Volcanic Suites and 

Related Cauldrons of 

Timber Mountain-Oasis 

Valley Caldera Complex, 

Southern Nevada 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 919 

Prepared in cooperation with the 

U.S. Atomic Energy Commission 

VOLCANIC SUITES AND 

RELATED CAULDRONS OF 

TIMBER MOUNTAIN-OASIS 

VALLEY CALDERA COMPLEX, 

SOUTHERN NEVADA 
Panorarmc v1ew of Timber Mountain caldera from poin1 southwest of Beatty Wash. Photograph by P. W. Lipman, 1964. 
. Ai 

I 

•

•. -4~ ~J . '"'' <F .. ~ .. 

Volcanic Suites and 

Related Cauldrons of 

Timber Mountain-Oasis 

Valley Caldera Complex, 

Southern Nevada 

By F. M. BYERS, JR., W. J. CARR, PAUL P. ORKILD, W. D. QUINLIVAN, 

and K. A. SARGENT 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 919 

Prepared in cooperation with the 

U.S. Atomic Energy Commission 

Stratigraphic, petrochemical, and 

structural relations of igneous rocks of 

Timber Mountain-Oasis Valley caldera complex, 

including calc-alkalic rocks from Silent Canyon caldera, 

southwestern Nevada volcanic field 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1976 
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1976 

Second printing 1984 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

l\lletric-English equivalents........................................................................................................... IX 

Abstract.......................................................................................................................................... I 

Introduction................................................................................................................................... 2 

Acknow ledgn1en ts...................................... ... .. . . . . . . ... . . . . . . . . . .. . ... . . . . . .. . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . ... .. .. . . . . . 2 

Non1enclature................................................................................................................................. 2 

(~eneral geologic relations............................................................................................................. 5 

Tuffs and lavas related to Sleeping Butte caldera........................................................................ 7 

Redrock Valley Tuff............................................................................................................... 7 

Crater Flat Tuff and intercalated lava................................................................................... 10 

Bullfrog Member............................................................................................................. ll 

Intercalated lava flow...................................................................................................... 14 

Prow Pass Member.......................................................................................................... 14 

Tuffs of Sleeping Butte and their relation to caldera wall................................................... 15 

Tuff of Tolicha Peak.............................................................................................................. 15 

Biotite-hornblende rhyolite lavas west of Split Ridge........................................................... 15 

Peralkaline rocks of Silent Canyon caldera.................................................................................. 16 

Stockade Wash Tuff and related calc-alkalic rocks associated with Silent Canyon caldera....... 16 

Paintbrush Tuff and rocks related to Claim Canyon cauldron................................................... 21 

General features...................................................................................................................... 21 

Original and redefinition of the Paintbrush Tuff......................................................... 21 

Bedded tuff....................................................................................................................... 22 

Geologic relations between welded ash-flow tuffs. lavas, and Claim Canyon 

cauldron....................................................................................................................... 24 

Lithology and field recognition ..................................... -................................................ 24 

Topopah Spring Member....................................................................................................... 25 

Pah Canyon Member and related pre-Pah Canyon lavas...................................................... 25 

Lavas and ash-flow tuff between Pah Canyon and Yucca Mountain Members................... 25 

Yucca Mountain Member....................................................................................................... 31 

Tiva Canyon Member............................................................................................................. 31 

Tuff breccias within Claim Canyon cauldron segment........................................................ 33 

Tuff of Pinyon Pass . .... .. .. ......... ............... .............. ...................... ........ .... . . .... .... . . .................. 35 

Post-Tiva Canyon rhyolite lavas............................................................................................ 35 

Cauldron subsidences related to eruption of ash-flow tuff members.................................... 36 

Possible magmatic resurgence of Claim Canyon cauldron................................................... 37 

Timber Mountain Tuff and rocks related to Timber Mountain caldera..................................... 38 

Original definition and redefinition of Timber Mountain Tuff.......................................... 38 

Rainier Mesa Member ...... . ........................ ... ......... ............ ............. .............. ......... ................. 39 

An1n1onia Tanks Member...................................................................................................... 43 

Tuff of Buttonhook Wash...................................................................................................... 47 

Tuffs of Crooked Canyon....................................................................................................... 49 

Debris. flows............................................................................................................................ 50 

Lavas petrologically related to Timber Mountain Tuff........................................................ 51 

Pre-Rainier Mesa Ia vas . .. .. .. .. .. . . .. .. .. .. .. .. . . . .. .. . .. .. . . .. .. .. .. .. .. . . .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. . 51 

Pre-Ammonia Tanks lavas in Timber Mountain caldera moat.................................... 52 

Intrusive rocks and relation to resurgent doming................................................................. 52 

Caldera collapses related to eruptions of Rainier Mesa and Ammonia Tanks Members .. .. 56 

Younger intracaldera rocks in Timber Mountain and Oasis Valley calderas.............................. 59 

Rhyodacitic and mafic lavas.................................................................................................. 59 

Tuffs of Fleur-de-lis Ranch and related rhyolite lavas.......................................................... 61 

Tuff of Cutoff Road and related rhyolite lavas..................................................................... 61 

Rhyolite lavas of Fortymile Cany-on...................................................................................... 62 

Sun1mary of geologic history ......... ............... .......... ........... ..... .... .. . .... ..... .. . ................ ...... . . ..... . . ... . 63 

References cited.............................................................................................................................. 67 

VII 
VIII CONTENTS 

ILLUSTRATIONS 

Page 

FRONTISPIECE. Panoramic view of Timber Mountain caldera from point southwest of Beatty Wash. 

FIGURE l. Map of Southwestern Nevada volcanic field showing location of Timber Mountain caldera area............................................... 3 

2. Index to original sources of geologic data and other maps for the Timber Mountain caldera area, Nevada Test Site, 

and adjacent region................................................................................................................................................................... 4 

3. Generalized schematic diagram through southwestern Nevada volcanic field............................................................................... 7 

4. Map showing areal distribution of Redrock Valley Tuff, members of Crater Fiat Tuff, and other volcanic rocks that 

are probably related to Sleeping Butte caldera......................................................................................................................... 8 

5. Graph showing modal and silica ranges of Redrock Valley, Crater Flat, and Stockade Wash Tuffs and petrologically 

related lavas ..... .. . .. . ............ ............... ............. ................ .. .......... .... .. ............ .............. ............ ..................... ........... ............ .... .... 12 

6. Map showing isopachs of Stockade Wash Tuff, Stockade Wash(?) Tuff, and tuff of Blacktop Buttes......................................... 17 

7. Chart showing stratigraphic relations and revisions of Stockade Wash, Paintbrush, and Timber Mountain Tuffs, Timber 

Mountain caldera and vicinity.................................................................................................................................................. 18 

8. Map showing areal extent of Topopah Spring and Tiva Canyon Members of Paintbrush Tuff and related lavas..................... 22 

9. Graph showing modal and silica ranges of units of Paintbrush Tuff and petrologically related lavas....................................... 26 

10. Map showing areal extent of Pah Canyon and Yucca Mountain Members of Paintbrush Tuff.. .................................... _............ 28 

ll. Generalized section A-A' through drill holes UE19f and UE19i, Silent Canyon caldera.............................................................. 30 

12. Generalized geologic map of Claim Canyon cauldron segment and vicinity .. . . .. . . .. . . . . ... . . .. . .. . . .. . . . . .. .. ... ........... .......... .. .. . . ... . .. ........ 33 

13. Geologic sections B-B' and C-C' through Claim Canyon cauldron segment.............................................................................. 34 

14. Generalized isopach map of Rainier Mesa Member of Timber Mountain Tuff............................................................................ 40 

15. Graph showing modal and silica ranges of units of Timber Mountain Tuff and petrologically related lavas............................ 42 

16. Generalized isopach map of Ammonia Tanks Member of Timber Mountain Tuff...................................................................... 44 

17. Photograph showing structural unconformity between uppermost high-silica rhyolite and underlying quartz 

latite of upper part of Ammonia Tanks Member..................................................................................................................... 47 

18. Map showing combined thicknesses of tuffs of Buttonhook Wash and Crooked Canyon and intrusive rocks on 

Timber Mountain resurgent dome............................................................................................................................................ 48 

19. Graphs showing phenocryst mineralogy of rhyolite lava of Windy Wash on opposite flanks of Timber 

Mountain caldera...................................................................................................................................................................... 51 

20. Photograph of rhyolite tuff dike that cuts basal quartz latite of Ammonia Tanks Member and dips inward 

along arcuate zone..................................................................................................................................................................... 53 

21. Graph showing modal and silica ranges of intrusive rocks on Timber Mountain compared with ranges of 

high-silica rhyolite and quartz latite subunits of upper part of Ammonia Tanks Member................................................... 54 

22. Generalized interpretive section across Timber Mountain resurgent dome.................................................................................... 55 

23. Section across Oasis Valley caldera segment.................................................................................................................................... 58 

24. Map showing distribution of younger intracaldera rocks of Timber Mountain-Oasis Valley caldera complex........................... 60 

25. Graph showing modal and silica ranges of younger intracaldera volcanic rocks of Timber Mountain-Oasis 

Valley caldera con1plex ............................................................................................................................................................ 62 

26. Sequence of north-south interpretive diagrams through Timber Mountain caldera..................................................................... 65 

TABLES 

Page 

TABLE l. Generalized stratigraphic nomenclature of tuff formations and members, and their indicated volcanic center........................... 5 

2. Nonmagnetic heavy minerals of Paintbrush and Stockade Wash Tuffs and related tuffs and lavas............................................. 21 

3. Thicknesses and K-Ar ages of Paintbrush Tuff units and genetically related lavas within Claim Canyon cauldron 

segment and outside on cauldron rim.......................................................................................................................................... 24 

4. Combined thin-section modes and computed accessory mineral percentages of local ash-flow tuff of 

Paintbrush Tuff and overlying pre-Tiva Canyon rhyolite lava.................................................................................................. 31 
Metric unit 

millimetre ( mm) metre (m) kilometre (km) 

square metre (m2) 

square kilometre (km2) 

hectare (ha) 

cubic centimetre (cm3) 

litre (1) 

cubic metre (m3) 

cubic metre cubic hectometre (hm3) 

litre 

litre 

litre 

cubic metre 

cubic metre 

gram (g) 

gram 

tonne (t) tonne 

CONTENTS IX 

METRIC-ENGLISH EQUIVALENTS 

English equivalent Metric unit English equivalent 

Length Specific combinations--Continued 

0.03937 inch (in) 

3.28 feet (ft) 

.62 mile (mi) 

Area 

10.76 

.386 

2.47 

Volume 

0.061 

61.03 

35.31 

.00081 

=810.7 

2.113 

1.06 

.26 

.00026 

6.290 

Weight 

o.o:m 

.0022 

1.1 

.98 

square feet (ft2) 

square mile (mi2) acres 

cubic inch (ina) 

cubic inches 

cubic feet (fta) acre-foot (acre-ft) 

acre-feet 

pints (pt) 

quarts (qt) gallon (gal) million gallons ( Mgal or 106 gal) barrels ( bbl) ( 1 bbl = 42 gal) 

ounce, avoirdupois (oz avdp) pound, avoirdupois (lb avdp) 

tons, short (2,000 lb) 

ton, long (2,240 lb) 

litre per second (l/s) .0353 cubic foot per second 

cubic metre per second 

per square kilometre 

[ (m3js) fkm2} 

metre per day (m/d) 

metre per kilometre 

(m/km) kilometre per hour 

(km/h) metre per second (m/s) metre squared per day (m2/d) 

cubic metre per second 

(m3/s) 

cubic metre per minute 

(m3fmin) 

litre per second (1/s) litre per second per metre [ (1/s)/m] 

kilometre per hour 

(km/h) metre per second (m/s) 

gram per cubic 

centimetre (g/cma) 

gram per square centimetre (g/cm2 ) 

91.47 

3.28 

:>.28 

.fH13 

3.28 

10.764 

= 22.826 

=264.2 

15.85 

4.8R 

.62 

2.237 

62.4~ 

2.04R 

cubic feet per second per 

square mile [ (ft3js)/mi2} feet per day (hydraulic conductivity) (ft/d) 

feet per mile (ft/mi) 

foot per second (ft/s) 

feet per second 

feet squared per day (ft2fd) 

(transmissivity) 

million gallons per day (Mgal/d) 

gallons per minute (gal/min) 

gallons per minute 

gallons per minute per foot 

[(gal/min) /ft} 

mile per hour (mi/h) 

miles per hour 

Specific combinations gram per square centimetre .0142 

pounds per cubic foot (lb/fP) 

pounds per square foot (lb/ft2) 

pound per square Inch (lb/in2) 

kilogram per square centimetre (kg/cm2) 

kilogram per square centimetre 

cubic metre per second 

(m3/s) 

0.96 

.98 

35.3 

atmosphere (atm} 

bar (0.9869 atm) 

cubic feet per second (ft3fs) 

degree Celsius ("C) 

degrees Celsius 

(temperature) 

Temperature 

1.8 degrees Fahrenheit ( •F) 

= [ ( 1.8 X •c) + 32] degrees Fahrenheit 

VOLCANIC SUITES AND RELATED CAULDRONS OF 

TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, 

SOUTHERN NEVADA 

By F. M. BYERS, JR., w. J. CARR, PAUL P. 0RKILD, 

W. D. QuiNLIVAN, and K. A. SARGENT 

ABSTRACT 

The Timber Mountain-Oasis Valley caldera complex occupies a 

slightly elliptical area about 40 km (25 mi) in maximum diameter in 

southern Nye County, Nev., and is a major part of the southwestern 

Nevada volcanic field, which includes peralkaline, alkali-calcic, calcalkalic, and calcic centers. Upper Miocene and lower Pliocene calcalkalic and alkali-calcic ash-flow sheets and petrologically related 

igneous rocks of the Timber Mountain-Oasis Valley caldera complex 

were erupted 16-9 m.y. (million years) ago and were associated with 

multiple cauldron subsidences. 

The newly named calc-alkalic Redrock Valley and Crater Flat Tuffs 

were associated with collapses 16-14 m.y. ago, possibly within the 

Sleeping Butte caldera, only a small part of which is still exposed. The 

Redrock Valley consists of only one known cooling unit, whereas the 

Crater Flat consists of two newly named members, the Bullfrog and the 

Prow Pass. 

The quartz-bearing Stockade Wash Tuff, formerly the Stockade Wash 

Member of the quartz-poor Paintbrush Tuff, is here raised to separate 

formational rank. It is a calc-alkalic tuff, erupted between 13.8 and 13.2 

m. y. ago as a late effusive from the dominantly peralkaline Silent Canyon 

caldera. The Stockade Wash is unrelated to the Paintbrush, in both its 

petrologic features and volcanic source, but is closely similar petrographically to two other late ash-flow sheets of limited extent within and 

around Silent Canyon caldera and also to 1,500 m (5,000 ft)of calc-alkalic 

tuffs and rhyolite lavas of Area 20, which fill Silent Canyon caldera. 

The Paintbrush Tuff is here restricted to include the quartz-free to 

quartz-poor bedded and intercalated welded ash-flow tuffs that occur 

between the underlying redefined Stockade Wash Tuff, or the tuffs and 

rhyolites of Area 20, and the overlying Timber Mountain Tuff. The 

Paintbrush is a major alkali-calcic volcanic sequence of genetically 

related bedded tuff and ash-flow tuff sheets that were erupted 13.2-12.5 

m.y. ago from the Claim Canyon cauldron center, the southernmost part 

of which is exposed in an arcuate segment at the south side of the 

complex. The ash-flow sheets consist of (I) the Topopah Spring Member 

near the base, {2) the Pah Canyon Member, (3) a local subsurface unit that 

may grade upward into a lava flow, (4) the Yucca Mountain Member, (5) 

the Tiva Canyon Member, which includes the intracauldron tuff of 

Chocolate Mountain, and (6) the intracauldron tuff of Pinyon Pass. 

The Claim Canyon cauldron segment exposes greatly thickened intracauldron facies of Paintbrush welded tuffs, intercalated lavas, and tuff 

breccia. The estimated volume of the Topopah Spring Member is about 

250 km3 (60 mi3), but may be much greater, as the unit is buried by 

younger welded tuffs and lavas within the exposed segment of the 

cauldron. The post-Topopah Spring ash-flow sheets of the Paintbrush 

Tuff and intercalated lavas are more than 1,500 m thick in the Claim 

Canyon cauldron segment. This thick post-Topopah Spring intracauldron filling implies considerable structural depression inside the 

cauldron. It seems unlikely that this much structural depression could 

have occurred within the cauldron following the eruption of only the 

Topopah Spring Member. We interpret the exposed Claim Canyon 

cauldron segment as probably recording continued episodic subsidence 

during or immediately following eruption of each ash-flow sheet of the 

Paintbrush. The maximum subsidence of Claim Canyon cauldron 

probably occurred during the late stages of eruption of I ,000 km3 (250 

mi3) of the Tiva Canyon Member. An alternative interpretation infers 

that subsidence at the Oasis Valley caldera segment was related to eruption of the Tiva Canyon. A thick tuff breccia along and near the wall of 

the Claim Canyon cauldron segment, however, grades laterally and 

vertically into welded masses of the Tiva Canyon and may mark a locus of 

Tiva Canyon volcanic vents. 

Cauldron resurgence possibly affected the Claim Canyon cauldron, as 

the tuffs filling the exposed Claim Canyon cauldron segment now stand 

topographically and structurally high; the buried larger part of the 

cauldron to the north has been dropped to great depth by collapses of the 

younger Timber Mountain caldera. Alternatively, the segment was raised 

with a much larger Yucca Mountain block during early broad doming of 

Timber Mountain caldera prior to eruption of the Timber Mountain 

Tuff. 

The lower Pliocene alkali-calcic Timber Mountain Tuff is redefined to 

include all quartz-bearing ash-flow tuff sheets and minor interbedded 

ash-fall tuff erupted about ll m.y. ago from the Timber Mountain 

caldera center. The tuff includes in ascending order, the Rainier Mesa 

and Ammonia Tanks Members, which are the two widespread ash-flow 

sheets of the original definition, and the newly added intracaldera tuffs of 

Buttonhook Wash and Crooked Canyon. The Ammonia Tanks Member 

has been further redefined to include the local tuffs of Cat Canyon and 

Transvaal, which are now known to be equivalent to part or all of the 

Ammonia Tanks. 

Two major subsidences of Timber Mountain caldera were associated 

with eruption of the Rainier Mesa Member, having a volume of more 

than 1,200 km3 (300 mi3), and with the eruption of the overlying 

Ammonia Tanks Member, having a volume of about 900 km3 (230 mi3). 

The tuff exposed on the central resurgent dome of Timber Mountain 

caldera is intracaldera Ammonia Tanks, more than 900 m (3,000 ft) thick, 

and was formerly mapped as tuff of Cat Canyon. Thicknesses in excess of 

450 m {1,500 ft), granophyric texture, and fluidal flow banding of both 

the Rainier Mesa and Ammonia Tanks Members within the Oasis Valley 

caldera segment suggest also partial collapse of that segment during 

eruption of the members, forming a large volcano-tectonic depression. 

The area that collapsed because of the eruption of the Rainier Mesa is 

somewhat larger than that which collapsed because of the Ammonia 

Tanks, as might be expected from the greater volume of the Rainier Mesa. 

The Timber Mountain resurgent dome in the central area of Timber 

Mountain caldera contains both inward-dipping high-silica rhyolite tuff 

dikes, possibly cone sheets, and an outward-dipping microgranitic ring 
2 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

dike approaching quartz latite composition (68 percent silica). These two 

rock types are petrologically similar to limiting compositions of the 

Ammonia Tanks M~mber and may have come from different levels of a 

zoned silicic magma chamber high in the crust. 

Post-Timber Mountain alkali-calcic tuffs and related lavas are 

confined to the Oasis Valley-Timber Mountain caldera complex and 

probably were erupted 11.0-9.5 m. y. ago from within the Oasis Valley 

caldera segment. 

Rhyolite lavas extruded from the rim and moat areas of Timber Mountain caldera concluded the activity of the caldera complex about 9 m. y. 

ago. 

INTRODUCTION 

The ash-flow sheets discussed in this report constitute 

the major stratigraphic units shown on the U.S. 

Geological Survey map of the Timber Mountain caldera 

area (Byers and others, 1976) in the Nevada Test Site region 

of southern Nevada. An attempt is made to relate the tuff 

sheets and petrologically similar lavas and intrusives to 

major volcanic centers of the Timber Mountain-Oasis 

Valley caldera complex (fig. 1). A closely related paper 

(Christiansen and others, 1976) emphasizes the structural 

setting and associated Tertiary volcanism in the region 

around the caldera complex. 

Present knowledge of the relations and distribution of 

volcanic rocks in the Nevada Test Site region of southwestern Nevada is based on 34 geologic quadrangle maps 

(fig. 2), about 30 exploratory drill holes, several Bouguer 

gravity (Don L. Healey, written commun., 1968)andaeromagnetic maps (G. D. Bath, written commun., 1968), more 

than 500 polarity determinations of natural remanent 

magnetism of rocks (G. D. Bath, written commun., 

1963-68), radiometric age determinations of 51 volcanic 

rocks (Kistler, 1968; Marvin and others, 1970), and more 

than 400 modal analyses of thin sections. 

During the concentrated effort of geologic mapping 

from 1960 to 1964, the volcanic rocks at the Nevada Test 

Site were grouped into formations because of the need to 

define stratigraphic units for the geologic maps (fig. 2). 

Ash-flow tuff sheets of similar mineralogy and chemistry 

were grouped into formations, the individual sheets being 

members of the formations (Smith, 1960a, p. 812-813; R. 

L. Smith, oral commun., 1960-68; Christiansen,. and 

others, 1968), but some stratigraphic divisions were based 

on limited knowledge regarding areal distributions of the 

units and their associations with major volcanic centers. 

In this paper the geologic and petrologic relations of the 

tuff sequences to lavas and intrusives at caldera centers are 

further emphasized in arriving at a logical basis for 

redefining some of the units. 

ACKNOWLEDGMENTS 

Several geologists have contributed materially to this 

report by supplying us with basic data and interpretation. 

R. L. Smith helped us define and solve many of the caldera 

problems through field conferences. D. L. Healey and G. 

D. Bath contributed gravity and aeromagnetic interpretation, which greatly aided in delineating the outlines 

of the buried caldera structures in figure 1. D. C. Noble 

contributed to the interpretation of volcanic history and 

location of the caldera boundaries. J. T. O'Connor 

supplied about 50 thin-section modes of the tuffs. Our 

colleagues R. E. Anderson, R. L. Christiansen, E. B. Ekren, 

and P. W. Lipman provided geologic information and 

stimulating discussion. We, however,. assume full 

responsibility for any errors or misinterpretations of the 

data. This work was fully supported by the U.S. Atomic 

Energy Commission. 

NOMENCLATURE 

In general, we follow guidelines of the ash-flow tuff 

nomenclature of Smith (1960a, p. 800-801; 1960b) and 

Ross and Smith (1961). For brevity in this report and on 

the geologic map of the Timber Mountain area (Byers and 

others, 1976) ash-flow tuff (Ross and Smith, 1961, p. 3) or 

simply the word "tuff" in the proper context carries the 

genetic connotation of ash-flow cooling unit of Smith 

( 1960a, p. 80 I), as well as that of ali tho logic term. A tuff or 

ash-flow cooling unit, in this connotation, was emplaced 

in an instant of geologic time and is therefore a timemarker as well as a lithologic unit (compare with Smith, 

1960b, p. 150). The plural term "tuffs," analogous to 

"lavas" or "lava flows," is used for two or more ash-flow 

cooling units mapped as a unit (for example, Christiansen 

and Lipman, 1965; Sargent, 1969; Rogers and others, 

1967). The singular term "tuff" is also used in formal 

stratigraphic nomenclature-for example, Paintgrush 

Tuff-in which it carries the dual genetic and lithologic 

implication just noted, inasmuch as the component tuff 

members and the informal tuffs are single ash-flow 

cooling units. "Ash-flow sheet" is also used descriptively 

in discussing the extensive cooling units of large volume 

associated with cauldron subsidence. 

Other types of tuff are indicated in this report and on the 

Timber Mountain map (Byers and others, 1976) by appropriate modifiers (compare with Ross and Smith, 1961, p. 

3), such as bedded tuff (descriptive) and ash-fall tuff 

(genetic). 

In Smith's ( 1960b, p. 157-158) summary discussion of 

the cooling unit and composite sheet, he implied the 

existence of complete cooling breaks between cooling 

units and partial cooling breaks within compound 

cooling units by giving criteria for recognizing types of 

hiatuses between ash-flow units. Recognition of the 

nature of these hiatuses, herein referred to as either a 

partial or complete cooling break, is necessary in order to 

define an ash-flow cooling unit as the basic stratigraphic 

unit with time-marker implications (Smith, l960a, pl. 1; 

Christiansen and others, 1968; Noble, Bath, and others 

1968, p. C6I). 
12 

NOMENCLATURE 

I I / . I? 

I 

i( 

\ ;' ·, I I '\ 'I I \~1 

~\ \ 

/) ' ' 

R 46 t: 47 

0 10 

0 10 20 KILOM ETRES 

EXPLANATION 

/ 

\ 

I 

\ 

) " , ' ' ' '\ ,_. /"';. 

' ' . '/ 

I:· 

I 

) .-----..... .. 

T1pp1pah 

, I 

-\. \ 

Point-~- \ 

"6612 "" ~· 

x Mme Mtn 

Lookout "'- Peak 

'" 5651 

I 

The Bench 

20MILES 

-r --r-1 Approximate outer limit of Timber MountainOasis Valley caldera complex. Includes 

Sleeping Butte and Claim Canyon segments. 

Dashed where indefinite 

....~.-1 ...Li.....L..~_._ Periphery of Timber Mountain resurgent dome 

3 

FIGURE I.-Southwestern Nevada volcanic field, Nye County, Nev., showing location of Timber Mountain caldera and other major volcanic 

centers. Area shown on the geologic map by Byers and others (1976) is shaded. 

A compositional subdivision of an ash-flow tuff sheet, 

characterized by a unique and laterally correlatable 

phenocryst assemblage, is called a subunit, following 

usage of Smith and Bailey (1966, p. ll) and Lipman, 

Christiansen, and O'Connor (1966). The term "caprock" 

(Lipman, Christiansen, and O'Connor, 1966, p. F5), 

although a topographic term, conveniently describes the 

more crystal-rich, commonly more mafic upper subunit of 

a compositionally zoned cooling unit. The caprock is so 

named because it commonly forms a resistant capping 
4 

NEVADA 

~ 

TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

116°00' 

I 

I 

.I 

-~ 

II Ill 

NORTHERN NEL LIS AIR FORCE BOMBING AND GUNNERY RANGE ---- -ttt- --~ 

(Ekren , Anderson, Rogers, and Noble, 1971) 

NORTHERN HALF 

BLACK MOUNTAIN 

(Rogers, Ekren , Noble, 

and Weir, 19681 

QUARTZITE MOUNTAIN 

(Rogers , Anderson, 

Ekren and O'Connor, 

1967) 

PAHUTE MESA 

(Orkild , Sargent, and 

Snyder, 1969) 

(Biankennagel and 

Weir, 1973) 

. , 

BELTED PEAK 

(Ekren, Rogers , 

Anderson and Botinelly 

1967) 

WH EELBARROW PEAK 

RAIN IER MESA AREA 

(Sargent and Orkild, 

1973) 

I 

I 

I 

I 

I 

I 

I 

-/ " OAK SPRING : I 

BLACK / DEAD HORSE QUARTET BUTTE 1 GROOM MINE I 

MOUNTAIN --· -r-· FLAT DOME (Rogers and I SW 

37 

SW !fRAIL RIDGE SILENT BUTTE" (Noble, (Sargent, Noble, 1969) j (Colton and I 

', (Noble and (Christiansen (Ekren, Anderson, Krushensky , Luft, CLIMAX STOCK I 

',, Christiansen, and No~l~, Orkild, and McKay and Gibbons, and (Houser and : ~;:~~ 1 

',,, 19681 1968) \ Hi:ichs, 1966) Ege'-1!67) !~~ve~~!~~ _';'.ll.'::2~~~: _ ~ I ',, I • 1 -- 1 [ -- -·o • ~~ ',, I THIRSTY \ j SCRUGHAM AMMONIA I RAINIER MESA JANGLE RIOdE' 

, I CANYON ~ TANKS : : (Gibbons, i ' I (O'Connor, : '\(B~~:~nd (Hinrichs, 1 1 Hinrichs, OA

1

K SPRING Ch(Barnes, 11 

',, 1

1 Anderson , 1

1 

Cummlngs, ' Krushensky, I 1 Hansen, and Barnes, rd•stBiansen,l 1

1 ' 1 Houser and an yers, , 1 and Lipman 1 , 19671 and Luft, 1 

1

1 Lemke, p I 19651 I' 

Gr;__LDFIELD, 45 mi (73 km) ',,,!=!¥: """"'19"'

6

"'

6

"") .,~: f=l===~= 67=l =!!l ~I ==1=

96

=

3

=)===f==oo=l=e,=1=96=3=!.J-----I-I, -r-ri: TIMBER MOUNTAINI!~;:u.-;;;;;ov-----l-~1~.,. BUCKBOARDll r

1 TIPPIPAH YUCCA FLAT PAIUTE .I 1 

1 CALDERA 1 THIRSTY .

1 

""" SPRING (Colton and RIDGE ~~ (B C I CANYON SE TIMBER MESA I I yers, arr, 2 (Byers, 1 (Orkild, McKay, (Byers and· 8 

Christiansen, : (Lipman, ~ MOUNTAIN 1 1963) 19661 Barnes, I "' Is:! 

Lipman, Orkild, 1 Quinlivan , ~ · (Carr a~d Rogers , 1 1967) ~~Zl and Quinlivan , Carr, and >I Quinlivan, Carr, and I YUCCA FLAT , ::l 

0

30' 

1976) : Anderson, ..; 19661 Luft, I (Fernald, Corchary, and I 1 

1 1966) eJ 1966} I Williams, 1968) I I -= ===w4=====fi==~~===-+- t-· --+---- 1------'---'"-"+'-' .:.::.:.:... __ +--- ,1---+_l--i-l! 37°00' 

I ~~ TOPOPAH TOPOPAH I PLUTONIUM I : 

' 

att y~ 

BULLFROG 

(Cornwall and 

Kleinhampl , 

1964) 

: i:l' SPRING NW SPRING : MINE YUCCA LAKE VALLEY I I 

c: I MOUNTAIN (Hinrichs I' : 2 (Christiansen (Orkild and I (Orkild, (McKeown, and 1 

1 ~ and Lipman, O'Connor, 1 

19681 1976) M K I 

I ~i 19651 19701 1 c ay, I I 

~-- l - J t -- 1960) .l: BARE MOUNTAIN 

FRENCHJAN ~ FLATI lo 

(Poole, g,u 

(Cornwall and Kleinhampl, 1\_ \,;') 1961) 

~ 

l TOPOPAH 

SPRING SW 

I

, (Lipman and 

McKay, 

19651 

i 

JACKASS 

FLATS 

(McKay and 

Williams, 

1964) 

SKULL 

MOUNTAIN 

(Ekren and 

Sargent, 

1965) 

! STRIPED HILLS SPECTER RANGE 

LATHROP WELLS (Sargent, NW 

(McKay and McKay, and (Sargent and 

CANE SPR ING 

(Poole, 

Elston, 

and Carr, 

19651 

CAMP DESERT 

ROCK 

(Hinrichs, 

1965) i ~~~ 

MERWRY ' 

I I 

I 

I 

1968) J Mercury 0 r- · ~argent, 19701 Burchfiel, 19701 Stewart, 1971) 

EXPLANATION Lathr~~. - . - -."' i : 

Area of geologiC map 

larger than ?'12-minu te 

quadrangle 

LATHROP WELLS "' 

(Burchhel, 1966) "'- LAS VEGAS, 72 "' mi (115 km) I I I 

0 10 15 20 MILES D Nevada Test Site 10 20 KILOMETRES 

F IGU RE 2.-Index showing original sources of geologic data and other maps published by the U.S. Geological Survey 

for the Timber Mountain caldera area , Nevada Test Site, and adjacent region. Mapped areas are identifi ed by author 

and da te and are listed in the references at the end of the text. 
GENERAL GEOLOGIC RELATIONS 5 

ledge, which is the result of a densely welded zone overlying a less welded zone in a compound cooling unit. In 

many places the contact between compositional subunits 

is gradational within a foot or two and commonly nearly 

coincides with the contact between welding and 

crystallinity zones. 

The Paintbrush Tuff and younger silicic volcanic rocks 

of the Timber Mountain-Oasis Valley caldera complex are 

alkali-calcic in the Peacock (1931) classification and do 

not readily fit some of the commonly used classifications 

(Rittmann, 1952; Nockolds, 1954; O'Connor, 1965). In 

order to emphasize the differences in chemical composition among these younger rocks of the Timber Mountain 

area (W. D. Quinlivan and P. W. Lipman, written 

commun., 1974), tuffs and lavas ranging from about 65 to 

72 percent Si02 are called quartz latites; rocks ranging 

from about 72 to 76 percent silica are called rhyolites or 

low-silica rhyolites to emphasize compositional range; 

and rocks in the range from about 76 to 78 percent silica are 

called high-silica rhyolites. The pre-Paintbrush volcanic 

rocks are normal calc-alkalic rhyolites, which are lower in 

total alkalis, particularly potassium, and slightly higher 

in lime than younger rocks of similar silica content. 

Peralkaline rocks (Shand, 1947, p. 229) are represented by 

the early effusives of Silent Canyon caldera and contain 

sodium in pyroxene and amphibole, as well as in feldspar. 

These rocks are considered briefly in this report. 

The terms "caldera" and "cauldron" are used in this 

paper to emphasize topographic expression. Caldera is 

used in the sense proposed by Williams (1941, p. 242) for a 

large, roughly circular or oval topographic depression in 

the central area of a volcano or volcanic complex. We also 

recognized filled and buried calderas. The Silent Canyon 

caldera no longer has the topographic form of a caldera, 

but it is fill ed with low-density volcanic rocks that make a 

large gravity anomaly having the shape of a caldera (D. L. 

Healey and C. H. Miller, written commun., 1967; Orkild 

and others, 1968, fig. 2). The term "cauldron" is used for a 

generally circular or oval, structural block which has subsided in the area of a volcanic or intrusive center but which 

has lost through subsequent erosion any topographic 

features of the former caldera. Our usage is virtually 

synonymous with "cauldron subsidence" as defined in the 

study of GlenCoe cauldron by Clough, Maufe, and Bailey 

(1909). The term "segment" is applied to a portion of a 

caldera or cauldron that does not have a circular form 

either because a part of it has been truncated by a later 

adjacent subsidence or because its original form has been 

severely modified by later structural movement. 

The term "volcanic center" is occasionally used herein 

for the entire volcanic edifice including the underlying 

magma chamber, the subsided cauldron, the surficial 

caldera, if preserved, and the extrusive vents that are 

preserved as dikes and vent breccias. The intimate association of central stocks, ring dikes, and cone sheets with 

cauldron structures has been emphasized many times in 

the literature, notably in review papers by Richey (1932), 

Richey, MacGregor, and Anderson (1961), Anderson 

(1936), Billings (1943), Buddington (1959, p. 680-685), 

Smith, Bailey, and Ross (1961), Branch (1966), Hamilton 

and Myers (1967, p. C6-C9), and Eggler (1968, p. 

1555- 1557). 

GENERAL GEOLOGIC RELATIONS 

The southwestern Nevada volcanic field (fig. 1) 

comprises upper Tertiary effusive rocks from the Timber 

Mountain-Oasis Valley caldera complex, rocks from peralkaline calderas in the northern part, and several minor 

satellite lava piles ranging from calc-alkalic to calcic 

(Noble and others, 1965; Christiansen and others, 1976). 

The ash-flow tuffs and related rocks discussed in this 

report constitute most of a thick upper Miocene and lower 

Pliocene calc-alkalic and alkali-calcic volcanic sequence 

shown on the geologic map of the Timber Mountain 

caldera area (Byers and others, 1976) and include all the 

known silicic eruptive products of the Timber MountainOasis Valley caldera complex. Peralkaline volcanic rocks 

of the Silent Canyon and Black Mountain calderas to the 

north (fig. I) intertongue and postdate, respectively, the 

tuffs and lavas of this report and have been discussed elsewhere (Noble and others, 1963, 1965; Sargent and others, 

1965; Christiansen and Noble, 1965; Noble, Sargent, and 

others, 1968; Christiansen and others, 1976). 

The newly named upper Miocene Redrock Valley and 

Crater Flat Tuffs (table 1) are calc-alkalic effusives of 

probably the oldest recognizable caldera within the 

TABLE I.-Generalized stratigraphic nomenclature of tuff formations 

and members as herein redefined and their indicated volcanic center 

[Minor intracaldera informal units are omitted] 

Age Volcanic 

cenler 

Pliocene Timber 

Mountain 

Caldera 

Claim 

Canyon 

Cauldron 

Miocene 

Silent 

Canyon 

Caldera 

Sleeping 

Butte 

Caldera 

1Geologic name redefined. 

2New geologic name. 

Formation Member 

or unit 

Timber Tuffs of Crooked Canyon' 

Mountain Tuff of Buttonhook Wash' 

Tuff' Ammonia Tanks Member' 

Rainier Mesa Member 

Paintbrush Tuff of Pinyon Pass 

Tuf£1 Tiva Canyon Member 

Yucca Mountain Member 

Pah Canyon Member 

Topopah Spring Member 

Stockade 

Wash 

Tuf£1 

Crater Flat Prow Pass Member2 

Tuf£2 Bullfrog Member2 

Redrock 

Valley 

Tuff2 
6 TIMBER MOUNT AIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

EXTRACALDERA 

AREAS 

CLAIM CANYON 

CAULDRON SEGMENT 

Pre-Redrock Valley rocks 

Paintbrush 

Tuff and 

re lated lavas 

Tuff of 

EXPLANATION 

~ Post-Timber Mountain Tuff rock of 

late Tertiary and Quaternary age 

Paintbrush Tuff 

Genetically related lavas 

t·· -~ :< :·:-: J Bedded tuff undivided 

Timber Mountain Tuff 

Genetically related lavas 

rimber Mountain-Oasis Valley caldera complex. Only a 

small part of this caldera is now exposed and is herein 

called the Sleeping Butte caldera segment (fig. 1). The 

redefined Stockade Wash Tuff (table l) is not a product of 

the Timber Mountain caldera complex but is a late calcalkalic ash-flow sheet of the dominantly peralkaline Silent 

Canyon caldera. The overlying Paintbrush Tuff, as herein 

Tuff breccia 

Debris flows and breccia 

Partial cooling break 

defined, consists of several quartz-poor densely welded 

ash-flow tuff cooling units that are lithologically and 

petrologically distinct from those of the formations above 

and below. Most of the units are exposed within the Claim 

Canyon cauldron segment (fig. I), where they are very 

thick and intertongue with petrologically similar rhyolite 

lavas. The uppermost formation, the Timber Mountain 
TUFFS AND LA VAS RELATED TO SLEEPING BUTTE CALDERA 7 

TIMBER MOUNTAIN 

RESURGENT DOME 

CALDERA 

Drill 

Paintbrush 

Tuff and 

related lavas 

SILENT CANYON 

Tuff and 

CALDERA 

Tuffs 

and 

lavas 

rhyolites ----------

of area 20 

r------------.z Lithic-rich 

tuff and 

lavas 

PrePaintbrush 

rocks 

<( 

1-

z 

:J 

0 

~ 

Local tuffs and 

lavas of per a I ka I i ne 

composition 

Grouse Canyon Member 

of Belted Range Tuff 

FIGURE 3 (left and above).-Generalized schematic diagram through southwestern Nevada volcanic field, showing geologic relations between 

ash-flow tuff sheets and related rocks discussed in this report. A few minor units omitted. Length of diagram about 50 km (30 mi). 

Tuff, contains common to abundant quartz phenocrysts 

and consists of two widespread coextensive members, the 

Rainier Mesa and the overlying Ammonia Tanks. Within 

parts of the Timber Mountain caldera these two members 

thicken, intertongue with rhyolite lavas, and are overlain 

by intracaldera informal units of the Timber Mountain 

Tuff, as herein redefined. Post-Timber Mountain tuffs 

and lavas are of relatively small volume and are largely 

confined to the Timber Mountain- Oasis Valley caldera 

complex. The stratigraphic and geologic relations of the 

ash-flow tuffs and intercalated rhyolite lavas in the southwestern Nevada volcanic field are shown in figure 3. 

TUFFS AND LAVAS RELATED TO SLEEPING 

BUTTE CALDERA 

REDROCK VALLEY TUFF 

The Redrock Valley Tuff, the oldest ash-flow sheet 

related to the Timber Mountain-Oasis Valley caldera 

complex, is here named after exposures in Redrock Valley 

(fig. 4). The least altered and most complete exposure, 

however, is on the west side of Yucca Flat at hill 5504, 2.4 

km ( 1.5 mi) eastofthe head of Redrock Valley (fig. 4) in the 

northern part of the Tippipah Spring quadrangle (Orkild, 

1963) and is designated the type locality. The unit, as now 
8 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

-

-

-

-

D 

-

D 

UE2Df 

0 

EXPLANATION 

Biotite-hornblende rhyolite west of 

Split Ridge 

Tuffs of Sleeping Butte 

Prow Pass Member (underlain by 

Bullfrog Member) 

Rhyolite lava flow between mern bers 

Bullfrog Member 

Bullfrog Member (underlain by Redrock Valley Tuff) 

Redrock Valley Tuff 

Zero isopach of ash-flow tuff 

Limit of information 

Drill holes used for control - Number 

mentioned in text 

0 5 10 

0 5 10 15 

.... MOUNT 

. JACKSON 

15 20 MILES 

20 KILOMETRES 

.• MOUNTAIN .•'' 

, .. 45 ' -

TO LICH A 

PEA K 

-. 

''"''""\ 

~ ..... 

FIGURE 4.- Areal distribution of Redrock Valley Tuff, members of Crater Flat 

known, consists of only one ash-flow tuff cooling unit and 

has been informally called "tuff of Redrock Valley" 

(Marvin and others, 1970, p. 2659, 2667). The Redrock 

Valley Tuff is the oldest known volcanic rock that 

originated from the Timber Mountain- Oasis Valley 

caldera complex. 

The known extent of the Redrock Valley Tuff is shown 

in figure 4. At hill 5504 the unit is approximately 125 m 
TUFFS AND LAVAS RELATED TO SLEEPING BUTTE CALDERA 9 

4 5' <5' 

.> ~ 

~ "' 

~ 

~ 

·-.. l i 

"< s 4 ~;-~," · ~ 

QUARTZ M 

' ' ' ".... 

' ' 

, ............ ······~-... , 

OASIS 

VALLEY 

CALDERA 

SEGMENT 

SILENT CANYON 

CALDERA 

MESA 

c~NYON 

.'-. i 

"'"")i~ 

(/ I 

I ,, 

<5' 

37° 

Tuff, and other volcanic rocks that are probably re lated to Sleeping Butte caldera. 

(400 ft) thick. It reaches a maximum thickness of 418 m 

(1 ,370ft) where penetrated in test wellS and is about300 m 

(1,000 ft) thick in two wells west and south of Rainier 

Mesa. Where penetrated in other drill holes (fig. 4), it 

ranges in thickness from about 30 to 210m (100-700 ft). 

The tuff is exposed at the surface in three widely 

scattered localities besides the one northwest of Yucca Flat. 

The most significant of these is a small outcrop 5 km (3 mi) 
10 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

south of Sleeping Butte near the intersection of Sleeping 

Butte and Oasis Valley caldera segments (fig. 4); this 

exposure is the only one known on the west side of the 

caldera complex. The northernmost exposure at the south 

end of the Belted Range was locally mapped in Quartet 

Dome quadrangle (fig. 2; Sargent and others, 1966) as 

"ash-flow tuff of Kawich Valley." North of Frenchman 

Flat the nonwelded distal edge of the Redrock Valley Tuff 

is exposed in a few fault blocks along the border between 

Nye and Lincoln Counties. 

Little is known of the original total extent of the 

Redrock Valley Tuff, but if the unit was as widespread as 

suggested by the scattered outcrops and drill holes, as 

much as 360 km3 (90 mi3) may be present outside the 

Timber Mountain-Oasis Valley caldera complex, 

assuming an average thickness of 120m (400ft) over 3,000 

km2 (1,200 mi2). The volume of tuff buried beneath the 

Timber Mountain-Oasis Valley caldera complex would 

not increase this figure significantly in view of the inferred 

wide areal extent and the uncertainties involved. 

The Redrock Valley Tuff rests on Paleozoic rocks in the 

Eleana Range or on a thin, bedded unnamed tuff sequence 

immediately overlying the Fraction Tuff (Rpgers and 

others, 1967; Marvin and others, 1970) in drill holes of the 

Rainier Mesa and Yucca Flat areas shown in figure 4. The 

tuff is overlain with local unconformity by bedded tuff, as 

much as 60 m (200ft) thick, which is in turn overlain by the 

Bullfrog Member of the Crater Flat Tuff. 

The tuff at hill 5504 consists of a light-yellowish-gray 

nonwelded basal zone of shards, several metres thick, 

grading upward to a brown densely welded zone, which 

locally includes a thin dark-gray vitrophyre less than 5 m 

(15ft) thick. The upper 100m (330ft) becomes less welded 

with increasing crystallinity and is light purplish gray, 

mottled with red-the red coloration giving the name to 

Redrock Valley, which in turn is here reapplied to the tuff. 

Small light-colored pumice lenticles 1-2 em long in the 

devitrified zone contain abundant tiny (0.2-1.0 mm) 

biotite and hornblende phenocrysts that aid in identification of the unit. Quartz is absent to very sparse and, 

where present, commonly cannot be seen with a hand lens. 

The uppermost part of the tuff locally reflects vapor-phase 

type crystallization (Smith, l960b). The tuff has the zonation of a simple cooling unit. 

Petrographically, the rock can be distinguished from all 

other ash-flow tuffs at the Nevada Test Site by the criteria 

shown in figure 5. It typically contains hornblende, like 

the other calc-alkalic tuffs and lavas of the Sleeping Butte 

center. The lower part has a plagioclase to alkali feldspar 

phenocryst ratio of about 3: I and very little or no quartz. 

The upper part, in contrast, has about equal feldspar 

phenocrysts and sparse quartz. Phenocrysts seemingly 

increase slightly in the upper part, despite less compaction and welding. Small euhedral quartz microphenocrysts, commonly less than 0.5 mm and never exceeding 1.0 

mm, are diagnostic of the unit; two thin sections examined 

from the lowermost part of the tuff were quartz-free, but all 

sections cut from the upper part contain quartz. The 

division between the upper and lower parts is based on 

petrography and has not been recognized in the field. The 

twofold division can be recognized in surface samples and 

also in drill core from the area northwest of Yucca Flat. 

The K-Ar age of sanidine in the local basal vitrophyre of 

the Redrock Valley Tuff from the type locality at hill5504 

is 15.7 m.y., which is in good agreement with K-Ar ages of 

the overlying and underlying units (Marvin and others, 

1970). This age is close to the onset of volcanism at the 

Timber Mountain-Oasis Valley caldera complex and 

postdates nearly all the volcanic activity in the Northern 

Nellis Bombing and Gunnery Range to the north (Ekren 

and others, 1971 ). The thermal remanent magnetization is 

reverse polarity (G. D. Bath, written commun., 1968). Two 

chemical analyses of the tuff, one of the lower part and the 

other of the upper part, were made (W. D. Quinlivan and 

P. W. Lipman, written commun., 1974); the silica contents 

of these rocks are shown graphically in figure 5. 

CRATER FLAT TUFF AND INTERCALATED LAVA 

The Crater Flat Tuff is here named from exposures 

around the edges of Crater Flat (fig. 4), which is designated 

the type area of the formation. The tuff consists of a lower 

member, the Bullfrog Member (new), and an upper 

member, the Prow Pass Member (new), and local, 

unnamed intercalated breccia, bedded tuff, and ash-flow 

tuff. Both members and the intercalated breccia are well 

exposed at the southeast end of an unnamed hogback at 

south side of Crater Flat which is designated the type 

locality of the Crater Flat Tuff (fig. 4). This is the only 

known section in which both members are largely glassy 

and virtually unaltered and, therefore, amendable to K-Ar 

age dating. The thickness at the type locality totals about 

190m (620ft), of which the Bullfrog Member is 130m (430 

ft), intercalated breccia, 10 m (30 ft), and the Prow Pass 

Member, 50 m (160ft). 

The Crater Flat Tuff has been shown as "tuff of Crater 

Flat" on several U.S. Geological Survey 7~-minute quadrangle maps (fig. 2) covering the southern part of the 

Nevada Test Site (Christiansen and Lipman, 1965; Orkild 

and O'Connor, 1970; Poole, Elston, and Carr, 1965; 

Sargent and others, 1970). On the east side of the 

Wahmonie volcanic center (Poole, Elston, and Carr, 1965; 

Poole, Carr, and Elston, 1965) a local middle ash-flow tuff, 

similar to the underlying Bullfrog Member, is included 

with the formation. 

A local intercalated rhyolite lava in the south wall of the 

Oasis Valley caldera segment (fig. 4) is not included in the 

Crater Flat Tuff, in accordance with prior practice by 

authors of reports on Nevada Test Site geology. This lava 
TUFFS AND LAVAS RELATED TO SLEEPING BUTTE CALDERA 11 

is described here because it has about the same phenocryst 

ratios as the underlying Bullfrog Member-suggestive of a 

common origin. The lava has been altered in most places 

along with the underlying member and cannot conveniently be mapped separately. It has been included with 

the Crater Flat Tuff on the map of the Timber Mountain 

caldera area (Byers and others, 1976). 

Duplicate K-Ar ages on biotites from vitrophyre in the 

lower part of the Bullfrog Member are 14.0 and 13.0 m.y. 

For reasons previously discussed (Marvin and others, 1970, 

p. 2667), the 14.0-m.y. age is believed more reliable. The 

natural thermal remanent magnetism of both members of 

the Crater Flat Tuff is normal (G. D. Bath, written 

commun., 1965 ). 

BULLFROG MEMBER 

The Bullfrog Member, whose known distribution is 

shown in figure 4, is here named for the lower ash-flow 

sheet of the Crater Flat Tuff from exposures at Bullfrog 

Mountain (fig. 4). The type locality of the member is 

exposed on the mountain just north of the original Bullfrog mine, shown on the Bullfrog quadrangle (fig. 2) 

mapped by Cornwall and Kleinhampl (1964, pl. 4). These 

authors (1964, pl. 5, p. JlO) described the member as 

cooling unit 2 of their Bullfrog Hills caldera and showed a 

composite thickness of 280m (930ft) in the Bullfrog Hills. 

The Bullfrog Member is 130m (430ft) thick in the type 

locality of the Crater Flat Tuff at the south end of Crater 

Flat and is locally as much as 180m (600ft) thick in the 

northern part. Exposures of the member either are 

incomplete or were not mapped separately east of Crater 

Flat; therefore, maximum thicknesses are not known, but 

are probably as much as 150 m (500 ft) in paleovalleys. 

Thicknesses in Yucca Flat and in the Rainier Mesa area to 

the northwest do not exceed 120m (400ft). As the distal 

edges are approached, such as near the southwest corner of 

Lincoln County, the thickness of the unit ranges from 15 

to 30 m (50 to 100 ft). 

The unit is incompletely exposed south of Sleeping 

Butte (fig. 4), but probably the thickness does not exceed 60 

m (200 ft). From drill hole UE20j to UE20f, just outside 

and inside the Silent Canyon caldera, respectively, the unit 

decreases slightly from 170 to 130 m (560-430 ft) in 

thickness (Orkild and others, 1969, section A-A'). The 

unit probably predates the formation of Silent Canyon 

caldera, for there is no great difference in thickness 

between the two holes inside and outside the caldera, and 

the unit has been downdropped 2,660 m (8, 720 ft) to the 

east along later caldera faults (Orkild and others, 1969, 

section A-A'). 

The Bullfrog Member probably originally covered a 

larger area than that indicated on figure 4 and may have 

extended into the Death Valley area west of the Grapevine 

Mountains and southwesterly under the Amargosa Desert. 

The northwestern extent under Sarcobatus Flat is also 

unknown. If an original area of 6,500 km2 (2,500 mi2), 

twice that of the Redrock Valley Tuff, were covered, a 

maximum of 1,000 km3 (250 mi3) may have originally been 

present. 

The Bullfrog Member is commonly conformable on 

underlying bedded ash-fall tuff, which in turn separates 

the Bullfrog from the underlying Redrock Valley Tuff. In 

the Bullfrog Hills and on the eastern flank of the Grapevine Mountains (fig. 4), the member rests concordantly on 

bedded tuffaceous sandstone that overlies the Oligocene 

Titus Canyon Formation (Stock and Bode, 1935). At the 

type locality at Bullfrog Mountain, the member rests on 

cooling unit 1 of Cornwall andKleinhampl (1964). To the 

east along the southern border of its known extent, the 

member rests on the rocks of Pavits Springs (Poole, Elston, 

and Carr, 1965; Poole, Carr, and Elston, 1965). The 

member is overlain at the type locality of the Crater Flat 

Tuff by a monolithologic breccia composed of fragments 

of the member. It is overlain by the lava flow that is petrographically similar to the member in the south wall of the 

Oasis Valley caldera segment (fig. 4). Elsewhere the Bullfrog Member is overlain by bedded tuff of the Crater Flat, 

which, in turn, is overlain by the Prow Pass Member. 

Locally on the east flank of the Wahmonie volcanic center, 

a middle ash-flow petrographically similar to the underlying Bullfrog Member intervenes between the members. 

In drill hole UE20j on Pahute Mesa, the Bullfrog Member 

is separated from the overlying peralkaline Tub Spring 

Member of the Belted Range Tuff (Noble, Sargent, and 

others, 1968; Orkild and others, 1969) by 51.8 m ( 170 ft) of 

bedded ash-fall tuff. In drill hole UE20f, lavas of peralkaline composition occur below the member, and the 

tuff of Tolicha Peak occurs above the member. In 

exposures just north of Redrock Valley and in drill cores in 

the Rainier Mesa area the Bullfrog Member occurs as a 

local nonwelded tuff at or near the top of tunnel bed 1 of 

the Indian Trail Formation (D. L. Hoover, oral commun., 

1974). 

In lithology and petrography the calc-alkalic Bullfrog 

Member of the Crater Flat Tuff is generally similar to the 

Redrock Valley Tuff but differs in containing significant 

euhedral quartz phenocrysts (fig. 5) as long as 2.5 mm. The 

member has a feldspar compositional zonation with 

plagioclase significantly in excess of alkali feldspar in the 

lower part and feldspars subequal in the upper part. The 

common biotite flakes in small (1-3 em) white pumice 

lenticles are an identifying criterion which, along with 

readily visible quartz phenocrysts, serve to distinguish the 

member from the Redrock Valley Tuff. The ash -flow sheet 

is brown and glassy in the lower part at the type locality of 

the Crater Flat Formation at the south side of Crater Flat, 

but elsewhere the Bullfrog Member is light yellowish gray, 

slightly mottled with yellow and pale reddish brown and 

microcrystalline from base to top. The sheet, where the top 
12 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

MAJOR 

VOLCANIC 

CENTER 

STRATIGRAPHIC UNIT< SUBUNIT 

Number of thin sections (N) in parentheses 

PHENOCRYSTS 

AS PERCENTAGE 

OF TOTAL ROCK 

FELSIC PHENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

X 

LIJ 

_J 

a.. 

~ 

0 

{.) 

<! 

a: 

LIJ 

0 

_J 

<! 

{.) 

>-

LIJ 

_J 

_J 

<! 

> 

~ 

(/) 

<! 

0 

I 

z 

<( 

1-

z 

::::> 

0 

~ 

a: 

LIJ 

co 

~ 

i= 

Tuff of Blacktop Buttes 

(Hinricks and others, 1967) (3) 

Stockade Wash(?) Tuff 

(Stockade Wash(?) Member 

• 

• -----~f Sargent and1 others, 196~._)_(_2_) ___ '-t-----+----1 

· · Upper lavas 

<! 

a: 

w 

0 

(20 thin sections, 

5 dri II holes1 ) 

<.> of Area 20 (6 sections, B (Sargent 1969; Orkild Lower 3 drill holesl) 

>- and others, 1969) lavas 

~ Lower 

I 

• 

-

;i Tuffs and rhyolites { Upper part 

<.> part (7) 

l2 -----·----·----?---?--?'--1-----+--- LIJ 

-l Stockade Wash Tuff 

(i) of this paper (7) 

Tuffs and rhyolites of Area 

20-Lithic-rich ash-flow tuffs 

(Orkild and others, 1969) (9) 

_j 

D Quartz 

~ Plagioclase 

T I 

~ 

0 

Peralkaline sequence of soda rhyolite lava, Grouse Canyon Member of Belted Range Tuff, 

~ Alkali feldspar 

I 

and tuffs and lava and tuff of Dead horse Flat (Noble, Sargent, and others, 1968, Orkild and others, 1968, 1969) 

""· l. Biotite-hornblende 

rhyolite west 

of Split Ridge 

Upper 

lava (5) 

Lower 

• 

J. 

J lava (4) • -----?~-----?---------?---4---~~--~ 

Tuffs of Sleeping Butte (9) 

<! 

a: -----?----?·-----?------+----+-----! 

LIJ 

0 

_J 

<! 

{.) 

w 

II-

::::> 

co 

(9 

z 

a.. 

LIJ 

LIJ 

-l 

(/) 

Crater Flat Tuff 

and intercalated -< 

Prow Pass 

Member (10) 

Lava flow ( 5) 

lava Upper 

Redrock 

Valley Tuff 

part ( 12) 

Bullfrog-< 

Member 

Lower 

part (10) 

Upper part (5) 

l Lower part (5) 

1 Th1n-sect1on modes of at least 2 cores from each dnll hole 

were averaged to reduce scatter, because 'too few phenocrysts 

were counted per thin section. 

-

-

-

-I J 

0 10 20 0 

PERCENT 

I 

20 

I 

40 

I 

60 80 

I 

100 

FIGURE 5.-Modal and silica ranges of Redrock Valley, Crater Flat, and Stockade Wash Tuffs and petrologically related lavas in general 
0 

TUFFS AND LAVAS RELATED TO SLEEPING BUTTE CALDERA 

MAFIC PHENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

- Total mafics 

~ Biotite 

E3l Hornblende 

~ Pyroxene 

I I I 

(Hornblende present in heavy 

mineral suite; see table 2 

not present in thin section) 

(Hornblende present 

in heavy mineral suite; 

not present in thin section) 

SILICA PERCENTAGE RANGE 

Number of analyses (N) 

in parentheses 

(H 2 0 and C0 2 free) 

(0) 

(0) 

Glassy • ~~ 

5 

(Hornblende present 1n heavy 

mineral suite; see table 2 

not present in thin section) 

.•. , .•... ·j 

(2) 

I 

(1) 

(0) 

Peralkaline sequence of soda rhyolite lava, Grouse Canyon Member of Belted Range Tuff, 

(2) (4) 

Glassy 

I ~ (1) (2) 

and tuffs and lava and tuff of Dead horse Flat (Noble, Sargent, and others, 1968, Orkild and others, 1968, 1969) 

- (0) 

I 

(1) 

~ ~Clinopyroxene 

~ 

(2) in uppermost unit 

~ (2) 

J 

~Orthopyroxene 

Glassy 

I 

(1) 

(2) 

Glassy 

I I 

(1) (1) 

I 

I l I 

(1) 

I I 

(1) 

I I 

10 15 20 0 2.5 5.0 7.5 70 72 74 76 78 

PERCENT 

I 

(1) 

13 

80 

order of stratigraphic succession. Units with queried boundaries may be in reverse stratigraphic order from that shown or may intertongue. 
/ 

14 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

is not eroded, has an upper part that contains crystals of 

the vapor-phase zone in pumice. In the Bullfrog Hills, the 

member is silicified and locally mineralized with sulfides 

and gold (Cornwall and Kleinhampl, 1964, p. ]22) and is 

moderately welded. In the Yucca Flat area, northern 

Frenchman Flat, and in drill hole UE20j (fig. 4), the 

member is non welded to slightly welded and the original 

glass is now altered to zeolites. 

In drill hole UE20f (fig. 4) where the tuff member was 

penetrated between depths of 3,500 and 3,650 m ( ll ,500 

and 12,000 ft), the biotite has been completely replaced by 

chlorite, and the feldspars have been partly replaced by 

sericite. All these differing facies and degrees of alteration 

of the ash-flow sheet might cause great difficulty in 

correlation were it not for the lateral persistence of a 

unique phenocryst assortment (fig. 5), general stratigraphic position, and certain lithologic features, such as 

small, flattened biotite-bearing pumice lenticles not 

destroyed by alteration. 

The petrochemistry (W. D. Quinlivan and P. W. 

Lipman, written commun., 1974) of the Bullfrog Member 

is similar to that of the underlying Redrock Valley Tuff 

(fig. 5) in that the lower part of the ash-flow sheet is 

slightly more mafic and less silicic than the upper part. 

Although sampling has been limited, field observations 

suggest no sharp break between the upper and lower parts; 

the subdivision is arbitrary and based on petrography. 

INTERCALATED LAVA FLOW 

A brownish-gray fluidal lava flow overlying the Bullfrog Member but not part of the Crater Flat Tuff is exposed 

in fault blocks in the south wall of the Oasis Valley caldera 

segment (fig. 4). The lava pinches out a short distance 

south of the wall, for exposures of both members of the 

Crater Flat Tuff at Prow Pass and at the north end of 

Crater Flat include only intervening bedded tuff. A downfaulted portion of the lava is doubtless buried beneath 

younger volcanic rocks in the Oasis Valley caldera 

segment. The exposed part of the lava flow does not exceed 

60 m (200 ft) in thickness. 

The Prow Pass Member rests directly on the lava, but as 

much as 15m (50ft) of bedded tuff separates the lava from 

the underlying Bullfrog Member. 

The lava superficially resembles the underlying Bull 

frog Member, but its fluidal flow banding identifies it as a 

lava flow. The lower part locally includes a vitrophyre, 

whereas the brownish-gray uppermost part contains 

spheroidal white pumice with abundant fine biotite 

phenocrysts resembling the tuff. The thin-section modes 

of three specimens of the lava are well within the range of 

22 modes of the Bullfrog Member (fig. 5 ). Two chemical 

analyses of the lava (W. D. Quinlivan and P. W. Lipman, 

written commun., 1974) indicate that the devitrified 

specimen is several percent higher in Si02 content than is 

the specimen of the basal vitrophyre (fig. 5 ). 

PROW PASS MEMBER 

The Prow Pass Member of the Crater Flat Tuff is here 

named after Prow Pass, the type locality, at the north end 

of Yucca Mountain (fig. 4). At the type locality at Prow 

Pass, 15.2 m (50ft) of devitrified welded tuff constitutes the 

upper cooling unit of the Crater Flat. The member is 

slightly thicker and more densely welded in the northern 

part of the Crater Flat area than it is in the southern part. 

At the type locality of the Crater Flat Tuff (fig. 4), the 

member is 50 m (160ft) thick, but is partly glassy and only 

slightly welded. Near the eastern and southeastern edges of 

its extent, the Prow Pass Member is about 15m (50ft) thick 

and, like the Bullfrog Member, it is nonwelded and 

zeolitized. The member has not been recognized and 

probably is not present outside the Timber 

Mountain-Oasis Valley caldera complex north of the area 

shown in figure 4. The estimated extracaldera complex 

volume of the unit is only about 30 km3 (8 mi3), but the 

member may be several times thicker within the complex 

where an equal or greater volume could be present. 

The Prow Pass Member underlies all the constructional lavas of the calcic Wahmonie-Salyer volcanic center 

(Poole, Elston, and Carr, 1965; Poole, Carr, and Elston, 

1965; Noble and others, 1965) and the rhyolite lavas of the 

Calico Hills (Christiansen and Lipman, 1965). The age 

relations to the post-Crater Flat tuffs on the west rim of 

Sleeping Butte caldera segment are unknown from stratigraphic relations but the member may be older, on the 

basis of an inferred upward increase of phenocrystic alkali 

feldspar (fig. 5 ), typical of the later volcanic suites. The age 

relations to the peralkaline Tub Spring Member of the 

Belted Range Tuff (Sargent and others, 1965) are 

uncertain, but the member underlies a phenocryst-poor 

green peralkaline ash-fall tuff that underlies the Grouse 

Canyon Member of the Belted Range Tuff (fig. 5). 

The calc-alkalic Prow Pass Member has some general 

similarities in lithology to the Bullfrog Member, but it is 

thinner, generally less welded, and practically devoid of 

biotite. The unit is generally light gray, grayish pink, or 

moderate orange pink where devitrified, but is light brown 

where glassy at the type locality of the Crater Flat. 

Petrographically, it is a unique unit of all the silicic tuffs 

of the region in that it contains orthopyroxene as the 

dominant mafic phenocryst (fig. 5) and, in the extreme, 

almost lacy resorption of the quartz phenocrysts. No other 

ash-flow sheet in the region could be mistaken for this 

unit. However, orthopyroxene is preserved only in the 

glassy upper and lower parts of the cooling unit at the type 

locality of the Crater Flat Tuff; elsewhere the unit is 

devitrified and only smudgy opaque oxide pseudomorphs 
TUFFS AND LAVAS RELATED TO SLEEPING BUTTE CALDERA 15 

remain. The silica contents of two devitrified specimens 

are shown graphically in figure 5. 

TUFFS OF SLEEPING BUTTE AND THEIR RELATION TO 

CALDERA WALL 

Rocks under this heading comprise a sequence of three 

similar ash-flow tuff cooling units occupying the west rim 

of the Sleeping Butte caldera in the vicinity of Sleeping 

Butte (fig. 4). The caldera rim is delineated for several 

miles by the axis of an easterly dipping monocline in the 

tuff. The tuffs of Sleeping Butte are slightly more than 300 

m (1,000 ft) thick at Sleeping Butte, where the upper 

cooling unit is well exposed. Along the west rim of the 

caldera the monocline exposes 150-240 m (500-800 ft) of 

the tuffs. The outcrop area shown in figure 4 is delimited 

by the onlap of younger rocks. 

The monocline dips easterly under the peralkaline 

Grouse Canyon Member of the Belted Range Tuff and 

younger rocks, which fill the Sleeping Butte caldera 

segment. Near the southern end of the exposure, relatively 

unfractured but locally vertically dipping tuffs of Sleeping 

Butte probably onlap the highly fractured caldera wall 

consisting of the Redrock Valley Tuff and the Bullfrog 

Member of the Crater Flat Tuff, although the contact is 

not exposed. The local vertical attitude in the onlapping 

tuffs of Sleeping Butte strikes north, parallel to the 

inferred caldera wall, and is typical of the most easterly 

exposure. The steep initial dip of the tuffs probably 

occurred as they were compacted and plastered against the 

Sleeping Butte caldera wall. 

The inferred younger age relation of the tuffs of 

Sleeping Butte to the Prow Pass Member of the Crater Flat 

Tuff is suggested partly by the close stratigraphic 

succession of the units of the Crater Flat Tuff, and partly 

by the increase of alkali feldspar upward. The unfractured 

nature of the tuffs with respect to the highly fractured 

Bullfrog Member also indicates their younger age, 

probably as postcaldera tuffs draped over a wall composed 

of the Bullfrog Member and older rocks. 

The three ash-flow cooling units of the tuffs include a 

lower non welded to partly welded, phenocryst-poor shard 

tuff (not shown in figure 5 ), a middle purple welded tuff 

that contains fluidal flow banding where it dips inward 

(easterly) into the caldera, and an upper tuff, mainly 

exposed around Sleeping Butte. All the tuffs of Sleeping 

Butte are mostly light purplish gray, light brown, and 

microcrystalline; basal vitrophyres are practically nonexistent and the rocks are commonly mildly silicified and 

sericitized to the extent that former plagioclase phenocrysts are completely gone. Xenoliths are common and 

consist mainly of altered silicic volcanic rock and less 

common pilotaxitic intermediate and mafic lava. The 

fluidal flow banding of the middle unit, locally vertical, 

occurs only at its eastern margin where it dips into the 

inferred Sleeping Butte caldera. 

The thin-section modes of the middle and upper tuffs of 

Sleeping Butte are pooled graphically in figure 5. The 

lower shard-rich tuff is phenocryst-poor (2 to 3 percent 

phenocrysts) somewhat altered, and therefore not pooled 

with the overlying units. Its phenocryst percentages, based 

on only two sections, are roughly similar to those of the 

middle and upper unit; the chemical analysis (W. D. 

Quinlivan and P. W. Lipman, written commun., 1974) 

indicates a more silicic rock at 75.1 percent Si02 (waterfree) than the two phenocryst-rich specimens of the middle 

and upper units (fig. 5 ). The middle and upper units are 

similar in modal petrography except for sparse 

pigeonitic(?) clinopyroxene (low 2V, slightly pleochroic 

in pale green and yellow) in addition to hornblende in the 

upper unit. This is the oldest unit containing significant 

clinopyroxene phenocrysts. The presence of pigeonitic 

(low Ca) clinopyroxene in the highest stratigraphic unit of 

the tuffs of Sleeping Butte is analogous to the occurrence 

of orthopyroxene (low Ca) in the Prow Pass Member, the 

uppermost cooling unit of the Crater Flat Tuff. 

High potassium (W. D. Quinlivan and P. W. Lipman, 

written commun., 1974) in all three tuffs is due partly to 

very minor sericite replacing plagioclase, but the 

dominantly alkali feldspar phenocrysts indicate that most 

of the potassium is primary. Therefore, these youngest 

rocks of the Sleeping Butte caldera center approach the 

alkali-calcic composition of the overlying Paintbrush and 

Timber Mountains Tuffs and may be products of a late 

alkali enrichment in the upper part of the Sleeping Butte 

magma chamber. 

TUFF OF TOUCHA PEAK 

The tuff ofTolicha Peak (Noble and Christiansen, 1968; 

Rogers and others, 1968) is a local phenocryst-poor 

densely welded shard tuff, about 60 m (200 ft) thick, that 

occurs above the Bullfrog Member of the Crater Flat Tuff 

in drill hole UE20f (fig. 4; Orkild and others, 1969, cross 

section A-A'). The unit crops out in the north half of the 

Black Mountain IS-minute quadrangle (fig. 2) and in the 

southwest quarter of the quadrangle in the vicinity of 

Quartz Mountain and Tolicha Peak (fig. 1); it was 

penetrated in a drill hole in eastern Yucca Flat. The unit is 

distinctive lithologically and also petrographically in that 

it contains sparse quartz and plagioclase dominant over 

alkali feldspar. The tuff may be related to the Sleeping 

Butte caldera (Christiansen and others, 1976); however, E. 

B. Ekren (oral commun., 1974; Ekren and others, 1971, p. 

60) has evidence suggesting a source from the north in the 

Mount Helen area (fig. 4). 

BIOTITE-HORNBLENDE RHYOUTE LA VAS 

WEST OF SPLIT RIDGE 

Two calc-alkalic rhyolite lava flows and underlying 

petrographically similar ash-flow and ash-fall tuffs intertongue with peralkaline members of the Belted Range 
16 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

Tuff and related lavas (Sargent and others, 1965; Noble, 

Sargent, and others, 1968) in the triangular salient 

between Silent Canyon caldera, Timber Mountain 

caldera, and the peralkaline lava pile of Split Ridge (fig. 

4). These rocks occupy approximately the same stratigraphic position as the tuffs of Sleeping Butte; they also 

contain hornblende but are petrographically more mafic 

like the Redrock Valley Tuff (fig. 5). They are therefore 

generally similar to other calc-alkalic rocks grouped under 

Sleeping Butte caldera and are included here for 

convenience. Magnetic polarity determinations by G. D. 

Bath (written commun., 1965) indicate a reverse orientation in these rocks in contrast to normal remanent 

magnetization of the enclosing peralkaline rocks. 

The two lavas and underlying tuffs were called quartz 

latite flows and quartz latite tuff on the geologic map of 

the Ammonia Tanks quadrangle (fig. 2; Hinrichs and 

others, 1967); however, the one analyzed vitrophyre of the 

lower, more mafic, plagioclase-rich lava is chemically 

rather silicic at 73.9 percent Si02 (fig. 5 ), and total alkalis 

are 7 percent (W. D. Quinlivan and P. W. Lipman, written 

commun., 1974); hence these rocks are called rhyolite in 

this report and on the geologic map of the Timber Mountain area (Byers and others, 1976). Hinrichs, Krushensky, 

and Luft (1967) show a maximum thickness of 364 m 

(1,195 ft), comprising 114m (375ft) of underlying tuff, 174 

m (570 ft) of lower lava, and 76 m (250ft) of upper lava. 

The lower flow is a thick lenticular exposure in a short 

canyon tributary to Timber Mountain caldera; the over1 ying and underlying units are more stratiform and do not 

pinch out within the exposed area. 

The biotite-hornblende tuff-lava sequence west of Split 

Ridge is transected by green, peralkaline feeder dikes and 

vent breccias of the rhyolite complex of Split Ridge and is 

also overlain by greenish-gray peralkaline bedded tuffbreccia under the rhyolite lava of Split Ridge, which in 

turn underlies the Grouse Canyon, the upper member of 

the Belted Range Tuff. The sequence overlies greenishyellow peralkaline bedded tuff that in turn overlies the 

Tub Spring, the lower member of the Belted Range Tuff. 

The basal tuffs include two, possibly three, nonwelded 

ash-flow tuffs as much as 23 m (75 ft) thick, and intercalated bedded tuff. They are yellowish gray, weather 

lighter gray, and are typical of calc-alkalic zeolitized tuff 

stratigraphically between the Grouse Canyon Member 

and the Crater Flat Tuff in rhe area between Yucca Flat 

and Timber Mountain caldera (fig. 4). The phenocryst 

mineralogy of these basal tuffs is generally similar to that 

of the overlying biotite-hornblende rhyolite lavas. 

The lavas superficially resemble other calc-alkalic 

rhyolite lavas associated with the Timber Mountain-Oasis 

Valley caldera complex. Both lavas have a glassy breccia 

envelope, a dark basal vitrophyre, and a purplish-gray 

fluidal flow-banded middle part with conspicuous small 

phenocrysts,less than 2 mm, of feldspar, biotite, and hornblende (fig. 5 ). The occurrence of a few phenocrysts of 

sphene in every thin section of these lavas contrasts with 

the sporadic occurrence of sphene in other volcanic rocks 

of the Sleeping Butte caldera. 

PERALKALINE ROCKS OF SILENT CANYON 

CALDERA 

A complete description of the peralkaline rocks related 

to the Silent Canyon caldera center is beyond the scope of 

this paper; these rocks are described elsewhere (Sargent 

and others, 1965; Noble, Sargent, and others, 1968; Orkild 

and others, 1968; Orkild and others, 1969; Sargent, 1969; 

Noble and others, 1969; Noble, 1970; and Christiansen and 

others, 1976). These unique rocks, however, provide an 

extremely helpful datum for stratigraphic assignments of 

the rocks of the Timber Mountain-Oasis Valley caldera 

complex and, therefore, are mentioned briefly here. 

The comenditic Tub Spring and Grouse Canyon 

Members constitute, respectively, the lower and upper ashflow sheets of the Belted Range Tuff (Sargent and others, 

1965 ). Many peralkaline lava flows, including the rhyolite 

lava of Split Ridge, are genetically related to these sheets 

and occur within and around the Silent Canyon caldera 

center (Noble, Sargent, and others, 1968). The Tub Spring 

Member overlies the Crater Flat Tuff in drill hole UE20j 

(fig. 4), and the exposed uppermost part of the Tub Spring 

underlies the calc-alkalic biotite-hornblende rhyolitic 

rocks west of Split Ridge. The Grouse Canyon Member is 

much more widespread, covering about 8,000 km2 (3,000 

mi2) (Noble, Sargent, and others, 1968, p. 67), and 

represents the peak of peralkaline igneous activity. The 

maximum extracaldera thickness is about 90 m (300ft), 

and the original volume of the member is about 200 km3 

(50 mi3). The Grouse Canyon and related peralkaline 

rocks (fig. 5) separate the underlying calc-alkalic rocks of 

the Sleeping Butte caldera from overlying similar calcalkalic effusives that were erupted from the Area 20 center 

late in the development of Silent Canyon caldera. 

STOCKADE WASH TUFF 

AND RELATED CALC-ALKALIC ROCKS 

ASSOCIATED WITH SILENT CANYON CALDERA 

The Stockade Wash Member was first named by 

Hinrichs and Orkild (1961, p. D98) as a unit of the Oak 

Spring Formation from the thick exposure in Stockade 

Wash south of Rainier Mesa (fig. 6). The member was 

mapped in Rainier Mesa (Gibbons and others, 1963) and 

Tippipah Spring quadrangles (fig. 2; Orkild, 1963). Poole 

and McKeown (1962, p. C6l) raised the rank of the Oak 

Spring to group status and divided it into the Indian Trail 

and Piapi Canyon Formations. Their Piapi Canyon 

included, in ascending order, the Survey Butte, Stockade 

Wash, Topopah Spring, Tiva Canyon, and Rainier Mesa 
STOCKADE WASH TUFF AND ROCKS ASSOCIATED WITH SILENT CANYON CALDERA 

CALDERA 0 

0 0 

H U T F. 

0 

0 0 

0 

\ 

\ 

/ 

I 

:Tuff of Blackt~ ~ · Buttes ~: ""'-

~ / '":> ,-- :'\ _.: ·""·~· '-'· t -~ _)_:~· I 

• 

0 

- -0-?-

~.)I S•o<kode Wo.h{?) Toll •a]+ '

-·-·-·-

116' 

EXPLANATION 

Locality - Approximate thickness known 

Drill hole 

lsopachs, in metres - Queried where control is 

poor 

OAK SPRING ~ 

BUTTE- /-

MOUNTAIN 

JACKASS 

FLATS 

0 5 

. ... ~ 

::'c",.l, ........ 

, ...... •Ill',, 

MINE : -:. __ MTN ~ 

-=:,.1n•'''' 

WAHMONIE-SAL YER 

VOLCANIC CENTER 

0 0 

10 15 MILES 

---~--L--,----~L-_______ 1 

0 5 10 15 KILOMETRES 

FIGURE 6.-Isopachs of Stockade Wash Tuff, Stockade Wash(?) Tuff, and tuff of Blacktop Buttes in relation to Silent Canyon caldera. 

17 

Members. Later Orkild (1965) abandoned the Oak Spring 

Group and raised the Piapi Canyon to group, consisting 

of the Paintbrush and Timber Mountain Tuffs. His subdivision of these formations, which included the Stockade 

Wash Member at the base of the Paintbrush, is shown in 

the left column of figure 7. 
18 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

ORKILD (1965) 1 THIS REPORT 

NEVADA TEST SITEWEST AND NORTH AREAS 

NEVADA TEST SITE AND VICINITY 

OUTSIDE KNOWN CALDERA 

AREAS INCLUDING Y UCCA FLAT 

CLAIM CANYON CAULDRON 

SEGMENT AND PAINTBRUSH 

CANYON TUFF TYPE SECTION 

' ' Ammonia Tanks Member 4 z u.. Zu.. ::J u.. ::Ju.. Tuff of Transvaal 4 0 ::J O::J Tuff of Cat Canyon 4 :;;: f- :;;:fa:z a: z Bedded tuff w- (I) w 

CD <! co<! ~ f-

:;;:f- Rainier Mesa Member f= Rainier Me'sa Member f= 

Tiva Canyon Member Tiva Canyon Member u.. u.. u.. u.. ::J ::J f- u.. 

u.. 

::J 

fYucca Mountain Member f- Yucca Mountain I 

I 

(/) 

::J 

a: 

Bedded tuff 

I 

(/) 

::J 

a: 

CD 

fz 

~ 

Member (/) 

::J 

a: 

CD 

Bedded tuff fz tuff 

<! 

0. CD 

fz Pah Canyon Member 0. Pah Canyon 

~ Member 

0. 

Topopah Spring Member 

Stockade Wash Member 

Local informal units 

BELTED 

Bedded 

and ash.fiO\\, tuff 

ofArea20 

Local bedded tuff of 

peralkaline composition 

RANGE Grouse Canyon Member 

TUFF 

BELTED 

RANGE 

TUFF (peralkl ine composition) 

1Geometry of Orkild's (1965) diagram slightly altered in an attempt to make it 

fit new information presented here and still retain the stratigraphic relations in 

his diagram. 

'Noble. Krushensky, McKay. and Ege (1967). 

' Hinrichs, Krushensky, and Luft (1967). The Pah Canyon Member shown in 

their section A-A ' is a local unit of the Paintbrush Tuff that seemingly grades 

upward into the overlying pyroxene-bearing rhyolite lava. See text. 

'Ammonia Tanks Member as mapped by Orkild, Sargent, and Snyder (1969) in 

most places around the rim of Timber Mountain caldera is the upper part of the 

Ammonia Tanks of this report and the main part as mapped by Noble, 

Krushensky, McKay, and Eg< (1967). The tuff of Transvaal of Orkild·(l 965) is 

equivalent to all the lower part of the Ammonia Tanks Member as mapped by 

Noble, Krushensky, McKay, and Ege (1967). The intracaldera tuff of Cat Canyon 

as mapped by Carr and Quinlivan (1966) is equivalent to the entire Ammonia 

Tanks of this report and is a single compound cooling unit. See text. 

5The former Stockade Wash Member of the Paintbrush Tuff was placed above 

the tuffs and rhyolites of Area 20 by Orkild, Sargent, and Snyqer (19691 but is 

now known to be stratigraphically equivalent to some part. possibly near the 

middle, of the tuffs and rhyolites of Area 20. 

FIGURE 7 (above and right).-Stratigraphic relations and revisiOns of Stockade Wash, Paintbrush, and Timber Mountain Tuffs, 

Timber Mountain caldera and vicinity. Dark-shaded units intertongue with these named tuffs but are not part of them. Diagonal 

ruling indicates unit is not present; light shading, bedded tuff. 

The Stockade Wash Member is herein removed as the 

basal unit of the Paintbrush Tuff and raised to formational rank (second column, fig. 7) because it is lithologically and petrologically unlike the ash-flow tuff units 

of the Paintbrush and appears more closely related to the 

calc-alkalic tuffs and rhyolite lavas of Area 20 in Silent 

Canyon caldera. The areal distribution of the Srockade 

Wash Tuff and closely related units is shown in figure 6_ 

The tuff reaches its maximum thickness of slightly more 

than 120m ( 400 ft) along the west side of Rainier Mesa and 

also in the low hills north of Frenchman Flat (fig. 6) where 

Hinrichs and McKay (1965) mapped the unit as the 

Stockade Wash(?) Member. Subsequent field and petrographic work have established the identity of their unit in 

the northern Frenchman Flat area with that at the type 

section. The volume of the Stockade Wash Tuff is 
STOCKADE WASH TUFF AND ROCKS ASSOCIATED WITH SILENT CANYON CALDERA 19 

T HIS R EPORT ORKILD, SARGENT AND SNYDER 

(1969) AND OT HERS 23 

TIMBER MOUNTAIN CALDERA TYPE AREA PAHUTE MESA COMPOS ITE 

AND OASIS VALLEY CA L DERA SEGMENT SILENT CANYON CALDERA (ONLY) (INCLUDES SILENT CANYON 

CALDERA AND ADJACENT AREAS) 

Bedded 

· tuff Ash-fall and reworked tuff 

(bedded tuff) 

Ash-fall and reworked tuff 

(bedded tuff) 

Zu_ ' 

:Ju. 

O:J 

::2:fa:z 

~~ ::2:ffAmmoni a Ta nks 

Member 

Bedded tuff 

Rainier Mesa Member 

Pre-Timber Mountain 

volcanic rocks 

Upper part 

Lower part 

Debris flows 

(not exposed or penetrated 

in dril l hole) 

' z u_ :J u_ o:J 

::2: fa: z 

~<( 

2 ffu_ 

u_ 

:J 

fI 

(/) 

:J 

a: 

C!l 

fz 

<( 

c.. 

Ammon ia Tanks 

M ember 

Upper part 

Lower part 

Bedded tuff 

Rainier Mesa Member 

' Ammonia Main part 2 •

4 

Zu_ Tanks :Ju_ 

O:J Member Lower part 2 •

4 

2ra:z ~ollte I~.Z,T~ w- "" ln ra rriDat 

co<( Tuff of 

::2:f- Falcon Canyon 

-

f- Rainier Mesa Member 

Bedded ( ouart z·rich rhyolite lava ) ~ 

tuff Quartz-bearin rhyolite lava E 10t1te r o 1te ava -o 

~////////////////////~ ~ .><: 

Tiva Canyon "' 0.. " Member E 

"' 

~(~(&$0:v(~ .r:: 

u_ "' 2 u_ u 

:J < (/) 

f- Tuff of B lackto p Buttes 0 

I "' ~v:~:iltlr law 

> 

(/) ..J "' :J 

a: Pah Canyon Member 3 

C!l 

fz 

<( 

3~ 

c.. Bedded tuff 

Topopah Spring Member 

Bedded tuff 

Stockade Wash Member 5 

Tuffs and rhyolite lavas of Area 20 

(Stockade Wash Tuff not recognizable 

in Silent Cany on Caldera) Tuffs ar1d rhyolites of Area 20 

Lava and tuff of Dead horse Flat 

(peralkaline composition) Lava and tuff of Dead horse FIat 

BELTED 

RANGE 

TUFF 

probably between 20 and 40 km3 (5 and 10 mi3 ), depending 

on its largely unknown thickness and extent under Yucca 

and Frenchman Flats. 

The areal distribution pattern of the Stockade Wash, 

trending generally southeast from Silent Canyon caldera, 

is similar to that of the peralkaline ash fall of the Belted 

Range Tuff (Noble and others, 1968, fig. 2), which came 

from Silent Canyon caldera, and is unlike the distribution 

patterns of members of the Paintbrush Tuff (figs. 8, 9) 

which originated in the southern or southwestern part of 

the Timber Mountain-Oasis Valley caldera complex. The 

closest exposure of the Stockade Wash Tuff to the type 

section of the Paintbrush is about 15 miles. 

The Stockade Wash Tuff, where present in the Rainier 

Mesa and Yucca Flat areas, rests on zeolitic tuffs of the tuffs 

and rhyolite lavas of Area 20 but has not been recognized in 

drill holes inside Silent Canyon caldera (figs. 6 and 7). The 

lithic-rich ash-flow tuff in the lower part of the tuffs and 

rhyolite lavas of Area 20 within Silent Canyon caldera 

Grouse Canyon Member 

(pera lkaline composition) 

BELTED 

RANGE 

TUFF 

Grouse Canyon Member 

(Orkild and others, 1969) has also been recognized in 

several drill holes in Yucca Flat where it reaches a 

maximum thickness of 64 m (210ft). The lithic-rich ashflow tuff is separated from the overlying Stockade Wash by 

less than 10m (35ft) of zeolitic lithic-rich bedded tuff and 

rests on the Grouse Canyon Member of the Belted Range 

Tuff. The Stockade Wash Tuff is overlain by quartz-free 

friable glassy bedded tuff of the Paintbrush Tuff (Orkild, 

1965), although, locally, a few metres (10ft) of zeolitic 

lithic-rich bedded tuff occurs between the Paintbrush and 

the Stockade Wash. The best correlation of the Stockade 

Wash, based on petrography, is with the lower part of the 

lower rhyolite lava of the tuffs and rhyolites of Area 20 (see 

fig. 5 ), analogous to the relation of the Bullfrog Member 

of the Crater Flat Tuff with the petrographically similar 

lava (fig. 5 ). The Stockade Wash Tuff was probably 

erupted from the same magma as the lower lavas just 

before, in between, or just after the sequence of several 

lower rhyolite lavas of the tuffs and rhyolites of Area 20, 
20 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

which underlie the Paintbrush Tuff (fig. 7; Orkild and 

others, 1969). Therefore, where zeolitic quartz-bearing 

lithic-rich tuffs are present beneath or just above the 

Stockade Wash, they are assigned to the tuffs and rhyolite 

lavas of Area 20, rather than to the Paintbrush Tuff (fig. 7). 

The Stockade Wash Tuff is a simple cooling unit of 

massive nonwelded to partly welded ash-flow tuff having 

4-9 percent of small phenocrysts generally less than l mm 

in length. It is characterized in the field by randomly 

oriented orange pumice fragments that average half an 

inch irr length in a pale-yellowish-gray to light-yellowishbrown matrix. Locally the tuff is zeolitized at the base 

(Hinrichs and Orkild, 1961, p. D98; Gibbons and others, 

1963). Despite its partly welded character, the Stockade 

Wash Tuff is a cliff-former and locally south of Stockade 

Wash contains rosettes of closely spaced polygonal joints 

possibly localized by fumaroles during cooling. 

The Stockade Wash Tuff lithologically and mineralogically resembles two local massive non welded to very 

slightly welded simple cooling units, one of which is 

found outside the Silent Canyon caldera on the east and 

the other within the caldera (fig. 6). The one east of Silent 

Canyon caldera was mapped as Stockade Wash(?) Member 

of the Paintbrush Tuff in Quartet Dome quadrangle (fig. 

2; Sargent and others, 1966). This cooling unit is likewise 

raised in rank to Stockade Wash(?) Tuff. The other cooling 

unit that resembles the Stockade Wash is the tuff of 

Blacktop Buttes (cols. 5 and 6, fig. 7; Hinrichs and others, 

1967; Orkild and others, 1969), which is entirely confined 

within the Silent Canyon caldera (fig. 6). This tuff intertongues with the Paintbrush Tuff (as restricted in this 

paper) and was mapped with the Paintbrush on two 

geologic maps (Hinrichs and others, 1967; Orkild and 

others, 1969). Because of its lithologic similarity to the 

Stockade Wash, however, this tuff is herein regarded as 

local informal unit separate from the Paintbrush. 

The Stockade Wash and Stockade Wash(?) Tuffs and the 

tuff of Blacktop Buttes are all characterized by sparse to 

abundant dark-greenish-gray dense devitrified xenoliths 

of peralkaline volcanic rock. These xenoliths closely 

resemble the peralkaline rocks, including the devitrified, 

microcrystalline facies of the rhyolite of Split Ridge and 

the Grouse Canyon Member of the Belted Range Tuff 

from the floor and the rim of Silent Canyon caldera. We 

therefore infer that these units were extruded from vents 

inside or on the rim of Silent Canyon caldera. 

Average modal analyses of the Stockade Wash Tuff, 

Stockade Wash(?) Tuff, and tuff of Blacktop Buttes are 

compared in figure 5 with the lavas of the tuffs and 

rhyolites of Area 20; the intracaldera lavas are arbitrarily 

placed together. The modes suggest that the Stockade 

Wash and Stockade Wash (?) Tuffs each may generally 

correlate with the lower and the upper lavas, respectively, 

of the tuffs and rhyolites of Area 20. The Stockade Wash(?) 

Tuff, therefore, probably slightly postdates the type 

Stockade Wash and is a separate cooling unit. The tuff of 

Blacktop Buttes is known to be the youngest, for it postdates all the lavas of Area 20 (Orkild and others, 1969) and 

intenongues with two lavas related to the Paintbrush Tuff 

(cols. 5 and 6, fig. 7). In contrast with thin-section modes of 

Paintbrush Tuff units, the Stockade Wash and related 

tuffs contain common to abundant quartz and no 

clinopyroxene, although clinopyroxene has been found as 

a minor constituent of heavy-mineral separates. 

Percentages of nonmagnetic heavy mineral separates 

from the Stockade Wash Tuff and probable related calcalkalic units associated with the Silent Canyon caldera are 

compared with those separated from some units of the 

Paintbrush Tuff in table 2. The tuffs and rhyolite lavas of 

Area 20, the Stockade Wash Tuff, the Stockade Wash(?) 

Tuff, and the tuff of Blacktop Buttes are all characterized 

by a high allanite/ sphene ratio and very low content of 

augite in contrast to alkali-calcic tuff units of the Paintbrush. In the Topopah Spring Member, however, the 

paucity of both allanite and sphene makes the ratio 

insignificant for comparison. None of the three Paintbrush units contain orthopyroxene in the heavy mineral 

assemblage, but the Stockade Wash Tuff and tuff of 

Blacktop Buttes each contain l percent. The trace amounts 

of aegirine-augite in the tuffs and rhyolite lavas of Area 20, 

as well as in the Stockade Wash (?) Tuff and in the tuff of 

Blacktop Buttes, may have been derived from abundant 

peralkaline xenoliths. 

Chemical analyses of the lavas of Area 20 and the 

Stockade Wash Tuff were made (W. D. Quinlivan and P. 

W. Lipman, written commun., 1974); silica percentages of 

glass and divitrified rocks are plotted in figure 5. The lavas 

of the tuffs and rhyolites of Area 20 become progressively 

more silicic upward. The devitrified specimens are 

relatively enriched in silica with respect to the glassy 

specimens. 

In summary, the Stockade Wash and the tuff of Blacktop 

Buttes are removed from the Paintbrush Tuff and from the 

Pia pi Canyon Group. The Stockade Wash is raised in rank 

to a separate tuff formation, because it is lithologically and 

petrographically unlike any units in the Paintbrush Tuff 

but does show lithologic and petrographic affinities to late 

effusives occurring entirely within the Silent Canyon 

caldera. The Stockade Wash(?) Member of Sargent, Luft, 

Gibbons, and Hoover (1966), which occurs just east of and 

outside the Silent Canyon caldera, is also raised in rank 

but is retained as a queried unit because its petrographic 

features suggest that it correlates with a slightly higher 

part of the intracaldera sequence. The tuff of Blacktop 

Buttes (Hinrichs and others, 1967; Orkild and others, 

1968), the youngest ash flow entirely confined to the Silent 

Canyon caldera, is regarded as an informal unit not 

assigned to a formally named formation. 
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 21 

TABLE 2.-Nonmagnetic heavy minerals of Paintbrush and Stockade Wash Tuffs and related tuffs and lavas in order of stratigraphic succession 

[Stockade Wash Tuf£ and related units in italic type; r, mineral rare] 

Unit and subunll 

(number of 

specimens) 

~ ~ 

it c 

~ ; ~ -~ c. " " 0 

0 -~ iiD -5 ~ iii -< ~ 0 

Paintbrush Tuff: 

Tiva Canyon Member: 

Quartz latite (2) ... ........ 19 31 0 0 

Tuff of Blacktop 

Buttes (1) ............. 76 2.6 1.0 

Paintbrush Tuff: 

Local unit in 

hok lJE19f (I). 57 6 0 0 

Paintbrush Tuff: 

Topopah Spring Member: 

Quanz Ia tile (I) ..... ............... 80 18 0 0 

Stockade Wash(?) 

Tuff (1) .............. ..................... 229 1.6 r 0 

Stockade Wash Tuff (I) 72 0.4 0 1.0 

Rhyolite lavas of 

Area 20 (14) ....... 79 0.1 

Lithic-rich tuffs 

of Area 20 (8) .............. ...... ...... 42 0.1 0.3 0 

1Luw <.:Oillt'lll of al lan ue and probable lumt of detection of spht'nt' m Topopah Spnng 

not ~ignif nm t fi>r this comparison. 

PAINTBRUSH TUFF AND ROCKS RELATED TO 

CLAIM CANYON CAULDRON 

GENERAL FEATURES 

ORIGINAL AND REDEFINITION OF THE PAINTBRUSH TUFF 

The Paintbrush Tuff was defined by Orkild (1965, p. 

A49) from exposures in Paintbrush Canyon, a small gulch 

6.4 km (4 mi) northeast of Yucca Mountain (figs. l, 8). The 

most complete stratigraphic section, however, is on the 

north end of Yucca Mountain where four members are 

well exposed. The Topopah Spring and Tiva Canyon 

Members are widespread welded ash-flow sheets that were 

first defined as members of the former Oak Spring 

Formation (Hinrichs and Orkild, 1961, p. D97) in the 

Yucca Flat area (figs. l, 2). These members were subsequently included in the Piapi Canyon Formation of the 

Oak Spring Group (Poole and McKeown, 1962, p. C6l). 

The Yucca Mountain Member of the Piapi Canyon 

Formation was described by Lipman and Christiansen 

( 1964) and occurs between the Topopah Spring and Tiva 

Canyon Members (fig. 7) in the Yucca Mountain area (fig. 

8). Orkild (1965) included the Stockade Wash Member 

(Stockade Wash Tuff of this paper), the Topopah Spring 

Member, a newly defined Pah Canyon Member, and the 

Yucca Mountain and Tiva Canyon Members in his Paintbrush Tuff (fig. 7) of the Piapi Canyon Group. 

The Paintbrush Tuff is here redefined to include, in 

ascending stratigraphic order, the Topopah Spring, Pah 

Canyon, Yucca Mountain, and Tiva Canyon Members 

and similar, quartz-poor, informal ash-flow and ash-fall 

Volume percent of total heavy minera ls 

biJ 

c 

'5 ;.. ~ 

~-

~ 5 vs ~ -" ., .: ~ 0 

c 

~ 

-~ :~ 0. ~ ~ -~ .;V :0 c ~ c ·a .g " 3 R 0 E ·a. 0 -" ~ [ ' ~ 0. ·:<::! ;I) 

:t <J) < N -< \.) < 

9 18 12 9 1.3 0 0.7 

0.4 1.2 12 6 0 0 10 

0 24 8 6 0.8 0 0.3 

0 0 0.8 0.4 0 0 (') 

59 0.6 6.0 2.2 1.0 r 10 

18 0.2 3.4 6 2.2 0 17 

5.2 0.2 12 3.6 0.2 0 60 

r 33 21 1.2 1.8 +99 

2Biotite ~rccnta~;e disproponionately low with respect to other mafic minera ls owing 

to oxidation to me:tgnctite. 

tuffs that occur between the top of the underlying Stockade 

Wash Tuff, as herein defined, and the base of the overlying Rainier Mesa Member of the Timber Mountain 

Tuff, except for thin local bedded lithic-rich tuff that 

occurs just above the Stockade Wash (fig. 7). Excluded are 

the Stockade Wash Member, which was part of the 

original definition (Orkild, 1965 ), and also quartzbearing, lithic-rich tuffs of the tuffs and rhyolites of Area 

20, which occur between the Belted Range and Stockade 

Wash Tuffs and locally just above the Stockade Wash 

Tuff. In the Yucca Flat-Rainier Mesa area, these lithic-rich 

tuffs were included in the Paintbrush by Orkild (1965) as 

part of the now-abandoned Survey Butte Member (Poole 

and McKeown, 1962; Orkild, 1965) before the stratigraphic position of the lithic-rich tuffs in the subsurface of 

Area 20 was known (Orkild and others, 1968 and 1969). 

Where the Stockade Wash Tuff is missing, the lower 

contact of the Paintbrush can almost always be recognized 

within a few metres (several feet) as glassy quartz-poor 

friable tuff lying on zeoliticquartz-bearing lithic-rich tuff. 

In northeastern Yucca Flat and locally elsewhere in and 

around Yucca Flat (fig. 8), bedded tuff of the Paintbrush 

rests directly on the Belted Range Tuff or on a local thin 

peralkaline bedded sequence just above the Belted Range 

(Sargent and others, 1965; Noble and others, 1968). 

BEDDED TUFF 

The bedded tuff of the redefined Paintbrush, is generally 

recognized by the glassy state of contained shards and 

pumice lapilli and by its somewhat lesser induration than 

that of the underlying bedded tuff. The distinguishing 
22 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

EXPLANATION 

POST-TIVA CAN YON RHYOLITE 

LAVAS 

- Qu artz-bearing rhyolite lava 

- Biotite-bearing rh yolite lava 

TH ICK NESS OF T IV A CAN YO N 

0 .· .• O~I~~~'ER } 150-300 m In cludes tuffof Chocolate 

Mountain In Clatm Canyon 

>300 m cauldron segmen t 

UE19i 

Limit of Topopah Spring Member 

Limit of information other than 

ca ldera walls 

0 Drill h oles - Numbered holes d iscussed in tex t 

A___A' Line of geologic cross sec tion 

0 5 

0 5 10 

10 

15 

,,._ MOUNT 

JACKSON 

15 

20 KILOM ETRES 

= ...... ,,_ 

20 MILES 

, .. 45' 

TOLICHA 

PEA K A 

;, ... , 

FIG U RE R.-Areal ex tem of Topopah Spring and Tiva Canyon Members of Paimbrush Tuff and related lavas. Isopachs of Tiva Canyon 

cross sections shown 

petrographic fea ture of a ll except the uppermost part is the 

near absence of quartz, by comparison with the underlying and overlying rocks. In the uppermost 6-12 m (20-40 

ft) of the bedded tuff, quartz increases upward and becomes 

dominant in the overlying Rainier Mesa Member of the 

Timber Mountain Tuff. The lower part of the bedded tuff, 

as observed in cores of some drill holes in Yucca Flat (fig. 

8), has been locally zeolitized and indurated but, neverthe-
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 23 

'-'< s 4 

OUARTZ M 

' ' 

, .......... •''\ 

.•'' 

RANGE 

-.,_ 

OAK SPRING ~ BUTTE . .-

- - - - - ,.·----, -,i- - I 

,, .. · 

..... · ----\ ..... / 

Member show great thickness of its uppermost subunit (tuff of Chocolate Mountain) within Claim Canyon cauldron segment. Geologic 

in figures 11, 12, and 13 . 

less, contains fewer lithic fragments and much less quartz 

than the underlying lithic-rich tuffs of the tuffs and 

rhyolites of Area 20, At a few places along the east wall of 

Timber Mountain caldera (fig, 8) where the lithic-rich 

tuffs are glassy, the base of the bedded tuff of the Paintbrush is at the top of the Stockade Wash Tuff. Although 

induration by zeolitization does, in places, cross the lower 

stratigraphic contact of the bedded tuff, the relative 
24 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

abundance of quartz and lithic fragments is the best 

criterion for separating the bedded tuff of the Paintbrush 

from the underlying lithic-rich tuffs of the tuffs and 

rhyolites of Area 20. 

On the 7~-minute quadrangle maps (fig. 2), bedded tuff 

of the Paintbrush was not distinguished consistently from 

the underlying lithic-rich tuffs and from intertonguing 

bedded tuff of the Wahmonie Salyer volcanic center to the 

south (fig. I; Poole, Carr, and Elston, 1965). Moreover, on 

the north and south rim of Timber Mountain caldera, 

some beds within the Paintbrush Tuff coarsen into tuff 

breccia toward lava vents and intertongue with rhyolite 

lava. In some places the tuff breccia was included with the 

lava flows; in others, it was mapped as bedded tuff. On the 

geologic map of the Timber Mountain caldera area (Byers 

and others, 1976), the bedded tuff map unit includes 

bedded tuff of the Paintbrush, lithic-rich bedded and 

non welded ash-flow tuffs of the tuffs and rhyolites of Area 

20, tuff-breccia around rhyolite lava vents, and other 

minor local nonwelded ash flows, such as the tuff of 

Blacktop Buttes. The grouping together of all this lightcolored porous tuff sequence that occurs between the 

underlying Belted Range Tuff and the overlying Timber 

Mountain Tuff was necessitated by the difficulty in 

separating these various units in the field within the time 

allotted for the completion of maps at the Nevada Test 

Site. Drill cores have provided the means for positive 

separation and identification of these units in the subsurface, in contrast to their generally low profile in 

outcrop. 

GEOLOGIC RELATIONS BETWEEN WELDED ASH-FLOW 

TUFFS, LAVAS, AND CLAIM CANYON CAULDRON 

Welded ash-flow tuff members of the Paintbrush Tuff 

intertongue with genetically related, alkali-calcic lavas 

that are not part of the Paintbrush and are within the 

Claim Canyon cauldron segment, which is the exposed 

high-standing southern remnant of the now largely buried 

Claim Canyon cauldron (fig. 8). Eruption of the members 

of the Paintbrush Tuff resulted in cauldron subsidence in 

overlapping areas within the inferred limits of the Claim 

Canyon cauldron shown in figure 8. All four tuff members 

are probably present within the buried part of Claim 

Canyon cauldron. The lowermost member, the Topopah 

Spring, is not exposed within the cauldron segment, but a 

fragment of intracauldron facies that was included in the 

Timber Mountain caldera debris flow was cored at a depth 

of 1,402 m (4,600 ft) in drill hole UE18r (fig. 8); hence, the 

site of UE18r was probably within the cauldron. North of 

Timber Mountain caldera, lavas intercalated with ashflow tuff units are exposed in the wall of the caldera and 

were penetrated in drill holes north of the wall. Flow 

directions in the surface exposures indicate a southerly 

source (Cummings, 1964), probably from the rim area of 

the now-buried Claim Canyon cauldron within a few 

miles to the south. 

Welded ash-flow tuffs of the Paintbrush Tuff and intercalated lavas are several times thicker (table 3) inside the 

Claim Canyon cauldron segment than where they are 

exposed on the cauldron rim (fig. 8). A total thickness of 

more than 3,000 m (6,500 ft) of Paintbrush Tuff and intercalated lavas is present inside the Claim Canyon cauldron 

segment, but the Topopah Spring Member is not exposed. 

The tuff of Chocolate Mountain and the tuff of Pinyon 

Pass are informal map units of the Paintbrush Tuff 

entirely within the Claim Canyon cauldron segment (see 

fig. 7). The tuff of Chocolate Mountain (table 3) is an 

extremely thick intracauldron welded quartz latite 

caprock of the Tiva Canyon Member and, at one critical 

exposure, grades downward within a stratigraphic 

interval of about 50 feet into the underlying main part of 

the Tiva Canyon Member. The tuff of Pinyon Pass is a 

separate overlying cooling unit which closely resembles 

the mam part of the Tiva Canyon but is not considered 

part of it because of the complete cooling break between 

the units. 

TABLE 3.-Thicknesses and K-Ar ages of Paintbrush Tuff units and 

genetically related lavas within Claim Canyon cauldron segment 

and outside on cauldron rim 

[Modified !rom Byers and others (1969). K-Ar ages from Marvin and others (1970)] 

Cauldron rim Cauldron segmem 

Uni1s, in stratigraphic Thickness K-Ar age Thickness K-Ar age 

order (m) (.dO' yr) (m) (x l O' yr) 

Tuff of Pinyon Pass 0 ISO 

Tiva Canyon Member 

Tuff o£ Chocolate Moumain ... 600+ 12.5, 12.6 

Main sheet. 120 12.4, 12.4 300+ 12.6 

Yucca Moun Lain Member .... 60 335 

Lavas between Pah Canyon 

and Yucca Mountain 

Members .. 30 300 

Pah Canyon Member ... 90 200+ 

Pre-Pah Canyon lavas ... 0 150+ 

Topopah Spring Member . . . . . . . . . . . . . . . . . . . . 120 13.2 Not 

exposed 

O'>mposire thickness .. 420 2,000+ 

(1,400 ft) (6,500+ ft) 

LITHOLOGY AND FIELD RECOGNITION 

There are few unique criteria that can be used to distinguish individual welded tuff units of the Paintbrush 

Tuff in the field. Locally strong crystallization owing to 

slow cooling has destroyed primary textures, and so in 

places it is difficult to distinguish tuffs from intercalated 

lavas. The field geologist, following a long period of 

familiarization, eventually learns to distinguish, on the 

basis of very subtle differences, individual cooling units in 

a restricted area. Part of the problem of field identification 

arises from the fact that lithologic variations within 

individual welded tuff units tend to be similar for all units 

and to be much more conspicuous than variations between 

equivalent parts of separate units. 

All the welded ash-flow units of the Paintbrush Tuff 

and even the intercalated lavas have crystallization zones 
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 25 

similar to those described by Smith (1960b), and most of 

the lithologic contrasts observed in outcrop are related to 

these zones. Color, for example, is related to crystallization zones in the following way: dark gray to black, glassy; 

brownish gray, cryptocrystalline; light brownish gray to 

purplish gray, microcrystalline; and light gray, coarsely 

microcrystalline with granophyric texture in pumice 

lenticles. The light-gray granophyric tuffs are typical of 

the thick intracauldron sheets. The degree of welding is 

also similar for each ash-flow cooling unit and tends to be 

moderate to dense except at the very distal edges of the 

units. Notwithstanding these similarities there are a few 

features, such as lithic content, pumice content and size, 

and compositional zonations, that the experienced 

geologist can use in making field identifications. 

Generally modal analyses are necessary to identify a unit. 

TOPOPAH SPRING MEMBER 

The Topopah Spring Member of the Paintbrush Tuff 

has been rather thoroughly described by Lipman, 

Christiansen, and O'Connor (1966); therefore, only the 

salient features are summarized here. The Topopah 

Spring, whose distribution is shown in figure 8, extends 

generally east of the Tiva Canyon Member but overlaps 

the member by more than 50 percent. Variations in thickness, with a maximum thickness of nearly 275m (900ft) on 

the west flank of the CP Hills (fig. 8), are mainly the result 

of paleotopography. The member is not exposed inside 

the Claim Canyon cauldron segment but an unknown 

volume of it is buried there. The total extracauldron 

volume is about 160 km3 (40 mi3) (Lipman, Christiansen, 

and O'Connor, 1966); probably more than 250 km3 (60 

mi3) was erupted. 

The unit has a high-silica rhyolitic lower zone and a 

quartz latitic upper zone or cap rock. Near the south rim of 

the Claim Canyon cauldron segment (fig. 8) a middle 

xenolithic subunit containing granitic fragments 

separates the lower and upper parts (Lipman, 

Christiansen, and O'Connor, 1966, fig. 18). Densely 

welded quartz latitic bedded tuff, which occurs at 

Prospector Pass (fig. 8) just west of the cauldron segment, 

overlies the high-silica rhyolite zone and is probably very 

near the source area of the Topopah Spring area (Lipman, 

Christiansen, and O'Connor, 1966, p. F26). 

The modal compositions of the lower and upper 

compositional zones are shown in figure 9; the Topopah 

Spring is distinguished modally from all other members of 

the Paintbrush Tuff by the lack of sphene. The rhyolitic 

lower part of the Topopah Spring is also distinguished 

readily by a high plagioclase-to-alkali feldspar ratio, but 

the quartz latitic caprock of the Topopah Spring is distinguished from younger caprocks of the Paintbrush 

mainly by the absence of sphene. Chemical analyses of the 

different ash-flow units of the Topopah Spring Member 

were published by Lipman, Christiansen, and O'Connor 

( 1966, table 2); the silica ranges of the high-silica rhyolite 

and quartz latite are shown in figure 9. 

The Topopah Spring Member can also be identified by 

its normal thermal remanent magnetization (G. D. Bath, 

written commun., 1965 ), in contrast to reverse polarities of 

younger units of the Paintbrush Tuff. Its K-Ar age is 13.2 

m.y., in contrast to an average 12.5 m.y. for the Tiva 

Canyon Member (table 2). 

PAH CANYON MEMBER AND RELATED PRE-PAH LAVAS 

The Pah Canyon Member is lithologically similar to the 

quartz latitic caprock of the Topopah Spring Member but 

is a simple ash-flow cooling unit without compositional 

zoning. Also, its silica content-74 percent (fig. 9)-falls 

in the rhyolite range in contrast to that of the Topopah 

Spring caprock, which is in the quartz latite range of 69-72 

percent. Flattened pumice lenticles, less than 2 em in 

length, contrast with larger ones in the Topopah Spring. 

Sparse to common small cognate inclusions, mostly less 

than 0.5 em, are distributed throughout the Pah Canyon. 

The extracauldron extent of the Pah Canyon is only about 

200 km2 (75 mi2) (fig. 10). The extracauldron volume ofthe 

Pah Canyon does not exceed 20 km3 (5 mi3), but an equal 

or greater volume of the tuff must be contained within the 

Claim Canyon cauldron, for the average thickness of the 

tuff inside the cauldron is at least three times that of the 

tuff outside. The buried areal extent inside the cauldron is 

probably greater than that outside the cauldron. 

Two lava flows underlie the Pah Canyon within the 

Claim Canyon cauldron segment (figs. 3, 7). Their modes 

are pooled and are similar to but slightly more mafic than 

that of the Pah Canyon (fig. 9). These pre-Pah Canyon 

lavas are the lowest stratigraphically in the Claim Canyon 

cauldron segment and are also closest to the cauldron wall, 

near which they are brecciated and silicified. 

LAVAS AND ASH-FLOW TUFF BETWEEN PAH CANYON 

YUCCA MOUNTAIN MEMBERS 

One or more lava flows of similar composition were 

extruded on the Pah Canyon Member before eruption of 

the Yucca Mountain Member within and just outside the 

Claim Canyon cauldron segment (fig. 10). At about the 

same time a local ash-flow tuff and two overlying lava 

flows were poured northward into Silent Canyon caldera 

(Cummings, 1964; David Cummings, written commun., 

1964) from the now-buried Claim Canyon cauldron. The 

local ash-flow tuff to the north was penetrated only in drill 

holes UE19f and UE19i (fig. 10). On the north side of 

Timber l\1ountain caldera, the Pah Canyon and Yucca 

Mountain Members are not exposed or penetrated in drill 

holes, and so the local lavas and underlying tuff occur 

between the Topopah Spring and Tiva Canyon Members. 

The petrography (fig. 9) and cooling history of these local 

intercalated units on the north side, however, indicate that 
26 

..... 

..... 

:::l 

f-

~ :::l 

£ 

c 

m 

c... 

..... .,_ 

:::l 

f-

~ :::l 

£ 

c 

'(ii 

c... 

TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

I 

-< 

-< 

STRATIGRAPHIC UNIT-{ SUBUNIT 

Number of thin sections (N) in parentheses 

Quartz-bearing 

rhyolite lava (4) 

Post-Tiva Canyon -< rhyolite lavas 

Biotite-bearing 

rhyolite lava (5) 

? ? r 

Phenocryst-rich 

rhyolite caprock (5) 

Tuff of High-silica 

Pinyon Pass -< rhyolite (4) 

Rhyolite (3) 

r Tuff of Chocolate 

Mountain (8) 

(intracauldron) 

Quartz latite ( 13) 

Tiva Canyon ( extracau ldron) 

Member -< 

Rhyolite (13) 

High-silica 

rhyolite (10) 

Yucca Mountain Member ( 1 0) 

(essentially phenocryst-free) 

? 

HornblendeRhyolite lavas between bearing lavas (4) 

Yucca Mountain and < 

Pah Canyon Members Pyroxenebearing lavas ( 12) 

Pah Canyon Member ( 17) 

Pre-Pah Canyon rhyolite lavas(~) ,-? ? 

Quartz latite ( 11) 

1 Topopah Spring -< Member 

High-silica 

rhyolite (19) 

PHENOCRYSTS AS 

PERCENTAGE OF 

TOTAL ROCK 

I I 

-

• 

-

-

-

--

-

• 

I 

• 

-

-

-

-I I 

0 10 20 0 

FELSIC PHENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

0 

EJJ Quartz ~ Alkali feldspar 

~ Plagioclase 

20 

PERCENT 

40 60 80 100 

FIGURE 9.-Modal and silica ranges of significant units of Paintbrush Tuff and petrologically related lavas in order of stratigraphic 

sequences indicated by modal trends. Blank area in phenocryst column indicates absence of mineral. Modes of Topopah Spring 

and W. D. Quinlivan and P. W. Lipman (written commun., 1974). 
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 

MAFIC PHENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

- Total mafics [§ZJ Amphibole 

~ Biotite 

~-

~-

~-

-

0 

~ Clinopyroxene 

10 

PERCENT 

0 

(R) Resorbed 

!EJ 

(R) 

-!~ 

(R) 

?~ 

(R) Resorbed 

~ 

(R) 

~~ 

2.5 

NUMBER OF 

SPHENE PHENOCRYSTS 

PER THIN SECTION 

I 

D 

J 

0 20 40 68 

Sl LICA PERCENTAGE RANGE 

(H 2 0 and C0 2 free) 

Number of analyses (N) in parentheses 

I 

(2) 

~ (4) 

(1) 

(1) 

I 

(1) 

(3) 

(4) 

10) 

~ (6) 

~ (2) 

~ (4) 

(QJ 

(6) 

(8) 

70 72 74 

PERCENT 

succession. Queried successions between a few units inferred in part from structural relations and in part from general eruptive 

Member from Lipman, Christiansen, and O'Connor (1966). Silica percentages from Lipman, Christiansen, and O'Connor (1966) 

27 
28 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

EXPLANATION 

1:.// ::: :::_::j Yucca Mountain Member 

D 

0 UE19i 

RHYOLITE LAVAS BETWEEN 

YUCCA MOUNTAIN AND PAH 

CANYON MEMBER 

Hornblende-bearing lavas 

Pyroxene-bearing lavas 

Overlap of Yucca Mountain and 

Pah Canyon Members- Not shown 

at overlap with pyroxene-bearing 

lavas 

Pah Canyon Member 

Drill hole used for control- Numbered holes discussed in text 

A ___ A' Line of geologic cross section 

0 5 

0 5 10 

10 15 

15 20 KILOMETRES 

20 MILES 

TOUCH A 

PEAK ~ 

FIGURE 10.-Areal extent of Pah Canyon and Yucca Mountain Members of Paintbrush Tuff 

they were probably extruded between the times of eruption 

of the Pah Canyon and Yucca Mountain Members. Where 

both members are exposed just outside the Claim Canyon 

cauldron segment, coarse-bedded ash-fall tuff, as much as 

60 m (200ft) thick, related to lava venting occurs between 

the members, except where two erosional remnants of lava 

fronts are preserved: one at the north end of Yucca Mountain and the other 3.2 km (2 mi) south of the head of Pah 
_,;:: MOUNT 

HELEN 

QUARTZ M 

' ' ' "" ' '\. 

PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 

r~-·t·\} 

~HILLS~,/ l .... , ..... -·' 

WAHMONIE-SAL YER 

VOLCANIC CENTER 

NEVADA TEST SIT£ •"'~ ,,,•'"'~ l-.-·--·-- -·-·~,.,~n'-~,;~~·:y·'\. 

•'"'"""'"' SPECTER '\ 

/'"" RANGE .'\ 

and extent of lavas between the members. Geologic cross sections shown in figures II, 12, and 13. 

29 

Canyon (fig. 10). The local ash-flow tuff and all the lavas 

at this stratigraphic position on both north and south 

sides of Timber Mountain caldera have a thermal 

remanent magnetism that is reverse to the Earth's present 

field (G. D. Bath, written commun., 1965), and is typical of 

post-Topopah Spring units of the Paintbrush. 

The lavas between the Pah Canyon and Yucca Mountain Members in Claim Canyon cauldron segment are 
30 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

pyroxene bearing and are almost identical in modal petrography to the lower lava flow on the north side in Silent 

Canyon caldera. These lavas differ from the Pah Canyon 

in that sphene is more common and alkali feldspar phenocrysts are dominant over plagioclase; the modal ranges 

shown in figure 9 represent seven thin-section modes 

from the south side pooled with five from the north side of 

the Timber Mountain-Oasis Valley caldera complex. The 

74-percent silica content of the lower lavas indicates a 

rhyolitic composition similar to that of the Pah Canyon. 

The local ash-flow tuff penetrated in drill holes UE19i 

and UE19f (fig. 10) is closely associated with the overlying 

pyroxene-bearing lava flow. This ash flow is locally fused 

beneath the lava flow, as penetrated in drill hole UE19f 

(figs. 10, ll ). In drill hole UE19i, 4 km (2~mi) farther away 

from the inferred volcanic source, the two units are 

separated by nonwelded tuff, and a basal vitrophyre is 

present in the lava. Although the tuff resembles the Pah 

Canyon Member lithologically and was shown as Pah 

Canyon in a cross section by Hinrichs, Krushensky, and 

Luft (1967, section A-A'), both the tuff and the overlying 

pyroxene-bearing rhyolite lava (Byers and Cummings, 

1967) have similar phenocryst ratios (table 4). The subsurface tuff, moreover, differs modally from the Pah 

Canyon Member of the Paintbrush Tuff south of Timber 

Mountain caldera mainly in the sanidine/plagioclase 

phenocryst ratio (2: 1 ). The Pah Canyon Member has about 

equal sanidine and plagioclase phenocrysts in the mode 

and contains other petrographic features that relate it 

more closely to the underlying Topopah Spring Member. 

The petrographic evidence and close association with the 

overlying pyroxene-bearing lava indicate that the local 

subsurface tuff is probably slightly later than the Pah 

Canyon Member and therefore is considered a local 

informal unit of the Paintbrush Tuff. The close spatial 

and temporal association of the tuff and the lava, 

combined with their close petrographic similarity also 

suggest that they were probably erupted from the same 

Claim Canyon cauldron vent, probably just inside the 

present north wall of the Timber Mountain caldera. 

The upper, hornblende-bearing lava flows are exposed 

in the north wall of Timber Mountain caldera (fig. 10). 

They originally flowed only a short distance northward 

(Cummings, 1964; David Cummings, written commun., 

A A' 

S.S. W. 

7000'-

General direction of flowage from inferred source vent ~ N.N.W. 

6000'-

Drill hole 

UE19f 

PAHUTE MESA - -

Timber Mountain Tuff 

Pyroxene-bearing rhyolite lava flow 

Drill hole 

UE19i r 

Dense welded tuff 

5000'- /rades into lava (no 1-·-·-·-· .?------Basafvltrophyre___ --- -~ • Lava vitrophyre in lava) =- ----;:------------ ---:~ ~-------,= 

;~-:-:~~~:; ;;:·::~::=~-=~·=~-~~:~-~-=:=::: ~-------..:~~~-S-ub_s_u-rf-a-ce-tu_f_f _o_:f -=P:..::ai=-:n=tb=r..=u:...::sh:_:T~uf':f~~=-==-=-=:...::=-=~ 

4000'-

0 

0 

1-

.J 

::::> 

<( 

u. 

2000 

500 

Tuffs and rhyolites of Area 20 

filling Silent Canyon caldera 

4000 6000 FEET 

1000 1500 METRES 

Vertical exaggeration X 2 

FIGURE B.-Generalized section along A-A' through drill holes UE19f and UE19i, Silent Canyon caldera, showing relations between subsurface 

ash-flow tuff of Paintbrush Tuff and overlying, mineralogically similar pre-Tiva Canyon rhyolite lava. Vitrophyres shown in black; 

nonwelded to partially welded zones, stippled. Inferred transitional contact shown by dot-dash line. Line of section shown in 

figures 8, 10. 
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 31 

TABLE 4.-Combined thin-section modes and computed accessory mineral percentages of local ash-flow tuff of Paintbrush Tuff and overlying pre-Tiva Canyon rhyolite lava where penetrated in drill hole UE19f, Pahute Mesa (fig. 11) 

[Location of drill hole shown in fig. ll. N.d., not determined) 

S ~ Phenocrysts and accessory minerals as volume 

:::l ..., percent of total crystals 

~]---------------------------------------------------------------------- ; B ~ :: !!3 0 ''" ~ ~ ..:.t ~ e.~ :9 C'S g - ~~ ~E !::! ~ 'B ~oo. "' ~ ~ ~- Depth e .... .., - .... ~ ~ 5 ·a § ·= -;;; .s 

Unit 

QJ ~ ~ ~ \oiiJ ~g ~ ;::: ~ v tU- ~ e 

(m) f & < C: ~ 0 ~ ~ ~ ~ ~ 

Lava............................. 429 10 0 62 29 5.0 (3) 0.9 N.d. N.d. N.d. 8.8 

524 9 0 64 29 3.5 (3) 1.0 0.4 0.2 0.03 7.1 

607 11 0 61 32 5.1 0.6 .3 .1 .1 .03 7.1 

.................................................................................... .Insignificant or partial cooling break? .................................................................................... . 

Tuff............................. 655 14 1.9 64 29 2.5 .2 1.1 .3 .2 .03 5.7 

1Accessory mineral percentages computed from counts of heavy mineral separates using 

sphene as control to combine thin-section mode and heavy-mineral count. 

1964), as shown by the fact that they have not been 

penetrated by the extensive drilling in Silent Canyon 

caldera. Two lava flows of slightly different modal 

compositions are exposed. The volumetrically larger of 

the two lava flows is as much as 125m (400ft) thick and is 

separated from the underlying pyroxene-bearing rhyolite 

lava by 75 m (250ft) of coarse, near-vent tuff breccia. The 

overlying Tiva Canyon Member pinches out over the 

thickest part of the larger hornblende-bearing lava flow. 

There are two exposures of this flow separated by a thick 

paleovalley filling or source-vent filling of post-Tiva 

Canyon biotite-bearing lava (Byers and Cummings, 1967; 

Byers and others, 1976). Two thin-section modes from 

each of the two exposures are very similar and are pooled 

on figure 9. Two chemical analyses (W. D. Quinlivan and 

P. W. Lipman, written commun., 1974) indicate that the 

hornblende-bearing lava approaches high-silica rhyolite 

in composition (fig. 9). One thin section from the smaller 

flow at the same stratigraphic position slightly west of the 

larger flow is petrographically similar but contains a subordinate amount of quartz phenocrysts. 

YUCCA MOUNTAIN MEMBER. 

The Yucca Mountain Member (Lipman and 

Christiansen, 1964) is a simple cooling unit of uniform, 

nearly phenocryst-free, high-silica rhyolite (fig. 9) even 

where it is 335m (1,100 ft) thick within the Claim Canyon 

cauldron segment (table 3). Outside the Claim Canyon 

cauldron segment, a small thin lobe is present east and 

south of Pah Canyon (fig. 10) and a larger lobe with a 

maximum thickness of about 75 m (250ft) occupies about 

100 km2 (40 mi2) around northern Yucca Mountain and 

northern Crater Flat. Only about 8 km3 (2 mi3) is present 

outside the cauldron and roughly an equal volume can be 

inferred within the Claim Canyon cauldron segment. 

The member differs from all other units of the Paintbrush Tuff, in having less than 1 percent phenocrysts, 

largely sodic sanidine. Sparse small pumice lenticles and 

grayish-red xenoliths less than 1 em in length are strewn 

2lncludes opaque oxides not tallied separately. 

!Clinopyroxene altered in strongly devitrified interior of lava flow. 

through a matrix of shards or devitrified groundmass. The 

unit has distinct crystallization zones with brownish-gray 

to gray glassy upper and lower zones and a pale-purplishgray middle zone, which contains irregular white 

lithophysae where it is thick. 

The field and petrochemical evidence indicate that the 

Yucca Mountain Member postdates not only the 

clinopyroxene-bearing lavas but probably also the 

hornblende-bearing lavas exposed in the north wall of 

Timber Mountain caldera where the Yucca Mountain is 

absent. Between the Yucca Mountain and overlying Tiva 

Canyon Member, only a foot or two of fine-grained ashfall tuff, typical of the initial phase of the Tiva Canyon 

eruption, is nearly everywhere present; no coarse tuff 

breccia that would indicate nearby lava vents has been 

found between the members. This close stratigraphic 

association and the petrochemical trend toward increasing 

phenocrysts and decreasing silica content stratigraphically upward (fig. 9) indicate that the Yucca Mountain Member is an early eruptive phase of the same magma 

chamber that supplied the Tiva Canyon. Moreover, very 

little time, probably measurable in tens of years, elapsed 

between eruptions of the two members, for little if any 

erosion has been observed on the Yucca Mountain where it 

is overlain by the Tiva Canyon. The eruption of the Yucca 

Mountain, therefore, was probably very close to that of the 

Tiva Canyon Member in time and was almost certainly 

later than the extrusion of the hornblende-bearing 

rhyolite lava. 

TIV A CANYON MEMBER. 

The Tiva Canyon Member consists of a main part-a 

compositionally zoned extensive ash-flow sheet-and an 

upper thick quartz latite, the tuff of Chocolate Mountain 

(Christiansen and Lipman, 1965; Byers and others, 1976), 

which is herein included as part of the redefined Tiva 

Canyon Member. Outside the Claim Canyon cauldron 

segment (fig. 8), the main part of the Tiva Canyon, like the 

Topopah Spring Member, is compositionally zoned (fig. 
32 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

9); inside, the tuff of Chocolate Mountain overlies rhyolite 

of the lower main part of the Tiva Canyon above a thin 

transition zone characterized by crystal-rich quartz latitic 

pumice lenticles in a matrix of welded crystal-poor 

rhyolite. 

In its type section at Tiva Canyon (fig. 8), the member is 

107m (350ft) thick, and the maximum thickness, west of 

Beatty, is about 200m (700ft). The unit extends for about 

115 km (10 mi) along a broad arcuate belt outside the 

southern wall of the Claim Canyon cauldron segment. 

The distribution of the member is similar to that of the 

Topopah Spring Member, except that the Tiva Canyon 

extends not as far east and somewhat farther west-just 

barely into California. The Tiva Canyon was mapped by 

Cornwall and Kleinhampl ( 1964) as cooling unit 3 of their 

Bullfrog Hills caldera. In Silent Canyon caldera, two lobes 

of the Tiva Canyon each about 105 m (350 ft) thick in 

separate areas were penetrated in five drill holes (fig. 8). 

Inside the Claim Canyon cauldron segment the lower 

main part of the Tiva Canyon and the upper part, the tuff 

of Chocolate Mountain, aggregate more than 900 m (3,000 

ft) in thickness (table 2). The total area covered by the Tiva 

Canyon outside the cauldron is probably in excess of 2,600 

km2 ( 1,000 mi2) if the member extends northwest under 

Sarcobatus Flat (fig. 8). Inside the Claim Canyon 

cauldron, possibly as much as 520 km2 (200 mi2) of tuff 

having a thickness averaging more than 0.8 km (0.5 mi), is 

now largely buried under younger rocks of the Timber 

Mountain-Oasis Valley caldera complex. The total 

volume of the Tiva Canyon Member, therefore, may be in 

excess of 1,000 km3 (250 mi3). 

The Tiva Canyon Member is a compound cooling unit, 

predominantly gray to reddish-brown devitrified densely 

welded ash-flow tuff with minor light-brown and gray 

nonwelded to partially welded tuff at its base and top. 

Generally a few feet of genetically related light-gray ashfall tuff occurs at the base of the unit. In the lowermost part 

the Tiva Canyon is crystal-poor sanidine- and 

hornblende-bearing high silica rhyolite tuff; it grades 

upward into a middle crystal-poor rhyolite with biotite, 

which is in turn overlain by an upper (caprock) crystalrich quartz-latitic tuff (fig. 9). These three distinct 

compositional zones are laterally persistent over much of 

the sheet outside the Claim Canyon cauldron segment 

and, therefore, provide an intrasheet stratigraphy. 

The Tiva Canyon Member inside the cauldron segment 

is locally flow-laminated and contains flow folds. 

Upward, it grades in to normal welded tuff. This secondary 

flowage may have occurred where steep preemplacement 

topography marked the site of the cauldron wall. Welded 

caprock vitrophyre of the member is very thin in fault 

slices and blocks where it overlies tuff breccia near the 

cauldron wall, but locally the caprock vitrophyre grades 

downward into massive tuff breccia, possibly indicative of 

vent breccia related to the Tiva Canyon. The welded 

rhyolitic lower part thickens from a wedge-edge to nearly 

300 m (1,000 ft) downdip, 3.2 km (2 mi) away from the 

cauldron wall (fig. 8). A 15-m (50-ft) stratigraphic 

transition zone between the lower flow-laminated, 

phenocryst-poor rhyolite of the main part of the Tiva 

Canyon and the overlying tuff of Chocolate Mountain is 

marked within the Claim Canyon cauldron segment by an 

increase of dark-gray phenocryst-rich collapsed pumice 

lenticles of quartz latite, similar to those in the overlying 

tuff of Chocolate Mountain. No xenoliths of the main 

rhyolitic part of the Tiva Canyon have yet been found in 

the tuff of Chocolate Mountain, although sparse xenoliths 

of the Yucca Mountain Member and older rocks are 

present. Because of these gradational relations and the lack 

of Tiva Canyon xenoliths, the tuff of Chocolate Mountain must be the upperpartofonecompoundcoolingunit 

that includes the widespread main sheet of the Tiva 

Canyon. 

The tuff of Chocolate Mountain contains three compositionally similar subunits that were mapped by 

Christiansen and Lipman (1965); near the cauldron walls 

the subunits are separated by beds of yellow tuff breccia 

that thin and pinch out within a few kilometres northward 

(figs. 12, 13; Byers and others, 1976). The lower subunit is 

slightly more than 300m ( 1,000 ft) thick and is largely gray 

devitrified tuff with granophyric pumice. The middle and 

upper subunits are brown and purple devitrified tuff 

locally underlain by basal vitro ph yres indicative of partial 

cooling breaks. Petrographically, the tuff of Chocolate 

Mountain differs from the quartz latite caprock of the 

main part of the Tiva Canyon in containing abundant 

resorbed feldspar phenocrysts, a higher ratio of plagioclase to total feldspar phenocrysts (fig. 9), and sparse rather 

than very sparse hornblende phenocrysts. Although there 

is an overlap in content of silica (fig. 9) and other constituents (W. D. Quinlivan and P. W. Lipman, written 

commun., 1974) the petrographic differences indicate that 

the tuff of Chocolate Mountain is slightly more mafic than 

the main part of the Tiva Canyon outside the cauldron 

segment. At one locality within the cauldron segment, 

where a thick complete section is exposed in the south wall 

of Timber Mountain caldera (adjacent to the dikes 

between sections B-B' and C-C', fig. 12), rhyolitic Tiva 

Canyon grades upward through about 15 m (50ft) into 

typical extracauldron quartz latitic caprock of the Tiva 

Canyon. The transition upward to the typical tuff of 

Chocolate Mountain, however, is faulted out. 

A tabular mass of fluidal flow-banded tuff of Chocolate 

Mountain 6 m (20 ft) thick dips moderately and is intercalated with coarse tuff-breccia. Near the head of Claim 

Canyon (fig. 12), this mass shows chilling above and 

below and may be a dike. 
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 33 

D 

• . 

. 

Post-Rainier Mesa rocks 

Rainier Mesa Member and underlying lava 

Lava feeder dike 

U nconf ormi ty 

~ ~ Intracauldron tuffs of Chocolate 

Mountain and Pinyon Pass, and 

tuff breccia 

• ~] Pre-tuff of Chocolate Mountain, 

Paintbrush Tuff, and intercalated 

iava(LI <J) 

i, intracauldron 

e, extracauldron D Pre-Paintbrush rocks 

---- Fault- Dotted where concealed 

B--B' Geologic cross section 

TIMBER MOUNTAIN CALDERA 

0 2 3 MILES 

0 2 3 KILOMETRES 

FIGURE 12.-Generalized geologic map of Claim Canyon cauldron segment and vicinity, showing greater intensity of faulting of tuffs within 

cauldron than outside. Generalized from geologic map of Timber Mountain caldera (Byers and others, 1976). 

TUFF BRECCIAS WITHIN CLAIM CANYON 

CAULDRON SEGMENT 

Tuff breccias intertongue with and overlie the tuff of 

Chocolate Mountain (Christiansen and Lipman, 1966; 

Byers and others, 1976) but are not included with the 

Paintbrush Tuff. These breccias are more than 150m (500 

ft) thick along the south wall of the Claim Canyon 

cauldron segment but wedge out within a mile or two 

north of the wall. Only the larger outcrops are shown in 

figure 12 (see Byers and others, 1976); the breccias shown in 

cross sections B-B' and C-C' in figure 13 are interpreteted 

partly as vent breccias near the south wall of the cauldron. 

Within about 150m (500ft) of the cauldron wall, however, 

faulting and, locally, silicification have complicated interpretation of the origin of the breccia; there it is probably a 

landslide breccia consi~ting of rocks derived from the 

cauldron wall. 
34 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

The breccia from an area about 150m (500ft) to 0.8 km 

(0.5 mi) from the cauldron wall contains, in order of 

decreasing abundance, large blocks of intracauldron 

Yucca Mountain and Pah Canyon Member, pyroxenebearing sanidine-rich lava like that between the members, 

and sparse amounts of both members of Crater Flat Tuff. 

North of Crater Flat (fig. 12) this breccia locally includes a 

few giant blocks ofweldedrhyoliticTiva Canyon about30 

m (I 00 ft) thick and 90 m (300ft) long. These blocks dip 

steeply and are intricately crackled. The nonwelded 

6000' 

4000' 

2000' 

SEA 

LEVEL 

B 6000' 

2000' 

Yucca Mountain 

Pre-Paintbrush rocks 

Possible source vent of 

Tiva Canyon Member 

matrix of the breccia is also modally rhyolitic Tiva 

Canyon and, moreover, grades upward in a few places to 

unaltered moderately to steeply dipping densely welded 

Tiva Canyon quartz latite or tuff of Chocolate Mountain. 

The non welded matrix is not crackled or highly fractured 

and the densely welded quartz latite locally occuts as 

steeply dipping vitrophyre lenses with contacts gradational into the matrix. This part of the tuff-breccia may be 

an explosion breccia genetically related to eruptive vents 

of the Tiva Canyon Member. Lipman's (1976) model of 

CLAIM CANYON CAULDRON SEGMENT 

4000' 

2000' 

CLAIM CANYON CAULDRON SEGMENT 

6000' 

4000' 

2000' 

SEA 

LEVEL 

EXPLANATION 

Tuff of Pinyon Pass 

Yucca Mountain Member 

[;ti~.I:ii;!!J Pah Canyon Member liii!::t!Ii:~;!;l Intercalated lava flows 

fh\1~~!)] Bedded tuff of Paintbrush Tuff 

Topopah Spring Member 

~ ...,/ __ -------r 

2 

0 2000 

0 

Fault - Showing relative movement during cauldron subsidence (1) and inferred later 

movement during cauldron 

resurgence (2). Faults dashed 

where inferred 

4000 6000 8000 FEET 

1000 2000 METRES 

FIGURE 13.-Geologic sections through Claim Canyon cauldron segment, with critical units and faults projected above ground surface to 

indicate possible cauldron resurgence. Lines of sections shown in figure 12. Only major faults shown. 
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 35 

caldera collapse breccias in tuff associated with a calderaforming eruption would also fit the above 

observations-except for the presence of intracauldron 

blocks. 

Within the 150-m wide zone disturbed by faulting along 

the cauldron wall, a block of vertically dipping Topopah 

Spring Member was identified by thin-section examination. The contact of this breccia with the cauldron wall is 

locally exposed in Claim Canyon; it dips about 60° 

cauldronward and could be interpreted as the sole of a 

landslide. 

In detail, the explosion tuff breccia differs from the 

breccias along the cauldron wall in that it is much less 

altered and is composed of about 50 percent blocks as 

much as 150m (500ft) long in a matrix of grayish-yellow 

tuff containing 0-20 percent angular clasts smaller than 

block size. More than 80 percent of the blocks are derived 

from Pah Canyon and Yucca Mountain Members. Moreover, the blocks contain a light-gray devitrified facies of 

these members that is typical of these units inside the 

cauldron. This yellow tuff breccia is similar in appearance 

to that in rhyolite vent tuff breccia cones; moreover, the 

grayish-yellow frothy matrix of rhyolitic Tiva Canyon 

mineralogy locally grades into a dark-gray pumiceous 

scoria. We would interpret these yellow tuff breccias that 

contain intracauldron clasts as originally at or near vent 

sites, probably those that erupted the Tiva Canyon 

Member. 

Inward, about 2 km away from the cauldron wall (fig. 

12), the tuff breccia thins and intertongues with the tuff of 

Chocolate Mountain and lies between the tuffs of 

Chocolate Mountain and Pinyon Pass. More than 3 km (2 

mi) inward the tuff breccia tongues pinch out. These thin 

tongues of tuff breccia resemble the larger massive 

exposures to the south, except that the fragments are 

smaller and the tongues consist of a significant minor 

fraction of pre-Tiva Canyon rocks other than Pah Canyon 

and Yucca Mountain Members. 

TUFF OF PINYON PASS 

The tuff of Pin yon Pass is confined to the Claim Canyon 

cauldron segment and is here included as an informally 

named unit of the Paintbrush Tuff. The tuff is a multipleflow simple cooling unit as much as 150m (500ft) thick, 

distinguished in the field by its generally moderate, rather 

than dense, welding, lack of vitrophyres, and stratigraphic position above the tuff of Chocolate Mountain. 

The tuff of Pinyon Pass is mapped with the tuff of 

Chocolate Mountain in figure 12, but is shown separately 

in section C-C' (fig. 13), and on the geologic map (Byers 

and others, 1976). The lower nonwelded to partially 

welded rhyolitic zone, as much as 90 m (300 ft) thick, is 

commonly brilliant pink or light red where partially 

welded. Where the unit is thickest, however, the upper 

quartz latitic, most densely welded devitrified tuff is very 

much like the Chocolate Mountain in outcrop 

appearance; indeed, the flattened quartz latitic pumice 

lentils of the Pinyon Pass and Chocolate Mountain are 

identical in composition. The Pinyon Pass everywhere is 

concordant on the tuff of Chocolate Mountain or on tuff 

breccia that intertongues between the units. Very slight 

erosion of the upper part of the tuff of Chocolate Mountain before deposition of the tuff of Pinyon Pass is 

indicated locally by channeling and by absence of the nonwelded top of the tuff of Chocolate Mountain. 

The tuff of Pinyon Pass is a compositionally zoned 

compound cooling unit and petrographically resembles 

the main part of the Tiva Canyon Member outside the 

Claim Canyon cauldron segment. The lower rhyolitic 

subunit, however, contains sparse dark, phenocryst-rich 

quartz latite lenticles, as much as 15 em (6 in.) in length, 

typical of the underlying tuff of Chocolate Mountain. 

Hornblende is also very sparse or lacking (fig. 9) in the 

lower rhyolitic subunit of the tuff of Pinyon Pass, in 

contrast to the universal occurrence of hornblende and 

lack of biotite in the lower rhyolitic subunit of the Tiva 

Canyon Member. The middle unit isahigh-silicarhyolite 

that contains a little hornblende and could be mistaken for 

either the lower or middle part of the Tiva Canyon. The 

upper crystal-rich compositional zone is more silicic than 

the analogous part of the Tiva Canyon (fig. 9) but considerable overlap in phenocryst percentages makes distinction by means of thin-section modes highly uncertain. 

The stratigraphic position of the tuff of Pinyon Pass above 

the tuff of Chocolate Mountain is the best criterion for 

identification. Chemical analyses of the unit (fig. 9; W. D. 

Quinlivan and P. W. Lipman, written commun., 1974) 

corroborate the observed compositional zonation from 

rhyolite, upward to high-silica rhyolite, to low-silica 

rhyolite in the crystal-rich caprock. 

The close resemblance of the tuff of Pinyon Pass to that 

of the Tiva Canyon and the presence of Chocolate 

Mountain-type pumice lenticles in the lowermost part of 

the tuff of Pinyon Pass, despite slight local erosion of the 

underlying tuff of Chocolate Mountain, suggest that the 

tuff of Pinyon Pass was erupted within the Claim Canyon 

cauldron only a short time, perhaps within a few thousand 

years, after the Tiva Canyon Member was erupted. The 

lack of any bedded tuff breccia other than the single tongue 

that thins away from the cauldron wall suggests that no 

rhyolitic lava volcanism intervened between the tuff of 

Pinyon Pass and the underlying tuff of Chocolate Mountain. The tuff of Pinyon Pass, therefore, probably predates 

post-Tiva Canyon rhyolite lavas in the north wall of 

Timber Mountain caldera (fig. 8). 

POST-TIVA CANYON RHYOUTE LAVAS 

Two rhyolite lavas related to but not part of the Paint-
36 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

brush Tuff are exposed in the north wall of Timber 

Mountain caldera (fig. 8) above the Tiva Canyon Member. 

These lavas are similar in physical appearance except that 

the lower flow has visibly more and larger biotite flakes. 

Petrographically, they are easily distinguished, for the 

upper rhyolite lava has sparse small quartz euhedra, 

whereas the more mafic lower rhyolite lava has none. 

These two flows were mapped separately in the Scrugham 

Peak quadrangle (Byers and Cummings, 1967). 

Outcrop thicknesses of the individual flows average 

about 150 m (500 ft), but within a few miles downdip 

under Pahute Mesa the quartz-bearing flow attains 347m 

( 1,140 ft) in a drill hole. The flows extend a few miles north 

and northeast under Pahute Mesa, where they have been 

penetrated in drill holes (fig. 8). Drill records indicate that 

approximately 64 km2 (25 mi2) of the mesa is underlain by 

either one or both of the rhyolites. The thickening of these 

lavas under Pahute Mesa, as inferred for the earlier lavas 

intercalated with the Paintbrush, is doubtless related to 

the existence of the Silent Canyon caldera, into which 

these later lavas also flowed. 

The lower biotite-bearing lava exposures in the north 

wall of Timber Mountain caldera (fig. 8) are instructive 

relative to the source of the lava and the location of the 

Claim Canyon cauldron wall. The lava has fused underlying bedded tuffs and locally the basal vitrophyre zone 

cuts across the bedded tuff. Relict bedding visible below 

the flow within this vitrophyre indicates that the vitrophyre is a fused tuff. Tongues of vitrophyric agglutinated 

ash-fall tuff are also intercalated in the bedded tuff near the 

fused zone, suggesting a nearby source for these pyroclastics and the associated flow. Northerly flow azimuths 

(Cummings, 1964; David Cummings, written commun., 

1964) indicate the source to the south, probably just inside 

the present wall of Timber Mountain caldera. Petrographically, the lavas clearly are late effusives related to 

the Paintbrush Tuff (fig. 9). It is reasonable to assume that 

these flows were fed by radial fissures near the cauldron 

ring fault similar to feeders of the pre-Rainier Mesa lavas 

in the south wall of Timber Mountain caldera (fig. 12), 

thus providing additional corroborative evidence that the 

north wall of the Claim Canyon cauldron is probably 

buried within a few kilometres south of the north wall of 

Timber Mountain caldera. 

Petrographically and chemically, the two post-Tiva 

Canyon lavas are generally similar (fig. 9). Chemical analyses (W. D. Quinlivan and P. W. Lipman, written commun., 1974) reflect the minor petrographic differences 

between them. Probably of significance is the presence of 

euhedral quartz, ranging from 0.1 to 0.5 mm, amounting 

to several phenocrysts per thin section in the upper quartzbearing flow. The small euhedral character of these quartz 

phenocrysts and their presence in all thin sections 

examined indicates that the magma from which the 

quartz-bearing flow was extruded had just attained the 

quartz-feldspar cotectic curve (Tuttle and Bowen, 1958), 

marking the change from the predominantly quartz-free 

volcanic rocks to overlying quartz-rich rocks of the 

Timber Mountain Tuff and related lavas. 

CAULDRON SUBSIDENCES RELATED TO ERUPTION 

OF ASH-FLOW TUFF MEMBERS 

The fact that nearly all units of the Paintbrush Tuff and 

intercalated, petrologically similar lavas thicken greatly 

(table 3) within the Claim Canyon cauldron segment 

suggests accumulation within a caldera. The notable 

exception is the Topopah Spring Member, the basal unit 

of the Paintbrush Tuff; from study of other calderas in the 

area, we presume that it is present at depth beneath the 

thick infilling units of the cauldron (fig. 12). The great 

thickness of the post-Topopah Spring units within the 

Claim Canyon cauldron segment plus the areal 

distribution of the Topopah Spring Member (fig. 8) and 

also the presence of the middle xenolithic zone and the 

bedded upper vitrophyre within a few kilometres of the 

rim of the Claim Canyon segment (compare with Lipman, 

Christiansen, and O'Connor, 1966, fig. 18) indicate that 

the segment is an eroded portion of a cauldron whose 

earliest subsidence was probably associated with the 

eruption of the Topopah Spring Member. The northern 

boundary of this cauldron is probably not too far south of 

the present north wall of Timber Mountain caldera, for 

lavas related to the Paintbrush were poured northerly into 

Silent Canyon caldera (fig. 10) from vents at or just south 

of this wall. 

Additional suggestive evidence on the possible location 

of the north wall of Claim Canyon cauldron comes from a 

core taken at 1,402 m (4,600 ft) in drill hole UE18r. Debris 

flows associated with collapse of Timber Mountain 

caldera contain a few fragments of crystal-rich Topopah 

Spring caprock with granophyric pumice, typical of 

slowly cooled thick accumulations of ash-flow tuff within 

calderas. The occurrence of this granophyric Topopah 

Spring in the Timber Mountain caldera debris flows 

suggests that an intracauldron facies of the Topopah 

Spring Member was exposed within the present boundary 

of Timber Mountain caldera at the time of Timber 

Mountain caldera collapse. The northern limit of the 

collapse area associated with the eruption of the Topopah 

Spring Member, therefore, was probably north of drill 

hole UE18r, but south of the present north wall of Timber 

Mountain caldera (fig. 8). 

Cataclysmic subsidence of the Claim Canyon cauldron 

segment during eruption of the Tiva Canyon Member is 

indicated by the thick accumulation of the main part and 

the tuff of Chocolate Mountain of the Tiva Canyon within 

the cauldron. Within the Claim Canyon cauldron 

segment, the tuff of Chocolate Mountain intertongues 

with tuff breccias related to the Tiva Canyon (section 

C-C', fig. 13), possibly of vent breccia origin, for in 
PAINTBRUSH TUFF AND ROCKS RELATED TO CLAIM CANYON CAULDRON 37 

many places they are modally like the Tiva Canyon and 

locally grade into welded Tiva Canyon quartz latite or tuff 

of Chocolate Mountain. These tuff breccias are coarse and 

thick just inside the cauldron wall but are thin and become 

finer away from the wall of the Claim Canyon cauldron 

segment. They were possibly largely derived from nearwall vents that were active during the emplacement of the 

Tiva Canyon. Moreover, these breccias contain pre-Tiva 

Canyon blocks of intracauldron facies as young as the 

Yucca Mountain Member, which immediately underlies 

the Tiva Canyon Member and may be considered an early 

eruptive phase of the Tiva Canyon. In a very few places 

within a kilometre of the cauldron wall, giant crackled 

blocks of dense welded Tiva Canyon occur in the massive 

tuff breccia and may have been autobrecciated in place 

during emplacement and subsequent collapse. Alternatively, these blocks may have been an early solidified 

phase of the Tiva Canyon that calved off the caldera rim 

during late stages of eruption and collapse, similar to the 

origin of the caldera breccias of the western San Juan 

Mountains, Colorado (Lipman, 1976). Under either 

hypothesis, however, the cauldron tuff breccias that are 

seen today just within the southern border of the Claim 

Canyon segment are a product of volcanism related to the 

Tiva Canyon and not to the Topopah Spring Member. 

Indirect evidence also suggests that the Claim Canyon 

cauldron segment is a part of the collapse area related to 

eruption of the Tiva Canyon. The main part of the Tiva 

Canyon within the segment locally contains fluidal flow 

folds like a rhyolite lava, suggesting flowage while a 

caldera wall slope was being formed during emplacement 

and compaction related to welding. Only within the 

cauldron segment have flow folds been observed within 

the Tiva Canyon. 

The walls of the two caldera collapses associated with 

the eruptions of the Topopah Spring and Tiva Canyon 

Members are probably not far apart but are not readily 

identified because later structural movements in the 

vicinity of the cauldron wall have obscured evidence of 

earlier collapses. Evidence of the wall related to Topopah 

Spring collapse is probably preserved in the vicinity of 

Claim Canyon just outside the fault that repeats the 

thickened Pah Canyon and the rhyolite lava under it 

(section C-C', fig. 12). Unfortunately, this part of the wall 

is covered by the alluvium flooring Claim Canyon. 

Unraveling the sequence of events along the cauldron wall 

probably can be accomplished only by detailed geologic 

mapping at a scale significantly larger than that (1:24,000) 

of the quadrangle maps (fig. 2). 

Specific collapses related to the eruptions of the Pah 

Can yon and Yucca Mountain Members are also difficult to 

document. They and the intercalated lavas have a much 

greater thickness, fivefold to sixfold, inside the cauldron 

than outside; the total thickness of the units between the 

Topopah Spring and Tiva Canyon Members is 180m (600 

ft) on the cauldron rim (table 3) compared to about 1,000 m 

(3,300 ft) within the cauldron segment. We would 

interpret from sections B-B' and C-C''(fig. 13) that the top 

of the Topopah Spring within the main part of the 

cauldron segment would be at least 820 m (1,000 m minus 

180 m) below its top outside just after collapse related to 

the Yucca Mountain Member. 

The preceding line of argument becomes even more 

compelling when the difference in thicknesses of the Tiva 

Canyon, inside and outside the cauldron, namely about 

800 m (2,600 ft), is added to the 820 m (2, 700 ft) net 

difference in thickness between the Topopah Spring and 

Tiva Canyon Members (table 3). That is, assuming no 

resurgence of the cauldron, the top of Topopah Spring 

must have been more than 1,600 m (about 1 mi) below its 

altitude on the rim of the Claim Canyon cauldron after the 

eruption of the Tiva Canyon Member. It is difficult to conceive of this much cauldron subsidence relief on the 

Topopah Spring immediately after its eruption, because 

the unit has a relatively small volume (200 km3) outside the 

cauldron. The Claim Canyon cauldron more likely 

preserves a record of episodic subsidence (fig. 13) 

associated with eruption of each of the members of the 

Paintbrush Tuff. Analogous to the caldera relief of the 

Topopah Spring Member of the Paintbrush Tuff, the 

Grouse Canyon Member of the Belted Range Tuff within 

the Silent Canyon caldera is today about 2 km below its 

elevation outside the caldera (Orkild and others, 1968, 

1969), owing to later subsidence related to eruptions of 

many post-Grouse Canyon units. 

In a related paper, Christiansen and others (1976) 

consider the regional structure and the overall pattern of 

distribution and thicknesses of members of the Paintbrush Tuff and suggest the Oasis Valley caldera segment, 

in contrast to the Claim Canyon cauldron segment, as the 

main area of collapse related to eruption of the Yucca 

Mountain and Tiva Canyon Members. The differences 

between the conclusions expressed by these authors (four 

of whom are also authors of the present report) and those 

expressed here are minor and involve mainly emphasis on 

possible contrasting source areas between the two lower 

and the two upper ash-flow tuff members of the 

Paintbrush Tuff. 

P08SIBLE MAGMATIC RESURGENCE 

OF CLAIM CANYON CAULDRON 

The Claim Canyon cauldron segment, in contrast to 

nearly all other penecontemporaneous cauldron 

structures of the southwestern Nevada ash-flow field, 

today stands topographically high, like many central 

Nevada cauldron structures that have been elevated by 

both resurgence and basin-range faulting. The notable 

local exception is the Timber Mountain resurgent dome to 

be described later. The cauldron segment is not 
38 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

structurally high in the sense that older rocks are exposed 

within it; in fact, generally younger rocks of the 

Paintbrush Tuff are exposed within the cauldron 

segment, analogous to the resurgent doming of the 

Ammonia Tanks Member of the Timber Mountain Tuff 

within Timber Mountain caldera. The present authors 

will attempt to point out some geometric relations that in 

our opinion suggest that magmatic resurgence of the 

Claim Canyon cauldron occurred prior to emplacement of 

the Rainier Mesa Member and its petrologically related 

underlying lavas, an idea already outlined in an abstract 

(Byers and others, 1969). Christiansen and others (1976) 

present structural evidence that the present position of the 

Claim Canyon cauldron segment is not due to magmatic 

resurgence related to Claim Canyon cauldron. 

The geologic map shown on figure 12, which is 

generalized from the geologic map of Timber Mountain 

caldera (Byers and others, 1976), is included in the present 

report merely to show the great intensity and trend of 

faults within the cauldron segment, especially in that part 

where the tuffs of Chocolate Mountain and Pinyon Pass 

(K-Ar age, 12.5 m.y.) of late Paintbrush age are exposed. 

The Rainier Mesa Member (K-Ar age, 11.3 m.y.) and the 

underlying petrologically similar lava are affected mainly 

by basin-range faults that are approximately radial to the 

intensely faulted wall of the Claim Canyon cauldron 

segment; that is, the basin-range faults strike more northeasterly where the wall faults trend northwesterly at the 

west end of the segment. As pointed out by Christiansen, 

Lipman, Orkild, and Byers (1965, p. B46), movement on 

these radial basin-range type faults also occurred during 

broad doming of the Timber Mountain volcanic center, as 

well as during regional basin-range extension. Many of 

these radial faults can indeed be traced into the cauldron 

segment, but most terminate against the circumferential 

cauldron faults. 

Closely spaced circumferential faults and radial cross 

faults shown in figure 12 are localized within the cauldron 

segment and probably are representative of the much 

larger Claim Canyon cauldron, most of which is buried by 

the later collapses of Timber Mountain caldera and the 

Oasis Valley caldera segment. The strong intensity of this 

fault system confined within the cauldron segment is 

interpreted as a marginal fault pattern related to resurgent 

doming, and it bears close similarity to the fault pattern in 

the ring fracture zone at the periphery of Timber 

Mountain resurgent dome (Carr, 1964, fig. 2; Carr and 

Quinlivan, 1968, fig. 1; Byers and others, 1976). That this 

resurgence took place before the eruptions of pre-Rainier 

Mesa lavas and the Rainier Member is shown by the near 

absence of circumferential faults in these later units (fig. 

12). 

Further strongly suggestive evidence that the Claim 

Canyon cauldron resurged following eruption of the 

Paintbrush Tuff is the structurally high position of the 

Pah Canyon Member and its reasonable former extension 

upward, prior to erosion (fig. 13). Also the main part and 

the tuff of Chocolate Mountain of the Tiva Canyon are 

structurally higher within the cauldron than outside the 

cauldron or just inside the wall (section B-B', fig. 13). 

Minor faults of very little displacement are not shown in 

the cross sections in order to emphasize the stratigraphy 

and structural position of the units. The present authors 

are also aware that these observed relations between the 

intracauldron and extracauldron rocks could also have 

come about, without magmatic resurgence, by extensional 

basin-range faulting related to regional right-lateral transcurrent movement along the Walker Lane (Locke and 

others, 1940)-the cauldron segment remaining 

structurally high, while the radial extracauldron blocks 

subsided. We do not favor this alternative hypothesis as the 

local control, because the intense circumferential fault 

pattern within the cauldron segment indicates more movement than in the blocks outside and, moreover, is typical 

of an outer arcuate fault pattern of a magmatically 

resurgent dome. 

TIMBER MOUNTAIN TUFF AND ROCKS RELATED 

TO TIMBER MOUNTAIN CALDERA 

ORIGINAL DEFINITION AND REDEFINITION OF 

TIMBER MOUNTAIN TUFF 

The Timber Mountain Tuff (Orkild, 1965, p. A46-A47) 

was originally defined to include the composite sequence 

shown in the left column of figure 7. This sequence 

comprised the Rainier Mesa Member at the base 

successively overlain by the tuff of Cat Canyon, the tuff of 

Transvaal (tuff of Camp Transvaal of Lipman, 

Quinlivan, and others, 1966), and the Ammonia Tanks 

Member. Thin bedded tuff sequences at the base of the ashflow members were included with the overlying member. 

The Timber Mountain Tuff is here redefined to include 

all quartz-bearing ash-flow tuffs and thin ash-fall tuffs 

erupted about 11 m.y. ago (Marvin and others, 1970) from 

the Timber Mountain caldera center-the type area 

(Orkild, 1965). The tuff includes in ascending order, the 

Rainier Mesa and Ammonia Tanks Members of the 

original definition and, as here redefined, the tuff of 

Buttonhook Wash (Carr and Quinlivan, 1966) and the tuff 

of Crooked Canyon (Byers and others, 1976). The known 

areal distribution of the two newly added informal units is 

confined within the Timber Mountain-Oasis Valley 

caldera complex (fig. 7). Outside the complex, in all 

surface exposures, the Ammonia Tanks Member is the 

upper unit of the Timber Mountain Tuff, as in the 

original definition (Orkild, 1965). The Ammonia Tanks 

Member is redefined on the basis of field and petrographic 

studies to include the tuff of Cat Canyon and the tuff of 

Transvaal (fig. 7, first column; Orkild, 1965; Carr and 

Quinlivan, 1966; Lipman, Quinlivan, and others, 1966). 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 39 

These units are equivalent to part or all of the Ammonia 

Tanks, as mapped in many of the 7l-2-minute quadrangles 

of the Nevada Test Site region (fig. 2). Thin sequences of 

bedded ash-fall tuffs that are intercalated with the ash-flow 

units are included in the Timber Mountain Tuff and were 

generally included with the overlying ash-flow unit on 

quadrangle maps of the Nevada Test Site (fig. 2). A bedded 

ash-fall sequence above the Ammonia Tanks Member was 

penetrated in a few drill holes in Yucca Flat (fig. 7, column 

2). This bedded tuff, however, is less than 5 m (15ft) thick 

and has not been recognized in surface exposures. 

Quartz phenocrysts as much as 3 mm long are common 

to abundant in both the ash-flow and associated thin ashfall tuffs, in contrast to the absence or near-absence of 

quartz in the underlying and overlying stratigraphic 

units. An exception to this general statement is the uppermost several meters of the underlying bedded tuff, in 

which quartz crystals increase in abundance and size 

upward to the Paintbrush-Timber Mountain contact. 

Several lithologic units, closely associated with the 

Timber Mountain Tuff in genesis or in time, were not 

included in the original definition (Orkild, 1965) and are 

here likewise excluded-analogous to the exclusion of 

similar units from the Paintbrush Tuff. These units 

include debris flows that intertongue with the Rainier 

Mesa Member, alkali-calcic rhyolite lavas, and tuff dikes 

and other intrusives on Timber Mountain resurgent 

dome. The ash-flow tuff members and informal tuff units 

of the Timber Mountain Tuff and their relations to coeval 

debris flows, lavas, and intrusive rocks, are discussed on 

following pages. 

RAINIER MESA MEMBER 

The Rainier Mesa Member, the basal member of the 

Timber Mountain Tuff, was originally described by 

Hinrichs and Orkild (1961, p. D97) from excellent 

exposures at Rainier Mesa (fig. 14). These authors 

included the Rainier Mesa Member as the uppermost 

member of the Oak Spring Formation. Poole and 

McKeown (1962, p. C6l), a year later, included the Rainier 

Mesa Member as the uppermost unit of their Pia pi Canyon 

Formation of the Oak Spring Group. Still later, Orkild 

(1965, p. A49) defined Timber Mountain Tuff (fig. 7) and 

placed the Rainier Mesa Member at the base. 

An isopach map of the Rainier Mesa Member 

generalized from Byers and others ( 1968) is shown in figure 

I4. The Rainier Mesa is more than 500 m (I ,500 ft) thick, 

and the base is not exposed in the west wall of Timber 

Mountain caldera, where the unit is densely welded and is 

strongly devitrified with granophyric texture in the 

pumice. The coarse textural features suggest that at least 

another hundred metres may be present in the unexposed 

lower part in the west wall. A thickness of 396m ( 1,300 ft) 

was penetrated in a drill hole on Pahute Mesa. The total 

volume of the Rainier Mesa Member, as calculated from 

the isopach map, is in excess of 1,200 km3 (300 mi3). 

The upper quartz latitic caprock of the Rainier Mesa 

Member was locally mapped along the wall of Timber 

Mountain caldera as the tuff of Falcon Canyon (Hinrichs 

and others, 1967; Orkild and others, I969). This usage is 

not retained. This upper quartz latitic subunit is exposed 

within the caldera near the northeastern wall at test well 8 

(fig. 14) and also in the bottom of Beatty Wash where it 

cuts the southwestern wall of the caldera. In both localities 

the quartz latite intertongues with debris flows (figs. 3 and 

7). The debris flows pinch out northward in the southwest caldera wall and are absent where 450+ m (I ,500+ ft) of 

Rainier Mesa Member is exposed in the Transvaal Hills 

(fig. 14). Here the quartz latitic subunit of the Rainier 

Mesa is slightly thicker than elsewhere outside the caldera, 

but only about 15 m (50 ft) of quartz latitic caprock was 

penetrated by drill hole UEI8r. The upper quartz latitic 

subunit was emplaced during late stages of caldera 

collapse (Byers and others, 1969, p. 94-95), as exhibited by 

the intertonguing relations with the debris flows. 

The Rainier Mesa Member is a compositionally zoned 

compound cooling unit consisting of an extensive highsilica rhyolite tuff overlain with a partial cooling break by 

a considerably thinner quartz latitic tuff that is restricted 

to the vicinity of Timber Mountain caldera. The rhyolitic 

subunit, where exposed outside the caldera, is commonly 

pink nonwelded shard tuff at the base grading upward 

into densely welded black vitrophyre (Byers and others, 

1968, p. 93). The vitrophyre grades upward into dense 

brown to pale-red devitrified tuff with common white 

flattened pumice lenticles, ranging from 2.5 to 10 em (l-4 

in.). In the west wall of Timber Mountain caldera (fig. I4), 

the devitrified zone is coarser than elsewhere and the 

pumice is granophyric. In general, the devitrified zone 

grades upward into a light-gray crystalline zone with 

spherulitic sanidine and cristobalite in the pumice, typical 

of Smith's (1960b) vapor-phase zone. The vapor-phase 

zone of the high-silica rhyolite subunit commonly grades 

within a few feet into a vitrophyric or dark-brown cryptocrystalline quartz latite, which in turn grades upward into 

a second light-colored vapor phase zone, visible in only a 

few places because of erosion or colluvial cover. Dark 

quartz latitic pumice lenticles in the quartz latite are somewhat larger than the white rhyolitic pumice in the 

underlying rhyolite. Where the Rainier Mesa Member is 

396 m ( l ,300 ft) thick in the drill hole in the west part of 

Silent Canyon caldera, the main cooling unit described 

above is underlain by a nonwelded phenocryst-poor pink 

shard tuff about ISO m (500ft) thick. The phenocryst ratios 

are similar to those of the lower rhyolitic part of the overlying main cooling unit of the Rainier Mesa. These two 

tuffs may be separate cooling units, and the Rainier Mesa, 

like the overlying Ammonia Tanks Member, may be a 

composite sheet (Smith, I960a, p. 812). 
40 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPL EX, NEVADA 

EXPLANATION 

THICKNESS OF RAINIER MESA 

: ~:~:::E: 300-450 m 

> 450 m 

PRE-RAINIER MESA RHYOLITE 

I 

UE18r 

0 

LL__A' 

LAVAS 

High-silica rhyolite lava 

Rhyolite lavas of Windy Wash 

Feeder dike 

Limit of information other than 

caldera walls 

Drill holes used for control - Numbered holes discussed in text 

Line of geologic cross section 

0 5 

0 5 10 

.... MOUNT 

\_ JACKSON 

10 15 20 MILES 

15 20 KILOM ETRES 

FIGURE 14.-Generalized isopach map of the Rainier Mesa Member of Timber Mountain Tuff and 

The unique petrography of the Rainier Mesa Member 

distinguishes it from cooling units of the underlying 

Paintbrush Tuff, as well as from overlying units of the 

Timber Mountain Tuff (fig. 15 ). Analyses of 10 high-silica 

rhyolite samples show an average of about 77 percent 

silica, a percentage similar to that of the subunits of the 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 41 

distribution of petrologically related, pre-Rainier Mesa lavas. Geologic cross section shown in figure 23. 

Paintbrush Tuff (W. D. Quinlivan and P. W. Lipman, 

written commun., 1974, table 6; Lipman, 1966). The 

Rainier Mesa rhyolite, however, is crystal rich and 

contains quartz, in contrast to crystal-poor quartz-free 

rhyolites of the Paintbrush Tuff. The quartz latitic 

caprock contains a mixture of hornblende-rich and 
42 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

FE LSIC PH ENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

Quartz 

STRATIGRAPHIC UNIT -(SUBUNIT 

Number of thin sections (N) 

in parentheses 

PHENOCRYSTS AS PERCENTAGE 

OF TOTAL ROCK 

~ Alkal i feldspar 

Plagioclase 

I I I I I 

I Upper tuff ( 7) 

Tuffs of 

Crooked Canyon ~ 

L Lower tuff (8) -

li->''.''1 

~ ~ 

{ Quartz latite ( 14) 

Buttonhook 

Tuff of 

Wash 

High-silica rhyolite (3) 

-

-

!>,,,,,,,,.,,,,,,,, 

~,0'/##/#_#tj 

r Rhyolite (5) (mostly 

"' high-silica rhyolite) c. 

Q; 

c. 

c. 

-

Quartz latite ( 31) Ammonia :::J 

Tanks 

Member ~ Rhyolite (41) (inc ludes 

'" c. high-silica rhyolite) 

Q; 

s 

0 

_J Quartz latite (27) 

Pre-Ammonia Tanks 

rhyolite lavas 

Quartz 

(9) 

latite (5) 

-

- (intracaldera) 

Rainier Mesa Quartz latite ( 12) 

Member -< (extracaldera) 

(::· ··· ..•. ... ·.·.·.·.-.,, 

High-silica 

rhyolite (37) 

I High-sili ca rhyolite 

Pre-Rainier 

Mesa lavas ~ Rhyolite 

lavas 

lavas 

( 5) 

of 

-

r,::' :,1 L Windy Wash (9) 

- ~ 

c__ __ c__l~ __ c__I __ L___ I __ L___ I _ _ I I I I I 

0 10 20 30 40 50 0 20 40 60 80 1 00 

PERCENT 

FIGURE l5.-Modal and silica ranges of units of Timber Mountain Tuff and petrologically related lavas in order of stratigraphic succession. 

(!962), Lipman (1966), and W. D. Quinlivan 

clinopyroxene-rich mafic scoria pumices with subordinate quartz-rich rhyolitic pumice similar to that in the 

underlying rhyolite subunit (Byers and others, 1968, p. 95 ). 

The Rainier Mesa Member, like the Topopah Spring 

Member, differs from the overlying ash-flow units of the 

Timber Mountain Tuff in containing no sphene. The 

ratio of plagioclase to total feldspar phenocrysts is also 

higher in the Rainier Mesa high-silica rhyolite than in the 

rhyolites of overlying ash-flow units of the Timber 

Mountain Tuff. This feldspar phenocryst variation is 

similar to that in rhyolitic subunits of members of Paintbrush Tuff (fig. 9). Sanidine phenocrysts in the high-silica 

rhyolitic subunit of the Rainier Mesa are relatively 

potassic and have cryptoperthite rims in contrast to microperthitic sodic sanidine and anorthoclase in the rhyolites 

of younger cooling units of the Timber Mountain Tuff 

(O'Connor, 1963). 

The Rainier Mesa Member of the Timber Mountain 

Tuff is the same unit as cooling unit 4 of Cornwall and 

Kleinhampl ( 1964, p. J10) in the Bullfrog Hills (fig. 14). 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 43 

MAFIC PHENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

-Total mafics EJl Hornblende 

~ Biotite ~ NUMBER OF SILICA PERCENTAGE RANGE 

(Recalculated H, 0 and CaC0 3 free) 

Number of analy.ses (N) in parentheses 

Clinopyroxene SPHENE PHENOCRYSTS 

(RI Resorbed PER THIN SECTION 

-

• 

0 10 15 

PERCENT 

a 

(R) 

""'~ (R) 

0 

I 

I 

0 10 20 64 66 

I I 

(0) 

10) 

(0) 

(18) 

(4) 

I 

(1) 

(6) 

I I 

68 70 72 

PERCENT 

I I I 

I 

Ill 

I 

(1) 

124) 

~ (3) 

~ 

~ (2) 

~ 

I (3) I I 

74 76 78 

Moda l analyses by F. M. Byers, W. D. Quinlivan, and J. T. O 'Connor, and from Cornwall (1962, ta ble 2). Silica percentages from Cornwall 

and P. W. Li)Jman (written commun., 1974). 

The K-Ar age is ll.l m.y. (Marvin and others, 1970), and 

the thermal remanent magnetization is reverse (G. D. 

Bath, written commun., 1965). 

AMMONIA TANKS MEMBER 

The Ammonia Tanks Member was defined by Orkild 

(1965) as the uppermost unit of the Timber Mountain 

Tuff; its type locality is at Ammonia Tanks (fig. 16). 

Because of evidence to be presented in the following paragraphs, the Ammonia Tanks is here redefined to include 

the tuff of Cat Canyon on Timber Mountain resurgent 

dome (Carr and Quinlivan, 1966) and the tuff of Transvaal 

(Orkild, 1965; tuff of Camp Transvaal of Lipman, 

Quinlivan, and others, 1966). (See fig. 7). An intracaldera 

section of the Ammonia Tanks is displayed at West Cat 

Canyon (fig. 16), where the tuff of Cat Canyon consists of a 

lower part of two mapped subunits, unit A and unit C 

(Lipman, Quinlivan, and others, 1966, section B-B'), 

which are more than 600 m (2,000 ft) thick and which are 

overlain by an upper parl (unitE and unit F) about 300m 
44 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

~~? r---------~"'r·,~··----~--.----~--------~ '·~··~-- ------------------­ sroNEWALL 

EXPLANATION 

THICKNESSES OF AMMON IA 

TANKS MEMBER- ISOPACHS 

DASHED WHERE APPROXIMATELY LOCATED 

0-150m 

I50-300 m 

300-450 m 

450-600 m 

>600 m 

- Pre-Ammonia Tanks rhyolite lavas 

UE18r 

Limit of information 

Inferred buried wall of caldera that 

formed with eruption of Ammonia 

Tanks Member 

0 Drill holes used for control - Numbe red holes discussed in text 

A A' --- Geologic section 

0 5 

0 5 10 

.... MOUNT MOUNTAIN ~~/ ~ JACKSON 

~,,. 

10 15 20 MILES 

15 20 KILOMETRES 

FIGURE 16_-Generalized isopach map of Ammonia Tanks Member of Timber Mountain Tuff, showing great thickness within Timber Mounshown in 

(I ,000 ft) thick. An extracaldera section is in the Transvaal 

Hills (fig. 16; see also fig. 23), where two mapped subunits 

of the tuff of Camp Transvaal total about 150m (500ft) in 

thickness (Lipman, Quinlivan, and others, 1966) and 

constitute the lower part of the Ammonia Tanks Member 

(Byers and others, 1976, section B-B' ). The upper part of 

the Ammonia Tanks at this section rests with slight 

angular discordance but without a complete cooling break 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 45 

tain caldera , and associated lava flows that are petrologically similar to Ammonia Tanks high-silica rhyolite subunits. Geologic cross section 

figure 23. 

on the lower part of the Ammonia Tanks (Byers and 

others, 1976, section B-B'). 

A revised isopach map of the Ammonia Tanks Member 

(fig. 16; compare with Byers and others, 1968, fig. 5) 

includes the tuffs of Cat Canyon and Transvaal. The 

Ammonia Tanks is more than 900 m (3,000 ft) thick 

beneath the Timber Mountain resurgent dome and 

slightly more than 450 m (1,500 ft) thick at Oasis 
46 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

Mountain just inside the wall of the Oasis Valley caldera 

segment. Elsewhere, the member is generally less than 150 

m (500 ft) thick. The total volume of the redefined 

Ammonia Tanks Member is about 900 km3 (230 

mi 3)-somewhat less than the volume of the Rainier Mesa 

Member. 

The following field and subsurface relations show that 

the Ammonia Tanks Member outside Timber Mountain 

caldera is the same cooling unit as the tuff of Cat Canyon 

on the Timber Mountain resurgent dome inside the 

caldera. The tuff of Buttonhook Wash overlies the 

Ammonia Tanks with complete cooling break both on the 

resurgent dome and on the west rim of Timber Mountain 

caldera. The stratigraphic succession penetrated in drill 

hole UE18r (fig. 16) on the buried north flank of the dome 

includes from top to bottom: the tuff of Cat Canyon (now 

part of the Ammonia Tanks Member), rhyolite lava, debris 

flows, and the upper part of the Rainier Mesa Member. 

This succession is the same as in surface exposures outside 

the collapse area of the caldera associated with the 

eruption of the Ammonia Tanks (fig. 16). The Ammonia 

Tanks in the drill hole is directly correlative with the lower 

part of the tuff of Cat Canyon in surface exposures several 

kilometres ( a few miles) south on Timber Mountain 

dome. The contact between the Ammonia Tanks Member 

and the overlying tuff of Buttonhook Wash is marked by 

about 10 em (several inches) of fine ash-fall tuff at 

exposures within and outside the caldera. The contact 

zone with the underlying rhyolite lava consists of about a 

metre (a few feet) of bedded ash-fall tuff, both inside and 

outside the area of subsidence associated with the eruption 

of the Ammonia Tanks Member (fig. 16). Moreover, the 

rhyolite lava is a high-silica rhyolite, petrologically 

similar to high-silica rhyolite of the Ammonia Tanks (fig. 

15 ). Where the rhyolite lava is missing, the Ammonia 

Tanks is separated from the Rainier Mesa Member by 

debris flows and bedded tuff inside the caldera and by not 

more than 20 m (70 ft) of thick-bedded tuff outside the 

caldera. The Ammonia Tanks Member, therefore, is 

tightly bracketed inside and outside the caldera by the 

underlying units and by the overlying tuff of Buttonhook 

Wash. 

The Ammonia Tanks Member is compositionally far 

more complex than the Rainier Mesa Member. The 

Ammonia Tanks is probably a composite sheet as defined 

by Smith (l960a, p. 812- 813), for a local complete cooling 

break in the extracaldera Ammonia Tanks on Pahute 

Mesa (Noble and others, 1967) must be represented by one 

of several partial cooling breaks at the intracaldera section 

on Timber Mountain dome. The Ammonia Tanks 

Member is divided into upper and lower parts in order to 

show fault displacement on the geologic map of the 

Timber Mountain caldera area (Byers and others, 1976). 

These two parts can be further subdivided on the basis of 

composition into quartz latite and (high-silica) rhyolite, 

as shown in figure 15. The tuff of Cat Canyon (now part of 

the Ammonia Tanks), originally included six map units 

on Timber Mountain resurgent dome (Carr and 

Quinlivan, 1966). In general, our lower quartz latite 

includes Carr and Quinlivan's units A and B; our lower 

rhyolite, their units C and D; our upper quartz latite, their 

unitE; and our upper rhyolite, their unit F; complex intertonguing of these units on the dome, however, makes 

direct correlation difficult in some localities. Chemical 

analyses of 25 intracaldera specimens and 23 specimens of 

extracaldera specimens were made available by W. D. 

Quinlivan and P . W. Lipman (written commun., 1974). 

The four compositional subunits of the Ammonia 

Tanks Member (fig. 15) are not everywhere present outside 

the area of Ammonia Tanks subsidence (fig. 16), and are 

not precisely correlative over the entire extent of the 

Ammonia Tanks. A few extra lenses of the upper quartz 

latite intertongue with the upper rhyolite on Timber 

Mountain dome (Carr and Quinlivan, 1966; 1968, p. 101). 

In the Dead Horse Flat quadrangle (fig. 2) on eastern 

Pahute Mesa, Noble, Krushensky, MfKay, and Ege (1967) 

found a complete cooling break between their lower part, a 

quartz latite, and their main part, consisting of rhyolite 

overlain by quartz latite. In the area of the extracaldera 

section on the west rim of Timber Mountain caldera, 

Lipman, Quinlivan, Carr, and Anderson (1966) mapped a 

10° angular unconformity between a lower rhyolitic part, 

which shows a compound cooling zonation (their tuff of 

Camp Transvaal), and an upper part, which consists of 

rhyolite and quartz latite (their Ammonia Tanks 

Member). These two parts are seemingly fused together in 

many places but locally a few centimetres of nonfused 

bedded tuff separates them (P. W. Lipman, oral commun., 

1969). On the east flank of Timber Mountain dome a 

gentle angular discordance with a partial cooling break 

occurs between a faulted quartz latite and an overlying 

unfaulted local high-silica rhyolite, both within the upper 

part of the Ammonia Tanks (fig. 17). 

The lowest compositional subunit (fig. 15), where 

present, is a quartz latitic vitrophyre with sparse mafic 

lava xenoliths, especially in the western outcrop area of 

the Ammonia Tanks (fig. 16). This subunit extends 

eastward to Shoshone Mountain and eastern Pahute Mesa 

(fig. 16). In a few localities bordering the tuff dike zone on 

the eastern flank of Timber Mountain dome, the presumed 

equivalent subunit is a moderate brown devitrified tuff 

with conspicuous biotite; the base is not exposed. The 

overlying subunit, the rhyolite of the lower part (fig. 15 ), is 

typically a light-gray high-silica rhyolite with sparse 

mafic lava xenoliths and wedge-shaped phenocrysts of 

sphene. On Timber Mountain dome this subunit is 

compositionally gradational through hundreds of feet 

downward through low-silica rhyolite into the underlying quartz latite. 

Peripheral to Timber Mountain in the northern moat 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 47 

FIGURE 17.-Structural unconformity between uppermost high-silica rhyolite and underlying quartz latite of upper part of Ammonia Tanks 

Member. North wall of tributary to Brushy Canyon on east flank of Timber Mountain resurgent dome. Timber Mountain caldera wall 

in far right background. Timber Mountain dome had begun to rise (left or west side of photograph) before uppermost unit involved 

in doming was extruded, probably through tuff dike zone. The upper (light) unit here has nearly a complete cooling break with the 

faulted dark welded quartz latite tuff below, whereas a short distance to the west there is only a partial cooling break. 

area of Timber Mountain caldera (fig. 16), a thin extracaldera rhyolite, less than 15m (50ft) thick, is composed of 

a pink nonwelded basal zone grading up through a few 

feet of partly welded gray vitrophyre into an upper lightgray devitrified zone. This rhyolite reappears as the lower 

subunit of the local main part on eastern Pahute Mesa 

(Noble and others, 1967). Thin-section modes of this 

subunit, a high-silica rhyolite, are pooled with the lower 

rhyolitic subunit of Timber Mountain dome (fig. 15) 

because the two subunits have similar stratigraphic 

positions within the Ammonia Tanks. This high-silica 

rhyolite is overlain by an upper quartz latite (as many as 

three quartz latites occur at this stratigraphic position on 

Timber Mountain dome) that can scarcely be distinguished lithologically or petrographically from the 

lower quartz latite of the Ammonia Tanks (fig. 15). 

Locally between Ammonia Tanks and test wellS (fig. 16) 

this upper quanz latite subunit becomes more mafic than 

elsewhere, having relatively large phenocrysts, including 

clinopyroxene, as much as 2.5 mm, and biotite books, as 

much as 4 mm, which are poikilitic with apatite and 

zncon. 

The uppermost subunit, the upper rhyolite (fig. 15), is 

found only on the east flank of Timber Mountain dome 

(fig. 17). The rhyolite is largely white, devitrified, and has 

spherulitic pumice, characteristic of the vapor phase zone. 

It is petrographically identical to most of the high-silica 

rhyolite dikes of the tuff dike zone and contains brown 

xenoliths of the lower quartz latite subunit, which the 

dikes intrude. 

The average of several K-Ar ages determined on the 

Ammonia Tanks Member is 11.1 m.y. (Kistler, 1968; 

Marvin and others, 1970). The Ammonia Tanks Member 

is equivalent to cooling unit 5 of Cornwall and 

Kleinhampl (1964, p. J10) in the Bullfrog Hills (fig. 16). 

Both the extracaldera and intracaldera facies of the 

Ammonia Tanks have a normal thermal remanent 

magnetization (G. D. Bath, written commun., 1965). 

TUFF OF BUTTONHOOK WASH 

The tuff of Buttonhook Wash of Carr and Quinlivan 

( 1966) is a thin compositionally zoned cooling unit above 

the Ammonia Tanks Member and is here included with 

the redefined Timber Mountain Tuff. The best exposures 
48 

37" 

00' 

111,,_.. 

BULLFFfOG 

HILLS.-'' ,'' 

TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

MESA 

..... "'''111111;-;,11,,,,,, __ 

0 

0 

CRATER 

FLAT 

10 20 Ml LES 

10 20 KILOMETRES 

EXPLANATION 

Thickness of intracaldera tuffs- Dashed 

where approximately located 

Intrusives cutting resurgent domeDirection of dip in degrees 

L____A ' Geologic cross section 

-

::j\1: 

FIGURE !B.-Combined thicknesses of tuffs of Buttonhook Wash and Crooked Canyon and intrusive rocks on Timber Mountain 

resurgent dome. Tuff of Buttonhook Wash has not been recognized in eastern Timber Mountain caldera and overlying 

tuffs of Crooked Canyon have not been recognized in the Oasis Valley caldera segment. Geologic cross section shown 

in figure 22. 

are in Buttonhook Wash and in West Cat Canyon, on the 

southwest flank of Timber Mountain dome (fig. 18). 

Canyon is shown in figure 18. Both tuffs are largely 

confined to the western part of the Timber 

Mountain-Oasis Valley caldera complex. The tuff of 

Buttonhook Wash thins over Timber Mountain resurgent 

The distribution of the tuff of Buttonhook Wash 

together with that of the overlying tuffs of Crooked 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 49 

dome, mainly by pinchout of.its quartz latite caprock (fig. 

3). The thickness of the tuff of Buttonhook Wash on the 

western flank of the dome is about 60 m (200 ft); if the 

downdip thickening is projected under the caldera moat, 

the tuff may thicken to more than 150m (500ft). The tuff 

of Buttonhook Wash is found locally above the Ammonia 

Tanks Member in Oasis Valley and apparently just 

outside the Oasis Valley caldera segment in the eastern 

Bullfrog Hills, where the tuff may have flowed a short 

distance against an eroded wall. The tuff of Buttonhook 

Wash is not present in the intracaldera drill hole UE18r 

(fig. 18), nor in the eastern and northern moat areas of 

Timber Mountain caldera. 

The tuff of Buttonhook Wash is separated from the 

underlying Ammonia Tanks Member on Timber 

Mountain dome by about 10 em of fine ash-fall tuff, and 

the lower part is glassy and non welded, indicating a brief 

but complete cooling break. On the west wall of Timber 

Mountain caldera, the tuff of Buttonhook Wash is also 

separated from the underlying Ammonia Tanks by about 

10 em of ash-fall tuff. On Timber l\tlountain dome the tuff 

of Buttonhook Wash is separated from the overlying tuffs 

of Crooked Canyon by a very slight angular unconformity that is probably related to resurgent doming of 

Timber Mountain. Less than 60 em of bedded shard tuff 

separates the two units. 

The tuff of Buttonhook Wash consists of a lower lightgray moderately welded high-silica rhyolite and an upper 

reddish- to purplish-brown densely welded quartz latite 

that is locally vitrophyric. The high-silica rhyolite is 

indistinguishable from high-silica rhyolites of the 

Ammonia Tanks Member (fig. 15). The quartz latite 

caprock, however, is distinguished from Ammonia Tanks 

quartz latites by ( 1) plagioclase consistently in excess of 

alkali feldspar phenocrysts (fig. 15), (2) clinopyroxene in 

excess of biotite, and (3) all phenocrysts rarely exceeding 

1.5 mm in contrast to phenocrysts as much as 4.0 mm long 

in the Ammonia Tanks. Sparse hornblende where the tuff 

is not oxidized also distinguishes the tuff of Buttonhook 

Wash from the upper quartz latite of the Ammonia Tanks. 

These petrographic features of the quartz latite caprock 

make the tuff of Buttonhook Wash an excellent stratigraphic marker. 

The high Si02 content of the quartz latite, nearly 73 

percent (fig. 15 ), is probably due to slight silicification of 

the devitrified rock. 

One K-Ar age of 10.5 m.y., determined on sanidine by 

Kistler ( 1968, p. 254), is available from the tuff of Buttonhook Wash. Kistler (1968) referred to the unit as "tuff of 

Cat Canyon, upper cooling unit," a logical designation at 

that time. This K-Ar age is believed to be slightly young 

because of slight sericitization of the rock (Marvin and 

others, 1970, p. 2666). The slight thinning of the tuff onto 

the Ammonia Tanks Member (average K-Ar age of 11.1 

m.y.) over the resurgent dome suggests that resurgent 

doming had already started before eruption of the tuff of 

Buttonhook Wash. However, this slight angular discordance is not believed to represent a major time break, 

for slight angular discordances without a complete 

cooling break are present within the Ammonia Tanks 

Member on the east flank of the dome (fig. 17). 

TUFFS OF CROOKED CANYON 

The tuffs of Crooked Canyon are here included as an 

uppermost intracaldera unit of the redefined Timber 

Mountain Tuff (fig. 7) as.shown on the map of the Timber 

Mountain caldera area (Byers and others, 1976). On the 

east flank of Timber Mountain dome the unit was mapped 

as tuff of Buttonhook Wash by Carr and Quinlivan (1966) 

and by Byers, Rogers, Carr, and Luft (1966). The exposed 

nonwelded distal edges of two ash-flow tuffs constituting 

the tuffs of Crooked Canyon are each less than 8 m (25ft) 

thick where they onlap the northeast flank of the dome 

(fig. 18) in Crooked Canyon. Downdip in the subsurface 

off the northern flank of Timber Mountain, the tuffs 

thicken rapidly to more than 250 m (800 ft) toward the 

outer edge of the caldera and become partly welded; they 

were penetrated in the interval from 335 to 593 m ( 1, I 00 to 

1,945 ft) in drill hole UE18r in the northern moat of the 

caldera (figs. 3, 18). The total thickness of the upper tuff in 

the drill hole is about 180m (600ft); the lower tuff, 75 m 

(245 ft) thick, was not cored. The great increase in 

thickness, from the exposed flank of Timber Mountain 

dome outward to drill hole UE18r, indicates that the tuffs 

of Crooked Canyon were emplaced after the resurgent 

central dome was well advanced. 

The tuffs of Crooked Canyon overlie the tuff of Buttonhook Wash or the Ammonia Tanks Member with a minor 

angular unconformity (fig. 3) and overlie the tuff of 

Buttonhook Wash in the central graben of the Timber 

Mountain resurgent dome. The contact zone between the 

tuffs of Crooked Canyon and the underlying Ammonia 

Tanks Member or tuff of Buttonhook Wash consists of 1 m 

or less of uniformly finegrained ash-fall shard tuff; moreover, the contact zone between the two ash-flow tuffs of 

Crooked Canyon contains 10-60 em of fine ash-fall tuff, in 

contrast to as much as 18m of variegated thin-bedded fine 

to coarse ash-fall tuff separating the tuffs of Crooked 

Canyon from the overlying post-Timber Mountain 

cooling units of the caldera fill (Byers and others, 1976). 

No compositional zoning in either of the tuffs of 

Crooked Canyon is apparent. The lower tuff is a simple 

non welded to moderately welded cooling unit of uniform 

rhyolitic composition, almost everywhere devitrified and 

pinkish to purplish gray, except at the distal edges 

flanking Timber Mountain dome. It contains fewer and 

smaller phenocrysts, with slightly more mafic phenocrysts than the high-silica rhyolite in the upper part of the 

Ammonia Tanks (fig. 15). Near the tuff dike zone, it 
50 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

contains distinctive brown xenoliths characteristic of the 

lower quartz latite of the Ammonia Tanks Member in the 

tuff dike zone. A few thin tuff dikes (fig. 18), identical in 

composition to the lower tuff, also contain these brown 

xenoliths, indicating the probable source of the tuff (see 

footnote description in Byers and others, 1966). 

The upper tuff is orange brown, glassy, and nonwelded 

near its pinchout on the northeast flank of Timber 

Mountain dome and likewise contains brown xenoliths 

from the lower quartz latite of the Ammonia Tanks in the 

tuffdike zone, suggesting a possible source similar to that 

of the lower tuff. The upper tuff contains, in addition, 

diagnostic phenocryst-poor glass xenoliths and 

phenocryst-rich pumice lumps. In drill hole UE18r, the 

upper tuff is partly welded and, except for the glass 

xenoliths and its stratigraphic position above the lower 

tuff and the Ammonia Tanks, is almost indistinguishable 

petrographically from similar parts of the Ammonia 

Tanks (fig. 15 ). 

The K-Ar age of the tuffs of Crooked Canyon is 

bracketed by the 10.5-m.y. age of the tuff of Buttonhook 

Wash and by the K-Ar age of 9.5 m.y. on the overlying tuff 

of Cutoff Road (Lipman, Quinlivan, and others, 1966; tuff 

of the caldera fill of Kistler, 1968, p. 254). The 10.5-m.y. KAr age of the tuff of Buttonhook Wash, however, is 

probably slightly young because of alteration of the rock 

(Marvin and others, 1970, p. 2666). More likely the K-Ar 

age of the tuffs of Crooked Canyon is close to II m.y., as 

suggested by the stratigraphic relations: less than 60 em of 

ash-fall occurs between the tuffs of Crooked Canyon and 

the underlying units of the Timber Mountain Tuff, 

whereas about 18 m (60 ft) of thin-bedded ash-fall tuff 

separates the overlying, 9.5-m.y., unit from the tuffs of 

Crooked Canyon near their pinchout on the east flank of 

Timber Mountain dome. On the west side of the Oasis 

Valley caldera segment, about 300m (1,000 ft) of volcanic 

rocks, comprising bedded tuff, three ash-flow tuff cooling 

units, and a lava flow, occurs within this same 

stratigraphic interval in caldera fill (fig. 23 ). Moreover, the 

cooling units within the 300-m caldera-fill interval are 

petrologically dissimilar from the underlying quartzbearing tuffs of Crooked Canyon and other units of the 

Timber Mountain Tuff as herein defined. From this stratigraphic and petrologic evidence, the tuffs of Crooked 

Canyon apparently are close in K-Ar age to the Ammonia 

Tanks Member, possibly about 11 m.y. 

The petrologic and inferred close-time relations 

between the upper units of the Timber Mountain Tuff are 

consistent with the relatively small volume of the tuffs of 

Crooked Canyon, their lack of compositional zoning, 

their generally nonwelded to partly welded character, and 

their stratigraphic position as the youngest tuffs of the 

Timber mountain eruptive sequence. All these features 

indicate a marked decrease in the eruptive activity of the 

Timber Mountain magma chamber (fig. 3) following 

voluminous eruptions of the Rainier Mesa and Ammonia 

Tanks Members and the onset of resurgent doming of 

Timber Mountain. 

DEBRIS FLOWS 

The term "debris flows" as used here designates clastic 

deposits emplaced by rapid flowage of coarse and fine 

detritus. Such deposits are exposed within Timber 

Mountain caldera near the eastern and northern walls and 

within the Oasis Valley caldera segment in the Transvaal 

Hills and near the west wall just west of Oasis Mountain 

(figs. 14, 16). The unit overlies and locally intertongues 

with the uppermost part of the Rainier Mesa Member in 

the vicinity of test well 8 and in Beatty Wash where it 

crosses the southern end of the Transvaal Hills (fig. 14) but 

is not considered part of the redefined Timber Mountain 

Tuff. The debris flows underlie the Ammonia Tanks 

Member and pre-Ammonia Tanks rhyolite lava in drill 

hole UE18r (fig. 3). The unit has not been found outside 

Timber Mountain caldera or the Oasis Valley caldera 

segment. 

The debris flows are best exposed on the east side of the 

Timber Mountain caldera south of test well 8 (fig. 16) 

where the unit consists mostly of blocks of welded tuff as 

much as 6 m (20 ft) long in a yellowish-gray to grayishorange tuffaceous matrix. The lower part of the deposit is 

nonsorted and nonstratified and apparently represents a 

single debris flow, but the highest part of the unit contains 

well-sorted and bedded lenses of reworked tuff which 

separate several thinner flows. These thinner flows 

contain more fine tuffaceous matrix with fewer angular 

cobbles than does the single flow, and the thinner flows 

may have been emplaced as successive mudflows. All rock 

types present in the caldera wall, including the lower 

rhyolitic subunit of the Rainier Mesa Member, are present 

in the coarse detritus. Densely welded tuff of the Paintbrush Tuff, Grouse Canyon Member of the Belted Range 

Tuff, and rhyolite of the lower part of the Rainier Mesa 

Member occur as blocks in the debris flows along the east 

wall of Timber Mountain caldera; fragments of rhyolite 

lavas predominate inside the north wall where pre- and 

post-Tiva Canyon rhyolite lavas are exposed. In the 

southern part of the Transvaal Hills, fragments of intracauldron Paintbrush Tuff are most abundant and indicate 

a source from the Claim Canyon cauldron segment. The 

debris flows are exposed in places inside the Oasis Valley 

caldera segment west and south of Oasis Mountain and 

contain an assemblage of blocks similar to those of the 

debris flows inside Timber Mountain caldera. 

Debris flows related to caldera collapse were penetrated 

in drill hole UE18r from a depth of 1,183 to 1,442 m (3,880 

to 4,730 ft). They are overlain by pre-Ammonia Tanks 

rhyolite lava and underlain by the Rainier Mesa Member. 

Blocks many feet across are contained in a tuffaceous 

matrix. Most of the debris is welded tuff of the Paintbrush 

Tuff, but rhyolite lava intercalated with Paintbrush and 
TIMBER MOUNTAIN TllFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 51 

blocks of rhyolitic Rainier Mesa are also present. No rocks 

older than Paintbrush Tuff were identified. 

The stratigraphic position of the debris flows, their 

intertonguing relation with the uppermost Rainier Mesa 

quartz latitic tuff, the presence of rhyolitic blocks from the 

lower part of the Rainier Mesa, and finally the location of 

the exposed ends of the flows just within caldera walls all 

indicate that the bulk of the debris flows were generated 

catastrophically within a short period of time as a result of 

caldera collapse related to the eruption of the Rainier Mesa 

Member. The debris flows contain assorted discrete blocks 

of the lower rhyolitic subunit of the Rainier Mesa, 

indicating the presence of already solidified Rainier Mesa 

in the caldera wall. This relation further suggests that the 

Rainier Mesa Member is a composite ash-flow sheet in 

Smith's (1960a) terminology. 

LAVAS PETROLOGICALLY RELATED TO 

TIMBER MOUNTAIN TUFF 

PRE-RAINIER MESA LAVAS 

Lava flows underlie the Rainier Mesa and Ammonia 

Tanks Members. The lava flow immediately under each 

member is petrographically similar to the member which 

overlies it. The pre-Rainier Mesa lavas (fig. 14) are 

divisible into two petrographic types: low-silica rhyolite 

lavas with sphene-the rhyolite of Windy Wash of 

Christiansen and Lipman ( 1965 )-and high -silica 

rhyolite lavas without sphene and with few mafic phenocrysts, petrographically and chemically like the highsilica rhyolitic tuff of the Rainier Mesa Member (fig. 15 ). 

The stratigraphic relations between these two rhyolites are 

not certain, but the fact that the high-silica rhyolite is 

petrographically more similar to the overlying Rainier 

Mesa Member than to the rhyolite of Windy Wash suggests 

that the high-silica lava postdates the rhyolite of Windy 

Wash. Moreover, the rhyolite of Windy Wash bears some 

petrologic affinity to the underlying Tiva Canyon 

Member of the Paintbrush Tuff and to the post-Tiva 

Canyon rhyolite lavas, mainly in sphene content and in 

the compositiOn of opaque iron-titanium oxides 

(Lipman, 1971). The rhyolite lavas of Windy Wash 

commonly have a dark basal vitrophyre overlain by a palered devitrified interior. Feeder dikes of these flows are 

exposed as radial dikes in the south wall of Timber Mountain caldera (Byers and others, 1976), and one dike can 

actually be traced into the flow on the rim of the caldera. A 

rhyolite lava identical in lithology and chemistry (W. D. 

Quinlivan and P. W. Lipman, written commun., 1974) to 

the rhyolite of Windy Wash was penetrated in two drill 

holes in Silent Canyon caldera (fig. 14). This lava was 

called "quartz-rich lava of Scrugham Peak quadrangle" 

by Byers and Cummings ( 1967) and by Orkild, Sargent, 

and Snyder ( 1969; see composite diagram of their usage in 

fig. 7). The nearly identical thin-section modes of flows in 

these two areas on opposite sides of Timber Mountain 

caldera are shown in figure 19. The close similarity in 

composition of the lavas in these two areas suggests that 

the lavas were erupted at about the same time from the 

same compositional layer in the underlying magma 

chamber. 

The high-silica rhyolite lava crops out on the southwest side of the Timber Mountain-Oasis Valley caldera 

complex in the vicinity of Beatty (fig. 14). It has a typical 

glassy envelope with a light-gray devitrified interior. The 

lava is rather sparsely porphyritic and contains about 

equal numbers of quartz and sanidine phenocrysts; the 

sanidine has cryptoperthite(?) reaction rims similar to 

SOUTH 17 miles NORTH 

FLANK FLANK 

(5 specimens) (4 specimens) 

Ql 30 RANGE WITHIN 

:2 

J 

SAMPLE SUITE "iii 

a; 

D a. Sphene 20 

~ ~ ~ 

. 

Apatite u 

0 10 Ql ~ ~ Zircon .0 

E w El ::::1 Allanite (perrierite?) z 0 

"' 5 

~ u 4 0 

c: 

[] Ql 

.c: Biotite a. 3 

0 

~ Ql 2 Hornblende 3' 

c: 

Ql 

~ c.. 0 

60 "' Ql D ........ 

D u Alkali feldspar ~£ u .... (Percentage of phenocrysts) 0 0 c c 40 Ql "' 

. 

D .c ..... 

crcore LJcore Plagioclase a. 0 

0 Ql (Percentage of phenocrysts) Cl 

Ql ~ Cl c 20 I ~ ~ I Rim 

Estimated mol percent c .... Ql I Rim Ql anorthite in plagioclase ~ a. Qlc.. 0 phenocrysts 

2: 0 

30 

, ."'- I u Total phenocrysts 

e "' 

~ 

, 

(Percentage of total rock) 

<ti u 

0 

0 ~ 20 

~ .... .c: Quartz a. 0 ..... (Percentage of phenocrysts) 

Ql 0 

ClQl 

"' Cl 10 [] Total mafic phenocrysts c ~ Ql c u Ql (Percentage of phenocrysts) 

Qj ~ m c.. Ql 

c.. D 

0 

FIGURE 19.-Phenocryst mineralogy of rhyolite lava of Windy Wash 

(pre-Rainier Mesa lavas) on opposite flanks of Timber Mountain 

caldera. Lava flow on north flank penetrated in drill holes (figs. 

3, 14). 
52 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

those of sanidine phenocrysts in the Rainier Mesa 

Member. 

The pre-Rainier Mesa rhyolite lavas correlate in part 

with "rhyolite flows and intrusives" of Cornwall and 

Kleinhampl (1964, p. Jll; pl. 4). A K-Aragedeterminedon 

the high-silica rhyolite under the Rainier Mesa Member 

east of Beatty is 11.3 m.y. (Kistler, 1968, p. 255 ), the same 

age as the Rainier Mesa Member (Marvin and others, 1970, 

p. 2261). 

PRE-AMMONIA TANKS LAVAS IN TIMBER MOUNTAIN 

CALDERA MOAT 

The pre-Ammonia Tanks lavas are exposed near the 

north and southwest walls of Timber Mountain caldera 

(fig. 16) beneath thin glassy exposures of the Ammonia 

Tanks Member. These outcrops are in the outer annular 

moat which subsided during the eruption of the Rainier 

Mesa Member but which was not affected by collapse 

related to the eruption of the Ammonia Tanks Member 

(fig. 16). The lavas in the northern moat area of Timber 

Mountain caldera were called "rhyolite lavas of Timber 

Mountain caldera moat" on the map of the Scrugham 

Peak quadrangle (Byers and Cummings, 1967; fig. 2). In 

addition to the surface exposures, a lava flow was 

penetrated in the interval from 1,082 to 1,183 m (3,550 to 

3,880 ft) in drill hole UE18r (figs. 3, 16) between the 

Ammonia Tanks Member and the underlying debris flows 

which partly filled the collapse area related to eruption of 

the Rainier Mesa Member. 

The pre-Ammonia Tanks lavas are glassy in the lower 

parts and have subjacent locally fused bedded tuff. The 

interiors of the flows are purplish gray laminated to light 

gray microcrystalline with conspicuous quartz phenocrysts. The upper parts are light gray, porous, glassy, and 

commonly brecciated, typical of most rhyolite lavas. The 

lavas contain sphene and are petrochemically (W. D. 

Quinlivan and P. W. Lipman, written commun., 1974) 

generally similar to rhyolitic parts of the Ammonia Tanks 

but contain fewer phenocrysts. Thin-section modes (fig. 

15) of the northern lava are the same as those of the 

southern lava (fig. 16), indicating a close magmatic and 

temporal relationship similar to that between the 

northern and southern bodies of the rhyolite of Windy 

Wash of pre-Rainier Mesa age (fig. 19) already described. 

The cycle consisting of eruption of lava followed by 

eruption of tuff petrologically similar to the lava appears 

to have been repeated. 

A core of the uppermost zeolitized brecciated part of preAmmonia Tanks rhyolite lava from drill hole UE18r 

contained about 1 percent phenocrysts, consisting of 

sparse quartz, sanidine, and 1 phenocryst of sphene. This 

nearly aphyric lava may have been extruded at a slightly 

different time than the lavas exposed in the northern and 

southern moat areas of Timber Mountain caldera, but the 

sample may not be truly representative. 

The age of the pre-Ammonia Tanks rhyolite lavas is 

bracketed by K-Ar ages of 11.3 m.y., determined on the 

Rainier Mesa Member (Marvin and others, 1970), and 11.1 

m.y., determined on specimens of the Ammonia Tanks 

Member of the Timber Mountain Tuff (Kistler, 1968). The 

thermal remanent magnetization is reverse like that of the 

underlying Rainier Mesa but unlike that of the overlying 

Ammonia Tanks Member (G. D. Bath, written commun., 

1965 ). 

INTRUSIVE ROCKS AND RELATION TO 

RESURGENT DOMING 

Intrusive rocks on Timber Mountain resurgent dome 

that are largely post-Ammonia Tanks comprise an outer 

tuff-dike zone, an inner microgranite-porphyry ring dike 

(Carr, 1964, p. Bl7), and intrusive rhyolite mainly in the 

central part of the dome (fig. 18). These intrusive rocks are 

petrologically closely similar to either high-silica 

rhyolitic or quartz latitic subunits of the Timber 

Mountain Tuff; these similarities, together with position 

and field relations of the intrusive rocks on Timber 

Mountain resurgent dome, imply a close genetic relation 

to the Timber Mountain Tuff. Presumably the tuffs, the 

related lavas, and the intrusive masses were derived from 

the same underlying magma that res urged and caused the 

doming of Timber Mountain. 

The tuff dike zone crops out on the outer eastern flank of 

the Timber Mountain dome and parallel to it (fig. 18). The 

dikes dip steeply inward toward the center of Timber 

Mountain (fig. 20). They are mostly high-silica rhyolite in 

composition and range in thickness from 0 to 8 m (26ft). 

They intrude a distinctive brown devitrified facies of lower 

quartz latite of the Ammonia Tanks in an area about 150m 

(500 ft) wide. Some dikes appear to be feeders to the 

uppermost high-silica rhyolite subunit of the Ammonia 

Tanks, for this subunit is not only petrographically 

identical with these dikes (fig. 21), but also contains the 

same distinctive brown xenoliths of the lower quartz latite 

subunit of the Ammonia Tanks. These dikes have a sharp 

contact with the Ammonia Tanks but show no evidence of 

chilling, indicating that the Ammonia Tanks was still hot 

at the time of intrusion. 

A few dikes are petrologically similar to the two tuffs of 

Crooked Canyon, which also contain xenoliths of 

Ammonia Tanks quartz latite. The slightly finer grain size 

of these dikes, at the contact indicates chilling and 

suggests that the dikes were intruded after the Ammonia 

Tanks had cooled somewhat. Although it is not possible to 

distinguish petrographically between dikes that may have 

been feeders for either the lower or upper tuff of the tuffs of 

Crooked Canyon, the presence of xenoliths in the tuffs 

indicates that their vent source was probably the tuff dike 

zone. No modal ranges of these dikes are shown in figure 

21, because the feldspar phenocrysts are too altered to 

allow distinguishing sanidine and plagioclase. 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 53 

FIGURE 20.-Rhyolitic tuff dike cuts basal quartz latite of Ammonia 

Tanks Member and dips inward (westerly) along arcuate zone 

around the east flank of Timber Mountain dome. Dikes were probably feeders for uppermost high-silica rhyolite of Ammonia Tanks 

and tuffs of Crooked Canyon. Flattened pumice ( P; barely visible) 

and biotite foliation in quartz latite host rock are parallel to walls 

of dike. Note fragments of dark quartz latite tuff in dike and white 

(recrystallized) nonflattened pumice in quartz latite host rock. Exposure in Brushy Canyon (fig. 18). 

The microgranite porphyry ring dike consists of several 

alined discordant bodies (fig. 18) that intrude the lower 

part of the Ammonia Tanks Member. The dike is in an 

inner ring-fracture zone closer than the tuff dike zone to 

the apex of the Timber Mountain dome. Here the 

Ammonia Tanks is extremely thick (fig. 16) and may have 

extended as much as 600 m (2,000 ft) above the highest level 

penetrated by the ring dike. The contacts of the various 

bodies are abrupt, and the porphyry shows no change in 

texture or mineralogy at the borders. The lack of chilled 

borders suggests that the Ammonia Tanks host rock was 

still hot when the dikes were emplaced. Several of the dikes 

and irregular bodies broaden downward, presumably to 

connect at depth. The ring-dike system is generally 

parallel to an arcuate fault zone, a short distance to the 

southeast. Both the arcuate fault zone and also the ring 

dike dip steeply southeast (fig. 18), away from the central 

part of Timber Mountain resurgent dome. The intrusion 

of the porphyry dike probably accompanied the resurgent 

doming of Timber Mountain (Carr, 1964; Carr and 

Quinlivan, 1968, p. 103-104). 

The microgranite porphyry is nearly uniform in general 

appearance, texture, and color; small bodies differ only 

slightly in texture from the largest. It is medium- to lightgray nonfoliated fairly homogeneous porphyritic rock 

containing about 50 percent phenocrysts of abundant 

feldspar as much as I em across, minor biotite, and clinopyroxene (fig. 21) in a very finely granular groundmass. 

Quartz and sphene phenocrysts are absent. The large 

feldspar phenocrysts consist of small plagioclase cores less 

than 3 mm in size, rimmed by a thick jacket of alkali 

feldspar. The groundmass is largely quartz and alkali 

feldspar, 0.1 to 0.2 mm in size and partly as granophyric 

intergrowths. The microgranite porphyry is almost 

chemically identical with the more mafic quartz latitic 

upper part of the Ammonia Tanks that contains 66-68 

percent silica (W. D. Quinlivan and P. W. Lipman, 

written commun., 1974). The microgranite porphyry, 

however, had attained a more advanced stage of crystallization, as evidenced by (l) larger and slightly more 

abundant phenocrysts (fig. 21) and (2) more abundant 

alkali feldspar in the form of thick outer jackets enclosing 

plagioclase in the larger phenocrysts. Mafic quartz latites 

of the Ammonia Tanks and also the microgranite 

porphyry contain little if any quartz and sphene. The 

name "microgranite porphyry" was applied to these rocks 

prior to obtaining chemical analyses (W. D. Quinlivan 

and P. W. Lipman, written commun., l9H). 

In addition to microgranite porphyry dikes, pipelike 

and plug like bodies of rhyolite, and probably quartz latite, 

intrude the upper part of the Ammonia Tanks Member on 

Timber Mountain dome. Contact zones are commonly 

glassy and gradational through a horizontal zone of as 

much as 90 m (300 ft); considerable mixing and 

remobilization occurred between the glassy intrusives and 

the tuff subunits of the Ammonia Tanks. A few intrusives 

are coextensive with small extrusive domes that rest on, 

and show a complete cooling break from, the upper part of 

the Ammonia Tanks. Two petrologic types in different 

structural settings on the dome are included in the 

intrusive rhyolite. The rhyolite of Parachute Canyon (fig. 

18), probably a quartz latite, is on the north flank of the 

dome near the inner ring-fracture zone, which is not well 

exposed here. In contrast, the rhyolite lavas of East Cat 

Canyon are composed of high-silica rhyolite, and were 

extruded from vents along faults of the central graben of 

the resurgent dome. 

The rhyolite of Parachute Canyon is a plug dome, as 

evidenced by vertical foliation near steep contacts. Vertical 

foliation is lacking in areas where the rhyolite reached the 

surface and flowed onto the upper part of the Ammonia 

Tanks Member. The interior of the body is light gray and 

porphyritic, and contains abundant small lithophysae, 

but it is encircled by a darker, finer grained border zone 

that is locally glassy, particularly on its northwest side 

where it apparently flowed onto the Ammonia Tanks. The 

border zone appears to consist of a few hundred metres of 
54 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

FELSIC PHENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

ROCK UNIT J SUBUNIT 

Number of thin sect ions (N) 

in parentheses 

PHENOCRYSTS AS PERCENTAGE 

OF TOTAL ROCK 

[2] Quartz ~ Alkali feldspar 

~ Plagioclase 

high si lica (3) 

Intrusive rhyolite ---------1 

{ 

East Cat Canyon; 

Parachute 

Canyon (3) 

Microgranite porphyry 

ring dike (4) 

Tuff-dike zone high -

silica rhyolite (4) 

Ammonia Tanks 

Member -

Upper part { 

High-silica 

rhyolite (5) 

Quartz Latite (31) 

0 

I I 

-

-

-

I I I 

10 20 30 I 

40 

I 

,,,,.,,,,,,,.,,,,,,,,,,,,,,,,,,,,,,:j 

~ 

V'''''''''''''d 

~ 

Wffffff&M 

I 

Ll "" 

I I I 

50 0 20 40 60 80 

PERCENT 

I 

100 

FIGURE 21.-Modal and silica ranges of intrusive rocks on Timber Mountain compared with ranges of high-silica rhyolite and quartz latite 

Member of Timber Mountain Tuff. Silica analyses from 

Ammonia Tanks near the apex of the Timber Mountain 

resurgent dome (fig. 18). The pipes are localized along the 

faulted margin of a northwest-trending central graben, 

described by Carr and Quinlivan (1968, p. 105). The 

central parts of the pipes are light gray and devitrified, 

whereas the border zones are generally glassy or finely 

spherulitic. In a few places a crystallized transitional 

border zone as much as 15 m (50 ft) wide contains 

alternating bands of rhyolite having different phenocryst 

contents and probably represents a zone of intermixed 

intrusive high-silica rhyolite and remobilized quartz latite 

wallrock of the Ammonia Tanks Member. The modal 

range of three specimens and the silica content of 76.8 

percent approach the composition of a typical high-silica 

rhyolite of the Ammonia Tanks Member (fig. 21). 

The inferred relations among the Ammonia Tanks 

Member in the Timber Mountain resurgent dome, the 

petrologically related intrusives, and an inferred underlying compositionally zoned magma chamber are shown 

in figure 22. The two-layer compositional zoning of the 

upper part of a large magma chamber is based on models 

suggested by Quinlivan and Lipman (1965), by Lipman, 

Christiansen, and O'Connor (1966), and by Smith and 

Bailey ( 1966). The inward-dipping tuff dikes formed from 

high-silica rhyolite magma near the apical part of the 

magma chamber and are somewhat analogous to cone 

sheets (Anderson, 1936). The rhyolite intrusives of East 

mixed flow-banded rhyolite and probably fused mobilized 

quartz latite wallrock of the upper part of the Ammonia 

Tanks Member. Parts of the intrusive mass are quartz rich, 

and others are quartz poor, thus indicating a pronounced 

internal compositional variation that may reflect 

composite intrusions. 

The porphyritic central part of rhyolite of Parachute 

Canyon, whose mode is shown in figure 21, is more likely a 

quartz latite. Phenocrysts of alkali feldspar, as much as 8 

mm across, enclose cores of subordinate plagioclase, with 

minor embayed quartz and biotite, all in a microcrystalline groundmass. This central part petrographically shows a striking textural and modal similarity 

to the microgranite porphyry dike (fig. 21 ), except for 

smaller groundmass crystals and somewhat fewer phenocrysts, and it probably represents a more chilled finegrained upward extension of similar microgranite 

porphyry at depth. The quartz and sphene in the central 

part may represent mixture at one stage or another of 

chilled rhyolitic tuff of the Ammonia Tanks Member or of 

similar magma composition. The Parachute Canyon vent 

is also located near the inner ring-fracture zone on the 

flank of the Timber Mountain dome, a structural position similar to that of the porphyry ring dike on the south 

side (fig. 18). 

Pipelike intrusives of high-silica rhyolite, one grading 

upward into a plug dome, cut the upper part of the 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 55 

MAFIC PHENOCRYSTS AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

- Total mafics 

~ Biotite 

PERCENT 

[[] Hornblende 

f22l Clinopyroxene 

(R) Resorbed 

I 

(R) 

~ 

b ;_ 0//fl ///1 

(R) 

0 2.5 5.0 

NUMBER OF SPH ENE 

PHENOCRYSTS 

PER THIN SECTION 

0 

SILICA PERCENTAGE RANGE 

(H 2 0 and C0 2 free) 

Number of analyses (N) in parentheses 

I 

(1) 

(0) 

(0) 

I 

(1) 

1--1 

(6) 

(From lower part) 

(18) 

70 72 74 76 78 

PERCENT 

subunits of upper part of Ammonia Tanks Member. Age relations uncertain except that intrusive rocks generally postdate Ammonia Tanks 

W. D. Quinlivan and P. W. Lipman (written commun., 1974). 

A SOUTH 

M I LES 

2 

SEA 

LEVEL 

2 

3 

D 

TIMBER MOUNTAIN RESURGENT DOME 

CENT RAL GRABEN 

EXPLANATION 

Post-Timber Mountain rocks 

Post-Ammonia Tanks in tracaldera tuffs 

of Buttonhook Wash and Crooked 

Canyon of Timber Mountain Tuff 

NORTH 

CALDERA MOAT 

Ammonia Tanks Member of Timber 

Mountain Tuff 

Pre-Ammonia Tanks rocks 

2 

SEA 

LEVEL 

3 

FIGURE 22.-Generalized interpretive section across Timber Mountain resurgent dome, showing inferred relations between compositionally 

zoned magmatic source of Timber Mountain Tuff and related intrusives cutting Timber Mountain resurgent dome. Line of section shown 

in figure 18. 
56 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

Cat Canyon, localized along marginal faults of the central 

graben, also formed from the high-silica rhyolite in the 

upper part of the magma chamber_ The outward-dipping 

microgranite porphyry ring dike formed from quartz 

latitic magma, which averaged about 68 percent silica, and 

lay below the high-silica rhyolite interface_ 

This idealized model of the magma chamber would 

probably be attained prior to and reestablished a short 

time after the voluminous extrusion of the Ammonia 

Tanks Member with its complex compositional gradations of high-silica rhyolite and quartz latite_ Intermixing 

of these two compositions within the Ammonia Tanks 

Member inside the Timber Mountain caldera indicates 

disturbance of the interface (fig_ 22) and probably the 

extrusion of the Ammonia Tanks Member from many 

vents, as originally suggested by Quinlivan and Lipman 

(1965 )-

CALDERA COLLAPSES RELATED TO 

ERUPTIONS OF RAINIER MESA AND 

AMMONIA TANKS MEMBERS 

Discussion of areas of collapse related to eruptions of the 

Rainier Mesa and Ammonia Tanks Members of the 

Timber Mountain Tuff has been deferred until all the 

petrologically related quartzose igneous rocks that are 

defined as belonging to the Timber Mountain caldera 

center have been described_ Timber Mountain caldera, as 

shown in figure 1 and other illustrations of this report, was 

originally defined not on the basis of its extrusive products 

but as a volcano-topographic feature in the Williams 

(1941) sense_ The perspective diagram (see frontispiece) 

illustrates the extent of a subcircular caldera wall 

enclosing a moatlike annular area surrounding the central 

dome. Only in the northwestern part, where Thirsty 

Canyon has been incised, is the caldera wall lacking. 

These topographic features, indicative of a caldera and 

central resurgent dome, were recognized by R. L. Smith as 

early as 1960, and the name Timber Mountain was applied 

to the caldera and resurgent dome after the topographically prominent central mountain. There are, 

however, not one but two voluminous coextensive ashflow sheets, petrologically similar and erupted within a 

few hundred thousand years of one another, that are distributed peripherally to Timber Mountain caldera. Did 

Timber Mpuntain caldera as defined topographically 

result as a collapse feature related mainly to eruption of 

the Rainier Mesa Member, or is it a composite caldera 

resulting from two or more eruptions of Timber 

Mountain Tuff, or, still a third possibility, did other 

adjacent areas subside, producing a volcano-tectonic 

depression, during eruption of the members? These alternatives and their possible combinations are considered 

next. 

The known areal extents of both the Rainier Mesa and 

Ammonia Tanks Members are closely similar, as a 

comparison of figures 14 and 16 will show. The extracaldera thickness of the Rainier Mesa Member, however, is 

locally two to three times thicker than the Ammonia 

Tanks in former topographic depressions such as Silent 

Canyon caldera; elsewhere the Rainier Mesa Member is 

generally 50-100 percent thicker than the Ammonia 

Tanks (Byers and others, 1968, p. 91 and 93). Inasmuch as 

the intracaldera thickness of the Rainier Mesa Member is 

not known, only a minimum volume of 1,200 km3 (300 

mi3) of the Rainier Mesa Member can be inferred, 

compared with a more accurately known total volume of 

900 km3 (230 mi3) of the Ammonia Tanks. The intracaldera thickness of the Rainier Mesa might well be twice 

that of the Ammonia Tanks, and the total volume of the 

Rainier Mesa Member might well be nearly twice that of 

the Ammonia Tanks. Obviously, the approach of 

comparing the extracaldera volumes of the members with 

the areas and volumes of collapse is fraught with many 

unknowns, including the intracaldera thickness of the 

Rainier Mesa. A further complicating factor is the 

possibility that an inner Ammonia Tanks ring-fracture 

zone may have also been loci for some collapse along an 

inner zone during eruption of the Rainier Mesa Member. 

As a first approach, however, it seems likely that the area of 

collapse related to the Rainier Mesa eruption would be 

somewhat larger than that related to the Ammonia Tanks 

eruption. Fortunately, more direct evidence bearing on 

this problem is available. 

The area of collapse related to the eruption of the 

Ammonia Tanks Member is considered first, because 

thicknesses, facies, and other geologic relations are somewhat better known than those parameters of the Rainier 

Mesa Member. The areas of collapse related to eruption of 

the Ammonia Tanks Member are inferred mainly from 

sequences more than 300 m (1,000 ft) thick that are 

strongly devitrified and contain granophyric pumice. 

Inside Timber Mountain caldera the main area of collapse 

is outlined in figure 16. Another probable area of 

Ammonia Tanks collapse is indicated in the Oasis Valley 

caldera segment by the 450-m-thick (1,500-ft) strongly 

devitrified Ammonia Tanks exposed on Oasis Mountain. 

The thick granophyric facies of the Ammonia Tanks 

Member on Timber Mountain resurgent dome was 

penetrated in drill hole UE18r (fig. 3), but core from the 

drill hole lacks the upper part of the Ammonia Tanks, 

suggesting that the area occupied by Timber Mountain 

dome further collapsed within an innermost ring-fracture 

zone, inside the area of outer collapse enclosed by the 300-

m isopach (fig. 16). The area around the large mass of preAmmonia Tanks rhyolite lava (fig. 16) is obviously 

outside the caldera related to Ammonia Tanks collapse, 

although it is inside the prominent north wall of Timber 

Mountain caldera. Between this north wall and the 

inferred wall related to Ammonia Tanks collapse (fig. 16), 

the Ammonia Tanks Member is glassy, partly welded, and 
TIMBER MOUNTAIN TUFF AND ROCKS RELATED TO TIMBER MOUNTAIN CALDERA 57 

less than 60 m (200 ft) thick-typical of the extracaldera 

facies. 

Similar relations occur just inside the south wall of 

Timber Mountain caldera along a narrow annular zone 

about 1.5 km wide in which a pre-Ammonia Tanks lava 

flow is overlain by thin glassy Ammonia Tanks (fig. 16). 

This narrow zone, obviously, was not within the collapse 

area related to the Ammonia Tanks Member, but this zone 

is within the caldera wall continuous with that related to 

the eruption of the Rainier Mesa Member. 

Oasis Mountain (fig. 16) occurs in a large westerly 

salient or "scallop" of the Oasis Valley caldera segment. 

The mountain is composed of at least 450 m (1,500 ft) of 

dense devitrified ash-flow tuff of the Ammonia Tanks 

having very low porosity. The general attitude of the 

Ammonia Tanks as determined by the basal contact (fig. 

23) and by a prominent parting in the middle is 30°-35° 

eastward, but in the upper part stretched granophyric 

pumice dips 70°-80°. The upper part probably slid 

downdip, possibly as a result of caldera collapse farther 

east in the caldera segment. 

The Ammonia Tanks thins to about 150m (500ft) over 

moderately westward-dipping Rainier Mesa in the 

Transvaal Hills (fig. 23) on the east edge of the Oasis 

Valley caldera segment; a 10°-15° angular discordance 

occurs between the Ammonia Tanks and Rainier Mesa. 

Locally, an angular unconformity as much as 10° occurs 

between the westerly dipping upper and lower parts of the 

Ammonia Tanks. The thinner upper part chilled to a 

vitrophyre and is locally "frozen" to the thicker more 

devitrified lower part. The local small angular 

discordance and the implied partial cooling break indicate 

minor contemporaneous westward tilting during 

emplacement of the Ammonia Tanks Member. 

In summary, the area of collapse related to the eruption 

of the Ammonia Tanks was mainly peripheral to the 

Timber Mountain resurgent dome, because that is where 

the Ammonia Tanks is thickest and where the resurgence 

took place. High-silica rhyolite lavas that petrologically 

resemble parts of the Ammonia Tanks are present in the 

Timber Mountain caldera moat just outside the area of 

collapse. Thicknesses of more than 450 m (1,500 ft), 

steeply dipping stretched pumice at Oasis Mountain, and 

a local 10° angular unconformity within gently westward 

dipping Ammonia Tanks in the Transvaal Hills suggest 

that the floor of the Oasis Valley caldera segment partially 

subsided and tilted westward with the eruption of the 

Ammonia Tanks. This subsidence, however, was 

considerably less than that peripheral to Timber 

Mountain dome. 

The approach to the problem of the area of Rainier 

Mesa subsidence is more indirect and perhaps less 

convincing than in the case of the Ammonia Tanks. 

Several years ago D. C. Noble (written commun., 1966) 

suggested that the area of collapse related to the eruption 

of the Rainier Mesa Member included not only Timber 

Mountain caldera, but also Oasis Valley caldera segment 

and most of Sleeping Butte segment as well. There was 

little direct evidence to support his hypothesis at the time, 

but as geologic mapping and field checking became 

completed in the caldera segments west of Timber 

Mountain caldera, the hypothesis of a larger area of 

subsidence related to the Rainier Mesa eruption has 

become more attractive. 

There can be little doubt that the topographically 

defined Timber Mountain caldera was the main site of 

collapse caused by eruption of the Rainier Mesa Member. 

Under the description of debris flows, emphasis was 

placed on the unique intertonguing relation of the debris 

with the uppermost part of the Rainier Mesa Member, and 

on the fact that the debris flows lap onto the north and east 

walls of the topographically defined Timber Mountain 

caldera. The debris flows, moreover, contain blocks 

locally derived from the adjacent caldera wall. Clearly 

then, the debris flows, at least the basal parts of them that 

intertongue with the uppermost Rainier Mesa, record the 

late stages of collapse related to the eruption of the Rainier 

Mesa (Byers and others, 1968). The onlapping relation of 

the debris flows to the caldera wall outlines the limits of 

structural movement related to extrusion of the Rainier 

Mesa. Although the Rainier Mesa and the debris flows are 

buried by younger deposits elsewhere around the 

continuation of the caldera wall, the continuation of the 

wall and onlapping thin Ammonia Tanks at a few places 

just inside the wall indicate that Timber Mountain 

caldera, as defined herein, was also the site of collapse 

associated with eruption of the Rainier Mesa Member as 

well as the Ammonia Tanks. 

The Transvaal Hills (fig. 14) form the exposed west wall 

of Timber Mountain caldera as topographically 

defined-yet here the Rainier Mesa Member is more than 

450 m ( 1,500 ft) thick, and the base is not exposed. The unit 

has a gray granophyric texture, typical of thick ash-flow 

tuffs. Debris flows intertongue with the uppermost 

Rainier Mesa and overlie it in the same relation as within 

Timber Mountain caldera. Although the Rainier Mesa 

dipsabout25° W., suggestingadipoutwardaway from the 

caldera wall, the debris flows exposed between the Rainier 

Mesa and the Ammonia Tanks thin and pinch out northward in the Transvaal Hills along a line about 5 km (3 mi) 

north of the south wall of the Oasis Valley caldera 

segment. Moreover, the debris flows are exposed under the 

Ammonia Tanks for a few miles westward just inside the 

south wall of Oasis Valley caldera segment, analogous to 

the relations along the north and east walls of Timber 

Mountain caldera. 

At Oasis Mountain (figs. 14, 16), in the westerly salient 

or "scallop" of the Oasis Valley caldera segment, the thick 

eastward-dipping Ammonia Tanks rests on at least 90 m 

(300 ft) of debris flows, whose base is not exposed (see fig 
58 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX. NEVADA 

FEET 

6000 

A WEST 

Geologic relations 

projected under 

alluvium from 

outcrop to south 

CALDERA ~ 

Oasis Mountain 

4ooo~==~~~~R~1Mf=~f:J~A 

SEA 

LEVEL 

2000 

BASIN-RANGE FAULTS 

OASIS VALLEY CALDERA SEGMENT 

Tuff of Fleur-de-lis Ranch 

EXPLANATION 

Alluvium and minor intercalated tuff 

Tuff of Cutoff Road 

Rhyolite lava of West Cat Canyon - lntertongues with tuffs of Fleur-<le-lis 

Ranch 

Tuffs of Fleur-<le-lis Ran ch - Includes 

underlying tuffaceous sedimentary 

rocks 

Timber Mountain Tuff 

Tuff of Buttonhook Wash 

Ammonia Tanks Member 

Rainier Mesa Member 

Debris flow - Intertongues with uppermost part of Rainier Mesa Member 

FIGURE 23 (above and right).-Geologic section across Oasis Valley ca ldera segment, showing great thickness of Rainier Mesa and Ammonia 

Tanks Members and presence of debris flows of Timber Mountain caldera on east and west sides of segment. Line of section shown 

in figures 14 and 16. Geology of Oasis Valley area from P. P. Orkild, K. A. Sargent, and R. L. Christiansen (unpub. da ta, 1971). 

23 ). The debris flows contain rhyolite blocks of the Rainier 

Mesa and Tiva Canyon Members similar in lithology to 

the same units exposed in the Bullfrog Hills to the west. 

Again, these relations are analogous to those inside 

Timber Mountain caldera, where blocks in the debris 

flows are derived from the adjacent wall. In fugure 23, we 

have interpreted the western limit of Rainier Mesa 

collapse to lie just west of Oasis Mountain. The deeper 

part of the cauldron subsidence was probably a few miles 

farther east under Oasis Valley, as indicated by a strong 

gravity gradient (D. L. Healey, written commun., 1969). 

Alternatively, the thick exposure of the Rainier Mesa 

sheet in the Transvaal Hills may not have accumulated in 

its own cauldron subsidence, but may have accumulated 

in an earlier Oasis Valley segment of the Claim Canyon 

cauldron, as suggested by Christiansen and others (1976). 

We recognize this probability, because the outline of the 

buried Claim Canyon cauldron shown in figures 8 and 10 

includes the Transvaal Hills. However, the additional 

evidence, just cited, concerning the extent of the debris 

flows around the edges of Oasis Valley caldera segment 

suggests partial collapse during the Rainier Mesa 

eruptions. 

Also suggesting that the west wall of the subsidence 

associated with the Rainier Mesa eruption may be west of 

the Transvaal Hills are the two, Rainier Mesa-related, 

high-silica rhyolite lavas just east and north of Beatty (fig. 

14). Other lavas previously discussed, especially the preAmmonia Tanks lavas (fig. 16), are generally within a few 

kilometres of the cauldron subsidence associated with the 

petrologically related tuffs. The high-silica rhyolite petrologically related to the Rainier Mesa Member (fig. 15) in 

the Bullfrog Hills (fig. 14) would be more than 15 km (9 

mi) away from a caldera wall located in the Transvaal 
YOUNGER INTRACALDERA ROCKS, TIMBER MOUNTAIN AND OASIS VALLEY CALDERAS 59 

OASIS VAL LEY CA LDERA SEGMENT 

Tuff of Buttonhook Wash 

Oasis Valley 

____________ Tma __- ...--

Tmr ~ ---- - ---- ..-- -- -------- Pre·Rainier 

-- Mesa rocks 

------------------ ----

0 

0 2 

Hills. ll is far more likely, on the basis of comparison of 

the location of other rhyolite lavas to known related 

cauldron subsidences, that the Oasis Valley caldera 

segment collapsed during eruption of the Rainier Mesa. 

The availability nearby of high-silica rhyolite magma 

indicates that the volatile-rich magma near the top of 

magma chamber probably underlay Oasis Valley. 

D. C. Noble (written commun., 1966) interpreted the 

westward-dipping thick Rainier Mesa Member in the 

Transvaal Hills to be the tilted remnant of a central 

resurgent dome. According to the resurgent dome hypothesis, post-Rainier Mesa magmatic pressure related to an 

underlying mobile body of batholithic proportions may 

have caused the uplift and westward tilt, possibly forming 

a central resurgent dome that foundered with the eruption of the Ammonia Tanks Member. Christiansen and 

others (1976), however, emphasize several observations 

that are detrimental to the hypothesis. 

In conclusion, we believe the evidence strongly suggests 

that westward tilting and partial subsidence of the Oasis 

Valley caldera segment may have occurred along with subsidence of the Timber Mountain caldera during eruptions 

of both the Rainier Mesa and Ammonia Tanks Members. 

These repeated composite subsidences would thereby 

outline a volcano-tectonic depression, as originally 

defined by van Bemmelen (1930, 1939). The Timber 

Mountain caldera and its adjoining Oasis Valley caldera 

segment probably overlay the apical part of a magma 

chamber of batholithic magnitude. 

Transvaal Hills 

Pre-R ainier 

Mesa rocks 

BASIN-RANGE FAULTS 

2 M I LES 

3 KILOMETRES 

EAST A' 

KILOMET RES 

TIMBER 

MOUNTAIN 

CALDERA 

CALDERA 

FAULT 

2.0 

1. 5 

SEA 

LEVE L 

0.5 

1. 0 

YOUNGER INTRACALDERA ROCKS IN TIMBER 

MOUNTAIN AND OASIS VALLEY CALDERAS 

Following the climactic eruptions and resurgence at the 

Timber Mountain center there was a brief period of 

relative quiescence with local small lakes in the caldera 

moat. The earliest bedded tuffs on the flank of the dome 

are sandy, well sorted, and cross-laminated. These features 

suggest that the ash-fall tuff was reworked by water and 

that little, if any, volcanic activity took place. Moreover, 

the earliest mafic lavas are palagonitized, which suggests 

interaction with water. Locally as much as 30m (100ft) of 

sediments accumulated, including calcareous sandstone 

and siltstone in both Oasis Valley and Timber Mountain 

calderas. Upon renewal of volcanic activity, the composition of the first intracaldera tuffs and lavas had drastically 

changed from highly silicic rhyolites of the Timber 

Mountain Tuff, to plagioclase-rich low-silica rhyolites, 

rhyodacite, and mafic lavas. The chemical character, 

however, of the post-Timber Mountain rocks remained 

alkali-calcic. 

RHYODACITIC AND MAFIC LAVAS 

The oldest post-Timber Mountain rocks to be extruded 

on the flank of the resurgent dome were intermediate to 

mafic lavas and relatively restricted in volume. Two of the 

largest areas of these lavas are on the east and south flanks 

of Timber Mountain dome (fig. 24). The smaller lava flow 

on the southern flank is an olivine-bearing trachyandesite 

(latite of Lipman, Quinlivan, and others, 1966) which has 
60 

3 

00' 

TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

ftllt,,,,,, 

'} 

0 

CRATER 

FLAT 

116.30' 

H U T F. MESA 

10 20 MILES 

0 10 20 KILOMETRES 

EXPLANATION 

D Rhyolite lavas of Fortymile Canyon 

- Rhyolite lavas of Beatty Wash 

D Rhyolit~ lavas of West Cat Canyon 

Rhyodacitic and mafic lavas - Relations 

with above lavas uncertain 

Thicknesses of tuffs of Cutoff Road and 

Fleur-<le-lis Ranch - Isopachs dashed 

where inferred 

0-150 m } Pattern dropped at 

150-300 m overlap with lavas 

Boundary of Timber Mountain resurgent 

dome 

FIGURE 24.-Distribution of younger intracaldera rocks of Timber Mountain-Oasis Valley caldera complex. Rhyolite lavas 

of Beatty Wash in northern moat are inferred from drill-hole data and negative aeromagnetic anomaly. 
YOUNGER INTRACALDERA ROCKS, TIMBER MOUNTAIN AND OASIS VALLEY CALDERAS 61 

10-percent phenocrysts consisting of 90-percent plagioclase and 10-percent olivine, biotite, clinopyroxene, and 

orthopyroxene. The larger lava flow on the east flank is 

more exposed and has been more extensively sampled than 

the smaller flow. The larger lava flow is a phenocryst-rich 

rhyodacite whose mode and silica content are shown 

graphically in figure 25. A complete chemical analysis of 

this rock was furnished by W. D. Quinlivan and P. W. 

Lipman (written commun., 1974). 

TUFFS OF FLEUR-DE-LIS RANCH 

AND RELATED RHYOLITE LAVAS 

The tuffs of Fleur-de-lis Ranch are most completely 

exposed on the west side of Oasis Valley just west of the 

Fleur-de-lis Ranch (fig. 24) where three petrographically 

similar ash-flow tuff cooling units include an intercalated lava flow between tuff units 2 and 3. Tuff units 1 

and 2 crop out only at this locality where the aggregate 

thickness of all three tuffs and the lava is about 300 m 

(I ,000 ft). Tuff unit 3 and the underlying lava extend 

southward; Oasis Valley is a narrow gorge where it cuts 

through these units. The only other place where a similar 

tuff is exposed is beneath a lava plug dome in an old vent 

south of Beatty Wash (fig. 24). Here a partly fused tuff, 

possibly a vent-related hot ashfall, dips inward toward the 

center of the plug dome and encloses it. 

The lavas at both localities are modally closely similar 

to the enclosing tuffs of Fleur-de-lis Ranch and also to the 

rhyolite lava at West Cat Canyon (figs. 24, 25), where the 

tuffs are absent. For convenience, all these lavas are 

designated the rhyolite lavas of West Cat Canyon. 

The tuffs of Fleur-de-lis Ranch and the related rhyolite 

lavas of West Cat Canyon reston the oldest bedded tuff and 

sediments that fill Oasis Valley and Timber Mountain 

calderas and are overlain by the tuff of Cutoff Road, which 

is included with the tuffs of Fleur-de-lis Ranch in figure 

24. Both the tuffs and the associated lavas are probably 

thicker and more extensive at depth under the Oasis Valley 

caldera segment. The tuffs have not been found flanking 

Timber Mountain dome, but the presence of the modally 

similar rhyolite lava at West Cat Canyon on the southwest 

flank of the dome (fig. 24) suggests the possibility of a concealed onlap (Byers and others, 1976, sec. B-B'). 

The similar modes of the tuffs and lavas are shown 

graphically in figure 25. Individual specimens vary somewhat with respect to total phenocrysts, but the most 

common phenocrysts are plagioclase, biotite, and 

clinopyroxene-very similar to those of the dominant 

mafic pumice in the quartz latitic caprocks of the Paintbrush and Timber Mountain Tuffs. One specimen from 

the intercalated lava west of Fleur-de-lis Ranch north of 

Oasis Valley gorge contains hornblende and clinopyroxene; specimens collected south of the gorge contain 

only clinopyroxene. Possibly two separate lava flows at 

the same stratigraphic position were sampled. Chemical 

analyses of the uppermost tuff at Fleur-de-lis Ranch and 

the rhyolite lava at West Cat Canyon are from W. D. 

Quinlivan and P. W. Lipman (written commun., 1974). 

The high alkalis and silica and high normative quartz and 

alkali feldspars with low normative anorthite indicate 

rhyolitic compositions, in contrast to more quartz latitic 

compositions of crystal-rich caprocks of Paintbrush and 

Timber Mountain Tuffs. The magnetic polarity of the 

upper tuff is normal (G. D. Bath, oral commun., 1964). 

TUFF OF CUTOFF ROAD AND RELATED RHYOLITE LAVAS 

The tuff of Cutoff Road and the rhyolite lavas of Beatty 

Wash, which are compositionally equivalent, are the 

youngest ash-flow tuff and related lava confined to the 

Timber Mountain and Oasis Valley calderas. The tuff 

occurs in the Timber Mountain caldera moat area and 

around the exposed edges of the Oasis Valley caldera 

segment (fig. 24). The rhyolite lavas of Beatty Wash are 

exposed in Beatty Wash in the southern part of Timber 

Mountain caldera moat (fig. 24) and extend eastward 

under younger rhyolite lavas of Fortymile Canyon. A lava 

flow of the same modal petrography crops out in the 

bottom of Fortymile Canyon (fig. 24) beneath the rhyolite 

lavas of Fortymile Canyon but is included with these 

overlying rhyolite lavas on the geologic map of the 

Timber Mountain caldera area (Byers and others, 1976). 

Another lava flow, about 150 m (500 ft) thick, was 

penetrated in drill hole UE18r in the northern caldera 

moat; its inferred extent is based partly on a negative aeromagnetic anomaly (G. D. Bath, written commun., 1968) 

and partly on structural interpretation of the moat area. 

The tuff of Cutoff Road does not exceed 60 m (200ft) in 

thickness where exposed; commonly it is less than 30 m 

( l 00 ft) and non welded, except in Oasis Valley on the west 

side of the Oasis Valley caldera segment. Near the distal 

nonwelded edge on the east flank of Timber Mountain 

dome, a maximum thickness of 26 m (85 ft) is exposed 

where the tuff dips gently eastward under cover. The 

rhyolite lavas of Beatty Wash are 150m (500ft) thick in the 

northern moat and greater than 60 m (200ft) thick in the 

southern moat. The lavas are probably a significant 

fraction of the total, owing to the greatly reduced volume 

of tuff erupted from post-Timber Mountain Tuff vents of 

the Timber Mountain-Oasis Valley caldera complex. 

The tuff of Cutoff Road locally on laps the rhyolite lava 

of Beatty Wash in Beatty Wash, but there is less than 3m 

( l 0 ft) of coarse locally derived ash-fall tuff between the 

units. The tuff overlies the tuffs of Fleur-de-lis Ranch or its 

associated lavas, but the rhyolite lava of Beatty Wash has 
62 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

STRATIGRAPHIC \ SUBU S UNIT _j NIT 

FELSIC PHENOCRYSTS 

AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

MAFIC PHENOCRYSTS 

AS PERCENTAGE 

OF TOTAL PHENOCRYSTS 

Number of thin £ections 

IN) in parentheses 

PHENOCRYSTS 

AS PERCENTAGE 

OF TOTAL ROCK 

Cil Quartz - Total mafics D Hornblende 

~ Plagioclase ~ Alkali Feldspar ~Biot ite [223 Clinopyroxene 

"' 

:g "'c u "'0 c > ·ro c ~ 

"' c u ~ 

"' 0 :::::> 

L~ 0 § u. foe 

~ E > ·- "' ~ - c 

"' 0 ~ > 

g. .S .<: "' a: E 

~ 

"' > 

"' _J 

Rhyolite lava ':Z" of Lipman 

and others (1966) (3) 

- ?'------ Rhyolite lava overlying 

mafic lavas of Dome 

Mountain (2) 

Rhyolite lava of 

Pinnacles Ridge (4) 

Rhyolite lava of 

Comb Peak (4) 

Rhyolite lava of 

Waterpipe Butte (5) 

Rhyolite lava of 

Burieq Canyon (2) 

Clhyolite lava of 

Delirium Canyon (2) 

Rhyolite lava of 

Vent Pass (5) 

Tuff of Cutoff Road (7) 

(Tuff probably intertongues 

with unit be low) 

Rhyolite lavas of Beatty Wash (6) 

Tuffs of Fleur-de-lis Ranch 17) 

(Tuffs intertongue With 

unit below) 

Rhyo l ite lavas of West Cat Canyon (7) 

- -?'--- - '----- Rhyodacite lava on 

Timber Mountain dome (4) 

0 

• 

'--t-----+?----t 

• 

-

- -I 

-

-

'-t--- t---?---1 

I I -I 

10 20 30 

D I 

r2Z2Zl ~ 

D - = 

ES:J-

~-

0 0 5 10 

F IG URE 25.-Modal and silica ranges of younger, intracaldera volcanic rocks of Timber Mountain- Oasis Valley caldera complex, 

uncertain field relation. Rhyolite lava "V" (Lipman, Quinlivan, and others, 1966) may correlate with rhyolite of Pinnacles 

cryst is absent. Silica analyses from W. D. Quinlivan and P. W. Lipman (written commlln., 1974). 

not been observed in direct stratigraphic contact with these 

units. The tuff of Cutoff Road and the rhyolite of Beatty 

Wash are inferred to be slightly younger than the tuffs of 

Fletir-de-lis Ranch and the rhyolite lavas of West Cat 

Canyon, as indicated partly by superposition of the tuff 

units and partly by inferred near-equivalence in time of 

petrographically similar tuff and rhyolite lava. 

The thin-section modes of the tuff of Cutoff Road and of 

the rhyolite lavas of Beatty Wash are nearly identical, even 

in the relative abundance of sphene (fig. 25 )- The only 

slight difference is the presence of sparse quartz in a few of 

the tuff thin sections. Both rocks are rhyolites having 

identical silica content at 74.5 percent (fig. 25) and therefore can be correlated as one stratigraphic unit. 

A mean K-Ar age on the tuff of Cutoff Road is 9.5 m.y. as 

reported by Kistler ( 1968, table I, "tuff ofthe caldera fill"). 

The magnetic polarity for both the tuff and the rhyolite of 

Beatty Wash is reversed from the Earth's present field (G. 

D_ Bath, written commun., 1968). 

RHYOLITE LAVAS OF FORTYMILE CANYON 

The last activity of the Timber Mountain center resulted 

in a sequence of rhyolitic lava flows, domes, and closely 

associated air-fall pyroclastic rocks erupted from a zone 

along and just outside the south and west rims of the 

Timber Mountain caldera. The lavas flowed maiuly 

southward across the outer flank of the caldera structure 

but also flowed into the caldera moat, and at one place 

lapped onto the Timber Mountain resurgent dome (fig. 

24). These rocks have been designated collectively the 

rhyolite lavas of Fortymile Canyon (Christiansen and 

Lipman, 1965; Orkild and O'Connor, 1970). 

The rhyolite lavas of Fortymile Canyon include eight 

known individual lava flows and domes (fig. 25), each 

associated with a sequence of pyroclastic rocks, mainly 

bedded tuffs. These flow units and their tuffs have been 

mapped individually on the 7\1!-minute quadrangle maps 

(fig. 2; Christiansen and Lipman, 1965; Orkild and 

O'Connor, 1970). No stratigraphic thickness is 

meaningful for the unit as a whole because of the limited 

areal extent and the large variations in thickness of individual flows. Flows are as thick as 300m (l ,000 ft) where they 

fill old valleys. The pyroclastic rocks in most instances 

thin outward from the vent areas, where piles of tuff and 

agglomerate associated with a single flow are as much as 
SUMMARY OF GEOLOGIC HISTORY 63 

NUMBER OF 

SPHENE PHENOCRYSTS 

PER THIN SECTION 

1:-:-:----.··· ····· ····.·.·-·.:-·-: ! 

0 10 20 30 

SILICA PERCENTAGE RANGE 

(Recalculated H, 0 and CO, free) 

Number of analyses (N) in parentheses 

v, vitrophyre; c, microcrystalline and groundmass 

c 

I 

----r---~--~--+---+-~( 1 

1-? c 

I 

Ill 

64 66 

101 

v c 

~ 

121 

v c 

~ 

121 

c 

I 

Ill 

121121 

vcvc 

(Lavaljoi ~ 

(Dike) 

vvcc 

~ (4) 

cP . ~ henocrysts-nch 

131118 percent)lava 

c 

I 

Ill 

c 

I 

Ill 

c 

I 

Ill 

vc 

I 

121 

68 70 72 74 

PERCENT 

76 78 

in order of strat igraphic succession, except as no ted . Query indi ca 1cs 

Ridge (Christiansen a nd Lipman, 1965, 1966). Rl a nk indica tes pheno250 m (800 ft) thick. The rhyolite lavas of Fortymile 

Canyon contain about 20 km3 (5 mi3) of erupted material. 

The content of silica nad quartz phenocrysts in the 

rhyolite lavas of Fortymile Canyon increases upward from 

quartz-poor flows at the base to quartz-rich flows at the top 

(fig. 25 ). The quartz trend seems to repeat a similar trend in 

the Paintbrush-Timber Mountain Tuff sequence. The 

silica content of the lavas increases gradually upward to 

high-silica rhyolite (fig. 25 ), also similar to the trend in the 

tuffs and rhyolites of Area 20 that fill the Silent Canyon 

caldera. The petrochemical trend of these lavas toward 

high quartz and silica continues the trend of the older 

postresurgence lavas and tuffs in the caldera moat. 

Rhyolite "V" of Lipman, Quinlivan, Carr, and Anderson 

(1966) is a high-silica rhyolite (figs. 24, 25) with thinsection modes almost identical to those of the rhyolite lava 

of Pinnacles Ridge (fig. 25 ), suggesting either approximate contemporaneity or the tapping of high-silica 

rhyolite magma near the top of the magma chamber at 

different times. 

The rhyolite lavas of Fortymile Canyon are petrochemically and structurally related to volcanism of the 

Timber Mountain center and are probably the last eruptive products of that episode. Their age is bracketed by a KAr age date of 9.5 m.y. on the underlying tuff of Cutoff 

Road and by a K-Ar date of 7.5 m.y. on the overlying 

Thirsty Canyon Tuff (Kistler, 1968) from the Black 

Mountain caldera center. The petrochemical trend of the 

rhyolite lavas both repeats the main eruptive sequence of 

the Timber Mountain-Oasis Valley caldera complex and 

continues the trend of the younger intracaldera rocks, 

thereby concluding the activity related to the Timber 

Mountain center about 9 m.y. ago. 

SUMMARY OF GEOLOGIC HISTORY 

Prior to about 16 m.y. ago the area of the Timber 

Mountain-Oasis Valley caldera complex was probably a 

terrane of basin-range-type block-faulted mountains of 

relatively low relief, separated by valleys partially filled 

with alluvium and tuff. The vents from which the tuff was 

erupted were outside the area. The youngest of these old 

tuffs, the calc-alkalic Fraction Tuff, is slightly more than 

16 m.y. old (all K-Ar dates from Kistler, 1968, or Marvin 

and others, 1970); it flowed into the ancestral valleys of the 

Timber Mountain-Oasis Valley area from vents to the 

north (Ekren and others, 1971). 

The earliest known volcanic activity in the Timber 

Mountain-Oasis Valley caldera complex was the eruption 

of predominantly hornblende-bearing calc-alkalic ashflow and bedded tuffs and subordinate rhyolite lavas, 

probably related to the Sleeping Butte caldera whose 

northwest wall is exposed in northern Oasis Valley. Little 

is known of the extent of this early caldera, and for that 

reason only the extracaldera extrusive products are shown 

in diagram l of figure 26. The principal extrusives from 

the Sleeping Butte caldera include the Redrock Valley and 

Crater Flat Tuffs. These ash flows possibly constituted as 

much as 1400 km3 (350 mi3) of tuff deposited around and 

within the Timber Mountain-Oasis Valley caldera 

complex. The Bullfrog Member of the Crater Flat Tuff is 

the most extensive, possibly extending into Death Valley 

to the west. The extracaldera extent of the overlying Prow 

Pass Member is on the south side of the complex. The 

Prow Pass Member is the only known tuff with orthopyroxene as the chief mafic phenocryst. These eruptive 

events took place 16 to 14 million years ago. 

A major collapse within the Silent Canyon caldera 

(diagram 2, fig. 26) occurred 13.8 m.y. ago with the 

eruption of the peralkaline Grouse Canyon Member of the 

Belted Range Tuff. The Silent Canyon caldera is not part 

of the Timber Mountain- Oasis Valley caldera complex, 

from which only calc-alkalic and alkali-calcic rocks are 

known. Following the early peralkaline eruptions, the 

Silent Canyon caldera was the source of calc-alkalic rocks, 

mainly the tuffs and rhyolite lavas of Area 20 inside the 

caldera and the Stockade Wash Tuff outside the caldera. 

The Stockade Wash was formerly the lowermost member 

of the alkali-calcic Paintbrush Tuff but differs litho-
64 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

SOUTH 

TIMBER MOUNTAIN CALDERA 

' 

Tp 

pTr 

Tp 

pTr 

CLAIM CANYON 

CAULDRON SEGMENT 

,-----"-----, 

Tp ·- :t- CLAIM 

~~ Tp-

~· Tp 

~\.____; \ pTr 

Magma 

Subsidence 

Subsidence 

Resurgence(?) 

CANYON CALDERA 

CLAIM CANYON 

CAULDRON ;rs 

+ Subsidence 

SILENT 

CANYON 

CALDERA 

Tra 

SILENT 

CANYON 

CALDERA 

Tp 

TPT Tra 

Tp ~ :: ~ 

~~ 

-;' 4 V pTr \ «<= 

Magma 

SILENT 

Ts CANYON 

Approximately 40 km (25 mil 

7 

Extrusion of pre-Ammonia Tanks lavas followed by eruption 

of Ammonia Tanks Member and tuff of Buttonhook Wash. 

Further cauldron subsidence 

11.1 m.y. 

6 

Eruption of Rainier Mesa Member accompanied by multiple 

collapse and emplacement of debris from caldera wall onto 

floor of caldera and into ring fissures 

11.3 m.y. 

5 

Broad doming accompanied by extrusion of pre-Rainier Mesa 

lavas 

11.3-12 m.y. 

4 

Possible resurgence of Claim Canyon cauldron at early stage 

of magmatic doming 

12-12.5 m.y. 

3 

Eruption of Paintbrush Tuff and related lavas with recurrent 

subsidence within Claim Canyon cauldron 

12.5-13· m.y. 

2 

Eruption of Grouse Canyon Member and main collapse within 

Silent Canyon caldera followed by eruption of tuffs and 

rhyolites of Area 20 inside caldera and by eruption of 

Stockade Wash Tuff outside caldera. Episodic cauldron 

subsidence 

13-14 m.y. 

1 

Eruption of pre -Grouse Canyon tuffs and lavas on eroded preTertiary rocks. Probable subsidences at Sleeping Butte 

caldera west of line of section 

14-16 m.y. 
SOUTH 

SUMMARY OF GEOLOGIC HISTORY 

TIMBER 

MOUNTA IN 

TIMBER MOUNTAIN 

Further crystallization of magma 

TIMBER MOUNTAIN 

RESURGENT DOME 

NORTH 

Pahute 

Mesa 

10 

Present Timber Mountain caldera after deposition of ca ldera 

fi II and erosion 

0-9 m.y. 

9 

Extrusion of intraca ldera tuffs and lavas, concluding activity 

of ca ldera complex. Crystallization of magma chamber 

into graniti c batholith 

9-11 m.y. 

8 

Magmatic resurgen ce, forming Timber Mountain dome accompanied by ring di kes, intrusive rh yolite, and tuff dikes 

65 

leone sheets?) and followed by eruption of tuffs of Crooked 

Canyon 

11 m.y. 

Resurgence 

EXPLANATION 

P'~ #9 Post caldera-complex rock-Largely caldera fi ll 

!NTRACALDERA TUFFS AND LAVAS 

; 

Rhyolite lavas of Fortymile Canyon and feeder 

dike 

Tuff of Cutoff Road 

Rhyolite lavas of Beatty Wash and West Cat 

Canyon, an d feede r dike 

Tmc 

TIMBER MOUNTAIN TUFF AND RELATED 

IGNEOUS ROCKS 

Tuffs of Crooked Canyon 

Microgranite porphyry ring dike, intrusive rhyolite, 

and tuff dike 

Tuff of Buttonhook Wash and Ammonia Tanks 

Member 

Pre-Ammonia Tttnks rhyolite lavas and feeder dike 

Rainier Mesa Member and associated debris flows 

and breccia 

Pre-Rainier Mesa rhyolite lavas and feeder dike 

ROCKS OF CLAIM CANYON CAULDRON 

Paintbrush Tuff and related rhyolite lavas and 

feeder dike 

ROCKS OF SILENT CANYON CALDERA 

.---.- - -, Stockade Wash Tuff and tuffs and rhyolites of 

Ts Tra Area 20 

Ts, Stockade Wash Tuff (extracaldera) 

Tra, tuffs and rhyolites of Area 20 (intracaldera) 

Grouse Canyon Member of Belted Range Tuff 

PRE-GROUSE CANYON ROCKS 

Crater Flat Tuff 

Pre-Belted Range tuffs, lavas, and volcaniclastic 

sedimentary rocks-Intercalated with Crater 

Flat and Redrock Valley Tuffs 

Redrock Valley Tuff 

Pre-Tertiary rocks 

FIGURE 26 (left and above).-Sequence of interpretive diagrams through Timber Mountain caldera, illustrating volcanic history of 

the Timber Mountain-Oasis Valley caldera complex. 
66 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

logically and petrographically from members of the Paintbrush Tuff, as now defined, and is more closely related in 

its petrography to the intracaldera tuffs and rhyolites of 

Area 20. The xenolithic inclusions of intracaldera Grouse 

Canyon and the areal distribution of the Stockade Wash 

indicate a source within the Silent Canyon caldera. The 

total erupted volume of the Stockade Wash was about 

20-40 km3 (5-I 0 mi3). Recurrent minor subsidences 

probably occurred within Silent Canyon caldera as the 

thick sequence of tuffs and rhyolites of Area 20 

accumulated inside the caldera, with the result that the 

underlying Crater Flat Tuff was eventually downdropped 

more than 2 km (diagram 2, fig. 26). 

The members of the Paintbrush Tuff and petrologically related alkali-calcic lavas were erupted from the 

Claim Canyon cauldron (diagram 3, fig. 26) resulting in a 

caldera of unknown extent. Only a small segment of the 

Claim Canyon cauldron is now exposed, but the general 

size and location of the collapse structure can be inferred, 

from distribution of tuffs and lavas north and south of 

Timber Mountain caldera, to be about 25 km (15 mi) in 

diameter and to be centered several kilometres west of the 

present summit of Timber Mountain. The oldest member 

of the Paintbrush Tuff, the Topopah Spring, was erupted 

about 13 m.y. ago with a total volume of about 250 km3 (60 

mi3). Recurrent subsidence within the cauldron followed 

eruption of the Topopah Spring Member of the Paintbrush Tuff, and probably a minor collapse followed the 

eruption of the Pah Canyon and Yucca Mountain 

Members. Within the caldera they accumulated to several 

times their thickness outside the caldera. The Tiva 

Canyon Member, representing the climactic eruption in 

the evolution of the Claim Canyon cauldron, flowed out 

about 12.5 m.y. ago mainly to the west of the Claim 

Canyon cauldron. The tuff of Chocolate Mountain, which 

is about I ,000 m (3,300 ft) thick, was erupted as a late 

quartz latitic phase of the Tiva Canyon and was confined 

within the Claim Canyon cauldron, suggesting that 

collapse was occurring during its eruption, although 

Christiansen and others ( 1976) infer a collapse to the west 

of the site of Oasis Valley. Tuff breccia, similar petrographically to the Tiva Canyon, was generated at vents 

along or near the wall of the Claim Canyon cauldron 

segment. The volume of the Tiva Canyon, including the 

intracauldron tuff of Chocolate Mountain, may have 

totaled as much as 1,000 km3 (250 mi3). The youngest 

separate cooling unit of the Paintbrush, the tuff of Pinyon 

Pass, followed very soon after the Tiva Canyon was 

confined to the newly formed ca ldera. 

After eruption of the ash-flow sheets of the Paintbrush 

Tuff and after the formation of the caldera, intricate 

faulting occurred within the Paintbrush cauldron, similar 

to that on Timber Mountain. The fault pattern is interpreted by the present authors (see also Christiansen and 

others, 1976) as owing to magmatic resurgence with uplift 

of the cauldron block from its former subsided position 

(diagram 4, fig. 26). The entire Paintbrush cauldron later 

participated in a broad magmatic doming preliminary to 

the culminating stages of volcanism-the eruption of ashflow sheets of the Timber Mountain Tuff. Quartz-bearing 

pre-Rainier Mesa rhyolite lavas were extruded from 

fractures probably related to this broad doming (diagram 

5, fig. 26). Gas-charged silicic rhyolitic magma 

accumulated at the top of the domical chamber, and tuffs 

were assimilated from the roof as crystallization proceeded 

lower in the chamber. 

The Timber Mountain Tuff chapter of the caldera 

complex began 11.3 m.y. ago with eruption of the 

voluminous Rainier Mesa Member from the central part of 

the broad dome that had formed over the magma chamber. 

The domed roof over the magma chamber ruptured 

sufficiently to cause a considerable reduction of pressure 

and to trigger vesiculation which greatly increased the 

volume of the gas-charged magma escaping through 

fissures. As the eruption proceeded, collapse of the roof 

occurred, and the Rainier Mesa accumulated to a greater 

thickness inside the subsidence area than au tside (diagram 

6, fig. 26). As the caldera deepened with continuing 

eruption a more crystal-rich, but less gas-charged, quartz 

latitic magma was tapped, whose eruptions were largely 

confined within the caldera. Poorly sorted debris flows 

containing large blocks of welded tuff and lava slid off the 

newly formed oversteepened caldera walls and intertongued with the upper quartz latitic part of the Rainier 

Mesa Member. A total volume of about 1,200 km3 (300 mi3) 

of tuff had been extruded by this time, and a large volcanotectonic depression formed, including the Timber 

Mountain caldera and probably the adjacent Oasis Valley 

caldera segment. There is little evidence to indicate 

whether or not there was central resurgent doming of the 

Rainier Mesa Member within this depression, except for 

the westward-tilted block in the Transvaal Hills west of 

Timber Mountain caldera . 

During the brief interval between the eruptions of the 

Rainier Mesa Member and Ammonia Tanks Members of 

the Timber Mountain Tuff (11.3-11.1 m.y.), preAmmonia Tanks rhyolite lavas were extruded within the 

caldera that had resulted from the Rainier Mesa eruptions 

(diagram 7, fig. 26). 

The Ammonia Tanks Member of the Timber Mountain 

Tuff was erupted 11.1 m .y. ago. Nearly half its total 

volume of 900 km3 (230 mi 3) probably accumulated within 

a concomitantly subsiding caldera, as evidenced by the fact 

that no debris flows or breccias are known to intertongue 

with the tuff. This episode of cauldron subsidence of 

Timber Mountain caldera occurred within an area somewhat smaller than the Rainier Mesa collapse area 

(diagram 7, fig. 26) and was centered farther south, with 
REFERENCES CITED 67 

the result that a crescent-shaped northern moat block did 

not participate in the Ammonia Tanks subsidence. The 

Ammonia Tanks eruptions reflect compositional trends 

with time that were more complex than earlier ash-flow 

tuff eruptions. In the ash flows of the Ammonia Tanks 

that were spread outside the caldera the trends progress 

upward from quartz latite to rhyolite and back to quartz 

latite. This general sequence outside the Ammonia Tanks 

collapse area was represented within the collapse area by a 

much greater thickness of complex intertonguing rhyolite 

and quartz latite, suggesting multiple source vents and 

concomitant eruption and collapse. 

Central resurgent doming of the Ammonia Tanks 

immediately occurred during the emplacement of the late 

intracaldera units of the Timber Mountain Tuff. The 

doming may even have begun before the final ash-flows of 

the Ammonia Tanks had been extruded. The tuff of 

Buttonhook Wash was erupted very shortly after the 

Ammonia Tanks and was confined within the Timber 

Mountain-Oasis Valley caldera complex. During or just 

before this eruption about 11 m.y. ago, magmatic 

resurgence continued and resulted in slight moderate 

doming of the Ammonia Tanks and intrusions of ring 

dikes, rhyolite plugs, tuff dikes, and possibly cone sheets 

into the dome (diagram 8, fig. 26). Doming continued and 

the weakly welded tuffs of Crooked Canyon of the Timber 

Mountain Tuff were erupted into the newly formed 

Timber Mountain caldera moat at a late stage of the 

doming. A northwest-trending apical graben formed at 

the crest of the resurgent dome owing to stretching of the 

domed mass. 

The culminating chapter in the volcanic history of the 

Timber Mountain-Oasis Valley caldera complex was now 

over; subsequent eruptions were small and confined to the 

caldera moat (diagram 9, fig. 26). The tuffs of Fleur-de-lis 

Ranch and Cutoff Road and related lavas were erupted 11 

to 9.5 m.y. ago and were confined within the Timber 

Mountain-Oasis Valley caldera complex. They probably 

had source vents to the west of Timber Mountain caldera 

in the Oasis Valley caldera segment. Local caldera 

collapses probably occurred within the segment 

accompanying these eruptions, accounting for the low 

relief of the segment and the caldera fill that postdates the 

tuff of Cutoff-Road. Finally, the rhyolite lavas of Fortymile Canyon accumulated on the rim and in the moat of 

Timber Mountain caldera and concluded the volcanic 

history of the complex. 

The geologic history subsequent to the last volcanic 

activity of the Timber Mountain-Oasis Valley caldera 

complex includes the partial filling of the caldera moat 

with mafic lavas, ash-flow tuffs and lavas from the 

neighboring Black Mountain center, and the deposition of 

fan gravels (diagram 10, fig. 26). The Timber Mountain 

caldera walls and the resurgent dome were somewhat 

modified by erosion. 

REFERENCES CITED 

Anderson, C. A., 1941, Volcanoes of the Medicine Lake highland, California: California Univ. Dept. Geol. Sci. Bull., v. 25, p. 347-422. 

Anderson, E. M., 1936, The dynamics of the formation of cone sheets, 

ring dikes, and cauldron subsidences: Royal Soc. Edinburgh Proc., 

v. 56, pt. 2, p. 128-157. 

Barnes, Harley, Christiansen, R. L., and Byers, F. M., Jr., 1965, Geologic map of the Jangle Ridge quadrangle, Nye and Lincoln 

Counties, Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-363. 

Barnes, Harley, Houser, F. N., and Poole, F. G., 1963, Geology of 

the Oak Spring quadrangle, Nye County, Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-214. 

Bemmelen, R. W., van, 1930, The origin of Lake Toba (North 

Sumatra): Pacific Sci. Cong., 4th, Batavia-Bandoeng (Java) 1929, 

Proc., v. 2a, p. 115-124. 

__ 1939, The volcano-tectonic origin of Lake Toba (North 

Sumatra): De Ingenieur Ned.-Indie, v. 6, no. 9, p. 126-140. 

Billings, M. P., 1943, Ring-dikes and their origin: New York Acad. 

Sci. Trans., ser. 2, v. 5, no. 6, p. 131-144, 

Branch, C. D., 1966, Volcanic cauldrons, ring complexes, and associated 

granites of the Georgetown Inlier, Queensland: Australia Bur. 

Mineral Resources, Geology and Geophysics Bull. 76, 158 p. 

Buddington, A. F., 1959, Granite emplacement with special reference 

to North America: Geol. Soc. America Bull., v. 70, p. 671-747. 

Burchfiel, B. C., 1966, Reconnaissance geologic map of the Lathrop 

Wells 15-minute quadrangle, Nye County, Nevada: U.S. Geol. 

Survey Misc. Geol. Inv. Map I-474. 

Byers, F. M., Jr., and Barnes, Harley, 1967, Geologic map of the Paiute 

Ridge quadrangle, Nye and Lincoln Counties, Nevada: U.S. Geol. 

Survey Geol. Quad. Map GQ-577. 

Byers, F. M., Jr., Carr, W. J ., Christiansen, R. L., Lipman, P. W., 

Orkild, P. P., and Quinlivan, W. D., 1976, Geologic map of the 

Timber Mountain caldera area, Nye County, Nevada: U.S. Geol. 

Survey Misc. Geol. Inv. Map I-891. 

Byers, F. M., Jr., Christiansen, R. L., Lipman, P. W., MarVin, R. 

F., and Orkild, P. P., 1969, Cauldron collapse and resurgence of 

Paintbrush volcanic center, southern Nye County, Nevada, in 

Abstracts for 1968: Geol. Soc. America Spec. Paper 121, p. 493. 

Byers, F. M., Jr., and Cummings, David, 1967, Geologic map of the 

Scrugham Peak quadrangle, Nye County, Nevada: U.S. Geol. 

Survey Geol. Quad. Map GQ-695. 

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: Geol. Soc. America 

Mem. llO, p. 87-97. 

Byers, F. M., Jr., Rogers, C. L., Carr, W. J., and Luft, S. J., 1966, 

Geologic map of the Buckboard Mesa quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-552. 

Carr, W. J., 1964, Structure of part of the Timber Mountain dome 

and caldera, Nye County, Nevada; in Geological Survey research 

1964: U.S. Geol. Survey Prof. Paper 501-B, p. B16-B19. 

Carr, W. J., and Quinlivan, W. D., 1966, Geologic map of the Timber 

Mountain quadrangle, Nye County, Nevada: U.S. Geol. Survey 

Geol. Quad. Map GQ-503. 

__ 1968, Structure of Timber Mountain resurgent dome, Nevada 

Test Site, in Nevada Test Site: Geol. Soc. America Mem. 110, 

p. 99-108. 

Christiansen, R. L., and Lipman, P. W., 1965, Geologic map of the 

Topopah Spring NW quadrangle, Nye County, Nevada: U.S. 

Geol. Survey Geol. Quad. Map GQ-444. 

__ 1966, Emplacement and thermal history of a rhyolite lava flow 

near Fortymile Canyon, southern Nevada: Geol. Soc. America 

Bull., v. 77, p. 671-684. 
68 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

Christiansen, R. L., Lipman, P. W., Carr, W. J., Byers, F. M., Jr., 

Orkild P. P., and Sargent, K. A., 1976, The Timber MountainOasis Valley caldera complex of southern Nevada: Geol. Soc. 

America Bull. v. 87. 

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 Basin-Range structure, in Geological 

Survey research 1965: U.S. Geol. Survey Prof. Paper 525-B, p. 

B43-B48. 

Christiansen, R. L., and Noble, D. C., 1965, Black Mountain volcanism 

of southern Nevada [abs.]: Geol. Soc. America Spec. Paper 82, 

p. 246. 

___ 1968, Geologic map of the Trail Ridge quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-774. 

Christiansen, R. L., Noble, D. C., Orkild, P. P., and Sargent, K. A., 

1968, Cogenetic sequences and source areas in silicic volcanic 

terranes [abs.]: Geol. Soc. America Spec. Paper 101, p. 39. 

Clough, C. T., Maufe, H. B., and Bailey, E. B., 1909, The cauldronsubsidence of Glen Coe and the associated igneous phenomena: 

Geol. Soc. London Quart. Jour., v. 65, p. 611-678. 

Colton, R. B., and McKay, E. J., 1966, Geologic map of the Yucca 

Flat quadrangle, Nye County, Nevada: U.S. Geol. Survey Geol. 

Quad. Map GQ-582. 

Colton, R. B., and Noble, D. C., 1967, Geologic map of the Groom 

Mine SW quadrangle, Nye and Lincoln Counties, Nevada: U.S. 

Geol. Survey Geol. Quad. Map GQ-719. 

Cornwall, H. R., 1962, Calderas and associated volcanic rocks near 

Beatty, Nye County, Nevada, in Engel, A. E. J., James, H. L., 

and Leonard, B. F., eds., Petrologic studies (Buddington volume): 

Geol. Soc. America, p. 357-371. 

Cornwall, H. R., and Kleinhampl, F. J., 1961, Geology of the Bare 

Mountain quadrangle [Nye County], Nevada: U.S. Geol. Survey 

Geol. Quad. Map GQ-157. 

Cornwall, H. R., and Kleinhampl, F. J., 1964, Geology of Bullfrog 

quadrangle and ore deposits related to Bullfrog Hills caldera, Nye 

County, Nevada, and Inyo County, California: U.S. Geol. Survey 

Prof. Paper 454-J, p. Jl-J25. 

Cummings, David, 1964, Eddies as indicators of local flow direction 

in rhyolite, in Short papers in geology and hydrology: U.S. Geol. 

Survey Prof. Paper 475-D, p. D70-D72. 

Eckel, E. B., 1968, Development of geologic knowledge at Nevada 

Test Site, in Nevada Test Site: Geol. Soc. America Mem. llO, 

p. 5-10. 

Eggler, D. H., 1968, Virginia Dale Precambrian ring-dike complex, 

Colorado-Wyoming: Geol. Soc. America Bull., v. 79, p. 1549-1563. 

Ekren, E. B., Anderson, R. E., Orkild, P. P., and Hinrichs, E. N., 

1966, Geologic map of the Silent Butte quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-493. 

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. Geol. Survey Prof. Paper 

651, 91 p. 

Ekren, E. B., Rogers, C. L., Anderson, R. E., and Botinel1y, Theodore, 

1967, Geologic map of the Belted Peak quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-606. 

Ekren, E. B., and Sargent, K. A., 1965, Geologic map of the Skull 

Mountain quadrangle, Nye County, Nevada: U.S. Geol. Survey 

Geol. Quad Map GQ-387. 

Fernald, A. T., Corchary, G. S., and Williams, W. P., 1968, Surficial 

geologic map of Yucca Flat, Nye and Lincoln Counties, Nevada: 

U.S. Geol. Survey Misc. Geol. lnv. Map 1-550. 

Gibbons, A. B., Hinrichs, E. N., Hansen, W. R., and Lemke, R. W., 

1963 Geology of the Rainier Mesa quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-215. 

Hamilton, Warren, and Myers, W. B., 1967, The nature of batholiths: 

U.S. Geol. Survey Prof. Paper 554-C, 30 p. 

Hinrichs, E. N., 1968, Geologic map of the Camp Desert Rock quadrangle, Nye County, Nevada: U.S. Geol. Survey Geol. Quad. Map 

GQ-726. 

Hinrichs, E. N., Krushensky, R. D., and Luft, S. J., 1967, Geologic 

map of the Ammonia Tanks quadrangle, Nye County, Nevada: 

U.S. Geol. Survey Geol. Quad. Map GQ-638. 

Hinrichs, E. N., and McKay, E. J., 1965, Geologic map of the 

Plutonium Valley quadrangle, Nye and Lincoln Counties, Nevada: 

U.S. Geol. Survey Geol. Quad. Map GQ-384. 

Hinrichs, E. N., and Orkild, P. P., 1961, Eight members of the Oak 

Spring Formation, Nevada Test Site and vicinity, Nye and Lincoln 

Counties, Nevada, in Short papers in the geologic and hydrologic 

sciences: U.S. Geol. Survey Prof. Paper 424-D, p. D96-Dl03. 

Holmes, Arthur, 1965, Principles of physical geology [revised ed.]: 

New York, Ronald Press Co., 1288 p. 

Houser, F. N., and Poole, F. G., 1960, Preliminary geologic map of 

the Climax stock and vicinity, Nye County, Nevada: U.S. Geol. 

Survey Misc. Geol. lnv. Map 1-328. 

Kistler, R. W., 1968, Potassium-argon ages of volcanic rocks in Nye 

and Esmeralda Counties, Nevada, in Nevada Test Site: Geol. Soc. 

America Mem. 110, p. 251-262. 

Lipman, P. W., 1966, Water pressures during differentiation and 

crystallization of some ash-flow magmas from southern Nevada: 

Am. Jour. Sci., v. 264, p. 810-826. 

___ 1971, Iron-titanium oxide phenocrysts in compositionally zoned 

ash-flow sheets from southern Nevada: Jour. Geology, v. 79, p. 

438-456. 

___ 1976, Caldera collapse breccias in the western San Juan Mountains, Colorado: Geol. Soc. America Bull., v. 87. 

Lipman, P. W., and Christiansen, R. L., 1964, Zonal features of an 

ash-flow sheet in the Piapi Canyon Formation, southern Nevada, 

in Geological Survey research 1964: U.S. Geol. Survey Prof. Paper 

501-B, p. B74-B78. 

Lipman, P. W., Christiansen, R. L., and O'Connor, J. T., 1966, A 

compositionally zoned ash-flow sheet in southern Nevada: U.S. 

Geol. Survey Prof. Paper 524-F, p. Fl-F47. 

Lipman, P. W., and McKay, E. J., 1965, Geologic map of the Topopah 

Spring SW quadrangle, Nye County, Nevada: U.S. Geol. Survey 

Geol. Quad. Map GQ-439. 

Lipman, P. W., Quinlivan, W. D., Carr, W. J., and Anderson, R. 

E., 1966, Geologic map of the Thirsty Canyon SE quadrangle, 

Nye County, Nevada: U.S. Geol. Survey Geol. Quad. Map 

GQ-489. 

Locke, Augustus, Billingsley, P. R., and Mayo, E. B., 1940, Sierra 

Nevada tectonic patterns: Geol. Soc. America Bull., v. 51, 

p. 513-540. 

Luft, S. J., 1964, Mafic lavas of Dome Mountain, Timber Mountain 

caldera, southern Nevada, in Geological Survey research 1964: 

U.S. Geol. Survey Prof. Paper 501-D, p. Dl4-D21. 

Marvin, R. F., Byers, F. M., Jr., Mehnert, H. H., Orkild, P. P., and 

Stern, T. W., 1970, Radiometric ages and stratigraphic sequence 

of volcanic and plutonic rocks, southern Nye and western Lincoln 

Counties, Nevada: Geol. Soc. America Bull., v. 81, p. 2657-2676. 

McKay, E. J., and Sargent, K. A., 1970, Geologic map of the Lathrop 

Wells quadrangle, Nye County, Nevada: U.S. Geol. Survey Geol. 

Quad. Map GQ-883. 

McKay, E. J., and Williams, W. P., 1964, Geology of the Jackass Flats 

quadrangle, Nye County, Nevada: U.S. Geol. Survey Geol. Quad. 

Map GQ-368. 

McKeown, F. A., 1976, Geologic map of the Yucca Lake quadrangle, 

Nye County, Nevada: U.S. Geol. Survey Geol. Quad Map 

GQ-1327. 
REFERENCES CITED 69 

Noble, D. C., 1970, Loss of sodium from crystallized comendite welded 

tuffs of the Miocene Grouse Canyon Member of the Belted Range 

Tuff, Nevada: Geol. Soc. America Bull., v. 81, p. 2677-2688. 

Noble, D. C., Anderson, R. E., Ekren, E. B., and O'Connor, J. T., 

1963, Thirsty Canyon Tuff of Nye and Esmeralda Counties, 

Nevada, in Short papers in geology and hydrology: U.S. Geol. 

Survey Prof. Paper 475-D, p. D24-D27. 

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

1968, Zonal relations and paleomagnetism of the Spearhead and 

Rocket Wash Metnbers of the Thirsty Canyon Tuff, southern 

Nevada, in Geological Survey research 1968: U.S. Geol. Survey 

Prof. Paper 600-C, p. C6l-C65. 

Noble, D. C., and Christiansen, R. L., 1968, Geologic map of the 

southwest quarter of the Black Mountain quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Misc. Geol. Inv. Map I-562. 

Noble, D. C., Haffty, Joseph, and Hedge, C. E., 1969, Strontium and 

magnesium contents of some natural pera1kaline silicic glasses 

and their petrogenetic significance: Am. Jour. Sci., v. 267, 

p. 598-608. 

Noble, D. C., Kistler, R. W., Christiansen, R. L., Lipman, P. W., 

and Poole, F. G., 1965, Close association in space and time of 

alkalic, calc-alkalic, and calcic volcanism in southern Nevada 

[abs.]: Geol. Soc. America Spec. Paper 82, p. 143-144. 

Noble, D. C., Krushensky, R. D., McKay, E. J., and Ege, J. R., 1967, 

Geologic map of the Dead Horse Flat quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-614. 

Noble, D. C., Sargent, K. A., Mehnert, H. H., Ekren, E. B., and Byers, 

F. M., Jr., 1968, Silent Canyon volcanic center, Nye County, 

Nevada, in Nevada Test Site: Geol. Soc. America Mem. 110, 

p. 65-75. 

Nockolds, S. R., 1954, Average chemical composition of some igneous 

rocks: Geol. Soc. America Bull., v. 65, p. 1007-1032. 

O'Connor, J. T., 1963, Petrographic characteristics of some welded 

tuffs of the Piapi Canyon Formation, Nevada Test Site, Nevada, 

in Short papers in geology and hydrology: U.S. Geol. Survey Prof. 

Paper 475-B, p. B52-B55. 

___ 1965, A classification for quartz-rich igneous rocks based on 

feldspar ratios, in Geological Survey research 1965: U.S. Geol. 

Survey Prof. Paper 525-B, p. B79-B84. 

O'Connor, J. T., Anderson, R. E., and Lipman, P. W., 1966, Geologic 

map of the Thirsty Canyon quadrangle, Nye County, Nevada: 

U.S. Geol. Survey Geol. Quad. Map GQ-524. 

Orkild, P. P., 1963, Geology of the Tippipah Spring quadrangle, Nye 

County, Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-213. 

___ 1965, Paintbrush Tuff and Timber Mountain Tuff of Nye 

County, Nevada, in Cohee, G. V., and West, W. S., Changes in 

stratigraphic nomenclature by the U.S. Geological Survey, 1964: 

U.S. Geol. Survey Bull. 1224-A, p. A44-:A5l. 

___ 1968, Geologic map of the Mine Mountain quadrangle, Nye 

County, Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-746. 

Orkild, P. P., Byers, F. M., Jr., Hoover, D. L., and Sargent, K. A., 

1968, Subsurface geology of Silent Canyon caldera, Nevada Test 

Site, Nevada, in Nevada Test Site: Geol. Soc. America Mem. 110, 

p. 77-'-86. 

Orkild, P. P., and O'Connor, J. T., 1970, Geologic map of the 

Topopah Spring quadrangle, Nye County, Nevada: U.S. Geol. 

Survey Geol. Quad. Map GQ-849. 

Orkild, P. P., Sargent, K. A., and Snyder, R. P., 1969, Geologic map 

of Pahute Mesa, Nevada Test Site and vicinity, Nye County, 

Nevada: U.S. Geol. Survey Misc. Geol. Inv. Map I-567. 

Peacock, M. A., 1931, Classification of igneous rock series: Jour. 

Geology, v. 39, p. 54-67. 

Poole, F. G., 1965, Geologic map of the Frenchman Flat quadrangle, 

Nye, Lincoln, and Clark Counties, Nevada: U.S. Geol. Survey 

Geol. Quad. Map GQ-456. 

Poole, F. G., Carr, W. J., and Elston, D.P., 1965, Salyer and Wahmonie 

Formations of southeastern Nye County, Nevada, in Cohee, G. 

V., and West, W. S., Changes in stratigraphic nomenclature by 

the U.S. Geological Survey, 1964: U.S. Geol. Survey Bull. 

1224-A, p. A36-A44. 

Poole, F. G., Elston, D. P., and Carr, W. J., 1965, Geologic map of 

the Cane Spring quadrangle, Nye County, Nevada: U.S. Geol. 

Survey Geol. Quad Map GQ-455. 

Poole, F. G., and McKeown; F. A., 1962, Oak Spring Group of the 

Nevada Test Site and vicinity, Nevada, in Short papers in geology, 

hydrology, and topography: U.S. Geol. Survey Prof. Paper 

450-C, p. C60-62. 

Quinlivan, W. D., and Lipman, P. W., 1965, Compositional variations 

in some Cenozoic ash-flow tuffs, southern Nevada, in Abstracts 

for 1964: Geol. Soc. America Spec. Paper 82, p. 342. 

Ratte, J. C., and Steven, T. A., 1967, Ash flows and related volcanic 

rocks associated with the Creede caldera, San Juan Mountains, 

Colorado: U.S. Geol. Survey Prof. Paper 524-H, 58 p. 

Richey, J. E., 1932, Tertiary ring structures in Britain: Geol. Soc. 

Glasgow Trans., v. 19, p. 42-140. 

___ 1961, British regional geology; Scotland-The Tertiary volcanic 

districts [3d ed.], with revision by MacGregor, A. G., and Anderson, 

F. W .: Edinburgh, Scotland, Dept. Sci. and Indus. Research Geol. 

Survey and Museum, 120 p. 

Rittmau, Alfred, 1952, Nomenclature of volcanic rocks: Bull. 

Volcano!. I, ser. 2, v. 12, p. 75-102. 

Rogers, C. L., Anderson, R. E., Ekren, E. B., and O'Connor, J. T., 

1967, Geologic map of the Quartzite Mountain quadrangle, Nye 

County, Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-672. 

Rogers, C. L., Ekren, E. B., Noble, D. C., and Weir, J. E., 1968, Geologic map of the northern half of the Black Mountain quadrangle, Nye County, Nevada: U.S. Geol. Survey Misc. Geol. 

Inv. Map I-545. 

Rogers, C. L., and Noble, D. C., 1969, Geologic map of the Oak Spring 

Butte quadrangle, Nye County, Nevada: U.S. Geol. Survey Geol. 

Quad. Map GQ-822. 

Ross, C. S., and Smith, R. L., 1961, Ash-flow tuffs-their origin, geologic relations and identification: U.S. Geol. Survey Prof. Paper 

366, 81 p. 

Sargent, K. A., 1969, Petrography and heavy minerals of three groups 

of rhyolitic lavas, Pahute Mesa, Nevada Test Site, in Geological 

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

Cl8'-C24. 

Sargent, K. A., Luft, S. J., Gibbons, A. B., and Hoover, D. L., 1966, 

Geologic map of the Quartet Dome quadrangle, Nye County, 

Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-496. 

Sargent, K. A., McKay, E. J., and Burchfiel, B. C., 1970, Geologic 

map of the Striped Hills quadrangle, Nye County, Nevada: U.S. 

Geol. Survey Geol. Quad. Map GQ-882. 

Sargent, K. A., Noble, D. C., and Ekren, E. B., 1965, Belted Range 

Tuff of Nye and Lincoln Counties, Nevada, in Cohee, G. V., and 

West, W. S., Changes in stratigraphic nomenclature by the U.S. 

Geological Survey, 1964: U.S. Geol. Survey Bull. 1224-A, 

p. A32-A36. 

Sargent, K. A., and Orkild, P. P., 1973, Geologic map of the Wheelbarrow Peak-Rainier Mesa area, Nye County, Nevada: U.S. Geol. 

Survey Misc. Geol. Inv. Map I-754. 

Sargent, K. A., and Stewart, J. H., 1971, Geologic map of the Specter 

Range NW quadrangle, Nye County, Nevada: U.S. Geol. Survey 

Geol. Quad. Map GQ-884. 

Shand, S. J., 1947, Eruptive rocks, [3d ed.]: New York, John Wiley 

and Sons, 488 p. 

Smith, R. L., 1960a, Ash flows: Geol. Soc. American Bull., v. 71, 

p. 795-842. 
70 TIMBER MOUNTAIN-OASIS VALLEY CALDERA COMPLEX, NEVADA 

___ l960b, Zones and zonal variations in welded ash flows: U.S. 

Geol. Survey Prof. Paper 354-F, p. 149-159. 

Smith, R. L., and Bailey, R. A., 1966, The Bandelier Tuff-A study 

of ash-flow eruption cycles from zonal magma chambers: Bull. 

Volcano!., v. 29, p. 83-103. 

Smith, R. L., Bailey, R. A., and Ross, C. S., 1961, Structural evolution of the Valles caldera, New Mexico. and its bearing on the 

emplacement of ring dikes, in Short papers in the geologic and 

hydrologic sciences: U.S. Geol. Survey Prof. Paper 424-D, 

p. Dl45-Dl49. 

Stock, Chester, and Bode, F. D., 1935, Occurrence of lower Oligocene 

mammal-bearing beds near Death Valley, California: Nat. Acad. 

Sci. Proc., v. 21, no. 10, p. 571-579. 

Tuttle, 0. F., and Bowen, N. L., 1958, Origin of granite in the light 

of experimental studies in the system NaAlSi30 11KA1Si30 11 -Si02

-

H20: Geol. Soc. America Mem. 74, 153 p. 

Williams, Howel, 1941, Calderas and their origin: California Univ. 

Dept. Geol. Sci. Bull., v. 25, p. 239-346. 

u U.S. GOVERNMENT PRINTING OFFICE, 1976-677-340/95 

Byers and others-TIMBER MOUNTAIN-OASIS VALLEY, CALDERA COMPLEX, NEVADA-Geological Survey Professi