STRUCTURAL STYLES AND EXTENSIONAL HISTORY OF THE ALABAMA APPALACHIANS INTERPRETED FROM SEISMIC REFLECTION PROFILES

Two major domains of normal faults occur beneath and partly in front of the southern Appalachian fold-thrust belt, one northeast trending and the other northwest trending. The northeast-trending normal faults occur parallel to and beneath the Valley and Ridge province. In the northeastern corner of Alabama a major basement-involved half graben is filled with growth sediments of Cambrian and possibly older age. The master normal fault and major rider blocks dip to the southeast. The youngest growth unit is the Cambrian Conasauga Formation. The Rome Formation is greatly expanded in the graben and pre-Rome sediments may be present. Extensional basin geometry and basin stratigraphy exert significant controls over Appalachian structures. There is no evidence of reactivation of this trend during Appalachian thrusting.

Northwesterly trending normal faults are present throughout the Black Warrior basin in front of the Appalachian thrusts, both at the surface and in the subsurface. Northeast of Tuscaloosa all the normal faults appear to be thin skinned with a lower detachment in the lower part of the Pottsville Formation. Southwest of Tuscaloosa, the normal faults have lower detachments in the lower Pottsville, at the base of the Knox, or deep in the basement. Some faults on this trend show evidence of modest growth during the deposition of the Rome and Conasauga Formations. One major fault has substantial growth during Knox deposition. New normal faults formed and others were reactivated after deposition of the youngest preserved Pennsylvanian Pottsville sediments. The major structures are brittle half grabens with little associated drape folding.

All the NE- and many of the NW-trending normal faults formed during the opening of the Iapetus ocean and some on the NW trend remained active throughout Knox deposition. The post-Knox unconformity might be the result of uplift of the rift shoulders. Appalachian anticlines show evidence of minor growth during Pottsville deposition and thrusts cut normal faults in the Pottsville, indicating that these normal faults formed before the climax of the Alleghanian orogeny. Post-Pennsylvanian reactivation of the NW set of normal faults and the formation of new thin-skinned faults within the trend is attributed to Ouachita thrust loading.