# Blackshare Formation

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The **Blackshare Formation** is a [geologic formation](/source/Formation_(stratigraphy)) exposed in the [Hagan](/source/Hagan,_New_Mexico) Basin west of the [Ortiz Mountains](/source/Ortiz_Mountains) of [New Mexico](/source/New_Mexico). It is estimated be to of [Miocene](/source/Miocene) age.[1][2][3]

## Description

The Blackshare Formation consists primarily of [sandstone](/source/Sandstone) and [conglomerate](/source/Conglomerate_(geology)) with minor [mudstone](/source/Mudstone). The conglomerate forms lenticular bodies, with [clasts](/source/Clast) consisting mostly of [Ortiz porphyry](/source/Ortiz_porphyry_belt) with small amounts of [hornfels](/source/Hornfels) and [chert](/source/Chert). The sandstone layers tends to be more coarse at their base. The [dip](/source/Dip_(geology)) of the beds (which is to the northeast) decreases from 16 degrees to 1 degree from the base to the top of the formation. [Paleocurrents](/source/Paleocurrent) confirm deposition by stream flow from the [Ortiz Mountains](/source/Ortiz_Mountains) to the east. The total thickness is over 1,260 metres (4,130 ft).

The formation is divided into four informal [members](/source/Member_(stratigraphy)), described as a sandstone [facies](/source/Facies_(geology)), a conglomeratic sandstone facies, a conglomerate facies, and a muddy sandstone facies. The formation [interfingers](/source/Interfingering) with the underlying [Tanos Formation](/source/Tanos_Formation) and is overlain with [angular unconformity](/source/Angular_unconformity) by the [Tuerto Formation](/source/Tuerto_Formation).[1]

An [ash bed](/source/Volcanic_ash) about 200 metres (660 ft) below the top of the formation has a [radiometric age](/source/Radiometric_dating) of 11.65 ± 0.38 million years ([Ma](/source/Ma_(unit))).[1]

The formation is interpreted as deposition by streams flowing from the east into a [closed basin](/source/Endorrheic_basin).[1] The progressive tilting of the beds, with the lower beds showing significant dip while the uppermost are nearly horizontal, may be connected to the uplift of the [Sandia Mountains](/source/Sandia_Mountains) to the south while the sediments making up the formation were being deposited.[4]

## History

The beds making up this formation were first assigned to the Santa Fe Formation by Stearns in 1953.[5] They were designated as the Blackshare Formation of the [Santa Fe Group](/source/Santa_Fe_Group_(geology)) by Connell *et al.* in 2002.[1]

## References

1. Connell et al. 2002.

1. Cather et al. 2002.

1. Williams & Cole 2007.

1. House, Kelley & Roy 2003.

1. Stearns 1953.

### Bibliography

- Cather, Steven M.; Connell, Sean D.; Lucas, Spencer G.; Picha, Mark G.; Black, Bruce A. (June 2002). ["Geologic Map of the Hagan Quadrangle, Santa Fe County, New Mexico"](https://geoinfo.nmt.edu/publications/maps/geologic/ofgm/downloads/50/HaganReport.pdf). *New Mexico Bureau of Geology and Mineral Resources Open-file Digital Geologic Map*. **OF-GM 050**. Retrieved 17 August 2020.
- Connell, Sean D.; Cather, Steven M.; Dunbar, Nelia W.; McIntosh, William C.; Peters, Lisa (November 2002). ["Stratigraphy of the Tanos and Blackshare Formations (lower Santa Fe Group), Hagan embayment, Rio Grande rift, New Mexico"](https://geoinfo.nmt.edu/publications/periodicals/nmg/24/n4/nmg_v24_n4_p107.pdf). *New Mexico Geology*. **24** (4): 107–120. [doi:10.58799/NMG-v24n4.107](https://doi.org/10.58799/NMG-v24n4.107). Retrieved 9 June 2020.
- House, M. A.; Kelley, S. A.; Roy, M. (October 2003). "Refining the footwall cooling history of a rift flank uplift, Rio Grande rift, New Mexico: THERMOCHRONOMETRY OF RIO GRANDE RIFT". *Tectonics*. **22** (5): n/a. [doi:10.1029/2002TC001418](https://doi.org/10.1029/2002TC001418)
- Stearns, Charles E. (1953). "Tertiary geology of the Galisteo-Tonque area, New Mexico". *Geological Society of America Bulletin*. **64** (4): 459. [Bibcode:1953GSAB...64..459S](https://ui.adsabs.harvard.edu/abs/1953GSAB...64..459S). [doi:10.1130/0016-7606(1953)64\[459:TGOTGA](https://doi.org/10.1130/0016-7606(1953)64%5B459:TGOTGA%5D2.0.CO;2)2.0.CO;2]
- Williams, Paul L. & Cole, James C. (2007). ["Geologic Map of the Albuquerque 30' x 60' Quadrangle, North-Central New Mexico"](https://pubs.usgs.gov/sim/2007/2946/downloads/pdf/2946_pamphlet_508.pdf). *USGS Scientific Investigations Map*. **2946**. Scientific Investigations Map. [doi:10.3133/sim2946](https://doi.org/10.3133/sim2946). [hdl:2027/mdp.39015049951992](https://hdl.handle.net/2027/mdp.39015049951992). Retrieved 17 August 2020.

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Adapted from the Wikipedia article [Blackshare Formation](https://en.wikipedia.org/wiki/Blackshare_Formation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Blackshare_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.