Dakota Formation
The Dakota is a sedimentary geologic unit name of formation and group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and shales deposited in the Mid-Cretaceous opening of the Western Interior Seaway. The usage of the name Dakota for this particular Albian-Cenomanian strata is exceptionally widespread; from British Columbia and Alberta to Montana and Wisconsin to Colorado and Kansas to Utah and Arizona. It is famous for producing massive colorful rock formations in the Rocky Mountains and the Great Plains of the United States, and for preserving both dinosaur footprints and early deciduous tree leaves.
| Dakota Formation / Group | |
|---|---|
| Stratigraphic range: Cenomanian, around | |
Road cut into the lower Dakota Group at crest of Dinosaur Ridge, near Golden, Colorado | |
| Type | Geological formation or group |
| Sub-units | Type Location: Formation (Nebraska/Iowa): Woodbury Romeroville Sandstone Pajarito Formation Mesa Rica Sandstone |
| Underlies | Graneros Shale (sometimes included in the Colorado Group of the Great Plains or the Mancos Shale of the Southwest) |
| Overlies | Precambrian (Sioux Quartzite), Permian, Early Cretaceous (Lytle), and Jurassic (Morrison Formation) |
| Lithology | |
| Primary | varying proportions of terrestrial sandstone, mudstone, and clay, with beds of shallow marine mud |
| Other | marine shale, lignite, coal |
| Location | |
| Coordinates | 42.307°N 96.454°W |
| Approximate paleocoordinates | 38.9°N 66.4°W |
| Region | Great Plains, Rocky Mountains, Colorado Plateau, Rio Grande rift valley |
| Country | United States Canada |
| Type section | |
| Named for | Dakota City, Nebraska |
| Named by | Meek and Hayden |
| Year defined | 1862 |
Dakota Formation (the United States) Dakota Formation (Nebraska) | |
Owing to extensive weathering of older rocks during the Jurassic and Triassic, the Dakota strata lie unconformably atop many different formations ranging in age from Precambrian to Early Cretaceous. With a few local exceptions, it is the oldest Cretaceous unit exposed in the northern Great Plains, including Kansas, Nebraska, Iowa, Minnesota, and Wisconsin, as well as the Desert Southwest. It generally consists of sandy, shallow marine or beach deposits with marine-influenced mudflat sediments, and occasional stream deposits.