GEOLOGIC MAP OF THE SOUTHERN 1/2 OF RED HILL AND EDGEFIELD 7.5-MINUTE QUADRANGLES, EDGEFIELD COUNTY, SOUTH CAROLINA

The South Carolina Geological Survey has produced new 1:24,000-scale geologic maps of the southern ½ of Red Hill and Edgefield Quadrangles, Edgefield County, South Carolina. Mapping was cooperatively funded through the STATEMAP component of the USGS National Cooperative Geologic Mapping Program.

The geology of Edgefield and Red Hill quadrangles consists of four distinct lithotectonic elements, which include: (1) Neoproterozoic­–Cambrian Carolina terrane (Ct), (2) Late Paleozoic Modoc shear Zone (MSZ), (3) Pennsylvanian­–Permian Edgefield granite (PPeg), and (4) Eocene–Holocene sediments of the Atlantic Coastal Plain. The Ct consists of greenschist facies intermediate to felsic volcanic rocks (Persimmon Fork Formation), and siliciclastic sedimentary and mafic volcanic rocks (Emory and Richtex Formations). A tonalitic stock of unknown age intrudes the Persimmon Fork-Emory contact. Hornfelsic textures in the surrounding rocks indicate the stock is younger than the Ct. The stock resembles the Devonian Clouds Creek igneous suite mapped to the NE.

The MSZ is a component of the Eastern Piedmont fault system, forms the southern boundary of the Ct, and consists of a deformed, garnet-zone amphibolite facies paragneiss unit containing interlayered garnet-bearing quartzite, mica-garnet schist, and biotite-garnet quartzofeldspathic gneiss. The paragneiss also contains sheets of mylonitic granitic orthogneiss. The MSZ strikes N70°E and dips moderately NW. Where shear fabrics are recognized, kinematic indicators demonstrate top-to-the-northeast dextral motion. A mineral lineation plunges gently NE-SW. A distinct, locally sillimanite-bearing, biotite-garnet schist separates the paragneiss unit from the PPeg, which intrudes the southernmost boundary of the MSZ. The schist may represent the northern boundary of a separate, enigmatic Savannah River terrane.

Structural analysis of ductile and brittle fabric elements defines potentially four distinct phases of deformation, including three phases of folding, and dextral transpression related to Paleozoic orogenesis. Whole-rock, major- and trace-element analysis of Ct metavolcanic rocks supports previous work that the terrane is remnant of a subduction-related volcanic arc that likely developed on a substrate of an older orogenic volcanic arc.