Download Crustal Evolution of the GreatBasin and the Sierra

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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, W C 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
The Great Basin harbors at least sixty Tertiary
calderas and inferred sources of tuff and several tens
of thousands of cubic kilometers of ash-flow deposits,
making it one of the greatest manifestations of
prolonged ash-flow volcanism in the terrestrial rock
record. Some individual calderas are exposed east to
west across three or four mountain ranges. Simplecooling-unit outflow tuff sheets cover areas of tens of
thousands of square kilometers and range to as much
as hundreds of meters thick. During the "ignimbrite
flareup" from about 31 to 22 Ma, when most of the
ash-flows were erupted, extrusion of lava in the Great
Basin was minor, widely scattered, and did not form
major edif:ces such as composite volcanoes. Typical
volcanic sections consist of multiple ash-flow tuff
cooling units from nearby caldera sources and only
local lava flows and pyroclastic-surge and -fall
deposits.
Regional extension was minimal during most of
the ignimbrite flareup. However, local extension
occurred before the flareup and major extensional and
local strike-slip faulting beginning in the early
Miocene affected many parts of the Great Basin,
including the Caliente and Kane Spring Wash caldera
complexes where synvolcanic faults form many
caldera margins.
INTRODUCTION
The purpose of this brief review and field trip
roadlog is to sample the results of the waxing and
waning of the Great Basin ignimbrite flareup. For a
more detailed report on Great Basin volcanism during
the Tertiary, see Best and others (l989a). The support
of the National Science Foundation through grants
EAR-8604195, -8618323, and -8904245 to M.G. Best
and E.H. Christiansen is gratefully acknowledged. 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
The pre-volcanic underpinning of the Great
two feldspars, quartz, Fe-Ti oxides, and biotite are
common. Rhyolite tuff occurs throughout the
Basin i~ a terrane containing late Precambrian,
"""Phaneiozoic," d local Mesozoic sedimentary rocks
Tertiary, whereas huge volumes of dacite ash flows
were erupted about 31 to 27 Ma and high-temperature
e ormed during compressional episodes in Paleozoic
and Mesozoic Eras and intruded locally by Mesozoic
trachydacite magmas containing phenocrysts of
granitic plutons. After widespread erosion in late
plagioclase and pyroxene erupted from many centers
Cretaceous and early Tertiary time which produced a
mostly about 27 to 23 Ma. After 17 Ma, alkaline
profound unconformity and, in some places, early
metaluminous to mildly peralkaline magmas
Tertiary sedimentation, volcanism began in the
containing Fe-rich pyroxene and olivine, sanidine,
Eocene about 43 Ma in northern Nevada and Utah
and quartz phenocrysts began to be erupted.
and swept southward along an arcuate, roughly
east-west front, reaching southern Nevada by middle
Miocene time.
The inventory of Cenozoic rocks in the Great
Basin by Stewart and Carlson (1976; see also Best
and Christiansen, 1991; Figs. 3 and 4) clearly shows
the products of the late Oligocene-early Miocene
ignimbrite flareup; the volume of resulting ash-flow
deposits in the Great Basin is not widely appreciated
but is an order of magnitude larger than in the well
known San Juan and Mogollon-Datil fields in the
eastern Cordillera. However, the Great Basin harbors
only a fraction of the volume of ash flow tuffs in the
Sierra Madre Occidental of Mexico. During the
ignimbrite flareup in the Great Basin, the volume of
extruded lava was minor compared to ash-flow
deposits and was less than the volume of lava
extruded before and after the flareup. Scarce
Oligocene debris flows indicate a general lack of
large volcanic edifices in contrast to, for example, the
San Juan volcanic field.
Until the early Miocene, at about 24 Ma, calcalkaline volcanism had produced a large volume of
dacite to rhyolite ash-flow tuff and subordinate
high-potassium andesite and dacite and rhyolite lava;
basalt appears only after 22 Ma (Barr and others,
1992). During the next 8 m.y., explosive volcanism
waned and a broader compositional spectrum, but stilI
dominated by rhyolite, appears in the overall volcanic
record. Basaltic volcanism has been a significant
aspect of Great Basin activity after about 13 Ma,
partieularly along the eastern and western margins of
the region but also locally in the center (McKee and
Noble, 1986). Many silicic tuffs and lavas younger
than about 17 Ma are peralkaline or topaz-bearing
(Noble and Parker, 1975; Christiansen and others,
1986).
After the middle Miocene, the general east-west
orientation of magmatic zones changed to north-south
(Best and others, 1980; Stewart, 1983), probably
reflecting a fundamental change in the state of stress
.
in the lithosphere (Best, 1988).
Extensional tectonism in the Great Basin during
Tertiary time was episodic (e.g., Taylor and others,
1989), was intense in some areas (e.g., Proffett,
1977; Moores and others, 1968; Gans and others,
1989) and moderate in others (e.g., the southern
Pancake Range, Snyder and others, 1972), and in
general correlates poorly in space and time with
volcanism (Best and Christiansen, 1991). In and near
the Caliente and Kane Springs Wash caldera
complexes (Figs. 1 and 2) the main extensional
episode was in early to middle Miocene time (Scott,
1990; Rowley and others, 1992), concurrent with
caldera volcanism, as Great Basin ash-flow activity
waned. During the ignimbrite flareup, however,
sparse clastic deposits and few angular discordances
in outflow volcanic sections show that regional
tectonic extension in the Great Basin as a whole was
limited.
CALDERAS
Recognition of calderas is hampered not only
by erosion and burial beneath younger deposits, but
also in the Great Basin by widespread post-volcanic,
and local synvolcanic (Caliente area and Stillwater
Range), faulting that has dismembered the calderas
into small segments, blurring their margins and
internal structure. Geographic centering within the
outflow sheet may be misleading as outflow lobes are
commonly not radially symmetric about the source
caldera (e.g., Windous Butte Formation, Best and
others, 1989a). Topographic margins are poorly
known even for some of the better located calderas.
Piles of tuff as much as 2-3 km thick are an obvious
indicator of a caldera (Ekren and others, 1973; Best
and others, 1989a, Figs. R29, R32-R38), but some
demonstrable proximal outflow tuff deposits ponded
in older calderas and on downthrown sides of
synvolcanic extensional growth faults are also thick
(Dixon and others, 1972; Best and others, 1989b,
Figs. 5B and 5C). Dense compaction and widespread
propylitic alteration of compound or multiple cooling
units comprising the intracaldera tuff make it more
resistant to erosion relative to the caldera wall rocks
and hence causes the development of inverted
topography that is a common clue to the existence of
the caldera. Megabreccia and "rafts" of internally
shattered but nonetheless stratigraphically coherent
rock, locally more than 2 km across and hundreds of
meters thick, occur within a few kilometers of some
caldera walls (Bonham and Garside, 1979, p. 40;
McKee, 1976; Best and others, 1989a, Figs. R12,
R24, R25, R36, and R38).
Caving of the unstable caldera escarpment
enlarges the perimeter of a caldera so that topographic
diameters can be several kilometers greater than the
ring-fault system (Best and others, 1989a, Fig. R32;
Best and others, 1989b, Fig. 5B). Younger postcaldera collanse denosits may completely fill and even
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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
most older Great Basin calderas precludes evaluation
of resurgence; however, it is apparent in the Indian
Peak caldera (Best and others, 1989b) but not in the
relatively well exposed Mt. Jefferson caldera (Boden,
1992) and middle Miocene Kane Springs Wash
caldera (Novak, 1984). Pre-caldera collapse
tumescence is equally difficult to evaluate. Late-stage,
ring-fault controlled extrusion of lava domes is not
manifest in some well-mapped very large Great Basin
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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
,.=
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Clover Creek caldera margin, in north, concealed only, intracalderaBauers TuffMember of Condor Canyon
Formation is patterned, Hachures on caldera side of margin
Delamarcaldera margin dashed where approximate, dotted where concealed, intracaldera Hiko Tuffis patterned.
Hachures on caldera side of margin
Buckboard Canyoncaldera margin, intracaldera tuffof Rainbow Canyon is patterned. Hachures on caldera side of
margin
Kane Springs Wash caldera margin,dotted where concealed. Hachures on caidera side of margin
Narrow Canyon caldera margin. Hachures on caldera side of margin
North and east boundaryof aeromagnetic anomaliyassociated with Narrow Canyon caldera
Paleozoic rocks
Fault, dashed where approximately
located, dotted where concealed
•
~~~i~'Je~olcanic rocks,
Roads
G
Stock of Jumbo Wash
Fieldtripstop numbers
_~~Q_~~~~~;~~~7~ary
Mining Districts: Helene, H; Delamar, D; Taylor, T; Pennsylvania, P
.~_._~_ 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
Ridge and Indian Peak calderas) but occurs in some
rhyolite centers (e.g., Caliente caldera complex).
ASH-FLOW TUFF
By far the largest volume of volcanic rock of
late Oligocene-early Miocene age in the central Great
Basin is ash-flow tuff that is variably welded in
simple, compound, and multiple cooling units.
Regionally extensive outflow tuff sheets are
commonly tens and in some places hundreds of
meters thick; several are presently found over areas
exceeding 10,000 km 2 and the two largest sheets, the
Windous Butte and Wah Wah Springs, each cover
about 40,000 km 2 (after compensation for 50% postvolcanic east-west crustal extension these areal extents
are reduced to about two-thirds).
Most Great Basin ash-flow tuffs are rhyolite,
but range to dacite, trachyte, and sparse andesite and
latite (lUGS classification, Le Maitre, 1989). Pre-17
Ma tuffs are potassic and calc-alkaline, but many
middle Miocene tuffs have especially high FeO/(FeO
+ MgO) ratios (about 0.9) and are mildly
peralkaline. Phenocryst concentrations in Great Basin
tuffs are as much as 40% and in rare cases 50%
(e.g., Harmony Hills Tuff). All dacite and andesite
tuffs are crystal-rich. Quartz and two feldspars are the
most common phenocrysts. Mafic phases in
calc-alkaline tuffs typically include biotite and
magnetite; less common are hornblende, ilmenite,
augite, and hypersthene and, in peralkaline tuffs,
Fe-rich phenocrysts including olivine, sodic
amphibole, and pyroxene. Most tuffs contain trace
amounts of apatite and zircon. Titanite (sphene)
occurs in a few tuffs and trace amounts of allanite,
perrierite, and/or chevkinite has been found in several
rhyolite tuffs.
Most ash-flow sheets show normal
compositional zoning (usually based on 'whole-rock
samples) toward a more mafic upper part which
contains larger phenocrysts. The Pahranagat
Formation is laterally zoned as well and the Windous
Butte Formation has a normally zoned rhyolite
_
outflow that trends without a significant compositional
gap to dacite intracaldera tuff.
precision 4°ArrAr dating and quantitative
determination of thermoremanent-magnetization
direction. The 40ArrAr technique provides a
precision better than 1% (Dalrymple and Duffield,
1988; Deino and Best, 1988; Deino, 1989; McIntosh
and others, 1992) allowing greater stratigraphic
resolution than conventional K-Ar methods with
uncertainties of about 3 % (McKee and Silberman,
1970).
Although calc-alkaline rhyolite magmas were
erupted throughout the entire span of volcanic activity
in the Great Basin, other compositions were also
erupted from multiple centers during particular time
intervals (e.g., Anderson and Ekren, 1968;
Armstrong and others, 1969; Scott and others, 1971;
Noble, 1972; McKee, 1976). Three time-dependent
compositional types of ash-flow tuff (Christiansen and
Best, 1989; Best and others, 1989a; Noble and
Parker, 1974) will be examined on the field trip. The
Monotony compositional type consists of crystal-rich
dacite (compare the "monotonous intermediates" of
Hildreth, 1981) and was named for the Monotony
Tuff of southeastern Nevada. This type includes five
large volume outflow sheets and one intracaldera tuff
whose aggregate volume exceeds 12,000 knr' that
were emplaced from about 31 to 27 Ma. The second
compositional type, the Isom, was named after the
Isom Formation in the southeastern Great Basin and
was erupted from several centers east to west across
the province mostly 27 to 23 Ma as relatively thin
(10-20 m), densely welded sheets. This type consists
of calc-alkaline trachydacite that contains less than
20% phenocrysts of plagioclase, pyroxene, and Fe-Ti
oxides and has unusually high concentrations of Ti02 ,
K20, and Zr compared to other rocks of similar Si02
and CaO content (Christiansen and Best, 1989). The
third compositional type is unnamed but is
represented in the peralkaline rhyolite tuffs associated
with the middle Miocene Kane Springs Wash caldera.
Multiple magma systems forming each compositional
type apparently had similar sources and partial
melting and crystallization histories in similar tectonic
environments.
Paleomagnetism was used to distinguish
between and/or correlate ash-flow sheets by Noble
and others (1968) and Gromme and others, (1972) in
the Great Basin, and by Mclntosh and others (1992)
in the Mogollon-Datil volcanic field. The method
(e.g., Best and others, 1989a; Scott and others, in
press a) depends upon the fact that during cooling an
ash-flow deposit acquires a stable thermoremanent
magnetization parallel to the geomagnetic field, thus
preserving a sampling of both kinds of its time
variations. These are secular variation of the direction
having a characteristic time of one to a few centuries
and amplitude of 10-20° of arc, and complete
reversals of polarity, which occur at random but with
a characteristic frequency of a few thousand
centuries. If, after correction for post-emplacement
tectonic tilting (in most instances by measurement of
the eutaxitic compaction foliation), two ash-flow
sheets have paleomagnetic directions that are opposite
in polarity or are significantly different from each
other, the ash-flows cannot have erupted at the same
time. The converse hypothesis, that if two sheets have
the same paleomagnetic direction they are
contemporaneous, can only be assigned a probability
of truth depending on how different their common
direction is from the most likely field direction, that
of the geocentric axial dipole.
CORRELATION
CENTRAL NEVADA CALDERA COMPLEX
Because of pervasive faulting and consequent
erosion throughout the Great Basin, the distribution of
dismembered outflow sheets cannot be determined by
geologic mapping alone, necessitating application of
reliable correlation techniques. We and our coworkers
have found that in most cases position in stratigraphic
sequence combined with phenocryst concentration
ratios serve as primary correlation criteria; other
petrographic features and compositional aspects
determined in the laboratory are useful in certain
cases. Two powerful techniques, that are especially
helpful in establishing synchroneity of compositionally
dissimilar outflow and intracaldera tuffs, are high
This cluster of calderas, one of four considered
on the field trip, includes ten calderas and two
indefinite source areas (Fig. 1, Table 1). Our
petrologic, paleomagnetic, and chronologie investigations together with field reconnaissance have
somewhat refined the work two decades ago of E.B.
Ekren and W.D. Quinlivan and their associates in the
U.S. Geological Survey (see references in Best and
others (1989a) and Gardner and others (1980)).
The oldest magmas erupted at 35.3 Ma to form
the Stone Cabin Formation were equilibrated near the
QFM (quartz-fayalite-rnagnetite) buffer; succeeding
magmas which formed the outflow and intracaldera
tuffs of the Windous Butte Formation, Monotony
Tuff, and the titanite-bearing tuff of Orange Lichen
Creek were progressively more oxidized. The cycle
was repeated with eruption of the 26.7-Ma magma of
the lower member of the Shingle Pass Tuff, which
has phenocrysts of quartz, magnetite, and Fe-rich
olivine, succeeded again by more oxidized magmas,
culminating in the titanite-bearing Fraction Tuff
erupted at 18.3 Ma. We have no explanation as yet
for these two cycles of increasing degree of oxidation
of the magmas. The compositionally zoned Stone
Cabin magmas equilibrated at relatively low water
fugacity and high pressure, about 4 kb (based on
hornblende and feldspar geobarometers; Radke,
1992), or a depth of about 15 km. The 31.3 Ma
Windous Butte Formation and related intracaldera tuff
is a compositionally zoned sequence ranging from
first erupted rhyolite in the outflow sheet to last
erupted dacite (66 to 75 % SiO~ forming the
intracaldera pile. Phillips (1989) interpreted this
sequence to have erupted at 750 to 790°C; his
hornblende compositions suggest equilibration at less
than 4 kb. The dacite Monotony Tuff (27.3 Ma) is
consistently more mafic (68 to 70 % SiO~ than the
Windous Butte outflow sheet and lacks abundant
sanidine, but appears to have last equilibrated at
lower temperature (750°C) and pressure (3 kb)
(Phillips, 1989). Major eruptions of compositionally
zoned rhyolite magma from 26.7 to 26.0 Ma formed
at least four rhyolite cooling units of the Shingle Pass
Tuff. The lowest is mineralogically distinctive,
containing quartz, two feldspars, Fe-rich olivine and
pyroxene and only small quantities of biotite and
amphibole (Nielsen, 1992); like the older Stone
Cabin, the oldest Shingle Pass magma was relatively
dry and last equilibrated near QFM at low pressures
(about 3 kb) and 730 to 770°C. In contrast, the
uppermost magma of the Shingle Pass Tuff
crystallized at higher fugacities of oxygen and was
quite wet, lacking quartz and anhydrous mafic phases.
The outflow tuff of the Pahranagat Formation erupted
at 22.6 Ma is laterally and vertically zoned from
high- to low-silica rhyolite.
Long intervals of time prior to eruption of the
Windous Butte, Monotony, and Pahranagat magmas
were punctuated by many very small extrusions of
andesite, dacite, and rhyolite lava flows and by small
rhyolite ash-flow eruptions.
INDIAN PEAK CALDERA COMPLEX
This complex of southward migrating eruptive
loci lying astride the Nevada-Utah state line has been
described by Best and others (1989b). Early eruptions
of rhyolite at about 33 Ma were followed by three
dacite ash flows of the Monotony compositional
type--forming the Cottonwood Wash Tuff, the Wah
Wah Springs Formation, and the Lund Formation-whose aggregate volume is at least 8,000 knr'. The
Wah Wah Springs Formation erupted at about 30 Ma
is unique among Great Basin tuffs because hornblende
dominates over biotite in most samples. After the
second and third dacite eruptions, zoned rhyolite ash
flows of an order-of-magnitude smaller volume were
emplaced within the older calderas. Several cooling
units of the trachydacite Isom Formation were erupted
late in the history of the Indian Peak magmatic system
at about 27 Ma. If a related caldera for the Isom
exists, it has been concealed beneath younger tuffs
and lava flows at the southeast margin of the Indian
Peak caldera complex.
Evolved mafic magma, some of which leaked to
the surface as local andesite lava flows, combined
with silicic crustal material in vigorously convecting
magma chambers to produce the voluminous dacite
magmas. Mixing processes are indicated by relatively
high concentrations of compatible elements (e.g., Ca,
Sr, Cr) compared to other magmas from the volcanic
field at a given silica concentration. High Sr- and low
Nd-isotope ratios suggest that the silicic end member
came from the continental crust. The late trachydacite
IS0111 magmas were probably produced by fractiona.l
crystallization of andesite with minimal assimilation
of crustal material, perhaps because the low melting
temperature fraction was already removed from the
crust or crystallization was enhanced by the strong
thermal contrast with wall rocks during the waning
stages of magmatism. The Isom tuffs have high
concentrations of incompatible elements and but low
pyroxene- and feldspar-compatible elements (Co, Sr,
Ca), as well as lower Sr- and higher Nd-isotope ratios
than the older dacitic tuffs erupted from the Indian
Peak caldera complex. Nonetheless, both types of
magma appear to have stalled, partially crystallized,
and then erupted from upper crustal magma
chambers. Estimates of temperature and pressure for
crystallization of phenocrysts in the Wah Wah Springs
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
Early work in the Caliente caldera complex
(Figs. 1 and 2) include the stratigraphic studies of
Williams (1967) and reconnaissance mapping by
Noble and others (1968), Noble and McKee (1972),
and Ekren and others (1977). Recent and current
work (referred to in the following pages) consists of
quadrangle mapping by P.D. Rowley and R.E.
Anderson, isotopic dating by L.W. Snee and H.H.
Mehnert, geophysics by H.R. Blank, igneous
petrology by L.D. Nealey (e.g., Nealy and others, in
press), isotope geochemistry by D.M. Unruh (e.g.,
Unruh and others, in press), and paleomagnetism by
C.S. Grornme, M.R. Hudson, and J.G. Rosenbaum.
The Caliente caldera complex consists of
numerous inset calderas (Fig. 2), four of which we
have named (Rowley and Siders, 1988). The oldest
known so far is the Clover Creek caldera, the source
of the densely welded Bauers Tuff Member (22.7 Ma)
and perhaps the slightly older similar Swett Tuff
Member, both of the Condor Canyon Formation
(Rowley and others, in press b). A fault-bounded
mass of this caldera is exposed on the northern side
of Caliente. The Delamar caldera, the source of the
moderately welded Hiko Tuff (18.6 Ma; Taylor and
others, 1989), makes up the western end of the
Caliente caldera complex, as Ekren and others (1977)
discovered. It IS possible that different parts of the
Hiko Tuff and the similar Racer Canyon Tuff erupted
from separate but virtually simultaneous vents in the
Caliente caldera complex, including a vent south of
the ghost town of Delamar (Fig. 2). The Buckboard
Canyon caldera is probably a trap-door caldera that
subsided on -its northern side, near Caliente, and
erupted the small, poorly welded tuff of Rainbow
Canyon (15.6-15.2 Ma; Mehnert and others, 1989).
Other tuff deposits have no well defined caldera;
these include the following: The moderately welded
Racer Canyon Tuff (19.2?-18.7 Ma; L.W. Snee,
written commun., 1991) probably derived from the
eastern end of the complex; the minor, poorly welded
tuff of Tepee Rocks (17.8 Ma; L.W. Snee, written
commun., 1991) probably derived from east of the
Delamar caldera but produced little surviving outflow
tuff; the small, poorly welded tuff of Kershaw
Canyon (15.5?-14.0 Ma) interbedded with outflow
units of the Kane Wash Tuff south of Caliente with a
presumed source southeast of town (its outflow is
confined to an area within 15 km of the northern side
of the Caliente caldera complex); the minor,
moderately welded tuff of Sawmill Canyon, a thin
outflow unit intertongued within the lower part of the
tuff of Kershaw Canyon presumably derived from a
source southeast of Caliente; the small-volume,
moderately welded tuff of Etna (14.0 Ma; Rowley
and others, 1991) consisting mostly of outflow in and
north of the Caliente caldera complex and overlying
the tuff of Kershaw Canyon and derived from a
source 9 km south-southwest of Caliente, in the
Helene lineament (Fig. 2) on the southern margin of
the Delamar caldera; the small, poorly welded Ox
Valley Tuff (12.6-11.4 Ma; H.H. Mehnert, written
commun., 1980) consisting of outflow deposits
confined mostly to the Bull Valley Mountains in Utah
and probably derived from the southeastern part of
the Caliente caldera complex (according to Anderson
and Hintze (1991) and unpublished mapping by R.E.
Anderson). All known tuffs from the Caliente
complex are rhyolite, those including and older than
the tuff of Tepee Rocks are low-silica varieties and
the younger units are high silica.
Three episodes of extensional deformation took
place in the Caliente area. The oldest is poorly
constrained except that it predates volcanism which
began at about 31 Ma, is inferred to be Tertiary, and
resulted in detachment faults (see Taylor and Bartley,
1992 and included references). The second, main
episode began before 19 Ma, based on dated dikes in
faults zones, and continued to about 12 Ma; 25 Ma
plutons in the Chief district probably also formed
during this extensional episode (Rowley and others,
1992), so that it coincides with the duration of
caldera-related magmatism 23 to 13 Ma, if not
exceeds it. Extension during this episode was by
complexly intermixed dip-slip and strike-slip faulting
and warping. Faults of highly variable dip direction
and amount show complex fault-slip characteristics
developed during the main phase of extension
(Michel-Noel and others, 1990). One result of the
synchronous extension and caldera development is
that caldera margins are largely bounded by faults,
some" of which are related to east-west
lineaments. The best example is the Delamar caldera,
which is largely bounded on the north by some faults
of the Timpahute lineament of Ekren and others
(1977) and on the south by some faults of the Helene
lineament (Fig. 2). Another result of the synchronous
extension and magmatism is the creation of gold
deposits in fault and breccia zones. These relations
are similar to those in the Walker Lane (Hardyman
and Oldow, 1991), one of the most important gold
belts in the country. The third episode of extension
was basin-range faulting, inferred to be post-l0 Ma,
when the present topography dominated by
north-south ranges formed beyond the limits of the
Caliente caldera complex (Anderson, 1989).
of the members consist of sanidine, quartz,
hedenbergitic clinopyroxene, fayalitic olivine, zircon,
chevkinite, and Fe-Ti oxides.
Nearby epithermal gold districts include the
small Chief mining district north of Caliente (Rowley
and others, 1990; Rowley and Shroba, 1991; Rowley
and others, 1991; Rowley and others, 1992), the
major Delamar (Ferguson) mining district just
southwest of the caldera complex (Swadley and
Rowley, 1992), the Taylor (Easter) mine, and the
Pennsylvania and Goldstrike on the south and
southeast sides of the complex.
The northern and western margin of the Kane
Springs Wash caldera in the Delamar Mountains has
been affected by small offsets on northwest-striking
dextral faults. A small stock (post-18.6 and pre-14.7
Ma; marked by two Is, Fig. 2) has been offset about
one km by one of these faults, but the caldera wall
indicates less than a few hundred meters of offset on
the same fault. In contrast, the eastern part of the
Kane Spring Wash caldera in the Meadow Valley
Mountains is tilted as much as 30° eastward, cut by
northeast-striking sinistral-oblique and dip-slip faults,
and injected by dikes feeding post-collapse
caldera-filling volcanism (Harding, 1991; Harding
and others, in press). The contrasting degree and style
of deformation in the western and eastern parts of this
30 km x 13 km caldera demonstrates the
heterogeneity of extensional deformation typical in
this part of the Basin and Range (Scott, 1990).
KANE SPRINGS WASH CALDERA COMPLEX
Early work in the Delamar and Meadow Valley
Mountains began with correlations of ash-flow tuffs
by Cook (1965) that lead to petrologic studies of the
alkaline metaluminous to mildly peralkaline ash-flow
tuffs and minor lavas erupted from the Kane Springs
Wash caldera and related sources (e.g., Noble, 1968;
Noble and Parker, 1974; Novak, 1984; Novak and
Mahood, 1986). In addition to interdisciplinary
studies by the same workers as on the Caliente
complex listed above, detailed mapping of the
complex is by R.B. Scott, W C Swadley, A.E.
Harding, W.R. Page, and E. H. Pampeyan and
petrological studies by R.B. Scott, S.W. Novak, and
L.D. Nealey. Mapping within and near the Kane
Springs Wash caldera was aided by previous work
(Novak, 1985; Pampeyan, 1989; and quadrangle maps
referred to by Scott and others, in press a). We now
recognize that the Kane Springs Wash caldera is the
youngestof a complex that includes the Narrow
Canyon caldera (Scott and others, 1991) and probably
two others that have not been located, as suggested by
Novak (1984). The Kane Wash Tuff has a volume of
about 150 knr': it has been redefined to consist only
of the three outflow cooling units derived from the
Kane Springs Wash caldera (Scott and others, in press
b). The older, single cooling unit of the metaluminous
Grapevine Spring Member (14.7 Ma) and the
younger, mildly peralkaline Gregerson Basin
Member, which consists of two nearly identical
cooling units (14.55 and 14.4 Ma, L.W. Snee,
written commun., 1991), were erupted from a magma
chamber zoned from peralkaline rhyolite near 820°C
at the top to a dominant volume of mafic trachyte
near lOOO°C (Novak and Mahood, 1986). Zirconium
contents of the Grapevine Spring range from 600 to
800 ppm whereas those of the Gregerson Basin range
from 800 to 1300 ppm. Phenocrysts in rhyolitic parts
One precursor to the Kane Springs Wash
caldera is the Narrow Canyon caldera (Scott and
others, 1991). Remnants of its margin, caldera-wall
breccia, and megabreccia are exposed within Narrow
Canyon (NCC in Fig. 2). Although the compositional
trends and phenocryst mineralogy in the associated
metaluminous to mildly peralkaline rhyolites are
similar to those of the Kane Springs Wash caldera,
the Narrow Canyon caldera differs in that its margin
has been thoroughly dismembered by
northeast-striking sinistral faults, displays a shallow
resurgent trachytic intrusion, and erupted very limited
outflow tuff. Caldera outflow, intracaldera equivalents
to the outflow units, and post-collapse caldera-filling
volcanic units are difficult to distinguish because
exposures of Narrow Canyon caldera units under the
blanket of Kane Wash Tuff are limited.
No calderas have been found for the
pre-14.7-Ma Sunflower Mountain Tuff or the pre15.6-Ma Delamar Lake Tuff (Scott and others, in
press a) that underlie the Kane Wash Tuff and overlie
the Hiko Tuff in the southern Delamar Mountains.
However, thicknesses of these units suggest that their
sources lie, respectively, to the southwest and to the
northwest of the Kane Springs Wash caldera. Both the
Sunflower Mountain and Delamar Lake are alkaline
metaluminous rhyolites and contain phenocrysts
similar to those of the Kane Wash Tuff.
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)
144 and 145 Ma
# GREGERSON BASIN MBR
TUFFS OF KERSHAW CANYON and
SAWMILL CANYON (1)
14.5 Ma rhyolite
two zoned cooling units, trachyte top
comendlte below that contuins q, 5. c, I. i
# GRAPEVINE SPRING MBR 14 7 Ma
rhyolite 10-25q 60-755 5-1Oc Sf 0-5i
zoned
1O~35tOI
TUFFS FROM NARROW CANYON CALDERA
? Ma peralkaline rhyolite
TUFF OF RAINBOW CANYON (Buckboard
Canyon)
15.6 Ma
rhyolite
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
#PAHRANAGAT FORMATION (Kawich) 22.6 Ma
21-49q 22-42s 16-42p 1-Gb O-Ih Il-j c 1-1-51101
: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
22.2± Ma
anoesue-rracnvandcsue
in Bull Valley Mounlains area)
0-3s 55-70p 10-20b 0-15h O-Sc 40-60101
zoned rhyolite
rhyolite
TUFF OF BIG TEN PEAK (Big Ten Peak) 25 Ma
TUFF OF GOBLIN KNOBS
(Goblin Knobs)
25.4 Ma
dacile
(unnamed)
25.4 Ma
zoned low-Si rhyolile to dacite lS-30q 5~25s 40-70p 5-15b 0-5h 10-30101
Q sci
O'SUNFLOWER MOUNTAIN TUFF (?) 147$ Ma
rhyolite 30-55q 40-655 If C If f 5-20101
O'DELAMAR LAKE TUFF
(?) 15.6 $ Ma
rhyolite
2S·50q 40-705 If C 10-25101
0~10q
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 LUNAR CUESTA
150M FORMATION (See tcomotej trachydaclte 70-aOp 5-20c 5-2010t
# HOLE-IN-THE-WALL MBR
? Ma
? Ma 3 or more cooling units
TUFF OF HAMLIGHT CANYON
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
.. 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
2 or more coofingunils
# 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
TUFF OF HOT CREEK CANYON
(HOI Creek)
29.7 Ma
rhyolite
RIPGUT FORMATION
(MI. Wilson)
O'LUND FORMATION
(While Rock)
? Ma
zoned rhyolite
27.9 Ma dacile
O'WAH WAH SPRINGS FORMATION
CIndian Peak) between 29.6 and 31.1 Ma
dacite 0-11q 52-68p 5-12b 14-32h 0-4c 11-5010'
(?)
? 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
#- COTTONWOOD WASH TUFF
Tuff unit in bold if seen on field trip. # regionaBy extensive outflow shee~. $ These dates may be 0.4 Ma 100 y<?ung. Name ot Source caklera in parentheses, Queried where indelinilelylocated. Only Ihe
members of the Isom Formation listed here could have come from the indianPeak caldera complex area. Proportions 01 phenocrysts With respect 10 100% as q, Quartz, s, sanidine, p, plagioclase,b, blctke,
h. hornblende, c. pyroxene, I, ilmenite, I, Fa-rich olivine, I, titanite (sphene), lot, proportion 01 total phenocrysts in whole rock,
FJELD TRIP ROAD LOG
DAY ONE Saturday May 22, 1993
We will see some of the calderas in the central
Nevada caldera complex and some outflow ash-flow
sheets in "outflow alley" associated with it and the
Indian Peak caldera complex. A stop will be made to
examine a highly extended terrane and associated
coarse clastic deposits of Pliocene age in the Grant
Range.
Tonopah, a Shoshone or Paiute Indian word
meaning "greasewood spring", was a famous bonanza
mining camp from about 1900 to 1930. Bonham and
Garside (1979) report that epithermal veins of silver
sulfides and sulfosalts are cut by and are overlain by
several rhyolite domes that form the prominent hills
around Tonopah. Mineralization and alteration seem
to be associated with a probable caldera that formed
during eruption of the Fraction Tuff at 19.8 Ma (not
to be confused with the Fraction Tuff emplaced at
18.3 Ma on the Nellis Air Force Bombing and
Gunnery Range).
Interval miles
Cumulative miles
0.0 0.0 Road log begins at intersection of US 95
and 6 in the SE part of Tonopah. Head E out
of town on US 6.
5.5 5.5 Intersection NV 376. Continue E.
1.4 6.9 Intersection of road to right (S) to Tonopah
airport and small refinery which processes
crude from Railroad Valley field.
5.0 11.9 Intersection of road to S to Tonopah Test
Range, home of the Stealth fighter plane, on
the Nellis Air Force Range. Continue E.
4.7 16.6 Red-brown outflow tuff of Pahranagat
Formation deposited at 22.6 Ma in hills on both
sides of highway for next two miles.
3.2 19.8 Rhyolite dome to N. To NE, brown cliff
above white tuff is Pahranagat outflow, capped
by mafic lava flow in small hill. Hills ahead are
of intracaldera tuff of Lunar Cuesta deposited at
25.4 Ma in its unnamed caldera.
8.4 28.2 Hills from 9 to 12 o'clock are cooling
units of intracaldera tuff of Lunar Cuesta,
zoned low-silica rhyolite to dacite.
3.1 31.3 Across Stone Cabin Valley to SE is the
Kawich Range, which forms most of the
exposed part of the Kawich caldera that
collapsed at 22.6 Ma as the Pahranagat ash
flow was erupted. The northern margin of the
caldera lies just S of the highway where it
5.9
7.6
5.0
7.5
7.1
passes between the Kawich Range and the Hot
Creek Range to the N. On the W flank of the
Hot Creek Range, to the left of the most
northerly, black lava-capped hill, is a sliver of
the Kiln Canyon caldera which formed during
deposition of the tuff of Orange Lichen Creek
at 26.8 Ma. To N, prominent W-dipping cuesta
is made of cooling units of trachydacite ashflow tuff of the Isom compositional type.
Beyond cuesta is the high Monitor Range,
which harbors the Big Ten Peak caldera.
37.2 Intersection of gravel road to right (S) to
Golden Arrow and Silver Bow mining districts
near margin of Kawich caldera.
44.8 Summit. To E and less than one km S of
highway at N end of Kawich Range is the
topographic margin of the Kawich caldera,
marked by about 30 m of tuff breccia overlain
by more than one km of multiple cooling units
of caldera-filling ash-flow tuff, which contain
lithic blocks as much as 20 m in diameter and
by sedimentary deposits (Gardner and others,
1980). These intracaldera rocks of the
Pahranagat Formation overlie propylitically and
argillically altered Monotony Tuff emplaced at
27.3 Ma and exposed on either side of the
highway. Large masses of broken Paleozoic
rock lie within the Monotony and are
interpreted to be slide blocks within the
Monotony source--the Pancake Range caldera.
49.8 Warm Springs roadhouse (abandoned, but
has hot bath and telephone!) and intersection
with NV 375. To NE is aptly named Pancake
Range and to S of NV 375 is the Reveille
Range. Continue on US 6.
57.3 To E across Hot Creek Valley in far
distance through pass between Pancake and
Reveille Ranges is the Quinn Canyon Range,
location of the Quinn Canyon caldera that was
the source of at least four cooling units of the
Shingle Pass Tuff deposited between 26.7 and
26.0 Ma. At 1 o'clock is the topographic wall
of the Pancake Range caldera, source of
Monotony Tuff (Fig. 3).
64.4 STOP 1 (Best, Gromme, and Deino)
Pull off highway into rest stop S of Blue Jay
maintenance station for overview of Central
Nevada caldera complex. This stop lies within
the oldest recognized caldera in the complex,
the Williams Ridge, whose collapse was
initiated by eruption of rhyolite ash flows that
formed the Windous Butte Formation around
the caldera and continued with eruption of
Figure 3. Only exposed topographic margin of the
Pancake Range caldera, which was the source of
the Monotony Tuff. As recognized by Ekren and
others (1974), this margin (arrow) is defined by
the post-Monotony Shingle Pass Tuff and
overlying tuff of Lunar Cuesta (cliff) on the right
banked against and capping the pre-Monotony
tuff of Halligan Mesa and overlying tuff of
Palisade Mesa on the left.
dacite that formed the intracaldera tuff of
Williams Ridge and Morey Peak. Across Hot
Creek Valley to the N in the Hot Creek Range
horst, the entire eastern escarpment, about 1
km high beneath Morey Peak, is intracaldera
tuff. Just W of Morey Peak is the N-S-trending
eastern margin of the Hot Creek caldera (John,
1987). Additional fill in the Williams Ridge
caldera consists of landslide deposits and local
sedimentary material plus subsequent local lava .
flows and a much greater volume of several
rhyolite ash-flow deposits derived from the
local magma system. To the NE, two of these
rhyolite tuffs with a combined thickness of
about 300 m make up most of Palisade Mesa.
The upper unit, aptly named the tuff of Palisade
Mesa (emplaced at 29.6 Ma), has a spectacular
system of columnar joints evident from the
highway to the N. Due E of this stop, a 240 mthick section of Shingle Pass Tuff and overlying
outflow tuff of Lunar Cuesta is banked
disconformably against the pre-Monotony tuffs
(Fig. 3). Ekren and others (1974) interpreted
this to be a segment of the topographic wall of
the caldera formed as Monotony dacite ash
flows were erupted at 27.3 Ma. Stewart and
Carlson (1976) designated it the Pancake Range
caldera.
Continue NE on highway.
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
only intracaldera dacite tuff, which is exposed
just S of hole. This intracaldera tuff of
Williams Ridge and Morey Peak has the same
paleomagnetic direction and 40 Arj39Ar age
(mean of determinations on two samples of
31.32 ± 0.08 Ma), within analytical
uncertainty, as the outflow Windous Butte
(mean of two, 31.31 ± 0.11 Ma).
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.
1.4 90.0 Intersection with gravel road S to Lunar
Crater. The surrounding youthful alkaline basalt
field of lava flows, about 70 cinder cones and
at least two maars, developed since 6 Ma along
a NNE-striking fissure system (Scott and Trask,
1971). Lunar Crater is a spectacular maar
easily reached from this turnoff. Nearly every
lava flow and ejecta deposit contains
megacrysts and Ti-Al-rich xenoliths of gabbro
and c1inopyroxenite, but only three vents
erupted Cr-rich spinel peridotite of mantle
origin (Menzies and others, 1987). Late
Cenozoic volcanism is concentrated along the
margins of the Great Basin and its only
expression within the interior is here in the
Lunar Crater field (Fitton and others, 1988).
Youthful "Black Rock" lava flow to E contains
abundant megacrysts of olivine, pyroxene, and
feldspar.
9.1 99.1 Black Rock Summit. To NE near relay
tower are large slide masses of shattered and
altered Paleozoic carbonate rock resting on
altered intracaldera tuff of the Windous Butte
Formation within a few kilometers of the
margin of the Williams Ridge caldera
(Quinlivan and others, 1974). For next four
miles, highway follows a Quaternary basalt lava
flow; hills on either side of highway are of
altered intracaldera tuff and Paleozoic rock.
3.5
3.4
1.7
7.7
8.0
6.0
3.8
102.6 Topographic margin of Williams Ridge
caldera near here.
106.0 STOP 3 (Christiansen) Two cooling
units of Monotony Tuff; vitrophyre of lower
unit lies at highway level. The Monotony
appears to have been deposited directly on
Paleozoic rocks here; the absence of the
Windous Butte Formation might imply uplift
near the Williams Ridge caldera prior to its
deposition.
107.7 Lockes ranch.
115.4 To N is the E flank of the Pancake
Range where alternating light- and dark-brown
layers are cooling units of the Stone Cabin
Formation (Radke, 1992). The Railroad Valley
oil field produces from a pre-Windous Butte,
post-Stone Cabin tuff and the upper Eocene to
lower Oligocene clastic Sheep Pass Formation.
123.4 Red and brown weathering Ragged
Ridge to E is a steep NE tilted section of
Oligocene rhyolite lava flows, clastic rocks,
and overlying regional tuff sheets including, in
ascending order, the Stone Cabin, Windous
Butte (on skyline), and Wah Wah Springs
Formations and Shingle Pass Tuff. Overlying
these sheets is a thick (as much as 3 km)
sequence of clastic sedimentary rocks, tuffs,
and gravity-slide masses of the Pliocene Horse
Camp Formation (Moores, Scott, and Lumsden,
1968). The significant post-volcanic extensional
faults in the Grant Range contrast sharply with
the few faults cutting the flat-lying strata of the
Pancake Range to the W.
129.4 Community of Currant. Turn SE on
gravel road passing N end of Ragged Ridge.
133.2 STOP 4 (Scutt) To NW is a low ridge
within the Horse Camp Basin that contains the
lowest of a series of tectonic breccia slices that
were emplaced as gravity slides during
deposition of the Pliocene Horse Camp
Formation (Moores, Scott, and Lumsden,
1968). These slices consist of brecciated
31.3-Ma Windous Butte Formation and 33-Ma
Railroad Valley Rhyolite. To SW, units in
Ragged Ridge dip 40-70° eastward, flooring the
Horse Camp depocenter. Dips in the Horse
Camp Formation decrease upsection from 70°
to 30°. To NE is a hill consisting of tectonic
slices of highly attenuated, brecciated, but
stratigraphically ordered Cambrian to Devonian
strata within the upper part of the Horse Camp
sediments. To SE, Red Mountain contains
brecciated slices of the Mississippian Joana
0.7
4.3
8.7
Limestone, Railroad Valley Rhyolite, slightly
younger sedimentary and ash-flow tuffs, and
the 34.5-Ma Stone Cabin and Windous Butte
Formations, tectonically emplaced within the
Horse Camp sequence. Farther E, across the
Ragged Ridge fault that borders the Horse
Camp Basin in the Horse Range, several
allochthonous sheets of Paleozoic and Tertiary
rocks overlie autochthonous Paleozoic strata, a
relationship typical of detachment-style
extensional tectonics. Presumably, these
detachments fed the growing Horse Camp
depocenter during the Pliocene. Scott (1965;
1966; also W.E. Brooks, written commun.,
1992) noted that volcanic strata in Ragged
Ridge have K 20 contents in the range of 8 to
10 % in contrast to relatively unaltered ranges
of 4 to 6 % elsewhere. This metasomatism
could have happened as the highly attenuated
strata of Ragged Ridge, buried under about 3
km of Pliocene sediments, were juxtaposed via
a basal detachment over warmer middle crust,
resulting in hydrothermal circulation that caused
alkali exchange (compare Walker and others,
1992).
133.9 Horse Camp Formation; optional stop.
138.2 Calloway Well and Stone Cabin,
namesakes and type sections of the formations.
of those names. The rhyolitic Calloway Well
Formation (in low hills immediately behind
cabin and well), named by Moores, Scott, and
Lumsden (1968), is a sequence of bedded
pyroclastic-surge and -fall deposits alternating
with massive ash-flow tuff emplaced at 35.3
Ma. The overlying Stone Cabin Formation,
which has the same 40ArP9Ar age within
analytical uncertainty, will be examined at the
next stop.
146.9 STOP 5 (Best, Christiansen, Deino,
Gromme) Intersection of 4X4 trail to left (N).
Hill to NE is brown, 210-m-thick, Stone Cabin
Formation overlain in distance by the 280m-thick Windous Butte Formation seen here as
a thin ledge of black vitrophyre. Complete
sections of these two rhyolite tuffs can be seen
by hiking to N around W side of hill where the
bottom part of the Stone Cabin contains 10-20
m of black vitrophyre underlain by several m of
white to salmon-colored, weakly welded
ash-flow tuff; beneath the Stone Cabin is a few
tens of m of cross-bedded, well-sorted gray
sandstone overlying Paleozoic rocks. By
continuing around hill to E, one can go up
section through the E dipping Stone Cabin into
the overlying conformable Windous Butte,
which consists of a few m of weakly welded
salmon-colored ash-flow tuff overlain by black
vitrophyre and then brown devitrified tuff, both :
densely welded. Note the absence of bedded
plinian ash-fall deposits at the base of these two
tuff sheets. On the SE side of the road, capping
the low hill, is the dacitic, hornblende-rich Wah
Wah Springs Formation derived from the Indian,
Peak caldera in the Indian Peak caldera
.
complex (Fig. 1). The lowest Wah Wah Springs
here is a slabby weathering welded tuff that
grades upward into about one m of dark gray
vitrophyre and then into devitrified red-brown
tuff. In the low rolling hills to the E are three,
thin, devitrified, densely welded ash-flow tuff
cooling units. The first is the red-brown lower
member of the Shingle Pass Tuff containing
conspicuous phenocrysts of sanidine and lesser
plagioclase and a few Fe-rich pyroxene and
olivine manifest by rusty spots; this unit across
a small gully is purplish and grussy weathering.
The overlying cooling unit of unknown
stratigraphic identity is a brown tuff containing
obvious shards and sparse tiny smokey quartz
phenocrysts. It is overlain by the upper member
of the Shingle Pass Tuff, which here is a
purplish-gray tuff containing light gray pumice
and phenocrysts of two feldspars and biotite.
Continue E on gravel road.
12.8 159.7 Intersection with NV 318. Turn left (N).
14.5 173.2 Enter farming community of Lund.
5.2 178.4 Lanes Ranch Motel, our overnight
accommodation. Community of Preston lies in
grove of trees to W.
DAY TWO
Sunday May 23, 1993
An extraordinarily complete, well-exposed
section of regional ash-flow outflow sheets will be
examined in White River Narrows along NV 318.
The sheets in this "outflow alley" section lie between
their sources in three nearby caldera complexes: the
Central Nevada to the west, the Indian Peak to the
east, and the Caliente to the southeast. This section,
and the one at stop 5 yesterday, are representative of
numerous outflow volcanic sections emplaced during
the ignimbrite flareup in the central and southeastern
Great Basin which contain little or no sediment and
angular discordances indicative of regional synvolcanic extension (compare Gans and others, 1989,
Fig. 18 and Best and Christiansen, 1991, Fig. 6).
As time permits, late afternoon stops will be
made near Caliente as an introduction to the Caliente
caldera complex to see the Clover Creek caldera
(source of the Bauers Tuff Member of the Condor
Canyon Formation) and the Delamar caldera (Hiko
Tuff).
0.0
0.0 Roadlog begins at Lanes Ranch Motel.
Head S on NV 318.
4.3 4.3 Lund.
15.4 19.7 Intersection with gravel road on W that
goes to stop 5.
8.7 28.4 Intersection with gravel road on E that
goes to Shingle Pass where an exceptional
section of volcanic units (Best and others,
1989a, p.118), including the Shingle Pass Tuff,
is well exposed.
5.5 33.9 Sunnyside ranch.
23.1 57.0 Intersection with gravel road on E that
goes to Bristol Wells and Pioche.
17.6 74.6 STOP 6 (Gromme, Best, and
Christiansen) Intersection with gravel road on
W to petroglyphs. Exposed in hill to W (Fig. 4)
are, in ascending order, the Petroglyph Cliff
Ignimbrite (named and briefly described by
'Cook, 1965), Monotony Tuff, tuff of Hamilton
Spring (Taylor and others, 1989), and a
hornblende-pyroxene andesite lava flow. The
Petroglyph Cliff (27.3-27.9 Ma), legitimately a
welded tuff breccia, is one of the oldest of the
Isom-compositional-type tuffs in the Great
Basin and is unusual because of its abundant
lapilli- and block-size fragments. This cooling
unit is found only here and in two ranges to the
E and probably vented nearby. The distal part
of the Monotony outflow sheet here is fairly
thick but poorly welded. The Hamilton Spring
is another Isom type tuff that may correlate,
according to paleomagnetic data (Scott and
others, in press a), with the Baldhills Tuff
Member of the Isom Formation that is widely
exposed to the E into Utah.
Continue S on highway.
0.9 75.5 Site of a famous photograph and diagram
by Cook (1965, Fig. 29) showing a pinch-out
of ash-flow sheets over the stubby toe of a thick
hornblende-augite andesite lava flow. Cook
concluded that, due to the differential and
diminishing compaction of each successively
deposited tuff, the topographic relief over the
lava flow diminished as each younger ash flow
was emplaced. Assuming the top of each flow
NORTH
SOUTH
lava flow I
M
P -
R
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.
1.0
1.2
was initially horizontal, he calculated about
50 % compaction.
76.5 STOP 7 (Gromme, Christiansen, and
Rowley) At the N end of White River Narrows
are four ash-flow sheets consisting of, in
ascending order starting at the road cut, a
crystal-rich rhyolite tuff of unknown
stratigraphic identity, upper member of the
Shingle Pass Tuff, thin, densely welded
Hole-in-the-Wall Tuff Member of the 1som
Formation, and thick (104 m) Leach Canyon
Formation containing spectacular columnar
joints. The three named units had three
different sources, the Central Nevada caldera
complex to the NW, possibly the Indian Peak
complex to the NE, and Caliente caldera
complex to the SE, respectively.
Continue S on highway.
77.7 STOP 8 (Rowley, Christiansen, and
Best) At the S end of White River Narrows are
five regional ash-flow sheets overlying Leach
Canyon, which here is a pink, lithic-rich
devitrified tuff. These five tuff sheets are, in
ascending order: Condor Canyon Formation
consisting of Swett and overlying Bauers Tuff
Members, Pahranagat Formation, Harmony
Hills Tuff (exposed only in a small hill), and
Hiko Tuff, which has large columnar joints.
The Leach Canyon Formation emplaced at 23.8
Ma was suggested, on the basis of distribution
and isopach data, to have been derived from the
northern part of the Caliente caldera complex
(Williams, 1967), but no source has been
identified; perhaps it is under the Panaca basin
between Pioche and Caliente. The Condor
Canyon Formation was derived from the Clover
Creek caldera of the Caliente complex. The
Harmony Hills Tuff (between 22.7 and 21.7
Ma; Rowley and others, 1989) has been
proposed to come from the Caliente caldera
complex (Ekren and others, 1977) but more
likely it was derived from the Bull Valley
Mountains to the E in Utah (Blank, 1959;
Blank and others, 1992).
Continue S on highway.
11.0 88.7 Outcrops to E are of Hiko Tuff and
underlying Bauers Tuff Member of the Condor
Canyon Formation. The same units are exposed
for next two miles through Hiko Narrows.
8.9 97.6 Community of Hiko (Paiute word for
"white man's town")
4.7 102.3 Veer left, continuing on NV 318.
0.6 102.9 Turn left (E) on US 93 toward Caliente.
2.8 105.7 Pass through Hiko Range. Crystal-rich
rhyolite of the Hiko Tuff, which weathers into
bulbous forms much like granite, is overlain by
thin, middle Miocene, densely welded Kane
Wash Tuff.
10.5 116.2 Pahroc Summit Pass. To S is South
Pahroc Range where Hiko outflow is as much
as 350 m thick; to N is North Pahroc Range.
R.B. Scott has mapped east-striking strike-slip
faults near the road here that he interprets to be
part of the Timpahute lineament of Ekren and
others (1976).
Continue E across Delamar Valley.
10.2 126.4 To S, gravel road to ghost town of
Delamar, on the Wedge of the Delamar Range,
and center of the Ferguson mining district, an
epithermal gold deposit that was Nevada's most
productive at the turn of the century.
2.8 129.2 Road cut of Cambrian Highland Peak
Formation on left and hills of the same unit to
left and right.
0.5 129.7 Cross NNE-striking Seven Oaks Spring
fault, which separates Highland Peak Formation
on W from both sedimentary rocks (left of
road) and intracaldera Hiko Tuff (right of road)
on E. The sharp peak at 10 o'clock is underlain
by a Cambrian section ranging from red
Zabriskie Quartzite at road level to Highland
Peak Formation at the top. The Zabriskie,
however, is highly faulted just N of the road
against masses of intracaldera Hiko to the S.
Thus, the margin of the Delamar caldera here,
as in many other places, is a fault, not a typical
collapsed structure.
1.4 131.1 Road bends to right and dirt road on left
leads along an ENE-trending valley underlain
by the same caldera margin. Continue on US
93 ESE through intracaldera Hiko Tuff on both
sides of road which contains numerous
interbeds, some as much as 100 m thick, of
volcanic mudflow breccia--typical of the upper
part of the Delamar intracaldera fill. Some
interbeds are megabreccia masses from
sloughing of caldera walls, but most clastic
beds are of dacite to andesite flow rocks, and
the deposits are interpreted to be from
stratovolcanoes near the caldera rim that were
synchronous with caldera volcanism and
faulting.
1.7 132.8 Oak Springs Summit, crest of the
Delamar Mountains.
3.5 136.3 Roadcuts on left in Newman Canyon
show intertonguing intracaldera beds of
mudflow breccia, megabreccia, and Hiko Tuff.
The Delamar caldera was subsequently
complexly faulted, mostly during the main, or
latest Oligocene to middle Miocene, episode of
extension along mostly NNW-striking obliqueslip faults. About 200 m farther E the valley
widens into a basin of upper Tertiary gravels
and sand. Cross the major NNW-striking
oblique-slip Dula Canyon fault, which probably
has several kilometers of right-slip and
unknown amounts of down-to-the-E normal
slip.
3.0
1.8
0.7
0.5
0.7
0.1
1.5
0.6
139.8 Road passes through vertical cuts on
both sides consisting of the gray and pink,
E-dipping, 3 m-thick tuff of Etna. The tuff is
sandwiched between tan sedimentary rocks of
Newman Canyon. Several hundred meters N of
here, tan sedimentary rocks of this unit that. _
overlie tuff of Etna contain a bed of waterlaid
pumice deposited at 13.8 Ma (H.H. Mehnert,
unpub. data, 1991; Rowley and others, 1991),
thus constraining the age of the tuff of Etna.
Note numerous NNW-striking oblique-slip
faults in roadcuts. Continue E, tuff of Etna
thickens and caps mesas.
141.6 Toreva block of tuff of Etna on right. In
gulch to left, behind houses and junk cars, the
Newman Canyon detachment fault dips gently
toward road, just above the dark spur. Left of
the dark spur, in the main gulch, this fault cuts
off a high-angle oblique left-slip fault, the
Gravel Pit fault of Rowley and others
(1992). This fault has at least 2 km of left slip,
and in the main gulch here it is intruded by a
dike of the porphyry of Meadow Valley Wash
emplaced at 19.4 Ma (Snee and others, 1990;
Rowley and others, 1992).
142.3 Enter town limits of Caliente.
142.8 Church on left. Intersection with NV
317 on right. Continue along US 93 through
town to just past the post office.
143.5 Turn right, heading E across Union
Pacific Railroad tracks, then right (S) past
stores.
143.6 Turn left and head up small canyon
along a dirt road through a thick, steeply
NE-dipping fanglomerate sequence that contains
white and-yellow air-fall tuff beds correlated
with the tuff of Kershaw Canyon.
145.1 At the pass, take a sharp right r:'R)
along dirt road to overlook near relay tower.
145.7 STOP 9 (Anderson and Rowley)
Overview of the geology of the N margin of the
Caliente caldera complex and synchronous
faults. The Clover Creek caldera is best
displayed NE of Caliente, in the canyon of
Clover Creek. This caldera, recognized by
Rowley and Siders (1988), is the source of the
Bauers Tuff Member (22.7 Ma; Best and
others, 1989a) of the Condor Canyon
Formation; about 400 m of intracaldera
rhyolitic Bauers is exposed on the N wall of the
canyon just E of town. W of that, in Meadow
Valley Wash and W of Caliente, are two N- to
2.2
0.3
1.2
NNE-striking oblique-slip faults, each with at
least 2 km of left slip. E and N of the 400 m
exposures of Bauers is a NNW oblique-slip
fault with at least 2 km of right slip and 0.5 km
of normal slip that enters Meadow Valley Wash
3 km N of Caliente. A S-dipping low-angle
normal fault that contains the tuff of Etna (14
Ma according to dating by H.H. Mehnert) in its
hanging wall, passes under the scarp here. The
Delamar caldera (Rowley and Siders, 1988),
the source of the rhyolitic Hiko Tuff (18.6 Ma;
Taylor and others, 1989), is inset into the
Clover Creek caldera and underlies us but its
nearest exposures are in English Canyon 2 km
to our E. Tan post-Etna clastic sedimentary
rocks that show lesser deformation upward in
their section are seen in most directions.
Return to US 93.
147.9 Turn right (N) on US 93.
148.2 Turn right (E) just past the row houses,
then, in 50 m, right (SE) again beyond the
tracks on a dirt road that runs along the N side
of the Union Pacific tracks, heading E up
Clover Creek canyon. Thick, gently.N-dipping
intracaldera Bauers makes up all the walls of
the canyon.
149.4 STOP 10 (Anderson and Rowley)
Wash of English Canyon passes under the
railroad tracks. Examine Bauers 0.5 km up (S)
English Canyon to the fault contact between
Bauers on the N and the same post-Hiko
fanglomerate sequence exposed E of Caliente.
The fault strikes E and dips S and exhibits
oblique-slip (right-lateral and normal) in
exposures to the E. The Bauers in the footwall
here dips north, but within 1 km to the east,
attitudes swing around and Bauers dips S in the
hanging wall, defining an asymmetrical
anticline interpreted to have formed by folding
along this major fault. The fault and numerous
other nearby faults and folds can be shown by
structural analysis to represent the same
deformation. The fault also marks the N
topographic margin of the Delamar caldera
along which subsidence occurred during and
after emplacement of Hiko Tuff; the margin is
thus unlike anything described in the literature
because it clearly has significant oblique slip.
This fault is one of several E-W faults and
plutons (including the one that controls the
Chief mining district NW of Caliente) within an
E-W zone at least 5 km wide that Ekren and
others (1976) called the Timpahute
lineament. Another 1 km upstream is
underlying, more densely welded intracaldera
Hiko and intertongued caldera megabreccia
deposited in the Delamar caldera from
landslides off the oversteepened fault/caldera
margin.
Return to US 93 and Caliente for overnight.
DAY 3
Monday May 24, 1993
The first part of the day will continue to
provide a brief overview of the Caliente caldera
complex by way of a traverse southward from
Caliente to Elgin through Rainbow Canyon along NV
317 to see the Buckboard Canyon caldera (tuff of
Rainbow Canyon) and several outflow tuffs derived
from other parts of the complex. Much of the story of
the complex, however, is about the structural
geology, for magmatism was in large part
synchronous with the main episode of extensional
deformation.
The second part of the day we will examine the
Kane Springs Wash caldera which has been sliced by
the left-lateral oblique-slip Kane Springs Wash fault,
and the Narrow Canyon caldera, a peralkaline to
metaluminous precursor to the Kane Springs Wash
caldera which has been highly deformed by
high-angle normal faults and by northwest-striking
dextral faults.
0.0
0.4
0.6
0.5
0.0· Start log at intersection of US 93 and NV
317 across from the church. Turn left (SE) onto
NV 317 and head S down Rainbow Canyon
following Meadow Valley Wash.
0.4 Jagged hill on E is the deformed hanging
wall of the low-angle, S-dipping normal fault
mentioned at stop 9. Most rocks are of tuff of
Etna and underlying and overlying clastic
sedimentary rocks.
1.0 Tuff of Etna, a moderately welded,
rhyolite outflow tuff is exposed on the E side of
the road in a rollover in the hanging wall of a
low-angle fault. Source of Etna is about 8 km
SSW of here. Two outflow cooling units of the
moderately to densely welded peralkaline
Gregerson Basin Member of the Kane Wash
Tuff (14.7 Ma; Scott and others, in press a) are
interbedded with clastic sedimentary rocks
under the tuff of Etna just S of this stop.
1.5 On the E, clastic sedimentary rocks under
the tuff of Etna intertongue to the S with white
nonwelded rhyolite ash-flow tuff that we call
the tuff of Kershaw Canyon. The tuff of Etna is
the uppermost resistant bed at most places in
northern Rainbow Canyon. Look W to see
several of its cooling units in exposures on the
other side of the Canyon.
0.5 2.0 STOP 11 (Rowley, Anderson, L.D.
Nealey, D.M. Unruh, Scott, and Harding).
Road to E goes to Kershaw-Ryan State
Recreation Area (closed). Walk 0.7 km E along
the road to examine tuff of Kershaw Canyon
and to view a fault that offsets two cooling
units of the Gregerson Basin Member and
intertongued outflow(?) tuff of Kershaw
Canyon. The Gregerson Basin units were
derived from the Kane Springs Wash caldera.
As first noted by R.E. Anderson (see Bowman,
1985 and Michel-Noel and others, 1990) the
NNW-striking, high-angle, oblique-slip fault on
the N wall of the' canyon offsets the lower
cooling unit twice as much as it offsets the
upper cooling unit, whereas the tuff of Etna on
top has virtually no offset. The upward
decreasing amount of offset is due to recurrent
movement along the fault, which is not a
typical Gulf-Coast-type growth fault because
much of the offset is right-lateral. The latest
movement on the fault reflects some of the
youngest movement during the main episode of
extension in the area, which began at least
before 19 Ma and continued to at least 12
Ma. As we walk back to the vehicles, look
ahead to the SWan the E wall of Rainbow
Canyon where the fan array of tuff of Kershaw
Canyon and intertongued Gregerson Basin
Member resting on mostly pink tuff of Rainbow
Canyon to the S. Dips are progressively
greater in progressively older rocks, due to
recurrent movement on nearby oblique-slip
faults.
1.2 3.2 Stock pond and ranch to E; mobile homes
and Union Pacific warehouse to W. Here,
Buckboard Canyon enters Rainbow Canyon
from the W. Turn right onto former NV
317. Cross tracks; rugged gulch straight ahead
contains another major high-angle NW-striking
oblique-slip fault that passes under us and
continues SE through the N side of a canyon
behind us.
004 3.6 STOP 12 (Rowley, L.D. Nealey, and
D.M. Unruh) Most rocks here and around us
belong to the poorly to nonwelded intracaldera
rhyolite tuff of Rainbow Canyon. The tuff here
is in its source, the Buckboard Canyon caldera
004
0.6
1.0
0.2
0.3
(Rowley and Siders, 1988) that is interpreted to
be a trap-door type caldera in which most
subsidence was on its N side; the N margin is
buried by younger rocks but is inferred to be
near Caliente. The tuff has a 40 Arj39Ar age of
15.6 Ma and a K-Ar age of 15.2 Ma. The
rocks SW of the major fault in the gulch
mentioned above have been downthrown and
include the two Gregerson Basin cooling units
(also just S of the homes and warehouse) and,
under those, a local thick, brown, poorly to
moderately welded outflow ash-flow tuff near
the bottom of Buckboard Canyon that we call
the tuff of Sawmill Canyon. These three
cooling units are interbedded with the tuff of
Kershaw Canyon and are overlain by the tuff of
Etna capping the S wall of Buckboard Canyon.
As is typical of most of these NNW
oblique-slip faults, the rock sequences on
opposite sides of the fault display different
facies of the same stratigraphic units. Thus the
rocks above the tuff of Rainbow Canyon N of
the major fault are much thinner and include
only one Gregerson Basin cooling unit and only
minor amounts of the tuff of Kershaw
Canyon. Examine tuff of Rainbow Canyon,
which makes up all exposures near this stop
except for a large, badly deformed horse of
Hiko Tuff in the zone of the major fault.
Return to NV 317.
4.0 Turn right (S); continue down Rainbow
Canyon on NV 317.
4.6 Sawmill Canyon enters Rainbow Canyon
on E. Tuff of Sawmill Canyon, two cooling
units of the Gregerson Basin Member, and tuff
of Etna occur in canyon wall to W. From here
to a point about 2 km to the S, observe the
same units interbedded with thick tuff of
Kershaw Canyon on E canyon wall broken by
numerous faults. The facies on the E wall of
Rainbow Canyon is thicker than that on the W
wall; probably a buried NE-striking fault with
strike-slip offset is beneath us.
5.6 As we round the corner and head westerly,
note the NW-striking high-angle fault ahead that
cuts the tuff of Rainbow Canyon low in the
canyon wall but does not offset the tuff of Etna
on the mesa top.
5.8 Former railroad stop of Etna. The tuff of
Etna caps all surrounding mesas.
6.1 A NW-striking fault at 3 o'clock dropped
the tuff of Rainbow Canyon on the N against
the Hiko Tuff on the S. At 1:30, note a
0.3
1.0
0.7
0.6
0.5
spectacular fault along which the tuff of
Rainbow Canyon on W is dropped downward
against the Hiko Tuff on the E. The Hiko on
the E side is overlain disconformably by the
tuff of Rainbow Canyon; this contact may be
thought of as the floor of the Buckboard
Canyon trapdoor caldera, and we infer that the
tuff of Rainbow Canyon here is much thinner
than it was in the N part of the caldera, as at
the last stop.
6.4 Roadcuts of Hiko Tuff on both sides of
road.
7.4 Dula Canyon enters Rainbow Canyon from
the W. The canyon is underlain by the major '
NNW-striking oblique-slip Dula Canyon fault I
that can be traced the length of the Caliente 7.51
"
minute quadrangle. The amount of right slip
has not been determined but is doubtless greater'
for older rocks than younger and for Hiko Tuff
must be at least a couple of kilometers. The
rocks on the SW side of the fault are much
different from those to the NE and consist
mostly of rhyolite domes and lava flows.
8.1 Chokecherry Canyon enters Rainbow
Canyon from the W. Spectacular exposures of a .
rhyolite dome and its underlying pyroclastic
cone occur several hundred meters up the
canyon. A rough dirt road that starts here leads
up the canyon and then SW for about 5 km to
the Taylor (Easter) Mine, an epithermal gold
deposit consisting of gold-bearing quartz veins
along a N-dipping normal fault and adjacent
hydrothermally altered intracaldera Hiko Tuff.
8.7 Baldy Mountain to the Wand exposures
extending into Chokecherry Canyon are
underlain by the rhyolite mentioned above. The
high fault scarp to the E, with the Dula Canyon
fault at its base, is underlain by the Acklin
Canyon rhyolite dome, which rests on
intracaldera Hiko Tuff. Sharp brown hill ahead
is of andesite lava flows that were deposited
outside (S of) the Delamar caldera. The hill is
bounded on its W side by a NNW-striking
left-slip fault under Meadow Valley Wash and
on its E by an almost N-striking right-slip fault.
9.2 S-dipping contact between the andesite (on
the N) and outflow Hiko Tuff occurs in the E
roadcut here. The Dula Canyon fault is about 1
km to the E, on the other side of this small
mountain of Acklin Canyon rhyolite flows and
tuffs. Most rocks to Ware rhyolite flows and
tuffs of the Baldy Mountain dome; farther W
this dome rests on intracaldera Hiko.
0.2
9.4 Taylor Mine Canyon enters Rainbow
Canyon from the W. Old shell of a concrete
building in cottonwood trees at 3 o'clock is the
former pump house for a water line from
Meadow Valley Wash over the Delamar
Mountains to the town and mines of the
Delamar (Ferguson) district, about 18 km to the
WSW. Because the water line was installed
midway through the life of the district the gold
in the quartzite was originally mined and milled
dry, and thus the Delamar district is known as
the "widow maker" because of deaths from
silicosis. The town survived 17 years until 1909
and had a population of as many as 2000 people
(Ferris, 1991). The district, located just SW of
the Delamar caldera (Fig. 2), is an epithermal
system in massive Lower Cambrian quartzite
and has many geologic similarities to the small ,
Chief gold district NW of Caliente (Rowley and
others, 1992). The gold at both places was
deposited in quartz veins along fractures in
quartzite, which shattered during the main
episode of Tertiary extensional faulting that
accompanied magmatism and associated
convective hydrothermal activity.
10.5 Rock Springs Canyon enters Rainbow
Canyon from the W. Light-gray, nonwelded
rhyolite ash-flow tuff visible about 1 km to the
Ware possible correlatives with pyroclastic
deposits of the Baldy Mountain dome. Rock
Springs Canyon narrows dramatically 2 km to
the SW where the 400-m-high gorge consists of
the vertically flow-foliated vent facies tuff of
Etna. The vent cut through intracaldera Hiko
Tuff at the S edge of the Delamar caldera. The
margin of the caldera is an E-W fault zone
within the E-W Helene lineament (Rowley and
others, in press a) which is at least 5 km wide
and characterized across this width by almost
continuous rhyolite that conceals the actual
caldera margin. In general the rhyolite N of the
fault/caldera margin consists of extrusive
products (domes and tuff), whereas the rhyolite
S of the margin consists of the deeply eroded
and hydrothermally altered intrusive vents for
domes and tuff that were removed by erosion.
Almost all vents are marked by dikes that
mostly strike E-W and clearly intruded along
other faults within the lineament. At the SW
edge of the Delamar caldera, the E-W rhyolite
dikes and faults continue W into the Delamar
district. A buried silicic pluton is inferred for
the dikes under the district and is hypothesized
I
1.1
to be the heat pump for the convective overturn
of groundwater and thus for the mineralization.
1.3 11.8 Pass under the railroad; ranch on NW.
Most rocks are hydrothermally altered; local
anomalous gold has been reported and, in a few
places, mined. Here we are probably S of the
Delamar caldera. Based on an analogy with the
gold deposits at Delamar, the potential for gold
in this E-W faulted rhyolite zone would seem to
be good, but we have not yet sampled the rocks
here.
3.1 14.9 Pass under the railroad. Former railroad
stop of Boyd about 1 km N of here. We
continue S out of the deformed and
hydrothermally altered rocks of the Helene
lineament and into a simple, gently S-dipping
homoclinal sequence of volcanic rocks,
including, from base upward, dark-brown
outflow tuff of the Gregerson Basin Member of
the Kane Wash Tuff, a sequence of
yellowish-tan, nonwelded rhyolite ash-flow tuff
that is possibly tuff of Kershaw Canyon,
and dark-brown, moderately welded outflow of
the tuff of Etna.
3.9 18.8 Rainbow Canyon narrows as Meadow
Valley Wash and the road pass through thick
outflow tuff of Etna. Five overlying basalt lava
flows are in turn overlain by thick fluvial
sedimentary rocks. Although not dated, the
basalts probably are correlative with flows in a
similar stratigraphic position on the east side of
the Meadow Valley Mountains dated by H.H.
Mehnert (unpublished data, 1983) at 13.2 and
13.3 Ma. The sedimentary sequence that forms
steep hillsides and cliffs on either side of the
canyon as it opens up around Elgin formed in a
basin considerably larger and older than the
current structural basin. Small cliffs on left
formed by three layers of young peralkaline(?)
ash-flow tuffs(?) are apparently the youngest
silicic volcanic rocks in the area.
1.8 20.6 Intersection with dirt road to the two
inhabited houses remaining in Elgin on the
north side of Meadow Valley Wash. Continue
SE.
0.6 21.2 End of pavement on NV 317.
0.2 21.4 Turn left (S) onto gravel road that climbs
SW out of Rainbow Canyon through thick
sedimentary sequence.
0.4 21.8 On the left, NW-dipping sedimentary
rock, basalt, and tuff are deformed against a
strand of the Kane Springs Wash normal fault
that makes the SE border of the Elgin structural
basin. These deformed basalts were dated by H.
H. Mehnert (unpublished data, 1980) at 8 Ma
and correlate well with basalts found in the
fluvial deposits on the E side of the Meadow
Valley Mountains, also dated by Mehnert at 8.1
Ma. Thus, the original depositional basin for
these fine-grained fluvial sediments persisted at
least 5 million years, a surprisingly long
period. The strand of the Kane Springs Wash
fault contains subhorizontal slickenlines that
indicate a left-lateral oblique sense of motion;
as we will see farther S at Stop 13, 4-7 km of
sinistral offset has been established by the offset
of the margins of the Kane Springs Wash
caldera. It is likely that the Elgin structural
basin is related to this motion; note that the
NE-striking Kane Springs Wash fault joins a
WNW-striking fault and a N-striking fault at
the S end of the exposures of Paleozoic rocks
NE of Elgin (Fig. 2).
0.9 22.7 On right, new roadcuts expose steeply
NE-dipping sedimentary rocks that are cut by
. NW-dipping normal faults.
1.9 24.6 At crest between Kane Springs Valley
and Meadow Valley Wash note that the
sedimentary rocks are fine grained even at this
high topographic elevation; the nearby
coarser-grained rocks exposed at lower
topographic levels to the northwest may require
either undetected structures or local channels.
Continue straight on NV 317 past crossroads
and down Kane Springs Valley, the Meadow
Valley Mountains are on the left (E), the
Delamar Mountains on the right (W). The low
pass in the Meadow Valley Mountains is
underlain by the less resistant Harmony Hills
Tuff; SW of the pass, the NE wall of the E part
of the Kane Springs Wash caldera coincides
with the higher part of the mountains that are
underlain by the more resistant caldera-filling
rhyolites within the caldera. Farther down Kane
Wash Valley, observe intracaldera equivalents
of the Kane Wash Tuff characterized by the
darker exposures at the base of the Meadow
Valley Mountains; the yellowish rocks and the
rocks above them are post-collapse
caldera-filling tuff and rhyolitic lava flows
(Harding, 1991; Harding and others, in press).
On the right, a NE-trending low ridge of
outflow tuff from the Narrow Canyon caldera is
exposed in the foreground. In the background
are E-dipping slopes of the 13.3(?)-Ma basalt
capping the tuff of Etna and the Kane Springs
8.2
0.5
Wash Tuff. Pass the northern wall of the Kane
Springs Wash caldera in the Delamar
Mountains on the way to next stop.
32.8 Turn left at faint 4X4 trail toward
Meadow Valley Mountains.
33.3 STOP 13 (Harding and Scott) Walk to
exposures of intracaldera and caldera-filling
0.5
units, S wall of Kane Springs Wash caldera,
6.6
wall breccia, precaldera units and (perhaps)
outflow units outside the caldera wall. Also
walk along the trace of the Kane Springs Wash
fault to see low-angle slickenlines indicating
sinistral oblique slip. The footwall block
.
contains the contact between the uppermost and ;
underlying intracaldera units equivalent to the
1.9
two outflow cooling units of the Gregerson
Basin Member of the Kane Wash Tuff. The
bluish- to greenish-gray thin layers at the
quenched base of the upper cooling unit are like
0.3
the layers found at the base of its outflow
tooling unit. See caldera wall breccia just S of
the saddle formed by a hanging-wall block of
basalt and post-collapse caldera-filling rhyolite
flows. and tuffs. Here, the wall consists of a
pre-caldera trachyte flow containing
conspicuous alkali feldspar phenocrysts. Crude
bedding in the breccia dips NE into the caldera.
Above the breccia, the intracaldera ash-flow
tuff has been quenched against the breccia in
similar manner as seen elsewhere within and
outside the caldera. Walk out of the caldera
toward the SE; at the crest and down the slope
to the W, note a large block of the Grapevine
2.1
Spring Member of the Kane Wash Tuff,
1.6
enveloped by quenched vitrophyre of
intracaldera Gregerson Basin, that sloughed off
the caldera wall. At the wash at the base of the
hill, an aphyric dike cuts the caldera wall
2.8
parallel to the range front fault and passes
outside the caldera into the lower part of the
1.7
pre-14.7-Ma Sunflower Mountain Tuff. Climb
through the pre-caldera trachyte flow into the
Grapevine Spring Member. On the way back to
0.6
vehicles, walk through the caldera to observe
several post-collapse, caldera-filling, rhyolitic
ash-flow tuffs, rhyolite flows, and a basalt
flow. Note that the nonwelded tuff of this
0.7
sequence is superficially similar to the
nonwelded base of the Sunflower Mountain
Tuff; this similarity contributed to past
difficulties in recognizing the caldera in the
1.0
Meadow Valley Mountains. Below this
sequence is the upper part of the youngest
cooling unit of the intracaldera Gregerson Basin
that contains cognate inclusions and fiamme of
mafic trachyte in a somewhat more trachytic
matrix. Similar evidence for a layered magma
chamber can be found in the tops of the outflow
cooling units.
33.8 Rejoin NV 317 and turn right (NE).
40.4 Turn right just past corral on left toward
low pass in Meadow Valley Mountains.
Harmony Hills Tuff and Hiko Tuff in the
saddle dip steeply to the NE, away from the N
wall of the caldera, which lies to the right (S).
Dikes of a pre-caldera trachyte (about 14.7 Ma)
cut these ash-flow tuffs.
42.3 At water tank in low pass, turn sharply
right on 4X4 trail and drive up an alluvial-fan
veneer on a pediment developed on the
Harmony Hills Tuff.
42.6 STOP 14 (Harding and Scott) Park at
end of trail and walk W a few hundred meters
to exposures of the N wall of the Kane Springs
Wash caldera. Everywhere along this segment
of the wall, breccias and intercalated ash-fall
tuffs dip 10-40° S toward the caldera,
perpendicular to the trace of the wall.
Erosionally resistant caldera-filling rhyolite lava
flows and ash-flow tuffs dip 20-40° SE and are
repeated by several NW-dipping normal faults
parallel to the Kane Springs Wash range-front
fault. Nonresistant Harmony Hills and Hiko
Tuffs in the wall dip 50-60° N away from the
caldera, forming the valley below us.
44.7 Rejoin NV 317 and turn right.
46.3 At crest between Kane Springs Valley
and Meadow Valley Wash, turn left (NW)
toward the Delamar Mountains, driving through
dissected pediment above older Tertiary
sedimentary basin fill.
49.1 To left are dip slopes of 13.3(?)Ma
basalt.
50.8 Cross basalt. To right in valley are
rounded exposures of tuff of Etna.
51.4 Turn left. On right, exposures of Hiko
Tuff underlie outflow sheets of the Narrow
Canyon caldera, which in turn underlie the tuff
of Etna; the Kane Wash Tuff is missing here.
52.1 Distal edge of upper cooling unit of the
Gregerson Basin Member is plastered on a hill
of Hiko Tuff. These chilled relationships are
similar to those we saw earlier in the
intracaldera Gregerson Basin.
53.1 Cross wash. On left (S) is a ridge of
outflow Kane Wash Tuff. Although only 10 km
0.2
0.7
from the caldera, the Kane Wash Tuff is less
than 100 m thick. On right are exposures of
nearly horizontal Hiko Tuff, against which the
Kane Wash Tuff is thinned, possibly because of
tumescence prior to collapse of the nearby
Narrow Canyon caldera.
53.3 Cross exposures of a pyroxeneplagioclase andesite(?) flow breccia that
underlies the Hiko Tuff.
60.0 STOP 15 (Scott) Wall of the Narrow
Canyon caldera marked by megabreccia of the
Hiko Tuff. Outflow and intracaldera tuffs have
Zr contents in the range of 500-1050 ppm, only
slightly lower than those of the Kane Wash
Tuff (600 to 1300 ppm). Although the outflow
has limited extent, intracaldera tuff is exposed
in the N wall of the Kane Springs Wash caldera
beneath the pre-caldera trachyte flow at stop
13. The caldera wall has been fragmented
tectonically by NW-striking, dextral-slip faults
and high-angle normal faults; in fact, the
narrow, trough-like segment of caldera-filling
tuffs between caldera wall breccia may indicate
that this caldera, like several similar features
within the Caliente caldera complex, formed
essentially coevally with extension. If so, the
structural depressions created during extension
may have acted as loci of eruption and/or as
topographic depressions in which eruptions
pooled. Pre-existing strain and perhaps stress
conditions in the upper crust never allowed the
classic "cookie cutter" style caldera to form;
instead regional faults control the shape of
collapse in the caldera. A resurgent trachytic
intrusive dome occurs on the western side of
the Delamar Mountains (centered at R on Fig.
2 about 10 km west of here). This dome is
centered on a large 420 nanoTesla positive
magnetic anomaly and what is interpreted to be
the thickest pool of intra caldera material forms
a 500-800 nanoTesla negative anomaly about 4
km SW of this stop (Blank and Kucks, 1989)
(the northern and eastern boundary for these
anomalies are shown as the dashed and dotted
line in Figure 2). Caldera-filling ash-flow tuffs
dip as much as 80 0 away from the dome on the
Nand E sides. The Kane Wash Tuff pinches
out abruptly 2 km to the SE of the dome.
Megabreccia of Hiko Tuff is exposed in the
stream bed, post-collapse caldera-filling tuffs
are on either side, and Kane Wash Tuff caps
the surrounding hills. About 250 meters
downstream exposures of intracaldera
7.8
megabreccias and extracaldera andesite flow
breccias are only a few tens of meters
apart. The age of the Narrow Canyon caldera is
between that of the Hiko Tuff (18.6 Ma) and
that of the Grapevine Spring Member (about
14.7 Ma).
Return to NV 317.
67.8 END OF TRIP Turn right to rejoin US
93 to reach Las Vegas about 2.5 hours away;
turn left to return to Caliente and northern and
western destinations.
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