Abstract

Planktonic and benthonic foraminifera from two sections of the Hilliard Formation, one in the type area near Kemmerer, Wyoming and the other in the Snider Basin have been examined and tentatively correlated with foraminifera from two Niobrara sections, one in Centennial Valley near Laramie, Wyoming and the other near Pueblo, Colorado. The evidence from these sections suggest that the Niobrara and Hilliard Formations are essentially time equivalent and range in age from late Turonian to late Campanian, and probably early Maestrichtian. Correlations are based on appearances and extinctions of planktonic foraminifera and environmentally controlled migrations of planktonic foraminifera such as occur during major transgressions. These techniques have illustrated that planktonic foraminifera can be extremely useful for correlation of the Cretaceous of the Western Interior and have raised some questions concerning previous correlations.