Uranium-series dating of mollusks and corals, and age of Pleistocene deposits, Chesapeake Bay area, Virginia and Maryland

Geologic mapping in conjunction with uranium-series dating of fossil mollusks and corals suggests that the low-lying ( < 17 m in altitude) terrace deposits in the central and southern Chesapeake Bay area include two main depositional sequences, each of which represents a high stand of the sea in late Pleistocene time. The older depositional sequence includes the Accomack and Omar beds of the Delmarva area, the fossiliferous deposits along the lower Rappahannock River, and the Norfolk Formation deposits west of the Suffolk scarp. These beds have yielded a single reliable coral age estimate of 184,000?20,000 years B.P., suggesting an early late Pleistocene age. The younger sequence, including the type beds of the Norfolk Formation and equivalent strata east of the Suffolk scarp, has yielded several coral ages ranging from about 62,000 to 86,000 years B.P. (including ages from our samples and previously reported age estimates); thus, it is clearly late Pleistocene in age. Groupings of ages obtained from our quahog analyses also suggest two transgressive sequences; however, the estimated quahog ages are consistently younger than ages based on coral samples from the same and equivalent stratigraphic units. Stratigraphic, paleoclimatic, and geomorphic data suggest that the estimated uranium-series age of 71,000?7,000 years B.P. for the type beds of the Norfolk, obtained by averaging our coral dates, may be too young by as much as several tens of thousands of years. A postulated equivalency of the type Norfolk beds, upper Pleistocene deposits near Charleston, S.C. (apparent uranium-series age = 95,000?5,000 years), and deposits in the Caribbean area thought to represent the highest sea stand during the last interglacial period (apparent age, 125,000?10,000 years) implies diagenetic modification of coralline material possibly in part because of regional differences in depositional and postdepositional environments.