APPALACHIAN-OUACHITA THRUST BELT: ALONG-STRIKE CHANGES IN STRATIGRAPHIC COMPOSITION AND STRUCTURAL STYLE

The late Paleozoic Appalachian-Ouachita thrust belt bends abruptly around the Alabama promontory of southeastern Laurentia (North America), reflecting the shape of the older rifted continental margin. The northeast-striking Appalachian thrust front truncates the northwest-striking Ouachita thrust belt, consistent with diachroneity of foreland subsidence and synorogenic clastic deposition. Abrupt along-strike changes in detachment level, stratigraphic composition, and structural style coincide with the thrust-belt junction. Appalachian thrust sheets are detached near the base of the Paleozoic cover succession; a massive stiff layer of lower Paleozoic passive-margin carbonates controls structural style of imbricate thrusts. In contrast, the Ouachita allochthon consists of off-shelf deep-water sedimentary facies in internally disharmonic thrust sheets. The Ouachita allochthon rests on an upper-level detachment above authochthonous, lower Paleozoic passive-margin carbonates equivalent to those within Appalachian thrust sheets.

Although broken by rift-stage faults, basement beneath the Appalachian thrust belt dips gradually southeastward at shallow depths, indicating low-amplitude, long-wavelength subsidence of the foreland. In contrast, deeper basement beneath the Ouachita thrust belt reflects relatively high-amplitude, short-wavelength subsidence. In the Black Warrior foreland basin, in the foreland corner between the intersecting thrust belts, a late Paleozoic synorogenic clastic wedge thickens southwestward toward the Ouachita thrust front, and the top of the underlying passive-margin carbonate succession dips southwestward beneath the Ouachita thrust front. Southwestward foreland deepening toward the Ouachita thrust front results in along-strike deepening along the younger Appalachian thrust front. The younger, structurally shallower, stratigraphically lower Appalachian detachment truncates the older, structurally deeper, stratigraphically higher Ouachita allochthon and the southwestward-deepening Ouachita foreland basin.