RHENIUM-OSMIUM GEOCHEMISTRY OF THE WOODFORD SHALE: IMPLICATIONS FOR BASIN CONNECTIVITY

The Late Devonian to Early Mississippian was a time of global deposition of organic carbon-rich shales associated with a distinctive paleogeography, with large areas of epicontinental seaways and continental sags in present-day North America, Europe and Australia. This time interval recorded a ~70m fall in sea level that resulted in basin restriction and widespread ocean anoxia.

The Woodford Shale in the Permian Basin, west Texas is one of several Upper Devonian shale formations in North America and Europe. Rhenium-osmium (Re-Os) elemental abundances and isotope compositions were obtained for four sample sets from a core in the western Permian Basin that spans the entire time interval. The Re–Os isotopic data yield Re–Os dates of 379.0 ± 7.9 Ma (initial ¹⁸⁷Os/¹⁸⁸Os (IOs) = ~0.29; middle Frasnian); 371.5 ± 5.8 Ma (IOs = ~0.40; Famennian); 364.0 ± 13 Ma (IOs = ~0.69; Famennian); and 357.9 ± 5.3 Ma (IOs = ~0.47; Tournaisian).

The Frasnian and Famennian sample sets display systematically higher ¹⁸⁷Re/¹⁸⁸Os and more radiogenic ¹⁸⁷Os/¹⁸⁸Os values up section. The Tournaisian sample set possesses ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os values similar to and greater than the three older sample sets. The IOs values for the Permian Basin, with the exception of the uppermost Famennian sample set, are similar to time-correlated sections of the Appalachian and Peace River Basins of North American and the Rhenohercynian Basin of Europe. This suggests that although the Permian Basin became restricted as a result of a ~70m fall in sea level, the Upper Devonian and Early Mississippian ocean connectivity remained between regional and global basins.

The youngest Woodford Shale interval represents the period of greatest restricted ocean circulation and relatively oxidizing water column, which would predict low Re-Os enrichment. However, the Tournaisian sample set possesses the highest Re and Os abundances and a mix of organic matter type compared to the older three sections. The Woodford Shale shows no evidence for changes in sedimentation rate, salinity, pH, and / or temperature, but does display a shift from dominantly Type II to a mixed Type II – III organic matter. As a result we propose that Re and Os enrichment and fractionation in organic-rich sedimentary rocks is, in part, controlled by organic matter type.