SOUTHWARD MIGRATION OF MID-TERTIARY VOLCANISM: RELATIONS IN THE COCHETOPA AREA, NORTH-CENTRAL SAN JUAN MOUNTAINS, COLORADO

In the late Eocene-Oligocene, just prior to structural development of the Rio Grande rift, eruptive style, composition, and age of volcanism migrated southward from central Colorado into New Mexico. At any locus of this progression, volcanism began with quiescent emplacement of intermediate-composition lavas from central volcanoes, followed by voluminous explosive eruptions and caldera collapse. New studies in the Cochetopa area exemplify complexities resulting from nonsynchronous formation of similar eruptive sequences from geographically disparate centers. The oldest mid-Tertiary unit, rheomorphic Wall Mountain Tuff (36.3 Ma; Ar-Ar sanidine age) erupted from the Mt Princeton area 80 km NE, is overlain by thick sequences of intermediate lavas erupted locally. Capping the lavas are: (1) Bonanza Tuff, a reversely zoned ash-flow sheet (crystal-rich dacite upward into crystal-poor rhyolite), erupted from Bonanza caldera 50 km to the NE at 32.5 Ma; then (2) newly recognized tuff of Saguache Creek, atypically alkalic crystal-poor rhyolite erupted at 31.9 Ma from Cochetopa Park. Tuff of Saguache Creek had previously been misidentified as Sapinero Mesa Tuff (27.9 Ma) from the west San Juans. These tuffs bracket emplacement of intermediate-composition lavas in the NE San Juans (36.3-32.5 Ma) as older than compositionally similar Conejos Formation to the south and west (mostly >32.5 Ma). Tuff of Saguache Creek is directly overlain in most places by the enormous Fish Canyon Tuff (27.6 Ma), with ~4 my depositional hiatus, but east of Cochetopa Park newly recognized compositionally zoned "tuff of Luders Creek" occupies this interval. Cochetopa caldera is morphologically beautifully preserved due to late passive (non-eruptive) collapse during discharge of the ~26.5 Ma Nelson Mountain Tuff, from an "underfit" depression within the San Luis caldera complex 30 km to the SW. Cochetopa thus is a large-scale Oligocene analog for the 1912 Valley of Ten Thousand Smokes eruption from Novarupta crater in the Aleutians, during which main caldera collapse was 12 km distant at Katmai volcano. Cochetopa was a site of recurrent caldera subsidence associated with intermittent explosive activity for about 6 my; other multicyclic calderas are known in the San Juans and elsewhere, but such a long time span is exceptional for a single site.