Fish Canyon Tuff

Hornblende andesite 

Hill 9519 Hornblende andesite Andesite flow

East wall,

North Pass

caldera

INTRACALDERA UNITS OUTFLOW UNITS

Saguache 

Creek Tuff

Andesite flows ("Conejos") Precursor and reworked tuffs

WEST EAST

Figure 8. Photograph showing east wall of North Pass caldera, viewed obliquely from the south across Saguache valley (along Colorado Hwy 114). Outflow Saguache 

Creek Tuff and underlying lavas and tuffs of Conejos Formation are abruptly truncated along caldera wall, then are onlapped by ponded caldera-filling dacite and 

andesite lavas (~30 Ma) and overlying Fish Canyon Tuff.

ASH-FLOW SHEETS AND

RELATED ROCKS LAVAS AND RELATED ROCKS VOLCANIC SEDIMENTARY ROCKS INTRUSIONS

CORRELATION OF MAP UNITS

Holocene

Pleistocene QUATERNARY

Nelson Mountain caldera cycle

COCHETOPA

PARK

CALDERA

and

SAN LUIS CALDERA

COMPLEX (26.9 Ma)

SOUTH RIVER

CALDERA CYCLE

(27.4 Ma)

BACHELOR CALDERA 

CYCLE (27.5 Ma)

LA GARITA CALDERA

CYCLE (28.0 Ma)

PREVOLCANIC ROCKS

Cretaceous

MESOZOIC

Jurassic

PRECAMBRIAN

Oligocene

TERTIARY

Wason Park Tuff

Carpenter Ridge Tuff

Conejos Formation, lower units 

WESTERN SAN JUAN

CALDERAS (28.3 Ma)

CALDERA 

RELATIONS

(age)1

Nelson Mountain Tuff

Cebolla Creek Tuff

Rat Creek Tuff

VOLCANOES PREDATING

NORTH PASS & BONANZA

CALDERAS (33–34 Ma)

SURFICIAL DEPOSITS

GLACIAL DEPOSITS

Caldera-margin fanglomerate

Intracaldera tuffaceous rocks

Caldera-margin landslide breccia

Sapinero Mesa Tuff

Local lava flows and volcaniclastic rocks

NORTH PASS CALDERA

CYCLE (32.2 Ma)

BONANZA CALDERA

CYCLE (33.2 Ma)

Wall Mountain Tuff (36.9 Ma)

Badger Creek Tuff (34.1 Ma) 

QUATERNARY 

AND TERTIARY

Jurassic

Fill of the Cochetopa Park caldera

SAWATCH RANGE

CALDERAS

Miocene

POSTCALDERA

ROCKS

PALEOZOIC

OLD SURFICIAL DEPOSITS

Huerto Andesite

Fish Canyon Tuff

Intrusions of uncertain age and

affinity (precaldera and caldera)

Cochetopa Dome Rhyolite

Basalt of point Benny

Andesite of Lone Tree Gulch

Tuff of Big Dry Gulch

Andesite of hill 9519

Conejos Formation, upper units (~30 Ma) 

Debris-flow deposits of Buffalo Pass Campground

Luders Creek Tuff

Saguache Creek Tuff

Volcanics of Cochetopa Hills

(fill of North Pass caldera)

Dacite of East Pass Creek

Rhyolite of Taylor Creek

Andesite, undivided

Bedded rhyolitic tuff

Andesitic conglomerate

Dacitic conglomerate

Tuff of Spanish Creek

Upper andesite

Bonanza Tuff

Andesite of Big Dry Gulch

Bedded tuff

Rhyolite of Barret Creek

Andesite of Mountain Lion Creek

Upper volcaniclastic rocks

[Dotted boundaries indicate gradational welding or other emplacement contacts within eruptive units.

Some major formation names are labeled. Some units of a formation are divided by emplacement type]

Hinsdale Formation

1

Ar-Ar ages generalized from Lipman and McIntosh (2008)

Qal Qc

Ql

Qf

Qfo

Qt

Qlo

Qm Qg

Qgo

Twsb

Tws

Twsn

QTl QTf QTg

Thb

Tcv

Tev

Trdn

Trr

Tw

Tcr

Thu

Tndn

Tnd

Tnr

Tnn

Tfc

Tci

Tia Tid

Tca

Tcap Tcd Tcfr

=g

Km

=m

 Tpbm Tpbs Tpbr Tpbd

Tcrn

Tfcn

Tfcb

Tbpv

Tbpf

Tla Tlav

Tbg

Tdc Tdb Tdbm

Taha

Tahh

Tld

Tlr

Tscl

Tscc

Tsc Tscn

Tnpd

Tnpa

Tst

Tsd

Tsa

Tba

Tbd

Tbrl

Tir

Tcsa

Kd

Jj

Jm

Tab

Tbt

Tnpr

Tbfa

Tbfc

Tal

|l

Tcuv

Tts Ttsn

Tpbf Tpba

Tcdu

Tcdc

Tcdm Tcdx

Tcdl

Tct

Tcht

Tbc

Twm

Tcc

Tbpi

Tpt

Tcdi

Tpff Tpcf

LIST OF MAP UNITS

[Some unit exposures on the printed or plotted maps are too small to distinguish the color 

for unit identification. These units are labeled where possible; unlabeled units are 

attributed in the database]

SURFICIAL DEPOSITS

Alluvium (Holocene)

Colluvium (Holocene)

Landslide deposits (Holocene and Pleistocene)

Alluvial-fan deposits (Holocene)

Talus (Holocene)

Older landslide deposits (Pleistocene)

Older alluvial-fan deposits (Pleistocene)

GLACIAL DEPOSITS

Moraine and till (Pleistocene; Pinedale glaciation)

Glacial-outwash and other terrace gravels (Pleistocene; Pinedale 

glaciation)

Older glacial-outwash and other stream gravels (Pleistocene; 

pre-Pinedale glaciation)

OLD SURFICIAL DEPOSITS

High-level landslide or debris-flow deposits (Quaternary and Tertiary?)

Deeply dissected alluvial-fan deposits (Quaternary and Tertiary?) 

High-level gravel deposits (Quaternary and Tertiary?)

MIOCENE POSTCALDERA VOLCANIC ROCKS

Hinsdale Formation

Mafic flows

OLIGOCENE VOLCANIC ROCKS

ROCKS OF COCHETOPA PARK CALDERA AND SAN LUIS CALDERA COMPLEX

Rocks of the Nelson Mountain Caldera Cycle

Caldera-Fill Flows and Intrusions

Cochetopa Dome Rhyolite

Upper lava flow

Crystal-rich flow

Middle lava flow

Glassy flow breccia

Lower lava flow

Intrusions

Caldera-Fill Volcaniclastic Rocks

Caldera-margin fanglomerate

Fanglomerate with clasts of Fish Canyon Tuff

Fanglomerate and sandstone, with clasts of andesitic and rare dacitic 

lava derived from the Conejos Formation

Intracaldera tuffaceous sediments

Caldera-margin landslide breccias

Massive breccia with clasts solely of Fish Canyon Tuff

Breccia with sparse clasts of Carpenter Ridge Tuff

Breccia with clasts of Saguache Creek Tuff

Breccia with clasts of Conejos rhyolite lava

Breccia with clasts of Conejos dacite lava

Breccia with clasts of Conejos andesite lava

Nelson Mountain Tuff

Nonwelded to partly welded dacite

Welded silicic dacite and transitional rhyolite-dacite

Welded to partly welded rhyolite

Nonwelded to partly welded rhyolite and silicic dacite

Rocks Erupted from the Cebolla Creek Caldera 

Cebolla Creek Tuff

Weakly welded mafic dacite

Rocks Erupted from the Rat Creek Caldera

Rat Creek Tuff

Weakly welded dacite

Nonwelded rhyolite

ROCKS ERUPTED FROM SOUTH RIVER CALDERA

Wason Park Tuff

YOUNGER INTERMEDIATE-COMPOSITION LAVA FLOWS

Andesite of Mountain Lion Creek

ROCKS ERUPTED FROM BACHELOR CALDERA

Carpenter Ridge Tuff

Nonwelded to partly welded tuff

Welded rhyolite tuff

ROCKS OF LA GARITA CALDERA CYCLE

Huerto Andesite

Olivine-bearing andesite and hornblende-bearing andesite-dacite, 

undivided

Fish Canyon Tuff

Fault-scarp breccia, with clasts of Fish Canyon Tuff

Widespread welded outflow ignimbrite sheet

Nonwelded to partly welded tuff 

OLDER TUFFS RELATED TO WESTERN SAN JUAN CALDERAS

Sapinero Mesa Tuff

Fault-scarp breccia, with clasts of Sapinero Mesa Tuff

Welded outflow tuff sheet

Nonwelded to partly welded distal tuff

CONEJOS FORMATION, UPPER UNITS

Upper volcaniclastic rocks of the Conejos Formation

Rhyolite of Barret Creek

Coarsely porphyritic rhyolite

Aphyric to sparsely porphyritic rhyolite

Basalt of point Benny

Lava flow

Cone scoria

Vent dike and lava pond

Andesite of Lone Tree Gulch (Saguache valley)

Lava flows

Volcaniclastic rocks

Tuff of Big Dry Gulch

Andesite of Big Dry Gulch

Debris-flow deposits of Buffalo Pass Campground

Distal conglomerate

Mesobreccia and conglomerate

Megabreccia blocks

Andesite of hill 9519

Aphyric andesite

Hornblende andesite

CALDERA-RELATED ROCKS PREDATING THE WESTERN AND CENTRAL 

CALDERA CLUSTERS

North Pass Caldera Cycle

Luders Creek Tuff

Welded dacite

Nonwelded to partly welded rhyolite

Volcanics of Cochetopa Hills

Volcaniclastic deposits

Bedded rhyolitic tuff

Dacitic conglomerate

Andesitic conglomerate

Postcaldera lava flows

Dacite of East Pass Creek

Rhyolite of Taylor Canyon

Andesite, undivided

Saguache Creek Tuff

Landslide breccia of hill 9943

Densely welded crystal-rich rhyolite 

Nonwelded to densely welded crystal-poor rhyolite

Nonwelded crystal-poor rhyolite

Bedded tuff

Tuff of Spanish Creek

Welded tuff

Nonwelded tuff

Intrusions of Uncertain Affinity

Andesite

Dacite

Rhyolite

Bonanza Caldera Cycle

Postcaldera lavas and related rocks

Upper andesite

Bonanza Tuff

Lower dacite

Lower rhyolite, densely welded

VOLCANIC ROCKS PREDATING BONANZA CALDERA

Conejos Formation, Lower Units

Intermediate-composition granitoid intrusions

Lava flows and proximal breccias 

Andesite

Andesite scoria

Plagioclase andesite

Dacite

Rhyolite

Volcaniclastic rocks

Rhyolitic tuff

Tan to light-gray rhyolitic tuff and tuffaceous sandstone

Pink tuff of Home Gulch

Early volcaniclastic sedimentary deposits

EARLY IGNIMBRITES ERUPTED FROM SAWATCH RANGE CALDERAS

Badger Creek Tuff

Wall Mountain Tuff

PREVOLCANIC ROCKS

MESOZOIC SEDIMENTARY ROCKS

Mancos Shale (Cretaceous)

Dakota Sandstone (Cretaceous)

Morrison Formation (Jurassic)

Junction Creek Sandstone (Jurassic)

PALEOZOIC SEDIMENTARY ROCKS

Paleozoic limestone (lower Paleozoic)

PRECAMBRIAN ROCKS

Granitic intrusive rocks, undivided

Metamorphic rocks, undivided

Contact—Dashed where approximately located; dotted where gradational. 

Internal contacts delineate some conspicuous lava flows or gradational 

ash-flow cooling breaks within map units. No contact shown where 

boundary between map units is uncertain or indefinite or where mapping 

stops

Fault—Dashed where approximately located; dotted where concealed or 

occupied by intrusive rocks. Bar and ball on downthrown side

Quaternary slump block—Approximately located; hachures on downthrown 

side

Unconformity along caldera wall—Dashed where approximately located; 

dotted where concealed

Cochetopa (26.9 Ma)

La Garita (28.0 Ma)

North Pass (32.1 Ma)

Strike and dip of beds

Inclined

Horizontal

Strike and dip of foliation

Inclined

Vertical

Horizontal

Hydrothermally altered rock—Areas of conspicuous supergene acid-sulfate 

alteration, mainly due to dissolution of pyrite, in vicinity of subvolcanic 

intrusions

Age determination—In Ma (table 3, CD-ROM). 39Ar/40Ar (b, biotite; h, 

hornblende; ob, obsidian; gm, groundmass separate; no abbreviation, 

sanidine)

32.48±0.05

15

10

Qal

Tnpa

Tscl

Tscc

Tsc

Tscn

Tbt

Tts

Ttsn

Tia

Tid

Tir

Tba

Tbd

Tbrl

Tci

Tca

Tcsa

Tcd

Tcfr

Tcv

Tct

Tcht

Tev

Tbc

Twm

Km

Kd

Jm

Jj

|l

=g

=m

Tcap

Qc

Ql

Qf

Qt

Qlo

Qfo

Qm

Qg

Qgo

QTl

QTf

QTg

Thb

Tcdu

Tcdc

Tcdm

Tcdx

Tcdl

Tcdi

Tpff

Tpcf

Tpt

Tpbf

Tpbm

Tpbs

Tpbr

Tpbd

Tpba

Tndn

Tnd

Tnr

Tnn

Tcc

Trdn

Trr

Tw

Tal

Tcrn

Tcr

Thu

Tfcb

Tfc

Tfcn

Twsb

Tws

Twsn

Tcuv

Tbfc

Tbfa

Tbpf

Tbpv

Tbpi

Tla

Tlav

Tbg

Tab

Tdc

Tdb

Tdbm

Taha

Tahh

Tld

Tlr

Tst

Tsd

Tsa

Tnpd

Tnpr

Geologic Map of the Cochetopa Park and North Pass Calderas, Northeastern San Juan Mountains, Colorado

By

Peter W. Lipmam

2012

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3123

Sheet 2 of 2

Pamphlet and CD-ROM accompany map

Printed on recycled paper

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

For sale by U.S. Geological Survey, Information Services, Box 25286, 

Federal Center, Denver, CO 80225, 1–888–ASK–USGS

Digital files available at http://pubs.usgs.gov/sim/3123/

Suggested Citation: Lipman, P.W., 2012, Geologic map of the Cochetopa 

Park and North Pass calderas, northeastern San Juan Mountains, Colorado: 

U.S. Geological Survey Scientific Investigations Map 3123, 48 p., 2 sheets, 

scale 1:50,000. (Available at http://pubs.usgs.gov/sim/3123/.)

Figure 10. Generalized geologic map of San Luis complex in relation to Cochetopa Park caldera, showing present-day 

caldera margins (unconformities between precaldera and caldera-fill rocks; thick colored lines, dotted where concealed) 

and inferred topographic rim of Cochetopa Park caldera. Modified from Lipman (2006, fig. 5C). Calderas of the San 

Luis complex: CC, Cebolla Creek; NM, Nelson Mountain; RC, Rat Creek. Tfc, Fish Canyon Tuff; FCT, clasts of Fish 

Canyon Tuff in caldera-margin landslide deposits. Geographic names: BC, Baldy Cinco; CD, Cochetopa Dome; MeM, 

Mesa Mountain; MM, Mineral Mountain; NP, North Pass; RCD, Razor Creek Dome; SaP, Saguache Park; SL, San 

Luis Peak; SP, Stewart Peak; STM, Sawtooth Mountain. C. D., Continental Divide.

?

?

C. D.

C. D.

MeM

BC

SL

SP

MM

Cathedral

o

106°45’

o

107°

SaP

La Garita Mtns

N M

C C

R C

BACHELOR

LA GARITA

(north)

Resurgent 

 dome 

Nutras Creek

 Dacite

RCD

STM

CD

FCT

landslide

& faults

Tuffaceous

sediments

FCT landslide

Cochetopa

Park

caldera

Topographic

rim

NP

38°15'

Cochetopa Creek 

Graben

Los Pinos Graben

0 5 10 KM

Intermediate-composition

lava and breccia

Tfc

Tfc

Tfc

38°

Precambrian and Mesozoic

SP

AT

FC

5,000 km3

SM CR

LJ

WM

GP SC

BC B

Sawatch Range Trend

S.E. & W. SJ

Transition Loci Central SJ

(Bcr)

Latir Lake City

CP

NM

WP SMT

SR

16,000

Cumulative volume, km3

0

12,000

8,000

4,000

SP

500

1,000

38

Volume (individual tuff sheets), km3

0

36 34 32 30 28 26 24 22

Age, in m.y.

4,000 km3/m.y.

250 km3/m.y.

1,000 km3/m.y.

2,000 km3/m.y.

NM

AT

VOLCANIC

LOCI

Figure 7. Age-distance-volume plot, showing southward progression of Tertiary ignimbrite-caldera volcanism in the Southern Rocky Mountain 

Volcanic Field. Ignimbrite abbreviations; AT, Amalia Tuff; B, Bonanza Tuff; BC, Badger Creek Tuff; Bcr, rhyolite of Barret Creek flow field; CP, 

Chiquito Peak Tuff; CR, Carpenter Ridge Tuff; FC, Fish Canyon Tuff; GP, Grizzly Peak Tuff; LJ, La Jara Canyon Tuff; NM, Nelson Mountain 

Tuff; SC, Saguache Creek Tuff; SM, Sapinero Mesa Tuff; SMT, Snowshoe Mountain Tuff; SP, Sunshine Peak Tuff; SR, tuff of Stirrup Ranch; 

WM, Wall Mountain Tuff; WP, Wason Park Tuff. SJ, San Juan.

Stewart Peak Volcanics (5)

Baldy Cinco Dacite (4)

Nelson Mountain Tuff, outflow (7)

Mineral Mountain Rhyolite (1)

Cebolla Creek Tuff (3)

Rat Creek Tuff (4)

Nelson Mountain Tuff, 

 intracaldera (4)

South River Volcanics (1)

Wason Park Tuff (1)

Blue Creek Tuff (1)

Carpenter Ridge Tuff (3)

Fish Canyon Tuff (3)

Chiquito Peak

 Tuff (2)*

Crystal Lake Tuff (2)*

Masonic Park 

 Tuff (1)

SANLUISCOMPLEX

Fisher Dacite (4)

Snowshoe Mountain Tuff, 

 intracaldera (5)

Snowshoe Mountain Tuff, outflow (2)

Younger

Older

CREEDE

Cochetopa Dome Rhyolite

Post-caldera

lavas

29 28 27 26 25

Age (Ma), before present

29 28 27 26 25

Age (Ma), before present

Incremental

heating

Laser

fusion

(2)

Biotite

Sanidine

Hornblende

This report

(sanidine)

(4)

(5)

(2)

(5)

(4)

(8)

(3)

(2)

(4)

(2)

(2)

Figure 6. Summary of 40Ar/39Ar age determinations for tuff sheets and lavas of the northeastern and central San Juan 

region, illustrating the narrow range in ages obtained for multiple samples of individual eruptive units, in comparison 

to previously published results. Diagram modified from Lipman (2000, fig. 11). Units are listed in stratigraphic order, 

but sequence of samples plotted for each unit is not significant. Caldera-fill lavas listed in italic type. New determinations by sanidine single-crystal laser fusion are in magenta; bar indicates range in age for multiple samples; number of 

samples in parentheses; samples calibrated to Fish Canyon Tuff at 28.02 Ma. Prior ages (in black, mostly from 

Lanphere, 1988, 2000, and written commun.) are recalibrated to Fish Canyon Tuff at 28.02 Ma. Ages marked by 

asterisks are from Lipman and others (1996) and Bove and others (2001). Dashed horizontal lines connect phenocryst 

phases from single samples; numbers of separately sampled localities analyzed for each unit are listed in parentheses. 

Dashed vertical line (blue), at 27 Ma, provides visual guide for rapid sequence of eruptive events during San LuisCochetopa and Creede caldera cycles, as documented by the new sanidine age determinations (magenta).

Nelson

caldera

Northern

La Garita caldera

North Pass

caldera

Cochetopa Park

caldera

ShC

CD

STM

SP SaC

LPC

RCD

CC

TM

Los Pinos graben

LBB SM

NP

CCn

Cochetopa graben

Figure 5. Oblique view (Google Earth), looking north at Cochetopa and North Pass calderas. The topographic rim of Cochetopa Park caldera (about 30 km across) 

is morphologically well preserved (dashed green lines): along its northwest and north sides, high points on the rim are marked by Sawtooth Mountain (STM) and 

Razor Creek Dome (RCD); the east rim is along the Continental Divide, which defines the boundary with the older North Pass caldera farther to the east; and the 

south boundary also follows the Continental Divide where the Cochetopa Park caldera merges with the northern segment of the La Garita caldera. The northeasttrending Los Pinos graben formed at the time of collapse at La Garita, and the parallel Cochetopa graben, which breaches the north wall of La Garita caldera and 

forms a low segment of Cochetopa rim, is filled by thick tongue of Nelson Mountain Tuff. This lobe of densely welded tuff was channeled northeast from its 

eruptive source within the San Luis caldera complex, at lower left of image, and flowed into the Cochetopa Park caldera where it thins to only a few meters of 

nonwelded tuff in the southeastern caldera basin. Other geographic localities: CC, upper Cochetopa Creek; CCn, Cochetopa Canyon (outlet from Cochetopa Park 

caldera, carved in Precambrian granitic rocks); CD, Cochetopa Dome (sequence of crystal-poor lava flows of petrologically evolved rhyolite that accumulated 

within Cochetopa Park caldera shortly after its collapse); LBB, Long Branch Baldy (high point along Continental Divide, where 33-Ma Bonanza Tuff banks against 

high-standing erosional remnants of a large andesitic stratocone); LPC, Los Pinos Creek; NP, North Pass (followed by Colorado Hwy 114); SaC, Saguache Canyon 

(outlet from La Garita caldera, carved in >32-Ma intermediate-composition lavas, inherited from Oligocene topography); ShC, Sheep Creek; SM, Sargents Mesa 

(high flat along the Continental Divide, locally capped by Bonanza Tuff); SP, Saguache Park (Carpenter Ridge and Wason Park Tuffs, banked against the northern 

La Garita caldera wall); TM, Table Mountain (high-standing erosional remnant of Nelson Mountain Tuff that ponded within the La Garita caldera).

BONANZA

CALDERA

33.2 Ma

Cochetopa

graben

NORTH PASS

CALDERA

33.2 Ma

COCHETOPA

LA GARITA CALDERA

28.0 Ma

NELSON 

CALDERA

CALDERA

 26.9 Ma

MARSHALL

 CALDERA

 33.9 Ma

(Lipman, 2006)

(Lipman, 2006)

NP cycle

Quaternary sediment

Rio Grande rift sediment

Younger lava flows

Caldera-fill sediment, slide breccia

Postcaldera lavas (flow ages, 1 oldest)

Nelson Mtn Tuff (26.9 Ma)

Central San Juan ignimbrites (Fish Canyon 

(Tfc) and Carpenter Ridge Tuffs)

Postcaldera lava and tuffs

Saguache Creek Tuff (32.25 Ma)

Bonanza Tuff (33.2 Ma)

Intermediate-composition intrusions

Older lavas (Conejos Formation)

Pre-Tertiary rocks

Caldera topographic rim

Margin of caldera fill; dotted 

where concealed

Concealed graben fault 

Qal

Ts

Ts

CochetopaNM cycle

pT

EXPLANATION

Figure 4. Generalized geologic map and cross section, northeast San Juan region, showing recent geologic mapping (2000–2008). Some rock units grouped by 

caldera cycle: NP, North Pass; NM, Cochetopa-Nelson Mountain. Geographic localities: HH, Houghland Hill; HL, Houselog Creek; JC, Jacks Creek volcano; 

LBB, Long Branch Baldy; ML, Mount Lion; RCD, Razor Creek Dome; SaM, Sargents Mesa; SM, Sawtooth Mountain; TM, Trickle Mountain. The Nelson 

caldera is youngest subsidence of the San Luis caldera complex. Rectangular grid, boundaries of 7.5' quadrangle maps.

3

2

1

pT

pT pT

pT

pT

pT

pT

Qal

Qal

Ts

Ts

pT

12

8

4

A

Vertical Exaggeration, 3.5

pT

Sawtooth

Mtn

pT

pT

pT

Elevation (feet x 1,000)

0 10 20 KM

12

8

4

A'

Tfc ?

pT

Tsc ?

NORTH PASS

CALDERA

COCHETOPA CALDERA

Continental

Divide

Saguache

Creek

Cochetopa

Creek

Houghland

Cochetopa 

Hill

Dome

Landslide

breccia

Wall Mtn Tuff

 (not shown 

on map)

A

A'

Needle Creek

intrusion

(34.4 Ma)

Rhyolite of Barret

Creek (29.8 Ma)

xSaguache

x

Tracy

Mtn

x

x

SM RCD

x

LBB

x

SaM

Bonanza

Tuff

x

TM

x

JC

HL

x

HH

SAN LUIS VALLEY

ML

AREA

OF MAP

38°

107° 106°

38.5°

Lake City

(23.0)

Lost

Lakes

(28.5)

UC

(28.6)

Silverton

(27.6)

Uncompahge

(28.3)

SJ

(28.3)

Summitville

(29.0)

R C

La Garita

 (28.0)

Creede

 (26.9)

Bachelor

 (27.55)

C C

South 

River 

La Garita

 South

Platoro

(29.8-28.4)

(28.0)

 N M

(26.9)

COCHETOPA PARK

(26.9)

MAP AREA

NORTH

PASS

(32.2)

Bonanza

 (33.2)

Marshall

(33.9)

Gunnison valley

Central San Juan

map area

(Lipman, 2006)

San Juan

basin

San Luis Valley

Figure 2. Map showing calderas (green, brown, purple, and dark red calderas discussed in this report; red, other calderas) in the 

San Juan volcanic region (blue) and location of map area. Dotted lines, inferred locations of caldera margins where concealed by 

younger rocks. Caldera ages (in parentheses) are in Ma. Caldera abbreviations: CC, Cebolla Creek; NM, Nelson Mountain; RC, 

Rat Creek; SJ, San Juan; UC, Ute Creek.

108°

38°

37°

106° 107°

0 10 20 30 40 KM

UA

Conejos Formation

(35–33)

Pre-Tertiary rocks (mainly Precambrian)

BT (33.2)

VC

SCT ??

Rhy fl

Dacite flows

(32.2)

LCT

Debrisflow 

breccia

Saguache

Creek Tuff

(32.25)

EAST

WEST E. WALL,

NORTH PASS

CALDERA

Fish Canyon

Tuff (28.0)

Tuff of

Big Dry

Gulch (30.5)

Andesite of

Lone Tree

Gulch (30.2)

F.G.

Andesite

(30.0)

Tuff of

Spanish Cr

(32.5)

Hornblende

andesite,

Hill 9519

Figure 9. Schematic east-west stratigraphic section, along Saguache valley into the North Pass caldera. Ages of units, in Ma 

(parentheses). BT, Bonanza Tuff; F.G., fine-grained (andesite); LCT, Luders Creek Tuff (32.17 Ma); Rhy fl, intracaldera 

flows (32.2 Ma); SCT, Saguache Creek Tuff (intracaldera); UA, upper andesite of Bonanza center; VC, volcaniclastic 

conglomerates (intracaldera).

Approx. original limit

 of volcanic rocks

Approx. original limit

 of volcanic rocks

l l l l l l

l l l l l l l l l l l l l

l l l l l l l l l l l l

l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l

l l l l l l l

l l l l l

l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l

l l l

l l l l l l l l l l l

l l l l l l l l

l l l l l l l l l l

l l l l l l l l

l l l

l l l l

l l l l l l l l l

l l l l l

l l l

l l l l l l l

l l l

l l l l l l l l l l l l

39-Mile

 volcanic 

area

West Elk

locus

San

Juan

volcanic

locus

MP

GP

MP

Questa-Latir 

 volcanic 

 locus

F r o n t

Ra n g e

South

Park

S a w a t c h

Rge

We t

M ts

S a n g r e

d e

Cris

t o

Mts

Spanish

Peaks

Tu s a s

M ts

Colorado

 Springs

Denver

COLORADO

NEW MEXICO

San Luis

Valley

NP C

Cr

LGn

LGs

Cr

SL

Bz

Pl

SR

S

TG

0 60 Mi

0 100 km

108° 106° 104°

40°

38°

Granitic intrusion 

Mid-Tertiary volcanic areas

Sedimentary fill of Rio Grande rift

Late rift basalts

Mount Princeton batholith

Trend of Colorado Mineral Belt

Caldera

Area of

Figure 4

EXPLANATION

Figure 1. Map of Southern Rocky Mountain volcanic field, showing ignimbrite calderas, major erosional remnants, 

and inferred original extent of mid-Tertiary volcanic cover, caldera-related granitic intrusions, and later sedimentary 

fill of the Rio Grande rift zone. Arrows indicate trend of Late Cretaceous to early Tertiary (Laramide) intrusions of the 

Colorado Mineral Belt. Calderas: Bz, Bonanza; C, Cochetopa Park; Cr, Creede; GP, Grizzly Peak; LGn, La Garita 

north segment; LGs, La Garita south segment; TG, The Gate; Pl, Platoro; S, Silverton; SL, San Luis complex; SR, 

South River. Modified from McIntosh and Chapin (2004); inferred original limit of volcanic rocks modified from 

Steven (1975); intrusions from Tweto (1979) and Lipman (1988, 2000).