In July, U.S. Geological Survey (USGS) scientists Wayne Baldwin (St. Petersburg, FL) and Ilya Buynevich (Woods Hole, MA) collected more than 10 km of ground-penetrating-radar (GPR) profiles as part of the South Carolina Coastal Erosion Study.
The continuous, high-resolution subsurface records collected with GPR help scientists to visualize the geometry, thickness, and continuity of various sedimentary layers.
The recent GPR recordscollected in Myrtle Beach and North Myrtle Beach along the north coast of South Carolinareveal remnants of ancient landforms created when barrier islands migrated landward with rising sea level and became welded to the mainland.
This survey complements mapping of the inner continental shelf offshore northern South Carolina by scientists from the USGS' Woods Hole Field Center, led by William Schwab, in collaboration with scientists from the USGS' St. Petersburg Science Center (St. Petersburg), Coastal Carolina University, and the Scripps Institution of Oceanography.
Accurate mapping of the inner continental shelf and adjacent areas of the coast is critical to understanding how geologic frameworkthe distribution of rock and sedimentaffects large-scale coastal behavior.
The GPR data collected in July will be used to link the shallow onshore geology of the area with nearshore seismic records collected last May.
The attenuation of the GPR system's electromagnetic radar signal by saltwater, though one of the major limitations of the technique, was not an issue in the recent surveys, which were conducted on the upper part of the beach and adjacent areas. The penetration was commonly 2 to 4 m, in some places more than 6 m.
Subsurface records from Myrtle Beach show a prominent shallow reflectormost likely the top of the Pleistocene beachrock, or sand that has been cemented by calcium carbonate, possibly during exposure to air and ground water during times of lower sea level.
Above the shallow reflector are Holocene beach and dune sedimentary deposits and recent artificial fill. Along North Myrtle Beach, numerous buried inlets and ephemeral runoff channels, or "swashes," were identified, some filled with more than 5 m of sand.
Beneath the roads and parking lots, shore-normal transects revealed several buried remnants of beach ridges and intervening wetland deposits.
The records are being postprocessed and incorporated into the existing geographic-information-system (GIS) data base. In the future, limited coring efforts will be required to ground-truth the key reflections and improve depth calculations on the geophysical images.
The results of this study will be presented at the special theme session on paleo-inlets at the Northeastern-Southeastern Sections Joint Meeting of the Geological Society of America, which will take place near Washington, D.C., in March 2004.
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