Green River Formation


Green River Formation

Infobox Rockunit
name = Green River Formation


caption =
type = Geological formation
prilithology = varied
otherlithology = see text
namedfor = Green River (Utah)
namedby =
region = Rocky Mountains
country = United States
coordinates =
unitof =
subunits =
thickness =
extent =
area =
age = Eocene, Ma|48
The Green River Formation is an Eocene geologic formation that records the sedimentation in a series of intermountain lakes. The sedimentary layers were formed in a large area of interconnecting lakes, named for the present-day Green River, a tributary of the Colorado River. 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, a larger area in the southwest 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". Fossil Butte National Monument in Lincoln County, Wyoming is located in a part of the formation known as "Fossil Lake" because of the abundance of exceptionally well preserved fish fossils found in the area.

Lithology and formation

The formation of the intermountain lake environment in the area during the Eocene was a result of the late Cretaceous Sevier orogeny to the west and the uplift of the Rocky Mountains during the Paleogene Laramide orogeny. The Eocene sedimentary basins thus received sediments from all directions. The Uinta uplift shed sediments north, east and south into the basins. The Wind River Mountains of west central Wyoming provided sediments from the north into the Green River basin. The Front Range, the Park Range and the Sawatch Range of the Colorado Rockies provided sediments into the basins from the east. The Uncompahgre geanticline and the San Juan Mountains provided sediments from the south. To the west were the Wasatch Mountains of Utah and the ranges of eastern Idaho.

The lithology 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. Volcanic ash layers within the various sediments from the active Absaroka Volcanic field to the north in the vicinity of Yellowstone and the San Juan volcanic field to the southeast provide dateable horizons within the sediments.

The trona (hydrated sodium bicarbonate carbonate) beds of Sweetwater County, Wyoming are noted for a variety of rare evaporite minerals. The beds are 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).

Cyclicity

The beds display a pronounced Milankovic cycleicity, with the precession, obliquity, and eccentricity orbital components all clearly detectable. This enables the beds to be internally dated with a high degree of accuracy, and astrochronological dates agree very well with radiometric dates.cite journal
author = Meyers, S.R.
year = 2008
title = Resolving Milankovitchian controversies: The Triassic Latemar Limestone and the Eocene Green River Formation
journal = Geology
volume = 36
issue = 4
pages = 319–322
url = http://www.unc.edu/~smeyers/pubs/Meyers_2008.pdf
accessdate = 2008-04-24
doi = 10.1130/G24423A.1
]

Fossil zones

d lime muds containing abundant fish and other fossils. These are easily split along the layers to reveal the fossils. This thin zone represents some 4000 years of deposition. The second fossil zone, the "split fish layer", is an unlaminated layer about six feet thick that also contains abundant detailed fossils, but is harder to work because it is not composed of fissile laminae.

The limestone matrix is so fine-grained that fossils include rare soft parts of complete insects and fallen leaves in spectacular detail. More than twenty-two orders of insects are represented in the Green River 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 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, including the scales of "Borealosuchus", 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. There was even an indigenous freshwater stingray, "Heliobatis". Approximately sixty 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 armadillo-like mammal, "Brachianodon westorum", with some scattered vertebrae of others, like the dog-sized "Meniscotherium" and "Notharctus", one of the first primates. The earliest known bats ("Icaronycteris index" and "Onychonycteris finneyi"), already full-developed for flight, are found here. 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.

Millions of fish fossils have been collected from the area.

Oil shale

The Green River Formation contains the largest oil shale deposits in the world. The 213 billion tons of oil shale contain an estimated 2.38 x 1011m3 (1.5 trillion US barrels) of shale oil. [ [http://pubs.usgs.gov/sir/2005/5294/pdf/sir5294_508.pdf USGS Scientific Investigations Report 2005–5294, "Geology and Resources of Some World Oil-Shale Deposits"] ]

ee also

* List of fossil sites "(with link directory)"

References

* "Geologic Atlas of the Rocky Mountain Region", Rocky Mountain Association of Geologists, 1972, Denver Colorado
* King, Philip B., 1977, "The Evolution of North America", Revised edition, Princeton University Press
*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.
* [http://www.mindat.org/loc-4235.html Minerals of the Green River Formation, Sweetwater County, Wyoming, Mindat]
* [http://www.ucmp.berkeley.edu/tertiary/eoc/greenriver.html Green River Formation] UCMP Berkeley website
* [http://www.coloradomtn.edu/campus_rfl/staff_rfl/kohls/eocene.shtml Eocene fossils from the Green River Formation]
* [http://www.fossilmuseum.net/Fossil_Galleries/GreenRiverFish.htm Fossil fish of the Green River Formation]
* [http://www4.ncsu.edu/~dtksepka/Green%20River%20Avifauna.html Fossil birds of the Green River Formation]
* [http://paleodb.org/cgi-bin/bridge.pl?action=displayCollectionDetails&collection_no=37809 Paleobiology Database: Icaronycteris Type Locality: Wasatchian, Wyoming, aka Fossil Lake, Green River]
* [http://paleodb.org/cgi-bin/bridge.pl?action=displayCollectionDetails&collection_no=49626 Paleobiology Database Green River Quarry: Kimmeridgian - Tithonian, Utah]
* [http://paleodb.org/cgi-bin/bridge.pl?action=displayCollectionDetails&collection_no=60584 Paleobiology Database: BYU Locality #712, Uintah Basin, Green River Formation, Utah: Eocene - Eocene, Utah]
* Green River Formation and Shale Oil, Research Brief by Rand [http://www.rand.org/pubs/research_briefs/RB9143/index1.html]


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