Evolution of Subsurface Appalachian-Ouachita Fold-Thrust Belt Beneath Gulf Coastal Plain

William A. Thomas

A continuous belt of deformed rocks defines the curved trace of the Appalachian-Ouachita fold-thrust belt beneath the Gulf coastal plain between Appalachian outcrops in Alabama and Ouachita outcrops in Arkansas. Both the structural style and stratigraphic composition of the belt change along trend in central Mississippi. The Appalachian succession of Cambrian-Mississippian shallow shelf facies and Mississippian-Pennsylvanian shallow marine to deltaic foreland-basin clastic-wedge rocks contrasts with the Ouachita succession of Ordovician-Mississippian deep-water offshelf facies and Mississippian-Pennsylvanian turbidite clastic-wedge strata. Appalachian structures, in response to a thick competent carbonate unit, are large-scale, internally coherent thrust sheets detached n ar the base of the Paleozoic section. In contrast, Ouachita structures include internally disharmonic, less continuous thrust sheets in a section characterized by thick incompetent units, and a central uplift of slaty lower Paleozoic rocks. The Ouachita thrust faults and central uplift follow a cratonward-convex arcuate trace from the Arkansas outcrops to east-central Mississippi. The thrust front approximately parallels a large down-to-the-south basement fault system that displaces foreland-basin rocks, and the central uplift evidently is thrust over downthrown shelf-facies rocks. Appalachian thrust faults can be traced in a cratonward-concave curve into central Mississippi where west-trending Appalachian thrusts diagonally truncate southeast-trending Ouachita structures. The distributi n of rock types in the subsurface thrust sheets indicates that the thrust front crosses the old shelf edge diagonally eastward from the deep-water facies into the shelf facies. Clastic-wedge stratigraphy and structural geometry document: (1) initial thrusting along the Ouachita front in Mississippi during the Mississippian, (2) progression of thrusting westward to the Ouachita outcrops during the Pennsylvanian, and (3) thrusting along the Appalachian structures during the Pennsylvanian.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.