The geology of

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 ...

includes thick sequences of

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

and

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

sedimentary rocks overlying ancient

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Earth ...

and

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

crystalline basement rock. Eastern Montana has considerable oil and gas resources, while the uplifted Rocky Mountains in the west, which resulted from the

Laramide orogeny The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the o ...

and other tectonic events have locations with metal ore.

Geologic History, Stratigraphy & Tectonics

The oldest rocks in Montana are part of the

Wyoming Craton The Wyoming Craton is a craton in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah. Also called the Wyoming Province, it is the init ...

in the center and east of the state, primarily between Livingston and Red Lodge, as well as small areas in the Little Belt Mountains around Neihart and the core of the Little Rocky Mountains south of Harlem. Drill cores indicate that these rocks underlie much of the Great Plains. A106, Glacier National Park, Montana, USA, Bowman Lake, 2004

A106, Glacier National Park, Montana, USA, Bowman Lake, 2004

The Pony Group, Cherry Creek Group and the Stillwater Complex are examples of Precambrian metamorphic rock units The first two groups are made up of folded and metamorphosed marine sedimentary rocks which were folded, metamorphosed. They were subsequently intruded by

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is che ...

diabase Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-graine ...

and

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

. The Stillwater Complex, by contrast, is made up of ultrabasic igneous rocks exposed at the surface in Park, Stillwater, and Sweet Grass Counties. Together, these three rock formations formed between 2.54 and 1.69 billion years ago. The Pony Group and Cherry Creek Group are overlain by the Belt Series sedimentary rocks, which deposited after a period of erosion that left a large unconformity. These shallow water rocks from the

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

range between 35,000 and 50,000 feet thick, overlain by additional Middle Cambrian sedimentary rocks. Iceberg Lake Ridge (4158171784)

Belt rocks make up the mountains around St. Mary Lake at Glacier National Park. Deposition took place in a broad trough extending southeast to the Big Snowy Mountains as well as to the southwest through Three Forks, Whitehall, the Highland Mountains, and Armstead. Regional low-grade metamorphism altered

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

, carbonates and silty

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

argillite :''"Argillite" may also refer to Argillite, Kentucky.'' Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts ...

and

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

. Geologists divide the rocks into four units, from oldest to youngest the pre-Ravalli rocks, the Ravalli, the Piegan, and the Missoula groups. In some locations, late Precambrian sills, dikes and lava flows are found. The Sunshine Mine in the Coeur d'Alene district of Idaho, which is in correlates with the Ravalli Group. Uranium mineralized in the rock 1.19 million years ago, giving a young age for limit for the surrounding rocks.

Paleozoic (539-251 million years ago)

Thick sequences of sedimentary rock deposited during the

, reaching thicknesses of up to 10,000 feet. Sixteen miles west of Missoula and further west, all Paleozoic rocks have eroded away. To the east, Paleozoic rocks are exposed and upturned along the Rocky Mountains. Most Paleozoic rocks originated in a marine environment, particularly dolomite and limestone together with shale, siltstone, sandstone, and evaporites such as gypsum, anhydrite and salt.

Mesozoic (251-66 million years ago)

During the

, one mile thick sedimentary rocks accumulated across 55 percent of the state. They are exposed at the surface due to the Black Hills uplift and the Cedar Creek Anticline. Marine rocks alternate with terrestrial sandstone, indicating a string of

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

and regression events. In the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

, the

began to assemble the Rocky Mountains, intruding granite in Ravalli County as well as the Boulder batholith and Tobacco Root batholith.

Tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

and

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...

lava flows resulting from volcanic eruptions is found interbedded in sedimentary rocks.

Cenozoic (66 million years ago-present)

Continued uplift due to the Laramide orogeny drove accelerated erosion in the

. Eastern Montana lowlands were covered in up to 4000 feet of sediment. The Fort Union Formation in the east, made up of the Tongue River, Lebo and Tullock members is a remnant 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 ...

deposition. It is overlain by Wasatch Formation

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

and stream conglomerate which has been significantly eroded. Intermontane valleys preserve thick sequences of

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

and

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

rocks that are rare in other parts of the Eastern Plains. These sediments vary considerably, from claystone, mudstone and siltstone to bentonite, diatomaceous earth, coal and tuff. The Flaxville Formation is the youngest rock unit Montana from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58sillimanite Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut. Occurrence ...

and

kyanite Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks gener ...

in the Cherry Creek Group, while chromite is common in the Stillwater Complex that spans Stillwater and Sweet Grass counties. The western third of Montana has sedimentary rocks from the Paleozoic and Mesozoic which are potential sources of phosphate rock, limestone, silica, crushed stone and clay. In places bentonite has been mined altered volcanic ash beds in Cenozoic intermontane sedimentary basins. Central Montana has metal ore deposits that have been mined in the Little Rocky Mountains, the North Moccasin Mountains, the Judith Mountains and the Little Belt Mountains. The region also has significant production of oil and natural gas from the Cat Creek anticline, the Kevin-Sunburst dome and the Sweet Grass arch. Mesozoic sandstones are a primary reservoir rock. Some Cretaceous sandstone, for example The third Cat Creek sandstone at the base of the Kootenai Formation, the Virgelle sandstone member of the Eagle sandstone, and the Fox Hill sandstone, are valuable ground water aquifers all dating to the Cretaceous. The Kootenai Formation also contains clay usable for brick and tile. Much of central Montana is underlain by Jurassic and Cretaceous coal beds. Although there are no metal ore deposits in the eastern third of the state, the region holds more than 90 percent of Montana's coal reserves. The coal is part of the Paleocene Fort Union Formation, particularly the Tongue River Member. The eastern region produces petroleum, principally in the western Williston Basin and the Cedar Creek anticline. Most hydrocarbons come from Paleozoic rocks, although some are sourced from Cretaceous rock. Bentonite is mined from Cretaceous beds in Carter County.http://mbmg.mtech.edu/sp28/geology.htm

External links

*http://www.gravmag.com/gibson-geology.pdf *https://ngmdb.usgs.gov/Prodesc/proddesc_16363.htm *http://www.mbmg.mtech.edu/mbmgcat/public/ListCitation.asp?pub_id=11596& *https://serc.carleton.edu/research_education/mt_geoheritage/sites/augusta_choteau/stratigraphy.html *https://mrdata.usgs.gov/geology/state/map-us.html

References

{{Geology of the United States by political division