2012

U.S. Department of the Interior

U.S. Geological Survey

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

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. Cochetopa Canyon

NE-trending tongue,

Nelson Mountain Tuff

Razor Creek Dome

11,520 ft

(northwest caldera rim)

Cochetopa Dome, 11,132 ft

(intracaldera rhyolite flows)

Green Mountain,10,930 ft

(northeast caldera rim)

Caldera-filling tuffs and volcaniclastic sedimentary rocks
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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
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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 2

6. Summary of 40Ar/39Ar age determinations.........................................................................sheet 2

7. Age-distance-volume plot...................................................................................................sheet 2

8. Photograph showing east wall of North Pass caldera...................................................sheet 2

9. Schematic east-west stratigraphic section, along Saguache valley into the North Pass 

caldera ...........................................................................................................................sheet 2

10. Generalized geologic map of San Luis complex in relation to Cochetopa Park caldera......

.........................................................................................................................................sheet 2

Tables

1. Regional ignimbrites and calderas of the Southern Rocky Mountain Volcanic Field........41

2. Summary of new 40Ar/39Ar age determinations for Tertiary volcanic rocks .........................42

3. Compilation of 40Ar/39Ar age determinations for Tertiary volcanic rocks...................................

......................................................................................................................................CD-ROM

4. Representative chemical analyses of Tertiary volcanic rocks ..............................................44

5. Compilation of chemical analyses of Tertiary volcanic rocks....................................CD-ROM

6. Characteristic features of ignimbrite sheets in the central and northeastern San Juan 

region.......................................................................................................................................48
1

Introduction

The San Juan Mountains in southwestern Colorado 

(fig. 1, sheet 2) have long been known as a site of exceptionally voluminous mid-Tertiary volcanism (Cross and Larsen, 

1935; Larsen and Cross, 1956), including at least 22 major 

ignimbrite sheets (each 150–5,000 km3) and associated caldera 

structures active at 33–23 Ma (fig. 2, sheet 2; table 1, Steven 

and Ratté, 1965; Lipman and others, 1970; Steven and Lipman, 

1976). Recent volcanologic and petrologic studies in the San 

Juan region have focused mainly on several ignimbrite-caldera 

systems: the southeastern area (Platoro complex: Lipman, 1975; 

Dungan and others, 1989; Lipman and others, 1996), western 

calderas (Uncompahgre-Silverton-Lake City: Lipman and 

others, 1973; Hon and Lipman, 1989; Bove and others, 2001), 

and the central cluster (La Garita-Creede calderas: Lipman and 

others, 1989, 1997; Riciputi and others, 1995; Lipman, 2000, 

2006; Schmitz and Bowring, 2001; Bachmann and others, 2002, 

2007; Parat and others, 2005). 

Far less studied has been the northeastern San Juan 

region (fig. 2), which occupies a transition between earlier 

volcanism in central Colorado and large-volume younger 

ignimbrite-caldera foci farther south and west (fig. 1). Other 

than mineral-resource studies of small areas, much of this 

northeastern area previously had been mapped only in broad 

reconnaissance for the Colorado State geologic map (Tweto 

and others, 1976; Tweto, 1979). The present map is based on 

new field coverage of volcanic rocks in seventeen 7.5' quadrangles in northeastern parts of the volcanic field (fig 3, sheet 

1; fig. 4, sheet 2), high-resolution age determinations for 120 

new sites (table 2; table 3, CD-ROM; Lipman and McIntosh, 

2008), and petrologic studies involving several hundred new 

chemical analyses (table 4; table 5, CD-ROM). Among topics 

explored are previously little-understood complexities of 

pyroclastic eruption and emplacement, geometric interrelations between caldera subsidence and resurgence, petrologic 

diversity of sequential ignimbrite eruptions, recurrent eruption of intermediate-composition lavas after caldera-forming 

events, associated regional fault development, volume relations 

between ignimbrite eruptions and associated calderas, growth 

of subvolcanic batholiths, and involvement of mantle-derived 

mafic materials in magma-generation processes. This mapping and the accompanying lab results (1) document volcanic 

evolution of the previously unrecognized North Pass caldera 

and the morphologically beautifully preserved but enigmatic 

Cochetopa basin (fig. 5, sheet 2), including unique features not 

previously described from ignimbrite calderas elsewhere; (2) 

provide evidence for a more rapid recurrence of large ignimbrite eruptions than previously known elsewhere; (3) quantify 

the regional time-space-volume progression from the earlier 

Sawatch magmatic trend southward into the San Juan region; 

and (4) permit more rigorous comparison between the broad 

mid-Tertiary magmatic belt in the western U.S. Cordillera and 

the type continental-margin arc volcanism in the central Andes. 

This pamphlet is intended (1) to provide a guide to the geologic evolution of the Cochetopa and North Pass calderas (modified from Lipman and McIntosh, 2008) within a conceptual 

framework of caldera processes and the regional setting for 

these newly interpreted caldera systems within the Southern 

Rocky Mountain volcanic field of mid-Tertiary age (Lipman, 

2007) and (2) to offer brief descriptions for each of the 100-plus 

rock units depicted on the accompanying geologic map.

Ignimbrite Caldera Cycles and Eruptive 

Processes

As previously interpreted for silicic volcanism in the San 

Juan Mountains and elsewhere, lavas, volcaniclastic sedimentary rocks, and intrusions emplaced concurrently with 

ignimbrites in the northeastern San Juan region define multiple 

caldera cycles, based on affinities of geographic distribution 

and stratigraphic sequence, isotopic age and paleomagnetic pole 

positions, and petrologic character. Interpretation of some units 

remains ambiguous, especially assignment of certain lavas and 

tuffs to the late stages of one cycle versus inception of the next 

one. The revised analysis of caldera development and regional 

volcanic history (table 1) builds upon earlier syntheses for the 

San Juan region (Lipman and others, 1970, 1978; Steven and 

Lipman, 1976; Lipman, 2000), utilizing general concepts of 

caldera geometry and eruptive cycles summarized by Smith and 

Bailey (1968) and Lipman (1984, 1997, 2007). These overview 

papers provide a framework for broader understanding of the 

map relations depicted on this map.

Briefly, many large ignimbrite calderas, such as those 

in the San Juan region, form at sites of preceding volcanism 

that records shallow accumulation of caldera-related magma. 

Large eruptions (>50–100 km3

 of ignimbrite magma) cause 

caldera collapse concurrently with volcanism, as indicated by 

thick intracaldera ignimbrite fill and interleaved collapse slide 

breccias. Volumes of intracaldera and outflow tuff tend to be 

subequal; correlation between them is commonly complicated 

by contrasts in abundance and size of phenocrysts and lithic 

fragments, degree of welding, devitrification, alteration, and 

even chemical composition of magmatic material. Structural 

boundaries of calderas commonly are single ring faults or composite ring-fault zones that dip steeply. Scalloped topographic 

walls beyond the structural boundaries of most calderas are due 

to secondary gravitational slumping during subsidence. The 

area and volume of caldera collapse are roughly proportional 

to the amount of erupted material. Postcollapse volcanism may 

occur from varied vent geometries within ignimbrite calderas; 

ring-vent eruptions are most common in resurgent calderas, 

reflecting renewed magmatic pressure. Resurgence within 

calderas may result in a symmetrical dome or more geometrically complex forms. In addition to resurgence within single 

calderas, broader magmatic uplift occurs within some silicic 

volcanic fields, reflecting isostatic adjustment to emplacement 

of associated subvolcanic batholiths. Large intrusions related to 

resurgence are exposed centrally or along the margins of some 

deeply eroded calderas. Hydrothermal activity and mineralization accompany all stages of ignimbrite magmatism, becoming 

dominant late during caldera evolution. Much rich mineralization formed millions of years later than caldera collapse, when 
2

the caldera served primarily as a structural control for late intrusions and associated hydrothermal systems.

Methods of Study

Geologic mapping, initiated to characterize the geologic 

setting for the Creede Scientific Drilling Project (Bethke and 

Hay, 2000), was gradually expanded to cover the entire central 

caldera cluster and adjacent areas to the northeast as multiple 

stratigraphic, structural, geochronologic, and volcanologic problems emerged (fig. 2). In addition to the published geologic map 

of the central San Juan caldera complex (Lipman, 2006), field 

study of the Cochetopa and North Pass caldera areas by Lipman 

totaled about 30 weeks, mainly between 2000 and 2005; assistance during initial fieldwork was provided by Joel Robinson 

(2000) and Pierre Christe (2001). Continuing field studies (starting in 2006) farther to the east are focused on interpretation 

of the Bonanza caldera system (fig. 4) and a newly identified 

caldera fragment exposed east of the Continental Divide that 

probably continues into upper Marshall Creek (Gregory and 

McIntosh, 1996). Small parts of the map area, especially preTertiary rocks in northwestern quadrangles, were recompiled 

from published maps (Hedlund and Olson, 1974; Olson and 

others, 1975; Olson, 1976a,b; Olson and Steven, 1976a,b); all 

Tertiary volcanic rocks and other areas that required significant 

reinterpretation were remapped (fig. 3). Fieldwork and primary 

map compilation were at a scale of 1:24,000, using 7.5' topographic quadrangle maps as base materials (fig. 3); these data 

are included as digital files in the database accompanying this 

report. In total, 102 map units are distinguished. Most of the 

fieldwork preceded use of GPS for locations; some Quaternary 

deposits in flat-lying areas were compiled by photogrammetric 

methods. Two recently mapped 7.5' quadrangles (Stork and 

others, 2006, 2007) are adjacent to the northwest corner of the 

map area.

The digital files were compiled using ArcMap GIS software, mainly by Joel Robinson. The geologic map data were 

transferred to the 1:50,000-scale county map series by GIS 

methods to provide a more legible base than would have been 

possible by photo reduction of the 1:24,000-scale quadrangle 

maps. In the Description of Map Units, needed references 

to map locations are by abbreviations of the 7.5' quadrangle 

names, as listed on figure 3; for example, the Cochetopa Park 

quadrangle is referenced as CP. Some unit descriptions, age 

determinations, and geologic interpretations are based partly on 

features from adjacent quadrangles to the south (Lipman, 2006) 

or east (Bonanza area, fieldwork in progress) of the map area 

(fig. 3); these are also referenced by quadrangle abbreviations 

where appropriate. 

Volcanic rock names are used in general accord with the 

IUGS classification system (Le Bas and others, 1986); in particular, the term “silicic dacite” is used for rocks designated quartz 

latite in many prior publications. Most of the volcanic rocks 

constitute a high-K assemblage that is transitional between 

subalkaline and alkaline suites, similar to those at other Tertiary 

volcanic fields in the Southern Rocky Mountains. For simplicity 

and continuity with previous usage, such modifiers as "high-K" 

or "trachy" are omitted from most rock names. Names, divided 

on the basis of percent SiO2

, are <52, basalt; 52–57, basaltic 

andesite; 57–62, andesite; 62–66, dacite; 66–70, silicic dacite 

(formerly quartz latite); 70–75, rhyolite; >75, silicic rhyolite (all 

compositions for bulk-rock analyses recalculated to reported 

summations volatile-free, all FeO as Fe2O3). Phenocryst assemblages distinguish many of the major ignimbrite sheets (table 6). 

Cited chemical and petrographic data include 130 new majoroxide and trace-element analyses for samples from the map area 

(table 4; table 5, CD-ROM), supplemented by data for regional 

units from Lipman (2004). Sanidine compositions, initially 

determined from x-ray cell parameters (Lipman, 1975) and later 

by electron-microprobe analysis (Lipman and Weston, 2001; 

Lipman, unpub. data, 2000–2007), have also provided useful 

discriminants to test correlations among some ignimbrite sheets. 

Mineral-chemical data for other phenocrysts (biotite, augite) 

show less variation among San Juan ignimbrites.

Critical to many of the interpretations presented here is 

the large array (120 localities; in total, 125 mineral and groundmass ages, table 3, CD-ROM) of new high-resolution 40Ar/39Ar 

age determinations for volcanic rocks in the San Juan region 

(Lipman and McIntosh, 2008). Age results for sites within the 

northeastern San Juan map area (81 samples) are plotted on the 

map. Especially useful have been single-crystal laser-fusion 

ages (SCLF) on sanidine phenocrysts, obtained wherever this 

phase was present; additional determinations are incrementalheating plateau ages for biotite and hornblende phenocrysts 

and for groundmass concentrates from lavas lacking datable 

phenocryst phases. The ages were determined at the New 

Mexico Geochronology Research Laboratory by methods 

similar to those described in McIntosh and Chapin (2004). 

Multiple samples were dated for most major volcanic units to 

evaluate correlations and to improve precision of pooled ages. 

For most single-crystal laser-fusion ages, 10–15 feldspar grains 

were analyzed for each site; as many as 36 grains were dated 

for a few stratigraphically critical sites. All new and previously 

published ages are calibrated to Fish Canyon Tuff at 28.02 Ma. 

Analytical uncertainties for individual samples are reported 

to two standard deviations (95% confidence level); pooled 

sanidine ages for multiple sites from a single unit are listed as 

weighted means, with uncertainties as the standard error (Se). 

Interpreted preferred ages (table 2; fig. 6, sheet 2) are influenced by stratigraphic relations, supplemented by additional age 

determinations from beyond the map area; interpretive problems 

with ages for some units are discussed in Lipman (2000) and 

Lipman and McIntosh (2008).

Cited paleomagnetic-pole directions for the San Juan rocks 

are from published sources (Tanaka and Kono, 1973; Diehl 

and others, 1974; Rosenbaum and others, 1987; Lipman and 

McIntosh, 2008).

In addition to the data summarized in the pamphlet, 

complete analytical results for the new 40Ar/39Ar determinations, whole-rock chemistry of volcanic units present in 

the northeastern San Juan region, and supplemental photographs of major rock units and caldera features are included 

on the CD-ROM that accompanies the printed report or at 
3

http://pubs.usgs.gov/sim/3123/. Successful completion of 

fieldwork and interpretations of the caldera history have been 

strongly dependent on the productive multi-year collaborative 

studies with William McIntosh of the New Mexico Institute of 

Mining and Technology, principle results of which have been 

reported elsewhere (Lipman and McIntosh, 2008). In addition 

to technical collaboration with numerous other geologists, as 

noted or referenced in this report, I also thank many friends in 

the San Juan region who provided diverse hospitality, logistical 

support, help with back-country and property access, and other 

assistance during fieldwork: especially John and Patty Judson 

of Quarter-Circle Circle Ranch in Cochetopa Park; Billy, Curt, 

and Lee Ann Cadwell of Cathedral Creek Ranch; Robert Sharp 

of the Old Agency Ranch along Los Pinos Creek; and Pip and 

Aaron Conrad of the Rafiki Ranch near Villa Grove.

Regional Framework

Mid-Tertiary volcanic deposits once formed continuous cover across much of the Colorado Rocky Mountains and 

into northern New Mexico (fig. 1), constituting a composite 

Southern Rocky Mountain volcanic field (SRMVF), for which 

the San Juan region is the largest erosional remnant (Steven, 

1975; Lipman, 2007). Subareas of the SRMVF, dismembered 

by subsequent erosion, have commonly been described as 

multiple separate volcanic fields (San Juan, Sawatch, Thirtynine 

Mile, Latir, West Elk, Central Colorado, and others) rather 

than as time-space transgressive magmatic foci within a large 

composite field.

Activity in the SRMVF peaked between 37 and 26 Ma 

(McIntosh and Chapin, 2004; Lipman, 2007; Lipman and 

McIntosh, 2008). Dominantly intermediate-composition lavas 

and breccias (andesite-dacite) erupted from widely scattered central volcanoes were characteristic early phases of 

SRMVF activity. Major volcanic foci, initially established 

by clustered stratocones, became eruption sites for about 30 

caldera-associated ignimbrites of more silicic compositions 

(table 1; fig. 7, sheet 2), in response to increased magmatic 

input focused at these sites. Composite volumes of the earlyintermediate volcanoes are large; in the San Juan region stratigraphic sequences commonly are more than a kilometer thick, 

and total volume, estimated at 25,000 km3

 (Lipman and others, 

1970), exceeds that of the later-erupted ignimbrites by about 50 

percent. 

The original areal extent of the overall SRMVF appears 

to have exceeded 100,000 km2, with a total volume of volcanic 

deposits greater than 50,000 km3 (Lipman, 2007). Peak magmatic volumes in the SRMVF, associated with ignimbrite eruptions, define a general (if imperfect) progression (fig. 7) from 

early eruptions along the Sawatch Range in central Colorado 

(37–33 Ma; McIntosh and Chapin, 2004), southward into the 

San Juan region (32–27 Ma; Lipman and others, 1970), and 

later to the 25-Ma Latir-Questa magmatic center in northern 

New Mexico (Lipman, 1988; Johnson and others, 1989) and the 

23-Ma Lake City caldera in the far western San Juan Mountains 

(Lipman, 1976; Bove and others, 2001). 

Mid-Tertiary volcanic rocks of the northeastern San Juan 

Mountains lie along the broad boundary between Precambriancored uplifts of the Southern Rocky Mountains, which 

formed by contraction during low-angle plate subduction in 

Late Cretaceous and Early Tertiary time, and less-deformed 

Paleozoic and Mesozoic sedimentary rocks along the northeast 

flank of the Colorado Plateau. These differences in geologic 

setting exerted important controls on structural and morphologic 

evolution during volcanism. To the south and west in the San 

Juan region, ignimbrite volcanism generated a well-stratified 

plateau, interrupted by local complexities associated with calderas. The original mid-Tertiary volcanic terrain was much like 

the Altiplano of the central Andes, because voluminous eruptions buried and subdued most of the pre-existing topography. 

In contrast, northeastern parts of the San Juan volcanic accumulation lap onto rugged paleotopography associated with earlier 

Tertiary uplifts of the Southern Rocky Mountains. Valleys, 

developed during erosion of the early Tertiary uplifts and 

during the growth of central volcanoes prior to large ignimbrite 

eruptions, strongly influenced the distribution of subsequently 

emplaced volcanic deposits (Chapin and Lowell, 1979; Steven 

and others, 1995). Some major paleovalleys survived the entire 

period of volcanism, and many present-day San Juan drainages 

are directly inherited from the mid-Tertiary landscape, leading 

to our fieldwork expression “once a valley, always a valley.” 

Most conspicuous within the map area is the course of lower 

Saguache Creek, which empties into the San Luis Valley at the 

town of Saguache (fig. 4). This modern drainage coincides with 

a broad paleovalley between Oligocene stratocones to the north 

and south. All the major Oligocene ignimbrites of the region 

were deposited preferentially along this paleovalley. Similarly, 

the present-day basin of Cochetopa Park is similarly inherited 

morphology from an Oligocene caldera.

Exposed pre-Tertiary rocks also provide useful information 

on paleotopography at the inception of volcanism and subsequent events. Notably, small windows of Precambrian granite, 

but no Mesozoic sedimentary rocks, are exposed around margins of the North Pass caldera, showing that this caldera formed 

within a prevolcanic highland, probably a southern continuation 

of the Sawatch Range uplift of Late Cretaceous age. In eastern 

parts of the map area, considerable paleorelief existed on the 

Precambrian rocks prior to burial by Tertiary volcanism. Farther 

west and north, gently dipping Mesozoic sedimentary rocks are 

widely present beneath the volcanic cover, reflecting proximity 

to the Colorado Plateau.

Due to greater paleorelief, the stratigraphic record of 

sequential eruptions is less complete in the northeastern San 

Juan Mountains than in central parts of this volcanic region. 

Many volcanic deposits in the northeast sector accumulated 

in deep valleys, which were incompletely filled and then 

re-excavated by erosion between successive eruptions. The 

regional ignimbrites, rather than forming a stratified plateau, 

can be preserved in inverted topographic order, with earlier tuff 

sheets capping ridges and younger units exposed at lower levels 

within paleovalleys. In many places, welded tuffs are exposed 

as isolated scabs, unconformably against slopes of paleovalleys, 

without stratigraphic continuity between sequential deposits. 
4

Frequent miscorrelations of ignimbrite units in previously 

mapped local areas of the northeastern San Juan region have 

resulted from such complexities, as well as from limited exposures due to forest cover, incomplete knowledge of the regional 

eruptive sequence, and inadequate recognition of petrographic 

distinctions among tuff sheets.

Erosional postvolcanic dissection to depths of as much as 

1.5 km in the San Juan region has exposed diverse features of 

intracaldera ignimbrite and associated caldera-fill deposits that 

accumulated to multikilometer thickness within concurrently 

subsiding caldera structures. The calderas display a variety of 

postcollapse structures, and caldera-forming events produced 

complex fault geometries that localized late mineralization, 

especially at Platoro, Creede, and several western San Juan 

calderas. Most of the San Juan calderas have been severely 

eroded, exposing internal deep structures of the caldera, and their 

identification has been dependent on detailed geologic mapping. 

In the map area, however, the near-pristine Oligocene morphology of the Cochetopa Park caldera has been exceptionally 

preserved because of rapid infilling by moat sediments, which 

were preferentially eroded only during the past few million 

years. Cochetopa Park is also marked by a large negative gravity 

anomaly (Plouff and Pakiser, 1972; Drenth and Keller, 2004). As 

a result, this feature has long been recognized as a major caldera 

(Steven and Lipman, 1976), even though its association with 

a specific ignimbrite eruption has remained obscure (Lipman, 

2000, p. 37). In contrast, the newly recognized North Pass 

caldera is largely concealed beneath erosion-resistant calderafilling lavas that form an inverted present-day topography, in the 

Cochetopa Hills along the present-day Continental Divide.

Early Northern Volcanism Along the Sawatch 

Trend

Volcanism in northern parts of the SRMVF began at least 

as early as 38 Ma, as recorded by andesitic lavas and breccias in 

the Thirtynine Mile volcanic area (McIntosh and Chapin, 2004). 

Caldera sources for all early-erupted (37–33 Ma) large ignimbrites of the SRMVF (table 1) form a north-northwesterly alignment along the crest of the Sawatch Range (fig. 1). The first 

major regional ignimbrite was the far-traveled Wall Mountain 

Tuff (Chapin and Lowell, 1979) erupted at 36.9 Ma (McIntosh 

and Chapin, 2004) from vents above the 25- x 35-km Mount 

Princeton batholith (Johnson and others, 1989; Lipman, 2007), 

the largest Tertiary intrusion in the Rocky Mountains (fig. 1, 

table 1). Distal Wall Mountain Tuff spread eastward more than 

100 km from its source, onto the High Plains between Denver 

and Colorado Springs (Chapin and Lowell, 1979). This ignimbrite (Twm) has now been recognized as local valley-fill deposits in the west-central map area (photo 1, CD-ROM), 80–90 km 

southwest of Mount Princeton, which represent the only known 

localities west of the Sawatch Range. 

North of the Mount Princeton batholith is the Grizzly Peak 

caldera and associated tuff (Fridrich and others, 1998), active at 

34.3 Ma (McIntosh and Chapin, 2004). Nested within the south 

margin of the Princeton batholith is the slightly younger Mount 

Aetna caldera (fig. 1), source of the 34.1-Ma Badger Creek Tuff 

(Epis and Chapin, 1974; Johnson and others, 1989) and associated intrusions (34.1–29.6 Ma; McIntosh and Chapin, 2004). 

Small remnants of Badger Creek Tuff (Tbc) are preserved at the 

northeast corner of the map area. 

Just east of the map area, small exposures of megabreccia, mapped by Olson (1983) in upper Marshall Creek at the 

northeast margin of the San Juan Mountains, were interpreted 

by Gregory and McIntosh (1996) as collapse breccia within an 

inferred Marshall caldera. Recent studies east of the Continental 

Divide have located a previously unrecognized caldera margin, 

at a striking topographic feature known as "The Gate" near the 

boundary between the Bonanza and Mount Ouray quadrangles 

(fig. 4); thick welded tuff of intracaldera type banks against 

Precambrian wall rocks (P. Lipman, unpub. data, 2008). This 

caldera margin continues across Marshall Pass and connects 

with the megabreccia in Marshall Creek, but much of the 

Marshall caldera is concealed beneath younger lava flows and 

truncated to the southeast by the Bonanza caldera. The Marshall 

caldera appears to have been the source of the 33.9-Ma Thorn 

Ranch Tuff, a widespread ignimbrite in the Thirtynine Mile 

volcanic remnant to the northeast (Epis and Chapin, 1974; 

McIntosh and Chapin, 2004). Surprisingly, no preserved outflow remnants of Thorn Ranch Tuff or other ignimbrite sheet 

that could have erupted from this caldera have been found 

farther west or south in the San Juan Mountains.

The more completely preserved Bonanza center, including the caldera source of the Bonanza Tuff at 33.2 Ma (photo 2, 

CD-ROM), also lies within the San Juan Mountains (fig. 4) but 

is equally appropriately considered the youngest and southernmost caldera along the Sawatch trend (Varga and Smith, 1984; 

McIntosh and Chapin, 2004). The Bonanza and The Marshall 

calderas mark the beginning of caldera progression to the southwest into the core of the San Juan volcanic region. The Bonanza 

center contains voluminous andesite erupted before and after 

ignimbrite eruptions, as well as postcaldera intrusions.

San Juan Volcanic Region

The San Juan Mountains are the largest erosional remnant of the SRMVF (fig. 1). Preserved rocks of the San Juan 

region occupy an area of more than 25,000 km2 and have a 

volume of about 40,000 km3. They cover a varied basement 

of Precambrian to early Tertiary rocks along the uplifted and 

eroded west margin of the Late Cretaceous to early Tertiary 

(Laramide) uplifts of the Southern Rocky Mountains and 

adjoining portions of the San Juan Basin on the eastern 

Colorado Plateau (fig. 1). The San Juan region is one of many 

loci of Tertiary volcanic activity—including the Sierra Madre 

Occidental, Trans-Pecos, Mogollon-Datil, Absaroka, Challis, 

and Lowland Creek fields—that developed along the eastern 

Cordilleran margin of the North American plate, probably in a 

complex response to changing subduction geometry along its 

west margin.

As mid-Tertiary volcanism migrated southward from the 

Sawatch Range trend, widely scattered intermediate-composition centers (dominantly andesite, lesser dacite, minor rhyolite) 

erupted lava flows and flanking volcaniclastic breccias in the 
5

San Juan region starting at 35–34 Ma (Lipman and others, 1970; 

Lipman and McIntosh, 2008). The intermediate-composition 

assemblage, which constitutes about two-thirds the volume 

of the preserved volcanic assemblage, is widely overlain by 

numerous large-volume compositionally diverse ignimbrite 

eruptions and associated caldera collapses (Steven and Lipman, 

1976). After about 26 Ma, volcanism shifted to a bimodal 

assemblage dominated by trachybasalt and silicic rhyolite, concurrent with the inception of regional extension during establishment of the Rio Grande rift.

Large ignimbrite eruptions began in the northeast San Juan 

Mountains with eruption of the newly recognized Saguache 

Creek Tuff (Tsc) from the North Pass caldera at 32.2 Ma 

(Lipman and McIntosh, 2008). Ignimbrite eruptions gradually 

became widespread in the southeast San Juan region at about 

29.4 Ma (Platoro caldera complex), followed shortly by eruptions mainly of crystal-poor rhyolite from western San Juan 

calderas (Steven and Lipman, 1976; Bove and others, 2001). 

Ignimbrite activity progressively focused in the central San Juan 

Mountains, leading to eruption of the enormous Fish Canyon 

Tuff (5,000 km3

 of monotonously uniform crystal-rich dacite) 

and collapse of the 35 x 75 km La Garita caldera at 28.0 Ma 

(Lipman, 2000, 2006; Bachmann and others, 2002). In the central San Juans, seven major eruptions of compositionally diverse 

ignimbrite, with volumes of 100–1,000 km3, erupted during a 

1.1-m.y. interval from calderas nested within the enormous La 

Garita caldera (fig. 2). At each caldera, initial post-ignimbrite 

volcanism consisted of intermediate-composition to silicic lavas 

and breccias that variably filled the caldera depressions, some to 

overflowing. 

The eruptive and subsidence history of the most northerly of the central San Juan systems, the San Luis complex 

from which three regional ignimbrite sheets were erupted in 

rapid succession at 26.9 Ma (Lipman and McIntosh, 2008), is 

closely intertwined with development of the Cochetopa Park 

caldera (figs. 2, 4). In particular, the final large eruption, which 

produced the Nelson Mountain Tuff, appears to have triggered 

subsidence both at the eruption site in the San Luis complex and 

at Cochetopa Park, as discussed below. 

Early Lavas and Intrusions: SaguacheCochetopa Region (35–32 Ma)

As elsewhere in the San Juan region, ignimbrite sheets 

and other rocks associated with calderas in the SaguacheCochetopa region overlie thick lava sequences that erupted 

from large central volcanoes (fig. 4). In comparison with the 

early-intermediate assemblage farther south and west in the 

San Juan Mountains, (1) the northeastern area contains higher 

proportions of proximal lavas and breccias relative to distal 

laharic conglomerates and other volcaniclastic rocks (photo 3A, 

CD-ROM ), and (2) dacite and rhyolite are more voluminous 

components of the dominantly andesitic lava assemblage.

These early flows are broadly correlative with the Conejos 

Formation elsewhere in the San Juan region (Lipman and 

others, 1970; Dungan and others, 1989), but several earlyerupted tuff sheets are interstratified with lavas of ages that 

predate all ignimbrite units from farther south and west in the 

San Juan Mountains. These interstratified tuff sheets, including several from caldera centers along the Sawatch trend, and 

datable sanidine phenocrysts in some silicic lavas help define 

the eruptive history of early lavas in more detail than has been 

possible elsewhere in the region. The lavas that are interstratified with the early tuff sheets, as well as andesitic and dacitic 

lavas that ponded within ignimbrite calderas throughout the San 

Juan region and are described as late phases of individual caldera cycles, are compositionally indistinguishable from earliererupted lavas of the Conejos assemblage and are concurrently 

interpreted to constitute eruptive continuity in the dominantly 

intermediate-composition regional volcanic field (SRMVF).

In the northeastern San Juan region, lavas are locally 

exposed over a vertical range of more than 1,000 m from along 

Saguache Creek to the Continental Divide, although thicknesses 

decrease northward toward the Gunnison valley. Total thickness 

and volume is far greater for the early lavas than for interstratified and overlying ignimbrite sheets. At least eight eruptive 

centers for Conejos lavas lie within the area of figure 4, but 

several of the best preserved are east of the detailed geologic 

map. These centers probably include the earliest eruptions in 

the San Juan region, with activity becoming younger to the west 

and south:

(1) A large composite stratocone or cluster of early 

Conejos-age volcanoes, within which the Bonanza caldera 

is centered, is overlain by the 33.2-Ma Bonanza Tuff on its 

flanks and must be among the oldest of these centers, although 

detailed mapping and 40Ar/39Ar dating are incomplete for 

much of the area. These lavas, locally designated the Rawley 

Andesite (Burbank, 1932; Varga and Smith, 1984), include 

dacite and rhyolite flows low in the section, resting directly on 

Precambrian basement. An upper lava sequence of the Bonanza 

center, including caldera-filling flows, is also dominantly 

andesitic; its age is bracketed as 33–32 Ma by the underlying 

Bonanza Tuff and overlying 32.2-Ma Saguache Creek Tuff.

(2) South of the Saguache valley, a thick lava pile marks 

the north flanks of another large central volcano of Conejos age, 

also east of the map area but separate from the Bonanza center. 

The Bonanza Tuff wedges out within these southern lavas, 

above a sequence of dark andesite flows, but below capping 

lavas of porphyritic dacite on Tracy Mountain (fig. 4, LG). A 

petrographically unusual dacite flow high in this assemblage, 

containing sanidine phenocrysts as large as 2 cm across, yielded 

a 40Ar/39Ar age of 32.72±0.09 Ma, consistent with its inferred 

position beneath the Saguache Creek Tuff. 

(3) Well-exposed radial dikes and outward-dipping andesitic lavas just east of the North Pass caldera define the core of 

another eastern volcano centered in the Jacks Creek drainage 

(fig. 4, LMNE; Bruns and others, 1971) that is about the same 

age as Rawley Andesite at Bonanza. The Jacks Creek volcano 

predates eruption of the Bonanza Tuff, which laps out against 

its eroded flanks and onto dikes in its core; in-progress 40Ar/39Ar 

determinations for lavas and intrusions of this center are mostly 

in the range 34–35 Ma (W. McIntosh and P. Lipman, unpub. 
6

data, 2007–08), among the oldest dated rocks from the San Juan 

region. Andesitic to rhyolitic lavas that are capped by scattered 

small erosional remnants of Bonanza Tuff as far east as the 

Continental Divide just north of the North Pass caldera (fig. 4, 

NP) also must be roughly correlative to the Rawley Andesite. 

(4) Farther northwest of the Continental Divide, the large 

(3.5 x 9 km) Needle Creek intrusion (fig. 4, WB) is exposed at 

near roof level and is texturally variable, ranging from nearly 

aphanitic andesite to fine-grained granodiorite. This body in 

places appears to have grown as coalesced dikes; elsewhere, 

subhorizontal upper contacts suggest growth as a laccolithic 

complex. Despite its northeasterly trend, aligned with ignimbrite-related structures farther southwest, phases of this intrusion have yielded hornblende 40Ar/39Ar ages of 34.3–34.4 Ma 

(table 2), suggesting that it constitutes the early-Conejos-age 

source of the large surrounding sequence of proximal andesitic 

lavas. This intrusion and associated dikes (photo 3B, CD-ROM) 

likely mark the core of another large andesitic stratovolcano. 

A nearby dacite flow, low in the lava assemblage, has sanidine 

40Ar/39Ar ages of 34.0–34.1 Ma. The Needle Creek intrusion is 

also spatially associated with voluminous younger silicic lava 

flows within the upper Conejos Formation (rhyolite of Barret 

Creek; 29.6–30.1 Ma; fig. 4), an unusual assemblage for the 

precaldera assemblage. Unroofed southern parts of the Needle 

Creek intrusion are overlain by unaltered Fish Canyon Tuff, 

indicating that deep erosion occurred prior to eruption of this 

ignimbrite at 28 Ma. 

(5) The near-horizontal roof zone of another large highlevel intrusion (at least 3.5 km across), composed of variably 

altered porphyritic dacite and fine-grained granodiorite, is 

exposed along lower canyon slopes in upper Long Branch 

(SM). This body, with an isotopic age of about 33 Ma (table 2), 

was erosionally unroofed (or nearly so) prior to eruption of the 

Bonanza Tuff at 33.2 Ma and appears likely to constitute the 

late-intruded core of another early volcanic edifice, east of the 

Needle Creek intrusion. 

(6) The core of yet another large central volcano that consists dominantly of andesite is marked by an elliptical pluton of 

porphyritic hornblende andesite, the Sawtooth intrusion (1 x 1.5 

km across), and associated smaller plugs and dikes farther south 

that intrude Conejos lavas and breccias on the northwest inner 

slope of the Cochetopa Park caldera (photo 3C, CD-ROM). A 

cluster of northwest-trending dikes of compositionally similar 

hornblende andesite cut Conejos rocks exposed on the floor 

of the caldera across the valley of Cochetopa Creek to the east 

(RD). Both groups of hornblende-bearing intrusions appear to 

be central parts of a large stratocone that is flanked to the northwest and northeast by the lava piles on Sawtooth Mountain and 

Razor Creek Dome (fig. 4, ST, RD). The age of this volcano is 

convincingly bracketed only by the underlying Wall Mountain 

Tuff (Twm, 36.9 Ma) and overlying Sapinero Mesa Tuff (Tws, 

28.3 Ma). The volcano may be largely or entirely younger than 

the Saguache Creek Tuff (Tsc, 32.2 Ma), but field and age 

relations remain ambiguous. The capping flow on Sawtooth 

Mountain yielded a hornblende age of 32.00±0.09 Ma, but 

a dike cutting the Conejos lava sequence low on the south 

flank of Razor Creek Dome has a hornblende 40Ar/39Ar age of 

34.61±0.16 Ma. Erosional remnants of Saguache Creek Tuff 

rest directly on prevolcanic rocks around the northwest flank of 

Sawtooth Mountain, and poor exposures of this tuff on heavily 

timbered north slopes appear to be overlain by andesite lava 

flows on both sides of upper East Beaver Creek (SH). In this 

case, the entire Sawtooth Mountain lava sequence would have 

been emplaced during the interval 32.2–32.0 Ma. Alternatively 

(and seemingly less likely), this edifice might predate and have 

been severely eroded prior to eruption of the Saguache Creek 

Tuff, which then banked unconformably against its northwest 

flank.

(7) A highly altered Conejos-age intrusion of porphyritic biotite-plagioclase dacite that forms an erosional basin 

within a structural ridge between two northeast-trending 

grabens, exposed in Blue Creek along the southwest margin 

of Cochetopa Park caldera (CS), is a probable center for thick 

andesitic lavas that underlie the 32.2-Ma Saguache Creek Tuff 

along the Continental Divide and separate Cochetopa Park 

from La Garita caldera to the south (fig. 4). These altered 

rocks are surrounded and covered by unaltered Fish Canyon 

and Carpenter Ridge Tuffs. West of the map area, along the 

north margin of La Garita caldera, some dacite and rhyolite 

lavas are as young as 29.7 Ma (Lipman, 2006), but direct 

stratigraphic relations to Saguache Creek or Bonanza Tuffs are 

absent. 

(8) A dike-like plug of nearly aphanitic basalt at point 

Benny (NP; triangulation station herein named point Benny), 

about 5 km east of the North Pass caldera, is an exceptionally mafic late Conejos intrusion (Tbpi) that forms the vent 

for a basaltic cone and an associated basaltic flow (photo 4, 

CD-ROM). A mean groundmass 40Ar/39Ar age of 30.2±0.1 Ma 

and onlap by Fish Canyon Tuff show that this vent and flow 

are part of the Conejos Formation (upper units), rather than 

related to compositionally similar Miocene basalt (Hinsdale 

Formation).

Saguache Creek Tuff and North Pass 

Caldera Cycle (32.2 Ma)

A recurrent problem in studies of San Juan volcanism has 

been establishing reliable regional correlations for multiple 

widespread but discontinuously exposed ignimbrite units of 

similar petrologic characteristics. Among such puzzles has been 

the inferred extreme northeast extent of the Sapinero Mesa Tuff 

(Tws), a large-volume ignimbrite erupted from the western 

San Juan caldera cluster (fig. 10 of Steven and Lipman, 1976). 

Since the late 1960s, the Sapinero Mesa has been inferred to 

include densely welded crystal-poor rhyolite ignimbrite that 

crops out widely in the valley of Saguache Creek, as much as 

90 km from the western San Juan caldera cluster (Bruns and 

others, 1971; Simon and Wendlandt, 1999). This distribution of 

the Sapinero Mesa is highly asymmetric to the source caldera, 

and the ignimbrite in the Saguache valley is atypically thick and 

densely welded, compared to similarly distal portions of this 

unit elsewhere. 
7

Such concerns, along with recognition of problems 

with published interpretations of the Cochetopa Park caldera (Lipman, 2000, p. 37), led to tentative inference that the 

rhyolitic ignimbrite in Saguache Creek might be a separate 

ignimbrite erupted from this more proximal caldera. Initial 

results from restudy of the Cochetopa area demonstrated (1) 

that the ignimbrite in Saguache Creek was indeed a different ignimbrite, now named the Saguache Creek Tuff (Lipman 

and McIntosh, 2008) and distinguishable from the Sapinero 

Mesa Tuff by hand-lens petrography (absence of biotite), 

more alkalic rock and mineral compositions, and isotopic ages 

(Lipman and Calvert, 2003), but (2) that at least the last major 

subsidence at Cochetopa Park was too young to be associated 

with eruption of this ignimbrite (postdating emplacement of 

the Fish Canyon Tuff, which overlies the welded ignimbrite in 

Saguache Creek). Detailed mapping in Cochetopa Park and farther east has now documented that the previously unrecognized 

North Pass caldera (figs. 4, 5) was the source of the Saguache 

Creek Tuff. 

As interpreted here, the North Pass caldera is a filled 

depression, about 15 km across, that straddles the present-day 

Continental Divide in the Cochetopa Hills (fig. 4, NP). This 

caldera cycle includes precursor tuffs and lavas of probable 

small volume and limited areal extent, the regional calderarelated Saguache Creek Tuff, a thick sequence of lava flows 

and volcaniclastic rocks that filled the caldera, and the lateerupted Luders Creek Tuff that may have erupted from within 

central parts of this caldera that are now covered by younger 

ignimbrites from the San Juan central caldera complex. Small 

preserved areas of younger andesite flows (~30 Ma) may be 

late parts of the North Pass cycle or just continued background 

flux of intermediate-composition magmatism. An alternative proposed caldera source for the Saguache Creek Tuff, 

30 km farther northeast along the Continental Divide (Turner 

and others, 2003), instead appears to mark the core of a large 

andesitic stratovolcano that predated the 33-Ma Bonanza Tuff; 

no mapped geologic features support the presence of a major 

ignimbrite caldera in this area.

In contrast to Cochetopa Park, no well-defined gravity low 

is associated with the North Pass caldera, perhaps due to the 

presence of thick dense caldera-filling lavas and (or) presence 

of relatively mafic subvolcanic intrusions. Similar lack of strong 

gravity expression characterizes several other calderas along 

the east side of the San Juan Mountains, including Bonanza and 

Platoro calderas (Plouff and Pakiser, 1972).

High-resolution 40Ar/39Ar dating (Lipman and McIntosh, 

2008) shows that the entire North Pass caldera cycle, including 

a possible precursor ignimbrite of small volume, the culminating eruption of the Saguache Creek Tuff, post-caldera lavas, 

and late eruption of the Luders Creek Tuff, developed within a 

brief interval at 32.5–32.2 Ma (table 2). For the subsequent 4 

m.y., volcanism was relatively quiescent within and adjacent 

to the caldera area, limited to a few small andesite lava flows, 

and erosion was dominant. In many places, the 28-Ma Fish 

Canyon Tuff rests directly on Saguache Creek Tuff or occupies 

paleovalleys eroded below depositional levels of the Saguache 

Creek. 

Precursor(?) Lavas (32.2 Ma) and Tuff of Spanish

Creek (32.5 Ma)

Three small areas of distinctive rhyolitic ignimbrite, 

the tuff of Spanish Creek, are exposed within short distances 

east and north of the North Pass caldera; this unit has yielded 

ages only a few hundred thousand years older than the main 

ignimbrite eruption from this caldera. These crystal- and 

quartz-rich tuffs (sanidine 40Ar/39Ar ages of 32.5 Ma) overlie 

the assemblage of early lavas, are younger than the Bonanza 

Tuff, and seem plausibly interpreted as initial eruptions broadly 

associated with the North Pass cycle. Some of these tuffs were 

initially considered potential candidates for distal deposits from 

Sawatch Range centers, but their younger ages preclude such 

correlations. Breccia clasts from the tuff of Spanish Creek are 

present along the northwest margin of the North Pass caldera, 

which helps constrain the timing of caldera collapse.

A large rhyolite lava dome centered between Archuleta 

and Los Creeks (CP) and an east-west-trending stubby dike-like 

body of altered dacite porphyry just west of Cochetopa Pass at 

the southwest margin of the North Pass caldera (NP) have both 

yielded 40Ar/39Ar ages of about 32.2 Ma (table 3, CD-ROM). 

Although these ages are analytically indistinguishable from 

the time of caldera formation, both units appear to be truncated 

unconformably by the caldera topographic wall and caldera-fill 

lavas, indicating late-precaldera emplacement.

Saguache Creek Tuff (32.25 Ma)

The Saguache Creek Tuff (Tsc), as named by Lipman and 

McIntosh (2008), is a distinctively alkalic crystal-poor rhyolitic 

ignimbrite (72.3–75.4% SiO2) that is widely preserved in the 

Saguache basin (photo 5, CD-ROM), as well as in scattered erosional remnants farther northwest. It is especially well exposed 

along Saguache Creek along the east edge of the map area, 

where it overlies the Conejos Formation and is overlain by Fish 

Canyon Tuff. Its newly mapped distribution is roughly symmetric around the North Pass caldera, if allowance is made for 

more Oligocene erosion in the northwest sector and widespread 

cover by younger rocks to the southwest. On published U.S. 

Geological Survey (USGS) geologic quadrangle maps, many of 

the scattered northwestern outcrop areas, which are now reliably 

identified as Saguache Creek Tuff by composition and isotopic 

age, were previously miscorrelated with diverse regional ignimbrites from the central caldera cluster.

The maximum exposed thickness of the tuff, 50–75 m, 

occurs along the axial Saguache paleovalleys. Although widely 

eroded elsewhere, the areal extent and thickness of preserved 

densely welded exposures yield an estimated 250 km3

 of 

magmatic volume for the outflow sheet. If this tuff is thickly 

ponded within its source caldera, as is typical of other large 

San Juan ignimbrites, its total volume (neglecting downwind 

ash) could be ~500 km3. Where thickness of the Saguache 

Creek Tuff exceeds a few tens of meters, the interior of the 

sheet is densely welded (photos 5A,B, CD-ROM), commonly 

with gray pumice lenses (to 10 cm) in a purple-brown matrix. 

A central zone of spherical to lenticular gas cavities 5–10 cm 
8

in longest dimension is common where densely welded (photo 

5D, CD-ROM), similar to other crystal-poor ignimbrites in the 

region. In places, the Saguache Creek is densely welded nearly 

to its base, but the lower 5–10 m commonly is nonwelded white 

to tan tuff. Where not eroded, 10–20 m of upper nonwelded 

vapor-phase to glassy tuff is widely present, though typically 

largely covered by talus and slope wash from overlying units 

(photo 5B, CD-ROM). 

The Saguache Creek Tuff widely rests on early (35–33 Ma) 

andesitic lavas, but as much as 10–20 m of water- and windreworked tuffaceous sediments are present locally between 

andesite and overlying ignimbrite along the axial Saguache 

paleovalley (photo 5C, CD-ROM). In the paleovalley, this 

ignimbrite is widely overlain conformably by Fish Canyon 

Tuff that is 4 m.y. younger, with only thin intervening tuffaceous sediments. Weakly welded biotitic rhyolitic ignimbrite 

that appears to be distal Sapinero Mesa Tuff and discontinuous 30-Ma andesite lavas are also present locally between the 

Saguache Creek and Fish Canyon Tuffs along Saguache Creek. 

To the northeast, the Saguache Creek laps out against Bonanza 

Tuff and overlying andesite on the flank of the Bonanza caldera 

center. In contrast to the conformable sequence in the Saguache 

paleovalley, the discontinuous erosional remnants of Saguache 

Creek Tuff typically cap ridges and mesas to the northwest (HG, 

IR), while Fish Canyon Tuff was deposited topographically 

lower, within re-excavated paleovalleys. In places, paleolandside and talus of Saguache Creek clasts are overlain by Fish 

Canyon Tuff along walls of such paleovalleys, documenting 

times of valley erosion.

No in-place Saguache Creek Tuff or older rocks are 

exposed within central parts of the North Pass caldera, but an 

exceptionally thick (>200 m) elliptical exposure of tuff interpreted as the wedge edge of an intracaldera accumulation is 

banked against the western topographic margin (NP). This high 

ridge of Saguache Creek Tuff dips 15°–25° into the caldera and 

is overlain by caldera-fill units. In contrast, proximal outflow 

ignimbrite sheets are typically absent or thin on high-standing 

caldera rims elsewhere in the San Juan region. Only small 

scattered patches of outflow Saguache Creek Tuff, seemingly 

remnants of discontinuous deposits against high-standing 

irregular paleotopography, bank against older rocks of the 

Conejos Formation at varied topographic levels adjacent to the 

west caldera margin. Along the east margin of the North Pass 

caldera (TM), small remnants of outflow Saguache Creek Tuff 

are truncated abruptly and are covered by the lava sequence 

that is interpreted to have filled the caldera (fig. 8, sheet 2). All 

dated caldera-filling dacite and rhyolite lavas have yielded ages 

similar to, or younger than, the Saguache Creek Tuff (table 2).

Major phenocrysts (3–10% total) of the Saguache Creek 

Tuff are sodic low-barium sanidine (Or51Ab46An2.7Cn0.4) and 

plagioclase; biotite is absent or extremely sparse. Otherwise 

similar-appearing crystal-poor rhyolitic ignimbrites of the San 

Juan region, such as the Sapinero Mesa and Carpenter Ridge 

Tuffs, contain biotite phenocrysts, have more potassic sanidine 

compositions (typically Or60-65), and have less alkalic majorand trace-element compositions (lower Na, Zr, and light REE 

such as La and Ce; higher Sr and Ba: table 4). Despite broad 

similarities to other crystal-poor rhyolite tuffs of the area, these 

details show that the Saguache Creek Tuff is compositionally 

unique in the San Juan region. The eruption age is tightly constrained at 32.25±0.05 Ma, from the weighted mean of six new 

laser-fusion sanidine 40Ar/39Ar determinations from geographically diverse sites (table 2). The Saguache Creek Tuff has a 

reversed paleomagnetic direction, based on sample sites along 

Saguache Creek and west of the North Pass caldera (Lipman 

and McIntosh, 2008, table 3).

Formation of the North Pass Caldera

Eruption of the Saguache Creek Tuff from a nonresurgent 

caldera in the North Pass area, which was then filled by younger 

lavas, is documented by the distribution of this ignimbrite, 

abrupt truncations of regional units, local hydrothermal alteration along caldera margins, and the ages and compositions of 

caldera-filling rocks. The regional distribution of the Saguache 

Creek Tuff is broadly symmetric around flanks of the caldera, as 

shown by erosional remnants preserved as much as 15 km east, 

10 km south, and 30 km northwest (fig. 4). Absence of distal 

exposures to the southwest is due to widespread cover by young 

volcanic units of the central caldera cluster. Small isolated 

paleohills of Precambrian granitic rocks exposed around the 

margins of the caldera-filling assemblage, but nowhere within 

the depression, also help define limits of the North Pass caldera. 

Rocks filling the North Pass caldera, as well as the entire 

volcanic sequence east of the Continental Divide, dip gently 

eastward (5°–10°), probably in response to regional tilting along 

the entire west flank of the San Luis Valley segment of the Rio 

Grande rift zone (fig. 1). As a result, deeper older caldera-fill 

units crop out along the west margin of the North Pass caldera; 

younger deposits are preserved to the east. 

Caldera subsidence at North Pass postdates emplacement 

of the Saguache Creek Tuff, which is truncated along exposed 

sectors of both the east and west caldera margins, and predates 

the Fish Canyon Tuff. The caldera-margin unconformity that 

defines surface expression of the North Pass caldera, although 

unimpressive around much of its perimeter because precaldera 

Conejos (Rawley) andesite and dacite are lithologically similar to the caldera-filling lavas and volcaniclastic rocks, is well 

exposed on the east side just east of Sheep Creek, a tributary of 

Saguache Creek (TM). Here bleached and otherwise hydrothermally altered Conejos lavas and overlying Saguache Creek Tuff 

are abruptly truncated along a steep westward-facing contact, 

against which are banked thick ponded flows of andesite, overlain by anomalously thick and densely welded Fish Canyon Tuff 

(fig. 8; photo 6, CD-ROM). 

A similar truncation of regional units, including Saguache 

Creek Tuff, is associated with local hydrothermal alteration 

along the west caldera margin, although relations are less clear 

on this flank because of fault complexities and more limited exposures. On this side of the caldera, in roadcuts along 

Colorado Hwy 114 (northwest NP), highly disrupted Conejos 

rocks and small masses of welded Bonanza Tuff and tuff of 

Spanish Creek are intermixed with nonwelded Saguache Creek 

Tuff on scales too complex to depict even on a detailed geologic 
9

map (photo 7, CD-ROM ); the assemblage is interpreted as 

caldera-margin megabreccia. At the mouth of Pine Creek and 

in Hwy 114 roadcuts just to the north, blocks of tuff of Spanish 

Creek, along with fragments of Bonanza Tuff, probably represent more finely comminuted caldera-collapse breccia. The 

North Pass caldera-fill assemblage is also notably less altered 

than adjacent Conejos lavas and volcaniclastic rocks that define 

the east caldera margin east of lower Sheep Creek and the west 

margin along Lujan Creek (NP). 

The caldera was filled with thick dacite lava flows and 

local rhyolite domes (volcanics of Cochetopa Hills) that 

have isotopic ages similar to the 32-Ma Saguache Creek Tuff 

(photo 8, CD-ROM ). Otherwise lithologically similar precaldera lavas of the Conejos Formation and Rawley Andesite 

are largely or entirely older (33–35 Ma) in areas adjacent 

to the North Pass caldera. In addition, laharic volcaniclastic 

rocks within the caldera area, which mainly contain clasts of 

intermediate-composition lavas, locally contain sparse fragments of Saguache Creek Tuff, which requires that these 

deposits are also younger than otherwise similar clastic rocks of 

the Conejos Formation that underlie this ignimbrite adjacent to 

the caldera. 

The caldera-filling lavas are resistant to erosion and support 

a segment of the present-day Continental Divide; as a result, the 

caldera remains largely buried. The thickness of exposed caldera 

fill exceeds 500 m, but erosion has been insufficient to expose 

underlying rocks. As a result, neither the total subsidence nor the 

likely presence of thick intracaldera Saguache Creek Tuff can be 

evaluated from present-day exposures. 

Caldera-filling Lavas (Volcanics of Cochetopa 

Hills, 32.2 Ma)

The volcanics of Cochetopa Hills are a diverse assemblage 

of lava flows and volcaniclastic deposits that accumulated 

rapidly within the North Pass caldera after subsidence during 

eruption of the Saguache Creek Tuff. The bulk of the lavas 

at exposed levels are flows of porphyritic dacite that overlie 

diverse volcaniclastic rocks and scattered lava domes of crystalpoor rhyolite that have compositions similar to the Saguache 

Creek Tuff (table 4).

Massive flows of gray to tan crystal-rich dacite (Tnpd, 

dacite of East Pass Creek) are the dominant exposed fill in the 

caldera. The thickest and most widespread accumulations are 

north of North Pass (photo 8, CD-ROM ), where multiple flows 

are locally mappable, based on zones of basal vitrophyre and 

upper carapace breccia. Individual flows are as thick as 200 

m; total exposed thickness is greater than 500 m. These flows 

are petrographically fairly uniform, containing 20–30 percent 

phenocrysts of blocky plagioclase (to 1 cm), biotite, and clinopyroxene ± hornblende; sanidine is absent. Compositionally, 

this unit is typical dacite of the region (SiO2

, 61.4–66.0%). 

Biotite plateau 40Ar/39Ar ages for three samples from the main 

area of exposure are 32.07–32.31 Ma, consistent with eruption 

soon after the Saguache Creek Tuff at 32.25±0.05 Ma (table 2). 

A small outlier of similar dacite that is banked unconformably 

against a slope along the inferred south margin of the North 

Pass caldera (GC) yielded slightly older ages of 32.82±0.08 Ma 

(bio) and 32.75±0.15 Ma (hbl); interpretation of this flow as 

part of the caldera fill remains ambiguous. 

Scattered lava domes of phenocryst-poor gray to light-tan 

rhyolite (Tnpr, rhyolite of Taylor Canyon), exposed locally 

beneath the dacite of East Pass Creek, are also interpreted as fill 

of the North Pass caldera. Flow layering, glassy carapace breccia, and ramp structures near upper surfaces are typical (photo 

8A, CD-ROM). Analyses from several flows, exposed along the 

west caldera wall and farther to the east (74–76% SiO2

; 1–5% 

sanidine, plagioclase), are similar to the Saguache Creek Tuff 

in lacking biotite phenocrysts and containing elevated levels of 

incompatible trace elements. One flow yielded a sanidine age of 

32.15±0.10 Ma. Exposed thickness of the rhyolite is as much as 

150 m.

At lowest exposed levels, laharic conglomerate, sandstone, and water-reworked tuff (Tsa) are interlayered with 

and underlie the lava flows within the North Pass caldera. 

Most widespread are crudely bedded laharic breccias, along 

with better bedded conglomerate, that mainly contain clasts of 

dark-gray andesite and dacite in weakly indurated lighter-gray 

sandy matrix. Outcrops are rare; the unit typically forms loose 

cobbles on subdued slopes. These rocks, likely recording rapid 

postcollapse erosion of Conejos (Rawley) lavas along the inner 

caldera slopes, closely resemble volcaniclastic sediments of the 

older Conejos Formation. A key to identification as a younger 

volcaniclastic deposit is the presence of rare clasts of Saguache 

Creek Tuff. 

Thin deposits of white to light-tan bedded rhyolitic tuff 

(Tst), deposited in shallow-lake or low-energy-stream environments, are preserved discontinuously along the west caldera 

margin (NP), where they interfinger with and overlie older 

caldera-fill units. These deposits resemble younger tuffaceous 

sedimentary fill of the Cochetopa Park caldera to the west, but 

their higher topographic level and location along margins of the 

North Pass caldera suggest that they are late sedimentation in 

this caldera, probably in conjunction with eruption of rhyolitic 

lava flows or the Luders Creek Tuff. Obscure gully exposures 

along the west side of the Continental Divide demonstrate that 

at least some lake beds underlie the Luders Creek Tuff. The preserved thickness is less than 30 m, consistent with rapid filling 

of the North Pass caldera by the volcanics of Cochetopa Hills 

without any persistent lake or other sediment-filled basin.

Luders Creek Tuff (32.17 Ma)

A distinctive compositionally zoned ignimbrite, the Luders 

Creek Tuff (named by Lipman and McIntosh, 2008), crops out 

on the Continental Divide along the west side of the North Pass 

caldera. The Luders Creek Tuff grades from weakly welded tan 

rhyolite (Tlr: 72–73% SiO2; 5–10% plagioclase and sanidine 

phenocrysts, sparse biotite) upward into densely welded dark 

dacite (Tld: 66–68% SiO2; 20–30% plagioclase, sanidine, biotite, sparse clinopyroxene). It appears to be a late eruptive unit 

of the North Pass cycle, based on its location and age. 

Four sanidine 40Ar/39Ar ages for the Luders Creek Tuff 

(weighted mean: 32.17±0.04 Ma) are only slightly younger 
10

than the Saguache Creek Tuff (32.25±0.02 Ma), providing a 

tight age bracket for the entire sequence of lavas and volcaniclastic deposits within the North Pass caldera (table 2; table 

3, CD-ROM). Limited preserved distribution of the Luders 

Creek Tuff suggests probable eruption either from within the 

North Pass caldera beneath subsequent cover by Fish Canyon 

and Carpenter Ridge Tuffs or from within adjacent parts of the 

Cochetopa Park area. 

The Luders Creek Tuff is similar in appearance and phenocryst mineralogy to the much younger Nelson Mountain Tuff 

(26.9 Ma), with which this ignimbrite had been correlated previously (Cochetopa Creek Tuff of Steven and Lipman, 1976, fig. 

24). The small remaining areas of in-place Luders Creek Tuff 

overlie dacite and rhyolite flows (volcanics of Cochetopa Hills), 

and widespread laharic debris-flow deposits of Buffalo Creek 

Campground (Tdb), derived from Luders Creek Tuff, overlie 

the Saguache Creek Tuff and underlie Fish Canyon Tuff across 

much of the Saguache paleovalley. Some areas of shattered 

megablocks in the debris-flow deposit, as much as 25 m across 

(photo 8C, CD-ROM), are sufficiently large and conspicuous to 

map separately (Tdbm). The volume of Luders Creek Tuff in the 

debris-flow deposits exceeds that of preserved in-place exposures. The origin of this large debris flow is unknown, but it is 

locally interleaved with ~30-Ma andesitic lava flows and, therefore, ~2 m.y. younger than eruption of the Luders Creek Tuff.

Late Andesite Flows and Associated Rocks 

(30.0–30.5 Ma)

Several small areas of andesitic lava and a thin partly 

welded dacite tuff of similar age, deposited at about 30 Ma 

along the east side of the North Pass caldera (fig. 9, sheet 2), 

may be late phases broadly associated with this caldera cycle 

or, alternatively, local phases of the regional flux of dominantly 

intermediate-composition lava eruptions (Conejos Formation, 

upper units).

Oldest are a porphyritic hornblende andesite on hill 9519 

(Tahh: 58.1% SiO2) and an overlying aphyric andesite flow 

(Taha) that bank thickly against the sloping east wall of the 

caldera at Sheep Creek (figs. 8, 9, southwest TM). The thick 

hornblende andesite and its inclined columnar-jointed contact 

against the caldera wall somewhat resemble an intrusive body, 

but emplacement as a ponded lava flow is indicated by the presence of elliptical vesicles, even within the most massive parts 

of the unit, and by continuity with subhorizontal eastern parts 

of the flow that directly overlie Saguache Creek Tuff where 

the lava overflowed the caldera margin. These flows, which 

are overlain by the widespread laharic breccia derived from the 

Luders Creek Tuff, have yielded 40Ar/39Ar ages of 30.41±0.79 

Ma (hornblende) and 29.98±0.31 Ma (groundmass concentrate), 

respectively (table 2; table 3, CD ROM). The ponding of these 

lavas, as well as the anomalously thick and densely welded 

character of overlying Fish Canyon Tuff, demonstrate that the 

east side of the North Pass caldera remained incompletely filled 

at the time of their eruption, 2 m.y. after initial caldera collapse.

A thin weakly-welded ignimbrite of crystal-poor dacite 

(Tbg, tuff of Big Dry Gulch) that has restricted areal extent 

and small volume along the upper Saguache valley (fig. 9) is 

a distinctive local marker unit above the Saguache Creek Tuff 

and the laharic debris-flow deposit with Luders Creek clasts and 

below andesitic lavas. This gray ignimbrite sheet (67.1% SiO2

) 

contains 3–5 percent phenocrysts of plagioclase and abundant 

biotite; sanidine is absent, in contrast to otherwise similarappearing distal portions of crystal-poor regional ignimbrites 

such as Sapinero Mesa (Tws) or Carpenter Ridge Tuffs (Tcr). Its 40Ar/39Ar plateau age of 30.47±0.08 Ma (biotite) links it in time 

to the late andesite that may represent final stages of the North 

Pass cycle.

Several well-stratified andesitic lava flows (59% SiO2) 

and associated volcaniclastic rocks (Tla, Tlav, andesite of Lone 

Tree Gulch) separate the tuff of Big Dry Gulch from overlying Fish Canyon Tuffs along the paleo-Saguache valley, on the 

southeast flank of Trickle Mountain and adjacent areas (photo 9, 

CD-ROM). Individual flows are 5–10 m thick; maximum total 

thickness is about 50 m. A groundmass-concentrate 40Ar/39Ar 

age of 30.21±0.17 Ma links this andesite assemblage closely 

to the andesite flows ponded against the east wall of the North 

Pass caldera, but the location of these flows well to the east 

suggests that they represent continued regional flux of intermediate-composition volcanism unrelated to any caldera cycle.

Chaotic breccia and some possibly intact lava flows of 

dark-gray crystal-poor andesite (Tab, andesite of Big Dry 

Gulch) form rugged massive outcrops in upper reaches of Big 

Dry Gulch, a southern tributary of Saguache Creek (LM). Much 

of this unit appears to have been emplaced by chaotic landslides; some parts may be primary eruptive deposits, perhaps on 

a proximal flank of the source eruptive edifice. These masses of 

andesite breccia remained topographic highs at time of eruption 

of the Fish Canyon Tuff. Age and emplacement relations are 

unknown relative to the andesite of Lone Tree Gulch; they may 

represent proximal and more distal portions of the same eruptive sequence. 

A flow of silicic alkalic basalt (Tbpf, Tpbv: 52.0% SiO2), 

erupted from a vent (point Benny, NP) centrally located within 

the North Pass caldera (fig. 4), also overlies the laharic breccia 

deposit containing Luders Creek clasts (photo 4, CD-ROM). This 

finely olivine-phyric basalt contains widely distributed sparse 

xenocrysts of quartz and feldspar, derived from disaggregated 

fragments of Precambrian granite. This flow and vent intrusion 

(Tbpi) are the only known Oligocene basalt in the San Juan 

region. The weighted mean 40Ar/39Ar age of 30.22±0.10 Ma for 

groundmass-concentrate determinations on five samples, sampled 

to bracket diverse amounts of Precambrian xenocrystic detritus, 

is consistent with stratigraphic position beneath Fish Canyon Tuff 

and also with eruption nearly concurrent with the late andesite 

ponded against the east wall of the North Pass caldera.

Northern Rhyolite-Dacite Dome Field 

(29.8 Ma)

Spatially unrelated to the North Pass caldera system, but 

broadly concurrent with the late-erupted andesitic lavas just 
11

described, is a cluster of silicic domes and flows centered on 

upper Barret Creek, in the north-central part of the map area 

(fig. 4). These rocks range from nearly aphyric rhyolite (Tbfa: 

1–5% sanidine and plagioclase, sparse biotite; 73–76% SiO2) 

to crystal-rich silicic dacite and low-Si rhyolite (Tbfc: 15–35% 

plagioclase-sanidine-quartz-biotite; 67–74% SiO2). Individual 

flows are 50 to 150 m thick; preserved exposures of the composite flow field cover an area of about 150 km2

. The range of 

laser-fusion 40Ar/39Ar ages for sanidine phenocrysts from four 

separate lava flows is 29.63–29.85 Ma; biotite from a fifth 

flow yielded a slightly older age of 30.08±0.09 Ma (table 2). 

These silicic flows lap onto the eroded core of an older andesitic volcano, including a central intrusion (northwest WB) 

of fine-grained granodiorite (pooled 40Ar/39Ar plateau age of 

34.36±0.07 Ma for hornblende from three sites) that is the largest early intrusion (5 x 10 km) in the San Juan region. 

The rhyolite-dacite dome field at Barret Creek constitutes 

the only sizable area of silicic volcanism, anywhere in the San 

Juan region, in the time interval between peak activity of the 

North Pass cycle at 32 Ma and inception of ignimbrite eruptions 

from the Platoro caldera complex in the southeastern mountains at 29.4 Ma. The most dacitic lavas are compositionally 

and petrographically similar to the voluminous Fish Canyon 

Tuff, documenting that generation of this regionally distinctive magma type began relatively early in the San Juan region, 

well north of the area that would become La Garita caldera (the 

34.0-Ma Badger Creek Tuff, erupted from a caldera source in 

the Sawatch Range, is also petrologically similar to the Fish 

Canyon Tuff). The Barret Creek lavas, which possibly represent 

an aborted or failed caldera system, provide a further bridge 

in the transition between earlier ignimbrite activity along the 

Sawatch trend and younger silicic centers in the San Juan region.

Younger Eruptions from Other San Juan 

Calderas (28.3–26.9 Ma)

The northeastern San Juan Mountains were formerly blanketed by regional Oligocene ignimbrites erupted from areas to 

the south and west, and widespread remnants of this cover have 

survived erosion. Locally erupted intermediate-composition 

lava flows and volcaniclastic rocks interfinger with these ignimbrites within the map area. The following sections summarize 

new age data and aspects of these tuffs that bear on development of the North Pass and Cochetopa Park calderas.

Sapinero Mesa Tuff

The Sapinero Mesa Tuff (Tws), a crystal-poor rhyolite erupted from the western San Juan caldera cluster at 

28.27±0.06 Ma (table 2), is among the largest ignimbrites in 

the San Juan region, with an estimated volume of 1,000 km3

or more (Lipman and others, 1973; Bove and others, 2001). 

With the unit now recognized as Saguache Creek Tuff excluded 

from this ignimbrite, the preserved distribution of Sapinero 

Mesa Tuff is mainly confined to scattered erosional remnants 

in the western and northern map area. In these distal areas, the 

Sapinero Mesa Tuff is rarely more than 20 m thick, only partly 

welded, and light gray in color due to vapor-phase crystallization; phenocrysts are sparse, small, and more broken than in 

more proximal sites. 

Small areas of lithologically similar ignimbrite are also 

discontinuously present just east of the map area along the 

Saguache paleovalley, especially in the tributary of Houselog 

Creek (LG), where the tuff has a similar sanidine 40Ar/39Ar age 

(28.20±0.06 Ma) and reverse paleomagnetic pole position as the 

Sapinero Mesa Tuff farther west (Lipman and McIntosh, 2008). 

This apparently distal eastern area of thin Sapinero Mesa Tuff 

probably was channeled down the Saguache paleovalley, resulting in an overall distribution much like that depicted previously 

(Steven and Lipman, 1976, fig. 10), even though the thick 

densely welded ignimbrite sheet formerly identified as Sapinero 

Mesa in this area is now recognized to be the older Saguache 

Creek Tuff.

Fish Canyon Tuff

The crystal-rich dacitic Fish Canyon Tuff (Tfc), erupted 

from the 75- x 35-km La Garita caldera at 28.02 Ma (fig. 2), is 

the most voluminous ignimbrite in the SRMVF, with a volume 

greater than 5,000 km3 (Lipman and others, 1970; Lipman, 

2000; Bachmann and others, 2002). Seven additional large 

ignimbrite sheets erupted from within La Garita caldera during 

the next 1.1 m.y. (table 1), forming the central caldera cluster 

(Lipman, 2000, 2006). The north margin of La Garita caldera 

is just south of the map area (fig. 2), and proximal Fish Canyon 

Tuff originally blanketed most or all of the northeastern San 

Juan region, banking out locally against paleotopographic 

highs such as the early central volcanoes. Within the North 

Pass caldera (photo 10, CD-ROM), the Fish Canyon Tuff is as 

much as 300 m thick, exceptionally densely welded, and widely 

preserved, indicating that the subsidence basin had been incompletely filled. Eruption of the Fish Canyon Tuff is also inferred 

to be closely intertwined with early development of the unusual 

caldera structure at Cochetopa Park, as discussed later. 

Carpenter Ridge and Wason Park Tuffs

The Carpenter Ridge Tuff (Tcr), another >1,000-km3

ignimbrite erupted at 27.55±0.05 Ma from the Bachelor caldera 

centrally within La Garita caldera (fig. 2), also once covered 

much of the northeastern San Juan region as a widespread 

thin sheet of densely welded crystal-poor rhyolite (photo 11, 

CD-ROM). This crystal-poor rhyolitic ignimbrite commonly 

contains a near-basal black vitrophyre and a central lithophysal 

zone, even where only a few tens of meters thick. In addition 

to its stratigraphic position above the Fish Canyon Tuff, the 

Carpenter Ridge can be distinguished in outcrop from the otherwise similar-appearing Saguache Creek Tuff by the presence 

of biotite phenocrysts. Proximal Sapinero Mesa Tuff closely 

resembles the Carpenter Ridge, but the distal Sapinero Mesa 

within the map area is only partly welded and lacks lithophysae. 

The Carpenter Ridge Tuff provides an important stratigraphic 
12

marker that helps distinguish faults and structures associated 

with eruption of the Fish Canyon Tuff from younger features 

formed during later subsidence at Cochetopa Park.

The Wason Park Tuff (Tw), a crystal-rich rhyolite erupted 

at 27.38±0.05 Ma from the South River caldera of the central San Juan cluster (fig. 2), mainly ponded within La Garita 

caldera, but locally escaped to the northeast following paleovalleys that breached the caldera wall (photo 12, CD-ROM). The 

most northeastern preserved outcrops of the Wason Park tuff 

are at Sheep Creek (northwest TM), against the east margin of 

the North Pass caldera, which demonstrates that this side of the 

caldera remained a topographic low nearly 5 m.y. after initial 

subsidence. 

Younger Intermediate-Composition Lava 

Flows

Several mesa-capping erosional remnants of aphanitic to 

sparsely porphyritic andesitic to dacitic lava southeast of the 

North Pass caldera (younger lava flows, fig. 4) were erupted 

concurrently with ignimbrites from the central caldera cluster. 

Older flows that overlie the Fish Canyon Tuff in the vicinity of 

Mount Lion (photo 13, CD-ROM; LM) have yielded ages of 

28.0–27.6 Ma (7 samples) on groundmass separates (Lipman 

and McIntosh, 2008); flows on the west flank of Lake Mountain 

are overlain by a thin remnant of Carpenter Ridge Tuff (27.55 

Ma), stratigraphically consistent with the isotopic results. 

These flows are thus equivalent to the Huerto Andesite (Thu), 

a major unit within and adjacent to the La Garita caldera to the 

southwest. 

Other mesa-capping flows that overlie the Carpenter Ridge 

Tuff directly south of the North Pass caldera (fig. 2) have ages 

of 27.51–27.46 Ma, consistent with eruption before the Wason 

Park Tuff at 27.38 Ma. These flows, which have no stratigraphic 

counterparts in northern parts of the central caldera cluster, are 

informally designated the andesite of Mountain Lion Creek 

(Tal). Most of these localities were previously interpreted as 

Miocene Hinsdale Formation (Tweto, 1979), but only one small 

mesa cap of Hinsdale lava has been confirmed in the northeastern San Juan region—a flow on Houghland Hill just east of the 

map area (fig. 4; LMNE) dated at 21.81±0.21 Ma.

Rocks of the San Luis Caldera Complex

Three compositionally diverse ignimbrites and associated lavas of the San Luis caldera complex (photos 14–16, 

CD-ROM), nested along the northwest side of La Garita 

caldera (figs. 2, 10), are the youngest major eruptions from 

the central San Juan caldera cluster, other than the Snowshoe 

Mountain Tuff from the Creede caldera farther south (Lipman, 

2000, 2006). New age results show that these three ignimbrites 

were erupted geologically rapidly between 26.91±0.02 and 

26.90±0.02 Ma (table 2, weighted means), a time span too brief 

to resolve reliably by 40Ar/39Ar dating (Lipman and McIntosh, 

2008). The eruptive history of the San Luis caldera complex 

(Lipman, 2000) is summarized briefly, because the last-erupted 

Nelson Mountain Tuff is interpreted as closely intertwined with 

subsidence at Cochetopa Park. 

Rat Creek Tuff

The initial ignimbrite erupted from the San Luis caldera 

complex, the Rat Creek Tuff, is compositionally zoned (73–65% 

SiO2) from weakly indurated light-tan crystal-poor rhyolite (Trr) 

upward into welded crystal-rich dacite (Trdn). The Rat Creek 

Tuff is the smallest of the San Luis ignimbrites in estimated 

volume (~150 km3) and the most restricted in known extent, 

and its source is the least understood (Lipman, 2000, p. 32–33). 

Within the map area, preserved exposures of the Rat Creek 

Tuff are limited to weakly welded distal ignimbrite along the 

northeast-trending Los Pinos graben (fig. 5; RC, CS). Eight 

samples from geographically widespread sites have a weightedmean laser-fusion sanidine age of 26.91±0.02 Ma.

Cebolla Creek Tuff

The Cebolla Creek Tuff (Tcc) is a distinctive compositionally uniform ignimbrite (photo 15, CD-ROM) of gray to lightbrown, weakly vapor-phase-devitrified mafic dacite (62–66% 

SiO2; 25–40% phenocrysts of plagioclase, biotite, hornblende, 

sparse clinopyroxene). In contrast to other dacitic tuffs of the 

San Luis caldera complex, the Cebolla Creek lacks sanidine, 

and hornblende is abundant. Hornblende and biotite plateau 

ages for this unit (27.07–27.13 Ma) are about 0.2 m.y. older 

than sanidine ages for the underlying Rat Creek Tuff (table 2). 

Such slightly older ages for these mafic minerals relative to 

sanidine are fairly common among dated volcanic rocks in the 

San Juan region (Lipman and McIntosh, 2008). 

Like the underlying Rat Creek Tuff, exposures of outflow 

Cebolla Creek deposits are weakly welded and are limited to 

ponded fill within the northern La Garita caldera and along 

the Los Pinos graben (fig. 5). These lower ignimbrite sheets 

were widely eroded prior to eruption of the overlying Nelson 

Mountain Tuff, which rests directly on older units in many 

places within the general distribution area of the Rat Creek 

and Cebolla Creek Tuffs. The source caldera for the Cebolla 

Creek Tuff, although now largely concealed beneath the Nelson 

Mountain caldera source and flanking postcollapse volcanoes, 

appears to be the largest among the three calderas of the San 

Luis complex, with dimensions of 12 x 17 km at present erosional levels (fig. 10, sheet 2). 

Nelson Mountain Tuff

This unit includes a regional ignimbrite sheet and thick 

intracaldera fill; like the Rat Creek Tuff, it grades upward in 

composition (73–63% SiO2) from weakly welded crystal-poor 

rhyolite (Tnr) to densely welded dark crystal-rich dacite (Tnd). 

The Nelson Mountain Tuff is the most densely welded and 

voluminous outflow deposit erupted from the San Luis caldera 

complex (estimated >500 km3), and its eruption was followed 
13

by growth of large volcanoes on the east and west caldera flanks 

(Stewart Peak Volcanics, Baldy Cinco Dacite), roughly concurrent with resurgent uplift of the caldera interior (Steven and 

Lipman, 1976; Lipman, 2000, 2006). 

Exposed intracaldera Nelson Mountain Tuff is more than 

800 m thick on San Luis Peak, and erosional remnants of the 

outflow sheet are preserved widely around southwest and southeast sides of the San Luis caldera complex, where this ignimbrite spread across northern parts of La Garita caldera (Lipman, 

2006). A northeastern outflow lobe of the Nelson Mountain was 

able to cross the La Garita caldera wall, following paleovalleys 

of the Los Pinos and Cochetopa Creek grabens (figs. 4–5; RC, 

CS) into Cochetopa Park caldera (photos 14, 16A, CD-ROM), 

and a weakly welded distal facies of this unit (photo 16B, 

CD-ROM) reached the southeast side of the Cochetopa Park 

caldera (fig. 4). The northeast lobe was previously considered to 

be a separate younger ignimbrite sheet (tuff of Cochetopa Creek 

or Cochetopa Park Tuff) interpreted to have erupted from the 

Cochetopa Park caldera (Steven and Bieniewski, 1977; Olson 

and Steven, 1976a; Olson, 1988), but more detailed mapping, 

paleomagnetic pole positions, and age determinations have 

documented its correlation as outflow Nelson Mountain Tuff 

(Lipman, 2000, p. 37). 

New single-crystal sanidine age determinations for the 

Nelson Mountain Tuff, both outflow and intracaldera (table 2; 

Lipman and McIntosh, 2008), have resolved some enigmatic 

problems concerning this unit. Previously determined 40Ar/39Ar 

incremental-heating plateau ages on multiple-grain sanidine 

concentrates, although analytically reproducible, differed by as 

much as 1 m.y. among samples from different geographic localities and for intracaldera versus outflow parts of this ignimbrite 

unit (fig. 6; discussion in Lipman, 2000). The new single-crystal 

laser-fusion ages document the presence of sanidine xenocrysts, 

and also grains with degassed melt inclusions, that yielded 

scattered ages substantially older or younger than the average 

for closely grouped grains in individual samples (Lipman and 

McIntosh, 2008). When statistically anomalous crystals are 

excluded, sanidine from eight outflow samples yield a weighted 

mean age of 26.90±0.02 Ma, and two samples of intracaldera 

Nelson Mountain have a mean of 26.91±0.04 Ma (table 2). 

The new mapping has also shown that many areas of ridgecapping ignimbrite north of the Los Pinos graben and extending 

nearly to the Gunnison valley, which were depicted as Nelson 

Mountain Tuff and (or) tuff of Cochetopa Creek (Cochetopa 

Park Tuff) on previously published geologic quadrangle maps 

(Olson, 1976a,b; Olson and Steven, 1976a,b; Olson, 1988), 

instead consist entirely of diverse pre-Nelson-Mountain regional 

units, including at various sites the Wall Mountain, Saguache 

Creek, Fish Canyon, and Carpenter Ridge Tuffs. The Nelson 

Mountain Tuff and earlier tuffs from the San Luis complex 

may well have originally been deposited over sizable areas 

to the north, but as the youngest and topographically highest 

deposits, they have been entirely eroded. No tuff of Cochetopa 

Creek or Nelson Mountain Tuff remains preserved north of the 

Cochetopa Park caldera.

There is a sizable discrepancy between the volume of 

the Nelson Mountain Tuff and the amount of subsidence at its 

eruptive source. The widespread outflow and thick intracaldera 

accumulation of the Nelson Mountain Tuff (500 km3

) constitutes the largest of the three San Luis ignimbrites in eruptive 

volume (table 1). In contrast, the subsidence basin at its eruptive 

source within the San Luis caldera complex is anomalously 

small relative to the estimated eruptive volume. The basin 

defined by the unconformity between wall rocks and caldera fill 

is 10 x 15 km (fig. 10), but the southern third of the basin is a 

truncated paleovalley, as documented by mapping and mineralexploration data (Lipman, 2000). The paleovalley is well 

outside of the structurally subsided block, which has estimated 

maximum dimensions of only about 8 x 10 km and an area of 

about 65 km2. About 8 km average caldera subsidence would 

thus be needed to accommodate the 500 km3 erupted volume 

of the Nelson Mountain Tuff, but the asymmetric trap-door 

subsidence geometry at this caldera (Lipman, 2000) permits 

only about 500 m of subsidence on the north side of the caldera 

and no more than 1.5–2 km at the more deeply subsided south 

side. This apparent inconsistency between eruptive and subsidence volumes associated with eruption of the Nelson Mountain 

Tuff is interpreted as the result of complex interrelations with 

subsidence at the much larger Cochetopa Park caldera centered 

30 km to the northeast. 

Cochetopa Park Caldera: Relation to 

Nelson Mountain Tuff (26.9 Ma)

Cochetopa Park (cover photo) is among the larger and 

most morphologically obvious caldera basins (about 30 km in 

diameter) in the region (photos 17–19, CD-ROM), but its depth 

of subsidence appears to be less than for other San Juan calderas. Subsidence was only about 300 m along the south margin, 

where constrained by downdropped Saguache Creek Tuff and 

underlying rocks on the caldera floor (SA), just south of the 

map area (Lipman, 2006). Subsidence was as much as 700–800 

m on the north side where precollapse lavas on the caldera floor 

are structurally coherent, as shown by the continuity of andesite 

dikes and lava flows (photos 3C, 20, CD-ROM). In contrast, 

geometric modeling of other large San Juan calderas suggests 

structural subsidence of 3–5 km (Lipman, 1997), similar to 

caldera-fill sections exposed in tilted calderas elsewhere in the 

western United States (Seager and McCurry, 1988; Lipman 

1993; John, 1995; John and others, 2008).

The earliest caldera-fill deposits at Cochetopa Park are 

thick massive aprons of coarse landslide breccia, banked 

against the inner caldera slopes (photos 20, 21, CD-ROM). 

This distribution is in contrast to slide breccias at other more 

deeply subsided calderas in the San Juan region, where the 

landslide deposits spread widely across the subsided basin and 

interfinger with thick intracaldera tuff. The location of slide 

breccias at Cochetopa Park, hanging on slopes of the inner 

caldera wall, is consistent with the relatively limited subsidence 

interpreted from offsets of the caldera floor. In places, distal 

Nelson Mountain Tuff directly overlies the landslide deposits 

(photo 16B, CD-ROM), documenting subsidence at Cochetopa 
14

Park and rapid slump failures of the oversteepened inner walls 

to generate the slide breccias during eruption of this ignimbrite, 

before it reached distal areas of deposition.

The landslide deposits reflect lithologies on the adjacent 

caldera rims: around the north side, breccia consisting of andesitic clasts (Tpba) is dominant, along with areally restricted lenses 

of monolithologic breccia derived from Saguache Creek Tuff 

(Tpbs); around the south and east sides are monolithologic slide 

breccias of shattered Fish Canyon Tuff (Tpbf; photo 21A–D, 

CD-ROM), in places containing sparse clasts derived from 

Carpenter Ridge Tuff (Tpbm; photo 21E, CD-ROM). Confident 

identification of the crystal-poor biotite-bearing rhyolite clasts as 

derived from the Carpenter Ridge Tuff, rather than lithologically 

similar Sapinero Mesa Tuff, is critical evidence in interpreting 

the time of major caldera subsidence. Sanidine 40Ar/39Ar ages 

on two clasts from the west and northeast caldera sectors are 

27.59±0.05 and 27.48±0.05 Ma (table 2; Lipman and McIntosh, 

2008), which confirm identity as Carpenter Ridge Tuff. 

Some massive slide breccias of monolithologic Fish 

Canyon debris, characterized by closely packed clasts with little 

or no interstitial matrix fines (photos 21B–C, CD-ROM), were 

initially considered candidates for a hot emplacement mechanism, but random paleomagnetic pole orientations of clasts for 

sites both on west and southeast sides of the caldera are inconsistent with this hypothesis (Lipman and McIntosh, 2008, table 2). 

The caldera was further filled by fanglomerates along 

caldera walls (photos 22, 23, CD-ROM) and a thick central pile 

of crystal-poor silicic lava flows of Cochetopa Dome Rhyolite 

(photos 18, 19, 24, CD-ROM), here named for compositionally distinctive rocks previously described only briefly (Steven 

and Lipman, 1976, p. 27; Lipman and McIntosh, 2008). The 

rhyolite flows on Cochetopa Dome overlie and are surrounded 

by thin nonwelded pyroclastic flow deposits, airfall ash, and 

tuffaceous sediments (cover photo). These lavas, the most petrologically evolved Oligocene rhyolites in the San Juan region 

(table 4), have 40Ar/39Ar ages (weighted mean of eight samples: 

26.86±0.03) that are only slightly younger than the Nelson 

Mountain Tuff (26.90±0.02). Although subsidence is relatively 

modest in outer parts of the caldera, based on the floor geometry, exposures are inadequate to exclude deeper subsidence 

centrally within this little-eroded caldera. Cochetopa Park is 

also the site of a large Bouguer gravity low (25–30 mgal; Plouff 

and Pakiser, 1972; Drenth and Keller, 2004), suggesting the 

presence of thick low-density fill in central parts of the caldera 

and (or) a shallow subcaldera silicic pluton. 

The morphologic preservation of the Cochetopa Park 

caldera is especially striking (fig. 5; cover photo; photo 17, 

CD-ROM). It is among the few calderas in the SRMVF for 

which the original topographic rims can still be reconstructed 

with confidence (fig. 4). Survival of the constructional geometry at Cochetopa Park caldera is due to a combination of 

(1) late development, thereby less obscured by subsequent 

volcanism; (2) relatively shallow subsidence and only a modest 

volume of postcaldera volcanism, thus never filled by dense 

rocks that would resist erosional excavation; (3) partial fill by 

weakly indurated tuffaceous sediments that have been exhumed 

geologically recently; and (4) presence of erosion-resistant 

rocks (Precambrian) at the present-day (and probable Tertiary) 

drainage outlet (Cochetopa Canyon) that has limited downcutting and dissection of the caldera basin. The contrasting lack of 

morphologic expression at the North Pass caldera reflects the 

absence of all these factors.

The time of collapse at Cochetopa Park caldera and identity of an associated regional ignimbrite have been long-standing problems (Steven and Lipman, 1976, p. 22–25; Lipman, 

2000, p. 37). Cochetopa Park caldera was previously interpreted 

as associated with eruption of the tuff of Cochetopa Creek 

(Steven and Bieniewski, 1977; Olson and Steven, 1976a; Olson, 

1988), but all rocks south of Cochetopa Park caldera formerly 

interpreted as tuff of Cochetopa Creek are now known to be 

Nelson Mountain Tuff (Lipman, 2000). My mapping also shows 

that all rocks previously interpreted as tuff of Cochetopa Creek 

north of the caldera are diverse ignimbrite units older than the 

Nelson Mountain Tuff. Based on clasts in the slide breccias 

deposited along the caldera wall, the timing of last major subsidence at Cochetopa Park is now firmly established as postdating eruption of the Fish Canyon and Carpenter Ridge Tuffs. 

All younger large-volume ignimbrite sheets were demonstrably 

erupted from vents to the south, within the central San Juan 

caldera cluster. As a result of these reinterpretations, no tuff of 

Cochetopa Creek or other obvious late ignimbrite sheets remain 

that could have had an eruptive source from the Cochetopa Park 

caldera, even though this large morphologic and structural basin 

clearly formed by subsidence.

As reinterpreted here, the Cochetopa Park area has been a 

site of recurrent structural events associated with several large 

eruptions, perhaps including the Luders Creek Tuff at 32.2 Ma, 

followed by graben formation and possible modest subsidence during Fish Canyon eruptions of La Garita cycle at 28.0 

Ma. Indicators for events associated with eruption of the Fish 

Canyon Tuff include (1) formation of the northeast-trending Los 

Pinos and parallel Cochetopa Creek grabens (figs. 4, 5) during 

or after eruption of the Fish Canyon Tuff, but prior to eruption 

of the Carpenter Ridge Tuff at 27.55 Ma (Lipman, 2000); (2) 

development of fault-scarp breccias in Fish Canyon Tuff where 

it is downdropped along the west margin of Cochetopa Park 

caldera (west CS), prior to emplacement of the Carpenter Ridge 

Tuff; (3) exceptionally thick densely welded Fish Canyon Tuff 

(<300 m) that is locally preserved where deposited against steep 

slopes along the southwest margin of Cochetopa Park caldera; 

and (4) post-Fish Canyon andesite flows northeast of Cochetopa 

Park, which predate the Carpenter Ridge Tuff and are analogous 

in stratigraphic position to the caldera-related Huerto Andesite 

to the south. Thus, the Cochetopa Park area may have been the 

site of a northerly fourth segment of caldera subsidence during 

eruption of the Fish Canyon Tuff, directly on line with the 

composite La Garita structure (fig. 2), which then would have a 

north-south extent of about 100 km. No deep subsidence in the 

Cochetopa Park area seems likely at that time, however, as indicated by the apparent absence of thick ponded Carpenter Ridge 

Tuff that would have accumulated within the resulting basin.

Much of the record of structural development in the 

Cochetopa Park area during the Fish Canyon eruptions is concealed by the sizable later subsidence event that formed most of 
15

the presently exposed features of the caldera basin. This beautifully preserved structure owes its modern appearance largely 

to passive collapse during discharge of the 26.9-Ma Nelson 

Mountain Tuff (~500 km3), from the underfit depression within 

the San Luis caldera complex 30 km to the southwest. Evidence 

is strong that the Nelson Mountain Tuff erupted from the San 

Luis complex (Lipman, 2000) but is completely lacking for any 

venting from the Cochetopa area. The preserved distribution 

of outflow Nelson Mountain Tuff is symmetrical around the 

San Luis complex, while Cochetopa Park is at the northeastern 

distal limit of the distribution of this tuff. Intracaldera Nelson 

Mountain Tuff is >800 m thick, densely welded, and propylitically altered within the San Luis caldera system (Lipman, 2000, 

2006), while preserved exposures of the Nelson Mountain 

Tuff become nonwelded and wedge out within eastern parts of 

Cochetopa Park caldera. Neither Nelson Mountain nor the earlier Fish Canyon Tuff accumulated to a multi-kilometer thickness at Cochetopa Park, nor did they acquire the welding and 

alteration features typical of intracaldera deposits within other 

San Juan calderas. Relatively shallow caldera collapse is also 

indicated by the widespread accumulation of landslide and talus 

breccias high along the inner caldera walls, rather than farther 

downslope and across the caldera floor as observed at more 

deeply subsided San Juan calderas (Lipman, 1976, 2000). 

During the Nelson Mountain eruption, magma is interpreted to have moved from the Cochetopa to San Luis areas 

along a southwest-trending dike system, expressed at the 

surface by the large Cochetopa graben (Lipman and McIntosh, 

2008). Cochetopa Park caldera must have begun to subside 

before or early during eruption of the compositionally zoned 

Nelson Mountain Tuff (crystal-poor rhyolite to crystal-rich 

dacite), as evidenced by deposition of rhyolite and thick dacite 

within western parts of the caldera (photos 14, 16A, CD-ROM), 

after landslides had already been generated from oversteepened caldera walls. Such passive (noneruptive) subsidence at a 

site distant from the associated ignimbrite vent is uncommon. 

Previously, such a configuration appears to have been thoroughly documented only for the much smaller 1912 eruption (~13 km3

 magma volume) from Novarupta crater in the 

Aleutians, where the main caldera collapse was 10 km distant 

at Katmai volcano (Hildreth and Fierstein, 2000). A geometrically somewhat similar analog, involving linked subsidences, 

may be the paired ignimbrites that erupted concurrently from 

the Rotorua and Ohakuri calderas in the Taupo zone of New 

Zealand (Gravley and others, 2007). Another geometrically 

comparable configuration could be the genetically enigmatic 

Mono Basin in eastern California, which is connected to the 

Long Valley caldera by a zone of extensional faults; the Bishop 

Tuff erupted from Long Valley at 0.76 Ma, without known 

concurrent eruptive activity at Mono Basin 20 km to the north 

(Bailey and others, 1976).

Structure

Structural features of the northeastern San Juan 

Mountains involve complex interactions between regional 

northeast-trending graben systems that were influenced by 

structural grain in the Precambrian craton; diverse localized 

faulting associated with volcanism, especially the large calderas; and effects of regional extension associated with inception 

of the north-south Rio Grande rift zone (Lipman, 2000, fig. 14).

Northeast-Trending Graben Faults

Several linear grabens and other faults adjacent to calderas 

appear largely to have been established during segmented subsidence of La Garita caldera, then passively buried by younger 

ignimbrite sheets and lava flows (photos 11A, 14, CD-ROM). 

These include the Los Pinos and Cochetopa Creek grabens 

(figs. 4, 5), which connect the northern La Garita segment to 

the Cochetopa Park caldera at the southwest margin of the map 

area (RC, CS), and probable obscure faults of similar trend 

that localized the elongate high-level intrusion (34.4 Ma) in the 

Needle Creek area (WB). 

Caldera Faults

Many faults are present within the map area, but erosion 

levels are insufficient to expose large-displacement ring faults 

related to deep caldera collapse, such as those present within 

the Lake City and Silverton calderas in the western San Juan 

Mountains (Steven and Lipman, 1976). Exposed peripheral 

faults of the Cochetopa Park caldera can accommodate only 

modest subsidence; either larger faults are present more centrally within the caldera, or total subsidence was more modest 

than at most other San Juan calderas. 

At Cochetopa Park, peripheral arcuate faults along the 

northwest and south margins of the caldera basin, which have 

displacements down toward the caldera interior and bound subsided caldera floor rocks of the Conejos Formation, appear to 

permit, at most, a few hundred meters of subsidence. The southern arcuate fault (CP, south of the map area; Lipman, 2006) 

has a maximum of about 250 m of displacement, as marked 

by downdropped Saguache Creek Tuff. On the northwest side 

of the caldera, a series of faults step down Precambrian and 

Mesozoic units in increments of a few tens of meters each 

toward the caldera interior; these are oriented appropriately to 

accommodate a portion of the 400–600 m maximum subsidence of Conejos lavas and breccias that make up the outer 

caldera floor in this area. Some of these faults may mark hinge 

zones that accommodated initial caldera downsag along zones 

of relatively modest peripheral subsidence, compared to other 

more deeply subsided San Juan calderas. Much of the northern 

arcuate fault system is interpreted to lie concealed beneath subsidence-related landslide breccia and fanglomerate (units Tpba, 

Tpcf; photo 20, CD-ROM). Some monolithologic slide breccias 

appear to represent shattered slump blocks that detached along 

faults associated with inward-sagging caldera walls, then moved 

semicoherently partway down into the subsiding basin.

No peripheral faults are exposed in eastern Cochetopa 

Park, but a linear array of north-trending faults along the west 

margin, north of McDonough Reservoir (CS, RC), appears 

related, at least in part, to recurrent caldera subsidence. This 
16

set of faults is designated the Ralston Creek fault zone, for the 

prominent tributary of Los Pinos Creek that transects the fault 

array where it is widest and best developed. These faults clearly 

displace the Sapinero Mesa Tuff and older units of the Conejos 

Formation, but complex relations with the Fish Canyon Tuff 

suggest offsets closely associated with emplacement of this 

ignimbrite. The Ralston Creek faults merge to the south with 

structures of the Los Pinos graben that predate emplacement 

of the Carpenter Ridge Tuff, and much of their displacement 

may have been concurrent with subsidence adjacent to north 

margins of the composite La Garita caldera during eruption of 

the Fish Canyon Tuff. Such an interpretation is supported by 

intricate distribution of shattered Fish Canyon Tuff (Tfcb) along 

some fault segments, which merge with adjacent segments that 

appear concealed beneath coherent welded tuff of the same 

ignimbrite. Relations between brecciated and coherent tuff are 

gradational, and in places lower brecciated zones in the Fish 

Canyon are overlain by unbroken tuff, suggesting that some 

brecciation resulted from downslope detachment motion within 

fault-bounded blocks. In places, the brecciated Fish Canyon 

Tuff mapped as fault related (Tfcb) is also difficult to distinguish in appearance from the monolithologic caldera-margin 

landslide breccia (Tpbf) that formed during subsidence of the 

Cochetopa Park caldera, and some areas of breccia may have 

been misinterpreted. Additionally, the Ralston Creek fault array 

merges with the projection of northern peripheral faults of the 

Cochetopa Park caldera, and displacements are progressively 

down to the east (toward the caldera). Thus, these structures 

may have moved recurrently and accommodated subsidence 

of this caldera during eruption of the Nelson Mountain Tuff. 

Vegetative cover and steep slopes obscure many features of 

these intricate faults, but exposures are sufficient to document 

structural relations that are unlike caldera-related faults elsewhere in the region.

No exposed faults are clearly associated with the North 

Pass caldera. Small faults, with northerly trends along the west 

margin of this caldera (NP), drop rocks interpreted as part of 

the caldera-fill sequence down to the east, toward the caldera. 

These faults may continue farther to the north and south than 

mapped, along linear valleys where contact relations are 

obscured by surficial deposits and heavy vegetation. The linear 

trends and small displacements seem plausible for a regional 

structure. Nevertheless, these faults appear to die out in or be 

covered by tuffaceous sediment, interpreted as volcaniclastic fill 

of the North Pass caldera, suggesting that their movement may 

have been concurrent with caldera formation. In contrast, the 

large Sheep Creek fault system along the east side of the North 

Pass caldera is interpreted as primarily a regional extensional 

feature associated with the Rio Grande rift system. 

Rio Grande Rift Faults and Associated Volcanism

During late stages of San Juan volcanism, the Rio Grande 

rift zone became active within the present-day San Luis Valley 

area to the east of the San Juan Mountains (fig. 1; Lipman and 

Mehnert, 1975), and a few north- to northeast-trending faults 

within the map area have appropriate geometry and timing to 

reflect such regional tectonism. Regional structural trends of the 

volcanic rocks in the map area—flat in the western part, hinged 

to the east near the present-day Continental Divide, and dropped 

mainly down to the west by north-trending faults that have 

displacements antithetic to the main San Luis Valley segment 

of the rift zone—are strikingly similar to general trends in the 

southeastern San Juan region (Lipman, 1975). This structural 

style reverses just to the northeast of the map area, where volcanic rocks of the Bonanza center are tilted to the west, along 

faults that appear to constitute a structural transition toward 

the Sawatch Range-Upper Arkansas Valley segment of the Rio 

Grande rift zone.

Within the map area, northerly trending faults of the Sheep 

Creek zone (photo 12, CD-ROM) are the most prominent and 

largest rift-related structures (TM, west LM). Some of these 

faults cut the andesite of Mountain Lion Creek (Tal), dated at 

about 27.5 Ma east of Hat Mountain (GC), which documents 

relatively late movement. Several smaller faults of similar trend, 

in the Grouse Creek area farther to the west, displace the Fish 

Canyon Tuff and older rocks. Thinning, nondeposition, and 

(or) erosional removal of regional ignimbrite sheets, erupted 

from the central caldera complex (Carpenter Ridge, Wason 

Park, Nelson Mountain), on the uplifted east side of the Sheep 

Creek fault zone suggest that these structures initially may have 

become active during peak ignimbrite volcanism. The central 

sector and maximum offset along the Sheep Creek fault zone 

also coincide closely with the east margin of the North Pass 

caldera, indicating that structures of the caldera margin influenced location of the younger rift-related faults or that incipient rift faulting was already underway at the time of caldera 

subsidence.

Bimodal mafic-silicic volcanism, which regionally accompanies Miocene development of the Rio Grande rift system 

(Lipman and Mehnert, 1975), may have begun relatively early 

in the northeastern San Juan region. The highly-evolved rhyolite 

lavas on Cochetopa Dome and associated intrusive bodies, 

dated at 26.9–27.1 Ma, are only a few hundred thousand years 

younger than mesa-capping olivine-andesite flows within the 

map area (Tal, andesite of Mountain Lion Creek); these rocks 

may define the beginning of the transition to the bimodal suite 

that characterizes volcanism associated with the Rio Grande 

rift. Several small intrusions of high-silica rhyolite (Tcdi) are 

similar in composition and age to the flows of Cochetopa Dome 

Rhyolite within the caldera. These include a stubby dike elongated north-northeast, exposed at Deer Hill (RD) at the north 

margin of caldera-fill fanglomerate, and a similarly aligned 

trend of three equant plugs farther to the north, the largest of 

which is the conspicuous knob of Prosser Rock (RD; photo 25, 

CD-ROM). The more northerly trends of the rhyolite plugs, in 

contrast to the northeastern grain of the earlier caldera-related 

graben faults and associated Conejos-age intrusions, are an 

appropriate orientation for inception of rift-related regional 

extension.
17

DESCRIPTION OF MAP UNITS

[Locations are indicated by abbreviations (fig. 3) for 7.5' quadrangles. For major volcanic units, phenocryst 

percentages are listed, with principal phases in order of abundance: bi, biotite; cpx, clinopyroxene; hbl, 

hornblende; pl, plagioclase; qz, quartz; sn, sanidine]

SURFICIAL DEPOSITS

Qal Alluvium (Holocene)—Silt, sand, gravel, and peaty material in valley bottoms. Locally includes 

small deposits of colluvium (Qc) and talus (Qt) at margins of valley bottoms

Qc Colluvium (Holocene)—Poorly sorted silt- to boulder-sized material on slopes and in steep valleys. Locally includes small alluvial, talus, landslide, and glacial-moraine deposits

Ql Landslide deposits (Holocene and Pleistocene)—Lobate accumulations of poorly sorted soil 

and rock debris on slopes marked by hummocky morphology and downslope-facing scarps. 

Derived from bedrock and glacial deposits. Includes small earth-flow, block-slump, and 

block-slide deposits

Qf Alluvial-fan deposits (Holocene)—Generally poorly sorted material, ranging in size from silt to 

boulders, that grades into modern colluvium in upper parts of drainages. Only large low-angle fans are shown; smaller steep fan deposits are mapped with colluvium (Qc). Fan deposits 

largely predate outwash of Pinedale glaciation and are mostly derived from unglaciated 

drainages

Qt Talus (Holocene)—Angular rock fragments as much as 1 m in diameter forming talus cones, 

talus aprons, and scree slopes. Locally well sorted. Grades into colluvium (Qc) as sand and 

silt content increases

Qlo Older landslide deposits (Pleistocene)—Landslide deposits in which original hummocky morphology has become subdued due to erosion and sedimentation. Eroded landslide material 

forming high-standing erosional remnants 

Qfo Older alluvial-fan deposits (Pleistocene)—High-standing erosional remnants of dissected alluvial fans, consisting mainly of volcanic clasts

GLACIAL DEPOSITS

Qm Moraine and till (Pleistocene; Pinedale glaciation)—Terminal and lateral moraines; thick 

valley-bottom till. Poorly sorted and generally unstratified clay, silt, and sand containing 

cobbles and boulders. Characterized by hummocky or ridgy topography. Some till has been 

mapped with colluvium (Qc) 

Qg Glacial-outwash and other terrace gravels (Pleistocene; Pinedale glaciation)—Moderately 

sorted to well-sorted stratified terrace and fan deposits consisting of sand, gravel, and wellrounded boulders. In places, difficult to distinguish reliably from alluvial-fan terraces (Qf)

Qgo Older glacial-outwash and other stream gravels (Pleistocene; pre-Pinedale glaciation)—Deposits preserved as local erosional remnants of high-standing outwash terraces. Preserved 

mainly along ridge between Cochetopa Creek and Van Tassel Gulch (CP, CS). In places, 

difficult to distinguish reliably from older alluvial-fan deposits (Qfo)

OLD SURFICIAL DEPOSITS

QTl High-level landslide or debris-flow deposits (Quaternary and Tertiary?)—Erosional patches high on valley walls of Long Branch (SM), containing large blocks of Bonanza and 

Saguache Creek Tuffs; puzzling erosional hill of unconsolidated bouldery deposit, containing clasts of dacitic Nelson Mountain Tuff (Tnd) and tuffaceous breccia (CS). Emplacement 

possibly related to subsidence of the Cochetopa Park caldera. Thickness, >50 m

QTf Deeply dissected alluvial-fan deposits (Quaternary and Tertiary?)—Mainly angular clasts of 

Precambrian metamorphic rocks (=m) in Iris and Iris NW quadrangles. In places, difficult 

to distinguish from float derived from underlying Precambrian bedrock; in others, merges 

with younger colluvium (Qc). Previously mapped on the Iris quadrangle as Tertiary gravel 

deposits and inferred to underlie the Fish Canyon Tuff (Olson, 1976b); mapped on the Iris 

NW quadrangle as Rat Creek Tuff (Hedlund and Olson, 1974) 
18

QTg High-level gravel deposits (Quaternary and Tertiary?)—Local erosional remnants of old 

stream gravels containing mainly volcanic clasts, especially in northeastern map area. 

Merges with and in places difficult to distinguish from older alluvial-fan deposits (Qfo)

MIOCENE POSTCALDERA VOLCANIC ROCKS

Hinsdale Formation—The Hinsdale Formation is an assemblage of silicic alkalic basalt (trachybasalt) and basaltic-andesite lavas (50–60% SiO2

) erupted from widely scattered vents, 

with volumetrically subordinate silicic rhyolite erupted as lava domes and tuffs. These rocks, 

emplaced intermittently between about 26.5 Ma and 14 Ma in the central and northeastern 

San Juan Mountains, constitute a bimodal volcanic suite that accompanied regional extension along the Rio Grande rift zone (Lipman and Mehnert, 1975)

Thb Mafic flows—Small mesa-capping erosional remnants of fine-grained dark-gray basaltic andesite that overlie Wall Mountain Tuff (Twm) along the ridge crest between Sugar and Right 

Creeks (northwestern SH). Small sparse olivine phenocrysts, partly altered to rusty brown 

iddingsite, are typical; these lavas appear less altered and more mafic than adjacent andesite 

mapped as flows of the Conejos Formation along the same ridge. Tentatively included in 

the Hinsdale Formation, in continuity with prior assignment by Olson and others (1975); 

lacking isotopic ages, may be an older unit. The only basaltic flow in the northeastern San 

Juan region with a confirmed Hinsdale age (40Ar/39Ar groundmass age: 21.81±0.21 Ma) caps 

Houghland Hill, just east of the map area (LG). Thickness 0–50 m

OLIGOCENE VOLCANIC ROCKS

ROCKS OF COCHETOPA PARK CALDERA AND SAN LUIS CALDERA COMPLEX

The San Luis caldera complex (photo 26, CD-ROM) is the composite source for three successive 

large ignimbrite eruptions (fig. 10) and associated overlapping caldera-subsidence events that successively produced the Rat Creek, Cebolla Creek, and Nelson Mountain Tuffs (Lipman, 2000, 2006). After 

each ignimbrite eruption, the associated caldera was variably filled by andesitic to rhyolitic lavas. Recent 

isotopic ages on sanidine phenocrysts by single-crystal laser-fusion 40Ar/39Ar methods tightly bracket the 

three regional ignimbrites at 26.90 Ma (Lipman and McIntosh, 2008), greatly improving the age resolution 

for this eruptive sequence compared to prior isotopic results (fig. 6). Cochetopa Park caldera is interpreted 

as having last subsided passively during eruption of the Nelson Mountain Tuff from the San Luis complex, 

after which it was filled by volcaniclastic sediments and rhyolitic lavas

Rocks of the Nelson Mountain caldera cycle

The Nelson Mountain Tuff (photo 16, CD-ROM) is the most voluminous outflow deposit erupted 

from within the San Luis caldera complex, and its eruption was followed by growth of large postcaldera 

volcanoes on its east and west flanks (Stewart Peak Volcanics, Baldy Cinco Dacite), roughly concurrent 

with resurgent uplift of the caldera interior (Lipman, 2000). Our mapping shows that the Cochetopa Park 

caldera (probably polycyclic, with some structural subsidence during La Garita caldera cycle and eruption of the Fish Canyon Tuff) last subsided passively during early stages of the Nelson Mountain eruption, 

then was flooded by later-erupted Nelson Mountain Tuff erupted from vents within the San Luis complex. 

No evidence exists to indicate that any large pyroclastic eruptions vented directly from within Cochetopa 

Park caldera; prior proposals for eruption of a tuff of Cochetopa Creek or Cochetopa Park Tuff of Olson 

(1988) from this caldera were based on regional miscorrelations between different geographic sectors of the 

Nelson Mountain Tuff (Lipman, 2000). Subsidence at Cochetopa was relatively modest in scale, no more 

than about 1 km vertically, compared to other San Juan ignimbrite calderas, and a downdropped segment 

of floor rocks (mainly Conejos Formation) is exposed on the north side of the caldera. The caldera is less 

deeply dissected than most other San Juan calderas, in part because of lower overall elevation and because 

the Precambrian granitic rocks in Cochetopa Canyon, the morphologic outlet, are relatively resistant to 

erosion. As a result, the filling history of the caldera is known only for late events, and the thickness of fill 

cannot be estimated for central parts of the caldera

Caldera-Fill Flows and Intrusions

After the last subsidence at the Cochetopa Park caldera during eruption of the Nelson Mountain Tuff 

from vents within the San Luis complex 30 km to the southwest, a sequence of viscous rhyolitic lava flows 
19

constructed the morphologic feature known as Cochetopa Dome near the center of the caldera (photos 18, 

24, CD-ROM). The rhyolite flows overlie diverse volcaniclastic rocks that accumulated soon after caldera 

collapse, but some upper caldera-fill sedimentary deposits may have interfingered with the lava flows and 

filled intervening low moat areas between the dome and the inner caldera walls. Because the dome lavas 

are resistant and the volcaniclastic rocks are relatively readily eroded, Cochetopa Park caldera retains much 

of its primary morphology, with peripheral high points near the original topographic rim, a low sedimented 

moat, and a central constructional lava pile (fig. 5) 

Cochetopa Dome Rhyolite—Three voluminous lava flows of phenocryst-poor high-silica rhyolite (lower, middle, and upper flows: Tcdl, Tcdm, Tcdu) form the bulk of Cochetopa Dome 

(photo 19, CD-ROM). A small area of crystal-rich rhyolite (Tcdc), exposed only as float, 

is a probable fourth flow. The main lava units contain locally mappable carapace breccias 

(Tcdx). In bulk-sample and phenocryst chemistry, these lavas are the most petrologically 

evolved rocks within the map area or anywhere among the voluminous Oligocene volcanic 

rocks of the San Juan region. In the sequence of main lava units on Cochetopa Dome, total 

phenocrysts decrease upward and sanidine compositions become more sodic as degree of 

magmatic fractionation increases (defined mainly from trace-element data, Lipman, 2004; 

table 5, CD ROM). North of Cochetopa Park caldera, small rhyolite plugs (Tcdi) that intrude 

older volcanic and prevolcanic rocks are similar in age and composition to the lavas on 

Cochetopa Dome and are probably additional parts of the same caldera-related magmatic 

system. Outcropping ridges on Cochetopa Dome have a pronounced northeast-trending 

grain, parallel to graben structures to the southwest and the elongate Needle Creek intrusion 

to the northeast, but no faults or other structural controls for these trends were recognized 

during mapping on the heavily vegetated slopes of Cochetopa Dome

Tcdu Upper lava flow—The highest flow on Cochetopa Dome is virtually aphyric light-gray highsilica rhyolite (76.3–76.9% SiO2

) preserving small nodules of nonhydrated obsidian (Apache 

tears) along a basal zone of perlitic glass; distribution of the obsidian Apache tears, which are 

present only in the upper flow, was helpful in tracing the basal contact of this lava through 

heavily forested slopes. 40Ar/39Ar age on obsidian, 26.91±0.20. Maximum thickness, >250 m

Tcdc Crystal-rich flow—Tan rhyolite (76.5–76.6% SiO2) containing 20–30% small phenocrysts 

(sn, pl, bi). Exposed only as float on southwest side of Cochetopa Dome, along a timbered bench between middle and upper main flows. Compositions of sanidine phenocrysts 

(Or56–58Ab42–43An0.6–0.8Cn0.1) are distinctly more sodic than those for underlying crystal-poor 

rhyolite flows. 40Ar/39Ar age on sanidine, 26.81±0.04 Ma. Thickness, 0–50 m

Tcdm Middle lava flow—Gray to tan rhyolite (76.5–77.0% SiO2; 1–2% sn, pl, qtz, bi) forming flowlaminated to massive cliffs around south side of Cochetopa Dome. Compositions of sparse 

sanidine phenocrysts (Or59–60Ab38–40An1.0–1.3Cn0.05–0.1) are slightly more sodic than those for 

the underlying lower flow. 40Ar/39Ar age on sanidine, 26.88±0.05 Ma. Maximum thickness, 

200 m

Tcdx Glassy flow breccia—Locally mappable green-gray carapace breccia especially well exposed 

on northeast flank of the middle flow (Tcdm). Contains angular fragments to 1 m across. 

Thickness, 0–20 m

Tcdl Lower lava flow—Tan flow-layered rhyolite (75.3–75.8% SiO2; 2–3% sn, pl, qtz, bi) exposed 

only low on the east flank of Cochetopa Dome, where steeply dipping pressure ramps mark 

the top of this flow. Compositions of sanidine phenocrysts (Or60–63Ab36–38An1.0–1.2Cn0.1–0.3) 

are more potassic than those in overlying flows, but less than in caldera-related Nelson 

Mountain Tuff. This flow is the most phenocryst rich and the least geochemically evolved; it 

previously was mapped as landslide deposits (Tweto and others, 1976), perhaps because the 

ramp structures were interpreted as slump blocks. 40Ar/39Ar age on sanidine, 26.99±0.14 Ma. 

Thickness, >125 m 

Tcdi Intrusions—Massive to flow-layered gray to tan crystal-poor rhyolite (75–79% SiO2; 1–2% 

san, pl, qtz) forming equant plugs and elongate dike-like masses. Central parts of the Prosser 

Rock intrusion (RD) contain as much as 20% angular to rounded fragments of dark-gray 

aphanitic andesite in varying stages of disaggregation (photo 25, CD-ROM). 40Ar/39Ar ages 

on sanidine: Prosser Rock intrusion, 27.14±0.05 Ma for sample of uniform flow-laminated 

rhyolite and 27.08±0.05 Ma for rhyolitic portion of andesite-charged sample; dike-like intrusion at Sage Park (RD), 27.07±0.06 Ma. The slightly older ages suggest that these intrusions 
20

may have been precursors to the compositionally similar eruptions at Cochetopa Dome, 

rather than direct equivalents

Caldera-Fill Volcaniclastic Rocks

After deposition of the Nelson Mountain Tuff within the concurrently subsiding caldera at Cochetopa 

Park, the resulting basin was filled by a varied assemblage of landslide tongues from the oversteepened 

wall, caldera-margin fanglomerate, and fine-grained tuffaceous sediments that accumulated in the moat 

area, in part concurrently with eruption of rhyolitic lava flows (Cochetopa Dome Rhyolite) centrally within 

the caldera 

Caldera-margin fanglomerate—Alluvial-fan deposits derived by stream erosion of the inner 

caldera walls (photo 22, CD-ROM). The fanglomerate is only weakly indurated, outcrops 

are rare, and most occurrences are cobble-strewn slopes. These deposits contain varying 

clast assemblages, reflecting diverse lithologies along different sectors of the caldera walls. 

The caldera-margin fanglomerates largely overlie but locally interfinger with the intracaldera tuffaceous sediments that dominate sedimentation in the moat (photo 19B, CD-ROM). 

The fanglomerate may be largely younger than the Cochetopa Dome Rhyolite, but no direct 

stratigraphic relation is preserved

Tpff Fanglomerate with clasts of Fish Canyon Tuff—Andesitic clasts are dominant, but scattered 

clasts of Fish Canyon Tuff (Tfc), as well as rare clasts of dacitic Luders Creek Tuff (Tld), 

are conspicuous. Best exposures are along Archuleta Creek (CP), along the east margin of 

Cochetopa Park caldera. Maximum thickness, >200 m

Tpcf Fanglomerate and sandstone, with clasts of andesitic and rare dacitic lava derived from 

the Conejos Formation—Typical caldera-fill deposits low along north caldera wall (photo 

23, CD-ROM). Best exposures are low along ridges between Homestead and Townsite 

Gulches (ST). Maximum thickness, >200 m

Tpt Intracaldera tuffaceous sediments—White to gray tuffs and water-reworked sediments, including laminated lake-bed deposits, deltaic shallow-water sandstone in medial areas of the 

caldera moat, and coarser river-channel sandstone and local conglomerate in sites more 

proximal to inner walls of the caldera. Exposures are discontinuous and limited in vertical 

and lateral extent, due to weak induration of most of the unit and low topographic relief in 

the moat area. Widespread zeolitic alteration of originally glassy ash. 40Ar/39Ar ages on sanidine: 26.90±0.04 Ma for pumice blocks east of Cochetopa Dome; 26.83±0.05 Ma for small 

nonwelded ignimbrite deposit south of the dome. Based on phenocryst content, eruption of 

the dated tuff samples may have been associated with emplacement of the middle flow on 

the dome. Maximum exposed thickness, >50 m

Caldera-margin landslide breccias—Massive landslide deposits are widely distributed along 

lower parts of the gently sloping inner caldera wall (photo 21, CD-ROM). Most are monolithologic, or nearly so, and record dominant lithologies high on the adjacent sector of the 

caldera rim at the time of subsidence. Sparse matrix is microbreccia of the same lithologies 

as the clasts. Individual clasts are commonly <1 m, but large clasts are locally present, and 

a few seemingly coherent megablocks are as much as several hundred meters across. In 

places, at upper limits of map units, the landside breccias become more coherent and difficult to distinguish from in-place exposures of the same lithology exposed on the caldera 

wall. Several megablocks of andesite are parts of the slide breccia near upper map limits of 

this unit on the southeast slope of Sawtooth Mountain (ST), and a large slab of flow-layered 

dacite is enclosed by slide breccia along Dry Gulch at about 10,000 ft elev (CS-ST boundary). Emplacement of collapse-related landslide deposits along the inner caldera slope at 

Cochetopa contrasts with the breccia sheets that spread widely across the caldera floor and 

intertongue with intracaldera ignimbrite deposits at other calderas of the San Juan region and 

elsewhere (Lipman, 1976, 2000). This difference probably reflects shallower subsidence (on 

the order of 1 km or less) at the noneruptive Cochetopa Park caldera, in contrast to deeper 

subsidence (2–5 km) documented at many other calderas that were primary eruptive centers 

for associated ignimbrite eruptions

Tpbf Massive breccia with clasts solely of Fish Canyon Tuff—Monolithologic breccias consisting of Fish Canyon clasts (Tfc) are exposed nearly continuously along the south and east 
21

margins of the caldera basin. Clast-supported texture, sparse matrix fines, angular blocks, 

and unfilled voids are consistent with modest transport distance. These slide deposits locally 

form rugged cliffs of well-indurated breccia (photo 21, CD-ROM), for example along the 

Saguache Park road in Mexican Joe Gulch (CP and adjacent SA to the south), where they 

previously were mapped as in-place Fish Canyon Tuff (Steven and others, 1974). Small 

patches of similar breccia, preserved on the west caldera margin along both sides of Los 

Pinos Creek (CS), are difficult to distinguish from other breccia deposits (Tfcb) of Fish 

Canyon clasts in the same area, which are inferred to have formed earlier in association with 

faulting along a graben related to subsidence of La Garita caldera. Thickness, 0–200 m

Tpbm Breccia with sparse clasts of Carpenter Ridge Tuff—Locally exposed breccia dominated 

by Fish Canyon fragments but containing mixed clast lithologies (photo 21D, CD-ROM); 

typically less indurated than the monolithologic breccia (Tpbf) and exposed mainly as float. 

Distributed discontinuously along the north margin of the caldera, roughly concordantly 

with preserved outcrops of in-place Carpenter Ridge Tuff (Tcr) on the caldera rim. The 

mixed-breccia unit is best exposed west of the Samora Creek road (northeast CP), along the 

Wolverine Creek trail (south RD), and in a small roadcut along Los Pinos Creek road (CS). 

Presence of Carpenter Ridge clasts in the landslide breccia assemblage is critical evidence 

for timing of the last major subsidence at Cochetopa Park caldera. The Carpenter Ridge 

clasts are densely welded red-brown tuff, characterized by sparse biotite phenocrysts. Sparse 

clasts of densely welded Saguache Creek Tuff (Tsc) are also locally mixed with dominant 

Fish Canyon blocks in the landslide deposits; clasts of Saguache Creek Tuff are distinguishable from Carpenter Ridge clasts in the field by absence of biotite. Identification of Carpenter Ridge clasts in critical localities has been confirmed by rock and phenocryst (sanidine) 

chemical analyses and by isotopic ages concordant with dated in-place samples of this tuff 

(less alkalic and 4.5 m.y. younger than Saguache Creek Tuff). 40Ar/39Ar age on sanidine from 

a Carpenter Ridge clast along the Los Pinos Creek road, 27.59±0.05 Ma; for clast west of 

Samora Creek, 27.48±0.05 Ma. Thickness 0–25 m

Tpbs Breccia with clasts of Saguache Creek Tuff—A large breccia lens, consisting of monolithologic fragments of densely welded red-brown Saguache Creek Tuff (Tsc) lies low along the 

north caldera wall on the ridge south of Homestead Gulch (ST). This lens is interpreted as 

a landslide deposit emplaced into Cochetopa Park caldera, based on its location along the 

inner caldera slope and its low topographic elevation relative to scattered preserved outcrops 

of in-place Saguache Creek Tuff north of the caldera. Lithologically similar breccias consisting of Saguache Creek Tuff (Tscl), which were emplaced in paleovalleys by landslides active prior to eruption of the Fish Canyon Tuff, are present north of the caldera basin, and the 

Homestead Gulch locality could possibly be such a deposit. Thickness, 0–50 m

Tpbr Breccia with clasts of Conejos rhyolite lava—Small breccia lenses consisting of gray to tan 

crystal-poor rhyolite (Tcfr; 2–5% pl, bi). Best developed north of Archuleta Creek (CP) and 

in Myers Gulch (RD). Thickness, 0–100 m

Tpbd Breccia with clasts of Conejos dacite lava—Small breccia lenses consisting of gray to tan 

porphyritic dacite (Tcd; 15–30% pl, bi, cpx/bh). Best developed along northwest caldera 

margin, between Dry Gulch and Willow Creek (CS-ST boundary). Thickness, 0–60 m

Tpba Breccia with clasts of Conejos andesite lava—Breccia consisting of fragments of dark-gray 

to brown andesite; widespread along the northern caldera margin. Phenocrysts are typically 

sparse, and clasts closely resemble in-place andesite lavas and volcaniclastic rocks of the 

Conejos Formation (Tca) higher on the inner caldera wall. Deposits are typically massive, 

but crude stratification locally parallels inward-dipping caldera-wall slopes. In places, difficult to distinguish from in-place volcaniclastic breccia of the Conejos Formation (Tcv). 

Thickest and most massive downslope from present-day high points on the north caldera rim 

(Sawtooth Mountain, Razor Creek Dome; photo 23, CD-ROM). Thickness, 0 to >100 m

Nelson Mountain Tuff

Caldera-forming eruption of the Nelson Mountain Tuff generated a regional ignimbrite sheet and thick 

intracaldera fill; the tuff grades upward in composition from rhyolite to dacite (73–63% SiO2). Typical 

sanidine compositions are Or62–67Ab30–34 An1.1–1.3Cn1.6–2.5 (Lipman and Weston, 2001), becoming more potassic and less homogeneous upward in section as bulk compositions change from rhyolite to dacite. Map 

subunits are primarily compositional and welding zones, rather than strictly sequential stratigraphic units. 
22

Normal magnetic polarity, with relatively steep inclination becoming nearly vertical high in some sections (Lipman, 2000, fig. 10). Weighted mean of single-crystal 40Ar/39Ar ages (sn) from ten geographically 

diverse localities, 26.90±0.03 Ma (Lipman and McIntosh, 2008)

Tndn Nonwelded to partly welded dacite—Uppermost Nelson Mountain Tuff, both within and locally 

outside the Cochetopa Park caldera (photo 18, CD-ROM). Yellowish-tan to gray, porous, 

vapor-phase-crystallized to glassy. Grades down into more welded dacitic Nelson Mountain 

Tuff (Tnd). Crystal rich; phenocrysts similar to welded dacite (Tnd). Mostly poorly exposed; 

especially well preserved on low ridge (photo 16B, CD-ROM) between Mexican Joe Gulch 

and Monchego Creek (CP). Thickness, 0–30 m

Tnd Welded silicic dacite and transitional rhyolite-dacite—Transitional rhyolite-silicic dacite 

welded zone (67–72% SiO2; 15–25% pl>sn, bi>cpx, qz). Characterized by less potassic 

sanidine compositions (Or64–67) than mafic dacite of the dominant intracaldera facies (San 

Luis caldera complex). Forms brownish-gray main cliff-forming welded caprock of outflow Nelson Mountain Tuff, commonly with black vitrophyre (3–5 m) near base. Contains 

pumice lenses of both crystal-poor rhyolite and crystal-rich dacite. Grades downward into 

rhyolite (Tnr). Particularly thick and well exposed in narrows of Cochetopa Creek (photos 

14, 16A, CD-ROM) along southwest caldera wall (CS). Thickness, 20–100 m 

Tnr Welded to partly welded rhyolite—Light-gray and tan to light-reddish-brown welded ignimbrite (71–74% SiO2

; 5–15% pl>sn>bi>>cpx). Dark-gray pumice fiamme, characterized by 

flattening ratios <10:1, are commonly glassy and set in orange-brown devitrified matrix, 

forming colorful “halloween” rock (glassy black pumice lenses in devitrified red-brown matrix). Locally intensely zeolitized or argillically altered. Grades into less welded ignimbrite 

(Tnn). Exceptionally thick and well exposed along distal tongue of Nelson Mountain Tuff 

that flowed down the Cochetopa Creek graben into Cochetopa Park caldera (CS). Thickness, 

0–50 m

Tnn Nonwelded to partly welded rhyolite and silicic dacite—Gray porous pumiceous tuff containing common small angular fragments of andesitic lava. In most sections, originally glassy; 

now widely argillized or zeolitized. Some thick accumulations are vapor-phase crystallized, 

even where nonwelded. Commonly poorly exposed due to weak induration and to cover by 

talus from welded caprock (Tnd). Flow-unit partings, locally marked by thin crystal-rich 

surge beds and accretionary lapilli, are present in some well-exposed sections. Phenocrysts 

(typically 5–10% pl, sn>bi) are similar to welded rhyolite (Tnr). Thickness, 10–200 m; 

greatest where accumulated along west margin of Cochetopa Park caldera south of Los 

Pinos Creek (CS)

Rocks Erupted from the Cebolla Creek Caldera

Eruption of the compositionally distinctive Cebolla Creek Tuff (photo 15, CD-ROM) caused subsidence of a large caldera within the San Luis complex, now largely concealed beneath the Nelson Mountain 

caldera and flanking postcollapse volcanoes, all lying southeast of the map area (Lipman, 2006)

Cebolla Creek Tuff—Distinctive compositionally uniform ignimbrite sheet of mafic dacite 

(photo 15, CD-ROM), characterized by abundant phenocrystic hornblende relative to augite 

and by lack of sanidine. Basal crystal- and lithic-rich surge beds, about 1 m thick, are well 

exposed at Wheeler Monument (Wheeler Geologic Area) just south of map area (Lipman, 

2006). Previously mapped as part of the Nelson Mountain or Rat Creek Tuffs (Steven, 1967; 

Steven and Ratté, 1973; Steven and Bieniewski, 1977). Weakly welded deposits were widely 

eroded prior to eruption of Nelson Mountain Tuff, which directly overlies older units in 

many places within the general distribution area of the Cebolla Creek. Distinctive normal 

magnetic direction (Lipman, 2000, fig. 10). Weighted mean of four 40Ar/39Ar ages (hbl, bio), 

27.10±0.0.07 Ma, is anomalously slightly older than ages on sanidine from underlying Rat 

Creek Tuff

Tcc Weakly welded mafic dacite—Gray to light-brown, glassy to weakly vapor-phase-devitrified 

ignimbrite (62–66% SiO2; 25–40% pl>>bi, hbl>>cpx) typical of proximal outflow sections. 

Contains orange-brown to black pumice fragments (2–5 cm across) and abundant small (1–3 

cm across) andesitic lithics. Exposed only as small erosional remnants in southwest map 

area. Thickness, 0–25 m
23

Rocks Erupted from the Rat Creek Caldera

The earliest ignimbrite sheet of the San Luis assemblage, the Rat Creek Tuff (photo 15, CD-ROM), is 

the smallest in volume, the most restricted in known areal extent, and the least understood. Evidence for the 

Rat Creek caldera (Lipman, 2000, p. 32–33) is also limited, because of cover by younger rocks

Rat Creek Tuff—Compositionally zoned ignimbrite sheet (65–73% SiO2) of relatively modest 

volume, generally poorly exposed beneath Nelson Mountain Tuff. Within study area, only 

exposures are nonwelded to partly welded ignimbrite along southwest map margin (RC). 

Weakly welded deposits were widely eroded prior to eruption of Cebolla Creek Tuff, which 

directly overlies older units in many places within general distribution area of the Rat Creek 

Tuff. Weighted mean 40Ar/39Ar age (sn) from eight sample sites, 26.91±0.02 Ma (Lipman 

and McIntosh, 2008). Remanent magnetic polarity, normal

Trdn Weakly welded dacite—Gray tuff (68–72% SiO2, silicified; 25% pl>sn>bi>>cpx), characterized by vapor-phase matrix crystallization. Preserved as float, without real outcrops, in a few 

small areas along Los Pinos Creek in vicinity of McDonough Reservoir Number 2 (CS). No 

direct stratigraphic relation to the regional ignimbrite sheets erupted from the San Luis complex is preserved; correlation with Rat Creek based largely on lithology, isotopic age, and 

topographic position, far below caprock rim of Nelson Mountain Tuff south of Los Pinos 

Creek. 40Ar/39Ar age, 26.88±0.06 Ma. Thickness, 0–25 m

Trr Nonwelded rhyolite—Poorly exposed light-gray to yellow pumiceous tuff (70–73% SiO2; 

5–20% pl, sn>bi), commonly containing several percent centimeter-size fragments of 

andesitic to rhyolitic rocks. Typical sanidine composition is Or62Ab34An1.5Cn2.5 (Lipman and 

Weston, 2001). Glassy, ranging to zeolitic or argillically altered tuff. Grades upward into 

silicic dacite. Difficult to distinguish from rhyolitic Nelson Mountain Tuff (Tnn) in absence 

of intervening Cebolla Creek Tuff (Tcc). Exposed only on small hill near southwest corner 

of map area (RC); more widespread to the southwest. Thickness, 0–50 m

ROCKS ERUPTED FROM SOUTH RIVER CALDERA

South River caldera (fig. 2), resulting from eruption of the Wason Park Tuff, is defined by a steep caldera-wall unconformity that juxtaposes the caldera-filling South River Volcanics against flat-lying regional 

ignimbrites, now exhumed along the arcuate drainages of Red Mountain and Goose Creeks (Lipman, 2000, 

2006)

Tw Wason Park Tuff—Distinctive regional ignimbrite sheet of crystal-rich rhyolite, locally grading 

upward into silicic dacite (68–74% SiO2; 10–30% pl, sn>>bi>cpx) (Ratté and Steven, 1967; 

Webber, 1988). Red-brown devitrified interior of cooling unit is characterized by white 

collapsed pumice lenses as much as 0.3 m long, consisting of intergrown alkali feldspar 

and tridymite. More phenocryst rich and alkalic than other San Juan rhyolitic ignimbrite 

sheets (Lipman and others, 1978); characterized by distinct tabular morpholology and sodic 

composition (Or55–60) of sanidine phenocrysts. Preserved within map area only as small 

erosional remnants: along Still Creek within Los Pinos graben (RC), as a sinuous paleovalley fill (photo 12, CD-ROM ) at the mouth of Saguache canyon (LM), and as scattered small 

patches along Sheep Creek (TM); in these areas, 70.0–70.3% SiO2. Weighted mean 40Ar/39Ar 

age (sn) for five sample sites, 27.38±0.05 Ma (Lipman and McIntosh, 2008). Remanent 

magnetic polarity, reverse (D 174.7°, I –16.0°) in Sheep Creek. Thickness within map area, 

0–30 m

YOUNGER INTERMEDIATE-COMPOSITION LAVA FLOWS

Mesa-capping mafic flows that are compositionally similar to slightly older lavas of the Huerto Andesite (Thu) and are difficult to distinguish reliably at some sites. These younger lavas constitute a compositional transition between the dominantly andesitic Oligocene volcanism and more basaltic lavas of the 

Miocene Hinsdale Formation (Lipman and others, 1970; Lipman and Mehnert, 1975). The lavas mapped 

here as the andesite of Mountain Lion Creek were previously mapped with the Hinsdale Formation (Tweto 

and others, 1976; Tweto, 1979), but Hinsdale-age rocks are now known only at a single locality in the 
24

northeastern San Juan Mountains (Houghland Hill (LG), just east of the map area; Lipman and McIntosh, 

2008; table 2)

Tal Andesite of Mountain Lion Creek—Mesa-capping erosional remnants of fine-grained dark-gray 

lava flows of silicic olivine andesite (trachyandesite, 61–62% SiO2) that overlie Carpenter 

Ridge Tuff (Tcr) at the head of Mountain Lion Creek and on Hat Mountain (GG, LM). Small 

sparse olivine phenocrysts, partly altered to rusty brown iddingsite, are typical. 40Ar/39Ar 

ages on groundmass separates from three sites at head of Mountain Lion Creek group tightly 

at 27.46–27.51 Ma, suggesting emplacement just prior to eruption of the Wason Park Tuff 

(Tw) at 27.38±0.05 Ma (Lipman and McIntosh, 2008). These flows appear to occupy a 

stratigraphic position similar to the lava flows of McClelland Mountain, as mapped within 

the southern Bachelor caldera 40 km to the southwest (Lipman, 2006). An additional site 

of olivine-bearing lava (Round Park-Hat Mountain area) yielded an older groundmass age 

(27.81±0.16 Ma), but this flow may predate the Carpenter Ridge Tuff and thus be part of the 

Huerto Formation (Thu). Thickness 20–30 m

ROCKS ERUPTED FROM BACHELOR CALDERA

The Bachelor caldera, entirely enclosed within central parts of La Garita caldera (fig. 2), was the 

source of the Carpenter Ridge Tuff (photos 10A, 11, CD-ROM), the second largest eruptive unit (~1,000 

km3) from the central caldera cluster. Thick intracaldera Carpenter Ridge Tuff, which consists dominantly 

of phenocryst-poor rhyolite, forms the host rocks for the economically important vein mineralization of the 

Creede district, within the central caldera cluster (Steven and Ratté, 1965; Lipman and others, 1989)

Carpenter Ridge Tuff—Widespread caldera-forming ignimbrite sheet, containing complex 

welding and compositional zonation (67–74% SiO2), especially within Bachelor caldera. 

Volumetrically dominant rhyolite, containing 3–5% phenocrysts (pl>sn>bi), grades locally 

upward to silicic dacite (Mammoth Mountain Member) containing as much as 30% phenocrysts in proximal sites. In comparison to other crystal-poor rhyolites within the map area, 

Carpenter Ridge sanidine compositions (typically Or64–66Ab31–33An1.2–1.5Cn1.0–1.2) are much 

more potassic than in Saguache Creek Tuff and less barium-rich than in the Sapinero Mesa 

Tuff. Weighted mean 40Ar/39Ar age (sn) for four sample sites, 27.55±0.05 Ma (Lipman and 

McIntosh, 2008). Remanent magnetic polarity, reverse

Tcrn Nonwelded to partly welded tuff—Light-tan cavernous-weathering upper part of unit, locally 

forming distinctive vuggy knob outcrops of vapor-phase crystallized tuff. Sufficiently thick 

and distinctive to map separately where preserved along the downthrown side of the Lake 

Mountain fault zone (TM). Compositionally and mineralogically similar to the welded rhyolite (Tcr). Thickness, 0–15 m

Tcr Welded rhyolite tuff—Dominantly phenocryst-poor tan rhyolite (Lipman and others, 1970; 

Whitney and others, 1988), widely characterized by black basal vitrophyre and conspicuous 

central lithophysal zone (photo 11, CD ROM). Forms widespread thin outflow sheet east 

of the Cochetopa Hills, where ash flows crossed the northeast rim of La Garita caldera and 

spread out in the headwaters of present-day Saguache Creek, and smaller erosional remnants 

elsewhere. Exceptionally thick (as much as 150 m), where ponded along Los Pinos graben 

(RC), which subsided during formation of La Garita caldera. In this area, the Carpenter 

Ridge Tuff was initially confused with similar-appearing Sapinero Mesa Tuff (Tws) because 

of its low depositional level, topographically below exposures of the Fish Canyon Tuff on 

shoulders of the graben. Thickness, commonly 25–50 m, as much as 150 m where ponded in 

Los Pinos graben

ROCKS OF LA GARITA CALDERA CYCLE

The enormous La Garita caldera (35 x 75 km across; 100 km if additional subsidence occurred in 

the Cochetopa Park area) and its eruptive products define the overall geometry of the central San Juan 

caldera cluster (fig. 2), all later calderas having formed within La Garita structure (Lipman, 2000, 2006). 

Its associated ignimbrite sheet, the enormous Fish Canyon Tuff, which consists of monotonously uniform 

phenocryst-rich dacite (at least 5,000 km3

), has long been recognized as among the world’s largest ignimbrite units (Lipman and others, 1970; Steven and Lipman, 1976; Bachmann and others, 2002)
25

Huerto Andesite—Andesitic lava flows and volcaniclastic rocks (Larsen and Cross, 1956; 

Askren and others, 1991; Parat and others, 2005) form remnants of large shield volcanoes 

and flanking erosional debris, mainly within and adjacent to La Garita caldera. Thickest and 

best developed around and within southern parts of the caldera. Compositionally similar 

high-level lava flows within the map area (photo 13, CD-ROM) are correlated with the 

Huerto, on the basis of similar composition, stratigraphic position, and isotopic age

Thu Olivine-bearing andesite and hornblende-bearing andesite-dacite, undivided—Gray 

sparsely porphyritic olivine-bearing lavas and breccias, in places containing resorbed 

xenocrysts of quartz and feldspar, are overlain by Carpenter Ridge Tuff on east slope of 

Lake Mountain (LM). Lavas on Mount Lion, which are probably continuous with the Lake 

Mountain flows across Houselog Creek to the southeast, include a lower flow (56.7% SiO2; 40Ar/39Ar age 27.96±0.11 Ma) and a xenocrystic upper flow (61.9% SiO2

; 40Ar/39Ar age 

27.64±0.12 Ma). Lavas characterized by phenocrystic hornblende (60–63% SiO2; 15–20% 

pl>hbl, cpx), directly overlie Fish Canyon Tuff on some hills in eastern map area, without 

intervening Carpenter Ridge Tuff. Particularly conspicuous is the thick flow on Trickle 

Mountain (62.4% SiO2; 40Ar/39Ar age on hornblende, 27.92±0.19 Ma). Some of these flows 

alternatively might be correlative with the andesite of Mountain Lion Creek (Tal) that 

yielded slightly old ages (not uncommon for hornblende and groundmass ages), or even constitute early-erupted lavas of the Hinsdale Formation. Maximum thickness, 100 m on Trickle 

Mountain, 50 m on Lake Mountain

Fish Canyon Tuff—A single large cooling unit of nonwelded to densely welded gray to lightbrown ignimbrite of compositionally uniform silicic dacite (66–68% SiO2

; 40–50% pl>>sn, 

qtz, bi>hbl), preserved as much as 100 km beyond rims of La Garita caldera. Characterized 

by simple welding zonation except in thick proximal sections, where compound welding is locally developed. Pumice fragments are obscure in many exposures of devitrified 

tuff, and lithic fragments typically are sparse. Densely welded tuff is massive (photo 10, 

CD-ROM), but upper parts commonly have slabby jointing parallel to obscure compaction 

foliation. Rare dark pumice lenses, which lack large phenocrysts and resemble crystal-poor 

rhyolite in the field, contain abundant finely milled crystal fragments and are compositionally identical to the more common pumice fragments of crystal-rich dacite. Sparse 

resorbed pinkish quartz, accessory sphene, and hornblende without augite are distinctive 

phenocrysts (Lipman, 1975; Whitney and Stormer, 1985). Sanidine phenocrysts (typically 

Or72–74Ab23–24An1Cn1.5) are relatively potassic and conspicuously zoned, with barium (Cn) 

varying 1–4 mol percent (Lipman and others, 1997; Bachman and others, 2002). 40Ar/39Ar 

age, 28.02 Ma (currently accepted as geochronologic standard; Renne and others, 1998). 

Magnetic polarity, normal. Maximum thickness within map area, 100 m 

Tfcb Fault-scarp breccia, with clasts of Fish Canyon Tuff—Narrow zones of brecciated Fish 

Canyon Tuff, developed locally along intricate system of normal faults where the Los Pinos 

graben merges with the west margin of Cochetopa Park caldera (CS). Interpreted as having formed concurrently with eruption of the Fish Canyon Tuff and subsidence of La Garita 

caldera, because faults of the Los Pinos graben predate emplacement of the Carpenter Ridge 

Tuff (Tcr). May provide evidence for early subsidence (or at least volcano-tectonic processes) 

within the Cochetopa Park area during the Fish Canyon eruptions. Difficult to distinguish in 

some outcrops from younger landslide breccia with clasts of Fish Canyon Tuff (Tpbf) associated with late subsidence of the Cochetopa Park caldera during eruption of Nelson Mountain 

Tuff; some associated faults may have become reactivated during this later subsidence 

Tfc Widespread welded outflow ignimbrite sheet—Light-gray variably welded tuff, commonly 

forming massive rounded outcrops with columnar and sheet cooling joints. Continuously 

outcropping and exceptionally thick (to 300 m) where ponded within southeast side of North 

Pass caldera. Similarly thick and banked abruptly against andesitic lavas of the Conejos 

Formation southeast of McDonough Park (RC), a feature which may record early subsidence 

in the Cochetopa Park area during eruption of the Fish Canyon Tuff. More discontinuously 

preserved and variable in thickness along northern parts of map area because of deposition 

along broad north-trending paleovalleys. Thickness, 0–300 m

Tfcn Nonwelded to partly welded tuff—Poorly exposed weakly indurated lower parts of distal ignimbrite sheet. Sufficiently thick and widespread to map separately only along lower Razor 

Creek and in Parlin Flats area (HG). Thickness, 0–25 m 
26

OLDER TUFFS RELATED TO WESTERN SAN JUAN CALDERAS

Distal facies of several regional ignimbrite sheets erupted from western San Juan sources (Lipman 

and others, 1973; Steven and Lipman, 1976; Bove and others, 2001) are exposed discontinuously north of 

the central caldera cluster (Olson, 1974; Steven and Hail, 1989). These include three lithologically similar 

phenocryst-poor rhyolite ignimbrites (Blue Mesa, Dillon Mesa, and Sapinero Mesa Tuffs). Only the Sapinero Mesa Tuff is present within the map area

Sapinero Mesa Tuff—Nonwelded to partly welded gray-brown to red-brown rhyolitic ignimbrite 

sheet (72–74% SiO2; 3–5% pl>sn>bi), locally containing black basal vitrophyre, erupted 

from the composite Uncompahgre-San Juan caldera (Steven and Lipman, 1976). Typically 

thin and discontinuously preserved. Where thickest (west of map area), contains a well-developed central lithophysal zone (as much as 10–20 m thick), similar to that in the Carpenter Ridge Tuff. Most widespread of the three phenocryst-poor western ignimbrite sheets; 

only one present within map area. Difficult to distinguish from Carpenter Ridge Tuff (Tcr), 

except by stratigraphic position relative to Fish Canyon Tuff and by locally distinct welding 

and crystallization textures. Distinguished in field by presence of sparse biotite phenocrysts 

from otherwise similar-appearing Saguache Creek Tuff (Tsc); also contains more potassic 

and barium-rich sanidine (average, Or61Ab33An2Cn4). Partly welded distal outcrops distinguished in field from similar-appearing tuff of Big Dry Gulch (Tbg) by presence of sanidine. 40Ar/39Ar age (sn), 28.20±0.06 Ma in Houselog Creek (LG). Magnetic polarity, reverse: 

D 116.1°, I –65.4° in Houselog Creek. Maximum thickness, 25 m

Twsb Fault-scarp breccia, with clasts of Sapinero Mesa Tuff—Narrow zone of breccia along 

south normal fault of the Los Pinos graben, where this fault crosses Blue Creek (CS). Clasts 

of densely welded tuff occupy zone 5–10 m wide, bounded to south by altered Conejos-age 

intrusive porphyry and to north by unaltered Fish Canyon Tuff

Tws Welded outflow tuff sheet—Red-brown to tan welded tuff, containing small flattened pumice 

lapilli. Best developed east of Monument Creek (SH) and north of upper Los Pinos Creek 

(RC, CS). Thickness, 0–75 m

Twsn Nonwelded to partly welded distal tuff—Weakly indurated gray to tan distal tuff, commonly 

vapor-phase crystallized, that is discontinuously preserved and exposed mostly as slopemantling clasts in northwestern areas along Gunnison valley (HG) and along Right Creek 

(SH), in southwest map area (GC), and east of the present map area in Houselog Creek (LG) 

Thickness, 0–25 m

CONEJOS FORMATION, UPPER UNITS

Locally erupted lavas and tuffs that overlie the Saguache Creek Tuff and other units of the North Pass 

caldera cycle. These diverse rocks are stratigraphically equivalent to upper parts of the Conejos Formation farther west and south in the San Juan region, where early Oligocene ignimbrite sheets that provide 

regional stratigraphic marker horizons are absent. An unusual cluster of thick rhyolitic-dacitic lava flows 

and domes, about 6 km across in preserved areal extent and centered in Barret Creek (WB), may record 

a failed caldera system (Lipman and McIntosh, 2008). Several small areas of basaltic to andesitic lava 

flows and a thin dacitic ignimbrite, preserved within and adjacent to the North Pass caldera, are as much 

as 2 m.y. younger than the main eruptive units of this caldera. These rocks may represent prolonged late 

activity of the caldera cycle or, alternatively, may be local expressions of continuing regional flux of 

intermediate-composition eruptions (upper units of the Conejos Formation), unrelated to any specific 

ignimbrite-caldera cycle

Tcuv Upper volcaniclastic rocks of the Conejos Formation—Crudely bedded to well-bedded conglomerates, sandstones, and mudflow breccias, containing rounded clasts of dark andesite 

and dacite in lighter-gray sandy matrix. Overlie Saguache Creek Tuff (Tsc) along canyon 

of Cochetopa Creek (IR). Stratigraphically equivalent to andesitic conglomerate in the 

volcanics of Cochetopa Hills (Tsa), volcaniclastic rocks on the andesite of Lone Tree Mesa 

(Tlav), and lithologically similar conglomerates (too thin to show at map scale) interleaved 

with the Saguache Creek and Sapinero Mesa Tuffs along Saguache Creek in the eastern map 

area. Thickness, as much as 50 m
27

Rhyolite of Barret Creek—Flows and domes of aphyric to porphyritic rhyolite and silicic dacite 

clustered in upper Barret Creek and along the divide toward Razor Creek to the south (WB). 

Overall thickness, as much as 400 m

Tbfc Coarsely porphyritic rhyolite—Massive light-gray flows and domes of low-silica rhyolite 

and dacite (68–75% SiO2) containing 20–30% phenocrysts of plagioclase, sanidine, and 

biotite. Basal black vitrophyre zones are locally as much as 25 m thick; carapace breccia is 

preserved in places. 40Ar/39Ar ages (sn), 29.63–29.85 Ma (samples from three flows). Thickness of individual flows, 0–300 m

Tbfa Aphyric to sparsely porphyritic rhyolite—Light-gray to tan flow-layered rhyolite (71–75% 

SiO2) typically containing less than 5% phenocrysts of plagioclase and biotite. Devitrified 

and glassy rocks are interleaved along steeply dipping ramp structures at tops of some flows. 

40Ar/39Ar age (sn), 29.79±0.05 Ma (flow on Green Mountain, WB). Thickness of individual 

flows, 0–250 m

Basalt of point Benny—Flow of fine-grained dark-gray silicic basalt (52.0% SiO2) and an associated eroded vent dike and aggutinated vent scoria at point Benny (named hill top, NP), 

centrally located within the North Pass caldera (photo 4, CD-ROM). Basalt is finely olivinephyric and contains widely distributed sparse xenocrysts of quartz and feldspar, derived 

from disaggregated fragments of Precambrian granite. Despite the xenocrystic component, 

this flow is the most mafic Oligocene volcanic unit yet documented from anywhere in the 

San Juan region. Weighted mean 40Ar/39Ar age of 30.22±0.10 Ma for groundmass determinations on five samples (Lipman and McIntosh, 2008) is consistent with stratigraphic position 

beneath Fish Canyon Tuff, also with eruption nearly concurrent with the late andesite of hill 

9519 that ponded against the east wall of the North Pass caldera

Tbpf Lava flow—Several small erosional remnants of a single large basaltic lava flow. Fish Canyon 

Tuff fills paleovalleys eroded into the basalt, in places cropping out at levels below that of 

the lava flow. Thickness, 0–30 m

Tbpv Cone scoria—Aggutinated dark-gray to red-brown scoria at point Benny, interpreted as marking eroded cone from which basalt flow erupted. Thickness, 0–50 m

Tbpi Vent dike and lava pond—Basaltic dike and massive lava at crest of point Benny are interpreted as eroded vent deposits. The near-vertical dike, 10–20 m wide, cuts upward through 

scoria wall rock and flares into a broader mass of dense basalt interpreted as remnants of 

lava lake within a crater

Andesite of Lone Tree Gulch (Saguache valley)—Several andesitic lava flows and associated 

volcaniclastic rocks (photo 9, CD-ROM) lie between the tuff of Big Dry Gulch (Tbg) and 

overlying Fish Canyon Tuff (Tfc) along the axis of paleo-Saguache Creek, on the southeast 

flank of Trickle Mountain and adjacent areas (TM-LM boundary). A whole-rock 40Ar/39Ar 

age of 30.21±0.17 Ma links this andesite closely to the slightly older andesite flows of hill 

9519 that are ponded against the east wall of the North Pass caldera, but the location of these 

flows well to the east suggests that they represent continued regional flux of intermediatecomposition volcanism unrelated to any caldera cycle

Tla Lava flows—Dark-gray flows of sparsely porphyritic vesicular andesite-dacite (59% SiO2). 

Most individual flows are 5–10 m thick; maximum total thickness, 50 m

Tlav Volcaniclastic rocks—Thin locally exposed lenses of reworked crudely bedded to well-bedded 

laharic conglomerates, sandstones, and mudflow breccias, containing clasts of dark andesite 

in light-gray sandy matrix. Mainly preserved along mesa northwest of Trickle Mountain. 

Reliably distinguishable from older volcaniclastic units, such as distal debris-flow deposits 

of Buffalo Pass Campground (Tdc) or volcanics of Cochetopa Hills (Tsa), only where separated by interlayered tuff of Big Dry Gulch (Tbg). Thickness, 0–15 m

Tbg Tuff of Big Dry Gulch—Nonwelded to partly welded gray-brown to red-brown dacitic ignimbrite 

(67% SiO2; 3–5% pl> bi>>cpx), locally containing black basal vitrophyre. Erupted from 

unknown source. Forms a distinctive local marker unit above the Saguache Creek Tuff (Tsc) 

and laharic debris-flow deposit with clasts of Luders Creek Tuff (Tdb) and below the andesite 

of Lone Tree Gulch (Tla). Distinguished from similar-appearing Sapinero Mesa Tuff (Tws) by 

absence of sanidine, presence of sparse clinopyroxene phenocrysts and more abundant biotite, 

isotopic age, and magnetic direction. Because this distinction was recognized only late during 

fieldwork, assignment of some isolated small exposures may be erroneous. 40Ar/39Ar age (biotite plateau), 30.47±0.08 Ma; magnetic direction, D 175.8°, I –37.6°. Maximum thickness, 20 m
28

Tab Andesite of Big Dry Gulch—Dark-gray crystal-poor andesite as chaotic breccia and some possibly intact lava flows. Forms rugged outcrops of massive monolithologic breccia in upper 

reaches of Big Dry Gulch (LM). Much of unit appears to have been emplaced as landslide 

material; parts may be primary eruptive deposits, perhaps on proximal flank of source eruptive edifice. Formed paleotopographic highs at time of eruption of the Fish Canyon Tuff. 

Age and emplacement relations are uncertain relative to the tuff of Big Dry Gulch (Tbg) 

or andesite of Lone Tree Gulch (Tla); these two andesite units may represent proximal and 

more distal portions of a single eruptive sequence. Thickness, 0–50 m

Debris-flow deposits of Buffalo Pass Campground—Variable fragmented poorly sorted 

deposit, derived largely from Luders Creek Tuff exposed in the vicinity of the present-day 

Continental Divide. Clast size decreases, clast roundness increases, and proportion of Luders 

Creek Tuff decreases relative to andesitic clasts with distance eastward from source region, 

as transport in debris flows gives way to more fluidized mudflow and stream transport. 

Generation process, 2 m.y. after main phases of the North Pass caldera cycle, is uncertain; 

perhaps emplaced in response to some late structural events near concealed source for eruption of Luders Creek Tuff

Tdc Distal conglomerate—Thin locally exposed lenses of reworked crudely bedded to 

well-bedded stream conglomerate and sandstones, containing dominant clasts of dark 

andesite and sparse clasts derived from the Luders Creek Tuff in light-gray sandy matrix. 

In southeastern part of map area (LM) and farther to east. Reliably distinguishable from 

younger volcaniclastic rocks, such as those in the andesite of Lone Tree Gulch (Tlav), only 

where separated by interlayered tuff of Big Dry Gulch (Tbg). Thickness, 0–15 m

Tdb Mesobreccia and conglomerate—Poorly sorted mixed clasts dominated by light-tan rhyolite 

and dark-gray dacite fragments derived from the Luders Creek Tuff; clasts of andesitic lava 

increase in relative abundance eastward. In western parts of the unit, angular slabs of darkgray dacitic Luders Creek Tuff are as much as 10 m across; in places, distinction is difficult 

between essentially in-place Luders Creek Tuff and material that has suffered significant 

transport in secondary debris flows. Thickness, 0–75 m

Tdbm Megabreccia blocks—Dense masses of healed breccia derived from Luders Creek Tuff (Tld) 

and perhaps some dacitic lava (dacite of East Pass Creek, Tnpd), forming rugged knobs 

within the fine-grained debris-flow deposit over a small area near the Buffalo Pass Campground (photo 8C, CD-ROM). Coherent knobs defining individual megablocks within the 

debris-flow deposit are as much as 25 m across. Brecciation within the knobs is so severe 

that the original character of the rocks is widely obscured (photo 8D, CD-ROM); the brecciation and rehealing predated debris-flow transport of the megablocks to their present sites

Andesite of hill 9519—Thick flow of porphyritic hornblende andesite on hill 9519 and overlying aphyric andesite, banked against the east wall of the North Pass caldera at Sheep Creek 

(TM), constitute late-erupted fill within this caldera (photo 6, CD-ROM)

Taha Aphyric andesite—A single dense dark-gray lava flow (57% SiO2) that lacks phenocrysts. 

Conglomeratic sedimentary debris, including clasts of Precambrian rocks, is locally present 

as float, raveling out from between the aphyric flow and underlying hornblende andesite 

(Tahh). Age, 29.98±0.31 Ma (whole rock); thickness, 0–20 m

Tahh Hornblende andesite—Massive thick dark-gray lava flow of porphyritic andesite (20% pl, 

hbl). Where banked along the caldera wall, the massive hornblende andesite (58% SiO2) and 

its inclined columnar-jointed margin somewhat resemble an intrusive body. Emplacement as 

a ponded lava flow is indicated by presence of elliptical vesicles, even within the most massive interior of the unit, and by continuity with subhorizontal eastern parts of the flow that 

directly overlie Saguache Creek Tuff where the lava overflowed the caldera wall. 40Ar/39Ar 

age, 30.41±0.79 Ma (hbl). Thickness, 0–250 m

CALDERA-RELATED ROCKS PREDATING THE WESTERN AND CENTRAL CALDERA 

CLUSTERS

Several ignimbrites, exposed widely in the map area, were erupted from northeastern sources and 

predate the large ignimbrite sheets erupted from the central and western caldera clusters. These ignimbrites 

include the Saguache Creek and Luders Creek Tuffs erupted from the North Pass caldera at 32.2 Ma, as 

well as the Bonanza Tuff erupted from the Bonanza caldera at 33.2 Ma. They have ages older than much 
29

of the precaldera Conejos Formation that has been dated farther south and west in the San Juan region, yet 

they overlie andesitic-dacitic lavas that are compositionally similar to the younger Conejos rocks to the 

south. Where interfingering relations with datable ignimbrites are absent, lower and upper components of 

the overall Conejos assemblage cannot readily be separated. The northeastern ignimbrite units also define 

an age and geographic progression toward caldera-related volcanism of early Oligocene age in central 

Colorado (McIntosh and Chapin, 2004; Lipman, 2007)

North Pass Caldera Cycle

The newly recognized North Pass caldera, source of the recently identified Saguache Creek Tuff (Lipman and Calvert, 2003; Lipman and McIntosh, 2008), is nearly concealed beneath caldera-filling lavas and 

tuffs, erupted from the same general area (volcanics of Cochetopa Hills, Luders Creek Tuff), and younger 

ignimbrites from central San Juan sources (Fish Canyon and Carpenter Ridge Tuffs). All rocks of this caldera cycle, including the precursor tuff of Spanish Creek, appear to have erupted during a brief interval at 

about 32 Ma. Thus, in age and geographic location, this caldera defines a transition between older ignimbrite calderas aligned north-south along the Sawatch Range, of which Bonanza at 33 Ma is the youngest 

and most southerly, and younger caldera systems located more centrally within the San Juan volcanic field

Luders Creek Tuff—Compositionally zoned cooling unit, changing upward from weakly 

welded light-tan rhyolite ignimbrite to densely welded dark-gray dacite (named by Lipman 

and McIntosh, 2008). Previously described as the “tuff of Luders Creek” (Lipman, 2006). 

Exposed only in small areas along the Continental Divide (northeastern SP, NP), where it is 

underlain by the caldera-filling volcanics of Cochetopa Hills and overlain by Fish Canyon 

Tuff. Similar in appearance, phenocryst mineralogy, and chemical zoning to outflow Nelson 

Mountain Tuff, with which the Luders Creek Tuff had been correlated previously (Steven 

and Lipman, 1976, fig. 24). Presence of laharic debris, derived from Luders Creek Tuff, 

below the Fish Canyon Tuff and above the Saguache Creek Tuff, initially demonstrated that 

this tuff is a separate older ignimbrite, now confirmed by isotopic ages. Limited preserved 

distribution of this unit suggests probable eruption from within the North Pass caldera, beneath subsequent cover by Fish Canyon and Carpenter Ridge Tuffs, or from within the area 

of the younger Cochetopa Park caldera. Maximum thickness within map area, 75 m

Tld Welded dacite—Upper dark-gray welded zone (66–68% SiO2; 20–30% pl>sn, bi>cpx). 

Sanidine phenocrysts (Or56Ab42An2.0Cn0.3) commonly contain Ba-rich rims (to Or56Ab35 

An2.6Cn6.4). Exposed only as small slumped and severely rotated outcrops of brownish-gray 

caprock, which closely resembles upper parts of outflow Nelson Mountain Tuff (Tnd). 

Contains pumice lenses of both crystal-poor rhyolite and crystal-rich dacite. Grades downward into rhyolite (Tlr). Distributed widely as clasts in debris-flow deposits of Buffalo Peak 

Campground. 40Ar/39Ar ages (sn), 32.15±0.06 Ma from outcrops at the head of Luders Creek, 

32.15±0.05 Ma from clast in laharic debris-flow deposit (Tdb). Thickness, 10–20 m

Tlr Nonwelded to partly welded rhyolite—Lower zone of weakly welded light-tan tuff (72–73% 

SiO2; 5–10% pl, sn>>bi), exposed only as float on hill slopes beneath welded dacite (Tld). 

Sanidine phenocrysts are more sodic and homogeneous (Or49Ab49An1.7Cn0.1) than in overlying later-erupted dacite. 40Ar/39Ar ages (sn), 32.15±0.06 Ma and 32.22±0.05 Ma. Thickness, 

0–75 m 

Volcanics of Cochetopa Hills

North Pass caldera-fill deposits. A diverse assemblage of volcaniclastic deposits and lava flows that 

accumulated largely within the North Pass caldera, soon after subsidence associated with eruption of 

the Saguache Creek Tuff (Tsc). Dacitic lava flows (Tnpd) constitute volumetric bulk of unit (photo 8, 

CD-ROM)

Volcaniclastic deposits—Sandstone, bedded tuff, and conglomerate that are interlayered with 

and underlie the lava flows within and near the North Pass caldera

Tst Bedded rhyolitic tuff—White to light-tan tuff deposited in shallow-lake or low-energy stream 

environments. Preserved discontinuously along west caldera margin, where overlies older 

caldera-fill units (volcanics of Cochetopa Hills). Deposits resemble tuffaceous sedimentary 

fill (Tct) of the young Cochetopa Park caldera to the west, but their higher topographic level 

and location along margins of the North Pass caldera suggest that they are most plausibly 
30

interpreted as late sedimentation during filling of this caldera, probably in conjunction with 

eruption of rhyolitic lava flows (Tnpr) or the Luders Creek Tuff. Preserved thickness, 0–30 m 

Tsd Dacitic conglomerate—Reworked crudely bedded conglomerate and mudflow breccia, containing clasts of light-gray porphyritic dacite (Tnpd) in weakly indurated lighter-gray sandy 

matrix. Exposed only in small areas near the Continental Divide near the southwest caldera 

margin (NP). Typically present as loose cobbles on subdued slopes. Thickness, 0–20 m

Tsa Andesitic conglomerate—Mostly reworked crudely bedded to well-bedded conglomerate, 

sandstone, and mudflow breccia, containing clasts of dark andesite and sparse dacite in 

weakly indurated lighter-gray sandy matrix. Exposed mainly in upper Luders Creek and 

its tributaries (NP). Outcrops are rare; unit typically present as loose cobbles on subdued 

slopes. Closely resembles volcaniclastic sediments of Conejos Formation (Tcv) and difficult 

to distinguish in many localities. Critical to identification as a younger volcaniclastic assemblage is presence of rare clasts of Saguache Creek Tuff (Tsc), most prominently on north 

side of Luders Creek (NP) just west of overlying Fish Canyon Tuff. Sparse clasts of crystalrich dacitic ignimbrite (Tbc, or Tts?) are also present locally in unit along ridge between 

Luders and Cantonment Creeks. Beyond margins of the North Pass caldera, unit is difficult 

to distinguish from some younger volcaniclastic rocks of upper parts of the Conejos Formation (Tdc, Tlav), where intervening units are absent

Postcaldera lava flows—Andesitic, dacitic, and rhyolitic flows that filled the North Pass caldera 

and widely overflowed its rims

Tnpd Dacite of East Pass Creek—Massive flows of gray to tan crystal-rich dacite (62–66% SiO2; 

20–30% pl>bi>cpx). Dominant exposed fill of the North Pass caldera. Thickest and most 

widespread accumulations are north of North Pass, where multiple thick flows are marked 

by basal vitrophyre and upper carapace-breccia zones. 40Ar/39Ar age (bi) from three flows 

within main body of this unit, 32.07–32.31 Ma. Individual flows are as thick as 200 m; total 

thickness, 0–500 m

Tnpr Rhyolite of Taylor Canyon—Scattered lava domes of phenocryst-poor gray to light-tan rhyolite (74–76% SiO2

; 1–5% sn, pl). Flow layering, glassy carapace breccia, and ramp structures near upper surfaces are typical. At least one flow, exposed along west caldera wall in 

Corduroy Creek (NP), is mineralogically and chemically distinctive, similar to the Saguache 

Creek Tuff (Tsc) in lacking biotite phenocrysts and containing elevated incompatible traceelement contents. 40Ar/39Ar age (sn) on flow in Corduroy Creek, 32.15±0.10 Ma. Exposed 

thickness, 0–150 m

Tnpa Andesite, undivided—Local flows of dark-gray phenocryst-poor andesite (1–10% pl, cpx). 

Mainly exposed along west caldera margin. Thickness 0–50 m

Saguache Creek Tuff—Crystal-poor rhyolite ignimbrite (named by Lipman and McIntosh, 

2008), distributed discontinuously over much of the map area (photo 5, CD-ROM). Similar in appearance and phenocryst content to proximal Sapinero Mesa Tuff (Tws), to which 

this unit had been previously assigned in adjacent areas (Bruns and others, 1971; Steven 

and Lipman, 1976, fig. 10; Olson, 1988). More recently described as the “tuff of Saguache Creek” by Lipman and Calvert (2003) and Lipman (2006). The Saguache Creek lacks 

biotite, has more sodic low-barium sanidine (Or50–51.5Ab45–47An2.6–2.8Cn0.5), has different 

trace-element bulk compositions, and is more densely welded than the distal Sapinero Mesa 

within the map area. Also resembles, but different in similar ways, the upper rhyolite of 

underlying Bonanza Tuff that is exposed just east of the map area (LG: Bruns and others, 

1971; Varga and Smith, 1984). Weighted mean 40Ar/39Ar age (sn) for Saguache Creek Tuff 

is 32.25±0.05 Ma for six determinations from separate localities, significantly younger than 

the Bonanza Tuff (33.2 Ma; McIntosh and Chapin, 2004). Eruption of the Saguache Creek 

Tuff from North Pass caldera is supported by the preserved distribution of this unit, semisymmetrically around the North Pass area where not hidden by younger rocks (to the south 

and west), and by stratigraphic relations with postcaldera lavas. Magnetic polarity, reverse; 

D 168°, I –55° in Saguache and Salaya Creeks. Thickness, 0–75 m

Tscl Landslide breccia of hill 9943—Weakly indurated monolithologic breccia consisting solely of 

clasts of welded Saguache Creek Tuff. In addition to exposure on hill 9943 (head of Muddy 

Creek, ST), exposures of similar breccia occupy paleovalleys along East Sheep Creek (TM) 

and Long Gulch (HG). In Long Gulch, Fish Canyon Tuff laps onto the slide breccia, demonstrating its Oligocene age. Thickness, 0–40 m
31

Tscc Densely welded crystal-rich rhyolite—Local upper zone of densely welded dark-gray tuff 

(73.5% SiO2; 15–20% pl, sn), exposed as small outcrops along Mexican Joe Gulch (northern 

SP) and on hill 10,076 (ST). Thickness, 0–15 m 

Tsc Nonwelded to densely welded crystal-poor rhyolite—Variably welded light-tan to red-brown 

tuff (74–76% SiO2; 5–10% pl, sn). Locally grades up into more densely welded crystal-rich 

rhyolite (Tscc). Thickness, 0–50 m 

Tscn Nonwelded crystal-poor rhyolite—Light-gray upper part of ignimbrite sheet (photo 5B, 

CD-ROM) that is locally sufficiently thick and distinctive to be mapped as separate zones. 

Thickness, 0–35 m

Tbt Bedded tuff—White to light-gray weakly indurated bedded tuff and sandstone (photo 5C, 

CD-ROM). Includes reworked fluviatile and lake-bed deposits, as well as thin ash-fall and 

nonwelded ignimbrite deposits. Exposed below Saguache Creek Tuff mainly in basin (Alkali 

Spring) west of Trickle Mountain (TM) and locally farther east along Saguache Creek. 

Thickness, 0–75 m

Tuff of Spanish Creek—Crystal-rich rhyolite ignimbrite exposed locally beneath the Saguache 

Creek Tuff (Tsc) and interpreted as possible precursor eruption of the North Pass cycle, 

based on distribution and similar ages

Tts Welded tuff—Nonwelded gray to densely welded pale-brown crystal-rich rhyolite ignimbrite 

(73–75% SiO2; 20–30% pl, sn, qtz, bi, trace hbl), preserved only in small areas on north 

side of Spanish Creek (NP) and north of Samora Creek (WB). Thick vitrophyre and rheomorphic flow layering where banked against steep paleotopography north of Samora Creek. 

Clasts of this unit are present in breccias along the North Pass caldera margin, along the 

mouth of Pine Creek and in roadcuts along Colorado Highway 114 (NP). Broadly resembles Fish Canyon Tuff in welding appearance, crystallization textures, and phenocryst assemblage, but has modestly lower total phenocryst content, more abundant quartz and sanidine, and more silicic bulk composition. 40Ar/39Ar ages (sn) in Spanish Creek, 32.48±0.05 

Ma (vitrophyre) and 32.55±0.05 Ma (overlying devitrified tuff); north of Samora Creek, 

32.40±0.05 Ma; and in collapse breccia along the northwest margin of North Pass caldera, 

32. 45±0.05 Ma

Ttsn Nonwelded tuff—White to light-gray nonwelded crystal-rich ignimbrite (15–25% pl, sn, qtz, 

bi), exposed locally beneath Saguache Creek Tuff on both sides of Saguache Creek near 

mouth of Big Dry Gulch (TM-LM boundary). 40Ar/39Ar age (sn), 32.49±0.6 Ma on nonwelded tuff in lower Squaw Creek (LM). Tentatively correlated with welded ignimbrite in 

Spanish Creek, based on similar stratigraphic position, phenocryst mineralogy, and age. 

Thickness, 0–25 m

Intrusions of Uncertain Affinity

Fine-grained intrusive rocks, mostly dikes, lacking definitive features of areal distribution or petrology 

that permit confident correlation with a stratigraphically constrained volcanic sequence. Many of these are 

probably associated with Conejos-age volcanoes, but some are likely younger

Tia Andesite—Dark-gray dikes and other small intrusive bodies, typically containing small phenocrysts (pl>cpx, hbl)

Tid Dacite—Gray dikes and larger intrusive bodies, typically phenocryst rich, containing pl>bi>hbl, 

cpx. One large dacite intrusion with a near-horizontal roof is well exposed at the head of 

Long Branch (SM). An intrusion at Cochetopa Pass (NP) that is truncated by the southwest 

margin of North Pass caldera yielded an 40Ar/39Ar age (sn) of 32.19±0.13 Ma; its emplacement may have been a precursor event of the North Pass caldera cycle. Another probable 

dacite intrusion in upper Razor Creek (RD) may alternatively be a thick near-vent flow/

dome associated with the rhyolite of Barret Creek

Tir Rhyolite—Light-gray to tan dikes, containing pl>bi, qtz

Bonanza Caldera Cycle

The southwest rim of the Bonanza caldera, source of the 33-Ma Bonanza Tuff, lies only about 5 km 

northeast of the map area (BZ, KM; fig. 4), and rocks associated with this eruptive center constitute major 

volcanic accumulations in the eastern map area. Bonanza is the southernmost and youngest of a chain of 

four ignimbrite caldera centers (Grizzly Peak, Princeton, Aetna, Bonanza) aligned north-south along the 
32

Sawatch Range that marks the west shoulder of the present-day Rio Grande rift zone in central Colorado 

(fig. 1)

Postcaldera lavas and related rocks—A thick sequence of dominantly andesitic lava flows that 

accumulated within the Bonanza caldera after subsidence and spread across proximal outflow Bonanza Tuff (Varga and Smith, 1984). Preserved mainly east of the map area. Without 

intervening Bonanza Tuff, this lava assemblage is difficult to distinguish from underlying 

rocks of the Conejos Formation, and some of the Conejos in central and western parts of 

map area include units equivalent to the upper andesite of the Bonanza cycle

Tba Upper andesite—Dark-gray to brown andesite and dacite, containing sparse small phenocrysts 

(pl, hbl, cpx). Within map area, preserved only as a small erosional remnant of andesite that 

overlies Bonanza Tuff on a ridge spur east of junction, forks of Long Branch (SM ). Thickness, 25 m

Bonanza Tuff—Outflow ignimbrite sheet displaying complexly gradational welding and compositional zonation: from densely welded lower rhyolite and dacite (photo 2, CD-ROM), 

upward into less welded rhyolite, and, locally, an upper welded dacite. Upper rhyolite and 

dacite are exposed only east of the map area. Weighted mean age for 14 samples throughout regional extent, 33.2 Ma (McIntosh and Chapin, 2004; W. McIntosh, written commun., 

2007)

Tbd Lower dacite—Mainly densely welded dark-gray to brown crystal-rich dacite welded tuff 

(63–66% SiO2; 25–35% pl>>bi, sn, cpx, qtz). Preserved as discontinuous erosional remnants 

of paleovalley fills along east margin of map area; more widespread farther east. Sanidine 

composition: Or65Ab31An2.3Cn2.2. 40Ar/39Ar age (sn), 33.17±0.06 Ma, on densely welded tuff 

that may be a megabreccia block along the northwest margin of North Pass caldera (NP, 

along Hwy 114). Thickness, 0–100 m

Tbrl Lower rhyolite, densely welded—Red-brown crystal-poor rhyolite (72–74% SiO2; 5–10% 

sn>>pl>>bi, hbl). Compared to other crystal-poor biotite-bearing rhyolite tuffs in the map 

area (Tws, Tcr), rhyolitic Bonanza Tuff has a much higher sanidine:plagioclase phenocryst 

ratio and contains small phenocrysts of hornblende in addition to biotite. Sanidine composition Or53Ab45An1.9Cn0.03. Within map area, only a single small area of outcrops on ridge 

spur east of junction at forks of Long Branch (northern SM). Locally thick in lower part of 

intracaldera fill (BZ); probably correlates with widespread outflow rhyolitic Bonanza Tuff to 

east (Gribbles Park Tuff of McIntosh and Chapin, 2004). 40Ar/39Ar age (sn), 33.19±0.05 Ma 

in Long Branch. Thickness, about 25 m

VOLCANIC ROCKS PREDATING BONANZA CALDERA

Conejos Formation, Lower Units

Thick accumulations of dominantly andesite and dacite as lava flows and proximal breccias that were 

erupted from central volcanoes, surrounded by voluminous aprons of volcaniclastic debris (photo 3A, 

CD-ROM) emplaced as mudflow and stream-fan deposits (Lipman, 1975; Colucci and others, 1991). These 

intermediate-composition lavas, breccias, and associated intrusions, which underlie caldera-related ignimbrites throughout the San Juan region, constitute the volumetric bulk of the volcanic deposits. Subsequently 

deeply eroded, the early eruptive deposits are now discontinuously exposed as surviving topographic highs 

along margins of the ignimbrite calderas. In distal areas, locally difficult to distinguish from younger suites 

of compositionally similar intermediate-composition lavas, which were emplaced concurrently with the 

ignimbrite eruptions. Compositional subunits among Conejos lavas have been divided in varying detail, depending on degree of local diversity, quality of exposures, and mapping scale. In the northeastern San Juan 

Mountains proximal lavas tend to become more silicic upward, with biotite-bearing dacite and low-silica 

rhyolite especially abundant on the northeast side of Cochetopa Park caldera. Much of this unit, which 

underlies the Bonanza Tuff (33.2 Ma) in the eastern map area, is older than widespread Conejos rocks to 

the south and west, which appear to have erupted largely in the interval 33–30 Ma. In the vicinity of the 

Bonanza caldera, just east of the map area, lower parts of the Conejos Formation that are overlain by the 

Bonanza Tuff have long been designated the “Rawley Andesite” (Burbank, 1932; Varga and Smith, 1984), 

but compositionally similar pre-Bonanza lavas erupted from multiple centers are the dominant volume of 

the Conejos Formation across much of the map area. Maximum exposed thickness, about 800 m along the 
33

north flanks of Middle Baldy (WB) or Sawtooth Mountain (ST); sections as much as 2.3 km thick have 

been penetrated by petroleum exploration drilling just east of the map area (Gries, 1985; Brister and Gries, 

1994)

Tci Intermediate-composition granitoid intrusions—Fine-grained equigranular to porphyritic 

monzonite and granodiorite (pl>>cpx>>bi), grading irregularly into porphyritic andesite. 

Occurs as small stocks, dikes, and rare sills representing the intrusive cores of Conejos 

volcanoes. An intrusion exposed in Alkali Creek (ST) probably marks the core of a large 

stratovolcano; present-day topographic highs of Sawtooth Mountain and Razor Creek Dome 

represent the northwest and northeast flanks of this volcano, as indicated by primary dips of 

lavas and other proximal deposits along both sides of Cochetopa valley. A dike along Hwy 

114 (RD) probably associated with this center has a 40Ar/39Ar age (hbl) of 34.81±0.16 Ma. 

The large elongate Needle Creek intrusion, a texturally diverse composite body including 

sill- and dike-like components (photo 3B, CD-ROM), probably represents the core of another large edifice, for which West Baldy constitutes the eroded southern flank. Hornblende 

40Ar/39Ar determinations from three sites in this intrusion (WB) yielded ages of 34.28–34.42 

Ma

Lava flows and proximal breccias—Flows and flow breccias of aphanitic to porphyritic lava 

from many volcanic centers, some of which probably lie within subsided areas of calderas 

(photos 20, 27, CD-ROM). Primary dips preserved on flanks of volcanoes are as steep as 30° 

Tca Andesite—Finely porphyritic to aphanitic dark-gray lava flows and breccia (55–61% SiO2; 

5–15% pl>cpx, hbl), constituting the most voluminous Conejos Formation lithology. Individual flows are 5–40 m thick; multiple-flow accumulations are as much as several hundred 

meters thick on volcano flanks 

Tcsa Andesite scoria—Reddish-brown agglutinated scoria, marking a largely buried vent exposed 

on hill 10,943 (CS), along ridge crest 2 km south of Agency Peak. Similar unmapped scoria 

is present locally elsewhere in the Conejos Formation. Thickness, 50 m

Tcap Plagioclase andesite—Distinctive porphyritic dark-gray lavas (56–60% SiO2; 15–30% 

pl>>cpx), characterized by tabular phenocrysts of plagioclase as much as 1 cm across 

Tcd Dacite—Gray porphyritic flow-layered lava and breccia, characterized by light color and 

conspicuous biotite phenocrysts (62–68% SiO2; 20–30% pl>>bi, cpx, hbl). Massive, locally 

with flow layering or basal vitrophyre. Widely distributed, but much less abundant than 

andesites of Conejos Formation. Commonly high in the Conejos lava sequence. In proximity to the North Pass caldera, locally difficult to distinguish reliably from younger dacite of 

the caldera fill (Tnpd). At the south boundary of the map area, near Carnero Pass, a petrologically unusual dacite flow, containing large sanidine phenocrysts (to 2 cm), yielded an 

40Ar/39Ar age (sn) of 32.71±0.10 Ma. In Needle Creek (WB), a thick sanidine-bearing dacite 

flow low in the Conejos lava sequence has 40Ar/39Ar ages (sn, two sites) of 34.10±0.05 and 

33.98 ±0.08 Ma, similar to the granodiorite intrusion (Tci) that cuts the lava sequence in this 

area. Farther west, a dacite flow, which caps Sawtooth Mountain (ST) and is interpreted to 

be an upper part of a volcanic construct that may postdate the Saguache Creek Tuff, has an 40Ar/39Ar age (hbl) of 32.00±0.09 Ma. Thickness, 0–200 m 

Tcfr Rhyolite—Light-gray to tan flow-layered lava and breccia (70–76% SiO2; 3–5% pl>bi). 

Rare crystal-rich flows (to 15–20% phenocrysts) contain sparse high-K sanidine 

(Or69–71Ab26–27An0.5–1.0Cn2–3). Especially common high in the Conejos Formation northeast of 

North Pass caldera (SM-TM boundary). Overlain by Bonanza Tuff, so distinctly older than 

petrologically similar crystal-poor rhyolite of the Barret Creek assemblage (Tbfa). These 

flows may represent initial silicic magmatism precursor to explosive ignimbrite eruptions 

from Bonanza center. A large rhyolite dome along Archuleta Creek near the southwest rim of 

North Pass caldera (CP), which has an 40Ar/39Ar age (sn) of 32.17±0.06 Ma, may represent 

precursor volcanism of the North Pass caldera cycle. An isolated sanidine-bearing rhyolite 

flow, near the entry to Cochetopa Canyon (ST) yielded an age of 37.13±0.09 Ma . This is the 

oldest silicic volcanic rock thus far dated from the San Juan region, similar in age to the voluminous Wall Mountain Tuff (Twm) erupted farther north in the SRMVF. Thickness, 0–125 m

Tcv Volcaniclastic rocks—Mostly reworked crudely bedded to well-bedded conglomerates, 

sandstones, and mudflow breccias, containing clasts of dark andesite and dacite in light-gray 

sandy matrix (photo 3A, CD-ROM). Primary dips are typically <5°. Mainly distal deposits 
34

on lower slopes of volcanic constructs; most abundant in the northern map area. Volumetrically less abundant, relative to proximal lavas, than in preserved Conejos sequence farther 

west and south in the San Juan region. Maximum thickness, about 200 m on east flank of 

Sawtooth Mountain (ST)

Rhyolitic tuff—Small local units of nonwelded phenocryst-poor rhyolitic tuff (75–76% SiO2; 

1–3% pl>>bi) are poorly exposed near base of the Conejos Formation in the northern map 

area and stratigraphically higher along the septum of Conejos rocks exposed between North 

Pass and Cochetopa Park calderas. Some of the rhyolitic tuff is areally associated with compositionally similar rhyolitic lava flows of Conejos age, but eruptive sources have not been 

identified

Tct Tan to light-gray rhyolitic tuff and tuffaceous sandstone—Weakly indurated and poorly 

exposed tuff, variably altered to clays and zeolites. Locally thick along inner east wall of 

Cochetopa Park caldera (CP-NP boundary) and in the northeastern map area, especially in 

vicinity of the Needle Creek intrusion (north WB). Maximum thickness, ~125 m north of 

Archuleta Creek (east CP)

Tcht Pink tuff of Home Gulch—Distinctive nonwelded ignimbrite of nearly aphyric rhyolite 

exposed low in the Conejos Formation along north margin of Cochetopa Park caldera. Small 

angular fragments of andesite and sparse Precambrian rocks are conspicuous; in Home 

Gulch (RD), locally are as large as 0.5 m. Sparse small crystal fragments are mostly xenocrysts. Overlain at mouth of Alkali Creek (ST) by rhyolitic lava flow with sanidine age of 

37.13±0.09 Ma. Maximum thickness, 100 m

Tev Early volcaniclastic sedimentary deposits—Conglomerates and sandstones containing fragments of intermediate-composition lavas and some Precambrian clasts. Includes coarse 

boulder conglomerate (clasts to 1 m) west of Long Branch (SM). Records erosion from 

paleohills of Precambrian rocks early during Tertiary volcanism. Thickness, 0–20 m

EARLY IGNIMBRITES ERUPTED FROM SAWATCH RANGE CALDERAS

Tbc Badger Creek Tuff—Distinctive gray partly to densely welded ignimbrite (67–69% SiO2; 

25–35% pl>bi,>sn, hb, qtz). Present as slump masses in northeast corner of map area 

(SS); larger outcrop areas are present farther east along Marshall Creek (PP). Resembles 

Fish Canyon Tuff (Tfc) in lithology, mineral composition (for example, potassic sanidine: 

Or72Ab25An0.8Cn2.8), and rock chemistry, but erupted at 34.1 Ma (McIntosh and Chapin, 

2004) from the Mount Aetna caldera in the Sawatch Range (fig. 1). Thickness, 0–40 m

Twm Wall Mountain Tuff—Distinctive red-brown densely welded ignimbrite (photo 1, CD-ROM), 

product of the oldest large pyroclastic eruption in the Southern Rocky Mountains. Crystalrich rhyolite (70–75% SiO2

; 20–30% sn, pl>bi), characterized by large tabular phenocrysts 

of unaltered sanidine (typically Or61–62Ab34–35An2–3Cn1–3) and widely argillized plagioclase. 

Large lenses of compacted pumices (to 0.5 m) commonly weather as cavities that resemble 

gas cavities (photo 1B, CD-ROM); widespread rheomorphic flowage of this unit (Chapin 

and Lowell, 1979) produces ramp structures in upper parts, with dips of compaction foliation locally approaching vertical (photo 1A, CD-ROM). Unit is widespread in central 

Colorado, reaching the High Plains between Denver and Colorado Springs, but had not 

previously been recognized west of the Rio Grande rift zone. In the northern map area, 

scattered erosional remnants are preserved in paleovalleys, resting directly on pre-Tertiary 

rocks; some of these had previously been variably mapped as Sapinero Mesa Tuff (Olson 

and Steven, 1976a) and Cochetopa Park Tuff (Olson and others, 1975). Erupted in the 

Sawatch Range 70–90 km to the northeast, probably from a caldera that was above the 

Mount Princeton batholith (Johnson and others, 1989; Lipman, 2007). 40Ar/39Ar age (sn) in 

Bead Park (ST), 36.95±0.15 Ma (Lipman and Calvert, 2003), 36.85±0.08 Ma (Lipman and 

McIntosh, 2008). Magnetic polarity, normal (as determined in Bead Creek, with portable 

fluxgate magnetometer). Thickness, 0–100 m

PREVOLCANIC ROCKS

Mesozoic and rare Tertiary sedimentary deposits are widely distributed along the north boundary 

of the map area, beneath the Oligocene volcanic rocks of the San Juan region; large areas are concealed 
35

beneath widespread landslide deposits. The sedimentary sequence is underlain by Proterozic granite that 

intrudes metasedimentary and metavolcanic rocks

MESOZOIC SEDIMENTARY ROCKS

Km Mancos Shale (Cretaceous)—Dark-gray to brown silty shale, too poorly indurated to make good 

exposures. Widely covered by landslide deposits. Preserved thickness, 0–200 m

Kd Dakota Sandstone (Cretaceous)—Light-gray to brown sandstone, with interbeds of friable 

siltstone and platy sandstone in upper part. Medium to coarse grained, locally crossbedded, 

well indurated. Thickness, 15–70 m

Jm Morrison Formation (Jurassic)—Varigated green-gray to red-brown mudstone, siltstone, andplaty sandstone. Thickness, 25–100 m

Jj Junction Creek Sandstone (Jurassic)—Light-yellow to gray eolian quartzose sandstone; fineto medium grained, locally crossbedded, and silicified. Thickness, 10–50 m

PALEOZOIC SEDIMENTARY ROCKS

Pzl Paleozoic limestone (lower Paleozoic)—Gray to tan limestone and limey sandstone, exposed as 

concentrated boulders in small area west of lower Long Branch (SS). Similar rocks crop out 

more widely along Marshall Creek, just northeast of map area (Olson, 1983)

PRECAMBRIAN ROCKS

=g Granitic intrusive rocks, undivided—Medium-grained pink to tan, locally weakly gneissic 

intrusive rock of Proterozoic age (photo 28, CD-ROM), containing about 30–40% microcline, 25% plagioclase, 20–30% quartz, and 3–10% biotite. Mapped and described in more 

detail by Olson (1976a,b, 1988). Published ages are in range 1,676–1,713 Ma (Bickford and 

others, 1988)

=m Metamorphic rocks, undivided—Interlayered metasedimentary and metavolcanic rocks of 

Proterozoic age, including foliated schist, phyllite, quartzite, metabasalt, and metarhyolite. 

Mapped and described in more detail by Olson (1988) and maps referenced 

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40
41

Table 1. Regional ignimbrites and calderas of the Southern Rocky Mountain Volcanic Field. (Updated and simplified from 

Lipman, 2007, table 1.)

Ignimbrite Caldera

Tuff Percent SiO2 Rock, phenocrysts1 Vol (km3

) Age (Ma) Name Area (km)

West San Juan Mountains

Sunshine Peak Zoned, 76–68 Qtz, sodic san 200–500 23.0 Lake City 15 x 18

Latir Mountains, New Mexico

Amalia 76–77 Peralk: qtz, sodic san 500 25.1 Questa 14 x >15

Central San Juan Mountains

Snowshoe Mountain 62–66 Xl-rich dacite >500 26.9 Creede 20 x 25

Central San Juan Mountains: San Luis caldera complex

Nelson Mountain Zoned, 74–63 Xp rhy; xl dacite >500 26.9 San Luis-Cochetopa 9 x 9, 20 x 25

Cebolla Creek 61–64 Xl dacite, hbl, no san 250 26.9 San Luis complex 14 x 16

Rat Creek Zoned, 74–65 Xp rhy - xl dacite 150 26.9 San Luis complex 9 x 12

Wason Park Zoned, 72–63 Xl rhyolite - dacite >500 27.40 South River 20 x 20

Central San Juan Mountains

Blue Creek 64–68 Xl dacite, no san 250 27.45 (concealed) --

Carpenter Ridge Zoned, 74–66 Xp rhy - xl dacite >1,000 27.55 Bachelor 25 x 30

West San Juan Mountains

Crystal Lake 72–74 Xp rhyolite 50–100 27.6 Silverton 20 x 20

Central San Juan Mountains

Fish Canyon 66–68 Xl dacite, hbl,qtz 5000 28.0 La Garita 35 x 75

Southeast San Juan Mountains: Treasure Mountain Group

Chiquito Peak 64–67 Xl dacite, sanidine 500–1,000 28.4 Platoro 18 x 22

Central San Juan Mountains

Masonic Park 62–66 Xl dacite, no san 500 28.4 (concealed) --

West San Juan Mountains

Sapinero Mesa 72–75 Xp rhyolite >1,000 28.3 Uncompahgre-San Juan 20 x 45

Dillon Mesa 72–75 Xp rhyolite 50–100 28.5 Uncompahgre? 20 x 20

Blue Mesa 72–74 Xp rhyolite 200–500 28.5 Lost Lakes (buried) 10 x 10

Ute Ridge 66–68 Xl dacite, sanidine 250–500 28.6 Ute Creek 8 x 8

Southeast San Juan Mountains: Treasure Mountain Group

South Fork 68–70 Xl dacite, sanidine 50–100 29.0 Platoro/Summitville? 8 x 12?

Ra Jadero 64–66 Xl dacite, sanidine 150 29.0 Summitville? 8 x 12

Ojito Creek 67–70 Xl dacite, no san 100 29.5 Summitville? 8 x 12

La Jara Canyon 66–68 Xl dacite, no san 500–1,000 29.5 Platoro 20 x 24

Black Mountain 67–69 Xl dacite, no san 200–500 29.9 Platoro --

Northeast San Juan Mountains

Saguache Creek 73–75 Alkali rhyolite, no bio 250–500 32.2 North Pass 15 x 17

Bonanza 74–63 Zoned: rhy-dac-rhy-dac 1,000? 33.2 Bonanza-Gribbles 15 x 20

North-South Sawatch Range Trend

Thorn Ranch 77 Xl rhyolite 50–100? 33.9 Marshall 10 x 10?

Badger Creek 69–70 Xl dacite 250? 34.0 Mount Aetna 10 x 15

Grizzly Peak Zoned, 77–57 Xl rhy-dacite-andesite 500? 34.3 Grizzly Peak 15 x 17

Wall Mountain 70–73 Xl rhyolite 1,000+/- 36.9 Mount Princeton 15 x 30?

1

Abbreviations: bio, biotite; dac, dacite; hbl, hornblende; qtz, quartz; rhy, rhyolite; san, sanidine; XL, crystal-rich; XP, crystal-poor
42

Table 2. Summary of new 40Ar/39Ar age determinations, northeastern and central San Juan region. (See table 3 (CD-ROM) for full analytica

samples. Ages were determined at New Mexico Institute of Mining and Technology, Socorro.)

Unit1,2 Material Analyzed2 Analysis type3 No. sites Age (Ma)4 ±Se/2s5 Comments

HINSDALE FORMATION

Basalt flow Groundmass Plateau 1 21.81 ±0.21 Overlies Carpenter Ridge Tuff

CREEDE CALDERA

Fisher Dacite Sanidine SCLF 2 26.77 0.04 Post-resurgence, Fisher Mountain

Fisher Dacite Sanidine SCLF 2 26.82 ±0.05 Pre-resurgence, west flank Snowshoe 

Snowshoe Mountain Tuff Sanidine SCLF 4 26.87 ±0.02 Both intracaldera and outflow location

SAN LUIS-COCHETOPA CALDERA CYCLE

Baldy Cinco Dacite Sanidine SCLF 4 25.89–26.16 Section at Baldy Cinco

Cochetopa Dome Rhyolite Sanidine SCLF 8 26.86 ±0.03 Four lava flows and moat-tuff samples

Intrusions of silicic rhyolite Sanidine SCLF 3 27.10 ±0.04 Intrusions north of Cochetopa caldera

Nelson Mountain Tuff, intracaldera Sanidine SCLF 2 26.91 ±0.04 Small crystals

Nelson Mountain Tuff, outflow Sanidine SCLF 8 26.90 ±0.02

Cebolla Creek Tuff Bio and hbl Plateau 4 27.10 ±0.07 Bio and hbl ages slightly too old

Rat Creek Tuff (dacite) Sanidine SCLF 8 26.91 ±0.02

Dacite flow of McKenzie Mountain Biotite Plateau 1 27.08 ±0.08 Probable precursor lava

OLDER UNITS, INCLDUING IGNIMBRITES RELATED TO CENTRAL AND WESTERN SAN JUAN CALDERAS

Wason Park Tuff Sanidine SCLF 5 27.38 ±0.05

Andesite of Mountain Lion Creek Groundmass Plateau 3 27.46–27.51 Overlies distal Carpenter Ridge Tuff

Carpenter Ridge Tuff Sanidine SCLF 4 27.55 ±0.05

Crystal Lake Tuff Sanidine SCLF 1 27.41 ±0.11 Bove and others (2001): 27.62±0.07

Huerto Andesite (lava flows) Hbl and gm Plateau 5 27.64–28.32

Fish Canyon Tuff Sanidine SCLF 28.02 Standard age: Renne and others (1998Masonic Park Tuff Biotite Plateau 1 28.89 ±0.08 From Lanphere (1998)

Sapinero Mesa Tuff Sanidine SCLF 4 28.27 ±0.06 Bove and others (2001): 28.38±0.03 M

Dillon Mesa Tuff Sanidine SCLF 3 28.42 ±0.08 Bove and others (2001): 28.58±0.04 M

Blue Mesa Tuff Sanidine SCLF 2 28.46 ±0.04 Bove and others (2001): 28.58±0.07 M

Ute Ridge Tuff Sanidine SCLF 1 29.12 ±0.05 Bove and others (2001): 28.81±0.05 M

BARRET CREEK RHYOLITE-DACITE DOME COMPLEX

Rhyolite and dacite lava flows Sanidine SCLF 4 29.63–29.85 Failed caldera site?
43

Table 2. Summary of new 40Ar/39Ar age determinations.—Continued

Unit1,2 Material Analyzed2 Analysis type3 No. sites Age (Ma)4 ±Se/2s5 Comments

LAVAS & TUFF, EAST SIDE OF NORTH PASS CALDERA

Basalt of Point Benny Groundmass Plateau 5 30.22 ±0.10 Most mafic early lava in San Juan reg

Andesite of Lone Tree Gulch Groundmass Plateau 1 30.21 ±0.17

Tuff of Big Dry Gulch Biotite Plateau 1 30.47 ±0.08 No sanidine, excess Ar biotite spectru

Aphanitic andesite, hill 9519 Groundmass Plateau 1 29.98 ±0.31

Hornblende andesite flow, hill 9519 Hornblende Plateau 1 30.41 ±0.79 Poor plateau

NORTH PASS CALDERA CYCLE

Luders Creek Tuff Sanidine SCLF 4 32.17 ±0.04

Volcanics of Cochetopa Hills Dacite of East Pass Creek Biotite Plateau 3 32.07–32.31 Main area of caldera-fill flows

 Dacite of East Pass Creek (?) Bio and hbl Plateau 1 32.82 ±0.08 At south caldera wall(?)

 Rhyolite of Taylor Canyon Sanidine SCLF 1 32.15 ±0.10

 Rhyolite breccia, Taylor Canyon Sanidine SCLF 1 32.44 ±0.08 Possibly landslide breccia from a prec

Saguache Creek Tuff Sanidine SCLF 6 32.25 ±0.05

Precursor? lava flow and intrusion Sanidine SCLF 2 32.18 ±0.07 At southeast margin of North Pass cal

Precursor? tuff of Spanish Creek Sanidine SCLF 4 32.50 ±0.03

EARLY LAVAS, TUFFS, & INTRUSIONS

Rhyolite flow, Cebolla Creek Biotite Plateau 1 31.49 ±0.14 Below Blue Creek Tuff

Dacite, Sawtooth Mountain Biotite Plateau 1 32.00 ±0.09 Summit lava flow

Porphryitic dacite, Carneros Pass Sanidine SCLF 1 32.72 ±0.09 Highest flow (below Tfc)

Long Branch Intrusion Bio and hbl Plateau 1 32.98–33.01 Roof zone of large pluton of porphyri

Bonanza Tuff Sanidine SCLF 1 33.17 ±0.06 Mean age, 33.2 Ma (McIntosh and Ch

Needle Creek Intrusion Hornblende Plateau 3 34.28–34.42

Hornblende-rich dike Hornblende Plateau 1 34.61 ±0.16 Intrudes Cochetopa caldera-floor ande

Conejos xl dacite (san-bearing) Sanidine SCLF 2 34.04 ±0.06 Beneath sequence of andesites

Wall Mountain Tuff Sanidine SCLF 1 36.85 ±0.08 Mean age, 36.9 Ma (McIntosh and Ch

1Bold type, regional ignimbrite sheets2Mineral abbreviations: bio, biotite; gm, groundmass separate; hbl, hornblende; san, sanidine3Analysis type: Plateau, incremental-heating analysis; SCLF, single-crystal laser fusion4Ages for multiple samples are weighted means or ranges5For SCLF, standard error of the weighted mean; for plateau ages, 2 s.d. (95% confidence level)
44

Table 4. Representative chemical analyses for Tertiary volcanic rocks, northeastern San Juan Mountains, southwestern Colorado. 

determined by WDXRF (Taggart and others, 1987); minor elements were by EDXRF methods (Siems, 2000).

[Abbreviations: bio, biotite; hbl, hornblende; pl, plagioclase; san, sanidine; Cr, Creek; Hwy, Highway; Mtn, Mountain; Tfc, Fish Canyon Tuff]

Lithology, location Unit

label

Sample 

No.

Latitude Longitude X-ray fluorescence (WDXRF); weight % (calculated to 

Deg Min Deg Min SiO2 TiO2 Al2O3 FeTO3 MgO CaO Na2O 

Hinsdale Formation (21.8–27.5 Ma)

Lava, Powderhorn trail Thb 98L-22 38 4.67 107 6.88 54.45 1.57 15.89 8.55 3.61 6.66 4.15 

Basaltic andesite lava flow, Houghland Hill Thb 00L-1 38 7.43 106 17.68 50.14 1.86 14.83 11.53 7.30 8.03 2.99 

Cochetopa Dome Rhyolite (26.9 Ma)

Apache tears, base, upper flow Tcdu 00L-31-A 38 13.55 106 43.92 76.31 0.11 12.79 0.82 <0.10 0.41 3.75 

Fine-grain crystal-rich flow Tcdc 01L-25 38 12.92 106 42.88 76.48 0.09 13.00 0.71 <0.10 0.32 4.27 

Middle flow, lower part Tcdm 01L-27 38 12.60 106 43.00 76.44 0.11 12.89 0.83 <0.10 0.46 4.08 

Lower flow, east side Tcdl 00L-33 38 12.83 106 40.55 75.84 0.13 13.01 0.81 <0.10 0.53 3.89 

Andesite of Mountain Lion Creek

Basaltic andesite, Hat Mountain Tal 03L-10 38 1.57 106 31.70 61.28 0.86 15.37 5.63 2.24 4.74 4.27 

Carpenter Ridge Tuff (27.5 Ma)

Devitrified interior, roadcut Hwy 114 Tcr 02L-20-A 38 10.48 106 28.63 75.11 0.21 13.11 1.26 0.08 0.75 3.49 

Clast landslide breccia, Samora Creek Tcr 02L-3 38 14.25 106 38.33 74.44 0.23 13.53 1.40 0.07 0.89 3.66 

Huerto Formation

Dacite flow, Trickle Mtn Thu 02L-58 38 9.12 106 23.42 62.42 0.89 15.78 5.06 1.50 3.45 4.44 

Dacite flow, south side Spruce Mtn Thu 05L-46 38 1.12 106 21.51 63.63 0.82 15.30 5.24 1.48 3.49 3.99 

Andesite flow, lower, east Mount Lion Thu 03L-44A 38 3.67 106 23.50 56.87 1.20 15.73 7.70 2.95 5.99 4.03 

Sapinero Mesa Tuff (28.25 Ma)

Los Pinos Cr road, distal, most dense welded Tws 00L-21B 38 11.87 106 50.87 75.14 0.26 12.66 1.46 0.22 1.07 3.13 

Partly welded distal tuff, Houselog Creek Tws 03L-45 38 5.93 106 21.95 73.54 0.27 13.67 1.41 0.19 0.77 3.30 

Pre-FCT flows and tuffs (~30 Ma)

Andesite, dike, Point Benny Tbpv 03L-42 38 10.07 106 32.77 52.00 1.75 16.22 9.65 4.21 7.47 3.71 

Andesite of Lone Tree Gulch Tla 05L-40 38 9.38 106 22.41 59.01 1.18 16.29 7.67 1.49 4.68 4.02 

Tuff of Big Dry Gulch Tbg 03L-1 38 9.70 106 24.60 67.11 0.60 16.43 3.01 1.21 2.34 2.39 

NORTH PASS CALDERA CYCLE 

Luders Creek Tuff (32.2 Ma)

Crystal-rich dense welded upper tuff, Luders Cr Tld 02L-26 38 10.15 106 34.93 66.93 0.36 16.73 2.71 0.57 1.74 4.59 

Crystal-poor part welded lower tuff, Luders Cr Tlr 02L-28 38 9.97 106 35.52 72.50 0.27 15.04 1.36 0.20 2.03 3.12 

Volcanics of Cochetopa Hills

Dacite of East Pass Creek (32.2 Ma)

Dacite flow, North Pass Tnpd 03L-26 38 11.80 106 32.28 61.40 0.74 17.11 5.57 1.89 4.46 3.79 

Dacite flow, Luders Creek Tnpd 03L-41 38 14.38 106 35.15 65.95 0.51 16.03 2.96 0.86 2.39 4.20 

Bio-hbl dacite flow, Fourmile Creek Tnpd? 05L-3 38 10.58 106 34.37 65.57 0.49 17.57 3.69 1.03 3.77 3.44 

Caldera-related rhyolite lava flows

Crystal-poor rhyolite, Corduroy Creek Tnpr 04L-4 38 10.57 106 36.52 76.95 0.18 12.42 0.46 <0.10 0.46 3.42 

Crystal-poor rhyolite, Benny Creek Tnpr 05L-43 38 10.67 106 33.57 75.03 0.19 12.89 1.05 <0.10 0.53 4.01 

Saguache Creek Tuff (32.25 Ma)

Upper welding zone, Pine Creek Tsc 00L-24 38 12.40 106 36.42 72.41 0.22 14.75 1.37 <0.10 0.66 4.39 

Above Bonanza Tuff, Cross Creek Tsc 02L-24 38 11.73 106 19.83 73.62 0.23 13.94 1.37 0.15 0.65 4.20 

Densely welded rhyolite, Saguache Park Tsc 02L-32 38 2.12 106 38.38 74.45 0.21 13.78 1.47 0.13 0.53 4.10 

Lower crystal-rich dark phase, Bear Creek Tsc 02L-11 38 13.48 106 29.25 73.72 0.21 13.90 1.30 0.14 0.59 4.20 

Densely welded rhyolite tuff, Soldier Park Tsc 02L-33 38 22.47 106 52.82 72.98 0.23 14.56 1.45 0.10 0.64 4.32 

Precursor tuff of Spanish Creek(32.5 Ma)

Densely welded crystal-rich rhyolite, Spanish Creek Tts 02L-34-A 38 14.50 106 32.55 73.00 0.24 14.37 1.79 0.48 1.36 3.50 

Densely welded crystal-rich rhyolite, Spanish Creek Tts 02L-34-B 38 14.70 106 32.63 75.13 0.20 13.34 1.48 0.31 1.10 3.46 
45

U.S. Geological Survey analyses (1998–2005) of nonmineralized rocks from the northeastern San Juan region. Major oxides were 

reported total without LOI) EDXRF, ppm 

K2O P2O5 MnO LOI Total Zn Rb Sr Y Zr Nb Pb Th Ba La Ce Nd V Cr Ga Ni

3.45 0.71 0.12 1.94 99.16 112 84 996 24 283 27 1340 84

2.68 0.49 0.14 2.87 99.99 101 53 653 21 160 21 5 <2 686 23 54 26 220 225 22 140

5.47 <0.05 0.11 0.67 99.76 58 206 <2 23 131 31 32 17 8 37 67 26 12 4 20 <2

4.61 <0.05 0.22 0.75 299.50 52 251 10 19 111 36 34 22 127 38 63 24 6 <3 19 3

4.71 <0.05 0.08 0.71 299.21 60 189 10 21 127 25 29 14 28 40 74 27 15 5 17 3

5.09 <0.05 0.05 0.84 99.36 32 178 26 17 133 23 31 12 105 56 103 33 7 <3 19 <2

4.08 0.57 0.08 0.45 99.12 98 88 1130 17 317 20 19 8 2010 92 154 79 116 65 24 30

5.40 0.19 0.08 0.81 99.68 42 156 80 25 201 19 23 19 470 59 102 42 18 <3 15 7

5.31 0.22 0.08 0.99 99.84 43 152 103 26 209 20 26 18 552 57 105 37 19 <3 16 5

4.66 0.65 0.07 0.50 98.92 97 112 1060 15 426 35 21 8 2180 111 182 88 88 10 24 14

4.35 0.44 0.09 0.67 98.83 82 105 767 13 292 20 14 <4 1860 97 158 102 96 41 16 5

4.12 0.70 0.11 0.82 99.41 98 85 1210 18 265 23 14 6 1810 74 130 65 148 155 22 56

5.11 0.08 0.05 2.03 99.19 41 123 190 24 230 20 22 22 1130 62 106 47 24 3 14 3

6.05 0.07 0.05 1.27 99.33 35 144 142 24 247 22 26 26 1050 70 105 39 25 5 15 4

3.45 0.75 0.13 1.33 99.33 108 64 1190 24 204 24 11 <4 1420 51 96 56 236 85 23 45

3.45 0.58 0.18 2.35 98.55 92 67 1010 17 255 18 10 <4 1860 67 125 75 153 47 18 20

6.07 0.15 0.10 6.23 99.39 65 143 332 32 365 19 20 15 979 54 101 48 36 <5 17 5

5.48 0.18 0.11 0.79 99.42 67 98 357 28 416 20 29 12 1620 114 186 78 34 5 20 8

5.30 0.10 0.04 6.90 99.96 44 154 319 23 266 22 24 11 984 79 128 53 28 5 15 6

4.09 0.38 0.10 1.79 99.54 74 87 875 24 267 18 24 10 1540 66 106 54 101 8 19 11

4.57 0.22 0.07 0.81 97.77 57 99 609 26 264 20 27 12 1620 82 121 62 61 5 19 7

4.45 0.35 0.12 2.10 100.48 58 115 573 23 282 14 12 <4 1700 51 91 49 9 7 15 3

5.21 <0.05 0.01 0.96 99.11 21 117 29 30 248 56 24 11 289 86 149 5 14 inf 15 24

4.96 0.09 0.08 0.70 98.83 39 123 32 22 201 25 26 10 309 85 146 65 29 <5 11 2

5.66 0.08 0.07 0.98 99.61 60 129 52 29 289 27 30 14 430 100 177 69 8 <3 19 2

5.39 0.09 0.11 0.97 99.75 73 125 50 32 282 27 29 13 391 97 169 60 28 <3 17 6

5.34 0.09 0.04 1.28 100.14 73 124 43 28 272 24 28 12 365 88 147 51 14 <3 17 6

5.32 0.09 0.06 0.70 99.53 70 124 42 33 292 27 30 14 307 90 158 71 18 <3 17 6

5.38 0.08 0.04 1.07 99.77 48 123 64 39 295 27 21 14 459 107 151 77 16 <3 16 7

5.17 0.12 0.07 2.61 100.11 48 130 223 15 143 18 31 11 727 48 80 34 22 10 19 6

4.76 0.10 0.05 1.77 99.93 40 100 191 16 136 17 27 10 707 49 73 37 18 11 16 7
46

Table 4. Representative chemical analyses for Tertiary volcanic rocks.—Continued

Lithology, location Unit

label

Sample 

No.

Latitude Longitude X-Ray Fluorescence (WDXRF); weight % (calculated to 

Deg Min Deg Min SiO2 TiO2 Al2O3 FeTO3 MgO CaO Na2O 

OLDER NORTHEASTERN TUFFS & LAVAS

Bonanza Tuff (33.2 ma)

Densely welded interior ''upper'' tuff, Poison Gulch Tbd 02L-21-B 38 9.30 106 16.33 73.73 0.22 12.70 1.34 0.19 2.89 3.78 

Vitrophyric lower dacite, Findley ridge area Tbd 02L-22-B 38 9.23 106 16.30 64.28 0.74 17.31 3.87 1.03 2.95 3.83 

Conejos Formantion (including Rawley Andesite)

Cebolla-Los Pinos Creeks 

Flow-laminated interior, large flow, Cebolla Creek Tcfr 95L-5 38 6.63 107 1.88 73.47 0.20 14.15 0.83 0.24 1.22 3.81 

Flow-laminated dacite lava, Los Pinos Pass Tcd 99L-3 38 6.58 106 58.17 65.68 0.74 16.90 3.71 0.61 3.16 4.23 

Cochetopa Park area

Intrusions

Hornblende-rich dike (Conejos), Hwy 114 Tci 03L-31 38 17.25 106 45.00 56.94 1.06 16.37 8.35 3.61 5.95 3.63 

Granodiorite intrusion, upper Needle Creek Tci 03L-17 38 19.70 106 34.12 59.97 0.78 16.52 6.52 2.67 5.39 3.90 

Bio-hbl intrusion, Left Hand Needle Creek Tci 05L-12 38 20.62 106 31.78 61.98 0.69 16.51 5.43 1.68 4.19 4.02 

Dike, Needle Rock Tci 03L-20A 38 21.53 106 33.17 60.06 0.87 16.31 6.85 2.69 4.95 3.91 

Hbl-bio-pl dike, Barret Creek Tci 05L-15 38 22.69 106 35.40 59.19 0.85 16.56 6.88 2.92 5.16 3.71 

Lava flows

Crystal-poor rhyolite, Green Mountain Tbfa 04L-21 38 8.33 106 21.15 76.13 0.12 12.69 0.90 <0.10 0.93 3.48 

Crystal-rich rhyolite flow, Barret Creek Tbfc 04L-5 38 10.57 106 36.52 74.28 0.24 13.45 1.56 0.43 1.23 2.89 

Quartz-rich rhyolite, south of Spanish Creek Tcfr 02L-13 38 13.45 106 31.42 73.15 0.25 14.11 1.77 0.39 1.43 3.84 

Dacite flow, Luhan Creek (Tfc mineralogy) Tcd 01L-10 38 15.05 106 34.10 70.96 0.29 15.32 2.12 0.38 1.59 4.06 

Coarse porph dacite (san), south of Carnero Pass Tcd 05L-45 37 59.73 106 25.99 65.57 0.70 15.30 4.60 1.13 2.93 3.59 

Dacite lava, Sawtooth Mtn, top flow Tcd 01L-29 38 16.43 106 51.90 63.20 0.72 16.21 5.67 2.00 4.20 3.56 

Fine-grain andesite, summit Razor Creek Dome Tca 01L-19 38 18.97 106 42.95 60.59 0.84 16.63 6.79 2.08 5.08 4.04 

Dense aphyric mafic lava, Middle Baldy Tca 02L-38 38 18.88 106 29.88 57.11 1.10 15.89 8.00 3.71 6.28 3.60 

Olivine-rich mafic lava, Middle Baldy Tca 02L-39 38 18.87 106 30.30 53.12 1.06 15.08 9.44 7.36 8.16 3.13 

Wall Mountain Tuff (36.9 Ma)

Rheomorphic tuff, Alkali Creek Twn 01L-22 38 17.72 106 46.63 74.68 0.41 13.71 1.57 0.31 0.53 2.22 

Rheomorphic tuff, Bead Creek Twn 01L-30 38 20.12 106 50.30 69.90 0.50 15.48 2.19 0.30 1.25 3.68 
47

reported total without LOI) EDXRF, ppm 

K2O P2O5 MnO LOI Total Zn Rb Sr Y Zr Nb Pb Th Ba La Ce Nd V Cr Ga Ni

5.10 0.09 0.03 3.16 100.07 39 237 82 26 202 46 24 52 106 68 108 30 16 4 16 7

5.16 0.31 0.05 1.21 99.53 77 126 659 30 528 25 25 21 1720 101 183 92 76 6 21 9

4.59 0.08 0.06 0.71 98.66

4.23 0.30 0.07 1.19 99.64

3.00 0.52 0.11 1.65 99.54 95 58 958 23 216 13 18 7 1260 46 81 43 163 47 22 26

3.19 0.36 0.12 1.94 99.41 77 61 865 22 221 19 21 5 1540 58 97 56 109 26 22 14

3.67 0.44 0.11 1.24 98.71 80 67 985 22 264 12 17 <4 2250 68 114 82 88 18 17 3

3.22 0.41 0.08 1.26 99.35 84 56 918 23 212 11 20 6 1620 48 85 49 131 36 21 20

3.08 0.49 0.20 2.16 99.05 75 56 882 25 281 14 14 <4 1710 47 97 68 112 19 16 3

4.47 0.05 0.04 0.69 98.81 32 98 203 15 111 27 29 <4 1300 62 89 61 13 <1.1 16 <0.6

4.95 0.08 0.04 1.84 99.12 21 117 29 30 248 56 24 11 289 86 149 5 14 inf 15 24

4.60 0.14 0.05 0.75 99.74 39 102 301 15 183 15 26 9 972 52 87 45 21 9 17 6

4.74 0.12 0.05 0.79 99.64 79 104 363 15 193 17 26 8 1110 49 79 34 26 9 18 3

4.79 0.27 0.08 1.32 98.96 51 126 478 33 317 22 17 8 1210 74 137 68 99 87 14 3

3.93 0.33 0.10 1.89 99.91 72 96 775 23 223 15 24 12 1390 49 91 51 86 19 19 12

2.97 0.40 0.10 0.80 99.53 84 58 980 22 214 12 16 7 1540 49 90 57 107 42 21 18

3.28 0.64 0.12 0.54 99.73 101 73 1180 28 318 24 20 15 1420 75 136 66 167 56 24 44

1.94 0.41 0.14 1.16 99.84 92 34 769 24 155 12 8 <2 954 25 55 46 197 610 18 211

6.15 0.07 0.03 2.24 99.67 32 184 158 29 383 22 29 35 1580 100 161 77 30 7 15 5

6.08 0.10 0.04 1.13 99.53 53 195 250 41 482 31 40 42 1520 135 197 108 32 5 19 5
48

Table 6. Megascopic characteristic features, major ignimbrite sheets in the central and northeastern San Juan region.

Ignmbrite sheet1 Composition2 Textures and phenocrys

Snowshoe Mountain Tuff Mafic dacite Phenocryst rich; densely welded within caldera, weakly 

Nelson Mountain Tuff Low-Si rhyolite⇒dacite Compositionally zoned; weakly welded crystal-poor to 

Cebolla Creek Tuff Mafic dacite Typically weakly welded; abundant hornblende >> aug

Rat Creek Tuff Low-Si rhyolite⇒dacite Compositionally zoned; weakly welded rhyolite to den

Wason Park Tuff Rhyolite Phenocryst-rich rhyolite; tabular sanidine phenocrysts

Blue Creek Tuff Dacite Phenocryst rich; sanidine is absent (contrast with Mam

Carpenter Ridge Tuff

Mammoth Mountain Member (upper) Dacite Phenocryst rich; sanidine is common (contrast with Blu

Outflow and lower intracaldera tuff Low-Si rhyolite Phenocryst-poor; common basal vitrophyre, central lith

Crystal Lake Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff, but less weld

Fish Canyon Tuff Dacite Distinctive light-gray, phenocryst-rich; resorbed quartz

Sapinero Mesa Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff, but generally 

Dillon Mesa Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff, but generally 

Blue Mesa Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff, but generally 

Ute Ridge Tuff Dacite Phenocryst-rich; contains sparse sanidine (in contrast to 

Masonic Park Tuff Dacite Phenocrysts similar to Blue Creek Tuff; typically less w

Luders Creek Tuff Low-Si rhyolite⇒dacite Compositionally zoned; resembles Nelson Mountain Tu

Saguache Creek Tuff Low-Si rhyolite Resembles Carpenter Ridge and Sapinero Mesa Tuffs, 

Bonanza Tuff Zoned complexly Local basal xl-poor rhyolite, lower xl dacite, upper rhy

Badger Creek Tuff Dacite Crystal rich; resembles Fish Canyon Tuff

Wall Mountain Tuff Rhyolite Crystal-rich, large blocky sanidine; locally complexly r

1Bold type, ignimbrite sheets of Cochetopa-North Pass area2Si, SiO2

3xl, crystal