The Ouachita Core: An Inverted Rift?

Raymond P. Sorenson Cimarex, Tulsa, OK RSorenson@cimarex.com

Contemporary models for the origin of the Ouachita Mountains include Eocambrian rifting during the breakup of Rodinia and the formation of the Iapetus Ocean, deep water deposition throughout most of the Paleozoic Era, and formation of allocthonous thrust sheets during a Pennsylvanian plate collision.

These models reasonably explain the geology of the northern Ouachita core areas and the foreland Arkoma Basin in Oklahoma and Arkansas, but are inconsistent with several geological features to the south in the Ark-La-Tex province. Problems include:

• the unknown position of the Eocambrian shelf edge, the postulated dividing line between Arbuckle and Ouachita depositional facies
• the awkward orientation of the southern Oklahoma and Mississippi Valley failed rifts relative to the postulated continental margins
• the sparcity of Pennsylvanian igneous activity
• the lack of a recognized trench or subduction zone
• structural elevation of the core areas relative to the region immediately to the south (>20,000')
• southward directed structures within the core areas
• decreasing thermal maturity south of the core areas
• periodic evidence for a southern sediment source
• Desmoinesian-Permian open marine deposition immediately south of the core areas, at a time when the mountain elevation should have been at its maximum.

For this presentation, the Ouachita trough will be portrayed as a series of failed rifts located landward of the Eocambrian continental margin. These formed a locus for deep water deposition within what was otherwise a shallow continental shelf during the Early Paleozoic, and were transformed into a major depocenter for clastic sedimentation in the Mississippian and Early Pennsylvanian. Compressive stress developed during the Pennsylvanian, but a direct plate-to-plate collision did not occur along this portion of the Appalachian-Ouachita-Marathon continental margin. The rift bounding faults were zones of weakness that inverted, causing major uplifts that were geographically largely limited to the boundaries of the original grabens. Regions to the south of the uplifts were only modestly deformed, allowing a quick resumption of marine conditions.