Re-Evaluation of the Albian-Cenomanian Boundary in the U.S. Western Interior Based on Dinoflagellate Cysts

The position of the Albian-Cenomanian boundary in the U.S. Western Interior Basin has been the subject of debate because the ammonites and foraminifers that define the boundary are endemic. Traditionally, the boundary, as defined in Europe by planktonic foraminifers and ammonites, is correlated with the last occurrence of the ammonite genus, Neogastroplites [Reeside, J.B., Cobban, W.A., 1960. Studies of the Mowry Shale (Cretaceous) and contemporary formations in the United States and Canada. U.S. Geological Survey Professional Paper 355, 126 pp]. More recently, the boundary was correlated with the first occurrence of Metengonoceras teigenensis [Cobban, W.A., 1951. Colorado shale of central and northwestern Montana and equivalent rocks of Black Hills. American Association of Petroleum Geologists Bulletin 35, 2170-2198]. These ammonites are associated with bentonites, the ages of which have been extrapolated to the type region of France to date the base of the Cenomanian from the Western Interior Basin. However, since cosmopolitan dinoflagellates are common to this region and the European reference sections where the boundary is defined, they can be used to reevaluate the position of the Albian-Cenomanian boundary in the Western Interior Basin. In our study, 224 samples from 29 outcrop sections in Montana, Wyoming, Colorado, Oklahoma and New Mexico were analyzed for dinoflagellate cysts, as well as other palynomorphs, foraminifers, bivalves and ammonites; these fossils were used for graphic correlation. The recovery and preservation of the dinoflagellate cysts varied from poor to good, and diversity varied from low to moderate. Typical Late Albian to Early Cenomanian taxa, including Ovoidinium verrucosum, Ovoidinium scabrosum and Palaeohystrichophora infusorioides, dominate the assemblages; however, dinoflagellate ranges in the five sections in which the neogastroplitid zones are defined (Arrow Creek, Ayers Ranch, Belt Butte, Geyser, Teigen) suggest correlation with the uppermost Albian. Dinoflagellate ranges were confirmed in additional Montana, Wyoming and northern Colorado sections by a few diagnostic taxa (Aptea polymorpha, Apteodinium grande, Batioladinium jaegeri, Luxadinium propatulum, Chichaouadinium vestitum), and they were graphically correlated with published European ranges. The result is that the Albian-Cenomanian boundary correlates with the 97 million year old Clay Spur Bentonite