Depositional and Structural Histories of Richmond and Taylorsville Triassic Rift Basins: ABSTRACT

Abstract

The Richmond and Taylorsville basins of Virginia display similar structural and sedimentological histories, which differ somewhat from those of other eastern North American rift basins. Both basins formed during the middle Carnian in response to the initial opening of the North Atlantic Ocean. Both are westward tilted half grabens whose border faults share a reactivated Paleozoic basement thrust (the Hylas zone). A dipmeter log from the Bailey 1 well (Richmond basin) shows that bedding dips toward the border fault and increases with depth. These relationships are interpreted to reflect syndepositional rotation and tilting of the basin above a listric normal fault. Unlike the red beds typical of other eastern North American rift basins, sediments of the Richmond and Taylorsville basins are mostly unoxidized gray sandstones and siltstones. The sediments of both basins can be divided into three facies associations. The lowest is a poorly sorted, coarse-grained, lithic arkose rapidly deposited in alluvial fans and braided streams. This was succeeded by sand, silt, and mud of a lower End_Page 1449------------------------------ gradient, fluvial-paludal setting. Organic-rich black shales accumulated in lakes (oxbows?) and overbank deposits. An influx of cross-bedded fluvial sands, possibly triggered by renewed border faulting on the west, characterizes the final stage of basin fill. These sediments represent fan-delta systems that prograded northeastward across the lacustrine-paludal lowlands. End_of_Article - Last_Page 1450------------