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.[4] In the language of the native
Ute people, Wasatch means "mountain pass" or "low pass over high range."[5][6] According to
William Bright, the mountains were named for a
Shoshoni leader who was named with the Shoshoni term wasattsi, meaning "blue heron".[7]
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 300 feet (91 m) thick. The Wasatch Formation ranges from about 3,000 feet (910 m) in the western part of the Uinta Basin, thinning to 2,000 feet (610 m) in the east.[8] In the Silt Quadrangle of
Garfield County, Colorado, the formation overlies the
Williams Fork Formation.[3] The formation is exposed in the
Desolation and
Gray Canyons pertaining to the Colorado Plateau in northeastern Utah,[9] and in
Flaming Gorge National Recreation Area at the border of southwestern Wyoming and northeastern Utah.[10]
In Montana, the formation overlies the Fort Union Formation and is overlain by the
White River Formation.[14] There is a regional, angular unconformity between the Fort Union and Wasatch Formations in the northern portion of the Powder River Basin.[15]
Subdivision
Many local subdivisions of the formation exist, the following members have been named in the literature:[13]
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.[22] 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.[23] 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).[24]
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.[17]
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 90 metres (300 ft) thick. These sandy strata of the Molina Member form continuous, erosion-resistant benches that extend to the north of the type section for approximately 25 kilometres (16 mi). 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.[17]
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 1,500 metres (4,900 ft) were gathered for U-Pb geochronological analysis. The detrital zircon age spectrum ranged from 1433-2957 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.[25] Cobbles and pebbles in the Wasatch are rich in
feldspathic rock fragments, with individual samples containing as much as 40 percent,[26] derived from erosion of the
Precambrian core of the Bighorn Mountains.[24] Part of the feldspar has been replaced by
calcite cement.[27]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.[18]
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.[19] Deformation in the upper part of the formation has been interpreted as the result of the last phase of uplift during the
Laramide orogeny.[28]
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.[32] 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 25 °C (77 °F) and temperature variations were minimal during this time.[33]
At the
Fossil Butte National Monument, the Wasatch Formation preserved
ichnofossils attributed to
arthropods and described as Lunulichnus tuberosus.[34] 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.[35]
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.[35]
Fossil content
Among the following fossils have been found in the formation:[2]
The Wasatch Formation is a
tight gasreservoir rock in the
Greater Natural Buttes Field in the
Uinta Basin of Utah and Colorado. The formation is characterized by
porosity ranging from 6 to 20% and
permeability of up to 1
mD. Based on 409 samples from the Wasatch Formation, average porosity is 8.75 percent and average permeability is 0.095 mD.[8] The production rates after 2 years are 100–1,000 mscf/day for gas, 0.35–3.4 barrel per day for oil, and less than 1 barrel per day for water. The water:gas ratio ranges from 0.1 to 10 barrels per million standard cubic feet, indicating that free water is produced along with water dissolved in gas in the reservoir.[120] Oil in the
Bluebell-Altamont Field in the Uinta Basin and gas in the
Piceance Creek Field in the Piceance Basin are produced from the Wasatch Formation.[121]
As of May 2019, tight gas from the Wasatch Formation and underlying
Mesaverde Group has been produced more than 1.76 trillion cubic feet (TCF) of gas from over 3,000 wells in the
Uinta Basin, mostly from the
Natural Buttes gas field in the eastern part of the basin. In the
Piceance Basin, the Mesaverde Group and Wasatch Formation produced more than 7.7 TCF from over 12,000 wells, mostly from the central part of the basin.[122]
Mining
Coal
Coal is mined from the Wasatch Formation in Wyoming. Together with the Fort Union Formation, the Wasatch Formation represents the thickest coal bed deposits in the state.[123]
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Sanborn, Albert F. 1981. Potential petroleum resources of northeastern Utah and northwestern Colorado, 255–266. Western Slope (Western Colorado), Epis, R. C.; Callender, J. F.; (eds.), New Mexico Geological Society 32nd Annual Fall Field Conference Guidebook.
Strait, S. G.; P. A. Holroyd; C. A. Denvir, and B. D. Rankin. 2016. Early Eocene (Wasatchian) rodent assemblages from the Washakie Basin, Wyoming. PaleoBios 33. 1–28. .
Holroyd, P. A., and B. D. Rankin. 2014. Additions to the latest Paleocene Buckman Hollow local fauna, Chappo Member of the Wasatch Formation, Lincoln County, southwestern Wyoming. Palaeontologia Electronica 16. 26. .
Hutchison, J. H. 2013. New turtles from the Paleogene of North America. In D. B. Brinkman, P. A. Holroyd, J. D. Gardner (eds.), Morphology and Evolution of Turtles, 477–497. ;
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