Geologic history and stratigraphy of the Triassic-Jurassic Culpeper Basin, Virginia: Summary

R. C. LINDHOLM Department of Geology, George Washington University, Washington, D.C. 20052

INTRODUCTION Olsen and others, 1978). Unique among ognized the "Border Conglomerate forma- these lacustrine deposits are the analcime- tion, the Manassas sandstone and the Bull The Culpeper Basin in northern Virginia rich beds of the Lockatong Formation in the Run shales." The sandstones lying near the extends south from the Potomac River for a Newark Basin (Van Houten, 1964). Coal eastern edge of the basin were named by distance of more than 145 km (90 mi). It swamps were restricted to some of the ba- Roberts the "Manassas sandstone." He occupies the western part of the Piedmont sins in North Carolina and southern considered the basal conglomerates that province and is bounded on the west by the Virginia. Basalt flows are common features underlie the sandstones to be part of the Blue Ridge province. The western margin of in the northern basins, but not in those to "Border Conglomerate formation." The ex- the basin is marked by a steep east-dipping the south. Diabase intrusions occur in most tensive shale sequence lying west of (and normal fault; the sedimentary and volcanic of the basins. stratigraphically above) the Manassas strata dip westward into the fault. The sequence in the Culpeper Basin be- sandstone was designated by Roberts as the The Culpeper Basin is but one of many gins with coarse clastic rocks at the base, "Bull Run shales." In the Culpeper Basin such basins in North America where passing upward into fine-grained clastic the main body of the "Border Conglomer- sedimentation extended from Late Triassic rocks. On the west side of the basin the ate formation" lies west of the "Bull Run to Early Jurassic time (Cornet, 1977). fine-grained clastic rocks are overlain by a shales" and is stratigraphically above the Other such basins include the Newark in younger sequence of coarse clastics, includ- shales. New Jersey, the Gettysburg in Pennsyl- ing extensive conglomerates. The conglom- vania, the Hartford in Connecticut, the erates are in turn overlain by multiple basalt 1950 to Present Deerfield in Massachusetts, and the Fundy flows and interbedded sedimentary units. A in Nova Scotia. The southernmost basins conglomeratic unit, containing intercalated Except for Roberts' work in the 1920s, contain only rocks of Late Triassic age; they black shale beds, caps the sequence. the rocks in the Culpeper Basin were largely include the Durham, Sanford, Wadesboro, Diabase intrusions are abundant through- ignored by geologists until the 1950s. and Dan River Basins in North Carolina out the basin. These plutons are surrounded Mapping done between 1950 and 1974 and the Danville and Richmond Basins in by extensive metamorphic aureoles of added details of local geology but was too southern Virginia (Cornet, 1977). black, green, and gray hornfels. restricted geographically to add significant- Red beds deposited on broad alluvial ly to a more complete understanding of the plains dominate in most of these basins. PREVIOUS WORK stratigraphy of these rocks. Lacustrine black shale and limestone repre- In 1977, Lee published a major revision sent a facies common to most of the basins, 1883 to 1928 of Culpeper Basin stratigraphy. Except for but these beds generally make up only a the lower sandstone sequence, his nomen- small part of the total sequence (Hubert and The first comprehensive study of the Cul- clature represents a drastic departure from others, 1978; Wheeler and Textoris, 1978; peper Basin was by Roberts (1928). He rec- Roberts' well-established terminology.

The complete article, of which this is a summary, appears in Part II of the Bulletin, v. 90, no. 11, p. 1702-1736.

Geological Society of America Bulletin, Part I, v. 90, p. 995-997, 1 fig., November 1979, Doc. no. 591102.

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PROPOSED STRATIGRAPHIC Basin north of the Rappahannock River. conglomerate units are separated into sev- NOMENCLATURE FOR ROCKS IN This follows Roberts' (1928) use of Manas- eral members on the basis of differences in THE CULPEPER BASIN sas Sandstone in reference to these rocks, clast lithology (Lindholm and others, but it differs from Lee's (1977) usage, 1979). Triassic-Jurassic rocks in eastern North which includes the Reston Member as part Leesburg Conglomerate Member. The America have traditionally been assigned to of the Manassas Sandstone. Plagioclase-rich extensive carbonate pebble conglomerate the Newark Group. Olsen (1978) has arkosic sandstones compose two-thirds of that crops out around the town of Leesburg suggested that the term Newark be elevated this unit, with most of the rest being in Loudoun County and northward across to the status of supergroup, and he desig- plagioclase-rich siltstone. Paleocurrent data the Potomac River into Maryland is herein nated the rocks in several of the basins as a (cross-bedding) indicates a western source named the Leesburg Conglomerate Mem- separate group. To date no formal term area. This is consistent with the abundant ber of the Bull Run Formation. This con- exists for the rocks in the Culpeper Basin, plagioclase in these rocks, suggesting deri- glomerate is what Lee (1977, p. C6) re- although Cornet (citing Olsen, 1976, as an vation from Chilhowee and Catoctin ferred to as the Leesburg Limestone Con- informal source) has referred to these rocks metamorphic rocks now exposed in the glomerate Member of the Bull Run Forma- as composing the Culpeper Group. The Blue Ridge west of the Culpeper Basin. tion. In typical exposures the rock consists Triassic-Jurassic boundary lies below the ot gray carbonate pebbles set in a red lowermost basalt flow, although the precise Bull Run Formation sand-silt matrix. In the northern area position is not known (Cornet, 1977). The dolomite pebbles dominate over limestone scheme here follows as closely as possible I propose Bull Run Formation as the pebbles, whereas the opposite is true in the that established by Roberts (1928) and gen- name for the sequence that is bounded southern area. Carbonate pebbles were de- erally agrees with the informal nomencla- below by the Manassas Sandstone and rived from lower Paleozoic carbonates ex- ture used by Cornet (1977; see Fig. 1). above by the lowermost basalt flow in the posed on the east flank of the Blue Ridge overlying Buckland Formation. This is anticlinorium. Reston Formation equivalent to "Bull Run shale" as used by Goose Creek Conglomerate Member. Roberts (1928). Fluvial red plagioclase-rich The Goose Creek Conglomerate Member of I propose that the Reston Member of the mudstones and siltstones dominate the Bull the Bull Run Formation is herein named Manassas Sandstone as defined by Lee Run Formation. Most are massive to from exposures on the south side of Goose (1977) be changed to the Reston Forma- mottled, although some are laminated or Creek in Loudoun County. Greenish and tion. This conforms more closely to cross-bedded. Dessication cracks are com- grayish pebbles composed of fine-grained Roberts' original stratigraphic scheme. The mon throughout. Thin gray "lacustrine" silicates make up 72% of the clasts in the Reston Formation is the basal unit in the lutites are scattered throughout the section Goose Creek Conglomerate Member. These Culpeper Basin and is present along the (Cornet, 1977). As with the Manassas clasts were derived from metamorphic eastern margin of the basin, north of Bull Sandstone, paleocurrent data and mineral- rocks of the Catoctin and Chilhowee For- Run. The composition of the Reston For- ogy indicate a western source area. mations now exposed in the Blue Ridge mation indicates derivation from nearby province west of the Culpeper Basin. Piedmont terrane. Conglomeratic Members of the Cedar Mountain Conglomerate Mem- Bull Run Formation ber. The Cedar Mountain Conglomerate Manassas Sandstone Member is herein named from exposures on Numerous conglomeratic bodies are Cedar Mountain, which is located 14 km I use Manassas Sandstone to include only present in the Culpeper Basin. Those above (8.7 mi) southwest of the town of Culpeper the sequence dominated by sandstones the Manassas Sandstone and below the in Culpeper County. This unit consists of which overlies the Reston Formation and lowest basalt flow are herein included as pebble to boulder conglomerates composed crops out in the eastern part of the Culpeper members of the Bull Run Formation. These of 60% to 95% greenstone clasts derived from the Catoctin Formation. Barboursville Conglomerate Member. TRIASSIC 1 JURASSIC The Barboursville Conglomerate Member is herein named for exposures south of Bar- boursville in Orange County. This unit oc- curs as isolated masses scattered through- THI S LINDHOL M

PAPE R out the southernmost part of the Culpeper Reston Manassas Bull Run "ormation Buckland Fm. Waterfall Fm. Fm. Sandstone Congl. Basin south of the Robinson River. The Members Barboursville Conglomerate Member con- Robert s

Border Manassas Bull Run 192 8 sists of nearly equal amounts of clasts com- Border Cong Congl. Sandstone Shale posed of fine-grained silicates and arkosic sandstone. Source rocks for the Bar- Manassas Sandstone Bull Run Formation

Balls Bluff 1 9

Le e boursville conglomerates were the Pre- Reston Sandstone Leesburg 7 Siltstone 7 Member Member Ls. Congl. Basaltic- Flow- Bea ing-ciaslic- Member cambrian Catoctin and Fauquier Forma- Member tions. 19 7 Corne t

Fo mation K Formation B-J Formation A 7 Buckland Formation 1 1

Figure 1. Stratigraphic nomenclature proposed in this paper compared with that of The Buckland Formation is herein named Roberts (1928), Lee (1977), and Cornet (1977). This illustration is Figure 4 in the accom- from exposures along US 29-211 in the panying article in Part II. vicinity of Buckland, Prince William

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County. It includes a sequence of basalt Run Formation. On the west side of the ba- limited exposures due to intensive weather- flows and interbedded sedimentary units. sin, the fine-grained clastic rocks interfinger ing and little topographic relief. Nonethe- The Buckland Formation crops out in an with and are overlain by a sequence of less, I feel that the divisions are usable and arcuate belt concave toward the western coarse clastic rocks, including the con- that the terminology conforms, as best it border fault and extends northward from glomeratic members of the Bull Run For- can, to that used in this area for more than the Rappahannock River. The sedimentary mation. This second flood of coarse clastic half a century. rocks in the Buckland Formation are domi- sediments was probably caused by renewed nantly sandstone, mudstone, mud shale, movement along the western border fault. REFERENCES CITED and silt shale. Lacustrine beds, including The Bull Run Formation is overlain by black shale and limestone, lie between the multiple basalt flows and interbedded Cornet, B., 1977, The palynostratigraphy and lowermost two flows. sedimentary units assigned to the Buckland age of the Newark Supergroup [Ph.D. the- Formation. Black, calcareous, fish-bearing sis]: University Park, Pennsylvania State University, 505 p. Waterfall Formation lacustrine shales are present between the Hubert, J. F., 1978, Guide to the redbeds of cen- two lowermost flows. These are the oldest tral Connecticut: Society of Economic The Waterfall Formation is herein named lacustrine beds in the Culpeper Group, Paleontologists and Mineralogists, Eastern for exposures in the vicinity of the commu- although Cornet (1977) has described Section, 1978 Field Guide (University of Massachusetts Department of Geology and nity of Waterfall, Prince William County, numerous thin gray shale beds throughout Geography Contribution No. 32), 127 p. especially in fields northwest of Waterfall. It the Bull Run Formation as being lacustrine. Lee, K. Y., 1977, Triassic stratigraphy in the lies adjacent to the western border fault, in These gray shales probably represent stand- northern part of the Culpeper Basin, Fauquier and Prince William Counties. The ing water (for example, ponds and lakes on Virginia and Maryland: U.S. Geological Waterfall Formation contains interbedded a river flood plain) but not permanent lakes Survey Bulletin 1422-C, 17p. Lindholm, R. C., 1978, Tectonic control of like those in which the black shale of the conglomerate, sandstone, and shale units. sedimentation in Triassic-Jurassic Culpeper The conglomerate is dominantly made up Buckland Formation was deposited. The Basin, Virginia: American Association of of clasts composed of fine-grained silicates Waterfall Formation overlies the upper Petroleum Geologists Bulletin, v. 62, p. 537. and quartzite clasts. Source rocks were basalt in the Buckland Formation and con- Lindholm, R. C., Hazlett, J. M., and Fagin, S. W., 1979, Geology of Triassic-Jurassic mainly Catoctin and Chilhowee metamor- tains conglomeratic beds intercalated with Culpeper Group Conglomerates, Virginia: phic rocks of the Blue Ridge province to the lacustrine deposits similar to those in the Journal of Sedimentary Petrology (in press). west. Fish-bearing lacustrine beds (black Buckland Formation. The lacustrine beds Olsen, P. E., 1978, On the use of the term calcareous shales) are present in the upper may have been deposited after the basin Newark for Triassic and Early Jurassic half of the Waterfall Formation. floor had been tilted to the west, thereby rocks of eastern North America: Newsletter ponding runoff in a topographic low be- of Stratigraphy, v. 7, p. 90-95. Olsen, P. E., and others, 1978, Cyclic change in CONCLUSIONS tween the elevated area west of the border Late Triassic lacustrine communities: Sci- fault and the elevated region to the east ence, v. 201, p. 729-733. The character of Culpeper Group sedi- (produced by the regional eastward tilting Roberts, J. K., 1928, The geology of the Virginia of the basin) (Lindholm, 1978). Triassic: Virginia Geologic Survey Bulletin mentary rocks was strongly influenced by 29, 205 p. syntectonic activity. Although deposition of The stratigraphic scheme presented in Van Houten, F. B., 1964, Cyclic lacustrine the Reston Formation may have been con- this paper attempts to retain as fully as sedimentation, Upper Triassic Lockatong trolled by faulting along the eastern margin possible the well-established terminology Formation, central New Jersey and adjacent of the basin, paleocurrent data suggest a used by Roberts (1928), as well as to utilize Pennsylvania, in Merriam, D. F., ed., Sym- posium on Cyclic Sedimentation: Kansas western source for the overlying Manassas new data gained during the past 5 yr. The Geological Survey Bulletin 169, v. 11, Sandstone. The sandstones may represent most clearly delineated stratigraphic unit is p. 495-531. an early period of movement along the the package of basalt flows and interbedded Wheeler, W. H., and Textoris, D. A., 1978, east-dipping normal fault that borders the sedimentary units. This is a basic element in Triassic limestone and chert of playa origin in North Carolina: Journal of Sedimentary basin to the west. As the western source Cornet's (1977) informal system. In the un- Petrology, v. 4-8, p. 765-776. area was lowered by weathering and ero- derlying Triassic units I have attempted to sion, the east-sloping alluvial surface be- separate major conglomeratic units and came progressively less steep, and stream those units dominated by either sandstone competency was reduced. This resulted in or shale. The difficulties in mapping these MANUSCRIPT RECEIVED BY THE SOCIETY DE- CEMBER 26, 1978 the change from sandstone upward to the sedimentary units include (1) lateral and REVISED MANUSCRIPT RECEIVED APRIL 19,1979 shale, siltstone, and mudstone in the Bull vertical gradational boundaries, and (2) MANUSCRIPT ACCEPTED JUNE 27, 1979

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