Organic carbon production and preservation in response to sea-level changes in the Turonian Carlile Formation, U.S. Western Interior Basin

A primary sea-level control over the distribution of total organic and carbonate carbon and organic matter type can be inferred in the early to middle Turonian Carlile Formation (Fm.), Western Interior Basin, United States. The conceptual model relies on chemo- and lithostratigraphic correlations of lower to mid-Turonian strata in the central KWIS, supported by ammonite biostratigraphy, and is based primarily on lithologic, gamma-ray spectrometric, and geochemical facies analysis of the USGS Portland No. 1 Core from central Colorado, the Amoco Rebecca Bounds No. 1 Core from western Kansas, and the Hawarden Core from northwestern Iowa. Sedimentation in the central marine axial basin of the Cretaceous Western Interior Seaway (KWIS) during the Turonian mostly reflects deposition by pelagic settling and from nepheloid layers with winnowing by bottom currents. Relatively high % total organic carbon (TOC), % carbonate (CaCO₃) and Rock-Eval pyrolysis hydrogen index (HI) values correspond to transgressive or highstand episodes within the overall regressive sequence, whereas low values of these parameters characterize regressive intervals. The lower Fairport Shale Member of the Carlile Fm and coeval strata in Iowa were deposited during a second-order sea-level highstand, the waning stages of the Greenhorn cyclothem. An overall shallowing- and coarsening-upward sequence characterizes the overlying majority of the Carlile Fm. This trend is punctuated by a short-term transgressive episode with associated retrograde facies and a disconformity. Earlier studies document relatively high productivity during the Turonian. Nutrient input to the seaway, required to sustain water-column productivity, is difficult to account for solely by riverine inputs; thus, a model of transgressive flooding of preconditioned, oxygen-deficient, nutrient-rich water from the global ocean into the KWIS is invoked. This advection of nutrients and low-oxygen water also helped to create broadly distributed dysoxic to anoxic conditions in the seaway, which would otherwise have been difficult to maintain in a relatively well-mixed, shallow sea. As the seaway regressed, river-supplied sea-surface nepheloid layers provided sufficient nutrient inputs and occasionally established temporary stratification of the water column, and thus contributed to maintaining an environment poised to produce and preserve organic matter.