Cochiti Formation

The Cochiti Formation is a geologic formation exposed near the southwest Jemez Mountains of northern New Mexico. Its age is estimated as 10 to 2.6 million years, corresponding to the middle Miocene to Pliocene.

Cochiti Formation Stratigraphic range: Miocene to Pliocene, 10–2.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Cochiti Formation in road cut at Kasha-Katuwe Tent Rocks National Monument
Type	Formation
Unit of	Santa Fe Group
Sub-units	Lookout Park Gravel Member
Overlies	Keres Group
Lithology
Primary	Sandstone
Other	Conglomerate
Location
Coordinates	35.673°N 106.453°W / 35.673; -106.453
Region	New Mexico
Country	United States
Type section
Named for	Cochiti Pueblo
Named by	Bailey, Smith, and Ross
Year defined	1969

Description

The formation consists of volcanic sand and gravel eroded off the Keres Group of the southern Jemez Mountains. [1] The formation is restricted to sedimentary strata composed purely of volcaniclastic sediments that overlie Keres Group volcanic rocks and correlative volcanic sediments south of the Jemez Mountain and are in turn overlain by Pliocene and early Pleistocene gravels.

Members

The Lookout Park Gravel Member is a stratified cobble to boulder gravel composed almost entirely of Keres Group volcanic debris that underlies high level geomorphic surfaces sloping to the southeast. It is interbedded with basalt with an age of 2.5 million years. [2]

History of investigation

The formation was first defined by Bailey, Smith, and Ross in 1969 as part of their work establishing the stratigraphy of the Jemez Mountains. The formation was named for exposures near Cochiti Pueblo. [3]

Smith and Lavine argued in 1996 that the original definition was inconsistent and that volcanic and sedimentary basin stratigraphers had worsened the problem by their different approaches to regional stratigraphy.[4] For example, Goff et al. mapped volcaniclastic beds of the San Miguel Mountains as Cochiti Formation based on the original definition of the formation, but noted the inconsistency.[5] Smith and Lavine identified a mappable, distinct stratigraphic unit that included most of the original exposures mapped by Smith, Bailey, and Ross in 1970[6] and excluded most strata added by other investigators.[4]

Chamberlin et al. tentatively assigned the Gravel of Lookout Park as a member of the Cochiti Formation in 1999.[2]

Footnotes

1. Smith et al. 1969, pp.2-8
2. Chamberlin et al. 1999
3. Smith et al. 1969, pp.8-9
4. Smith and Lavine 1996
5. Goff et al. 2002
6. Smith et al. 1970

References

• Chamberlin, R.M.; Pazzaglia, F.J.; Wegmann, K.W.; Smith, G.A. (Jun 1999). "Geologic Map of the Loma Creston Quadrangle, Sandoval County, New Mexico" (PDF). New Mexico Bureau of Geology and Mineral Resources Open Map Series. 25. Retrieved 17 May 2020.
• Goff, Fraser; Gardner, Jamie N.; Reneau, Steven L. (2002). "Geology of the Frijoles quadrangle, Los Alamos, Sandoval, and Santa Fe Counties, New Mexico". New Mexico Burea of Geology and Mineral Resources Open File Geologic Map Series. 42. Retrieved 16 May 2020.
• Smith, Gary A.; Lavine, Alexis (1996). "What is the Cochiti Formation?" (PDF). New Mexico Geological Society Field Conference Series. 47: 219–224. Retrieved 17 May 2020.
• Smith, R.L.; Bailey, R.A.; Ross, C.S. (1969). "Stratigraphic Nomenclature of ' Volcanic Rocks in the Jemez Mountains, New Mexico" (PDF). GEOLOGICAL SURVEY BULLETIN (1274-P). Retrieved 4 May 2020.
• Smith, R.L.; Bailey, R.A.; Ross, C.S. (1970). "Geologic map of the Jemez Mountains, New Mexico". U.S. Geological Survey Miscellaneous Investigations Map Series (I-571). Retrieved 16 May 2020.