ABSTRACT

For more than three-quarters of a century, the classic Fort Union nonmarine molluscan fauna of F.B. Meek and F.V. Hayden served as the basis for correlating the Paleocene from the Dakotas, as part of the first standard reference section, to elsewhere in the Western Interior. The biostratigraphy and thus the precise age of this fauna has remained virtually undetermined. This situation has been exacerbated by conflicting opinions on the lithostratigraphy. This study provides a reinterpretation of the age of the upper Missouri River section designated by Meek and Hayden as the Fort Union Group, which includes the upper parts of the Bullion Creek (in North Dakota) and Tongue River (in Montana) Formations and the overlying lower part of the Sentinel Butte Formation. Studies of the mollusks and mammals in the Williston Basin have shown that this stratigraphic interval in the type area of the Fort Union Group is upper Paleocene, based on an age interpretation of a middle Tiffanian (Ti3 and Ti4) for the faunal succession. Study of many of the molluscan taxa from elsewhere in the northern plains demonstrates that they have relatively restricted temporal ranges, suggesting that previous correlations of the classic Fort Union fauna represented a biochronologic unit of much shorter duration than interpreted by earlier workers. Continuing studies at new localities along the upper Missouri River indicate that the composite assemblage representing the Fort Union fauna of Meek and Hayden may be divisible into an even more meaningful biostratigraphic zonation.