"' 

Calderas of the San Juan 

Volcanic Field, 

Southwestern Colorado 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 958 

• 
Calderas of the San Juan 

Volcanic Field, 

Southwestern Colorado 

By THOMAS A. STEVEN and PETER W. LIPMAN 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 958 

Eighteen major ash-flow tuff sheets 

were deposited and perhaps as many 

related calderas developed during 

emplacement of an underlying shallow 

batholith in late Oligocene time 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:I976 
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Steven, Thomas August, 191 ?-

Calderas of the San Juan volcanic field, southwestern Colorado. 

(Geological Survey Professional Paper 958) 

Bibliography: p. 

Supt. of Docs. no.: I 19.16:958 

1. Calderas-Colorado-San Juan Mountains. 2. Volcanic ash, tuff, etc.-Colorado-San Juan Mountains. 3. BatholithsColorado-San Juan Mountains. 

I. Lipman, Peter W., joint author. II. Title. III. Series: United States Geological Survey Professional Paper 958. 

QE524.S85 551.2'1'09788'38 75-619377 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-001-02783-6 
CONTENTS 

Page Page 

Abstract .......................... ............... ......................... .. .................... . 1 

1 

3 

4 

4 

6 

7 

7 

8 

Central San Juan caldera complex- Continued 

Introduction ........ ..................... ........ ... ................................... ...... . Bachelor caldera ......................... ....................... ................. .. . . 19 

20 

22 

22 

General geology ........................................................................... . Mammoth Mountain(?) caldera ............ ................................ . 

Early eastern calderas ....... ......................... .............................. ... .. . Source of the Wason Park Tuff .......................................... . . 

Platoro and Summitville calderas ........................................ . San Luis and Cochetopa Park calderas ............................... . 

Bonanza caldera .................................................... ................ . Compound subsidence of the San Luis caldera ........... . 25 

26 

27 

27 

27 

29 

30 

30 

30 

31 

31 

32 

33 

34 

Western San Juan caldera complex .. ................... ........................ . Subsidence of the Cochetopa Park caldera ................... . 

Ute Creek caldera .......................... ........................................ . Resurgence of the San Luis caldera .............................. . 

Lost Lake caldera (buried) ................................................... . Postcaldera lavas ............................................................ . 

San Juan, Uncompahgre, and Silverton calderas ................ . 10 

10 

11 

12 

12 

12 

13 

16 

16 

18 

Creede caldera ....................... ............................................... . . 

Uncompahgre and San Juan collapses ......................... . Late general magmatic uplift. ............................................. . . 

Postcollapse lavas and sediments ....... ........................... . Block-faulted area ................................................................. . 

Intracaldera ash-flow tuffs ... ............................. ............ . Discussion .............................. ............. ......................................... . 

Silverton collapse ............................ ........................ ...... . Development of the batholith .............................................. . 

Resurgent doming .................................................. ....... . Relation of ash-flow eruption and caldera subsidence ........ . 

Lake City caldera ............. ..................................................... . Differentiation in local cupolas ........................................... . 

Central San Juan caldera complex .............................................. . Resurgence ..... ...................................................................... . . 

Mount Hope caldera ....................................... .. .................... . Mineralization ....................................................................... . 

La Garita caldera ................ ...................................... ............ . References cited .......................... ........................................ .......... . 

FIGURES 1-25. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22. 

23. 

24. 

25. 

ILLUSTRATIONS 

Maps: Page 

Calderas in the San Juan volcanic field in relation to Bouguer gravity field........................................................................... 2 

Generalized geology of the Platoro and Summitville calderas............. ...................................................................................... 5 

Distribution of La Jara Canyon Member of Treasure Mountain Tuff in relation to Platoro caldera................ ..................... 6 

Distribution of Ojito Creek and Ra Jadero Members of Treasure Mountain Tuff in relation to Summitville caldera.......... 6 

Geologic map of Ute Creek and Lost Lake calderas.... .............................................................................................................. 8 

Distribution of Ute Ridge Tuff in relation to Ute Creek caldera............................. .......... ....................................................... 9 

Distribution of Blue Mesa Tuff in relation to Lost Lake caldera.................... ....... .......... ......................................................... 9 

Distribution of Dillon Mesa Tuff in relation to Uncompahgre caldera............ ........................................................................ 10 

Western San Juan caldera complex after subsidence related to eruption of the Sapinero Mesa Tuff...................................... 11 

Distribution of Sapinero Mesa Tuff in relation to San Juan and Uncompahgre calderas....................................................... 12 

Distribution of Crystal Lake Tuff in relation to Silverton caldera............................................................................................ 13 

Western San Juan caldera complex after subsideflce of the Silverton caldera and general resurgence.................................... 14 

Generalized geology of the western San Juan caldera complex showing distribution of rocks related to the Lake City 

caldera ........................................................................................................................................................................ ,.......... 15 

Generalized geology of the Mount Hope caldera and adjacent areas........................................................................................ 17 

Distribution of the upper member of Masonic Park Tuff in relation to Mount Hope caldera................................................ 18 

Restored Bachelor and La Garita calderas.................................................................................................................................. 20 

Distribution ·of the Fish Canyon Tuff in relation to the La Garita caldera.............................................................................. 21 

Distribution of Carpenter Ridge Tuff in relation to Bachelor caldera...................................................................................... 21 

Distribution of Mammoth Mountain Tuff in relation to probable source ar~a........ .. .. ... ....................... .................................. 22 

Distribution of Wason Park Tuff in relation to possible source area ......................... .. ............................................................ 22 

Generalized geology of the San Luis and Cochetopa Park calderas in relation to remnants of the Bachelor and La 

Garita calderas...................................................................................................................................................................... 23 

Generalized geology of the Creede and San Luis calderas in relation to remnants of the Bachelor and La Garita calderas.. 24 

Distribution of Nelson Mountain Tuff in relation to San Luis caldera................................................................................... 26 

Distribution of the Cochetopa Park Tuff in relation to Cochetopa Park caldera.................................................... ................. 26 

Areas where erosional remnants of Snowshoe Mountain Tuff are preserved in relation to the Creede caldera .......... ~ ....... _. ... 28 

TABLE 

Page 

TABLE 1. Ash-flow stratigraphy in the San Juan volcanic field. ...................................................................................................................... 4 

III 

CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN 

COLORADO 

By THOMAS A. STEVEN and PETER W. LIPMAN 

ABSTRACT 

Calderas in the San Juan volcanic field in southwestern Colorado 

formed largely in late Oligocene time (30-26 m.y. ago) in response to 

recurrent large-volume ash-flow eruptions. The ash-flow deposits overlie a coalescing assemblage of early Oligocene (35-30 m.y.) andesitic 

stratovolcanoes that formed the southwest part of a widespread 

composite volcanic field in the southern Rocky Mountains. A nearly oneto-one relationship exists between large-scale pyroclastic eruptions and 

calderas: 18 major ash-flow sheets have been identified; 15 calderas are 

known, 2 are postulated on indirect evidence, and another one may 

possibly be identified in the northeast part of the volcanic field. In 

general, the different caldera cycles confirm the successive stages of 

development described by Smith and Bailey in 1968, except that few 

calderas demonstrate all stages of activity. On the other hand, almost 

every stage is exceptionally well developed in one or more of the calderas. 

The development of the calderas is believed to chronicle the emplacement of successive segments of an underlying shallow batholith that is 

indicated by a major gravity low having sharp marginal gradients. Early 

calderas in the eastern part of the field formed in areas of clustered 

andesitic volcanoes and are not clearly associated with the main gravity 

low. These early calderas are believed to have formed above local highlevel magma chambers that developed in the roots of the volcano clusters 

before the m~in body of the batholith rose to shallow depths. Postcollapse volcanics are largely of andesitic composition, indicating that 

only limited volumes of silicic differentiates formed at the tops of these 

chambers and that these differentiates were depleted by the ash-flow 

eruptions. 

The western San Juan caldera complex also formed in an area of 

clustered earlier andesitic volcanoes, but are above the western part of the 

batholith indicated by gravity data. Large volumes of silicic differentiates 

formed within the batholith and were spread widely by ash-flow 

eruptions. Five calderas formed within a period of about 2 m.y. (in the 

interval from 29 to 27 m.y. ago), and contrasting lithologies of the ashflow sheets related to the calderas require sequential development of 

cupolas and magmatic differentiation within them. Postsubsidence lavas 

that were erupted after emplacement of the most voluminous of these 

ash-flow sheets were largely of mafic quartz latite to andesite compositions, indicating temporary depletion of silicic differentiates in the 

source magma chamber. In the same area, a sixth caldera formed 4-5 m. y. 

later in response to eruption of a petrologically distinct ash-flow tuff 

believed to have had an origin different from the earlier ash-flow tuffs. 

Development of the central San Juan caldera complex began about 28 

m.y. ago, during the period of ash-flow eruptions and caldera collapses 

in the western San Juan Mountains, and was largely complete by the end 

of the Oligocene, 26 m.y. ago. During this 2-m.y. span, recurrent pyroclastic eruptions caused deposition of eight major ash-flow sheets and 

formation of at least seven calderas. The calderas are above the main 

eastern segment of the gravity low and are believed to mark the 

culminating upward movement of magma in this part of the batholith. 

Contrasting lithologies of sequential ash-flow sheets, which were derived 

from clustered and, in some places, nested caldera sources, require rapid 

development of successive cupolas above the batholith of and local 

differentiation within them. Most of the postsubsidence lavas that were 

erupted late in the different caldera cycles are coarsely porphyritic quartz 

latites compositionally related to the associated ash-flow tuffs; 

apparently even the most voluminous ash-flow eruptions did not deplete 

the silicic differentiates at the top of this part of the batholith. Concurrent eruption of andesitic rocks from scattered volcanoes not closely 

associated with the calderas is evidence of the presence of more mafic undifferentiated magma at depth, however. 

The life span of the batholithic magma chamber, as indicated by ashflow eruptions and caldera subsidence, appears to have been brief. 

Voluminous andesitic material was erupted from widely scattered centers 

throughout early Oligocene time (35-30m.y. ago). Toward the end of this 

period, the first local magma chambers 10-30 km across had risen to 

shallow depths beneath some of the major volcano clusters, and had 

differentiated sufficiently to supply large volumes of silicic .ash. Within 

the next 4 m.y. (30-26 m.y.), the main batholith rose to shallow depths in 

segments indicated by the main caldera complexes; vast quantities of ash 

were erupted and numerous calderas collapsed into the partly evacuated 

magma chambers. However, within another 4 m.y. (by 22 m.y. ago), the 

batholith had congealed sufficiently to allow a younger, petrologically 

distinctive magma to penetrate to comparably shallow depths and retain 

its compositional identity. 

INTRODUCTION 

The San Juan Mountains, southwestern Colorado (fig. 

1 ), consist mainly of volcanic rocks that form the largest 

remnant of a major composite volcanic field that covered 

most of the southern Rocky Mountains in middle Tertiary 

time (Steven and Epis, 1968; Steven, 1975). This remnant is 

an eroded volcanic plateau (Steven, 1968), in which 

coalescing early andesitic volcanoes were widely overlain 

by the silicic tuffs of 18 major and several minor ash-flow 

sheets and by related lavas and breccias (Lipman and 

others, 1970). The sources of all the larger ash-flow sheets 

were near-surface magma chambers that were rapidly 

evacuated during voluminous pyroclastic eruptions, 

thereby causing collapse of overlying calderas. This paper 

reviews the history of the ash-flow field and its related 

calderas and summarizes the general relations that seem 

common to most individual cycles of pyroclastic eruption 
2 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

0 

---.-- Fault-Bar and ball on downthrown side 

c:::J Caldera 

(.::=.:.) Buried or inferred caldera 

10 20 30 40 50 Kl LOMETRES 

EXPLANATION 

--300- Bouguer gravity contours-Hachures point 

toward closed gravity lows. Contour 

interval 10 milligals. Gravity data from 

Plouff and Pakiser (1972, fig. 3) 

DENVERo 

COLORADO 

San Juan 

volcanic field 

LOCATION OF SAN JUAN VOLCANIC FIELD 

CALDERAS (In order 

of increasing agel 

LC Lake City 

C Creede 

CP Cochetopa Park 

SL San Luis 

8 Bachelor 

LG La Garita 

MH Mount Hope 

S Silverton 

SJ San Juan 

UN Uncompahgre 

L Lost Lake 

U Ute Creek 

SM Summitville 

P Platoro 

Bz Bonanza 

FIGURE I.-Calderas in the San Juan volcanic field (patterned) in relation to Bouguer gravity field. 
GENERAL GEOLOGY 3 

and caldera subsidence in the San Juan field. 

The sequence of events we have determined for each of 

the calderas conforms well to the succession of stages in the 

development of a typical resurgent caldera described by 

Smith and Bailey (1968), although few of the San Juan 

calderas demonstrate all stages of activity. On the other 

hand, almost every stage is exceptionally well developed in 

one or more of the San Juan calderas. 

GENERAL GEOLOGY 

The general evolution of the San Juan volcanic field has 

been described by Lipman, Steven, and Mehnert (1970) 

and will be outlined only briefly here. Volcanic activity 

began in latest Eocene or earliest Oligocene time, probably 

between 40 and 35 m.y. ago. 1 The early rocks are largely 

intermediate in composition (andesite, rhyodacite, and 

mafic quartz latite) and were erupted from many scattered 

stratovolcanoes. These volcanoes were especially active in 

the interval from 35 to 30 m.y. ago, and the products 

derived from them coalesced into a composite volcanic 

field covering more than 25,000 km2. 

About 30 m.y. ago the character of volcanic activity 

changed markedly to predominantly pyroclastic 

eruptions, and large-volume quartz latitic and rhyolitic 

ash flows spread widely from many centers. Most of the 

larger sheets show evidence of compound cooling, and 

evidently were formed by many individual ash flows that 

followed one another in rapid succession. The earliest ash 

flows came largely from the northeastern and southern 

parts of the SanJuan field, and were erupted ~rom clusters 

of the early stratovolcanoes; caldera collapse resulting 

from the ash-flow eruptions largely destroyed the upper 

parts of these volcanoes. Postsubsidence eruptions around 

these early calderas were largely of intermediatecomposition lavas and breccias that commonly are 

virtually indistinguishable from the early intermediate 

rocks of the composite volcanic field. 

Beginning about 29 m.y. ago, ash-flow eruptions broke 

out in the western part of the San Juan volcanic field 

where five calderas formed in less than 2 m.y. The first two 

of these calderas are largely covered and are imperfectly 

understood, but the last three evolved in a manner 

generally similar to the early calderas farther east. They 

developed in an area of clustered andesitic central 

volcanoes whose vent areas were largely destroyed by 

caldera subsidence. Postsubsidence eruptions here were 

also mainly of intermediate-composition lavas and 

breccias that closely resemble those of the early volcanoes. 

About 28 m.y. ago, while ash flows were still erupting 

and calderas forming in the western San Juan Mountains, 

1 Except where otherwise noted, all specific age designations are from Lipman, Steven, and 

Mehnert (1970) or Steven, Mehnert, and Obradovich (1967). 

major pyroclastic eruptions began in the central part of 

the San Juan volcanic field. A sequence of eight major ashflow sheets formed, and caldera subsidences have been 

identified or inferred at all the ash-flow source areas. Postsubsidence eruptions around most of the central San Juan 

calderas were of viscous quartz-latitic and rhyolitic lavas 

closely related in composition to the ash-flow tuffs. 

Although some more mafic volcanoes were active during 

this same interval, they were not closely associated in space 

with the developing calderas. Ash-flow activity terminated 

in the central San Juan Mountains about 26.5 m.y. ago. 

In early Miocene time, about 25 m.y. ago, the character 

of the erupted material changed from the andesitic and 

derivative rocks that formed the early San Juan volcanoes 

and succeeding ash-flow tuffs to fundamentally basaltic 

materials with some associated high-silica alkali-rich 

rhyolites. This change approximately coincided with 

inception of basin-and-range faulting in the adjacent San 

Luis Valley segment of the Rio Grande trough (Lipman 

and Mehnert, 1975). Fundamentally basaltic eruptions in 

the San Juan field continued intermittently until about 5 

m.y. ago. The only large-volume rhyolitic ash-flow tuff 

deposited during the period of fundamentally basaltic 

activity is the Sunshine Peak Tuff, about 22.5 m.y. old 

(Mehnert and others, 1973a), which formed from ash flows 

that accumulated in and around the concurrently 

developing Lake City caldera in the western part of the San 

Juan volcanic field. 

In all, 15 calderas are now known in the San Juan 

volcanic field, and indirect evidence suggests that at least 

two and perhaps three more exist. 

A large negative Bouguer gravity anomaly underlies the 

area containing most of the calderas (fig. 1 ), and is believed 

to reflect a major underlying batholith (Plouff and 

Pakiser, 1972). Sharp gradients at the margins of the 

anomaly indicate that the top of the batholith is relatively 

shallow. The change from eruption of intermediatecomposition rocks by widely scattered early stratovolcanoes to eruption of the more silicic ash flows 

probably took place as this batholith rose and 

differentiated beneath the central part of the field. When 

the roofs of the more differentiated and gas-charged 

cupolas of the batholith failed, great volumes of ash were 

erupted rapidly, and unsupported segments collapsed to 

form the calderas. The sequential development of the 

calderas is believed to reflect the progressive emplacement 

of the different high-level plutons of a composite 

batholith. 

The calderas in the San Juan volcanic field became 

comprehensible to us only after the complex stratigraphy 

of the related ash-flow units and associated rocks was 

determined by regional mapping of the Durango 1 °x2° 

quadrangle (Steven, Lipman, Hail, and others, 1974) and 
4 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

adjacent parts of the Montrose 1 °x2° quadrangle. This 

regional work has led to major revisions in stratigraphic 

nomenclature established by earlier studies of local areas 

(Steven, Lipman, and Olson, 1974; Lipman and others, 

1973). The volcanic stratigraphy of the ash-flow units as 

understood in 1975 is given in table 1. For a more complete 

summary of the total volcanic stratigraphy, the reader 

should consult the above references and particularly the 

explanation of the Durango quadrangle map. 

EARLY EASTERN CALDERAS 

The oldest calderas in the eastern part of the San Juan 

field-the Bonanza caldera and the nested Platoro and 

Summitville calderas-are widely separated and the 

related ash-flow sheets do not overlap. Thus, the relative 

ages of eruption and caldera development at the two 

centers are uncertain (table 1 ). All these cycles are younger 

than the dated rocks in the early intermediate-composition 

volcanoes (34.7-31.1 ni.y.) and are older than the Fish 

Canyon Tuff (27.8 m.y.). The only age relations among 

these rocks that can be told directly by superposition are 

those between the several members of the Treasure 

Mountain Tuff that are derived from the Platoro and 

Summitville calderas. K-Ar ages of the Treasure Mountain Tuff related to the Platoro and Summitville calderas 

appear older than those obtained from tuffs related to any 

of the calderas in the western San Juan Mountains, but 

present data do not permit interpretation of age relations 

between the Bonanza caldera and any of the other older 

calderas. 

Both the Bonanza caldera and the Platoro and Summitville calderas formed within clusters of earlier andesitic 

stratovolcanoes, and they are on or just outside of the 

margin of the shallow batholith that gravity data suggest 

underlies the San Juan volcanic field (fig. 1) (Plouff and 

Pakiser, 1972). The Bonanza caldera is located near the 

northeast end of a narrow gravity low that extends eastnortheast from the main anomaly; this caldera probably is 

localized above a satellitie pluton. The Platoro and 

Summitville calderas are outside the sharp gradient along 

the southeast side of the main gravity low, and thus are 

probably not above the near-surface part of the main 

batholith. Quite possibly these early calderas developed 

above local high-level magma chambers that formed in the 

roots of earlier volcanoes before the main batholith had 

risen to its present near-surface position. 

PLATORO AND SUMMITVILLE CALDERAS 

The Platoro and Summitville calderas in the southeastern part of the volcanic field (fig. 1) constitute a 

composite collapse structure about 20 km in diameter that 

formed as a result of recurring eruptions of ash flows of the 

Treasure Mountain- Tuff (Lipman and Steven, 1970; 

Lipman, 1975a, b). The Platoro caldera and the nested 

younger Summitville caldera (fig. 2) formed within a 

cluster of six or seven intermediate-composition stratovolcanoes of the Conejos Formation. These central 

volcanoes had been extensively eroded and the intervening basins filled with the resultant detritus, producing 

a widespread low-relief surface. Ash-flow activity began 

30-29 m.y. ago when at least 500 km3 of phenocryst-rich 

quartz-latitic ash that now constitutes the LaJara Canyon 

Member of the Treasure Mountain Tuff was erupted from 

sources in the Summitville-Platoro region and spread 30-

40 km in all directions (fig. 3). Caldera collapse began 

before these eruptions were complete, and the late ash 

flows forming the La Jara Canyon Member ponded within 

the collapsing caldera to a thickness of more than 800 m. 

Similar concurrent eruption and collapse characterized 

other large calderas in the San Juan Mountains, when the 

TABLE 1.-Ash-flow stratigraphy in the San juan volcanic field 

Ash-flow unit 

Sunshine Peak Tuff ...................................... . Snowshoe Mountain Tuff ............................ . Cochetopa Park Tuff.. .................................. . Nelson Mountain Tuff ................................. . Rat Creek Tuff .............................................. . Wason Park Tuff .......................................... . Mammoth Mountain Tuff.. .......................... . Carpenter Ridge Tuff ................................... . Crystal Lake Tuff ..... .................................... . Fish Canyon Tuff .................. ....................... . Masonic Park Tuff (upper member) ............ . Sapinero Mesa Tuff ...................................... . 

Dillon Mesa Tuff .......................................... . Blue Mesa Tuff ............................................. . Ute Ridge Tuff ............................................. . Treasure Mountain Tuff: 

Ra Jadero Member ................................. . Ojito Creek Member .............................. . La Jara Canyon Member ....................... . Bonanza Tuff ................................................ . 

Estimated 

volume 

(km') 

100-500 

>500 

<100 

>500 

<100 

100-500 

100-500 

>500 

25-100 > 3,000 

>500 

> 1,000 

25-100 

100-500 

>500 

100-500 

40-70 

>500 

Dominant composition 

Silicic rhyolite ........................................... . Quartz latite ............................................... . Zoned rhyolite to quartz latite .................. . .... do ........................................................... . ·Rhyolite ..................................................... . . ... do ........................................................... . Zoned rhyolite to quartz latite .................. . Rhyolite, locally zoned to quartz latite .... . Rhyolite ..................................................... . Quartz latite ............................................... . . ... do ........................................................... . Rhyolite ..................................................... . 

Age (m.y.) 

22.5 

26.5(?) 

> 26.4 < 26.7 > 26.4 < 26.7 

>26.4 < 26.7 

>26.4 <26.7 

26.7 

>26.7 <27.8 

>26.7 <27.8 

27.8 

28.2 

>27.8<28.4 

.... do ............................................................ >27.8<28.4 .... do ............................................................ > 27.8 28.4 

Quartz latite................................................ 28.4 

.... do......... ................................................... >29.1 < 29.8 .... do ............................................................ > 29.1 < 29.8 .... do............................................................ 29.8 .... do ............................................................ > 27.8 

Related caldera 

Lake City. 

Creede. 

Cochetopa Park. 

San Luis. 

Early stage San Luis. 

Unknown. 

Do. 

Bachelor. 

Silverton. 

La Garita. 

Mount Hope. 

Uncompahgre and 

San Juan. 

Uncompahgre(?) . Lost Lake(?) . Ute Creek. 

Summitville 

Do . Platoro . Bonanza . 
EARLY EASTERN CALDERAS 5 

Precaldera andesitic rocks 

(Conejos Formation) 

Precaldera andesitic rocks 

(Conejos Formation) 

0 5 10 15 Kl LOMETRES I I I I 

FIGURE 2.-Generalized geology of the Platoro and Summitville calderas. COntrol moderate to good where boundaries of calderas are shown 

by solid symbols; conjectural where shown by open symbols. 

roofs of the magma chambers lost support before 

eruptions were completed. 

The thick La jara Canyon tuffs within the Platoro 

caldera are topographically and structurally high as a 

result of resurgent uplift shortly after collapse. Early 

resurgence is demonstrated by the presence in the core of 

the caldera of monolithologic talus breccias that were 

derived from the La jara Canyon Member and that intertongue with lavas that filled the structural moat adjacent 

to the resurgent block. The resurgent core forms a nearly 

unbroken block that dips homoclinally to the southwest, 

in contrast with the fractured domical uplifts that 

characterize many other known resurgent calderas. 

After resurgence was virtually complete, the marginal 

moat of the Platoro caldera was filled by as much as I km 

of dark andesitic lavas and interbedded volcaniclastic 

sedimentary rocks of the lower member of the Summitville Andesite. These andesitic lavas, in essence, represent 
6 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

0 100 Kl LOMETRES 

FIGURE 3.-Distribution of La Jara Canyon Member (diagonal lines) 

of Treasure Mountain Tuff in relation to Platoro caldera (p) and 

San Juan volcanic field (shaded). Base for figures 3, 4, 6-8, 10, II, 

15, 17-20, and 23-25 from U.S. Geological Survey, Colorado State 

map, 1:500,00, 1968. 

a continuation of the same type of volcanic activity that 

characterized the development of the Conejos Formation, 

with which they are readily confused in the field. 

A younger collapse structure, the Summitville caldera, 

occupies the northern part of the Platoro caldera (fig. 2). 

This caldera apparently formed when ash-flows 

constituting the upper sheets of the Treasure Mountain 

Tuff, including the Ojito Creek and Ra Jadero Members, 

were erupted after the Platoro caldera moat was nearly 

filled by lavas of the lower member of the Summitville 

Andesite. The Ojito Creek and Ra Jadero tuffs (fig. 4) are 

nearly coextensive with those in the La Jara Canyon 

Member and their constituent ash must have been erupted 

from the same general area. Although the volumes of these 

upper two members are much less than that of the Lajara 

Canyon Member (table 1), they are large enough to suggest 

associated caldera collapse (Smith, 1960, fig. 3). In 

.addition, the Summitville caldera is indicated by ( 1) a 

large-displacement (800+ m) arcuate fault marking the 

main ring-fracture fault on the southeast side of the 

caldera (fig. 2), (2) fragmentary exposures of the topographic wall, especially on the northeast side, and (3) the 

concentration of the products of late igneous activity and 

mineralization around the margins of the caldera. The 

caldera probably was not resurgent, but was filled by thick 

lavas of the upper member of the Summitville Andesite, 

and many key geologic relations are largely buried. In 

0 100 Kl LOMETRES 

FIGURE 4.-Distribution of Ojito Creek and Ra Jadero Members 

(diagonal lines) of Treasure Mountain Tuff in relation to Summitville caldera (S) and San Juan volcanic field (shaded). 

addition, extensive late intrusion and hydrothermal 

alteration further obscured relations. 

Porphyritic rhyodacitic to rhyolitic lavas and 

genetically related dikes and granitic stocks were emplaced 

repeatedly around the margins of the Platoro and Summitville calderas during the interval between 29 and 20 m. y. 

ago, with dated events at 29.1, >26.7<27.8, 25.8, 22.8, and 

20.2 m.y.; these shallow intrusions and associated rocks 

locally were hydrothermally altered and mineralized: 

BONANZA CALDERA 

A caldera in the vicinity of the old mining camp of 

Bonanza in the northeast part of the San Juan volcanic 

field has been postulated by Karig (1965), Mayhew (1969), 

Bruns (1971), Knepper and Marrs (1971, p. 261), and 

others. 

The oldest rocks in the Bonanza area are andesitic to 

latitic flows and breccias of the Rawley Andesite and 

Hayden Peak Latite (Burbank, 1932; Mayhew, 1969) that 

form an interfingering assembJage. We interpret these 

rocks to be the near-source facies of one or more local 

volcanoes equivalent in age to the Conejos Formation 

near the Platoro area. These rocks are overlain by the 

Bonanza Tuff, a densely welded quartz-latitic ash-flow 

tuff that once formed a widespread sheet over much of the 

northeastern part of the San Juan volcanic field. Near 

Bonanza, remnants of this sheet must have been deeply 

faulted into the older andesitic pile, probably as a result of 

caldera subsidence. Younger andesitic flows and breccias 
WESTERN SAN JUAN CALDERA COMPLEX 7 

cover at least some of the Bonanza Tuff that occurs within 

the subsided block (Knepper and Marrs, 1971, pl. 1). 

Few generalizations can be made about the Bonanza 

caldera from available published data. However, we 

consider it significant that, as in the Platoro caldera 

complex, subsidence took place within an area of older 

intermediate-composition volcanoes and that postsubsidence lavas are predominantly andesitic 1n 

composition. 

WESTERN SAN JUAN CALDERA COMPLEX 

Six calderas in the western San Juan Mountains are 

located above the main western lobe of the gravity low that 

has been interpreted to represent a shallow batholith. The 

first five of these calderas, the Ute Creek, Lost Lake, San 

Juan, Uncompahgre, and Silverton, formed within the 

brief span of about 2 m.y., and the five related ash-flow 

deposits superficially resemble each other enough to have 

once been included within a single formation (the nowabandoned Gilpin Peak Tuff of Luedke and Burbank, 

1963). 

Of these five calderas, only the Ute Creek caldera is not 

clearly within the gravity low, but instead is located at the 

sharp gradient along the south side of the gravity low (fig. 

1 ). This structure may have formed above a local magma 

chamber that either was too deep or, after ash-flow 

eruptions, did not retain enough relatively light silicic 

differentiate at its top to be detected by gravity measurements. The other four early calderas are well within the 

area of the gravity low, and, of these, three are closely 

clustered and have overlapping cycles of developme~t. 

These cycles are believed to represent high-level 

magmatism and volcanic eruption related to the main 

upward movement of magma in this western part of the 

batholith. 

The last subsidence structure, the Lake City caldera, 

formed about 5 m.y. after the earlier calderas subsided, and 

the associated ash-flow tuff contrasts markedly in 

composition and appearance with the earlier ash-flow 

tuffs. During the 5-m.y. interval, the main batholith is 

believed to have congealed sufficiently for a petrologically distinct batch of magma to penetrate to shallow 

depths and still retain its compositional identity. 

UTE CREEK CALDERA 

The southern margin of the caldera that formed during 

eruption of the Ute Ridge Tuff is exposed for about 3 km 

along the canyon of Ute Creek (figs. 5 and 6). Elsewhere, 

the eastern and northern topographic wall of the caldera 

can be located closely at only two places, one near the 

mouth of Ute Creek and the other along the south side of 

Pole Creek Mountain, near the eastern end of the 

mountain. The downdropped block within these limits 

may have subsided as a trapdoor that has no ring-fracture 

zone along its west side. 

Along Ute Creek, intracaldera Ute Ridge Tuff, more 

than 300 m thick, is juxtaposed against Precambrian 

melasyenite and quartzite and an older unnamed crystalpoor ash-flow tuff of Tertiary age, whereas an outflow 

layer of the Ute Ridge Tuff is only about 130m thick and 

extends out over an irregular surface cut on the older rocks. 

The probable, caldera-boundary fault between the thick 

section of Ute Ridge Tuff and the adjacent rocks is 

occupied by a quartz monzonite intrusive (fig. 5); another 

similar intrusive cuts the thick section of Ute Ridge Tuff 

north of lower Ute Creek. 

In the vicinity of Ute Creek, a younger ash-flow sheet, 

the Blue Mesa Tuff, unconformably overlies the Ute Ridge 

Tuff, quartz monzonite intrusives, and Precambrian 

rocks. A chilled vitrophyre at the base of the Blue Mesa is 

in contact with all these older rocks, and north of Ute 

Creek a thin lens of andesite occurs locally along the 

unconformity. Development of the structural discordance 

between the thick section of Ute Ridge Tuff and the 

juxtaposed older rocks thus closely accords in time with 

eruption of the Ute Ridge Tuff, and seems most easily 

accounted for by caldera subsidence related to that 

eruption. Subsidence concurrent with eruption is 

indicated by the thick section of Ute Ridge Tuff within the 

downdropped block, in contrast with the relatively thin 

outflow sheet. Although we saw no evidence for postsubsidence resurgence, two quartz monzonite intrusive 

bodies along and near the southern margin apparently 

were emplaced late in the Ute Creek caldera cycle. 

The topographic caldera wall along the east side of the 

downdropped block is closely controlled near the mouth 

of Ute Creek, where the top of a buried hill of older 

andesitic volcanic breccias (San Juan Formation) is 

exposed a few hundred metres east of the thick section of 

Ute Ridge Tuff that lies within the caldera. Several kilometres northwest, across the Rio Grande, another buried 

hill of andesitic breccias of the San Juan Formation is 

exposed along the south side of Pole Creek Mountain. The 

Ute Ridge and Blue Mesa Tuffs wedge out against this 

hill, which may represent a remnant of the northeast 

caldera wall. 

Thick Ute Ridge Tuff is excellently exposed along the 

Rio Grande inside the west-facing semicircular arc 

described by the intrusive-filled fault along Ute Creek and 

the two buried hills of andesite breccia. The rude layers 

representing the many successive ash flows in the Ute 

Ridge Tuff dip about 5° east as part of a much later 

regional tilting that affected the whole western San Juan 

area, but otherwise are undeformed westward from the 

arcuate wall for 12-15 km. The tops of at least two buried 

hills of Precambrian crystalline rocks extending well up 

into the Ute Ridge Tuff are exposed along the Rio Grande 

3-9.5 km west of the arcuate wall. 

These incompletely exposed relations suggest that the 
8 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

0 5 10 Kl LOMETRES 

FIGURE 5.-Geologic map of Ute Creek (U) and Lost Lake (L)calderas. p£r, Precambrian rocks; o. precaldera volcanic rocks; u, Ute Ridge 

Tuff; b, Blue Mesa Tuff; y, postcaldera volcanic rocks; black, intrusive rocks. Open rectangles indicate approximate boundaries of postulated 

buried calderas. 

source of the Ute Ridge Tuff was in a hilly area cut on 

older Precambrian rocks and Tertiary andesitic rocks, and 

that the pyroclastic eruptions caused concurrent 

subsidence of a trapdoor block that was faulted along the 

south, east, and north but probably only down warped on 

the west. The fragmentary structural and topographic 

wall exposed in places along Ute Creek and Pole Creek 

Mountain reflects the abrupt boundaries of subsidence in 

these directions, whereas the downwarped(?) western 

margin is obscured by the relatively flat lying upper part ot 

the Ute Ridge Tuff exposed along the Rio Grande. 

LOST LAKE CALDERA (BURIED) 

A distinctive arcuate drainage pattern along the upper 

Rio Grande (fig. 5) reflects a buried caldera related to 

eruption. of the ash composing the Blue Mesa Tuff. All 

rocks in this area dip regionally about 5° east and 

northeast; eastward across the arcuate drainage pattern, 

however, the dips increase to 10°-15° and then flatten again 

to form an arcuate monocline. Fracturing related to this 

monocline provided a zone of weakness later etched out by 

stream erosion. The monocline involves the Dillon Mesa 

and Sapinero Mesa Tuffs and younger units in this area, 

and reflects minor late subsidence around the western 

periphery of an older buried caldera. 

The western margin of the largely buried Lost Lake 

caldera closely follows the arcuate monocline. The wall of 

the caldera is closely controlled near the mouth of Ute 

Creek, where a hill of older andesitic mudflow breccia 

protrudes up into the ash-flow section and represents a 
WESTERN SAN JUAN CALDERA COMPLEX 9 

0 100 KILOMETRES 

~---~--~--~L_~ 

FIGURE 6.-Distribution of Ute Ridge Tuff (diagonal lines) in relation 

to Ute Creek caldera (U) and San Juan volcanic field (shaded). 

wedge-shaped septum between the Ute Creek and Lost 

Lake calderas (fig. 5). Farther north, the northwestern wall 

of the Lost Lake caldera follows Lost Trail Creek for about 

5 km; this segment of the caldera is marked, on the west 

side of the creel~, by early, intermediate-composition 

andesitic rocks in the wall of the caldera and, on the east 

side, by caldera-filling rocks. No constraints are known on 

the east side of the caldera, however, and the projected 

margin (fig. 5) is drawn simply to close out the eastern side 

of a circular area that conforms to the arcuate monocline 

and drainage pattern to the west. The Lost Lake caldera, 

like several others in the San Juan volcanic field, possibly 

may have subsided as a trapdoor block with an incomplete 

ring-fracture zone and hinged eastern margin. 

The oldest rocks exposed within the Lost Lakes caldera 

are the nearly identical Dillon Mesa and Sapinero Mesa 

Tuffs. These rocks form the lower parts of prominent cliffs 

along the Rio Grande, and are best exposed along the 

north side of the river opposite the mouth of Ute Creek. At 

this locality, the lower cliffs consist of Dillon Mesa Tuff; 

the upper 150m of this unit is well exposed but, as the base 

is everywhere below the level of the river, its total thickness 

is not known-although it probably greatly exceeds the0-

1 00 m of Dillon Mesa Tuff exposed on Pole Creek 

Mountain west of the caldera. The overlying Sapinero 

Mesa Tuff, which forms the upper cliffs, is about 130m 

thick, about the same as it is on Pole Creek Mountain. 

Evidently the Dillon Mesa ponded within and passively 

filled the caldera, and the succeeding Sapinero Mesa Tuff 

was deposited across it without appreciable thickening. 

The Lost Lake caldera cuts off the Ute Ridge Tuff (fig. 

5 ), and in turn was largely filled by the Dillon Mesa Tuff. 

These stratigraphic relations limit the time of caldera 

subsidence to the period during which the ash of the Blue 

Mesa Tuff was erupted. Of all the major ash-flow sheets in 

the upper Rio Grande area, only the Blue Mesa Tuff lacks 

an exposed source. The Blue Mesa is a widespread sheet 

(fig. 7) that pinches out against Precambrian rocks in the 

Needle Mountains on the south, extends westward to the 

eroded edge of the volcanic rocks near Telluride, and 

wedges out to the north against older volcanic rocks north 

of the Gunnison River, more than 80 km north of Ll-J.e 

upper Rio Grande area. To the east, the Blue Mesa Tuff is 

covered by younger rocks and its distribution is unknown 

(Steven, Lipman, Hail, and others, 1974). The Blue Mesa 

Tuff and older rocks are sufficiently well exposed north 

and west of the upper Rio Grande area that a caldera 

source can be eliminated in most of these areas. The source 

therefore most likely lies within the covered part of the 

sheet-and thus in the Lost Lake caldera. 

0 100 Kl LOMETRES 

FIGURE 7.-Distribution of Blue Mesa Tuff (diagonal lines) in relation 

to Lost Lake caldera (L,) and San Juan volcanic field (shaded). 

The Blue Mesa Tuff is a uniform-textured, moderately 

phenocryst poor, densely welded ash-flow tuff over most of 

its exposed extent. In the upper Rio Grande area, however, 

the phenocrysts are larger and more abundant than elsewhere, and the unit is weakly compositionally zoned, 

becoming more mafic upward. A rhyodacitic lava flow 

also occurs locally between the Blue Mesa and the over-
10 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

1 ying Dillon Mesa Tuff on the north side of Pole Creek 

Mountain and just west of the arcuate monocline; this is 

the only known locality of lava-flow activity at this 

horizon. These areally restricted variations suggest a 

nearby source for the Blue Mesa Tuff. 

No evidence was seen for resurgent doming of the Lost 

Lake caldera after subsidence. However, only the upper 

parts of the caldera fill can be seen, and the oldest rocks 

exposed are along the margins of the caldera, where the 

effects of resurgence would have been minimal. Thus, a 

low dome could exist a depth and not be reflected by the 

younger rocks that cover the caldera area. 

SAN JUAN, UNCOMPAHGRE, AND SILVERTON CALDERAS 

The geologic history of the caldera complex in the 

western San Juan Mountains has recently been reinterpreted by Lipman, Steven, Luedke, and Burbank (1973), 

and the discussion here is taken largely from them. The 

western San Juan Mountains were the site of a cluster of 

intermediate-composition central-vent volcanoes in early 

Oligocene time, 35-30 m.y. ago. The near-source facies of 

these volcanoes consists of complex accumulations of 

lavas, breccias, and pyroclastic debris, which pass laterally 

into coalescing volcaniclastic aprons consisting predominantly of mudflow breccias and, at the margins, of 

conglomeratic and other stream-worked debris. The 

clustered volcanoes formed part of the great field of early 

intermediate-composition volcanic rocks that covered 

much of the southern Rocky Mountains in early 

Oligocene time (Lipman and others, 1970; Steven and 

Epis, 1968; Steven, 1975). 

Ash-flow sheets from nearby sources at the Ute Creek 

and Lost Lake calderas covered the lower flanks and 

coalescing outflow aprons of these volcanoes, and wedged 

out against highlands built up around the central vents. 

UNCOMPAHGRE AND SAN JUAN COLLAPSES 

The volcanic. cycles that led to the development of the 

San Juan, Uncompahgre, and Silverton calderas probably 

began when the relatively small volume of material 

constituting the Dillon Mesa Tuff (table 1; fig. 8) was 

erupted shortly before 28 m.y. ago. No direct evidence can 

be marshalled to tie this eruption with any specific 

increment of subsidence, but the unit seems to be radially 

distributed around the area of the Uncompahgre caldera, 

and is thickest and most densely welded near the margins 

of this possible source area. Renewed eruptions of similar 

rhyolitic ash-:flow tuff material about 28 m.y. ago led to 

widespread emplacement of the Sapinero Mesa Tuff, and 

to simultaneous collapse of the Uncompahgre and San 

Juan calderas (fig. 9). 

The Sapinero Mesa Tuff spread widely (fig. 10) from its 

source area at these calderas, and had an estimated volume 

in excess of 1,000 km3• It has been tracednortheastwardforJ 

more than 90 km, northward 65-70 km, and south0 100 KILOMETRES 

FIGURE B.-Distribution of Dillon Mesa Tuff (diagonal lines) in relation to Uncompahgre caldera (U) and San Juan volcanic field 

(shaded). 

eastward 40-45 km. Apparently it wedged out southward 

against the older highland of Precambrian rocks in the 

Needle Mountains. The western flank of the volcanic pile 

has been removed by erosion, and the original extent of the 

Sapinero Mesa Tuff in this direction is not known. 

Outflow Sapinero Mesa Tuff is generally less than 100m 

thick, whereas the intracaldera Eureka Member is at least 

700 m thick in places and the base is only locally exposed. 

This disparity in thickness between outflow and intracaldera facies is common among the San Juan calderas, 

and is interpreted to indicate subsidence concurrent with· 

eruption; the intracaldera accumulation is generally 

somewhat younger than most of the outflow sheet. 

Approximate contemporaneity of subsidence of the 

Uncompahgre caldera and eruption of the Sapinero Mesa 

Tuff can be demonstrated convincingly on the basis of 

geologic evidence (Lipman and others, 1973), and similar 

general relations of intracaldera and outflow 

accumulations of Sapinero Mesa Tuff for the San Juan 

caldera imply a similar timing. Subsidence of the two 

calderas was accompanied by widespread landsliding and 

avalanching of the steep caldera walls, and abundant 

debris from these sources is interleaved marginally with 

the intracaldera ash-flow accumulations (Eureka 

Member) in both calderas. By this process the topographic 

walls of the two calderas actually merged locally to form a 

single depression with a distor!ed dumbbell shape: the two 

main subsided blocks were joined across a low divide when 

the septum between them largely caved away (fig. 9). 

The simultaneous development of adjacent but separate 
WESTERN SAN JUAN CALDERA COMPLEX 11 

0 5 10 15 Kl LOMETRES 

FIGURE 9.-Sketch map of the western San Juan caldera complex after subsidence related to eruption of the Sapinero Mesa Tuff. Control 

moderate to good where boundaries are shown by solid symbols; conjectural where shown by open symbols. 

calderas was followed by merged later stages of the local 

caldera cycles. The calderas were resurgently domed 

together over an extended period of time, while the 

depressions were being filled with lavas and sediments 

from local sources and with ash-flow tuffs from concurrently developing calderas to the east. Renewed ashflow eruptions forming the Crystal Lake Tuff during this 

period of resurgence and caldera filling led to subsidence 

of a local trapdoor block, the Silverton caldera, nested 

within the older San Juan caldera. These generally concurrent developments will be discussed separately in the 

folbwing paragraphs, although the events are closely 

related and are manifestations of the same basic volcanic 

processes. 

POSTCOLLAPSE LAVAS AND SEDIMENTS 

The first postsubsidence eruptions within the San Juan 

and Uncompahgre calderas were mostly of viscous, 

coarsely porphyritic lavas of predominantly rhyodacitic 

and quartz-latitic composition. These heaped up in the 

vicinity of their vents to form prominent local domes and 

thick flows, surrounded by lower areas where pyroclastic 

and reworked debris accumulated as bedded tuffs and 

volcaniclastic sediments. Some of these lower areas 

apparently were the sites of local ponds in which finely 

stratified deposits formed. This assemblage of thick 

porphyritic flows and associated bedded deposits is best 

displayed within the San Juan caldera, where it has been 

called the Burns Formation. It can be recognized as a 
12 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

0 100 KILOMETRES 

FIGURE 10.-Distribution of Sapinero Mesa Tuff (diagonal lines) in 

relation to San Juan (S) and Uncompahgre (U) calderas and San 

Juan volcanic field (shaded). 

distinctive assemblage across the low divide into the 

northwest part of the Uncompahgre caldera; eastward, 

sedimentary rocks predominate and there are only minor 

porphyritic lavas. 

The lavas filling the depression changed in 

composition upward to dark fine-grained andesite flows. 

These lavas were appreciably more fluid than the viscous 

porphyries of the Burns Formation, and the resulting 

flows are thinner and more widespread. Sedimentation in 

low areas continued, and the resulting bedded rocks are 

indistinguishable from many of those in the underlying 

Burns Formation. Local volcanic activity diminished later 

in the period of infilling, and volcaniclastic sedimentary 

rocks compose the upper layers of fill. The sequence of 

predominant andesitic flows grading upward into predominant sedimentary beds is best displayed in northeastern parts of the San Juan caldera, where the flows were 

originally called the pyroxene andesite unit and the overlying bedded rocks the Henson Tuff (Cross and others, 

1905, 1907). Later, Luedke and Burbank (1963) combined 

both of these units into a redefined Henson Formation. 

Across the north side of the Uncompahgre caldera, the 

proportion of andesite flows to sedimentary rocks 

decreases eastward, and in the northeast part of the 

Uncompahgre caldera, the whole section generally 

equivalent to the Burns and Henson Formations consists 

largely of similar-appearing sedimentary rocks. 

Rocks equivalent to the caldera-filling Burns and 

Henson Formations are found outside the calderas only on 

the southeast side, where andesitic flows and breccias 

overlie the Sapinero Mesa Tuff and underlie younger ashflow units derived from sources to the east. 

INTRACALDERA ASH-FLOW TUFFS 

The Burns and Henson rocks forming the bulk of the fill 

i~ the San Juan and Uncompahgre calderas are overlain 

by a sequence of ash-flow tuffs, mostly from caldera 

sources in the central San Juan Mountains, interlayered 

with locally derived sedimentary and volcanic rocks. The 

basal unit of this upper sequence is the Fish Canyon Tuff, 

derived from the La Garita caldera source area about 27.8 

m.y. ago. Overlying ash-flow units consist in sequence of 

the Crystal Lake Tuff (derived from the Silverton caldera), 

the Carpenter Ridge Tuff (from the Bachelor caldera), 

Wason Park Tuff (caldera source not identified), and 

Nelson Mountain Tuff (from the San Luis caldera). 

Locally derived sediments closely similar to those in the 

Henson and Burns Formations separate all these ash-flow 

units at one place or another, and locally derived lavas of 

generally intermediate composition intervene between the 

Crystal Lake and Carpenter Ridge Tuffs, the Carpenter 

Ridge and Nelson Mountain Tuffs, and overlie the Nelson 

Mountain Tuff within and adjacent to the Uncompahgre 

caldera. 

SILVERTON COLLAPSE 

Eruption of the Crystal Lake Tuff (fig. 11) during the 

period of caldera filling resulted in trapdoor subsidence of 

the Silverton caldera within the older San Juan caldera 

(fig. 12). A block 15 km across, consisting in its upper parts 

of thick sections of Burns and Henson lavas and 

sediments, was displaced more than 600 m downward 

along its southern margin, whereas a sharp monocline cut 

by a deeply infaulted graben marks its northeastern 

margin. Even in the present deeply dissected topography, 

the core of the trapdoor block stands nearly as high as the 

ash-flow deposits in the adjacent terrain, suggesting that 

no great thickness of Crystal Lake Tuff could ever have 

existed there. This suggestion accords with relations noted 

at other lesser calderas in the San Juan volcanic field 

related to small- to moderate-volume ash flow eruptions, 

where subsidence usually followed eruption and in places 

did not form complete ring-fracture zones. 

RESURGENT DOMING 

Resurgence of the San Juan, Uncompahgre, and 

Silverton calderas differed in several important aspects 

from resurgence at many of the other calderas in the San 

Juan volcanic field. Although it apparently began shortly 

after subsidence, uplift of the volcanic source areas 

probably continued for more than a million years through 

the period of infilling by locally derived lavas and 

sediments and by ash-flow tuffs from extraneous sources. 

As uplift continued rhyolitic magma regenerated beneath 

the San Juan caldera and erupted catastrophically as the 

ash-flows of the Crystal Lake Tuff, with related sub-
WESTERN SAN JUAN CALDERA COMPLEX 13 

0 100 KILOMETRES 

FIGURE 11.-Distribution of Crystal Lake Tuff (diagonal lines) in relation to Silverton caldera (S) and San Juan volcanic field (shaded). 

sidence of the Silverton caldera. Structures that formed 

during this later subsidence appear to merge with some of 

the structures formed by uplift and seem to have developed 

concurrent! y. 

Early resurgence is indicated by local angular unconformities between slightly tilted and eroded welded tuffs in 

the intracaldera Eureka Member of the Sapinero Mesa 

Tuff and overlying intermediate-composition lavas in the 

Burns Formation, as first noted by Luedke and Burbank 

( 1968, p. 183 ). The extent of this early resurgence cannot be 

determined from the sparse evidence now available. Later 

magmatic uplift is indicated by the broad doming of the 

whole area of the San Juan and Uncompahgre calderas 

that affected the younger caldera fill as well. This broad 

uplift apparently was episodic, inasmuch as the ash-flow 

units overlying the Burns and Henson Formations appear 

to have been confined to a moat area around at least the 

eastern and northern margins of the uplifted core of older 

rocks, and yet in turn were tilted by further uplift of the 

core. 

Longitudinal distention fractures along the crest of the 

broad domal uplift formed the deeply infaulted Eureka 

graben, which extends northeastward from the San Juan 

caldera into the uncollapsed septum between the San Juan 

and Uncompahgre calderas, where it is cut off abruptly by 

the much younger Lake City caldera. This graben offsets 

the caldera-filling Burns and Henson Formations as much 

as 0.5 km along the major faults, and, in places, graben 

faults cut small remnants of the younger Crystal Lake 

Tuff. 

At it's southwest end, the trend of the Eureka graben 

changes sharply, turning from northeast to northwest and 

giving the downfaulted area the distinctive bootlike shape 

shown in figure 12 (Burbank, 1951; BurbankandLuedke, 

1969). The intersection of the northeast-trending 

Sunnyside fault and northwest-trending Ross Basin fault 

at the instep of the boot is intensely mineralized and is the 

site of the highly productive Sunnyside mine. This area 

has been studied in detail by many geologists and is well 

illustrated on a geologic map by Burbank and Luedke 

( 1969, pl. 2). The fractures and veins pass smoothly around 

a sharp right-angle bend, and seem to have formed concurrently. To the northwest, as shown in the same 

illustration, several important veins and faults begin at the 

footwall of the Ross Basin fault as northeast-trending 

fractures generally parallel to the leg of the boot, but after 

several kilometres they swing sharply east and southeast to 

parallel the instep of the boot. This change in direction 

would again seem to require coexistent distentional 

stresses related to both major trends of the Eureka graben. 

Whereas the northeast-trending fractures are along the 

crest of the broad dome of the simultaneously uplifted San 

Juan and Uncompahgre calderas, the northwest-trending 

fractures that define the foot of the boot are within the 

monoclinal hinge area along the north side of the younger 

Silverton caldera. The northwest-trending fractures thus 

appear to represent distentional fractures related to 

trapdoor subsidence of this block. Thus, in order for these 

different fault systems to have merged so completely, 

eruption of the Crystal Lake Tuff and subsidence of the 

Silverton caldera must have been concurrent with a major 

episode of uplift of the older calderas. Later or continued 

uplift of the broad dome is suggested by several minor 

northeast-trending faults that cut the foot of the Eureka 

graben boot (Burbank and Luedke, 1969, pl. 2). 

Except for i~ having been formed during the period of 

general uplift, the Silverton caldera itself shows no 

evidence of beihg resurgently domed. Development of the 

spectacular radial and other intricate vein and dike 

patterns just outside the northwest and southeast margins 

of the Silverton caldera (Burbank, 1941; Varnes, 1963) may 

have resulted from this special stress environment. No 

other San Juan caldera has similar intricate patterns of 

fracture-related structures. 

LAKE CITY CALDERA 

After about a 4-m.y. period of greatly reduced volcanic 

activity in the western San Juan Mountains, ash-flow 

eruptions that produced the Sunshine Peak Tuff began 

about 22.5 m.y. ago in the Lake City area (Lipman and 

others, 1973; Mehnert and others, l973a). Outflow 

Sunshine Peak Tuff must have been deposited widely in 

the region, although only small erosional remnants have 

survived. Concurrently with these eruptions, an elliptical 

block about 12 by 15 km across, nested within the southern 
14 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

380 

General form of 

resurgent dome 

EXPLANATION 

?- Fault-Hachures on downthrown 

side. Queried where restored 

0 5 10 15 Kl LOMETRES 

FIGURE 12.-Sketch map of the western San Juan caldera complex after subsidence of the Silverton caldera and general resurgence. 

Control moderate to good where boundaries are shown by solid symbols; conjectural where shown by open symbols. 

part of the older Uncompahgre caldera, subsided to form 

the Lake City caldera (fig. 13 ). The last-erupted part of the 

Sunshine Peak accumulated to a thickness of as much as 1 

km within the concurrently subsiding caldera, and great 

landslide avalanches caved off the caldera wall, .spread 

across the caldera floor, and intertongued with the 

accumulating ash flows. 

The ring fault along which this collapse occurred is 

continuously exposed for about 300° of arc around the 

caldera; it is nearly everywhere a single fault, typically 

marked by only a metre or so of gouge and minor hydrothermally altered rock. It is exposed over topographic 

relief of as much as 600 m, and is everywhere steep, 

dipping inward from 75° to nearly vertical. 

Shortly after the ash-flow eruptions ceased, lava flows 

and domes of viscous silicic quartz latite, fed largely from 

vents along the ring fault, accumulated around >the 

margins of the caldera floor. These lavas tend to overlie the 

Sunshine Peak Tuff conformably and dip quite steeply in 

places. They probably largely predate resurgent doming, 

although the effects of doming on them are difficult to distinguish reliably from primary dips of the flows. An eastnortheast-trending line of rhyolite intrusives was 

emplaced in the moat area of the older Uncompahgre 

caldera, north of the Lake City caldera. These rocks are 

closely similar in composition to the Sunshine Peak Tuff, 

but apparently were emplaced somewhat more recently, 

with one intrusive yielding a K-Ar age of about 18 m.y. (H. 

H. Mehnert, written commun., 1974). 
WESTERN SAN .JUAN CALDERA COMPLEX 15 

Postcaldera basalts 

~=r'""V"'~ 

lcK" =? ~ ~=<f" v?, ~~ 

~ ~ ~~=~~ "' Intrusive rocks related tf1 """"'' ~ 

to Lake City caldera ~,/! y UNCOMPAHGRE ', 

~A f CALDERA 

~~~ t' ff J-Ianson Creek 

Topographic rim ~c::<f~ ~ 

c::<t-x ' 

fi'~ 

~ 

#'#',;:? 

I // 

} / 

j / 

I I 

~c::<t 

~ j # Sunshine Peak Tuff 

~~ f U SILVERTON .11 

~ CALDERA F 

~ If 

\ 1: p~ 

\ / / tf 

' ~""""' / f '\, ~"""' === ="""/ ~ ~= ~~ =""""' r ~ Structural margin di' 

~ cb>r ~~ -~d!= ""'~ cb" ~ EXPLANATION 

0 5 10 15 Kl LOMETRES 

Fault-Hachures on downthrown 

side, where known 

FIGURE 13.-Generalized geology of the western San Juan caldera complex showing distribution of rocks related to the Lake 

City caldera. 

Resurgence of the Lake City caldera produced a simple 

dome characterized by outward dips of 20°-25° on its flanks 

and a northeast-trending apical graben over its distended 

crest. The trend of this graben reflects reactivation of the 

trends of the earlier Eureka graben system, which was 

related to resurgence of the Uncompahgre and San Juan 

calderas. Most of the mapped faults of the Lake City 

resurgent structure have relatively small displacements-10-50 m-and some seem to be little more 

than cracks that localized weak hydrothermal alteration. 

Chaotic caldera -collapse breccias are widely exposed 

below the intracaldera tuffs on the southwest side of the 

Lake City caldera, and the resurgence appears to have been 

somewhat asymmetrical, with maximum uplift in this 

area. Resurgence resulted from upward movement of 

magma which crystallized as a shallow stock of granite 

porphyry. 

Gently dipping upper contacts of the granite porphyry 

are exposed at the bottoms of several deep erosional valleys 

within the core of the Lake City caldera, and the most 

intensely altered rock within the resurgent dome occurs 

around margins of the granite porphyry. Parts of the 

northern ring fault are occupied by a short, thick, 

discontinuous ring dike, which broadens and becomes 

coarser grained downward. The top of the granite 

porphyry is within l km of the top of the Sunshine Peak 

Tuff indicating crystallization at very shallow depths 

beneath the resurgent dome. 

Most of the original topographic wall of the Lake City 

caldera has been eroded, and only a few small remnants are 
16 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

preserved southwest of Lake City. The present valleys of 

Henson Creek and the upper Lake Fork of the Gunnison 

River, which define a striking elliptical drainage pattern 

just outside the structural boundary of . the Lake City 

caldera, are not directly controlled by any major fault 

structures. They more likely developed in debris that 

accumulated in a low topographic moat along the margin 

of the resurgent dome of the Lake City caldera outside the 

limits of the subsided block, and were subsequently superimposed onto older rock$ of the caldera wall. 

CENTRAL SAN JUAN CALDERA COMPLEX 

The central San Juan caldera complex developed above 

the eastern part of the gravity anomaly (fig. 1) between 28.2 

and 26.5 m.y. ago. Within this brief span of less than 2 

m.y., eight major ash-flow sheets were erupted, at least 

seven calderas formed, and numerous local volcanic rock 

units accumulated. These caldera cycles thus had an 

average duration of only about a quarter of a million 

years, and in at least one case, the cycles of two nearby 

calderas overlapped in time. 

We believe that these intense pyroclastic eruptions and 

resulting caldera subsidences marked the culmination of 

upward movement of magma to form the eastern part of 

the underlying batholith. As the roof above this segment of 

the batholith became progressively thinner, it was 

breached by numerous ash-flow eruptions of great 

magnitude, and related calderas subsided at the ash-flow 

source areas. The development of the numerous clustered 

and partly overlapping calderas that followed one another 

in rapid succession, and the common marked contrasts 

between lithologies-ranging from phenocryst-rich 

quartz hitites to phenocryst-poor rhyolites-of successive 

ash-flow deposits, required rapid congealing of the upper 

parts of the local cupolas and equally rapid reestablishment of new cupolas and magmatic differentiation within 

them. The pyroclastic eruptions and caldera subsidences 

apparently ceased when the upper part of the batholith 

congealed to a thickness sufficient to contain the 

magmatic pressures. 

MOUNT HOPE CALDERA 

The Mount Hope caldera (fig. 14) is almost completely 

buried by younger rocks, primarily the Fish Canyon Tuff, 

but fragmentary evidence suggests that it was the source of 

two phenocryst-rich quartz latite ash-flow sheets of the 

Masonic Park Tuff. The lower sheet has been recognized 

only within 10-15 km of the caldera, but the upper sheet 

(fig. 15) extends 30 km west of the caldera, to the margin of 

the San Luis Valley 50 km east, and into New Mexico 75 

km southeast (Steven, Lipman, Hail, and others, 1974). 

The topographic wall along the north margin of the 

Mount Hope caldera is exposed for about 6 km. This wall 

cuts sharply across the lower ash-flow sheet of the Masonic 

Park Tuff, andesitic flows and breccias of the Sheep 

Mountain Andesite, thick quartz latite flows of the 

volcanics of Leopard Creek, and the upper ash-flow sheet 

of the Masonic Park Tuff. Total relief along the exposed 

section of this wall is about 500 m. The caldera is filled 

with Fish Canyon Tuff that is at least 1.4 km thick and 

extends over adjacent rocks on the south, east, and northeast sides of the caldera. Adjacent to the caldera, the Fish 

Canyon Tuff generally rests directly on the upper sheet of 

the Masonic Park Tuff. 

The exposed segment of the topographic wall indicates 

that the caldera subsided after the ash flows forming the 

widespread upper sheet of Masonic Park Tuff were 

erupted (28.2 m.y. ago), and the basin formed by subsidence was in turn filled passively by the next succeeding 

major ash-flow sheet, the Fish Canyon Tuff (27.8 m.y. 

old). In addition to these stratigraphic constraints on the 

age of the caldera, other evidence, as follows, indicates that 

the Masonic Park Tuff was derived from a source within 

the Mount Hope caldera: ( l) the position of the caldera 

well within the Masonic Park ash-flow sheet (fig. 15) and 

adjacent to the thickest sections of this formation; (2) intertonguing of the Mas0nic Park Tuff with andesite lavas of 

the Sheep Mountain Andesite and with other locally 

derived lavas near the rim of ~he Mount Hope caldera; and 

(3) the presence near the south and southwest edges of the 

caldera of bedded welded tuffs at the base of the upper ashflow sheet of the Masonic Park that are believed to 

represent agglutinated ash fall, a depositional process 

only likely close to the eruptive source (Lipman, l975a, 

fig. 26). 

An earlier complex history of eruptions from the same 

general source is indicated by scattered exposures northwest and west of the Mount Hope caldera and in the highly 

faulted area southwest of the caldera (fig. 14). In these 

areas, the several individual ash flows within the 

composite older ash-flow sheet of Masonic Park Tuff 

intertongue with dark andesite flows and breccias of the 

Sheep Mountain Andesite that apparently formed a 

number of local volcanoes near the western margin of the 

later Mount Hope caldera. Just northwest of the Mount 

Hope caldera, th~ intertongued assemblage of Masonic 

Park Tuff and Sheep Mountain Andesite is overlain by 

thick coarsely porphyritic quartz latite lavas (volcanics of 

Leopard Creek), and these in turn are overlain by the thick 

upper sheet of Masonic Park Tuff (Steven and Lipman, 

1973). 

We interpret these relations to indicate early concurrent 

eruptions of quartz latitic ash flows of Masonic Park Tuff 

and andesitic lavas and breccias from local Sheep 

Mountain volcanoes. The ash-flow eruptions were 

apparently of small volume, and it is not known whether 

they were accompanied by caldera subsidence. Viscous 

quartz-latitic lavas forming a thick local assemblage 

(volcanics of Leopard Creek) were erupted along the 
CENTRAL SAN JUAN CALDERA COMPLEX 17 

Younger volcanics 

0 

Older volcanics 

0 

0 5 10 15 KILOMETRES 

FIGURE 14.-Generalized geology of the Mount Hope caldera and adjacent areas. o. precaldera (older) volcanic rocks; Tmp, 

Masonic Park Tuff; Tsm, Sheep Mountain Andesite; Tf, Fish Canyon Tuff; Th,, Huerto Formation; Tl, volcanics 

of Leopard Creek; Tc, Carpenter Ridge Tuff; y, volcanic rocks younger than Carpenter Ridge Tuff. Open rectangles 

outline the buried caldera. Heavy lines indicate faults; bar and ball or hachures on downthrown sides; dotted where 

buried. 

northwest side of the postulated source area following 

accumulation of the older Masonic Park ash-flow tuffs 

and Sheep Mountain andesitic flows and breccias. 

Renewed eruptions of quartz-latitic ash flows identical in 

composition and appearance to the older sheet of Masonic 

Park Tuff spread great volumes of tuff in all directions 

from the source area, thereby forming the widespread 

upper sheet of Masonic Park Tuff and resulting in subsidence of the Mount Hope caldera. 

The Mount Hope caldera was not resurgently domed 

immediately after subsidence to any discernible extent; the 

Fish Canyon Tuff filled a hole more than 1.4 kmdeepand 

the floor of the fill is not exposed. Later broad uplift of the 

caldera area is indicated by the progressive overlap of 

younger ash-flow units onto the eastern side of the caldera 

fill (fig. 14). This uplift apparently was more a trapdoor 

uplift than a dome, with the western side preferentially 

uplifted. This later uplift probably was roughly concurrent with major faulting west and southwest of the 

Mount Hope caldera that will be discussed later. The 

Mount Hope caldera forms a type generally transitional 

between the earlier and later calderas of the San Juan field. 

Mount Hope is like the earlier calderas in its associated 

andesitic volcanism and the quartz-latitic composition of 
18 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

0 100 KILOMETRES 

FIGURE 15.--"-Distribution of the upper member of Masonic Park Tuff 

(diagonal lines) in relation to Mount Hope caldera (M) and San 

Juan volcanic field (shaded). 

the erupted tuffs; yet this caldera is well within the main 

gravity anomaly (fig. 1) and is associated with more silicic 

lava flows, as represented by the volcanics of Leopard 

Creek. 

LA GARITA CALDERA 

The La Garita caldera (fig. 16), the largest in the San 

Juan volcanic field, ranks among the great calderas in the 

world. The original topographic basin was at least 40 km 

across from north to south, and the subsided block within 

this basin appears to have been nearly 30 km across. The 

original east-west diameter of the caldera was somewhat 

less, but the precise dimensions cannot be established 

because the western part of the La Garita caldera has since 

been destroyed by later caldera subsidences. 

The La Garita caldera is located near the center of the 

San Juan volcanic field, above the east-central part of the 

triangular-shaped batholith indicated by gravity data (fig. 

1). Subsidence took place 27.8 m.y. ago (Lipman and 

others, 1970, p. 2340), in response to eruption of more than 

3,000 km3 of the phenocryst-rich quartz-latitic ash of the 

Fish Canyon Tuff (fig. 17). This unit spread over an area of 

more than 15,000 km2; it is 30-200 m thick over wide areas 

and accumulated to a thickness of more than 1.4 km 

within the concurrently subsiding caldera. 

Whereas many of the older San Juan calderas formed 

near clusters of earlier andesitic volcanoes (the Bonanza, 

Platoro-Summitville, San Juan, Uncompahgre, and 

Mount Hope calderas), any such relationship is more 

difficult to determine for the La Garita caldera because of 

its size. The intrusive core of an older volcano is exposed 

along the eastern wall of the La Garita caldera, ;:md other 

volcanic centers are near the east, west, and north sides of 

the caldera, but exposures are inadequate to indicate 

whether these volcanoes were concentrated near the 

caldera. 

A more important localizing factor may have been the 

position of the La Garita caldera with respect to the underlying batholith. The caldera occupies much of the roof 

above the broadest part of the batholith indicated by the 

gravity anomaly (fig. 1 ); this part of the roof was repeatedly disrupted by ash-flow eruptions and caldera subsidences throughout the less than 2 m.y. that remained of 

the period of intermediate to silicic volcanic activity. The 

recurrent volcanic activity and related subsidences here 

must have reflected equally active high-level magmatism 

throughout the eastern part of the batholith. 

Little can be reconstructed of the early development of 

the La Garita caldera. The floor of the caldera is nowhere 

exposed, and the eastern and northern margins that 

survived later caldera subsidences are marked by a 

structural moat deeply filled with younger volcanic 

deposits. Little air-fall ash occurs below the outflow sheet, 

so premonitory eruptions seem to have been minor. In 

addition, no compositional zoning was noted anywhere in 

the outflow sheet of Fish Canyon Tuff, and the basal rocks 

seem indistinguishable from those elsewhere in the unit. 

Apparently, large volumes of ash were erupted quite 

suddenly from a major chamber containing relatively 

homogeneous phenocryst-rich quartz-latitic magma. 

The disparity in thickness between ·the intracaldera 

(more than 1.4 km thick) and outflow parts (generally less 

than 200m thick) of the Fish Canyon Tuff requires subsidence concurrent with eruption. Compaction foliation 

and rude layering, marked in places by less-welded 

partings, are virtually parallel throughout the caldera fill, 

and indicate that block subsidence was concurrent with 

filling. 

Little igneous activity or sedimentation seems to have 

occurred immediately after subsidence, and the first 

demonstrable event was broad resurgent doming of the 

core. The crest of the dome was relatively flat over an area 

at least 5 km across, and the preserved eastern and northeastern flanks dip 5°-15° radially outward from the crest. 

Minor tensional faults separate flat from inclined 

segments of the core, and also separate inclined segments 

that dip in different directions. Maximum relief from the 

northeast moat to the crest of the dome was more than 1.4 

km. The western and southwestern flanks of the resurgent 

dome have since caved into younger calderas, and have 

been covered by younger volcanic units. 

Most of the moat is deeply filled by younger volcanic 

rocks, or has been destroyed by younger caldera subsidences. The lower fill, along the northeast side of the 
CENTRAL SAN JUAN CALDERA COMPLEX 19 

caldera, consists of Carpenter Ridge Tuff, in places 

showing evidence of having accumulated in shallow 

water. Minor tuffaceous sedimentary rocks exposed locally 

in this area probably represent deposition by small streams 

or in local ponds. 

No late ring-fracture igneous activity is evident along 

the eastern or northern sides of the La Garita caldera, but 

the major rhyolite dome along Miners Creek, partly 

exposed in' a deep canyon 6 km west of Creede (Steven and 

Raw~, 1965), is older than the Carpenter Ridge Tuff, and 

may have erupted through the western ring-fracture zone 

of the La Garita caldera. 

The eastern wall of the La Garita caldera apparently 

stood especially high, and most subsequent units from the 

central San Juan caldera complex were trapped against it. 

Elsewhere, the young~r ash flows overtopped the La 

Garita caldera rim and spread widely. This high part of 

the caldera wall marks the truncated end of a line of major 

older andesitic volcanoes that extended west-northwest 

from the Summer Coon center 10 km north of Del Norte 

(Lipman, 1968), through the Baughman Creek and upper 

La Garita Creek centers, to the Sky City center exposed in 

the eastern topographic wall of the caldera (fig. 16). These 

volcanoes were deeply eroded prior to eruption of the Fish 

Canyon Tuff, but protruded through all but the youngest 

subsequent ash-flow units. 

BACHELOR CALDERA 

The Bachelor caldera, the source of rhyolite ash flows 

that formed the widespread Carpenter Ridge Tuff (fig. 18), 

collapsed along the west margin of the La Garita caldera 

(fig. 16). The age of the Carpenter Ridge Tuff and of the 

Bachelor caldera is bracketed by K-Ar ages of 27.8 m.y. for 

the older Fish Canyon Tuff and 26.7 m.y. for the younger 

Mammoth Mountain Tuff. 

The outflow Fish Canyon and Carpenter Ridge Tuffs 

are separated by andesite flows and breccias of the Huerta 

Formation that formed a series of volcanoes near the 

present edge of volcanic rocks south and southwest of the 

Bachelor caldera. After eruption of the andesites, the area 

southwest of the Mount Hope caldera (fig. 14) was 

intricately downfaulted toward the caldera complex, as 

described later. During the Huerta eruptions and later 

faulting, magma accumulated in a newly developed highlevel chamber beneath the west side of the La Garita 

caldera, and differentiated to a silicic phenocryst-poor 

rhyolite which erupted to form the Carpenter Ridge Tuff. 

Southeast of the caldera, the Carpenter Ridge is locally 

compositionally zoned from rhyolite upward into quartz 

latite. 

Eruption of the Carpenter Ridge Tuff was accompanied 

by subsidence of an oval-shaped caldera about 15 by 25 km 

across. Tuff more than 1.5 km thick accumulated within 

the subsiding caldera, whereas the outflow sheet generally 

is less than 400 m thick. As in other calderas in the San 

Juan Mountains, these relations are believed to indicate 

subsidence concurrent with eruption. 

The floor and eastern wall of the Bachelor caldera are 

exposed locally northeast of Creede. In this area, a rough 

fault-block topography marked by tectonically shattered 

rocks, fault scarps, and talus breccia developed on the 

older rocks (Steven and Ratte': 1965, p. 17). These fault 

blocks underlie the Bachelor caldera fill and form the 

eastern caldera wall, where they mark the progressive 

breaking down of the resurgent core of the La Garita 

caldera toward the younger subsidence feature to the west. 

In part these fault blocks may represent inward sliding of 

the Bachelor caldera wall toward the main ring-fracture 

zone during Carpenter Ridge eruptions. 

Similar relations on the west side of the caldera may be 

represented by the intertonguing of Shallow Creek Quartz 

Latite with the intracaldera Bachelor Mountain Member 

of the Carpenter Ridge Tuff, as described by Steven and 

Ratte(l965, p. 23). This intertonguing was mapped in the 

early 1950's before any of the calderas in the central San 

Juan Mountains had been recognized. In retrospect it 

seems plausible that the layers of Shallow Creek Quartz 

Latite within the Bachelor caldera fill may instead be 

avalanche and landslide breccia derived from a major 

quartz latite dome (fig. 8) that formed the nearby oversteepened caldera wall, a relationship which has been 

found in many other intracaldera fills in the San Juan 

Mountains. This suggestion needs to be checked in the 

field. 

Three small ash flows resembling Fish Canyon Tuff 

appear to intertongue with the intracaldera Bachelor 

Mountain Member of the Carpenter Ridge Tuff on the east 

side of the Bachelor caldera (Steven and Ratte'; 1965, p. 18). 

These relations, if valid, indicate that neighboring 

cupolas on the underlying batholith were sufficently 

locally restricted to permit contrasting magmas to coexist 

locally and earlier magma types to persist 'Yhile the 

Carpenter Ridge rhyolite was differentiating. 

The core of the Bachelor caldera was res urgently domed 

shortly after subsidence, and a fragmented cross section of 

the dome can be seen in the north wall of the younger 

Creede caldera. The top of the Carpenter Ridge Tuff is 

more than 700 m higher at the crest of the dome than it is 

near the eastern and western ring-fracture zones. 

Tensional fractures that formed during doming dropped 

local blocks nearly 500 m near the crest of the uplift, and 

longitudinal normal faults, including the ancestral 

Amethyst fault in the Creede mining district (Steven and 

Ratte'; 1965, p . 55-56), extended north-northwest down the 

axis of the uplift. These faults marked the initial 

fracturing of an area that was recurrently broken during 

later volcanic episodes, and was widely mineralized at the 

time when the ores of the Creede mining district were 

deposited. 
20 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

380 

0 5 10 15 Kl LOMETRES 

FIGllRE 16.-Restored Bachelor and La Garita calderas. Control moderate to good where boundaries are shown 

by solid symbols; conjectural where shown by open symbols. 

Small plugs of rhyolite cut the intracaldera tuffs near 

the center of the Bachelor caldera. These plugs are most 

abundant in the rocks that are cut by tensional faults and 

that were pervasively brecciated during resurgent doming. 

Evidence is ambiguous, but the rhyolite plugs may have 

been emplaced before brecciation and, thus, prior to 

resurgence. No postresurgence igneous activity related to 

the Bachelor caldera cycle has been recognized. 

The shattered intracaldera Bachelor Mountain Member 

of the Carpenter Ridge Tuff on the resurgent dome was 

altered during the waning stages of the Bachelor caldera 

cycle. The shattered tuff was intensely silicified and 

largely healed to a massive rock. The K20 content of the 

silicified rock increased markedly from about 5 percent to 

as much as 11 percent(Ratte and Steven, 1967, p. H13); this 

increase is reflected by abundant very fine grained 

orthoclase in the matrix. In the northern part of the 

Bachelor caldera, the intracaldera tuffs were variably 

bleached and altered, and disseminated pyrite was 

erratically introduced. No significant economic mineral 

deposits related to the Bachelor caldera cycle are known, 

however. 

MAMMOTH MOUNTAIN(?) CALDERA 

Indirect evidence suggests that subsidence took place 

during eruption of the Mammoth Mountain Tuff, but the 
CENTRAL SAN JUAN CALDERA COMPLEX 21 

0 100 KILOMETRES 

FIGURE 17.-Distribution of the Fish Canyon Tuff (diagonal lines) in 

relation to La Garita caldera (G) and San Juan volcanic field 

(shaded). 

postulated caldera has been largely destroyed by the 

younger Creede caldera and covered by younger rocks. 

The Mammoth Mountain eruptive cycle began with 

local episodic pyroclastic eruptions that formed a 

sequence originally called Farmers Creek Rhyolite by 

Steven and Ratte (1964, 1965) and Ratte and Steven (1967); 

subsequent regional work has indicated that these rocks 

were primarily local products of premonitory eruptions in 

the Mammoth Mountain cycle (Steven, Lipman, Hail, and 

others, 1974; Steven and Ratte, 1973) that were deposited 

just northeast of the postulated source beneath the 

younger Creede caldera. The early episodic pyroclastic 

eruptions progressed into a pulsating, nearly continuous, 

ash-flow eruption that flooded (fig. 19) the lower parts of 

the rough topography left by subsidence and resurgence of 

the La Garita and Bachelor calderas (fig. 1 ). Most of these 

Mammoth Mountain ash flows welded into coherent, 

dense tuff that shows obscure compound cooling (Ratte 

and Steven, 1967, p. H18-H33). 

The Mammoth Mountain Tuff accumulated to a thickness of more than 500 m in the moat of the resurgent 

Bachelor caldera, where it is strongly zoned from 

phenocryst-poor rhyolite at the base to phenocryst-rich 

quartz latite at the top (Ratte and Steven, 1964, 1967). The 

Mammoth Mountain Tuff wedges out against and is 

absent over the top of the resurgent core of the Bachelor 

caldera, and was contained on the northeast and southwest by the outer topographic wall of the caldera. A 300- to 

400-m-thick sheet of densely welded quartz latite tuff of the 

0 100 Kl LOMETRES 

FIGURE lB.-Distribution of Carpenter Ridge Tuff (diagonal lines) in 

relation to Bachelor caldera (B:) and San Juan volcanic field (shaded). 

Mammoth Mountain Tuff extends southeast from the 

postulated source within the subsequently formed Creede 

caldera to the southeast topographic wall of the La Garita 

caldera where it wedges out abruptly. Remnants of crystalrich Mammoth Mountain Tuff 5-20 m thick are preserved 

near the Continental Divide 35-40 km south of Creede, 

indicating that the unit spread widely in this direction 

over the smooth top of Carpenter Ridge Tuff. 

Near the postulated source, the products of the 

Mammoth Mountain eruptive cycle show structural 

relations that may indicate nearby caldera subsidence · 

related to the ash-flow eruptions. The rudely layered rocks 

at the base of the section-the Farmers Creek Tuff 

(Farmers Creek Rhyolite of Steven and Ratte, 1965 )-and 

most of the overlying phenocryst-poor tuff are inclined 

10°-15° NE. The dip of this compaction foliation flattens 

in the younger quartz-latitic rocks; the change in dip is 

about coincident with the change in lithology and takes 

place within a vertical span of about 50 m. This change in 

dip is interpreted to reflect structural disturbance during 

Mammoth Mountain eruptions, a structural disturbance 

perhaps related to caldera collapse at the source vents. 

Most of any Mammoth Mountain caldera that may have 

existed was destroyed by the younger Creede caldera. The 

size or shape of the postulated caldera is unknown, except 

that it must be within the topographic wall of the Creede 

caldera, which exposes pre-Mammoth Mountain rocks on 

its northern, eastern, and southwestern segments. 

Numerous viscous quartz-latitic lava flows that probably 
22 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

0 100 KILOMETRES 

FIGURE 19.-Distribution of Mammoth Mountain Tuff (diagonal lines) 

in relation to probable source area (S) and San Juan volcanic field 

(shaded). 

accumulated near their own vents overlie the Mammoth 

Mountain Tuff east and northeast of the Creede caldera 

and may represent the outer parts of flows and domes 

erupted marginally around the postulated Mammoth 

Mountain caldera. 

SOURCE OF THE WASON PARK TUFF 

The Wason Park Tuff is a simple cooling unit of rhyolitic ash-flow tuff that spread widely over the central San 

Juan volcanic field (fig. 20) and seems centered on the 

younger Creede caldera (fig. 1 ). North of the Creede 

caldera, the Wason Park fills the upper parts of the same 

rough topography that confined the Mammoth Mountain Tuff. It is largely confined to the moats around theresurgent La Garita and Bachelor calderas where it rests on 

thick tongues of Mammoth Mountain Tuff, and it thins or 

wedges out laterally against the resurgent cores or topographic walls of these calderas. To the west, south, and 

southeast, however, the Wason Park Tuff spread widely as 

a thin sheet on top of older ash-flow sheets. 

The Wason Park Tuff has a volume greater than 100 

km3 (Ratte and Steven, 1967, p. H34) and is sufficiently 

voluminous that subsidence should have resulted at the 

source (Smith, 1960, fig. 3), but no caldera structure is 

exposed within the area covered by the sheet. The only area 

large enough for subsidence related to Wason Park Tuff is 

under the younger Creede caldera, which is near the center 

of the area of distribution of the thickest and most densely 

welded Wason Park Tuff. White pumice blocks characteristic of the Wason Park Tuff are larger near the Creede 

0 100 KILOMETRES 

FIGURE 20.-Distribution of Wason Park Tuff (diagonal lines) in relation to probable source area (S) and San Juan volcanic field (shaded). 

caldera than in the distal parts of the sheet (Ratte and 

Steven, 1967, fig. 16), and may indicate proximity to 

source. A local lava flow closely similar in lithology to the 

densely welded Wason Park Tuff underlies the Wason 

Park along the northeast rim of the Creede caldera (Ratte 

and Steven, 1967, fig. 15B), again suggesting a nearby 

source. 

SAN LUIS AND COCHETOPA PARK CALDERAS 

Although completely separate subsidence structures, 

the San Luis and Cochetopa Park calderas (figs. 21 and 22) 

are so intertwined in evolution that they are considered 

together. Furthermore, the San Luis caldera appears to be 

a compound structure consisting of two overlapping 

subsided blocks that formed in sequence during separate 

periods of ash-flow eruptions. 

Stratigraphic uncertainties preclude confident interpretation of the evolution of these calderas. The history of 

past usages of the Rat Creek, Nelson Mountain, and 

Cochetopa Park as rock-stratigraphic units chronicles the 

confusion that has stemmed from attempts to correlate 

virtually identical rock types from area to area; nearly 

every report dealing with these units that has been published since 1964 treats them differently. This report 

continues this confusing practice because none of the 

previous usages permits reconstruction of a coherent 

history of evolution of the calderas. 

In its type locality in the Creede mining district (Steven 

and Raw~, 1964, 1965 ), the Rat Creek Tuff consists largely 

of soft white nonwelded to slightly welded zeolitized tuff, 
CENTRAL SAN JUAN CALDERA COMPLEX 23 

CALDERA 

COCHETOPA PARK 

CALDERA 

Fault (hachures on downthrown side 

0 5 

LA GARITA 

CALDERA 

10 

...... ~ 

~ ...... ............ 

~"" Topographic rim~ 

\. 

' .>o \ ' ;oo \ \ ;9,.... \ 

'?o ' \ · \ \ 0~ ~ ' ~ ~ I ~ J-structural margin / 

I I I ; 

I r~ 

I r' 

I I 

I t 

I f 

15 Kl LOMETRES 

FIGURE 21.-Generalized geology of the San Luis and Cochetopa Park calderas in relation to remnants of the Bachelor (B) 

and La Garita calderas. Control moderate to good where boundaries are ·shown by solid symbols; conjectural where 

shown by open symbols. E, general area of the Equity mine; S L P, San Luis Peak. 

with a ledge of densely welded tuff 10-30 m thickjustabove 

the middle. The Rat Creek is overlain by Nelson Mountain 

Tuff, a densely welded quartz latite containing 15-30 

percent phenocrysts. Steven and Ratte (1965, p. 37) 

included in the Nelson Mountain the large mass of 

propylitized densely welded tuff (the Equity Quartz Latite 

as used by Emmons and Larsen, 1923) that fills the lower 

part of the San Luis caldera, although they could not 

prove physical continuity. 

After extensive regional mapping in the central and 

northern San Juan Mountains, Steven, Lipman, and 

Olson (1974, p. A80) concluded that the densely welded 

rocks in the San Luis caldera were probably the intracaldera equivalent of the Rat Creek Tuff, and called them 

the Equity Member of that formation. Overlying densely 

welded tuffs on the riorth and northeast sides of the caldera 

were correlated with the lithologically identical Nelson 

Mountain Tuff at its type locality. These overlying welded 
24 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

380 

CALDERA 

~ 

~ 

~ 

~ 

\ 

\ 

\ 

' \ 

\ 

\ 

0 5 

LA GARITA 

CALDERA 

\-structural margin 

I 

I 

I 

I 

Fault (hachures on downthrown side) / 

10 

I 

I 

I 

I 

I 

I 

I 

15 KILOMETRES 

l 

FIGURE 22.-Generalized geology of the Creede and San Luis calderas in relation to remnants of the Bachelor (8) and 

La Garita calderas. Control moderate to good where boundaries are shown by solid symbols; conjectural where shown 

by open symbols. PR, Point of Rocks volcano; S, Spar City. 

tuffs extend into their apparent source area in the 

Cochetopa Park caldera to the northeast. Younger postsubsidence tuffs filling the Cochetopa Park caldera were 

called the Cochetopa Park Member of the Nelson 

Mountain Tuff. 

Continued work in the northwestern San Juan 

Mountains has shown that something is wrong with this 

interpretation of the stratigraphic assemblage. Major 

densely welded ash-flow tuffs in the Lake City area (40 km 

northwest of Creede) almost certainly are equivalent w the 
CENTRAL SAN JUAN CALDERA COMPLEX 25 

Nelson Mountain Tuff at its type locality in the Creede 

mining district, but details of distribution and volume 

make it highly unlikely that they were derived from the 

Cochetopa Park caldera. This forces the following tentative conclusions: 

I. The correlation of Nelson Mountain Tuff in its type 

locality in the Creede district with lithologically 

identical tuffs northeast of the San Luis caldera 

probably is wrong. 

2. The original correlation of Nelson Mountain Tuff 

with the densely welded (Equity) tuffs in the San Luis 

caldera by Steven and Ratte ( 1964, 1965) probably is 

correct. The Equity Member is therefore removed from 

the Rat Creek Tuff and assigned to the Nelson 

Mountain Tuff. 

3. The densely welded tuffs north and northeast of the San 

Luis caldera, apparently derived from the Cochetopa 

Park caldera, constitute a separate unit younger than 

the Rat Creek and Nelson Mountain Tuffs. We therefore remove the Cochetopa Park Member from the 

Nelson Mountain Tuff and raise it to formational rank 

as the Cochetopa Park Tuff. 

The interpretations that follow assume that these tentative conclusions are generally correct, but repeated 

examinations of critical field relations and petrographic 

studies have failed to develop the firm criteria for distinguishing these units that will be required for confident 

evaluation of our evolutionary model. 

COMPOUND SUBSIDENCE OF THE SAN LUIS CALDERA 

As currently (1975) understood, the Rat Creek Tuff 

appears to be a relatively low volume assemblage of poorly 

welded rhyolitic ash-flow tuffs confined largely to the 

vicinity of the Creede mining district (Steven and Ratte, 

1965 ). Several factors imply a nearby source: ( 1) at least one 

local volcano of Rat Creek age is well exposed in the heart 

of the Creede district (Steven and Ratte, 1965, p. 35); (2) a 

local ledge of densely welded tuff occurs just above the 

middle of the Rat Creek Tuff within the Creede district; 

and (3) a small subsidence structure (caldera) of Rat Creek 

age is located along the northwest side of the Creede 

district. This last structure, described below, is here 

considered an early stage in the development of the 

compound San Luis caldera (fig. 21). 

The regional sheet of Rat Creek Tuff ranges widely in 

thickness because of rough underlying topography. It is 

150-200 m thick through the central part of the Creede 

district, but to the northeast it wedges out completely 

against an eroded fault scarp cutting the resurgent core of 

the La Garita caldera. North of the Creede district:, the Rat 

Creek is cut off by the topographic wall of the main San 

Luis caldera. 

In the early stage of the San Luis caldera along Miners 

Creek northwest of the Creede district (fig. 21), soft 

zeolitized tuffs that are physically continuous with type 

Rat Creek Tuff fill a partly exposed subsidence structure 

that has a steep arcuate fault along the south and southwest sides. To the north and northeast, this caldera is 

largely covered by younger rocks and relations are almost 

totally obscured; the map pattern (figs. 21 and 22) implies 

that the northern part of the caldera caved into the main 

San Luis caldera which developed shortly afterward. 

Soft tuffs of the Rat Creek accumulated to a thickness of 

more than 350m within the early caldera and are at least 

twice as thick as nearby correlative outflow tuffs. The 

intracaldera tuffs are flat lying and show no evidence of 

resurgence. The faulted margin is largely obscured by 

massive landslides, but it can be estimated to within a few 

metres along one gully where it appears to be very steep. 

E;_lsewhere the fault is indicated by juxtaposition of thick 

intracaldera Rat Creek Tuff and flat-lying older rocks in 

the wall. Two intrusives were emplaced along or near the 

faulted margin. One is a major neck, more than I km 

across, that clearly occupies the fault; the other forms a low 

knob, 500 m across, completely surrounded by landslide 

debris. 

Several features indicate a Rat Creek age for the early 

caldera. The wall outside the fa~lted margin includes 

rocks as young as Wason Park Tuff, which is the next 

oldest ash-flow unit in the San Juan Mountains, and 

locally may include the overlying andesite of Bristol Head 

which is directly beneath the Rat Creek Tuff just east of the 

early caldera. The local ledge of densely welded tuff in the 

upper part of outflow Rat Creek Tuff extends westward to 

the vicinity of the early caldera, but is nowhere found 

within the caldera, although the area of its projected 

position is well exposed. However, the caldera margin, 

whose development probably accounts for this 

discordance, is covered by surficial debris and younger 

lavas. The caldera fill is overlain by typical Nelson 

Mountain Tuff that extends from its type locality 

westward across the area of the early San Luis caldera. The 

Nelson Mountain forms an unbroken rim across the trend 

of the older faulted caldera margin without any change in 

thickness or evidence of deformation. 

The main San Luis caldera (figs. 21, 22, and 23) is now 

thought to have subsided concurrently with eruption of 

the Nelson Mountain Tuff, and at least 1.5 km of 

phenocryst-rich quartz latite accumulated within the subsiding basin, whereas the outflow sheet nearby is only 

about 300 m thick. This thick mass of intracaldera ash 

welded into a dense, nearly homogeneous rock with only a 

few local less-welded partings. Later propyllitic alteration 

further homogenized the rock and obscured partings. The 

base of the intracaldera Nelson Mountain Tuff is exposed 

locally within the north-central part of the caldera core 
26 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

0 100 Kl LOMETRES 

FIGURE 23.-Distribution of Nelson Mountain Tuff (diagonal lines) 

in relation to San Luis caldera (S) and San Juan volcanic field 

(shaded). 

where densely welded quartz latite directly overlies a local 

hill, probably a volcanic dome, of older rhyolite. Some 

probable Carpenter Ridge Tuff is exposed on the north 

flank of this hill, but its relations with the adjacent 

rhyolite are obscure. No soft tuff representative of Rat 

Creek ash flows is exposed on this buried hill, but the 

lower parts of the caldera floor where such tuff deposits 

might more reasonably be expected are nowhere exposed. 

Subsidence of the main San Luis caldera produced a 

broad basin nearly 15 km across. Intermediate to silicic 

lavas and breccias of the volcanics of Stewart Peak 

accumulated within the northern and eastern parts of this 

basin, and were locally accompanied by deposits of stream 

and lake sediments (figs. 21 and 22). Most intracaldera 

lavas around the eastern side of the caldera are rhyodacite 

and quartz latite, but perlitic rhyolite flows are abundant 

along the north margin. All known dike and neck feeders 

for the intracaldera lavas are within the caldera core. The 

eastern margin of the caldera block again subsided locally 

during eruption of the intracaldera lavas, as indicated by a 

fault with at least 500 m of throw that places the lower part 

of the volcanics of Stewart Peak within th.e caldera against 

older Fish Can yon Tuff in the wstll. The fault passes under 

unbroken Cochetopa Park Tuff to the north, and ledges of 

this younger unit representing successive ash flows both to 

the north and south intertongue with younger lavas on the 

Stewart Peak sequence, providing an upper age limit for 

the renewed subsidence. 

SUBSIDENCE OF THE COCHETOPA PARK CALDERA. 

Ash flows of the Cochetopa Park Tuff that were 

deposited in the San Luis caldera area (fig. 1) during 

accumulation of the intracaldera volcanics of Stewart 

Peak had their source in the Cochetopa Park area, some 30 

km northeast. Minor phenocryst-poor rhyolite ash accumulated near the source, but the eruptions soon changed 

to crystal-rich quartz latite that is seemingly identical with 

the Nelson Mountain Tuff. The Cochetopa Park ash flows 

followed the nearly filled moat around the northern side of 

the La Garita caldera (fig. 1) and inter-tongued with the 

volcanics of Stewart Peak within the San Luis caldera (fig. 

24). 

0 100 KILOMETRES 

FIGURE 24.-Distribution of the Cochetopa Park Tuff (diagonal lines) 

in relation to Cochetopa Park caldera (C) and San Juan volcanic 

field (shaded). 

In contrast with the larger San Juan calderas, where 

subsidence occurred concurrently with ash-flow 

eruptions, the Cochetopa Park caldera collapsed after 

major eruptions had ceased, as indicated by lack of 

thickening of the intracaldera tuff. In further contrast with 

the larger calderas, the Cochetopa Park caldera did not 

form a complete circular structure, but subsided as a trapdoor bounded by a horseshoe-shaped fault. The southwestern margin did not fracture, but merely bent downward and tilted eastward. A trough, probably representing 

a graben, formed along the northwest side of the caldera 

core, and a tilted, northeast-trending ridge of Cochetopa 

Park Tuff dipping 5°-10° E. extended beyond the middleof 

the caldera (fig. 21 ). An inner ring-fracture zone can be 

postulated to account for major subsidence (fig. 21 ). 
CENTRAL SAN JUAN CALDERA COMPLEX 27 

Maximum displacement on the northeastern side of the 

trapdoor is 700-800 m, as judged from the height of the 

topographic wall in this direction. 

After the inner trapdoor block had subsided, or perhaps 

concurrently with subsidence, the walls of the caldera 

slumped inward along arcuate faults that nearly surround 

the downfaulted parts of the caldera. The breakaway zone 

near the hingeline of the trapdoor on the west side of the 

caldera is a splintered area of many faults and jumbled 

relations between blocks. Elsewhere, the slumped blocks 

are bounded by curved, linking faults that are concave 

toward the subsided trapdoor. Locally along the northeast 

side, no slumping took place. 

Minor pyroclastic eruptions after subsidence of the 

Cochetopa Park caldera deposited local moderately to 

densely welded ash-flow tuffs near the hinge line and thick 

nonwelded pumiceous ash-flow tuff within the caldera 

east of the medial ridge (fig. 21 ). The trough (graben?) 

northwest of the medial ridge was filled with sandy 

tuffaceous stream deposits and a few layers of air-fall tuff. 

The two facies merge in the northeastern part of the 

caldera about on trend with the medial ridge. Layering in 

the tuffaceous caldera fill is flat and shows no resurgence 

of the caldera core. In places the caldera fill covers arcuate 

faults of the outer slumped zone and provides an upper 

limit on the age of that faulting. 

A thick rhyolite flow with a prominent black vitrophyre 

at its base was erupted through the caldera fill about on 

trend with the medial ridge, and eroded remnants still 

persist as the feature called Cochetopa Dome. 

RESURGENCE OF THE SAN LUIS CALDERA 

Whereas minor resurgence of the San Luis caldera may 

have preceded or accompanied eruption of the intracaldera volcanics of Stewart Peak, most resurgence took 

place later. Densely welded layers of the Cochetopa Park 

Tuff, representing many successive ash flows from the 

Cochetopa Park caldera, intertongue with the upper 

volcanics of Stewart Peak and are involved in this 

resurgent uplift. North of the caldera, the Cochetopa Park 

Tuff is inclined less than 10° in various directions, but 

near the caldera margin the layers are bent up along a 

curving hingeline and dip 20°-25° radially outward from 

the resurgent core. In part, this hingeline is marked by a 

fault with little displacement and the change in dip is 

sharp; elsewhere no fault is apparent and the change in dip 

is less abrupt. South of the caldera, resurgence is locally 

marked by an abrupt change in dip from flat layers of 

Nelson Mountain Tuff outside the caldera to layers 

dipping 15° or so southward off the dome within the 

caldera. Near the Equity mine in the northern part of the 

Creede district, however, local resurgence uplifted a 

triangular block, 1.5-3 km on a side, nearly a kilometre 

(Steven and Ratte, 1965). 

This resurgence was somewhat asymmetrical to the San 

Luis caldera. The small remnant of the early subsided 

block along the southwest side of the caldera is not domed, 

and evidence of resurgence is apparently limited to the 

main caldera and to an area extending about a kilometre 

beyond the eastern and northeastern structural margin of 

the caldera. The edge of resurgent uplift on the north side 

of the San Luis caldera is just inside the outer topographic wall, and possibly is outside the buried structural 

margin. 

The minimum structural relief caused by resurgence is a 

little less than a kilometre, as indicated by the elevation 

difference between the top of flat-lying Nelson Mountain 

Tuff outside the caldera and the top of San Luis Peak 

within the caldera. On the north side of the caldera, this 

difference is about 1.4 km, whereas on the south side it is 

about 0. 7 km; as discussed later, this contrast reflects 

tilting caused by late general uplift of the roof of the 

batholith that underlies the central part of the San Juan 

volcanic field. 

POSTCALDERA LAVAS 

Resurgence of the San Luis caldera was followed by 

development of a line of volcanoes that extends westward 

from the caldera about 14 km. The eastern part of these 

postcaldera volcanic rocks, as shown in figures 21 and 22, 

consists of coarsely porphyritic lavas and breccias (the 

quartz latite of Baldy Cinco) that were erupted from many 

local vents, some within the western part of the San Luis 

caldera. Over most of their extent, these lavas and breccias 

rest on flat ledges of densely welded Nelson Mountain 

Tuff or Cochetopa Park Tuff, but on the east they abut and 

wedge out against tilted volcanics of Stewart Peak and 

Nelson Mountain Tuff in the resurgent core of the San 

Luis caldera. Apparently most of the western third of the 

caldera was once covered by these rocks. Lavas of similar 

age and composition (quartz latite of Rambouillet Park) 

(Steven, 1967) extend farther southwest, toward the southeast margin of the Uncompahgre caldera. 

CREEDE CALDERA 

The Creede caldera (fig. 22) formed in response to 

eruption of the Snowshoe Mountain Tuff about 26.5 m.y. 

ago and is the youngest subsidence structure in the central 

San Juan caldera complex. Its form is clearly reflected in 

the modern landscape. (See frontispiece, Ratte and Steven, 

1967, for a color panorama of the Creede caldera.) The 

excellently preserved topographic form of this structure 

has resulted primarily from burial of all but the higher 

parts of the resurgent core by stream and lake sediments of 

the upper Oligocene Creede Formation that were not 

removed by erosion until late Cenozoic time (Steven, 1968, 

p. 114). Development of the Creede caldera was considered 

in detail by Steven and Ratte (1965, p. 58-62), and Smith 

and Bailey (1968, p. 625-626) used it as one of their seven 

examples of typical resurgent cauldrons. 
28 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

The Creede caldera subsidence was localized along the 

southwest margin of the La Garita caldera (fig. 22); it 

obliterated the south part of the Bachelor caldera and the 

larger part of any calderas related to eruptions of the 

Mammoth Mountain and Wason Park Tuffs. 

Initial eruptions of the phenocryst-rich quartz latite 

forming the Snowshoe Mountain Tuff spread a thin, 

poorly welded sheet over the flat surfaces of earlier ashflow tuffs in adjacent areas (fig. 25 ). Subsidence began 

shortly thereafter and proceeded concurrently with 

eruption. More than l. 4 km, and perhaps more than 2 km, 

of nearly uniform crystal-rich ash accumulated within the 

subsiding basin; most of this is densely welded, but a few 

partings are less welded. 

0 100 KILOMETRES 

FIGURE 25.-Areas where erosional remnants of Snowshoe Mountain 

Tuff are preserved (diagonal lines) in relation to Creede caldera (C) 

and San Juan volcanic field (shaded). 

Tongues of talus and rockfall breccia (Steven and Ratte, 

1965, p. 42) extend into at least tbe upper 700 m of the 

intracaldera Snowshoe Mountain Tuff along the western 

side of the caldera, and probably are present but 

unexposed elsewhere within the intracaldera tuffs. These 

breccias clearly indicate subsidence concurrent with 

accumulation of the Snowshoe Mountain Tuff, and 

demonstrate that the developing caldera wall exposed 

rocks as old as Wason Park Tuff at least episodically 

during subsidence. The successive rude layering and 

compaction foliation in the Snowshoe Mountain Tuff are 

virtually parallel, and indicate that the core of the caldera 

sank as a coherent mass, rather than in piecemeal blocks. 

Final subsidence left a basin 12-15 km across, with steep 

walls rising l-1. 4 km above the flat floor. These walls were 

unstable, and great masses fell off to form tongues of rockfall breccia that extended over the caldera floor. The northeast wall of the caldera, where densely welded Mammoth 

Mountain and Wason Park Tuffs overlie soft tuffs, was 

particularly susceptible to avalanching, and a broad 

crescent-shaped scallop, 8 km wide and 3 km deep, 

developed in the outer wall. Breccias derived from the 

northeast wall of the caldera (Steven and Rattl, 1965, p. 42-

43) have been identified more than halfway across the 

caldera, 8-l 0 km from their source. 

The core of the Creede caldera was strongly domed after 

subsidence, and the center of the dome was uplifted more 

than 1.5 km above the structural moat left around the 

periphery. The eastern part of the uplift is a simple half 

dome with the layers of Snowshoe Mountain Tuff dipping 

radially outward 25°-45°. A deep north-trending graben 

extends across the center of the uplift; displacement is 

minor near the north and south ends of the graben, but 

exceeds 700 m near the center of the dome. Internal 

structure of the graben is complex (Steven and Ratte, 1965, 

pl. l ), but its general form is a fractured keystone block 

across the distended top of the resurgent dome. The west 

part of the resurgent core was less regularly uplifted; the 

northern flank was tilted northwest and broken by 

faulting and the western flank was tilted generally westward and was progressively more uplifted toward the 

south. 

A small dome of auto brecciated rhyolite (Point of Rocks 

volcano) formed along the west side of the Creede caldera 

at some time just preceding, during, or immediately after 

resurgent doming. Pebbles of similar rhyolite have been 

found in some avalanche breccias caught in jumbled fault 

blocks along the keystone graben, suggesting that this 

volcano may have formed before resurgence. On the other 

hand, the eruption may have followed resurgent uplift, 

inasmuch as a local exposure shows the feeding neck to be 

nearly vertical where it cuts across strongly inclined layers 

of Snowshoe Mountain Tuff. 

Resurgence of the Creede caldera was followed by 

eruption of flows and domes of Fisher Quartz Latite at 

places around the periphery and by deposition elsewhere 

of the stream and lake sediments and travertine of the 

Creede Formation. Closely accordant K-Ar age determinations of Fisher lavas indicate that this stage in the 

Creede caldera cycle took place about 26.4 m.y. ago. Most 

of the postcaldera lavas were erupted from centers northeast and south of the caldera. Ash-flow deposits in the concurrently deposited Creede Formation are most abundant 

toward the south, indicating that Fisher centers in this 

direction supplied much of the ash that elsewhere was 

reworked into the predominant stream-and lake-sediment 

facies. Travertine in the Creede Formation was deposited 

widely around the periphery of the caldera; consideration 

of the timing of deposition and of carbon isotope ratios led 
CENTRAL SAN JUAN CALDERA COMPLEX 29 

Steven and Friedman (1968, p. B32-B33) to conclude that 

the carbonate was derived from underlying sedimentary 

carbonate units by resurgent magma rising into the roots 

of the Creede caldera. The Creede Formation is presently 

exposed over a vertical range of more than 700 m; the 

bottom of the basin of deposition is not exposed and the 

top of the formation is eroded. The original thickness of 

the formation was more than a kilometre and was possibly 

as much as 1.4 km. 

The final major stage in structural disruption that has 

been recognized in the vicinity of the Creede caldera was 

strong local faulting accompanied by mineralization in 

the Creede mining district adjacent to the north margin of 

the caldera (Steven and Ratte, 1965) and in the Spar City 

mining district along the south margin of the caldera 

(Steven, 1964). Steven (1972) and Steven and Eaton (1975) 

have interpreted the faulting and mineralization in the 

Creede district to have resulted from local intrusion of a 

stock into the roots of a preexisting broken zone. This 

intrusion may have been the terminal stage of the Creede 

caldera cycle or, more probably, may have been a later 

unrelated event, inasmuch as K-Ar age determinations on 

adularia from the OH vein in the Creede district indicate 

that mineralization took place there about 24.6 m.y. ago 

(M. A. Lanphere, P. M. Bethke, P. B. Barton, written 

commun., 1973), nearly 2 m.y. after caldera subsidence. 

LATE GENERAL MAGMATIC UPLIFT 

A broad zone extending from the Platoro caldera 

complex northwest through the central San ] uan caldera 

complex was broken by normal faults late in the period of 

caldera development. Faults that developed at this time 

extend along the crest of the eastern part of the gravity low 

(fig. I); these faults are thought to reflect uplift and 

distention related to a general buoyant rise of the whole 

eastern part of the batholith, where evidence for concurrent magmatism is strong. 

Faulting took place in two general segments-the Rio 

Grande graben (figs. 1 and 22) extending southeast from 

the central caldera complex, and the Clear Creek graben 

(fig. I) extending tangentially northwest from the caldera 

complex. Some faults in the Rio Grande graben were 

recurrently active, particularly near the northwest end, 

where we recognized several increments of displacement 

related to the progressive breakdown of the southwestern 

half of the La Garita caldera. Recurrent movement is also 

evident at the southeast end of the graben system, where 

Miocene basalts of the Hinsdale Formation are locally 

involved. The main graben faulting, which integrated 

some of the earlier faults into a regional pattern, took 

place during the Creede caldera cycle. Many of the faults 

cut Nelson Mountain Tuff and are in turn overlapped by 

unbroken Fisher Quartz Latite that was erupted late in the 

Creede caldera cycle. The northwest-trending faults are 

generally parallel to the trend of the underlying batholith. 

This same part of the batholith was the source of repeated 

ash-flow eruptions and caldera subsidences during and 

just before graben development, indicating high-level 

activity in the underlying magma chamber. Some Rio 

Grande graben faults terminate to the southeast against a 

north-trending fault that closely parallels the eastern 

margin of the gravity low-again implying a close 

relationship between the faulting and distention of the 

roof above an active segment of the batholith. 

As shown on figure I, the Clear Creek graben (Steven, 

1967) extends tangentially northwest from the southwest 

side of the central San Juan caldera complex. Its trend is 

parallel to the southwest side of the gravity low in the 

south-central part of the volcanic field. Where the graben 

extends across the gravity low toward the north, it 

separates an area of then-recent ash-flow eruptions and 

caldera subsidences to the east from an area of older ashflow eruptions and caldera subsidences to the west. The 

graben reflects distention of the roof of the batholith, and 

it also marks relative uplift of the area to the east (Steven, 

1967), toward the area of concurrently active magmatism. 

To the north, faulting changes from a graben to a westfacing normal fault and then to a northwest-facing 

monocline that bends around the northwest side of the San 

Luis caldera and dies out as it approaches the margin of 

the gravity low. As discussed earlier, the Nelson Mountain 

Tuff is about 0. 7 km higher to the south of the San Luis 

caldera than it is to the north-a discordance believed to 

measure some of the late general uplift of the area above 

the batholith. 

The Clear Creek graben began to develop during the 

closing stages of the San Luis caldera cycle, and may have 

been about concurrent with the Creede caldera cycle. The 

west-facing normal fault at the north end of the graben 

already existed when the quartz latite of Baldy Cinco was 

erupted, inasmuch as lavas of this unit poured over and 

covered a scarp related to the fault. Relations are 

ambiguous near the Creede caldera at the southeast end of 

the graben, where widespread glacial till obscures much of 

the bedrock. We saw no evidence that the graben existed 

when the Creede Formation was being deposited, yet none 

of the faults can be demonstrated to cut the Creede 

Formation. At least one fault on trend with the Clear Creek 

graben cuts Fisher Quartz Latite flows in the Spar City 

mining district on the south side of the Creede caldera, but 

this might reflect late reactivation of an earlier fault. 

The Rio Grande and Clear Creek grabens are thus 

believed to reflect general distention and minor buoyant 

uplift of the eastern part of the batholith, where intense 

ash-flow activity and caldera subsidence demonstrated 

concurrent high-level magmatism in the underlying 

batholith. The faulting dies out southeastward and northwestward as it approaches the margins of the related 

gravity low, and no related graben faults of this age are 
30 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

present in the western part of the gravity low where ashflow eruptions and caldera subsidences are older. 

However, the intricate pattern of veins and small faults 

that radiate outward froin the Silverton caldera may reflect 

analogous broad uplift during waning stages of volcanism 

in that area. 

BLOCK-FAULTED AREA 

An area west and southwest of the Mount Hope caldera 

is also broken by faults that were recurrently active during 

the period of caldera formation. This structurally somewhat anomalous area seems localized where the south 

margin of the gravity low, and presumably of the subvolcanic batholith (fig. 1 ), extends southwest across a thick 

wedge of Paleozoic-Mesozoic sedimentary rocks marking 

the northern extension of the San Juan Basin (fig. 1). The 

faults have the general pattern of a short ladder (fig. 14), 

with two northeast-trending faults bounding an area cut 

into narrow blocks by irregularly northwest trending steep 

faults. Several periods of movement can be discerned. 

The oldest faulting followed deposition of the intertongued Masonic Park welded tuffs and Sheep Mountain 

andesitic lavas, and preceded deposition of the Fish 

Canyon Tuff. A later period of faulting is indicated in the 

eastern part of the highly faulted area, where andesitic 

breccias and minor flows in the Huerta Formation thin 

abruptly from more than 1,500 m to about 70 m thick 

eastward across a buried scarp on the Fish Canyon Tuff. 

This scarp probably marks a local fault that was active 

between deposition of the Fish Canyon Tuff and the 

Huerta Formation. 

The main faulting followed eruption of the andesitic 

lavas and breccias of the Huerta Formation. The laddershaped pattern of faults formed at this time and the general 

result was to depress the area toward the caldera area to the 

north. These faults are superimposed on the steep gravity 

gradient along the south side of the underlying batholith 

(fig. 1 ), and the general effect of the faulting was to warp a 

local segment of the roof downward toward the batholith 

across its southwestern margin. This faulting did not 

coincide with any particular ash-flow eruption, but rather 

followed accumulation of episodically erupted andesitic 

lavas and breccias at a number of local volcanic centers. 

The movement thus probably resulted from magmatic 

movements within the batholith and distention along its 

south margin. This distention may have been in part 

localized by the relatively incompetent prevolcanic sedimentary sequence in this area. 

Many of the faults that formed during the main postHuerto faulting were reactivated later, after the Carpenter 

Ridge Tuff accumulated. In places, this reactivated movement was in the same direction as the earlier movement, 

and in other places, the direction of displacement was 

reversed. Most of the post-Carpenter Ridge faulting 

appears to have reflected minor readjustments 1n the 

already broken roof of the batholith. 

The youngest faults within this area reflect shallow 

gravity sliding of the south flank of the volcanic pile out 

over underlying Cretaceous shales. Fracturing took place 

along crescentic faults facing southward toward the edge 

of the volcanic plateau (fig. 14). The rocks enclosed within 

these faults were dropped downward toward the south, 

and tilted northward 30° or more by concurrent rotation. 

These faults are related to the present erosional scarp 

along the south side of the volcanic plateau, and are not 

directly related to the older faults that resulted from 

magmatic movement during late Oligocene volcanic 

activity. 

DISCUSSION 

DEVELOPMENT OF THE BATHOLITH 

Calderas and related subsidence structures in the San 

Juan volcanic field are situated within a marked negative 

gravity anomaly that is believed to reflect an underlying 

shallow batholith. Successive ash-flow eruptions and 

caldera collapses in the volcanic pile above this batholith 

probably mark the local culminations of upward movement of magma: when the roof of an individual chamber 

became so thin that it failed, voluminous ash was erupted 

rapidly and a caldera collapsed into the partly evacuated 

magma chamber. Resurgent upwardmovementofmagma 

domed many of the calderas and caused extrusion of lavas 

along the earlier formed structures. High-level 

magmatism, as manifested by volcanic eruptions and 

related structural adjustments, diminished at each center 

as the underlying cupola crystallized. 

Using this model, we can trace the development of the 

high-level batholith beneath the San Juan volcanic field 

from the histories of the successive calderas. The early 

calderas are widely scattered and are either on the margins 

of the gravity low or on outward projections from it. Most 

of these early calderas developed within clusters of older 

andesitic stratovolcanoes, and the postsubsidence 

eruptions were commonly of intermediate lavas similar to 

the older andesites. We interpret these calderas to have 

developed above isolated high-level cupolas of magma 

that developed in the roots of older volcanoes before the 

main body of the batholith rose to its present position. The 

upper parts of these cupolas differentiated to quartz latite 

and low-silica rhyolite, which formed the ash-flow tuffs 

associated with the early calderas, but the quantity of 

silicic material was limited, and the postsubsidence lavas 

were from the underlying relatively undifferentiated 

andesi tic magma. 

The later calderas and associated structures are above 

the main body of the batholith as indicated by gravity data. 

At least 12 separate calderas formed within about 3 m.y. 

This intens.e activity is believed to reflect the rise of the 
DISCUSSION 31 

batholith in two main high-level segments corresponding 

to the two main caldera complexes. The segment beneath 

the western San Juan Mountains rose to shallow depths 

about a million years before the central San Juan segment, 

but high-level magmatic activity in each segment overlapped in time. 

The top of the batholithic mass of magma was 

extensively differentiated-to phenocryst-poor rhyolite in 

the upper parts of local cupolas, and to large volumes of 

phenocryst-rich quartz latite beneath. Ash-flow eruptions 

in the western San Juan Mountains depleted the more 

silicic differentiates in the upper parts of the magma 

chambers, and the postsubsidence lavas were more mafic 

intermediate-composition rocks from progressively 

greater depths in the chambers. In the central San Juans, 

however, even the largest ash-flow eruptions did not 

exhaust the differentiated material in the underlying 

chambers, and the postsubsidence lavas that were erupted 

around the calderas are typically porphyritic quartz latites 

which are related in composition to the associated ash flow 

tuffs. Local andesitic volcanoes not closely associated with 

the calderas tapped deeper, little-differentiated parts of the 

batholith throughout the period of ash-flow eruptions 

and caldera subsidences. 

The Lake City caldera in the western part of the San 

Juan volcanic field, which is related to eruptions of the 

petrologically distinct alkali rhyolite of the Sunshine Peak 

Tuff, collapsed about 4 m.y.later than the youngest major 

eruptions of andesitic and derivative rocks from the central 

and western parts of the San Juan volcanic field. This late 

caldera is believed to have formed above a later high-level 

magma chamber unrelated to the earlier segments of the 

batholith. 

These interpretations suggest some time limitations on 

the magmatic life span of a shallow, composite batholith 

100 km across. After about 5 m.y. (during the period from 

35 to 30 m.y. ago) of intensive eruption of andesitic lavas 

from many scattered centers, local magma chambers 10-30 

km across had risen to shallow depths beneath some of the 

larger volcano clusters, and had differentiated sufficiently 

to supply large-volume eruptions of silicic ash. Within the 

next 4 m.y. (30-26 m.y.) the batholith evolved from this 

collection of scattered chambers to a broad shallow mass of 

extensively differentiated magma, and then to virtual 

dormancy. Within another 4 m.y. (26-22 m.y.), the lower 

part of the batholith had congealed sufficiently to permit a 

younger, petrologically distinctive body of magma to 

work its way up to similar shallow depths, while still 

retaining its compositional identity. 

RELATION OF ASH-FLOW ERUPTIONS AND CALDERA 

SUBSIDENCE 

An essentially one-to-one relationship exists between 

major eruptions of ash flows and subsidence of calderas in 

the San Juan volcanic field. Unresolved is the volume of 

ash required to make this maxim operative. Most of the 

ash-flow sheets we have mapped have either very small 

(<10 km3) or moderate (100-500 km3) to large (>500 km3) 

volumes; caldera subsidence is known or suspected to have 

been associated with all the moderate-volume sheets and 

invariably accompanied the large-volume sheets. Several 

of the calderas associated with moderate-volume ash-flow 

sheets did not form complete circular collapse structures, 

but subsided as trapdoors hinged on one side. Such 

calderas generally were not resurgently domed after 

subsidence. Calderas associated with large-volume sheets, 

on the other hand, generally are complete subcircular 

structures, and commonly were resurgently domed after 

collapse. 

All large-volume ash-flow units are much thicker 

within the calderas, typically by an order of magnitude, 

than in the surrounding outflow areas. This relationship 

is most easily explained by subsidence concurrent with 

eruption. Many of the large-volume units are compositionally zoned from early rhyolite to later quartz latite. In 

most, the rhyolite is confined to the base of the outflow 

sheet, commonly near the source, and the exposed intracaldera tuff is entirely quartz latite. This suggests that by 

the time ash-flow eruptions had removed sufficent magma 

to cause collapse, the rhyolitic material at the top of the 

chamber was usually exhausted. Alternatively, initiation 

of collapse may have disrupted the vent systems and caused 

tapping of deeper, less differentiated parts of the magma 

chamber. No generalization can be made concerning the 

beginning of subsidence relative to composition of the ash 

being erupted, however, as the relative volumes of 

rhyolitic versus quartz-latitic ash vary widely from one 

sheet to another. At the extremes, no rhyolite at all has 

been found at the base of the Fish Canyon Tuff, which 

forms the largest sheet in the San Juan field, whereas 

rhyolite dominates in both the outflow and intracaldera 

facies of the large-volume Sapinero Mesa and Carpenter 

Ridge Tuffs. 

DIFFERENTIATION IN LOCAL CUPOLAS 

Evidence for independent differentiation at separate 

volcanic source areas supports the idea that the successive 

.calderas developed above local cupolas that projected 

above the general top of the batholith. This postulate is 

inherent in our interpretation of the development of the 

widely scattered early calderas, but appears valid for the 

clustered calderas in the western and central San Juan 

complexes as well. 

In the western San Juan caldera complex, contrasting 

phenocryst-rich quartz-latitic Ute Ridge Tuff and 

phenocryst-poor rhyolitic Blue Mesa Tuff were erupted 

sequentially from calderas only a few kilometres apart, 

and individual magma chambers with separate 
32 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

differentiation seem required. The later series of eruptions RESURGENCE 

of rhyolitic Dillon Mesa and Sapinero Mesa Tuffs, 

progressively more mafic lavas and breccias of the Burns Two broad types of magmatic uplift and general uplift 

and Henson Formations, and rhyolitic Crystal Lake Tuff, related to calderas in the San Juan volcanic field have been 

recognized-in one, the uplift was closely confined to the all from within the confines of the San Juancaldera and the immediately adjacent area and occurred Uncompahgre-Silverton caldera cluster, chronicle the 

sequence of depletion of the differentiated magma at the soon after collapse; in the other, however, broader uplift 

top of one major cupola, establishment of another cupola, involved major segments of the roof of the batholith and 

occurred over an extended period. These two types of further differentiation, and, finally, renewed eruptions of 

regenerated rhyolite. Inasmuch as only about 2 m.y. magmatic uplift merge and are, in places, difficult to 

distinguish. intervened between eruptions of the Ute Ridge and Crystal 

Lake Tuffs, these sequential high-level magmatic Virtually none of the smaller calderas in the San Juan 

processes must have progressed rapidly. volcanic field show evidence of resurgent doming after 

· subsidence. Fairly clear-cut relations indicating no 

The major La Garita caldera in the central San Juan resurgent doming are found at the Summitville, Ute 

Mountains occupied much of the roof of the eastern Creek, Silverton, and Cochetopa Park calderas, as well as 

segment of the batholith. The associated ash flows show at the buried or postulated Lost Lake, Mammoth 

virtually no evidence of compositional zoning; evidently, Mountain, and Wason Park calderas. Of these, the 

little highly silicic material had accumulated at the top of Silverton, Cochetopa Park, and probably the Ute Creek 

the broad magma chamber that supplied the enormous calderas subsided as trapdoor blocks with horseshoequantities of ash for the Fish Canyon Tuff. The magma shaped incomplete ring-fracture zones interrupted by 

chambers that developed successively along the west side monoclinal hinges on one side; the buried Lost Lake 

of the La Garita caldera, however, were strongly caldera possibly may have subsided in a similar manner. 

differentiated. Rhyolite is a major constituent in the The Summitville caldera is deeply buried by a fill of 

Carpenter Ridge, Mammoth Mountain, and Wason Park andesite flows, and the postulated Mammoth Mountain 

Tuffs, which followed in succession from closely and Wasson Park calderas were largely or completely 

associated source areas. Each of these units has distinctive destroyed by younger subsidences. 

phenocryst characteristics requiring separate develop- Most major San Juan calderas were resurgently domed 

ment, and the first two are strongly compositionally zoned shortly after subsidence. The Platoro, La Garita, Bachelor, 

upward from phenocryst-poor rhyolite to phenocryst-rich . San Luis, Creede, and Lake City calderas display such 

quartz latite. All three of these units evidently developed doming particularly well, and the Bonanza caldera may 

under conditions that permitted local differentiation belong to this group. These are the typical resurgent 

within sequentially formed restricted chambers. cauldrons so well described by Smith and Bailey (1968), in 

The later Rat Creek, Nelson Mountain, Cochetopa 

Park, and Snowshoe Mountain Tuffs are from more 

widely scattered centers, for which local cupolas can 

readily be postulated. Except for the rhyolitic Rat Creek 

Tuff, these units are mostly composed of closely similar 

phenocryst-rich quartz latite, although the Cochetopa 

Park Tuff has some rhyolitic ash at its base. The overlap in 

time of the areally separate San Luis and Cochetopa Park 

caldera cycles also indicates concurrently developing, 

separated cupolas containing individually differentiated 

batches of magma. 

Considering the number of calderas that developed 

sequentially within the approximately 4 m.y. of ash-flow 

activity above the San Juan batholith, differentiation must 

have been relatively rapid to generate silicic magma at the 

tops of the individual cupolas. This is particularly true for 

the succession of Fish Canyon, Carpenter Ridge, 

Mammoth Mountain, and Wason Park Tuffs, where the 

related calderas and source areas apparently overlap and 

the individual high-level magma chambers had to develop 

and differentiate in sequence. 

which uplift was a definite stage in the caldera cycle (stage 

V) and was closely confined to the collapsed block and a 

narrow belt around the margin. Uplift clearly seems to 

have resulted from the rise of a central pluton beneath the 

subsided block, and often was accompanied by escape of 

magma along the marginal ring-fracture zones. The 

uplifted blocks within these calderas range from nearly 

symmetrical domes cut by tensional fractures (the Creede, 

Lake City, San Luis, and probably La Garita and Bachelor 

calderas) to tilted trapdoor blocks (the Platoro caldera). 

Less well defined is the long-continued uplift of the area 

of the western San Juan caldera complex. Although some 

typical resurgence-in the sense described by Smith and 

Bailey (1968)-of both the San Juan and Uncompahgre 

calderas may have taken place, the main uplift was of long 

duration and involved an oval-shaped area that included 

both calderas. It began shortly after collapse of the San 

Juan and Uncompahgre calderas, continued through 

filling by locally derived lavas and sediments of the Burns 

and Henson Formations, eruption of the Crystal Lake tuff 

and resultant subsidence of the Silverton caldera, and 

further filling by ash flows from intracaldera sources in 
DISCUSSION 33 

the central San Juan Mountains. Quite possibly the total 

period of recurrent uplift spanned a million or more years. 

This long-continued activity may have resulted in part 

from general buoyant uplift by the major magma chamber 

that underlay the whole western caldera complex. 

Another example of late general uplift is the Mount 

Hope caldera. We saw no evidence for postcollapse 

resurgence, and the succeeding Fish Canyon Tuff appears 

to have passively filled a deep depression. After filling, 

however, the whole area of the caldera was broadly 

upwarped, and so several of the overlying ash-flow sheets 

wedged out laterally against a low dome in the caldera 

area. Again, slight buoyant uplift above either a persisting 

cupola of magma, or a later renewed incursion of magma, 

would account for the relations seen. This type of uplift, 

and to a lesser extent that displayed in the Uncompahgre 

and San Juan calderas in the western part of the volcanic 

field, seems transitional to the broad buoyant uplift that 

affected the whole eastern part of the shallow batholith, as 

described in the section on "Late magmatic uplift." 

MINERALIZATION 

Only about a third of the calderas in the San Juan 

volcanic field are significantly mineralized (Steven, 

Luedke, and others, 1974). These calderas all had complex 

postsubsidence histories involving recurrent intrusion 

and extrusion of magma along the ring-fracture zones and 

related grabens. Some mineralization may have taken 

place during terminal stages of the associated caldera 

cycle, but the association of mineralization with a given 

caldera cycle generally seems tenuous, and the caldera 

seems principally to have provided fractures that guided 

later igneous intrusion and hydrothermal activity. 

The Creede mining district occupies a radial graben on 

the north side of the Creede caldera. Faulting began 

during resurgence of the Bachelor caldera, and was 

reactivated several times later during the volcanic history 

of the area. The last faulting took place either late in the 

Creede caldera development cycle or shortly thereafter, 

when a generally equidimensional area 5-6 km across just 

outside the Creede caldera wall was broken, probably by 

intrusion of a stock at depth (Steven and Eaton, 1975). 

Important silver, lead, zinc, copper, and gold ores were 

deposited widely in fractures in the heart of the broken 

area (Steven and Ratt{, 1965 ). The Creede caldera subsided 

about 26.5 m.y. ago, whereas the ores were deposited about 

24.6 m.y. ago (M.A. Lanphere, P.M. Bethke, and P. B. 

Barton, written commun., 1973). These dates are perhaps 

too widely separated for the mineralization to be 

considered a terminal phase of the Creede caldera cycle of 

development and should, instead, be considered a later, 

unrelated event. 

Mineralization around the nested Platoro and Summitville calderas is even less closely tied to the caldera cycles. 

The calderas formed 29-30 m.y. ago in response to the 

repeated ash-flow eruptions of the Treasure Mountain 

Tuff (Lipman and Steven, 1970; Lipman, 197 5a). The 

ring-fracture zones of these calderas were the sites of 

repeated igneous intrusion that began shortly after caldera 

subsidence and continued to about 20 m.y. ago. 

Significant mineral deposits seem to be concentrated 

where ring fractures of the Summitville and Platoro 

caldera complex intersect a regional northwest-trending 

fault zone (fig. 1) in the Summitville, Stunner, and Platoro 

mining districts. Other mineralized areas in or near this 

complex, at Crater Creek, Jasper, and Cat Creek, are also 

localized by caldera-margin faulting and igneous activity. 

The ores at Summitville have been dated by K-Armethods 

as 22.4 m.y. old (Mehnert, Lipman, and Steven, 1973b), 

; and thus are much too young to be tied to the main caldera 

cycles. 

The western San Juan Mountains have a complex 

history of mineralization with several distinct periods of 

ore deposition extending over an interval of about 15 m.y. 

in late Tertiary time (Lipman and others, 1976). In the 

Lake City area, scattered mineral deposits, currently of 

limited economic significance, occur in the intrusive cores 

of intermediate-composition stratovolcanoes that were 

active between 35 and 30 m.y. ago, before any of the ashflow eruptions that resulted in caldera collapses. 

Significant vein and disseminated mineralization 

occurred within northern parts of the Uncompahgre 

caldera after it collapsed about 28 m.y. ago, but before 

collapse of the Lake City . caldera during eruption of the 

Sunshine Peak Tuff 22.5 m.y. ago. This timing is 

indicated by mineralized areas cut off by the younger 

subsidence and fragments of mineralized rock in landslide debris within adjacent parts of the Lake City caldera 

fill. Additional vein and disseminated mineralization 

occurs within and adjacent to the Lake City caldera, and is 

therefore younger than 22.5 m. y. old. 

Major veins that follow faults of the Eureka graben (fig. 

13) between the Lake City and Silverton calderas seem also 

to have formed after collapse of the Lake City caldera, 

although the graben faults existed as pre-Lake City 

structures. Later mineralization seems indicated by the 

following field relations: (1) We saw no altered rocks or 

vein fragments in landslide breccias within the Lake City 

caldera adjacent to the Eureka graben, although veins and 

mineralized rocks along graben faults extend to the 

structural margin of the caldera. (2) At Engineer Pass on 

the northwest side of the Eureka graben, silicic quartz 

porphyry intrusions that cut the Sunshine Peak Tuff, 

which was erupted from Lake City caldera, occur at the 

junction of a hydrothermally altered breccia pipe with 

associated mineralized veins and have yielded K-Ar and 

fission-track ages of 15-22 m.y. (H. H. Mehnert and C. N. 

Naeser, written commun., 1974). These silicic quartz 

porphyry intrusives, which form a well-defined northeasttrending belt extending about 40 km from northwest of 
34 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

Silverton to north of Lake City, range from quartz latite to 

silicic rhyolite and granite. They are petrologically 

distinct from dated intrusions of the main cycle of ash 

flows and caldera collapse, which occurred at about 28 

m.y., but they have close petrographic affinities to 

magmas of the 22.5-m.y. Lake City cycle. 

In the productive Red Mountain district, on the west 

side of the Silverton caldera, intense alteration and 

breccia-pipe mineralization are similarly associated with 

silicic quartz-porphyry intrusions that have been dated at 

22-23 m.y. (H. H. Mehnert and C. N. Naeser, written 

commun.,_ 1974), suggesting that mineralization in this 

area is young and genetically unrelated to the Silverton 

caldera cycle. The ages of the major vein systems southeast and northwest of the Silverton caldera are less certain, 

but at least some of the major veins southeast of Silverton 

cut quartz-porphyry intrusions similar to those in the 

Engineer Pass and Red Mountain areas, and replacement 

ores associated with the rich veins on the northwest side of 

the Silverton caldera have yielded adularia K-Ar ages of Ill 7 m. y. (F. S. Fisher and H. H. Mehnert, written commun., 

1974). Thus, much-perhaps all-of the productive 

mineralization structurally associated with the Silverton 

caldera appears to have occurred at least 6-10 m.y. later 

than formation of the caldera 28 m.y. ago. The caldera 

appears to have acted mainly as a structural control, with 

some mineralization genetically associated with quartzporphyry intrusions that are petrologically distinct from 

volcanic rocks erupted during the caldera-forming 

process. 

The primary function of calderas in mineralization thus 

appears to be the preparation of zones of weakness in the 

roofs of major magma~hambers. If conditions at depth are 

favorable, som~ of t~ese zones are the sites of recurrent 

igneous intrusion and extrusion, locally accompanied by 

hydrothermal activity and mineralization. Whether this 

activity is the terminal stage of a specific caldera cycle, or is 

later and generally independent, seems accidental at our 

present incomplete state of knowledge. 

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___ 1975, Middle Tertiary volcanic field in the southern 

Rocky Mountains, in Curtis, B. F., ed.,. Cenozoic history of the 

southern Rocky Mountains: Geol. Soc. America Mem. 144, p. 

75-94. 

Steven, T . A., and Eaton, G. P., 1975, Geologic environment of ore 

deposition in the Creede district, San Juan Mountains, Colorado: 

Econ. Geology, v. 70, No. 6, p. 1023-1037. 

Steven, T. A., and Epis, R. C., 1968, Oligocene volcanism in southcentral Colorado, in Epis, R. C., ed., Cenozoic volcanism in the 

southern Rocky Mountains: Colorado School Mines Quart., v. 

63, no. 3, p. 241-258. 

Steven, T. A., and Friedman, Irving, 1968, The source of travertine 

in the Creede Formation, San Juan Mountains, Colorado, in 

Geological Survey research 1968: U.S. Geol. Survey Prof. Paper 

600-B, p. B29-B36. 

Steven, T. A., and Lipman, P. W., 1973, Geologic map of the Spar 

City quadrangle, Mineral County, Colorado: U.S. Geol. Survey 

Geol. Quad. Map GQ-1052. 

Steven, T. A., Lipman, P. W., Hail, W. J., Jr., Barker, Fred, and 

Luedke, R. G., 1974, Geologic map of the Durango quadrangle, 

southwestern Colorado: U.S. Geol. Survey Misc. Geol. Inv. Map 

1-764 . 

-Steven, T. A., Mehnert, H. H., and Obradovich, J. D., 1967, Age ot 

volcanic activity in the San Juan Mountains, Colorado, in Geological Survey research 1967: U.S. Geol. Survey Prof. Paper 

575-D, p. D47-D55. 

Steven, T. A., Lipman, P. W., and Olson, J. C., 1974, Ash-flow stratigraphy and caldera structures in the San Juan volcanic field, 

southwestern Colorado, in Cohee, G. V., and Wright, W. B., 

Changes in stratigraphic nomenclature by the U.S. Geological 

Survey, 1972: U.S. Geol. Survey Bull., 1394-A, p. A75-A82. 

Steven, T. A., Luedke, R. G., and Lipman, P. W., 1974, Relation 

of mineralization to calderas in the San Juan volcanic field, southwestern Colorado: U.S. Geol. Survey Jour. Research, v. 2, no. 

4, p. 405-409. 

Steven, T. A., and Ratte'; J. C., 1964, Revised Tertiary volcanic... 

sequence in the central San Juan Mountains, Colorado, in Short 

papers in geology and hydrology: U.S. Geol. Survey Prof. Paper 

475-D, p. D54-D63. 

___ 1965, Geology and structural control of ore deposition 

in the Creede district, San Juan Mountains, Colorado: U.S. Geol 

Survey Prof. Paper 487, 90 p. 

___ 1973, Geology of the Creede quadrangle, Mineral and 

Saguache Counties, Colorado: U.S. Geol. Survey Geol. Quad. Map 

GQ-1053. 

Varnes, D. J., 1963, Geology and ore deposits of the South Silverton 

mining area, San Juan County, Colorado: U.S. Geol. Survey Prof. 

Paper 378-A, 56 p. 

u U.S. GOVERNMENT PRINTING OFFICE, 1976-677-340/52