Stratigraphy, Preliminary 

Petrology, and some

- Structural Features of 

Tertiary Volcanic Rocks

 in the Gabbs Valley 

and Gillis Ranges, 

Mineral County, Nevada

n K o :i, o <; I c: A i. s u » v K y « u i. r. i; 7 i \ i 4 >i

-4.

Stratigraphy, Preliminary 

Petrology, and some 

Structural Features of 

Tertiary Volcanic Rocks 

in the Gabbs Valley 

and Gillis Ranges, 

Mineral County, Nevada

By E. B. EKREN, F. M. BYERS, JR., R. F. HARDYMAN, 

R. F. MARVIN, and M. L. SILBERMAN

GEOLOGICAL SURVEY BULLETIN.1464

Stratigraphy and petrologic features of voluminous 

ash-flow tuffs that were deposited in the Walker 

Lane and were then subjected to severe en echelon, 

right-lateral strike-slip faulting

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretory

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Stratigraphy, preliminary petrology, and some structural features of Tertiary volcanic rocks

in the Gabbs Valley and Gillis Ranges, Mineral County, Nevada. 

(Geological Survey Bulletin 1464) 

Bibliography: p. 52

1. Geology, Stratigraphic Tertiary. 2. Volcanic ash, tuff, etc. Nevada Mineral Co. 

3. Geology-Nevada-Mineral Co. I. Ekren, Einar Bartlett, 1923- II. Series: United

States Geological Survey Bulletin 1464. 

QE75.B9 [QE691] 557.3'08s [557.93'51] 78-11872

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-03261-9
CONTENTS

	Page

Abstract .. . ... . . ... i

Introduction ----.-------------.-----------..-- - - 2

Previous work 2

Geologic setting 3

Stratigraphy and geochronology . - - -- --. 10

Lavas of Giroux Valley 10

Benton Spring Group 11

Mickey Pass Tuff -- - .. . - 15

Guild Mine Member . 15

Weed Heights Member 24

Singatse Tuff------------------------------- 26

Petrified Spring Tuff - -- -- - 28

Age of the Benton Spring Group 29

Ash-flow tuffs between the Mickey Pass and Singatse Tuffs 29

Local units between the Blue Sphinx Tuff and the Benton Spring Group -- 31

Intermediate lavas - 31

Sedimentary rocks 31

Ash-flow tuffs - -. . -..- . .. 32

Blue Sphinx Tuff .---.- - 33

Intermediate lavas of Nugent Wash area 36

Hu-pwi Rhyodacite ------ -- ------ 36

Nugent Tuff Member - - - 37

Ghost Dance Lava Member 38

Intermediate lavas of Poinsettia Spring area 39

Poinsettia Tuff Member 40

Age and distribution of Hu-pwi Rhyodacite 41

Tuff of Copper Mountain 41

Rocks of Mount Ferguson - - - -- - - 43

Tuff of Redrock Canyon 44

Tuffaceous sedimentary rocks and landslipped debris 46

Lavas of intermediate composition 46

Intrusive rocks - -. -- .- . -- - -- - --- - 47

Preliminary petrology and volcano-tectonic origin of the thick volcanic section - 48

Significance of phenocryst resorption 49

Significance of a virtual lack of compositional zoning in the Hu-pwi Rhyodacite and reverse zoning in tuffs of the Benton Spring Group -- 50 

Relationship of volcanic center to Walker Lane 51

Acknowledgments. . - .-.. -..- ......- .. .. ... .- 52

References cited 5 2

in
IV CONTENTS

ILLUSTRATIONS

Page

FIGURE 1. Schematic diagram showing relationships of principal volcanic

units in the Gabbs Valley and Gillis Ranges 5

2. Map showing type and reference localities of volcanic units described

in this report ...... ... .......................... g

3. Map showing inferred original distributions of Singatse Tuff and ; 

Guild Mine Member of Mickey Pass Tuff 8

4. Bar graphs showing volume of phenocrysts and abundance of six 

crystal components of principal volcanic rocks of Gabbs Valley 

and Gillis Ranges, Mineral County, Nev. 15

5. Photograph of faulted outcrops of the lower two formations of the ± 

Benton Spring Group - -- - - - - 18

6. Graph showing phenocryst variations from base to top of Guild 

Mine Member of Mickey Pass Tuff at reference locality and 

vicinity in the Gabbs Valley Range and at type locality of the r~ 

member in the Singatse Range 22

7. Photograph of the Guild Mine Member of the Mickey Pass Tuff at

the reference locality just north of Todd Mountain 23

8. Graph showing phenocryst variations in the Weed Heights Member 

of the Mickey Pass Tuff at the type locality of both in the Singatse

Ran IXdllge fro 

 

. ____ 

 

_ __ 

 

_ 

 OC £t*j

9. Graphs comparing phenocryst variations of Singatse Tuff at ^ 

Calavada Summit in the Gabbs Valley Range with Singatse Tuff 

at the type locality ---- -- 30 '-'

10. Photograph northeastward showing Blue Sphinx monolith the

type locality of the Blue Sphinx Tuff 34

11. Photomicrograph of thin section of Blue Sphinx Tuff showing

intensely resorbed quartz phenocryst 35

12. Photomicrograph of tuff of Redrock Canyon showing shattered,

intensely resorbed quartz phenocryst 45

TABLES

TABLE 1. Principal Tertiary volcanic units in the Gabbs Valley and Gillis

Ranges - - - - - ---- 4

2. Analytical data for age determinations of Tertiary volcanic rocks 12

3. Chemical analyses and norms of Mickey Pass Tuff, Singatse Tuff, 

and tuff of Copper Mountain from Singatse Range and Gabbs 

Valley Range- ---- ---- - 20
STRATIGRAPHY, PRELIMINARY PETROLOGY, 

AND SOME STRUCTURAL FEATURES

OF TERTIARY VOLCANIC ROCKS

IN THE GABBS VALLEY AND GILLIS RANGES,

MINERAL COUNTY, NEVADA

By E. B. EKREN, F. M. BYERS, JR., R. F. HARDYMAN, 

R. F. MARVIN, and M. L. SILBERMAN

ABSTRACT

A composite thickness of more than 2,000 m of Tertiary (22 m.y. to 27 m.y.) subalkaline, genetically related, ash-flow tuffs and intercalated lava flows of intermediate 

composition is exposed in the Gabbs Valley and Gillis Ranges in west-central Nevada. 

The two oldest ash-flow units can be traced for distances of more than 150 km. Six 

regionally extensive tuff sheets, locally separated by lava flows, are recognized: the 

Mickey Pass and Singatse Tuffs (names defined by Proffett and Proffett, 1976 and 

adopted in this report; included here together with the Petrified Spring Tuff in the 

Benton Spring Group, a new name), the herein-named Blue Sphinx Tuff, the hereinnamed Nugent and Poinsettia Tuff Members of the Hu-pwi Rhyodacite, and the tuff 

of Copper Mountain. In contrast to most tuff sheets that grade upward from silicic to 

intermediate composition, several of these units in western Nevada grade upward 

from bases of intermediate composition to silicic tops. One unit displays a remarkable 

change upward from rhyodacite at the base through quartz latite to rhyolite at the 

top.

The tuff sheets show northwesterly elongated distribution patterns that reflect a 

pretuff topographic trough. This trough coincides with the present-day Walker Lane 

and probably reflects the Walker Lane as a distinct rift zone prior to ash-flow tuff 

eruptions. In early Miocene time (at least 23 m.y. ago), after many of the tuff sheets 

were erupted, the region was subjected to conjugate strike-slip faulting along 

northwest- and northeast-trending faults. Most of the movement was right-lateral 

along a series of en echelon faults with northwest trends. This faulting continued into 

Quaternary time as indicated by fault scarps and shutter ridges in alluvium.

The eruption of tuff and lava in volumes comparable to that in the Gabbs Valley 

and Gillis Ranges normally gives rise to a large cauldron complex, but no such complex appears possible within this area. The possibility exists that, instead of a faultbounded cauldron complex, the massive extrusions formed a southeasterly elongated 

depression or depressions whose axes were more or less coincident with the pretuff 

topographic trough. The fact that many tuffs contain intensely resorbed phenocrysts 

of quartz, plagioclase, and, in some units, alkali feldspar suggests that the magma 

was erupted from levels that were undersaturated with respect to water. Such undersaturation could indicate eruption from great depths. This possibility is suggested 

also by the unique zonation of the tuffs from bases of intermediate composition to 

silicic tops. Perhaps these depths were too great to allow the development of normally 

zoned magma chambers and well-defined cauldrons.
2 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

INTRODUCTION

The geology of the Gabbs Valley and Gillis Ranges, Mineral County, Nev., is being studied as part of the U.S. Geological Survey's 

Earthquake Hazards Reduction Program. Previous geologic work 

(see next section) and the high seismicity of the area (Ryall and 

others, 1966; Gumper and Scholz, 1971; Slemmons, 1967) indicated 

that the area might be suitable for an earthquake control experiment. 

To provide the geologic and tectonic information deemed necessary 

for confirmation of suitability, detailed geologic mapping was started 

in the late spring of 1973.

One of the first objectives of the mapping program was to acquire a 

working knowledge of the stratigraphy of the voluminous Tertiary 

volcanic rocks exposed in the ranges. This report is a product of that 

phase of the work.

PREVIOUS WORK

The Gabbs Valley and Gillis Ranges and a large area surrounding 

these ranges in Mineral and Nye Counties, Nev., are the sites of 

pioneering studies by H. G. Ferguson and S. W. Muller that began in 

1922 and continued intermittently through 1949. The most significant results of these investigations are in Muller and Ferguson (1939) 

and Ferguson and Muller (1949); these investigations, along with 

those of Silberling and Roberts (1962), and Nielsen (1965), concentrated almost exclusively on stratigraphy and structure of Mesozoic 

sedimentary rocks and batholithic rocks. Very little attention was 

paid to the extensive and thick Tertiary volcanic and sedimentary sequences until 1956, when D. C. Ross of the U.S. Geological Survey 

began investigating the geology of Mineral County as part of a 

cooperative program between the Nevada Bureau of Mines and the 

U.S. Geological Survey. The work of Ross, who was later accompanied by F. J. Kleinhampl of the USGS, was of necessity reconnaissance in nature; they grouped the volcanic rocks into four broad 

categories (Ross, 1961).

In the late 1960's, Kleinhampl, M. L. Silberman, and R. Kopf reconnoitered the Gabbs Valley and Gillis Ranges and other parts of 

Mineral County in greater detail as part of the Nevada State geologic 

map project (Stewart and Carlson, 1974). This work led to a better 

understanding of the ages of the major Tertiary lithologic units.

Recently Proffett and Proffett (1976) described and named various 

ash-flow tuffs in the Yerington district of the Singatse Range northwest of the Gabbs Valley and Gillis Ranges. The first draft of our 

manuscript was completed prior to learning that the Yerington 

report was being prepared. We subsequently revised our manuscript 

to avoid, as much as possible, duplicating the names and excellent
GEOLOGIC SETTING 3

descriptions of Proffett and Proffett (1976); we have adopted the 

nomenclature of two of their formally named ash-flow tuff formations, the Mickey Pass and Singatse Tuffs.

These tuffs and others named and described herein occur also in the 

Wassuk Range, where they have been briefly described but not formally named by Bingler (1973).

GEOLOGIC SETTING

The Gabbs Valley and Gillis Ranges lie within the Walker Lane as 

originally defined by Locke, Billingsley, and Mayo (1940). The two 

ranges, which contain Tertiary volcanic rocks more than 2,000 m 

thick (table 1 and fig. 1), are cut by five well-exposed northwesttrending right-lateral strike-slip faults having a combined displacement of at least 32 km (fig. 2). The fault that bounds the eastern flank 

of the Wassuk Range (fig. 3) probably is an oblique-slip fault that 

may provide an additional 5-10 km of right-lateral displacement. In 

addition to the regional strike-slip faults, virtually thousands of 

moderate-angle (30°-60°) normal and oblique-slip faults occur that, 

in general, strike parallel to the strike-slip faults and greatly extend 

the Tertiary strata. Detachment and bedding-plane faults occur at 

several stratigraphic horizons within the Tertiary section where there 

is marked contrast in lithologic competency. These are low-angle 

faults (0°-30°) that, in general, displace younger strata over older. 

They probably resulted from the same stresses that produced the 

strike-slip faults (Hardyman and others, 1975). In the Gabbs Valley 

and Gillis Ranges a major detachment fault separates the bulk of the 

Tertiary volcanic section from the underlying Mesozoic "basement." 

This "uncoupling" of Tertiary strata from the basement rocks was 

first noted by F. J. Kleinhampl, R. Kopf, and M. L. Silberman during 

mapping for the Nevada State geologic map (Stewart and Carlson, 

1974).

The Mesozoic basement rocks in this region include thick Triassic 

and Jurassic marine sedimentary and volcanic rocks that were thrust 

faulted and tightly folded during the Jurassic Period (Ferguson and 

Muller, 1949). These rocks are extensively intruded by Mesozoic 

batholithic rocks that range in composition from diorite to granite. 

The batholithic rocks are spatially and temporally part of the Sierra 

Nevada. We have obtained dates of 158 ± 4 m.y. on biotite and 155 ± 

7 m.y. on hornblende from granodiorite at Copper Mountain (fig. 2). 

These dates indicate a late Middle or early Late Jurassic age. According to the chronology scheme of Lanphere and Reed (1973), the 

granodiorite was intruded during "epoch B" of Mesozoic-Cenozoic 

plutonism (158-132 m.y. ago), and between the Yosemite and the Inyo Mountains intrusive epochs of Evernden and Kistler (1970).
TABLE \.-Principal Tertiary volcanic units in the Gabbs Va

Ranges, Mineral County, Nev.

Age

Miocene

Oligocene or 

Miocene

0) 

B

.»

0

K-Ar age

(m.y.)

15-23.5

22-23

23

25-26

24-28

26-27

Rock unit

Rocks of Mount Ferguson 

Lavas of intermediate composition 

Tuff of Redrock Canyon 

Tuffaceous sedimentary rock and 

debris

Tuff of Copper Mountain

2

a"§

£

rt

D. 

3

K

Poinsettia Tuff Member

Ghost Dance Lava Member/ 

Intermediate lavas of 

Poinsettia Spring area

Nugent Tuff Member

Intermediate lavas of Nugent Wash 

area

Blue Sphinx Tuff

Tuff of Gabbs Valley equivalent to 

Blue Stone Mine Tuff of Proffett 

and Proffett (1976)

Sedimentary rocks

Intermediate lavas between the Blue 

Sphinx and Singatse Tuffs

Benton Spring Gr.

Petrified Spring Tuff 

Singatse Tuff 

Mickey Pass Tuff 

Weed Heights Member 

Guild Mine Member

Lavas of Giroux Valley

Thickness 

(meters)

0-900

0-100

0-500

0-100

0-500

0-80

0-300

0-80

0-30

0-200

0-75 

100-300

0-70 

70-300

0-30

Tentative class

Principally rhyoda

quartz latite

Quartz latite and r

Rhyodacite

Rhyodacite

Rhyodacite

Rhyodacite

Quartz latite

Rhyolite

Rhyodacite and qu

latite

Rhyodacite, quartz 

and rhyolite

Rhyodacite and qu
Rocks of 

Mount Ferguson Lavas of intermediat

Tuff of Copper 

Mountain

Tuff of 

Copper Mountain

Tu

Pomsettia 

Tuff Member

Ghost Dance 

Lava Member

Nugent 

Tuff Member

Intermediate lavas of Nugent Wash 

Miocene

or 

Oligocene Tuff of Gabbs Valley Tuff of Gabbs Valley=Blue Stone Mine Tu

Petrified 

Spring Tuff

Petrified Spring Tuff

Lavas of intermediate compos

Weed Heights Member (whm)

Lavas of Giroux Valley (Igv)

'Lavas of intermediate composition and tuffaceous sedimentary rocks intertongue with but are 

2 Of Proffett and Proffett (1976)

FIGURE 1. Schematic diagram showing relationships of principal volcanic units i
STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

39°00

RAW HIDE 5 KM.

R. 31VS E R 32 E

38°30'

FIGURE 2. Map showing type and reference localities of volcanic units described in 

this report. Heavy lines, strike-slip faults; arrows show direction of relative movement. Dashed where uncertain; queried where unknown. Also shown are southern 

limits of preserved outcrops of younger tuff units. The displacement indicated by 

right-lateral shift of Mesozoic rocks along the fault that passes west of Benton 

Spring is almost exactly equal to the shift of the Blue Sphinx Tuff about 9 km. 

MP-1, -2, reference localities of Guild Mine Member of Mickey Pass Tuff; GV, out-
GEOLOGIC SETTING

H 35 E 118°00'

crops of lavas of Giroux Valley; PS, type locality of Petrified Spring 

Tuff; BS, type and reference localities of Blue Sphinx Tuff; P, type 

locality of Poinsettia Tuff Member of Hu-pwi Rhyodacite; N, type 

locality of Nugent Tuff Member of Hu-pwi Rhyodacite; GD, type 

locality of Ghost Dance Lava Member of Hu-pwi Rhyodacite; CM, 

outcrops of tuff of Copper Mountain. Base from AMS 1:250,000 

Walker Lake, Nev., Calif., 1957 (revised 1969).
8 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

120° 119°

rt ^

^ Va..."

- pnx--r- ' y ....^^ <.c--- u^^H-^?-t^~^\ x

M, pP/ -<Vv 

s>

FIGURE 3. Map showing inferred original distributions of Singatse Tuff and Guild 

Mine Member of Mickey Pass Tuff. Also shown are reconstructed distributions of 

Blue Sphinx Tuff and Hu-pwi Rhyodacite. The boundaries are queried where data 

are poor, and the boundaries of Blue Sphinx and Hu-pwi are projected considerable 

distances southwestward from preserved outcrops. (Compare with fig. 2.) We 

believe that the patterns shown here for the tuff sheets represent the maximum
GEOLOGIC SETTING

118°

V\ .f.;».f..l SsSgv

distributions reasonable. Also shown are strike-slip faults (heavy lines, dashed 

where inferred); arrows show direction of movement. Map is based on our own 

observations, those of Proffett and Proffett (1976), Bingler (1978), unpublished data 

supplied by E. C. Bingler, H. F. Bonham, Jr., and R. C. Speed (oral and written communs., 1975, 1976), and Gilbert and Reynolds (1973). Area of figure 2 shown by rectangle. Base from U.S. Geological Survey 1:1,000,000 Nevada (1965).
10 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

The Mesozoic basement rocks had been deeply eroded and the 

paleotopography in the area of study was subdued when the first ashflow tuffs of Oligocene age swept over the terrane. The distribution of 

these tuffs (fig. 3), however, indicates control by a regionally extensive northwest-trending topographic trough. We infer that this 

trough reflects the Walker Lane (fig. 2) as a distinct rift zone before 

the advent of ash-flow tuff eruptions. That faulting or rifting continued during the period of ash-flow tuff eruptions is indicated by the 

Poinsettia Tuff Member of the Hu-pwi Rhyodacite in the Gabbs 

Valley Range. The tuff was deposited there, mostly in a faultcontrolled structural and topographic trough. The fault that bounds 

this tuff on its southwest side is definitely pre-Poinsettia in age and it 

trends N. 65° W., an orientation that is parallel to the axes of 

distribution of all the tuffs (fig. 3). This fault is older than about 23 

m.y. and the pre-Poinsettia movements could have been, in part, 

strike-slip. Occurrences of landslipped debris beneath the tuff of 

Redrock Canyon in a zone 14 km long betweeen the two easternmost 

faults (figs. 2 and 3) suggest that strike-slip faulting did, indeed, commence at least 23 m.y. ago. That this faulting did not start much 

earlier is indicated by all the displacements along the faults shown in 

figure 2. As nearly as can be estimated, the displacements are the 

same in Tertiary strata as in the basement rocks. Shutter ridges and 

fault scarps in alluvium indicate that strike-slip faulting continued 

into Quaternary time.

STRATIGRAPHY AND GEOCHRONOLOGY

LAVAS OF GIROUX VALLEY

The oldest Tertiary volcanic rocks in the Gabbs Valley and Gillis 

Ranges comprise lava flows, flow breccias, and tuff breccias, all of intermediate composition. These rocks are rarely more than 30 m thick 

and they are exposed discontinuously. They are informally called the 

lavas of Giroux Valley after andesitic and latitic lava flows near the 

head of Giroux Valley at Calavada Summit (fig. 2). In the vicinity of 

Calavada Summit, these lavas underlie the Mickey Pass Tuff but are 

in low-angle detachment-fault or bedding plane-fault contact with 

the overlying younger tuff. In the southern Gillis Range, rocks included with the lavas of Giroux Valley are more silicic than those in 

the Gabbs Valley Range and comprise conspicuously porphyritic 

quartz latite and rhyodacite. 1 The lavas of Giroux Valley have not 

been recognized north of about lat 38 °37'. It is problematical whether 

the absence of these lavas to the north and over broad areas in the 

south is due to nondeposition, erosion prior to ash-flow tuff eruption, 

or omission by low-angle detachment faulting. We consider that the

'Names are tentative and are based on phenocryst mineralogy no chemical analyses are available.
STRATIGRAPHY AND GEOCHRONOLOGY 11

first possibility is the most likely and that the lavas were never contiguous over great distances.

The lavas of Giroux Valley consist of two or more lava flows (or 

flow-breccia sequences) in nearly all exposures, and in the southern 

Gillis Range they include a thin stratum of biotite-rich tuff breccia 

that yields a K-Ar age of 26.7 ± 0.9 m.y. (table 2). The andesitic and 

calc-alkaline latitic lavas and flow breccias in the Gabbs Valley 

Range are typically stony (devitrified), somber gray on fresh surfaces, and weather dark brownish gray and dark purplish gray. The 

quartz latite and rhyodacite lavas of the Gillis Range, however, are 

entirely glassy in some exposures, whereas in others they are completely devitrified except for thin glassy (vitrophyric) zones near the 

base and top. Phenocrysts are mostly small, rarely exceeding 3 mm, 

and, in general, they compose less than 30 percent of the volume of 

the rock (fig. 4).

BENTON SPRING GROUP

The Benton Spring Group is here named after Benton Spring (figs. 

2 and 5) in T. 9 N., R. 34 E., the type locality, and comprises in 

ascending order the Mickey Pass, Singatse, and Petrified Spring 

Tuffs. The Petrified Spring Tuff is not present at Benton Spring, but 

all three formations are well exposed in fault blocks east of the spring 

and within 2 km northwest of Calavada Summit, on Nevada State 

Highway 23 (fig. 2); Calavada Summit, in T. 9 N., R. 35 E., is here 

designated a reference locality for the Benton Spring Group. The 

group name is proposed because the three formations appear to be 

closely associated in time, in petrography, in distribution, and, 

perhaps, in source. The close time association between the three units 

is inferred from K-Ar age data and from general petrologic 

similarities. No significant stratigraphic units occur between the 

tuffs other than bedded tuff or thin (0-30 m), local nonwelded tuffs of 

similar petrographic character. There is little indication of significant 

erosion between the three tuff formations in the Gabbs Valley and 

Gillis Ranges, but the abundance of petrified fragments of large trees 

between the Singatse and Mickey Pass Tuffs, together with the occurrence of lignite beds and various tuffaceous sedimentary rocks in 

the Singatse Range (Proffett and Proffett, 1976), indicates that at 

least several thousand years elapsed between the eruption of the two 

units.

Of the three tuff formations assigned to the Benton Spring Group 

in the Gabbs Valley Range, the two youngest the Petrified Spring 

and Singatse Tuffs are multiple-flow simple cooling units; the 

Mickey Pass Tuff at the base consists of two cooling units. The oldest 

of these, the Guild Mine Member, is a compound cooling unit. With
12 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

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rH

0

d

o

o

d

mCM 

CM

d

(V

CO

 Q

s

a

JS

I

o

-2's

 o

B

m

i

o

CO 

CM

Gabbs aboutValley Range, 1.6 Petrified Spring, km SSW. of

118°07'W. 38°38'N., 

Q

CO

ciS)

CO

d

+1m

CO

CM

CO 

IN

d

CM

CO

cn

CM

00 Tj*

CO

2 

o

CQ

B 

0

B

a

0

"S

OS "o

1

CQ

CO

COm

CM

Q
Hu-pwi Rhyodacite:

Poinsettia Tuff Member,

D2395B cooling unit 2 .............. Biotite ..........

2306-7B ........... do ........... Biotite ..........

2306-7P ........... do ........... Plagioclase ......

D2543B Ghost Dance Lava Member . Biotite ..........

D2572B Nugent Tuff Member ...... Biotite ..........

MLS-73-9 Tuff of Gabbs Valley ......... Sanidine ........

MLS-73-10 ............ do ............ Sanidine ........

8.40 2.886

8.33

8.41 2.811

8.36

0.823 0.2635

0.825

8.70 3.014

(avg)

7.41 2.508

(avg)

7 91 2 941

7.92

10.26 3.981

10.27

0.72 

0.64 

0.72 

0.68 

0.64 

0.61 

0.82 
TABLE 2. Analytical data for age determinations of Tertiary volcanic rocks, Gabbs Valley and 

Sample No. Lithologic unit

Mickey Pass Tuff:

qooQ i rvp A n

Mineral dated

Biotite ...........

K2 0 *Ar<0 

( 1 0 ""/moles/gram)

10.63 4.363

10.63 

1.08 0.3878

1.06 

8.05 3.244

7.98

10.37 

8.03 3.133

8.00

(avg) 

5.53 2.112

*Ar<°/Ar10 

0.87 

0.82 
STRATIGRAPHY AND GEOCHRONOLOGY 15

the possible exception of the Singatse Tuff, each cooling unit in the 

group is characterized by a similar petrochemical zonation from 

rhyodacite or quartz latite in its lower parts to quartz latite or 

rhyolite in its upper parts. Biotite, pyroxene, and (or) hornblende are 

abundant in all three units. In general, quartz increases in size and 

albundance upward in each unit, and ferromagnesian minerals 

decrease by the elimination of pyroxene and most hornblende.

MICKEY PASS TUFF

The Mickey Pass Tuff was named by Proffett and Proffett (1976) 

for exposures just northeast of Mickey Pass (name of Proffett and 

Proffett), its type locality (sec. 13, T. 13 N., R. 24 E.), in the Singatse 

Range. At the type locality, two cooling units compose the formation, 

and these were named the Guild Mine and Weed Heights Members 

by Proffett and Proffett (1976). These three formal rock-stratigraphic 

names are herein adopted. The two members are respectively named 

after the Guild placer mines in sec. 18, T. 13 N., R. 25 E. and after 

Weed Heights, located about 4 km east of Mickey Pass in sec. 17, T. 

13 N., R. 25 E. These two localities are the type localities for the two 

members. The Guild Mine Member is as much as 550 m thick in the 

Singatse Range; the Weed Heights Member, in contrast, is only 

60-120 m thick.

GUILD MINE MEMBER

The Guild Mine Member is present throughout the Gabbs Valley 

and Gillis Ranges and ranges in thickness from less than 60 m to as 

much as 300 m. According to Proffett and Proffett (1976) the member 

occupies a deep southeast-trending paleovalley in the Singatse Range 

(fig. 3), where it may be as much as 900 m thick. We have tentatively 

identified the Guild Mine Member as far to the east as the northern 

Cedar Range (fig. 3, Cedar Mountain). The rock there is about 70 m 

thick and consists entirely of rhyodacite. (See following paragraph.) 

It contains considerably more lithic fragments than in the Gabbs 

Valley and Gillis Ranges, not unusual for a distal part of an ash-flow 

sheet. The member occurs as erosional remnants under much younger 

lavas of intermediate composition in the northern Garfield Hills, and, 

according to R. C. Speed (written commun., 1976), remnants occur as 

far south as the eastern Excelsior Mountains (fig. 3). Thin sections 

supplied by Speed and modally analyzed by F. M. Byers indicate that 

both the Guild Mine Member and Singatse Tuff are probably present 

there. According to M. W. Reynolds (oral commun., 1976), the 

member is present in the Pine Grove Hills (fig. 3); the remnants 

preserved there are thin and were deeply eroded prior to the onset of 

intermediate lava volcanism. Reynolds reported that although there 

is no indication that the Guild Mine is lapping onto higher
16 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

INTERMEDIATE 

LAVAS OF

MOUNT

FERGUSON

TUFF OF 

COPPER

MOUNTAIN

£

o

< 

o

0

>-

X

<r

Q_

^

X

POINSETTIA

TUFF

MEMBER

GHOST DANCE

LAVA

MEMBER

NUGENT

TUFF

MEMBER

INTERMEDIATE

LAVAS OF

NUGENT WASH

AREA

BLUE

SPHINX

TUFF

INTERMEDIATE

LAVAS BETWEEN

THE BLUE SPHINX

TUFF AND BENTON

SPRING GROUP

Andesite or latite

Latite

*! 1

or rhyodacite Rhyodacite

|x-| | i

an - BiJ

Quartz latite, Base Quartz latite, Middle Rhyolite, Upper

X; X'J I -XI _

j^jSfi n jfciLaJ L iitf^u n

Rhyodacite, Lowest, unit 1 Rhyodacite, Middle, unit 3 Rhypdacte or quartz latit 

+\ r± Uppermost, unit 5

fTTTLJ 1 TmJ 1 g^fe: ': : fmlJ 1 Rhyodacite

V.

1 I

nil-1 1

Rhyodacite, Lower, Rhyodacite, Upper, white pumice part

brown pumice part

_JmJ Jl

Rhyodacite

Rhyodacite or quartz 

inlatite 

Lower Iflm (local) Quartz latite, Lower 10m Quartz |atile Upper

Rhmu v*^

rv r~.

miLJ n 1 fcd^: _M£ : :ifni n1 !

y°dacite Rhyodacite

_

m \

an'

FIGURE 4. Volume of phenocrysts and abundance of six crystal componCounty,

topography to the southwest, the overall thinness suggests that the 

preserved outcrops are close to the distal end of the sheet. 

The member is absent in the northeastern part of the area shown in 

figure 2.

The Guild Mine Member of the Mickey Pass Tuff is a compound 

cooling unit (Smith, 1960), displaying alternating zones of dense and 

moderate welding. Pumice clasts, ranging in length from about 1 cm 

to as much as 25 cm, are abundant throughout the cooling unit. 

Where the tuff is densely welded, however, the pumice fragments are 

commonly extensively flattened. They more closely resemble flow 

laminae and lenticules in a rhyolite lava. The member has a persistent 

black basal vitrophyre that ranges in thickness from about 1 m to as 

much as 5 m. This vitrophyre generally is rhyodacite, but where the 

entire cooling unit is thin the rhyodacite may be missing and the
STRATIGRAPHY AND GEOCHRONOLOGY

Q.

O 

OC

0

13

IT 

Q_

00

2

O

z

CO

oit

ii J^

Q. Q_

UJ

00

z ?

00

.t£=?

Uoo

2 <f

Q.

GUILD

MINE

MEMBER

LAVAS OF

GIROUX

VALLEY

RhyodaciteJSasal vitrophyre Rhyodac te, M ddle

I;!; M_T1 ^_::

Quart; latite, Basal vitrophyre

rrn

W:; :': L fl

Quartz lat te, Upper

rrm 1 1 i5wM-:-fHll 1 1

Quartz latite, Upper iiiiE n

Rhyodacite, Basal v trophyre

Rhyolite, Upper Quartz>

Quartz latite, M ddle Kspar n some zones

 J~1 5»

-i _ L_n ilk n

Rhyodacite lava Rhyodacite lava 

Rhyodacite or Quartz latite tuff South Gillis Range . , , .. 

, . « . «-ii- n i i UdDDS Valley nailQc breccia, South ~ Gillis Ranae m

 n i 1 ^=M

17

EXPLANATION

ents of principal volcanic rocks of Gabbs Valley and Gillis Ranges, Mineral

Nev.

basal vitrophyre is rhyolite. Lithic fragments, principally of lava of 

intermediate composition, are numerous in the basal 30 m of the unit; 

but, except for pumice, fragments of all kinds are exceedingly rare 

above this basal zone. The paucity of lithic fragments and the 

presence of extremely flattened pumice are useful criteria for 

distinguishing the Guild Mine Member from the Singatse Tuff.

Both members of the Mickey Pass Tuff show gradations upward 

from bases rich in plagioclase and ferromagnesian minerals to tops 

poor in ferromagnesian minerals and rich in alkali feldspar and 

quartz. Such changes in composition are much more extreme in the 

Guild Mine Member than in the Weed Heights Member. The basal 

part of the Guild Mine is rhyodacite and the upper part is alkali 

rhyolite (table 3, figs. 4 and 6) according to the classification scheme 

of Young. E. J. Young (Segerstrom and Young, 1972) has found a
18 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

FIGURE 5. View eastward of faulted outcrops of lower two formations of Benton 

Spring Group at Benton Spring, which is located in small gulley just beyond trailer 

and pickup truck, mp, Mickey Pass Tuff;s, Singatse Tuff. Zone in photograph lies 

just east of a major strike-slip fault (see fig. 2), and faults in photograph (heavy 

lines) strike parallel to main fault. They are interpreted as oblique-slip faults 

(arrows, direction of movement).

felsic-mafic ratio (SiO 2+K2O+Na 2O)/(FeO+Fe 2O 3+MgO+CaO) to be 

very useful in classifying igneous (and possibly metamorphic) rocks. 

Using data by Nockolds (1954) on average chemical compositions of 

igneous rocks, felsic-mafic ratios are distributed as follows:

Rock type Felsic-mafic ratio 

. __ >50

 , ^j ^jj 

. __ 15 25

. ___.__.. 10 -15

7 in

....... c 7

^ j

2

1 O

........................ 1.4- 2.1

. - - < 1.4

Note: This chart reflects minor changes made by E. J. Young in 1976 (written commun.).

In the Singatse Range and at Calavada Summit in the Gabbs 

Valley Range (fig. 6) the lower rhyodacite part is about 100 m thick 

and the upper rhyolite is 200-400 m thick. The sharp change in 

phenocryst content (fig. 6) probably coincides with a contact between 

ash flows, but no cooling break is indicated.
STRATIGRAPHY AND GEOCHRONOLOGY 19

The ferromagnesian minerals constitute as much as 30 percent of 

the volume of the total phenocrysts in the rhyodacite but only about 

4 percent in the rhyolite. In the rhyodacite, biotite and pyroxene vary 

in proportions from about 2:1 to 1:2. Clinopyroxene and orthopyroxene occur in subequal amounts. Hornblende occurs in only trace 

amounts in most thin sections of rhyodacite but some thin sections 

contain as much as 1.5 percent; this contrasts with the Singatse Tuff, 

which is rich in hornblende. Biotite is the sole fresh ferromagnesian 

mineral in the rhyolite but a few ubiquitous pseudomorphs probably 

are after pyroxene.

The rhyodacite zone generally lacks quartz and contains sparse 

alkali feldspar. Both minerals are consistently present in the gradational zone between the basal rhyodacite and the rhyolite, and these 

minerals increase in abundance and size upward. Most quartz 

phenocrysts are less than 3 mm but grains as large as 4 mm are not 

uncommon in the upper part. Quartz is moderately embayed and 

resorbed. As will become evident later, the size and the degree of embayment and resorption of quartz grains are critical features that aid 

in distinguishing among several cooling units.

Two reference localities are designated herein for the Guild Mine 

Member of the Mickey Pass Tuff. The first locality is 1.6 km north of 

Todd Mountain, just north of Nevada State Highway 23, on the 

southwestern flank of the Gabbs Valley Range (fig. 2, MP-1). The 

second locality is a northeast-trending fault block in the southern 

Gillls Range about 3.2 km northwest of the Kincaid gold mill and 

about 3.2 km due north of U.S. Highway 95 (fig. 2, MP-2). Two 

reference localities are designated because the tuff displays extremely contrasting megascopic and mineralogic features in the two areas. 

At the Todd Mountain locality (fig. 7), the Guild Mine Member is 

about 300 m thick and includes a lower rhyodacite zone that is nearly 

100 m thick. Except for the basal vitrophyre, the cooling unit is incipiently hydrothermally altered and displays numerous bleached 

(tan and yellow) zones along near-vertical joints. The bleached zones 

in the lower part of the tuff range in width from a few centimeters to 

several meters and they thicken upward so that most of the upper 

part is bleached (fig. 7). The rock between the bleached zones is rustybrown and this color too is secondary. It is due principally to the oxidation of ferromagnesian minerals by hydrothermal solutions and 

subordinately to the oxidation of finely disseminated hydrothermally 

introduced pyrite. The color and overall appearance of the tuff at this 

reference locality are fairly typical of the unit throughout the Gabbs 

Valley and Gillis Ranges, although in places the rock is altered 

throughout and is pastel shades of yellow, pink, and green.

At the second reference locality, in contrast, the tuff is fresh, thin 

(about 60 m thick), and, in most places, lacks the rhyodacite zone. The
TABLE 3. Chemical analyses and norms (in weight percent) of Mickey Pass, Singatse, and Co

Gabbs Valley Ranges

[For approximate modal analyses of zones sampled for chemical analyses see graphs (figs. 4, 6, 8, 

Field No. 

Sample 1

675 

A

270 

B

295 

C

661 

D

Chemical composition

SiO2 ....................

Al O

Fe2O3 ...................

FeO ....................

MgO ...................

CaO ....................

Na O

K2O ....................

TiO2 ....................

p n

MnO ...................

ZrO2 ....................

S. ......................

Total ...............

...... 66.34

...... 16.68

...... 2.37

...... 1.21

...... 1.15

...... 3.14

...... 4.10

...... 4.12

...... 0.60

...... 0.16

...... 0.07

...... 0.02

...... 0.03

...... 99.99

74.96 

13.78 

1.56 

0.14 

0.33 

1.19 

2.94 

4.73 

0.25

0.12 

0.01 

0.01

100.02

77.75 

12.41 

1.20

0.10 

0.77 

3.16 

4.39 

0.15

0.06 

0.01 

0.01

100.01

69.43 

16.79 

1.51 

0.68 

0.66 

2.58 

4.25 

3.45 

0.51 

0.10 

0.04 

0.01

100.01

1
Normative composition

Q............................ 18.716 36.745 40.812 25.307 

C............................ 0.153 1.673 1.071 1.621 

Z............................ 0.031 0.016 0.016 0.015 

OR .......................... 24.324 27.953 25.926 20.364 

AB .......................... 34.743 24.844 26.700 35.930 

AN .......................... 14.510 5.880 3.806 12.152 

WO ......................... 0 0 0 0 

EN .......................... 2.863 0.812 0.242 1.635 

MT.......................... 2.302 0.082 0 0.829 

HM ......................... 0.784 1.498 1.199 0.936 

IL ........................... 1.131 0.475 0.113 0.974 

TN .................... ...... 0 0 0 0 

RU .......................... 0 0 0.092 0 

AP .......................... 0.389 0 0 0.243 

PR .......................... 0.058 0.020 0.020 0 

 

Total .................... 100.002 99.999 99.998 100.006 

 

Differentiation index ......... 77.783 89.541 93.438 81.601 

 

Samples A through G are from Singatse Range, analyses by Anaconda Copper Company (Proffett and Proffett, 1976); A, 

fluorescence except MgO and Na2O by flame photometry, MnO by calorimetric method, FeO and S by wet chemical method

A12O3 , TiO2 , total Fe as FeO, and MnO by X-ray fluorescence; analyst, J. S. Wahlberg. MgO, CaO, Na2O, K2O by atomic absor

percent minus H2O and CO2 . Samples B, C, E, F, and G are slightly altered based on CO2 contents of 0.38-0.56 percent. Fo

Sample descriptions: 

A. Basal vitrophyre of Guild Mine Member, Mickey Pass Tuff. 

B. Guild Mine Member 167 m above base.

C. Guild Mine Member, near top.

D. Basal vitrophyre of Weed Heights Member, Mickey Pass Tuff.

E. Weed Heights Member, near base.

F. Weed Heights Member, near top.

G. Singatse Tuff, near middle.

H. Tuff of Copper Mountain, near top.
22 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

60 40 20 

PERCENT OF TOTAL PHENOCRYSTS

60 40 20 

PERCENT OF TOTAL PHENOCRYSTS

FIGURE 6. Phenocryst variations from base to top of Guild Mine Member of 

Mickey Pass Tuff. A, at reference locality and vicinity in the Gabbs Valley 

Range (composite section at and in vicinity of reference locality north of Todd 

Mountain). B, at type locality of the member in the Singatse Range (continuous 

section). Symbols on graphs are approximate stratigraphic positions of 

1,000-point modal analyses of single thin sections.
STRATIGRAPHY AND GEOCHRONOLOGY 23

 ?# ; !. -jfi.h' .-' ;-~-'.i»:> *m*

FIGURE 7. View to northwest of the Guild Mine Member of the Mickey Pass Tuff at 

the reference locality of the member just north of Todd Mountain (see fig. 2). Here 

the member (gm) is separated by detachment-fault zones (enclosed by black lines) 

from the underlying Triassic Luning Formation ("fil). Vehicle gives scale. Zone b, 

fault breccia that includes fragments of quartz-rich densely welded tuff that is not 

part of the Guild Mine Member; zone g, scattered granite boulders set in slickensided clay gouge that was probably formed in part from residual regolith. Lack of 

sorting in this clay zone and ubiquitous slickensides attest fault movement. The 

basal vitrophyre of the Guild Mine Member here is badly broken and forms the 

slope. Farther to the west, however, the vitrophyre is intact and forms a cliff that is 

separated from the underlying Mesozoic batholithic rocks by a zone of gouge and 

breccia about 2 m thick. The breccia includes sheared wedges of densely welded tuff 

lacking quartz. Guild Mine Member here dips gently away from, and to the right of, 

the observer.
24 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

rock is light to medium reddish gray on fresh fracture and weathers 

brown and brownish gray. A minor detachment or bedding-plane 

fault occurs at the base of the tuff in this locality, and both the partially welded base and overlying vitrophyre are cut out locally along 

the fault zone. In these places the foliation in the tuff dips very steeply into the gently dipping fault zone.

In both reference localities the Guild Mine Member rests alternately on either Mesozoic rocks or the lavas of Giroux Valley and is 

overlain by the Singatse Tuff.

According to Proffett and Proffett (1976, p. 16) the Guild Mine 

Member is magnetically normal.

WEED HEIGHTS MEMBER

The Weed Heights Member (Proffett and Proffett, 1976) of the 

Mickey Pass Tuff is a simple cooling unit 60-120 m thick. It persists 

as a continuous, albeit much faulted, stratum throughout the northern Gillis and Wassuk Ranges where it underlies the Singatse Tuff 

and overlies the Guild Mine Member, but it is extremely discontinuous in the Gabbs Valley Range, having been recognized with certainty only in the vicinity of Calavada Summit.

Although the Weed Heights Member is preserved in various fault 

blocks in the northern Gillis Range, nowhere is a complete and 

unaltered section available. Thin sections from scattered outcrops, 

however, indicate that the member displays virtually the same 

mineralogic variations in the Gillis Range as it does in the Singatse 

Range (fig. 8). The member is buff to lavender to reddish brown and is 

characterized by abundant white pumice fragments (Proffett and 

Proffett, 1976). It is mostly weakly welded and the top 70 m is virtually nonwelded. The Weed Heights grades upward from quartz 

latite at the base to alkali rhyolite near the top (table 3). It displays a 

gradual upward increase in volume of quartz and alkali feldspar and a 

corresponding decrease in plagioclase (fig. 8). The percentage of ferromagnesian minerals fluctuates considerably from base to top. 

These minerals are biotite, sparse hornblende, and pseudomorphs of 

iron oxide and chlorite(?) that appear to be entirely after pyroxene. 

The fresh biotite and altered pyroxene occur in subequal amounts. In 

general, the phenocrysts are small. Most quartz and alkali feldspar 

crystals are 2 mm long, or less, and plagioclase crystals are rarely 

more than 3 mm long. Most of the small quartz grains, however, are 

moderately embayed and resorbed, and some hand specimens contain 

a few grains that are so riddled with holes that they qualify for the 

classification "worm eaten" in the vernacular of fieldnote descriptions.

In the southern Gabbs Valley Range, specifically at Calavada Sum-
STRATIGRAPHY AND GEOCHRONOLOGY 25

Top of Weed 

Heights Member

60 40 20 

PERCENT OF TOTAL PHENOCRYSTS

FIGURE 8. Phenocryst variations in the Weed Heights Member 

of the Mickey Pass Tuff at the type locality of both in the 

Singatse Range (continuous section). Symbols mark locations of 

1,000-point modal analyses of single thin sections. Upper 30 m 

has been determined to be a separate partially welded cooling 

unit (J. M. Proffett, oral commun., 1976).

mit, a zone of alternating densely and partially welded tuff occurs 

above the Guild Mine Member of the Mickey Pass Tuff and below the 

Singatse Tuff that we tentatively had considered to be part of the
26 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

Singatse Tuff. This tentative conclusion was based on the lack of 

precise evidence of complete cooling breaks within the zone and between the zone and unequivocal Singatse Tuff. Now, on the basis of 

mineralogic evidence, we conclude that the lowest 20-30 m, consisting of densely welded tuff grading upwards to partially welded 

tuff, is the Weed Heights Member and therefore the cooling break 

with the overlying tuff has to be complete. The mineralogic evidence 

consists of virtually identical percentages and ratios of phenocrysts 

with the Weed Heights at the type locality, identical sizes of various 

phenocrysts, and the occurrence of a few grains of "worm-eaten" 

quartz having not only the same size but the same degree of embayment as the quartz grains in the tuff at the type locality. This tuff is 

fresh and, in contrast to the rock at the type locality, contains 

unaltered pyroxene consisting of subequal hypersthene and 

clinopyroxene.

The basal vitrophyre of the Weed Heights is as much as 3 m thick, 

but it is discontinuous. It forms the lowest of three conspicuous ribs 

between the Guild Mine Member and the Singatse Tuff at Calavada 

Summit. Two samples from the discontinuous vitrophyre at 

Calavada Summit indicate that the Weed Heights Member is 

magnetically normal (G. D. Bath, written commun., 1975).

SINGATSE TUFF

The Singatse Tuff adopted in this report was named by Proffett 

and Proffett (1976) for exposures on Singatse Peak in the Singatse 

Range west of Yerington, where the Singatse is a multiple-flow simple cooling unit 240-410 m thick. They designated exposures in sec. 

13, T. 13 N., R. 24 E. as the type area. It is brown to red brown, 

moderately to densely welded and crystal rich, containing abundant 

plagioclase, quartz, sanidine, biotite, and hornblende phenocrysts 

(Proffett and Proffett, 1976). It is quartz latite in composition from 

base to top in the Singatse Range and displays only slight variations 

in phenocryst mineralogy. Unlike the Mickey Pass Tuff, the Singatse 

Tuff is characterized throughout by abundant lithic fragments. 

These fragments, however, are much more numerous in some flows 

within the cooling unit than in others. The fragments compose as 

much as 10 percent of the rock and they range in length from less 

than 1 cm to more than 10 cm. The fragments are principally lavas of 

intermediate composition but they include Mesozoic granitic rocks, 

and variegated carbonate and clastic rocks.

Unaltered Singatse Tuff in the Gabbs Valley and Gillis Ranges is a 

mirror image in outcrop of the Singatse at the type locality and is 

nearly a mirror image in mineralogic profile (fig. 9). However, if the
STRATIGRAPHY AND GEOCHRONOLOGY 27

assumption is valid that the middle rhyodacitic rib-forming zone (see 

section, Ash-flow tuffs between the Mickey Pass and Singatse Tuffs) 

is genetically part of the Singatse at Calavada Summit, then the 

Singatse there differs significantly from the rocks exposed in the Yerington district. Like the members of the Mickey Pass, the formation 

displays a gradation upward from rhyodacite at the base to quartz 

latite at the top. The change from rhyodacite to quartz latite, 

however, is abrupt and, as discussed previously, coincides closely 

with a partial or complete cooling break.

In the Gabbs Valley and Gillis Ranges, the Singatse Tuff commonly has a black basal vitrophyre 1-4 m thick. The vitrophyre grades 

upward to densely welded light-red or reddish-gray devitrified tuff, 

wliich weathers pale brown and brownish gray. Pumice clasts are 

abundant throughout the cooling unit and are conspicuous in both 

altered and unaltered rock. The top 100 m is characteristically rich in 

flattened white pumice fragments about the size of silver dollars. 

Where the Singatse Tuff is altered, it is typically purple or purplish 

gray, weathering to dark purple and dark purplish gray.

As shown in figures 4 and 9, there is little mineralogic and, 

presumably, little chemical variation from base to top in the Singatse 

Tuff proper. Analyzed chemically, a sample from the middle of the 

Singatse at Yerington falls well within the quartz latite rock type of 

E. J. Young's classification (Segerstrom and Young, 1972). The most 

distinctive mineralogic feature is the occurrence of hornblende, which 

in many thin sections from the lower one-third of the unit is as abundant as biotite. There is less hornblende upwards, however, as a 

result of both alteration and gradual reduction in volume. Pyroxene 

occurs in a few thin sections near the base.

The Singatse Tuff together with the Guild Mine Member of the 

Mickey Pass Tuff comprise the most widespread ash-flow tuffs in the 

area under investigation. The Singatse occurs throughout the Gabbs 

Valley and Gillis Ranges, the northern Wassuk Range (E. C. Bingler, 

oral commun., 1975), the northeastern Excelsior Mountains, and, according to Proffett and Proffett (1976), as far west as the Pine Nut 

Mountains (fig. 3). To the southeast it is present in the northern part 

of Cedar Mountain (fig. 3) where it is estimated to be at least 60 m 

thick; and, according to R. C. Speed (written commun., 1976), it is as 

much as 100 m thick in the Pilot Mountains where it has been dated 

at 27 m.y.

Samples of Singatse Tuff from the type locality (Proffett and Proffett, 1976), the northern Wassuk Range, the northern Gillis Range, 

and Calavada Summit (including the rhyodacite and blob zone; see 

section, Ash-flow tuffs between the Mickey Pass and Singatse Tuffs), 

are magnetically normal (G. D. Bath, written commun., 1976).
28 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA 

PETRIFIED SPRING TUFF

The Petrified Spring Tuff is here named after Petrified Spring in 

the southern Gabbs Valley Range. The formation is not exposed in 

the immediate vicinity of the spring, but is locally exposed in several 

northwest-striking fault blocks in the southwest quarter of T. 9 N., R. 

35 E. (fig. 2). The base and top of the Petrified Spring occur in only 

one locality, designated herein as the type locality. This exposure (fig. 

2, PS) is in the NW1/* sec. 31, T. 9 N., R. 35 E., and is 4 km south of 

Petrified Spring.

At the type locality the Petrified Spring Tuff is underlain by two 

local thin and discontinuous nonwelded ash-flow cooling units, each 

about 15 m thick, and a discontinuous well-bedded air-fall tuff as 

much as 10 m thick. The nonwelded ash-flow tuff is mostly white and 

rich in lithic fragments of intermediate flow rock and rhyolite. Locally, the Petrified Spring rests directly on the Singatse Tuff. At the 

type locality the Petrified Spring Tuff is directly overlain by the Blue 

Sphinx Tuff, but 2 km to the north it is overlain unconformably by 

the much younger lavas of Mount Ferguson. At the base of the lava it 

is evident that much bedding-plane or detachment-fault movement 

has taken place, but we infer that most of the omission of strata is 

due to prelava erosion.

The Petrified Spring Tuff is about 75 m thick and consists of a 

basal vitric partially welded dark-gray zone as much as 5 m thick that 

is rich in white pumice clasts as long as 0.5 m. The dark-gray vitric 

tuff grades upward into a thick (as much as 15 m) medium-gray to 

black soft vitrophyre that contains black vitrophyric pumice as long 

as 0.5 m. The vitrophyre grades upward into moderately welded 

pinkish-gray partly devitrified tuff that weathers pinkish brown and 

yellowish brown. Lithic fragments of intermediate lava are conspicuous and abundant throughout the formation. The Petrified 

Spring is the most mafic of the three formations included in the Benton Spring Group, but it displays the same vertical variation in 

mineralogy as the underlying formations. Near the base the Petrified 

Spring is rhyodacite, and it is virtually lacking in quartz and alkali 

feldspar phenocrysts. Near the top, however, the rock is quartz latite; 

quartz crystals as much as 5 mm long compose 10 percent of the total 

phenocrysts. Volume of alkali feldspar phenocrysts increases upward 

to as much as 23 percent of total phenocrysts; these crystals may be 

as large as 4.5 mm. Plagioclase phenocrysts are as large as 6 mm and 

many of these are extremely poikilitic and charged with glass. The 

large feldspar phenocrysts are characteristic of this unit.

The Petrified Spring Tuff is sporadically preserved beneath the 

Blue Sphinx Tuff in the Gabbs Valley Range north of Benton Spring 

and locally preserved in the southern part of the northern Gillis
STRATIGRAPHY AND GEOCHRONOLOGY 29

Range. Its absence in places appears to be due to both omission by 

very low angle faulting and by pre-Blue Sphinx Tuff erosion. The 

unit is magnetically reversed (G. D. Bath, written commun., 1976).

AGE OF THE BENTON SPRING GROUP

Potassium-argon ages for the Guild Mine Member of the Mickey 

Pass Tuff range from 26.3 m.y. (excluding the low 24.4-m.y. date on 

plagioclase) to 28.0 m.y. (table 2), and agree very well with ages for 

the Guild Mine Member from the Singatse Range listed by Proffett 

and Proffett (1976). An age of 26.7 m.y. also was obtained from lava 

of Giroux Valley beneath the Guild Mine Member, and two dates 

(25.0 m.y. and 26.1 m.y.) were obtained from tuff of Gabbs Valley 

that lies stratigraphically above the Benton Spring Group. These 

dates suggest that all the tuffs in the Benton Spring Group could 

have been erupted within a period of 1 m.y. or less.

ASH-FLOW TUFFS BETWEEN THE MICKEY PASS 

AND SINGATSE TUFFS

Studies by John Proffett (oral commun., 1976) that postdate the 

descriptions given in Proffett and Proffett (1976) indicate that a partially welded quartz latite or rhyolite tuff at the top of the Weed 

Heights Member in the Yerington district, formerly considered to be 

part of the poorly welded top of the Weed Heights, constitutes a 

separate and distinct cooling unit. Modes of two thin sections from 

this cooling unit are plotted on figure 8. A single thin section from 

the vicinity of Calavada Summit suggests that this unnamed unit occurs there also, but, if so, it is extremely local.

In addition to this unit, however, another higher cooling unit occurs at Calavada Summit that is widespread in the Gabbs Valley 

Range. This higher cooling unit is rhyodacite at the base and 

vitrophyric (fig. 9). The vitrophyric zone forms the middle rib of the 

three ribs previously mentioned. This zone grades upward to slope 

forming weakly welded light-gray tuff and thence upward to 

moderately welded light-brown tuff containing large (as much as 30 

cm long) pumice fragments, some of which are black glass, others 

gray and devitrified. The large-pumice or "blob" zone forms the third 

and highest rib (fig. 9). It is composed of quartz latite or silicic 

rhyodacite and contains as much as 13 percent quartz and 22 percent 

sanidine. In contrast, the lower rhyodacite is without quartz and contains only 5-10 percent sanidine. The blob zone is separated from the 

Singatse Tuff proper by a zone of weakly welded tuff 10-20 m thick. 

The pumice lapilli in this zone are only slightly flattened and a partial 

or complete cooling break is indicated. Whatever the actual nature of 

the break, it seems reasonable to assume that very little time elapsed
30 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

300

200

100

: / rit-l

/ £ I '.03 S

/

*- I 3 O>

2? ' » «

*J ' '

^'

Basal vitrophyre

*> '

%

80 60~ 40 20 

PERCENT OF TOTAL PHENOCRYSTS

60 ~ 40 20 0 

PERCENT OF TOTAL PHENOCRYSTS

FIGURE 9. Phenocryst variations of A, Singatse Tuff at Calavada Summit in the 

Gabbs Valley Range (composite section), compared with B, Singatse Tuff at the 

type locality, Singatse Range (continuous section). Also shown are phenocryst 

plots of three rib-forming zones between the Guild Mine Member and the Singatse 

Tuff at Calavada Summit. The lowest rib and overlying partially welded tuff (not 

sampled) are correlated with the Weed Heights Member of the Mickey Pass Tuff. 

The upper two ribs are regarded as parts of a single rhyodacite compound cooling 

unit. The cooling break between this unit and the Singatse Tuff proper may not be 

complete. The rhyodacite unit is not present in the Singatse Range.
STRATIGRAPHY AND GEOCHRONOLOGY 31

between the eruption of the blob zone and the main ash-flows of the 

Singatse Tuff, and that the zone is genetically related to the 

Singatse.

LOCAL UNITS BETWEEN THE BLUE SPHINX TUFF 

AND THE BENTON SPRING GROUP

Following the eruption of the uppermost tuff of the Benton Spring 

Group, local units and thin tuffs originating outside the area were 

emplaced. Sequences of intermediate lavas, tuffaceous sedimentary 

rocks, and ash-flow tuffs at least 50 m thick, and locally more than 

200 m thick, form a composite section between the Benton Spring 

Group and the overlying Blue Sphinx Tuff in the Gabbs Valley and 

Gillis Ranges near and to the north of Nugent Wash. The units are 

discontinuous, due to omission by both erosion and faulting. For example, in a single fault slice north of Nugent Wash as many as four 

thin (less than 30 m thick) cooling units of welded tuff are present between the Blue Sphinx Tuff and the Benton Spring Group. In adjacent fault slices, several or all units may be absent.

INTERMEDIATE LAVAS

Lavas beneath the Blue Sphinx Tuff and above the Benton Spring 

Group range in composition from rhyodacite to quartz latite (fig. 4). 

They commonly are lavender or blue gray. The lavender color indicates incipient. hydrothermal (propylitic) alteration. Sparse fresh 

rocks are medium to dark gray. Most lavas in this sequence are conspicuously porphyritic, containing euhedral plagioclase as much as 1 

cm long. Some lavas contain a few phenocrysts of quartz.

The lavas in at least one locality in the central part of the Gabbs 

Valley Range may be as much as 400 m thick. In most places, 

however, they comprise a stratum less than 30 m thick, and in several 

areas, as previously mentioned, they are absent.

SEDIMENTARY ROCKS

Sedimentary rocks consisting mostly of even-bedded lacustrine 

siltstones are present locally between the Blue Sphinx Tuff and the 

intermediate lavas just described. Thin-bedded air-fall tuffs, in some 

places pisolitic, occur locally within the sedimentary rocks. These 

beds were deposited in one or more shallow lakes and at one time 

possibly were continuous over a broad region northwest of Nugent 

Wash. The rocks probably reflect regional subsidence related to extrusion of the Benton Spring Group. They commonly are extensively 

altered; only locally is it possible to determine that the ash-fall beds 

originally were rich in biotite. No exposures of the sedimentary rocks 

are known to be more than about 30 m thick.
32 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

Southeast of Nugent Wash the lacustrine sedimentary rocks intertongue with fluvial sandstone, coarse gravel, and biotite-rich air-fall 

tuff. The entire sequence is mostly less than 20 m thick and it pinches 

out 2-3 km south of the latitude of Poinsettia Spring (fig. 2). Most 

clasts in all these rocks include fragments of intermediate lava but 

locally they include fragments of Singatse Tuff and boulders of preTertiary quartzite.

ASH-FLOW TUFFS

As many as four ash-flow cooling units are present northwest of 

Nugent Wash. From base to top these units include a biotite-rich ashflow tuff of quartz latite composition, a rhyolite ash-flow tuff, a 

biotite-rich quartz latite ash-flow tuff that is modally similar to the 

basal unit, and a second rhyolite ash-flow tuff that contains conspicuous shards. The biotite tuffs appear to thin northward; in contrast, the rhyolite tuffs thicken northward. The older biotite tuff is 

lithologically similar to the Petrified Spring Tuff and possibly correlates with that unit.

The two rhyolite tuffs are part of the "tuff of Gabbs Valley," a sequence of ash-flow tuffs (Ekren and Byers, 1976) best exposed on the 

north and east sides of Gabbs Valley (fig. 2). These two thin cooling 

units are inferred to have been extruded onto the Gillis Range and 

northern Gabbs Valley Range when the Gabbs Valley volcanic center 

lay far northwest of its present position. The two tuffs are very 

similar modally and megascopically. They are crystal poor, containing fewer than 10 percent phenocrysts, principally alkali feldspar. 

Quartz occurs as rare tiny grains less than 2 mm long. Ferromagnesian minerals, principally biotite, are sparse; they are rarely intact 

because of the extensive and pervasive alteration of the tuffs. The 

older rhyolite tuff contains conspicuous, well-flattened pumice 

fragments as much as 15 cm long. The younger unit is only partially 

welded, in contrast to the older; shards in the younger unit are visible 

with the hand lens.

Both tuffs are reddish brown where fresh, but throughout the area 

the reddish-brown color is rare because of intense alteration. In most 

places the tuffs are bleached white, light gray, or pastel shades of 

green or pink. They both contain numerous fragments of crystal-poor 

rhyolite lava and tuff and moderately crystal-rich fragments of intermediate lava.

The tuff of Gabbs Valley occurs in the Yerington district in the 

Singatse Range where it has been named the Blue Stone Mine Tuff 

(Proffett and Proffett, 1976). We prefer to continue to use the informal "tuff of Gabbs Valley" for the sequence because the ash flows 

originated there (Ekren and Byers, 1976) and there they are best
STRATIGRAPHY AND GEOCHRONOLOGY 33

developed. Furthermore, detailed stratigraphic studies in the region 

of Gabbs Valley will undoubtedly define very complex stratigraphic 

relationships of this important ash-flow field, and the strata may 

warrant member-, formation-, and even group-rank names. The adoption of a formal name for exposures of poorly welded ash flows far 

removed from the source area predictably will prove cumbersome.

BLUE SPHINX TUFF

The Blue Sphinx Tuff is here named for the Blue Sphinx, a natural 

monolith of Blue Sphinx Tuff located on the eastern flank of the 

Gabbs Valley Range just south of the Golden Pen gold mine in sec. 

33, T. 11 N., R. 32 E. (fig. 2), its type locality. The monolith is aptly 

named because it is blue to lavender and closely resembles an Egyptian sphinx (fig. 10). The type locality of the Blue Sphinx Tuff includes the monolith. Because the base of the tuff is poorly exposed in 

this vicinity, an excellent exposure just north of Nugent Wash in sec. 

18, T. 11 N., R. 32 E. (fig. 2) is designated here as a reference locality. 

At the reference locality, the Blue Sphinx Tuff rests on thin-bedded 

tuffaceous and lacustrine sedimentary rocks, but at the type locality 

these underlying rocks are exposed only in fault slivers south of the 

monolith. The Blue Sphinx Tuff in other localities, however, rests on 

thin biotite-rich air-fall tuff, the tuff of Gabbs Valley, the Petrified 

Spring Tuff, or directly on the intermediate lavas previously described. At the type locality, the Blue Sphinx Tuff is part of a fault 

block dropped down on the east side of the range by a northweststriking fault (fig. 2).

At the type locality, hornblende-rich intermediate lavas of Nugent 

Wash overlie the Blue Sphinx Tuff. At the reference locality, in contrast, the Blue Sphinx is overlain by the Nugent Tuff Member of the 

Hu-pwi Rhyodacite, and only thin rubble of intermediate lavas 

separate the two tuff sequences.

The Blue Sphinx Tuff is a multiple-flow simple cooling unit of 

quartz latite. Typically, it is blue or lavender, a result of mild 

hydrothermal alteration. In the rare exposures where the tuff is fresh, 

it is pink or pinkish gray on fresh fracture and weathers pale brown 

and medium brown. In the southern Gabbs Valley Range, the Blue 

Sphinx contains few lithic fragments. In the northern Gabbs Valley 

Range, however, for example at the type locality, the tuff contains 

abundant lithic fragments. Fragments consist principally of two 

types: (1) lavender and purple intermediate lava mostly nearly 

aphyric but some with conspicuous large plagioclase phenocrysts and 

(2) dark-brownish-green-weathering, greenish-gray on fresh fracture, 

highly altered tuff(?) containing sparse quartz phenocrysts. Type 2 

rocks have been almost entirely replaced by calcite, chlorite, and
34 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

FIGURE 10. View looking northeastward at Blue Sphinx monolith the type locality 

of the Blue Sphinx Tuff. Dark outcrops to right of monolith are hornblende lava 

that overlies the Blue Sphinx Tuff; dark outcrops to left are Nugent Tuff Member of 

Hu-pwi Rhyodacite, which overlies the hornblende lava. Layered strata in 

background are cooling units of the Poinsettia Tuff Member, which rests on intermediate lavas of the Poinsettia Spring area. A major northwest-trending strikeslip fault (see fig. 2) separates the strata in foreground from those in the 

background.

epidote. At the reference locality, the type locality, and Calavada 

Summit, the Blue Sphinx Tuff is 30-90 m thick, but it possibly is as 

much as 300 m thick in the northern Gillis Range.

The Blue Sphinx Tuff contains about 35 percent phenocrysts. It 

shows a slight tendency to become more silicic upwards. At Calavada 

Summit, one of the few localities where the tuff is not altered, it contains about 13 percent quartz from base to top but, in contrast, alkali 

feldspar increases upward from about 20 percent at the base to as 

much as 34 percent near the top. Ferromagnesian minerals, consisting of subequal hornblende and biotite, constitute 8-14 percent of 

the total phenocrysts. Clinopyroxene occurs locally. The ferromagnesian minerals show little variation in volume from base to top.

The most distinguishing characteristic of the Blue Sphinx is its 

quartz phenocrysts (fig. 11). These are so intensely resorbed that 

many grains are sieves. They range in length from about 2 mm to as 

much as 6 mm, and although they constitute only about 5-15 percent 

of the phenocrysts (fig. 4), they nevertheless serve to distinguish the 

Blue Sphinx from all other major welded tuff units in the region. It is 

worthy of mention that a stratigraphically higher tuff (informally 

called tuff of Redrock Canyon, see later pages) contains quartz
STRATIGRAPHY AND GEOCHRONOLOGY 35

FIGURE 11. Photomicrograph of thin section of Blue Sphinx Tuff showing intensely 

resorbed quartz phenocryst. Most phenocrysts in this view are plagioclase crystals 

that are partly replaced by calcite and clay. A cavity lined with secondary quartz 

and filled with birefringent clay lies at lower left. Plane polarized light.

phenocrysts as large and as resorbed as those in the Blue Sphinx but 

the tuff of Redrock Canyon is restricted to the eastern part of the 

Gabbs Valley Range and contains so many granite lithic fragments 

there that it is easily distinguished from the areally extensive Blue 

Sphinx. The overall composition of the Blue Sphinx and the 

presenceof resorbed quartz suggest that the formation is transitional 

in composition as well as in time between the Benton Spring Group 

and the Hu-pwi Rhyodacite.

The Blue Sphinx Tuff is a major mappable unit throughout the 

Gabbs Valley and Gillis Ranges. It is thick and crops out over broad 

areas in the northern Wassuk Range (E. C. Bingler, oral commun., 

1975). It does not occur in the Singatse Range, but Proffett and Proffett (1976, p. 23) reported an ash-flow tuff with "sieve-textured 

resorbed quartz" about 7.25 miles S. 71° E. of Yerington that is in 

the same stratigraphic position as the Blue Sphinx. The Blue Sphinx
36 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

has not been observed at Cedar Mountain and vicinity (fig. 3). The 

formation is younger than the tuffs of Gabbs Valley, which are dated 

at about 25 m.y. (table 2), and it is older than the Hu-pwi Rhyodacite, 

which has been dated at 22-23 m.y.

INTERMEDIATE LAVAS OF NUGENT WASH AREA

Dark-gray lavas of intermediate composition overlie the Blue 

Sphinx Tuff near Blue Sphinx monolith and over broad areas northwest and southeast of Nugent Wash. The most widespread lava is 

hornblende latite or rhyodacite. The lava is 30-50 m thick, dark gray 

on fresh fracture, weathering to dark greenish brown and black. It 

contains about 10 percent by volume of conspicuous phenocrysts of 

hornblende as much as 6 mm long and inconspicuous plagioclase that 

rarely exceeds 3 mm in length.

In several localities south of Nugent Wash, the hornblende lava is 

overlain by dense andesitic lava as much as 30 m thick that is 

moderately altered in all exposures. This lava closely resembles 

basalt in outcrop. It is dark gray to black where relatively fresh and 

dark purple where altered. It grades upward without a visible cooling 

break from massive-weathering andesitic phenocryst-poor rock containing sparse small phenocrysts of pyroxene and plagioclase into 

rhyodacitic flow breccia containing abundant large (as much as 8 

mm) phenocrysts of plagioclase, pyroxene, and hornblende, and 

biotite as large as 5 mm. The phenocryst-rich breccia, in turn, is 

overlain by the pumice-rich Nugent Tuff Member of the Hu-pwi 

Rhyodacite.

HU-PWI RHYODACITE

The herein named Hu-pwi Rhyodacite consists of a sequence of 

rhyodacite tuff and lava that is rich in ferromagnesian minerals and 

nearly lacking in quartz, and totals at least 1,000 m in thickness. The 

formation is named after exposures in the vicinity of Hu-pwi Wash, 

the type locality (T. 12 N., R. 30 E.), which drains the northeastern 

flank of the northern Gillis Range (fig. 2). The Hu-pwi Rhyodacite 

consists of three formal members and one informal member in 

ascending order the Nugent Tuff Member, Ghost Dance Lava 

Member, intermediate lavas of the Poinsettia Spring area that probably are coeval with the Ghost Dance, and Poinsettia Tuff Member. 

The Hu-pwi Rhyodacite is the equivalent of the tuff and breccia of 

Gallagher Pass of Proffett and Proffett (1976).

The three formal members are exposed in fault blocks between 

Nugent Wash and Hu-pwi Wash (fig. 2), and they occur together only 

in the type locality of the Ghost Dance Lava Member.
STRATIGRAPHY AND GEOCHRONOLOGY 37

NUGENT TUFF MEMBER

The oldest member of the Hu-pwi Rhyodacite is the Nugent Tuff 

Member, here named after exposures in and near Nugent Wash in the 

northern Gabbs Valley Range. The type locality is designated as 

Nugent Wash in T. 11 N., R. 32 E., about 1 km east of R. 31 E. (fig. 2). 

In this vicinity the Nugent Tuff Member rests on thin-bedded redbrown ferruginous tuffaceous sandstone 1-5 m thick or on rubble of 

lavas of intermediate composition less than a few meters thick, which 

in turn rests on the Blue Sphinx Tuff. Southeast of Nugent Wash the 

member overlies either the Blue Sphinx Tuff or the intermediate 

lavas of the Nugent Wash area. The member is overlain in the Gillis 

Range by the Ghost Dance Lava Member, but in the Gabbs Valley 

Range it is overlain by a variety of strata including intermediate 

lavas of the Poinsettia Spring area, Poinsettia Tuff Member, tuff of 

Redrock Canyon, and local unsorted landslipped debris.

The Nugent Tuff Member is a compound cooling unit that displays 

little variation in phenocryst mineralogy from base to top; however, 

it displays marked megascopic variation. In the northeastern Gillis 

Range it is 300 m thick, but it thins drastically to 70 m thick in the 

vicinity of Ghost Dance Ridge (fig. 2), where it is entirely vitrophyre. 

There the tuff contains small pumice and abundant (as much as 40 

percent) cognate fragments of the same mineralogy as the enclosing 

matrix. These fragments are somewhat compressed, are black in 

color, and in places are as much as 0.3 m long. A few cognate fragments are distinctively flow banded. The abundance of these 

fragments imparts a tuff-breccia appearance to the rock. The thinness and glassy nature together with the tuff-breccia character suggest that the tuff in this locality may be near the distal end of the 

unit. This conclusion is supported by absence of the tuff in the northern Wassuk Range (E. C. Bingler, oral commun., 1975). In the northeastern part of the Gillis Range and throughout the Gabbs Valley 

Range the tuff averages at least 300 m thick and is densely welded 

and devitrified throughout except for a local thin basal vitrophyre. 

Throughout this area it is a compound cooling unit consisting of 

many ash flows. In the basal two-thirds it is characterized by brown 

pumice fragments that contrast markedly with the gray matrix. 

Pumice fragments rarely exceed 3 cm in length, and some zones in the 

basal two-thirds are nearly devoid of pumice fragments. The top third 

consists of ash-flow tuffs that contain gray pumice fragments, which 

are lighter in color than the matrix, and less common brown pumice 

fragments. The gray pumice fragments are as much as 20 cm long. 

The contact between the upper ash-flow tuffs containing large gray 

pumice fragments and the lower ash-flow tuffs containing brown 

pumice fragments is marked by a zone of distinctively bedded welded
38 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

air-fall tuff as much as 1.5 m thick and by numerous lava fragments 

of intermediate composition that occur in the basal 10 m of the upper 

tuff. A complete cooling break may not be indicated, however, 

because the pumice clasts above, below, and within the bedded sequence are well flattened, and the tuff is completely devitrified.

Unaltered devitrified Nugent Tuff Member is typically medium 

gray on fresh fracture, weathering pale brown and brown. Where 

moderately altered, the tuff is dark gray, weathering to dark 

brownish or dark purplish gray.

The Nugent Tuff Member commonly consists of as much as 50 percent phenocrysts (fig. 4) and, of these, 15-30 percent are ferromagnesian crystals principally biotite and clinopyroxene in approximately equal proportions. Hornblende and orthopyroxene rarely exceed a few grains per thin section. The ferromagnesian crystals are 

large; biotite occurs in books as large as 4 mm; pyroxenes and hornblendes are as long as 5 mm. Plagioclase phenocrysts vary considerably in size; some ash flows of the Nugent contain plagioclase 

that rarely exceeds 4 mm, but other flows contain plagioclase that are 

mostly 5-8 mm long. Most large plagioclase phenocrysts are 

poikilitic and, surprisingly for a densely welded tuff, are little shattered and broken. Few of the more than 50 thin sections counted contain quartz or alkali feldspar. Some thin sections, however, contain as 

many as three grains of quartz and seven grains of alkali feldspar. 

The Nugent Tuff Member is magnetically reversed (G. D. Bath, written commun., 1976).

GHOST DANCE LAVA MEMBER

The Ghost Dance Lava Member is here named after Ghost Dance 

Ridge in the Gillis Range (fig. 2), but the type locality is in Hidden 

Wash in T. 11 N., R. 30 E., about 3.2 km east of the ridge. There the 

member overlies the Nugent Tuff Member and underlies the Poinsettia Tuff Member. The contacts between the members, however, are 

low-angle detachment faults.

The rhyodacitic lava of the Ghost Dance Lava Member is virtually 

identical in phenocryst mineralogy to the rhyodacitic tuff in the 

underlying Nugent Tuff Member and overlying Poinsettia Tuff 

Member; thus the contrasting rock types merely reflect changing 

physical conditions in the magma chamber. The basal part of the 

Ghost Dance Lava Member is a flow breccia of variable thickness, 

generally about 10 m. This zone consists of dark-brown to black 

glassy rhyodacite fragments, commonly flow banded, that range in 

length from about 1 cm to blocks as large as 1 m. These fragments are 

set in a brownish-gray glassy lava matrix that composes approximately 60 percent of the rock. The brownish-gray flow breccia grades
STRATIGRAPHY AND GEOCHRONOLOGY 39

upward into black glassy flow breccia and thence upward into 

devitrified gray flow-banded lava displaying ramp structures and 

flow folds. Layers in the flow-banded rock are alternately phenocrystrich and phenocryst-poor, and the phenocrysts in the layers range 

markedly in length from only a few millimeters in some layers to 

more than 5 mm in others.

The maximum known thickness of the Ghost Dance Lava Member 

in the northern Gillis Range is about 100 m; it ranges from a feather 

edge to as much as 30 m thick in the northern Wassuk Range, according to E. C. Bingler (oral commun., 1975). Bingler also reported 

that in the Wassuk Range it is difficult to demonstrate a cooling 

break between the lava and the overlying Poinsettia Tuff Member. 

He believed that the lava and tuff sequences there constitute a single 

compound cooling unit.

The Ghost Dance Lava Member is confined to the northern Gillis 

and Wassuk Ranges and this distribution (keeping in mind a probable right-lateral slip of 5-10 km between the two ranges) probably 

corresponds very closely to the original distribution. The Ghost 

Dance Lava Member probably was fed simultaneously from several 

vents that were located in an area that lies between the northern 

parts of the two ranges.

INTERMEDIATE LAVAS OF POINSETTIA SPRING AREA

An apparently very local pile of lava of intermediate composition, 

at least 400 m thick, is exposed near Poinsettia Spring (fig. 2). The 

base of the pile is not exposed but we infer that the lavas overlie the 

Nugent Tuff Member of the Hu-pwi Rhyodacite. The lava sequence is 

overlain by the Poinsettia Tuff Member. These relations indicate that 

the sequence probably is the time equivalent of the Ghost Dance 

Lava Member but the lavas are not included in that member because 

they constitute such lithologically diverse flows.

Near Poinsettia Spring the rocks are propylitically altered and a 

northwest-trending fracture there is veined with cinnabar. The lava 

exposed at the cinnabar mine near the spring is conspicuously porphyritic, containing large hornblende prisms as much as 1.5 cm long, 

biotite books as much as 4 mm long, and plagioclase as much as 1 cm 

long. This rock contains a grain or two of quartz per hand specimen 

and probably i£ quartz latite in composition. Most of the lava in the 

pile, however, is femic rhyodacite, containing generally small 

phenocrysts. These include prisms of hornblende, clinopyroxene, and 

orthopyroxene that rarely exceed 3 mm long and plagioclase that also 

rarely exceeds 3 mm long. Some flows contain only pyroxene 

phenocrysts. The lavas of Poinsettia Spring, although in general finer 

grained and richer in pyroxene than the formal members of the Hupwi Rhyodacite, are similar to the enclosing tuffs and, like the Ghost
40 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

Dance Lava Member, represent a relatively quiescent stage in the 0 

eruptive cycle of the underlying magma chamber. >'

Bedding-plane or detachment faults are important structural 

features near Poinsettia Spring and eastward toward Gabbs Valley ^'' 

(fig. 2), and these were avenues for hydrothermal solutions. In several 

localities the lavas adjacent to the faults are intensely altered.

$ -,

POINSETTIA TUFF MEMBER

The Poinsettia Tuff Member is here named for exposures in T. 10 « 

N., R. 33 E., the type locality, about 2 km south-southeast of Poinsettia Spring (fig. 2, P). The tuff overlies alternately the Nugent Tuff ' 

Member or the intermediate lavas of the Poinsettia Spring area. Near ^ 

the type locality the tuff is overlain by lavas of Mount Ferguson. 

Although the upper contact of the Poinsettia Tuff Member is a fault * 

contact in this vicinity, the uppermost cooling unit of the Poinsettia v, 

displays a partially welded top, which indicates that very little of the 

cooling unit has been removed by faulting or prelava erosion. To the 

northwest, however, between Poinsettia Spring and Copper Mountain, the Poinsettia is conformably overlain by the tuff of Copper 

Mountain. The omission of this member at Poinsettia Spring * 

therefore suggests that considerable erosion did occur prior to the ex- ^ 

trusion of the oldest lava of Mount Ferguson.

In the vicinity of the type locality it is apparent that the Poinsettia 

Tuff Member was largely confined to a structural and topographic 

trough. The tuff is bounded on the southwest side by an old fault ' 

scarp formed of Nugent Tuff Member and older tuffs and lavas. This *, 

fault zone is 10 m or so wide and the intensity of shearing suggests 

that it may be a strike-slip fault, or at least that it had some component of strike-slip motion. The fault strikes about N. 65 ° W., an orien- * 

tation that is parallel to the axes of distribution of all the tuff sheets 

exclusive of the tuff of Copper Mountain. On the northeast side the 

tuff laps up against the lavas of the Poinsettia Spring area. The zone v: 

of onlap is linear and can be traced for a distance of about 9 km trending about N. 70 ° W. This relationship suggests that this tuff boundary is controlled also by a fault, but that this fault zone was buried by ^ 

the onlapping tuff sheet.

The Poinsettia Tuff Member consists of five partially to densely 

welded multiple-flow simple cooling units. The four basal cooling >. 

units are of rhyodacite composition and the top cooling unit is of 

quartz latite composition. The basal cooling unit is as much as 200 m 

thick. The other units range in thickness from about 30 m to 150 m. 

All five units are largely devitrified but much of this devitrification 

probably is due to postcooling groundwater alteration because 

pumice fragments in all units are characteristically perlitic. The units
STRATIGRAPHY AND GEOCHRONOLOGY 41

dip 20°-70° to the northeast and are greatly extended by obscure 

moderate- to low-angle southwest- and west-dipping faults. Because 

of the obscure nature of the faults, the five cooling units appear, at 

first glance, to compose a single compound cooling unit several hundred meters thick.

The four rhyodacite cooling units are virtually identical, containing 

the same phenocryst abundance and assemblage. They are the same 

color light gray or light reddish gray on fresh fracture, weathering 

to pale brownish gray and brown. The uppermost quartz latite cooling unit is reddish gray on fresh fracture and weathers medium 

brown or reddish brown. This unit contains both quartz and alkali 

feldspar (fig. 4) and is the only unit to do so. It also contains a grain or 

two of sphene per thin section.

AGE AND DISTRIBUTION OF HU-PWI RHYODACITE

All three formal members of the Hu-pwi Rhyodacite have been 

dated by K-Ar methods (table 2). Exclusive of a plagioclase date, 

these ages are in good agreement and indicate that the formation is 

22.6-23.3 m.y. old.

The Nugent Tuff and Poinsettia Tuff Members resemble each other 

so closely that in isolated outcrops one cannot be certain which 

member is present. Because of this close similarity, the distribution 

of Hu-pwi Rhyodacite shown in figure 3 encompasses both members. 

The Nugent Tuff Member has been recognized with certainty only in 

the northern Gabbs Valley and Gillis Ranges, and, as previously mentioned, the local abrupt thinning of the member in the Gillis Range 

together with its absence in the northern Wassuk Range (E. C. 

Bingler, oral commun., 1975) suggest that the Nugent Member was 

much more restricted areally than the older tuff units and, perhaps, 

the younger Poinsettia Tuff Member as well. The possibility exists 

that the Nugent Member was confined entirely to a depression that 

formed in response to the extrusion of the older tuffs. According to J. 

M. Proffett (oral commun., 1975), however, the tuff and breccia of 

Gallagher Pass (in Proffett and Proffett, 1976) in the Singatse Range 

could correlate with either the Nugent Tuff Member or the Poinsettia 

Tuff Member. Therefore, the possibility cannot be ruled out that the 

Nugent Tuff Member is more widespread than we have suggested.

TUFF OF COPPER MOUNTAIN

The tuff of Copper Mountain is informally named for exposures 

about 0.8 km east of Copper Mountain (fig. 2, CM). The best exposure 

is the first prominent north-striking fault block east of Copper Mountain. This block is the most westerly of a series of repeated fault
42 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

blocks that are controlled by west-dipping normal or oblique-slip 

faults that dip 15 °-45 ° W. The tuff of Copper Mountain in each block 

dips 50 0-80 0 E. In the first fault block the tuff is very steeply dipping above a fault that dips about 15 ° W. and that places the Copper 

Mountain on the underlying quartz and alkali feldspar cooling unit of 

the Poinsettia Tuff Member of the Hu-pwi Rhyodacite. The basal 

vitrophyre, although dipping on the average about 80° E., is so 

repetitiously repeated by faults, each fault having less than a meter 

or so of displacement, that it appears from a distance to be dipping 

very gently. The tuff of Copper Mountain in this locale is capped by a 

small outlier of lava of Mount Ferguson. This lava, however, is 

detached at the base and the former top of the tuff is missing either 

as a result of faulting or prelava erosion. Despite the complex 

faulting, the tuff of Copper Mountain retains a part of the nonwelded 

base beneath the steeply dipping vitrophyre.

The tuff of Copper Mountain is a simple cooling unit of quartz 

latite. In various fault blocks in the vicinity of Copper Mountain, 60 

meters to as much as 100 m of tuff are preserved. The tuff is light 

gray on fresh fracture and weathers brownish gray and brown. It is 

characterized by a persistent black vitrophyre, about 3 m thick, at 

the base, which overlies nonwelded to partially welded light-gray tuff 

that is at least 6 m thick. The tuff above the vitrophyre is moderately 

to densely welded and devitrified. Pumice fragments are conspicuous 

from base to top.

The volume of phenocrysts ranges from about 28 to 44 percent. 

Quartz grains constitute from 3 to 25 percent of the total 

phenocrysts. They are as much as 5 mm long and are moderately 

resorbed and embayed. The tuff carries abundant accessory sphene. 

These grains are visible with a hand lens, and as many as 15 grains 

occur in a single thin section. The accessory mineral is less abundant 

in the middle part of the exposure than at the base and top. The abundance of sphene distinguishes the Copper Mountain from all other 

tuffs in the region; without this accessory it could easily be mistaken 

for the Singatse Tuff.

Like the Singatse Tuff, the ferromagnesian minerals in the tuff of 

Copper Mountain are principally biotite and hornblende but clinopyroxene is ubiquitous and as abundant as hornblende in some thin 

sections. The ferromagnesian minerals constitute from 11 percent to 

as much as 20 percent of the total phenocrysts.

In most exposures the tuff of Copper Mountain does not display a 

pronounced tendency to become more silicic upwards. In general, ferromagnesian minerals decline from base to uppermost exposures, but 

there is no corresponding increase of quartz and alkali feldspar. In 

the fault block near Copper Mountain, quartz actually diminishes in 

volume upwards from about 14 percent in the basal vitrophyre to
STRATIGRAPHY AND GEOCHRONOLOGY 43

about 3 percent in the topmost exposure. Alkali feldspar and 

plagioclase fluctuate in volume, showing no systematic variations. 

Three kilometers east of Copper Mountain, however, rock at the top 

of the exposure is rich in quartz and alkali feldspar, and we infer that 

the rock there includes ash-flow tuff that was removed by erosion or 

faulting in the fault blocks nearer Copper Mountain. Rock having 

this quartz- and alkali feldspar-rich mineralogy was sampled for 

chemical analysis in Gabbs Valley on the east flank of the northern 

Gabbs Valley Range (table 3, sample H). According to the classification of Young (see p. 18), the rock is rhyolite in composition.

The tuff of Copper Mountain is exposed discontinuously beneath 

the lavas of Mount Ferguson in several fault blocks starting from 

about the latitude of Poinsettia Spring to north and northeast of the 

ghost town of Rawhide (fig. 2). It is poorly exposed in the Pilot 

Cone-Rawhide area, but it is well exposed in the Monte Christo 

Mountains along the west flank of Gabbs Valley (fig. 2). In the exposures northeast of the Rawhide area and in the Monte Christo 

Mountains, the tuff of Copper Mountain rests directly on rhyolite 

tuffs of Gabbs Valley, which, in turn, rest directly on Mesozoic rocks. 

The absence of all other tuffs, which are thick and widespread in the 

Gabbs Valley and Gillis Ranges to the west, must be due to: (1) 

original deposition of those tuffs within a northwest-trending trough 

that lay southwest of the Monte Christo-Gabbs Valley area and (2) 

juxtaposition of two separate and distinct ash-flow fields by the right 

lateral strike-slip faults. The tuff of Copper Mountain, which overlaps 

both fields, must have been extruded after the northwest-trending 

trough was more or less filled, and, in addition, must have had a more 

northerly source than the other tuffs described in this report. According to E. C. Bingler (1978), the tuff of Copper Mountain probably correlates with his Santiago Canyon Tuff, which is thick and widespread 

in the Carson City-Silver City area.

There is no evidence of extensive erosion between the Poinsettia 

Member and the tuff of Copper Mountain. The two units are of 

similar age (about 23 m.y.) because intermediate lava that directly 

overlies the Copper Mountain has been dated at about 22 m.y. (table 

2, samples 2384-IP and 2384-1H). The eruption of the tuff of Copper 

Mountain marked the end of main ash-flow tuff volcanism that persisted in the region for a period of 3-4 m.y.

ROCKS OF MOUNT FERGUSON

Although lava of intermediate composition makes up most strata 

exposed in and adjacent to Mount Ferguson (fig. 2), and is the sole 

lithology in many places, other rock types are locally important. 

These include the tuff of Redrock Canyon, a densely to partially weld-
44 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

ed ash-flow tuff that appears to be confined to the Gabbs Valley 

Range, and a thick sequence of tuffaceous sedimentary rocks including landslipped debris and rock that is principally lacustrine in 

origin. The lacustrine strata suggest subsidence possibly related to 

ash-flow eruptions. The landslipped debris, on the other hand, probably reflects early movements along the strike-slip faults.

TUFF OF REDROCK CANYON

The distinctive ash-flow tuff in the Mount Ferguson sequence is 

here called informally the tuff of Redrock Canyon. The tuff is a compound cooling unit that may be as much as 300 m thick. It is locally 

exposed from the vicinity of Petrified Spring northwestward nearly 

to the latitude of Poinsettia Spring (fig. 2). The tuff rests unconformably on a great variety of rocks. Near Petrified Spring it rests, in 

places, on Singatse Tuff and Blue Sphinx Tuff. One kilometer to the 

north it rests on coarse landslipped debris; 3 km farther north it rests 

on lavas of intermediate composition that intertongue with debris 

and are typical of the Mount Ferguson pile; near Redrock Canyon 

(fig. 2) tuff of Redrock Canyon rests on debris that includes 

fragments of Poinsettia Tuff Member.

The tuff of Redrock Canyon, although apparently of local extent, is 

nevertheless an interesting unit because it combines the 

characteristics of the Mickey Pass and Blue Sphinx Tuffs. The basal 

50-70 m is densely to partially welded, dark-brown rhyodacite that is 

rich in lithic fragments and that contains as much as 25 percent ferromagnesian phenocrysts. These include 5-13 percent of clinopyroxene 

and orthopyroxene, a trace to 6 percent of biotite, and a trace to 6 percent of hornblende. The rhyodacite contains only a few grains of 

quartz and alkali feldspar per thin section, and the quartz is not 

noticeably embayed. The rhyodacite contains abundant plagioclase, 

consisting of about equal amounts of fresh euhedral crystals and extremely embayed and poikilitic crystals. These last are virtual sieves 

that are charged with glass. The initial flows of the rhyodacite were 

apparently very hot because a persistent homogeneous black 

vitrophyre occurs at the base, and in several places the underlying 

tuffaceous sedimentary rocks are baked to a depth of 1 m or more. A 

characteristic feature of the rhyodacite that proved to be a valuable 

criterion for identification in the field is the tendency of the pumice in 

the basal vitrophyre and the immediately overlying devitrified rock 

to weather to pits.

The rhyodacite grades upward to partially welded, brownish-gray 

quartz latite that also is rich in lithic fragments; however, in contrast 

to the rhyodacite, whose fragments are almost exclusively lava of intermediate composition, the fragments in the quartz latite include
STRATIGRAPHY AND GEOCHRONOLOGY 45

numerous clasts of Mesozoic granite, some of which are as much as 

3 m long.

The quartz grains in the quartz latite are as large as 7 mm; the 

grains larger than about 2 mm (fig. 12) are as embayed and resorbed 

as those in the Blue Sphinx, but most grains smaller than about 2 

mm show very little embayment. The quartz latite contains an occasional grain of sphene per thin section, but sphene is absent in the 

rhyodacite. This absence, together with a different assemblage of 

lithic fragments, suggests that the quartz latite and rhyodacite were 

erupted from different parts of the magma chamber as well as from 

different levels of the chamber.

Biotite from the tuff of Redrock Canyon yields an age of 23.5 ±0.6 

m.y. (table 2).

FIGURE 12. Photomicrograph of tuff of Redrock Canyon showing shattered, intensely resorbed quartz phenocryst. The three parts of the grain are in optical continuity. 

Crossed nicols.
46 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA *' 

TUFFACEOUS SEDIMENTARY ROCKS AND LANDSLIPPED DEBRIS &

Tuffaceous sedimentary rocks and landslipped debris intercalated 

with lavas of intermediate composition crop out locally beneath the .* 

tuff of Redrock Canyon. These rocks appear to be confined to the 

same general area as the tuff of Redrock Canyon. The landslipped * 

debris, however, persists northward for several kilometers beyond 

the outcrops of sedimentary rocks and the tuff of Redrock Canyon. 

The debris is restricted to a zone between the two easternmost strike- * 

slip faults, and we infer that it reflects early movements along these . 

faults.

The sedimentary rocks are mostly thin bedded and include tuf- ^ 

faceous lacustrine siltstones, reworked air-fall tuff, and conglomerate. The beds average about 8 cm in thickness and are pastel 

shades of yellow, green, and gray. The bedded sequence exclusive of 

landslipped debris is as much as 200 m thick. The debris consists of ^ 

well-cemented, unsorted boulders, cobbles, and pebbles of lava of intermediate composition and welded tuff, locally including the < 

Poinsettia Tuff Member of the Hu-pwi Rhyodacite. Most fragments 

are angular to subangular and undoubtedly include landslide 

material. The debris is not continuous and varies considerably in 

thickness. In places, it is at least several tens of meters thick.

LAVAS OF INTERMEDIATE COMPOSITION

In the vicinity of Mount Ferguson, lavas that range in composition +t 

from trachyandesite or latite to quartz latite crop out beneath, 

within, and above the tuffs and sedimentary rocks previously 4' 

described. These lavas, although similar to the localized piles (fig. 1) 

that we infer were fed from the same magma source as the ash-flow 

tuffs, are so regional in extent that, with the possible exception of the 

oldest flows that predate the tuff of Redrock Canyon, they cannot be * 

genetically related to the ash-flow tuffs. The lavas were erupted more 

or less simultaneously over a large part of the Great Basin and adja- * 

cent parts of the Sierra Nevada. (See, for example, Proffett and Proffett, 1976; Anderson and Ekren, 1968; Gilbert and Reynolds, 1973.)

The lavas in detail show a considerable range in color, but they present an overall aspect of somber gray. Despite their hardness they ± 

tend to weather to rounded ridges and slopes. This is especially true 

of flow-brecciated rock. The latites that border on andesite in com- x 

position are the most competent rocks and have the greatest tendency 

to weather to steep slopes and cliffs. They are dark gray on fresh fracture and weather dark brownish gray and black. The rocks are porphyritic and the phenocrysts are set in a pilotaxitic groundmass. The 

principal ferromagnesian phenocrysts are clinopyroxene and orthopyroxene. The occurrence of both minerals is generally apparent

**
STRATIGRAPHY AND GEOCHRONOLOGY 47

in hand specimen because the clinopyroxene is green and the orthopyroxene is brown commonly cinnamon brown. Phenocrysts of 

labradorite rarely exceed a few millimeters in length and these are 

mostly dark and indistinct in hand specimen. The latite lavas are not 

continuous over great distances and they are not confined to any particular horizon within the pile. They are estimated to compose less 

than 50 percent of the overall volume of the lava sequence.

Rocks considered to be rhyodacite and quartz latite (pending confirmation by chemical analyses) show all gradations from near latite 

on the one hand to near rhyolite on the other. All rocks are porphyritic and contain pilotaxitic groundmasses. Some flows contain 

quartz phenocrysts; none contain alkali feldspar phenocrysts. The 

dominant ferromagnesian mineral is hornblende, which may be as 

large as 2.5 cm by 4 mm in some flows. Nearly all rocks examined in 

thin section contain clinopyroxene and orthopyroxene. Pyroxene is 

nearly as abundant as hornblende in some flows; in others it constitutes only a tiny fraction of the ferromagnesian assemblage. Some 

rocks, which we consider to be quartz latite in composition, contain 

abundant biotite. Phenocryst bar graphs of several lavas are shown 

in figure 4.

Available data indicate a considerable range in age for the lavas of 

Mount Ferguson. A basal lava that rests on the tuff of Copper Mountain near the mouth of Nugent Wash in T. 11 N., R. 32 E., yielded an 

age of 22.5±0.6 m.y. on hornblende and 21.4±0.6 m.y. on 

plagioclase. A sample of one of the youngest flows on the eastern 

flank of Mount Ferguson yielded an age of 15.0±0.5 m.y. on 

plagioclase (table 2).

INTRUSIVE ROCKS

Intrusive dikes, plugs, and small stocks occur in several localities 

in the Gabbs Valley and Gillis Ranges. These rocks range in composition from basalt to rhyolite. Most of the larger masses are of intermediate composition; hornblende is the principal ferromagnesian 

mineral. Textures of the intermediate rocks vary considerably. Some 

rocks of rhyodacitic composition are conspicuously porphyritic with 

phenocrysts as long as 1 cm that are set in holocrystalline but very 

fine grained groundmasses. Other rocks of dioritic composition are 

equigranular but mostly fine grained.

A K-Ar age determination on an aphyric basaltic andesite or black 

latite that was intruded along one of the strike-slip faults in the 

Gabbs Valley Range yielded an age of 5.8±0.2 m.y. on whole rock. 

Biotite from a rhyolite dike in the Gabbs Valley Range and from 

rhyolite vent breccia in the Garfield Hills yielded ages of 19.2±0.7 

m.y. and 18.1 ±0.6 m.y., respectively (table 2).
48 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

PRELIMINARY PETROLOGY AND VOLCANO-TECTONIC ORIGIN 

OF THE THICK VOLCANIC SECTION

The marked petrographic similarity of ash-flow tuffs and intercalated lava flows of intermediate composition in the Gabbs Valley 

and Gillis Ranges suggests that many, if not most, volcanic units 

were derived from a common magma source. A single substratum or 

large magma chamber probably was the source of nearly all the rocks. 

If present, this chamber would be located somewhere between Yerington and the northern Gabbs Valley Range, because the strata are 

thickest in this region and the region encompasses the center of 

distribution of the various ash-flow sheets (fig. 3). Mapping of this 

area, however, by ourselves, John Proffett, E. C. Bingler, and H. F. 

Bonham, Jr., failed to disclose a cauldron complex or any structures 

that can logically be interpreted as cauldrons. All of the ranges in the 

area expose large outcrops of Mesozoic rocks and the basins that are 

sufficiently large to partly conceal a caldera for example, Gabbs 

Valley and Walker Lake either lie outside the tuff distribution area 

or they are bounded by such extensive outcrops of Mesozoic rocks 

that a concealed center within the valleys appears extremely unlikely.

Because of deep erosion that stripped the volcanic rocks from large 

parts of the ranges, it is difficult to calculate the volumes of the tuff 

sheets. Assuming average thicknesses of 100 m for the major ashflow sheets whose distributions are plotted in figure 3, the Guild 

Mine Member of the Mickey Pass Tuff has a volume of 1,100 km3 ; the 

Singatse has about the same; and the combined Hu-pwi Rhyodacite 

and the Blue Sphinx Tuff somewhat more. These calculations are 

rough, and for one reason or another these volumes may be excessive.

Even allowing for errors, however, the volumes must be enormous, 

and had cauldrons formed they would be huge either very deep or 

extremely broad and of such magnitude that if they were bounded by 

arcuate faults they should be easily discerned. We believe that instead of fault-bounded cauldrons, a broad saucer-shaped volcanotectonic depression developed that was elongated in a northwest 

direction and centered on what is now the northern parts of the 

Gabbs Valley, Wassuk, and Gillis Ranges. Within this depression 

various broad "sags" or basins developed that were, in part, fault 

controlled. The main evidence for the broad depression is the 

presence of composite sections consisting of as much as 2,000 m of 

ash-flow tuff and 1,000 m or more of lava; the evidence for local 

basins is the presence of lake sediments at two horizons within the 

ash-flow tuff and lava sections. The depression and local basins must 

have developed concomitantly with the eruption of the volcanic 

rocks. The present-day outcrop pattern of the tuff units (fig. 2) suggests that the younger units were increasingly confined to the more
PETROLOGY AND VOLCANO-TECTONIC ORIGIN 49

central part of the sagging depression. The depression was subsequently segmented by northwest-trending strike-slip faults, and the 

segments were shifted in a right-lateral sense.

The apparent lack of a fault-bounded cauldron complex in the 

source area for tuffs and lavas whose combined volume exceeds many 

hundreds of cubic kilometers must be explained. We believe that the 

logical explanation for the lack of such a complex is that the rocks 

were erupted from depths that were too great to allow fault-bounded 

calderas to form. That this is not an unreasonable concept is shown 

by analogy with results of underground nuclear testing at the 

Nevada Test Site. Explosion of a nuclear device at any depth 

underground creates a cavity. Where the device is at shallow depth a 

subsidence crater forms at the surface above the underground cavity 

within a few minutes or a few hours after the explosion. Where the 

device is exploded at deeper levels no surface subsidence crater forms 

(Houser, 1970), but sags develop over some of the deep sites. The extrapolation of these observed facts to caldera mechanics has been 

suggested by various geologists who have worked at the Nevada Test 

Site, notably by K. A. Sargent (oral commun., 1977).

Three features are present in the tuffs of the Gabbs Valley and 

Gillis Ranges that we believe suggest eruption from a deeply buried 

source. One feature is the presence of strongly resorbed quartz and 

feldspar phenocrysts, another is the virtual lack of compositional 

zoning in the Hu-pwi Rhyodacite, and a third feature is the unusual 

reverse zoning exhibited by formations in the Benton Spring Group.

SIGNIFICANCE OF PHENOCRYST RESORPTION

The resorption of phenocrysts indicates that the magma was 

undersaturated with respect to water (Robertson and Wyllie, 1971; 

Steiner and others, 1975; Lipman, 1966; and Noble, 1970). The undersaturation, in turn, indicates that the magma chamber was positioned at a deep water-deficient level of the crust. Noble (1970) 

pointed out that resorption in intratelluric crystals takes place if 

pressure is reduced when magma is undersaturated with respect to 

water, but not when the magma is water saturated. He perhaps was 

the first to consider the possibility that ash-flow tuffs containing 

strongly resorbed phenocrysts may have been derived from great 

depths, and, thus, that their eruption may not be accompanied by 

caldera collapse.

The presence of resorbed phenocrysts alone, however, cannot be used 

as bona fide evidence of great depth of eruption because resorbed 

phenocrysts occur in rocks that were erupted from chambers at 

shallow depths. The tuffs from the Timber Mountain caldera, for example, that were erupted from the resurged caldera contain 

moderately resorbed quartz phenocrysts (Byers and others, 1976).
50 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

Some of the lavas that were erupted from the Platoro caldera (Lipman, 1975) have strongly resorbed quartz phenocrysts, as do some of 

the lavas that were erupted from cauldrons on the Mogollon Plateau 

(Rhodes, 1976). Some cauldron-forming tuffs with resorbed 

phenocrysts apparently were derived from magmas that began to 

crystallize at deep water-undersaturated levels and then moved upward to spend considerable time in shallow depth chambers. This has 

been carefully documented in the Superstition-Superior volcanic area 

of Arizona by Stuckless and O'Neill (1973): 87Sr/86Sr of the tuffs and 

the associated mafic and silicic lavas in the Superstition-Superior 

area indicate that the parent magma was derived below the base of 

the Precambrian granitic crust probably at depths greater than 22 

km and they inferred that crystallization began at those depths. 

Their studies further showed, however, that two of the ash-flow tuffs 

and one lava are greatly enriched in 87Sr and have largely reequilibrated under P-T (pressure-temperature) conditions of a shallow 

magma chamber. Presumably, the eruption of tuffs from this environment caused the development of Superstition-Superior cauldron 

complex (Sheridan and others, 1970; Peterson, 1968). Apparently, 

therefore, resorbed phenocrysts may or may not be indicative of deep 

(non-caldera forming) eruptions and the vital factor is whether or not 

the magma moved into a shallow-seated "halfway house" prior to 

tuff eruptions. We believe that in the area of the Gabbs Valley and 

Gillis Ranges the magma probably did not reside for any lengthy 

periods at shallow depth because of the zoning relationships.

SIGNIFICANCE OF A VIRTUAL LACK OF COMPOSITIONAL ZONING

IN THE HU-PWI RHYODACITE AND REVERSE ZONING

IN TUFFS OF THE BENTON SPRING GROUP

Compositional zoning of ash flows from shallow-seated chambers 

generally shows a simple upward succession from high-silica to lowsilica rhyolite or from mafic-poor rhyolite to mafic-rich quartz latite 

(Byers and others, 1976; Lipman and others, 1966; Smith and Bailey, 

1966). This vertical zoning in the ash flows is generally attributed to 

reflecting vertical compositional zoning caused by differentiation in 

the magma chamber the first eruptions tapped the upper silica-rich 

levels of the chamber and subsequent eruptions tapped deeper, more 

mafic and less silicic levels.

The tuffs in the Gabbs Valley and Gillis Ranges do not show the 

compositional zoning characteristic of shallow-seated chambers. 

They either display no zoning (Hu-pwi Rhyodacite) or zoning that is 

the reverse of zoning characteristic of shallow chambers. The latter 

could be produced by a compositionally zoned chamber being tapped 

initially at a deep level and, as eruptions proceeded, at progressively 

higher levels; or by magma levels sinking to the level of the tap.
RELATIONSHIP, VOLCANIC CENTER TO WALKER LANE 51

** Although this theory is plausible, it seems unlikely that this

, ., mechanism could have occurred several times; further, it probably

has little merit because of the likelihood that magma chambers are

always tapped from the top or from the sides near the top. The top of

*- the chamber at any depth is the part that is under the least confining 

pressure, and the immediately overlying rocks would certainly tend

* to be more fractured than the rocks on the flanks of the chamber. Different P-T conditions and chamber configurations in a deep chamber 

versus a shallow one must account for the differences in zoning.

Shallow-depth chambers are known to have domed overlying surface rocks prior to ash-flow eruptions (Christiansen and others, 1965; 

Smith and Bailey, 1968). Lipman (1966) suggested that resurgent

* doming would be geometrically improbable if the upper part of the 

magma chamber had the proportions of a thick cylinder. On the other 

hand, a magma chamber at great depth may have the shape of a thick

* cylinder; at least there would be less potential for a domical roof and 

less potential for a laccolithic shape because of greater confining

* pressures. Certainly, extensive vertical compositional zoning in a

a, shallow chamber suggests that there may be an extensive floor.

Perhaps the deeper chambers are not extensively floored. Vertical 

zonation due to fractional crystallization would be disrupted by con-

; vection currents and new silicic melt flowing upward from the 

melting source. The rising melt probably would tend to occupy the 

central part of the chamber and press the early crystal fraction

^ against the sides and top. In the case of the Hu-pwi Rhyodacite the

eruptions in three broad stages (three members) were unable, until

- * the very last gasp, to exhaust the copious rhyodacite magma concen-

« trated at the top and sides of the chamber. In the case of the more 

voluminous Guild Mine Member the eruptions exhausted the 

rhyodacite melt and reached deeply into the siliceous rhyolite

v. interior.

RELATIONSHIP OF VOLCANIC CENTER TO 

WALKER LANE

The Gabbs Valley Range-Gillis Range-Wassuk Range volcanic 

center is another volcanic source area that lies along Walker Lane. 

These volcanic centers to the southeast include Tonopah, Goldfield, 

Cactus Range, Stonewall Mountain, Mount Helen, Black Mountain, 

Silent Canyon, Timber Mountain-Oasis Valley, Calico Hills, and 

Wahmonie (Ekren and others, 1971; Christiansen and others, 1977). 

Centers occur on either side of the lane throughout the Great Basin 

but they occur in considerably greater numbers along the lane.

The Walker Lane throughout Nevada (fig. 2) is a broad zone 50-70 

km wide that is marked by changes in range trends, structural and
52 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

topographic discontinuities, several major right-lateral faults and 

oroclinal bends. Unlike the San Andreas fault zone, no single fault or 

closely alined fault zone can be mapped continuously throughout the 

lane (Christiansen and others, 1977). Conceivably, tensional fractures 

related to deep-seated right-lateral faulting tapped an already hot 

substratum under the thin crust of the Great Basin and could have 

localized the volcanic centers.

ACKNOWLEDGMENTS

We are indebted to E. C. Bingler and H. F. Bonham, Jr., of the 

Nevada Bureau of Mines for supplying data and thin sections from 

the Wassuk Range and Carson River areas, and to R. C. Speed of 

Northwestern University for data, rock slabs, and thin sections from 

the Excelsior and Pilot Mountains. We wish to thank J. M. Proffett 

of Anaconda Copper Company for guiding us through the volcanic 

section in the Yerington area of the Singatse Range. We are indebted 

also to M. W. Reynolds of the U.S. Geological Survey for data in the 

Pine Grove Hills. We have benefited from discussions with our colleagues P. W. Lipman, R. E. Anderson, K. A. Sargent, M. A. Kuntz, 

and D. H. Mclntyre. The manuscript was technically reviewed by 

P. D. Rowley, A. T. Fernald, and F. Maldonado.

Part of R. F. Hardyman's work in the northern Gillis Range was 

financed by Penrose Research Grant No. 1744-73 from the 

Geological Society of America.

REFERENCES CITED

Anderson, R. E., and Ekren, E. B., 1968, Widespread Miocene igneous rocks of intermediate composition, southern Nye County, Nevada, in E. B. Eckel, ed., Nevada 

Test Site: Geological Society of America Memoir 110, p. 57-63.

Bingler, E. C., 1973, Oligocene welded tuff sequence in the northern Wassuk Range, 

central-western Nevada [abs.]: Geological Society of America Abstracts with Programs, v. 5, no. 1, p. 12.

___1978, Abandonment of the Hartford Hill rhyolite tuff and new formation names 

for middle Tertiary ash-flow tuffs in the Carson City-Silver City area, Nevada: 

U.S. Geological Survey Bulletin 1457-D, 19 p.

Byers, F. M., Jr., Carr, W. J., Orkild, P. P., Quinlivan, W. D., and Sargent, K. A., 

1976, Volcanic suites and related cauldrons of Timber Mountain-Oasis Valley 

caldera complex, southern Nevada: U.S. Geological Survey Professional Paper 

919, 70 p.

Christiansen, R. L., Lipman, P. W., Orkild, P. P., and Byers, F. M., Jr., 1965, Structure of the Timber Mountain caldera, southern Nevada, and its relation to basinrange structure, in Geological Survey Research 1965: U.S. Geological Survey Professional Paper 525-B, p. B43-B48.

Christiansen, R. L., Lipman, P. W., Carr, W. J., Byers, F. M., Jr., Orkild, P. P., and 

Sargent, K. A., 1977, Timber Mountain-Oasis Valley caldera complex of southern 

Nevada: Geological Society of America Bulletin, v. 88, no. 7, p. 943-959.
REFERENCES CITED 53

Dalrymple, G. B., and Lanphere, M. A., 1969, Potassium-argon dating Principles, 

techniques, and applications to geochronology: San Francisco, W. H. Freeman 

and Co., 258 p.

Ekren, E. B., Anderson, R. E., Rogers, C. L., and Noble, D. C., 1971, Geology of northern Nellis Air Force Base Bombing and Gunnery Range, Nye County, Nevada: 

U.S. Geological Survey Professional Paper 651, 91 p.

Ekren, E. B., and Byers, F. M., Jr., 1976, Ash-flow fissure vent in west-central Nevada: 

Geology, v. 4, no. 4, p. 247-251.

Evernden, J. F., and Kistler, R. W., 1970, Chronology of emplacement of Mesozoic 

batholithic complexes in California and western Nevada: U.S. Geological Survey 

Professional Paper 623, 42 p.

Ferguson, H. G., and Muller, S. W., 1949, Structural geology of the Hawthorne and 

Tonopah quadrangles, Nevada: U.S. Geological Survey Professional Paper 216, 

55 p.

Gilbert, C. M., and Reynolds, M. W., 1973, Character and chronology of basin 

development, western margin of the Basin and Range province: Geological 

Society of America Bulletin, v. 84, p. 2489-2509.

Gumper, F., and Scholz, C., 1971, Microseimicity and tectonics of the Nevada seismic 

zone: Seismological Society of America Bulletin, v. 61, no. 5, p. 1413-1432.

Hardyman, R. F., Ekren, E. B., and Byers, F. M., Jr., 1975, Cenozoic strike-slip, 

normal, and detachment faults in northern part of Walker Lane, west-central 

Nevada [abs.]: Geological Society of America Abstracts with Programs, v. 7, no. 

7, p. 1100.

Houser, F. N., 1970, A summary of information and ideas regarding sinks and collapse, Nevada Test Site: Available from U.S. Dept. of Commerce, National 

Technical Information Service, Springfield, Va. 22161, as Report USGS-474-41, 

129 p.

Ingamells, C. O., 1970, Lithium metaborate flux in silicate analysis [with French and 

German summaries]: Analytica Chimica Acta, ', 52, p. 323-334.

Lanphere, M. A., and Reed, B. L., 1973, Timing of Mesozoic and Cenozoic plutonic 

events in circum-Pacific North America: Geological Society of America Bulletin, 

v. 84, p. 3773-3782.

Lipman, P. W., 1966, Water pressures during differentiation and crystallization of 

some ash-flow magmas from southern Nevada: American Journal of Science, 

v. 264, no. 10, p. 810-826.

___1975, Evolution of the Platoro caldera complex and related volcanic rocks, 

southeastern San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 852,128 p.

Lipman, P. W., Christiansen, R. L., and O'Connor, J. T., 1966, A compositionally 

zoned ash-flow sheet in southern Nevada: U.S. Geological Survey Professional 

Paper 524-F, 47 p.

Locke, A., Billingsley, P., and Mayo, E. B., 1940, Sierra Nevada tectonic pattern: 

Geological Society of America Bulletin, v. 51, p. 513-539.

Muller, S. W., and Ferguson, H. G., 1939, Mesozoic stratigraphy of the Hawthorne 

and Tonopah quadrangles, Nevada: Geological Society of America Bulletin, v. 50, 

p. 1573-1624.

Neilsen, R. L., 1965, Right-lateral strike-slip faulting in the Walker Lane, west-central 

Nevada: Geological Society of America Bulletin, v. 76, p. 1301-1308.

Noble, D. C., 1970, Significance of phenocryst resorption for the intratelluric 

crystallization and eruptive history of silicic pyroclastic rocks [abs.]: Geological 

Society of America Abstracts with Programs, v. 2, no. 2, p. 125.

Nockolds, S. R., 1954, Average chemical compositions of some igneous rocks: Geological Society of America Bulletin, v. 65, no. 10, p. 1007-1032.
54 REFERENCES CITED

Peterson, D. W., 1968, Zoned ash-flow sheet in the region around Superior, Arizona, 

in Southern Arizona Guidebook 3 Geological Society of America Cordilleran 

Section, 64th Annual Meeting, Tucson, 1968: Arizona Geological Society, 

p. 215-222.

Proffett, J. M., Jr., and Proffett, B. H., 1976, Stratigraphy of the Tertiary ash-flow 

tuffs in the Yerington district, Nevada: Nevada Bureau of Mines and Geology 

Report 27, 28 p.

Rhodes, R. C., 1976, Petrologic framework of the Mogollon Plateau volcanic ring complex, New Mexico Surface expression of a major batholith: New Mexico 

Geological Society Special Publication No. 5, p. 103-112.

Robertson, J. K., and Wyllie, P. J., 1971, Rock-water systems, with special reference 

to the water-deficient region: American Journal of Science, v. 271, p. 252-277.

Ross, D. C., 1961, Geology and mineral deposits of Mineral County, Nevada: Nevada 

Bureau of Mines Bulletin 58, 98 p.

Ryall, A., Slemmons, D. B., and Gedney, L. D., 1966, Seismicity, tectonism, and 

surface faulting in the western United States during historic time: Seismological 

Society of America Bulletin, v. 56, no. 5, p. 1105-1135.

Segerstrom, K., and Young, E. J., 1972, General geology of the Hahns Peak and Farwell Mountain quadrangles, Routt County, Colorado, with a discussion of Upper 

Triassic and pre-Morrison Jurassic rocks, by G. N. Pipiringos: U.S. Geological 

Survey Bulletin 1349, 63 p.

Sheridan, M. F., Stuckless, J. S., and Fodor, R. V., 1970, A Tertiary silicic cauldron 

complex at the northern margin of the Basin and Range province, central 

Arizona, U.S.A.: Bulletin of Volcanology, v. 34, no. 3, p. 649-662. [197.1]

Silberling, N. J., and Roberts, R. J., 1962, Pre-Tertiary stratigraphy and structure of 

northwestern Nevada: Geological Society of America Special Paper 72, 58 p.

Slemmons, D. B., 1967, Pliocene and Quaternary crustal movements of the Basin and 

Range province, U.S.A., in Sea level changes and crustal movements of the 

Pacific Eleventh Pacific Scientific Congress, Tokyo, 1966, Symposium 19: 

Osaka City University Journal of Geosciences, v. 10, p. 91-103.

Smith, R. L., 1960, Zones and zonal variations in welded ash flows: U.S. Geological 

Survey Professional Paper 354-F, p. 149-159. [1961]

Smith, R. L., and Bailey, R. A., 1966, The Bandelier tuff A study of ash-flow eruption 

cycles from zonal magma chanbers: Bulletin of Volcanology, v. 29, p. 83-103.

____1968, Resurgent cauldrons, in R. R. Coats and others, eds., Studies in volcanology [Williams volume]: Geological Society of America Memoir 116, p. 613-662.

Steiner, J. C., Jahns, R. H., and Luth, W. C., 1975, Crystallization of alkali feldspar 

and quartz in the haplogranite system NaAlSi3O8 -KAlSi3O8 -SiO2 -H2O at 4 kb: 

Geological Society of America Bulletin, v. 86, p. 83-97.

Stewart, J. H., and Carlson, J. E., compilers, 1974, Preliminary geologic map of Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-609.

Stuckless, J. S., and O'Neill, J. R., 1973, Petrogenesis of the Superstition-Superior 

volcanic area as inferred from strontium- and oxygen-isotope studies: Geological 

Society of America Bulletin, v. 84, p. 1987-1997.

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