Morrison Formation | |
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Stratigraphic range: Upper Jurassic (Kimmeridgian to Tithonian), | |
Type | Geologic formation |
Underlies | Cedar Mountain Formation, Cloverly Formation, Lakota Formation, Burro Canyon Formation |
Overlies | Summerville Formation, Beclabito Formation, Curtis Formation, Bell Ranch Formation, Sundance Formation |
Thickness | Up to 200 m |
Lithology | |
Primary | Mudstone |
Other | Sandstone, siltstone, limestone |
Location | |
Coordinates | 39°39′04″N 105°11′17″W / 39.651°N 105.188°W |
Approximate paleocoordinates | 40°24′N 53°12′W / 40.4°N 53.2°W |
Region | Central North America: Arizona Colorado Idaho Kansas Montana Nebraska New Mexico North Dakota Oklahoma South Dakota Texas Utah Wyoming |
Country | United States[1] |
Extent | |
Type section | |
Named for | Morrison, Colorado |
The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone, and limestone and is light gray, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.
It is centered in Wyoming and Colorado, with outcrops in Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. Equivalent rocks under different names are found in Canada.[2] It covers an area of 1.5 million square kilometers (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east, and much of its western paleogeographic extent was eroded during exhumation of the Rocky Mountains.
It was named after Morrison, Colorado, where some of the first fossils in the formation were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope. In Colorado, New Mexico, and Utah, the Morrison Formation was a major source of uranium ore.