Geologic history and stratigraphy of the Triassic-Jurassic Culpeper Basin, Virginia
'R. C. LINDHOLM Department of Geology, George Washington University, Washington, D.C. 20052
Geological Society of America Bulletin. Part II. v. 90, p. 1702-1736,9 figs., 1 table, November 1979, Doc. no. M91102.
9 ,The Culpeper Basin is one of the many RJTRODUCT ION basins in North America in which sedimen-
The Culpeper Basin in northern Virginia tation extended from Late Triassic to
extends from just south of the Albemarle- . Early Jurassic time (Cornet, 1977).
County--Orange County line northeastward to Other such basins include the Newark in
the Potomac River, a distance of more than New Jersey, the Gettysburg in Pennsyl-
148 km (90 mi).. The basin continues north- vania, the Hartford in Connecticut, the
ward across the Potomac and terminates just Deekfield in Mas%achusetts, and the Fundy
southwest of Frederick, Maryland. It occu- in Nova Scotia. The southernmost basins
pies the western part of the Piedmont contain only Upper Ttiassic rocks. They,
province. On the west it is bounded by an include the Durham, Sanford, Wadesboro,
* east-dipping normal fault; the sedimentary and Dan River Basins in North Carolina
and volcanic strata dip westward into the and the Danville &d Richmond Basins in
faultr (Fig. 1). The rocks fn this area southern Virginia (Cornet. 1977).
traditionally have been considered part of Red beds deposited on broad alluvial
the Newark Group and referred to as Trias- plains dominate in most of these basins. I lr sic, but Cornet's (1977) palynological Lacustrine black shale and limestone rep-
study indicates that they range from Up$er resent a facies common to most of the t. Triassic to Lower Jurassic. basins, but these bgds generally make up 1702
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\ /
Figure 1. Locality map of study area. li = Washington, D.C.,
B = Baltimore, PR = Potomac River, A = AppalachianMountains,
SV = Shenandoah Valley (Great Valley in Maryland), BR = Blue Ridge,
I P = Piedmont, CP = Coastal Plain. Block diagram is highly
schematic.
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ondy a small part of the total sequence surrounded by extensive contact metamorphic
(Hubert and others, 1978; Wheeler and aureoles composed of bblack, gray, and
Textoris, 1978; Olsen 'and others, 1978). green hornfels (Pig. 2).
Unique among these lacktrine deposits are There are several reasons for current
the analcime-*d beds of the Lockatong interest in Triassic-Jurassic rocks in
Formation in the Newark Basin (Van Houten, eastern North Amer,ica. These rocks were
1964). Coal swamps were restricted to some 'deposited during the early phase of con-
of the basins in North Carolina and tinental break-up leading to formation
southern Virginia. Basalt flows are com- of the7-proto-Atlantic. As such, they
mon features in the northern basins, but play a role in the continuing development
not in those to the south. Diabase in- of plate-tectonics theory. In addition, L trusions are present in most of the basins. Triassic-Jurassic deposits north of the
The sequence in the Culpeper Basin be- CulpepCr Basin contain traces of uranium
gins with coarse clastic rocks at.the base, (McCauley, 1961; Klemic, 1962). Iaterest
passing upward into fine-grained clastic of local jurisdictions in ground water
rocks. On the west side of the basin the has prompted the need for a better kder
lutites are overlain by a younger sequence standing of rocks in the northern part
Y of coarse clastic rocks, including exten- of the Culpeper Basin, an area,which is
sive conglomerates deposited as alluvial undergoing rapid urbanization. For these
fans that spread eastward- from the high- reasons a clearer understanding of Cul- l lands to the west (Lindholm, 1978a).* The peper stratigraphy is required.
conglomerates are in turn overlain by My study of'this area began in 1974
ioultiple basalt flows and interbedded and has been concentrated on the sedi- I sedimentary units. A conglomeratic unit, nentology and structural geologyT within
with intercalated black shale, caps the the basin (Lindholm, 1977a, 1977b, 1978a,
sequence. Diabase intrusions are abundant , 1978b). Durixig the early part of this
throughout-the basin. These plutons are project I used soil maps as a framework \/
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Figure 2. Map showing distribution of igneous and metamorphic rocks
in Culpeper Basin. C = Culpeper, L = Leesburg, I4 = Manassas, T = Thoroughfare
Gap. Stratigraphic units on index map: M = Pianassas Sandstone, CR = Bull
Run Formation, B<= Buckland Formation, bJ = Waterfall Formation. Figure 2
appears on the folloving frame.
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0ASALT - Ilows
DIABASE - intrusives
HORNFELS-contact zone
GREEN and GRAY SHALE-Iacustrine
BLACK SHALE with FOSSIL FISH lacustrine
Figure 2.
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Figure 3 is a simplified version of this Culpeper Basin.
I map. In this paper I present a strati- Roberts (1928) interpreted the sequence
graphic framework consistent with my as having accumulated as a "valley" fill
present knowledge of these'rocks: with conglomerate at the base, passing
upward into sandstone and finally shale PREVIOUS WORK at the top. This interpretation was 1883 to 1928 based on the ptesence along the basin
Early work on the lower Mesozoic rocks margins of conglomerate that .grades later-
in Virginia was concentrated in the Rich- ally into sandstone toward the center of
mond Basin in the southern part of the the basin and shale in the medial area.
state (Rogers, 1884; Fontaine, 1883).,. Such an interpretation would make the
The first comprehensive study of the Cul- conglomerates on the eastern edge of the
peper Basin (called the "Potomac area" by basin the same age as those on the west-
Roberts) was by Roberts (1922, 1923, 1928), ern edge.
as part of his general study of "Triassic" The persistent westward dip of the
rocks in Virginia. He recognized three beds clearly indicates that rocks to the
major lithologic units: the "Border Con- west, in the downdip direction, are pro-
glomerate formation, the Manassas sandstone gressively younger than those in the east.
and. the Bull Run shales." He applied This puts the conglomeratic sequence on
these names to all of the Triassic basins the western margin at the top of the c in Virginia. To that extent these terms section and that on the eastern margin at
were overextended, but when applied to the bottom of the section. This inter- 1 the sequence expoded within the Culpeper pretation was rejected by Roberts. but was Basin they still have limited value. Cer- accepted by some of his contemporaries
tainly they should not be used as formal (for example, Stose, 1927) and virtually
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Conglomerate Members of the BULL RUN FM. Waterfall Fm.
Leesburg
Buckland Fm.
Goose .Creek
Bull Run Fm.
Cedar M t n.
Manassas S&t.
Barboursvi Ile
Reston Fm.
Figure 3. M&I showing stratigraphic units in Culpeper Basin. (A) Northern area between
Potomac River and Rappahannock River. (B) Southern area south of Rappahannock Rik. * Conglomerate lembers of Bull Run Formation are arranged in legend by .geographic location
(that is, tcff td\ north and bottom to south)'; see Figure 6 for approximate stratigraphic
position of these units. Intrusive igneous rocks and faults (other than western border
fault) are not shwon. B = Buckland, Ba = Barboursville, C = Culpeper, L = Leesburg,
M = Manassas, Rf = Raccoon Ford, W = Waterfall. Figure 3 is continued on the following
frame .
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Figure 3. (Continued)
3- 15
.- .. . la
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, all others since, insludiqg Cornet (1977), erate formation" codfains a variety of
whose palynological study has provided lithologic types.
the first comprehensive biostratigraphic 1950 to Present framework for thes.e rocks.
The sandstones lying near the eastern , Except for Roberts' work in the 1920s,
edge of the basin were named by Roberts the *rockg itthe Culpeper Basin were '
(1928, p. 25) as the "Manassas sandstone." largely ignored by geologists untii the
' He considered the basal conglomerates that 1950s. Mapping by Bennison,and Milton
underlie the sandstones to be part of the . (19541, Lindskold (1961), Toewe (1966), and
I1 Border Conglomerate formation." The ex- Eggleton (1975) added details of local
tensive shale sequence lying west of (and geology, but these studies were too restric-
stratigraphically above) the "Manassas ted geographically to add significantly to.
sandstone" was designated by Roberts (1928, a more complete understanding of the stra-
p. 39) the "Bull Run shales." In the tigraphy of 'these rocks.
Culpeper Basin the main body of the The absence of a detailed description ,) ,) "Border Conglomerate format ion" lies west of the basalt flows exposed in the western I. of the "Bull Bun shales" and is strati- part of the area, north,of the Rappahan- graphically above the- shales. Roberts nock River and sout'h of the town of Lees- L recognized several "classes of conglom- burg, is a prime example of how little
erates." He noted that the "limestone con- progress was made in understanding the rocks
glomerates ... a member of the Border in the Culpeper Basin since Roberts' work.
Conglomerate formation[,] is the northern The flows have generally been mapped as
one in Virginia and is confined altogether intrusive sills (Roberts, 1928; Furcron,
t to the western margin of the Triassic in 1939; Toewe, 1966), although the extrusive
Loudoun County" (Roberts, 1928, p. lo). character of certain parts of the sequence
\ South of the outcrop area of the "lime- was recognized (Ward and Roberts, 1938;
stone conglomerate," the "Border Conglom- , Toewe, 1966). Only McCollum (1971) men-
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tioned 'in a brief note that there were five glomerate, sandstone, siltstone, shale,
flow units inthe.sequence and that they. basalt flows, and locally coal and inlpure
have a total thickness of more than 1,680 m limestone'' (Lee, 1977, p. C-6), and he
(5,500 ft), including the intercalated sed- divided the formation into the Leesburg
imentary rock. Limestone Conglomerate Member and the
In 1977 Lee published a major revision basaltic flow--bearing clastic rocks mem-
of Culpeper Basin stratigraphy. Except for ber. This sequence generally lies above
the lower sandstone sequence, his nomen- Roberts' Bull Run shales and lithologically
clature represents a drastic departure , quite different from it. For the most part I from Roberts' well-established terminology. Lee's Bull Run Formatioq lies within
Lee (1977, p. C-3) adopted the term "Manas- Roberts ' "Border Cong1ome;ate formation''
sas Sandstone, " with essentially-the same exposed along the western margin of the
meaning as Roberts (1923, 1928), but Lee basin (see Fig. 4).
included in it,'the lower conglomeratic Lee (1977, p. C-5) htroduced the term
Reston Member along with the Sandstone Mem- "Balls Bluff Siltstone" for the sequence
ber, the latter being equivalent to Roberts' that Roberts called the Bull Run shales.
Manassas Sandstone. The name comes from "the rocks eFposed at I 9 - ,Unfortunately; Lee's (1977, p. C-6) the Balls Bluff National Cemetery in the
Bull Run Formation does not similarly con- Waterford quadrangle, Loudoun County,
form to Roberts' original usage, even Va.," which are described as '!mostly cal-
though Lee wrote th3t "the Bull RUJI Forma- ci6iltite that commonly contains thin
tion was originally called the Bull Run streaks and layers of gray to blackish-
Shale and named after rocks exposed at the - .. 'gray limestone, which is locally oolitic, Bull Run ,battle field (Manassas National gray dolomitic and light-gray and very
Battlefield Park), Prince William County, dark red, highly calcareous, feldspathic,
Va.'! He included in the Bull Run Formation micaceous, very fine to medimgrained
'I a very heterogeneous assemblag? of con- sandstone. " The sequence exposed near the
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TRIASSIC I JURASSIC I I I m-l r . mt -I
Reston Manassas Bull Run Formation ~ "0 Buckland Fm. Waterfall Fm. Fm. Sandstone '. Cong I. g 6 Members . $2 Manassas Border Cowl BorderCongl. . I Sandstone
Manassas Sands tone Bull Run Formation Bluff Gr Reston Sandstone Leesburg -4a Member Member Ls. Congl. Basaltic- Flow-Bearing-Clastic- Member -4 a ., Member - Gl? Formation K Formation B-J Formation A 2
Figure 4. Stratigraphic nomenclature proposed in this paper compared with that of
Roberts (1928), Lee (1977); and Cornet (1977). ~
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Balls BlpCemetGry is a poor choice to boungary lies below the lowermost basalt
characterize the red shales originally Elow, although its precise position is
called the Bull Run shales, because the se- not known.
quence is probably the distal part of the The following scheme follows as closely
limestone pebble conglomerates that lie to as possible that established by Roberts - the west, and it bears little resemblance (1928) and generally agrees with the in-
to the Bull Run shales of Rpberts. formal nomencldture used by Comet (1977; see Fig. 3). Major modifications involving PROPOSED, STRATIGRAPHIC NOMENCLATURE FOR strata in the upper part of the section are ROCKS IN THE CULPEPER BASIN required because of recent work that shows
Triassic-Jurassic rocks in eastern a more complex picture than envisioned by
North America have traditionally been as- Roberts .
signed to the Newark Group. Olsen (1978) Stratigraphic thickness is b?sed on
$as suggested that the termNewark be ele- calculations made using dip and outcrop
vated to the status of supergroup, and he width.. This assumes that therk has been
designated the rocks in several of the no repetition by intrabasinal faults.
basins as a separate grsup. To date, no . For several reasons, I have not attempted
formal term exists for the rocks in the to establish type sections for the units
Culpeper Basin, although Comet .(citing discussed 'in this paper. First, the units
Olsen, 1976, as an informal source) has are generally quite thick and laterally
referred to these rocks as making up the variable. Second, long and continuous ex-
Culpeped Group. On the basis of Comet's posures 'are uncommon. Rather than de-
(1977) palynological study, the rocks in scribing' type sections that' would have
the Culpeper Basin range between the middle little regional value, I 'have presented
I of 'the Norian Stage (185 m.y. B.P.) to the localities I believe are representative
later part of the Pliensbqchian Stage , of the unit.
(170 m.y. B.P.). The TriassiclJurassic
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to the National Center of the U.S. Geologi- Reston Formation v cal Survey; at the cuts 0.6 km (0.4 mi) due
I propose that the Reston Member of the west of the juncture of Reston Avenue and
Manassas Sandstone as defined by Lee (1977) the old railroad; and along the roadcuts
be changed to the Reston Formation. This at the intersection of Reston Avenue and ,
conforms more closely to Roberts' original Baron Cameron Avenue. I'
stratigraphic scheme, especially as it al- Description. Roberts (1928) descrlbed
lows retention of "Manassas" solely for the conglomerate north of Herndon as con-
the overlying sandstones. The Reston For- taining abundant schist pebbles and that
mation is the pasal unit of'the Culpeper south of Hemdon as being dominated-by '
Group. quartz,lpebbles. Eggleton (1975) noted '
Distribution. Roberts (1928) noted that that' in exposures south. of Herndon the
conglomerates are present along the eastern conglomerate contains mostly quartz pebbles
margin of the basin, beginning from a point near the'kop and more abundant gneissic and
3.3 km (2 mi) north of Herndon and extend- granitic pebbles .lower in the section.
ing south to Bull Run, a distince of 22.5 Intraformational cong'lomerate is also
km (14 mi). This distribution is essential- present, as are interbedded lenses of mud-
ly the same as that shown by Lee (1977, stone and arkosic sandstone. Eggleton
P1. U). The outcrop belt is generally also suggested that the conglomerate be- I less than 1.6 km (1 mi) and pinches out to comes coarser downward in the sec'tion. the north and south. The rock is poorly Angular biocks ^.as much as 1 m across of / lithified, and good exposures are diffi- phyllitic material are present near the 7' cult to find. Lee (1977, p. C44) noted eastern contact with Piedmont metamorphic
I the followfng exposures: "on Reston Ave- rocks, eosed along Reston Avenue about
nue at the bridge over the Dul1es"Airport 600 m (1,970 ft) west of the Dulles Air .- Access Road; along the north cut face of port Road.
Sunrise Valley. Drive at the north entrance The composition of the Reston F?rmation
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terrane. Whether the source area was east km (1.9 to 3.1 mi) wide. Southwestward
of the present basin margin or to the west to Bristersburg, for a distance of 8.8
(the area now buried by Triassic rocks) is km (5.5 mi), sandstones that interfinger
Unkn0WI-I. with lutites are found near the sastern
Thickness. Lee (1977) gave a thickness margin of the basin, but they are not suf-
of 22 m (72 ft) for this unit. Its thick- ficiently abundant to warrant assignment
ness in a core taken from a well drilled of these rocks to the Manassas Sandstone.
in the site of the U.S. Geological Survey DescriptiE. The upper and lower con-
National Center (in Reston) is approximately tacts with the overlying Bull Run Forma-
20 m (65 ft) (Larson, 1978). tion and the underlying Reston Formation
are gradational. This, plus the absence Manassas Sandstone of continuous exposures, makes a precise
I use Manassas Sandstone to include determination of lithologic composition
only the sequence dominated by sandstones difficult, but on the basis of sampling
which overlies the Reston Formation and of numerous outcrops I estimate that
crops out in the eastern part of the Cul- plagioclase-rich arkosic sandstones make
peper Basin north of the Rappahannock River. up two-thirds of this unit, with most of
This follows Roberts' (1928) use of the rest being plagioclase-rich siltstone.
Manassas Sandstone in reference to these Lenses of conglomerate, containing both
rocks, but it differs from Lee's (1977) intraformational and extraformational
usage, which includes the Reston Member as clasts, occur locally. About 40% of the
part of the Manassas Sandstone. sandstones are medium to very coarse
Distribution. The, Manassas Sandstone grained; the remainder ore equally divided
extends southward from the Potomac River between fine and very fine grained. Most
for 51.5 km (32 mi) along the eastern of the sandstones are red, although some
margin of the basin to about lat 38°37'30"N. are gray.
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of the best and most accessible exposures thickest unit in the Culpeper Basin and
/ include roadcuts along Braddock Road (Route occupies the medial area bn the northern
620) 300 m (1,000 ft) southeast of Big part of the basin. In the southern part .
Rocky Run, and east of Manassas Park along of the basin, where neither the Manassas
cuts along the Southern Railroad 160 m (525 Formation nor the Buckland Format ion
ft) south of Russia Branch. occurs, the Bull Run Formation is the only
Paleocurrent data (cross-bedding) indi- unit present.
cates a western source area (Fig. 5C). Distribution. The Bull Run Formation
Thickness. The gradational character of conformably overlies and interf ingers
the contacts and the paucity of continuous with the ?kinassas Sandstone in the northern
exposures make determination of thickness area. Southward from lat 37'37'30"N, it
difficult, but a maximum ranging between is in contact with the Piedmont Gqocks along
800 and 900 m (2,700 and 3,000 ft) for the eastern margin of the basin. Westward
the area south of the Potomac Rivcr is it is overlain by tne lowermost basalt flow
reasonable. This agrees with the values in the overlying formation.
given by Lee (1977, p. C-5). Typical exposures include outcrops along
Horsepen Run, 400 m (1,300 ft) south of Bull Rur, Formation the access road to Dulles International
I propose Bull Run Formation as the Airport; and numerous outcrops in the
name for the sequence that is bounded be- Manassas National Battle Zeld Park, es-
low by the Manassas Sandstone and above.by pecially roadcuts along U.S. Route 29-211
the lowermost basalt flow in the overlying southwest of Bull Run.
Buckland Formatiori. This is equivalent to Descripte. Red plagioclase-rich
"Bull Run shale" as used by Roberts (1928) mudstones 2nd siltstones dominate the
but also includes conglomeratic units below Bull Run Formation. Most are massive or
the lowermost basalt flow in the Buckland mottled, although soine are laminated or
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I BULL RUN FM,
Scale L 0--10 mi 0 15 hp
Figure 5. Fluvial paleocurrents in Culpeper Basin.
(A) Buckland Fprmation’. (B) Bull Run Formation. (C) Manassas Formation. Heavy Arrow- shows outcrop vector mean; outcrop located at0 end of arrow. Thin arrow with circle indicates
cross-bedded siltstone. Shaded are in B is stratigraphic 1 interval where paleocurrents are from west t-o east.
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cross-bedded. Dessication cracks are com- of the Bull Run Formation. Although they \. mon throughout. Very fine grained sand- are not necessarily contemporaneous, most
stones make up less than 5% of this unit of these conglomerates are relatively high
qdare concentrated in the upper part of in the formation, and crop out in the
the sequence and in the lower part southwest western part of the basin (Fig. 6). In
of the-outcrop belt of the Manassas Forma- the southern part of the area, south of
tion. Thin gray "lacustrine" lutites are the Rapidan River, conglomerates geaerally
scattered throughout the section (Cornet, occupy the lowermost part of the exposed \ 1977). Bull Run Formation (Fig. 6A).
As. with the Xanassas Sandstone, paleo- In addition to being diverse in strati-
current data indicate a western source graphic position and outcrop area, the cbn-
area (Fig. 5B). glomerates are compositionally quite
,Thickness. The Bull Run Formation is' varied in terms of clast lithologies (Lind-
the thickest unit in the Culpeper Basin; holm and others), 1978, 1979). It is on
it has a maximum thickness of about 5,100 the basis 'of clast lithology that I have , m (16,800 ft) in the area north of the separated the conglomerates into several
Rappahannock River. members. The variation in composition is clearly a funqtion of provenance. Alluvial Conglomeratic Members of the Bull hn < Format ion fans spreading eastward from the fault- produced highlands-west of the basin Numerous conglomeratic Iddie;, ,which .deposited most of these conglomerates.
include intercalated shale aad sandstone Viscous debris f lobs were probably/ respon-
lenses, occur .in the Culpeper Basin. Their sible for deposition of these poorly sorted
general charactel; apg distribution were de- sediments (Fagin, 1977; Hazlett, 1978).
scribed by Roberts (1928). Those above Leesburg Cdnglomerate Member the Manassas Sandstone agd below the lowest
basalt flow are herein included as members The extensive carbonate pebble conglomera
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METERS FEET
Figrrre 6. Sections showing straLigraphic position of conglomeratic members
of Bull Run Formation: R = Barboursville, CM = Cedar Mountain, GC = Goose Creek,
L = Leesburg. Diagonal pattern indicates missing section either removed by
li faulting or occupied by intrusive bodies. Position of conglomerate was determined \ by calculating stratigraphic interval (using outcrop width and dip) from base
of lowennost flow in Buckland Formation and/or base of Bull Run Formation.
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that crops out around the town of Leesburg Leesbutg, northward to a point 0.7 km (0.4
in Loudoun County-and northward across the mi) south of Lucketts on U.S. 15. Several
Potomac River into Maryland is herein named large conglomerate bodies lie stratigraphic-
the Leesburg Conglomerate Member of the ally below the one previously described and
Bull Run Formation. TMs conglomerate is are separated from the overlying conglomeral
that Roberts (1928, p. Lo--ll) referred to by red mudstones typical of the Bull Run
as the "limestone conglomerbte" membei. of Formation. The total distance along strike
the Borddr Conglomerate formation, and Lee of the Leesburg Conglomerate Member outcrop
,(1977, p. C6) called it the Leesburg belt in Virginia is 15.3 km (9.5 mi).
Limestone Conglomerate Member of the Bull Excellent exposcres abound iq the area
Run Formation. It should be pointed out - north of Leesburg. Especially good is a
again that the rest of Lee's Bull Run' For- roadcut northeast of Leesburg at the inter
mation i-cludes the rocks above the 'con- section of U.S. 15 Bypass and the road
glomerates, whereas the Bull Run Formation leading eaaward to the Balls Bluff National
as used in this paper includes the mud- Cemetery and outcrops along an unnamed
stones below and adjacent to the conglomer' stream 0.6 km (0.4 mi) east of U.S. 15, at ., ate8 These conglomerates have been the point where Route 661 crosses the
quarried in Maryland and sold locally under stream.
the name "Potomac marble" and "Calico Description. In typical exposures the
marble" (Merrill, 1891, p. 92--93). Shannon rock consists of gray carbonate pebbles
(1925) described an area of contact meta- set in a red sand-silt matrix. Locally
morphism of the conglomerate adjacent to the matrix is gray, and in some cases
a large diabase pluton southeast of Lees- there is sparry calcite cement between the
burg. clasts. In the' northern area dolomite Distribution. The conglomerate is pebbles dominate over. limestone pebbles, present cogt inuously from approximately the whereas the opposite is true in the
southern leg of the U.S.' 15 Bypass south of southem area (Hazlett, 1978; Lindholm
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and others, 1?78,, 1979). This contrasts Goose Creek ConglomerAte Member with most previous descriptions, in which
limestone pebbles are indicated as being The Goose Creek Conglomerate Member of
the dominant lithology throughout the con- the Bull Run Formation is herein named from
glbmerate (Robgrts, 1928; Lee, 1977). exposures on the south side of Goose Creek
Carbonate pebbles were derived from lower 2 km (1.2 mi) east of the confluence of *
Paleozoic limestones and dblcimites. Goose Creek and Little River in Loudoun
Although it is generally not obvious County. -It is not clear whether this
in the fields and pastures where the con- sequence is part of Roberts' (1928) "arkose
glomerates crop out, massive red mudstones cong$merate"; the localities he mentioned
metres to several tens of metres thick are occur in what I am calling the Waterfall I interbedded with the conglom.erate beds. Formation (discussed below), a unit that
Roadcuts along U.S. 15 north of Leesburg, lies above the youngest .basalt flow and is
1 3 from the point of intersection with U.S. 15 therefore much younger than the Bull Run
Bypass, southward for'more than 1 km clear- Formation. Although Lee's descriptlon is
ly show these relationships. terse, it is clear from his map (Pl. 1 A)
Thickness. Along its outcrop belt in that the conglomerates of the Goose Creek
Loudoun County the conglomerate occupies Member are in the lower part of his basaltic
a stratigraphic interval about 1,800 m flow--bearing clastic member of the Bull
(5,900 ft) thick, although exposures Run Formation. Toewe (1966, p. 7) described
farther to the south are younger than those the outcrops along Goose Creek and referred
to the-north (see Fig. 6, C and D). Be- to them simply as "quartz conglomerate;" a
cause faulting has removed the upper part rock that "is expdsed over much of the
of the northern section, it is not poSsible southem portion of the Leesburg Quadrangle. " .. to determine whether the stated thickness On his map these conglomerates are not
is valid for any single locality. differentiated from the "Limestone con-
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glomerate"; both are combined under "con- ' Blue Ridge Province west of the Culpeper
glome rat e. '' Basin. About 17X are quartzite clasts, de-
Distribution. The Goose Creek Conglomer- rived from the Weverton Formation. The
ate Member outcrop belt, which is less than remainder are vein quartz, granitic pegma- 8 3. km (1.9 mi) wide, begins north of Hay- tite, limestone, and mafic rock fragments.
mrket in Prince William County and extends Thickness. Although the paucity of out-
northward for 32'km (20 mi) to a point just crops precludes an accurate determination,
south of the southernmost outcrops of the the thickness exceeds 900 m (3,000 ft). a Leesburg Conglomerate Member. The con- Cedar Mountain Conglomerate Member glomeratic bodies are irregular in shape
and generally extend for 1.6 to 8 km The Cedar Mountain Conglomerate Member '
(1 to 5 mi) along strike; they are sepa- is herein named from exposures on Cedar
rated laterally from one adother Ey sand- Mountain, located 14 km (8.7 mi) southwest
stones Ad mudstones. This pattern may .of the town of Culpeper in Culpeper County.
reflect the presence of several different Cedar I Mountain was described by Roberts
alluvial fans at the time of deposition. (1928, p. 20) as being ''the most significant
In addition to the outcrops along Goose topographic feature" developed on any con-
Creek, there are excellent exposures in s glomerate in the Culpeper Basin, The Cedar 1. pastures north of Route 601, 1 km (0.6 mi) Mountain Conglomerate was called the "Trap
east of the intersection with U.S. 15 in conglomerate" by Roberts and was included
Prince William County . as one class in the Border Conglomerate.
Description. Greenish ' and grayish Distribution! The main body of the 6 pebbles composed,of f ine-grained silicates Cedar Mountain Conglomerate Member occu- b make up 72% of the clasts in the Goose pies a belt 31 km (19 mi) long and about
Creek Conglomerate Member. These clasts 3 km (1.7 mi) wide, along the western mar-
were derived from the Catoctin and Chil- . gin of the basin in Ciulpeper County, ep
howee metamorphic rocks now exposed in the tending between the Rappahannock River and
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area along the Rapidan River extends from able. Fresh exposures show that the matrix
the town of Rapidan northeastward to Rac- is brick red. South of Culpeper in an ex-
coon Ford, a distance of 13 km (8 mi.). tensive zone .of, contact metamorphism, the
These rocks lie along the southeastern mar- matrix is medium gray. s gin of the basin and are separated from Thickness. Although exact thicknesses
the main body by a massive diabase pluton are difficult to determine, I estimate the
d and an unknown thickness of red lutite; thickness to range between 240 and 1,400 m
they are therefore older than conglomerates (800 and 4,600 ft).
exposed to 'the west. Barboursville Conglomerate Member In addition to the exposures in Cedar
Mountain, there are excellent outcrops The Barboursville Conglomerate Member
# along the Southern Railroad cut near its is herein named for exposures near the' '
intersection with U.S. 522, southeast of original Barbour Estate south of Barbours-
Culpeper, as well as in an abandoned quarry ville in Orange County. This unit consists
on the south side of the Rapidan River near of isolated masses scattered throughout the Raccoon Ford, Orange County. southernmost. part of the Culpeper Basin Description. The Cedar Nourkain Member south of the Robinson River. In general
consists of pebble to boulder conglomerates these rocks were included by Roberts (1928,
composed of 60% to 95% greenstone ;lasts p. 17--18) in his schist conglomerates,
derived from the Catoctin Formation (Fagin, although his description of the outcrops
1977; Lindholm and others, 1978, 1979). in the southern part of the Culpeper Basin
Other clasts derived from clude epidosite and hematitic breenstone Distribution. Good exposures south of in addition to sandstone' and quartz clasts Barboursville are on an east-facing slope 3 r of uncertain origin. Fagin (1977) noted 100 m (330 ft) west of Blue Run, due west that on weathered surfaces the clasts and of the original Barbour Estate (designated Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 I )4 as the Barbour Ruins on the Barboursville 1,300 m' (4,2(60 ft) thick, and relatively 7 ~ 112-minute quadrangle). Other exposures little of this is, conglomerate. The maxi- are in open pasture lands. south of the mum thickness of the Ba&ursville Con- sewage disposal site along Laurel Run and glomerate Member is estimated to be 150 along the Southern Railroad 1.6 km (1.0 mi) m (490 ft). ' northeast of Montpelier Station; both lo- Buckland Format ion calities are in Orange County. The rail- 'road-cut exposure is deeply weathered, but The Buckland Formation is herein named it is quite extensive. 'from exposures along V.S. 29-211 in the Description. The Barboursville Con- vicinity of Buckland, Prince William * glomerate. Member comprises nearly equal County. Lee (1977, p. C-6) has named this. amounts of.clasts composed of fine-grained sequence as the basaltic flow-bearing silicates and equigran'ular quartzose- clastic member of the Bull ;$.. Formation. feldspathic clasts. The latter category is It includes a sequence of basalt flows and dominantly arkosic sandstone, alihough it interbedded sedimentary units. The de- may include a'cmplement of medium-grained tailed stratigraphy of this foruption, . granitic fragments. In oily one sample especially of the sedimentary Units, is weke foliated ietamorphic rock fragments not known, and no formal subdivision into .'I ,.noted. The matrix is generally dark grayish members is made here. However, I use here maroon. Sources for the Barbouksville an informal classification in which the conglomerates were the Catoctin and Fauquier basalt-flow units are designated I through Format ions. V, withII being the oldest and V the Thickness. Because of the very irregu- youngest. . Several of the flow units contain 9 I. lar outcrop pattern and ill-defined beddinp,, two flows separated by a relatively thin it is difficult to estimate an accurate seamentary sequence. The thicker sedi- thickness. The total sedimentary column mentary units between major flow units are * south of the Rapidan River is approximately referred to by denoting the flows between Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1 which they occur (for example, sedimentary ,rich in diopside, orthoclase, and quartz. unit 1-11, between flow units I and 11). Well-developed columnar jointing and With the exception of flow unit 1, the vesicular zones at the flow tops are cow flows-are for the most part laqerally ,con- mon features in well,-exposed sections. tinuous. Flow unit I, especially along its Minor thermal effects, primarily ,darkening southern extremity in Fauquier County, of red sedimentary rocks below the flows, ., , crops.. out as isolated bodies generally less is also pres6nt; these zones are generally / than 0.5 km long in the strike direction. less than 1 m thick. This phenomenon is probably the result of The sedimentary rocks in the Buckland Jurassic erosion,,although nonaccumulatson, Formation are dominantly sandstone, mud- ,of.basalt over topographic highs is also stone, mud shale, an@ silt shale. Lacus: possible. trine beds, including black shale and Distribution. The Buckland Formation limestone, lie between flow units I and I1 crops out in an arcuate belt concave to- (localities 4, 5, and 9 of Baer and Martin, ward the border fault, and it extends from 1949). The only conglomerates in the Buck- ,\ the Rappahannock River northward to a point land Formation lie between flow units I and 5.1 km (3.2 mij south of Leesburg in I1 in the northern part of the outcrop belt. Loudoun County. The length of this area Thickness. The mean thicknegs of the - Q is 64 km (40 mi), and it has a maximum Buckland Formation is 1,670 m (5,480 ft). Q width of 4.6 km (2.9 mi). The best ex- Figure 8 shows a columnar section of the posed section of the Buckland Formation B,uckland Formation. The thickness of indi- 1 I occurs along U.S. 29-211 (Fig. 7). Other vidual flows increases .from south to north representative Gcaiities are given in (Fig. 9). Table 1. . Waterfall Formation Description. The basalt is generally fine ta..medium crystalline' (0.1 to 0.5 nun) The Waterfall Formation is herein named and locally contains a pe'gmatidc phase' for exposures in' the vicinity of,the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1726 Figure 7: Map showing strati’- graphic sequence within Buckland t Formation as exposed along U.S. 29-211-15 (double easf-west ‘line near center of map) near Buckland, Prince William County and Fauquier County (Thoroughfare Gap 7g-minute quadrangle). Basalt flow units indicated by numerals I through V. See Figure 3 for location. Figure 7 is continued on the following frame. pre-R wqterfal I q tz i t.e 0 Fm. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1727 V iv 111 II 1 - Bull Run BUCKLAND FM. Fm. Pigure 7. ' (Continued) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1728 TABLE 1. SELECTED LOCALITIES OF UNITS WITHIN BUCKLAND FORMATION Stratigraphic Locality Comments Unit 1. Flow Unit V Rt. 701 2.15 km west of intersection with Fine-grained sediment fillink U.S. 15; Prfnce Wm Co.; Middleburg quad. fissures in tqp of flow 2. Sedimentary Just west of Rt. 630 at Chesfnut'Lick; Green and brown mud shale a'nd Unit IV-V Prince Wm. Co'.; Middleburg quad. thin sandstone beds; possible lacustrine uuit , 3. Flow Unit IV Rt. 630 0.2 km south of Chestnut Licki Nonvesicular basalt near top of Prince GJm. Co.; Middleburg quad. flow , 4. Sedimentary Numerous outcrops'northwest and south- Medium-grained sandstone and.muc Unit' 111-IV west of Silver Lhke; Prince Wm. Co.; shale with some showing small- , Thoroughfare Gap quad. scale cross-bedding 5. Frow Unit 111 Large quamy owned by-W.W. Sanders, Jr., Nearly complete section qhrough east of Rt. 643, 2.5 km nbrthwest of lower flow; amygdaloidal zones Casanov'a; Fauquier Co. ; Catlett quad. and columnar jointidg in top of flow in'northernmost pdt 6. Flow Unit 111 Rt. 701 0.85 km west of U.S. 15; Prince Base of lower flow showing them Wm. Co.; Middleburg quad. bffects on underlying mud shale and sandstone 7. Sedimentary Numerous outcrops along Turkey Run 1.5 Siltstone and sandstone Unit 11-111 km northwest of Casanova; Fauquier Co. ; Catlet t quad. 8. Sedimenta.ry Rt. 701 0.60 Ian west of U.S. 15; Prince Silt shale showing small-scale Unit 11-111 Wm. Co.; Middleburg quad. cross-bedding 9. Flow Unit 11. Rt. 701 0.45 km west of U.S. 15; Prince Vesicular zone at top of bpper Wm. Co.; Middleburg quad. flow 10. Flow Unit I1 Rt. 684 0.15 km south of South Run; Vesicular basalt at top ,of lower Prince Wm. Co.; Thoyoughfare Gap quad. flow 11. Flow Unit I1 Southern Railrbad cut 0.30 km southeast' Fault breccia in basalt; fault of Casanova where, Rt. 602 crosses track; trends N20°W Fauquier Co.; Catlett quad. 12. Sedimentary Licking Creek 0.4 km northwest of Rt. 602; Lacustrine black shale and lime- Unit' 1-11 Fauquier Co. : Midland quad. stone with greenish 6dstone 13. Sedimentary Rt. 602 0.3 northeast of Licking Creek; Silt shale showing small-scale Unit I- 11 Fauquier Co. ; Midland quad. cross-bedding,' burrowing, and dessication cracks 14. Flow Unit I Rt. 6jl 0.1 km west of Gleedsville; Vesicular basalt at gop of. flow' Loudoun Co.; Leesburg quad. 15. Flow Unit I Southeast side of U.S. 15 0.4 km southwest Base of lower flow showing them; of intersection with Rt. 55; north-facing effects on underlying shale; ex- slop& south of service building; Prince cellent spher~idalweathering Wm. Co.; Thoroughfare Gap quad. -Note: Numbers are keyed to columnar section shown in Figure 8. Flow. Unit refers to unit composed mainly of one or more basalt flows; Unit I is the oldest and V the youngest. Sedimentary units are designated by the flow units between which they occur. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1'729 Basaltic . Flow* Locality No. Sca I e Unit i1 12 13 Figure 8. Columnar section of BuckIand *t4- Fqrmation showing mean thickness of. various unjts. BasaLt flow units are indicated by numerais I through V. Sedimentary units * are indicated by dash pattern. Localities of typical exposures are shown by numerals 15 1 'through 15; th.&se are described in Table 1. 16 17' 8 9 10 111 112 113 114 115 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1730 feet neters 500 1,500 m 400 Yl w’ z ‘& 1,000 300 V zY b’ 2 00 5 00 100 C I I I1 I Ill I I I 0 ABI) C Ds E F GH/ K; LMN 0 P QR’S. SOUTH ’ ’ 36 mi ’ NORTH - 60 hrn - - . Figure 9. Thickness of basalt flow units I through V, calculared using dip and outcrop width along section A-A (shown on index map). Flow units I1 and I11 eacb contain two separate flows. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1' r ' community of Waterfali, Prince Willjab north of New Balti.moTe in Fauqder County County, especially in fields north Qf ahd an extepsive (b& deeply weathered) Route 630, 0.: km (0.2 mi7 northwest of roadcut 0.45 km (0;28 'mi) west of Route 1 Waterfall. Part of the Waterfall Formation 600 on a dirt rpad leading into the Bull is inc1udeql.h what Roberts (1928, p. Run Estates [intersection of tbe dirt road, 15--16)* called the arkose conglomerate. and'Route 600 is 0.9 km (0.6 mi) south of His description of this rock is quite dif- the Intersection of Routes 600 and 7011. ferent 'from mine. Part of this discrepancy The outcrop in the Bull Run Estates is is probably due to the fact tbat fresh less than 100 m (330 ft) east of the border outcrops.are not common, and, as Rob'erts fault. 'indicated, these conglomerates are for Descriptionr Th'e Waterfall Formation the most part deeply weathered. contains interbedded conglomerate, sand- c Distribution. The Waterfall Formation.. stone, and shale units. The Conglomerate lies adjacent to the western border Eault a beds, which range in thickness from less and extends northward from'- just south of than 1 m to several. tens of 'metres, are 4 * the community of Broken Hill in Fauquier dominantly composed of clasts of fine- I County to the Bull Run Estates in Prince grained silicates and quartzite and are William County. This arc" is 18 km (11 mi) very similar to the southern part of the ' long and has a maximum width of 2.3 km G,oose Creek Member of the Bull Run Forma- (1.4 mi). tion, which crops out to the northeqst. . In addition to the outcrops near.Water- Source rocks for the Waterfall' conglvmer- 6 fall, excellent exposures occur in the ates were mainly Catoctin and Chilhowee idillbrook. Quarry south of route 55, 1.2 metamorphic rocks of the Blue Ridge km (0.7 mi) east of Thoroughfare Gap in , province to the west. Locally basalt Prince Wil.liam County. Other localities clasts are abundant inucating erosion include outcrops in the stream valley 0.3 of Jur'assic flows. In addition, six of km (0.2 mi) southeast of Beulah Church the.nine fish-bearing lacustrine beds Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1 37 (black calcbreous shales) described by Baer > the finegrained clastic rocks interfinger and'Martin (1949) are present in the upper with axid are overlain by a sequence of '1 I half of the Waterfall Formation. cosrse clastic rocks, including the cm- Thickness .' The &ximum thickness .of glomeratic members of the Bull Run Forma- the Waterfall Formation is cal,culated to tion. This second flooddof coarse clastic 4- be'1,500 m (5,000 ft) in the vicinity of sediment' was' probably caused by renewed '1 Waterfall. movement along the western border fault. The Bull Run Formation is overlain by CONCLUSIONS multiple basalt flows and interbedded The character of Culpeper Group sedi- sedimentary units assigned to the Buckland mentary rob was strongly influenced by Formation. Black, calcareous, fish-bearing I ' syntectonic gctivity. Although depositiou lacustrine shales are present bet,ween the of the Reston Formation may have been con- two lowermost .flows. These are the oldest trolled by faulting along the eastdm mar- true la'custrine beds in the Culpeper Gdoup, gin of the basin, paleocurrent data suggest although Comet (1977) has described numer- a western source for the overlying Manassas, ous thih gray shale Ueds'throughdut the 1 'San.dstone. . The sandstones may represent an Bull' Run Formation as being lacktrine. .. \ early period of movement along the east- These gray shales probably represent stand- \ dipping normal fault that. borders the ing water (for example, ponds and lakes on 'r basin to the west. As ,the western source a -river Hood plain) but not permanent / area was lowered by weathering and erosion, lakes like thosy in which tke black shale the east-sloping alluvial sqrface became of the Buckiand Formation was deposited. progressively less 'steep, and stream compe- The Waterfall Formation overlies the, upper , tency was reduced. This result*ed in the basalt in ,the Buckland Formation and con- change from sandstone upward to the shalet tains conglomeratic beds intercalated with siltstone, and mudstone in the Bull- Run lacustrine deposits similar to those in Formation. On the west side of rhe basin, thk Bucklind Formation. The lacustkine beds Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1 33~~ ~~ may save been deposited after the basin I half a,century. floor ha-d been tilted to the west, thereby I ACKNOWLEDGMENTS ponding runoff in a topographic low between the elevated area west of the border fault I thank 'all of the Sandowners. who so 0 and the elevated region to the east (pro- kindly allowed me on their property dur;ing ) I \ duced by the regional eastward tilting of the course of this project... I also thank the basin;.- Lindholm, l978a). the Terrain ikidysis Center, U.S. Army The,stratigraphic scheme presented in Engineers Topographic Laboratories, Ft . this paper attempts to retain as fully as Belvoir, Virginia, for ietting me use an possible the well-established terminology enlarging unit without which it would have used by Roberts (1928), as we3i as to use been impossible to prepare the numerous new data gained during the past 5 yir. Th'e maps involved in' this study. I am greatly most clearly delinkated stratigraphic tmit' indebted to the following for reading the , is the pqckag? of basalt flows and inter- manuscript for this paper and making . ' bedded sedimentary units. This is a basic numerous help,ful comments: G. L. Bain, .. c element in Cornet's (1977) informal system, A. G. Coates, B. Cornet, S. W. Fagin, Beyond chat, I have attempted to separate A. J. Froelich, J. M. ,Hazlett, K. Y. Lee, major conglomeratic units and those units J. Reinhardt, R. P. Volckmann, and R. E. dominated by either sandstone or shale. Weems (each reader does not necessarily' The difficdties in mapping these sedimen- -agree with all aspects of.this paper, tab Fits include: (1) lateral and vert:: however). Research funds were provided cal gradational boundaries, and . (2) limiD$d by the George Washsngton University Com- exposures due to intensive weathering, aad mittee on Research between 1974 and 1979. little tdpographic relief. Nonetheless, ' REFERENCES ' CITED I feel that the divisions are usable and that the terminoAogy Conforms as best it Baer, F. M., and Martin, W. H., 1942, Some can to that used in this area for more than new finds of fossil ganoids in the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 1 34 Virginia Triassic: Science, v. 110, rangle, Virginia: Virginia Geological .. p. 684-686. Survey Bulletin 54, 94 p. Bennison, A. P., and Milton, C., 1954, Pre- Hazlett, J. M., 1978, Petrology and prove- P liminary geologic map of the Fairfax, nance of the Triassic Limestone Con- Virginia, and part of the Seneca, Vir- glomerata in the vicinity of Leesburg, ginia-Maryland quadrangles: U. S. Geo- Virginia [M. S. thesis] : Washington, logical Sukey Open-File Report, scale D.C., George Washington University, 1:62,500, 1 sheet. 100 p. Comet, B., 1977, The palynoscratigraphy Hubert, J. F., and others, 1978, Guide and Age of the .Newark Supergroup [Ph;D. to the redbe'ds of central Connecticut: thesis]: University Park, Pennsylvania Society Economic Paleontologists and State University, 505 6. Mineralogists,' Eastern Section, 1978 Eggleton, R: E., 19.75,. Preliminary geologic , Field Guide (University of Massachusetts map of the Hemdon, quadrangle,' Virginia: Department of Geology and Geography U. S. Geolpgical Survey Open-File Xeport Contribution No. 32), 127 p. 75-386, scale 1:24,000, 1 sheet. Klemic, H., 1962, Uranium occurrences in Fag&, S. W., 1977, The geology of the sedimentary rocks .of Pennsylvania: Culpeper Basin within Culpeper- County, U.S. Geological Survey Bdletin 1107-Da Virginia [M. S. thesis] : Washington, p. 243-288. D.C., George Washington University,! Larson, J., D., .1978, Hydrology of the 06 ( 103 p. servation well site at the United States Fontaine, W. M., 1883, Contribufions to Geological Survey National Center, ' the knowledge of the older Mesbzoic Reston, Virginia: U.S. Geological Sur- flora of Virginia: -U.S. Geological vey Open-File Report 78-144, 35 p. Survey Monograph 6, 144 p. Lee, K. Y:, 1977, Ttiassic stratigraphy in i Furcron, A. S., 1939, Geology and mineral the northern part of the Culpeper Basin, \ resources of the Warrenton (15') quad-. Virginia and Maryaand: U.S. Geological Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11_Part_II/1702/3433846/i0016-7606-90-11-1702.pdf by guest on 30 September 2021 P 5 Survey Bulletin 1422-C, 17 p. Abstracts with Program, p. 4--5. L Lindholm, R. C., 1977a, Geology of Juras- Lindholm, R. C., Hazlett, J. M., and sicTriassic Cdlpeper Basin, North of Fagin, S. W., 1979, Geology of Triassic- Rappahannock River, Virginia: American Jurassic conglomerates in the Culpeper Association of Petroleum Geologists Basin, Virginia: Journal of Sedimentary Annual Meeting, Abstracts with Program, Petrolog); (in press). p. 112. Lindskold, J. E., 1961, Geology and fietrog- - 1977b, The Culpeper Basin, Virginia: A raphy of the Gainsville, Virginia, case study in which Triassic tectonic quadtangle [M.S. ,thesis]: 'Washington, l .patterns were inherited from pre,existing D.C., George Washington University, structural fabrics: GeoX\ogical Society '50 p. of America Abstracts with Programs, v. XcCauley, J. F., 1961, Uranium in Pennsyl- \ 9, p. 1070. vania: Pennsylvania Topographic and >. - 1978a, Tectonic control of sedimentation Geologic Survey Bulletin 1443, 71 p. in Triassic-Jurassic Culpeper Basin, McCollum, M. B., 1971, Basalt flows in the Virginia: ,American Association of Pe- hriassic Culpeper Bas&, Virginia: / troleum Geologists Bulletin, v. 62, Geolzgical Society of America Bulletin, p. 537. V. 82, p. 2331--2332. - 1978b, Triassic-Jurassic faulting in Merrill, G. P., 1891, Stones for building eastern Nwth America: A model'based and decoration: New York, John Wiley Qn pre-Triassic structures: Geology, ti Sons, 453 p. 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