SUBSURFACE STRATIGRAPHY OF THE GRAND STRAND COAST IN SOUTH CAROLINA

The Grand Strand of South Carolina consists of an arcuate coastline extending 90 kilometers from Debordieu Island to the North Carolina border. This northern coastal segment is dominated by mainland beaches that are in places attached to eroding Pleistocene headlands. These early to late Pleistocene age sediments are generally 5 to 10 meters thick and underlie a thin veneer of present day beach/dune and nearshore marine deposits. However, sediment thicknesses in excess of 25 meters have been recorded resulting from the infilling of fluvial paleovalleys. The lack of a significant modern day fluvial drainage system in this area limits the amount of "new" sediments transported and deposited on the coast. Hence these sediment-starved beaches must rely on sediment derived from the erosion of older sedimentary deposits either exposed along the coast or as hardgrounds exposed in the immediate shoreface zone. Eight borings, using rotasonic drilling techniques, have been utilized to characterize the stratigraphy of the nearshore sedimentary units that underlie the Grand Strand coastline. In the northern portion of the study area, the Quaternary deposits are underlain by Cretaceous age sediments of the Peedee Formation. These sediments consist of variably indurated clayey sands of marine origin. Cores collected to the south penetrated Paleocene Black Mingo Group lithologies. These units consist of less consolidated sandstones, siltstones and mudstones that were deposited as beach/dune, nearshore marine, and back barrier lagoonal facies. It is believed that the subsurface stratigraphy has a profound effect on the erosional/accretional trends that are currently being observed along this stretch of coastline.