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retired

Abstract

The 26.9-Ma Creede caldera is a large resurgent ignimbrite-subsidence structure that hosts world-class epithermal Ag-Pb-Zn deposits set within the San Juan locus of the Southern Rocky Mountain volcanic field (SRMVF). This field guide reviews the understanding of the SRMVF and central San Juan magmatic locus and then provides a summary of the Oligocene Creede Formation, the caldera moat fill, and ancient Lake Creede, which occupied the caldera for ~100 k.y. after the caldera-forming eruption. The summary of the volcanic setting and Creede Formation provides a framework for discussing the hydrothermal system that deposited Ag-Pb-Zn ores in the Creede district ~1.8 m.y. after the caldera formed. This guide also highlights controversial aspects of the Lake Creede environment, such as the duration of the intracaldera sedimentation, proposed pseudomorphs after ikaite (CaCO3·6H2O) in the lake sediments and tufa, and the paleolimnology of ancient Lake Creede.

Introduction

The 26.9-Ma Creede caldera in the central San Juan Mountains, Colorado, is a remarkably preserved example of a large resurgent caldera associated with world-class epithermal Ag-Pb-Zn deposits (Steven and Ratté, 1965; Bethke and Hay, 2000). Creede caldera is the youngest of nine subsidence structures to have formed in the central San Juan caldera cluster between 28.6 and 26.9 Ma. As such, Creede caldera represents the culminating phase of late Oligocene volcanic eruptions in the San Juan magmatic locus of the regional Southern Rocky Mountain volcanic field (SRMVF) in Colorado and New Mexico. Late Neogene uplift of the southern Rocky Mountains and incision by the Rio Grande led to exhumation of soft, intracaldera sediments within Creede caldera, while preserving its constructional morphology (Rye et al., 2000). In 1991, Creede caldera was the site of a U.S. Continental Scientific Drilling Project that aimed to test models for involvement of caldera lake fluids in the genesis of the 25-Ma silver- and base-metal ores in the Creede mining district north of Creede caldera (Bethke and Lipman, 1987), and better understand caldera-hosted lake processes within a silicic volcanic terrane (Bethke and Hay, 2000).

The San Juan magmatic locus of the SRMVF has served as a laboratory for the study of continental magmatic processes for more than 100 yr (Lipman, 2007). Despite enormous effort, stratigraphic relationships continue to be refined (Lipman, 2000, 2006). The insight gained from detailed study of this region has clarified many processes common to silicic caldera systems, especially in regard to collapse mechanisms and pet-rologic evolution.

The Creede Formation is one of the best-exposed and well-documented intracaldera sedimentary successions in the world (Larsen and Crossey, 1996; Bethke and Hay, 2000). Detailed study shows that some features of the Creede Formation differ significantly from those in other calderas (e.g., extensive travertine [tufa] deposits) (Steven and Friedman, 1968; Larsen and Crossey, 1996; Rye et al., 2000), high proportion of sediment versus lava fill, and calcite pseudomorphs after ikaite in travertine and volcaniclastic mud (Larsen, 1994a). Intracaldera sedimentary successions provide detailed archives of the evolution of the postcollapse caldera environment (Bailey et al., 1976; Nelson et al., 1994; Larsen and Crossey, 1996; Manville et al., 2009; Murphy et al., in press) and, as such, inform understanding of caldera processes and their hazards (Manville et al., 2009). Detailed studies of the Creede Formation in core from the scientific drilling project (CCM-1 and CCM-2) and extensive surface exposures have greatly expanded understanding of caldera-lake environments and relationships to volcanic processes, but also present many unresolved problems (Bethke and Hay, 2000).

This field guide reviews the volcanic settings of the SRMVF and central San Juan magmatic locus, summarizes the formation of Creede caldera and evolution of Oligocene Lake Creede, and provides a road log of the field-trip stops. The field guide summarizes current understanding of processes in this dynamic ancient volcanic environment and hopefully stimulates future directions of research and inquiry.

The Southern Rocky Mountain Volcanic Field

The Southern Rocky Mountain volcanic field in Colorado and New Mexico is perhaps the best-preserved large-volume late Paleogene volcanic field in Cordilleran North America, containing widespread lavas and multiple thick, extensive ignimbrites, as well as exposing subvolcanic granitoid intrusions (Fig. 1). The composite SRMVF, now erosionally dissected, is among several discontinuous sites of eastern Cordilleran magmatism in the late Paleogene, continuing southward through the Mogollon-Datil region in New Mexico (Elston, 1984; Ratté et al., 1984; McIntosh et al., 1992), into Trans-Pecos, Texas (Henry and Price, 1984), and the vast Sierra Madre Occidentale of northern Mexico (McDowell and Clabaugh, 1979; Ferrari et al., 2007; McDowell and McIntosh, 2012). The SRMVF is also broadly similar in age, eruption volume, caldera size, magma composition, and eruption duration to the ignimbrite flareup farther west in the Great Basin (Best et al., 2013, 2016; Henry and John, 2013). The SRMVF is also comparable in similar parameters to large young ignimbrite terranes such as the well-documented Altiplano-Puna volcanic complex of the Central Andes (de Silva, 1989; de Silva and Gosnold, 2007; Salisbury et al., 2011), although exposed more completely in the third dimension because of uplift and tilting along flanks of the Rio Grande rift and resulting deep erosional dissection. This GSA field trip will briefly explore general features of the SRMVF, the central caldera complex of the San Juan magmatic locus, and Creede caldera as a volcanologic framework for the evolution of Lake Creede.

Figure 1.

Map of Southern Rocky Mountain volcanic field, showing ignimbrite calderas, major erosional remnants and inferred original extent of late Paleogene volcanic cover, caldera-related granitic intrusions, and later sedimentary fill in asymmetric grabens of the Rio Grande rift zone. Graben asymmetry and boundary-fault geometry reverse from east-dipping in the San Luis Valley segment to west-dipping in the Sawatch Range-Upper Arkansas segment to the north. Blue dashed lines indicate major bounding faults of asymmetrical rift grabens. A diffuse structural-transition boundary lies south and east of the Bonanza area (green dashed line). Arrows indicate trend of Late Cretaceous-early Paleogene (Laramide) intrusions of the Colorado Mineral Belt. Calderas: B—Bachelor; Bz—Bonanza; C—Cochetopa Park; Cr— Creede; GP—Grizzly Peak; LGn—La Garita north segment; LGs—La Garita, south segment; M—Marshall; MA—Mount Aetna; NP—North Pass; Pl—Platoro; S—Silverton; SL—San Luis complex; SR—South River. Geographic locations: BP—Buffalo Peaks; SC—Summer Coon volcano; SK—Storm King Mountain; WMT—distal Wall Mountain Tuff on High Plains. Modified from McIntosh and Chapin (2004); inferred original limit of volcanic rocks modified from Steven (1975); intrusions from Tweto (1979) and Lipman (1988, 2000).

Figure 1.

Map of Southern Rocky Mountain volcanic field, showing ignimbrite calderas, major erosional remnants and inferred original extent of late Paleogene volcanic cover, caldera-related granitic intrusions, and later sedimentary fill in asymmetric grabens of the Rio Grande rift zone. Graben asymmetry and boundary-fault geometry reverse from east-dipping in the San Luis Valley segment to west-dipping in the Sawatch Range-Upper Arkansas segment to the north. Blue dashed lines indicate major bounding faults of asymmetrical rift grabens. A diffuse structural-transition boundary lies south and east of the Bonanza area (green dashed line). Arrows indicate trend of Late Cretaceous-early Paleogene (Laramide) intrusions of the Colorado Mineral Belt. Calderas: B—Bachelor; Bz—Bonanza; C—Cochetopa Park; Cr— Creede; GP—Grizzly Peak; LGn—La Garita north segment; LGs—La Garita, south segment; M—Marshall; MA—Mount Aetna; NP—North Pass; Pl—Platoro; S—Silverton; SL—San Luis complex; SR—South River. Geographic locations: BP—Buffalo Peaks; SC—Summer Coon volcano; SK—Storm King Mountain; WMT—distal Wall Mountain Tuff on High Plains. Modified from McIntosh and Chapin (2004); inferred original limit of volcanic rocks modified from Steven (1975); intrusions from Tweto (1979) and Lipman (1988, 2000).

The SRMVF has long been studied as a site of late Paleo-gene silicic volcanism on especially voluminous scales (Cross and Larsen, 1935; Larsen and Cross, 1956; Lipman et al., 1970; Epis and Chapin, 1974; Steven and Lipman, 1976; Johnson et al., 1989; McIntosh and Chapin, 2004; Lipman, 2007), including at least 25 ignimbrite sheets (each 150-5000 km3) and associated calderas active at 37-23 Ma (Tables 12). In places, virtually pristine volcanic morphology has been exhumed by recent erosion; elsewhere, rugged topography and structural tilting expose multi-km volcanic sections, down into upper levels of subvol-canic intrusions. Small granitoid plutons, many spatially and temporally associated with ignimbrite calderas, are exposed at near-roof level (Table 2).

Summary of Regional Ignimbrites, Caldera Sources, and 40Ar/39Ar Ages, Southern Rocky Mountain Volcanic Field (Updated and Simplified from Lipman, 2015, his Table 1)

Table 1.
Summary of Regional Ignimbrites, Caldera Sources, and 40Ar/39Ar Ages, Southern Rocky Mountain Volcanic Field (Updated and Simplified from Lipman, 2015, his Table 1)

Characteristic Features, Ignimbrite Sheets of Central and Northeastern San Juan Region (From Lipman et al., 2015, their Table 2)

Table 2.
Characteristic Features, Ignimbrite Sheets of Central and Northeastern San Juan Region (From Lipman et al., 2015, their Table 2)
Ignimbrite sheet*CompositionTextures and phenocrysts§
Snowshoe Mountain TuffMafic dacitePhenocryst rich; densely welded within caldera, weakly welded outflow
Nelson Mountain TuffLow-Si rhyolite=daciteCompositionally zoned; weakly welded crystal-poor to dense crystal-rich
Cebolla Creek TuffMafic daciteTypically weakly welded; abundant hornblende >> augite is distinctive
Rat Creek TuffLow-Si rhyolite= daciteCompositionally zoned; weakly welded rhyolite to dense dacite
Wason Park TuffRhyolitePhenocryst-rich rhyolite; tabular sanidine phenocrysts
Blue Creek TuffDacitePhenocryst rich; sanidine is absent (contrast with Mammoth Mountain Member)
Carpenter Ridge Tuff
- Mammoth MountainDacitePhenocryst rich; sanidine is common (contrast with Blue Creek Tuff)
Member (upper)
- Outflow and lowerLow-Si rhyolitePhenocryst poor; common basal vitrophyre, central lithophysal zone
intracaldera tuff
Crystal Lake TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but less welded within map area
Fish Canyon TuffDaciteDistinctive light-gray, phenocryst-rich; resorbed quartz, hornblende, absence of augite
Sapinero Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Dillon Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Blue Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Ute Ridge TuffDacitePhenocryst rich; contains sparse sanidine (in contrast to Masonic Park Tuff)
Masonic Park TuffDacitePhenocrysts similar to Blue Creek Tuff; typically less welded
Luders Creek TuffLow-Si rhyolite= daciteCompositionally zoned; resembles Nelson Mountain Tuff
Saguache Creek TuffLow-Si rhyoliteResembles Carpenter Ridge and Sapinero Mesa Tuffs, but lacks phenocrystic biotite
Bonanza TuffZoned complexlyLocal basal xl-poor rhyolite, lower xl dacite, upper rhyolite, local upper xl dacite
Thorn Ranch TuffZoned complexlyIntracaldera alternation of rhyolite & dacite; outflow mainly high-Si rhyolite
Badger Creek TuffDaciteCrystal rich; resembles Fish Canyon Tuff
Wall Mountain TuffRhyoliteCrystal-rich, large blocky sanidine; locally complexly rheomorphic
Ignimbrite sheet*CompositionTextures and phenocrysts§
Snowshoe Mountain TuffMafic dacitePhenocryst rich; densely welded within caldera, weakly welded outflow
Nelson Mountain TuffLow-Si rhyolite=daciteCompositionally zoned; weakly welded crystal-poor to dense crystal-rich
Cebolla Creek TuffMafic daciteTypically weakly welded; abundant hornblende >> augite is distinctive
Rat Creek TuffLow-Si rhyolite= daciteCompositionally zoned; weakly welded rhyolite to dense dacite
Wason Park TuffRhyolitePhenocryst-rich rhyolite; tabular sanidine phenocrysts
Blue Creek TuffDacitePhenocryst rich; sanidine is absent (contrast with Mammoth Mountain Member)
Carpenter Ridge Tuff
- Mammoth MountainDacitePhenocryst rich; sanidine is common (contrast with Blue Creek Tuff)
Member (upper)
- Outflow and lowerLow-Si rhyolitePhenocryst poor; common basal vitrophyre, central lithophysal zone
intracaldera tuff
Crystal Lake TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but less welded within map area
Fish Canyon TuffDaciteDistinctive light-gray, phenocryst-rich; resorbed quartz, hornblende, absence of augite
Sapinero Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Dillon Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Blue Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Ute Ridge TuffDacitePhenocryst rich; contains sparse sanidine (in contrast to Masonic Park Tuff)
Masonic Park TuffDacitePhenocrysts similar to Blue Creek Tuff; typically less welded
Luders Creek TuffLow-Si rhyolite= daciteCompositionally zoned; resembles Nelson Mountain Tuff
Saguache Creek TuffLow-Si rhyoliteResembles Carpenter Ridge and Sapinero Mesa Tuffs, but lacks phenocrystic biotite
Bonanza TuffZoned complexlyLocal basal xl-poor rhyolite, lower xl dacite, upper rhyolite, local upper xl dacite
Thorn Ranch TuffZoned complexlyIntracaldera alternation of rhyolite & dacite; outflow mainly high-Si rhyolite
Badger Creek TuffDaciteCrystal rich; resembles Fish Canyon Tuff
Wall Mountain TuffRhyoliteCrystal-rich, large blocky sanidine; locally complexly rheomorphic
*

Bold type—ignimbrite sheets of Bonanza map area.

Si—SiO2.

§

xl—crystal.

The original areal extent of the overall SRMVF appears to have exceeded 100,000 km2, with a total volume of volcanic deposits greater than 60,000 km3 (Lipman, 2007; Lipman and Bachmann, 2015). Peak magmatic volumes in the SRMVF, associated with ignimbrite eruptions, define a general progression, from early eruptions along the trend of the Sawatch Range in central Colorado (37-34 Ma), southward into the San Juan region (33-27 Ma), and later to the 25-Ma Latir-Questa locus in northern New Mexico and the 23-Ma Lake City caldera in the western San Juan Mountains (Fig. 2). Geophysical data document the presence of several composite subvolcanic batholiths that encompass most calderas of the SRMVF (Plouff and Pakiser, 1972; Cordell et al., 1985; Drenth et al., 2012; Lipman and Bachmann, 2015).

Figure 2.

Age-location-volume plot, showing southward progression of ignimbrite-caldera volcanism in the SRMVF (revised from Lipman, 2007). Vertical bars, volumes of individual ignimbrites, scale on left axis (data from Table 1); stippled area, increasing cumulative eruptive volume (right axis). Inset, slopes corresponding to different cumulative eruption rates. Abbreviations: AT—Amalia Tuff; B—Bonanza Tuff; BC—Badger Creek Tuff; BM—Blue Creek Tuff; CP—Chiquito Peak Tuff; CR—Carpenter Ridge Tuff; FC—Fish Canyon Tuff; GP—Grizzly Peak Tuff; LJ—La Jara Canyon Tuff; NM—Nelson Mountain Tuff; SC—Saguache Creek Tuff; SM—Sapinero Mesa Tuff; SMT—Snowshoe Mountain Tuff; SP—Sunshine Peak Tuff; TR—Thorn Ranch Tuff; WM—Wall Mountain Tuff; WP—Wason Park Tuff. S.E. & W. SJ—southeast and west San Juan (figure from Lipman et al., 2015).

Figure 2.

Age-location-volume plot, showing southward progression of ignimbrite-caldera volcanism in the SRMVF (revised from Lipman, 2007). Vertical bars, volumes of individual ignimbrites, scale on left axis (data from Table 1); stippled area, increasing cumulative eruptive volume (right axis). Inset, slopes corresponding to different cumulative eruption rates. Abbreviations: AT—Amalia Tuff; B—Bonanza Tuff; BC—Badger Creek Tuff; BM—Blue Creek Tuff; CP—Chiquito Peak Tuff; CR—Carpenter Ridge Tuff; FC—Fish Canyon Tuff; GP—Grizzly Peak Tuff; LJ—La Jara Canyon Tuff; NM—Nelson Mountain Tuff; SC—Saguache Creek Tuff; SM—Sapinero Mesa Tuff; SMT—Snowshoe Mountain Tuff; SP—Sunshine Peak Tuff; TR—Thorn Ranch Tuff; WM—Wall Mountain Tuff; WP—Wason Park Tuff. S.E. & W. SJ—southeast and west San Juan (figure from Lipman et al., 2015).

The ignimbrite-caldera systems of the San Juan Mountains that constitute the largest preserved erosional remnant of the SRMVF have been a continuing laboratory for volcanologic and petrologic research: these include the southeastern caldera complex (Platoro: Dungan et al., 1989; Lipman et al., 1996), western calderas (Uncompahgre-Silverton-Lake City: Hon and Lipman, 1989; Bove et al., 2001; Kennedy et al., 2016), and central caldera cluster (La Garita-Creede calderas: Lipman, 2000, 2006; Bachmann et al., 2002, 2007). Older eruptive centers for SRMVF ignimbrite volcanism (Stop 1.1) are in the Sawatch Range to the north (Shannon, 1988; Toulmin and Hammarstrom, 1990; Frid-rich et al., 1991; Zimmerer and McIntosh, 2012a; Mills and Coleman, 2013) and the younger Questa-Latir locus to the south (Lipman, 1988; Tappa et al., 2011; Zimmerer and McIntosh, 2012b).

As summarized more fully elsewhere (Lipman, 2007, and references therein), dominantly intermediate-composition lavas and associated breccias (andesite, dacite) of the Oligocene Conejos Formation were voluminous precursors to most ignim-brite eruptions, and eruption of similar andesite to dacite lavas continued concurrently with the major ignimbrites, commonly filling caldera depressions (Steven and Lipman, 1976). At the San Juan locus, the central volcanoes and their clastic aprons (~25,000 km3) constitute almost two-thirds of total volcanic volume (Lipman et al., 1970). Basaltic lavas are virtually absent, despite repeated searches for mafic end-member compositions. Volcanic loci migrated from north to south in the SRMVF, both for intermediate-composition lava eruptions and for ignimbrites (Fig. 2, Table 1; Lipman, 2007); the southward migration is parallel to that documented for eruptive centers in the Basin and Range region, probably related to disruption of the subducted Farallon plate (Stewart et al., 1977; Lipman, 1980; Henry and John, 2013).

The earliest well-documented regional ignimbrite in the northern SRMVF (Table 1) (Stop 1.1, Fig. 1.1), was the far-traveled Wall Mountain Tuff at 37 Ma (Chapin and Lowell, 1979; Zimmerer and McIntosh, 2012a); the southernmost, and among the youngest ignimbrites, was the Amalia Tuff, erupted from the Questa caldera at 25.1 Ma (Lipman, 1988; Tappa et al., 2011; Zimmerer and McIntosh, 2012b). Geographically and temporally between these early and late centers, the San Juan region contains the largest preserved erosional remnant of the composite Oligo-cene volcanic field (Larsen and Cross, 1956; Steven et al., 1974). The San Juan locus is notable for the large number of high-volume compositionally diverse ignimbrites (cumulatively, ~15,000 km3) and associated caldera collapses, at least 18 in only the 3-m.y. interval 30.1-26.9 Ma (Table 1). Unzoned uniform crystal-poor rhyolite, crystal-rich dacite (“monotonous intermediates”), and ignimbrites that grade from initially erupted rhyolite upward into dacite are present is subequal numbers in the SRMVF. Sizable precursor Plinian-fall deposits have not been recognized beneath any of these ignimbrite types, contrary to some recent inferences (e.g., Gregg et al., 2012; Cashman and Giordano, 2014).

San Juan Magmatic Locus

The San Juan Mountains are the largest erosional remnant of the SRMVF (Fig. 1). Preserved volcanic rocks occupy an area of more than 25,000 km2 and had an original volume of ~40,000 km3. They cover a varied basement of Precambrian to early Paleogene rocks along the uplifted and eroded west margin of the Late Cretaceous to early Paleogene (Laramide) uplifts of the southern Rocky Mountains and adjoining eastern parts of the Colorado Plateau (Fig. 1). As late Paleogene volcanism migrated southward from the Sawatch Range (Fig. 2), widely scattered intermediate-composition centers erupted lavas and flanking vol-caniclastic breccias starting at 35-34 Ma (Lipman et al., 1970; Lipman and McIntosh, 2008). These rocks, which constitute about two-thirds of the volume of the preserved volcanic assemblage, are widely overlain by large ignimbrites associated with caldera collapses (Steven and Lipman, 1976).

After initial eruptions from Marshall and Bonanza calderas at 33.9 and 33.1 Ma in the northeastern San Juan Mountains (Fig. 1, Table 2), ignimbrite activity migrated to the southwest, with eruption of the Saguache Creek Tuff from the North Pass caldera at 32.2 Ma (Lipman and McIntosh, 2008; Lipman, 2012), then to the southeast San Juan region at 30.1 Ma (inception of Platoro caldera complex), followed shortly by eruptions mainly of crystal-poor rhyolitic ignimbrites from western calderas (Steven and Lipman, 1976). Ignimbrite activity progressively focused in the central San Juan Mountains (Stop 1.2; Tables 12), leading to eruption of the enormous Fish Canyon Tuff (5000 km3 of monotonously uniform crystal-rich dacite) and collapse of the 35 × 75 km La Garita caldera at 28.0 Ma (Lipman, 2000, 2006; Bachmann et al., 2002). In the central San Juan Mountains, seven more eruptions of compositionally diverse ignimbrite, with volumes of 100-1000 km3, erupted during the 1.1-m.y. interval from 28.0 to 26.9 Ma from calderas nested within La Garita caldera (Figs. 12). At ca. 26 Ma, magmatism shifted to a bimodal assemblage dominated by trachybasalt and silicic rhyolite, concurrent with the inception of regional extension along the Rio Grande rift.

Central San Juan Caldera Complex

This GSA field trip obliquely transects the central part of the San Juan volcanic field; major emphasis is on depositional, structural, and petrologic features within the central caldera cluster (Fig. 3, Table 1), especially the caldera-filling deposits and epithermal vein mineralization near Creede, Colorado. The following summary is updated from Lipman (2000), with substantial interpretive revisions based largely on implications of high-precision 40Ar/39Ar single-crystal sanidine ages reported by Lipman and McIntosh (2008).

Figure 3.

Geologic map of central caldera cluster, San Juan volcanic locus, Colorado. Abbreviated geographic names: LH—Lake Humphreys; MM—Mineral Mountain; SL—San Luis Peak. Calderas of San Luis complex: CC—Cebolla Creek; NM—Nelson Mountain; RC—Rat Creek. Ash-flow sheets: BC—Blue Creek Tuff; CR—Carpenter Ridge Tuff; WP—Wason Park Tuff. C. D.—continental divide. Generalized from Lipman (2006).

Figure 3.

Geologic map of central caldera cluster, San Juan volcanic locus, Colorado. Abbreviated geographic names: LH—Lake Humphreys; MM—Mineral Mountain; SL—San Luis Peak. Calderas of San Luis complex: CC—Cebolla Creek; NM—Nelson Mountain; RC—Rat Creek. Ash-flow sheets: BC—Blue Creek Tuff; CR—Carpenter Ridge Tuff; WP—Wason Park Tuff. C. D.—continental divide. Generalized from Lipman (2006).

Andesitic to rhyolitic volcanic rocks of the central San Juan region, along with associated epithermal ores, have been studied intermittently since early in the twentieth century (Emmons and Larsen, 1923; Cross and Larsen, 1935; Larsen and Cross, 1956), including detailed mid-century study of the Creede mining district by Steven and Ratté (1965, 1973) and regional field and volcanological studies (Lipman et al., 1970; Steven et al., 1974; Steven and Lipman, 1976). While much had been learned during these studies about the evolution of several complex caldera clusters within the San Juan locus of the SRMVF, from which at least 22 major ignimbrite sheets (each 150-5000 km3) were erupted at 30-23 Ma (Lipman, 1989), mapping and other research initiated in support of the Creede Scientific Drilling Project (Bethke and Lipman, 1987; Bethke and Hay, 2000) provided additional major insights for stratigraphic sequence, duration of volcanism, eruptive processes, magmatic evolution, and structure in the central part of the San Juan volcanic locus (Lipman, 2000, 2006, 2007). Other recent volcanologic and petrologic studies have shed further light on eruptive processes, relations between volcanism and associated batholith growth in the upper crust, and magmatic evolution of the central San Juan magmatic locus (Riciputi et al., 1995; Bachmann et al., 2002, 2014; Bachmann and Bergantz, 2003; Parat et al., 2005; Drenth et al., 2012; Lipman and Bachmann, 2015). Accordingly, the present field guide constitutes a substantial revision and update of one prepared in conjunction with a prior field excursion (Lipman et al., 1989).

In the central San Juan Mountains (Fig. 3), eruption of at least 8800 km3 of dacitic-rhyolitic magma as nine major ignim-brite sheets (individually 150-5000 km3) was accompanied by recurrent caldera subsidence between 28.6 and 26.9 Ma (Lipman et al., 1989; Lipman, 2000; Lipman and McIntosh, 2008). Voluminous andesitic-dacitic lavas and breccias were erupted from central volcanoes prior to the ignimbrite eruptions (Stop 1.3), and similar lava eruptions continued within and adjacent to the calderas during the period of more silicic explosive volcanism (Lip-man et al., 1978; Colucci et al., 1991; Parat et al., 2005). Exposed calderas vary in size from 10 to 75 km in maximum dimension, the largest calderas being associated with the most voluminous eruptions (Table 1). Caldera-subsidence structures that likely accompanied initial explosive eruption of the Masonic Park Tuff are entirely concealed beneath younger rocks and structures in the central San Juan region. The giant La Garita caldera (35 x 75 km) collapsed in three successive northward-migrating segments during eruption of the Fish Canyon Tuff at 28.0 Ma (Fig. 3). After collapse of La Garita caldera, seven additional explosive eruptions and calderas formed inside the La Garita depression within about 1 m.y. (Table 1). Erosional dissection to depths of as much as 1.5 km has exposed diverse features of intracaldera ignimbrite and interleaved caldera-collapse landslide deposits that accumulated to multi-km thickness within concurrently subsiding caldera structures. The calderas display a variety of postcollapse resurgent uplift structures, and caldera-forming events produced complex fault geometries that localized late mineralization, including the epithermal base- and precious-metal veins of the well-known Creede mining district (Steven and Ratté, 1965; Wetlaufer et al., 1979; Foley et al., 1993; Barton et al., 2000a). Most of the central San Juan calderas have been variably disrupted by younger volcanic events and deeply eroded; as a result their identification is dependent on detailed geologic mapping. In contrast, the primary volcanic morphology of the youngest ignimbrite center, the symmetrically resurgent Creede caldera, has been exceptionally preserved because of rapid infilling by moat sediments of the Creede Formation, which were preferentially eroded only during the past few m.y.

The six calderas of the central cluster, as in the other calderas of the volcanic field, formed within a locus of precaldera volcanoes that are discontinuously preserved as remnant topographic highs along the caldera walls (Fig. 3). The Masonic Park Tuff, erupted from an entirely buried caldera at ca. 28.6 Ma, is transitional in age, petrology, and caldera location between rocks of the southeastern complex and those of the younger central cluster (Stop 1.4). It is a compositionally uniform silicic dacite that, except for the absence of sparse sanidine, petrologi-cally resembles the overlying Chiquita Peak Tuff erupted from Platoro caldera, with which it was previously confused (Lip-man et al., 1996).

The voluminous and seemingly homogeneous 28.0-Ma Fish Canyon Tuff (Stop 1.2; 5000 km3) is associated with formation of the 35 X 75 km La Garita caldera, the largest in the field (Stop 1.4). Distinctive petrologic features of this silicic dacitic tuff (66%-68% SiO2) indicate that its phenocrysts grew at relatively high pressures, in contrast to most other tuff units of the field (Whitney and Stormer, 1985; Bachmann et al., 2002). All the later ignimbrite sheets of the central cluster were erupted from smaller calderas aligned north-south along the western side of La Garita caldera, suggesting that these were postcollapse moat volcanoes of the La Garita cycle.

Eruption of the Carpenter Ridge Tuff (27.55 Ma) from Bachelor caldera produced a laterally and vertically complex ignimbrite sheet that has been the cause of much interpretive confusion in the region (Lipman et al., 1989). In places, the deposit is compositionally zoned from rhyolite (Stop 1.2) upward into silicic dacite (74%-66% SiO2), containing a burst of more mafic and alkalic magma (61%-63% SiO2) along the same horizon as the abrupt change in dominant eruptive composition (Lipman, 1975; Bachmann et al., 2014). Widespread tuff, once included as part of the Mammoth Mountain Tuff (Ratté and Steven, 1967; Steven et al., 1974), is now recognized as a separate major ignimbrite sheet, the Blue Creek Tuff erupted from a concealed source beneath the younger Creede caldera. The overlying Wason Park Tuff (27.38 Ma), a relatively crystal-rich and alkalic rhyolite, was erupted from South River caldera, which is largely covered by younger lavas (Lipman, 2000).

The San Luis caldera complex is a composite feature that was the source for three major sheets: Rat Creek, Cebolla Creek, and Nelson Mountain Tuffs. The Rat Creek and Nelson Mountain Tuffs contain broadly similar phenocryst assemblages and are compositionally zoned from rhyolite to dacite (74%-65% SiO2), while the intervening Cebolla Creek is a distinctively uniform mafic dacite that contains abundant hornblende but lacks sanidine. Eruption of the Nelson Mountain Tuff from a relatively small depression (~8 x 10 km) within the caldera complex was accompanied by concurrent subsidence of the larger Cochetopa caldera 25 km to the northeast (Lipman and McIntosh, 2008). All three ignimbrites associated with the San Luis complex erupted within an interval too brief to distinguish within analytical uncertainty, between 26.91 ± 0.02 and 26.90 ± 0.03 Ma (Lipman and McIntosh, 2008).

The Snowshoe Mountain Tuff and associated Creede caldera (Stop 2.1) have long been considered to be the youngest in the central cluster (Steven and Ratté, 1965; Steven and Lipman, 1976), even though stratigraphic evidence to support such a sequence has proved frustratingly elusive and mostly indirect (Lipman, 2000, his Table 7). This interpretation is now more firmly supported by multiple high-precision 40Ar/39Ar age determinations (Lipman and McIntosh, 2008), showing that the Snowshoe Mountain Tuff was erupted at 26.87 ± 0.02 Ma, just a few tens of thousand years later than that of the next youngest ignimbrite (Nelson Mountain Tuff). The Snowshoe Mountain Tuff is a relatively mafic dacite (62%-66% SiO2). It has some compositional affinities to the Fish Canyon, including a relatively high-pressure origin interpreted from its phe-nocrysts. Intracaldera Snowshoe Mountain Tuff is more than 1.8 km thick on the steep-sided resurgent dome with no base exposed (Steven and Ratté, 1965, p. 59). In contrast, the outflow sheet is only partly welded, less than 100 m thick, and preserved mainly where capped by mafic lavas south of South Fork and along the continental divide near South River caldera (Lipman, 2006).

The central caldera cluster offers exceptional opportunities to study the process of resurgent doming. La Garita, Bachelor, and Creede calderas all have fairly symmetrical and structurally simple resurgent domes. Dips on the flanks of La Garita caldera are relatively gentle (mostly less than 15°). In contrast, the flanks of the resurgent dome within Creede caldera dip as steeply as 45°. The crest of the resurgent dome of Bachelor caldera appears to be eccentrically located north of the center of this caldera. In addition, lineate and rheomorphic deformation of fiamme (flattened pumices) deep in the caldera-filling Carpenter Ridge Tuff on flanks of the resurgent dome suggests that definition of the domical structure began rapidly after the eruption, while the intracaldera ignimbrite remained sufficiently hot to deform viscously. The San Luis caldera complex resurged asymmetrically, as an incompletely understood trap-door uplift, somewhat similar to that at Platoro caldera in the southeastern caldera complex. Additionally, resurgence at the San Luis caldera complex was eccentric to the subsidence basin, with the north margin of uplift marked by a hinge zone 2-5 km beyond preserved caldera-fill deposits. In contrast, exposed levels in South River caldera provide no evidence for any resurgent uplift.

Tuffs and lavas associated with the central caldera cluster tend to be more silicic than those of the southeastern caldera complex; even more silicic compositions are present in the western San Juan region. Several of the caldera cycles, especially Bachelor and northern La Garita, had limited post-subsidence lava activity prior to eruption of the next major ignimbrite unit. Nevertheless, the same general compositions prevail, and the presence of local andesitic to dacitic lavas that interleave between all major ignimbrites of the central San Juan cluster documents the continued presence of mafic magmas during differentiation of the magmas associated with ignimbrite eruptions (Lipman et al., 1978; Riciputi et al., 1995; Lipman, 2000; Parat et al., 2005; Lipman and McIntosh, 2008).

Mineralization in the central caldera cluster was mainly confined to the Creede district (Steven and Ratté, 1965). Ore deposits are localized by caldera structures (keystone graben faults across the resurgent dome of Bachelor caldera and near walls of San Luis and Creede calderas), but are ~1 m.y. younger than any associated volcanic deposits (Bethke et al., 1976). No sizeable shallow plutonic intrusions are exposed in the mining district, but such bodies are inferred to be present at shallow depth to provide heat sources for the hydrothermal systems responsible for the mineralization. Several small granitoid bodies exposed low within the uplifted core of the San Luis caldera probably are high points on a larger resurgent intrusion.

Formation of Creede Caldera

Creede caldera, which subsided during eruption of the Snow-shoe Mountain Tuff at 26.87 ± 0.02 Ma (Lipman and McIntosh, 2008), has spectacularly preserved constructional morphology for an Oligocene volcano (Stop 2.1), and many of its major features were initially well documented by Steven and Ratté (1965) and Steven and Lipman (1976).

Caldera Geometry

Creede caldera is a symmetrically resurgent caldera (Fig. 4) that has structural dips to 45° on flanks of its domical uplift and a well-developed apical graben (Steven and Ratté, 1965). Its near-pristine constructional morphology is largely due to erosional exhumation of its sedimentary moat fill by the Rio Grande during the past few million years (Steven et al., 1995; Rye et al., 2000). The well-preserved morphology and intracaldera sedimentary deposits, explored by drilling in the caldera moat for mineral exploration and for the U.S. Continental Scientific Drilling Program, provide special insights into the three-dimensional structure of this representative medium-size plate-subsidence caldera. Creede caldera is similar in overall geometry to other well-studied resurgent calderas in the western United States such as Valles (New Mexico), Timber Mountain (Nevada), and Long Valley (California). Although not directly exposed, a ring fault ~16 km in diameter is inferred from arcuate trends of postcollapse lava vents and fossil spring deposits (travertine), as well as from the resurgent doming along confocal boundaries (Steven and Lipman, 1976; Lipman, 1984). The larger diameter of the topographic caldera rim relative to the structural margin provides evidence for large-scale enlargement of the caldera by gravitational landsliding during the eruption, then modestly modified by erosion (Larsen and Crossey, 1996), to form the “collar” volume between ring-fault structural boundary and topographic wall (Lipman, 1997, Fig. 3).

Figure 4.

Generalized geologic map of Creede caldera, showing approximate location of eroded topographic caldera rim, present-day extent of caldera-fill deposits, inferred buried ring fault, and late normal faults during resurgent doming (Deep Creek graben) and mineralization (Creede graben). Modified from Lipman (2000).

Figure 4.

Generalized geologic map of Creede caldera, showing approximate location of eroded topographic caldera rim, present-day extent of caldera-fill deposits, inferred buried ring fault, and late normal faults during resurgent doming (Deep Creek graben) and mineralization (Creede graben). Modified from Lipman (2000).

An important geometric constraint on the level of intracal-dera fill is the presence of a thick scab of intracaldera Snowshoe Mountain Tuff, plastered against the southern topographic rim over an exposed elevation range from 3370-3750 m (11,05012,300 ft) just below Fisher Mountain (Fig. 5). These exposures indicate that the Snowshoe Mountain, at a late stage during its eruption, filled Creede caldera to a level ~1.7 km higher than that of the post-eruption caldera floor as constrained by the Creede scientific drilling (Hulen, 1992; Heiken et al., 2000). The differences in elevation for the depositional top of the intracaldera tuff document large-scale subsidence near the end of the Snowshoe Mountain eruptions, events that perhaps are also responsible for large-scale late landsliding discussed in the next section.

Figure 5.

Simplified cross sections through the southern margin of Creede caldera showing interpreted sequence of events during caldera subsidence and post-eruption volcanism and sedimentation. (A) Maximum height of intracal-dera Snowshoe Mountain Tuff, late during eruption, after structurally bounded caldera had become enlarged by earlier landslide slumping. (B) Further major subsidence near end of eruptions, causing additional landsliding of highest intracaldera Snowshoe Mountain Tuff and adjacent wall rocks. (C) Resurgent doming of caldera floor, filling of resulting moat by sediments of Creede Formation and lavas of Fisher Dacite, and subsequent erosion to present-day land surface. Location of section shown in Figure 4.

Figure 5.

Simplified cross sections through the southern margin of Creede caldera showing interpreted sequence of events during caldera subsidence and post-eruption volcanism and sedimentation. (A) Maximum height of intracal-dera Snowshoe Mountain Tuff, late during eruption, after structurally bounded caldera had become enlarged by earlier landslide slumping. (B) Further major subsidence near end of eruptions, causing additional landsliding of highest intracaldera Snowshoe Mountain Tuff and adjacent wall rocks. (C) Resurgent doming of caldera floor, filling of resulting moat by sediments of Creede Formation and lavas of Fisher Dacite, and subsequent erosion to present-day land surface. Location of section shown in Figure 4.

A minimum subsidence depth for Creede caldera can be estimated from the combined >3.5 km vertical dimensions of the initially unfilled topographic caldera (1.7 km) and exposed intracaldera tuff (>1.8 km). Only volumetrically minor landslide breccias are interleaved with tuff exposed on the resurgent dome near the center of the caldera, suggesting that much additional landslide debris must be concealed at depth. The modeled height of a lithic debris fan within the fill, needed to account for the collar volume, is ~2 km, suggesting total subsidence of 4-5 km (Lipman, 1997). Analogous geometries of slide-breccia accumulations as thick fans near caldera margins, wedging into thinner layers in the caldera interior, are well exposed at several western U.S. calderas that are more eroded, such as Lake City and Grizzly Peak in Colorado, and the Tucson Mountains in Arizona (Lipman, 1984, and references therein).

Caldera-Collapse Landslide Deposits

Landslide deposits have long been recognized locally inter-stratified with upper parts of the intracaldera Snowshoe Mountain Tuff on the resurgent dome (Steven and Ratté, 1965), demonstrating their deposition prior to resurgence. Additional large-scale landslide deposits have since been identified and provide further support for enlargement of the topographic Creede caldera and subsequent fill by sediments (Lipman, 2000).

On the west flank of the resurgent dome, north of McCall Creek, minor breccia lenses interfinger with upper parts of the Snowshoe Mountain Tuff, but most breccia deposits overlie the uppermost weakly welded Snowshoe Mountain Tuff on the outer flanks of the resurgent dome. The dominant clast type, especially in the lowermost breccia, is dark-gray andesite, probably derived from Bristol Head on the caldera wall to the west. Other locally abundant clast types are Wason Park Tuff and crystal-poor rhyolite tuff of the intracaldera Carpenter Ridge Tuff (Bachelor Mountain Member).

The most spectacularly exposed landslide breccia is the rugged outcrop on the northwest flank of Snowshoe Mountain (Stop 2.2), locally known as “Point of Rocks” (Figs. 4 and 6), that previously was interpreted as a brecciated postcaldera rhyolitic lava dome (Steven and Ratté, 1965, p. 43). Further study shows, however, that Point of Rocks is a resurgently tilted and slightly faulted layer of silicified monolithologic breccia, consisting solely of crystal-poor welded tuff derived from the Willow Creek welding zone of intracaldera Carpenter Ridge Tuff. Flattened pumice textures within the rhyolite breccia clasts are obscure in the most accessible exposures at Point of Rocks, but are readily visible nearby. Between the rhyolite breccia layer and weakly welded massive intracaldera Snowshoe Mountain Tuff at Point of Rocks is a thin intervening lens of brecciated phenocryst-rich dacite. This brecciated dacite was likely derived by sliding from lava higher on the topographic rim, and several meter-scale beds of lithic-rich tuff that are interpreted to represent waning of Snow-shoe Mountain eruptions.

Figure 6.

Road map for central San Juan region. Road map shows trip route from Del Norte to Creede, locations of ignimbrite calderas, and prominent geographic features. Uncon-formable margins of caldera-fill lava and tuff are solid lines where exposed and short dashed lines where concealed. Geologic base map from Steven et al. (1974).

Figure 6.

Road map for central San Juan region. Road map shows trip route from Del Norte to Creede, locations of ignimbrite calderas, and prominent geographic features. Uncon-formable margins of caldera-fill lava and tuff are solid lines where exposed and short dashed lines where concealed. Geologic base map from Steven et al. (1974).

Scientific drilling in the caldera moat penetrated thick breccias composed of clasts of welded rhyolite tuff derived from the Willow Creek zone, which are interpreted as correlative with the Point of Rocks breccia. The breccia deposits overlie flat-lying intracaldera tuff at elevations of 2255 m (7397 ft) in drill hole CCM-1 and 2029 m (6656 ft) in drill hole CCM-2 (Hulen, 1992; Heiken et al., 2000). The greater depth in CCM-2 is presumed to be the result of reactivation of faulting within the earlier-formed Creede graben, which extends northward into Bachelor caldera (Fig. 4). These depths are 1.5-1.7 km below the average elevations of high points on the preserved caldera topographic rim at 3.7-3.8 km (12,200-12,500 ft), providing constraints on the original postcollapse depth of the unfilled caldera moat, now partly filled by postcaldera sediments and lavas. Together with the outcrops of similar breccia at Point of Rocks and north of McCall Creek, the drill holes provide evidence that a sheet of landslide breccia derived from cliffs of Willow Creek zone on the caldera wall spread over much of the northwest quadrant of the caldera floor, resting on uppermost Snowshoe Mountain Tuff. The most probable source for such a widespread breccia sheet would have been from the caldera wall near the mouth of Willow Creek, where the Creede caldera wall has a pronounced northward embayment at the structurally weak intersection with keystone graben faults of Bachelor caldera.

Clasts from the breccia deposits at Point of Rocks have normal magmatic alkali ratios, closely similar to outflow Carpenter Ridge Tuff. In contrast, all bedrock samples of Willow Creek type, from even the most distant possible source on the northeast wall of Creede caldera (between Dry Gulch and Farmers Creek), show substantial alkali exchange (Ratté and Steven, 1967; Lip-man, personal observ.). The compositions from the Point of Rocks breccias, in comparison to the Willow Creek welding zone of intracaldera Carpenter Ridge Tuff along the northern wall of Creede caldera, support the interpretation of Sweetkind et al. (1993) that the potassic alteration in the Creede mining district was a precursor to the late faulting and major mineralization along the Creede graben, rather than during resurgence and early (“ancestral”) faulting associated with the Bachelor caldera cycle.

Postcollapse Intracaldera Sedimentation and Volcanism

Following cessation of the Snowshoe Mountain eruptions, Creede caldera was a steep-walled flat-floored closed basin 20-25 km in rim diameter that filled rapidly with volcaniclas-tic sediments, primary volcanic deposits, and water (Larsen and Crossey, 1996; Heiken et al., 2000; Larsen and Nelson, 2000). Early caldera-filling processes included mass wasting from unstable caldera walls, as documented by conglomerates and sedimentary breccia deposits interbedded with fine-grained alluvial and shallow-lake sediments of the Creede Formation (Larsen and Nelson, 2000), emplacement of lavas of Fisher Dacite, erosional recycling of caldera-fill deposits as the caldera floor resurged, and gradual filling of the closed basin by surface waters and sediments of the Creede Formation (Barton et al., 2000a). In both the scientific drilling cores (Hulen, 1992; Heiken et al., 2000), sedimentary deposits of the Creede Formation become dominant above distinctive monolithologic breccia deposits composed of fragments from the Willow Creek welding zone of the Bachelor Mountain member of the Carpenter Ridge Tuff. At lower elevations in both scientific drilling boreholes, bedded tuffaceous sandstone makes up small percentages of a transitional succession that consists dominantly of nonwelded tuff of Snowshoe Mountain lithologic type and interlayered landslide breccia (Lipman and Weston, 1994). Most of these lower beds appear to be primary surge and fall deposits, probably emplaced rapidly during waning eruptions of Snowshoe Mountain Tuff, whereas others record initial brief periods of alluvial sedimentation, transitioning into the lacustrine depositional environment of the main Creede Formation.

Creede Formation

The sedimentary and interbedded pyroclastic deposits that accumulated within the Creede caldera basin comprise the Creede Formation (Figs. 7 and 8) (Emmons and Larsen, 1923; Steven and Ratté, 1965; Larsen and Crossey, 1996; Finkelstein et al., 1999; Heiken et al., 2000; Larsen and Nelson, 2000). The Creede Formation consists largely of intermediate to felsic fallout ash, finely interlaminated tuffaceous siltstone and limestone, and thin-bedded to massive sandstone and conglomerate (Fig. 7), representing shallow- to deep-water lake deposits in the central moat (Larsen and Crossey, 1996). Breccia, conglomerate, and sandstone were deposited along the margins of the caldera and resurgently uplifted core during the latter phases of caldera fill, and represent ancient colluvial, alluvial, and deltaic environments around the margins of a shallowing caldera lake (Larsen, 1994b; Larsen and Crossey, 1996). Massive to laminated limestone tufa (historically termed “travertine”) locally cross-cut strata, but are commonly concordant with the stratified deposits of the Creede Formation within the moat succession and are interpreted to represent deposits of mineral springs active during sedimentation (Fig. 7).

Figure 7.

Generalized map of Creede caldera showing the distribution of stratified rocks and travertine in the Creede Formation (modified from Larsen and Crossey, 1996). The locations of measured sections and core sites (CCM prefix) used in Figure 8 are shown. The line connects the sections and cores in the order shown in Figure 8. Sections and localities discussed in the text are also presented. The geologic base map for this figure is from Steven and Ratté (1965) and not updated to reflect more recent studies by Lipman (2000, 2006).

Figure 7.

Generalized map of Creede caldera showing the distribution of stratified rocks and travertine in the Creede Formation (modified from Larsen and Crossey, 1996). The locations of measured sections and core sites (CCM prefix) used in Figure 8 are shown. The line connects the sections and cores in the order shown in Figure 8. Sections and localities discussed in the text are also presented. The geologic base map for this figure is from Steven and Ratté (1965) and not updated to reflect more recent studies by Lipman (2000, 2006).

Figure 8.

Correlation of sections measured and cores described in the Creede Formation (modified from Larsen and Crossey, 1996). Numbers beneath sections indicate the height (in meters) of the base of the section relative to core CCM-2. The sections are aligned to the I tuff because it more closely represents an isochronous surface than the base of the Creede Formation. Pseudomorphs in the lacustrine facies are interpreted to be after ikaite (see text).

Figure 8.

Correlation of sections measured and cores described in the Creede Formation (modified from Larsen and Crossey, 1996). Numbers beneath sections indicate the height (in meters) of the base of the section relative to core CCM-2. The sections are aligned to the I tuff because it more closely represents an isochronous surface than the base of the Creede Formation. Pseudomorphs in the lacustrine facies are interpreted to be after ikaite (see text).

Attempts to determine the duration of Creede Formation sedimentation by dating ash beds from the scientific drilling cores have yielded inconsistent results, likely due to contamination of ash beds by older volcaniclastic material. Lanphere (2000) had concluded that the maximum duration of Creede Formation sedimentation was 0.66 m.y., based on his age determinations for the Snowshoe Mountain Tuff (26.92 Ma) and young flows of Fisher Dacite (26.26 Ma) that overlie the highest-elevation beds in the Creede Formation. More rapid accumulation for the sedimentary section and resurgent doming (50-100 k.y.) is now indicated by higher-precision 40Ar/39Ar ages for the pre-resurgence McCall Creek flow (26.82 ± 0.05 Ma) in comparison to the flows at Fisher Mountain (26.77 ± 0.04 Ma) that are stratigraphically above the Creede Formation (Lipman and McIntosh, 2008).

Surface exposures of the Creede Formation are extensive along the course of the Rio Grande through the caldera moat; however, no more than 100 m of section is exposed at any location. Post-depositional faulting, mainly associated with the Creede graben, has offset Creede Formation strata in much of the northern part of the moat, making lithological correlation difficult. Larsen and Nelson (2000) used the heavy mineral composition of fallout ash, lithostratigraphic markers, and geophysical logs to correlate between the CCM-1 and -2 cores, which was integrated with Larsen’s (1994b) heavy mineral content and lithostratigraphically based correlation of surface sections in the moat (Fig. 8). Correlation of surface exposures to the cores suggests as much as 200 m of offset in the Creede Formation along faults (Larsen and Crossey, 1996), primarily those of the Creede graben. Heiken et al. (2000) and Finkelstein et al. (2000) argue for a slightly modified correlation of the two cores and an absence of fault offset. Heiken et al. (2000) based their correlation on the presence of hydrovolcanic shard textures and crystal content, correlating the I tuff in CCM-2 with the H tuff in CCM-1. Finkelstein et al. (2000) used crystal chemistry (which Heiken et al., 2000, argued was non-diagnostic) and shard morphology to correlate the I tuff in CCM-2 to the G tuff in CCM-1. Neither of these authors completed detailed surface-exposure studies. Furthermore, absence of fault offset between the two cores would imply that faults of the Creede graben (Steven and Ratté, 1965; Lipman, 2000) did not displace the Creede Formation. Despite these differences in correlation interpretations, all authors agree in the general increase in vitric character and felsic composition in the upper part of the Creede Formation in cores and exposures.

The principal sedimentary facies in the Creede Formation were described in detail by Larsen (1994a), Larsen and Crossey (1996), Larsen and Smith (1999), Finkelstein et al. (1999), and Larsen and Nelson (2000), and are only summarized in the section below (see Table 3). The facies distributions are interpreted to represent depositional environments present in the caldera moat during the postcollapse depositional history, of which both highstand and lowstand conditions are shown diagrammatically in Figure 9 (Larsen and Crossey, 1996). Most of the lacustrine deposits include a lesser component of tuffaceous sandstone facies interbedded with more predominant laminated facies (Table 3) that can be observed at Stops 2.3-2.7 of this field trip. The laminated facies, deposited from suspension settling (Larsen and Crossey, 1996), is present throughout the lacustrine deposits and helps to distinguish alluvial from shallow- to deep-water lacustrine deposits. Laminated facies include tuffaceous siltstone, micrite, micritic peloids, and calcite pseudomorphs after ikaite (Larsen, 1994a). Ikaite, CaCO36H2O, is metastable with respect to calcite at near-surface conditions, but is observed in numerous cold-water (< 4 °C) marine and lacustrine settings. Pyrite and organic matter are prominent in the laminated facies, especially where carbonate laminae are present. Alternating carbonate and tuffaceous laminae may reflect seasonal sedimentation, but irregularity in thickness of tuffaceous laminae and complexity in the composition and internal structure of carbonate laminae suggest that other processes are also important. The suspension laminae are interbedded at a cm scale with fine-grained, tuffaceous sandstones that represent a continuum of depositional processes that include: (1) turbidity-current deposition below wave base (graded tuffaceous sandstones), (2) turbidity-current or flood-lag deposition with silty drapes (sandstone-siltstone couplets) that were reworked by storm waves (pinch-and-swell sandstones), and (3) deposition by oscillatory wave processes (laminated and wave-rippled sandstones). The turbidites show little internal structure in weathered exposures, such as those at Stops 2.3, 2.5, and 2.7 of this field guide, but show partial to complete Bouma sequences in core. Folded beds comprising these fine-grained facies are common and are interpreted to be slump deposits; the thickest slump interval is 29 m and conformably overlain by laminated facies at section FR-1 (Fig. 7).

Descriptions and Interpretations of Depositional Facies of the Creede Formation

Table 3.
Descriptions and Interpretations of Depositional Facies of the Creede Formation
FaciesDescriptionInterpretation and modern analogs
Laminated siltstoneSandy tuffaceous siltstone and carbonate siltstone.Suspension fallout sedimentation; comparable
Carbonate constituents include micrite, elongatemodern deposits found in Fayetteville Green Lake,
micritic peloids, and calcite pseudomorphs after ikaiteNew York (Ludlam, 1969), and Lake Zurich,
(Larsen, 1994b).Switzerland (Kelts and Hsü, 1978).
Graded tuffaceousSilt to very-coarse-grained, pebbly, tuffaceousTurbidite deposition based on the grading
sandstonesandstone with distribution normal grading. Internalcharacteristics (Lowe, 1982), presence of Bouma
stratification is not prominent in outcrop, but Boumasubdivisions, and intercalation with laminated
subdivisions Ta, Tb, Tc, Td, and Te are observed infacies. Graded, structureless turbidites known
core.from numerous modern lakes (Ludlam, 1969;
Anderson et al., 1985).
Tuffaceous sandstone-Very-fine- to coarse-grained tuffaceous sandstoneThese deposits are interpreted to be associated with
siltstone coupletslaminae overlain by a silt lamina.the passing of turbidity currents: the sandstone
could be either a lag deposit or plane bed, and the
silt drape represents suspension settling from the
overlying turbid water.
Cross-laminatedVery-fine- to very-coarse-grained, moderately sortedWave-rippled beds have symmetrical, bifurcating
tuffaceous sandstonewave-ripple and pinch-and-swell sandstone. Pinch-crests typically formed under oscillatory flow
and-swell laminae are 1 to 2 cm thick and generallyconditions. Pinch-and-swell beds may form by
lack internal stratification.both wave-related (Smoot and Lowenstein, 1991)
or density-current-related (Sturm and Matter,
1978) processes.
Horizontal-laminatedVery-fine- to very-coarse-grained, moderately sorted,These units are comparable to sands deposited as
tuffaceous sandstonelaminated sandstone.sand sheets under upper-flow-regime conditions
(Allen, 1984).
Horizontal-bedded andFine- to very-coarse-grained, pebbly, moderately toHorizontal-bedded units represent sheetflood
cross-bedded pebblypoorly sorted, scour-and-fill, low-angle trough, anddeposits laid down in either subaerial or shallow,
sandstoneplanar cross-bedded and horizontal-beddedlacustrine settings, analogous to those described
sandstone. Beds are composed of lithic andby Blair and MacPherson (1994) and Smoot and
pyroclastic grains but lack intraclasts.Lowenstein (1991). Scour-and-fill and low-angle-
trough cross-beds are similar to low-angle cross-
stratified deposits laid down during high-discharge
events in shallow, braided streams (Picard and
High, 1973).
Pebbly, calcareousCoarse- to very-coarse-grained, pebbly sandstone andSimilar deposits are described in lake-margin settings
sandstone andconglomerate with planar foresets that dip basinward.where detritus worked by stream and wave
conglomerate with planarThe sandy beds are composed of moderately to wellprocesses cascades down lakeward-dipping
foresetssorted sand with oolitic coatings.foresets (Smoot and Lowenstein, 1991).
Epsilon cross-beddedFine- to very-coarse-grained pebbly sandstone andThe geometry of these units is characteristic of
sandstones and pebblepebble to cobble conglomerate with crude to well-channel-fill sequences dominated by lateral
to cobble conglomeratedeveloped epsilon cross-bedding. This faciesaccretion processes (Allen, 1984). These facies
invariably fills channel-form erosional features inare almost exclusively observed interbedded with
finer-grained facies. The erosional channels are 1 tolaminated lacustrine facies rather than sandy
2 m deep and 9 to 23 m across, and are, in somefluvial facies. Similar types of deposits are noted
cases, nested. The channel-fill facies fine upwardin both ancient and modern submarine fan
and laterally, into graded beds and laminated units inchannels (Clark and Pickering, 1996), and are
some cases.ascribed to channel avulsion and migration
processes.
Crudely beddedSandy, pebble to boulder, poorly sorted, crudely beddedThe poor sorting, crude stratification, and channelized
conglomerateconglomerate, as tabular beds and channel fill.nature suggest deposition in high-gradient stream
channels (Rust, 1978).
Graded, massiveBeds are generally tabular and 5 to 150 cm thick, butSediment-gravity-flow deposition (see text).
sandstone andsome channel-fill beds are observed. Three types are
conglomerate or brecciadistinguished:
1) Massive, lithic-rich pebble sandstone and pebble
to boulder conglomerate, non-graded, with lithic
pebbles, cobbles, and, less commonly, boulders
in a sandy matrix.
2) Massive, intraclast-rich pebble to boulder
conglomerate and breccia, basal coarse-tail
reverse grading, with a sandy, calcareous
(pulverized intraclast) matrix.
3) Massive, mixed-composition pebble sandstone
and pebble to cobble conglomerate, typically with
stratification in the upper part of the beds. The
beds have flat to convex-down bases and flat to
convex-up tops; erosional or load deformation
features are common along basal contacts.
Bedded, fine-grained tuffLight-colored, fine-grained, massive and laminated bedsInterpreted as pyroclastic fallout based on the pure
within the lacustrine strata.vitric-ash composition, fine grain size, and
moderate sorting characteristics (Fisher and
Schmincke, 1984).
FaciesDescriptionInterpretation and modern analogs
Laminated siltstoneSandy tuffaceous siltstone and carbonate siltstone.Suspension fallout sedimentation; comparable
Carbonate constituents include micrite, elongatemodern deposits found in Fayetteville Green Lake,
micritic peloids, and calcite pseudomorphs after ikaiteNew York (Ludlam, 1969), and Lake Zurich,
(Larsen, 1994b).Switzerland (Kelts and Hsü, 1978).
Graded tuffaceousSilt to very-coarse-grained, pebbly, tuffaceousTurbidite deposition based on the grading
sandstonesandstone with distribution normal grading. Internalcharacteristics (Lowe, 1982), presence of Bouma
stratification is not prominent in outcrop, but Boumasubdivisions, and intercalation with laminated
subdivisions Ta, Tb, Tc, Td, and Te are observed infacies. Graded, structureless turbidites known
core.from numerous modern lakes (Ludlam, 1969;
Anderson et al., 1985).
Tuffaceous sandstone-Very-fine- to coarse-grained tuffaceous sandstoneThese deposits are interpreted to be associated with
siltstone coupletslaminae overlain by a silt lamina.the passing of turbidity currents: the sandstone
could be either a lag deposit or plane bed, and the
silt drape represents suspension settling from the
overlying turbid water.
Cross-laminatedVery-fine- to very-coarse-grained, moderately sortedWave-rippled beds have symmetrical, bifurcating
tuffaceous sandstonewave-ripple and pinch-and-swell sandstone. Pinch-crests typically formed under oscillatory flow
and-swell laminae are 1 to 2 cm thick and generallyconditions. Pinch-and-swell beds may form by
lack internal stratification.both wave-related (Smoot and Lowenstein, 1991)
or density-current-related (Sturm and Matter,
1978) processes.
Horizontal-laminatedVery-fine- to very-coarse-grained, moderately sorted,These units are comparable to sands deposited as
tuffaceous sandstonelaminated sandstone.sand sheets under upper-flow-regime conditions
(Allen, 1984).
Horizontal-bedded andFine- to very-coarse-grained, pebbly, moderately toHorizontal-bedded units represent sheetflood
cross-bedded pebblypoorly sorted, scour-and-fill, low-angle trough, anddeposits laid down in either subaerial or shallow,
sandstoneplanar cross-bedded and horizontal-beddedlacustrine settings, analogous to those described
sandstone. Beds are composed of lithic andby Blair and MacPherson (1994) and Smoot and
pyroclastic grains but lack intraclasts.Lowenstein (1991). Scour-and-fill and low-angle-
trough cross-beds are similar to low-angle cross-
stratified deposits laid down during high-discharge
events in shallow, braided streams (Picard and
High, 1973).
Pebbly, calcareousCoarse- to very-coarse-grained, pebbly sandstone andSimilar deposits are described in lake-margin settings
sandstone andconglomerate with planar foresets that dip basinward.where detritus worked by stream and wave
conglomerate with planarThe sandy beds are composed of moderately to wellprocesses cascades down lakeward-dipping
foresetssorted sand with oolitic coatings.foresets (Smoot and Lowenstein, 1991).
Epsilon cross-beddedFine- to very-coarse-grained pebbly sandstone andThe geometry of these units is characteristic of
sandstones and pebblepebble to cobble conglomerate with crude to well-channel-fill sequences dominated by lateral
to cobble conglomeratedeveloped epsilon cross-bedding. This faciesaccretion processes (Allen, 1984). These facies
invariably fills channel-form erosional features inare almost exclusively observed interbedded with
finer-grained facies. The erosional channels are 1 tolaminated lacustrine facies rather than sandy
2 m deep and 9 to 23 m across, and are, in somefluvial facies. Similar types of deposits are noted
cases, nested. The channel-fill facies fine upwardin both ancient and modern submarine fan
and laterally, into graded beds and laminated units inchannels (Clark and Pickering, 1996), and are
some cases.ascribed to channel avulsion and migration
processes.
Crudely beddedSandy, pebble to boulder, poorly sorted, crudely beddedThe poor sorting, crude stratification, and channelized
conglomerateconglomerate, as tabular beds and channel fill.nature suggest deposition in high-gradient stream
channels (Rust, 1978).
Graded, massiveBeds are generally tabular and 5 to 150 cm thick, butSediment-gravity-flow deposition (see text).
sandstone andsome channel-fill beds are observed. Three types are
conglomerate or brecciadistinguished:
1) Massive, lithic-rich pebble sandstone and pebble
to boulder conglomerate, non-graded, with lithic
pebbles, cobbles, and, less commonly, boulders
in a sandy matrix.
2) Massive, intraclast-rich pebble to boulder
conglomerate and breccia, basal coarse-tail
reverse grading, with a sandy, calcareous
(pulverized intraclast) matrix.
3) Massive, mixed-composition pebble sandstone
and pebble to cobble conglomerate, typically with
stratification in the upper part of the beds. The
beds have flat to convex-down bases and flat to
convex-up tops; erosional or load deformation
features are common along basal contacts.
Bedded, fine-grained tuffLight-colored, fine-grained, massive and laminated bedsInterpreted as pyroclastic fallout based on the pure
within the lacustrine strata.vitric-ash composition, fine grain size, and
moderate sorting characteristics (Fisher and
Schmincke, 1984).
Figure 9.

Diagrammatic sketches showing the distribution of deposi-tional environments during: (A) highstand and (B) lowstand lake conditions (from Larsen and Crossey, 1996).

Figure 9.

Diagrammatic sketches showing the distribution of deposi-tional environments during: (A) highstand and (B) lowstand lake conditions (from Larsen and Crossey, 1996).

Three distinct types of pebbly conglomerate and sandstone are interbedded with the tuffaceous sandstone and laminated facies. (1) Facies 1 is massive, lithic-rich pebble sandstone and pebble to boulder conglomerate with coarse-tail reverse grading and protrusion of the clasts above the bed surface interpreted as debris-flow deposits (Lowe, 1979; Middleton and Southard, 1984). (2) Facies 2 is massive, intraclast-rich pebble to boulder conglomerate and breccia that are gradational to folded and slumped along the margins and interpreted to be debris-flow deposits derived from slumping. (3) Facies 3 is massive, mixed-composition pebble sandstone and pebble to cobble conglomerate, typically with stratification in the upper parts of the beds (Table 3). The massive internal structure, grading characteristics of the lower to middle parts of beds, and convex-up profiles are typically associated with deposition by debris-flow processes (Lowe, 1979; Smith and Lowe, 1991), but some of these characteristics may also be produced by deposition from high-density turbidity currents (Kneller and Branney, 1995). Facies 3 conglomerates and sandstones form multi-bed units that are commonly observed basinward of known or inferred paleovalleys and are interpreted as deposits of sub-lacustrine fan and fan-delta deposits (Fig. 9) (Larsen and Smith, 1999). Sublacustrine-fan deposits are prominent in the cliffs at Stop 2.3 and fan-delta deposits are present in upper exposures at Stop 2.5 of this field trip.

Interspersed within and gradational to the lacustrine deposits are fallout tuff units, 15 cm to 15 m thick. The tuff beds are gray to white, fine-grained, crystal-vitric to vitric ash that are massive at the base and are commonly overlain by laminated and thin tuffaceous turbidite deposits. Crystals of plagioclase, sani-dine, biotite, and Fe-Ti oxide minerals are present in all tuffs, but most of the lower tuffs (B through G) are rich in hornblende whereas the upper tuffs (H through L2) contain significantly less hornblende. Aside from the A tuff, which is interpreted to be the fallout from the caldera-forming eruption, all other fallout tuffs are interpreted to have been deposited in a perennial lake by a combination of suspension fallout and ash-laden turbiditic flows; an excellent example is exposed at Stop 2.7 of this field trip. Based on the mineralogy, chemical composition, and spatial and temporal proximity, fallout tuffs within the lacustrine succession are interpreted to have originated from Fisher Dacite volcanoes (Larsen, 1994b; Heiken et al., 2000). Ash-flow tuff is less common in the Creede Formation, but ash-flow tuff is inter-bedded with lacustrine deposits in the southeastern part of the caldera (Larsen, 1994b), which we will see at Stop 3.1 of this field trip. The ash-flow tuffs are discussed more in a subsequent section on Fisher Dacite. The overall paucity of pumice lapilli in Creede Formation strata may reflect the importance of hydrovol-canic processes (Heiken et al., 2000); however, pumice may have floated during the eruptive event and later sunk after becoming waterlogged (Larsen, 1994b).

Alluvial and lake-margin deposits are composed of inter-bedded fine- to very coarse-grained, laminated and ripple cross-laminated sandstone; green, sandy siltstone and mudstone; and mud to very coarse-grained, pebbly, graded or scour-and-fill sandstone (Table 3). These deposits are most common at the base of both CCM cores and along the margin of the caldera moat, especially in the uppermost exposures. Debris-flow conglomerate and breccia beds are also present in some intervals. Analysis of wave-rippled facies in outcrop yields maximum water depths of 1-6 m (Larsen and Crossey, 1996). Wave-worked shoreline gravels, in some cases with ooid-like carbonate grain coating, and Gilbert-style deltaic foresets and bottomsets are present locally and are associated with alluvial fan and fan delta deposits. Horizons of silt- and sand-filled desiccation cracks, as much as 7 cm deep, are present in the lower parts of both CCM-1 and -2, but nearly absent in surface exposures. Weakly developed paleo-sols with root traces and petrified wood are present in uppermost exposures of scour-and-fill cross-bedded stream deposits and adjacent wave-rippled nearshore sandstone in the Creede Formation on Bachelor Mountain and in the area of sections 4UR-1 and -2 (Fig. 7). Talus and debris-flow breccias are locally present along the margins of the caldera moat, in some cases interstratified with laminated lacustrine deposits, suggesting a lacustrine debris apron setting.

Tufa facies include massive, laminated, thinolitic, and stro-matolitic varieties that form mounds and fissure ridges within the moat basin and intergrade with sandstone and conglomerate along the caldera margins (Figs. 7 and 9). The laminated, stro-matolitic, and massive facies have fibrous and micritic textures, which are typical of algal and bacterial precipitates (Larsen and Crossey, 1996). The thinolitic textures are similar to those observed at Pyramid and Mono Lakes in the western Great Basin, United States (Shearman et al., 1989) and are also interpreted as pseudomorphic after ikaite (Larsen, 1994a). Smoot and Benson (1998) note the presence of thinolite grains in Pyramid Lake muds, suggesting that Pyramid Lake also shows an association of tufa and strata-bound pseudomorphs after ikaite. Tufa west of the town of Creede contains intervening chalcedony interpreted by Rye et al. (2000) as being syndepositional, yielding a precipitation temperature of 45 ± 9° C based on 518O measurements. Bedded silica is locally present in the Creede Formation adjacent to the Bachelor paleovalley, suggesting precipitation from localized silica-rich springs; however, most silica in the travertine is in the form of post-depositional opal C-T (cristobalite-tridymite) or quartz (Larsen, 1994b; Larsen and Crossey, 2000).

The lacustrine facies have yielded a variety of leaf and insect fossils (Axelrod, 1987; Wolfe and Schorn, 1989; Cuffey et al., 1982), but no shelly fauna or vertebrate fossils have been reported. Plant and insect fossils may be observed at Stops 2.5 and 2.7 of this field trip. In addition, petrified wood is also observed in alluvial strata in several locations. Using a floristic approach, Axelrod (1987) interpreted the paleoclimate within the moat basin to have been warm and semi-arid; however, Wolfe and Schorn (1989), applying a physiognomic analysis to Axelrod’s (1987) data, interpreted the flora to represent a cool, montane climate, similar to that present in the region today. The presence of pseudomorphs after ikaite is most consistent with the interpretations of Wolfe and Schorn (1989).

The Creede Formation strata experienced post-depositional faulting in the Creede graben, which resulted in the youngest lacustrine strata being down-dropped west of the town of Creede (Figs. 7 and 8) and substantial offset between boreholes CCM-1 and CCM-2 (Larsen, 1994b). Other faulting affected the Creede Formation in the Antlers Park area and in the southeastern part of the moat basin. The Creede Formation also experienced silicate burial diagenesis under a high geothermal gradient (Finkelstein et al., 2000; Larsen and Crossey, 2000) and at least two hydro-thermal alteration events, one associated with mineralization in the Creede mineral district (Larsen and Crossey, 2000) and another in the Antlers Park area (Finkelstein et al., 2000; Larsen and Crossey, 2000). Faulting and coarse-grained facies in the Creede Formation appear to have significantly focused hydro-thermal fluid flow, resulting in low-temperature hydrothermal alteration (Larsen and Crossey, 2000).

Evolution of the Caldera Moat Basin and Ancient Lake Creede

The volcanic eruption, lake-level and basin-fill history are shown in Figure 10 (Larsen and Crossey, 1996). The dacitic through rhyolitic composition of the fallout ash is inferred from observed crystal mineralogy and heavy mineral fractions (Larsen, 1994b). Lake level is inferred from facies distributions in core and outcrop (Fig. 9), with the dashed portion of the lake-level curve based almost entirely on evidence of lacustrine incursions observed in surface exposures and cores from the Bachelor paleovalley. The basin-fill history integrates observations from throughout the moat, but mainly in the northern part of the basin. The postcollapse record inferred from the cores and surface exposures of landslide breccia suggests that as much as 200 m of fallout ash, landslide breccia, laharic breccias, and alluvial deposits initially filled the basin (Larsen and Nelson, 2000).

Figure 10.

Diagrammatic sketch showing volcanic, lake-level, and basin-fill history (modified from Larsen and Crossey, 1996). Dashed part of lake-level history is derived solely from sequences in Bachelor paleovalley and is not corrected relative to CCM-2 stratigraphy. Basin history cross section represents a schematic north (right)-south (left) cross section through site CCM-2. Ts is Snowshoe Mountain Tuff; Tov is older volcanic rocks; stippled pattern is alluvial deposits; lined pattern is lacustrine deposits; blocks are landslide breccia. Also shown are periods I, II, and III of Rye et al. (2000).

Figure 10.

Diagrammatic sketch showing volcanic, lake-level, and basin-fill history (modified from Larsen and Crossey, 1996). Dashed part of lake-level history is derived solely from sequences in Bachelor paleovalley and is not corrected relative to CCM-2 stratigraphy. Basin history cross section represents a schematic north (right)-south (left) cross section through site CCM-2. Ts is Snowshoe Mountain Tuff; Tov is older volcanic rocks; stippled pattern is alluvial deposits; lined pattern is lacustrine deposits; blocks are landslide breccia. Also shown are periods I, II, and III of Rye et al. (2000).

Following resurgence, Lake Creede filled the steep-margined moat basin and deepened quickly (Fig. 10) (Barton et al., 2000a). Rye et al. (2000) used stable C and O isotope data to divide the preserved lacustrine record into three periods. Based on stable C and O isotope data of lacustrine carbonate (Fig. 11) (Rye et al., 2000) and lacustrine facies in the two CCM cores, the early lake lacked stratification and was subject to frequent ash fallout. During period I of the lake (Fig. 12) little carbonate was deposited, all of which was micrite. Stable O isotope data from lacustrine carbonate show increasingly heavy values in core CCM-2 between 390 and 290 m depth, along with an increase in total carbonate, peloidal carbonate, and pseudomorphs after ikaite. The peloids, which are elongate micritic masses, as much as 10 mm long and 0.2 mm wide, are interpreted as brine shrimp fecal pellets (Larsen and Crossey, 1996; Finkelstein et al., 1999) and reflect the isotopic composition of the productive zone. The ikaite formed in the mud and thus the isotopic signal from this phase reflects the pore waters rather than open waters, as indicated by their heavy O isotopic composition (Rye et al., 2000). During period II, the lake is interpreted to reflect continuous stratification with dense, somewhat saline, isotopically heavy bottom waters (monimolimnion) overlain by more dilute, well-mixed isotopically light surface waters (mixolimnion; Rye et al., 2000). Stable C isotope data from lacustrine carbonates indicate that the zone of primary production extended from the mixolimnion into the monimolimnion, resulting in inversely correlated stable C and O isotope relationships (Fig. 11) (Rye et al., 2000). During this period, Lake Creede shows the first indications of lake-level fall with wave reworking of lacustrine strata in basin margin sections and CCM-1, and progradation of lacustrine fan and fandelta deposits into the lake (Fig. 10) (as observed at Antlers Park, Stop 2.3 of this field trip). During the transition from period II to III the lake level rebounded; however, during period III almost all stable O and C isotope data from the lacustrine carbonate reflect precipitation from the dense, isotopically heavy monimolimnion (Rye et al., 2000). Although increasing lake surface area through time may have in part balanced the evaporation creating a smaller lake (Barton et al., 2000a, 2000b), the absence of a depositional record originating in the mixolimnion (Fig. 12) suggests that the lower lake levels in the latter part of Lake Creede’s history were driven in part by lower effective moisture in the watershed. During the latter part of period III, lake level fell again resulting in progradation of fan-delta and lacustrine fan strata from the Bachelor paleovalley as far south as CCM-2 and the low hills to the north of the borehole site (Stop 2.5). Although lake level rose again during the interval of the M tuffs, it fell again soon after. During the remaining recorded history of the Lake Creede, alluvial strata built into the basin with a smaller lake occupying central parts of the basin. The topographic rim of the caldera basin broadened as weathering and stream incision removed sediment from the surrounding highlands and plateau (Fig. 10). Brief lacustrine highstands are recorded in surface exposures and core in the Bachelor paleovalley, but no chemical or isotopic data are available to evaluate lake conditions. Lake Creede is interpreted to have become a small, shallow lake late in its history (Barton et al., 2000a), prior to draining through a surface-water outlet. The most likely surface outlet for the lake existed to the southeast at an elevation of 3050 m, where a Miocene Hinsdale basalt flow follows a paleovalley extending to the southeast (Steven et al., 1995; Larsen and Crossey, 1996).

Figure 11.

Stable carbon and oxygen isotope compositions of lacustrine carbonates from CCM-2 showing calculated 513CCO2 and 518OH2O values of fluids for assumed temperatures (from Rye et al., 2000, their figure 9). Tie lines connect core sample data with fluid composition fields interpreted to reflect early (period I) and late (period III) water compositions. Maximum 513C values were calculated assuming the dissolved inorganic carbon was from volcanic CO2 added directly to the lake at the travertine springs (see Rye et al., 2000, for further information).

Figure 11.

Stable carbon and oxygen isotope compositions of lacustrine carbonates from CCM-2 showing calculated 513CCO2 and 518OH2O values of fluids for assumed temperatures (from Rye et al., 2000, their figure 9). Tie lines connect core sample data with fluid composition fields interpreted to reflect early (period I) and late (period III) water compositions. Maximum 513C values were calculated assuming the dissolved inorganic carbon was from volcanic CO2 added directly to the lake at the travertine springs (see Rye et al., 2000, for further information).

Figure 12.

Schematic diagrams showing interpreted limnologic structure of Lake Creede during each evolutionary period (from Rye et al., 2000, their figure 11).

Figure 12.

Schematic diagrams showing interpreted limnologic structure of Lake Creede during each evolutionary period (from Rye et al., 2000, their figure 11).

The chemical evolution of Lake Creede has long been speculated upon (Steven and Ratté, 1965; Bodine et al., 1987), especially with regard to the origin of the Creede mineral district ore deposits (Bethke and Rye, 1979; Barton et al., 2000a, 2000b). The stable C and O isotope studies of Rye et al. (2000) indicate isotopic enrichment from evaporative processes; however, little information is provided regarding the chemical evolution. Fin-kelstein et al. (1999) argue that the calcite pseudomorphs after ikaite are instead pseudomorphic after gypsum; however, such an interpretation is inconsistent with the replacement fabric of these pseudomorphs (Shearman and Smith, 1985; Larsen, 1994a; Larsen and Crossey, 2001). Finkelstein et al.’s (1999) argument that primary thenardite (Na2SO4) is trapped in clay minerals seems implausible given the diagenetic and hydrothermal alteration of these sediments (Larsen and Crossey, 2000; Finkelstein et al., 2000) and the evidence of early diagenetic or possibly primary pyrite in the Creede Formation sediments (Ilchik and Rumble, 2000). The sulfur content of the lake water was largely partitioned into sulfide, as evident by abundant pyrite in the lacustrine strata, rather than sulfate in the anoxic monimolimnion waters, especially considering the plentiful organic matter as a reducing agent (Larsen and Crossey, 2000, 2001). The extensive travertine deposits in the Creede Formation are anomalous, especially for a caldera lake. Rye et al. (2000) argue that CO2-rich fluids discharging into the lake mixed with Ca-bearing monimolimnion waters resulting in ikaite during the winter months and calcite during warmer months. This model is consistent with the association of ikaite with CO2-rich spring discharge into lakes and ocean basins (Shearman et al., 1989; Larsen, 1994a). Evidence for increased alkalinity through time is limited to the presence of radiating alkali feldspar clusters interpreted to be pseudomorphic after phillipsite (Larsen and Crossey, 2000), an alkaline zeolite known to form in saline, alkaline lakes. Alkalinity may have been kept in check by H2SO4 added during frequent ash fall events into the lake and watershed. No sodium and chloride evaporative minerals are preserved in the Creede Formation, nor is there convincing evidence that they precipitated in the strata; thus, these solutes would have become concentrated in the lake water through time. Although groundwater leakage from the bottom of the basin (Larsen and Crossey, 1996) is plausible, no clear evidence for it exists. Taken together, the evidence points toward Lake Creede becoming smaller, shallower, and more saline through time, with a diminished volume of dilute mixolimnion waters overlying saline monimolimnion waters. Given the complex volcanic and spring input into Lake Creede, its chemical evolution seems to have been complex and difficult to explain by simple models of chemical evolution in topographically closed lake basins (Eug-ster and Hardie, 1978; Jones and Deocampo, 2005).

Volcanic Clast Petrology of the Creede Formation

Coarse clastic facies of the Creede Formation, deposited against lower slopes of the caldera walls and in lacustrine fan and fan-delta facies, are important sources of information on caldera-wall morphology and mechanisms of caldera collapse, and also record the lithologies present high on the walls at the time of subsidence. Particularly informative is the paleovalley fill on Bachelor Mountain, north of Creede (Steven and Ratté, 1965, p. 48-49), which contains cobble conglomerates with clasts locally greater than 0.5 m in diameter. This channel in part follows the downdropped keel of the Creede graben that was initially blocked out during resurgence of the Bachelor caldera. In addition to direct structural control, the paleovalley may have been enlarged by fault-controlled landslide failure during subsidence of Creede caldera, but was further modified by stream erosion concurrent with Creede Formation deposition. Clasts collected from upper reaches of the paleovalley (surface exposures and mineral-exploration drill core) include: (1) abundant cobbles of distinctive Wason Park Tuff that form wall rock at the local channel level, (2) clasts of porphyritic andesite that closely resembles andesite of Bristol Head that overlies the Wason Park in nearby exposures, and (3) clasts of phenocryst-rich dacitic welded tuff that resembles Rat Creek or Nelson Mountain Tuff (Lipman and Weston, 1994). All the clast types are affected by intense alkali exchange and potassium metasomatism, comparable to that in the underlying bedrock of intracaldera Carpenter Ridge Tuff (Bachelor Mountain Member). These relations, along with the absence of alkali exchange in landslide breccia clasts exposed on the Snowshoe Mountain resurgent dome, indicate that the main episode of K-metasomatism postdated formation of Creede caldera.

Despite potassic alteration of the crystal-rich tuff clasts from the Bachelor paleovalley, microprobe analyses of their unaffected sanidine phenocrysts are relatively sodic, compositions that are plausible for the Rat Creek or Nelson Mountain Tuffs, but not the Snowshoe Mountain Tuff (Lipman and Weston, 1994). These cobbles thus support interpretations that the Nelson Mountain and/or Rat Creek Tuffs existed on the north rim of Creede caldera prior to its collapse and erosional enlargement. Lithic fragments from surface-exposed Creede Formation on the northeast side of Creede caldera also include distinctive fragments of partly welded hornblende-rich tuff (Larsen, 1994b), which have no surface analogs other than Cebolla Creek Tuff, further suggesting that eruptions from the San Luis caldera complex predated formation of Creede caldera.

Abundant clasts of silicified Willow Creek welding zone, derived from the Point of Rocks landslide breccia, are incorporated in conglomeratic beds of the Creede Formation exposed across the Rio Grande northwest of Point of Rocks (Steven and Ratté, 1965). Presence of these clasts documents that resurgent uplift had proceeded sufficiently vigorously to provide a voluminous source of erosional debris to the Creede Formation by the time sediments had accumulated to the topographic height of these river-level outcrops, ~350 m above the pre-resurgence caldera-fill level as constrained by the scientific drilling.

Fisher Dacite

Lavas and associated breccias of porphyritic dacite (62%-66% SiO2; 25%-35% plagioclase, biotite, clinopyroxene, hornblende, sparse large sanidine) completely fill the southern to southeastern moat of Creede caldera, and additional lavas are present in the eastern moat north of Wagon Wheel Gap, on the western flank of Snowshoe Mountain south of McCall Creek, and on the southwestern caldera wall near Cliff Creek (Fig. 4). In contrast to earlier calderas of the central cluster, postsubsidence eruptive activity was volumetrically subordinate to sedimentation as represented by the Creede Formation.

Much of the southern area previously mapped as Fisher Quartz Latite (renamed Fisher Dacite: Lipman, 2000) between Red Mountain and Goose Creeks (Steven et al., 1974) is now recognized as older South River volcanics. These lavas, which fill South River caldera, overlap in composition with the Fisher Dacite, and contacts between the two assemblages are uncertain in places. In addition, on the east flank of South River Peak, small erosional remnants of andesite that overlie outflow Snowshoe Mountain Tuff must be broadly correlative with the main areas of Fisher Dacite to the north, even though these lavas are lithologi-cally similar to the underlying South River volcanics.

The McCall Creek flow appears to have erupted early, on the flat caldera floor prior to uplift on the flank of the resurgent dome, as indicated by fairly uniform thickness of the lava, by downslope dips that are semi-conformable with the tilted underlying Snowshoe Mountain Tuff, and by its unconformable onlap by untilted sedimentary beds of the Creede Formation. This flow also yielded a relatively old 40Ar/39Ar age, 26.82 ± 0.05 Ma (Lipman and McIntosh, 2008). The Wagon Wheel Gap flow, which banks against the eastern topographic wall of Creede caldera, also erupted relatively early, as demonstrated by exposures down to present-day valley bottoms and by onlapping sediments of the Creede Formation.

Lavas in the southern moat have not been subdivided, but several flows are present, with vents likely present beneath or near surviving topographic highs such as Fisher and Copper Mountains. East of Copper Mountain, a sequence of multiple thick flows aggregating nearly 1 km of section is exposed virtually continuously down to Goose Creek, where early-erupted dacite lavas rest directly on rocks older than Creede caldera. The variable elevations of these flows and the lack of exposed inter-bedded Creede Formation indicate recurrent lava eruptions as the caldera moat filled. In the large cirque on the SE slopes of Fisher Mountain, two thick dacite flows are separated by bedded laharic breccias. These flows and breccias may be the youngest parts of the Fisher Dacite, based on their high topographic levels, above the highest known exposures of Creede Formation at ~10,900 ft at the head of Lime Creek, and their relatively young 40Ar/39Ar age (26.77 ± 0.04 Ma: Lipman and McIntosh, 2008).

A few nonwelded ignimbrites and tephra-fall tuffs of dacitic to rhyolitic composition intertongue with sediments of the Creede Formation both in surface exposures and in scientific drilling core. Individual ash-flow units are commonly less than 10 m thick (Larsen and Crossey, 1996, 2000; Heiken et al., 2000); they appear to be limited in areal extent and relatively small in volume. These likely represent explosive eruptions associated with the Fisher Dacite, based on their similar phenocryst mineralogy and absence of other identified eruptive vents. Prior to the Creede scientific drilling effort, it seemed possible that ignimbrites from the San Luis caldera complex might be present at depth in the caldera moat, interfingered with the Creede Formation, but the major San Luis tuff sheets are absent in the two Creede drill holes. The cobbles in the Bachelor paleovalley that are apparently derived from these tuff sheets provide additional evidence that the San Luis tuff sheets predate formation of Creede caldera.

Filling History of the Caldera Basin in Relation to Caldera Resurgence

Conglomeratic sediments of the Creede Formation lap unconformably onto resurgently tilted Snowshoe Mountain Tuff on flanks of the resurgent dome in several places. Dips are locally as steep as 10-15°, but are plausible as primary deposi-tional attitudes. No resurgently tilted bedded deposits have been confidently identified from stratigraphic levels above the Point of Rocks landslide deposit. Barton et al. (2000a) argued that resurgence was delayed based on the time required to erode the Bachelor paleovalley prior to lacustrine deposition. If landslide breccia of the Willow Creek unit in the CCM cores and at Point of Rocks (Lipman, 2000) originated, at least in part, from the Bachelor paleovalley, then this valley may have been created during caldera collapse, and little time would have been required for incision. Although sedimentation must have commenced before resurgence was complete, any such deposits have been stripped from exposed parts of the dome.

Caldera resurgence was insufficiently prolonged to permit erosion of the Point of Rocks landslide breccia and other breccia deposits prior to accumulation of Creede sediments to present-day river level, 350 m above the postcollapse caldera floor. Such rapid resurgence is consistent with well-constrained age data from several young caldera systems, which resurged within 100 k.y. or less after subsidence; these include Long Valley (Bailey et al., 1976), Yellowstone (Christiansen, 1984), Toba in Indonesia (de Silva et al., 2015), and Chegem in Russia (Lipman et al., 1993; Gazis et al., 1995).

Even if resurgence commenced immediately upon cessation of Snowshoe Mountain eruptions, rapid erosion of uncon-solidated ash blanketing the caldera basin 20-25 km in diameter probably could have kept the growing moat filled with sediment, maintaining pace with early resurgence. For example, erosion of an ash blanket only a few tens of meters thick, comparable to that interleaved with landslide breccia between the top of the welded Snowshoe Mountain Tuff and the main body of Creede Formation in the scientific drill holes, would have been volumetrically adequate to fill the entire Creede moat to the level of the present-day Rio Grande.

Relationship of Ancient Lake Creede to the Creede Ore Deposits

A major impetus for scientific drilling in Creede caldera was to test the hypothesized link between fluids formed during deposition of the Creede Formation and ore-forming processes (Bethke and Rye, 1979; Bethke and Lipman, 1987; Bethke and Hay, 2000). Data from the drill cores and surface studies of the Creede Formation generally support the hydrothermal deposition model (Fig. 13: Bethke and Rye, 1979; Barton et al., 2000b). The principal components of the hydrothermal model that require sources from the Creede Formation are: (1) water enriched in 18O, (2) bacterially reduced sulfur, (3) dissolved organic compounds, and (4) saline fluids. Isotopically enriched lake water is indicated by the stable isotopic composition of lacustrine carbonate and tufa; however, the enrichment in the ore fluids is less than that inferred from the carbonate isotope data (Rye et al., 2000). Pyrite in the Creede Formation shows evidence of extreme isotopic enrichment by bacterial processes (Bethke et al., 2000; Ilchik and Rumble, 2000), consistent with sulfur isotope values observed in the Creede mineral district. The carbonate deposition in the Creede Formation is attributed largely to algal and bacterial processes (Larsen and Crossey, 1996; Finkelstein et al., 1999), although brine shrimp were also important in the metabolic history of the organic matter. The salinity of the Lake Creede waters is poorly established and likely evolved through the depositional history; evidence from brine shrimp (Finkelstein et al., 1999), authigenic mineralogy (Larsen and Crossey, 2000), isotopic enrichment (Rye et al., 2000), and hydrologic balance (Barton et al., 2000a) strongly argues for increased salinity through time. Coarse-grained facies in the Creede Formation proximal to the margin of the caldera and extending basinward from the Bachelor paleovalley, as well as offset and deformation of Creede Formation strata along Rio Grande graben faults, provide ample conduits for fluid migration out of and into the basin (Larsen and Crossey, 2000); thus, permitting favorable pathways for hydro-thermal fluid flow.

Figure 13.

North-south cross section through the northern part of Creede caldera and Bachelor and San Luis calderas showing conceptual model for the Creede hydrothermal district (from Barton et al., 2000b). Arrows show hypothesized circulation of cool meteoric (M), cool lacustrine (L) pore waters, other water sources (R), and hot magmatic waters (P).

Figure 13.

North-south cross section through the northern part of Creede caldera and Bachelor and San Luis calderas showing conceptual model for the Creede hydrothermal district (from Barton et al., 2000b). Arrows show hypothesized circulation of cool meteoric (M), cool lacustrine (L) pore waters, other water sources (R), and hot magmatic waters (P).

Barton et al. (2000b) envision that a small pluton was intruded beneath the northern part of the Creede district 1.8 Ma after collapse of the caldera (Fig. 13). Intrusion may have been accompanied by tectonic adjustments along existing structures, perhaps including further displacements along the Creede graben, thus creating pathways for fluid migration in the volcanic rocks and within the Creede Formation. Heated groundwater and gaseous emanations from the pluton created a convecting plume of mixed waters comprising three major components: magmatic fluid expelled from magma, dilute meteoric water from adjacent highlands, and saline formational fluids from the Creede Formation. The descending saline formational fluids reacted with wall rocks along the flow path and then mixed with magmatic waters while rising buoyantly above the pluton. Although a remnant of ancient Lake Creede is illustrated in Figure 13, the lake need not have existed by the time of mineralization. The mixed waters rose above the pluton, and boiled and mixed with cool meteoric waters, driving the alteration of host rocks and precipitation of the Ag, Au, and base-metal ores. These mineralizing fluids ultimately flowed down-gradient into Creede caldera through the Bachelor paleovalley and along faults where they cooled further, resulting in disseminated mineralization in the coarse-grained deposits of the paleovalley and widespread silica and illite precipitation in upper parts of the Creede Formation (Larsen and Crossey, 2000). Although many details of the ore-forming process remain uncertain, such as the source of chlorine and the origin of radiogenic lead in the deposits (Barton et al., 2000b), the generalities of the hydrothermal model fit well with the field, petrological, and geochemical data.

Field-Trip Route

After the drive from Denver through the Front Range of the southern Rocky Mountains, to the northern Rio Grande rift at Buena Vista and continuing south into the San Luis Valley rift segment, the field-trip route (Fig. 6) obliquely transects central parts of the San Juan volcanic locus. Heading northwest from Del Norte at the west margin of the San Luis Valley, the trip follows a paleovalley up the present-day Rio Grande valley to South Fork, between constructional highs of several precaldera Conejos-age volcanoes. From South Fork, the route then follows the Rio Grande graben, an extensional feature related to regional uplift during growth of the volcanic field and inferred emplacement of an underlying composite batholith (Steven and Lipman, 1976). The canyon of the Rio Grande cuts a spectacular section through the east wall of the large La Garita caldera, the source of the Fish Canyon Tuff. At Wagon Wheel Gap, we enter Creede caldera, the best-preserved resurgent caldera of the San Juan volcanic field and the source of the 26.87-Ma Snowshoe Mountain Tuff. The trip follows the caldera moat northwest to the town of Creede, where we examine complex fill deposits within the caldera and structural features of the Creede mining district. Descriptions for several field-trip stops are modified from Lip-man et al. (1989). Published geologic maps especially pertinent to the field trip include the Del Norte and central San Juan maps, at scales of 1:48,000 and 1:50,000, respectively (Lipman, 1975, 2006). All GPS locations are based on NAD27 coordinates, as used on USGS 7.5’ topographic maps for the San Juan region.

■ Day 1. (En Route: Denver to South Fork) Introduction to Southern Rocky Mountain Volcanic Field

Depart Denver on U.S. 285 west toward Buena Vista, Colorado. Along U.S. 285 between mileposts 214 and 215 (east of Johnson Village), turn right on County Road 304 into Collegiate Peaks recreation site, to overlook at picnic area.

Stop 1.1. Rio Grande Rift-Sawatch Range Overlook (38°49.00’N, 106°05.17’W)

View of the Sawatch Range to the west, across the Upper Arkansas segment of the Rio Grande rift (Fig. 1.1). Directly ahead is Mount Princeton (4327 m [14,196 ft]). In the Sawatch Range, the Princeton batholith and adjacent upper-crustal granitoid plutons of late Paleogene age are interpreted as solidified late-stage magmatic phases for several early caldera-related regional ignimbrites of the SRMVF, including Wall Mountain Tuff (ca. 37 Ma) and Badger Creek Tuff (ca. 34 Ma).

Figure 1.1.

Upper Arkansas segment of Rio Grande rift and Sawatch Range. The rift valley is an asymmetrical graben, with the main bounding fault on the west side, at base of the Sawatch Range. High point is Mount Princeton (elev. 4327 m [14,196 ft]), central within the 35-37-Ma batholith that is inferred to underlie the now erosionally removed caldera source of the 37-Ma Wall Mountain Tuff. Photograph by Peter Lipman.

Figure 1.1.

Upper Arkansas segment of Rio Grande rift and Sawatch Range. The rift valley is an asymmetrical graben, with the main bounding fault on the west side, at base of the Sawatch Range. High point is Mount Princeton (elev. 4327 m [14,196 ft]), central within the 35-37-Ma batholith that is inferred to underlie the now erosionally removed caldera source of the 37-Ma Wall Mountain Tuff. Photograph by Peter Lipman.

Return to U.S. 285 and continue south to Del Norte, via CO 112.

Outflow Ignimbrite Stratigraphy: Del Norte to South Fork

This trip segment provides a brief introduction to the regional stratigraphy of the central San Juan region where well-stratified outflow ignimbrite sheets thinly veneer thick accumulations of intermediate-composition lava and breccia (Conejos Formation) that erupted from multiple stratovolca-noes. An initial focus is on the characteristic end-member types of ignimbrite in the San Juan Mountains: compositionally uniform crystal-rich dacite (Fish Canyon Tuff) and crystal-poor rhyolite (Carpenter Ridge Tuff), underlain by Conejos andesitic lavas along the east margin of the San Juan Mountains. At the village of South Fork, views provide perspectives on interfin-gering of ignimbrites from the Platoro caldera complex with those from the central caldera cluster.

Route begins in Del Norte, Colorado (2403 m [7884ft]), at junction of U.S. 160 and CO 112 (Fig.1.2A). Proceed north 3.1 mi on CO 112 (Oak St.). Turn left (north) onto Rio Grande County Road (RG) 33 and proceed 1.2 mi to gravel track (5203) on the left that leads to outcrops (Elephant Rocks).

Figure 1.2.

Fish Canyon Tuff (FCT) at Elephant Rocks, along west margin of San Luis Valley. (A) Geologic map of Del Norte area (Lipman, 1976). Southeast flank of Summer Coon volcano: Tsa—andesite breccia; Tsd—dacite lava; Tsi—intermediate-composition dike; Tsri—rhyolite dike. Conejos Formation: Tcv—andesitic lavas of uncertain source; Tvs—volcanic sandstone and conglomerate. Ignimbrites from central caldera cluster: Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff. Ignimbrites from Platoro caldera complex: Ttc—Chiquita Peak Tuff; Ttr—Ra Jadero Tuff. Surficial deposits: Qal—alluvium; Qc—colluvium; Qf—alluvial-fan deposits; Qo—glacial outwash deposits. (B) Fish Canyon Tuff at Elephant Rocks. Rounded granitoid-appearing outcrops are typical of this ignimbrite where a moderately welded outflow sheet crops out in weathered exposure. View to west. Photograph by Kenzie Turner.

Figure 1.2.

Fish Canyon Tuff (FCT) at Elephant Rocks, along west margin of San Luis Valley. (A) Geologic map of Del Norte area (Lipman, 1976). Southeast flank of Summer Coon volcano: Tsa—andesite breccia; Tsd—dacite lava; Tsi—intermediate-composition dike; Tsri—rhyolite dike. Conejos Formation: Tcv—andesitic lavas of uncertain source; Tvs—volcanic sandstone and conglomerate. Ignimbrites from central caldera cluster: Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff. Ignimbrites from Platoro caldera complex: Ttc—Chiquita Peak Tuff; Ttr—Ra Jadero Tuff. Surficial deposits: Qal—alluvium; Qc—colluvium; Qf—alluvial-fan deposits; Qo—glacial outwash deposits. (B) Fish Canyon Tuff at Elephant Rocks. Rounded granitoid-appearing outcrops are typical of this ignimbrite where a moderately welded outflow sheet crops out in weathered exposure. View to west. Photograph by Kenzie Turner.

Stop 1.2. Fish Canyon Tuff (FCT) at Elephant Rocks (37°43.75’N, 106°18.64’W)

This stop illustrates typical exposures of the distinctive crystal-rich dacitic FCT ignimbrite (Fig. 1.2B) of enormous volume (~5000 km3) erupted at 28.02 Ma from La Garita caldera; the FCT is compositionally uniform in bulk-sample scale but contains a crystal assemblage that is exceptional mineralogically and chemically complex (Bachmann et al., 2002; Charlier et al., 2007). The outflow sheet is a single large cooling unit of light-gray to tan tuff, still preserved as much as 125 km beyond rims of La Garita caldera. It is characterized by a simple welding zona-tion except in thick proximal sections, especially in the South Fork area, where compound welding zones are well developed (as many as six cooling subunits mapped locally: Lipman, 2006). At this and other exposures, the pumice fragments are difficult to discern and lithic fragments typically are sparse. Densely welded tuff is massive, weathering to rounded granitoid-like outcrops, but upper less-welded parts commonly have slabby jointing parallel to weakly developed compaction foliation. Despite the enormous volume of this ignimbrite, its bulk composition is uniformly silicic dacite (66%-68% SiO2; 40%-50% pl>>sn, bi>hbl: Lipman, 1975; Whitney and Stormer, 1985; Bachmann et al., 2002). Sparse resorbed pinkish quartz, accessory sphene, and hornblende without augite are a distinctive phenocryst assemblage among ignimbrites erupted from the San Juan region. Sani-dine phenocrysts (typically Or72-74Ab23-24An1Cs15) are relatively potassic and conspicuously zoned in Ba, with Cs varying 1-4 mol%. Other crystal-rich dacites erupted from San Juan calderas contain clinopyroxene with little or no hornblende, lack quartz or titanite, and most also lack sanidine. Among all ignimbrites in the SRMVF, only the much older Badger Creek Tuff (34.1 Ma), erupted from the Mount Aetna caldera in the Sawatch Range to the north (Fig. 1), petrologically resembles the Fish Canyon Tuff.

Return to RG 33 and continue north 1.3 mi, to junction with Rio Grande-A; turn right (east) and proceed ~0.9 mi to Stop 1.3.

Stop 1.3. Basal Vitrophyre of Crystal-Poor Carpenter Ridge Tuff (CRT) (37°44.89’N, 106°17.45’W)

This stop at a ridge-crest roadcut shows representative outflow of this dominantly phenocryst-poor rhyolitic ignimbrite of large volume (~1000 km3), erupted from Bachelor caldera at 27.55 Ma. The CRT is typically densely welded, widely characterized by a conspicuous basal vitrophyre (individual glass shards discernable by hand lens at this outcrop), and in places by a central lithophysal zone. Much of the outflow ignimbrite sheet is uniform crystal-poor rhyolite, densely welded nearly to its base, but in places, it contains more complex welding and compositional zonations, especially within Bachelor caldera. The volumetrically dominant rhyolite contains 3%-5% phenocrysts (pl>sn>>bi); in its caldera and locally elsewhere, it grades upward to silicic dacite and dacite (67%-74% SiO2) containing as much as 30% pheno-crysts. Sanidine compositions are less potassic than in the FCT and somewhat variable, mostly Or62-67Ab35-31An2-1Cs1-4 (Whitney et al., 1988; Riciputi, 1991). The CRT is the only densely welded crystal-poor rhyolite of large volume erupted from the central caldera cluster, but it can be megascopically difficult to distinguish with confidence from lithologically similar regional rhyolites that erupted from western calderas (Blue, Dillon, and Sapinero Tuffs) and northeastern calderas (Thorn Ranch, rhyolitic Bonanza, and Saguache Creek Tuffs). Outflow CRT rhyolite as seen here can be compared with its depositional equivalent deep within Bachelor caldera, as exposed above the town of Creede.

Retrace route ~6.5 mi to Del Norte and continue west 0.4 mi on U.S. 160, to the large outcrop south of the road at the western edge of the town. Access is from the parking lot at the visitor center.

Stop 1.4. Conejos Formation: Hornblende-Andesite Lava (Optional) (37°40.73’N, 106°21.65’W)

This stop features a representative lava of the Conejos Formation, on the distal flank of a volcanic center near the northeast rim of the Platoro caldera complex to the south. The regional Conejos Formation consists dominantly of andesite and dacite lavas and proximal breccia erupted from central volcanoes, surrounded by voluminous aprons of volcaniclastic debris emplaced as mudflow and stream-fan deposits (Lipman et al., 1970; Lipman, 1975; Colucci et al., 1991). These thick accumulations of intermediate-composition lava and breccia, which underlie the caldera-related ignimbrite sequence that dominates the central caldera cluster of the San Juan Mountains, constitute the volumetric bulk of the San Juan volcanic locus. Subsequently deeply eroded, these early eruptive deposits are now discontinuously exposed as surviving topographic highs along margins of the ignimbrite calderas. Compositional subunits among Conejos Formation lava have been mapped in varying detail, depending on degree of local diversity and quality of exposures, but remain much less studied than the overlying ignimbrites. Proximal lavas tend to become more silicic upward, with biotite-bearing dacite and low-silica rhyolite especially abundant on the northeast side of La Garita caldera. Northern units of the Conejos Formation, which underlie the tuff of Saguache Creek (32.20 Ma) and the Bonanza Tuff (33.12 Ma) northeast of the central caldera cluster, are mostly older than the widespread Conejos Formation rocks to the south, which are mainly 33-30 Ma (Lipman et al., 1970; Lipman and McIntosh, 2008). Maximum exposed thickness of Conejos Formation rocks is ~800 m along south margins of the central caldera cluster; sections as thick as 2.3 km have been penetrated by petroleum exploration drilling in the San Luis Valley to the east (Gries, 1985; Brister and Gries, 1994).

Continue west along the Rio Grande on U.S. 160,16 mi to South Fork, Colorado.

The lower slopes on both sides of the Rio Grande valley are lavas and volcaniclastic rocks of the Conejos Formation. Higher on the heavily vegetated south side are ignimbrites of the Treasure Mountain Group, erupted from the Platoro caldera complex. To the north, most hills (Twin Peaks especially conspicuous) are southern flanks of the Summer Coon and Embargo Creek volcanoes (ca. 33-32 Ma: Lipman, 1968; Parker et al., 2005; Poland et al., 2008). Approaching South Fork, the prominent high mesa to the north (Agua Ramon Mountain) is capped by cliffs of Fish Canyon Tuff, above Chiquita Peak Tuff (from Platoro) and underlying Conejos lavas.

At South Fork, bear right on CO 149; just after crossing Rio Grande bridge, turn right on Hwy. 25 (River Club Dr.). Continue 0.9 mi to entry of River Club condominiums to Stop 1.5.

Stop 1.5. South Fork Overview (37°40.59’N, 106°38.27’W)

Stratigraphic and structural relations between outflow ignimbrites erupted from the Platoro caldera complex and those from the central San Juan caldera cluster (Fig. 1).

Adjacent cliff outcrop is brecciated flow of plagioclase andesite (Conejos Formation). Palisades to the northwest (up the Rio Grande valley) are thick Masonic Park and Fish Canyon Tuffs (Fig. 1.3A). The Rio Grande follows faults of a northwest-trending graben system that drops Fish Canyon Tuff on the southwest side of the river down against Masonic Park Tuff on the north. The South Fork of Rio Grande marks a narrow depositional corridor along which crystal-rich dacitic ignimbrites from Platoro caldera interfinger with and wedge out against Masonic Park Tuff; at ca. 28.7 Ma, the initial dacitic ignimbrite erupted from the central caldera complex (Lipman et al., 1996). To the northeast and southeast of South Fork village, Chiquita Peak Tuff is the main ignimbrite beneath the Fish Canyon Tuff, but the Chiquita Peak wedges out deposi-tionally against distal Masonic Park Tuff. The low cliffs to the south across the mouth of the South Fork are Conejos Formation breccia, overlain by a thin wedge of distal Masonic Park Tuff and then by Chiquita Peak Tuff. Older ignimbrites of the Treasure Mountain Group that occur beneath these two units to the south and east are absent this far northwest. The prominent ridge nose northeast of the town (Point Baxter: 2850 m [9349 ft]) contains an informative well-exposed section: several thick welding zones of Masonic Park Tuff are overlain by thin Chiquita Peak Tuff that thins and completely wedges out only 1.5 km farther along the ridge, where it is capped by Fish Canyon Tuff (Fig. 1.3B).

Figure 1.3.

South Fork overview. (A) Geologic map of South Fork area (from Lipman, 2006). Units: Tca— andesitic lavas of the Conejos Formation; Tfc—Fish Canyon Tuff; Tmp— Masonic Park Tuff (dotted lines indicate partial cooling breaks 1-3); Ttc—Chiq-uita Peak Tuff; Tts—South Fork Tuff; Qal— alluvium; Qc—colluvium; Ql— landslide deposits. (B) View of Point Baxter, ridge northwest of South Fork village, showing boundary zone between similar-appearing dacitic ignimbrites (Masonic Park, Chiquita Peak Tuffs). Along Point Baxter ridge, thin Chiquita Peak Tuff wedges out just to the northwest, between thick underlying Masonic Park and overlying Fish Canyon Tuffs. In contrast, Masonic Park Tuff thins and ends abruptly southeast of South Fork, and these two ignimbrites overlap only along a narrow corridor a few km wide, wherever exposed, all the way south beyond Wolf Creek Pass. Photograph by Peter Lipman.

Figure 1.3.

South Fork overview. (A) Geologic map of South Fork area (from Lipman, 2006). Units: Tca— andesitic lavas of the Conejos Formation; Tfc—Fish Canyon Tuff; Tmp— Masonic Park Tuff (dotted lines indicate partial cooling breaks 1-3); Ttc—Chiq-uita Peak Tuff; Tts—South Fork Tuff; Qal— alluvium; Qc—colluvium; Ql— landslide deposits. (B) View of Point Baxter, ridge northwest of South Fork village, showing boundary zone between similar-appearing dacitic ignimbrites (Masonic Park, Chiquita Peak Tuffs). Along Point Baxter ridge, thin Chiquita Peak Tuff wedges out just to the northwest, between thick underlying Masonic Park and overlying Fish Canyon Tuffs. In contrast, Masonic Park Tuff thins and ends abruptly southeast of South Fork, and these two ignimbrites overlap only along a narrow corridor a few km wide, wherever exposed, all the way south beyond Wolf Creek Pass. Photograph by Peter Lipman.

Return to CO 149, and continue west along the Rio Grande valley 3.7 mi to Stop 1.6.

Several major faults of the Rio Grande graben system follow the valley. As a result, the welded tuffs on the southwest side are entirely Fish Canyon Tuff, but on the northeast side the lower two-thirds of the slope is uplifted Masonic Park Tuff, capped by Fish Canyon Tuff. The Rio Grande graben is among several extensional fault systems peripheral to the central caldera complex, interpreted as related to modest late uplift above a solidifying subvolcanic batholith centered beneath the calderas (Steven and Lipman, 1976).

Stop 1.6. Rio Grande Canyon: La Garita and Creede Caldera Walls, Masonic Park Tuff (37°41.91’N, 106°41.69’W)

Pull off on the right, by the Rio Grande Forest entry sign, for a view of the eastern topographic wall of La Garita caldera and stratigraphic relations between ignimbrite sheets (Fig. 1.4). The eastern topographic wall descends the broad side valley ahead to the northwest. On the caldera rim above, Fish Canyon Tuff, forming the uppermost major columnar-jointed cliff, rests on thick Masonic Park Tuff, which shows compound cooling of alternating welded ledges and less welded benches. To the west, three younger ignimbrite sheets are ponded within La Garita caldera.

Figure 1.4.

Rio Grande Canyon: La Garita and Creede caldera walls, Masonic Park Tuff. (A) Geologic map of Rio Grande canyon (from Lipman, 2006). Units: Tbc—Blue Creek Tuff; Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff; Tmp—Masonic Park Tuff (dotted lines indicate partial cooling breaks; Tw—Wason Park Tuff; Qc— colluvium; Ql—landslide deposits; Qt—talus. (B) East topographic wall of La Garita caldera, in Rio Grande canyon between South Fork and Creede, which truncates multiple cooling units of flat-lying Masonic Park Tuff and proximal outflow of Fish Canyon Tuff. Caldera fill includes slump block of Fish Canyon Tuff (FC), and depositionally overlying Blue Creek, Carpenter Ridge (not labeled), and Wason Park Tuffs that wedge out against caldera wall. View to northwest. Photograph by Peter Lipman.

Figure 1.4.

Rio Grande Canyon: La Garita and Creede caldera walls, Masonic Park Tuff. (A) Geologic map of Rio Grande canyon (from Lipman, 2006). Units: Tbc—Blue Creek Tuff; Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff; Tmp—Masonic Park Tuff (dotted lines indicate partial cooling breaks; Tw—Wason Park Tuff; Qc— colluvium; Ql—landslide deposits; Qt—talus. (B) East topographic wall of La Garita caldera, in Rio Grande canyon between South Fork and Creede, which truncates multiple cooling units of flat-lying Masonic Park Tuff and proximal outflow of Fish Canyon Tuff. Caldera fill includes slump block of Fish Canyon Tuff (FC), and depositionally overlying Blue Creek, Carpenter Ridge (not labeled), and Wason Park Tuffs that wedge out against caldera wall. View to northwest. Photograph by Peter Lipman.

High along the distant ridge crest is thick columnar-jointed Wason Park Tuff that ponded within the caldera wedges out against the Fish Canyon at about the same elevation as the similarly columnar Fish Canyon Tuff capping the nearer ridge along the topographic wall. Beneath the Wason Park, another thick dacitic ignimbrite sheet, the Blue Creek Tuff, wedges against the petrologically similar Masonic Park units. The Blue Creek here is another phenocryst-rich silicic dacite, containing 25%-45% phenocrysts of plagioclase, biotite, and augite—petrographically similar to the adjacent Masonic Park Tuff. Both ignimbrites are also characterized by crude layering, due to combined effects of compound cooling and subtle internal flow-unit discontinuities. The Blue Creek Tuff was long confused with the dacitic Mammoth Mountain upper part of the Carpenter Ridge Tuff (Ratté and Steven, 1967; Steven et al., 1974; Lipman et al., 1989), but presence of a basal vitrophyre at this site and elsewhere, as well as absence of sanidine phenocrysts, demonstrates that it is a discrete ignimbrite sheet (Lipman, 2000). Underlying the Blue Creek Tuff, though difficult to see from this vantage, is a thin lens of rhyolitic Carpenter Ridge Tuff, which provides definitive age control on timing of the caldera-wall unconformity in this side valley. As additional critical evidence of caldera-wall complexity, perched anomalously lower along the caldera wall is a shattered mass of outflow Fish Canyon Tuff, ~1 km across, and interpreted to have slumped partway down the wall during collapse (Fig. 1.4).

End of Day 1 trip. Retrace route to South Fork, Colorado.

■ Day 2. Creede Caldera and Ancient Lake Creede

Start.Junction of Highways U.S. 160 and CO 149 in South Fork, Colorado. Set odometer to 0.0 km (0.0 mi.). Turn on CO 149 and drive to Creede, Colorado.

(4.8 km, 3.0 mi.) Masonic Park Tuff forms the cliffs to the right.

(13.7 km, 8.5 mi.) Roadcut in prominent bend is Masonic Park Tuff. Overlying bench is Blue Creek Tuff against Masonic Park Tuff on the La Garita caldera wall.

(16.4 km, 10.2 mi.) Lake beds of La Garita caldera are on the right. We will visit this location on Day 3 of the field trip.

(17.5 km, 10.9 mi.) Thick section of Blue Creek Tuff is on the northeast side of Blue Creek; tuff is ponded within La Garita caldera. Ahead to the right is the Wagon Wheel Gap lava dome on the east wall of Creede caldera.

(20.6 km, 12.8 mi.) Wagon Wheel Gap lava dome is on the right. Ahead in the valley are low hills of the caldera-filling moat sediments of the Creede Formation.

(27.8 km, 17.3 mi.) Crossing the Rio Grande at Wason Ranch.

(32.2 km, 20.0 mi.) Entering Creede, Colorado.

(33.0 km, 20.5 mi.) Intersection of CO 149 with Mineral County Rd. 504. Turn right and ascend through the low hills of the Creede Formation up to the intersection of the Bachelor paleo-valley with Creede caldera wall. Continue 2.4 mi on County Rd. 504 to intersection with Forest Service Rd. 504.1B.

Stop 2.1. Panoramic View of Resurgent Creede Caldera (37°51.67’N, 106°56.63’W)

(36.9 km, 22.9 mi.) At this stop we are looking south toward Snowshoe Mountain (3411 m [11,192 ft]), from the mouth of the Bachelor paleovalley along the northern margin of Creede caldera. Snowshoe Mountain, the high point on the Creede resurgent dome (Fig. 2.1), is surrounded by a topographically low moat followed by the Rio Grande, which has excavated tuffaceous sediments and lake beds of the Creede Formation. Flanks of Snowshoe Mountain are dip slopes of as much as 45° in the 26.87-Ma Snowshoe Mountain Tuff. Below is the town of Creede and the Bulldog Mountain mine. In the far distance is Fisher Mountain (3919 m [12,858 ft]), capped by late postcaldera lavas (27.77 Ma) of Fisher Dacite. The entire northern half of the Creede caldera wall is visible, extending from Bristol Head (3873 m [12,706 ft]) on the west, through the north wall where we are standing, around to the east at Wagon Wheel Gap.

Figure 2.1.

Panoramic view of resurgent Creede caldera. Creede caldera, as viewed from its north wall (Bachelor Road, at Windy Gulch). Baldy Mountain (elev. 3806 m [12,488 ft]) is on southwest wall; Wagon Wheel Gap is at east wall. Fisher Mountain (elev. 3921 m [12,865 ft]) is sequence of postcaldera lavas (Fisher Dacite) on south wall. Snowshoe Mountain is high point on resurgently uplifted caldera floor, defining a symmetrical dome of intracaldera Snowshoe Mountain Tuff, with a keystone graben outlined by northward drainages, and with flanks dipping as steeply as 45° (Steven and Ratté, 1965). Rio Grande flows into caldera moat from the west, arcs around north side of the resurgent dome, and exits at Wagon Wheel Gap. The river has preferentially eroded weakly indurated moat-fill sediments of the Creede Formation, exhuming much of the Oligocene caldera morphology. Town of Creede in lower left of photo. View to south. Photograph by Peter Lipman.

Figure 2.1.

Panoramic view of resurgent Creede caldera. Creede caldera, as viewed from its north wall (Bachelor Road, at Windy Gulch). Baldy Mountain (elev. 3806 m [12,488 ft]) is on southwest wall; Wagon Wheel Gap is at east wall. Fisher Mountain (elev. 3921 m [12,865 ft]) is sequence of postcaldera lavas (Fisher Dacite) on south wall. Snowshoe Mountain is high point on resurgently uplifted caldera floor, defining a symmetrical dome of intracaldera Snowshoe Mountain Tuff, with a keystone graben outlined by northward drainages, and with flanks dipping as steeply as 45° (Steven and Ratté, 1965). Rio Grande flows into caldera moat from the west, arcs around north side of the resurgent dome, and exits at Wagon Wheel Gap. The river has preferentially eroded weakly indurated moat-fill sediments of the Creede Formation, exhuming much of the Oligocene caldera morphology. Town of Creede in lower left of photo. View to south. Photograph by Peter Lipman.

In the far distance southeast is North Mountain (3878 m [12,723 ft]), at Summitville, Colorado, consisting of thick dacite flows and shallow intrusions (as young as 20.7 Ma) adjacent to the north margin of Platoro caldera. In the far distance southwest is Baldy Mountain (2806 m [12,488 ft]), the top of a thick lava/breccia pile of Huerto Andesite that filled the southwestern part of La Garita caldera. The drainage of Deep Creek, conspicuous on the north side of Snowshoe Mountain, follows the down-dropped keystone graben on the crest of the Snowshoe Mountain dome. The large outcrops on the Bachelor caldera wall, to the east, are the Willow Creek zone of Bachelor Mountain Member, representing a small part of the thick intracaldera accumulation of the Carpenter Ridge Tuff within Bachelor caldera.

After the stop, return downhill to the town of Creede.

(40.7 km, 25.3 mi.) Intersection of Mineral County Rd. 504 with CO 149, turn right, passing many exposures of the Creede Formation along the way.

(50.4 km, 31.3 mi.) Seven-Mile Bridge and intersection with Middle Creek Rd. (Mineral County Rd. 523). Rugged cliff on left is Point of Rocks. Turn left (south) onto Middle Creek Rd.

Stop 2.2. Top of Intracaldera Snowshoe Mountain Tuff at Point of Rocks (37°47.53’N, 106°58.82’W)

(50.7km, 31.5 mi.) Park vehicles carefully along road shoulder, without blocking traffic. Outcrops along this road display partly to densely welded tuff at the top of the thick intracaldera accumulation of Snowshoe Mountain Tuff on the northwest flank of resurgent dome; this stratigraphic level of the intracaldera ignimbrite is correlative with units penetrated at the bottoms of the two Creede scientific drill holes from 1991. The Snowshoe Mountain here displays crudely bedded meter-scale variations in lithic content and pumice abundance that probably result from fluctuating energetics during waning of the ignimbrite eruption. Sparse sanidine crystals from this dacitic tuff (67.6% SiO2) yielded a single-crystal 40Ar/39Ar age (26.91 ± 0.15 Ma: Lipman and McIntosh, 2008).

The overly rugged outcrops of white silicic rhyolite (76% SiO2), locally known as “Point of Rocks” (Fig. 2.2), were previously interpreted as a brecciated post-caldera rhyolitic lava dome (Steven and Ratté, 1965, p. 43), but these rocks are the most spectacularly exposed landslide breccia in Creede caldera. They form a resurgently tilted and slightly faulted layer of silicified monolithologic breccia, consisting solely of crystal-poor welded tuff derived from the Willow Creek welding zone of intracaldera Carpenter Ridge Tuff. Flattened pumice textures within breccia clasts are difficult to discern in the most accessible exposures at Point of Rocks, but are readily visible nearby. Between the rhyo-lite breccia layer and weakly welded massive intracaldera Snow-shoe Mountain Tuff at Point of Rocks is a thin intervening lens of brecciated phenocryst-rich dacite. The brecciated dacite was likely derived by sliding from a lava of uncertain location higher on the topographic rim.

Figure 2.2.

Top of intracaldera Snowshoe Mountain Tuff, at Point of Rocks. Upper part of intracaldera Snowshoe Mountain Tuff (Tsm), in partial fault contact with caldera-landslide breccia consisting of monolithologic clasts of intracaldera Carpenter Ridge Tuff (Willow Creek welding zone), on northwest flank of Creede resurgent dome at Point of Rocks. Above treeline in distance are La Garita Mountains, exposing the resurgently uplifted block of intracaldera Fish Canyon Tuff. Photograph by Peter Lipman.

Figure 2.2.

Top of intracaldera Snowshoe Mountain Tuff, at Point of Rocks. Upper part of intracaldera Snowshoe Mountain Tuff (Tsm), in partial fault contact with caldera-landslide breccia consisting of monolithologic clasts of intracaldera Carpenter Ridge Tuff (Willow Creek welding zone), on northwest flank of Creede resurgent dome at Point of Rocks. Above treeline in distance are La Garita Mountains, exposing the resurgently uplifted block of intracaldera Fish Canyon Tuff. Photograph by Peter Lipman.

Return toward Creede on CO 149, to Antlers Rio Grande Lodge; turn left into the resort and park.

Stop 2.3. Creede Formation Exposures at Antlers Park (37°48.84’N, 106°58.59’W)

(53.6 km, 33.3 mi.) At this location, we will see spectacular cliff exposures of the Creede Formation along the Rio Grande. If permitted by the owners of the lodge (you must obtain permission in advance if you are taking this trip on your own), walk across the swinging bridge to the west side of Rio Grande and follow the trail up the hill. Stop along the trail to examine rubbly exposures of Creede Formation and walk ~0.36 km (0.22 mi) to the top of hill to the overlook. Access to the cliffs isnotoriously treacherousand will not be done unless river-level access is permitted. Stratigraphic section AP-1 (Figs. 7 and 8) was measured along the cliffs. Typical facies include laminated tuffaceous siltstone and limestone, turbiditic sandstone, and pebbly sediment-gravity-flow deposits (Fig. 2.3), along intervals of slump folded deposits and debris-flow deposits formed therefrom. We may have time to walk up the hill to view low exposures of travertine with excellent thinolite preservation.

Figure 2.3.

Creede Formation strata at section AP-1. Lensoid package of pebbly sediment-gravity-flow deposits (pinching out toward Rio Grande) and overlying turbiditic tuffaceous sandstone and laminated tuffaceous siltstone. Rock hammer for scale (circled). Photograph by Gary A. Smith.

Figure 2.3.

Creede Formation strata at section AP-1. Lensoid package of pebbly sediment-gravity-flow deposits (pinching out toward Rio Grande) and overlying turbiditic tuffaceous sandstone and laminated tuffaceous siltstone. Rock hammer for scale (circled). Photograph by Gary A. Smith.

Return to vehicle and drive back to CO 149.

(53.9 km, 33.5 mi.) Turn left on CO 149.

Stop 2.4. View of Antlers Park (Lunch Stop) (37°49.04’N, 106°58.27’W)

(54.4 km, 33.8 mi.) Drive down the road to the building (or as close as access permits). This is the site of the CCM-1 borehole. The borehole penetrated through the Creede Formation and terminated in the underlying Snowshoe Mountain Tuff at a depth of 418.3 m. Core recovery was excellent (>99%). The borehole was logged with nine geophysical tools prior to borehole closure (Larsen and Nelson, 2000). The core is currently housed at the U.S. Geological Survey Core Research Center in Denver (see Latysh, this volume; high-resolution core photos are available at https://my.usgs.gov/crcwc/). Also visible from this location are the lensoidal packages of coarse-grained sediment forming lacustrine lobes (Fig. 2.4) (Larsen and Smith, 1999). The view affords a nice location for lunch.

Figure 2.4.

Sketch of Antlers Park cliffs showing three prominent systems of sublacustrine-fan lobes (from Larsen and Smith, 1999).

Figure 2.4.

Sketch of Antlers Park cliffs showing three prominent systems of sublacustrine-fan lobes (from Larsen and Smith, 1999).

Return to CO 149.

(54.6 km, 33.9 mi.) Turn left on CO 149.

Stop 2.5. Creede Formation Exposures at Airport Hill (37°49.75’N, 106°55.10’W)

(59.0 km; 37.1 mi.) Exposures along the road comprise mostly laminated tuffaceous siltstone and limestone, turbiditic tuffaceous sandstone, and debris-flow and slump deposits derived from fine-grained lacustrine strata. Some of the longest pseudo-morphs after ikaite were recovered in this area with lengths as long as 10 cm (Fig. 2.5A). Microstromatolites are present on some laminae surfaces. This location is also known as “Fossil Butte” for the abundance of evergreen, shrub, and rare deciduous tree and insect fossils found along the laminae surfaces (Fig. 2.5B). These exposures and those overlying are locally folded and faulted due to post-depositional Creede graben faults. Being within the graben, the strata in sections CW-2 and CW-4 (Fig. 8) are some of the youngest exposures of Lake Creede lacustrine strata and record the upper part of period III discussed by Rye et al. (2000). Overlying strata on the low hills to the north of section CW-4 contain channelized deposits, ripple cross-laminated tuffaceous sandstone, and Gilbert delta foresets, reflecting low to intermediate lake levels above the M tuff. Due south of Airport Hill, ~0.4 km (0.25 mi) is the location of the CCM-2 borehole. The borehole penetrated to a depth of 708.2 m and terminated in the underlying Snowshoe Mountain Tuff (Larsen and Nelson, 2000); core recovery was also >99%. A suite of nine geophysical tools was used to log the borehole prior to the hole being filled.

Figure 2.5.

Creede Formation strata at Airport Hill (Fossil Butte). (A) Elongate calcite pseudomorphs after ikaite in tuffaceous siltstone (lens cap is 6 cm in diameter). (B) Tuffaceous siltstone with conifer and deciduous shrub fossils on partings (pen is 14 cm long). Photographs by Dan Larsen.

Figure 2.5.

Creede Formation strata at Airport Hill (Fossil Butte). (A) Elongate calcite pseudomorphs after ikaite in tuffaceous siltstone (lens cap is 6 cm in diameter). (B) Tuffaceous siltstone with conifer and deciduous shrub fossils on partings (pen is 14 cm long). Photographs by Dan Larsen.

Return to vehicle and drive east on CO 149 through Creede.

(65.3 km, 40.6 mi.) Intersection of CO 149 with Forest Service Rd. 801. Turn left.

(65.5 km, 40.7 mi.) Bear right at Forest Service Rd. 801 intersection. Low hills north of Forest Service Rd. expose multiple tuffs, of which the I and J tuffs are the thickest white bands observed from the road. The beds dip to the southwest and are offset downthrown to the west along a major fault that follows Dry Gulch. Several minor faults, some down to east and others down to west, offset the beds on these low hills.

Stop 2.6. Creede Formation Tufa and Strata along Caldera Margin (37°49.68’N, 106°53.34’W)

(67.6 km; 42.0 mi.) Park at Farmer’s Creek trailhead. Walk along Farmer’s Creek Trail to the east ~1.0 km (0.6 mi.) and then hike up hills to tufa exposures along hills. The lower 4 m of exposed tufa was likely deposited during the time of the H tuff (a deformed white ash is locally present in the tufa); the white ash bed overlying the travertine on the hill is the I tuff. Much of the lower part of the tufa is laminated facies with intervening tufa breccia beds and channel fill. Some of the laminated tufa has desiccation features and teepee-like structures; thinolite crystals are rare and only present in the upper 1.5 m. The upper 5 m of exposed tufa is mostly massive breccia with local pockets of radiating thinolite crystals (Fig. 2.6), as much as 20 m long. The observations in the tufa at this location are consistent with the lower part of the tufa being deposited during the first lowstand of Lake Creede, with localized exposure and channeling of the laminated tufa facies and general absence of thinolite. Increasing lake level during formation of the upper part of the tufa is evident by common thinolite and massive base of slope tufa breccia.

Figure 2.6.

Thinolitic travertine in Creede Formation near Farmer’s Creek Trail. (A) Radiating geometry of clusters of pseudomorphs (lens cap is 6 cm in diameter). (B) Close-up image from the same outcrop showing well-preserved pseudomorphic crystal geometry. Photographs by Dan Larsen.

Figure 2.6.

Thinolitic travertine in Creede Formation near Farmer’s Creek Trail. (A) Radiating geometry of clusters of pseudomorphs (lens cap is 6 cm in diameter). (B) Close-up image from the same outcrop showing well-preserved pseudomorphic crystal geometry. Photographs by Dan Larsen.

Return to the trail and hike to the top of the first rise to examine strata in the gully. This is section WR-3, which exposes strata below tuff I through tuff L and records a period of generally higher lake levels and the modulation of the lake chemistry from period II to III (Rye et al., 2000). The lower 55 m of this section are mainly laminated tuffaceous siltstone and limestone, turbiditic tuffaceous sandstone, and debris-flow and slump deposits comprised of fine-grained lacustrine facies; however, intervals of pebbly sediment gravity-flow deposits are present from 40 to 55 m. Overlying a covered interval, laminated tuffa-ceous siltstone and limestone, turbiditic sandstone, and pebbly sediment gravity flows are present interbedded with wave-rippled tuffaceous sandstone and bedded limestone. The latter facies is associated with the start of the lake-level fall prior to the L tuff, which is present as a thin resistant bed at the top of the exposure. Pseudomorphs after ikaite increase in abundance upward in the section at the expense of finely laminated limestone.

Return to vehicle at trailhead and drive back to CO 149 on Forest Service Rd.

(69.8 km, 43.4 mi.) Intersection of Forest Service Road 801 with CO 149. Turn left.

Stop 2.7. Basin Center Exposures of Creede Formation, Intruded by Miocene Hinsdale Dike (37°49.26’N, 106°53.29’W)

(71.5 km; 44.4 mi.) Park on the right side of the road and hike along the river to the west to the cliff exposures. At this location, basin central strata including the I tuff and overlying lacustrine deposits are exposed and intruded by a mafic dike of the Miocene Hinsdale Formation. The Creede Formation strata on either side of the dike cannot be correlated, indicating that offset occurred during intrusion or that the intrusion followed a preexisting fault. The I tuff is exposed at the base of the section and is 7 m thick, comprising a series of thin to thickly bedded turbiditic tuff interbedded with laminated tuff (Fig. 2.7A). The base of the I tuff is not exposed, indicating that it may be much thicker than the measured 7 m. The I tuff is directly overlain by 7.75 m of laminated tuffaceous siltstone and limestone (Fig. 2.7B), tuffaceous turbiditic sandstone, and intraclast-rich debris-flow deposits derived from slope failure and slumping. This section demonstrates well the thickening of ash beds in basin center locations, similar to that observed in core CCM-2.

Figure 2.7.

Fallout tuff and laminated Creede Formation strata at section WR-1. (A) White fallout tuff comprising laminated and turbiditic beds (pencil sharpener is 3 cm across). (B) Typical finegrained facies comprising tuffaceous siltstone and limestone laminae and tuffaceous turbiditic sandstone (lens cap is 6 cm in diameter). Photographs by Dan Larsen.

Figure 2.7.

Fallout tuff and laminated Creede Formation strata at section WR-1. (A) White fallout tuff comprising laminated and turbiditic beds (pencil sharpener is 3 cm across). (B) Typical finegrained facies comprising tuffaceous siltstone and limestone laminae and tuffaceous turbiditic sandstone (lens cap is 6 cm in diameter). Photographs by Dan Larsen.

(99.0 km, 61.5 mi.) Return to South Fork on CO 149 for dinner and lodging.

■ Day 3. Return to Denver

Start.Junction of Highways U.S. 160 and CO 149 in South Fork, Colorado. Set odometer to 0.0 km (0.0 mi.). Turn on CO 149 and drive toward Creede, Colorado.

(17.9 km, 11.1 mi.) Intersection of CO 149 with Goose Creek Rd. Turn left. Exposures of Creede Formation are on the right. Exposures of Carpenter Ridge and breccia along the margin of Bachelor caldera are on the left.

Stop 3.1. Upper Creede Formation at 4UR Ranch, Pyroclastic and Alluvial Facies (37°44.81’N, 106°50.07’W) You must obtain permission in advance from the landowner to access this property if you are taking this trip on your own.

(20.8 km; 12.9 mi.) Park on the right. At this location, we will see Creede Formation strata proximal to the lavas of Fisher Dacite at Fisher Mountain and Wagon Wheel Gap. In general, the strata are coarser-grained, alluvial to colluvial deposits; however, lacustrine and lacustrine margin strata are also observed in the upper parts of several measured sections in the 4UR area. The L tuffs are present in the middle of the 4UR stratigraphic sections; thus, the lacustrine sections likely correspond to the documented lake-level rise after the time of the L tuffs (Fig. 10). The lowest part of the section at this site is an upthrown fault block of tan weathered lacustrine strata. The basal bed is poorly exposed and is interpreted as a lithic-rich ash-flow tuff that entered into the lake and was diluted by lake water. On the west side of the fault is a 10 m section of greenish-gray, tuffaceous poorly sorted sandstone and conglomerate with horizontal and scour-and-fill cross-bedding, typical of high-energy braided streams and fans. The greenish-gray strata are overlain by tan weathered laminated tuffaceous siltstone and limestone, some with pseudomorphs after ikaite, and laminated to massive tufa, indicating the return of full lacustrine conditions. Several pyroclastic flow deposits are interbedded in the strata in the 4UR area and pumice is typically more common in all types of strata; both of these features are consistent with the more proximal volcaniclastic setting in this area.

Return to vehicle and turn around to return to CO 149.

(23.7 km, 14.7 mi.) Intersection of Goose Creek Rd. with CO 149. Turn right.

Stop 3.2. Lacustrine Sediments of La Garita Caldera Moat Fill (37°45.80’N, 106°47.33’W)

(25.1 km; 15.6 mi.) Park to the right, off of the highway. Beware of traffic in this area. The La Garita lacustrine strata at this exposure comprise laminated tuffaceous siltstone and turbiditic sandstone, similar to those observed in the Creede Formation. More curious, however, is the sparse presence of highly weathered, but discernable pseudomorph after ikaite casts. Whatever set of conditions favored the formation of ikaite in Lake Creede had previously occurred in La Garita caldera lake. Much more work needs to be done on the moat sediments of other calderas in the San Juan Mountains to more fully understand the chemical and climatic framework for these ancient lakes.

Start the return trip to Denver, through South Fork, Del Norte, and Saguache, stopping en route near Villa Grove for a brief discussion of Bonanza caldera (earliest and northernmost part of San Juan magmatic locus) in comparison to Creede caldera.

Stop 3.3. Panoramic Overview, Southeast Side of 33.12-Ma Bonanza Caldera (38°15.05’N, 105°57.43’W)

U.S. Hwy 285, N of Villa Grove. This distant vantage provides broad perspectives on the geographic scale of the deeply eroded Bonanza caldera and its major structural elements (Lip-man et al., 2015), offering intriguing comparisons and contrasts with Creede caldera. Looking clockwise, on the skyline are: the south rim of the caldera, a faulted fragment of Precambrian granite on the southeast wall at Clayton cone, the valley of Kerber Creek containing a caldera ring fault, and the elongate resurgent dome from Hayden Peak across to Whale Hill. Low grassy, sage-covered slopes in the near distance are dissected old rift-fill fan deposits, perhaps as old as the Neogene Dry Union Formation (Santa Fe Group). Mount Princeton, across Poncha Pass to the north, is a batholith of Paleogene granodiorite that is inferred to underlie the source of the 37-Ma Wall Mountain Tuff (Fig. 1; Stop 1.1 on first day).

Acknowledgments

This field guide is dedicated to the pioneering work of Thomas Steven, whose insight into relations between caldera processes and mineral deposits at Creede, Colorado, has inspired both of the authors. Tom’s work also motivated many other geologists, especially the studies by Phil Bethke, Robert Rye, and Paul Barton regarding the genesis of epithermal ore deposits in the Creede district and their relationship to the caldera setting. The detailed observations and carefully constructed ideas and models of these scientists serve as benchmarks for future work by geoscientists. The field guide manuscript was greatly improved by thorough reviews of Benjamin Murphy and Todd LaMaskin, as well as editorial comments by Steve Keller. Many thanks are extended to the owners of Antler’s Lodge, Wason Ranch, and 4-UR Ranch for graciously allowing us to access their land.

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Figures & Tables

Figure 1.

Map of Southern Rocky Mountain volcanic field, showing ignimbrite calderas, major erosional remnants and inferred original extent of late Paleogene volcanic cover, caldera-related granitic intrusions, and later sedimentary fill in asymmetric grabens of the Rio Grande rift zone. Graben asymmetry and boundary-fault geometry reverse from east-dipping in the San Luis Valley segment to west-dipping in the Sawatch Range-Upper Arkansas segment to the north. Blue dashed lines indicate major bounding faults of asymmetrical rift grabens. A diffuse structural-transition boundary lies south and east of the Bonanza area (green dashed line). Arrows indicate trend of Late Cretaceous-early Paleogene (Laramide) intrusions of the Colorado Mineral Belt. Calderas: B—Bachelor; Bz—Bonanza; C—Cochetopa Park; Cr— Creede; GP—Grizzly Peak; LGn—La Garita north segment; LGs—La Garita, south segment; M—Marshall; MA—Mount Aetna; NP—North Pass; Pl—Platoro; S—Silverton; SL—San Luis complex; SR—South River. Geographic locations: BP—Buffalo Peaks; SC—Summer Coon volcano; SK—Storm King Mountain; WMT—distal Wall Mountain Tuff on High Plains. Modified from McIntosh and Chapin (2004); inferred original limit of volcanic rocks modified from Steven (1975); intrusions from Tweto (1979) and Lipman (1988, 2000).

Figure 1.

Map of Southern Rocky Mountain volcanic field, showing ignimbrite calderas, major erosional remnants and inferred original extent of late Paleogene volcanic cover, caldera-related granitic intrusions, and later sedimentary fill in asymmetric grabens of the Rio Grande rift zone. Graben asymmetry and boundary-fault geometry reverse from east-dipping in the San Luis Valley segment to west-dipping in the Sawatch Range-Upper Arkansas segment to the north. Blue dashed lines indicate major bounding faults of asymmetrical rift grabens. A diffuse structural-transition boundary lies south and east of the Bonanza area (green dashed line). Arrows indicate trend of Late Cretaceous-early Paleogene (Laramide) intrusions of the Colorado Mineral Belt. Calderas: B—Bachelor; Bz—Bonanza; C—Cochetopa Park; Cr— Creede; GP—Grizzly Peak; LGn—La Garita north segment; LGs—La Garita, south segment; M—Marshall; MA—Mount Aetna; NP—North Pass; Pl—Platoro; S—Silverton; SL—San Luis complex; SR—South River. Geographic locations: BP—Buffalo Peaks; SC—Summer Coon volcano; SK—Storm King Mountain; WMT—distal Wall Mountain Tuff on High Plains. Modified from McIntosh and Chapin (2004); inferred original limit of volcanic rocks modified from Steven (1975); intrusions from Tweto (1979) and Lipman (1988, 2000).

Figure 2.

Age-location-volume plot, showing southward progression of ignimbrite-caldera volcanism in the SRMVF (revised from Lipman, 2007). Vertical bars, volumes of individual ignimbrites, scale on left axis (data from Table 1); stippled area, increasing cumulative eruptive volume (right axis). Inset, slopes corresponding to different cumulative eruption rates. Abbreviations: AT—Amalia Tuff; B—Bonanza Tuff; BC—Badger Creek Tuff; BM—Blue Creek Tuff; CP—Chiquito Peak Tuff; CR—Carpenter Ridge Tuff; FC—Fish Canyon Tuff; GP—Grizzly Peak Tuff; LJ—La Jara Canyon Tuff; NM—Nelson Mountain Tuff; SC—Saguache Creek Tuff; SM—Sapinero Mesa Tuff; SMT—Snowshoe Mountain Tuff; SP—Sunshine Peak Tuff; TR—Thorn Ranch Tuff; WM—Wall Mountain Tuff; WP—Wason Park Tuff. S.E. & W. SJ—southeast and west San Juan (figure from Lipman et al., 2015).

Figure 2.

Age-location-volume plot, showing southward progression of ignimbrite-caldera volcanism in the SRMVF (revised from Lipman, 2007). Vertical bars, volumes of individual ignimbrites, scale on left axis (data from Table 1); stippled area, increasing cumulative eruptive volume (right axis). Inset, slopes corresponding to different cumulative eruption rates. Abbreviations: AT—Amalia Tuff; B—Bonanza Tuff; BC—Badger Creek Tuff; BM—Blue Creek Tuff; CP—Chiquito Peak Tuff; CR—Carpenter Ridge Tuff; FC—Fish Canyon Tuff; GP—Grizzly Peak Tuff; LJ—La Jara Canyon Tuff; NM—Nelson Mountain Tuff; SC—Saguache Creek Tuff; SM—Sapinero Mesa Tuff; SMT—Snowshoe Mountain Tuff; SP—Sunshine Peak Tuff; TR—Thorn Ranch Tuff; WM—Wall Mountain Tuff; WP—Wason Park Tuff. S.E. & W. SJ—southeast and west San Juan (figure from Lipman et al., 2015).

Figure 3.

Geologic map of central caldera cluster, San Juan volcanic locus, Colorado. Abbreviated geographic names: LH—Lake Humphreys; MM—Mineral Mountain; SL—San Luis Peak. Calderas of San Luis complex: CC—Cebolla Creek; NM—Nelson Mountain; RC—Rat Creek. Ash-flow sheets: BC—Blue Creek Tuff; CR—Carpenter Ridge Tuff; WP—Wason Park Tuff. C. D.—continental divide. Generalized from Lipman (2006).

Figure 3.

Geologic map of central caldera cluster, San Juan volcanic locus, Colorado. Abbreviated geographic names: LH—Lake Humphreys; MM—Mineral Mountain; SL—San Luis Peak. Calderas of San Luis complex: CC—Cebolla Creek; NM—Nelson Mountain; RC—Rat Creek. Ash-flow sheets: BC—Blue Creek Tuff; CR—Carpenter Ridge Tuff; WP—Wason Park Tuff. C. D.—continental divide. Generalized from Lipman (2006).

Figure 4.

Generalized geologic map of Creede caldera, showing approximate location of eroded topographic caldera rim, present-day extent of caldera-fill deposits, inferred buried ring fault, and late normal faults during resurgent doming (Deep Creek graben) and mineralization (Creede graben). Modified from Lipman (2000).

Figure 4.

Generalized geologic map of Creede caldera, showing approximate location of eroded topographic caldera rim, present-day extent of caldera-fill deposits, inferred buried ring fault, and late normal faults during resurgent doming (Deep Creek graben) and mineralization (Creede graben). Modified from Lipman (2000).

Figure 5.

Simplified cross sections through the southern margin of Creede caldera showing interpreted sequence of events during caldera subsidence and post-eruption volcanism and sedimentation. (A) Maximum height of intracal-dera Snowshoe Mountain Tuff, late during eruption, after structurally bounded caldera had become enlarged by earlier landslide slumping. (B) Further major subsidence near end of eruptions, causing additional landsliding of highest intracaldera Snowshoe Mountain Tuff and adjacent wall rocks. (C) Resurgent doming of caldera floor, filling of resulting moat by sediments of Creede Formation and lavas of Fisher Dacite, and subsequent erosion to present-day land surface. Location of section shown in Figure 4.

Figure 5.

Simplified cross sections through the southern margin of Creede caldera showing interpreted sequence of events during caldera subsidence and post-eruption volcanism and sedimentation. (A) Maximum height of intracal-dera Snowshoe Mountain Tuff, late during eruption, after structurally bounded caldera had become enlarged by earlier landslide slumping. (B) Further major subsidence near end of eruptions, causing additional landsliding of highest intracaldera Snowshoe Mountain Tuff and adjacent wall rocks. (C) Resurgent doming of caldera floor, filling of resulting moat by sediments of Creede Formation and lavas of Fisher Dacite, and subsequent erosion to present-day land surface. Location of section shown in Figure 4.

Figure 6.

Road map for central San Juan region. Road map shows trip route from Del Norte to Creede, locations of ignimbrite calderas, and prominent geographic features. Uncon-formable margins of caldera-fill lava and tuff are solid lines where exposed and short dashed lines where concealed. Geologic base map from Steven et al. (1974).

Figure 6.

Road map for central San Juan region. Road map shows trip route from Del Norte to Creede, locations of ignimbrite calderas, and prominent geographic features. Uncon-formable margins of caldera-fill lava and tuff are solid lines where exposed and short dashed lines where concealed. Geologic base map from Steven et al. (1974).

Figure 7.

Generalized map of Creede caldera showing the distribution of stratified rocks and travertine in the Creede Formation (modified from Larsen and Crossey, 1996). The locations of measured sections and core sites (CCM prefix) used in Figure 8 are shown. The line connects the sections and cores in the order shown in Figure 8. Sections and localities discussed in the text are also presented. The geologic base map for this figure is from Steven and Ratté (1965) and not updated to reflect more recent studies by Lipman (2000, 2006).

Figure 7.

Generalized map of Creede caldera showing the distribution of stratified rocks and travertine in the Creede Formation (modified from Larsen and Crossey, 1996). The locations of measured sections and core sites (CCM prefix) used in Figure 8 are shown. The line connects the sections and cores in the order shown in Figure 8. Sections and localities discussed in the text are also presented. The geologic base map for this figure is from Steven and Ratté (1965) and not updated to reflect more recent studies by Lipman (2000, 2006).

Figure 8.

Correlation of sections measured and cores described in the Creede Formation (modified from Larsen and Crossey, 1996). Numbers beneath sections indicate the height (in meters) of the base of the section relative to core CCM-2. The sections are aligned to the I tuff because it more closely represents an isochronous surface than the base of the Creede Formation. Pseudomorphs in the lacustrine facies are interpreted to be after ikaite (see text).

Figure 8.

Correlation of sections measured and cores described in the Creede Formation (modified from Larsen and Crossey, 1996). Numbers beneath sections indicate the height (in meters) of the base of the section relative to core CCM-2. The sections are aligned to the I tuff because it more closely represents an isochronous surface than the base of the Creede Formation. Pseudomorphs in the lacustrine facies are interpreted to be after ikaite (see text).

Figure 9.

Diagrammatic sketches showing the distribution of deposi-tional environments during: (A) highstand and (B) lowstand lake conditions (from Larsen and Crossey, 1996).

Figure 9.

Diagrammatic sketches showing the distribution of deposi-tional environments during: (A) highstand and (B) lowstand lake conditions (from Larsen and Crossey, 1996).

Figure 10.

Diagrammatic sketch showing volcanic, lake-level, and basin-fill history (modified from Larsen and Crossey, 1996). Dashed part of lake-level history is derived solely from sequences in Bachelor paleovalley and is not corrected relative to CCM-2 stratigraphy. Basin history cross section represents a schematic north (right)-south (left) cross section through site CCM-2. Ts is Snowshoe Mountain Tuff; Tov is older volcanic rocks; stippled pattern is alluvial deposits; lined pattern is lacustrine deposits; blocks are landslide breccia. Also shown are periods I, II, and III of Rye et al. (2000).

Figure 10.

Diagrammatic sketch showing volcanic, lake-level, and basin-fill history (modified from Larsen and Crossey, 1996). Dashed part of lake-level history is derived solely from sequences in Bachelor paleovalley and is not corrected relative to CCM-2 stratigraphy. Basin history cross section represents a schematic north (right)-south (left) cross section through site CCM-2. Ts is Snowshoe Mountain Tuff; Tov is older volcanic rocks; stippled pattern is alluvial deposits; lined pattern is lacustrine deposits; blocks are landslide breccia. Also shown are periods I, II, and III of Rye et al. (2000).

Figure 11.

Stable carbon and oxygen isotope compositions of lacustrine carbonates from CCM-2 showing calculated 513CCO2 and 518OH2O values of fluids for assumed temperatures (from Rye et al., 2000, their figure 9). Tie lines connect core sample data with fluid composition fields interpreted to reflect early (period I) and late (period III) water compositions. Maximum 513C values were calculated assuming the dissolved inorganic carbon was from volcanic CO2 added directly to the lake at the travertine springs (see Rye et al., 2000, for further information).

Figure 11.

Stable carbon and oxygen isotope compositions of lacustrine carbonates from CCM-2 showing calculated 513CCO2 and 518OH2O values of fluids for assumed temperatures (from Rye et al., 2000, their figure 9). Tie lines connect core sample data with fluid composition fields interpreted to reflect early (period I) and late (period III) water compositions. Maximum 513C values were calculated assuming the dissolved inorganic carbon was from volcanic CO2 added directly to the lake at the travertine springs (see Rye et al., 2000, for further information).

Figure 12.

Schematic diagrams showing interpreted limnologic structure of Lake Creede during each evolutionary period (from Rye et al., 2000, their figure 11).

Figure 12.

Schematic diagrams showing interpreted limnologic structure of Lake Creede during each evolutionary period (from Rye et al., 2000, their figure 11).

Figure 13.

North-south cross section through the northern part of Creede caldera and Bachelor and San Luis calderas showing conceptual model for the Creede hydrothermal district (from Barton et al., 2000b). Arrows show hypothesized circulation of cool meteoric (M), cool lacustrine (L) pore waters, other water sources (R), and hot magmatic waters (P).

Figure 13.

North-south cross section through the northern part of Creede caldera and Bachelor and San Luis calderas showing conceptual model for the Creede hydrothermal district (from Barton et al., 2000b). Arrows show hypothesized circulation of cool meteoric (M), cool lacustrine (L) pore waters, other water sources (R), and hot magmatic waters (P).

Figure 1.1.

Upper Arkansas segment of Rio Grande rift and Sawatch Range. The rift valley is an asymmetrical graben, with the main bounding fault on the west side, at base of the Sawatch Range. High point is Mount Princeton (elev. 4327 m [14,196 ft]), central within the 35-37-Ma batholith that is inferred to underlie the now erosionally removed caldera source of the 37-Ma Wall Mountain Tuff. Photograph by Peter Lipman.

Figure 1.1.

Upper Arkansas segment of Rio Grande rift and Sawatch Range. The rift valley is an asymmetrical graben, with the main bounding fault on the west side, at base of the Sawatch Range. High point is Mount Princeton (elev. 4327 m [14,196 ft]), central within the 35-37-Ma batholith that is inferred to underlie the now erosionally removed caldera source of the 37-Ma Wall Mountain Tuff. Photograph by Peter Lipman.

Figure 1.2.

Fish Canyon Tuff (FCT) at Elephant Rocks, along west margin of San Luis Valley. (A) Geologic map of Del Norte area (Lipman, 1976). Southeast flank of Summer Coon volcano: Tsa—andesite breccia; Tsd—dacite lava; Tsi—intermediate-composition dike; Tsri—rhyolite dike. Conejos Formation: Tcv—andesitic lavas of uncertain source; Tvs—volcanic sandstone and conglomerate. Ignimbrites from central caldera cluster: Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff. Ignimbrites from Platoro caldera complex: Ttc—Chiquita Peak Tuff; Ttr—Ra Jadero Tuff. Surficial deposits: Qal—alluvium; Qc—colluvium; Qf—alluvial-fan deposits; Qo—glacial outwash deposits. (B) Fish Canyon Tuff at Elephant Rocks. Rounded granitoid-appearing outcrops are typical of this ignimbrite where a moderately welded outflow sheet crops out in weathered exposure. View to west. Photograph by Kenzie Turner.

Figure 1.2.

Fish Canyon Tuff (FCT) at Elephant Rocks, along west margin of San Luis Valley. (A) Geologic map of Del Norte area (Lipman, 1976). Southeast flank of Summer Coon volcano: Tsa—andesite breccia; Tsd—dacite lava; Tsi—intermediate-composition dike; Tsri—rhyolite dike. Conejos Formation: Tcv—andesitic lavas of uncertain source; Tvs—volcanic sandstone and conglomerate. Ignimbrites from central caldera cluster: Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff. Ignimbrites from Platoro caldera complex: Ttc—Chiquita Peak Tuff; Ttr—Ra Jadero Tuff. Surficial deposits: Qal—alluvium; Qc—colluvium; Qf—alluvial-fan deposits; Qo—glacial outwash deposits. (B) Fish Canyon Tuff at Elephant Rocks. Rounded granitoid-appearing outcrops are typical of this ignimbrite where a moderately welded outflow sheet crops out in weathered exposure. View to west. Photograph by Kenzie Turner.

Figure 1.3.

South Fork overview. (A) Geologic map of South Fork area (from Lipman, 2006). Units: Tca— andesitic lavas of the Conejos Formation; Tfc—Fish Canyon Tuff; Tmp— Masonic Park Tuff (dotted lines indicate partial cooling breaks 1-3); Ttc—Chiq-uita Peak Tuff; Tts—South Fork Tuff; Qal— alluvium; Qc—colluvium; Ql— landslide deposits. (B) View of Point Baxter, ridge northwest of South Fork village, showing boundary zone between similar-appearing dacitic ignimbrites (Masonic Park, Chiquita Peak Tuffs). Along Point Baxter ridge, thin Chiquita Peak Tuff wedges out just to the northwest, between thick underlying Masonic Park and overlying Fish Canyon Tuffs. In contrast, Masonic Park Tuff thins and ends abruptly southeast of South Fork, and these two ignimbrites overlap only along a narrow corridor a few km wide, wherever exposed, all the way south beyond Wolf Creek Pass. Photograph by Peter Lipman.

Figure 1.3.

South Fork overview. (A) Geologic map of South Fork area (from Lipman, 2006). Units: Tca— andesitic lavas of the Conejos Formation; Tfc—Fish Canyon Tuff; Tmp— Masonic Park Tuff (dotted lines indicate partial cooling breaks 1-3); Ttc—Chiq-uita Peak Tuff; Tts—South Fork Tuff; Qal— alluvium; Qc—colluvium; Ql— landslide deposits. (B) View of Point Baxter, ridge northwest of South Fork village, showing boundary zone between similar-appearing dacitic ignimbrites (Masonic Park, Chiquita Peak Tuffs). Along Point Baxter ridge, thin Chiquita Peak Tuff wedges out just to the northwest, between thick underlying Masonic Park and overlying Fish Canyon Tuffs. In contrast, Masonic Park Tuff thins and ends abruptly southeast of South Fork, and these two ignimbrites overlap only along a narrow corridor a few km wide, wherever exposed, all the way south beyond Wolf Creek Pass. Photograph by Peter Lipman.

Figure 1.4.

Rio Grande Canyon: La Garita and Creede caldera walls, Masonic Park Tuff. (A) Geologic map of Rio Grande canyon (from Lipman, 2006). Units: Tbc—Blue Creek Tuff; Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff; Tmp—Masonic Park Tuff (dotted lines indicate partial cooling breaks; Tw—Wason Park Tuff; Qc— colluvium; Ql—landslide deposits; Qt—talus. (B) East topographic wall of La Garita caldera, in Rio Grande canyon between South Fork and Creede, which truncates multiple cooling units of flat-lying Masonic Park Tuff and proximal outflow of Fish Canyon Tuff. Caldera fill includes slump block of Fish Canyon Tuff (FC), and depositionally overlying Blue Creek, Carpenter Ridge (not labeled), and Wason Park Tuffs that wedge out against caldera wall. View to northwest. Photograph by Peter Lipman.

Figure 1.4.

Rio Grande Canyon: La Garita and Creede caldera walls, Masonic Park Tuff. (A) Geologic map of Rio Grande canyon (from Lipman, 2006). Units: Tbc—Blue Creek Tuff; Tcr—Carpenter Ridge Tuff; Tfc—Fish Canyon Tuff; Tmp—Masonic Park Tuff (dotted lines indicate partial cooling breaks; Tw—Wason Park Tuff; Qc— colluvium; Ql—landslide deposits; Qt—talus. (B) East topographic wall of La Garita caldera, in Rio Grande canyon between South Fork and Creede, which truncates multiple cooling units of flat-lying Masonic Park Tuff and proximal outflow of Fish Canyon Tuff. Caldera fill includes slump block of Fish Canyon Tuff (FC), and depositionally overlying Blue Creek, Carpenter Ridge (not labeled), and Wason Park Tuffs that wedge out against caldera wall. View to northwest. Photograph by Peter Lipman.

Figure 2.1.

Panoramic view of resurgent Creede caldera. Creede caldera, as viewed from its north wall (Bachelor Road, at Windy Gulch). Baldy Mountain (elev. 3806 m [12,488 ft]) is on southwest wall; Wagon Wheel Gap is at east wall. Fisher Mountain (elev. 3921 m [12,865 ft]) is sequence of postcaldera lavas (Fisher Dacite) on south wall. Snowshoe Mountain is high point on resurgently uplifted caldera floor, defining a symmetrical dome of intracaldera Snowshoe Mountain Tuff, with a keystone graben outlined by northward drainages, and with flanks dipping as steeply as 45° (Steven and Ratté, 1965). Rio Grande flows into caldera moat from the west, arcs around north side of the resurgent dome, and exits at Wagon Wheel Gap. The river has preferentially eroded weakly indurated moat-fill sediments of the Creede Formation, exhuming much of the Oligocene caldera morphology. Town of Creede in lower left of photo. View to south. Photograph by Peter Lipman.

Figure 2.1.

Panoramic view of resurgent Creede caldera. Creede caldera, as viewed from its north wall (Bachelor Road, at Windy Gulch). Baldy Mountain (elev. 3806 m [12,488 ft]) is on southwest wall; Wagon Wheel Gap is at east wall. Fisher Mountain (elev. 3921 m [12,865 ft]) is sequence of postcaldera lavas (Fisher Dacite) on south wall. Snowshoe Mountain is high point on resurgently uplifted caldera floor, defining a symmetrical dome of intracaldera Snowshoe Mountain Tuff, with a keystone graben outlined by northward drainages, and with flanks dipping as steeply as 45° (Steven and Ratté, 1965). Rio Grande flows into caldera moat from the west, arcs around north side of the resurgent dome, and exits at Wagon Wheel Gap. The river has preferentially eroded weakly indurated moat-fill sediments of the Creede Formation, exhuming much of the Oligocene caldera morphology. Town of Creede in lower left of photo. View to south. Photograph by Peter Lipman.

Figure 2.2.

Top of intracaldera Snowshoe Mountain Tuff, at Point of Rocks. Upper part of intracaldera Snowshoe Mountain Tuff (Tsm), in partial fault contact with caldera-landslide breccia consisting of monolithologic clasts of intracaldera Carpenter Ridge Tuff (Willow Creek welding zone), on northwest flank of Creede resurgent dome at Point of Rocks. Above treeline in distance are La Garita Mountains, exposing the resurgently uplifted block of intracaldera Fish Canyon Tuff. Photograph by Peter Lipman.

Figure 2.2.

Top of intracaldera Snowshoe Mountain Tuff, at Point of Rocks. Upper part of intracaldera Snowshoe Mountain Tuff (Tsm), in partial fault contact with caldera-landslide breccia consisting of monolithologic clasts of intracaldera Carpenter Ridge Tuff (Willow Creek welding zone), on northwest flank of Creede resurgent dome at Point of Rocks. Above treeline in distance are La Garita Mountains, exposing the resurgently uplifted block of intracaldera Fish Canyon Tuff. Photograph by Peter Lipman.

Figure 2.3.

Creede Formation strata at section AP-1. Lensoid package of pebbly sediment-gravity-flow deposits (pinching out toward Rio Grande) and overlying turbiditic tuffaceous sandstone and laminated tuffaceous siltstone. Rock hammer for scale (circled). Photograph by Gary A. Smith.

Figure 2.3.

Creede Formation strata at section AP-1. Lensoid package of pebbly sediment-gravity-flow deposits (pinching out toward Rio Grande) and overlying turbiditic tuffaceous sandstone and laminated tuffaceous siltstone. Rock hammer for scale (circled). Photograph by Gary A. Smith.

Figure 2.4.

Sketch of Antlers Park cliffs showing three prominent systems of sublacustrine-fan lobes (from Larsen and Smith, 1999).

Figure 2.4.

Sketch of Antlers Park cliffs showing three prominent systems of sublacustrine-fan lobes (from Larsen and Smith, 1999).

Figure 2.5.

Creede Formation strata at Airport Hill (Fossil Butte). (A) Elongate calcite pseudomorphs after ikaite in tuffaceous siltstone (lens cap is 6 cm in diameter). (B) Tuffaceous siltstone with conifer and deciduous shrub fossils on partings (pen is 14 cm long). Photographs by Dan Larsen.

Figure 2.5.

Creede Formation strata at Airport Hill (Fossil Butte). (A) Elongate calcite pseudomorphs after ikaite in tuffaceous siltstone (lens cap is 6 cm in diameter). (B) Tuffaceous siltstone with conifer and deciduous shrub fossils on partings (pen is 14 cm long). Photographs by Dan Larsen.

Figure 2.6.

Thinolitic travertine in Creede Formation near Farmer’s Creek Trail. (A) Radiating geometry of clusters of pseudomorphs (lens cap is 6 cm in diameter). (B) Close-up image from the same outcrop showing well-preserved pseudomorphic crystal geometry. Photographs by Dan Larsen.

Figure 2.6.

Thinolitic travertine in Creede Formation near Farmer’s Creek Trail. (A) Radiating geometry of clusters of pseudomorphs (lens cap is 6 cm in diameter). (B) Close-up image from the same outcrop showing well-preserved pseudomorphic crystal geometry. Photographs by Dan Larsen.

Figure 2.7.

Fallout tuff and laminated Creede Formation strata at section WR-1. (A) White fallout tuff comprising laminated and turbiditic beds (pencil sharpener is 3 cm across). (B) Typical finegrained facies comprising tuffaceous siltstone and limestone laminae and tuffaceous turbiditic sandstone (lens cap is 6 cm in diameter). Photographs by Dan Larsen.

Figure 2.7.

Fallout tuff and laminated Creede Formation strata at section WR-1. (A) White fallout tuff comprising laminated and turbiditic beds (pencil sharpener is 3 cm across). (B) Typical finegrained facies comprising tuffaceous siltstone and limestone laminae and tuffaceous turbiditic sandstone (lens cap is 6 cm in diameter). Photographs by Dan Larsen.

Summary of Regional Ignimbrites, Caldera Sources, and 40Ar/39Ar Ages, Southern Rocky Mountain Volcanic Field (Updated and Simplified from Lipman, 2015, his Table 1)

Table 1.
Summary of Regional Ignimbrites, Caldera Sources, and 40Ar/39Ar Ages, Southern Rocky Mountain Volcanic Field (Updated and Simplified from Lipman, 2015, his Table 1)

Characteristic Features, Ignimbrite Sheets of Central and Northeastern San Juan Region (From Lipman et al., 2015, their Table 2)

Table 2.
Characteristic Features, Ignimbrite Sheets of Central and Northeastern San Juan Region (From Lipman et al., 2015, their Table 2)
Ignimbrite sheet*CompositionTextures and phenocrysts§
Snowshoe Mountain TuffMafic dacitePhenocryst rich; densely welded within caldera, weakly welded outflow
Nelson Mountain TuffLow-Si rhyolite=daciteCompositionally zoned; weakly welded crystal-poor to dense crystal-rich
Cebolla Creek TuffMafic daciteTypically weakly welded; abundant hornblende >> augite is distinctive
Rat Creek TuffLow-Si rhyolite= daciteCompositionally zoned; weakly welded rhyolite to dense dacite
Wason Park TuffRhyolitePhenocryst-rich rhyolite; tabular sanidine phenocrysts
Blue Creek TuffDacitePhenocryst rich; sanidine is absent (contrast with Mammoth Mountain Member)
Carpenter Ridge Tuff
- Mammoth MountainDacitePhenocryst rich; sanidine is common (contrast with Blue Creek Tuff)
Member (upper)
- Outflow and lowerLow-Si rhyolitePhenocryst poor; common basal vitrophyre, central lithophysal zone
intracaldera tuff
Crystal Lake TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but less welded within map area
Fish Canyon TuffDaciteDistinctive light-gray, phenocryst-rich; resorbed quartz, hornblende, absence of augite
Sapinero Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Dillon Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Blue Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Ute Ridge TuffDacitePhenocryst rich; contains sparse sanidine (in contrast to Masonic Park Tuff)
Masonic Park TuffDacitePhenocrysts similar to Blue Creek Tuff; typically less welded
Luders Creek TuffLow-Si rhyolite= daciteCompositionally zoned; resembles Nelson Mountain Tuff
Saguache Creek TuffLow-Si rhyoliteResembles Carpenter Ridge and Sapinero Mesa Tuffs, but lacks phenocrystic biotite
Bonanza TuffZoned complexlyLocal basal xl-poor rhyolite, lower xl dacite, upper rhyolite, local upper xl dacite
Thorn Ranch TuffZoned complexlyIntracaldera alternation of rhyolite & dacite; outflow mainly high-Si rhyolite
Badger Creek TuffDaciteCrystal rich; resembles Fish Canyon Tuff
Wall Mountain TuffRhyoliteCrystal-rich, large blocky sanidine; locally complexly rheomorphic
Ignimbrite sheet*CompositionTextures and phenocrysts§
Snowshoe Mountain TuffMafic dacitePhenocryst rich; densely welded within caldera, weakly welded outflow
Nelson Mountain TuffLow-Si rhyolite=daciteCompositionally zoned; weakly welded crystal-poor to dense crystal-rich
Cebolla Creek TuffMafic daciteTypically weakly welded; abundant hornblende >> augite is distinctive
Rat Creek TuffLow-Si rhyolite= daciteCompositionally zoned; weakly welded rhyolite to dense dacite
Wason Park TuffRhyolitePhenocryst-rich rhyolite; tabular sanidine phenocrysts
Blue Creek TuffDacitePhenocryst rich; sanidine is absent (contrast with Mammoth Mountain Member)
Carpenter Ridge Tuff
- Mammoth MountainDacitePhenocryst rich; sanidine is common (contrast with Blue Creek Tuff)
Member (upper)
- Outflow and lowerLow-Si rhyolitePhenocryst poor; common basal vitrophyre, central lithophysal zone
intracaldera tuff
Crystal Lake TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but less welded within map area
Fish Canyon TuffDaciteDistinctive light-gray, phenocryst-rich; resorbed quartz, hornblende, absence of augite
Sapinero Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Dillon Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Blue Mesa TuffLow-Si rhyoliteSimilar to rhyolitic Carpenter Ridge Tuff, but generally less welded within map area
Ute Ridge TuffDacitePhenocryst rich; contains sparse sanidine (in contrast to Masonic Park Tuff)
Masonic Park TuffDacitePhenocrysts similar to Blue Creek Tuff; typically less welded
Luders Creek TuffLow-Si rhyolite= daciteCompositionally zoned; resembles Nelson Mountain Tuff
Saguache Creek TuffLow-Si rhyoliteResembles Carpenter Ridge and Sapinero Mesa Tuffs, but lacks phenocrystic biotite
Bonanza TuffZoned complexlyLocal basal xl-poor rhyolite, lower xl dacite, upper rhyolite, local upper xl dacite
Thorn Ranch TuffZoned complexlyIntracaldera alternation of rhyolite & dacite; outflow mainly high-Si rhyolite
Badger Creek TuffDaciteCrystal rich; resembles Fish Canyon Tuff
Wall Mountain TuffRhyoliteCrystal-rich, large blocky sanidine; locally complexly rheomorphic
*

Bold type—ignimbrite sheets of Bonanza map area.

Si—SiO2.

§

xl—crystal.

Descriptions and Interpretations of Depositional Facies of the Creede Formation

Table 3.
Descriptions and Interpretations of Depositional Facies of the Creede Formation
FaciesDescriptionInterpretation and modern analogs
Laminated siltstoneSandy tuffaceous siltstone and carbonate siltstone.Suspension fallout sedimentation; comparable
Carbonate constituents include micrite, elongatemodern deposits found in Fayetteville Green Lake,
micritic peloids, and calcite pseudomorphs after ikaiteNew York (Ludlam, 1969), and Lake Zurich,
(Larsen, 1994b).Switzerland (Kelts and Hsü, 1978).
Graded tuffaceousSilt to very-coarse-grained, pebbly, tuffaceousTurbidite deposition based on the grading
sandstonesandstone with distribution normal grading. Internalcharacteristics (Lowe, 1982), presence of Bouma
stratification is not prominent in outcrop, but Boumasubdivisions, and intercalation with laminated
subdivisions Ta, Tb, Tc, Td, and Te are observed infacies. Graded, structureless turbidites known
core.from numerous modern lakes (Ludlam, 1969;
Anderson et al., 1985).
Tuffaceous sandstone-Very-fine- to coarse-grained tuffaceous sandstoneThese deposits are interpreted to be associated with
siltstone coupletslaminae overlain by a silt lamina.the passing of turbidity currents: the sandstone
could be either a lag deposit or plane bed, and the
silt drape represents suspension settling from the
overlying turbid water.
Cross-laminatedVery-fine- to very-coarse-grained, moderately sortedWave-rippled beds have symmetrical, bifurcating
tuffaceous sandstonewave-ripple and pinch-and-swell sandstone. Pinch-crests typically formed under oscillatory flow
and-swell laminae are 1 to 2 cm thick and generallyconditions. Pinch-and-swell beds may form by
lack internal stratification.both wave-related (Smoot and Lowenstein, 1991)
or density-current-related (Sturm and Matter,
1978) processes.
Horizontal-laminatedVery-fine- to very-coarse-grained, moderately sorted,These units are comparable to sands deposited as
tuffaceous sandstonelaminated sandstone.sand sheets under upper-flow-regime conditions
(Allen, 1984).
Horizontal-bedded andFine- to very-coarse-grained, pebbly, moderately toHorizontal-bedded units represent sheetflood
cross-bedded pebblypoorly sorted, scour-and-fill, low-angle trough, anddeposits laid down in either subaerial or shallow,
sandstoneplanar cross-bedded and horizontal-beddedlacustrine settings, analogous to those described
sandstone. Beds are composed of lithic andby Blair and MacPherson (1994) and Smoot and
pyroclastic grains but lack intraclasts.Lowenstein (1991). Scour-and-fill and low-angle-
trough cross-beds are similar to low-angle cross-
stratified deposits laid down during high-discharge
events in shallow, braided streams (Picard and
High, 1973).
Pebbly, calcareousCoarse- to very-coarse-grained, pebbly sandstone andSimilar deposits are described in lake-margin settings
sandstone andconglomerate with planar foresets that dip basinward.where detritus worked by stream and wave
conglomerate with planarThe sandy beds are composed of moderately to wellprocesses cascades down lakeward-dipping
foresetssorted sand with oolitic coatings.foresets (Smoot and Lowenstein, 1991).
Epsilon cross-beddedFine- to very-coarse-grained pebbly sandstone andThe geometry of these units is characteristic of
sandstones and pebblepebble to cobble conglomerate with crude to well-channel-fill sequences dominated by lateral
to cobble conglomeratedeveloped epsilon cross-bedding. This faciesaccretion processes (Allen, 1984). These facies
invariably fills channel-form erosional features inare almost exclusively observed interbedded with
finer-grained facies. The erosional channels are 1 tolaminated lacustrine facies rather than sandy
2 m deep and 9 to 23 m across, and are, in somefluvial facies. Similar types of deposits are noted
cases, nested. The channel-fill facies fine upwardin both ancient and modern submarine fan
and laterally, into graded beds and laminated units inchannels (Clark and Pickering, 1996), and are
some cases.ascribed to channel avulsion and migration
processes.
Crudely beddedSandy, pebble to boulder, poorly sorted, crudely beddedThe poor sorting, crude stratification, and channelized
conglomerateconglomerate, as tabular beds and channel fill.nature suggest deposition in high-gradient stream
channels (Rust, 1978).
Graded, massiveBeds are generally tabular and 5 to 150 cm thick, butSediment-gravity-flow deposition (see text).
sandstone andsome channel-fill beds are observed. Three types are
conglomerate or brecciadistinguished:
1) Massive, lithic-rich pebble sandstone and pebble
to boulder conglomerate, non-graded, with lithic
pebbles, cobbles, and, less commonly, boulders
in a sandy matrix.
2) Massive, intraclast-rich pebble to boulder
conglomerate and breccia, basal coarse-tail
reverse grading, with a sandy, calcareous
(pulverized intraclast) matrix.
3) Massive, mixed-composition pebble sandstone
and pebble to cobble conglomerate, typically with
stratification in the upper part of the beds. The
beds have flat to convex-down bases and flat to
convex-up tops; erosional or load deformation
features are common along basal contacts.
Bedded, fine-grained tuffLight-colored, fine-grained, massive and laminated bedsInterpreted as pyroclastic fallout based on the pure
within the lacustrine strata.vitric-ash composition, fine grain size, and
moderate sorting characteristics (Fisher and
Schmincke, 1984).
FaciesDescriptionInterpretation and modern analogs
Laminated siltstoneSandy tuffaceous siltstone and carbonate siltstone.Suspension fallout sedimentation; comparable
Carbonate constituents include micrite, elongatemodern deposits found in Fayetteville Green Lake,
micritic peloids, and calcite pseudomorphs after ikaiteNew York (Ludlam, 1969), and Lake Zurich,
(Larsen, 1994b).Switzerland (Kelts and Hsü, 1978).
Graded tuffaceousSilt to very-coarse-grained, pebbly, tuffaceousTurbidite deposition based on the grading
sandstonesandstone with distribution normal grading. Internalcharacteristics (Lowe, 1982), presence of Bouma
stratification is not prominent in outcrop, but Boumasubdivisions, and intercalation with laminated
subdivisions Ta, Tb, Tc, Td, and Te are observed infacies. Graded, structureless turbidites known
core.from numerous modern lakes (Ludlam, 1969;
Anderson et al., 1985).
Tuffaceous sandstone-Very-fine- to coarse-grained tuffaceous sandstoneThese deposits are interpreted to be associated with
siltstone coupletslaminae overlain by a silt lamina.the passing of turbidity currents: the sandstone
could be either a lag deposit or plane bed, and the
silt drape represents suspension settling from the
overlying turbid water.
Cross-laminatedVery-fine- to very-coarse-grained, moderately sortedWave-rippled beds have symmetrical, bifurcating
tuffaceous sandstonewave-ripple and pinch-and-swell sandstone. Pinch-crests typically formed under oscillatory flow
and-swell laminae are 1 to 2 cm thick and generallyconditions. Pinch-and-swell beds may form by
lack internal stratification.both wave-related (Smoot and Lowenstein, 1991)
or density-current-related (Sturm and Matter,
1978) processes.
Horizontal-laminatedVery-fine- to very-coarse-grained, moderately sorted,These units are comparable to sands deposited as
tuffaceous sandstonelaminated sandstone.sand sheets under upper-flow-regime conditions
(Allen, 1984).
Horizontal-bedded andFine- to very-coarse-grained, pebbly, moderately toHorizontal-bedded units represent sheetflood
cross-bedded pebblypoorly sorted, scour-and-fill, low-angle trough, anddeposits laid down in either subaerial or shallow,
sandstoneplanar cross-bedded and horizontal-beddedlacustrine settings, analogous to those described
sandstone. Beds are composed of lithic andby Blair and MacPherson (1994) and Smoot and
pyroclastic grains but lack intraclasts.Lowenstein (1991). Scour-and-fill and low-angle-
trough cross-beds are similar to low-angle cross-
stratified deposits laid down during high-discharge
events in shallow, braided streams (Picard and
High, 1973).
Pebbly, calcareousCoarse- to very-coarse-grained, pebbly sandstone andSimilar deposits are described in lake-margin settings
sandstone andconglomerate with planar foresets that dip basinward.where detritus worked by stream and wave
conglomerate with planarThe sandy beds are composed of moderately to wellprocesses cascades down lakeward-dipping
foresetssorted sand with oolitic coatings.foresets (Smoot and Lowenstein, 1991).
Epsilon cross-beddedFine- to very-coarse-grained pebbly sandstone andThe geometry of these units is characteristic of
sandstones and pebblepebble to cobble conglomerate with crude to well-channel-fill sequences dominated by lateral
to cobble conglomeratedeveloped epsilon cross-bedding. This faciesaccretion processes (Allen, 1984). These facies
invariably fills channel-form erosional features inare almost exclusively observed interbedded with
finer-grained facies. The erosional channels are 1 tolaminated lacustrine facies rather than sandy
2 m deep and 9 to 23 m across, and are, in somefluvial facies. Similar types of deposits are noted
cases, nested. The channel-fill facies fine upwardin both ancient and modern submarine fan
and laterally, into graded beds and laminated units inchannels (Clark and Pickering, 1996), and are
some cases.ascribed to channel avulsion and migration
processes.
Crudely beddedSandy, pebble to boulder, poorly sorted, crudely beddedThe poor sorting, crude stratification, and channelized
conglomerateconglomerate, as tabular beds and channel fill.nature suggest deposition in high-gradient stream
channels (Rust, 1978).
Graded, massiveBeds are generally tabular and 5 to 150 cm thick, butSediment-gravity-flow deposition (see text).
sandstone andsome channel-fill beds are observed. Three types are
conglomerate or brecciadistinguished:
1) Massive, lithic-rich pebble sandstone and pebble
to boulder conglomerate, non-graded, with lithic
pebbles, cobbles, and, less commonly, boulders
in a sandy matrix.
2) Massive, intraclast-rich pebble to boulder
conglomerate and breccia, basal coarse-tail
reverse grading, with a sandy, calcareous
(pulverized intraclast) matrix.
3) Massive, mixed-composition pebble sandstone
and pebble to cobble conglomerate, typically with
stratification in the upper part of the beds. The
beds have flat to convex-down bases and flat to
convex-up tops; erosional or load deformation
features are common along basal contacts.
Bedded, fine-grained tuffLight-colored, fine-grained, massive and laminated bedsInterpreted as pyroclastic fallout based on the pure
within the lacustrine strata.vitric-ash composition, fine grain size, and
moderate sorting characteristics (Fisher and
Schmincke, 1984).

Contents

GeoRef

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