CONCHOSTRACAN EVIDENCE FOR THE AGE OF FOOTPRINT-BEARING STRATA IN THE CULPEPER CRUSHED STONE QUARRY (SOUTHERN CULPEPER BASIN NEAR STEVENSBURG, VIRGINIA)

The Culpeper Crushed Stone Quarry has exposed the lowest seven cyclic lake-bed sequences of the Groveton Member of the Bull Run Formation known from the southern Culpeper basin area. These sequences can be traced several miles northward along strike from the quarry, but beyond that distance poor outcrop, intrusive diabase bodies, and faulting prevent accurate field correlation with lake-bed strata exposed to the north in the central Culpeper basin. Abundant dinosaur and crurotarsian reptile footprints occur within the quarry on two bedding surfaces that lie immediately beneath the gray strata of the second and fifth lake-bed sequences, respectively. Conchostracans have been found directly on the lower (second lake-bed cycle) footprint-bearing layer in the Culpeper Quarry and also within strata of the second lake bed immediately above the quarry section (ninth lake-bed cycle in the quarry region). The lower conchostracan horizon has yielded abundant Shipingia hebaozhaiensis; thus it lies within the S. hebaozhaiensis conchostracan biozone (Alaunian = middle Norian). The upper layer yielded both S. hebaozhaiensis and Redondestheria grovetonensis; thus it lies within the R. grovetonensis biozone (lower Sevatian = lower upper Norian). Therefore, the middle-upper Norian boundary lies within the stratigraphic interval between the second and ninth lake beds in and near Culpeper Quarry.

In the central Culpeper basin near Manassas, Virginia, about 40 lake-bed cycles are known from the Groveton Member, and the base of the R. grovetonensis zone is above the 20th lake-bed cycle there. Because the base of the R. grovetonensis zone occurs at least as low as the ninth lake-bed cycle in the southern Culpeper basin, we infer that at least 12 of the lowest 20 lake-bed cycles of the Groveton Member, present in the central Culpeper basin, are missing in the southern basin region. It in not clear whether this is due to an undetected unconformity within the Triassic strata of the southern Culpeper basin, to southward facies changes from lake to playa flat depositional environments, or to the oldest Groveton Member lake beds in the southern basin being concealed by faulting.