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 thick. The Wasatch Formation ranges from about in the western part of the Uinta Basin, thinning to in the east. 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: 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 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. 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.
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. == Paleontological significance ==