Role of the Birmingham basement fault in thin-skinned thrusting of the Birmingham anticlinorium, Appalachian thrust belt in Alabama

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The Birmingham anticlinorium is a northeast-striking, northwest-verging, thrust-related fold associated with the Jones Valley and Opossum Valley frontal thrust ramps in the southern Appalachian thin-skinned sedimentary thrust belt in Alabama. The amplitude of the southeastern, trailing limb of the anticlinorium (Cahaba synclinorium) is greater than that of the forelimb (Coalburg syncline). The 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 Birmingham basement fault. The geometry, kinematics, and mechanics of structure of the Birmingham anticlinorium reflect stratigraphic variations in mechanical properties within the Cambrian to Pennsylvanian stratigraphic succession; the stratigraphic variations record episodic synsedimentary movement 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 less than 800 m thick northwest of the Birmingham basement graben, but is a succession of shale and thin-bedded limestone more than 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. The massive carbonate regional stiff layer, the 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 flexural subsidence associated with Taconic tectonic loading of the southeastern margin of Laurentia. 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 localizes both the leading edge of the mushwads of shale-dominated Conasauga Formation and frontal ramps of the regional décollement.