EFFECTS OF LOCAL AND REGIONAL ANTECEDENT GEOLOGY ON THE MODERN INNER CONTINENTAL SHELF: SOUTHERN LONG BAY, SOUTH CAROLINA

Swath bathymetry, side-scan sonar and high-resolution seismic reflection, groundtruthed with grab samples, vibracores and seafloor video, have been used to map the surficial and sub-bottom geology of the inner continental shelf of southern Long Bay, SC (i.e. between Murrells and North Inlets). These data are analyzed to reveal the integral role that antecedent geology has played in the area’s geomorphologic evolution.

The study area is situated on the southwest flank of the Cape Fear Arch, a structural high on the Carolina Platform. Uplift of the arch is thought to have prevented post-Tertiary subsidence, and caused the southward migration of fluvial systems towards Winyah Bay. Extensive paleo-fluvial incision and truncation of Tertiary strata at the seafloor are the combined results of structural influence and fluctuation of sea level. Despite a common lack of fluvial sediment supply, southern Long Bay contrasts the bay’s northern reaches, having greater surficial sediment accumulation. A likely cause of this discrepancy is the observed net southward littoral drift within the bay. Depositional centers associated with Murrells and North inlets, to the northeast and southwest, contain thick deposits of sediment overlying a well-defined regional ravinement surface. These deposits have been reworked by modern hydrodynamics into detached northeastward shore oblique trending ridges. Underlying paleo-fluvial strata tend to outcrop in swales separating the ridges. The central portion of the study area is characterized by a series of moderately developed shoreface attached ridges, extending ~7 to 9 m water depth. Between ~9 to 11 m, ridges become less developed, commonly interrupted by outcrops of lithified strata, passing into a featureless zone of non-deposition. Poor development of ridges in this transitional zone suggests that sediment supply and hydrodynamics are limiting factors in ridge maintenance at these depths. Within the non-depositional zone, a thin veneer of sediment overlies dipping Tertiary and paleo-fluvial strata. A prevalence of ripples (l ~50 cm) is indicative of wave influence. Seaward of this zone, in water depths greater than 10 m, another series of moderately developed detached ridges exists. These bedforms are inferred to be shoreface ridges stranded after transgression.