Hinsdale Formation

Hinsdale Formation
Stratigraphic range: Neogene 26.8–4.4  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Hinsdale Formation at its type locality in Colorado (Devils Lake, Powderhorn Wilderness)
Type	Formation
Overlies	Los Pinos Formation [1]
Thickness	1,200 ft (370 m)
Lithology
Primary	Basalt, andesite, rhyolite
Location
Coordinates	37°47′13″N107°19′08″W / 37.787°N 107.319°W / 37.787; -107.319
Region	Colorado New Mexico
Country	United States
Type section
Named for	Hinsdale County, Colorado
Named by	Charles Whitman Cross
Year defined	1911
Hinsdale Formation (the United States) Show map of the United States Hinsdale Formation (Colorado) Show map of Colorado

The Hinsdale Formation is a geologic formation exposed in southwestern Colorado and northern New Mexico. ^[2] It has a radiometric age of 4.4 to 26.8 million years, corresponding to the Neogene period. ^[3]

Description

The Hinsdale Formation is a bimodal volcanic formation, containing silica-poor olivine basalt and high-silica rhyolite with only small quantities of volcanic rock of intermediate composition. ^[4] The sequence reflects assimilation and fractional crystallization (AFC) of a primitive basalt magma. ^[5] Total thickness is in excess of 1,200 feet (370 m). ^[2]

The formation is the youngest volcanic formation of the San Juan volcanic field, and is separated from older units by a significant erosional surface. ^[2] Radiometric ages range from 4.4 to 26.8 million years. ^[3] The formation once formed an extensive thin veneer over the San Juan volcanic field before itself being eroded. ^[5]

The change to bimodal association coincided with the transition from Laramide compression to Rio Grande rift extension, a pattern seen elsewhere in the western United States. ^[5]

Economic geology

The rich mineralization of the San Juan Volcanic Field has been attributed to early intrusions of the Hinsdale Formation. ^[6]

History of investigation

The formation was first named the Hinsdale Volcanic Series by Charles Whitman Cross in 1911 for exposures in Hinsdale County, Colorado. ^[2] Larsen redefined the unit as the Hinsdale Formation and included the Los Pinos Member. ^[7] Barker removed the Los Pinos as its own formation in 1958. ^[8]

Footnotes

1. Manley 1981.
2. Cross 1911.
3. Thompson & Machette 1989.
4. Lipman, Steven & Mehnert 1970.
5. Parker et al. 2005.
6. Bethke et al. 1976.
7. Cross & Larsen 1935.
8. Barker 1958.

References

• Barker, Fred (1958). "Precambrian and Tertiary geology of Las Tablas quadrangle, New Mexico". New Mexico Bureau of Mines and Mineral Resources Bulletin. 45. Retrieved 28 June 2020.
• Bethke, P. M.; Barton, P. B.; Lanphere, M. A.; Steven, T. A. (1 October 1976). "Environment of ore deposition in the Creede mining district, San Juan Mountains, Colorado; II, Age of mineralization". Economic Geology. 71 (6): 1006–1011. Bibcode:1976EcGeo..71.1006B. doi:10.2113/gsecongeo.71.6.1006.
• Cross, Whitman (1911). "Geology of area near Lake City, Colorado". In Irving, J.D.; Bancroft, Howland (eds.). Geology and ore deposits near Lake City, Colorado: U.S. Geological Survey Bulletin 478. pp. 18–32. Retrieved 31 August 2020.
• Cross, Whitman; Larsen, E.S. (1935). "A brief review of the geology of the San Juan region of southwestern Colorado". U.S. Geological Survey Bulletin. 843. Retrieved 31 August 2020.
• Lipman, Peter W.; Steven, Thomas A.; Mehnert, Harald H. (1970). "Volcanic History of the San Juan Mountains, Colorado, as Indicated by Potassium–Argon Dating". Geological Society of America Bulletin. 81 (8): 2329. Bibcode:1970GSAB...81.2329L. doi:10.1130/0016-7606(1970)81[2329:VHOTSJ]2.0.CO;2.
• Manley, Kim (1 December 1981). "Redefinition and description of the Los Pinos Formation of north-central New Mexico". GSA Bulletin. 92 (12): 984–989. Bibcode:1981GSAB...92..984M. doi:10.1130/0016-7606(1981)92<984:RADOTL>2.0.CO;2.
• Parker, D.F.; Ghosh, A.; Price, C.W.; Rinard, B.D.; Cullers, R.L.; Ren, M. (January 2005). "Origin of rhyolite by crustal melting and the nature of parental magmas in the Oligocene Conejos Formation, San Juan Mountains, Colorado, USA". Journal of Volcanology and Geothermal Research. 139 (3–4): 185–210. Bibcode:2005JVGR..139..185P. doi:10.1016/j.jvolgeores.2004.08.015.
• Thompson, Ren A.; Machette, Michael N. (1989). "Geologic Map of the San Luis Hills Area, Conejos and Costilla Counties, Colorado" (PDF). U.S. Geological Survey Miscellaneous Investigations Series Map. I-1906. doi:10.3133/i1906 . Retrieved 31 August 2020.