Thermopolis Shale
| Thermopolis Shale Stratigraphic range: Late Albian, ~103–98.5 Ma |
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|---|---|
| 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.
The Western Interior Seaway was an inland sea that existed from the Late Jurassic (161.2 ± 4.0 to 145.5 ± 4.0 million years ago [Ma]) to the end of the Paleogene (66 to 23.03 Ma). It existed in the middle of North America, extending from the Arctic Ocean to the Gulf of Mexico. It was roughly 3,000 miles (4,800 km) long and 1,000 miles (1,600 km) wide. The seaway was relatively shallow, with a maximum depth estimated at 660 to 1,640 feet (200 to 500 m).
A foreland basin existed just to the east of the Sevier orogenic belt, which was inundated by the Western Interior Seaway. A forearc on the western side of the basin made this deeper than the eastern side, encouraging the build-up of sediment and, in time, sedimentary rock. Erosion of the Western Cordillera also contributed to the build-up of sedimentary rock on the western edge of the basin, while the more low-lying area to the east provided much less. Changes in the amount, type, rate, and other aspects of the sedimentation were caused by uplift, subsidence, sea level changes, and other factors. The water in the basin made at least two major advances and one major retreat during the Cretaceous, adding complexity to the rock and permitting the creation of riverine, marsh, and estuarine rock in addition to the principal shallow and deep marine rock.
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