GROUND PENETRATING RADAR INVESTIGATION OF AN EMERGENT MID-PLEISTOCENE ESTUARINE SHORELINE COMPLEX ON MARYLAND'S EASTERN SHORE, CHESAPEAKE BAY, MARYLAND, USA

The eastern margin of Chesapeake Bay in Dorchester County, Maryland, is a broad (~ 40 km) platform, bounded on the north and southeast by the Choptank and Nanticoke Rivers, respectively. The platform slopes gently from MSL at the bay edge to ~ +10 m MSL east of Cambridge, Maryland. Owens and Denny (1979, 1986) mapped the surficial geology of the area as the Pleistocene age, bay bottom sediments of the Kent Island Formation, exposed in the upper reaches of the platform, and as low, 1 m-relief, bay margin-subparallel sand ridges immediately adjacent to the bay. The modern salt marsh overlies the Kent Island Formation in much of the lower reaches of the platform, landward of the bay-edge sand ridges. In several locations on the platform are deposits of the Holocene to Pleistocene age, Parsonsburg Sand. This unit is suspected of having multiple origins, from estuarine to fluvial to eolian.

In this investigation we present ground penetrating radar and lithologic evidence to delineate the linear Parsonsburg Sand deposits in the northeastern part of the county as a stranded estuarine shoreline complex, composed of at least three highstand units emplaced during climate-driven sea level cycles during the mid Pleistocene. The Parsonsburg Sand deposit in this location forms a nearly east-west trending ridge with 5 meters of relief, rising from its contact with the Kent Island Formation at + 10 m MSL to ~ + 15 m MSL along its crest. The thin (5 meters or less) highstand deposits identified herein cut into the underlying Miocene Pensauken Formation and overtop one another, attaining modern elevations of + 14 m, + 11 m, and + 13 m MSL, in succession. Shoreface and nearshore facies and structures of these highstand deposits are preserved in the Parsonsburg Sand, and are traced into their equivalent bay bottom facies in the adjacent Kent Island Formation. Correlation of these deposits with other regional and global coastal units of MIS 11 age is likely.