T112 Marine Redox Evolution: Controls and Consequences (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

PRELIMINARY CHEMOSTRATIGRAPHY OF THE PEARSALL FORMATION, LOWER CRETACEOUS, LA SALLE COUNTY, SOUTH TEXAS

Paper number: T112-7
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Oceanic anoxic events (OAEs) of the Cretaceous represent unique episodes in Earth history. The Aptian-age Pearsall Formation of South Texas represents sedimentation in a mixed carbonate-siliciclastic system, and may preserve a record of paleoceanographic conditions during a portion of OAE-1. The objective of the current project is to develop and interpret stratigraphic changes in the geochemistry of the Pearsall Formation for the purposes of 1) understanding the paleoceanography of South Texas during a unique period in Earth history, 2) developing regional correlation using multiple cores, and 3) refining the stratigraphy for the oil and gas industry. Methods used to carry out the project include: real-time geochemical analysis using x-ray fluorescence (XRF) of the core face, TOC (total organic carbon) and TIC (total inorganic carbon) analysis, x-ray diffraction (XRD) of bulk samples and clay fractions, and stable carbon isotope analysis of organic and inorganic constituents.

Preliminary XRD results from the Tidewater Wilson Core, La Salle County, TX, indicate that the Pearsall Formation is dominated by quartz and calcite, with variable concentrations of gypsum, and generally minor concentrations of illite, kaolinite, albite, and pyrite. The gypsum content complicates interpretations that assume the overwhelming majority of the calcium is in the calcite phase---an assumption that can be made elsewhere in the Pearsall, and in the Cenomanian-Turonian Eagle Ford (middle-late Cretaceous of South Texas). Earlier studies of the Pearsall from a neighboring county (Bee) indicate that TOC values range from ~0.2% up to 3%. TOC results from the Tidewater core are forthcoming and will be presented, along with stable isotopic results from TOC and calcite. Notably, unlike some other TOC-enriched intervals, the Pearsall is not enriched in redox-sensitive trace elements (RSTEs). Further characterization and interpretation of the high-TOC, low-RSTE Pearsall will be undertaken.