Revised volcanic history of the San Juan, Uncompahgre, Silverton, and Lake City calderas in the western San Juan Mountains, Colorado

The sequence of mid-Cenozoic volcanic events in the western San Juan Mountains is closely analogous to that elsewhere in the San Juan volcanic field. The Lake Fork, Picayune, and San Juan Formations were erupted from a cluster of central volcanoes from 35 to 30 m.y. ago, when dominant activity shifted to more silicic ash-flow eruptions with accompanying caldera collapses. The Uncompahgre and San Juan calderas, each about 20 km across, formed mainly from eruption of the 28-m.y.-old Sapinero Mesa Tuff. Collapse occurred concurrently with eruption, and intracaldera tuffs accumulated to a thickness of more than 700 m. Both calderas were resurgently domed together; the northeast-trending Eureka graben formed along the distended crest of that dome. The Uncompahgre caldera was then flooded by several 27- to 28-m.y.-old ash-flow sheets from easterly sources, and also by one apparently erupted from the Silverton caldera nested within the older San Juan caldera. The Lake City caldera, located within the older Uncompahgre caldera, formed about 22.5 m.y. ago in response to eruption of the Sunshine Peak Tuff.