LATE DEVONIAN BASIN REDOX CONDITIONS REVEALED BY URANIUM GEOCHEMISTRY

Typically, in black shales, uranium (U) contents display a linear relationship to total organic carbon (TOC). This association is a result of the mechanisms responsible for the incorporation of U into the sediment during the deposition and remineralization of organic matter. As such, the U content in black shales has been used as a proxy for redox conditions and organic carbon burial. However, some shales display uncharacteristically low U content despite having high TOC content. Other shales have large enrichments of U relative to TOC. Here we examine the U to TOC ratios and U-isotope composition (δ²³⁸U) of three Late Devonian-Early Mississippian shales: the Woodford Shale, the Cleveland Shale, and the Bakken Shale. The low average U/TOC ratio and the relatively high δ²³⁸U values of the Cleveland Shale suggest deposition along the basin margin with periods of reduced bottom water oxygenation, likely due to fluctuations in the location of the pycnocline. Higher U/TOC ratios and higher δ²³⁸U characterize the Woodford Shale which suggests deposition in an unrestricted setting with intermittent sulfidic conditions. In the Bakken Shale cores, high U/TOC ratios, and exceptionally high δ²³⁸U values indicate intense metal draw-down into sediments under persistently sulfidic waters. These three nearly time-equivalent shales, while lithologically similar, have distinct geochemical compositions which indicate decidedly different depositional environments.