Trace Elements and Basin Processes: Woodford Shale, Permian Basin, West Texas
Harris, Nicholas B.
The trace and minor element geochemistry of the Upper Devonian Woodford Shale, Permian Basin, are analyzed in order to gain insights into paleoceanographic conditions and processes during deposition of this formation, which is a major source of hydrocarbons in the basin and an analog for many other Upper Devonian shales in North America and elsewhere. Stratigraphic relationships suggest that the Woodford was deposited during a second-order sea level fall, culminating in the deposition of cherts and organic-poor, highly bioturbated mudstones.
The Woodford lacks enrichment in many trace metals, in contrast to many organic-rich shales. Only Mo, U, S and Se are significantly enriched. Other redox sensitive elements are depleted or similar to average shale composition, including Pb, Bi, Cr, Ti, Cu, Zn, Co, and V. Multivariate factors analysis identified associations between elements, including groupings of: rare earth elements; elements enriched in granitic crust; silica, varying antithetically with elements in carbonate minerals; organic carbon, Mo and U; V; phosphate; Fe and S. Noteworthy among the results are the different behavior of redox-sensitive element V in comparison to Mo and U, suggesting different precipitation mechanisms or varying dependence on reservoir effects. A strong basin reservoir effect is noted among several redox-sensitive elements, including Mo, Cu and Ni, which likely accounts for the depletion of Cu and Ni.
A strong redox effect is noted in the TOC/Ptot ratio at the same depth where Mo/TOC raios indicate a short-term significant fall in sea level. This is interpreted to approximate the Frasnian - Famennian boundary, which was marked by an abrupt long-term transition to an anoxic water column, based on the TOC/Ptot data. This suggests that anoxia was induced by isolation of the basin from the global ocean; somewhat surprisingly, there is little change in organic carbon content at this boundary. Re-Os geochronology for the Woodford section is similar to correlative sections of the Appalachian and Peace River Basins of North American and the Rhenohercynian basin of Europe. This indicates that although the Permian Basin became restricted during the upper Devonian and early Mississippian, ocean connectivity remained between regional and global basins.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013