The Chugwater Formation is a mapped bedrock unit consisting primarily of red

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

, in the states of

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

, and

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

in the United States. It is recognized as a geologic formation in Colorado and Montana, but as a Group (set of formations), the Chugwater Group, in Wyoming. Despite its presence below the highly studied

Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltston ...

, the Chugwater receives little attention.

Description

The most noticeable feature on a large scale is the brick-red color, caused by

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

of iron minerals in the rock. This color is periodically interrupted by streaks and spots of reduced

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

, a light bluish-gray shade. Near the top of the formation is a thick layer of

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

of very high quality. The whole rock is interrupted by gypsum

veins Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal c ...

as well as having a disrupted texture because of the precipitation of gypsum crystals after deposition of the rock. The Chugwater consists mainly of

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

and shales with interspersed sandstones. While this composition will lend itself to breaking easily and not outcropping, the shear size of the formation causes there to be large outcrops. Though most

sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different parti ...

are not visible due to the gypsum interruption, there are a few examples of

ripple marks In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water ( current or waves) or directly by wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple ...

and occasional

crossbedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The Principle ...

. Examples of infilled

mudcrack Mudcracks (also known as mud cracks, desiccation cracks or cracked mud) are sedimentary structures formed as muddy sediment dries and contracts.Jackson, J.A., 1997, ''Glossary of Geology'' (4th ed.), American Geological Institute, Alexandria, VA ...

s of irregular shape, are common. The main constituent of the Chugwater is

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, which helps to account for its unusual hardness. Feldspars are also present, though in lesser amounts, as are

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

and gypsum. Small flecks of a dark, opaque mineral have been tentatively identified as

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

. The minerals are cemented by a non-carbonate

matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...

, probably siliceous. The Chugwater is a prime example of

red beds Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain t ...

, commonly deposited during the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

and Triassic periods.

Stratigraphy

The Chugwater Group is divided into various formations, depending on the basin in which it occurs. It is mapped in the following basins within the Rocky Mountains:

Denver Basin The Denver Basin, variously referred to as the Julesburg Basin, Denver-Julesburg Basin (after Julesburg, Colorado), or the D-J Basin, is a geology, geologic structural basin centered in eastern Colorado in the United States, but extending into sou ...

Wind River Basin The Wind River Basin is a semi-arid intermontane foreland basin in central Wyoming, United States. It is bounded by Laramide orogeny, Laramide uplifts on all sides. On the west is the Wind River Range and on the North are the Absaroka Range and t ...

Bighorn Basin The Bighorn Basin is a plateau region and intermontane basin, approximately 100 miles (160 km) wide, in north-central Wyoming in the United States. It is bounded by the Absaroka Range on the west, the Pryor Mountains on the north, the Bigho ...

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 ...

, Greater

Green River Basin Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a combin ...

, Laramie Basin. For example, in the Wind River and Greater Green River basins, the formations (in ascending stratigraphic order) are: Red Peak Formation, Crow Mountain Formation, Jelm Formation, and

Popo Agie Formation The Popo Agie Formation ( )Locally "po-PO-zha" according to Don Pitcher, 2006, ''Moon Handbooks Wyoming,'' p. 26/ref>More ambiguous transcription of "po-po-zsha" at is a Triassic geologic Formation (geology), formation that outcrop, crops out in ...

. However, in the

, the formations are: Red Peak Formation, Alcova Limestone, Crow Mountain Sandstone, unnamed red beds, and

.Cavaroc, V.V., and Flores, R.M., 1991, Red beds of the Triassic Chugwater Group, southwestern Powder River basin, Wyoming, IN Evolution of sedimentary basins; Powder River basin: U.S. Geological Survey Bulletin, 1917-E, p. E1-E17
link to report
/ref>

Age

Relative age dating of the Chugwater places it in the

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

Period.

References

{{Chronostratigraphy of Colorado, Mesozoic state=expanded, Paleozoic state=expanded Sandstone formations of the United States Triassic United States Geologic groups of Wyoming Geologic formations of Montana Geologic formations of Colorado Shale formations of the United States Limestone formations of the United States