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Exploring the Origin of Carbonate Lenses Within Distal Delta Front Sediments of the Turonian Frontier Formation, Northern Uinta Basin, Utah


A series of discrete carbonate lenses were discovered in distal delta front siltstones and sandstones of the Frontier Formation of the northern Uinta Basin, Utah. As delta fronts are often locations of periodic sediment input pulses, in situ carbonate precipitation is anomalous. However, several studies of offshore marine siliciclastic facies have revealed the presence of localized carbonate accumulations (e.g. Tropic Shale carbonate nodules of SE Utah; Kiel et al. 2012, in Geology, 40: 839-842). Such deposits are often linked to microbial activity around methane seeps. In this study, we consider sedimentological relationships and apply petrographic observations and carbon isotope data to investigate the origin of the carbonate horizons, in particular whether or not their presence records methane seeps. In outcrop, carbonate lenses occur as thin, bedding-concordant, internally brecciated horizons > 1 m thick and 1 – 10 m long. These bodies ubiquitously occur within distal delta front facies consisting of interbedded siltstones and thin sandstones dominated internally by flat- to low-angle laminations and capped by current ripple cross-lamination (hyperpycnites). Cone-in-cone structures and bedding parallel, fibrous calcite veins (beef) are also observed within adjacent sandstone bodies. Samples were collected from several locations in the vicinity of Dinosaur National Monument (Utah and Colorado), which were subjected to standard petrographic and cathodoluminesence (CL) microscopy. In thin section, carbonate bodies are composed of botryoids of acicular needles and other fabrics suggestive of microbial involvement in precipitation. Intracrystalline brown residue is also commonplace and may be indicative of the presence of hydrocarbons. A bright red-orange CL character indicates high manganese concentrations, which implies mineral precipitation under reducing conditions. Additional carbon isotope composition data will provide insight into carbon sources and carbonate precipitation mechanisms. Carbonate precipitated during and as a consequence of sulfate reduction or methanogenesis is typically isotopically heterogeneous and characterized by variable and extreme dC values. The potential role of methane in carbonate precipitation contributes to a more complete understanding of the Frontier Formation as a total petroleum system (with source, reservoir, seal, and flow pathways) throughout the Uinta Basin region.