Graneros Shale | |
---|---|
Stratigraphic range: | |
Type | Formation |
Unit of | Colorado Group (Great Plains) Benton Group (Colorado, South Dakota) Mancos Group (Colorado, New Mexico) |
Sub-units | See text |
Underlies | Greenhorn Formation |
Overlies | Dakota Formation |
Thickness | 114–1,000 feet (35–305 m) |
Lithology | |
Primary | Shale |
Other | Sandstone, limestone |
Location | |
Coordinates | 38°16′35″N 104°42′44″W / 38.2763°N 104.7121°W |
Region | Colorado, Iowa, Kansas, Minnesota, Montana, New Mexico, Nebraska, North Dakota, South Dakota, Wyoming |
Country | United States |
Type section | |
Named for | Graneros Creek, Walsenburg quadrangle, Pueblo Colorado |
Named by | G.K. Gilbert (from R.C. Hills) |
Year defined | 1896 |
The Graneros Shale is a geologic formation in the United States identified in the Great Plains as well as New Mexico that dates to the Cenomanian Age of the Cretaceous Period.[1] It is defined as the finely sandy argillaceous or clayey near-shore/marginal-marine shale that lies above the older, non-marine Dakota sand and mud, but below the younger, chalky open-marine shale of the Greenhorn. This definition was made in Colorado by G. K. Gilbert and has been adopted in other states that use Gilbert's division of the Benton's shales into Carlile, Greenhorn, and Graneros. These states include Kansas, Texas, Oklahoma, Nebraska, and New Mexico as well as corners of Minnesota and Iowa.[2][3] North Dakota, South Dakota, Wyoming, and Montana have somewhat different usages — in particular, north and west of the Black Hills, the same rock and fossil layer is named Belle Fourche Shale.[4][5]
The Graneros Shale was named by Gilbert (1896, p. 564) for 200 to 210 feet of argillaceous or clayey shale lying between the top of the Dakota and the base of the overlying Greenhorn Limestone." "[In northwestern Nebraska and around the Black Hills] the thicknesses are for the combined Mowry and Belle Fourche shales but, as noted above, only the Belle Fourche is lithologically equivalent to the Graneros.
The lithologic equivalent of the Graneros in the Black Hills and surrounding areas is the Belle Fourche Shale. Considerable similarity exists between these two formations, and, if the name Graneros is to be perpetuated in the Black Hills region, it would be best used to replace the name Belle Fourche over which the former has priority.