves, must be considered. I t i s h igh ly u n l i k e l y t h a t t s u - namis from t h e ocean would cause s p e c t a c u l a r l y damaging e f f e c t s i n t h e Juneau a rea , such a s t hose experienced elsewhere by t h e tsunamis caused by t h e 1964 Alaska ear thquake. The previous d i scuss ion of f a u l t s p o i n t s out t ha t t h e Lynn Canal-Chatham S t r a i t f a u l t ( f i g . 1) is probably a s t r i k e - s l i p f a u l t t h a t had r i g h t - l a t e r a l movement ( see p. 7 ) . Tsunamis are not known t o be caused by such h o r i z o n t a l f a u l t movements; v e r t i c a l displacement is considered necessary f o r t h e i r gene ra t ion (P la fke r , 1969, p. 138). Consequently, s t r i k e - s l i p movement along t h e Lynn Canal-Chatham S t r a i t f a u l t probably would not cause a tsunami. However, P l a f k e r (1969, p. 139) mentions t h a t unusual water d i s tu rbances i n l akes , f i o r d s , and r i v e r s not p h y s i c a l l y r e l a t e d t o t h e e p i c e n t e r of t h e 1964 Alaska e a r t h - quake" * * * may have been genera ted by i n e r t i a l e f f e c t s of t h e water bodies as t h e land mass was d i sp l aced h o r i z o n t a l l y beneath them. Hori- zonta l movement of a deep s teep-s ided bas in o r f i o r d , i f it occurred f a s t enough, would be expected t o impart p o t e n t i a l energy t o a contained water mass by changing i t s s u r f a c e conf igu ra t ion * * *. Thus, because o f i t s i n e r t i a , water would t end t o p i l e up above i t s o r i g i n a l l e v e l along shores oppos i te t o t h e d i r e c t i o n o f displacement , and it would s imul taneous ly be lowered along shores i n t h e d i r e c t i o n of displacement." P l a f k e r s t a t e s f u r t h e r "For a given amoun-t: and r a t e o f displacement , t h e e f f e c t of h o r i z o n t a l movement on t h e water mass would be p r o p o r t i o n a l l y g r e a t e s t where o r i e n t a t i o n of shores i s normal t o t h e d i r e c t i o n o f ho- r i z o n t a l movement and r e l a t i v e l y s teep bas in s i d e s permi t ted t h e maximum energy t o be t r a n s f e r r e d from t h e bas in t o t h e water mass."

I n c f f c c t , t h c n , t i i c J u ~ l e a u n c a cannot bc considered f r e c from abnormal- l y high and d e s t r u c t i v e water waves i f , i n t h e f u t u r e , t h e r e i s a s eve re ear thquake c e ~ l t c r e d alorlg the Lynn Canal-Chatham S t r a i t f a u l t . Although the preceding d i scuss ions of f a u l t s suggest t h a t t h e Lynn Canal-Chatham S t r a i t f?,ult mn- no 1or:;:r bc a c t i v e , t h o h i s t o r i c a l se i smic record f o r t h i s clrc:l i s s o ~ h o r t , 21:d t h e ca l - l ab i l i ty of p r e d i c t i ~ l g csr thquakcs and t h e i r e f f e c t s so unce r r s in , t h a t recoyni t ion of t h c p o s s i b i l i t y of abnormal water waves occurr ing i n t h e a r ; ~ Is o n l y prudc.nt. The National Ocean Survey maintains a n c t o f obscl'i.:~;icn s t s t i o n s t ha t permit warnings t o t h e r c s i d c l ~ t s o f area.: cxpccted t o be a f f c c t e d by tsunamis from d i s - t a n t sources , but only t h e ~ n d i v i d u a l s ' awareness o f t h e p o t e n t i a l danger from a11norr:ral w ~ t t u r \iLt\.es caused by r e l a t i v e l y nea r s eve re ear thquakes can he lp prevent damage from such waves.

SUbBLARY EVIII~UAT I O N OF RELATIVE P R O B A B I L I T Y OF OCCUKKEXCE OF CERTAIN HAZ+ARDOUS GEOLOGIC EVENTS

The foregoing d i scuss ions make it obvious t h a t t h e r e a r e no c r i t e r i a e s t a b l i s h e d a t t h e present time t h a t w i l l r evea l t h e s p e c i f i c form, t ime, o r p l ace of occurrence of hazardous gcologic even t s . Judgments on my p a r t , however, based p a r t l y on q u a n t i t a t i v e t e s t s and p a r t l y on s u b j e c t i v e reasoning as a r e s u l t of s tudying t h e geologic m a t e r i a l s i n t h e Juneau a r e a , place se lec ted gcologic hazards i n t o f i v e c a t e g o r i e s o f r e l a t i v e p r o b a b i l i t y of occurrence. The general range of p r o b a b i l i t y from almost impossible t o almost ce r ta in i s a r b i t r a r i l y i nd ica t ed by t h e numbers 1 t o 5, r e s p e c t i v e l y . Within t h e d e t a i l e d d i scuss ions t h a t fo l low i n t h e remaindexof therepor t c e r t a i n hazards a r e d iscussed regard ing t h e i r r e l a - t i o n s h i p s t o each geologic formation. Table 3 l is ts what i n my judgment are t h e r e l a t i v e p r o b a b i l i t i e s of occurrence of s e l e c t e d geologic events .

GEOMORPHOLOGY

Phys iographica l ly , t h e Juneau a r e a c o n s i s t s of three uni ts--mountains , c o a s t a l benches along t h e f i o r d s and bays, and f l o o r s of s t ream agd r i v e r v a l l e y s . The s lopes of t h o mountains a r e gene ra l ly s t eep ; 35'-45 s l o p e s a r e p reva len t , but even s t e e p e r s lopes a r e common. The s t e e p s l o p e s merge into more g e n t l e s lopes n e a r s e a l e v e l along t h e f i o r d s , s o t h e s e v a l l e y s have t h e appearance o f having r e c e n t l y been g l a c i a l l y shaped and smoothed. The U-shape, however, was developed as a r e s u l t of t h e lower mountain s lopes being covered by v a l l e y - f i l l i n g s u r f i c i a l d e p o s i t s . These deposi ts f i l l t h e deep bedrock-walled f i o r d conta in ing Gast ineau Channel and provide t h e v a l l e y with a f l a t f l o o r . G l a c i a l i c e d i d smooth a t l e a s t t h e upper pa r t of t h e f i o r d wa l l s , but se ismic d a t a i n - d i c a t e t h a t t h e mountainsides cont inue downward a t t h e same s lope ang le t o form a V-shaped bedrock t rough a t depth (Gene Rusnak, w r i t t e n cornmun., 1967). The o r i g i n a l f l o o r of t h e v a l l e y i s shotm t o be as much as 600 feet below t h e f l o o r of t h e modern channel.

Well-defined and prominent topographic benches extend from south of the town of Douglas northward t o Outer Po in t . Two s u r f a c e s s epa ra t ed by bedrock r i d g e s o r knobs t h a t p r o j e c t through t h e s u r f i c i a l d e p o s i t s

" + "

Table 3 . --;-<s ~ z ~ ~ c pll;,-s, s s j r f g C ~ : i y p g i . l c ; ' ~ o,'' i:C:~"::.::lIU;;,.;: zn'j L':3ze:?'?L?~L:cl'

huznrdous gmZogic events in t h e Juneau arleu vi-tirix 230 y~,::.rr,~s

Eart h ,pakss ~ o i : ; . b - ~ ~ i t ~ d

Earthquake of c::li:nitude 6 o r g rea te r w i t 1 1 ep i cen te r a t Juneau-------------------------------------------------- 1

Earthquake o f magnitude 6 o r g r c a t e r wi th e p i c e n t e r w i t h i n 50 r : i les J; .~pnu-------------------------------- 3

E a r t h q r ~ t ~ k e of : ; .2g~~i t : id2 6 o r g rea te r with e p i c e n t e r t i i t I z i n 100 ljlilcs of jurleau ------------- -- ----- ----------- 5

T ; ~ E of hazard

Movement along faults in Juneau area----------------------- 1

Massive l a n d s l i d e s i n glacioniarine d e p o s i t s s i m i l a r t o l a n d s l i d e s t h a t occurred i n t h e Bootlegger Cove Clay i n t h e r\ncl~orage area d u r i ~ g t h e Elarch 1964 earthquakc----------------------------------------------- 1

Delta-front slides i n t o water as r e s u l t o f e a r t h - quake, causing waves wi th r a p i d runups i n excess of 5 feet------------------------------------------------ 3

Tsunamis i n Gastineau Channel with rapid runups in excess of 5 feet----------------------------------------- 2

Tsunamis i n Lena Cove, Auke Bay, F r i t z Cove, Tee Harbor, and along North Douglas Island and rapid runups i n excess of 5 feet------------------------------- 3

Debris f lows along e x i s t i n g o r new channels on mountain s lope above t h e Gastineau Avenue-Franklin S t r e e t area---------------------------------------------- 5

Massive rocks l ide-ava lanches along mountain fronts--------- 4

I s o l a t e d r o c k f a l l s along e x i s t i n g t a l u s cones, and as unexpected occurrences elsewhere----------------------- 5

Damage from severe shaking causcd by ear thquake of magnitude 6 o r g r c a t e r wi th e p i c e n t e r w i th in 100 miles of Juneau------------------------------------------ 3

Compaction and se t t l emen t of wa te r - sa tu ra t ed depos- i t s from shaking o f ground i n response t o e a r t h - quake of magnitude 6 o r g r e a t e r with epicenter within 100 m i l e s of Juneau------------------------------- 3

Q 1 rn'? :, u p r o b a b i l i t y ranges from 1 fimpo:siblc) t o 5 (almost c e r t a i n )

provide a s t a i r s t e p appearance t o t h e lower s l o p e s of t h e mountains on Douglas I s l a n d along Gastincnu Channel. The lower s u r f a c e extends t o a l t i t ~ i d c s of about 200 feet above sea l e v e l . . These benches a r e t h e re- s u l t o f depos i t i on of subaqueous sediments over anc ien t wave-cut surfaces on bedrock followed by u p l i f t o f t h e land as t h e weight of mel t ing g l a c i e r s decreased.

The l a r g e t r i b u t a r y s t reams on t h e mainland, such as Salmon, Lemon, and Montana Creeks, genera l ly have broad evenly s lop ing a l l u v i u m - f i l l e d v a l l e y s a t t h e i r lower ends . Upstream, however, t h e s t reams flow through narrow bedrock gorges, which more o r l e s s mark t h e p re sen t l i m i t o f p o s s i b l e u rban iza t ion .

GEOLOGY INTERPRETATIONS

The fol lowing d i scuss ion c o n s i s t s p r i m a r i l y of in format ion supplementary t o t h a t presented i n t a b u l a r form. For t h i s reason , t h e geologic forma- t i o n s a r e d iscussed i n t h e same o rde r i n which t h e y appear i n t h e t a b l e .

The d i s t r i b u t i o n and n a t u r e o f t h e s u r f i c i a l d e p o s i t s i n t h e v i c i n i t y of Juneau are shown on p l a t e 1, s h e e t s I and 11. Regional bedrock s t u d i e s c u r r e n t l y are being made by D . A. Brew and A. B. Ford, U.S. Geological Survey; consequent ly, f o r t h e purposes of t h i s r e p o r t t h e bedrock i s u n d i f f e r e n t i a t e d on t h e accompanying geglogic map. Southeas te rn Alaska, which l i e s wi th in an a c t i v e t e c t o n i c b e l t t h a t extends around t h e P a c i f i c Ocean, has been t e c t o n i c a l l y a c t i v e s i n c e t h e e a r l y Paleozoic. I t was s u b j e c t t o "intermittent marine d e t r i t a l c l a s t i c , carbonate , and vo lcan ic depos i t i on from e a r l y Paleozoic through l a t e Mesozoic t imett (Brew and o t h e r s , 1966, p. 149). The Juneau area i s immediately unde r l a in by l aye red greenstone, graywacke, s l a t e , g reensch i s t , and metavolcanic f low b r e c c i a t h a t a r e "mostly of Mesozoic age, perhaps as young as Ea r ly Cre- taceous" (Loney and o t h e r s , 1967, p . 521). These rocks l i e exposed where Quaternary g l a c i e r s have scraped and removed r e s i d u a l s o i l s from a long t h e mountain s lopes . A t lower a l t i t u d e s , however, t h e bedrock i s g e n e r a l l y obscured by over ly ing unconsol idated m a t e r i a l s of l a t e P l e i s tocene and Holocene age.

The unconsol ida ted m a t e r i a l s of Quaternary age are subdivided i n t h i s r e p o r t i n t o groups of depos i t s of s i m i l a r o r i g i n s , though p o s s i b l y of d i f f e r e n t ages. These groups a r e manmade f i l l , muskeg, mass-wasting d e p o s i t s , g l a c i a l d e p o s i t s , a l l u v i a l d e p o s i t s , d e l t a i c d e p o s i t s , beach d e p o s i t s , marine d e p o s i t s , and glaciomarine d e p o s i t s . With t h e excep- t i o n of t h e f i r s t f i v e c a t e g o r i e s named, a l l o r p a r t o f t h e o t h e r deposits o r i g i n a t e d because of depos i t i on r e l a t e d t o changes i n sea l e v e l r e s u l t i n g f i r s t from t h e depress ion and then from t h e subsequent rebound of land owing t o t h e advance and r e t r e a t , r e s p e c t i v e l y , o f t h e l a s t widespread g l a c i a t i o n i n sou theas t e rn Alaska.

Sometime p r i o r t o 12,000 years ago, t h e land was depressed a t l e a s t 500 f e e t and, l o c a l l y , a s much a s 700 f e e t below modern sea l e v e l i n t h e

Juneau region. Micro- and macrofoss i l s i n t h e glaciomarine d e p o s i t s , and radiocarbon da re s determined from them, provide t h e evidence and t ime of such submergence. Pebbles , cobbles , , and boulders , i n a sandy mat r ix conta in ing t h e s h e l l s , c o n s t i t u t e much o f t h e glaciomarine depos- i t s preserved i n the Juneau area. Deltas and beaches accumulated from streams and t i d a l ~ i a t e r s ; t hese are preserved today as d e p o s i t s r a i s e d s e v e r a l hundred f e e t above modern s e a l e v e l ,

Holocene m a t e r i a l s i n t h e Juneau a rea , as shown on t h e geologic map, a r e both g l a c i a l and nonglac ia l i n o r i g i n . I c e - l a i d as well a s f l u v i a l de- p o s i t s r ep re sen t t h e m a t e r i a l s t h a t accumulated as a r e s u l t o f t h e presence o f piedmont o r v a l l e y g l a c i e r s . These d e p o s i t s a r e found along major s t reams on t h e mainland and Douglas I s land . Nonglacial d e p o s i t s inc lude al luvium underlying f lood p l a i n s and t e r r a c e s , and i n d e l t a s b u i l d i n g i n t o blendenhall and Auke Lakes, and i n t o Gast ineau Channel, Beach and b a r depos i t s occur i n some p l aces along t h e shores of t h e channel , bays, and coves. I n t e r t i d a l m a t e r i a l s are along t he p r e s e n t shores i n arcas of moderate o r weak o f f sho re c u r r e n t s , and a l s o u n d e r l i e s u r f a c e s l i f t e d s l i g h t l y above h igh t i d e l e v e l s i n c e t h e t u r n o f t h e century . bluskeg o v e r l i e s and obscures p o r t i o n s of some of t h e P l e i s tocene and Holocene d e p o s i t s i n t he Juneau a r e a .

Deposi ts accumulated from mass-wasting processes a r e widespread and a r e evidence o f a continued p o t e n t i a l major geologic r i s k i n t h e Juneau a r e a . Lands l ides t h a t inc lude r o c k s l i d e avalanches and d e b r i s f lows, and accu- mulat ions o f talus near t h e base of s l o p e s suggest t h a t uns t ab le mountain s l o p e s surround t h e Juneau urban a reas . Loose and weathered rock, and r e s i d u a l d e b r i s , l i e on s t e e p s lopes awaiting Oome t r i g g e r t o send them moving toward the f o o t of t h e s lopes .

Bedrock

Bedrock, shown on map as u n i t (b ) , of t h e Juneau a r e a inc ludes layered greenstone, graywacke, s l a t e , g r e c n s c h i s t , and metavolcanic flow b r e c c i a (Knopf, 1912; Buddington and Chapin, 1929; Barker, 1957; and D . A . Brew, w r i t t e n commun., 1965). Upvalley from t h e Mendenhall G lac i e r and beyond my mapped a r e a is one major source for t h e p i e c e s of metamorphic s c h i s t s , gne i s se s , and a coarse-grained hornblende qua r t z d i o r i t e contained i n t h e s u r f i c i a l d e p o s i t s (D. A. Brew, w r i t t e n commun., 1965). A s i m i l a r s u i t e o f rocks, e a s t o f Juneau, l i e s ad jacent t o o r i n t h e Gold Creek drainage (Sainsbury, 1953).

The age of t h e layered rocks i n t h e Juneau r eg ion ranges from late T r i a s s i c t o Ea r ly Cretaceous (P la fke r , 1962, p l . 10; Brew and o thers , 1966, f i g s . 8-2 , 8-10, and 8-11). The i n t r u s i v e rocks e a s t o f t h e a r e a mapped i n t h i s r e p o r t a re of Ear ly Cretaceous age (Brew and o t h e r s , 1966, fig. 3 and p. 153) and Eocene age (Forbes and Engels, 1970, p. 583).

Two principal rock groups are present on t h e mainland and Douglas I s l and , according t o Buddington and Chapin (1929, p l . I) . Graywacke, s l a t e , and conglomerate extend along t h e e n t i r e length of t he e a s t e r n s i d e o f

Douglas I s l and and along t h e mainland northwestward from Juneau. Green- s t o n e and p reensch i s t (green p h y l l i t e ) are interbedded with black and ?-ray s l a t y p h y l l i t e along t h e mainland shore of Gast ineau Channel south of Juneau. The i s l a n d s i n Auke Bay and F r i t z Cove and t h e mainland n o r t h from Auke Bay have been mapped as augi te -bear ing vo lcan ic flow b r e c c i a of J u r a s s i c ( ? ) t o Cretaceous (?) age by Barker (19.57).

The bedrock t ends t o form n e a r l y v e r t i c a l bluffs along shores and moder- a t e l y s t e e p s lopes along much o f t h e mountainsides. Flass wast ing and heavy ~ a i n f a l l have removed most of t h e g l a c i a l d e p o s i t s from t h e s e s lopes so t h a t rock i s gene ra l ly a t t h e su r f ace a t most p l aces above 500 f e e t above s e a l e v e l . Steep t o v e r t i c a l a lcoves are common on s l o p e s above a r e a s of r o c k f a l l s , rock avalanches, and some t a l u s cones,

The phys ica l p r o p e r t i e s of t h e bedrock in f luence t h e s t a b i l i t y of t h e mountain s l o p e s and u s e of t h e land a t the base of t h e mountains. Sev- e r a l broad g e n e r a l i z a t i o n s may c a l l a t t e n t i o n t o what I cons ide r t o be c r i t i c a l a s p e c t s of land use along o r below s t e e p bedrock s lopes . The f o l i a t i o n o r bedding of t h e layered rocks on t h e mainland s t r i k e n e a r l y p a r a l l e l t o t h e t r e n d of t h e s t e e p s l o p e s along Gast ineau Channel. These p l a n a r f ~ a f u r e ~ ~ g e n e r a l l y d i p northeastward i n t o t h e mountainside from about 30 t o 75 . A j o i n t s e t t h a t s t r i k e s almost pe rpend icu la r ly t o t h e f o l i a t i o n o r bedding i; well developed, and g e n e r a l l y d i p s northwestward at between 55' and 80 . Another important j o i n t s e t s t r i k e s n e a r l y p a r a l l e l t o t h e f o l i a t i o n and l aye r ing , bu t d i p s southwestward a t about 65'. The r e s u l t o f t h e combination of p l a n a r f e a t u r e s and j o i n t s i s t h a t t h e bedrock r e a d i l y breaks i n t o l a r g e b locks which can become loosened and uns t ab le on t h e steep slopes. Such l a r g e b locks formed by t h e s e in- t e r s e c t i n g f r a c t u r e s a r e loosened even f u r t h e r by t r e e r o o t s t h a t grow i n the openings and push t h e blocks apart .

On Douglas Is land, t h e l aye r ing o f the rocks which i s somewhat e a s i e r t o s e e because of t h e conspicuous p a r t i n g s i n s l a t e a l s o s t r i k e s t o t h e

0 d northwest and d i p s northeastward about 60 t o 65 i n many p l aces ; t h e d i p i s l o c a l l y s t e e p e r . These beds a r e cu t by j o i n t s e t s t h a t have v a r i - able d i r e c t i o n s ; some s t r i k e no r theas t and d i p southeastward, o t h e r s strike n o r t h e a s t and d i p northwestward. ,

Erosion of t h e bedrock i s g e n e r a l l y c o n t r o l l e d by weathering along t h e p l a n a r f e a t u r e s , which a r e t h e weakest aspec t of most of t h e bedrock. Weathering gene ra l ly progresses along t h e s e i n c i p i e n t f r a c t u r e s , and developes openings and zones of weakness along which water and g r a v i t y , s i n g l y o r i n combination, can loosen and erode t h e rock. The micaceous g reensch i s t above Juneau weathers and erodes e a s i l y , and s c h i s t o s e par- t i c l e s accumulate a s c:ayey colluvium. The bedrock on t h e s t e e p moun- t a i n s lopes has been scraped by past g l a c i e r s t h a t moved ac ros s t h e area. Sainsbury (1953) r e p o r t s r e s i d u a l s o i l s on bedrock a t i n t e r s t r e a m d i v i d e s a t a l t i t u d e s of 3,000 f e e t and h ighe r , but t h e bedrock i s s c a r c e l y weathered on t h e g l a c i a t e d mountainsides.

Bedrock i s more r e s i s t a n t t o shaking from earthquakes than any o f t h e o t h e r geologic m a t e r i a l s i n t h e Juneau a r e a . Although damage t o bu i ld - i ngs from se ismic v i b r a t i o n i s no t a geologic e f f e c t a long , t h e frequency and ampli tude, which a f f e c t bu i ld ings , depend on geologic f a c t o r s . The s u b s o i l i s important i n t h i s respect , and, o t h e r t h i n g s be ing equal , it i s widcly accepted t h a t v i b r a t i o n i s l e s s i n a r e a s o f bedrock than i n a r e a s of t h i c k unconsol idated m a t e r i a l s . D i r ec t se i smic damage is h i g h l y s e l c c t i v c , and poor cons t ruc t ion p r a c t i c e s and s t r u c t u r a l weaknesses a r e qu ick ly revea led by ear thquakes (Berg and Stratta, 1964, p. 58), whether on bedrock o r on unconsol idated ma te r i a l s .

The wave-cut benches on bedrock along t he channel , bays, and coves a re i n a r e a s t h a t a r e s u s c e p t i b l e t o tsunami waves. The low f l a t bedrock benches along t h e shore of Auke Bay, on blendenhall Peninsula , and on Auke Cape are p a r t i c u l a r l y s u s c e p t i b l e because of t h e i r openncss t o t h e waters extending no r th and n o r t h w e s t . The bedrock bench bounding t h e Lena Cove a r e a on Point Lena i s a l s o exposed t o open waters . Waves t h a t o r i g i n a t e d t o t h e no r th o r northwest probably would s t r i k e t h e s e a r e a s wi th f u l l fo rce .

Large rocks l ide - avalanche d e p o s i t s i n t h e Juneau a r e a a r e proof of p a s t r o c k f a l l a c t i v i t y , but r o c k f a l l p o t e n t i a l a long t h e mountains v a r i e s from p lace t o place. Most prev ious r o c k f a l l s o r i g i n a t e d on s l o p e s gen- e r a l l y free of dense vegeta t ion and l e f t s c a r s t h a t co inc ide wi th p l anes of weakness along j o i n t s e t s o r a t boundaries between d i f f e r e n t rock types. Some mountain areas a r e unde r l a in by rocks t h a t have t i g h t j o i n t s and smooth d e b r i s - f r e e s lopes . Such s lopes a r e more s table than slopes where broken rock i s being pushed a p a r t by t r e e r o o t s o r i s slowly moving downhill .

While most o f t h e bedrock s lopes i n t h e Juneau a r e a l i e some d i s t a n c e from Gast ineau Channel, l a r g e r o c k f a l l s could reach t h e water and cause waves. The f i o r d country of Norway i s noted f o r nonearthquake-related massive r o c k f a l l s and s l ides t h a t have caused g i a n t waves and destruc- t i o n along shores . In p l aces i n Norway t h e r i s k of r o c k f a l l s i s of such concern t h a t l a r g e loose rocks , many of which a r e being pushed a p a r t by t r ee r o o t s (as i n t h e Juneau a r e a ) , have been secured by cab le s anchored t o s o l i d rock (Bjerrurn and J o r s t a d , 1968, p. 7 ) . Inspec t ion o f t h e s lopes above t h e urbanized a r e a s of t h e Juneau Borough probably would l o c a t e s i m i l a r l a r g e and uns t ab le fragments t h a t we l l could be secured i n a simila 'x manner.

S u r f i c i a l d e p o s i t s

Manmade f i l l

Manmade f i l l , as used i n t h i s r e p o r t , c o n s i s t s of e a r t h m a t e r i a l s re - worked by man, an3 s o l i d waste discarded by man. These m a t e r i a l s are mapped a s d e b r i s from mining and m i l l i n g ope ra t ions t h a t have been p laced i n dumps (md), u n d i f f e r e n t i a t e d m a t e r i a l s g e n e r a l l y used f o r highway and cons t ruc t ion f i l l ( m f ) , and accumulations of s o l i d waste and rubbish placed i n dumps (mw).

Mine dumps (md)

P'ine dumps c o n s i s t o f waste from mining and m i l l i n g ope ra t ions . They a r e p r i m a r i l y mixtures o f angular fragments o f s l a t e , greens tone , d i o r i t e , g r e e n s c h i s t , and ve in quart:. Flany of t h e p i e c e s a r e 4-10 inches i n longcs t d imension; o ther p ieces came through t h c stamp m i l l and range i n s i z e from less than 2 inches i n diameter t o sand,

' The m i x dumps along Gold Creek a r e r e l a t e d t o m i l l s , many of which have disappeared. The dwnps along Gast ineau Channel nea r t h e Alaska-Juneau m i l l a r e t h e l a r g e s t i n t h e a r ea . Smaller dumps a r e near t h e Treadwell proper ty on Douglas I s l and sou theas t o f Douglas, P i les of rock t h a t form e longate t e r r a c e s o r mu l t ip l e r i d g e s and mounds a r e c h a r a c t e r i s t i c of t h e mine dumps. The beach a t Douglas is made up most ly of m i l l t a i l i n g s .

The Alaska-Juneau t a i l i n g s dump i n Gast ineau Channel i s t h e t h i c k e s t mine dump i n t h e a r e a . Fathometer t r a v e r s e s revea led t h a t t h e margin o f t h e A - J dump extends about 100 f e e t below sea l e v e l (Robert D . Mi l l e r , 1967). The th i ckness of t h e dump above high t i d e i s not d e f i n i t e l y known but it exceeds 20 f e e t i n most p l aces . Other dumps a r e t h i n n e r and probably veneer h i l l s i d e s t o a th i ckness of 20-50 f e e t but extend seve ra l hundred f e e t down s lope . Small i s o l a t e d dumps along Gold Creek and on Douglas Is land are probably l e s s than 40 f e e t t h i c k ,

Much o f t h e d e b r i s i n mine dumps i s weathered and decomposed. The l a r g e r fragments i n t h e A-J dump were n o t processed thraugh t h e stamp m i l l , and p i e c e s 4-6 inches and l a r g e r a r e common. Mine dumps on n e a r l y l e v e l ground a r e composed o f weathered rock fragments , and f ine -g ra ined mater i - a l seems t o be d i s t r i b u t e d throughout t h e d e p o s i t .

I n f i l t r a t i o n i s g e n e r a l l y good t o e x c e l l e n t i n t h e c o a r s e r mine dumps, and poor i n t h e i n t e n s e l y weathered dumps, The surface o f t h e l a r g e A-J dump i s gene ra l ly sandy, b u t apparent ly t h e bulk o f t h e dump con- sists of blocky fragments , so t h e r e probably are openings between rock fragments which al low free flow o f water , Runoff is good on t h e Eine- gra ined &mps w i t h s t e e p s lopes and f a i r t o poor on t h e l a r g e dumps a long Gast ineau Cnannel.

The f ine-gra ined dumps a r e e a s i l y eroded by concent ra ted flow bu t r e s i s t sheet wash. The l a r g e dumps along Gast ineau Channel r e s i s t shee t wash and wave eros ion . T ida l c u r r e n t s have winnowed t h e dump ma te r i a l and formed broad f l a t s t h a t can be seen at low t i d e between the large A - J mine dump and Snowslide Creek dump.

The l a r g e A - J dump provides s a t i s f a c t o r y foundat ions f o r o i l t anks and o t h e r s t r u c t u r e s under s t a t i c condi t ions . The gene ra l coarseness of the m a t e r i a l , coupled wi th compaction and se t t l emen t over many y e a r s , seems t o have r e s u l t e d i n a good foundat ion, The f ine -g ra ined dumps probably a r e l e s s s a t i s f a c t o r y foundat ion m a t e r i a l s , and d i f f e r e n t i a l se t t lement should be expected.

Severe v i b r a t i o n shakes loose ly consol ida ted m a t e r i a l s more than dense ly compacted ma te r i a l s . Mater ia l i n mine dumps i s gene ra l ly l o o s e l y p laced , on ly moderately compacted, and con ta ins many openings between rock f r a g - ments. During an earthquake on February 1 2 , 1954, the A - J dump s e t t l e d i n s eve ra l p l aces and Liaste conveyors were wrecked, appa ren t ly by shaking (U.S. Coast and Geodetic Survey, 1934, p . 3 8 ) . P r o f i l e s ac ros s Gast ineau Channel from t h e s lopes of t h e A - J dump show s l o p e conf igu ra t ions t h a t may i n d i c a t e subaqueous s l i d i n g of blocks marginal t o t h e dump (Robert D . b l i l l e r , 1967) . A severe ear thquake might r e s u l t i n marginal slumping and s l i d i n g , as well a s s e t t l emen t from compaction, of dump d e p o s i t s o therwise s t a b l e under q u i e t condi t ions .

The l a r g e dumps o f t h e A - J mine and sma l l e r dumps of t h e Treadwell mine sou theas t o f Douglas a r e a l l a t o r near s h o r e l i n e . The impact of wave runup on t h e s e d e p o s i t s would be s i m i l a r t o t h a t d i scussed under younger d e l t a d e p o s i t s (p. 6 7 ) . A low runup would r i s e on t h e s lopes of t h e dumps but probably would not overtop them. If t h e rock fragments i n t h e A-J dumps a r e i n poin t - to-poin t con tac t , t h e s e d e p o s i t s should be h igh ly permeable, and high pore p re s su res would be u n l i k e l y t o develop wi th in t h e dump a s a r e s u l t o f r ap id drawdown of water l e v e l a s p a r t o f v i o l e n t o s c i l l a t i o n s of t h e channel waters during se i che o r tsunami a c t i v i t i e s . If t h e depos i t i s no t r e l a t i v e l y permeable, pore pressure w i l l i n c r e a s e as a r e s u l t of any drawdown.

a F i e l d examinations o f t h e s lopes o f mine dumps showed l i t t l e evidence o f slumps o r s l i d e s . I f s t e e p c u t s a r e made i n dumps, however, r a v e l i n g and slumping would probably occur . The mine dumps along t h e s t e e p moun- t a i n s lopes a r e p o t e n t i a l l y uns t ab le and any d i s r u p t i o n of t h e toes of t h e s e d e p o s i t s probably w i l l cause slumps and s l i d e s ,

The l a r g e A - J mine dump is ' used i n p a r t a s a g o l f course; t h e dump by t h e m i l l on t h e s lopes nea r Juneau has been used a s a source o f f i l l . The mine dump over ly ing t h e Snowslide Creek d e l t a i s used a s a t r a s h and garbage dump.

Und i f f e ren t i a t ed f i l l (mf)

The composition of u n d i f f e r e n t i a t e d f i l l is h i g h l y v a r i a b l e from p lace t o p l ace . Most highway f i l l s a r e mixtures o f rock , s i l t y sand, grave l , and s o i l ob ta ined from nearby c u t s and borrow p i t s ; many of t h e s e f i l l s were emplaced yea r s ago. Modern cons t ruc t ion p r a c t i c e s wherever f e a s i b l e now r e s t r i c t f i l l t o m a t e r i a l s t h a t a r e not s u s c e p t i b l e t o f r o s t a c t i o n ; sandy g rave l and o ther m a t e r i a l s having a low s i l t content a r e now commonly used. In some l o c a l i t i e s , glaciomarine d e p o s i t s have been used f o r f i l l because of l a ck o f o t h e r t ypes of m a t e r i a l s nearby; such an area i s between F r i t z Cove and Peterson Creek on Douglas I s l and . Angular fram- ments from t h e A - J dump c o n s t i t u t e t h e most common f i l l m a t e r i a l beneath much of Juneau, but o t h e r r e a d i l y a v a i l a b l e m a t e r i a l s , i nc lud ing sawdust i n t h e sawmill a r e a along p a r t o f t h e Juneau wa te r f ron t , a r e a l s o p r e s e n t . The f i l l under t h e a i r p o r t i s mostly f i n e gra ined and much of it was obtained from borrow p i t s i n sandy younger d e l t a d e p o s i t s (Qdy) ad j acen t t o t h e runway.

blost of t he areas of f i l l a re on n e a r l y f l a t l y ing o r g e n t l y s lop ing ground. These d e p o s i t s should have good s lope s t a b i l i t y and should not f a i l by l ands l id ing . The margins o f t h i c k f i l l s having s t e e p embankments, however, may slump o r s l i d e owing t o overloading t h a t would exceed t h e shear s t r e n g t h o f t h e f i l l m a t e r i a l . An example of poor f i l l m a t e r i a l with low s lopc s t a b i l i t y i s a sawdust f i l l i n t h c SE!aSE14 sec . 2 3 , T . 41 S . , R. 67 E . , along Gast ineau Channel. Sawdust accumulated on t h e t i d a l f l a t s over many years of sawmill opera t ion . New e a r t h f i l l from t h e A - J dump was placed a t t h e s i t e of t h e t imber p i le -suppor ted sawmill during cons t ruc t ion of t h e new f r e i g h t d i s t r i b u t i o n depot of t h e Alaska Steamship Co. About 4 f e e t o f t h e sawdust and o t h e r d e b r i s l y ing on t h e t i d a l f l a t s was bur ied by t h e new f i l l (Robert Kil lewich, o r a l commun. t o J . A. FIcConaghy, U.S. Geol. Survey, Nov. 15, 1966). Four t o s i x thousand cubic yards of t h e new f i ' l l a t t h i s s i t e s l i d out on t h e saw- d u s t i n t o 40 f e e t o f water i n Gast incau Channel on Oct. 15, 1966 (Daily Alaskan Empire, Oct. 15, 1966). A severe ear thquake would probably cause f i l l d e p o s i t s t o shake v i o l e n t l y , f r a c t u r e i n p l a c e s , and s l i d e on s t e e p s lopes o r where t h e embankments of t h i c k f i l l s a r e unsupported, o r where f i l l i s on weak m a t e r i a l .

The l a r g e s t amount of manmade f i l l u n d e r l i e s t h e Juneau a i r p o r t and shopping a r e a where it forms a l a r g e f l a t pad, Around Juneau, u n d i f f e r - e n t i a t e d f i l l was placed around s t r u c t u r e s o r i g i n a l l y b u i l t on p i l e s d r iven i n t o i n t e r t i d a l f l a t s , beach d e p o s i t s , and younger d e l t a d e p o s i t s . Thc o r i g i n a l s h o r e l i n e a s determined from o l d e r topographic maps and turn-of - the-century photographs matches t h e shoreward boundary of t h e f i l l a s shown on t h e geologic map. The f i l l extends channelward around t h e docks and bu i ld ings along Gast ineau Channel, and covers t h e lower p a r t of t h e Gold Creek f a n and i s emplaced over t h e younger d e l t a . Elsewhere i n t h e a r e a , only l a r g e o r very prominent a r e a s of f i l l a r e mapped; i n - numerable small f i l l s have been emplaced along roads , s t r e e t s , and bu i ld - ing pads. F i l l e d a r e a s a r e gene ra l ly f l a t where used f o r bu i ld ings bu t form t e r r a c e s o r ramps under roadways.

Thickness o f f i l l v a r i e s from p lace t o p l ace . F i l l s on ly a few f e e t t h i c k emplaced a s a pad f o r cons t ruc t ion a r e no t mapped. In some p a r t s of t h e f i l l on t h e Gold Creek fan d e l t a , where t h e A - J dump m a t e r i a l was used , t h e t h i c k n e s s exceeds 25 fee t (Frankle t and Swedell, 1969).

Phys ica l p r o p e r t i e s o f t h e f i l l vary from p lace t o place, depending on m a t e r i a l s used and method of emplacement. The d e n s i t y is v a r i a b l e . Older f i l l s were placed without compaction, whereas new f i l l i s gene ra l ly compacted dur ing placement by use o f smooth-wheeled v e h i c l e s and sheeps- f o o t r o l l e r s , Some a r e a s needing f i l l have been used t o d i spose of t rees , s o i l , and muskeg removed from c l ea red a reas . Such p r a c t i c e s , which r e s u l t i n d i f f e r e n t i a l d e n s i t i e s and s t r e n g t h s o f f i l l , seem t o be confined t o small nonprofessional o p e r a t i o r ~ s by i n d i v i d u a l s .

In most p laces , o t h e r than a r e a s of coarse angular rocks obta ined from t h e A - J dump, excavat ion and d r i l l i n g i n f i l l i s g e n e r a l l y very easy . The c o a r s e r m a t e r i a l s make d r i l l i n g d i f f i c u l t because of t h e looseness o f t h e ind iv idua l pieces i n the f i l l ,

Coarse f i l l has e x c e l l e n t i n f i l t r a t i o n c h a r a c t e r i s t i c s , Fine f i l l gene ra l ly has poorer i n f i l t r a t i o n c h a r a c t e r i s t i c s , and water s t a n d s a f t e r r a i n s . Surface runoff i s slow over broad a r e a s o f n e a r l y l e v e l f i l l .

Where f i l l i s dominantly coarse m a t e r i a l , e ros ion seems t o be s l i g h t ; f i n e r gra ined m a t e r i a l s seem s u b j e c t t o shee t wash and gu l ly ing where flow i s concent ra ted . F i l l t h a t i s s u b j e c t t o l a t e r a l scour by s t reams i s e a s i l y eroded un le s s p ro t ec t ed by r i p r a p .

Most a r e a s of f i l l t h a t ticre observed during t h i s s tudy seem t o provide s t a b l e foundat ions f o r l i g h t s t r u c t u r e s if p rope r ly compacted. In a r e a s where glaciomarine d e p o s i t s are used f o r f i l l , s t a b i l i t y seems b e t t e r i f f i l l i s emplaced and compacted with optimum moisture i n d ry weather.

Response of f i l l t o se i smic v i b r a t i o n s (shaking) w i l l vary from p l a c e t o p l ace . S tud ie s o f damage i n a r e a s s p e c i f i c a l l y a f f e c t e d by t h e 1964 Alaska ear thquake revea led t h a t a r e a s of f i l l e d ground gene ra l ly were more s e v e r e l y a f f ec t ed than were ad jacent n a t u r a l s u r f i c i a l d e p o s i t s . Highway f i l l s of coarse-grained sand and grave l were gene ra l ly more s t a b l e t han t h o s e composed of f ine-gra ined sand and s i l t . F i l l s placed over f i ne -g ra ined sediments subsided more than t h o s e on coarse-grained sediments. The f i l l depos i t placed a t t h e southern end o f Auke Lake o v e r l i e s swampy pea ty d e p o s i t s , and it probably would be deformed and con to r t ed i f a s t r o n g ear thquake occurred i n t h e Juneau area. Cracks as wide a s one-half inch i n t h e runway a t t h e Juneau a i r p o r t were r epo r t ed t o have developed as a r e s u l t of t h e 1964 Alaska ear thquake (Von Hake and Cloud, 1966, p. 54) . The se ismic response o f f i l l unde r l a in by i n t e r - t i d a l d e p o s i t s (Qts) and younger d e l t a d e p o s i t s (Qdy) w i l l be s i m i l a r t o t h a t of t hose d e p o s i t s (see p. 68 , 81).

Most of t h e manmade f i l l d e p o s i t s a r e placed a t o r nea r water l e v e l a long Gast ineau Channel and F r i t z Cove and ad jacent t o Auke Lake. If s e i c h e waves o r s e i smic sea waves a f f e c t e d t h e s e a r e a s t h e f i l l s would be inundated (see p. 69) .

Waste dump (mw)

Three d e p o s i t s of s o l i d waste and rubbish a r e shown on t h e map; (1) t h e dump near t h e mouth of Lemon Creek, (2) t h e wrecked-auto dump along Gast ineau Channel sou theas t o f Salmon Creek, and (3) t h e o l d A - J dump on the Snowslide Creek d e l t a . A fou r th a r ea , t h e Glory Hole a t Treadwell , sou theas t o f Douglas, was being considered by l o c a l a u t h o r i t i e s f o r waste d i sposa l a t t he t ime o f mapping. The Lemon Creek depos i t i s l a n d f i l l and has a n e a r l y l e v e l s u r f l c e . The o t h e r two a r e a s a r e s u r f a c e d e p o s i t s , and have a hummocky su r f ace composed o f junk. The t h i c k e s t d e p o s i t i s a dump covering most of t h e Snowslidc Creek d e l t a , where a s much as 25 feet of trash has accumulated. The o t h e r d e p o s i t s probably a r e thinner. Many small waste p i l e s were not mapped.

San i t a ry l a n d f i l l i s formed by t h e b u r i a l beneath s o i l o f l o o s e l y compacted waste ma te r i a .1~ . Surface waste d e p o s i t s a r e extremely loose and voids are abundant bctwccn p i eces . The d e p o s i t s conscqucnt ly have a low d e n s i t y and a r e extremely compressible.

S a n i t a r y l a n d f i l l dumps va ry from easy t o d i f f i c u l t t o excavate w i th heavy power equipment. The surface accumttlations are e a s i l y moved and excavated by heavy power equipment. D r i l l i n g ranges from easy t o d i f f i c u l t because of bur ied conc re t e , c a r s , and logs .

I n f i l t r a t i o n i s slow t o r a p i d depending on type of e a r t h used f o r b u r i a l ; it i s cxtrcmcly r a p i d i n su r f ace waste p i l e s . Runoff i s g e n e r a l l y slow because o f t h e nea r ly l e v e l su r f ace of l a n d f i l l a r e a s , t h e l o o s e l y compacted n a t u r e of t h e bur ied d e p o s i t s , and t h e jumbled n a t u r e o f s u r f a c e accumulations. Erosion i s h i g h i f l a n d f i l l s i t e s are sub jec t ed t o s t ream o r concent ra ted su r f ace runof f .

Waste dumps g e n e r a l l y provide ve ry poor foundat ions . Se t t lement i s ex- c e s s i v e , and d i f f e r e n t i a l movement could cause s t r u c t u r a l damage t o bu i ld ings p laced on waste dumps. These m a t e r i a l s a r e probably as l o o s e l y compacted as any depos i t i n t h e a r e a and would t h u s be seve re ly a f f e c t e d by ear thquake v i b r a t i o n . The Lemon Creek and Snowslide Creek dumps a r e placed over water -sa tura ted f ine -g ra ined d e l t a i c d e p o s i t s . The expect- able i n t e n s e shaking of t h e s e underlying d e p o s i t s w i l l be t r a n s f e r r e d t o and perhaps ampl i f ied wi th in t h e waste-dump m a t e r i a l s . Also t o be expected would be d i f f e r e n t i a l s e t t l emen t caused by shaking,

Loosely compacted waste d e p o s i t s have very poor s l o p e s t a b i l i t y in c u t s and excavat ions , and dumps placed along bluffs a r e h i g h l y s u s c e p t i b l e t o l a n d s l i d i n g because of very low shear s t r e n g t h .

Waste dumps can be converted t o r e c r e a t i o n a l use a f t e r t h e land is rs- claimed. The low d e n s i t y and uncompacted n a t u r e of waste dumps makes them u n s u i t a b l e f o r gene ra l cons t ruc t ion uses.

Muskeg

Muskegs around Juneau a r e commonly r e f e r r e d t o as s l o p e muskeg, r a i s e d muskeg, and f l a t muskeg. Slope muskegs r e s u l t from the accumulation o f v e g e t a t i v e ma te r i a l on s lop ing land under extremely wet cond i t i ons and develop b e s t where t h e t e r r a i n i s low and h i l l y (Dachnowski-Stokes, 1941, p. 3-4; Heusser, 1960, p. 4 7 ) . Sedge marshes a r e g e n e r a l l y t h e pa ren t m a t e r i a l of t h e s lope muskegs. The v a l l e y o f Kowee Creek on Douglas Island con ta ins slope muskegs. Raised muskegs develop under l e s s wet cond i t i ons i n a s t r o n g l y acid environment and t h e absence of minera l n u t r i e n t s so t h a t moss can grow and accumulate as pea t (Dachnowski- Stokes, 1941, p , 4) . Convex s u r f a c e s are t y p i c a l . hloisture f a l l s on t h e muskeg surface r a t h e r than being suppl ied from t h e water t a b l e o r from streamflow (Heusser, 1960, p. 4 8 ) . Raised muskegs occur i n t h e Montana-Windfall Creek a r e a , a t Sunny Po in t , i n the flat a r e a s above t h e town of Douglas along Lawson and P a r i s Creeks, and on t h e f l a t s n e a r

Johnson Creek near t h e no r th end of Douglas I s l and . Small shal low pools a r e s c a t t e r e d throughout t h e s e muskcgs. F l a t muskegs a r e c l o s e l y r e - l a t e d t o s lope muskeg but arc l imi t ed t o lowlands, edges of l akes , and v a l l e y s where t h e s t ream water i s s l i g h t l y a c i d and poor i n s o l u b l e minera ls (Dachnowshi-Stokes, 1931, p . 4 ; Hcusser, 1960, p. 49) . T h e i r s u r f a c e s a r e f l a t t o s l i z h t l y concave.

Peat (Qmk)

Peat and o t h e r p l a n t d e b r i s i n va r ious stages o f decay c o n s t i t u t e the muskegs i n t h e Juneau area. These muskegs con ta in very dark brown woody, f i b r o u s peat and humus, a s wel l a s v e g a t a t i v e l a y e r s t h a t con ta in p i e c e s of wood. Peat on t h e edge.of ponds o r over ly ing beach d e p o s i t s i s s i l t y . P lan t seeds and p o l l e n spores are l o c a l l y wel l p reserved . De- t a i l e d b o t a n i c a l and s t r a t i g r a p h i c d e s c r i p t i o n s o f s e l e c t e d pea t depos- i t s i n t h e Juneau area a r e r epo r t ed by Dachnowski-Stokes (1941, p. 24-32), Rigg (1937, p. 194-195), and Ileusser (1960, f i g . 24, p. 154). Ages of t h e p e a t s i n d i f f e r e n t muskegs va ry . The o l d e s t i s b a s a l pea t from t h e d i v i d e between Montana Creek and Windfall Creek, which i s da ted a t 10,0004400 yea r s B.P. (Heusser, 1960, p. 97) . This d e p o s i t is a t an a l t i t u d e o f about 800 f e e t , a s determined from t h e a l t i m e t e r i n a h e l i - cop te r used i n t h e course of my mapping, and is one of t h e topographica l - l y h ighes t pea t samples da ted . Muskegs a r e s c a t t e r e d throughout most of t he mapped a r e a on t o p of t h e glaciomarine and g l a c i o f l u v i a l d e p o s i t s , bedrock, and l e s s commonly on younger outwash and o t h e r d e p o s i t s . Peat i n t h e muskegs i s covered by mosses, tussocks o f sedges, and s c a t t e r e d growths of scrubby t imber . Beds o f pea ty m a t e r i a l l e s s than 5 f e c t t h i c k generally are n o t mapped, e s p e c i a l l y where muskegs o v e r l i e most o l d e r r a i s e d beach d e p o s i t s , t h i n and continuous (Qbe) on Douglas I s l and and t h e mainland. The muskeg t ends t o form r a t h e r f l a t t o s l i g h t l y domed s u r f a c e s .

The deepes t muskegs con ta in more than 10 fect of p e a t and a r e t h o s e domed o r r a i s e d muskegs on t h e Montana Creek-Windfall Creek d i v i d e , a t Sunny Poin t , i n t h e Lena Beach a r e a , i n t h e f l a t a r e a s above t h e town of Douglas, e s p e c i a l l y along Lawson and P a r i s Creeks, i n t h e broad muskeg a r e a near Johnson Creek near t h e no r the rn end of Douglas Is land, and i n t h e muskeg a r e a near t h e n o r t h end of Auke Lake. Muskegs g e n e r a l l y l e s s than 6 f e e t deep occur i n mountain v a l l e y s , such a s t h e Kowee Creek va l - l e y on Douglas I s l and . These peat d e p o s i t s g e n e r a l l y a r e mapped wherever t h e y a r e ex t ens ive , r e g a r d l e s s of t h e i r t h i ckness . Peat on t h e wide f l a t s on t h e e a s t s i d e of Douglas I s l and i s g e n e r a l l y 2 - 3 f e e t t h i c k and o v e r l i e s t h i n sandy g r a v e l l y beach d e p o s i t s ; t h e s e a r e a s o f shal low muskeg a r e no t mapped because p o r t r a y a l of t h e under ly ing geologic u n i t s t h e r e is more important f o r cons t ruc t ion and planning.

No phys ica l -proper ty t e s t s were made of pea t as p a r t o f t h i s s tudy . F i e l d observa t ions i n d i c a t e t h a t almost everywhere it i s s a t u r a t e d , s o f t , spongy, and sub jec t t o h igh compaction under loads. F i e l d and l abo ra to ry tests made by t h e Alaska IIighway Department, as par t of a m a t e r i a l s i n - v e s t i g a t i o n i n t h e Mendenhall v a l l e y , showcd t h a t undis turbed pea t i n t h e muskeg on t h e no r the rn s i d e of Auke Lake contained more than 500

percent moisture (expressed as a percentage o f t h e d r y weight) (Ray D . Miller , D i s t r i c t Materials Engineer, Juneau D i s t r i c t , Alaska State Iligh- way Deyt . , w r i t t e n cornmun., 1964) . The dry weight of t h i s pea t was 11.5 lbs pe r cu f t and t h e wet weight was 60.7 l b s p e r cu ft. Such a h igh water content permi ts 3 h igh degree of compaction and flowage of t h c peat under loads. Fcat i s commonly removed and t h e excavat ion back- f i l l e d w i t h grave l so as t o avoid f r o s t heaving,

Peat can e a s i l y be excavated with hand o r power equipment. Deep excava- t i o n s tend t o have Ket f l o o r s and water seeps from t h e wa l l s . Where earthmovers 1 1 3 ~ ~ move3 over muskeg a r e a s r epea t ed ly , t h e bcar ing s t r e n g t h of t h e pea t diminishes and a muskeg-quagmire can r e s u l t . Thin d e p o s i t s o f pea t can be removed by t ractor-drawn s c r a p e r s and earthmovers o r by dozers ; t h i c k e r d e p o s i t s r e q u i r e backhoe o r d r a g l i n e . Peat i s e a s i l y d r i l l e d bur support i s r equ i r ed f o r t h e d r i l l r i g and t h e d r i l l ho l e r e q u i r e s cas ing .

Muskegs a r e gene ra l ly wet, except dur ing per iods of prolonged drought when t h e water t a b l e i s lowered. \\%en t h e pea t i s wet, i n f i l t r a t i o n through i t is slow and s tanding water i s common; seeps a r e g e n e r a l l y found a t t h e contac t with t h e underlying d e p o s i t s , Surface runoff i s slow because of t h e n e a r l y f l a t o r very g e n t l y s lop ing su r f ace . When t h e pea t i s dry , i n f i l t r a t i o n i n t o it i s more r a p i d , but quick s a t u r a t i o n o f t h e upper p a r t o f t h e pea ty m a t e r i a l slows f u r t h e r i n f i l t r a t i o n . A l - though t h e small s t reams t h a t c r o s s some muskegs a r e s l i g h t l y i n c i s e d , l a t e r a l d ra inage through t h e upper p a r t o f t h e pea ty m a t e r i a l is slow and t h e muskeg a r e a remains wet.

Peat has l i t t l e bear ing s t r e n g t h , e s p e c i a l l y when s a t u r a t e d . Di f fe ren- t i a l compaction could cause structures b u i l t on p l a t fo rms on th ick muskeg t o s e t t l e unevenly. In a r e a s o f t h i n pea t d e p o s i t s , excavat ion and back- f i l l i n g with more s t a b l e ma te r i a l a r e advisable ; ca i s son o r p i l e foo t ings should be used. Road cons t ruc to r s g e n e r a l l y u se excavat ion and b a c k f i l l - i ng methods i n muskeg a r e a s ; bu t i n a r e a s where muskeg i s over 10 f e e t deep and t h e pea t i s not excavated, grave l b l anke t s 4-5 f e e t t h i c k a r e placed over t h e muskeg t o avoid roadbed problems (Munson, 1964, p . 5-6).

Each muskeg and i t s organic content should be eva lua ted i n d i v i d u a l l y as t o i ts probable behavior during an ear thquake because of t h e many types of underlying d e p o s i t s , each of which has a d i f f e r e n t se i smic response t h a t would be t r a n s m i t t e d t o t h e muskeg. The response o f a muskeg a r e a t o a seve re earthquake commonly would be i n t e n s e because it is loose , porous, and gene ra l ly s a t u r a t e d . Seismic shaking gene ra l ly i s much more i n t e n s e i n loose sediments than i n bedrock, and it l a s t s longer i n unconsol ida ted wet m a t e r i a l s than i n unconsol idated d r y sediments. Peat deposi ts under la in by unconsol idated m a t e r i a l s , such as outwash, are more s u s c e p t i b l e t o v i b r a t i o n than peat unde r l a in by bedrock. Road embankments and o t h e r f i l l ma te r i a l placed on t h i c k p e a t probably would crack i n response t o v i b r a t i o n , a s would pea t reclaimed by d r a i n i n g t h e muskeg a r e a s [Hansen, 1965, p . A27). Dense m a t e r i a l s t end t o subs ide into less dense underlying sediments when shaken (Kachadoorian, 1968,

p. C19). IIigh~ia)' f i l l p1:rccd olpcr muskcg thus could b6 expected t o f r a c t u r e , conlpact d i f f e r e n t i a l l y , flow l a t e r a l l y , and bu lge upward around t h e edges of t h e f i l l ma te r i a l 3s a r e s u l t o f prolongcd shaking dur ing ear thquahcs. If f r o ~ c n , muskeg ovcr unconsohlidntcd d e p o s i t s t h a t were s a t u r a t e d but not f rozen would a c t as a coherent and competent l a y e r , a conciition fou:i~l t o bc t y p i c a l o f f r a c t u r e d ground dur ing t h e 1964 e a r t h - quakc (Coul t c r and ? .I igl iaccio, 1966, p . C 2 Z ) . The nuskeg would then be more l i k e l y t o f r a c t u r e and e j e c t water and sediment as spou t s o r b o i l s (Lemke, 1967) . Peat d e p o s i t s gene ra l ly a r e s u f f i c i e n t l y above s e a and l ake l e v e l s t o be ou t of cianger o f tsunamis o r s e i ches . I f a tsunami o r s e i che invaded Gastineau Channel, a low runup on shore could inundate t h e margin o f t h e muskeg f l a t on Douglas I s l and oppos i t e Sunny Poin t .

Peat s t ands i n n e a r l y v e r t i c a l c u t s when f r e s h l y excavated, even when wet. Water d r i p s and seeps down cut f aces i n s a t u r a t e d peat ; l a r g e r seeps are comnlon a t t h e base of muskeg i n a cu t , and highly decomposed pea t l a y e r s s l~unp o r flow i n t ime. Dr i e r f i b r o u s pea t w i l l s t and in - d e f i n i t e l y i n v e r t i c a l c u t s . Peat depos i t s t end t o be undercut by wave o r cu r r en t e ros ion of underlying unconsol idated m a t e r i a l s , and i n such cases f a l l as blocks. Peat does not a s a r u l e s l i d e of i t s own accord, because t h e f i b r o u s m a t e r i a l gene ra l ly holds t h e depos i t t oge the r .

Sphagnum moss i s gene ra l ly t he pea t commercially p r e f e r r e d as a humus- forming product , a l though sedge p e a t s a l s o are used. Fibrous and sedge p e a t s consis t of underground stems of g r a s s l i k e p l a n t s and have horizon- tal laminat ion, whereas t hose der ived from sphagnum moss a r e cha rac t e r - i z e d by small columnar lumps and v e r t i c a l aggrega tes and are p r e f e r r e d f o r use as s t a b l e bedding o r f o r packing and shipping small p l a n t s (Dachnowski-Stokes, 1941, p . 7 ) . Slope muskegs do no t develop a cont inuous cover of sphagnum moss, but ins tead are composed of sedges, heaths, and pa tches o f sphagnum moss. Raised muskegs g e n e r a l l y have sedge i n t h e lower pa r t and l a y e r s o f sphagnum moss a t t h e t o p , l o c a l l y s epa ra t ed by wood (Dachnowski-Stokes, 1941, p. 26-27); t h e sphagnum moss is con- s idered by Dachnowski-Stokes (1941, p. 30-31) t o be well suixed f o r com- merc ia l use . fie eva lua t e s t h e Montana Creek muskeg depos i t a s exceeding i n ex t en t and amount t h e moss pea t a v a i l a b l e a t e i t h e r t h e Sunny Po in t muskeg o r t h e Lena Beach muskeg. F l a t muskegs g e n e r a l l y have l i t t l e commercial use .

Mass-wasting d e p o s i t s

Mass-wasting d e p o s i t s , as mapped i n t h e a r e a of t h i s s tudy , inc lude colluvium (Qc) , t a l u s (Qta) , debr is - f low d e p o s i t s (Qf 1) , rocks l ide-avalanche d e p o s i t s (Qra), u n d i f f e r e n t i a t e d l a n d s l i d e s (Qsl) , and c o l l u v i a l ( ? ) diamicton (Qud). While some of t h e s e d e p o s i t s are contemporaneous i n age, i n t o t a l they span t h e time i n t e r v a l from very r ecen t t o p r e h i s t o r i c , and probably extend back as f a r as e a r l y Holocene o r l a t e P le i s tocene times. As used i n t h i s s tudy , mass-wasting d e p o s i t s include d e p o s i t s some au tho r s would s e p a r a t e i n t o m a t e r i a l s o r i g i n a t i n g from mass movement and mass t r a n s p o r t , as wel l a s from mass-wasting (Fa i rb r idge , 1968

p. 688-700). In cach of t h e above-mentioned geologic d e p o s i t s g r a v i t y p l ays an important p a r t i n t h e accunlulation of m a t e r i a l s i n t o mappable d e p o s i t s . Katcr, snow, and p o s s i b l y a i r a l s o were involved i n t h e t r a n s p o r t o f some o r a l l of t h e s e d e p o s i t s t o a g r e a t e r o r l e s s e r degree.

Neathering w c a k e ~ ~ s t h e m a t e r i a l s exposed a t t h e s u r f a c e of t h e e a r t h , t h e r e b y al lowing these mater ia ls t o be more s u s c e p t i b l e t o g r a v i t a t i o n a l i n f luence r e s u l t i n g i n downslope movement. Colluvium (Qc), rocks l ide -

. avalanche d e p o s i t s (Qra), und i f f e ren t i a t ed l a n d s l i d e s (Qsl) , and c o l l u - v i a l [ ? ) diamicton (Qud) a r e most s t r o n g l y inf luenced s o l e l y by g r a v i t y . Talus (Qta) and debris-£ low deposi ts (Qf 1) r ep re sen t d c p o s i t s t h a t are in f luenced a l s o i n p a r t by water and(or) snow.) A l l o f t h e s e d e p o s i t s r ep re sen t accumulations of geologic m a t e r i a l s a f t e r t r a n s p o r t .

Transpor t can be slow o r f a s t , t h e m a t e r i a l s d r y o r wet, t h e a r e a s i n - volved l a r g e o r smal l , and t h e movement can be r ep resen ted by creep , s l i d e , flow, o r f a l l . Colluvium (Qc) i n t h e Juneau a r e a r e p r e s e n t s slow t r a n s p o r t of weathered o r unconsol idated m a t e r i a l s t o t h e lower p a r t s of s lopes by g r a v i t y , supplcrnented by moisture i n t h e form of water, snow, o r i c e r e s u l t i n g i n some movement by s lope o r shee t wash, and s o i l creep. Ta lus (Qta) he re inc ludes r o c k f a l l t a l u s ( i nd iv idua l p i eces t h a t f a l l , bounce, and r o l l t o t h e bottom of s lopes ) , a l l u v i a l t a l u s ( p a r t i c l e s of a l l s i z e s t r anspor t ed by water ; t h e t a l u s accumulates g e n e r a l l y a s a r e s u l t of heavy r a i n s and mel t ing snow and commonly d i s p l a y s narrow flow channels , n a t u r a l l evees , and gouged channels through brushy v e g e t a t i o n ) , and avalanche t a l u s (gene ra l ly a s a r e s u l t o f snow avalanches i n t h e

.

Juneau a r e a ) . Debris-flow d e p o s i t s (Qfl) f o r t h e most p a r t r e p r e s e n t water -sa tura ted loose r e s i d u a l m a t e r i a l s t h a t moved r a p i d l y from s t e e p s lopes . Rockslide-avalanche d e p o s i t s (Qra) g e n e r a l l y r ep re sen t extreme- l y r a p i d downslope t r a n s p o r t from s t e e p bedrock c l i f f s . J o i n t s e t s seem

I t o be important i n t h e weakening and u l t i m a t e r e l e a s e of masses of bedrock; s t r e s s r e l e a s e may be t h e dominant f a c t o r i n t h e r o c k s l i d e avalanches. The l a r g e mass of rock s lowly s l i d e s downward along d ipping j o i n t p lanes as a u n i t , bur breaks i n t o l a r g e and small fragments a s t he mass acce l e r - a t e s down t h e s teep mountain s lope . Slopes as s t e e p a s 70' axe common at t h e source of t h e s e r o c k s l i d e avalanches. Und i f f e ren t i a t ed l a n d s l i d e s (Qsl) a r e gene ra l ly be l i eved t o be of a r a p i d l y moving but b a s i c a l l y d ry type. They are formed by t h e downward and outward movement of s lope- forming mater ia l ! composed of n a t u r a l rock, s o i l s , manmade f i l l s , o r com- b i n a t i o n s of t h e s e m a t e r i a l s [Varnes, 1958, p. 20). C o l l u v i a l ( ? ) diamic- t o n (Qud) i s t h e mass-wasting depos i t l e a s t understood by me. The o r i g i n g

of t h e s e d e p o s i t s is problemat ica l , but I b e l i e v e t h a t most of them were formed by c o l l u v i a l p roces ses , inc luding creeping , f lowing, and s l i d i n g .

Col luvium (Qc)

The composition o f colluvium v a r i e s from p l a c e t o p l ace . The a r e a s mapped a s colluvium inc lude t a l u s and waterborne slope-wash depos i t s t h a t axe t o o small t o show s e p a r a t e l y a t t h e s c a l e o f t h e map, as wel l as s o i l and rock fragments. The bedrock t h a t u n d e r l i e s the s lopes pro- v ides most of t h e l a r g e r fragments i n colluvium, and s o i l and g l a c i a l

or glaciomarine s u r f i c i a l d e p o s i t s mixed with t h e pieces of bedrock make up t h e colluvium on t h e lower mountain s l o p e s i n t h e Juneau a r e a . In gene ra l , greenstone and mica-r ich g reensch i s t a r e t h e most common bedrock types i n t h e d e p o s i t s along Gastineau Channel on t h e mainland. A t t h e nor thern end of Douglas I s l and , colluvium contains fragments o f green- s tone , s l a t e , and volcanic Elow breccia . O f p a r t i c u l a r i n t e r e s t i s t h e colluvium covering t h e s lopes i n t h e v i c i n i t y of Kelson S t r e e t i n Juneau. The mat r ix o f t h e depos i t here i s r i c h i n mica, much o f it weathered t o c l a y , which was derived from a g reensch i s t t h a t forms t h e westward face o f t h e r idge between Mount Roberts and bIount blaria. An exposure along t h e Glac i e r Highway ac ros s from t h e Chi ldrens ' Home r e v e a l s g reensch i s t fragments nccunulatcd over a pea ty humus l a y e r . Age i s Holocene.

The a r e a l ex t en t of colluvium is l a r g e l y a r b i t r a r y a s mapped. Only s i z - a b l e d e p o s i t s o f colluvium a r e mapped; sma l l e r d e p o s i t s e x i s t bu t a r e not mappable a t t h e s c a l e of t h e map. Much of t h e d e l i n e a t i o n o f co l lu - vium was done by i n t e r p r e t a t i o n o f a e r i a l photographs. Slopes underlain by colluviun are gene ra l ly s t e e p a t the t o p and curve and become flatter downward. Deposi ts were mapped on t h e s l o p e s t o an a r b i t r a r y he igh t where t h e bedrock appeared t o be f ree o f s u r f i c i a l cover . The th i ckness o f colluvium i s h ighly v a r i a b l e and is more than 15 f e e t i n some p l aces . Deposi ts on s t e e p s lopes a r e t h i n n e r .

Large rock fragments i n colluvium l i e near t h e angle of repose on s t e e p s lopes , wi th t he f l a t s i d e s of t h e fragments p a r a l l e l t o each o t h e r ; spaces between fragments may be f i l l e d with c layey o r humic ma t t e r , o r by sma l l e r p i eces of rock. On lower, more g e n t l e s l o p e s t h e colluvium con ta ins a g r e a t e r amount of f ine-gra ined mat r ix ; much of t h i s f i n e m a t e r i a l has been a l t e r e d t o a clayey mixture of humus and s i l t and sand. Th i s kind of colluvium o v e r l i e s t h e mica-r ich g reensch i s t on h i l l s i d e s south of Mendenhall v a l l e y .

The looseness o f most colluvium permi ts r a p i d i n f i l t r a t i o n o f water . In some p l aces where bedrock o r o t h e r impermeable l a y e r s u n d e r l i e t h i n co l - luvium, sp r ings and seeps appear , e s p e c i a l l y i n road c u t s . Runoff i s r a p i d on upper s lopes , but becomes much s lower on t h e lower s lopes . Erosion i n colluvium i s r a p i d where water flows down s lopes f r e e of vege- t a t i o n . The f ine-gra ined colluvium der ived from g reensch i s t is e s p e c i a l l y s u s c e p t i b l e t o such e ros ion .

Colluvium is a poor foundat ion m a t e r i a l ; it t ends t o c reep downslope even on the more g e n t l e lower mountain s lopes . The high r a i n f a l l i n the Juneau a r e a l u b r i c a t e s t h e p l a t e y fragments i n colluvium, and l i g h t s t r u c - t u r e s founded on such m a t e r i a l move out of plumb over a pe r iod of t ime. Thin colluvium should be removed and bu i ld ings placed on f i rmer underlying mate r i a l s .

Colluvium i s genera l ly uns t ab le even under ear thquake- f ree cond i t i ons because of t h e s t e e p s lopes on which it occurs and because o f t h e p l a t y n a t u r e o f t h e fragments i n t h e depos i t . Creep, a very slow downward movement shown by t r e e t r u n k s bent downslope, is t h e norm r a t h e r t han

t h e except ion. l!'eathcrcd mica p l a t e s i n colluvium on t h e h i l l s i d e s s o u t h o f !lcndcnhnll v a l l e y a r e s tacked as a dcck of cards and have a t e n d e n c y t o s l i d e over each o t h e r . IIcnvy r a i n f a l l can cause slumps and r a p i d l y niovjng s l i d e s and d e b r i s f l c l w s i n colluvium. Colluvium on s l o p e s

-,o stecp2-r. th:!n -71 i s i n rla11scr o f sl i c ! l ; ~ ~ ; \ \ i i ~ . i l the cohe.sive~icss o f t h e material i s d e s t r o y e d o r d i s r u p t e d , whatever t he cause (Suanston, 1970, p. 1 4 ) . Excavarions i n colluvium c r e a t e p o t e n t i a l l y uns t ab le cond i t i ons upslope and r., . :L .~ c I n 1 . n ~ - c l i n g , ~ a s h i n g , o r s l i d i n g .

Stro2g 'earthcpskc v l b r n t i o ~ l s w i l l i nc rease t h e g r a v i t a t i o n a l e f f e c t s on t h e cal!uvial d e p o s i t s , could cause some displacement downslope, and might evcn cause quick-moving l a n d s l i d e s . The d e b r i s flows on Flount Roberts o r i z i . n n t e d i n p a r t i n colluvium t h a t became extremely s a t u r a t e d and t h a t l o s t in te rna l cohesion. Consequcntly, colluvium on steep s lopes would be v e r y s u s c e p t i b l e t o earthquake-induced s l i d i n g , especially i f t h e ear thquake occurred d u r i n g o r a f t e r pe r iods of prolonged heavy r a i n s .

Danger from i s o l a t e d r o c k f a l l s i s high along s lopes covered by cnl luviu; :~. Numerous large angular rock fragments l y i n g wi th in t h e t r e e s bea r t e s t i - mony t o t h e s u s c e p t i b i l i t y t o rocks f a l l i n g from t h e h ighe r s lopes . See the d i scuss ion under d e b r i s f low (Qfl) and bedrock (b) . Talus (Qta)

Rock f r a g ~ n q t s i n t.4?luscs arc. l o c a l l y der ived and cons i s t of micaceous .-: : - , + i ~t , nr~d metavolcanics t h a t range i n s i z e g r c r n ~ c ! i < s t , I--- -

froci l/-i ir.~'. b . . l ; ~ i : t ierivcd from s l a t e o r s c h i s t o s e rocks t o 10 f e e t where der ived from ha rde r blocky rocks. A l l of the t a l u s e s are of Holo- cene age, ~ n d most t a l u s e s are s t i l l accumulating d e b r i s today. Some t a l u s e s , however, are i n a c t i v e . Such t a l u s e s a r e g e n e r a l l y covered w i t h t r e e s 60-150 years o l d . These t a l u s e s a r e not completely s t a b i l i z e d , however, as ind ica t ed by some t runks as l a r g e as 40 inches i n diameter t h a t a r e bent upslope as t h e t r e e a t tempts t o main ta in a v e r t i c a l t runk . Being r e l a t i v e l y i n a c t i v e , t h e s e d e p o s i t s a r e no t r ece iv ing present-day accumulations of rock fragments by any of t h e t a l u s processes descr ibed e a r l i e r (p. 3 4 ) . Such i n a c t i v e t a l u s e s a r e shown on t h e geologic map by a d i agona l ly l i n e d o v e r p r i n t .

An a r e a o f i n a c t i v e t a l u s is well exposed i n an excavation f o r a home i n t h e NEGSWA sec. 15, T . 41 S . , R . 67 E . , and a long t h e Glac i e r Highway northwest from Norway Po in t . Th i s area i s c u r r e n t l y s t ab l e , but s e v e r a l layers of p l a t y g r e e n s c h i s t - r i c h t a l u s between a t l e a s t two p e a t and woody beds suggest r ecu r ren t cyc l e s of t a l u s accumulations. The ground t h e r e is g e n e r a l l y wet, and sp r ings seep on t h e s lope . The peaty beds are a s t h i c k as 2 f e e t b u t contain g reensch i s t fragments i n l a y e r s 2 inches t h i c k . The o l d age of t h i s t a l u s i s ind ica t ed by t h e l a rge 40-inch-diameter t r e e s growing on t h e d e p o s i t , as well a s t h e i n t e r l a y e r - i ng o f t a l u s and pea ty beds, which suggest d e p o s i t i o n a l cond i t i ons t h a t no longer e x i s t .

There a r c many t a l u s e s a t t h e base of s t e e p s lopes i n t h e mapped a r e a . Ta luses elsewhere i n t h e a r e a a r e fewer and l e s s conspicuous. Some l a r g e t a l u s e s have coalesced along t h e s t e e p s lopes t o form cont inuous aprons t h a t cover broad areas o f r ;~ountainside. Smaller t a l u s e s a r e r e - s t r i c t e d t o riarrow troughlike o r r i bbon l ike d e p o s i t s on f o r e s t e d s lopes . Ta luses range i n th i ckness from a few f e e t i n t h e upslope p a r t of t h e depos i t t o probably more than 10 f e e t a t t h e base. Exposures along Gold Creek and Gastineau Channel r evea l t h i cknesses o f 8-10 f e e t .

The t a l u s e s along p a r t of Gold Creek and south from Juneau along Gas t i - neau Channel are mica r i c h , and rock fragments l i e wi th f l a t s u r f a c e s p a r a l l e l wi th each o t h e r . Some d e p o s i t s have i n t e r l o c k i n g blocky p i eces , b u t open spaces are abundant betwgen l a r g e r fragments i n n e a r l y a l l t a l u s e s .

Rock fragments f a l l o r r o l l from c l i f f s and s t e e p s l o p e s t o t h e base o f s lopes \<here they gene ra l ly l i e a t t h e angle of repose . Source a r e a s f o r some of t h e t a luses are shown on t h e geologic map by a s ca rp symbol. Arrows shown below some of t h e sca rps i n d i c a t e known o r a n t i c i p a t e d pa ths o f f a l l i n g rock fragments . The rocks g e n e r a l l y are r e l e a s e d unexpectedly from high on t h e s lopes , so t h e rock fragments can have an extremely h igh v e l o c i t y t h a t c a r r i e s some of them beyond t h e mapped e x t e n t of t h e depos- i t . Taluses a r e mapped above u n d i f f e r e n t i a t e d landsl ide d e p o s i t s (Qls) i n some p l aces where t h e rocks cont inue t o r a v e l from bedrock faces even though most of t h e o r i g i n a l d e b r i s moved as a l a n d s l i d e . Ta luses composed of small fragments can be e a s i l y excavated and g e n e r a l l y d r i l l e d without t r o u b l e . The d e p o s i t s having c o a r s e r fragments would be moder- a t e l y d i f f i c u l t o r d i f f i c u l t t o excavate wi th power equipment and d i f f i - c u l t t o d r i l l because of t he l a r g e loose p i eces .

The coarseness and hardness o f rock fragments g e n e r a l l y makes t a l u s e s on ly s l i g h t l y s u s c e p t i b l e t o e ros ion i n t h e p o r t i o n s low on t h e s lopes . Ta luses t h a t have a f ine-gra ined mat r ix , such a s t hose along t h e s lopes of Mount Roberts south o f downtown Juneau, a r e more susceptible t o ero- s i o n from heavy r a i n f a l l and a r e g u l l i e d . Creep i s common; s l o p e s formed by excavat ions r a v e l and s l i d e . Excavations i n a c t i v e o r i n a c t i v e t a l u s e s gene ra l ly w i l l exceed t h e n a t u r a l angle of repose o f t h e m a t e r i a l , reduce the s t a b i l i t y , and may cause s l i d e s .

Ta luses a r e unsu i t ab l e l o c a t i o n s f o r s t r u c t u r e s because they are uns t ab le and because blocks o f rock s t i l l f a l l on most of them from time t o t ime. In case of a s t rong earthquake rock fragments on s l o p e s above t a l u s e s probably would be dis lodged and would r o l l and bound down t h e s t e e p moun- t a i n s i d e . The t a l u s i t s e l f i s uns t ab le , and shaking dur ing ear thquakes elsewhere has caused t a l u s e s t o move downslope. Roads a t t h e lower mar- g i n s of t a l u s could be blocked by s l i d e s o f t a l u s . S t r u c t u r e s such as water t roughs , b u i l t a c ros s t a l u s e s , probably would be damaged. The water f lowing from such a broken t rough would s a t u r a t e t h e m a t e r i a l below and probably cause a d e b r i s s l i d e o r d e b r i s f low,

Debris-flow d e p o s i t s (Qfl)

Debris-flow d e p o s i t s , as mapped i n t h e Juneau a r e a , inc lude d e p o s i t s o f d e b r i s flows o r d e b r i s avalanches, one d e b r i s s l i d e , and one sand f low, and a11 c o n s i s t of materials of va r ious s i z e g r a d a t i o ~ l s t h a t moved r ap id - l y under w e t condi t ions (i'arnes, 1958). Four of t h e f i v e mapped d e b r i s flolvs along t h e s lopes of Mount Roberts a r e dark g ray and c o n s i s t o f l o c a l l y der ived t a b u l a r g reensch i s t fragments mixed with a few rounded boulders of g r a n i t i c rock i n a ma t r ix of f i n e r m a t e r i a l . The o t h e r ma_npsd d e b r i s f law \/as der ived l a r g e l y from colluvium and s o i l and added d e b r i s from houses, r e t a i n i n g wa l l s , and o t h e r s t r u c t u r e s . The d e b r i s s l i d e , loca ted i n t h e Salmon Creek v a l l e y , c o n s i s t s o f s h e l l - r i c h d i a - micton. The sand flow, l oca t ed along lower Salmon Creek, i s e n t i r e l y brownish-gray sand and sandy g rave l . The age of a l l t h e s e d e p o s i t s is Holocene, and a l l the d e b r i s f lows but one occurred dur ing h i s t o r i c t ime ,

The debr i s - f low d e p o s i t s a long t h e s lopes o f Mount Koberts near F i r s t S t r e e t i n sou theas t Juneau and along Gast ineau Avenue and F rank l in S t r e e t inc lude a t l e a s t f o u r f lows. One of t h e f o u r d e p o s i t s mapped in- c ludes two flows t h a t occurred 32 yea r s a p a r t . The d e p o s i t s form narrow bands o f rubble t h a t have a bulbous t o fan-shaped lower t e y i n u 8 where not modified by cons t ruc t ion . The flows extend down t h e 35 -45 s lope of Mount Roberts i n narrow g u l l i e s o r t r e e - c l e a r e d flow t r a c k s . A t t h e i r heads some o f t h e g u l l i e s b i f u r c a t e n e a r t h e s lop ing r i d g e t o p , 1,000- 1,500 f e e t above Gast ineau Channel, The debris-f low d e p o s i t s form r i d g e s i n t h e lower p a r t t h a t are d i s t i n c t l y d i f f e r e n t from t h e g e n e r a l l y smooth s t e e p s lopes of t h e mountainside. These d e p o s i t s vary i n t h i c k n e s s , bu t range from 5 t o a t l e a s t 20 f e e t .

The d e b r i s s l i d e i s along t h e southern s i d e o f Salmon Creek v a l l e y and extends from below t h e flume t o creek l e v e l . The pa th o f t h e d e b r i s s l i d e i s f l o o r e d by smooth bedrock. Large i s o l a t e d masses t h a t moved as u n i t s remain i n the upper p a r t of t h e t rough, b u t s a t u r a t e d m a t e r i a l formed a hummocky lobe a t t h e base. The t h i c k n e s s o f t h i s flow i s about 10-12 f e e t . The sand flow is nea r t h e mouth of Salmon Creek a long an o l d road alin'ement and a t t h e no r the rn en$ of an o ld b r idge . The p r e s e n t s l o p e i s near t he angle o f repose , 30 -3s0, bu t seems t o be s t a b l e now and i s covered by shrubbery. Thickness of t h e flow i s unknown,

Densi ty of t h e debr i s - f low d e p o s i t s a long Mount Roberts probably is g r e a t e r t han t h a t of t h e m a t e r i a l s from which t h e flows were de r ived . The r e l a t i v e l y undisturbed source mater ials--col luvium, broken rock , and so i l - -gene ra l ly con ta in numerous voids . A f t e r water d r a i n s from a d e b r i s flow, t h e depos i t b e c o r ~ s s t a b i l i z e d , compacts, and i s f i r m e r and l e s s porous than t h e undis turbed m a t e r i a l s . The sand i n t h e flow along lower Salmon Creek, however, probably is about a s loose a s t h e undis turbed sand i n t h e b l u f f . The ma te r i a l i n the d e b r i s s l i d e i n Salmon Creek came from a glaciomarine depos i t t h a t i s f i rm and hard when d ry , bu t which flows e a s i l y when wet. As it d r i e s t h e m a t e r i a l seems t o become as f i rm as t h e o r i g i n a l depos i t .

a A l l t h e d e b r i s flows recognized i n t h e Juneau a r e a occurred a f t e r sudden o r unusual amounts of water were added t o t h e m a t e r i a l forming t h e s teep .lopes. Those along )lount Roberts moved a f t e r being sub jec t ed t o prolonged and in t ens ive r a i n f a l l . .4 d e b r i s f low on Sept. 28, 1918, followed 7.45 inchcs o f r a i n i n 3 days, and t h e f low on Jan. 2 2 , 1920, followed 6.30 inches of r a i n i n 3 days. A c a t a s t r o p h i c d e b r i s f low on Nov. 2 2 , 1936, t h a t caused 14 dea ths followed 3.85 inches of r a i n i n 24 hours , and followed a per iod o f heavy r a i n s t h a t amounted t o 18 inches i n Octo- b e r an: provided an a d d i t i o n a l 25 inches through h'overnber. A d e b r i s flow on Oct. 51, 1949, followed 2.55 inches o f r a i n w i th in 24 hours , and one i n l a t e October 1952 f o l l o r ~ e d a prolonged per iod o f r a i n dur ing which t ime only f i v e o f 77 days had no p r e c i p i t a t i o n (U.S. Weather Bur. , 1918- 58) .

The s lopes of blount Roberts are covered by broken rock , colluvium (which here is a mixture of rock fragments and weathered and loosened debr i s from p a s t g l a c i a t i o n s ) , and s o i l . The lower s lopes a r e most ly between

0 35' and 45 , the upper s lopes a r e commonly SO o r s t e e p e r , and g r a v i t y normally causes colluvium on such s t e e p s l o p e s t o move s lowly downslope. In add i t i on , t h e j o i n t s i n t h e s l a t y s h i s t o s e rocks form p lanes of weak- nes s t h a t d i p s t e e p l y outward from t h e mountain and provide loose p i eces o f rock. Sa tu ra t ion of such m a t e r i a l s r e s u l t s i n sudden movements as d e b r i s f lows.

The 1968 d e b r i s s l i d e i n t h e Salmon Creek v a l l e y probably r e s u l t e d from .. water escaping from a flume t h a t c ros ses a r a v i n e a t t h e head o f t h e d e b r i s s l i d e . ghe glaciomarine diamicton t h e r e became s a t u r a t e d and

L moved down a 28 bedrock s l o p e t o c reek l e v e l .

The r a i n s on Nov. 22 , 1936, a l s o caused t h e sand flow downstream from the bedrock gorge nea r t h e mouth of Salmon Creek. Two sand flows came down i n about t h e same a r e a , bu t t he second f i l l e d and blocked t h e chan- n e l of Salmon Creek so t h a t t h e water swept around t h e b r idge and over t h e road. The br idge was swept away dur ing t h e n igh t of Nov. 23 (The Daily Alaska Empire, Nov. 23 and 24, 1936).

The d e b r i s flow along Gold Creek and i n Evergreen Bowl appa ren t ly occurred before Juneau was s e t t l e d ; t h i s assumption i s based on t h e presence of mature t r e e s i n t e r s p e r s e d w i t h decaying logs on t h e d e p o s i t . The d e b r i s flow seems t o have been der ived from t h e m a t e r i a l brought down from Mount Juneau as p a r t of t h e r o c k s l i d e avalanche.

The debris-f low d e p o s i t s have a h igh a f f i n i t y f o r water . They would be extremely uns t ab le i n t h e event o f s t rong se ismic shak ing i n t h e Juneau a r e a while t h e deposi t , were s a t u r a t e d . The upper p a r t s of t h e flow d e p o s i t s probably would move downslope and conceivably could ove r r ide o r even extend beyond t h e lower p a r t s of t h e d e p o s i t s . In a d d i t i o n , l oose s u r f i c i a l depos i t s high on t h e s l o p e would be shaken loose i f t h e v ib ra - tions were of long du ra t ion , causing new f lows o r s l i d e s t o move down t h e t r a c k s of previous d e b r i s flows. The a r e a ad j acen t t o South Frank- lin and Gastineau Avenues i n Juneau probably i s e s p e c i a l l y s u s c e p t i b l e

t o carthquahc-induced flows. If t h e d e p o s i t s were d r y a t t h e t ime of an earthqtiakc , t he re probably \could be some s h i f t i n g and d i f f e r e n t i a l compac t ion , and poss ib ly somc movement dormslope. The sand depos i t along Salmon Creek i s p a r t i c u l a r l y s u s c e p t i b l e t o d ry flowage from shaking.

The recurrence of d e b r i s flows along t h e lower s l o p e s o f Elount Roberts sugges ts t h a t upslope condi t ions f avo r t h e occurrence of s i m i l a r flows i n t h e f u t u r e . In an e f f o r t t o b e t t e r understand t h e s lope cond i t i ons , e s p e c i a l l y t h e s t a b i l i t y , a long t h e mountainside, I made a t r a v e r s e dol.w.:clrd from about 1,000 f e e t above Gast ineau Avenue. The mountainside,

0 which i s co~rnonly as s t e e p as 50 by measurement, a l s o inc ludes small c l i f f s t h a t r equ i r ed r a p e l l i n g i n o rde r t o descend s a f e l y . Even the tree-covered po r t ions of t h e upper 400 f e e t of t h e s lope were s o s t e e p t h a t ropes were used f o r safety.

In a d d i t i o n t o t h e s teepness o f t h e mountainside, two geologic f a c t o r s c o n t r i b u t e s i g n i f i c a n t l y t o the high s l o p e i n s t a b i l i t y . These a r e t h e type of rock and t h e f r a c t u r e s r e l a t e d t o weak zolles i n t h e rock. So f t p l a t y mica-rich g reensch i s t u n d e r l i e s most of t h e upper par t of t h e mountainside. Th i s rock d i s i n t e g r a t e s when weathered, and t h e small f l a t mica f l a k e s accumulate with wir~dblown silt t o form a t h i n loose a

cover on t h e bedrock. P ieces of broken rock, from 6 inches t o 5 o r 6 f e e t i n l eng th , l i e p reca r ious ly on t h e s t e e p s lopes i n t h e mixture of mica f l a k e s , sma l l e r rock p i eces , an3 s i l t . The f o l i a t i o n o r l aye r ing of t h e i s - p l a c e s o f t rock d i p s g e n e r a l l y away from Gast ineau Channel a t

; a about 47 , but i n t h e n e a r l y v e r t i c a l f a c e s of t h e small c l A f f s t h e l a y e r s a r e bent outward and i n p l aces a c t u a l l y d i p about 10 toward t h e channel ( f ig . 6 ) . Where t h e layers arc bent , they p a r t and t h e rock . .. becomes extremely s u s c e p t i b l e t o weathering and dislodgment.

Fractures c o n s i s t i n g of j o i n t s o r s e t s of j o i n t s also weaken t h e rock and reduce s lope s t a b i l i t y . I hypothesize that t h e f r a c t u r e s i n t h i s s lope may be t h e r e s u l t o f cont inuing r e l e a s e of s t r e s s confined i n t h e rock. The absence of conf in ing p re s su re i n t h e d i r e c t i o n o f Gast ineau Channel may al low t h e rock t o expand about p a r a l l e l t o t h e e x i s t i n g s lope and open cracks along p l anes of weakness i n t h e rock. If t h e s t r e s s - r e l e a s e concept a p p l i e s t o t h i s s lope , t h e c racks may cont inue t o en l a rge and new c.les develop over t h e yea r s as t h e s t r e s s i s s lowly r e l eased .

The j o i n t f r a c t u r e s i n t e r s e c t t h e l aye r ing o f t h e rock t o form a criss- c r o s s set o f cracks. Three j o i n t s e t s seem t o be dominant i n t h e a r e a above Gastineau Avenue. One j o i n t set s i r i k e s N . 80' W. gnd has a n e a r l y v e r t i c a l d ip . Another s e t s t r i k e s N. 85 E. and d i s 80 northward; F t h e l a s t o f t h e s e domirant j o i n t sets s t r i k e s N. 35 W . , almost p a r a l l e l t o the mountainside, and d i p s southwestward at 75'. Some of t h e p i e c e s o f rock between t h e s e numerous i n t e r s e c t i n g f r a c t u r e s a r e l oose , and o t h e r s w i l l loosen with t ime . Moisture moving along t h e open cracks between rock fragments he lps weaken t h e rock, and t r e e s send r o o t s i n t o t h e c racks and thereby t end t o a c c e l e r a t e t h e sepa ra t ion of t h e rock fragments. Figure 6 shows a l s o t h e r e l a t i o n s h i p o f some o f t h e j o i n t s t o t h e f o l i a t i o n o r l aye r ing and t h e c h a r a c t e r i s t i c p a t t e r n t h a t r e s u l t s from t h e i n t e r s e c t i n g f r a c t u r e s .

Troughs 10-20 f c c t dccp and 15-30 f e e t wide o r i g i n a t e near t h e r i d g e above t h e Gast ineau Avenue-Franklin S t r e e t a r ea . Severa l of t h e s e t roughs merge and extend down t h e f a l l - l i n e t o t h e base of t h e mountain. i a e s e t roughs mark a r e a s where t h e e a r t h m a t e r i a l s o f t h e mountainside have s l i d o r f l o ~ i c d do\inslopc toward Juneau. F rac tu re s along se t s of j o i n t s seemingly c o n t r o l development of these t roughs , and the wa l l s a r e genera l ly jo in r surfaces. In tense ly cracked and broken rocks pro- j e c t from t h e wa l l s ; s i m i l a r p i e c e s have f a l l e n and have accumulated on t h e bedrock f l o o r s o f t h e t roughs . Numerous l a r g e blocks of rock l i e i n a rubb~y mixture of snaller rock fragments , s i l t , s o i l , and f a l l e n t r e e s . Most of t h i s d e b r i s l i e s a t ang le s s t e e p e r t han t h e angle o f repose , and consequent ly the ~ u b b l e i s very unstable. Pieces of t r e e t runks , l imbs, and r o o t s apparent ly act t o r e s t r a i n t h e mass from moving downslope. Th i s debr i s is so loose , and l y i n g on such s t e e p s l o p e s , t h a t my a s s i s t a n t and I avoided walking on it f o r f e a r of s t a r t i n g r o c k s l i d e s .

On t h e lower p a r t of t h e mountainside, t h e t roughs a r e p a r t i a l l y f i l l e d wi th t h e uppermost p o r t i o n s o f d e b r i s flows t h a t moved down i n t o t h e r e s i d e n t i a l area below. Along Gast ineau Avenue, r i d g e s mark t h e h i s t o r - i c a l d e b r i s f lows; between t h e s e r i d g e s a r e o l d e r t ree-covered p r e h i s - t o r i c t r anspor t ed m a t e r i a l , he re mapped as u n d i f f e r e n t i a t e d l a n d s l i d e d e p o s i t s (Qs l ) , i n d i c a t i n g t h a t s l i d e s and flows have occurred over a long per iod of t ime.

It i s d i f f i c u l t t o p r e d i c t whether f u t u r e f lows w i l l move e x c l u s i v e l y along t h e pa ths e s t a b l i s h e d by e a r l i e r f lows , Tracks of prev ious f lows, as marked by t h e t roughs , would h e l p concen t r a t e heavy r a i n f a l l . How- ever , t o my knowledge, on ly t h e 1952 flow followed a p a t h e s t a b l i s h e d by an e a r l i e r flow, i n t h i s ca se one t h a t occurred i n 1920. A l l o t h e r f lows apparent ly o r i g i n a t e d i n ma te r i a l t h a t apparent ly had n o t p rev ious ly f a i l e d by d e b r i s flowage.

I t is even more d i f f i c u l t t o p r e d i c t s p e c i f i c a l l y when t h e flows w i l l occur . Heavy p ~ o l o n g e d r a i n f a l l preceded each o f t h e known d e b r i s f lows. The f a l l of the year i s the most common season f o r the f lows t o move. The recorded amount o f r a i n f a l l f o r d i f f e r e n t occurrences o f d e b r i s f lows, however, ranged from about 2% inches i n 24 hours , n e a r t h e end of almost 2 months cf continuous r a i n f a l l , t o 7% inches o f r a i n i n 3 days. Swanston (1970, p. 14) s tud ied t h e mechanics of d e b r i s f lows i n shal low permeable t i l l s o i l s i n blaybeso Creek v a l l e y on Pr ince of WalesoIsland, southeas t of t h e Juneau a rea . He considered a s l o p e angle of 37 t o be c r i t i c a l and t h a t m a t e r i a l s i n s l o p e s s teeper than t h a t a r e i n imminent danger of s l i d i n g when t h e cohesiveness of t h e s o i l i s destroyed o r d i s rup ted . I consider s lope s t a b i l i t y of t h e t h i n c o l l u v i a l ma te r i a l over t h e bedrock t o be of s p e c i a l concern, and a c a r e f u l eva lua t ion should be made of t h e p o s s i b l e e f f e c t cons t ruc t ion might have on the s t a b i l i t y of t h e s lope-covering d e p o s i t s , e s p e c i a l l y i n t h e areas between t h e t r a c k s o f known flows.

A s t h e above d i scuss ion b r i n g s o u t , the mountainside above t h e Gast ineau Avenue-Franklin S t r e e t a r e a seems t o e x i s t under geologic and topographic condi t ions t h a t , i n t ime, reduce t h e cohesiveness of t h e s o i l o r rock,

and t h a t r e s u l t i n t he accumulation of loose rubble on extremely s t e e p s lopes . Spec i f i c triggering a c t i o n s t h a t cause d e b r i s flows remain unknotcn. Physical and crivironmental cond i t i ons do change with t ime, however. Pe r iod ic heavy r a i n s permeate t h e rubble , and t h e moisture content may rcach t h e poin-t: o f extreme s a t u r a t i o n where t h e rubble w i l l florv of i t s o m : i ~ i ~ h t . Trees g ~ o u on t h c mountainside, but cvcn tua l ly d i e , Though t h e roots of a gro\<ing t r e e can push rock s l a b s apart and cause p icces t o f a l l , many r o o t s bind l a r g e s l a b s o f bedrock and h e l p s t a b i l i z e t h e s l ope . Dcnth and decay o f such a t r e e removes any s t r e n g t h from in t e r tw ined r o o t s and reduces s lope stability. Wind t h a t commonly accclmpnnies heavy autumn r a in s ma>. be an important t r i g g e r i n g a c t i o n . Slabs of rock as l a r g e a s 10 f e e t long were seen he ld i n p l ace by f i n e - grained mixtures of weathered rock , s i l t , and humus f i l l i n g c racks . These massive rocks a l s o act a s w a l l s t o r e s t r a i n loose m a t e r i a l l y i n g upslope. If a t r e e growing from a crack is blown down dur ing a heavy windstorm, i t seems conceivable t o me t h a t t h e rock s l a b might be d i s - lodged and f a l l , thereby al lowing t h e loose m a t e r i a l t o move down t h e mountainside. Some of t h e s l a b s would f a l l f r e e as much as 50 fee t before landing on t he accumulation of saturated rubb le . The weight and s t r i k i n g force o f a l a r g e f a l l i n g rock could s t a r t t h e d e b r i s f lowing downs lope.

The b r i e f examination of t h e mountainside above south Juneau r e i n f o r c e s my b e l i e f t h a t d e t a i l e d examination of t h e s lope should be a prerequi - s i t e before any major urban changes a r e made i n t h e Gast ineau Avenue- Frankl in S t r e e t a r e a . Excavations f o r low-grade f i l l m a t e r i a l w i l l reduce t h e s t a b i l i t y o f t h e upslope m a t e r i a l . The h igh a f f i n i t y of t h e d e p o s i t s f o r water makes any such f i l l m a t e r i a l s u s c e p t i b l e t o slumping wi th s a t u r a t i o n . No p r a c t i c a l u s e i s known f o r t h e debr i s - f low d e p o s i t s , e s p e c i a l l y where on t h e lower s l o p e s of t h e mountain. Open green space probably i s t h e b e s t u se f o r t h e debr i s - f low depos i t s , because o f t h e p o s s i b i l i t y r h a t f u t u r e f lows may move down t h e same rou te s .

Rockslide-avalanche d e p o s i t s (Qra)

A s used i n t h i s r e p o r t , a rocks l ide-ava lanche is a rock avalanche t h a t either f a l l s a s h o r t d i s t a n c e and then avalanches downslope, o r s t a r t s a s a s l i d e and becomes an avalanche. The rock t h a t f a l l s may s t a r t as one block, o r s e v e r a l , bu t repea ted impact g e n e r a l l y causes it t o d i s - i n t e g r a t e as it moves downslope. I t qu i ck ly becomes a mass of s l i d i n g , r o l l i n g , and bounding rock d e b r i s . Rockslide-avalanche d e p o s i t s c o n s i s t of jumbled rock fragments of many s i z e s . Some a r e as l a r g e a s 30 f e e t across ,but 5 - t o 8-foot s i z e s a r e most common. Each avalanche depos i t is p r i n c i p a l l y o f one rock type; t h e depos i t on Douglas I s l and i s composed mostly of blocks of a greenish metamorphosed p o r p h y r i t i c d ike rock; t h e su r f ace p a r t o f t h e greenish rock weathers t o a reddish-brown r i n d a b o u t , & inch t h i c k rhat i s coated by a light-brown l a y e r . The d e p o s i t s on t h e mainland were der ived from greenstone o r g reensch i s t . A l l of t h e rocks l ide-ava lanche d e p o s i t s appa ren t ly are of Holocene age.

There are f i v e rocks l ide-ava lanche d e p o s i t s i n t h e v i c i n i t y of Juneau. One is on t h e o u t s k i r t s of Juneau; t h e rocks l ide-ava lanche s t a r t e d on

t h e s i d e o f bluu~lt Juneau, where a l a r g e s c a r can be seen, crossed t h e Gold C r o c k v a l l e y , and rose more than 180 f e e t on t h e oppos i te s lope where t h e depos i t now p a r t l y covers a bedrock r i d g e t h a t connects Mount Plaria t o Slount Roberts . Spencer (1906, p. 83) recognized t h e l a r g e de- p o s i t i n Gold Creok v a l l e y a s an anc ien t s l i d e o r avalanche t h a t dnm~ned Gold Crech. Th i s dcpos i t i n Gold Creck v a l l e y is a t l e a s t 38 f e e t t h i c k and i s so massive and t h e fragments so l a r g e t h a t t o t h e casua l observer the d c b r i s looks l i k e knobs o f bcdrock surrounded by s u r f i c i a l m a t e r i a l . High on t h e bedrock r i d g e l a r g e s c a t t e r e d blocks form a depos i t about 300 fcc t tcidc. The absence o f a cont inuous cover of d e b r i s leads me t o concludc t h a t these very 13rqe p ieces bounded up t h e s lope a s a resu l t o f energy provided from f a l l i n g , r a t h e r t han from being c a r r i e d on an a i r cushion. A d i s t i n c t and abrupt margin i s t y p i c a l of t h e d e p o s i t . The leading edge of t h e avalanche p ro j ec t ed o f f t h e blount Maria-blount Roberts r i d g e and cont inued dohn t o t h e s i t e of Juneau. I s o l a t e d angu- l a r fragments 2 f e e t o r more i n l a r g e s t dimension provide evidence t h a t t h e avalanche reached a t l e a s t a s f a r a s t h e upper p a r t o f 6 t h S t r e e t . Building and grad ing over t h e yea r s probably removed o r bu r i ed most of t h e fragments.

This massive rocksl ide-avalanche blocked Gold Creck completely and formed a l ake (Spencer, 1906, p. 84) . The avalanche depos i t extends about 38 fee t above t h e concre te flume a t the lower end of Last Chance Basin. T e r r a i n on t h e south s i d e o f Gold Creek c o n s i s t s o f r i d g e s and e longa te channels i n t h e v a l l e y and l a r g e boulders covering t h e s l o p e o f Mount Maria. The r i d g e s and channels a r e i n t e r p r e t e d a s be ing caused by e ros ion a s Gold Creek overflowed t h e avalanche dam.

The d e b r i s flow d e p o s i t a long Gold Creek n e a r and i n Evergreen Bowl i s i n t e r p r e t e d a s being t h e r e s u l t o f breakthrough o f the avalanche dam and r a p i d dra inage of t h e l ake behind t h e dam causing a d e b r i s flow. Large 8-foot boulders i n t h e debr i s - f low m a t e r i a l a r e no t g r e e n s c h i s t , bu t a r e f i ne -g ra ined rock s i m i l a r t o t h a t found i n t h e avalanche depos i t a long Gold Creek and on the r i d g e between Mount Maria and Mount Roberts.

J o i n t fractures on the s lopes of Mount Juneau apparent ly weakened the rock and provided t h e proper circumstances f o r a massive r o c k s l i d e - avalanche. Examination on t h e r i d g e j u s t below t h e t o p of Mount Juneau d id nor show any zones o f mine ra l i za t ion t h a t might have weakened t h e rock. In s t ead , t h e rock i s hard and f i rm bu t f r a c t u r e d along wel l - def ined j o i n t s e t s . The j o i n t s t h a t appa ren t ly a r e most r e l a t g d t o t h e rocksi ide avalanche $re t hose t h a t s t r i k e N. 65: W. and d i p 83 NE, s t r i k e N. 85 W . and d i p 75 S. , and t h a t s t r i k e N. 50 E. and d i p 75' SE. These j o i n z s a r e e i t h e r p a r a l l e l t o t h e f a c e of t h e avalanche s c a r on the mountain, o r i n t e r s e c t each o t h e r o r t h e f o l i a t i o n t o provide f r a c - t u r i n g of t h e rock under ly ing t h e s lope . T h e c a u s e o f such r o c k s l i d e s may be weathering along t h e s e f r a c t u r e s over long pe r iods o f t ime, o r may be continued expansion o f t h e f r a c t u r e s as a r e s u l t o f p o s s i b l e s t r e s s r e l e a s e a c t i v e s i n c e d e g l a c i a t i o n o f t h e Juneau a r e a . One t r i g - g e r may have been a s t rong ear thquake i n p r e h i s t o r i c t ime t h a t involved what i s now the Juneau area.

The s p e c i f i c age of t h i s rocksl ide-avalanche is unknown. Trees 15-20 inches i n diameter t h a t a r e growing on r o t t i n g t runks a r e about 60 years o ld . L~lrgcr d i n ~ ~ i c t e r s tumps, i n various s t a g e s o f decay, a l s o are on t h e avn lanche d~>; .os i t . Phot3gr,zphs of J u n e a u taken n c a r t h e t u r n o f t h e century suggc j t : t h a t t h e r i d g e between :.:aunt >laria and blount Roberts was probably logged about 60 yea r s ago, thus accounting f o r t h e r e l a t i v e l y young t r e e s groii ing on t h e avalanche depos i t . Some o f t h e o l d e r t r e e t r unks a r e i n advanced s t a g e s of decay. Assuming t h a t a t l e a s t 150 o r 200 years a r e requi red i n t h e Juneau a r e a f o r decay, a very rough e s t i m a t e f o r the rocksl ide-avalanche i s about 200-250 years.

A l a r g e and impressive rocksl ide-avalanche temporar i ly dammed Lemon Creek i n p r e h i s t o r i c t ime. T h i s s l i d e had i t s source on t h e sou theas t e rn f l ank o f Heintzleman Ridge . A jumbled mass o f rock fragments, some as l a r g e as 50 f e e t i n l eng th , l i e s on t h e no r the rn end of a bedrock r i d g e and extends southward t o f i l l p a r t of the v a l l e y behind t h e r idge . Canyon Creek was also blocked, and a temporary l ake probably f i l l e d p a r t of t h e v a l l e y upstream from t h e avalanche; however, no depos i t s were loca t ed t h a t might r ep re sen t t h e remnants o f such a lake. Rock sp ines s t a n d promi- n e n t l y a t t h e south edge o f t h e avalanche d e p o s i t , and they form conspicuous landforms when viewed from t h e Lemon Creek t r a i l where i t t r a v e r s e s a bedrock r idge . The f o l i a t i o n i n t h e sp ines matches t h a t o f t h e bedrock i n t h e r idge west o f t h e avalanche depos i t . Consequently, t h e s e sp ines a r e r e a l l y bedrock i n p l ace and apparent ly r e f l e c t e ros ion along f o l i a t i o n and j o i n t f r a c t u r e s by waters t h a t overflowed from t h e dammed lake, The s t r i k e s of t h e l a r g e fragments i n t h e avalanche mass, however, vary and inc ludes N. S O 0 W., N. 40" E . , and N. 45' W . , and t h e d i p s a l s o are v a r i a b l e and i n both eastward and westward d i r e c t i o n s , The age of t h i s rocksl ide-avalanche depos i t i s n o t known, bu t seemingly mature trees cover t h e s u r f a c e of t h e d e b r i s ; thus it i s a t l e a s t s e v e r a l hundred years o ld .

An equa l ly impressive rocksl ide-avalanche depos i t l i e s on t h e northwest s i de o f Salmon Creek, n e a r t h e junc t ion of t h e t r a i l and f o o t br idge over t h e c reek , where b locks o f rock 5-6 feet ac ros s cover t h e sur face . Spaces between the b locks a r e gene ra l ly open but some con ta in f i n e sand and s i l t . The terminus o f t h e depos i t extends along Salmon Creek f o r about 2,000 feet . The avalanche extended i n t o t h e creekbed downstream from t h e f o o t br idge b u t has been l a r g e l y removed by t h e stream. The depos i t apparent ly came from a s c a r r e d a r e a on t h e lower s l o p e o f Blackerby Ridge. Mature t r e e s cover t h e s l i d e d e b r i s .

The avalanche depos i t n e a r t h e a i r p o r t is r e l a t i v e l y small but extends as f a r downslope a s t h e Glac i e r Highway. The source a r e a is a n e a r l y v e r t i c a l 400-foot bedrock c l i f f above a bench t h a t is 70 feet above t h e road. The ma te r i a l i n t h i s d e p o s i t probably f e l l f r e e from t h e f a c e of t h e c l i f f and became a rockfa l l -ava lanche . Large angular fragments as l a r g e a s 30 f e e t ac ros s r e s t on the bench. A lobe of t h e depos i t which conta ins fragments a s large a s 10 f e e t can be seen a t t h e edge o f t h e road. Mature t r e e s a l s o cover t h i s depos i t .

a On Douglas I s l a n d , n l a r g e rocksl ide-avalanche depos i t extends 2,500 feet outuard from a s c a r on Slount Anderson and ends on a f l a t t i s h bench. A s e r i e s of lobnte r i d g e s confines swamps and small ponds wi th in t h e avalanche depos i t . Blocks o f moss-covered rock as l a r g e a s 10 feet form grotesque shapes i n t l lc dim l i g h t o f t h e dense f o r e s t , The avalanche depos i t ends ab rup t ly wi th d i s t i n c t edges. The r o o t s of a p a r t i a l l y decayed t r e e found on t h i s depos i t were entwined around t h e avalanche fragments; growth and decay of the t r e e probably r e p r e s e n t s s e v e r a l hundred years . The o l d e s t l i v i n g t r e e found i n t h e same a r e a o f t h e avalanche d e p o s i t , horzeuer, i s only about 60 years o ld . Attempts t o f i n d long-time r e s i d e n t s who might remember t h e rocksl ide-avalanche were unsuccessful .

This rocksl ide-avalanche apparent ly r e s u l t e d from t h e s e p a r a t i o n of d ike rock from ad jo in ing black s l a t e . The d i k e approximately p a r a l l e l e d t h e f ace o f t h e c l i f f . The weathering r i n d suggests prolonged weakening o f t h e rock, b u t t h e t r i g g e r f o r t h e avalanche i s unknown.

I n f i l t r a t i o n i n rocksl ide-avalanche depos i t s i s g e n e r a l l y good i n t h e a r eas of l a r g e fragments, bu t f a i r t o poor i n a r eas where many smal l p i eces o r f i n e ma te r i a l p a r t i a l l y f i l l l a r g e voids i n t h e avalanche depos i t . The swamps and ponds on t h e Douglas I s l and depos i t probably l i e on very t h i n avalanche d e b r i s and r e f l e c t t h e more impermeable n a t u r e o f t h e d e p o s i t s underlying t h e rocksl ide-avalanche. Runoff on a l l depos i t s i s fa i r t o very good, depending on p a r t i c l e s i z e , i n f i l t r a t i o n r a t e , and s l o p e conf igura t ion .

Severe ground v i b r a t i o n a s soc i a t ed with a major ear thquake could cause rock f a l l s from t h e c l i f f s above t h e fou r known avalanche d e p o s i t s and poss ib ly a l s o on s t e e p s l o p e s elsewhere i n the f i o r d a reas . A s t r o n g shock can be expected t o send e i t h e r i nd iv idua l fragments o r l a r g e masses down t h e s lopes . A l l s t e e p unvegetated s l o p e s e s p e c i a l l y should be con- s ide red p o t e n t i a l l y hazardous.

Because t h e p i eces and fragments w i th in rocksl ide-avalanche d e p o s i t s a r e mostly loose and jumbled, t h e depos i t s gene ra l ly a r e uns t ab le . The upslope p a r t s of t h e d e p o s i t s a r e probably less s t a b l e t han t h e lower p a r t s . Excavation can r e s u l t i n embankments t h a t w i l l r a v e l , and it may loosen large, rock fragments. Excavation and o t h e r earth-moving a c t i v i t i e s can a l t e r t h e equi l ibr ium i n t h e depos i t s .

Und i f f e ren t i a t ed l a n d s l i d e s (Qsl)

Und i f f e ren t i a t ed l a n d s l i d e s i n t h e Juneau a r e a c o n s i s t of l a r g e and small + depos i t s of heterogeneous mixtures o f e a r t h m a t e r i a l s . Included a r e de-

p o s i t s o f rock d e b r i s t h a t moved downslope a s p a r t of massive snow avalanches and t h a t l i e beyond t h e margins o f t a l u s d e p o s i t s , o r d i r e c t l y a t the base of s t e e p t a l u s - f i l l e d chutes . Most o f t h e l a n d s l i d e d e p o s i t s a r e mixtures of bedrock fragments i n a s i l t y sand mat r ix o f most ly s o i l and other s u r f i c i a l depos i t s . Blocks o f rock as l a r g e as 30 f e e t ac ros s occur

i n d i v i d u a l l y o r i n c l u s t e r s a t t h e base o f some mountain s l o p e s , and rocks 5-10 f c c t long are common i n s l i d e d e b r i s i n t h e v a l l e y s of Gold, blcGinnis, and Nugget Creeks. The s l i d e i n Lemon Creek val ley, and most o f t h e s l i d e depos i t s i n south Juneau, inc lude p i eces of grcenschist 6 inches t o 1 f ~ c t i n length i n a mat r ix o f sand-r ich o rgan ic m a t e r i a l . The age of these s l i d e s i s Holocene, and some are h i s t o r i c a l l y young.

Some l a n d s l i d e depos i t s occur i n p l aces along s lopes border ing Gast ineau Channel, bu t most seem t o b e along t h e nor thern s l o p e s of Gold, Salmon, Lemon, and Nugget Creek v a l l e y s . The l a r g e s l i d e on blcGinnis Mountain, hoizever, l i e s on t h e c a s t ~ i d c o f hIcGinnis Creek. Small l a n d s l i d e s along Gast ineau Channel have a r c u a t e d e b r i s r i dges t h a t seem t o merge upslope i n t o t a l u s o r are bounded by bedrock s lopes . The e longa te s l i d e s along Lemon and Salmon Creeks a r e narrow and have f l a t t o concave surfaces t h a t extend upslore behond t h e mapped area. The l a rge s l i d e s along Kugget and Gold Creeks are fan-shaped and extend upslope t o f i l l narrow bedrock t r o u g l ~ s o r chutes .

Most l ands l ides i n t h e Juneau a r e a apparent ly r e s u l t from l o s s o f s h e a r s t r e n g t h by loosening of rock along joint f r a c t u r e s i n response t o s t r e s s r e l e a s e cont inuing s i n c e r e t r e a t o f t h e g l a c i a l i c e i n l a t e P l e i s tocene time, Other f a c t o r s t h a t may be important a r e weathering o f j o i n t su r - faces or of mineral ized zones p a r a l l e l t o c l i f f s o r s t e e p s lopes and l o s s of f r i c t i o n because of water f i l l i n g j o i n t f r a c t u r e s . The l a n d s l i d e depos i t a long FIcGinnis Creek l i e s below a V-shaped scar on t h e s lope leading down t o t h e s l i d e depos i t . Some o f t h e j o i n t sets on t he r i d g e above t h e c l i f f a r e almost p a r a l l e l t o t h e c l i f f face . The jo in t se ts that s t r i k e N. 43' W. and d i p 78' NE. and t h o s e t h a t s t r i k e N. 35' W. and d i p 55" SW. a r e almost p a r a l l e l t o t h e v e r t i c a l c l i f f f a c e a t t h e t o p o f the l a n d s l i d e a rea . Another s e t strikes N. 45' E. and d i p s 75' SW. and t r a n s e c t s t h e o t h e r j o i n t s a l lowing t h e f r e e i n g o f l a r g e blocks o f rock.

The McGinnis Creek s l i d e may have s t a r t e d as a r o c k f a l l when part o f t h e rock i n t h e s t e e p west f a c e o f McGinnis Mountain f e l l f r e e l y f o r some d i s t a n c e be fo re s l i d i n g down t h e mountain. This s l i d e appa ren t ly i s i n an a r e a of r e c u r r i n g r o c k f a l l s and s l i d e s ; t h e l a s t major s l i d e r epo r t ed was about 1949 when blocks 10-15 f e e t long dammed a l ake 30-40 f e e t wide and about 300 f e e t long , according t o M r . R. E. Reed ( o r a l commun., 1968) who was ope ra t ing a mining claim along McGinnis Creek a t t h e time. Dust was seen along t h e s teep f ace on Mount McGinnis dur ing my mapping i n 1968 and may have been caused by small r o c k f a l l s , S i m i l a r l y , t h e s l i d e s on t h e s lopes o f Bul la rd Mountain along t h e Lower Basin o f Nugget Creek may a l s o have s t a r t e d a s r o c k f a l l s .

Bullard Mountain was not v i s i t e d on t h e ground, bu t a s seen from an a i r - p lane , t h e f a c e o f t he c l i f f on t h e mountain seems t o be c o n t r o l l e d by j o i n t p lanes . The s t r i k e of t h e c l i f f a p ~ a r e n t l y i s roughly p a r a l l e l t o t h e N, 43" W. and N. 35' W . j o i n t s e t s t h a t seemingly c o n t r o l t h e development o f t h e c l i f f on McGinnis Elountain. Landsl ide s c a r s a r e wel l developed on Bul la rd Mountain and have narrow V-shaped funnels that lead t o t h e s l i d e d e b r i s below.

A s i m i l a r V-shaped scar i s upslope from t h e s l i d e i n Gold Creek t h a t occurred i n September 1901, T h i s s l i d e apparent ly was caused by heavy ~ L n f a l l , and probably was a wet s l i d e o r d e b r i s avalanche. I t may have bccn t ~ . i g ; ~ r c . d by 'ol:~stiri:; L I ~ t h e Ebrler 5iinc (I!:~ily Al:~ska Dispatch, Scpt. 74 , 1901) . Silt re1; l t ionsl l ip of t h i s s l i d e t o j o i n t s e t s i s no t knbizn, bu t tlie t rend of one o f t h e c l i f f faces above' the s l i d e n e a r l y matches t h e N. 65" W. s t r i k e o f one o f t h e c o n t r o l l i n g j o i n t s e t s o f the ;4ount Juneau rocksl ide-avalanche source area.

l'he development of a c l i f f above t h e s l i d e i n Lemon Creek a l s o seems t o be rcl;rtpcl t o rile j o i n t sc t s i n t h e rock a t t h e southwest end of 1Icint:le- man Ridgc, ~ C * L ; \ C C ~ ~ >!cnden?1a11 and Lcnion Creek v a l l e y s . One j o i n t s e t s t r i k e s >;. 73' E. and i s a1n:ost p a r a l l e l with the f a c e of the c l i f f i n sec. 2 2 , T. 40 S . , R. 66 E . , i n d i c a t e d by a hachure on t h e geologic map.

The age o f t h i s l a r g e s l i d e i n Lemon Creek v a l l e y can be es t imated a s 5cf T '; 31.-!- :' 5i'O years o l d . One o f t h e s tunps of a t r e e logged s i n c e 1962 i s 1;eli 2rcscrved and r evea l s more than 512 r ings . This stump was roo ted i n t h e s l i d e depos i t r i c h i n g reensch i s t and greenstone fragments t h a t cover oldex d e l t a dcpos i t s (Qdo). The mature f o r e s t covering t h e s l i d e t r a c k on t h e s lopes o f Heintzlcman Ridge a l s o suppor ts such an age.

The u n d i f f e r e n t i a t e d l a n d s l i d e depos i t s exposed between t h e debris-f low depos i t s along Gastineau Avenue and Frankl in S t r e e t may be o l d e r deb r i s - flow depos i t s . Because t h e s e ma te r i a l s a r e mostly bu r i ed by t h e h i s - t o r i c a l debris-f low depos i t s t h e i r o r i g i n can only be assumed. For t h i s reason, t h e s e d e p o s i t s , though r i c h i n l a r g e angular g reensch i s t fragments a s a r e t h e debris-f low d e p o s i t s , a r e grouped with t h e u n d i f f e r e n t i a t e d l ands l ides . The s t a b i l i t y of t h e s u r f i c i a l depos i t s and bedrock upslope from Gastineau Avenue is descr ibed i n d e t a i l under debris-f low d c p o s i t s CQfll.

Some of t h e small s l i d e depos i t s elsewhere along Gast ineau Channel may have been t r anspor t ed a s a p a r t o f snow avalanches o r as d e b r i s t h a t s l i d down t h e snow-covered su r f aces of t h e avalanche t r a c k s . The i s o l a t e d a r c u a t e r i dges a t t h e bottom o f t h e bedrock s lopes o r s l i d e s c a r s sugges t such methods o f mMvement t o me. The lack o f mature t r e e s i n s l i d e s c a r s above some of t h e s l i d e d e p o s i t s sugges ts r e c u r r i n g snow o r rock avalanches t h a t prevent t r e e growth. Snow avalanches f r equen t ly move downslope from Mount Juneau and t e rmina te i n t h e Highland a rea . The l a s t l a r g e one in thatarea occurred on March 2 2 , 1962 (Daily Alaska Empire, Mar. 2 2 , 1962). Older snow avalanches have extended beyond the houses i n t h e Highland a r e a and onto the former t i d e f l a t s of Gast ineau Channel i n an a r e a now occupied by a motel.

Gentle s lopes on l a n d s l i d e dcpos i t s are s l i g h t l y s u s c e p t i b l e t o sheetwash o r g u l l y wash; vege ta t ion cover he lps p r o t e c t s lopes . Some o f t h e s t e e p e r slopes erode by g u l l y wash, e s p e c i a l l y t hose s lopes n e a r a c t i v e t a l u s e s . Narrow rock t roughs concen t r a t e runoff coming from high on t h e s lopes i n t o small streams which erode t h e l a n d s l i d e depos i t s . Landsl ide d e p o s i t s t h a t blocked s treams, such a s Salmon and Gold Creeks, have been e x t e n s i v e l y eroded.

Landslide depos i t s a r e l oose ly compacted. Those ly ing on s t e e p s lopes probably w i l l s e t t l e o r s l i d e downslope i f a severe ear thquake shakes t h e a r e a ; and some rock masses o r i nd iv idua l rocks high on t h e s l o p e s may f a l l (see over lay 1, sheets 1 and 2 ) .

The upper s lopes o f s l i d e s are uns tab le . However, t he lower s l o p e s a r e f l a t t e r and a r e u sua l ly more s t a b l e . \ 'here sp r ings develop, t h e s l o p e s t a b i l i t y decreases . Excavations i n t h e lower par ts of t h e l a n d s l i d e d e p o s i t s can fu r the r reduce s t a b i l i t y and may r e a c t i v a t e t h a t p a r t o f t h e s l i d e above t h e c u t . Excavations t h a t l eave large blocks hanging i n t h e wal l s of t h e c u t s are e s p e c i a l l y hazardous. Heavy r a i n f a l l can cause t h e s e l a r g e fragments t o loosen and f a l l . Removal o f t h e lower p a r t of any s l i d e depos i t on a s t e e p s lope can cause renewed s l i d e movement; such r e a c t i v a t i o n would be e s p e c i a l l y l i k e l y dur ing heavy r a i n f a l l s .

The abundance of l a n d s l i d e depos i t s p o i n t s ou t t h e tendency f o r rocks t o f a l l and s l i d e i n t h e Juneau area. Overlay 1 shows a r e a s o f pas t and p o t e n t i a l l a n d s l i d e hazards .

Co l luv ia l l?l diamicton IQud)

The c o l l u v i a l ( ? ) diamicton c o n s i s t s of pale-yellow (SY 7 / 3 g d r y , t o o l i v e (5Y - 5/3) wet , cohesive heterogeneous mixtures oT s i l t , clay, sand, pebbles , and s c a t t e r e d cobbles ( t a b l e 4 , no. 7). S la te i s t h e most common rock type , and t h e s l a t e fragments have random o r i e n t a t i o n s i n t h e depos i t s . Judged from small and s c a t t e r e d exposures , t h e d e p o s i t s seem t o be massive. Surfaces on broken pieces of the diamicton a r e rough and porous, and angular fragments o f s l a t e 1/4-1/2 inch long p ro t rude from the su r f ace . Dry bulk d e n s i t y determined for one sample i s 130 pcf.

The age o f t h e depos i t s may range from l a t e P l e i s t o c e t o l a t e Holocene.

These d e p o s i t s are shown on t h e geologic map only a long Lawson, Kowee, and Eagle Creeks on Douglas I s land . The d e p o s i t a long Kowee Creek was t h e only one examined; it forms a r idge 50 f e e t h igh along t h e northwest s ide o f the creek. The o t h e r depos i t s shown on t h e map were recognized on a e r i a l photographs.

As mentioned under "Mass wasting," I b e l i e v e t h a t most o f t h e s e d e p o s i t s were formed by c o l l u v i a l p rocesses , inc luding creep, flowing, and s l i d i n g . The yel lowish c o l o r extends throughout t h e m a t e r i a l , and probably repre- s e n t s ox ida t ion o f t h e parent ma te r i a l be fo re that m a t e r i a l was reworked i n t o t h i s depos i t . The absence o f macro- o r mic ro fos s i l s , t h e f a c t t h a t the depos i t can be t r a c e d upval ley as h igh a s 1,000 f e e t above s e a l e v e l , and t h e apparent absence o f Eluvia l o r l a c u s t r i n e bedding e l i m i n a t e most a l t e r n a t e o r i g i n s , except poss ib ly g l a c i a l .

A g l a c i a l o r i g i n i s l e s s s a t i s f a c t o r y than t h e concept o f formation by c o l l u v i a l processes . The c o l l u v i a l ( ? ) diamicton does n o t seem t o con ta in

a ~ o l o r codes from Munsell S o i l . Color Char t s , 1954 ed., Munsell Color Co., Inc., Balto. , Md. Color numbers are used i n t h e d e s c r i p t i o n s o f se- l e c t e d depos i t s where such des igna t ion i s an a i d t o f i e l d i d e n t i f i c a t i o n .

4 9

s t o n e s l a r g e r than pebbles . In c o n t r a s t , g l a c i a l till t y p i c a l l y has a wide assortment of coarse m a t e r i a l s . Such a ,w ide s i z e range would b e expected on Douglas I s l and because o f t h e v a s t number o f gran i te -boulder e r r a t i c s elsewhere on t h e i s l a n d . The s u r f a c e o f t h e depos i t has a r idge form b u t it is n~ore l i k e l y e ros iona l than depos i t i ona l . The ages of t h e glaciomarine depos i t s and o l d e r d e l t a depos i t s , which are topo- g raph ica l ly lower, r e q u i r e t h a t i f t h i s depos i t i s , however, g l a c i a l it i s r e l a t e d t o t h e waning phases of t h e Cord i l l e r an i c e s h e e t , and i s o l d e r than 10,000 years.

G lac i a l depos i t s

G lac i a l depos i t s i n t h e Juneau a r e a inc lude p i t t e d outwash depos i t s [Qop), moraine (Qrn), younger outwash d e p o s i t s (Qoy), l a t e glacial-outwash d e p o s i t s (Qol) , o l d e r g l a c i a l (?) alluvium (Qprn), and o l d e r till (Qpt). Mendenhall v a l l e y conta ins t h e l a r g e s t amount o f g l a c i a l d e p o s i t s mapped i n t h e Juneau a rea , b u t o t h e r va l l eys , such a s t hose o f Fish Creek and Kowee Creek on Douglas I s l and , conta in a l a r g e r v a r i e t y o f depos i t s . S imi l a r ly , t h e v a l l e y s of Gold, Lemon, Salmon, Nugget, Montana, Eagle, and F a l l s Creeks, on t h e mainland and Douglas I s l and , con ta in some o r a l l o f t h e above depos i t s . These depos i t s range i n age from l a t e P l e i s - tocene t o Holocene. The o l d e r till (Qpt) i s probably t h e o l d e s t s u r f i c i a l depos i t mapped; it probably was depos i ted p r i o r t o and overr idden by t h e l a t e P le i s tocene i c e shee t . The o l d e r g l a c i a l ( ? ) a l luvium (Qpm) under- lies depos i t s t h a t a r e o l d e r than 10,000 yea r s . The l a t e glacial-outwash d e p o s i t s (Qol) c o n s i s t of m a t e r i a l s t h a t a r e r e l a t e d t o t h e d e p o s i t i o n a l a l l u v i a l cyc l e t h a t followed t h e d e g l a c i a t i o n o f t h e Juneau a r e a i n l a t e P l e i s tocene o r e a r l y Holocene time. The moraine (@) was depos i t ed by i c e and t h e younger outwash depos i t s (Qoy) and p i t t e d outwash d e p o s i t s (Qop) were depos i ted by g l a c i a l meltwater dur ing Neoglacial o r younger t imes.

P i t t e d outwash depos i t s (Qop)

P i t t e d outwash depos i t s c o n s i s t o f l ight-brownish-gray (2.5Y 6/21 medium t o coa r se sand and f i n e t o medium grave l con ta in ing p e b b l e s a n d cobbles. Granules and pebbles 1/2-1 inch i n diameter a r e t h e most p reva len t s izes , and cobbles and boulders a r e scarce . Broken p i eces of g r a n i t e , g n e i s s , and greenstone, and g r a i n s of quar tz and dark micas and hornblcndes g ive a salt-and-pepper appearance t o t h e d e p o s i t s , The p i t t e d outwash d e p o s i t s a r e of very l a t e Holocene age and were formed dur ing a very r e c e n t r e t r e a t o f t h e Mendenhall G lac i e r . Ice covered t h e a r e a behind t h e t h i r d rnofainal r i d g e from t h e f r o n t (see geologic map, p l . 1 ) i n 1909-10 (Knopf, 1912). P a r t o f t h e a r e a now unde r l a in by t h i s outwash depos i t was s t i l l covered by i c e i n 1942 (U.S. Gcol. Survey topographic map, Juneau 8-2, 1947 ed.) . By 1948, t h e a r e a was i c e f r e e (U.S. Geol. Survey topographic map, Juneau B-2, 1962 ed.) ; consequent ly most of t h e p i t t e d outwash d e p o s i t s accumulated between 1942 and 1948.

As t h e i c e terminus o f Mendenhall G lac i e r r e t r e a t e d from i t s 1909-10 p o s i t i o n , Nugget Creek appa ren t ly flowed from beneath t h e g l a c i e r , as i t does today, and depos i ted sand and grave l over s t agnan t g l a c i a l i c e . The 1909-10 nap o f Knopf (1912) shows such a s u b g l a c i a l flow, a s does t h e map showing t h e 1932 i c e p o s i t i o n (U.S. Geol. Survey topographic map, Juneau B-2, ed, 1947). blendenhall Lake has expanded s i n c e 1942 and Nugget Creek now flows i n t o t h e lake . The p i t t e d outwash depos i t s were abandoned a s a channel and became p a r t o f t h e shore on Mendenhall Lake. I c e b locks bur ied oy t ho outwash sand and grave l s lowly melted and formed depress- ions i n an o therwise r e l a t i v e l y smooth su r f ace .

This s i n g l e depos i t o f p i t t e d outwash occurs along Mendenhall Lake n e a r t h e U.S. Fores t Serv ice V i s i t o r s ' Center a t t h e upper end o f Flendenhall va l l ey . Moss and flowers cover t h e depos i t d i scont inuous ly along with i s o l a t e d clumps of a lder and small evergreens. The s u r f a c e o f t h e deposit i s graded t o t h e channel used by Steep Creek i n 1948. S teep Creek has been d ive r t ed northward s i n c e t h a t t ime and now flows from n e a r t h e powerhouse across t h e p i t t e d outwash i n t o Mendenhall Lake. An ice-cored s t eep - s ided kame (not d i f f e r e n t i a t e d on t h e map) s t i l l e x i s t e d i n 1971 a t t h e edge of t h e p i t t e d outwash depos i t on t h e shore of t h e lake.

Moraine (Qm)

Moraine i s an accumulation of g l a c i a l d r i f t wi th a d i s t i n c t i v e and char- a c t e r i s t i c topographic expression. A s mapped i n t h e Juneau a r e a , moraines a r e composed o f loose till and s t and a s a r c u a t e r i d g e s ac ros s Mendenhall va l ley , and extend as e longate r i dges l a t e r a l l y along t h e mountainsides. In a d d i t i o n , moraines form s m a l l e r arcuate r i dges i n c i rques ; such mor- a i n e s a r e mapped i n c i rques a t the heads of s t reams t r i b u t a r y t o F ish Creek and i n t h e c i r q u e a t t h e head o f Kowee Creek, bo th on Douglas Is- land. The moraines i n Mendenhall v a l l e y are predominantly unsor ted mix- t u r e s o f l i gh t -g ray s i l t - r i c h g r a v e l l y sand t o sandy coa r se g rave l con- t a i n i n g rounded cobbles and boulders ; i n a few p l a c e s , however, t h e moraines c o n s i s t mostly of boulders . The c i r q u e moraines a r e composed almost e n t i r e l y o f boulders .

The moraines i n Mc idenhall v a l l e y have t e x t u r e s i n p a r t dependent upon t h e p o s i t i o n of t he moraine r e l a t i v e t o anc i en t streamflow. The ma t r ix o f most of t h e depos i t s i n t h e a r cua te morainal system has a s a l t - and - pepper appearance caused by numerous dark minerals . D i o r i t i c and g r a n i t i c fragments, a s wel l a s greenstone, gne i s s , schist, and qua r t z make up most of t h e ma te r i a l s i n t h e moraines. The v a r i a b l e t e x t u r e o f t h e s e m a t e r i a l s ranges from s i l t y g r a v e l l y sand t o s c a t t e r e d boulders 6-10 feet ac ros s and i s c h a r a c t e r i s t i c of much o f t h e western and c e n t r a l morainal a rc . The depos i t s i n t h i s po r t ion probably were dropped d i r e c t l y from t h e i c e with l i t t l e modi f ica t ion o r s o r t i n g by flowing water. A cobble- r ich sandy grave l , blanketed by a concent ra t ion o f semiround boulders 6-12 feet i n diameter , forms a n e a r l y continuous boulder r i d g e as p a r t o f t h e morainal a r c on t h e e a s t s i d e of t h e va l l ey . S i m i l a r concen t r a t ions of l a r g e boulders form t h e l a t e r a l moraine along t h e mountain s l o p e , and, i n fac t , mark t h e h ighes t morainal r i dge n e a r Nugget Creek. These boulder

accumulations are shown on t h e geologic map by an ove rp r in t . The presence o f grave l w i th in p a r t o f t h i s boulder - r idge a r e a i n d i c a t e s wa te r s o r t i n g ; t h e l a r g e boulders probably r o l l e d down from t h e mountainsides, bu t I b e l i e v e they were concent ra ted by s treams flowing on o r marginal t o t h e i c e a t t hose places.

The l a r g e s t moraines mapped are i n Mendenhall va l l ey . The outermost two moraines form d i s t i n c t and prominent r i dges s epa ra t ed by outwash. These r idges can be t r a c e d t o t h e v a l l e y wa l l s and i n t o conspicuous l a t e r a l moraines. A t h i r d , i n n e r moraine, i s l e s s prominent, bu t a l s o i s a rcua te . A broad a r e a o f mounds, r i d g e s , and k e t t l e s , l o c a l l y i n t e r r u p t e d by outwash channels , extends upval ley from t h e t h i r d moraine. There, r i dges t r end almost a t r i g h t angle t o t h e a r cua te t r e n d o f t h e o u t e r moraines and g ive t h e i n n e r moraines a d i s t i n c t i v e s t r i p e d appearance. K e t t l e lakes g e n e r a l l y l i e on t h e c r e s t s o f a l l t h e a r c u a t e r i dges .

Smaller and less d i s t i n c t moraines l i e i n t h e a l p i n e c i rques ; on ly those on Douglas I s l and a t t h e head o f Kowee Creek and i n c i rques a t t h e heads of s t reams t r i b u t a r y t o Fish Creek were mapped. In gene ra l , t h e smal l c i rque moraines c o n s i s t o f boulder r i dges made d i s t i n c t i v e by t r e e growth. These r idges a r e low, only about 1-3 f e e t h igh; t h e t r a c e of t h e moraines is most v i s i b l e a f t e r a l i g h t snowfal l t h a t b l anke t s t h e shal low troughs between moraines.

The th i ckness of t h e moraines v a r i e s from p l a c e t o p l a c e i n Mendenhall va l l ey . Ridges forming t h e end moraines are some of t h e t h i c k e r depos i t s , and t h e till i s a t l e a s t 80 f e e t thick i n some p a r t s o f t h e end moraine a r e a where it o v e r l i e s bur ied pea ty woody beds ( see f i g . 7).

The minimum age o f t h e moraines i n t h e Mendenhall v a l l e y i s based on t h e date of rees tab l i shment o f vege ta t ion and t h e i n f e r r e d s t a b i l i z a t i o n o f t h e moraine as t h e i c e r e t r e a t s . Lawrence (1950, p. 203) cons iders t h a t t h e outermost moraine became s t a b i l i z e d about 1767-69. There i s no morainal ma te r i a l downvalley from t h e outermost moraine, and t h i s moraine apparent ly r ep re sen t s t h e f a r t h e s t advance o f t h e Mendenhall G l a c i e r of Neoglacial age.

The Neoglaciat ion is be l i eved t o have s t a r t e d i n t he Juneau a r e a about 3,500 yea r s ago (Heusser, 1953, p. 637; 1960, p. 186). A radiocarbon d a t e of 2,800;200 B.P. (sample W-2379, Meyer Rubin, w r i t t e n commun., 1970) from one o f s e v e r a l stumps depos i ted i n morainal d e b r i s on t h e west s i d e of Mendenhall Lake sugges ts t h a t t h e Mendenhall G l a c i e r advanced from an e a r l i e r upval ley p o s i t i o n p r i o r t o 3,000 years ago and had reached wi th in 2 miles o f t h e p o s i t i o n o f i t s mid-18th-century t e r - minus a t l e a s t by about 2,800 years ago. A radiocarbon d a t e of 1,970+250 B.P. (sample W-1989, bleyer Rubin, w r i t t e n commun., 1968) was obtained from pea t i n a horizon conta in ing logs and stumps which under- l i e s t h e morainal depos i t s and o v e r l i e s l a t e g l a c i a l outwash (Qol) and is exposed along t h e south bank of Mendenhall Lake. Thus, t h e ice ap- parently reached a p o i n t about 1 1/2 miles upval ley from t h e 18th-century

t e rmina l moraine about 2,000 yea r s ago. It i s not known when t h e g l a c i e r reached the te rmina l moraine o r how long t h e ' i c e f r o n t was a t t h a t p o s i t i o n before t h e g l a c i e r s t a r t e d t o recede.

The t o t a l r eces s ion of t h e g l a c i e r a s o f 1962 from t h e outermost moraine s i n c e about 1767-69 was about 2 1/4 mi les . Knopf (1912) showed t h e 1909-1910 p o s i t i o n as being about 1 mi le upval ley from t h e mid-18th- century moraine. The withdrawal o f an a d d i t i o n a l 1 mi le t o t he 1948 p o s i t i o n r e f l e c t s a much-accelerated r e t r e a t . Lawrence (1950, p. 202) pointed out t h a t "Tile 1931 pos i t i on . . . s t a n d s about halfway between t h a t o f 1909-1910 and t h a t o f 1948, an i n d i c a t i o n of a r a t h e r uniform r a p i d r a t e f o r t h e p a s t 40 years." Between 1948 and 1962, t h e l a t e s t p o s i t i o n shown on t h e geologic map, t h e i c e f r o n t r e t r e a t e d an a d d i t i o n a l q u a r t e r of a mile . The morainal d e b r i s l y i n g above t h e approximately 2,000-year-old peat along t h e south edge o f Mendenhall Lake was s t i l l covered by i c e u n t i l 1942. In 1968, only t h e e a s t e r n one-half mi le of t h e g l a c i e r f r o n t s t i l l extended i n t o Mendenhall Lake; t h e remainder of t h e i c e f r o n t was on land , and sandy bouldery morainal d e b r i s was accumulating over t h e r e c e n t l y uncovered smoothed-bedrock su r f ace .

The ages of t h e moraines i n t h e c i rques on Douglas I s l a n d are based on t h e growth r i n g s i n t r e e s growing on t h e boulder r i d g e s ; on t h a t b a s i s , t h e outermost moraines i n t h e c i rques seem t o be o l d e r than t h e moraines i n Mendenhall va l l ey . Those moraines a t t h e head o f Kowee Creek and t h e t r e e s growing on t h e muskeg-covered till a long t h e shore o f Cropley Lake a r e a t about 1,700 f e e t above s e a l e v e l . Cores o f t r e e s on moraines i n t h e c i rque a t t h e head of Kowee Creek r e v e a l r i n g s t h a t sugges t t ha t t h e t r e e s on t h e outermost boulder r i dge s t a r t e d growing about 400-435 yea r s ago; co re s of t r e e s on t h e next r i dge upva l l ey i n d i c a t e an age of about 300 years f o r t h e s t a r t o f t r e e growth. A t r e e on t h e shore of Cropley Lake revea led r i n g s sugges t ing growth s t a r t e d a t l e a s t 400 yea r s ago.

Trees i n c i rques above Cropley Lake show o l d e r ages i n a c i r q u e a t 2,300 feet than i n a c i r q u e a t about 3,000 f e e t above s e a l e v e l . The 2,300- f o o t c i rque con ta ins s e v e r a l boulder r i d g e s ; t h e outermost r i d g e n e a r t h e bedrock th re sho ld a t t h e o u t l e t of t h e c i rque suppor ts t r e e s t h a t are a t l e a s t 400 yea r s old. Trees on t h e outermost moraine i n t h e 3,000- foot c i rque s t a r t e d growing only about 270 yea r s ago, and the t r e e s on the next moraine upval ley s t a r t e d growing about 100 yea r s l a t e r .

Absence of a permanent i c e f i e l d on Douglas I s l and , such a s is found on t h e mainland, sugges ts t h a t t h e g l a c i e r s o f l a t e P l e i s tocene age d i d not survive through t h e prolonged warm c y c l e preceding t h e Neoglacial g l a c i a t i o n . The boulder r i dges accumulated a t t h e margins o f snow o r ice t h a t developed dur ing Neoglacial t ime i n t h e s e a l p i n e c i rques . Whether t h e s e c i rques contained only permanent snow f i e l d s o r a c t i v e l y moving g l a c i e r s i s n o t known. I t i s p o s s i b l e t h a t t h e r i dges may be accumulations o f boulders t h a t f e l l from t h e c i rque wa l l s and r o l l e d across t h e snow f i e l d s t o accumulate a s low morainel ike r i d g e s c a l l e d p r o t a l u s ramparts. I f small c i r q u e g l a c i e r s developed, t h e r i d g e s r ep re sen t d e b r i s dropped along t h e margins o f s lowly r e t r e a t i n g g l a c i e r s .

A scvcra earthquake i n t h e Juneau area probably would cause the moraines ir t h e blendenhall v a l l e y t o r a v e l , slump, and compact d i f f e r e n t i a l l y . In a r eas where t h e water t a b l e is n e a r t h e s u r f a c e , prolonged shaking of t h e depos i t s probably would cause ground f rac tu r ing and might produce sand spouts and sand b o i l s . Large boulders l y ing on the morainal s lopes could be expected t o r o l l . Homes built: on t h e s lopes o f morainal r i d g e s might be damaged by d i f f e r e n t i a l compaction o f t h e morainal d e p o s i t s and 5y -,oiling boulders .

If an earthquake caused l a n d s l i d e s and ice f a l l s i n t o Flendenhall Lake, l a r g e waves might r e s u l t . Also, an ear thquake might cause t h e f r o n t s of d e l t a s t o s l i d e and c r e a t e waves t h a t could c r o s s the lake and inundate t h e west shore a rea . The waves could r e f l e c t back toward the campground a r e a b u i l t on low-lying glacial moraine d e p o s i t s ( see p. 69).

G lac i e r faces were s h a t t e r e d and i c e was thrown outward elsewhere i n Alaska during t h e 1964 earthquake [Waller, 1966, p. 4) . Large masses of g l a c i a l i c e f a l l i n g i n t o an i c e - f r e e l a k e could cause waves t h a t might c r o s s t h e l ake and r i s e onto t h e oppos i t e shore. The shore d i r e c t l y op- p o s i t e t h e p o r t i o n o f the blendenhall G lac i e r reaching i n t o Mendenhall Lake inc ludes a h igh morainal b l u f f and t h e much lower p i t t e d outwash depos i t (Qop). A U.S. Fores t Se rv i ce campground i s s i t u a t e d on low morainal topography along t h e southwestern sho re , b u t is p ro t ec t ed solriewhat by t h e younger d e l t a deposi t (Qdy) t h a t p r o j e c t s i n t o t h e lake

. a between t h e campground a r e a and t h e i c e f r o n t . Nevertheless , a wave capable o f a 15-foot runup could over top t h e wa te r - l eve l d e l t a and con- t i n u e on t o t h e campground, though i n a somewhat a t t enua ted form. The continued r e t r e a t o f t h e Mendenhall Glacier a s su re s t h e complete ground- ing o f t h e i c e f r o n t i n t h e fo re seeab le fu tu re , which w i l l reduce t h e chance o f such waves.

Slope s t a b i l i t y o f the g l a c i a l moraine d e p o s i t s i s gene ra l ly good. Slopes range from 10' t o as much a s 60" i n t h e b l u f f s a long t h e sho re of Mendenhall Lake; the genera l range i s between 20" and 35" most p l aces . The measured angle o f repose o f t h e s e d e p o s i t s i s gene ra l ly between 30' and 35"; consequently, s lopes steeper t h a n 35' a long t h e l ake c o n s t a n t l y r a v e l and slump. The waves on t h e l ake tend t o remove t h e d e b r i s , keeping t h e s lope s t e e p and uns tab le .

Younger outwash d e p o s i t s (Qoy)

The younger outwash depos i t s c o n s i s t p r i n c i p a l l y o f gray s i l t y f i n e sand. Boulders 7-12 f e e t i n diameter a r e p r e s e n t i n the outwash d e p o s i t s n e a r the moraines i n F lendenh~l l va l l ey . D i o r i t i c rock, g r a n i t e , gne i s s , s c h i s t , with t h e i r contained dark minera ls such as mica and hornblende, and green- s t o n e and qua r t z a r e t h e p r i n c i p a l c o ~ i s t i t u e n t s of the deposi t s . Par t ic le size v a r i e s with l o c a t i o n ; t h e c o a r s e s t s ize g e n e r a l l y i s n e a r e r the g l a - c i e r and along modern channels t h a t follow anc ien t pa ths o f l a r g e r s t reams. The c o a r s e s t outwash depos i t s bound t h e eastern s i d e of the broad Menden- h a l l va l l ey , where they grade from cobble and coa r se pebble g rave l t o

boulders ncar t h e lake , t o pebble grave l and sandy cobble g rave l n e a r Ilendcnhaven, and t o f i ne o r medium sand n e a r t h e Glac i e r Highway. Severa l d e p o s i t i o n a l s u r f a c e s r i s e above t h e modern s t ream channels . One such s u r f a c e is 4 f e e t h jghe r thzn t h e cobble-gravel s u r f a c e near blendenhaven and i s unde r l a in by g r a y mediun sand with pebbles . A s t i l l h i g h e r s u r - face along t h e blendenhall River n e a r blontana Creek i s unde r l a in by very f i n e sand and s i l t i n t e r l a y e r e d with pea t and humus beds. An e x c e l l e n t exposure o f such a depos i t i s a long t h e blendenhall River channel a t a bend west of blendenhaven.

Ma te r i a l s i n t h e outwash channels wi th in t h e blendenhall moraines are c o a r s e r and younger than t h e outwash d e p o s i t s i n f r o n t o f the outermost moraine. Outwash depos i t s between t h e morainal r i dges gene ra l ly a r e composed of cobbles and s c a t t e r e d boulders i n coarse sand, and a r e we l l exposed nea r t h e junc t ion o f the Montana Creek Road and t h e road t o Mendenhall campground. The depos i t i n t h e channel on t h e edge o f t h e Mendenhall River i s a f i n e t o medium sand. S imi l a r grading occurs on t h e o t h e r s i d e of t h e r i v e r where meltwater channels r e p r e s e n t o l d channels o f Nugget and Steep Creeks.

Outwash depos i t s i n t h e v a l l e y o f Nugget Creek seem t o be l i t h o l o g i c a l l y very s i m i l a r t o t hose i n blendenhall v a l l e y . T e x t u r a l l y , t hey seem t o be mainly sandy pebble g rave l t h a t conta ins some cobbles and a few boulders .

Records of t e s t we l l s and water w e l l s i n Mendenhall v a l l e y (J, A. McConaghy, U. S. Geol . Survey, w r i t t e n commun. , 1967) i n d i c a t e t h a t a r e l a t i v e l y widespread pea t ox carbonaceous l a y e r s epa ra t e s t h e younger outwash depos i t s from an o l d e r outwash d e p o s i t t h a t i s somewhat coa r se r . The pea t l a y e r extends beneath p a r t i f not a l l o f t h e moraines a s w e l l , and i s shown on t h e fence diagram, f i g u r e 7.

Upright sheared t r e e s roo ted i n p e a t o r carbonaceous m a t e r i a l a r e exposed beneath 12 f e e t o f outwash depos i t s a long t h e blendenhall River. A rad io- carbon de termina t ion on wood from one o f t h e s e t r e e s shows a dea th date of 8605260 B. P. (about 1,100 A.D.) (sample W-1947, Meyer Rubin, U.S. Geol. Survey, w r i t t e n commun., 1968). The l o c a t i o n i s shown on t h e geologic map. The outwash d e p o s i t s i n channels w i t h i n the moraines a r e younger; most o f t h e a r e a was covered by i c e a s l a t e as 1900-1910, and S teep Creek and Nugget Creek flowed along some o f t h e inner-moraine outwash channels as l a t e as 1942.

The outwash depos i t s s lope downvalley wi th a more o r l e s s even su r f ace . Outwash channels wi th in t h e moraines i n t h e upper blendenhall v a l l e y t r e n d p a r a l l e l t o a r cua te r i dges . Outwash depos i t s i n f r o n t of t h e moraines form a broad s h e e t t h a t u n d e r l i e s t h e v a l l e y f l o o r a s f a r a s Gast ineau Channel. The blendenhall River , entrenched i n t o t h e outwash d e p o s i t s about 12 f e e t , and sma l l e r s t reams have l e f t several s u r f a c e s across t h e v a l l e y which are bounded by sca rps 1-4 f e e t high.

A small amount o f younger outwash depos i t s i s mapped i n t h e v a l l e y of Nugget Creek, e a s t of Mendenhall Glac ie r . I t has a f l a t s u r f a c e s l i g h t l y i n c i s e d by Nugget Creek. l

The younger outwash d e p o s i t s appa ren t ly accumulated dur ing t h e f i n a l I

Neoglacial i c e surge as a r e s u l t o f t h e increased ca r ry ing capac i ty o f L

t h e s t reams flowing from advancing g l a c i e r s . The outwash bu r i ed humus l aye r s and f o r e s t growing on o l d e r autwash depos i t s t h a t had accumulated be fo re t h e l a t e s t Neoglacial advance. Streams flowing from t h e Mendenhall G lac i e r , from g l a c i e r s a t t h e heads of Kugget an3 McGinnis Creeks, as we l l as g l a c i e r s t r i b u t a r y t o Montana Creek, c a r r i e d sand and f i n e grave l on to t h e broad su r f ace of t h e o l d e r outwash depos i t s . The s i l t , f i n e sand, and muskeg i n t h e a r e a above t h e confluence o f ?lantana Creek and t h e Mendenhall River probably accumulated from overbank f looding along Flontana Creek. The presence of thc l a t e Neoglacial Flendenhall G l a c i e r probably s t a b i l i z e d t h e depos i t i on i n t h e main v a l l e y , whereas t h e absence o f l a r g e v a l l e y - f i l l i n g g l a c i e r s w i th in t h e Montana Creek dra inage r e s u l t - ed i n seasonal f loods o r extremely high runof f s . Pre l iminary examination o f seeds and p l a n t s from s e v e r a l pea t beds w i th in t h e Mendenhall v a l l e y i n d i c a t e s a fresh-water and t e r r e s t r i a l environment ( E s t e l l a Leopold, w r i t t e n commun., 1969). These f o s s i l s i n d i c a t e t h a t t h e v a l l e y f l o o r was above s e a l e v e l throughout l a t e Neoglacial time.

The radiocarbon age o f one o f t h e bur ied trees a long t h e Mendenhall River channel (p.56) r e s t r i c t s t h e s t a r t o f r a p i d depos i t i on of t h e younger outwash t o about 860 years ago (about 1,100 A.D.) Whether t h i s depos i t i on s t a r t e d be fo re t h e Neoglacial Mendenhall G lac i e r reached t h e p o s i t i o n of t h e end moraine i s not known, The age of te rmina t ion o f most outwash depos i t i on i n t h e Mendenhall v a l l e y i s apparent ly i n d i c a t e d by two widely sepa ra t ed t r e e s t h a t s t a r t e d growing about 290-300 years ago. One o f t he o l d e s t s t ands of t r e e s on t h e outwash i s a long t h e channel o f t h e Menden- h a l l River, i n t h e SE1/4SW1/4SW1/4 sec . 18, T. 40 S . , R. 66 E. , where one spruce about 38 inches i n d iameter was cored and revea led 250 growth r i n g s wi th the c e n t e r a t l e a s t 2 i nches f a r t h e r i n t o t h e t r e e from t h e end of the core. Judged from the c loseness of t h e r i n g s i n the l as t 1 1 / 2 inches o f t h e core , about 50 r i n g s a r e probably represented by t h e missing 2 i n - ches of core. Thus, an age of about 300 yea r s seems reasonable f o r t h i s t r e e . A s i m i l a r age was r epo r t ed by Lawrence (1950, p. 203) a s be ing represented by a t r e e c u t i n 1948 i n t h e SW1/4 sec . 30, T. 40 S., R. 66 E. , where t h e Mendenhall Loop Road c ros ses Duck Creek. The growth o f t h a t t r ee s t a r r e d about 290 yea r s ago a s of 1971. These o ld t r e e s were grow- i n g be fo re t h e i c e r e t r e a t e d from the end moraine; however, t h e r ad io - carbon age of t h e tree along t h e Mendenhall River channel and t h e ages of these cored t r e e s r e s t r i c depos i t i on o f t h e widespread younger outwash s h e e t i n f r o n t o f t h e moraines t o about a 560-year i n t e r v a l .

The water t a b l e i n a r e a s unde r l a in by t h e younger outwash d e p o s i t s ranges from 2 t o about 10 f ee t below t h e ground su r f ace . Although the depos i t s a r e e a s i l y excavated by hand t o o l s and power equipment, d r a g l i n e s and backhoe t r enche r s are used t o excavate below t h e water t a b l e . I t is easy t o d r i l l through t h e f ine-gra ined m a t e r i a l s bu t ho le s need t o be cas cd .

I n f i l t r a t i o n i n t o outwash d e p o s i t s is g e n e r a l l y good, except where t h e water t ab le i s extremely h igh o r b a r r i e r s o f l enses o f s i l t - s i z e o r clay-size ma te r i a l are presen t . Surface runoff ranges from poor t o good, depending on s lope and amount o f vege ta t ion . The g e n t l e s lope o f t h e v a l l e y confines e x t r c r n e l ~ ~ r a p i d r u n o f f t o e s t a b l i s h e d s t ream courses . After periods of extreme and continuous r a i n f a l l o r snowmelt, water s t ands i n many p l aces having a r e l a t i v e l y t h i c k cover o f vege ta t ion .

Most e ros ion t a k e s p l ace i n banks o f s t reams, where t h e younger outwash depos i t s are very s u s c e p t i b l e t o unde rcu t t i ng and caving; such e ros ion i s cornmon along t h e hiendenhall River . The depos i t s gene ra l ly absorb sheetwash runoff and u s u a l l y a r e n o t eroded where t h e s u r f a c e i s f la t . Vegetat ion cover a lso reduces t h e tendency t o e rode , b u t where t h e depos i t s a r e c l e a r o f vege ta t ion and n e a r t h e t e r r a c e s c a r p s , g u l l i e s may be eroded by running water .

Many o f t h e r e a c t i o n s o f water -sa tura ted f ine-gra ined a l l u v i a l depos i t s t o the 1964 Alaska ear thquake elsewhere, r e a c t i o n s such as compaction, f r a c t u r i n g o f ground, and spout ing of ejected water r i c h i n sand and s i l t , could occur i n Mendenhall v a l l e y i n t h e event of a s t r o n g eaxth- quake (see p.69). The Mendenhall v a l l e y c o n s i s t s o f more than 100 f e e t o f t h i c k v a l l e y f i l l beneath t h e younger outwash ( f i g . 7 ) . Mendenhall v a l l e y should be considered h i g h l y s u s c e p t i b l e t o damage from severe earthquakes because shaking i s much g r e a t e r and damage more widespread i n a r eas o f t h i c k unconsol idated depos i t s than i n bedrock a reas ( see overlay 2 ) . I was t o l d t h a t ground waves were v i s i b l e i n Mendenhall v a l l e y during t h e 1958 Lituya Bay ear thquake and t h e 1964 ear thquake, b u t this could n o t be confirmed. In the lower p a r t o f t h e v a l l e y t h e outwash grades i n t o t h e Mendenhall River d e l t a t h a t extends i n t o Gast i - neau Channel and F r i t z Cove. Any l a r g e l a t e r a l movement o f t h e o u t e r s l o p e of t he d e l t a would probably cause l a t e r a l spreading and f r a c t u r i n g i n t h e ground s u r f a c e upvalley.

Younger outwash d e p o s i t s a r e n e a r sea l e v e l i n t h e lower par t of t h e Mendenhall va l l ey . If l a r g e tsunamis were t o e n t e r F r i t z Cove and Gastineau Channel, t hey probably would cover t h a t p a r t o f t h e outwash depos i t s near t h e Glac i e r Highway. In 1964, some de l t a i c d e p o s i t s e l s e - where i n Alaska were engulfed by se i che and tsunami waves; i n a d d i t i o n , b a c k f i l l waves from s l i d i n g o f d e l t a f aces swept upval ley over t h e land. (See p. 69 .)

Vertical t o n e a r l y v e r t i c a l s lopes a s much a s 10 f e e t h igh a r e common i n g rave l p i t s i n t h e younger outwash depos i t s . Slumping and r a v e l i n g occur where t h e d e p o s i t s s t a n d unsupported. Slope s t a b i l i t y i s poor i n c u t s below the water t a b l e , and t h e outwash depos i t s tend t o slump o r wash. The d e p o s i t s a r e no t susceptible t o l a r g e l ands l id ing .

Late glacial-ourwash depos i t s ($01)

Late glacial-outwash depos i t s c o n s i s t of l i gh t -g ray (N7 t o 5Y 6/1) sand and pebble t o cobble g rave l , s i l t y sand, and locally,-brownigh s i l t and

a very f i n e sand. !lost o f t h e rocks i n t h e depos i t s were der ived l o c a l l y ; s la te i s t h e dominant rock t y r e on Douglas I s l a n d , and g r e e n s c h i s t , greenstone, and g r a n i t e are t h e dominant rock types on t h e mainland. The l a r g e r fragments o f greenstone and g r a n i t e a r e gene ra l ly subround t o round, and those of s l a t e and g reensch i s t a re t a b u l a r ; p i eces sma l l e r than cobbles a r e genera l ly subangular t o subround. The depos i t s a t some l o c a l i t i e s , such as i n t h e Salmon Creek v a l l e y , con ta in only s i l t , but elsewhere they inc lude f i rmly cemented crossbedded sand and grave l . Most bedding i s even and p a r a l l e l s t h e s u r f a c e o f t h e depos i t . Sand u n d e r l i e s a l a y e r o f humus, p e a t , and wood beneath t h e youngest g l a c i a l moraine a t t h e n o r t h end of blendenhall va l l ey . Peat from t h i s l a y e r was dated a s 1,970+250 years o ld (sample W-1989, bleyer Rubin, U.S. Geol. Survey, w r i t t e n commun., 1968). Most of t h e o t h e r l a t e glacial-outwash d e p o s i t s a r e probably much more than 2,000 years o ld ; i n many p l aces t hey a r e graded t o , and apparent ly contemporaneous with, t h e o l d e r d e l t a depos i t s (Qdo) o f l a t e P l e i s tocene o r e a r l y Holocene age.

The l a t e glacial-outwash d e p o s i t s a r e conspicuous i n t h e v a l l e y s o f F ish , Lawson, Kowee, Eagle, F a l l s , Gold, Lemon, and Salmon Creeks, and a r e present i n one exposure n e a r t h e upper end o f ?lendenhall v a l l e y . Most of t h e s e d e p o s i t s extend upva l l ey from t h e o l d e r d e l t a depos i t s , apparent ly t o a l t i t u d e s wel l above 600 f e e t . The downvalley-sloping s u r f a c e s of t h e depos i t s a r e r e l a t i v e l y smooth and i n most p laces uneroded. The depos i t n e a r Kowee Creek i s h i g h e r than the ad jacent muskeg-covered bedrock, and forms an anomalous topographic f e a t u r e w i th in the valley. The d e p o s i t s mapped along Gold Creek l i e behind and a r e graded t o bed rock- r i s e r s . The shape o f the depos i t a long upper Gold Creek i n S i l v e r Bow Basin i s p a r t l y obscured by an a l l u v i a l f an depos i ted by Icy Creek. Both the Lemon and Salmon Creek d e p o s i t s have smooth s u r f a c e s graded downstream. A series of t e r r a c e s has been c u t i n t o t h i s depos i t a long Lemon Creek, t h e h i g h e s t of which i s a t t h e same a l t i t u d e a s t h e o lder d e l t a s u r f a c e below t h e bedrock gorge. The Salmon Creek d e p o s i t l i e s upval ley from bedrock r idges and has f l a t - s u r f a c e d channels t h a t wind around bedrock h i l l s .

The l a r g e s t expanse o f l a t e glacial-outwash depos i t s i s mapped along Fish Creek on Douglas Is land. Examined only by reconnaissance, t h i s water- depos i ted ma te r i a l apparent ly covers most of t h e v a l l e y f l o o r . Numerous boulders , many o f g r a n i t e , cover t h e eroded s u r f a c e o f t h e depos i t . P a r t o f t h i s depos i t may c o n s i s t o f d e l t a i c beds. A scarp over 70 feet h igh (shown on t h e geologic map by a t e r r a c e - s c a r p symbol i n s ecs . 1 3 and 24, T. 41 S., R. 66 E.) i s a prominent topographic f e a t u r e i n t h e lower p a r t o f Fish Creek v a l l e y and may be t h e terminus o f a d e l t a . Channels 10-25 f e e t deep a r e eroded i n t o t h e otherwise l e v e l s u r f a c e upstream from t h e scarp. Below the sca rp , t h e outwash i s r i c h i n cobble- and boulder -s ized rocks on t h e su r f ace . No deep exposures o f m a t e r i a l were found.

The l a t e g l a c i a l outwash probably accumulated from reworking o f o l d e r v a l l e y f i l l by water flowing from mel t ing remnants o f t h e l a t e P l e i s tocene ice. This wast ing i c e probably had been p a r t of a broad i ce s h e e t , an extension of t h e Juneau I c e f i e l d , t h a t by n a t u r e was b a s i c a l l y c l ean ice.

In l a t e P l e i s tocene time as t h e i c e margin r e t r e a t e d toward t h e p r e s e n t i c e f i e l d l o c a l i t y , an i s o l a t e d and r a p i d l y mel t ing i c e f i e l d was l e f t on Douglas I s land . The mel t ing i c e on the mainland provided t h e water f o r t h e conspictlous l a t e glacial-outwash depos i t s i n Gold Creek, Lemon Creek, and Salmon Creek v a l l e y s . The d e p o s i t s i n Last Chance Basin, a long Gold Creek, are interbedded grave l and s i l t beds that o v e r l i e fo s s i l -bea r ing glaciomarine depos i t s . The uppermost beds t h e r e may have been depos i ted i n a b a s i n dammed by t h e p r e h i s t o r i c rocksl ide-avalanche, as Spencer pos tu la t e s (1906, p. 79), but I f e e l t h a t t h e rocks l ide - avalanche i s much t o o young t o have dammed t h e l a t e glacial-outwash depos i t s . Test we l l s d r i l l e d i n t h e bas in pene t r a t ed two l a y e r s of grave l separa ted by a c layey s i l t t h a t may be par t o f a l ake depos i t (Waller, 1959, p. 3 ) . Gravels behind a bedrock b a r r i e r i n Lemon Creek v a l l e y a l s o o v e r l i e s i l t (Spencer, 1906, p. 119). Salmon Creek flows e n t i r e l y on bedrock below t h e concre te dam forming Salmon Reservoi r , bu t sand and s i l t a r e abundantly exposed a t a l t i t u d e s a s h igh a s 500 f e e t above sea l e v e l where t h e Salmon Creek t r a i l and powerhouse flume a r e l o - cated i n c u t s i n t o o r through d e p o s i t s o f t h e l a t e g l a c i a l outwash.

Whether a l l t h e depos i t s mapped as l a t e g l a c i a l outwash are o f the same age o r are even n e c e s s a r i l y p a r t of t h e same d e p o s i t i o n a l c y c l e i s no t known. The outwash i n t h e v a l l e y s of Lemon and Salmon Creeks on t h e mainland, and i n the v a l l e y s o f Kowee, Eagle, and Lawson Creeks, on Douglas I s l and , a l l seem t o be graded t o the o l d e r d e l t a d e p o s i t s (Qdo).

a The l a t e g lac ia l outwash along F i sh Creek on Douglas I s l and , does n o t grade t o such a d e l t a . The s c a r p i n t h e lower p a r t o f F i sh Creek v a l l e y may r ep re sen t t h e o u t e r boundary of a depos i t t h a t may be an o l d e r d e l t a depos i t (Qdo), but i f s o , t h i s p a r t i c u l a r d e p o s i t i s a t l e a s t 100 feet h ighe r than o l d e r d e l t a d e p o s i t s mapped elsewhere on Douglas I s land . The o l d e r de l ta depos i t s have been da ted a t about 10,000-12,000 yea r s old (see p.70); t h u s , t h e l a t e g l a c i a l outwash i s l a t e s t P l e i s tocene i n age i f it i s graded t o o l d e r d e l t a deposi ts .

Older g l a c i a l (?) alluvium (Qpm)

me o l d e r g l a c i a l ( ? ) a l luvium gene ra l ly c o n s i s t s o f a brownish-gray very f i n e sand i n t e r l a y e r e d with sandy pebble g rave l t h a t i s reddish t o yellowish brown. No s h e l l s o r s h e l l fragments were seen i n outcrops of t h e u n i t , The g rave l i s o l d e r than the ove r ly ing diamicton i n t h e g lac io- marine depos i t , f i r s t phase, and thus is more than about 10,000 years old.

The o l d e r g l a c i a l ( ? ) alluvium is exposed only upval ley from t h e bedrock gorge on Montana Creek b u t south o f t h e mouth of McGinnis Creek. It disconformably u n d e r l i e s t h e glaciomarine d e p o s i t s , first phase [Qmb), and t h e contac t s lopes downstream.

An exposure of t h e o l d e r g l a c i a l ( ? ) alluvium i n t h e b l u f f a long Montana Creek i n t h e SW1/4SW1/4NE1/4 sec. 2, T. 40 S., R. 65 E. , r e v e a l s 9 feet of sandy grave l beneath t h e glaciomarine diamlcton, but even more impor tan t ly , shows t h e g rave l over ly ing a 2-foot- thick i n t e n s e l y

compacted pea ty , woody zone. The exposure was discovered by Car l Blanchard, U.S. Geological Survey, and c a l l e d t o my a t t e n t i o n . I t may be one of t h e most geo log ica l ly s i g n i f i c a n t exposures r e l a t i v e t o t h e g lac ia l h i s t o r y i n t h e Juneau area . The compressed pea t sugges ts a s u r f a c e t h a t prcdatcs the depress ion and submergence of t h e land owing t o t h e weight: o f g l a c i a l ice . Ithcther t h e gravel above t h e p e a t i s outwash depos i ted i n advance o f t h e i c e encroadlment, outwash depos i ted a s t h e i c e r e t r e a t e d , o r some o t h e r form o f al luvium i s no t known. Pea t samples have been submit ted t o 1J.S. Geological Survey l a b o r a t o r i e s f o r C14 and po l l en and seed de termina t ions .

Poss ib le o l d e r g l a c i a l ( ? ) alluvium i s r epo r t ed from t e s t w e l l s elsewhere i n t h e Juneau area . Sand and g rave l d e p o s i t s u n d e r l i e glaciomarine diamicton i n Last Chance Basin a long Gold Creek v a l l e y (Waller, 1959, t a b l e 1 ) and u n d e r l i e s i l t y c layey glaciomarine depos i t s i n lower Salmon Creek v a l l e y (J. A. McConaghy, w r i t t e n commun., 1967) and along Auke Creek nea r t h e beach a t Auke Bay (C. L. Sainsbury, w r i t t e n commun., 1964).

The bedrock a t t h e gorge along lower Montana Creek r i s e s above an o l d e r v a l l e y f l o o r , and perhaps was a b a r r i e r t o depos i t i on o f t h e alluvium. Similar bedrock r i d g e s occur a t t h e mouths o f a l l major s t reams (o the r than t h e Mendenhall River) i n t h e Juneau area . It is p o s s i b l e t h a t t h e o lde r g l a c i a l ( ? ) alluvium accumulated i n a l ake dammed behind t h e r idge . L i t t l e i s known about t h e d e p o s i t i o n a l environment o f t h i s depos i t .

Older till (Qpt)

Older till i s mapped only along t h e v a l l e y s o f Kowee and Fish Creeks and i n t h e c i rques of s t reams t r i b u t a r y t o Fish Creek on Douglas I s land . The till c o n s i s t s o f a greenish- t o brownish-gray c layey , s i l t y , sandy, and pebbly ma te r i a l . The till l i e s p l a s t e r e d aga ins t bedrock a long t h e s lopes of t h e v a l l e y s . Rock t r p e s are predominantly l o c a l ; greenstone, a r g i l l i t e , and graywacke are most common; i s o l a t e d pebbles o r cobbles of g r a i n i t e a r e inc luded i n t h e depos i t .

The till has no d i s t i n c t i v e land form. T igh t ly p l a s t e r e d till i s exposed i n Kowee Creek v a l l e y along t h e s k i t r a i l , and i n slump scars a long t h e v a l l e y wall n e a r t h e c i rque . T h i s - t i l l seems t o r ep re sen t depos i t i on by an o l d e r i c e be fo re t h e westward expansion o f t h e i c e moving from t h e Juneau I c e f i e l d .

A s i m i l a r l y ha rd and i n t e n s e l y compacted till l i e s p l a s t e r e d above bedrock i n the c i r q u e ho ld ing Cropley Lake, and a g a i n s t t h e v a l l e y wa l l above t h e l ake and i n two much h i g h e r c i rques t h a t hang above t h e v a l l e y . Exposures a r e ra re ; two o f t h e b e s t exposures are i n excavat ions made during cons t ruc t ion of the two small dams b u i l t t o r a i s e t h e l e v e l of the n a t u r a l l y dammed Cropley Lake. About 3 f e e t o f pea ty m a t e r i a l o v e r l i e s t h e t ill i n t h e s e excavat ions . Bedrock i s n o t exposed beneath t h e 2-3 feet o f t i l l , and the th i ckness o f t h e o l d e r t i l l i s unknown.

Though not everywhere exposed, t h e o l d e r t ill appa ren t ly u n d e r l i e s many o f the muskegs i n Kowee and Fish Creek v a l l e y s . Probes pene t r a t ed only about 3 fee t o f peat i n muskegs between Cropley Lake and F i d ~ Creek before meeting r e f u s a l . 11e h0110ri end o f t h e probe contained t ill . Sca t t e r ed exposures i n Koriee Creek v a l l e y suggest t h a t t h e o l d e r t i l l extends near ly t o t h e lo \ i e r end o f t h e v a l l e y ; t h e l o c a t i o n o f t h e lower boundary of t h i s depos i t i s a r b i t r a r y .

A l l u v i a l depos i t s

A l luv ia l d e p o s i t s , a s mapped, c o n s i s t o f m a t e r i a l s t h a t are i n t e r p r e t e d as having gene ra l ly accumulated from flowing water . Modern alluvium (Qal), fan depos i t s ( Q f ) , and t e r r a c e depos i t s (Qt) a r e found along flowing s treams, bu t no t n e c e s s a r i l y along t h e same streams. Rubble d e p o s i t s (Qar) a r e accumulations of coarse fragments o f rock f i l l i n g o r blocking d ry g u l l i e s o r flowing s treams t h a t a r e much t o o weak t o move such l a r g e p ieces .

Modern alluvium (Qal)

Modern alluvium is der ived from bedrock and s u r f i c i a l depos i t s w i th in t h e dra inage bas in o f each s t ream, and thus i t s composition v a r i e s from p lace t o p lace . Alluvium along t h e Mendenhall River c o n s i s t s most ly o f grani te and d i o r i t e pebbles , cobbles , and small boulders t h a t form poin t -bars a long t h e r i v e r channel. Alluvium along Duck and Jordan Creeks, w i th in Mendenhall v a l l e y , ranges from pebble g rave l upstream t o coa r se sand n e a r t h e Glac i e r Highway. The s t reams flow on sand and g rave l i.n most o t h e r places on t h e mainland, except fox segments o f s t e e p g rad ien t a long mountain s t reams where boulders a r e dominant and gene ra l ly l i e d i r e c t l y on bedrock. Cobbles and boulders of g r a n i t i c rock a s l a r g e as 3 f e e t make up most of t h e alluvium a long Fish Creek. Much o f Fish Creek i s entrenched i n bedrock gorges where t h e alluvium i s n e a r l y absent . Else- where on Douglas I s l and s t reams a r e s h o r t , s t e e p , and s w i f t ; on ly s c a t - t e r e d cobbles and boulders accumulate along t h e s e s t reams. Age o f t he modern alluvium i s l a t e Holocene.

Thickness of modern alluvium i s v a r i a b l e and ranges from a few f e e t t o probably s e v e r a l t e n s o f f e e t . Alluvium a t t h e mouth o f t h e Mendenhall River i s more than 6 f e e t t h i c k . Remnants o f alluvium 3 f e e t o r more t h i c k a r e preserved a s broad f l a t s i n t h e lower p a r t of F i sh Creek v a l l e y marginal t o t h e boulder concent ra t ions along t h e s t ream channel. Modern alluvium along Lemon Creek v a r i e s i n t h i ckness bu t i s probably more than 5 f e e t t h i ck .

Modern alluvium gene ra l ly i s a poor foundat ion m a t e r i a l ( see over lay 2 ) ; it forms a broad p l a i n , which has a low g rad ien t . I t g e n e r a l l y i s composed o f r e l a t i v e l y loose d e p o s i t s t h a t have a h igh water t a b l e . Some alluvium is entrenched s l i g h t l y by streams and has a water t a b l e lower than depos i t s where s t reams flow d i r e c t l y on t h e s u r f a c e o f t h e p la in . Alluvium with such a lower water table would provide s l i g h t l y b e t t e r foundat ion condi t ions , bu t it is l e s s d e s i r a b l e f o r b u i l d i n g s i t e s t han d r i e r and more compacted depos i t s .

Very t h i n d e p o s i t s o f coa r se modern alluvium on bedrock probably would respond t o an ear thquake by t r a n s m i t t i n g t h e waves wi th l i t t l e i n t e r n a l displacement. Thick, wa te r - sa tu ra t ed d e p o s i t s , however, l i k e t hose depos i t s mapped along lower Lemon Creek and a t t h e lower end o f t h e Mendenhall v a l l e y , would f r a c t u r e , compact, and poss ib ly e j e c t sediment- laden water a s descr ibed under t h e younger outwash d e p o s i t s (Qoy) (see p. 58 ) and e s p e c i a l l y under t h e younger d e l t a d e p o s i t s (Qdy) ( s ee p. 69 ) . The lower p a r t s o f l a r g e s t ream v a l l e y s t h a t e n t e r t i dewa te r s a r e suscep- t i b l e t o s e i che waves o r tsunamis. Entrenched s t ream channels and low t e r r a i n ad jacent t o the s treams make easy pa ths f o r se i smic s e a waves.

Fan depos i t s (Qf)

Al luv ia l fans form a t t h e mouths o f most s t reams t h a t flow from s t e e p mountain s lopes onto f l a t t e r ground on t h e mainland and Douglas I s land . Fans form where s t reams e n t e r a body of wa te r on ly i f t h e r e i s an onland break i n s lope from s teep t o more g e n t l e g rad ien t . The fans a r e composed of sand and pebbles mixed t o g e t h e r i n vary ing amounts. S l a t e and o t h e r f o l i a t e d rocks seem t o be t h e most prominent rock types and form t a b u l a r angular fragments 1-3 inches ac ros s ; s i l t y sand, coa r se sand, o r granules f i l l t h e spaces between rock fragments. Grani te fragments l o c a l l y seem t o be present i n amounts out of p ropor t ion t o t h e r e l a t i v e e x t e n t o f g r a n i t e i n t h e source a reas and may r e f l e c t reworking o f upstream g l a c i a l depos i t s . This is e s p e c i a l l y t rue of t h e fan of Auke Nu Creek n e a r Mendenhall v a l l e y and t h e f an o f F ish Creek on Douglas I s land . Ma te r i a l from Lake Creek, which shares a f an with Auke Nu Creek, i s composed a l - most exc lus ive ly o f p l a t y s l a t e fragments wi th no granite boulders . The fan depos i t s are a l l o f Holocene age and t h e d e p o s i t s a r e accumulating a t t h e p re sen t time.

Large depos i t s which extend landward from d e l t a s a r e mapped as fan de- p o s i t s i n t h i s r epo r t . Gold Creek fan and t h e f a n of Lake and Auke Nu Creeks a r e such fans. Figure 8 shows t h e r e l a t i o n s h i p s between fans , del tas , and fan d e l t a s . The geologic map shows only t h e prominent f a n depos i t s ; some small fans a r e n o t mapped. Almost a l l fans s l o p e 5"-10" downslope. The f r o n t of t h e d e l t a p o r t i o n o f t h e f a n along Lake and Auke Nu Creeks s lopes 25". The s u b a e r i a l s lope of each fan, however, depends on the abruptness o f change i n g rad ien t where t h e s t ream e x i t s from t h e mountain s l o p e , as we l l a s on t h e volume and v e l o c i t y of t h e stream. Some fans extend upward i n t o the mountain v a l l e y s a s a l l u v i a l cones where they have s t e e p e r g rad ien t s . Most o f t h e d e p o s i t s shown on the map a r e i nd iv idua l fans, b u t along t h e e a s t e r n s i d e o f Mendenhall v a l l e y t h e fans have coalesced t o form a s e r i e s o f undula t ing su r f aces .

The l a r g e r t h e f an and s t e e p e r t h e upstream g r a d i e n t , t h e t h i c k e r i s t h e fan depos i t . A t e s t h o l e d r i l l e d i n t o d e p o s i t s o f t h e Lake Creek a l l u v i a l fan pene t r a t ed 80 f e e t of grave l before e n t e r i n g glaciomarine depos i t s (J. A. McConaghy, w r i t t e n cornmun., 1967). The d e p o s i t s accumulate t o b u i l d the f an shape as s treams wander from s i d e t o s i d e ; t h e s t ream channels become blocked as m a t e r i a l i s dropped and t h e s t ream s h i f t s

from p lace t o place. If an a c t i v e s t ream channel is shor tened o r o the r - wise steepened, t h e stream w i l l erode the fan t o a d j u s t t o t h e new g rad ien t .

Rock types i n fans vary acccrd ing t o t h e rocks i n t h e dra inage a r e a through which t h e streams flow. The d e p o s i t s a r e s a t u r a t e d below t h e water t a b l e , which v a r i e s i n depth from p lace t o place. In d e p o s i t s t h a t c o n s i s t c h i e f l y o f angular s l a t e fragments, p l a t e s o r ch ips o f s l a t e tend t o l i e f l a t l i k e cards i n a deck o r are s tacked l i k e s h i n g l e s and i n c l i n e d a g a i n s t t h e direc- t i o n o f flow. The voids bet\<een s l a t e fragments gene ra l ly a r e only par - t i a l l y f i l l e d by f i n e r m a t e r i a l . The looseness permi ts r ap id i n f i l t r a t i o n under normal condi t ions . Streams on a l l u v i a l fans are gene ra l ly confined t o a s i n g l e channel dur ing normal flow, b u t a t t imes o f h igh d ischarge t h e channel may s h i f t , be abandoned i n f avo r of a new one, o r flow i n s e v e r a l channels.

Under normal condi t ions t h e foundat ion c a p a b i l i t y o f a l l u v i a l fan depos i t s i s good. Heavy s t r u c t u r e s commonly r e q u i r e spread foo t ings o r dr iven- f r i c t i o n p i l e s ; l i g h t s t r u c t u r e s gene ra l ly a r e placed on concre te mats o r foo t ings . The extremely loose c h a r a c t e r o f t h e d e p o s i t decreases wi th depth, and gene ra l ly excavat ions 5 o r 6 f e e t deep f o r houses f i n d firmer ground f o r foot ing . Basements a r e imprac t i ca l i n d e p o s i t s having a high water t ab le .

The l o o s e r f an d e p o s i t s would r e a c t v i o l e n t l y t o shaking caused by a * seve re earthquake (see ove r l ay 2 ) . The par t s of fans t h a t a r e n e a r a s t ream o r lake o r form t h e apex would be very s u s c e p t i b l e t o shaking, f r a c t u r i n g , and perhaps water-sediment e j e c t i o n . Most s u s c e p t i b l e would be such p a r t s of t h e Gold Creek f an , t he Lake Creek-Auke Nu Creek fan, and t h e F ish Creek fan. S t rong shaking could cause l i q u e f a c t i o n and(or) s l i d e s i n sandy deposi t s along f an -de l t a f r o n t s . The water t a b l e i s unusual ly h igh i n the a l l u v i a l f an depos i t s along t h e e a s t e r n side o f Mendenhall v a l l e y and along Montana Creek. The cond i t i ons a t t h e s e p l aces a r e s i m i l a r t o t h o s e o f Fo res t Acres n e a r Seward, Alaska, where sand b o i l s , sand spou t s , and ground f r a c t u r i n g damaged homes during t h e 1964 ear thquake (Lemke, 1967, p. E34-E40). There i s less prob- a b i l i t y o f such v i o l e n t f r a c t u r i n g and spout ing i n fans with lower water t a b l e s . Nevertheless v i b r a t i o n would be h i g h e r on fan d e p o s i t s than on b e t t e r consol ida ted depos i t s , and s t r o n g shaking, f r a c t u r i n g , d i f f e r e n t i a l s e t t l i n g , and compacting should be expected.

Slopes i n excavat ions i n fan depos i t s w i l l r a v e l i f v e r t i c a l c u t s are made. This i s e s p e c i a l l y t r u e o f the upper 15 o r 20 f e e t o f t h e fan depos i t . A t g r e a t e r depths a more compact depos i t i s reached t h a t probably would s t and b e t t e r i n s t e e p e r excavat ions i f t h e water t a b l e i s below t h e f l o o r of t h e excavat ion. Because t h e f an m a t e r i a l s vary from place t o p l ace , a s does t h e water t a b l e , i nd iv idua l s i t e s should be c a r e f u l l y i n v e s t i g a t e d be fo re excavat ions are planned. S l i d i n g of t h e subaqueous d e l t a f r o n t s o f fans is d iscussed on p. 68.

Terrace d e p o s i t s ( Q t )

Terrace depos i t s a r e a l l u v i a l d e p o s i t s t h a t s t and above t h e s u r f a c e of modern alluvium (Qal] along entrenched s tream channels. In gene ra l , t h e t e r r a c e depos i t s a r e l i t h o l o g i c a l l y s i m i l a r t o al luvium, b u t may be somewhat coarser . In most p l aces , the t e r r a c e depos i t s a r e r i c h i n hard g r a n i t i c rocks. The t e r r a c e depos i t n e a r the mouth o f Lemon Creek i s a s i l t y f i n e sand; f a r t h e r upstream n e a r t h e bedrock gorge, t h e t e r r a c e depos i t i s sandy cobble grave l . The t e r r a c e depos i t s a long upper Montana Creek a r e mostly rounded boulders and cobbles , and those a long t h e lower p a r t o f Fish Creek c o n s i s t o f wel l - sor ted cobble and boulder grave l . The age o f t h e depos i t s i s l a t e Holocene.

Although t e r r a c e depos i t s a r e shown on t h e geologic map only along Montana, Peterson, Gold, Lemon, and Fish Creeks, s i m i l a r depos i t s t o o small t o map a l s o a r e p re sen t along o t h e r s t reams. Some t e r r a c e de- p o s i t s form two o r more d i s t i n c t s u r f a c e s t h a t a r e s epa ra t ed by sca rps 1-4 f e e t high.

Streams o f t h e Juneau area a r e now graded t o t i dewa te r . A s t h e land i n the Juneau a r e a r o s e fol lowing t h e l a s t major r eg iona l g l a c i a t i o n , s t reams cut i n t o t h e i r own alluvium. The t ime and r a t e o f t h e r i s e o f t h e land t h a t caused t h i s downcutting a r e d iscussed on p. 8 2 .

Rubble depos i t s (Qar)

The rubble depos i t s gene ra l ly c o n s i s t o f angular , l o c a l l y der ived blocks of s l a t e , greenstone, and metavolcanic rocks , and round t o subround boulders o f g r a n i t e . Some o f t h e p i eces are as l a r g e a s 12 f e e t ac ros s . Rock fragments i n t h e depos i t l i e a g a i n s t each o the r , and t h e spaces between l a r g e fragments a r e only p a r t l y f i l l e d wi th s m a l l e r rocks. The age probably i s Holocene.

These rubble d e p o s i t s probably r ep re sen t more than one depos i t i ona l o r i g i n and age, b u t it seems l i k e l y t h a t most of them were formed by t o r r e n t i a l f l oods . The l a r g e g r a n i t e boulders probably a r e g l a c i a l e r r a t i c s . The l a r g e s t ream discharge r equ i r ed t o form t h e d e p o s i t s could be r e l a t e d t o an ep isode o f g r e a t e r p r e c i p i t a t i o n than cha rac t e r - izes t h e p re sen t c l imate . Some o f t h e depos i t s extend downslope t o a l t i t u d e s below 500 f e e t , and thus a r e below t h e marine l i m i t . These rubble depos i t s must have accumulated a f t e r t h e land had s t a r t e d t o rebound i s o s t a t i c a l l y and had r i s e n r e l a t i v e t o modern sea l e v e l .

The depos i t s cover t h e f l o o r s o f narrow rav ines and b l anke t some of t h e s lopes a t t h e n o r t h end o f Douglas I s l and and t h e mainland between Lemon Creek and Tee Harbor. The jumbled landform i s unmistakable and bea r s no resemblance t o t h e smoother s lopes developed on t h e ad j acen t depos i t s . Lena Creek flows almost e x c l u s i v e l y through such a rubbly depos i t . The rubble depos i t s a long Gold Creek n o r t h e a s t o f Juneau s t a r t ab rup t ly where the rav ines s teepen toward Gold Creek. Deposi ts on t h e Mendenhall Penin- s u l a a l s o seem t o have abrupt margins n e a r t h e upper ends o f r av ines .

D e l t a i c d e v o s i t s

1, d e l t a i s an a l l u v i a l depos i t t h a t forms where s t reams drop t h e i r loads o f s o l i d p a r t i c l e s as t h e r e s u l t o f decreased s t ream v e l o c i t y where the flowing water c n t e r s a body of water. Deposi t ion i s g r e a t e s t when t h e s t ream l o a d i s h ighes t and l e a s t when t h e s t ream load i s lowest. As t he d e l t a s lowly b u i l d s outward, t h e uppermost beds gene ra l ly remain above high water . Streams flowing on d e l t a s gene ra l ly wander and change chaanels un less a r t i f i c i a l l y confined, such as Gold Creek which is re- stricted t o a flume. Channel change gene ra l ly t akes p l ace dur ing pe r iods of high stream flow. As a d e l t a grows, t h e landward po r t ions tend t o en l a rge and a combination fan and d e l t a is b u i l t ( f i g . 8). Gold Creek has such a fan d e l t a .

Deltaic depos i t s , t hose sediments t h a t form a d e l t a , a r e s epa ra t ed i n t o two groups i n t h i s r e p o r t . Younger d e l t a d e p o s i t s (Qdy) a r e those s e d i - ments i n modern d e l t a s , whereas t h e o l d e r d e l t a depos i t s (Qdo) are those sediments t h a t f o n e d d e l t a s dur ing t h e t ime when t h e land was s t i l l depressed from t h e weight o f t h e l a t e P l e i s tocene i c e cover and s e a l e v e l was h ighe r r e l a t i v e t o t h e land than a t t h e p re sen t time. Consequently, t h e s e o l d e r d e l t a s now are found s e v e r a l hundreds o f f e e t above modern s e a l e v e l .

Younger d e l t a d e p o s i t s (Qdy)

The younger d e l t a depos i t s range from f i n e t o coarse ma te r i a l . S l a t e , greenstone, and g r a n i t e a r e the most common rock types. The younger d e l t a depos i t s a r e gene ra l ly o v e r l a i n by i n t e r t i d a l s i l t s o r fill m a t e r i a l ; consequently, they a r e n o t e a s i l y seen. Test h o l e s d r i l l e d and augered by t h e M a t e r i a l s Division of t h e Alaska Highway Department as p a r t of s e v e r a l roadway and b r idge i n v e s t i g a t i o n s show t h a t most d e l t a depos i t s a r e gene ra l ly composed o f f i n e sand o r sandy grave l mixed with m a l l amounts o f s i l t , and that: they become f i ne r and more dense with depth (Munson and S l a t e r , 1963; Munson and Rasmussen, 1966; Frankle t and Rasmussen, 1969; S l a t e r and Grahek, 1970; S l a t e r and Palczer, 1970).

The larger d e l t a s such as those o f Mendenhall River , Salmon Creek, Lemon Creek, and Gold Creek, conta in m a t e r i a l s which have a wide range i n t ex - t u r e and l i t ho logy . Light-gray g r a n i t i c rock fragments a r e very common i n most of t h e d e l t a depos i t s along t h e mainland. Dark s l a t e s and green- s tones a r e dominant i n t h e Douglas I s l and younger d e l t a d e p o s i t s , whose t e x t u r e ranges from f i n e sand t o a pebble grave l t h a t l o c a l l y con ta ins cobbles and boulders . \fiere s l a t e predominates, more than 60 percent of t h e p i eces a r e t h i n and p l a t y . The greenstone fragments gene ra l ly a r e more blocky and subrounded. Cobbles and boulders a r e common i n t h e younger d e l t a o f Eagle Creek, whereas t h e d e l t a o f Lawson Creek i s mostly sandy grave l (blunson and F rank le t , 1963a). These d e l t a s a r e Holocene and axe growing and en la rg ing a t p re sen t .

Del tas are mapped along Gast ineau Channel and F r i t z Cove and blendenhall L-ke. The d e l t a s have a t y p i c a l l y t r i a n g u l a r o r fan shape i n plan. The upper su r f ace o f t h e d e l t a s l o p e s g e n t l y toward t h e water , and t h e modern s t ream clzannel may bc incised from 1 t o 5 f e e t . During pe r iods o f high d ischarge , s t rcams m2y erode l a t e r a l l y and s h i f t t h e i r channels. New channels a r e a l s o formed when t h e s t ream s h i f t s p o s i t i o n a t t h e apex of t h e d e l t a . The f r o n t of t h e d e l t a i s below water. The d e l t a s i n Gast ineau Chmnel,and F r i t z Cove a r e encroaching on t h e channelways i n t h e f i o r d an? have s o f i l l e d ti le c o n s t r i c t e d n o r t h end of t h e channel t h a t dredging i s necessary from t i m e t o t ime t o maintain a boa t channel u sab le a t high t i d e .

The a rcua te d i s t a l o u t l i n e and broad l a t e r a l ex t en t o f most o f t h e d e l t a s a r e we l l exposed a t minus t i d e s , except f o r t h e Mendenhall River d e l t a , which remains obscured. Fathometer t r a v e r s e s ac ros s f r o n t s o f d e l t a s i n Gastineau Channel show s l o p e s o f about 25" (R. D. T.liller, 1967), Bathy- me t r i c contours on topographic maps suggest s i m i l a r s lopes .

Most o f the in format ion a v a i l a b l e concerning t h e th i ckness o f t h e younger d e l t a depos i t s comes from d r i l l i n g a l r e a d y c i t e d . Judging from i n t e r p r e t a - t i o n o f d r i l l r eco rds , I t h i n k t h a t t h e l a r g e d e l t a s n e a r water l e v e l a r e more than 50 f e e t t h i c k .

S l i d i n g o f d e l t a f r o n t s was common dur ing t h e 1964 Alaska ear thquake. Large s l i d e s occurred a t Seward (Lemke, 1967, p. E30); Valdez (Coul te r and Migl iacc io , 1966, p. C15); and W h i t t i e r (Kachadoorian, 1965, p. B16- B17). Ground v i b r a t i o n of s u f f i c i e n t s t r e n g t h and du ra t ion i n f i n e - grained saturated d e p o s i t s can inc rease pore p re s su re , produce liquefac- t i on , and reduce t h e shear s t rength of t h e d e l t a i c m a t e r i a l s . Massive s l i d e s may r e s u l t .

Sa tura ted unconsol idated m a t e r i a l s similar t o t hose o f t h e younger d e l t a i c depos i t s responded dur ing t h e 1964 ear thquake by compacting and subs id ing , l a te ra l spreading, f r a c t u r i n g , and spout ing sediment-laden water . Compac- t i o n of s i l t and sand n e a r Portage r e s u l t e d i n l and subsidence o f 4-5 f e e t (Seed, 1964, p. 3 7 ) . Writ ing of subsidence, probably o f about 2-3 f e e t i n Snow River v a l r e y n o r t h of Setiard, Lernke (1967, p. E40) a l s o mentions t h a t some o f the subsidence i n t he Snow Creek v a l l e y n e a r Seward may have been caused by l a t e r a l spreading from the 'ou tward movement o f a d e l t a i n nearby Kenai Lake. Simi l a r l a t e r a l spreading of f l a t o r n e a r l y f l a t ground toward an unconfined d e l t a f ace is be l i eved by Coul te r and Mig l i acc io (1966, p. C21) t o have caused f r a c t u r e s t h a t t rended ac ros s t h e v a l l e y par - a l l e l t o t h e d e l t a f a c e a t Valdez during t h e 1964 earthquake. In a d d i t i o n , ground f i s s u r e s develop &hen se ismic energy i s t r a n s m i t t e d through sediments and i s transformed i n t o surface waves which s t r e t ch and compress t h e su r - face mate r i a l s u n t i l t hey f r a c t u r e . F i s su re s caused i n t h i s way occurred i n f rozen ground a t Valdez (Coulter and Fl igl iaccio, 1966, p. C25) .

Sediment-laden water spewed from t h e ground a t va r ious places i n Alaska during t he 1964 earthquake. Vibra tory compaction o f s a t u r a t e d sediments

forced water f r e e d from t h e i n t e r g r a i n voids t o t h e s u r f a c e a s sand spouts and b o i l s (Seed, 1963, p. 3 7 ) . Such emissions g e n e r a l l y occurred a t i s o l a t e d p o i n t s dur ing t h e earthquake. In a d d i t i o n , cont inued undula- t o r y ground moverncnt pumped ?and, s i l t , and water upward, where it emerged along f i s s u r e s i n a s e r i e s o f p u l s a t i n g surges @lacelwane, 1947, p. 17; Coul te r and Migl iacc io , 1966, p. C25; Lemke, 1967, p. E39). Buildings on o r nea r such f i s s u r e s were s t r u c t u r a l l y damaged a t Valdez (Coulter and I l i g l i acc io , 1966, p. C30-C35), and Seward (Lemke, 1967, p. E13), and s e v e r a l homes i n Forest Acres, n e a r Seward, were p a r t l y f i l l e d by sand from spou t s , b o i l s , and f i s s u r e s .

' Most of t h e damaging water waves dur ing t h e 1964 Alaska ear thquake have been ascr ibed t o subaqueous s l i d e s o r tsunamis. Waves generated l o c a l l y t h a t were apparent ly not r e l a t e d t o t e c t o n i c movement, tsunamis, o r subaqueous s l i d e s were r epo r t ed over much o f t h e a r e a a f f e c t e d by t h e e a r t h - quake. How many o f t h e s e waves were se i smic s e i c h e s a r e n o t known, and t h e importance o f s e i smic se i ches r e l a t i v e t o s h o r e l i n e inundat ion i s no t s p e c i f i c a l l y understood.

Juneau appa ren t ly was bu t s l i g h t l y a f f e c t e d by t h e tsunami caused by t h e earthquake i n 1964. A r e s i d e n t l i v i n g on t h e b l u f f above Auke Bay noted t h a t the water o s c i l l a t i o n a t Auke Bay was out of phase wi th t h a t be ing r epor t ed from Gast ineau Channel a t Juneau [Keith Har t , o r a l commun., 1968). Spaeth and Bexkman (1967, t a b l e 4) show 7 feet as be ing the maximum r i s e o f t h e water above a n t i c i p a t e d t i d a l h e i g h t s over a per - iod of many hours of f l u c t u a t i o n s a t t h e t i d e gage a long Gast ineau Channel a t Juneau.

Another i n c i d e n t was a t t r i b u t e d t o a tsunami i n Gast ineau Channel. A f loat plane f l ipped over i n the water and sank i n t h e harbor a t Douglas as a r e s u l t o f t h e unusual t i d a l a c t i o n (Juneau Dai ly Alaska Empire, Mar. 29, 1964). Thus, although we l l w i t h i n the s h e l t e r e d waters of the Ins ide Passage, Gast ineau Channel a t Juneau was e v i d e n t l y affected by sea waves o r i g i n a t i n g many miles away.

Wilson and Tbrum (1968, p. 363-372) d i scuss t h e a b i l i t y of s t r u c t u r e s t o withstand tsunamis, damage t o harbor s t r u c t u r e s , p r o t e c t i v e measures, and s a f e t y s t anda rds i n a r e a s t h a t may be a f f e c t e d by tsunamis. They report t h a t o l d , one-story wooden frame bu i ld ings had poor r e s i s t a n c e t o tsunamis i n c o a s t a l communities h i t by t h e se i smic s e a waves i n 1964. Concrete block and r e in fo rced concre te s t r u c t u r e s , on t h e o t h e r hand, gene ra l ly withstood t h e tsunamis. Wave damage can a l s o be reduced i f bu i ld ings have deeply embedded foot ings .

A dramatic consequence o f subaqueous s l i d i n g o f d e l t a f r o n t s dur ing t h e 1964 ear thquake was b a c k f i l l waves. These waves formed as water rushed in to the void c r e a t e d when t he m a t e r i a l s l i d out . Backfill waves caused much of t h e damage t o b u i l d i n g s and o t h e r s t r u c t u r e s on the d e l t a s a t Valdez (Coulter and Migl iacc io , 1966, p. C14) and Seward (Lemke, 1967, p. E4-E5; p. €41). Water from a b a c k f i l l wave ran up a s h igh as 30 f e e t

above a s l i d e s c a r p i n Kenai Lake (McCulloch, 1966, p. A7, A17). Sl ide - iliduced waves a l s o move outward and inundate low shores oppos i t e t h e s l i d e . Such LGaves on Kenai Lake ran up t o a he igh t o f 25-35 f e e t i n t h e c e n t e r o f the wave-washcd area, and t o he igh t s o f 11-13 f e e t on t h e s i d e s (bIcCulloch, 1966, p. AS) .

Mendenhall Lake con ta ins a p o t e n t i a l danger i n t h e form o f s l ide- induced waves from earthquakes. The l ake has a depth o f 200 f e e t and covers a s u r f a c e a r e a of s l i g h t l y more than 1 square mi le (Barnwell and Boning, 1968, p. 1 ) . S l i d e s could occur i n t h e d e l t a o f Nugget Creek a t t h e no r theas t edge o f t h e l ake , and i n t h e o l d S teep Creek outwash d e l t a a long t h e sou th -cen t r a l shore o f t h e lake. The l a t t e r d e l t a i s oppos i te a f o r e s t e d shore , and waves c r e a t e d by a s l i d e probably would cause no s e r i o u s damage. The a r e a oppos i t e the Nugget Creek d e l t a inc ludes t h e U.S. Fores t Serv ice V i s i t o r s 1 Center and parking l o t . If t h e d e l t a f r o n t s l i d o u t , waves could endanger t h e campground, which i s loca t ed on land that i n many p l aces i s l e s s than 15 f e e t above l a k e l e v e l . Although Mendenhall Lake i s sma l l e r than Kenai Lake, it i s deeper , and waves a t l e a s t a s high as those a t Kenai Lake probably could be developed from s l i d e s .

Older d e l t a d e p o s i t s (Qdo)

The o l d e r d e l t a d e p o s i t s c o n s i s t of o l ive-gray (5Y 5/2) sandy g rave l and g r a v e l l y sand, conta in ing v a r i a b l e amounts of -s i l t and c l ay . The depos i t s are made up most ly o f d e l t a i c f o r e s e t beds t h a t con ta in cobbles , boulders , and s h e l l fragments. Layers of gray diamicton 2-10 f e e t t h i c k a r e in te rbeddcd wi th f o r e s e t s o f grave l t h a t a r e exposed i n a p i t i n t h e SW1/4SE1/4 sec . 21 , T. 40 S. , R. 65 E . , near t h e Auke Bay f e r ry te rmina l ( t a b l e 4 , no. 5) . S h e l l s and s h e l l fragments a r e widespread, and many unbroken ba rnac l e s a r e a t t ached t o cobbles i n n a t u r a l growth posit ions. The i n t e r l a y e r e d diamicton beds a r e t e x t u r a l l y s i m i - l a r t o those descr ibed under glaciomarine depos i t s . These diamictons form ledges having n e a r l y v e r t i c a l f aces . The d e l t a depos i t a t t h e grave l p i t mentioned-above has an eroded s u r f a c e t h a t t r u n c a t e s t h e gravel; t h i s s u r f a c e is ove r l aSn by a massive diamicton r i c h i n ba rnac l e s h e l l s which, i n t u r n , i s o v e r l a i n by a second sequence of d e l t a i c g rave l s .

Barnacle s h e l l s a t t ached t o cobbles i n t h i s upper diamicton have a rad io- carbon age o f 12,730+500 years (sample W-1830), and s h e l l s w i th in t h e diamictons i n t h e lower d e l t a i c beds have an age of 12,880+500 years (sample W-1831, Meyer Rubin, U.S. Geol. Survey, w r i t t e n commun., 1966). These ages, though from m a t e r i a l s s epa ra t ed by an e ros iona l su r f ace , suggest t h a t a l l o f t h e d e p o s i t s a r e o f about t h e same age.

The o l d e r d e l t a d e p o s i t n e a r t h e mouth o f Gold Creek, i n Juneau, con- t a i n s s i m i l a r l y i n t e r l a y e r e d beds o f g ray i sh diamicton. The age of ba rnac l e s h e l l s i n t h e s e beds is 10,880+340 years (sample W-1829, Meyer Rubin, w r i t t e n commun., 1966).

A well-developed l a y e r , 4 f e e t t h i c k , o f gray very compact and indura ted d i r n i c t o n i n a grave l p i t a t Eagle Creek, on Douglas I s l a n d , con ta ins boulders , cobbles , and s h e l l s ( t a b l e 3 , no. 6) and unconformably o v e r l i e s d e l t a i c sand and g rave l . This l a y e r appears t o be h o r i z o n t a l and b l anke t s an eroded s u r f a c e t h a t t r u n c a t e s t h e underlying f o r e s e t beds. Boulders and cobbles seem t o be concent ra ted i n t h e lower two- th i rds of t h e diamicton. S h e l l s i n t h e diarnicton a r e 9,1501800 years o l d (sample W-2395; Meyer Rubin, w-itten conlmun., 1970).

The o l d e r d e l t a d e p o s i t s occur p r i m a r i l y along Gast ineau Channel a t t h e mouths of some t r i b u t a r y va l l eys . In a d d i t i o n , an ex tens ive o l d e r d e l t a depos i t i s preserved i n t h e lower p a r t o f Montana Creek v a l l e y . A small d e l t a exposed i n t h e grave l p i t nea r t h e Alaska f e r r y te rmina l appa ren t ly i s r e l a t e d t o an e a r l i e r drainageway t h a t has s i n c e been abandoned. Sur- p r i s i n g l y , no d e l t a depos i t was found i n t h e lower p a r t o f t h e F ish Creek v a l l e y , which i s one o f t h e l a r g e r v a l l e y s on Douglas I s l and , even though d e l t a s e x i s t on some o f the sma l l e r v a l l e y s o f Douglas I s l and . The d e l t a s occur a t he igh t s o f a s much a s 500 f e e t above sea l e v e l .

Many analyses of t h e o l d e r d e l t a depos i t s a r e included i n p r o j e c t r e p o r t s o f t h e Alaska S t a t e Highway Department. Most d e l t a i c d e p o s i t s a r e loose and v a r i a b l e i n composition. Boulders as l a r g e as 2 f e e t ac ros s and i s o - l a t e d fragments a s l a r g e a s 10 f e e t make up about 20 percent o f t h e o l d e r d e l t a depos i t s . S i l t and c l a y gene ra l ly make up l e s s than 10 percent of any depos i t . Samples from diamicton l a y e r s had d ry bulk d e n s i t i e s o f a s much as 140 l b s p e r cu f t . The diamicton on t o p o f t h e f o r e s e t beds a t Eagle Creek on Douglas I s l and is even more dense, and has a dry bulk d e n s i t y o f 150 l b s p e r cu f t .

The o l d e r d e l t a d e p o s i t s i n t h e Juneau a r e a &cumulated i n ;he marine waters o f Gastineau Channel and Auke Bay when t h e land was lower r e l a t i v e t o sea l e v e l ( f i g . 10) . The land subsequent ly r o s e r e l a t i v e t o s e a l e v e l and l e f t t hese depos i t s perched along mountain f r o n t s a s high as 250 t o about 500 feet i n a l t i t u d e . Such h e i g h t s i n d i c a t e t h e p o s i t i o n o f t h e anc i en t s e a + l e v e l r e l a t i v e t o modern s e a l e v e l a t t h e t ime t h e d e l t a s were formed. The maximum he igh t o f t h e d e l t a s does not r ep re sen t t h e marine l i m i t , which i s be l i eved t o be cons iderably h ighe r , b u t apparent ly r ep re sen t s one o f perhaps s e v e r a l temporary and r e l a t i v e l y s t a b l e p o s i t i o n s dur ing i s o s t a t i c rebound. The diamictons interbedded wi th t h e f o r e s e t beds of t h e d e l t a may have s e t t l e d out of marine water dur ing i n t e r v a l s when s treams were no t b r ing ing coa r se m a t e r i a l t o t h e d e l t a , o r they may repre- sent mudflows der ived from s a t u r a t e d glaciomarine d e p o s i t s upstream. A t Eagle Creek, t h e m a t e r i a l s i n t h e l a y e r t h a t i s draped over t h e t runca ted d e l t a i c beds appears t o be graded; t h e c o a r s e r m a t e r i a l is concent ra ted i n the lower two-thirds o f t h e depos i t . Such grading has been descr ibed as being c h a r a c t e r i s t i c o f mudflows elsewhere (Crandel l , 1952; Mullineaux and Crandel l , 1962, p. 857-858). Such an o r i g i n , however, would r e q u i r e t h a t t h e mudflow came t o Tes t on a s l o p e o f 30"-35". The diamictons on top o f t h e d e l t a i c sand and g rave l may a l s o r ep re sen t a pe r iod when t h e rise of e u s t a t i c s e a l e v e l exceeded t h a t o f i s o s t a t i c rebound o f t h e land

and t h e t i d a l waters temporar i ly submerged t h e d e l t a i c depos i t . If t h i s occurred, the coarse materials i n t h e diamictons could have been der ived from melt ing i c e blocks f l o a t i n g i n t h e f i o r d . The r e l a t i v e l y young age of 9,1SO+S00 years B.P. (sample W-2395, ?Ieyer Rubin, U.S. Geol. Survey, w r i t t e n commun., 1970) f o r t h e blanket-forming diamicton a t Eagle River suppor ts t h e concept o f a temporary e u s t a t i c r i s e o f s e a l e v e l .

The o l d e r d e l t a s t h a t l i e a t an a l t i t u d e o f about 250 f e e t , o r s l i g h t l y lower, a r e t h e d e l t a s t h a t con ta in t h e upper blanket-forming diamictons. These diamictons a r e be l i eved t o r e l a t e t o a pause i n t h e rebound o f t h e land dur ing i s o s t a t i c readjustment . This 250-foot l e v e l i s c o n s i s t e n t throughout t h e Juneau reg ion f o r d e l t a i c p re se rva t ion and appears t o be reg ional i n na tu re . Del tas occur a t a s i m i l a r a l t i t u d e i n t h e Skagway and Haines a r eas (Yehle and Lemke, w r i t t e n commun., 1972; Lemke and Yehle, 1972b). This pause i n rebound i s d iscussed i n somewhat more d e t a i l under glaciomarine depos i t s .

Prolonged shaking fxom a severe earthquake n e a r Juneau could cause compaction and d i f f e r e n t i a l s e t t l emen t i n p a r t s o f t h e r e l a t i v e l y loose o l d e r d e l t a depos i t s . Some of t h e f ron t31 s lopes o f t h e s e o l d e r d e l t a s are a t angles o f 30"-35', about t h e angle of repose f o r sand and grave l . Such s lopes could s l i d e o r flow from prolonged shaking, and headward s top ing by t h e s e s l i d e s could des t roy par t s of t h e o l d e r d e l t a depos i t s near t h e s e s t e e p s lopes .

Rockfal ls and o t h e r types o f l a n d s l i d e s have encroached upon some of the o l d e r d e l t a depos i t s i n t h e p a s t . Blocks o f bedrock have f a l l e n o r r o l l e d onto t h e o l d e r d e l t a a long Auke Bay. A p r e h i s t o r i c l a n d s l i d e covers par t of t h e o l d e r d e l t a a t Lemon Creek. Heintzleman Ridge is t h e apparent source and should be examined be fo re r e s i d e n t i a l development o f t h e Lemon Creek d e l t a .

The o l d e r de l ta depos i t s a r e p o t e n t i a l sources o f good q u a l i t y grave l . Tests f o r s p e c i f i c uses a r e necessary f o r each d e l t a depos i t . The o l d e r d e l t a depos i t a t Eagle River is now be ing u t i l i z e d and s i t e r e p o r t s of t h e Alaska S t a t e Highway Department show t h a t t e x t u r e and composition are v a r i a b l e , and t h a t weathering and decomposition a r e ex t ens ive i n some zones. Mater ia l s from t h i s p i t have been used f o r po r t ions o f t h e North Douglas Road and t h e small-boat ha rbo r a t Douglas @funson, 1963). The o l d e r d e l t a ' d e p o s i t a t Gold Creek has a l s o been developed b u t was n o t being used a t the t i m e of t h i s i n v e s t i g a t i o n . An inact ive g rave l p i t is loca ted i n t h e o l d e r de l t a depos i t a t Montana Creek.

The broad flat s u r f a c e s and easy excavat ion make s e v e r a l of t h e o l d e r de l t a depos i t s a t t r a c t i v e s i t e s f o r urban expansion. I t should be noted , however, t h a t t h e permcable n a t u r e o f t h e d e l t a i c sands and g rave l s could permit waste f l u id s t o move through t h e depos i t and t o contaminate wells.

Beach. d e p o s i t s

Beach d e p o s i t s , as mapped i n t h e Juneau a r e a , inc lude modern beach depos i t s (Qby) , s p i t depos i t s (Qb) , young r a i s e d beach depos i t s (Qrb) , o l d e r r a i s ed beach d e p o s i t s ( t h i n , continuous) (Qbe), and o l d e r r a i s e d beach depos i t s ( t h i c k , l o c a l ) (Qbo). A beach i s a "zone o f unconsol ida ted ma te r i a l bounded t o seaward by t h e junc t ion o f land and s e a a t low t i d e and,, t o landward, by a d e f i n i t e change i n ma te r i a l o r physiographic form, such as 'a s c a c l i f f , o r by a l i n e o f permanent vegetat ion" (Stokes and Varnes, 1955, p. 13) . Beaches a r e ephemeral cons t ruc t iona l s h o r e l i n e f e a t u r e s whose form and s i z e can change a s t h e d i r e c t i o n and i n t e n s i t y o f waves and long-shore c u r r e n t s change and a s t h e amount o f t r a n s p o r t a b l e ma te r i a l changes.

Beach depos i t s formed by wave and shore c u r r e n t s t h a t c a r r i e d and depos- i t e d ma te r i a l . Rivers and s t reams brought some o f t h e m a t e r i a l i n from d i s t a n t sources , bu t some came from t h e e ros ion o f adjacent deposits . Most o f t h e boulders and cobbles and some l a r g e blocks were eroded from nearby glaciomarine d e p o s i t s o r o t h e r coa r se s u r f i c i a l m a t e r i a l s . A few o f t he very l a r g e blocks probably f e l l from nearby bedrock c l i f f s o r promontories.

Beaches formed a t d i f f e r e n t t imes a f t e r t h e g l a c i e r i c e l e f t t h e Juneau area . As a r e s u l t , t h e r e a r e beach depos i t s a t d i f f e r e n t a l t i t u d e s above present sea l eve l ; these r e f l e c t depos i t i on dur ing i s o s t a t i c u p l i f t i n response t o t h e removal of t h e weight from g l a c i a l i c e . The modern beach depos i t s (Qby) and t h e s p i t depos i t s (Qb] are be ing depos i ted a t the presen t time a t t h e modern s e a l e v e l . The young r a i s e d beach d e p o s i t s (Qrb), r e f l e c t beaches t h a t have been r a i s e d above s e a l eve l by rebound i n t h e last few hundred years . The o l d e r beach depos i t s ( t h i n and continuous) (Qbe), and t h c o l d e r r a i s e d beach d e p o s i t s ( t h i ck and l o c a l ) (Qbo) probably accumulated i n p a r t contemporaneously but, also, i n part sequentially. The o lde r r a i s e d beach depos i t s u n d e r l i e pea t beds dated a t from about 8,200 t o 5,700 years o ld . The o l d e s t r a i s e d beach depos i t s u n d e r l i e pea t da ted a t about 7,200 years o ld on t h e Mendenhall Peninsula; t hey a r e be l i eved t o be o l d e r a t o t h e r l o c a l i t i e s . Both o l d e r r a i s e d beach depos i t s o v e r l i e glaciomarine depos- t s dated as being about 10,000 y e a r s o ld .

Separat ion of t h e two o l d e r r a i s e d beach depos i t s (Qbe) and (Qbo) i n t h e f i e l d , and a s shown on t h e geologic map, is based most ly on landform and d i s t r i b u t i o n . The o l d e r r a i s e d beach ( t h i n and cont inuous) formed as broad expanses on gen t ly s lop ing s u r f a c e s of t h e under ly ing glaciomarine depos i t s . Such d e p o s i t s extend f o r mi les along t h e s h o r e l i n e ; t h e i r broad l a t e r a l ex ten t as a c o n t h u o u s b l anke t reaching from t h e s e a c l i f f s t o a s high as 600 feet above sea level apparent ly r e f l e c t s a uniform accumula- t i o n during rebound. T h i s depos i t , i n t h e broad sense, i s e a s i l y t r a c e d along t h e s h o r e l i n e and r a i s e d seacoas t and i s recognizable from p lace t o place by i t s almost uniform th ickness and brown t o reddish co lor . The o l d e r r a i s e d beach depos i t s ( thickand l o c a l ) , however, a r e more r e s t r i c t e d i n t h e i r occurrence; they a r e found i n narrow topographic settings t h a t are conducive t o g r a v c l l y accumulations from high-energy waves. These

o l d e r r a i s e d beach d e p o s i t s ( t h i c k and l o c a l ) a r e a t d i f f e r e n t a l t i t u d e s a + d i f f e r e n t p laces , which sugges t s t o me t h a t t h e s e d e p o s i t s accumulated a t d i f f e r e n t t imes i n p l aces where s e a l e v e l , t e r r a i n , and wave d i r e c t i o n and energy were a l l balanced. Whereas some of t h e s e o l d e r t h i c k and l o c a l depos i t s are t r u l y beaches, a s a l o n g t h e e a s t e r n shore n e a r Tee Harbor, some of t h e depos i t s may be bay-mouth b a r s t h a t accumulated between head- lands , such a s t h e d e p o s i t s on blendenhall Peninsula and n e a r Auke Lake.

Modein beach d e p o s i t s (Qby)

Modern beach depos i t s c o n s i s t o f brownish t o dark-grayish f i n e sand and pebble- t o boulder -s ize grave l . Some depos i t s a r e composed almost en- t i r e l y of whole and broken s h e l l s . S l a t e , greenstone, and flow b r e c c i a are t h e predominant rock types . G r a n i t i c rocks , gene ra l ly of boulder o r cobble s i z e , a r e scattered along t h e beaches. Overpr in ts are used on the map t o show predominant fragment s i ze where accumulations a r e uniform i n s i z e . Some beach s l o p e s c o n s i s t e n t i r e l y o f beveled bedrock; t h e s e a reas a r e mapped as bedrock extending seaward from t h e s h o r e l i n e . The beach depos i t s a r e of Holocene age and a r e accumulating today.

Beaches occur s p o r a d i c a l l y along most o f t h e shores i n t h e Juneau a rea . Very s t e e p b l u f f s and deep channels a r e n o t conducive t o beaches, b u t most s h o r e l i n e s have beach depos i t s t h a t can be seen a t l e a s t dur ing low t i d e . Beaches are we l l developed a t bays and coves b u t a r e l e s s p e r s i s t e n t along e longa te channels. Beach s lopes average about lo0,

Fine-grained depos i t s a r e loose and uncompacted. The cobbles and pebbles a r e wedged toge the r i n a ma t r ix of sand and s i l t . Broken s h e l l s l oose ly f i l l t h e spaces between s t o n e s on some beaches. Stones a r e t i g h t l y wedged toge the r i n beaches of l a r g e boulders , b u t some boulders r o l l under foo t and wi th the t i d e . Blocks 10-20 f e e t on a s i d e a r e s c a t t e r e d on the beaches.

Deposition and e ros ion a r e i n ba lance along most modern beaches away from t h e mouths of r i v e r s . Beaches can be eroded by s t r o n g t i d a l o r long-shore c u r r e n t s , ' o r by waves dur ing s torms. Large boulders o r angular blocks fa l len from nearb, bedrock outcrops may s h i f t i n response t o s torm waves and t i d e s , b u t do n o t move s i g n i f i c a n t d i s t a n c e s . Cons t ruc t ion of ha rbo r s , docks, e t c . , can change t h e shore c u r r e n t s and a l t e r t h e previous ba lance between e ros ion and depos i t i on .

Beach depos i t s provide a poor foundat ion. Where t h e r e a r e l a r g e boulders , t h e i r looseness r e s u l t s i n an uns t ab le foundat ion. Most beach d e p o s i t s are l e s s t h m 5 f e e t t h l c k and can be r e a d i l y removed s o t h a t foo t ings can b e placed on bedrock. Bedrock gene ra l ly provides an e x c e l l e n t founda- t i o n , bu t t h e r e are o t h e r prohlenis i n b u i l d i n g s t r u c t u r e s on beaches. In add i t i on t o p e r i o d i c inundat ion by high-high t i d e s and s torm waves, beaches a r e s u s c e p t i b l e t o tsunamis and t o s l i de -gene ra t ed and o t h e r abnormal wav* .

L o c a l i t i e s most l i k e l y t o be a f f e c t e d by a tsunami are those t h a t are exposed t o broad a reas o f open water . Auke Bay and t h e no r the rn end o f Douglas I s land a r e exposed t o long reaches of open water , and Tee Harbor, which faces northward, would b e extremely s u s c e p t i b l e i f waves approached from t h e nor th . The p r e d i c t i o n o f tsunamis o r o t h e r abnormal waves i s beyond t h e scope o f t h i s r e p o r t ; the f a c t t ha t such waves can be caused by earthquakes makes beach depos i t s unsu i t ab l e f o r r e s idences (see p. 69).

The boulder and cobble beaches probably would respond s t r o n g l y t o s eve re se i smic v ib ra t ion . The rounded and ovoid fragments could r o l l and move but t h e general t h i n n e s s of t h e beach d e p o s i t s would prevent much compac t ion o r severe shaking. Where beach depos i t s a r e t h i c k e r i n s h e l t e r e d i n l e t s o r coves, t hey might be s e v e r e l y shaken and compacted, and they might s l i d e o r flow.

S p i t depos i t s (Qb)

S p i t s a r e embankments b u i l t by waves and c u r r e n t s extending from land and terminating i n open water . Because s p i t s a r e c l o s e l y r e l a t e d t o beaches, s p i t depos i t s a r e included with beach d e p o s i t s ,

Most s p i t depos i t s a r e gray pebbly sand and sandy grave l . In p l aces they a r e composed o f pebble t o boulder g rave l . The rock types are most ly a r g i l l i t e and flow b r e c c i a i n s p i t s i n t h e North Douglas Is land-Outer Poin t a r e a , and most ly a r g i l l i t e and greenstone elsewhere. Local ly t h e depos i t s conta in some boulders o f g r a n i t i c rock. Overpr in ts a r e used on t h e geologic map t o show t h e predominant fragment s i z e where accumulations are uniform i n s i z e . S p i t s a r e forming now.

S p i t depos i t s a r e b e s t developed n e a r p o i n t s o f land , a long i r r e g u l a r s h o r e l i n e s near t h e mouths of s t reams, and between shore and o f f sho re rock promontories. Severa l s p i t s are on t h e n o r t h po r t ion o f Douglas I s l and along F r i t z Cove n e a r Outer Point. The l a r g e s t depos i t is a concent ra t ion o f pebbles and cobbles t h a t extends outward from t h e mainland n e a r F ish Creek t o Entrance Poin t , a bedrock promontory. Though mapped a s a s p i t , t h i s depos i t forms a b a r r i e r beach between the mainland and Entrance Poin t . The l a r g e s t mainland s p i t i s on t h e south s i d e o f Lena Point . A l l s p i t s a r e asymmetrical r i d g e s , gene ra l ly l e s s than 12 f e e t high. above mean s e a l e v e l , t h a t have a s t e e p e r landward s l o p e and more g e n t l e seaward s lope . The c r e s t l i n e s range from a few inches t o s e v e r a l f e e t i n width.

The th icknesses o f s p i t d e p o s i t s a r e v a r i a b l e . Some b a r s on t i d a l f l a t s are only a few f e e t t h i c k ; o t h e r s a r e i n l o c a t i o n s s u b j e c t t o s t r o n g e r c u r r e n t s and a r e larger and thicker, The thickest d e p o s i t known is i n t h e b a r a long t h e south s i d e o f Lena Poin t and c o n s i s t s o f about 16 f e e t o f sandy gravel t h a t o v e r l i e s bedrack.

S p i t s form where t i d a l sho re c u r r e n t s and s torm waves move sand and gravel l a t e r a l l y along sho re t o an a r e a where t r a n s p o r t energy i s de- c reased for some reason and par t of the load i s dropped. General ly

t h e ma te r i a l is provided by s treams e n t e r i n g t h e body o f water ; t hus , rrqst s p i t s a r e n e a r t h e mouths o f s t reams. Strong shore c u r r e n t s and t i d a l c u r r e n t s a l s o rework and t r a n s p o r t beach d e p a s i t s l a t e r a l l y and form s p i t s a t j u t t i n g po in t s of land t h a t a f f e c t cu r r en t flow, o r where two shore currents meet. S p i t d e p o s i t s a r e eroded as wel l a s depos i ted by sho re and s torm waves. Strong storm waves can modify t h e shape and s i z e o f a s p i t b a r , o r even remove t h e b a r temporari ly . Strong runoff i n s t reams n e a r s p i t s can cause l a t e r a l s cour o f t h e s p i t s , e s p e c i a l l y duririg low t i d e s .

S p i t depos i t s a r e a r a t h e r minor geologic u n i t i n t h e Juneau a r e a and not gene ra l ly u t i l i z e d f o r cons t ruc t ion s i t e s . If s p i t s a r e used, however, t h e i r looseness and nearness t o s e a l e v e l r e q u i r e understanding o f t h e depos i t s1 shortcomings. Besides be ing e a s i l y reshaped o r com- p l e t e l y removed dur ing s torms o r s t r o n g shore-cur ren t a c t i v i t y , t h e depos i t s a r e s u s c e p t i b l e t o inundat ion from l a r g e storm waves o r s e i s - mica l ly induced s e a waves. The s u r f a c e o f a s p i t depos i t i s c l o s e t o s e a l e v e l and thus has l i t t l e p r o t e c t i o n from abnormal waves. Such loose d e p o s i t s would be h igh ly s u s c e p t i b l e t o shaking and compaction dur ing a s t r o n g ear thquake i n t h e Juneau area . F rac tu re s t h a t might provide o u t l e t s f o r water and sediment e j e c t i o n might form, though t h e r e l a t i v e t h i n n e s s of t h e depos i t might reduce t h e amount o f compac- t i o n and f r a c t u r i n g . However, some o f t h e s p i t s o v e r l i e t h i c k t i d a l depos i t s whose r e a c t i o n s t o a s t r o n g ear thquake might overshadow t h e r e a c t i o n s of t h e s p i t depos i t s .

Young r a i s e d beach d e p o s i t s (Qrb)

Young r a i s e d beach d e p o s i t s a r e g e n e r a l l y composed o f brown t o gray f i n e sand and pebble t o cobble grave l . Fragments of p l a t y s l a t e make up t h e d e p o s i t s a t most p l aces , b u t broken s h e l l fragments are common; graywacke supplements s l a t e i n d e p o s i t s on Spuhn I s l and , and rounded p i eces of greenstone and flow b r e c c i a s make up depos i t s n e a r Outer Poin t and Tee Harbor. Well-developed podzol s o i . 1 ~ having an A2 horizon (a bleached whi te ashy-appearing l a y e r ) a r e found on some, b u t n o t all, o f the r a i s e d beach depos i t s . A well-developed yellowish-brown t o reddish clay-enriched B horizon u n d e r l i t s t h e A2 hor izon i n d e p o s i t s a long Peterson Creek, n e a r Outer Point on Douglas I s land . A l l o f the raised beaches a r e o f l a t e Holocene age, bu t no t a l l depos i t s were formed a t t h e same time. The access road t o the beach a t Lena Cove l i e s between two r a i s e d beach r i d g e s , t h e i n n e r (shoreward) o f which has a b e t t e r developed s o i l than does t h e seaward r i dge (Freeman Stevens, o r a l comrnun., 1968). Trees on t h e seaward r idge are about 175 years o ld , whereas a mature f o r e s t covers t h e i n n e r r idge . A t r e e growing Letween t h e two r i d g e s was cored and i s more than 398 years o ld ; thus, t h e i n n e r beach r idge is more than 400 yea r s old.

Raised beach d e p o s i t s i n t h e Juneau a r e a are a t t he heads of Tee Harbor and Lena Cove, and n e a r Outer Poin t on t h e no r the rn par t o f Douglas I s land . Raised beach d e p o s i t s have two topographic forms. Some a r e narrow beach r idges , asymmetrical, and have t h e s t e e p e r s l o p e on t h e leeward s ide .

Others are broad expanses with r e l a t i v e l y smooth s u r f a c e s t h a t s l o p e g e n t l y upward and extend in l and n e a r Outer Point and on Spuhn I s l and .

These d e p o s i t s accumulated by normal wave and shore c u r r e n t a c t i o n and by occas iona l storm waves. A l l o f t h e s e d e p o s i t s i n t h e Juneau a r e a f a c e an open o r semi-open expanse o f water . The a s ~ m e t r i c a l beach r idges developed a t t h e heads o f coves, where waves depos i ted m a t e r i a l i n e longate a r cua te r i dges more o r l e s s p a r a l l e l t o t h e shape o f t h e cove. The two r i d z e s st Lena Cove a r e examples. Both o f t h e s e r idges may b e storm bezches, bu t t h e g r e a t age d i f f e r e n c e sugges ts t h a t a t l e a s t t h e i n n e r r i dge i s a normal beach t h a t r e f l e c t s u p l i f t o f t h e land i n r e l a t i o n t o water l e v e l during t h e l a s t few hundred years . The d e p o s i t s n e a r Outer Poin t a r e exposed t o many mi les o f open water t o t h e n o r t h and probably r ep re sen t storm-beach accumulations tha t have been e l eva t ed s i n c e they were formed.

Raised beach depos i t s f a c e open o r n e a r l y open waters t o t h e no r th . These broad expanses of water provided t h e f e t c h f o r waves t o develop t h e energy t o form d e p o s i t s n e a r Outer Poin t and on Spuhn I s l a n d , b u t they a l s o expose t h e d e p o s i t s t o p o s s i b l e tsunamis coming from t h e nor th . Most o f t h e young r a i s e d beaches a r e l e s s than 20 f e e t above water l e v e l and would b e very s u s c e p t i b l e t o inundat ion by l a r g e tsunamis. The n e a r e s t known subaqueous ear thquake e p i c e n t e r i s o f f Icy Poin t a long Icy S t r a i t s , about 100 mi les from Juneau ( f i g . 3 ) . Admiralty I s l a n d l i e s between t h e Juneau a r e a and Icy S t r a i t s , which i s t h e main opening t o the Gulf o f Alaska t o t h e west. Presumably, then , any se i smic s e a wave coming from t h e open s e a o r t h e Gulf o f Alaska would be a t t enua ted by I

t h e time it reached t h e Juneau-Douglas I s l and area. The p o s s i b i l i t y of tsunamis caused by h o r i z o n t a l movement along t h e Lynn Canal-Chatham S t r a i t f a u l t was d iscussed on p. 18.

Older r a i s e d beach depos i t s , thi.n and continuous, (Qbe)

Older raised beach depos i t s , t h i n and cont inuous, c o n s i s t o f very-dark- reddish-brown (lOYR 3/23 t o yellowish-brown (IOYR 4/51 pebble gravel conta in ing some sand b u t very l i t t l e s i l t , and p r a c t i c a l l y no c l a y ( t a b l e 4, 'no. 8 ) . The c o l o r of moist m a t e r i a l i s b lack ( l O Y R 7/1) .

I

Indiv idua l p i eces of s la te , greenstone, and graywacke are g e n e r a l l y t a b u l a r and l i e w i th t h e i r f l a t s i d e s p a r a l l e l t o each o t h e r and t o t h e s u r f a c e of the under ly ing depos i t . Subangular t o subround edges a r e t y p i c a l , bu t a few fragments have sharp edges. Sand and some s i l t mixed with pea t from t h e over ly ing t h i n muskeg f i l l t h e spaces between t h e p ieces . This organ ic ma t t e r obscures t h e n a t u r a l dark-gray and green I

c o lo r s o f t h e rocks and g ives t h e depos i t s a reddish hue. Light-gray (10YR - 8/1) l a y e r s a p p a r n t l y r e l a t e d t o podzol ic s o i l development a r e

I I

conspicuous i n some exposures. Medium t o f i ne sand t h a t u n d e r l i e s t he I

gravel is gene ra l ly ye l lowish brown ( I O Y R 4/51 b u t is l o c a l l y gray (N 4/0). These o l d e r r a i s e d beach deposi ts , t h i n a n d continuous, are commonly- 1

under la in by glaciomaxine depos i t s .

These beach d e p o s i t s 'were l i f t e d a s t h e land emerged i n response t o i so - s t a t i c rebound during and a f t e r t h e l a t e P l e i s tocene r e t r e a t o f t h e g l a c i e r s . The i r ages may be almost a s g r e a t as those o f t h e under ly ing glaciomarine depos i t s . Judged from radiocarbon d a t e s from pea ty ma te r i a l over ly ing t h e s e beach d e p o s i t s , however, t h e e a r l i e s t da ted accumulation o f o l d e r t h i n and continuous r a i s e d beach g rave l , a long t h e sho re o f Auke Lake, i s o l d e r than 8,2801350 yea r s B.P. [sample W-2258; Meyer Rubin, w r i t t e n commun., 1969), and t h e youngest da ted g rave l , n e a r Douglas, accumulated a l o n g t h e r i s i n g s h o r e l i n e s about 5,730 years ago (sample W-1949; Meyer Rubin, w r i t t e n commun., 1968).

Such radiocarbon da t e s suggest beach development throughout t h e en t i re per iod o f shore e ros ion and u p l i f t i n Holocene time. As t h e land rebounded i s o s t a t i c a l l y , t h e glaciomarine depos i t s were eroded and t h e coa r se f r ag - ments c o l l e c t e d t o form beach depos i t s . That t h e s u r f a c e s on t h e beach depos i t s a r e g e n e r a l l y evenly s lop ing sugges ts a uniform r a t e o f land u p l i f t and a r a t e of beach depos i t accumulation t h a t kep t pace with t h e u p l i f t . The pea t may o r may n o t have s t a r t e d t o accumulate immediately a s the t i d a l waters receded from t h e rebounded land; consequent ly, t h e r e may be unknown time i n t e r v a l s between t h e formation o f t h e da t ed pea t s and t h e underlying beach g rave l s . Evidence t h a t pea t does accumulate quickly on s t i l l - a c t i v e beaches i s suggested by t h e p l a n t m a t e r i a l f i l l i n g t h e spaces between pebbles and cobbles on t h e modern beach along Auke Bay nea r t h e o u t l e t o f Auke Creek. S a l t - t o l e r a n t p l a n t s t h e r e grow below mean sea l e v e l and a r e inundated d a i l y by t i d e s ; such p l a n t s probably grew i n e a r l i e r t imes, and thus t h e a l t i t u d e s o f b a s a l pea t samples and t h e i r dates may c l o s e l y r e p r e s e n t t he posit ion o f sea l e v e l and t h e t ime o f - beach development a t t h a t a l t i t u d e .

Older raised beach d e p o s i t s , t h i n and cont inuous, a r e found a long both s i d e s o f Gast ineau Channel, a long Fritz Cove, Auke Bay, Lena Cove, and along t h e lower reaches o f Montana Creek. T h e i r s u r f a c e s s l o p e gen t ly , 10 ' -15~, bu t l o c a l l y a r e i n t e r r u p t e d by sca rps l e s s t han 10 f e e t high. The depos i t s extend from the s e a b l u f f , o r s t ream channel , t o t h e moun- t a i n s i d e . The s u r f a c e s on t h e s e depos i t s approximately r e f l e c t t h e s u r f a c e s on t h e underlying depos i t s ( p r o f i l e C - C t , f i g . 9 ) . The depos i t s extend more than 600 f e e t above sea l e v e l n e a r Kowee Creek on Douglas I s l and , and elsewhere have been mapped t o an a l t i t u d e o f 500 f e e t . Peat (Qmk) and f o r e s t humus o v e r l i e t h e s e beach d e p o s i t s , s o t h a t t h e t h i n beach depos i t s a r e exposed only along b l u f f s o r i n excavat ions.

Bedrock along t h e s h o r e l i n e p r i o r t o and dur ing i s o s t a t i c rebound provided some o f t h e fragments found i n t h e o l d e r r a i s e d beach d e p o s i t s , t h i n and cont inuous, a s d i d t he glaciomarine d e p o s i t s whose coa r se f rag- ments were reworked i n t o the beach depos i t s . The preponderance of l o c a l rocks i n t h e s e r a i s e d beach g r a v e l s , however, sugges ts t h a t bedrock was t h e prime source o f t h e g rave l . Some o f t h e pea ty humus t h a t i s i n t e r - mixed with t h e p l a t y pebbles and sand s i f t e d down from above i n t o open spaces , b u t some a l s o accumulated a s p a r t o f t h e beach grave l .

Older r a i s e d beach d e p o s i t s , t h i n and cont inuous, a r e gene ra l ly loose and uncompacted and a r e t hus very s u s c e p t i b l e t o shaking dur ing an earthquake. Broad depos i t s probably would r e a c t by some l a t e r a l movement o f t h e p l a t y p i eces and, poss ib ly , some compaction. The depos i t s a r e t h i n , s o compac- t i o n would be s l i g h t . I t i s u n l i k e l y t h a t a l a r g e b u i l d i n g would be founded e n t i r e l y on o r i n such a t h i n d e p o s i t , bu t i f i t were, ground cracking and d i f f e r e n t i a l s e t t l i n g might r e s u l t .

Older r a i s e d beach depos i t s , t h i c k and l o c a l , (Qbo)

The o l d e r r a i sed beach d e p o s i t s , t h i c k and l o c a l , c o n s i s t o f o l ive-gray (5Y 5/21 t o pale-brown (10YR 6 / 3 1 pebble, cobble, o r boulder grave l i n a saEd matrix. Sla te , g raywzke , greenstone, flow b r e c c i a , and g r a n i t e a r e dominant rock types . This coarse m a t e r i a l l o c a l l y o v e r l i e s a medium t o coarse sand which conta ins abundant she1 1 fragments s c a t t e r e d through t h e sand and a l s o concent ra ted i n l aye r s . Beach depos i t s i n a r e a s exposed t o long expanses of open water have t h e c o a r s e s t m a t e r i a l s ; t h e depos i t extending along t h e e a s t s i d e o f Tee Harbor is t h e c o a r s e s t i n t h e Juneau area. Here, subround t o round l o c a l l y der ived greenstone and f low-breccia boulders 2-3 f e e t i n diameter a r e concent ra ted i n a conspicuous depos i t . The broad depos i t n e a r Outer Poin t on Douglas I s l and i s s t a i n e d a very dark brown (10YR 3 / 2 ) , b u t t h e cobbles b leach almost whi te a f t e r t h e over ly ing th inmuskeg i s scraped from t h e depos i t . Elsewhere, most of t h e deposits are pebble t o granule size and con ta in i s o l a t e d g r a n i t e cobbles and boulders . The depos i t between Auke Lake and Montana Creek has numerous cobbles and boulders of g r a n i t e ; t hey a r e e s p e c i a l l y abundant along t h e base o f a bedrock knob t h a t extends above t h e su r f ace of the o l d e r beach depos i t .

These depos i t s a r e o f e a r l y Holocene age. They a r e younger t han t h e 10,630f500 B.P. radiocarbon d a t e obta ined from s h e l l s i n t h e underlying glaciomarine d e p o s i t s , t h i r d phase (Qme) (sample W-2263, Meyer Rubin, w r i t t e n commun., 1969) and o l d e r than t h e 7,210+300 y e a r B.P. d a t e obta ined on p e a t t h a t o v e r l i e s t h e o l d e r raised t h i c k and l o c a l beach depos i t s 180 f e e t above MLLW along t h e Engineers Cutoff on Mendenhall Peninsula (sample W-1832, Meyer Rubin, w r i t t e n commun., 1966).

Except f o r t h e depos i t i n Juneau below Mount Maria, t h e o l d e r r a i s e d beach d e p o s i t s , t h i c k and l o c a l , occur only i n t h e no r the rn p a r t o f t h e mapped area . The most ex t ens ive depos i t forms a broad s u r f a c e nea r Outer Point . I t extends upslope t o h e i g h t s o f as much a s 200 f e e t above s e a level, A t Tee Harbor, a s i m i l a r depos i t c o n s i s t s o f conspicuous coarse boulders t h a t l i e along t h e mountain s l o p e as h igh as 50 f e e t above the Glac i e r Highway. F a r t h e r n o r t h , t h e depos i t forms a s e r i e s o f benches. A f i n e r t e x t u r e d equ iva l en t depos i t u n d e r l i e s much of t h e s u r f a c e between Tee Harbor and Lena Cove. Older t h i c k and l o c a l r a i s e d beach depos i t s occur on Coghlan I s l and and blendenhall Peninsula and have s t e e p s lopes t o t h e nor th . The depos i t a t t h e southern edge o f Auke Lake s lopes from a c r e s t gen t ly southward toward t h e Mendenhall v a l l e y , and more s t e e p l y northward on a g rad ien t o f about 20' t o t h e shore o f Auke Lake.

The depos i t shown o n . P l a t e 1, shee t I, below Mount Maria i n Juneau merges wi th t h e o l d e r d e l t a depos i t t h a t forms Evergreen Bowl. I t i s i n t e r p r e t e d as a r a i s e d beach depos i t r a t h e r than p a r t o f t h e d e l t a because it l i e s h ighe r than t h e d e l t a depos i t , and because i t s s u r f a c e i s graded toward t h e d e l t a depos i t . These two condi t ions suggest t o me t h a t t h i s o l d e r thick and l o c a l r a i s e d beach depos i t accumulated from a sou rce d i f f e r e n t from t h a t o f t h e d e l t a . The two d e p o s i t s may have been n e a r l y contemporaneous, bu t t h e o l d e r d e l t a was depos i ted by waters flowing from Gold Creek, and t h e o l d e r t h i c k and local r a i s e d beach depos i t was formed by waves and c u r r e n t s from Gast ineau Channel. This p a r t i c u l a r r a i s e d beach deposit s lopes from t h e base of t h e r i d g e between Mount Maria and Mount Roberts toward downtown Juneau. The m a t e r i a l i n t h i s d e p o s i t i s gene ra l ly pebble s i z e o r smal le r . Cobbles occur b u t r a r e l y , and then i n d i s c r e t e l a y e r s r a t h e r t han wi th in sandy beds. S h e l l fragments occur i n t h e de- p o s i t . Figure 9 shows t h e r e l a t i o n s h i p o f t h i s depos i t t o t h e o l d e r d e l t a depos i t , and t h e l i t h o l o g y a s recorded from d r i l l h o l e no. 1.

Elsewhere, t h e o l d e r r a i s e d beach d e p o s i t s , t h i c k and l o c a l , were formed by waves and c u r r e n t s a long shore and i n shallow water between h ighland promontories. Bedrock and glaciomarine depos i t s were t h e source m a t e r i a l s . The coa r se d e p o s i t s on t h e e a s t s i d e o f Tee Harbor probably o r i g i n a t e d i n r o c k f a l l s from t h e ad jo in ing c l i f f s . A t some o t h e r l o c a l i t i e s , such a s along t h e bedrock h i l l n e a r Auke Lake, g r a n i t e boulders were concent ra ted a f t e r being eroded from nearby glaciomarine depos i t s . Some o f t h e g rave l s mapped a s o l d e r r a i s e d beach depos i t s , t h i c k and l o c a l , may have been formed a s nearshore bay-mouth b a r s , The asymmetrically shaped o l d e r r a i s e d beach d e p o s i t s , t h i c k and l o c a l , on Coghlan I s l and , Mendenhall Peninsula , and a t t h e southern end o f Auke Lake may be examples o f such bars. Thei r asymmetry may be due t o t h e d i r e c t i o n o f s t rong-energy waves.

Marine d e p o s i t s

Marine depos i t s i nc lude t h e modern i n t e r t i d a l depos i t s (Qts) t h a t extend from below t h e edge o f t h e water a t low t i d e shoreward t o t h e beach, and the emergent i n t e r t i d a l d e p o s i t s (Qe) t h a t have been r a i s e d by rebound of t h e land i n response t o modern i s o s t a t i c u p l i f t apparent ly cen te red around Glac i e r Bay (Hicks and Shof-nos, 1965, p. 3318). I n t e r t i d a l de- p o s i t s a r e formed by t i d a l c u r r e n t s which t r a n s p o r t sediment from t h e mouths of r i v e r s t o t h e margins and f l o o r s of channels and bays. High t i d e s and waves r e l a t e d t o s torm t i d e s l o c a l l y erode t h e s e unconsol idated d e p o s i t s and r e d i s t r i b u t e t h e ma te r i a l .

I n t e r t i d a l d e p o s i t s (Qts)

I n t e r t i d a l depos i t s a r e composed o f gray t o dark-gray sandy s i l t , s i l t y g r a v e l l y sand, and sandy g rave l ; s h e l l s a r e s c a t t e r e d throughout t h e depos i t s . The c o a r s e r p a s t s o f t h e d e p o s i t s a r e mostly n e a r t h e mouths of streams. The d e p o s i t s a r e accumulating a t t h e p re sen t time.

The i n t e r t i d a l d e p o s i t s extend along the shores o f t h e f i o r d s and bays of t h e Juneau area. Broad gen t ly s lop ing t i d a l f l a t s are revea led a t low o r minus t i d e s , and a r e gene ra l ly obscured a t high t i d e s . The i n t e r t i d a l d e p o s i t s extend outward under water . The depos i t s shown on t h e geologic map a r e those exposed dur ing normal high t i d e s . The s u r f a c e on t h e i n t e r - t i d a l depos i t s s lopes toward t h e c e n t e r o f Gast ineau Channel a t about 5' n e a r shore , and s teepens o f f sho re t o about 17' (R. D. M i l l e r , 1967). The broades t expanse o f t h e d e p o s i t s i s n e a r t h e mouth of t h e Mendenhall River .

The ma te r i a l s i n t h e i n t e r t i d a l d e p o s i t s d i f f e r from p l a c e t o p l ace ; t hey a r e gene ra l ly n o n p l a s t i c s i l t t o sandy g rave l . A t many p l aces t h e upper 5-20 f e e t i s loose , whereas i n many o t h e r p l aces t h e m a t e r i a l is compact and dense from t h e s u r f a c e downward. Ma te r i a l s e l e c t e d as r e p r e s e n t i n g t h e poores t foundat ion condi t ions a long t h e proposed G l a c i e r Expressway shows a void r a t i o o f 1.405 (Frankle t and Rasmussen, 1969, p. 15). Such a void r a t i o i n d i c a t e s t h a t t h e voids make up more than h a l f o f t h e volume of t h e depos i t ; t hus , it i s a very loose porous ma te r i a l . The same sample had a n a t u r a l moisture content of 53 pe rcen t , a l i q u i d l i m i t of 33 pe rcen t , and a n a t u r a l dry d e n s i t y o f 72 pcf . This sample was c o l l e c t e d fram 2 fee t below t h e ground surface. Samples c o l l e c t e d elsewhere contained l e s s water and had a g r e a t e r dens i ty .

Var i a t ions i n t e x t u r e and d e n s i t y make t h i s depos i t a poor foundat ion (see over lay 2 ) . Most of t h e i n t e r t i d a l depos i t s i n downtown Juneau are covered by manmade f i l l (mf) on which c~rnrnercial and p r i v a t e bu i ld ings have been cons t ruc ted . The r e s u l t s o f t e s t s made by t h e Alaska S t a t e Highway Department of f i l l emplaced over t h e i n t e r t i d a l d e p o s i t s showed a maximum se t t l emen t o f 3 1 / 2 feet and an average o f 2.9 f e e t s e t t l e m e n t under 26 f e e t o f f i l l (Frankle t and Rasmussen, 1969, p. 9-27) .

I n t e r t i d a l depos i t s were compacted and f r a c t u r e d elsewhere i n Alaska dur- i n g the 1964 earthquake. Compaction o f m a t e r i a l s s i m i l a r t o t h e i n t e r - t i d a l depos i t s caused power po le s t o tilt o r f a l l (Eckel , 1967, p, B19) and highways t o f r a c t u r e o r subs ide . Long-continued shaking caused c e n t e r l i n e f r a c t u r e s i n highways where dense roadway m a t e r i a l subsided i n t o l e s s dense underlying sediments which then flowed out from beneath t h e f i l l (Kachadoorian, 1968, p. C19). Frac tures along t h e edges of t h e roadway and p a r a l l e l t o t h e road were a l s o caused by t h i s compaction and flowage. The roadway f i l l l o c a l l y subsided d i f f e r e n t i a l l y and f r a c - t u r e s t r a n s v e r s e t o t h e roadway d i sp l aced t h e road, caused waves i n t h e roadway, and dropped the filled approaches to br idges . F i l l placed over t h e i n t e r t i d a l depos i t s a t Juneau probably would r e a c t i n a s i m i l a r manner . Some p l aces i n t h e Juneau a r e a have f i l l emplaced wi th an abrupt edge 10 f e e t high o r more s t and ing above t h e i n t e r t i d a l s u r f a c e , L a t e r a l spreading o f t h e f i l l from overloading and compaction o f t h e i n t e r t i d a l depos i t s by the f i l l dur ing cons t ruc t ion should be expected.

The l o c a t i o n o f t h e i n t e r t i d a l depos i t s a t s e a l e v e l makes them h igh ly s u s c e p t i b l e t o inundat ion by se i ches o r tsunamis (see p. 58) .

Footings of many bui ld ings a r e founded i n t h e i n t e r t i d a l depos i t s along Gastineau Channel. blost bui ld ings a r e on p i l e s , o the r s a r e on f i l l e d ground placed over i n t e r t i d a l deposi t s . The deposi t s south o f Juneau hdve slopes t h a t abrupt ly steepen outward from t h e shore l ine (R. 0. Mil le r , 1967). .4 dock b u i l t on p i l i n g between t h e present A-J dump and the Snow- s l i d e Creek d e l t a s l i d i n t o t h e channel on Jan. 15, 1930 (Daily Alaskan Empire, Jan. 16, 1930) ; a submarine l ands l ide was given as t h e cause. The same a r t i c l e r e f e r r e d t o a s i m i l a r "cave-in1' 700 f e e t f a r t h e r nor th 2 years e a r l i e r . Such occurrences suggest t h a t the i n t e r t i d a l deposi t s are highly suscep t ib le t o s l i d i n g .

Emergent i n t e r t i d a l deposi t s (Qe)

Emergent i n t e r t i d a l deposi t s a r e composed o f gray (5YR 6/1) cohesive sandy s i l t s t h a t contain some c lay-s ize p a r t i c l e s b u s i t t l e coarse mater ia l . Plant roo t s and occasional s h e l l s he lp t y p i f y the deposi t a s an o lde r i n t e r t i d a l deposi t . The co lo r of moist ma te r i a l is very dark gray (SY 3/1) b u t appears greenish t o t h e eye. This deposi t is genera l ly massive-but loca l l y has t h i n laminat ions. A "swampyll appearance o f t h e su r face of t h e deposi t i s c h a r a c t e r i s t i c and, when moist, the mate r i a l genera l ly bends before breaking with subangular o r subconchoidal f r ac - t u r e s . The age o f t h i s deposi t is l a t e Holocene.

Emergent i n t e r t i d a l deposi t s occur near the mouths of major streams. Thei r surfaces genera l ly s lope gent ly t o mean t i d e l eve l and a r e commonly covered by grass. These deposi t s extend t o about 30 f e e t above mean ;. lower low water i n t h e Mendenhall va l l ey , and about 20 f e e t elsewhere. Small spruce trees dot t h i s emerged surface near t h e Mendenhall River.

7:

Runoff paths form d e n d r i t i c p a t t e r n s cu t a s much as 2 f e e t i n t o the deposi t s .

The deposi t s a r e probably l e s s than 10 f e e t th ick a t most p laces . They o v e r l i e t h e younger outwash and younger d e l t a deposi t s i n t h e lower p a r t of Mendenhall va l l ey , as well a s t h e glaciomarine deposi t s along t h e western p a r t of t h e va l l ey nor th of the Glacier Highway. Tes t holes a t t h e s i t e of a sand p i t nea r t h e a i r p o r t showed a maximum of only 2 f e e t o f t i d a l ma te r i a l above 47 f e e t of sand of t h e younger d e l t a deposi t s (Munson and Rasmursen, 1966, p. 15).

S i l t i s the predominant s i z e composing t h e emergent i n t e r t i d a l depos i t ; sand and c lay-s ize p a r t i c l e s make up most of t h e remainder o f the ma te r i a l i n the deposi t . S i l t averages 61 percent , sand 22 percent , c l ay - s i ze p a r t i c l e s 15 percent , and gravel only 2 percent i n samples t e s t e d . I t i s t h e high s i l t content t h a t causes t h e mater ia l t o bend before breaking. Although the deposi t beromes s l i p p e r y when wet, t h e two samples t e s t e d were too sandy f o r t h e i r p l a s t i c i t y index t o be determined.

The emergent i n t e r t i d a l deposi t s accumulated i n t h e i n t e r t i d a l zone j u s t as modern i n t e r t i d a l deposi t s a r e forming today. The emergent i n t e r t i d a l deposi t s probably a r e i n p a r t e s tua r ine because many streams ca r ry sand and, gravel i n t o t h e f io rds . Maps prepared i n the ea r ly 1900's show t h e

a reas now covered by t h e s e emergent d e p o s i t s t o have been sub jec t ed t o d a i l y t i d a l inundat ions (Spencer, 1906; Knopf, 1912). Thus, t h e depos i t ~ ? p a r e n t l y accumulated between lower low t i d e and mean h igh t i d e p r i o r t o 1900. U p l i f t o f t h e Juneau a r e a r e l a t i v e t o s e a l e v e l has been c a l - culated t o be 1.31 cm per year (Hicks and Shofnos, 1965, p. 3318). Accordingly, t h e i n t e r t i d a l f l a t s should have been e l eva t ed about 2.5 feet s i n c e 1909, which appears t o be about t h e amount o f a c t u a l emergence as based on t h e depth o f channels eroded by s t reams i n t o t h e i n t e r t i d a l deposit:.

The emergent i n t e r t i d a l d e p o s i t s are t h i n and cover t h i c k e r unconsol idated ma te r i a l s . These underlying d e p o s i t s , t h e younger d e l t a d e p o s i t s (Qdy) a t t h e mouths o f s t reams e n t e r i n g Gast ineau Channel, and t h e younger ourwash (Qoy) upstream from t h e mouth o f the Mendenhall River, w i l l con- t r o l t h e response o f t h e emergent i n t e r t i d a l depos i t s t o an earthquake.

Glaciomarine d e p o s i t s

The glaciomarine depos i t s a r e t h e most widespread b u t geo log ica l ly t h e l e a s e understood m a t e r i a l s i n t h e Juneau area . The d e p o s i t s normally a r e composed of heterogeneous t i l l - l i k e mixtures o f c l a y , s i l t , sand, and grave l -s ized p a r t i c l e s and abundant remains o f broken and unbroken molluscs , ba rnac l e s , and foramini fe ra ; i n p l aces , t hey a l s o con ta in s c a t t e r e d cobbles and boulders . Because o f t h e genera l l ack o f s o r t i n g and bedding, and because o f t h e presence o f s c a t t e r e d boulders o f l o c a l and fo re ign rock types i n a ma t r ix o f f o s s i l i f e r o u s s i l t y sand, t h e s e m a t e r i a l s have been c a l l e d marine till i n t h e past. In t h i s r e p o r t , however, t h e depos i t s are c a l l e d diamictons. A diamicton is a poor ly s o r t e d o r unsor ted sediment t h a t c o n s i s t s o f p a r t i c l e s larger than sand i n a mat r ix o f sand, s i l t , and c lay-s ized p a r t i c l e s ; t h e term i s noncommittal as t o how t h e depos i t was formed. T t was o r i g i n a l l y devised ( F l i n t and o the r s , 1960a, 1960b) t o provide a d e s c r i p t i v e term f o r deposits of unsor ted t e x t u r e ; t h a t i s , depos i t s conta in ing a heterogeneous mixture of p a r t i c l e s i z e s a s descr ibed above, which cannot be shown t o have been depos i ted by g l a c i e r s .

Diamictons s i m i l a r t o t hose i n t h e Juneau a r e a occur at a l t i tudes o f less than 700 f e e t alorlg t h e c o a s t s o f sou theas t e rn Alaska, B r i t i s h Columbia, and t h e S ta te o f Washington. Armstrong and Brown (1954) and Easterbrook (1963), among o t h e r s , have d iscussed t h e poss ib l e o r i g i n s f o r t h e s e de- p o s i t s . Armstrong and Brown (1954, p. 357-358) sugges t t h a t plowing o f t h e s e a f l o o r by g l a c i a l ice, depos i t i on of d e b r i s from f l o a t i n g i c e , and submarine l a n d s l i d e s , s lope wash, o r t u r b i d i t y c u r r e n t s could account f o r t h e combination o f coa r se g rave l , s t ones , and f o s s i l s i n a f ine -g ra ined matr ix. They cons ider ~ s l f - i c e , berg- ice , and s e a - i c e a s types o f f l o a t - ing i c e that could t r a n s p o r t sand and g ~ a v e l i n t o marine waters where l i v i n g s h e l l f i s h could be bu r i ed by m a t e r i a l s from me l t ing i c e .

The submarine l a n d s l i d e and slope-wash theory r equ i r e s s l i d i n g o r washing of prev ious ly depos i ted m a t e r i a l s , and would r e s u l t i n i n t e r m i t t e n t depos i t ion and perhaps even a d i s t i n c t i v e l aye r ing o f depos i t s . Transpor t

by t u r b i d i t y cu r ren t s would s a t i s f y the t e x t u r a l requirements f o r d ia - micton, but such depos i t s probably would be confined t o small areas.

Easterbrook (1963, p. 1474) evaluated t h e s e processes and concluded t h a t t h e f o s s i l i f e r o u s diamictons i n t h e nor thern p a r t o f t h e Puget Lowland near S e a t t l e , were formed beneath s h e l f - i c e and berg-ice. Shel f - ice is one o f t h e r e s u l t s of th inning of a broad i c e sheet along a seacoas t . The i c e sheet i s assumed t o have o r i g i n a l l y been grounded on t h e s e a floor, but a s the i c e thinned it l o s t contac t with t h e f l o o r and became a f l o a t i n g mass,similar t o t h e i c e f r ing ing the Antarc t ic Continent. Berg i c e i s i c e t h a t ca lves from v a l l e y g l a c i e r s wherever t h e i c e reaches the s e a o r o the r body of water.

Cer ta in l i m i t a t i o n s apply t o t h e o r i g i n of t h e glaciomarine deposi t s a t Juneau and poss ib ly throughout much of southeastern Alaska. First t h e s i m i l a r i t y of ages f o r t h e glaciomarine diamictons, f o r the o l d e r d e l t a deposi t s perched along the s lopes of the f i o r d s , and f o r t h e peat i n a lp ine val leys r equ i res t h a t t h e Gastineau Channel a r e a and the mountain va l l eys were g l a c i e r f r e e a t t h e time o f deposi t ion. Second, t h e presence o f v a s t amounts of coarse mater ia l over a broad region requ i res an e f fec - t i v e process by which t h e sand, gravel , and boulders could be t ranspor ted and deposited. Third, most of these depos i t s a r e massive, dense, and compact. Fourth, a marine o r f i o r d environment i s necessary because a l l of t h e diamictons contain marine f o s s i l s .

The acceptance of an open-water environment along Gastineau Channel and o t h e r f i o r d s i n t h i s region imposes problems regarding t h e d i s t r i b u t i o n of l a r g e amounts of coarse ma te r i a l s found i n the widespread glaciomarine diamictons. One of the most obvious so lu t ions t o t h e problem of transpor t of such mater ia l - - la rge v a l l e y - f i l l i n g g l a c i e r s t h a t moved through the f io rds of southeas tern Alaska--does no t f i t the known environmental condi t ions , The numerous o lde r d e l t a s t h a t a r e preserved t o he ights o f 200-250 f e e t o r more above modern s e a l e v e l a t t e s t t h a t t h e f i o r d s were f r e e of g l a c i e r s when t h e d e l t a s formed. The ages of the d e l t a s and t h e diamictons overlap. Nevertheless, some form of i c e t r anspor t seems necessary t o move and d i s t r i b u t e the coarse debr is throughout s o much of t h e region. This would seem t o leave us with s h e l f - i c e o r berg- ice f o r t r anspor t and deposi t ion.

During a discussion with D. M. Hopkins i n July 1971, i n which Hopkins emphasized t h a t a f i o r d environment does not lend i t s e l f t o development of c l a s s i c a l f l o a t i n g s h e l f - i c e , it was concluded t h a t a seasonal development of sea-ice, o r i c e t h a t bu i lds outward from t h e shores each win te r and t h a t l o c a l l y covers small lagoons and coves, could provide s i m i l a r deposi t ional r e s u l t s . Rather than carry ing debr is from a large land- bound i c e mass, however, t h e sea-ice would carry t h e ma te r i a l deposi ted by streams flowing from the land onto t h e i ce . This debr i s o f a l l s i z e s would be accumulated during early sp r ing runoff before t h e shore-ice breaks up. Eventually t h e i c e would separa te from shore, f l o a t away, and when it melts o r breaks, t h e debr is would drop i n t o t h e marine waters of t h e f io rds . Thus coarse mater ia l would be intermixed with the s o f t , s a tu ra ted , f iner-gra ined bottom deposi t s i n t h e f iord .

Transport by seasonal s ea - i ce supplemented by c o n t r i b u t i o n s from berg- ice might have provided t h e coa r se p a r t i c l e s found i n t h e glaciomarine de- p o s i t s now exposed along Gast ineau Channel and elsewhere along t h e shores o f t h e Juneau area. The u t i l i z a t i o n o f berg- ice r e q u i r e s t h a t c e r t a i n o f t h e l a r g e v a l l e y g l a c i e r s o f t h e reg ion continued t o t e rmina te i n t h e open waters o f Fiords. I l e se v a l l e y g l a c i e r s may have been t h e l a t e P le i s tocene o r e a r l i e s t Holocene ances to r s of some o f t h e g l a c i e r s p r e s e n t l y flowing from i c e f i e l d s i n p a r t s o f sou theas t e rn Alaska.

I t i s no t known which v a l l e y s e n t e r i n g Lynn Canal conta ined t h e s e ancient g l a c i e r s . The l a r g e va l l eys i n t h e Juneau a r e a do no t con ta in l a t e r a l moraines, ox any o t h e r geomorphic evidence t h a t t h e Mendenhall, Herbert , o r Eagle G lac i e r s could have been sources f o r such berg- ice . If much o f t h e coarse d e b r i s i n t h e diamicton came from be rg - i ce , t h e g l a c i e r s must have been h e a v i l y laden wi th g rave l , and t h e r e must have been numerous a c t i v e g l a c i e r s t e rmina t ing i n t h e s e a i n o r d e r t o provide t h e v a s t amount of coarse m a t e r i a l found i n t h e glaciomarine depos i t s .

The hypothes is o f t r a n s p o r t of coarse d e b r i s by e i t h e r s e a - i c e o r berg- ice, o r both, i s no t f r e e o f problems. The p r i n c i p a l problem with each type o f i c e i s t h e seasonal and t h e r e f o r e expected c y c l i c n a t u r e of t h e depos i t ion . Sea-ice seemingly would drop i t s load dur ing l a t e spring and summer; subsequent depos i t i on would tend t o be r e l a t i v e l y f r e e o f coa r se fragments. Berg-ice seemingly would drop i t s load a s l o c a l i z e d accumulations o f coarse ma te r i a l . Nei ther s i t u a t i o n is r e f l e c t e d i n e i t h e r t h e t e x t u r e o r d i s t r i b u t i o n of t h e coarse m a t e r i a l throughout t h e diamicton i n t h e Juneau area . A t t h i s t ime, however, t h e r e seems t o be no b e t t e r explana t ion o f t h e presence o f t h e coarse fragments than by t h e i r t r a n s p o r t and depos i t i on by sea- and berg-ice.

The glaciomarine diamictons a r c subdivided i n t o four u n i t s on t h e geo- l o g i c map, Three of t h e s e glaciomarine depos i t s a r e recognizable mappable ma te r i a l s . The boundaries between t h e s e d e p o s i t s a r e somewhat a r b i t r a r y because o f t h e lack of continuous exposures. These t h r e e u n i t s a r e i d e n t i f i e d i n exposures on t h e basis o f t h e i r t e x t u r e and sequence of depos i t i on r e l a t i v e t o t h e emerging o f land i n response t o i s o s t a t i c adjustment a f t e r t h e massive l a t e - P l e i s t o c e n e i c e cover had melted. They are des igna ted as glaciomarine d e p o s i t s , f i r s t phase (Qmb); g l ac io - marine depos i t s , second phase (Qms) ; and glaciomarine d e p o s i t s , t h i r d phase (Qme). The fou r th u n i t i s u n d i f f e r e n t i a t e d glaciomarine d e p o s i t s (Qmu). A l l o f t h e s e d e p o s i t s apparent ly accumulated between about 12,000 and 9,800 yea r s ago. Figure 10 provides diagrammatic ske tches i n t e r p r e t i n g t h e manner o f accumulation o f t h e t h r e e phases o f glaciomarine depos i t s .

Judging from a v a i l a b l e f i e l d and l abo ra to ry d a t a , I c u r r e n t l y v i s u a l i z e t h e accumulation o f t h e d i f f e r e n t phases o f t h e glaciomarine depos i t s a s be ing gene ra l ly s e q u e n t i a l , b u t a l s o i n p a r t simultaneous. The f i r s t phase of glaciomarine depos i t i on was t h e accumulation o f s t o n y diamicton during t h e maximum l a t e P l e i s tocene depress ion o f t h e land and during the f i rs t p a r t of emergence o f t h e land a f t e r t h e i c e s h e e t had r e t r e a t e d

and marine waters were reoccupying the f i o r d s i n t h e Juneau region. A s t h e land s lowly rebounded and t h e h ighe r p a r t s o f t h e glaciomarine de- p o s i t s g radua l ly emerged above water , waves reworked t h e m a t e r i a l and *>pos i ted a t h i n b lanket o f beach g rave l s (Qbe) on t h e emerged depos i t ion- a l su r f ace , S l i g h t l y more than midway through t h e t o t a l amount of emergence a slowing o r h a l t o f unknown dura t ion apparent ly occurred. The o l d e r d e l t a depos i t s (Qdo), now about 200-250 f e e t above p re sen t s e a l e v e l , probably were graded t o t h e s e a l e v e l a t t h a t time. Also, dur ing t h i s pause, waves eroded t h e glaciomarine d e p o s i t s , f i r s t phase, and formed low r i s e r s o r small cscarpments along t h e sho re l ine . Although t h e glaciomarine depos- i ts , second phase, probably were formed dur ing t h i s s t i l l s t a n d , t h e i r exac t mode of o r i g i n i s n o t c l e a r . They a r e be l ieved t o r e p r e s e n t chiefly l o c a l depos i t i ona l f e a t u r e s r e l a t e d t o h ighe r energy waves than were t h e first and t h i r d phase glaciomarine d e p o s i t s , and hence, t hey a r e i n t e x - p r e t e d as having formed a s b a r r i e r b a r depos i t s . The p o s s i b i l i t y t h a t t h e s e second phase d e p o s i t s , however, a l s o may have formed i n o t h e r ways i s b r i e f l y d iscussed under glaciomarine d e p o s i t s , second phase.

While t h e r i s e i n sea l e v e l appa ren t ly exceeded land u p l i f t , r e l a t i v e l y q u i e t waters existed along the shores . I t was a t t h i s t ime t h a t t h e diamicton on t o p of t h e o l d e r d e l t a s descr ibed e a r l i e r and shown on f i g u r e 10 was deposi ted. The glaciomarine d e p o s i t s , t h i r d phase, accum- u l a t e d as land u p l i f t a c c e l e r a t e d and exceeded t h e r i s e o f s e a l eve l . Eventual ly t he land r o s e a t l e a s t 500 f e e t and l o c a l l y as much a s 750 f e e t above p r e s e n t s e a l e v e l . The t h i r d phase d e p o s i t s a r e g e n e r a l l y more f i n e gra ined than t h e f i r s t phase depos i t s , and probably r ep re sen t a nearshore o r t i d a l zone depos i r iona l environment. The t h i r d phase depos i t s a r e i n p a r t reworked from t h e f i rst phase depos i t s .

The u n d i f f e r e n t i a t e d glaciomarine d e p o s i t s (Qrnu) a r e d e p o s i t s t h a t a r e n o t exposed b u t whose su r f aces appear t o be continuous wi th t h e s u r f a c e s of e i t h e r t h e first o r t h i r d phase depos i t s .

Marine f o s s i l s occur i n d e p o s i t s o f a l l t h ~ e e phases. Unbroken a s we l l as broken pelecypods and o t h e r molluscs , a s wel l a s ba rnac l e s , have been recovered from t h e f i rs t and t h i r d phase depos i t s . Only broken fragments of such s h e l l s have been recovered from t h e second phase d e p o s i t s . In- t e r p r e t a t i o n s o f t h e environment o f t h e molluscs and o f t h e foramini fe ra by Warren Addicott (wr i t t en commun., 1966) and Ruth Todd and Doris Low (wr i t t en commun., 19671, r e s p e c t i v e l y , i n d i c a t e shal low water ranging

between low t i d e and a maximum depth of SO fathoms. Lists o f t h e s e f o s s i l s a r e shown i n t a b l e 5.

In 1958, t h e l a t e Don J. M i l l e r c o l l e c t e d molluscan and foramini fe ra1 samples from d e p o s i t s i n t h e Juneau a rea , The f o s s i l s i d e n t i f i e d from these samples a r e l i s t e a i n t a b l e s 6 and 7. The c o l l e c t i o n sites a r e shown on t h e geologic map by t h e numbers l i s t e d i n t h e t a b l e s . In h e r eva lua t ion of t h e fo ramin i f e ra i n t hose samples, Ruth Todd (wr i t t en comrnun., 1959) considered t h e assemblages t o i n d i c a t e marine depos i t i on , and a t t h a t t ime be l i eved t h a t t h e fo ramin i f e ra "could have l i v e d a t depths from 0 t o 100 fathoms o r even more.''

Table 5.--Fossils coZZect~d from massive gZaciomarine deposits i n the Juneau, Alaska, vicinity by R. D. M< ZZer

[Numbers i n columns i n d i c a t e phase of glaciomarine depos i t i on as used i n t h i s r epo r t ]

3 1 Localities-

1/ I d e n t i f i c a t i o n s by W. 0 . Addicot t (wr i t t en commun., 1966) *

2/ I d e n t i f i c a t i o n s by Ruth Todd and Doris Low ( w r i t t e n commun., 1967) - 3/ Loca l i t y numbers s t a r t i n g wi th M r e f e r t o U.S. Geol. Survey Ceno-

zo ic i o c a l i t y numbers on f i l e with t h e U.S. Geol. Survey. Other numbers are U.S. Geol. Survey s t a t i o n numbers as r e f e r r e d to i n t h e field notebooks of t h i s s tudy.

4/ Sample c o l l e c t e d from D r i l l Hole 4, at 12.7 feet below the su r f ace . Drill- ole 4 i s shown on t h e cross s e c t i o n s i n f i g u r e 9.

88 (no p. 89)

M2650

1 .! Pelecypods-.

ChZamys mbida hinds; (Carpenter) 3

CZinocardiwn cil iatwn (Fabricius) 3

HiateZZa arc t i ca forma phoZadis (Linnb) 3

HiateZZa sp. (?)

Macoma ca Zearea Gme 1 i n 3

P'ya tmncata Linnd 3

273

---~

1

1

1

*

M2652

1

1

1

1

1

487

1

1

1

as tropodsi!

Buccinwn sp . 3

Cryptonatica cZausa (Broderip and 3 Sowerby)

Fus i tg ton oregonensis (Redfield)

Neptunea Zyrata (Gme 1 i n )

Vermetid

~ a r n a c l e s y 2 1 Foraminifera- .

CassiduZina teret is Tappan

EZphCdim cZavatwn Cushman

E l p h i d i m f r i g i d m Cushman

Pyrgo ZuoernuZa (Schwager)

Virgu Zina (7 )

4/ DH4-

1

1

1

1

1

1

Table 6. --MoZZuscan fossils coZZected from g Znciomurine deposits in 2958 in the Juneau, Alaska, vicinity, by Don J . Miller

[Numbers i n columns ind ica te phase o f glaciomarine deposi t ion a s used i n t h i s r e p o r t . ]

Local i t ies ~ 2 1 0 1 ~12111 ~12121 bI214 1~1216 l ~ 2 1 7 1 ~ 2 4 2 1 ~ 2 4 3

Pelecypods: / I I I I / Astarte aZaskensis Dall

I I I I I I I CZirwcardim ciliatwn (Fabricius) / 11 1 3 1 1 3 1

3

hinopsida serr4cata (Carpenter )

Cardid

ChZmnys m b i d a (Hinds) - .

Chlamys rubida hindsi (Carpenter)

?CZinocardiwn ciZiatwn (Fabricius) 1

1 r

2

2

I I I I I I I

I I I I I I I Macoma c f . M. calcurea Gemlin 1 1 1

3 1 3

CZCnocardium nuttaZZi (Conrad)

CycZocardia ventricosa (Gould)

HiateZZa arctica (LinnC)

1 3

1

Macoma caZcarea Gmelin

Macoma inquinata (Deshayes)

Macoma incongma (Martens)

Macoma brota Dall I I I 1 I I I

2

2

Macoma brota Zipara Dall I I 1 I I 1 I

1 3

Mya t m c a t a Linnt I I I I I I I

3

3

3

3

NucuZana fossa (Baisd) 1 1 1 1 2 1 - I I I I I I I

1

3 ~ u m ~ a n a sp .

I I I I I I I

1

1 1 1

flucuZa cf. 1V. tenuis (Montagu)

Panomya arnpZa Pal l

PoZynemamussiwn alaskense (Dal 1 )

Po Zynernmrmssiwn davidsoni (Dall)

I 1 I I I I I

Tresus capm (Gould) 3

I

3

3

I

1 2 1

Protothaca cf. P. stminea (Conrad)

Saxidonrus giganteus (Deshayes)

Serripes grontandicus (Bruguiere)

1

3

3 3

2

3

3

3

3

3

1

1

3

3

1

1

3

3

3

3

3

3

3

Table 6.--iz:oZZt~scan fossZZs coZZected from gZac<omarine deposits i n 2958 i n the Juneau, AZaska, vicinity, by Don J. MiZZer--Continued

M210

Gastropods :

?Buccinwn g Zacia Ze ~ i n n d 1

Buccinwn gZaciaZe LinnC --

Buccinwn gZaoiaZa Linnt--weakly sculp tured form -

cozus s p .

CoZus cf, C. huZZi Dall

M211 M212

2

- -

2

Cryptonatica cZausa (Broderip and Sowerby) -

Fusitriton sp . Lepeta concentrica (Middendorff)

Littorina sitkana P h i l l i p i

Neptunea Zirata Gme lin

Neptunea s p . 1

Odostomia sp.

?Po ZCnices sp . PoZinices sp . mopebela s p .

Functuxe ZZa major Dal 1

TrichotropCs boreaZis (Broderip and Sowerby)

Trophonopsis pacificus ( D a l l )

T~ophonopsis? c f . T . Zatus (Dall)

TwboniZZa sp.

EI214

3

3

3

3

M243

_I_

3

3

3

3

3

3

3

3

3

M217

3

3

3

3

3

3

1

1

bi216

1

1

1

M242

1

1

1

2

2

I

3

3

3

3

Table 7 . --Forminifera coZZected from glaciumarine deposits i n 2958 i n the Juneau, AZaska, v i c in i t y by Don J . MiZZer

[Numbers i n columns indicate phase of glaciomarine deposit ion as used i n t h i s repor t . Fossils i d e n t i f i e d by Ruth Todd (writ ten commun., 1959) *Tentat ive identification]

BENTHONIC

ASTRORHIZIDAE

genus ?

MILIOLIDAE

QuinqueZocuZim aggzutinata Cushman

Q. akneriana d 1 Orbigny

Q. frigida Parker

Q. staZkeri Loeblich and Tappan

T. tficauYinata d' Orbigny

Pyrgo ZucemuZa (Schwager)

LAGEN IDAE

Lagena graci Z Zima (Seguenz a)

BULIMINIDAE

BuZimine ZZa e Zegantissima (d Orbigny)

GZobobuZCmina amicuZata (Bailey)

VirguZina fus i fomis (Williamson)

BoZivina decussata Brady

B. pacifica Cushman and McCulloch

Fissuz-4na sp. (globular, slit aper ture) 1

ROTALIIDAE

Bucce ZZa frigida (Cushman)

ELPHIDIIDAE

E l p h i d i m bmt l e t t i Cushman

E l p h i d i m c lavatm Cushman

EZphidium frigidum Cushman

EZphidieZZa nit ida Cushman

k k k k k k k k

1

1

1

1

1 1 2 3 1 1 1 1 1 3

1 2

1 2

1 2

m m m m ~ c o r - r . + c o r l W r . C O r l 0 0 N r U d ( U

m d r - a u a 4

1 2 3 1

2

1 1 2 3 1

2

1 1 2 3 1

3

3

I

~ ~ s ~ ~ ~ s l ~ p $ 0 3 0 3 0 0 0 0 D 3 M o O o 3 M o 3 LnInmLOmmLnLnLnLn

1

111

1

1 1 1 3

1 1 1 3

3

P

1

1 3

13

3

3

3

3

1 3

3

Table 7.--Foraminifera coZZected from ~Zaciornmine deposits i n 2958 i n the Juneau, Alaska, vicinity by Don J . Miller--Cont inued

mmL'-

ANObtALINI DAE

Cibicides ZobatuZus (Walker and Jacob) -- Dyoeibieides biseriaZis Cushman and Valentine - -- --

NONIONIDAE

NonCon Zabradorim (Daws on) --- -

IVonConeZZa turqida Williamson, var.

digi tata NBrvang

Pseudononion aurieuZa (Heron-Allen and Earland) I1 l 2 Astrononion gaZZowayi Loeblich and Tappan

A. steZZigermm (dlOrbigny) ----"--I+ CASSIDULINIDAE I I

Ca~siduZina isZandica Nbrvang 1

C. t e r e t i s Tappan 1 1

@istomineZZa exigua (Brady)

PLANKTONIC

GLOBIGERINIDAE I I

As a r e s u l t o f a reg ional s tudy o f foramini fe ra i n sou theas t e rn Alaska and B r i t i s h Columbia, Roberta K. Smith (1970, p. 692) concluded t h a t water depths were more r e s t r i c t i v e , and t h a t . t h e fo ramin i f e ra faunas ,

she c o l l e c t e d tiere r e p r e s e n t a t i v e of water Itprobably l e s s t han 30 meters deep * * *."

On t h e b a s i s o f t h e pa l eon to log ic eva lua t ions , I i n t e r p r e t t h e depth o f water i n t h e Juneau area t o have been r e l a t i v e l y shal low a t t h e t ime of t h e depos i t ion o f a l l of t h e glaciomarine diamictons, probably l e s s than t h e 30 meters suggested above.

Glaciomaxine d e p o s i t s , f i r s t phase (Qmb)

The glaciomarine d e p o s i t s , f i r s t phase, c o n s i s t of gray ( N 5 ) t o l i gh t -g ray (SY 7/1) cohesive compact diamicton. They are heterogeneous mixtures of sand, s i l t , g r a v e l , and c l a y ( t a b l e 4 , no. 2) which con ta in pebbles , cobbles , and boulders , some a s l a r g e a s 10 f e e t across . They a l s o con ta in broken and whole s h e l l s o f marine molluscs , some o f which a r e a r t i c u l a t e d . Foraminifera abound, a s do ba rnac l e s , many o f which a r e a t t ached t o rocks i n t h e o r i g i n a l p o s i t i o n o f growth. The m a t e r i a l when moist is dark gray (N 3/0) o r very dark gray (5Y 4/1) and appears dark b l u i s h o r black on rF'c outcrops. These d e p o s i t s form most o f t h e ma te r i a l t h a t l o c a l wel l d r i l l e r s c a l l "blue clay.I1 Rock types inc lude greenstone, s l a t e , graywacke, metavolcanics , and g r a n i t e ; most of the l a r g e r p i eces a r e e i t h e r g r a n i t e ox dense metavolcanic rock. In t h e depos i t s above t h e confluence o f Montana Creek wi th McGinnis Creek, clay and s i l r averages 51 pe rcen t , and the c l a y - s i l t - s a n d s i z e s a r e more evenly d i s t r i b u t e d than i n other d e p o s i t s i n t h e area ( t a b l e 4 , no. 3) . Below t h e confluence with McGinnis Creek, t he depos i t a long Montana Creek v a l l e y coarsens t o a grave l i n a c l a y and s i l r mat r ix .

Severa l radiocarbon d a t e s were obta ined on shel ls from t h e glaciomarine depos i t s , f i r s t phase. One s h e l l sample, from n e a r t h e base o f t h e depos i t a t an a l t i t u d e o f about 80 feet n e a r t h e junc t ion o f Fritz Cove Road and the Glac i e r Highway, i s 10,6402300 years o l d (sample W-1827, Meyer Rubin, w r i t t e n commun., 1966). Another s h e l l sample, t aken from an outcrop a t an a l t i t u d e of about 400 f e e t i n the Salmon Creek v a l l e y , has an age o f 11,920+1,000 yea r s (sample W-2396, Meyer Rubin, w r i t t e n commun., 1970).

This diamicton crops out a long t h e s i d e s of Gast ineau Channel, a long Fritz Cove, Auke Bay, Lena Cove, and t h e Tee Harbor a r e a , and u n d e r l i e s much of downtown Juneau. In a d d i t i o n , it extends a long Montana Creek upval ley from Tolch Rock, a s we l l as up the Fish Creek v a l l e y . Foss i l - i f e rous c l a y , assumed t o be p a r t o f t h e f i rst phase d e p o s i t s , i s found 110 f e e t below t h e ground su r f ace in Last Chance Basin a t an a l t i t u d e o f 280 feet (Waller, 1959, t e s t ho le no. 4 , t a b l e l), and beneath a g rave l i n t h e lower-middle p a r t o f t h e Lemon Creek v a l l e y (Spencer, 1906, p. 119).

The s u r f a c e o f t h e diamictons adjacent t o Gast ineau Channel s l o p e s 10°-150 upward toward t h e mountainsides from sea b l u f f s o r from sca rps t L a t s e p a r a t e t h e f i r s t phase diamicton from t h e ove r ly ing younger t h i r d phase diamicton t h a t extends dormslope. Local ly, h i l l s o f bedrock p r o j e c t through t h i s surface and i n t e r r u p t t he broad even s lope. Most o f t h e sca rps b e t ~ e e n the t \ i o diamictons a r e composed o f f i r s t phase diamicton, but l o c a l l y t h e scarps may be bedrock o r bedrock mantled by t h e diamicton.

In nos t p laces t h i s diamicton i s mapped gene ra l ly t o a h e i g h t o f about 500 f e e t above s e a l eve l a s determined from very poor and spo rad ic exposures and i n f e r r e d frorn pronounced changes i n topography. However, ma te r i a l t h a t conta ins s h e l l fragments and t h a t is thought t o be a p a r t o f t h i s f i rst phase diamicton extends up t o an a l t i t u d e o f 750 f e e t i n t h e SW1/4SW1/4 sec. 27, T. 39 S. , R. 65 E. , i n a b lu f f a long upper Montana Creek. This i s t h e h ighes t f o s s i l i f e r o u s glaciomarine depos i t recognized i n t h e Juneau area . Elsewhere on t h e mainland and a t some p l aces on Douglas I s l and , first phase diamicton may b e a s high a s 600 f e e t above s e a l e v e l .

The diamicton i n t h e f i r s t phase i s more massive than t h a t i n t h e t h i r d phase, and l a y e r s and l enses of f i n e sand a r e r a r e . Sand i n t h e f i r s t q

phase depos i t s a l s o i s s l i g h t l y c o a r s e r than t h a t i n t h e t h i r d phase diamicton, and grave l i s more common ( t a b l e 4, no. 2) . Cores from t e s t ho le s d r i l l e d i n t o t h e f i r s t phase diamicton revea led an extremely dense and t i g h t ma te r i a l . Core recovery was almost 100 percent i n t h i s m a t e r i a l , b u t no s i z e g rada t ion o r depos i t i ona l breaks were seen. Foraminifera were

, a found i n a l l o f t h e ho le s a t a l l depths. Although sand i s t h e p r i n c i p a l p a r t i c l e s i z e ( t a b l e 4 , no. 2 ) , combined c l a y and s i l t averages 38 percent, and t h e m a t e r i a l t ends t o become s l i p p e r y when w e t , even though the p l a s t i c i t y index is g e n e r a l l y 8 o r l e s s . Liquid l i m i t s a r e a s h igh a s 22 pe rcen t , b u t i n 1 3 samples t h e average is 19 percent . The phys i ca l p r o p e r t i e s of t h e u n i t a r e n o t uniform and show both l a t e r a l and v e r t i c a l v a r i a t i o n s , For example, samples t e s t e d show extremes of c l a y s i ze from 7 t o 21 percent , s i l t s i z e 11 t o 33 pe rcen t , sand s i z e 20 t o 65 pe rcen t , and g rave l s i z e 5 t o 38 percent . The P roc to r dry d e n s i t y determined f o r one outcrop sample was 127.5 pc f , with an optimum mois ture o f 10 pe rcen t ; t h e sample became mushy a t 16-percent moisture content . The optimum moisture <s n e a r t h e average n a t u r a l moisture content o f 12.5 pe rcen t , as determined from 1 2 o t h e r samples. Samples c o l l e c t e d from slumping and flowing diamicton contained about 25 percent moisture. Dry bulk d e n s i t i e s o f 15 samples c o l l e c t e d from outcrops a long t h e sho re o f the Gastineau Channel average 129.1 pcf but range from 112.1 t o 143.2 pcf . A sample from along Montana Creek, below i t s confluence wi th McGinnis Creek, had a dry bulk d e n s i t y of 127.2 pc f . Samples of cores from t e s t ho les i n d i c a t e a h i g h e r d e n s i t y and a lower moisture content than from outcrop samples. Cores from d r i l l ho l e 3 , which was loca t ed behind the elementary school a t t h e no r the rn edge o f Douglas, had a maximum d e n s i t y o f 149.6 pcf a t 11 f e e t below t h e s u r f a c e , bu t had an average dry bulk d e n s i t y o f 136.9 pcf. The lowest bulk d e n s i t y was found i n t h e co re sample a t 22 f e e t below t h e sur face . The average n a t u r a l moisture content was 7.4 percent . Numerous samples o f co re s from d r i l l h o l e 4 , by the Methodist Church i n Juneau, had an average dry bulk d e n s i t y o f 146.1 pcf

and an average mois ture content o f 5.8 pe rcen t ; a sample from about 24.5 f e e t had l e s s t han 1 percent n a t u r a l mois ture , whi le samples n e a r t h e bottom o f t h e ho le , between 61 and 61.5 f e e t , had n a t u r a l mois ture con ten t s oi only about 2-4 percent . Two samples o f core m a t e r i a l ob ta ined from t h i s t e s t ho le between 19-22 f e e t , and a t 54 f e e t below the s u r f a c e , i n confined compression t e s t s showed angles o f i n t e r n a l f r i c t i o n o f 55" and 63' r e - spec t ive ly . The sample cores from d r i l l ho l e 5 , behind t h e S t a t e Cap i to l Building, showed an aveTage dry bulk d e n s i t y of 146.1 pcf and an average moisture content of 7 . 3 percent .

The reason f o r t h e inc reased denseness of t h e m a t e r i a l s i n t h e lower p a r t of t h e t e s t holes i s not known. I t could be t h e resu l t of t i d a l s i f t i n g dur ing depos i t i on , which caused very f i n e ma te r i a l t o d i s p l a c e water and f i l l most vo ids ; it might be repea ted v i b r a t i o n , p o s s i b l y from earthquakes; it might be t h e r e s u l t o f normal conso l ida t ion from t h e weight o f ove r ly ing m a t e r i a l , d e s s i c a t i o n of diamicton, o r it might be t h a t t h i s denser d i a - micton i s an o l d e r glaciomarine depos i t t h a t was over r idden by a P l e i s tocene i c e s h e e t and thereby overconsol ida ted .

The compactness, cohesiveness , s tony c h a r a c t e r , and d e n s i t y o f the diamicton, a long with t h e abundance o f large boulders , makes excavat ion d i f f i c u l t even by heavy power equipment. In some p l aces t h e toughness o f t h e m a t e r i a l and t h e tendency f o r t h e m a t e r i a l t o break i n t o l a rge cohesive masses makes excavat ion extremely d i f f i c u l t . D r i l l i n g i s slow because o f t h e l a r g e number o f cobbles and boulders .

Concentrated flows o f water, such as from broken flumes o r d ive r s ion of stream channels , cut i n t o and deeply erode t h e diamicton. Very s t e e p

.r

sided g u l l i e s , 5-20 f e e t deep, have been formed by t h i s k ind o f concen- t rated flow, J u s t such a g u l l y has been cut along Gold Creek n e a r Ever- green Bowl where water from a flume has eroded through the o l d e r d e l t a depos i t s , and t h e g u l l y i s entrenched i n t o t h e glaciomarine d e p o s i t s , f i rs t phase, Sheet wash, however, erodes t h e depos i t s on ly s l i g h t l y , even on s t e e p s lopes .

Seismic response o f t h e undis turbed glaciomarine d e p o s i t s , first phase, t o a severe ear thquake probably w i l l b e e s s e n t i a l l y l i k e t h a t o f nearby bedrock, ch ie f ly hecause o f t h e n a t u r a l dryness and h igh bulk d e n s i t i e s of t h e depos i t s . The high angles of i n t e r n a l f r i c t i o n suggest that t h e ma te r i a l i s s t a b l e under s t a t i c cond i t i ons and t h a t it would probably remain s o i f shaken by an earthquake. A sample from a d r i l l h o l e was determined s o n i c a l l y t o have a s h e a r modulus o f 89,910 p s i , which i s n e a r e r t h a t o f s o l i d rock than any other unconsol idated s u r f i c i a l depos i t i n t h e Juneau a rea .

Diamicton used f o r f i l l , however, might r e a c t d i f f e r e n t l y . Disturbance of t h e ma te r i a l by excavat ion, dumping, and improper compaction could e a s i l y i nc rease t h e a f f i n i t y f o r water o f such poor ly s o r t e d depos i t s . Such an a f f i n i t y could r a i s e t h e n a t u r a l mois ture content f a r above t h e normal range o f 5-7 percent s o t h a t t h e l i q u i d l i m i t would be approached o r exceeded. In such a s t a t e t h e ma te r i a l very l i k e l y would compact,

and subs ide o r flow i f sub jec t ed t o s t r o n g se i smic shaking, a s d i d poorly s o r t e d and poorly compacted m a t e r i a l elsewhere dur ing t h e 1964 Alaska earthquake (Kachadoorian, 1968, p. C43).

Dry diamicton o f t h e g l ac iona r ine d e p o s i t s , f i r s t phase, i s very s t a b l e i n n a t u r a l ox c u t s lopes . Sa tu ra t ed diamicton i s very uns t ab le and tends t o slump, flow, o r move a s small s l i d e s . The d e b r i s s l i d e i n Salmon Creek below t h e flume v i v i d l y i n d i c a t e s how t h e ma te r i a l behaves when s a t u r a t e d . S l i d e s and s l~unps of the f i r s t phase diamicton have l o c a l l y blocked t h e Glac i e r Highway a f t e r heavy r a i n s .

Glaciomarine d e p o s i t s , second phase (Qms)

The glaciomarine d e p o s i t s , second phase, are very hard compact cohesive diamictons t h a t a r e gray (SY 6 /1 , 2.SY 6/1) when dry and very dark gray (E 3/0) when moist. ~ e x t u r 2 1 l y , they-are heterogeneous and c o n s i s t dominantly of g r a v e l , wi th l e s s e r amounts o f sand, s i l t , and c l a y t h a t con ta in boulders a s l a r g e as 15 inches i n diameter ( t a b l e 4 , no. 4 ) . Although fragments o f s h e l l s a r e widely s c a t t e r e d through t h e d e p o s i t , no complete s h e l l s were found. Weak s t r a t i f i c a t i o n i s ev iden t i n some p l aces but t h e d e p o s i t s a r e gene ra l ly massive. Unmapped t h i n sandy grave l , which may be a beach d e p o s i t , l o c a l l y o v e r l i e s t h e glaciomarine d e p o s i t s , second phase. A radiocarbon d a t e o f 9,800+300 y e a r s B.P. (sample W-2392, Meyer Rubin, w r i t t e n commun., 1970) determined from s h e l l s i n a depos i t a t t h e mouth o f Cove Creek, i n t h e no r the rn p a r t o f Douglas I s l and , sugges ts t h a t t h e t ime o f formation o f t h e s e d e p o s i t s may have overlapped t h a t o f t h e t h i r d phase depos i t s .

The second phase d e p o s i t s a r e not a s widespread as t h e o t h e r glaciomarine depos i t s . They occur only i n s e v e r a l s e p a r a t e a r eas . Most a r e found a long t h e shores of Gast ineau Channel, Auke Bay, Auke Lake, and F r i t z Cove; d e p o s i t s a l s o occur nea r Indian Cove and extend between Lemon and Salmon Creeks. The l a r g e s t d e p o s i t accumulated as a s e r i e s o f knobs and r i d g e s at the no r the rn end o f Douglas I s land , south o f Outer Poin t n e a r Pe ter - son Creek. Between Lemon and Salmon Creeks, second phase d e p o s i t s form a r i d g e t h a t i s more o r l e s s s inuous ; a sma l l e r r i d g e p r o j e c t s away from t h e main r i d g e t o form Vanderbi l t H i l l . The depos i t s a t Cove Creek, on no r the rn Douglas I s l and , and along Montana Creek are o f l i m i t e d a r e a l e x t e n t and a r e more d e l t a i c i n form.

Gravel is t h e p r i n c i p a l s i z e o f ma te r i a l i n t h e 1 1/2-inch o r sma l l e r s i z e s i n t h e glaciomarine depos i t s , second phase, a l though t h e sand content may be n e a r l y as h igh ( t a b l e 4 , no. 4). Textura l extremes o f t h e samples c o l l e c t e d show p a r t i c l e ranges as fol lows: c l a y , 0-26 per- cen t ; s i l t , 1-24 pe rcen t ; sand 16-62 pe rcen t ; and grave l 17-63 percent . Because t h e f ine-gra ined p a r t i c l e s f i l l t h e spaces between t h e coa r se fragments, t h e depos i t is hard and firm. These f i n e s may have been t h e r e s u l t of s i f t i n g by waves o r t i d a l waters . The o u t e r 1-2 inches of t h e s u r f a c e o f outcrops i s c h a r a c t e r i s t i c a l l y hardened by dry sandy s i l t , which r e s i s t s pene t r a t ion when s t r u c k wi th a pick. The m a t e r i a l i s non- plast ic because of t h e r e l a t i v e l y low silt and high sand content . The

@ Proctor dry d e n s i t y o f two samples from second phase m a t e r i a l i s 141.7 pcf and 142.0 pcf . One c h a r a c t e r i s t i c i n p a r t i c u l a r s e t s t h e s e second- phase depos i t s a p a r t from t h e o t h e r m a t e r i a l s ; t h i s m a t e r i a l has an extre:ne a f f i n i t y f o r water and, when wetted, qu ick ly l a s e s cohesion and i n t e r n a l s t r e n g t h . The n a t u r a l moisture content o f two samples from t h e same outcrop i s 2.98 percent and 3.00 percent . Optimum mois ture content o f o t h e r saniples from two l o c a l i t i e s was about 6.5 pe rcen t , and t h e s e samples became wet and soupy wi th on ly 7-8 percent moisture; t h u s , t h i s ma te r i a l has a very c r i t i c a l moisture po in t . As a r e s u l t , a r t i f i c i a l o r n a t u r a l overwett ing o f t h e m a t e r i a l causes l o s s of i n t e r n a l cohesion, and any unconfined m a t e r i a l e i t h e r flows o r t u r n s i n t o a soupy mass. I f such a depos i t i s l e f t undis turbed t o d r a i n , t h e s u r f a c e o f t h e m a t e r i a l f i r s t hardens,and wi th cont inued dry ing the remainder of t h e m a t e r i a l slowly rega ins enough i n t e r n a l cohesion t o be almost as f i r m a s undisturbed ma te r i a l .

The mode of o r i g i n o f t he glaciomarine depos i t s , second phase, i s s t i l l somewhat enigmatic. The i r coarse-grained n a t u r e , t h e i r apparent prox- i m i t y t o a r eas o f e i t h e r p r i o r h igh wave energy o r t o t h e v i c i n i t y o f s t reams capable o f providing l a r g e s u p p l i e s o f coa r se g rave l , t h e i r conten t o f s c a t t e r e d broken s h e l l s and t h e i r c h a r a c t e r i s t i c depos i t i ona l landforms l e d me t o cons ider t h e s e d e p o s i t s t o be o l d b a r r i e r bars ( f i g . 10). The f i n e r p a r t i c l e s probably accumulated by s i f t i n g i n t o and f i l l i n g open spaces between t h e g rave l p a r t i c l e s e i t h e r dur ing i n t e r v a l s of r e l a t i v e l y q u i e t water o r because o f t h e f l u c t u a t i n g c u r r e n t s o f d a i l y t i d e s . It is a l s o recognized t h a t t h e s e second phase depos i t s could be remnants o f an o l d e r , l a t e P l e i s tocene glaciomarine depos i t o r , perhaps, eroded remnants of till t h a t p r o j e c t through t h e glaciomarine depos i t s , f i r s t phase. If t h e s e a r e o l d e r depos i t s , apparent ly t hey were depos i ted e i t h e r from i c e i n t o marine water , a s evidenced by the broken s h e l l s , o r they r ep resen t eroded remnants o f reworked g l a c i a l depos i t s . Absence of recognizable till elsewhere i n t h e Juneau area below a t l e a s t 500 f e e t above modern s e a l e v e l , and t h e c l o s e associat ion of t h e glaciomarine depos i t s , second phase, t o t hose d e p o s i t s o f t h e f i r s t and t h i r d phases, l eads me t o d iscount t h e t i l l o r i g i n o f t h e diamicton i n t h e glaciomarine d e p o s i t s , second phase.

Second phase d e p o s i t s where dry probably would r e a c t t o shaking much like bedrock. Although t h e glaciomarine d e p o s i t s , second phase, have been used f o r road f i l l and embankments, a f t e r prolonged r a i n f a l l p a r t s of t h e s e f i l l s have become s a t u r a t e d and have flowed l a t e r a l l y . Prolonged s t r o n g se ismic v i b r a t i o n o f poorly compacted wet m a t e r i a l probably a lso would r e s u l t i n s i m i l a r flowage and compaction and subsidence o f t h e f i l l ma te r i a l .

Glaciomarine d e p o s i t s , t h i r d phase (Qme)

The t h i r d phase o f glaciomarine d e p o s i t s c o n s i s t s o f l i gh t -g ray (SY 7/1) massive compact t o punky diamicton; s p e c i f i c a l l y i t i s a heterogeneous mixture, i n o r d e r of abundance, o f sand, s i l t , c l ay , and g rave l ( t a b l e 4 , no. 1 ) . I s o l a t e d cobbles and boulders a r e s c a t t e r e d throughout the d e p o s i t s

l o c a l l y . Stems and l eaves , some o f which a r e carbonized, a r e conspicuous a i n some p l a c e s , as a r e she l l fragments and whole molluscan s h e l l s , many of which remain a r t i c u l a t e d ; fo ramin i f e ra a r e a l s o common. The c o l o r of moist m a t e r i a l i s very dark gray (5Y 4/1) and appears dark b lue o r black on t h e outcrop. T h i s diamicton ?s a l s o a p a r t o f t h e m a t e r i a l t h a t l o c a l wel l d r i l l e r s c a l l "blue c lay ." Rock t r p e s inc lude l o c a l l y de- r i v e d greenstone, s l a t e , graywacke, g r a n i t e , and some e r r a t i c p i eces of sandstone and l imestone, o r marble. Most o f t h e l a r g e boulders a r e e i t h e r g r a n i t e o r dense metavolcanic rocks. S h e l l s c o l l e c t e d from t h e diamicton i n t h e glaciomarine d e p o s i t s , t h i r d phase, nea r Auke Lake have a rad io- carbon age of 10,630+500 yea r s B.P. (sample W-2263, Meyer Rubin, w r i t t e n cornmun., 1969). S h e l l s from a diamicton t h a t p a r t l y u n d e r l i e s an undated o l d e r d e l t a depos i t (Qdo) near Kowee Creek, were a l s o da ted by radiocarbon methods, and gave an age of 9,700+800 yea r s (sample W-2393, Meyer Rubin, w r i t t e n commun., 1970); t h i s i s t h e youngest radiocarbon d a t e on g l a c i a - marine depos i t s a t Juneau. This young d a t e from a t h i r d phase diamicton t h a t i s now c l o s e t o s e a l e v e l suggests t h a t t h e r i s e o f s e a l e v e l occurred a t n e a r l y t h e same r a t e as t h e r i s e o f t h e land.

The glaciomarine d e p o s i t s , t h i r d phase, crop out i n t e r m i t t e n t l y along both s i d e s o f Gast ineau Channel, a long F r i t z Cove, Auke Bay, Lena Cove, Tee Harbor, and t h e lower reaches o f Montana Creek i n Fiendenhall va l l ey . These d e p o s i t s gene ra l ly veneer p r e e x i s t i n g s u r f a c e s t h a t s l o p e upward a t about 1 0 ~ - 1 5 " from t h e modern beaches o r s e a b l u f f s t o t h e mountainsides. The s u r f a c e of t h e depos i t along lower Montana Creek s lopes downstream under a cover of muskeg and f o r e s t . Scarps t h a t a r e gene ra l ly no more than 10 f e e t high commonly s e p a r a t e t h e t h i r d phase depos i t s from t h e somewhat o l d e r , h ighe r , and more ex tens ive glaciomarine d e p o s i t s , f i r s t phase, (Qmb). The t h i r d phase depos i t s reach an a l t i t u d e o f about 200 f e e t . Thin and continuous o t h e r r a i s e d beach d e p o s i t s (Qbe) and muskeg (Qrnk) o f v a r i a b l e t h i ckness o v e r l i e and gene ra l ly obscure t h i s glaciomarine m a t e r i a l almost everywhere except i n b l u f f s o r excavat ions. The t h i r d phase d e p o s i t s a r e g e n e r a l l y 4-12 f e e t t h i c k .

The t h i r d phase diamicton d i f f e r s from t h e o t h e r diamictons i n s e v e r a l ways: t h i n lamina t ions occur l o c a l l y , p l a n t remains a r e common, and t h e bulk d e n s i t y i s lower. The t h i r d phase diamictons gene ra l ly appear massive i n outcro:, b u t some exposures show t h e m a t e r i a l s t o c o n s i s t o f laminae and t h i n l a y e r s o f very f i n e sand t h a t p a r a l l e l t h e s l o p e o f t h e su r f ace . Although composed p r i n c i p a l l y o f f i n e sand ( t a b l e 4 , no. I ) , t h e s e depos i t s show an average c l a y and s i l t content of 46 pe rcen t ; consequent ly , the m a t e r i a l becomes unctuous when wet, even though t h e p l a s t i c i t y index i s l e s s than 8.

The phys i ca l p r o p e r t i e s o f the t h i r d phase depos i t s vary somewhat from p l a c e t o p lace . Samples t e s t e d dur ing t h i s i n v e s t i g a t i o n show extremes of clay from 0 t o 32 pe rcen t , s i l t 4 t o 49 percent , sand 26 t o 57 percent, and grave l 0 t o 66 pe rcen t ; gene ra l ly grave l is r a r e , however. A Proc to r dry d e n s i t y of 118.0 pcf was determined, which f o r one sample c o l l e c t e d about 3 f e e t below t h e t o p of t h e depos i t a long t h e southern shore of Auke Lake, i s the lowest d ry d e n s i t y o f any o f t h e glaciomarine

depos i t s . This sample a l s o had an optimum moisture con ten t o f 12.5 per - cen t and became mushy a t 18 percent . Su rp r i s ing ly , both o f t h e s e per-

k centages a r e l e s s t han t h e average n a t u r a l moisture content o f t h i s sample, which was 21-24 percent . Dry bulk d e n s i t i e s o f o t h e r samples average 116.8 pcf and show extremes of 108.4 and 123.3 pcf . Ten samples c o l l e c t e d 3-4 f e e t below t h e t o p of t h e glaciomarine d e p o s i t s , t h i r d phase, elsewllere i n t he Juneau a r e a revealed an average n a t u r a l mois ture content o f about 17 pe rcen t , with extremes o f 6.2 percent and 42 percent . Severa l samples c o l l e c t e d from t h e s u r f a c e o f wet and f lowing m a t e r i a l contained 35-55 percent moisture. Thus, some samples can r e t a i n m o i s t u ~ e i n excess o f SO percent even though t h e l i q u i d l i m i t , t h e a r b i t r a r y l i m i t between the p l a s t i c and l i q u i d s t a t e s o f a m a t e r i a l , a s determined from n i n e samples i s g e n e r a l l y cons iderably l e s s , averaging about 21 percent . These data show t h i s deposit t o be extremely s e n s i t i v e t o moisture.

The upper few inches of an undisturbed depos i t can become s a t u r a t e d r a t h e r e a s i l y , b u t excess mois ture apparent ly does no t r e a d i l y p e n e t r a t e below t h e s e few inches. For example, one sample c o l l e c t e d from s t a b l e m a t e r i a l immediately below t h e flowing ma te r i a l contained only 19 pe rcen t n a t u r a l moisture. Disturbed m a t e r i a l becomes s a t u r a t e d e a s i l y , however, and when t h i s happens flowage can r e s u l t .

The low d e n s i t y and h igh s i l t and sand content o f t h i s depos i t g e n e r a l l y permit very easy excavat ion and d r i l l i n g a s compared t o t h e o t h e r g l ac io - marine depos i t s ; occas iona l l a r g e boulders may be encountered, however. If wet, t h e depos i t becomes very s o f t and heavy equipment can become bogged down. D r i l l i n g equipment g e n e r a l l y needs a p l a t fo rm t o hold t h e r i g . Wet m a t e r i a l s removed from excavat ions as s p o i l d r a i n r e a d i l y and become hard and firm i n d ry weather b u t w i l l flow dur ing wet weather.

Natura l exposures o f diamictons i n t h e glaciornarine d e p o s i t s , t h i r d phase, resist sheetwash, b u t g u l l i e s develop where t h e m a t e r i a l s a r e exposed t o concentrated running water . Most undis turbed depos i t s a r e covered by vege ta t ion , which he lps reduce e ros ion . Sheetwash e ros ion is minor along s lopes of a r t i f i c i a l cu t s . When excavated m a t e r i a l i s p i l e d o r dumped, i t s high a f f i n i t y f o r water causes s a t u r a t i o n dur ing pe r iods of wet weather, and flowage can occur.

The foundat ion s t a b i l i t y o f t h e s e deposits is poor t o f a i r . Observat ions of bu i ld ings under cons t ruc t ion show that some s t r u c t u r e s a r e b u i l t on concre te foo t ings p laced d i r e c t l y on t h e 4-12-foot-thick d e p o s i t s , o t h e r s t r u c t u r e s a r e placed on p i l e s o r c o n c r e t e - f i l l e d ca i s sons d r i l l e d t o f i rmer underlying m a t e r i a l s .

Low d e n s i t y o f t h i s m a t e r i a l , t o g e t h e r wi th a r e l a t i v e l y h igh n a t u r a l moisture content and low optimum mois ture , sugges ts t h a t t h i s m a t e r i a l would be p o t e n t i a l l y uns t ab le i n i t s n a t u r a l s t a t e i f p laced under dynamic cond i t i ons , and it would be s u b j e c t t o damaging r e a c t i o n s when sub jec t ed t o prolonged shaking from a seve re ear thquake i n t h e Juneau area. Sediments t h a t have been excavated and used f o r f i l l , i f poor ly compacted, may subs ide , f r a c t u r e , and f a i l by flowage i f sub jec t ed t o

strong seismic shaking (Kachadoorian, 1968, p. C43). The average n a t u r a l moisture of 17 percent i s nea r t h e average l i q u i d l i m i t o f 21 percent . A s i n t e r p r e t e d from t h e P roc to r t e s t , t h e broad optimum mois ture curve s l o p e s gen t ly t o t h e mushy p o i n t ( t h e r e i s no sharp' s teep-s ided break) s o t h a t f r a c t u r i n g and colnpnction t a u l d be more l i k e l y than massive s l i d i n g . In a d d i t i o n , t h e genera l t l i inness o f the depos i t he lps reduce t h e r i s k o f massive block s l i des , such a s t hose t h a t occurred a t Anchorage dur ing t h e Alaska ear thquake o f 1964.

F i e ld observa t ions of t h i s diamicton r e v e a l t h a t it s t a n d s we l l under s t a t i c condi t ion i n n a t u r a l exposures and i n n e a r l y v e r t i c a l wa l l s o f shallow excavat ions hen d r y o r moist , but no t when wet. Gentle s lopes permit g r e a t e r i n f i l t r a t i o n and wet t ing of t h e depos i t s , and flowage may r e s u l t . I f i n f i l t r a t i o n o f water t a n be prevented, t h e r i s k o f flowage should be reduced, Slumps and small s l i d e s a r e r a r e i n n a t u r a l m a t e r i a l because of i t s cohcsivencss, Slumps along b lu f f s have occurred where waste m a t e r i a l s overloaded t h e edges of t h e b l u f f s . I n a d d i t i o n , loose excavated ma te r i a l t h a t was pushed over b l u f f s has become s a t u r a t e d wi th water and has flowed and slumped.

Glaciomarine depos i t s , t h i r d phase, on Douglas I s l and appa ren t ly were used t o produce b r i c k s during t h e e a r l y days of Juneau. The beach along t h e no r theas t e rn p a r t o f Douglas I s l and conta ins many fragments of t h e s e b r i c k s .

Undi f fe ren t ia ted glaciomarine depos i t s (Qmu)

Areas i n which t h e r e a r e no exposures, b u t which a r e thought t o be under- l a in by diamicton of e i t h e r f i r s t o r second phase glaciomarine d e p o s i t s , or both , are shown on t h e geologic map as u n d i f f e r e n t i a t e d glaciomarine depos i t s . These a reas a l s o may l o c a l l y inc lude s u r f i c i a l d e p o s i t s of o t h e r kinds, a s well as bedrock.

Und i f f e ren t i a t ed glaciomarine depos i t s a r e p re sen t on t h e Mendenhall Peninsula , no r th of Auke Bay and Auke Lake, and i n t h e Lena Cove a r e a , where they l i e upslope from t h e glaciomarine d e p o s i t s , t h i r d phase (Qme), or between t h e shore and t h e mountainside. The depos i t s g e n e r a l l y l i e below t h e 300-foot contaur , b u t i n t h e Waydelich Creek a r e a no r th of Auke Bay, u n d i f f e r e n t i a t e d depos i t s a r e mapped t o an a l t i t u d e of about 500 feet.

SELECTED REFERENCES I

Ailen, C . R . , S t . h a n d , Pierre, Richter , C . F . , and Norquist, J . M . , 1965, Rela t ionship between seismicity and geologic s t r u c t u r e i n t h e southern Ca l i fo rn ia region: Seismol. Soc. America Bull . , v . 55, p. 753-797.

Armstrong, 3 . E . , and Brown, W . L . , 1954, Late Wisconsin marine d r i f t a;,d associa ted sediments of t h e lower Fraser Valley, B r i t i s h Columbia, Canada: Geol. Soc. America Bul l . , v. 65, p. 349-364.

Barker, Fred, 1957, Geology of t h e Juneau (B-3) quadrangle, Alaska: U.S. Ceol. Survey Geol. Quad. Map GQ-100.

Barnwell, W. W . , and Boning, C . W . , 1968, Water resources and s u r f i c i a l geology of the blendenhall Valley, Alaska: U.S. Geol. Survey ~ydrol. Inv. At las HA-259, 6 p.

Berg, G. V . , and S t r a t t a , J . L . , 1964, Anchorage and t h e Alaska ear rh- quake of March 27, 1964: New York, Am. I ron and Steel Inst . , 83 p ,

Bjerrum, Laur i t s , and J d r s t a d , F . A . , 1968, S t a b i l i t y of rock s lopes i n Norway: Norwegian Geotech. I n s t . Pub. 79, p. 1-11.

Boucher, Gary, and F i t c h , T. J . , 1969, Microearthquake s e i s m i c i t y of t h e Denali f a u l t : Jou r . Geophys. Research, v. 74, no. 27, p. 6638-6648,

Brew, D. A . , Loney, R. A . , and Muffler, L. J . , 1966, Tectonic h i s t o r y of southeas tern Alaska: Canadian I n s t . Mining and Metallurgy Spec. Vol. 8 , p. 149-170.

Buddington, A. F . , and Chapin, Theodore, 1929, Geology and mineral deposits of southeas tern Alaska: U.S. Geol. Survey Bull . 800, 398 p.

Canada Department o f Energy, Mines and Resources, Seismological Service , 1953, 1955, 1956, 1961-1963, 1966, 1969, 1970 [Canadian earthquakes, 1841-19653: Dominion Observatory, Ottawa Pubs.

Coul ter , H. W . , and Migliaccia, R . R . , 1966, Ef fec t s o f t h e earthquake of March 2 7 , 1964, a t Valdez, Alaska: U.S. Geol. Survey Prof . Paper 542-12, p , C1-C36.

Crandell , D. R . , 1952, Some f e a t u r e s of mudflow depos i t s [abs,]: Geol. Soc. America Bul l . , v. 68, p t . 2 , p. 1821.

Dachnowski-Stokes, A. P . , 1941, Peat resources i n Alaska: U.S. Dept. Agr icul ture Tech. Bull. 769, 84 p.

Davis, T. N . , and Echols, Carol , 1962, A t a b l e o f Alaskan earthquakes, 1788-1961: Alaska Univ. Geophys. Ins t . [Rept. Ser.] UAG-R131 (Geophys . Research Rept . 8) , [44 3 p .

Deevey, E. S., and Flint, R. F., 1957, Pastglacial hypsithsrmal interval: Science, v. 125, p. 182-184.

D r u ~ y , W. H., 1956, Bog flats and physiographic processes in the upper Kuskokwim River region, Alaska: IIarvard Univ., Gray Herbarium Contr. 178, 150 p.

Easterbraok, D. J., 1963, Late Pleistocene glacial events and relative sea-level changes in the northern Puget Lowland, Washington: Geol. Soc. America Bull., v. 74, p. 1465-1484.

Eckel, E. B,, 1967, Effects of the earthquake of March 27, 1964, on the air and water transport, communications, and utilities systems in south-central Alaska: U.S. Geol. Survey Prof. Paper 545-B, p. B1-B27.

Eppley, R. A., 1965, Earthquake history of the United States--Pt. 1, Stronger earthquakes of the United States (exclusive of California and western Nevada): U.S. Coast and Geod. Survey Spec. Pub. 41-1, revised ed. (through 19631, 120 p. Originally published 1938.

Fairbridge, R. W., ed., 1968, The encyclopedia of geomorph~logy-- Encyclopedia of earth sciences series, V. 3: New Yoxk, Reinhold Book Corp., 1295 p.

Flint, R. F., Sanders, J. E., and Rodgers, John, 1960a, Symmictite--a name for nonsorted terrigenous sedimentary rocks that contain a wide range of particle sizes: Geol, Soc, America Bull., v. 71, p. 507-510.

1960b, Diamictite, a substitute term for symmictite: Geol. Soc. America Bull., v. 71, p. 1809-1810.

4 0 Forbes, R. B., and Engels, J. C., 1970, K / ~ r ~ ~ age relations of the Coast Range Batholith and related rocks of the Juneau Ice Field area, Alaska: Geol. Soc, America Bull., v. 81, no. 2, p. 579-584.

Franklet, G. A , , 1963, Materials site investigation--Mendenhall Loop Road alignment, sawmill pit extension: Alaska Dept. Highways, Code 33-004-62, Project No. 5-0966(3), 10 p ,

Franklet, G. A., and Rasmussen, J. D., 1969, Centerline soils and materials investigations--Vanderbilt Hill-Norway Point, Sta. 6+00-Sta. 195+60.95: Alaska Dept. Highways, Eng. Geology Sec. , Materials Div., Project No. F-095-8[3), 27 p.

Franklet, G. S., and Swedell, R. M., Jr., 1969, Centerline soils--Juneau outer drive, Phase 11, Sta. "0" 74+80.00 to Sta. "0" 85+38.40, and Juneau main street, Sta. "E" 11.~00.00 to Sta. "Eft 21+00.00: Alaska Dept, Highways, Eng. Geology Sec., Materials Div,, Project No. F-095-4 (2) (Juneau outer drive), and Project No. U-093-l(2) (Juneau main street), 9 p,

Grantz, Arthur, 1966, S t r i k e - s l i p f a u l t s i n Alaska: U.S. Geol. Survey open- f i l e r e p o r t , 82 p.

Hamilton, Warren, and Myers, W. B . , Cenozoic t e c t o n i c s of t h e Western United S t a t e s : Rev. Geophysics, v . 4, p . 505-549.

Hansen, W. R . , 1965, Ef fec t s o f t h e earthquake of March 27, 1964, a t Anchorage, Alaska: U.S. Geol. Survey Prof . Paper 542-A, p. A1-A68.

Heck, N . H . , 1958, Continental United States and Alaska (exclusive of Ca l i fo rn ia and w e s t e n Nevada), Pt. 1 o f Earthquake h i s t o r y of t h e United S t a t e s : U.S. Coast and Geod. s=vey [Pub.] 41-1, revised ed. (through 19561, 80 p. Revised by R. A. Eppley, 1958; o r i g i n a l l y pub. 1938.

Heusser, C. J . , 1953, Radiocarbon da t ing of t h e thermal maximum i n southeas tern Alaska: Ecology, v. 34, no. 3, p. 637-640.

1960, Late-Pleis tocene environments of North P a c i f i c North America: Geog. Soc. Spec. Pub. 35, 308 p.

Hicks, S. D . , and Shofnos, William, 1965, The determinat ion of land emergence from sea l e v e l observat ions i n southeas tern Alaska: Sour. Geophys. Research, v. 70, no. 14, p. 3315-3320.

In t e rna t iona l Conference Building O f f i c i a l s , 1970, Uniform bu i ld ing code--1970 Edi t ion: In t e rna t . Conf. Bldg. O f f i c i a l s , Pasadena, C a l i f . , v. 1, 651 p.

In t e rna t iona l Seismological Centre, 1967-1970, Regional catalogue o f earthquakes [1964-19661: Edinburgh, Scot land.

Jessen , K., 1949, S tudies i n l a t e Quaternary depos i t s and f l o r a - h i s t o r y of I re land: Royal I r i s h Acad. Proc., no. 52, sec. B, p. 85-290.

Kachadoorian, Reuben, 1965, E f f e c t s of t h e earthquake of March 27, 1964, a t Whi t t ie r , Alaska: U.S. Geol. Survey Prof . Paper 542-B, p. B1-B20.

1968, Effec ts of t h e earthquake of March 27, 1964, on t h e Alaska highway system: U.S. Geol. Survey Prof. Paper 545-C, p. C1-C66.

Knopf, Adolph, 1912, The Eagle River region , southeas tern Alaska: U.S. Geol. Survey Bull, 502, 61 p.

Lawrence, D. B., 1950, Glac ier f l u c t u a t i o n f o r s i x c e n t u r i e s i n south- e a s t e r n Alaska and i t s r e l a t i o n t o s o l a r a c t i v i t y : Rev. Geophysics, v. 40, no. 2 , p. 191-223.

Lemke, R. W., 1967, Effects o f t h e earthquake of March 2 7 , 1964, a t Seward, Alaska: U.S. Geol. Survey Prof . Paper 542-E, p. El-E43.

Lemke, R . 'W., and Yehle, L . A , , 1972a, Regional and o t h e r genera l factors bear ing on eva lua t ion of ear thquake and o t h e r geologic --

hazards t o c o a s t a l comnunities o f sou theas t e rn Alaska: U.S. Gcol. Survey o p e n - f i l e r e p o r t , 99 p.

- 1972b, Reconnaissance engineer ing geology o f t h e Haines a r e a , Alaska, w i t h emphasis on eval.uation of earthquake and o t h e r geologic hazards: U.S. Geol. Survey o p e n - f i l e r e p o r t , 109 p.

Lomi t7 , Chinna, 1967, Time series and ear thquake p r e d i c t i o n , - i n IBhI s c i e n t i f i c computing symposium on environmental s c i ences , Yorktown Heights , N . Y . , 1966, Proc.: Iihite P l a i n s , New York, IBM Data Processing Div., p. 129-141.

Loney, R . S . , Brew, D . A,, and Lanphere, M . A , , 1967, Post-Paleozoic rad iometr ic ages and t h e i r re levance t o f a u l t movements, no r the rn sou theas t e rn Alaska: Geol. Soc. America Bul l . , v . 78, p. 511-526.

Macelwane, J. B . , 1947, \$%en t h e e a r t h quakes: Milwaukee, Wis., Bruce Pub. Co., 288 p.

McCulloch, D. S . , 1966, Sl ide- induced waves, s e i ch ing , and ground f r a c t u r i n g caused by t h e ear thquake o f March 2 7 , 1964, a t Kenai Lake, Alaska: U.S. Geol. Survey Prof . Paper 543-A, p. A1-A41.

McGarr, Arthur , and Vorhis , R. C . , 1968, Seismic se i ches from t h e March 1964 Alaska ear thquake: U.S; Geol. Survey Prof . Paper 544-E, p. El-E43.

Miller, D. J . , 1960, Giant waves i n Lituya Bay, Alaska: U.S. Geol. Survey Prof . Paper 354-C, 86 p.

Miller, Ray D. , and Munson, R . J . , 1962, Ma te r i a l s s i t e inves t iga t ion - - Tee Harbor t o Shr ine Creek paving p r o j e c t : Alaska Dept. Highways, Project No. F-095-4(12), 7 p.

Miller, Robert D . , 1967, P r o f i l e s showing conf igu ra t ion and probable bottom d e p o s i t s a s i n t e r p r e t e d from fathometer t r a v e r s e s ac ros s and along p a r t s o f Gastineau Channel, near Juneau, Alaska: U.S. Geol. Survey o p e n - f i l e r e p o r t , 1 sheet.

Milne, W. G . , 1963, Se i smic i ty of western Canada: Bol. Bibl iog. Geof is ica y Oceanografia Am., v. 3, p t . Geof i s i ca , p . 17-40.

1967, Earthquake e p i c e n t e r s and s t r a i n r e l e a s e i n Canada: Canadian Jour . Eal'h Sci., v. 4 , no. 5, p. 797-814.

Milne, W, G,, and Davenport, A . G . , 1969, D i s t r i b u t i o n o f ear thquake r i s k i n Canada: Seismol. Sac. America Bul l . , v. 59, p. 729-754.

bIorner, Nils-Axel, 1969, Eustatic and climatic changes during the last 15,000 years: Geologie en Mijnbouw, v. 48(4), p. 389-399.

Mullineaux, D. R., and Crandell, D , R., 1962, Recent lahars from Mount St. Helens, Nashington: Geol. Soc. America Bull., v. 73, p. 855-870.

Munson, R. J., 1961a, Borrow pit investigation--vicinity of Auke Bay Lumber Company sawmill, blendenhall Loop Road: Alaska Dept, High- ways, Project No. 0966(3), 12 p.

1961b, blaterials investigation--blendenhall Loop Road: Alaska Dept . Highways, Project No. 0966 (33, 39 p .

1962, Materials site investigation--Steep Creek bank protec- tion, blendenhall Glacier area: Alaska Dept, Highways, Project No. 0968(2), 4 p.

1963, Centerline soils and materials sites investigation-- North Douglas Road north 3.5 miles, Kowee Creek to Sta. 191+58.9: Alaska Dept. Highways, Project No. S-0959(2), 43 p.

1964, Centerline soils and materials site investigation-- North Douglas Road, Sta . 191+38.S-Sta. 428+95.2: Alaska Dept. Highways, Project No. S-0959 (2), 63 p ,

Munson, R. J., and Franklet, G. A . , 1963a, Centerline soils and materials site investigation--Douglas urban: Alaska Dept. Highways, Project NO. F-095-8(2), 27 p.

1963b, Centerline soils and materials site investigation-- Brotherhood Bridge approaches, Glacier Highway at Mendenhall River: Alaska Dept. Highways, Project No. F-095-4 (Is), 19 p .

Munson, R. J., and Rasmussen, J. D., 1965, Centerline soils and marerials sites investigation--Juneau outer drive, phase 11: Alaska Dept. Highways, Project No. F-095-4(2), 19 p ,

1966, Centerline soils and materials sites investigation-- Glacier Highway, Smith's Dairy-Brotherhood Bridge, Sta. 352+49.36- Sta. 421+93.58: Alaska Dept. Highways, Project No. F-095-8(3), 22 p.

Munson, R. J., and Slater, W. H., 1963, Centerline soils and materials sites investigation--Juneau outer drive, Phase I: Alaska Dept. Highways, Project No. F-095-4 (2), 39 p.

'1964, Centerline soils and materials sites investigation-- Juneau outer drive, Phase 11: Alaska Dept. Highways, Project No. F-095-4 (2), 23 p.

Page, Robert, 1969, Late Cenozoic movement on the Fairweather fault in southeastern Alaska: Geol. Soc; America Bull., v. 80, p. 1873-1878.

Pla fke r , George, 1962, Geologic i n v e s t i g a t i o n s of proposed power-s i tes a t Sheep Creek, Carlson Creek, and Turner Lake, Alaska: U.S. Geol. Survey Bull . 1031-F, p. 127-138.

1969, Tectonics o f t h e ?larch 27 , 1964, Alaska earthquake: U.S. Geol. Survey Prof . Paper 543-1, 174 p .

Richter , C . F . , 1958, Elementary seismology: San Franc isco , C a l i f . , W. H . Freeman and Co., 768 p .

Rich te r , D . H . , and 31atson, N. A . , J r . , 1971, Qcaternary f a u l t i n g i n t h e e a s t e r n Alaska Range: Geol. Soc. America Bul l . , v. 8 2 , p. 1529-i540.

Rigg, G . B . , 1937, Some r a i s e d bogs o f southeas te rn Alaska wi th no te s on f l a t bogs and muskegs: Am. Jou r . Botany, v. 24, p. 194-198.

Rogers, G . C . , 1972, A microearthquake survey i n northwest B r i t i s h Columbia and sou theas t Alaska: Geol. Soc. America Abs t r ac t s wi th Programs, v. 4 , no. 3, p. 226.

Sainsbury, C. L . , 1953, Geology of t h e btount Olds-Clark Peak a r e a , Juneau v i c i n i t y , Alaska: Colorado Univ., unpub. t h e s i s , 48 p.

Sainsbury, C. L . , and Twenhofel, W. S. , 1954, Fau l t p a t t e r n s i n south- e a s t e r n Alaska [ a b s . ] : Geol. Soc. America Bul l . , v. 65, no. 12, p. 1300.

S t . Amand, Pierre, 1954, Tec tonics o f Alaska as deduced from seismic data [abs.]: Geol. Soc. America Bul l . , v. 65, p. 1350.

1957, Geological and geophysical syn thes i s o f t h e t e c t o n i c s of p o r t i o n s of B r i t i s h Columbia, t h e Yukon T e r r i t o r y , and Alaska: Geol. Soc. America Bul l . , v. 68, no. 10, p. 1343-1370.

Seed, E. B. , 1964, S o i l engineer ing problems: Earthquake and Geologic Hazards Conf., San Francisco, C a l i f . , Dec. 7-8, 1964, C a l i f o r n i a Resources Agency, p . 37-43.

Slater, W. H . , and Grahek, bl. E . , 1970, Foundation r e p o r t , Lemon Creek bridge no. 1197: Alaska Dept. Highways, Eng. Geology Sec. , Ma te r i a l s Div., P ro j ec t No. F-095-8(3), 4 p .

S l a t e r , W. H. , and Pa lcze r , T. L . , 1970, Foundation r e p o r t , Salmon Creek bridge no. 1188: Alaska Dept. Ilighways, Eng. Geology Sec., Materials Div,, P r o j e c t No. F-095-8(3), 4 p.

Smith, R. K . , 1970, Late g l a c i a l fo ramin i f e ra from sou theas t Alaska and British Columbia and a worldwide h igh no r the rn l a t i t u d e shallow- water faunal province: Archives Sci . , v. 23, f a s c i c u l e 3 , p, 675-701.

Spaeth, M. G . , and Berkman, S. C . , 1967, The tsunami o f March 28, 1964, as recorded a t t i d e s t a t i o n s : U.S. Coast and Geod, Survey Tech. Bull. 33, 86 p.

Spencer, A, C . , 1306, The Juneau gold b e l t , Alaska: U.S. Geol. Survey Bull, 287, 161 p.

Stokes, W. L . , and Varnes, D. J . , 1955, Glossary o f se lec ted geologic terms: Colorado Sc i . Soc. Proc., v. 16, 165 p.

a

Swanston, D . N . , 1970, Mechanics o f debr i s avalanching i n shallow till s o i l s of southeast Alaska: U.S. Dept, Agriculture, Forest Service Research Paper PNW-103, 1 7 p.

Tarr , R. S . , and Martin, Lawrence, 1912, The earthquakes a t Yakutat Bay, Alaska, i n September 1899: U.S. Geol, Survey Prof. Paper 69, 135 p.

Tobin, D . G . , and Sykes, L . R . , 1968, Seismici ty and t e c t o n i c s o f t h e nor theas t P a c i f i c Ocean: Jou r . Geophys. Research, v. 73, no. 1 2 , p. 3821-3846.

Tocher, Don, and h l i l l e r , D . J., 1959, Fie ld observations on e f f e c t s o f Alaska earthquake o f 10 J u l y 1958: Science, v . 129, no. 3346, p. 394-59s.

Twenhofel, W. S. , and Sainsbury, C. L . , 1958, Fault p a t t e r n s i n southeastern Alaska: Geol, Soc. America Bull. , v. 69, no, 11, p. 1431-1442.

U.S. Coast and Geodetic Survey, 1930-1969, United S t a t e s earthquakes [annual volumes f o r t h e years 1928-19671.

1964-1970, Preliminary determination o f ep icen te r s : Monthly Listing, Jan. 1964-Dec. 1969.

1966, P r inc ip les underlying the i n t e r p r e t a t i o n of seismograms: Spec; Pub. 254, 50 p.

1969, Hypocenter data f i l e [computer p r in tou t shee t s f o r the period January 1961-July 1969 covering l a t 4a0-750 N . , long 12O0- 14s0 W.].

U,S. Weather Bureau, 1918-1958, Climatological da ta , Alaska sec t ion [monthly] 1917-1957.

Varnes, D. J., 1958, Landslide types and processes, Chap. 3 oE Eckel, E. B., ed., Landslides and engineering p r a c t i c e : Natl. Research Council, Highway Research Board, Spec. Rept. 29 , p . 20-45.

Von Hake, C. A . , and Cloud, W. K . , 1966, United S t a t e s earthquakes, 1964: U.S. Coast and Geod. Survey, 91 p.

IYallcr. I?. > I . , 19.59, Sun!l:iary o f t e s r - d r i l l i n g resu l t s i n Last Chance Basin, Juncau, Alashn: .113ska Dept. Iicalth, San i t a t ion and Eng. Sec . , Ifater I iyc l ru l . Data, no. 8, 1 7 p . ; U.S. Geol. Survey open-fi le r e p o r t , 25 p .

Kallc17, 11. .If., 1366, Effects o f t h e ?I:irch 1964 Alaska earthquake on t h c hydrology of south-cent ra l A l a s k a : U.S. Geol. Survey Prof . Paper 544-.4, p . A1-A28.

Jliilson, B. IV., and Tdrum . \ l f , 1968, The tsunami o f t h e Alaskan ear thquake, 1964, engineering evalua t ion: U.S. Army Corps of Engineers, Coas ta l Eng. Itcscarch Ccntcr, Tech. Ilen. no. 25, 401 p.

I,OOC:, t . . J . , 1 - 3 u , 1112 Fr lncc : , x i l l L . . >au,ld, i113~'r;a, e a r t h q u a k e o f 1964 and :,<;orshocks: U.S. Coast and Gcod. Survey, Pub. 10-3, v . 1 , 263 p.

Kood, t i . 0 . , and Xcucann, Frr?n!;, 1931, X s d i f i e d l \ le rca l l i i n t e n s i t y sca le o f 1931: Seismol. Soc. America Bul l . , v. 21 , p. 277-283.

hrri g h t , F . E . , and Wright , C. W. , 1905, The Ketchikan and iyrangell mining d i s t r i c t s , Alaska: U.S. Geol. Survey Bull. 347, 210 p.

F i g u ~ c 1 . - - l , o c n t i o r ~ o f s e l e c t e c l f n a j n r f a u l t s i n s o u t h e a s t e r n Al r i sha tha t : m i g h t c a u s e ea~- thc!uakcs t h a t c o u l d a f f e c t t h e ,Juneau a r e a . F a u l t l o c a t i o n s from 'l'wenhofel and Sainsbul -y l l Y S 8 ) , Tobirl and Sykcs

1 (190S), and K i c h t e r and ) l n t son (1971) .

~inlrnum - about 98 miles F i g u r e 3. --Locat ion of epi c e n t e r s and approximate magnitude o f ea r th -

quakes in southeastern A l a s k a and adjacent regions for the p e r i o d 1899- 1969 i n c l u s i v e . (Data f lhorn Davis and Echols , 1962; Tobin and Sykes, 1968; U.S. Coast and Ceo~lc t ic Survey; Canada D e p t . Energy, Mines, and Resources; 2nd l n t e r n a t l . Sei s~nol . ~ e n ' t r e . ) Campiled and drawn by L. A . Yehle (Lemke and Yehlc,1972a). 1 Admi r l l l t y I s l a n d , 2 Berners Bay.

Datesand magnitudesof e m earthquakes of magnitude 2 6

Designation Date Magnitude on msp (Universal Time)

September 4, 1899 September 10, 1899 September 10, 1899 October 9, 1900 Hay 15, 1908

July 7 , 1920 A p r i l 10, 1921 October 2 4 , 1927 February 3 , 1944 August 3, 1945

February 28, 1948 August 22, 1949 October 31, 1949 March 9, 1952 November 17, 1956 July 10, 1958

ZONE 1 - Hinor damage: d i s t a n t earthquakes may cause damage t o structures w i t h fundamental periods greater than 1.0second; corresponds to inten- s i t i e s V and V I o f the HM* Scale

ZONE 2 - Moderate damage: corresponds t o i n t e n s i t y V I I o f the MH* Scale

ZONE 3 - Major damage: corresponds t o in tens i ty V l l l and higher o f the MM* Scale

*Modified Mercal l i I n t e n s i t y Scale o f 1931

J I;igui-e 4.--Seismi: r d n e map of A laska . blodifiecl from 1

t h e 1970 cd t i o n of t h e Uniform Building Code [ I n t e r n a t i o n 1 Conference o f Uui l d i n g Officials, , 1970) . . \ f t e r I.emkc and Yehlc ( 1 9 7 2 n ) . i

E X P U T I O N Hodif ied from deecr ipt ions accompanying the r e f d c

probabil i ty map for A l a e k a by U. S. A m y , Cotpo of Engineere, Alaska District; map prepared tn 1937 and rev-ed in 1965 (Warren George, written comp~m., 1968; 1971).

Minor 3.0-4,5 Moderate 4.5-6.0

> 6.W

*Largest inrtmmented earthquakes of the world

B Haines C Hoonah D S i t k a E Petersburg F Wrangell G Ketchikan

_- H Metlakatla

After Lemke and Yehle {197xa]

Figure 5.--Seismic probab*Tl~ map for most of Alaska as modified from U.S. Army Corps of Engineers, Alaska Dis tr ic t .

.a Channel

Figure 6.--Sketch showing r e l a t i o n s h i p s of f o l i a t i o n and l a y e r i n g and j o i n t s e t s t o each o t h e r in t h e mountain slope above the Gastineau Axrenue- F r a n k l i n Street area , Juneau , Alaska. The numerous f r a c t u r e s shown t e n d t o weaken t h e bedrock s l o p e and t o p rov ide access r o u t e s f o r ground water and r o o t s . Tree r o o t s can push t h e rock pieces apa r t and increase t h e instability of t h e mountainside,

Figure 8.--Diagrammatic s k e t c h e s showing t h e relationships berween fans , deltas, and f a n d e l t a s ,

First Phase

w Ancestral Gastintau Channel E

&n+lrrb aso I c v r l a+ s t a r t 04 d. o*r f rmw bnd - - I - - --.-f .--a . L- .* 'A- -

Figure 10.--Diagrammatic sketches showing i n t e r p r e t a t i o n s of manner'of glaciomarine d e p o s i t i o n and accumtilat ion o f r e l a t e d deposits during t h e t h r e e d e p o s i t i o n a l phases of the glac ion ia r ine depos i t s in t h e Juneau, Alaska, area. Qmb, glaciomarine deposits, firsr phase; Qms, glaciomarine deposi ts , second phase; Qme, glaciomarine deposits, third phase; Qdo, o l d e r r a i sed d e l t a ; Qbe, a l d e r raised beach deposits (thin and cont inuous) ; d, diamicton layer on t o p of delta depasi t s ,

86

' . ,. . . \

- , I . . , ...

. . DEPARTMENT. OF TYE, INT;ERIOR . I ._ . : . . . ,

I ' ' OPEN FILE UNITED STATES ,GEOLOGI'CAL SURVEY . a .. ,1972

, U.S. . , Geological Survey

OPEN F I L E ~EPOR.T

+ ' . * (u ' E " : , This. i l l u s . t r a t i o n ,is p re l im ina ry L b \t .E i , &d has no t been e&trpd ar * Y E . 0 . , ,,. . 1 ,r. . .

' Sr reviewed <or -conformity ' w i th

" "3 ! j \ S r J . ,

\ /. " . . \ I Geologica l , Survey s t a n d a r d s .

Sand 1' * . .

SIZE RANGE OF CLAY, .SILT, SAND, AND GRAVEL '

. C 1 ay <0. 004' Am ' v e r y f i n e 0.0625-0.125 mm S i l t 0,004-0.0625 mm f i n e 0.125-0.25 m m 9 Sand 0.0625-2.0 mm Sand medium ' 0.25-0.5 mm Gravel 2.0-32 mm c o a r s e 0.5-1.0 mm

. , v e r y c o a r s e 1.0-2.0.mm

Table 4.--Average p e r c e n t a g e o f p a l - t i c l e s p a s s i n g 14-inch s c r e e n from s e l e c t e d g e o l o g i c u n i t s . U n i t s Qme and Qmb: a r e composite res i i l t ' s of 10-15 k a m ~ l e i ; o t h e r u n i t s a r e composite r e s u l t s o f 1-5 samples .

i united states department of the interior...

Documents