GEOLOGIC MAPPING OF THE NEARSHORE, NORTHERN SOUTH CAROLINA

In 1999, the US Geological Survey, in cooperation with the South Carolina Sea Grant Consortium, began a program to produce geologic maps of the inner shelf and shoreface off northern South Carolina using high-resolution sidescan-sonar, swath bathymetry and subbottom profiling techniques. Goals of the investigation are to determine the regional-scale sand-resource availability needed for planned beach-nourishment programs, to investigate the role that inner-shelf morphology and shallow stratigraphy play in the evolution of this coastal region, and to provide base maps for use in proposed biologic habitat studies. The study area extends from the North Carolina border south to the area off Winyah Bay, SC and from the shoreline to 10 km offshore. Preliminary analysis of these data depict an area of sea floor dominated by outcropping strata of Cretaceous and Tertiary coastal-plain strata and younger channel-fill (fluvial) strata. Holocene sediment cover is relatively thin or absent over most of the study area with the exception of a large sand body (about 36 million cubic meters) located off northern Myrtle Beach. Analysis of the sediment distribution patterns and bedform orientations suggest that (a) modern sediment is likely derived from erosion of the inner shelf (b) the net sediment transport direction is toward the southwest (with local variations), and (c) the cross-shelf component of sediment flux affects coastal evolution.