CENOMANIAN TO CAMPANIAN CARBON ISOTOPE CHEMOSTRATIGRAPHY FROM THE WESTERN INTERIOR BASIN, USA

Late Cretaceous carbon isotope curves generated for localities in Europe, Asia, and the deep-sea records have provided the foundation for development of a contiguous, intercontinental chemostratigraphic framework. Despite the development of carbon isotope records from selected stratigraphic intervals in the Western Interior Basin, however, a comprehensive delta C-13 record comparable to those developed in Europe and Asia, and from ODP sites, had not yet been completed. This study reports a new, high-resolution Cenomanian to Campanian carbon isotope record for the central Western Interior Basin of North America that makes a key contribution to the intercontinental correlation. The curve is correlated to the well-developed molluscan biostratigraphic framework for the Western Interior Basin, as well as a revised geochronology based on integration of new radioisotopic dating and astrochronology developed in the same core records from where the delta C-13 data were derived. The new delta C-13 record reflects not only major perturbations in the global carbon cycle, such as the mid-Cenomanian Event and the Cenomanian-Turonian Ocean Anoxic Event 2, but also smaller excursions that have been recognized in records from Europe and Asia. This shows that the Western Interior shallow epeiric sea was not isolated for any appreciable part of Cenomanian to early Campanian time-it consistently recorded changes in the global marine carbon cycle observed elsewhere. A critical attribute of the chemostratigraphic dataset produced in this study is its linkage with a revised Late Cretaceous time scale. Assignment of revised ages to Western Interior carbon isotope events that are globally expressed will allow export of the improved time scale to localities within and outside the Western Interior where similar carbon isotope records have been generated. This chronostratigraphic tool will allow a number of stratigraphic and geochemical hypotheses to be more rigorously tested.