INCISED VALLEY FILL AND HIGHSTAND DEPOSITS IN THE INDIAN RIVER WATERSHED, DELAWARE

The coastal geology and geomorphology of Indian River Bay, Delaware has been shaped by repeated late Pleistocene sea level fluctuations that are recorded in the sediments. Approximately 3.2 km of ground-penetrating radar, 1.6 km of electrical resistivity, and 65 km of marine chirp seismic data were collected to provide a detailed stratigraphic framework of the area in support of the development of a hydrologic model of the watershed. In conjunction with cores collected previously by other geologists, the geophysical transects were used to construct a stratigraphic cross-section from White Neck to the modern shoreline (about 7 km). Seven major stratigraphic units are identified in the cross-section. The basal unit is the Pliocene Beaverdam Formation which underlies the entire area and subcrops shallowly in the western part of the study area. Pleistocene sediments overlie the Beaverdam Formation and in some cases fill primary incised valleys that existed prior to the last glacial maximum (Stage 2). Holocene paludal and estuarine deposits now fill the lowstand valley of Indian River, although upper Pleistocene sediments are exposed along the shallow flanks by lateral ravinement. Several sets of middle to late Pleistocene shorelines form the sandy higher ground in the eastern third of the watershed. The Holts Landing field site on White Neck, along the south bank of Indian River Bay, contains shorelines that are tentatively identified as pre-Stage 5 (older than 125 ka). The orientation of the inferred shorelines at Holts Landing, when correlated with potential shorelines on Long Neck, on the north bank, suggests that the mouth of Delaware Bay had a much different configuration than at present. Field sites on Cedar Neck and at Fresh Pond are tentatively interpreted as Stage 5 shorelines (125-75 ka), although this will be tested with OSL dating. The final unit recognized consists of possible Stage 3 (60-30 ka) shorelines which lie closest to the modern Atlantic coast. The proposed stratigraphic framework not only provides more details on the geology and geomorphology of Indian River Bay, but also makes advances in understanding sea level history and shoreline geomorphology on the Delmarva coast.