Fish Creek Mountains Tuff 

And Volcanic Center, 

Lander County, Nevada

GEOLOGICAL SURVEY PROFESSIONAL PAPER 681
Fish Creek Mountains Tuff 

And Volcanic Center, 

Lander County, Nevada

By EDWIN H. McKEE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 681

Description of a new formation (a composite 

ash-flow sheet), and a discussion of its 

source in the Fish Creek Mountains of 

central Nevada

UNITED STATES GOVERNMENT PRINTING OFFICE, W ASH INGTON : 1 9 7 0
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog-card No. 70-608-605

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

Washington, D.C. 20402 Price 35 cents (paper cover)
CONTENTS

Page

Abstract __.___________.__ __ _..... 1

Introduction _. . 1

Acknowledgments _._. - _ . ._. 1

Physiography and general geology _. ..._._.. 1

Geologic relations of the Fish Creek Mountains Tuff ... 3

Rocks older than Fish Creek Mountains Tuff ............ 4

Rocks younger than Fish Creek Mountains Tuff ...... 4

Fish Creek Mountains Tuff .................................................. 7

Distribution and dimensions of the ash-flow sheet. 7

Type locality and alternate localities ........................ 8

Nature of the ash-flow sheet............. ............_.......... 8

Lower cooling unit . 9

Upper cooling unit - ..... .............-......................... 10

Lithology -....- ........................................... ....... 10

Fish Creek Mountains Tuff Continued 

Upper cooling unit Continued

Chemistry 

Remanent magnetization .................. .

Age ............................-..... .... ...... . . .

Tertiary structure in the Fish Creek Mountains 

Pre-Basin and Range structure ....... ...........

Basin and Range faults ............-....-... ........

Miocene to Holocene geologic history ...................

Sequence of events 

Interpretation of the Fish Creek Mountains 

volcanic center . _ . .

Summary ________________________. 

Selected references 

Page

10

12

13

13

13

14

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15

15

17

ILLUSTRATIONS

FIGURE 1. Map showing the general location of the Fish Creek Mountains and other topographic and

cultural features in the region, with index map..... ...................... .......... _ 

2. Shaded relief map of the Fish Creek Mountains showing the saucerlike shape of the range. 

3. Aerial view of the Fish Creek Mountains showing the saucerlike shape of the range 

4. Mosaicked aerial photograph of the southern part of the Fish Creek Mountains .

5. Geologic map of the Fish Creek Mountains- ------- - - -

6-9. Photographs of:

6. Fish Creek Mountains Tuff, lithic unit at the mouth of Dacies Canyon . . .....

7. Fish Creek Mountains Tuff, lithic unit viewed west from the mouth of Dacies Canyon.

8. Fish Creek Mountains Tuff at the east margin of the range . -

9. Two hand specimens of Fish Creek Mountains Tuff ... ...

10. Trilinear plot of normative molecular Ab, An, and Or .... ................ .......... 

11. Schematic possible history of the Fish Creek Mountains volcanic center. 

Page

2

3

3

5

6

9

10

11

12

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16

TABLES

TABLE 1. Description and estimate of amount of matrix and of crystal, lithic, and pumice fragments at five 

horizons through the lithic-bearing ash-flow cooling unit of the Fish Creek Mountains Tuff at the 

mouth of Dacies Canyon, west side of Fish Creek Mountains. . 

2. Modes of four samples of Fish Creek Mountains Tuff..... . . -

3. Chemical analyses of two samples of Fish Creek Mountains Tuff and the "average calc-alkali,

rhyolite and rhyolite-obsidian" from Nockolds (1954, table 1).... 

4. Analyses of Rb, Sr, and K from sanidine separates of Fish Creek Mountains Tuff ........ ................. .

5. Mean remanent magnetization directions, Fish Creek Mountains Tuff ................. ... ~ 

6. Radiometric dates of Fish Creek Mountains Tuff .-. 

Page

11

11

12

13

13

III

FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER,

LANDER COUNTY, NEVADA

By EDWIN H. McKEE

ABSTRACT

The Fish Creek Mountains in central Nevada are the site 

of an early Miocene volcano (about 24 million years old on 

the basis of potassium-argon and fission-track ages) from 

which was erupted about 75 cubic miles of crystal-rich rhyolitic ash-flow tuff, here named the Fish Creek Mountains Tuff. 

The formation appears to be a composite sheet, although 

most of it was erupted during a short period of time and 

cooled as a large, nearly homogeneous body. Two cooling 

units that make up the sheet are separated by a cooling break 

and can be distinguished in most places by the presence of a 

large amount of lithic debris in the lower unit. The ashflow 

spread outward uniformly in all directions from a vent area 

in the south-central part of the range. Collapse of this vent 

area probably accompanied eruption, and the depression so 

formed acted as a trap for continued eruption of similar tuff. 

Evidence of resurgence of the vent area is unclear, although 

tectonic activity probably related to the final phase of volcanism is evident. Several long, arcuate faults that circumscribe about half of a "core" area of leached and tectonically 

chaotic tuff may be remnants of a caldera structure.

INTRODUCTION

Numerous ash-flow sheets have been recognized in 

central Nevada, and the distribution of many of 

them has been delineated fairly accurately. The 

source area of most of these units, however, has not 

been found, although a definite effort has been 

made to locate volcanic centers, especially those with 

collapse caldera structures.

The south-central part of the Fish Creek Mountains in Lander County, Nev., is the source of a thick, 

but areally restricted composite sheet of ash-flow 

tuff that is not found in any adjacent region of central Nevada, except a short distance to the west in 

the Tobine Range. The restricted occurrence of this 

tuff in the Fish Creek Mountains is evidence that 

its source was within the boundary of the present range; its thickness and distribution pattern 

within these mountains support this conclusion. 

Inconclusive evidence suggests that there was some

subsidence of the vent area during eruption and perhaps some resurgence at a later time.

Although about 75 cubic miles of ash-flow material 

was erupted, it did not spread more than 10 miles 

from its source; this suggests that it was low-energy 

eruption. This ash-flow sheet formed a lens-shaped 

body about 3,000 feet thick at its center and 15-20 

miles in diameter.

ACKNOWLEDGMENTS

John H. Stewart, of the U.S. Geological Survey, 

visited the Fish Creek Mountains with the author 

a number of times and offered many valuable suggestions on interpretation of the geology. Charles W. 

Naeser, of the U.S. Geological Survey, collected samples of welded tuff and determined their age by the 

fission-track method. A summary of his data is 

included. C. Sherman Gromme, also of the U. S. Geological Survey, visited the area and collected drill 

samples for studies of remanent magnetization. The 

result of this work is also included. Donald C. Noble, 

of Harvard University, offered stimulating discussion, and he and Ted G. Theodore of the U.S. Geological Survey critically reviewed the manuscript. 

Fieldwork was done in cooperation with the Nevada 

Bureau of Mines as part of the Lander County map 

project.

PHYSIOGRAPHY AND GENERAL GEOLOGY

The Fish Creek Mountains, approximately 40 

miles southwest of the town of Battle Mountain, 

Nev. (fig. 1), form a circular-shaped range that covers an area of about 200 square miles and has an 

average relief of about 1,500 feet. The range is surrounded by broad alluvial valleys except on the 

southwest near the north end of the Augusta Mountains; here some low hills and a canyon separate the 

two ranges. The circular shape of the Fish Creek 

Mountains, in particular its broad arcuate southern
FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

117°30' 117'00' 116°30'

40°30'

40°00'

41 °f~ ! Winnemucca. Iliattle

I 1_ _ _______jMountain i I» ">.'' ii i

FIGURE 1. Map showing the general location of the Fish Creek Mountains and other topographic and cultural features 

the region, with index map. The dark shaded area on inset is the area of the geologic map (fig. 5).
GEOLOGIC RELATIONS

edge, contrasts sharply with the narrow and linear 

shape of the northeast-trending ranges typical of 

surrounding regions in central Nevada.

In profile the range is crudely saucer shaped (see 

figs. 2 and 3), having a rim of fairly uniform height 

about 1,500 feet above the surrounding valley floor

FIGURE 2. Shaded relief map of the Fish Creek Mountains 

showing the saucerlike shape of the range.

and a broad shallow central basin several hundred 

feet lower than the rim. The southern and western 

part of the range form the highest and widest part of 

the saucer rim, and in this region the Fish Creek 

Mountains are eroded into very rugged badland topography that is associated with a drainage pattern 

that is dendritic (fig. 4).

Most of the Fish Creek Mountains are made of a 

distinctive ash-flow sheet early Miocene by radiometric dating (see section on "Age") that occurs 

only in or near the range (fig. 5). This ash-flow 

sheet lies unconf ormably on older rocks in the northern and western part of these mountains; its basal 

contact is not exposed in the central, eastern, or

FIGURE 3. Aerial view, looking west, of the Fish Creek 

Mountains showing the saucerlike shape of the range. The 

central basin is filled with tuffaceous sedimentary rocks 

(white); the Fish Creek Mountains Tuff forms the rim.

southern part where the sheet is thickest. Locally 

this unit is overlain by sedimentary rocks and at a 

few places by a second, thin, crystal-poor ash-flow 

sheet.

The Fish Creek Mountains Tuff (named in this 

paper) shows little lithologic variation, but the presence or lack of stratification and the local occurrence of lithic debris within the tuff are characteristics that can be mapped (fig. 5).

Faulting or other tectonic structure is difficult to 

see within the range because of the homogeneous nature of the welded tuff. However, at least two subparallel arcuate linear features more than 10 miles 

long that are undoubtedly faults are clearly visible 

on aerial photographs of the southern part of the 

range (fig. 4). These faults, which are at right angles to the regional northeast trend of Basin and 

Range faulting in this part of central Nevada, are 

most probably related to the volcanic evolution of 

the Fish Creek Mountains rather than to later Basin 

and Range faulting.

GEOLOGIC RELATIONS OF THE 

FISH CREEK MOUNTAINS TUFF

The Fish Creek Mountains Tuff lies unconformably on a variety of older rocks ranging in age from 

Pennsylvanian to Oligocene. In most places, the 

unconformity is angular, but where the tuff lies on a 

nearly flat-lying Oligocene welded tuff, there is no 

structural evidence of an unconformity. The overlapping of the Fish Creek Mountains Tuff clearly indicates that the unconformity exists, however, and a 

difference in age of nearly 10 million years between
FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

the two ash-flow units bears out this conclusion. The 

fact that the Fish Creek Mountains Tuff spread over 

a surface of some relief is shown by local thick accumulations of weakly welded to nonwelded tuff at the 

base of the formation; the thickness indicates this 

material must have filled shallow depressions in the 

old topographic surface. The lithic-bearing portion of 

the tuff, the lowest unit in the vicinity of the mouth 

of Dacies Canyon (fig. 5), has an especially thick 

basal zone of nonwelded tuff and probably filled a 

small basin at this site.

ROCKS OLDER THAN FISH CREEK MOUNTAIN TUFF

The oldest rocks in the Fish Creek Mountains are 

strata of Pennsylvanian and Permian age that make 

up a series of hills along the western edges of the 

range; they are separated from the Fish Creek 

Mountains Tuff by an angular unconformity. It 

seems likely that Paleozoic rocks underlie that tuff 

in most of the southern part of the range also, 

although the basal contact is not exposed in this part 

of the mountains. South of the Fish Creek Mountains, at the north end of the Augusta Mountains, 

the Fish Creek Mountains Tuff locally lies on the 

Triassic Augusta Mountain Formation.

Coarse-grained granite rock of presumed Middle 

Jurassic age underlies the Fish Creek Mountains Tuff 

at one place in the north-central part of the range. On 

the northeastern edge of the Fish Creek Mountains, 

a series of dark lava flows and lenticular tuffaceous 

sedimentary rocks of Oligocene(?) age crop out 

beneath the Fish Creek Mountans Tuff; the tuff 

lies unconformably on both of these older Tertiary 

rock types and in places overlaps the contact between 

these units. Stratigraphic relations between similar 

units in the Shoshone Range about 5 miles east of 

the Fish Creek Mountains and K-Ar dates on rocks 

associated with them suggest that the older lava 

flows and sedimentary rocks probably are about 10 

million years older than the Fish Creek Mountains 

Tuff.

The Caetano Tuff (Gilluly and Masursky, 1965), a 

widespread ash-flow unit in areas east of the Fish 

Creek Mountains, locally underlies the Fish Creek 

Mountains Tuff in the northern part of the range. 

Both units here are flat or nearly flatlying so that 

the contact appears conformable. Mapping on a regional scale, however, shows that the units have a 

different geographic distribution, the Fish Creek 

Mountains Tuff lapping onto the Caetano Tuff only 

in a small area in the northern part of the Fish 

Creek Mountains. Potassium-argon ages of the two 

units (32.3 m.y. for Caetano Tuff in the Fish Creek

Mountains, McKee and Silberman, 1970; 24.3 m.y. by 

K-Ar and fission-track dating for Fish Creek Mountains Tuff) support a hiatus of nearly 10 m.y.

ROCKS YOUNGER THAN FISH CREEK MOUNTAINS TUFF

In the area where the Fish Creek Mountains Tuff 

is thickest, and throughout most of the region where 

it is found, it is the youngest unit. Only in the north, 

north-central, and western parts of the range are 

there younger rocks. The oldest of these younger 

units is an ash-flow cooling unit questionably identified as Bates Mountain Tuff, a formation widespread southeast of the Fish Creek Mountains 

(Sargent and McKee, 1969; McKee and Stewart, 

1970). The Bates Mountain (?) Tuff is the same 

age (24.0 m.y. in the Fish Creek Mountains by fission-track dating (McKee and Naeser, 1970)) as the 

underlying Fish Creek Mountains Tuff (24.3 m.y. by 

K-Ar and fission-track dating; see section on "Age) 

within the resolution of the radiometric dates. This 

unit forms a thin (about 50 ft thick) veneer on the 

Fish Creek Mountains Tuff at several localities in 

the northern part of the range and probably was 

once more extensive. There is no evidence, however, 

that it ever spread across what is now the central 

and southern Fish Creek Mountains. The overlapping of Bates Mountain (?) Tuff on Fish Creek 

Mountains Tuff in the northern part of the range and 

its absence in the central and southern parts, where 

the Fish Creek Mountains Tuff is thickest, suggests 

that this thin tuff spread unconformably around and 

on the edge of a wedge-shaped body of Fish Creek 

Mountains Tuff.

Sedimentary rocks, mostly tuffaceous sandstone 

and mudstone, crop out in a shallow basin in the central part of the Fish Creek Mountains (figs. 3 and 

5). These strata dip gently away from the margins 

of the basin, reaching a maximum thickness of several hundred feet near the center of the basin. No 

fossils have been found in these rocks, but their general similarity to vertebrae-bearing beds in nearby 

regions (including Jersey Valley to the west, the 

Shoshone Range to the east, and Reese River Valley 

localities to the southeast) suggests that they are 

late Miocene or early Pliocene (Gilluly and Gates, 

1965, p. 88). On the basis of petrographic evidence, 

Deffeyes (1959, p. 35) speculated that they are older 

than late Miocene and early Pliocene but was unable 

to establish a definite age for them. They lie unconformably on the Fish Creek Mountains Tuff (earliest Miocene). It seems likely that a hiatus of several 

million years, perhaps as much as 10 m.y., separates 

the tuffaceous sedimentary rocks from the Fish 

Creek Mountains Tuff.
GEOLOGIC RELATIONS

5

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 a
FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

117°30'

40" 15' 

Geology from McKee (1969) and 

McKee and Stewart (1969 a, b.

FIGURE 5. Geologic map of the Fish Creek Mountains.
FISH CREEK MOUNTAINS TUFF

EXPLANATION

Flows, cinder cones, and small

Stratified tuffaceous siltstone

Dark-colored igneous rocks

Flows and shallow intrusive 

bodies. Andesite to quartz 

latite. Age uncertain

Fish Creek Mountains Tuff

Crystal-rich rhyolitic welded 

tuff. Wavy lines; massive, 

much altered and tectonically 

chaotic. Circles; lithic-rich 

cooling unit. Uniform tone; 

massive to layered; well-developed columnar joints

BB

Caetano Tuff

Crystal-rich guartz-latite welded tuff

Undifferentiated igneous rocks and

tuffaceous sedimentary rocks

Sedimentary, metamorphic, and 

igneous rocks

Fault

Dashed where inferred; dotted where concealed; 

bar and ball on downthrown side

Fault scarp or fault-line scarp

Sample locality 

1-5, radiometric age 

A and B, remanent magnetization 

a and b, chemical analysis

These tuffaceous sedimentary rocks are overlain 

in the center of the basin by an olivine basalt flow 

that covers about 1.5 square miles and is about 50 

feet thick. Other small olivine basalt flows, cinder 

cones, and dikes crop out in a northeast-trending 

belt along the northern edge of the range. These 

basalts intrude or lie on all older rock types and are 

the youngest rocks in the region. The cinder cones 

are only slightly eroded, retaining their original profile, and samples of the basalt are fresh and show 

little signs of weathering. The age of the olivine basalts is unknown but probably is late Tertiary or 

Quaternary.

A belt of dark intrusive and extrusive rock of 

andesitic to quartz latitic composition of unknown age 

crops out along most of the western side of the Fish 

Creek Mountains. These rocks lie along the inferred 

northern projection of several long arcuate faults 

which ring the southern part of the range. The time 

of emplacement of the dark-colored igneous rocks 

may be related to the time of eruption of the Fish 

Creek Mountains Tuff or to subsequent tectonic activity, or these rocks may be younger and related to 

later Basin and Range faulting. The fact that they 

lie about 2 miles from the range front and may be 

associated with the pre-Basin and Range arcuate 

faults suggests that the andesites and quartz latites 

are related to the development of the Fish Creek 

Mountains volcanic center and are possibly early 

Miocene in age.

FISH CREEK MOUNTAINS TUFF

DISTRIBUTION AND DIMENSIONS OF 

THE ASH-FLOW SHEET

Fish Creek Mountains Tuff is here named for a 

composite ash-flow sheet that makes up the entire 

southern part of the Fish Creek Mountains. The 

tuff crops out within the range in a circular area 

about 16 miles in diameter. This tuff is also found in 

three other localities, all less than 5 miles from the 

southern edge of the Fish Creek Mountains and it 

occurs in the Tobin Range at localities as far as 

10 miles from the Fish Creek Mountains. Fish Creek 

Mountains Tuff is exposed over an area of about 200 

square miles. It seems unlikely that the tuff ever 

spread much beyond the area in which it is now 

found, as no outcrops of the tuff are known outside 

this area, although rocks of the same age are widespread in nearby regions.

The thickness of the formation ranges from a 

maximum of about 3,000 feet in the south-central 

part of the Fish Creek Mountains to about 100 feet 

at the margins of the range. This change in thickness occurs across a distance of about 8 miles.
8 FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

The present shape of the unit is approximately 

that of a lens. The original volume of tuff, assuming 

a radius of about 16 miles and an average thickness 

of about 2,000 feet, was about 75 cubic miles. Compared with other welded ash-flow sheets in Nevada 

for which enough information is known to determine 

approximate volumes, the Fish Creek Mountains 

Tuff is a medium-sized unit. Ash-flow sheets with a 

volume of more than 100 cubic miles are not uncommon, and a few units had original volumes of several hundred cubic miles (for example, the Windous 

Butte Tuff of Cook, about 500 cu mi (Cook, 1965); 

the Rainer Mesa Member of the Tinier Mountain 

Tuff, more than 285 cu mi (Byers and others, 

1968)).

TYPE LOCALITY AND ALTERNATE LOCALITIES

Excellent exposures of the formation are found almost anywhere within the Fish Creek Mountains. 

The type locality for Fish Creek Mountains Tuff is 

designated the exposures at the U.S. Government 

radio facility (see U.S. Geol. Survey, Mount Moses, 

Nev., 15-minute quadrangle), SW^4, sec. 19, T. 27 N., 

R. 41 E. Outcrops of the tuff, which form the 1,000- 

foot cliff directly west of the radio facility, are typical of the unit. An alternate or reference section,

which is thinner and shows a more pronounced layering but also representative of the formation, is on 

the east side of the range along Fish Creek Canyon 

at long 117° 15', lat 40° 12'.

NATURE OF THE ASH-FLOW SHEET

The Fish Creek Mountains Tuff is a composite 

ash-flow sheet consisting of at least two ash-flow 

sequences separated locally by a complete cooling 

break that literally becomes a partial cooling break 

and finally dies out altogether (terminology of ashflow units follows Smith (1960) as supplemented by 

Noble, Bath, Christiansen, and Orkild (1968)). 

The cooling break can be traced for about a mile before it disappears and the upper and lower units 

merge. This cooling break is between a unit characterized by the occurence of locally abundant lithic 

fragments and an overlying lithic-f ree ash flow.

At the mouth of Dacies Canyon (fig. 5), where 

the relationship between cooling units is best seen, a 

lower cliff composed of the welded and devitrified 

part of the lithic-bearing unit is separated from an 

overlying cliff of nonlithic tuff by about 10 feet of 

nonwelded tuff. This nonwelded tuff is gradational 

upward into the upper unit and is the basal part of 

the ash flow. That no vapor-phase alteration was

TABLE 1. Description and estimate of amount of matrix and of crystal, lithic, and pumice fragments at five horizons through 

the lithic-bearing ash-flow cooling unit of the Fish Creek Mountains Tuff at the mouth of Dacies Canyon, west side 

Fish Creek Mountains

[The vertical section is completely gradational and these horizons are representative of parts of the series]

Thickness 

(ft)

Top

10

26

25

25-75

0-300

(average

100)

Estimate of percentage per unit area (no correction for compaction and porosity)

Description

Densely welded

crystal-rich

tuff; collapsed

pumice faint

but visible.

Densely welded

eutaxitic

crystal-rich

tuff. (See

fig. 6 A.)

Moderate to

densely welded

tuff with eutaxitic structure.

Characterized

by "blebs" of

glass and

lenticular

cavities.

Slightly welded to

nonwelded.

Pumice oriented

and partially

collapsed.

(See fig. 6B.)

Nonwelded, soft,

weakly consolidated rock.

(See fig. 6C.)

Matrix

Percentage Characteristics Percenta

55-60 Flattened shards <5

visible on wet

surface only.

Devitrified.

55 Flattened shards 5

are visible.

Devitrified.

55 Shards are oriented 10

and slightly

flattened. Some

vapor-phase

alteration.

50 Shards and small 15

pumice fragments

oriented

horizontally,

not flattened.

40 White ash, shards, 25

and small pumice

fragments. No

preferred

orientation.

Lithic fragments Pumice fragments

ige Characteristics Percentage Characteristics Percentage

% in. or less __ . __ .

% in. or less,

average % % in.

1% in. or less.

average % % in.

3 in. or less,

avg. % % in.

Volcanic rock

types. Scoria and

dark lava types.

4 in. or less,

avg. 1 % in.

.10 Completely flattened 30

and devitrified.

15 Blebs and fiamme 25

of black glass.

20 Blebs of black 15

glass.

30 2 in. or less. Some 5

partially flattened

but retain "woody"

structure.

35 Blocks of "woody" Trace

tubular pumice,

unoriented and

unfiattened.

Crystals

Type

Sanidine 15

percent. Smoky

quartz 15

percent.

Sanidine 12

percent. Smoky

quartz 12

percent.

Sanidine and smoky

quartz.

Smoky quartz and

sanidine, mostly

masked by matrix.

Smoky quartz and

sanidine, mostly

masked by matrix.

Base
FISH CREEK MOUNTAINS TUFF

* * IL *. I «a^- *-1L*v^'*w v -'^Ita

FIGURE 6. Fish Creek Mountains Tuff, lithic unit at the 

mouth of Dacies Canyon, in stratigraphic sequence (downward A-C) through about the lower two-thirds of the unit. 

The pen is 6 inches long. A, Moderately to densely welded. 

Eutaxitic texture with relatively few small lithic fragments. 

B, Slightly welded to nonwelded. Pumice orientated and 

slightly flattened. Moderate number of lithic fragments, 

most less than 2 inches in diameter. C, Completely nonwelded. Large blocks of pumice and numerous lithic fragments up to 4 inches in diameter.

observed in these soft rocks suggests that in this 

place the lower unit was completely cool before the 

overlying ash flow was deposited. This cooling break 

can be mapped for about a mile in the vicinity of 

Dacies Canyon, but it becomes less discernible and 

ultimately cannot be found in the southern part of 

the range, where the main body of the ash flow was 

deposited.

LOWER COOLING UNIT

The lower cooling unit is a lenticular body of nonwelded to densely welded tuff characterized by numerous large blocks of pumice and relatively large 

xenoliths, as much as 4 inches in diameter, of volcanic rock of intermediate composition. The volcanic 

fragments are concentrated near the base of this 

unit and are largest and most numerous in the lower 

nonwelded part of the tuff. The unit becomes progressively more densely welded upward, and near the top, 

where it is most densely welded, it contains only a 

scattering of small lithic fragments. The difference 

in lithology between the top of the unit and its base 

is so great that were it not for the gradational middle part, one would not suspect that the two rock 

types are from the same ash-flow cooling unit. Detailed studies were made at five horizons in vertical 

alinement on a section of the lithic-bearing unit at 

the mouth of Dacies Canyon. At each point a 2-foot 

square was measured, and the percentage of lithic, 

pumice, and crystal fragments estimated. The degree 

of welding was also observed, and in the more densely 

welded parts of the tuff, the crystal content was 

ascertained. No correction is made in the relative 

percentage of lithic fragments and matrix with regard to the amount of compaction. Had this correction been made on the assumption of several-fold 

compaction indicated by the flattening of pumice, the 

relative amount of lithic debris in the nonwelded to 

weakly welded lower part of the tuff would be increased proportionally. The relative percentages at 

the five horizons through the cooling unit are given 

in table 1, and three horizons typical of intermediate 

parts of the tuff are shown in figure 6. The lower 

two-thirds of the unit is nonwelded to moderately
10 FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

welded tuff (fig. 7). The concentration of the lithic 

fragments and their lateral and vertical gradation in 

this unit indicate that they probably represent a local concentration related either to preeruption topography or to contamination of one of the initial 

flows on eruption.

FIGURE 7. Fish Creek Mountains Tuff, lithic unit viewed 

west from the mouth of Dacies Canyon. About two-thirds of 

the cooling unit shown. The upper, densely welded part has 

been eroded from the top. Approximately 500 feet of nonwelded to weakly welded tuff exposed.

UPPER COOLING UNIT

The general appearance of the upper cooling unit 

of the Fish Creek Mountains Tuff in the south-central part of the range, where it is thickest, differs in 

gross aspect from that at the margins of the range, 

where it is thinner. This difference is due to the development of persistent subhorizontal to horizontal 

layering, as well as to well-formed columnar joints 

normal to this stratification (fig. SA, B) in the unit 

at the margin. The thick, massive central part of the 

tuff, structureless and otherwise devoid of any feature that would influence the erosion pattern, has 

eroded into deeply incised badland topography. Within the central region there are also large areas of less 

resistant, highly leached and altered Fish Creek 

Mountains Tuff. However, rocks representative of 

the thick central tuff and the thinner margin tuff 

usually cannot be distinguised from each another in 

hand specimen or thin section.

LITHOLOGY

The Fish Creek Mountains Tuff is a light-gray to 

pink crystal-rich (greater than 30 percent crystals) 

rock that is usually structureless (fig. 9A) and 

rarely shows eutaxitic structure except in thin section or on wetted polished surfaces. At a few localities, however, eutaxitic texture is obvious in the outcrop (fig. SB). Most of the tuff is hard, dense rock 

containing about equal amounts of smoky quartz and 

sanidine as crystals and crystal fragments as much 

as 1 mm in diameter. A very few completely altered 

mafic phenocrysts are present in the rock, and in 

some places a few lithic fragments can be seen on an 

outcrop of several square feet. Zones of vitrophyre 

and vapor-phase alteration are rare or nonexistent 

and weakly welded parts of the tuff are also uncommon and not obvious in the field. Point counts on 

thin sections, however, reveal variations in the degree of welding. The most densely welded rock is 

about half matrix (devitrified glass) and half crystal 

phenocrysts; the least welded of the samples studied 

contains about 25 percent crystals and 75 percent 

matrix and voids per unit area. A typical piece of 

Fish Creek Mountains Tuff is made up of about 65 

percent matrix and 35 percent phenocrysts. Modes 

based on 1,000 points counted on thin sections of 

four samples are shown in table 2. Two samples are 

"typical" or average Fish Creek Mountains Tuff; the 

other two represent more or less welded varieties. 

Heavy minerals separated from three samples are 

zircon and apatite. They make up appreciably less 

than 1 percent of the rock.

The quartz phenocrysts in most rocks are smoky 

to black, but in some places are colorless. Localities 

with smoky or clear quartz show no obvious correlation with position in the volcanic pile or with degree 

of welding. The development of smoky quartz may be 

related to the heating or radiation history of the 

rock, but this is uncertain.

CHEMISTRY

The Fish Creek Mountains Tuff is of rhyolitic 

composition. Two whole-rock analyses of densely 

welded devitrified tuff are given in table 3, and normative Ab, An, and Or in molecular proportions are 

plotted on a ternary diagram (fig. 10). The analyses 

are similar to the "average calc-alkali rhyolite and 

rhyolite-obsidian" of Nockolds (1954), shown in table 3 and figure 10 for comparison, although they are 

richer in silica and poorer in total iron oxides, magnesia, lime and titania. The triangular plot of normative Ab, An, and Or shows that the samples of Fish 

Creek Mountains Tuff are within the rhyolite portion of the diagram as defined by O'Conner (1965, 

fig. 3) for volcanic rocks from southern Nevada. 

Within the rhyolite field, a subfield delineated by 

normative feldspar values from eight analyses of 

two rhyolite welded tuffs given by O'Conner is 

shown. The Fish Creek Mountains Tuff lies within 

this subfield.
FISH CREEK MOUNTAINS TUFF 11

FIGURE 8. Fish Creek Mountains 

Tuff at the east margin of the 

range. The columnar joints cut 

the layering and are pervasive 

through the entire sequence. B 

is a closer view of tuff shown in 

A.

TABLE 2. Modes of four samples of Fish Creek Mountains

Tuff

[Based on 1,000 points counted per thin section]

Composition, in percent

Densely 

welded "typical" 

sample

Densely 

welded "typical" 

sample

Densely 

welded

Moderately 

welded

Matrix and voids:

Devitrified pumice 

and glass shards.... 64

Crystal phenocrysts:

Sanidine ________ 20

Quartz _________ 15

Plagioclase ______ 1

Total _________ 36

68

17

14

1

32

50

29

19

2

50

74

15

10

26

TABLE 3. Chemical analyses of two samples of Fish Creek 

Mountains Tuff and the "average calc-alkali rhyolite and 

rhyolite-obsidian" from Nockolds (1954, table 1).

[Samples analyzed by methods described in U.S. Geol. 

supplemented by atomic-absorption method ; analyzed 

of Leonard Shapiro]

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

A12O3-.----

FeaOq

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

MgO--.___.........

CaO .... ..-.-

Na2O.....-.-.-.

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

H20+.-..-....-..

H2O ...............

TiO2------

P205 ------

MnO.-....-..-..

C02 ...................

Fish Creek

Mountains 

Tuff : fig. 5, a

75.3

12.3

1.1

.20 

.12 

.61 

4.2 

4.9 

.68 

.30 

.14

<.05

Fish Creek

Mountains 

Tuff, fig. 6, b

74.0 

13.6 

.84 

.24 

.15 

.84 

3.3 

5.4 

.38 

.82 .

.17 

.04 

.02 

<.05 .

Survey Bull. 1144-A, 

under the direction

"Average 

calc-alkali rhyolite 

and 

rhyolite-obsidian' ' 

of Nockolds (1954)

73.66 

13.45 

1.25 

.75 

.32 

1.13 

2.99 

5.35 

.78

.22

.07 

.03
12 FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

:";?p?S¥^S&

. v,*^^'.*--\;5.^r- *' , ;&& '

4^Sl^fev^

^VS8fc>'- s :.i.r'* :.v''i""* * ' ' ^ms?t-^^-" ". '- ' *-*^^s^- v ,*:- £ ^«y t-jMfl*>>Tr^:^%58

'fui.i 25 mm

j^S;-^ 

;'^S&s i^^..,^a^^i.?:'*

«S^i*^«fiS!3SfiS

FIGURE 9. Two hand specimens of Fish Creek Mountains Tuff. Dark phenocrysts are smoky quartz. Both specimens contain 

about 35 percent crystals of quartz, sanidine, and a little plagioclase. A, Eutaxitic structure not developed. B, Strong 

eutaxitic structure parallel to the base of the specimen.

Sr, Rb, and K values for phenocrystic sanidine 

from three samples of Fish Creek Mountains Tuff 

are given in table 4. From these values, approximate 

Rb and Sr contents for the whole rock can be estimated if the weight percent of sanidine phenocryts 

and groundmass material is known (see table 2) and 

the amount of Sr and Rb in the groundmass of the 

rock can be inferred. The estimated whole-rock values are 65 ppm Sr and 265 ppm Rb, values based on 

an average of the three Rb, Sr, and K determinations 

shown in table 4, an average modal content of 26 percent sanidine and 53 percent groundmass with a K^O 

content of 4.7 percent (table 3) and 20 ppm Sr, and 

the assumption that the K:Rb ratio of glass divided 

by that of coexisting sanidine is about 0.24 (D.C. 

Noble and C. E. Hedge, unpub. data).

The calculated Sr value (65 ppm) is relatively low 

compared with that for many quartz latitic volcanic 

rocks in the Great Basin and is comparable with subalkaline rhyolites from southern Nevada (Noble and 

Hedge, 1969). The Rb value (265 ppm) is relatively 

high for silicic rocks (Heier and Adams, 1965, Ewart 

and others, 1968).

Although both the Rb and Sr values suggest 

appreciable differentiation by fractional crystallization, the rocks do not appear to be as fractionated as 

many of the Sr-poor peralkaline and subalkaline 

rocks found at the margins of the Great Basin 

(Noble, Haffty, and Hedge, 1969).

TABLE 4. Analyses of Rb, Sr, and K from 

sanidine separates of Fish Creek Mountains Tuff

[Rb and Sr determined by Donald C. Noble using 

X-ray fluorescence methods. K^O analyzed by Lois B. 

Schlocker using flame-photometer methods]

Sample 

(fig. 5)

1

3

4

Rb

(ppm)

154

170

215

Sr 

(ppm)

200

219

182

K

(percent)

7.85

7.63

7.43

REMANENT MAGNETIZATION

Thirteen drill samples from the upper cooling unit 

of the Fish Creek Mountains Tuff from two localities in the range were collected by Sherman Gromme, 

and remanent magnetization of the samples was 

determined in the U.S. Geological Survey's remanent
TERTIARY STRUCTURE IN FISH CREEK MOUNTAINS 13

20 80

60 40

20

Ab 20 40 60 80 Or

FIGURE 10. Normative molecular Ab, An, and Or. Normative classification after O'Conner 1965 (fig. 3). Nos. 1 and 

2, Fish Creek Mountains Tuff; No. 3, the "average calcalkali rhyolite and obsidian" of Nockolds (1954, table 1). 

Shaded area is the approximate field defined by eight 

samples of two rhyolite ash-flow tuffs from southern 

Nevada shown in O'Conner (1965, fig. 3).

magnetization laboratory. The results of the determinations are shown in table 5.

The fields of the two directions (D and I) overlap 

and the remanent magnetization is the same at both 

localities. These data strongly suggest that the two

TABLE 5. Mean remanent magnetization directions, 

Fish Creek Mountains Tuff

Sample

(fig. 5)

A

B

N

10

3

D

344.3°

344.0°

I

36.7"

39.1°

«95

2.5°

3.3°

H

200.00

100.00

N = number of specimens. 

D = declination, east of north. 

/ = inclination, downward. 

a95 = radius of 95-percent confidence cone around

mean directions.

H = peak alternating field, in oersteds, used to 

remove unstable magnetization.

parts of Fish Creek Mountains Tuff were contemporaneous in cooling, and it seems most likely that they 

were also erupted at nearly the same time.

AGE

The age of the Fish Creek Mountains Tuff is considered to be early Miocene on the basis of radiometric dates. Three K-Ar age determinations were 

made on sanidine in samples from different places 

in the mountains, and two samples were dated on 

zircon using the fission-track method. Samples 1-4 

are from the upper cooling unit; sample 5 is from the 

lower unit. The average age of the five samples date, 

as shown in table 6, is 24.3 m.y., which is early Miocene (Harland and others, 1964).

TABLE 6. Radiometric dates of Fish Creek Mountains Tuff

[Average of dates is 24.3 m.y.]

Sample

(fig. 5)

Method of 

dating

Age Mineral 

(m.y.) used Reference

1 K-Ar .. ._.__......_ 23.6 Sanidine..

2 Fission-track ___ 23.9 Zircon _

3 K-Ar _ __ __..._._ 24.4 Sanidine..

4 ... do _..._.._......_ 24.5 do. ...

5 Fission-track ___ 25.0 Zircon......

Average' 24.3 

...... McKee and Silberman, 1970.

_.. McKee and Naeser, 1970. 

...... McKee and Silberman, 1970.

...._ Do.

_.... McKee and Naeser, 1970.

The standard deviation of this group of dates is 

0.6. The range of the mean age at the 95-percent confidence level is 0.7 m.y., as determined by a standard 

table of cumulative t distribution. From these age 

determinations, the samples of Fish Creek Mountains Tuff can be considered to be 24.3 ± 0.7 m.y.

TERTIARY STRUCTURE IN THE 

FISH CREEK MOUNTAINS

The generally massive nature of the Fish Creek 

Mountains Tuff makes it difficult to recognize structure within the range. At various places along the 

edges of these mountains, where the tuff is thin and 

where its contact with older rocks serves as a datum, 

faults can be observed, but in most places within the 

tuff, few features exist that can be used to demonstrate faulting or folding. Probable faults have been 

mapped locally in the eastern part of the range, 

where the horizontal layering or jointing offers some 

control; however, in most of the range, where the 

layering is not developed, faults have not been 

mapped. Aerial photographs, however, .offering an 

overall view of the body of tuff, show several persistent linear features, not obvious on the ground, that 

probably are faults (fig. 4).

PRE-BASIN AND RANGE STRUCTURE

Two or more linear arcuate faults parallel the 

curved southern edge of the Fish Creek Mountains 

for at least 10 miles. Inspection of these faults in the 

field shows that they are zones about 100 feet wide 

of crushed and leached Fish Creek Mountains Tuff 

bounded on either side by relatively fresh tuff. Because they are bounded by identical material, it is 

impossible to determine the amount of displacement 

or even the relative movement along these faults.
14 FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

The trace of these faults across topography indicates 

that they are steeply dipping and their arcuate shape 

suggests that movement, if any, was approximately 

vertical, with the central part of the range moving 

relatively up or down, or perhaps both ways at different times.

The two most prominent of these faults lie half a 

mile or more from the topographic edge of the range 

and are cut at places by drainage flowing outward 

from the center of the range. They are not related to 

the present physiography of the Fish Creek Mountains. Younger, generally north-trending Basin and 

Range faults are responsible for the present topography of the Fish Creek Mountains and surrounding 

ranges of central Nevada. The arcuate faults in the 

southern part of the Fish Creek Mountains trend at 

right angles to this regional northerly grain.

The projection of one of the arcuate faults as it 

curves around the southwestern part of the Fish 

Creek Mountains is denned by a belt of andesitic to 

quartz latitic igneous rocks about 1.5 miles from the 

western edge of the range (fig. 5). It is possible that 

these rocks were emplaced along the zone of weakness 

caused by the arcuate fault and that the fault is now 

obscured by the intrusions and small extrusions.

The central and south-central part of the Fish 

Creek Mountains (the "core" of the range) is a deeply 

dissected region underlain by tuff characterized by irregular zones of leaching, minor alteration, brecciation, and variable welding. No definite faults can be 

mapped, but the overall chaotic aspect of the rocks 

suggests that this area has been subjected to intense 

tectonism.

BASIN AND RANGE FAULTS

The Fish Creek Mountains are uplifted along their 

western edge by well-defined Basin and Range faults. 

Scarps can be seen in the field and a well-marked 

north-northeast-trending line at the base of the range, 

clearly visible on air photographs, probably is the 

trace of the most recent fault. Uplift on this western 

Basin and Range fault is about 1,000 feet or more and 

has elevated the Fish Creek Mountains Tuff as well as 

about 1,000 feet of the underlying Paleozoic basement. 

The tuff-basement contact exposed here because of 

Basin and Range faulting is not seen anywhere else in 

the southern half of the Fish Creek Mountains, although it is seen in the northern part of the range and 

at the north end of the Augusta Mountains. (See section on "Rocks older than Fish Creek Mountains 

Tuff.")

Basin and Range faulting at other places in the 

range is less obvious, or the faults have much less displacement. Short faults with vertical displacement of 

several hundred feet or less displace other units in the 

northern part of the range, north of the northernmost 

outcrops of Fish Creek Mountains Tuff. The line of 

Pleistocene or Holocene basalt cones and flows along 

the northwest edge of the range probably represents 

eruption along the western Basin and Range fault 

zone; this line of eruptions is about 10 miles long and 

generally lines up (although it trends more northeasterly) with the fault scrap along the southwestern 

part of the range (see preceding section). The eastern 

edge of the range is not defined by Basin and Range 

faults, although several linear north-northeast-trending canyons in this part of the range may follow the 

trace of, or have been caused by, block faulting. 

About 5-7 miles east of the Fish Creek Mountains, 

the western scarp of the Shoshone Range is a very 

prominent north-northeast-trending fault; and in the 

low hills between the Fish Creek Mountains and the 

Shoshone Range in the Reese River Valley, there are 

well-defined parallel faults of smaller displacement.

MIOCENE TO HOLOCENE GEOLOGIC HISTORY 

SEQUENCE OF EVENTS

Eruption of Fish Creek Mountains Tuff began in 

early Miocene time (24.3±0.7 m.y. by radiometric 

dating). (See section on "Age.") The first of the ashflows probably spread over an area of slight relief, but 

local thick and lenticular accumulations of nonwelded 

to weakly welded lithic-rich tuff of the basal 

units suggest that some of the tuff ponded in shallow 

depressions. Eruption continued for an undetermined period of time, but radiometric ages on both 

high and low units in the series (maximum radiometric age 25.0, minimum 23.6 m.y.) suggest that the 

eruptive phase probably lasted for about 1 m.y. Remanent magnetization of two samples from different levels in the pile suggests that much of the tuff 

erupted and cooled at the same time. The Fish Creek 

Mountains Tuff spread outward from its source in 

the area that is now the south-central part of the 

Fish Creek Mountains more or less uniformly for 

about 10 miles in all directions. Tectonic adjustment, 

probably of collapse type, must have taken place 

during eruption, and the thick, massive tuff in the 

south-central part of the mountains represents tuff 

trapped in this caldera-type depression.

The dark andesitic to quartz latitic rocks were 

probably emplaced along the western part of the caldera fault zone shortly after subsidence, although 

these rocks may not be related to the Fish Creek 

Mountains Tuff volcanism at all.

Fish Creek Mountains Tuff accumulated to a thickness of more than 3,000 feet near its source in the
SUMMARY 15

south-central part of the range, but less than 100 feet 

of tuff was deposited within a distance of about 10 

miles from the source. This pile of tuff remained as a 

topograpjiic high for at least a million years; during 

this time a thin ash-flow sheet of Bates Mountain (?) 

Tuff (radiometric age in the Fish Creek Mountains 

about 24 m.y.) spread around the southern, eastern, 

and northern part of the Fish Creek Mountains volcanic center. There is no evidence that this tuff sheet 

ever completely overlapped the region that is now 

the Fish Creek Mountains.

Following deposition of the Bates Mountain ashflow sheet in early Miocene time, there is no record of 

geologic events in the Fish Creek Mountains until the 

deposition of tuffaceous sedimentary rocks of late 

Miocene (?) or early Pliocene (?) age. These strata accumulated in a shallow basin within the range and in 

nearby areas to the east, south, and west. Local units 

of conglomerate in some of these sedimentary sequences and the fact that basins in which they could 

accumulate developed suggest that the first stage of 

tectonism related to Basin and Range structure began 

at this time. Basin and Range faulting responsible for 

the present physiography of central Nevada subsequently "blocked out" the Fish Creek Mountains as 

they are today. The main faulting of this type in the 

Fish Creek Mountains is along their western edge. 

Olivine basalt flows and cones, many of which were 

erupted along the major western Basin and Range 

fault, are the last volcanic rocks deposited in the Fish 

Creek Mountains.

Recent erosion has deeply incised the south-central 

part of the range in the vicinity of the source of the 

Fish Creek Mountains Tuff. This area is particularly 

susceptible to erosion because of pervasive alteration 

as well as the presence of a large amount of soft, 

partially to moderately welded tuff.

INTERPRETATION OF THE 

FISH CREEK MOUNTAINS VOLCANIC CENTER

The thickness and distribution patterns of the Fish 

Creek Mountain Tuff strongly suggest that the source 

of this ash-flow sheet was from the south-central part 

of the Fish Creek Mountains. Lithologic variations 

and zones of alteration within the tuff, as well as 

large tectonic features, support this conclusion and 

offer clues to the nature of the volcanic center. No 

single line of evidence is conclusive, and some of the 

obvious features seen today, such as the basin in the 

center of the range, the sedimentary beds within this 

basin, and the capping basalt flow, are not related to 

the Fish Creek Mountains volcanic center. These features postdate the Fish Creek Mountains Tuff by as

much as 10 m.y. and belong to a later chapter in the 

geologic history of the range.

A series of schematic diagrams showing what the 

author considers to be the most likely history of the 

Fish Creek Mountains volcanic center are shown in 

figure 11. The major elements in this model of the 

evolution of the volcano are: (1) Accumulation of a 

thick pile of tuff erupted as a low-energy ash-flow 

sheet (Noble, 1969) from a central vent area. The 

margins of the pile are thin and show horizontal 

joints which probably are partings between ash flow 

pulses. (2) Probable subsidence of the central part of 

the tuff pile during eruption and subsequent filling 

and possible overflowing of the collapsed area by tuff. 

Tuff in this area is nonstratified, and the collapse area 

is characterized by chaotic structure, alteration of the 

tuff, and irregular zones of welding. The long arcuate 

faults that circumscribe the southern part of this central core are probably traces of the original collapse 

zone. (3) Possible resurgence of the central core of 

the tuff pile along the same zones of weakness developed on collapse. Resurgence might account for elevation of the core area and the beginning of the deep 

erosion of this part of the range. Later Basin and 

Range faulting, however, has elevated the mountains 

to their present height, and most erosion is related to 

this uplift. The belt of andesitic to quartz latitic rocks 

in the western part of the range may have been emplaced along the fracture formed by collapse of the 

source area of the Fish Creek Mountains Tuff. If it 

was so emplaced, the time of emplacement was probably not long after the eruption and subsequent 

collapse of the volcano.

SUMMARY

The Fish Creek Mountains in central Nevada are 

composed almost entirely of a single probably composite ash-flow sheet, the Fish Creek Mountains Tuff, 

considered to be of early Miocene age. This sheet is 

found only in this range or within a few miles of its 

edge. In the south-central part of the range, the formation is more than 3,000 feet thick, is devoid of 

structure except for some columnar jointing, and appears to be pervasively faulted. Irregular zones of 

alteration and differential welding also characterize 

the thick central part of the tuff body. Around the 

margins of the range a strong layering is present; 

many layers of tuff can be seen, all of which are cut 

by well-formed columnar joints which pass from one 

layer to the next without change.

In the southern part of the range, several long arcuate faults, trending approximately at right angles to 

the regional north-south topographic grain of central
16 FISH CREEK MOUNTAINS TUFF AND VOLCANIC CENTER, LANDER COUNTY, NEVADA

sw ARCUATE FAULT

10 MILES

FIGURE 11. Schematic possible history of the Fish Creek Mountains volcanic center. Horizontal and 

vertical scale approximate. 1, Initial eruption a large volume (approximately 75 cu mi) of tuff 

erupted nearly simultaneously. Layering more pronounced at the margins. Approximately 24 m.y. 

ago. 2, Collapse during eruption the collapse structure filled with tuff from continued eruption. 

Central collapse are structurally chaotic and highly altered. Approximately 24 m.y. ago. 3, Possible 

resurgence following eruption elevation of the core area and beginning of deep erosion. Possible 

intrusion of andesitic to quartz latitic rocks long the faults on the western edge of the core area. 

Slightly less than 24 m.y. ago. 4, Present-day topography the Fish Creek Mountains are elevated 

along north-northeast-trending Basin and Range faults. Such faults are especially obvious on the 

west side of the range.
SELECTED REFERENCES 17

Nevada, separate the central massive and tectonically 

chaotic and altered core area of tuff from the marginal layered tuff. These faults, which are not related 

to more recent Basin and Range faulting, are considered remnants of collapse or resurgent structures 

formed at the time of eruption (about 24 m.y. ago). 

The ash-flow sheet probably was erupted from vents 

within the south-central part of the Fish Creek 

Mountains.

The geologic history of the Fish Creek Mountains 

volcanic center started with the eruption of a voluminous (about 75 cu mi) ash-flow sheet. The eruption 

was probably of a low-energy type, for the ash flow 

spread only a short distance from its source and 

formed a relatively thick pile of tuff. Subsidence of 

the source area probably accompanied eruption and 

served to futher localize the ash flow near its source. 

The total duration of ash-flow eruption probably was 

less than several thousand years, and there was no 

later ash-flow activity from the volcanic center.

SELECTED REFERENCES

Byers, F. M., Jr., Orkild, P. P., Carr, W. J., and Quinlivan, 

W. D., 1968, Timber Mountain Tuff, southern Nevada, 

and its relation to cauldron subsidence, in Nevada test 

site, studies in geology and hydrology: Geol. Soc. America 

Mem. 110, p. 87-97.

Cook, E. F., 1965, Stratigraphy of Tertiary volcanic rocks in 

central Nevada: Nevada Bur. Mines Kept. 11, 61 p.

Deffeyes, K. S., 1959, Late Cenozoic sedimentation and tectonic development of central Nevada: Princeton Univ., 

Princeton, N.J., Ph.D. thesis, 118 p.

Ewart, A., Taylor, S. R., and Capp, A. C., 1968, Trace and 

minor element geochemistry of the rhyolitic volcanic 

rocks, central North Island, New Zealand: Contr. Mineralogy and Petrology, v. 18, p. 76, 107.

Gilluly, James, and Gates, Olcott, 1965, Tectonic and igneous 

geology of the northern Shoshone Range, Nevada, with 

sections on Gravity in Crescent Valley, by Donald Plouff, 

and Economic geology, by K. B. Ketner: U.S. Geol. Survey Prof. Paper 465, 153 p.

Gilluly, James, and Masursky, Harold, 1965, Geology of the 

Cortez quadrangle, Nevada, with a section on Gravity and 

aeromagnetic surveys, by D. R. Mabey: U.S. Geol. Survey 

Bull. 1175, 117 p.

Harland, W. B., Smith, A. G., and Wildock, Bruce, eds., 1964, 

The Phanerozoic time-scale A symposium dedicated to 

Professor Arthur Holmes: Geol. Soc. London Quart. 

Jour., v. 120s, supp., 458 p.

Heier, K. S., and Adams, J. A. S., 1964, Geochemistry of the 

alkali metals, in Ahrens, L. H., Press, Frank, and Runborn, S. K., eds., Physics and chemistry of the Earth, v. 5: 

New York, McGraw-Hill, p. 253-381.

McKee, E. H., 1969, Geologic map of the Mount Moses and 

southern part of the Buffalo Springs quadrangles, Lander 

County, Nevada: U.S. Geol. Survey open-file map, scale 

1:62,500.

McKee, E. H., and Naeser, C. W., 1970, K-Ar and fission track 

dates of Tertiary ash-flow sheets, comparison and evaluation of the methods: [abs.] : Geol. Soc. America, Cordilleran Sec.-Paleont. Soc., Pacific Coast Sec., 66th Ann. 

Mtg., Hayward, Calif., 1970, Program, pt. 3, p. 118.

McKee, E. H., and Silberman, M. L., 1970, Geochronology of 

Tertiary igneous rocks in central Nevada: Geol. Soc. 

American Bull., v. 81, no. 8, p. 2317-2328.

McKee, E. H., and Stewart, J. H., 1969a, Geologic map of the 

McCoy quadrangle, Lander County, Nevada: U.S. Geol. 

Survey open-file map, scale 1:62,500.

 1969b, Geologic map of the Cedars quadrangle, Lander 

County, Nevada: U.S. Geol. Survey open-file map, scale 

1:62,500. 

-1970, Stratigraphy and potassium-argon ages of some

Tertiary tuffs from Lander and Churchill Counties, central Nevada: U.S. Geol. Survey Bull. 1311-B. [In press.]

Noble, D. C., 1969, Energy of eruption, timing of subsidence, 

and the distribution and form of welded pyroclastic deposits [abs.]: Geol. Soc. America, Cordilleran Sec.-Paleont. Soc., Pacific Coast Sec., 65th Ann. Mtg., Eugene, 

Oreg., 1969, Program, pt. 3, p. 48.

Noble, D. C., Bath, G. D., Christiansen, R. L., and Orkild, 

P. P., 1968, Zonal relations and paleomagnetism of the 

Spearhead and Rocket Wash Members of the Thirsty 

Canyon Tuff, in Geological Survey research, 1968: U.S. 

Geol. Survey Prof. Paper 600-C, p. C61-C65. .

Noble, D. C., Haffty, Joseph, and Hedge, C. E., 1969, Strontium and magnesium contents of some natural peralkaline 

silicic glasses and their petrogenetic significance: Am. 

Jour. Sci., v. 267, no. 5, p. 598-608.

Noble, D. C., and Hedge, C. E., 1969, Sr87 /Sr86 variations 

within individual ash-flow sheets, in Geological Survey 

research, 1969: U.S. Geol. Survey Prof. Paper 650-C, 

p. C133-C139.

Nockolds, S. R., 1954, Average chemical compositions of some 

igneous rocks: Geol. Soc. America Bull., v. 65, no. 10, 

p. 1007-1032.

O'Conner, J. T., 1965, A classification for quartz-rich igneous 

rocks based on feldspar ratios: U.S. Geol. Survey Prof. 

Paper 525-B, p. B79-B84.

Sargent, K. A., and McKee, E. H., 1969, The Bates Mountain 

Tuff in Northern Nye County, Nevada: U.S. Geol. Survey 

Bull. 1294-E, 12 p.

Smith, R. L., 1960, Zones and zonal variations in welded ash 

flows: U.S. Geol. Survey Prof. Paper 354-F, p. 149-159.

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