Stratigraphic and Volcano-Tectonic

T"* i A * r* /^* -A T^T A *TH pf* 1 Relations of Crater Flat Tuft and 

Some Older Volcanic Units, 

Nye County, Nevada

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1323
Stratigraphic and Volcano-Tectonic 

Relations of Crater Flat Tuff and 

Some Older Volcanic Units, 

Nye County, Nevada

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

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1323

Prepared in cooperation with the 

U.S. Department of Energy

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1986
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Carr, Wilfred James, 1926-

Stratigraphic and volcano-tectonic relations of Crater Flat Tuff and some older volcanic units, Nye County, 

Nevada.

(Geological Survey professional paper ; 1323)

"Prepared in cooperation with the U.S. Department of Energy."

Bibliography: p.

Supt.ofDocs.no.: 119.16:1323

1. Volcanic ash, tuff, etc. 2. Geology, Stratigraphic Miocene. 3. Petrology Nevada Nye County. I. 

Byers, F. M. (Frank M.), 1916- . II. Orkild, Paul P. III. United States. Dept. of Energy. IV. Title. V. 

Series.

QE461.C374 1985 552'.23 84-600019

For sale by the Branch of Distribution

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425 

Denver, CO 80225
CONTENTS

Page

1

1

3

3

3

7

9

............................ 9

Rhyolite lava ...................... 9

Lithic Ridge Tuff ................... 10

Dacite intrusive and flow breccia ............. 13

Crater Flat Tuff and related lava ............ 14

Tram Member ..................... 14

Type locality .................... 14

Drill hole USW-G1 ................ 14

Distribution and thickness ............ 14

Abstract .............................

Introduction ...........................

General geologic relations ....................

Stratigraphic relations ......................

Tuff of Yucca Flat .....................

Older ash-flow and bedded tuffs of drill hole USW-G1 .

Quartz latite .........................

Lithic Ridge Tuff and petrographically similar rhyolite lava

Stratigraphic relations Continued

Crater Flat Tuff and related lava Continued 

Tram Member Continued

Lithology and petrography ............ 17

Age and relation to other units ......... 17

Bullfrog Member and petrographically similar lava . 18 

Prow Pass Member .................. 18

Magnetic properties of Crater Flat Tuff and subjacent units 21

Calderas related to Crater Flat Tuff and subjacent units . 21

Geophysical evidence .................. 21

Distribution and thickness of the tuffs ....... 22

Geologic relations ................... 24

Relation between Tram and Grouse Canyon Members 25

Pre-Crater Flat Tuff caldera ............. 27

References ............................ 27

ILLUSTRATIONS

Page

FIGURE 1. Index map showing location of southwestern Nevada volcanic field and some associated calderas ........... 2

2. Chart showing Stratigraphic relations of Crater Flat Tuff .................................. 4

3. Map showing distribution and thickness of tuff of Yucca Flat ............................... 5

4. Generalized cross section showing Stratigraphic relations of tuff of Yucca Flat and Lithic Ridge Tuff at Lithic Ridge . . 6

5. Generalized cross section showing Stratigraphic relations of tuff of Yucca Flat and Lithic Ridge Tuff at Area 16 tunnel

below Tippipah Point ..................................................... 6

6. Chart showing modal ranges of outcrop samples of Crater Flat Tuff and associated volcanic units .......... 8

7. Geologic cross section through drill holes USW-G1 and J-13 ................................ 9

8. Chart showing petrographic variation of Crater Flat Tuff and older units in drill hole USW-G1 ........... 10

9-12. Sketch maps showing:

9. Dacitic to rhyolitic lava flows and intrusive rocks of pre-Crater Flat Tuff age in the southwest Nevada Test

Site region ...................................................... 11

10. Distribution and thickness of Lithic Ridge Tuff ................................... 12

11. Distribution and thickness of Tram Member of Crater Flat Tuff ......................... 15

12. Geologic relations of Tram Member of Crater Flat Tuff in Tram Ridge-Beatty Wash area .......... 16

13. Geologic section sketch from northwest to southeast through Tram Ridge, showing relations of Tram Member to lava

flows and possible cauldron structure ........................................... 17

14. Map showing distribution and thickness of Bullfrog Member of Crater Flat Tuff..................... 19

15. Map showing distribution and thickness of Prow Pass Member of Crater Flat Tuff ................... 20

16. Bouguer gravity map of the NTS region, showing the regional gravity low associated with several caldera complexes . 23

17. Aeromagnetic map of southern Crater Flat area, showing caldera boundary and buried resurgent dome of the Bullfrog

caldera ............................................................. 24

18. Sketch map of the southwestern NTS region, showing the relation of the Crater Flat^Prospector Pass caldera complex

to geologic features ..................................................... 26

TABLE

TABLE 1. Remanent magnetic directions of Crater Flat Tuff, Lithic Ridge Tuff, and tuff of Yucca Flat

Page

22

in

STRATIGRAPHIC AND VOLCANO-TECTONIC RELATIONS OF 

CRATER FLAT TUFF AND SOME OLDER VOLCANIC UNITS,

NYE COUNTY, NEVADA

By W. J. CARR, F. M. BYERS, Jr., and PAUL P. ORKILD

ABSTRACT

The Crater Flat Tuff is herein revised to include a newly recognized lowest unit, the Tram Member, exposed at scattered localities 

in the southwest Nevada Test Site region, and in several drill holes 

in the Yucca Mountain area. The overlying Bullfrog and Prow Pass 

Members are well exposed at the type locality of the formation near 

the southeast edge of Crater Flat, just north of U.S. Highway 95. 

In previous work, the Tram Member was thought to be the Bullfrog 

Member, and therefore was shown as Bullfrog or as undifferentiated 

Crater Flat Tuff on published maps. The revised Crater Flat Tuff 

is stratigraphically below the Topopah Spring Member of the 

Paintbrush Tuff and above the Grouse Canyon Member of the Belted 

Range Tuff and is approximately 13.6 m.y. old.

Drill holes on Yucca Mountain and near Fortymile Wash penetrate 

all three members of the Crater Flat as well as an underlying quartzpoor unit, which is herein defined as the Lithic Ridge Tuff from exposures on Lithic Ridge near the head of Topopah Wash. In outcrops 

between Calico Hills and Yucca Flat, the Lithic Ridge Tuff overlies 

a Bullfrog-like unit of reverse magnetic polarity that probably correlates with a widespread unit around and under Yucca Flat, referred 

to previously as Crater Flat Tuff. This unit is here informally designated as the tuff of Yucca Flat. Although older, it may be genetically 

related to the Crater Flat Tuff.

Although the rocks are poorly exposed, geophysical and geologic 

evidence to date suggests that (1) the source of the Crater Flat Tuff 

is a caldera complex in the Crater Flat area between Yucca Mountain 

and Bare Mountain, and (2) there are at least two cauldrons within 

this complex one probably associated with eruption of the Tram, 

the other with the Bullfrog and Prow Pass Members. The complex 

is named the Crater Flat-Prospector Pass caldera complex. The 

northern part of the Yucca Mountain area is suggested as the general 

location of the source of pre-Crater Flat tuffs, but a caldera related 

to the Lithic Ridge Tuff has not been specifically identified.

INTRODUCTION

The U.S. Geological Survey has conducted research 

in support of the Nevada Nuclear Waste Storage Investigations of the U.S. Department of Energy in the 

southwest Nevada Test Site (NTS) region since 1977. 

From 1978 to 1982, regional geophysical investigations, 

geologic mapping, regional geologic reconnaissance, and

data from several drill holes, principally on Yucca 

Mountain (fig. 1), have defined a thick Miocene volcanic 

sequence under Yucca Mountain and Crater Flat. In 

the lower part of the sequence, below the Crater Flat 

Tuff (Byers and others, 1976a and 1976b), are several 

volcanic units that were unrecognized prior to 1978. 

Some of these units crop out outside the deep volcanic 

subsidence area beneath Crater Flat and northern 

Yucca Mountain, and have been correlated with the 

subsurface units by petrographic methods. The purposes of this paper are to revise the stratigraphic position and content of the Crater Flat Tuff, to suggest 

its volcano-tectonic setting, and to describe the older 

volcanic units in order to better define the subsurface 

structure in the southwestern NTS area.

The authors acknowledge valuable support from colleagues in the U.S. Geological Survey, especially Gordon D. Bath and J. G. Rosenbaum, who provided magnetic polarity determinations of the tuffs. Margaret 

Baldwin, of Fenix and Scisson, Inc., Mercury, Nev., 

made a number of determinations of magnetic characteristics of the units. A geologic map of the southwestern NTS area compiled by Florian Maldonado (1985) 

uses informal nomenclature for the new stratigraphic 

units established formally in this report. Extensive 

studies of geologic relations in the adjoining Bare 

Mountain quadrangle by two of the authors of this report, Orkild and Carr, and by P. W. Lipman and W. 

D. Quinlivan have been essential in establishing stratigraphic relations of the ash-flow tuffs and intercalated 

lavas. Detailed lithologic logs of drill hole USW--G1 by 

Spengler and others (1981), of drill hole USW--G2 by 

Florian Maldonado and S. L. Koether (written commun., 1982), and of drill holes USW-G3 and USW-H6 

by R. B. Scott (written commun., 1982) also provided 

important data for this report.
CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

117°00' 11B°00'

37° 

3D'

STONEWALL 

CALDERAf?) COMPLEX

36° 

30'

\

CRATER FLAT- 

\ PROSPECTOR PASS 

\ CALDERA COMPLEX 

\

\

\

&<& ^te

wL£ %t\\

V Pahrump

0 50 KILOMETERS 

J

0 25 MILES

FIGURE 1. Locations of southwestern Nevada volcanic field (shaded) and some associated calderas.
CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

GENERAL GEOLOGIC RELATIONS

The western part of the NTS is within the southwestern Nevada volcanic field (Christiansen and others, 

1965; Byers and others, 1976b; Christiansen and others, 

1977), which consists of a thick sequence of Tertiary 

silicic volcanic rocks about 7-15 m.y. (million years) old, 

roughly centered around the Timber Mountain-Oasis 

Valley and Silent Canyon caldera complexes (Noble and 

others, 1968). The field extends from east of Yucca Flat 

westward to about the California-Nevada State line, 

and from approximately Skull Mountain and U.S. Highway 95 northwestward to the vicinity of Goldfield. The 

regional geologic setting is outlined by Christiansen and 

others (1977, p. 943-946).

The volcanic stratigraphic units defined in this paper 

underlie the extensive, well-exposed ash-flow sheets of 

the Paintbrush and Timber Mountain Tuffs (fig. 2), 

which originated from the Timber Mountain-Oasis Valley caldera complex described by Byers and others 

(1976a, b). The older, pre-Paintbrush volcanic rocks 

were not given the intensive study that the younger, 

better exposed ones received (Byers and others, 

1976b). The Crater Flat Tuff, then thought to consist 

of only the Bullfrog and Prow Pass Members, was recognized to be widespread in the southwestern part of 

the NTS and the adjacent area to the west (Byers and 

others, 1976b, fig. 4). Recent geologic and geophysical 

work and the results of deep drilling on Yucca Mountain (fig. 1) now lead us to believe that an important 

volcano-tectonic depression, which may be more than 

3,000 m (10,000 ft) deep and may include several buried 

calderas, underlies Crater Flat and part of Yucca 

Mountain (Snyder and Carr, 1982).

The revised Crater Flat Tuff is now known to be 

younger than the Grouse Canyon Member of the Belted 

Range Tuff (fig. 2), and the pre-Grouse Canyon unit 

that was previously correlated with the Bullfrog Member in the Yucca Flat and Pahute Mesa areas of the 

northern NTS is reduced to informal status and here 

called the tuff of Yucca Flat.

The period of intense volcanism discussed in this report spans about 1.5 m.y., roughly between 14.5 and 

13 m.y. ago (fig. 2). Existing age control and the 

number of volcanic units indicate that major eruptive 

episodes occurred, on the average, about every 100,000 

years during this period.

STRATIGRAPHIC RELATIONS 

TUFF OF YUCCA FLAT

The tuff of Yucca Flat is here designated as an informal unit consisting of a quartz-bearing ash-flow sheet 

present around and beneath Yucca Flat (fig. 3). The 

unit may be an early precursor to the members of the 

Crater Flat Tuff, as it closely resembles the Bullfrog 

Member petrographically, but it has not been found in 

southwestern NTS or in drill holes at Yucca Mountain. 

The tuff of Yucca Flat was erroneously correlated with 

the Crater Flat Tuff (Byers and others, 1976b, p. 8-9; 

Dixon and others, 1976, p. A50). It is a useful! stratigraphic marker in the subsurface of east-central Yucca 

Flat, Pahute Mesa, and Rainier Mesa (Orkild and 

others, 1968), and crops out on both the east and west 

sides of Yucca Flat, as well as to the southeast in Scarp 

and Nye Canyons (fig. 3).

The tuff of Yucca Flat occurs stratigraphically between tunnel beds 1 and 2 in east-central Yucca Flat 

(Dixon and others, 1976) and in the Rainier Mesa tunnel area (Steele and Fairer, 1978; Fairer and Townsend, 1979). Tunnel beds 1 through 4 (Gibbons and 

others, 1963) occur stratigraphically between the Redrock Valley Tuff (fig. 2), which has a potassium-argon 

age of 15.7 m.y., and ash-fall beds of the Grouise Canyon Member of the Belted Range Tuff of 13.8 m.y. age1 

(Gibbons and others, 1963; Marvin and others, 1970; 

Byers and others, 1976b). The oldest peralkaline ashfall beds in tunnel bed 2 are believed to correlate with 

the earliest peralkaline lava, the rhyolite of Kawich 

Valley, dated at 14.8 m.y. (Marvin and others, 1970). 

Thus, from available data, the age of the tuff of Yucca 

Flat is probably about 15.0 m.y.

The stratigraphic position of the tuff of Yucca Flat 

is shown on figures 2, 4, and 5. At Lithic Ridge (figs. 

3 and 4), the tuff of Yucca Flat is overlain by a local 

quartz latitic tuff and the Lithic Ridge Tuff. Below Tippipah Point (fig. 5), the tuff of Yucca Flat rests on 

the Redrock Valley Tuff and is overlain successively 

by a quartz-sanidine tuff, not recognized elsewhere, the

'A K-Ar age determination of 13.6 ±0.5 m.y. has been made on alkali feldspar from 

a boulder of porphyritic welded tuff of the Grouse Canyon collected from beneath the Crater 

Flat Tuff type locality at U.S. Highway 95 (R. F. Marvin, written commun., 1980), suggesting that an average of 13.7 m.y. may be a better age for the Grouse Canyon.
CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

Approximate 

age 

(m.y.)

Western Bullfrog

Hills area

(1)

Crater Flat-Yucca

Mountain area

(2)

Lithic Ridge 

(3)

Frenchman FlatCane Spring area 

(4)

Yucca Flat area 

(5)

South edge of

Pahute Mesa

(6)

11-13 Timber Mountain and Paintbrush Tuffs

Thin tuff beds related to Wahmonie 

or Salyer Formations present locally

Wahmonie 

Formation

Same as areas 

(2) and (3)

Stockade Wash Tuff

Tuffs and lavas of Calico Hills

Prow Pass 

Member

Grouse Canyon Member of Belted Range Tuff

Rhyolite lava 

of Split Ridge

Dacite lava 

and plugs

13.8 Lithic Ridge Tuff3 4

13.9 Rhyolite and quartz latite 

lavas and dikes Quartz latitic tuff4 5

Tub Spring Member 

of Belted Range Tuff

14.5

Formerly tuff of Mara Wash in Cane Spring area.

2Grouse Canyon Member of Belted Range Tuff occurs as boulders beneath Bullfrog Member near south 

end of Crater Flat.

4ln Frenchman Flat-Cane Spring aree (column 4), occurs only on south slopes of Skull Mountain. 

Along south edge of Pehute Mesa (column 6), forms part of biotite-hornblende rhyolite west of Split Ridge 

(unit Tht, Byers and others, 1976a).

In Frenchman Flat-Cane Spring area (column 4), known only at Nye Canyon. 

Exact relations with units beneath Crater Flat Tuff are unknown. 

7Formerly Crater Flat Tuff in Yucca Flet and Pahute Mesa areas (columns 5 and 6).

Tuff of Yucca Flat7

117° 116°

\

NEVADA TEST 

SITE

x 5

50 KILOMETERS

0 25 MILES

GENERAL LOCATION OF 

STRATIGRAPHIC COLUMNS

FIGURE 2. Stratigraphic relations of Crater Flat Tuff and associated units in Nevada Test Site region. Numbers in column heads

refer to localities shown on inset.

Lithic Ridge Tuff, the Grouse Canyon Member datum, 

bedded tuffs, and the Paintbrush Tuff, which forms 

Tippipah Point. Ash-fall tuff of the Grouse Canyon is

present between the Lithic Ridge Tuff and the overlying Paintbrush Tuff about a mile north of the section 

below Tippipah Point.
37°30 116W

37°15'

37<W

UE25b 

160

TIMBER MOUNTAINOASIS VALLEY 

CALDERA COMPLEX

EXPLANATION

X Outcrop locality 

o Drill hole locality;

thickness in meters

100 Isopach, dashed where uncertain; 

interval 100 meters

20 MILES

30 KILOMETERS

FIGURE 3. Distribution and thickness of tuff of Yucca Flat, Nye County, Nev.
CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

Topopah Spring Member 

of Paintbrush Tuff

7° dip

 _

Lithic Ridge Tuff-243.8 maters

I 

5DD METERS

FIGURE 4. Stratigraphic relations of tuff of Yucca Flat and Lithic Ridge Tuff at Lithic Ridge.

WEST EAST

Tippipah 

Point

Datum of ash-fall tuff associated with 

the Grouse Canyon Member (13.7 m.y.) 

(present in section a short distance 

to the north)

Approximately 1200 meters (3600 feet) 

FIGURE 5. Stratigraphic relations of tuff of Yucca Flat and Lithic Ridge Tuff at Area 16 tunnel below Tippipah Point.
OLDER ASH-FLOW AND BEDDED TUFFS OF DRILL HOLE USW-G1

Four successive ash falls at Nye Canyon, at the eastern border of the Nevada Test Site (fig. 3), are apparently petrographically related to the tuff of Yucca Flat, 

the quartz latitic units, the Grouse Canyon Member, 

and the Bullfrog Member, respectively. At Scarp Canyon (fig. 3), the tuff of Yucca Flat is interbedded with 

tuffaceous sediments of the rocks of Pavits Spring 

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

Elston, 1965), about 45 m (150 ft) stratigraphically 

below the base of the Bullfrog Member of the Crater 

Flat Tuff. At Pahute Mesa, in drill holes UE20J and 

UE20f (fig. 3), the tuff of Yucca Flat underlies the Tub 

Spring Member of the Belted Range Tuff and the tuff 

of Tolicha Peak, respectively (Byers and others, 1976b, 

p. 14-15). In drill holes of eastern Yucca Flat, the tuff 

of Yucca Flat also underlies the Tub Spring and the 

tuff of Tolicha Peak, which are included in tunnel bed 

2. Thus, the tuff of Yucca Flat has the same stratigraphic relations over a large area extending from 

Scarp and Nye Canyons northwestward beneath 

Rainier and Pahute Mesas.

The tuff of Yucca Flat consists of zeolitic and nonwelded ash-flow tuff in the Yucca Flat area, but is moderately welded and devitrified at Lithic Ridge. At Nye 

Canyon, the correlative ash fall is vitric. The tuff of 

Yucca Flat petrographically resembles the Bullfrog 

Member of the Crater Flat Tuff but differs from it by 

having (1) few if any resorbed ("wormy") quartz 

phenocrysts, (2) spherulitic rhyolite lithic fragments, 

particularly where xenoliths are abundant, and (3) a 

general lack of pilotaxitic quartz latitic to dacitic lava 

fragments. Apparently the pilotaxitic lavas, intrusives, 

and minor tuffs of intermediate composition postdate 

the tuff of Yucca Flat (fig. 6). The spherulitic rhyolitic 

tuff or lava fragments, as seen under 100 x magnification, contain very thin (a few microns or less), brown, 

filamentous mafic crystallites resembling hair. These 

fragments very closely resemble those seen in the older 

tuffs of drill hole USW-G1 and in the Lithic Ridge Tuff 

and are similar to those in the Tram Member of the 

Crater Flat Tuff, but they have not been noted in other 

ash-flow tuffs rich in lithic fragments, such as those 

of Area 20 for example (Byers and others, 1976b). The 

occurrence of similar fragments in several stratigraphic 

units suggests a common source area, not only for the 

fragments but perhaps for the tuffs as well. The principal reasons for placing the tuff of Yucca Flat stratigraphically beneath the older tuffs at Yucca Mountain 

are its content of silicic phenocryst assemblages and 

its lack of intermediate-composition lithic fragments, 

which are common in other pre-Crater Flat tuffs.

It should be noted that pre-Grouse Canyon ash-flow 

tuffs of the Sleeping Butte area (fig. 3), mapped by 

Paul P. Orkild and K. A. Sargent (written commun., 

1971), are probably not related to the tuff of Yucca 

Flat and the Crater Flat Tuff as intimated by Byers 

and others (1976b, p. 15). The lithic fragments are different in those rocks, and the tuff at Sleeping Butte 

that resembles the tuff of Yucca Flat and the Bullfrog 

Member contains common sphene in every thin section 

examined. Further study is necessary to determine the 

relationship, if any, of the tuffs in the Sleeping Butte 

area to the Crater Flat or older units.

OLDER ASH-FLOW AND BEDDED TUFFS OF 

DRILL HOLE USW-G1

These tuffs were penetrated beneath Yucca Mountain 

in drill hole USW-G1 (figs. 7 and 8) from a depth of 

1,508 m (4,946.4 ft) to 1,829 m (6,000 ft), the bottom 

of the hole. The sequence is described in detail in the 

drill hole report by Spengler and others (1981), and 

only the principal petrographic features are mentioned 

here. The older tuffs of USW-G1, with one or two possible exceptions, are known only in the subsurface 

under Yucca Mountain. Spengler and others (1981) 

have divided the older tuffs into three units from the 

top down: unit A, a thick, quartz-rich ash-flow tuff (fig. 

8); unit B, a transitional unit of several thin ash flows 

in which quartz decreases and plagioclase phenocrysts 

increase downward; and unit C, which consists of several ash-flow tuffs separated by reworked tuffaceous 

sediments and which contains minor alkali feldspar and 

quartz but relatively abundant plagioclase and mafic 

phenocrysts (fig. 8). The older tuffs in hole USW-G1 

are hydrothermally altered, but the alteration has not 

completely erased the primary minerals or their 

pseudomorphs.

Certain significant petrographic features of these 

older tuffs are noteworthy. The lowest unit, C, from 

1,656 m (5,434 ft) to the bottom of the hole at 1,829 

m (6,000 ft) contains several ash-flow tuffs characterized by relatively abundant plagioclase and lesser 

amounts of biotite, hornblende, and clinopyroxene, in 

decreasing order of abundance. Pilotaxitic lava lithics 

of quartz latitic composition predominate in the lithic 

assemblages of unit C, suggesting a possible coincidence of eruptive source areas of quartz latite lava and 

ash-flow tuff.

The transition unit, B, has less quartz and alkali feldspar than unit A, but also has a higher proportion of 

pilotaxitic lava lithic fragments of intermediate composition.
00

STRATIGRAPHIC PHFNnrRYcTc FELSIC 

UNIT rntiNUUMY&it> PHENOCRYSTS

^^ Quartz (Q) 

(Number of thin ^ P|ag ioc|ase 

sections counted KXXXS AH , * u in parentheses) ^ Alkali feldspar

.£!»!: ..P.....; ' ..?°'fc..A ,..'.." /*/ '.. « .,<..: *..,...*. .!:

1 *|lJufJfrQ9':^em$er^ 'i

Rhyolite lava 

(6)

l^fcr* j

Dacite intrusive 

(or rhyodacite) 

(1)

Lithic Ridge Tuff 

(10)

Rhyolite lava 

(3)

Quartz latite 

(6)

Tuff of Yucca Flat 

(8)

 J

M *.'P | . : -1 --m^m

, i^^^ '(Strongly r,^,0rb|idl '" ' -;'' '|^^j^^ '.{Strongly .f^sorbed/I '* 

1 (Sj%5$3Sfi8'$!«ii1' :< 

 .::'... ...... "-^ -.V. . ^ ...'7*,. '' / . ..... . ,

] ': ! :^^^s . i

l^^^l ' '

mm

BS^S^^8883

i. I^^WIISSS^SSJ'; -,

:. " S^^^^i « " , w ,

1 (Resorbed, xenocrystic) XQ

1

-g 

.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.|

p&8p&8&3

1 ^^

^t]

B88S

«r.;,; .;.;.;

MAFIC ACCESSORY 

PHENOCRYSTS MINERALS

1 Biotite(B) MM Sphene ^ Hornblende(H) R&883 Zircon3 Orthopyroxene and/or 1 1 Allanite 

clinopyroxene

11, ..... 4 j*

J | $ L

i

* i

n LJ

. Orthopyroxene 

B and clinopyroxt

(Trace of clinopyro)

3 Clinopyroxene

&WX&i

R$^$$^$^$^

w| Clinopyroxene

i

e

ne

ene)

!

tz^

. ,}.

jw*8*1 ., V ';

/ /I/- r*/

i'^iS: " ^ ;

!.

i

(Apatite common, 

i microphenocrysts)

1 

Baa '

^ '

^iiil

^^^888^^^8I

^i^xj

^^^88^

r*

DIAGNOSTIC 

XENOLITHS

Rhyolitic volcanic 

rocks

|:;:j:v:;:;:| Intermediate composition lava

PL g«Mudston^ 5

; IXendMths rare} ;

(None)

g, t '! 'I?-' " ! " *

^%5S;::S^y!!:?ft:^;;^::^S:^: i

; 'B^fefl.*yltri|ted..rfty6})fe-: ;|t

(None)

1- 1

mmwmmmm

(None)(None)

1

m

0 10 20 30 0 10 20 30 40 50 60 70 80 90 0 5 10 15 20 0 5 10 15 20 25 0 5 10 15 20 25 3

1 of Crater PERCENT PERCENT OF TOTAL PHENOCRYSTS NUMBER OF GRAINS NUMBER OF XENOLITHS 

Flat Tuff OF ROCK PER THIN SECTION PER THIN SECTION

o

6

M 

»^

i

D OLDERVOLC

C UN

*Hi t?g

o 

o cj*

^ t?g

I

FIGURE 6. Modal ranges of outcrop samples of Crater Flat Tuff and associated volcanic units of southwest Nevada.
LITHIC RIDGE TUFF AND SIMILAR LAVA

NW

METERS 

2000 ,

YUCCA 

MOUNTAIN

INFERRED

PRE-CRATER FLAT

TUFF CALDERA

BOUNDARY FORTYMILE

WASH 

Qa J-13

1000^

SE

FEET 

 6000

 4000

 2000

Quaternary alluvium 

Paintbrush Tuff (Miocene): 

Tiva Canyon Member

Topopah Spring Member

1 2 MILES 

EXPLANATION

Bedded tuff of Calico Hills (Miocene) 

Crater Flat Tuff (Miocene): 

Prow Pass and Bullfrog Members

Tram Member

Dacite flow breccia (Miocene) 

Lithic Ridge Tuff (Miocene) 

Older Tertiary tuffs 

Mesozoic and Paleozoic rock

FIGURE 7. Geologic cross section between drill holes USW-GI and J-13 in the Yucca Mountain area, Nye County, Nev. Shaded area

is below static water level.

Unit A is a simple cooling unit of silicic, quartz-rich 

ash-flow tuffs, very similar to the superjacent Tram 

Member of the Crater Flat Tuff (fig. 8). Unit A, however, has fewer lithic fragments than parts of the 

Tram, and among these fragments, spherulitic rhyolite 

is dominant and pilotaxitic lava of intermediate composition is subordinate. The Tram Member has the opposite lithic abundances. The lithic content is the best 

means of distinguishing this unit from the Tram. Also, 

mafic phenocrysts are slightly less abundant in unit A 

than in the Tram.

QUARTZ LATITE

The volcanic rocks included under this heading underlie the Tram Member and the Lithic Ridge Tuff and 

are widespread at this general stratigraphic position. 

As mentioned, these rocks may have been a source of 

the pilotaxitic xenoliths of unit C in the older ash-flow 

tuffs of drill hole USW-GI, as well as those in the 

Lithic Ridge Tuff and the Tram Member of the Crater 

Flat Tuff. The quartz latite unit is mainly lava, but 

quartz latitic ash-flow tuff, possibly correlative with the 

lavas, occurs beneath the Lithic Ridge Tuff at Lithic 

Ridge, and similar ash fall is found at Nye Canyon and 

in the subsurface beneath Yucca Flat in unit 2 of the 

tunnel beds (Gibbons and others, 1963).

Quartz latite lava is exposed under the Lithic Ridge 

Tuff at Bullfrog Mountain (fig. 1) and at Tram Ridge

near Beatty Wash (fig. 9). A similar lava flow also is 

exposed above a tuffaceous sedimentary sequence south 

of Black Marble Hill, at the south end of Bare Mountain, near U.S. Highway 95 (figs. 1 and 9). Quartz latite 

dikes in the eastern part of Bare Mountain, adjacent 

to Crater Flat, gave the same potassium-arg-on age 

(13.9 m.y.; R. F. Marvin, written commun., 1980) as 

the pre-Lithic Ridge Tuff lava flows in Beatty Wash.

The phenocryst assemblage of the quartz latite lava 

is characterized by about 80 percent plagioclase and 20 

percent mafic phenocrysts, which consist mainly of biotite and hornblende, with or without clinopyroxene (fig. 

6). Zircon is present in all specimens and apatite is common. A few microphenocrysts of allanite and sphene 

occur in some specimens. Chemical analyses may show 

that some of these rocks are rhyolites, but their 

phenocryst assemblage is sufficiently mafic to warrant 

the name quartz latite, in order to distinguish these 

rocks from the more silicic rhyolites containing quartz 

and alkali feldspar phenocrysts.

LITHIC RIDGE TUFF AND

PETROGRAPHICALLY SIMILAR

RHYOLITE LAVA

RHYOLITE LAVA

The rhyolite lava (fig. 6) and the Lithic Ridge Tuff 

are discussed under the same heading in order to emphasize their petrographic similarity and probable
10

DEPTH, IN 

METERS UNIT

549.1

CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

PLAGIOCLASE

750

1000

1250

1500

1750

1828.8

Tcp

Tcb

Td

Tdl

Tlr

Tt

DEPTH, IN

MAFIC FEET 

PHENOCRYSTS

1801.5

 2000

 2500

 3000

 3500

 4000

 4500

 5000

 5500

6000

PERCENT OF ROCK 

*See figure 7 for explanation 

of unit symbols

PERCENT OF TOTAL PHENOCRYSTS

FIGURE 8. Petrographic variation of Crater Flat Tuff and older units in drill hole USW-GI on Yucca Mountain, Nye County, Nev.

genetic association. Similar associations between lava 

and ash-flow tuffs are common within the Paintbrush 

and Timber Mountain Tuffs (Byers and others, 1976b, 

p. 24, 25, 51-52, 61). The rhyolite lava that resembles 

the Lithic Ridge Tuff is exposed only in a small area 

at Tram Ridge, near Beatty Wash, where the rhyolite 

overlies a quartz latite lava (fig. 9). The rhyolite lava 

is separated from the overlying Lithic Ridge Tuff by 

several meters of bedded ash-fall tuff. Biotite from both 

lavas yielded the same age: 13.9 m.y. (R. F. Marvin, 

written commun., 1980).

The rhyolite lava is characterized by a plagioclase-alkali feldspar ratio of about 2:1, like that of the Lithic

Ridge, by small amounts of quartz and biotite, and by 

rare hornblende. As in the Lithic Ridge Tuff, at least 

five microphenocrysts of sphene occur in each thin section of all three specimens examined.

LITHIC RIDGE TUFF

The Lithic Ridge Tuff is here named after the excellent exposure at Lithic Ridge2 (figs. 4 and 10), its type 

locality, where approximately 250 m (800 ft) of non2"Lithic Ridge" is a newly established geographic name that is not shown on older 

topographic maps of the area.
LITHIC RIDGE TUFF AND SIMILAR LAVA

45' 30'

11

EXPLANATION

Exposed bedrock

Area of exposure of lava flow or plug

Dike

Area of lava flows penetrated by drill holes

fLACK 

MAfiBLE HILL

TYPE LOCALITY

CRATER FLAT

TUFF

USW G1

o Drill hole penetreting pre-Crater Flat Tuff lavas

36°45 -

10 MILES

FIGURE 9. Sketch map of dacitic to rhyolitic lava flows and intrusive rocks of pre-Crater Flat Tuff age in the southwestern Nevada

Test Site region.

welded to partially welded tuff is exposed. Lithic Ridge 

is located 1 km southeast of Topopah Spring in the Topopah Spring TVa-minute quadrangle. The Lithic Ridge 

Tuff is one of the most extensive of the units discussed 

in this report, as shown on figure 10. Furthermore, 

it is the only unit older than the Paintbrush Tuff known 

to occur in both the northern and southern parts of 

the NTS region. The unit was completely penetrated 

at depths below about 1,200 m (4,000 ft) in drill holes 

USW-G1, USW-H1, USW-G2, and USW-G3, where 

thicknesses of 303, 274, 207, and 304 m (994, 899, 679, 

and 997 ft) are reported (fig. 10; Spengler and others, 

1981; Florian Maldonado, written commun., 1982; and 

R. B. Scott, written commun., 1982). The upper part 

of the unit was penetrated in drill hole UE25b-l at 

1,207 m (3,960 ft) and in drill hole J-13 at 981 m (3,200 

ft) (fig. 7). At Bullfrog Mountain (fig. 1), about 70 m 

(230 ft) of hydrothermally altered Lithic Ridge Tuff 

dips northeastward just above the shaft of the Bullfrog 

Mine, where the unit was formerly thought to constitute the lower part of the Bullfrog Member (Byers and 

others, 1976b).

The Lithic Ridge Tuff crops out in a small area at 

the southern edge of Pahute Mesa, in the canyon between Blacktop Buttes and Split Ridge (fig. 10). The 

unit is overlapped by younger tuffs, so its base is not 

exposed and its total thickness is unknown at this locality, but it occurs stratigraphically above quartz-latitic 

tuff, which rests on the Tub Spring Member of the 

Belted Range Tuff, and below the rhyolite lavas of Split 

Ridge, which are beneath the Grouse Canyon Member. 

This locality is just southeast of the edge of the Silent 

Canyon caldera, the source area of the Tub Spring and 

Grouse Canyon Members of the Belted Range Tuff. The 

unit could be much thicker to the northwest within the 

collapse area of the Silent Canyon caldera, which is related to the Tub Spring Member.

At the type locality of the Lithic Ridge (figs. 4 and 

10), the unit was originally included in the lower part 

of the tuff of Calico Hills (Orkild and O'Connor, 1970;
HI

I

H3J,VHO QNV H3Q1O OINV01OA ' 3AN ' VQVA3N 
DACITE INTRUSIVE AND FLOW BRECCIA 13

Byers and others, 1976a). At Lithic Ridge the unit 

rests on a thin, quartz-latitic ash-flow tuff that is similar petrographically both to the tuff penetrated near 

the bottom of USW-G1 at 1,829 m (6,000 ft) and to 

the quartz-latite lava at Bullfrog Mountain and at Tram 

Ridge (figs. 1 and 10). In drill hole USW-G2, the Lithic 

Ridge Tuff is underlain by altered quartz-latitic lava. 

At the type locality at Lithic Ridge (fig. 4), the unit 

is intruded by dacite plugs and dikes somewhat similar 

petrographically to the dacite flow breccias that overlie 

the unit in drill holes USW-G1, -HI, and -G2. In holes 

UE25b-l and J-13, a thin, bedded-tuff interval separates the Lithic Ridge Tuff from the overlying Tram 

Member of the Crater Flat. In exposures south of Skull 

Mountain a thin ash-flow tuff, probably the Lithic 

Ridge, is intercalated in tuffaceous sedimentary rocks 

of Pavits Spring (Poole, Elston, and Carr, 1965) about 

90 m (300 ft) below the base of the Bullfrog Member.

The Lithic Ridge Tuff at the type locality has about 

3 percent phenocrysts and a few percent lithic fragments in the lower part, and about 9 percent phenocrysts and 10 percent lithic fragments in the upper 

part. The groundmass is white and zeolitic, and, in drill 

cores from holes in Yucca Mountain, most of the tuff 

is slightly to partially welded, zeolitized, and argillized. 

Near the top, the tuff is moderately welded at Yucca 

Mountain and contains 18 percent phenocrysts (fig. 8). 

At the Pahute Mesa locality the Lithic Ridge is partly 

vitric and contains well-preserved hornblende and 

clinopyroxene. The lithic-fragment content generally 

ranges from about 5 to 25 percent, and no occurrences 

are known in which the lithic content is less than 3 

percent.

The persistence of the lithic-fragment content and 

the phenocryst assemblage make the Lithic Ridge Tuff 

an excellent marker unit, readily identifiable from a 

single thin section. The lithic-fragment assemblage is 

marked by predominant pilotaxitic lava and subordinate 

spherulitic rhyolite. The rhyolite contains brown crystallite filaments that resemble hair under a magnification of 100 x. The felsic phenocryst ranges are unusual 

and are similar to those of the underlying, cogenetic 

rhyolite lava (fig. 6), and, moreover, at least five microphenocrysts of sphene, or its alteration products, occur 

in every thin section. Rare hornblende occurs only as 

microphenocrysts, in contrast to the greater abundance 

and larger size of hornblende crystals in the tuff of 

Yucca Flat and the Bullfrog Member of the Crater Flat 

Tuff. Although the quartz content of the Lithic Ridge 

overlaps slightly with that of members of the Crater 

Flat Tuff, nearly all lithic-rich samples from the Tram 

Member lack spherulitic lithics and have more quartz. 

Quartz phenocrysts of the Bullfrog and Prow Pass

Members are not only more abundant than those in the 

Lithic Ridge, but are highly resorbed as well.

As shown by exposures at Tippipah Point (fig. 5), 

the Lithic Ridge Tuff is loosely bracketed between the 

Redrock Valley Tuff, dated at 15.7 m.y., and the 

Grouse Canyon Member, dated at 13.7 m.y. In Beatty 

Wash, the Lithic Ridge rests on lavas dated at 13.9 

m.y., so the age of the Lithic Ridge is about 13.8 m.y.

DACITE INTRUSIVE AND FLOW BRECCIA

Dacite (Spengler and others, 1981, p. 36 and fig. 4) 

flow breccia was penetrated between the Tram Member 

of the Crater Flat Tuff and the underlying Lithic Ridge 

Tuff in holes USW-G1, -G2, -HI, -H5, and -H6, but 

is absent in holes USW-G3, UE25b-lH, and J-13 (fig. 

7). The dacite is thickest, about 250 m (800 ft), in drill 

hole USW-H6 (fig. 9). In holes USW-G1 and -HI the 

dacite flow breccia is about 120 m (400 ft) thick, 

whereas in USW-G2 it is much thinner about 35 m 

(115 ft) thick. Lava is present in the interior of the 

flow breccia. Thus, the unit thickens to the northwest 

beneath Yucca Mountain and pinches out southeastward.

The dacite intrusive plug that cuts the Lithic Ridge 

Tuff (fig. 4) at Lithic Ridge is apparently truncated by 

the present erosional surface, which may in part include 

an exhumed surface older than the tuffs of Calico Hills. 

The dacite intrusive rocks do not cut units younger 

than the Lithic Ridge Tuff.

The petrography of the dacite flow breccia penetrated in hole USW-G1 is shown graphically on figure 

8. Phenocrysts constitute about 13 percent of the rock, 

and 70 to 80 percent of the phenocrysts are plagioclase. 

The remaining phenocrysts are all mafic, consisting of 

about equal amounts of hornblende and clinopyroxene. 

Apatite microphenocrysts, typical of lavas of intermediate composition, are fairly common, and a few 

grains of zircon occur in one of the thin sections. The 

groundmass of the vitrophyre is perlitic, indicating an 

intermediate to rhyolitic composition. Norms calculated 

from chemical analyses of two samples of the flow breccia in USW-G1 show 20 percent normative quartz 

(Spengler and others, 1981), indicating the rock is dacite rather than andesite (O'Connor, 1965).

The dacite intrusive at Lithic Ridge was originally 

mapped as andesite (Orkild and O'Connor, 1970), but 

it is even more silicic than the flow breccia in the drill 

holes and may actually be a rhyodacite. (See fig. 6.) 

One thin section of the unit showed that phenocrysts 

constitute only 8 percent and that about 60 percent of 

the phenocrysts are plagioclase and the remaining 40 

percent are mafic minerals. However, the mafic miner-
14 CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

als include about equal amounts of biotite and 

hornblende and have subordinate orthopyroxene and 

clinopyroxene, suggesting a slightly more silicic composition than that of the flow breccia penetrated in USWGl. Microphenocrysts of apatite are present but zircon 

is rare. The phenocryst composition of the dacite plug 

at Lithic Ridge is generally like that of some lavas of 

the younger Wahmonie and Salyer Formations, but all 

lavas of the Wahmonie are phenocryst-rich (Poole, 

Carr, and Elston, 1965, p. A41-A43).

CRATER FLAT TUFF AND RELATED LAVA

The Crater Flat Tuff was named by Byers and others 

(1976b) from exposures around Crater Flat. The best 

exposure, near U.S. Highway 95 (fig. 9), includes the 

Bullfrog and Prow Pass Members separated by a 

monolithologic breccia of Bullfrog clasts in a matrix of 

nonwelded Bullfrog tuff. The underlying Tram Member, defined in this report, is not present at this locality. A tuff that is exposed mainly in fault blocks between the north end of Crater Flat and Beatty Wash 

was formerly correlated with the Bullfrog (Byers and 

others, 1976b) but is now recognized as the Tram. Locally a lava flow, similar in phenocryst mineralogy to 

the Bullfrog, occurs between the Bullfrog and the Tram 

in the area between Beatty Wash and Crater Flat (fig. 

9; Byers and others, 1976b, fig. 4, p. 14).

The three ash-flow units of the Crater Flat Tuff are 

present in drill holes on Yucca Mountain, in well J-13, 

and at Mara Wash (fig. 11; Poole, Elston, and Carr, 

1965). Although the presence of three ash-flow tuffs at 

Mara Wash had been known for some time, Byers and 

others (1976b, p. 10) regarded the middle tuff, now 

known to be Bullfrog, as a local cooling unit. The lowermost tuff unit at Mara Wash probably correlates with 

the Tram.

TRAM MEMBER

TYPE LOCALITY

The Tram Member, the oldest ash-flow tuff of the 

Crater Flat Tuff, is here named from exposures on the 

north slope of Tram Ridge3 (figs. 1, 12 and 13). The 

type locality is designated as along Beatty Wash northeastward from the outcrops where the Tram Member 

rests unconformably on the Lithic Ridge Tuff and on 

the rhyolite and quartz latite lavas under the Lithic 

Ridge (figs. 12 and 13). The top of the Tram is eroded 

in the upthrown fault block northwest of the large, pos3Tram Ridge" is a newly established geographic name that is not shown on older 

topographic maps of the area.

sible caldera fault under Tram Ridge (fig. 13). On the 

southeast side of this fault block, the Tram is overlapped by the Paintbrush and Timber Mountain Tuffs 

(fig. 12). The large northeasterly trending possible caldera ring fracture fault under Tram Ridge displaces the 

Paintbrush but not the Timber Mountain Tuff.

The total thickness of the Tram Member is not 

known at the type locality because of erosion on the 

upthrown block and concealment by the Bullfrog Member and younger rocks on the downthrown block, but 

the minimum thickness is estimated with the aid of the 

cross section shown on figure 13. The strike and dip 

shown on figure 12, however, are not representative 

of the entire exposure of the Tram along Beatty Wash. 

The minimum thickness exposed is estimated to be at 

least about 180 m (600 ft) including approximately 60 

m (200 ft) of shard-rich nonwelded to partially welded 

tuff in the lower part. The thickness of the Tram in 

the downfaulted possible caldera block may be much 

greater.

DRILL HOLE USW-G1

Drill hole USW-G1 at Yucca Mountain (fig. 9; Spengler and others, 1981) penetrated the Tram Member 

from 804.5 to 1,073.5 m (2,639.4-3,522.0 ft), a total 

thickness of 269.0 m (882.6 ft). The Tram in hole USWGl is overlain by a thin sequence of bedded tuff beneath the Bullfrog Member and underlain by bedded 

tuff overlying the dacite flow breccia of USW-G1. The 

lower half of the Tram in hole USW-G1 is nonwelded 

to partially welded, and rhyolite, quartz latite, and dacite lava xenoliths constitute as much as 50 percent of 

the rock. These xenoliths contain sparsely disseminated 

pyrite, and the tuff matrix is locally zeolitic and argillic. 

The upper half of the unit is partially to moderately 

welded and devitrified and has only 3-5 percent lava 

xenoliths. For a complete description of the Tram Member penetrated in hole USW-G1, see Spengler and 

others (1981, p. 17-18).

DISTRIBUTION AND THICKNESS

The Tram Member is distributed in an east-west pattern from Mara Wash near Frenchman Flat to Daylight 

Pass on the Nevada-California State line, where there 

are at least two complete exposures of nonwelded to 

partially welded Tram, 50 m (164 ft) or less in thickness. Only the upper few meters of the nonwelded ashflow tuff are exposed at Mara Wash (fig. 11). The member was penetrated in all the deep drill holes in the 

Yucca Mountain area, including well J-13. In most of 

the outcrop exposures, the base of the unit is either 

covered or cut out by faulting, except at the type locality near Tram Ridge, where the top has been eroded.
37°30'

37°15'

37<W

UE25b 

160

TIMBER MOUNTAIN

OASIS VALLEY 

CALDERA COMPLEX

TRAM RIDGE 

TYPE LOCALITY

> I LINCOLN

|COUNTY i_ _. 

EXPLANATION

70 Outcrop locality; 

thickness in meters

Drill hole locality; 

thickness in meters

 700 Isopach, dashed where uncertain; 

interval 100 meters

] I

I 

] 10

10I

I 

20

20 MILES 

I I

I 

30 KILOMETERS

O

I

FIGURE 11. Distribution and thickness of Tram Member of Crater Flat Tuff in Nevada Test Site region, southwestern Nevada.
16 CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

0 100 200 300 METERS 

EXPLANATION

Tpr

Tp

Alluvium 

Colluvium

Gravel and tuffaceous

sediments 

Debris flow and breccia of

Tiva Canyon Member of

Paintbrush Tuff

Tuff of Buttonhook 

Wash

Rainier Mesa Member 

Tuff breccia

Pre-Rainier Mesa rhyolite 

lava flows

Paintbrush Tuff

> QUATERNARY

Timber

Mountain

Tuff

TERTIARY 22

 A 

23

Bullfrog Member I 

Tram Member I 

Lithic Ridge Tuff

Rhyolite and quartz latite 

lavas

CONTACT

FAULT-Bar and ball 

on downthrown side. 

Dashed where inferred, 

dotted where concealed

Strike and dip of foliation 

Strike and dip of beds

Location of K-Ar dated 

samples

Crater Flat Tuff

Bedded tuff

> TERTIARY

FIGURE 12. Geologic relations of Tram Member of Crater Flat Tuff in the Tram Ridge-Beatty Wash area, southwestern Nevada.
CRATER FLAT TUFF AND RELATED LAVA 17

NORTHWEST SOUTHEAST

CALDERA MARGIN?

PROSPECTOR

PASS

CAULDRON 

SEGMENT

VERTICAL EXAGGERATION APPROXIMATELY 1.5

0 500

I

1000 METERS 

J

1000 2000

I 

3000 FEET

FIGURE 13. Geologic section from northwest to southeast through 

Tram Ridge, showing relations of Tram Member to lava flows and 

possible cauldron structure. See figure 12 for explanation of symbols.

The Tram is apparently thickest in the northern Crater 

Flat area, where it has the crystalline texture characteristic of thick intracaldera sequences. However, the 

greatest minimum thickness actually exposed there is 

only about 250 m (825 ft).

In general, the bedded tuff interval between the 

Tram and the Bullfrog is several meters thicker than 

the bedded tuffs between the Bullfrog and Prow Pass 

Members.

LITHOLOGY AND PETROGRAPHY

The Tram Member, as penetrated in most of the drill 

holes at Yucca Mountain, can be subdivided into a 

lower lithic part, containing abundant primarily rhyolitic and intermediate composition lava xenoliths, and 

an upper part much poorer in lithic fragments. The 

Tram Member generally contains more xenoliths than 

the overlying Bullfrog, but this criterion is not everywhere reliable, and recourse must often be made to 

petrography to differentiate the two. Lithic parts of 

the Tram are similar megascopically to the underlying 

Lithic Ridge Tuff, but contain much more quartz and 

only very rare sphene. At the type locality at Tram 

Ridge, near northern Crater Flat, and near the distal 

edges of the member (fig. 11), lithic fragments are 

much less common than in drill cores from under Yucca 

Mountain, and for this reason the unit is difficult to 

distinguish from the Bullfrog in outcrop or hand specimens.

Near northern Crater Flat and south of Skull Mountain, the bedded tuffaceous interval between the Tram

and the Bullfrog contains a distinctive bed, about a 

meter thick, of colorful to white opalized or jasperized 

tuffaceous siltstone that is a useful stratigraphic 

marker.

Identification of the Tram can be confirmed by thinsection modal analysis. In drill hole USW-G1, the Tram 

has considerably more quartz phenocrysts (fig. 8) than 

the Bullfrog, and they are only slightly resorbed, in 

contrast to the strongly resorbed phenocrysts in the 

Bullfrog. As shown on figure 6, however, a comparison 

of the quartz phenocryst content indicates considerable 

overlap between ranges of outcrop samples of the Tram 

and the Bullfrog. The plagioclase-alkali feldspar ratio 

is generally a little higher in the Bullfrog than in the 

Tram, and hornblende is more abundant.

The Tram also has considerably fewer spherulitic 

rhyolite xenoliths than the underlying Lithic Ridge 

Tuff. In drill hole USW-G1, unit A of the older ashflow tuffs is very similar to the Tram. Unit A differs 

from the Tram only in containing few or no rhyolite 

or intermediate lava xenoliths, and in containing many 

spherulitic rhyolite fragments that have hairlike crystallites similar to those seen in the Lithic Ridge Tuff 

and the tuff of Yucca Flat.

As discussed later, the Tram is also distinct from 

other Crater Flat Tuff units in having reversed remanent magnetism.

AGE AND RELATION TO OTHER UNITS

No potassium-argon age determinations have been 

made on the Tram, mainly because of hydrothermal alteration, but its age is well established from relations 

to other units. However, relations to the Grouse Canyon Member of the Belted Range Tuff, which is a widespread stratigraphic marker in the northern part of the 

NTS, dated at about 13.7 m.y. (see footnote 1, above), 

are not precisely known, except that the two units are 

approximately the same age. The Lithic Ridge Tuff at 

Tippipah Point (fig. 5) is beneath the Grouse Canyon 

horizon as mapped by Orkild (1963). A boulder bed that 

occurs beneath the Bullfrog Member at the Crater Flat 

Tuff type locality near U.S. Highway 95 (fig. 9) contains welded Grouse Canyon and Tram clasts. Another 

occurrence of smaller and much less abundant boulders 

of Belted Range Tuff affinities occurs stratigraphically 

between the Tram and Bullfrog Members in the Daylight Pass area near the California-Nevada State line 

(fig. 11). Lava just beneath the Tram and the Lithic 

Ridge in Beatty Wash has been dated at 13.9 m.y. (R. 

F. Marvin, written commun., 1980). It appears that the
18 CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

Tram and the Grouse Canyon are nearly the same 

age about 13.7 m.y. Discussion presented in the concluding section suggests that the Tram may be the 

younger of the two.

BULLFROG MEMBER AND PETROGRAPHICALLY 

SIMILAR LAVA

The Bullfrog Member was originally named and defined as the lower of two members of the Crater Flat 

Tuff (Byers and others, 1976b, p. 11-14), but we now 

know it as the middle member and designate the petrographically similar Tram as the lower member. The 

distribution of the Bullfrog is similar to that of the 

Tram Member (figs. 11 and 14), except that the 

Bullfrog occurs over a larger area. Near its western 

known limits the Bullfrog is somewhat thicker and 

more densely welded than the Tram. At its eastern 

limits near Scarp Canyon, the Bullfrog is nonwelded 

to partially welded, and at Nye Canyon only 10 m (35 

ft) of related ash-fall tuff is present.

The Bullfrog Member under Yucca Mountain consists 

of nonwelded to partially welded, zeolitic upper and 

lower parts and a central zone, 30-90 m (100-300 ft) 

thick, of moderately to densely welded tuff. At Prow 

Pass, only the upper 70 m (230 ft) of welded Bullfrog 

is exposed, and it contains an assortment of xenoliths, 

including plagioclase porphyritic rhyodacite(?) and 

granitic rock not seen elsewhere. At one locality at 

Prow Pass, a train of xenoliths the size of small boulders occurs along a parting about 10 m (35 ft) below 

the top of the Bullfrog. Locally, in a few zones observed in the drill holes, the Bullfrog contains rare 

mudstone xenoliths, but these are more typical of the 

overlying Prow Pass. At the type locality of the Crater 

Flat (fig. 14), the Bullfrog is partly vitric and nonwelded to densely welded. It is overlain by a breccia 

of welded, devitrified Bullfrog clasts in a nonwelded, 

glassy, Bullfrog-like matrix.

Petrographically, the Bullfrog Member generally contains about twice as much plagioclase as alkali feldspar, 

and about 10 to 25 percent of the phenocrysts are 

quartz (fig. 6). Most of the quartz, however, is moderately to strongly resorbed. The mafic minerals are principally biotite and subordinate hornblende, but 

hornblende or its pseudomorphs occur in almost every 

thin section examined. The overlying Prow Pass Member also contains sparse hornblende, but its general 

lack of biotite and its content of orthopyroxene make 

it distinctive (fig. 6). Inasmuch as the Prow Pass also 

has resorbed quartz, distinguishing the two units where 

the mafic phenocrysts are altered can be difficult.

Rhyolite lava that occurs between the Tram and the 

Bullfrog east of Tram Ridge (fig. 14; Byers and others,

1976b, p. 9) is petrographically similar to the Bullfrog. 

As seen on figure 6, the lava petrographically resembles both the Bullfrog and the Tram in phenocryst 

mineralogy, but its hornblende content is more like that 

of the Bullfrog.

Two potassium-argon dates were obtained (Marvin 

and others, 1970) from the Bullfrog Member, namely 

14.0 m.y. on biotite and 13.0 m.y. on sanidine (Marvin 

and others, 1970; Byers and others, 1976b). Previously, 

we had accepted the older age of 14.0 m.y. because 

the Bullfrog was thought to underlie the Grouse Canyon Member, then dated at 13.8 m.y. (Marvin and 

others, 1970). With the present knowledge, it seems 

best to average the two ages at 13.5 m.y.

PROW PASS MEMBER

The Prow Pass Member of the Crater Flat Tuff was 

defined by Byers and others (1976b, p. 14-15) as the 

upper of two ash-flow tuffs that crop out at Prow Pass 

(fig. 15) beneath the tuffs and rhyolite lavas of Calico 

Hills. Elsewhere, in the drill holes at Yucca Mountain, 

and at the type locality of the Crater Flat Tuff, the 

Prow Pass invariably rests on several meters of bedded 

tuff above the Bullfrog Member; no rhyolite lava has 

yet been found between the two members. The distribution and thickness of the Prow Pass is shown on figure 15. The Prow Pass is thinner than the Bullfrog almost everywhere the two members are exposed (figs. 

14 and 15).

Inasmuch as the Prow Pass Member overlies the 

Bullfrog, it must also be stratigraphically higher than 

the Grouse Canyon Member and the tuffs of Sleeping 

Butte (fig. 2). The Prow Pass and the Bullfrog underlie 

the Wahmonie and Salyer Formations (Poole, Carr, and 

Elston, 1965) at Mara Wash and also underlie the tuffs 

of Calico Hills at Prow Pass and in the drill holes on 

Yucca Mountain.

Lithologically, the Prow Pass and the Bullfrog are 

similar and would be difficult to distinguish were it not 

for the fact that the two units occur in sequence in 

most areas. At its type locality, the Prow Pass is exposed along a north-south strike length of about 1 km 

(0.6 mile), and from north to south the unit thickens 

from about 45 m (150 ft) to 75 m (250 ft) and becomes 

more densely welded. Near its southernmost exposure 

at Prow Pass, the member is 77 m (252 ft) thick and 

is underlain by 16 m (52 ft) of thick-bedded ash-fall tuff. 

The lower 13.5 m (45 ft) of the member is nonwelded 

and separated from the overlying part by about 0.3 m 

(1 ft) of thin-bedded ash-fall tuff. The lower part of 

the Prow Pass contains sparse reddish-brown mudstone 

fragments, similar to those seen in the lower part of 

the thicker Prow Pass in the Yucca Mountain drill
37°30' new

37°15'

37°00'

UE25b 

160

TIMBER MOUNTAINOASIS VALLEY 

CALDERA COMPLEX

EXPLANATION

70 Outcrop locality; 

thickness in meters

TYPE LOCALITY 

CRATER FLAT TUFF

Drill hole locality; 

thickness in meters

-100 Isopach, dashed where uncertain; 

interval 100 meters

20 MILES 

_|

0

»I

HE

\ 

30 KILOMETERS

FIGURE 14. Distribution and thickness of Bullfrog Member of Crater Flat Tuff in the Nevada Test Site region, southwestern Nevada.
37°30 11BW

37°15'

37°00'

TIMBER MOUNTAIN- f| T4M

OASIS VALLEY 

CALDERA COMPLEX

TYPE LOCALIT 

CRATER FLAT TUFF

>/

EXPLANATION

X Outcrop locality;

thickness in meters

UE25b o Drill hole locality;

thickness in meters

100 Isopach, dashed where uncertain; 

interval 100 meters

20 MILES 

I

0 10 20 30 KILOMETERS 

FIGURE 15. Distribution and thickness of Prow Pass Member of Crater Flat Tuff in the Nevada Test Site region, southwestern Nevada.

O

o 

o

o1O O
CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA 21

cores, but no bedded interval separates the two parts 

in the drill holes. At Prow Pass, rare granitic xenoliths 

were observed in the upper part, which is locally 

welded and contains a basal vitrophyre 0.1 m thick.

The generalized phenocryst mineralogy of 14 outcrop 

samples of the Prow Pass is shown on figure 6. Both 

the Prow Pass and the Bullfrog contain 10 to 20 percent 

phenocrysts, and both contain highly resorbed 

("wormy") quartz phenocrysts. The feldspar ratio in the 

Prow Pass is generally subequal, in contrast to plagioclase dominance in the Bullfrog, but there is some overlap. The main features that distinguish the Prow Pass 

from the Bullfrog are its content of mudstone fragments, as mentioned above, the presence of orthopyroxene phenocrysts, and the scarcity of biotite 

and hornblende. Rare pilotaxitic lava xenoliths are also 

found in the Prow Pass.

The age of the Prow Pass is bracketed by average 

potassium-argon ages of 13.5 m.y. for the underlying 

Bullfrog and 13.4 m.y. for the overlying tuffs of Calico 

Hills (Marvin and others, 1970).

MAGNETIC PROPERTIES OF CRATER FLAT 

TUFF AND SUBJACENT UNITS

Magnetic characteristics of welded tuffs and lava 

flows can be useful in correlation of units, especially 

where oriented drill core is available. Such data are 

also necessary for interpretations of aeromagnetic maps 

over complex volcanic rock terrain. Table 1 gives preliminary data (courtesy of J. G. Rosenbaum, U.S. 

Geological Survey) for some of the major prePaintbrush Tuff units in the southwestern NTS area. 

The samples are demagnetized drill core and oriented 

outcrop samples.

The tuff of Yucca Flat has been determined to be 

magnetically reversed (table 1). Drill cores from Yucca 

Flat support this determination, but few measurements 

of declination are presently available.

Remanent magnetism of the Lithic Ridge Tuff measured to date is variable; the remanence is apparently 

weak and has a low inclination and an intermediate direction. Lava flows underlying the Lithic Ridge and 

the Tram, however, seem to have a consistently reversed direction, based on field measurements made 

with a portable fluxgate magnetometer and on a few 

oriented samples drilled and measured in the laboratory. The dikes on Bare Mountain are also reversed.

The Tram Member has a relatively consistent reversed magnetization (table 1), even in outcrops that 

are not densely welded, and is therefore easily distinguished from the Bullfrog and Prow Pass Members, 

both of which have normal remanence.

CALDERAS RELATED TO CRATER FLAT TUFF 

AND SUBJACENT UNITS

The principal purpose of this report is to revise the 

stratigraphy of the Crater Flat Tuff, but new data and 

geologic relationships also provide a perspective on 

source areas and structural setting of the prePaintbrush Tuff volcanic units in the southwestern NTS 

area. Therefore, a brief discussion follows of the volcano-tectonic history as it is presently understood.

The probable source area of the Crater Flat Tuff, 

a large structural depression best expressed topographically in southern Crater Flat, is here named the 

Crater Flat-Prospector Pass caldera complex (fig. 1). 

The northern part of this feature, believed to be the 

source of the Tram Member, is named the Prospector 

Pass4 caldera segment; the name is taken from a pass 

between Beatty Wash and northern Crater Flat in 

northwesternmost Yucca Mountain. (See Byers and 

others, 1976b, fig. 4.) The northeastern part of the 

Prospector Pass segment is truncated by the younger 

Timber Mountain-Oasis Valley caldera complex.

Three main lines of evidence are significant in identifying structures associated with eruption of these 

tuffs: (1) geophysical data, (2) distribution and thickness of the ash-flow tuffs and their associated lava 

flows, and (3) key geologic relations in the postulated 

caldera area.

GEOPHYSICAL EVIDENCE

Many new geophysical data are available from recent 

investigations of the region surrounding Yucca Mountain. The most significant for volcano-tectonic interpretation include gravity, magnetic, and seismic surveys.

Detailed gravity data show that Crater Flat itself is 

a large compound gravity low (Snyder and Carr, 1982), 

which is a southward extension of a regional low associated with several caldera complexes, including Silent Canyon (Orkild and others, 1968) and Timber 

Mountain (fig. 16). Although calderas do not invariably 

produce distinct gravity lows, most calderas in southern Nevada are areas of relatively low density crustal 

rocks, including thick accumulations of tuff and alluvium. The absence of coherent Paleozoic and Proterozoic sedimentary rocks beneath the calderas can also 

contribute to a negative gravity signature.

The gravity low at Crater Flat is elongate northsouth but is separated into north and south areas by 

a septum across central Crater Flat (Snyder and Carr,

^'Prospector Pass" is a newly established geographic name that is not shown on older 

topographic maps of the area.
22 CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA 

TABLE l. Remanent magnetic directions of Crater Flat Tuff, Lithic Ridge Tuff, and Tuff of Yucca Flat

[Unconnected for structural rotation]

Unit

Crater Flat Tuff: 

Prow Pass Member 

Bullfrog Member 

Tram Member 

Lithic Ridge Tuff 

Tuff of Yucca Flat 

Number of 

localities

5 

4 

7 

4 

2

Inclination

Average

+41° 

+37°

-55° 

2.450

( 3 )

Range

+35° 

+33° 

-43° 

+19° 

-13°

to 

to 

to 

to 

i

+48° 

+44°

-75° 

-57° 

-32°

Declination

Average

344° 

3° 

160° 

234° 

( 3 )

Range

1 307° 

341° 

144° 

225.5° 

190°

to 11° 

to 13° 

to 181° 

to 248° 

, 193°

1 Some of the variation may be due to rotation of oriented core.

2 If one probably anomalous inclination is discarded, the average inclination is +4°

3 Because only two samples were obtained from outcrop, averages would have little 

validity. Average inclinations in core from drill holes 03J9 and OE7ba in Yucca Flat are 

-27° and -45°, respectively. Average declinations could not be determined from core.

1982). The general configuration of the low is like that 

of a sector graben a collapse segment on the flanks 

of a volcanic edifice. However, the section penetrated 

by drill hole USW-VH1 (Carr, 1982) in south-central 

Crater Flat showed that it is unlikely the hole is in 

a sector graben related to the Timber Mountain-Oasis 

Valley caldera complex; no unusual thickening of the 

Paintbrush Tuff occurs, and it is not downfaulted to 

great depths, at least not at USW-VH1. Gravity data 

(Snyder and Carr, 1982) suggest there are more than 

3,000 m (10,000 ft) of volcanic and Tertiary sedimentary 

rocks in the Crater Flat depression. As drill hole 

USW-VH1 showed, the Paintbrush Tuff has a normal 

thickness and lithology for the area, and the base is 

at a depth of only 867 m (1,833 ft); therefore, the thickening of the volcanic rocks must occur in the older 

units.

Detailed, draped aeromagnetic surveys of the Crater 

Flat area (U.S. Geological Survey, 1978, 1979), provided one of the first clues that some previous correlations of Crater Flat Tuff units were incorrect. Study 

of the aeromagnetic map also revealed several significant anomalies (fig. 17), one of which was drilled to 

762 m (2,500 ft) at USW-VH1. This circular positive 

magnetic anomaly in southern Crater Flat, about 7 km 

(4.3 mi) across, is thought to represent a resurgent 

dome that formed in the caldera after eruption of the 

Bullfrog Member (Carr, 1982, p. 8-9).

Aeromagnetic evidence for a possible caldera related 

to the Tram Member in northern Crater Flat is not

definitive, but a general magnetic low is present in the 

area, accentuated by strongly negative anomalies wherever the Tram Member is exposed (U.S. Geological 

Survey, 1978). Because of highly complex structure and 

hydrothermal alteration, especially in the northern part 

of the Prospector Pass segment, aeromagnetic and 

other geophysical evidence of caldera structure is tenuous.

Several kinds of seismic evidence support the concept 

of a Crater Flat caldera complex. Multiple recordings 

of seismic waves from nuclear explosions under Pahute 

Mesa show traveltime delays of nearly 0.5 seconds 

across Crater Flat, compared to the region south of 

Crater Flat (W. D. Mooney and Lynn Hoffman, U.S. 

Geological Survey, written commun., 1981). An unreversed seismic line recorded a shot from near Beatty 

west-east across Crater Flat and Yucca Mountain (W. 

D. Mooney, written commun., 1982); this method also 

detected significant traveltime delays in Crater Flat. 

The presence of lower velocity rocks in Crater Flat 

supports the interpretation that a deep, tuff-filled depression exists in that area.

DISTRIBUTION AND THICKNESS OF THE TUFFS

Isopach information presented earlier (figs. 11, 14, 

and 15) clearly shows the geographic relation between 

the probable source area in Crater Flat and the thickness and distribution of the three members of the Crater Flat Tuff. The east-west elongation of distribution
37°30'

37°00'

36°30'

117°00'

CALDERAS RELATED TO CRATER FLAT TUFF

H6°30' new

23

40 50 KILOMETERS 

~ I

30 MILES

FIGURE 16. Generalized Bouguer gravity map of the NTS region, showing the regional gravity low associated with several 

caldera complexes. Contour interval 10 mGal. A, Cathedral Ridge caldera; B, Black Mountain caldera; C, Silent Canyon 

caldera; D, Timber Mountain-Oasis Valley caldera complex; E and F, Crater Flat-Prospector Pass caldera complex. Locations 

of drill holes USW-VH1, -Gl, and -G2 also shown. Based on gravity studies by R. W. Saltus (written commun., 1982).
24 CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

116°37'30" 11B°30

BULLFROG MEMBER 

CALDERA BOUNDARY?

3 MILES

FIGURE 17. Aeromagnetic map of southern Crater Flat area, showing caldera boundary and the proposed resurgent dome of the 

Bullfrog caldera. Contour interval 100 gammas. From U.S. Geological Survey (1978, 1979).

of the Crater Flat is similar to that of the Paintbrush 

Tuff (Byers and others, 1976b), but, significantly, none 

of the members of the Crater Flat are found north of 

latitude 37° N. Lobes of these tuffs could be concealed 

beneath the Timber Mountain-Oasis Valley caldera 

complex, however. Owing to extensive cover by 

younger units, data on the thickness of the Crater Flat 

are spotty, and many thicknesses shown on the isopach

maps (figs. 11, 14, and 15) are minima. There is enough 

control, however, to show that the thickest part of the 

Tram is in the northern Crater Flat area, and the thickest part of the Bullfrog is in the general area from 

north of Bare Mountain to Crater Flat. Drill hole 

USW-VH1 in south-central Crater Flat bottomed in 

the Bullfrog Member after 141 meters (463 ft) were 

penetrated (Carr, 1982). Textures and welding of the 

Bullfrog in continuous core from this hole strongly 

suggest much more of the member lies beneath the section penetrated. As mentioned previously, aeromagnetic data support this conclusion.

The Prow Pass Member appears to be thickest in 

the Yucca Mountain area (fig. 15), but the relatively 

thin sequence penetrated within the postulated caldera 

at drill hole USW-VH1 seems to argue against the 

caldera concept. However, the postulated resurgent 

dome in the Bullfrog caldera in the USW-VH1 area 

could cause the observed thinning of the Prow Pass. 

The exact location of a collapse related to the Prow 

Pass remains a question; the total volume of the Prow 

Pass is not great, so a large or deep collapse area is 

not required.

GEOLOGIC RELATIONS

Geologic relations that help associate caldera locations and structures with individual members of the 

Crater Flat Tuff are both subtle and discontinuous because exposures of the members are few. One of the 

most useful clues is the distribution of lava flows 

erupted during the magmatic episodes that generated 

the tuffs. As described previously, several dacitic to 

rhyolitic lavas and intrusive bodies are exposed in the 

southwestern NTS area and penetrated by several drill 

holes at Yucca Mountain (fig. 9). These rocks all predate the Bullfrog Member, and many are beneath the 

Lithic Ridge Tuff. Scattered exposures of lavas and 

dikes flank Crater Flat (fig. 9), except on the southeast 

side, where the only pre-Bullfrog rocks exposed are 

those at the type locality of the Crater Flat Tuff. Dikes 

on Bare Mountain show a slightly arcuate pattern, concave toward the postulated caldera complex, suggestive 

of outer ring-fracture intrusions. As mentioned earlier, 

these dikes are petrographically similar to the other 

rhyodacitic bodies, and one has been dated at 13.9 

m.y., identical in age to the lavas beneath the Lithic 

Ridge Tuff at Beatty Wash. In addition, some of the 

dikes on Bare Mountain are composite, consisting of 

a quartz latitic or rhyodacitic border phase and a central, quartz-rich, rhyolitic phase, the same sequence observed in the lavas in Beatty Wash.

The abundance of lithic fragments, including many 

lava clasts, in the Tram and the Lithic Ridge, especially
CALDERAS RELATED TO CRATER FLAT TUFF 25

in the drill holes at Yucca Mountain, shows that these 

tuffs were erupted through a terrain containing thick 

lava flows. As flows are not present southeast of hole 

USW-G1, we infer that the sources of the Tram and 

Lithic Ridge Members lie to the west or northwest, 

perhaps in the general area between Crater Flat and 

Beatty Wash.

At the type locality of the Crater Flat Tuff (fig. 14), 

a monolithologic breccia deposit of welded, devitrified 

Bullfrog Member occurs at the top of the Bullfrog. The 

lithology of the breccia fragments in that deposit is unlike that of any other local Bullfrog exposure. Small 

faults that cut the Bullfrog at this locality do not offset 

the overlying Prow Pass. These features suggest that 

abrupt tectonic movements accompanied the eruption 

of the Bullfrog Member and are a further indication 

that this locality is near the collapsed caldera associated 

with eruption of the Bullfrog. About 3 km (1.7 mi) to 

the north, at the southern extremity of the caldera 

complex, are several faults that trend northeast, parallel to the postulated caldera wall (F on fig. 18) and 

downthrown toward it.

Although, certainly, not all basalts in the Great Basin 

are associated with calderas, there is a strong tendency, especially for the more voluminous basalt flows, 

to occupy caldera ring fracture zones. The basalts of 

Crater Flat (Crowe and Carr, 1980) appear to be an 

example of flows that were localized by the deep-seated 

fracture system of the caldera but probably reached the 

surface along northeast-trending, riftlike fracture systems. Another basalt that may be a clue to caldera location is the group of lO-m.y.-old (R. F. Marvin, written 

commun., 1980) dikes on Yucca Mountain (fig. 18). One 

of these dikes occurs along a north-striking fault, but 

several others occur in fractures that form a northwesttrending zone. Taken together, the dikes suggest an 

arcuate pattern, slightly concave to the west-southwest, and we suggest that their location was controlled 

by a buried outer ring fracture related to the Prospector Pass segment of the caldera (fig. 18).

Scattered exposures of the Tram Member, between 

Beatty Wash and the north end of Crater Flat, are 

hydrothermally altered and broken by numerous small 

faults. The texture, welding, and crystallization of the 

Tram in these exposures are like those of very thick 

intracaldera ash-flow tuffs, such as the Ammonia Tanks 

Member of the Timber Mountain Tuff, as exposed on 

Timber Mountain (fig. 14), a resurgent dome in the center of the Timber Mountain caldera (Byers and others, 

1976a, b). These characteristics, the fault pattern, and 

the relatively high structural position of the outcrops 

all suggest that this may be an eccentric resurgent area 

in the Prospector Pass segment of the caldera.

RELATION BETWEEN TRAM AND 

GROUSE CANYON MEMBERS

The discovery of welded tuff boulders of the Grouse 

Canyon and Tram Members beneath the Bullfrog Member established the stratigraphic relationship between 

the Bullfrog and the Grouse Canyon, but the exact 

Tram-Grouse Canyon volcano-tectonic and stratigraphic 

succession remains obscure. The following discussion, 

however, outlines present reasoning for tentatively 

concluding that the Tram is younger than the Grouse 

Canyon.

At the Crater Flat type locality near U.S. Highway 

95, the boulder bed beneath the Bullfrog contains very 

large clasts of Tram and Grouse Canyon material, some 

as much as 1.8 m (6 ft) in diameter; virtually no other 

units are represented in this deposit. In general, it resembles a flood-emplaced debris flow, and transport of 

such large boulders for any appreciable distance 

suggests a catastrophic flood or a relatively fluid volcanic mudflow, possibly caused by a sudden release of 

a large volume of water by dam breaching. The nearest 

Grouse Canyon welded tuff exposures are 65 km (40 

mi) north of the boulder bed exposure, but as outlined 

earlier, the Tram caldera is probably located in the 

northern part of Crater Flat. Its southern wall is postulated to be in north-central Crater Flat, about 15 km 

(10 mi) north of the boulder bed outcrops. The Belted 

Range caldera, source of the Grouse Canyon, is about 

80 km (50 mi) to the north, beneath Pahute Mesa. Had 

the Tram eruption and caldera collapse preceded the 

Grouse Canyon eruption, it seems unlikely that the 

Grouse Canyon boulders could have been transported 

across the Tram caldera in northern Crater Flat. Boulders of Grouse Canyon have also been noted at a locality about 3 km (1.8 mi) up Beatty Wash from the Tram 

type locality. The boulders occur as a thin lag deposit 

on top of the Tram Member and are preserved beneath 

the Rainier Mesa Member, now nearly stripped away 

at that location. It is reasonable to speculate that a 

lobe of the Grouse Canyon Tuff was present in the 

Beatty Wash-northern Crater Flat area, possibly as far 

south as the southern edge of the subsequent Tram 

caldera. We speculate that, after the Tram caldera collapsed, its south wall consisted largely of Tram Member 

on the rim, underlain by Grouse Canyon Member. A 

large lake could then have filled the Tram caldera and 

may have overflowed by breaching the south wall in 

a catastrophic flood or debris flow, which carried large 

boulders of Tram and some Grouse Canyon material 

southward about 15 km (10 mi) to their present location 

near U.S. Highway 95. Although highly speculative, 

this sequence of events seems a logical way to account 

for the known relationships.
26 CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

116M5' 11 PISCRATER FLATPROSPECTOR PASS-: 

CALDERA 

COMPLEX

QUATERNARY AND TERTIARY ALLUVIUM 

PLEISTOCENE AND PLIOCENE BASALTS 

TERTIARY VOLCANIC ROCKS 

UPPER PALEOZOIC CLASTIC ROCKS

MIDDLE AND LOWER PALEOZOIC CARBONATE ROCKS

LOWER PALEOZOIC AND PRECAMBRIAN 

CLASTIC ROCKS

EXPLANATION

 CONTACT

FAULT Dotted where concealed; bar and ball on downthrown 

side

> CALDERA WALL Mostly inferred and approximately located

-LOW-ANGLE FAULT, THRUST OR GLIDE PLANE-Sawteeth on 

upper plate; dotted where concealed

p* POSSIBLE RESURGENT DOME 

X MIOCENE BASALT DIKES

(F) LOCATION OF FAULTS THAT HAVE TRENDS APPROXIMATELY 

^-^ PARALLEL TO POSTULATED CALDERA WALL

 DRILL HOLE

FIGURE 18. Relation of the Crater Flat-Prospector Pass caldera complex to generalized geologic features of the southwestern Nevada Test

Site region.
CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA 27

Further evidence for the Tram being post-Grouse 

Canyon is the fact that the top of the Tram is well 

exposed at several places between northern Crater Flat 

and Beatty Wash, and no Grouse Canyon outcrops have 

been found between it and the other Crater Flat Tuff 

members. In contrast, the base of the Tram is exposed 

in only one tiny area in the same region.

PR&-CRATER FLAT TUFF CALDERA

Drilling at Yucca Mountain revealed a thick sequence 

of pre-Crater Flat tuff, including the Lithic Ridge Tuff. 

In addition, several lava flows possibly associated with 

the Lithic Ridge were penetrated by the drill holes 

(figs. 7, 8, and 10). As the Crater Flat Tuff shows no 

abrupt thickening in the area around drill holes USWGl and USW-G2 that corresponds with a gravity low, 

it is unlikely that these units are within their own 

source area. Gravity data indicate (Snyder and Carr, 

1982), however, that more than a kilometer of tuff is 

present in this area beneath the bottom of the drilled 

section. Obviously, some sort of volcano-tectonic 

depression exists in the northwestern Yucca Mountain 

area, but its origin and boundaries are largely unknown. We suggest that the north-trending, slightly arcuate fault system that lies just east of the drill holes 

and the NTS boundary at Yucca Mountain may be an 

expression of a reactivated ring-fracture zone, whose 

depressed area may lie somewhere to the west of this 

fault system (figs. 7 and 17). As there is little evidence 

of ash-flow tuffs older than the Lithic Ridge in the 

southwest NTS area, other than in the drill holes at 

Yucca Mountain, the volume of these older units may 

not have been large, and hence a well-defined caldera 

may not have formed. However, the abrupt westward 

thickening of pre-Tram dacitic lavas between drill holes 

USW-G1 and USW-H6 (fig. 9) suggests the possibility 

that the Lithic Ridge caldera could lie just to the west 

or northwest of Yucca Mountain, perhaps in the same 

general area as the Prospector Pass cauldron segments.

If the Lithic Ridge Tuff is correctly identified at Pahute Mesa (table 1), it is the only pre-Paintbrush Tuff 

unit known to extend well north and south of lat 37° 

N. (fig. 10). The tuff of Yucca Flat, however, does have 

a distribution somewhat similar to that of the Lithic 

Ridge, and it is quite possible it lies below the 1,829-m 

(6,000-ft) depth drilled at Yucca Mountain. If the tuff 

of Yucca Flat produced a caldera, it may lie north of 

Yucca Mountain, partly within and concealed by the 

younger Timber Mountain caldera complex.

REFERENCES

Byers, F. M., Jr., Carr, W. J., Christiansen, R. L., Lipman, P. 

W., OrMld, P. P., and Quinlivan, W. D., 1976a, Geologic map 

of the Timber Mountain caldera area, Nye County, Nevada: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I891, scale 1:48,000.

Byers, F. M., Jr., Carr, W. J. ( OrMld, P. P., Quinlivan, W. D., 

and Sargent, K. A., 1976b, Volcanic suites and related cauldrons 

of Timber Mountain-Oasis Valley caldera complex, southern 

Nevada: U.S. Geological Survey Professional Paper 919, 70 p. 

Carr, W. J., 1982, Volcano-tectonic history of Crater Flat, southwestern Nevada, as suggested by new evidence from drill hole USWVH1 and vicinity: U.S. Geological Survey Open-File Report 82- 

457, 23 p.

Christiansen, R. L., 1979, Cooling units and composite sheets in relation to caldera structure, in Ash-flow tuffs: Geological Society 

of America Special Paper 180, p. 29-42.

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. Geological SurveyTrofessional Paper 525-B, p. B43-B48.

Christiansen, R. L., Lipman, P. W., Carr, W. J., Byers, F. M., 

Jr., Orkild, P. P., and Sargent, K. A., 1977, The Timber Mountain-Oasis Valley caldera complex of southern Nevada: Geological Society of America Bulletin, v. 88, p. 943-959. 

Crowe, B. M., and Carr, W. J., 1980, Preliminary assessment of 

, the risk of volcanism at a proposed nuclear waste repository in 

the southern Great Basin: U.S. Geological Survey Open-File Report 80-557, 15 p.

Dixon, G. L., Sargent, K. A., and Carr, W. J., 1976, Abandonment 

of the Indian Trail Formation and distribution of certain equivalent ash-flow tuffs, eastern Nevada Test Site, in Cohee, G. V., 

and Wright, W. B., Changes in stratigraphic nomenclature by 

the U.S. Geological Survey, 1975: U.S. Geological Survey Bulletin 1422-A, p. A49-A54.

Fairer, G. M., and Townsend, D. R., 1979, Geological investigations, 

in U.S. Geological Survey investigations in connection with the 

Mighty Epic event: U.S. Geological Survey report USGS-474- 

228, 191 p.; available only from U.S. Department of Commerce, 

National Technical Information Service, Springfield, VA 22161. 

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. Geological Survey Geologic Quadrangle Map GQ215, scale 1:24,000.

Maldonado, Florian, compiler, 1985, Geologic map of the Jackass 

Flats area, Nye County, Nevada: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1519 [in press]. 

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: Geological Society of America Bulletin, v. 81, p. 2657-2676.

Noble, D. C., Sargent, K. A., Mehnert, H. H., Ekren, E. B., and 

Byers, F. M., Jr., 1968, Silent Canyon volcanic center, Nye 

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based on feldspar ratios, in Geological Survey research, 1965: 

U.S. Geological Survey Professional Paper 525-B, p. B79-B84.
28 CRATER FLAT TUFF AND OLDER VOLCANIC UNITS, NYE COUNTY, NEVADA

Orkild, P. P., 1963, Geology of the Tippipah Spring quadrangle, Nye 

County, Nevada: U.S. Geological Survey Geologic Quadrangle 

Map GQ-213, scale 1:24,000.

Orkild, P. P., Byers, F. M., Jr., Hoover, D. L., and Sargent, K. 

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Orkild, P. P., and O'Connor, J. T., 1970, Geologic map of the Topopah Spring quadrangle, Nye County, Nevada: U.S. Geological 

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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. Geological Survey Bulletin 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. 

Geological Survey Geologic Quadrangle Map GQ-455, scale 

1:24,000.

Snyder, D. B., and Carr, W. J., 1982, Preliminary results of gravity 

investigations at Yucca Mountain and vicinity, southern Nye 

County, Nevada: U.S. Geological Survey Open-File Report 82- 

701, 36 p.

Spengler, R. W., Byers, F. M., Jr., and Warner, J. B., 1981, Stratigraphy and structure of volcanic rocks in drill hole USW-G1, 

Yucca Mountain, Nye County, Nevada: U.S. Geological Survey 

Open-File Report 81-1349, 50 p.

Steele, S. G., and Fairer, G. M., 1978, Geological investigations, in 

U.S. Geological Survey Investigations in connection with the Dining Car event, U12e.l8 tunnel, Rainier Mesa, Nevada Test Site: 

U.S. Geological Survey report USGS-474-246 (Area 12-49), 568 

p.; available only from U.S. Department of Commerce, National 

Technical Information Service, Springfield, VA 22161.

U.S. Geological Survey, 1978, Aeromagnetic map of Lathrop Wells 

area, Nevada: U.S. Geological Survey Open-File Report 78-1103.

 1979, Aeromagnetic map of the Timber Mountain area,

Nevada: U.S. Geological Survey Open-File Report 79-587.

U. S. GOVERNMENT PRINTING OFFICE 1986: 676-047/26,026