PALEOCEANOGRAPHIC AND ESTUARINE INFLUENCES ON SEDIMENTATION IN A WORLD-CLASS UNCONVENTIONAL SOURCE ROCK-RESERVOIR SYSTEM: DEVONIAN SHALE IN THE SOUTHERN MIDCONTINENT, USA

A range of depositional and geochemical variables were analyzed in core and outcrop to critically assess the origin of the Woodford Shale and the Chattanooga Shale in the southern Midcontinent of Oklahoma and southwestern Missouri. These strata are dominated by organic-rich mudrocks and novaculitic chert that were deposited where an ancient oxygen minimum zone (OMZ) intersected the southern edge of Laurussia. Chert with phosphate nodules dominates the Famennian section near the Ouachita embayment and provides evidence for oceanic upwelling along the Arkoma shelf and the Southern Oklahoma Aulacogen. Soft-sediment deformation provides evidence for unstable slopes in this region, and synsedimentary extension indicates that aulacogen structure was active during Woodford deposition.

North of the aulacogen and Arkoma shelf, facies distribution was influenced by incised valleys that underlie Woodford-Chattanooga strata. Ripple cross-laminae, inverse grading, and normal grading indicate that argillaceous mud was deposited by currents related to upwelling, storms, and internal waves. An unusual conodont lag contains bundled laminasets indicative of spring-neap tidal cycles, and the transition from chert-bearing facies to silty argillite indicates that flocculation and pelletization were important processes affecting sedimentation on the inner shelf. Organic-bound trace elements (V, Ni), pyrite forms, and paleobiological parameters indicate that anoxia and dysoxia were widespread and were most intense in the aulacogen, which may have been near the core of the OMZ. The distribution of clay-bound elements (Cr, Co) suggests that flocculation occurred in more oxygenated settings, and shoreward increase of Th levels points toward increased sediment input from terrestrial settings. Hence, estuarine flushing is interpreted as an important mechanism that helped deliver siliciclastic material to areas where the inner shelf was in the OMZ.