GEOCHEMISTRY OF THE BARNETT FORMATION FROM THREE TEXAS HILL COUNTRY CORES

Three drill cores from McCulloch, San Saba, and Brown Counties, TX, were assessed for their bulk inorganic and organic geochemical compositions in order to place constraints on environmental conditions during deposition of the Barnett Formation. Samples were taken at approximately 1-foot intervals in each core, for a total of 160 samples. TOC values range from ~0.3% to 10.3%, %S values range from 0.3% to 30% with core averages ranging between 1.6% and 2.7%, and total inorganic carbon (%TIC) values range from 0% to 10.5% with core averages of ~1.8%. Unlike the Barnett Formation in the Fort Worth Basin, the Hill Country strata contain very little or no dolomite, but possess significant quantities of illite, quartz, calcite, pyrite, and Ca-phosphate. Concentrations of trace elements (e.g., Cu, Ni, Zn, U, and Mo) suggest that their enrichments are both hydrogenous and biogenous. Molybdenum enrichments are elevated above average gray shale, in some cases by a factor of 20 or greater, suggesting that sulfidic conditions occurred during deposition. This is also supported by the non-normal marine data pattern on a graph of TOC vs. %S, which indicates that the sediments were deposited under anoxic conditions. Within two of the cores from San Saba and Brown counties, the majority of samples show that ~82% of the sample Fe is in the form of pyrite. The third core (MC-1, McCulloch County) indicates that only about 56% of the Fe is in the form of pyrite. The overall low enrichment of Mo, relative to the large amount of TOC suggests that the depositional environment was hydrographically restricted. Initial results and interpretations from a petrographic study will also be presented and discussed.