SHOREFACE ARCHITECTURE IN A SEDIMENT STARVED SYSTEM: UPDATE FROM INTERTIDAL VIBRACORES AND SHORE PERPENDICULAR CHIRP DATA

A series of geophysical surveys conducted along a 90 km section of the northern South Carolina coast have sought to define the architecture of the shoreface and to quantify the geometry and volume of sediment in the modern active beach system along this sediment-starved region. This study was completed as part of the USGS-South Carolina Sea Grant Coastal Erosion Study. Recently, shore perpendicular high-resolution CHIRP profiles were collected from the nearshore bar to the inner shelf, to link with onshore boreholes and ground penetrating radar surveys along the study area. Eighteen vibracores, ranging from 0.73-6.70 m long, were obtained from the lower intertidal zone along the coast to better resolve the base of the active sediment lens and directly groundtruth the CHIRP profiles. These data have substantially improved and updated our understanding of shoreface architecture.

Within each core, sediment characteristics were classified based on facies and the base of the active modern sediment lens was interpreted. An extensive time series of beach profiles from the region (wading depth-1986 to present; long profiles 1993-present) supported the interpretation and defined the base of recent beach nourishment projects. Stratigraphic contacts observed in intertidal cores and onshore borings were used to project reflector geometries observed in the shore perpendicular CHIRP data across the beach.

The active beach exists as a very thin (0-5 meters) and locally patchy veneer eroding into a range of Quaternary, Tertiary, and Cretaceous age strata. In many areas, older underlying strata exist within the shoreface, and their mechanical/geotechnical characteristics result in various expressions of relief. Outcrops of heavily indurated deposits form high relief scarps and ledges observed in beach profiles. Unconsolidated Quaternary sediments, as well as nearshore and shallow marine deposits, often exhibit subtle departures from otherwise concave upward geometries of long beach profiles.

The active beach lens geometry (thickness, shape, continuity) and relationship to the inner shelf varies along the coast and correlates well with local stratigraphy and geologic evolution.