INVESTIGATING PRE-CRETACEOUS BASEMENT ROCKS BENEATH THE SOUTHEASTERN U.S. COASTAL PLAIN

A USGS project with state and university partners is beginning to extend coastal-plain basement geologic mapping and related research from southern NJ to VA (geologic map in production), southward to an area centered on SC and GA. This project serves as a model for mapping pre-Cretaceous basement rocks beneath other parts of the Atlantic Coastal Plain.

Pre-Cretaceous terranes and rift basins concealed beneath sediments of the southeastern U.S. Coastal Plain occupy a large part of the continental crust and are among the last frontiers of regional geologic research in the U.S. These terranes include (listed southeastward) subsurface extensions of the peri-Gondwanan Carolina superterrane (including rocks of the Kiokee and Belair belts, Neoproterozoic igneous complexes beneath the Savannah River Site, and sparsely drilled areas of the superterrane near the coast), enigmatic rocks of the Charleston terrane, the >100 km-wide Suwannee suture zone, and accreted Gondwanan crust of the Suwannee terrane. The early Mesozoic South Georgia basin and smaller rift basins are also concealed beneath coastal-plain sediments in this area. Subsurface geologic mapping will be conducted using petrographic, geochemical, and geochronologic (U-Pb zircon, ⁴⁰Ar/³⁹Ar) analyses of drill cores and cuttings in the context of regional aeromagnetic, gravity, and seismic data. Geophysical research will include the collection of new, high-resolution (400-m flight spacing) aeromagnetic and radiometric (K, U, Th) data that will help delineate geologic structures over the Charleston, SC, region.

Potential applications of the resulting maps include identification of subsurface mineral, energy, and groundwater resources at accessible depths. The top of pre-Cretaceous rocks, as the base of the coastal-plain aquifer system, is an important boundary for groundwater flow. Radiogenic, heat-producing granites insulated by coastal-plain sediments are potential sources of low-temperature geothermal energy. Buried Mesozoic rift basins show limited potential for sources of natural gas or as repositories for deep CO₂ storage. Mapping of the basement geology is pertinent for infrastructure and engineering applications, and seismic-hazard assessment which may include young faults related to the 1886 Mw~7 Charleston earthquake.