CORRELATING LATE PLEISTOCENE DEPOSITS ON THE COASTAL PLAIN OF VIRGINIA WITH THE GLACIAL-EUSTATIC SEA-LEVEL CURVE

Late Pleistocene sediments mapped along Virginia?s southeastern coastal plain record deposition by high-frequency cycles during isotope stages 5-to-1. Correlations between several of these units both south (S) and east (E) of the Chesapeake Bay have proven difficult due to insufficient dating methods. To improve these regional correlations, two members of the Tabb Formation on Virginia?s southside and two formations on the Southern Delmarva Peninsula were selected for stratigraphic analyses. Ground penetrating radar provided geologic details in areas where samples were collected for OSL dating. The stratigraphically older units, Sedgefield Member of the Tabb Formation (S) and the Butlers Bluff Member of the Nassawaddox Formation (E), yielded OSL dates of approximately 70 (stage 5a) and 50ka (stage 3), respectively. The Butlers Bluff data matches previous dates from coral in the Sedgefield; however, the dates from the Sedgefield were discarded due to a high concentration of U and Th indicating post-deposition migration into the sample site. The younger units, Poquoson Member of the Tabb Formation (S) and the Wachapreague Formation (E), yielded OSL dates of approximately 43ka and 42ka (stage 3), respectively. Altitudes of marine depositional surfaces for these units range from +12 to +3m. However, established glacial-eustatic sea-level curves, along with these dates and others reported for these units, suggests the units were deposited when sea level oscillated between +3 to ?25m. If both the sea-level curves and the dates are correct, then an uplift of at least 10 to 15m must have occurred in order to explain the altitudes of these units. These discrepancies may arise from isostatic adjustments of the forebulge peripheral to the Laurentide ice sheet that would have been at maxima during isotope stages 6 and 2. If peripheral bulges develop and decay more slowly than eustatic sea-level responds to high-frequency climate change, then these depositional units may mark times when a minor spike of sea level occurred while the bulge was relatively relaxed.