GEOLOGIC MAP OF THE SOUTH BOSTON 30X60-MINUTE QUADRANGLE, VIRGINIA AND NORTH CAROLINA

A 1:100,000-scale geologic map of the South Boston, VA-NC, 30X60-minute quadrangle provides geographically-referenced digital geologic information for the Piedmont along the I-85 and U.S. 58 corridors and in the Roanoke River watershed, including Kerr Reservoir and Lake Gaston. The Raleigh terrane on the eastern side of the map includes Neoproterozoic to early Paleozoic(?) polydeformed gneisses and schists equivalent to the informally named Raleigh gneiss and Macon formation farther south in NC. The Carolina terrane in the central part of the map contains Neoproterozoic metavolcanic and metasedimentary rocks at greenschist facies in the Carolina slate belt and at amphibolite facies west of the slate belt. The slate-belt structure mapped across the area, although locally complicated, is generally an anticlinorium of Hyco Formation units flanked on the west and east by synclinoria, which are cored by the overlying Aaron and Virgilina Formations. Amphibolite-facies gneisses in the western part of the Carolina terrane include the Neoproterozoic South Boston gneiss and Country Line complex (informal names), and extend north-northeast across the map. Farther west, the Milton terrane contains polydeformed metavolcanic and metasedimentary gneisses as young as Ordovician(?). Crosscutting relations and fabrics in mafic to felsic plutonic rocks constrain the timing of Neoproterozoic to late Paleozoic deformations across the Piedmont. A 5-9 kilometer-wide band of late Paleozoic mylonite zones (Nutbush Creek and Lake Gordon) and syntectonic granite (Buggs Island pluton) separates the Raleigh and Carolina terranes. Another 5-8 kilometer-wide band of late Paleozoic tectonism (which includes the Hyco shear zone, the Clover shear zone, syntectonic granitic sheets, and amphibolite-facies gneisses in the western part of the Carolina terrane) was the locus for early Mesozoic normal faults. The early Mesozoic Scottsburg, Randolph, and Roanoke Creek basins along these faults are interpreted as erosional keel remnants of a formerly much larger rift basin. Abrupt changes in fluvial terrace levels on hills and slopes near the Roanoke, Dan, and Staunton Rivers suggest possible late Cenozoic tectonism.