LATE CRETACEOUS-CENOZOIC BRITTLE FAULTING BENEATH THE WESTERN SOUTH CAROLINA COASTAL PLAIN: REACTIVATION OF THE EASTERN PIEDMONT FAULT SYSTEM

The Eastern Piedmont fault system (Hatcher and others, 1977) that extends from Alabama to Virginia is characterized by long, narrow zones of Paleozoic ductile deformation marked by prominent, elongate magnetic anomalies. Later brittle deformation is associated with many of the faults. In South Carolina the Augusta and Modoc faults are part of the fault system. The Augusta fault is exposed at the Savannah River but passes beneath the Coastal Plain less than 3 miles to the northeast. The trace of the fault was inferred from magnetic anomalies. Recent geologic mapping of Piedmont inliers between Horse Creek and the North Edisto River shows silicified breccias that are associated with the Augusta fault where it crosses Horse Creek, Shaw Creek, South Edisto River, McTier Creek, and North Edisto River. At McTier Creek, south of Monetta, movement on the brittle fault has offset the Piedmont-Upper Cretaceous contact thirty feet up to the south. East of McTier Creek, map patterns of Eocene and Miocene formations (Nystrom and others,1986) north of the fault end abruptly at the fault trace. At other localities map patterns either terminate north of the fault or narrow where the formations cross the structure. The correlation indicates fault movement younger than the map units.

A related fault, the Davis Pond fault, occurs between the Augusta fault and the Modoc fault that it parallels. It is marked by a linear magnetic anomaly and, at least locally, is the boundary between Piedmont map units. Near Davis Pond, south of Batesburg, brittle faulting associated with the ductile structure offsets the Piedmont-Upper Cretaceous contact 20 feet up to the north, truncating the Upper Cretaceous. The Piedmont block between the Augusta and Davis Pond faults dropped down relative to the northwest and southeast. The Davis Pond fault trends N60E on strike with and connected by linear magnetic anomaly with a silicified breccia just south of Lake Murray dam. The Augusta, Davis Pond, and Modoc faults are integral elements of the Eastern Piedmont fault zone with long complex histories. Offsetting on the later brittle faults, though only a few tens of feet, continued into the late Cenozoic, effecting present day topography. The young faults remain zones of weakness where further movement could occur.