DECIPHERING THE ROLE OF ANTECEDENT GEOLOGY IN SHAPING MODERN COASTAL EVOLUTION: EXAMPLES FROM THE DELMARVA PENINSULA
A three-dimensional model for the Delaware and Maryland Atlantic shoreface is being constructed from existing core data and nearly 700 line-kilometers of previously uninterpreted 3.5 kHz and boomer (0.2-2.2 kHz) seismic-reflection profiles and new, high-resolution Chirp (2-12 kHz) profiles collected in the extreme nearshore zone and back-barrier waterways. Preliminary results show that outside of incised valleys, Holocene sediments form only a thin veneer above Pleistocene units along much of the Delaware coast. Geostatistical analysis of historical shoreline-change data from Maryland and Delaware revealed a positive correlation between alongshore changes in the geologic framework and erosion rates. The lowest shoreline-retreat rates were found adjacent to resistant Pleistocene headlands, as expected, but similarly low rates were also observed in two areas where earlier Holocene units provide localized sediment sources. One of these regions, located on Assateague Island (Md.), may represent the first evidence of an earlier Holocene (4,500? ybp) highstand event preserved on the Delmarva Peninsula. Additional geophysical data (Chirp and ground-penetrating radar) and shallow cores will be collected in 2001 to explore the origin of this proposed earlier Holocene barrier island.