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The late Pleistocene of Virginia's outer coastal plain consists of sediments dated to marine isotope stages (MIS) 5 and 3. Two members from the Tabb Formation south of the Chesapeake Bay in southeastern Virginia and two formations east of the bay on the southern Delmarva Peninsula were dated using optically stimulated luminescence (OSL). The stratigraphically older Butlers Bluff Member yielded OSL ages of 70 ka (62–78 ka) (MIS-5a), and the younger Poquoson Member and Wachapreague Formation, MIS-3 ages of approximately 43 ka (33–50 ka) and 42 ka (33–54 ka), respectively. These shoreface and near-shore geologic units reached maximum altitudes ranging from 3 to 12 m above present sea level, and were deposited when established glacial-eustatic sea-level curves suggest that sea levels were significantly lower than present by approximately 40 m. If these new ages and the sea-level curves are correct, there must have been regional uplift of more than 40 m, probably due to isostatic adjustments of forebulges peripheral to North American ice sheets when they were at their maxima during MIS-6 and MIS-2. If the late MIS-6 forebulge collapse continued throughout MIS-5 and MIS-4, we propose that regional land elevations may have been low enough for deposition to occur during the lower eustatic sea levels of MIS-3. During late MIS-3, the units experienced renewed uplift followed by subsidence to present-day elevations. If this paraglacial region is not yet in isostatic equilibrium and still requires further forebulge subsidence, this could explain the present-day altitude and age discrepancies associated with these relict marine deposits. |
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