Green River Formation
| Green River Formation Stratigraphic range: Eocene, 53.5–48.5 Ma |
|
|---|---|
| Type | Geological formation |
| Lithology | |
| Primary | varied |
| Other | see text |
| Location | |
| Region | Rocky Mountains |
| Country | United States |
| Type section | |
| Named for | Green River |
The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in the dry season. Each pair of layers is called a varve and represents one year. The sediments of the Green River Formation present a continuous record of six million years. The mean thickness of a varve here is 0.18 mm, with a minimum thickness of 0.014 mm and maximum of 9.8 mm.
The sedimentary layers were formed in a large area named for the Green River, a tributary of the Colorado River. The three separate basins lie around the Uinta Mountains of northeastern Utah:
Fossil Butte National Monument in Lincoln County, Wyoming is in a part of the formation known as Fossil Lake because of its abundance of exceptionally well preserved fish fossils.
The formation of intermontane basin / lake environments during the Eocene resulted from mountain building and uplift of the Rocky Mountains (late Cretaceous Sevier orogeny and the Paleogene Laramide orogeny). Tectonic highlands supplied the Eocene sedimentary basins with sediment from all directions: the Uinta Mountains in the center; the Wind River Mountains to the north; the Front Range, Park Range and Sawatch Range of the Colorado Rockies to the east; the Uncompahgre Plateau and the San Juan Mountains to the south and finally, the Wasatch Mountains of Utah and the ranges of eastern Idaho to the west.
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