The Piedmont of South Carolina and Georgia is a complex mosaic of exotic terranes of uncertain provenance. Farther south and east, these terranes form the basement beneath several kilometers of Cretaceous and Cenozoic sedimentary rocks, commonly referred to as the Atlantic Coastal Plain. The distribution and geologic history of this hidden crystalline basement can be inferred only on the basis of limited exposures at the margins of the Coastal Plain onlap, aeromagnetic lineaments that define basement trends in the subsurface, and core data from wells that penetrate basement.

During the past 40 years, basement cores aggregating more than 6 miles (10,000 m) have been recovered from 57 deep wells at the Department of Energy's Savannah River Site. These cores provide the only known samples of basement terranes that lie southeast of the Fall Line in central South Carolina. Cores from the 57 deep wells, along with structural trends defined by aeromagnetic lineaments, allow us to define four distinct units within the basement beneath the Coastal Plain: (1) the Crackerneck Metavolcanic Complex (greenstones and felsic tuffs, all metamorphosed under greenschist- facies conditions), (2) the Deep Rock Metaigneous Complex (mafic to felsic volcanic and plutonic rocks metamorphosed under lower amphibolite–facies conditions), (3) the Pen Branch Metaigneous Complex (amphibolites, garnet amphibolites, garnet- biotite schists, and gneiss), and (4) the Triassic Dunbarton Basin Group, a sedimentary unit that fills a northeast-trending graben beneath younger sedimentary rocks of the Atlantic Coastal Plain.

All of the metaplutonic and metavolcanic rocks have calc-alkaline fractionation trends, consistent with formation in subduction-related arc terranes at convergent margins. Zircon U-Pb crystallization ages of ca. 626 Ma to 619 Ma, however, show that the Deep Rock and Pen Branch complexes do not correlate with the younger Carolina terrane (570–535 Ma) or Suwannee terrane (ca. 550 Ma). The Deep Rock and Pen Branch Metaigneous Complexes may be a continuation of Proterozoic basement that forms the older infrastructure of the Carolina arc. The contact between the Crackerneck Metavolcanic Complex (= Persimmon Fork Formation?) and the Deep Rock and Pen Branch Metaigneous Complexes thus may be equivalent to the angular unconformity between the Uwharrie Formation and the Virgilina sequence. On the basis of their compositions and ages, we tentatively correlate these rocks with the Hyco Formation in southern Virginia and central North Carolina. The Hyco Formation constitutes the infrastructure of the Carolina terrane in Virginia and North Carolina, where it was affected by the ca. 600 Ma “Virgilina” orogeny. The rocks of the Deep Rock and Pen Branch Metaigneous Complexes may have formed the arc infrastructure of the Carolina Slate belt in South Carolina, detached by later tectonic events, or may have formed the Late Proterozoic arc infrastructure at another location in the arc that has been moved into its current location by transcurrent motions. Limited age and isotopic data suggest that none of these rocks correlate with the Suwannee terrane of North Florida and southern Georgia.

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