U.S. Department of the Interior Scientific Investigations Map 3394 U.S. Geological Survey Sheet 2 of 2 Pamphlet accompanies map

CORRELATION OF MAP UNITS LIST OF MAP UNITS 05.6' 05.4' 05.2' Tbas Squirrel Gulch andesite Ttmc Megabreccia, Proterozoic clasts dominant Figure 15. A, Geologic map of [Some unit exposures on the printed or plotted maps are too small to distinguish the A A A 10,300 [Dotted unit-box boundaries indicate gradational welding or other emplacement contacts within eruptive units. Some major formation sheet-like mass of flow-layered 10,200 A Tbav Sawatch-Range Ignimbrite (Erupted from Aetna Caldera) A A color for unit identification. These units are labeled where possible; unlabeled units Volcaniclastic rocks rhyolite (see B, C, photo 10.3), A R R names are labelled. Some units of a formation are divided by emplacement type] R R are attributed in the database] Tbc Badger Creek Tuff A Bonanza Tuff interpreted as rheomorphic AM R A M SURFICIAL DEPOSITS GLACIAL DEPOSITS lower Bonanza Tuff (unit Tbrr), A R 10,000 SURFICIAL DEPOSITS Dacite Conejos Formation, Lower Units Q along upper Schoolhouse Gulch R R A A R R Tbdu M A R R Qal Qc Ql Qf Qt Qr Qal Alluvium (Holocene) Upper outflow dacite Early lavas and proximal breccias (GG). Letter symbols mark A R A A Holocene 14.8' R QUATERNARY locations of GPS-located R AR Tcd Dacite R R R A Qlo Qfo Qm Pleistocene Qc Tbd Main dacite 10,100 R A R Colluvium (Holocene) outcrops and areas of monolith- A A A R R R R M A Tcap Plagioclase andesite ologic float inferred to reflect A 9900 Tbdn Nonwelded dacite 10,000 R A Ql Landslide deposits (Holocene) immediately underlying R R R R Tca R R R Tfbd Andesite bedrock: A, andesite; D, dacite; A FLOW-LAYERED R A MIOCENE POSTCALDERA ROCKS Qf Alluvial-fan deposits (Holocene) Intracaldera dacite fracture fill R 9900 R RHYOLITE M, large blocks of andesite, in R Rhyolite Tcab Andesite breccia 9800 (Rheomorphic R A Qt matrix of nonwelded rhyolite A Bonanza Tuff?) Talus (Holocene) R A SEDIMENTARY ROCKS OF THE RIO GRANDE RIFT ZONE Tbru Upper rhyolite tuff; MBR, mega- and 9700 A R R 9800 Tcv Volcaniclastic rocks A R Qfo A R A A A Older alluvial-fan deposits (Pleistocene) meso-breccia of andesite R RR Ttra Tdup Tbr Main intracaldera rhyolite A A R R Jacks Creek Volcano (~34.5 Ma) blocks surrounded by rhyolite 9600 A Tdu Miocene TERTIARY Qlo Older landslide deposits (Pleistocene) A A A Tdua matrix; Q, colluvium; R, flow-lay- R R Travertine Tbrl Lower rhyolite Intrusions 14.6' A M ered rhyolite. Topographic Q A A A A A 9700 Dry Union Formation GLACIAL DEPOSITS Tjir Rhyolite A MBR Tbrb Brecciated rhyolite contour interval, 50 feet; thick 9500 Qr Rock glacier (Holocene) gray lines, graded gravel roads ANDESITE Tjid Dacite A A Q Tbrr Rheomorphic rhyolite of the Elk Horn Ranch subdivi- A AA ROCKS OF THE SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD Qm Till (Pleistocene; Pinedale glaciation) R CALDERA Tjia Andesite sion. B, Highly irregular blocks A Tbrn Nonwelded rhyolite A (outlined in orange; dashed Tree Gulch Lane R RELATIONS A MIOCENE POSTCALDERA ROCKS A R 1 Tjig Granodiorite where approximate) of andesite 9600 (age) IGNIMBRITE SHEETS AND RELATED ROCKS LAVAS AND RELATED ROCKS VOLCANIC SEDIMENTARY ROCKS INTRUSIONS Tfbr Intracaldera rhyolite fracture fill R Ttra Travertine (Miocene?) laharic breccia, enclosed within A A A Schoolhouse Gulch Lavas and related rocks R Intracaldera landslide breccia flow-laminated rheomorphic A Dry Union Formation 9400 D Tjd Dacite rhyolite tuff. Although flow D A Tbmu Mixed clast or undivided megabreccia 9500 Tdu A A A POSTCALDERA Thb Dry Union Formation, undivided Tjds laminated, the rhyolite contains 14.4’ Small-phenocryst dacite 9300 A ROCKS Miocene Tbma Andesite- and dacite-clast megabreccia Tjdb centimeter-size angular R A Hinsdale Formation Tdup Dry Union Formation, Precambrian clasts dominant R andesite fragments (hiking pole, A A Tjap Plagioclase andesite A A Tbmp Plagioclase-andesite-clast megabreccia ~0.9 m). C, Closer view of Tdua Dry Union Formation, volcanic clasts dominant A Tja Andesite contact (dashed line) between Q Tbmc Proterozoic-clast megabreccia SOUTH RIVER Tw Hinsdale Formation block of andesite laharic A CALDERA Tjab Andesite cone breccia breccia and enclosing dense Wason Park Tuff Thb Mafic lavas VOLCANIC ROCKS PREDATING BONANZA CALDERA Q (27.4 Ma) rhyolite that remains fluidal at Conejos Formation, Middle Units Tjfr Rhyolite D the contact. Several small 9300 OLIGOCENE VOLCANIC ROCKS A fragments of andesite enclosed Tracy Volcano (<32.5?–33.5 Ma) Tjv Volcaniclastic rocks 9500 Tcrn IGNIMBRITES AND ASSOCIATED ROCKS OF THE CENTRAL SAN JUAN in the fluidal rhyolite are 14.2’ D DACITE BACHELOR 050 100 METERS CALDERA COMPLEX Intrusions PREVOLCANIC ROCKS circled. CALDERA Tcr 0 100 200 FEET A Rocks Erupted from South River Caldera (27.4 Ma) Tyir Rhyolite (27.5 Ma) Carpenter Ridge Tuff LOWER TERTIARY SEDIMENTARY ROCKS Tw Wason Park Tuff Tyid Dacite Tev Conglomerate and sandstone (Eocene?) B Rocks Erupted from Bachelor Caldera (27.55 Ma) Tyia Andesite PALEOZOIC SEDIMENTARY ROCKS Thu Thv Carpenter Ridge Tuff Tyip Plagioclase andesite Upper Paleozoic sedimentary rocks Huerto Andesite Tcrn Nonwelded ignimbrite LA GARITA CALDERA Tyig Granodiorite s Sharpsdale Formation (Pennsylvanian) Tfc CYCLE (28.0 Ma) Tcr Welded rhyolite ignimbrite Caldera-filling(?) rocks k Kerber Formation (Pennsylvanian) Tfcn Rocks of La Garita Caldera Cycle (28.0 Ma) Tyca Caldera andesite Lower Paleozoic sedimentary rocks Fish Canyon Tuff Huerto Andesite Tydt Dacite caldera-wall talus Ml Leadville Limestone (Mississippian) Tsav Thu Olivine andesite and hornblende andesite-dacite, undivided Tyat Andesite caldera-wall talus and landslide breccia Mlb Brecciated and silicified rocks Andesitic conglomerate Thv Bedded breccia and conglomerate Flank lavas Dc Chaffee Group (Devonian and lower Mississipian) Twsn Fish Canyon Tuff WESTERN SAN JUAN Tyv Volcaniclastic rocks Of Fremont Dolomite (Upper Ordovician) CALDERAS (28.3 Ma) Sapinero Mesa Tuff Tfc Welded outflow ignimbrite sheet Tydu Upper dacite Tfcn Nonwelded to partly welded ignimbrite Oh Harding Quartzite (Middle Ordovician) Tyau Upper andesite Local Volcaniclastic Rocks of Saguache Paleovalley Om Manitou Limestone (Lower Ordovician) Tyap Plagioclase andesite C Tla Tsav Andesitic conglomerate l Lower Paleozoic rocks, undivided Andesite of Lone Tree Gulch Older Ignimbrite Related to Western San Juan Calderas (28.3 Ma) Tyds Small-phenocryst dacite PRECAMBRIAN ROCKS Twsn Sapinero Mesa Tuff Tya Andesite a Aplite of Alder Creek Tbg Conejos Formation, Upper Units (~30 Ma) Tyaq Quartz-xenocrystic andesite g Granitoid intrusive rocks, undivided CONEJOS FORMATION, Tuff of Big Dry Gulch Tla Andesite of Lone Tree Gulch (Saguache Valley) Upper units (~30 Ma) Tyab Andesite breccia m Metamorphic rocks, undivided Tdc Tbg Tuff of Big Dry Gulch Tyfr Rhyolite u Precambrian crystalline rocks, undivided Debris-flow deposit of Debris-flow deposit of Buffalo Pass Campground Buffalo Pass Campground Tysr Rhyolitic sedimentary rocks Tdc Distal conglomerate EXPLANATION OF MAP SYMBOLS Dacite of Red Rock Canyon CALDERA-RELATED ROCKS PREDATING THE WESTERN AND CENTRAL Contact—Dashed where approximately located; dotted where grada- CALDERA CLUSTERS Tyd Dacite lavas LOCAL LAVAS AND VOLCANICLASTIC ROCKS tional. Internal contacts (dash-dot) delineate some conspicuous lava flows or ash-flow cooling breaks within map units. No contact North Pass Caldera Cycle (32.2 Ma) Tydb Dacite breccia shown where boundary between map units is uncertain or indefinite Saguache Creek Tuff Sargents Mesa Volcano Fault—Dashed where approximately located; dotted where concealed or Tsc Tscn Tscn Nonwelded crystal-poor rhyolite Tsa Andesite-dacite lava occupied by intrusive rocks. Bar and ball on downthrown side, where movement known NORTH PASS CALDERA Saguache Creek Tuff Tsc Partly to densely welded crystal-poor rhyolite Rawley Volcanic Complex (~33.2–33.8 Ma) CYCLE (32.2 Ma) Low-angle (thrust?) fault—Teeth on upper block Tbt Tbt Bedded rhyolitic tuff Intrusions Bedded rhyolitic tuff Crest of elliptical resurgent dome Rocks of the Bonanza Caldera Cycle (33 Ma) Trig Granodiorite Quaternary slump block—Dashed where approximately located. Granitoid intrusions Trid Dacite Hachures on downthrown side SOUTHWEST NORTHEAST Spring Creek intrusion Lavas and related rocks Monoclinal hinge zone—Dotted where concealed. Triple hachures Fish Canyon Tuff (28 Ma) Tas Trv Volcaniclastic rocks TERTIARY Aplitic granite indicate direction of uplift 28 Tas Tgs Tag Ttg Tta Tgu Ted Sapinero Mesa Tuff Tgs Porphyritic granite Trfr Rhyolite Unconformity along caldera wall—Dashed where approximately Granitoid intrusions Oligocene located; dotted where concealed Tag Porphyritic granite of Alder Creek Trt Rhyolite tuff Bonanza (33.12 Ma) Turquoise Mine intrusion Tbdh Tbs Trdk Sanidine-bearing dacite Marshall (33.9 Ma) Ttg Granodiorite Tbdk Trdx Dacite shatter breccia CONEJOS FORMATION, upper lava units Bonanaza caldera topographic rim—Dashed where approximately (mostly southwest of Bonanza map area) Tta Intrusive andesite located; dotted where concealed. Erosionally modified Tbfd Trd Dacite 30 Tbav Tbfs Tgu Granitoid rocks, undivided Syncline Trds Small-phenocryst dacite Tbfr Tblr Ted Eagle Gulch Dacite Strike and dip of beds BONANZA CALDERA Volcaniclastic rocks Trap Plagioclase andesite CYCLE (33 Ma) Tbab Tbam Tba Tbap Tbas Intrusions of uncertain age and affinity 40 Inclined Tir Tid Tiap Tia Trax Andesite shatter breccia Volcanics of Cochetopa Hills (North Pass) Postcaldera lavas Tir Rhyolite Vertical Bonanza Tuff Intrusions of uncertain age and Tra Andesite Saguache Creek Tuff (32 Ma) affinity (precaldera and caldera) Tid Dacite Strike and dip of foliation 32 Tras Andesite scoria Postcaldera volcanics (Bonanza) Tiap Plagioclase andesite 35 Inclined Summer (upper andesite, dacite, Porphyry Peak Rhyolite, and others) Tbdu Rocks of the Marshall Caldera Cycle (33.8–33.9 Ma) Coon Tia Andesite Vertical Bonanza Tuff (33 Ma) Tbru Tbru Postcollapse caldera-filling rocks Postcaldera lavas and volcaniclastic rocks Trend of lineation—Showing bearing and plunge. Defined by elongate million years Age, Tracy Tbd Tbdn Tbd Tbdn Tbr Tfbd Tbmu Tbma Tbmp Tbmc Tmd Dacite lavas, undivided Rawley volcano Postcaldera volcanics (Marshall) CONEJOS (Sheep Mountain Dacite, andesite, other dacite) Tbs Lake-bed sedimentary rocks pumice lenses or flow structures. May be combined with foliation (Rawley Andesite, Tbrl Tbrl Tbrb Tbrr Tbrn Tfbr Intracaldera landslide breccia FORMATION Tmdf Small-phenocryst dacite symbol includes dacite and rhyolite) Tbdh Hayden Peak Dacite (undivided) Thorn Ranch Tuff (34 Ma) Outflow tuff Intracaldera tuff Fracture-fill tuff 34 Tmap Plagioclase andesite lavas Tbdk K-feldspar dacite Dacitic tuff (Tfbd) Jacks Tma Andesite lavas, undivided Creek Tbfd Dacite lavas Rhyolitic tuff (Tfbr) Tyca Tydt Tyat Tmr Rhyolite lava Tbfs Finely porphyritic dacite lavas Sample location—X-ray fluorescence (XRF) chemical analyses in Tyv Tydu Tyau Tyir Tyid Tyia Tyip Tyig Tmds Sheep Mountain Dacite tables 3, 5. 39Ar/40Ar ages shown in millions of years (Ma) on map Tbfr Porphyry Peak Rhyolite Tyap Tyds Tya Tyaq Tyab Tyfr Tysr and in tables 4, 6. Sample abbreviation at end of age: b, biotite; gm, [No lavas preserved?] Tmv Volcaniclastic rocks groundmass separate; h, hornblende; no abbreviation, sanidine. Tyd Tydb Tblr Rhyolite of Laughlin Gulch 36 (Table 5, 6 available at https://doi.org/10.3133/sim3394) CONEJOS Tracy volcano ( <32.5?–33.5 Ma) Andesitic rocks Tmp Pitch-Pinnacle Formation FORMATION, XRF Tsa Upper dark andesite Thorn Ranch Tuff Middle units 34.08 40 39 ±0.13b Ar/ Ar Wall Mountain Tuff (37 Ma) (~33.2–33.8 Ma) Sargents Mesa volcano Tbab Ttr Densely welded tuff Breccia cone and spatter REGIONAL LAVA UNITS INTRACALDERA LAVA UNITS 34.08 40 39 Ttrn ±0.13b XRF and Ar/ Ar Trv Trfr Trt Tbam Andesite flows Non-welded to partly welded tuff Figure 9. Diagrammatic stratigraphic relations between regional ignimbrites and early interme- Trig Trid diate-composition lavas (Conejos Formation), northeast San Juan region. Thin-dashed lines delimit Trdk Trdx Trds Trap Trax Tras Ttma Megabreccia, andesite clasts dominant Hydrothermally altered rock—Areas of conspicuous supergene Trd Tra Tba Sparsely porphyritic andesite lavas acid-sulfate alteration, mainly due to dissolution of pyrite, in vicinity locally mapped lava units in relation to associated calderas. Thick-dashed lines (vertical), approx- Rawley volcanic complex (~33.2–33.8 Ma) imate age spans of mapped central volcanoes along east flank of the San Juan Mountains. Tbap Plagioclase-rich andesite lavas Ttmb Megabreccia, clasts of Badger Creek Tuff of subvolcanic intrusions

11 A B Tmd Tmdf Tmap Tma Tmr Tmds Tmv Tmp No. Mean 32 MARSHALL CALDERA Unit (sanidine age, except as noted) Postcollapse caldera-filling rocks Trachydacite Samples Age (Ma)* CYCLE (33.8–33.9 Ma) Ttr Ttrn Ttma Ttmb Ttmc Thorn Ranch Tuff Intracaldera breccia 10 Rhyolite NORTH PASS CALDERA CYCLE Saguache Creek Tuff 6 32.25±0.05 SC T

BONANZA CALDERA CYCLE Post- SAWATCH-RANGE Tbc 9 Trachyandesite Resurgent intrusions caldera IGNIMBRITE Badger Creek Tuff (34.1 Ma) Fine-grain granodiorite/andesite (sn, gm) 4 32.83±0.21 lavas Aplitic granite 2 33.28±0.06 Late postcollapse lavas CONEJOS 33 Tcd Tcap Tca Tcab Tcv Tjd Tjds Tjap Tja Tjab Tjfr Tjv Tjir Tjid Tjia Tjig Dacite of Hayden Peak (gm) 2 32.66±0.18 FORMATION, Tjdb 8

Megacrystic dacite 2 32.76±0.02 Bonanza cycle Lower units Early lavas and proximal breccias Jacks Creek volcano (~34.5 Ma) Rawley

O, in weight percent Caldera-fill lavas (tilted) Granitic

2 complex BZ T Sanidine dacite 3 33.03±0.03 intrusions Basaltic Porphyry Peak Rhyolite 4 33.04±0.04 Marshall

O + Na 7 PREVOLCANIC ROCKS 2 Andesite-dacite (bio, gm) 4 33.0–33.3 caldera

K trachy EXPLANATION Bonanza Tuff lavas

andesite Intracaldera 4 33.05±0.06 Age, Ma Tev Silicic intrusions Bonanza Tuff Eocene? TERTIARY West-side dacite 13 33.12±0.03 Intermediate intrusions Rawley complex Dacite East-side rhyolite (Gribbles Park Tuff) 6 33.14±0.09 TR T cycle 6 Tracy Marshall s Postcaldera silicic lavas Tracy volcano Precaldera central volcanoes Upper Andesite volcano Postcaldera intermed. lavas Jacks Cr volcano Sargents Mesa volcano (bio, gm) 4 32.99±0.15 34 k Paleozoic Rawley volcanic complex (gm) 8 33.3–33.7 5 Precursor(?) rhyolitic lavas 2 33.73±0.09 BC T Mlb 50 60 70 80 Ml PALEOZOIC OLDER VOLCANIC ROCKS SiO , in weight percent Dc 2 Tracy volcano (sn, bio, gm) 10 31.6–33.7 l Lower Figure 6. Alkali-silica diagram, showing compositional variations in rocks of the Bonanza map area. Volcanic and intrusive rocks of the Bonanza caldera Marshall caldera cycle Of Paleozoic Jacks cycle are modestly more alkalic than those from the preceding central volcanoes (Tracy, Jacks Creek). Postcollapse lavas (bio, gm) 6 33.4–33.6 Creek Oh Thorn Ranch Tuff 5 33.93±0.10 volcano Om Aetna caldera, Badger Creek Tuff 2 34.06±0.15 Pre-Bonanza Jacks Creek volcano (bio, gm) 8 34.2–35.3 Cochetopa-North Pass Bonanza Caldera map area (Lipman, 2012) map area (this map) 35 a g 107° 106° *Ages are normalized to Fish Canyon Tuff at 28.02 Ma u Proterozoic PRECAMBRIAN 108° m 1 40 39 Ar-Ar ages generalized from Lipman and others (2015) valley Figure 7. A. Summary of Ar/ Ar age determinations for igneous rocks of the Bonanza region, illustrating the narrow weighted-mean ages obtained for ison multiple samples of individual ignimbrites, lavas, and intrusions of the Bonanza cycle. Abbreviations: bio, biotite; gm, groundmass; sn, sanidine. Data Gunn Marshall from tables 3, 6. B. Eruptive chronology of the Bonanza region shows precursors to eruption of the Bonanza Tuff, including several central volcanoes and (33.9) Marshall caldera, source of the Thorn Ranch Tuff at 33.9 Ma. Abbreviations: BC T, Badger Creek Tuff; BZ T, Bonanza Tuff; SC T, Saguache Creek Tuff; TR Bonanza T, Thorn Ranch Tuff. Lake City map area (33.2) (Lipman, 1976a) COCHETOPA PARK (26.9) NORTH Southeast 106°15' 106°07'30" 106° 106°30' 106° 105°30' 105° PASS 39°30' and 0 5 10 KILOMETERS EXPLANATION (32.2) West San Juan Central Poncha Qal Quaterary surficial deposits EXPLANATION VOLCANIC Sawatch Range Trend Transition Loci San Juan Latir Lake City Pass CC 0 5 MILES Qal Uncompahge LOCI Post-Bonanza ignimbrites Caldera boundary [60+ km] Distance from Bonanza Caldera (28.3) NM The Resurgent intrusions TR Thorn Ranch Tuff Bonanza Tuff, areal 38° (26.9) LALa GARITA Garita FC Gate RIM Lake City RRC C (27.5) 3 PO gr Granite distribution—Dashed (28.0) 4,000 km3/m.y. 2,000 km3/m.y. 5,000 km TO GP Gribble’s Park/Bonanza Tuff (23.0) boundaries approximate San Luis Valley gd Granodiorite 38°22'30" 65 (Rio Grande rift) Silverton Bachelor SP a Andesite (27.6) (27.55) AT MARSHALL * 80 SJ 3 35 Grizzly Peak 1,000 km /m.y. NM CALDERA 55 45 Egd Eagle Gulch Dacite (28.3) S Sheep * * cauldron Lost 16,000 Mountain * * Post-Bonanza lavas—Andesite to rhyolite Lakes Creede a UC n WM B 3 SM CR BONANZA (28.5) 250 km /m.y. * Bonanza Tuff—Rhyolite to silicic andesite Buffalo (28.6) (26.9) 1,000 Peak CALDERA 39° L

Porphyry Whale Hill 75 Del Norte OLIGOCENE Peaks u Antora 55 * Pre-caldera lavas—Andesite to rhyolite SC i map area 3 s Peak 13 gr Andesite and dacite (Lipman, 1976c) Triad THIRTYNINE MILE V

s 60 SOUTH South a Monoclinal t Mount Ridge 5 15 l Rhyolite VOLCANIC AREA

* 3 l 16 u RIVERRiver l a PC hinge gd Princeton 12,000 F e Sargents Figure 14A Sheep Mountain Dacite batholith y LJ g * Guffey La Garita CP Mesa n 30 [60+ km] La Garita i Thorn Ranch Tuff—Rhyolite to dacite South R Central San Juan South 25 * a Summitville Paleozoic sedimentary rocks map area (Lipman, 2006) (28.0) Post- 45 Mount Aetna (29.0) Bonanza 20 * Egd * Caldera- Precambrian granitic rocks cauldron Platoro andesite floor NM 20 Flagstaff * * lavas Bonanza caldera—Dashed where approximate; dotted where concealed TR (29.8–28.4) TR Mountain 35 Resurgence crest C 25 Eroded topographic rim [Eroded] GP WP 8,000 38°15' 30 Salida SMT AT 38°30' GP Canon 500 Pre-Bonanza Ring faults—Bar and ball on downthrown block. Dashed where approximate; City Kerber Creek Platoro map area andesite * * dotted where concealed Mostly [Eroded] Lower Conejos TR SC Cumulative volume, km and dacite * Marshall caldera—Dashed where approximate (Lipman, 1974) East Gulch Tuff map area SP 5 35 55 37° San Juan Wall unconformity (Lipman, 1975) (individual tuff sheets), km Volume 35 rhyolite distribution Basin * Fault—Bar and ball on downthrown block. Dashed where approximate; Marshall BC Creek 4,000 40 dotted where concealed Bonanza Caldera Caldera BONANZA Monoclinal hinge Mostly Figure 14B Jacks Creek Thorn Ranch Tuff TUFF (33.12 Ma) volcano Resurgent crest [60+ km] dacite M distribution Rosita RI O 20 Inclined bedding—Showing strike and dip Saguache (Bcr) T OP Saguache NZA SAN JUAN NA Some caldera megabreccia sites Findley Gulch Peak O VOLCANIC S B 38° a * [80+ km] g 16 Photograph Site—Includes figure number; C, cover. Bar indicates view direction FIELD 0 u 0 a 5 [Eroded/covered] 0 10 20 30 40 50 KILOMETERS c h Dike—Jacks Creek volcano 38 36 34 32 30 28 26 24 22 HH e 0 10 20 MILES C [Covered] 0 10 20 30 MILES re Cross section location—See cross sections in figure 14 ek Qal 38°07'30" 0 15 30 KILOMETERS Age, in millions of years [Mapped 2006-11] Figure 8. Age-location-volume plot, showing southward progression of Tertiary ignimbrite-caldera volcanism in the Southern Rocky Mountain Volcanic Figure 10. Generalized geologic map of Bonanza and Marshall calderas, showing major stratigraphic units, structural Field (revised from Lipman, 2007). Colored vertical bars, volumes of individual ignimbrites, scale on left axis (data from table 1); shaded area, increasing features, and locations of cross sections (fig. 14). Margin of Marshall caldera (unconformity between caldera-fill and Figure 12. Areal extent of preserved erosional remnants of Bonanza Tuff (modified from McIntosh and Chapin, 2004). Figure 2. Map showing calderas (maroon, purple, green, and brown calderas discussed here; red, others) of the San Juan volcanic region (blue) cumulative eruptive volume (right axis). Inset, slopes corresponding to different cumulative eruption rates. Abbreviations: AT, Amalia Tuff; B, Bonanza precaldera rocks), dashed where approximately located. Approximate topographic rim of Bonanza caldera inferred from Eastern areas are dominated by crystal-poor rhyolite, while western area consists mainly of crystal-rich dacite. Figure also and location of map area. Dotted lines, inferred locations of caldera margins where concealed by younger rocks. Caldera ages (in parentheses) Tuff; BC, Badger Creek Tuff; CP, Chiquito Peak Tuff; CR, Carpenter Ridge Tuff; FC, Fish Canyon Tuff; GP, Grizzly Peak Tuff; LJ, La Jara Canyon Tuff; Bcr, distribution of intracaldera rocks and from erosionally modified present-day morphology. Rectangular grid, boundaries shows inferred distribution of the eastern outflow of Thorn Ranch Tuff and East Gulch Tuff. GP, Gribbles Park/Bonanza Tuff; are in Ma. Caldera abbreviations: CC, Cebolla Creek; NM, Nelson Mountain; RC, Rat Creek; SJ, San Juan; UC, Ute Creek. Published map areas are rhyolite of Barret Creek flow field; NM, Nelson Mountain Tuff; SC, Saguache Creek Tuff; SM, Sapinero Mesa Tuff; SMT, Snowshoe Mountain Tuff; SP, of 7.5' quadrangle maps. Abbreviations: HH, Houghland Hill; PC, Peterson Creek; SC, Spring Creek. TR, Thorn Ranch Tuff. outlined and labeled. Sunshine Peak Tuff; TR, Thorn Ranch Tuff; WM, Wall Mountain Tuff; WP, Wason Park Tuff.

A Intracaldera rhyolite and dacite tuff, BROAD CREST OF RESURGENT DOME UAV (granitic intrusive rocks exposed just beyond plane of section) SOUTHWEST ~2.5 km thick NORTHEAST A B MO Marshall Creek Salida Feet, above Flagstaff FLOOR Whale Hill, 12,162’ Sangre de Cristo Range DERA sea level Mountain CAL The Gate PO EXPLANATION 12,000 12,000 MARSHALL CALDERA CALDERA RIM Burned Mountain

f Alluvial-fan deposits CALDE v v v v v v v SM Caldera-floor andesite-dacite (a) R SaM AP A INTRACALDERA CALDERA RING FAULTS FLO PP g Glacial moraine OR r Near-vertical northeast flank, RM Andesite mbreccia (m) m Caldera-collapse megabreccia Creek Wagonbox resurgent dome 10,000 Kerber Creek g Creek Brewery 10,000 WH r Creek Slaughterhouse d Dacitic Bonanza Tuff a m d Resurgent Resurgent Middle Creek hinge d Post-collapse r f r Rhyolitic Bonanza Tuff dacite (d) r granodiorite intrusion r FM Bz San Luis Valley lavas d aplitic granite intrusion  a m d a Andesitic lava flows 8,000 8,000 a HP  Precambrian rocks Rhyolite

(~2.5 km intracaldera Bonanza Tuff; >1 km postcaldera lavas = ~4–5 km total caldera subsidence) 0 2 1 KILOMETER Dacite 0 0.25 0.5 MILES ~1.5 km (5,000 ft) displacement? Figure 14. Cross sections, B showing structures and WEST EAST stratigraphic relations across Bonanza caldera; locations SOUTH NORTH Villa Grove shown on fig. 10. A. South- SOUTH Topographic rim NORTH west topographic rim, across MARSHALL CALDERA Kerber Creek South caldera South flank Gribbles Park Tuff rim ~600 m (2,000 ft) resurgent dome crest of resurgent dome ~500 m Little KC displacement (Whale Hill), to west margin Feet, above Meters,above Intracaldera previously sea level sea level of San Luis Valley (segment of Malpais andesite, 33.0 UP Jacks Creek Fish Canyon T, 28.0 mapped as simple sequence: EXPLANATION 9,000 2,750 Rio Grande rift zone) shows Saguache Creek T, 32.2 Prior intracaldera D large ring-fault displace- andesite-dacite-rhyolite, Upper Paleozoic (clastic rocks) volcano, Fine-grain andesite thickness, estimated: 300 m ments, great thickness of Rawley lavas, ~33.7–33.3 with many fault repeats D ~35.3–34.2 Bonanza Tuff, (Varga and Smith, 1984); S Sharpsdale/Kerber Formations S Kerber Creek L S intracaldera Bonanza Tuff, 33.15 (Burbank, 1932) now 2.5 km Precambrian L L steep dips on flanks of Lower Paleozoic (carbonate rocks) 8,000 granite F F 2,450 Post-collapse lavas of Marshall caldera Precambrian resurgent dome, and inferred L Leadville F M M granite Precambrian location of subcaldera M Landslide breccia D Dyer granite granitoid intrusions. B. South SP (mainly andesite) caldera margin, showing Proterozoic granitic rocks F Fremont 7,000 2,150 Paleozoic Thorn Ranch Tuff (intracaldera), 33.9 displacements across ring rocks (?) M Manitou Formation faults. Figure 13. Generalized regional thickness and compositional variations in the Bonanza Tuff. Outflow ignimbrite is mainly early erupted rhyolite to the east, mainly Figure 5. Oblique view of Bonanza caldera area, viewed from the south. Line definitions: (long dashed where approximate, short dashed later-erupted dacite to the west. Intracaldera tuff contains multiple compositional zones and is ponded to thickness of at least 2.5 km, as exposed on west flank of where concealed) orange, erosionally modified remnants of Bonanza topographic caldera rim; black, caldera-subsidence ring faults; yellow, 0 1 2 KILOMETERS Figure 11. Diagrammatic cross section through concealed south margin of Marshall caldera, showing inferred relations of caldera-filling volcanic units. resurgent dome. Intracaldera tuff was previously mapped as a simple andesite-dacite-rhyolite sequence with many fault repeats (Burbank, 1932). Intracaldera tuff crest of elliptical resurgent dome; green, margin of Marshall caldera. Topographic-name abbreviations: AP, Antora Peak; Bz, town of Bonanza; Possible remnants of the topographic wall are marked by an east-west alignment of paleohills of Precambrian rocks. Post-collapse lavas and volcani- thickness was previously estimated at 300 m (Varga and Smith, 1984) but is currently considered 2.5 km thick. A. Photograph showing densely welded intracaldera dacite, 0 0.5 1 MILE FM, Flagstaff Mountain; HP, Hayden Peak; MO, Mount Ouray; PO, Poncha Pass; PP, Porphyry Peak; RM, Round Mountain; SaM, Sargents Mesa; clastic rocks of this caldera cycle are onlapped by lavas of the Rawley volcanic complex, then by younger ignimbrites (Bonanza, Saguache Creek, and with flattened pumice lenses as long as 0.5 m that define a down-dip lineation, east side road above Squirrel Gulch. B. Photograph showing lower rhyolite, densely welded SM Sheep Mountain; SP, Saguache Peak; UAV, upper Arkansas Valley; UP, Ute Pass; WH, Whale Hill. Imagery from Google Earth (9/24/13). Fish Canyon Tuffs) and interleaved lavas. Ages, million years (Ma); T, Tuff. to its base without any intervening tephra deposits, deposited against caldera-wall andesite at Windy Point (above white dashed line). See figure 10 for photo location.

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 This map or plate also is offered in an online, digital version. Users should be aware that, because of differences in rendering Geologic Map of the Bonanza Caldera Area, Northeastern San Juan Mountains, Colorado processes and pixel resolution, some slight distortion of scale may occur when viewing the online version on a computer screen or when printing it on an electronic plotter, even when it is viewed or printed at its intended publication scale For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1–888–ASK–USGS By Digital files available at https://doi.org/10.3133/sim3394 and https://doi.org/10.5066/P911OL4Q. Printed on recycled paper Suggested citation: Lipman, P.W., 2020, Geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey Scientific Investigations Map 3394, pamphlet 73 p., 2 sheets, scale 1:50,000, Peter W. Lipman ISSN 2329-1311 (print) https://doi.org/10.3133/sim3394. ISSN 2329-132X (online) Associated data for this publication: Robinson, J.E., and Lipman, P.W., 2020, Database for the geologic map of the Bonanza caldera 2020 https://doi.org/10.3133/sim3394 area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey data release, https://doi.org/10.5066/P911OL4Q.