Spectral Gamma-Ray Signatures Across the Devonian/Carboniferous Boundary, Woodford Shale, Oklahoma

Jim Puckette, Darwin Boardman, Mark Bryan, Jacqueline Berryman, Robert Berryman, and Daniel Hurst
Oklahoma State University, Stillwater, OK

Spectral gamma-ray profiles of the Woodford Shale and the correlative Chattanooga Shale provide evidence to support the placing of the Devonian/Carboniferous boundary within these seemingly lithologically indistinct units. Widely spaced Woodford outcrops (35 to 75 km apart) located in southern Oklahoma and Chattanooga shale outcrops in the Ozarks were measured, described and spectral gamma-ray surveyed. In the Woodford, the abundance and size of phosphate nodules increase toward the Devonian/Carboniferous (Mississippian) boundary. Sections with the more complete biostratigraphically constrained records of Mississippian Woodford deposition (Over, 1990) contain a boundary bed that is distinctly lighter colored and less organic-rich than beds immediately above or below. Gamma-ray spectrometric profiles indicate a dramatic drop in radioactivity in the boundary bed and a marked shift in U, Th and K ratios that may be traceable in the subsurface. Anomalous radioactivity, decreased organic matter, and lighter color at the boundary reflect an increased rate of deposition and sediment dilution and/or deposition in shallower, more oxygenated water. The disappearance of phosphate is attributed to decreased water depth and a shift of deposition away from the upwelling zone. In less complete Woodford sections that are missing biostratigraphic zones due to erosion or nondeposition, the boundary bed is absent and the resulting anomalous gamma-ray signature is not evident. These abbreviated Woodford sections are similar to Chattanooga ones in that a decrease in total radioactivity and U/Th and U/K ratios coincides with the contact between dark Woodford shale and the overlying green-gray pre-Welden/pre-Compton shale of Mississippian Kinderhookian age.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas