Investigation of the Geologic Framework of the Grand Strand Coast in South Carolina

The Grand Strand consists of a continuous 100 km arcuate shoreline extending from Winyah Bay to Little River, South Carolina. This coastal segment of South Carolina is dominated by mainland beaches, which are attached to eroding Pleistocene headlands. Pleistocene and Holocene age deposits generally form a relatively thin veneer of unconsolidated sediments that overlie early Tertiary or late Cretaceous sedimentary units. These older indurated to semi-consolidated deposits are exposed as "hardgrounds" in the immediate shoreface zone. Wave erosion of Quaternary deposits and the underlying older strata provides a varied sand source for this sediment-starved coastal segment. Therefore, knowledge of the geologic framework of the lower coastal plain and the inner continental shelf is extremely important in understanding long and short-term coastal changes that affect this region. Sixteen borings drilled to a maximum depth of sixty feet, as well as 150 additional data points derived from geophysical well logs and existing core data, have been utilized to characterize the near-surface stratigraphy and define the Holocene unconformity beneath the Grand Strand. Pleistocene and Holocene sediments analyzed from these borings suggest deposition in beach ridge/spit, tidal inlet, back barrier, nearshore marine and fluvial/deltaic paleoenvironments. To the south, these younger sediments overlie an erosional surface incised into fine-grained shelfal sand, silt, and clay strata of the Paleocene Black Mingo Group, whereas to the north, these deposits unconformably overlie sandy mudstones and siltstones of the late Cretaceous Peedee Formation. Coast-parallel and perpendicular seismic surveys confirm the presence of buried fluvial channels that were incised into these older stratigraphic units by ancestral Piedmont rivers during Pleistocene sea level low stands. The identification of nearshore morphological features, coupled with the sedimentological characteristics and thickness of modern shelfal deposits, are important criteria in defining both the sediment budget and sediment transport processes affecting this coastal segment. Through further integration of land-based data and offshore geophysical surveys, a comprehensive model of the geologic framework is being developed to better understand both the geologic evolution and the modern day processes that influence the Grand Strand coastline.