PRE-OUACHITA EPEIROGENIC UPLIFT AND SYN-TECTONIC SEDIMENTATION ON THE OZARK PLATEAU

Stratigraphic evidence in the Ozarks region points toward a major episode of epeirogenic uplift during the early Devonian that persisted through the early Carboniferous, pre-dating the late Carboniferous Ouachita orogeny. Devonian and Mississippian fault movements modified paleo-topography so that erosional truncations and lapout surfaces separate distinctive tectono-stratigraphic sequences. The lower to middle Paleozoic succession (middle Ordovician through middle Silurian) on the southern margin of Laurentia was gently downwarped and progressively truncated below the middle and upper Devonian, and the crest of the Ozarks was uplifted, denuded, and karstified prior to middle Devonian. Middle and upper Devonian mixed carbonates and siliciclastics lap out on the northern flank, and upper Devonian black shale blanketed the western region. Although it is difficult to assess the amount of denudation, exotic clasts from impact breccias in the Decaturville structure indicate that the lower to middle Paleozoic succession was comparable to the preserved record on the eastern flank of the St. Francois Mountains.

Subsequently, Devonian successions were truncated below the regional sub-Mississippian unconformity. During the early and middle Mississippian, fault movements generated accommodation, initiated slumps, and gently folded sedimentary successions. Pre-Ouachita episodes of tectonism marked the onset of convergence between southern Laurentia and the approaching Ouachita allochthon. Crustal loading, flexure, and structural movements along re-activated Precambrian faults provided the impetus for syn-tectonic sedimentation and backstepping of the shelf margin from southern to northern Arkansas, northeastern Oklahoma, and southwestern Missouri.