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Home , Amalia Tuff, Cochetopa Hills, La Garita Caldera, Rhyolite

Scientific Investigations Map 3123 U.S. Department of the Interior Sheet 2 of 2 U.S. Geological Survey Pamphlet and CD-ROM accompany map

CORRELATION OF MAP UNITS LIST OF MAP UNITS Debris-flow deposits of Buffalo Pass Campground [Dotted boundaries indicate gradational welding or other emplacement contacts within eruptive units. [Some unit exposures on the printed or plotted maps are too small to distinguish the color Tdc Distal conglomerate 0 5 10 KM Some major formation names are labeled. Some units of a formation are divided by emplacement type] for unit identification. These units are labeled where possible; unlabeled units are 107° attributed in the database] Tdb Mesobreccia and conglomerate 38°15' 106°45’ SURFICIAL DEPOSITS SURFICIAL DEPOSITS Tdbm Megabreccia blocks Precambrian and Mesozoic GLACIAL DEPOSITS Qal Alluvium (Holocene) Qal Qc Qf Qt Holocene of hill 9519 Ql Qc Intermediate-composition Colluvium (Holocene) Taha Aphyric andesite RCD Qfo Qm Qg QUATERNARY and Pleistocene Ql Landslide deposits (Holocene and Pleistocene) Qlo Qgo Tahh andesite STM Topographic Qf Alluvial-fan deposits (Holocene) OLD SURFICIAL DEPOSITS -RELATED ROCKS PREDATING THE WESTERN AND CENTRAL rim Qt Talus (Holocene) CALDERA CLUSTERS

C. D. QTl QTf QTg QUATERNARY North Pass Caldera Cycle AND TERTIARY Qlo Older landslide deposits (Pleistocene) CALDERA Luders Creek Qfo Older alluvial-fan deposits (Pleistocene) FCT RELATIONS ASH-FLOW SHEETS AND Tld NP Welded landslide 1 AND RELATED ROCKS VOLCANIC SEDIMENTARY ROCKS INTRUSIONS CD (age) RELATED ROCKS Tfc & faults Cochetopa GLACIAL DEPOSITS Tlr Nonwelded to partly welded rhyolite Park Tuffaceous Thb Qm Moraine and till (Pleistocene; Pinedale glaciation) POSTCALDERA Volcanics of caldera sediments ROCKS Miocene Qg Glacial-outwash and other terrace gravels (Pleistocene; Pinedale Volcaniclastic deposits glaciation) Tst Bedded rhyolitic tuff Tpff Tpcf ? o Qgo Older glacial-outwash and other stream gravels (Pleistocene; 38° pre-Pinedale glaciation) Tsd Dacitic conglomerate Caldera-margin fanglomerate FCT landslide Los Pinos Graben ? Tfc Tcdu OLD SURFICIAL DEPOSITS Tsa Andesitic conglomerate QTl High-level landslide or debris-flow deposits (Quaternary and Tertiary?) Postcaldera lava flows Tcdc Cathedral Graben Tpt Tnpd Dacite of East Pass Creek Tcdm Tcdx QTf Deeply dissected alluvial-fan deposits (Quaternary and Tertiary?) Nutras Creek C C SaP Tnpr Rhyolite of Taylor Canyon Dacite Tcdl QTg High-level gravel deposits (Quaternary and Tertiary?) C. D. COCHETOPA SP PARK Rhyolite Intracaldera tuffaceous rocks Tnpa Andesite, undivided LA GARITA MIOCENE POSTCALDERA VOLCANIC ROCKS MM CALDERA Tcdi Saguache Creek Tuff (north) Tpbf Tpbm Tpbs Tpbr Tpbd Tpba Hinsdale Formation o Fill of the Cochetopa Park caldera Tscl Landslide breccia of hill 9943 SL Caldera-margin landslide breccia Thb flows R C N M Tfc and Tndn Tscc Densely welded -rich rhyolite BC Nelson Mountain caldera cycle VOLCANIC ROCKS Resurgent Tnd Tsc Nonwelded to densely welded crystal-poor rhyolite dome SAN LUIS CALDERA Tnr ROCKS OF COCHETOPA PARK CALDERA AND SAN LUIS CALDERA COMPLEX La Garita Mtns Tscn Nonwelded crystal-poor rhyolite COMPLEX (26.9 Ma) Tnn Rocks of the Nelson Mountain Caldera Cycle MeM Tbt Bedded tuff Nelson Mountain Tuff Caldera-Fill Flows and Intrusions Tuff of Spanish Creek BACHELOR Tcc Cochetopa Dome Rhyolite Tcdu Upper lava flow Tts Welded tuff Cebolla Creek Tuff Figure 10. Generalized geologic map of San Luis complex in relation to Cochetopa Park caldera, showing present-day Tcdc Crystal-rich flow Ttsn Nonwelded tuff caldera margins (unconformities between precaldera and caldera-fill rocks; thick colored lines, dotted where concealed) Trdn and inferred topographic rim of Cochetopa Park caldera. Modified from Lipman (2006, fig. 5C). of the San Intrusions of Uncertain Affinity Trr Tcdm Middle lava flow Luis complex: CC, Cebolla Creek; NM, Nelson Mountain; RC, Rat Creek. Tfc, ; FCT, clasts of Fish BC CD MeM Tia Andesite Canyon Tuff in caldera-margin landslide deposits. Geographic names: , Baldy Cinco; , Cochetopa Dome; , Rat Creek Tuff Tcdx Glassy flow breccia Mesa Mountain; MM, Mountain; NP, North Pass; RCD, Razor Creek Dome; SaP, Saguache Park; SL, San Tid Dacite Luis Peak; SP, ; STM, Sawtooth Mountain. C. D., Continental Divide. SOUTH RIVER Tw Tcdl Lower lava flow CALDERA CYCLE Tir Rhyolite (27.4 Ma) Wason Park Tuff Tcdi Intrusions Bonanza Caldera Cycle Tal Caldera-Fill Volcaniclastic Rocks Postcaldera lavas and related rocks Andesite of Mountain Lion Creek Caldera-margin fanglomerate Tba Upper andesite Tpff Fanglomerate with clasts of Fish Canyon Tuff Tcrn Bonanza Tuff BACHELOR CALDERA Tpcf Fanglomerate and sandstone, with clasts of andesitic and rare dacitic Tcr CYCLE (27.5 Ma) lava derived from the Conejos Formation Tbd Lower dacite Carpenter Ridge Tuff Tpt Intracaldera tuffaceous sediments S.E. & W. SJ Tbrl Lower rhyolite, densely welded VOLCANIC Trend Transition Loci Central SJ Latir Lake City Caldera-margin landslide LOCI Thu VOLCANIC ROCKS PREDATING BONANZA CALDERA Tpbf Massive breccia with clasts solely of Fish Canyon Tuff Huerto Andesite FC Conejos Formation, Lower Units 3 Tpbm 5,000 km Tfcb Breccia with sparse clasts of Carpenter Ridge Tuff Tci Intermediate-composition granitoid intrusions 3 2,000 km3/m.y. CYCLE (28.0 Ma) Tfc Tpbs Breccia with clasts of Saguache Creek Tuff 4,000 km /m.y. Lava flows and proximal breccias SP AT 16,000 Tfcn Tpbr Breccia with clasts of Conejos rhyolite lava Tca Andesite NM WM SM CR Fish Canyon Tuff 1,000 3 Tpbd Breccia with clasts of Conejos dacite lava Tcsa Andesite scoria 1,000 km /m.y.

Twsb Tcap andesite 3 Tpba Breccia with clasts of Conejos andesite lava 250 km3/m.y. WESTERN SAN JUAN Tws Nelson Mountain Tuff Tcd Dacite 12,000 3 CALDERAS (28.3 Ma) LJ CP Twsn Tndn Nonwelded to partly welded dacite Tcfr Rhyolite Sapinero Mesa Tuff Tnd Welded dacite and transitional rhyolite-dacite Tcv Volcaniclastic rocks Tbfc NM Tnr Welded to partly welded rhyolite Rhyolitic tuff GP SC 8,000 500 WP SMT AT Tbfa Tct Tan to light-gray rhyolitic tuff and tuffaceous sandstone Tnn Nonwelded to partly welded rhyolite and silicic dacite Rhyolite of Barret Creek Cumulative volume, km Rocks Erupted from the Cebolla Creek Caldera Tcht Pink tuff of Home Gulch SP

Tcuv (individual tuff sheets), km Volume Tbpf Cebolla Creek Tuff Tev Early volcaniclastic sedimentary deposits Tbpi BC B 4,000 Tbpv Tcc Weakly welded mafic dacite EARLY ERUPTED FROM SAWATCH RANGE CALDERAS SR of point Benny Rocks Erupted from the Rat Creek Caldera Tbc Badger Creek Tuff (Bcr) Tla Tlav Rat Creek Tuff Twm Wall Mountain Tuff Andesite of Lone Tree Gulch Trdn Weakly welded dacite 0 0 38 36 34 32 30 28 26 24 22 PREVOLCANIC ROCKS Trr Nonwelded rhyolite Age, in m.y. Tbg TERTIARY MESOZOIC SEDIMENTARY ROCKS Tuff of Big Dry Gulch Oligocene ROCKS ERUPTED FROM SOUTH RIVER CALDERA Km Mancos Shale () Tw Wason Park Tuff Figure 7. Age-distance-volume plot, showing southward progression of Tertiary -caldera volcanism in the Southern Rocky Mountain Tab . Ignimbrite abbreviations; AT, Amalia Tuff; B, Bonanza Tuff; BC, Badger Creek Tuff; Bcr, rhyolite of Barret Creek flow field; CP, Kd Dakota Sandstone (Cretaceous) Andesite of Big Dry Gulch YOUNGER INTERMEDIATE-COMPOSITION LAVA FLOWS Chiquito Peak Tuff; CR, Carpenter Ridge Tuff; FC, Fish Canyon Tuff; GP, Grizzly Peak Tuff; LJ, La Jara Canyon Tuff; NM, Nelson Mountain Upper volcaniclastic rocks Tuff; SC, Saguache Creek Tuff; SM, Sapinero Mesa Tuff; SMT, Snowshoe Mountain Tuff; SP, Tuff; SR, tuff of Stirrup Ranch; Tal Andesite of Mountain Lion Creek Jm Morrison Formation () WM, Wall Mountain Tuff; WP, Wason Park Tuff. SJ, San Juan. Tdc Tdb Tdbm Local lava flows and volcaniclastic rocks ROCKS ERUPTED FROM BACHELOR CALDERA Jj Junction Creek Sandstone (Jurassic) Debris-flow deposits of Buffalo Pass Campground Carpenter Ridge Tuff PALEOZOIC SEDIMENTARY ROCKS Taha Tcrn Nonwelded to partly welded tuff |l Paleozoic limestone (lower Paleozoic) Tahh Tcr Welded rhyolite tuff PRECAMBRIAN ROCKS Andesite of hill 9519 =g Granitic intrusive rocks, undivided Conejos Formation, upper units (~30 Ma) ROCKS OF LA GARITA CALDERA CYCLE Huerto Andesite =m Metamorphic rocks, undivided Tld Thu -bearing andesite and hornblende-bearing andesite-dacite, Age (Ma), before present Tlr undivided 29 28 27 26 25 Fish Canyon Tuff Contact—Dashed where approximately located; dotted where gradational. Luders Creek Tuff Internal contacts delineate some conspicuous lava flows or gradational Tfcb Fault-scarp breccia, with clasts of Fish Canyon Tuff Younger ash-flow cooling breaks within map units. No contact shown where CREEDEFisher Dacite (4) Tst (2) Tfc Widespread welded outflow ignimbrite sheet boundary between map units is uncertain or indefinite or where mapping Older Bedded rhyolitic tuff stops Snowshoe Mountain Tuff, Tfcn Nonwelded to partly welded tuff (2) Tnpd Tsd Fault—Dashed where approximately located; dotted where concealed or intracaldera (5) OLDER TUFFS RELATED TO WESTERN SAN JUAN CALDERAS occupied by intrusive rocks. Bar and ball on downthrown side Dacite of East Pass Creek Dacitic conglomerate Snowshoe Mountain Tuff, outflow (2) Sapinero Mesa Tuff Quaternary slump block—Approximately located; hachures on downthrown (2)

Tnpr Post-caldera Twsb Fault-scarp breccia, with clasts of Sapinero Mesa Tuff side Rhyolite of Taylor Creek Baldy Cinco Dacite (4) lavas (4) Tws Welded outflow tuff sheet Unconformity along caldera wall—Dashed where approximately located; Cochetopa Dome Rhyolite (8)

(fill of North Pass caldera) Tnpa Tsa dotted where concealed Volcanics of Cochetopa Hills Volcanics Twsn Nonwelded to partly welded distal tuff Stewart Peak Volcanics (5) Andesite, undivided Andesitic conglomerate Cochetopa (26.9 Ma) CONEJOS FORMATION, UPPER UNITS SAN Tscl La Garita (28.0 Ma) NORTH PASS CALDERA Tcuv Upper volcaniclastic rocks of the Conejos Formation Nelson Mountain Tuff, (2)

CYCLE (32.2 Ma) intracaldera (4) Tscc North Pass (32.1 Ma) LUIS Rhyolite of Barret Creek Strike and dip of beds Tbfc Tsc Tscn Coarsely rhyolite 10 Nelson Mountain Tuff, outflow (7) Inclined Tbfa Aphyric to sparsely porphyritic rhyolite COMPLEX Saguache Creek Tuff Horizontal (5) Basalt of point Benny Mineral Mountain Rhyolite (1) Tbt Strike and dip of Tbpf Lava flow 15 Bedded tuff Inclined Cebolla Creek Tuff (3) (3) Tbpv Cone scoria Vertical Rat Creek Tuff (4) Tts Ttsn Tbpi Vent and lava pond (4) Horizontal Andesite of Lone Tree Gulch (Saguache valley) South River Volcanics (1) Tuff of Spanish Creek Tla Lava flows Hydrothermally altered rock—Areas of conspicuous supergene acid-sulfate Wason Park Tuff (1) Tid Tir (5) Tba Tia alteration, mainly due to dissolution of pyrite, in vicinity of subvolcanic Intrusions of uncertain age and Tlav Volcaniclastic rocks intrusions Blue Creek Tuff (1) Upper andesite affinity (precaldera and caldera) 32.48±0.05 Incremental Laser Tbg Tuff of Big Dry Gulch Age determination—In Ma (table 3, CD-ROM). 39Ar/40Ar (b, ; h, Carpenter Ridge Tuff (3) BONANZA CALDERA (4) heating fusion CYCLE (33.2 Ma) Tbd hornblende; ob, ; gm, groundmass separate; no abbreviation, Tab Andesite of Big Dry Gulch Crystal Lake Tuff (2)* Biotite Tbrl ) Sanidine Bonanza Tuff Fish Canyon Tuff (3) Hornblende Chiquito Peak This report (2) Tuff (2)* (sanidine) Tcsa Tci Masonic Park VOLCANOES PREDATING Tcv Tuff (1) Tct Tcap Tcd Tcfr NORTH PASS & BONANZA CALDERAS (33–34 Ma) Tcht Tca 29 28 27 26 25 Age (Ma), before present Conejos Formation, lower units East wall, North Pass 40 39 INTRACALDERA UNITS OUTFLOW UNITS Figure 6. Summary of Ar/ Ar age determinations for tuff sheets and lavas of the northeastern and central San Juan Tev caldera region, illustrating the narrow range in ages obtained for multiple samples of individual eruptive units, in comparison Tbc Hornblende andesite Saguache Creek Tuff to previously published results. Diagram modified from Lipman (2000, fig. 11). Units are listed in stratigraphic order, SAWATCH RANGE Badger Creek Tuff (34.1 Ma) Hill 9519 Fish Canyon Tuff Andesite flow Hornblende andesite but sequence of samples plotted for each unit is not significant. Caldera-fill lavas listed in italic type. New determina- CALDERAS WEST EAST tions by sanidine single-crystal laser fusion are in magenta; bar indicates range in age for multiple samples; number of Twm samples in parentheses; samples calibrated to Fish Canyon Tuff at 28.02 Ma. Prior ages (in black, mostly from Wall Mountain Tuff (36.9 Ma) Lanphere, 1988, 2000, and written commun.) are recalibrated to Fish Canyon Tuff at 28.02 Ma. Ages marked by Andesite flows ("Conejos") Precursor and reworked tuffs asterisks are from Lipman and others (1996) and Bove and others (2001). Dashed horizontal lines connect phases from single samples; numbers of separately sampled localities analyzed for each unit are listed in parentheses. PREVOLCANIC ROCKS Dashed vertical line (blue), at 27 Ma, provides visual guide for rapid sequence of eruptive events during San Luis- Cochetopa and Creede caldera cycles, as documented by the new sanidine age determinations (magenta). Km Cretaceous Kd MESOZOIC Jm Jurassic Jj

|l PALEOZOIC WEST E. WALL, NORTH PASS EAST Figure 8. Photograph showing east wall of North Pass caldera, viewed obliquely from the south across Saguache valley (along Hwy 114). Outflow Saguache Debris- F.G. CALDERA Andesite of =g 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 Saguache flow Andesite Fish Canyon Tuff of Lone Tree 1Ar-Ar ages generalized from Lipman and McIntosh (2008) PRECAMBRIAN andesite lavas (~30 Ma) and overlying Fish Canyon Tuff. breccia Creek Tuff Big Dry (30.0) Tuff (28.0) Gulch (30.2) UA =m (32.25) Gulch (30.5) LCT BT (33.2) Dacite flows Hornblende (32.2) Tuff of Creek andesite, Conejos Formation Mtn Divide Spanish Cr Cochetopa Hill 9519 (35–33) Sawtooth A COCHETOPA CALDERA Continental NORTH PASS A' Rhy fl (32.5) Wall Mtn Tuff CALDERA

(not shown Hill VC 12 Landslide Cochetopa 12 on map) Dome Creek Pre-Tertiary rocks (mainly Precambrian) Saguache

breccia Houghland 108° 106° 104° SCT ?? 40° 8 8 0 60 Mi pT Tfc ? Tsc ? pT pT Figure 9. Schematic east-west stratigraphic section, along Saguache valley into the North Pass caldera. Ages of units, in Ma 0 100 km 4 pT pT 4 (parentheses). BT, Bonanza Tuff; F.G., fine-grained (andesite); LCT, Luders Creek Tuff (32.17 Ma); Rhy fl, intracaldera 0 10 20 KM SCT UA VC F Elevation (feet x 1,000) flows (32.2 Ma); , Saguache Creek Tuff (intracaldera); , upper andesite of Bonanza center; , volcaniclastic r Vertical Exaggeration, 3.5 o conglomerates (intracaldera). n t

107° 106° 38.5° R pT a n pT Ts g 106° e 107° pT

S 108°

l pT

l

l a AREA

GP l South l w l l MAP AREA l l l a Park OF MAP t pT c Colorado Marshall h Gunnison valley 39-Mile Springs (33.9) Ts volcanic Bonanza CCn RCD LBB SM area (33.2) MARSHALL MP Rhyolite of Barret STM

COCHETOPA PARK l CALDERA Approx. original limit l Creek (29.8 Ma) West Elk l l (26.9) l NORTH 33.9 Ma Qal of volcanic rocks R locus PASS LBB BONANZA g x e (32.2) pT A x CALDERA SM RCD Needle Creek SaM 33.2 Ma CD C C x intrusion NP

TG Bz Uncompahge x

l

l

l (34.4 Ma) l

l l

Area of l W (28.3) ShC l

l N M LPC l

C l e l 38° La Garita Figure 4 l NP l t (26.9)

l l Lake City R C

l l l S (28.0) Bonanza

l

l l l l l l

l

l l

l a l

l

l (23.0)

l

l

l l

l l

l 3

l n Tuff l

l

l M

l l l

l Cochetopa Park l l l l g North Pass l l

38° l 1 pT

l l l t

l l l r Bachelor

l SL l l

l s l e Silverton COCHETOPA l

l l l l LGn

l l l l l

l l l (27.55) 2 x

l l pT

l

l l (27.6) caldera

l l caldera l l

l SJ

S l JC

l

l l Cr

l l l l l

l l l l l

l l l l (28.3) l l l l NORTH PASS

l l Lost l l l l l l Cr San

l Lakes CALDERA l l l UC Creede x

l l l Juan 26.9 Ma

l (28.5) (26.9) CALDERA

l (28.6) TM l HH

l SR l l

l graben l l volcanic l l l d Cochetopa x l l l San Luis l l e

l l locus

l LGs l

l l 33.2 Ma

l l Los Pinos graben l l l l l

l Valley l

l l A' Approx. original limit l l l l South HL l l SP l Pl l River of volcanic rocks l l ML SaC l l xSaguache Central San Juan Spanish La Garita LA GARITA CALDERA Northern

South Qal SAN LUIS VALLEY C Cochetopa graben

map area (28.0) 28.0 Ma Tracy r Peaks

(Lipman, 2006)

i NELSON Mtn La Garita caldera

s COLORADO (Lipman, 2006) x t Summitville CALDERA

o (29.0) 38° T NEW MEXICO u (Lipman, 2006)

s Platoro TM l Nelson l a l

s l Questa-Latir (29.8-28.4)

l volcanic M l EXPLANATION caldera l l l l M locus San Juan t s l CC t s basin Qal pT 37° Quaternary sediment Central San Juan ignimbrites (Fish Canyon Pre-Tertiary rocks (Tfc) and Carpenter Ridge Tuffs) Ts EXPLANATION sediment Postcaldera lava and tuffs Caldera topographic rim

Granitic intrusion MP Mount Princeton batholith cycle NP 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) Younger lava flows Ts Saguache Creek Tuff (32.25 Ma) 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

Mid-Tertiary volcanic areas Trend of Margin of caldera fill; dotted 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

l

l where concealed l

l Caldera-fill sediment, slide breccia Bonanza Tuff (33.2 Ma) south boundary also follows the Continental Divide where the Cochetopa Park caldera merges with the northern segment of the La Garita caldera. The northeast- Sedimentary fill of Rio Grande rift l l Caldera l l trending 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 0 10 20 30 40 KM Concealed graben fault Late rift Postcaldera lavas (flow ages, 1 oldest) Intermediate-composition intrusions 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

NM cycle

Cochetopa- CC CCn Figure 1. Map of Southern Rocky Mountain volcanic field, showing ignimbrite calderas, major erosional remnants, Nelson Mtn Tuff (26.9 Ma) Older lavas (Conejos Formation) nonwelded tuff in the southeastern caldera basin. Other geographic localities: , upper Cochetopa Creek; , Cochetopa Canyon (outlet from Cochetopa Park and inferred original extent of mid-Tertiary volcanic cover, caldera-related granitic intrusions, and later sedimentary caldera, carved in Precambrian granitic rocks); CD, Cochetopa Dome (sequence of crystal-poor lava flows of petrologically evolved rhyolite that accumulated fill of the Rio Grande rift zone. Arrows indicate trend of Late Cretaceous to early Tertiary (Laramide) intrusions of the within Cochetopa Park caldera shortly after its collapse); LBB, Long Branch Baldy (high point along Continental Divide, where 33-Ma Bonanza Tuff banks against Colorado Mineral Belt. Calderas: Bz, Bonanza; C, Cochetopa Park; Cr, Creede; GP, Grizzly Peak; LGn, La Garita Figure 2. Map showing calderas (green, brown, purple, and dark red calderas discussed in this report; red, other calderas) in the Figure 4. Generalized geologic map and cross section, northeast San Juan region, showing recent geologic mapping (2000–2008). Some rock units grouped by high-standing erosional remnants of a large andesitic stratocone); LPC, Los Pinos Creek; NP, North Pass (followed by Colorado Hwy 114); SaC, Saguache Canyon north segment; LGs, La Garita south segment; TG, The Gate; Pl, Platoro; S, Silverton; SL, San Luis complex; SR, San Juan volcanic region (blue) and location of map area. Dotted lines, inferred locations of caldera margins where concealed by caldera cycle: NP, North Pass; NM, Cochetopa-Nelson Mountain. Geographic localities: HH, Houghland Hill; HL, Houselog Creek; JC, Jacks Creek ; (outlet from La Garita caldera, carved in >32-Ma intermediate-composition lavas, inherited from Oligocene topography); ShC, Sheep Creek; SM, Sargents Mesa South River. Modified from McIntosh and Chapin (2004); inferred original limit of volcanic rocks modified from younger rocks. Caldera ages (in parentheses) are in Ma. Caldera abbreviations: CC, Cebolla Creek; NM, Nelson Mountain; RC, LBB, Long Branch Baldy; ML, Mount Lion; RCD, Razor Creek Dome; SaM, Sargents Mesa; SM, Sawtooth Mountain; TM, Trickle Mountain. The Nelson (high flat along the Continental Divide, locally capped by Bonanza Tuff); SP, Saguache Park (Carpenter Ridge and Wason Park Tuffs, banked against the northern Steven (1975); intrusions from Tweto (1979) and Lipman (1988, 2000). Rat Creek; SJ, San Juan; UC, Ute Creek. caldera is youngest subsidence of the San Luis caldera complex. Rectangular grid, boundaries of 7.5' quadrangle maps. La Garita caldera wall); TM, Table Mountain (high-standing erosional remnant of Nelson Mountain Tuff that ponded within the La Garita caldera).

Geologic Map of the Cochetopa Park and North Pass Calderas, Northeastern , Colorado 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, By 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 Peter W. Lipmam Park and North Pass calderas, northeastern San Juan Mountains, Colorado: U.S. Geological Survey Scientific Investigations Map 3123, 48 p., 2 sheets, Printed on recycled paper 2012 scale 1:50,000. (Available at http://pubs.usgs.gov/sim/3123/.)