PRE-OUACHITA TECTONISM, DEVELOPMENT OF A BACK-STEPPING SHELF MARGIN, AND SYN-TECTONIC SEDIMENTATION (MIDDLE DEVONIAN THROUGH MISSISSIPPIAN) ON SOUTHERN LAURENTIA: A REGIONAL SYNTHESIS OF THE OZARKS

Southern Laurentia in the Ozarks region has been characterized as a Late Proterozoic rift shoulder that developed into a passive margin; docking with the Ouachita allochthon during the Early Pennsylvanian led to development of the Arkoma Basin and Ozark Uplift. Yet, why were pre-Mississippian strata progressively truncated to the north and west? Why was the Chattanooga Shale preserved in southwestern Missouri and northwestern Arkansas but eroded below the Mississippian in the Boston Mountains? Why did deep-water Mississippian carbonates accumulate in southern Missouri on peritidal lower Paleozoic strata? We consider that much of the epeirogenic uplift and denudation of the Ozarks preceded the Ouachita orogeny, occurring in the Early to Middle Devonian, and structural evolution persisted through Mississippian and Early Pennsylvanian time.

The onset of convergence led to development of the Arkansas Novaculite foredeep, and in the Ozarks, loading and flexure resulted in progressive truncation of much of the post-lower Ordovician to pre-Mississippian succession. Up to 500 m of strata were cut out, yet impact structures, isolated sink-fills, and faults record the presence of stratigraphic units that are missing regionally. Upper Ordovician to middle Silurian clasts are incorporated into the Decaturville breccia (pre-Devonian impact) in lower Ordovician country rock. Middle Devonian marine sandstone filled a sink in lower Ordovician dolomite near Rolla. Upper Devonian Chattanooga Shale blanketed the eroded western Ozark platform. Most of this unit was removed below the sub-Mississippian unconformity in the central Ozarks but was preserved in an embayment that we informally refer to as the Northwest Arkansas Basin. Clasts of Chattanooga Shale have been recovered from the Weaubleau breccia (mid-Mississippian impact), a thin interval is preserved along the Highlandville Fault, along the lower Buffalo River, and in sink-fills in northern Arkansas.

During the Early Mississippian, continued flexure resulted in back-stepping of the shelf margin, so deep-water facies accumulated on erosional remnants or peritidal lower Paleozoic strata. Major faults in southern Missouri are associated with some anomalous units of sandstone, limestone, or olistoliths, providing a record of syn-tectonic sedimentation.