LITHOLOGIC AND SPECTROMETRIC EVIDENCE FOR THE MISSISSIPPIAN/DEVONIAN BOUNDARY IN THE WOODFORD SHALE

Detailed measuring and sampling of the historically characterized “homogeneous” Woodford shale reveal lithologic patterns and gamma-ray spectrometric evidence to support the placing of the Mississippian-Devonian boundary within this seemingly lithologically indistinct unit. Widely spaced Woodford outcrops (35 to 75 km apart) located in southern Oklahoma were measured and described on the decimeter scale. Spectral gamma-ray surveys for each outcrop section were compared to lithologic descriptions and biostratigraphic data to establish the position of the Mississippian/Devonian boundary.

Our preliminary results suggest the following lithologic characteristics. (1) The relative abundance of phosphate and the size of individual phosphate nodules increase toward the top of the Devonian, but decrease significantly or are absent in the lowermost Mississippian beds, (2) Chert, which is prevalent in the Devonian, diminishes or disappears across the boundary and does not return for several meters in the sections that contain the most complete record of Mississippian Woodford deposition, and (3) The boundary bed 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. The anomalous radioactivity, decreased organic matter, and lighter color at the boundary may indicate a more rapid rate of deposition and sediment dilution and/or deposition in shallower, more oxygenated water. The disappearance of phosphate can be attributed to decreased water depth and a shoreward shift of facies away from the upwelling zone. Likewise, the number of certain pelagic forms (radiolarians) and the resultant accumulation of siliceous tests would decrease in shallower water and thereby impact chert development. Recurring typical dark Woodford shale with interbedded chert at a position several meters above the Mississippian/Devonian boundary in the more complete sections signifies a return to deeper-water deposition.