Green-River-Formation

The Green River Formation is a rock association ( formation ) from the time of the Eocene in North America.

The Green River Formation is named after the Green River, a tributary of the Colorado River in the United States. It consists of the deposits originally interrelated in connection of large-scale mountain lakes. The deposition area of the formation consisted of three sub-basins, which encircle the located in the northeast of the State of Utah Uinta Mountains:

• An Eastern in Northwest Colorado
• The larger area Gosiute Lake basin - at the southwest corner of Wyoming, in the immediate northern edge of the Uintas
• As well as the all surpassing in size Lake Uinta Basin in northeastern Utah and western Colorado.

The Fossil Butte National Monument in Lincoln County, Wyoming, is located in an area of ​​the Green River Formation, which is called Fossil Lake - here an amazing number of extremely well-preserved fossilized fish was discovered.

Lithology and formation conditions

Cause of the formation of these inter- montane mountain lakes was further already taking place in the West during the Upper Cretaceous Sevier orogeny, and then the block-like rise of the Rocky Mountains during the mixing Lara orogeny in the Paleogene. This explains the taking place from all directions sediment in the subbasin - by the gradual emergence of the central Uintas the sediments were poured into north, east and south. From the north of the entry in the Green River Basin was carried out by the formation of the Wind River Mountains in western Wyoming. The Front Range, the Park Range and the Sawatch Range of the Colorado Rockies gave the subbasin their sediment load from the East. The arching of the Uncompahgre Plateau and the San Juan Mountains of sediments brought from the south. Further to the west finally lay the Wasatch Mountains of Utah and the mountain ranges of the eastern Idaho.

Lithologically these continental sediments are quite versatile - it change sandstones, mudstones, siltstones, oil shale, coal seams, saliferous Evaporitlagen well as a variety of lacustrine limestones and dolomites from each other. Intermediate volcanic ash layers from the north, near Yellowstone nearby Absaroka Volcanic Field and the southeastern San Juan Volcanic Field form datable reference positions within the sedimentary sequence.

The trona layers of Sweetwater County, Wyoming are known for their variety of rare Evaporitmineralen. They are type locality for seven rare minerals: Bradleyit, Ewaldit, Loughlinit, McKelveyit - (Y), Norsethit, Paralabuntsovit -Mg and Shortite. It can also Nahcolith, dawsonite and even moissanite ( SiC) are emerging.

Milankovitch cycles

The layer sequence of the Green River Formation has a strong Milankovitch cyclicity, the Earth's orbital parameters precession orbit, orbital inclination and eccentricity are clearly detectable. Therefore, the individual layer elements can be dated with great accuracy - the astro chronological results are in this case very well with the radiometric information is the same.

Fossil records

In fossil -prone area around Fossil Lake in southwestern Wyoming pits two very fine-grained Tonsteinlagen in appearance, containing a variety of preserved complete and detailed fossils. These layers form a fossil deposit of Eocene - otherwise only the very rare conditions to be met for the mass meeting of organisms and their undisturbed fossilization were given here. The most productive position, the so-called 18 inch layer (18 -inch layer ) consists of a sequence of laminated calcareous mudstones or gewarvten, which is extremely rich in fish and other animals. The clays can be easily split parallel to its stratification, then reveals their fossil content. But this relatively thin layer corresponds to a period of 4000 years sedimentation history. The second fossil location, called the split fish layer is a about two meters thick, unlaminated layer also contains tons of detail preserved fossils. But it is more difficult to work since it is compact.

The limestone matrix is so fine -grained that the fossils themselves reproduce still rare soft tissue completely preserved insects and fallen leaves detail. More than 22 insect orders are alone in the Green River collection of the Smithsonian Institution in Washington DC present.

The Green River fossils are about 48 million years old, but were deposited over a period of several million years away - documented case was the transition from the humid climate of the early Eocene to the slightly drier mid- Eocene. The climate was humid and subtropical hot back then, so even were crocodiles who can not tolerate frost, exist. The lakes were surrounded by plane trees forests, scattered palm fronds were discovered.

Due to the changing with time Seenkofigurationen each Green River archaeological site is unique in its character. Thus, the lakes in the relatively flat landscape could move away lightly with tectonic movements quite rapidly over former river deltas, the sediment came from the Uinta highland and the Rocky Mountains to the east and north.

The deposits were formed under anoxic conditions in the fine-grained lime mud of the lakes. Lack of oxygen slowed the bacterial decay, and lasted scavengers away, so that palm, fern and sycamore leaves were totally undisturbed and preserved in fine-grained sediment - a few leaves even still show the insect damage suffered during their growth. Insects were perfectly preserved, even delicate wing membranes of dragonflies and spider webs are still visible.

Also vertebrates were fossilized, including the scales of the crocodile Borealsuchus which äuf a first early indication the ruling during the Eocene in western North America mild climate provided. Fish are everywhere - Knightia, a simple count of the herring fish that sometimes occurs in such dense layers on, giving the impression as if a swarm penetrated in oxygen-poor areas and then died there. This species is well known in all fossil lovers and also commercially widespread on Fossil exchanges etc.. Among the fish finds there is also Heliobatis, an endemic freshwater stingrays.

A total of approximately sixty Wirbeltiertaxa were discovered in the Green River Formation. Besides the fish are represented:

• At least eleven species of reptiles ( including snakes Boavus idelmani )
• Some bird species, including the early bird geese Presbyornis
• Softshell turtles
• Brachianodon westorum, an armadillo -like mammal
• Meniscotherium the size of a dog (eddy residues)
• Notharctus, one of the first primates (eddy residues)
• The earliest known and already fully capable of flying bats, Icaronycteris index and Onychonycteris finneyi.

Discovery of the fossil deposit

Dr. John Evans had already collected in 1856 and described for the first time in the strata of the Green River Formation fish fossils. The Artzuweisung was as Knightia eocaena. Edward Drinker Cope also collected extensively in the area and brought in 1871 a publication on its fish finds out. Since then, millions of fish fossils have been discovered here.

Oil shale

The Green River Formation contains the largest deposits of oil shale worldwide. The amount of shale oil contained is estimated at 213 billion tons.