A NEW CHRONOSTRATIGRAPHIC FRAMEWORK FOR EXPLORING LATERAL VARIABILITY AND DIACHRONEITY OF EAGLE FORD- AND AUSTIN CHALK-EQUIVALENT STRATA IN WEST TEXAS

The Ernst Member of the Boquillas Formation of West Texas was deposited in shallow waters (above storm wave base) near the Tethyan aperture of the Cretaceous Western Interior Seaway (KWIS) immediately following regionwide subaerial exposure of the Buda Limestone, and has partial lateral equivalence to both the Eagle Ford Group and overlying Austin Chalk of south and central Texas (where the Eagle Ford Group is a prolific, unconventional shale play). The Ernst Member type section at Ernst Tinaja, Big Bend National Park (BBNP), Texas is an important reference section for understanding stratigraphic relations between Eagle Ford Group- and Austin Chalk-equivalent strata in West Texas and northern Mexico.

A new chronostratigraphic framework integrating four U-Pb Chemical Abrasion-Isotope Dilution Thermal Ionization Mass Spectrometry (CA-IDTIMS) bentonite ages from the Ernst Member and six bentonite ages from the Eagle Ford Group at Lozier Canyon with biostratigraphic proxies (ammonites, inoceramids, foraminifera and calcareous nannofossils), hand-held spectral gamma ray (HHSGR), hand-held X-Ray Fluorescence (HHXRF) and δ¹³C_carb measurements constraining (spatially and temporally) stratal surfaces, isotopic events and biozones in West Texas is presented. In BBNP, the Ernst Member preserves neither the OAE2 positive carbon isotope excursion nor the Cenomanian/Turonian boundary. Chemostratigraphic analysis of U, Mo, V, Zr, Ti, Ca, Sr, K, Al and Si/Ti identify carbonate and TOC-rich intervals whereas biostratigraphic proxies and CA-IDTIMS bentonite ages, in conjunction with lithostratigraphic data, delineate a Late Turonian Austin Chalk-equivalent section (52.7-84.6 m; 173-277.5 ft) at Ernst Tinaja, BBNP.