Old Earth Ministries Online Earth History Curriculum

       Fossil Butte National Monument in Lincoln County, Wyoming is in a part of the formation known as Fossil Lake because of its abundance of exceptionally well preserved fish fossils.

Lithology and Formation

     The formation of intermontane basin / lake environments during the Eocene resulted from mountain building and uplift of the Rocky Mountains (late Cretaceous Sevier orogeny and the Paleogene Laramide orogeny). 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. Volcanic ash layers within the various sediments from the then 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 Green River Formation, is the type locality for eight rare minerals: bradleyite, ewaldite, loughlinite, mckelveyite-(Y), norsethite, paralabuntsovite-Mg, shortite and wegscheiderite. It also has a natural occurrence of moissanite (SiC) and 23 other valid mineral species.

Cyclicity

     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.  The high number of annual layers (over 13 million) has drawn much criticism from young earth creationists, since the interpretation that these are annual layers means the lake took more than 13 million years to form, which does not fit into the young earth timescale.  Young earth objections have focused on claims that the layers were individual storm events, but they have no proof of their claims.  Given the astrochronological and radiometric agreement, there is little doubt that they are annual layers.

     Individual layers are called varves.  These vary in thickness from 0.014 millimeters to 9.8 millimeters, depending on the composition. The average thickness being 0.18 mm.  There are four different rock types represented in the layers; three grades of oil shale (low, moderate, and rich), and fine-grained limy sandstone.  Deposition rates have been calculated, with the average coming out to 2,200 years (to accumulate one foot of sediment).  According to these rates and the thickness of the formation, it took about 6,500,000 years to accumultate the over 13 million layers of the Green River Formation. (Data source).

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 split fish layer—consists of a series of laminated about 6 feet thick 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.   This thin zone represents some 4000 years of deposition. The second fossil zone, the 18 inch layer, is an unlaminated layer about 18 in (46 cm) 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.

Discovery of the Fossil Beds

     The first documented records of (invertebrate) fossils from what is now called the Green River Formation are in the journals of early missionaries and explorers such as S.A. Parker, 1840, and J.C. Fremont, 1845. Geologist Dr. John Evans collected the first fossil fish, described as Culpea humilis (later renamed Knightia eocaena), from the Green River beds in 1856. Edward Drinker Cope collected extensively from the area and produced several publications on the fossil fish from 1870 onwards. Ferdinand Vandeveer Hayden (geologist-in-charge of the United States Geological and Geographical Survey of the Territories, the fore-runner of the USGS) first used the name "Green River Shales" for the fossil sites in 1869.

     Millions of fish fossils have been collected from the area, commercial collectors operating from legal quarries on state and private land have been responsible for the majority of Green River vertebrate fossils in public and private collections all over the world.

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 × 10¹¹ m³ (1.5 trillion US barrels) of shale oil.

Trona and Nahcolite

     The unusual chemistry of the lakes in which it was deposited makes the Green River Formation a major source of sodium carbonate salts. In southwest Wyoming the formation contains the world's largest deposits of trona, and in Colorado, the world's largest deposits of nahcolite.

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Source: Green River Formation