REFINED CHRONOSTRATIGRAPHIC MAPPING PROVIDES NEW INSIGHTS INTO THE LATE-PLEISTOCENE RELATIVE SEA-LEVEL HISTORY OF THE DELMARVA PENINSULA

Contradictory interpretations of Late Pleistocene (120–40 ka) sedimentary deposits along the US Mid-Atlantic Coast have hindered the development of a reliable and reproducible regional sea-level curve for the last glacial cycle. We hypothesize this uncertainty arises in part from misinterpretation of the depositional history of several key geologic units emplaced during periods when sea level reached between 0.5 and 8.0 m higher than present. This study presents a depositional model for the southern Delmarva Peninsula, with specific emphasis on transgressive, highstand, and regressive deposits preserved above modern mean sea level (MSL) along the Virginia Eastern Shore. It relies on new and compiled morphological (LiDAR), geophysical (ground-penetrating radar), stratigraphic (sediment cores), geochemical (carbon content), and geochronological (optically stimulated luminescence [OSL], amino acid racemization [AAR]) data. Results suggest that preserved Late Pleistocene coastal deposits capture four separate periods of sea-level highstand. Specifically, we find that the Wachapreague Formation (Fm.) (the youngest of the Pleistocene units) was constructed during two sequential highstands: its western component (2–4 m MSL; ca. 70 ka[OSL, AAR]) is characterized by seaward-prograding beach and foredune ridges, whereas the subaerial eastern ≤ 600 m of the Wachapreague Fm. (the ‘Upshur Neck Member’; 2.5–0.5 m MSL) is characterized by preserved washover, dune, and channel-fill deposits. We interpret the Upshur Neck Member as a transgressive barrier-island system partially welded to the regressive western Wachapreague at ca. 50 ka (OSL). Further, we reinterpret the older Butler's Bluff Member (the uppermost wave-built facies of which are located 8–10 m MSL) and Joynes Neck Sand, both located immediately west of the Wachapreague Fm., as a remnant highstand southward-prograding spit and coastal lag deposit, respectively. These both formed at ca. 80 ka, in part from sediments eroded from the up-dip Accomack Member (10–14 m MSL), which dates to ca. 120 ka or older. This new chronostratigraphic interpretation refines the depositional model for the southern Delmarva and improves reconstructions of sea-level change along the US Mid-Atlantic during the last glacial cycle.