Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada (GSA Field Guide 11)

Field Guide 11
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edited by Ernest M. Duebendorter and Eugene I. Smith
Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona,
California, and Nevada
Geology Department Frier Hall
USA
University of Nevada, Las Vegas 4505 S. Maryland Parkway
Las Vegas, Nevada 89154-4010 USA
3300 Penrose Place, P.O. Box 9140 Boulder, Colorado 80301-9140 USA
2008
ii
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Published by The Geological Society of America, Inc. 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA www.geosociety.org
Printed in U.S.A.
Field guide to plutons, volcanoes, faults, reefs, dinosaurs, and possible glaciation in selectedareas of Arizona, California, and Nevada / edited by Ernest M. Duebendorfer, Eugene I. Smith. p. cm. -- (Field guide ; 11) Includes bibliographical references. ISBN: 978-0-8137-0011-3 (pbk.) 1. Geology--Arizona. 2. Geology--California. 3. Geology--Nevada. I. Duebendorfer, Ernest M. II. Smith, Eugene I. (Eugene Irwin), 1944-
QE85.F54 2008 557.9--dc22 2008006898
Cover: Spectacular geology in the Lake Mead area just west of Las Vegas. The River Mountains volcanic section (foreground in Nevada) and the Wilson Ridge pluton (on the skyline to the east in Arizona) represent a linked volcanic-plutonic system separated by the Saddle Island detachment fault. The mesa is Fortification Hill capped by 5.8 m.y. old basalt. Photo by Eugene Smith, May 2006.
10 9 8 7 6 5 4 3 2 1
iii
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
1. The mid-Miocene Wilson Ridge pluton and River Mountains volcanic section, Lake Mead area of Nevada and Arizona: Linking a volcanic and plutonic section . . . . . . . . . . . . 1 Denise Honn and Eugene I. Smith
2. Late Paleozoic deformation in central and southern Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Pat Cashman, Jim Trexler, Walt Snyder, Vladimir Davydov, and Wanda Taylor
3. Active tectonics of the eastern California shear zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Kurt L. Frankel, Allen F. Glazner, Eric Kirby, Francis C. Monastero, Michael D. Strane, Michael E. Oskin, Jeffrey R. Unruh, J. Douglas Walker, Sridhar Anandakrishnan, John M. Bartley, Drew S. Coleman, James F. Dolan, Robert C. Finkel, Dave Greene, Andrew Kylander-Clark, Shasta Marrero, Lewis A. Owen, and Fred Phillips
4. Ediacaran and early Cambrian reefs of Esmeralda County, Nevada: Non-congruent communities within congruent ecosystems across the Neoproterozoic-Paleozoic boundary . . . . 83 Stephen M. Rowland, Lynn K. Oliver, and Melissa Hicks
5. Magmatism and tectonics in a tilted crustal section through a continental arc, eastern Transverse Ranges and southern Mojave Desert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Andrew P. Barth, J. Lawford Anderson, Carl E. Jacobson, Scott R. Paterson, and Joseph L. Wooden
6. Cenozoic evolution of the abrupt Colorado Plateau–Basin and Range boundary, northwest Arizona: A tale of three basins, immense lacustrine-evaporite deposits, and the nascent Colorado River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 James E. Faulds, Keith A. Howard, and Ernest M. Duebendorfer
7. Interpretation of Pleistocene glaciation in the Spring Mountains of Nevada: Pros and Cons . . . 153 Jerry Osborn, Matthew Lachniet, and Marvin (Nick) Saines
8. Quaternary volcanism in the San Francisco Volcanic Field: Recent basaltic eruptions that profoundly impacted the northern Arizona landscape and disrupted the lives of nearby residents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 S.L. Hanson, W. Duffield, and J. Plescia
9. The Spirit Mountain batholith and Secret Pass Canyon volcanic center: A cross-sectional view of the magmatic architecture of the uppermost crust of an extensional terrain, Colorado River, Nevada-Arizona . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Nicholas P. Lang, B.J. Walker, Lily L. Claiborne, Calvin F. Miller, Richard W. Hazlett, and Matthew T. Heizler
10. Devonian carbonate platform of eastern Nevada: Facies, surfaces, cycles, sequences, reefs, and catastrophic Alamo Impact Breccia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 John E. Warme, Jared R. Morrow, and Charles A. Sandberg
11. Dinosaurs and dunes! Sedimentology and paleontology of the Mesozoic in the Valley of Fire State Park . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Joshua W. Bonde, David J. Varricchio, Frankie D. Jackson, David B. Loope, and Aubrey M. Shirk
iv Contents
v
Preface
Welcome to Las Vegas! This guidebook has been prepared in conjunction with the 2008 combined Cor- dilleran and Rocky Mountain Sections meeting of the Geological Society of America. This volume contains background information and road logs for eleven fi eld trips in Nevada, Arizona, and California.
Southern Nevada and adjoining areas contain a rich geologic history spanning the interval from the Paleoproterozoic to the present. Las Vegas lies at or near several critical geological junctures and localities including the structural boundary between the Colorado Plateau and Basin and Range, the physiographic boundary between the Great Basin and the southern Basin and Range, the eastern margin of the Sevier fold- and-thrust belt, the tectonically active Death Valley area, tilted and faulted volcanic-plutonic systems expos- ing the upper part of the crust, and the enigmatic “amagmatic zone.”
Field trips in this volume span the geologic record from the Ediacaran (late Neoproterozoic) to the Holo- cene. Steve Rowland, Lynn Oliver, and Melissa Hicks will lead participants to three of the best examples of Ediacaran and Early Cambrian reefs in North America (Chapter 4). A trip led by John Warme, Jared Morrow, and Charles Sandberg (Chapter 10) examines the long-lived Devonian shallow-water carbonate platform and features a visit to the spectacular Alamo Impact Breccia. Middle Mississippian to late Permian tectonism as recorded by regional unconformities, folding, thrusting, and the stratigraphic record is the focus of a trip led by Pat Cashman, Jim Trexler, Walt Snyder, Vladimir Davydov, and Wanda Taylor (Chapter 2).
Andy Barth, Lawford Anderson, Carl Jacobson, Scott Paterson, and Joe Wooden bring us into the Meso- zoic with an overview of the tectonic evolution of a tilted section through the upper and middle crust of the Cretaceous Cordilleran arc (Chapter 5). Cretaceous sedimentary rocks deposited in the foredeep of the Sevier fold-and-thrust belt and their dinosaur fossils are the topic of a trip led by Joshua Bonde, David Varricchio, Frankie Jackson, David Loope, and Aubrey Shirk (Chapter 11).
The Cenozoic is well represented by six different trips. Nick Lang, B.J. Walker, Lily Claiborne, Calvin F. Miller, Rick Hazlett, and Matt Heizler (Chapter 9) examine spectacular cross-section view of the Miocene Spirit Mountain batholith and a coeval, and possibly related, eruptive center (Secret Pass) in the Colorado River extensional corridor. Another volcano-plutonic complex, the River Mountains–Wilson Ridge igneous system, which was dismembered by the Saddle Island detachment fault is the destination of a trip led by Denise Honn and Gene Smith (Chapter 1). Jim Faulds, Keith Howard, and Ernie Duebendorfer examine synextensional basins that constrain the timing of the structural demarcation between the Colorado Plateau and the Basin and Range (Chapter 6). Jerry Osborn, Matthew Lachniet, and Nick Saines weigh the evidence for and against Pleistocene glaciation in the Spring Mountains of southern Nevada in Chapter 7. The cultural effects of some of the youngest volcanism in the continental United States outside the Cascades is the focus of a trip by Sarah Hanson, Wendell Duffi eld, and Jeffrey Plescia (Chapter 8) to the San Francisco volcanic fi eld near Flagstaff, Arizona. Finally, Kurt Frankel and a cast of thousands bring us up to date with a look at the active tectonics of the eastern California shear zone with discussions regarding signifi cant discrepancies between long-term slip rates and the current rate of strain accumulation along active faults (Chapter 3).
With fi eld trips ranging from old to the present, the middle crust to the surface, from tectonics to paleontology, and from volcanism to glaciation, this volume offers something for everyone.
Ernest M. Duebendorfer Eugene I. Smith
Map of the Nevada, California, Arizona, and Utah areas visited in these fi eld trips showing locations of trips by number.
1
INTRODUCTION
The study of an igneous system is limited by exposure and preservation of the rock record. In most cases, only a portion of the system is exposed (i.e., volcanic or plutonic) and therefore only part of the magmatic history can be studied. Based on work done over the past 20 years, we interpret the River Mountains volcanic section of southern Nevada and the Wilson Ridge Pluton in northwestern Arizona as volcanic and plutonic segments of the same igneous system (Fig. 1). The connection between the River Mountains volcanic section and the Wilson Ridge pluton is based on structure, lithology, mineralogy, geochemistry, and geochronology. This fi eld trip will visit both the River Mountains and Wilson Ridge and will emphasize links between the volcanic and plutonic sections.
The River Mountains volcanic section–Wilson Ridge pluton igneous system crops out at the northern end of the Colo- rado River extensional corridor, a north-south trending area of southern Nevada, western Arizona and eastern California that underwent up to 100% extension between ca. 23 and 12 Ma. In the northern part of the corridor, volcanic rocks of Tertiary age lie on Precambrian crystalline rocks and locally a thin conglomerate containing sedimentary and crystalline clasts. Paleo- zoic and Mesozoic sedimentary sections are missing and were probably stripped from a rising structural arch (the Kingman Arch) during late-Cretaceous, early Tertiary time (Faulds et al., 2001). The arch plunges gently to the north (~15°) and termi- nates against the Lake Mead fault system just north of Lake Mead. In the Colorado River extensional corridor, magmatism migrated to the north, pre-dating crustal extension by about 1 m.y. (Faulds et al., 2001).
The Geological Society of America Field Guide 11
2008
The mid-Miocene Wilson Ridge pluton and River Mountains volcanic section, Lake Mead area of Nevada and Arizona:
Linking a volcanic and plutonic section
Denise Honn* Eugene I. Smith*
Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154-4010, USA
ABSTRACT
This fi eld trip will visit the River Mountains volcanic section (12.17 ± 0.02 to 13.45 ± 0.02 Ma) and Wilson Ridge pluton (13.10 ± 0.11 Ma) in southern Nevada and northwestern Arizona. Although this volcanic-plutonic system was disrupted by the Saddle Island detachment fault during Miocene crustal extension, there are convinc- ing lithological, mineralogical, geochemical and geochronological indicators that suggest a cogenetic relationship. The trip consists of 17 stops that emphasize evidence that links the volcanic and plutonic sections. In addition we will visit the Saddle Island detachment fault at its type locality on Saddle Island.
Keywords: plutonic rocks, volcanoes, Lake Mead, petrology, geochronology.
*[email protected], [email protected]
Honn, D., and Smith, E.I., 2008, The mid-Miocene Wilson Ridge pluton and River Mountains volcanic section, Lake Mead area of Nevada and Arizona: Linking a volcanic and plutonic section, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 1–20, doi: 10.1130/2008.fl d011(01). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.
2 Honn and Smith
Mid-Miocene Wilson Ridge pluton and River Mountains volcanic section 3
RIVER MOUNTAINS VOLCANIC SECTION
The River Mountains volcanic section (12.17 ± 0.02 to 13.45 ± 0.02 Ma, 40Ar/39Ar whole-rock and mineral dates; Faulds et al., 1999) composed of mainly dacite, andesite, basalt and rhyolite is locally intruded by hypabyssal dacite plugs and a quartz monzonite stock. Smith (1982; 1984) suggested that the River Mountains are composed of at least four volcanoes that were juxtaposed by mid-Tertiary strike-slip faulting related to the left- lateral Lake Mead fault system:
1. The River Mountains stratovolcano and related satellitic dacite, rhyolite and basalt volcanoes. The stratovolcano is cored by the River Mountains quartz monzonite stock, which is surrounded by a zone of altered volcanic rocks cut by numerous dikes. The stock contains many xenoliths of basalt and dolomite. Dikes of porphyritic dacite radiate from the plug. The stock is chemically equivalent to rocks of the Wilson Ridge pluton and may represent the detached apex of one of the Wilson Ridge intrusions. Rocks above the intrusion are altered and mineralized andesite and plutonic rock cut by numerous dacite dikes that emanate from the River Mountains stock. The blue sodium amphibole, magnesio-riebeckite, occurs along
fractures and coatings on rocks of the quartz monzonite stock and surrounding altered volcanic rocks. Magnesio- riebeckite is also found in fractures and thin veins in various phases of the Wilson Ridge pluton and within the Colorado River extensional corridor appears to be unique to this volcanic-plutonic system.
2. The Bootleg Wash section just north Boulder City, Nevada, composed from base to top of a section of andesite fl ows, volcaniclastic breccia, and fl ow-banded dacite fl ows.
3. The Red Mountain section formed by highly altered andesite and dacite fl ows, volcaniclastic rocks, and local granitic intrusions. On Red Mountain, andesite fl ows and breccia are interleaved along numerous low-angle faults. The Red Mountain section is separated from the River Mountains stratovolcano by a northwest-striking fault (probably strike slip) and may represent highly altered volcanic and plutonic rocks related to the Boul- der City pluton (14.17 ± 0.6 Ma; NAVDAT (http://navdat. kgs.ku.edu/); 13.8 Ma K/Ar age reported by Anderson et al., 1972).
4. The Casino dacite just east of Railroad Pass is characterized by andesite and dacite fl ows and a thin moderately welded ash-fl ow tuff.
Qts - Tertiary and younger sediments
Tmf - Fortification Hill basalt
Thc - Hamblin-Cleopatra volcanic rocks
Tgp - Unassigned volcanic rocks
Powerline road volcanic rocks Tpd - dacite Tpd - basalt and andesite
Tbv - Bootleg Wash volcanic rocks
River Mountains Stratovolcano Trs - quartz monzonite stock Tsv - andeste and dacite
Quaternary
Mesozoic
Paleozoic
Precambrian
Tbc - Boulder City Pluton Wilson Ridge Pluton Twrh - hypabyssal phase Twrm - medium grained phase Twrc - coarse grained phase Twrg - red feldspar granite phase Tid - diorite phase
Thd - Tuff of Hoover Dam and Dam Conglomerate Tbw - Boulder Wash volcanic rocks Tbr - Breccia
Tpm - Patsy Mine volcanic rocks
K-Tpp - Paint Pots intrusive rocks
Mz-p - Pennsylvanian through Mesozic rocks
M-Pc - Precambrian through Mississippian rocks
Faults - dotted where concealed or inferred ball on downthrown side
Contacts
Dikes
WILSON RIDGE PLUTON (ABSTRACTED FROM LARSEN AND SMITH, 1990)
The Wilson Ridge pluton is an epizonal to hypabyssal calc- alkaline pluton that formed ca. 13.10 ± 0.11 Ma (40Ar/39Ar hornblende date; Faulds et al., 1999) during a period of mid-Miocene extension. Faulting and erosion have provided a cross section of the pluton in plan view. Geobarometric data and geologic constraints indicate the pluton has been tilted 17° to the north (Met- calf et al., 1993). The apex of the pluton, just south of Boulder Canyon (Lake Mead), Nevada, is composed of hypabyssal quartz monzonite and dacite cut by numerous dikes of rhyolite, dacite and basalt. The base of the pluton is 20 km to the south where quartz monzodiorite, monzodiorite, and diorite are in low-angle intrusive contact with Precambrian basement. The pluton was separated from comagmatic volcanic rocks in the River Mountains by movement along the Saddle Island fault system which includes the Saddle Island detachment, Hamblin Bay, and Eldorado faults (Weber and Smith, 1987; Duebendorfer et al., 1990). The age of detachment is estimated to be younger than ca. 13.5 Ma based on the inference that detachment faulting must postdate the formation of the Wilson Ridge pluton and River Mountain volcanic suite (Duebendorfer et al., 1990). The River Mountains now lie approximately 20 km to the west of the pluton.
The Wilson Ridge pluton is composed of the Teakettle Pass suite consisting of foliated monzodiorite and quartz monzodiorite, unfoliated quartz monzonite, and the older Horsethief Canyon diorite. The Teakettle Pass suite comprises the main phase of the Wilson Ridge pluton (80 km2 outcrop area). The major minerals of the coarse-grained quartz monzonite (the dominant phase of the Teakettle Pass suite) are quartz (20%), orthoclase (25%), plagioclase (40%), and subhedral prismatic hornblende (<5%) (Larsen, 1989). The secondary phase of the Teakettle Pass suite is medium- grained quartz monzodiorite and monzodiorite. Major minerals of this phase are plagioclase (45%), interstitial quartz and orthoclase (<20%), subhedral to euhedral biotite and hornblende (40%) (Larsen, 1989). Accessory minerals ubiquitous to the Teakettle Pass suite include sphene (2–4%), apatite, and zircon.
The Teakettle Pass suite intrudes the Horsethief Canyon diorite in the southern portion of the Wilson Ridge pluton. The Horsethief Canyon diorite (4 km2 outcrop area) is composed of
plagioclase (50%), hornblende (35%), biotite (10%), anhedral quartz and orthoclase (<10%), megascopic 1–4 mm diameter sphene (2%–4%), and trace amounts of apatite (Larsen, 1989). The diorite is also present as angular to rounded xenoliths within the Teakettle Pass suite.
Intermediate rocks of the Teakettle Pass suite contain abun- dant basalt and diorite enclaves. Basaltic enclaves are lensoidal and pillow-like and commonly have crenulate and fi ne-grained margins. The enclaves probably represent blobs of mafi c liquid that commingled and mechanically mixed with felsic magma to produce the intermediate rocks of the pluton. Basaltic enclaves commonly occur as inclusion-rich zones that represent synplutonic mafi c dikes injected into a quartz monzonite host. Mafi c magma was entrained and mechanically broken down by magmatic fl ow shear. A continuum in shape exists from enclaves that are bulbous and ellipsoidal to thin, tabular mafi c selvages and schlieren and ultimately to the mafi c component in foliated quartz monzodiorite and monzodiorite (Larsen, 1989). Diorite enclaves have angular contacts with host rocks and are interpreted as xenoliths.
Saddle Island Detachment Fault
The Saddle Island detachment fault (Smith, 1982) cut the River Mountains volcanic section–Wilson Ridge pluton igneous system and moved the River Mountains volcanic section approximately 20 km to the west of the Wilson Ridge pluton (Weber and Smith, 1987) (Fig. 2). The detachment is exposed on Saddle Island on the west shore of Lake Mead just east of the River Mountains. Based on lithology and structure, Sewall (1988) divided the upper plate of the Saddle Island detachment fault into three fault-bounded domains. Domain one includes the Lower Cambrian Tapeats Sandstone and Pioche Shale and the overlying Miocene Horse Spring Formation. This lithology of this domain is very similar to exposures, interpreted as the upper plate of the Saddle Island detachment, on the east side of Wilson Ridge at the Cohenour Mine and in Petroglyph Wash (Feuerbach, 1986). On Saddle Island domain one is cut by mid-Miocene hypabyssal dacite and hornblende dacite. Domain two includes Pre- cambrian amphibolite, gneiss, mica schist, quartz monzonite, and pegmatite intruded by mid-Miocene dacite, hypabyssal dacite, and diorite. The Saddle Island detachment is the lower boundary
Saddle Island
Boulder Basin
River Mountains
Lake Mead
W E
Figure 2. Cross section through from the River Mountains on the west to Detri- tal Wash on the east. The Saddle Island detachment fault crops out on Saddle Island east of the River Mountains and on Arch Mountain east. Adapted from Duebendorfer et al. (1990).
of this domain. Domain three includes Precambrian basement similar to that of domain two; however, domain three has few mid-Miocene intrusions. Dacite and hypabyssal dacite intrusions of Saddle Island contain rounded porphyritic basaltic enclaves commonly with embayed and cuspate margins. This texture resulting from magma commingling is common within both the River Mountains volcanic section and in the southern part of the Wilson Ridge pluton.
The zone of detachment faulting between the upper and lower plates consists of a 2-m-thick microbreccia or ultracataclasite that overlies a 30-m-thick zone of chlorite phyllonite. The lower plate contains variably mylonitized amphibolite, gneiss, and pegmatite. The degree of shearing (mylonitization) in the lower plate increases upward toward the detachment zone. A complete description of the Saddle Island detachment is provided in Duebendorfer et al. (1990).
Evidence for the Volcano-Pluton Link
Based on lithology, mineralogy, geochemistry, geochronology, and structure, the following observations support a link between Wilson Ridge and the River Mountains.
1. Mafi c enclaves are present in the Teakettle Pass suite and Horsethief Canyon diorite of the Wilson Ridge pluton. Larsen (1989) and Larsen and Smith (1990) demon- strated that these enclaves are chemically similar to mafi c dikes of the Wilson Ridge pluton and to basalt fl ows of the River Mountain volcanic section (Fig. 3) (Table 1).
2. Relatively immobile trace elements Th (4.2–25.1 ppm), Hf (3.3–6.6 ppm), and Ta (0.9–1.7 ppm) from 88 samples from the pluton and 32 samples from the volcanic section are tightly clustered and overlap (Fig. 4) (Table 1). Samples from nearby igneous systems plot outside of this cluster (Duebendorfer et al., 1990; Feuer- bach, 1986; Larsen, 1989; Larsen and Smith, 1990). Also, chondrite-normalized rare-earth element distribu- tions for Saddle Island dacite and Wilson Ridge volcanic and plutonic rocks overlap but are different from rare- earth element values for the nearby Boulder City pluton (Duebendorfer et al., 1990) (Fig. 5). On a 87Sr/86Sr versus SiO
2 plot, rocks of the Wilson Ridge pluton and
the River Mountains volcanic suite form a linear trend with 87Sr/86Sr increasing with increasing SiO
2 (Fig. 6)
(Duebendorfer et al., 1998). 3. Rocks of the River Mountains and Wilson Ridge pluton
have been dated using several techniques, including K- Ar (sanidine, biotite and whole rock), 40Ar/ 39Ar (sanidine, biotite, hornblende, and whole rock) and 206U/238Pb (zircon). 40Ar/ 39Ar dates from the Wilson Ridge pluton and River Mountains volcanic suite overlap and range from 13.45 ± 0.02 to 12.17 ± 0.02 (40Ar/ 39Ar whole-rock and mineral dates; Faulds et al., 1999). Preliminary 206Pb/238U zircon dates (LA-ICPMS) from 106 to 40 µm spots on 49 zircons suggest a complex multiphase system active
for 4.2 million years (based on a zircon core-rim pair) to a maximum of 7.2 million years (based on two zircon rim dates 18.9 ± 0.8 to 13.1 ± 0.6 Ma) (Honn et al., 2007) (Fig. 7). K-Ar and 40Ar/ 39Ar sanidine, biotite, and hornblende whole-rock dates represent emplacement ages within the crust and therefore refl ect a short history for the system. In contrast, 206U/238Pb zircon dates refl ect the entire history as well as the age of antecrysts and
Rb Ba Th U K La Ce Sr P Nd Sm Eu Ti Yb Lu
Ro ck
/P ri
m it
iv e
M an
tl e
Sun/McDon. 1989-PM
100,000
10,000
1,000
100
10
1
0.1
basalt from the RM Mafic dikes of the WRP basaltic enclaves WRP
Figure 3. Trace element distribution diagram comparing the basalt from the River Mountains (RM), mafi c dikes and basaltic enclaves of the Wilson Ridge pluton (WRP). Data from Larsen (1989).
Th Ta
WRP RM BCP
Figure 4. Ternary diagram refl ecting the tight clustering of immobile elements Th, Hf, and Ta from the Wilson Ridge pluton (WRP) and Riv- er Mountains volcanic section (RM). The Boulder City pluton (BCP) does not fi t in this tight cluster. Data from Duebendorfer et al. (1990); Feuerbach (1986); Smith et al. (1990); and Weber and Smith (1987).
6 Honn and Smith
T A
B LE
1
10
100
1000
R EE
River Mtns. Wilson Ridge White Hills Aztec Wash Boulder City
-6
-7
-8
-9
-10
-11
-12
-13
SiO2
(A.) (B.)
Figure 5. Rare earth element distribution diagram for the Wilson Ridge pluton (light stippling), Boulder City pluton (heavy stippling) and hypabyssal dacites from Saddle Island (line). SiO
2 ranges are given for
Boulder City pluton and Wilson Ridge pluton samples. Adapted from Duebendorfer et al. (1990).
Figure 6. (A) The River Mountains volcanics and Wilson Ridge pluton rock are distinct from nearby volcanic and plutonic rocks in terms of their 87Sr/86Sr and epsilon Nd. (B) The positive correlation between SiO
2 and 87Sr/86Sr suggests that the River Mountains volcanics and the Wil-
son Ridge pluton are cogenetic. This single linear array suggests magma mixing between two endmembers created the volcanic-plutonic suite. Adapted from Duebendorfer et al. (1998).
RM stock
(Millions of Years Ago)
Figure 7. Preliminary LA-ICPMS U-Pb dates from zircon for the Wilson Ridge pluton (WRP) and River Mountains (RM). Dates are based on individual 40 micron spots. Dates older than 19 Ma are considered xenocrysts.
10 Honn and Smith
100 microns 100 microns
A
B
D
C
E
F
Figure 9. Cartoon of petrogenetic model for the evolution of the River Mountains–Wilson Ridge igneous system (adapted from Larsen, 1989; Larsen and Smith 1990) with general stop areas within the system (A) River Mountains overlook area, (B) Kingman Wash, (C) Horsethief Canyon, (D) River Mountains stock and border zone, (E) Saddle Island detachment fault, (F) northern River Mountains volcanic section.
Figure 8. Cathodoluminescent images of zircons used in U-Pb LA-ICPMS dating with xenocrystic cores. (A) Zircon has at least two major dissolution events (dashed lines). (B) Zircon has an angular core (dashed line) and small (10 microns in diameter) melt inclusion (black oval).
xenocrysts. Textural evidence in cathodoluminescence images was used to recognize xenocrysts and inherited cores (Fig. 8), which give ages from 1517.5 ± 11.2 Ma to 21.3 ± 0.8 Ma. Many of these older ages have larger error bars and likely refl ect analytical errors (Honn et al., 2007).
4. The rare occurrence of magnesio-riebeckite (a sodic amphibole) in the Wilson Ridge pluton was studied by Potts (2000), but has only recently been noted in the River Mountains volcanic section. In both areas, magnesio- riebeckite occurs as coatings on fracture surfaces, vein- lets, dense stockwork veining, and diffusive alteration fronts. Potts (2000) concluded that magnesio-riebeckite in the Wilson Ridge pluton formed due to subsolidus Na-metasomatism. The metasomatism was related to hydrothermal alteration of the pluton involving Na-rich fl uids with meteoric origins (based on δ18O values of the amphibole). The timing of magnesio-riebeckite formation in the pluton is constrained by the high-angle faults that were active about 10 Ma, the surfaces of which are coated with magnesio-riebeckite, and the emplacement of the Fortifi cation Hill Basalt (4.61–5.89 Ma), which contains no magnesio-riebeckite (Potts, 2000).
CONCLUSIONS
Linked by structure, rock type, enclaves, geochemistry, and geochronology, the River Mountains volcanic suite and Wilson Ridge pluton are a rare example of a well-preserved and exposed igneous system that can be used to study magma chamber processes. Figure 9 is a schematic diagram from Larsen (1989) of the combined River Mountains–Wilson Ridge pluton igneous system that has been modifi ed to show the position of fi eld trip stops within the River Mountains volcanic section–Wilson Ridge pluton magmatic system.
DAY 1
Area A. River Mountains Trail Overlook
This is a short (2.4 km) hike from the River Mountains Trail parking lot in Boulder City to a peak of in the southern River Mountains for an introduction to the local geology, the River Mountains stratovolcano, and an outcrop of magnesio-riebeckite alteration (Figs. 10 and 11).
Stop 1. River Mountains Overlook Once the trail enters the canyon, it follows the contact
between the Red Mountain volcanic section (west side) and the River Mountains stratovolcano on the right (east) side of the wash. The contact is a northwest-striking fault (probably strike slip). The Red Mountain volcanic section may represent highly altered volcanic and plutonic rocks related to the Boulder City pluton (13.88 ± 0.1 Ma; Faulds et al., 1999).
From the peak, the eastern skyline shows, from north to south, the River Mountains stratovolcano and the River Moun- tains stock. Across Lake Mead is the Wilson Ridge pluton. The prominent fl at-topped mesa in front of the Wilson Ridge pluton is Fortifi cation Hill, capped by basalt dated at 5.89 ± 0.08 Ma (Feuerbach et al. 1991). Tomorrow morning, we will be driving in Kingman Wash along the south side of Fortifi cation Hill.
Andesite and dacite fl ows form the fl anks of the River Mountains stratovolcano and are cut by dacite sills and dikes that radiate from the River Mountains stock. Many of the dikes grade from fi ne-grained quartz monzonite near the stock to dacite at their distal ends.
Alteration of the stratovolcano is primarily argillic and ferric; mineralization is characterized by barite, specular hematite, and manganese oxide. Although the Paleozoic section is gener- ally missing (perhaps eroded during the uplift of the Kingman Arch), remnants of a Paleozoic carbonate section of Cambrian and Mississippian age is preserved and intruded by the quartz monzonite stock. Epidote, tremolite, and garnet are locally pro- duced in these contact zones.
Stop 2. Magnesio-Riebeckite exposures On our way back we will stop at an outcrop of magnesio-
riebeckite (sodic amphibole) within the River Mountains stratovolcano (Fig. 10). The Wilson Ridge pluton is the only other documented occurrence of magnesio-riebeckite in the Colorado River extensional corridor. In the Wilson Ridge pluton, magnesio-riebeckite formed by Na-metasomatism related to subsolidus hydrothermal alteration. δ18O (Potts, 2000) data suggest that the fl uids involved in metasomatism were meteoric (δ18O ranging from −5 to +10). Tomorrow we will see the extensive riebeckite alteration in the Wilson Ridge pluton.
DAY 2
Area B. Kingman Wash
The purpose of this stop is to see the main phase of the pluton, a late stage dike, riebeckite alteration, and younger volcanic units. Starting at the Kingman Wash road located 4 km east of Hoover Dam. Turn off of U.S. 93 onto Bureau of Land Manage- ment (BLM)–approved backcountry road 70 and turn right onto road 70C. See Figures 10 and 12 for area and stop locations.
Stop 3. Magnesio-Riebeckite Outcrops along the last mile (1.6 km) of road 70C (6.75
miles or 10.9 km from the turn off of U.S. 93) have extensive magnesio-riebeckite alteration on fracture surfaces and in veins. These exposures of magnesio-riebeckite are evidence for the subsolidus sodic alteration. Quartz monzonite of the pluton is fi ner grained here than in the main phase of the pluton. Based on the amount of tilting of the pluton, we are in the mid-level of the pluton at this location but closer to the margins than in Stop 4 (Fig. 10).
12 Honn and Smith
0 1 mi.5N MN
12.5O .50 1 km
114o51.000' W 114o50.000' W
Area A
Figure 11. Topographic map with locations of stops for areas (A) River Mountains (RM) trail overlook and (D) River Mountains stock and border zone. Stops 1, 2, 7, and 8: 1—River Mountains overlook; 2—magnesio-riebeckite outcrop; 7—River Mountains stock; 8—border zone.
14 Honn and Smith
Stop 4. End of the Road Campsite At the end of road 70C, the main phase of the Wilson Ridge
pluton is medium- to coarse-grained quartz monzodiorite. This is the mid-level of the pluton (Fig. 10); magma commingling textures are present but not as common as near the base of the pluton in Horsethief Canyon (Area C). A late-stage dike crops out approximately 100 m up the trail (the now closed road) to the north. The dike is porphyritic rhyolite with plagioclase phenocrysts up to 1 cm in length.
Stop 5. View of Fortifi cation Hill (Abstracted from Metcalf et al., 1993)
Backtrack along road 70C. The black mesa to the northwest is Fortifi cation Hill. Pliocene basalts in the Lake Mead area defi ne the Fortifi cation Hill volcanic fi eld (Feuerbach et al., 1993). Basalts of this fi eld are subalkalic and alkalic and erupted between 5.89 and 4.7 Ma (K-Ar plagioclase separate dates; Feuerbach et al., 1991). Magmatic activity in the fi eld ended with the formation of low-volume alkali basalt centers in northwest Arizona along U.S. 93 and in Petroglyph Wash and near Boulder Beach.
The Fortifi cation Hill–Lava Cascade group forms a chain of at least 6 vents with a length of 25 km. The chain appears to be controlled by north-northwest–striking faults that bound the west side of the Black Mountains. Vents occur both on the margin and in the interior of the range. The escarpment that forms the cap of Fortifi cation Hill is composed of over 100 fl ows of olivine basalt and scoria. Cinder cones aligned in a north-south direc- tion are intruded by dikes and plugs and served as the source for the fl ows. The Fortifi cation Hill cinder cones represent a node of intense volcanic activity along a north-trending en echelon dike system. Dike orientation is coplanar with the east-dipping mid- Miocene Fortifi cation fault zone.
Area C. Horsethief Canyon
The purpose of this stop is to see the commingling textures near the fl oor of the pluton between the Teakettle Pass suite and Horsethief Canyon diorite. After returning to U.S. 93 drive 2 miles (3.2 km) east and turn onto on BLM-approved backcountry road make a left turn onto road 66 (just before the White Rock Canyon turn off) and continue to its end (4.75 miles; 7.64 km) at the dry falls. See Figures 10 and 13 for stop locations.
Stop 6. Campground at Dry Falls/End of the Road (Abstracted from Larson, 1989, Larson and Smith, 1990, and Metcalf et al., 1993)
The spectacular magma-commingling textures on the dry falls at the entrance to the canyon are just a taste of what’s to come. We will use the trail up the hill on the north side of the dry falls to get into the canyon. We are now near the fl oor of the pluton (Figure 10). On a 1–2 km hike up the canyon we will see exposures of the Horsethief Canyon diorite intruded by basalt (now enclaves), magmas of the Teakettle Pass suite (quartz monzonite) as well as xenoliths of diorite within monzodiorite.
The complete spectrum from discrete xenoliths to commingled magmas to completely mixed rock in the Teakettle Pass suite is exposed in this canyon.
Enclaves are lensoidal, fusiformal, tabular, pillow shaped, commonly have crenulate margins and fi ne-grained borders, and are locally boudinaged. Many enclaves show a weak internal foliation that is subparallel with the foliation in the host and with the mesoscopic foliation defi ned by the alignment of the enclaves themselves. Enclaves are typically 20–50 cm long and are rarely isolated; more typically they cluster in enclave- rich tabular zones that display strong fl ow foliation near their margins.
Enclave zones occur throughout the pluton in the intermediate phases of the Teakettle Pass suite. In two dimensions, enclave zones range from <1 m by 5 m to 10 m by 500 m. A continuum in shape exists from enclaves that are bulbous and ellipsoidal to those that are thin, tabular mafi c schlieren and ultimately to the mafi c component in foliated quartz monzodiorite and monzodiorite.
Chilled borders, crenulate margins, boudinage, and pillow- like geometries of mafi c enclaves of the Teakettle Pass suite strongly suggest that they were liquid or semi-liquid at the time of their incorporation in the felsic host. The abundance of lensoidal and phacoidal inclusions and the lack of strain within the inclusions and most phases of the Teakettle Pass suite indicate that enclaves were deformed while still partially molten. The lack of sharply bounded, tabular dikes with chilled margins suggests that dike injection occurred before the felsic host was crystal- lized enough to sustain a fracture. Dike emplacement occurred continuously as the pluton cooled. Late-stage dikes are tabular, cut all phases of the pluton except felsic dikes, and are colinear with high-angle normal faulting.
DAY 3
Area D. The River Mountains Stock and Border Zone
This is a short (1.6 km) hike to outcrops of the River Moun- tains stock, border zone, and an overlook of the River Mountains stratovolcano (Fig. 10 and 11).
Stop 7. The River Mountains Stock (Abstracted from Smith, 1984)
Heading east on U.S. 93 toward Hoover Dam from Boul- der City, turn north (left) onto BLM-approved backcountry road 76 then turn left onto road 77 and continue on to the break in slope. The River Mountains stock is a plug in the conduit of the River Mountains stratovolcano. The stock is a composite pluton composed of fi ne- to medium-grained quartz monzonite. Near the top of the stock the rock resembles fi ne-grained dacite. The stock has radiating dikes of monzonite that intrude a border zone of altered and mineralized volcanic rock. The stock contains several large blocks (10 m in size) of Paleozoic carbonate, Tertiary andesite, and basalt.
16 Honn and Smith
Stop 8. The Border Zone (Abstracted from Smith, 1984) Head back down road BLM-approved backcountry road 77
until the canyon narrows about (0.5 miles; 0.8 km). The border zone surrounding the River Mountains stock is a complex transition zone of several porphyritic dacite dikes that intrude the highly altered and mineralized andesite of the River Mountains stratovolcano as well as Paleozoic carbonate blocks. The contact between border zone and volcanic rocks of the River Mountains stratovolcano is gradational.
Optional Area E. Saddle Island Detachment The purpose of visiting this area is to see outcrops of the
Saddle Island detachment fault and rocks in the upper plate (Fig. 10 and 14).
Optional Stop 9. Saddle Island Detachment Turn east off of Lake Shore Road onto the paved road to
Southern Nevada Water System Treatment and Pumping Plant operated by the Southern Nevada Water Authority (SNWA). Not only does Saddle Island house the water treatment plant, two of the intakes for Las Vegas’ water supply are located on the east side of the island. Because of the strategic importance of the island, SNWA restricts access. We will visit Saddle Island only if we obtain permission from SNWA. After passing through the security checkpoint, drive across the causeway to Saddle Island and park at the turnout just before the road turns south.
Optional Stop 11. Detachment Fault The purpose of this stop is to view the zone of intense shear-
ing and the microbreccia just below the detachment fault. We will hike about 0.5 km to the northeast to the saddle at the crest of the Saddle Island ridge. The hike begins in weakly foliated Precambrian amphibolite cut by scattered dikes of muscovite- bearing pegmatite. In some areas the Precambrian amphibolite is weakly foliated and contains hornblende, plagioclase, and chlorite as dominant minerals. In other areas, amphibolite displays a strong near-horizontal mylonitic foliation. Thin zones of brittle shearing are superimposed on the mylonitic fabric. In these zones, the rock develops a schistose fabric and earlier textures are rarely preserved.
The reddish rocks on the north side of the saddle are brec- ciated Tertiary dacites that form the lowermost part of the upper plate on Saddle Island. The detachment fault is located about half way up the ridge at the color change (green rock = lower plate; red rock = upper plate). On the south side of the saddle, amphibolite displays a weak mylonitic foliation, but little evidence of brittle shearing is present. About 20 m below the fault, amphibolite begins to show the effects of brittle shearing. As the fault is approached, the lower-plate rock progressively becomes more intensely sheared and is converted into chlorite schist. The strike of the brittle foliation is N30E (coplanar with the detachment). Notice the small-scale, lens-like pattern of foliation and the boudin of quartz-feldspar rock (intensely sheared pegmatite dikes?).
Just below the fault, a black, fi ne-grained microbreccia or ultracataclasite is exposed. This rock displays strong ferric alteration (hematite). The microbreccia marks the uppermost part of the lower plate and represents the most intense shearing along the detachment fault. The fault itself is not exposed in the saddle. There is an excellent exposure, however, on the east side of island, just below the saddle.
Optional Stop 12. Upper Plate Rocks Continue walking to the north along the crest of the Saddle
Island ridge. The three fault bounded domains of Sewall (1988) in the upper plate are divided into four lithological units (Due- bendorfer et al., 1990). These are: (1) Precambrian amphibolite, schist, gneiss, and granite; (2) Precambrian basement intruded by dikes and small plugs of quartz monzonite and hypabyssal dacite and diorite; (3) a Lower Cambrian section consisting of the Tapeats Sandstone, Bright Angel Shale, and Bonanza King Dolomite that strike northwest and is vertical to overturned; and (4) conglomerate and megabreccia of Tertiary (?) age (possibly correlative with the Rainbow Gardens Member of Horse Spring Formation) containing clasts (<10 m) of Paleozoic carbonate and Precambrian basement. The conglomerate is intruded by hornblende-quartz monzonite and basalt.
These terranes are fault slices that are bounded above and below by low-angle faults and display a reverse stratigraphic order. Precambrian rocks form the structurally highest terrane and Tertiary conglomerate forms the lowest. Geochemical data demonstrate that the intrusive rocks of Tertiary age on Saddle Island are correlative with the Wilson Ridge pluton.
Area F. Northern River Mountains Volcanic Section: Basaltic Flows, Dacite Domes, Flows, and Mafi c Enclaves
Park at the Longview Scenic Wayside located approximately 14.5 km (9 miles) east of the Lake Mead National Recreation Area entrance station on Lake Mead Drive. Walk south across the highway to the old Lake Shore Drive now part of the River Mountains Trail System. Walk northwest on old Lake Shore Drive to the fi rst dirt road; turn left (south).
This 3 km hike provides a view of the diverse volcanic section in the northern River Mountains. Named the Powerline Road volcanic section by Smith (1984), the section includes lower Powerline Road dacite fl ows and breccia intruded by numerous basalt dikes and rhyolite domes; middle Powerline Road andesite and basalt, and upper Powerline Road rhyolite domes and pyroclastic rocks. See Figures 10 and 15 for stop locations.
Stop 13. The hike starts at the base of the section volcaniclastic
rocks intruded by plagioclase-biotite dacite of the lower Power- line Road. Numerous dikes of pyroxene, olivine basalt cut this section. Rhyolite dome of the upper Powerline Road just to the west of the road contain fl ow-banded rhyolite and a marginal
18 Honn and Smith
vitro phyre. The road passes directly through the center of a fl ow- banded rhyolite dome. The rhyolite contains quartz and sanidine as well as sparse biotite.
Stop 14. After passing through the dome, the trail (now a wash)
turns to the west. The stratigraphic section to the north of the wash contains middle Powerline Road pyroxene-olivine basalt and agglomerate overlain by rhyolite breccia shed from nearby domes. After 0.4 km the trail again turns to the south.
Stop 15. Uphill to the east is a faulted section containing middle
Powerline Road basalt (some with clinopyroxene phenocrysts 1–2 cm in size) overlain by pyroclastic surge, fl ow, and cara-
pace from nearby upper Powerline Road domes. Back in the wash, we will pass the contact of middle Powerline Road basalt with lower Powerline Road dacite, a rhyolite dome (lower Powerline Road), and basalt dikes in lower Powerline Road dacite.
Stop 16. Continue walking to the end of the road. Here lower Pow-
erline Road dacite domes are faulted against the purple-colored dacite two (Tdp2) of Smith (1984). This unit contains numerous basalt enclaves some of which have crenulate margins. Originally thought to be a thick dacite fl ow, recent fi eld work indicates that it represents a shallow intrusion that lies stratigraphically above lower Powerline Road volcaniclastic rocks but below lower Power line Road dacite domes.
114o48.000' W
10 upper plate
Figure 14. Topographic map with locations of stops for area (E) Saddle Island. Stops 9 and 10: 9—detachment fault outcrop; 10—outcrop of upper plate rocks.
Stop 17. If time permits, continue walking along the wash to view the
interior of a fl ow-banded rhyolite dome and a basalt dike containing a large block of Precambrian gneiss.
Walk back along the wash and return to the Longview Scenic Wayside.
ACKNOWLEDGMENTS
Our understanding of the Wilson Ridge pluton and River Mountains has been infl uenced by many people over the last 30 years. We especially thank former University of Nevada, Las Vegas, students Dan Feuerbach, Terry Naumann, Jim Mills, Lance Larson, Deborah Potts, Angela Sewall, Mike Weber, and Ed Eschner, who completed research projects that directly affected our understanding of the area. Adam Simon analyzed
the preliminary 206U/238Pb in zircons by LA-ICP-MS at ETH Zurich. Discussions with Ernie Anderson, Sue Beard, Jim Faulds, Ernie Duebendorfer, Rod Metcalf, Adam Simon, and Terry Spell were very valuable. Sue Beard reviewed the manuscript and provided helpful comments. Lastly, we thank Ernie Duebendorfer for his efforts as co-editor of the fi eld guide and his comments on the manuscript.
REFERENCES CITED
Anderson, R.E., Longwell, C.R., Armstrong, R.L., and Marvin, R.F., 1972, Sig- nifi cance of K-Ar ages of Tertiary Rocks from the Lake Mead region, Nevada-Arizona: Geological Society of America Bulletin, v. 83, p. 273– 288, doi: 10.1130/0016-7606(1972)83[273:SOKAOT]2.0.CO;2.
Anderson, R.E., Barnhard, T.P., and Snee, L.W., 1994, Roles of plutonism, mid- crustal fl ow, tectonic rafting and horizontal collapse in shaping the Mio- cene strain fi eld of the Lake Mead area, Nevada and Arizona: Tectonics, v. 13, p. 1381–1410.
166
basalt-rhyolite section
basalt-dacite contact
114o52.000' W
13
14
Figure 15. Topographic map with locations of stops for area (F) northern River Mountains volcanic section. Stops 11–14: 11—dissected rhyolite dome; 12—basalt-rhyolite contact; 13—pyroclastic surge deposit, dome carapace, and fault; 14—dacite intrusion with mafi c enclaves.
20 Honn and Smith
Armstrong, R.L., 1966, K-Ar dating using neutron activation for Ar analysis: granitic plutons of the eastern Great Basin, Nevada and Utah: Geochimica et Cosmochimica Acta, v. 30, no. 6, p. 565–600.
Armstrong, R.L., 1970, Geochronology of Tertiary igneous rocks, eastern Basin and Range province, western Utah, eastern Nevada, and vicinity, U.S.A: Geochimica et Cosmochimica Acta, v. 34, no. 2, p. 203–232.
Duebendorfer, E.M., Sewall, A.J., and Smith, E.I., 1990, The Saddle Island Detachment fault, an evolving shear zone in the Lake Mead area of southern Nevada, in Wernicke, B., ed., Mid-Tertiary extension at the latitude of Las Vegas: Geological Society of America Memoir 176, p. 77–97.
Duebendorfer, E.M., Beard, L.S., and Smith, E.I., 1998, Restoration of Tertiary extension in the Lake Mead region, southern Nevada: The role of strike- slip transfer zones, in Faulds, J.E., and Stewart, J.H., eds., Accommoda- tion Zones and Transfer Zones: The regional segmentation of the Basin and Range province: Geological Society of America Special Paper 323, p. 127–148.
Faulds, J.E., Smith, E.I., and Gans, P., 1999, Spatial and temporal patterns of magmatism and extension in the Northern Colorado River Extensional Corridor, Nevada and Arizona: A preliminary report, in Faulds, J.E., ed., Cenozoic Geology of the Northern Colorado River Extensional Corridor, Southern Nevada and Northwestern Arizona: Economic implications of regional segmentation structures: Nevada Petroleum Society 1999 Field Trip Guidebook, Reno, Nevada, p. 171–183.
Faulds, J.E., Feuerbach, D.L., Miller, C.F., and Smith, E.I., 2001a, Cenozoic evolution of the northern Colorado River extensional corridor, southern Nevada and northwest Arizona: in Erskine, M.C., Faulds, J.E., Bartley, J.M., and Rowley, P.D., eds., The geologic transition, high plateaus to Great Basin: a symposium and fi eld guide: the Mackin volume: Pacifi c Section of the American Association of Petroleum Geologists Publication GB 78 (also Utah Geological Association Publication 30), p. 239–272.
Feuerbach, D.L., 1986, Geology of the Wilson Ridge pluton; a mid-Miocene quartz monzonite intrusion in the northern Black Mountains, Mohave County, Arizona, and Clark County, Nevada [M.Sc. thesis]: University of Nevada, Las Vegas, 79 p.
Feuerbach, D.L., Smith, E.I., Shafi quallah, M., and Damon, P.E., 1991, New K-Ar dates for mafi c late-Miocene to Pliocene volcanic rocks in the Lake Mead area, Arizona and Nevada: Isochron West, p. 17–20.
Feuerbach, D.L., Smith, E.I., Walker, J.D., and Tangeman, J.A., 1993, The role of the mantle during crustal extension: constraints from geochemistry of volcanic rocks in the Lake Mead area, Nevada and Arizona: Geological Society of America Bulletin, v. 105, p. 1561–1575, doi: 10.1130/0016- 7606(1993)105<1561:TROTMD>2.3.CO;2.
Honn, D.K., Simon, A.S., Smith, E.I., and Spell, T.L., 2007, The River Moun- tains volcanic section–Wilson Ridge pluton, a long-lived multiphase mid- Tertiary igneous system in southern Nevada and northwestern Arizona, USA: Eos (Transactions, American geophysical Union), v. 88, p. 52.
Howard, K.A. and John, B.E., 1987, Crustal extension along a rooted system of imbricate low-angle faults: Colorado River extensional corridor, Califor- nia and Arizona, in Coward, M.P., Dewey, J.F., and Hancock, P.L., eds., Continental Extensional Tectonics: Geological Society of London Special Publication 28, p. 299–311
Larsen, L.L., 1989, The origin of the Wilson Ridge pluton and its enclaves, northwestern Arizona; implications for the generation of a calc-alkaline intermediate pluton in an extensional environment [M.Sc. thesis]: Univer- sity of Nevada, Las Vegas, 81 p.
Larsen, L.L., and Smith, E.I., 1990, Mafi c enclaves in the Wilson Ridge Pluton, northwestern Arizona: Implications for the generation of a calc-alkaline intermediate pluton in an extensional environment: Journal of Geophysi- cal Research, v. 95, p. 17693–17716.
Metcalf, R.V., Smith, E.I., and Mills, J.G., 1993, Magma mixing and commingling in the northern Colorado River extensional corridor; constraints on the production of intermediate magmas—Part I, in Lahren, M.M., Trexler, J.H., Jr., and Spinosa, C., eds., Crustal evolution of the Great Basin and Sierra Nevada: Reno, Nevada, University of Nevada, p. 35–55.
Potts, D.A., 2000, Sodium metasomatism and magnesio-riebeckite mineralization in the Wilson Ridge Pluton [M.Sc. thesis]: University of Nevada, Las Vegas, 104 p.
Sewall, A.J., 1988, Structure and geochemistry of the upper plate of the Saddle Island Detachment, Lake Mead, Nevada [M.Sc. thesis]: University of Nevada, Las Vegas, 84 p.
Smith, E.I., 1982, Geology and geochemistry of the volcanic rocks in the River Mountains, Clark County, Nevada and comparisons with volcanic rocks in nearby areas, in Frost, E.G., and Martin, D.L. eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River Region, California, Arizona and Nevada: San Diego, California, Cordilleran Publishers, p. 41–54.
Smith, E.I., 1984, Geologic map of the Boulder City quadrangle, Nevada: Nevada Bureau of Mines and Geology, Map 81, 1:24,000, 1 sheet.
Smith, E.I., Feuerbach, D.L., Naumann, T.R., and Mills, J.G., 1990, Mid-Mio- cene volcanic and plutonic rocks in the Lake Mead area of Nevada and Arizona; Production of intermediate igneous rocks in an extensional environment, in Anderson, J.L., ed., The nature and origin of Cordilleran magmatism: Geological Society of America Memoir 174, p. 169–194.
Sun, S.S., and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts; implications for mantle composition and processes, in Saunders, A.D., and Norry, M.J., eds., Magmatism in the ocean basins: Geological Society of London Special Publication 42, p. 313–345.
Weber, M.E., and Smith, E.I., 1987, Structural and geochemical constraints on the reassembly mid-Tertiary volcanoes in the Lake Mead area of southern Nevada: Geology, v. 15, p. 553–556, doi: 10.1130/0091- 7613(1987)15<553:SAGCOT>2.0.CO;2.
MANUSCRIPT ACCEPTED BY THE SOCIETY 30 JANUARY 2008
Printed in the USA
21
INTRODUCTION
We have recognized a complex history of late Paleo- zoic deformation throughout the Great Basin by focusing on the internal stratigraphy and structure of the “Antler foreland basin” and the “Antler overlap sequence” in central Nevada (Fig. 1). The latest Devonian-Mississippian “Antler foreland basin” comprises a two-part stratigraphy: (1) The initial fore-
land basin fi ll (signaling the collapse of the continental margin) is latest Devonian through mid-Mississippian in age (Poole and Sandberg, 1977). (2) These foreland strata are deformed, and unconformably overlapped by Upper Mississippian through lower Pennsylvanian strata ( Silberling et al., 1997). The Penn- sylvanian-Permian “Antler overlap sequence,” so named because it unconformably overlies rocks interpreted to have been deformed during the Antler orogeny, also contains angular
The Geological Society of America Field Guide 11
2008
Pat Cashman Jim Trexler
Department of Geological Sciences and Engineering, M.S. 172, University of Nevada, Reno, Nevada 89557, USA
Walt Snyder Vladimir Davydov
Department of Geosciences, Boise State University, Mailstop-1535, 1910 University Drive, Boise, Idaho 83725-1535, USA
Wanda Taylor Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154-4010, USA
ABSTRACT
In central Nevada, a series of angular unconformities records protracted orogenic activity between middle Mississippian and late Permian time. These unconformities are regional, and can be correlated with lithofacies boundaries at their distal edges. Both the unconformities and the tectonically created sedimentary basins they bound are best expressed in a north-south belt of localities from Winnemucca south to the Las Vegas area.
This paper briefl y describes seven localities where rocks display both structural and stratigraphic features related to one or more of these unconformities and their related tectonic events. At Edna Mountain, the record is both stratigraphic and structural, and is mostly from the Pennsylvanian. At Carlin Canyon, we will look at both Mississip- pian and Pennsylvanian folding, thrusting, and unconformities. In the Diamond Range, we will see evidence that Pennsylvanian folding is regionally important. At Secret Can- yon, the record is mostly of Permian deformation and sedimentation. In the Hot Creek Range, we will see southern versions of Mississippian stratigraphy, and thrusting that is late Paleozoic in age. In the Timpahute Mountains, complex faulting is also believed to be late Paleozoic.
Keywords: Nevada, upper Paleozoic tectonics, unconformities, upper Paleozoic deformation.
Cashman, P., Trexler, J., Snyder, W., Davydov, V., and Taylor, W., 2008, Late Paleozoic deformation in central and southern Nevada, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 21–42, doi: 10.1130/2008.fl d011(02). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.
22 Cashman et al.
unconformities, thus recording several different Pennsylvanian and Permian deformation events.
Detailed biostratigraphy enables us to recognize and correlate unconformities between mountain ranges, and is the key to determining the extent and signifi cance of late Paleozoic deformation. We have adopted a scheme for naming the late Paleozoic unconformities that is analogous to that used in the Mesozoic of the Four Corners region (Pipiringos and O’Sullivan, 1978; Trex- ler et al., 2003). Application of this unconformity scheme across much of Nevada reveals that the location of the most intense deformation has changed with time.
In this paper, we summarize the stratigraphic and structural evidence for late Paleozoic deformation at seven Nevada localities (Fig. 2). For each, we present a brief summary of the stratigraphic units involved, a synthesis of the geometry and kinematics recorded by the structures, and the detailed age control that makes unraveling the story possible. The results of previous workers are the basis for many of these summaries. In particular, we acknowledge the work of Silberling et al. (1997) and Tosdal (unpublished mapping) in the northern Pinyon Range, and Dott (1955) in the Adobe Range. Recent thesis research at the Univer- sity of Nevada, Reno, by Danielle Villa (2007) at Edna Mountain
4
Highway Ls. Highway Cong.
Figure 1. Tectonostratigraphic units and basins, northeastern and east-central Nevada. Time scale and numerical ages from House and Gradstein (2004), Davydov et al (2004), and Wardlaw et al (2004), all in Gradstein et al. (2004).
Late Paleozoic deformation in central and southern Nevada 23
and Jeremy McHugh (2006) at Luther Waddles Wash in the Hot Creek Mountains was key to understanding these areas.
RATIONALE FOR UNCONFORMITY SCHEME
Tectonically generated unconformities are stratigraphic boundaries for genetic basin sequences; they are a consequence of the orogenic creation of accommodation space. Orogenic uplift or downwarp is rapid relative to basin response times, and thus these unconformities bracket times when bedrock was eroded and sediment deposited as a response to tectonism. We have pro- posed the testable hypothesis that tectonically generated unconformities can serve as regional temporal markers (e.g., Snyder et al., 2002; Trexler et al., 2003). We have adopted a scheme of unconformities (Fig. 1); each is defi ned at a locality where the subjacent strata are deformed by an event not recorded by overlying strata. Two key observations make this scheme work: (1) it is important to choose localities where units above and below the unconformity are as close in age as possible, because succes- sive overprinting of deformation events is expected; and (2) each unconformity is expected to be less angular with distance from the tectonic disturbance, and in even more distal areas the unconformity may be recorded as only a facies change, or not recorded at all. The latter observation implies that one can potentially identify the area of orogenic activity by looking at the intensity of sub-unconformity deformation.
SUMMARY OF GEOLOGY AT EDNA MOUNTAIN
Stratigraphy of Interest Here
Strata at Edna Mountain (Fig. 3) comprise three genetic packages, including (in ascending structural order) the Lower Paleozoic Preble Formation, upper Paleozoic “overlap” carbonate and siliciclastic strata, and the Golconda allochthon. The units of interest for our purposes are in the second group, and include strata deposited on the Preble Formation. These are, from oldest to youngest, the (Morrowan) Iron Point Conglomerate (new name, Villa, 2007) the (Morrowan) Highway Limestone and overlying, undated Highway Conglomerate, the (Virgillian) Antler Peak Formation, and the (Guadalupian) Edna Mountain Formation (Erickson and Marsh 1974b, 1974c). The newly named Iron Point Conglomer- ate was previously mapped as the Battle Formation (Erickson and Marsh 1974b, 1974c); however, it underlies the Highway Lime- stone, which is Morrowan in age. The type Battle Formation, near Battle Mountain, contains a thin limestone of Atokan age (Saller and Dickinson, 1982; Theodore, 2000), and clasts of mid-Atokan limestone; it is no older than Atokan. It is therefore 3–5 million years younger than the Iron Point Conglomerate.
Unconformities Here
All of the stratigraphic contacts at Edna Mountain are unconformities except the gradational contact between the Iron Point
Conglomerate and overlying Highway Limestone (Fig. 3). The oldest late Paleozoic unit at Edna Mountain is the Iron Point Con- glomerate; it occurs in the upper plate of the Iron Point fault, and its depositional base is not is not preserved here. Iron Point Con- glomerate beds grade upward into the Highway Limestone. The upper contact of the Highway Limestone is a karsted, erosional surface (C5), overlain by the “Highway Conglomerate” (Villa et al., 2007), (previously thought to be interbedded with the High- way Limestone [Erikson and Marsh, 1974b, 1974c]). The “High- way Conglomerate” is therefore renamed as a formation in its own right. It occurs in the cores of west-verging synclines in the Highway Limestone, suggesting that deposition was structurally controlled, and that the conglomerate postdates the mid-Penn- sylvanian west-vergent folding (Villa et al., 2007; Villa, 2007). There is an angular unconformity between the Highway Lime- stone and the overlying Antler Peak Formation (C6); the High- way Conglomerate is not preserved at this contact. The Antler Peak also unconformably overlies the folded and metamorphosed Preble Formation along the west fl ank of Edna Mountain, where the base of the Antler Peak contains clasts of Preble phyllite. The Edna Mountain Formation lies with angular unconformity (P4) on the Antler Peak Limestone and Preble Formation.
Structure
Several late Paleozoic fold sets, each truncated by one of the unconformities, can be recognized in the “Antler overlap sequence” at Edna Mountain (Fig. 4): West-southwest–verging, asymmetric to overturned folds in the Highway Limestone are unconformably overlain by Antler Peak Formation that does not exhibit this deformation. Open, east-trending folds in the Ant- ler Peak Formation and older units predate the unconformably overlying Edna Mountain Formation. Northeast-trending, south- east-verging folds are developed in the Edna Mountain Forma- tion and all underlying units. Another late Paleozoic structure, the low-angle Iron Point fault, also occurs at Edna Mountain. Originally interpreted to be a thrust fault (Erickson and Marsh 1974b, 1974c), it has been reinterpreted as a top-to-the-northeast low-angle normal fault (Villa, 2007; Villa et al., 2007). Motion along the Iron Point fault post-dates the west-vergent folding and predates the folding around east-trending axes.
Implications and Signifi cance
At least one of the unconformities and deformation events recorded in the “Antler overlap sequence” at Edna Mountain appears to correlate with a similar feature already identifi ed in Carlin Canyon 100 km to the east (Trexler et al., 2004), thus establishing it as regional in extent. The base of the Iron Point Conglomerate is not preserved at Edna Mountain, so the age and possible correlation of the basal unconformity are constrained only as Morrowan or older (C3 or C2); the C2 unconformity underlies the correlative section at Carlin Canyon. Tight, west- verging folding is developed in Morrowan-Atokan rocks at both
24 Cashman et al.
Figure 2. Regional map with localities discussed in the text.
Carlin Canyon and Edna Mountain; the oldest undeformed unit overlying these rocks at Edna Mountain is the Antler Peak Lime- stone. The unconformity below the Antler Peak correlates with the regional C6 unconformity, and constrains the age of west-vergent deformation to at least older than Missourian and younger than Atokan. In addition, we suggest that the unconformity between the Highway Limestone and the Highway Conglomerate is the regional C5 unconformity at the base of the Morrowan; if so, this places a tighter age constraint on the west-vergent deformation.
SUMMARY OF GEOLOGY AT CARLIN CANYON
Unconformities Here
Only the boundary between the Tonka and Moleen-Tomera formations is conformable; all the rest, including an unconformity within the Strathearn Formation, are angular unconformities (Fig. 1). The Melandco-Tonka formation boundary is an angular unconformity (C2), particularly well expressed at the west end of the canyon. The Tonka-Moleen-Tomera sequence represents continuous sedimentation from coarse siliciclastics through interbedded carbonate strata. These strata are folded and faulted, and unconformably overlain by the lower (Virgil- lian) Strathearn Fm (C6) The upper Strathearn (Sakmarian) lies with low-angle discordance (P1) on the lower Strathearn and subjacent strata. The Buckskin Mountain Formation is apparently discordant on the upper Strathearn Formation, here recording the P2 boundary.
Structure
Three fold sets, at least two of which are truncated by an unconformity and thus unequivocally late Paleozoic, can be recognized in the upper Paleozoic section at Carlin Canyon (Trex- ler et al., 2004): West-northwest–verging, overturned folds and imbricate thrust faults in the Moleen and Tomera formations are unconformably overlain by the (lower) Strathearn Formation. Open, northeast-plunging folds in the lower Strathearn are erosionally trimmed, then overlain by the upper Strathearn. North- trending, subhorizontal folds are developed in the upper Strat- hearn; their age is not constrained here.
Fortuitous exposure of fossiliferous limestone here provides tight constraints on several of the important tectonic unconformities. The C2 unconformity at the base of the Tonka Forma- tion clearly documents deformation of the initial Antler foreland basin strata, in an angular relationship also seen throughout the Pinyon Range and 100 km to the south in the Diamond Range (see below, and Trexler and Nitchman, 1990; Trexler et al., 1991; Silberling et al., 1997). West-vergent thrusting and overturned folding of Pennsylvanian strata beneath the C6 unconformity records thin-skinned contraction along the continental margin during late Desmoinesian time. Low-angle erosional unconformities P1 and P2 document limited but signifi cant deformation in the early Permian in this locality. The structure of this area is documented in detail in Trexler et al. (2004).
SIDE EXCURSION TO FERDELFORD CANYON— SUMMARY
Two genetically different packages are preserved at Fer- delford Canyon. The stratigraphy here is the same as at Carlin
P4
Iron Point fault
Figure 3. Stratigraphy at Edna Mountain (see Figure 2 for location). This fi gure is from Villa (2007).
26 Cashman et al. 469000
469000
469500
469500
470000
470000
470500
470500
471000
471000
471500
471500
Iron Point Conglomerate
Tertiary basalt
Quaternary units
Figure 4. Geologic map of Edna Mountain, based on mapping by Erickson and Marsh (1974a, b) and modifi ed and expanded by Villa (2007).
DevonianMississippianPennsylvanianPermian Leonardian
28 Cashman et al.
Canyon (Fig. 5). Older stratigraphy includes limestone of the Devonian Devil’s Gate Formation, upper Devonian Webb Formation shale and argillite and lower-middle Mississippian Melandco Formation sandstone, shale, and thin conglomerates. The Webb is Famennian in age (Smith and Ketner, 1978) and the Melandco is not dated here. These units are folded and, in some localities, thrust faulted (Jansma and Speed, 1993, Sil- berling et al., 1997, Tosdal, unpublished mapping). Overlying this folded stratigraphy is the subhorizontal Tonka Formation conglomerate and sandstone, compositionally and texturally similar to the Tonka Formation at Carlin Canyon. The Tonka Formation is dated here as Chesterian, based on macrofossils (Smith and Ketner, 1978).
Unconformities Here
Two signifi cant unconformities are preserved here: the C1 unconformity between the passive-margin Devonian carbonates and the synorogenic, lower Mississippian clastics, and the C2, middle-upper Mississippian unconformity. Strata of the Melandco and Webb formations are deformed and moderately to steeply dipping below the angular C2 unconformity overlain by gently dipping Tonka Formation.
Structure
Detailed mapping of the Melandco and Webb formations in Ferdelford Canyon documents east-verging folding and thrust faulting (Jansma and Speed, 1993; Silberling et al., 1997; Tosdal, unpublished mapping). Although angular relationships at the C2 unconformity are preserved in several ranges, this locality is the most accessible and best documented example of the deforma- tional style and kinematics of the mid-Mississippian deformation (Silberling et al., 1997; Trexler et al., 2003).
Sedimentation in the initial Antler foreland is documented here by the Woodruff and Webb formations (upper Devonian through lowest Mississippian strata) lying unconformably (C1) on strata of the middle Paleozoic miogeocline (e.g., the Devonian Devil’s Gate Formation). The shales and argillites of the Woodruff and Webb are the best evidence for tectonically driven subsidence of the continental margin related to initial emplacement of the Robert Mountains allochthon (Poole and Sandberg, 1977; Murphy et al., 1984). The synorogenic sediments—like those exposed at Ferdelford Canyon—provide better information about the timing of the Antler orogeny than do the fi eld relationships for the age of the Roberts Mountains thrust in most areas.
This locality also provides kinematic information about middle Mississippian deformation of Antler foreland strata. This deformation has been recognized throughout the Pinyon Range (e.g., Johnson and Pendergast, 1981).
SUMMARY OF GEOLOGY AT THE DIAMOND MOUNTAINS
At Three-Mile Canyon in the Diamond Mountains, the stratigraphy consists of the lower-middle Mississippian Dale Canyon Formation, the upper Mississippian Diamond Peak Formation, the Pennsylvanian Ely Formation, and unnamed Permian units Artinskian through Guadalupian in age (Fig. 6) (Larson and Riva, 1963). The contact (C3) between the Diamond Peak and Ely formations is gradational. Upper Diamond Peak Formation has interbedded limestones that yield Chesterian microfossils (Trexler and Nitchman, 1990; Trexler et al., 1991). The Ely For- mation comprises well-bedded, cyclothemic limestones that span the lower and middle Pennsylvanian (Larson and Riva, 1963). Overlying the Ely carbonates are a series of upper Pennsylvanian and Permian unnamed units, separated by regional unconformities (Larson and Riva, 1963; Van Hofwegen, 1995) (see Fig. 6).
Unconformities Here
The C2 unconformity separates lower and middle Missis- sippian Dale Canyon Formation from overlying Diamond Peak Formation (Trexler et al., 2003). This unconformity is angular and is identical to the C2 unconformity in Carlin Canyon. The C3 through C6 unconformities here are either expressed as lithostratigraphic boundaries or are trimmed by younger unconformities. The C3 unconformity here is the lithostratigraphic boundary between the Diamond Peak and Ely formations. Dott (1955) recognized upper and lower members of the Ely Forma- tion; we would correlate the boundary between these members with the C4 unconformity. Van Hofwegen documents lower Strathearn Formation (Missourian-Virgillian) above the Ely, and this boundary is the regional C6 unconformity. The P2 overlap strata here are unnamed Leonardian silty and sandy carbonates. Finally, at the top of the range, Guadalupian sandstone and conglomerate caps the section above the P4 unconformity. The large, overturned syncline that forms the Diamond Range has been interpreted as Mesozoic, but predates the Cretaceous Newark Canyon Formation, preserved near Eureka.
Structure
Our reconnaissance work at Three-Mile Canyon documents folding in the Ely Formation that is erosionally trimmed at the basal contact (C6) of the overlying strata of lower Strathearn age. Pressure solution cleavage is locally developed in the Ely, but not in the overlying Permian units. Subsequent deformation, including a probable fault-bend fold related to a Mesozoic thrust ramp, overprints the Pennsylvanian deformation, and more structural data are needed before this can be confi dently removed. However, preliminary results suggest that the pressure solution cleavage dips steeply east after later deformation is removed; this
Late Paleozoic deformation in central and southern Nevada 29 115°50'
39 °5
Diamond Peak Fm.
N
Figure 6. Geologic map and stratigraphy at Three-Mile Canyon, Diamond Range. Map is from Larson and Riva (1963).
30 Cashman et al.
would be consistent with west-vergent deformation. This area is a target of future work by the authors.
This area was the fi rst locality where upper Paleozoic deformation along the western craton margin was documented using the rec- ognition of folded angular unconformities (Trexler and Nitchman, 1990; Trexler et al., 1991). Here, as elsewhere along the deformation belt in central Nevada, the Roberts Mountains allochthon and the Roberts Mountains thrust are very close-by to the west. The implication is that whatever basement structure caused the Roberts Mountains thrust to cut up to the surface at this longitude has also controlled the location and intensity of subsequent deformation.
SUMMARY OF THE GEOLOGY AT SECRET CANYON
Stratigraphic Framework
The Secret Canyon succession preserves ~900 m of Mississip- pian–late Early Permian strata (Fig. 7; only lower 640 m shown). The lower 70 m (base not measured) of Mississippian (Cheste-
rian) Diamond Peak Formation is overlain by a 603 m succession of Lower Permian silty micrite to micritic siltstone, fi ne-grained sandstone, wackestone and packstone strata (the Carbon Ridge Formation of Nolan et al. [1956]), and ~300 m of Lower to Middle (?) Permian conglomerate. This conglomerate was originally considered to be part of the Cretaceous Newark Canyon Formation (Nolan et al., 1956), but the gradational contact with the underlying Carbon Ridge Formation, and rare fusulinids within limestone lenses in the lowermost portion led Steele (1959) and Bissell (1962) to suggest the conglomerate is part of the Carbon Ridge Formation. However, Nolan and Brew (1971) and Strawson (1981), Schwarz (1987) and Baines et al., (1989) have noted the similarity of the conglomerate with the Garden Valley Formation in the Sulfur Spring Range and suggested that this would be a more appropriate name. Here, we adopt the original name “Carbon Ridge Formation” for the entire fi ne-grained sequence and “Garden Valley Formation” as an acceptable name for the upper conglomerate unit.
Unconformities
The lowest Permian unit, which we call Unit 1, is “upper Strathearn” in age (upper Asselian–lower Sakmarian) and rests on
Figure 7. Stratigraphy at Secret Canyon (see Figure 2 for location).
the Diamond Peak conglomerates along the P1 unconformity. The equivalent of Unit 1 in the Sulfur Spring Range is solely Asselian in age. The upper contact of Unit 1 is the P2 unconformity which displays 3 m of erosional relief along 100 m of strike. Ammonoids and conodonts recovered from the lower portion of Unit 2 document the abrupt change from the shallow marine succession of Unit 1 to the relatively deeper water depositional environment of these strata. Units 2 through 4 are a shallowing-upwards succession of latest Sakmarian-Artinskian age (late Wolfcampian–early Leonardian) strata. The base of Unit 5 is the P3 unconformity. Unit 5 is poorly exposed, and consists predominantly of silty micrite/micritic siltstone, micritic, and very fi ne sandstone, with occasional wackestone, packstone and grainstone event beds. The abundance of Phycosiphon grazing traces and in situ biota consisting mainly of small inarticulate brachiopods is reminiscent of Unit 2. Units 5–7 refl ect another shallowing-upward succession. The P4? unconformity separates the conglomerates of Unit 8 from the underlying units. We speculate that the P5 unconformity may mark the base of the dominantly conglomerate facies of Unit 8B.
Structure
The stratigraphic section at Secret Canyon is a homocli- nal succession on the east limb of a larger anticline. This fold is part of a complex regional structural confi guration refl ecting both Mesozoic and Cenozoic events. The P1 unconformity is slightly angular suggesting, but not proving, some pre-P1 tilting. No major folds have been documented within the section except immediately subjacent to the P3 unconformity.
The unconformities at Secret Canyon record a succession of Late Pennsylvanian to Early Permian tectonic events (e.g., Sny- der et al., 1991; Trexler et al., 2004). The Ely Limestone and the lower Strathearn Formation that were seen at Carlin Canyon are entirely missing at Secret Canyon. Just 25 km to the south in the Pancake Range, ~1000 m of Ely is preserved above the Diamond Peak Formation and below the Permian section. The P1 unconformity is thus interpreted to refl ect tectonically controlled dif- ferential uplift and subsidence. The P2 boundary between Units 1 and 2 refl ects an abrupt change in depositional settings and marks the tectonic initiation of the “Dry Mountain Trough” (Stevens, 1977). The origin of the P3 boundary between Units 4 and 5 is unresolved, but a tectonoeustatic mechanism cannot be ruled out. Because they refl ect deposition intervals of 400,000 yr or less, the 4th and 5th order sequences of Units 2, 3, and 4 are interpreted to be eustatic in origin. The P4 unconformity correlates with the generation of the Phosphoria and Park City basins. The uppermost part of the Garden Valley Formation in the Sulfur Spring Range has yielded Smithian (mid-Early Triassic) conodonts, thus the Garden Valley (Unit 8) at Secret Canyon may host another unconformity that correlates with TR1; this is the subject of an ongoing investigation.
SUMMARY OF THE GEOLOGY AT LUTHER WADDLES WASH, HOT CREEK MOUNTAINS
Strata in this part of the Hot Creek Range (Fig. 8) occur in four thrust sheets, each of which contains a distinctive (but as yet unnamed) Mississippian section and therefore records a different part of the Mississippian continental margin (McHugh, 2006). (1) The Big Cow (westernmost) thrust sheet (Fig. 9) contains an Ordovician-Devonian section and fault-bounded early Mississip- pian and Permian limestones; other parts of the upper Paleozoic section are not exposed. The Mississippian limestones contain graded and scoured beds interpreted as clastic (quartz sand-bearing) and carbonate turbidites. The Permian section includes limestones and dolomites. The Big Cow thrust is unconformably overlain by Early Triassic clastic rocks comprising a basal conglomerate and overlying shale, siltstone and quartz arenite (Fig. 9). (2) The Fishhook Ridge thrust sheet (Fig. 9) contains Devonian carbonates overlain by the Woodruff Formation; the latter represents the Antler foreland basin. This section is overlain by an unnamed, mid-Mississippian (Visean) limestone which is a bioclastic grainstone. It is unusual because other mid-Mississippian rocks in the region (e.g., the Melandco and Dale Canyon formations) are clastic. There is also a fossiliferous coarse-grained Pennsylvanian limestone in this thrust plate; it has been displaced along a later low-angle extensional fault, and its original stratigraphic context is not known. (3) The footwall to both of these thrusts is silicifi ed and hydrothermally altered Pennsylvanian-Permian crystalline limestone of the White Horse thrust sheet (Fig. 9). (4) The structurally lowest thrust sheet, the Orange Lichen footwall (Fig. 9), is a late Mississippian (Chesterian) turbidite section containing both siliciclastic (both quartz- and chert-clast–bearing) and carbonate turbidites. It matches the Eleana Formation on the Nevada Test Site ~250 km to the south in both composition and age, and is the farthest north this unit is known to occur.
Unconformities Here
We have not done detailed stratigraphic, sedimentologic or biostratigraphic work on these rocks, so we do not know how many, or which, upper Paleozoic unconformities are preserved here. The age control to date is primarily from conodonts and macrofossils, and much of it is unpublished (see Tables 1–4 in McHugh, 2006, for a compilation and references).
Structure
Two thrust faults, one east-directed (i.e., foreland-vergent) and the other northwest-directed (i.e., hinterland-vergent) place older Paleozoic rocks over the same Pennsylvanian and Permian footwall section in this area. Superposed folds demonstrate that the east-directed thrusting was the fi rst of these two deformation events; stratigraphic relationships bracket it between Permian and
32 Cashman et al.
Fishhook Ridge thrust sheet
White Horse thrust sheet
1. Early Triassic Ammoniods (G. Klapper) 2. Permian conodonts (A. Harris)
Early Mississippian conodonts (G. Klapper) Ordovician conodonts (R.J. Ross, Jr.)
5. Mesozoic? Ammoniods (E.H. Yockelson) 6. Penn.-Triassic (likely Penn.)conodonts
(A. Harris) 7. Early Triassic conodonts (A. Harris)
8. Pennsylvanian conodonts (M. Kurka) 9. Osage-Meramec conodonts (M. Kurka)
10. mid-Devonian brachiopods (H.W. Dodge, Jr.) 11. Early-mid Devonian crinoids and brachiopods
(J.G. Johnson and J.T. Dutro, Jr.) 12. middle Devonian brachiopods (J.G. Johnson) 13. Early Devonian brachiopods (J.T. Dutro, Jr.) 14. Early to Late Mississippian conodonts
(E.H. Yockelson)
Eureka quartzite (Oe)
silicified crystalline limestone
mudrock, shale, and chert
dolostone with minor limestone
fine-grained, light brown, (where fresh) crystalline dolostone
light grey, recrystallized dolostone
light colored, med. to fine-grained, well-cemented qtz arenite
locally fossiliferous wackestone and grainstone
pale grey bioclastic limestone
STRATIGRAPHIC UNITS:
exposed at surface
Biostratigraphic age control
Figure 8. Stratigraphy at Luther Waddle’s Wash, Hot Creek Range (see Figure 2 for location) from McHugh (2006).
Early Triassic. A structurally lower east-directed thrust fault jux- taposes two very different Mississippian units, placing mid-Mis- sissippian limestone over Late Mississippian clastic and carbonate turbidites of the Eleana Formation. An additional, poorly exposed, thrust fault in the northern part of the area appears to place Penn- sylvanian-Permian limestone and conglomerate over Early Trias- sic rocks, documenting a later, post-Early Triassic thrusting event.
Our work has documented a signifi cant east-directed thrust fault of late Paleozoic age in Luther Waddles Wash; it is over- printed by a northwest-directed thrust fault that may also be late Paleozoic in age (McHugh et al., 2003; McHugh, 2006). Several unusual upper Paleozoic units in this area record depositional environments that were originally west of the Eleana submarine fan system (Trexler et al., 1996; Trexler and Cashman 1997), and are prime targets for further study.
SOUTHERN NEVADA
On this part of the trip, we will travel from Nye County through Lincoln County and end up in Clark County. The foci of this part of the trip are to: (1) consider the Schofi eld Pass fault and whether it is the southern continuation of the belt of Late
Paleozoic deformation, and (2) point out the post-Permian units and structures that cover and disturb the Late Paleozoic unconformities and structures (cf. Fig. 10). The latter point highlights why it has been diffi cult to identify evidence of Late Paleozoic deformation and unconformities in the region. A thorough understanding of the post-Permian tectonic and depositional history greatly aids in unraveling the Late Paleozoic history because it is important to generate an appropriate paleogeography.
Stratigraphy
The general stratigraphy in northwes

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Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada (GSA Field Guide 11)