MODERN AVAILABILITY AND HISTORIC SUPPLY OF SEDIMENT ALONG THE INNER-CONTINENTAL SHELF OFFSHORE NORTHEAST SOUTH CAROLINA

One goal of the South Carolina Coastal Erosion Study is to define the geologic framework along the inner-continental shelf offshore northeast South Carolina, in order to evaluate the role that pre-Holocene framework geology plays in the continuing evolution of the area. Data from a three-tiered, high-resolution geophysical survey (providing a dense grid of coincident CHIRP and BOOMER seismic, side-scan sonar and interferrometric sonar bathymetry) were used to achieve this goal. The three-dimensional results of this approach have allowed for detailed interpretation of the area’s surficial and shallow subsurface geology. Two striking observations emerge from this framework: 1) the current absence of any significant sediment supply or storage on the inner-shelf, and 2) the southwestward increase in size and complexity of paleochannels incised into the continental shelf strata of Paleocene and late Cretaceous age. When considered in a regional context, established by previously published Coastal Plain studies, these observations outline a fundamental shift in sediment supply to the area during the late Pleistocene. Progradational barrier strandplains were deposited across the Coastal Plain, between at least the late Pliocene and late Pleistocene, when sediment supply was high. During this time, the primary source of sediment was via fluvial transport associated with draining of the Piedmont by the Great and Little Pee Dee River system. This river system gradually migrated to the southern terminus of the area, where the majority of the sediment that it presently delivers is deposited in the Winyah Bay estuary. Geomorphologic processes associated with eustatic sea level fluctuations and relative uplift of the Mid-Carolina Platform High appear to be the mechanisms responsible for fluvial system migration. No major rivers have intersected the area since the late Pleistocene, and the primary source of sediment appears to be through “cannibalization” of Pleistocene mainland beaches, and excavation of the inner-continental shelf. Further work is underway, documenting the magnitudes and patterns of erosion generated as a result of complex interaction between varied lithologies within this framework and modern hydrodynamic processes.