Green River Formation

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 Whyoming 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, Whyoming, 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 theRocky 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 Winds 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 dandstones, mudstones, siltstones, oil shales, coal beds, saline environment 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 San Juan volcanic field to the southeast provide dateable horizons within the sediments.

== Fossil Zones ==

Within the Green River Formation of southwest Wyoming in the area known as Fossil Lake, two distinct zones of very fine-grained lime muds are particularly 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 undisturbed fossils. The most productive zone—called the 18 inch layer— consists of a series of laminated or varved 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.

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.