Early Paleozoic Continental Shelf to Basin Transition, Northern Virginia: Strasburg to Riverton, Virginia July 13, 1989

14 Pages·1.148 MB·English

by  Eugene K. Rader, J. Fred Read(auth.)

Early Paleozoic Continental Shelf to
Basin Transition, Northern Virginia
Strasburg to Riverton, Virginia
July 13, 1989
Field Trip Guidebook T221
Leaders:
Eugene K. Rader and}. Fred Read
American Geophysical Union, Washington, D.C.
Published 1989 by American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-87590-554-4
Printed in the United States of America
COVER: Shallow ramp and downslope buildups on carbonate ramp.
Ramp is homoclinal.
TABLE OF CONTENTS
Introduction. ...1
Stratigraphy. · .1
Pre-unconformity Early and Middle Ordovician rocks•..•.••••• .2
Middle Ordovician lower on-lap succession.. .3
Middle Ordovician off-lap succession••.••.• · .4
Middle and Upper Ordovician on-lap/off-lap succession.. •.4
Trip log•.•••.•....•...•..••••.•••.•.•••.•••••••••••••••.•• · .4
Stop 1. Tumbling Run Section, Middle Ordovician sequence•• .5
Stop 2. Oranda type section, Middle Ordovician off-lap succession. .7
Stop 3. Cabin Run section, Martinsburg Formation.•••••.•.•..•••• .8
Stop 4. Riverton section, Middle Ordovician on-lap succession••• .8
References cited••.•••••••.•••••••••.••••••••..•.••••.••••.••••••••. · .9
Leaders:
Eugene K. Rader
Division of Mineral Resources
P. O. Box 3667
Charlottesville, VA 22903
J. Fred Read
Department of Geological Sciences
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061
Acknowledgments
The writers would like to acknowledge the cooperation of Mr. John Elley of
The Riverton Corporation in permitting access to Stop 4 and to Robert C.
Milici and Donald C. Le Van for reviewing an early manuscript of this
publication.
IGC FIELD TRIP T221:
EARLY PALEOZOIC CONTINENTAL SHELF TO BASIN TRANSITION.
NORTHERN VIRGINIA
Eugene K. Rader
Virginia Division of Mineral Resources, Charlottesville
J. Fred Read
Virginia Polytechnic Institute and State University, Blacksburg
INTRODUCTION rocks of the Shenandoah Valley. In northern
and central Virginia (north of 380 N
The Ordovician transition from a latitude) the Middle Cambrian to Middle
passive-margin, as interpreted from the Ordovician carbonate rocks occur low on the
Beekmantown Group rocks. to foreland-basin limbs of the Massanutten synclinorium. The
deposition behind an active-margin. as synclinorium contains Middle Ordovician to
interpreted from New Market Limestone Middle Devonian. dominantly clastic rocks in
through Martinsburg Formation rocks. can be its trough area. Little North Mountain is
observed in four exposed sections in also the position of the mid-province
northern Virginia (Figure 1). A major structural front. Middle Cambrian carbonate
regional unconformity at the top of rocks (Elbrook Formation) are thrust over
Beekmantown Group marks the transition from Middle Devonian black shale (Marcellus Shale
carbonate deposition on a passive-margin or Millboro Shale) at the front, with a
shelf beneath the unconformity to homoclinal series of horse blocks of Upper Ordovician
ramp to foreland-basin deposition associated through Lower Devonian rocks in the fault
wi.th a convergent margin (Mussman and Read. zone. Stops 1 and 2 are on the western limb
1986). The unconformity probably is related of the Massanutten synclinorium; Stop 3 is
to island arc-continent or microplate in the trough area; and Stop 4 is on the
continent collision and uplift during the eastern limb of the synclinorium.
Early Middle Ordovician (Whitrockian 7)
(Shanmugam and Lash. 1982) coupled with
eustatic sea level lowering. This collision STRATIGRAPHY
can be considered an early phase of the
Taconic Orogeny of Rodgers (1971). Pre In the area of the field trip, cyclic
unconformity depocenters. one to the north carbonate rocks of the Beekmantown Group
of the field trip area in Maryland and underlie the unconformity. The strata
Pennsylvania and one to the south in record two on-lap/off-lap cycles at the
southwestern Virginia and Tennessee, may western exposures and a single on-lap/off
have influenced the development of the lap cycle at the eastern exposures. The
unconformity and the depositional fabric and change from unstable shelf to foreland basin
thickness of Middle Ordovician deposits is reflected in, the western, upper on
(Colton. 1970; Read. 1980; Mussman and Read. lap/off-lap cycle (Stop 2) and the onset of
1986)• turbidite deposition to the east (Stops 3
The four field trip stops are in the and 4). In the Strasburg area, on the more
Shenandoah Valley section of the Valley and stable shelf west of the Massanutten
Ridge physiographic province. The synclinorium. a sequence of carbonate and
Shenandoah Valley is bounded by the Blue fine- to coarse-grained clastic rocks
Ridge mountains on the east and by Little overlying the unconformity is interpreted to
North Mountain, the easternmost major. represent: (1) an upward-deepening (on-lap)
linear ridge of the Valley and Ridge unit (STOP 1), (2) an upward-shallowing
province. on the west. The Blue Ridge (off-lap) unit (STOPS 1 and 2). (3) an
mountains are also a structural front upward-deepening unit (STOPS 2 and 3; Figure
separating highly deformed and metamorphosed 2), and (4) an upward-shallowing unit (not
Lower Cambrian and Precambrian visited on this trip). The lowest on-lap
metasedimentary and metaigneous rocks from unit is composed of (in ascending order) New
the less-deformed. dominantly carbonate Market Limestone. Lincolnshire Limestone,
T221 :
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I __- - -WARRENco.
@)
~
SCALE
o
MILES
STOP 3
FIGURE 1 Road map of the Strasburg - Front Royal area showing the stop locations.
lap/off-lap sequence. The on-lap portion,
NW SE exposed at STOP 4, is interpreted to be more
basinward than the rocks exposed at STOPS 1
and 2. At Stop 4, the rock record is
interpreted as a upward-deepening sequence.
Stratigraphic units present are (in
ascending order): New Market Limestone;
Lincolnshire Limestone; Edinburg Formation,
SHELF FACIES Liberty Hall Member; and Martinsburg
r--;-l SKELETALGRAINSTONE, WACKESTONE, Formation, black shale member.
l.:=::::J AND SHALE
p::::;:::::r::j NODULAR,SKELETAL WACKESTONE, Stratigraphic correlation of the above
b::::::!:::d ANDSHALE
units with other areas in the Virginia
• SKELETAL MOUNDFACIES
D Valley and Ridge province is found in Rader
CHERTY SKELETAL WACKESTONE
(1982) and Le Van and Rader (1983).
~ LIME MUDSTONE(COMMONLYFENESTRAL),
~ PELLETGRAINSTONE,LESSER SKELETAL Regional stratigraphic correlations for the
LIMESTONE. BASAL RED ANDGRAY
MUDROCK IN BLACKFORD FORMATION Appalachian Valley and Ridge province are
found in (Patchen and others, 1985a and
BASIN FACIES
1985b)•
~ BLACK LIMESTONE ANDSHALE
Pre-unconformity Early and Middle Ordovician
FIGURE 2 Stratigraphic cross section of the Rocks
Middle Ordovician, northern Virginia;
approximate section location shown on Figure Beekmantown Group, Rockdale Run
3. Formation. The Rockdale Run Formation,
which underlies the Middle Ordovician
and Edinburg Formation (Lantz Mills, Liberty unconformity everywhere in the field trip
Hall). The off-lap unit is composed of the area, consists of limestone and dolomite
St. Lukes Member of the Edinburg Formation arranged in repetitive sequences, or cycles,
and the Oranda Formation. The upper on-lap 7 to 30 feet (2- 9 m) thick. The proportion
unit is represented by the black shale and of limestone to dolomite increases upward in
turbidites of the Martinsburg Formation. the formation. In some localities in
The upper Martinsburg is a shallowing-upward northern Virginia, where the ancient erosion
sequence with an eastern and southeastern surface has not been cut deeply into the
source. formation, the ratio of limestone to
On the eastern limb of the Massanutten dolomite in the upper 500 feet (150 m)
synclinorium, the stratigraphic sequence approaches 1 : 1. The carbonate sediments,
above the unconformity is a single on- which were deposited in peritidal
T221: 2
depositional environments, generally top of many of the fenestral units and may
"consist of, from top to bottom: (4) mark short periods of low sea-level.
crypta1ga1, laminated dolomite and fenestral
Lincolnshire Limestone. Overlying and
limestone (0 to 5.5 m thick); (3) thick,
interfingering with the peritidal New Market
laminated dolomite (1 to 4 m thick); (2)
rocks are the deeper water beds of the
massive and burrowed, thin-bedded dolomite
Lincolnshire Limestone, which ranges from 40
(0.5 to 3 m thick); (1) coarsely crystalline
to 250 feet (12-76 m) thick in the
dolomite and thrombo1ites (0.5 to 4 m
Shenandoah Valley. The Lincolnshire
thick)" (Mussman and Read, 1986, p. 284).
overlies the New Market disconformably at
The presence of silicified evaporite
many localities in northern Virginia, as at
nodules, abundant cryptalga1 lamination,
the Tumbling Run exposure (STOP 1). The
restricted faunas, and abundant early
Lincolnshire is composed of cherty,
dolomite indicate a semiarid tidal flat and
oncolitic skeletal wackestone/mudstone with
lagoonal setting and hypersaline conditions
thin beds of skeletal grainstone that were
(Mussman and Read, 1986). Each of these
deposited on a shallow ramp seaward of New
depositional sequences constitutes one cycle
Market tidal flats. The upper part of the
that was deposited in migrating, relatively
Lincolnshire grades upward into deeper ramp,
low-energy peritidal environments preserved
skeletal wackestone/mudstone deposits, with
on a slowly subsiding carbonate shelf.
some open-marine skeletal grainstone units,
The unconformity at Stop 1 is exgosed in
three very thin interbedded K-bentonites,
the creek bank, and overlain with 10 angular
and some well-developed. deeper water
discordance by New Market Limestone.
hardgrounds. Locally, the skeletal units
Regionally, erosional relief increases
thicken along strike into a small downslope
southwestward and eastward. In places, in
buildup of skeletal grainstone. On a
the southern and western parts of the
regional basis, similar but much larger
Appalachian basin, strata as old as Cambrian
buildups occur along strike further
are preserved beneath the unconformity.
southwest (Figure 3; Read, 1982). The deep
ramp buildups are commonly associated with
Middle Ordovician Lower On-Lap Succession
dark colored lime mudstone and shale. In
southwest Virginia there are several large
New Market Limestone. The New Market
shallow ramp buildups (Figure 3). In the
Limestone, 0 to 265 feet (0-80 m) thick,
Lincolnshire, the biota is diverse and
unconformably ~verlies the Rockdale Run
includes several species of brachiopods.
Formation everywhere in the field trip area.
trilobites. bryozoans, ostrocodes,
The unconformity is marked by limestone and
cephalopods, and algae. Locally the fossils
dolomite breccia, fine detrital dolomite, up
may be silicified. as at Strasburg Junction
to 30 feet (10 m) thick, or thin sequences
about 2 miles (3.2 km) north of STOP 1
containing dolomite lithoc1asts (as at Stop
(Whittington, 1960).
1). In many places in northern Virginia,
the New Market contains: (1) a lower
'11111111111111111 BASIN MARGIN
laminated dolomitic limestone (30-50 ft; 10
0/ ISOPACH- UNCONFORMITY TO BASE
15 m thick); and (2) fenestral limestone. ,..-'10 OF BENBOLT, LIBERTY HALL FMS.
Mussman (1982) recognizes up to 10 feet- (3 • BUILDUPS
m-) thick. three-part cycles in the lower -------LEADING EDGE OF
limestone unit. These cycles consist of, THRUST SHEET
from top to bottom: (3) dolomitic intraclast - - - TAZEWELL ARCH
limestone; (2) laminated limestone; (1) 9 IOOKM
skeletal limestone. A sparse fauna of
gastropods, ostracodes. algae. and the coral
Tetradium is almost entirely restricted to
the lower limestone. During initial
transgression of the gently sloping, low
relief Beekmantown surface the New Market
beds were deposited in intertidal- and N
shallow-subtidal settings characterized by
low to normal marine salinities and humid
1
climatic settings (Read, 1980).
At Stop 1, the top of the New Market
Limestone is marked by a scalloped erosion Isopach map, palinspastic base,
surface representing either differential in relation
solution or a tidal rock platform (Read and ramp, and buildups of Middle
Grover, 1977). These surfaces occur at the age.
T221: 3
Lower Edinburg Formation (Liberty Hall represent peritidal depositional
Formation, Lantz Mills Limestone). In environments. Thus, the lithologies record
northern Virginia. the Edinburg Formation shallowing of the basin or progradation of
includes the interbedded anoxic the shallow ramp from the northwest.
slope/basinal Liberty Hall and the deep
ramp/slope Lantz Mills lithofacies. The Oranda Formation, Stop 2. The Oranda
thickness of the Edinburg Formation in Formation, restricted to the western limb of
northern Virginia ranges from 400 to 500 the Massanutten synclinorium in northern
feet (122-152 m). On the western limb of Virginia, is characterized by argillaceous
the Massanutten synclinorium the proportion lime mudstone, wackestone, black shale, and
of Lantz Mills to Liberty Hall may approach K-bentonite beds. The thickness of the
1:1. However, on the eastern limb the Oranda ranges from 0 to 60 feet (0-18 m).
Liberty Hall is predominant. The Lantz Limestone beneath a K-bentonite bed may be
Mills is a nodular, argillaceous skeletal silicified. Fossils found on the upper
wackestone/mudstone with black shale surface of the silicified limestone appear
partings. Based on the stratigraphic to represent a buried community of
position of the Lantz Mills between shallow bryozoans, trilobites, brachiopods, and
ramp and basin facies. the lack of shallow cephalopods. This silty calcareous unit
water sedimentary structures, the presence represent an open-shelf facies, with
of diverse biotas. well-preserved fossils. sediment being transported from the north,
burrows, and abundant fine carbonate and west, and southwest as the basin filled.
terrigenous material the depositional
environment is interpreted to have been deep Middle and Upper Ordovician On-Lap/Off-Lap
ramp to slope (Read, 1980). To the north in Succession
Pennsylvania, this lithologic unit is mapped
as the Chambersburg Formation. The Liberty Martinsburg Formation. Stops 2. 3. and
Hall is an evenly thin-bedded, black lime 4. The basal 200 to 250 feet (60-75 m) of
mudstone and calcisiltite with interbedded the Martinsburg Formation is black,
black graptolitic shale. Millimeter size calcareous shale (slate 7) and lime
cubes of pyrite are common to abundant in mudstones, with K-bentonites near the base
the lime mudstone. especially at STOP 4. (Rader and Biggs, 1976). The bulk of the
Based on the stratigraphic position of the formation, approximately 2800 feet (853 m)
Liberty Hall between the deep ramp/ slope thick, is composed of medial- and proximal
and basinal facies, the presence of slump turbidite sequences. The Bouma cycles are
structures, allodapic grainstone, and base- and top-truncated. Typically, a cycle
discontinuities stongly suggests an anoxic begins with an upward-fining, medium- to
slope to basinal depositional environment. coarse-grained graywacke with a planar to
Interlayering of nodular Lantz Mills and channeled base. This unit is overlain by a
even-bedded Liberty Hall units may reflect fine- to medium-grained, ripple-bedded
changes in water depth or changes in graywacke which grades upward into a
dissolved oxygen of the bottom waters, which parallel-laminated, fine-grained graywacke.
were stratified in the basin to the east. The upper lithology in the cycle is a gray,
The sequence is well exposed at Stop 1. silty shale. Above the turbidite sequence
is a medium-grained sandstone about 165 feet
Middle Ordovician Off-Lap Succession (50 m) thick. The lower 100 feet (30 m)
contain marine fossils (Secrist and Evitt,
Edinburg Formation, St. Luke Limestone 1943). The Martinsburg Formation records a
Member, Stop 2. In the northern Shenandoah second major downwarping of the Ordovician
Valley the St. Luke Limestone Member at the foredeep. synchronous with the major Taconic
top of the Edinburg Formation ranges from 0 uplift to the east of the central
to 90 feet (0-27 m) thick. Skeletal Appalachians.
grainstone and wackestone comprise the lower
30 to 50 percent of the unit. The
lithologies are vexy similar to those found TRIP LOG
in the Lincolnshire Limestone. Above the
grainstone/wackestone is a massive, Participants should assemble at the
fenestral limestone similar to the upper Washington Convention Center. The trip
portion of the New Market. The presence of route follows Interstate Highway 66 West to
skeletal grainstone/wackestone with an open its junction with Interstate Highway 81.
marine fauna is interpreted to represent approximately 75 miles (120.7 km) to the
shallow ramp depositional environments. The west of Washington, D.C. Beginning in rocks
fenestral limestone is interpreted to of the Piedmont physiographic province. the
T221: 4
trip route crosses the Culpeper Triassic Blue Ridge province in this area.
basin, the western Piedmont and Blue Ridge
physiographic provinces to the first stop in Shenandoah River. The hill to the south
the Valley and Ridge province. of the west end of the Shenandoah River
bridge is a klippe of Rockdale Run dolomite
Washington Convention Center. The trip thrust over Rockdale Run dolomite. Rocks
begins at the convention center and proceeds exposed between the river and Exit 2,
via city streets to the Roosevelt Bridge Interstate Highway 66 are on strike with and
(Interstate Highway 66 West). This part of about 0.8 mile (1.3 km) north of the rocks
the route traverses schist, metagraywacke, expossed at STOP 4. West of the interchange
gneiss, phyllite, and phyllonite. and continuing to the junction of Interstate
highways 66 and 81, the route traverses
Tyson Corner Area. Unconsolidated rocks of the Martinsburg Formation (Rader
Cenozoic gravel and sand deposits overlie and Biggs, 1975; 1976). Approximately 3
the metamorphic rocks of the Piedmont miles (4.8 km) west of Exit 2 the mountain
province in this area. For approximately to the south is Massanutten Mountain. This
the next eleven miles (18 km) the route is the topographic feature in the complex
crosses the metamorphic rocks of the trough area of the Massanutten synclinorium.
Piedmont province (Drake and Froelich, The ridge is underlain by Upper Ordovician
1977). and Lower to Middle Silurian clastic rocks.
I-66 Rest Stop. Approximately 0.3 mile Junction of Interstate Highways 66 and
(0.48 km) west of the Rest Area, the route 81. At Interstate 81, Exit 76 proceed south
crosses into the Culpeper Triassic basin. on I-81 to Exit 75, approxinately 2 miles
The dusky red soils derived from rocks of (3.2. kIn).
the Newark Supergroup are very distinctive.
Siltstone, shale, sandstone, arkose, and Interstate 81, Exit 75. Junction of
u.s.
polymictic conglomerate are the typical rock Interstate 81 and Highway 11; proceed
types. Diabase dikes and sill-like bodies south on u.S. Highway 11 toward Strasburg.
intrude the sedimentary rocks of the basin. Continue through Strasburg on u.s. Highway
Hornfels are commonly associated with the 11 to the junction with State Road 601,
diabasic intrusions (Drake and Froelich, approximately 1 mile (1.6 km) south of
1977) . The route continues across this Strasburg.
basin to near the base of the Blue Ridge
Mountains, approximately 3.5 miles (5.6 km) Junction of U.S. Highway 11 and State
west of Haymarket. Road 601. Turn right (west) on 601, proceed
0.3 m.ile (0.48 km) an.d park on left side of
Blue Ridge Mountains. Approximately 3.5 road.
miles (5.6 km) west of Haymarket the route
crosses the faulted west boundary of the Stop 1. Tumbling Run Section, Middle
Culpeper basin into rocks of Lat€ Ordovician Sequence
Precambrian or Early Cambrian age. These
rocks of the Catoctin Formation are The rock column at this locality
greenstone, phyllite, quartzite, and quartz preserves a record of peritidal to basinal
muscovite schist. About 2 miles (3.2 km) environments in a vertical sequence of
east of Marshall the route crosses rocks. .Walking east on the road, up
metasedimentary and metaigneous rocks of section, and viewing the east-dipping rocks
Middle and Late Proterozoic age (Espenshade, is equivalent to traversing from an
1986). Near Linden the trip route crosses intertidal area across a homoclinal ramp to
into rocks of the Catoctin Formation on the the slope and into the basin. Beginning
west limb of the Catoctin-Blue Ridge along Tumbling Run to the north of the
anticlinorium (Lukert and Nuckols, 1976). bridge. the following units are exposed to
About 2 miles (3.2 km) west of Linden, the the east along the road: Rockdale Run
west foothills of the Blue Ridge are Formation; New Market Limestone (intertidal
underlain by sedimentary rocks of the Early to shallow subtidal); Lincolnshire Limestone
Cambrian Chilhowee Group (Rader and Biggs, (shallow to deep ramp); and Edinburg
1975). The lowland to the west is underlain Formation, Lantz Mills (deep ramp and slope)
by dolomite, limestone, and shale of Middle and Liberty Hall (anoxic slope and basin)
and Late Cambrian and Early Ordovician age. Members (Figures 4,5,6,7. 8, and 9).
Refer to Rader and Biggs (1975), Lukert and Return to U.S. Highway 11 via State Road
Nuckols (1976), and Espenshade (1986) for a 601. Turn left on U.s. Highway 11 and
detailed discussion of the geology of the proceed into Strasburg to the intersection
T221: 5