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Geologic History and Stratigraphy of the Triassic-Jurassic Culpeper Basin, Virginia: Summary

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Geologic History and Stratigraphy of the Triassic-Jurassic Culpeper Basin, Virginia: Summary 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11/995/3444397/i0016-7606-90-11-995.pdf by guest on 26 September 2021 996 R. C. LINDHOLM 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 LINDHOL S PAPE 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 M Members R Barboursville Conglomerate Member con- Robert Border Manassas Bull Run 192 sists of nearly equal amounts of clasts com- Border Cong Congl. Sandstone Shale 8 s posed of fine-grained silicates and arkosic sandstone. Source rocks for the Bar- Manassas Sandstone Bull Run Formation Balls Bluff 1 9 Le boursville conglomerates were the Pre- Reston Sandstone Leesburg 7 e Siltstone 7 Member Member Ls. Congl. Basaltic- Flow- Bea ing-ciaslic- Member cambrian Catoctin and Fauquier Forma- Member tions. 19 Corne 7 Fo mation K Formation B-J Formation A 7 Buckland Formation t 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.
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