ALBIAN/CENOMANIAN INTEGRATED BIOSTRATIGRAPHY FROM TEXAS TO WESTERN INTERIOR: NUMERICAL AGE CALIBRATION

The Albian/Cenomanian Stage boundary is defined at its GSSP by ammonites and the base of the planktic foraminifer, Rotalipora globotruncanoides. Its numerical age, however, is calibrated by bentonites in the U.S. Western Interior where these fossils are absent. Proxy endemic ammonites of Neogastroplites and Metengonceras have been used in place of the cosmopolitan species. Because of this uncertainty the numerical age calibration of this boundary shifted from 97.0 Ma to 98.9 Ma. This calibration was supported by radiometric dates from tuff beds with basal Cenomanian ammonites and planktic foraminifers in Japan.

Two lines of evidence test this age calibration, cosmopolitan marine dinoflagellate cysts and sequence stratigraphy between Texas and Wyoming. Dinoflagellates are common to both the WI and Europe. The key sections used in setting up the WI ammonite zones were re-described and re-sampled to define the dinoflagellate ranges in central Montana and Wyoming. This new database of dinoflagellates integrated with ammonites, foraminifers, and radiolaria supports the original correlation of the Clay Spur Bentonite near the Albian/Cenomanian boundary as defined in France. The FAD of Late Albian dinocysts, Ovoidinium verrucosum, Epelidosphaeridia spinosa, and Ovoidinium scabrosum, occur with the oldest neogastroplitid species in the Shell Creek Shale. The LAD of latest Albian dinocysts, Apteodinium grande, Batioladinium jaegeri, and Ovoidinium scabrosum, Luxadinium propatulum, and Chichaouadinium vestitum, are in the uppermost Mowry Shale with the youngest neogastroplitids and in the basal Belle Fourche Shale.

Three Late Albian Tethyan floodings from Texas into Wyoming deposited three regional sequences that correlate with cycles in the Washita Group in north Texas. There Tethyan ammonites define the Albian-Cenomanian boundary as it is in its GSSP. The three sequences record biofacies shifts of over 200 km. The Clay Spur Bentonite dated at 97.16±0.67 Ma caps the youngest sequence. This sequence correlates with the Stoliczkaia dispar Zone in Texas Washita sequence 5 and is overlain by basal Cenomanian ammonites.