Geologic Investigations Associated with Appalachian Ultradeep Core Hole (ADCOH) Site Study

J. L. Hopson, R. D. Hatcher, A. L. Stieve, M. Hartford

Geologic investigations related to the ADCOH site study are concentrated in several areas: (1) detailed geologic mapping of approximately 400 mi² of new area to add to the 3,000 mi² of existing detailed geologic mapping, (2) description and interpretation of cores from four shallow boreholes ranging in depth from 500 ft to 1,400 ft, and (3) structural interpretation of surface and reflection seismic data.

The eastern Blue Ridge Mountains (BR) and inner Piedmont (IP) consist of an assemblage of late Precambrian rift-related sedimentary and mafic volcanic rocks. These were metamorphosed during the early Paleozoic and intruded by plutons of different ages. The Chauga belt (CB) consists of an assemblage of late Precambrian to early Paleozoic(?) greenschist-facies clastic, carbonate, and volcanic rocks separating the two higher grade terranes of the BR and IP. The Brevard fault zone (BZ) is localized along the western edge of the CB. The BZ is a complexly reactivated zone of faulting exhibiting an episodic movement history that may span much of the Paleozoic. One of its last movement episodes brought slices of platform carbonate (Knox?) from beneath the Blue Ridge-inner Piedmont thrust shee to the surface along a brittle fault.

Lithologic units of the BR (including the 1.1-Ga Wiley gneiss basement), CB, and IP were encountered in the four shallow drill holes. The basement-cover contact was crossed numerous times in the lower third of the BR hole, indicating isoclinal folding of this boundary. It is uncertain whether the basement-cover contact is a folded fault or an unconformity. New geologic mapping has revealed a major late shear zone near this basement-cover contact on the southeastern flank of the Tallulah Falls dome.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.