UPPER PLEISTOCENE AND HOLOCENE GEOLOGY OF THE ST. HELENA ISLAND AREA, COASTAL SOUTH CAROLINA

Detailed 1:24,000 geologic mapping has defined the areal and three-dimensional distribution of upper Peistocene terraces and Holocene barrier-island systems in the St. Helena Island area, coastal South Carolina. Mapping was based on examination of surface exposures, on observation of cuttings from power-auger holes and hand-auger holes, and on geomorphic expression, elevation, radiometric dates and soil types. These factors were used to subdivide the depositional sequences, where possible, into marine, barrier-island complex, estuarine and mixed estuarine facies. Two upper Pleistocene terraces and two Holocene barrier-island systems are preserved and overlie Oligocene or Miocene deposits. The older upper Pleistocene terrace has its toe at 12 to 15 feet elevation, has dune fields as high as 24 feet, and is up to 60 feet thick. The younger upper Pleistocene terrace abuts the older terrace, has its toe at 6 to 8 feet elevation, has dunes as high as 15 feet and is up to 45 feet thick. The older Holocene system repeats depositional facies inland from the active barrier system. The younger Holocene system is the modern environment. The Holocene systems either abut, have altered or continue to alter both Pleistocene terraces to a maximum depth of 60 feet. The distribution of the Pleistocene and Holocene facies was or is controlled by sediment type and supply, elevation, and orientation to open water, inlets, rivers, prevailing winds and by the geomorphology of the pre-existing terraces. To a great extent, the geologic history of the Pleistocene terraces controls the location of the modern environment.