Blackshare Formation

Blackshare Formation
Stratigraphic range: Miocene, 23–11.6  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Formation
Unit of	Santa Fe Group
Underlies	Tuerto Formation
Overlies	Tanos Formation
Thickness	Over 1,260 m (4,130 ft)
Lithology
Primary	Sandstone, conglomerate
Other	Mudstone
Location
Coordinates	35°25′05″N106°18′25″W / 35.418°N 106.307°W / 35.418; -106.307
Region	New Mexico
Country	United States
Type section
Named for	Blackshare Ranch
Named by	S.D. Connell and S.M. Cather
Year defined	2002
Blackshare Formation (the United States) Show map of the United States Blackshare Formation (New Mexico) Show map of New Mexico

The Blackshare Formation is a geologic formation exposed in the Hagan Basin west of the Ortiz Mountains of New Mexico. It is estimated be to of Miocene age. ^{[1] [2] [3]}

Description

The Blackshare Formation consists primarily of sandstone and conglomerate with minor mudstone. The conglomerate forms lenticular bodies, with clasts consisting mostly of Ortiz porphyry with small amounts of hornfels and chert. The sandstone layers tends to be more coarse at their base. The dip 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 confirm deposition by stream flow from the Ortiz Mountains to the east. The total thickness is over 1,260 meters (4,130 ft).

The formation is divided into four informal members, described as a sandstone facies, a conglomeratic sandstone facies, a conglomerate facies, and a muddy sandstone facies. The formation interfingers with the underlying Tanos Formation and is overlain with angular unconformity by the Tuerto Formation. ^[1]

An ash bed about 200 meters (660 ft) below the top of the formation has a radiometric age of 11.65 ± 0.38 million years (Ma). ^[1]

The formation is interpreted as deposition by streams flowing from the east into a closed 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 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 by Connell et al. in 2002. ^[1]

References

1. Connell et al. 2002.
2. Cather et al. 2002.
3. Williams & Cole 2007.
4. House, Kelley & Roy 2003.
5. 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" (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" (PDF). New Mexico Geology. 24 (4): 107–120. doi: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 .
• 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. doi:10.1130/0016-7606(1953)64[459:TGOTGA]2.0.CO;2.
• Williams, Paul L.; Cole, James C. (2007). "Geologic Map of the Albuquerque 30' x 60' Quadrangle, North-Central New Mexico" (PDF). USGS Scientific Investigations Map. Scientific Investigations Map. 2946. doi: 10.3133/sim2946 . hdl:2027/mdp.39015049951992 . Retrieved 17 August 2020.