Description
The Hinsdale Formation is a Bimodal volcanism, 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 . 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 orogeny, 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 Formation, Los Pinos Member. Barker removed the Los Pinos as its own formation in 1958.Footnotes
References
* * * * * * * * {{cite journal , last1=Thompson , first1=Ren A. , last2=Machette , first2=Michael N. , title=Geologic Map of the San Luis Hills Area, Conejos and Costilla Counties, Colorado , journal=U.S. Geological Survey Miscellaneous Investigations Series Map , date=1989 , volume=I-1906 , doi=10.3133/i1906 , url=https://pubs.usgs.gov/mis/I-1906/pdf/I-1906(standard).pdf , accessdate=31 August 2020 Neogene Colorado Neogene formations of New Mexico Basalt formations Andesite formations Rhyolite formations