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A New Chronostratigraphic Framework for Eagle Ford- and Austin Chalk-equivalent Strata in west Texas: Implications for Basin Evolution and Diachroneity in Unconventional Reservoirs
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The Ernst Member within Big Bend National Park (BBNP), Brewster County, Texas provides unique opportunities for high-resolution chronostratigraphic study of Eagle Ford-equivalent strata across west Texas and northeastern Mexico, and is partially equivalent to the Eagle Ford Group in south and central Texas where it is a prolific, unconventional shale play. A new chronostratigraphic framework integrating three 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, hand-held spectral gamma ray (HHSGR), hand-held X-Ray Fluorescence (HHXRF) and δ¹³Ccarb measurements constraining (spatially and temporally) stratal surfaces, isotopic events and biozones in west Texas is presented. New ages for the Ernst Member type section in Ernst Tinaja, BBNP include 97.49 ± 0.12 Ma at 0.3 m (0.9 ft), 95.99 ± 0.15 Ma at 7.5 m (24.6 ft) and 91.16 ± 0.16 Ma at 52.1 m (171 ft) above the Buda Limestone/Ernst Member contact. In BBNP, the Ernst Member preserves neither the OAE2 positive carbon isotope excursion (CIE) nor the Cenomanian/Turonian (C/T) boundary. Chemostratigraphic analysis of U, Mo, V, Zr, Ti, Ca, Sr, K, Al and Si/Ti identify a low-TOC, carbonate-rich interval (0–4.8 m; 0–15 ft), a high-TOC interval (4.8–27.4 m; 15–90 ft), the OAE2 recovery period (27.4–33 m; 90–108 ft), Langtry Member-equivalent deposits (33–52.7 m; 108–173 ft) and, bounded by the Early Coniacian Allocrioceras hazzardi Zone above and bentonite age of 91.16 ± 0.16 Ma at 52.1 m (171 ft) below, a Late Turonian Austin Chalk-equivalent section (52.7–84.6 m; 173–277.5 ft) at Ernst Tinaja. Averaged correlation (COR)-derived sedimentation rate estimates range from 0.34–2.3 cm/kyr for Lower Eagle Ford-equivalent strata, 0.75–2.24 cm/kyr for Upper Eagle Ford-equivalent strata and 1.8–3.64 cm/kyr for Austin Chalk-equivalent strata.
Eagle Ford Group
Peavey, Eric James (2017). A New Chronostratigraphic Framework for Eagle Ford- and Austin Chalk-equivalent Strata in west Texas: Implications for Basin Evolution and Diachroneity in Unconventional Reservoirs. Master's thesis, Texas A & M University. Available electronically from