Lacustrine sequences in an early Mesozoic rift basin: Culpeper Basin, Virginia, USA

Abstract

Summary Lacustrine sequences in the Culpeper Basin, Virginia, USA were deposited in a half-graben dominated by fluvial sheetflood deposition. During periods of increased precipitation and runoff, lakes formed within the basin. Numerous lacustrine sequences, ranging from less than 1 m to more than 35 m thick, document the expansion and contraction of perennial freshwater and saline lakes controlled by Milankovitch-type cyclic climatic change. Many of the lacustrine sequences contain black, laminated shale, deposited in deep, anoxic water in stratified lakes. Most of the lakes were hydrologically closed, as indicated by the presence of evaporite crystal moulds and pseudomorphs in associated mudflat deposits.