# Picuris Formation

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Picuris_Formation
> Markdown URL: https://mediated.wiki/source/Picuris_Formation.md
> Source: https://en.wikipedia.org/wiki/Picuris_Formation
> Source revision: 1296564765
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

The **Picuris Formation** is a [geologic formation](/source/Geologic_formation) exposed in the eastern flank of the [Rio Grande rift](/source/Rio_Grande_rift) in northern [New Mexico](/source/New_Mexico).[1] It was deposited from the [late Eocene](/source/Late_Eocene) to [Miocene](/source/Miocene) [epochs](/source/Epoch_(geology)).[2]

## Description

The formation consists mostly of [tuffaceous](/source/Tuffaceous) [conglomerate](/source/Conglomerate_(geology)) and [sandstone](/source/Sandstone)[3] with a thickness of more than 1,200 feet (370 m).[1]

The formation is divided into three informal [members](/source/Member_(stratigraphy)).[1][3] These are a lower conglomerate member, a middle tuffaceous member, and an upper [volcaniclastic](/source/Volcaniclastic) member.[2] The lower member is a sandy or silty conglomerate containing mostly [Hondo Group](/source/Hondo_Group) [quartzite](/source/Quartzite) rock fragments ([clasts](/source/Clasts)) ranging in size from 2 millimetres (0.079 in) to over 2 metres (6.6 ft) in diameter. The conglomerate is greenish to pale yellow in color, poorly [sorted](/source/Sorting_(geology)), and moderately to well [rounded](/source/Roundness_(geology)). [Ar-Ar](/source/Argon%E2%80%93argon_dating) [geochronology](/source/Geochronology) indicates an age range from before 34.5 million years ([Ma](/source/Ma_(unit))) to less than 28.3 Ma.[3]

The middle member is a tuffaceous fine to pebbly sandstone. This is buff to yellowish in color, generally poorly [cemented](/source/Cementation_(geology)), and poorly sorted. However, some beds are highly silicified. The member contains some well-developed [normal grading](/source/Graded_bedding) sequences a few to tens of centimeters thick. There are some interbeds and channel fills of conglomerate containing local [Precambrian](/source/Precambrian) clasts, with more conglomerate towards the top. Some [ash](/source/Volcanic_ash) layers are [bioturbated](/source/Bioturbation). Ash beds from the [Amalia Tuff](/source/Amalia_Tuff) eruption have been identified within the member, with ages of 23 and 27 Ma, interpreted as pre- and post-[caldera](/source/Caldera) [pumices](/source/Pumice). This member is at least 150 metres (490 ft) thick. Based on Ar-Ar geochronology, this member was deposited from 28.3 to less than 23 Ma.[3]

The upper member is red to purple volcaniclastic conglomerate and sandstone, typically carbonate cemented. Individual bed [contacts](/source/Contact_(geology)) are abrupt and the lower bed is often scoured. A [basalt](/source/Basalt) clast from the base of the member has an age of 19.78 +/- 0.24 Ma, and the member as a whole spans the age range of less than 19.8 Ma to about 15 Ma. This member is overlain by the Dixon Member of the [Tesuque Formation](/source/Tesuque_Formation).[3]

Cabot included tuffaceous beds just north of Santa Fe in the Picuris Formation, but the [correlation](/source/Correlation_(geology)) of this Bishop's Lodge Member remains controversial, with some geologists keeping it with the Picuris Formation,[4] others assigning it to the Tesuque Formation,[5] and others more recently assigning it to the [Espinaso Formation](/source/Espinaso_Formation).[6]

The formation is of unusual interest in that it spans the transition from [Laramide](/source/Laramide_orogeny) crustal shortening to [Rio Grande rift](/source/Rio_Grande_rift) crustal extension.[2]

## History of investigation

The formation was first named by E.C. Cabot in 1938, likely for exposures in the [Picuris Mountains](/source/Picuris_Mountains) north of [Vadito, New Mexico](/source/Vadito,_New_Mexico).[1]

## Footnotes

1. Cabot 1938.

1. Aby, Bauer & Kelson 2004.

1. Bauer, Kenson & Aby 2005.

1. Galusha & Blick 1971.

1. Baldwin 1956.

1. Read et al. 2013.

## References

- Aby, S.B.; Bauer, P.W.; Kelson, K.I. (2004). ["The Picuris Formation: A late Eocene to Miocene sedimentary sequence in northern New Mexico"](https://nmgs.nmt.edu/publications/guidebooks/downloads/55/55_p0335_p0350.pdf). *New Mexico Geological Society Field Conference Series*. **55**: 335–350. Retrieved 31 August 2020.
- Baldwin, Brewster (1956). ["The Santa Fe group of north-central New Mexico"](https://nmgs.nmt.edu/publications/guidebooks/downloads/7/7_p0115_p0121.pdf). *New Mexico Geological Society Field Conference Guidebook*. **7**: 115–121. Retrieved 23 November 2020.
- Bauer, P.W.; Kenson, K.I.; Aby, S. (2005). ["Preliminary geologic map of the Penasco 7.5 minute quadrangle, Taos County, New Mexico"](https://geoinfo.nmt.edu/publications/maps/geologic/ofgm/downloads/62/PenascoReport.pdf). *New Mexico Bureau of Geology and Mineral Resources Open-File Geologic Map*. **OF-GM 62**. Retrieved 31 August 2020.
- Cabot, Edward C. (January 1938). "Fault Border of the Sangre de Cristo Mountains North of Santa Fe, New Mexico". *The Journal of Geology*. **46** (1): 88–105. [Bibcode:1938JG.....46...88C](https://ui.adsabs.harvard.edu/abs/1938JG.....46...88C). [doi:10.1086/624621](https://doi.org/10.1086/624621). [S2CID 128579201](https://api.semanticscholar.org/CorpusID:128579201)
- Galusha, Ted & Blick, John C. (1971). ["Stratigraphy of the Santa Fe Group, New Mexico"](https://digitallibrary.amnh.org/bitstream/handle/2246/1633/B144a01.pdf?sequence=1&isAllowed=y). *Bulletin of the American Museum of Natural History*. **144** (1). Retrieved 13 May 2020.
- Read, Adam S.; Rodgers, John B.; Ralser, Steven; Ilg, Brad R.; Kelley, Shari; Koning, Daniel J. (February 2013). ["Geologic map of Seton Village Quadrangle, Santa Fe County, New Mexico"](https://geoinfo.nmt.edu/publications/maps/geologic/ofgm/downloads/23/SetonVillageReport.pdf). *New Mexico Bureau of Geology and Mineral Resources Open-file Geologic Map*. **OF-GM 023**. Retrieved 31 August 2020.

---
Adapted from the Wikipedia article [Picuris Formation](https://en.wikipedia.org/wiki/Picuris_Formation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Picuris_Formation?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.