The Uncompahgre Formation is a geologic formation in Colorado. Its radiometric age is between 1707 and 1704 Ma, corresponding to the Statherian period.

History

The formation was first named by Charles Whitman Cross and Ernest Howe in 1905 for exposures around Uncompahgre Gorge near Ouray, Colorado. 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 and black phyllite 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. It is interpreted as metamorphosed marine and fluvial sandstone,

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

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 ...

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

detrital zircon geochronology Detrital zircon geochronology is the science of analyzing the age of zircons deposited within a specific sedimentary unit by examining their inherent radioisotopes, most commonly the uranium–lead ratio. Zircon is a common accessory or trace ...

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 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 peri ...

in northern New Mexico and the Mazatzal Group in Arizona.

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