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USGS Scientific Investigations Map 3394 Pamphlet Geologic Map of the Bonanza Caldera Area, Northeastern San Juan Mountains, Colorado By Peter W. Lipman Pamphlet to accompany Scientific Investigations Map 3394 WEST CALDERA RIM Windy Point Antora Peak Sheep Mountain Inner caldera wall Soutwest flank, resurgent dome LT AU F NG RI n) K tio E oca RE x. l C pro ER (ap RB KE 2020 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DAVID BERNHARDT, Secretary U.S. Geological Survey James F. Reilly II, Director U.S. Geological Survey, Reston, Virginia: 2020 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Lipman, P.W., 2020, Geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey Scientific Investigations Map 3394, 73 p., 2 sheets, scale 1:50,000, https://doi.org/10.3133/ sim3394. Associated data for this publication: Robinson, J.E., and Lipman, P.W., 2020, Database for the geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey data release, https://doi.org/10.5066/P911OL4Q. ISSN 2329-1311 (print) ISSN 2329-132X (online) Cover Photograph. West topographic rim of Bonanza caldera, as viewed to northwest, up valley of Kerber Creek. In distance, Antora Peak (center, 13,269’) is capped by densely welded dacitic Bonanza Tuff (33.12 Ma), while Windy Point (left, 12,800’) exposes thin layer of basal rhyolitic Bonanza Tuff that has undergone local rheomorphic flowage. Lower slopes of both high points are west-dipping sequence of interlayered andesitic lavas and volcaniclastic rocks that make up the erosionally modified inner wall of Bonanza caldera. Sheep Mountain (right distant skyline, 12,228’) is pre-Bonanza dacitic lava dome (33.78 Ma) that partly fills the older Marshall caldera, source of the 33.9-Ma Thorn Ranch Tuff. On left side of Kerber Creek, timbered slope is lower portion of southwest caldera wall; on right side of creek is dip slope on flank of postcollapse resurgent dome. The valley of Kerber Creek coincides with the main ring fault, along which more than 3 km of subsidence was accommodated during eruption of the Bonanza Tuff and concurrent caldera collapse. Contents Introduction.....................................................................................................................................................1 Ignimbrite Caldera Cycles and Eruptive Processes .................................................................................2 Methods ..........................................................................................................................................................3 Regional Framework of the Southern Rocky Mountain Volcanic Field ................................................5 Paleotopography and Structure .........................................................................................................5 Early Northern Volcanism along the Sawatch Trend ......................................................................7 San Juan Volcanic Region ..................................................................................................................7 Geologic Setting of Bonanza Caldera ........................................................................................................8 Pre-Tertiary Rocks in the Bonanza Region ................................................................................................8 Early Lavas and Intrusions (35–33 Ma).......................................................................................................8 Thorn Ranch Tuff and Marshall Caldera Cycle (33.9 Ma)......................................................................10 Bonanza Caldera Cycle (~33 Ma) ..............................................................................................................11 Pre-Bonanza Caldera Volcanism .....................................................................................................12 Bonanza Tuff ........................................................................................................................................12 Caldera Subsidence ...........................................................................................................................13 Caldera-Collapse Faults .....................................................................................................................14 Caldera-Floor Structures ...................................................................................................................15 Ignimbrite Fracture Fills .....................................................................................................................16 Ignimbrite and Subsidence Volumes ..............................................................................................16 Postcollapse Lavas and Intrusions ..................................................................................................17 Resurgent Uplift...................................................................................................................................18 An Additional Complexity ..................................................................................................................19 Initial and Present-Day Caldera Morphology ................................................................................20 Younger Eruptions from other San Juan Calderas (28.3–26.9 Ma) ......................................................22 Saguache Creek Tuff ..........................................................................................................................22 Sapinero Mesa Tuff ............................................................................................................................22 Fish Canyon Tuff ..................................................................................................................................22 Carpenter Ridge and Wason Park Tuffs ..........................................................................................22 Younger Intermediate-Composition Lavas ...............................................................................................23 Regional Structure .......................................................................................................................................23 Prevolcanic Structure ........................................................................................................................23 Rio Grande Rift and Transfer-Zone Geometry ................................................................................24 Alteration and Mineralization ....................................................................................................................26 Bonanza Mining District ....................................................................................................................26 Villa Grove Turquoise Mine ...............................................................................................................27 Acknowledgments .......................................................................................................................................27 Description of Map Units ............................................................................................................................27 Surficial Deposits ................................................................................................................................27 Glacial Deposits ..................................................................................................................................28 Miocene Postcaldera Rocks .............................................................................................................28 Oligocene Volcanic Rocks .................................................................................................................29 Ignimbrites and Associated Rocks of the Central San Juan Caldera Complex ..............29 Rocks Erupted from South River Caldera (27.4 Ma) ....................................................29 Rocks Erupted from Bachelor Caldera (27.55 Ma) ......................................................29 Rocks of La Garita Caldera Cycle (28.0 Ma) .................................................................30 Local Volcaniclastic Rocks of Saguache Paleovalley ................................................31 Older Ignimbrite Related to Western San Juan Calderas (28.3 Ma) ........................31 iii Conejos Formation, Upper Units (~30 Ma) ....................................................................32 Caldera-Related Rocks Predating the Western and Central Caldera Clusters ...............33 North Pass Caldera Cycle (32.2 Ma) ..............................................................................33 Rocks of the Bonanza Caldera Cycle (33 Ma) ..............................................................33
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