REACTIVATION HISTORY OF A BASEMENT FAULT: STRATIGRAPHIC AND STRUCTURAL CONTROLS ON LOCATION AND GEOMETRY OF A LARGE-SCALE APPALACHIAN FRONTAL THRUST RAMP

A large-scale, northwest-verging, frontal thrust ramp (Birmingham anticlinorium) in the thin-skinned Appalachian thrust belt in Alabama rises over a large (>2 km vertical separation), down-to-southeast, sub-décollement basement fault along the northwest boundary of the Birmingham basement graben. Stratigraphic variations document episodic reactivation of the basement fault prior to thin-skinned thrusting.

The regional décollement is in shaly facies of the Middle-Upper Cambrian Conasauga Formation; the overlying Upper Cambrian-Lower Ordovician (Knox Group) massive carbonates constitute the structural stiff layer. A thick ductile duplex (mushwad) of Conasauga shaly facies constrains the geometry of the stiff layer in the long trailing limb of the frontal ramp. Palinspastic restoration (using area balance) of the mushwad fills the Birmingham graben with thick Conasauga Formation, indicating ~2 km of pre-Knox normal separation along the northwest-boundary fault of the graben.

A regional sub-Middle Ordovician unconformity truncates the upper formations of the Knox Group in locations palinspastically within the Birmingham graben, documenting inversion of the basement graben during the Taconic (Blountian) orogeny. The magnitude of erosional truncation, along with thinning by onlap of the Middle-Upper Ordovician succession, indicates >600 m of inversion separation.

Small-amplitude angular truncation, synsedimentary faults, and soft-sediment deformation indicate episodic fault movement of small displacement from Silurian to Early Mississippian.

Upper Mississippian-Lower Pennsylvanian northeastward-prograding clastic-wedge deposits thicken abruptly southeastward (along stratigraphic strike) across the northwest-boundary fault of the Birmingham graben, and the clastic facies progrades farther northeast in the thicker successions. Synsedimentary slump faults document down-to-southeast paleoslopes. The difference in thickness indicates >1 km of vertical separation in normal reactivation of the basement fault.