Green River Formation

Green River Formation
Stratigraphic range: Eocene, 53.5–48.5 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Green River Formation exposed in the cliffs at Fossil Butte
Type	Geological formation
Lithology
Primary	varied
Other	see text
Location
Region	Rocky Mountains:  Colorado  Utah  Wyoming
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.^[1]

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 (north, east, and south) of northeastern Utah:

• an area in northwestern Colorado east of the Uintas
• a larger area in the southwest corner of Wyoming just north of the Uintas known as Lake Gosiute
• the largest area, in northeastern Utah and western Colorado south of the Uintas, known as Lake Uinta

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.