RECENT VOLCANISM NEAR DOTSERO, COLORADO

by

Robert F. Giegengack, Jr.

B.A., Yale University, 1960

A Thesis submitted to the faculty of the Graduate

School of the University of Colorado in Partial

fulfillment of the requirements for the Degree

Master of Science

Department of Geology

1962

i

Tliis Thesis for the M.S. degree by

Robert F. Glegengack, Jr.

has been approved for the

Department of Geology

by

C

C.

Date TV./.,., >r -

Gicgengack, Robert F. Jr. (M.S., Geology)

Recent Volcanism near Dotsero, Colorado

Thesis directed by Professor Warren 0. Thompson

The Dotsero Crater and associated volcanic rocks are located a

mile northeast of the junction of the Eagle and Colorado rivers at

Dotsero, Colorado. The area is underlain by Pennsylvanian and Permian

sedimentary rocks of the Minturn and Maroon formations in which simple

structures locally are truncated by an erosion_surface covered with a

series of Tertiarylaya flows.

One mile east of Dotsero, the Eagle River flood plain is overlain

and partially blocked by a younger basalt flow. The lava seems to have

issued from the vicinity of a deep, steep-sided explosion crater a mile

north of the Eagle River. This feature, known locally as the Blowout,

is developed principally in bedrock. The floor of the depression and

the area immediately surrounding it are covered with a thick mantle of

volcanic ash, which thins rapidly away from the source, locally lying

on the lava surface.

A charcoal sample, recovered from a tree which had been buried by

the falling ash, was dated by the Cl4 method, and found to be 4150 *

300 years old. The DotseroC_rater is thus the most recent documented

sou^xeoJ_jvo_Lcajiic_rocks in the state of Colorado.

The ash is recognizable as a distinct stratum in many of the flu-

vial deposits of the Eagle River and its tributaries; this radiocarbon

date may prove useful in dating post-Wisconsin periods of alluviation

and canyon- cut ting in the local watershed.

This abstract of about 250 words is approved as to form and content.I reconmend its publication.

S igned _ / a.-c-l c - &Instructor in charge of dissertation

5

CONTENTS

INTRODUCTION Page 1

ACKNOWLEDGEMENTS

TOPOGRAPHY AND DRAINAGE

PREVIOUS STUDY

STRATIGRAPHY OF THE PRE-QUATERNARY ROCKS

Minturn Formation

Maroon Formation

Unnamed Tertiary Rocks

STRUCTURE OF PRE-QUATERNARY ROCKS

STRATIGRAPHY AND STRUCTURE OF QUATERNARY ROCKS

Intrus ive Rocks

Extrusive Rocks

Lava Flow

Pyroclastic Rocks

Consolidated Lapilli Tuff

Lapilli Tuff

DATING OF THE QUATERNARY VOLCANIC ROCKS

INTERPRETATION

ECONOMIC GEOLOGY

BIBLIOGRAPHY

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ILLUSTRATIONS

Figure 1. Index Map Page 2

2. Vertical air photograph 5

3. View into crater 5

4. Dip slope of salt anticline 12

5. Sill intruding Maroon sandstone 15

6. Dike in Maroon sandstone 15

7. View north from southeast rin of Blowout 17

8. Lava outcrop on north wall of Blowout 18

9. View of lava flow surface 20

10. View of lava flow surface 20

11. Leading edge of basalt flow 21

12. Outcrop of consolidated lapilli tuff 23

13. Consolidated lapilli tuff 25

14. Consolidated lapilli tuff 25

15. View southwest from northeast rim of Blowout .... 26

16. Lapilli tuff outcrop 29

17. Lapilli tuff outcrop 29

18. Bomb sag in lapilli tuff 31

19. Bomb sag in lapilli tuff 31

20. Volcanic ash in alluvial fan 34

21. Volcanic ash in alluvial fan 34

22. Waterworked volcanic ejecta 36

Plate 1. Geologic Map of Dotsero Region In Pocket

2. Geologic Map of the Dotsero Crater and LavaF low In Pocket

3. Stratigraphic Relationships ofPre-Quaternary Rocks

4. Generalized Structure across the DotseroRegion 13

I

INTRODUCTION

The Dotsero Crater and associated volcanic rocks are located

mediately northeast of the junction of the Eagle and Colorado rivers at

Dotsero in Eagle County, Colorado. Route U.S. 6 & 24 crosses the lava

flow 3 miles east of the point where the road emerges from the upper

end of Glenwood Canyon.

The crater and adjacent area were napped in August and early Septem-

ber of 1961. Most of the field work was limited to the triangle of

land defined on the south by the Eagle River, on the northwest by the

Colorado River, and by Trail Gulch to the east. Field data recorded on

aerial photographs later were transferred to a base map, which consists

of a 1:31680 U.S. Forest Service sheet enlarged to 1:15840 by means of

a Saltzmami projector. A more detailed, large-scale map of the crater

and Recent volcanic rocks was prepared from the photographs.

The bedrock geology, while not essential to an understanding of

the volcanism, was studied to orient the crater accurately with respect

to regional stratigraphy and structure.

A cursory petrographic study of the volcanic rocks was undertaken.

ACKNOWLEDGEMENTS

The writer is indebted to Dr. Warren 0. Thompson, who suggested

the project and supervised the research, and to Dr. William C. Bradley,

who also served as advisor. Meyer Rubin, of the Radiocarbon Laboratory

of the U.S. Geological Survey in Washington, D.C. provided the radio-

carbon date. Henry E. Holt spent a day in the field with the writer in

June 1961 and offered much helpful advice. Appreciation is extended to

those members of the faculty and student body at the University of

C O L O R A D O

MILES!

BLOWOUT

Fig. 1. Index map of the Dotsero region.

• "

I

Colorado whose interest, curiosity, and many suggestions inspired much

of vhat appears herein.

TOPOGRAPHY AND BRASSAGE

The drainage pattern in the area studied is controlled by a salt-

cored anticline breached by the Eagle River, which flours along the fo

axis. North of the river, drainage is consequent on the dip slope of

the north limb of the anticline. Host of this water finds its way to

the Colorado River to the north and northwest; in at least one instance

headvard growth of a short, steep tributary of the Eagle River has cap-

tured a. portion of the Colorado drainage.

All of the streams in the area are intermittent, with the excep-

tion of the Eagle and Colorado rivers, both through streams which derive

touch of their volume from the Cora and Park ranges to the east, and

Deep and Sweetwater creeks, tributaries of the Colorado, which drain

areas of the White River Uplift on the northwest. Gypsum Creek, which

occupies a broad, flat valley in the southeast part of the area, flows•

off the north side of Red Table Mountain and enters the Eagle River at

Gypsum.

The Dotaero Crater occupies a short, steep gully tributary to the

Eagle River a mile northeast of Dotsero. The gulch can be traced up-

stream beyond the crater, but the explosion that formed the depression

effectively blocked any through drainage. Locally known as the Blowout,

this feature is an oval-shaped, conical depression 2600 feet long and

2000 feet vide. The nearly flat floor, composed of fragments of vol-

canic and country rock, lies 400-600 feet below the rim. The crater is

developed principally in bedrock; the sides of the Blowout slope inward

at 30-33°, apparently the angle of repose of the pyroclastic fragments,

except where cliffs of sedimentary rock have steepened the slope of the

walls. Such angle of repose slopes are very unstable, and minor slides

iJr

1

Figs. 2 and 3. Left: Vertical air photo of the Dotsero Graterand associated "lava flow. Right: View into crater, lookingsouthwest.

are of almost daily occurrence. Vegetation on the crater vails is

sparse and, except in cases where it is firmly anchored to bedrock,

seems unable to contend with the constant sliding.

PREVIOUS STUDY

The Blowout and associated lava flow were briefly described by

R. E. Landon in 1933. The existence of the crater had been documented

by van Diest in 1888 and by Arthur Lakes in 1890. Lakes' account des-

cribee a day's excursion to the site, and includes several drawings and

an accurate discussion of the distribution of the volcanic rocks. M. R.

Campbell in 1922 noted the existence of the lava flow; he also reported

the presence of a "dark hill" a short distance north of the flow and

visible from the river, which he took to be the source of the lava.

Mention of the Lava flow and associated vent appears in Guide to the

Geology of_ Colorado 1960 as an entry in the road log of the West Central

Colorado field trip. Commentary is by N. Wood Bass. The crater is on

the Glenwood Springs 1-degree quadrangle, which was mapped by members

of the U.S. Geological Survey during the years 1946-1953*. The Blowout

is located on the southeast quadrant of the sheet, due to appear some-

time in 1962.

STRATIGRAPHY OF THE PRE-QUATERNARY ROCKS

This part of the Central Colorado Trough is underlain by Penn-

sylvanian and Permian sedimentary rocks of the Minturn and Maroon

formations in which simple structures locally are truncated by an ero-

sion surface which is covered with a series of Tertiary lava flows.

. Wood Bass, personal communication, 1962.

PLATE 3: STRATI GRAPHIC REIATIONSHIPS°f** PRE-QUATERNARY ROCKS

7" GGAVFLS

8

Hlnturn Formation

The Mlnturn formation consists of a series of interbedded silty

sandstones, gypsiferous siltstones and shales, and pure gypsum strata.

Mica flakes, chiefly muscovite, are present In abundance and give cer-

tain of the bedding planes a characteristic metallic glitter. Large,

veil-developed crystals of selenite occur in the lower part of the se-

quence. The section is a uniform ivory to light tan color; a heavy

rain darkens the rock, as much water seems to be absorbed by the gypsum,»•

but a day or two of sunlight evaporates the moisture and bleaches the

rock once again.

The Minturn formation is not very resistant to erosion, being ex-

tremely soft, relatively soluble, and readily disaggregated; hence it

cannot support much topographic relief. It forms prominent features

only when held up by overlying, more resistant strata, as in the cliffs

along the Eagle and Colorado rivers.

Toward the top of the section, Individual strata of sandstone and

siltstone, a dark brick-red In color, occur with increasing frequency,

grading upward into the even-bedded silty sandstone redbeds of the

Maroon formation. The Jacque Mountain limestone, which serves in other

areas to define the Minturn-Haroon boundary, is not present in this sec-

tion. As a result, the contact was defined as the top of the transition

zone, where the color changes subtly from dark brick-red to a more

brilliant red-orange. This demarcation is not readily discernible on

the outcrop; the subtlety of transition is best appreciated with the

perspective of distance, and was mapped from the other side of the can-

yon.

The writer does not propose that such a contact has any great time

or lithologic significance; it is, however, a horizon that can be recog-

nized in other nearby areas and may serve in crude correlation.

Maroon Formation

The Maroon formation is a series of bright red sandstones and silt-

stones containing layers of silty red shale. Hear the top of the sec-

tion a single horizon of gray-green crystalline limestone, a foot or so

in thickness, is exposed. The mica flakes, so striking in the Mintum,

are present in the Maroon formation as well, but do not seen to comprise

as large a percentage of the rock. In this locality, the Maroon dif-

fers from the Minturn formation principally in its tmifonn red color

and its lack of gypsum.

Large-scale cross-bedding is evident in some exposures of the

Maroon formation, and certain of the strata exhibit fine, contorted

laminae. No fossils were observed in these formations within the mapped

area.

Unnamed Tertiary Rocks

Simple structures in the sedimentary section are truncated by an

erosion surface locally veneered witih well-rounded stream gravels.

Particles are of pebble to cobble size and predominantly of granitic

and metamorphic composition. This surface at one time oust have been

more extensive than it is now, as the nearest source of such material

is the Park and Gore ranges Similar erosion surfaces

«< b strej3BB_dgPoaits have begnrecognized under_middle Tertiary- v

lava flows at many localities in Colorado, even as far away as the San

10

Juan Mountains*. This deposit may fall within the interval of the

Browna' ark formation, a Miocene (?) sequence of interbedded stream

gravels and lava flows which is a prominent member of the Tertiary sec-

tion exposed in the Flattops area <R. Kucera, 1962). A similar gravel-

bearing surface beneath a Tertiary lava flow has been described by

P. B. Schmidt (1961).

In the Dotsero area this erosion surface is overlain and preserved

by a aeries of basaltic lava flows 35-50 feet thick. The lava holds upj *

the cliff on the north aide of the Eagle River and, where not resting

on gravel, lies with angular unconformity on a gently deformed surface

of Hinturn and Maroon sediments. Very little of the original structure

of the flow is preserved; the upper surface has been removed by erosion

and weathering has destroyed any large-scale structure. Bo distinctive

joint pattern was observed. Exposed sections of the lava are weathering

by exfoliation of spheroidal boulders which, with angular, frost-wedged

chunks of relatively fresh basalt, litter the slopes below the outcrop.

Some such talus slopes include rounded cobbles weathered out from the

erosion surface beneath the basalt.

If the erosion surface is time-equivalent to other such pre-basalt

gravels in western Colorado and belongs in the Browns Park interval,

the Tertiary section in the Dotsero area is Miocene (?) in age. Dormer

(1949) describes a similar occurrence in the McCoy area and assigns a

Miocene age to the sequence on the basis of a fossil dog found in a

volcanic tuff and described by J. A. Wilson.

Kft

*Wilbur S. Burbank, personal communication, 1961.

SYBDCTDBE OS PEE-QUATEBBAEY ROCKS

Small-scale, complex convolutions, consisting of oft-repeated, in

places quite symmetrical synclines and anticlines, were observed in

highly gypsiferous zones of the Minturn formation. Such structure is

not expressed in the overlying strata, and is interpreted as a result

of gypsum flovage, perhaps induced by differential pressure caused by

removal of part of the overburden by erosion.

,As described above, the Eagle River runs along the axis of an. anti-

cline in the sedimentary rocks. The north limb of the anticline dips

away from the river at 13-15° and is capped with remnants of the

Tertiary lava flow on vhich the dip slope la preserved. The sooth limb

has a more gentle dip, rangfag from 4 to 7°. In the absence of lava

immediately south of die Eagle River little topographic expression of

the fold is seen. The anticline seems to be genetically related to

flowage of gypsum. It becomes less well-defined as it is traced up-

stream along the Eagle River, and disappears entirely near the Gore

Range where the Minturn formation undergoes a facies change from evapo-

ritic to detrital material. Much, if not all of the movement has taken

place since the Miocene (?), as the Tertiary baaalt is tilted as steeply

as any stratum involved in the fold near Dotsero. The solution of the

question as to whether the anticline predates the Eagle River or was

formed by the swelling of anhydrite due to absorption of river water is

beyond the scope of this paper.

The presence of a north-south trending fault Just west of Trail

Gulch is inferred from the abrupt change in elevation of the basalt

outcrops.

12

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Fig. 4. Dip slope of salt anticline preserved onsurtace of Tertiary lava flow. View west along thenorth limb of the fold.

I

PLATE 4: GENERALIZED STRUCTURE*- DOT5ERO REGIONo'cross

f'"*-V

CONTACTMAZOON

An east-dipping monocline lies along the Colorado River north of

Dotsero, and a broad, gentle swell in the sediments which parallels the

course of Trail Gulch projects up the valley of Gypsum Creek'. This

arch is not readily discerned from the attitude of the highly contorted

beds in the Mintura formation, but becomes evident from a projection of

the pre-basalt surface and coasideratioa of regional structure. These

broad structural trends are diagramed schematically ia plate 4.

STBAHCRAPHY AHD 3HEUCTDEE Of QOAIEHHMT BOCKSi "

Superimposed upon the basenent of earlier rocks is a sequence of

Quaternary deposits, consisting of stream alluvium interbedded with a

small, thin lava flow and overlain by a layer of volcanic ash. A smalt

intrusive body which crops out within the Blowout seems to be related

to the Quaternary volcanism.

Intrusive Rocks

High on the north vail of the Blowout a small intrusive body is ex-

posed. This pluton consists of a sill 3 to 5 feet thick and 60 feet

long connected to a dike 18 inches wide which rises into the Maroon for-

mation to a point 20-25 feet above the top of the sill. The outcrop

terminates in the country rock and does not seem to be connected to the

other volcanic rocks, at least in the plane of the exposure.

The rock is a uniform steel blue in color and is finely vesicular

throughout. Petrographic examination discloses phenocrysts of olivine,

clinopyroxene, and magnetite, and microlitic laths of plagiocLase, held

in a very fine groundmass. The olivine is Fa24> or chrysotile; the

plagioclase was identified as bytownite, Any^. The phenocrysts are

quite unaltered and present a fresh, euhedral appearance, with the

15

1a

Fig. 5. Sill intruding Maroon sandstone in northwall of Blowout.

Fig. 6. Dike connected with sill in Maroon sand-stone. Exposure is in north wall of Blowout,(see Fig. 5.)

'fi

16

V-

exception of the magnetite, a small amount of which has altered to hema-

tite. A thin surface coating of limonite was observed in areas of the

outcrop. Following Wahlstrom (1955), this rock is termed an olivine

basalt.

Extrusive Rocks

Lava Flow

The lava flow is spread out on the Eagle River flood plain, where

it attains a thickness of 35 feet. It seems to have issued from the

mouth of a steep, narrow gulch in the north wall of the Eagle River

valley. Remnants of basalt plastered against the gully walls enable

the flow to be traced up the gulch to its head, where the basalt rem-

nants disappear into the end of the gulch which is here composed of

volcanic ash. In a direct line with the head of this gulch and about

100 feet farther north, the outcrop of the lava can be picked up again

where it is exposed in the south wall of the Blowout. Here the config-

uration of the lava outcrop is V-shaped, defining the form of a pre-lava

gulch which was continuous with the gully to the south Just described.

High on the north wall of the crater a much smaller, similarly V-shaped

lava body is exposed. The lava can be traced a few hundred feet north

of this outcrop, where it seems to feather out. The basalt flow is

everywhere in contact with sedimentary rock.

The distribution of lava seems to define the shape and course of a

pre-lava gulch, tributary to the Eagle River, that was filled and lined

with lava at the time of the eruption and served to funnel a quantity of

the molten basalt out onto the Eagle River flood plain. The lava proba-

bly issued from a vent in the bottom of the gulch somewhere within the

I

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17

1

Fig. 7. Vieu north from southeast rim of Blowout.Note lava "V" overlying red sandstone. Basalt in-trusion can be seen halfway up north wall.

18

Fig. 8. Lava outcrop on north wall of Blowout.

r 19

limits of the present Blowout. The intrusive body described above may

have been connected to this conduit. The lava briefly filled die gulch,

allowing a small amount to ooze upvalley from the vent; the balance of

the molten fluid ran down the canyon through a V-shaped funnel of basalt

which adhered to the gully walls. The lava flowed across an alluvial

fan at the mouth of the gulch and out onto the flood plain. That the

gulch was filled with lava at one time is evidenced by the depth of the

V in the south wall of the crater aad die height of basalt remnants

plastered against the valley walla. It seems likely that the eruption

was of the non-violent, fissure type.

Petrographically, the flow is identical with the intrusive basalt

described above.

The surface of the lava flow, both in exposures in the gulch and

out on the flood plain* is highly vesicular. The scoriaceous zone is

as much as 8-10 feet deep and is quite fresh in appearance. A central

zone of dense, finely vesicular blue-gray lava overlies a basal layer

of blocky, fractured basalt containing inclusions of country rock. Much

of the vesicular surface is littered with loose blocks of scoria which

shift and roll when stepped on and produce a metallic clinking sound

when struck together. The vesicular surfaces are fresh and sharp

enough to destroy a pair of shoes in short order. While not readily

visible from the ground, pressure ridges concentric about the mouth of

the gulch can be observed on air photos. Large upended blocks of lava

sitting on the flow surface provide 15-20 feet of relief. The appear-

ance of the flow is typical of so-called "aa" lavas described from

Recent and historic eruptions in Hawaii (ffentworth and MacDonald 1953).

mih-g

Figs. 9 and 10. 2 views of the lava flow surface. Left: lookingeast up valley of Eagle River. Right: view across flow to thenorth. Gypsiferous hills, mantled with volcanic ash, appear in thebackground.

jj VI r"-*-* -• •'" "~ " "*- '-"i'r" "-T"

21

Fig. 11. Leading edge of basalt flow being undercutby Eagle River. Note depth of vesicular zone. Thinstringers of volcanic ash appear on gypsiferous hillsin background.

The lava flow may have pushed the Eagle River over to the south

side of its valley, but there is no evidence that it ever dammed the

river. No lava was found in place on the south bank. Material as loose

and poorly consolidated as the lava flow could easily have been removed

by the river; the sheer volume of the flow, however, probably was effec-

tive in crowding the river out of its channel.

The river has cut into the periphery of the lava front; fresh,

crystalline, blue-gray basalt is exposed at the river's edge and it is

here that an estimate of the depth of the vesicular zone can be made.

The leading edge of the lava is under water; apparently some alluviation

has occurred since the extrusion.

The lava flow is overlain by an alluvial fan composed of material

carried from the gulch down which the lava flowed. Much of the fan con-

sists of reworked boulders of lava derived from the V-shaped lining of

the gully; near the top of the section and close to the mouth of the

gulch the percentage of sedimentary rock particles is greater. The

intermittent stream that occupies the gulch now has cut through the lava

along much of its length and at present is downcutting in the sediments.

Pyroclastic Rocks

Consolidated Lap i 1 1 i Tuff Where the V-shaped lava body crops out in the

south wall of the Blowout, the basalt is overlain by a section of bedded,

consolidated volcaniclastic rock. This material is quite heterogeneous;

it contains lapilli-sized fragments of pyroclastic basalt and pieces of

country rock of both the Minturn and Maroon formations, which range

from large angular boulders and cobbles to rounded, pellet-shaped

pebbles of the sediments. It may also contain fragments of the basal-

tic lava, impossible to distinguish from the pyroclastic particles.

22I!:

It

ft

23

Fig. 12. Close-up view of outcrop of consolidatedlapilli tuff. The texture of the lower part of thesequence in this photograph is reminiscent of theappearance of the overlying lapilli tuff. Note thewell-rounded cobble of Maroon sandstone.

A very fine, brown clay matrix fills interstices, but does not seem to

be the cementing medium. Calcium carbonate, probably precipitated from

circulating ground waters, is present as well, but not in sufficient

quantity to be the cement.

With the exception of a single outcrop on the northeast rim of the

crater, this rock type seems to be restricted to a single exposure inL.

the south wall of the Blowout. Although the'outcrop is largely covered

with pyroclastic debris that has slumped down from the rim of the crater,i'

enough is still visible to indicate that there may be as much as 200

feet of the consolidated material.

The basaltic fragments that make up much of this rock are petro- ^

graphically similar to the dike and the lava flow.

The rock is uniformly bedded, but only crudely sorted. It is quite

porous, and can be disaggregated easily. Nonetheless, it resists ero-

sion, as it forms a series of scarps on the otherwise uniformly sloping

crater wall.

This rock is a clastic accumulation which may have been deposited <-

subaerially as a pyroclastic sediment, may have resulted from subaeque-

ous deposition within the lava-lined gulch, or may have accumulated as

a mudflow. In the latter two cases, erosion of the vesicular flow sur-

face may have provided the basaltic fragments present. The attitude of

the section may be a relic of mantle bedding in a pyroclastic deposit

or may be the result of a minor post-explosion collapse along the axis

of the Blowout.

The accumulation of 200 feet of material by stream deposition or

mudflow requires a substantial period of erosion; the petrographic simi-

larity of the rocks above and below this deposit seems to preclude such

fe:

W

Figs. 13 and 14. Left: view of outcrop of consolidated lapillituff. Right: this resistant material stands up in steep scarpsas in this exposure in the south wall of the Blowout.

N)

26 '-Hi

Fig. 15. View southwest from northeast rim of theBlowout. Note stratified outcrop of consolidatedlapilli tuff just below the south rim.

I

27

an interval. If the rock were the result of a general pyroclastic ex-

plosion, one would expect to find such material distributed over a wider

area, but it occurs only within the confinement of the lava-lined valley.

The best explanation seems to require a directed pyroclastic explosion

of limited volume that lacked the power to blow material ouch above the

bottom of the gulch. Such an eruption would have filled the gully in

short order but left no record up on the divides. Wentworth and .*.

MacDonald (1953) report (pp. 22-23) that basaltic eruptions in Hawaii> *

are characterized by a few hours of extrusion of fluid lavas; activity

then often becomes restricted to a short stretch of the fissure, over

which a "spatter-and-cinder cone" is built. True cinder and bombs nay

be extruded from such a feature. Often such pyroclastic particles re-

tain enough heat to be welded together upon landing; a continual series

exists, one end member of which is the case in which the pyroclastic

fragments fuse into a lava flow. The other extreme results in a clastic

accumulation in which no welding occurs. The deposit described above

probably resulted from a spatter-and-cinder cone eruption which retained

enough heat to consolidate the component clasts but still "pMrvf?iw

its clastic texture.

Wahlstrom (1955) would term this rock a lapilli tuff, since most

of the clastic fragments are between 4 and 32 mm. in diameter. Fisher

(1960) has proposed the term lapillistone for pyroclastic rocks whose

particles lie between 2 and 64 mm. in diameter. The indurated nature

of this deposit merits the additional modifier consolidated.

Lapilli Tuff All the earlier volcanic rocks are blanketed by a layer

of unconsolidated, in part fragmental, volcanic ejecta. This material

appears in fluvial deposits of the Eagle River and tributaries as far

IIpIi

I'll

east as Gypsum, but does not occur more than a mile west of die Blowout.

The ejecta seem to be diickest at the crater's rim, where excavation

has exposed 60 feet of section, which thins rapidly away from die de-

pression. Exposures in Trail Gulch are about 4 inches thick. The over-

all distribution suggests that the material was ejected from the

immediate vicinity of die crater at times when strong west winds were

blowing.

The section exposed in die Ideal Lava Products Company excavation '

on the southeast rim of die crater is a light bluish gray in color and

stands in steep scarps in die quarry vails. The material is loose and

such cliffs are unstable; slight disturbance causes slumping to die |y

angle of repose. Bedding is diia and uniform, and parallels the config-

uration of preexisting topography. Such mantle bedding occurs on slopes

as steep as 30-33°, which angle seems to be die angle of repose of die jj!$

material.

The pyroclastic fragments are raostly of lapilli size, but particles

as fine as dust and others as large as 2 feet in lengdi are present.

Fragments of die country rock are interbedded widi die section. Some

of these are angular blocks as much as 18 inches in diameter; others

range down to sand and silt size. Fragments of bodi die Maroon sand-

stone, which is exposed in die crater walls, and die underlying Hinturn

lithology are present.

Certain strata seem to contain more country rock fragments than do

others; one bed in particular is composed of about 50Z red sandstone

particles. This horizon may represent a time of collapse of the crater

walls during explosion.

29

Fig. 16. Photograph of lapilli tuff outcrop, ex-posed in Ideal Lava Products Company quarry.

Fig. 17. Same as above. Note dip of mantle bed-ding, a relic of previous ground surface.

L ft

30

Bombs of many different shapes are abundant din the bedded ejecta;

some are typically fusiform, some are ropy, others are shapeless blobs.

Where exposed in the section, these projectiles indent the Under lying

strata in bond) sags; mantle bedding occurs in the overlying layers.

The bombs are darker than the lapilli and are strongly reminiscent of

the vesicular surface of the lava flow. The bombs are present only in

the immediate vicinity of the crater.

Like all the earlier volcanic rocks, the explosive phase contains> *

phenocrysts of olivine Fa24, clinopyroxene, and magnetite, and laths of

microlitic bytounite An^- The glassy matrix has a refractive index of

1.57, said to be about average for basaltic glass (Wahlatrom 1955, pg.

286). A very few grains of quartz were observed in thin sections of the

lapilli and bombs; those seen are clear and contain no inclusions.

Large quartz grains can be seen in hand samples of fh«» bombs; such crys-

tals are clear, glaasy, and minutely fractured. Many occur as long

prisms with euhedral, parallel sides, but the crystal terminations are

not well developed. An X-ray powder pattern determination by John P.

deNeufville of the Geophysical Laboratory in Washington D.C. has estab-

lished that these grains are alpha quartz. The presence of these crys-

tals exclusively in the pyroclastic rocks and especially in the bombs

suggests that they were included at the time of the explosion; in any

event they could not have developed through magmatic fractionation of

as basic a magma as the host basalt. The quartz is in much local demand

by collectors who think the crystals are diamonds. A shaft was sunk in

the flopr of the crater in 1898 in an attempt to locate the "mother

lode". The project was abandoned at a depth of 250 feet while the hole

| I

I

SI•

-

II

Figs. 18 and 19. 2 views of bomb sacs in the bedded lapilli tuff.The presence of such features confirms the origin of this depositas a gravity ash fall. V.J

was still in ejecta*.

The generic term volcanic ash could be applied to all products of

the final explosive phase of the eruption. Following Wahlstrom (1955),

this pyroclaatic deposit is called a lap ill! tuff where the bulk of the

volume consists of particles of Lapilli aize. Individual strata in

which bombs are abundant are termed agglomerates; other layers contain-

ing numerous sandstone fragments qualify as volcanic breccias. At some

distance from the source where the ejecta is of ash size the deposit isi "

called a tuff. Fisher's (1960) terra lapillistone seems to inply a

higher degree of induration than exists In this deposit.

g OF *pnt QpaXEBJaBX VOLCANIC

The lava flow lies with unconformity on all earlier rocks - the

Paleozoic sediments and a Quaternary fan and flood plain. The consoli-

dated lapilli tuff rests directly on the lava surface and is overlain

by deposits of volcanic ash. The petrographic similarity of all die

Recent volcanic rocks precludes significant erosional activity within

the interval of the eruption; the 3 stages of extrusion probably

occurred in such rapid succession that they represent but a moment of

geologic time.

The Eagle River at present is flowing on a confined flood plain

within a steep-walled valley. Development of meanders has undercut and

truncated a series of alluvial fans which are composed of material de-

rived from the intermittent erosion of tributary gulches. The Eagle

River presently is incised to a depth of 35-50 feet below a previous

level, as estimated from a projection of these alluvial surfaces.

***

*Wm. Randall, personal communication, 1962

33

It has been suggested (Capps, 1909; Landon, 1933) that these fans

are terraces composed of outwash gravels deposited during the Last

stage of Pleistocene glaclation. It seems more likely that such nateri-

1 was deposited by tributary streams at a time, when the Eagle Elver

was higher than at present, probably when it was so choked with glacial

debris that deposition was going on. Hear the center of the valley,

these fans contain numerous granitic and metamorphic cobbles; such o

material may have been introduced by a meandering Eagle River, saturated»*

with debris from a melting glacier upstream, and subsequently reworked

by the tributary streams. The profiles of these surfaces strongly sug-

gest a series of alluvial fans or flanking pediments rather than a

system of river terraces.

In either case, the present level of the Eagle River indicates a

period of downcutting which probably occurred after the-glaciers had

melted. The river then was relatively free of debris and bad energy to

spare for active erosion and removal of material. The tributary streams,

which produced the fans are now entrenched into these features.

The volcanic rocks are superimposed upon this latest valley profile.

The lava flow lies on the flood plain below the truncated fans. The

volcanic ash can be found on top of the flanking fans where in most

cases it is the highest stratum exposed; in other instances the tuff is

overlain by a thickness of reworked pyroclastic material. Trail Gulch,

which enters the Eagle River north of Gypsum, is incised into a flood

plain the projection of which seems to coincide with the surface of the

flanking fans. The presence of volcanic ash on the flood plain surface

indicates that downcutting had begun before the volcanic explosion.

II

i!

Figs. 20 and 21. Left: view of thin stratum of^volcanic ash over-lain by thickness of waterworked pyroclastic material. Exposure ison top of an alluvial fan adjacent to the Eagle River. Right: thinstratum of volcanic ash overlain by thickness of waterworked materialand alluvial material derived from erosion of Minturn formation.Section is in a cut on Trail Gulch Road.

^ ^ W>.

r 35

Some alluviation seems to have occurred since the time of die erup-

tion. As mentioned above, certain of the exposures of ash in fluvial

sequences are overlain by water-worked tuff. A aeries of alluvial fans

from intermittent streams tributary to Trail Gulch have been cut into

by Trail Gulch Road; in these exposures as much as 4 feet of alluvial

material overlies the volcanic ash. The leading edge of the lava flow

where it: is undercut by the Eagle River is presently under water. It

does not seem likely that alluviation in the headwaters of the local»•

watershed was prompted by such a minor change in Eagle River regimen;

deposition by the intermittent streams may have been initiated simply

by the abrupt introduction of a volume of pyroclastic material.

Accumulation of ejects around the north end of the Blowout damned

up 2 small intermittent streams which previously had flowed on down the

gulch now occupied by the crater. With their drainage blocked> these

gullies began to fill with material derived from the pyroclastic cover

and the red sediments in their source areas. Headword growth of a small

intermittent stream to a distance of 3000 feet from the north end of the

Blowout has incised a steep gulch in one of the filled stream valleys;

erosion here has cut down into the red rock to expose the complete see-

tion.

Field data suggest that the volcanic rocks were extruded some time

after the close of the last glacial period, the Wisconsin. A consensus

of authoritative opinion (Emiliani, 1955, 1958; Suess, 1956; Karlstrom,

1956) indicates that this event occurred between 6 and 9 thousand years

before the present. Alluviation of the Eagle River subsequent to the

extrusion may have occurred during the Recent, or "Little Ice Age"

(Karlstrom, 1956; Suess, 1956). The presence of 2 small hot springs on

1

I

36

u-;ft:

lifeits'

Fig. 22. Waterworked volcanic ejecta overlain byred clay soil. Section is in a gulch cut in thefilled valley north of the Blowout.

37

the Colorado River just west of Dotsero (George, 1920) suggests that

residual magmatic heat may still be present at depth. The amount of

erosion of the lava lining in the gulch below the Blowout and the ex-

tent of headward growth of incision in the filled valley above suggest

that some time has elapsed since volcanic activity ceased.

A charcoal sample was recovered from a tree which had been buried

by the falling ejecta. The tree was standing erect on the southeast

rim of the Blowout and was partially exposed by excavation in the Ideal1 *

Lava Products Company quarry. The three-dimensional structure of the

branching limbs was preserved and bedding in the lapilli was undisturbed

on all sides of the trunk. The tree was traced down to the red sand-

stone in which it probably was rooted; a collapse of the unstable column

of lapilli prevented further search for evidence of the root structure.

-"" The charcoal was dated by the Radiocarbon Laboratory of the U.S.

Geological Survey in Washington D.C. and yielded an age of 4150 - 300

years before the present. The Dotsero Crater—is-thus the most-recent:

source of volcanic rocks in the state of Colorado.

MTKKPRETATIOH

Inasmuch as this thesis is concerned primarily with the Quaternary

volcanic rocks, no discussion of earlier geologic history is included.

Comprehensive treatments of Pennsylvanian-Permian history of central

Colorado are in print, and there is no lack of publications discussing

Tertiary events. A description of pre-Quaternary history in the Dotsero

area adds nothing to what already is set forth. Accordingly, the fol-

lowing discussion is limited to events which have transpired since the

Eagle River valley attained its present form.

***

38

Basaltic lava veiled out of a vent in the bottom of a small gulch

on the north side of the Eagle River. The vent previously may have

been a source of some of the Tertiary volcanic rocks. The intrusive

body exposed in the north wall of the Blowout is, however, associated

with the Recent extrusion, as indicated by the petrographlc evidence.

Molten rock filled the gulch and adhered to the canyon vails; some of

it flowed up the gulch to a point beyond the upstream limit of the vent.

Host of the lava ran down the gulch and spread out unconfined on the»*

Eagle River valley bottom. The lava may have forced the Eagle River

out of its channel. There is no evidence that the flow ever dammed the

Eagle River; if it did, it must have done so only momentarily, until

the river was able to remove enough scoria to regain its old level. It

seems probable that the lava flow crowded the river to the south side

of the valley and then singly ran out of supply or solidified.

When the lava outpouring subsided, a spatter-and-clxtder cone prob-

ably rose over a part of the vent, and produced the consolidated lapilli

tuff which appears in the south wall of the Blowout.

Increase in volatiles within the magmatic mass beneath the vent

seems to have resulted in a pyroclastic explosion that blew out quanti-

ties of the .country rock and scattered sandstone fragments and volcanic

ejects over the immediate -area. Fine ash and volcanic dust was blown

as much as 6 miles east of the crater, where it is interstratified with

fluvial deposits. The explosion beheaded a small stream valley i&ich

previously had flowed southeast from the vicinity of the crater; the old

valley profile presently is represented by a low area in the east wall

of the Blowout. The present form of the crater is due to slump and col-

lapse of the walls; in view of this shape of the Blowout several

39

alternative hypotheses for the origin of the crater might be advanced.

It seems unlikely that the depression is due to subsidence of

country rock into a nyigma chamber or digestion of the sediments' by a

molten dike, as either of these mechanisms would be expected to produce

a change in magmatic composition of the final stages of the extrusion.

Soluble gypsum and/or anhydrite is present in the Hinturn formation

which underlies the material exposed in the crater. Sinkholes are not

uncommon in the area; a large, shallow sink is developed in the gypsif-j'

erous beds of the Hinturn 4 miles northeast of the Blowout. Bo sink- '.

holes are present anywhere in the Maroon formation in the mapped area.

It would seem extremely fortuitous for such & collapse to have occurred

at the exact point which all the field evidence dictates must be the

source of the volcanic rocks.

After volcanic activity ceased, erosion and mass-wasting altered

the form of the blowout and associated volcanic features. Slump on the

sides of the crater has reduced much of the wall surface to the angle .

of repose of the pyroclastic material; fragments of the red sandstone

have fallen into the crater and are contributing to its eventual filling.

Ueadward growth of a small, intermittent tributary of the Eagle River

currently is removing what lava remains in the gulch.

The Dotsero volcano certainly is no longer active, and may be ex-

tinct. The presence of 2 hot springs not far from Dotsero has been

mentioned; these may indicate the presence of residual heat at depth.

The temperatures of the 2 springs are 83 and 84° F.; their salinities

are approximately 11 °/oo (George, 1920). Thornbury (1956, pg. 489)

warns that "any volcano that has been active as recently as the Pleisto-

cene is potentially an active volcano." In view of this admonition, it

is proposed the Dotsero Crater be classified as dormant.

. ECONOMIC GEOLOGY

The ejecta currently is being exploited by the Ideal Lava Products

>any, who operate a quarry on the southeast rim of the Blowout. The

Lapilli tuff makes excellent lightweight aggregate and is fabricated

into cinder blocks at the company's plant in Dotsero. The Eagle County

Department of Public Works quarries ejecta from a steep, unstable slope

just south,, of the Blowout and has built up a stockpile of the material

lear highway 6 & 24. The cinders supposedly are spread on secondary

roads in Eagle County, but the quarry has not been worked recently and

the stockpile remains undisturbed.

BIBLIOGRAPHY

Allen, E., and Balk, R., 1954, Mineral Resources of Fort Defiance andTohatchi Quadrangles, Arizona and New Mexico: Bull. 36, State Bur. ofMines & Min. Resources, Hew Hex. Inst. of Mining and Technology.

Bass, N. Wood, 1958, Pennsylvania!* and Permian Rocks in the SouthernHalf of the White River Uplift, Colorado: in Symposium on PennsylvanianRocks of Colorado and adjacent areas, R.M.A.G., Denver Colorado.

_, and others, 1960, Geology of West-Central Colorado inGuide to the Geology of Colorado: G.S.A. , R.M.A.G. , C.S.S.

.Campbell, M.R. , 1922, Guidebook of the Western United States; Part E,the Denver and Rio Grande Western Route: U.S.G.S. Bull. 707, pp. 130-131.

Capps, S.R. , Jr, 1909, Pleistocene Geology of the Leadville Quadrangle,Colorado: U.S.G.S. Bull. 386, pp. 17-22.

Clippinger, Donn, and Gay, Walter, 1947* Pumice Aggregate in Hew Mexico;its Use and Potentialities: Hew Hex. Bur. of Mines and Min. ResourcesBull. 28.

t Cotton, C.A. , 1944, Volcanoes as Landscape Forms: Whitcombe and TombsLtd., Wellington, Hew Zealand, 416 p.

Darton, N.H., 1916, Explosion Craters: Scientific Monthly v. 3, p. 425.

Diller. J.S., 1887, Geology of the Lassen Peak District: 8th Amnml Re-port, U.S. Geological Survey.

Dormer, H.F., 1949, Geology of the McCoy area, Colorado: G.S.A. Bull.v. 60 pp. 1215-1248.

Emiliani, Cesar e, 1955, Pleistocene temperatures: Jour. Geol. v. 63,pp. 538-578.

_ _ , 1958, Paleo temperature Analysis of Core 280 andPleistocene Correlations: Jour. Geol. v. 66 pp. 264-275.

Endlich, P.M. ,-1876, On the Erupted Rocks of Colorado, 10th Annual Re-port, U.S. Geographical and Geological Survey of the Territories, pp.199-251.

Fenner, C.N., 1920: The Katmai Region, Alaska, and the Great Eruptionof 1912: Jour. Geol. v. 28, pp. 569-606.

Fisher, Richard J. , 1960, Proposed Classification of Volcanic lasticRocks: G.S.A. Program 1960 Annual Meetings.

_, and Wilcox, Say £.» 1960, Volcanic Clastic Bocks ofthe John Day Formation in Monument Quadrangle, Horth Central Oregon:G.S.A. Program 1960 Annual Meetings.

George, R.D., 1920, General Descriptions of the Hineral Springs inHineral Waters of Colorado: Colo. Geol. Survey Bull. 11, pp. 288 and289 and map.

Gilbert, G.K., 1890, Lake Bonneville, U.S.G.S. Monograph 1.

Gregory, H.E., 1915, The Igneous Origin of the "Glacial Deposits" onthe Havajo Reservation, Arizona and Utah: Am. Jour. Sci. 4th Series, v.40, pp. 97-115.

_, Geology of the Havajo Country: A Reconnaissance ofFarts of Arizona, Hew Mexico, and Utah, U.S.G.S. Professional Paper 93.

Jaggar, T.A., 1947, Origin and Development of Craters: G.S.A. Memoir 21,pp. 337-407.

Jahna, R.H., 1952, Calderas of the Pinacate Region, Sonora, Mexico (aba.)G.S.A. Bull. v. 63 no' , 12 part 2~pp. 1332-1333.

Karlstrom, T.N.V., 1956, Radiocarbon-based Pleistocene Correlations andWorldwide Climatic Change (abs): GG.S.A. Bull. v. 67, pg. 1711.

Rueera, Richard E., 1962, Late Cenozoic Structural and Geomorphic His-tory of the Park Range and White River Plateau, Horthwest Colorado:Rocky Mountain G.S.A. Program, 1962 Annual Meetings.

Lakes, Arthur, 1890, Extinct Volcanoes in Colorado: Am. Geol. v. 5, pp.40-43.

London, Robert E., 1933, Date of Recent Volcanism in Colorado: Am.Jour. Sci. Series 5 v. 25, pp. 20-24.

Lee, Willis T., 1907, Afton Craters of Southern New Mexico: G.S.A.v. 18, pp. 211-220.

MacDonald, G.A., 1943, The 1942 Eruption of Mauna Loa, Hawaii: Am.Jour. Sci., v. 241, pp. 241-256.

MacDonald, G.F., 1953, Anhydrite-Gypsum Equilibrium Relations: Am.Jour. Sci. v. 251 no. 12, pp. 884-898.

Moore, J.G., and Peck, D.L., 1962, Accretionary Lapilli in VolcanicRocks of the Western Continental United States: Jour. Geol. v. 70 no.2, pp. 182-194.

Nichols, R.L., 1939, Pressure Ridges and Collapse-Depressions on theMcCartys Basalt Flow, New Mexico: Trans. Am Geophys. Union v. 20, pp.432-433.

_, 1946, HcCartys Basalt Flow, Valencia County, Hew Bexico:G.S.A. Bull. 57, pp. 1049-1086.

Pirsson, L.V., 1915, The Microscopical Characters of Volcanic Tuffs:Am. Jour. Sci. Series 4 v. 40, pp. 191-211.

Beiche, Parry, 1937, The Toreva-Block, a Distinctive Landslide Type:Jour. Geol. v. 45, pp. 538-548.

_, 1940, The Origia of Kilbourne Hole, Hev Headco: Am.Jour. Sci. v. 238 no. 3, pp. 212-225.

Robinson, H.H., 1913, The San Francisco Volcanic Field: B.S.G.S. Pror-feasioaal Paper 76.

Riley, Paul, 1949, Geology of an Area along the Colorado River: H.S.Thesis, Univ. of Colorado, Unpublished.

Russell, I.C., 1897, Volcanoes of Borth America: The Mac ttLltan Co.,Hew York.

_, 1887, Quaternary History of Homo Valley, California:8th Annual Report, U.S.G.S-

Schmidt, P.B., 1961, The Geology of the State Bridge Area, Colorado:M.S. Thesis, Univ. of Colorado, Unpublished.

Shearer, Eugene, 1950, Geology of die Bed Dirt Creek Area: M.S. Thesis,Univ. of Colorado, Unpublished.

Shoemaker, B.M., 1953, Collapse Origin of the Diatxemes of the Savaio-Hopi Reservation: G.S.A. Bull. v. 64, pg. 1514.

'tSmith, Robert L., 1960, Ash Flows: G.S.A. v. 71 no. 6, pp. 795-842.

Smith, W.D., and Swartzlow, C.R., 1936, Mount Mazatna: Explosion versusCollapse: G.S.A. Bull. 47, pp. 1809-1830.

Smith, W.R., 1925, Aniakchak Crater, Alaska Peninsula: U.S.G.S. Pro-fessional Paper 132, pp. 139-145.

Stockdale, Paris, 1926, Moatlake, an Amazing Sinkhole; Jour. Geol. v.44, pp. 515-522.

Suess, H.E., 1956, Absolute Chronology of the Last Glaciatlon: Sci.,v. 123, pp. 355-357.

Tazieff, Haroun, 1950, L1eruption du Volcan Gituro (Kivu, Congo Beige):Congo Beige et Ruanda-Urundi, Direction Generale des Affaires Econotoique,Service Geologique, Hemoire no. 1.

Thornbury, W.D., 1956, Principles of Geonorphology, John Wiley and Sonsinc., Hew York.

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