Skip to main content
U.S. flag

An official website of the United States government

Database for the Geologic Map of the Bonanza Caldera Area, Northeastern San Juan Mountains, Colorado

March 10, 2020

The San Juan Mountains in southwestern Colorado have long been recognized as a site of exceptionally voluminous mid-Tertiary volcanism, including at least 24 major ignimbrite sheets (each 150-5,000 km3) and associated caldera structures active at 33-23 Ma. More 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), the central cluster (La Garita-Creede calderas). The northeast San Juan region that was far less studied until recently occupies a transition between earlier volcanism in central Colorado and the larger-volume younger ignimbrite-caldera foci farther south and west. The present study of the Bonanza area evaluates eruptive and magmatic processes of silicic Cordilleran volcanism in this northeast region, based on new geologic mapping (mainly summers of 2007-12) and concurrent petrologic and geochronologic analysis. The resulting map is based on new field mapping of volcanic rocks in thirteen 7.5-minute quadrangles in northeastern parts of the volcanic field, petrologic studies involving several hundred new chemical analyses, and high-resolution age determinations for about 130 sites. The Bonanza center contains exceptionally complete and diverse features of an ignimbrite caldera cycle, including voluminous andesite erupted before and after ignimbrite eruptions, complex compositional zonation within both the outflow ignimbrite sheet and the tuff concurrently ponded within the caldera, extensive portions of the ring-fault system that accommodated subsidence, thick compositionally diverse lavas that filled the caldera after subsidence, remnants of the original topographic caldera rim, widespread erosional exposure of caldera-floor features, and postcaldera granitic intrusions that generated a notably steep resurgent dome within the caldera. This geologic map database is accompanied by a report, which includes the formatted geologic map and explanatory pamphlet, available at The authors ask that users of the geologic map database cite both the database and the report: Report: Lipman, P.W., 2020, Geologic map of the Bonanza Caldera Area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey Scientific Investigations Map 3394, pamphlet 73 p., 2 sheets, scale 1:50,000, Database: Joel E. Robinson, and Peter W. Lipman, 2020, Database for the Geologic Map of the Bonanza Caldera Area, Northeastern San Juan Mountains, Colorado: U.S. Geological Survey,

Publication Year 2020
Title Database for the Geologic Map of the Bonanza Caldera Area, Northeastern San Juan Mountains, Colorado
DOI 10.5066/P911OL4Q
Authors Joel E Robinson, Peter W Lipman
Product Type Data Release
Record Source USGS Digital Object Identifier Catalog
USGS Organization USGS Volcano Science Center