| Benton Shale | |
|---|---|
| Stratigraphic range: Cretaceous | |
.jpg) Shale bluffs at Cow Island, Montana in the Missouri River Breaks adjoining Fort Benton, the type location of the Benton Shale. As seen in this image, the overlying Niobrara Formation is not as distinct from the Benton Shale as in the lower plains and these shales were later reclassified as Colorado Shale. The Benton name was abandoned here in favor of subdivision as Belle Fourche Shale, Greenhorn Limestone, and Carlile Shale. | |
| Type | Formation |
| Underlies | Niobrara Formation |
| Overlies | Dakota Sandstone |
| Lithology | |
| Primary | Shale, chalky shale, chalk beds |
| Other | many bentonite seams, septarians, selenite, occasional sandstone |
| Location | |
| Region | Montana, Wyoming, North Dakota, South Dakota, Colorado, Nebraska, Kansas |
| Country | United States |
| Type section | |
| Named for | Fort Benton, Montana |
| Named by | Meek, F.B. and Hayden, F.V. |
| Year defined | 1862 |
The Benton Shale (also Benton Formation or Benton Group) is a geologic formation name historically used in Montana, Wyoming, North Dakota, South Dakota, Colorado, Kansas, and Nebraska.[1] In the "mile high" plains in the center of the continent, the named layers preserve marine fossils from the Late Cretaceous Period. The term Benton Limestone has also been used to refer to the chalky portions of the strata, especially the beds of the strata presently classified as Greenhorn Limestone, particularly the Fencepost limestone.