Thomas A. Steven Robert G. Luedke Wilbur Burbank Peter W. Lipman 1973 <p>The sequence of mid-Cenozoic volcanic events in the&nbsp;western San Juan Mountains is closely analogous to that elsewhere in&nbsp;the San Juan volcanic field. The Lake Fork, Picayune, and San Juan&nbsp;Formations were erupted from a cluster of central volcanoes from 35 to&nbsp;30 m.y. ago, when dominant activity shifted to more silicic ash-flow&nbsp;eruptions with accompanying caldera collapses. The Uncompahgre and&nbsp;San Juan calderas, each about 20 km across, formed mainly from&nbsp;eruption of the 28-m.y.-old Sapinero Mesa Tuff. Collapse occurred&nbsp;concurrently with eruption, and intracaldera tuffs accumulated to a&nbsp;thickness of more than 700 m. Both calderas were resurgently domed&nbsp;together; the northeast-trending Eureka graben formed along the&nbsp;distended crest of that dome. The Uncompahgre caldera was then&nbsp;flooded by several 27- to 28-m.y.-old ash-flow sheets from easterly&nbsp;sources, and also by one apparently erupted from the Silverton caldera&nbsp;nested within the older San Juan caldera. The Lake City caldera,&nbsp;located within the older Uncompahgre caldera, formed about 22.5 m.y.&nbsp;ago in response to eruption of the Sunshine Peak Tuff.</p> application/pdf en U.S. Geological Survey Revised volcanic history of the San Juan, Uncompahgre, Silverton, and Lake City calderas in the western San Juan Mountains, Colorado article