Half lie radiometric dating
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 trona (hydrated sodium bicarbonate carbonate) beds of Sweetwater County, Wyoming are noted for a variety of rare evaporite minerals.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. Fish fossils of Diplomystus and Knightia are found in Fossil Lake but not in Lake Gosiute.Tectonic highlands supplied the Eocene sedimentary basins with sediment from all directions: the Uinta Mountains in the center; the Wind River Mountains to the north; the Front Range, Park Range and Sawatch Range of the Colorado Rockies to the east; the Uncompahgre Plateau and the San Juan Mountains to the south and finally, the Wasatch Mountains of Utah and the ranges of eastern Idaho to the west.The lithology of the 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 most productive zone—called the split fish layer—consists of a series of laminated or varved lime muds about 6 ft (1.8 m) thick, which contains abundant fish and other fossils.
These are easily split along the layers to reveal the fossils.
The beds display a pronounced cyclicity, 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.
Only Lake Gosiute has fossils of catfish (Ictaluridae and Hypsidoridae) and suckers (Catostomidae).
The catfish are found mostly in the deepest parts of the lake.
There were two genera of indigenous freshwater stingray, Heliobatis and Asterotrygon.