BASEMENT NORMAL FAULTS AND SUBSIDENCE OF THE BLACK WARRIOR FORELAND BASIN IN RESPONSE TO THE OUACHITA THRUST LOAD

The Black Warrior foreland basin (BWb) in Alabama and Mississippi is within a large-scale recess in the thrust front of the Appalachian-Ouachita orogen in the subsurface beneath the Gulf Coastal Plain. Paleozoic strata in the BWb dip southwestward toward the northwest-trending Ouachita thrust front within the regional thrust-belt recess. A grid of seismic reflection profiles documents the structure of Paleozoic strata and the top of Precambrian crystalline basement rocks in the BWb, Paleozoic strata in the frontal Ouachita thrust sheets, and Mesozoic-Cenozoic strata of the Gulf Coastal Plain. Three distinct stratigraphic units and corresponding interval velocities can be determined by correlation between seismic reflectors and deep wells. The lowermost stratigraphic unit is a carbonate-dominated succession of Lower Cambrian to Lower Mississippian rocks. Strong layered reflectors at the base of the carbonate succession overlie the Precambrian crystalline basement. A contrast in reflector character represents the contact between the Cambrian-Mississippian carbonate succession and an overlying Upper Mississippian-Pennsylvanian clastic succession. An angular discordance between reflectors represents the contact between the top of the Mississippian-Pennsylvanian clastic succession and the base of Mesozoic-Cenozoic strata of the Gulf Coastal Plain. Structural cross sections derived from the seismic reflection profiles are the basis for a structure contour map. The southwest-dipping strata in the BWb are broken by southwest-dipping normal faults and some antithetic faults. The faults displace the lower Paleozoic strata and underlying Precambrian basement, but some of the faults end upward in the Mississippian-Pennsylvanian clastic succession. The Ouachita frontal structures are southwest-dipping thrust faults, and the stratigraphy suggests correlation with the Mississippian-Pennsylvanian succession. The southwestward dip of the BWb and the dominantly down-to-southwest normal faults indicate subsidence of the foreland basin in response to emplacement of the Ouachita thrust sheets. The Mesozoic-Cenozoic strata of the Gulf Coastal Plain dip gently southwestward and overlie the structures of the BWb and Ouachita thrust belt at an angular unconformity.