Late Paleozoic (upper Mississippian) Organic-Rich Gas Shales of the North American Midcontinent

Dark gray-black organic-rich gas-producing shales are abundant in outcrop and subsurface in the North American Mid-Continent. Midcontinent Late Mississippian black shales include the Barnett Shale of Texas, Caney Shale of central and southern Oklahoma, and the Fayetteville Shale of Arkansas and northeast Oklahoma. The basinal equivalent to these shales, the Stanley Shale (Ouachita trough) is typified by coarser siliciclastics with significantly lower TOC values and is interpreted to represent deep marine turbidite deposition. The organic-rich black shales have complex age relationships within the outcrop region that are controlled by local structure, disconformities, as well as well pronounced lithofacies changes. These shales differ from those of the Woodford and Chattanooga in lacking significant chert beds or novaculite but are similar in containing black fissile organic-rich shale and non-skeletal phosphate. Marine condensed sections occur at numerous levels within the black shale succession and are characterized by glauconitic, granular phosphatic lags along with abundant and diverse conodont assemblages. Ammonoid-bearing diagenetic carbonate concretions (bullion) typify these black shales. These concretions occur at different levels and demonstrate a succession of ammonoid faunas that are regionally correlatable.

Faunal elements within these black shales include both pelagic and benthic components. The pelagic components include radiolarians, conodonts, ammonoids, and fish. Benthic faunas are present but stratigraphically restricted and consist of low diversity including acrotretid and Leiorhynchoidea brachiopods, bivalves (Caneyella), and gastropods (archaeogastropods). The presence of localized benthic faunal elements suggests dysoxic conditions alternated with anoxia.