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 FROM THE MAVERICK BASIN, LOWER CRETACEOUS, SOUTH TEXAS

Paper number: T112-3
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

The lower Cretaceous Pearsall Formation of the Maverick Basin, South Texas, represents sedimentation in a mixed carbonate-siliciclastic system during Aptian time, potentially preserving paleoceanographic information during a segment of Oceanic Anoxic Event-1 (OAE-1). The objective of the 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, and stable carbon isotope analysis of organic and inorganic constituents.

Preliminary geochemical results from the Comanche Ranch #34 drill core, recovered from Maverick County, Texas, suggest that the Pearsall Formation can be divided into at least three discrete units based on its %Ca (calcite) content and/or variability. The lower ~120 feet of strata preserve highly variable calcite concentrations (<10% to 90% CaCO3). The middle interval (80-feet thick) is characterized by slightly lower overall %Ca concentrations (~35% CaCO3) and much lower variability. The upper zone (~50 feet thick) records increased %Ca variability with slightly lower concentrations. TOC samples for the Comanche Ranch #34 core are forthcoming and will be presented. Earlier work on more thermally mature Pearsall strata from Bee County (outside the Maverick Basin) indicates that TOC values range from <1% up to 3%. Interestingly, the presumably TOC-enriched strata do not possess significant redox-sensitive trace element (RSTE) enrichments (e.g., Mo and U). We will put forth several scenarios under which the high-TOC, low-RSTE conditions may be explained in the greater context of the regional paleoceanographic conditions.