The Cody Shale is a
Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...
geologic
formation
Formation may refer to:
Linguistics
* Back-formation, the process of creating a new lexeme by removing or affixes
* Word formation, the creation of a new word by adding affixes
Mathematics and science
* Cave formation or speleothem, a secondar ...
. It is mapped in
Wyoming
Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the s ...
,
Idaho
Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...
, and
Montana
Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...
.
The formation is described by W.G. Pierce as follows: upper part is buff, sandy shale and thinly laminated buff sandstone; lower part is dark gray, thin-bedded marine shale.
[Pierce, W.G., 1997, Geologic map of the Cody 1 degree x 2 degrees quadrangle, northwestern Wyoming: U.S. Geological Survey, Miscellaneous Geologic Investigations Map I-2500, scale 1:250000.]
The formation is divided into many members that vary regionally. Alphabetically:
*Ardmore Bentonite Beds (WY)
*
Belle Fourche Member (MT, WY)
*
Carlile Member (MT, WY)
*Claggett Member (MT, WY)
*Eldridge Creek Member (MT)
*Gammon Ferruginous Member (MT, WY)
*
Greenhorn Calcareous Member (MT)
*
Niobrara Member (MT, WY)
*Sage Breaks Member (WY)
*Shannon Sandstone Member (MT, WY)
*Steele Member (WY)
*Sussex Sandstone Member (WY)
*Telegraph Creek Member (MT, WY)
*Wallace Creek Tongue (WY)
Certain members rise to formation rank in other areas; for example, the Greenhorn is classified as a formation in a number of states, particularly in Colorado and Kansas.
References
Shale formations of the United States
Cretaceous Montana
Cretaceous geology of Wyoming
{{US-geologic-formation-stub