PALEOCEANOGRAPHY AND PALEOENVIRONMENTAL CHANGES OF THE CENOMANIAN/TURONIAN BOUNDARY INTERVAL (94-93 MA): THE RECORD OF OCEANIC ANOXIC EVENT 2 IN THE WESTERN INTERIOR SEA

Cenomanian/Turonian (C/T) boundary marine strata of the Late Cretaceous Western Interior Sea (WIS) exhibit the distinctive isotopic signature of OAE 2 in bulk carbonate and organic carbon. The C/T boundary GSSP at Rock Canyon, in central Colorado, is characterized by alternating beds of light-colored limestone and darker-colored marlstone and calcareous shale. These lithologic couplets have been related to Milankovitch orbital cyclicity. Foraminiferal assemblages from numerous sections spanning the width of the seaway from Utah to Iowa, and from the central axis of the seaway to the Gulf of Mexico, indicate major perturbations in paleoceanographic conditions associated with eustatic sea level rise and global warming, with abrupt changes in the WIS at the onset of OAE 2. Despite the global nature of OAE 2, the record in the WIS indicates regional geological and oceanographic imprints, including the influence of southern Tethyan and northern Boreal water masses. While the onset of OAE 2 is elsewhere marked by elevated levels of organic carbon, it begins as an oxygenation event in the WIS, as sea level rise overcomes a bathymetric threshold at the southern aperture of the seaway. The organic-rich strata associated with the peak of OAE 2 in the WIS have been attributed to either enhanced surface water productivity or oxygen depletion of bottom waterrs as a result of fresh water runoff from both sides of the seaway and water column stratification. Building on the original model of Slingerland (1996), we propose an oceanographic model that invokes a cyclonic gyre operating in two different modes (strong and weak) to explain the lateral and temporal lithofacies and biofacies variations. The weakening and strengthening of gyre circulation and changes in nutrient availability in the surface water result in deposition of organic-rich marlstone/shale and limestone beds, respectively. The correlaton of the sequnce of events associated with OAE 2 in the WIS to equivalent strata in the UK and in the Demerara Rise Basin of the tropical Atlantic reveals the global nature of these events and importance of the WIS as a north-south paleoceanographic corridor in Northern Hemisphere circulation. The WIS may have also exported intermediate waters to the proto-Gulf of Mexico, Caribbean, and western Tethys during the C/T boundary interval.