LATE MISSISSIPPIAN GAS-PRODUCING SHALES OF THE NORTH AMERICAN MIDCONTINENT

Late Mississippian (upper Visean-Serpukhovian) black organic-rich gas- producing shales are abundant in outcrop and subsurface in North America. These shales are interpreted to represent a change in depositional motif from stable carbonate platform deposited under a greenhouse climatic regime to siliciclastic-rich cyclothemic sedimentation deposited in a Milankovitch Band icehouse setting.

These Late Mississippian organic-rich shales include the Barnett Shale, Fort Worth Basin; Caney Shale western Arkoma Basin, and the Fayetteville Shale of the eastern Arkoma Basin. The stratigraphic distribution of lithofacies and associated biofacies suggest episodes of shelfal anoxia punctuated by times of dysoxia . Commonly, these cycles of anoxia and dysoxia occur with a periodicity of 1-3 meters. We interpret the cycles to probably reflect Milankovitch Band cyclicity that caused minor eustatic fluctuations resulting from the buildup and demise of Gondwanaland glaciers. Faunal elements within the anoxic sediments are restricted and consist of only pelagic forms including radiolarians, conodonts, ammonoids, and fish debris. Benthic assemblages consisting of acrotretids and Leiorhynchoidea brachiopods, bivalves (Caneyella), and gastropods (archaeogastropods), and siliceous sponges are vertically restricted to sediments interpreted to have been deposited under dysoxic conditions.

These high frequency cycles also stack into probable third order cycles representing lower frequency long term eustasy. Both the Barnett Shale and Fayetteville Shale are separated by two distinct third order depositional sequences.