FRAMEWORK CONTROLS ON DISTRIBUTION OF SURFACE SEDIMENTS AND INCISED CHANNELS, NE SOUTH CAROLINA INNER SHELF

Closely gridded high-resolution seismic profiles and a continuous side-scan sonar mosaic from the inner shelf off the Grand Strand of South Carolina allow detailed mapping of the seafloor and interpretation of the structural and stratigraphic controls that influence the distribution and thickness of Holocene sediments. The inner shelf of this region is largely sediment starved owing to regional structural influences that have prevented post-Tertiary subsidence and have diverted fluvial sediment supply by causing rivers to flow inland parallel to the coast. From northeast to southwest the seafloor is characterized by outcrops of broadly folded strata on the western flank of the Cape Fear Arch that pass into southerly dipping strata that are truncated at the seafloor by repeated marine erosion. The dipping strata are incised by channels that are progressively larger to the southwest where they display characteristics of integrated drainage networks. The nested fills of these channels record a complex stratigraphic history of repeated periods of reoccupation and abandonment. The ages of the channels are unknown, but some probably are older than the Quaternary. The dipping beds and margins of the channels form outcropping hardgrounds with relief on the seafloor that results from differential erosion.

Surficial sediments are largely absent or are patchy and thin northeast of the inferred offshore contact between Tertiary and Cretaceous strata. The thickest accumulations of mobile sediments overlie a well-defined ravinement surface and form sand ridges that typically are asymmetrical and trend obliquely to the shoreline. In shallow water the ridges are attached to the beach and shoreface, but in deeper water (>10 m) they are detached from the beach and shoreface. Some of the thick accumulations of coarse clastic and biogenic sediments (sand, gravel, and shell) overlie or are on the margins of the deepest channels, or coincide with the inferred retreat path of the tidal delta at Murrels Inlet. However, in other areas, moderately thick accumulations of surficial sediments do not coincide with incised channels. Both the areas without surficial sediments and areas of prominent sand ridges provide a basis for interpreting the long-term, time-averaged hydrodynamic processes that influence the morphology and stability of the shoreface and inner shelf.