Burial of Authigenic Carbonate in Reducing Sediments of the Cenomanian/Turonian Western Interior Seaway

Early diagenetic carbonate cements (authigenic carbonate) have been hypothesized to form significant sinks for carbon during intervals of widespread oceanic anoxia. We examined rocks of the Eagle Ford Group in core (McMullen County, south Texas) and outcrop (Lozier Canyon and Antonio Creek, Terrell County, west Texas) to determine controls on authigenic carbonate formation. Rocks in both locations were deposited in shallow platform settings above storm wave base, with western samples showing evidence of greater current activity and occasional bioturbation under oxic conditions, particularly during the Turonian. Rocks from the south Texas core were deposited under almost uniformly anoxic conditions.

In outcrop, carbonate-rich lithologies occur as distinct beds interbedded with organic-rich shale, and as nodules or partially compacted nodular beds. Microscopically, carbonate occurs as foraminifer tests, inoceramid fragments, coarse void filling cements, disseminated silt-sized grains, and lenticular masses, with cements comprising 5-80% of rocks by volume. Foraminifer cavity fill defines a paragenetic sequence of early calcite followed by ±kaolinite, pyrite, and late calcite. This sequence is consistent with diagenesis under metal reducing to sulfate reducing conditions, occasionally in the presence of volcanic ash. Cements thus formed under a variety of pore fluid conditions likely controlled by anaerobic microbial metabolisms partially constrained by the redox state of the overlying water column.

Bulk carbonate carbon isotopic compositions record three negative excursions of up to 5‰ indicating early diagenetic incorporation of oxidized organic matter. Associated diagenetic Ba-rich celestine, SrSO₄, together with hydraulic lags of detrital celestine, indicate pervasive early dissolution of high-Sr carbonates in or around the sulfate reducing zone of the sediment column. Co-occurrence of negative isotopic excursions, celestine deposits, multiple ash beds, and apatite clast conglomerates and sandstones indicate episodes of overall sediment starvation. Authigenic carbonate burial was thus controlled primarily by anaerobic microbial processes, and constrained by the redox state of the water column and the duration of exchange with pore fluids.