SOURCE ROCK GENESIS AND PALEOREDOX CONDITIONS OF THE NIOBRARA FORMATION, MUSTARD HILL-GRAPE CREEK, CANON CITY, COLARADO

Organic carbon isotope (∂¹³C_org), total organic carbon (TOC) and elemental composition were measured for 175 outcrop samples of Niobrara Formation from Canon City, Colorado. The analyses were performed to understand the forcing functions behind enhanced organic carbon preservation and mechanisms responsible for sustained high organic carbon burial during deposition of the Niobrara Formation within the Cretaceous Western Interior Seaway (KWIS). Of particular interest is the interval containing the less studied ocean anoxic event (OAE 3). The ∂¹³C_org values from samples range from -29.62‰ to -24.61‰ relative to Vienna Pee Dee Bee while the TOC values ranges from 0.05% to 3.74%. Initial results indicate that organic matter is lacustrine/marine with high TOC intervals occurring almost exclusively within the Smokey Hill member. The Fort Hays Limestone member of the Niobrara Formation consists of bioturbated limestone that formed in oxygenated bottom water and minor shale beds that formed under lower oxygen conditions. These minor shale beds within the Fort Hays Member have higher TOC, as indicated by peaks on the TOC profile across the section. Increasing concentration of U, Th, V and Ba in the Smoky Hill Member correlate with higher TOC values and lower ∂¹³C values, which indicate changing ocean water chemistry and perhaps an OAE. Higher TOC values observed within upper Fort Hays and Smoky Hill suggest better preservation or high organic productivity. The OAE 3 corresponds to an interval with intermittently increasing TOC and corresponding enrichment of ∂¹³C_org within the Smokey Hill Member of the Niobrara Formation.