Coastal Basement Geology of the Southeastern U.S. Project

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This project, centered on South Carolina and Georgia, extends coastal-basement geologic mapping and related research south of a geologic map in production for pre-Cretaceous rocks beneath the Atlantic Coastal Plain from Virginia to southern New Jersey, and adds detail to a national-scale basement domain map constructed to delineate mineral-resource prospectivity. Plans include efforts to build on the potential as a prototype for mapping pre-Cretaceous basement beneath the entire Atlantic and Gulf Coastal Plains and possibilities for future cooperation with interested State geological surveys and university partners.

A map showing the Coastal Basement Geology of the Southeastern U.S. Project

The region of coastal-plain basement geologic mapping and related research is centered on South Carolina and Georgia.

Map created courtesy of Google Earth (USGS 2018, Public domain.) 

Pre-Cretaceous basement terranes and rift basins concealed beneath sediments of the Southeastern U.S. Coastal Plain are a large fraction of the continental mass and are among the last frontiers of regional geologic research in the U.S. These terranes include (listed southeastward) subsurface extensions of the Carolina terrane, Kiokee belt and Belair belt; Neoproterozoic volcanic arc terranes beneath the Savannah River Site; lesser known, concealed parts of the Carolina superterrane; enigmatic rocks of the Charleston terrane; the >100 km-wide Suwannee suture zone; and accreted Gondwanan (West African) crust of the Suwannee terrane. The largest Mesozoic rift basin in eastern North America (South Georgia basin) and smaller rift basins are also concealed beneath coastal-plain sediments in this area. Subsurface geologic mapping conducted using petrographic, geochronologic, and geochemical analyses of drill cores and cuttings in the context of regional aeromagnetic and gravity data can lead to breakthroughs in our understanding of their resource potential and geologic framework.

 

Applications are related to essential resources – mineral, energy, and groundwater resources, including critical minerals, metals, industrial minerals, and natural aggregate at accessible depths. The top of pre-Cretaceous bedrock is the base of coastal-plain aquifer systems. The pre-Cretaceous rocks are much less permeable than overlying sediments composing the coastal-plain aquifers and are separated from them by a mantle of low-permeability saprolite. The boundary between pre-Cretaceous rocks and overlying coastal-plain sedimentary units is thus an important boundary for the flow of ground water in the coastal-plain aquifers.  Radiogenic, heat-producing granites insulated by coastal-plain sediments are potential sources of low-temperature geothermal energy. Buried Mesozoic rift basins are potential energy resources for natural gas. The coastal basement geology is also relevant for applications such as deep supercritical CO2 storage in the South Georgia rift basin or underground storage of refrigerated natural gas in granite plutons. The basement geologic framework is also pertinent for infrastructure and engineering applications such as tunneling or seismic-hazard assessment and design of critical facilities such as nuclear power stations.

 

Initial work includes collaboration with scientists in the USGS Geology, Geophysics, and Geochemistry Science Center and the Earthquake Hazards Program in planning new high-resolution airborne geophysical surveys in the Charleston, SC, region. Aeromagnetic data will be used to image and characterize basement rocks and structures concealed beneath Coastal Plain sediments, including young faults that may be related to the magnitude ~7 Charleston earthquake of 1886. They will be combined with radiometric data (potassium, uranium, thorium) to image rare-earth-element-bearing heavy-mineral sands and characterize earth-surface materials as a foundation for future 3D geologic mapping.