Oligocene and Miocene Volcanic Rocks in the 

Central Pioche-Marysvale Igneous Belt, 

Western Utah and Eastern Nevada

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1433
Oligocene and Miocene Volcanic Rocks in the 

Central Pioche-Marysvale Igneous Belt, 

Western Utah and Eastern Nevada

Stratigraphy of the Volcanic Oligocene Needles Range Group 

in Southwestern Utah and Eastern Nevada

By MYRON G. BEST and S. KERRY GRANT

Miocene Magmatism and Tectonism in and near the Southern 

Wah Wah Mountains, Southwestern Utah

By MYRON G. BEST, HARALD H. MEHNERT, JEFFREY D. KEITH, 

and CHARLES W. NAESER

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1433A-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Best, Myron G. 

Oligocene and Miocene volcanic rocks in the central Pioche-Marysvale igneous belt, western Utah and eastern Nevada.

(U.S. Geological Survey professional paper; 1433-A-B)

Bibliography: p.

Supt. of Docs, no.: I 19.16:1433

1. Geology, Stratigraphic Oligocene. 2. Geology, Stratigraphic Miocene. 3. Volcanic ash, tuff, etc. Nevada Pioche 

Region. 4. Volcanic ash, tuff, etc. Utah Marysvale Region. 5. Needles Range Group (Utah and Nev.) 6. Magmatism 

Utah Wah Wah Mountains. 7. Geology Utah Wah Wah Mountains. I. Grant, S. Kerry. II. Title. III. Series: 

Geological Survey professional paper ; 1433-A-B. 

QE693.B47 1987 551.7 35 '097924 86-600019

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425 

Denver, CO 80225
CONTENTS

[Letters designate the chapters]

Page

(A) Stratigraphy of the volcanic Oligocene Needles Range Group in southwestern Utah,

by Myron G. Best and S. Kerry Grant .................................. 1

(B) Miocene magmatism and tectonism in and near the southern Wah Wah Mountains, 

southwestern Utah, by Myron G. Best, Harald H. Mehnert, Jeffrey D. Keith, and 

Charles W. Naeser ................................................... 29

III

Stratigraphy of the Volcanic 

Oligocene Needles Range Group 

in Southwestern Utah 

and Eastern Nevada

By MYRON G. BEST and S. KERRY GRANT

OLIGOGENE AND MIOCENE VOLCANIC ROCKS IN THE CENTRAL PIOCHEMARYSVALE IGNEOUS BELT, SOUTHWESTERN UTAH AND EASTERN NEVADA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1433-A

Several ash-flow cooling units with a total 

volume of more than 6,600 km3 make up two 

compositional cycles, each of which began 

with rhyolite and terminated with dacite
CONTENTS

Abstract ..........

Introduction ......

Acknowledgments

General geology ...

Stratigraphic nomenclature ..................

Change in Stratigraphic rank ...............

Abandonment of Wallaces Peak Tuff Member

General comment on crystal-rich dacite tuffs ...

Escalante Desert Formation .................

Marsden Tuff Member ....................

Rhyolite flow member .....................

Andesite flow member ....................

Lamerdorf Tuff Member ...................

Beers Spring Member .....................

Cottonwood Wash Tuff .....................

Wah Wah Springs Formation ................

Outflow tuff member .....................

Intracaldera member ......................

Page

3

4

4

4

5

6

6

7

7

8

8

8

9

9

9

12

12

12

Page

Ryan Spring Formation .............................. 13

Greens Canyon and Mackleprang Tuff Members ....... 13

Lava flow members ................................ 14

Lund Formation .................................... 14

Tuff member ..................................... 14

Andesite flow member ............................. 15

Distribution, volume, and sources of Needles Range Group 15

Escalante Desert Formation ........................ 15

Cottonwood Wash Tuff ............................. 17

Wah Wah Springs Formation ....................... 17

Indian Peak caldera .............................. 18

Utah sector ................................... 19

Nevada sector ................................. 21

Resurgence ................................... 21

Total volume of Wah Wah Springs Formation ....... 22

Ryan Spring Formation ............................ 22

Lund Formation .................................. 23

White Rock caldera .............................. 24

Summary .......................................... 25

References cited .................................... 26

ILLUSTRATIONS

FIGURE Al.

A2.

A3.

A4-A10.

A5. 

A6. 

A7. 

A8. 

A9. 

A10.

Page

5

6

10

16

17

18

19 

Greens Canyon Tuff Member of Ryan Spring Formation .................................... 22

Mackleprang Tuff Member of Ryan Spring Formation ...................................... 23

Lund Formation ...................................................................... 24

Index map showing localities referred to in text .............................................

Chart showing Stratigraphic relations and nomenclature of the Oligocene Needles Range Group ....

Geologic map and idealized cross section of the northeastern segment of the Indian Peak caldera ... 

Maps showing distribution and thickness of units in the Needles Range Group:

A4. Marsden Tuff Member of Escalante Desert Formation and location of Pine Valley caldera

Lamerdorf Tuff Member of Escalante Desert Formation .............................

Cottonwood Wash Tuff .........................................................

Wah Wah Springs Formation and location of Indian Peak caldera

All. Map showing nested sources of tuff cooling units in the Needles Range Group .......................... 26

TABLES

TABLE Al. Modal composition of samples of the Three Creeks Tuff Member of the Bullion Canyon Volcanics ....

A2. Isotopic ages of dacite tuffs in the Needles Range Group ......................................

A3. Presently exposed areal extents and estimated volumes of ash-flow tuffs of the Needles Range Group

Page 

7

12 

15
OLIGOCENE AND MIOCENE VOLCANIC ROCKS IN THE CENTRAL 

PIOC HE-MARYS VALE IGNEOUS BELT, WESTERN UTAH AND EASTERN NEVADA

STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP

IN SOUTHWESTERN UTAH

By MYRON G. BEST and S. KERRY GRANT

ABSTRACT

Dominantly explosive mid-Oligocene magmatic activity 33-28 m.y. 

ago along the southern Utah-Nevada border produced several ash-flow 

cooling units with a total volume of at least 6,600 km3 and formed 

the large Indian Peak caldera complex. These units constitute two 

eruptive-compositional cycles, each of which began with crystal-poor, 

lithic, low-silica rhyolite tuff and culminated with a larger volume of 

crystal-rich, mostly nonlithic dacite tuff. Pyroxene andesite flows were 

erupted from many widely scattered vents contemporaneously with 

first-cycle rhyolite flows and tuffs, and locally with second-cycle tuffs. 

Five major pyroclastic units deposited during the two eruptive cycles 

are here accorded formation rank (some were previously assigned 

member rank) within the revised Needles Range Group (formerly the 

Needles Range Formation). From the base upward, these formations 

are the Escalante Desert Formation (newly added to group), the Cottonwood Wash Tuff (formerly a member), and the Wah Wah Springs 

Formation (formerly a member) in the older cycle, and the Ryan Spring 

Formation (new unit) and the Lund Formation (formerly a member) 

in the younger cycle. The Wallaces Peak Tuff (formerly a member in 

the Needles Range Formation) is herein abandoned because of 

synonymy with the Three Creeks Tuff Member of the Bullion Canyon 

Volcanics, whose source lies far to the east in south-central Utah.

The Escalante Desert Formation consists of at least two low-silica 

rhyolite ash-flow tuff cooling units, rhyolite and pyroxene andesite 

lava flows, and local lenses of epiclastic deposits. The crystal-poor, 

lithic rhyolite tuff units are given formal status as the older Marsden 

Tuff Member (new unit) and the younger Lamerdorf Tuff Member. 

The source of the Marsden, whose total volume is about 300 km3, was 

the Pine Valley caldera, a poorly defined collapse structure extending 

from the Indian Peak Range (southern Needle Range of older usage) 

to the southern Wah Wah Mountains in Utah. The source of the 

Lamerdorf may also have been the same caldera, but direct evidence 

is lacking. The Beers Spring Member is assigned to the Escalante 

Desert as the upper member.

The 500-km3, 30.6-m.y.-old Cottonwood Wash Tuff, deposited after 

the Escalante Desert Formation, is essentially a simple cooling unit 

of crystal-rich dacite tuff. Like all of the crystal-rich dacite tuff units 

of the Needles Range Group, it has about 40 percent phenocrysts,

about half of which are plagioclase. In the Cottonwood Wash Tuff, 

large phenocrysts of biotite and lesser quartz are characteristic. The 

source of the Cottonwood Wash Tuff appears from indirect evidence 

to be concealed beneath a broad, mostly alluviated area immediately 

west of the Utah-Nevada State line.

The first eruptive-compositional cycle of the Needles Range Group 

culminated 29.5 m.y. ago with the eruption of at least 1,500 km3 of 

crystal-rich dacite tuff of the outflow tuff member of the Wah Wah 

Springs Formation, which spread over an area of at least 22,000 km2 

in Utah and Nevada. The tuff is distinctively rich in hornblende and 

poor in quartz, and it is the only reversely magnetized tuff in the 

Needles Range Group. The Indian Peak caldera produced by this eruption had an estimated area of at least 1,200 km2 and is partly filled 

by 2,400 km3 or so of the intracaldera member of the Wah Wah 

Springs Formation, which comprises landslide and fault debris that 

sloughed off the topographic wall as well as lithic dacite tuff.

Two cooling units of crystal-poor, lithic-rich rhyolite ash-flow tuff 

the Greens Canyon and Mackleprang Tuff Members of the Ryan 

Spring Formation, all newly named in this report initiated the second cycle. The more than 200 km3 of ash-flow tuff is almost entirely confined within the Indian Peak caldera, where its source was 

located. The pattern of deposition implies that the core of the older 

caldera had been resurgently uplifted and block faulted before the 

Ryan Spring Formation was deposited. Andesitic and rhyolitic lava 

flows of the Ryan Spring Formation occur locally.

The 27.9-my.-old crystal-rich dacite tuff of the Lund Formation completely filled the resurgently domed Indian Peak caldera to depths of 

several hundred meters and spilled beyond to cover an area of about 

11,000 km2 in Utah and Nevada with a volume of about 1,600 km3 . 

It is the most quartz-rich unit in the Needles Range Group and the only 

one that contains trace amounts of sphene. The tuff is a compound 

cooling unit within the Indian Peak caldera and within the younger 

White Rock caldera, which partly eclipses the Indian Peak caldera on 

the southwest and is the source of the Lund. Perhaps as much as 2 

km of tuff and intercalated landslide breccia are exposed along the 

northwest margin of the White Rock caldera, whose estimated area 

was 2,000 km2. Pyroxene andesite lava flows very locally cap the tuff.

3
VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

INTRODUCTION

Since the pioneering efforts of J. Hoover Mackin and 

his students in the 1950's and early 1960's, the crystalrich, dacitic tuffs1 of the Needles Range Group of 

Oligocene age have been recognized as one of the most 

widespread and voluminous sequences of Cenozoic ashflow deposits in the Western United States. Mackin 

(1960; see also 1963) and Cook (1965) noted that these 

lithologically distinctive rocks crop out from the central High Plateaus of Utah westward across the eastern 

Great Basin into central southern Nevada (fig. Al). 

Their areal extent was estimated to be at least 

34,000 km2 and their volume to be 6,500 km3.

A direction for future investigations of the Needles 

Range tuffs was indicated in a statement by Mackin 

(1963, p. 77): "* * * the significant point is differences 

in thickness, lithology, and distribution of different 

Needle Range ignimbrites suggest that they may have 

been erupted in different places. The answer to this and 

most other problems relating to the Needles Range Formation depends on member-by-member mapping." Virtually all areas in Utah where Needles Range units occur 

have now been mapped, mostly at a scale of 1:24,000, 

under the auspices of the U.S. Geological Survey. 

Thomas A. Steven, Peter D. Rowley, John J. Anderson, 

and Charles G. Cunningham have been responsible for 

much of the effort in the High Plateaus and into the 

easternmost Great Basin, and we, along with Lehi F. 

Hintze, have had responsibility in western Utah. Adjacent parts of Nevada have not been mapped in comparable detail, but reconnaissance work in these areas 

by Cook (1965), Gromme and others (1972), Ekren and 

others (1977), and Best (unpublished) provides important clues to the distribution and types of tuff and their 

possible sources. Available isotopic ages bracket the age 

of the Needles Range units between about 33 and 

28 m.y., and paleomagnetic studies have shown that all 

but one of the tuff sheets the Wah Wah Springs have 

normal magnetic polarity (Gromme and others, 1972; 

Shuey and others, 1976).

The purpose of this report is to summarize what is 

currently known regarding the lithologic character, 

thickness, distribution, and source of the units in the 

Needles Range Group, and to revise their stratigraphic 

nomenclature.

ACKNOWLEDGMENTS

This study would have been impossible without the 

fruitful insights, patient encouragement, and unflagging

1Unless otherwise indicated, the term "tuff' in this chapter implies lapilli ash-flow tuff.

logistical efforts of Thomas A. Steven. Lehi F. Hintze 

first introduced the senior author to the geology of the 

eastern Great Basin in the late 1960's and has continued 

to provide valuable assistance and a listening ear ever 

since. Bart J. Kowallis of Brigham Young University 

kindly provided fission-track ages. Many other geologists too numerous to mention here have provided 

assistance and constructive discussions over the years.

GENERAL GEOLOGY

The inception of Cenozoic volcanism in southwestern 

Utah approximately 34 m.y. ago followed a long period 

of erosion that had begun with the warping, folding, and 

thrusting of pre-Tertiary sedimentary strata during the 

Sevier and Laramide orogenies (Rowley and others, 

1979). The rocks from this earliest volcanic episode have 

calc-alkalic affinities and are discontinuous and widely 

scattered; locally they occur in east-west paleovalleys 

carved into sedimentary strata to depths of a few hundred meters, rarely to as much as a kilometer. Into one 

of these paleovalleys, at Sawtooth Peak in the southern 

Mountain Home Range, 300 m of ash-flow tuff with 

abundant phenocrysts of quartz, plagioclase, biotite, 

and minor sanidine and hornblende was deposited. This 

tuff was designated the Sawtooth Peak Formation by 

Conrad (1969). The location of his type section, 

Sawtooth Peak (Best and Hintze, 1980a), is here clarified as in NEV4 sec. 15, T. 28 S., R. 18 W. A potassiumargon age on biotite determined in 1983 by S. H. Evans, 

Jr., at the University of Utah is 33.5±1.2 m.y., and a 

fission-track age on zircon from the same sample is 

33.6±1.8 m.y. as determined in 1984 by Bart Kowallis 

at Brigham Young University. The sample was collected at lat 38°25'38" N. and long 113°5113" W.

Volcanic rocks of the Needles Range Group, deposited 

next, flooded the landscape to depths as great as a 

kilometer and filled source calderas to depths of at least 

three kilometers. After deposition of these volcanic 

rocks, much of southwestern Utah and southeastern 

Nevada must have been a rather featureless plain, 

because densely welded trachytic tuffs of the Isom Formation generally only a few tens of meters thick overlie 

the Needles Range Group throughout the region. However, along the northern margin of the Escalante Desert 

these Isom tuffs (average K-Ar age about 26.0 m.y.; 

Fleck and others, 1975, adjusted for new decay constants of Dalrymple, 1979) are hundreds of meters thick 

and locally contain large compacted pumice blocks with 

secondary flowage features, suggesting a nearby source 

(Rowley and others, 1979; Best, 1987).

Early Miocene ash-flow tuffs of the Quichapa Group 

(Williams, 1967) are virtually absent north of lat. 38 ° N.,
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP

39'

Beers Tunne.

f% Spring ~-\ < J

xl I 

Sawtooth''^ T ;

Peak i 3 .sTd

K,,n ff\\ ^Buckhorn ^<5

Is TOV^T ia?"*" 

(@N'K)

FIGURE Al. Location of features referred to in text. Elevations in feet (1 ft = 0.3048 m).

except for the densely welded Bauers Tuff Member of 

the Condor Canyon Formation, which is a thin but 

widespread sheet. However, the Quichapa Group and 

other associated Miocene rocks form a thick sequence 

to the south and especially to the southwest, where the 

sources of the regional ash-flow sheets probably lay in 

the Caliente cauldron complex (Ekren and others, 1977; 

Williams, 1967). North of the Escalante Desert, episodic 

Miocene magmatism erupted two bimodal silicic-mafic 

associations from many local vents without forming a 

caldera (Best and others, this volume, chap. B). A period 

of east- to northeast-trending extensional block faulting 

accompanied the early Miocene bimodal activity 

23-18 m.y. ago.

STRATIGRAPHIC NOMENCLATURE

Crystal-rich dacite tuffs of Oligocene age have long 

served as key stratigraphic markers in southwestern 

Utah and southeastern Nevada because of their wide

geographic extents and distinctive compositions. Mapping by many geologists in southwestern Utah has 

distinguished four widespread crystal-rich ash-flow 

units, which were included as members in the Needles 

Range Formation of Mackin (1960). As summarized by 

Best and others (1973), these stratigraphic units are, 

from oldest to youngest, Cottonwood Wash, Wah Wah 

Springs, Lund, and Wallaces Peak. In southeastern 

Nevada, Cook (1965) recognized as many as five distinctive Needles Range units, three of which are the Cottonwood Wash, Wah Wah Springs, and Lund.

As originally described by Mackin (1960), the type 

locality of the Needles [sic] Range Formation was the 

Needle Range in western Utah. This geographic designation was used on all maps published by the U.S. 

Geological Survey until 1974. Since that time, the 

southern part of the Needle Range has been called the 

Indian Peak Range, and the northern part the Mountain Home Range. We will use the former name when 

the whole mountain range is being discussed, and the 

newer names in referring to specific parts.
VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

CHANGE IN STRATIGRAPHIC RANK

Although Mackin's original definition of the Needles 

Range Formation included only crystal-rich dacite tuffs 

(Best and others, 1973), one of us (Grant) realized in the 

late 1970's that associated crystal-poor rhyolite tuffs 

were integral parts of the Needles Range magma 

system; some are interstratified with the crystal-rich 

tuffs and all share source areas with the crystal-rich 

tuffs. Two compositional cycles can be recognized in the 

erupted parts of the Needles Range magma system, 

each beginning with crystal-poor rhyolite and culminating with a greater volume of crystal-rich dacite 

(fig. A2). It is now apparent that the stratigraphic rank 

of the whole assemblage of apparently cogenetic tuffs 

and minor lava flows should be that of a group, so that 

each major unit is a formation that has its own interpretable history of origin and emplacement.

The Needles Range Group thus defined consists in 

ascending order of the Escalante Desert Formation 

(newly assigned to group), the Cottonwood Wash Tuff, 

the Wah Wah Springs Formation, the Ryan Spring Formation (new name), and the Lund Formation. Our 

justification for omitting the Wallaces Peak Tuff from 

the Needles Range Group is outlined in the next section. The new formational status of the Cottonwood 

Wash, Wah Wah Springs, and Lund applies wherever 

they are exposed. The type locality and the upper and 

lower boundaries of the Cottonwood Wash Tuff are 

those given by Best and others (1973) for its prior status 

as a member unit. Type sections and boundaries of the 

chiefly rhyolitic Escalante Desert and Ryan Spring Formations are described in later sections.

The development of the Indian Peak caldera, which 

was the source of the Wah Wah Springs Formation, can 

be traced by dividing this formation into two members. 

The outflow tuff member of the formation is here defined as the ash-flow tuff sheet that lies almost entirely 

outside the limits of the caldera and whose catastrophic 

eruption led to initial caldera collapse. Only a few remnants of this member are found inside the topographic 

rim of the caldera. The outflow tuff member includes 

all of those rocks formerly assigned by Best and others 

(1973) to the Wah Wah Springs Tuff Member of the 

Needles Range Formation of Mackin (1960); definitions 

of the type locality, upper and lower boundaries, and 

lithologic character of the previous member now apply 

to the newly defined outflow tuff member. The intracaldera member of the formation will be defined and 

described in a later section.

The White Rock caldera, the source of the Lund Formation, has only recently (1984) been discovered; most 

of its definition, as well as detailed elucidation of constituent members of the Lund Formation, must await

Mackleprang \ 

Tuff Member \

FIGURE A2. Stratigraphic relations and nomenclature of the 

Oligocene Needles Range Group.

future mapping in Nevada. The tuff member of the 

Lund Formation encompasses rocks formerly assigned 

by Best and others (1973) to the Lund Tuff Member of 

the Needles Range Formation of Mackin (1960); the type 

section, the definitions of upper and lower boundaries, 

and the lithologic character of the previous member now 

apply to the newly defined tuff member of the Lund Formation. The andesite member will be defined in a later 

section in this chapter.

ABANDONMENT OF THE 

WALLACES PEAK TUFF MEMBER

Best and others (1973) named the youngest member 

of the Needles Range Formation of Mackin (1960) the 

Wallaces Peak Tuff Member for exposures at Wallaces 

Peak on the east flank of the Wah Wah Mountains, 

45 km west of Milford. It is a crystal-rich dacite tuff 

with unusually large (as much as 4 mm long), euhedral 

plagioclase phenocrysts and is mineralogically similar 

to the newly defined outflow tuff member of the Wah 

Wah Springs Formation, except that it has less quartz 

and no augite. Unlike the outflow tuff member, though, 

it has normal magnetic polarity. Although the Wallaces 

Peak is not isotopically dated, its stratigraphic position 

in western Utah between the Lund Formation and the 

Isom Formation fixes its age between 27.9 and 26.0 m.y.
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP

Unlike the other Needles Range units, it appears to have 

a source well east of the Indian Peak caldera complex. 

Indeed, westernmost exposures of the Wallaces Peak 

Tuff are along the eastern flank of the Wah Wah Mountains. Its easternmost known exposure is in Minersville 

Canyon, 25 km southeast of Mttford, where it lies between the Lund and Isom Formations.

Examination of the Three Creeks Tuff Member of the 

Bullion Canyon Volcanics, which is widely exposed in 

the Marysvale volcanic field of south-central Utah (Cunningham and others, 1983), indicates that the Wallaces 

Peak Tuff Member is the same unit. The type area of 

the Three Creeks Tuff Member is in Clear Creek Canyon, 20 km northwest of Marysvale, where it forms a 

thick compound cooling unit in and near its source trapdoor caldera (Steven and others, 1979; Steven, 1981). 

This thick section of tuff was believed by Mackin (1963) 

to be his Needles Range Formation. Subsequently, 

Caskey and Shuey (1975) showed that only the lowermost part had the requisite reverse magnetic polarity 

and mineralogical composition to be the then recognized 

Wah Wah Springs Tuff Member. Most of the overlying 

section, with normal polarity, was perceived by Caskey 

and Shuey (1975) to be of local derivation and was 

named by them the Clear Creek Tuff Member. As this 

name was preempted by prior use, Steven and others 

(1979) renamed the unit the Three Creeks Tuff Member 

of the Bullion Canyon Volcanics.

The most striking megascopic similarity between the 

Wallaces Peak Tuff Member, recognized in western 

Utah, and the Three Creeks Tuff Member, in central 

Utah, is their content of abundant large, euhedral 

plagioclase phenocrysts. Both have normal magnetic 

polarity and similar mineralogical composition (table 

Al). Seven isotopic ages on the Three Creeks Tuff scatter widely between 31.4 and 26.2 m.y. (Steven and 

others, 1979) but do show that the unit is older than 

the 26.0-m.y.-old Isom Tuff, which overlies the Wallaces 

Peak Tuff in western Utah.

Because the Wallaces Peak Tuff Member of western 

Utah and the Three Creeks Tuff Member of central 

Utah appear to be the same unit and because the latter 

has a demonstrated source far distant from the sources 

of the other units of the Needles Range Group, the 

Wallaces Peak Tuff is abandoned.

GENERAL COMMENT ON 

CRYSTAL-RICH DACITE TUFF UNITS

All three of the crystal-rich dacite ash-flow tuff units 

of the Needles Range Group (table Al) contain approximately 40 percent crystals, and about one-half of these 

are complexly zoned, twinned plagioclase from 0.5 to

TABLE Al. Modal composition of samples of the Three Creeks Tuff 

Member of the Bullion Canyon Volcanics

[About 1200 points were counted through one thin section per sample. All samples contain 

trace amounts of apatite and zircon. Multiple samples from one locality are Ksted in order 

of ascending stratigraphic position. The relatively crystal-poor sample from south of 

Wallaces Peak is from the basal vitrophyre]

Constituent

Clear Creek

Canyon near

type area

Minersville Wallaces South of 

Wallaces

Canyon 1 Peakl 2 peak 1

Matrix 3 -

Plagioclase

Hornblende

Biotite -

Fe-Ti oxide

Quartz 

47

40

8

3

1

<1

52

33

10

3

2

<1

51

33

10

3

2

1

61

26

9

2

2

<1

55

26

11

5

1

2

60

23

11

H

1

1

58

29

6

5

1

1

66

17

12

3

2

<1

49

36

8

4

1

1

locality of formerly designated Wallaces Peak Tuff.

2 Best and others (1973).

3 Includes lithic fragments.

3 mm long. Combinations of quartz, biotite, and hornblende constitute most of the remaining crystals. All 

of the tuffs contain generally less than 3 percent total 

augite and equant Fe-Ti oxides, and trace amounts of 

apatite and zircon. Foreign lithic fragments in the tuffs 

are rare, except for volcanic clasts in tuff of the intracaldera member of the Wah Wah Springs Formation. 

Lapilli and local blocks of light-colored pumice are prominent and contrast with the gray, red, pink, or brown 

ashy matrix of the dacite tuffs. On an anhydrous basis, 

the rocks contain 65-69 weight percent SiO2.

Thick, simple cooling units of the Needles Range 

dacite tuffs near their sources typically have a gray to 

black, densely welded vitrophyre from 1 to 10 m thick 

near the base. This is overlain by red-brown, slightly 

devitrified tuff, whose degree of welding and compaction decreases upward. Porous, partially welded tops 

of the tuffs are pink, buff, or light gray. The actual base 

of any ash-flow deposit is rarely exposed, and the interval below the vitrophyre is generally only a few 

meters thick, at most. Where exposed, the base is generally a nonstratified tuff containing phenocrysts that 

are finer than those in other parts of the unit.

ESCALANTE DESERT FORMATION

The Escalante Desert Formation is herein made a part 

of the Needles Range Group because the magmas that 

created it were derived from essentially the same area, 

and, we believe, from the same magma system that produced the other formations in the group. The Escalante 

Desert Formation was named and described by Grant 

(1978) for a sequence of crystal-poor, lithic-rich, rhyolite 

ash-flow tuff units underlying the Needles Range Formation of Mackin (1960) and overlying a local unnamed
8 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

conglomerate at the type section on the northeast flank 

of hill 6535, sec. 6, T. 32 S., R. 14 W., in the Lund, Utah, 

71/2-minute quadrangle (Grant and Best, 1979b). At this 

locality near the southeastern end of the Wah Wah 

Mountains, the Cottonwood Wash Tuff is absent and 

the outflow tuff member of the Wah Wah Springs Formation rests directly on a local bed of volcanic conglomerate included within the Escalante Desert Formation. 

Mapping in nearby areas has disclosed the presence 

of andesite and rhyolite lava flows that interfinger with 

the rhyolite tuffs of the Escalante Desert Formation 

(table Al). Accordingly, the Escalante Desert Formation is here expanded to include a local unnamed 

andesite flow member, a rhyolite lava flow member, and 

minor epiclastic deposits. Because the type section of 

Grant (1978) for the formation does not include the lava 

flow units, a principal reference section for the formation is established about 8 km northwest of his type section in sees. 21, 22, 27, and 28, T. 31 S., R. 15 W., in 

the northeast corner of the Mountain Spring Peak, 

Utah, 7 1/2-minute quadrangle (Best, 1979; Best, Morris, 

and others, 1987), where the Escalante Desert Formation is several hundred meters thick and overlies Cambrian carbonate rocks and local unnamed Tertiary 

conglomerates and underlies the outflow tuff member 

of the Wah Wah Springs Formation. An additional 

reference section that is designated here contains the 

upper, clastic member as well as the two rhyolite tuff 

members but lacks the andesite and rhyolite lava flow 

members. It lies in the SWVa sec. 1, T. 29 S., R. 19 W., 

along the unimproved dirt road between Ryan Spring 

and Indian Peak in the Miners Cabin Wash, Utah, 

7 1/2-minute quadrangle in the northern Indian Peak 

Range (fig. A3) (Best and others, 1979; Best, Hintze, 

and Holmes, 1987). Here, the Escalante Desert Formation overlies Ordovician carbonate rocks and is overlain 

by landslide and fault breccias of the intracaldera 

member of the Wah Wah Springs Formation formed 

during subsidence of the Indian Peak caldera.

MARSDEN TUFF MEMBER

The basal member of the Escalante Desert Formation 

is herein named the Marsden Tuff Member for its occurrence at Marsden Spring (fig. Al), which is also designated the type section, in sec. 25, T. 31 S., R. 15 W., 

in the Lund, Utah, 7 1/2-minute quadrangle (Grant and 

Best, 1979b). There, the member rests on a local unnamed conglomerate and consists of three cooling units 

that formed the lower member of the Escalante Desert 

of Grant (1978) and are altogether about 110 m thick. 

It is overlain by the Lamerdorf Tuff Member. The 

Marsden consists of white to pale-gray, green, orange,

and pink, densely welded ash-flow tuff with less than 

6 percent phenocrysts (decreasing downward) of sodic 

plagioclase, quartz, and a trace of biotite, hornblende, 

and iron-titanium oxides. The tuff is generally easily 

eroded and poorly exposed because of pervasive, closely spaced fractures. Weak alteration to sericite and to 

carbonate and epidote minerals is widespread. Fragments of gray carbonate rock, green phyllite, and white, 

red, pink, and purple orthoquartzite are characteristic 

of these rocks and increase in abundance downward to 

as much as 12 percent near the base of the member. The 

quartzite clasts were probably derived from the thick 

upper Precambrian to Lower Cambrian quartzites that 

underlie much of southwestern Utah. At Marsden 

Spring the lithic fragments in the member are as much 

as 6 cm in diameter. In the reference section for the 

member designated in this report as north of Indian 

Peak (fig. A3) in SW1A sec. 1 and sec. 12, T. 29 S., R. 19 

W. clasts of carbonate rock and quartzite are locally 

as much as 15 cm across. At the type section of the 

member and westward for a few kilometers a local 

crystal-rich, plagioclase-biotite tuff several meters thick 

forms the base of the unit.

RHYOLITE FLOW MEMBER

The Marsden Tuff Member is generally overlain by 

either rhyolite or andesite lava flow members. Locally, 

the rhyolite flows interfinger with the tuff, as between 

Ryan Spring and Indian Peak (fig. A3). The rhyolite 

flow member consists of flow-layered lava that is 

variegated red, purple, brown, or rarely gray and contains 5-10 percent phenocrysts of plagioclase, quartz, 

and minor biotite and hornblende. The rhyolitic rocks 

contain about 73 weight percent SiO2 on an anhydrous 

basis.

ANDESITE FLOW MEMBER

Andesite lava flows extruded from many vents 

throughout the Needle Range and southern Wah Wah 

Mountains generally lie above the Marsden Tuff 

Member and the rhyolite flow member but locally intertongue with them. These lava flows are here informally designated as the andesite flow member of the 

Escalante Desert Formation. Locally, the lava flows are 

as much as 350 m thick, as in SE1/^ sec. 15, T. 31 S., 

R. 15 W., in the Mountain Spring Peak, Utah, 

7 1/2-minute quadrangle in the southeastern Wah Wah 

Mountains (Best, 1979; Best, Morris, and others, 1987), 

where the member lies between the Marsden and
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 9

Lamerdorf Tuff Members of the E sealante Desert Formation. The andesitic rocks have phenocrysts of plagioclase, augite, and hypersthene in a partly glassy matrix 

containing the same minerals plus iron-titanium oxides; 

they have a low Mg/Fe ratio and about 59 weight percent SiO2.

LAMERDORF TUFF MEMBER

The Lamerdorf Tuff Member of the Escalante Desert 

Formation was named and described by Campbell 

(1978). As used here, the member consists of as many 

as three cooling units that are somewhat more crystalrich than the Marsden Tuff Member. The type locality 

of the member lies in the central part of the Lamerdorf 

Peak, Utah, 71/2-minute quadrangle (Abbott and others, 

1983), in sees. 1, 2, and 12, T. 29 S., R. 16 W., and sees. 

5, 7, and 8 in T. 29 S., R. 15 W., where two cooling units 

are separated by lava flows of the andesite flow member 

and have a total thickness of about 100 m. The densely 

to partially welded tuffs of the Lamerdorf are mottled 

gray, orange, red, purple, or buff and contain 10-15 percent plagioclase, 1-3 percent biotite and trace amounts 

of hornblende, quartz, sanidine, and Fe-Ti oxides. Lightcolored pumice lapilli and dark volcanic fragments, 

which compose 5-25 percent of the tuff, cause the mottling. The upper cooling unit has about 69 weight percent SiO2, and the lower has about 71 percent, on an 

anhydrous basis. A biotite from the Lamerdorf has a 

potassium-argon age of 32.3±1.1 m.y., determined by 

H. H. Mehnert in the U.S. Geological Survey's Denver 

laboratories; the sample was collected at lat 38°18'45" 

N. and long 113°29'47" W.

BEERS SPRING MEMBER

Based on his work in the central Needle Range, Conrad (1969) defined a heterogeneous sequence of tuffs, 

sandstones, and conglomerates lying below the Needles 

Range Formation of Mackin (1960) as the Beers Spring 

Formation. Constituent units, in ascending order, were 

(1) a densely welded ash-flow tuff with phenocrysts of 

plagioclase and pyroxene, (2) a coarse sandstone and 

conglomerate, and (3) crystal-poor tuff overlain by sandstone and conglomerate. Subsequent mapping by Best 

(1976) and Campbell (1978; see also Best, Hintze, and 

Holmes, 1987) showed that Conrad's member 1 is the 

Isom Formation and member 2 is a local epiclastic 

deposit that contains clasts of the Isom Formation and 

therefore cannot predate the Needles Range Formation 

of Mackin (1960). Hence, Campbell (1978) abandoned 

Conrad's members 1 and 2 and made his member 3 the

Beers Spring Member of the Escalante Desert Formation. This revision is adopted here.

The type section of the Beers Spring Member lies in 

sees. 15 and 16, T. 26 S., R. 18 W., around Beers Tunnel Spring (fig. Al) in the Halfway Summit, Utah, 

7 Ms-minute quadrangle (Best and Hintze, 1980b). The 

Beers Spring Member here underlies the Cottonwood 

Wash Tuff and overlies the Sawtooth Peak Formation 

or the Lamerdorf Tuff Member of the Escalante Desert 

Formation. At its type section, the Beers Spring 

Member is about 50 m thick. The lower part of the 

member is a loosely welded, white to pink, somewhat 

lithic-rich, ash-flow tuff with about 10 percent phenocrysts of plagioclase, quartz, and biotite; the top is a 

conglomerate or volcanic debris flow with clasts of 

greenish-brown porphyritic andesite.

Elsewhere in the Needle Range and the Wah Wah 

Mountains, the Beers Spring Member consists chiefly 

or exclusively of well-sorted, evenly bedded, friable, 

greenish-gray sandstone, which is generally several 

meters thick but locally ranges up to more than 100 m. 

A reference section for this member is selected 1-2 km 

south of Ryan Spring, just east of and along an unimproved road leading to Indian Peak in sec. 1, T. 29 S., 

R. 19 W., where the sandstone is about 60 m thick and 

lies between the Lamerdorf Tuff Member of the 

Escalante Desert Formation and breccias of the intracaldera member of the Wah Wah Springs Formation.

COTTONWOOD WASH TUFF

The Cottonwood Wash Tuff was formally defined by 

Best and others (1973) as a member of the Needles 

Range Formation of Mackin (1960) but is here elevated 

to formation rank within the Needles Range Group.

The Cottonwood Wash Tuff appears to be a simple 

cooling unit of crystal-rich, dacitic lapilli ash-flow tuff 

containing about 25 percent plagioclase; large books of 

biotite as much as 6 mm in diameter; sparse broken, embayed quartz phenocrysts that are almost as large; and 

minor hornblende, green augite, and equant irontitanium oxides.

Exposures of moderately welded ash-flow tuff near 

the northern distal margin of the Cottonwood Wash 

Tuff, northeast of Crystal Peak, Utah, are locally 

underlain by compositionally similar air-fall and surge 

deposits several meters thick, which are also included 

in the unit. Tuffaceous sedimentary deposits in the Mercury quadrangle in southwestern Nevada include as 

much as 9 m of reworked tuff, which contains abundant 

phenocrysts of plagioclase, quartz, and biotite as much 

as 5 mm in diameter, has a revised potassium-argon age
10 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

Paleozoic sedimentary 

rocks, undivided

<:[^^^

Outer ting fault of

Indian Peak caldera,

major ring fault of older

Pine Valley caldera

EXPLANATION

>:Qa-',- Quaternary alluvium

Tertiary sedimentary and volcanic rocks Mostly

volcanic in southeast and sedimentary in north 

Lund Formation

Ryan Spring Formation

Intracaldera member 

Twip, granodiorite porphyry 

Twib, breccia 

Twir, rhyolite 

Twit, tuff

Outflow tuff member

Cottonwood Wash Tuff

Beers Spring and Lamerdorf 

Tuff Members

Marsden Tuff Member 

and andesite flow member

Rhyolite flow member 

Sawtooth Peak Formation 

Silurian and Ordovician rocks 

Cambrian rock

Wah Wah

- Springs

Formation

I

Escalante

Desert

Formation

Needles

- Range

Group OLIGOCENE

 Contact

 Fault Dotted where concealed;

bar and ball on downthrown side

 ' Strike and dip of bedding

 Strike and dip of foliation

FIGURE A3. Geologic map (facing) and idealized cross section (above) of the northeastern segment of the Indian Peak caldera in the easttilted northern Indian Peak Range and southernmost Mountain Home Range (central Needle Range) showing topographic rim and shelf, 

ring faults, and caldera fill. Cross section is more or less parallel to the strike of the volcanic units and shows the topographic wall, 

ring faults, and caldera fill prior to resurgent uplift. Minor faulting that is possibly related in part to resurgence is omitted.
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP

113°52'30"

11
12 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

of 30.1 ±0.9 m.y. (Barnes and others, 1982), and is probably correlative with the Cottonwood Wash Tuff. These 

are the only two localities known where the Cottonwood 

Wash Tuff includes deposits not formed from ash-flows. 

The Cottonwood Wash Tuff has an average age of 

30.6 m.y. based on four potassium-argon determinations 

(table A2).

WAR WAR SPRINGS FORMATION

The Wah Wah Springs Tuff Member of the Needles 

Range Formation was named by Mackin (1960) and was 

retained with that stratigraphic rank by Best and 

others (1973). Subsequent mapping in the Needle Range 

and White Rock Mountains has shown that the unit 

consists of two cogenetic units related to evolution of 

the source caldera of the formation. Accordingly, the 

Wah Wah Springs Tuff Member is here elevated in rank 

to formation, the appellation "Tuff" is dropped, and the 

formation is made part of the Needles Range Group. 

The two informal members within the Wah Wah 

Springs Formation are here defined to be the outflow 

tuff member and the intracaldera member.

No single exposure or group of nearby exposures 

shows both members of the Wah Wah Springs Formation together. The nature of the large Indian Peak 

caldera precludes their close juxtaposition. Accordingly. a type locality for the formation showing stratigraphic relations of constituent members is the entire 

Miners Cabin Wash, Utah, 71/2-minute quadrangle and 

sees. 21, 27, and 28, T. 29 S., R. 18 W., in the adjoining 

Buckhorn Spring 71/2-minute quadrangle (figs. Al and 

A3; Best and others, 1979; Best, Hintze, and Holmes, 

1987).

OUTFLOW TUFF MEMBER

The type locality of the Wah Wah Springs Tuff 

Member of the Needles Range Formation of Mackin 

(1960) and Best and others (1973) represents only the 

newly defined informal outflow tuff member of the Wah 

Wah Springs Formation. The stratigraphic boundaries 

of the new informal member are the same as those of 

Mackin's Wah Wah Spring Tuff Member. The deposit 

is generally a simple cooling unit, wherever exposed.

In addition to abundant plagioclase, the outflow 

member contains prominent hornblende, lesser biotite, 

and minor quartz, green augite, and equant irontitanium oxides (Dinkel, 1969).

The average of 16 potassium-argon ages for the outflow tuff member is 29.5 m.y., which is only slightly

TABLE A2. Isotopic ages ofdacite tuffs in the Needles Range Group

[Older determinations adjusted to new decay constants (Dalrymple, 1979). All ages are by 

K-Ar method except for one fission-track age on zircon]

Source of data Material Age±2o 

(m.y.)

Lund Formation

Armstrong (1970); sample 83^B BiotiteH. H. Mehnert (unpub. data, Biotite1982). 1 

M. G. Best and B. J. Kowallis Zircon 

(unpub. data, 1984).

Average27.9±1.2 

27.9±1.0

27.8±1.0 

27.9

Wah Wah Springs Formation

J. D. Obradovich

(unpub. data, 1965). 2

Armstrong (1970):

Sample 159B -

Sample 159G -

Sample 102C -

Sample 170B -

Sample 183B -

Lemmon and others (1973) 

Fleck and others (1975) -

T. A. Steven

(unpub. data, 1981). 3

Noble and McKee (1972) -

Average -

Biotite 

Hornblende 

Biotite 

Hornblende 

Hornblende ~ 

-- Biotite 

Glass -

-- Biotite with

hornblende.

 Biotite -----

Hornblende

with biotite.

-- Biotite 

Hornblende 

-- Biotite 

Biotite 

Hornblende 

-- Biotite 

30.5±1.2

30.6±1.2

28.7±1.1

29.8±1.2

27.2±1.1

30.0±1.2

28.1|±1.2

31 -5±1.2

30.4±1.2

29.2±1.6

30.1±1 .2

30.0±1.2

29.5±1.0

28.il±1.0

28.3±1.7

28.7±1.3

29.5

Cottonwood Wash Tuff

Armstrong (1970):

Sample 101E 

Sample 83^AB 

Sample 83HAH 

nioi/i te

 Biotite -

 Biotite -

-- 30.4±1.2

- 31.5±1.2

- 31.4±1.8

-- 30.6

'Sample from east flank of Wah Wah Mountains at lat 

38°21'32" N., and long 113°31'i»0n W.

Determined on three samples collected by R. D. Hose 

in the Confusion Range. See Fleck and others (1975).

3Analyses by H. H. Mehnert on a sample from the 

Crystal Peak area, Utah.

less than the widely quoted age of 29.7 m.y. from the 

study of Armstrong (1970), which used fewer determinations and older decay constants (table A2).

INTRACALDERA MEMBER

The informal intracaldera member of the Wah Wah 

Springs Formation includes dacite tuff, breccia, intrusive granodiorite porphyry, and minor rhyolite and 

andesite, which were emplaced within the Indian Peak
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 13

caldera during and just after its collapse. The type 

locality for the member is near Indian Peak in sees. 

19-21 and 27-30, T. 29 S., R. 18 W., in the southern 

Miners Cabin Wash and Buckhorn Spring 7V£-minute 

quadrangles (fig. A3; Best and others, 1979; Best, 

Hintze, and Holmes, 1987) and sees. 8-10, T. 30 S., R. 18 

W., in the adjacent Atchison Creek and Pinto Spring 

7V2-minute quadrangles (Grant and Best, 1979a; Best, 

Grant, and others, 1987). Tuff and lava flows of the 

Escalante Desert Formation underlie the intracaldera 

member; tuff of the Ryan Spring Formation overlies the 

member. In the Indian Peak Range the intracaldera 

member is at least 2 km thick.

The intracaldera member is a compound cooling unit 

consisting of crystal- and lithic-rich, dacite ash-flow tuff. 

It is orange brown to olive gray, densely welded, and 

generally contains about 10 percent lapilk'-size clasts of 

dark volcanic rock; locally near the northeastern segment of the inner caldera ring fault (fig. A3) these clasts 

are as much as 2 m in diameter and compose half or 

more of the tuff. Phenocrysts constitute almost half of 

the tuff and are mostly plagioclase with lesser amounts 

of hornblende, biotite, quartz, augite, and iron-titanium 

oxides. The amount of quartz is slightly more than in 

the outflow tuff member.

Breccias south of the inner ring fault, near Indian 

Peak, are mostly monolithologic accumulations of 

volcanic fragments, chiefly of the Lamerdorf Tuff 

Member and Cottonwood Wash Tuff, whereas near Arrowhead Pass (fig. Al) clasts are Paleozoic sedimentary 

rocks and tuff from the Sawtooth Peak Formation. 

These breccias represent debris slides and talus deposits 

shed off the unstable scarp of the ring fault. Between 

the northeastern segment of the ring fault at Indian 

Peak and the topographic wall of the caldera 10 km to 

the north the intracaldera member consists of commonly cataclastic monolithologic breccias that were pervasively sheared and pulverized. These breccias are 

believed to have formed in slabs that slid from the 

northward-retreating caldera wall into the depression 

along low-angle detachment surfaces.

Local bodies of rhyolite and andesite occur within the 

sequence of tuff and breccia that fills the inner part of 

the caldera; they may be sills, lava flows, or landslide 

breccias.

Granodiorite porphyry that intrudes the intracaldera 

tuff is olive to brownish gray and has the same phenocrysts as the tuff, except that they are somewhat larger 

and unbroken. Propylitically altered rocks are widespread. The main body of intrusive rock, 7 km south 

of Indian Peak (fig. A3), crops out over an area of at 

least 12 km2 as described by Grant (1979). The porphyry locally forms dikes and plugs along the inner ring 

fault of the Indian Peak caldera where, in places, it is

vertically foliated and appears to grade texturally into 

intracaldera tuff. In other places, where the intrusions 

cut Paleozoic carbonate rocks, the rock is more 

leucocratic and variable in texture.

RYAN SPRING FORMATION

Two cooling units of crystal-poor, lithic-rich rhyolite 

ash-flow tuff, as well as local pyroxene andesite and 

rhyolite lava flows, lie between the intracaldera member 

of the Wah Wah Springs Formation and the younger 

Lund Formation within the topographic depression of 

the Indian Peak caldera. The two rhyolite cooling units 

are quite similar in composition to the ash-flow tuff 

units in the older Escalante Desert Formation; only 

differences in color, nature of lithic fragments, and 

stratigraphic position allow the two doublets to be 

distinguished in the field. As proposed by Rauch (1975), 

these younger rhyolite tuff units and associated lava 

flows are here formally designated the Ryan Spring Formation. At its type section west and north of Ryan 

Spring in NEV* sec. 35, and NWV* sec. 36, T. 28 S., R. 19 

W., in the Miners Cabin Wash quadrangle (fig. A3; Best 

and others, 1979; Best, Hintze, and Holmes, 1987), the 

formation consists of the two tuff members and is as 

much as 560 m thick, overlies breccias of the intracaldera member of the Wah Wah Springs Formation, 

and underlies the Lund Formation. Because this type 

section does not include the rhyolite and andesite lava 

flow members, a principal reference section for the formation is designated 2-8 km south of Arrowhead Pass 

in sees. 19, 30, and 31, T. 31 S., R. 17 W., and sees. 24 

and 25, T. 31 S., R. 18 W., in the central part of the 

Steamboat Mountain, Utah, 7 1/2-minute quadrangle 

(Best, Grant, and others, 1987). In this locality the formation is a discontinuous sequence consisting of the 

tuff members, pyroxene andesite flows, and capping 

altered rhyolite flows of the andesite and rhyolite flow 

members; the formation is underlain by altered rocks 

of the intracaldera member of the Wah Wah Springs 

Formation and to the south is overlain by the Lund 

Formation.

GREENS CANYON AND 

MACKLEPRANG TUFF MEMBERS

The lower simple cooling unit of the Ryan Spring Formation is here named the Greens Canyon Tuff Member 

for Greens Canyon, in the Miners Cabin Wash quadrangle, whose north and east slopes are in part underlain 

by the member. This unit resembles the Marsden Tuff 

Member of the Escalante Desert Formation in the
14 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

small size (generally <1 mm) and abundance of crystals 

(12 percent plagioclase, 2 biotite). However, in the 

Greens Canyon Member volcanic clasts predominate 

over carbonate and quartzite; lapilli- to small block-size 

pieces of red tuff from the Wah Wah Springs Formation are common. Weathered surfaces on the member 

typically have holes a centimeter or so in diameter surrounded by haloes lighter in color than the orange- to 

pink-brown matrix. These holes apparently formed by 

weathering of baked carbonate inclusions. The upper 

part of the member is a crystal-poor tuff containing no 

pumice lapilli or lithic fragments and locally is overlain 

by several meters of volcanic debris flow and sandstone. 

The type section is in sec. 6, T. 29 S., R. 18 W., where 

the member overlies breccias and tuffs of the intracaldera member of the Wah Wah Springs Formation 

and is overlain by the younger Mackleprang Tuff 

Member of the Ryan Spring Formation.

The younger simple cooling unit or tuff member of 

the Ryan Spring Formation is here named the 

Mackleprang Tuff Member for the old Mackleprang 

Homestead 3 km due east of Ryan Spring (fig. A3). This 

member is virtually identical in most respects to the 

Lamerdorf Tuff Member of the Escalante Desert 

Formation only position in the stratigraphic sequence 

positively distinguishes them. It is distinguished from 

the Greens Canyon Member by its larger (up to 2 mm) 

and slightly more abundant phenocrysts of plagioclase 

and biotite (14 and 3 percent, respectively) and by a 

more strongly developed compaction foliation defined 

by varicolored, flattened pumice lapilli. The member is 

a thin (less than 50 m), densely welded simple cooling 

unit at its type section, just northeast of Ryan Spring 

in NWV4 sec. 36, T. 28 S., R. 19 W. In the clifflike exposures just south of Buckhorn Spring in the quadrangle of that same name (fig. A3; Best and others, 

1979; Best, Hintze, and Holmes, 1987), 8 km east of 

the type section, the member is somewhat thicker and 

is a variably welded compound cooling unit. This locality, in sees. 34 and 35, T. 28 S., R. 18 W., is designated as a reference section for the Mackleprang. 

Here, the unit is sandwiched between the lilac gray, 

almost wholly vitric tuff of the Greens Canyon Tuff 

Member and the overlying Lund Formation. Locally, 

the Mackleprang includes well-sorted sandstone below 

and above the tuff.

LAVA FLOW MEMBERS

Nonvesicular, gray to black andesite lava flows are 

widespread in the principal reference locality for the 

Ryan Spring Formation, 2-8 km south of Arrowhead 

Pass in the central part of the Steamboat Mountain

quadrangle. These flows, containing phenocrysts of 

plagioclase, augite, hypersthene, and locally, near the 

base of the unit, hornblende, are informally designated 

as the andesite flow member of the Ryan Spring Formation. Well sorted brown sandstone occurs locally in 

the sequence of flows. The unit overlies altered tuff of 

the intracaldera member of the Wah Wah Springs Formation and interfingers with tuffs of the Ryan Spring 

Formation (fig. A2).

The andesite flow member is locally overlain in the 

area south of Arrowhead Pass by rhyolite lava flows 

that are informally designated as the rhyolite flow 

member of the Ryan Spring Formation. The rhyolite is 

varicolored and locally strongly flow layered and has 

sparse phenocrysts of plagioclase, biotite, potassiumrich feldspar, and quartz. Zircon from the rhyolite 

yielded a fission-track age of 28.4± 1.2 m.y. This determination, on a sample from lat 38°6'25" N. and long 

113°47'27" W., was made by Bart J. Kowallis at 

Brigham Young University in 1984.

LUND FORMATION

The Lund Tuff was formally defined by Best and 

others (1973) as a member of the Needles Range Formation of Mackin (1960) but is here raised in 

stratigraphic rank to a formation within the Needles 

Range Group, and the appellation "Tuff" is dropped 

because other rock types occur. Much has yet to be 

learned about the formation and its source the White 

Rock caldera through future mapping in eastern 

Nevada. A reference section for the formation, 

designated in this report, is in the southern Steamboat 

Mountain, Utah, 7 1/2-minute quadrangle (Best, Grant, 

and others, 1987) in sees. 6 and 7, T. 32 S., R. 17 W., 

and sees. 11 and 12, T. 32 S., R. 18 W., where the formation lies between the Ryan Spring and Isom Formations. There, two informal members can be recognized, 

a tuff member and an andesite flow member.

TUFF MEMBER

The tuff member of the Lund Formation is here defined to be equivalent to the Lund Tuff Member of Best 

and others (1973). The type section, stratigraphic 

boundaries, and lithologic description of the former 

Lund Tuff Member now apply to the new tuff member. 

A reference section designated for the member is in sees. 

9 and 10 and the NV4 sec. 16, T. 5 N., R. 67 EM in the 

Schoolmarm Basin, Nevada, 7 1/2-minute quadrangle, 

where it is a thick intracaldera compound cooling unit 

locally intercalated with lenses of breccia that formed
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 15

as landslides sloughed off the nearby topographic wall 

of the White Rock caldera. Clasts in the breccias are 

of Paleozoic carbonate rocks and granite of unknown 

age. The base of this thick unit is not exposed, but it 

is overlain by the Isom Formation. Thick sections of 

the tuff member within the older Indian Peak caldera 

are also compound cooling units, but outside the White 

Rock and Indian Peak calderas it is essentially a simple 

cooling unit.

The tuff member of the Lund Formation is distinguished from the other crystal-rich dacite tuffs of the 

Needles Range Group by relatively abundant (5-10 percent) quartz, more biotite than hornblende, and most 

diagnostic of all, trace amounts of sphene (Kreider, 

1970). Fragments of volcanic rock are only locally 

conspicuous in the tuff. The average isotopic age is 

27.9 m.y. (table A2).

ANDESITE FLOW MEMBER

A thick section of pyroxene andesite lava flows occurs in the type section of the Lund Formation and is 

here designated as an informal member. The lava flows 

are gray to black, nonvesicular, and locally flow-layered 

and have phenocrysts of plagioclase, augite, and hypersthene. Tuff of similar composition and lenses of brown, 

well sorted sandstone appear locally in the member. 

This unit is juxtaposed against the Ryan Spring Formation and the tuff member of the Lund Formation 

along the topographic wall of the White Rock caldera. 

An unusually thick section of the Isom Formation 

overlies the andesite flow member within the caldera 

depression.

DISTRIBUTION, VOLUME, AND SOURCES 

OF NEEDLES RANGE GROUP

Data concerning the distribution and volume of formations in the Needles Range Group have a high degree 

of uncertainty because the formations are concealed by 

younger volcanic deposits, especially in the southern 

part of the area of figure Al (see Stewart and others, 

1977), and by alluvial valley fill in at least half of the 

area of the Great Basin. Uneven pre-emption topography and local post-Oligocene uplifts, which led to local 

nondeposition and erosion of Needles Range rocks, are 

additional hinderances in determining original area! extents and volumes of the tuff sheets. Thicknesses of formations in the Needles Range Group in Nevada are still 

more uncertain because of the paucity of detailed mapping and because of possible difficulties in stratigraphic

TABLE A3. Presently exposed, known areal extents and estimated 

volumes of ash-flow tuffs of the Needles Range Group

[From figures A4 to A10 (corrected for an assumed 40 percent east-west crustal extension 

since the Oligocene). Volume of Cottonwood Wash Tuff does not include the unknown 

amount in its source caldera]

Unit

Lund Formation:

Outside caldera 

Ryan Spring Formation: 

Mackleprang Tuff Member - 

Greens Canyon Tuff Member -- 

Wah Wah Springs Formation:

Intracaldera member -

Cottonwood Wash Tuff 

Escalante Desert Formation:

Lainerdorf Tufi Member

Area 

(km 2 )

2,000 

11,000

MOO 

400

1,200 

22,000 

5,000

2,000 

1,000

Volume 

(km 2 )

Unknown 

1,600

100 

150

2,400 

1,500 

500

100 

300

Total- >6,650

*The area of the outflow tuff member of the Wah Wah 

Springs Formation eclipses that of all other tuffs.

correlation. Correcting the estimated areal extents and 

volumes of the formations for post-Oligocene east-west 

crustal extension during development of the Basin and 

Range province reduces numerical values; local 

estimates of extension range from 100 percent or more 

to 20-30 percent for the Great Basin area of Nevada 

and Utah (Stewart, 1980). Tilts of Oligocene volcanic 

deposits in the Needle Range and Wah Wah Mountains 

of southwestern Utah average close to 25 °, which implies an extension there of 50 percent. Crustal thicknesses of the eastern Great Basin and adjacent Colorado 

Plateaus (summarized in Wannamaker, 1983) indicate 

an extension of 50 to 75 percent. We will assume 40 percent in the discussion that follows.

The areal extents of the tuffs shown on figures A4-A7 

are conservatively drawn around known exposures. 

Areas listed in table A3 are reduced 40 percent from 

those measured on the figures, to correct for east-west 

crustal extension. Volume estimates in table A3 have 

been made by dividing the known areas of distribution 

into sectors in which the thickness has been estimated 

or taken from measured sections.

ESCALANTE DESERT FORMATION

The two rhyolite ash-flow tuff sheets in the Escalante 

Desert Formation are exposed in the Needle Range and 

the Wah Wah Mountains. Thick sections of propylitically altered tuff of the Marsden Tuff Member (fig. A4), 

containing abundant lapilli- and block-sized clasts of
16 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

MARSDEN TUFF 

MEMBER

FIGURE A4. Distribution and thickness of the Marsden Tuff Member of Escalante Desert Formation. Solid squares show occurrences 

of rhyolite flow member. Gray pattern shows known exposures; numbers indicate thickness in meters. Diagonally ruled area is inferred original distribution of tuff. Zeros indicate absence of member. Filled circles show structural margin of Pine Valley caldera 

from which tuff erupted. (See also cross section on fig. A3.)

sedimentary rocks, occur immediately north of Indian 

Peak and also at the southeastern end of the Wah Wah 

Mountains. These sections probably lie within a source 

caldera, whose northern margin seems to be represented 

in part by a major east-west fault that lies west of Ryan 

Spring (figs. A3 and A4). About 600 m of vertical movement took place along this fault during subsidence of 

the younger Indian Peak caldera, but the Paleozoic rock 

units across it are offset about 1,900 m. The 1,300-m 

difference in displacement is believed to reflect collapse 

of the earlier caldera during eruption of the Marsden 

Tuff Member. This conclusion is supported by deposits 

found just south of the fault: here, wedges of sandstone, 

breccia, and poorly bedded debris flows, containing 

clasts of Ordovician sedimentary rocks within matrices 

rich in volcanic material, overlie but also interfinger 

with the Marsden Tuff Member. These wedges of clastic 

material are thickest near the postulated ring fault and

pinch out 4 km to the south; they are interpreted to consist of debris wasted off the unstable wall of the caldera, 

where Ordovician rocks were exposed. No Marsden Tuff 

Member is exposed north of the fault.

The source caldera for the Marsden Tuff Member of 

the Escalante Desert Formation is here named the Pine 

Valley caldera because it straddles the valley of that 

name between the Indian Peak Range and Wah Wah 

Mountains (fig. Al).

The known areal extent of the Lamerdorf Tuff 

Member of the Escalante Desert Formation (fig. A5) is 

about the same as that of the Marsden Tuff Member, 

but its measured thicknesses and apparent volume are 

less. No direct evidence for the location of its source exists. However, the tuff partly covers the Pine Valley 

caldera, so its source could be that caldera or, possibly, 

one nested within it and now lying beneath alluviumcovered Pine Valley.
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 17

J151

LAMERDORF TUFF 

MEMBER

FIGURE A5. Distribution and thickness of the Lamerdorf Tuff Member of Escalante Desert Formation. Symbols as on figure A4.

COTTONWOOD WASH TUFF

Ash flow tuff of the crystal-rich Cottonwood Wash 

Tuff (fig. A6) has an estimated volume of about 

500 km3, based on known area! extent (table A3). For 

this volume, a fairly large caldera would be expected, 

but no direct evidence points to its specific location. In 

the Mountain Home Range, where the Cottonwood 

Wash and its basal vitrophyre are thickest, no breccias 

or even lithic clasts occur within the tuff, and no system 

of faults compatible with a fault-bounded caldera 

depression have been recognized. To the west, across 

a 40-km-wide alluvial valley, sections of the Cottonwood 

Wash in the Fortification Range are about 200 m thick, 

have a basal vitrophyre several meters thick, and are 

unique in containing sparse lapilli and small blocks of 

volcanic xenoliths. Characteristics of the Cottonwood 

Wash in both the Fortification Range and the Mountain Home Range indicate proximity to the source, as 

does their more or less central location within the known

area of the tuff sheet. It appears likely, therefore, that 

the source lies concealed beneath the intervening broad 

alluvial valley, but whether it is a fault-bounded caldera 

or a downwarped sag (compare Steven and others, 1984) 

cannot be said. In either case, an additional volume of 

tuff possibly equal to that outside the source lies buried 

within it.

WAH WAH SPRINGS FORMATION

Eruption of the widespread and voluminous outflow 

tuff member of the Wah Wah Springs Formation led 

to subsidence of the Indian Peak caldera. The minimum 

area of exposure of the outflow tuff member, as shown 

on figure A7, is about 22,000 km2; this is surely less 

than the original extent of the ash-flow deposit, because 

a significant part of the sheet is obscured by younger 

volcanic cover southwest of the Indian Peak caldera. 

The known volume (table A3) is on the order of 

1,500 km3.
VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

COTTONWOOD WASH 

TUFF

FIGURE A6. Distribution and thickness of the Cottonwood Wash Tuff. Dark-gray area is probable location of source of the tuff. Symbols as on figure A4.

In areas to the southeast and south of the Indian 

Peak caldera, partially welded outflow tuff is thin to 

locally absent. South of the Escalante Desert the 

outflow tuff is only 10-17 m thick and lies immediately below the Isom Formation or occurs within a sequence of clastic sediments and lacustrine limestones 

that has been included within the upper part of the 

Claron Formation (Blank, 1959; Rowley and others, 

1979, p. 5). An unpublished fission-track date of 

29.0± 1.7 m.y. on zircon by C. W. Naeser confirms these 

remote exposures as outflow tuff member, a conclusion 

previously drawn on the basis of phenocryst composition by Blank.

The original size and shape of the Indian Peak caldera 

cannot be determined accurately from present exposures. The northeast segment of the caldera is exposed 

in the central Needle Range, where an east-tilted and 

somewhat faulted section of rocks several kilometers 

thick around Indian Peak and northward to Sawtooth 

Peak reveals the details of the northeastern ring fault 

zone, topographic shelf and rim, and caldera fill (figs.

A3 and A7). The southeast and northwest segments of 

the caldera are indirectly constrained by exposures of 

tuff and breccia of the intracaldera member, by the 

distribution of pre-Oligocene sedimentary rocks, and by 

permissive structural relations. No control exists for its 

southwestern and western perimeter.

The Indian Peak caldera as defined by its inferred ring 

fault zone is about 70 km across east-west. Prior to eastwest crustal extension, assumed to be 40 percent, the 

original east-west dimension was about 50 km; inasmuch as the north-south dimension was at least 30 km, 

the area of the caldera was about 2,000 km2. Accommodating crustal extension makes the original outline 

of the caldera more nearly equant.

INDIAN PEAK CALDERA

This section summarizes what is known about the 

evolution and present characteristics of the source 

caldera of the Wah Wah Springs Formation.
40'

STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP

115° 114° 113°

19

WAH WAH SPRINGS 

FORMATION

EXPLANATION

Intracaldera member 

Outflow tuff member 

Inferred original extent

Ring fault of Indian Peak caldera 

Dashed where inferred

Topographic rim of Indian Peak caldera 

Dashed where inferred

Locality of paleomagnetic sample

FIGURE A7. Distribution and thickness of Wah Wah Springs Formation and location of Indian Peak caldera. Thickness values in meters. 

Open squares are sites where paleomagnetic samples were collected in Nevada by Gromme and others (1972) and in Utah by Shuey 

and others (1976). Location of the Miocene Caliente cauldron complex after Ekren and others (1977). Elevation contours at 5,500, 

7,500, 9,500, and 11,500 feet (1,676, 2,286, 2,896, and 3,505 m).

UTAH SECTOR

The most direct evidence for the Indian Peak caldera 

lies in the east-tilted central Needle Range, around Indian and Sawtooth Peaks and the area to the west (fig.

A3). North of a generally east-west line through 

Sawtooth Peak, the Lund Tuff is no more than 55 m 

thick and is locally absent between the underlying 

outflow tuff member of the Wah Wah Springs Formation and the overlying Isom Formation. To the south
20 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

of this line, in this same stratigraphic interval, is a 2-kmthick section of breccias of the intracaldera member of 

the Wah Wah Springs Formation and overlying ashflow tuffs of the Ryan Spring Formation and Lund Formation. This 2-km-deep depression, which formed after 

eruption of the outflow tuff member but before deposition of the Ryan Spring Formation, must represent the 

topographically enlarged source caldera of the Wah 

Wah Springs Formation. Although a significant normal 

fault, exposed west of Ryan Spring (fig. A3), parallels 

the east-west topographic rim, it had only about 600 m 

of offset during subsidence of the caldera. The major 

ring fault that controlled most of the caldera subsidence 

in this area lies farther to the south, 2 km north of Indian Peak, where Paleozoic sedimentary rocks and an 

overlying thick sequence of volcanic rocks of the Sawtooth Peak and Escalante Desert Formations are juxtaposed against a section of the intracaldera member of 

the Wah Wah Springs Formation that is at least 2.5 km 

thick. The intracaldera member is similarly thick for as 

much as 14 km farther south through the Indian Peak 

Range.

Breccias of the intracaldera member of the Wah Wah 

Springs Formation exposed north of the inner, major 

caldera ring fault document sloughing of landslide and 

fault masses off the unstable topographic wall of the 

deepening caldera as this escarpment retreated some 

10 km northward from the major ring fault (figs. A3 and 

A7). This topographic shelf, which was cut into older 

rock units and covered by intracaldera breccias, probably sloped into the caldera, but later resurgent uplift 

(see below) and post-Oligocene faulting have modified 

its original attitude. The breccias of the intracaldera 

member are mostly monolithologic rock types of 

volcanic units normally occurring between the outflow 

tuff member of the Wah Wah Springs Formation and 

the Marsden Tuff Member of the Escalante Desert Formation; breccia of the Cottonwood Wash Tuff is 

especially prominent. Widespread cataclastic fabrics in 

these breccias show that not all of the masses caving 

off the caldera wall were landslides or loose talus falls: 

many must have been fairly intact fault slices capable 

of generating sufficient frictional drag to create 

throughgoing seams of finely pulverized cataclastic 

material.

Within the sequence of breccias north of the inner ring 

fault are lenses of Wah Wah Springs tuff. Some of these 

lenses are apparently of the outflow tuff member, are 

densely welded, and lack the common complementary 

less welded upper part of the deposit; these may be 

decapitated fault slices derived from the northwardretreating escarpment. Other lenses of lithic-free tuff 

are neither typical outflow tuff nor intracaldera member 

and may represent local accumulations of Wah Wah

Springs type eject a that were intermittently expelled 

during caldera collapse and were quickly buried and 

densely welded during accompanying breccia 

emplacement.

The intracaldera member of the Wah Wah Springs 

Formation is well exposed for 18 km south of the inner 

ring fault in the horst that forms the backbone of the 

Indian Peak Range. In the horst, the intracaldera 

member consists of a compound cooling unit of ash-flow 

tuff with intercalated wedges of essentially monolithologic breccia, intrusive granodiorite porphyry, and 

minor rhyolite and andesite. The wedges of breccia 

generally pinch out southward within a kilometer or so 

of the inner ring fault. Collapse of the caldera, slope 

failure along the fault escarpment, and expulsion of 

ejecta were obviously episodic. The clasts in the breccia consist chiefly of recognizable pre-Wah Wah Springs 

rock types, although some wedges of breccia near the 

fault contain clasts of exotic volcanic rock several 

meters across (compare the megabreccias of Lipman, 

1976). Breccias south of the inner ring fault are not 

cataclastic and must therefore represent accumulations 

of landslide and loose talus debris that fell off the 

unstable scarp of the fault. Intrusions of porphyry 

within the intracaldera member appear to have formed 

from Wah Wah Springs magma rising into higher levels 

of the hot pile of tuff filling the caldera. At least part 

of this intrusion may have caused the resurgent uplift 

of the caldera floor (described below).

The topographic rim of the Indian Peak caldera is 

mostly concealed in the area 4 km south of Sawtooth 

Peak (fig. A3). Exposures of locally brecciated rock of 

the Cottonwood Wash Tuff and of strongly welded but 

unusually thin outflow tuff member of the Wah Wah 

Springs Formation are barely visible on the northern 

margin of an extensive area covered by the thick, 

caldera-filling Lund Formation. This compound cooling 

unit and the overlying Ryan Spring Formation conceal 

the inner ring fault on the east flank of the Indian Peak 

Range.

The topographic rim and ring fault system of the Indian Peak caldera are buried beneath late Cenozoic 

basin fill in southern Pine Valley (fig. Al). In the 

southern Wah Wah Mountains a normal extracaldera 

sequence of Needles Range rocks overlies Paleozoic 

sedimentary rocks, and no intracaldera member of the 

Wah Wah Springs Formation is exposed. However, 

stratigraphic thickness relations in the Lund Formation 

around The Tetons are such that the topographic rim 

could have been located just to the west.

The perimeter of the Indian Peak caldera probably 

swings southwestward across the southern tip of the 

Indian Peak Range, but its exact location is obscured 

by Miocene magmatic units (Best and others, this
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 21

volume, chap. B) and other more recent deposits. A 

northeast-striking normal fault through Arrowhead 

Pass (fig. Al) separates a section at least 2 km thick 

of tuff and breccia of the intracaldera member on the 

north from a section mainly of tuff only one-tenth as 

thick to the south (Best, Grant, and others, 1987). Breccia near but north of the fault has clasts of tuff of the 

Sawtooth Peak Formation, but a slab of shattered 

Ordovician and Silurian sedimentary rocks, a square 

kilometer or so in area, also lies north of the fault, within 

the caldera. The fault through Arrowhead Pass appears 

to have had a complex history and was not the outermost ring fault. The 200-m-thick section of intracaldera 

tuff south of the fault is overlain by discontinuous lava 

flows and tuffs of the Ryan Spring and Lund Formations, and by Miocene volcanic rocks, indicating postcaldera uplift, partly during resurgence of the caldera 

but also during block faulting and magmatic activity 

in the Miocene (Best and others, this volume, chap. B).

NEVADA SECTOR

Only reconnaissance field work has been done in the 

northern White Rock Mountains and Wilson Creek 

Range in Nevada, building upon the 1:250,000 map of 

Ekren and others (1977). Although Miocene rocks cover 

much of this area, tuffs of the Lund and Ryan Spring 

Formations are exposed together with almost 100 km2 

of tuff and minor breccia of the intracaldera member 

of the Wah Wah Springs Formation.

Southeast of the Atlanta gold mining district, intensely silicified Paleozoic carbonate rocks are juxtaposed 

against tuffs of the intracaldera member of the Wah 

Wah Springs and the Ryan Spring Formation to the 

south. Nearby unaltered Paleozoic rocks are overlain by 

latite flows containing phenocrysts of plagioclase and 

smaller amounts of pyroxene, biotite, and hornblende. 

One biotite specimen collected at lat 38°27'38" N. and 

long 114°15'10" W. has a potassium-argon age of 

31.8±1.2 m.y. (determination by S. H. Evans, Jr., at 

the University of Utah in 1982). West of Atlanta, latite 

flows are juxtaposed against altered rocks, in part of 

the Ryan Spring Formation, to the south. These relations indicate the topographic margin of the Indian 

Peak caldera lies near Atlanta. Until more study is made 

it is impossible to tell whether the widespread 

hydrothermal activity around Atlanta is related to a 

ring-fault system bounding the caldera or to a later 

north-striking fault system. Five to six kilometers 

southeast of Atlanta, slabs of brecciated lower Paleozoic 

carbonate rocks and quartzite rest on tuff of the Wah 

Wah Springs Formation; at the base of the slabs the 

clasts are separated by a matrix of the tuff. These

"jostled" slabs are interpreted to have fallen from the 

nearby wall of the caldera into the depression during 

collapse and now rest as erosionally resistant caps on 

the tuff. The tuff appears to be of the outflow member, 

but more or less continuous exposures of the formation 

southward lead into more typical tuffs of the intracaldera member. There are obviously significant variations in the type of tuff in the intracaldera member 

within the caldera and it is not unlikely that some of 

it could closely resemble the outflow tuff member.

No direct evidence for the location of the ring fault 

or topographic wall of the caldera has been found 

elsewhere in Nevada, so its western and southwestern 

perimeter is conjectural.

RESURGENCE

At no place within the Indian Peak caldera have any 

lake beds or significant areas and thicknesses (more 

than a few meters) of any other sedimentary material 

been recognized between the Wah Wah Springs Formation and the caldera-filling volcanic deposits of the Ryan 

Spring and Lund Formations. If such sediments were 

ever deposited they must have been eroded away prior 

to emplacement of the younger volcanic rocks because 

of resurgent uplift of the floor of the caldera.

Evidence that is more direct for resurgence in the 

Utah sector of the caldera can be seen in the distribution and thickness of the Ryan Spring Formation (figs. 

A8 and A9). The Greens Canyon Tuff Member lies in 

a small basin bounded to the north by the topographic 

wall and to the south by another topographic high, 

which was probably the margin of the resurgent core 

of the caldera. A moat between the resurgent core and 

topographic wall is also expressed in the younger 

Mackleprang Tuff Member (fig. A9), and variations in 

its thickness on the order of several hundred meters 

south of Indian Peak reflect deposition on the blockfaulted uplift. South of the fault through Arrowhead 

Pass, post-caldera uplift appears to have locally exceeded 2 km, inasmuch as Paleozoic basement rocks are 

exposed beneath an unusually thin intracaldera 

member. Part of this uplift may have coincided with the 

extrusion of lava flows in the Ryan Spring Formation 

onto the thin intracaldera member.

The intrusive porphyry of the intracaldera member was 

probably emplaced during resurgence and may have 

caused the resurgenca The main intrusion of porphyry 

is exposed west of the Cougar Spar Mine (fig. A3). South 

of this intrusion, as far as Arrowhead Pass, compaction 

foliation in tuff of the intracaldera member dips southward from 20° to 60°. These dips presumably reflect 

tilting of the tuff over a large buried mass of porphyry.
22 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

J151

GREENS CANYON 

TUFF MEMBER, 

RYAN SPRING 

FORMATION

FIGURE A8. Distribution and thickness of the Greens Canyon Tuff Member of the Ryan Spring Formation. The tuff was confined on 

the north by the topographic wall of the Indian Peak caldera, on the south by its resurgent uplift, and on the east by a north-striking 

fault escarpment (heavy line). Symbols as on figure A4.

TOTAL VOLUME OF WAH WAH SPRINGS FORMATION

Minimum measured thicknesses of tuff and minor 

breccia in the intracaldera member in the Indian Peak 

Range and the northeastern Wilson Creek Range are 

2-3 km. If a conservative value of 2 km is used for the 

entire caldera, the volume of tuff in the intracaldera 

member is about 2,400 km3. The calculated total 

volume of ash-flow tuff in the Wah Wah Springs Formation, including both outflow and intracaldera 

members, is therefore about 4,000 km3.

Relations shown on figure A3 along the northeastern 

segment of the caldera indicate a total drawdown of 

roughly 3 km, or perhaps more, depending on how much 

resurgent uplift was compensated for, as magma was 

withdrawn from the underlying magma chamber. If this 

drawdown is applied to the entire caldera, the volume 

of extruded magma must have been at least 3,600 km3. 

The estimated volumes of extruded magma and tuff are

in close agreement. Unevaluated factors, such as the 

amount of subsidence on subsidiary faults outside the 

main ring fault, the location of the western and southern 

margins of the caldera and therefore its area, the porosity of partially welded tuff, the volume of the landslide 

debris in the intracaldera member, and the volume of 

air-fall deposits far from the caldera, all enter into the 

volume considerations.

RYAN SPRING FORMATION

The Ryan Spring Formation (figs. A8 and A9) lies 

almost wholly within the Indian Peak caldera, which 

probably includes its source. The Greens Canyon Tuff 

Member in the lower part of the formation is confined 

to an area along the northern margin of the caldera 

where a moat existed between a resurgently uplifted 

core and the topographic wall. The member thins
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 23

J151

Beers Tunne 

Spring

\ - 

VZ

MACKLEPRANG TUFF

MEMBER, RYAN SPRING

FORMATION

FIGURE A9. Distribution and thickness of the Mackleprang Tuff Member of the Ryan Spring Formation. The tuff pinched out over the

resurgent uplift of the Indian Peak caldera. Symbols as on figure A4.

significantly eastward across a north-south fault near 

Sawtooth Peak, which offsets the topographic wall of 

the caldera about 4 km north-south (fig. A3). Although 

some displacement along this fault postdates deposition of the Lund Formation and thus is also younger 

than the Ryan Spring Formation, some could have 

preceded deposition of the Greens Canyon Tuff 

Member, or even been contemporaneous with its eruption, in which case the fault could be a bounding fault 

for a segment of a source caldera. Preliminary mapping 

of areas in the northern White Rock Mountains has 

disclosed thick sections of the Greens Canyon, suggesting proximity to a caldera wall.

The Mackleprang Tuff Member of the Ryan Spring 

Formation does not thin across the north-south fault 

near Sawtooth Peak but wedges out a short distance 

to the south; the member is absent a few kilometers east 

and south of Indian Peak. The Mackleprang Tuff 

Member reappears farther south of Indian Peak and 

thickens to as much as several hundred meters east and

south of Arrowhead Pass. These thickening and thinning relations probably reflect deposition of the tuff in 

a moat between the resurgently uplifted and block 

faulted core and the topographic wall of the Indian Peak 

caldera.

LUND FORMATION

The Lund Formation (fig. A10) is almost as extensive 

east-west as the outflow tuff member of the Wah Wah 

Springs Formation, but is more restricted north and 

south. The thickest sections of this compound cooling 

unit, which commonly have a basal black vitrophyre 

several meters thick, are in the northeastern moat of 

the Indian Peak caldera and the northwestern part of 

the White Rock source caldera. Had the earlier depression of the Indian Peak caldera not existed, the Lund 

Formation would undoubtedly have spread far to the 

north, as did the outflow tuff member of the Wah Wah 

Springs Formation (fig. A7).
24 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

I15-) 114° 113°

FIGURE A10. Distribution and thickness of the Lund Formation. Heavy, hachured double Une indicates known location of topographic 

wall of the White Rock caldera; lighter, plain double Une shows inferred approximate location. Other symbols as on figure A4.

The tuff of the Seaman Range in southern Nevada 

(Ekren and others, 1977) is similar to the Lund in 

composition and occupies the same stratigraphic position. Whether the Lund Formation and tuff of the 

Seaman Range are actually parts of the same sheet 

or are similar, virtually contemporaneous deposits 

from separate sources can only be decided by future 

investigations.

WHITE ROCK CALDERA

The catastrophic eruption of about 1,600 km3 of the 

dacite tuff member of the Lund Formation caused the

collapse of the White Rock caldera. Only small 

segments of its eastern and northwestern margins are 

clearly exposed.

A segment of the topographic wall is exposed just 

west of Wilson Canyon in the southern Indian Peak 

Range, 8-9 km south of Arrowhead Pass. A calderafilling sequence about 1.2 km thick of the andesite flow 

member of the Lund Formation and Isom Formation 

is banked against older rocks of the Lund, Ryan Spring, 

and Wah Wah Springs Formations (Best, Grant, and 

others, 1987). A lens of brecciated tuff member of the 

Lund Formation that is completely out of stratigraphic position lies beneath the caldera fill along the
STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 25

topographic wall. Ash flows of the Isom Formation 

spilled northward beyond the topographic rim, where 

their thickness is an order of magnitude less than inside the caldera depression. A thick section of post-Isom, 

early Miocene lava flows conceals older units southward 

beyond Modena, so the nature of the rocks filling the 

eastern segment of the White Rock caldera is unknown.

Preliminary mapping in the White Rock Mountains 

discloses an unusually thick section of the Isom Formation just south of about lat 38 °15' N.; this is permissive 

evidence for the approximate location of the topographic 

wall of the White Rock caldera.

Field relations 8 km northwest of Mount Wilson, in 

the reference locality of the tuff member of the Lund 

Formation, clearly indicate proximity to the structural 

margin of the White Rock caldera. A northeast-tilted 

compound cooling unit of the tuff member, which may 

be as much as 2 km thick, contains intercalated lenses 

of breccia whose clasts are of Paleozoic carbonate rocks 

and granite of unknown age. Generally, in this wellexposed reference locality, the tuff member has sparse 

clasts, locally almost a meter across, that have generally the same composition as the tuff but are finer grained 

or flow-layered, or have other textural contrasts.

Many drill holes in the Pioche mining district encountered volcanic rocks beneath Cambrian sedimentary rocks, or an alternation of the two (Park and others, 

1958; Gemmffl, 1968). Tschanz and Pampeyan (1970, 

p. 125, 127) interpret these rocks to be a "megabreccia 

composed of slices and blocks of Tertiary volcanic rocks 

and Cambrian limestone" that "is either a thrust breccia, a chaotic sedimentary megabreccia, or a megabreccia produced by gravity sliding. Small dikes or dikelike 

masses of volcanic rocks with the microscopic characteristics of welded tuff occur * * * These are believed to be downfaulted remnants of blocks dragged 

beneath a postvolcanic thrust plate. This * * * has 

since been eroded or else it has slid off the flanks of the 

range to form the megabreccia." Alternatively, these 

puzzling relations can be interpreted to indicate the close 

proximity of the Pioche district to the unstable wall of 

a caldera along which Cambrian rocks sloughed into the 

depression to cover and intermingle as breccias with 

either precaldera volcanic rocks or concurrently erupting tuff. Only a few churn-drill cuttings of volcanic 

rocks from these holes, drilled in the late 1940's and early 1950's, have been located. The volcanic cuttings are 

outflow tuff of the Wah Wah Springs Formation and a 

slightly altered tuff whose petrographic character and 

fission-track age on zircon are most compatible with the 

Lund Formation. Until additional drill-hole material can 

be examined and detailed mapping of volcanic rocks 

done around Pioche, there is some uncertainty whether 

the postulated caldera is the White Rock.

SUMMARY

Ash-flow tuffs of the newly revised Needles Range 

Group cover an area of at least 22,000 km2 and probably much more; they have an aggregate volume of at 

least 6,600 km3 and possibly as much as 10,000 km3. 

They were erupted from clustered, overlapping sources 

(fig. All) along the southern Utah-Nevada border between 33 and 28 m.y. ago. The group consists of seven 

cooling units in two compositional cycles. Each cycle 

began with crystal-poor, lithic-rich rhyolite and 

culminated with more voluminous crystal-rich dacite. 

Andesite lavas were extruded more or less contemporaneously.

The first cycle began about 33 m.y. ago with eruption of the rhyolitic Marsden and Lamerdorf Tuff 

Members of the Escalante Desert Formation. The Pine 

Valley caldera collapsed during the extrusion of the 

Marsden and, possibly, Lamerdorf Tuff Members. 

Many compositionally similar rhyolite flows were extruded from vents near the margin of the caldera at 

about this same time. More widely scattered vents 

throughout the Needle Range and the Wah Wah Mountains were the source of an even greater number of contemporaneous pyroxene andesite lava flows. Thin, 

widely scattered deposits of sand and local thick lenses 

of gravel accumulated in topographic lows between the 

lava flows, forming the Beers Spring Member of the 

Escalante Desert Formation. Then followed two 

voluminous eruptions of crystal-rich dacite tuff. At least 

500 km3 of the 30.6-m.y.-old Cottonwood Wash Tuff 

was erupted from a source that appears to be concealed 

beneath a broad alluvial valley near the northern margin 

of the younger Indian Peak caldera.

Dacite tuffs of the Wah Wah Springs Formation were 

erupted 29.5 m.y. ago from the Indian Peak caldera, 

which straddles the Utah-Nevada border. Although this 

caldera is mostly concealed by younger deposits, it appears to have had an area of at least 1,200 km2 at the 

time of collapse and its northeastern perimeter generally coincides with the northeastern perimeter of the 

smaller, earlier Pine Valley caldera. The Indian Peak 

caldera collapsed after the expulsion of about 

1,500 km3 of the outflow tuff member, which spread 

over an area of at least 22,000 km2 in Nevada and 

Utah to depths as great as 500 m. Continued eruption 

of the Wah Wah Springs tuffs and intermittent caving 

of the unstable caldera wall produced tuff and breccia 

in the intracaldera member that filled the depression 

inside the major ring fault to a depth of at least 2 km. 

The northeastern segment of the topographic shelf and 

wall of the caldera retreated as much as 10 km back 

from the major ring fault as the central floor of the 

depression subsided about 3 km. The total volume of
26

39'

VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

113°

FIGURE Al 1. Nested sources of tuff cooling units in the Needles Range Group from figures A4 to A10. The Marsden Tuff Member and 

possibly the Lamerdorf Tuff Member of the Escalante Desert Formation were derived from the Pine Valley caldera. Tuffs of the 

Wah Wah Springs and the Ryan Spring Formations were derived from the Indian Peak caldera. The Lund Formation originated 

from the White Rock caldera. Approximate location of the source of the Cottonwood Wash Tuff is shown by shaded area.

the Wah Wah Springs Formation is estimated to be on 

the order of 4,000 km3, which ranks this unit among 

the great pyroclastic sheets in the world.

The second cycle of eruptive activity began sometime 

after the core of the Indian Peak caldera had risen 

resurgently at least several hundred meters. Two smallvolume cooling units of rhyolite ash-flow tuff of the 

Ryan Spring Formation had sources within the area of 

the older Indian Peak caldera; rhyolite and andesite lava 

flow members of the Ryan Spring Formation occur in 

the southeastern part of the caldera.

Crystal-rich dacite ash-flows of the 27.9-m.y.-old Lund 

Formation then ponded as a compound cooling unit to 

depths of at least 600 m within the northeastern moat 

of the Indian Peak caldera and spread widely to the east 

and west in Utah and Nevada, covering an area of about 

11,000 km2. The source of this 1,600 km3 volume of

tuff was the White Rock caldera, which had an 

estimated area of about 2,000 km2 and partly eclipses 

the southwestern part of the older Indian Peak caldera. 

The volume of the Lund Formation within the White 

Rock caldera is unknown but possibly equals the 

outflow.

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STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP 27

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stratigraphy, Sevier-Cove Fort area, central Utah: Utah Geology, 

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Conrad, O. G., 1969, Tertiary volcanic rocks of Needles Range, western 

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29, 28 p.

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Nevada: Nevada Bureau of Mines Report 11, 61 p.

Cunningham, C. G., Steven, T. A., Rowley, P. D., Glassgold, L. B., 

and Anderson, J. J., 1983, Geologic map of the Tushar Mountains 

and adjoining areas, Marys vale volcanic field, Utah: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

I-1430-A, scale 1:50,000.

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from old to new constants: Geology, v. 7, p. 558-560.

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1-1041, scale 1:250,000.

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of mid-Tertiary volcanism in High Plateaus region of Utah, in 

Cenozoic geology of southwestern High Plateaus of Utah: 

Geological Society of America Special Paper 160, p. 53-61.

Gemmill, Paul, 1968, The geology of the ore deposits of the Pioche 

district, Nevada, in Ridge, J. D., ed., Ore deposits of the United 

States, 1933-1967 (The Graton-Sales Volume): American Institute 

of Mining, Metallurgical, and Petroleum Engineers, p. 1128-1147.

Grant, S. K., 1978, Stratigraphic relations of the E seal ante Desert 

Formation near Lund, Utah: Brigham Young University Geology 

Studies, v. 25, p. 27-30.

___1979, Intrusive rocks of the Indian Peak Range, Utah, in 

Newman, G. W., and Goode, H. D., eds., Basin and Range Symposium and Great Basin Field Conference: Rocky Mountain 

Association of Geologists and Utah Geological Association 

Guidebook, p. 339-344.

Grant, S. K., and Best, M. G., 1979a, Geologic map of the Pinto Spring 

and part of the Atchison Creek quadrangles, Beaver and Iron 

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9 p., 1 pi.

___1979b, Geologic map of the Lund quadrangle, Iron County, 

Utah: U.S. Geological Survey Open-File Report 79-1655, 7 p., 1 pL

Gromme, C. S., McKee, E. H., and Blake, M. C., Jr., 1972, Paleomagnetic correlations and potassium-argon dating of middle 

Tertiary ash-flow sheets in the eastern Great Basin, Nevada 

and Utah: Geological Society of America Bulletin, v. 83, 

p. 1619-1638.

Kreider, J. E., 1970, The Lund ash-flow tuff: Rolla, Mo., University 

of Missouri, M.S. thesis, 73 p.

Lemmon, D. M., Silberman, M. L., and Kistler, R. W., 1973, Some 

K-Ar ages of extrusive and intrusive rocks of the San Francisco 

and Wah Wah Mountains, Utah: Utah Geological Association 

Publication 3, p. 23-26.

Lipman, P. W., 1976, Caldera-collapse breccias in the western San Juan 

Mountains, Colorado: Geological Society of America Bulletin, v. 

87, p. 1397-1410.

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___1963, Reconnaissance stratigraphy of the Needles Range 

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Preliminary geologic map and cross sections of The Tetons 

quadrangle and adjacent part of the Observation Knoll 

quadrangle, Beaver and Iron Counties, Utah: U.S. Geological 

Survey Open-File Report 82-778, 13 p., 3 pis., scale 1:24,000.

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volcanic units of the Caliente volcanic field, Lincoln County, 

Nevada, and Washington County, Utah: Isochron/West, no. 5, 

p. 17-24.

Park, C. F., Jr., Gemmill, Paul, and Tschanz, C. M., 1958, Geologic 

map and sections of the Pioche Hills, Lincoln County, Nevada: 

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Needle Range, Beaver County, Utah: Rolla, Mo., University of 

Missouri, M.S. thesis, 58 p.
28 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

Rowley, P. D., Steven, T. A., Anderson, J. J., and Cunningham, C. G., 

1979, Cenozoic stratigraphic and structural framework of southwestern Utah: U.S. Geological Survey Professional Paper 1149, 

22 p.

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paleomagnetism of the Needles Range Formation, Utah and 

Nevada: American Journal of Science, v. 276, p. 954-968.

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Utah: Brigham Young University Geology Studies, v. 28, pt. 3, 

p. 1-7.

Steven, T. A., Cunningham, C. G., Naeser, C. W., and Mehnert, H. H., 

1979, Revised stratigraphy and radiometric ages of volcanic rocks 

and mineral deposits in the Marys vale area, west-central Utah: 

U.S. Geological Survey Bulletin 1469, 40 p.

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of the Marysvale volcanic field, west-central Utah: Journal of 

Geophysical Research, v. 89, p. 8751-8764.

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fault blocks, Western United States: Geological Society of 

America Bulletin, v. 91, p. 460-464.

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mineral depostis, Nevada and Utah: Geological Society of America 

Bulletin, v. 88, p. 67-77.

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deposits of Lincoln County, Nevada: Nevada Bureau of Mines and 

Geology Bulletin 73, 187 p.

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mineral resources in the northern Basin and Range province: 

Geothermal Resources Council Special Report 13, p. 345-362.

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Group, southwestern Utah and southeastern Nevada: Seattle, 

Wash., University of Washington, Ph. D. thesis, 139 p.
Miocene Magmatism and 

Tectonism in and near the 

Southern Wah Wah Mountains, 

Southwestern Utah

By MYRON G. BEST, HARALD H. MEHNERT, 

JEFFREY D. KEITH, and CHARLES W. NAESER

OLIGOCENE AND MIOCENE VOLCANIC ROCKS IN THE CENTRAL PIOCHEMARYSVALE IGNEOUS BELT, WESTERN UTAH AND EASTERN NEVADA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1433-B

Radiometric ages and field and chemical data 

document two episodes of bimodal magmatism 

and one accompanying pulse of block faulting
CONTENTS

Page

Abstract ................................................................... 31

Introduction ................................................................ 31

Acknowledgments ........................................................ 33

Generalized volcanic history .................................................. 33

Early Miocene Blawn Formation ............................................... 35

Rhyolitic tuffs and trachyandesitic lava flows ................................ 35

Rhyolitic lava flows and intrusions ......................................... 38

Mid-Miocene Steamboat Mountain Formation ................................... 39

Rhyolitic tuffs and flows .................................................. 40

Mafic flows ............................................................. 40

Petrochemistry of rhyolites ................................................... 40

Tectonism .................................................................. 40

Alteration and mineralization ................................................. 42

Discussion and conclusions ................................................... 44

Tectonism .............................................................. 44

Magmatism ............................................................. 45

References cited ............................................................. 45

Appendix Descriptions of samples and sample localities .......................... 47

ILLUSTRATIONS

Page

FIGURE Bl. Generalized geologic map of the Pioche-Marysvale igneous belt ............................................ 32

B2. Generalized aeromagnetic map of the Pioche-Marysvale igneous belt ....................................... 33

B3. Tertiary stratigraphy of the southern Wah Wah Mountains and the Indian Peak Range ...................... 34

B4. Geologic map of the central part of the Pioche-Marysvale belt ............................................ 37

B5. Geologic map and cross section of the area around The Tetons ............................................ 43

TABLES

Page

TABLE Bl. Radiometric ages of igneous rocks in the Wah Wah Mountains and Indian Peak Range ....................... 36

B2. Chemical composition of mafic to intermediate Miocene lava flows from the Wah Wah Mountains .............. 38

B3. Chemical composition of Miocene rhyolite from the southern Wah Wah Mountains and the Indian Peak Range ... 41

Appendix. Descriptions of samples and sample localities in the Wah Wah Mountains and nearby areas, southwestern Utah . 47
OLIGOCENE AND MIOCENE VOLCANIC ROCKS IN THE CENTRAL 

PIOCHE-MARYSVALE IGNEOUS BELT, WESTERN UTAH AND EASTERN NEVADA

MIOCENE MAGMATISM AND TECTONISM IN AND NEAR 

THE SOUTHERN WAH WAH MOUNTAINS, SOUTHWESTERN UTAH

By MYRON G. BEST, HARALD H. MEHNERT, JEFFREY D. KEITH, and CHARLES W. NAESER

ABSTRACT

Magmatic and tectonic activity in the central part of the PiocheMarysvale igneous belt in southwestern Utah during the Miocene contrasts sharply with that during the Oligocene. Great volumes of chiefly 

intermediate calc-alkalic magma erupted to form large calderas in the 

Oligocene, but beginning about 23 million years (m.y.) ago, smaller 

volumes of bimodal mafic-silicic magmas erupted from many local 

centers. This first pulse of bimodal magmatism lasted until 18 m.y. 

ago and was characterized by potassic trachyandesite lava flows 

associated with mostly younger rhyolite tuffs and lava flows (some 

topaz bearing) and shallow porphyritic intrusions, one of which hosts 

disseminated molybdenum and tungsten. After a hiatus of about 

5 m.y., bimodal magmatism resumed for about a million years 

(13-12 m.y. ago), when mafic lava flows containing less potassium and 

silicon than the earlier mafic flows were emplaced along with uniformly 

high-silica, lithophile-rich, topaz-bearing rhyolite flows and tuffs. These 

mid-Miocene rhyolites form a northeast-trending belt just south of 

the similarly trending belt containing early Miocene rhyolitic rocks. 

Hydrothermal activity accompanied both episodes of silicic 

magmatism.

During the Oligocene, southwestern Utah and the adjacent part of 

Nevada appear to have been tectonically inactive; successive ash-flow 

deposits progressively smoothed the prevolcanic topography, and so 

the latest Oligocene (26-m.y.-old) Isom Formation now forms a thin 

sheet no more than a few tens of meters thick throughout most of 

the area. However, in the early Miocene, northeast-trending normal 

faulting began in the central part of the Pioche-Marysvale igneous 

belt. At the same time, many northeast-trending, subvertical rhyolite 

dikes were emplaced, indicating local northwest-southeast extension 

of the crust in contrast to the east-west directed extension of the last 

10 m.y. or so that has produced northerly trending basins and ranges 

in the eastern Great Basin.

INTRODUCTION

Recent papers have focused attention on the geophysical and tectonomagmatic character of broad, generally

east-trending belts of mineralization in western Utah 

and eastern Nevada that cut across boundaries of major geologic provinces, including the Colorado Plateaus, 

the Basin and Range province, the Sevier erogenic belt, 

and the Mesozoic metamorphic belt (fig. Bl, inset; see 

Rowley and others, 1978; Shawe and Stewart, 1976; 

Stewart and others, 1977). These mineral belts are noted 

for valuable deposits of gold, silver, lead, zinc, copper, 

iron, molybdenum, and tungsten and are associated 

with east-trending belts of Tertiary igneous rocks that 

decrease in age from north to south. Broad aeromagnetic highs with superimposed short-wavelength 

anomalies characterize the igneous and mineral belts 

(fig. B2).

In southwestern Utah and adjacent parts of Nevada, 

one of these belts has been called the Pioche mineral 

belt by Shawe and Stewart (1976); this name earlier had 

been applied only to the Nevada portion of the belt by 

Roberts (1966), and the Utah portion had been 

designated the Wah Wah-Tushar belt by Hilpert and 

Roberts (1964). The zone of magmatic rocks that was 

designated the Pioche-Marysvale igneous belt by 

Rowley and others (1979) generally coincides with the 

mineral belt. This belt, however defined, trends eastnortheasterly from a point west of Pioche, Nev., 

through the southern Wah Wah Mountains of western 

Utah, the Tushar Mountains between Beaver and 

Marysvale, to the western part of the Colorado Plateaus 

(fig. Bl; compare the aeromagnetic map of fig. B2). A 

subparallel belt of igneous rocks and mineral deposits 

lies about 50 km south of the Pioche-Marysvale belt and 

includes near its eastern end the Iron Springs district 

west of Cedar City.

31
32 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

39'

ARIZONA

0 20 40 60 80 100 KILOMETERS i____i____i____i____i____i

r NEVADA 7 ~T~7 ^1

<&f / &/£&' ~\

, V^-;«/ / -P T /^^/ o W/{//ff\&\

EXPLANATION

\ 3f-^ x

Pioche mineral belt \ r Jl

QUATERNARY ALLUVIUM \K

TERTIARY VOLCANIC ROCKS

TERTIARY GRANITIC ROCKS

MESOZOIC AND PALEOZOIC SEDIMENTARY ROCKS

 CONTACT Approximately located 

±^Z STRIKE-SLIP-FAULTS

FIGURE Bl. Generalized geology of the Pioche-Marysvale igneous belt of Rowley and others (1978) and surrounding areas. Note the 

northeast-striking, left-lateral strike-slip faults and the single northwest-striking, right-lateral fault south of Pioche. Geology based on 

Hintze (1980) and Stewart and Carlson (1978); inset after Shawe and Stewart (1976).

Shawe and Stewart (1976, p. 226) drew attention to 

major northeast- to east-northeast-striking high-angle 

faults in the Nevada segment of the Pioche-Marysvale 

belt that are clearly oblique to the northerly oriented 

normal faults that bound most of the present basins and 

ranges. These oblique faults tend to be en echelon and 

stepped to the right as though produced by a left-lateral 

shear couple. Strike-slip faults with left lateral displacement occur south of Pioche (fig. Bl; see also Ekren and 

others, 1977).

Rowley and others (1978) defined the east-trending 

Blue Ribbon lineament, which largely overlaps the 

Pioche-Marysvale belt and is based in part upon the 

distribution of Tertiary rhyolite bodies and deposits of 

fluorspar, uranium, and tungsten.

Recent mapping at a scale of 1:24,000 and compilations at 1:50,000, combined with radiometric dating in 

the southern Wah Wah Mountains and the Indian Peak 

Range,1 has established a framework of tectonic, magmatic, and hydrothermal events that sheds light on the 

character and evolution of at least the central part of 

the Pioche-Marysvale belt. This framework also clarifies 

broader aspects of the Cenozoic tectono-magmatic 

development of the Colorado Plateaus-Basin and Range 

transition zone.

The Indian Peak Range is the southern part of what was called the Needle Range on 

maps published prior to 1974. The northern part of the Needle Range is now called the Mountain Home Range.
MIOCENE MAGMATISM AND TECTONISM, SOUTHERN WAH WAH MOUNTAINS

113o 112°

33

ARIZONA

0 20 4D

EXPLANATION 

Magnetic intensity in gammas:

> 11,300 

11,300-11,100 

11,100-10,900 

10,900-10,700 

< 10,700

FIGURE B2. Generalized aeromagnetic map of the Pioche-Marysvale igneous belt and surrounding area. Modified from Zietz and others

(1976, 1978).

ACKNOWLEDGMENTS

Many individuals made this investigation possible. 

Thomas A. Steven and C. G. "Skip" Cunningham 

helped in many ways, including stimulating visits in the 

field, coordination of logistical details, and considerable 

encouragement over the years. Ernest Anderson advised us on faulting. Students in the Brigham Young 

University summer field geology course did preliminary mapping. Fred Nelson assisted with the chemical 

analyses, and Julie Barrott Willis, Stanley Evans, and 

Bart Kowallis helped with radiometric dating. For 

constructive comments on the manuscript we are indebted to Ernest Anderson, James Baer, Eric Christiansen, Kerry Grant, Lehi Hintze, Hal Morris, and Tom 

Steven.

GENERALIZED VOLCANIC HISTORY

In the region between Pioche and Marysvale, regional 

ash-flow sheets and locally derived lava flows and 

volcaniclastic deposits of Tertiary age rest on a surface 

of moderate relief, generally less than a few hundred 

meters, carved into Paleozoic and Mesozoic sedimentary 

strata that were deformed during the latest Cretaceous 

Sevier orogeny (fig. B3). Paleovalleys now filled with 

thick volcanic deposits are generally aligned more or 

less east-west; one of these on the west flank of the Wah 

Wah Mountains was especially deep about 1,200 m 

(Abbott and others, 1981). Another deep paleovalley cut 

into Ordovician carbonate rocks at Sawtooth Peak in 

the southern Mountain Home Range contains 300 m of 

the Sawtooth Peak Formation.
34 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

AGE 

(m.y.) 

 10

22

26 -

30 i

34

Steamboat Mountain Formation

Blawn Formation

Ryan Spring Formation

Escalante Desert Formation

Basement of local 

Mesozoic and Paleozoic rocks

Sawtooth Peak Formation

FIGURE B3. Tertiary stratigraphy of the southern Wah Wah Mountains and the Indian Peak Range, southwestern 

Utah. Regional ash-flow sheets are stippled. The Condor Canyon Formation comprises regional ash-flow sheets 

whose source may have been south of Pioche, Nev. (Williams, 1967); in the southern Wah Wah Mountains 

and the Indian Peak Range, only the Bauers Tuff Member of the Condor Canyon is present.
MIOCENE MAGMATISM AND TECTONISM, SOUTHERN WAH WAH MOUNTAINS 35

Volcanic rocks of the Needles Range Group, deposited 

next after the Sawtooth Peak, flooded the landscape in 

southwestern Utah and the adjacent part of Nevada to 

depths as great as a kilometer and filled nested source 

calderas that straddle the State line to much greater 

depths. This group, which has been newly revised in 

chapter A of this volume, consists of more than 

6,600 km3 of ash-flow tuffs and minor lava flows 

representing two eruptive-compositional cycles. Each 

cycle began with eruption of crystal-poor, low-silica 

rhyolites followed by greater volumes of crystal-rich 

dacite. First-cycle rhyolite tuffs and lava flows of the 

Escalante Desert Formation erupted about 33 m.y. ago 

were succeeded by dacite ash flows 30.6 and 29.5 m.y. 

ago that formed the Cottonwood Wash Tuff and the 

Wah Wah Springs Formation. Second-cycle rhyolite 

ash-flow tuffs of the Ryan Spring Formation followed 

by dacite of the Lund Formation were emplaced by 

28 m.y. ago.

The 26-m.y.-old2 (Fleck and others, 1975) Isom Formation consists of a sequence of trachytic ash-flow tuffs 

that is generally less than a few tens of meters thick 

yet crops out over much of southwestern Utah and extends into Nevada. It is thus apparent that extrusion 

of the Needles Range tuffs had produced a flat terrain 

outside the Indian Peak caldera and that negligible tectonic disturbance and erosion had occurred by the end 

of the Oligocene.

Elsewhere in southwestern Utah and the adjacent 

part of Nevada, regional ash-flow sheets of the Quichapa 

Group were deposited 25-22 m.y. ago (Armstrong, 

1970; Fleck and others, 1975) on tuffs of the Isom Formation. Their source was probably the Caliente 

cauldron complex south of Pioche, Nev. (Williams, 1967; 

Ekren and others, 1977). However, only the northeastern part of the most widespread sheet the Bauers Tuff 

Member of the Condor Canyon Formation (22.3 m.y., 

Fleck and others, 1975) is found in the Wah Wah 

Mountains and the mountain ranges on either side. The 

Bauers sheet of low-silica rhyolite tuff is generally less 

than 20 m thick but firmly welded.

In early Miocene time, volcanic activity shifted from 

eruption of the earlier, generally intermediate composition magmas to a more bimodal assemblage of mafic 

and silicic compositions. Between 23 and 18 m.y. ago, 

many local magmatic centers were active in the central 

part of the Pioche-Marysvale belt, producing the mafic 

lava flows and rhyolite lava flows and tuffs of the Blawn 

Formation, named and described in this report. After 

a hiatus of about 5 m.y., bimodal magmatic activity was 

renewed and mafic lava flows and topaz-bearing rhyolite

Potassium-argon ages published prior to 1977 have been adjusted for new radiometric 

constants (Dalrymple, 1979).

extrusions of the 13- to 12-m.y.-old Steamboat Mountain Formation (adopted as proposed by Thomson and 

Perry in 1975) were erupted. Alteration and mineralization accompanied both episodes of silicic acitivity. 

These two bimodal associations and related early 

Miocene tectonism in the central part of the PiocheMarysvale belt are the main subject of this paper.

EARLY MIOCENE BLAWN FORMATION

In contrast to the dominantly intermediatecomposition calc-alkalic magmas extruded chiefly as 

regionally extensive ash flows during Oligocene time, 

extruded Miocene magmas in the central part of the 

Pioche-Marysvale belt have smaller volumes and more 

restricted compositions and comprise an essentially 

bimodal assemblage of silicic and mafic rocks emplaced 

around many local magmatic centers. No caldera formed 

during this Miocene activity.

A bimodal assemblage of silicic intrusive and extrusive rocks and potassium-rich mafic lava flows 

emplaced between 23 and 18 m.y. ago (table Bl) is here 

designated the Blawn Formation. The unit is named 

from Blawn Wash, a major drainage through the 

southern Wah Wah Mountains (fig. B4), where a nearly complete sequence of the Blawn Formation overlies 

the Isom Formation and underlies unaltered midMiocene mafic lava flows of the 13- to 12-m.y.-old 

Steamboat Mountain Formation. (See later section.) 

Silicic rocks of the formation are also exposed in the 

nearby Blawn Mountain area; these and some older 

rocks in the area have been altered to alunite and clay 

minerals and may also contain small deposits of 

uranium, tin, molybdenum, and beryllium (Lindsey and 

Osmonson, 1978). The type section for the formation 

lies near the intersecting corners of the Lamerdorf Peak, 

Frisco, The Tetons, and Blue Mountain quadrangles at 

38°15' N. and 113°30' W. in sec. 35, T. 29 S., R. 15 W. 

(Abbott and others, 1983; Best, Morris, and others, 

1987; Weaver, 1980), where it is about 600 m thick. Wellexposed sections are found along the southeast flank 

of the Wah Wah Mountains northeast of Blawn Wash 

(Abbott and others, 1983).

RHYOLITIC TUFFS AND 

TRACHYANDESITIC LAVA FLOWS

Early Miocene bimodal volcanic activity that produced the Blawn Formation in the southern Wah Wah 

Mountains began with deposition of rhyolitic volcaniclastic rocks. At the base, 3-20 m of stratified volcanic
36 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

TABLE Bl. Radiometric ages of igneous rocks in the Wah Wah Mountains and nearby areas, southwestern Utah 

for descriptions of samples and localities. Potassium analyses by X-ray fluorescence at Brigham Young University, Provo, Utah. Ages determined by H. H. Mehnert at

U.S. Ueological Survey laboratory, Denver, Colo., except as noted. Decay constants: K \e=0.581X10 /yr; ^=4.962X10 "7yr; "K/Ktotal= 1.167 X1U '. Loaders ( ), not determined]

Locality

»T Rock type * ., ,_, No. JP North

latitude

West 

longitude

Radiogenic ** 0 Ar

K n ..... ...

(wt. pet) Moles/g

x1(f 10

Percent of 

total Ar

Age + 2a 

(m.y.)

Steamboat Mountain Formation

1

2

3

4

5

6

7

8

9

1 10

Rhyolite 

 do 

 . do i 

 do 

 'do- 

 do 

 rl ,-, 

"" UO " _ 

Trachyandesite 

Mugearite 

 -do 

s

s

s

s

s

s

s

w

w

w

38°04'02"

38°05'17"

38°00'20"

38°02'18"

38°05'10"

38°04'30"

38°01 '08"

38°19 f 57"

38°15'38"

38°18'53"

113°38

113oin

113042

113043

1130411

113034

113°46

113027

113°27

113°26

 43"

'22"

 H 3 Tt

'26"

1 4511

'30"

MO"

« 0 i|»

'52"

'22"

7.03,

7.66,

7.66,

6.60,

7.79,

5.92,

5.

3.76,

2.68,

2.68,

7.19

7.67

7.60

6.57

7.73

5.85

49

3.73

2.68

2.85

1.317

1.383

1.317

1.214

1 .411

1.069

1.097

.666

.498

.5422

82.2

79.8

81.3

48.5

82.4

61.5

18.9

73.9

62.0

49.2

12

12

11

12

12

12

13

12

12

13

.8±0.5

.5±0.4

.9'*0.5

.8±0.5

.6±0.4

.6*0.5

.8±1 .5

.3±0.5

.9±0.5

.3±0.3

Alunitic alteration

11

12

Alunite 

-- do - .50° 1o

113°31 '

113°18'

6.0<

4.5 1

), 6.09

4, 4.55

1 Q 1 Q

1 .480

42.4 

8i,

20.6±0.9

22.5±5.7

Blawn Formation

13a

13b

14

2 15

2 16

2 17

18

19

2 20

21

22

i 23

24

25

26

27

1 28

* 29

2 30

Rhyolite 

_ JJQ _ __

 do " ^ 

 do 

 do 

 do -

 do"- --

 do 

- do . 

 do -

 do 

 do 

 do - 

 do' »

 do 

Trachyandesite* 

 * do ' 

Granite 

s

s

s

z

z

z

s

b

z

s

b

b

s

P

b

s

w

w

z

38°15'00"

38°15'00"

38°12'50"

38°26'47"

38°12'45"

38 0 12'50"

38°12'50"

38°12'50"

38°10'20"

38°16'16"

38°15'59"

38°18'41"

38°14'45"

38°28'30"

38°28'30"

37°59'19"

38°22'55"

37°59 t OO"

38°20'30"

113°48

113°48

113°33

1 13°13

113051

113°48

113°48

113°48

113°50

113°25

113°31

113°33

113°34

113°18

1 1 3°18

114°03

113°29

113°47

113°09

 10"

'10"

 45"

 50"

 00"

 15"

'15"

'15"

'30"

'47"

 01"

'35"

 45"

'55"

'55"

 43"

'57"

'39"

 13"

7.72,

8.09,

4.38,( :

( :

( :

7.35,

8.47,( :

5.47,

8.37,

5.44,

8

QC. .OD ,

1.

7.

4.62,

3.28,

4.(!

7.82

8.05

4.25

i \

z \

i \

7.30

8.52

z )

5.78

8.36

5.50

8

Q )i .OH

98

60

4.68

3.35

70Z)

2.457

2.452

1.146

( 2 )

( 2 )

( 2 )

2.187

2.688 ( 2 )

1.847

2.687

1.697

_ __ Q d. * bo*-

.6122

2.534

. 

1 .118

1.5259 ( 2 )

70

76

84

(

(

(

81

74

(

81

74

29

74

66

80

38.0,

23

32

(

.8

.2

.6

2 )

2 )

2 )

.4

.0

2 )

.2

.9

.6

.0

.0

.0

42.5

.4

.0

2 )

21 .8±0.9

21.0±0.8

18.3±0.7

18.9±1 .0

19. Oil .1

20.6±1 .3

20.6±0.7

21 .8±0.8

21 .5*1 .1

22.1 ±0.8

22.2±0.8

21 .5±0.8

20.2i0.9

3 21.3±0.4

3 23.0±0.4

21 .0±1.3

23.2*1 .0

22.5±1 .1

21 .6±2.3

*Type of material dated: w = whole rock; s = sanidine; b = biotite; z = zircon; p = plagioclase.

'Age determined by M. G. Best at U.S. Geological Survey laboratory at Menlo Park, Calif. 

2 Fissioni-track age by C. W. Naeser, U.S. Geological Survey, Denver, Colo. K/Ar data not applicable. 

3 Age corrected for new decay constants (Dalrymple, 1979). 

"Age determined by Stanley Evans, University of Utah, Salt Lake City.

sand and pumice and 0-4 m of crudely stratified air-fall 

tuff underlie a weakly welded, crystal-poor, somewhat 

lithic ash-flow tuff. Thickest sections (60-200 m) of the 

ash-flow tuff are exposed on the east flank of the Wah 

Wah Mountains (Abbott and others, 1983), and deposits 

a few meters thick have been recognized at two localities

on the eastern slopes of the Indian Peak Range. The 

presently exposed extent of this tuff is an elliptical area 

of 800 km2 surrounding Pine Grove in the southern 

Wah Wah Mountains; a conservative estimate of its 

original volume is 25 km3 (Keith, 1980). Large lithic 

fragments (20-60 cm) lie between the Plinian air-fall and
MIOCENE MAGMATISM AND TECTONISM, SOUTHERN WAH WAH MOUNTAINS

WOO' 113°30'

37

1131)0'

15 20 25 30 KILOMETERS

EXPLANATION 

Steamboet Mountain Formation (12-13 m.y.)

Rhyolite flows 

Blawn Formation (1B-22 m.y.)

Rhyolite flows

Rhyolite dike

Intrusive silicic porphyry

Oligocene rhyolite flows

Miocene!?) and Oligocene granite, quartz monzonite

 Fault Dotted where concealed 

+ + Topographic rim of Indian Peak caldera

FIGURE B4. Distribution of Miocene silicic rocks and major high-angle faults within ranges in the central part of the Pioche-Marysvale 

belt, southwestern Utah. Pluses enclose the Indian Peak caldera complex (chapter A of this volume). Data for White Rock Mountains 

from Keith (1980), Hintze (1980), and unpublished reconnaissance mapping by M. G. Best. Data for the Shauntie Hills-Milford area 

from Lemmon and Morris (1979a, b) and Felt (1981). Data for Indian Peak Range, Mountain Home Range, and Wah Wah Mountains 

from Abbott and others (1983); Best (1987); Best, Grant, and others (1987); Best, Hintze, and others (1987); Best, Morris, and others 

(1987); Grant and Best (1979); and Weaver (1980).

underlying deposits, whereas smaller (<10 cm) 

fragments occur in the ash-flow tuff. Fragments include 

conspicuous pieces of tuff from the Lund and Wah Wah 

Springs Formations and other volcanic rocks containing phenocrysts of plagioclase; less common fragments 

include quartzite, olive-colored phyllite, and varitoned 

gray carbonate rocks similar to Lower Cambrian units 

exposed in the Wah Wah Mountains.
38 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

The ash-flow tuff unit near the base of the Blawn Formation contains small (<2 mm), phenocrysts of quartz, 

sanidine, plagioclase, and biotite (in order of decreasing 

abundance) and trace amounts of hornblende, iron-titanium oxides, and dark-red almandine-spessartine garnet. 

The garnet is especially notable because it is indistinguishable in chemical composition, size, relative abundance, 

and nature of minute inclusions from garnets within intrusive rocks in Pine Grove (Keith, 1982; Abbott and 

others, 1983). These observations and the fact that the 

outcrops of the tuff surround Pine Grove led Keith (1982) 

to conclude that this ash-flow tuff represents an eruptive facies of the granitic intrusive system in Pine Grove 

that is host to a disseminated molybdenum-tungsten 

deposit at depth. Extrapolation of the east-dipping 

volcanic succession exposed on the east flank of the Wah 

Wah Mountains westward over Pine Grove indicates 

about 1.3 km of roof rock has been stripped off the intrusive complex since it vented to the surface.

The age of both the tuff and the Pine Grove intrusive 

rocks is bracketed by the 23.2±1.0-my. potassium-argon 

age of the overlying trachyandesite lava flow (see below) 

and the 26-m.y.-old Isom Formation beneath.

Overlying the garnet-bearing tuff in the southeastern 

part of the Wah Wah Mountains is a sequence of porphyritic trachyandesite lava flows. Phenocrysts include 

small, sparse olivine grains, almost invariably replaced 

by iddingsite, and more abundant crystals of augite, 

hypersthene, and plagioclase. Biotite occurs locally in 

glomeroporphyritic clots. Chemically, these potassiumrich mafic flows range from trachyandesite to latite (Le 

Maitre, 1976): their SiO2 content ranges from 62 to 54 

weight percent and their K2O content ranges from 4.7 

to 2.2 weight percent (table B2).

A sample from the base of the thick (200 m) section 

of potassium-rich mafic lava flows in the Blawn Formation on the east side of the Wah Wah Mountains has a 

potassium-argon age of 23.2±1.0 my. (table Bl, sample 

28). Three kilometers to the south, a lens of Bauers Tuff 

Member of the Condor Canyon Formation with an age 

of 22.3 m.y. appears to lie within the sequence of mafic 

flows of the Blawn Formation.

Overlying these potassium-rich mafic lava flows in the 

Blawn Formation of the southeastern Wah Wah Mountains are loosely consolidated rhyolitic deposits similar 

to those beneath the lavas but lacking garnet. Crystalpoor ash-flow tuffs dominate, but beds of air-fall ash and 

pumice, reworked water-laid tuffaceous sands, and basesurge deposits also occur. That these deposits were 

derived from local volcanic centers is indicated by the 

presence of abundant angular xenoliths, locally a meter 

or so in diameter, consisting of tuffs from the Lund and 

Wah Wah Springs Formations and assorted, probably 

cogenetic rhyolite flow rocks.

TABLE B2. Chemical composition of mafic to intermediate Miocene 

lava flows from the Wah Wah Mountains, southwestern Utah

[Data in weight percent. Analyses by X-ray fluorescence spectrometry using the Nourish and 

Hutton (1969) method and several international rock standards. See appendix for descriptions of samples and localities]

Major 

oxide

Trachyandesite flows 

in Blawn Formation

Sample Sample Sample 

28 29 31

Mugearite flow

in Steamboat

Mountain

Formation,

sample 10

Si0 2 - 

Ti0 2 - 

A1 2 0 3 - 

Fe 2 0 3 1 - 

MnO -

60.5

.96

15.9

7.42

.05

55.0

14.5 

8.0 

.13

58.7

.94

15.4

7.2

.09

52.6

1.89

16.4

9.72

J.y 

6.01

2

r\

3

-5 O

.32 

.33

o.u

6.50

3.3

4.70 

.56

3-0 

6.65

3.44

3.10 

.34

4. U

6 no

3.9

2.79 

.84

MgO 

CaO 

Na 2 0- 

K 2 0 

P,0, 

Total 101.29 100.10 99.65 99.82

'Total iron expressed as Fe 2 0,

In the Shauntie Hills, Indian Peak Range, and 

southern White Rock Mountains, sequences of rhyolitic 

tuffs and lavas and intermediate-composition lava flows 

similar to those in the Wah Wah Mountains occur in the 

same stratigraphic position above the Isom Formation 

(Lemmon and Morris, 1979a, b; Felt, 1981; Lemmon and 

others, 1973; Best, Grant, and others, 1987; Keith, 1980).

RHYOLITIC LAVA FLOWS AND INTRUSIONS

Rhyolitic lava flows mostly overlie but locally interfinger with garnet-free tuffs, and similar-appearing 

rhyolite intrusions are contemporaneous with or slightly 

older than the flows (table Bl). All silicic bodies in the 

Blawn Formation have various combinations of 

phenocrystic quartz, sanidine, plagioclase, biotite, and, 

rarely, hornblenda Some also have clots of micrographic 

granite. Marginal vitrophyre and breccia are common 

around felsitic cores of several of the silicic extrusions.

A variety of flows and intrusions of the Blawn Formation is exposed in the southern Wah Wah Mountains. 

Plutons in Pine Grove are largely concealed granitic 

bodies that have combinations of phenocrysts of quartz, 

alkali feldspar, plagioclase, and biotite several 

millimeters in diameter set in a felsitic aphanitic matrix 

(Keith, 1982). Hydrothermal solutions have altered 

these rocks to assemblages of quartz, sericite, kaolinite, 

montmorillonite, carbonate minerals, and chlorite. A 

disseminated molybdenum-tungsten ore deposit lies at
MIOCENE MAGMATISM AND TECTONISM, SOUTHERN WAH WAH MOUNTAINS 39

a depth of 900 m to more them 2,000 m in the intrusive 

complex (Abbott and others, 1983). South and east of 

Pine Grove, weakly porphyritic, flow-layered rhyolite 

containing biotite phenocrysts forms small dikes at the 

head of Willow Creek (fig. B4) and small flows east of 

the dikes. Small, commonly propylitically altered 

trachyandesite dikes and quartz-rich pebble dikes are 

present near rhyolite dikes. Biotite samples from two 

dikes have potassium-argon ages of about 22 m.y. (table 

Bl, samples 22 and 23); a flow dome in the same area 

has a potassium-argon age on plagioclase of 21.3± 

0.5 m.y. and on biotite of 23.0±0.5 m.y. (Lemmon and 

others, 1973). In the large pile of rhyolite lava flows east 

of Blawn Wash, phenocrysts range in abundance from 

approximately 20 percent to nil and in size from 1 cm 

to 1 mm. Gem quality red beryl is locally present, and 

one flow capping the pile has abundant vapor-phase 

topaz crystals several millimeters long. Sanidine from 

rhyolite in the lower part of the pile has a potassiumargon age of 22.1±0.8 m.y. (table Bl, sample 21). 

Rhyolite dikes and some flows west, south, and 

southeast of The Tetons are characterized by abundant 

(25 percent) large phenocrysts of feldspar (up to 2 cm) 

and quartz; these bodies tend to be parallel to northeaststriking faults and are commonly altered.

Slightly porphyritic, gray to purple, flow-layered 

rhyolite of the Blawn Formation crops out near the 

Staats mine and at The Tetons (Lindsey and Osmonsen, 

1978; Best, Morris and others, 1987). Exposures near 

the Staats mine are of a shallow intrusion, or intrusions, 

and associated pyroclastic deposits, whereas only lava 

flows are exposed at The Tetons. Phenocrysts in these 

rocks are small, generally 2 mm or less across, and consist of smoky quartz, sanidine, plagioclase, and sparse 

biotite. Lithophysal cavities and vugs contain quartz 

and, less commonly, topaz and fluorite; rare vapor-phase 

garnet occurs at The Tetons (E. H. Christiansen, oral 

commun., 1981). The rhyolite near the Staats mine has 

anomalously high concentrations of lithium, beryllium, 

niobium, molybdenum, and tin. Sanidines from these 

bodies give potassium-argon ages of about 20 and 

18 m.y. (table Bl, samples 24 and 14).

In the Indian Peak Range, strongly porphyritic flows 

and feeder dikes containing phenocrysts of sanidine (up 

to 6 cm across), quartz, plagioclase, biotite, and hornblende occur southeast of Indian Peak. The sanidine 

phenocrysts have an age of 21 m.y. (table Bl, sample 

13). South of Indian Peak the rhyolite occurs as flow 

domes in downdropped fault blocks that flank the central horst of the range. Feeder-dike swarms within the 

horst follow two different northeast trends (fig. B4); the 

northern lava flows and dike swarm trend eastnortheast and have potassium-argon and zircon fissiontrack ages of 19 to 22 m.y. (table Bl, samples 16-19).

The southern flows and dike swarm trend northeast; a 

single fission-track age on zircon from one of the dikes 

is 21.5 m.y. (sample 20). Propylitized mafic rock containing xenocrysts of quartz and feldspar occurs locally along the margins of dikes in the northern swarm.

Only one of several small, flow-layered rhyolite flow 

domes and shallow intrusions of the Blawn Formation 

in the Shauntie Hills (fig. B4) has been dated (table Bl, 

sample 15). Granitic intrusions farther to the east near 

Milford have discordant potassium-argon ages (Lemmon and others, 1973). Additional analytical work is 

currently under way by T. A. Steven, H. H. Mehnert, 

and C. W. Naeser to clarify chronologic relations in what 

appear to be older (Oligocene?) intrusions heated by 

Miocene magmatic activity. However, the Moscow 

granite pluton west-southwest of Milford is clearly of 

Blawn age (table Bl, sample 30).

Shallow porphyritic intrusions in the Stateline area 

have been described by Keith (1980). One that cuts early 

Miocene rhyolite flows and tuffs and trachyandesite 

flows has an age of 21.0±1.3 m.y. (table Bl, sample 27). 

Some intrusions are characterized by conspicuous 

layers of contrasting composition that may reflect mixing of compositionally contrasting magmas. Rhyolite 

to rhyodacite intrusions occur in and around Cottonwood Canyon. Phenocrysts range from 5 to 45 percent 

in abundance and include quartz, sanidine, plagioclase, 

biotite, and minor iron-titanium oxides, zircon, allanite, 

and sphene. Intrusions that have more abundant 

phenocrysts of plagioclase and biotite have less silica 

and weaker flow foliation the same correspondences 

seen in silicic bodies in the Wah Wah Mountains.

MID-MIOCENE 

STEAMBOAT MOUNTAIN FORMATION

The Steamboat Mountain Formation is a bimodal 

assemblage of igneous rocks similar in some respects 

to those constituting the Blawn Formation. However, 

it is distinctly younger (13-12 m.y.); the topaz rhyolite 

lava flows that constitute its chief rock type lie farther 

south in the Indian Peak Range, Wah Wah Mountains, 

and Shauntie Hills (fig. B4); and the mafic and silicic 

compositions of its rocks are both more pronounced, so 

the assemblage is more truly bimodal.

The name Steamboat Mountain Formation was first 

used by Thomson and Perry (1975) for mid-Miocene 

rhyolite in their report on the Stateline mining district 

(fig. B4), and it is so used in this report. The type section designated in this report lies 4 km east of Steamboat Mountain, just north of Typhoid Spring in the 

Bible Spring quadrangle (Best, Grant, and others, 1987), 

where rhyolitic tuffs are overlain by topaz rhyolite
40 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

lava flows. At Steamboat Mountain the aggregate 

thickness of these flows is about 500 m.

Compared to the fairly widespread potassium-rich 

mafic flows in the Blawn Formation, mafic flows of the 

Steamboat Mountain Formation are more restricted in 

extent; they occur chiefly on the eastern side of the 

southern Wah Wah Mountains and as scattered small 

remnants in the Shauntie Hills.

RHYOLITIC TUFFS AND FLOWS

Silicic volcanism of Steamboat Mountain age began 

with the explosive eruption of gas-charged magma 

that spread pyroclastic debris around many vents in 

the Steamboat Mountain and Broken Ridge areas. 

Most of these deposits are of weakly consolidated ashflow tuffs, but local air-fall, base-surge, and waterreworked tuffs are also evident. Nearby sources for 

these volcaniclastic deposits are suggested by the local 

occurrence of abundant large (a meter or so in diameter) lithic fragments, most commonly of the Lund 

Formation and of flow-layered felsitic or vitrophyric 

rhyolites. Abundant, large, cognate fragments of 

rhyolite appear to have formed by the crumbling of extrusive flow and dome margins or by explosive 

disintegration at the vent. Petrographically, Steamboat 

Mountain tuffs are generally indistinguishable from 

Blawn tuffs.

Most rhyolite lava flows of the Steamboat Mountain 

Formation range from light gray and moderately porphyritic (phenocrysts compose 25 percent of rock) to 

purple-gray, strongly flow-layered, and nearly aphyric 

with felsitic, aphanitic groundmasses. Exposed margins 

of lava flows and rare feeder dikes around the flanks 

of Broken Ridge are red, black, and green perlitic 

vitrophyres. Phenocryst-poor flow-layered extrusions 

are commonly spherulitic; lithophysae and vugs are 

lined with quartz and less abundant topaz. Phenocrysts, 

generally less than 3 mm in diameter, are chiefly smoky 

quartz and sanidine; biotite and plagioclase are rare, and 

minute amphiboles were found in only one sample a 

vitrophyre. Many Steamboat Mountain rhyolite lava 

flows are petrographically indistinguishable from Blawn 

lava flows at the Staats mine, The Tetons, and in the 

thick pile east of Blawn Wash,

Within the limits of analytical error, all dated Steamboat Mountain rhyolites (table Bl, samples 1-7) are 

12.4 m.y. old.

Slightly younger rhyolites (11.6 and 10.3 m.y.) described by Rowley and others (1978) occur in the southern Shauntie Hills (fig. B4) and on the southeast side 

of Escalante Valley.

MAFIC FLOWS

A mesa-capping sequence of mafic flows of the Steamboat Mountain Formation with ages of 13 to 12 m.y. 

(table Bl, samples 8-10) overlies rhyolitic lava flows and 

tuffs of the Blawn Formation on the east flank of the 

southern Wah Wah Mountains in the southwestern 

quarter of the Frisco 15-minute quadrangle (Abbott and 

others, 1983). The northern, younger flows are of 

aphyric trachyandesite, whereas the southern, older 

flows are of somewhat potassic mugearite (table B2; see 

Le Maitre, 1976). Clotted phenocrysts of plagioclase, 

augite, and olivine are characteristic of the mugearite 

flows. Petrographically similar flow rocks occur as 

boulders in erosional remnants of alluvial fans on the 

east side of Steamboat Mountain, indicating that 

similar flows once existed in the southern Indian Peak 

Range. Basaltic rocks with ages of 13 to 11 m.y. crop 

out elsewhere in southwestern Utah and the adjacent 

part of Nevada (Best and others, 1980).

PETROCHEMISTRY OF RHYOLITES

The silicic lava flows of the Blawn and Steamboat 

Mountain Formations range in composition from near 

average rhyolite (Le Maitre, 1976) to high-silica rhyolite 

that contains topaz and generally lacks mafic minerals 

(table B3). Flows of the 23- to 18-m.y.-old Blawn Formation span the entire composition range, whereas all 

dated samples of 13- to 12-m.y.-old Steamboat Mountain rhyolite that have been analyzed have high silica 

content. Though not determined in our samples, Christiansen (1980) shows that fluorine occurs in concentrations of 0.25 to 0.50 weight percent in the high-silica 

topaz rhyolites of both early and middle Miocene age 

from Broken Ridge and the Staats pluton.

The high-silica rhyolites that have more than 75 

weight percent SiO2 have, as expected, significantly 

lower TiO2 , MgO, CaO, Sr, and Ba than the less silicic 

rhyolites in the Blawn Formation that contain more 

abundant phenocrysts of mafic minerals and plagioclase. In the almost wholly felsic high-silica rhyolites, 

titanium, manganese, and magnesium are present only 

in trace amounts, whereas rubidium and niobium are 

especially abundant.

TECTONISM

The tectonic pattern of the central part of the PiocheMarysvale belt is a composite of north-south- and 

northeast-southwest-trending features. The faultbounded Wah Wah Mountains and Needle Range,
TABLE B3. Chemical composition of Miocene rhyolite from the southern Wah Wah Mountains and the Indian Peak Range, sou

[Analyses by X-ray fluorescence spectrometry using the Norrish and Hutton (1969) method for major and minor elements (except Na2O) and using 

dards were GH, MIN-G, and G2. See appendix for descriptions of samples and localities]

_ . . Steamboat Mountain Formation 

Constituent T 2 3 u 5

Si0 2 76.2 76.0 75.4 75.2 75.7 

Ti0 2 .07 .09 .06 .08 .10 

A1 2 0 3 12.33 12.2 12.4 12.4 12.5 

Fe 2 0 3 3 1 .2 1 .2 1 .2 1.1 1.1 

MnO .08 .09 .07 .03 .06

MgO .03 .04 .03 .~04 .06 

CaO .24 .05 .52 .54 .50 

Na 2 0 4.0 4.0 3.8 3-7 3.85 

K 2 0 4.92 5.0 4.93 4.94 4.88

Total 99.07 98.67 98.41 98.03 98.65

6

75.3 

.09 

12.5 

1 .1 

.07

.18 

.64 

4.0 

4.85

98.73

7 l 32 ' 33 14 1 34

Major oxides (data in percent)

76.4 76.8 77.3 75.8 75.8 

.08 .11 .10 .06 .06 

12.2 12.3 12.2 12.4 12.8 

1 .2 1 .16 1 .19 1.1 1 .3 

.05 .09 .08 .11 .05

.03 .10 .06 .05 .11 

.48 .91 .45 .62 .67 

3.2 4.21 3-63 3-7 4.06 

4.88 3.25 4.57 4.72 4.60

98.42 98.83 99.58 98.56 99.43

pressed pellets of pure re>ck powdea- 

Blawn Formation

13 21

74.5 76.1 

.28 .05 

12.8 12.5 

1.78 1.17 

.05 .14

.21 .05 

.69 .62 

3.7 4.2 

5.41 4.64

99.42 99.47

35

73-9 

.20 

13.0 

1.36 

.05

.34 

1 .21 

4.1 

5.05

99.21

Trace elements (parts per million)

Rb 555 415 515 275 375

Sr 15 10 10 10 15

Zr 225 205 185 180 200

Nb 125 90 110 60 85

Ba 20 20 15 15 30

515 

10 

165 

85 

20

'Analyses from Christiansen (1980). Sample 

U, 20; Hf, 7; Ta, 6; Th, 54; Cs , 87; La, 42; Ce , 

the following, in ppm: U, 14; Hf, 6; Ta, 10; Th, 

2Stratigraphic identity uncertain; flow lies 

top of pile of Blawn-age rhyolite flows. 

3Total iron expressed as Fe 2 0 3 .

270 595 690 572 

20 17 20 18

200 530 215

ob 1(0

it? o

200 645 

65 5 

310 155 

40 115 

395 25

530 

75 

214

340

32 (his WW-9) also contains 0.47 weight percent F and the 

94. Sample 33 (Christiansen 's STC-4) also contains 0.53 w

50; Be, 5; Cs , 5; La, 20; Ce, 50. 

immediately below 12.9±0.5-m.y.-old mugearite flow (sampl
42 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

aligned more or less north-south, are truncated at their 

southern ends by the northeast-trending northern 

margin of the Escalante Valley basin (figs. Bl and B4). 

The southern termini of these ranges are linked by a 

prominent zone of northeast-striking high-angle faults 

that coincides with a belt of early and middle Miocene 

silicic magmatic activity. Our data show the northeasttrending faulting was chiefly early Miocene. Development of the more conspicuous, northerly trending 

basins and ranges typical of this part of the Basin and 

Range province apparently followed the early Miocene 

event.

The pattern of northeast-striking faults of early 

Miocene age is displayed on figure B4. The virtually unfaulted segment of the Wah Wah Mountains north of 

Pine Grove, where few Tertiary volcanic rocks are exposed, contrasts sharply with the segment to the south, 

where numerous subvertical, northeast-striking faults 

cut voluminous Oligocene and Miocene volcanic rocks. 

This swarm of faults continues southwestward into the 

southernmost Indian Peak Range.

These northeast-striking faults, as well as the one 

northwest-striking fault zone extending from Blue 

Mountain to Blawn Mountain in the Wah Wah Mountains, have vertical displacements as great as 1 km. The 

northeast-striking faults are slightly oblique to the 

overall trend of the Pioche-Marysvale belt (fig. Bl inset) because of a more northerly strike. A crude en 

echelon pattern is apparent. Although no unequivocal 

strike-slip displacement of map units has been found, 

several well-exposed subvertical fault surfaces along the 

Bible Spring fault zone, in the low hills linking the 

southern termini of the Indian Peak Range and the Wah 

Wah Mountains, have strongly developed subhorizontal slickensides. The emplacement of the northeaststriking rhyolite dikes around the Cima mine in the Wah 

Wah Mountains and the Cougar Spar mine in the Indian Peak Range more or less contemporaneously with 

faulting (see below) indicates at least a local northwestsoutheast direction of extension.

The age of the northeast-striking faults is well constrained. Oligocene volcanic rocks and overlying early 

Miocene Blawn Formation are cut and tilted, whereas 

13- to 12-m.y.-old rhyelites of the Steamboat Mountain 

Formation are generally not faulted and lie unconformably on top of the older extrusions. At The Tetons (fig. 

B5) the constraints are even tighter because unfaulted 

18-m.y.-old Blawn rhyolite rests unconformably on 

tilted, eroded Oligocene units; older Blawn units and the 

22.3-m.y.-old Bauers Tuff Member of the Condor Canyon Formation are tilted in fault slices 4 km to the 

southeast.

Additional evidence for early Miocene block faulting 

is provided by deposits of chaotic megabreccia and 

associated coarse-cemented alluvium along the 

northeast-striking Bible Spring fault zone (fig. B4) that 

contain abundant clasts of 22.3-m.y.-old Bauers Tuff 

Member. These deposits, whose total area of exposure 

is about 4 km2, appear to represent landslide debris 

that sloughed off uplifted fault blocks. The deposits 

underlie virtually unfaulted, subhorizontal, 13- to 

12-m.y.-old rhyolite lava flows of the Steamboat Mountain Formation.

Locally, as at the northeast end of the Bible Spring 

fault zone, altered Steamboat Mountain rhyolite flows 

are cut by faults that might be reactivated early 

Miocene fractures. Minor structures on slickenside surfaces in the zone, indicating left-lateral movement, are 

well preserved in intensely silicified rocks, which apparently were produced by mid-Miocene hydrothermal 

activity that preceded the horizontal movement. 

Northeast-striking, left-lateral faults occur in the Star 

Range just west of Milford (Lemmon and Morris, 1979b) 

and in Lincoln County, Nevada (Ekren and others, 

1977), south of Pioche (fig. Bl).

ALTERATION AND MINERALIZATION

Hydrothermally altered and mineralized rocks are 

closely associated with silicic igneous rocks and with 

northeast-striking faults. Locally, Paleozoic carbonate 

rocks have been bleached and silicified, producing jasperoid. Rhyolitic tuffs are locally zeolitized or opalized. 

All volcanic units except the 13-m.y.-old mugearite 

flows are argillized or silicified at one place or another, 

either pervasively or selectively along fracture systems 

visible in single exposures. Considerable jasperoid occurs on Blawn Mountain (Lindsey and Osmonson, 1978; 

Whelan, 1965) and for a few kilometers east and northeast, where hills of alunitized tuffs stand in valleys 

underlain by less altered volcanic units. The more 

porous rocks in the upper part of the Lund Formation 

and the loosely consolidated tuffs and reworked 

deposits of the Blawn Formation are generally the most 

intensely altered, especially where they lie in graben 

wedges. Cinnabar and native sulfur occur in altered 

rhyolitic tuffs and fault-juxtaposed Paleozoic rocks at 

the Cima mine (fig. B4). Steeply dipping veins of quartz 

carbonate, some a meter or so wide, occur in many areas 

of argillically altered volcanic rocks, especially 

southeast of The Tetons (fig. B5).

Intracaldera tuff in the Wah Wah Springs Formation 

within the Indian Peak caldera in the Indian Peak
MIOCENE MAGMATISM AND TECTONISM, SOUTHERN WAH WAH MOUNTAINS 43

2 KILOMETERS

J

EXPLANATION

Quaternary alluvium

Blawn Formation:

Younger (18-m.y.-old) rhyolite flow

Younger (18-m.y.-old) tuff

Older (20-to 22-m.y.-old) rhyolite

flow 

Older (20-to 22-m.y.-old) tuff Includes

Bauers Tuff Member of Condor Canyon

Formation

Isom and Lund Formations 

Wah Wah Springs Formation

Cottonwood Wash Tuff 

Escalante Desert Formation 

Mxft Mesozoic and Paleozoic rocks, undivided

TERTIARY

TERTIARY

 Contact

T High-angle normal fault Ball and bar on downthrown side; dotted where concealed

xxx Steeply dipping quartz-carbonate vein 

ff Strike and dip of foliation

FIGURE B5. Geologic map and cross section of the area around The Tetons, Beaver County Utah. Modified from Best, Morris, and

others (1987).
44 VOLCANIC ROCKS IN THE PIOCHE-MARYSVALE IGNEOUS BELT, UTAH AND NEVADA

Range was widely propylitized about 29.5 m.y. ago, 

shortly after the caldera subsided. These propylitized 

rocks were locally overprinted by Miocene alteration 

related to silicic magmatism.

Keith (1980) has reviewed the nature and distribution 

of mineralized rocks from the Wah Wah Mountains to 

the White Rock Mountains. Gold, silver, and lesser 

amounts of lead, copper, tellurium, mercury, molybdenum, and uranium occur in quartz-carbonate vein 

systems in the Stateline area; gangue minerals in these 

veins include adularia, fluorite, pyrite, and ironmanganese oxides. Fissure-replacement deposits of lead, 

zinc, copper, silver, and traces of gold have been mined 

from sedimentary rocks adjacent to Oligocene quartz 

monzonite intrusions related to the Indian Peak caldera 

in the Indian Peak Range and adjacent to late Miocene 

porphyritic granite intrusions in Pine Grove; disseminated molybdenum and tungsten have recently been 

discovered at depth in the Pine Grove plutonic rocks. 

Fluorite has been produced from the brecciated and 

argillized carbonate rocks at the margin of the topaz 

rhyolite intrusion at the Staats mine and from veins and 

breccia zones in igneous rocks at the Cougar Spar and 

other nearby mines. The only recorded production of 

uranium has come from the margin of the intrusion at 

the Staats mine, where Lindsey and Osmonson (1978) 

indicate a potential also exists for valuable concentrations of tin, molybdenum, and beryllium. Economic 

deposits of uranium, thorium, and molybdenum are 

possible in porous tuffs at the base of topaz rhyolite 

flows and in quartz-carbonate veins and their wall rocks 

in the Stateline district, where high concentrations of 

these elements (U, 1,300 ppm; Mo, 7,600 ppm) have been 

found.

Early Miocene potassium-argon ages have been 

obtained on alunite from the NG claims at the head 

of the Blawn Wash and from White Mountain in the 

Shauntie Hills (table Bl, samples 11 and 12). Much of 

the alteration in the Indian Peak Range and Wah Wah 

Mountains took place during Blawn igneous activity 

because of the intimate association between altered 

rocks and extrusive and intrusive rhyolite centers of 

that age. A preliminary potassium-argon age on sericite 

from the porphyry of Pine Grove (J. T. Abbott, written commun., 1979) correlates with the Blawn igneous 

episode. The 13- to 12-m.y.-old rhyolite flows of the 

Steamboat Mountain Formation are generally unaltered and locally rest on altered older rocks, as east of 

Broken Ridge. Elsewhere, though, these younger flows 

are also altered, indicating that the younger silicic 

magmatism was also accompanied by hydrothermal 

activity.

DISCUSSION AND CONCLUSIONS

Early to middle Miocene magmatic, hydrothermal, 

and tectonic activity in the central part of the PiocheMarysvale belt in southwest Utah was clearly confined 

to a northeast-trending belt about 30 km wide that extends from the Shauntie Hills southwestward at least 

90 km into Nevada. All three types of activity were 

manifest in the early Miocene during emplacement of 

the Blawn Formation, whereas the middle Miocene 

magmatic-hydrothermal episode lacked conspicuous 

tectonism. The younger silicic activity that created the 

Steamboat Mountain Formation paralleled the northeast trend established some 5-10 m.y. earlier but was 

displaced a few kilometers to the south. Both magmatic 

pulses were bimodal.

TECTONISM

Zoback and others (1981) have proposed that a westsouthwest to east-northeast direction of crustal extension prevailed uniformly throughout the Western 

United States 20-10 m.y. ago. This tensional direction 

conflicts with the orientation of the early Miocene 

(23-18 m.y. old) northeast-trending dikes and faults in 

the central part of the Pioche-Marysvale belt. Perhaps 

a slightly earlier period is represented in the belt, or 

perhaps there was a local spatial perturbation in the 

regional stress field. Such a local perturbation could be 

caused by the buttressing effect of a large Oligocene 

batholith of calc-alkalic affinity that must underlie the 

Indian Peak caldera complex (fig. B4). Alternatively, or 

in addition, a localized perturbation may have resulted 

because the central part of the Pioche-Marysvale belt 

is close to a northeast-trending Paleozoic-Mesozoic 

depositional hinge line, which roughly follows the 

western margin of the Colorado Plateaus (fig. Bl, inset).

Anderson and Ekren (1977) and Zoback and others 

(1981) emphasize that since about 10 m.y. ago the direction of crustal extension in the Great Basin has been 

west-northwest to east-southeast. This deformation 

field has been chiefly responsible for the northerly 

trending basins and ranges of the eastern Great Basin. 

However, in the central part of the Pioche-Marysvale 

belt the early Miocene northeast trend appears to truncate the supposedly more recent northerly trending 

ranges.

It is obvious that "normal faults in the Basin and 

Range province form a complex geometry and history 

that cannot be accomodated by a single applied field 

of stress either in time or space" (Anderson and Ekren, 

1977, p. 389).
MIOCENE MAGMATISM AND TECTONISM, SOUTHERN WAH WAH MOUNTAINS 45

MAGMATISM

Early and middle Miocene high-silica, high-alkali 

rhyolite flows and related intrusive porphyries in the 

central part of the Pioche-Marysvale belt are enriched 

in lithophile elements. Potentially economic deposits of 

molybdenum, tungsten, tin, beryllium, uranium, and 

niobium exist. (See, for example, Tucker and others, 

1981).

Middle Miocene rhyolites of the Steamboat Mountain 

Formation have fairly uniform compositions, consisting 

almost wholly of felsic, topaz-bearing flows and minor 

tuffs. In contrast, the silicic rocks of the older Blawn 

Formation range in composition from highly evolved 

rhyolites identical to those in the Steamboat Mountain 

Formation (having very low strontium, barium, 

titanium, and magnesium) to less evolved rhyolites and, 

very locally in the Pine Grove intrusive rocks, dacitic 

rocks (Keith, 1982).

Christiansen (1981) shows that widespread topaz 

rhyolites in the Western United States are almost all 

of post-Oligocene age. They were emplaced in a bimodal 

association with mafic igneous rocks during crustal extension in areas underlain by a thick sialic crust that 

had been previously subjected to calc-alkalic magmatism. He argues that high-fluorine parental magmas 

were derived by partial melting of deep crustal rocks 

that had lost hydrous components during the earlier 

calc-alkalic activity. Subsequent differentiation of these 

fluorine-bearing magmas led to further enrichment of 

fluorine and other lithophile elements in apical parts of 

high-level magma chambers.

The aeromagnetic map (fig. B2) suggests that a large 

magmatic body (or bodies), now solidified and still deeply buried, once fed the many small silicic extrusions and 

shallow intrusions of Miocene age in the central part 

of the Pioche-Marysvale belt. It is impossible to decide 

what precluded an explosive, caldera-forming eruption 

of large volumes of this silicic magma (Hildreth, 1981), 

but it is tempting to speculate that the early Miocene 

extensional fracturing continuously permitted small 

amounts of magma to leak to the surface, thereby relieving pressure from a crustal magma chamber that otherwise might have erupted in large volume and collapsed 

(Bacon, 1982). However, there is little compelling 

evidence that major crustal extension continued during middle Miocene time, when extrusions of silicic 

lavas and pyroclastic material again formed at many 

centers not related to a caldera.

The difficulty in explaining, on the basis of available 

data, the southward shift to younger, more uniformly 

evolved silicic magma extrusions is a complementary

problem to that of accounting for the space-timecomposition pattern in Miocene mafic activity in southwestern Utah (Best and others, 1980). East- to 

northeast-trending belts of mafic extrusions dated at 

24-20, 14-10, 9-5, and 2 m.y. that extend across the 

Great Basin and into the western part of the Colorado 

Plateaus shift southward about 120 km in the central 

to eastern part of the Pioche-Marysvale belt (Best and 

others, 1980). The early Miocene lava flows in the north 

are not strictly basalts but are relatively rich in 

potassium and have intermediate silica contents. In contrast, the younger, more southerly mafic extrusions 

span the range of compositions from tholeiitic to alkali 

olivine basalts and include mugearite and hawaiite as 

well as andesite. Generally, potassium concentrations 

are lower in younger mafic lavas. We have not determined whether any other mantle-incompatible elements 

are less common in the younger lavas.

We are just beginning to see some of the details of 

intriguing space-time-composition patterns in the episodic bimodal magmatism that accompanied a complicated period of tectonism along the border between 

the Colorado Plateaus and the Great Basin. The relationship of this Miocene tectonomagmatic activity and 

of the Pioche-Marysvale belt as a whole to other easttrending belts of magmatism and mineralization in the 

Western United States and to possible deep continental or upper mantle phenomena must be a subject for 

future speculation.

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Zietz, Isidore, Gilbert, F. P., and Kirby, J. R., Jr., 1978, Aeromagnetic 

map of Nevada: U.S. Geological Survey Geophysical Investigations Map GP-922.

Zietz, Isidore, Shuey, Ralph, and Kirby, J. R., Jr., 1976, Aeromagnetic 

map of Utah: U.S. Geological Survey Geophysical Investigations 

Map GP-907.

Zoback, M. L., Anderson, R. E., and Thompson, G. A., 1981, Cainozoic 

evolution of the state of stress and style of tectonism of the Basin 

and Range province of the western United States: Royal Society 

of London Philosophical Transactions, ser. A, v. 300, no. 1454, 

p. 407-434.
MIOCENE MAGMATISM AND TECTONISM, SOUTHERN WAH WAH MOUNTAINS 

APPENDIX. Descriptions of samples and sample localities in the Wah Wah Mountains and nearby areas, southwestern Utah

[Sample numbers correspond to those used in tables Bl, B2, and B3]

47

Sample 

No.

1

2

3

i*

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

Field No.

BBS-8-229-2

BBS- 5- 23-1 

BBS- 5-25- 5~

BBS-7-88-4 

BBS-7-93-1 

nor y O "

STM-7-153-1

LAM-956-1 

LAM-9-103-3

WAH-65 

M244 

80-S-31B 

BUCK- 8- 179- 2

TET-9-43-2 

82-S-16 

M726 

PNT-8-137-1

PNT-8-137-1

PNT-8-137-1

PUT- 8- 136-1

LAM- 1-38- 2 

LAM- 9-74-9 

WAH-PD 

 ( l ) 

 ( 2 ) 

 <«) 

 ( *) 

WAH-66 

BAN- 8- 165- 2

83-3-1 

LAM- 9-1 04- 5

WW-9 

WW-10 

STC-4 

TET-9-77-1 

LAM-9-104-4

Lithologic description

Felsic, porphyritic, topaz-bearing rhyolite

flow.

 do 

Perlitic, vitrophyric base of topaz-bearing

rhyolite flow.

Felsic, porphyritic rhyolite flow 

 do 

rhyolite flow.

Perlitic, vitrophyric base of rhyolite flow

Trachyandesite flow 

Mugearite flow 

 do 

Alunitic alteration 

 do 

Strongly porphyritic felsic dike containing

amphibole phenocrysts.

Flow-layered felsic topaz-bearing rhyolite

flow.

Flow-layered rhyolite 

Altered rhyolite dike containing

disseminated pyrite.

Perlitic, vitrophyric rhyolite flow 

 do 

 do 

Argillized rhyolite dike 

Felsic, porphyritic rhyolite flow 

Flow-layered felsic rhyolite plug 

Felsic rhyolite dike 

Intrusive plug feeder and extrusive

rhyolite flow.

Rhyolite flow 

 do 

Rhyolite intrusion 

Basal trachyandesite flow 

Lava flow 

Slightly altered granite of Moscow Canyon 

Trachyandesite flow 

Green perlitic vitrophyre flow 

Felsic rock from same outcrop as 35 

Felsic porphyritic rhyolite 

Strongly porpyhritic, felsic flow with

phenocrystys of feldspar and smoky quartz

as much as 1 cm across.

Biotite-bearing perlitic vitrophyre flow 

Locality description

Mountain Spring 

S. side of Sawmill Canyon 

W. end of North Peaks 

NE. of White Cliff 

Ponderosa Park 

Four-Mile Wash 

S. end of Wilson Canyon 

Near Willow Creek, 4 km E. of

Baudino Ranch.

E. side of Wah Wah Mts between

Blawn Mtn and Miners Hill

Reservoir.

E. side of Wah Wah Mts, 4 km

W. of Brimstone Reservoir.

NE. of Blawn Mtn in NG claims

E. end of White Mtn 

E. side of Indian Peak Range,

6 km SE. of Indian Peak.

The Tetons 

SE. flank San Francisco Mts 

6 km S. of Indian Peak and 1

km NW. of Cougar Spar mine.

2 km E. of Cougar Spar mine 

 do 

 do 

5 km S. of Cougar Spar mine 

5 km W. of Miners Hill Reservoir, E. flank Wah Wah Mts.

3 km NE. of Blawn Mtn 

5 km SE. of Pine Grove 

Staats mine 

Along Willow Cr, E. flank of

Wah Wah Mts.

 do 

Serviceberry Canyon 

E. side of Wah Wah Mts 

NE. side Escalante Desert 

In Moscow Canyon 

In Willow Cr, 2 km SE. of

Baudino Ranch.

SE. end of Broken Ridge, SWK,

sec. 2, T. 32 S., R. 16 W.

 do 

Near Staats mine 

W. side of Blawn Wash, SW^

sec. 14, T. 30 S., R. 15 W.

Near Willow Cr, E. side of

Quadrangle

Bible Spring.

Do.

Do.

Do.

Do.

Peak.

Steamboat Mountain.

Frisco.

Do.

Do.

Lamerdorf Peak.

Frisco.

Buckhorn Spring.

The Tetons.

Milford.

Pinto Spring.

Do.

Do.

Do.

Do.

Frisco.

Lamerdorf Peak.

Do.

The Tetons.

Frisco.

Do.

Deer Lodge Canyon.

Frisco.

Bannion Spring.

Milford.

Frisco.

Mountain Spring

Peak.

Do.

The Tetons .

Do.

Frisco.

38

LAM-2- -doLAM-9-103-2 Felsic porphyritic, topaz-rich flow at top 

of pile of rhyolite just below 12.9-m.y.- 

old mugearite (sample 9).

Wah Wah Mts, SEJ; sec. 1,

T. 29 S., R. 15 W. 

Near Willow Cr on E. side of

Wah Wah Mts, NE^ sec. 31,

T. 28 S., R. 14 W. 

E. flank of Wah Wah Mts -

Do.

Do.

'From Rowley and others (1978).

2 From Lemmon and others (1973).

'From Keith (1980).

* U.S. GPO: 1987 775-W7/6B006