The Uncompahgre Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

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 ...

. Its radiometric age is between 1707 and 1704 Ma, corresponding to the

Statherian The Statherian Period (; , meaning "stable, firm") is the final geologic period in the Paleoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically. Th ...

period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

History

The formation was first named by Charles Whitman Cross and Ernest Howe in 1905 for exposures around Uncompahgre Gorge near

Ouray, Colorado Ouray ( ) is a List of municipalities in Colorado#Home rule municipality, home rule municipality that is the county seat of Ouray County, Colorado, United States. The city population was 898 as of the 2020 United States census, 2020 census. The ...

. They grouped it with the Vallecito Conglomerate into the Needle Mountains Group, but this was abandoned by Tweto in 1977. Karl Karlstrom and coinvestigators informally raised the Uncompahgre Formation to group rank in 2017, assigning the Vallecito Conglomerate as a member.

Geology

The Uncompahgre Formation is a sequence of

quartzite Quartzite is a hard, non- foliated metamorphic rock that 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 tecton ...

and black

phyllite Phyllite ( ) is a type of foliation (geology), foliated metamorphic rock formed from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. I ...

some in thickness. It is exposed in a curving belt from the northwestern to northeastern

Needle Mountains The Needle Mountains are a subrange of the San Juan Mountains of the Rocky Mountains located in the southwestern part of the U.S. State of Colorado. Much of the range is protected in the Weminuche Wilderness of the San Juan National Forest. The ...

with outliers near Ouray and

Rico, Colorado Rico is a town in Dolores County, Colorado, United States. It was settled in 1879 as a silver mining center in the Pioneer Mining District; today it functions as a historic and tourism site. The population was 288 at the 2020 census. Its curr ...

. It is interpreted as metamorphosed marine and fluvial

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 ...

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.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

, and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

. The formation overlies plutons with an age of 1707 Ma and

detrital zircon geochronology Detrital zircon geochronology is the science of geochronology, analyzing the age of zircons deposited within a specific sedimentary rock, sedimentary unit by examining their inherent radioisotopes, most commonly the uranium–lead dating, uranium ...

confirms a maximum age of 1709 Ma. Ar⁴⁰Ar/³⁹Ar dating of muscovite in an intruding dike gives a minimum age of 1704 Ma, allowing the age of the Uncompahgre Formation to be tightly constrained. Dating of contact metamorphism aureoles show the formation was deformed by metamorphosis at around 1460 to 1400 Ma, concurrent with the

Picuris orogeny The Picuris orogeny was an orogenic event in what is now the Southwestern United States from 1.43 to 1.3 billion years ago in the Calymmian Period of the Mesoproterozoic. The event is named for the Picuris Mountains in northern New Mexico and ...

. The formation is thought to be correlative with the

Ortega Formation The Ortega Formation is a geologic formation that crops out in most of the mountain ranges of northern New Mexico. Detrital zircon geochronology establishes a maximum age for the formation of 1690-1670 million years ( Mya), in the Statherian per ...

in northern

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

and the Mazatzal Group in

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

Footnotes

References

* * * * * {{cite journal, last1=Tweto , first1=Ogden , year=1977 , title=Nomenclature of Precambrian rocks in Colorado , journal=Contributions to Stratigraphy: U.S. Geological Survey Bulletin, 1422-D , pages=D1–D22 , url=https://books.google.com/books?id=Xc-Nn-oIHm0C&q=Tweto,+Ogden,+1977,+Nomenclature+of+Precambrian+rocks+in+Colorado,+IN+Contributions+to+stratigraphy++U.S.+Geological+Survey+Bulletin,+1422+D,+p.+D1+D22.&pg=PA1 , access-date=21 May 2020 Mesoproterozoic geology of Colorado