PETROGRAPHIC AND STABLE ISOTOPE ANALYSIS OF CARBONATES IN THE EAU CLAIRE FORMATION:

The Eau Claire Formation is the lowest member of the Upper Sauk Sequence within the Illinois Basin in the Midwestern United States. This unit is characterized by several different fabrics including carbonate-rich sandstones, siltstones, and shales. The composition, texture, and mineralogy of the Eau Claire are important to consider not only for understanding of the depositional and burial environments, but also because the Eau Claire is the primary seal for a major potential carbon capture and sequestration (CCS) system in the Midwestern United States. The Eau Claire contains abundant carbonates that comprise up to ~30% of the rock and includes: up to 26% calcite, up to 21% dolomite, and up to 14% ankerite. These carbonates include detrital shells and grains, authigenic crystals, and multiple textures of cements including micrites and spars. The non-carbonate mineralogy is mostly composed of quartz, k-feldspar, and illite/glauconite and to a lesser extent chlorite, plagioclase, and localized iron oxide. Preliminary data from core-derived samples in White Co., IN show carbonate isotopic values that range from -2.71 to -0.62 ‰ for δ¹³C (relative to PDB) and δ¹⁸O values of -9.37 to -7.44‰. These samples include texturally and isotopically distinct groups including 3 siltstone samples with isotopically heavier sparry cements, and 4 muddy siltstone samples with isotopically lighter micrite cements. Multiple analyses of different textures of carbonates from a single sample show variation of 0.56‰ for δ¹³C and 1.28‰ for δ¹⁸O. Overall, the samples show more stratigraphic variability in isotope composition than micro-scale textural variability, although these relationships are based on preliminary data and continue to be investigated in samples from multiple localities. The carbon and oxygen stable isotope composition of the carbonate grains and cements within the Eau Claire are used to identify related phases and to constrain the nature of the fluids that precipitated the carbonates. Fluids responsible for carbonate mineralization in the Eau Claire may be related to Mississippi Valley-type mineralization, as was seen in the correlative Bonneterre Formation, or may include Cambrian seawater, basinal brines, meteoric water, and/or multiple fluid/temperature mixing systems.