ORGANIC MATTER PRODUCTION AND PRESERVATION DURING T-R CYCLES OF THE SMOKY HILL MBR. (MIDDLE CONIACIAN-MIDDLE SANTONIAN), MANCOS SHALE, CRETACEOUS WESTERN INTERIOR SEAWAY

The Upper Cretaceous Niobrara cyclothem (upper Turonian-lower Campanian) is a major transgressive-regressive cycle of the Western Interior Seaway reflecting interactions among eustatic sea level change, regional tectonic events, and sediment supply in an actively subsiding foreland basin. These strata provide a unique window into sediment deposition and paleoceaonographic conditions in an epicontinental seaway. The response of organic matter production and burial to these forcings is presented here. Geochemical analyses of the Smoky Hill and basal Cortez Members of the Mancos Shale at its principal reference section at Mesa Verde N.P. CO, reveal relationships among organic matter abundance and composition, paleoceanographic conditions inferred from macrofossils, and sea level change.

At Mesa Verde, the Smoky Hill is broadly characterized as dark-gray, foraminifer-rich calcareous shale, mudstone, and marlstone. Bulk geochemical properties, including %TOC, %CaCO₃, and C/N, reflect changes in organic matter delivery and preservation, and are closely correlated to inferred water-depth and/or distance from shore. Proximity to the western paleo-shore appears to exercise a primary control over the sedimentary elemental and isotopic composition with secondary influence from redox-sensitive diagenetic processes and autochthonous microbial production, which in turn may reflect higher-order sea-level fluctuations.

The Smoky Hill-basal Cortez sequence records two higher-order transgressive cycles of carbonate-rich mud correlative with the lower limestone (upper Coniacian) and middle chalk (middle Santonian) of the Smoky Hill Mbr., Niobrara Formation, along Colorado Front Range, and the C and B chalks, respectively, in the D-J Basin. Each transgressive pulse is recorded by a distinctive 2-part signature of: 1) peaks of TOC and C/N with very low molluscan abundance and diversity, interpreted as incursions of an oxygen minimum zone during transgression, followed by 2) a sharp decrease in TOC, marked increase in molluscan diversity, development of calcarenites and/or an ino-oyster biostrome, and peak d¹³C_org values, interpreted as maximum flooding surfaces. A fossil-poor sandy interval in the lower Santonian separates the two transgressive intervals (T3, R3, and T4 of Molenaar, 1983).