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Wasatch Formation

Geologic formation in the western United States

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Clinker-bed breccia in Wyoming, USA. Clinker is a scarce, odd rock. It occurs adjacent to former coal bed horizons. Coal is combustible (that’s why coal is used as an energy source in human society).
James St. John · CC BY 2.0

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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.

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. In the language of the native Ute people, Wasatch means "mountain pass" or "low pass over high range." According to William Bright, the mountains were named for a Shoshoni leader who was named with the Shoshoni term wasattsi, meaning "blue heron".

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. 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 Montana and Idaho in the north across Utah 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.

In Montana, the formation overlies the Fort Union Formation and is overlain by the White River Formation. There is a regional, angular unconformity between the Fort Union and Wasatch Formations in the northern portion of the Powder River Basin.

Subdivision

Many local subdivisions of the formation exist, the following members have been named in the literature:

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. 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. 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).

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.

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.

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 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. Cobbles and pebbles in the Wasatch are rich in feldspathic rock fragments, with individual samples containing as much as 40 percent, derived from erosion of the Precambrian core of the Bighorn Mountains. Part of the feldspar has been replaced by calcite cement. 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.

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. Deformation in the upper part of the formation has been interpreted as the result of the last phase of uplift during the Laramide orogeny.

Correlations

The basal part of the Wasatch Formation is equivalent to the Flagstaff Formation in the southwest part of the Uinta Basin. The Wasatch Formation is correlated with the Sentinel Butte and Golden Valley Formations of the Williston Basin.

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