Chugwater formation
Encyclopedia
The Chugwater Formation is a mapped bedrock unit consisting primarily of red sandstone
, in the states of Wyoming
, Montana
, and Colorado
in the USA. It is recognized as a Formation in Colorado and Montana, but as a Group (sequence of Formations) in Wyoming .
Due to its lack of fossil
s and its presence below the highly-studied Morrison Formation
, the Chugwater receives little attention.
is a thick layer of gypsum
of very high quality. The whole rock is interrupted by gypsum veins
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
or very fine-grained durable sandstone
; it is quite hard and does not break easily. Though most sedimentary structures
are not visible due to the gypsum interruption, there are a few examples of ripple marks
and occasional crossbedding. Examples of infilled mudcrack
s of irregular shape, are common.
The main constituent of the Chugwater is quartz
, which helps to account for its unusual hardness. Feldspars are also present, though in lesser amounts, as are hematite
and gypsum. Small flecks of a dark, opaque mineral have been tentatively identified as biotite
. The minerals are cemented by a non-carbonate matrix
, probably siliceous.
The Chugwater is a prime example of red beds
, commonly deposited during the Permian
and Triassic periods.
, Wind River Basin
, Bighorn Basin
, Powder River Basin
, Greater Green River Basin, 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. However, in the Powder River Basin
, the formations are: Red Peak Formation, Alcova Limestone, Crow Mountain Sandstone, unnamed red beds, and Popo Agie Formation.
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, in the states of Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
, Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
, and Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
in the USA. It is recognized as a Formation in Colorado and Montana, but as a Group (sequence of Formations) in Wyoming .
Due to its lack of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s and its presence below the highly-studied Morrison Formation
Morrison Formation
The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is 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, siltstone and limestone and is light grey, greenish...
, the Chugwater receives little attention.
Description
The most noticeable feature on a large scale is the brick-red color, caused by oxidation of iron minerals in the rock. This color is periodically interrupted by streaks and spots of reduced iron, a light bluish-gray shade. Near the top of the formationis a thick layer of gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
of very high quality. The whole rock is interrupted by gypsum veins
Vein (geology)
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation...
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
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
or very fine-grained durable sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
; it is quite hard and does not break easily. Though most sedimentary structures
Sedimentary structures
Sedimentary structures are those structures formed during sediment deposition.Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand the depositional...
are not visible due to the gypsum interruption, there are a few examples of ripple marks
Ripple marks
In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.- Defining ripple cross-laminae and asymmetric ripples :...
and occasional crossbedding. Examples of infilled mudcrack
Mudcrack
Mudcracks are sedimentary structures formed as muddy sediment dries and contracts.- Formation :...
s of irregular shape, are common.
The main constituent of the Chugwater is quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, which helps to account for its unusual hardness. Feldspars are also present, though in lesser amounts, as are hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
and gypsum. Small flecks of a dark, opaque mineral have been tentatively identified as biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
. The minerals are cemented by a non-carbonate matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
, probably siliceous.
The Chugwater is a prime example of red beds
Red beds
Red beds are sedimentary rocks, which typically consist 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 thin beds of conglomerate, marl, limestone, or some combination of...
, commonly deposited during the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
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 BasinDenver Basin
The Denver Basin, sometimes also called the Julesburg Basin, Denver-Julesburg Basin , or the D-J Basin, is a geologic structural basin centered in eastern Colorado in the United States, but extending into southeast Wyoming, western Nebraska, and western Kansas...
, Wind River Basin
Wind River Basin
The Wind River Basin or Shoshone Basin is a semi-arid intermontane structural basin in central Wyoming, USA. It is bounded by Laramide uplifts on all sides. On the west is the Wind River Range and on the North are the Absaroka Range and the Owl Creek Mountains...
, Bighorn Basin
Bighorn Basin
The Bighorn Basin is a plateau region and intermontane basin, approximately 100 miles wide, in north-central Wyoming in the United States. It is bounded by the Absaroka Range on the west, the Bighorn Mountains on the east, and the Owl Creek Mountains and Bridger Mountains on the south...
, Powder River Basin
Powder River Basin
The Powder River Basin is a geologic region in southeast Montana and northeast Wyoming, about east to west and north to south, known for its coal deposits. The region supplies about 40 percent of coal in the United States. It is both a topographic drainage and geologic structural basin...
, Greater Green River Basin, 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. However, in the Powder River Basin
Powder River Basin
The Powder River Basin is a geologic region in southeast Montana and northeast Wyoming, about east to west and north to south, known for its coal deposits. The region supplies about 40 percent of coal in the United States. It is both a topographic drainage and geologic structural basin...
, the formations are: Red Peak Formation, Alcova Limestone, Crow Mountain Sandstone, unnamed red beds, and Popo Agie Formation.