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Late Ordovician Stable Carbon Isotope Values of the Red River Fm., Williston Basin: Local Environmental Effects and Diagenetic Overprints


The upper Katian Red River Formation is an overall shallowing (and “brining”) upward supersequence reaching a thickness of more than ~200 meters in the epicratonic Williston Basin depocenter in North Dakota. This study is based on a detailed bed-by-bed analysis of three thickest and stratigraphically most complete cores of formation in North Dakota. Bulk carbonate powders of 223 limestone and dolomite samples collected at 30–90 cm intervals were analyzed for stable carbon isotopes. Forty thin-sections representative of major facies from the studied cores were petrographically analyzed, and twelve non-covered thin-sections were examined using cathodoluminescence microscopy. Carbon-isotope values of shallow-marine Red River carbonates vary from -3‰ to +1.7‰ in limestone, and from -0.7‰ to +2‰ in dolomite; such similar values suggest that carbon was rock buffered, and could be representing the original marine δ13C record. The δ13C trend can be subdivided into eight stages characterized by three major excursions to positive δ13C values that can be correlated across the Williston Basin. The first excursion with an amplitude of up to+2‰ occurs in C laminated member. The second excursion reaches amplitude of +2‰ within the B burrowed member. Both excursions occur within the upper Katian Aphelognathus divergens conodont Zone. The third excursion with an amplitude of +2‰ to +4‰ occurs in the middle part of the A interval. The different overall trend and absolute δ13C values between the Williston Basin and the Cincinnati Region suggest a lack of exchange between waters of the Williston Basin and the Midcontinent, and consequently the influence of different regional environmental processes characteristic of a large, periodically isolated Williston Basin, coupled with a different diagenetic histories. Fine crystal size, lack of zoning, and non-obliterated porosity in the dolomites sampled for stable isotopes, coupled with their intimate association with the overlying evaporites, suggests a hypersaline-brine, early replacive origin for the bulk of the dolomite. Dark orange to red luminescence of both laminated and burrowed dolomites suggests their similar dolomitization history, and diagenetic stabilization under reducing conditions. The investigated interval in the upper part of the Red River Formation in North Dakota provides one of the most complete upper Katian (post-Waynesville excursion and pre-Hirnatian excursion) δ13C records available in North America.