Conodont biostratigraphy of the upper Osagean (lower Visean) Ozark uplift, southern midcontinent U.S.A

Oil and gas exploration in Mississippian strata of the southern Midcontinent has increased interest in the lateral and vertical trends of reservoir facies and generated a need for more refined facies models. This study establishes a biostratigraphic framework for the Boone Group (upper Osagean), Mississippian Subsystem based on a bed by bed analysis of the Reeds Spring Formation and Bentonville Formation (formerly known as the Burlington-Keokuk Formation). Three outcrops along US Highway 65 in northwestern Arkansas (Boone County, Arkansas), and one outcrop along US Highway 71 (Benton County, Arkansas) were sampled for conodonts and acetate peel petrography. Conodont biozones were found to be unconstrained by facies and support the hypothesis that upper Osagean carbonate wedges are time transgressive. The middle "texanus" zone is found within the Reeds Spring Formation in Cherokee County, Oklahoma (middle to outer ramp facies) and Bentonville Formation in Benton County, Arkansas (inner ramp facies). This relationship clearly demonstrates multiple facies occur within one biozone, with higher energy facies in more proximal settings and lower energy facies in more distal settings. These higher and lower energy facies were previously called Burlington-Keokuk Formation and Reeds Spring Formation, respectively. Conodont biostratigraphy was used to pick the approximate position of the "contact" between the Reeds Spring and Bentonville formations, but higher-frequency sampling is necessary to clarify the boundary at some outcrops. The integration of the results of this study with previous biostratigraphic investigations (Thompson and Goebel, 1969; Thompson and Fellows, 1970) is helping improve our understanding of depositional processes and facies relationships in the upper Osagean of the southern Midcontinent.