Structural Transect of the Central Appalachian Fold-and-Thrust Belt: Harpers Ferry, West Virginia to Cumberland, Maryland July 15, 1989

24 Pages·2016·2.41 MB·English

by  Arthur P. Schultz

Stmctural Transect of the Central
Appalachian Fold-and-Thrust Belt
Harpers Ferry, West Virginia to Cumberland, Maryland
July 15, 1989
Field Trip Guidebook T227
Leaders:
Arthur P Schultz Robert C. McDowell
Associate Leader: Howard Pohn
American Geophysical Union, Washington D.C.
COVER Disharmonic broken folds in alternating limestones and
shales of the Silurian McKenzie Formation, Stop 6.
Leaders:
Arthur Schultz and Robert McDowell
U.S. Geological Survey
926 National Center
Reston, VA 22092
Published 1989 by American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-81590-621-4
Printed in the United States of America
IGC FIELD TRIP T22'1:
STRUCTURAL TRANSECT OF THE CENTRAL APPALACmAN FOLD-AND-THRUST BELT
Arthur P. Schultz and Robert C. McDowell
U.S. Geological Survey, Reston, Virginia
INTRODUCTION
Folded and faulted- rocks of Proterozoic through deformation, metamorphism, and igneous
Permian age record both the sedimentary and intrusion (Williams and Hatcher, 1987). Mesozoic
tectonic evolution of the central Appalachian rifting followed the last major compressional
fold-and-thrust belt. This trip will focus on the episode, the Alleghanian orogeny. Finally,
styles of deformation in the major lithotectonic passive margin sedimentation, associated with
units that have been affected by the multiple mid-Atlantic oceanic spreading, began in the
accretionary and compressional phases of the Cretaceous and continues to the present (Fig. 3).
Appalachian orogen. In the field trip area the rocks of the fold-and
The seven stops of this trip will allow us to thrust belt were deformed by the Late
compare structural features and lithotectonic Pennsylvanian(?) and Permian Alleghanian
units across four of the six major physiographic orogeny.
and structural provinces of eastern North Washington, D.C. is located in the Atlantic
America. The six provinces are the Atlantic Coastal Plain province, consisting of Cretaceous
Coastal Plain, Piedmont, Blue Ridge, Great to Holocene siliciclastic and minor carbonate
Valley, Valley and Ridge, and Allegheny Plateau. rocks. These rocks form an oceanward-thickening
The trip will begin in the Blue Ridge structural wedge that was deposited on the crystalline and
province, a major structural boundary between metasedimentary rocks of the eastern Inargin of
polydeformed accreted terranes of the Piedmont the Appalachian orogen during the formation of
province to the east and less deformed the present passive continental rnargin. Rocks of
Appalachian rocks to the west. At the field trip the Coastal Plain province onlap rocks of the
stops we will examine the regional structural Piedmont province. The present easternmost
setting, local structural features, subsurface edge of continuous Coastal Plain rocks, known as
interpretations, and will review the development the fall line, is generally the westernmost limit
of geologic thought concerning the genesis of the to major shipping along the eastern North
features examined. Several of· the stops are of American coast and therefore the location of
cultural·and historical interest as well. most of the large cities of eastern United States.
The Piedmont province is a zone of "suspect"
geologic terranes, i.e., their structural and
GENERAL GEOLOGIC SETTING stratigraphic affinity to the Appalachian basin
rocks to the west has yet to be established
The Appalachian orogen can be subdivided into (Williams and Hatcher, 1987). These rocks are of
six major physiographic and geologic provinces highly varied lithologies, deformational styles,
(Fig. 1, 2 and 3). These provinces consists of and grades of metamorphism. The "suspect"
approximately parallel northeast trending belts terranes were accreted to the eastern margin of
that differ in lithofacies, structure and the Appalachian orogen during several major
topography. Within these belts are rocks which westward-directed compressional events (Fig. 3).
record cycles of mountain building and tectonic A series of Mesozoic rift basins are exposed in
quiescence that occurred from the Late the Piedmont province or are buried beneath
Proterozoic to the present (Fig. 3). These cycles rocks of the Coastal Plain province. These basins,
include rifting of the Middle Proterozoic which contain continental and lacustrine
continental crust followed by several sediments, are found at several places along the
accretionary episodes throughout the Late east coast of North America (Fig. 1).
Proterozoic and Paleozoic. Three major Gently rolling to relatively flat topography is
compressional orogenies, the Ordovician Taconic, characteristic of the Coastal Plain and Piedmont
the Devonian Acadian and the Carboniferous provinces and areas underlain by the Mesozic
Alleghanian have been correlated with the major basins. In sharp contrast is the Blue Ridge
phases of accretion (Williams and Hatcher, 1987). province, west of the Piedmont. It is a belt of
These three orogenies affected different parts mountains which include the highest elevations of
of the Appalachian orogen and were responsible eastern North America.
for basin-wide sedimentation, structural
T227: 1
n,
1 0m ea
8 1k D e
BR21 ····~C-,._-=:....'.:~~'.'...S,. C-, nderandambrian,Permian;pAnticlin
C. m O'::__....-'~..' - mKulan,C-CnianandroadTo
,NY-------------- 1AA..., D. o50ktI NUFUS43~,,"~_D~2··;·.·..:'>.:::'-::s.O~_~'-.-:.......','·······1----.,..-<'.'~.,'.'.."':::0~~-----,'< regionalcrosssection(modifiedfronmapareTriassic.pC-PrecambriassippianandDevonian,P-Pennsylvault,BTS-BroadTopSynclinorium,BCreekSyncline.
IL.- P BTS ~~~MD__,,--~SO"--.'····:0':'.,C---...-...- p,andgeneralizedon.Opencirclesovonian,MD-MissiNorthMountainFant,GCS-Georges
MI,I III INJ BTA5~.~~~-=:~",..::~"~-.'.......--~"..--- map,triproutemaStopsoncrosssectiOrdovician,D-Deensyncline;NMF-AF-AlleghenyFro
MAP WUA :;.~...=:~...~.~--- hicindexonofTripilurianandMassanuttanticline,
INDEX GCSAF7P6~~:-MD-.-.'"--~Y\:~.,..~............'.'C- pC FIGURE1Physiograp1986).Notethepositio-Ordovician,SO-SBR-BlueRidge,MS-WMA-WillsMountain
..,I'\)I'\)-.1 I'\)
SYSTEM GROUP or FORMATION
.eQ.n. Perm. Dunkard Group .: .
0 Monongahela and Conemaugh Fms. ---------
en e:
..Q.-... .e~.n eQ:) Alleghany Formation
0 Q.
,~~i'~{~:J
.Q-) Pottsville Formation
7 [ 'to- ...---+-------------------1'_--=-=-:1
e: -
.0. Mauch Chunk Formation - --
.c
as en
0 .e-n Greenbrier Limestone
~ -- - --
Pocono Formation •.•'•1••.-t...t...'.•
~ ~.:~;.
Catskill Group ..
4 [ e:
.a-a 1t--••-,•-••-••
c: Onondaga Limestone I 1 1
>0 Oriskany Sandstone l' "
Q)
5 c Helderberg Group
Keyser Limestone
..ae-.s: Tonoloway, Wills Creek and
6 McKenzie Formations
-
.~- ..............,
UJ Tuscarora,Rose Hill,Keefer Fms. ._.'_.·._.·_.·.:1
/
.·.·.·:.·::7
e: Junita and Martinsburg Fms. '------_""'1
.a-s T T T
.C-J Chambersburg Limestone IT II I
> 1 1
.0. New Market and Lenoir Ls.
3 [ "C
0
Beekmantown Group
Conococheague Limestone
e: Elbrook Limestone /T/ T7
..a-.s. "7 7
Waynesboro Formation
.c Tomstown Dolomite 7 7"
E T 7
2 ... 0as ChSilahnodwsetoenGe,roHupa:rpAernstieFtoarmmation, -~-~.-I-I·-1-•-I:--•
1 and Weverton Formation :",:':'::-:',)
Pre- Catoctin Formation
camb.
basement
FIGURE 2Stratigraphic section.
T227: 3
western cratonic
ocean
continental crust
---M-- - - - ~/
Late Precambrian - Lower Ordovician
carbonates 1" ,rI
clastics 'f'··;3
ACCRETED TERRANE
metamorphics ~
eastern uplifted sediment source
_.-I..... -
~---
........M
~
Middle Ordovician - Silurian
NW
SE
"
- - _.M - - _ ",
FIGURE 3 Block diagrams illustrating the tectonic development of Eastern North America
(Cook and others, 1983). Late Precambrian-Early Ordovician Atlantic-type passive margin
with normal faults in the basement and a carbonate bank extending over a wide area, and
cratonic sediment source to the west. Ordovician-Silurian phases of accretion and
thrusting with sediment source from tectonic highlands on the east and clastic wedges in
the basin. Present configuration of basin following last major compressional event and
subsequent erosion with passive margin sedimentation.
T227: 4
Structurally, the Blue Ridge province separates with a variety of faults (Mitra, 1987). Although
polydeformed metamorphosed rocks of the most folds approach a parallel geometry, many
accreted terranes on the east from less deformed are strongly disharmonic. This disharmony
Appalachian foreland basin deposits on the reflects contrasts in rock competency during
west. Although the Blue Ridge province is shortening. Throughout most of the Valley and
allochthonous, the metasedimentary rocks which Ridge, well developed solution cleavage in
overlie the allochthonous crystalline basement carbonates and preferred alignment of micas in
are stratigraphically continuous with Paleozoic shales are the result of the early-layer parallel
rocks of the Great Valley, Valley and Ridge and shortening which preceeded folding (Mitra,
Allegheny Plateau provinces to the west. The 1988). Most of the more competent units in the
grade of metamorphism and intensity of strain Valley and Ridge were shortened by extensive
decreases westward from the Blue Ridge minor faulting.
province, chiefly as a function of distance from In the field trip area, the search for economic
the accreted eastern margin of the Appalachian quantities of oil and gas has led to extensive
orogen. seismic exploration. This exploration has
Thick Cambrian and Ordovician dolomites and confirmed models of thin-skinned thrust
limestones are exposed in the Great Valley tectonics now generally accepted for the
physiographic province of the Appalachians. This Appalachian fold-and-thrust belt (Kulander and
carbonate sequence is one of the most extensive Dean, 1986). All major thrusts are thought to
ancient platform deposits known (Demicco and root in a master decollement above the
Mitchell, 1982). These rocks are up to 3,500 m crystalline base·ment (Fig. 1). Thrusting and
thick, and were deposited on a huge early associated ramping has produced rootless
Paleozoic platform that fringed the craton of structures. Major decollement zones throughout
eastern North America. From east to west the Valley and Ridge are located in relatively
across the Great Valley, Valley and Ridge, and incompetent shales of Early Cambrian, Late
Allegheny Plateau provinces, facies within the Ordovician, and Devonian age.
Cambrian and Ordovician carbonates generally Rocks of the Allegheny Plateau province lie to
indicate a change from deep basin to carbonate the west of the Valley and Ridge. The boundary
ramp to tidal flat depositional environments. between the two, known as the Allegheny
Rocks of the Great Valley are thrust-faulted structural front, is a transition from high
and folded with intensity of deformation amplitude, short wavelenth folds of the Valley
decreasing from east to west. Thermal and Ridge to long wavelength, low amplitude
rnaturation data (Epstein and others, 1977 ) folds and flat lying rocks of the plateau. In the
indicate sUbgreenschist facies. The deformation southern Appalachians the boundary is a major
occurred during the late Paleozoic Alleghanian thrust fault, whereas in the central Appalachians,
orogeny and is characterized by thin-skinned along the field trip traverse, the structural front
folding and faulting above decollements. is a zone of intense, small scale folding and
Well-developed karst topography is faulting on the vertical to overturned limb of
characteristic of the Appalachian Great Valley, Inajor folds. In general, Carboniferous
and many famous caverns are found in this area (Mississippian and Pennsylvanian) and Permian
in Virginia. The limestones and dolomites are rocks are exposed in the Allegheny Plateau. The
important economic deposits for building stone relatively flat-lying Carboniferous sandstones are
and for raw material for the fertilizer, cement, resistant to erosion and form many of the high,
and steel industries. flat-topped bluffs characteristic of the Plateau.
The Appalachian Valley and Ridge province is Major folds in the Plateau are thought to have
west of the Great Valley. In the field trip area formed in response to shortening on blind thrusts,
(Fig. 1), the boundary is lnarked by the leading that· is, thrust faults which do not reach the
edge of the North Mountain thrust sheet shown at present surface and which are rooted in the
"NMF" on the cross section. Rocks of the North Valley and Ridge province.
Mountain thrust sheet in the Great Valley are
folded and faulted lower Paleozoic carbonates.
West of the North Mountain sheet, in the Valley STOP DESCRIPTIONS
and Ridge, are folded and faulted, middle to
upper Paleozoic siliciclastic and carbonate Stop 1. Crest of Blue Ridge.
rocks. The characteristic topography of the
Valley and Ridge is a result of differential Stop 1 (Fig. 1 cross section and Fig. 4) is
weathering of linear belts of rocks that have located approximately 1km west of Keys Gap on
been repeated by late Paleozoic folding and West Virginia Rqute 9 at a roadside scenic
thrusting. Folds and·faults occur at many scales overlook. The overlook is just west of the crest
in the Valley and Ridge. Folding in the Valley of the Blue Ridge. The Shenandoah River can be
and Ridge is related to shortening associated seen 250 m below. At this location, the river is
T221: 5
Blue Ridge
Potomac R.
Stop
EXPLANATION
Triassic rocks ~
Ordovician and Cambrian c:J
carbonate rocks Stop 1
Cambrian and Precambrian ~
volcanics and siliciclastic rocks
Precambrian granitic basement ~
Potomac R.
Great
10-
1- i-
t ;.
i- + 1-
1-
t -+
i- N
--\. i
1- 1- -+ I
+
i-
-+ -+
'1'
i-
~
i- +
-{ 0 10 km
I.-..-..J
BIue Ridge antielinorium
Stops 1,2
A
................."....-. 10 km
o 10 km
I I
FIGURE 4 Tectonic map (modified from Mitra and Lukert, 1982) and generalized cross
section (modified from Mitra, 1987) of the Blue Ridge area.
T221: 6
the boundary between the Blue Ridge and the Stop 1 is on the overturned western limb of the
Great Valley provinces. Alarge dolomite quarry, Blue Ridge anticlinorium (Fig. 4). The Blue
which can be seen on the northeast side'of the Ridge anticlinorium is about 400 km long and
river, is in the basal carbonate of the Great extends from near Roanoke, VA. into
Valley sequence, the Tomstown Dolomite. If Pennsylvania. The anticlinorium contains
visibility is good, the mountains of the Valley and Proterozoic Grenville age (1.1 BY) granitic rocks
Ridge province can be seen beyond the Great in its core and Late Proterozoic(?) through
Valley. Cambrian clastics on its flanks. Rocks were
Outcrops in the roadcuts at the overlook are of deformed under greenschist:grade conditions at
the Lower Cambrian Weverton Formation. Here, temperatures of 350 C and 3.5 kb (Mitra, 1987).
the upper part of the Weverton consists of cross The Blue Ridge anticlinorium characteristically
bedded, pebbly, coarse:grained sandstones (Fig. has an overturned northwest limb and an upright
4) and interbedded phyllitic fine:grained eastern limb. Within the core of the
sandstones and shales. The Weverton Formation, anticlinorium are several thrust faults that
100-200 meters thick, is the lowest unit of the separate different types of granitic rocks
basal Cambrian clastic sequence. This sequence (Mitra,1987). Numerous 2nd- and 3rd-order
was deposited unconformably upon Late isoclinal recumbent folds are characteristic
Proterozoic rift-facies metabasalts of the of the overturned limb of the anticlinorium
Catoctin Formation and billion-year-old in this area. The western limb has a normal
Grenville crust (Fig. 2 and 3). The Weverton stratigraphic contact between the Late'
clastics were derived from exposed North Proterozoic and Cambrian clastic rocks and the
American crust to the west and were deposited overlying Cambrian and Ordovician carbonates of
along the eastern margin of the continent in a the Great Valley. Elsewhere, the Blue Ridge
series of rift basins related to the opening of the thrust separates these units. Subsurface
proto-Atlantic (Iapetus) ocean.
FIGURE 5 Overturned, cross-bedded pebbly quartzites of the Cambrian Weverton Formation
at stop 1.
T227: 7
interpretations, based in part on seismic data solution and dislocation creep as important
(Fig. 4) show the Blue Ridge anticlinorium as a penetrativ·e deformation mechanisms associated
major ramp anticline (Harris, 1979) that has with the isoclinal folding.
overridden Cambrian rocks. Displacement of the
Blue Ridge- increases from about 30 km here to Stop 2. Harpers Ferry National Historical Park,
over 200 km in the southern Appalachians West Virginia
(Harris, 1979). Deformation occurred during the
late Paleozoic Alleghanian orogeny (Mitra, 1987). Harpers Ferry is located at the juncture of
Rocks of the Weverton Formation exposed in the the Potomac and Shenandoah Rivers in a
the road cuts are overturned and dip about 25 spectacular gorge through the Blue Ridge, where
degrees to the southeast (Fig. 5). Phyllitic beds West Virgina, Virgina and Maryland come
have a well defined cleavage. Microscopic together (Fig. 1). Stop 2 is both geological and
structural analysis (Mitra, 1987) of Weverton historical.
quartzite samples from near here show pressure
FIGURE 6 Ruins of bridges destroyed during the great flood of
1936 at Harpers Ferry. The cliffs in the background are composed
of phyllite and sandstones of the Cambrian Harpers Formation.
T221: 8