ROLE OF THE BIRMINGHAM BASEMENT FAULT IN THIN-SKINNED THRUSTING OF THE BIRMINGHAM ANTICLINORIUM, ALABAMA

The northeast-striking, northwest-verging Birmingham anticlinorium is associated with the Jones Valley and Opossum Valley frontal thrust ramps in the southern Appalachian thin-skinned sedimentary thrust belt in Alabama. Amplitude of the southeastern, trailing limb of the anticlinorium (Cahaba synclinorium) is greater than that of the forelimb (Coalburg syncline). Depth to the top of basement rocks beneath the décollement is greater beneath the Cahaba synclinorium than beneath the Coalburg syncline, commensurate with vertical separation of the top of basement along the down-to-southeast Birmingham basement fault at the northwest boundary of the Birmingham basement graben. The frontal ramp that controls the geometry of the Birmingham anticlinorium rises northwestward over the down-to-southeast basement fault.

The geometry, kinematics, and mechanics of the Birmingham anticlinorium reflect stratigraphic variations in mechanical properties within the Paleozoic succession; the stratigraphic variations record episodic synsedimentary movements on the Birmingham basement fault. The regional décollement at the Birmingham anticlinorium is in the Middle to lower Upper Cambrian Conasauga Formation, which is dominantly a massive carbonate and <800 m thick northwest of the Birmingham graben, but is a succession of shale and thin-bedded limestone >2,000 m thick within the graben. Ductile deformation of the thick, shale-dominated succession localized the Palmerdale and Bessemer mushwads (ductile duplexes) along strike beneath the Birmingham anticlinorium; a massive carbonate regional stiff layer (Cambrian-Ordovician Knox Group) forms the mushwad roof. The upper part of the Knox Group was eroded as a consequence of inversion of the graben during the Taconic orogeny. Small-scale synsedimentary structures indicate minor structural reactivation from Late Ordovician through Early Mississippian. Thickness and facies variations in Upper Mississippian-Lower Pennsylvanian synorogenic clastic-wedge deposits document large-scale reactivation of the down-to-southeast Birmingham basement fault. The Birmingham basement fault localized the mushwads and the large-amplitude frontal ramps as the Appalachian décollement propagated toward the foreland.