GEOLOGIC FACIES MAP OF THE SOUTH CAROLINA COAST: WHERE DETAILED GEOLOGIC MAPPING AND DATABASES MEET

In South Carolina, detailed geologic mapping at 1:24,000-scale is the basis for compiling regional scale maps. This new coastal compilation was produced using detailed geologic maps of 76 7.5-minute quadrangles and represents more than 30 years of mapping effort. Map data were collected from surface exposures, subsurface borehole drilling and lithologic logging. Facies assignments were determined from lithologic interpretations and include estuarine (mud, fine sand, and possible shell material), barrier island (fine-medium sand and shell material with possible cross- or planar-bedding), and fluvial (fine to very coarse sand with massive or graded bedding). In the past these units were referred to as formations, morphostratigraphic units or terrace-formations- all now classified as alloformations.

Previous work by Earl Sloan, C.W. Cooke, D.J. Colquhoun, J.R. DuBar were refined by R.E. Weems and W.R. Doar, III during their extensive mapping campaign. One mapping goal was to provide reliable geologic maps for industry, researchers, land-use planners, the general public and land-use historians. For example, estuarine deposits commonly have low relief and drain poorly making them suitable for impounding water. A local land-use historian used our new regional compilation to identify areas likely used for impounding water at historic rice plantations.

To generate this compilation, detailed geological mapping data was assembled using GIS and the new USGS database model “Geologic Map Schema” (GeMS). GeMS takes GIS map data, including all information on standard geologic maps – geologic units, descriptions, ages, field observations, etc. – and relates it back to geospatial data. Then using GIS, the geologic units were queried, grouped, and colored by age and depositional environment. The maps were compiled in a short period of time and the output is this facies map. This compilation demonstrates the power of robust map databases populated with detailed geologic data and GIS.