Detailed Fingerprints of Global Sea-level Change Revealed in Upper Devonian / Mississippian Woodford Shale of South-Central Oklahoma
Stanley T. Paxton, Anna M. Cruse, and Alischa M. Krystyniak
Oklahoma State University, Stillwater, OK
Gamma-ray logging profiles in organic-rich shale commonly contain intervals in which readings exceed 150 API units. These intervals of elevated gamma-ray, or “hot streaks”, are considered stratigraphic condensed sections; relatively long periods of continuous geological time amalgamated in and represented by a relatively a thin slice of stratigraphic section. Inspection of these profiles, however, reveals that “hot streaks” contain much more stratigraphic information than heretofore recognized. We have prepared full-scale displays of gamma-ray profiles from subsurface well logs through the marine Woodford Shale and made comparisons with spectral gamma-ray profiles collected at outcrop with a handheld spectrometer. Most of the Woodford has gamma-ray readings >150 API units. Some intervals of the formation contain up to 110 ppm uranium and approach 1000 API units. Our analyses indicate that the gamma-ray response in the Woodford is clearly dictated by the uranium in the shale. Noted by past researchers, and verified by our work, the distribution and intimate association of uranium with the shale suggests the source of the uranium was seawater. Therefore, the uranium enrichment was likely caused by diffusion from Devonian/Mississippian seawater into the upper 10-20 cm of the underlying, chemically-reducing seafloor mud. Once in the mud, uranium would have been fixed in the sediments through chemical reduction, thereby establishing a concentration gradient in the porewaters to further drive diffusion. In this sense, the concentration of uranium in the muddy substrate would be linked to sedimentation rate, with extremely slow or starved sedimentation exhibiting the most abundant uranium concentrations.