HANGINGWALL-TO-DETACHMENT TRANSITION AT A THIN-SKINNED EXTENSIONAL DETACHMENT FAULT IN THE EASTERN BLACK WARRIOR BASIN, ALABAMA

How do hangingwall normal faults merge into a lower detachment? Models range from faults with displacements abruptly truncated at the lower detachment, to faults that maintain near-constant displacement as they join the detachment, requiring significant rollover folding of the hangingwall. The Pennsylvanian Pottsville Formation in the Black Warrior foreland basin is broken by numerous, long, northwest-trending normal faults of Alleghanian age. There is virtually no associated rollover folding, just tilting. Both domino-style half grabens and full grabens are represented. Area-depth relationships derived from detailed cross sections across coalbed methane fields and an underground coal mine in the eastern part of the basin indicate that the faults are thin-skinned, with a lower detachment in the lower Pottsville Formation. Field mapping along the upturned southeastern edge of the basin shows the detachment to be in the Fayette coal cycle of the lower Pottsville, as predicted, and shows its relationship to the hangingwall structures. Nowhere do the normal faults penetrate below the Fayette cycle to offset the upturned basal Pottsville sandstone ridge. A temporary exposure in a new roadcut across the upper part of the Fayette revealed a highly deformed zone that might be the detachment itself. Large domino-style normal faults of the basin lose stratigraphic separation as they approach the detachment and new small domino faults that die out stratigraphically upward and downward appear between them. No rollover folding is present. The extensional displacement thus appears to become distributed over a thick zone of pure shear at the base of the fault block as the detachment is approached, suggesting a new model of “soft-bottomed” fault blocks. The normal faults are generally regarded as being caused by the flexural extension of the crust during the formation of the Ouachita foredeep and, consequently, have previously all been inferred to penetrate deep into the crust. We envision the thin-skinned displacement on the lower detachment in the eastern part of the basin as continuing to the southwest where it connects in the subsurface to faults that penetrate basement, forming a regional ramp-flat fault geometry.