Green River Formation

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Green River catchment area

The Green River Formation is a rock formation ( formation ) from the Eocene period 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 of originally connected large lakes. The formation's deposit area comprised several basins that girdled the Uinta Mountains in northeast Utah :

  • the Fossil Basin in southwest Wyoming , where Fossil Lake was originally formed
  • the larger Green River Basin and the Washakie Basin also in southwest Wyoming, on the immediate northern edge of the Uintas; here was Lake Gosiute
  • as well as the Uinta Basin in northeast Utah and the Piceance Creek Basin in northwest Colorado with the larger than all other Lake Uinta .

The Fossil Butte National Monument in Lincoln County, Wyoming, located in Fossil Basin. There are extremely well-preserved fossilized fish in very high numbers.

Lithology and educational conditions

In total, the Green River Formation covers more than 65,000 km² and is on average 600 m thick. In the eastern distribution area in the former Lake Uinta , it reaches its maximum thickness of over 2150 m. Lithologically , these continental lake sediments are quite diverse - sandstones , claystones , siltstones , oil shale , coal seams , salt-bearing evaporite layers and a multitude of limnic limestone and dolomites alternate with one another. Intermediate volcanic ash layers from the Absaroka Volcanic Field further north, near Yellowstone , and the San Juan Volcanic Field south-east form datable reference positions within the sedimentary sequence. The entire formation can be broken down into 14 subunits, six of which are located in Lake Uinta , five in Lake Gosiute and three in Fossil Lake . The sediment sequence is often interrupted by deposits of fluvial origin, which in the lower area can be attributed to the Wasatch Formation , in the upper area, however, to the Bridger Formation . They show that the lakes did not exist all the time, but fell dry at times. The reason for the formation of these intermontane mountain lakes was the Sevier mountain formation , which took place further to the west during the Upper Cretaceous , and then the block-like rise of the Rocky Mountains during the Laramian mountain formation in the Palaeogene . This is the reason for the sediment entering the sub-basins from all directions - the gradual emergence of the central Uintas caused the sediments to be poured in north, east and south. From the north, the entry into the Green River Basin took place through the emerging Wind River Mountains in western Wyoming. The Front Range , the Park Range and the Sawatch Range of the Colorado Rockies supplied the sub-basins with their sediment load from the east. The bulges of the Uncompahgre Plateau and the San Juan Mountains brought sediment deposits from the south. Finally, further to the west were the Wasatch Mountains of Utah and the ridges of eastern Idaho.

The Trona strata of Sweetwater County , Wyoming are known for their variety of rare evaporite minerals. They are type locality for seven rare minerals: Bradleyit , Ewaldit , Loughlinit , McKelveyit- (Y) , Norsethit , Paralabuntsovit-Mg and Shortite . It can also nahcolite , dawsonite and even moissanite (SiC) draw near.

Milanković cycles

The sequence of layers of the Green River Formation has a pronounced Milanković cyclicity , the orbital parameters of the earth's orbit, precession, orbital inclination and eccentricity can be clearly demonstrated. Therefore, the individual layers can be dated with great accuracy - the astro-chronological results agree very well with the radiometric data.

Fossil sites

The fossilized fish Diplomystus (left) and Knightia (right) from the Green River Formation

Fossil finds are known from large parts of the Green River Formation. Significant finds come from the Laney Member of Lake Gosiute and from the Parachute Creek Member of Lake Uinta . But the Fossil Butte Member in Fossil Lake is outstanding . Here two very fine-grain limestone layers appear, which contain a variety of fully and detailed fossils . These layers form a fossil deposit from the Eocene - the conditions for the mass encounters of organisms and their undisturbed fossilization, which otherwise only rarely had to be met, were given here. One of the two productive layers, the so-called 18 inch layer , consists of a sequence of laminated calcareous clay stones that are extremely rich in fish and other animals. The clay stones can easily be split parallel to their layers in order to reveal their fossil content. The second fossil layer , the so-called sandwich bed layer (also called split fish layer ), is an approximately two to four meter thick, also finely laminated layer that contains vast amounts of fossilized fossils.

The lime matrix is ​​so fine-grained that the fossils reproduce even the rare soft parts of completely preserved insects and fallen leaves in great detail. More than 22 orders of insects are present in the Smithsonian Institution's Green River Collection in Washington DC alone .

The perch-like Priscacara liops , about 11.5 cm in length

The Green River fossils from Fossil Butte, Laney and Parachute Creek Member are between 52 and 46 million years old, i.e. they were deposited over a period of several million years - the transition from the humid climate of the early Eocene was documented to the somewhat drier middle Eocene. The climate at that time was humid and subtropical warm, so that even crocodiles that cannot tolerate frost could exist. The lakes were framed by plane trees , and isolated palm fronds were discovered.

Finds of palm fronds are evidence of a subtropical climate

Due to the lake configurations changing over time, each Green River site is unique in its character. The lakes in the relatively flat landscape were able to move quickly over former river deltas by means of slight tectonic movements; the sediment load came from the Uinta highlands and the Rocky Mountains in the east and north.

The deposits formed under anoxic conditions in the fine-grained lime sludge of the lakes. A lack of oxygen slowed down bacterial decay and also kept scavengers away so that palm, fern and plane tree leaves could be embedded and preserved undisturbed in the fine-grained sediment - some leaves even show the insect damage suffered during their growth. Insects have been completely preserved, even the delicate wing membranes of dragonflies and cobwebs can still be seen.

Vertebrates were also fossilized, including the scales of the crocodile Borealsuchus , which provided a first early indication of the mild climate prevailing in western North America during the Eocene. Fish are omnipresent - Knightia , a fish belonging to the herrings, is sometimes found in such dense layers that it seems as if a school has penetrated into an oxygen-deprived area and then perished there. This species is well known to all fossil lovers and is also widely used commercially on fossil fairs, etc. Fish finds include Heliobatis , an endemic freshwater stingray .

In total, approximately sixty vertebrate taxa have been discovered in the Green River Formation. Besides the fish are represented:

Discovery of the fossil deposit

John Evans first collected and described fish fossils in the layers of the Green River Formation in 1856. The species was assigned as Knightia eocaena . Even Edward Drinker Cope collected extensively in this area and brought out in 1871 a publication on its fish finds. Since then, millions more of fish fossils have been discovered here.

Oil shale

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

See also

Individual evidence

  1. ^ A b H. Paul Buchheim, Robert A. Cushman Jr. and Roberto E. Biaggi: Stratigraphic revision of the Green River Formation in Fossil Basin, Wyoming: Overfilled to underfilled lake evolution. Rocky Mountain Geology 46 (2), 2011, pp. 165-181
  2. ^ A b M. Elliot Smith, Alan R. Carroll and Brad S. Singer: Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States. Geological Society of America Bulletin 120 (1/2), 2008, pp. 54-84
  3. ↑ a b Lance Grande: The lost world of Fossil Lake. Snapshot from deep time. University of Chicago Press, Chicago and London, 2013, pp. 1–425 ISBN 978-0-226-92296-6 (pp. 1–16)
  4. ^ Meyers, SR: Resolving Milankovitchian controversies: The Triassic Latemar Limestone and the Eocene Green River Formation . In: Geology . 36, No. 4, 2008, pp. 319-322. doi : 10.1130 / G24423A.1 . Retrieved April 24, 2008.
  5. ↑ USGS Scientific Investigations Report 2005–5294, Geology and Resources of Some World Oil-Shale Deposits (PDF; 2.4 MB)

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