An Ignimbrite Caldera from the Bottom Up: Exhumed Floor and Fill of the Resurgent Bonanza Caldera, Southern Rocky Mountain Volcanic GEOSPHERE; V

Home , Amalia Tuff, Antora Peak, Cochetopa Hills, Hinsdale Formation, La Garita Caldera, Marshall Pass

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GEOSPHERE An ignimbrite caldera from the bottom up: Exhumed floor and fill of the resurgent Bonanza caldera, Southern Rocky Mountain volcanic GEOSPHERE; v. 11, no. 6 field, Colorado doi:10.1130/GES01184.1

1 2 2 24 figures; 4 tables; 4 supplemental files Peter W. Lipman , Matthew J. Zimmerer , and William C. McIntosh 1U.S. Geological Survey, Menlo Park, California 94025, USA 2New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico 87801, USA CORRESPONDENCE: [email protected]

CITATION: Lipman, P.W., Zimmerer, M.J., and McIntosh, W.C., 2015, An ignimbrite caldera from the bottom up: Exhumed floor and fill of the resurgent ABSTRACT while tilting and deep erosion provide three-dimensional exposures of intra- Bonanza caldera, Southern Rocky Mountain volcanic caldera fill, floor, and resurgent structures. The absence of Plinian-fall deposits field, Colorado: Geosphere, v. 11, no. 6, p. 1902–1947, Among large ignimbrites, the Bonanza Tuff and its source caldera in the beneath proximal ignimbrites at Bonanza and other calderas in the region is doi:10.1130/GES01184.1. Southern Rocky Mountain volcanic field display diverse depositional and struc- interpreted as evidence for early initiation of pyroclastic flows, rather than lack tural features that provide special insights concerning eruptive processes and of a high eruption column. Although the absence of a Plinian deposit beneath Received 26 February 2015 Revision received 25 June 2015 caldera development. In contrast to the nested loci for successive ignimbrite some ignimbrites elsewhere has been interpreted to indicate that abrupt Accepted 14 August 2015 eruptions at many large multicyclic calderas elsewhere, Bonanza caldera is rapid foundering of the magma-body roof initiated the eruption, initial caldera Published online 2 October 2015 an areally isolated structure that formed in response to a single ignimbrite collapse began at Bonanza only after several hundred kilometers of rhyolitic eruption. The adjacent Marshall caldera, the nonresurgent lava-filled source tuff had erupted, as indicated by the minor volume of this composition in for the 33.9-Ma Thorn Ranch Tuff, is the immediate precursor for Bonanza, but the basal intracaldera ignimbrite. Caldera-filling ignimbrite has been largely projected structural boundaries of two calderas are largely or entirely separate stripped from the southern and eastern flank of the Bonanza dome, exposing even though the western topographic rim of Bonanza impinges on the older large areas of caldera-floor as a structurally coherent domed plate, bounded caldera. Bonanza, source of a compositionally complex regional ignimbrite by ring faults with locations that are geometrically closely constrained even sheet erupted at 33.12 ± 0.03 Ma, is a much larger caldera system than pre- though largely concealed beneath valley alluvium. The structurally coherent viously recognized. It is a subequant structure ~20 km in diameter that sub- floor at Bonanza contrasts with fault-disrupted floors at some well-exposed sided at least 3.5 km during explosive eruption of ~1000 km3 of magma, then multicyclic calderas where successive ignimbrite eruptions caused recurrent resurgently domed its floor a similar distance vertically. Among its features: subsidence. Floor rocks at Bonanza are intensely brecciated within ~100 m (1) varied exposure levels of an intact caldera due to rugged present-day inboard of ring faults, probably due to compression and crushing of the sub- topography—from Paleozoic and Precambrian basement rocks that are in- siding floor in proximity to steep inward-dipping faults. Upper levels of the truded by resurgent plutons, upward through precaldera volcanic floor, to floor are locally penetrated by dike-like crack fills of intracaldera ignimbrite, a single thickly ponded intracaldera ignimbrite (Bonanza Tuff), interleaved interpreted as dilatant fracture fills rather than ignimbrite vents. The resur- landslide breccia, and overlying postcollapse lavas; (2) large compositional gence geometry at Bonanza has implications for intracaldera-ignimbrite vol- gradients in the Bonanza ignimbrite (silicic andesite to rhyolite ignimbrite; ume; this parameter may have been overestimated at some young calderas

60%–76% SiO2); (3) multiple alternations of mafic and silicic zones within a elsewhere, with bearing on outflow-intracaldera ratios and times of initial cal- single ignimbrite, rather than simple upward gradation to more mafic compo- dera collapse. Such features at Bonanza provide insights for interpreting cal sitions; (4) compositional contrasts between outflow sectors of the ignimbrite deras universally, with respect to processes of caldera collapse and resurgence, (mainly crystal-poor rhyolite to east, crystal-rich dacite to west); (5) similarly inception of subsidence in relation to progression of the ignimbrite eruption, large compositional diversity among postcollapse caldera-fill lavas and resur- complications with characterizing structural versus topographic margins of cal- gent intrusions; (6) brief time span for the entire caldera cycle (33.12 to ca. deras, contrasts between intra- versus extracaldera ignimbrite, and limitations 33.03 Ma); (7) an exceptionally steep-sided resurgent dome, with dips of 40°– in assessing volumes of large caldera-forming eruptions. Bonanza provides a 50° on west and 70°–80° on northeast flanks. Some near-original caldera mor- rare site where intact caldera margins and floor are exhumed and exposed, For permission to copy, contact Copyright phology has been erosionally exhumed and remains defined by present-day providing valuable perspectives for understanding younger similar calderas in Permissions, GSA, or [email protected]. landforms (western topographic rim, resurgent core, and ring-fault valley), some of the world’s most active and dangerous silicic provinces.

108°W 106° 104° 40° N 060 mi INTRODUCTION 0100 km Denver F r The composite Southern Rocky Mountain volcanic field (SRMVF) (Fig. 1) N o n t has long been studied as a site of mid-Tertiary silicic volcanism on especially

voluminous scales (Cross and Larsen, 1935; Larsen and Cross, 1956; Lipman

R et al., 1970; Epis and Chapin, 1974; Steven and Lipman, 1976; McIntosh and a n WMT Chapin, 2004), including at least 28 ignimbrite sheets (each 150–5000 km3) and g

S e

l South

GP l a associated calderas active at 37–23 Ma (Tables 1 and 2). Ignimbrite-caldera sys- l w l l Park l l a t tems of the San Juan Mountains, constituting the largest preserved erosional c Colorado h BP 39-Mile Springs remnant of the SRMVF, have been a special focus for recent volcanologic and volcanic petrologic research: southeastern calderas (Platoro complex: Dungan et al., MP area MA 1989; Lipman et al., 1996), western (Uncompahgre-Silverton–Lake City: Hon Approx. original limit l West Elk l l and Lipman, 1989), and the central cluster (La Garita–Creede calderas: Lipman, of volcanic locusrocks R GunnisoGunnisonn g x e 2000, 2006; Bachmann et al., 2002, 2007). Much less examined have been Bo- Area of Fig. 2 nanza and adjacent Marshall calderas in the northeast San Juan region (Figs.

M Bz

l W

l 2 and 3), which define a transition from earlier volcanism in central Colorado l

l l l e

C l l t l to the larger-volume younger ignimbrite-caldera foci farther southwest (Fig. 1). l NP

l l S

l l

l l l l l l l

l l

l a l

l l

l l M

l Saguachex n Other than mineral-resource studies of small areas (e.g., Scott et al., 1975; Van l

l Saguache

l l

l l l Structural

l l g t

38° l l

l l s

l l

l l l r l SL

l l LO boundary Alstine, 1975; Olson, 1988), most of the northeastern San Juan Mountains until

l l LGn .

l l e

l l

l l l l l l

l SK l

l l l San Luis

S l l

B l

l l l l recently had been examined only in reconnaissance for the Colorado State

l l l

l l ll l l l l

l l l l l l SC l l

l Cr geologic map (Tweto et al., 1976; Tweto, 1979). l San l

l l l l l

l Juan

l Rio Grande rift

l SR l Existence of a caldera has long been inferred in the Bonanza area, based on l l l

l l l volcanic d l l l l Va e

l l locus gravity data (Karig, 1965) and regional reconnaissance studies (Steven and Lip-

lLGs l l l

l l l l l l l

l l ley segme l man, 1976; Varga and Smith, 1984), but detailed geologic mapping, petrologic

Approx. l l l l l l Pl of volcanic rock l l l information, and geochronologic data have been sparse. The only previously Spanish published geologic maps for any part of Bonanza caldera were the pioneering

o C

riginal limi Peaks r report on the Bonanza mining district by Patton (1916) and a more detailed

nt, i

s Colorado t study of the district at a scale of 1:12,000 (Burbank, 1932). These studies dis-

o New Mexico T tinguished the major local rock units for a relatively small area and provided

s t s l Questa-Latir information on mine workings, but both were undertaken before development a l s

l volcanic of modern concepts for ignimbrite volcanism and associated caldera sub M l l l l M locus t t s s sidence. Several theses at the Colorado School of Mines in the late 1960s and . . early 1970s, which focused on the Bonanza area as a “ground-truth” test area Explanation for remote-sensing data, provided lithologic information of varying detail for Granitic intrusion MP, Mount Princeton batholith areas adjacent to the mining district (Bridwell, 1968; Kouther, 1969; Mayhew, Mid-Tertiary volcanic areas

Trend of Colorado Mineral Belt 1969; Knepper and Marrs, 1971; Perry, 1971; Marrs, 1973). These studies in-

Sedimentary fill of Rio Grande Rift l cluded recognition that the unit named Bonanza latite and interpreted as a l l l Caldera l l thick lava sequence by Patton (1916), who carefully described the presence Late rift basalts Regional structural attitude of abundant small fragments of andesite and puzzled over its “rhyoclastic” Figure 1. Map of Southern Rocky Mountain volcanic field, showing ignimbrite calderas, major erosional remnants texture, consisted of welded tuff (Bruns et al., 1971). and inferred original extent of mid-Tertiary volcanic cover, caldera-related granitic intrusions, and later sedimentary Concurrently, a detailed stratigraphy for outflow ignimbrites (ash-flow fill in asymmetric grabens of the Rio Grande rift zone. Graben asymmetry and boundary-fault geometry reverse from tuffs) was developed in the Thirtynine Mile volcanic area farther to the northeast-dipping in the San Luis Valley segment to west-dipping in the Sawatch Range–Upper Arkansas segment to the north. Blue dashed lines, major bounding faults of asymmetrical rift grabens. A diffuse structural-transition boundary east (Fig. 1; Chapin and Epis, 1964; Epis and Chapin, 1968, 1974), but with in- lies south and east of the Bonanza area (green dashed line). Arrows indicate trend of Late Cretaceous–early Tertiary sufficient regional control to locate eruptive sources unambiguously. More (Laramide) intrusions of the Colorado Mineral Belt. Calderas: Bz—Bonanza; B—Bachelor; C—Cochetopa Park; Cr— recent geochronologic and paleomagnetic studies of the distal ignimbrites in Creede; GP—Grizzly Peak; LGn—La Garita north segment; LGs—La Garita, south segment; M—Marshall; MA—Mount Aetna; Pl—Platoro; S—Silverton; SL—San Luis complex; SR—South River. Geographic locations: BP—Buffalo Peaks; the Thirtynine Mile area, and comparisons with intracaldera ignimbrites and LO—Lookout Mountain; SK—Summer Coon volcano; SK—Storm King Mountain; WMT—distal Wall Mountain Tuff on associated rocks along the Sawatch trend as far south as Bonanza, demon- High Plains. Location of Figure 2 indicated by rectangle. Modified from McIntosh and Chapin (2004); inferred original limit of volcanic rocks modified from Steven (1975); intrusions from Tweto (1979) and Lipman (1988, 2000).

strated that one of the eastern ignimbrites, the Gribbles Park Tuff of Epis and Chapin (1974), is indistinguishable in age and paleomagnetic direction from proximal Bonanza Tuff (McIntosh and Chapin, 2004). Despite these discoveries, uncertainties have continued about stratigraphic and structural relationships

Table DR2. 40Ar/39Ar sanidine laser-fusion data. in the Bonanza area and about the mid-Tertiary ignimbrite sources (Grizzly 40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1 (x 10-3) (x 10-15 mol) (%) (Ma) (Ma) Peak, Princeton, and Aetna) that are aligned along the Sawatch Range to the SUPPLEMENTAL TABLE 1. Summary of 40Ar/39Ar analytical data, rocks of Bonanza caldera area. Complete analytical data and probability plots listed in Data-Repository Tables

Sample Unit Location LatitudeLongitude Lab No.Mineral Method# Age(Ma) ±2s* Comments (supplemental notes numbered)

HINSDALE FORMATION 00L-1 Basalt flow Houghland Hill 38 o07.43' 106o17.68' 54993-01 GroundmassPlateau 21.81±0.21 Dense, nonvesicular; overlies CRT; fairly flat spectrum (Lipman and McIntosh, 2008, Table 2) 06L-34, Sanidine, J=0.0007521±0.05%, D=1.004±0.001, NM-205C, Lab#=56946 north (Fig. 1; Shannon, 1988; Fridrich et al., 1991; McIntosh and Chapin, 2004). WASON PARK TUFF Weighted mean age 27.38±0.05 (Lipman and McIntosh, 2008, Table 2) X0229.90 0.0111 19.18 3.56845.981.032.59 0.10 CARPENTER RIDGE TUFF Weighted mean age 27.55±0.05 (Lipman and McIntosh, 2008, Table 2)

HUERTO ANDESITE (Northern equivalent) X0434.03 0.0035 32.65 3.362146.6 71.6 32.78 0.13 05L-46Hbl andesite flow Caps Spruce Mtn 38 o01.02' 106o21.51' 55987-01 HornblendePlateau 25.68±0.79 Probably below CRT; too young, but poor "forced" plateau (Lipman and McIntosh, 2008, Appendix Table 1) o o Our study of the Bonanza area evaluates eruptive and magmatic processes 06L-17Xenocrystic andesiteMill Creek 38 03.28' 106 20.12' 56988 Sanidine SC Mean 27.79±0.06 Above FCT; amazing texture, sanidine xenocrystic? (Lipman and McIntosh, 2008, Appendix Table 1) o o o X0629.76 0.0140 18.05 3.20336.482.132.84 0.11 06L-19-BOlivine andesiteStorm King Mtn 37 57.02' 106 24.90' 56986-01 GroundmassPlateau 28.00±0.24 Above FCT; shown as Hinsdale on Durango 2 map (Lipman and McIntosh, 2008, Appendix Table 1) 06L-19-AOlivine andesiteStorm King Mtn 37 o57.02' 106o24.91' 56982-01 GroundmassPlateau 28.14±0.13 Above FCT; shown as Hinsdale on Durango 2o map (Lipman and McIntosh, 2008, Appendix Table 1) 09 25.08 0.0173 1.484 2.07929.698.333.12 0.08 FISH CANYON TUFF Weighted mean age 28.02 Standardized age

SAPINERO MESA TUFF Bove et al. (2001): 28.38±0.03 (recalc, to FCT @28.02) 05 25.45 0.0126 2.617 5.32340.697.033.18 0.07 o o of silicic Cordilleran volcanism, based on new geologic mapping (mainly sum- 03L-45Partly welded rhyolite Houselog Creek38 05.93' 106 21.95' 54944 Sanidine SC Mean 28.20±0.06 (Lipman and McIntosh, 2008, Appendix Table 1) 10 25.13 0.0132 1.395 2.51338.898.433.23 0.08 CONEJOS FORMATION, UPPER UNITS 05L-40Andesite of Lone Tree GulchS of Lone Tree Gul38 o09.38' 106o22.41' 56013-01 GroundmassPlateau 30.21±0.17 Above tuff of Big Dry Gulch (Lipman and McIntosh, 2008, Table 2) o o 08 25.30 0.0148 1.979 3.91334.597.733.23 0.07 03L-46Tuff of Big Dry GulchBig Dry Gulch38 07.08' 106 24.25' 54999-01 BiotitePlateau 30.47±0.08 No san; excess-Ar bio spectrum (Lipman and McIntosh, 2008, Table 2) 40 39 SAGUACHE CREEK TUFF Weighted mean age 32.25±0.05 (Lipman and McIntosh, 2008, Table 2) 01 25.03 0.0154 1.015 3.00333.298.833.24 0.08 mers of 2007–2011), high-precision Ar/ Ar age determinations (130 localities; BONANZA CALDERA CYCLE 14 25.26 0.0143 1.660 1.25235.798.133.30 0.12 RESURGENT INTRUSIONS Turquoise Mine intrusion 11 25.10 0.0147 1.126 1.53534.898.733.30 0.11 1 08L-41Fine-grain andesite (intr) Betw Peterson & Kelly Cr 38 o17.62' 106o03.53' 58868-01 GroundmassIsochron 32.83±0.21 Isochron 40/36 value indicates excess argon, but isochron & plateau age (33.06±0.21) are indistinguishable 08L-4AAplitic granodioriteRidge, N of Kelly Cr 38 o17.63' 106o03.71' 58869-01 GroundmassIsochron31.91 ±0.28 Grades into coarser granodiorite; age seems young, probable slow cooling; plateau age, 32.80±0.22 Ma (1) 12 29.28 0.0150 15.21 4.56534.084.733.33 0.09 138 mineral and groundmass ages: Fig. 4, Table 3, Supplemental Tables 1 , Alder Creek intrusion 08L-21Porphyritic graniteS of Alder Cr 38 o20.47' 106o05.45' 59100-01 BiotitePlateau 33.02±0.12 South of Alder Creek; distinct from pC aplite; small exposure, within altered area 15 25.94 0.0171 3.751 2.60129.895.733.38 0.10 Spring Creek intrusion 06L-34Pophyritic aplite S of Spring Creek 38o19.43' 106o03.36' 56946 K-feldspar SC Mean 33.26±0.07 Large K-spar; patchy incipient argillic alteration 2 3 06L-33Near-aphyric rhyolite Spring Creek38 o19.39' 106o04.08' 56991 K-feldspar SC Mean 33.30±0.09 Large K-spar phenos; some micro-perthite. Weighted mean is for "youngest" population. Oldest age is 37.3 Ma 03 25.69 0.0101 2.774 1.67750.796.833.43 0.11 08L-16Interior granite 1 Spring Creek38 o19.72' 106o03.47' 58815-01 K-feldspar Plateau34.36 ±0.08 Too old, but complex spectrum; K-feldspar is microperthite (2) 2 , and 3 ), and chemical and petrographic data including new major-oxide 11L-8Interior graniteSpring Creek38 o19.22' 106o04.73' 60877-01 K-feldspar ------[no plateau] Discordant age spectrum suggests slow cooling, ~34-33 to 32 Ma; isochron age, 33.14±0.05 Ma, but large MSWD 07 33.22 0.0133 28.26 5.77038.474.933.43 0.11 Weighted Mean (n=2) 33.28 ± 0.06

West-side intrusions X1328.03 0.0160 10.37 2.90232.089.133.57 0.10 09L-22Granodiorite Elkhorn Gulch38 o18.15' 106o06.22' 59152-01 Sanidine Isochron 33.32±0.05 Isochron 40/36 value indicates excess argon, but isochron & plateau age (33.38±0.08) indistinguishable 4 11L-30 Granodiorite Rawley Gulch38 o19.04' 106o08.06' 60878-01 K-feldspar Isochron 33.03±0.08 Plateau age slightly older than BZT. Isochron suggests excess argon, but best estimate for cooling age Mean age ± 2 n=11 MSWD=1.16 36.4 ±11.7 33.26 0.07 and trace-element analyses for ~280 samples (Table 4, Supplemental Table 4 ).

06L-33, Sanidine, J=0.0007476±0.06%, D=1.004±0.001, NM-205L, Lab#=56991 Methods are summarized in the Appendix and Supplemental Tables. Although 1Supplemental Table 1. Summary of 40Ar/39Ar ana 02 25.13 0.0167 0.8386 3.47830.599.033.25 0.09 08 25.60 0.0145 2.300 5.91135.297.333.31 0.06 01 25.68 0.0132 2.517 4.59338.897.133.33 0.08 surface exposures are not exceptional (heavy vegetation and widespread talus lytical data, rocks of Bonanza caldera area. Please visit X0626.07 0.0153 3.201 7.68833.496.433.58 0.07 X1325.27 0.0141 0.3125 1.96236.299.633.64 0.14 http://dx .doi .org /10 .1130/GES01184 .S1 or the full-text X0925.36 0.0137 0.5349 8.98437.199.433.68 0.06 cover on some steep slopes), the Bonanza center displays unusually complete article on www.gsapubs.org to view Supplemental X0725.53 0.0144 0.7576 3.84635.599.133.81 0.08 X0425.56 0.0146 0.8080 5.79335.099.133.83 0.07 and diverse features of a large ignimbrite caldera cycle. These include volu- X1426.84 0.0148 3.064 4.94534.596.634.65 0.07 Table 1. X1026.40 0.0158 1.333 5.91832.398.534.75 0.07 X1227.23 0.0134 3.940 4.22638.095.734.82 0.08 minous andesite and more silicic lavas erupted before the ignimbrite erup- X1127.78 0.0165 3.603 2.76530.996.235.67 0.11 X0327.38 0.0127 1.376 2.48040.298.536.02 0.11 tions, complex compositional zonations within both the outflow sheet and tuff Table DR1. 40Ar/39Ar step-heating analytical data. X1528.98 0.0153 6.530 2.74133.393.336.12 0.12 X0528.37 0.0150 1.511 3.97034.098.437.27 0.09 ID Power 40 39 37 39 36 39 39 K/Ca 40 39 Age ±1 Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar Mean age ± 2 n=3 MSWD=0.19 34.8 ±8.3 33.30 0.09 concurrently ponded within the caldera as a single ignimbrite unit, extensive (Watts or temp) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma) 10L-13, Sanidine, J=0.0025077±0.01%, IC=1.03182±0.0013, NM-246C, Lab#=60865 portions of the ring-fault system that accommodated caldera subsidence, thick 08L-4A, wr, 17.1 mg, J=0.00101±0.07%, D=1.004±0.001, NM-222E, Lab#=58869-01 X12B 7.885 0.0011 2.18411.698465.2 91.8 32.41 0.02 XA 2.5 452.9 24.00 1411.3 1.37 0.0218.4 0.7 68.85 3.75 04B 8.656 0.0023 4.76221.114217.1 83.7 32.45 0.02 XB 3.0 244.9 22.37 747.6 2.62 0.023 10.51.9 47.14 2.16 iC 3.5 122.2 -2.3428 349.8 4.28 - 15.24.0 33.57 1.29 11B 8.005 0.0008 2.55127.985638.6 90.6 32.46 0.01 compositionally diverse lavas that filled the caldera after subsidence, eroded iD 4.0 70.51 -0.0473 175.5 6.79 - 26.57.4 33.67 0.48 13B 7.609 0.0014 1.19529.718363.4 95.4 32.49 0.01 E 4.5 36.95 -0.0004 63.20 9.85 - 49.5 12.2 33.00 0.22 05B 8.264 0.0006 3.39615.592926.8 87.8 32.51 0.02 F 6.0 28.81 0.0534 35.90 14.8 9.6 63.2 19.4 32.88 0.16 01B 8.007 0.0008 2.52621.092616.6 90.7 32.51 0.01 remnants of the original topographic caldera rim, widespread exhumation of G 8.0 31.10 0.1482 43.79 37.1 3.4 58.4 37.5 32.81 0.14 07B 7.608 0.0006 1.16920.125853.8 95.5 32.51 0.01 iH 10.0 35.01 0.1092 57.71 29.4 4.7 51.3 51.9 32.44 0.18 03B 7.872 0.0001 2.06119.0823585.492.332.52 0.01 XI 18.0 40.89 0.1166 74.52 79.2 4.4 46.2 90.6 34.09 0.21 caldera-floor features, and postcaldera intrusions that core a notably steep re- XJ 25.0 43.10 0.0538 80.78 19.3 9.5 44.6 100.0 34.71 0.23 14B 7.867 0.0001 2.04110.2074316.492.332.52 0.02 08B 8.483 0.0005 4.11243.8041124.385.732.54 0.01 Integrated age ± 2 n=10 204.8 1.0 33.92 0.46 X09B 7.720 0.0002 1.51129.2282418.094.232.57 0.01 Plateau ± 2 steps C-H n=6 MSWD=1.76 102.236 5.000±4.578 49.9 32.80 0.22 surgent dome within the caldera (Fig. 5). 40 36 Isochron±2 steps A-J n=7 MSWD=4.28 Ar/ Ar= 309.8±2.2 31.91 0.28 Among broader topics examined are some complexities of pyroclastic erup- 08L-41, wr, 16.6 mg, J=0.0010169±0.07%, D=1.004±0.001, NM-222E, Lab#=58868-01 3 40 39 XA 2.5 1228.9 4.499 4054.4 0.485 0.11 2.50.4 56.45 11.75 Supplemental Table 3. Ar/ Ar sanidine laser-fusion XB 3.0 296.4 1.222 937.0 1.21 0.42 6.61.4 35.69 2.68 tion and emplacement, geometric relations between caldera subsidence and XC 3.5 137.0 1.117 397.4 1.72 0.46 14.42.9 35.76 1.24 data. Please visit http://dx .doi .org /10 .1130/GES01184 XD 4.0 77.47 2.328 195.4 2.75 0.22 25.75.2 36.26 0.88 XE 4.5 50.56 1.773 107.9 3.27 0.29 37.38.0 34.27 0.66 resurgence, petrologic diversity of sequential eruptions that formed a single ig- Xi F 6.0 42.78 2.467 80.65 6.94 0.21 44.8 13.8 34.86 0.40 .S3 or the full-text article on www.gsapubs.org to G 8.0 31.35 0.3913 44.18 25.0 1.3 58.5 35.0 33.33 0.17 H 10.0 26.88 0.2851 29.71 43.0 1.8 67.4 71.3 32.95 0.11 view Supplemental Table 3. nimbrite, volumes of outflow and intracaldera ignimbrite in relation to caldera I 25.0 28.13 0.7588 33.91 34.0 0.67 64.6 100.0 33.05 0.14 Integrated age ± 2 n=9 118.40.73 33.45 0.40 Plateau ± 2 steps G-I n=3 MSWD=1.72 102.0 1.3 ±1.1 86.2 33.06 0.21 size and inception of subsidence, recurrent eruption of intermediate-composi- 40 36 Isochron±2 steps A-I n=8 MSWD=1.76 Ar/ Ar= 299.6±1.9 32.83 0.21 SUPPLEMENTAL TABLE B. CHEMICAL COMPOSITIONS, ROCKS OF BONANZA CALDERA AREA SampleMap 40Ar/39Ar Latitude Longitude [ANALYSES, NORMALIZED TO ORIGINAL TOTALS W/O VOLATILES]

Number Quad Unit Rock type LocationAge DegMin DegMin SiO2 TiO2 Al2O3 FeTO3 MgOCaO Na2OK2OP2O5 MnOLOI TOTALZnRbSrY Zr Nb Pb Th Ba La Ce Nd Cu VU Cr Cs Ga Mo Ni Sc HINSDALE FORMATION 00L-1LG Thb Basaltic lava flowHoughland Hill 21.81 38 7.35 106 17.68 50.14 1.86 14.83 11.537.308.032.992.680.490.142.87 99.99 101 53 653 21 160 21 5<2 686 23 54 26 46 220 <2 225 <3 22 3 140 tion lavas after caldera-forming events, emplacement of subvolcanic plutons, WASON PARK TUFF (27.38 Ma) 08L-21, bt, 8.14 mg, J=0.0015573±0.15%, D=1.005±0.001, NM-225L, Lab#=59100-01 02L-50TM Tw Densely welded tuff Sheep Creek 27.38 38 9.85 106 27.50 # 70.34 0.40 15.30 2.16 0.42 1.53 4.00 5.24 0.12 0.08 0.63 99.60 49 144 314 27 309 18 20 14 1300 65 11158732 4614 17 10 CARPENTER RIDGE TUFF (27.55 Ma) 27.55 XA 650.0 221.1 0.0333 718.5 1.77 15.34.0 0.6 24.66 3.26 08L-30 CH TcrDensely welded crystal-poor C.D., W of Windy Peak 38 20.75 106 17.50 73.76 0.22 13.65 1.25 0.12 0.72 3.72 5.50 0.03 0.07 0.54 99.03 45 158 80 27 203 19 25 21 443 57 103 38 1106 31504 LATE MAFIC FLOWS (HUERTO ANDESITE) XB 750.0 25.66 0.0080 46.97 7.60 64.2 45.93.3 32.76 0.37 06L-11 LG Thu? Dark fine-grain flow Tracy Mountain382.81 106 19.14 63.91 0.60 16.79 4.98 1.57 4.28 3.35 4.03 0.41 0.11 1.74 100.03 59 78 695 22 227 12 15 <4 1550 47 89 51 565<4<514<23 06L-17 LG Thu Xenocrystic f.g. dark flowMill Creek 27.79 38 3.28 106 20.12 65.42 0.71 15.57 4.60 1.19 3.17 3.95 4.57 0.47 0.06 1.09 99.71 69 106 719 12 287 20 16 <4 1890 97 161 92 28 68 42516<23 06L-19-BLO Thu Andesite flow, above TfcStorm King Mtn 28.00 37 57.02 106 24.91 55.40 1.16 15.93 8.45 4.61 6.75 3.48 3.08 0.63 0.13 1.43 99.61 96 51 1230 22 260 17 11 <4 2010 73 137 76 55 173 <4 182 17 251 magnitude and rates of caldera resurgence, relations to regional extensional XC 850.0 13.86 0.0025 6.472 31.2 207.1 86.2 14.3 33.22 0.09 06L-19-ALO Thu Andesite flow, above TfcStorm King Mtn 28.14 37 57.02 106 24.90 57.50 1.09 15.64 7.58 3.92 6.02 3.68 3.47 0.53 0.11 0.87 99.55 90 63 1090 18 259 16 11 <4 1760 63 121 70 46 150 <4 133 16 <2 32 SAPINERO MESA TUFF (28.27 Ma) XD 920.0 12.60 0.0027 2.233 25.6 188.6 94.8 23.2 33.21 0.08 03L-45 LG Twsn Partly welded distal tuff Houselog Creek 28.20 38 5.93 106 21.95 73.54 0.27 13.67 1.41 0.19 0.77 3.30 6.05 0.07 0.05 1.27 99.33 35 144 142 24 247 22 26 26 1050 70 105 39 4256 5154 CONEJOS FORMATION, Upper Units XE 1000.0 12.70 0.0050 2.479 16.1 101.8 94.2 28.9 33.28 0.08 Andesite of Lone Tree Gulch 05L-40 LMNE Tla Andesite flow South of Lone Tree Gulch 30.21 38 9.38 106 22.41 59.01 1.18 16.29 7.67 1.49 4.68 4.02 3.45 0.58 0.18 2.35 98.55 92 67 101017 255 18 10 <4 1860 67 125 75 41 153 <4 47 18 20 Tuff of Big Dry Guch faulting, involvement of mantle-derived mafic components in magma genera- XF 1075.0 13.01 0.0085 3.621 14.3 60.1 91.8 33.9 33.20 0.10 03L-1TM Tbg Partly welded tuff Alkali Gulch 30.47 38 9.70 106 24.60 67.11 0.60 16.43 3.01 1.21 2.34 2.39 6.07 0.15 0.10 6.23 99.39 65 143 332 32 365 19 20 15 979 54 101 48 8366<5 17 5 SAGUACHE CREEK TUFF (32.25 Ma) 32.25 XG 1110.0 12.80 0.0099 3.589 13.2 51.3 91.7 38.5 32.67 0.09 00L-3CPTsc Crystal-poor welded tuff Corduroy Creek3811.72 106 37.68 72.48 0.22 14.60 1.34 0.16 1.19 3.94 5.24 0.05 0.12 6.14 99.36 66 221 164 32 265 25 28 12 397 88 165 61 <2 15 <2 <3 13 18 32 00L-23-ANPTsc Lower welding zone Pine Creek 38 12.62 106 36.40 72.28 0.24 14.76 1.50 0.15 0.75 4.22 5.53 0.09 0.06 1.08 99.58 69 127 83 31 279 26 28 14 482 85 148 53 3175<3 3193<2 00L-24 NP TscUpper welding zone Pine Creek 38 12.40 106 36.42 72.41 0.22 14.75 1.37 <0.10 0.66 4.39 5.66 0.08 0.07 0.98 99.61 60 129 52 29 289 27 30 14 430 100 177 69 <2 83<3 4193 2 H1180.0 12.78 0.0236 3.181 25.9 21.6 92.7 47.6 32.95 0.08 01L-32A ST Tsc Lower cooling unitUpper Rock Creek38 19.30 106 49.33 73.50 0.21 14.28 1.36 0.14 0.68 4.19 5.39 0.09 0.08 1.13 99.84 56 123 53 38 280 25 27 13 396 95 160 64 3383<3 517<16 01L-32B ST TscUpper cooling unit Upper Rock Creek38 19.35 106 49.43 72.26 0.22 14.87 1.43 0.12 0.67 4.45 5.55 0.07 0.07 0.49 99.64 69 128 52 33 303 27 31 15 399 93 167 62 3195<3 4172 4 02L-24 LMNE TscUnit above Bonanza Tuff Cross Creek 38 11.73 106 19.83 73.62 0.23 13.94 1.37 0.15 0.65 4.20 5.39 0.09 0.11 0.97 99.75 73 125 50 32 282 27 29 13 391 97 169 60 3283<3 8176 I 1210.0 12.49 0.0248 2.092 46.4 20.6 95.1 63.9 33.04 0.06 02L-32 SA TscDensely welded rhyolite Saguache Park 38 2.12 106 38.38 74.45 0.21 13.78 1.47 0.13 0.53 4.10 5.34 0.09 0.04 1.28 100.14 73 124 43 28 272 24 28 12 365 88 147 51 <2 14 4<3617 6 tion, time-space-volume-compositional progressions in the SRMVF, and com- J 1250.0 12.24 0.0035 1.223 68.9 144.1 97.1 88.0 33.04 0.05 BONANZA CALDERA CYCLE RESURGENT INTRUSIONS Alder Creek intrusion K 1300.0 12.18 0.0011 1.016 26.2 484.2 97.5 97.2 33.04 0.07 08L-21 WH Tag Porphyritic granite Alder Creek 33.02 38 20.47 106 5.45 72.31 0.45 14.07 1.86 0.49 0.38 3.32 6.00 0.06 0.02 0.75 98.96 28 274 186 23 326 47 19 36 545 39 77 28 18 32 94 18 Spring Creek intrusion 08L-16 WH TasInterior granite Spring Creek 34.36 38 19.72 106 3.47 72.62 0.28 13.97 1.83 0.47 0.82 4.01 4.89 0.09 0.09 0.56 99.06 60 177 158 23 203 34 22 31 468 74 11933423 72 19 04 L 1680.0 12.92 0.0019 3.630 7.87 269.3 91.7 100.0 32.97 0.12 11L-8WHTas Interior granite Spring Creek 38 19.22 106 4.73 72.84 0.29 13.92 1.74 0.38 0.64 4.15 4.69 0.09 0.08 0.76 98.83 40 178 164 22 220 35 25 29 465 66 107 33 7196 31733 parisons with continental-margin volcanism elsewhere. 06L-33 WH Tas Porphyritic aplite Spring Creek 33.30 38 19.39 106 4.08 74.35 0.28 13.74 1.74 0.35 0.58 3.95 4.88 0.17 0.05 1.01 100.09 127 184 97 15 183 35 30 <2 362 76 130 39 <2 13 4<513<22 06L-34 WH Tas Sparsely porphyritic aplite Spring Creek 33.26 38 19.43 106 3.36 77.60 0.17 12.60 0.74 <0.10 0.24 3.61 5.12 0.10 0.03 0.77 100.20 36 233 10 10 128 45 36 <2 22 58 106 30 <2 14 5<513<22 Turquoise Mine intrusion Integrated age ± 2 n=12 285.0 55.0 33.01 0.16 08L-4A WH Ttg Aplitic granodioriteRidge, N of Kelly Creek 31.91 38 17.63 106 3.71 67.05 0.66 14.92 3.50 1.41 1.99 3.62 5.45 0.23 0.08 1.27 98.91 72 193 398 30 432 46 28 36 942 71 143 56 27 59 11 15 21 68 08L-4B WH TtgGranodiorite Ridge, N of Kelly Creek38 17.63 106 3.71 62.42 0.60 17.41 4.54 0.72 3.81 4.55 3.72 0.38 0.09 1.26 98.24 94 72 985 24 266 17 14 8 1752 63 124 57 3483 21907 08L-14 WH TtgGranodiorite Peterson Creek 38 17.47 106 3.47 55.13 1.51 15.92 8.57 4.33 6.03 3.95 3.68 0.66 0.15 1.31 99.94 114 124 717 35 403 39 18 19 1082 81 170 74 88 186 571214819 08L-41 WH TtaIntrusive andesiteß 32.83 38 17.62 106 3.53 54.14 1.61 15.62 9.20 4.29 6.12 3.35 3.46 0.67 0.15 0.18 98.61 131 120 836 38 472 43 17 22 1227 89 192 81 62 207 625222519 Plateau ± 2 steps H-L n=5 MSWD=0.32 175.2 149.9 ±389.9 61.5 33.02 0.12 06L-35 WH TtgCoarse gabbroTurquois Mine, Peterson Cr 38 17.89 106 3.38 55.17 1.49 15.91 8.71 3.68 5.57 3.30 4.11 0.72 0.14 1.92 98.81 85 161 751 35 530462041350 110 223 115 101 169 52121418 07L-35 WH TtaFine-grain diorite-andesite SE of Hayden Pk 38 15.82 106 5.20 55.90 1.44 16.42 7.77 2.89 6.20 3.20 4.48 0.65 0.17 2.50 99.11 105 173 811 35 466 41 15 20 1094 92 186 80 44 163 726191416 40 36 West-side intrusions Ar/ Ar= 07L-52 WH Tgu Granodiorite Greenback Gulch38 15.70 106 5.95 58.88 1.11 16.27 7.09 2.74 4.47 3.43 4.48 0.41 0.12 1.55 99.02 91 170 634 32 364 34 18 25 1115 82 166 67 75 136 913191412 Isochron±2 steps A-L n=12 MSWD=3.74 292.2±2.8 33.09 0.11 09L-22 WH Tgu Granodiorite (K feldspar)Elkhorn Gulch 33.32 38 18.15 106 6.22 56.11 1.29 16.68 8.17 4.05 6.36 3.94 3.03 0.58 0.16 3.61 100.37 102 86 1012 27 261 22 15 15 1163 60 124 57 57 177 438213717 11L-30 BZ Tgu Granodiorite (K feldspar)Rawley Gulch 33.03 38 19.04 106 8.06 55.68 1.29 16.96 8.02 3.96 5.58 3.93 3.19 0.57 0.14 1.98 99.31 11687 975 27 206 22 16 13 1358 63 129 60 55 186 423223214 Eagle Gulch Dacite 09L-23 WH Ted Eagle Gulch Dacite Ridge, S of Elkhorn Gulch38 17.50 106 6.77 67.44 0.49 16.39 2.75 0.67 1.61 4.12 6.07 0.18 0.09 1.24 99.80 66 111 318 24 511212514 1247 81 152 58 6314 51868 11L-12 WH Ted Eagle Gulch Dacite Ridge, betw Eagle&Elkhorn38 16.82 106 7.26 68.10 0.51 16.05 2.94 0.23 0.76 4.46 5.94 0.21 0.03 1.06 99.21 36 123 290 19 463 20 19 13 1248 48 11138437 47 18 55 07L-34 WH Ted? Plag-bio dacite SE ridge, Hayden Pk 38 16.05 106 5.61 67.33 0.56 16.19 2.53 0.56 1.55 4.55 5.08 0.13 0.15 1.50 98.64 94 123 482 32 330 25 26 13 2014 69 124 52 0224 21905 Dikes and small intrusions 08L-16, kspar, 14.98 mg, J=0.0010236±0.05%, D=1.004±0.001, NM-222H, Lab#=58815-01 11L-2BZTid Dacite intrusion (Ted, Tmds?) Slaughterhouse cirque 38 19.63 106 12.82 62.12 0.80 17.26 4.30 1.63 2.70 4.28 5.07 0.30 0.11 1.41 98.56 80119 732 33 553 25 22 21 1557 96 199 84 13 77 57 18 79 [repeat analysis] 62.66 0.81 17.39 4.55 1.65 2.75 4.32 5.11 0.30 0.11 1.41 99.67 82 121 740 33 563 25 23 21 1574 93 193 83 14 78 66 20 79 11L-13 BZ TidDacite intrusion (Ted, Tmds?) Head, Kerber Cr 38 20.05 106 11.56 66.43 0.59 15.90 3.35 1.17 2.16 4.24 4.88 0.20 0.08 1.85 99.01 64 153 523 26 393 27 27 32 1142 78 147 60 19 54 78 19 66 06L-40 GG TidDacite intrusion Kerber Creek junction 38 12.90 106 4.43 63.71 0.70 16.18 4.14 1.54 3.17 3.68 4.71 0.34 0.08 2.94 98.26 53 104 443 27 332 2716<2 1720 65 11955968 <4 815<23 12L-6BZTia Andesite dike Upper Squirrel Gulch38 19.43 106 8.55 61.60 0.96 16.90 5.65 1.86 4.61 3.00 4.66 0.37 0.10 4.42 99.72 90 119 651 27 287 20 24 17 1355 57 1195119118 33 21 414 12L-7BZTia Andesite dike Upper Kerber Creek38 19.15 106 9.32 57.60 1.16 17.20 7.05 2.66 6.59 3.28 3.31 0.44 0.13 4.98 99.42 96 69 802 29 234 16 15 9 1084 47 97 46 15 152 21 20 216

POSTCOLLAPSE LAVA FLOWS & LOCAL TUFFS 10L-13 BZ Welded tuff of uncertain affinity Flagstaff Mtn, NE ridge 32.51 38 16.72 106 11.83 76.90 0.22 12.40 1.34 0.51 0.26 0.92 6.72 0.05 0.03 2.66 99.34 #46 200 129 17 198 42 34 49 477 15 85 11 115102 13 4 Dacite of Hayden Peak 07L-53 WH Tbdh Dacite of Hayden Peak Hayden Peak, east 32.58 38 16.47 106 6.01 69.73 0.45 15.42 1.99 0.31 0.86 4.52 5.77 0.06 0.06 1.03 99.18 69 144 26131 378 28 30 11 1825 57 11650010 42 17 05 REGIONAL FRAMEWORK 09L-15 WH Tbdh Dacite of Hayden Peak NW trib Greenback Gulch 32.76 38 16.28 106 6.71 70.55 0.43 15.54 1.75 0.22 0.83 4.61 5.61 0.06 0.06 0.90 99.66 75 142 242 31 390 30 34 13 1689 60 123 49 1851 18 36 2 40 39 08L-6WH Tbdh Dacite of Hayden Peak Hayden Peak, N ridge 34.08 38 16.80 106 6.20 69.22 0.45 15.65 1.78 0.24 0.78 4.56 5.59 0.06 0.27 0.72 98.60 83 140 262 33 383 29 31 13 1889 63 131 52 0852 19 07 Late dacite flows 10L-51 BZ Tbdk Large-san dacite flow/intrusion? Head, Squirrel Gulch38 21.98 106 8.65 64.23 0.68 17.58 3.91 0.61 2.46 4.01 5.72 0.25 0.08 1.50 99.54 75 125 534 35 553 24 23 22 1508 100 208 87 9634 31938 Supplemental Table 2. Ar/ Ar step-heating ana 08L-43 MO Tbdk Large-sanidine dacite flow Poncha Creek 32.79 38 24.28 106 9.26 66.14 0.64 15.66 3.70 0.69 2.44 4.26 4.58 0.26 0.06 0.73 98.43 70 128 57123 257 30 18 21 1180 63 119422271414 19 13 6 08L-44 BZ Tbdk Large-sanidine dacite flow Silver Creek 32.76 38 21.39 106 10.24 65.65 0.69 15.50 3.99 1.65 3.22 4.05 4.24 0.27 0.07 1.16 99.33 77 122 583 23 254 29 20 20 1063 63 112411879419 20 11 8 08L-31 BZ Tbfd? Altered dacite (or l Trp?) Head, Squirrel Gulch38 20.79 106 9.02 72.27 0.58 16.55 1.07 0.46 0.47 1.71 5.22 0.20 0.02 3.28 98.54 23 160 155 27 392 28 24 35 908 82 163 61 11 49 10 619 lytical data. Please visit http://dx .doi.org/10 .1130 Mid-Tertiary volcanic deposits once were continuous across much of the /GES01184.S2 or the full-text article on www.gsapubs 4Supplemental Table 4. Chemical compositions, rocks .org to view Supplemental Table 2. of Bonanza caldera area. Please visit http://dx .doi Colorado and northern New Mexico mountains (Fig. 1), constituting the com- .org/10 .1130/GES01184 .S4 or the full-text article on posite Southern Rocky Mountain volcanic field (SRMVF), for which the San www.gsapubs.org to view Supplemental Table 4. Juan region is the largest erosional remnant (Steven, 1975; Lipman, 2007). Subareas of the SRMVF, dismembered by subsequent erosion, have commonly been described, somewhat misleadingly, as separate volcanic fields (San Juan, Sawatch, Thirtynine Mile, Latir, West Elk, and Central Colorado), rather than as time-space transgressive magmatic foci within a large composite field.

TABLE 1. SUMMARY OF REGIONAL IGNIMBRITES, CALDERA SOURCES, AND 40AR/39AR AGES, SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD Ignimbrite Caldera

SiO2 Volume Age Area Tuff (%) Rock and phenocrysts (km3) (Ma)Name (km) Western San Juan Mountains Sunshine Peak Zoned, 76–68 Qtz, sodic san200–500 23.0 Lake City 15 × 18 Latir Mountains, New Mexico Amalia 76–77 Peralkaline: qtz, sodic san500 25.1 Questa 14 × >15 Central San Juan Mountains Snowshoe Mountain 62–66 Xl-rich dacite>50026.9 Creede20× 25 Central San Juan Mountains: San Luis caldera complex Nelson Mountain Zoned: 74–63 Xp rhy; xl dacite >500 26.9 San Luis–Cochetopa9× 9, 20 × 25 Cebolla Creek 61–64 Xl dacite, hbl, no san250 26.9 San Luis complex14× 16 Rat Creek Zoned: 74–65 Xp rhy–xl dacite 15026.9San Luis complex9× 12 Central San Juan Mountains Wason Park Zoned: 72–63 Xl rhyolite–dacite >500 27.4 South River20× 20 Blue Creek 64–68 Xl dacite, no san250 ~27.45 [concealed] — Carpenter Ridge Zoned: 74–66 Xp rhy–xl dacite >1000 27.55Bachelor25× 30 Western San Juan Mountains Crystal Lake 72–74 Xp rhyolite 50–100 27.6 Silverton 20 × 20 Central San Juan Mountains Fish Canyon 66–68 Xl dacite, hbl,qtz 5000 28.0 La Garita35× 75 Western San Juan Mountains Sapinero Mesa 72–75 Xp rhyolite >1000 28.3 Uncompahgre–San Juan 20 × 45 Dillon Mesa 72–75 Xp rhyolite 50–100 28.5 Uncompahgre? 20 × 20 Blue Mesa 72–74 Xp rhyolite 200–500 28.5 Lost Lakes (buried) 10 × 10 Ute Ridge 66–68 Xl dacite, sanidine 250–500 28.6 Ute Creek 8× 8 Southeast San Juan Mountains (Treasure Mountain Group) Chiquito Peak 64–67 Xl dacite, sanidine 500-1000 28.6 Platoro 18 × 22 Central San Juan Mountains Masonic Park 62–66 Xl dacite, no san500 28.7 [concealed] — Southeast San Juan Mountains (Treasure Mountain Group) South Fork 68–70 Xl dacite, sanidine 50–100 28.8 Platoro/Summitville?8× 12? Ra Jadero 64–66 Xl dacite, sanidine 15028.9Summitville? 8× 12 Ojito Creek 67–70 Xl dacite, no san100 ~29.5Summitville? 8× 12 La Jara Canyon 66–68 Xl dacite, no san500–100030 Platoro 20 × 24 Black Mountain 67–69 Xl dacite, no san200–500 30.1 Platoro — Northeast San Juan Mountains Saguache Creek 73–75 Alkali rhyolite, no bio250–500 32.2 North Pass 15 × 17 Bonanza Zoned: 76–60 Zoned: rhy-dac-rhy-dac 1000 33.1 Bonanza15× 20 Thorn Ranch 77– Xl dacite; Xp rhyolite 250–500?33.9Marshall Creek 10 × 10? North-South Sawatch Range Trend Badger Creek 69–70 Xl dacite 250? 33.9 Mount Aetna10× 15 Grizzly Peak Zoned: 77–57 Xl rhy-dacite-andesite 500? 34.3 Grizzly Peak 15 × 17 Wall Mountain 70–73 Xl rhyolite 1000±36.9Mount Princeton15× 30? Note: Updated and simpliﬁed from Table 1 in Lipman (2012). Rhy—rhyolite; dac—dacite; san—sanidine; bio—biotite; qtz—quartz; hbl—hornblende; Xl—crystal-rich; Xp—crystal-poor; dashes indicate no data.

TABLE 2. CHARACTERISTIC FEATURES, IGNIMBRITE SHEETS OF CENTRAL AND NORTHEASTERN SAN JUAN REGION Ignimbrite sheet Composition Textures and phenocrysts Snowshoe Mountain Tuff Mafic dacite Phenocryst rich; densely welded within caldera, weakly welded outflow Nelson Mountain Tuff Low-Si rhyolite⇒dacite Compositionally zoned; weakly welded crystal poor to dense crystal rich Cebolla Creek Tuff Mafic dacite Typically weakly welded; abundant hornblende >> augite is distinctive Rat Creek Tuff Low-Si rhyolite⇒dacite Compositionally zoned; weakly welded rhyolite to dense dacite Wason Park Tuff Rhyolite Phenocryst-rich rhyolite; tabular sanidine phenocrysts Blue Creek Tuff Dacite Phenocryst rich; sanidine is absent (contrast with Mammoth Mountain Member) Carpenter Ridge Tuff Mammoth Mountain Member (upper) Dacite Phenocryst rich; sanidine is common (contrast with Blue Creek Tuff) Outflow and lower intracaldera tuff Low-Si rhyolite Phenocryst poor; common basal vitrophyre, central lithophysal zone Crystal Lake Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff, but less welded within map area Fish Canyon Tuff Dacite Distinctive light-gray, phenocryst-rich; resorbed quartz, hornblende, absence of augite Sapinero Mesa Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff but generally less welded within map area Dillon Mesa Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff but generally less welded within map area Blue Mesa Tuff Low-Si rhyolite Similar to rhyolitic Carpenter Ridge Tuff but generally less welded within map area Ute Ridge Tuff Dacite Phenocryst rich; contains sparse sanidine (in contrast to Masonic Park Tuff) Masonic Park Tuff Dacite Phenocrysts similar to Blue Creek Tuff; typically less welded Luders Creek Tuff Low-Si rhyolite⇒dacite Compositionally zoned; resembles Nelson Mountain Tuff Saguache Creek Tuff Low-Si rhyolite Resembles Carpenter Ridge and Sapinero Mesa Tuffs but lacks phenocrystic biotite Bonanza Tuff Zoned complexly Local basal xl-poor rhyolite, lower xl dacite, upper rhyolite, local upper xl dacite Thorn Ranch Tuff Zoned complexly Intracaldera alternation of rhyolite and dacite; outflow mainly high-Si rhyolite Badger Creek Tuff Dacite Crystal rich; resembles Fish Canyon Tuff Wall Mountain Tuff Rhyolite Crystal-rich, large blocky sanidine; locally complexly rheomorphic

Note: Bold type—ignimbrite sheets of Bonanza map area; Si—SiO2; xl—crystal.

Activity in the SRMVF peaked between 37 and 26 Ma (McIntosh and Chapin, batholiths that encompass most calderas of the SRMVF (Plouff and Pakiser, 2004; Lipman, 2007; Lipman and McIntosh, 2008). Dominantly intermediate- 1972; Drenth et al., 2012; Lipman and Bachmann, 2015). The SRMVF is among composition lavas and breccias (andesite-dacite), erupted from widely scat- several discontinuous loci of intense Tertiary volcanic activity—including the tered central volcanoes, were characteristic early phases of SRMVF activity. Sierra Madre Occidental, Trans-Pecos, Mogollon-Datil, Absaroka, Challis, and Major volcanic foci, initially established by clustered stratocones, became Lowland Creek fields—that developed along the eastern Cordilleran margin of eruptive sites for ~28 caldera-associated ignimbrites of more silicic composi- the North American plate, probably in a complex response to changing sub- tions (Table 1 and Fig. 6), in response to increased magmatic input. Composite duction dynamics along the western plate boundary (Lipman, et al., 1972). volumes of the early-intermediate volcanoes are large; in the San Juan region, stratigraphic sequences of the early volcanic rocks commonly are more than a kilometer thick, and total volume (~25,000 km3; Lipman et al., 1970) is nearly Early SRMVF Volcanism along the Sawatch Trend twice that of the later-erupted ignimbrites. The original areal extent of the overall SRMVF appears to have exceeded Eruptions beginning in northern parts of the SRMVF at least as early as 100,000 km2, with a total volume of volcanic deposits greater than 60,000 38 Ma (Epis and Chapin, 1974; McIntosh and Chapin, 2004) are the precursor km3 (Lipman, 2007; Lipman and Bachmann, 2015). Peak magmatic volumes framework for ignimbrite volcanism in the Bonanza area. Caldera sources for in the SRMVF, associated with ignimbrite eruptions, define a general (if im- the early (37–33 Ma) large ignimbrites (Table 1) are aligned north-northwest- perfect) progression, from early eruptions along the trend of the Sawatch erly along the crest of the Sawatch Range (Fig. 1), but preserved exposures of Range in central Colorado (37–34 Ma), southward into the San Juan region the outflow tuff sheets are limited, especially west of the present-day Conti- (33–27 Ma), and later to the 25-Ma Latir-Questa locus in northern New Mexico nental Divide. The stratigraphy, regional distribution, and eruptive history of and the 23-Ma Lake City caldera in the western San Juan Mountains (Fig. 6). these ignimbrites remain incompletely documented; main features are sum- Geophysical data document the presence of several composite subvolcanic marized briefly here.

GEOSPHERE | Volume 11 | Number 6 Lipman et al. | An ignimbrite caldera from the bottom up Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/6/1902/4333789/1902.pdf 1906 by guest on 29 September 2021 Research Paper a chetop Creek Co ntinental A Divide Mt n Co Wall Mtn Tuff wtooth COCHETOPA CALDERA A′

(not shown Sa NORTH PASS CALDERA Landslide Cochetopa Hill 12 on map) 12 Dome Creek

breccia Houghland Saguache 8 8 pT Tfc ? Tsc ? pT pT pT 4 pT pT 4

0 10 Elevation Vertical Exageration = 3.5 20 Km Feet x 1000 (Lipman, 2012) 38.5o pT N N pT Ts BONANZA MAP AREA pT Figure 2. Generalized geologic map and cross section, northeast San Juan region, pT pT based on recent geologic mapping (2000– Ts 2011). Some rock units grouped by caldera cycle: North Pass (NP); Cochetopa–Nelson MARSHALL Mountain (NM). Geographic localities: Rhyolite of Barret CALDERA HH—Houghland Hill; HL—Houselog Creek; Creek (29.8 Ma) 33.9 Ma Qal JC—Jacks Creek volcano; LBB—Long BONANZA x Branch Baldy; ML—Mount Lion; RCD— pT A x LBB CALDERA SM RCD Needle Cr 33.1 Ma Razorback Dome; SaM—Sargents Mesa; x SaM intrusion x SM—Sawtooth Mountain; TM—Trickle (34.4 Ma) A Mountain. The Nelson caldera is the young- OP X est subsidence of the San Luis caldera 3 Villa Grove complex. Dashed rectangular grid, bound 1 pT COCHET aries of 7.5′ quadrangle maps. Generalized 2 x Qal JC pT rift-related regional structural tilts (in NORTH PASS green) are gentle (10°–20°). Green-dashed CALDERA 26.9 Ma x line, area of Cochetopa–North Pass caldera CALDERA TM HH x map (Lipman, 2012); Blue-dashed line, Bo- VALLEY nanza map area (this study); red-dashed Cochetopa graben 32.2 Ma line, northeast corner of central San Juan HL map area (Lipman, 2006). ML A' XSaguache LUIS LA GARITA CALDERA Tracy Qal NELSON 28.0 Ma Mtn CALDERA x SAN 38o o (Lipman, 2006) o 107 W 0 10 20 KM 106

Qal Quaternary sediment e Caldera-fill sediment, slide breccia Bonanza Tuff (33.12 Ma) Ts Rio Grande rift sediment Postcaldera lavas (flow ages, 1 oldest)

NM cycl Granitoid intrusions

Younger lava flows Cochetopa- Nelson Mtn Tuff (26.90 Ma) Caldera topo rim Central San Juan ignimbrites Older lavas (Conejos Formation) Margin of caldera fill; (Fish Canyon (Tfc) and Carpenter Ridge Tuffs) pT dotted where concealed Pre-Tertiary rocks Postcaldera lava and tuffs Concealed graben fault

Cycle Tsc Saguache Creek Tuff (32.25 Ma)

MO Marshall Cr Salida eek The Gate Sang PoP re de Cr

Marshall caldera isto R SaM v v AP SM ang e

v v v RM iddle Creek hinge M WH

FM Bz San

Luis Figure 3. Oblique view of Bonanza cal- HP Valley dera area (Google Earth), viewed from the south. Dashed lines: orange—erosionally modified remnants of Bonanza topographic caldera rim; black—caldera- subsidence ring faults; yellow—crest of elliptical resurgent dome; green— margin of Marshall caldera. Abbrevi- ations: AP—Antora Peak; Bz—town of Bonanza; FM—Flagstaff Mountain; HP—Hayden Peak; KC—Kerber Creek; MO—Mount Ouray; PP—Porphyry Peak; PoP—Poncha Pass; RM—Round Moun- tain; SaM—Sargents Mesa; SM—Sheep Villa Grove Mountain; SP—Saguache Mountain; Kerber Cree k UP—Ute Pass; WH—Whale Hill.

Little KC

The first major regional ignimbrite was the far-traveled Wall Mountain North of the Mount Princeton batholith, the spectacularly exposed Grizzly Tuff (Chapin and Lowell, 1979), a relatively crystal-rich rhyolite (Table 2). It Peak caldera and associated intracaldera tuff (Fridrich et al., 1991) formed at erupted at 36.9 Ma, probably from a now completely eroded caldera above 34.3 Ma (McIntosh and Chapin, 2004), but the presumed equivalent outflow the 25 × 35-km Mount Princeton batholith (Fig. 1 and Table 1: Shannon, tuff sheet has been almost completely eroded. Nested within the southern 1988; Lipman and Bachmann, 2015; alternative interpretation in Mills and margin of the Princeton batholith is the slightly younger Mount Aetna caldera Coleman, 2013). Distal Wall Mountain Tuff is preserved on the High Plains, (Shannon, 1988), source of the 34.1-Ma Badger Creek Tuff (Epis and Chapin, more than 150 km east from its source (Chapin and Lowell, 1979). In con- 1974; Shannon et al., 1987) and associated intrusions (34–29.6 Ma; Zimmerer trast, a few small erosional remnants northwest of Cochetopa Park (Fig. 1), and McIntosh, 2012; Mills and Coleman, 2013). At the south end of the Sawatch 70–80 km southwest of Mount Princeton, are the only known localities west trend, Marshall caldera (within the Bonanza area: Fig. 2) was the source for the of the Sawatch Range for this large ignimbrite (Lipman and McIntosh, 2008; 33.9-Ma Thorn Ranch Tuff that is preserved mainly in the Thirtynine Mile region Lipman, 2012). east of the Rio Grande rift (Epis and Chapin, 1974; McIntosh and Chapin, 2004).

32.0 31.58±0.09 Saguache Creek Tu o* (six samples: 32.25±0.05) 32.2 o BONANZA CALDERA CYCLE 32.4 Postcollapse lavas * Dacite of Hayden Peak (2) x Megacrystic dacite (2) + Dacite (3) 32.6 o Rhyolite (7) * Sargents Mesa Volcano o Andesite (2) Intrusion (2) * Andesitic lava (4) x * x * 32.8 o * * Resurgent Figure 4. Summary of 40Ar/39Ar de- o + o intrusions (6) terminations for igneous rocks of the 33.0 + +o Bonanza-Marshall caldera area, illustrating * ++ x o o o o o the narrow ranges of ages obtained for MARSHALL CALDERA: + x o + +* +++ x multiple samples of individual ignimbrites, o ++ lavas, and intrusions of the Bonanza cycle 33.2 Upper dacite lava (2) o+++ and precursor eruptions. Data from Supple- o mental Table 1 (best quality ages indicated o * o Sheep Mountain Dacite (1) o by bold type). Precursors to eruption of the o * o Bonanza Tuff include several central volca- 33.4 * (33.12±0.03-out ow) noes (Jacks Creek, Rawley, Tracy, and Sar- gents Mesa) and Marshall caldera, source of E, million years o x Intracaldera (3) the Thorn Ranch Tuff at 33.9 Ma. In brackets:

AG (33.93±0.10) * o number of dated samples per unit. Broad x Intracaldera (5) + horizontal lines and gray rectangles repre- 33.6 sent weighted-mean ages and analytical + uncertainties (95% confidence [2σ]) for major regional ignimbrites. Uncertainties are x o o Tracy Volcano relatively large for biotite and groundmass 33.8 oo determinations; much lower for recent sani- o x o o o Rhyolitic lava (2) o dine ages, especially those determined with o Dacitic lava (5) the Argus VI mass spectrometer (green xx o* Andesite lava (1) font, on Supplemental Table 1). 34.0 o x Rawley Complex o o * Andesite (4) o * 34.2 o* + Dacite (2) o Rhyolite (5) o Badger Dated phase: * Creek o x + Sanidine 34.4 Biotite Hornblende Jacks o Groundmass Creek * * 34.6 * Volcano (8)

34.8 STRATIGRAPHIC SEQUENCE

TABLE 3. SUMMARY OF NEW 40Ar/39Ar AGE DETERMINATIONS, BONANZA AREA AND NORTHEASTERN SAN JUAN REGION Material Analysis No. Age Unit analyzed type sites (Ma)* ±Se/2σ† Comments HINSDALE FORMATION Basalt flow Groundmass Plateau 1 21.81 ±0.21 Overlies Carpenter Ridge Tuff HUERTO ANDESITE Andesite flow Sanidine SCLF127.79 ±0.06Sanidine xenocrysts; overlies Fish Canyon Tuff Andesite flows Groundmass Plateau 2 28.00–28.14 Overlies Fish Canyon Tuff SAPINERO MESA TUFF Sanidine SCLF128.20 ±0.06 BARRET CREEK RHYOLITE–DACITE DOME COMPLEX Rhyolite and dacite lava flows Sanidine SCLF 4 29.63–29.85 Failed caldera site? CONEJOS FORMATION, UPPER UNITS Basalt of Point Benny Groundmass Plateau 5 30.22 ±0.10Most mafic early lava in San Juan region Andesite of Lone Tree Gulch Groundmass Plateau 1 30.21 ±0.17 Tuff of Big Dry Gulch Biotite Plateau 1 30.47 ±0.08No sanidine; excess Ar biotite spectrum Aphanitic andesite, hill 9519 Groundmass Plateau 1 29.98 ±0.31 Hornblende andesite hill 9519 Hornblende Plateau 1 30.41 ±0.79Poor plateau NORTH PASS CALDERA CYCLE Luders Creek Tuff Sanidine SCLF432.17 ±0.04 Volcanics of Cochetopa Hills Dacite of East Pass Creek Biotite Plateau 3 32.07–32.31 Main area of caldera-fill flows Rhyolite of Taylor Canyon Sanidine SCLF132.15 ±0.10 Rhyolite breccia, Taylor Canyon Sanidine SCLF132.44 ±0.08Possibly landslide breccia from precursor flow Saguache Creek Tuff Sanidine SCLF632.25 ±0.05 Precursor? lava flow and intrusion Sanidine SCLF 2 32.18 ±0.07 At southeast margin of North Pass caldera Precursor? tuff of Spanish Creek Sanidine SCLF432.50 ±0.03 BONANZA CALDERA CYCLE Resurgent intrusions Turquoise Mine Groundmass Plateau 1 32.83 ±0.21 Granodiorite and andesite Alder Creek Biotite Plateau 1 33.02 ±0.12Porphyritic granite West side K-feldspar SCLF233.02 ±0.08 Granodiorite Spring Creek K-feldspar SCLF233.28 ±0.06Aplitic granite Postcollapse lava flows and local tuffs Distal weakly welded tuffs Sanidine SCLF232.7–32.8May be distal facies of late caldera-fill lavas Dacite of Hayden Peak Groundmass Plateau 2 32.66 ±0.18On crest of resurgent dome Megacrystic dacite Sanidine SCLF332.76 ±0.02 Along north caldera wall “Malpais” andesite Groundmass Plateau 1 31.76 ±0.18W of caldera; age too old, overlies Bonanza Tuff Caldera-fill lava flows Rhyolite cobbles (paleovalley fill) Sanidine SCLF 2 32.98 ±0.01 Small-sanidine dacite Sanidine SCLF433.03 ±0.03 Porphyry Peak rhyolite Sanidine SCLF533.03 ±0.04 Squirrel Gulch and other andesite Bio, groundmass Plateau 3 33.0–33.3West flank of resurgent uplift Bonanza Tuff Intracaldera Sanidine SCLF 3 33.05 ±0.06 Outflow Sanidine SCLF13 33.12 ±0.03 Precursor lavas Sargents Mesa volcano Bio, groundmass Plateau 4 32.99 ±0.15Age too young; flows underlie Bonanza Tuff Rawley volcano (dacite, rhyolite) Sanidine SCLF733.73 ±0.09 (continued)

TABLE 3. SUMMARY OF NEW 40Ar/39Ar AGE DETERMINATIONS, BONANZA AREA AND NORTHEASTERN SAN JUAN REGION (continued) Material Analysis No. Age Unit analyzed type sites (Ma)* ±Se/2σ† Comments MARSHALL CALDERA CYCLE Pitch-Pinnacle Formation Sanidine SCLF433.5–33.9Tuffaceous lake sediments Caldera-fill lava flows Biotite Plateau 2 33.4–33.6 Sheep Mountain Dacite Sanidine SCLF133.89 ±0.07 Thorn Ranch Tuff Bimodal population: 33.83 (n = 4), 34.08 (n = 4) Intracaldera Sanidine SCLF533.87 ±0.11 Outlfow Sanidine SCLF334.01 ±0.17 BADGER CREEK TUFF Sanidine SCLF234.06 ±0.15 McIntosh and Chapin, 2004 CONEJOS FORMATION, EARLY (undivided) North side Bio, groundmass Plateau 2 33.5–35.0 South side Sanidine (dacite) SCLF132.72 ±0.09 JACKS CREEK VOLCANO Andesite to rhyolite flows and dikes Bio, groundmass Plateau 10 34.2–34.6 CENTRAL VOLCANOES SOUTH OF SAGUACHE PALEOVALLEY Tracy volcano San, Bio, Hbl SCLF, Plateau 8 31.6–33.7? Beidell Creek volcano Sanidine SCLF133.39 ±0.07 Note: Analytical data for individual samples listed in Supplemental Table 4 (see text footnote 4) (a few ages from table 3 in Lipman, 2012). Bold—regional ignimbrite sheets, major caldera units. Mineral abbreviations: Bio—biotite; Hbl—hornblende; San—sanidine. Analysis type: SCLF—single-crystal laser fusion; Plateau—incremental-heating analysis. *Ages are calculated relative to the neutron-flux monitor FC-2, with an assigned age of 28.02 Ma. Ages for multiple samples are weighted means or ranges. †For SCLF, standard error of the weighted mean; for plateau ages, 2 s.d. (95% confidence level).

During the present study, it became clear that two previously described and San Juan Volcanic Region named regional tuffs in the Thirtynine Mile area, the East Gulch and Stirrup Ranch Tuffs of Epis and Chapin (1974; McIntosh and Chapin, 2004), are not The San Juan Mountains are the largest erosional remnant of the SRMVF large discrete ignimbrites, thereby increasing the recurrence intervals between (Fig. 1). Preserved volcanic rocks occupy an area of more than 25,000 km2 early SRMVF eruptions (Fig. 6). The previously described East Gulch Tuff (Epis and have a volume of ~40,000 km3. They cover a varied basement of Pre- and Chapin, 1974) is the non-welded to weakly welded basal cooling zone of cambrian to early Tertiary rocks along the uplifted and eroded west margin the Thorn Ranch Tuff, as clearly exposed at several sections, including sites of the Late Cretaceous to early Tertiary (Laramide) uplifts of the Southern sampled for geochronologic and paleomagnetic determinations (McIntosh Rocky Mountains and adjoining eastern parts of Colorado Plateau (Fig. 1). As and Chapin, 2004). The transition upward into the densely welded cliff-forming mid-Tertiary volcanism migrated southward from the Sawatch Range (Fig. 6), interior of the Thorn Ranch Tuff is completely gradational, and the isotopic widely scattered intermediate-composition centers erupted lavas and flanking ages and paleomagnetic pole directions are indistinguishable. The Stirrup volcaniclastic breccias starting at 35–34 Ma (Lipman et al., 1970; Lipman and Ranch unit is a local sheet of coarse monolithologic breccia, consisting entirely McIntosh, 2008). These rocks, which constitute about two-thirds the volume of of angular clast-supported blocks, with only minor interstitial matrix of com- the preserved volcanic assemblage, are widely overlain by large ignimbrites minuted tuff. All the blocks are densely welded Wall Mountain Tuff, as identi- associated with caldera collapses (Steven and Lipman, 1976). fied by similar rock chemistry, phenocryst populations, and 40Ar/39Ar ages. The After ignimbrite eruptions from Marshall and Bonanza calderas at 33.9 and absence of bedding and random orientation of large clasts (to several meters) 33.1 Ma in the northeastern San Juan Mountains (Fig. 6 and Table 3), magmatic in this unit are closely comparable to some monolithologic welded-tuff brec- activity migrated southwest with eruption of the Saguache Creek Tuff from the cias in central Nevada that have been interpreted as products of catastrophic North Pass caldera at 32.2 Ma (Lipman and McIntosh, 2008; Lipman, 2012), then dam-burst–type floods resulting from short-term blockage of paleovalleys by to the southeast San Juan region at 30.1 Ma (inception of Platoro caldera com- volcanic rocks or landslides (Henry, 2008), and a similar origin seems likely for plex), followed shortly by eruptions mainly of crystal-poor rhyolitic ignimbrites the Stirrup Ranch deposit. from western calderas (Steven and Lipman, 1976). Ignimbrite activity progres-

TABLE 4. REPRESENTATIVE COMPOSITIONS, VOLCANIC AND INTRUSIVE ROCKS OF BONANZA CALDERA AREA Latitude Longitude Sample Map 40Ar/39Ar Number Quad Unit Rock type Location Age Deg Min Deg Min Hinsdale Formation 00L-1 LG Thb Basaltic lava flow Houghland Hill 21.81 38 7.35 106 17.68 BONANZA CALDERA CYCLE (33 Ma) Late Intrusions 11L-8 WH Tas Interior granite Spring Creek 38 19.22 106 4.73 06L-34 GG Tas Sparsely porphyritic aplite Spring Creek 33.26 38 19.43 106 3.36 08L-4B WH Ttg Granodiorite Ridge, N of Kelly Cr 38 17.63 106 3.71 08L-41 WH Tta Intrusive andesite Ridge, N of Kelly Cr 33.06 38 17.62 106 3.53 06L-35 GG Ttg Coarse gabbro Peterson Cr 38 17.89 106 3.38 Postcaldera lava flows 08L-17 BZ Tbfr Crystal-poor rhyolitePorphyry Peak 38 21.35 106 9.09 07L-53 WH Tbdh Dacite of Hayden Peak Hayden Peak, east 32.64 38 16.47 106 6.01 08L-44 BZ Tbdk Large-sanidine dacite flow Silver Creek 32.76 38 21.39 106 10.24 13L-4 BZ Tbfd Upper dacite flow Slaughterhouse road 38 18.14 106 10.39 10L-19B LMNE Tbam “Malpais” andesite Bear Creek 38 14.84 106 19.52 07L-56 BZ Tbas Squirrel Gulch Andesite Kerber Creek 33.38 38 19.29 106 9.58 Bonanza Tuff (33.15 Ma) Outflow west 09L-8A KM Tbdu Bonanza T, upper dacite Findley Ridge 38 7.88 106 11.98 09L-8B KM Tbru Bonanza T, upper rhyolite Findley Ridge (33.07) 38 7.85 106 11.97 09L-8D KM Tbd Bonanza T, mid lower dacite Findley Ridge (33.41) 38 7.78 106 11.83 09L-33B LN Tbd Mafic scoria, resorb K-feld Jacks Cr tributary38 12.09 106 21.07 11L-10A KM Tbd Mafic scoria, upper unit (no K-f) Findley Ridge 38 7.84 10611.93 Outflow east (“Gribble Park Tuff”) 08L-33A JH Tbrl Lower crystal-poor rhyolite Two Creek section(33.09) 38 36.49 105 46.79 Rawley volcanic complex (~33.3-33.8 Ma) 08L-26 GG Trfr Rhyolite flow N of Kerber junction 33.81 38 13.09 106 4.79 08L-25 GG Trd Basal dacite flow Kerber Creek 38 13.06 1064.69 07L-3 KM Trap Dark plag andesite Ute Pass road 33.35 38 10.47 106 8.08 MARSHALL CALDERA CYCLE (34 Ma) Lava fill of Marshall caldera (~=Rawley volcanic complex) 10L-27 CH Tmfr Rhyolite flow Duncan Creek 38 21.38 106 18.62 08L-35 BZ Tmds Sheep Mountain Dacite Sheep Mountain 32.89 38 21.58 106 11.68 07L-22 KM Tra Aphanitic andesite Mill Gulch 32.61 38 12.56 106 14.81 Thorn Ranch Tuff-Distal outflow 09L-10C BM Ttr Lower, partly welded S Tallahassee Cr 33.90 38 31.18 105 33.65 Intracaldera 08A-2 BZ Ttr Crystal-poor rhyolite Silver Creek 38 21.62 106 10.79 08L-39 MO Ttr Dacite, ridge crest Starvation Creek 33.89 38 23.10 106 12.89 (continued)

TABLE 4. REPRESENTATIVE COMPOSITIONS, VOLCANIC AND INTRUSIVE ROCKS OF BONANZA CALDERA AREA (continued) (ANALYSES, NORMALIZED TO ORIGINAL TOTALS W/O LOI) Sample

Number SiO2 TiO2 Al2O3 FeTO3 MgO CaO Na2OK2OP2O5 MnO LOI TOTALZnRbSrYZr Nb Pb Ba La Ce Nd VGaNi Hinsdale Formation 00L-1 50.14 1.86 14.83 11.53 7.30 8.03 2.99 2.68 0.49 0.14 2.87 99.99 10153 653 21 160 21 5 686 23 54 26 220 22 140 BONANZA CALDERA CYCLE (33 Ma) Late Intrusions 11L-8 72.84 0.29 13.92 1.74 0.38 0.64 4.15 4.69 0.09 0.08 0.76 98.83 40 178 164 22 220 35 25 465 66 107 33 19 17 3 06L-34 77.60 0.17 12.60 0.74 <0.10 0.24 3.61 5.12 0.10 0.03 0.77 100.20 36 233 10 10 128 45 36 22 58 106 30 14 13 2 08L-4B 62.42 0.60 17.41 4.54 0.72 3.81 4.55 3.72 0.38 0.09 1.26 98.24 94 72 985 24 266 17 14 1752 63 124 57 48 19 0 08L-41 54.14 1.61 15.62 9.20 4.29 6.12 3.35 3.46 0.67 0.15 0.18 98.61 131 120 836 38 472 43 17 1227 89 192 81 207 22 25 06L-35 55.17 1.49 15.91 8.71 3.68 5.57 3.30 4.11 0.72 0.14 1.92 98.81 85 161 751 35 530 46 20 1350 110 223 115 169 21 18 Postcaldera lava flows 08L-17 74.67 0.23 13.73 0.80 0.20 0.41 3.19 5.55 0.03 0.01 1.26 98.83 41 198 37 16 203 39 28 106 41 67 23 8170 07L-53 69.73 0.45 15.42 1.99 0.31 0.86 4.52 5.77 0.06 0.06 1.03 99.18 69 144 261 31 378 28 30 1825 57 1165010170 08L-44 65.65 0.69 15.50 3.99 1.65 3.22 4.05 4.24 0.27 0.07 1.16 99.33 77 122 583 23 254 29 20 1063 63 11241792011 13L-4 64.61 0.70 16.71 4.13 0.70 2.83 4.17 4.72 0.28 0.16 0.94 99.03 77.1 141 646 29 362 28 20 1579 74 138 57.184218 10L-19B 58.69 1.10 17.18 6.66 2.47 5.30 3.66 3.49 0.45 0.12 0.92 99.119072 931 26 215 16 14 1465 43 97 47 133 20 14 07L-56 58.85 0.93 17.11 6.26 2.81 5.39 3.75 3.77 0.38 0.11 2.74 99.35 88 70 831 23 332 17 16 1493 64 130 57 127 19 11 Bonanza Tuff (33.15 Ma) Outflow west 09L-8A 61.76 0.82 17.64 4.98 1.21 3.64 4.03 5.14 0.44 0.07 1.15 99.73 90 119 680 37 446 25 24 1362 94 186 82 88 19 6 09L-8B 74.14 0.22 13.06 1.35 0.26 1.09 4.02 5.14 0.05 0.07 0.96 99.41 41 241 53 29 208 43 26 126 56 1123029182 09L-8D 63.49 0.81 17.27 3.82 1.06 4.24 3.98 4.96 0.25 0.04 1.60 99.91 65 196 710 30 431 35 25 1175 80 156 61 82 20 11 09L-33B 63.91 0.63 17.29 3.31 0.96 2.50 4.04 5.85 0.23 0.09 1.04 98.81 80 125 593 35 600 23 27 1730 105 207 89 54 19 5 11L-10A 59.72 0.95 17.77 5.35 1.94 4.33 3.91 4.61 0.43 0.13 0.84 99.14 91 103 825 32 530 21 22 1809 92 178 82 109 18 7 Outflow east (“Gribble Park Tuff”) 08L-33A 74.57 0.28 13.07 1.23 0.26 0.75 3.80 5.09 0.08 0.04 1.29 99.16 51 238 84 29 209 47 31 201 68 1113527180 Rawley volcanic complex (~33.3-33.8 Ma) 08L-26 71.23 0.31 14.74 1.34 0.09 0.41 4.12 6.46 0.03 0.04 0.54 98.78 49 137 11131 355 31 38 1158 66 126 51 6184 08L-25 62.91 0.82 16.52 4.69 1.64 3.63 3.98 4.64 0.36 0.10 1.68 99.29 80 101 591 25 325 24 20 1406 62 122 52 96 20 12 07L-3 58.04 1.08 16.30 6.83 3.01 5.88 3.82 3.29 0.44 0.09 1.43 98.79 91 77 889 25 261 18 15 1369 54 106 47 168 19 54 MARSHALL CALDERA CYCLE (34 Ma) Lava fill of Marshall caldera (~=Rawley volcanic complex) 10L-27 76.12 0.11 12.89 0.73 0.09 0.68 3.56 5.24 0.02 0.05 0.64 99.48 27 195 63 10 97 24 27 109 32 49 19 3183 08L-35 65.25 0.45 15.94 2.70 1.22 3.28 3.77 6.30 0.16 0.12 3.62 99.19 56 108 364 22 479 20 22 1310 72 142 55 29 16 0 07L-22 56.58 0.99 17.63 7.04 2.66 5.70 4.37 3.63 0.61 0.13 0.65 99.35 108731150 28 350 19 17 1712 80 162 69 140 21 1 Thorn Ranch Tuff-Distal outflow 09L-10C 74.82 0.19 13.27 1.22 0.45 0.71 2.33 5.82 0.03 0.05 4.52 98.89 65 233 45 32 162 33 35 11647923811172 Intracaldera 08A-2 73.09 0.26 13.96 1.48 0.41 1.19 1.35 6.95 0.05 0.11 2.38 98.84 114 249 52 29 248 31 19 246 76 145 55 10 19 0 08L-39 66.24 0.37 15.85 2.21 0.33 3.45 3.89 6.60 0.10 0.06 3.00 99.10 69 113 183 25 458 24 27 663 95 179 66 15 18 0 Note: Determined by X-ray fluorescence methods, at Washington State University GeoAnalytical Laboratory, mainly 2006-12.

Figure 5. West topographic rim of Bonanza caldera, as viewed to northwest, up valley of Kerber Creek. In distance, Antora Peak (cen- WEST CALDERA RIM ter, 13,269 ft, 4044 m) is capped by densely Windy Point Antora Peak welded dacitic Bonanza Tuff (33.12 Ma), while Windy Point (left, 12,800 ft, 3901 m) Sheep Mountain exposes thin layer of basal rhyolitic Bo- nanza Tuff that has undergone local rheo morphic flowage. Lower slopes of both high points are west-dipping sequence of interlayered andesitic lava flows and vol caniclastic rocks that make up the erosionally modified inner wall of Bonanza caldera. Inner caldera wall Sheep Mountain (right, 12,228 ft, 3727 m) is SW ank, resurgent dome pre-Bonanza dacitic lava dome (33.89 Ma) that partly fills the older Marshall caldera, source of the 33.9-Ma Thorn Ranch Tuff. On left side of Kerber Creek, timbered slopes are lower portion of the southwest caldera , wall; on right side of creek is dip slope on flanks of the postcollapse resurgent dome. ox. location The valley of Kerber Creek coincides with Appr ring fault the main ring fault (concealed beneath Quaternary surficial deposits), along which more than 3 km of subsidence was accom- rber Creek Ke modated during eruption of the Bonanza Tuff and concurrent caldera collapse. Photo location shown on Figure 9.

sively focused in the central San Juan Mountains (Tables 1 and 2), leading to thick Tertiary volcanic cover of the SRMVF; (2) ring-fault subsidence and up- eruption of the enormous Fish Canyon Tuff (5000 km3 of monotonously uniform lift structures of the strongly resurgent Bonanza caldera are exposed at deep crystal-rich dacite) and collapse of the 35 × 75 km La Garita caldera at 28.0 Ma levels down to the floor of this geometrically complex caldera; (3) widespread (Lipman, 2000, 2006; Bachmann et al., 2002). In the central San Juans, seven alteration and vein mineralization obscure stratigraphy and structure within more eruptions of compositionally diverse ignimbrite, with volumes of 100– the Bonanza mining district; and (4) postvolcanic extension has produced nor- 1000 km3, erupted during the 1.1-m.y. interval from 28.0 to 26.9 Ma from cal- mal faults, stratal tilting, and accommodation-zone structures at the junction deras nested within La Garita caldera (Figs. 1 and 2). At ca. 26 Ma, magmatism between the San Luis Valley and Upper Arkansas rift segments of the Rio shifted to a bimodal assemblage dominated by trachybasalt and silicic rhyolite, Grande rift zone (Fig. 3). Recognition and interpretation of the multiple epi- concurrent with the inception of regional extension along the Rio Grande rift. sodes of faulting and other structures are further hindered by heavy vegetation and limited outcrops in many areas, by lack of well-defined stratigraphy within GEOLOGIC SETTING OF THE BONANZA CALDERA the massive ignimbrites filling the Bonanza and Marshall calderas, by uncertain distinctions among lava sequences of multiple ages that have similar lith- The Bonanza area is more complex structurally than most other sectors ologies, and by difficulties in distinguishing effects of fault offsets from strati- of the San Juan Mountains: (1) deep erosion has exposed prevolcanic struc- graphic discontinuities resulting from non-planar deposition of volcanic units tures and paleotopography in Paleozoic and Proterozoic rocks beneath the in deep paleovalleys. Several unconformities along flanks of large paleovalleys

S.E. & W. SJ Latir Lake City VOLCANIC Sawatch Range Trend Transition Loci Central SJ LOCI FC 3 4,000 km3/m.y. 2,000 km3/m.y. 5,000 km SP AT NM Figure 6. Age-location-volume plot, show- 1,000 km3/m.y. 16,000 ing southward progression of Tertiary WM B SM CR ignimbrite-caldera volcanism in the South- 3 1000 250 km /m.y. ern Rocky Mountain volcanic field (SRMVF) (revised from Lipman, 2007). Vertical bars—

3 volumes of individual ignimbrites, scale on left axis (data from Table 1); stippled area—

km 12,000 increasing cumulative eruptive volume LJ CP (right axis). Inset—slopes corresponding to different cumulative eruption rates. Abbre- 3 viations: AT—Amalia Tuff; B—Bonanza Tuff; BC—Badger Creek Tuff; CP—Chiquito Peak , km NM Tuff; CR—Carpenter Ridge Tuff; FC—Fish GP WP SMT AT 8000 Canyon Tuff; GP—Grizzly Peak Tuff; LJ—La 500 Jara Canyon Tuff; BCr—rhyolite of Barret olume

v Creek flow field; NM—Nelson Mountain TR SC

BM ve Tuff; SC—Saguache Creek Tuff; SM—Sapin- SP ero Mesa Tuff; SMT—Snowshoe Mountain Tuff; SP—Sunshine Peak Tuff; TR—Thorn BC 4000 Ranch Tuff; WM—Wall Mountain Tuff; WP— umulati lume (individual tuff sheets),

C Wason Park Tuff; SJ—San Juan. Vo

0 0 38 34 30 26 22 Age (m.y.)

in the Bonanza area previously have been represented on regional maps as In contrast, volcanic accumulations in the Bonanza area lap onto rugged large faults (e.g., Tweto, 1979). In the present study, faults have been depicted paleotopography associated with the Late Cretaceous to early Tertiary (Lara- only where evidence for displacement is clear; some unmapped structures un- mide) uplifts. Large paleovalleys, which developed during erosion of the early doubtedly exist in places where field relations are inadequately convincing. Tertiary uplifts and during the growth of central volcanoes prior to ignimbrite eruptions, strongly influenced the distribution of subsequently emplaced vol- Paleotopography of the Bonanza Area canic deposits (e.g., Chapin and Lowell, 1979; Steven et al., 1995). Some major paleovalleys survived the entire period of volcanism, and many present-day Mid-Tertiary volcanic rocks of the Bonanza area lie along the broad bound- drainages are inherited from the mid-Tertiary landscape, leading to our field- ary between Precambrian-cored uplifts of the Southern Rocky Mountains and work expression “once a valley, always a valley.” less deformed Paleozoic and Mesozoic sedimentary rocks along the northeast Pre-Tertiary rocks also provide useful information on paleotopography at margin of the Colorado Plateau. These contrasts in geologic setting exerted the inception of volcanism and subsequent events in the Bonanza area. Nota important controls on depositional, structural, and morphologic evolution. bly, Precambrian basement and small windows of Paleozoic sedimentary To the south and west in the San Juan region, gently dipping Mesozoic sedi- rocks, but no Mesozoic rocks, are exposed around margins of the Marshall and mentary rocks semi-conformably underlie the volcanic sequence, and the ig- Bonanza calderas, showing that they formed within a prevolcanic highland, nimbrite sheets form a stratified plateau, interrupted mainly by local structures probably a southern continuation of a Late Cretaceous uplift in the vicinity of associated with caldera and central-volcano constructs. The original mid-Ter- the present-day Sawatch Range. Farther west, gently dipping Mesozoic sedi tiary volcanic terrain was much like the Altiplano of the central Andes (de Silva mentary rocks are widely present beneath the volcanic cover (Tweto, 1979; et al., 2006), because voluminous eruptions buried and subdued much of the Lipman, 2012), reflecting proximity to the Colorado Plateau. Absence of early preexisting topography. Tertiary sedimentary deposits along the unconformity at the base of the vol-

canic sequence in the map area, such as preserved farther south and west in Early Lavas (35–33 Ma) the San Juan region (Blanco Basin Formation and Telluride Conglomerate), is further evidence that the prevolcanic paleosurface stood topographically high As elsewhere in the San Juan region, ignimbrite sheets and other rocks and was primarily a region of erosion rather than deposition. associated with calderas in the Bonanza region overlie thick lava sequences Due to the greater paleorelief, the stratigraphic record of sequential erup- erupted from large central volcanoes (Figs. 2 and 6), recording initial as- tions is less complete in the Bonanza area than to the southwest. Many vol- sembly of upper-crustal magma bodies preparatory to caldera-scale ex- canic deposits in the northeast sector accumulated in broad valleys, which plosive eruptions. Sections of the early lavas as thick as 2.3 km have been were incompletely filled and then re-excavated by erosion between successive penetrated by petroleum exploration drilling southeast of the Bonanza area eruptions. The regional ignimbrites, rather than forming a stratified plateau, (Gries, 1985; Brister and Gries, 1994). In comparison with the early-interme- in places are preserved in inverted topographic order, with earlier tuff sheets diate assemblage farther south and west in the San Juan Mountains, lava capping ridges and younger units exposed at lower levels within paleovalleys. thicknesses in the northeastern area are more variable due to the rugged In many places, welded tuffs are preserved as isolated scabs, unconformable prevolcanic paleotopography. The assemblage in the Bonanza region also against slopes of paleovalleys, without stratigraphic continuity between se- contains greater proportions of proximal lavas and breccias relative to distal quential deposits. Frequent miscorrelations of ignimbrite units in previously laharic conglomerates and other volcaniclastic rocks, and dacite and rhyo mapped parts of the northeastern San Juan region resulted from such com- lite are more voluminous components of the dominantly andesitic lava plexities, as well as from limited exposures due to forest cover, incomplete assemblage. Rocks of the central volcanoes are broadly correlative with the regional knowledge of the regional eruptive sequence, and inadequate recognition of Conejos Formation (Lipman et al., 1970; Colucci et al., 1991), but several petrographic distinctions among tuff sheets. early-erupted tuff sheets from calderas along the Sawatch trend are interstrati fied with lavas that predate all ignimbrites from farther south and west in the Pre-Tertiary Rocks in the Bonanza Area San Juan Mountains (Fig. 7). The ignimbrites and sparse sanidine-bearing lavas help define the eruptive history of early lavas in more detail than possible elsewhere in the region. The lavas that are interstratified with the early tuff Proterozoic and Paleozoic rocks that crop out widely around eastern and sheets, as well as andesitic and dacitic lavas that ponded within ignimbrite cal- northern margins of the Bonanza caldera define a rugged paleosurface of high- deras throughout the San Juan region, are compositionally indistinguishable lands and valleys that were repeatedly partially buried and re-excavated during from earlier-erupted lavas of the Conejos assemblage and document eruptive mid-Tertiary volcanism. The Proterozoic rocks include large areas of coarsely and compositional continuity during growth of the SRMVF. porphyritic granodiorite, intruded into diverse metasedimentary and metavol- On a regional scale, total thickness and volume of the early lavas are far canic country rocks of quartzo-feldspathic gneiss, schist, and amphibolite. greater than for interstratified and overlying ignimbrite sheets. In the Bonanza Paleozoic sedimentary rocks preserved between Precambrian basement area, lavas are locally exposed over a vertical range of more than 1000 m from and overlying Tertiary volcanic rocks provide important evidence for pre- and along Saguache Creek to the Continental Divide, but thickness tends to de- synvolcanic structural events. Lower Paleozoic strata are dominantly marine crease northward toward the Gunnison Valley and eastward toward the San carbonate deposits (Burbank, 1932; Cappa and Wallace, 2007); these have long Luis Valley segment of the Rio Grande rift zone. Paleohills of basement rocks been subdivided into several formations that provide high-resolution struc- project through the volcanic cover more commonly than farther to the south tural markers. Upper Paleozoic rocks are thick continental clastic deposits that and west in the San Juan region. Within the Bonanza caldera area (Fig. 2), the record concurrent uplift (Ancestral Rocky Mountains) and erosion; distinctive early lavas thin against prevolcanic paleohills, and in some paleovalleys, Bo- subunits are lacking, and detailed subdivision has not been possible. nanza Tuff was deposited directly against Proterozoic granite. The pre-Tertiary rocks were further deformed, uplifted, and deeply eroded Parts of at least five major eruptive centers for pre-Bonanza lavas, all equiv- during Laramide compressional growth of the Rocky Mountains in the Late alent to rocks of the Conejos Formation farther southwest, lie within the Bo- Cretaceous and early Tertiary, and the mid-Tertiary volcanic rocks were depos- nanza caldera area, and others are present farther west (Lipman, 2012). These ited mainly on Precambrian rocks with only local surviving remnants of the centers probably include the earliest eruptions in the San Juan region (Table 3; Paleozoic sequence. The largest areas of exposed Paleozoic strata, on both Supplemental Table 1). Eruptions shifted west and south with time. The Jacks sides of lower Kerber Creek, define a south-plunging anticline and an E-W– Creek volcano (34.6–34.2 Ma) and andesitic and dacitic lavas (33.9–33.4 Ma) trending zone of faulting (Fig. 3). These features have been interpreted as Lara that filled Marshall caldera (eruptive source of the Thorn Ranch Tuff) were fol- mide structures (Burbank, 1932, p. 37–42; Cappa and Wallace, 2007, p. 38, 45), lowed by the Rawley cluster of volcanoes (33.7–33.3 Ma) that formed a vol but (below) we infer that they largely resulted from subsidence and subse- canic highland within which Bonanza caldera is centered. A thick andesite pile quent resurgence of Bonanza caldera. of poorly constrained age (ca. 33.2 Ma?) accumulated along Sargents Mesa

SOUTHWEST NORTHEAST

28 Sapinero Mesa T

CONEJOS FORMATION: Upper lava units (mostly southwest of Bonanza map area) 30

Volcanics of Cochetopa Hills (North Pass) Figure 7. Diagrammatic stratigraphic relations between regional ignimbrites and early intermediate-composition lavas 32 (Conejos Formation), northeast San Juan region. Thin-dashed lines delimit locally Summer Postcaldera volcanics (Bonanza) (”Upper Andesite”, dacite, Porphyry Peak Rhyolite, etc.) mapped lava units in relation to associ- Coon ated calderas. Thick dashed lines (vertical), approximate age spans of mapped central volcanoes along east flank of the San Juan Mountains. Tracy e (m.y. ) Rawley volc. complex Postcaldera volcanics (Marshall) (”Sheep Mtn Dacite”, andesite, other dacite)

Ag CONEJOS (“Rawley Andesite,” includes dacite and rhyolite) FORMATION 34

Jacks (undivided) Creek

36 [No lavas preserved?]

REGIONAL LAVA UNITS INTRACALDERA LAVA UNITS

along the Continental Divide west of Bonanza, and the large Tracy volcano The northern wall of Marshall caldera is exposed continuously from The south of the Saguache valley (ca. 33.7–31.6 Ma) erupted intermediate-compo- Gate northwestward into upper Marshall Creek (Figs. 3 and 9), where it projects sition lavas both before and after the Bonanza Tuff. southward, as constrained by exposed Proterozoic and Paleozoic rocks farther west. Its southwestern margin is concealed beneath caldera-filling lavas and younger volcanic rocks (33.9–33.4 Ma), but an east-west alignment of paleohills Thorn Ranch Tuff and Marshall Caldera Cycle (33.9 Ma) of Proterozoic rocks and parallel exposures of Bonanza and Saguache Creek Tuffs just to the north provide limits to the southern extent of Marshall cal- Eruption of the Thorn Ranch Tuff from the previously unmapped Marshall dera (Fig. 9). Southeastern margins appear to have been deeply buried beneath caldera (Figs. 2 and 3) was the most proximal antecedent to the ignimbrite younger lavas and clastic rocks of this caldera cycle and the Rawley volcanic from Bonanza 0.8 m.y. later, and features of this caldera provide the basic complex, then truncated by Bonanza caldera. Based on these relations, Mar- stratigraphic and morphologic framework for interpreting the younger vol shall caldera is estimated to be ~15 × 20 km, with a subsided area of ~250 km2. canic activity. The thick sequence of intermediate-composition lavas that filled No postcollapse resurgence is evident at Marshall caldera, and much of its Marshall caldera merge with precursor lavas of the Bonanza cycle, and Bo- structure is concealed beneath lavas that overflowed the collapse area. The nanza caldera caved away the southeast margin of the earlier caldera. Along caldera is exposed at relatively shallow levels, revealing only upper parts of with the slightly younger Saguache Creek Tuff (32.2 Ma; Lipman and McIntosh, the intracaldera ignimbrite, overlain by postcollapse lavas. Exposed landslide 2008), these three major ignimbrite eruptions and associated calderas of the megabreccia, in upper Marshall Creek, is high in the intracaldera ignimbrite, northeast San Juan Mountains bracket a major geographic transition, from directly overlain by caldera-fill andesite. This geometry suggests abrupt sub- earlier SRMVF activity along the Sawatch Range trend, into the locus of peak sidence late during the eruption, after sustained slower subsidence during ac- eruptive activity in the San Juan region (Figs. 1 and 5). cumulation of the thick ignimbrite exposed at The Gate, which contains abun- Probable presence of an ignimbrite caldera in upper Marshall Creek was dant lithic fragments as much as 0.5 m across but lacks discrete interleaved initially inferred by Thomas Steven (1986, written commun.), based on re- megabreccia lenses. interpretation of published geologic mapping in the Marshall Pass mining The postcollapse lavas merge to the south and west with petrologically district (Olson, 1983). Olson’s detailed map depicts a lithologic assemblage indistinguishable andesite and dacite of the Rawley volcanic complex (as (welded tuff interleaved with volcanic breccia, overlain by tuffaceous lake- old as 33.7 Ma) that are precursor to the Bonanza caldera cycle. The post- bed deposits) similar to that within ignimbrite calderas elsewhere in the San collapse lavas of Marshall caldera also interfinger with lake-bed deposits of Juan Mountains (Steven and Lipman, 1976). Subsequently, during study of the Pitch-Pinnacle Formation (Olson, 1983; Gregory and McIntosh, 1996). This fossil flora in lake sediments of the Pitch-Pinnacle Formation, Gregory and sedimentary assemblage is thickest beyond the northwest margin of Marshall McIntosh (1996) dated the welded tuff in upper Marshall Creek, noted asso- caldera, in an embayment that appears to have been a broad paleovalley dis- ciated megabreccia of intracaldera type, and suggested that this assemblage rupted by caldera-forming events and then occupied by a lake concurrently marked a caldera source for the 33.9-Ma Thorn Ranch Tuff preserved widely with eruption of caldera-filling lava flows. in the Thirtynine Mile area to the northeast (Epis and Chapin, 1974; McIntosh Small remnants of partly welded dacite ignimbrite, interpreted as proxi- and Chapin, 2004). mal Thorn Ranch Tuff, on the basis of ages and compositions (Tables 3 and 4; Our present study augments these interpretations, demonstrating that Supplemental Tables 1 and 4) similar to the intracaldera accumulation and eruptions from Marshall caldera were major precursors to inception of the widespread outflow east of the Rio Grande rift zone, are preserved locally east Bonanza eruptive cycle. Although largely concealed beneath younger lavas and northeast of Marshall caldera. Perplexingly, no outflow remnants of Thorn and partly truncated by Bonanza caldera, Marshall caldera is the oldest ig- Ranch Tuff or any other ignimbrite sheet that could be related to Marshall cal- nimbrite source in the San Juan region. Along the previously unrecognized dera have been identified farther west or south in the San Juan Mountains, northeast margin of this caldera, intracaldera Thorn Ranch Tuff, >400 m thick even though basal volcanic strata are exposed widely in contact with underly- with no base exposed, banks against Proterozoic wall rocks at a geographic ing Mesozoic and Proterozoic rocks (Lipman, 2012). Because of these limited feature known as “The Gate” (Figs. 8 and 9), east of Marshall Pass. This exposures, the eruptive volume of the Thorn Ranch Tuff cannot be determined steep unconformity has been depicted on regional maps as a contact of in detail, but the large area of Marshall caldera (>250 km2), the presence of andesite lava against the Proterozoic basement (Tweto et al., 1976; Cappa intracaldera tuff more than 400 m thick with no base exposed, and preserva- and Wallace, 2007), but densely welded, lineated tuff dips parallel to cal- tion of outflow tuff at least 70 km east of the source caldera (Fig. 10; McIntosh dera-wall contacts at The Gate and contains large fragments of Proterozoic and Chapin, 2004, Fig. 2D) indicate a major ignimbrite eruption, estimated as rock derived from the wall. Dips in the Thorn Ranch Tuff decrease, from 250–500 km3. locally near-vertical at the caldera wall, to as low as 20°–25° at 100–200 m Marshall caldera is eroded less deeply than Bonanza, lacks exposed co- distance from the contact. magmatic intrusions, and exposes only upper portions of the caldera-filling

TOPOGRAPHIC WALL (unconformity), MARSHALL CALDERCALDERAA Intracaldera Thorn Ranch Tuff (33.9 Ma)

Precambrian Figure 8. Topographic wall of Marshall caldera, at The Gate. Densely welded Thorn granite gneiss Ranch Tuff, more than 300 m thick with no base exposed, is depositionally banked against Precambrian granite gneiss at this prominent topographic feature in upper Silver Creek. Photo location shown on Figure 9.

ignimbrite. This geometry is consistent with Rio Grande rift-related regional Existence of a caldera in the Bonanza area was first inferred from a local westward tilting (~10°–20°) of a large structural block that includes the Sawatch gravity low (Karig, 1965), followed by recognition that the Bonanza latite of Pat- Range and continues south nearly to the valley of Saguache Creek (Fig. 1). ton (1916) and Burbank (1932) was welded tuff likely erupted from the Bonanza area (Bruns et al., 1971). As an outgrowth of exploration for porphyry deposits at depth below the mineralized veins at Bonanza, Varga and Smith (1984) synthe- BONANZA CALDERA CYCLE (ca. 33 MA) sized available regional data, augmented by new chemical and isotopic analy ses, as evidence for a trap-door caldera of relatively modest size (~8 × 12 km), in Bonanza caldera, source of the 33.12-Ma Bonanza Tuff (~1000 km3), is which the subsided block was tilted westward and hinged on its east side. De- the southernmost and youngest of the ignimbrite centers aligned along the tails of this interpretation are problematic, however, because lavas that overlie Sawatch Range trend (Fig. 1). In contrast to the multicyclic nested caldera loci the intracaldera ignimbrite are tilted steeply westward along with the underlying for successive eruptions at many large-volume ignimbrite flare-ups elsewhere, ignimbrite. Such a relationship would require the caldera to have subsided after Bonanza is an areally isolated collapse structure that formed in response to a emplacement of the lavas, rather than accompanying the ignimbrite eruption. single ignimbrite eruption. Although the western topographic rim of Bonanza Bonanza caldera is here reinterpreted as a much larger resurgent caldera, impinges on Marshall caldera, projected structural boundaries of two calderas ~20 × 25 km across, in which the caldera floor was uplifted steeply after sub appear to be largely or entirely separate. sidence (Figs. 3 and 9). The previously inferred eastern hinge is now inferred

GEOSPHERE | Volume 11 | Number 6 Lipman et al. | An ignimbrite caldera from the bottom up Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/6/1902/4333789/1902.pdf 1919 by guest on 29 September 2021 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/6/1902/4333789/1902.pdf Research Paper N 15 ′ 38° geologic maps (Figs. 13 and 14). Rectangular grid (outlined in red) represents boundaries of 7.5 Bonanza caldera inferred from distribution of intracaldera rocks and from erosionally modified present-day morphology. Rectangles indicate location of detailed 15). Margin of Marshall caldera (unconformity between caldera-fill and precaldera rocks) is dashed where approximately located. Approximate topographic rim of Figure 9. Generalized geologic map of Bonanza and Marshall calderas, showing major stratigraphic units, structural features, and locations of cross sections (Fig.

OLIGOCENE Sa M M rgents esa esa gr P J acks Creek a- aleo ignimbrites gd vo vo rocks B andesit B d onanza onanza P P lcano lcano Qa l

ost- ost- z 5

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rh e a

Monoclinal y Monoclinal HH HH Eg Quaterar Bonanza Pr Sheep Mountain Dacit P Thorn R P P 5

aleozoic sedimentar ost-Bonanza lavas (andesit ost-Bonanza ignimbrites

ecambrian granitic rocks d Pr

Saguache Saguache Saguache Saguache 15 ecaldera lavas (andesit M M a, andesite; Egd, Eagle Gulch Dacite gr Resurgent intrusions: c c , granite; gd, granodiorite; arshall arshall alder alder

anch

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ndley Gulch ndley e- Fi agstaff Mt n Mt n rh 35 rh p yolite)

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Qal To To te e

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rp Bonanza Bonanza

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45 Tu

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* 17 reek * 16

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[Mapped 2006-11]

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GEOSPHERE | Volume 11 | Number 6 Lipman et al. | An ignimbrite caldera from the bottom up 1920 Research Paper

106°30′ 106° 105°30′ 105°W

39°30′

AREAL EXTENT, PRESERVED EROSIONAL REMNANTS, 33.1233.2 MAMA BONANZABONANZA TUFFTUFF [distance from caldera rim] 39°

Figure 10. Areal extent, preserved erosional remnants of Thorn Ranch and Bo- nanza Tuffs. Modified from McIntosh and [60+60+ km]km Chapin (2004). Eastern areas are dominated by crystal-poor rhyolite, while western area consists mainly of crystal-rich dacite. Figure also shows inferred distri- ] bution of the eastern outflow Thorn Ranch Tuff. Numbered localities reference sam- [Eroded ples dated by McIntosh and Chapin (2004). 38°30′ Mostly [Eroded]

rhyolite

Mostly [60+ km] dacite

38° N [80+ km] [Eroded/covered] [Covered] [Modified from Mcintosh and Chapin, 2004]

to be the crest of a large elliptical resurgent uplift (Whale Hill dome) within a Pre-Bonanza Caldera Volcanism caldera block that occupies much of the Bonanza map area. Small remnants of east-dipping Bonanza Tuff are preserved on deeply eroded eastern flanks of the The composite volcanic highland in which the caldera formed is dominated Whale Hill dome, but the east margin of the caldera lies concealed beneath the by intermediate-composition to silicic lavas from several eruptive centers San Luis Valley segment of the Rio Grande rift zone. As a result of the larger cal- (Table 4; Supplemental Table 4). Silicic flows (dacite as well as rhyolite) high in dera size and revised correlation of the outflow ignimbrite, the combined intra the precaldera assemblage are more abundant at Bonanza than at most other caldera and outflow volume of Bonanza Tuff is estimated at ~1000 km3 (Table 1), sites of ignimbrite eruptions in the San Juan region and appear to record a pro- in contrast to the estimate of less than 50 km3 by Varga and Smith (1984). longed history (33.7–33.2 Ma) of magma assembly and evolution toward evolved

compositions. Volcaniclastic deposits interlayered with the lava sequence are continued by others (e.g., McIntosh and Chapin, 2004). In contrast, our work less abundant than in similar younger assemblages in the San Juan Mountains, shows that all compositional variations in both intracaldera and outflow Bo- consistent with the inference of a precaldera constructional highland. nanza Tuff occur within a single ignimbrite sheet characterized by compound Presence of a central highland at Bonanza, prior to eruption of the Bonanza welding zonations. Tuff, is confirmed by deposition of the ignimbrite in broad radial paleovalleys, West and south of Bonanza caldera, the outflow ignimbrite sheet is domi- as especially preserved on the west and south flanks of the caldera (Varga nantly dark- brown crystal-rich dacite, but some proximal sections contain gra- and Smith, 1984). Effects of any precursor tumescence in the Bonanza area dational transitions from densely welded lower crystal-poor rhyolite, a thick are difficult to evaluate but would have been subordinate to constructional central zone of dacite, somewhat less welded upper rhyolite, and, locally, an processes in generating the overall present-day geometry of the precaldera upper welded dacite (Figs. 11 and 12). The compositional transitions corre- volcanic sequence. Much of the highland constitutes fill of the earlier Marshall spond imperfectly with welding breaks. For example, the change from main caldera, and many dated lavas are closer in age (33.7–33.5 Ma) to that caldera dacite to upper rhyolite is associated with a bench-forming zone of weaker than to Bonanza. Only on Sargents Mesa and along the south flank of Bonanza welding between dual cliffs of densely welded tuff, where well exposed along caldera, near Saguache Peak, do relatively low-precision whole-rock dates on lower Saguache Creek (e.g., Lipman et al., 2013, Stop 1-7), but the upward gra- andesitic lavas (ca. 33.1–33.3 Ma) approach the age of the Bonanza cycle. dational decrease in crystal content and increase in silica mainly occurs high in the lower cliff rather than coincident with the zone of least welding. Rare part- ings defined by abrupt changes in pumice or lithic size or abundance within Bonanza Tuff the ignimbrite (Fig. 11A) are interpreted as recording pulsation of eruption intensity or ignimbrite-emplacement dynamics during essentially continuous In contrast to the one-sided easterly distribution of preserved Thorn Ranch ignimbrite deposition. No evidence was observed for existence of significant Tuff in relation to its source from Marshall caldera, the Bonanza Tuff is pre- time breaks during deposition, such as interlayered bedded-surge, tephra-fall, served widely both to the east and west of its caldera (Fig. 10). Any deposition or fluviatile ash deposits in either the outflow ignimbrite or in the thick intra- of this ignimbrite in the vicinity of the Sawatch Range to the north has been caldera accumulation. completely eroded, however, and it has been covered by younger volcanic de- In contrast, outflow Bonanza Tuff, which is preserved 80 km beyond the cal- posits or eroded south and west of the Saguache paleovalley. dera to the east (Fig. 10), was described until recently as a separate rhyolitic ig- The Bonanza Tuff is characterized by large and complex lateral and vertical nimbrite, the Gribbles Park Tuff (Epis and Chapin, 1974; McIntosh and Chapin, variations in chemical and phenocryst compositions (Table 4; Supplemental 2004). The eastern outflow sheet consists dominantly of light-colored, crys- Table 4), from dark crystal-rich silicic andesite and dacite (25%–35% plagio tal-poor rhyolite but contains a thin internal zone of red-brown dacite, at least

clase, biotite, augite; 60%–66% SiO2) to light-colored, crystal-poor rhyolite at the well-exposed sequence at Two Creek (38°36.6′N, 105°46.7′W), which has that consists mainly of fine ash (~5% sanidine, plagioclase, biotite; 73%–76% been used as a reference section (Epis and Chapin, 1974; McIntosh and Chapin,

SiO2). A distinctive feature of the dacitic tuff, in both intracaldera and outflow 2004; Supplemental Tables 1 and 4). Thus, the eastern outflow ignimbrite has deposits, is the presence of several percent of small angular lithics (typically a similar general gradational compositional zonation as to the west, but with a several cm or less), dominantly fragments of andesitic lava. Flattened pumice much higher proportion of rhyolite to dacite. Total volume of the eastern out- fiamme are conspicuous in the dacite, commonly 5 cm or longer (Figs. 11 B and flow must be large, at least several hundred cubic kilometers. This unit is not 11C). In contrast, lithic fragments are rare in the outflow rhyolite, and fiamme simply correlative with the upper rhyolite zone in proximal western sections are small and obscure. Uniquely among rocks of the SRMVF, even mafic dacite as proposed by McIntosh and Chapin (2004, p. 230), who recommended “that

(62%–64% SiO2) has phenocrysts of sanidine, while some rhyolite contains the name Bonanza Tuff be restricted to the lower crystal-rich dacitic unit, and trace hornblende and titanite. that Gribbles Park Tuff be used for the upper rhyolitic unit.” However, our study Prior studies had distinguished a lower dacite (latite) and upper rhyolite, indicates that all the compositional zones in the outflow ignimbrite, both to starting with Burbank (1932), but the eruptive significance of the compositional west and east, are within a single variably welded ignimbrite sheet, displaying change remained uncertain. From work on the southwest flank of the Bonanza compound cooling (Smith, 1960) but without depositional breaks, for which center, Bruns et al. (1971, p. 187) concluded that the “Bonanza Tuff consists of the name Bonanza has precedence (Patton, 1916; Burbank, 1932). Accordingly, two cooling units” but did not describe the contact. Varga and Smith (1984) Gribbles Park Tuff is not used as a stratigraphic name. later described the Bonanza Tuff as consisting of two sheets, each a single cool- The thick intracaldera Bonanza Tuff contains even more complex compo- ing unit, not compositionally comagmatic, and erupted from geographically sitional and welding variations within a single ignimbrite unit (Fig. 11). Where separate vents. Neither of these studies specifically discussed the presence or a complete ignimbrite section (2.5 km thick) from caldera-floor rocks to post- absence of evidence for a time break, such as bedded tuff or other sedimentary collapse lavas is exposed on the west flank of the resurgent dome (Fig. 13), units, between the two Bonanza cooling units, but the implied interpretation rhyolite and dacite compositional zones interfinger as many as 13 times, as was the existence of two separate ignimbrites, an interpretation that has been identified by variations in crystal content and groundmass color; dacitic tuff

Figure 11. Regional thickness, compositional and textural variations in the Bo- nanza Tuff. Outflow ignimbrite is mainly early-erupted rhyolite to the east, mainly later-erupted dacite to the west. Intra caldera tuff contains multiple compositional zones; ponded to thickness of at least 2.5 km, as exposed on west flank D of the resurgent dome. (A) Parting within welded dacite, between relatively pumice-poor tuff and underlying tuff that contains larger flattened pumice blocks (weathered-out cavities) at Findley Ridge E (at site of sample 08L-8C: Fig. 14). Bound- INTRACALDERA ary is inferred to record a pulsating insta- IGNIMBRITE bility in eruptive discharge or fluctuation in emplacement kinematics. (B) Densely A welded intracaldera dacite, with elongate pumice lenses (to 0.5 m) that define a B down-dip lineation, east side road above Squirrel Gulch. (C) Typical densely welded Andesite dacite on Antora Peak. Flattened pumice lenses are 5–15 cm long. (D) Lower rhyolite Rhyolite unit, densely welded to its base without intervening tephra deposits, deposited C E against caldera-wall andesite at Windy Dacite Point. (E) Lower rhyolite unit: weakly weathered surface on hand sample, showing fluidal welding despite containing abundant andesite fragments. Same location as D.

WESTERN OUTFLOW

B ~ 500 m A Prior intracaldera thickness, est: 300 m (Varga and Smith, 1984); Intracaldera ignimbrite mapped previously now 2.5 km as simple sequence: andesite-dacite-rhyolite, with many fault repeats (Burbank, 1932) EASTERN OUTFLOW

Landslide breccia (mainly andesite) D [”Gribbles Park Tu ”]

Biotite-dacite lava Dacite 09L-7 Rhyolite 8A

8B Dacite

Bonanza

Tuff 8D

Andesite lava

Figure 12. Complex compositional and welding zonations in proximal Bonanza Tuff, reference section along ridge on west side of Findley Gulch (KM). The Bonanza Tuff is ~175 m thick in this section. Circles indicate locations of chemically analyzed samples (Table 4; Supplemental Table 4). Variable dips in cliff-forming zones of densely welded tuff reflect differential compaction in proximity to margin of paleovalley. The lower rhyolite is absent in this paleovalley fill but crops out below the main dacite in ignimbrite exposures 6 km to the southwest. Photo location shown on Figure 9.

is volumetrically dominant, but the eruptive sequence began and ended with and locally rheomorphic. The lower rhyolite is especially fluidal and lava-like rhyolite. The incomplete caldera-fill section on the steeply dipping north- on the west caldera rim (Figs. 11D and 11E) and along the steeply dipping east flank also contains multiple rhyolite-dacite alternations and is at least northeast flank of the resurgent dome. Pumice lenses, where distinguishable 1.5–2 km thick, including thick interleaved landslide-breccia deposits (Fig. 14). in fluidal and rheomorphic zones (both rhyolitic and dacitic), commonly define Other than contacts against landslide breccias, all compositional and weld- a prominent lineation with prolate elongation ratios of as much as 20:1. Linea- ing variations are gradational. The multiple zones of dacitic tuff are typically tions are especially well developed low in the thick ignimbrite section that is densely welded (Figs. 11B and 11C); many of the rhyolite zones appear to be so well preserved on the west flank of the resurgent dome; there, they typically less welded, although degree of welding is more difficult to evaluate because plunge nearly directly down dip. of common absence of large pumice and/or fiamme lenses and widespread The weighted-mean age for sanidine from intracaldera Bonanza Tuff intense alteration. (33.05 ± 0.06 Ma) is marginally younger than that for the outflow ignimbrite The initially erupted lower rhyolite is relatively thin and small in volume 33.12 ± 0.03 Ma). Although within analytical uncertainty, this difference might within the caldera, varying greatly in thickness within short distances laterally reflect prolonged cooling of the multi-kilometer-thick intracaldera accumula- and probably reflecting pre-eruption volcanic topography. This zone is up to tion or possible effects of later alteration. ~100 m thick locally on parts of the southwest flank of the resurgent dome, but its thickness is highly variable; it is absent in other nearby exposures and almost the entire exposed north flank. The dacitic tuff is typically densely welded Postcollapse Lavas and Intrusions (Figs. 11B and 11C); in places where propylitically altered, the intracaldera dacite is identifiable as welded tuff only by the typical presence of abundant After the ignimbrite eruption, compositionally diverse lavas ranging from small angular fragments of andesitic lava. Much of the rhyolite is only moder- andesite to high-silica rhyolite (Table 4; Supplemental Table 4) filled the caldera ately welded, but some deep intracaldera zones are densely welded to fluidal, to overflowing and spread across adjacent slopes to the northwest and south-

106°10′ 106°7.5′W

h Line 1 (Fig. 17A)

Rawley Gulc

Squirrel Creek

Kerber Creek Copper Gulc

Wagon Box Gulc

Slaughterhouse Cree

Elkhorn Gulc

BONANZA 30° 17.5′

v N v

Contour interval: 80 feet

MAP UNITS Tbds

Tbru 0 1 2 KM v

Qal Qc Qt Ql Qm Tbap Ted v v v v v Trap

v v

v Tbd Tbr Tbmav v v Tbmpv v Quaternary surficial deposits Tbas Tbrl Tra Postcollapse lavas and intrusions Bonanza Tuff Caldera-floor lavas (Rawley complex)

Figure 13. Geologic map of the Kerber Creek valley near the town of Bonanza, showing part of the west flank of the Whale Hill resurgent dome. West-dipping section of intracaldera Bonanza Tuff (~2.5 km thick) is overlain conformably by tilted postcollapse lavas, indicating that at least much of the resurgence postdated eruption of the lava fill. Within the thick intracaldera Bonanza Tuff, multiple compositional and welding zones interfinger with landslide-breccia lenses. Only one small mapped fault disrupts the tilted intracaldera ignimbrite and lava sequence. Location of figure is shown by rectangle on Figure 9. Red-dashed line indicates position of central part of the Line 1 cross section (Fig. 17A). Hachured lines—Quaternary slump areas (hachures on downthrown side). Red-stipple pattern—prominently altered areas of bleached and oxidized rocks, resulting from supergene weathering of pyrite. Surficial deposits: Qal—alluvium; Qc—colluvium; Qt—talus; Ql—landslide deposits; Qm—moraine. Postcollapse lavas: Tbds—dacite lava; Tbap—plagioclase-andesite; Tbas—Squirrel Creek Andesite. Intrusion: Ted—Eagle Gulch Dacite. Bonanza Tuff (a single ignimbrite): Tbru—upper rhyolite; Tbd—main dacite; Tbr—interleaved rhyolite; Tbma—interleaved breccia of fine-grained andesite; Tbmap—interleaved breccia of plagioclase andesite lava; Tbrl— lower rhyolite. Caldera-floor lavas (Rawley volcanic complex): Trap—plagioclase andesite; Tra—andesite.

(segment of Rio Grande MAP UNITS SAN L

UIS QalQc Ql 38° V ALLEY 22.0′ Quaternary surficial deposits

N v

ri v

) v v ft v v

Alder Creek . 17A zone v Tdv u? ig v ) Rio Grande rift ll

Line 1 (F

v v

x x x v v v v

x x v v v

v v

v v v Tbrr x v Tbd Tbr x x Tbma Tbmp Tbmu x v v v v v Tbrl Bonanza Tuff

Trap

38° Tra 21.0′ Caldera-floor lavas (Rawley complex)

pCa

Aplite (Proterozoic)

38 21.0′ 106°4.0′W 106°2.5′ Contour interval: 80 feet 0 1 Km

Figure 14. Geologic map of the Alder Creek area, showing steeply-dipping units on the northeast flank of the Whale Creek resurgent dome. The caldera-filling ignimbrite (Bonanza Tuff) and inter leaved landslide breccias consistently dip 60°–80° to the northeast, and the exposed partial section of the ignimbrite (no top preserved) is as much as 1.8 km thick. Location of figure is shown by rectangle on Figure 9. Red-dashed line indicates position of northeast end of the Line-1 cross section (Fig. 17A). Red-stipple pattern, prominently altered areas of bleached and oxidized rocks, resulting from supergene weathering of pyrite. Surficial deposits: Qal—alluvium; Qc—colluvium; Ql—landslide deposits. Rio Grande rift fill: Tdu—Dry Union Formation. Intracaldera Bonanza Tuff (a single ignimbrite): Tbd—main dacite; Tbr—interleaved rhyolite; Tbrr—rheomorphic and fluidal rhyolite; Tbma—landslide megabreccia of fine-grained andesite lava; Tbmap—landslide megabreccia of plagioclase-andesite lava; Tbmap—landslide megabreccia of mixed lithologies; Tbrl—lower rhyolite. Caldera-floor lavas (Rawley volcanic complex): Trap—plagioclase-andesite; Tra—andesite.

west. Postcollapse lavas are not preserved on the more eroded eastern side of depth. Several small exposures of similar granodiorite to andesite crop out the caldera, but a distinctive thick lava sequence of crystal-poor silicic dacite farther west, along tributaries of Kerber Creek, and mineral-exploration drilling

(69%–70% SiO2) on Hayden Peak (Fig. 9) appears to have accumulated in a paleo on Manitou Mountain (Fig. 3) penetrated granitoid rocks at depths of ~1 km valley carved deeply through intracaldera Bonanza Tuff. Ages of the upper lava (Cook, 1960; Gordon Gumble, 2006, written commun.). sequence of the Bonanza center, including caldera-filling flows, are bracketed at A pluton of aplitic to porphyritic granite exposed at near-roof levels in an 33.12–32.25 Ma by the underlying Bonanza Tuff and overlying Saguache Creek ~3 × 4 km area of upper Spring Creek is compositionally similar to postcaldera Tuff, but the ~1-km-thick intracaldera sequence capped by Porphyry Peak Rhyo- lavas of the Porphyry Peak Rhyolite. Roof zone and margins of the Spring Creek

lite on the west flank of the Whale Hill dome was emplaced rapidly, by 33.03 Ma intrusion are aplitic porphyry (74%–77% SiO2), containing 15% euhedral K-feld- (mean of five samples; Supplemental Table 1). The lowermost postcollapse spar; deeper interior portion of the exposed intrusion are also porphyritic but

lavas (Squirrel Gulch Andesite) directly overlie upper-rhyolitic ignimbrite of have a medium-grained matrix that is modestly less silicic (72%–73% SiO2). the Bonanza Tuff and were tilted steeply along with the underlying tuff during Microperthitic and weakly argillized K-feldspar from the granitoid intrusions later resurgent uplift. In contrast, some late caldera-filling lavas (Porphyry Peak has not been datable with precision comparable to volcanic sanidine, but the Rhyolite and sanidine-bearing dacite) may be tilted somewhat less, as based on somewhat varied cooling ages (32.8–33.3 Ma; Table 3; Supplemental Table 1) contact geometry (primary depositional attitudes are difficult to constrain pre- overlap those from the Bonanza Tuff and postcaldera lavas. These cooling cisely in these massive viscous flows that commonly form widespread talus on ages, together with field relationships, indicate emplacement of the resurgent steep vegetated slopes). The tilted intracaldera lavas at Bonanza, erupted prior plutons shortly after caldera eruption. Numerous other caldera systems dis- to and during resurgence, thus are analogous to voluminous early postcaldera play similarly rapid timing (e.g., review by Lipman and Bachmann, 2015). 3 rhyolites at Long Valley (~100 km , as much as 600 m thick), also erupted during A roughly concordant sill-like body of uniform dacite (67%–68% SiO2), resurgence within ~100 k.y. after caldera collapse (Bailey et al., 1976). the Eagle Gulch Dacite (latite of Burbank, 1932), was intruded between cal- In contrast to many other large ignimbrite calderas, only small deposits of dera-floor andesitic lavas and basal intracaldera Bonanza Tuff along a north- lake sediments or small late pyroclastic eruptions are preserved in the post east-trending belt ~8 km long from Kerber Creek to the north slope of Elkhorn collapse fill at Bonanza. The rarity of intracaldera volcaniclastic rocks at Bo- Peak (Fig. 9). The sill-like shape and uniform texture and composition of the nanza is consistent with rapid accumulation of the lavas, largely or entirely Eagle Gulch Dacite differ from the more diverse granitoid intrusions, which are prior to major resurgent uplift. The time span between the Bonanza ignimbrite interpreted as uppermost levels of a vertically extensive composite intrusion eruption at 33.12 ± 0.03 Ma and that of Porphyry Peak Rhyolite margin that coring the Whale Hill resurgent dome. overflowed the north caldera margin by 33.03 ± 0.04 Ma indicates that at least 1 km of lava accumulated in the northwestern caldera area within ~90 k.y. (as little as 20 k.y. and no more than 160 k.y., based on the 2-sigma uncertainties CALDERA EVOLUTION AND STRUCTURE for the weighted-mean ages). Compositionally and texturally diverse intrusions varying from gabbroic Despite detailed complexities, overall evolution of Bonanza caldera has to silicic granitoid rocks intruded the caldera floor and lowest ignimbrite fill, the simplifying attribute of involving only a single large ignimbrite eruption forming widely scattered exposures that are inferred to represent an irregular and attendant caldera formation. The diverse exposure levels provide special roof zone of a more continuous composite body at slightly greater depth. The insights concerning timing of caldera subsidence, distribution of subsidence largest intrusions crop out on the deeply eroded eastern side of the Whale faults, caldera-floor structures, geometry of resurgent uplift, and ignimbrite Hill resurgent dome. Texturally diverse areas of granodiorite (56%–62% and subsidence volume estimates.

SiO2) and intergradational finer-grained phases, which form the 3 × 7 km exposed area of the Turquoise Mine intrusion on the eastern side of the dome (Fig. 9), are compositionally similar to postcollapse andesite and dacite pre- Inception of Caldera Subsidence served on the western flank. Fine-grained intrusive phases, covering areas as much as several hundred meters across, form bold outcrops of dense dark The asymmetric areal distributions of the outflow rhyolite and dacite tuffs

andesite (55%–56% SiO2), characterized by rectilinear jointing unlike the from Bonanza caldera provide key information on paleotopography and timing hackly fractures that characterize most andesite lavas in the Bonanza area. of initial caldera collapse (Fig. 15). At inception of the eruption, some barrier on Some larger areas of the finer-grained phases are composite, containing in- the west side, either prevolcanic structural highlands or earlier volcanic con- ternal contacts between subunits differing in phenocryst abundance, size, or structs, must have impeded ignimbrite flow; early-erupted rhyolitic ash spread mode. The areal abundance of intrusive andesite, comingled with coarser mainly to the east. Beginning of caldera collapse late during eruption of the granodiorite, is interpreted as representing the roof zone of a large inter lower rhyolite appears to have disrupted the western barrier, accompanied mediate-composition intrusion that would be less heterogeneous at greater by increased eruptive draw-down and/or tapping a new sector of a compo-

East Figure 15. Schematic caldera-collapse model, indicating possi- West ble paleotopographic controls on contrasting distributions of Precaldera volcanic construct outflow Bonanza Tuff, even though tuff erupted from a laterally A Eruption of lower extensive layered or zoned magma body. (A) Initial eruptions of rhyolite (from vents on the east side of the future caldera?) Bonanza rhyolite spread widely to the east, perhaps blocked from westward flow Crystal-poor rhyolite by the precaldera construct of the composite Rawley volcano complex and similar accumulations of intermediate compo- Crystal-rich dacite-andesite sition lava that filled the earlier Marshall caldera. The lower Bonanza rhyolite is thin and discontinuous within the caldera area, indicating that subsidence began only late during erup- East West tion of this phase. (B) Dacitic phases of the Bonanza ignimbrite eruption ponded thickly within the subsiding caldera and B Eruption of main spread far to west but are largely absent in the eastern outflow Caldera structural block Bonanza dacite sheet. A possible interpretation is that inception of caldera subsidence and perhaps shifting vent locations provided access for pyroclastic flows to cross west flanks of the Bonanza center. An alternative could be the presence of separate rhyolite and dacite magma bodies in close proximity beneath the caldera.

sitionally complex reservoir that led to initial discharge of voluminous dacite. Interfingered with the alternating zones of rhyolite and dacite ignimbrite Dacite was then able to spread widely to the west while accumulating thickly within Bonanza caldera are many irregular lenses of brecciated precaldera within the subsiding caldera, concurrently with intermittent further eruptions rocks (Figs. 9, 13, and 14), both mesobreccia (Fig. 16) and much larger masses of rhyolitic tuff. The asymmetrical distribution of early- versus later-emplaced of little-broken massive lava, which are interpreted as landslide debris derived compositional phases within the outflow of a single ignimbrite sheet appears from caldera walls that had become oversteepened during subsidence. Indi- somewhat analogous to eruptive processes well documented for eruption of vidual blocks in some lenses are larger than outcrops and are termed mega- the Bishop Tuff from a zoned magma body (Hildreth and Wilson, 2007). An- breccia (Lipman, 1976b). Some breccias are heterolithologic on outcrop scale, other example of contrasts in composition and distribution between early- and but in other large areas, blocks are compositionally uniform lava. The most late-erupted ignimbrite is the Lunar Cuesta Tuff in central Nevada (Best et al., voluminous breccia, locally as much as several hundred meters thick, is low 2013b, fig. 59 and text). The Nevada ignimbrite has a simple reverse composi- in the caldera fill, close to or in direct contact with caldera-floor rocks (Figs. 13 tional zonation, however, in contrast to the multiple compositional oscillations and 14); boundaries between breccia and floor can be obscure. The deep brec- in the Bonanza Tuff. cia is best developed along the southwest and north margins of the central re- The relatively modest thicknesses and limited areal extent of the lower- surgent uplift, in proximity to caldera ring faults and the inner wall. Exposures rhyolite zone within the Bonanza caldera provide the primary documentation are especially good on the dry southwest-facing slopes of Kerber Creek valley. that caldera collapse began relatively late during this phase of the ignimbrite Much less breccia appears to have reached central areas of the caldera floor, eruption. If subsidence had accompanied inception of the eruption, or even as exposed along the crest of the resurgent dome, and the landslides appear to triggered initial magma expulsion as proposed in some models for large ig- have thinned with distance from the inner caldera walls. The voluminous deep nimbrite calderas (e.g., Sparks et al., 1985; Lindsay et al., 2001; Gudmunds- breccias, in places deposited directly on caldera-floor rocks, are interpreted to son, 2008; Gregg et al., 2012; Cashman and Giordano, 2014), a much greater record catastrophic initial caldera collapse during the later stages of eruption thickness and volume of the early rhyolitic ignimbrite should have ponded of the lower rhyolite zone. Thus, intracaldera landslide breccia at the base of within Bonanza caldera. If a volume of early rhyolite tuff comparable to that an intracaldera ignimbrite sequence need not necessarily document caldera in the eastern outflow sheet (estimated at 200–300 km3) had accumulated con- collapse (or vent enlargement) concurrently with eruption inception. currently within Bonanza caldera, the thickness of intracaldera early rhyolite Smaller lenses of meso- and megabreccia interfinger at higher horizons of would have been greater than 1 km. Even though later stages of the Bonanza the intracaldera Bonanza Tuff (Figs. 13 and 17A), indicating that caldera walls eruption were accompanied by concurrent caldera subsidence, the outflow became oversteepened intermittently during subsidence, but not as severely volume of lower rhyolite alone is comparable to that of several uniform dacite as during the initial collapse. Most breccia consists of andesite and dacite frag- ignimbrites in the SRMVF (Table 1) and elsewhere, suggesting that no simple ments from the Rawley complex, but Proterozoic debris is also present, espe- correlations exist between eruptive volumes or magma compositions and in- cially within northern sectors of the caldera fill. A Proterozoic source for land- ception of caldera subsidence. slides along this sector would have required deep early subsidence (>1 km),

Figure 16. Typical mesobreccia in road cuts, Rawley Gulch. (A) Equant rounded block of andesite lava, ~0.7 m in diameter, surrounded by light-gray matrix of Bonanza Tuff and additional andesite fragments. The light color of the tuff indicates that it was originally only weakly welded and probably glassy, due to quenching against the abundant clasts of wall-rock andesite. (B) Closer view of mesobreccia texture, showing wide range of andesite-clast sizes dispersed in lighter-colored tuffaceous matrix. Photo location shown on Figure 9.

cutting down below the volcanic fill of Marshall caldera. Another major sub- more thickly down slope. Such tilting and asymmetrical collapse have been sidence event late during the eruption is recorded by the ~1000-m difference in documented at other ignimbrite calderas (Carr and Quinlivan, 1968; Lipman, elevation between high remnants of Bonanza Tuff, plastered against the west- 1984, 1997; Branney and Kokelaar, 1994) and in analogue models, especially for ern caldera wall, between Antora Peak and Windy Point (Fig. 5) and the upper- early downsagging and trap-door subsidence in small-volume eruptions (Cole most intracaldera tuff within the caldera structural block that subsided along et al., 2005; Acocella, 2007). Weakly asymmetrical subsidence at Bonanza is the Kerber Creek ring fault (Fig. 17A). also suggested by rheomorphic structures in rhyolitic tuff deep in the caldera Varied thickness and lateral extent among the multiple interfingering zones fill and by strongly prolate compaction and elongation of large pumice lenses of rhyolite and dacite in the intracaldera ignimbrite probably reflect diverse in dacitic tuff (Fig. 11B). Such flowage structures seem likely in calc-alkaline ig- factors, including surface irregularities on the pre-eruption lava assemblage, nimbrites only when deposited on a slope (e.g., Chapin and Lowell, 1979; John depositional slopes generated by caldera-wall landslide deposits, and mildly et al., 2008), especially in a dynamic environment of increasing steepness as asymmetric caldera subsidence. Caldera-floor morphology probably mainly could occur during caldera collapse. affected distribution and thickness of the lower rhyolite zone. The varied Despite this evidence for modest asymmetry at times during subsidence thickness and extent of many compositional zones higher in the intracaldera at Bonanza, the overall geometry is coherent subsidence of a structural block accumulation are spatially unrelated to breccia lenses. These variations sug- ~15 × 20 km across, accommodated along peripheral ring faults. Dips of folia- gest that the ignimbrite depositional surface became weakly tilted in varied tion defined by pumice fiamme (most 35°–55°) do not vary significantly or sys- directions during the course of caldera subsidence, and tuff accumulated tematically upward through the 2.5-km section of the intracaldera ignimbrite

In tracalder Broad crest of resurgent dome SOUTHWEST ~2.5a km rhyoli [granitic intrusive rocks exposed just beyond plane of section] NORTHEAST Flagstaff Caldera te & Caldera oor Whale Hill, 12,162′ Mountain rim 3,650 thick da 12,000 Creek cit Burned Mtn Caldera oo ox e tuf Caldera- oor andesite-dacite (a) CALDERA RING FAULTS y Creek f, r

gonb Intracaldera ignimbrite, ndesite breccia (m) r A Wa tilted to near vertical Kerber Creek 10,000 g Brewer 3,050 r Slaughterhouse Creek a br by resurgence r d Resurgent Resurgent d Post-collapse dacite (d) f lavas r d granodiorite intrusion aplitic granite intrusion pC a r m d 2,450 a 8,000 1 Kilometer [~2.5 km intracaldera Bonanza Tuff; >1 km postcaldera lavas = ~4–5 km total caldera subsidence] Meters [No vertical exaggeration] UNITS: a, andesitic lavas; d, dacitic Bonanza Tu; f, alluvial-fan deposits; g, glacial morraine; m, caldera-collapse megabreccia; pC, Precambrian rocks; r, rhyolitic Bonanza Tu above A sea level

Ring fault displacements, UPPER PALEOZOIC B (clastic rocks): south caldera margin ~1.5 km S, Sharpsdale/ displacement? Kerber Formations AR LOWER PALEOZOIC (carbonate rocks): L, Leadville; D, Dyer; Figure 17. Cross sections, showing structures and stratigraphic relations across SOUTH Bonanza caldera; locations are on generalized map (Fig. 9), but much more NORTH F, Fremont; ~450 m stratigraphic detail is illustrated by the sections. (A) Line 1: Southwest topo- S caldera S flank M, Manitou Fm graphic rim, across crest of resurgent dome (Whale Hill), to west margin of rim displacement resurgent dome San Luis Valley (segment of Rio Grande rift zone): shows large ring-fault dis- 9000 2750 placements, great thickness of intracaldera Bonanza Tuff, steep dips on flanks of resurgent dome, and inferred location of subcaldera granitoid intrusions. (B) Line 2: South caldera margin, showing displacements across ring faults. Tilt-

S S Kerber Cr L S ing of units reflects some combination of rotational block faulting during cal- Precambrian L granite L F dera subsidence, as well as possible uplift at south margin of resurgent dome. 8000 M Precambrian F 2450 F granite M Precambrian M granite

7000 2150 No V. E. Feet, Meters, above above S.L. S.L.

section exposed on the west flank of the resurgent dome (Fig. 13), indicating kilometers outboard of the major ring faults accommodated additional sub that the overall subsidence did not involve sustained progressive tilting. No sidence, slumping of large blocks along the south and west caldera margins, major fault offsets have been recognized within caldera-floor lavas of the main and modest inward rotation. As much as ~4 km of stratigraphic offset occurs subsided block; subsidence was dominantly piston style, not piecemeal. along the concealed ring faults on the west side of the caldera, as constrained by a cross section from Whale Hill to Flagstaff Mountain (Fig. 17A). Net offset Caldera-Collapse Faults along the Kerber Creek fault diminishes farther to the south, approaching zero at the junction with Little Kerber Creek and the intersection with the anticlinal Caldera collapse during eruption of the Bonanza Tuff was primarily accom- crest of the Whale Creek dome (Figs. 3 and 9); the decreased displacement modated along ring faults that are largely concealed beneath surficial depos- along this southern fault segment is interpreted to result from uplift during its along Kerber Creek and its tributaries (Fig. 9). Fault strands up to several resurgence. To the north, caldera faults are largely concealed beneath col-

lapse-related megabreccia and later lavas of the Bonanza eruptive cycle; it gular and rounded clasts are matrix supported, but despite areal proximity to remains unclear whether subsidence was as deep as to the south and west. megabreccia at the base of the intracaldera Bonanza Tuff, no tuffaceous com- The few other SRMVF calderas that are sufficiently deeply eroded to expose ponent is present in the shatter breccia. Although fault planes, slickensides, or bounding ring faults display varied structural patterns. The multiple intercon- other evidence of offset are sparse, the close proximity of the shatter breccias nected fault strands associated with the single ignimbrite eruption at Bonanza to the main caldera-collapse faults suggests that they formed during subsid- (Fig. 9) differ from the predominantly single ring-fault strand that is well ex- ence (or resurgence), perhaps due in part to hydraulic fracturing. Temperatures posed at Lake City, or from the nested faults at Grizzly Peak and probably at must have remained low during fracturing, however, as evidence is absent for Mount Aetna (Lipman, 1975; Shannon, 1988; Fridrich et al., 1991). silicification or other mineral precipitation from high-temperature hydrother- Andesitic and dacitic lavas of the caldera floor that are adjacent to ring faults mal fluids such as described along some faults elsewhere (Caine et al., 2010, along Kerber Creek and to the south are locally severely shattered, involving and references therein). Compositions of breccia matrix differ little from bulk textures and structures that seemingly have not been widely recognized at cal- andesite compositions, other than modestly variable alkali ratios and higher deras elsewhere. Angular blocks mostly less than 0.5 m across are juxtaposed, loss on ignition values (Supplemental Table 4). Alternatively, and perhaps with only minor matrix of comminuted lava (Fig. 18). In many exposures, finely more likely, shattering of the floor rocks may have resulted from compression shattered fragments fit together without large-scale rotation or other move- and crushing of the subsiding structural caldera block in proximity to steeply ment, and such rocks grade into more massive lavas of the caldera floor within inward-dipping ring faults. The shatter breccias at Bonanza somewhat resem- 100–200 m away from mapped ring faults. In some zones, breccias with an- ble the “collar breccia” at Indian Peak caldera, southern Nevada-Utah (Best

ABB

Figure 18. Shatter breccia adjacent to inferred ring fault along Kerber Creek. (A) Brecciated caldera-floor andesite along margin of resurgent dome, adjacent to valley of Kerber Creek; distant ridges form the eroded topographic rim of caldera (upper left). (B) In detail, crushed fragments fit closely together, without abundant fine-grained matrix or evidence for major shearing. Photo location shown on Figure 9.

et al., 2013a, especially their fig. 40), interpreted by these authors as having Such deep levels of caldera structure have rarely been observed on a com- formed by landsliding and ring-fault–related fracturing. parable areal scale elsewhere, where floor rocks are seen mainly in oblique Also interpreted as related to caldera subsidence are several east-trending cross sections through structurally disrupted and tilted caldera remnants, as arcuate faults south of Kerber Creek, which juxtapose Paleozoic sedimentary in the Great Basin of the western United States (e.g., Best et al., 2013a, 2013b; formations against Proterozoic rocks (Fig. 9). These were described by Burbank Henry and John, 2013). Caldera-floor levels of greater areal extent are well ex- (1932, p. 39–40, Plate 3) as low-angle thrust faults of prevolcanic age, but no posed at several well-documented large ignimbrite calderas of Paleozoic age; specific evidence was cited for a thrust interpretation, other than the presence e.g., Ordovician Scafell and Glen Coe calderas in Great Britain (Branney and of Proterozoic granite on high ridges south of the southward-dipping Paleo- Kokelaar, 1994; Moore and Kokelaar, 1998) and Permian Sesia and Ora calderas zoic strata exposed low along Kerber Creek. Detailed tracing of faults across in northern Italy (Quick et al., 2009; Sbisà, 2010; Willcock et al., 2013), but these ridges and gullies, with better base-map and geographic positioning system differ from Bonanza in important aspects, including morphologic preservation, (GPS) control than was possible for Burbank, documents generally steep dips, eruptive history, and tectonic setting. The record of explosive volcanism at at least 45°–60°, although vertical relief is insufficient to quantify fault dip all these sites comes mainly from thick intracaldera accumulations that have precisely. Accordingly, rather than low-angle thrusts active during Laramide been deeply eroded; little or no outflow ignimbrite or features of near-sur- compression, these faults are here interpreted as large-scale block slumps of face caldera morphology are preserved. All four Paleozoic calderas are inter- mid-Tertiary age, related to peripheral subsidence and failure along oversteep- preted to have formed in extensional tectonic regimes, where regional faults ened walls along the south margin of Bonanza caldera (Fig. 17B, line 2). Ro- strongly influenced subsidence geometry. None of the Paleozoic calderas are tation during slumping could have produced southward dips of the Paleozoic resurgently domed. Both British Ordovician calderas are polycyclic, each hav- strata that crop out at low elevations. Additionally, at least some component ing erupted multiple large ignimbrites, separated by significant time breaks as of the southward dips in the prevolcanic rocks likely resulted from tilting along documented by phreatomagmatic and sedimentary interbeds. Thus, recurrent the lower south flank of the large resurgent dome within the caldera. subsidence was likely, perhaps at differing loci within the overall caldera complex, and the documented piecemeal-style disruption of their floors may have Caldera-Floor Structure resulted from composite disruption during the multiple collapse events. The thick and texturally spectacular Ora ignimbrite has been interpreted as a single Particularly revealing within Bonanza caldera are the areally widespread deposit, erupted sequentially from two separate caldera loci that each sub- exposures of structurally coherent caldera-floor lavas and basal deposits asso- sided relatively coherently, but locations and nature of original caldera walls ciated with the ignimbrite eruption and caldera collapse. Other than the thick and bounding faults seem widely obscured by postvolcanic regional faults. dipping section of Bonanza Tuff on the lower west flank, virtually the entire resurgently domed caldera floor has been erosionally exhumed at stratigraphic Ignimbrite Fracture Fills levels close to original contacts with the basal intracaldera ignimbrite, over an area ~10 × 15 km across constituting much of the ring-fault–bounded caldera Where well exposed, the tuff matrix in much of the caldera-collapse brec- structural block (Fig. 9). cia is only weakly welded and distributed as highly irregular seams (Fig. 16), At Bonanza, stratigraphic levels close to the original caldera floor are ex- but in some outcrops tuffaceous crack fills have dike-like shapes, are strongly posed for about two-thirds of the area of the ring-fault–bounded structural welded, and contain steeply dipping pumice fiamme (Fig. 19). The crack fills core (the little-broken thick sequence of overlying intracaldera ignimbrite cov- are relatively thin (typically <1–2 m), discontinuous (commonly traceable only ers ~15% of the west flank of the resurgent dome, while on the eastern side, for a few tens of meter), and irregular in shape and trend. In places, fiamme- surficial deposits of the San Luis Valley conceal an additional 15%–20%). Ex- rich welded tuff grades along strike into flow-laminated crystal-poor rhyolite posures of the stratigraphic transition from caldera-floor lavas to ignimbrite that lacks obvious fragmental textures; a few parallel dike-like bodies con- fill at Bonanza are especially good on the relatively dry and weakly vegetated sist entirely of rhyolite without lithic fragments or other surviving pyroclastic south-facing slopes above Kerber Creek. In this area fractured but seemingly textures. coherent thick sequences of andesite and dacite lavas merge imperceptibly The best exposed fracture fills of highly welded and rheomorphic rhyo- upward into shattered outcrops of similar lava, between which irregular crack litic tuff, on slopes north of Kerber Creek, tend to be parallel to adjacent cal- fills and pockets of dacitic and rhyolitic tuff form matrix between megabreccia dera ring faults, but all identified fracture fills are located near the transition blocks. Within the near-floor megabreccia, most good outcrops consist of from caldera floor upward into caldera-fill megabreccia and matrix tuff. No erosion-resistant intermediate-composition lavas. Much of the matrix tuff is comparable fracture-fill tuff has been found at deeper exposed levels of cal- weakly welded, lithic rich, and exposed only as fragments on slopes. As a dera-floor lavas or in underlying Paleozoic and Proterozoic rocks. The highly result, contacts between caldera floor versus caldera-fill megabreccia can be welded to fluidal rhyolite in the dike-like fracture fills in places merges with broadly gradational and locatable only approximately in many places. areas of less welded tuffaceous matrix in the megabreccia and with larger

A pockets and lenses of more uniform lower rhyolite of the intracaldera Bonanza Tuff that dip conformably with the flanks of the resurgent dome. Most fracture fills with identified pyroclastic textures consist of crystal-poor rhyolite (only Topo rim one small outcrop of dacite fracture-fill tuff was found), similar to that of the Inner caldera wall early-erupted lower rhyolite phase of the ignimbrite sheet as would be anticipated if caldera subsidence began during this stage of the eruption. Similar local pods of pumiceous to fluidal rhyolite are present elsewhere on flanks of the resurgent dome, especially within large northern areas interpreted as megabreccia, but these areas are more vegetated and exposures are limited. Many of the crack-fill tuffs appear originally to have been glassy, even where Kerber Creek (ring-fault)Andesite most welded and fluidal, as suggested by well-preserved relict pumice and

shard textures and by extreme alkali exchange (K2O/Na2O ratios, commonly Welded 3-10) compared to analyzed samples of the outflow and intracaldera ignimbrite tuff (vertical) (typical ratios, 1.3-1.5: Supplemental Table 4). Some of the ignimbrite crack fills at Bonanza could be interpreted as intrusive dikes of fluidal rhyolite or vent fissures for ignimbrite eruptions, but their stratigraphic distribution, lateral textural variations, and compositions suggest that they are best interpreted as surficial fills between blocks of early caldera-collapse megabreccia, injected down into cracks that opened dilatantly during caldera subsidence. Dike-like pyroclastic bodies at several ignimbrite and caldera settings elsewhere have been similarly interpreted as dilatant B crack fills (Lipman, 1964; Branney and Kokelaar, 1994, p. 525; Best et al., 2013a, C p. 920). Somewhat similar to the Bonanza crack fills are wider and more laterally continuous pyroclastic intrusions that have been discussed elsewhere as possible ignimbrite vent structures: e.g., a welded-tuff fissure near a caldera margin in central Nevada (Ekren and Byers, 1976), the tuff dike at Mount Aetna Andesite caldera in the Swatch Range (Shannon et al., 1987), elongate steep bodies of lithic welded tuff in the Grizzly Peak caldera (Fridrich et al., 1991), pyroclastic dikes at Fairview Peak and Caetano calderas in Nevada (Henry and John, 2013, p. 977, 993), some tuff-filled fissures at the Scafell caldera in the British Lake District (Branney and Kokelaar, 1994, p. 526), or the Big Butte pyroclastic complex in Montana (Houston and Dilles, 2013, p. 1407). Interpretation of the Bonanza fracture fills as marking primary eruptive sites seems improbable, however, because (1) these structures are localized near the interface between caldera-floor lavas and overlying ignimbrite and megabreccia fill, (2) in places steeply dipping fracture-filling tuff is traceable continuously into the lower rhyolite zone of intracaldera Bonanza Tuff, (3) the fracture fills are relatively small and discontinuous in comparison to pyro clastic dikes described elsewhere, and (4) the predominance of rhyolitic compositions in the fracture fills seems inconsistent with interpretation as primary Figure 19. Dike-like bodies of vertically dipping Bonanza Tuff, interpreted as filling dilatant frac- vents because the peak stages in the Bonanza eruption were dominated by tures in upper part of caldera-floor lava and megabreccia sequence. (A) View to southwest, up the valley of Kerber Creek, which follows the main caldera ring fault; crack fill is rib of densely dacitic ignimbrite. For interpretation of the Bonanza crack fills as eruptive sites, welded tuff, with near-vertical contact between crack-filling tuff and country rock of andesitic the vent geometry would have been an areally-widespread diffuse network of lava, on lower flank of resurgent dome; high distant ridges are eroded topographic rim of cal- weakly interconnected small fractures, without preservation of sizable discrete dera. This and some nearby crack fills are semi-parallel to the NW trend of the adjacent Kerber Creek ring fault, but others are at high angles. (B, C) Details of steeply dipping foliation defined eruptive loci along the caldera ring faults. by fiamme in the tuff. Photo location shown on Figure 9.

Absence of Eruptive Precursors? without concurrently generating a high thermal column of buoyantly convect- ing ash at the vent(s), analogous to that in smaller historic eruptions. The well-preserved morphology and structures at Bonanza, especially the Alternatively, the presence or absence of a proximal basal Plinian deposit exposed caldera floor and segments of the topographic rim, provide unusual may be related to timing of initial ignimbrite generation. A rate of magma dis- opportunities to evaluate proximal precursor activity prior to a major ignimbrite charge that was high from inception of the eruption could generate large pyro- eruption in the SRMVF. Notably, no silicic deposits have been recognized, either clastic flows rapidly, such that proximal ash and pumice fall would settle and lavas or Plinian tephra, that closely preceded the ignimbrite eruption. mix with the moving pyroclastic flow, without forming an underlying fallout Several lavas of crystal-poor, sanidine-bearing rhyolite, high in the layer; Plinian fallout deposits would be preserved only distally beyond the ig- caldera-floor lava assemblage along east and south slopes of the resurgent nimbrite sheet. Such a process was observed on a small scale on 7 August 1980 dome and on the proximal south flank of Bonanza caldera, were initially con- at Mount St. Helens, where the pyroclastic flow was emplaced while the Plinian sidered potential candidates for ignimbrite precursors because of their similar column continued to rise, and proximal tephra fell to the ground only after ter- modal mineralogy to rhyolitic Bonanza Tuff. However, these lavas have mod- mination of movement in the pyroclastic flow (Christiansen and Peterson, 1981, estly different trace-element compositions than the ignimbrite (Supplemen- their fig. 15). Contrasting crystal contents in matrix versus pumice indicate that tal Table 4), their sanidine phenocrysts are more potassic, and 40Ar/39Ar ages large volumetric proportions of ash were elutriated during eruption of several (ca. 33.7 Ma; four sites) are ~0.5 m.y. older (Table 3; Supplemental Table 1). well-studied large ignimbrites that lack underlying Plinian-fall deposits; the win- Other rhyolitic lavas high in the precaldera lava sequence (Fig. 9) lack sanidine nowed fine ash must have been deposited on the moving pyroclastic flow and phenocrysts and cannot be dated precisely, but these also have trace-element at more distal sites (e.g., Lipman, 1967; Walker, 1972; Folkes et al., 2011; Chesner, compositions dissimilar to rhyolitic Bonanza Tuff and appear to be typical of 2012). Such an explanation for the absence of Plinian layers beneath large ig- the Conejos-type rhyolites that are distributed sparsely but widely in the north- nimbrites has also been proposed briefly by Branney and Kokelaar (2002, p. 7 east San Juan region (e.g., Lipman, 2012). Despite the diverse erosional levels and their fig. 6.5). For the SRMVF ignimbrites that lack associated proximal-fall at Bonanza, exposures of the precaldera assemblage are far from complete, deposits, distal Plinian tephra may exist among the many ash beds in the Oligo- and precursor lavas could remain hidden. However, the absence of silicic erup- cene White River Formation on the High Plains, Colorado and Wyoming (e.g., tions of appropriate composition or age suggests that the rhyolitic Bonanza Prothero, 1996; Larson and Evanoff, 1998), although no systematic studies have magma was assembled shortly before inception of the ignimbrite eruptions, thus far attempted detailed correlations. Similarly, as much as several hundred with few if any lava precursors reaching the surface. cubic kilometers of fallout ash in the midcontinent has been interpreted as cor- In addition to apparent absence of precursor silicic lavas at Bonanza, no relative with individual large dacitic ignimbrites of mid-Tertiary age erupted in initial tephra-fall deposits of Plinian type are preserved beneath the ignimbrite. the southern Great Basin (Best et al., 2013a, 2013b). The deep dissection of the caldera fill at Bonanza locally provides well-exposed contacts between basal ignimbrite (dacite, as well as rhyolite) and underlying caldera-floor rocks. Both on flanks of the resurgently domed caldera floor and Resurgent Uplift along the western caldera rim, proximal Bonanza Tuff is commonly welded to its base and directly overlies precaldera lavas, without intervening bedded After the ignimbrite eruption and emplacement of at least most caldera- tephra (Fig. 11D). filling lava flows, the caldera floor was arched into a spectacularly large and Similarly, no thick or widespread Plinian fall deposits have been recognized steep-sided resurgent dome that is gently arcuate to the east (Figs. 3, 9, 17A, beneath other large ignimbrite sheets in the SRMVF, including those with rhyo and 20). Erosion has stripped most intracaldera Bonanza Tuff from the floor litic or zoned compositions. Plinian deposits also appear to be rare or absent lavas along the crest of the dome, which is well defined at present by the in association with compositionally diverse ignimbrite eruptions in other calc- gentle upland surface on Whale Hill (Fig. 20). Small remnants of subhorizontal alkaline Cordilleran systems, such as the southern Great Basin ignimbrite field Bonanza Tuff preserved along the crest of the resurgent structure at Round (Best et al., 2013a, 2013b), the central Andes (Sparks et al., 1985; Lindsay et al., Mountain and Elkhorn Peak provide critical constraints on caldera-floor geom 2001; de Silva et al., 2006), or the 74-ka Toba eruption in Indonesia (Chesner, etry and structure along highest parts of the dome. Notably, caldera-floor lavas 2012). This contrast with thick Plinian deposits at well-known younger calderas are at elevations above 3700 m on the crest of the resurgent dome, as high such as Long Valley, Yellowstone, and Taupo raises questions about recent as comparable units on the west topographic rim (Fig. 17A). This relation in- proposals that precursor tephra deposits typically form at inception of rhyo- dicates ~3.5 km resurgent uplift, equal to the subsidence documented by the litic ignimbrite eruptions but not during large dacitic eruptions, in response thick caldera-fill section of ignimbrite and pre-resurgent lavas on the west flank to contrasting mechanisms and timing of caldera subsidence (summarized by of the dome. If thickness of the caldera fill centrally within the caldera had been Cashman and Giordano, 2014, and references therein). Explosive discharge of similar to the west-flank section, the original dome crest would have had an hundreds to thousands of cubic kilometers of silicic magma seems improbable elevation of ~7000 m above present-day sea level.

Round Mountain Pikes Peak (dacitic Bonanza Tuff) Whale Hill Elkhorn Hayden Peak Porphyry Peak (dacitic (late lava flows) Peak Sangre de Cristo Mountains Bonanza Tuff) (rhyolitic San Luis Valley Bonanza Tuff) (Rio Grande rift)

Caldera-floor lavas SE caldera rim

Postcollapse rhyolite dacitic Bonanza Tuff

Postcollapse lavas Moraine and landslide

Concealed ring fault Andesite lavas (on west caldera wall) Andesite lavas Moraine & landslide (on west caldera wall)

Bonanza Tuff, on west caldera rim

Figure 20. Panoramic view across crest of Whale Hill resurgent dome, toward San Luis Valley (Rio Grande rift zone), from west rim of Bonanza caldera. Intracaldera Bonanza Tuff is as much as 2.5 km thick, dips 45°–55° on flank of resurgent dome. Photo location shown on Figure 9.

Dips on flanks of the Whale Hill dome are uniquely steep and variable continuation of the elongate Whale Hill dome. Because the Proterozoic rocks compared to resurgent uplifts at other well-documented ignimbrite calderas: in the core of the anticline are directly overlain by Tertiary lava flows, with- 40°–60° on the west side of the dome where widespread preservation of intra out intervening Paleozoic sedimentary strata, this area must have also been a caldera tuff provides robust structural control, nearly vertical along parts of prevolcanic high. Perhaps a more open fold in the prevolcanic rocks influenced the northeast and southwest flanks (Figs. 13, 14, and 17A), but typically only the location of postcaldera resurgence, or alternatively this area was simply a 20°–30° on the southeast flank. This asymmetry may result in part from vari- paleohighland along a northwestern erosional truncation of Paleozoic strata. able tilting of the Bonanza region, within a zone of structural transfer between Aspects of the resurgent caldera structure at Bonanza were anticipated by segments of the Rio Grande rift zone. The dome appears to have been largely Burbank (1932, p. 42–43), who concluded that “arching and tilting of the forma- bounded and partly accommodated by ring faults that initially formed during tions . . . is believed to have been initiated by the intrusion of a large body of caldera subsidence. Resurgence is inferred to have been caused by emplace- molten lava . . . The crust was consequently bulged upward, blocks of it were ment of multiple intrusions centrally within the caldera, including the grano tilted in different directions.” Prior mapping depicted a highly intricate mosaic diorite to granite bodies that are exposed on the eastern flank of the dome. of rectilinear faults in the mining district (Burbank, 1932, plate 1), but many of The south margin of the resurgent dome is especially well constrained by the depicted faults were required to accommodate an overly simplified strati- the southeast-plunging Proterozoic-cored anticline and flanking Paleozoic sedi graphic sequence, without adequate available concepts of ignimbrite-eruption mentary strata (Figs. 9 and 17B) that are stratigraphically much more precisely and caldera-filling processes. defined than the precaldera lava succession. The southern anticline has been In the current study, only a few faults with documentable displacement previously interpreted as a prevolcanic (Laramide) structure (Burbank, 1932; have been identified confidently within the resurgent dome. Evidence for siz- Tweto et al., 1976; Cappa and Wallace, 2007). Dips of immediately overlying able fault displacements has been elusive on heavily vegetated slopes where volcanic strata including Bonanza Tuff, although less widely measurable, lo- talus is widespread, outcrops rare, reliable stratigraphic-marker horizons cally are nearly as steep on the fold flanks and have a similar asymmetry as sparse, and underground mines no longer accessible. More faults than have the well-stratified Paleozoic sedimentary strata (Fig. 21), however, requiring been mapped are likely within the resurgent block, but major fault repetitions that much of the tilting was postcaldera. At least the main development of the seem unlikely. In contrast to the work by Burbank (1932), Patton (1916, p. 63), fold accordingly must be Tertiary in age; it is here interpreted as the southern with access to more of the underground mine workings in the Bonanza dis-

Manitou Limestone Fremont Limestone Figure 21. Intracaldera Bonanza Tuff at Bonanza Soda Springs Creek, dipping steeply on Leadville southwest flank of resurgent dome. Dips Limestone are conformable with underlying Paleozoic pC (dacite)Tuff Dacite (intrusion?) strata, exposed on ridge at left of image. Lens of andesite blocks between upper Paleozoic rocks and dacite appears to be Upper caldera-collapse landslide breccia. Dacite Paleo appears to be an irregular sill-like intrusion, sandston zoic rather than precaldera lava flow. pC—Pre- e cambrian granite. Photo location shown on Figure 9. Andesite

trict, noted that “While minor faults involving a movement of a few inches average resurgence rate (3.5 cm/yr) at Bonanza is roughly similar to that well or, at most, a few feet are of common occurrence, no evidence of faulting on constrained for uplift of the Samosir Island resurgent dome at Toba caldera a large scale has been discovered.” Beyond the mining district, relatively co- for the interval since ca. 34 ka, declining from ~4.9 cm/yr to <1 cm/yr, with a herent subsidence and subsequent resurgence of the caldera floor are well suggested long-term average of 2–3 cm/yr (de Silva et al., 2015). documented in the southern caldera area, where detailed structural control Several tantalizing features suggest that resurgence at Bonanza may have is provided by the high-resolution stratigraphy of the regional lower Paleo- been rapid, while deep parts of the caldera-fill ignimbrite remained hot and duc- zoic formations. These strata dip steeply on flanks of the resurgent dome but tile: (1) extreme fluidal welding and lava-like flowage of the lower rhyolite unit are traceable continuously across the nose of its south-plunging anticlinal ter- at the base of the thick intracaldera ignimbrite accumulation (Figs. 11D and 11E); mination without sizable fault displacements other than by the major caldera (2) down-dip trends of prolate fiamme lineations in fluidly welded ignimbrite on ring faults. flanks of the dome; (3) apparently limited brittle-fault disruption of caldera-floor Despite the scarcity of mapped faults on the resurgent dome at Bonanza, levels in the Whale Hill dome, in comparison to complex keystone and other some disruption likely accompanied uplift, as at other well-studied resurgent faults in resurgently domed calderas elsewhere; (4) lower dips in upper lavas of calderas that are characterized by keystone grabens and other uplift-related the caldera fill than in initially erupted lavas and underlying Bonanza Tuff, sug- faults (e.g., Valles, Creede, Timber Mountain, Lake City, and Cerro Galan: Smith gesting rapid resurgence concurrently with the accumulation of these lavas as and Bailey, 1968; Steven and Ratté, 1973; Byers et al., 1976; Lipman, 1976a; documented by isotopic ages; and (5) marginally younger sanidine ages from Folkes et al., 2011). The apparently more limited fault disruption of caldera floor intracaldera Bonanza Tuff than from outflow portions (Fig. 4), compatible with and resurgent dome at Bonanza may be related to formation of this caldera prolonged slow cooling deep in the intracaldera ignimbrite. in response to a single ignimbrite eruption; the other resurgent calderas just noted are nested within earlier ignimbrite subsidence structures, and prior disruption of the subsided areas may have contributed to more complex fractur- Initial and Present-Day Caldera Morphology ing and larger displacements during resurgence. The relatively limited faulting at Bonanza may also partly account for the modest mineralization there, in A notable feature of Bonanza is the geologically recent exhumation of comparison to otherwise analogous epithermal vein systems in SRMVF cal- the western side of the caldera to morphology closely approximating its pri- dera settings such as Creede and Silverton. mary volcanic features (Fig. 5), while concurrently exposing deep levels on Uplift of the Whale Hill dome was geologically rapid: 3.5 km at caldera-floor its eastern slopes, including caldera floor, the pre-Bonanza lava assemblage, level in less than 100 k.y. (Table 3), as bracketed by ages of the tilted caldera- and underlying Paleozoic sedimentary strata and Precambrian basement filling Bonanza Tuff (33.12 Ma) and Porphyry Peak Rhyolite (33.03 Ma). The rocks (Fig. 9). By late in Oligocene time, the Bonanza caldera area was likely

buried to considerable depth by continued local volcanism and by the many scabs of intracaldera tuff have survived erosion on the southeast flank of the younger ignimbrites erupted from farther southwest in the San Juan region. Whale Hill dome, and erosion has widely cut through the precaldera lavas of More recently, rift-related tilting and deep erosion into the rugged present-day the caldera floor down into Paleozoic and Precambrian basement rocks. landforms have exposed the interior of the caldera at levels from Precambrian The east margin of Bonanza caldera is largely concealed beneath alluvial fill basement, through the Paleozoic sedimentary sequence, precaldera interme- of the San Luis Valley in the northern Rio Grande rift. However, any geometri diate-composition lavas, the entire section of intracaldera tuff, overlying post- cally simple arcuate projection of the Kerber Creek ring fault and the morpho- caldera lavas, and associated resurgent intrusions. logic topographic rim suggests that the original east caldera margin lay close Despite the later depositional, erosional, and structural complexities, parts to the major bounding normal faults at the west base of the present-day San- of the present-day topography have been eroded to features notably similar gre de Cristo Range from south of Poncha Pass to east of Villa Grove (Figs. 3 to those of the Oligocene caldera. These include the high present-day topo- and 9). In a small erosional patch low along the steep west-facing range front graphic crest of the Whale Hill resurgent dome (Fig. 20), even though almost southeast of Poncha Pass (Fig. 9), dacitic Bonanza Tuff and andesitic lavas lap entirely stripped of intracaldera Bonanza Tuff; the entire ~3.5 km thickness of onto Proterozoic metamorphic rocks (Van Alstine, 1975), perhaps defining a caldera-filling tuff and postcollapse lavas on the west flank of the dome (Figs. distal wedge of intracaldera ignimbrite lapping out against the eastern cal- 13 and 17); western tributaries to Kerber Creek that coincide broadly with an dera wall. Because the Bonanza eruption preceded formation of the rift zone original moat between inner caldera wall and flank of the resurgent uplift and uplift of the Sangre de Cristos, thus permitting the ignimbrite to spread (Fig. 9); and the high western ridges of Antora Peak (13,269 ft, 4044 m), Windy widely to the east (Fig. 10), this patch of Bonanza Tuff at low elevation along Point (11,900 ft, 3627 m), Flagstaff Mountain (12,072 ft, 3680 m), and farther the mountain front would have been much deeper prior to rift formation and south (Fig. 5) that coincide roughly with the original topographic rim of the uplift of the Sangre de Cristo block. Alternatively, these small exposures might caldera. Although more modified by erosion, the continuation of high ridges be interpretable as preserving proximal outflow of the Bonanza Tuff, if future southeast across Ute Pass to Saguache Peak (10,550 ft, 3215 m) and then east- studies were to document major rift faults farther east, higher within exposed ward (Fig. 3) seems likely to represent further approximations of the caldera Precambrian rocks on the west-facing slope of the Sangre de Cristo Range. rim (Fig. 9). The caldera elements viewed up Kerber Creek valley, from toward Combining remnants of the original topographic rim on west and south Antora Peak (Fig. 5), appear broadly similar to those right after completion of sides of Bonanza with projection across the San Luis Valley to the inferred east the Bonanza resurgence, except perhaps that the caldera-moat valley would wall against the northern Sangre de Cristo Range defines an approximate over- have been largely filled by postcollapse lavas. Thus, the morphology on the all topographic caldera ~20 × 25 km across. This topographic caldera would western side of Bonanza caldera is broadly comparable to younger San Juan have been larger than the structurally subsided floor, because of landsliding calderas such as Creede and Cochetopa Park, where later Cenozoic erosion and block slumping from oversteepened inner walls during the continued ig- has exhumed Oligocene caldera morphology that is even more completely nimbrite eruption (Lipman, 1997). The arcuate valley of Kerber Creek, under- preserved (Steven and Ratté, 1973; Steven and Lipman, 1976; Lipman and lain by the main arcuate ring-fractures that accommodated caldera subsidence McIntosh, 2008). (Fig. 5), appears to follow the original morphologic moat between the inner Large segments of the west caldera rim are still capped by thick Bonanza caldera wall that had been enlarged by landsliding and the south and west Tuff, although some of these west-dipping exposures may represent high rem- flanks of the resurgent dome. To the north and northwest, the structural bound- nants of caldera-filling ignimbrite, ponded between the ring-fault zone and the ary, and even the topographic rim, are largely obscured by fill of postcollapse caldera-wall “collar” (Lipman, 1997) that had been enlarged by landsliding lava flows, but the presence of thick megabreccia in this sector suggests deep during caldera collapse. The incomplete section of Bonanza Tuff, with its top subsidence and proximity to a structural boundary at greater depth. Projection eroded, is more than 300 m thick on the northwest flank of Antora Peak; such from the better constrained south and west sides suggests that the ring faults a great thickness of proximal ignimbrite seems unlikely to have accumulated bound an elliptical structural caldera ~15 × 20 km across. About two-thirds of high on the preexisting edifice of the Rawley volcanic complex prior to caldera this structural area is exposed at present-day levels of caldera floor; ~20% is collapse. When the geometry of the west-rim ignimbrite remnants is corrected covered by intracaldera ignimbrite and overlying lavas, mainly on the west for their 15°–20° westward dips, they appear as westward-thinning scabs, side of the resurgent dome, and ~15% is concealed beneath sediments in the banked against older intermediate-composition lavas of the inner caldera wall. San Luis Valley. Similar high-fill remnants of intracaldera ignimbrite, preserved as continued The resulting three-dimensional preservation of morphologic, stratigraphic, subsidence dropped the central caldera floor to greater depths, are preserved and structural caldera features at Bonanza includes shallow postcollapse fill at other San Juan calderas such as Lake City and Creede (Lipman, 1976a; Lip- and only modestly modified segments of the morphologic topographic rim man, 2000; Figs. 13 and 14). and inner caldera walls, down through the entire intracaldera-filling ignimbrite In contrast, the eastern Bonanza caldera is exposed at much deeper levels, and megabreccia, to ring faults, resurgent intrusions, and prevolcanic base- and accordingly little of the caldera morphology is preserved. Only a few small ment, with resulting constraints on eruptive and structural evolution during

a single caldera cycle. Such extensive exposure of these diverse caldera ele- of >3 km for the relatively coherent equant block within the ring fault (>2 km ments is unique in the Southern Rocky Mountain volcanic field and perhaps of intracaldera ignimbrite, overlain by >1 km of caldera-filling lava). Compara- among large Cordilleran-arc calderas associated with ignimbrite flareups ble multi-kilometer subsidence is also documented by minimum thickness of elsewhere. The beautifully preserved young calderas of the Altiplano volcanic caldera-filling ignimbrite and postsubsidence lavas at other large mid-Tertiary complex and Cerro Galan in arid high regions of the South American Andes calderas such as Lake City and Grizzly Peak in the SRMVF (Lipman, 1975, 2007; lack exposure of deep levels of intracaldera fill and floor rocks or any associ- Fridrich et al., 1991) and elsewhere in the U.S. Cordillera (Henry and John, ated subvolcanic intrusions. In the semi-arid Great Basin of the western United 2013; Best et al., 2013a, 2013b), but the extent of exposed caldera floor at Bo- States, tilting by younger faults provides exceptional oblique exposures of nanza seems unique. mid-Tertiary ignimbrite caldera fragments and their intracaldera deposits while A corollary implication of the high-standing caldera floor on the Bonanza preserving widespread outflow equivalents, but overall caldera geometry is resurgent dome is that intracaldera ignimbrite volume could be overestimated obscured by the extensional disruption and sediment-filled basins between and potentially yield overly large ratios of intracaldera versus outflow accu- dismembered volcanic sections. Deeply dissected Ordovician calderas (Scafell, mulations, if based on exposed maximum topographic relief of the resurgent Snowdonia, and Glen Coe) in Great Britain display structurally complex cal- uplift as assumed for some little-eroded young calderas (e.g., Lindsay et al., dera floors and ignimbrite fill, but features directly associated with individual 2001; Salisbury et al., 2011). For Bonanza, ring-fault locations, subsidence eruptions are difficult to separate at these multicyclic ignimbrite centers, and depth (~3.5 km, from thickness of intracaldera ignimbrite and postcollapse the associated outflow ignimbrites have been completely eroded (Branney lava fill), and similar height of resurgent uplift are relatively well constrained. and Kokelaar, 1994; Moore and Kokelaar, 1998). These then define proportions of intracaldera ignimbrite versus uplifted caldera floor in the resurgent structure. Based on a simple ellipsoidal model for uplift at Bonanza, caldera-floor rocks form about one-third of total dome vol- Ignimbrite and Subsidence Volumes at Resurgent Calderas ume. Somewhat analogously, intracaldera volume of the 2.1-Ma climactic ignimbrite at the superbly preserved Cerro Galan caldera in the central Andes is As elsewhere in the SRMVF, estimates of the magmatic volume erupted as now estimated at 315 km3, reduced from an earlier estimate of 500 km3 (Sparks ignimbrite are highly approximate at Bonanza, because of incomplete expo- et al., 1985) largely due to recognition of high-standing basement rocks on the sure, variable thickness due to deposition on irregular paleotopography, and southern flank of the dome (Folkes et al., 2011). extensive subsequent erosion. Total eruptive volume of the Bonanza Tuff is es- In contrast, volumes of outflow ignimbrites may tend to be underestimated timated at ~1000 km3 (Table 1), based on intracaldera and outflow distribution because of common rapid erosion, especially for weakly welded deposits. The and thickness. The volume of intracaldera ignimbrite is estimated to be greater Thorn Ranch Tuff, as well as the three large early Oligocene ignimbrites erupted than 500 km3, based an area of 250 km2 of the subsided structural block (~15 × from Sawatch Range calderas (Wall Mountain, Grizzly Peak, and Badger Creek 20 km across), and an average tuff thickness of 2 km or greater. While much Tuffs), are nearly or completely absent west of their source calderas in compari of the intracaldera tuff has been eroded, its thickness is as much as 2.5 km in son to widely exposed areas to the east (McIntosh and Chapin, 2004). The few the complete section on the west flank of the resurgent dome and more than small western remnants of Wall Mountain Tuff, even though ~70 km southwest 1.5 km in the steeply dipping incomplete section on the northeast side (Figs. of its inferred source above Mount Princeton, are still thick (to >100 m) and 13, 14, and 17A). Additional ignimbrite fill that would have accumulated in the rheomorphically welded (Lipman, 2012); accordingly, this ignimbrite originally “collar” area between the inner topographic wall and the ring fault, but its would likely have been about as voluminous and widespread to the west as volume would have been partly counterbalanced by the caldera-wall landslide its much more widely preserved eastern distribution. This present-day asym- deposits within the structural block. The preserved extent of outflow Bonanza metry suggests that erosion was rapid along higher slopes of Laramide-age Tuff, still densely welded at least 40 km west from caldera rim (Lipman, 2012) Rocky Mountain uplifts, soon after ignimbrite eruptions from sources in the and 70 km to the east (Fig. 10; McIntosh and Chapin, 2004) and typically several Sawatch Range. In comparison, ignimbrites erupted from farther west and tens to as much as a hundred meters thick, yield an estimated volume of sev- south in the San Juan region, where the volcanic rocks were deposited mainly eral hundred cubic kilometers for each sector. Thus, even though deposition on less deformed strata of the northeastern Colorado Plateau, tend to be pre- along paleovalleys, erosional removal of original deposits to the north, and served widely in all directions around their caldera sources (Steven and Lip- cover by younger volcanic rocks to the southwest in the San Juan region pre- man, 1976). Even there, however, weakly welded upper zones of the ignimbrite clude more detailed estimates for Bonanza, intracaldera and outflow volumes sheets are commonly eroded, down to dense interiors, and eruptive volume thus seem roughly subequal, a crude approximation that appears to hold for may be underestimated. In an impressively detailed evaluation of magmatic many large ignimbrites (Lipman, 1984; Mason et al., 2004; Folkes et al., 2011). volumes for the young Cerro Galan ignimbrite, Folkes et al. (2011) estimated In comparison, the volume of total structural subsidence at Bonanza cal- that the exposed outflow deposit (108 km3) constitutes only about a quarter dera is also estimated at 750–1000 km3, based on average vertical subsidence of the original outflow volume (486 km3). Thus, much of the deposit has been

lost in only 2.1 m.y., even in a highly arid area where the dominant erosion has Rocks of the Bonanza cycle (Table 4) are more geochemically diverse and been by wind. modestly more alkalic than younger volcanic assemblages associated with Because of such uncertainties, inferences about ratios between intracaldera large ignimbrite eruptions farther south and west in the San Juan region (Fig. and outflow ignimbrite volumes should be viewed only as rough approxima- 22). In comparison to other San Juan ignimbrites, the Bonanza Tuff is char- tions (e.g., discussion in Mason et al., 2004, p. 737), to be applied with caution acterized at any equivalent silica content by elevated incompatible elements for interpretations such as time of initial caldera formation. Inception of col- such as total alkalis, Zr, Rb, La, and Ce (Fig. 22). The least altered samples of lapse appears to correlate roughly with eruption size; small calderas collapsing Bonanza Tuff, especially outflow dacitic phases, are also higher in alkalis and late during the eruption, larger ones forming earlier (reviewed by Cashman incompatible trace elements than otherwise compositionally similar precal- and Giordano, 2014). Initial subsidence at some large calderas has even been dera and postsubsidence lavas and intrusions of the Bonanza cycle. inferred to precede and trigger eruption of voluminous dacitic ignimbrites The Bonanza Tuff contains the broadest bulk-compositional (60%–76%

from low fountains without generating fallout deposits, as proposed for Cerro SiO2) range of any ignimbrite erupted from SRMVF calderas, and its crystal

Galan and some other large calderas (Sparks et al., 1985; Gudmundsson, 2008; cargo is distinctive. Rhyolitic Bonanza Tuff is more silicic (to 76% SiO2) than Gregg et al., 2012; Cashman and Giordano, 2014). However, the cited geologic the other Oligocene ignimbrites, and its phenocrysts are nearly unzoned and evidence for these partly model-based interpretations (absence of proximal in apparent equilibrium with a rhyolitic magmatic liquid. Crystal-poor rhyo-

fallout deposits beneath the ignimbrite, estimated large intracaldera volumes lite (2%–5% phenocrysts) contains sodic low-Ba (Cn< 0.1) sanidine of restricted

relative to the associated outflow tuff, and high viscosity of crystal-richdacite compositional range (Or51–54), along with accessory titanite and hornblende in magma) seems open to alternatives. Thick intracaldera accumulations of com- some samples. In contrast, feldspars in dacitic Bonanza Tuff display complex positionally uniform ignimbrites document that the associated caldera col- disequilibrium zoning and resorption features (Figs. 23 and 24). Composition-

lapsed at some stage during an eruption but provide no direct stratigraphic ally variable sanidine with much higher Ba contents (Or55–70; Cn< 0.1– > 6 ) is pres- evidence for inception time; at least some large ignimbrites of crystal-rich dac- ent both in bulk samples and in nonfragmented pumice fiamme of low-silica

ite generated voluminous distal fallout deposits (Best et al., 2013a, 2013b); and dacite (62%–64% SiO2), a more mafic host composition than known for this some were highly mobile, comparable to crystal-poor rhyolites. For example, mineral elsewhere in the region. The diversity of sanidine compositions and distal Fish Canyon Tuff, the archetypal “monotonous-intermediate” ignimbrite textures in dacitic Bonanza Tuff are similar but more extreme, in comparison (Hildreth, 1981), is preserved along the Arkansas River Valley, 125 km north- to other dacitic ignimbrites and lavas of the SRMVF, indicating that magma east of the rim of La Garita caldera (McIntosh and Chapin, 2004, p. 228), and a mixing, pressure changes, and variations in volatile components occurred similar distance south of the caldera at Las Tablas, New Mexico (Zimmerer and commonly during prolonged processes of magma assembly (Lipman et al., McIntosh, 2011). For the Bonanza ignimbrite, compositional variations demon- 1978; Bachmann et al., 2002, 2014). strate that caldera collapse began only after voluminous eruption of the east- Bulk dacite samples, although dominated by pumices of low-silica dacite,

ern outflow ignimbrite. Later-erupted intracaldera tuff ponded to multi-kilome commonly also contain sparse dark scoria of silicic andesite (60%–62% SiO2) ter thickness; intracaldera and outflow ignimbrite volumes appear roughly that lack sanidine, and the lowest welded zone in the Findley Ridge section subequal, but proximal precursor fallout is absent. (Fig. 14) consists entirely of transitional andesite-dacite that lacks sanidine. Dacite zones also commonly contain minor proportions of light-gray pumices with only sparse crystals and more silicic appearance (but too small to sample MAGMATIC EVOLUTION AT BONANZA and analyze). The diverse sanidine compositions, both in bulk-ignimbrite samples and individual pumice lenses (Fig. 24) also document both pre-eruption Bonanza caldera was localized within the large edifice of the composite magma mingling and further mechanical mixing of rhyolite and dacite during Rawley volcanic complex (ca. 33.7–33.3 Ma), which probably recorded early eruption and emplacement processes. At least three discrete magma compo- incremental growth and petrologic evolution of the upper-crustal magma sitions are resolvable from pumice and bulk-sample compositions; evaluation body that culminated in an ignimbrite eruption. Similar precursor sequences of potential additional compositions transitional from dacite to silicic rhyolite are well documented elsewhere in the San Juan region and other Cordilleran is impeded by the small size and dense welding of rhyolitic pumices. Multiple ignimbrite centers (Lipman, 1984, 2007). Although even the most pristine rocks pumice compositions have also been described from other SRMVF ignimbrites from the Bonanza system display effects of weak alteration, especially for mo- such as Carpenter Ridge, Sunshine Peak, and Grizzly Peak Tuffs (Lipman, 1975; bile elements such as the alkalis, the analytical data (Supplemental Table 4) are Fridrich and Mahood, 1987; Hon and Lipman, 1989; Bachmann et al., 2014). sufficiently abundant to define chemical trends and contrasts with other parts Such mixed pumice populations and bulk compositional variations have of the SRMVF. Detailed petrologic study of the Bonanza eruptive sequence is been diversely interpreted to record (1) variable drawdown depth during fluc- currently in progress (e.g., Memeti and Lipman, 2014), and only brief aspects tuating eruption dynamics from a single compositionally zoned or layered of magmatic evolution are noted here. magma body (Smith, 1979; Blake, 1981; Bacon and Druitt, 1988; Hildreth and

Wilson, 2007), or alternatively (2) tapping multiple isolated magma pockets Bonanza Tuff are inferred to record fluctuating drawdown depths that tapped in close lateral or vertical proximity (Maughan et al., 2002; Eichelberger et al., vertically stacked compositional zones in the source magma body, as the in- 2000; Ellis and Wolff, 2012; Cashman and Giordano, 2014). For Bonanza and tensity of eruptive discharge pulsated or opening of new fractures during sub other SRMVF ignimbrites, the overall progression from early rhyolite to large sidence tapped different melt levels. The compositional and textural diversity of volumes of later-erupted dacite, along with structural evidence for coherent this ignimbrite suggests eruption from a crystal-poor silicic cap and underlying near-equant subsidence and resurgence of a relatively intact intracaldera block more crystal-rich dacite, only a few kilometers thick, that accumulated above (floor and ignimbrite fill), are here interpreted as primarily recording vertical more mafic and vertically extensive mushy magma, similar to that inferred for compositional variations within a single relatively well-interconnected shallow other ignimbrites (Smith, 1979; Hildreth, 1981; Bachmann and Bergantz, 2004). lens of eruptible magma, rather than tapping multiple discrete bodies of more The contrasting compositions between eastern and western areas of pre- homogeneous magma (Fig. 15). The alternating rhyolite and dacite zones in the served outflow Bonanza Tuff (Fig. 11) could suggest an alternative mechanism,

11.0 A Trachydacite 10.0

9.0 Trachyandesite

Figure 22. Plots of bulk-rock analyses for eight %) 8.0 the Bonanza area (data from Supplemen- tal Table 4), compared to data fields for the 27‑Ma Creede caldera (numbers of analy- 2 7.0 Basaltic Rhyolite ses, in parentheses). (A) Total alkali-silica diagram. (B) Zirconium-silica diagram. trachy Dacite Much of the considerable scatter among 2 6.0 andesite the Bonanza samples is due to varying Na O + K (w Andesite magma compositions, as also reflected by diverse phenocryst assemblages and compositions, but some scatter also results 5.0 from deep burial and widespread weak 50 60 70 80 alteration. Overall, the Bonanza Tuff and closely associated postcollapse lavas and 600 intrusions of the caldera cycle are more alkalic and enriched in incompatible ele- B ments such as Zr, in comparison to earlier nearby volcanoes such as Jacks Creek and Tracy (Fig. 9). Lavas of the Rawley volcanic complex, which constitute the most direct precursors to the Bonanza ignimbrite erup- 400 tion, tend to be transitional between the CREEDE CALDERA earlier volcano and rocks of the Bonanza Postcollapse lavas (26) cycle. All the volcanic rocks of the Bonanza and area are more alkalic and enriched in in- Snowshoe Mtn Tuff (18) compatible elements, in comparison to

Zr (ppm) the Creede caldera and other ignimbrite 200 systems farther southwest in the San Juan region (Creede data from Lipman, 2006).

0 50 60 70 80

SiO 2 (weight %)

A B 5.0–5.5.0–5.99 9.7– 10.6 Figure 23. Diverse mineral-disequilibria tex- 4.9–5.4 tures, dacitic rocks of the Bonanza cycle. Electron-backscatter images of sanidine crystals show complex resorption textures, 0.6–1.7 compositional variations, and overgrowth of crystal cores. Brighter areas are more Ba rich; numbers indicate ranges in mol% celsian content (Cn) for cores and rims. Hori zontal scale bars: 1 mm length. (A) Sani dine from a dacitic Bonanza pumice lens (09L-33C s2) contains a core with multiple 2.1–3.6 faint compositional zones that are truncated along a resorption boundary, then overgrown by a well-defined Ba-rich rim with euhedral crystal faces. (B) Another sanidine crystal (09L-33C s6) from the same pumice lens displays conspicuously different resorption and overgrowth textures: wormy-textured resorption boundaries C within the crystal core as well as on the D outer surface are overgrown by a Ba-rich rim of varied thickness. (C) Sanidine crystal from a postcaldera dacite lava (10L-2 s8) contains a resorbed core and outer surface, both overgrown by Ba-rich rims. (D) Rounded core (08L-44 s5) of a sanidine 2.8–3.9 megacryst (15 mm) from a late dacite lava appears compositionally homogeneous but is jacketed by rapakivi-like rim of finely 0.9–1.8 intergrown quartz and sodic plagioclase (Ab69–73). Other crystal-rich dacites in the 0.4–0.8 Southern Rocky Mountain volcanic field (SRMVF) typically show similarly complex mineral-disequilibrium textures.

Pl+ Qtz

involving concurrent eruption of large discrete magma bodies that differed in stable access to separate discrete magma bodies throughout the course of composition and were laterally isolated (Eichelberger et al., 2000), as appears a sustained large-volume eruption, and the full spectrum of mafic to silicic to have been common in the extensional environment of the Taupo zone (e.g., magma continued to be available after the ignimbrite eruption on both flanks Gravley et al., 2007; Allan et al., 2012; Bégué et al., 2014). Similar interpretive is- of the resurgent dome (lavas on the west and resurgent intrusions on the east). sues have been much discussed for the 0.76-Ma Bishop Tuff, from Long Valley Also lacking at the time of the Bonanza eruption was any strong extensional caldera (Hildreth and Wilson, 2007; Gualda and Ghiorso, 2013, and references environment such as characterizes Taupo. therein). Such a geometry of multiple dispersed magma bodies seems less The voluminous but short-lived postignimbrite magmatism at Bonanza, appropriate at Bonanza; however, Bonanza is a single large caldera without including caldera-filling lavas and resurgent intrusions, also was characterized evidence for satellitic subsidence or vent sites, generation of the many com- by compositional and textural diversity (Table 4 and Fig. 22). Caldera-filling

positional alternations within the ignimbrite would have required prolonged flows range from andesite (56% SiO2) to silicic rhyolite (76% SiO2), emplaced

Figure 24. Electron-microprobe compositions of sanidine from the Bonanza Tuff, showing large compositional variations in dacite samples compared to the rhyolite. (A) Individual sanidine A crystals from a single dacitic pumice lens vary by as much as 10 mol% Or (orthoclase) and >5 mol% Cn (celsian). Celsian content increases as Or decreases, while Ab (albite) and An (anorthite), not plotted, remain more nearly constant at 29–31 and 1.5–2.0 mol%, respectively. The highest Cn values (~10%) for crystal s6 are along a thin fracture and may be due to late alteration; see Figure 23B. (B) Multiple sanidine grains from a bulk sample of crystal-poor upper rhyolite (proximal west side) are more sodic than any dacite compositions in A, have only a

narrow range in potassium (Or50.5–54), and are characterized by very low Ba contents (Cn0 –0.1),

many below analytical detectability (~Cn0.03). (C) Lower rhyolite and upper dacite from a distal Cn (mol %) east-side section (Two Creek) also have contrasting sanidine populations. Sanidine from the rhyolite again is less potassic than the dacite and very low in barium, as in B. Sanidine in the dacite has a large compositional range, including a population in which Cn increases as Or decreases, similar to the trend defined by the single pumice lens in A. A secondary compositional population is lower in Or and very low in Cn, similar to sanidines from the underlying rhyolite; this population is interpreted to record mechanical mixing of some rhyolite with the dominant

dacite (bulk-sample composition is 66.8% SiO2) during magma rise and eruption. B

within ~105 yr after the ignimbrite eruption. Resurgent intrusions that were assembled and solidified in about the same time span similarly range from mafic

granodiorite and andesite (54%–62% SiO2) to aplitic granite (76% SiO2). These later magmas of the Bonanza cycle, while also more alkalic than rocks of the Cn (mol %) central San Juan caldera cycles, are somewhat lower in incompatible elements than samples of Bonanza Tuff at similar silica contents (Fig. 22). Preliminary radiogenic-isotope data for Bonanza rocks (Memeti and Lip- man, 2014) indicate relatively limited ranges in Sr, Nd, and especially low Pb values, compared to the multiple-cycle ignimbrite eruptions from Platoro and central San Juan calderas farther south and west, consistent with the relatively C brief duration of the Bonanza cycle as constrained by 40Ar/39Ar ages (Table 3). The Bonanza lead values are relatively low (206Pb/204Pb = 17.4–17.9), and their range is closer to those for the early-intermediate lavas that preceded caldera cycles than to ignimbrites elsewhere in the San Juan region. Such low lead values have long been interpreted as reflecting incorporation of lower crust depleted in U and Th during Proterozoic craton assembly (Lipman et al., 1978;

Johnson, 1991; Riciputi et al., 1995). Cn (mol %) The great compositional diversity and recurrence of andesite as magmatic products during the temporally brief caldera cycle at Bonanza document the capacity of large upper-crustal magmatic systems to evolve rapidly, especially when recurrently recharged by arrival of new batches of mafic magma. The dominant andesitic to rhyolitic magmas are interpreted to have been gener- Or (mol %) ated by rise of voluminous mantle-derived basalt that provided heat to as- similate variable amounts of lower crust, as the evolving magmas crystallized and fractionated (e.g., Lipman et al., 1978; Hildreth and Moorbath, 1988; John- SUMMARY son, 1991; DePaolo et al., 1992; Riciputi et al., 1995; Farmer et al., 2008). The absence of erupted basalt, prior to onset of regional extension, shows that Bonanza caldera displays diverse structural and compositional features primitive mantle melts were unable to penetrate a developing upper-crustal that provide special insights concerning ignimbrite eruptive processes. Bo- batholith at Bonanza, probably because warm near-solidus batholithic rocks nanza, source of a compositionally complex regional ignimbrite sheet erupted were a barrier to rise of hot mafic magma without attendant cooling, crystal- at 33.12 ± 0.03 Ma, is a subequant structure ~20 km diameter that subsided lization, and stagnation. >3 km during eruption of ~1000 km3 of ignimbrite. In contrast to the multicyclic

caldera loci for many large-volume ignimbrite flareups elsewhere, Bonanza is ring faults. In-place caldera floor grades into caldera-fill megabreccia with an areally isolated collapse structure that formed in response to a single ig- boundaries that can be located only approximately. Dike-like ignimbrite crack nimbrite eruption. A large body of high-precision 40Ar/39Ar ages shows that the fills of strongly welded tuff, which penetrate upper levels of the caldera floor entire Bonanza caldera cycle, including ignimbrite, postcollapse lavas, most and have steeply dipping fiamme, are interpreted to have originated mainly resurgence, and associated intrusions, occurred within the relatively brief in- as surficially generated fills between blocks of early caldera-collapse mega- terval from 33.12 ± 0.03 to 33.03 ± 0.04 Ma. Later erosion has exhumed some breccia into cracks that opened dilatantly during caldera formation, rather than near-original caldera morphology that is expressed by present-day landforms originating as ignimbrite vents. (western topographic rim, resurgent core, and ring-fault valley), while tilting The absence of tephra-fall deposits beneath the Bonanza Tuff and other and deep erosion provide exceptional three-dimensional exposures of fill, SRMVF ignimbrites is interpreted to indicate early generation of pyroclastic floor, and resurgent structures. Caldera-fill ignimbrite has been largely stripped flows, rather than evidence for lack of a Plinian eruptive column. A discharge from the southern and eastern flank of the dome, exposing large areas of cal- rate that was high from inception of the eruption could have produced rapid dera floor as a coherently domed plate broken by only small-displacement ignimbrite flowage at the onset of the eruption, such that any proximal Plinian faults, and bounded by ring faults with locations that are geometrically closely pumice and ash could fall directly into the moving pyroclastic flow, no basal constrained even though largely concealed beneath valleys. fall layer would be deposited proximally, and tephra-fallout deposits would be The Bonanza Tuff displays extreme compositional gradients (silicic andesite preserved only distally beyond the ignimbrite sheet. Although the absence of

to rhyolite; 60%–76% SiO2), multiple alternations of rhyolite and dacite zones a Plinian deposit beneath some ignimbrites elsewhere has been interpreted rather than simple upward gradation from silicic to mafic, and compositional to indicate that abrupt rapid foundering of the magma-body roof initiated the contrasts among outflow sectors (mainly crystal-poor rhyolite to east and eruption, initial caldera collapse began at Bonanza only after several hundred crystal-rich dacite to west). Varied thickness and lateral extent among the multi- kilometers of rhyolitic tuff had erupted, as indicated by the small thickness and ple interfingering zones of rhyolite and dacite within the intracaldera ignimbrite modest volume of the lower rhyolite within the caldera. reflect multiple factors, including surface irregularities on the pre-eruption lava After completion of the ignimbrite eruption and accumulation of andesitic assemblage, depositional slopes generated by caldera-wall landslide deposits, to rhyolitic lavas in the caldera basin, the floor and fill of Bonanza caldera were and weakly asymmetric caldera subsidence. The relatively modest thicknesses arched into an notably high and steep-sided dome by continued magma rise, and limited areal extent of the lower-rhyolite zone within the caldera provide forming plutons of granite and granodiorite that are exposed at roof levels. Re- rigorous constraints on timing of initial collapse; it must have begun relatively surgence uplifted the pre-ignimbrite caldera floor to levels as high as the eleva- late during this phase of the Bonanza ignimbrite eruption. If subsidence had ac- tion of the same lava units on the western caldera rim, leading to widespread companied inception of the eruption, or even triggered initial magma expulsion present-day erosional exposure of caldera-floor rocks in the caldera interior. as inferred in some ignimbrite calderas, a greater thickness and volume of the Estimates of intracaldera ignimbrite volume at little-eroded young calderas, early rhyolite should have ponded within Bonanza caldera. based on morphology of resurgent uplifts without information on caldera-floor For Bonanza and other SRMVF ignimbrites, the overall progressions from geometry, might yield misleadingly large estimated ratios of intracaldera ver- early rhyolite to later-erupted dacite, along with structural evidence for coher- sus outflow ignimbrite accumulations. The insights provided by Bonanza cal- ent near-equant subsidence and resurgence of a relatively intact intracaldera dera and its ignimbrite should be broadly applicable to processes of silicic block (floor and ignimbrite fill), are interpreted as primarily recording verti- Cordilleran magmatism elsewhere, with respect to ignimbrite eruptive pro- cal compositional variations within a single well-connected shallow lens of cesses, style and timing of caldera subsidence, complications with character- eruptible magma, rather than tapping multiple isolated bodies of more homo izing structural versus topographic margins of calderas, contrasts between geneous magma. The alternating rhyolite and dacite zones in the Bonanza Tuff intra- versus extracaldera ignimbrite, scales of caldera resurgence, and the are inferred to record variable drawdown that tapped a vertical sequence of limitations in assessing magma volumes associated with large caldera-form- compositional zones in the source magma body as the intensity of eruptive ing eruptions. Bonanza provides a rare site where both caldera margins and discharge fluctuated. caldera floor are exhumed and exposed, providing valuable perspectives for Particularly revealing at Bonanza caldera are the widespread exposures of understanding similar young calderas in some of the world’s most active and caldera-floor volcanic rocks and basal deposits associated with the ignimbrite dangerous silicic provinces. eruption and caldera collapse. The structurally coherent floor at Bonanza contrasts with fault-disrupted floors at other well-exposed multicyclic calderas where successive ignimbrite eruptions caused recurrent subsidence. Notable ACKNOWLEDGMENTS at Bonanza are zones of floor-rock shattering and brecciation within ~100 m in- We especially thank Andrea Sbisà, who provided outstanding mapping assistance during long field days in the summers of 2009–2011. Lisa Peters and numerous students at the New Mex- board of ring faults, perhaps due to compression and crushing of the subsiding ico Geochronology Research Laboratory helped with mineral separates and data collection. De- floor in proximity to steep inward-dipping faults adjacent to caldera-collapse tailed perceptive manuscript reviews were provided by U.S. Geological Survey (USGS) reviewer

David John, Geosphere reviewers Mike Branney, John Dilles, and Darrel Gravley, and editors Jan the first ~10%–20% of the 39Ar released. Despite petrographic screening to avoid glassy matrix ma- Lindsay and Shan de Silva. We also thank many friends in the northeastern San Juan region who terial, the young ages probably reflect40 Ar loss from low retentive sites, such as poorly crystallized provided diverse hospitality, logistical support, help with back-country and property access, and or otherwise altered interstitial sites where potassium was concentrated. Inclusion of these young, other assistance. These include John and Patty Judson of Quarter-Circle Circle Ranch in Coche- low-temperature steps commonly yields integrated ages with large uncertainties and, in some topa Park; Forest, Billy, Curt, and Lee Ann Cadwell of Cathedral Creek Ranch; and especially for cases, ages that conflict with the established stratigraphy. the recent years of Bonanza studies, Pip and Aaron Conrad of the Rafiki Ranch near Villa Grove, Chemical and petrographic data for the study area include new major-oxide and trace-ele- who have created a virtual “Rafiki Ranch Geologic Research Station” and hosted many visiting ment analyses for about 280 samples, determined by X-ray fluorescence methods (Johnson et al., earth scientists. 1999) at the Geoanalytical Lab, Washington State University (Table 4; Supplemental Table 4), supplemented by data for regional units (Lipman, 2006, 2012). Sample selection for analytical work was complicated within Bonanza caldera, due to widespread propylitic metamorphism of deeply buried volcanic rocks, along with local acid-sulfate hydrothermal alteration and supergene APPENDIX: FIELD AND LABORATORY METHODS leaching of disseminated pyrite. Accordingly, wherever possible, units were sampled from outflow areas beyond the caldera. Because the ignimbrites are typically densely welded, most analyses are Geologic mapping of western fringes of the Bonanza area, initiated to resolve interpretive for bulk samples; only a few localities of dacitic Bonanza Tuff contain pumice lenses sufficiently problems within the adjacent Cochetopa–North Pass calderas (Lipman, 2012; Lipman et al., 2013), large to sample and analyze separately (noted on Table 4). During preparation for analyses, bulk was gradually expanded to cover Bonanza and adjacent areas to the northeast (Fig. 2) as multiple ignimbrite samples were coarse crushed, and any discernable lithic fragments were discarded. stratigraphic, structural, geochronologic, and volcanologic complexities emerged. Detailed geo- All major-oxide analyses were recalculated volatile free, to sum to originally reported analytical logic mapping of the Bonanza-Marshall caldera area (field compilation on 1:24,000 topographic totals. Despite potential alteration issues, most samples yielded plausible magmatic values, even quadrangle base maps; intended for publication as a USGS SIM-series map, 1:50,000 scale), is a for especially mobile elements such as the alkalis (e.g., Fig. 22). Effects of alteration are evident, primary basis for documenting many of the interpretations developed here. however, in the modestly greater data scatter on data plots than typical of young volcanic suites. Critical to many interpretations presented here are the high-precision 40Ar/39Ar age deter- A few obviously anomalous samples with suspect compositions (marked in red, on Supplemental minations for volcanic rocks in the Bonanza area (130 localities; 138 mineral and groundmass Table 4) were omitted from plots of chemical data. ages: Table 3; Supplemental Tables 1–3), including 28 determinations for regional units that were Mineral compositions determined by routine electron-microprobe analysis on polished thin published previously (McIntosh and Chapin, 2004; Lipman and McIntosh, 2008). All ages were sections (Lipman, unpubl. data, 2006-2011; methods similar to those of Lipman and Weston, 2001), determined at the New Mexico Geochronology Research Laboratory by methods similar to those have proved useful discriminants to test correlations among some ignimbrite sheets, as well as described in McIntosh and Chapin (2004); additional details on sample preparation and irradiation, providing information on processes of magmatic evolution. instrumentation, analytical parameters, age calculations, and probability plots are listed in Sup- plemental Tables 2 and 3. Especially high precision ages were obtained for samples analyzed with the multi-collector Argus VI mass spectrometer, starting in 2012. A single-detector MAP 215‑50 REFERENCES CITED was used to determine isotopic ratios prior to 2012. All new and previously published ages are calibrated to Fish Canyon Tuff (sanidine) at 28.02 Ma, for consistency with prior reports. 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