STRATIGRAPHIC ARCHITECTURE AND TECTONIC CO-EVOLUTION OF THE LATE TRIASSIC TAYLORSVILLE AND RICHMOND RIFTS, VIRGINIA

Subsurface data from the Taylorsville and Richmond basins including; 11,000 m of well cuttings, 6,800 m of continuous core, and more than 275 km of seismic reflection profiles, reveals that the rifts contain two unconformity bounded sequences. The initial phase of rifting, termed Sequence 1 (S1), occurred during the Carnian on local normal faults that formed small basins distributed throughout the Piedmont of eastern Virginia. The small, sub-regional half-graben of S1 filled first with a) fluvial coarse sandstone and conglomerate, and then by; b) deep lacustrine, deltaic, and paludal deposits indicative of persistent basin flooding as the rift deepened. The stratigraphic pattern of S1 is remarkably similar in the Richmond and Taylorsville basins, although the Richmond basin contains more substantial coal beds. Fault length-displacement scaling relationships suggest that the Richmond and Taylorsville Basins were isolated sub-regional-scale half-grabens that were not connected during this stage of rifting, but rather shared similar patterns of sedimentation. As crustal extension continued the local normal faults increased in length and some of the faults linked to become the dominant, regional-scale faults while other faults ceased to accommodate extension. The fill of Sequence 2 (S2) is characterized by: a) lower, coarse sandstone and conglomerate interval; b) a middle interval of lacustrine, fluvial-deltaic, and paludal (coal) sedimentation; and c) a long, upper, interval of shallow lacustrine and fluvial rocks including cyclic, carbonate paleosols. S2 in the Richmond basin is substantially thinner than S2 in the Taylorsville basin because subsidence in the Richmond basin ended in the Carnian, while the Taylorsville rift continued to subside into the Norian. The stratigraphic architecture of the rifts controls possible source rock and reservoir relationships important for hydrocarbon production. The proximity of the basins to highly populated metropolitan areas in the mid-Atlantic region makes the possibility of natural gas production especially intriguing.