327 Paleoecology of late Pleistocene marine sediments of S. Virginia

The late Pleistocene faunas of southeastern Virginia are essentially modern in character, with approximately 97 percent of the 332 species extant. The fossils are contained in two transgressive-regressive depositional cycles, assigned here to the Acredale and Powells Crossroads formations, two new stratigraphic units. One new species of gastropod, Cingula norfolkensis, is described. 

Paleoecologic analyses of Acredale assemblages and sediments indicate a cyclic event beginning with estuarine conditions and comparatively cool temperatures during the deposition of the basal Great Bridge Member. More open bay or inlet conditions and warmer temperatures were established during deposition of the superjacent lower part of the Norfolk Member, giving way to a sublittoral shelf environment and warm water temperatures at peak transgression during deposition of the upper part of the Norfolk Member. Overlying faunas indicate shallowing for the lower part of the Kempsville Member with temperatures similar to those indicated for the upper portion of the Norfolk. The upper part of the Kempsville Member deposits are a complex of beach sands, channel fill, and very shallow sublittoral environments with drastically reduced temperatures.

The younger Powells Crossroads Formation, representing a second transgressive-regressive cycle, is less extensively developed, but two members can be recognized in the study area. Cool temperatures and estuarine conditions prevailed during deposition of the Londonbridge Member. Warm temperatures and sublittoral shelf environments prevailed during deposition of the lower portion of the Sand Bridge Member. Temperatures remained warm during deposition of the upper part of the Sand Bridge Member, while a lowering of sea level produced a complex of beach sands, washover fans, and channels dominated by intertidal species.

This pattern of sea level and inferred temperature variation for the Virginia late Pleistocene section is compatible with the oxygen isotope and sea surface temperature patterns from deep sea cores from isotope stage 5 — 70,000 to 125,000 years before the present.

Year published: 1987