Paleozoic Synsedimentary Structural Deformation in Appalachian Fold-Thrust Belt of Alabama and West Georgia

Benjamin A. Ferrill

The Appalachian fold-thrust belt consists of allochthonous Paleozoic sedimentary cover rocks transported by late Paleozoic Alleghenian thrust faults. Local variations in stratigraphy in Alabama and west Georgia indicate episodic, synsedimentary, structural movement during much of the Paleozoic. To provide a base map for interpreting prethrust stratigraphy, a new palinspastic map was constructed from a series of balanced and restored cross sections.

Local thickness variations suggest that synsedimentary deformation was associated with high-angle basement faults, and some basement faults are confirmed by seismic data. Two major phases of synsedimentary basement fault movement account for most of the local thickness variation: (1) Early and Middle Cambrian, and (2) Early Mississippian to Early Pennsylvanian. The Upper Cambrian-Lower Ordovician Knox Group is a regionally pervasive carbonate unit that reflects tectonic quiescence. The interval from Middle Ordovician to Lower Mississippian is characterized by thin, laterally variable, shallow marine units. The section includes several unconformities that locally coalesce, thus completely removing this succession. Different expressions of facies and unconformities locally suggest both ynsedimentary and postdepositional structural basins. Minor reactivation of basement faults evidently occurred during the middle Paleozoic.

Relative locations of early synsedimentary structures and later Alleghenian thin-skinned structures suggest both strike-parallel and cross-strike basement faults that locally correspond to frontal and lateral decollement ramps, respectively. Thus, Paleozoic basement-related, thick-skinned, synsedimentary structures probably controlled the location and geometry of some decollement ramps generated during Alleghenian thrusting.

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