DEPOSITIONAL HABITAT FOR PALEOZOIC MUDROCK INTERPRETED FROM ANALYSIS OF SPECTRAL GAMMA-RAY RESPONSE

Analysis of spectral gamma-ray (SGR) response in mudrocks provides detailed information about sediment provenance, sedimentation, and depositional environment within ancient interior seaways. For the present study, SGR data were obtained from outcrop, well logs, and core for Devonian, Mississippian, Pennsylvanian, and Permian mudrocks in the southern mid-continent of North America. Collectively, the rocks represent a host of different depositional settings. To prepare the data for analysis, the K (%), U (ppm), and Th (ppm) values were converted to API units through use of a standard equation employed by the geophysical well-logging community. Next, the data were normalized in terms of relative contributions to total calculated API and the results plotted on a ternary diagram with K, U, and Th at each apex. The data trends and grouping of populations suggest the following: Mudrocks known to have clear continental affinity and deposited under oxic conditions (Permian redbeds) are observed to be enriched in K and Th relative to U. In contrast, Devonian and Pennsylvanian mudrocks from deep marine settings and deposited under anoxic and/or euxinic conditions (shales that are finely laminated, organic-rich, and pyrite-bearing) are enriched in uranium relative to Th and K. Because the abundance of U in black shale can be partly attributed to the duration of starved sedimentation on the ancient seafloor (which favors diffusion), data that plot close to the U pole on the triangle represent extremely condensed stratigraphic sections. Mississipian- and Pennsylvanian-age gray-shale populations fall intermediate on the ternary diagram to the marine black shale and the Permian redbed mudstone. Low-uranium mudrock that is enriched in K relative to Th is interpreted to be indicative of sediment undergoing rapid denudation of a crystalline-to-low-grade-metamorphic hinterland. Th-enrichment in mudrock, common to Permian redbeds and Pennsylvanian underclays, is suggestive of derivation from a provenance or depositional setting that has undergone deep weathering or leaching. Based on K-U-Th, the Upper Devonian/Lower Mississippian Chattanooga Shale of Oklahoma/Kansas appears to have been deposited in a more proximal-marine setting relative to the time-equivalent Woodford Shale of south-central Oklahoma.