VOLCANIC-ASH MEDIATED NUTRIENT SUPPLY AND REDOX TRANSITION WITHIN THE CRETACEOUS NIOBRARA FORMATION, WESTERN INTERIOR SEAWAY, COLORADO

The Cretaceous Niobrara Formation is divided into the Fort Hays Limestone Member and Smoky Hill Shale Member based on an upward change in lithofacies from limestone to shale/marl and chalk. In contrast, a change in concentrations of redox sensitive trace elements and total organic carbon (TOC) is used to separate the Niobrara into two principal chemostratigraphic intervals that differ from traditional lithostratigraphy. Interval 1 contains the Fort Hays Limestone Member and the lowest chalk-marl or shale of the Smoky Hill Member, whereas Interval 2, contains the remaining marls and chalks that make up the Smoky Hill Member. Interval 1 is intensely bioturbated, has lower concentrations of TOC, U, Mo, and V, enhanced Mn concentrations, and comparatively low enrichment factors for Cu, Zn, U, V, and Mo, characteristics indicative of oxic conditions during deposition that precluded preservation of organic matter. Interval 2 has a concurrent increase in concentrations of redox sensitive trace elements Mo, V, and U, with TOC, and decreased concentration of Mn, inferring anoxic to sulfidic paleodepositional conditions that preserved organic matter. The redox transition between Interval 1 and 2 coincides with decreased bioturbation and heightened nutrient supply sustained in part by recurring volcanic ashfalls as evidenced by bentonite beds in Interval 2. Ash deposition occurred during sea level highstand that favored chalk deposition, and lowstand when marls were deposited. Sustenance of high productivity under reducing conditions through both marl and chalk deposition was likely facilitated by density stratification of interacting masses of colder northern water and warmer southern water in the Western Interior Seaway. Increased nutrients in Interval 2 boosted primary productivity, carbon production and export, and increased oxidant demand over a long duration of time (>3 Ma) that is coincident with Ocean Anoxic Event 3 (OAE 3), the youngest global anoxic event.