Research Paper 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 vol canic 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 multi cyclic 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- topog raphy—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 intra caldera-ignimbrite vol- gradients in the Bonanza ignimbrite (silicic andesite to rhyolite ignimbrite; ume; this parameter may have been overestimated at some young cal deras 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. © 2015 Geological Society of America 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 1902 by guest on 29 September 2021 Research Paper 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 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 l 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 l l a l l l 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 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 l 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 u l l 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 .