Paleoecology and sedimentology of a late triassic lake, culpeper basin, Virginia, U.S.A

Abstract

Late Triassic and Early Jurassic lacustrine sedimentary rocks in the Culpeper Basin, northern Virginia and Maryland, U.S.A., locally contain abundant fossil ostracodes ( Darwinula sp.), conchostracans (clam shrimp, Cyzicus sp. and Cornia sp.), fish, and plant fragments. Stromatolites, pelecypods, notostracans (tadpole shrimp, Triops cf. cancriformis), and insects are known from some Late Triassic beds, and freshwater gastropods have been reported from the Early Jurassic of the Culpeper Basin. A 7-m thick, Late Triassic, lacustrine sequence exposed in the central part of the Culpeper Basin records the history of a perennial saline lake. The lake deposits consist chiefly of olive gray mudstone and claystone, with interbedded sandstone and siltstone, and stromatolitic limestone in the upper part. Evaporite crystal molds and pseudomorphs are present locally in the central part of the sequence, indicating saline conditions. Red laminated and cross-laminated sandstone and siltstone beds at the top and near the bottom of the sequence were deposited by fresh water inflow to the lake. Marginal lacustrine deposits associated with these fluvial redbeds contain abundant faunal remains, including ostracodes, conchostracans, notostracans, and scarce fish remains and insects. These organisms were restricted to relatively fresh water near the lake margins; faunal remains are absent from the central, saline part of the lake sequence.