Graneros Shale
The Graneros Shale is a geologic formation in the western United States, east of the Rocky Mountains.[1][2] It preserves fossils dating back to the Cenomanian Age of the Cretaceous period.[3]
| Graneros Shale Stratigraphic range: Cenomanian | |
|---|---|
 Graneros Shale at outlet of El Vado Reservoir, New Mexico, USA | |
| Type | Formation |
| 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 | 38.2763°N 104.7121°W |
| Region | CO,IA,KS,MN,MT,NM,NE,ND,OK,SD,WY |
| Country |  United States |
| Type section | |
| Named for | Graneros Creek, Walsenburg quadrangle, Pueblo Colorado |
| Named by | R.C.Hills |
| Year defined | 1896 |
  Graneros Shale (the United States)  Graneros Shale (Colorado) | |
Description
The Graneros Shale is primarily dark gray shale with minor sandstone and limestone. It rests on the Dakota Group and is in turn overlain by the Greenhorn Limestone.[4][5] The thickness varies from 114–1,000 feet (35–305 m).[3][6]
The northern part of the formation has been divided into (in ascending stratigraphic order) the Skull Creek shale, the Newcastle sandstone, the Nefsy shale, the Mowry shale, and the Belle Fourche shale.[6] The southern part is divided into (in ascending stratighraphic order) a lower barren member, the Thatcher Limestone Member, an upper fossiliferous member, a marker bentonite bed, and an Ostrea beloiti bed. Ammonites from the Thatcher Limestone Member are indicative of Cenomanian age.[3]
Fossils
The upper part of the formation contains abundant oyster fossils[3] and the ammonoids Tarrantoceras sellardsi Adkins, Desmoceras, Anthoceras, and Borissiakoceras. The mollusks Inoceramus rutherfordi Warren, Ostrea beloiti Logan, and Turrilites acutus americanus are also present. Fossils become more scarce in Wyoming. [7]
History of investigation
The unit was first named by R.C. Hills in 1896 for exposures in eastern Colorado.[4] By 1938 it had been mapped into eastern Wyoming, southeastern Montana, South Dakota, Nebraska, Kansas, and northeastern New Mexico. [8]
References
- Siliciclastic and calcareous sedimentary rocks of early Late Cretaceous age in the Western Interior of the United States have been assigned to, in ascending order, the Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and their lateral equivalents (including members of the Frontier Formation and overlying formations). Merewether, E. Allen; Cobban, William A.; Obradovich, John D. (2007). "Regional disconformities in Turonian and Coniacian (Upper Cretaceous) strata in Colorado, Wyoming, and adjoining states - Biochronological evidence". Rocky Mountain Geology. 42 (2): 95–122. doi:10.2113/gsrocky.42.2.95.
- "Geologic Unit: Graneros". National Geologic Map Database: Geolex. United States Geologic Survey.
- Cobban, W.A.; Scott, G.R. (1972). "Stratigraphy and ammonite fauna of the Graneros Shale and Greenhorn Limestone near Pueblo, Colorado". U.S. Geological Survey Professional Paper. Professional Paper. 645. doi:10.3133/pp645. Retrieved 3 September 2020.
- Gilbert, G.K. (1896). "The underground water of the Arkansas Valley in eastern Colorado". U.S. Geological Survey Annual Report. 17 (2): 551–601. Retrieved 3 September 2020.
- Ziegler, Kate E.; Ramos, Frank C.; Zimmerer, Matthew J. (2019). "Geology of Northeastern New Mexico, union and Colfax Counties, New Mexico: A Geologic Summary" (PDF). New Mexico Geological Society Field Conference Series. 70 (4): 47–54. Retrieved 1 September 2020.
- Collier, A.J. (1922). "The Osage oil field, Weston County, Wyoming". U.S. Geological Survey Bulletin. 736-D: 71–110.
- Cobban, W.A. (1988). "Professional Paper". USGS Professional Paper. 1473. doi:10.3133/pp1473. Retrieved 3 September 2020.
- Wilmarth, M.G. (1938). "Lexicon of geologic names of the United States (including Alaska)". U.S. Geological Survey Bulletin. 896 (1–2). Retrieved 3 September 2020.