| Thermopolis Shale | |
|---|---|
| Stratigraphic range: Late Albian, ~ | |
| Type | Formation |
| Unit of | Colorado Group and Dakota Formation |
| Sub-units | Upper Thermopolis Member, Muddy Sandstone Member, Lower Thermopolis Member, "Rusty Beds" Member |
| Underlies | Mowry Shale |
| Overlies | Kootenai Formation (Cloverly Formation equivalent) |
| Thickness | 320 to 450 feet (100 to 140 m) |
| Lithology | |
| Primary | Shale |
| Other | Bentonite, Claystone, Lignite, Mudstone, Sandstone, Siltstone |
| Location | |
| Region | Western Interior Basin |
| Country | United States |
| Type section | |
| Named for | Thermopolis, Wyoming |
| Named by | Charles T. Lupton |
| Location | Big Horn County, Wyoming |
| Year defined | 1916 |
| Coordinates | 44°32′N 107°59′W / 44.53°N 107.99°W |
| Country | United States |
The Thermopolis Shale is a geologic formation which formed in west-central North America in the Albian age of the Late Cretaceous period. Surface outcroppings occur in central Canada, and the U.S. states of Montana and Wyoming. The rock formation was laid down over about 7 million years by sediment flowing into the Western Interior Seaway. The formation's boundaries and members are not well-defined by geologists, which has led to different definitions of the formation. Some geologists conclude the formation should not have a designation independent of the formations above and below it. A range of invertebrate and small and large vertebrate fossils and coprolites are found in the formation.