Early Jurassic sedimentation of a rift-valley lake: Culpeper basin, northern Virginia

Abstract

Gray, calcareous, lacustrine strata of Early Jurassic age crop out adjacent to the faulted western margin of the Culpeper basin, a Mesozoic graben dominated by fluviogenic red beds. Sedimentological, mineralogical, and paleontological aspects reveal a distinct zonation of littoral, sublittoral, and profundal deposits in the lacustrine sequence. Reconstructed physiographic profiles of the lake exhibit shore-terrace, slope, and basin-plain subenvironments. Dominant sedimentary processes in these domains were, respectively, sediment agitation by waves; deposition from suspension, slumping, and proximal sediment gravity flows; and suspension and distal turbidity current deposition. Deposition of the lacustrine sediments occurred in a eutrophic, calcite-precipitating, permanently or normally stratified lake. The lake occupied an elongate, actively subsiding sub-basin adjacent to, or possibly straddling, the western border fault. The sedimentary fill of the sub-basin is characterized by several cyclic fluvio-lacustrine sequences which average 150 m in thickness and closely resemble sedimentary cycles of other Newark rift basins. The maximum areal extent of the lake may have exceeded 3,000 km2, with an average maximum depth of ∼35 m. Proterozoic and lower Paleozoic strata of the Blue Ridge anticlinorium to the west constituted the major source of clastic sediments in the lacustrine sequence, but uptilted basalt flows and interbedded sedimentary units of the Culpeper Group simultaneously shed detritus from the east.