TESTING THE ASTRONOMICAL TIME SCALE FOR OCEANIC ANOXIC EVENT 2, AND ITS EXTENSION INTO CENOMANIAN STRATA OF THE WESTERN INTERIOR BASIN (U.S.A.)

The Cenomanian/Turonian Boundary interval (CTB; ~94 Ma) is characterized by widespread deposition of organic-carbon rich strata and a carbon isotope excursion of ~4 permil. It is also associated with the extinction of a variety of molluscs, planktic foraminifera and nannoplankton. This major perturbation to the Earth System, known as Oceanic Anoxic Event 2 (OAE 2), has been the focus of more than three decades of scientific inquiry, resulting in the accumulation of a tremendous array of geochemical, paleobiologic and sedimentologic data from sites spanning the deep ocean to epicontinental seaways. High-resolution time scales for the OAE 2 are required to estimate meaningful rates of biogeochemical and paleobiologic change from such datasets, and to test hypotheses for the paleoceanographic mechanisms that underlie the event. Previous work developed an astronomical time scale for the OAE 2 in the Western Interior Basin (WIB), near the C/T GSSP in Pueblo, Colorado, where a rich radioisotopic, biostratigraphic and chemostratigraphic framework exists within the rhythmically-bedded Bridge Creek Limestone Member. The purpose of the present study is (1) to test the veracity of the published CTB astrochronology at another location within the WIB, (2) to extend the astrochronology into the earlier Cenomanian, if possible, and (3) to integrate the orbital time scale with published and emerging radioisotopic and chemostratigraphic data from the WIB. In contrast to previous cyclostratigraphic work, which relied on the analysis of a grayscale record (a proxy for carbonate content), this study utilizes X-ray fluorescence (XRF) core scanning to generate high-resolution elemental data that record climate-sensitive lithogenic (Al, Si, Ti, K), biogenic (Ca), and authigenic (S, Fe/Al) processes. In addition to its cyclostratigraphic application, the new data set will provide the first continuous (5 mm resolution) analysis of a wide suite of biogeochemical proxies through the OAE 2 interval in the WIB.