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Crustal Evolution of the GreatBasin and the Sierra Nevada

Edited by Mary M. Lahren and James H. Trexler, Jr., Department of Geological Sciences, University of Nevada, Reno, NV 89557

and

Claude Spinosa Department of Geosciences, Boise State University, Boise, ID 83745

Field Trip Guidebook for the 1993 Joint Meeting of the Cordilleran/Rocky Mountain Sections of the Geological Society of America Reno, Nevada, May 19-21, 1993

Published by Department Geological Sciences Mackay School of Mines University of Nevada, Reno Reno, Nevada 89557 OLIGOCENE-MIOCENE CALDERA COMPLEXES, ASH-FLOW SHEETS, AND TECTONISM IN THE CENTRAL AND SOUTHEASTERN GREAT BASIN

Myron G. Best Brigham Young University, Provo, Utah 84602 Robert B. Scott, Peter D. Rowley, WC Swadley, R. Ernest Anderson U.S. Geological Survey, Denver, Colorado 80225 C. Sherman Gromme U.S. Geological Survey, Menlo Park, California 94025 Anne E. Harding University of Colorado, Boulder, Colorado 80309 Alan L. Deino Geochronology Center, Institute of Human Origins Berkeley, California 94709 Eric H. Christiansen, David. G. Tingey, Kim R. Sullivan Brigham Young University, Provo, Utah 84602

ABSTRACT Regional extension was minimal during most of the ignimbrite flareup. However, local extension The Great Basin harbors at least sixty Tertiary occurred before the flareup and major extensional and calderas and inferred sources of tuff and several tens local strike-slip faulting beginning in the early of thousands of cubic kilometers of ash-flow deposits, Miocene affected many parts of the Great Basin, making it one of the greatest manifestations of including the Caliente and Kane Spring Wash caldera prolonged ash-flow volcanism in the terrestrial rock complexes where synvolcanic faults form many record. Some individual calderas are exposed east to caldera margins. west across three or four mountain ranges. Simple­ cooling-unit outflow tuff sheets cover areas of tens of INTRODUCTION thousands of square kilometers and range to as much as hundreds of meters thick. During the "ignimbrite The purpose of this brief review and field trip flareup" from about 31 to 22 Ma, when most of the roadlog is to sample the results of the waxing and ash-flows were erupted, extrusion of lava in the Great waning of the Great Basin ignimbrite flareup. For a Basin was minor, widely scattered, and did not form more detailed report on Great Basin volcanism during major edif:ces such as composite volcanoes. Typical the Tertiary, see Best and others (l989a). The support volcanic sections consist of multiple ash-flow tuff of the National Science Foundation through grants cooling units from nearby caldera sources and only EAR-8604195, -8618323, and -8904245 to M.G. Best local lava flows and pyroclastic-surge and -fall and E.H. Christiansen is gratefully acknowl- deposits. edged. We appreciate helpful reviews by C. Chapin, D.A. John, R.F. Hardyman, and E.H. McKee. Tuffs older than about 17 Ma in the Great "Basin are high-potassium calc-alkaline rhyolite, GEOLOGIC SETTING dacite, and sparse andesite in which phenocrysts of two feldspars, quartz, Fe-Ti oxides, and biotite are The pre-volcanic underpinning of the Great common. Rhyolite tuff occurs throughout the Basin i~ a terrane containing late Precambrian, Tertiary, whereas huge volumes of dacite ash flows """Phaneiozoic," d local Mesozoic sedimentary rocks were erupted about 31 to 27 Ma and high-temperature e ormed during compressional episodes in Paleozoic trachydacite magmas containing phenocrysts of and Mesozoic Eras and intruded locally by Mesozoic plagioclase and pyroxene erupted from many centers granitic plutons. After widespread erosion in late mostly about 27 to 23 Ma. After 17 Ma, alkaline Cretaceous and early Tertiary time which produced a metaluminous to mildly peralkaline magmas profound unconformity and, in some places, early containing Fe-rich pyroxene and olivine, sanidine, Tertiary sedimentation, volcanism began in the and quartz phenocrysts began to be erupted. Eocene about 43 Ma in northern Nevada and Utah and swept southward along an arcuate, roughly volcanism (Best and Christiansen, 1991). In and near east-west front, reaching southern Nevada by middle the Caliente and Kane Springs Wash caldera Miocene time. complexes (Figs. 1 and 2) the main extensional episode was in early to middle Miocene time (Scott, The inventory of Cenozoic rocks in the Great 1990; Rowley and others, 1992), concurrent with Basin by Stewart and Carlson (1976; see also Best caldera volcanism, as Great Basin ash-flow activity and Christiansen, 1991; Figs. 3 and 4) clearly shows waned. During the ignimbrite flareup, however, the products of the late Oligocene-early Miocene sparse clastic deposits and few angular discordances ignimbrite flareup; the volume of resulting ash-flow in outflow volcanic sections show that regional deposits in the Great Basin is not widely appreciated tectonic extension in the Great Basin as a whole was but is an order of magnitude larger than in the well limited. known San Juan and Mogollon-Datil fields in the eastern Cordillera. However, the Great Basin harbors CALDERAS only a fraction of the volume of ash flow tuffs in the Sierra Madre Occidental of Mexico. During the Recognition of calderas is hampered not only ignimbrite flareup in the Great Basin, the volume of by erosion and burial beneath younger deposits, but extruded lava was minor compared to ash-flow also in the Great Basin by widespread post-volcanic, deposits and was less than the volume of lava and local synvolcanic (Caliente area and Stillwater extruded before and after the flareup. Scarce Range), faulting that has dismembered the calderas Oligocene debris flows indicate a general lack of into small segments, blurring their margins and large volcanic edifices in contrast to, for example, the internal structure. Geographic centering within the San Juan volcanic field. outflow sheet may be misleading as outflow lobes are commonly not radially symmetric about the source Until the early Miocene, at about 24 Ma, calc­ caldera (e.g., Windous Butte Formation, Best and alkaline volcanism had produced a large volume of others, 1989a). Topographic margins are poorly dacite to rhyolite ash-flow tuff and subordinate known even for some of the better located calderas. high-potassium andesite and dacite and rhyolite lava; Piles of tuff as much as 2-3 km thick are an obvious basalt appears only after 22 Ma (Barr and others, indicator of a caldera (Ekren and others, 1973; Best 1992). During the next 8 m.y., explosive volcanism and others, 1989a, Figs. R29, R32-R38), but some waned and a broader compositional spectrum, but stilI demonstrable proximal outflow tuff deposits ponded dominated by rhyolite, appears in the overall volcanic in older calderas and on downthrown sides of record. Basaltic volcanism has been a significant synvolcanic extensional growth faults are also thick aspect of Great Basin activity after about 13 Ma, (Dixon and others, 1972; Best and others, 1989b, partieularly along the eastern and western margins of Figs. 5B and 5C). Dense compaction and widespread the region but also locally in the center (McKee and propylitic alteration of compound or multiple cooling Noble, 1986). Many silicic tuffs and lavas younger units comprising the intracaldera tuff make it more than about 17 Ma are peralkaline or topaz-bearing resistant to erosion relative to the caldera wall rocks (Noble and Parker, 1975; Christiansen and others, and hence causes the development of inverted 1986). topography that is a common clue to the existence of the caldera. Megabreccia and "rafts" of internally After the middle Miocene, the general east-west shattered but nonetheless stratigraphically coherent orientation of magmatic zones changed to north-south rock, locally more than 2 km across and hundreds of (Best and others, 1980; Stewart, 1983), probably meters thick, occur within a few kilometers of some reflecting a fundamental change in the state of stress caldera walls (Bonham and Garside, 1979, p. 40; in the lithosphere (Best, 1988). . McKee, 1976; Best and others, 1989a, Figs. R12, R24, R25, R36, and R38). Extensional tectonism in the Great Basin during Tertiary time was episodic (e.g., Taylor and others, Caving of the unstable caldera escarpment 1989), was intense in some areas (e.g., Proffett, enlarges the perimeter of a caldera so that topographic 1977; Moores and others, 1968; Gans and others, diameters can be several kilometers greater than the 1989) and moderate in others (e.g., the southern ring-fault system (Best and others, 1989a, Fig. R32; Pancake Range, Snyder and others, 1972), and in Best and others, 1989b, Fig. 5B). Younger post­ general correlates poorly in space and time with caldera collanse denosits may completely fill and even + ,/ 1+ 1 I /•..•... ··············l zl kiloMeters 100 "'Ie < -l >- 1):>- ",.--">, ••••.•~UREKA~ 01" I' DELTA / ~ __+-'-~~~_\~-~~'~_:~~--- __

+ + 37i\1 \ 112\0/

Figure 1. Caldera margins and areal extents of some outflow tuff sheets in the southeastern Great Basin (Table 1). Caldera margins (heaviest lines) dashed where approximately located; dotted lines indicate indefinite source areas. Calderas and sources in the Central Nevada caldera complex include the Broken Back 2 (BB), source of Stone Cabin Formation (S), Williams Ridge (W), Hot Creek (H), Pancake Range (P), Kiln Canyon (KC), Big Ten Peak (BT), unnamed caldera source of tuff of Lunar Cuesta (L), Kawich (K), Goblin Knobs (G), Quinn Canyon Range (Q), and Cathedral Ridge (CR) and in the Indian Peak caldera complex the source of the Cottonwood Wash Tuff (C), Indian Peak caldera (IP), White Rock (WR), Mt. Wilson (MW), and source of some members of the Isom Formation (I). The Caliente Caldera complex (C) includes inset calderas shown in Figure 2 and discussed in text. To the south are the Kane Springs Wash caldera complex (KS) and nearby Narrow Canyon caldera (NC) and two unnamed sources. Medium lines outline composite extent of outflow tuff sheets related to the Central Nevada complex (dots, CNCC), extent of the Wah Wah Springs Formation (dashes, WWS), which almost eclipses all other outflow sheets in the Indian Peak ash-flow field, and extent of the Bauers Tuff Member of the Condor Canyon Formation (dash and dot, BTM) which essentially eclipses all of the sheets related to the Caliente caldera complex. Lightest lines are highways. Stars and numbers are locations of field trip stops on first two days. overflow the caldera depression. Dismemberment of 1992) and middle Miocene Kane Springs Wash most older Great Basin calderas precludes evaluation caldera (Novak, 1984). Pre-caldera collapse of resurgence; however, it is apparent in the Indian tumescence is equally difficult to evaluate. Late-stage, Peak caldera (Best and others, 1989b) but not in the ring-fault controlled extrusion of lava domes is not relatively well exposed Mt. Jefferson caldera (Boden, manifest in some well-mapped very large Great Basin + Timpahute +

,:' :--'0 5mi ../ ...1 --JL..-"",---+-_~....I 5

Figure 2. Generalized geologic map of the western part of the Caliente caldera complex and the Kane Springs Wash caldera complex and location of field trip stops (stars) 9 through 15. Geology after Ekren and others (1977), modified where significantly changed by our new mapping, and by our interpretations of gravity and aeromagnetic anomalies (Blank and Kucks, 1989) for some margins of the Caliente caldera complex and the Kane Springs Wash caldera complex. EXPlANATION , .= Clover Creek caldera margin, in north,concealed only, intracalderaBauers TuffMember of Condor Canyon "~~T?i Formation is patterned, Hachures on caldera side of margin Delamarcaldera margin dashed where approximate,dotted where concealed, intracaldera Hiko Tuffis patterned. ··.It. Hachureson caldera side of margin Buckboard Canyoncaldera margin, intracaldera tuffof Rainbow Canyon is patterned. Hachures on caldera side of margin

KaneSpringsWash caldera margin,dotted where concealed. Hachures on caidera side of margin

Narrow Canyoncaldera margin. Hachures on caldera side of margin

North and east boundaryof aeromagnetic anomaliyassociated with Narrow Canyon caldera

Fault, dashed where approximately Paleozoicrocks located, dottedwhere concealed

• ~~~i~'Je~olcanic rocks, Roads

G Stock ofJumbo Wash Fieldtripstop numbers Mining Districts: Helene, H; Delamar, D; Taylor, T; Pennsylvania, P _~~Q_ ~~~~~;~~~7~ary .~_._~_ Calie."lte C;§:lgJn.. E; res..tJ~e~t d0.!'1El..<>!1'J~rro"", Q.aIlYQ.n ~jc:I~~-"L _

calderas that erupted dacite magma (e.g., Williams middle Miocene tuffs have especially high FeO/(FeO Ridge and Indian Peak calderas) but occurs in some + MgO) ratios (about 0.9) and are mildly rhyolite centers (e.g., Caliente caldera complex). peralkaline. Phenocryst concentrations in Great Basin tuffs are as much as 40% and in rare cases 50% ASH-FLOW TUFF (e.g., Harmony Hills Tuff). All dacite and andesite tuffs are crystal-rich. Quartz and two feldspars are the By far the largest volume of volcanic rock of most common phenocrysts. Mafic phases in late Oligocene-early Miocene age in the central Great calc-alkaline tuffs typically include biotite and Basin is ash-flow tuff that is variably welded in magnetite; less common are hornblende, ilmenite, simple, compound, and multiple cooling units. augite, and hypersthene and, in peralkaline tuffs, Regionally extensive outflow tuff sheets are Fe-rich phenocrysts including olivine, sodic commonly tens and in some places hundreds of amphibole, and pyroxene. Most tuffs contain trace meters thick; several are presently found over areas amounts of apatite and zircon. Titanite (sphene) exceeding 10,000 km2 and the two largest sheets, the occurs in a few tuffs and trace amounts of allanite, Windous Butte and Wah Wah Springs, each cover perrierite, and/or chevkinite has been found in several about 40,000 km2 (after compensation for 50% post­ rhyolite tuffs. volcanic east-west crustal extension these areal extents are reduced to about two-thirds). Most ash-flow sheets show normal compositional zoning (usually based on 'whole-rock Most Great Basin ash-flow tuffs are rhyolite, samples) toward a more mafic upper part which but range to dacite, trachyte, and sparse andesite and contains larger phenocrysts. The Pahranagat latite (lUGS classification, Le Maitre, 1989). Pre-17 Formation is laterally zoned as well and the Windous Ma tuffs are potassic and calc-alkaline, but many Butte Formation has a normally zoned rhyolite outflow that trends without a significant compositional precision 4°ArrAr dating and quantitative gap to dacite intracaldera tuff. determination of thermoremanent-magnetization direction. The 40ArrAr technique provides a Although calc-alkaline rhyolite magmas were precision better than 1% (Dalrymple and Duffield, erupted throughout the entire span of volcanic activity 1988; Deino and Best, 1988; Deino, 1989; McIntosh in the Great Basin, other compositions were also and others, 1992) allowing greater stratigraphic erupted from multiple centers during particular time resolution than conventional K-Ar methods with intervals (e.g., Anderson and Ekren, 1968; uncertainties of about 3 % (McKee and Silberman, Armstrong and others, 1969; Scott and others, 1971; 1970). Noble, 1972; McKee, 1976). Three time-dependent compositional types of ash-flow tuff (Christiansen and Paleomagnetism was used to distinguish Best, 1989; Best and others, 1989a; Noble and between and/or correlate ash-flow sheets by Noble Parker, 1974) will be examined on the field trip. The and others (1968) and Gromme and others, (1972) in Monotony compositional type consists of crystal-rich the Great Basin, and by Mclntosh and others (1992) dacite (compare the "monotonous intermediates" of in the Mogollon-Datil volcanic field. The method Hildreth, 1981) and was named for the Monotony (e.g., Best and others, 1989a; Scott and others, in Tuff of southeastern Nevada. This type includes five press a) depends upon the fact that during cooling an large volume outflow sheets and one intracaldera tuff ash-flow deposit acquires a stable thermoremanent whose aggregate volume exceeds 12,000 knr' that magnetization parallel to the geomagnetic field, thus were emplaced from about 31 to 27 Ma. The second preserving a sampling of both kinds of its time compositional type, the Isom, was named after the variations. These are secular variation of the direction Isom Formation in the southeastern Great Basin and having a characteristic time of one to a few centuries was erupted from several centers east to west across and amplitude of 10-20° of arc, and complete the province mostly 27 to 23 Ma as relatively thin reversals of polarity, which occur at random but with (10-20 m), densely welded sheets. This type consists a characteristic frequency of a few thousand of calc-alkaline trachydacite that contains less than centuries. If, after correction for post-emplacement 20% phenocrysts of plagioclase, pyroxene, and Fe-Ti tectonic tilting (in most instances by measurement of oxides and has unusually high concentrations of Ti02, the eutaxitic compaction foliation), two ash-flow

K20, and Zr compared to other rocks of similar Si02 sheets have paleomagnetic directions that are opposite and CaO content (Christiansen and Best, 1989). The in polarity or are significantly different from each third compositional type is unnamed but is other, the ash-flows cannot have erupted at the same represented in the peralkaline rhyolite tuffs associated time. The converse hypothesis, that if two sheets have with the middle Miocene Kane Springs Wash caldera. the same paleomagnetic direction they are Multiple magma systems forming each compositional contemporaneous, can only be assigned a probability type apparently had similar sources and partial of truth depending on how different their common melting and crystallization histories in similar tectonic direction is from the most likely field direction, that environments. of the geocentric axial dipole.

CORRELATION CENTRAL NEVADA CALDERA COMPLEX

Because of pervasive faulting and consequent This cluster of calderas, one of four considered erosion throughout the Great Basin, the distribution of on the field trip, includes ten calderas and two dismembered outflow sheets cannot be determined by indefinite source areas (Fig. 1, Table 1). Our geologic mapping alone, necessitating application of petrologic, paleomagnetic, and chronologie investi­ reliable correlation techniques. We and our coworkers gations together with field reconnaissance have have found that in most cases position in stratigraphic somewhat refined the work two decades ago of E.B. sequence combined with phenocryst concentration Ekren and W.D. Quinlivan and their associates in the ratios serve as primary correlation criteria; other U.S. Geological Survey (see references in Best and petrographic features and compositional aspects others (1989a) and Gardner and others (1980)). determined in the laboratory are useful in certain cases. Two powerful techniques, that are especially The oldest magmas erupted at 35.3 Ma to form helpful in establishing synchroneity of compositionally the Stone Cabin Formation were equilibrated near the dissimilar outflow and intracaldera tuffs, are high QFM (quartz-fayalite-rnagnetite) buffer; succeeding magmas which formed the outflow and intracaldera INDIAN PEAK CALDERA COMPLEX tuffs of the Windous Butte Formation, Monotony Tuff, and the titanite-bearing tuff of Orange Lichen This complex of southward migrating eruptive Creek were progressively more oxidized. The cycle loci lying astride the Nevada-Utah state line has been was repeated with eruption of the 26.7-Ma magma of described by Best and others (1989b). Early eruptions the lower member of the Shingle Pass Tuff, which of rhyolite at about 33 Ma were followed by three has phenocrysts of quartz, magnetite, and Fe-rich dacite ash flows of the Monotony compositional olivine, succeeded again by more oxidized magmas, type--forming the Cottonwood Wash Tuff, the Wah culminating in the titanite-bearing Fraction Tuff Wah Springs Formation, and the Lund Formation-­ erupted at 18.3 Ma. We have no explanation as yet whose aggregate volume is at least 8,000 knr'. The for these two cycles of increasing degree of oxidation Wah Wah Springs Formation erupted at about 30 Ma of the magmas. The compositionally zoned Stone is unique among Great Basin tuffs because hornblende Cabin magmas equilibrated at relatively low water dominates over biotite in most samples. After the fugacity and high pressure, about 4 kb (based on second and third dacite eruptions, zoned rhyolite ash hornblende and feldspar geobarometers; Radke, flows of an order-of-magnitude smaller volume were 1992), or a depth of about 15 km. The 31.3 Ma emplaced within the older calderas. Several cooling Windous Butte Formation and related intracaldera tuff units of the trachydacite Isom Formation were erupted is a compositionally zoned sequence ranging from late in the history of the Indian Peak magmatic system first erupted rhyolite in the outflow sheet to last at about 27 Ma. If a related caldera for the Isom erupted dacite (66 to 75 % SiO~ forming the exists, it has been concealed beneath younger tuffs intracaldera pile. Phillips (1989) interpreted this and lava flows at the southeast margin of the Indian sequence to have erupted at 750 to 790°C; his Peak caldera complex. hornblende compositions suggest equilibration at less than 4 kb. The dacite Monotony Tuff (27.3 Ma) is Evolved mafic magma, some of which leaked to consistently more mafic (68 to 70 % SiO~ than the the surface as local andesite lava flows, combined Windous Butte outflow sheet and lacks abundant with silicic crustal material in vigorously convecting sanidine, but appears to have last equilibrated at magma chambers to produce the voluminous dacite lower temperature (750°C) and pressure (3 kb) magmas. Mixing processes are indicated by relatively (Phillips, 1989). Major eruptions of compositionally high concentrations of compatible elements (e.g., Ca, zoned rhyolite magma from 26.7 to 26.0 Ma formed Sr, Cr) compared to other magmas from the volcanic at least four rhyolite cooling units of the Shingle Pass field at a given silica concentration. High Sr- and low Tuff. The lowest is mineralogically distinctive, Nd-isotope ratios suggest that the silicic end member containing quartz, two feldspars, Fe-rich olivine and came from the continental crust. The late trachydacite pyroxene and only small quantities of biotite and IS0111 magmas were probably produced by fractiona.l amphibole (Nielsen, 1992); like the older Stone crystallization of andesite with minimal assimilation Cabin, the oldest Shingle Pass magma was relatively of crustal material, perhaps because the low melting dry and last equilibrated near QFM at low pressures temperature fraction was already removed from the (about 3 kb) and 730 to 770°C. In contrast, the crust or crystallization was enhanced by the strong uppermost magma of the Shingle Pass Tuff thermal contrast with wall rocks during the waning crystallized at higher fugacities of oxygen and was stages of magmatism. The Isom tuffs have high quite wet, lacking quartz and anhydrous mafic phases. concentrations of incompatible elements and but low The outflow tuff of the Pahranagat Formation erupted pyroxene- and feldspar-compatible elements (Co, Sr, at 22.6 Ma is laterally and vertically zoned from Ca), as well as lower Sr- and higher Nd-isotope ratios high- to low-silica rhyolite. than the older dacitic tuffs erupted from the Indian Peak caldera complex. Nonetheless, both types of Long intervals of time prior to eruption of the magma appear to have stalled, partially crystallized, Windous Butte, Monotony, and Pahranagat magmas and then erupted from upper crustal magma were punctuated by many very small extrusions of chambers. Estimates of temperature and pressure for andesite, dacite, and rhyolite lava flows and by small crystallization of phenocrysts in the Wah Wah Springs rhyolite ash-flow eruptions. tuff prior to eruption are 850°C and <2 kb and for the Isom tuffs they are 950°C and <2 kb. CALIENTE CALDERA COMPLEX outflow unit intertongued within the lower part of the tuff of Kershaw Canyon presumably derived from a Early work in the Caliente caldera complex source southeast of Caliente; the small-volume, (Figs. 1 and 2) include the stratigraphic studies of moderately welded tuff of Etna (14.0 Ma; Rowley Williams (1967) and reconnaissance mapping by and others, 1991) consisting mostly of outflow in and Noble and others (1968), Noble and McKee (1972), north of the Caliente caldera complex and overlying and Ekren and others (1977). Recent and current the tuff of Kershaw Canyon and derived from a work (referred to in the following pages) consists of source 9 km south-southwest of Caliente, in the quadrangle mapping by P.D. Rowley and R.E. Helene lineament (Fig. 2) on the southern margin of Anderson, isotopic dating by L.W. Snee and H.H. the Delamar caldera; the small, poorly welded Ox Mehnert, geophysics by H.R. Blank, igneous Valley Tuff (12.6-11.4 Ma; H.H. Mehnert, written petrology by L.D. Nealey (e.g., Nealy and others, in commun., 1980) consisting of outflow deposits press), isotope geochemistry by D.M. Unruh (e.g., confined mostly to the Bull Valley Mountains in Utah Unruh and others, in press), and paleomagnetism by and probably derived from the southeastern part of C.S. Grornme, M.R. Hudson, and J.G. Rosenbaum. the Caliente caldera complex (according to Anderson and Hintze (1991) and unpublished mapping by R.E. The Caliente caldera complex consists of Anderson). All known tuffs from the Caliente numerous inset calderas (Fig. 2), four of which we complex are rhyolite, those including and older than have named (Rowley and Siders, 1988). The oldest the tuff of Tepee Rocks are low-silica varieties and known so far is the Clover Creek caldera, the source the younger units are high silica. of the densely welded Bauers Tuff Member (22.7 Ma) and perhaps the slightly older similar Swett Tuff Three episodes of extensional deformation took Member, both of the Condor Canyon Formation place in the Caliente area. The oldest is poorly (Rowley and others, in press b). A fault-bounded constrained except that it predates volcanism which mass of this caldera is exposed on the northern side began at about 31 Ma, is inferred to be Tertiary, and of Caliente. The Delamar caldera, the source of the resulted in detachment faults (see Taylor and Bartley, moderately welded Hiko Tuff (18.6 Ma; Taylor and 1992 and included references). The second, main others, 1989), makes up the western end of the episode began before 19 Ma, based on dated dikes in Caliente caldera complex, as Ekren and others (1977) faults zones, and continued to about 12 Ma; 25 Ma discovered. It IS possible that different parts of the plutons in the Chief district probably also formed Hiko Tuff and the similar Racer Canyon Tuff erupted during this extensional episode (Rowley and others, from separate but virtually simultaneous vents in the 1992), so that it coincides with the duration of Caliente caldera complex, including a vent south of caldera-related magmatism 23 to 13 Ma, if not the ghost town of Delamar (Fig. 2). The Buckboard exceeds it. Extension during this episode was by Canyon caldera is probably a trap-door caldera that complexly intermixed dip-slip and strike-slip faulting subsided on -its northern side, near Caliente, and and warping. Faults of highly variable dip direction erupted the small, poorly welded tuff of Rainbow and amount show complex fault-slip characteristics Canyon (15.6-15.2 Ma; Mehnert and others, 1989). developed during the main phase of extension Other tuff deposits have no well defined caldera; (Michel-Noel and others, 1990). One result of the these include the following: The moderately welded synchronous extension and caldera development is Racer Canyon Tuff (19.2?-18.7 Ma; L.W. Snee, that caldera margins are largely bounded by faults, written commun., 1991) probably derived from the some" of which are related to east-west eastern end of the complex; the minor, poorly welded lineaments. The best example is the Delamar caldera, tuff of Tepee Rocks (17.8 Ma; L.W. Snee, written which is largely bounded on the north by some faults commun., 1991) probably derived from east of the of the Timpahute lineament of Ekren and others Delamar caldera but produced little surviving outflow (1977) and on the south by some faults of the Helene tuff; the small, poorly welded tuff of Kershaw lineament (Fig. 2). Another result of the synchronous Canyon (15.5?-14.0 Ma) interbedded with outflow extension and magmatism is the creation of gold units of the Kane Wash Tuff south of Caliente with a deposits in fault and breccia zones. These relations presumed source southeast of town (its outflow is are similar to those in the Walker Lane (Hardyman confined to an area within 15 km of the northern side and Oldow, 1991), one of the most important gold of the Caliente caldera complex); the minor, belts in the country. The third episode of extension moderately welded tuff of Sawmill Canyon, a thin was basin-range faulting, inferred to be post-l0 Ma, when the present topography dominated by of the members consist of sanidine, quartz, north-south ranges formed beyond the limits of the hedenbergitic clinopyroxene, fayalitic olivine, zircon, Caliente caldera complex (Anderson, 1989). chevkinite, and Fe-Ti oxides.

Nearby epithermal gold districts include the The northern and western margin of the Kane small Chief mining district north of Caliente (Rowley Springs Wash caldera in the Delamar Mountains has and others, 1990; Rowley and Shroba, 1991; Rowley been affected by small offsets on northwest-striking and others, 1991; Rowley and others, 1992), the dextral faults. A small stock (post-18.6 and pre-14.7 major Delamar (Ferguson) mining district just Ma; marked by two Is, Fig. 2) has been offset about southwest of the caldera complex (Swadley and one km by one of these faults, but the caldera wall Rowley, 1992), the Taylor (Easter) mine, and the indicates less than a few hundred meters of offset on Pennsylvania and Goldstrike on the south and the same fault. In contrast, the eastern part of the southeast sides of the complex. Kane Spring Wash caldera in the Meadow Valley Mountains is tilted as much as 30° eastward, cut by KANE SPRINGS WASH CALDERA COMPLEX northeast-striking sinistral-oblique and dip-slip faults, and injected by dikes feeding post-collapse Early work in the Delamar and Meadow Valley caldera-filling volcanism (Harding, 1991; Harding Mountains began with correlations of ash-flow tuffs and others, in press). The contrasting degree and style by Cook (1965) that lead to petrologic studies of the of deformation in the western and eastern parts of this alkaline metaluminous to mildly peralkaline ash-flow 30 km x 13 km caldera demonstrates the tuffs and minor lavas erupted from the Kane Springs heterogeneity of extensional deformation typical in Wash caldera and related sources (e.g., Noble, 1968; this part of the Basin and Range (Scott, 1990). Noble and Parker, 1974; Novak, 1984; Novak and Mahood, 1986). In addition to interdisciplinary One precursor to the Kane Springs Wash studies by the same workers as on the Caliente caldera is the Narrow Canyon caldera (Scott and complex listed above, detailed mapping of the others, 1991). Remnants of its margin, caldera-wall complex is by R.B. Scott, W C Swadley, A.E. breccia, and megabreccia are exposed within Narrow Harding, W.R. Page, and E. H. Pampeyan and Canyon (NCC in Fig. 2). Although the compositional petrological studies by R.B. Scott, S.W. Novak, and trends and phenocryst mineralogy in the associated L.D. Nealey. Mapping within and near the Kane metaluminous to mildly peralkaline rhyolites are Springs Wash caldera was aided by previous work similar to those of the Kane Springs Wash caldera, (Novak, 1985; Pampeyan, 1989; and quadrangle maps the Narrow Canyon caldera differs in that its margin referred to by Scott and others, in press a). We now has been thoroughly dismembered by recognize that the Kane Springs Wash caldera is the northeast-striking sinistral faults, displays a shallow youngestof a complex that includes the Narrow resurgent trachytic intrusion, and erupted very limited Canyon caldera (Scott and others, 1991) and probably outflow tuff. Caldera outflow, intracaldera equivalents two others that have not been located, as suggested by to the outflow units, and post-collapse caldera-filling Novak (1984). The Kane Wash Tuff has a volume of volcanic units are difficult to distinguish because about 150 knr': it has been redefined to consist only exposures of Narrow Canyon caldera units under the of the three outflow cooling units derived from the blanket of Kane Wash Tuff are limited. Kane Springs Wash caldera (Scott and others, in press b). The older, single cooling unit of the metaluminous No calderas have been found for the Grapevine Spring Member (14.7 Ma) and the pre-14.7-Ma Sunflower Mountain Tuff or the pre­ younger, mildly peralkaline Gregerson Basin 15.6-Ma Delamar Lake Tuff (Scott and others, in Member, which consists of twonearly identical press a) that underlie the Kane Wash Tuff and overlie cooling units (14.55 and 14.4 Ma, L.W. Snee, the Hiko Tuff in the southern Delamar Mountains. written commun., 1991), were erupted from a magma However, thicknesses of these units suggest that their chamber zoned from peralkaline rhyolite near 820°C sources lie, respectively, to the southwest and to the at the top to a dominant volume of mafic trachyte northwest of the Kane Springs Wash caldera. Both the near lOOO°C (Novak and Mahood, 1986). Zirconium Sunflower Mountain and Delamar Lake are alkaline contents of the Grapevine Spring range from 600 to metaluminous rhyolites and contain phenocrysts 800 ppm whereas those of the Gregerson Basin range similar to those of the Kane Wash Tuff. from 800 to 1300 ppm. Phenocrysts in rhyolitic parts TABLE 1. STRATIGRAPHY AND PETROGRAPHY OF TUFFS RELATED TO CALDERA COMPLEXES

CENTRAL NEVADA INDIAN PEAK CALIENTE KANE SPRINGS WASH OX VALLEY TUFF (?) 12.5 Ma rhyolite 15-49q 55-80s 2-5p 2b 1-3h tc TUFF OF ETNA (?) 14± Ma mvome 20'30q 60-75s 1 p 2-3b tr h Ir c 20-50101 KANE WASH TUFF (Kane Springs Wash) # GREGERSON BASIN MBR 144 and 145 Ma two zoned cooling units, trachyte top TUFFS OF KERSHAW CANYON and comendlte below that contuins q, 5. c, I. i SAWMILL CANYON (1) 14.5 Ma rhyolite # GRAPEVINE SPRING MBR 14 7 Ma zoned rhyolite 10-25q 60-755 5-1Oc Sf 0-5i 1O~35tOI TUFFS FROM NARROW CANYON CALDERA ? Ma peralkaline rhyolite Q sci O'SUNFLOWER MOUNTAIN TUFF (?) 147$ Ma rhyolite 30-55q 40-655 If C If f 5-20101 TUFF OF RAINBOW CANYON (Buckboard Canyon) 15.6 Ma rhyolite O'DELAMAR LAKE TUFF (?) 15.6 $ Ma rhyolite 2S·50q 40-705 If C 10-25101 TUFF OF TEPEE ROCKS (?) 17.8 Ma rhyolile 3S-55q 20-405 1O-35p 1-7b Ir I # FRACTION TUFF (Cathedral Ridge) 18.3 Ma rhyolite :fI:HIKO TUFF (Delamar) 18.6 Ma zoned rhyolite 10-3Sq 15-355 30-65p 5-15b 0-5h Ir c If 130-40tOI # RACER CANyON TUFF (?) 18.7 Ma rhyolile 15-44q 5-405 15-BOp 1-13b 5h tr t 1/:HARMONY HILLS TUFF (source probably lies east of caldera complex in Bull Valley Mounlains area) 22.2± Ma anoesue-rracnvandcsue #PAHRANAGAT FORMATION (Kawich) 22.6 Ma zoned rhyolite 0~10q 0-3s 55-70p 10-20b 0-15h O-Sc 40-60101 21-49q 22-42s 16-42p 1-Gb O-Ih Il-jc 1-1-51101 CONDOR CANYON FORMATION O'BAUERS TUFF MBR (Clover Creek) 22.7 Ma zoned rhyolile Ir q 15-45s 35-70p 0-lOb Ir c 10-20101 O'SWETT TUFF MBR (Clover Creek?) ? Ma rhyoJile 65-85p 5-20b 5-15101 O'LEACH CANYON FORMATION (?) 23.8 Ma rhyolite 20-50q 5-40s 20-55p 0-15b tr h tr c Ir , 10·30101 TUFF OF BIG TEN PEAK (Big Ten Peak) 25 Ma rhyolite TUFF OF GOBLIN KNOBS (Goblin Knobs) 25.4 Ma dacile # TUFF OF LUNAR CUESTA (unnamed) 25.4 Ma zoned low-Si rhyolile to dacite lS-30q 5~25s 40-70p 5-15b 0-5h 10-30101

150M FORMATION (See tcomotej trachydaclte 70-aOp 5-20c 5-2010t # HOLE-IN-THE-WALL MBR ? Ma TUFF OF HAMLIGHT CANYON ? Ma 3 or more cooling units SHINGLE PASS TUFF (Quinn Canyon Range) rhyolite # UPPER COOLING UNIT 26.0 Ma tr q 30-40s 50-60p 5-15b lr h Ir c 5-10'0' ..INTERMEDIATE COOLING UNITS 26.4-28.5 Ma 0-5Q 25-505 35-60p 5-10b Ir h O-Sc 5-15101 # LOWER COOLING UNIT 26.7 Ma 5-15q 45-605 25-35p tr b Ir h 0-5 c 1 1 Ir a 10-20101 TUFF OF ORANGE LICHEN CREEK (Kiln Canyon) 26.8 Ma rhyoJile

41: BALD HILLS MaR 27.0 Ma 2 or more coofingunils .. MONOTONY TUFF (Pancake Range) 27.3 Ma locally 3 cooling units dacile 5-30q 0-15s 4.5-65p 5-20b 0-10 h o-ioc 10-60101

# PETROGLYPH CLIFF IGNIMBRITE (source nol located bullies between central Nevada and In"jian Peak caldera complexes) 27.6± Ma trachydacjte 60-BOp 20-30c 5-1010t

RIPGUT FORMATION (MI. Wilson) ? Ma zoned rhyolite O'LUND FORMATION (While Rock) 27.9 Ma dacile TUFF OF HOT CREEK CANYON (HOI Creek) 29.7 Ma rhyolite O'WAH WAH SPRINGS FORMATION CIndianPeak) between 29.6 and 31.1 Ma dacite 0-11q 52-68p 5-12b 14-32h 0-4c 11-5010' #- COTTONWOOD WASH TUFF (?) ? Ma dacile # WINDOUS BUTTE FORMAT'ON (Williams Ridge) 31.3 Ma intracaldera dacile 1O~30q 0-305 3S-GOp 5~ lOb 0-10h lr c 25-55101 outflow zoned rhyolite 10-40q 1D~45s lS-55p 0-20b 0-1011 tr c 20-50101 # PANCAI(E SUMMIT TUFF (Broken Back 2) 34.8 Ma rhyolile '# STONE CABIN FORMATION (?) 35.3 Ma zoned rhyolite coofing units 20-55q 0-45s 1O-60p 0-10b Ir h 23-52101

Tuff unit in bold if seen on field trip. # regionaBy extensive outflow shee~. $ These dates may be 0.4 Ma 100 yHot Creek Range to the N. On the W flank of the DAY ONE Saturday May 22, 1993 Hot Creek Range, to the left of the most We will see some of the calderas in the central northerly, black lava-capped hill, is a sliver of Nevada caldera complex and some outflow ash-flow the Kiln Canyon caldera which formed during sheets in "outflow alley" associated with it and the deposition of the tuff of Orange Lichen Creek Indian Peak caldera complex. A stop will be made to at 26.8 Ma. To N, prominent W-dipping cuesta examine a highly extended terrane and associated is made of cooling units of trachydacite ash­ coarse clastic deposits of Pliocene age in the Grant flow tuff of the Isom compositional type. Range. Beyond cuesta is the high Monitor Range, which harbors the Big Ten Peak caldera. Tonopah, a Shoshone or Paiute Indian word 5.9 37.2 Intersection of gravel road to right (S) to meaning "greasewood spring", was a famous bonanza Golden Arrow and Silver Bow mining districts mining camp from about 1900 to 1930. Bonham and near margin of Kawich caldera. Garside (1979) report that epithermal veins of silver 7.6 44.8 Summit. To E and less than one km S of sulfides and sulfosalts are cut by and are overlain by highway at N end of Kawich Range is the several rhyolite domes that form the prominent hills topographic margin of the Kawich caldera, around Tonopah. Mineralization and alteration seem marked by about 30 m of tuff breccia overlain to be associated with a probable caldera that formed by more than one km of multiple cooling units during eruption of the Fraction Tuff at 19.8 Ma (not of caldera-filling ash-flow tuff, which contain to be confused with the Fraction Tuff emplaced at lithic blocks as much as 20 m in diameter and 18.3 Ma on the Nellis Air Force Bombing and by sedimentary deposits (Gardner and others, Gunnery Range). 1980). These intracaldera rocks of the Pahranagat Formation overlie propylitically and Interval miles argillically altered Monotony Tuff emplaced at Cumulative miles 27.3 Ma and exposed on either side of the 0.0 0.0 Road log begins at intersection of US 95 highway. Large masses of broken Paleozoic and 6 in the SE part of Tonopah. Head E out rock lie within the Monotony and are of town on US 6. interpreted to be slide blocks within the 5.5 5.5 Intersection NV 376. Continue E. Monotony source--the Pancake Range caldera. 1.4 6.9 Intersection of road to right (S) to Tonopah 5.0 49.8 Warm Springs roadhouse (abandoned, but airport and small refinery which processes has hot bath and telephone!) and intersection crude from Railroad Valley field. with NV 375. To NE is aptly named Pancake 5.0 11.9 Intersection of road to S to Tonopah Test Range and to S of NV 375 is the Reveille Range, home of the Stealth fighter plane, on Range. Continue on US 6. the Nellis Air Force Range. Continue E. 7.5 57.3 To E across Hot Creek Valley in far 4.7 16.6 Red-brown outflow tuff of Pahranagat distance through pass between Pancake and Formation deposited at 22.6 Ma in hills on both Reveille Ranges is the Quinn Canyon Range, sides of highway for next two miles. location of the Quinn Canyon caldera that was 3.2 19.8 Rhyolite dome to N. To NE, brown cliff the source of at least four cooling units of the above white tuff is Pahranagat outflow, capped Shingle Pass Tuff deposited between 26.7 and by mafic lava flow in small hill. Hills ahead are 26.0 Ma. At 1 o'clock is the topographic wall of intracaldera tuff of Lunar Cuesta deposited at of the Pancake Range caldera, source of 25.4 Ma in its unnamed caldera. Monotony Tuff (Fig. 3). 8.4 28.2 Hills from 9 to 12 o'clock are cooling 7.1 64.4 STOP 1 (Best, Gromme, and Deino) units of intracaldera tuff of Lunar Cuesta, Pull off highway into rest stop S of Blue Jay zoned low-silica rhyolite to dacite. maintenance station for overview of Central 3.1 31.3 Across Stone Cabin Valley to SE is the Nevada caldera complex. This stop lies within Kawich Range, which forms most of the the oldest recognized caldera in the complex, exposed part of the Kawich caldera that the Williams Ridge, whose collapse was collapsed at 22.6 Ma as the Pahranagat ash initiated by eruption of rhyolite ash flows that flow was erupted. The northern margin of the formed the Windous Butte Formation around caldera lies just S of the highway where it the caldera and continued with eruption of 11.0 75.4 Turn left (N) onto gravel road and proceed toward Moores Station, a remnant of the stage-coach era. Mesa tilted toward us is capped by a 10-Ma basaltic lava flow (Ekren and others, 1973). 5.6 81.0 Turn sharp left onto dirt track through sagebrush and head SW. 0.5 81.5 STOP 2 (Best and Christiansen) A 1700-m drill hole here (Ekren and others, 1973) inside the Williams Ridge caldera penetrated Figure 3. Only exposed topographic margin of the only intracaldera dacite tuff, which is exposed Pancake Range caldera, which was the source of just S of hole. This intracaldera tuff of the Monotony Tuff. As recognized by Ekren and Williams Ridge and Morey Peak has the same others (1974), this margin (arrow) is defined by paleomagnetic direction and 40Arj39Ar age the post-Monotony Shingle Pass Tuff and (mean of determinations on two samples of overlying tuff of Lunar Cuesta (cliff) on the right 31.32 ± 0.08 Ma), within analytical banked against and capping the pre-Monotony uncertainty, as the outflow Windous Butte tuff of Halligan Mesa and overlying tuff of (mean of two, 31.31 ± 0.11 Ma). Palisade Mesa on the left. Turn around and return to highway. 6.1 87.6 Turn left (E) onto highway. To S are thin cooling units of Shingle Pass Tuff capped by outflow tuff of Lunar Cuesta. 1.0 88.6 Road cut in a 300 m-thick simple cooling unit of the Monotony Tuff. dacite that formed the intracaldera tuff of 1.4 90.0 Intersection with gravel road S to Lunar Williams Ridge and Morey Peak. Across Hot Crater. The surrounding youthful alkaline basalt Creek Valley to the N in the Hot Creek Range field of lava flows, about 70 cinder cones and horst, the entire eastern escarpment, about 1 at least two maars, developed since 6 Ma along km high beneath Morey Peak, is intracaldera a NNE-striking fissure system (Scott and Trask, tuff. Just W of Morey Peak is the N-S-trending 1971). Lunar Crater is a spectacular maar eastern margin of the Hot Creek caldera (John, easily reached from this turnoff. Nearly every 1987). Additional fill in the Williams Ridge lava flow and ejecta deposit contains caldera consists of landslide deposits and local megacrysts and Ti-Al-rich xenoliths of gabbro sedimentary material plus subsequent local lava . and c1inopyroxenite, but only three vents flows and a much greater volume of several erupted Cr-rich spinel peridotite of mantle rhyolite ash-flow deposits derived from the origin (Menzies and others, 1987). Late local magma system. To the NE, two of these Cenozoic volcanism is concentrated along the rhyolite tuffs with a combined thickness of margins of the Great Basin and its only about 300 m make up most of Palisade Mesa. expression within the interior is here in the The upper unit, aptly named the tuff of Palisade Lunar Crater field (Fitton and others, 1988). Mesa (emplaced at 29.6 Ma), has a spectacular Youthful "Black Rock" lava flow to E contains system of columnar joints evident from the abundant megacrysts of olivine, pyroxene, and highway to the N. Due E of this stop, a 240 m­ feldspar. thick section of Shingle Pass Tuff and overlying 9.1 99.1 Black Rock Summit. To NE near relay outflow tuff of Lunar Cuesta is banked tower are large slide masses of shattered and disconformably against the pre-Monotony tuffs altered Paleozoic carbonate rock resting on (Fig. 3). Ekren and others (1974) interpreted altered intracaldera tuff of the Windous Butte this to be a segment of the topographic wall of Formation within a few kilometers of the the caldera formed as Monotony dacite ash margin of the Williams Ridge caldera flows were erupted at 27.3 Ma. Stewart and (Quinlivan and others, 1974). For next four Carlson (1976) designated it the Pancake Range miles, highway follows a Quaternary basalt lava caldera. flow; hills on either side of highway are of Continue NE on highway. altered intracaldera tuff and Paleozoic rock. 3.5 102.6 Topographic margin of Williams Ridge Limestone, Railroad Valley Rhyolite, slightly caldera near here. younger sedimentary and ash-flow tuffs, and 3.4 106.0 STOP 3 (Christiansen) Two cooling the 34.5-Ma Stone Cabin and Windous Butte units of Monotony Tuff; vitrophyre of lower Formations, tectonically emplaced within the unit lies at highway level. The Monotony Horse Camp sequence. Farther E, across the appears to have been deposited directly on Ragged Ridge fault that borders the Horse Paleozoic rocks here; the absence of the Camp Basin in the Horse Range, several Windous Butte Formation might imply uplift allochthonous sheets of Paleozoic and Tertiary near the Williams Ridge caldera prior to its rocks overlie autochthonous Paleozoic strata, a deposition. relationship typical of detachment-style 1.7 107.7 Lockes ranch. extensional tectonics. Presumably, these 7.7 115.4 To N is the E flank of the Pancake detachments fed the growing Horse Camp Range where alternating light- and dark-brown depocenter during the Pliocene. Scott (1965; layers are cooling units of the Stone Cabin 1966; also W.E. Brooks, written commun., Formation (Radke, 1992). The Railroad Valley 1992) noted that volcanic strata in Ragged oil field produces from a pre-Windous Butte, Ridge have K 20 contents in the range of 8 to post-Stone Cabin tuff and the upper Eocene to 10 % in contrast to relatively unaltered ranges lower Oligocene clastic Sheep Pass Formation. of 4 to 6 % elsewhere. This metasomatism 8.0 123.4 Red and brown weathering Ragged could have happened as the highly attenuated Ridge to E is a steep NE tilted section of strata of Ragged Ridge, buried under about 3 Oligocene rhyolite lava flows, clastic rocks, km of Pliocene sediments, were juxtaposed via and overlying regional tuff sheets including, in a basal detachment over warmer middle crust, ascending order, the Stone Cabin, Windous resulting in hydrothermal circulation that caused Butte (on skyline), and Wah Wah Springs alkali exchange (compare Walker and others, Formations and Shingle Pass Tuff. Overlying 1992). these sheets is a thick (as much as 3 km) 0.7 133.9 Horse Camp Formation; optional stop. sequence of clastic sedimentary rocks, tuffs, 4.3 138.2 Calloway Well and Stone Cabin, and gravity-slide masses of the Pliocene Horse namesakes and type sections of the formations. Camp Formation (Moores, Scott, and Lumsden, of those names. The rhyolitic Calloway Well 1968). The significant post-volcanic extensional Formation (in low hills immediately behind faults in the Grant Range contrast sharply with cabin and well), named by Moores, Scott, and the few faults cutting the flat-lying strata of the Lumsden (1968), is a sequence of bedded Pancake Range to the W. pyroclastic-surge and -fall deposits alternating 6.0 129.4 Community of Currant. Turn SE on with massive ash-flow tuff emplaced at 35.3 gravel road passing N end of Ragged Ridge. Ma. The overlying Stone Cabin Formation, 3.8 133.2 STOP 4 (Scutt) To NW is a low ridge which has the same 40ArP9Ar age within within the Horse Camp Basin that contains the analytical uncertainty, will be examined at the lowest of a series of tectonic breccia slices that next stop. were emplaced as gravity slides during 8.7 146.9 STOP 5 (Best, Christiansen, Deino, deposition of the Pliocene Horse Camp Gromme) Intersection of 4X4 trail to left (N). Formation (Moores, Scott, and Lumsden, Hill to NE is brown, 210-m-thick, Stone Cabin 1968). These slices consist of brecciated Formation overlain in distance by the 280­ 31.3-Ma Windous Butte Formation and 33-Ma m-thick Windous Butte Formation seen here as Railroad Valley Rhyolite. To SW, units in a thin ledge of black vitrophyre. Complete Ragged Ridge dip 40-70° eastward, flooring the sections of these two rhyolite tuffs can be seen Horse Camp depocenter. Dips in the Horse by hiking to N around W side of hill where the Camp Formation decrease upsection from 70° bottom part of the Stone Cabin contains 10-20 to 30°. To NE is a hill consisting of tectonic m of black vitrophyre underlain by several m of slices of highly attenuated, brecciated, but white to salmon-colored, weakly welded stratigraphically ordered Cambrian to Devonian ash-flow tuff; beneath the Stone Cabin is a few strata within the upper part of the Horse Camp tens of m of cross-bedded, well-sorted gray sediments. To SE, Red Mountain contains sandstone overlying Paleozoic rocks. By brecciated slices of the Mississippian Joana continuing around hill to E, one can go up section through the E dipping Stone Cabin into As time permits, late afternoon stops will be the overlying conformable Windous Butte, made near Caliente as an introduction to the Caliente which consists of a few m of weakly welded caldera complex to see the Clover Creek caldera salmon-colored ash-flow tuff overlain by black (source of the Bauers Tuff Member of the Condor vitrophyre and then brown devitrified tuff, both : Canyon Formation) and the Delamar caldera (Hiko densely welded. Note the absence of bedded Tuff). plinian ash-fall deposits at the base of these two tuff sheets. On the SE side of the road, capping 0.0 0.0 Roadlog begins at Lanes Ranch Motel. the low hill, is the dacitic, hornblende-rich Wah Head S on NV 318. Wah Springs Formation derived from the Indian, 4.3 4.3 Lund. Peak caldera in the Indian Peak caldera . 15.4 19.7 Intersection with gravel road on W that complex (Fig. 1). The lowest Wah Wah Springs goes to stop 5. here is a slabby weathering welded tuff that 8.7 28.4 Intersection with gravel road on E that grades upward into about one m of dark gray goes to Shingle Pass where an exceptional vitrophyre and then into devitrified red-brown section of volcanic units (Best and others, tuff. In the low rolling hills to the E are three, 1989a, p.118), including the Shingle Pass Tuff, thin, devitrified, densely welded ash-flow tuff is well exposed. cooling units. The first is the red-brown lower 5.5 33.9 Sunnyside ranch. member of the Shingle Pass Tuff containing 23.1 57.0 Intersection with gravel road on E that conspicuous phenocrysts of sanidine and lesser goes to Bristol Wells and Pioche. plagioclase and a few Fe-rich pyroxene and 17.6 74.6 STOP 6 (Gromme, Best, and olivine manifest by rusty spots; this unit across Christiansen) Intersection with gravel road on a small gully is purplish and grussy weathering. W to petroglyphs. Exposed in hill to W (Fig. 4) The overlying cooling unit of unknown are, in ascending order, the Petroglyph Cliff stratigraphic identity is a brown tuff containing Ignimbrite (named and briefly described by obvious shards and sparse tiny smokey quartz 'Cook, 1965), Monotony Tuff, tuff of Hamilton phenocrysts. It is overlain by the upper member Spring (Taylor and others, 1989), and a of the Shingle Pass Tuff, which here is a hornblende-pyroxene andesite lava flow. The purplish-gray tuff containing light gray pumice Petroglyph Cliff (27.3-27.9 Ma), legitimately a and phenocrysts of two feldspars and biotite. welded tuff breccia, is one of the oldest of the Continue E on gravel road. Isom-compositional-type tuffs in the Great 12.8 159.7 Intersection with NV 318. Turn left (N). Basin and is unusual because of its abundant 14.5 173.2 Enter farming community of Lund. lapilli- and block-size fragments. This cooling 5.2 178.4 Lanes Ranch Motel, our overnight unit is found only here and in two ranges to the accommodation. Community of Preston lies in E and probably vented nearby. The distal part grove of trees to W. of the Monotony outflow sheet here is fairly thick but poorly welded. The Hamilton Spring DAY TWO Sunday May 23, 1993 is another Isom type tuff that may correlate, An extraordinarily complete, well-exposed according to paleomagnetic data (Scott and section of regional ash-flow outflow sheets will be others, in press a), with the Baldhills Tuff examined in White River Narrows along NV 318. Member of the Isom Formation that is widely The sheets in this "outflow alley" section lie between exposed to the E into Utah. their sources in three nearby caldera complexes: the Continue S on highway. Central Nevada to the west, the Indian Peak to the 0.9 75.5 Site of a famous photograph and diagram east, and the Caliente to the southeast. This section, by Cook (1965, Fig. 29) showing a pinch-out and the one at stop 5 yesterday, are representative of of ash-flow sheets over the stubby toe of a thick numerous outflow volcanic sections emplaced during hornblende-augite andesite lava flow. Cook the ignimbrite flareup in the central and southeastern concluded that, due to the differential and Great Basin which contain little or no sediment and diminishing compaction of each successively angular discordances indicative of regional syn­ deposited tuff, the topographic relief over the volcanic extension (compare Gans and others, 1989, lava flow diminished as each younger ash flow Fig. 18 and Best and Christiansen, 1991, Fig. 6). was emplaced. Assuming the top of each flow SOUTH NORTH

lava flow I

M

R P- STOP 7 petroglyphs STOP 6

Figure 4. Schematic diagram of volcanic section along west side of NV 318 north of the White River Narrows between stops 6 and 7. Vertically exaggerated diagram omits numerous slump blocks and Quaternary deposits. The south dipping contact between the two hornblende-pyroxene andesite lava flows is apparently depositional; if the contact were a fault, a complementary antithetic fault of just the right amount of displacement would be required to the south in order for the tuff of Hamilton Spring to have no net offset between stop 6 and the petroglyphs. Stratigraphic units in ascending order are: P, Petroglyph Cliff Ignimbrite; M, Monotony Tuff; H, tuff of Hamilton Spring; R, crystal-rich rhyolite tuff; S, lower member of Shingle Pass Tuff; I, Hole-in-the-Wall Member of the Isom Formation; L, Leach Canyon Formation; C, Condor Canyon Formation.

was initially horizontal, he calculated about identified; perhaps it is under the Panaca basin 50 % compaction. between Pioche and Caliente. The Condor 1.0 76.5 STOP 7 (Gromme, Christiansen, and Canyon Formation was derived from the Clover Rowley) At the N end of White River Narrows Creek caldera of the Caliente complex. The are four ash-flow sheets consisting of, in Harmony Hills Tuff (between 22.7 and 21.7 ascending order starting at the road cut, a Ma; Rowley and others, 1989) has been crystal-rich rhyolite tuff of unknown proposed to come from the Caliente caldera stratigraphic identity, upper member of the complex (Ekren and others, 1977) but more Shingle Pass Tuff, thin, densely welded likely it was derived from the Bull Valley Hole-in-the-Wall Tuff Member of the 1som Mountains to the E in Utah (Blank, 1959; Formation, and thick (104 m) Leach Canyon Blank and others, 1992). Formation containing spectacular columnar Continue S on highway. joints. The three named units had three 11.0 88.7 Outcrops to E are of Hiko Tuff and different sources, the Central Nevada caldera underlying Bauers Tuff Member of the Condor complex to the NW, possibly the Indian Peak Canyon Formation. The same units are exposed complex to the NE, and Caliente caldera for next two miles through Hiko Narrows. complex to the SE, respectively. 8.9 97.6 Community of Hiko (Paiute word for Continue S on highway. "white man's town") 1.2 77.7 STOP 8 (Rowley, Christiansen, and 4.7 102.3 Veer left, continuing on NV 318. Best) At the S end of White River Narrows are 0.6 102.9 Turn left (E) on US 93 toward Caliente. five regional ash-flow sheets overlying Leach 2.8 105.7 Pass through Hiko Range. Crystal-rich Canyon, which here is a pink, lithic-rich rhyolite of the Hiko Tuff, which weathers into devitrified tuff. These five tuff sheets are, in bulbous forms much like granite, is overlain by ascending order: Condor Canyon Formation thin, middle Miocene, densely welded Kane consisting of Swett and overlying Bauers Tuff Wash Tuff. Members, Pahranagat Formation, Harmony 10.5 116.2 Pahroc Summit Pass. To S is South Hills Tuff (exposed only in a small hill), and Pahroc Range where Hiko outflow is as much Hiko Tuff, which has large columnar joints. as 350 m thick; to N is North Pahroc Range. The Leach Canyon Formation emplaced at 23.8 R.B. Scott has mapped east-striking strike-slip Ma was suggested, on the basis of distribution faults near the road here that he interprets to be and isopach data, to have been derived from the part of the Timpahute lineament of Ekren and northern part of the Caliente caldera complex others (1976). (Williams, 1967), but no source has been Continue E across Delamar Valley. 10.2 126.4 To S, gravel road to ghost town of 3.0 139.8 Road passes through vertical cuts on Delamar, on the Wedge of the Delamar Range, both sides consisting of the gray and pink, and center of the Ferguson mining district, an E-dipping, 3 m-thick tuff of Etna. The tuff is epithermal gold deposit that was Nevada's most sandwiched between tan sedimentary rocks of productive at the turn of the century. Newman Canyon. Several hundred meters N of 2.8 129.2 Road cut of Cambrian Highland Peak here, tan sedimentary rocks of this unit that. _ Formation on left and hills of the same unit to overlie tuff of Etna contain a bed of waterlaid left and right. pumice deposited at 13.8 Ma (H.H. Mehnert, 0.5 129.7 Cross NNE-striking Seven Oaks Spring unpub. data, 1991; Rowley and others, 1991), fault, which separates Highland Peak Formation thus constraining the age of the tuff of Etna. on W from both sedimentary rocks (left of Note numerous NNW-striking oblique-slip road) and intracaldera Hiko Tuff (right of road) faults in roadcuts. Continue E, tuff of Etna on E. The sharp peak at 10 o'clock is underlain thickens and caps mesas. by a Cambrian section ranging from red 1.8 141.6 Toreva block of tuff of Etna on right. In Zabriskie Quartzite at road level to Highland gulch to left, behind houses and junk cars, the Peak Formation at the top. The Zabriskie, Newman Canyon detachment fault dips gently however, is highly faulted just N of the road toward road, just above the dark spur. Left of against masses of intracaldera Hiko to the S. the dark spur, in the main gulch, this fault cuts Thus, the margin of the Delamar caldera here, off a high-angle oblique left-slip fault, the as in many other places, is a fault, not a typical Gravel Pit fault of Rowley and others collapsed structure. (1992). This fault has at least 2 km of left slip, 1.4 131.1 Road bends to right and dirt road on left and in the main gulch here it is intruded by a leads along an ENE-trending valley underlain dike of the porphyry of Meadow Valley Wash by the same caldera margin. Continue on US emplaced at 19.4 Ma (Snee and others, 1990; 93 ESE through intracaldera Hiko Tuff on both Rowley and others, 1992). sides of road which contains numerous 0.7 142.3 Enter town limits of Caliente. interbeds, some as much as 100 m thick, of 0.5 142.8 Church on left. Intersection with NV volcanic mudflow breccia--typical of the upper 317 on right. Continue along US 93 through part of the Delamar intracaldera fill. Some town to just past the post office. interbeds are megabreccia masses from 0.7 143.5 Turn right, heading E across Union sloughing of caldera walls, but most clastic Pacific Railroad tracks, then right (S) past beds are of dacite to andesite flow rocks, and stores. the deposits are interpreted to be from 0.1 143.6 Turn left and head up small canyon stratovolcanoes near the caldera rim that were along a dirt road through a thick, steeply synchronous with caldera volcanism and NE-dipping fanglomerate sequence that contains faulting. white and-yellow air-fall tuff beds correlated 1.7 132.8 Oak Springs Summit, crest of the with the tuff of Kershaw Canyon. Delamar Mountains. 1.5 145.1 At the pass, take a sharp right r:'R) 3.5 136.3 Roadcuts on left in Newman Canyon along dirt road to overlook near relay tower. show intertonguing intracaldera beds of 0.6 145.7 STOP 9 (Anderson and Rowley) mudflow breccia, megabreccia, and Hiko Tuff. Overview of the geology of the N margin of the The Delamar caldera was subsequently Caliente caldera complex and synchronous complexly faulted, mostly during the main, or faults. The Clover Creek caldera is best latest Oligocene to middle Miocene, episode of displayed NE of Caliente, in the canyon of extension along mostly NNW-striking oblique­ Clover Creek. This caldera, recognized by slip faults. About 200 m farther E the valley Rowley and Siders (1988), is the source of the widens into a basin of upper Tertiary gravels Bauers Tuff Member (22.7 Ma; Best and and sand. Cross the major NNW-striking others, 1989a) of the Condor Canyon oblique-slip Dula Canyon fault, which probably Formation; about 400 m of intracaldera has several kilometers of right-slip and rhyolitic Bauers is exposed on the N wall of the unknown amounts of down-to-the-E normal canyon just E of town. W of that, in Meadow slip. Valley Wash and W of Caliente, are two N- to NNE-striking oblique-slip faults, each with at lineament. Another 1 km upstream is least 2 km of left slip. E and N of the 400 m underlying, more densely welded intracaldera exposures of Bauers is a NNW oblique-slip Hiko and intertongued caldera megabreccia fault with at least 2 km of right slip and 0.5 km deposited in the Delamar caldera from of normal slip that enters Meadow Valley Wash landslides off the oversteepened fault/caldera 3 km N of Caliente. A S-dipping low-angle margin. normal fault that contains the tuff of Etna (14 Ma according to dating by H.H. Mehnert) in its Return to US 93 and Caliente for overnight. hanging wall, passes under the scarp here. The Delamar caldera (Rowley and Siders, 1988), DAY 3 Monday May 24, 1993 the source of the rhyolitic Hiko Tuff (18.6 Ma; The first part of the day will continue to Taylor and others, 1989), is inset into the provide a brief overview of the Caliente caldera Clover Creek caldera and underlies us but its complex by way of a traverse southward from nearest exposures are in English Canyon 2 km Caliente to Elgin through Rainbow Canyon along NV to our E. Tan post-Etna clastic sedimentary 317 to see the Buckboard Canyon caldera (tuff of rocks that show lesser deformation upward in Rainbow Canyon) and several outflow tuffs derived their section are seen in most directions. from other parts of the complex. Much of the story of Return to US 93. the complex, however, is about the structural 2.2 147.9 Turn right (N) on US 93. geology, for magmatism was in large part 0.3 148.2 Turn right (E) just past the row houses, synchronous with the main episode of extensional then, in 50 m, right (SE) again beyond the deformation. tracks on a dirt road that runs along the N side of the Union Pacific tracks, heading E up The second part of the day we will examine the Clover Creek canyon. Thick, gently.N-dipping Kane Springs Wash caldera which has been sliced by intracaldera Bauers makes up all the walls of the left-lateral oblique-slip Kane Springs Wash fault, the canyon. and the Narrow Canyon caldera, a peralkaline to 1.2 149.4 STOP 10 (Anderson and Rowley) metaluminous precursor to the Kane Springs Wash Wash of English Canyon passes under the caldera which has been highly deformed by railroad tracks. Examine Bauers 0.5 km up (S) high-angle normal faults and by northwest-striking English Canyon to the fault contact between dextral faults. Bauers on the N and the same post-Hiko fanglomerate sequence exposed E of Caliente. 0.0 0.0· Start log at intersection of US 93 and NV The fault strikes E and dips S and exhibits 317 across from the church. Turn left (SE) onto oblique-slip (right-lateral and normal) in NV 317 and head S down Rainbow Canyon exposures to the E. The Bauers in the footwall following Meadow Valley Wash. here dips north, but within 1 km to the east, 0.4 0.4 Jagged hill on E is the deformed hanging attitudes swing around and Bauers dips S in the wall of the low-angle, S-dipping normal fault hanging wall, defining an asymmetrical mentioned at stop 9. Most rocks are of tuff of anticline interpreted to have formed by folding Etna and underlying and overlying clastic along this major fault. The fault and numerous sedimentary rocks. other nearby faults and folds can be shown by 0.6 1.0 Tuff of Etna, a moderately welded, structural analysis to represent the same rhyolite outflow tuff is exposed on the E side of deformation. The fault also marks the N the road in a rollover in the hanging wall of a topographic margin of the Delamar caldera low-angle fault. Source of Etna is about 8 km along which subsidence occurred during and SSW of here. Two outflow cooling units of the after emplacement of Hiko Tuff; the margin is moderately to densely welded peralkaline thus unlike anything described in the literature Gregerson Basin Member of the Kane Wash because it clearly has significant oblique slip. Tuff (14.7 Ma; Scott and others, in press a) are This fault is one of several E-W faults and interbedded with clastic sedimentary rocks plutons (including the one that controls the under the tuff of Etna just S of this stop. Chief mining district NW of Caliente) within an 0.5 1.5 On the E, clastic sedimentary rocks under E-W zone at least 5 km wide that Ekren and the tuff of Etna intertongue to the S with white others (1976) called the Timpahute nonwelded rhyolite ash-flow tuff that we call the tuff of Kershaw Canyon. The tuff of Etna is (Rowley and Siders, 1988) that is interpreted to the uppermost resistant bed at most places in be a trap-door type caldera in which most northern Rainbow Canyon. Look W to see subsidence was on its N side; the N margin is several of its cooling units in exposures on the buried by younger rocks but is inferred to be other side of the Canyon. near Caliente. The tuff has a 40Arj39Ar age of 0.5 2.0 STOP 11 (Rowley, Anderson, L.D. 15.6 Ma and a K-Ar age of 15.2 Ma. The Nealey, D.M. Unruh, Scott, and Harding). rocks SW of the major fault in the gulch Road to E goes to Kershaw-Ryan State mentioned above have been downthrown and Recreation Area (closed). Walk 0.7 km E along include the two Gregerson Basin cooling units the road to examine tuff of Kershaw Canyon (also just S of the homes and warehouse) and, and to view a fault that offsets two cooling under those, a local thick, brown, poorly to units of the Gregerson Basin Member and moderately welded outflow ash-flow tuff near intertongued outflow(?) tuff of Kershaw the bottom of Buckboard Canyon that we call Canyon. The Gregerson Basin units were the tuff of Sawmill Canyon. These three derived from the Kane Springs Wash caldera. cooling units are interbedded with the tuff of As first noted by R.E. Anderson (see Bowman, Kershaw Canyon and are overlain by the tuff of 1985 and Michel-Noel and others, 1990) the Etna capping the S wall of Buckboard Canyon. NNW-striking, high-angle, oblique-slip fault on As is typical of most of these NNW the N wall of the' canyon offsets the lower oblique-slip faults, the rock sequences on cooling unit twice as much as it offsets the opposite sides of the fault display different upper cooling unit, whereas the tuff of Etna on facies of the same stratigraphic units. Thus the top has virtually no offset. The upward rocks above the tuff of Rainbow Canyon N of decreasing amount of offset is due to recurrent the major fault are much thinner and include movement along the fault, which is not a only one Gregerson Basin cooling unit and only typical Gulf-Coast-type growth fault because minor amounts of the tuff of Kershaw much of the offset is right-lateral. The latest Canyon. Examine tuff of Rainbow Canyon, movement on the fault reflects some of the which makes up all exposures near this stop youngest movement during the main episode of except for a large, badly deformed horse of extension in the area, which began at least Hiko Tuff in the zone of the major fault. before 19 Ma and continued to at least 12 Return to NV 317. Ma. As we walk back to the vehicles, look 004 4.0 Turn right (S); continue down Rainbow ahead to the SWan the E wall of Rainbow Canyon on NV 317. Canyon where the fan array of tuff of Kershaw 0.6 4.6 Sawmill Canyon enters Rainbow Canyon Canyon and intertongued Gregerson Basin on E. Tuff of Sawmill Canyon, two cooling Member resting on mostly pink tuff of Rainbow units of the Gregerson Basin Member, and tuff Canyon to the S. Dips are progressively of Etna occur in canyon wall to W. From here greater in progressively older rocks, due to to a point about 2 km to the S, observe the recurrent movement on nearby oblique-slip same units interbedded with thick tuff of faults. Kershaw Canyon on E canyon wall broken by 1.2 3.2 Stock pond and ranch to E; mobile homes numerous faults. The facies on the E wall of and Union Pacific warehouse to W. Here, Rainbow Canyon is thicker than that on the W Buckboard Canyon enters Rainbow Canyon wall; probably a buried NE-striking fault with from the W. Turn right onto former NV strike-slip offset is beneath us. 317. Cross tracks; rugged gulch straight ahead 1.0 5.6 As we round the corner and head westerly, contains another major high-angle NW-striking note the NW-striking high-angle fault ahead that oblique-slip fault that passes under us and cuts the tuff of Rainbow Canyon low in the continues SE through the N side of a canyon canyon wall but does not offset the tuff of Etna behind us. on the mesa top. 004 3.6 STOP 12 (Rowley, L.D. Nealey, and 0.2 5.8 Former railroad stop of Etna. The tuff of D.M. Unruh) Most rocks here and around us Etna caps all surrounding mesas. belong to the poorly to nonwelded intracaldera 0.3 6.1 A NW-striking fault at 3 o'clock dropped rhyolite tuff of Rainbow Canyon. The tuff here the tuff of Rainbow Canyon on the N against is in its source, the Buckboard Canyon caldera the Hiko Tuff on the S. At 1:30, note a spectacular fault along which the tuff of 0.2 9.4 Taylor Mine Canyon enters Rainbow Rainbow Canyon on W is dropped downward Canyon from the W. Old shell of a concrete against the Hiko Tuff on the E. The Hiko on building in cottonwood trees at 3 o'clock is the the E side is overlain disconformably by the former pump house for a water line from tuff of Rainbow Canyon; this contact may be Meadow Valley Wash over the Delamar thought of as the floor of the Buckboard Mountains to the town and mines of the Canyon trapdoor caldera, and we infer that the Delamar (Ferguson) district, about 18 km to the tuff of Rainbow Canyon here is much thinner WSW. Because the water line was installed than it was in the N part of the caldera, as at midway through the life of the district the gold the last stop. in the quartzite was originally mined and milled 0.3 6.4 Roadcuts of Hiko Tuff on both sides of dry, and thus the Delamar district is known as road. the "widow maker" because of deaths from 1.0 7.4 Dula Canyon enters Rainbow Canyon from silicosis. The town survived 17 years until 1909 the W. The canyon is underlain by the major ' and had a population of as many as 2000 people NNW-striking oblique-slip Dula Canyon fault I (Ferris, 1991). The district, located just SW of that can be traced the length of the Caliente 7.51 the Delamar caldera (Fig. 2), is an epithermal minute quadrangle. The amount of right slip " system in massive Lower Cambrian quartzite has not been determined but is doubtless greater' and has many geologic similarities to the small ,

for older rocks than younger and for Hiko Tuff Chief gold district NW of Caliente (Rowley and I must be at least a couple of kilometers. The others, 1992). The gold at both places was rocks on the SW side of the fault are much deposited in quartz veins along fractures in different from those to the NE and consist quartzite, which shattered during the main mostly of rhyolite domes and lava flows. episode of Tertiary extensional faulting that 0.7 8.1 Chokecherry Canyon enters Rainbow accompanied magmatism and associated Canyon from the W. Spectacular exposures of a . convective hydrothermal activity. rhyolite dome and its underlying pyroclastic 1.1 10.5 Rock Springs Canyon enters Rainbow cone occur several hundred meters up the Canyon from the W. Light-gray, nonwelded canyon. A rough dirt road that starts here leads rhyolite ash-flow tuff visible about 1 km to the up the canyon and then SW for about 5 km to Ware possible correlatives with pyroclastic the Taylor (Easter) Mine, an epithermal gold deposits of the Baldy Mountain dome. Rock deposit consisting of gold-bearing quartz veins Springs Canyon narrows dramatically 2 km to along a N-dipping normal fault and adjacent the SW where the 400-m-high gorge consists of hydrothermally altered intracaldera Hiko Tuff. the vertically flow-foliated vent facies tuff of 0.6 8.7 Baldy Mountain to the Wand exposures Etna. The vent cut through intracaldera Hiko extending into Chokecherry Canyon are Tuff at the S edge of the Delamar caldera. The underlain by the rhyolite mentioned above. The margin of the caldera is an E-W fault zone high fault scarp to the E, with the Dula Canyon within the E-W Helene lineament (Rowley and fault at its base, is underlain by the Acklin others, in press a) which is at least 5 km wide Canyon rhyolite dome, which rests on and characterized across this width by almost intracaldera Hiko Tuff. Sharp brown hill ahead continuous rhyolite that conceals the actual is of andesite lava flows that were deposited caldera margin. In general the rhyolite N of the outside (S of) the Delamar caldera. The hill is fault/caldera margin consists of extrusive bounded on its W side by a NNW-striking products (domes and tuff), whereas the rhyolite left-slip fault under Meadow Valley Wash and S of the margin consists of the deeply eroded on its E by an almost N-striking right-slip fault. and hydrothermally altered intrusive vents for 0.5 9.2 S-dipping contact between the andesite (on domes and tuff that were removed by erosion. the N) and outflow Hiko Tuff occurs in the E Almost all vents are marked by dikes that roadcut here. The Dula Canyon fault is about 1 mostly strike E-W and clearly intruded along km to the E, on the other side of this small other faults within the lineament. At the SW mountain of Acklin Canyon rhyolite flows and edge of the Delamar caldera, the E-W rhyolite tuffs. Most rocks to Ware rhyolite flows and dikes and faults continue W into the Delamar tuffs of the Baldy Mountain dome; farther W district. A buried silicic pluton is inferred for this dome rests on intracaldera Hiko. the dikes under the district and is hypothesized to be the heat pump for the convective overturn basin. These deformed basalts were dated by H. of groundwater and thus for the mineralization. H. Mehnert (unpublished data, 1980) at 8 Ma 1.3 11.8 Pass under the railroad; ranch on NW. and correlate well with basalts found in the Most rocks are hydrothermally altered; local fluvial deposits on the E side of the Meadow anomalous gold has been reported and, in a few Valley Mountains, also dated by Mehnert at 8.1 places, mined. Here we are probably S of the Ma. Thus, the original depositional basin for Delamar caldera. Based on an analogy with the these fine-grained fluvial sediments persisted at gold deposits at Delamar, the potential for gold least 5 million years, a surprisingly long in this E-W faulted rhyolite zone would seem to period. The strand of the Kane Springs Wash be good, but we have not yet sampled the rocks fault contains subhorizontal slickenlines that here. indicate a left-lateral oblique sense of motion; 3.1 14.9 Pass under the railroad. Former railroad as we will see farther S at Stop 13, 4-7 km of stop of Boyd about 1 km N of here. We sinistral offset has been established by the offset continue S out of the deformed and of the margins of the Kane Springs Wash hydrothermally altered rocks of the Helene caldera. It is likely that the Elgin structural lineament and into a simple, gently S-dipping basin is related to this motion; note that the homoclinal sequence of volcanic rocks, NE-striking Kane Springs Wash fault joins a including, from base upward, dark-brown WNW-striking fault and a N-striking fault at outflow tuff of the Gregerson Basin Member of the S end of the exposures of Paleozoic rocks the Kane Wash Tuff, a sequence of NE of Elgin (Fig. 2). yellowish-tan, nonwelded rhyolite ash-flow tuff 0.9 22.7 On right, new roadcuts expose steeply that is possibly tuff of Kershaw Canyon, NE-dipping sedimentary rocks that are cut by and dark-brown, moderately welded outflow of . NW-dipping normal faults. the tuff of Etna. 1.9 24.6 At crest between Kane Springs Valley 3.9 18.8 Rainbow Canyon narrows as Meadow and Meadow Valley Wash note that the Valley Wash and the road pass through thick sedimentary rocks are fine grained even at this outflow tuff of Etna. Five overlying basalt lava high topographic elevation; the nearby flows are in turn overlain by thick fluvial coarser-grained rocks exposed at lower sedimentary rocks. Although not dated, the topographic levels to the northwest may require basalts probably are correlative with flows in a either undetected structures or local channels. similar stratigraphic position on the east side of Continue straight on NV 317 past crossroads the Meadow Valley Mountains dated by H.H. and down Kane Springs Valley, the Meadow Mehnert (unpublished data, 1983) at 13.2 and Valley Mountains are on the left (E), the 13.3 Ma. The sedimentary sequence that forms Delamar Mountains on the right (W). The low steep hillsides and cliffs on either side of the pass in the Meadow Valley Mountains is canyon as it opens up around Elgin formed in a underlain by the less resistant Harmony Hills basin considerably larger and older than the Tuff; SW of the pass, the NE wall of the E part current structural basin. Small cliffs on left of the Kane Springs Wash caldera coincides formed by three layers of young peralkaline(?) with the higher part of the mountains that are ash-flow tuffs(?) are apparently the youngest underlain by the more resistant caldera-filling silicic volcanic rocks in the area. rhyolites within the caldera. Farther down Kane 1.8 20.6 Intersection with dirt road to the two Wash Valley, observe intracaldera equivalents inhabited houses remaining in Elgin on the of the Kane Wash Tuff characterized by the north side of Meadow Valley Wash. Continue darker exposures at the base of the Meadow SE. Valley Mountains; the yellowish rocks and the 0.6 21.2 End of pavement on NV 317. rocks above them are post-collapse 0.2 21.4 Turn left (S) onto gravel road that climbs caldera-filling tuff and rhyolitic lava flows SW out of Rainbow Canyon through thick (Harding, 1991; Harding and others, in press). sedimentary sequence. On the right, a NE-trending low ridge of 0.4 21.8 On the left, NW-dipping sedimentary outflow tuff from the Narrow Canyon caldera is rock, basalt, and tuff are deformed against a exposed in the foreground. In the background strand of the Kane Springs Wash normal fault are E-dipping slopes of the 13.3(?)-Ma basalt that makes the SE border of the Elgin structural capping the tuff of Etna and the Kane Springs Wash Tuff. Pass the northern wall of the Kane cooling unit of the intracaldera Gregerson Basin Springs Wash caldera in the Delamar that contains cognate inclusions and fiamme of Mountains on the way to next stop. mafic trachyte in a somewhat more trachytic 8.2 32.8 Turn left at faint 4X4 trail toward matrix. Similar evidence for a layered magma Meadow Valley Mountains. chamber can be found in the tops of the outflow 0.5 33.3 STOP 13 (Harding and Scott) Walk to cooling units. exposures of intracaldera and caldera-filling 0.5 33.8 Rejoin NV 317 and turn right (NE). units, S wall of Kane Springs Wash caldera, 6.6 40.4 Turn right just past corral on left toward wall breccia, precaldera units and (perhaps) low pass in Meadow Valley Mountains. outflow units outside the caldera wall. Also Harmony Hills Tuff and Hiko Tuff in the walk along the trace of the Kane Springs Wash saddle dip steeply to the NE, away from the N fault to see low-angle slickenlines indicating wall of the caldera, which lies to the right (S). sinistral oblique slip. The footwall block . Dikes of a pre-caldera trachyte (about 14.7 Ma) contains the contact between the uppermost and ; cut these ash-flow tuffs. underlying intracaldera units equivalent to the 1.9 42.3 At water tank in low pass, turn sharply two outflow cooling units of the Gregerson right on 4X4 trail and drive up an alluvial-fan Basin Member of the Kane Wash Tuff. The veneer on a pediment developed on the bluish- to greenish-gray thin layers at the Harmony Hills Tuff. quenched base of the upper cooling unit are like 0.3 42.6 STOP 14 (Harding and Scott) Park at the layers found at the base of its outflow end of trail and walk W a few hundred meters tooling unit. See caldera wall breccia just S of to exposures of the N wall of the Kane Springs the saddle formed by a hanging-wall block of Wash caldera. Everywhere along this segment basalt and post-collapse caldera-filling rhyolite of the wall, breccias and intercalated ash-fall flows. and tuffs. Here, the wall consists of a tuffs dip 10-40° S toward the caldera, pre-caldera trachyte flow containing perpendicular to the trace of the wall. conspicuous alkali feldspar phenocrysts. Crude Erosionally resistant caldera-filling rhyolite lava bedding in the breccia dips NE into the caldera. flows and ash-flow tuffs dip 20-40° SE and are Above the breccia, the intracaldera ash-flow repeated by several NW-dipping normal faults tuff has been quenched against the breccia in parallel to the Kane Springs Wash range-front similar manner as seen elsewhere within and fault. Nonresistant Harmony Hills and Hiko outside the caldera. Walk out of the caldera Tuffs in the wall dip 50-60° N away from the toward the SE; at the crest and down the slope caldera, forming the valley below us. to the W, note a large block of the Grapevine 2.1 44.7 Rejoin NV 317 and turn right. Spring Member of the Kane Wash Tuff, 1.6 46.3 At crest between Kane Springs Valley enveloped by quenched vitrophyre of and Meadow Valley Wash, turn left (NW) intracaldera Gregerson Basin, that sloughed off toward the Delamar Mountains, driving through the caldera wall. At the wash at the base of the dissected pediment above older Tertiary hill, an aphyric dike cuts the caldera wall sedimentary basin fill. parallel to the range front fault and passes 2.8 49.1 To left are dip slopes of 13.3(?)Ma outside the caldera into the lower part of the basalt. pre-14.7-Ma Sunflower Mountain Tuff. Climb 1.7 50.8 Cross basalt. To right in valley are through the pre-caldera trachyte flow into the rounded exposures of tuff of Etna. Grapevine Spring Member. On the way back to 0.6 51.4 Turn left. On right, exposures of Hiko vehicles, walk through the caldera to observe Tuff underlie outflow sheets of the Narrow several post-collapse, caldera-filling, rhyolitic Canyon caldera, which in turn underlie the tuff ash-flow tuffs, rhyolite flows, and a basalt of Etna; the Kane Wash Tuff is missing here. flow. Note that the nonwelded tuff of this 0.7 52.1 Distal edge of upper cooling unit of the sequence is superficially similar to the Gregerson Basin Member is plastered on a hill nonwelded base of the Sunflower Mountain of Hiko Tuff. These chilled relationships are Tuff; this similarity contributed to past similar to those we saw earlier in the difficulties in recognizing the caldera in the intracaldera Gregerson Basin. Meadow Valley Mountains. Below this 1.0 53.1 Cross wash. On left (S) is a ridge of sequence is the upper part of the youngest outflow Kane Wash Tuff. Although only 10 km from the caldera, the Kane Wash Tuff is less megabreccias and extracaldera andesite flow than 100 m thick. On right are exposures of breccias are only a few tens of meters nearly horizontal Hiko Tuff, against which the apart. The age of the Narrow Canyon caldera is Kane Wash Tuff is thinned, possibly because of between that of the Hiko Tuff (18.6 Ma) and tumescence prior to collapse of the nearby that of the Grapevine Spring Member (about Narrow Canyon caldera. 14.7 Ma). 0.2 53.3 Cross exposures of a pyroxene­ Return to NV 317. plagioclase andesite(?) flow breccia that 7.8 67.8 END OF TRIP Turn right to rejoin US underlies the Hiko Tuff. 93 to reach Las Vegas about 2.5 hours away; 0.7 60.0 STOP 15 (Scott) Wall of the Narrow turn left to return to Caliente and northern and Canyon caldera marked by megabreccia of the western destinations. Hiko Tuff. Outflow and intracaldera tuffs have Zr contents in the range of 500-1050 ppm, only REFERENCES CITED slightly lower than those of the Kane Wash Tuff (600 to 1300 ppm). Although the outflow Anderson, R.E., 1989, Tectonic evolution of the has limited extent, intracaldera tuff is exposed Intermontane System, Basin and Range, in the N wall of the Kane Springs Wash caldera Colorado Plateau, and High Lava Plains, beneath the pre-caldera trachyte flow at stop Chapter 10, in Pakiser, L.C., and Mooney, 13. The caldera wall has been fragmented .. W.D., eds., Geophysical framework of the tectonically by NW-striking, dextral-slip faults continental United States: Geological Society of and high-angle normal faults; in fact, the America Memoir 172, p. 163-176. narrow, trough-like segment of caldera-filling Anderson, R.E., and Ekren, E.B., 1968, Widespread tuffs between caldera wall breccia may indicate Miocene igneous rocks of intermediate that this caldera, like several similar features composition, southern Nye County, Nevada: within the Caliente caldera complex, formed Geological Society of America Memoir 110, p. essentially coevally with extension. If so, the 57-63. structural depressions created during extension Anderson, R.E., and Hintze, L.F., 1991, Geologic may have acted as loci of eruption and/or as map of the Dodge Spring quadrangle, Lincoln topographic depressions in which eruptions County, Nevada and Washington County, pooled. 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This dome is Barr, D.L., Christiansen, E.H., Tingey, D.G., and centered on a large 420 nanoTesla positive Best, M.G., 1992, Time, space, and magnetic anomaly and what is interpreted to be composition patterns of mid-Cenozoic mafic to the thickest pool of intra caldera material forms intermediate composition lavas of the Great a 500-800 nanoTesla negative anomaly about 4 Basin, western U.S.A.: American Geophysical km SW of this stop (Blank and Kucks, 1989) Union (EOS) Transactions, v. 73, no. 43, p. (the northern and eastern boundary for these 658. anomalies are shown as the dashed and dotted Best, M.G., 1988, Early Miocene change in direction line in Figure 2). Caldera-filling ash-flow tuffs of least principal stress, southwestern U.S.: dip as much as 80 0 away from the dome on the conflicting inferences from dikes and Nand E sides. The Kane Wash Tuff pinches metamorphic core-detachment fault terranes: out abruptly 2 km to the SE of the dome. Tectonics, v. 7, p. 249-259. 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