Geologic Map of the Cochetopa Park and North Pass Calderas, Northeastern San Juan Mountains, Colorado By Peter W. Lipman Pamphlet to accompany Scientific Investigations Map 3123 Cochetopa Dome, 11,132 ft Razor Creek Dome (intracaldera rhyolite flows) 11,520 ft (northwest caldera rim) Green Mountain,10,930 ft (northeast caldera rim) NE-trending tongue, Cochetopa Canyon Nelson Mountain Tuff Caldera-filling tuffs and volcaniclastic sedimentary rocks Panoramic view of Cochetopa Dome (sequence of rhyolitic lava flows within caldera) and north walls of Cochetopa Park caldera, viewed from south. Cloud-obscured northwest caldera rim, at left of image, is Sawtooth Mountain (12,147 ft); northeast caldera rim is marked by Razor Creek Dome and Green Mountain on right side. Light-colored eroding outcrops in foreground are weakly indurated tuff and volcaniclastic sedimentary fill of the caldera basin. 2012 U.S. Department of the Interior U.S. Geological Survey This page intentionally left blank CONTENTS Introduction.....................................................................................................................................................1 Ignimbrite caldera cycles and eruptive processes .................................................................................1 Methods of study ...........................................................................................................................................2 Regional framework ......................................................................................................................................3 Early northern volcanism along the Sawatch trend .......................................................................4 San Juan volcanic region ....................................................................................................................4 Early lavas and intrusions: Saguache-Cochetopa region (35–32 Ma) .................................................5 Saguache Creek Tuff and North Pass caldera cycle (32.2 Ma) .............................................................6 Precursor(?) lavas (32.2 Ma) and tuff of Spanish Creek (32.5 Ma) ...............................................7 Saguache Creek Tuff (32.25 Ma) .........................................................................................................7 Formation of the North Pass caldera ................................................................................................8 Caldera-filling lavas (volcanics of Cochetopa Hills, 32.2 Ma) .......................................................9 Luders Creek Tuff (32.17 Ma) ...............................................................................................................9 Late andesite flows and associated rocks (30.0–30.5 Ma) ..........................................................10 Northern rhyolite-dacite dome field (29.8 Ma) ........................................................................................10 Younger eruptions from other San Juan calderas (28.3–26.9 Ma).......................................................11 Sapinero Mesa Tuff ............................................................................................................................11 Fish Canyon Tuff ..................................................................................................................................11 Carpenter Ridge and Wason Park Tuffs ..........................................................................................11 Younger intermediate-composition lava flows .......................................................................................12 Rocks of the San Luis caldera complex ...................................................................................................12 Rat Creek Tuff ......................................................................................................................................12 Cebolla Creek Tuff ...............................................................................................................................12 Nelson Mountain Tuf ..........................................................................................................................12 Cochetopa Park caldera: Relation to Nelson Mountain Tuff (26.9 Ma).....................................13 Structure........................................................................................................................................................15 Northeast-trending graben faults ....................................................................................................15 Caldera faults.......................................................................................................................................15 Rio Grande rift faults and associated volcanism ...........................................................................16 Description of Map Units ............................................................................................................................17 Surficial deposits ................................................................................................................................17 Glacial deposits ..........................................................................................................................17 Old surficial deposits .................................................................................................................17 Miocene postdaldera volcanic rocks ..............................................................................................18 Oligocene volcanic rocks ..................................................................................................................18 Rocks of Cochetopa Park caldera and San Luis caldera complex ...................................18 Rocks of the Nelson Mountain caldera cycle ..............................................................18 Caldera-fill flows and intrusions ...........................................................................18 Caldera-fill volcaniclastic rocks ............................................................................20 Nelson Mountain Tuff ..............................................................................................21 Rocks erupted from the Cebolla Creek caldera ...........................................................22 Rocks erupted from the Rat Creek caldera ..................................................................23 Rocks erupted from South River caldera ...............................................................................23 Younger intermediate-composition lava flows .....................................................................23 Rocks erupted from Bachelor caldera ...................................................................................24 Rocks of La Garita caldera cycle ............................................................................................24 Older tuffs related to western San Juan calderas ...............................................................26 Conejos Formation, upper units ...............................................................................................26 i Caldera-related rocks predating the western and central caldera clusters ...................28 North Pass caldera cycle ................................................................................................29 Volcanics of Cochetopa Hills .................................................................................29 Intrusions of uncertain affinity .......................................................................................31 Bonanza caldera cycle ....................................................................................................31 Volcanic rocks predating Bonanza caldera ..........................................................................32 Conejos Formation, lower units ......................................................................................32 Early ignimbrites erupted from Sawatch Range calderas ..................................................34 Prevolcanic Rocks ..............................................................................................................................34 Mesozoic sedimentary rocks ...................................................................................................35 Paleozoic sedimentary rocks ...................................................................................................35 Precambrian rocks ....................................................................................................................35 References Cited..........................................................................................................................................35 Figures 1. Map of Southern Rocky Mountain volcanic field ............................................................sheet 2 2. Map showing calderas in San Juan volcanic region and location of map area .......sheet 2 3. Map showing northeastern San Juan region and geologic mapping responsibilities .......... .........................................................................................................................................sheet 1 4. Generalized geologic map and section, northeast San Juan region ...........................sheet 2 5. Oblique view (Google Earth) of Cochetopa and North Pass calderas .........................sheet