The Fort Union Formation is a

geologic unit A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize ...

containing sandstones, shales, and coal beds in

Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

, and parts of adjacent states. In the

Powder River Basin The Powder River Basin is a geologic structural basin in southeast Montana and northeast Wyoming, about east to west and north to south, known for its extensive coal reserves. The former hunting grounds of the Oglala Lakota, the area is very s ...

, it contains important economic deposits of

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

, and coalbed methane.''Wyodak Coal, Tongue River Member of the Fort Union Formation, Powder River Basin, Wyoming: "No-Coal Zones" and Their Effects on Coalbed Methane Production'', by Mark Ashley, 2005
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Description

The Fort Union is mostly of

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

age and represents a time of extensive swamps as well as fluvial and

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

conditions. The rocks are more sandy in southwestern Wyoming and more coal-bearing in northeast Wyoming and southeast Montana, reflecting a general change from rivers and lakes in the west to swamps in the east, but all three environments were present at various times in most locations. Coal in the Fort Union in the Powder River Basin occurs mainly in the Tongue River Member, where as many as 32 coal seams total more than 300 feet in thickness. One such bed, the Wyodak Coal near

Gillette, Wyoming Gillette (, '' jih-LET'') is a city in and the county seat of Campbell County, Wyoming, United States. The town was founded in 1891 as a major railway town on the Chicago, Burlington and Quincy Railroad. The population was estimated at 32,030 a ...

, is as much as thick. Most of the coals in the Fort Union Formation are ranked

subbituminous Sub-bituminous coal is a lower grade of coal that contains 35–45% carbon. The properties of this type are between those of lignite, the lowest grade of coal, and those of bituminous coal, the second-highest grade of coal. Sub-bituminous coal ...

.''Cretaceous and Tertiary coals of the Rocky Mountains and Great Plains regions'', by R.M. Flores and T.A. Cross, 1991, ''in'' Economic Geology, U.S., Geological Society of America, Decade of North American Geology Series, vol. P-2, p. 547-571.

Fossil content

Mammals

Cimolestans

Leptictids

Marsupials

Primatomorphs

Ungulates

Reptiles

Birds

A partial ornithurine coracoid bone found in this formation is identical to others found in the older

Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...

. At present, this unnamed species is the only known individual

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

species that have survived the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...

.Longrich, N.R., Tokaryk, T. and Field, D.J. (2011). "Mass extinction of birds at the Cretaceous–Paleogene (K–Pg) boundary." ''Proceedings of the National Academy of Sciences'', 108(37): 15253-15257.

Crocodilians

Squamates

Testudines

Amphibians

Fish

Invertebrates

Bivalves

Gastropods

Plants

References

{{Chronostratigraphy of Colorado, Cenozoic state=expanded Geologic formations of Montana Geologic formations of North Dakota Geologic formations of Wyoming Cretaceous Montana Cretaceous geology of North Dakota Cretaceous geology of Wyoming Paleogene Montana Paleogene geology of North Dakota Paleogene geology of Wyoming Maastrichtian Stage of North America Danian Stage Cretaceous–Paleogene boundary Clarkforkian Lancian Puercan Tiffanian Torrejonian Paleontology in Wyoming Fluvial deposits Lacustrine deposits Sandstone formations of the United States Shale formations of the United States Coal formations Coal in the United States