| Abstract: | The late Turonian-early Campanian Niobrara Cycle of the Western Interior Seaway is the longest second-order cyclothem in the Upper Cretaceous. Despite this fact, it has received far less study than the preceding Cenomanian-Turonian Greenhorn cycle. Samples were collected in a 680 m section of Mancos Shale at Mesa Verde National Park, Colorado. The base of the Niobrara Cycle is represented by the Juana Lopez Member of the Mancos Shale in Colorado. The Cortez Member and overlying Point Lookout Sandstone define the end of the cyclothem.
Three, third-order transgressive cycles are recognized within the Niobrara cyclothem spanning a period of ~9 m.y. These cycles correlate with those defined by Molenaar (1983) across the Colorado Plateau, based on lithofacies and ammonite biozones. The cycles are recognized by increases in %CaCO3 and less clearly in % total organic carbon (TOC). They are designated Transgression 2 (T2), Transgression 3 (T3), and Transgression 4 (T4) after Molenaar (1983). T2 is represented by the upper Turonian Juana Lopez Member at the Mesa Verde site, T3 by the middle-upper Coniacian Smoky Hill Member, and T4 by the upper Santonian part of the Cortez Member. T2 is indicative of peak transgression within the cyclothem. Molenaar's appointed T1 transgression correlates to the lower Turonian Bridge Creek Member of the Greenhorn Cycle at Mesa Verde. At least two of the three transgressions of the Niobrara cyclothem are also recognized in events in the Upper Cretaceous of Utah. The correlations allow for a regional comparison to be drawn between different localities within the seaway. These cyclic episodes can be analyzed using foraminiferal, geochemical, and mineralogical data to develop a detailed paleoenvironmental record for the Niobrara cyclothem.
High planktic:benthic ratios (p:b) mark the transgressions of the Niobrara Cyclothem. These ratios reflect the advance of warm Tethyan water masses into the seaway. In addition, increased abundances of benthic infaunal taxa such as Neobulimina also coincide with the Coniacian Smoky Hill transgression. The marked increase in this benthic taxon suggests that it may be adapted to low oxygen conditions, like modern buliminids, and this is further supported by its association with the elevated levels of TOC, indicative of dysaerobic bottom waters likely due to the incursion of an Oxygen Minimum Zone (OMZ). Dysoxic to anoxic conditions in the seaway are also indicated by foraminiferal-free zones in the transgressive upper Santonian part of the Cortez Member. Low p:b ratios mark the regressive phases. Dysoxia in the seaway was caused by the incursion of an oxygen minimum zone with rising sea level, or by enhanced salinity stratification, particularly during regression. |