SEQUENCE OF THRUSTING AT THE OUACHITA–APPALACHIAN JUNCTION

The late Paleozoic Ouachita (OUA) and Appalachian (APP) thrust belts plunge from opposite directions beneath post-orogenic cover of the Gulf Coastal Plain (GCP). Off-shelf deep-water muddy facies are thrust over carbonate-shelf strata in the OUA; carbonate-shelf strata are imbricated in the APP. The distinct stratigraphic compositions empower use of drill samples to trace the thrust belts beneath the GCP. The OUA extends east from outcrops in Arkansas and curves southeast in Mississippi. The APP extends southwestward in Alabama, curves west, and truncates the OUA, indicating the sequence of thrusting. The intersection of OUA and APP defines the large-scale Alabama recess, which mimics the shape of the Alabama promontory of the Iapetan rifted margin.

The foreland subsidence history confirms the sequence of thrusting. Inside the Alabama recess, the Black Warrior basin (BWB) dips southwest beneath the OUA in Mississippi. Northeastward progradation and thinning of synorogenic clastic deposits indicate OUA tectonic loading and down-to-southwest subsidence along the southwest side of the Alabama promontory. A Mississippian carbonate ramp deepens southwestward and thins out into black shales; fining-upward sandstone-shale parasequences downlap northeastward onto the carbonate ramp. Lowermost Pennsylvanian fining-upward deltaic sandstone-shale parasequences in Mississippi pass eastward into a thick lagoonal shale succession that is bordered on the northeast by massive barrier-island sandstones in Alabama. These distinct patterns prevail from the BWB into trailing synclines in the APP, where thick Mississippian limestones on the northeast pinch out southwestward along APP strike. Facies and thickness distributions show that the Mississippian-Pennsylvanian deposits now in the APP originally were part of the Greater Black Warrior basin in the OUA foreland; OUA foreland deposits were later imbricated in the APP, consistent with the thrust sequence.

Diachroneity of terrane accretion parallels that in the thrust belts and foreland. Accretion of the Sabine terrane inside the Ouachita embayment of the rifted margin began before 328 Ma, as indicated by ages of tuffs. Dextral transpression of the Suwannee terrane truncated the corner of the Alabama promontory by 300 Ma, consistent with late thrusting of APP.