SEISMIC STRATIGRAPHY AND COASTAL DRAINAGE PATTERNS IN THE QUATERNARY SECTION OF THE SOUTHERN DELMARVA PENINSULA, VIRGINIA, USA

Seismic-stratigraphic analysis of the coastal zone and inner shelf of Virginia's southern Delmarva Peninsula has revealed three geochronologically significant surfaces of post-Tertiary age that impose a relative chronostratigraphic framework on Quaternary marine transgressive and regressive events. Characteristics of these surfaces indicate that two are sequence boundaries, and one is a ravinement surface. Lying at depths of 18-70 m (msl datum), the LP(b) surface (a late Pleistocene basal unconformity) represents the sequence boundary separating the Tertiary Chesapeake Group from the overlying late Pleistocene Nassawadox Formation. High relief (approximately 50 m) on the LP(b), surface is associated with large fluvial channels. Higher in the stratigraphic section, the LP(r) surface is found at depths of 6-28 m, and corresponds to a late Pleistocene transgressive, or ravinement surface. The surface dips southeastward with a regional dip of 0.04-degrees and has local relief of less than 2 m. The LP(r) surface may represent a ravinement which extended to the west side of the Chesapeake Bay prior to the development of the Nassawadox barrier spit. However, the LP(r) surface may steepen between the axis of the southern Delmarva Peninsula and the Holocene lagoon to form a shoreface attached to one of several known late Pleistocene shorelines. Lying at depths of 0-20 m, the H(b) surface is a basal unconformity that marks the Holocene sequence boundary. It deepens seaward, with maximum local relief of about 15 m, and has a topographic expression very similar to the present-day lagoonal drainage pattern. Maximum thicknesses of Holocene and Pleistocene sediments (12 and 70 m, respectively) are found above channels on the H(b) and LP(b) surfaces. The Pleistocene channels are large and limited in number and represent high-order channels of a drainage system that drained the Piedmont and Coastal Plain. The greater density of low-order stream channels on the H(b) surface suggests a relationship to much smaller drainage basins that were confined to the seaward part of the Coastal Plain east of the Delmarva Peninsula. These late Wisconsinan smaller H(b) channels do not re-occupy the former drain paths of the much larger high-order LP(b) channels.