RELATIONSHIP BETWEEN COMPRESSIONAL AND EXTENSIONAL STRUCTURES IN THE EASTERN BLACK WARRIOR FORELAND BASIN, ALABAMA

The Black Warrior basin of Alabama and Mississippi, the foreland basin of the Appalachian-Ouachita orogenic belt, is of Mississippian and Pennsylvanian age. The regional structure is a southwest-dipping wedge-shaped monocline that is turned up and truncated at the thrust front. The basin is cut by numerous normal faults, most of which formed above a detachment located below the economic coal cycles of the Pennsylvanian Pottsville formation; relatively few faults cut Mississippian and older units. Surface mapping along the thrust front between Tuscaloosa and Birmingham, Alabama, shows the normal faults are all thin-skinned, as originally inferred from the area-depth relationships from cross-sections of coalbed methane fields and coal mine maps. The extensional lower detachment is confirmed in outcrop along the upturned southeastern edge of the basin to be within the Fayette coal cycle in the upper portion of the lower Pottsville Formation. The extension occurred late in the Alleghanian orogeny because the normal faults cut the Pottsville coal cycles with little evidence of growth during deposition, and are cut by Appalachian thrusts. The map area contains fault-propagation-fold anticlines that have vertical forelimbs and gently sloping back limbs. The forelimbs are tectonically thickened due to small-scale thrusting between competent beds. Squeezing by the competent basal Pottsville Boyles sandstone quartzarenite in the forelimb syncline produces an out-of-syncline thrust that cuts the extensional lower detachment.