ORBITAL TIME SCALES, OCEAN ANOXIC EVENTS, AND CONTROLS ON THE ACCUMULATION OF ORGANIC CARBON IN THE CRETACEOUS WESTERN INTERIOR SEAWAY

Hemipelagic deposits of the late Cenomanian-early Turonian Greenhorn Formation in the Western Interior basin, North America, include the Hartland Shale Member, an organic carbon-rich black shale, and the Bridge Creek Limestone Member, a sequence of rhythmic couplets of organic carbon-poor limestone and organic carbon-rich marlstone. Over the years debate about the origin of organic enrichment in these units has included productivity, preservation, and dilution arguments. Because the rhythmic deposits of the Bridge Creek Limestone preserve a record of orbital forcing that can be resolved by spectral techniques, it is possible to develop an extremely high resolution time scale for the interval; this time scale can be extended to include the Hartland Shale and applied to test hypotheses of production, preservation, and dilution. In this study we employ the high resolution time scale to calculate accumulation rates for selected proxies in the Hartland and Bridge Creek Members (e.g., TOC, CaCO₃, Mo, Ti, and biomarkers such as n-alkanes, steranes, hopanes). Our objective in this talk is to examine modal switches in the basin that represent fundamental changes in the ocean-climate-sediment transport system. There are several candidates for such changes: (1) shift from clay-dominated black shale to carbonate dominated limestone-marlstone at the transition between the two members; (2) shift to enhanced carbon burial during the later part of the global Oceanic Anoxic Event II (expressed in the basin as a positive shift in d¹³C_org); and (3) proposed increase in expression of obliquity in the upper Bridge Creek. Analysis of these changes using the tools of accumulation rates and compound specific data has significant potential for deconvolving the controls on organic carbon burial, and for improving understanding of the larger scale controls on paleoceanography in ancient epicontinental seas.