Geochemical Constraints on the Depositional Environment of the Barnett Formation, Permian Basin, West Texas, USA

The Mississippian-age Barnett Formation of Texas represents a unique and widespread marginal marine environment characterized by slow overall deposition rates and relatively high organic matter accumulation. To date, most Barnett studies have focused on understanding deposition in the Fort Worth Basin of north-central Texas, and specifically, the focus has been on the organic characterization of the formation. The present study focuses on using bulk geochemical and petrographic results to define depositional conditions in West Texas. The upper ~50 meters of a core from Pecos County, Texas, was analyzed at a ~0.5-m sample spacing for major oxides, trace-metals, organic/inorganic carbon, sulfur, and bulk carbon and nitrogen isotopes. X-ray diffraction of select samples constrains stratigraphic changes in mineralogy. Bulk mineralogy is dominated by quartz, illite, and pyrite, with Ca-phosphate- and/or dolomite-rich intervals throughout the middle 25 meters of the sampled interval. Analysis of major element chemistry indicates four well-defined peaks in the Si/Al ratio, which indicate temporal variability in the relative deposition of detrital (clay) and biogenic (silica) components. The Si/Al peaks are broadly consistent with redox-sensitve trace-metal enrichments (Mo, U, Ni, Cr, Zn). An interesting aspect of the trace-metal record is that most samples are slightly to moderately enriched relative to average grey shale, but are not nearly as enriched as well-studied black shales deposited during the Devonian (e.g., Ohio Shale, New Albany Shale, Woodford Shale). This finding is consistent with previous work in the Fort Worth Basin, and suggests, based on the high organic accumulation, that the basin hydrography was highly restricted. TOC values range from ~1% to >6%, and the stable isotopic compositions of TOC and total nitrogen range from -24 to -30 permil, and +5.7 to +9 permil, respectively. These results will be interpreted in terms of our forthcoming study on the organic petrography of representative samples.