The Wasatch Formation (Tw)Shroba & Scott, 2001, p.3 is an extensive highly fossiliferous geologic formation stretching across several basins in Idaho, Montana, Wyoming, Utah and western Colorado.Wasatch Formation at Fossilworks.org It preserves fossils dating back to the Early Eocene period. The formation defines the Wasatchian or Lostcabinian (55.8 to 50.3 Ma), a period of time used within the NALMA classification, but the formation ranges in age from the Clarkforkian (56.8 to 55.8 Ma) to Bridgerian (50.3 to 46.2 Ma).
Wasatch fauna consists of many groups of mammals, including numerous genera of primates, artiodactyls, perissodactyls, rodents, carnivora, insectivora, hyaenodonta and others. A number of birds, several reptiles and fish and invertebrates complete the diverse faunal assemblages. Fossil flora and ichnofossils also have been recovered from the formation.
The formation, first named as Wasatch Group in 1873 by Ferdinand Vandeveer Hayden, was deposited in alluvial, fluvial and lacustrine environments and comprises sandstones, siltstones, mudstones and shales with coal or lignite beds representing wet floodplain settings.
The Wasatch Formation is an unconventional tight gas reservoir formation in the Uinta and Piceance Basins of Utah and the coal seams of the formation are mined in Wyoming. At the Fossil Butte National Monument, the formation crops out underlying the Green River Formation. In the Silt Quadrangle of Garfield County, Colorado, the formation overlies the Williams Fork Formation.Shroba & Scott, 2001, p.18
Description Definition The Wasatch Formation was first named as the Wasatch Group by Ferdinand Vandeveer Hayden in the 1873 edition of his original 1869 publication titled "Preliminary field report of the United States Geological Survey of Colorado and New Mexico: U.S. Geological and Geographical Survey of the Territories", based on sections in the Echo and Weber Canyons, of the Wasatch Mountains.Wasatch - USGS In the language of the native Ute people, Wasatch means "mountain pass" or "low pass over high range."Wasatch County - Utah History EncyclopediaVan Cott, 1990, p.390 According to William Bright, the mountains were named for a Shoshoni leader who was named with the Shoshoni term wasattsi, meaning "blue heron".Bright, 2004, p.549
Outcrops
At the base of Fossil Butte are the bright red, purple, yellow and gray beds of the Wasatch Formation. Eroded portions of these horizontal beds slope gradually upward from the valley floor and steepen abruptly. Overlying them and extending to the top of the butte are the much steeper buff-to-white beds of the Green River Formation, which are about thick. The Wasatch Formation ranges from about in the western part of the Uinta Basin, thinning to in the east. In the Silt Quadrangle of Garfield County, Colorado, the formation overlies the Williams Fork Formation. The formation is exposed in the Desolation and Gray Canyons pertaining to the Colorado Plateau in northeastern Utah, and in Flaming Gorge National Recreation Area at the border of southwestern Wyoming and northeastern Utah.
Extent
The Wasatch Formation is found across six states in the northwestern United States, from MontanaWasatch Formation - Montana - USGS and Idaho in the north across UtahWasatch Formation - Utah - USGS and Wyoming to Colorado in the southwest. The formation is part of several geologic provinces; the eponymous Wasatch uplift, Uinta uplift, Green River, Piceance, Powder River, Uinta and Paradox Basins and the Colorado Plateau sedimentary province and Yellowstone province.Wasatch "Group" - USGS
In Montana, the formation overlies the Fort Union Formation and is overlain by the White River Formation.Widmayer, 1977, p.18 There is a regional, angular unconformity between the Fort Union and Wasatch Formations in the northern portion of the Powder River Basin.Widmayer, 1977, p.105
Subdivision
Many local subdivisions of the formation exist, the following members have been named in the literature:
Member States Lithologies Notes Alkali Creek Tongue Wyoming Mudstones and sandstones Roehler, 1991, B16 Atwell Gulch Colorado Sandstones and mudstones Bullpen Wyoming Cathedral Bluffs Tongue Colorado, Wyoming Mudstones and sandstones Chappo Wyoming Cowley Canyon Utah Desertion Point Tongue Wyoming Hiawatha Colorado, Utah, Wyoming Kingsbury Conglomerate Wyoming Feldspathic conglomerates Knight Utah, Wyoming La Barge Wyoming Lookout Mountain Conglomerate Wyoming Conglomerates Luman Tongue Wyoming Luman Tongue Member at Fossilworks.org Main Body Wyoming Mudstones Main Body Member at Fossilworks.org Molina Colorado Sandstones Moncrief Wyoming Feldspathic conglomerates New Fork Tongue Wyoming Nightingale Wyoming Niland Tongue Colorado, Wyoming Ramsey Ranch Wyoming Red Desert Tongue Wyoming Renegade Tongue Colorado, Utah Shire Colorado Sandstones and mudstones Tunp Wyoming
Lithologies and facies In the Fossil Basin at the Fossil Butte National Monument, Wyoming, the Wasatch Formation consists primarily of brightly variegated mudstones with subordinate interbedded siltstones, sandstones, and conglomerates and represents deposition on an intermontane alluvial plain.Gunnells et al., 2016, p.984 In Mesa County, Colorado, the formation comprises interbedded purple, lavender, red, and gray claystones and shales with local lenses of gray and brown sandstones, conglomeratic sandstones, and volcanic sandstones that are predominantly fluvial and lacustrine in origin.Carrara, 2000, p.8 Along the western margin of the Powder River Basin, the Wasatch Formation contains two thick conglomeratic members (in descending order, the Moncrief Member and Kingsbury Conglomerate Member).Whipkey et al., 1991, D1
The Molina Member of the formation is a zone of distinctly sandier fluvial strata. The over- and underlying members of the Molina are the Atwell Gulch and Shire members, respectively. These members consist of infrequent lenses of fluvial-channel sandstones interbedded within thick units of variegated red, orange, purple and gray overbank and paleosol mudstones.Lorenz et al., 1996, p.3
The Molina Member represents a sudden change in the tectonic and/or climatic regimes, that caused an influx of laterally-continuous, fine, coarse and locally conglomeratic sands into the basin. The type section of the Molina is located near the small town of Molina on the western edge of the basin and is about thick. These sandy strata of the Molina Member form continuous, erosion-resistant benches that extend to the north of the type section for approximately . The benches are cut by canyons or "gulches", from which the Atwell Gulch and Shire Gulch members get their names. The Molina forms the principle target within the Wasatch Formation for natural gas exploration, although it is usually called the "G sandstone" in the subsurface.
Provenance Detrital zircons collected from the middle part of the formation in the Powder River Basin of Wyoming, where the Wasatch Formation reaches a thickness of more than were gathered for U-Pb geochronological analysis. The detrital zircon age spectrum ranged from 1433 to 2957 Ma in age, and consisted of more than 95% Archean age grains, with an age peak of about 2900 Ma. The 2900 Ma age peak is consistent with the age of Archean rocks at the core of the Bighorn Mountains. The sparse Proterozoic grains were likely derived from the recycling of Paleozoic sandstone units. The analysis concluded that the Wasatch sandstone is a first cycle sediment, the Archean core of the Bighorn uplift was exposed and shedding sediment into the Powder River Basin during time of deposition of the Wasatch Formation and the Powder River Basin Wasatch detrital zircon age spectra are distinct from the coeval Willwood Formation in the Bighorn Basin west of the Bighorn Mountains.Anderson et al., 2017 Cobbles and pebbles in the Wasatch are rich in feldspathic rock fragments, with individual samples containing as much as 40 percent,Whipkey et al., 1991, D5 derived from erosion of the Precambrian core of the Bighorn Mountains. Part of the feldspar has been replaced by calcite cement.Whipkey et al., 1991, D11 Glauconite is present in the Wasatch, although always in volumes of less than 1 percent of the grains. It most probably was derived from the nearby, friable, glauconite-bearing Mesozoic strata of the eastern Bighorn Mountains.Whipkey et al., 1991, D15
The presence of the Kingsbury Conglomerate at the base of the Wasatch Formation indicates that tectonic activity in the immediate vicinity of the Powder River Basin was intensifying. The conglomerate consists of Mesozoic and Paleozoic rock fragments. The lack of Precambrian fragments indicates that the metamorphic core of the Bighorn Mountains had not been dissected by this early deformation.Widmayer, 1977, p.11 Deformation in the upper part of the formation has been interpreted as the result of the last phase of uplift during the Laramide orogeny.Shroba & Scott, 2001, p.23
Correlations The basal part of the Wasatch Formation is equivalent to the Flagstaff Formation in the southwest part of the Uinta Basin.Sanborn, 1981, p.259 The Wasatch Formation is correlated with the Sentinel Butte and Golden Valley Formations of the Williston Basin.Widmayer, 1977, p.19Pocknall, 1987, p.30
Paleontological significance The Wasatch Formation is the defining formation for the Wasatchian, ranging from 55.8 to 50.3 Ma, within the NALMA classification. The Wasatchian followed the Clarkforkian stage (56.8-55.8 Ma) and is defined by the simultaneous first appearance of adapid and omomyid euprimates, hyaenodontid creodonts, perissodactyls and artiodactyls.Anemone & Dirks, 2009, p.116 The deposits of the formation were laid down during a period of globally high temperatures during the Paleocene-Eocene Thermal Maximum (PETM). Mean annual temperatures were around and temperature variations were minimal during this time.Pocknall, 1987, p.172
At the Fossil Butte National Monument, the Wasatch Formation preserved ichnofossils attributed to arthropods and described as Lunulichnus tuberosus. Trace fossils are common within the upper part of the Main Body Member. These traces occur in three distinct alluvial depositional settings: flood basin/alluvial plain, crevasse splay, and fluvial channel. Flood basin deposits (dominated by alluvial paleosols with pronounced color variegation) are characterized by common Planolites, rare Skolithos and small, meniscate plug-shaped burrows, possibly Celliforma.Zonneveld et al., 2006, p.88
Crevasse splay deposits (current-rippled to planar laminated, fine-grained sandstone to siltstone) are characterized by a mixed assemblage of vertical (Arenicolites, Skolithos, unwalled sinuous shafts, shafts with discoidal lenses of sediment), sub-vertical (Camborygma and Thalassinoides) and horizontal (Scoyenia, Rusophycus, Taenidium, Planolites and Palaeophycus) burrows. Large, vertically oriented burrows (Camborygma, cf. Ophiomorpha, Spongeliomorpha and Thalassinoides) are the dominant forms within fluvial channel deposits.
Fossil content Among the following fossils have been found in the formation:
Mammals Primates
Absarokius abbottiCovert & Hamrick, 1993 A. gaziniBown & Rose, 1987 Anaptomorphus aemulusWest & Dawson, 1973 Anemorhysis sublettensis Arapahovius gazini Artimonius nocerai A. witteriMuldoon & Gunnell, 2002 Cantius frugivorusHoney, 1988Gunnell et al., 2016, p.991 C. mckennai C. nunienus Carpodaptes cf. hazelae Chiromyoides minorGunnell, 1994 Chlororhysis knightensis Copelemur australotutusBeard, 1988 C. praetutus C. tutusGunnell et al., 2016, p.992 Ignacius frugivorus Loveina minuta L. sheai L. zephyriBown, 1979 Microsyops angustidens M. knightensis M. latidensGunnell et al., 2016, p.990 M. cf. elegans M. cf. scottianus Notharctus robinsoni N. venticolus Omomys carteri O. lloydi Plesiadapis dubiusHolroyd & Rankin, 2014