STRUCTURAL CHRONOLOGY OF THE BLACK WARRIOR FORELAND BASIN

The Black Warrior basin is a late Paleozoic foreland basin that formed in the recess between the Appalachian and Ouachita orogenic belts. The basin contains compressional and extensional structures, as well as joint-cleat networks. Analysis of field and geophysical data reveals a complex structural chronology. This complexity is the result of changing stress fields during assembly of Pangaea and interaction between the Appalachian and Ouachita orogens. The basin contains a thick, pre-orogenic succession of Cambrian-Mississippian carbonate rocks. Synorogenic siliciclastic rocks range in age from Late Mississippian to Pennsylvanian. Ouachita orogeny began along the southwest margin of the basin in the Late Mississippian, but Appalachian thrust and sediment loads did not impinge on the southeast part of the basin until Pennsylvanian time.

The basin is a southwest-dipping homocline that is deformed by northwest-striking normal faults and northeast-striking folds. Normal faults formed above a detachment with multiple ramps and flats. Some faults formed during Iapetan rifting, and others formed by Ouachita flexural extension. Appalachian folds have deformed the southeastern part of the homocline and are detached in Cambrian shale. Growth strata indicate early folding during the Morrowan, although major growth post-dates preserved stratigraphy. Normal faults form en echelon patterns along lateral ramps, suggesting temporal overlap between regional flexural extension and folding. Joints compose two distinct sets of systematic fractures, as do cleats. Regional E-NE trending joint and cleat sets formed under a continent-wide stress field, whereas fold-localized joint and cleat sets trend perpendicular to Appalachian fold axes. Field relationships demonstrate that regional fracture directions were established prior to major fold growth. Formation of the regional fractures requires an effective reversal of the stress field from that associated with Ouachita extension.