DECIPHERING THE ROLE OF ANTECEDENT GEOLOGY IN SHAPING MODERN COASTAL EVOLUTION: EXAMPLES FROM THE DELMARVA PENINSULA

A wide array of current topics in coastal geology require, at a fundamental level, a detailed model of the nearshore geologic framework. Among these are studies of sediment transport and sequence stratigraphy, as well as models of shoreline change occurring over societally relevant timescales. Shoreline-change studies rarely incorporate data related to framework geology, even though these data provide essential context for interpreting erosion rates in sediment-starved coastal areas. The Delmarva coastal zone, last studied intensively in 1970's and 1980's, is the site of a new wave of data collection and analysis. This research effort provides a prime opportunity to refine models of coastal evolution during transgression, and to develop new models to quantify the role of antecedent geology in controlling shoreline retreat and inner shelf geomorphology.

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.