The Green River Formation is an Eocene geologic formation that records the sedimentation in a series of intermontaine lakes. The sedimentary layers were formed in a large area of interconnecting lakes, named for the current Green River, a tributary of the Colorado. The area of the formation exists as three separate basins around the Uinta Mountains of northeastern Utah. An area in northwestern Colorado east of the Uintas known as Fossil Lake, the larger area in the southeast corner of Wyoming just north of the Uintas known as Lake Gosiute, and the largest area which lies in northeastern Utah and western Colorado south of the Uintas known as Lake Uinta.
The lithologies of these landlocked lake sediments is varied and includes sandstones, mudstones, siltstones, oil shales, coal beds, saline evaporite beds, and a variety of lacustrine limestones and dolostones.
The trona beds of Sweetwater County, Wyoming are noted for a variety of rare evaporite minerals. It is the type locality for seven rare minerals: bradleyite, ewaldite, loughlinite, mckelveyite-(Y), [[norsethite], paralabuntsovite-Mg, and shortite as well as an occurrence of moissanite (SiC).
Fossil zones
Two distinct zones within the Green River formed of very fine grained lime muds are particularily noted for preserving a variety of complete and detailed fossils. These layers are an Eocene Lagerstätte , a rare place where conditions were right for a rich accumulation of fossils. The most productive zone is called the 18 inch layer consists of a series of laminated or varved lime muds containing abundant fish and other fossils and are easily split along the layers to reveal the fossils. This thin aone represents some 4000 years of deposition. The second fossil zone, the spilt fish layer is an unlaminated layer about six feet thick also contains abundant detailed fossils, but is harder to work because of the lack of fisility.
The limestone matrix is so fine-grained that fossils include rare soft parts of complete insects and fallen leaves in spectacular detail. More than 22 orders of insects are represented in the collection at the Smithsonian Institution in Washington, D.C. alone.
The Green River fossils date about 48 mya, but cover several million years, including the transition between the moist early Eocene climate and the slightly drier mid-Eocene. The climate was moist and mild enough to support crocodiles, which do not tolerate frost, and the lakes were surrounded by sycamore forests. As the lake configurations shifted, each Green River location is distinct in character and time. The lake system formed over underlying river deltas and shifted in the flat landscape with slight tectonic movements receiving sediments from the Uinta highland and the Rocky Mountains to the east and north. The lagerstätten formed in anoxic conditions in the fine carbonate muds that formed in the lakebeds. Lack of oxygen slowed bacterial decomposition and kept the usual scavengers away, so leaves of palms, ferns and sycamores, some showing the insect damage they had sustained during their growth, were covered with fine-grained sediment and preserved. Insects were preserved whole, even delicate wing membranes and spider spinnerets.
Vertebrates were preserved too, besides the scales of Boreasuchus, the crocodile that was an early clue to the mild Eocene climate of Western North America. Fish are common. The fossils of the herring-like Knightia, sometimes in dense layers, as if a school had wandered into anoxic water levels and were overcome, are familiar to fossil-lovers and are among the most commonly available fossils on the commercial market. Approximately 60 vertebrate taxa in all have been found at Green River. Besides fishes they include at least eleven species of reptiles, and some birds and one slothlike mammal, Brachianodon westorum,. with some scattered vertebrae of others, like the dog-sized Meniscotherium and Notharctus one of the first primates. There is the earliest known bat, already full-developed for flight. Even a snake, Boavus idelmani found its way into a lake and was preserved in the mudstone.
Discovery of the fossil beds
A Dr. John Evans collected and had described the first fossil fish from the Green River beds in 1856. The specimen was identified as Knightia eocaena. Edward Drinker Cope collected extensively from the area and produced a publication on the fossil fishes in 1871.
Literally millions of fish fossils have been collected from the area.
References
- Geologic Atlas of the Rocky Mountain Region, Rocky Mountain Association of Geologists, 1972, Denver Colorado
- Gaggiano, Tom, The Green River Formation http://www.fossilnews.com/2000/grnrv/grnrv.html Accessed March 18, 2006.
- Carrol, Alan, 2001, Green River research project, http://www.geology.wisc.edu/~carroll/green_river.html Accessed March 18, 2006.
- Minerals of the Green River Formation, Sweetwater County, Wyoming, Mindat
- Green River FormationUCMP Berkeley website
- Eocene fossils from the Green River Formation
- Fossil fish of the Green River Formation