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Text and references to accompany Nevada Bureau of Mines and Geology Map 171
Geologic Map of the Northern Pequop Mountains,
Elko County, Nevada
by
Phyllis A. Camilleri
Department of Geosciences
Austin Peay State University
P.O. Box 4418
Clarksville, Tennessee 37044
2010
INTRODUCTION
The Pequop Mountains form a tilted cross section
through the Mesozoic to early Tertiary crust and display a
complex network of Mesozoic metamorphic and
contractional features and overprinting Cenozoic
extensional structures. Precambrian and Paleozoic
miogeoclinal strata and Tertiary volcanic and clastic rocks
are exposed within the range. Structurally, these rocks
form an east-tilted footwall block bounded on the west by
a west-dipping normal fault.
During the Mesozoic the Pequop Mountains occupied
a position in the hinterland of the Sevier fold and thrust
belt and miogeoclinal strata within the range strikingly
display the effects of hinterland shortening and
metamorphism. Locally, the Pequop Mountains are
situated in, and form the eastern margin of, a terrain of
miogeoclinal strata that underwent regional Barrovian
metamorphism, which peaked during the Late Cretaceous
(Camilleri and Chamberlain, 1997). This terrain includes
the Wood Hills and East Humboldt Range–Ruby
Mountains to the west (fig. 1). The Pequop Mountains
form the lowest grade (non-metamorphosed to lower
amphibolite facies), structurally shallowest part of the
terrain; ranges to the west expose progressively higher
grade and structurally deeper rocks (Camilleri and
Chamberlain, 1997; fig. 1). In addition to displaying the
effects of Mesozoic metamorphism, the Pequop Mountains
also provide a rare, superb cross section through two
hinterland thrust faults.
This paper focuses on the description and relative ages
of structures, whereas more regional tectonic and structural
interpretations of the Pequop Mountains have been
presented elsewhere (e.g., Camilleri and Chamberlain,
1997; Camilleri and McGrew, 1997; Camilleri, 1998;
Camilleri, 2009). The structure within miogeoclinal strata
in the Pequop Mountains is geometrically complex. The
most salient structures are a low-angle fault called the
Pequop fault and two thrust faults called the Independence
and unnamed thrusts (fig. 2). The Pequop fault is the most
important structure because it divides Paleozoic strata
within the range into two plates that have different
structural and metamorphic characteristics. An
unmetamorphosed Paleozoic section is present in the
hanging wall of the Pequop fault (shown shaded in fig. 2),
and a predominantly metamorphosed, ductiley deformed
section forms the footwall. Despite the foregoing
differences, Paleozoic strata in both the hanging wall and
footwall are cut by thrust faults: the Independence thrust in
the footwall and the unnamed thrust in the hanging wall
(fig. 2).
For simplicity, the structure of the Pequop Mountains
is described in four parts. The first part describes structure
in the footwall of the Pequop fault; the second, structure of
the Pequop fault and its hanging wall; the third, high-angle
faults; and fourth, range-bounding normal faults.
PREVIOUS WORK AND METHODS
The first detailed geologic study of the Pequop
Mountains was by Thorman (1970). He mapped, defined,
and correlated Paleozoic stratigraphy and mapped parts of
the Independence and unnamed thrusts and the Pequop
fault. Thorman (1970) also delineated a sequence of
metamorphosed strata that were unique at the time of his
work; they were stratigraphically unlike any previously
defined Precambrian-Paleozoic stratigraphic units in
eastern Nevada and therefore could not be correlated. He
suspected that the metamorphosed strata were Paleozoic in
age and tentatively suggested they were Ordovician to
Devonian (Thorman, 1970, p. 2432). Thorman was correct
in that the rocks are Paleozoic in age and are
stratigraphically unique to northeastern Nevada. However,
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Figure 1. Metamorphic map of the Pequop Mountains and environs. Map is modified after Camilleri and Chamberlain (1997). Diopside-
and tremolite-in isograds are for calc-silicate rocks and the sillimanite-in isograd is for metapelite.
the metamorphosed strata are Cambrian-Ordovician in age
rather than Ordovician-Devonian; in fact, the
metamorphosed sequence constitutes nearly the entire
Cambrian section. It was not until a little more than twenty
years after Thorman's work that new, unique Cambrian
formations in ranges due east of and adjacent to the
Pequop Mountains were named and type sections
delineated by McCollum and Miller (1991). The work of
McCollum and Miller (1991) allowed naming and
correlation of mappable metamorphosed units in the
Pequop Mountains with undeformed Cambrian units to the
east. Consequently, recognition of a new stratigraphy has
resulted in a new structural and stratigraphic picture of the
Pequop Mountains, which is presented herein.
Mapping of the Pequop Mountains was conducted
from 1988 to 1992. U.S. Geological Survey 1:24,000-scale
topographic quadrangles served as base maps. Tertiary
units and Quaternary surficial units were mapped
principally by using 1:24,000-scale color aerial
photographs.
STRATIGRAPHY
Rocks in the Pequop Mountains are divisible into
three general groups: 1) Proterozoic and Paleozoic strata in
the hanging wall and footwall of the Pequop fault, 2)
Eocene volcanic rocks and Miocene Humboldt Formation,
and 3) Tertiary to Quaternary surficial deposits. The three
groups are briefly discussed here; more detailed lithologic
descriptions are given in the description of map units.
Rocks in both the hanging wall and footwall of the
Pequop fault consist predominantly of carbonate with
lesser siliciclastic strata, but they differ with respect to
metamorphism and stratigraphy. Strata in the hanging wall
range in age from Ordovician to Permian, are
unmetamorphosed, and lack ductile deformation. Strata in
the footwall are Proterozoic to Permian with Mississippian
and older strata being predominantly metamorphosed and
ductiley deformed. In addition to the metamorphic contrast
across the Pequop fault, there are also a few stratigraphic
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Figure 2. Simplified map of the Pequop Mountains depicting major faults and stereograms of structural and fabric elements. Stereograms B and C are pi diagrams that depict the hinge line of
map-scale folds that fold foliation in the hanging wall of the Independence thrust. Attitude data presented in stereograms B and C are derived from the domains represented by the stippled
areas. Stereogram F depicts hinge lines of outcrop-scale folds that fold foliation in the hanging wall of the Independence thrust.
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differences. The most salient difference is the presence of
an eastern dolomitic facies of the Silurian Roberts
Mountains Formation in the footwall and the presence of a
western platy limestone facies of the Roberts Mountains
Formation in the hanging wall (Thorman, 1970; Berry and
Boucot, 1970; Sheehan, 1979). Another contrast is the
occurrence of sparse, very small Late Jurassic to
Cretaceous (?) felsic to intermediate composition
intrusions that are restricted to the footwall. The intrusions,
although sparse, are mostly in Cambrian strata, but a few
cut Ordovician rocks. Intrusions were not found in Silurian
and younger rocks.
The two oldest Cenozoic units in the Pequop
Mountains are a sequence of Eocene (41–39 Ma; Brooks et
al., 1995) volcanic rocks in the northeastern part of the
range and the Miocene Humboldt Formation in the
hanging wall of a normal fault on the western flank of the
range. The Eocene volcanic rocks are rhyolitic to andesitic,
and they depositionally overlap Paleozoic strata in the
hanging wall and footwall of the Pequop fault. The
Humboldt Formation is composed of clastic and
volcaniclastic rocks deposited on metamorphosed
Cambrian and Ordovician strata belonging to, and offset
from, metamorphosed strata in the hanging wall of the
Independence thrust. Clasts within the Humboldt
Formation appear to be largely derived from Paleozoic
strata (Thorman, 1970) in the hanging wall of the
Independence thrust. Some of the clasts are quite large, as
much as ~ 250 m long, and hence mappable (see map).
Mappable clasts are commonly fractured and are
predominantly derived from the Ordovician-Cambrian
Notch Peak Formation.
Quaternary and Tertiary surficial units mantle
Miocene and older rocks within the range. These units
include modern alluvium (unit ―Qa‖), alluvium and
sedimentary rocks (i.e., older alluvium) undivided (unit
―Qu‖) and pluvial deposits and alluvium undivided (unit
―Qla"). Pluvial deposits are present on the west and east
flanks of the Pequop Mountains. These deposits are
depositional remnants of separate lakes that once occupied
Independence and Goshute valleys. On the east flank of
the range, the contact between units ―Qla‖ and ―Qu‖ is
actually the approximate trace of a well-defined shore line
representing the high stand of the lake once occupying
Goshute Valley. The high-stand shoreline of the lake once
occupying Independence Valley was not mapped because
of poor definition of shore lines on the west flank of the
range.
STRUCTURE OF THE FOOTWALL OF
THE PEQUOP FAULT
There are three major structural features in the
footwall of the Pequop fault: a prograde metamorphic
fabric, the Independence thrust, and the Sixmile fault. The
metamorphic fabric is the oldest feature, and it is
transected by the Independence thrust. The Sixmile fault
(fig. 2) is inferred to be younger than the Independence
thrust.
Metamorphism
The Proterozoic-Paleozoic rocks in the footwall of the
Pequop fault range from unmetamorphosed to lower
amphibolite facies (fig. 3) and metamorphic grade
increases with stratigraphic depth. Metamorphism is
Barrovian style, but deformation accompanying
metamorphism was partitioned such that the rocks range
from unstrained to deformed. Consequently, the
metamorphic rocks display both regional and static
metamorphic fabrics, albeit static fabrics are scarce. The
age of metamorphism is bracketed between 154 and 84 Ma
(Camilleri and Chamberlain, 1997). The older age bracket
represents the age of a premetamorphic dike that is
deformed by the metamorphic fabric and the younger age
bracket represents a U-Pb metamorphic sphene age from
the Toano Limestone that is inferred to represent the time
of peak metamorphism (Camilleri and Chamberlain,
1997). The thermochronologic data indicate that the peak
of metamorphism was in the Late Cretaceous but that
metamorphism may have begun in the Late Jurassic.
Metamorphic Grade and Isograds
The distribution of metamorphic assemblages in
metapelite in the footwall of the Pequop fault allows
placement of biotite- and garnet-in isograds and a
boundary between unmetamorphosed and metamorphosed
rocks (figs. 3 and 4). In addition, calc-silicate assemblages
in siliceous dolomite allow placement of a tremolite-in
isograd, which coincides approximately with the garnet-in
isograd. The distribution of isograds and facies in the
footwall and hanging wall of the Independence thrust is
discussed separately below.
The transition zone between metamorphosed and non-
metamorphosed rocks in the footwall of Independence
thrust is placed between the last appearance of shale and
the first appearance of phyllite, which is between the
Mississippian Chainman Shale/Diamond Peak Formation
and the Ordovician Kanosh Shale (see cross section in fig.
3). The Chainman Shale/Diamond Peak Formation is
largely texturally unmetamorphosed, consisting of shale,
argillite, siltstone, and conglomerate. In contrast, the
Kanosh Shale is an argillite or a Q-SER-CHL phyllite and
therefore lies within the chlorite zone (see fig. 4 for
mineral abbreviations). The first appearance of biotite in
metapelite occurs below the Kanosh Shale but above the
upper Cambrian Dunderberg Shale. Consequently the
biotite-in isograd is placed just above the Dunderberg
Shale, which has a typical assemblage of BI-MU-CHL-
TOUR-ALL-PL-Q + CC/EP. The garnet-in isograd for
metapelite lies below the Dunderberg Shale but above the
Precambrian-Cambrian Prospect Mountain Quartzite.
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Figure 3. Metamorphic maps and cross section of the Pequop Mountains. The cross section is modified after Camilleri and Chamberlain
(1997). The map on the right depicts sample locations (boxes) of metamorphic rocks whose assemblages are listed in figure 4. Unit
symbols on the cross section are the same as on the geologic map with the exception of these: Op=Pogonip Group undivided and
OMu=Joana Limestone, Guilmette Formation, Lone Mountain Dolomite, Roberts Mountains Formation, Laketown Dolomite, and Fish Haven
Dolomite, undivided.
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Figure 4. List of mineral assemblages in metamorphic rocks in the
hanging wall and footwall of the Independence thrust.
ALL=allanite, BI=biotite, CC=calcite, CHL=chlorite, DO=dolomite,
EP=epidote, GT=garnet, HNB=hornblende, MU=muscovite,
PL=plagioclase, Q=quartz; SER=sericite, SPH=sphene, TLC=talc,
TOUR=tourmaline, TR=tremolite.
Micaceous layers within the Prospect Mountain Quartzite
contain the prograde assemblage Q-MU-BI-GT-PL. The
garnet-in isograd is inferred to coincide approximately
with the tremolite-in isograd for siliceous dolomite (see
GT-TR -in isograd in fig. 3). This inference is made
because the first appearance of tremolite in siliceous
dolomite (Toano Limestone) is in proximity of the first
appearance of garnet in the Prospect Mountain Quartzite
(see cross section in fig. 3).
The distribution of metamorphic facies and isograds in
the hanging wall of the Independence thrust is very similar
to that in the footwall but with a few minor differences.
The Dunderberg Shale in the footwall, and in most of the
hanging wall, is within the biotite zone and the biotite-in
isograd lies stratigraphically above the shale. However, in
the southern part of the hanging wall the biotite-in isograd
transects the shale, and the shale is locally within the
chlorite zone (fig. 3). The first appearance of tremolite in
siliceous dolomite in the hanging wall occurs in the Toano
Limestone, as it does in the footwall. Therefore, the
tremolite-in isograd in the hanging wall is also placed just
above the Toano Limestone. Moreover, although no
metapelites are exposed beneath this isograd, it is inferred
to coincide approximately with the garnet-in isograd as it
does in the footwall.
In summary, metamorphic grade within the
Proterozoic-Paleozoic section beneath the Pequop fault
increases with stratigraphic depth. Rocks range from
unmetamorphosed in middle Mississippian and younger
strata to lower amphibolite facies in Proterozoic-Cambrian
strata.
Metamorphic Fabric and Large-Scale
Deformation Accompanying
Metamorphism
Deformation during metamorphism accomplished
variable attenuation of stratigraphic units and resulted in
development of tectonites and large-scale pinch-and-swell
structure (Camilleri (1998). Metamorphosed strata
constitute rocks with a foliation only (S tectonite), rocks
with a foliation and a lineation (S-L tectonite [S1 and L1]),
and sparse rocks with a static fabric (no foliation or
lineation). Foliation is parallel or at very low angle to
bedding. Foliation that is at a low angle to bedding has a
consistent geometric relation to bedding: if bedding is
rotated to horizontal, foliation would consistently dip
gently to the west. Lineation is defined by minerals or
elongated grain aggregates with a dominant east to east-
southeast trend (fig. 2 D and E). The metamorphic rocks
also contain rare small-scale folds that are axial planar to
S1. These folds generally have amplitudes of a few
centimeters, are upright, and lack vergence, although an
easterly vergence was observed in few places. In addition,
some of the higher-grade rocks contain sparse macroscopic
pinch-and-swell structure and/or ptygmatically folded
calcite veins that are normal or at a very high angle to
foliation. Map-scale pinch-and-swell structure occurs only
in higher-grade Cambrian rocks and is manifest as a
gradational change in stratigraphic thickness along strike
(see map).
The character and distribution of fabric in the
metamorphic rocks changes with increasing metamorphic
grade. The changes are similar in the footwall and hanging
wall of the Independence thrust, with the exception of
where the metamorphic fabric first appears. Foliation in
the hanging wall of the Independence thrust first appears in
Ordovician-Silurian dolomite (unit SOlf). Foliation in the
footwall first appears in the Mississippian Joana
Limestone. However, foliation in the Joana Limestone is
restricted to the northernmost part of its mapped extent; to
the south, the Joana Limestone is unfoliated and the first
appearance of foliation occurs in the stratigraphically
underlying Devonian Guilmette Formation. The
predominant tectonite type in low-grade chlorite-zone
rocks in both the hanging wall and footwall of the
Independence thrust is an S tectonite whereas S-L
tectonites are scarce. In these low-grade rocks, foliation is
partitioned into sparse laterally discontinuous zones a few
centimeters to several meters thick. The foliated zones are
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Figure 5. Comparison of minimum stratigraphic thicknesses of
regionally metamorphosed and deformed Cambrian and
Ordovician sections in Pequop Mountains with a standard,
undeformed Paleozoic reference section. Both stratigraphic
columns are hung on the top of the Upper Cambrian Dunderberg
Shale for comparative purposes. The Mesozoic-Paleozoic
reference section was constructed using data from undeformed
sections in Pequop Mountains and nearby ranges (i.e., McCollum
and Miller, 1991; D. M. Miller, 1984; Glick, 1987; Thorman, 1970;
Robinson, 1961; and Fraser 1986). However, the thicknesses of
the Cambrian and Ordovician sections in the reference column are
derived from sections/type sections exposed in the Toano Range
and described by Glick (1987) and McCollum and Miller (1991).
The Toano Range is adjacent to, and due east of, the Pequop
Mountains.
separated by domains of undeformed, texturally
unmetamorphosed rock a few meters to tens of meters
thick. In higher-grade biotite- and garnet-zone rocks,
foliation is more penetrative, and although S tectonites are
the predominant tectonite type, S-L tectonites are more
abundant than they are in chlorite-zone rocks. The garnet-
and biotite-zone rocks define map-scale pinch-and-swell
structure with the swells being little deformed and the
pinched parts being penetratively deformed. In the thickest
(swell) parts of stratigraphic units, the distribution of fabric
resembles that described above for chlorite-zone rocks. In
contrast, fabric is most penetrative in severely attenuated
(pinch) parts. In both pinch-and-swell regions, there are
sparse static metamorphic fabrics attesting to the
partitioning of deformation during metamorphism. Rocks
with static metamorphic fabrics have the same prograde
mineral assemblages as tectonites whose metamorphic
minerals define lineation and/or foliation.
Development of the metamorphic fabric and pinch-
and-swell structure accommodated up to 50 percent
attenuation of stratigraphic section. This inference is based
on comparison of stratigraphic thicknesses of
metamorphosed Cambrian and Ordovician stratigraphic
units with undeformed but correlative units in the Toano
Range due east of the Pequop Mountains (fig. 5). The
thickest parts of metamorphosed units are generally close
to or comparable in thickness with the correlative but
undeformed units to the east. However, a comparison of
the thickness of the Cambrian-Ordovician section in the
footwall of the Independence thrust, which is the most
attenuated, with the thickness of the same but undeformed
section to the east indicates a negligible amount of
attenuation of Ordovician units and as much as 50 percent
local attenuation of the Cambrian section (fig. 5). This
suggests that the amount of attenuation increases with
stratigraphic depth and metamorphic grade.
Independence Thrust and Related
Structures
The Paleozoic section in the footwall of the Pequop
fault is transected by the Independence thrust, which in
map view juxtaposes rocks as old as Ordovician atop rocks
as young as Mississippian. The thrust has an overall ramp
geometry cutting footwall- and hanging-wall strata at a
moderate angle, but locally it refracts to a higher angle
where it cuts through more competent Upper Ordovician-
Lower Devonian units in the footwall (i.e., units Opl, Oe,
and DOu; see cross sections A–A’ and B–B’). The thrust
dips gently to the northeast, but locally it dips west where
it refracts.
The geometry of stratigraphic units in the footwall and
hanging wall of the thrust differ markedly. Stratigraphic
units and S1 in the footwall dip approximately 35° to 45°
to the east (fig. 2A) with local dip variations reflecting
gentle folding (e.g., see cross section in fig. 3).
Stratigraphic units and S1 in the hanging wall of the thrust
are deformed by outcrop-scale folds, map-scale northwest-
vergent back folds, and a back thrust called the Big
Springs thrust (fig. 2). Hanging-wall folds are
predominantly upright, have kink geometry, and in general
lack an axial planar cleavage. The hinge lines of map-scale
folds plunge gently to the northeast (see π axis in fig. 2B
and 2C). One of the map-scale folds appears to be a fault-
bend fold above Ordovician-Lower Devonian dolostone in
the footwall (see hanging wall anticline in cross section B–
B'). Hinge lines of outcrop-scale folds plunge gently in
northerly and southerly directions, but their trends vary
from north-northeast to northwest (fig. 2F). This variability
in trend may be a manifestation of curved hinge lines. The
geometries of outcrop-scale folds are diverse. Some appear
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Figure 6. A) Composite structure-contour map of the Pequop fault and the unnamed thrust. The contours are only depicted to the west of
the Long Canyon fault. The bold dashed line represents the approximate position of the line of intersection of the Pequop fault and the
thrust, which corresponds to the cutoff of the thrust fault in the plane of the Pequop fault. Arrow indicates position of cross section D-D’.
B) Map showing inferred geologic relationships beneath the hanging wall of Pequop fault. The map only depicts geology to the west of the
Long Canyon fault. Strike and dip symbols highlight geometry of units.
to be asymmetric and some define fault-bend or
propagation folds associated with small-scale thrust faults.
In addition, in a few places intense outcrop-scale folding
may have accommodated local thickening of units. For
example, the Dunderberg Shale in the hanging wall that is
in proximity of the thrust contact appears to be
substantially thickened (see map and cross section A–A').
Sparse outcrops of the shale in this area reveal locally
intense small-scale folding and crenulation of S1 and
bedding, which may account for a greater apparent
thickness. Moreover, some crenulations microscopically
exhibit solution features along their limbs indicating an
incipient ―S2‖. These types of crenulations, although
sparse, also occur in the footwall in proximity of the thrust.
The sense of slip on the Independence thrust can’t be
precisely constrained because no exposures of the thrust
surface were found. However, sense of slip is inferred to
be top-to-the-southeast, broadly perpendicular to hanging
wall map-scale fold hinges and the Big Springs thrust.
On the basis of cross-cutting relationships, the
Independence thrust is inferred to postdate the
metamorphic fabric but predate the Pequop fault.
Truncation and deformation of S1 by the Independence
thrust and structures in its hanging wall indicates that the
thrust postdates development of the ~ 84 Ma metamorphic
fabric (Camilleri and Chamberlain, 1997). Moreover, the
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Pequop fault appears to truncate gentle folds in the
hanging wall of the Independence thrust, and consequently
the thrust is inferred to predate the Pequop fault (see cross
sections D–D’ and E–E’).
Sixmile Fault
The Sixmile fault is a nearly vertical northeast
trending fault that cuts, and is exposed in, Cambrian and
Ordovician strata in the hanging wall of the Independence
thrust on the western flank of the range (see geologic map
and fig. 2). In map view, the fault terminates at a north-
trending high-angle fault that drops down the hanging wall
of the Pequop fault. However, the Sixmile fault is inferred
to be cut by the north-trending high-angle fault and to
continue eastward beneath the hanging wall of the Pequop
fault. Consequently, in map view, the Sixmile fault is
shown dotted (i.e., concealed) through the hanging wall of
the Pequop fault. Eastward continuation of the Sixmile
fault beneath the Pequop fault is geometrically required to
explain apparent structural discordance of units beneath
the Pequop fault. Figure 6B is an inferred map depicting
the Sixmile fault and stratigraphic units immediately
beneath and concealed by the hanging wall of the Pequop
fault. This map was constructed by projecting exposed
stratigraphic units in the footwall of the Pequop fault
beneath the hanging wall. The Sixmile fault is required to
explain the projected discordance of units to the north and
south of the fault (compare fig. 6B with the geologic map).
The sense of slip and amount of displacement along
the Sixmile fault can only be gauged in a relative sense
because the fault surface is not exposed and much of the
fault is concealed. The amount of displacement probably
increases eastward as suggested by the greater amount of
stratigraphic separation across the fault towards the east.
The apparent left-lateral sense of offset of contacts across
the fault in map view along the exposed portion of the fault
could be produced by dip slip with relative downward
motion of the northern block or by strike slip or a
combination thereof.
Several cross-cutting relationships suggest that the
Sixmile fault postdates the metamorphic fabric and
Independence thrust but predates the Pequop fault. First,
the Sixmile fault is inferred to be cut by, and therefore
older than, the Pequop fault. Second, the juxtaposition of
metamorphosed Ordovician strata with unmetamorphosed
Mississippian strata across the eastern end of the Sixmile
fault suggests that the fault is post metamorphic (see cross
section E–E’). Last, the Sixmile fault is inferred to
postdate the Independence thrust because it appears to
truncate a flexure in the hanging wall of the Independence
thrust (see fig. 6B).
Timing of Emplacement of Granitic
Intrusions
At least two different generations of granitic
intrusions are present in the Pequop Mountains. The oldest
generation predates metamorphism and is deformed by the
S1 metamorphic fabric. These intrusions are scarce and
were only found in the hanging wall of the Independence
thrust on the west flank of the range. Only one of the larger
bodies is shown on the map. The mapped intrusion yielded
a 154 Ma (Late Jurassic) U-Pb zircon age (Camilleri and
Chamberlain, 1997). The youngest generation of intrusions
are undeformed and cut the ~84 Ma metamorphic fabric
and therefore are constrained to be Late Cretaceous or
younger. These intrusives are most common in Lower
Cambrian strata in the footwall of the Independence thrust.
GEOMETRY OF THE PEQUOP FAULT
AND STRUCTURE IN ITS HANGING
WALL
The Pequop fault forms the base of a klippe that is cut
by younger, nearly vertical, small displacement faults (see
map and figs. 2 and 6). Structure contours on the Pequop
fault indicate that it has a gentle eastward dip (fig. 6A).
The presence of breccia adjacent to the Pequop fault and
the lack of ductile fault rock indicate that it is a brittle
fault.
The hanging wall of the Pequop fault consists of an
east-dipping sequence of Ordovician to Mississippian
strata that is thrust over east-dipping Permian strata along
the unnamed thrust. The unnamed thrust trends east-west,
dips at a moderate angle to the north, and is cut by and
therefore older than the Pequop fault (fig. 6A and cross
sections D–D’ and E–E’). Hanging-wall and footwall
cutoffs trend northeast in the plane of the thrust suggesting
a top-to-the-southeast sense of slip for the unnamed thrust.
Both relative and absolute age constraints can be
placed on the Pequop fault, but the relative and absolute
ages of the unnamed thrust are poorly constrained. The
Pequop fault is relatively younger than the Sixmile fault
but older than the 41–39 Ma volcanic rocks that
depositionally overlap strata in its hanging wall and
footwall. The Pequop fault must be Late Cretaceous or
younger because it emplaces unmetamorphosed strata atop
foliated metamorphosed rocks and therefore cuts the ~84
Ma metamorphic fabric. Thus, the age of the Pequop fault
is bracketed between 84 and 41 Ma. Although the
unnamed thrust is constrained to be older than the Pequop
fault, its age relative to structures in the footwall of the
Pequop fault are unconstrained. Camilleri and
Chamberlain (1997) infer that that the Pequop fault is a
top-to-the-west normal fault that cut two different
Paleozoic sections that were duplicated by thrust faulting
prior to slip along the Pequop fault.
10
HIGH-ANGLE FAULTS
The Pequop Mountains contain at least three
generations of vertical or high-angle normal faults (fig. 7).
The relative ages of these faults can be assessed by cross-
cutting relationships with the Pequop fault and the Eocene
volcanic rocks. The oldest generation is manifest by one
fault, the northeast-trending Sixmile fault, which is cut by
and therefore predates the Pequop fault. The middle
generation consists of at least two small-displacement
west-northwest and east-northeast trending faults that
transect the hanging wall of the Pequop fault along its
northern margin. These faults appear to be depositionally
overlapped by the 41–39 Ma volcanic rocks. Other
similarly oriented faults that transect the Tripon Pass
Limestone and Guilmette Formation in the footwall of the
Pequop fault to the north may be of the same age (see
geologic map and fig. 7). The youngest generation consists
of at least one fault, the Long Canyon fault (Thorman,
1970), which cuts the volcanic rocks and trends north to
north-northeast. Other linear-to-arcuate but broadly north-
trending high-angle faults whose relative ages are
unconstrained may belong to this generation as well. For
example, just south of Interstate 80, two north-trending
faults cut an east-northeast-trending fault that is inferred to
be a middle generation fault (fig. 7).
RANGE-BOUNDING NORMAL FAULTS
The west flank of the Pequop Mountains, is bounded
by two normal faults. The oldest is a down-to-the-west
listric normal fault. This fault cuts metamorphosed
Paleozoic strata in the range and contains metamorphosed
Cambrian and Ordovician strata depositionally overlapped
by clastic and volcaniclastic strata of the Miocene (?)
Humboldt Formation in its hanging wall. The Humboldt
Formation is both cut by and overlaps the fault, suggesting
that it was deposited in response to slip along the fault.
Moreover, the presence of abundant coarse-grained
sediment (gravel- to map-scale clasts) composed of
metamorphic rocks derived from the footwall of the fault
further suggests that the Humboldt Formation was
deposited as a consequence of relief generated by slip
along the fault. The youngest fault along the west flank of
the Pequop Mountains is an inferred high-angle fault that
cuts the northwestern margin of exposures of the
Humboldt Formation. This fault is not exposed but is
inferred on the basis of aerial photographs that revealed a
conspicuous Quaternary (?) scarp. This fault is likely part
of the modern range-bounding fault system.
SUMMARY: CHRONOLOGY OF
STRUCTURAL EVENTS
In the northern Pequop Mountains, at least six phases
of deformation can be distinguished by cross-cutting
relationships. These are the phases in chronologic order:
1) Regional metamorphism concomitant with the
production of S and S-L tectonites and attenuation of
stratigraphic units. Metamorphism is bracketed between
154–84 Ma (Camilleri and Chamberlain, 1997).
2) Transection of the metamorphosed Paleozoic section
by the Independence thrust accompanied by hanging-wall
shortening that was accommodated by thrusting along the
Big Springs thrust and folding.
3) Transection of the hanging wall (and probably the
footwall at depth) of the Independence thrust by the
Sixmile fault.
4) Emplacement of the unmetamorphosed Paleozoic
sequence atop the regionally metamorphosed section along
the Pequop fault. The unnamed thrust in the hanging wall
of the Pequop fault predates the Pequop fault, but its age
relative to events 1, 2, and 3 above is unconstrained.
5) Transection of strata in the hanging wall and footwall
of the Pequop fault by a set of small-displacement west-
northwest and east-southeast trending high-angle faults
followed by deposition of the 41–39 Ma volcanic rocks
across strata in the hanging wall and footwall of the
Pequop fault.
6) Formation of north-trending high-angle faults
including the Long Canyon fault and the range-bounding
listric normal fault. The relative ages between the high-
angle faults within the range and the listric range-bounding
fault are unknown.
11
Figure 7. Simplified map depicting three generations of high-angle faults in the Pequop Mountains. The Sixmile fault is shown as dotted
where it is concealed by the hanging wall of the Pequop fault and the Eocene volcanic rocks.
12
DESCRIPTION OF MAP UNITS
Preface
All thicknesses reported for Paleozoic strata represent
structural thickness rather than stratigraphic thickness.
Much of the Paleozoic section is ductiley attenuated and
metamorphosed and/or deformed by map- to outcrop-scale
folds and thrusts related to the post-metamorphic
Independence thrust. All of this deformation in
metamorphosed and non-metamorphosed strata has
resulted in laterally variable thicknesses of units.
Thicknesses are given only for formations whose bottoms
and tops are exposed and whose thicknesses can be
geometrically calculated from the map. These thicknesses
are estimates. In addition, several metamorphosed units
were mapped as combined units (e.g., Oplkc, Opba etc.).
This was done because, in places, metamorphism and
deformation made some adjacent formations
indistinguishable.
Qa Alluvium (Quaternary) Unconsolidated gravel,
sand, and silt deposited in intermittent streams and on
alluvial fans.
Qla Lacustrine deposits and alluvium, undivided
(Quaternary) Pluvial lacustrine deposits of gravel,
sand, and silt and alluvial deposits of gravel, sand, and silt
deposited in intermittent streams and on alluvial fans.
Qu Alluvium and sedimentary rocks, undivided
(Quaternary) Unconsolidated gravel, sand, and silt
deposited in intermittent streams and on alluvial fans, and
sparse light-gray to tan conglomerate, breccia, sandstone,
and siltstone.
Th Humboldt Formation (Miocene) Tan siltstone,
conglomerate, and sparse white vitric tuff. Conglomerate
contains abundant clasts of metamorphic rocks derived
predominantly from the hanging wall of the Independence
thrust. This unit also contains large mappable clasts of
Cambrian and Ordovician metamorphic rocks.
Tv Volcanic rocks (Eocene) Light-green to reddish-
brown volcanic and sedimentary rocks. Brooks et al.
(1995) report the presence of 1,200 meters of hornblende
rhyolite, dacite, rhyolite ash-flow tuff, andesite ash-flow
breccia, and sparse conglomerate in this unit. These
volcanic rocks have been dated by 40Ar/39Ar methods at 41
to 39 Ma (Brooks et al., 1995).
Pp Pequop Formation of Steele (1960) (Permian)
Light-gray limestone and silty limestone. Thickness of this
unit ranges from approximately 80 to 400 feet.
Ppf Pequop Formation of Steele (1960) and Ferguson
Mountain Formation, undivided (Permian) Tan to
light-gray silty limestone and fusilinid-bearing limestone
with sparse dolomite in the hanging wall of the Pequop
fault.
Pe Ely Limestone (Pennsylvanian) Light-gray
limestone with sparse chert; conglomerate with chert and
limestone clasts at the base. Thickness of this unit is as
much as 167 feet.
PPfe Ferguson Mountain Formation (Permian) and
Ely Limestone (Pennsylvanian), undivided Light-gray
limestone with sparse chert and silty limestone.
Mdpc Diamond Peak Formation and Chainman
Shale, undivided (Mississippian) Gray to tan to black
shale, sandstone, siltstone, chert-pebble conglomerate, and
sparse limestone. Thickness of this unit is at least 3,600
feet.
Md Dale Canyon Formation (Mississippian) Gray to
tan to black shale, sandstone, siltstone, chert-pebble
conglomerate, and sparse limestone.
Mj Joana Limestone (Mississippian) Light-gray-blue
cherty limestone and limestone or fine-grained calcite
marble. Thickness of this unit ranges from approximately
100 to 200 feet.
Mtp Tripon Pass Limestone (Mississippian) Basal,
bedded, black to gray-brown chert and overlying tan,
laminated limestone and sparse conglomerate. Limestone
contains sparse small-scale synsedimentary folds.
Thickness of this unit is as much as 1,200 feet.
MDgj Guilmette Formation (Devonian) and Joana
Limestone (Mississippian), undivided Cliff-forming
light-gray limestone with minor light-gray to tan
argillaceous limestone. The uppermost part of this unit
contains beds with abundant pelmetazoan fragments. C.H.
Thorman (personal communication, 1992) reports that
conodonts from these pelmetazoan-bearing beds indicate a
Mississippian age, and N. J. Silberling (personal
communication, 1999) indicates that these beds are
correlative with, and represent an erosional remnant of, the
Joana Limestone. Thickness of this unit is as much as
2,100 feet.
Dg Guilmette Formation (Mississippian and
Devonian) Cliff-forming light-gray limestone with
minor light-gray to tan argillaceous limestone. The
uppermost part of the Guilmette Formation in the footwall
of the Independence thrust contains limestone
conglomerate and argillaceous limestone; this distinctive
lithology appears to be absent from the Guilmette
Formation in the hanging wall of the Independence thrust
and in the hanging wall of the Pequop fault. Thickness of
this unit in the footwall of the Independence thrust is at
13
least 1,600 feet and in the hanging wall of the Pequop fault
it is as much as 1,915 feet.
DOu Simonson Dolomite, Lone Mountain Dolomite
(Devonian and Silurian), Roberts Mountains
Formation (Silurian), Laketown Dolomite (Silurian),
and Fish Haven Dolomite (Ordovician), undivided.
Ds Simonson Dolomite (Devonian) Laminated light-
gray to black dolostone with minor amounts of gray-green
laminated limestone. Red-brown-weathering, cross-bedded
dolomitic sandstone is present at the base of the Simonson
Dolomite in the hanging wall of the Independence thrust.
The Simonson Dolomite in the footwall of the
Independence thrust and in the hanging wall of the Pequop
fault does not contain sandstone. The contact between the
Simonson Dolomite and the overlying Guilmette
Formation is gradational and is placed at the base of cliff-
forming limestone assigned to the Guilmette Formation.
Thickness of this unit in the footwall of the Independence
thrust is as much as 1,230 feet and in the hanging wall of
the Pequop fault it is as much as 610 feet.
DSlm Lone Mountain Dolomite (Devonian and
Silurian) Light-gray to white, coarsely crystalline,
massive dolostone. The contact between the Lone
Mountain Dolomite and the overlying Simonson Dolomite
is gradational. Thickness of this unit in the footwall of the
Independence thrust is as much as 375 feet and in the
hanging wall of the Pequop fault it is at least 840 feet.
Srm Roberts Mountains Formation (Silurian) The
Roberts Mountains Formation in the footwall and hanging
wall of the Independence thrust is composed of thin-
bedded (platy) to thick-bedded, dark-grayish-purple,
calcareous dolostone and dolomite with sparse chert
nodules and lenses. In the hanging wall of the Pequop
fault, the bulk of the Roberts Mountains Formation
consists of platy light-grayish-purple limestone but the
basal part is composed of dark-gray dolostone, chert, and
dark-grayish-purple calcareous dolomite and dolostone.
Thickness of this unit in the footwall of the Independence
thrust is as much as 480 feet and in the hanging wall of the
Pequop fault it is as much as 490 feet.
SOlf Laketown Dolomite (Silurian), and Fish Haven
Dolomite (Ordovician), undivided Unit consists of
dark-gray cherty dolostone at the base (= Fish Haven
Dolomite; Thorman, 1970) and overlying layers of
alternating light-gray to dark-gray dolostone (= Laketown
Dolomite; Thorman, 1970). The contact between the
Laketown Dolomite and Roberts Mountains Formation is
gradational. Thickness of this unit in the footwall of the
Independence thrust is as much as 820 feet.
Oe Eureka Quartzite (Ordovician) Fine-grained
white quartzite with sparse gray streaks. The contact
between the Eureka Quartzite and the Fish Haven
Dolomite is sharp. Thickness of this unit in the footwall of
the Independence thrust is as much as 215 feet, and in the
hanging wall of the Pequop fault it is as much as 320 feet.
Divisions of the Pogonip Group
Opl Lehman Formation (Ordovician) In the hanging
wall and footwall of the Independence thrust, the Lehman
Formation is composed of light gray-green calcite marble
or limestone with brown-weathering silty layers. In
addition, the Lehman Formation in the hanging wall of the
Independence thrust on the east side of the Pequop
Mountains also contains dolomite beds near the top of the
unit. The Lehman Formation in the hanging wall of the
Pequop fault consists of unmetamorphosed, fossiliferous
gray-green limestone and silty limestone. The contact
between the Lehman Formation and the Kanosh Shale is
poorly exposed, but the contact between the Lehman
Formation and the Eureka Quartzite is sharp. Thickness of
this unit in the footwall of the Independence thrust is as
much as 500 feet, and in the hanging wall of the
Independence thrust it is at least 680 feet. The thickness of
this formation in the hanging wall of the Pequop fault is as
much as 500 feet.
Oplkc Lehman Formation, Kanosh Shale, and unit C,
undivided (Ordovician)
Opkc Kanosh Shale and unit C, undivided
(Ordovician)
Opk Kanosh Shale (Ordovician) The Kanosh Shale
in the footwall and hanging wall of the Independence
thrust consists of phyllite or argillite with subordinate
light-gray limestone, argillaceous limestone or fine-
grained marble and argillaceous limestone. The Kanosh
Shale in the hanging wall of the Pequop fault consists of
unmetamorphosed fissile shale that weathers into small
chips. Thickness of this unit in the hanging wall of the
Independence thrust is as much as 160 feet, and in the
hanging wall of the Pequop fault it is as much as 135 feet.
Opcb Unit C and unit B, undivided Present in the
hanging wall of the Pequop fault.
Opc Unit C (Ordovician) Light-gray to light-gray-
green, fine-grained, calcite marble or limestone and
micaceous calcite marble or argillaceous limestone with an
approximately 50-foot-thick grayish-yellow-weathering
quartzite or calcareous quartzite at the base. In various
places, unit C is texturally unmetamorphosed due to the
partitioning of strain. Unit C is present in the hanging wall
and footwall of the Independence thrust.
Opb Unit B (Ordovician) Light-gray to light-gray-
green micaceous calcite marble or silty limestone, calcite
marble or limestone and minor phyllite or argillite. In
14
various places, unit B is texturally unmetamorphosed
(unfoliated) due to the partitioning of strain. Unit B is
present in the hanging wall and footwall of the
Independence thrust. Thickness of this unit in the footwall
of the Independence thrust is as much as 800 feet, and in
the hanging wall of the Independence thrust it ranges from
approximately 240 to 900 feet.
Opba Unit B and unit A, undivided (Ordovician)
Opa Unit A (Ordovician) Light-grayish-green to
light-gray, white or cream-colored, micaceous calcite
marble with sparse chert or light-grayish-green limestone
with sparse chert. In various places, unit A is texturally
unmetamorphosed due to the partitioning of strain. Where
unmetamorphosed, a distinctive feature of unit A is the
presence of flat-pebble limestone conglomerate. Unit A is
present in the hanging wall and footwall of the
Independence thrust. Thickness of this unit in the footwall
of the Independence thrust is as much as 800 feet.
OCu Calcite marble, dolomite marble, and quartzite,
undivided (Ordovician and Cambrian)
OCnp Notch Peak Formation (Early Ordovician and
Late Cambrian) The Notch Peak Formation consists of
limestone and dolostone with bedding-parallel chert lenses.
However, dolostone does not appear to be entirely a
primary lithology because in many places limestone beds
can be traced laterally into dolostone. The Notch Peak
Formation in the footwall of the Independence thrust is
predominantly light gray and in outcrop its appearance
ranges from texturally unmetamorphosed to a very fine-
grained marble. In the hanging wall of the Independence
thrust, the color of the Notch Peak Formation varies
laterally from light-gray-green to black, and parts have
zebra-striping (alternating layers of wavy white coarse-
grained dolomite and finer-grained dark dolomite). Much
of the Notch Peak Formation in the hanging wall of the
Independence thrust is texturally unmetamorphosed;
however, on the extreme northwestern flank of the Pequop
Mountains, it is recrystallized to a medium-grained
dolomite marble. Thickness of this unit in the footwall of
the Independence thrust ranges from approximately 1,150
feet to as much as 1,960 feet, and in the hanging wall of
the Independence thrust it ranges from at least 400 feet to
1,300 feet.
Cd Dunderberg Shale (Late Cambrian) Phyllite or
schist and light-gray, micaceous calcite marble or
limestone. Thickness of this unit in the hanging wall of the
Independence thrust ranges from approximately 335 feet to
as much as 1,200 feet.
Com Oasis Formation (Late Cambrian), Shafter
Formation (Late and Middle Cambrian), Decoy
Limestone (Late and Middle Cambrian), and Morgan
Pass Formation (Middle Cambrian), undivided Light-
gray-green to light-gray-blue calcite marble, micaceous
calcite marble, and minor phyllite. Thickness of this unit
ranges from approximately 800 feet to as much as 1,675
feet.
Cdm Dunderberg Shale (Late Cambrian), Oasis
Formation (Late Cambrian) Shafter Formation (Late
and Middle Cambrian), Decoy Limestone (Late and
Middle Cambrian), and Morgan Pass Formation
(Middle Cambrian), undivided Schist or phyllite with
minor light-gray-green to light-gray-blue calcite marble
and micaceous calcite marble in uppermost part of unit (=
Dunderberg Shale) with underlying light-gray-green to
light-gray-blue calcite marble, micaceous calcite marble,
and minor phyllite and tan dolomite marble (= Oasis,
Shafter, Decoy, and Morgan Pass Formations). Thickness
of this unit is as much as 1,600 feet.
Ccl Clifside Limestone (Middle Cambrian) The
Clifside Limestone is composed of light-grayish-blue
limestone or fine-grained calcite marble with scarce
phyllitic layers. However, parts of the Clifside Limestone
in the hanging wall of the Independence thrust are variably
dolomitized. Where the Clifside Limestone is deformed, it
is a foliated fine-grained marble with a platy aspect. Where
undeformed, the Clifside Limestone consists of thick-
bedded limestone with oolites. The contact between the
Toano Limestone and the Clifside Limestone is gradational
and is placed at a color change in the rocks from light-
grayish-blue of the Clifside Limestone to the grayish-green
of the Toano Limestone. Thickness of this unit ranges
from approximately 250 feet in the footwall of the
Independence thrust to at least 2,600 feet in the hanging
wall of the thrust.
Ct Toano Limestone (Middle Cambrian) The Toano
Limestone consists of grayish-green, hornblende- or
tremolite-bearing calcite marble and grayish-green calcite
marble with 1-mm- to 1-cm-thick orange-weathering
phlogopite-bearing layers. The contact between the Killian
Springs Formation and Toano Limestone is gradational
and is placed at a color change in the rocks from black or
dark gray of the Killian Springs Formation to gray-green
of the Toano Limestone. Thickness of this unit is at least
320 feet.
Cks Killian Springs Formation (Middle and Early
Cambrian) The Killian Springs Formation consists of
graphitic, gray to black, tremolite-bearing marble, dark
micaceous quartzite, and biotite-muscovite schist. The
contact of the Killian Springs Formation with the
underlying Prospect Mountain Quartzite is gradational and
is placed at the lowermost marble or schist within the
Killian Springs Formation. Thickness of this unit is at
least 560 feet.
CZpm Prospect Mountain Quartzite (Early
Cambrian and Late Proterozoic) Dark-gray quartzite
15
with minor garnet-bearing micaceous layers.
Zmc McCoy Creek Group (Proterozoic) Unit shown
in cross section only.
Igneous Rocks
Kg Granitic pods (Cretaceous?) Undeformed,
hydrothermally altered granitic pods in the footwall of the
Independence thrust.
Jg Granitic dike (Jurassic) Foliated granitic dike in
the hanging wall of the Independence thrust. The dike
yields a U-Pb (zircon) age of ~154 Ma (Camilleri and
Chamberlain, 1997).
ACKNOWLEDGMENTS
Mapping of the Pequop Mountains was done as part of
a dissertation at the University of Wyoming. Mapping was
supported by grants from the Geological Society of
America, American Association of Petroleum Geologists,
Sigma Xi, Wyoming Geological Association, Shell Oil
Company, U. S. Geological Survey, and by National
Science Foundation grant EAR 87-07435 (awarded to A.
W. Snoke). Chevron USA loaned aerial photographs used
in this project. Publication of this map was supported by a
grant from the Geological Society of Nevada and an
Austin Peay State University Tower Grant. This work has
benefited from discussion and field visits with Linda
McCollum, Mike McCollum, David M. Miller, Arthur
Snoke, and Chuck Thorman. Helpful reviews of this
manuscript by Chris Henry, Connie Nutt, and Norm
Silberling, and field review of the map by Chris Henry and
Jim Trexler, are appreciated.
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