CHEMOSTRATIGRAPHY AND PALEOCEANOGRAPHY OF THE MISSISSIPPIAN BARNETT FORMATION, SOUTHERN FORT WORTH BASIN, TEXAS

The Mississippian (Visean-Serpukhovian) Barnett Formation is a lithologic unit composed primarily of laminated siliceous mudstone with some lime mudstone intervals. The Barnett Formation was deposited in the Fort Worth Basin, a marine foreland basin that formed as a result of the early Ouachita Orogeny from the collision of Laurasia and Gondwana.

The focus of the research project will better characterize the different aspects of the Barnett Formation paleodepositional environment in the southern Fort Worth Basin including: degree of basin restriction, redox conditions, organic character, and other geochemical changes along a northeast-southwest transect of the basin. This will be accomplished by examining the Barnett Formation chemostratigraphy and linking it with observed sedimentological changes in Barnett strata. Eight drill cores from four counties in Central Texas will be evaluated. Each core was scanned at a 1-foot (~0.3m) interval with an X-ray fluorescence (XRF) spectrometer to provide quantitative analysis of the following elemental concentrations: Mg, Al, S, Si, P, K, Ti, Ca, Mn, Fe, Mo, Cr, Co, Ni, Cu, Zn, Th, Rb, U, Sr, Zr, and V. In addition, total organic carbon (TOC), total inorganic carbon (TIC), percent nitrogen and sulfur data will be gathered from each core. At present, major and trace element XRF data are available from six cores. In addition, TOC, TIC, nitrogen and sulphur data is available for three cores.

Preliminary data show that the Barnett Formation from the cores is largely siliceous mudstone, based upon the Si/Al ratio. It is unclear at this point whether the silicon present is biogenic or in an inorganic mineral phase (e.g. quartz). Elevated concentrations of redox-sensitive trace elements including molybdenum, vanadium, chromium and uranium indicate that periods of euxinic (sulfidic) conditions existed during deposition of the formation. These conditions likely contributed to a high level of preservation for organic material. Total organic carbon (TOC) averages 4.3% for the three cores for which data are available. Major element (Ca) data for the cores show periodic changes in the provenance of material deposited in the basin. Major element data (Fe) for some of the cores show a thin volcanic ash layer that may be possible to use in correlation across the basin.