STRONTIUM ISOTOPES IN CARBONATE FROM THE EAGLE FORD GROUP AT THE CENOMANIAN-TURONIAN BOUNDARY IN NORTH-CENTRAL TEXAS: A POSSIBLE PROXY FOR SEAWATER COMPOSITION

The Eagle Ford Group and associated Cenomanian-Turonian (C-T) strata in Texas have been estimated to contain 8.5 billion barrels of oil and 66 trillion cubic feet of gas in continuous accumulations in onshore lands of the U.S. Gulf Coast region (Whidden et al., USGS Fact Sheet 2018-3033, https://doi.org/10.3133/fs20183033). The Eagle Ford Group was deposited from ~96 to 90 Ma encompassing the time of Ocean Anoxic Event 2. In order to improve understanding of the stratigraphy and regional correlations, shallow drill core penetrating the Eagle Ford Group has been sampled from three locations. The only core studied to date, USGS Gulf Coast #1 (GC-1), was drilled near Waco, Texas in 2015. Mudstones and marls of the Eagle Ford Group and underlying Pepper Shale were cored at depths from 159 to 408 feet. Sampling targeted white or light-colored material, corresponding to shells, foraminifera accumulations, and micritic calcite layers.

Samples were removed from the core by sawing. Carbonate-rich material and shells were isolated by breaking the samples into small fragments using stainless-steel tools under a binocular microscope. Samples were leached with 1M acetic acid for 1 h at room temperature, followed by a second leach with 3M acetic acid for 3 days at 120 °C. Both leachates were decanted and analyzed for strontium isotope composition by thermal ionization mass spectrometry; the final residues, dissolved in a mixture of HF and HNO₃, were also analyzed for Sr isotopes.

The first leach contained more radiogenic Sr with higher ⁸⁷Sr/⁸⁶Sr than the second leach, and the residues were much more radiogenic than both leachates. This is consistent with the concept of the first leach removing adhered, easily dissolved carbonate that may be post-deposition; in this case, we think, the second leach is the best representation of the composition of parent seawater. The results on residues show that addition of Sr derived from silicates to carbonate leachates would lead to erroneously larger ⁸⁷Sr/⁸⁶Sr values. The analyses of the second leachate indicate decreasing ⁸⁷Sr/⁸⁶Sr across the C-T boundary, partially consistent with the seawater ⁸⁷Sr/⁸⁶Sr curve. Some samples from the upper Eagle Ford show larger values indicating either a shift in seawater ⁸⁷Sr/⁸⁶Sr or that these samples are more affected by diagenesis or silicate-derived Sr.