Hinsdale Formation Explained
The Hinsdale Formation is a geologic formation exposed in southwestern Colorado and northern New Mexico. It has a radiometric age of 4.4 to 26.8 million years, corresponding to the Neogene period.
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. The sequence reflects assimilation and fractional crystallization (AFC) of a primitive basalt magma. Total thickness is in excess of 1200feet.
The formation is the youngest volcanic formation of the San Juan volcanic field, and is separated from older units by a significant erosional surface. Radiometric ages range from 4.4 to 26.8 million years. The formation once formed an extensive thin veneer over the San Juan volcanic field before itself being eroded.
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.
Economic geology
The rich mineralization of the San Juan Volcanic Field has been attributed to early intrusions of the Hinsdale Formation.
History of investigation
The formation was first named the Hinsdale Volcanic Series by Charles Whitman Cross in 1911 for exposures in Hinsdale County, Colorado. Larsen redefined the unit as the Hinsdale Formation and included the Los Pinos Member. Barker removed the Los Pinos as its own formation in 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 . 28 June 2020.
- Bethke . P. M. . Barton . P. B. . Lanphere . M. A. . Steven . T. A. . Environment of ore deposition in the Creede mining district, San Juan Mountains, Colorado; II, Age of mineralization . Economic Geology . 1 October 1976 . 71 . 6 . 1006–1011 . 10.2113/gsecongeo.71.6.1006. 1976EcGeo..71.1006B .
- Encyclopedia: Cross . Whitman . Charles Whitman Cross . 1911 . Geology of area near Lake City, Colorado . Irving . J.D. . Bancroft . Howland . Geology and ore deposits near Lake City, Colorado: U.S. Geological Survey Bulletin 478 . 18–32 . 31 August 2020.
- Cross . Whitman . Charles Whitman Cross . Larsen . E.S. . 1935 . A brief review of the geology of the San Juan region of southwestern Colorado . U.S. Geological Survey Bulletin . 843 . 31 August 2020.
- Lipman . Peter W. . Steven . Thomas A. . Mehnert . Harald H. . Volcanic History of the San Juan Mountains, Colorado, as Indicated by Potassium–Argon Dating . Geological Society of America Bulletin . 1970 . 81 . 8 . 2329 . 10.1130/0016-7606(1970)81[2329:VHOTSJ]2.0.CO;2. 1970GSAB...81.2329L .
- Manley . Kim . Redefinition and description of the Los Pinos Formation of north-central New Mexico . GSA Bulletin . 1 December 1981 . 92 . 12 . 984–989 . 10.1130/0016-7606(1981)92<984:RADOTL>2.0.CO;2. 1981GSAB...92..984M .
- Parker . D.F. . Ghosh . A. . Price . C.W. . Rinard . B.D. . Cullers . R.L. . Ren . M. . 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 . January 2005 . 139 . 3–4 . 185–210 . 10.1016/j.jvolgeores.2004.08.015. 2005JVGR..139..185P .
- Thompson . Ren A. . Machette . Michael N. . Geologic Map of the San Luis Hills Area, Conejos and Costilla Counties, Colorado . U.S. Geological Survey Miscellaneous Investigations Series Map . 1989 . I-1906 . 10.3133/i1906 . 31 August 2020.
