Geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado

The San Juan Mountains in southwestern Colorado have long been known as a site of exceptionally voluminous mid-Tertiary volcanism, including at least 22 major ignimbrite sheets (each 150–5,000 km³) and associated caldera structures active at 34–23 Ma. Recent volcanologic and petrologic studies in the San Juan region have focused mainly on several ignimbrite-caldera systems: the southeastern area (Platoro complex), western calderas (Uncompahgre-Silverton-Lake City), and the central cluster (La Garita-Creede calderas).

Far less studied has been the northeastern San Juan region, which occupies a transition between earlier volcanism in central Colorado and large-volume younger ignimbrite-caldera foci farther south and west. This map is based on new field coverage of volcanic rocks in thirteen 7.5' quadrangles in northeastern parts of the volcanic field, high-resolution age determinations for 130 sites, and petrologic studies involving several hundred new chemical analyses. This mapping and the accompanying lab results (1) document volcanic evolution of the deeply eroded Bonanza caldera that exposes unique features not previously described from ignimbrite calderas elsewhere, as well as the previously unstudied Marshall Pass caldera; (2) provide unique cross-sectional exposures of the steeply resurgent Bonanza caldera, from volcanic floor and underlying basement rocks through a complete 3.5-km-thick section of intracaldera ignimbrite and overlying compositionally diverse caldera-filling lavas; (3) document timing of caldera collapse concurrently with eruption of about 1,000 km³ of ignimbrite that oscillated in composition from mafic dacite to rhyolite; (4) quantify the regional time-space-volume progression from the earlier Sawatch magmatic trend southward into the San Juan region; and (5) permit more rigorous comparison between the broad mid-Tertiary magmatic belt in the western U.S. Cordillera and the type continental-margin arc volcanism of the central Andes in South America.