A NEW UPPER TRIASSIC DINOSAUR & REPTILE TRACK SITE, CULPEPER BASIN, PRINCE WILLIAM COUNTY, VIRGINIA

The Culpeper basin (CB) is an extensional fault basin located between Madison Mills, Virginia and Frederick, Maryland that had its origin during the initial phases of Mesozoic continental separation. During development, the CB was filled with detritus derived from uplifted pre-Mesozoic metamorphic & plutonic basement exposed along the rift margins. CB clastic rocks record deposition in a number of different subaerial environments including rift-margin alluvial fans, braided & meandering streams, overbank/floodplain areas, and lacusterine environments. In Virginia, these strata have historically yielded dinosaur and reptile tracks, gastroliths, rare bones, fish fossils, fresh-water invertebrates, and plant material. A new track site was discovered in Prince William County, Virginia, just SW of Manassas. The site lies in the upper 1/3rd of the early Norian Poolesville Member of the Manassas Sandstone. At the site, the Poolesville consists of shallowly westward dipping, micaceous, red or grey shales; red, parallel-bedded, silty shales with local ripple-drift cross lamination; and massively-bedded, to variably trough cross-bedded, dusky-red, fine to coarse-grained arkosic sandstone. Conglomeratic lithologies were not recognized. The Poolesville strata are consistent with deposition along floodplains bordering a sand-dominated fluvial system. Fossil material recovered at the Manassas site includes therapod and tetrapod footprints, fish fossils, and poorly preserved casts of wood and cycad fronds. The moderately well preserved dinosaur footprint impressions range in size from 0.5 cm to 15 cm and are most commonly preserved on slabs of mud-draped, arkosic, sandstone. The largest track-bearing slab measures 157 cm in length, weighs approximately one ton, and contains over 40 footprint impressions. Just over 8 km’s NE of the Manassas site, the Manassas Sandstone in Fairfax County has previously yielded footprints of Brachychirotherium parvum, Chirotherium lulli, and Plesiornis pilulatus. The new site has tentatively yielded footprints representing Kayentapus minor, Grallator, and Gregaripus. The stratigraphic position of the Manassas site appears to be at or just above the Fairfax County site, making it the second oldest horizon in the CB from which dinosaur tracks have been recovered.