The USGS/South Carolina Sea Grant Consortium Coastal Erosion Study is centered within Long Bay, a large sediment-starved embayment bound to the north by Cape Fear and associated shoal complexes, and to the south by Santee Delta/Cape Romaine (Figure 1). South Carolina's Grand Strand, the focus of this study, is centered on a 100-km arcuate stretch of coastline within the apex of Long Bay. This region has few large inlet/fluvial systems and is characterized by a limited influx of modern sediment.
Geophysical surveys, designed to define the geologic framework within the Grand Strand region, were conducted in 1999 - 2003. The survey area extends from seaward of breaking waves (< 1 km) to ~10 km offshore, and covers approximately 700 square kilometers of the inner-shelf and lower shoreface (Figure 1). High-resolution sidescan-sonar and interferometric bathymetric sonar systems were utilized to map the surficial sediment distribution and regional bathymetry; seismic-reflection systems were used to define the underlying geologic structure. Surface grab samples, cores, video and photographs were collected throughout the region to ground-truth the geophysics.
The inner-shelf off South Carolina's Grand Strand has a low-relief, gently dipping seaward slope with depths ranging from 4 meters in the nearshore to 14 meters offshore (Figure 2).
Bathymetric highs are present in several locations throughout the survey area; shoal complexes offshore of Waites Island and Murrells Inlet, a shore-oblique oblate feature trending NE-SW offshore of Myrtle Beach, and a ridge complex north of Winyah Bay, proximal to North Inlet. In general, these bathymetric highs correspond to regions on the inner-shelf that have some ( > 1 meter) accumulation of modern sediment.
The surficial character of the seafloor is somewhat variable within the Grand Strand region Figure 3). High-resolution sidescan-sonar systems were used to record the acoustic character of the seafloor. Variations in the strength of the return signal are represented as gray-scale values within an image; areas of low-backscatter (low-reflectance) are displayed as dark tones, areas of high-backscatter (high-reflectance) are displayed as light tones. Offshore of the Grand Strand, surficial grab samples and video show areas of low-backscatter to be characterized by fine-medium sand, silt, and mud.
Conversely, areas of high-backscatter are shown to be characterized by coarse sand, shell hash, hardground (outcrop), and gravel. In general, fine-medium sand, low-backscatter areas correspond to relative bathymetric highs, while high-backscatter areas dominated by coarse-sand and outcropping older strata correspond to relative bathymetric lows or areas of constant relief.
Boomer and chirp seismic-reflection systems were utilized to map underlying geologic structure offshore of the Grand Strand (Figure 4). Interpretation of the seismic records reveals several distinct units: Cretaceous/ Tertiary continental shelf strata influenced by regional tectonics, showing a gentle dip to the SE, with folding present in the northern portion of the survey area; a regional unconformity representing the latest marine transgression, or several regressive/transgressive cycles, marking the upper boundary to the underlying Cretaceous/Tertiary strata; paleochannels associated with old river systems which incised Cretaceous/Tertiary strata at low-stands of sea level; small paleochannels lying above the regional unconformity, most likely associated with local drainage of small tidal creeks/inlets and swales; and a patchy, discontinuous acoustically transparent modern sediment layer overlying the regional unconformity (Figure 5).
Regional tectonics, changes in sea-level through time, and modern oceanographic processes have shaped the character of the inner-shelf offshore of South Carolina's Grand Strand. Geophysical and sample data reveal the inner-shelf to be comprised of a patchy, discontinuous sand sheet, overlying a regional transgressive unconformity. Sediment thickness increases to the south (Figure 6). Large accumulations of sediment generally occur near inlet systems, and in some areas have been reworked by hydrodynamic processes to form shoal and ridge complexes. The shore-oblique sand body offshore of Myrtle Beach is an exception (Figure 7). Current research is focused on defining the origin of this feature. In areas devoid of modern sediment, or with only a thin veneer of sediment, Cretaceous/Tertiary units and paleochannel fill outcrop at the seafloor, primarily in large areas to the north and in troughs between sand ridges throughout the study area. Reworking of these older units by modern oceanographic processes generates a supply of sediment to this coastal region.