A NEW MODEL FOR THE TIMING AND STRUCTURAL DEVELOPMENT OF THE OZARKS UPLIFT

The timing of uplift on the Ozarks portion of the southern margin of Laurentia generally has been assumed to be Pennsylvanian in age, associated with the assembly of Pangaea, collision of the Ouachita allochthon, and development of the Arkoma foreland basin, but the Paleozoic stratigraphic record has never been fully integrated into a tectono-stratigraphic model. Evidence from regional unconformities and patterns of sedimentation indicate epeirogenic uplift began during middle Devonian time, persisted through the Mississippian, and culminated during the widely known Pennsylvanian Ouachita orogeny. Flexural uplift in the Ozarks was coupled with development of a foredeep on the Ouachita allochthon and deposition of the late Devonian to early Mississippian Arkansas Novaculite. Sinkhole fills and exotic clasts in impact structures in the Laurentian hinterland provide a window into the unusual concatenation of sub-Devonian and sub-Mississippian unconformities across the crest of the Ozarks plateaus. We argue that these pre-Pennsylvanian epeirogenic episodes of tectonism marked a shift from passive margin to convergence with movements along re-activated Precambrian faults. Movements associated with uplift provided a framework for syntectonic sedimentation and backstepping of the shelf margin onto the platform with relatively deep-water Devonian and early Mississippian strata deposited above peritidal lower Paleozoic strata.

Previous investigators have shown that the region is criss-crossed by conjugate sets of strike-slip faults. Much of the flexural uplift is associated with a compressional paleo-stress field with oblique convergence of the Ouachita allochthon. If the Ozarks were purely compressional, why are there so many grabens in Missouri? Like many other strike-slip provinces, the Ozarks experienced both shortening and extension in association with hard and soft linkages in transfer zones that trended obliquely to the dominant strike-slip faults. Active fault movements during Mississippian time directly affected deposition of the nascent carbonate ramp, delineating small intrashelf basins that became the locus of discrete carbonate slide and slump features as well as chaotically bedded units, which record syntectonic sedimentation.