Correlation of Sentinel Butte Shale in Western North Dakota

New stratigraphic and paleontologic evidence indicates that the Sentinel Butte shale, lately considered by some geologists to be of Wasatch (Eocene) age, is, as originally described and assigned, a dark sequence of strata in the upper part of the Fort Union formation (Paleocene). A diagrammatic cross section illustrates the writer's correlation of formations and members in the Paleocene and Eocene series of eastern Wyoming, Montana, and the Dakotas.