EFFECTS OF BASEMENT AND OUACHITA THRUST BELT STRUCTURES ON THE APPALACHIAN THRUST BELT IN MISSISSIPPI

In the subsurface beneath the Gulf Coastal Plain in east-central Mississippi, both the trace orientation and structural style of the Appalachian thrust belt (TB) dramatically change around the southeastern terminus of the Ouachita TB, which is truncated by the Appalachian TB. The Appalachian TB curves from the regional northeast trace to westward at the intersection of the two thrust belts and to northwest where Ouachita thrust sheets are in the Appalachian footwall. Farther west, the Appalachian TB bends to a west-southwest orientation, but cannot be traced farther because of poor seismic resolution and lack of deep wells.

The Appalachian thrust sheets are detached in Lower Cambrian strata and contain a distinctive Cambrian-Ordovician passive-margin carbonate succession. The Ouachita thrust sheets are detached above the passive-margin carbonates and contain a thick Carboniferous clastic succession. The thrust sheets in the Appalachian TB east of the intersection rest on an autochthonous footwall with a thin Lower Cambrian sedimentary cover above Precambrian crystalline basement. To the west, the Appalachian thrust sheets rest on an allochthonous footwall of thick Ouachita thrust sheets.

The top of Precambrian crystalline basement rocks dips southwestward beneath the Ouachita TB; large-magnitude down-to-southwest basement faults enhance the southwestward deepening of basement. Appalachian thrust sheets on the northeast are detached above relatively shallow basement but, to the west, are detached above thick Ouachita thrust sheets, which overlie much deeper basement. The structure of the basement reflects the Iapetan rifted margin, where the northwest-striking Alabama-Oklahoma transform bounds the southwest side of the Alabama promontory. The trends of basement structures and down-to-southwest subsidence toward the Ouachita TB parallel the Alabama-Oklahoma transform. In contrast, shallower basement and buried synrift basement grabens underlie the northeast-striking Appalachian TB northeast of the Ouachita TB.

The curves in strike and along-strike change in footwall structure of the Appalachian TB reflect controls by basement structure and by the Ouachita TB.