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DEFORMATION ASSOCIATED WITH PULASKI OVERTHRUSTINGIN THE PRICE MOUNTAIN AND EAST RADFORD WINDOWS,
MONTGOMERY COUNTY, SOUTHWEST VIRGINIA„·by
Arthur Philip“Schult;
— Thesis submitted to the Graduate Faculty of theVirginia Polytechnic Institute and State University
q in partial fulfillment of the requirements for the degree ofA
MASTER OF SCIENCEin
Geology ‘ _
APPROVED: ~
—_—W. D._L0§ryY
I üävid Gray J. Fred Readl
C. G. Tillman
April, l979_ Blacksburg, Virginia J
ACKNOWLEDGMENTS
The author wishes to thank Dr. w. D. Lowry and Dr. C. E.Sears for suggesting this research. Special thanks go to Dr. Lowryand Dr. David Gray for their generous contribution of both timeand energy to all aspects of the thesis. The author acknowledgesDr. C. G. Tillman and Dr. J. Fred Read for their critical readingof the thesis.
Finally, the author thanks his wife, , for constantsupport and encouragement.
ii
ITABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ........................ hiiLIST OF TABLES ......................... v
2LIST OF FIGURES ........................ viINTRODUCTION .......................... lGEOLOGIC SETTING lO
Sty-uctupe
Stpatjgpaphy . . „ • . „ „ • • . „.„. • . . . . . . „ . l2
Previous Study of Pulaski 'Qvepthpugt . . . . . . . . • . . . . . . . . . „ . . . . l4
TERMINOLOGY „ . . „ ...................... l8
ALLOCHTHONOUS ROCKS . . „ ................... 2O
Thrust Chaos and Tectonic Melange ·-··--······· 20
Previous work ..................... 20
General Structure ................... 2l
Fabric of the Melange ................. 23
Microstructure .................... 32Interpretation .................... 56
Conclusion ...................... 70
East Radford Allochthon ................. 70
Previous work ..................... 7l
Structure ..................E ..... 72
iii
_ TABLE 0F CONTENTS (Continued) ·
I Page
Interpretation and Conc1usion ............. 74
Max Meadows-Pu1aski Breccias ............... 75Previous work ..................... 75Structure ,...................... 78Microstructure .................... 81Interpretation and Conc1usion 97
PARAUTOCHTHONOUS ROCKS ..................... 99
Price Mountain window ................... 99Structure ....................... 99Microstructure 109Interpretation .................... 122
Conc1usion ...................... 123STRUCTURAL SYNTHESIS ...................... 124
Mechanics of Pu1aski Fau1ting ............... 124
Sequence of Deformation Associated withPu1aski Thrusting ................... 125
CONCLUSIONS .......................... 129REFERENCES ........................... 131VITA .............................. 135ABSTRACT
iv
ULIST OF TABLES
Page
1. Sequence of Deformation .......,........... 127
e v
ILIST OF FIGURES
Page
l. Regional Tectonic Map .................... 3
2. Sketch Map of Study Area .................. 6
3. Structure Sections ..................... 8
4. Stratigraphic Sections ................... l6
5. a. Outgrop Sketch of Pulaski _Decollement Melange .................. 24
b. Outcrop Sketch gf PulaskiDecollement Melange .................. 24
6. Pulaski Decollement Melange ................. 26
7. a. Detail of PulasgiaDecollement Melange .................. 28
b. Detail of PulaskiA
Decollement Melange ....T .............. 28
8. a. Boudins in the Melange ................. 3l
b. Disrupted Strata in the Melange ............. 3l
9. a. Microstructure of DeformedMelange Argillite ................... 34 _
b. Detail of Deformed MelangeArgillite ....................... 34
lO. Allochthon of Silurian Quartzite .............. 36
ll. a. Sedimentary Fabric in'aSilurian Quartzite .................. 39
b. Cataclastic Shear in aSilurian Quartzite .................. 39
l2. Disrupted Tourmaline Grain ina Cataclasite ....................... 4l
vi
_ LIST 0F FIGURES (Continued)
Page
13. Catac1astica11y Disrupted Fe1dsparGrain ........................... 43
14. Cumu1ative Frequency Curves ofCatac1asites ....................... 45
15. a. Si1urian U1tracatac1asites ............... 48
b. Si1urian U1tracatac1asites ............... 48
16. Cumu1ative Frequency Curves ‘of U1tracatac1asites ................... 50
17. Catac1astic Fabric in aMe1ange Boudin ...............E ....... 52
18. a. Quartzite—sha1e Contact in anU1tracatac1asite ................... 55
b. Sha1e Fragments in Catac1asticFabric . ......... . .............. 55
19. a. Tectonic Sty1o1ites in anU1tracatac1asite ................... 57
b. Tectonic Sty1o1ites in anU1tracatac1asite ................... 57
20. a. Sty1o1ite in a Limestone Boudin ............. 59
b. Sty1o1ite in a Fossi1iferousMudstone ....................... 59
21. a. Extension Fractures in aSi1ty Sandstone .................... 61
b. Extension Fractures in aShe11 Fragment .................... 61
vii
l LIST OF FIGURES (Continued)
Page
22. a. Strain—recovery New Grains inan U1tracatac1asite .................. 63
b. Strain-recovery New Grains inan U1tracatac1asite .................. 63
23. Histograms Showing Intracrysta11ineDeformation ........................ 66
24. Mississippian Strata in Pu1aski gDeco11ement Zone ..................... 6925. a. Exposures of the Pu1aski
Overthrust . ..................... 77b. Outcrop of the Max Meadows-
Pu1aski Breccias ................... 7726. a. Autobrecciation in the
E1br0ok Do1omite ................... 80b. Extension Fractures in the
E1br0ok Oo1omite ................... 8027. Fo1d in the Breccia Zone of
E1brook Do1omite ..................... 8328. Tectonic Breccia ...................... 8529. Sketch of Micro—Fabric of Autobreccia ............ 8730. Microstructure of D01omite Breccia ............. 8931. Orientation of Fau1t Rocks with Respect
to Pu1aski Thrust ..................... 9232. Rose Diagrams of Tectonic Sty1o1ites ............ 94
33. Pi Diagram of Tectonic Sty1o1ites .............. 96
viii
g LIST OF FIGURES (Continued)
Page
34. a. Outcrop Sketch of Fau1ts inMississippian Rocks .................. 101
b. Déco11ements in Mississippian’ Price Formation .................... 101
35. a. Norma1 Fau1t in PriceMountain Antic1ine .................. 104
b. Deformation A1ong Norma1 Fau1t ........ . .... 10436. a. Extension Fractures in
Window Rocks ..................... 106
b. Unusua1 Minera1 Segregationsin window Rocks .................... 106
37. a. Low Ang1e Thrust in PriceMountain Window .................... 108
b. Low Ang1e Thrust in PriceMountain Window .....i .·.....„ ......... 108
38. Microstructure of OverriddenMississippian Rocks .................... 112
39. a. Catac1astic Fabric inMississippian Quartzite ................ 114
b. Catac1astic Fabric inMississippian Quartzite ................ 114
40. Catac1astica11y Deformed Fe1dspar Grain ........... 117
41. Histograms of Deformed Fe1dspar Grain ............ 119
42. Kinked Mica in Zone of Catac1asis .............. 121
PLATE
Geo1ogic Map of Price Mountain and EastRadford windows ............_. ......... In
. ix
INTRODUCTION
In the Southern Appalachians, thrust faults of regional
extent have been recognized and studied for many years (Hayes, 1891;
Campbell, 1925; Butts, 1927; Rich, 1934; Rogers, 1949; King, 1970; lLowry, 1964, 1970; Gwinn, 1964; Harris, 1970, 1976; Milici, 1975;
Harris & Milici, 1977; Perry, 1978). However, details of many of
these faults and associated fabrics are lacking. Furthermore, an
‘ understanding of the specific deformation mechanisms active within
décollements during thrusting is limited.
Two general models for décollement deformation exist. The1 first involves "slip" and deformation within incompetent strati-
graphic units such as shales and gypsum (Rich, 1934; Gwinn, 1964;
Rogers, 1970; Harris & Milici, 1977). The incompetent units tend
to localize thrust décollements and accommodate thrusting by tight
folding and shearing. The second model involves décollement pro-
duced cataclastic deformation within competent rocks (Brock &
Engelder, 1977). The overthrusting occurs without the presence of
incompetent rocks in the decollement zone.
This study involves analysis of a portion of the greatU
Pulaski overthrust of the Southern Appalachians (Fig. 1). Rocks of
the Pulaski decollement zone have been studied in order to determine
the nature of the fault zone and to define the operative deformation
mechanisms. The aim is to contribute to our understanding of the
1
i 2
Figure 1, Major Fau1ts of the Overthrust Be1t
of Southwest Virginia
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deformation mechanisms which operate in the decollements of major
thrust faults.
Both competent and incompetent strata occur in the decollement
^ of the Pulaski thrust. These decollement fault rocks display a
variety of thrust—related fabrics defining specific modes of defor-
_ mation active prior to, during and after a major thrusting event.
Pulaski décollement deformation includes slip on incompetent strata,
cataclasis within competent rocks, formation of tectonic mélanges and
fault breccias and complex smaller scale folding and faulting. Micro-
scopically, competing deformation mechanisms on the décollement in-
· clude intergranular and intragranular deformation, micro-faulting,
extension fracturing and formation of stylolites. 'Traced almost 300 km along strike (Cooper, l970) the Pulaski
overthrust in southwest Virginia has a minimum horizontal displace-
ment of l8 km (Cooper, l970). This northwest directed thrust places
Cambrian and Ordovician carbonates and shales over Cambrian through
Mississippian sedimentary rocks. Thrusting was initiated during the
Alleghanian orogeny sometime during Middle to Late Mississippian time.
The present study is restricted to the Price Mountain and East
Radford windows, the most northwesterly windows in the Pulaski over-
thrust sheet (Figs. l, 2 and 3). Excellent exposures of the Pulaski
décollement occur in these two windows. The present study of the
Pulaski decollement zoneincludes:l.
mapping the distribution of décollement fault rocks
in the Price Mountain and East Radford area (Fig. 2),
5
Figure 2. Geoiogy of the Price Mountain and East
Radford Windows, Montgomery Co., Virginia
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Figure 3. Structure Sections of the Price Mountain
and East Radford windows
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9
Z. descriptions of typical outcrops of the Pulaski
décollement structure, including stratigraphic description and
3. microstructural analysis of décollement fault rocks.
Using the Pulaski fault as an example, this study defines the
complex interplay of deformation associated with thrust décollements.
Equally important to an understanding of thrust-related
deformation are the rocks overridden by a thrust sheet. Mapping,
outcrop description and microstructural analysis included in the
present study show that rocks overridden by thrust_sheets take an
active role in thrust-related tectonics. Overridden parautochthonous
rocks in the Price Mountain window of the Pulaski thrust (Fig. 2)
display deformation features typical of strata trapped below an
advancing thrust sheet.
Finally, by synthesizing the deformation patterns in
décollement rocks and in overridden rocks, a sequential model for the
overthrust belt of southwest Virginia is presented.
GEOLOGIC SETTING · ·
Structure
A series of imbricate thrusts, the Blue Ridge, Max Meadows
and Pulaski faults, and a series of reverse faults, the Fries, Salem,
Saltville, Narrows and St. Clair faults, dominate the regional
structure (Fig. l).( The most southeasterly overthrust, the Blue Ridge fault,
. places Precambrian basement rocks and Lower Cambrian clastic rocks
over Lower-Middle Cambrian rocks. Minimum horizontal displacement
on the Blue Ridge thrust is at least 8 km. in the Roanoke area.
, Rocks overridden by the Blue Ridge thrust are part of the
Max Meadow thrust sheet composed of Lower-Middle Cambrian shales
and carbonate rocks of the Rome Formation. Minimum horizontal dis-
placement on the Max Meadow thrust is 4.5 km.
1 The Max Meadow thrust sheet has, in turn, overridden theMiddle Cambrian rocks of the Pulaski overthrust, which has a minimum
horizontal displacement of l8 km (Cooper, l970). A series of windows(
in the Pulaski overthrust (Fig. l) expose overridden rocks ranging
in age from Cambrian to Mississippian. The overridden rocks of the
windows show that structures below the thrust sheet are dominated by
two major folds, the Christiansburg anticline and the Blacksburg
. synclinorium.
The Christiansburg anticline is a complex nappe-like fold
exposed in the Christiansburg and Ingles-Barringer Mountain windows
10
ll
((Fig. l). The fold is the first major structure northwest of the
Blue Ridge structural front.
To the northwest, the northwest limb of the complementary
Blacksburg synclinorium dips southeast below the northwest edge of
the Pulaski thrust (Fig. l). A smaller anticline on or near the
axis of the Blacksburg synclinorium is exposed in the Price Mountain
window of the Pulaski overthrust sheet (Fig. l). The area of the
present study is restricted to the most northwesterly windows in .
the Pulaski overthrust sheet, the Price Mountain and East Radford
windows (Fig. 2). In this area, the Middle Cambrian E brook dolomite,
which in most places forms the basal rock unit of the Pulaski over-
thrust, is all that remains of a once 3,600 meter thick thrust sheet.
Other allochthonous rocks in the study area (Fig. 2) include the de-
formed Cambrian through Mississippian rocks exposed within the
décollement of the Pulaski thrust and the large allochthon of
Ordovician, Silurian and Devonian strata exposed in the East Radford
window._Mississippianrocks exposed in the core of the Price Mountain
window show relatively little displacement with respect to the rocks
associated with the Pulaski fault. These parautochthonous rocks form
— a doubly plunging asymmetric anticline. In the study area (Fig. 2),
the anticline plunges west below the rocks of the Pulaski overthrust.
l2
StratigraphyAStratigraphy exerts important controls on the location of
thrust décollements and determines the style of deformation in the
décollements formed during overthrusting. In general, the strati—
graphy of the rocks of the overthrust belt of southwest Virginia
consistseofpackages of thick-bedded competent sandstones, limestones
and dolomites alternating with packages of incompetent thin—bedded
shales, limestones and sandstones and thick sequences of highly in-' competent shales. The following litho—tectonic units occur in the
overthrust belt of southwest Virginia (Fig. l):
l. Unconformably above l billion year old basement rocks are
a sequence of Lower Cambrian sandstones and shales of the Chilhowee
Group. Capped by a thick dolomite sequence of the Shady Formation,
the entire litho-tectonic unit behaves as a competent rock mass
resisting deformation during thrusting.
. 2. Lower Cambrian calcareous shales, shales and dolomites of
the Rome Formation lie conformably above litho—tectonic unit l and
occur conformably below a thick sequence of Cambro-Ordovician carbon-
ates. These very incompetent shales and associated limestones form
numerous basal detachment planes throughout the Southern Appalachians.
Litho-tectonic unit 2 responds to thrusting by extreme shortening from
tight folding and shearing.
3. A lO00-meter thick sequence of thick-bedded dolomite,
shaly dolomite and limestones form litho-tectonic unit 3. The
l3
formations of this unit include the Middle Cambrian Elbrook dolomite,
overlying dolomites, limestones and sandstones of the Knox Group and
the limestones of Middle Ordovician age. Litho—tectonic unit 3
behaves as a very competent structural mass resisting decollement
style deformation.
4. Unit 4 is a thick clastic wedge of Middle Ordovician to
Upper Ordovician sedimentary rocks including the Bays sandstone and
the correlative Moccasin Formation, Eggleston Formation and the thick
(350 meter) Martinsburg Formation. These thin interbedded shales,
limestones and sandstones contrast greatly with litho—tectonic unit
3. Numerous local decollements accompanied drag folding and shearing
of this rock unit.5. Unit 5 consists of a clastic sequence dominated by rela-
tively thick competent sandstones and minor shales. These rocks
represent clastic wedges formed during late Ordovician, Silurian and
early Devonian time. The formations include the Juniata, Tuscarora,(
Rose Hill and Keefer, as well as various Lower Devonian sandstones
and cherts. Like unit 3, this litho—tectonic rock mass resists
folding and detachment associated with thrust faulting.
6. Unit 6 is another thick clastic wedge formed during
Middle and Late Devonian time. It consists of very incompetent rocks
which localize thrust décollements. Strata over l000 meters thick, of
the Needmore, Millboro and Brallier Formations are dominated by shales
with minor thin—bedded sandstones. Numerous décollements and
l4
associated folding and shearing are found within this litho-tectonic
unit in the overthrust belt of the Southern Appalachians.
7. Massive sandstones and minor shales of Late Devonian
and Early Mississippian age form litho-tectonic unit 7. The Upper
Devonian "ChemungP Formation and the Lower Mississippian Price For-
mation make up most of this unit and respond to thrusting by resisting
décollement formation. The upper Price Formation, however, is in-
cluded in litho—tectonic unit 8.
8. Thin coal seams in the upper part of the Price Formation
combined with the overlying shales of the Lower Mississippian
Maccrady Formation localize the uppermost décollements in the over-
thrust belt of southwest Virginia. Shales and coals of Upper
Mississippian and Pennsylvanian age may have once been present in
the overthrust belt; however, these rocks have not been recognized
in the study area (Fig.
2).Rocksfrom several of the litho—tectonic units described above
are found within the study area. Fig. 4 shows two stratigraphic
columns of rocks occurring in and northwest of the study area.
Previous Studies of the Pulaski Thrust( The study of the Pulaski fault has a long and interesting
history. Campbell (l925), who named the Pulaski fault for its
passage through Pulaski, Virginia (Fig. l), attributed first recog-
nition of the overthrust to Henry D. and william B. Rogers as early
as l842. The Rogers brothers made the first known reference to the
Pulaski thrust and recognized its length to be over l00 miles (Rogers
15 ,
Figure 4. Stratigraphic sections. The New River section
is northwest of the Pu1aski overthrust; the
Maccrady is cut by the Pu1aski fau1t; and the
Honaker is cut by the Sa1tvi11e fau1t, farther
A northwest (Fig. 1). Price Mountain section is
from test we11; ho1e started in Price and bottomed
1 in the Moccasin. Martinsburg is be1ieved repeated
by fau1ting (Cooper, 1961).
16
THICKNESS: Feet (meters)SYSTEM FORMATION ———N§W——————-———-—§FTEE—-River Mountain
Mississippian Maccrady 1605 (490)
Price 975 (297) 730 (222)
"Chemung" 698 (212) 710 (216)Bra11ier 2699 (792) ' 1759 (536;• Mi11boro 850 259 2415 736D@v¤¤‘¤¤ Needmore 30 ( 9) - 50 ( 15)Huntersvi11e 30 ( 9) 124 ( 38)Rocky Gap 40 ( 12)
Keefer 140 ( 43) 130 ( 40)Si1urian Rose Hi11 127 ( 39) 185 ( 56) ·
. Tuscarora 100 ( 30) 163 ( 50)
Juniata 165 ( 50) 187 ( 57)Martinsburg 980 (299) 2743 (836)Egg1eston 158 ( 48)
Ordovician Moccasin 55 ( 17) 37 ( 11)Midd1e Ordovi-cian Timestone 529 (161)—————————————————————Knox Group-----555 2563 (781)———————————————————
No1ichuck 56 ( 17). Honaker E1brookCambrian on Pu1aski
b1ock)
Figure 4
17
& Rogers, 1842). Hayes (1891) recognized that the overthrust fau1ts
of the Southern Appa1achians were characterized by 1ow dips and great
disp1acements.
Campbe11 (1925) pub1ished the first comprehensive geo1ogic
map and structure sections of the overthrust be1t of southwest
Virginia. Campbe11 (1925) gave recognition to R. J. Ho1den, then
Head of the Geo1ogy Dept. at Virginia Po1ytechnic Institute and State
University for correct1y interpreting the structure of the Price
Mountain window.”
» Butts' (1933) map of the Va11ey and Ridge Province of Virginia
showed the Pu1aski thrust fau1t and the windows in the overthrust
b1ock. Cooper (1936, 1939, 1946, 1961, 1970) studied the Pu1aski and
re1ated thrusts in southwest Virginia, describing many of the inter-
esting features associated with the fau1ts. Lowry (1971) described
the regiona1 tectonic setting and structura1 evo1ution of the over-
thrust be1t of southwest Virginia.
TERMINOLOGY
The writer uses the following terms in this report in
the sense indicated:
l. allochthonous—-rocks that are structurally removed from
their original site of deposition; rootless; thrustfaulted.
2. autobreccia (Cooper, l970)—-a mechanically deformed rock
showing a complex intersecting pattern of extension fractures yet
maintaining original sedimentary continuity; extended fragments
have not rotated with respect to one another.
3. autochthonous—-rocks that are essentially in the same
structural position as when they were deposited; rooted.
4. cataclasis (Engelder, l974; Brock & Engelder, l977;
Sibson, l977)--a deformation mechanism occurring in the microstructure
of competent sandstones, arkosic sandstones, dolomites, etc.,
characterized by brittle fracture, rotation and comminution of
detrital grains. Rocks showing different degrees of cataclasis are
termed proto-cataclasites (l0—50%), cataclasites (50-90%), and”
ultracataclasites (90—l00%).
5. fault chaos (Noble, l94l)--a zone of tectonically folded,
faulted and mixed blocks, slices, imbricates, etc., of non—c0nformable
stratigraphic units in a thrust décollement zone; mega-breccia.
6. mélange (Moore and Wheeler, l978)--a distinctive rock
type showing a fabric characterized by isolated tectonic clasts of
l8
l9
relatively competent rock surrounded by highly deformed relatively
incompetent argillaceous matrix; mélange, in this report, implies
tectonic origin.
7. parautochthonous--rocks that have been displaced rela-
tively little with respect to their original site of deposition.
8. tectonic breccia (Cooper, l970)—-a mechanically deformed
rock mass consisting of breccia fragments of various sizes and shapes
mixed and welded together by a matrix of finely crushed rock.
éALLOCHTHONOUS ROCKS
Thrust Chaos and Tectonic Mé1ange
Parautochthonous rocks of the Price Mountain window areover1ain by a comp1exe1y deformed sequence of a11ochthonous strata
exposed in the déco11ement of the Pu1aski thrust. The a11ochthonsform a thrust chaos, and 1oca11y tectonic mé1anges. Rocks, of con-trasting competence in this zone define specific deformation mechanisms
acting prior to, during, and after Pu1aski thrusting. The dominant
deformation mechanisms inc1ude shearing, fo1ding, f1attening
associated with formation of mé1ange fabrics and catac1asis.
Previous work— Cooper (1961) in his description of the Pu1aski fau1t and
associated breccias noted the occurrence of Ptectonic erratics" nearthe western edge of the Price Mountain window. These a11ochthonous
b1ocks of strata represent "a detached thin s1ice of beds broken off
the tread of the overridden b1ock and carried forward by the advancing
thrust sheet" (Cooper, 1961). Cooper shows rocks of Ordovician,
Si1urian, and Devonian age caught between the Midd1e Cambrian E1brook
do1omite of the Pu1aski thrust sheet and the Mississippian rocks of
the Price Mountain window. In a 1ater report Cooper (1970) stated
that "significant quantities of the tread rocks have been dozed off
and worked into the breccias from p1ace to p1ace." Ear1ier mention
of thrust—re1ated erratics is given by Cooper (1939) in his description
· 20
2l
” of the Draper Mountain area (Fig. l) where a large irregular mass
of Devonian strata is exposed below the Elbrook dolomites of the
Pulaski thrust. Based on structural and stratigraphic relationships
in the area, Cooper (l939) concluded that the mass of Devonian rocks
is allochthonous and lies between the thrust sheet and autochthonous
rocks below. Cooper proposed the name "thrust erratic" for the
allochthon and called the contact of the Elbrook and the Devonian
strata a "pluck fault" to designate the genetic origin of the feature.
"Plucking" referred to the analogous situation developed in glacial
terrains where the overriding ice plucked bedrock as it advanced.
Butts (l933) shows a thin sliver of the Ordovician Martinsburg
Formation along the northwestern edge of the Price Mountain window
between the Elbrook and Mississippian rocks. However, he makes no
mention of its origin or significance.
Recent work by the writer has shown that tectonic mixing of
Cambrian through Mississippian rocks occurs in the décollement zone
of the Pulaski thrust in the Price Mountain and East Radford windows.
This mixing has formed a thrust chaos and tectonic mélange.
GeneraliStructure
In the study area, the fault chaos consists of imbricates of
highly deformed strata, generally stacked up in inverted stratigraphic
sequences and wedged between an overriding thrust plate and parau-
tochthonous rocks structurally below (Fig. 3). Allochthonous frag-
ments of at least nine stratigraphic units have been identified in the
chaos. These units include:
22
Maccrady Formation--Lower MississippianChemung and Brallier Formations—-Upper DevonianNeedmore(?) and Millboro Formations—-Middle and
Lower DevonianUndifferentiated sandstones-—Siluro-DevonianKeefer, Rose Hill and Tuscarora Formations—-SilurianMartinsburg Formation, Middle Ordovician limestones,
and Bays Sandstone-—~0rdovocianKnox Group including thé Cambrian Copper Ridge
Sandstone--Cambro——0rdovician JA typical exposure of the Pulaski thrust décollement occurs
along the northwest edge of the Price Mountain window (Figs. 5a, b).Here, the thrust chaos consists of tectonically juxtaposed imbricates
of Paleozoic age separated by faults. The exposure (Fig. 6) shows
brecciated Middle Cambrian Elbrook dolomite lying above a sequenceof folded and boudined Ordovician sandstones, shales and fossiliferouslimestones. A l- 2—nmter thick sequence of cataclastically dis-rupted Silurian quartzite occurs in the center of the Ordovicianrocks., The Ordovician sequence overlies a section of Devonianblack shales. The Devonian shales have, in turn, been mixed into
a mass of recumbent—folded Ordovician sandstones and shales (Figs.7a, b). Originally, hundreds of meters of strata separated these
stratigraphic units. Another exposure in this area (Fig. 5b) shows
Upper Cambrian Copper Ridge sandstone of the Knox Group on top of
Middle Ordovician shales and carbonates of the Martinsubrg Formation.At least 600 meters of geologic section is missing at the tectonic
contact of these two units. J
23
Figure 5
a. Exposure of the Pulaski thrust chaos and mélange of the
décollement zone. Outcrops are on Rt. 659 near the Stroubles
Creek waste Water Treatment Plant.
b. Exposure of the Pulaski décollement zone along the northwest edge
of the Price Mountain window.
24
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AND MELANGE ZONE@21 I20 :1:/4 8, Q 1rr: 12 ‘\rr:T.ggI 'L XT'IOI T. QT.
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25 U
Figure 6
Pulaski tectonic mélange in the Pulaski décollement along the
northwest edge of the Price Mountain window. The outcrop is
along Rt. 659 near the Stroubles Creek waste water Treatment
Plant. Outcrop sketch (Fig. 5a) shows the structure and strati—
graphy of this exposure. Visible in the photograph are the
black Devonian Millboro shales overlain by the boudined 0rdo—
vician Martinsburg limestones, sandstones and shales. The
. fractured blocks of dolomite at the top of the cut probably belong
to the Middle Cambrian Elbrook Formation.
26
27
Figure 7
a. Lower Part of exposure seen in (Fig. 6). Note that black
Devonian shales are folded around the gray-brown sandstones,shales and limestones of the Ordovician Martinsburg Formation.
Tectonism has all but obliterated the bedding in the
Martinsburg section.
b. Folded fault contact between the Ordovician Martinsburg
Formation and black Devonian shales in the Pulaski décollement
mélange. Photograph is just to the left of the writer in
Fig. 7a.
28
29
Fabric of the Mé1ange
Many of the imbricates of the fau1t chaos have a we11 deve1-
oped mé1ange fabric (Hsu, 1968, 1973, 1974; Cowan, 1974, 1978; B1ake
& Jones, 1974; Moore & whee1er, 1978). The Pu1aski fau1t mé1anges
consist of iso1ated "tectonic c1asts" of re1ative1y competent rock
surrounded by a fine-grained "fo1iated" matrix of 1ess competent
argi11ite.
In genera1, the Pu1aski mé1anges are mono-stratigraphic
(i.e. they have deve1oped within discrete stratigraphic interva1s of
various 1itho1ogic components). Each mono-stratigraphic mé1ange
"s1ice" is in contact with other mé1ange s1ices and/or massive b1ocks
of deformed sandstones or do1omites of the chaos (Figs. 5a, b).
The tectonic c1asts in the mé1anges range in size from sma11
pebb1es to bou1ders severa1 meters 1ong. They consist of sandstones,
si1tstones and 1imestones. In some cases, the c1asts are 1ozenge-
shaped boudins of the "choco1ate-b1ock" type. The boudins are sur-
rounded by deformed argi11ite (Fig. 8a). Their surfaces are irregu1ar,
and crenu1ated argi11ite is common1y squeezed into cracks and fo1ds
a1ong the surface. In most cases, the 1ongest dimension of the c1asts
para11e1s a penetrative "fo1iation" or fissi1ity whose p1anar fabric
(is visib1e at the outcrop (Fig. 8a).
Other tectonic c1asts in the mé1ange consist of angu1ar
fragments of competent rock surrounded by argi11ite (Fig. 8b). These
c1asts occur in zones of a1ternating sandstones and sha1es. The
30
Figure 8
a. Boudins of fossiliferous limestones of the Ordovician Martins-
burg Formation exposed in the décollement zone mélange of the
Pulaski thrust. Outcrop is in a railroad cut at the exposures
seen in Fig. 5b. Note the crenulated shales surrounding the
various sized boudins.
b. Deformation by fracturing and extension of interbedded sandstones
and shales of the Ordovician Martinsburg Formation in the
Pulaski décollement mélange. Note the variability in the sizes
of the tectonic clasts. Outcrop is same location as Fig. 7.
31
32
clasts show a "poorly sorted" fabric (Fig. 8b) and contrast with themore regular boudin-shaped clasts.
As mentioned above, the contacts between mélange slices arefolded (Fig. 5a) and faulted (Fig. 5a). Commonly, folds occur whollywithin a single mélange unit. The folds are usually recumbent andgenerally about 0.3 meter across. Faults which cut the mélange fabricare generally at a high angle to this fabric. Drag effects seen inthe mélange are associated with the faults (Figs. 5a, b).
Microstructure
Fissility of the argillite of the Pulaski décollement mélanges(Fig. 8a) is attributed to intense penetrative micro-faulting. In-ternal micro-slickensided surfaces with accompanying tectoniclineations are found on each cleavable plane down to the smallestfissile chip. when developed to maximum intensity, the argilliteshows a phyllite—like luster. These "slip—surfaces" now appearasstylolite—likeconcentrations of iron oxides which form an inter-
secting fabric (Fig. 9a). They are subparallel to the original
sedimentary texture. The fissility is best developed near the inter-faces of shale and sandstone or the shale and limestone where thelithologic contrast is greatest. Large blocks, tens of feet across,of sandstone (Fig. l0), limestone and dolomite in the Pulaski thrust
chaos show extensive fracturing with intersecting slickensided faultsurfaces and en echelon gash veins. In most cases, the sedimentary
fabric has been obliterated by thrust deformation.
33
Figure 9
a. Photomicrograph of deformed sha1e in the thrust chaos of the
Pu1aski déco11ement, a1ong the southwest edge of the Price
Mountain window, near the jucntion of Rts. 114 and 659. The
b1ack intersecting bands are micro-fau1ted "s1ip—surfaces."
The tan and red-brown grains are phy11osi1icates which make up
the majority of the rock fabric. The patches of gray are spaces
in the thin section. The samp1e is probab1y from the Devonian
Mi11boro Formation. Photomicrograph is crossed po1ars and
63X magnification. gf
b. Photomicrograph showing detai1 of a sing1e micro-fau1t s1ip—
surface from the thin section of Fig. 9a. Hematite is concen—
trated a1ong the 1ength of the s1ip-surface. Photomicrograph
is crossed po1ars and 160X magnification.
34
35
Figure 10
A11ochthon of catac1astica11y deformed Si1urian quartzite
in the déco11ement zone of the Pu1aski thrust, a1ong the south-
west edge of the Price Mountain window near the junction of
Rts. 114 and 659.
36
37
Allochthonous sandstones, arkosic sandstones and dolomites
have micro-cataclastic fabrics. within the sandstones and arkosic
sandstones of the Pulaski décollement zone chaos, cataclasis generates
an extremely poorly sorted fabric with angular grains and reduces
the mean grain size. Qualitatively, the effects are seen by comparing
fabric elements in thin sections of cataclastic fault rocks with
fabrics of non—cataclastically deformed stratigraphic equivalents
outside the thrust belt (compare Fig. lla with Fig. llb). Broken
and truncated detrital grains occur within the cataclastic shears
(Figs. llb, l2 and l3). Grain size is dramatically reduced and indi-
vidual fragments are very angular in shape (Figs. llb, l2). The
effects of cataclasis can be quantitatively portrayed using cumulative
grain-size frequency curves (Brock & Engelder, l977). The curves
(Fig. l4) are based on measurements of apparent longest axis of
detrital grains in Silurian quartzites in the décollement zone of
the Pulaski thrust. Curve l is generated from counts made on a non-
cataclastically deformed Silurian quartzite of the décollement zone.
This fabric is identical to the sedimentary fabric of the Silurian
rocks outside of the overthrust belt. Characteristically, curve l
shows a sharp slope indicating a well-sorted fabric, i.e., many grains
fall in a narrow size range. Curve 2 is based on grain-size measure-
ments of a cataclastic quartzite (Silurian) in the décollement zone
of the thrust and exposed along the southwest edge of the Price
N Mountain window. The change in mean grain size and the decrease in
38
Figure 11
a. Fabric of a non-catac1astica11y deformed Si1urian quartzite
from Gap Mountain, northwest of the Pu1aski overthrust sheet.
Photomicrograph is crossed po1ars and 63X magnification.
b. Si1urian quartzite from a Pu1aski déco11ement a11ochthona1ong the southwest edge of the Price Mountain window near
the junction of Rts. 114 and 659. A sing1e catac1astic shear
· cuts across the sedimentary fabric. Note the truncated de-
trita1 grains and the deve1opment of sma11er angu1ar grains in
the shear zone. Photomicrograph is crossed po1ars and
63X magnification.
39
40
Figure T2
Cataclastic shear in thin section of a Silurian quartzitefrom the same location as Fig. llb. Cataclasis generated anew population of smaller angular grains by fracturing andcomminution accompanied by rigid body rotation. The crushedtourmaline grain (yellow-brown fragments) attest to the severe
reduction in grain size during cataclasis; all the fragments
were once part of a single well-rounded detrital grain. Photo-micrograph is crossed polars and l60X magnification.
41
42
Figure l3
Photomicrograph of a cataclastically deformedplagioclasegrain
in an allochthonous mass of Devonian strata in the
décollement zone chaos of the Pulaski thrust along the northwest
edge of the Price Mountain window. The sample is an arkosic sand-stone probably of the Brallier Formation. Disruption of the twin
planes can be seen. Photomicrograph is crossed polars and
l60X magnification.
43
44
Figure 14. Cumu1ative frequency curves on fau1t rocks ofthe Pu1aski deco11ement zone. Each curve isbased on 300 measurements of apparent 1ongestdiameter of grains in thin section.
45
PercentE 99 1g 2·— 95GJE85¤·
70S 60Eg 30 1Q)
. .2 *5E3 5 4
E3U 0 200 I00 50 25 12 Microns
Groin size
Curve 1: Noncutcclostic qucrtziteCurve 2: Cotoclcistic quortziteCurve 3= Ultrocotcclustic qucrtzite
46
slope indicates the generation of a new population of smallergrainsand
development of a more poorly sorted fabric. Curve 3 is based on
counts of grain sizes from an ultracataclastic Silurian quartzite
of the décollement zone. Further reduction in mean grain size is
evident and continued development of a poorly sorted fabric is shown
by the decrease in the slope of the curve. All three curves are taken
from fault rocks that were essentially identical prior to deformation.
.The term "quartz fault gouge" was used by Brock and Engelder
(l977) to describe a "sandstone within which cataclastic deformation
· is so severe that any surviving grains are nearly surrounded by a
fine-grained matrix of crushed grains." Quartz fault gouges, or
ultracataclasites (Sibson, l977) occur throughout the Pulaski décole
lement zone. Within the ultracataclasites, the entire sedimentary
fabric is disrupted by cataclasis (Fig. l5a). Micro—fracturing and
subsequent rotation of broken fragments of detrital grains (Fig.
l5b) characterize the fabrics of these fault rocks.
Cumulative frequency curves, A, B, and C (Fig. l6) show the
characteristic decrease in mean grain size and the development of
a poorly sorted fabric. Also, the three curves were generated from
counts on three mutually perpendicular thin sections; thus, no pre-
ferred orientations of long axis of grains is generated during
cataclasis.Cataclastic deformation has been recognized in the tectonic
clasts of the Pulaski mélanges. Sandstone boudins in the mélange
show well—developed cataclastic and ultracataclastic fabrics (Fig. l7).
47
Figure l5
a. Photomicrograph of an ultracataclastic fabric developed in aSilurian quartzite allochthon in the décollement zone of thePulaski thrust along the southwest edge of the Price Mountainwindow near the junction of Rts. ll4 and 659. Complete dis-ruption of the original sedimentary fabric is evident. ‘Notethe angular grain shapes and the extremely poorly sorted fabricgenerated by cataclasis. Photomicrograph is crossed polars and
63X magnification.
b. A single disrupted detrital quartz grain in the ultracataclasite
(fig. l5a, north—central area). Micro-fracturing and subsequent
rotation of fragments away from the "host" grain can be seen.Photomicrograph is crossed polars and l60X magnification.
48
M
49
Figure l6. Cumulative frequency curves on fault rocks of the
Pulaski décollement zone. Each curve is based on 300measurements of apparent longest diameter of grainsseen in thin section. Curves A, B, C, are based oncounts from three mutually perpendicular thin sectionsfrom the same sample.
i ‘
I50
PercentE 99 1¤E esS CE es Brv”ö· 70 AE°»‘ 50Eä 30
I5 1E"5 5E:1U O ·
200 I00 50 25 I2 MicronsGroin size
Curve 1: Noncotoclcistic quortziteCurves A,B,C= Ultrocotoclostic quortzites
. 51
Figure 17
An u1tracatac1astic fabric deve1oped in a high1y deformed
boudin of the Ordovician Martinsburg Formation in the Pu1aski
déco11ement mé1ange a1ong the northwest edge of the Price Mountain
window, Photomicrograph is crossed po1ars and 25X magnification.
52
53
The contacts between these sandstone boudins and the surrounding
argillite (Fig. l8a) are extremely irregular. In many cases, argil-
lite shreds are mixed into the cataclastic fabric (Fib. l8b).
A feature distinctive of the ultracataclasites are tectonic
stylolites. These stylolites commonly occur within linear zones of
the finest comminuted material (Figs. l9a, b). The stylolites are
defined by residual concentrations of iron oxides. Stylolites in the
cataclastic rocks are usually of low amplitude and have a linear
morphology. Stylolites occur in other rocks in the décollement zone
of the Pulaski thrust. Fossiliferous limestone boudins in the tec-
tonic mélanges show well-developed stylolites (Fig. 20a) as do car-‘ bonate-rich mudstones of the mélange (Fig. 20b).
UMicro—extension fractures are common in rocks of the Pulaski
thrust chaos. These extension fractures are calcite-, dolomite- orquartz-filled. Some of the extension fractures show fiber—type
crystal growth but, in general, interlocking crystal types dominate.
Extension fractures usually occur with various crosscutting relation-
ships (Fig. 2la). They are also present in some of the fossiliferous
material of the thrust chaos (Fig. 2lb). Although cataclasis is the
dominant mode of thrust deformation, intracrystalline deformation
such as development of undulose extinction, deformation bands and/or
deformation lamellae and the formation of recrystallized new grains
(recovery) is evident in thin sections of the fault rocks on the
Pulaski décollement zone (Figs. 22a, b). In some instances, small
new grains have nucleated along distinct zones within a strained
54
Figure 18
a. Contact of a quartzite u1tracatac1asite and a deformedsha1e in the déco11ement zone chaos of the Pu1aski thrusta1ong the southwest edge of the Price Mountain window nearjunction of Rts. 114 and 659. The irregu1arities on the
1ower surface of the u1tracatac1asite were fi11ed with
crenu1ated sha1es which were squeezed into the spaces
created during catac1astic f1ow.
b. Photomicrograph of an u1tracatac1asite (Fig. 18a) showing sha1e
shreds that have been worked into the u1tracatac1astic quartz-
ite fabric at the contact zone. Compare the shreds seen in
this photomicrograph with the fabric of the deformed sha1es ofFig. 9a & b. Note a1so the dark bands or s1ip-surfaces that can
be seen in the sha1e shred in the upper 1eft corner of thephotograph. Photomicrograph is crossed po1ars and 25X
magnification.
55
56
Figure 19
a. Photomicrograph of an u1tracatac1asite from the Pu1aski
déco11ement exposed on the southwest edge of the Price
Mountain window near the junction of Rts. 114 and659.The
1inear b1ack zone is a tectonic sty1o1ite. Re1ative1y
1arge "1ate stage" euheda1 pyrite with hematite rims (center)
and fine1y disseminated minute crysta1s of secondary ca1cite
and/or do1omite can be seen. Photomicrograph is crossed po1ars
and 63X magnification.
b. Same thin section as in Fig. 19a but uncrossed po1ars. Photo-
micrograph is 63X magnification.
57
58
Figure 20
a. Stylolites developed in a fossiliferous boudin of the Ordo-vician Martinsburg Formation in the decollement of the
Pulaski thrust along the northwest edge of the Price Mountain
window near the Stroubles Creek waste water Treatment Plant.
Solution of the shell fragments and recrystallization of lime- ~
stone (lower right) can be seen. Photomicrograph is crossed
polars and 63Xmagnification.b.
Photomicrograph of a thin section from the Devonian Millboro
Formation. The sample is from the décollement mélange of the
Pulaski thrust along the northwest edge of the Price Mountain
window, near the Stroubles Creek waste Water Treatment Plant.
The fossil Styliolina is truncated by a solution stylolite, which
is, in turn, truncated by a calcite—filled fracture. Photo-
micrograph is crossed polars and 63X magnification.
59
60
IFigure 2l
a. Quartz—filled extension fractures in a thin section of theDevonian Brallier Formation. The sample is from a largeallochthon of Devonian strata exposed along the northwestedge of the Price Mountain window, near the junction ofRts. ll4 and 659. Photomicrograph is crossed polars and63X magnification.
b. Calcite—filled extension fractures developed in a phos-phatic Lingula—like shell fragment in an allochthonous
' block of the Ordovician Martinsburg Formation in the thrust ‘
chaos of the Pulaski décollement zone along the southwestedge of the Price Mountain window near the junction of Rts.ll4 and 659. Note the concentration of fractures at thepoint where a smaller fragment has pierced the edge of thelarger fragment. Photomicrograph is crossed polars and
T l60X magnification.
61
62
Figure 22
a. Strain-recovery grain growth along deformation bands in adetrital grain surrounded by an ultracataclastic quartzmatrix. Sample is from the Pulaski thrust décollement melangealong the southwest edge of the Price Mountain window. Photo-micrograph is crossed polars and l60X magnification.
b. Numerous small strain-recovery, recrystallized, new grainsgrowing along deformation bands in a host detrital grain inan ultracataclasite of the Pulaski décollement, along thesouthwest edge of the Price Mountain window. Photomicro-graph is crossed polars and l60X magnification.
0
63
64
detrital grain (Fig. 22a). These zones usually are subparallel to
deformation bands in the host grain (Fig. 22a). Commonly, the new
grains are quite small in comparison to the original detrital grains
in which they have nucleated (Fig. 22b).
To determine a possible correlation between cataclasis and
intracrystalline deformation, comparisons of thin sections from the
fault rocks and the foreland stratigraphic equivalents were made.
.Histograms (Fig. 23) show the important points. There is no signi-
ficant difference in the percentage of original detrital grains
showing intracrystalline deformation. Detrital grains in the folded
rocks of the Valley and Ridge, just northwest of the present edge
of the Pulaski overthrust, show undulose extinction, deformation bands
and lamellae as well as strain—recovery recrystallized new grains.
However, the histograms do not show the striking contrast that exists
between the actual number of individual new grains that nucleated in
the fault rocks versus the number of new grains in the rocks northwest
of the present edge of the Pulaski thrust. The cataclasites show
l0 to lO0 times more recrystallized new grains than the non-
cataclasites.
Interpretation (The structure and stratigraphy of the thrust chaos suggest
- that significant quantities of root zone and overridden rocks were
incorporated into the Pulaski décollement during thrusting. Probably,
the advancing Pulaski thrust cut across a previously folded and
faulted terrain somewhere in the root zone of the thrust, detaching(
65
Figure 23. Histograms showing comparison of intracrystallinedeformation of Silurian quartzites. Percent isbased on the number of detrital grains showingdeformation versus the total number of grains;300 grains were observed in each quartzite group.
66
UnduloseExtmctnon
too 3 _ .Deformotuon Bonds80 ond/or Lomelloe3*, 60 us
-0-„ gj 40 3*,, no20 cf 50 0A B 0 0 A B 0 0
Stroin RecoveryRecrystolluzed Groms _ A
3530 A N0n—cotocIosite, northwest
of thrust belt25 B Protocotoctoisite of the... Puloski decollement3*, 20 .0 C Cotoclosute of thedä', I5 Puloski décollement
D Ultrocotoclosite ofthe'OPuloski décollemeht50 A B 0 0
67
slices of overridden rocks and tectonically mixing them during thrust
transport. A new exposure of the Pulaski décollement zone occurs
along the northwest edge of the Price Mountain window. Figure 24
shows a large block of Mississippian strata (Maccrady Formation)
resting on top of a thick pile of carbonate breccias in the Pulaski
décollement. Field relations suggest the block is a partially
detached slice of window rock which has been rotated and Hdragged"
into the master décollement of the Pulaski thrust. This block probably
represents the initial stages of autochthonous rock incorporation into
the décollement zone. During transport, the faulted contacts be-
tween individual slices in the décollement were folded and faulted.
The fabric elements of the mélange, in particular the choco- °
late-block type of boudinage, appear to demonstrate a flattening
type of deformation. This probably occurred prior to detachment and
thrust transport of the mélange slices, perhaps on a highly attenuated
overturned limb of a root zone fold. Such structures have been
recognized in the root zone of the Pulaski thrust exposed in the
Christiansburg and Ingles-Barringer Mountain windows (Fig. l).
However, other evidence, such as the pervasive fissility and pene-
trative micro-fault fabric in the mélange argillite, support a
shearing type of deformation. Shearing would be expected on the dé-
collement of a large thrust fault. Cataclastic fault fabrics in the
competent rocks of the thrust décollement probably developed in the
initial stages of thrusting. The majority of the cataclasis may have
occurred prior to actual detachment and transport of the allochthonous
68
Figure 24. Exposure of the Pulaski decollement zone along the
northwest edge of the Price Mountain window. Note
the block of Mississippian strata overlying the
breccia. g
69
M ‘ Az äN649
ät-, 3 , LU(D~
5 z2 MI ‘“ 9<¤ " E K} 2E 2 2 6= 6 96“7‘Ü ° 2 2 6($3 · 9 Ü3 lg; ä Z § 'T’. Ü W! •§ g ä äE 22 2 ¤· E
E @999 9 2 9 E· : O E UÜ ‘ ÜÜA A
"}.¤i_.U)
gg 6 .9, 9 ·
70
rocks of the thrust chaos. This hypothesis is strengthened by
recognition of similar cataclastic fabrics in the rocks of the root
zone of the Pulaski thrust. Also, it is probable that maximum stress
in the competent strata, such as the thick Silurian quartzites, would
have occurred just prior to detachment. Following detachment, the
large blocks of competent material would probably have moved freely
with the more incompetent rocks caught in the décollement. Subsequent
deformation would have been largely accommodated by the incompetent
materials.
Conclusions ~Fault rocks in the décollement of the Pulaski thrust form a
fault chaos consisting of discrete imbricates of Paleozoic sedimentary
rocks. These rocks were incorporated into the moving Pulaski décol-
lement zone after detachment from root zone structures. Large blocks
of competent sandstone, arkosic sandstone, and dolomite of the thrust
chaos are highly fractured and display well-developed cataclastic
fabrics. Relatively incompetent strata in the Pulaski décollement,
such as thin—bedded shales, sandstones and limestones, have developed
a distinctive mélange fabric in response to thrust—related tectonics.
East Radford Allochthon
Important to the study of deformation associated with Pulaski
overthrusting are the exposures of rocks in the East Radford window
(Figs. l and 2). The rocks in the East Radford window display a style
7l
of deformation similar to that in rocks of the Pulaski décollement in
the Price Mountain window.
Previous work
The nature of the East Radford window has caused problems for
geologists for many years. Campbell's (l894) earliest work in the East
Radford area led him to conclude that rocks in the window were part
of the overriding Pulaski thrust sheet and equivalent to the Cambrian ‘
Rome Formation, which lies conformably below the Elbrook dolomites.
Years later, Campbell (l925) stated that the rocks in the East Radford
window were Devonian in age. His conclusion was based on the exposures
of typical black shales of Devonian age in the window and on the
presence of quartz conglomerates exposed along the eastern edge of the
window which overlie the black shales. Campbell (l925) reasoned,
incorrectly, that the quartz conglomerates are Mississippian in age
and conformably overlie the Devonian shales. Thus, he concluded that
the window was anticlinal in structure.
Butts (l933, l940) agreed with Campbell's interpretation of
the East Radford window. However, on his map, Butts (l933) shows only
the Devonian Brallier Formation in the window. Cooper (l96l)
recognized that the quartz conglomerates exposed along the eastern edge
of the window were Silurian in age and not Mississippian. Cooper
(l96l) believed the quartzites to be in normal stratigraphic contact
with the underlying Devonian sandstones and shales; thus, he had to
interpret the overall structure of the window to be an overturned
anticline.
72
Mapping by the present writer has shown structure of the
rocks exposed in the East Radford window to be far more complex than
previously recognized.
Structure
Unlike the parautochthonous Mississippian rocks in the Price
Mountain window, the rocks of the East Radford window are entirely
allochthonous with a minimum horizontal displacement of several kilo-
meters. The entire rock mass within the East Radford window is an
allochthon of the Pulaski thrust décollement zone, not unlike the
Ordovician, Silurian and Devonian allochthons around the edge of the
. Price Mountain window. The rocks of the East Radford window rest
in fault contact on parautochthonous Mississippian strata and are
overlain by Cambrian Elbrook dolomites (Fig. 3). The Silurian
quartzites in the window (Fig. 2) are not in stratigraphic contact
but, instead, are in fault contact with underlying Devonian sandstones
and shales (Fig. 3). The contact zone along the edge of the window
shows small patches of Ordovician Martinsburg strata wedged between
the Elbrook dolomites and underlying Silurian and Devonian rocks.
These rocks, too, are imbricates in the Pulaski décollement zone
(Fig. 3).
_U
A completely overturned section of Devonian strata is exposed
in the core of the East Radford window. This section includes parts
of the Devonian Chemung, Brallier and Millboro Formations, all of which
have been severely telescoped and deformed. The structure of the rocks
73
a in the window is complex, but is overall an asymmetric antiform com-
posed of a completely inverted stratigraphic section. The axial
surface of the antiform is folded along an open northwest—trending
fold.
Second order folds (about l00 meters across) occur throughout
. the structure. These folds are of various orientations and very
asymmetric. Third order folds (0.5 meters to several centimeters
across) are found in highly deformed fault zones within the window
structure. Faulting and folding is most intense near the edges of
the window where the Devonian rocks are closest to the contact of the
overlying Silurian, Ordovician and Cambrian rocks, respectively.
Mélange—type fabrics have developed in the thin—bedded Devonian shales
and sandstones near the contact zones. A gradual transition of
mélange from pinch—and-swell to relatively undeformed bedding occurs
T away from the contact zone. However, even rocks exposed in the core
of the structure show extensive thinning of the shale beds between the
more competent sandstones.
Allochthonous wedges of Silurian rocks (Figs. 2 and 3) show
extensive fracturing with smoothly polished, slickensided surfaces
throughout. Primary sedimentary features cannot be recognized. Micro-
scopically, the Silurian quartzites are cataclastically deformed.
Between the Silurian quartzites and the overlying Middle
Cambrian Elbrook dolomites are highly deformed wedges of shale and
limestone of the Ordovician Martinsburg Formation (Figs. 2 and 3).
The slices of Martinsburg Formation show well-developed mélange ‘
74
fabrics. where limestone occurs directly against the Elbrook con-
tact, it appears to have "flowed" into cracks and folds along the
lower surface of the dolomite.
V A well developed planar fabric is evident in the Ordovician
limestones. This fabric parallels the thrust plane surface and
V . consists of sub-parallel, slickensided surfaces with secondary cal-
cite growth. Internally, stylolites parallel these microfaults and
truncate calcite—filled extension fractures. The extension fractures
Vshow calcite fiber growth and twin lamellae, both distorted into the
plane of stylolite growth.
Interpretation and Conclusions .
The East Radford window exposes a complexly folded and faulted
· allochthon of Ordovician, Silurian and Devonian strata. The alloch-
thon represents a significant quantity of rock, detached from the
root zone of the Pulaski thrust and transported several miles to the
northwest.
Rocks in the East Radford window probably were folded and
faulted prior to detachment. Furthermore, these rocks show that at
least in some places the Pulaski décollement, typically at the base
of the Middle Cambrian Elbrook dolomite, was abandoned for other suit-
able stratigraphic horizons, i.e., the Devonian shales for example. A
minimum of l50 meters separates the Elbrook dolomite at the top of the
East Radford structure from the lowest exposed Devonian strata in the
75
core of the antiform. A fault at depth must separate the antiform
from parautochthonous rocks below. The style of deformation in
rocks of the East Radford window is similar to that of the décol—
lement allochthons in the Price Mountain window.
Max Meadows-Pulaski Breccias
_ Spectacular exposures of tectonic carbonate breccias occur
in the décollement zone around the edge of the Price Mountain and
East Radford windows (Figs. 25a, b). Like the Pulaski fault itself,
the study and interpretation of these deformed carbonate rocks has
a long and interesting history.
Previous work
Campbell (l894) interpreted the carbonate breccias to be the
product of "Paleozoic overlaps" which occurred during the deposition
of the carbonates (Cambrian and Ordovician). His interpretation is
quoted below:
In places it seems like a subaerial deposit, whilein others it appears like a true water-deposited con-glomerate. Its meaning is obscure, but the writeris of the opinion that it indicates the existence ofoverlaps in early Paleozoic time, probably during thedeposition of the Shenandoah limestone itself; thatthe limestone was folded and elevated above sea leveland formed cliffs, along which the masses and halfrounded fragments washed from the bank were recementedand formed these curious deposits of conglomerate.
Over twenty years later, Campbell (l925) was much less l
inclined to attribute the breccia to the geologic past. Campbell
(l925) postulated the breccias to have been formed by: l) deposition
76 _
Figure 25
a. View looking northwest across the floodplain of the New River
Valley. The white bluffs along the river, in the right middle
ground, are outcrops of folded, faulted and brecciated Elbrook
dolomites of the Pulaski overthrust sheet. The linear ridge
in the background exposes southeast dipping parautochthonous
- Mississippian strata at the edge of the Pulaski thrust.
b. Carbonate breccias, Max Meadows—Pulaski type, of the Pulaski
décollement zone exposed in a 20—meter high cliff along the
New River just east of the Radford Army Arsenal. Note the
vertical beds in the lower right corner of the photo and the
gently dipping beds above and to the left of the writer. The
zones showing no recognizable bedding are "tectonic" breccias
(Cooper, l946).
77
78
on the surface by springs, 2) deposition in the channels of streams
flowing on the surface, and 3) deposition in caverns and underground
channels. Campbell likened the deposits to the calcareous tufa de-
posits of Yellowstone Park.
Butts (l940) interpreted the carbonate breccias as "cave
fillings" formed from erosion and deposition of the carbonates byunderground streams running through the dolomites and limestones.
The tectonic nature of the carbonate breccias was first
recognized and described well by Cooper (l946, l96l, l970), who named
them the "Max Meadows-Pulaski Breccias" due to their association with
the Max Meadows and Pulaski thrust faults. Cooper (l946) recognized
two distinctive "facies" of the breccias, which represent end members
of a continuous series of rocks showing different degrees of cata-
clastic deformation. These two end members are the "autobreccias" and
the "tectonic breccias" (Cooper, l946).
Structure
A 20-meter thick zone of breccias exposed in the Norfolk &
Western Railway cut along the northwest edge of the Price Mountain
window (Fig. 25b) shows all stages of this breccia series.
Autobrecciation at this exposure involves extension fracturing
of thin—bedded dolomites with little or no rotation of the individual
fragments (Figs. 26a, b). The complex intersecting extension frac-
tures are filled with calcite and dolomite.
79
Figure 26
a. Thin—bedded dolomites of the Elbrook Formation showing i
initial stages of "autobrecciati0n" (Cooper, l946). These
vertical beds can be seen in the lower right corner of
Fig. 25b. · ‘
b. Dolomite- and calcite-filled extension fractures in
autobrecciated Elbrook dolomite shown in Fig. 26a.
80
( 8l
In contrast to the autobreccias, the tectonic brecciasconsist of broken fragments which have been rotated, comminuted,
rolled out and mixed into a mass containing other carbonate clastswithin finely ground-up dolomite and calcite (Fig. 28). Carbonate
clasts ranging in size from less than a centimeter to over 20 centi-meters in diameter occur in the tectonic breccias. The clasts arecemented together by fine-grained dolomite and calcite.
( The entire exposure (Fig. 25b) shows alternating zones oftectonic breccia and autobreccia. Within the thicker zones of tec-tonic breccia are "mega—clasts" of bedded dolomite surrounded bytectonic breccia. Some of these "mega—clasts" are over l0 meterslong and in cases are rotated and internally folded (Fig. 27).
Microstructure
Dolomites in the Max Meadows—Pulaski breccia zones have dis-‘ tinctive micro-fabrics similar in structural style to deformation
on the outcrop scale described previously.
A micro-autobreccia fabric (Fig. 29) consists of a complexnetwork of calcite- and dolomite-filled extension fractures which havedisrupted the sedimentary fabric. Sedimentary laminations and sty-lolites can be traced across the extension fractures (Fig. 29)indicating that no rotation of the fragments has occurred.
In contrast to the micro-autobreccias, micro-tectonic-breccias (Fig. 30) show cataclastic fabrics. Intersecting micro-shears disrupt dolomite grains, dragging them into zones of comminution
82
Figure 27
A broken recumbent fo1d in "autobrecciated" (Cooper, 1946),
thin-bedded E1brook do1omite. The exposure is c1ose to the out-
crop shown in Fig. 25b. Note the extension fracturing of the
do1omite beds, asymmetry of the fo1d and minor fau1ting.
83
84
Figure 28
Block of “tectonic breccia" (Cooper, l946) from the Pulaski
thrust décollement zone exposed at the location of Fig. 25b. The
photograph shows the various sizes and shapes of carbonate clasts
(brown and bluish) mixed in a matrix of pressure-solved dolomite
and calcite. Note the large clast of dolomite showing sedimentary
laminations.
85
86
Figure 29. Autobrecciation in a thin section of the Cambrian
Elbrook dolomite. Sample from an outcrop near
location of Fig. 25b. Non-shaded areas are
extension fractures filled with crystallinedolomite.
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Figure 30
Photomicrograph of a dolomite "mico—tectonic breccia" (Cooper,
l946) from the Pulaski decollement zone along the northwest edge of
the Price Mountain window. Angular fragments of dolomite were
plucked from the original sedimentary fabric along cataclastic shear
zones. The cataclastically generated matrix was then welded
together by pressure solution of dolomite. Photomicrograph is
crossed polars and 63X magnification.
89
90
and rotation (Fig. 30). Following the cataclasis, the angular poorly _
sorted fragments are "welded" together by pressure solution within the
fine-grained dolomite and calcite.
In places, the massive, tectonized dolomites of the décol—
lement zone show complex patterns of stylolites. These stylolites
are not simple bedding plane stylolites as shown by their complex
orientations (Fig. 32). Relating specific orientations to any knownU
or postulated stress fields associated with Pulaski thrusting is
difficult, because the host blocks have undergone rotation during
tectonic transport in the décollement. However, relative orientation
analysis of the stylolites in the dolomites of the décollement zone
show that they are not simple bedding parallel or bedding normal types.
Figure 32 shows 3 rose diagrams of relative orientations of stylolites
in a block of dolomite caught in the Pulaski décollement along the
southwest edge of the Price Mountain window. To measure the orien-
tations of stylolites, 3 mutually perpendicular faces were cut rela-
tive to the orientation of the Pulaski thrust at the collection site;
the interpreted fault plane is N45w/30Sw. Figure 3l shows diagram-
matically how the relative orientations were measured.
Figure 32 shows that stylolites occur in many orientations
with several sets of stylolites showing somewhat preferred Iorientations. In a few cases, stylolites could be traced across a
, slabbed face facilitating measurement of a stylolite surface. Figure
33 shows poles to stylolite surfaces in the dolomite sample. Two
clusters of points occur near the southern end of the diagram and a
91
Figure 31. Orientation of three mutua11y perpendicu1ar s1abbed
faces of a fau1t rock with respect to the Pu1aski
fau1t. P1us and minus signs refer to the re1ative
directions used in preparing the rose diagrams. Samp1e
was co11ected a1ong the southwest edge of the Price
Mountain window, near the junction of Rts. 114 and
659. Here, the Pu1aski fau1t strikes N45w and dips
35SW.
92
/Ä5trike af fault/ plane (N45W)
////
"‘·‘
x /xx; + —
/ décollemenlsurface
—°°'“ D harizanlal
Dip af faulf planej_“* —.. __
(30swl
93
Figure 32. Rose diagrams of tectonic stylolites in Pulaski fault
rocks. The length of the line equals the number ofi stylolites in that orientation.
94
I·45° +45°
~ 1
·90° +90°(55 stylolites)
Face A (perpendicular ta strike and dipof fault plane) (
00I
-45° +45°x /
-90° +90°(4l stylolites)
Face B (parallel to strike and dip of‘ fault plane)
O lU2
(number afl...—l.-J stylolites)
O0I
-4 ° 45°ä ,+
\\ „·9O° · +90°(67 stylalites)
Face C (parallel ta strike and perpendicularto dip of fault plane)
95
Figure 33. Pi diagram of poles to stylolite surfaces in Pulaski
fault rocks. The interpreted orientation of the
Pulaski thrust, N45w/35Sw, is included.
96
N
WE
s
97
scatter in the northeast quadrant. The plane of the fault at the
collection site is included,
Interpretation and Conclusion
The Max Meadow-Pulaski breccias define a cataclastic defor-
mation regime for the décollement of the Pulaski thrust. Cataclasis
occurs on both mesoscopic and microscopic scales. The two breccia
"facies," autobreccia and tectonic breccia, are end members in a
continuous series of cataclastic rocks of the Pulaski décollement
zone.Sequentially, the deformation history that formed the breccia
consist of:
l. extension of thin—bedded dolomite along the detachment
surface of the thrust during the early stages of faulting; this
forms the autobreccia fabric,
2. rotation and comminution of previously extended dolomite
in zones of cataclastically deforming rock; crushing forms the fine-
grained tectonic matrix of calcite and dolomite within which the
clasts are rotating; this forms the tectonic breccias,
3. dolomite clasts are fed to the zones of tectonic
breccias from along the edges of the autobreccia zones; as tectonic
breccia zones thicken, the autobreccia zones decrease in size,
4. thinned autobreccia zones begin to rotate as entire units
within thick zones of tectonic breccia, i.e., essentially as
98
"mega-clasts" of the breccia; folding of the thin slices of auto-
breccia occurs during transport,
5. following movement and cataclasis of the tectonic
breccia zones, the fine-grained rock "flour," consisting of dolomite
and calcite, undergoes pressure-solution, which welds the clasts
together into a coherent mass.
PARAUTOCHTHONOUS ROCKS
Price Mountain window I
Parautochthonous rocks of the Price Mountain window display
a décollement style of deformation similar to that described in other(
areas of the Southern Appalachians (Harris & Milici, l977). The window
rocks support a tectonic model which includes significant deformation
in overridden rocks as well as deformation in allochthonous thrust
sheets.Deformationfeatures of the parautochthonous Mississippian
strata in the Price Mountain window include both normal and thrust
faults, folding of at least three orders, stylolites, and quartz-
filled gash fractures.
Structure
The major structure exposed in the Price Mountain window is
an asymmetric doubly plunging anticline. In the study area (Fig. 2)
the fold plunges west beneath the overthrust sheet. Second and third
order folds occur near and on the hinge of the major anticline or
along fault zones within the window rocks. Locally and on both the
southeast and northwest limbs of the fold, the bedding is verticalI to overturned.
A roadcut in the northwest limb of the anticline near the
nose of the structure exposes the Mississippian Maccrady and Price
Formations (Fig. 34a). Structures at this outcrop are a series of
step-like normal faults and several low angle thrustfaults.99
100
Figure 34
a. Fau1ts on the northwest 1imb of the Price Mountain window,
exposed near the Stroub1es Creek waste water Treatment
P1ant.
b. Zone of deco11ements in the Mississippian Price Formation
on the southeast 1imb of the Price Mountain antic1ine, west
end of the Price Mountain window. Coa1 seams are shaded.
10]
Muss Pmcz FM Muss. mxccnnov FM__ / _ SE jj ~.
E,H·•A B‘
MISS MACCRADY ru. NW
0
8 Co so soo rau
30·j
25 .ä
W20___v5IO
*2 -= _, _2 ;.,, "TALUSO
~sw RR wncxn —‘
l02
Displacements on the faults range from less than 6 meters to
several hundred meters. Although many of the faults are discrete,
slickensided surfaces (Fig. 36a), other faults are thick deformation
zones (Fig. 35b) consisting of contorted, fractured and extensively
jointed rock (Fig. 35b). Quartz—filled gash fractures (Fig. 36a),
formed approximately perpendicular to bedding, are found in association
with the fault zones. An unusual segregation of dolomite and shale
(Fig. 36b) occurs in the Mississippian Maccrady Formation near the
faulted rocks.
A thrust fault near the hinge of the Price Mountain anticline
(Figs. 34a, 37a, b) places the massive sandstones of the Price
Formation northwest over younger shales and sandstones of the Mac-
crady Formation. Minimum displacement on the fault is several hundred „
meters. within the fault zone are disrupted sandstone beds and iso-
lated sandstone boudins, both surrounded by tightly folded shales
(Fig. 37a, b) that are cut by an intersecting network of smaller
shears. This thrust fault ramps sharply upward (Fig. 34a) through
the Price sandstones. A weakly defined fabric of crenulated shales
and isolated blocks of fractured sandstones parallel the fault
contact and truncate bedding planes (Fig. 34a).
The thrust fault can be traced to a zone of parallel décol—
lements within thin coal seams of the Mississippian Price Formation
on the southeast limb of the Price Mountain anticline (Fig. 34b).
Associated with the décollements are disharmonic second and third •
order recumbent folds. These folds are cut by numerous small shears
l03 _
Figure 35
a. Normal fault along Rt. 659, near the Stroubles Creek waste
water Treatment Plant. This fault occurs on the northwest
limb of the Price Mountain anticline. Rock hammer seen along
the lower edge of the photograph gives scale.
b. Deformation in the dropped block of the normal fault of
Fig. 35a. Note the pervasive jointing about perpendicular
to bedding.
l05
Figure 36
a. Quartz—filled gash veins in Maccrady siltstones along Rt.
659, near the Stroubles Creek waste water Treatment Plant.
The outcrop is in the northwest limb of the Price Mountain
anticline. The surface in the lower left of the photograph
is a small slickensided fault plane oriented at a high
angle to bedding.
b. Unusual segregations consisting of alternating layers of
shale and dolomite (tan colored) in the Maccrady Formation.
The exposure is along Rt. 659 in the northwest limb of the
Price Mountain anticline. Bedding in the photograph dips
to the right at about 30 degrees.
lO74
·
Figure 37
· a. Low angle thrust along Rt. 659 near the Stroubles Creek
Waste water Treatment Plant on the northwest limb of the
Price Mountain anticline. The Mississippian Price sandstones
(upper part of the photograph) have been thrust over the
Mississippian Maccrady Formation. Note the fissility in the
shales below the thrust contact. Displacement on this fault
is several hundred meters. .
”b. Another view of the low angle thrust (Fig. ). The hammer
head is parallel to the thrust contact. Highlydeformedshales
occur along the detachment surface; relative motion
was upper plate to the right. Fracture—formed boudins are
present in the Price sandstones above this detachment zone.
108
109
in the déco11ement zone. Characteristica11y, detached and rotated
fo1d hinges are juxtaposed to one another a1ong the anastamosing
déco11ement shears. This typica1 "broken formation zone" (Harris &
Mi1ici, 1977) has been described in association with déco11ements on
thrust fau1ts in other areas of the Southern Appa1achians.
In tné fau1t zone, the coa1 seams show extensive deformation
with f1owage in and around the hinges of the recumbent fo1ds. The
f1owage zones contain many sma11 c1ose1y spaced, asymmetric fo1ds.
Each fo1d is about 3 cm across with s1ickensided surfaces on both
1imbs. The asymmetry of these fo1ds indicate a northwest disp1ace-
ment on the fau1t. Many sma11 norma1 and thrust fau1ts occur in
the vicinity of the exposure (Fig. 34b).
Miscrostructure”
Micro-deformation features in the Mississippian rocks of the
Price Mountain window inc1ude fau1ting, extension fracturing, cata-
c1asis and formation of sty1o1ites. These features are 1oca1ized
near fau1t zones within the window rocks or deve1oped near the over-
riding Pu1aski thrust b1ock.
A fine-grained arkosic sandstone of the Maccrady Formation,
from parautochthonous strata at the present edge of the Price Mountain
window, is the first competent unit be1ow a 3 to 6 meter thick zone
of mixed a11ochthonous sha1es and catac1astic rocks of the Pu1aski
thrust déco11ement. Chrono1ogica11y, the deformation sequence seen
in this rock (Fig. 38) consists of:
110
1. formation of sty1o1ites para11e1 to origina1 sedi-
mentary 1aminations,, 2. micro—extension fracturing with quartz fi11ing,
I
3. micro—norma1 fau1ting and micro—thrust fau1ting,» 4. deve1opment of thin catac1astic zones a1ong the
micro—fau1ts.
The sty1e of micro—deformation para11e1s the mesoscopic
deformation seen in the outcrops of parautochthonous Mississippian
rocks (Fig. 34a). The micro—structura1 features described above
appear to be re1ated to the proximity of the overthrust sheet.
Comparison of simi1ar 1itho1ogies in the window rocks, at 1east 30
meters be1ow the overthrust contact and away from 1oca1 fau1ting,
showed no sty1o1ites, fracturing,micro—fau1ting or catac1asis.
Detached b1ocks of Mississippian strata, representing the
youngest rocks invo1ved in Pu1aski thrusting, occur about 6 ki1o—
meters north of the Price Mountain window. The transported rocks show
we11-deve1oped catac1astic fabrics (Fig. 39a). Rooted rocks
equiva1ent to the a11ochthonous b1ocks are catac1astica11y deformed
in a simi1ar way (Fig. 39b). Catac1asis in the rooted rocks indicates
that considerab1e deformation occurred prior to actua1 detachment
and transport.
Catac1astic fabrics occur in Mississippian rocks within
fau1t zones of the window, away from the over1ying Pu1aski thrust
sheet. These fabrics, deve1oped in the Mississippian Price sand-
stones, are simi1ar to those deve1oped in a11ochthonous Mississippian
strata.
111
Figure 38. Microstructure in a thin section of the Mississippian
. Maccrady Formation. Top of page is stratigraphic up.
Samp1e co11ected on the southeast 1imb of the Price
Mountain antic1ine, near the junction of Rts. 659
and 114.'
112
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sedimentcryIominotionsstylolitesexgenstionroc uresmicrofoultscotcclostic__ zonyes
x\“‘ ‘ "'
0 0.5 I.0 cm.
113
Figure 39
1 a. Cataciastic shears in a thin section of a quartz pebb1e
cong1omerate of the "Ho1iday Inn beds" (Lowry, 1971).
The samp1e occurs as a tectonic erratic severa1 mi1es
north of the Price Mountain window. These rocks are the
youngest strata known to have been invo1ved in Pu1aski
thrust fau1ting in the Price Mountain and East Radford
areas. Photomicrograph is crossed poiars and 63X
magnification.
b. Catac1astic fabric deveioped in rooted "Ho1iday Inn beds"
(Lowry, 1971) a1ong the southeast edge of the Price Mountain
antic1ine. Compare the grain size of the sing1e "undeformed"
grain seen here (Targest in the photomicrograph) with the_ grain sizes in the non-catac1astica11y disrupted patches of
fabric in Fig. 39a. Photomicrograph is crossed po1ars and
63X magnification. A
114
l|5
Cataclastic deformation in arkosic sandstones of the Price
Formation in the Price Mountain window is well documented by dis-
ruption of detrital feldspar grains. Grain fracturing and subsequent
rotation of the fragments occurs during cataclasis (Fig. 40).
To quantify the cataclastic deformation of the feldspar
grains, comparative grain counts were made on rocks with increasing
degrees of cataclasis (Fig. 4l). All the samples used in the study
were collected from a fault zone in the Price Mountain window (Figs.
37 a, b). Sample A and B are from rocks about 0.5 meters above the
fault zone. Sample C is at the contact of the fault zone and rela-
tively undeformed rock. Sample D is from the center of the fault
zone. All samples are from the Mississippian Price formation and are
mineralogically and petrologically identical. Polysynthetic-twinned
detrital feldspar grains were observed in thin sections of the four
rocks. Deformed grains (Fig. 40) and undeformed grains were counted
to calculate percentage of deformed grains versus the total. Histo-
grams (Fig. 4l) show a dramatic rise in percent of cataclastically
deformed feldspar grains of rocks caught in the fault zone. Almost
80 percent of all detrital feldspar grains are disrupted in the
fault zone.
Finally, detrital muscovite flakes in the cataclastically
deformed arkosic sandstones show bending, kinking and grain dis-
ruption in response to the faulting (Fig. 42).~
116
Figure 40
Photomicrograph of a catac1astica11y deformed p1agi0c1ase
grain in the Mississippian Price Formation of the parautochthonous
window rocks in the Price Mountain window. This samp1e is from a
fau1t zone on the northwest 1imb of the Price Mountain antic1ine, nearC
the Stroub1es Creek waste water Treatment P1ant (Rt. 659). Photo-
micrograph is crossed po1ars and 160X magnification.
117
118
Figure 41. Histograms showing comparative deformation ofdetrita1 fe1dspar grains. Fifty grains observedin each group.
H9
tooE¤‘•1::<¤EE .*6 soU
. *6E .aaSaa OCL A 6 c 0
A ond*B= 0.50 meters obove foult zoneC1 At the detochment surfoce of
the foultD: Boudin in the foult zone
· l20
Figure 42
Cataclastically disrupted (kinked) detrital muscovite flake
in a thin section of the Mississippian Price Formation. Outcrop
exposing the Price is along Rt. 659 just above the Stroubles Creek
waste water Treatment Plant on the northwest limb of the Price
Mountain anticline. Photomicrograph is crossed polars and
l60X magnification.
121
122
Interpretation
Pu1aski thrust sheet rocks are fo1ded into an antic1ine
para11e1 to the structure of the Price Mountain window. This
probab1y means that some fo1ding of the window rocks occurred after
emp1acement of the Pu1aski thrust sheet. Based on this assumption,
the chrono1ogica1 sequence of deformation in the parautochthonous
Mississippian window rocks is: _
1. detachment of Mississippian and younger strata from
parautochthonous window rocks during movement of the Pu1aski thrust
across the area; concurrent1y, catac1astic fabrics deve1op in rocks
c10se to the overriding thrust sheet; fo1ding and fau1ting occur in
the window rocks themse1ves,‘
2. emp1acement of the Pu1aski thrust sheet; major anti-
c1ina1 fo1d continues to deve1op in the Mississippian window rocks,
3. bedding para11e1 déco11ements deve1op in the incompetent
coa1 seams of the Mississippian Price Formation; these eventua11yB ramp from the southeast 1imb of the Price Mountain antic1ine upward
through younger strata and cut across the crest and northwest 1imb
of the antic1ine (Price sandstones are thrust over Maccrady sha1es
and sandstones); catac1astic fabrics deve1op in rocks a1ong these
fau1t zones a1ong with meso- and microscopic ·deformation features
inc1uding recumbent fo1ds, broken formation zones, micro-fau1ts,
sty1o1ites, etc,
4. norma1 fau1ts offset the thrust fau1ts.
123
Conc1usions
It is c1ear that parautochthonous rocks overridden by thrust
fau1ts are not "passive e1ements" (Harris & Mi1ici, 1977). Significant
deformation has occurred in the overridden rocks during thrust—
re1ated tectonics. Parautochthonous rocks in the Price Mountain
window p1ayed an active ro1e in the A11eghanian orogeny. As out-
1ined above, fo1ding and fau1ting have occurred in these rocks.
A comp1ex zone of subpara11e1 déco11ements are present in
the Mississippian Price Formation on the southeast 1imb of the Price
Mountain antic1ine. Associated with this detachment zone are numerous
sma11er fau1ts and fo1ds. A simi1ar zone of déco11ements in the
Ordovician Martinsburg Formation is suggested by data gathered from
the 300 meter test we11 dri11ed atop Price Mountain (Cooper, 1961;
Harris & Mi1ici, 1977). A1though some of this deformation probab1y
occurred prior to and during Pu1aski thrusting, a significant part of
the deformation undoubted1y occurred after emp1acement of the thrust
sheet.
STRUCTURAL SYNTHESIS
Mechanics of Pu1aski Fau1ting
The importance of a catac1astic deformation regime in the
déco11ement of the Pu1aski fau1t was shown by Cooper (1970). He
rejected the hypothesis (See Rogers, 1970) that Pu1aski fau1ting was
faci1itated by s1ippage a1ong a sa1t and gypsum 1ayer in the Cambrian
and Ordovician rocks of the stratigraphic section. Instead, he gave
amp1e evidence that catac1astic f1ow a1ong thick zones of brecciated
carbonate rocks faci1itated movement of the thrust sheet. This mode1
for Pu1aski thrusting is strengthened by the present writer's
recognition of the pervasive catac1astic deformation characteristic
of the déco11ement zone chaos. Catac1asis occurs on a11 sca1es from
the fracturing of individua1 detrita1 sand grains to the tectonic
mixing of a110chthons in the déco11ement chaos. This was the dominant
deformation mechanism associated with thrust movement.
Catac1asis a1one, however, does not characterize the déco1-
1ement zone of the Pu1aski thrust. Significant quantities of re1a-
tive1y incompetent rock, inc1uding thick s1ices of sha1es and thin-
bedded sandstones, sha1es and Timestones, occur in the déco11ement
_ zone. S1ip a1ong the interface of the competent and incompetent rocks
in the deco11ement undoubtediy faci1itated thrust movement. Further-
more, it is probabie that penetrative micro-fau1ting in the mé1ange
argi11ites a1so contributed to movement of the Pu1aski thrust.
124
l25
( Thus, the deformation features of the Pulaski décollement
described in this report combine the two models for décollement
deformation mechanisms. Cataclasis in the competent rocks of the
décollement, and "slippage," controlled by the incompetent rocks of
the décollement, combined to facilitate movement of the Pulaski thrust.
Each rock type developed a unique set of fault fabrics in response
to the deformation in the décollement zone.
Seguence of Deformation Associatedwith Pulaski Thrusting ·
Allochthonous rocks of the Pulaski décollement zone, including
the large allochthon in the East Radford window and the parautoch-
thonous rocks overridden by the Pulaski thrust, define a sequence of
deformation associated with a major thrusting episode during the
Alleghanian orogeny (Table l). The sequence of deformation events
as shown by this study has several important implications to our
understanding of the evolution of the overthrust belt of the Southern
Appalachians. These are: ·
l. complex folding and faulting preceded detachment of
the Pulaski thrust. These root zone folds were not passive elements
subordinate to growing thrusts but active tectonic structures
developing prior to the thrust faulting,
2. thrusting, with décollements confined to incompetent
stratigraphic units does not fully characterize the Pulaski fault.
Both competent and incompetent rocks occur on the décollement of
the Pulaski thrust.
126
Tab1e 1. Re1ative timing of deformation associated withPu1aski fau1ting.
AII0chth0n0us Rocks Parautochthonous Rocks1.C 'O¤‘• 3 m
C 1n O m _-1->‘·- 4-* z .C 'O rv4*, 3 G)-1-J C •r-V7 U GJ P'- VUOYU3 O >„c: COS- .CI(J °P-G)·1—*•r- r—·r—-U).C Uv- S-CL/I.C. w4-*1/IOWI- •P-S- O.•r- 4-*1/'IEr-—wC
L5 .C.Q 1--|·—*•r-MS-.3 •I-·Z v- gw *4—UCgUOw1/14-*
CI/7 w CCU OWCGJC ><<IJww1--QC
3 -1-*C •1·· P--I-*C)r—·S—L)I Q. 1-w 'OCOr—"OC!.C!Jw.CI- I 31- r·—·•r-L)3CE>·|-*+-*Q1 4-*(8 LI-E LI-4-*C3‘4-ZLJS-L);Q ¤_ . „ „ „ „
r-' 1- C\1 (V7 Ü1-1M
4-*C
.1 S- •r— I föID OU7 v- M S- 4-*Q. ¤.C M uv CD C
(/)•r···+-*U7 (U (U > 3I C'O<Dw 1-- CD 04-*0I- Ü, ms--m1— 3 c mzr-•
C S—S-.C3 Q. YU OGJ3
tg<[ AS- 4-*C-I->O 3 -/0--
3 I- *é“£°ä°"äi«;""“°‘6 "-1- •1·-.C G)
Lj 0 ,„_, mm E¤wEo 1n-1-> 1:cn(./7 _¥ VÖ UVP-C
<[ (/7
4-* r-·1-—U5L)$—.O 4-*'U1-4-*3O ¤_ <U'4—30‘<Uw‘<1JO¢'¤ wv-OCw
O O O O O I O
r—· C\1 (V7 Ü 1- G1 (*7
OLL.L1.!
L!.I U
4-) 'Y“
(5 O S- C•r·1-1 O) S. 4-* w FU•r- S1- -1-*
_ •——• .,.. U- 3 CI- 4, O m 31.1.1 "’ cw gov? za
3> s_ C CUI-I *1- •r··r-"·:. 5 UII |"“ 'I"
..1 ,„_ OS-OO S--U)
GJUISQ) UI SI- -/'___ M:} MCT- CQ)3 •r-S-U7•r-OL/7 CCDCD':-CI
II./7••— 1- Cr-•1-·
(Q) m +>m¢¤m -—u-•—>U "‘ä'“ääB "°33”
Q. 3 r— 3°' ¢¤o«¤¢¤¤¤¤ ccmZNLLCLLLJ L1..wL1..
J cd I6-? .-J cd
l28
3. if the initial detachment of the Pulaski fault formed at
the Elbrook-Rome contact, then subsequent décollement "splays"
occurred below the first detachment surface. This must be true, since
significant quantities of rock below the overriding Elbrook were
incorporated in the décollement. The structure of the East Radford
window shows that the Elbrook décollement surface was abandoned for
lower level décollements formed in overridden Devonian rocks (Fig. 3).
In a similar way, lower level décollements in the parautochthonous
rocks of the Price Mountain window (Fig. 3) formed after the
emplacement of the Pulaski thrust. Here, younger, lower—level
décollements appear to cut across older décollements which are
structurally above them.
CONCLUSIONS
Detailed study of the fault rocks in the décollement of thei
Pulaski thrust fault have shown that a complex interplay of competing
deformation mechanisms has occurred during thrusting. The deformation
mechanisms include:i
1. formation of a highly deformed fault chaos
2. extensive fracturing of competent strata v3. formation of melanges in incompetent strata
4. slip along competent- incompetent rock interfaces
5. slip within penetatively micro-faulted mélange argillite
6. folding and faulting of allochthons within the décollement
7. development of cataclastic fabrics
8. development of intracrystalline deformation fabrics
9. extension fracturing
10. formation of tectonic stylolites
This study has shown that characterization of deformation on
a décollement of a major thrust fault, such as the Pulaski overthrust,
as either "cataclasis" of "shale slippage" alone does not accurately
describe the complexities of thrust décollement deformation.
Finally, important to the understanding of the evolution of
the overthrust belt of southwest Virginia, the rocks of the Pulaski
fault and those rocks overridden by the thrust show that:
129
l30
a. folding and faulting preceded overthrusting
b. in some cases, lower—level décollements are younger than
structurally higher décollements and the lower, younger décollements
may cut older décollements
c. significant deformation occurs in overridden parautoch-
thonous rocks.
REFERENCES
Blake, M. C., Jr., and Jones, D. L., 1974, Origin of Franciscanmelanges in northern California, in Dott, R. H., Jr.,and Shaver, R. H., eds., Modern and ancient geosynclinalsedimentation: Soc. Econ. Paleontologists and Mineral-ologists Spec. Pub. 19, p. 345-357.
Brock, N. G., and Engelder, J. T., 1977, Deformation associated withthe movement of the Muddy Mountain overthrust in theBuffington window, southeastern Nevada: Geol. Soc. Am.Bullz, v. 88, p. 1667-1677.
1 Butts, Charles, 1927, Fensters in the Cumberland overthrust blockof southwestern Virginia: Va. Geol. Survey Bull.28, p. 12.', 1933, Geologic map of the Appalachian Valley of Virginiawith explanatory text: Va. Geol. Survey Bull. 42, map,
A p. 68.
, 1940, Geology-of the Appalachian Valley in Virginia(part 1): Va. Geol. Survey Bull. 52, p. 568.
Campbell, M. R., 1894, Paleozoic overlaps in Montgomery and PulaskiCounties, Virginia: Geol. Soc. Am. Bull., v. 5, p. 171-190.
j 1925, The Valley coal fields of Virginia: Va. Geol.Survey Bull. 25, p. 300.
Cooper, B. N., 1936, Stratigraphy and structure of the Marion area,Virginia: Va. Geol. Survey Bull. 46, p. 211. g, 1939, Geology and mineral resources of the Draper Mountain’area, Virginia: Va. Geol. Survey Bull. 55, p. 98.
. , 1946, Metamorphism along the "Pu1aski" fault in theAppalachian Valley of Virginia: Am. Jour. Sci., v. 244, p.95-104.
, 1961, Grand Appalachian excursion: Virginia Polytech.Inst. Eng. Ext. Ser., Geol. Gdbk. no. 1, p. 187, 78 plates,15 figures, 2 tables.
, 1970, The Max Meadows Breccias: A Reply, in Fisher, G. H.,et_al„ (eds.), Studies of Appalachian Geology: central andsouthern, Wiley-Interscience; p. 179-191.
131
132
Cooper, B. N., and Haff, J. C., 1940, Max Meadows fau1t breccia:Jour. Geo10Qy„ v. 48, p. 945-947.
Cowan, D. S., 1974, Deformation and metamorphism of the Franciscansubduction zone comp1ex northwest of Pacheco Pass, Ca1i-fornia: Geo1. Soc. Am. Bu11., v. 85, p. 1623-1634.
,1978, 0rigin of b1ueschist-bearing chaotic rocks intheFranciscanComp1ex, San Simeon, Ca1ifornia: Geo1. Soc. ;Am. Bu11., v. 89, p. 1415-1423. ·
Enge1der, J. T., 1974, Catac1asis and the generation of fau1t gouge:·”Geo1. Soc. Am. Bu11., v. 85, p. 1515-1522.
' Gwinn, V. E., 1944, Thin-skinned tectonics in the P1ateau andnorthwestern Va11ey and Ridge provinces of the centra1Appa1achians: Geo1. Soc. Bu11., v. 75, no. 9, p. 863-900.
Harris, L. D., 1970, Detai1s of thin—skinned tectonics in parts ofVa11ey and Ridge and Cumber1and P1ateau provinces of thesouthern Appa1achians, in_Fisher, G. N., and others, eds.,Studies of Appa1achian geo1ogy-centra1 and southern: NewYork, Interscience Pub1ishers, p. 161-173. .
, 1976, Thin-skinned tectonics and potentia1 hydrocarbontraps--i11ustrated by a seismic profi1e in the Va11ey andRidge province of Tennessee: U.S. Geo1. Surv. Jour.Research v. 4, p. 379-386.
Harris, L. D. and Mi1ici, R. C., 1977, Characteristics of thin-skinned sty1e of deformation in the southern Appa1achians,and potentia1 hydrocarbon traps: U.S. Geo1. Survey,P.P. 1018, p. 40.
Hayes, C. N., 1891, The overthrust fau1ts of the Southern Appa1achians,Geo1. Soc: Am. Bu11., v. 2, p. 141-154.
Hsu, K. J., 1968, Princip1es of mé1anges and their bearing on theFranciscan-Knoxvi11e paradox: Geo1. Soc. Am. Bu11., v. 79,p. 1063-1074., 1973, Mesozoic evo1ution of the Ca1ifornia Coast Ranges: A
second 1ook, in_De Jong, K. A. and Sho1ten R., eds., Gravity‘ and tectonics: New York, John Ni1ey and Sons, p. 379-396.
133
Hsu, K. J., 1974, Me1anges and o1istostromes, in_Dott, R. H., Jr.and Shaver, R. H., eds., Modern and ancient geosync1ina1sedimentation: Soc. Econ. Pa1eonto1ogists and Minera1-o1ogists Spec. Pub. 19, p. 321-333.
King, P. B., 1960, The anatomy and habitat of 1ow-ang1e thrustfau1ts: Am. Jour. Sci., v. 258-A, p. 115-125. ,
Lowry, W. D., ed., 1964, Tectonics of the southern Appa1achians:Virginia Po1ytech. Inst. Dept. Geo1. Sci. Mem. 1.
1971, Appa1achian overthrust be1t, Montgomery County,Southwestern Virginia, jn_V.P.I. & S.U., Dept. of Geo1.Sciences Gdbk. no. 5, p. 143-167.
i Mi1ici, R. C., 1975, Structura1 patterns in the southern Appa1achians-Evidence for a gravity s1ide mechanism for A11eghanian de-formation: Geo1. Soc. Am. Bu11., v. 86, no. 9, p. 1316-1320.
Moore, J. C., and whee1er, R. L., 1978, Structura1 fabric of amé1ange, Kodiak Is1ands, A1aska: Am. Jour. Sci., v. 278,p. 739-765.
Nob1e, L. F., 1941, Structura1 features of the Virgin Spring area,Death Va11ey, Ca1ifornia: Geo1. Soc. Am. Bu11., v. 52, p.941-999.
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Rich, J. L., 1934, Mechanics of 1ow-ang1e overthrust fau1ting asi11ustrated by Cumber1and thrust b1ock, Virginia, Kentucky,and Tennessee: Am. Assoc. Pet. Geo1. Bu11., v. 18, no. 12,p. 1584-1596.
”
Rogers, H. D., and Rogers, W. B., 1840, The physica1 structure ofthe Appa1achian Chain: Trans. Assoc. Am. Geo1. andNat., p. 483-494. ·
Rogers, John, 1949, Evo1ution of thought on structure of midd1e andsouthern Appa1achians: Am. Assoc. Pet. Geo1. Bu11., v. 33,no. 10, p. 1643-1654.
· 134
Rogers, John, 1970, The Pu1aski fau1t and the extent of Cambrianevaporites in the Centra1 and Southern Appa1achians, in
. Fisher, G. W., et_a1„ (eds), Studies of Appa1achian Geo1ogy:Centra1 and Southern, wi1ey-Interscience, p. 175-178.
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The vita has been removed fromthe scanned document
DEFORMATION ASSOCIATED WITH PULASKI OVERTHRUSTING
IN THE PRICE MOUNTAIN AND EAST RADFORD WINDOWS,
MONTGOMERY COUNTY, SOUTHWEST VIRGINIA.
byI °
. Arthur Philip Schultz
(ABSTRACT)
Within the overthrust belt of southwest Virginia, windows in
the Pulaski thrust sheet expose a variety of parautochthonous and
allochthonous rocks, ranging in age from Cambrian to Mississippian.
The décollement of the Pulaski fault as exposed in the Price
Mountain and East Radford windows is in places a thick, highly com-
plex, deformation zone. In this zone, a suite of carbonate breccias,
cataclastic quartzitic rocks and deformed shales occur. The rocks
of the décollement comprise a thrust chaos and tectonic mélange.
Mélange and chaos fabrics are typified by folding, faulting and
cataclasis.
The East Radford window exposes a complexly folded and
faulted antiform consisting of a telescoped, inverted stratigraphic
section of Ordovician, Silurian and Devonian rocks. The antiformal
structure is entirely allochthonous with minimum horizontal displace-
ment of several kilometers.
Deformation of the parautochthonous Mississippian rocks
below the Pulaski décollement in the Price Mountain window includes
thrust and normal faults, folds of at least 3 orders and cataclasis.
GEOLOGY OF THE PRICE MOUN TAl N AND EAST RADFORD WINDOWS, MONTGOMERY CO., VIRGINIA MISSISSIPPIAN
Mm Maccrady Fm. Mp Price Fm.
DEVONIAN Dch Chemung Fm. Db Brallier Fm. Om Millboro Fm.
SILURIAN
Ss Sandstones Undifferentiated
ORDOVICIAN
Omb Om I
Martinsburg Fm. 11 Middle Ordovician
Strike and dip of beds
Strike and dip of overturned beds
Strike of vertical bed 1
Thrust fault- teeth on upper plate
Axial trace of plunging anticline
Axial trace of plunging ,__~ sy~cline
Axial trace of plunging antiform of inverted section
............_ ______ _
Limes tones 11
CAMBRIAN @--@ Structure section
£cr Copper Ridge Sandstone (Knox Group)
£el Elbrook Fm.
*Includes: Mm, Dm, Ss, Omb, Oml, £cr
-\ (j)b
----
/ · /
Afl-T--tw_B --~· S C H U T Z ,
•••• • •