Paper No. 3
Presentation Time: 8:35 AM


THOMAS, William A., Geological Survey of Alabama, P.O. Box 869999, Tuscaloosa, AL 35486-6999,

The late Paleozoic Appalachian and Ouachita thrust belts plunge from the northeast and the west, respectively, beneath post-orogenic cover of the Gulf Coastal Plain. Projected structural convergence of the two thrust belts has attracted speculation over the years, and many regional maps continue to display interpretations that are demonstrably invalidated by abundant drill samples and seismic reflection surveys.

The Appalachian décollement in a shale-dominated weak layer underlies a Cambrian-Ordovician passive-margin carbonate stiff layer that controls geometry of large-scale thrust ramps; long, continuous seismic reflectors image coherent thrust sheets. In contrast, the Ouachita allochthon of off-shelf deep-water mud-dominated facies overlies the passive-margin carbonate facies, which remained in the footwall. Internally deformed and folded thrust sheets, disharmonic small-scale structures, slaty cleavage, and quartz veins characterize the interior of the Ouachita thrust belt. Upper-level sandstone stiff layers form broad open folds in the frontal Ouachitas. These strong contrasts in stratigraphic content and structural style enable tracing the Appalachian and Ouachita thrust sheets in the subsurface.

Numerous drill penetrations of massive carbonate rocks document the extent of the Appalachian thrust belt southwest beneath the Gulf Coastal Plain in western Alabama, curving to the west in eastern Mississippi; seismic reflection profiles show long internally coherent thrust sheets and high-amplitude thrust ramps. Similarly, drill penetrations of slaty mudrocks and vein quartz show the extent of the interior of the Ouachita thrust belt eastward in Arkansas, curving to the southeast in Mississippi. Seismic reflection profiles image the frontal open folds in a sandstone-shale succession but are chaotic in the interior Ouachitas. In eastern Mississippi, the west-striking Appalachian thrust front truncates southeast-striking Ouachita thrust faults. A northeast-prograding synorogenic clastic wedge in the Black Warrior basin in the Ouachita foreland extends southeastward along stratigraphic strike and is imbricated in the northeast-striking Appalachian thrust belt, in concert with the history of diachronous thrusting documented by cross-cutting structures.