Fort Union Formation

The Fort Union Formation is a geologic unit containing sandstones, shales, and coal beds in Wyoming, Montana, and parts of adjacent states. In the Powder River Basin, it contains important economic deposits of coal, uranium, and coalbed methane.

The Fort Union is mostly of Paleocene age and represents a time of extensive swamps as well as fluvial and lacustrine conditions. The rocks are more sandy in southwestern Wyoming and more coal-bearing in northeast Wyoming and southeast Montana, reflecting a general change from rivers and lakes in the west to swamps in the east, but all three environments were present at various times in most locations.

Coal in the Fort Union in the Powder River Basin occurs mainly in the Tongue River Member, where as many as 32 coal seams total more than 300 feet in thickness. One such bed, the Wyodak Coal near Gillette, Wyoming, is as much as 110 feet (34 m) thick. Most of the coals in the Fort Union Formation are ranked subbituminous.

A partial ornithurine coracoid bone found in this formation is identical to others found in the older Hell Creek Formation. At present, this unnamed species is the only known individual bird species that have survived the Cretaceous–Paleogene extinction event.

• List of fossiliferous stratigraphic units in Colorado
• List of fossiliferous stratigraphic units in Montana
• List of fossiliferous stratigraphic units in North Dakota
• List of fossiliferous stratigraphic units in Wyoming
• Uranium mining in Wyoming
• Guaduas Formation, contemporaneous coal-bearing formation of northern South America