U.S. Department of the Interior

U.S. Geological Survey

53 3'

Professional Paper 1701

Tephra Layers of Blind Spring Valley and Related 

Upper Pliocene and Pleistocene Tephra Layers, 

California, Nevada, and Utah: Isotopic Ages, 

Correlation, and Magnetostratigraphy
Tephra Layers of Blind Spring Valley 

and Related Upper Pliocene and 

Pleistocene Tephra Layers, California, 

Nevada, and Utah: Isotopic Ages, 

Correlation, and Magnetostratigraphy

Professional Paper 1701

U.S. Department of the Interior

U.S. Geological Survey

By Andrei M. Sarna-Wojcicki, Marith C. Reheis, Malcolm S. Pringle, 

Robert J. Fleck, Doug Burbank, Charles E. Meyer, Janet L. Slate, 

Elmira Wan, James R. Budahn, Bennie Troxel, and James P. Walker

Identification, chemical correlation, and isotopic dating of the widespread tephra 

layers of Blind Spring Valley allow correlation of associated sediments and tephra 

layers among numerous localities in the southwestern conterminous United States 

and provide a synoptic view of this large region during a short interval of late 

Cenozoic time.
ii 

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes 

only and does not imply endorsement of the U.S. Government.

United States Government Printing Office: 2005

For additional copies please contact:

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This report and any updates to it are available at:

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Text edited by Peter H. Stauffer

Production and design by Sara Boore

Manuscript approved for publication, August 23, 2004

Cataloging-in-publication data are on file with the Library of Congress (URL http://

www.loc.gov/).

FRONT COVER

Photograph of the tephra layers of Blind Spring Valley (massive white beds in foreground) in Pit 6 of the Cowan Pumice Mine, Blind 

Spring Valley, Calif. View to southeast, looking across Blind Spring Valley to Blind Spring Hill in the middle distance and the White 

Mountains in the far distance. USGS photo by A.M. Sarna-Wojcicki.
 iii

Contents

Abstract ................................................................................................ 1

Introduction .......................................................................................... 3

Tephra layers of Blind Spring Valley at the Cowan Pumice Mine ........................ 4

Tephra layers from other source areas in the western United States .................. 6

Acknowledgments .................................................................................... 6

Previous work .......................................................................................... 7

Analytical methods .................................................................................... 8

Field methods .................................................................................... 8

Laboratory preparation of samples for analysis ............................................. 8

Laser-fusion 40Ar/39Ar and conventional K-Ar analyses .................................... 8

Chemical analysis of volcanic glass shards ................................................ 9

Electron-microprobe (EM) analysis ................................................... 9

Energy-dispersive X-ray fluorescence (EDXRF) analysis ........................... 9

Instrumental neutron activation (INA) analysis ....................................... 9

Methods of data evaluation .................................................................. 12

Stratigraphic section at Cowan Pumice Mine—results of age analysis ..................... 12

Stratigraphic section in and below Pit 6 ...................................................... 12

Basal basalt flows ........................................................................ 12

Basal arkosic mudflows and the tuff of Benton Hot Springs ........................ 12

Tuffs of Blind Spring Valley ............................................................... 15

Lower tuffs of Glass Mountain ......................................................... 16

Tephra layers exposed in Pits 1 through 5 ................................................... 17

The Bishop ash ........................................................................... 17

Tuffs of Blind Spring Valley ............................................................... 17

Isotopic ages of the upper Glass Mountain tephra layers at Chalk Cliffs, at the south end of 

the Volcanic Tableland ..................................................................... 19

Interpretation of the isotopic ages at the Cowan Pumice Mine ................................. 21

Correlation of the tuffs of Blind Spring Valley to sites in the region ........................... 28

Northeastern Fish Lake Valley ............................................................... 28

Willow Wash section ........................................................................... 30

Yellowjacket Canyon, Chalfant Valley, east-central California ........................... 32

Waucoba lake beds, near Big Pine, Owens Valley, California ........................... 33

Correlation of the Tuffs of Blind Spring Valley and related tephra layers to distal sites in the 

western United States ..................................................................... 35

Confidence Hills, Death Valley, southeastern California .................................... 35

Lake Tecopa, southeastern California ...................................................... 38

Manix Basin, central Mojave Desert, southern California ................................. 39

Peninsular Ranges, California .................................................................. 41

Ventura Basin, southwestern coastal California ............................................. 41

Beaver Basin, west-central Utah ............................................................ 41

Discussion and conclusions ........................................................................ 41

References ............................................................................................. 49
iv 

Appendixes

Appendix 1. Analytical data for laser-fusion 40Ar/39Ar age determinations of tephra layers ...... 5

Appendix 2. Analytical data for conventional K-Ar analysis of obsidian clasts from the 

lower tephra layers of Glass Mountain at the Cowan Pumice Mine, Blind Spring 

Valley, Calif. .................................................................................... 6

Apendix 3. Analytical data for laser-fusion 40Ar/39Ar analysis of samples that were 

contaminated with detrital material ......................................................... 6

Figures

 1. Map showing sample localities of tuffs of Blind Spring Valley and related tephra 

layers, known distribution of tuffs of Blind Spring Valley, and locations of Glass

 Mountain and the Long Valley Caldera ...................................................... 3

 2. Generalized geologic map of the vicinity of Long Valley Caldera, Glass Mountain, 

Cowan Pumice Mine, and the south end of the Volcanic Tableland ..................... 4

 3. Map showing locations of pumice pits at Cowan Pumice Mine and cross sections 

A-A’ and B-B’ ................................................................................. 5

 4. Cross sections A-A’ and B-B’ in area of Cowan Pumice Mine, Blind Spring Valley, showing locations of samples in Pits 1-6 and 40Ar/39Ar ages .................................... 6

 5. Photograph showing exposure of tuffs of Blind Spring Valley and lower tuffs of 

Glass Mountain in Pit 6, Cowan Pumice Mine ............................................. 13

 6. Photograph of exposures in Pit 6 at Cowan Pumice Mine .............................. 13

 7. Stratigraphic section of exposure in Pit 6, Cowan Pumice Mine ........................ 14

 8. Photograph showing upper part of section in Pit 6, Cowan Pumice Mine ............ 16

 9. Photograph showing two thick, coarse pumice-fall tuffs of Blind Spring Valley at 

south wall of Pit 5, Cowan Pumice Mine ................................................... 17

 10. Stratigraphic section of tuffs of Blind Spring Valley at Pit 5, Cowan Pumice Mine ... 18

 11. Photograph of tuff of Blind Spring Valley in Pit 2, Cowan Pumice Mine ............... 19

 12. Composite stratigraphic section at Chalk Cliffs, south end of Volcanic Tableland ...... 25

 13. Stratigraphy of section exposed near Emigrant Pass, northeast Fish Lake Valley, 

Nevada .......................................................................................... 29

 14. Photograph showing tuff of Benton Hot Springs, near base of Rimrock section at 

Willow Wash ................................................................................. 30

 15. Stratigraphy of sections at Willow Wash, south of Fish Lake Valley, California-Nevada 31

 16. Photograph of Bishop Tuff near head of Yellowjacket Canyon ........................... 32

 17. Location map and stratigraphic section of Waucoba Lake Beds near Big Pine, 

California ....................................................................................... 33

 18. Magnetostratigraphy, stratigraphy, and tephrostratigraphy of the Confidence Hills section, Death Valley National Park, California ................................................ 36

 19. Correlation diagram of the tuffs of Blind Spring Valley and related tephra units ...... 39

 20. Photograph showing exposure of tuff of Blind Spring Valley in Beaver Basin, Utah ... 44

 21. Correlation diagram showing magnetostratigraphy and chronology of the tuffs 

of Blind Spring Valley and related tephra units at selected sites in the southwestern 

United States .................................................................................... 45

 22. Diagram showing chronology of tuffs of Blind Spring Valley and related tephra 

layers compared to chronology of near-source Glass Mountain rhyolites ............ 47
 v

Tables

 1. Chemical composition of volcanic glass shards, determined by electron-microprobe 

analysis, and ages of tephra layers sampled from quarry pits at the Cowan Pumice 

Mine, near Benton Hot Springs, California ................................................ 10

 2. Electron-microprobe analysis of glass of the Bishop Tuff and Bishop airfall pumice, 

compared with the younger set of tephra layers of Glass Mountain ..................... 20

 3. Chemical composition of volcanic glass shards, determined by electron-microprobe 

analysis, from middle Pleistocene and upper Pliocene tephra layers of northeastern 

Fish Lake Valley, Calif.-Nev. (Emigrant Pass area and vicinity) ........................... 22

 4. Electron-microprobe analyses of volcanic glass shards from middle Pleistocene 

and upper Pliocene tephra layers of the Willow Wash area, south of Fish Lake 

Valley, Calif.-Nev. .............................................................................. 23

 5. Energy-dispersive X-ray fluorescence analysis of volcanic glass from tephra layers ... 26

 6. Results of electron-microprobe analysis of glass from tephra layers in exposures 

of the Waucoba lake beds, east side of Owens Valley .................................... 34

 7. Glass chemical composition of tephra layers exposed in deformed alluvial and 

lacustrine sediments of the Confidence Hills, southern Death Valley, Calif. ............ 37

 8. Glass chemical composition of tephra layers from several sites in southern and 

southwestern California and in Beaver Basin, Utah ....................................... 40

 9. Glass chemical composition of the Huckleberry Ridge ash bed from sites in the 

western and central United States, as determined by electron-microprobe analyses ... 42

 10. Instrumental neutron-activation analysis of selected samples of the tuff of Blind 

Spring Valley, from the Cowan Pumice Mine (TTC-), and correlative or chemically 

related tephra layers erupted from the Glass mountain volcanic source area ......... 43
Tephra Layers of Blind Spring Valley and Related Upper 

Pliocene and Pleistocene Tephra Layers, California, 

Nevada, and Utah: Isotopic Ages, Correlation, and 

Magnetostratigraphy

By Andrei M. Sarna-Wojcicki1, Marith C. Reheis2, Malcolm S. Pringle3, Robert J. Fleck1, Doug Burbank4, 

Charles E. Meyer1

, Janet L. Slate2, Elmira Wan5, James R. Budahn2, Bennie Troxel6, and James P. Walker1

Abstract

Numerical ages have been determined for a stratigraphic sequence of silicic tephra layers 

exposed at the Cowan Pumice Mine in Blind Spring Valley, near Benton Hot Springs, east-central California, as well as at Chalk Cliffs, north of Bishop, Calif. The tephra layers at these sites 

were deposited after eruptions from nearby sources, most of them from near Glass Mountain, 

and some from unknown sources. The ages were determined primarily by the laser-fusion 

40Ar/39Ar method, mostly on sanidine feldspar; two were determined by conventional K-Ar 

analysis on obsidian clasts. These tephra layers, all underlying the Bishop ash bed and listed in 

order of concordant age and stratigraphic position, are:

Tephra Unit Method Material Age

Bishop Tuff (air-fall pumice) Ar/Ar sanidine 0.759±0.002 Ma*

Upper tuffs of Glass Mountain Ar/Ar sanidine 0.87±0.02 Ma

Upper tuffs of Glass Mountain Ar/Ar sanidine 1.13±0.19 Ma

Lower tuffs of Glass Mountain K-Ar obsidian 1.86±0.09 Ma (avg of 2 dates)

 Ar/Ar sanidine 1.92±0.02 Ma (avg of 2 dates)

Tuffs of Blind Spring Valley Ar/Ar sanidine 2.135±0.02 to

 sanidine 2.219±0.006 Ma (10 dates)

Tuffs of Benton Hot Springs Ar/Ar plagioclase 2.81±0.02 Ma

*Date published previously

The above tephra layers were also petrographically examined and the volcanic glass shards 

of the layers were chemically analyzed using the electron microprobe and, for some samples, 

instrumental neutron activation analysis and X-ray fluorescence. The same types of chemical and petrographic analyses were conducted on stratigraphic sequences of tephra layers of 

1

U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025

2

U.S. Geological Survey, Denver Federal Survey, Denver, CO 80225.

3

Scottish Universities Research and Reactor Center, Glasgow, Scotland

4

Department of Geological Science, University of California, Santa Barbara, CA 93106

5

945 Madison Street, Albany, CA 94706

6

Department of Geology, University of California, Davis, CA 95616
2 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

suspected upper Pliocene and Pleistocene age in several past and present depositional basins 

within the region outside of Blind Spring Valley. Chemical characterization, combined with 

additional dates and with magnetostratigraphy of thick sections at two of the distal sites, allow 

correlation of the tephra layers at the Cowan Pumice Mine with layers present at the distal sites 

and provide age constraints for other intercalated tephra layers and sediments for which age 

data were previously lacking. The identification at several sections of the widespread Huckleberry Ridge ash bed, derived from the Yellowstone eruptive source area in Wyoming, as well as 

a new 40Ar/39Ar age on this ash bed from a proximal locality, provide additional age constraints 

to several of the distal sections. The dated or temporally bracketed distal units, in order of concordant age and stratigraphic position, are:

Tephra Unit Method Material Age

Tephra layers of Glass 

Mountain (undiff.)

P-mag.*; correlation N/A <1.78 Ma

Tephra layers of Glass 

Mountain (lower)

P-mag.*; correlation N/A >1.78 , <1.96Ma 

Tephra layers of Emigrant 

Pass

P-mag.*; correlation N/A >1.96, <2.06 Ma

Tephra layer of Confidence 

Hills

Correlation, p-mag.* N/A >1.96, <2.06 Ma

Huckleberry Ridge ash bed Ar/Ar; correlation sanidine 2.063±0.007 Ma

Tephra layers of Blind 

Spring Valley

Correlation N/A 2.135 to 2.219 Ma

Tephra layers of the 

Badlands (upper)

Correlation; p-mag.* N/A >2.22, <2.57 Ma

Tephra layers of the 

Badlands (lower)

P-mag.*; correlation N/A >2.57, <2.89 Ma

Tephra layers of Benton Hot 

Springs

Ar/Ar; correlation plagioclase 2.89±0.03 Ma

*Magnetostratigraphic polarity determination

At the Cowan Pumice Mine, only a partial section of the eruptive record is preserved, 

but the best materials for laser-fusion 40Ar/39Ar and other isotopic dating methods were 

obtained. In the more distal Willow Wash and Confidence Hills sections, both persistent 

depositional basins for most of late Pliocene time, more complete sections of upper Pliocene 

tephra layers were preserved. In the region of Glass Mountain, the tephra layers that make up 

each of the mapped and dated pyroclastic units are multiple and complex, but a progressive 

simplification of the stratigraphy away from the source area was observed for more distal 

sites in southern and southwestern California and in Utah. This progressive simplification 

is attributed to both variable explosiveness and magnitude of individual tephra eruptions, as 

well as to variable dispersal of the tephra by winds during an eruptive episode. Lake beds 

present at several localities in the western Great Basin (Fish Lake Valley, Nev.; the Waucoba Road area near Big Pine, Calif.; and Confidence Hills, Death Valley, Calif.) are dated 

between ~2.15 and ~2.04 Ma and indicate that wetter or colder conditions than present 

existed at these sites. Age and correlation data presented here provide a chronostratigraphic 

framework for studies of late Neogene stratigraphy, tectonics, and environmental change in 

the southwestern United States.

Two Reunion magnetic events are bracketed between ~2.18 and ~2.17 Ma for the older 

(Reunion I) and between ~2.16 and ~2.14 Ma for the younger (Reunion II). A third, older 

excursion associated with these two is at 2.22 Ma, but it does not reach reversed magnetization.
Introduction

Tephrochronology consists of three main activities: (1) a 

search for relatively pure tephra layers in stratigraphic sections, 

followed by mapping of their areal distributions and determination of their stratigraphic relationships in the field, (2) the chemical and petrographic characterization of the layers, and (3) the 

determination of their ages by isotopic or other methods. The best 

materials for many types of isotopic analysis (40Ar/39Ar, K-Ar, 

fission-track) are found at or near the eruptive source areas—

coarse air-fall pumice layers containing large mineral grains 

and stratigraphically associated lava flows. However, complete 

stratigraphic sections are rarely preserved near the source areas 

because of rapid volcanic/tectonic deformation and accompanying stuctural complications, erosion, or burial by clastic sediments 

and volcanic flows. Hence, part of the eruptive record is commonly missing, while the remainder may be difficult to interpret 

or concealed. Long-lived sedimentary basins are sites where 

records of the larger widespread eruptions from multiple sources 

are preserved and their stratigraphic relations can be determined. 

Such basins are usually at some distance from volcanic centers. 

Tephra layers preserved in such basins are commonly thinner, 

finer-grained, and impure owing to reworking, and thus not ideal 

for isotopic dating. The grain-size characterstics of such tephra 

and associated sediments, however, are often suitable for paleomagnetic studies, and tephra layers in these basins can be characterized by their petrography and chemical composition.

The optimal strategy for developing regional chronostratigraphic frameworks using tephrochronology, therefore, is to 

use a combination of near-source airfall deposits and exposed 

sections or cores in downwind long-lived sedimentary basins. 

Employing this approach, this study uses tephrochronology 

to correlate and date a suite of upper Pliocene and Pleistocene 

tephra layers in California and Nevada. The tephra that formed 

these layers was erupted mainly from a long-lived volcanic 

source in east-central California, in the vicinity of Glass Mountain, as well as from one or more unknown eruptive centers in 

this region. Our strategy in this study was to (1) obtain isotopic ages on proximal tephra layers using primarily the laserfusion 40Ar/39Ar method, (2) correlate the tephra units from the 

proximal localities to distal locations in long-lived depositional 

basins by chemical and petrographic criteria and, where possible, to determine the magnetostratigraphy at the latter sites, 

and (3) integrate the isotopic-age, correlation, stratigraphic, and 

magnetostratigraphic data components to arrive at a regional 

chronostratigraphic framework for the upper Pliocene and Pleistocene sequences within the region that contains these layers.

Although this strategy now seems straightforward, we 

arrived at it piecemeal, by several different earlier studies conducted by different individuals over a period of about 25 years. 

This report encompasses results of several of these studies, 

mostly unpublished, carried on independently in several different 

depositional basins or within smaller areas of the study region.

We report here on the informally named tephra layers 

of Blind Spring Valley, at the Cowan Pumice Mine in Blind 

Spring Valley, about 17 km east of Glass Mountain, and 

related tephra layers that underlie and overlie the tephra layers of Blind Spring Valley. These tephra layers are correlated 

by means of their chemical composition and petrographic 

characteristics to other sites in the region within a few tens of 

kilometers from Glass Mountain and to more distant sites in 

California, Nevada, and Utah.

The region around Glass Mountain and Long Valley 

(fig. 1), in east-central California, has been a center of longlived silicic volcanism in late Cenozoic time. Repeated volcanic eruptions from this center, beginning perhaps as early as 

Willow Wash

Balcom Canyon

Corona

 Manix Basin

(Mojave Desert)

Confidence Hills

 (Death Valley)

 Waucoba Rd.

(Owens Valley)

Blind Spring

 Valley Glass Mt.

Nevada Utah 

California

Arizona Pacific

 Ocean

Beaver Basin

Fish Lake Valley Long Valley

 Caldera

KM

Figure 1. Localities at which tuffs of 

Blind Spring Valley and stratigraphically closely related tephra layers were 

sampled (small circles); location of Glass 

Mountain, near source of the tuffs of 

Blind Spring Valley (star), and location 

of Long Valley Caldera (oval outlined 

by dashes). Red shaded pattern represents known distribution of tuffs of Blind 

Spring Valley.

Introduction 3
4 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

~2.8 million years ago, have produced thick deposits of rhyolitic lava flows and tephra in the vicinity of Glass Mountain 

(Bailey and others, 1976; Metz and Mahood, 1985, 1991), and 

a widespread apron of roughly coeval tephra and volcaniclastic 

dome-collapse deposits to the north and northeast of Glass 

Mountain, referred to as the tuff of Taylor Canyon (Krauskopf 

and Bateman, 1977) (fig. 2). Volcanism at Glass Mountain culminated with the voluminous, climactic eruption of the Bishop 

Tuff at about 0.76 Ma, forming the Long Valley Caldera and 

truncating the southwestern flank of Glass Mountain along 

ring fractures, dropping it down to the southwest, where it now 

lies covered by younger volcanic rocks and sediments (Gilbert, 1938; Bailey and others, 1976; Metz and Mahood, 1985; 

Hildreth, 1979, Wilson and Hildreth, 1997). Thus, a section of 

unknown thickness of Glass Mountain rhyolites and equivalent 

tephra and volcaniclastic debris correlative with the tuff of 

Taylor Canyon of Krauskof and Bateman (1977) may be buried beneath the later Bishop Tuff and younger volcanic rocks 

that filled the subsided Long Valley Caldera. The rhyolites of 

Glass Mountain remain exposed on the east and northeast wall 

of the Long Valley Caldera and to the northeast. Silicic volcanism in the Long Valley Caldera resumed shortly after eruption of the Bishop Tuff and has continued intermittently until 

at least ~0.1 Ma (Bailey and others, 1976). Volcanic activity 

has continued in the western part of the Long Valley Caldera, 

at the Inyo Craters, and to the north at the Mono Craters, into 

Holocene time (Bailey and others, 1976; Lajoie, 1968).

Tephra Layers of Blind Spring Valley at the 

Cowan Pumice Mine

The tephra layers of Blind Spring Valley are exposed at the 

Cowan Pumice Mine in Blind Spring Valley, about 17 km east 

of glass Mountain (figs. 2, 3). These layers consist of coarse 

air-fall tephra and tephra layers partly reworked by sheetwash. 

The tephra layers of Blind Spring Valley are a complex unit 

consisting of several thick, coarse air-fall tephra layers. These 

pyroclastic layers have been previously correlated by Izett and 

others (1988) with the tuff of Taylor Canyon of Krauskopf and 

Bateman (1977). The tuff of Taylor Canyon at the type locality 

designated by Krauskopf and Bateman (1977), however, conFigure 2. Generalized geologic map of the area of Long Valley Caldera, Glass Mountain, the Cowan Pumice Mine, and south 

end of Volcanic Tableland (Chalk Cliffs locality). Compiled and revised from Metz and Mahood (1985, 1991), and Wilson and 

Hildreth (1997). Major roads shown in red.
sists of coarse, obsidian-bearing pumice air-fall and reworked 

tephra and other pyroclastic deposits correlative with, and 

temporally equivalent to, the rhyolites of Glass Mountain (Metz 

and Mahood, 1985; Metz and Mahood, 1991). Tephra layers 

correlated by us with the rhyolites of Glass Mountain on the 

basis of chemical composition and isotopic ages at the Cowan 

Pumice Mine overlie tephra layers of Blind Spring Valley, have 

younger isotopic ages and different chemical composition than 

the tephra layers of Blind Spring Valley. The tephra layers of 

Blind Spring Valley may have been erupted from the same 

area as the rhyolites of Glass Mountain. However, no proximal 

flows or tephra of the same age and composition as the tuffs of 

Blind Spring Valley are known to underlie the rhyolites of Glass 

Mountain at the source area near Glass Mountain.

The tuffs of Blind Spring Valley were well exposed in 

several quarry pits (filled in sometime shortly before the fall 

of 1995 and thus no longer accessible for study) at the Cowan 

Pumice Mine. They consist of thick beds of airfall pumice 

layers, intercalated with minor reworked tephra layers and 

tuffaceous alluvium, tuffaceous mudflows and debris flows, 

and reworked tuffaceous eolian deposits. Much of the area 

intervening between the pumice pits is poorly exposed and 

complicated by normal faulting, so correlation of tephra units 

among the pits is not straightforward (figs. 3, 4).

Pumice lapilli in tephra layers at the Cowan Pumice Mine 

contain large crystals of sanidine feldspar. Analysis of sanidines 

from pumice, rather than from the fine ashy matrix of distal 

tephra layers, allowed us to eliminate detrital feldspar grains 

from analysis. Laser-fusion 40Ar/39Ar analysis further allowed 

us to obtain ages on individual grains or small groups of several 

grains of a mineral (York and others, 1981; LoBello and others, 

1987; Dalrymple, 1989), to determine the degree of internal 

scatter for ages of each tephra layer, and thus to evaluate the age 

analysis for possible accidental contamination—presence in the 

sample of older xenoxrystic mineral grains torn from the walls 

of the magma chamber or volcanic vent during eruption. Lastly, 

overall coherence of the dates was checked against their relative 

stratigraphic positions, to test their consistency.

Because tephra at the Cowan Pumice Mine consists of 

coarse pumice fall deposits, finer-grained correlative layers are 

likely to be present at sites farther away. Ages determined on 

the proximal tephra layers can be correlated to the distal layers 

by means of their chemical and petrographic characteristics. We 

have performed petrographic analyses of the tephra layers, and 

Figure 3. Locations of pumice pits 

(black dots) at Cowan Pumice Mine 

and of cross-sections A-A’ and B-B’ 

(straight black lines; see fig. 4 ). 

Irregular black lines connecting Pits 

1-5 are surveyed lines. Base from 

U.S. Geological Survey Benton Hot 

Springs 7.5’ quadrangle (provisional 

edition, 1986).

Introduction 5
6 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

chemical analyses on volcanic glasses of the tephra layers, from 

the Cowan Pumice Mine, and results of these analyses were 

compared with those of tephra samples collected from other 

areas in the western conterminous United States. Fine-grained 

tephra layers of essentially the same chemical composition and 

age as those at the Cowan Pumice Mine are identified at multiple 

sites over a large area in the western United States. These correlations indicate that the Glass Mountain volcanic center was an 

important source of recurrent, explosive silicic volcanism in late 

Pliocene time and that tephra erupted from this general vicinity provides chronostratigraphic marker beds of regional extent, 

from the Pacific borderland to central Utah. The exact location of 

some of the sources active before about 2 Ma is still unknown.

Tephra Layers from Other Source Areas in the 

Western United States

The Huckleberry Ridge ash bed, erupted from the Yellowstone area of northwestern Wyoming and east-central 

Idaho (Christiansen and Blank, 1972; Izett, 1981; Izett and 

Wilcox, 1982; Sarna-Wojcicki and others, 1984), underlies the 

lower tuffs of Glass Mountain and the tuffs of Emigrant Pass 

and overlies the tuff of Blind Spring Valley at several localities in eastern California, western Nevada, and western Utah. 

This tephra layer provides an independent chronostratigraphic 

marker to test correlations based on chemical and petrographic 

characteristics.

Acknowledgements

We thank John D. Obradovich for providing us with a 

near-source sample of the Huckleberry Ridge Tuff (68-O-40). 

We thank Roy Bailey, Wes Hildreth, and Peter Stauffer of the 

U.S. Geological Survey, Menlo Park, California, for improving 

the content and clarity of the manuscript. We also thank Judy 

E. Fierstein for performing calculations to estimate the volume 

of the tuffs of Blind Spring Valley.

TTC-B-14,

 basalt

PIT 3 (west pit)

TTC-5,

2.14 ±0.01

PIT 2 (fault pit)

TTC-18,

2.16 ±0.02

PIT 6 (north pit)

projected south,

TTC-19 to 22,

2.17 ±0.02 to

2.19 ±0.04

TC-23, 24

1.86 ±0.09,

1.92 ±0.02 PIT 1 (east pit)

projected north,

TTC-7,

0.757 ±0.004

PIT 1 (east pit)

TTC-7,

0.757 ±0.004

A A'

Tuffs of Blind Spring Valley,

overlain by the lower tuffs of

Glass Mountain

v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v

? ?

v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v

B

B'

~70 m

Tuffs of Blind Spring Valley

v v v v v v v v v 

 v v v v v

 v v v v v

PIT 4

TTC-15,

2.16 ±0.02 

TTC-4

2.14 ±0.01

PIT 5

TTC-16, 17

2.16 ±0.01 to

2.18 ±0.01

Tuffs of Blind

Spring Valley

Bishop Tuff

fall pumice

Bishop Tuff

fall pumice basalt

basalt

basalt

100 meters

v v

Figure 4. Cross sections A-A’ and B-B’ in area of Cowan Pumice Mine, Blind Spring Valley, with locations and 40Ar/39Ar ages (in Ma) of 

samples in Pits 1-6. Presence and position of fault in cross-section B-B’ are inferred. See figure 3 for locations of sections.
Previous Work

Gilbert and others (1968) mapped and described the geology southeast of Mono Lake, Calif., and determined many KAr ages on late Tertiary volcanic units in this region. Among 

these were ages on basalts (4.2 to 2.6 Ma) that underlie the 

late Pliocene and Pleistocene silicic volcanic rocks erupted 

from the Long Valley-Glass Mountain volcanic center south 

and east of Mono Basin and an age for a rhyolite near the top 

of the Glass Mountain silicic flows (0.9±0.1 Ma).

Bailey and others (1976) studied the volcanism, structure, 

and geochronology of the Long Valley Caldera in east-central 

California. Although the focus of their study was the younger 

volcanic rocks of the Long Valley Caldera that postdated 

the eruption of the Bishop Tuff, they also obtained an age of 

1.92±0.05 Ma on a biotite rhyolite of Glass Mountain, in the 

lower part of the silicic volcanic pile, on the southwest side 

of Glass Mountain. This age, together with that of Gilbert and 

others (1968), defined at that time the range of Glass Mountain volcanism to between ~1.92 and ~0.9 Ma.

Krauskopf and Bateman (1977) named the tuff of Taylor 

Canyon after a canyon about 20 km northwest of the Cowan 

pumice mine (the site studied in this report). They mapped 

the tuff of Taylor Canyon as a dissected sheet of tephra layers 

spread over a broad area to the north and east of Glass Mountain.

Sarna-Wojcicki and others (1980, 1984), chemically 

analyzed and correlated a suite of tephra layers in eastern 

and southern California and proposed that these layers were 

erupted from the Long Valley-Glass Mountain volcanic 

province of east-central California. These authors showed that 

there was a systematic, age-related trend in the chemical composition of the volcanic glasses of this suite: an increase in the 

siderophile elements, and a decrease in the lithophile elements, 

from older to younger layers. They also correlated a tephra 

layer of the Long Valley-Glass Mountain type in the Waucoba 

Lake beds of Owens Valley with a layer in the Manix Basin 

of the Mojave Desert, and they correlated a closely underlying ash bed at the latter site with the Huckleberry Ridge ash 

bed, which had been previously dated elsewhere at about 2 Ma 

(Izett and Wilcox, 1982)

Izett (1981), and Izett and others (1988) analyzed two ash 

layers from the Cowan pumice mine, which were designated 

Taylor Canyon-P (lower) and -C (upper) beds. According to 

Izett and others (1988), a third bed, designated Taylor Canyon 

U, was the uppermost of three tephra layers from Yellowjacket 

Canyon, located about 6.5 km southeast of the Cowan Mine, 

and was considered to be the uppermost of the sequence of 

their Taylor Canyon beds. Their Taylor Canyon-P corresponds 

to TTC-6 of the tuff of Blind Spring Valley of this report, and 

Taylor Canyon-C corresponds to TTC-4 of the tuff of Blind 

Spring Valley, both in Pit 5 (p. 26 of Izett and others, 1988). 

Their Taylor Canyon-P and -C may also correspond to our 

TTC-21 and 22 of the tuff of Blind Spring Valley, respectively, 

in Pit 6 (p. 27 of Izett and others, 1988). Izett and others 

(1982, 1988) obtained an age of 2.10±0.02 Ma on the Taylor Canyon-C bed from the Cowan Pumice Mine, using the 

conventional K-Ar method on sanidine. These authors have 

also correlated the Taylor Canyon-C and -P ash beds to several 

other localities in the vicinity of Bishop, Calif. Moreover, they 

correlated Taylor Canyon-C to one or more ash beds in the 

Waucoba Lake beds of central Owens Valley (Izett and others, 

1988; their sample 79G25 may be equivalent to our WAC-6 

and WAC-7, and their sample 79G29 is probably equivalent to 

our W2A, in the Waucoba Lake Beds) and to a tephra layer in 

Beaver Basin west-central Utah, where the layer underlies the 

Huckleberry Ridge ash bed.

As mentioned above, Izett and others (1988) used the 

term “Taylor Canyon beds” for the thick, coarse air-fall tephra 

beds at the Cowan Pumice Mine, assuming that these were 

correlative to the tuff of Taylor Canyon of Krauskopf and 

Bateman (1977). In this report, we show that the tuff of Taylor 

Canyon of Krauskopf and Bateman at the type locality in Taylor Canyon is not correlative to the coarse air-fall tephra beds 

at Cowan Pumice Mine, because they differ in petrographic 

characteristics, chemical composition, and isotopic age. 

Consequently, the name “Taylor Canyon” is not appropriate 

for these coarse air-fall deposits at the Cowan Pumice Mine, 

and we renamed them informally as “the tuffs of Blind Spring 

Valley,” after the valley in which they are found.

Metz and Mahood (1985) conducted a geochronologic 

and petrologic study of Glass Mountain, the source area of 

Glass Mountain tephra and of the tuff of Taylor Canyon. This 

source is located about 17 km west of the Cowan Pumice 

Mine. Metz and Mahood obtained 38 K-Ar ages on lava 

flows, mostly on glass and felsite, and showed that the Glass 

Mountain volcanic center had been active from about 2.13 to 

about 0.79 Ma. Two episodes of volcanism were inferred for 

the Glass Mountain volcanic center by these authors: sporadic 

eruptions during an early episode of activity from about 2.1 

to 1.2 Ma and a later, more voluminous and sustained episode 

from 1.1 to 0.8 Ma. These authors confirmed observations by 

Sarna-Wojcicki and others (1984) that the eruptive products 

of the Long Valley-Glass Mountain volcanic area became less 

evolved with time, and that the subsequently erupted Bishop 

Tuff had the least evolved (though still highly silicic) composition of all units in this suite

Troxel (1986) described a thick section of deformed, upper 

Pliocene to early Pleistocene (?) lacustrine and fluvial sediments 

in the Confidence Hills of Death Valley, Calif., and Troxel and 

others (1986) presented chemical data for three of the thickest tephra layers, the lowermost of about 25 tephra layers that 

are present within this section. On the basis of the chemical 

composition of the volcanic glass shards, the basal of these three 

tephra layers was correlated with a tephra layer in the Waucoba 

lake beds in Owens Valley and with the tephra layers of the tuff 

of Blind Spring Valley. The middle layer was correlated with the 

Huckleberry Ridge ash bed (~2.0 Ma; Izett and Wilcox, 1982); 

the upper layer was chemically very similar to the lowermost 

layer, and Troxel and others speculated that the section may 

have been repeated by isoclinal folding. Data presented here 

identifies this third, uppermost tephra layer as one of the tuffs 

Previous Work 7
8 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

of Emigrant Pass, and discounts the possibility of isoclinal 

folding. In this study, we have examined the remaining tephra 

layers in the Confidence Hills section, and present analyses of 

those layers that are not altered or devitrified.

The section exposed in the Confidence Hills of Death 

Valley was subsequently studied by Beratan and Murray 

(1992), who further described its stratigraphy and lithology, 

and by Pluhar and others (1992), who determined its magnetostratigraphy. Pinning their chronology to the identified Huckleberry Ridge ash bed of Troxel and others (1986), Pluhar and 

others identified the Reunion events and the Olduvai Normal 

Subchron within the Matuyama Reversed Chron. Using the 

new, revised ages of these magnetostratigraphic datum levels 

and interpolation, these authors then obtained a revised age for 

the Huckleberry Ridge ash bed of 2.09 Ma.

Reheis and others (1991) presented data on the structure, 

stratigraphy, magnetostratigraphy, chemical composition, and 

tephrostratigraphy of about 55 tephra layers within a thick, 

composite late Cenozoic sedimentary and volcanic section at 

Willow Wash, south of Fish Lake Valley, about 90 km southeast of Glass Mountain, in eastern California. They tentatively 

identified several widespread tephra layers within this section 

that ranged in age from about 5.4 to 0.74 Ma, including tephra 

layers of the tuff of Blind Spring Valley. The lower tephra 

layers of Glass Mountain were also identified above the tuffs 

of Blind Spring Valley, near the top of the section. Tephra layers in the uppermost part of the section at Willow Wash were 

tentatively identified as reworked Bishop ash bed, but data 

presented below indicate that these layers are older than the 

Bishop ash bed, and are probably not younger than about 1.79 

Ma. Additional tephra layers were identified below the tuffs of 

Blind Spring Valley, including the tuffs of Benton Hot Springs 

(equivalent to the Rimrock ash beds), the Nomlaki and Putah 

Tuffs, and the tuffs of Mesquite Springs.

Reheis and others (1993) documented the petrographic 

and chemical characteristics, correlations, and ages of multiple 

tephra layers in late Pliocene sediments of northeastern Fish 

Lake Valley, Nev., about 70 km east of Glass Mountain, near 

Emigrant Pass. They identified the tuffs of Blind Spring Valley 

in lacustrine sediments, overlain by transitional lacustrine-alluvial sediments containing the Huckleberry Ridge ash bed (from 

the Yellowstone area of northwestern Wyoming and east-central Idaho), in turn overlain by the tuffs of Emigrant Pass, and, 

in turn, coarse fan gravels containing tephra layers of Glass 

Mountain. The latter, in turn, are overlain by the Bishop ash 

bed, present in beach sands of a younger, mid-Pleistocene lake.

Metz and Mahood (1991) described the development of 

the Long Valley magma chamber as recorded in the rhyolitic 

lavas of the Glass Mountain volcanic center. They present 

multiple major-, minor-, and trace-element analyses of lavas 

from this center. They have, as in their 1985 report, divided the 

rhyolites into two groups, an earlier eruptive episode of highly 

evolved but compositionally variable older lavas (~2.1 to 1.2 

Ma) and a later episode of slightly less evolved, but more 

homogenous lavas (~1.2 to 0.79 Ma). From comparison of 

the analyses in Metz and Mahood (1991) and those presented 

in this report, the rhyolites of Glass Mountain of Metz and 

Mahood (1991) correspond to the tuffs of Glass Mountain presented here almost in their entirety, with the exeption that the 

oldest dated rhyolite of Glass Mountain of Metz and Mahood 

(1991), OG-200, 2.13 Ma, may correlate with our tuffs of 

Emigrant Pass, which overlie the youngest tuffs of Blind 

Spring Valley (<2.135 Ma, >2.09 Ma) and the undated OFCT75-6 of Metz and Mahood (1991; a sample of R. Bailey, 

cited in the latter report), may correlate with the tuffs of Blind 

Spring Valley reported here.

Analytical Methods

Field Methods

Stratigraphic sections were measured and described in the 

pumice pits, and samples of tephra layers were collected for 

age and chemical analysis (figs. 2-4). Angular obsidian clasts 

were hand-picked from some tephra layers at the outcrops. 

Topographic transects were conducted between the pits (fig. 3) 

and stratigraphic cross sections drawn to correlate tephra units 

among the pits (fig. 4). Similar work was conducted at other 

sites within the region and at more distal sites, where putative 

correlative tephra layers were present (fig. 1, 2).

Laboratory Preparation of Samples for Analysis

In the laboratory, the samples were disaggregated or 

crushed and wet-sieved in plastic sieves using 100, 200, and 

325 nylon mesh screens (opening diameters of 150, 80, and 

45 , respectively). Plastic and nylon were used to avoid 

contamination with metals. Sized fractions were then cleaned 

chemically, first with 10% HCl, then very briefly with 8% 

HF, and in an ultrasonic bath in deionized water. They were 

then separated using magnetic and heavy-liquid methods into 

component parts—volcanic glass shards, magnetic minerals, 

nonmagnetic minerals, and lithic grains. Sanidine grains and, in 

a few instances, obsidian clasts were separated from the tephra 

samples for laser-fusion 40Ar/39Ar, and for K-Ar dating, respectively. Individual euhedral or glass-coated feldspar grains were 

hand-picked from the concentrates and further etched for several 

minutes with 10% HF for laser-fusion 40Ar/39Ar analysis.

Laser-fusion 40Ar/39Ar and Conventional K-Ar 

Analyses

Sanidine and plagioclase crystals were separated from 

pumice clasts, as described above, and hand-picked if necessary. After packaging and irradiation as described in SarnaWojcicki and others (2000), and after heating and cleanup, the 

samples were analyzed using the W argon laser system at the 

U.S. Geological Survey, Menlo Park, Calif., using methods 

described by Dalrymple (1989).
All ages reported herein are relative to 27.92 Ma for the 

TCR (Taylor Creek) sanidine 85G003 (Dalrymple and Duffield, 1988). Ages in the range 0 to 2.5 Ma originally calculated 

using this value of 27.92 Ma for the TCR (and the related value 

of 514.0 Ma for the MMhb-1) can be recalculated to other values of TCR or MMhb-1 used in the current literature. Thus, for 

example, the 0.759-Ma age of the Bishop Tuff (Sarna-Wojcicki 

and others, 2000) becomes 0.774 Ma using values recommended by Renne and others (1998). Errors are reported as 

the standard deviation of analytical precision, unless otherwise 

stated. Obsidian clasts separated from tephra layers were analyzed using methods described for conventional K-Ar analysis 

(for example, Dalrymple and Lanphere, 1969).

Analytical data for all laser-fusion 40Ar/39Ar runs are presented in appendix 1. Analytical data for the two conventional 

K-Ar runs are presented in appendix 2.

Chemical Analysis of Volcanic Glass Shards

Volcanic glasses separated from all tephra samples were 

analyzed by electron-microprobe (EM), and glasses from 

selected samples was also analyzed by energy-dispersive 

X-ray fluorescence (XRF) and instrumental neutron-activation analyses (INAA). Ages, stratigraphic and petrographic 

data, and chemical compositions of the tephra samples were 

compared with those of other tephra layers from the western 

United States, and correlations were noted if the requisite 

criteria for correlation were considered to be fully met (see 

section on methods of data evaluation, below).

Electron-microprobe (EM) Analysis

Methods used in EM analysis of glass shards are described 

in Sarna-Wojcicki and others (1984). The current database of the 

U.S. Geological Survey’s Tephrochronology Laboratory (Menlo 

Park) contains ~5000 EM analyses of tephra samples from the 

conterminous western United States that can be used for comparison and correlation. These EM analyses were determined over a 

period of ~25 years on three different instruments: ARL, SEQM, 

and JEOL8900 microprobes. With the JEOL8900 microprobe, 

we use a 10-l beam diameter, 10.05-nA, 15-KV beam current. 

Acquisition time is 10 seconds for Na, and 20 seconds for the 

remaining elements: Si, Al, Fe, Mg, Mn, Ca, Ti, and K. We use 

GSC (Corning Glass Standard) and An40 (anorthite) as standards, and RLS-132, a homogenous obsidian from La Puebla, 

Mexico, as an internal standard. The ZAF data-reduction program is used to obtain oxide concentrations. Replicate analyses 

of internal and external standards over the past 25 years indicate 

that analyses are mutually compatible, despite differences in 

instrumentation (table 1). Precision and accuracy of the analyses 

for each oxide are indicated by replicate analysis of RLS-132 

and by comparative values from wet-chemical analysis. Data 

for P, Cl, and Ba are available for some samples, but are generally not helpful in chemical characterization of the silicic tephra 

layers in this study using EM analysis. Similarly, MgO, MnO, 

and TiO2

 are not consistently useful in characterization of silicic 

tephra by this method owing to low concentrations in some of 

the volcanic glasses and consequently relatively large scatter 

in analyzed values (Sarna-Wojcicki and Davis, 1991). Despite 

these drawbacks, EM analysis makes it possible to determine the 

chemical composition of individual glass shards for the above 

major and minor oxides, and thus to determine the homogeneity 

of tephra samples and detect presence of multiple compositional 

modes (Sarna-Wojcicki and Davis, 1991).

Internal, polished surfaces of about 15 to 20 individual 

glass shards were analyzed for each sample. When EM analysis 

was not diagnostic for characterizing some tephra layers, or when 

other criteria for correlation such as numerical age, stratigraphic 

position, or petrographic characteristics were lacking or inadequate to achieve a reliable identification of a tephra layer, followup chemical analyses were conducted using energy-dispersive 

X-ray fluorescence or instrumental neutron-activation analysis.

Energy-dispersive X-ray Fluorescence (EDXRF) 

Analysis

About 0.5 g of pure volcanic glass was powdered and 

analyzed, together with previously analyzed standards, using 

a Kevex 0700/8000 EDXRF spectrometer. Methods used are 

described in Johnson and King (1987). Analyses included 

Fe2O3, MnO, CaO, TiO2, K2O, and Rb, Sr, Y, Zr, and Nb. In 

this type of analysis, cross-checks with results obtained by EM 

analysis are made for Fe2O3, CaO, and K2O, generally more precise concentrations are obtained for MnO and TiO2

 than in EM 

analysis, and the other minor and trace elements provide additional critical variables to discriminate among tephra layers. The 

current database of the U.S. Geological Survey’s Tephrochronology Project (Menlo Park) contains ~800 EDXRF analyses of 

tephra samples from the conterminous western United States; an 

additional ~700 analyses by wavelength-dispersive (WD) XRF 

analysis from the database of the University of Utah (collection 

of F. H. Brown and M. E. Perkins, Department of Geology and 

Geophysics) are available for comparison.

Instrumental Neutron Activation (INA) Analysis

Between 0.2 and 0.5 g of pure volcanic glass was powdered and analyzed using techniques described in Bowman and 

others (1973), Sarna-Wojcicki and others (1979), and Baedecker 

and McKown (1987). Currently, there are ~500 INA analyses of 

tephra samples in the database of the Tephrochronology Laboratory (Menlo Park) that can be used for comparison. These 

samples were analyzed over a period of about 20 years in three 

different laboratories: Lawrence Berkeley Laboratory of the 

University of California and the Radiochemistry Laboratories of 

the U.S. Geological Survey in Reston, Va., and in Denver, Colo. 

Of the three methods (EM, EDXRF, and INA), INA analysis is 

by far the most precise for a number of minor and trace elements. Analyses were made for 24 or more elements; between 

15 and 20 of these generally proved useful in characterizing 

Analytical Methods 9
Table 1. Chemical composition of volcanic glass shards determined by electron-microprobe analysis, and ages of tephra layers sampled from quarMine, near Benton Hot Springs, Calif. [Tephra layers are in stratigraphic order (from uppermost to lowermost), or in inferred stratigraphic position, based on projections into cross-sections. Oxide concentrations arecalculated to 100 percent. Original oxide totals before recalculation are given to indicate approximate degree of hydration of volcanic glass. About 15 individual glass shardComparative composition of a homogenous natural glass, RLS 132, used as an internal standard, is also given, providing information on precision and accuracy of the analyses

method, except for TTC-7*, which is by fission-track on zircons, and TTC-24*, which is by conventional K-Ar on obsidian clasts. Electron-microprobe chemical analyses by C

40Ar/39Ar analyses by M. A. Pringle; K-Ar analyses by J. A. Nakata; fission-track analyses by C. E. Meyer and Marta Woodward.] Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O 

Bishop airfall pumice bed; Pit 1 TTC-7 77.3 12.7 0.82 0.09 0.04 0.54 0.09 3.71 4.76 

TTC-7* 

Lower tuffs of Glass Mountain; Pit 6 TTC-24* 77.7 12.6 0.71 0.04 0.02 0.39 0.05 3.79 4.69 

TTC-23 77.6 12.6 0.71 0.03 0.03 0.32 0.03 3.58 5.10 

Tuffs of Blind Spring Valley Thick, coarse bed; Pit 2 TTC-18 77.9 12.5 0.59 0.05 0.10 0.35 0.07 3.45 5.09 

Thick coarse bed; Pit 3; TTC-5 77.3 12.8 0.59 0.05 0.09 0.35 0.10 3.80 4.93 

Thick, coarse bed; Pit 5 (south side) TTC-13 78.0 12.6 0.58 0.05 0.10 0.36 0.05 3.79 4.51 

TTC-4 77.5 12.6 0.59 0.03 0.09 0.37 0.06 4.13 4.61 

Thin tephra beds; Pit 5 (south side) TTC-12 77.7 12.7 0.62 0.06 0.09 0.38 0.07 3.81 4.58 

TTC-11 77.8 12.6 0.60 0.05 0.08 0.37 0.06 3.86 4.60 TTC-10 77.8 12.5 0.61 0.06 0.08 0.37 0.08 3.69 4.83 TTC-9A 77.5 12.8 0.59 0.05 0.10 0.34 0.07 3.87 4.65 TTC-9 77.6 12.7 0.58 0.04 0.10 0.34 0.07 3.69 4.88 

Thin airfall (?) beds; Pit 4 TTC-15 77.9 12.6 0.57 0.05 0.09 0.35 0.05 3.88 4.57 

Coarse, thick bed; Pit 5 (south side) TTC-8 77.5 12.8 0.58 0.05 0.11 0.34 0.05 3.69 4.88 

TTC-6 77.2 12.9 0.58 0.05 0.09 0.33 0.09 4.08 4.64 
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O 

Tuffs of Blind Spring Valley (continued)

Thick, coarse bed; Pit 5 (north side)

TTC-17 77.8 12.7 0.55 0.05 0.10 0.33 0.07 3.74 4.69 TTC-16 77.8 12.6 0.59 0.04 0.11 0.34 0.06 3.73 4.76 

Mostly thick, coarse airfall beds, interbedded with reworked tephra, debris flows or mud flows; Pit 6

TTC-22 77.8 12.6 0.59 0.04 0.05 0.34 0.06 3.73 4.78 TTC-21 77.7 12.6 0.60 0.05 0.07 0.32 0.06 3.33 5.32 TTC-20 77.7 12.6 0.58 0.05 0.06 0.31 0.05 3.38 5.29 TTC-19 (1) 77.4 12.6 0.59 0.05 0.07 0.32 0.06 3.85 5.04 TTC-19 (2) 77.5 12.5 0.59 0.05 0.07 0.33 0.06 3.91 4.89 

Lowermost bed of tuffs of Blind Spring Valley, reworked pumice tephra in debris flows or mudflows; Pit 6

TTC-25 77.9 12.5 0.56 0.03 0.06 0.31 0.06 3.58 4.99 

Tuffs of Benton Hot Spring

Lowermost airfall tephra, in arkosic mudflow deposits, below tuffs of Blind Spring Valley, above basalt; Pit 6

TTC-26A 76.1 13.4 0.77 0.09 0.05 0.64 0.07 3.46 5.41 TTC-26B 76.0 13.4 0.77 0.09 0.06 0.63 0.07 3.68 5.30 

Natural glass standard used as monitor (electron-microprobe analysis)

RLS 132 75.4 11.3 2.121 0.06 0.16 0.11 0.19 4.9 4.4 

(n=18)± 1 σ: 0.1 0.2 0.04 0.01 0.01 0.01 0.01 0.1 0.1 % σ: 0.2 1.4 1.9 17 6.3 9 5.3 2.7 1.4

Natural glass standard used as monitor (wet-chemical analysis)

RLS 132 75.7 11.4 2.121 0.05 0.15 0.12 0.21 5.3 4.5 

1Iron reported as FeO for the standard.

Table 1. Chemical composition of volcanic glass shards determined by electron-microprobe analysis, and ages of tephra layers sampled from quarMine, near Benton Hot Springs, Calif.—Continued[Tephra layers are in stratigraphic order (from uppermost to lowermost), or in inferred stratigraphic position, based on projections into cross-sections. Oxide concentrations arecalculated to 100 percent. Original oxide totals before recalculation are given to indicate approximate degree of hydration of volcanic glass. About 15 individual glass shardsample. Comparative composition of a homogenous natural glass, RLS 132, used as an internal standard, is also given, providing information on precision and accuracy of the 40Ar/39Ar method, except for TTC-7*, which is by fission-track on zircons, and TTC-24*, which is by conventional K-Ar on obsidian clasts. Electron-microprobe chemical afusion 40Ar/39Ar analyses by M. A. Pringle; K-Ar analyses by J. A. Nakata; fission-track analyses by C. E. Meyer and Marta Woodward.]
12 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

silicic tephra samples (Sarna-Wojcicki and Davis, 1991): Sc, Mn, 

Fe, Zn, Rb, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Dy, Yt, Lu, Hf, Ta, 

Th, and U. Small but systematic differences in analytical results 

were observed among laboratories and between successive sets 

of analyses, as determined on replicate analyses of sample splits 

submitted with each set. Corrections for these differences were 

made using regression analyses of replicate sample sets.

Methods of Data Evaluation

Analytical data on the chemical composition of volcanic 

glasses of tephra layers obtained in this study were first evaluated using the similarity coefficient of Borchardt and others 

(1972), and Borchardt (1974). The similarity coefficient is used 

as a guide to select a pool of candidate samples that are further evaluated in terms of the chemical and geological criteria 

for the closeness of a match. We used a computer program 

that compares any single sample with all previously analyzed 

samples for those elements that are considered the most reliable in chemical identification of tephra layers and that allow 

us to distinguish most clearly between tephra layers of similar 

composition (Sarna-Wojcicki and others, 1984; Sarna-Wojcicki 

and Davis, 1991). The program output lists the tephra samples 

that match most closely to the sample that is being evaluated for 

the specified set of elements, and these samples are ranked in 

order of the value of the similarity coefficient. The highest-ranking samples represent a pool of candidates for further evaluation 

for correlation. Selected samples and sample groups were also 

compared using cluster analysis (R-mode and Q-mode), with 

dendrograms (Parks, 1970, and XLSTAT software program).

The best matches from the initial comparison were run 

again using other element combinations, to evaluate the effects 

of possible post-depositional cation exchange and alteration. For 

example, one run will include the alkalies, while a second will 

not. Minor elements, such as Mg, Mn, and Ti, may be included 

in subsequent runs in various combinations if present in sufficient concentrations, to narrow the field of potentially correlative samples. The closest candidates were also run through 

the program to see if the rankings persisted in reverse comparisons (“knowns” against “unknowns”). The remaining pool of 

candidate samples was evaluated with regard to other criteria: 

independent age data, stratigraphic data, stratigraphic sequence 

relative to other tephra layers, and petrographic characteristics. 

For a more complete and detailed discussion of field criteria, 

petrographic characteristics, mineralogy, chemical analysis 

of glass, and data evaluation methods used in this report, see 

Sarna-Wojcicki and Davis (1991) and Sarna-Wojcicki (2000).

Stratigraphic Section at Cowan 

Pumice Mine—Results of Age Analysis

Tephra layers of the tuff of Blind Spring Valley are 

exposed in several pumice pits that are scattered over an area 

of about 1.5 km2 in the lower part of the western slope of the 

valley. The pits were excavated for commercial exploitation 

of the pumice. Exposures in 6 of these pits were examined in 

detail, measured, and sampled. A topographic transect was 

measured between the several quarry pits (fig. 3) and the 

tephra layers projected onto two cross-sections (fig. 4). A 

complication in deciphering the stratigraphic sequence in the 

pits is introduced by one or more normal faults that offset the 

sequence. Further complications are the low and variable dips 

of the units (~2o to ~12o), which are difficult to measure, the 

possible presence of undetected faults in intervening areas 

where the layers are not exposed, and possible inset relationships of younger into older units. Using a combination of the 

observed stratigraphic relationships in the several pits, the 

topographic survey, projections onto cross-sections, the glass 

chemistry of the tephra layers, and their isotopic ages, we 

derived a composite, though partly inferred, sequence for the 

tephra layers exposed in the quarry pits.

The thickest, most continuous section in this area is 

observed at pit 6 (figs. 5-8), and this section is used as the 

main reference for tying together the stratigraphy in the other 

pits (1-5)(figs. 3, 4). At and near pit 6, all three of the dated 

units, the tuffs of Benton Hot Springs, the tuffs of Blind 

Spring Valley, and the lower tuffs of Glass Mountain, as well 

as the basalt that underlies the entire section, are exposed.

Stratigraphic Section in and Below Pit 6

Basal Basalt Flows

The section exposed at Pit 6 of the Cowan Pumice Mine 

(figs. 3-8), about 25 m thick, is underlain ~250 m to the west 

by basalt flows that are undated at this locality but are presumed to be between 4.5 and 2.6 Ma on the basis of dates by 

Gilbert and others (1968) and Krauskop and Bateman (1977) 

on similar basalts. The basalt at this locality must be older 

than ~2.8 Ma, the age of the oldest tephra bed in the section 

above the basalt (see below). An age of 3.3 Ma was obtained 

on replicate analyses of whole-rock olivine basalts sampled in 

the same range of hills (Benton Range), several kilometers to 

the north of the study area, by Dalrymple and Hirooka (1965). 

The age of this latter basalt is compatible with the chronology 

we present here.

Basal Arkosic Mudflows and the Tuff of Benton 

Hot Springs

The basalt near Pit 6 is overlain by at least 5.5 m of 

arkosic mudflow deposits containing granitic and basaltic 

clasts; these are barren of any clasts of silicic flow rocks or of 

tephra. The basal arkosic unit is overlain by another arkosic 

mudflow unit consisting of several flows that contain abundant, rounded pumice lapilli and an ~0.5-m-thick tephra layer 

in the middle of the set. The basal 20 cm of this tephra layer 
Glass Mountain tephra,

 fall and reworked,

 TTC-23, 24

Mudflows, reworked

 tephra, and paleosols

Mudflows, reworked 

 tephra, and paleosols

Unit TTC-22,

 coarsest

 pumice fall

Unit TTC-21

Unit TTC-20

Tuffs of Blind Spring Valley

Unit TTC-19

Pumice falls:

Figure 6. Pit 6 at Cowan Pumice Mine, 

Blind Spring Valley, Calif. Same section 

as shown in figure 5, except view is to 

the north. The coarsest unit here, Unit 

TTC-22, is also probably the most widely 

dispersed, and is likely to correlate with 

distal localities in Beaver Basin, Utah (fig. 

20), and in the Ventura Basin of southwestern California. This view shows well 

the channeled top of unit TTC-22 and the 

upper part of the section containing the 

Glass Mountain tephra (see fig. 8).

Figure 5. Tuffs of Blind Spring Valley and lower tuffs of Glass Mountain exposed in Pit 6, Cowan 

Pumice Mine, Blind Spring Valley, California. View is to east. See figures 2 and 3 for location of 

Cowan Pumice Mine and figures 3 and 4 for location of Pit 6 and cross-section. See figure 6 for a 

view of this section looking north, which shows the tephra layers of Glass Mountain in the upper 

part of this section, and figure 7 for a stratigraphic description of this exposure.

Stratigraphic Section at Cowan Pumice Mine 13
14 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

Reworked pumice-lapilli ash layers with

 obsidian clasts to 2 cm

Reworked tuff or mudflow 35 cm thick

 containing clasts of pumice, light-gray

 felsite, and basalt

Coarse pumice-lithic tuff, 17 cm thick,

 containing clasts of pumice to 3 cm and

 obsidian clasts to 0.5 cm

Debris flow of reworked, lenticular pumice

 in muddy matrix containing basalt and

 obsidian clasts

Coarse pumice-lapilli and lithic fall tuff,

 3-4 m thick, containing pumice-lapilli

 clasts to 4-5 cm, lithics of light-gray

 felsite, and sparse obsidian granules. 

 Top eroded.

Reworked pumice-lapilli tuff

Pumice-lapilli fall tuff containing light-gray

 felsite, obsidian, and other lithic clasts.

 Top eroded.

Coarse-sand-sized pumice lapilli fall tuff,

 3 cm thick, laterally persistenty, uniform;

 drapes underlying surface

Coarse pumice-lapilli-lithic fall tuff con-

 taining gray felsite clasts; normally

 graded; pumice lapilli to 1 cm

Base not exposed in quarry face

Pumiceous, gravelly debris flow

Pure reworked tephra

Pumiceous, gravelly debris flow

Arkosic debris or mudflow containing

 poorly-vesiculated pumice lapilli

Pumice-lapilli fall tuff at base, with

 lapilli to 1 cm l.a.; reworked pumice-

 lapilli tuff above, with lapilli to 4-5 cm

Arkosic debris flow or mudflow,

 barren of tephra

Base not exposed

Vesicular basalt flows

Weathered granite

26

24

22

20

18

TTC-24, 1.857 ±0.085

TTC-23, 1.922 ±0.024

TTC-22, 2.191 ±0.017

TTC-21, 2.187 ±0.007

TTC-20, 2.176 ±0.008

TTC-19, 2.177 ±0.008

TTC-25, 2.219 ±0.006

16

14

12

10

8

TTC-26, 2.81 ±0.02

6

4

2

0

m Ages

(Ma)

 Lower tuffs of Glass

 Mountain

Tuffs of Blind Spring

 Valley

Tuffs of Benton Hot

 Springs

Basalt of Benton

 Range

Stratigraphy Units Lithology

(grain size is longest axis)

 ~3.30 (?) (inferred

 by correlation; not

 dated at this site)

Sampled at E-W cut, just above and east of Pit 6 Sampled in deep, N-S cut on E side of Pit 6 Sampled in ~E-W exposures S and W of Pit 6 In natural exposures in gully W of Pit 6

Figure 7. Stratigraphy of section in Pit 6, Cowan Pumice Mine, Blind Spring Valley, Calif.
is well-bedded and well-sorted airfall ash, containing poorly 

vesiculated pumice lapilli as large as 1-2 cm in diameter 

(layers TTC-26A, B). The upper, reworked 30 cm of this unit 

contains pumice clasts as large as 4 to 5 cm in diameter. The 

layering of this unit is obscured in places by krotovinas (filled 

animal burrows).

Unlike the tuff of Blind Spring Valley higher in this section, the pumice in the basal tephra layer (TTC-26A, B), here 

informally named the tuff of Benton Hot Springs, contains 

mostly plagioclase feldspar but no sanidine, and the glass 

shards contain less SiO2 and MnO, but more Al2O3, Fe2O3, 

MgO, CaO, and K2O than the tephra layers of the tuffs of 

Blind Spring Valley (table 1). This tephra layer also differs in 

petrographic characteristics from rhyolitic lavas and tephra 

layers erupted from Glass Mountain. It contains (1) dominantly tight, poorly vesiculated pumices with spherical to 

spindle-shaped vesicles and comagmatic biotite and magnetite, (2) abundant lithic fragments of volcanic porphyry with 

feldspar, biotite, and hornblende phenocrysts, and (3) free 

euhedral crystals and fragments of plagioclase, quartz, green 

hornblende, biotite, magnetite, and rare allanite. We obtained 

an age of 2.81±0.02 Ma on this unit, on plagioclase feldspar 

grains (table 1 ). This age is older than any previously reported 

on tephra or lava flows of Glass Mountain (Gilbert and others, 1968; Bailey and others, 1976; Metz and Mahood, 1985; 

Izett and others, 1988), and at first it was suspected that this 

age was too old because of contamination with detrital grains 

(Sarna-Wojcicki and Pringle, 1992). Examination of the data 

of 10 separate analyses of this sample, however (appendix 1), 

indicates that the ages are internally consistent and that they 

probably reflect the true age of this unit. Despite chemical and 

petrographic differences from tephra layers of the tuff of Blind 

Spring Valley and tuffs of Glass Mountain, the tuff of Benton 

Hot Springs bears a “family resemblance” to tephra layers 

erupted from the Glass Mountain source area and related eruptive sources in the southwestern part of Nevada.

Above the 0.5-m-thick tephra bed is a 5.3-m-thick mudflow containing abundant rounded pumice lapilli in a mudstone matrix. The arkosic debris flows, the 0.5-m-thick tephra 

bed, and the overlying pumiceous mudflow (fig. 7), are very 

similar to the “stream gravels near Benton Hot Springs” (Qsg) 

described by Krauskopf and Bateman (1977) and may very 

well be the same unit.

Tuffs of Blind Spring Valley 

Above the basal arkosic mudflows, and separated from 

them by a sharp, planar contact, is another 2-m-thick unit of 

mudflows, containing abundant, rounded pumice lapilli, and a 

20-cm-thick, rather pure, reworked tephra layer, with coarseto medium-grained pumice lapilli (layer TTC-25) in its middle 

part. The sample of this tephra layer contains (1) vitreous and 

silky, silvery pumice lapilli with tubular vesicles or capillaries, (2) abundant angular fragments of light-gray felsite and 

porphyry (rhyolite?) with biotite, quartz, and sanidine phenocrysts, and occasionally, biotite, and (3) mineral grains or fragments of sanidine and some cloudy white plagioclase, quartz 

(some with glass coating), euhedral biotite, and magnetite. 

Some hornblende is present; one grain was observed to have 

glass adhering. Some brown glass shards are present, together 

with small spherical obsidian cores charged with microlites. 

The chemical composition of glass shards and the mineral 

content of this layer are very similar to those of the tuffs of 

Blind Spring Valley found higher in the section (table 1). 

Sanidine feldspar separated from this layer yielded a weighted 

mean age (using the inverse of the variance) of 2.219±0.006 

Ma. This age is close to, but older than, the oldest ages 

obtained on rhyolite flows of Glass Mountain, ~2.13 Ma (Metz 

and Mahood, 1985).

Above the mudlow unit containing tephra layer TTC-25 

is an interval, 6.6-7.7 m thick, of several coarse and fine airfall 

tephra layers, with one ~0.6-m-thick layer of reworked tephra, 

that together represent the main part of the eruptive episode 

of the tuffs of Blind Spring Valley at this site (figs. 5, 6, 7). 

All airfall tephra layers in this interval, including a thin, 3-cm, 

finer grained one, were sampled to evaluate the possibility that 

finer grained, thin tephra layers may represent the fining edges 

of major tephra lobes that were transported from the eruptive 

source in directions other than toward the Cowan Pumice Mine.

The lowermost of the coarse tephra layers in this interval 

is a ~1.9-m-thick airfall tephra layer (TTC-19; figs. 5, 6, 7) 

containing mostly drawn-out pumice lapilli as much as 1-2 cm 

in diameter, with straight or tubular capillaries. This layer also 

contains angular lithic fragments of light-gray felsite, limonite-stained, poorly vesiculated pumice and felsite, and bipyramidal quartz, sanidine, biotite, and ilmenite. Some grains of 

all these minerals have glass coats. This mineralogy is typical 

of all the tuffs of Blind Spring Valley higher in the section at 

this locality. Rare grains of hornblende with adhering glass, or 

encased in glass shards, are also present, but these look frayed, 

rounded, or embayed, and may be accidental or comagmatic 

crystals that had crystallized earlier and then been partially 

resorbed prior to eruption.

Next upwards is a 3-cm-thick, finer-grained tephra layer 

(TTC-20; figs. 5, 6, 7) that drapes over small irregularities in 

the top of the underlying layer and maintains a remarkably 

constant thickness. It is very well sorted, normally graded, 

and undoubtedly of airfall origin. It contains mostly fine- to 

coarse-sand-size silky, silvery pumice lapilli with near-parallel 

alinement of elongate, capillary-like vesicles. Its mineralogy is 

the same as that of the underlying TTC-19, except that it contains fresh, pretty, euhedral crystals of ilmenite and distorted, 

lathlike crystals of comagmatic brown biotite. 

Above this 3-cm-thick layer is a 1.2-m-thick tephra layer, 

airfall in the lower half, reworked in the upper half (TTC-21; 

figs. 5, 6, 7). The upper half of the unit fills undulations in 

the topography of the underlying tephra layers, thinning and 

thickening laterally. This unit contains pumice lapilli that are 

similar in appearance and size to those of TTC-19. Some of 

the pumices contain very small, euhedral crystals or fragments 

of biotite, but the layer is otherwise similar in mineralogy to 

the other underlying layers of the Tuffs of Blind Spring Valley.

Stratigraphic Section at Cowan Pumice Mine 15
16 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

Next upward, resting on a sharp contact, is the thickest, 

coarsest tephra layer of the sequence (TTC-22), 3.5 to 4.6 m 

thick, composed of angular pumice clasts as much as 4 to 5 

cm in diameter. Other than its coarse grain size, this airfall 

layer is similar in all respects to the layers beneath. This layer 

is inferred by Izett and others (1988) to correlate with their 

Taylor Canyon-C; its coarse grain size and thickness suggest it 

is likely to be found in far distal localities, particularly toward 

the east (for example, in Beaver Basin, Utah).

Laser-fusion 40Ar/39Ar ages on the ~7-m-thick tephra 

interval of dominantly airfall tephra layers, proceeding upward 

in the section, are: 2.177±0.008 Ma (TTC-19); 2.176±0.008 

Ma (TTC-20); 2.187±0.007 Ma (TTC-21); and 2.191±0.017 

Ma (TTC-22, the thickest, coarsest layer)(table 1; appendix 1).

Lower Tuffs of Glass Mountain

Above the thickest, coarsest layer of the tuff of Blind Spring 

Valley (TTC-22), there is as much as 4 m of reworked and airfall 

tephra (figs. 5-8). The base of this part of the section consists of 

a mudflow 2-2.5 m thick containing lenses of reworked, rounded 

pumice and some large basalt clasts (perhaps derived from the 

underlying basalt) and smaller obsidian clasts. This unit is channeled into the top of the underlying tephra layer and swells and 

pinches out laterally. The mudflow is overlain by a sequence of 

thinly bedded, cross-bedded, reworked, and airfall tephra layers 

that we refer to here as the lower tuffs of Glass Mountain (figs. 

7, 8), on the basis of stratigraphy, chemical composition of the 

volcanic glass (which differs from that of the tuffs of Blind Spring 

Valley), and laser-fusion Ar/Ar and conventional K-Ar ages.

Above the mudflow is a 17-cm-thick, well-sorted, coarse 

pumice-lithic airfall tuff, containing pumice clasts as large as 3 

cm in diameter, and obsidian clasts as large as 0.5 cm in diameter 

(layer TTC-23). This layer contains, in addition to the preponderant pumice lapilli, abundant lithic fragments of light-gray felsite, 

obsidian clasts, and brown glass shards, as well as sanidine 

(some intergrown with ilmenite), free, abundant quartz (some as 

euhedral, bipyramidal crystals), biotite, ilmenite, magnetite, and 

rare zircon. This unit is overlain by a 35-cm-thick reworked ash 

or ashy mudflow, consisting of pumice lapilli and lithic fragments, including basalt, in a fine, tuffaceous muddy matrix. All 

overlying tephra layers at this site thicken to the east and are 

buttressed against the 35-cm-thick bed. These uppermost layers 

consist of reworked, cross-bedded pumice lapilli and lithic clasts, 

including abundant, coarse obsidian (layer TTC-24).

Within the section at Pit 6, macroscopic obsidian clasts 

appear first in the lower half of the tuff of Blind Spring Valley, within layer TTC-21, where they appear as sparse sandand granule-size clasts. Obsidian clasts increase in size and 

abundance upward in the section, and are coarsest and most 

abundant in the lower tuffs of Glass Mountain, at the level of 

layers TTC-23 and -24.

Four ages were obtained on the set of lower tuffs of Glass 

Mountain at Pit 6. Sanidine feldspar grains from layer TTC-23 

yielded 40Ar/39Ar ages of 1.936±0.030 Ma and 1.898±0.039 

Ma, for a weighted average of 1.922±0.024 Ma. Two convenDebris flow with pumice,basalt,

 and obsidian clasts, 2 - 2.5 m

 TTC-23, Pumice-

 lithic ash fall,17 cm,

 w/ obsidian clasts

Mudflow with

 pumice, 35 cm

Reworked pumice lapilli

ash layers with obsidian

clasts to 2 cm

Buttress

 unconformity

 Lower tuffs of

Glass Mountain

TTC-24

Figure 8. Upper part of section at Pit 6, Cowan Pumice Mine. View to north. Airfall pumice and ash, and reworked pumice 

lapilli ash layers of Glass Mountain, with direct ages of 1.92±0.02 Ma on TTC-23 (average of two laser-fusion Ar-Ar dates on 

sanidine), and 1.86±0.09 Ma on TTC-24 (average of two K-Ar dates on obsidian clasts, J.K. Nakata, written commun., 1987).
tional K-Ar analyses on obsidian clasts collected from two 

levels in layer TTC-24, gave ages of 1.799±0.050 Ma and 

1.981±0.073 Ma, for a weighted average age of 1.857±0.085 

Ma (table 1).

Tephra Layers Exposed in Pits 1 Through 5

The Bishop Ash

The uppermost part of the section at the Cowan Pumice 

Mine, exposed in Pit 1, about 1.35 km to the southeast of Pit 

6, is about 70-100 m stratigraphically above the level of the 

lower tuffs of Glass Mountain and the tuffs of Blind Spring 

Valley (figs. 3, 4). In Pit 1, a single very coarse tephra layer 

at least 3.7 m thick (the base is not exposed) contains pumice 

clasts as much as 15 cm in diameter (TTC-7). The chemical 

composition of the pumice glass and the mineralogy of this 

unit are different from the tuffs of Blind Spring Valley; they 

are more similar to the tuffs of Glass Mountain and to the 

much younger Bishop ash bed (Sarna-Wojcicki and others, 

1984)(table 1). A fisson-track age on zircon crystals separated 

from this unit yielded an age of 0.75±0.05 Ma (Meyer and 

others, 1991), and sanidine feldspar grains from the same 

pumice clasts analyzed by laser-fusion 40Ar/39Ar yielded an age 

of 0.757±0.004 Ma (Sarna-Wojcicki and others, 2000). Both 

ages are in good agreement with other ages of the Bishop Tuff 

and air-fall ash bed obtained by Sarna-Wojcicki and Pringle 

(1992), and Sarna-Wojcicki and others (2000). The volcanic 

glass of this pumice bed contains somewhat higher concentrations of iron, magnesium, calcium, and titanium than the basal 

Bishop air-fall pumice and the overlying Bishop ash flow tuff 

at the south end of the Volcanic Tableland (Chalk Cliffs) north 

of Bishop, and also higher concentrations of these elements 

than distal samples of the Bishop ash bed. Consequently, we 

suspect that this layer may have been erupted during later 

stages of the Bishop eruption (Wilson and Hildreth, 1997). 

Nevertheless, in light of its age, this coarse tephra layer (TTC7) is undoubtedly the Bishop Tuff.

The intervening section between Pits 1 and 6 is not 

exposed (figs 3, 4). Thus, we do not know whether tephra 

from younger eruptions of Glass Mountain (in the age range 

of ~1.8 to 0.76 Ma; Sarna-Wojcicki and others, 1980; 1984; 

Izett, 1981; Metz and Mahood, 1985; Izett and others, 1988) 

is present above the lower tuffs of Glass Mountain (TTC-24), 

and below the Bishop Tuff (TTC-7). Tephra layers within this 

interval, however, have been dated at other sites, for example 

at the Chalk Cliffs (see below).

Tuffs of Blind Spring Valley

Tephra layers from exposures in Pits 2 through 5 were 

analyzed regardless of thickness or grain size, to test the possibility that fine-grained layers might be marginal deposits from 

major eruptions, rather than products of small eruptions. Pit 5 

has the thickest and most varied stratigraphy of pits 2-5. The 

section at the south side of Pit 5 is ~5 m thick (figs. 9, 10) and 

consists of a coarse, thick layer at the base, with pumice lapilli 

as large as 1 cm in diameter (TTC-6, -8). This unit may be 

even thicker, but its base is not exposed. Above this are about 

1.2 m of well bedded, intercalated fine to coarse tephra layers 

(TTC-9 through 12) and well-sorted, tuffaceous silt and fine 

sand. These sediments may be in part lacustrine. Above these 

beds is a coarse, massive to poorly bedded air-fall tephra layer 

~2.8 m thick, containing pumice lapilli as large as 2-3 cm in 

diameter (TTC-4, -13). In Pit 2 (fig. 11), Pits 3 and 4, and the 

north side of Pit 5, only one or two tephra layers are exposed; 

these appear to be coeval with layers in Pit 6, on the basis of 

projections in the sections, isotopic ages, and chemistry of the 

volcanic glass.

TTC-13

TTC-8

TTC-9 through 12

Figure 9. Two thick, coarse pumice 

fall tuffs of Blind Spring Valley at 

south wall of Pit 5, Cowan Pumice 

Mine (samples TTC-8 and -13), separated by several thin, well-bedded, 

fine-grained tephra (samples TTC-9 

through -12), and a ~0.5-m-thick tan 

tuffaceous silt above the latter. See 

figure 10 for a stratigraphic description of this section. The fine-grained, 

middle part of this section may be 

lacustrine.

Stratigraphic Section at Cowan Pumice Mine 17
18 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

m Samples, Stratigraphy Lithology

 Age (Ma)

5

4

3

2

1

0

Caliche developed in top of unit

 parallel to eroded surface

Light gray, coarse pumice-lapilli fall tephra,

 with lapilli to 4-5 cm 

Gray-tan tuffaceous silt

 

Brown, poorly-sorted tuffaceous sand

Gray, well-sorted, medium- to coarse-grained

 sand-sized tephra 

Reversely graded tephra with poorly vesiculated

 pumice to 1.5 cm. Pumice fall (into water?)

Coarse, poorly sorted, sand-sized pumice

 lapilli to <1 cm.

Very coarse-sand-sized tephra layer

Very coarse sand-to-granule-sized, normally

 graded pumice lapill, to 1 cm.

Light gray pumice lapilli fall tephra, 

 lapilli to 1 cm.

Light gray, medium-sand-sized pumice lapill tuff

 containing angular lithic fragments of felsite 

TTC-13

TTC-4,

2.14 ±0.005 

TTC-12

TTC-11

TTC-10

TTC-9A

TTC-9

TTC-8

TTC-6

Thin tephra fall beds

 interbedded with

 silts and sands 

(grain size is longest axis)

Figure 10. Stratigraphy of tuffs of Blind Spring Valley at Pit 5, Cowan Pumice Mine, Blind Spring Valley, California.
Although tephra layers in Pits 2 through 5 are chemically 

and petrographically very similar to the tuffs of Blind Spring 

Valley in Pit 6 (table 1), glass shards of the layers in Pits 2-5 

have slightly higher contents of MnO and, on the average, 

slightly higher CaO than the tuffs of Blind Spring Valley in Pit 

6 (table 1). Results of EDXRF show little difference between 

the tuffs of Blind Spring Valley in Pit 6 and Pits 2-5. The 

laser-fusion 40Ar/39Ar ages on sanidines from samples in Pits 

2 through 5 range from 2.184 to 2.135 Ma. The older part of 

this range overlaps with the younger ages of the tuffs of Blind 

Spring Valley in Pit 6, but most of the ages of units in Pits 2 

through 5 are slightly younger (table 1; appendix 1).

Isotopic Ages of the Upper Glass 

Mountain Tephra Layers at Chalk Cliffs, at 

the South End of the Volcanic Tableland

At the Cowan Pumice Mine there are no exposures of 

upper Glass Mountain tephra layers in the age range of ~1.5 to 

~0.8 Ma, above the lower tuffs of Glass Mountain and below the 

Bishop air-fall pumice. Cross sections (fig. 4) indicate that about 

65-70 m of section are present above the lower tuffs of Glass 

Mountain, below the Bishop airfall pumice, but lack of exposures within the intervening 1.4 km and deep cover by younger 

alluvium prevented us from examining these sediments.

Several of the upper Glass Mountain tephra layers in the 

age range of ~1.0 Ma to ~0.8 Ma are present, however, beneath 

the Bishop Tuff and airfall pumice at the south end of the 

Volcanic Tableland, along the Owens River, north of Bishop, 

Calif. (Chalk Cliffs; Sarna-Wojcicki and others, 1980; 1984; 

1997; Izett, 1981; Izett and others, 1988)(figs. 2, 12). The 

chemical compositions of the volcanic glasses of these tephra 

layers (table 2) are similar both to that of the Bishop ash and 

to those of the lower tephra layers of Glass Mountain (tables 

1-4), although the calcium appears to be generally lower in the 

lower layers and higher in the higher layers. A new laser-fusion 

40Ar/39Ar age on the lowermost of three tephra layers exposed at 

this locality is 1.13±0.19 Ma (BT-15; sanidine)(appendix 1; fig. 

12). This sample may be contaminated with older detrital sanidine or xenocrysts, because the mean square of wighted deviates 

(MSWD), a ratio of the total error of the experiment divided by 

Figure 11. One of the tuffs of Blind Spring Valley, in Pit 2, is the thick, white pumice-fall unit in middle left. Sample TTC-18, 

from base of this unit, is dated 2.16±0.02 Ma by laser-fusion Ar/Ar on sanidine. This unit is probably the same as TTC-22 

in Pit 6, dated 2.19±0.02 Ma (fig. 5). As at TTC-22, this layer overlies reworked eolian tephra and paleosols and underlies 

the thick section of debris flows and reworked tephra containing basalt clasts. The top of the pumice unit is eroded and 

uneven, with the overlying debris flows and reworked tephra layers filling-in the uneven topography. Also see figures 4-7.

Isotopic Ages of the Upper Glass Mountain Tephra Layers at Chalk Cliffs, at the South End of the Volcanic Tableland 19
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O 

Bishop Tuff, fine-grained base of ash-flow, Insulating Aggregates Quarry, southeast end of Volcanic Tableland, south of Long Valley, C

BT-7A 77.63 12.48 0.74 0.03 0.05 0.42 0.07 3.87 BT-7B 77.57 12.48 0.73 0.02 0.05 0.43 0.07 3.88 BT-7C 77.54 12.60 0.75 0.03 0.03 0.45 0.05 3.86 

Bishop Tuff, basal pumice fall beneath ash flow, (same locality as BT-7, above), BT-8A 77.75 12.48 0.73 0.03 0.05 0.44 0.05 3.82 

BT-8B 77.40 12.73 0.72 0.04 0.04 0.45 0.06 3.34 BT-8C 77.48 12.68 0.71 0.03 0.02 0.45 0.06 3.85 

Bishop Tuff, basal pumice fall beneath ash-flow, southwest end of Volcanic Tableland, south of Long Valley, Calif. BT-11D2 77.17 13.13 0.76 0.02 0.04 0.42 0.05 3.73 

BT-11D1 77.47 12.75 0.74 0.03 0.04 0.41 0.04 3.76 BT-11C2 77.57 12.71 0.68 0.03 0.05 0.40 0.06 3.78 BT-11C1 77.71 12.59 0.72 0.03 0.04 0.42 0.06 3.67 BT-11B2, 77.71 12.71 0.74 0.03 0.04 0.39 0.05 3.67 BT-11B1 77.65 12.64 0.71 0.03 0.05 0.39 0.06 3.64 BT-11A2 77.80 12.55 0.71 0.03 0.04 0.40 0.06 3.80 BT-11A1 77.55 12.72 0.69 0.03 0.04 0.43 0.06 3.69 

Bishop Tuff, air-fall layer at top of section, Cowan Pumice Mine, east of Long Valley, Calif. TTC-7 77.30 12.65 0.82 0.09 0.04 0.54 0.09 3.71 

Ash of Glass Mountain (D), water-deposited, south-central end of Volcanic Tableland, south of Long Valley, Calif. BT-2A 77.27 12.90 0.79 0.03 0.03 0.43 0.06 3.68 

BT-2B 77.95 12.25 0.71 0.03 0.05 0.41 0.06 3.53 BT-2C 77.64 12.67 0.74 0.03 0.03 0.43 0.07 3.66 

Ash of Glass Mountain (E), air-fall, beneath and same locality as BT-2 BT-14A 77.21 12.78 0.75 0.03 0.04 0.43 0.06 3.77 

BT-14B 77.41 12.77 0.77 0.04 0.04 0.45 0.07 3.53 

Pumice of Glass Mountain (F), reworked, beneath and same locality as BT-14A, B BT-15 77.18 12.77 0.77 0.04 0.03 0.46 0.06 3.85 

Table 2. Electron-microprobe analysis of glass of the Bishop Tuff and Bishop airfall pumice (0.774 Ma), compared with the younger set of tephra layers of Gla[Values given are weight-percent oxide, recalculated to 100 percent fluid-free basis. Original oxide totals before recalculation (*) are given to indicate approximate degree of hy15 or more individual glass shards were analyzed for each sample. Electron-microprobe chemical analyses by C. E. Meyer (1977 to 1996). Compositions of volcanic glass froTaylor Canyon, both pumice and obsidian, from the type locality in Taylor Canyon, are also given for comparison (samples TCTL-4 and -5).]
the error expected from analytical errors alone, is quite large 

(appendix 1). An age of 0.88±0.07 Ma wa obtained on the 

stratigraphically next higher layer (BT-16)(SEM, standard error 

of the mean, is used here; see appendix 1). Six 40Ar/39Ar ages 

on feldspars grains obtained from a layer (BT-14) that overlies 

both of the latter layers (fig. 12) were scattered and considerably 

older than the ages of BT-15 and BT-16 (appendix 3). Consequently, sample BT-14 is considered to be contaminated. BT-14 

has been previously referred to as Glass Mountain ash D (SarnaWojcicki and others, 1984; Izett and others, 1988).

The chemical composition of the volcanic glass of these 

upper Glass Mountain tephra layers at the Chalk Cliffs locality 

is very similar to that of tephra layers and dome collapse breccias in the tuff of Taylor Canyon of Krauskopf and Bateman 

(1977) at the type locality in Taylor Canyon, north and northeast of Long Valley. Thus these two sets of tephra layers, both 

underlying the Bishop Tuff with small stratigraphic separation, 

are in all probability correlative (table 2).

Interpretation of the Isotopic Ages at 

the Cowan Pumice Mine

The age of the basal tephra layers in the sequence near Pit 

6 (2.81±0.023 Ma; TTC-26) precedes the oldest age of the tuffs 

of Blind Spring Valley by ~0.6 m.y. In light of its significantly 

older age and different petrographic and chemical characteristics, we attribute this tuff to a separate, earlier volcanic episode 

and give it the informal name of the tuff of Benton Hot Springs, 

named after a small town near the north end of Blind Spring 

Valley. The coarse grain size of the tuff of Benton Hot Springs 

at the Cowan Pumice Mine suggests that the volcanic source 

area was close by, possibly in the vicinity of Glass Mountain. 

However, to our knowledge, no rocks of this age are exposed at 

Glass Mountain; the oldest age obtained on volcanic-flow rocks 

from that source is 2.13 Ma (Metz and Mahood, 1985). The 0.6-

m.y. interval between the eruption of this oldest tephra layer and 

the tuffs of Blind Spring Valley represents a hiatus in the section 

at the Cowan Pumice Mine locality. Multiple tephra layers with 

broad chemical affinities to those of the tuffs of Blind Spring 

Valley and the tuffs of Glass Mountain were erupted during the 

period between 2.8 and 2.2 Ma and are present elsewhere in the 

region, but not within the Cowan Pumice Mine section. They 

are found, for example, in the Willow Wash section south of 

Fish Lake Valley (fig. 1, table 4; also see below). The sharp, planar contact above the tuffs of Benton Hot Springs is likely to be 

an unconformity. The section exposed in the pits of the Cowan 

Pumice Mine, situated on the western flank of Blind Spring Valley, is a condensed section. The depositional center of the valley 

is to the east of the pits, and tephra units correlative with those 

present in the pits are buried beneath younger deposits in the 

center of Blind Spring Valley. For this reason, the section at the 

Cowan Pumice Mine is likely to contain hiatuses.

The lowermost, reworked tephra layer ascribed to the 

Sample SiO

tuffs of Blind Spring Valley (TTC-25; 2.219±0.006 Ma) is 

2 Al O2 3 Fe O2 3 MgO MnO CaO TiO2 Na O K 2 O Total* 2

Ash of Glass Mountain (G), water-deposited, beneath and same locality as BT-14

BT-1 77.32 12.91 0.80 0.03 0.03 0.43 0.05 3.85 4.57 95.56BT-1CA 77.84 12.73 0.77 0.03 0.05 0.42 0.06 3.52 4.57 94.61BT-1CB 77.47 12.75 0.73 0.03 0.03 0.43 0.07 3.77 4.72 96.29BT-1B 77.59 12.57 0.74 0.03 0.03 0.40 0.06 3.75 4.83 95.50BT-1A 77.53 12.65 0.74 0.03 0.04 0.42 0.06 3.71 4.81 95.44

Pumice (5) and Obsidian (4) from tephra layers in the middle of the tuff of Taylor Canyon, at its type locality in Taylor Canyon (Krauskopf and Bateman, 1977)

Table 2.

TCTL-5 77.32 12.62 0.77 0.04 0.04 0.46 0.07 3.84 4.84 93.60TCTL-4 77.24 12.73 0.78 0.03 0.04 0.45 0.05 4.01 4.66 94.83

 Electron-microprobe analysis of glass of the Bishop Tuff and Bishop airfall pumice (0.774 Ma), compared with the younger set of tephra layers of Glass Mountain (~0.8 

to 1.2 Ma).—Continued

[Values given are weight-percent oxide, recalculated to 100 percent fluid-free basis. Original oxide totals before recalculation (*) are given to indicate approximate degree of hydration of volcanic glass. About 15 or more individual glass shards were analyzed for each sample. Electron-microprobe chemical analyses by C. E. Meyer (1977 to 1996). Compositions of volcanic glass from tephra layers of the Tuff of Taylor Canyon, both pumice and obsidian, from the type locality in Taylor Canyon, are also given for comparison (samples TCTL-4 and -5).]

Interpretation of the Isotopic Ages at the Cowan Pumice Mine 21
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2

Bishop ash bed: thick, coarse, reworked ash and tuff in nearshore sands of pluvial lake in Fish Lake Valley above alluvial gravels FLV-163-CS 77.6 12.9 0.79 0.02 0.05 0.41 0.06 3.5 4.

Tuffs of Glass Mountain (undifferentiated): fine-grained, thin-bedded tephra layers in alluvial gravels FLV-162-CS 77.4 12.9 0.77 0.03 0.07 0.46 0.07 3.4 5.

FLV-161-CS 77.5 12.8 0.71 0.03 0.06 0.45 0.06 3.5 5.FLV-160-CS 77.4 12.8 0.73 0.01 0.06 0.50 0.07 3.5* 4.

Lower (?) tuffs of Glass Mountain: fine-grained, lenticular tephra layers in alluvial gravels FLV-177-CS 77.1 12.5 0.70 0.03 0.03 0.40 0.05 2.8 5.

FLV-178-CS 77.7 12.5 0.70 0.03 0.03 0.40 0.05 3.1 5.

Tuffs of Emigrant Pass: fine-grained, lenticular tephra layers in alluvial gravels FLV-190-EP 77.6 12.4 0.65 0.05 0.10 0.29 0.07 3.3* 5.

FLV-189-EP 77.6 12.4 0.65 0.04 0.12 0.28 0.05 3.4 5.

Huckleberry Ridge ash bed: thick, fine-grained bed at transition between alluvial gravels (above) and lake beds (below) FLV-179-EP 76.6 12.1 1.72 0.01 0.04 0.59 0.11 2.8 6.

Tuffs of Blind Spring Valley: fine- to medium-grained air-fall and reworked tephra layers intercalated with lake beds (except for FLV-184-EP, which 

FLV-188-EP-A 77.9 12.1 0.58 0.05 0.09 0.31 0.05 3.1* 5.FLV-187-EP 77.6 12.3 0.59 0.05 0.08 0.36 0.08 3.0* 5.FLV-186-EP-A 77.1 12.6 0.60 0.06 0.09 0.31 0.07 3.2* 5.FLV-185-EP 77.3 12.7 0.57 0.05 0.09 0.33 0.07 3.0* 6.FLV-184-EP 77.3 12.8 0.58 0.06 0.04 0.78 0.09 2.7 5.FLV-183-EP 77.2 12.7 0.56 0.05 0.11 0.31 0.07 3.2* 5.FLV-182-EP-A 76.7 13.1 0.57 0.04 0.11 0.31 0.09 3.1* 6.FLV-181-EP-A 77.0 12.9 0.59 0.05 0.11 0.32 0.10 3.0 5.FLV-180-EP 77.1 12.9 0.59 0.04 0.11 0.32 0.10 3.0 5.

Table 3. Chemical composition, determined by electron-microprobe analysis, of volcanic glass shards from middle Pleistocene and upper Pliocene 

ern Fish Lake Valley, Calif.- Nev. (Emigrant Pass area and vicinity). [Samples are arranged in stratigraphic order, with youngest at top. Values given are in weight-percent oxide, recalculated to 100 percent. Original oxide totals 

to indicate ap-proximate degree of hydration of volcanic glass. About 15 individual glass shards were analyzed for each sample. C. E. Meyer, analyst.]
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O 

Badlands section

Tuffs of Glass Mountain

FLV-126-WW 77.8 12.1 0.78 0.05 0.03 0.45 0.07 3.7 5.0 FLV-125-WW 78.0 12.1 0.76 0.05 0.04 0.39 0.05 3.9 4.8 FLV-124-WW 77.6 12.6 0.76 0.02 0.05 0.34 0.06 4.0 4.6 

Lower tuffs of Glass Mountain

FLV-48-WW 77.4 12.8 0.71 0.04 0.06 0.32 0.06 3.8 4.8 FLV-47-WW 78.0 12.3 0.72 0.03 0.07 0.31 0.04 3.8 4.7 

Tuffs of Emigrant Pass

FLV-46-WW 77.9 12.4 0.68 0.02 0.08 0.30 0.07 3.8 4.7 FLV-45-WW 78.0 12.3 0.67 0.02 0.10 0.29 0.07 3.9 4.7 FLV-44-WW 77.8 12.6 0.64 0.04 0.10 0.29 0.05 3.9 4.5 

Tuffs of Blind Spring Valley

FLV-43-WW 78.0 12.3 0.56 0.03 0.08 0.32 0.06 3.8 4.8 FLV-42-WW 77.9 12.3 0.61 0.04 0.07 0.36 0.06 3.6 5.1 FLV-123-WW 77.5 12.6 0.59 0.03 0.08 0.35 0.08 3.8 5.0 FLV-122-WW 77.5 12.6 0.59 0.02 0.08 0.34 0.07 3.8 5.0 FLV-41-WW 77.7 12.7 0.55 0.04 0.11 0.31 0.09 3.6 5.0 FLV-40-WW 78.0 12.6 0.55 0.04 0.10 0.34 0.09 3.8 4.6 FLV-39-WW 77.6 12.6 0.53 0.06 0.09 0.35 0.10 2.8 5.9 

Tuffs of the Badlands (upper set)

FLV-38-WW 79.2 12.7 0.54 0.05 0.08 0.42 0.10 2.2 4.8 FLV-138-WW 76.8 13.0 0.56 0.04 0.09 0.42 0.09 2.7 6.4 FLV-37-WW 77.7 12.7 0.55 0.05 0.08 0.40 0.09 3.2 5.3 

Tuffs of the Badlands (lower set)

FLV-36-WW 77.5 12.8 0.58 0.05 0.09 0.43 0.11 3.7 4.8 FLV-35-WW 77.6 12.7 0.60 0.06 0.07 0.49 0.10 3.8 4.7 FLV-121-WW 77.6 13.0 0.65 0.04 0.06 0.57 0.07 2.9 6.2 

Table 4. Electron-microprobe analyses of volcanic glass shards from middle Pleistocene and upper Pliocene tephra layers of the Willow Wash areCalif.- Nev.[Values given are in weight-percent oxide, recalculated to 100 percent. Original oxide totals before recalculation are given to indicate approximate degree of hyAbout 15 or more individual glass shards were analyzed for each sample. Undet. - magnetic polarity undeterminable. C. E. Meyer, electron-microprobe analys
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O 

Badlands section Tuffs of the Badlands (lower set) FLV-34-WW 76.9 12.9 0.62 0.08 0.06 0.62 0.10 2.5 6.2 

FLV-33-WW 76.9 13.0 0.64 0.07 0.07 0.62 0.12 2.7 6.0 FLV-32-WW 77.2 12.9 0.61 0.07 0.08 0.62 0.12 2.8 5.7 

Tephra layer of unknown source (at base of section; no known correlative) FLV-31-WW 77.5 12.0 2.01 0.02 0.05 0.35 0.15 3.4 4.5 

Rimrock section Tuffs of the Badlands (upper set) FLV-29-WW 76.9 12.7 0.55 0.06 0.08 0.41 0.06 2.2 7.1 FLV-28-WW 77.5 12.6 0.53 0.06 0.08 0.39 0.09 2.2 6.7 FLV-27-WW 77.9 12.4 0.54 0.05 0.08 0.43 0.10 3.0 5.7 

Tuffs of the Badlands (lower set) FLV-26-WW 77.9 12.4 0.54 0.05 0.08 0.39 0.10 2.9 5.7 

FLV-25-WW 77.6 12.5 0.61 0.07 0.07 0.56 0.10 2.9 5.7 FLV-24-WW 76.7 13.0 0.67 0.07 0.07 0.63 0.08 3.2 5.6 

Tephra layer of unknown source (no known correlative) FLV-76-WW 77.9 12.2 0.86 0.11 0.02 0.46 0.18 3.2 5.1 

Tuff of Benton Hot Springs (tuffs of the Rimrock, or Rimrock ash) FLV-23-WW 75.7 13.9 0.77 0.08 0.09 0.63 0.07 4.1 4.6 

FLV-22-WW 75.5 14.0 0.78 0.08 0.10 0.64 0.08 4.2 4.6 

Table 4. Electron-microprobe analyses of volcanic glass shards from middle Pleistocene and upper Pliocene tephra layers of the Willow Wash areCalif.- Nev.—Continued [Values given are in weight-percent oxide, recalculated to 100 percent. Original oxide totals before recalculation are given to indicate approximate degree of hyAbout 15 or more individual glass shards were analyzed for each sample. Undet. - magnetic polarity undeterminable. C. E. Meyer, electron-microprobe analys
somewhat older than the oldest ages on the main sequence of 

overlying coarse tuffs of Blind Spring Valley in Pit 6 (TTC-19 

through TTC-22; 2.190-2.176 Ma; fig. 5, table 1). Thus, a dormant period of ~30 k.y. may have intervened between initial 

activity and the main eruptive episode. Again, the section at 

the Cowan Pumice Mine contains hiatuses, and any tephra 

that was erupted during this ~30-k.y. period could have been 

eroded or not deposited at this site (see below).

The four ages of the tuffs of Blind Spring Valley at Pit 6 

(TTC-19 through TTC-22) form a tight cluster in time and are 

not significantly different within analytical error (appendix 1; 

fig. 6). In fact, three of the ages (TTC-20, -21, and -22, dated 

2.176±0.008, 2.187±0.007, and 2.191±0.017 Ma, respectively) 

are reversed relative to their stratigraphic positions in the section, but the age differences between them are not significant 

within analytical error. The ages of these four layers together 

form a continuum with those obtained for tephra layers in 

Pits 2-5 (TTC-4,- 6, -15, -16, -17, -18; table 1), and appear 

to represent an episode of volcanic activity from about ~2.19 

to ~2.14 Ma. The tephra layers below the unconformity and 

Glass Mountain tephra in Pit 6, however, are slightly older 

than the layers in Pits 2 through 5. The oldest layers in Pit 6 

v v v v v v v v v v v v v

v v v v v v v v v v v v v

v v v v v v v v v v v v v

v v v v v v v v v v v v v

Ash flows

Chalky-white, fine-grained lenses of volcanic ash,

 to 30 cm thick, in fine-grained lake sediments

10-cm-thick airfall fine pumice-lapilli ash bed

10-cm-thick bed of contorted, impure ash

60-80-cm-thick bed of coarse, reworked sediments

 and tephra, with abundant clasts of obsidian

40-cm-thick, reworked tephra and fine lake sediments 

20-cm-thick lens of reworked, rounded pumice, with

 lapilli as much as 3 cm in diameter

60-cm-thick lake sediments and reworked tephra

[level of dirt road]

[~5 m of section omitted]

Multiple, reworked tephra layers

[level of Owens River]

BT-8, BT-11, 

 0.759 ±0.002 Ma*

BT-14 (contaminated

 sample; 52.9 - 1.36 

 Ma age range)

BT-16, 0.88 ±0.07 Ma

BT-15, 1.13 ±0.19 Ma

BT-1 (A-C)

* Standard error of the mean of multiple determinations from several sites, including BT-8 and BT-11.

Pumice fall

Bishop Tuff Tephra layers of Glass Mountain and lake deposits

Magnetic

 Polarity Normal Reversed

?

?

Figure 12. Composite stratigraphic section at Chalk Cliffs, south end of Volcanic Tableland, ~11 km north 

of Bishop, Calif., showing stratigraphy and new laser-fusion 40Ar/39Ar ages of the younger tephra layers of 

Glass Mountain, beneath the Bishop tuff. See figure 2 for location.

Interpretation of the Isotopic Ages at the Cowan Pumice Mine 25
26 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

are significantly older than the oldest layers in Pits 2 through 

5, suggesting that some section of the tuffs of Blind Spring 

Valley may have been cut out at Pit 6, at the top of the tuffs 

of Blind Spring Valley, below the tuffs of Glass Mountain. 

Slight differences in manganese concentration of glass shards 

of the tephra layers in Pit 6, as determined by the electron 

microprobe, are not apparent in EDXRF analysis (table 5), 

but may be significant. The tephra layers of the tuffs of Blind 

Sample K2O CaO TiO2 MnO Fe2O3 Rb Sr Y Zr Nb

Cowan Pumice Mine, Calif.

Pit 1; Bishop airfall pumice bed

TTC-7 5.69 0.69 0.073 0.046 0.89 183 15 18 77 21

Tuffs of Blind Spring Valley; Pit 2, thick, coarse bed 

TTC-18 5.82 0.50 0.064 0.089 0.55 252 0 25 88 42

Pit 3, thick coarse bed

TTC-5 6.56 0.46 0.064 0.102 0.61 263 0 31 92 45

Pit 4, thin airfall (?) beds

TTC-15 6.45 0.56 0.072 0.099 0.63 260 0 30 91 45

Pit 5 (south side), thick coarse bed

TTC-13 5.40 0.48 0.068 0.101 0.61 260 0 31 89 46

TTC-4 5.58 0.52 0.071 0.099 0.61 259 0 31 91 45

Pit 5 (south side), thin lake (?) beds

TTC-12 5.09 0.48 0.070 0.095 0.65 242 0 30 94 43

TTC-11 5.23 0.49 0.074 0.082 0.65 220 0 25 91 37

TTC-10 5.88 0.55 0.073 0.094 0.66 235 1 24 86 39

TTC-9A 5.81 0.53 0.072 0.099 0.63 260 0 29 89 44

TTC-9 5.94 0.50 0.077 0.100 0.64 260 0 30 88 45

Pit 5 (north side), thick, coarse bed

TTC-8 5.81 0.51 0.065 0.109 0.61 294 9 35 96 51

TTC-6 5.67 0.51 0.069 0.114 0.60 290 0 34 94 53

Pit 5 (north side), thick, coarse bed

TTC-17 6.15 0.58 0.070 0.114 0.61 285 0 33 89 46

TTC-16 6.07 0.54 0.069 0.114 0.62 286 0 37 93 50

Pit 6, mostly thick, coarse airfall, interbedded with mudflows (latter not analyzed); TTC-20 is 3-cm, thin ash bed

TTC-22 6.13 0.49 0.064 0.091 0.61 265 0 33 101 46

TTC-21 6.32 0.55 0.063 0.100 0.61 295 2 38 106 53

TTC-20 5.99 0.48 0.061 0.098 0.59 284 1 33 102 48

TTC-19 6.23 0.36 0.065 0.102 0.63 287 8 33 81 42

Other localities

Tephra layers in the Waucoba Lake Beds of Hopper (1947), Westgard Road locality (tuffs of Emigrant Pass, W3A, and tuffs of Blind Spring Valley, W2A); Waucoba Road locality (tuffs of Blind Spring Valley, WAC-7 and WAC-6), Owens Valley, Calif.

W3A 5.88 0.50 0.048 0.100 0.70 326 0 45 105 60

W2A 5.22 0.46 0.070 0.108 0.61 293 10 37 93 52

WAC-7 5.65 0.43 0.047 0.095 0.68 330 1 45 106 60

WAC-6 5.62 0.47 0.066 0.092 0.62 273 1 37 105 50

Table 5. Energy-dispersive X-ray fluorescence analysis of volcanic glass from tephra layers.

[Charles E.Meyer, Janet L. Slate, Andrei M. Sarna-Wojcicki, and Elmira Wan, analysts, U.S. Geological Survey, Menlo Park. Concentrations in weight percent for 

the oxides, and in parts per million atomic weight for the remaining elements. Analyses were done on a Kevex energy-dispersive X-ray fluorescence spectrometer.]
Sample K2

O CaO TiO2 MnO Fe2O3 Rb Sr Y Zr Nb

Other localities—continued

Confidence Hills, Southern Death Valley, Calif., (in strat. order)

Lower Tuffs of Glass Mountain

CHA86G9 5.35 0.54 0.061 0.044 0.76 182 1 19 88 25

CHA86G8 5.57 0.52 0.059 0.047 0.80 199 1 24 95 27

CHA86G7 5.43 0.56 0.057 0.056 0.77 191 0 21 85 28

CHA86W5 5.44 0.51 0.056 0.051 0.78 200 0 22 89 29

Tuffs of Emigrant Pass

CHA86BT3 5.20 0.48 0.051 0.105 0.71 346 0 44 99 60

Huckleberry Ridge ash bed

CHA86B2 5.11 0.68 0.107 0.044 1.83 197 4 77 225 52

Tuffs of Blind Spring Valley

CHA86BT1 5.34 0.50 0.057 0.096 0.64 291 1 35 88 49

Manix Basin, Mojave Desert, Calif.

Tuffs of Emigrant Pass (?)

MANIX-8 3.50 0.41 0.048 0.098 0.71 325 0 49 108 61

MANIX-4 3.62 0.39 0.053 0.102 0.70 327 9 44 99 58

MANIX-7 3.31 0.40 0.057 0.102 0.71 325 10 45 101 60

Balcom Canyon, Ventura Basin, Southwestern Calif.,

Tuffs of Blind Spring Valley

P-169Q 4.80 0.46 0.064 0.111 0.67 287 11 36 95 51

PICO-163Q 4.21 0.42 0.065 0.103 0.62 285 9 37 96 53

Huckleberry Ridge Ash Bed from several sites in the Western United States: Borchers Loc., Meade Co., Kansas (1), Beaver Basin, Utah (2), 

North-central Nevada (3), Confidence Hills, Death Valley (4), Lake Tecopa (5-7), Ventura Basin (8), Humboldt Basin, Calif. (9, 10), and Deep-Sea 

Drilling Project Hole 36 (11)

1. 68-W-98 5.30 0.66 0.115 0.041 1.75 199 5 77 217 55

2. BEAVER-1 5.91 0.67 0.106 0.043 1.84 211 18 86 236 54

3. VA-2-1982 5.84 0.62 0.096 0.040 1.79 207 12 85 224 55

4. CHA-86-BT-2 5.11 0.68 0.107 0.044 1.83 197 4 77 225 52

5. TECO-12B 5.25 0.65 0.111 0.046 1.91 201 16 83 251 53

6. TECO-2 5.15 0.65 0.124 0.046 1.91 185 25 73 240 46

7. TECO-1 5.79 0.69 0.126 0.046 1.96 189 7 74 253 53

8. PICO-157 4.48 0.61 0.105 0.045 1.85 199 14 83 241 53

9. SM-ASH-36 5.10 0.64 0.097 0.043 1.83 206 13 84 222 54

10. HPA-1 5.06 0.61 0.103 0.042 1.80 203 17 86 229 56

11. DSDP 36-4 5.42 0.62 0.119 0.045 1.88 203 19 84 247 58

Average of 11 5.31 0.65 0.110 0.044 1.85 200 14 81 235 54

 ± 1r 0.42 0.03 0.010 0.002 0.06 8 6 5 12 3

Table 5. Energy-dispersive X-ray fluorescence analysis of volcanic glass from tephra layers.—Continued

[Charles E.Meyer, Janet L. Slate, Andrei M. Sarna-Wojcicki, and Elmira Wan, analysts, U.S. Geological Survey, Menlo Park. Concentrations in weight percent for 

the oxides, and in parts per million atomic weight for the remaining elements. Analyses were done on a Kevex energy-dispersive X-ray fluorescence spectrometer.]

Interpretation of the Isotopic Ages at the Cowan Pumice Mine 27
28 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

Spring Valley in Pit 6 (TTC-19 through -22, and TTC-25), are 

otherwise not distinguished chemically from the set in Pits 2 

though 5 (TTC-4 through -6; -8 through -13; and -15 through 

-18). The ages of this combined group, consisting of ten dates 

(excluding the lowermost tephra layer assigned to the tuffs of 

Blind Spring Valley, TTC-25), represents an age range of about 

50 k.y. for the main eruptive episode of the tuffs of Blind Spring 

Valley. The lowermost layer having the chemical characteristics 

of the tuffs of Blind Spring Valley (TTC-25), but a somewhat 

older age, appears to be a precursor of this episode. The entire 

eruptive episode of the tuffs of Blind Spring Valley, then, lasted 

about 90 k.y. Stratigraphic evidence of significant elapsed time 

between eruptions is present in the section near and at Pit 6, 

between the lowermost layer of the tuff of Blind Spring Valley 

(TTC-25, ~2.22 Ma) and the next highest unit, the basal tuff 

exposed in Pit 6 (TTC-19, ~2.19 Ma). There is also stratigraphic 

evidence of some elapsed time between layer TTC-21 and the 

higher, coarsest layer TTC-22 (figs, 5, 6). The upper contact of 

layer 21 is eroded and overlain by ashy silts reworked by wind 

and water(?), with incipient soil development. The top of layer 

TTC-22 is also irregular and eroded and overlain by reworked 

pumice, ash, and mudflow deposits and wind-blown silt, with 

incipient soil development. These latter sediments were deposited after the deposition of the tuff of Blind Spring Valley, but 

before the deposition of the early tuffs of Glass Mountain.

The conventional K-Ar ages on obsidian clasts and 

laser-fusion 40Ar/39Ar ages on sanidine feldspar grains from 

the uppermost set of tephra layers at Pit 6 (TTC-23 and -24) 

are not significantly different from each other, within analytical error, but are significantly younger than the ages of the 

tuffs of Blind Spring Valley (table 1). We refer to these upper 

layers here as the lower tuffs of Glass Mountain, because of 

their similar age and glass chemical composition to tephra 

layers and flows erupted from Glass Mountain and their differences in composition from the tuffs of Blind Spring Valley. 

A weighted mean age for the four dates of the upper set of 

layers from Pit 6 is 1.90±0.04 Ma, with a 95% confidence 

interval of from 1.82 to 1.97 Ma. Some section is missing 

between the tuffs of Blind Spring Valley and the lower tuffs 

of Glass Mountain at Pit 6, as can be seen in the stratigraphy 

of the Emigrant Pass section at Fish Lake Valley (fig. 13; also 

see below). At the latter locality, the Huckleberry Ridge ash 

bed and the tuffs of Emigrant Pass are both present between 

the tuffs of Blind Spring Valley and the lower tuffs of Glass 

Mountain, but are absent at the Cowan Pumice Mine.

Correlation of the Tuffs of Blind Spring 

Valley to Sites in the Region

Northeastern Fish Lake Valley

A suite of 13 tephra layers intercalated with ancient lake 

beds and alluvial fan gravels are exposed in bluffs near the 

northeastern end of Fish Lake Valley, Nev., about 56 km east 

of the Cowan Pumice Mine (figs. 1, 13). Ten of the tephra 

layers are in the lower, dominantly lacustrine section of tuffaceous mudstones, siltstones, and sandstones, which provide 

evidence of a former lake in the now dry to marshy valley 

(Reheis and others, 1993). Nine of these lower tephra layers (FLV-180-EP through FLV-183-EP; FLV-185-EP through 

FLV-188-EP; and FLV-211-EP; table 3) are chemically and 

petrographically identical to the tuffs of Blind Spring Valley at 

the Cowan Pumice Mine (Pits 2 through 5; TTC-4 through 6; 

TTC-8 through 13; TTC-15 through 18; and, except for minor 

differences in MnO concentrations, TTC-19 through TTC-22 

at pit 6)(table 1). The tenth tephra layer of the lower set at Fish 

Lake Valley (FLV-184-EP) is chemically similar to the tuffs of 

Blind Spring Valley, but has an anomalously higher calcium 

content. A correlative of this tephra layer has not been found 

at the Cowan Pumice Mine or elsewhere.

Stratigraphically upward in the section from the lower 10 

tephra layers, there is a transitional interval of shallow-water 

lake beds with thin gravel beds, in turn overlain by alluvial 

fan gravel and poorly sorted sand beds. Within the dominantly 

lacustrine part of the transition interval (fig. 13), there is a 

0.5-m-thick, light gray, fine-grained vitric tephra layer (FLV179-EP). The glass composition of this unit matches well with 

that of the Huckleberry Ridge ash bed, erupted from the Yellowstone area of Wyoming and Idaho, about 1,600 km to the 

northeast (Izett, 1981; Sarna-Wojcicki and others, 1980, 1984; 

table 3). We conclude that this tephra layer is the Huckleberry 

Ridge ash bed, on the basis of the chemical composition of the 

glass shards, glass shard morphologies, the stratigraphic position of this bed, close above the tuffs of Blind Spring Valley, 

and the correlated ages of the underlying tuff of Blind Spring 

Valley and of the overlying older tuffs of Glass Mountain.

Conventional K-Ar ages determined previously on the 

Huckleberry Ridge ash bed are ~2.02 to 2.00 Ma (Christiansen 

and blank, 1972; Izett, 1981; Izett and Wilcox, 1982; J. D. 

Obradovich, written commun., 1986). In this study, we have 

determined an age of 2.063±0.007 Ma on sanidine feldspar 

grains from a more proximal exposure of the Huckleberry 

Ridge ash bed in the Yellowstone source area (appendix 1). 

Sanidine grains present in the Huckleberry Ridge ash bed near 

Emigrant Pass are too fine grained to date directly. Using the 

somewhat older monitor value for the Tuff of Taylor Creek of 

Renne and others (1998), the age of the Huckleberry Ridge 

ash bed recalculates to ~2.09 Ma.

Above the Huckleberry Ridge ash bed near Emigrant 

Pass, within the interbedded gravel and sand, are two lenticular tephra layers that we here refer to as the (informal) tuffs 

of Emigrant Pass (FLV-189-EP; FLV-190-EP). These tuffs are 

transitional in glass composition between the tuffs of Blind 

Spring Valley (below) and the lower tuffs of Glass Mountain 

(above). Thus, the tephra of the tuffs of Emigrant Pass was 

probably also erupted from the vicinity of Glass Mountain, 

as were those of the tuffs of Blind Spring Valley and Glass 

Mountain. Neither the tuffs of Emigrant Pass nor the Huckleberry Ridge ash bed have been found at the Cowan Pumice 
0

20

40

60

100

120

140

 Bishop ash (lapilli), reworked, FLV-57-CS, FLV-163-CS

5-15-cm-thick beds

FLV-162-CS

paleosol?

FLV-161-CS

FLV-160-CS

FLV-147-CS

fan gravel; ~20 m of section omitted 

FLV-190-EP

FLV-189-EP

current features

FLV-179-EP

FLV-188-EP

Thin altered tuff layers

FLV-211-EP

FLV-187-EP

ripple marks common

FLV-186-EP

FLV-185-EP

FLV-184-EP

mixed tephra and

angular gravel;

subaqueous slump?

FLV-183-EP

FLV-182-EP; ripple marks on top

FLv-181-EP; ripple marks on top

FLV-180-EP; ripple marks on top

Section at site 179-EP section at site 163-CS

Lacustrine Lacustrine Lacustrine ?

coarsens upward

Section at site 57-CS

Meters 

 tuffs of Emigrant Pass

 tuffs of Blind Spring Valley

Huckleberry Ridge ash

Lithology Explanation younger ash beds of Glass Mountain

Angular, coarse

 gravel

Pumiceous sand

 and fine gravel

Pebbly sandstone

Well sorted, tuffaceous

 sandstone

Interbedded mudstone

 and thin tuffaceous

 sandstone

Gypsiferous mudstone 

Tephra layer

Figure 13. Stratigraphy of section near Emigrant Pass, northeast Fish Lake Valley, Nevada. Slightly 

modified from Reheis and others (1993).

Correlation of the Tuffs of Blind Spring Valley to Sites in the Region 29
30 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

Mine, however. They were thus either not deposited at that site 

or were eroded after deposition. Stratigraphicaly higher up in 

the fan gravels above the tuffs of Emigrant Pass in northwestern Fish Lake Valley, south of the above-mentioned exposure, 

are several tephra layers that match on the basis of glass chemical composition with the tuffs of Glass Mountain (FLV-147-

CS; FLV-160-CS through FLV-162-CS; Reheis and others, 

1991). We cannot determine on the basis of glass composition 

whether these layers belong to the lower or upper tuffs of 

Glass Mountain. Their age is only loosely constrained between 

0.8 and 1.96 Ma on that basis, but magnetostratigraphy in the 

Willow Wash section to the south helps to constrain the ages 

of some of these layers more closely (see below). Above the 

tuffs of Glass Mountain, interbedded with alluvial fan gravels, 

is a 15-m-thick nearshore or beach deposit containing multiple 

thin layers of reworked Bishop pumice and ash (FLV-57-CS 

and FLV-163-CS)(Reheis and others, 1991; 1993).

Willow Wash Section 

The Willow Wash section is south of Fish Lake Valley, 

in east-central California, about 80 km to the southeast of the 

Cowan Pumice Mine and about 100 km southeast of Glass 

Mountain. The composite section is more than 800 m thick 

and consists of six linked, measured sections in fluvial, debrisflow, and lacustrine deposits (Reheis and others, 1991; Reheis, 

1992)(figs. 1, 14, 15). The area has been mapped in detail 

because of its stratigraphic and tectonic setting adjacent to the 

northern Furnace Creek Fault Zone, and the sections have been 

intensively studied because of the presence of numerous air-fall 

and reworked tephra layers—more than 55 layers have been 

sampled and analyzed. The three measured sections in the upper 

500 m of the Willow Wash section are late Pliocene in age; from 

bottom to top, they are called the Playa, Rimrock, and Badlands 

sections (fig. 15). At the base of the lacustrine deposits of the 

Playa section are two tephra layers (FLV-96- and -97-WW) identified as the ~3.4-Ma Putah and ~3.3 Ma Nomlaki Tuff Members 

of the Tehama Formation (Reheis and others, 1991; fig. 15).

About 175 m above the Putah and Nomlaki Tuff layers, at 

the top of the Playa section and near the base of the Rimrock 

section, are two closely spaced tephra layers (FLV-22- and 

-23-WW) that correlate chemically with the tuff of Benton Hot 

Springs, the basal tephra layers at the Cowan Pumice Mine 

(TTC-26A, B, 2.81 Ma; tables 1 and 4; fig. 7, 14, 15). We have 

obtained a direct 40Ar/39Ar laser-fusion age of 2.89±0.03 Ma 

on sample FLV-23-WW within the Rimrock section. This age 

is very similar, within the range of analytical errors, to that 

obtained on the tuff of Benton Hot Springs, 2.81±0.02 Ma, at 

the Cowan Pumice Mine (appendix 1).

We have attempted to obtain direct 40Ar/39Ar laser-fusion 

ages on feldspars contained in several tephra layers within 

the Willow Wash section. Most of these layers contained very 

small feldspar crystals that were too small to date successfully 

by this method. Attempts were made to pick out the coarsest 

feldspar crystals present in these layers, regardless of whether 

they had glass coatings or not, but most of these grains turned 

out to be detrital contaminants from older units (appendix 3).

Seven more tephra layers are present within an interval 

140 m thick above the tuff of Blind Spring Valley in the Rimrock section. Six tuff layers of this set (FLV-24-through -29-

WW) correlate chemically with nine tephra layers (FLV-32- 

through -34-, -121-, -35- through -37-, -138-, and -38-WW) in 

the nearby Badlands section (table 4 fig. 15). Because of the 

chemical similarity of their glass shards, it is not possible to 

correlate individual tephra layers between these two sections 

on the basis of EMA alone, but the tuffs can be correlated as 

a chemically relatively homogenous group between the two 

sections. We refer to this set of tephra layers as the (informal) 

tuffs of the Badlands. This set of tephra layers, like the tuff(s) 

Figure 14. Tuff of Benton Hot Springs, near base of Rimrock 

section at Willow Wash, California-Nevada. This tuff is referred 

to informally as the “Rimrock ash bed” in this area (Reheis and 

others, 1991). The tuff is dated here as 2.87±0.03 Ma by 40Ar/39Ar 

laser-fusion method. At the Cowan Pumice Mine in Blind Spring 

Valley, the same tuff, as determined by chemical correlation, is 

dated as 2.81±0.02 Ma by the same method. View to west. See 

also figure 15, showing the stratigraphy at this locality, and figure 

19, showing correlation of the Willow Wash section with other 

localities in the region.
/LDUVAI

2EUNION

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-AGNETOSTRATIGRAPHY 3TRATIGRAPHY

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"ADLANDS3ECTION

2IMROCK3ECTION

0LAYA3ECTION

.OMLAKI4UFF-EMBER4EHAMA&M	^-A

0UTAH4UFF-EMBER4EHAMA&M	^-A

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4UFFSOF'LASS-OUNTAIN

4UFFSOF%MIGRANT0ASS

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0EBBLYCROSSBEDDEDSANDSTONE

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-UDSTONEWITHINTERBEDDEDSANDSTONE

,ITHOLOGY

.ORMALMAGNETICPOLARITY

2EVERSEDMAGNETICPOLARITY

-AGNETOSTRATIGRAPHY

4EPHROSTRATIGRAPHY

.
0OSITIONOFSAMPLEINSECTIONMAGNETIC

ORIENTATIONNORMAL

2
0OSITIONOFSAMPLEINSECTIONMAGNETIC

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4EPHRASAMPLE

4EPHRALAYERSCORRELATEDBETWEENSECTIONS

-ETERS







Figure 15. Stratigraphy of sections at Willow Wash, south of Fish Lake Valley, California-Nevada.

Correlation of the Tuffs of Blind Spring Valley to Sites in the Region 31
32 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

of Benton Hot Springs, bears a family resemblance to the tuffs 

of Blind Spring Valley at the Cowan Pumice Mine.

A magnetic polarity transition, from normal below to 

reversed above, is present within the stratigraphic interval 

containing the tuffs of the Badlands, in both the Badlands and 

Rimrock sections, and was identified as the Gauss Normal 

Polarity Chron-Matuyama Reversed Polarity Chron boundary, on the basis of the correlated ages of the tephra layers in 

these sections (Reheis and others, 1991). This additional datum 

allows us to divide the tuffs of the Badlands into lower and 

upper sets. The lower set in the Badlands section (FLV-24-, -25-) 

and in the Rimrock section (FLV-32- through -34-, -121-, and 

-35-WW) is bracketed in age between ~2.89 Ma, below, and the 

Matuyama-Gauss polarity chron boundary, 2.57 Ma (McDougall and others, 1992), above. The upper set in the Badlands 

section (FLV-26- through -29-WW) and in the Rimrock section 

(FLV-37-, -138-, and -38-WW) is bracketed between 2.57 Ma, 

below, and the correlated age of the tuffs of Blind Spring Valley, 

~2.22 Ma, present higher in the section (fig. 15; table 4)

Many tephra layers are present within alluvial fan gravels 

above the tuffs of the Badlands. FLV-39- through -43-WW 

and FLV-122- and -123-WW correlate with the tuffs of Blind 

Spring Valley at the Cowan Pumice Mine and are thus ~2.22 

to 2.14 Ma. FLV-43-WW is a single, normally magnetized 

unit within reversely magnetized sediments of the Matuyama 

Reversed Chron, and thus it was probably deposited during 

one of the Reunion Normal magnetostratigraphic events. The 

Huckleberry Ridge ash bed has not been found in this section, 

but from our tephra correlations, we would have expected 

it to be situated above FLV-43-WW; the next higher layers, 

FLV-44- and -45-WW, correlate with the tuffs of Emigrant 

Pass, which closely overlie the Huckleberry Ridge ash bed in 

northeast Fish Lake Valley (figs. 13, 15; summary correlations 

shown in figs. 19, 21; table 4). Highest in the Badlands section 

is a set of six tephra layers that correlate well on the basis of 

their glass chemistry with the lower tuffs of Glass Mountain 

(FLV-46- through -48-WW, and FLV-124- through -126-WW). 

For example, FLV-47- and -48-WW match well with TTC-23 

and -24, the lower tuffs of Glass Mountain at the Cowan Pumice Mine (tables 1 and 4; figs. 7, 8, 15).

A reversed-to-normal paleomagnetic transition is present 

between tephra layers FLV-46-WW and FLV-47-WW and 

is identified as the basal boundary of the Olduvai Normal 

Subchron. The age of this subchron was recently estimated as 

between 1.98 and 1.96 Ma for the lower boundary and 1.79 to 

1.77 Ma for the upper (McDougall and others, 1992; Obradovich and Izett, 1992). Thus, FLV-46-WW must be younger than 

~2.06 Ma, the age of the Huckleberry Ridge ash bed, but older 

than ~1.97 Ma, the age of the base of the Olduvai Normal 

Subchron. The five highest tephra layers in the Badlands section, all normally magnetized, must then be bounded in age by 

the Olduvai Normal Subchron (~1.97 to ~1.78 Ma).

Yellowjacket Canyon, Chalfant Valley, 

East-central California

The tuffs of the Badlands section at Willow Wash are correlated to Yellowjacket Canyon, several kilometers southeast of 

the Cowan Pumice Mine. At Yellowjacket Canyon, three tephra 

layers are present in a stratigraphic section of fine-grained alluvium about 175 m thick (YJC-2, -3, and -4). On the basis of their 

stratigraphic position and electron-microprobe analysis of the 

glass, the lower two layers correlate with the older set mentioned 

above (~2.84 to ~2.57 Ma), while the uppermost third layer correlates with the younger set (~2.57 to ~2.22 Ma) and not with the 

“Tuff of Taylor Canyon” of Izett and others (1988; our tuffs of 

Blind Spring Valley). Near the head of Yellowjacket Canyon, the 

sequence is unconformably overlain by the Bishop Tuff (YJC-1; 

fig. 16, and summary correlations in figs. 19 and 21).

Figure 16. Bishop Tuff (ash-fall 

and ash-flow deposits) near head of 

Yellowjacket Canyon, east-Central 

California (see fig. 2), where sample 

YJC-1 was obtained. Angular clasts 

of local bedrock are entrained in the 

ash flow at the level of the geologist’s left hand and also higher in the 

unit, near top.
Waucoba Lake Beds, Near Big Pine, 

Owens Valley, California

Tephra layers correlative with the tuffs of Blind Spring 

Valley and Glass Mountain are present in ancient lake beds 

and overlying gravel exposed on the eastern side of Owens 

Valley, about 6 km northeast of the town of Big Pine, about 

70 km south of the Cowan Pumice Mine (figs. 1, 17; table 6). 

The layers are known from three localities in a triangular area, 

separated from each other by about 3 km, in the Waucoba 

Beds of Hopper (1947). A few of the layers appear to be clean, 

pure vitric ash beds, either direct airfall or water lain, but most 

beds are impure and probably reworked.

At one locality, near Westgard Road, two tephra layers 

were sampled from a sequence of lacustrine silts containing 

some sandy and pebbly beds (R.L. Hay, written commun., 

~1980). On the basis of electron-microprobe data, the lower 

of the two (W2A) matches well with the tuffs of Blind Spring 

Valley at the Cowan Pumice Mine (TTC-21) and with tuffs of 

Blind Spring Valley in the Emigrant Pass section of Fish Lake 

Valley, beneath the Huckleberry Ridge ash bed. Comparing 

EMA data, the upper of the two (W3A) matches equally well 

with tuffs just below the Huckleberry Ridge ash bed (tuffs of 

Blind Spring Valley) and with tuffs just above it (tuffs of Emi1 Km

!

Massive, silt-sized tephra layer; sharp upper and

 basal contacts

Well-bedded lacustrine silts, mostly non-volcanic

Medium sand-sized pumice tephra layer; massive

Well-bedded lacustrine silts

Very fine, well-bedded, uniform laminae of tuff and

 silty tuff with partings of silt. Upper half is tran-

 sitional into lacustrine silts; sharp basal contact

Well bedded, lacustrine silt, mostly non-volcanic

Fine pumice lapilli to 3 mm at base, overlain by fine

 tephra layer interbedded with fine pumice lapilli

 tephra, gradational upward into lacustrine silt;

 sharp basal contact.

Well-bedded lacustrine silt, mostly non volcanic

Fine, massive pumice lapilli tuff, lapilli 2-3 mm

 longest axis, at base; overlain by coarse pumice-

 lapilli tuff, 13 cm thick, with lapilli to 1.3 cm l.a.,

 and intebeds of fine tephra and tuffaceous or silty

 tephra, 0.6-1.3 cm thick. Gradational upward;

 sharp basal contact. 

WAC-7

WAC-6A

WAC-6

WAC-5

WAC-4

5

4

3

2

1

0

" Meters

Figure 17. Locality map (A) and 

stratigraphic section (B) showing positions of tephra layers and 

samples in the Waucoba Lake 

Beds, north of Big Pine, Owens 

Valley, Calif.

Correlation of the Tuffs of Blind Spring Valley to Sites in the Region 33
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K

Tuff of Emigrant Pass (W3A), and tuffs of Blind Spring Valley (?) (W2A) in lacustrine silts, locality 1, near Westgard Road W3A 77.5 12.4 0.65 0.03 0.09 0.32 0.05 3.61 5

W2A 77.4 12.5 0.60 0.03 0.08 0.34 0.08 3.30 5

Tuffs of Blind Spring Valley in lacustrine silts and sands (WAC-7, WAC-6), and uncorrelated tephra layers (WAC-5 and WAC-4), locality 2, along Waucoba Road; WAC-4 is altered WAC-7 77.2 12.8 0.59 0.03 0.09 0.37 0.06 4.16 4

WAC-6 77.2 12.7 0.60 0.04 0.06 0.34 0.09 3.98 4WAC-5 77.1 12.8 0.63 0.04 0.05 0.39 0.08 3.96 4WAC-4 74.9 16.8 1.89 5.53 0.00 0.82 0.18 0.21 0

Tuffs of Blind Spring Valley (DG-1, DG-3a?, DG-4), and uncorrelated tephra layer, possibly tuff of Emigrant Pass , (DG-2) in section transitional from 

and sands (below) to alluvial fan gravels (above), locality 3, along Waucoba Road DG-1 76.9 13.0 0.53 0.03 0.10 0.34 0.06 4.27 4

DG-2 76.9 12.9 0.68 0.03 0.06 0.34 0.05 4.30 4DG-3a 76.9 12.9 0.63 0.02 0.09 0.31 0.05 4.45 4DG-4 77.0 13.0 0.56 0.04 0.09 0.33 0.08 4.00 4

Table 6. Results of electron-microprobe analysis of glass from tephra layers in exposures of the Waucoba lake beds, east side of Owens Valley, ~6 [Samples of tephra layers are arranged according to stratigraphic position in each section, with youngest at top. Values given are weight-percent oxide, recaOriginal oxide totals before recalculation are given to indicate approximate degree of hydration of volcanic glass. About 15 or more individual glass shards sample. Electron-microprobe chemical analyses by C. E. Meyer.]
grant Pass). Some variation in the alkalies and iron preclude 

a more definitive correlation on the basis of EMA analysis. 

Results of INAA analysis of glass of this tephra layer, however, 

correlate it with the upper of two tephra layers overlying the 

Huckleberry Ridge ash bed in the Manix basin of the Mojave 

Desert (see the summary correlation diagrams, figs. 19, 21) and 

thus favor correlation of W3A with the tuffs of Emigrant Pass 

or the lower tuffs of Glass Mountain, both sets of which overlie 

the Huckleberry Ridge ash bed in northern Fish Lake Valley.

At a second locality, along Waucoba Road, five tephra 

layers are exposed in lacustrine silt and sand beds within 

~5 m of section (fig. 17). The basal layer (WAC-4) is an 

altered pumice lapilli tuff, unusually enriched in magnesium and depleted in alkalies (table 6). This layer does not 

match with any other layer we have studied, most likely 

because of its advanced degree of alteration. It is overlain by 

another pumice lapill tuff (WAC-5) that is somewhat similar 

in overall composition to the Bishop ash bed, although it 

has considerably less iron. This tuff must be older that the 

Bishop ash bed, however, as is indicated by the correlation of 

other tephra layers in this section. Above this tuff (WAC-5) 

is a fine-grained vitric tuff (WAC-6) that matches best with 

the tuffs of Blind Spring Valley (TTC-20, TTC-22). A thin, 

impure tuff overlying WAC-6 was not analyzed. At the top of 

the section is a massive, silty, fine-grained vitric tuff (WAC7; fig. 17) that also matches well with the tuffs of Blind 

Spring Valley (TTC-4, -5, and others).

In a third area southeast of the second locality, four 

tephra layers are exposed along a 5-km length of Waucoba 

Road. These tephra layers are presumed to be in stratigraphic order in a gently westward-dipping section of fan 

gravel that ovelies the Waucoba lake beds, but the section is 

poorly exposed and it is possible that part of the section is 

repeated by unrecognized faults. The lowermost of the four 

tuffs (DG-4), at the top of the lacustrine section, is directly 

overlain by fan gravel. This tuff matches well with the tuffs 

of Blind Spring Valley at the Cowan Pumice Mine (TTC-6) 

and in northeastern Fish Lake Valley near Emigrant Pass 

(FLV-185-EP). The next tuff up in the section (DG-3a), 

within the overlying fanglomerate, also matches the tuffs 

of Blind Spring Valley (for example, TTC-6). The third tuff 

(DG-2) is chemically similar to some of the uppermost tuffs 

at Willow Wash, within the Badlands section, but there are 

no secure matches. The highest tuff in this section (DG-1) 

again matches very well with the tuffs of Blind Spring Valley 

(TTC-17, -9, -9A) and correlative units at Willow Wash (for 

example, FLV-40-WW; tables 4, 6; see the summary correlation diagrams, figs. 19, 21).

Most of the tuffs in the Waucoba beds appear to correlate best with the tephra layers of Blind Spring Valley, thus 

placing them within an age range of ~2.22 to ~2.14 Ma, and 

possibly with the tuffs of Emigrant Pass, ~2.05-2.00 Ma, but 

the correlation of the remainder is uncertain. New work indicates that the Waucoba Lake Beds are overlain by fan gravels 

containing the Bishop ash bed (~0.76 Ma), and additional new 

work in the nearby Black Canyon indicates that a section very 

similar to that found at the Emigrant Pass section (see above) 

is present in that area (unpublished data, E. Kirby, A. SarnaWojcicki, M. Reheis, and D. Burbank, 2002).

According to Christopher Pluhar (University of California, Santa Cruz, written commun., 2004), all laustrine samples 

analyzed by him for paleomagnetic orientation from north 

of Waucoba Road in this region were reversed, and thus they 

must be older than 0.78 Ma, and also not of Olduvai age.

Correlation of the Tuffs of Blind Spring 

Valley and Related Tephra Layers to 

Distal Sites in the Western United States

Fine-grained tephra layers of the same chemical composition and age as the tuffs of Blind Spring Valley, and stratigraphically related tephra layers, are identified over a large 

area (~200,000 km2; fig. 1) in the western United States. There 

are no other known volcanic source areas of both similar age 

and composition in western North America; thus these finegrained layers can be safely inferred to correlate with the tuffs 

of Blind Spring Valley and its stratigraphically related layers. 

Taken together, these layers provide unique and identifiable 

chronostratigraphic reference datum levels for late Pliocene 

time in the western conterminous United States.

Confidence Hills, Death Valley, Southeastern 

California

Deformed lake beds of silt, clay, mud, and sand intercalated with gravel, gypsiferous beds, and halite in the Confidence Hills of southern Death Valley, southeastern California 

(Troxel and Butler, 1986; Troxel and others, 1986; Pluhar and 

others, 1992; Beratran and Murray, 1992), contain multiple 

tephra layers that correlate with tephra layers in several other 

sections discussed above and below (figs. 1, 18; table 7).

Samples of the basal tephra layers in the Confidence 

Hills (CHA86BT-1; CH88004 and CH88006) correlate on the 

basis of glass composition and stratigraphic position with the 

tuffs of Blind Spring Valley at the Cowan Pumice Mine. Good 

matches are also obtained, of course, with our identified layers 

of the tuffs of Blind Spring Valley in the Willow Wash section 

south of Fish Lake Valley (FLV-39 through 43-WW and FLV122, 123-WW) (fig. 15; tables 4, 7), and elsewhere.

Above the basal tuffs at the Confidence Hills locality is the 

Huckleberry Ridge ash bed (CHA86 BT-2; CH88007; table 7). 

This ash bed (actually an indurated tuff at this locality) is used 

as the reference point for correlation of the measured sections 

of Troxel and others (1986) and B. W. Troxel (written commun, 

1988) with that of Beratran and Murray (1992)(fig. 18). Note 

that the laser-fusion 40Ar/39Ar age we obtain for the Huckleberry 

Ridge ash bed is 2.056±0.007 Ma, and Pluhar and other’s (1992) 

age estimate of this tephra layer, based on their magnetostratigCorrelation of the Tuffs of Blind Spring Valley and Related Tephra Layers to Distal Sites in the Western United States 35
36 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

r

)

l

Figure 18. Magnetostratigraphy, stratigraphy, and tephrostratigraphy of a section exposed in the Confidence Hills, Death Valley National Park, California. Magnetostratigraphy determined by Pluhar and others 

(1992) in “Confusion Canyon” of Holt and Kirschvink (1995), and tephrostratigraphy of the same section, 

from Beratan and Murray (1992). The Huckleberry Ridge ash bed, sample 07, is common to both measured 

sections (Beratran and Murray, 1992; Troxell and others, 1986; this report) and is used as a reference for 

correlating the two sections and other tephra layers. Solid horizontal lines in tephrostratigraphy column 

are tephra layers. Dashed lines are time horizons connecting the same tephra layers or magnetostratigraphic boundaries. Sample numbers of tephra layers are prefixed by “CH880”; their analyses are given 

in table 7. Tuffs 02, 03, 05, 15, 22, 23, and 24 are altered and were not analyzed. Age control points used 

in interpolation of ages of undated horizons: top of the Olduvai Normal subchron—1.78 Ma; base of the 

Olduvai subchron—1.96 Ma; Huckleberry Ridge ash bed—2.09 Ma (our age of 2.057±0.008 Ma, multiplied by 

1.0142 to convert to the more commonly used mineral standard MMhb-1 of 520.4 Ma given in Sampson and 

Alexander, 1987); altered tuff 02—2.25 (?) Ma (age is based on the assumption that tuffs 02, 03, and 05 correlate, like samples 04 and 06, with the tuffs of Blind Spring Valley; we use the age of the lowermost tephra 

layer of the tuffs of Blind Spring Valley, 2.219±0.006 Ma, multiplied by 1.0142, as above).
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O 

Tuffs of Glass Mountain , undifferentiated (north side of gully)

CH88021 77.1 12.7 0.75 0.04 0.05 0.38 0.05 3.69 5.17 CH88020 77.1 12.7 0.77 0.03 0.03 0.41 0.07 3.69 5.25 CH88018A 77.1 12.7 0.74 0.03 0.04 0.35 0.06 3.70 5.24 CH88017 77.1 12.8 0.75 0.03 0.04 0.36 0.05 3.82 5.05 CH88016 77.0 12.9 0.74 0.03 0.03 0.36 0.05 3.65 5.23 

Tuffs of Glass Mountain , undifferentiated (south side of gully)

CH88014 76.8 12.9 0.76 0.02 0.07 0.32 0.06 3.71 5.32 CH88013 76.8 13.1 0.73 0.03 0.05 0.32 0.06 3.53 5.46 CH88012 76.8 13.1 0.71 0.02 0.07 0.30 0.05 3.68 5.25 

Lower Tuffs of Glass Mountain (north side of gully)

CH86G9 79.0 11.7 0.72 0.05 0.03 0.44 0.06 3.26 4.88 CH86G8 78.8 11.7 0.70 0.04 0.04 0.44 0.06 3.20 5.11 CH86G6 78.7 11.9 0.70 0.03 0.04 0.38 0.05 3.21 4.98 CH86W5 78.1 12.4 0.72 0.04 0.04 0.38 0.05 3.34 4.96 

Tuffs of Emigrant Pass (south side of gully)

CH88011 76.9 13.0 0.69 0.02 0.08 0.30 0.06 3.82 5.17 CH88010 76.8 13.1 0.64 0.02 0.09 0.29 0.05 3.84 5.14 CH88009 76.9 13.0 0.67 0.02 0.08 0.29 0.06 3.83 5.14 CH86BT-3 77.8 12.4 0.64 0.02 0.09 0.29 0.06 3.53 5.23 

Tuff of Confidence Hills (south side of gully)

CH88008 70.9 15.6 2.71 0.28 0.09 0.96 0.29 5.06 4.06 CH86GA4 72.4 14.9 2.57 0.24 0.08 1.02 0.30 4.71 3.86 

Huckleberry Ridge ash bed (south side of gully)

CH88007 75.9 12.8 1.79 0.02 0.03 0.59 0.12 3.61 5.09 CH86BT2 77.1 12.0 1.71 0.01 0.05 0.59 0.11 3.33 5.17 

Tuffs of Blind Spring Valley (south side of gully; CH88005 is an altered tuff, and was not analyzed)

CH88006 76.7 13.1 0.62 0.03 0.07 0.36 0.07 3.77 5.28 CH86BT1 77.8 12.4 0.58 0.02 0.09 0.33 0.06 3.55 5.17 CH88004 76.7 13.2 0.64 0.05 0.09 0.34 0.08 3.80 5.13 

Table 7. Glass chemical composition of tephra layers exposed in deformed alluvial and lacustrine sediments of the Confidence Hills, southern Deand others, 1986; this report).[Samples of tephra layers are arranged according to stratigraphic position in section (from uppermost to lowermost). Electron-microprobe chemical analyses by C. E. Meyer18) were not analyzed.]
38 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

raphy, is 2.09 Ma. The difference between their chronology and 

ours, about 34 k.y., may result from the different values used 

in the monitors for the age calibration of their magnetostratigraphic datum levels, and the 40Ar/39Ar ages we present here. 

If we multiply our age for the Huckleberry Ridge ash bed by 

1.0143, the correction factor between the two commonly used 

hornblende monitor values (Pringle and others, 1992; Renne 

and others, 1998; Sarna-Wojcicki and others, 2000), we obtain 

2.086 Ma, very close to the 2.09-Ma interpolated age for the 

Huckleberry Ridge ash bed of Pluhar and others (1992).

Overlying the Huckleberry Ridge ash bed is a tuff 

(CH88008, CHA86-GA4) having a glass chemical composition that we have not seen before in association with the 

Huckleberry Ridge ash bed or the tuff of Taylor Canyon. We 

informally name this tephra layer the tuff of Confidence Hills. 

The age of this tuff is estmated to be ~2.04±0.02 Ma, based 

on the age of the underlying Huckleberry Ridge ash bed, our 

isotopic chronology as correlated to the Confidence Hills, the 

overlying correlated tuffs of Emigrant Pass, and the corroborating magnetostratigraphy of Pluhar and others (1992). The 

tuff of Confidence Hills is derived from the Cascade Range 

volcanic province, as indicated by its chemical composition: 

high aluminum, iron, magnesium, calcium, and titanium concentrations, compared to tephra layers derived from the Glass 

Mountain and Yellowstone source areas.

Above the tuff of Confidence Hills are three tephra 

layers (CHA86 BT-3, which is the same layer as CH88009; 

CH88010 and CH88011) that match chemically with the tuffs 

of Emigrant Pass (FLV-189, 190-EP) near Emigrant Pass and at 

Willow Wash (FLV-44-, -45-, -46-WW) (figs. 13, 15, 18; tables 

3, 4, 7). Because of their chemical similarity to the basal tephra 

layer in the sequence at the Confidence Hills, Troxel and others 

(1986) had previously speculated (after a suggestion by A.M. 

Sarna-Wojcicki) that these layers might represent a repetition 

of the basal layer (CHA86BT-1, CH8004) by isoclinal folding (the beds in the Confidence Hills are vertical). The new 

chemical correlations and associated chronology, however, 

when compared with the stratigraphic sequence, chemical 

compositions, and ages of tephra layers at other sites discussed 

here, provide a better match and more satisfactory explanation 

for the sequence of chemical types in the Confidence Hills 

and rule out the possibility of isoclinal folding. The above 

notwithstanding, the tuffs immediately above and below the 

Huckleberry Ridge ash bed in the Confidence Hills and Emigrant Pass sections are very similar in composition and hard to 

distinguish on the basis of EMA analysis alone. For example, 

compare CH88004, which underlies the Huckleberry Ridge ash 

bed, with CH88009, CH88010, and CHD88011, which overlie 

it (table 7). The only differences are in the concentrations of 

magnesium and calcium, and these are slight and not definitive 

within the error of EMA analysis. Follow-up analyses of these 

tuffs by a more precise technique for minor and trace elements, 

such as INAA or ICP-MS (inductively coupled plasma mass 

spectroscopy), are desirable, to test for possible significant differences in other trace and minor elements in order to distinguish these layers with confidence.

Above the tuffs of Emigrant Pass at the Confidence 

Hills are several younger tephra layers. One set of samples 

from these was collected in 1986 (CH86-WA5, CH86-GA6, 

CHA86-GWA7, CH86-GWA8, CHA86-GA9), another in 1988 

(CH88012 through CH88014; CH88016 through CH88018-A; 

and CH88020 and CH99021). These tephra layers appear to 

grade, in terms of the chemical composition of their volcanic 

glasses, from tuffs that resemble the tuffs of Emigrant Pass, 

to tuffs that match in composition with the uppermost tephra 

layers in the Badlands section at Willow Wash, near Fish Lake 

Valley—that is, with the lower tuffs of Glass Mountain. For 

example, tephra layers CH88012 through CH88014 match 

well with tephra layers FLV-47-WW, -48-WW, and -125-WW 

at Willow Wash. These tephra layers undoubtedly all correlate 

with the tuffs of Glass Mountain. The lower of these tuffs at 

Confidence Hills are most similar to the lower tuffs of Glass 

Mountain at the Cowan Pumice Mine and to similar layers 

that are found above the tuffs of Emigrant Pass, in gravels that 

overlie the playa section containing the Huckleberry Ridge 

ash bed, in northeast Fish Lake Valley (figs. 8, 13). Combining the data from Troxel and others (1986), Pluhar and others 

(1992), and new analyses of the upper part of the Confidence 

Hills section presented here, the base of the Olduvai Normal 

Subchron within the Confidence Hills section is located above 

CH88011, which is reversed, and below CH88012, which is 

normal. As mentioned above, within the Willow Wash section 

the base of the Olduvai Normal Subchron is situated above 

FLV-46-WW, which is reversed, and below FLV-47-WW, 

which is normal (Reheis and others, 1991; figs. 15, 18).

Although the uppermost tephra layers in the Confidence 

Hills section (CH88012 through CH88021) and in the Willow 

Wash section (FLV-47-WW through FLV-126-WW) resemble 

the middle and upper tephra layers of Glass Mountain at Chalk 

Cliffs (BT-14, BT-15, and BT-16, ranging in age from ~1.2 

to 0.8 Ma; table 2, figs. 12, 15, 18), as well as the Bishop ash 

bed (TTC-7, ~0.76 Ma; table 1), they cannot be younger than 

about ~1.78 Ma, because they lie entirely within the Olduvai Normal Chron. Samples of nine additional tephra layers 

were collected throughout the Confidence Hills section in 

1988 (CH88001 through CH88003; CH88005; CH88015; 

CH88019; and CH88022 through CH88025; fig. 18), but the 

volcanic glasses of these turned out to be altered and were not 

analyzed.

Lake Tecopa, Southeastern California

The Huckleberry Ridge ash bed is present in beds of 

ancient Lake Tecopa, east of Death Valley, in southeastern 

California (Sarna-Wojcicki and others, 1980, 1984; Izett, 

1981). At most localities in Lake Tecopa, this tephra layer 

(Ash C of Sheppard and Gude, 1968) is altered to zeolites 

or authigenic potassium feldspar and cannot be identified 

by the methods used in this report. Only at one site, where 

this tephra layer has been uplifted within a small horst above 

correlative beds, has the unaltered Huckleberry Ridge tephra 
layer been identified (Sarna-Wojcicki and others, 1980, 1984, 

1987). Thin, white, altered tephra layers stratigraphically 

close below the Huckleberry Ridge ash bed are likely to be 

the tuff of Blind Spring Valley, but unaltered exposures of 

these layers have not been found in this area. Stratigraphically higher above the Huckleberry Ridge ash bed in the Lake 

Tecopa section, both below and above structureless mud beds 

that we interpret to be playa deposits, are thin tephra layers that are chemically similar to the lower and upper tephra 

layers of Glass Mountain, and these are overlain in turn by 

the Bishop ash bed (Sarna-Wojcicki and others, 1980, 1984). 

This sequence is very similar and in large part coeval with the 

upper part of the sequence of tephra layers near Emigrant Pass 

in northeastern Fish Lake Valley, Nev., and in the upper part 

of the Confidence Hills in Death Valley (figs. 13, 18; also see 

summary correlation diagrams, figs. 19, 21). At Lake Tecopa, 

the entire sequence is overlain by the Lava Creek B ash bed, 

erupted from the area of Yellowstone in Wyoming. The age of 

the Lava Creek B ash bed has been most recently revised to 

~0.64 Ma (Lanphere and others, 2002).

Manix Basin, Central Mojave Desert, Southern 

California

Three tephra layers are present in deformed lacustrine or 

playa deposits unconformably beneath late Pleistocene lake 

beds of the Manix basin, about 40 km east of Barstow, in the 

Mojave Desert of southeastern California (Sarna-Wojcicki 

and others, 1980, 1984; Izett, 1981; Izett and others, 1988; 

fig.1). The basal ash bed (MANIX-6), about 3 cm thick, is the 

Huckleberry Ridge ash bed (Sarna-Wojcicki and others, 1984). 

A white ash bed (the middle white ash bed of Sarna-Wojcicki 

and others, 1984; MANIX-7 and -8) above the Huckleberry 

ridge ash bed was previously correlated with the uppermost 

tephra layer at the first of three localities in the Waucoba Road 

area mentioned above (W3A) on the basis of INAA analysis 

of the glass. This tephra layer also correlates well in terms of 

glass chemistry (EMA) and sequence with the tuffs of Emigrant Pass (for example, with FLV-189- and -190-EP; and with 

CHA86BT-3; figs. 13, 18; tables 3 , 8).

Cowan

Pumice Mine

Bishop ash bed,

(covered

 interval)

2.81 Ma 

Bishop ash bed

Tuff(s) of Benton Hot Springs

RIMROCK

SECTION

BADLANDS

 SECTION

PLAYA SECTION

basalt ~3.5 Ma

Lower tuffs of Glass Mountain

Tuffs of Blind Spring

Valley

Tuffs of Emigrant Pass

Fish Lake

Valley

Willow

 Wash

Confidence

Hills

Tuffs of Glass Mountain (?)

(altered)

Lower tuffs of Glass Mountain

Tuffs of Emigrant Pass

Tuff of Confidence Hills

HRAB (Huckleberry Ridge ash bed)

Tuff(s) of Blind Spring Valley 

Tuff(s) of Blind Spring Valley, 

Tuff(s) of Blind Spring Valley (?)

(altered)

HRAB

Upper tuffs of the Badlands

Lower tuffs of the Badlands

Tuff(s) of Benton Hot Springs (Rimrock ash bed) 

 Tuff(s) of Benton Hot Springs,

Nomlaki Tuff

Putah Tuff (Members of the Tehama and Tuscan Formations) 

0.759 Ma

2.87 Ma

3.4 - 3.3 Ma 

2.22 - 2.14 Ma

 Upper tuffs

of Glass Mountain

100 m

Magnetic Polarity (where determined)

Reversed

Normal

Approx. Stratigraphic Thickness

Correlation between specific tuffs

Correlation between groups of tuffs

Correlation between magnetostratigraphic boundaries

Correlations

Figure 19. Correlation of the tuffs of Blind Spring Valley and other stratigraphically related units in the region of Glass Mountain and 

Long Valley, eastern California and western Nevada. See figures 1 and 2 for location of sections.

Correlation of the Tuffs of Blind Spring Valley and Related Tephra Layers to Distal Sites in the Western United States 39
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O 

Lake Tecopa, southeastern California. Lower tuffs of Glass Mountain TECO-23 76.8 13.0 0.77 0.03 0.04 0.37 0.07 3.42 

TECO-31 76.7 13.1 0.73 0.02 0.05 0.39 0.06 3.43 TECO-24 76.6 13.1 0.76 0.03 0.03 0.44 0.07 3.65 

Huckleberry Ridge ash bed TECO-1 76.5 11.9 1.93 0.03 0.03 0.64 0.14 2.85 

TECO-2 76.1 12.5 1.87 0.02 0.03 0.64 0.12 3.46 TECO-12B 76.3 12.4 1.75 0.03 0.05 0.61 0.14 3.46 

Manix Basin, Mojave Desert, Calif. Tuffs of Emigrant Pass (?) MANIX-7 77.9 12.6 0.66 0.03 0.06 0.30 0.05 4.77 

MANIX-8 78.4 12.0 0.68 0.02 0.09 0.34 0.07 4.68 

Huckleberry Ridge ash bed MANIX- 6 76.2 13.0 1.63 0.03 0.03 0.61 0.13 4.07 

Corona, Los Angeles Basin, Calif Tuffs of Blind Spring Valley 85-G-54 77.5 12.7 0.57 0.06 0.06 0.38 0.09 3.69 

Ventura Basin, Calif. Huckleberry Ridge ash bed PICO-157(1) 76.5 12.3 1.73 0.03 0.04 0.59 0.13 3.72 

PICO-157(2) 76.3 12.5 1.78 0.02 0.05 0.60 0.13 3.71 ASW-23-75-1 76.6 12.1 1.83 0.02 0.05 0.60 0.10 3.63 

Tuffs of Blind Spring Valley PICO-163Q 76.9 13.5 0.60 0.04 0.10 0.32 0.07 3.86 

PICO-169Q 77.6 12.6 0.58 0.04 0.10 0.33 0.06 3.87 

Beaver Basin, west-central Utah Lower tuffs of Glass Mountain (Last Chance Bench ash bed, 10 m above Huckleberry Ridge ash bed) LCB-1 77.4 12.6 0.74 0.04 0.04 0.43 0.06 4.04 

Huckleberry Ridge ash bed BEAVER-1 76.5 12.3 1.73 0.02 0.04 0.56 0.09 3.56 

Table 8. Glass chemical composition of tephra layers from several sites in southern and southwestern California and in Beaver Basin, Utah. [Electron-microprobe chemical analyses by C. E. Meyer. See table 1 for additional comments.]
Samples of the above two tephra layers were collected 

from localities on the north bank of the Mojave River. The tuff 

of Emigrant Pass (MANIX-7, 8) is also exposed on the south 

side of the Mojave River (MANIX 4). There, another tephra 

layer is present 12 to 15 m higher in the section (MANIX-2; 

the upper white ash bed of Sarna-Wojcicki and others, 1984). 

Neutron activation analysis indicates that the latter unit is 

broadly similar in glass chemistry to the tephra layers of Glass 

Mountain and the Bishop ash bed (table 8). We have no EMA 

analysis of this latter unit. Given its stratigraphic position, 

close above the tuffs of Emigrant Pass, it is most probably one 

of the tuffs of Glass Mountain.

According to Christopher Pluhar (University of California, Santa Cruz, written commun., 2003), the basal Huckleberry Ridge ash bed and the overlying middle white ash 

are reversely magnetized, while the upper white ash bed is 

normally magnetized. Thus, the middle white ash bed must be 

older than the base of the Olduval subchron, while the upper 

white ash bed is most likely of Olduvai subchron age. 

Peninsular Ranges, California

Lenses of tephra are present in thin marine sediments that 

cover a weathered, eroded surface cut in granitic rocks in the 

northern part of the Peninsular Ranges of southern California, 

in the Rancho Sobrante e San Jacinto area, north of Corona 

(D. Morton, written commun., 1982; fig. 1, table 8; sample 

85-G-54). The lenses of ash are preserved in what appear to 

be ancient tidal pools within a wave-cut marine platform. 

The sediments are overlain by a conglomerate. These tephra 

lenses match well with some tephra layers of the tuffs of Blind 

Spring Valley at the Cowan Pumice Mine (for example, with 

TTC-10), in the Willow Wash section (FLV-123-WW), and in 

the Waucoba lake beds (WAC-6)(tables 1, 4, and 5), and are 

here assigned to the tuffs of Blind Spring Valley.

Ventura Basin, Southwestern Coastal California

The Huckleberry Ridge ash bed has been identified in 

deformed and uplifted beds of the marine Pico Formation at 

South Mountain and Balcom Canyon, in the Ventura Basin 

(fig. 1; PICO-157, ASW-3-23-75-1; table 8; Sarna-Wojcicki 

and others, 1980, 1984, 1987), where the ash is situated 

stratigraphically far below a thin, white ash bed, estimated to 

be 1.5 Ma (Sarna-Wojcicki and others, 1984) and even farther 

below the Bailey ash bed (1.2 Ma, Izett and others, 1974; 

Sarna-Wojcicki and others, 1984). Both latter ash beds are correlated to the tuffs of Glass Mountain, and the Glass Mountain volcanic source area, on the basis of their glass chemical 

composition and age. The Huckleberry Ridge ash bed is 

fine-grained and only about 4 to 8 cm thick in this area. About 

97 m stratigraphically below the Huckleberry Ridge ash bed is 

a 1-cm-thick ash bed (samples PICO-162Q and PICO-169Q) 

that correlates well on the basis of its glass chemical composition with tuffs of Blind Spring Valley (table 8). Although this 

ash bed is stratigraphically far below the Huckleberry Ridge 

ash bed, the sedimentation rates determined for this section 

based on the age and stratigraphic position of datum levels 

higher in the section, above the Huckleberry Ridge ash bed 

(Sarna-Wojcicki and others, 1987), are high, ~0.72 mm/yr. An 

independent age estimate for this 1-cm-thick ash bed below 

the Huckleberry Ridge ash bed, based on the above sedimentation rate and the stratigraphic separation between the two 

units, is 2.19 Ma. This estimate is in good agreement with the 

isotopic age range of tuffs of Blind Spring Valley, supporting 

the identification of the PICO-162Q and –169Q as correlative 

with these tephra layers.

The range of chemical composition of the Huckleberry 

ridge ash bed as determined by EMA is given in table 9. Its 

age of 2.063±0.07 Ma, as determined in this study, is given in 

appendix 1.

Beaver Basin, West-central Utah

According to Izett and others (1988), a tephra layer about 

6 cm thick is present in Pliocene and Pleistocene sediments, 

about 30 m stratigraphically below the Huckleberry Ridge 

ash bed, in the Beaver Basin of west-central Utah (samples 

78G174, 78G167, 79W98, and 79W99 of Izett and others, 

1988; Machette and others, 1984; fig. 1), and this layer correlates well with their Taylor Canyon-C tephra layer at the 

Cowan Pumice Mine (samples 79G10 and 74W60; Izett and 

others, 1988; fig. 20). The INAA analysis of the samples from 

the Beaver Basin provides a convincing match with the tuff 

of Taylor Canyon of Izett and others (1988; our tuffs of Blind 

Spring Valley) at the Cowan Pumice Mine, and also with our 

INAA analyses of samples from the latter locality (TTC-5 

and TTC-4, from pits 3 and 5, respectively; table 10). The 

INAA data, combined with our 40Ar/39Ar ages on sanidine 

from samples TTC-4 and TTC-5 (table 1), provide a correlated 

age of ~2.14±0.01 Ma for this tephra layer. Izett and others 

(1982, 1988) had previously obtained a slightly younger date 

of 2.10±0.02 Ma by conventional K-Ar on sanidine from their 

Taylor Canyon-C layer. The Beaver Basin is ~540 km from the 

inferred source area of this tephra layer in eastern California; 

it is the farthest distal site of the tuffs of Blind Spring Valley 

(tuff of Taylor Canyon of Izett and others, 1988) found to date.

Our analysis of the Huckleberry Ridge ash bed from the 

Beaver Basin (BEAVER-1, table 9) corroborates those of Izett 

and others (1988). Our analysis of a thin, white tephra layer 

situated about 10 m stratigraphically above the Huckleberry 

Ridge ash bed in the Beaver Basin, the Last Chance Bench ash 

bed of Izett and others (1988), matches with the tuffs of Glass 

Mountain (sample LCB-1; tables 8, 9, 10).

Discussion and Conclusions

Tephra layers identified in this study on the basis of their 

glass chemical composition, combined with magnetostratiDiscussion and Conclusions 41
Sample SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O 

Huckleberry Ridge ash bed, reference sample from alluvium, Borchers faunal locality, Meade County, Kan. (1-4) (Izett and Wilcox, 1982, analyzed in this s

Emigrant Pass in northern Fish Lake Valley, Nev. (5); in lake beds, Confidence Hills, Death Valley, Calif. (6, 7); in lake beds, Pleistocene Lake Tecopa, Calif. (8-1

Mojave Desert, Calif. (11); in deep basinal marine sediments, Pico Fm., Ventura Basin, Calif. (12-14); and in lake beds, Beaver Basin, Uta

 1. 68W98(A) 77.0 12.3 1.66 0.02 0.04 0.58 0.10 3.2  2. 68W98(B) 76.7 12.5 1.69 0.02 0.03 0.58 0.10 3.3  3. 68W98(C) 75.9 12.6 1.75 0.02 0.04 0.65 0.11 3.7  4. 68W98(D) 76.7 12.3 1.71 0.02 0.05 0.58 0.12 3.4  5. FLV-179- EP7 76.6 12.1 1.72 0.01 0.04 0.59 0.11 2.8  6. CH88007 75.9 12.8 1.79 0.02 0.03 0.59 0.12 3.6  7. CH86BT2 77.1 12.0 1.71 0.01 0.05 0.59 0.11 3.3  8. TECO-1 76.5 11.9 1.93 0.03 0.03 0.64 0.14 2.9  9. TECO-2 76.1 12.5 1.87 0.02 0.03 0.64 0.12 3.5 10. TECO- 12B 76.3 12.4 1.75 0.03 0.05 0.61 0.14 3.5 11. MANIX- 6 76.2 13.0 1.63 0.03 0.03 0.61 0.13 4.1 12. PICO- 157(1) 76.5 12.3 1.73 0.03 0.04 0.59 0.13 3.7 13. PICO- 157(2) 76.3 12.5 1.78 0.02 0.05 0.60 0.13 3.7 14. ASW-23- 75-1 76.6 12.1 1.83 0.02 0.05 0.60 0.10 3.3 15. BEAVER- 1 76.5 12.3 1.73 0.02 0.04 0.56 0.09 3.6 Average, x(15) 76.5 12.4 1.75 0.02 0.04 0.60 0.12 3.4 

 ±2r 0.4 0.3 0.08 0.01 0.01 0.03 0.02 0.3 

Huckleberry Ridge ash bed, average composition of 21 samples, from the western and central United States Average, x(21) 76.4 12.4 1.74 0.02 0.04 0.59 0.12 3.6 

 ±2r 0.6 0.4 0.10 0.02 0.02 0.04 0.04 0.5 

Table 9. Glass chemical composition of the Huckleberry Ridge ash bed from sites in the western and central United States, as determined by elect[C. E. Meyer, analyst. See table 1 for additional explanation of analytical results.]
Sample no. Sc Mn Fe Rb Cs La Ce Nd Sm Eu Tb Dy Yb Lu Hf Ta Th 

Tuffs of Blind Spring Valley

TTC-5 3.03 639 0.40 251 7.8 18 44 19 4.4 0.06 0.71 4.8 3.1 0.43 4.3 3.2 22.3 TTC-4 3.14 638 0.41 259 7.9 18 46 19 4.5 0.07 0.72 5.0 3.2 0.40 4.5 3.4 22.7 TTC-6 3.36 694 0.40 281 9.0 17 48 22 4.8 0.06 0.82 5.6 3.5 0.48 4.6 3.6 24.2 

Tephra Layers in the Waucoba lake Beds of Hopper (1947), Waucoba Road Locality, in stratigraphic order, with WAC-7 youngest 

(WAC-6 correlates withTuffs of Blind Spring Valley, above)

WAC-7 4.77 685 0.46 325 11.0 16 48 24 5.8 0.03 1.03 6.8 4.3 0.63 5.4 4.5 29.7 WAC-6 3.25 637 0.43 266 8.3 18 49 21 4.6 0.07 0.77 4.7 3.3 0.45 4.5 3.4 23.2 WAC-5 3.07 559 0.42 233 6.9 21 54 22 4.4 0.08 0.68 4.4 2.8 0.39 4.3 2.9 20.9 

Tephra Layer in the Waucoba Lake Beds of Hopper (1947), east of Westgard Road Locality 

(correlates with WAC-6, above, and withTuffs of Blind Spring Valley, above)

WAC-10 3.24 599 0.42 267 8.4 19 49 21 4.6 0.07 0.77 5.0 3.2 0.46 4.6 3.4 23.5 

Tephra Layers in the Waucoba Lake Beds of Hopper (1947), Westgard Road Locality, in stratigraphic order with W3A youngest 

(W2A correlates with WAC-6, WAC-10, and with theTuffs of Blind Spring Valley)

W3A(1) 4.66 701 0.46 323 10.7 16 45 24 6.0 0.06 1.06 6.9 4.4 0.58 5.5 4.6 30.5 W3A(2) 4.62 676 0.45 320 10.5 15 48 24 5.7 0.02 1.00 6.5 4.1 0.58 5.3 4.4 29.2 W2A 3.31 702 0.40 285 9.5 17 48 21 4.8 0.07 0.83 5.2 3.4 0.48 4.6 3.7 25.2 

Tephra Layer in Alluvium of the Manix Basin, Mojave Desert: the Middle White Ash of Sarna-Wojcicki and others (1984) 

(correlates with WAC-7, and is also very similar to W3A)

MANIX-2 4.95 696 0.44 315 10.8 17 52 21 6.4 <0.01 1.00 6.7 4.5 0.64 5.9 4.7 30.2 MANIX-4 4.89 688 0.45 317 10.3 17 51 25 6.3 <0.01 1.08 7.0 4.3 0.56 5.7 4.6 29.5 MANIX-7 (1) 5.00 705 0.48 326 11.2 18 54 26 6.1 0.02 1.07 6.7 4.4 0.63 5.6 4.6 29.3 MANIX-7 (2) 4.86 698 0.46 327 11.1 18 52 27 6.3 0.02 1.04 7.1 4.5 0.60 5.8 4.7 29.7 MANIX-8 4.92 701 0.45 321 10.6 17 52 27 6.4 0.03 1.04 7.2 4.4 0.58 5.9 4.7 29.7 

The Huckleberry Ridge Ash Bed in the Manix Basin (MANIX-), Lake Tecopa (Teco-), Ventura Basin (ASW32375), and Humboldt Basin (Falor-2), California, and in the Sappa Form

Kansas (68W98)

MANIX-6(1) 1.77 287 1.21 201 3.7 92 181 73 13.8 0.69 2.20 13.9 7.9 1.08 9.5 3.5 29.9 MANIX-6(2) 1.75 298 1.19 212 3.8 92 179 73 13.6 0.71 2.20 14.1 8.0 1.03 9.3 3.6 30.2 Teco-1 1.75 305 1.25 189 2.8 102 189 78 14.3 0.81 2.00 13.4 7.1 1.00 9.0 3.0 27.8 Teco-2 1.80 310 1.28 192 3.2 102 201 80 14.3 0.89 2.00 13.6 7.6 1.05 9.6 3.3 28.1 Teco-12 1.70 306 1.20 197 3.1 96 179 66 13.3 0.78 2.10 13.6 7.3 0.97 8.8 3.3 28.7 ASW32375(1) 1.74 301 1.24 196 3.0 104 203 80 14.5 0.82 2.00 13.9 7.6 1.03 9.4 3.3 28.6 ASW32375(2) 1.80 308 1.24 204 3.3 104 201 81 14.7 0.85 2.10 13.7 7.5 0.99 9.7 3.5 30.2 68W98(1) 1.68 281 1.20 211 3.5 91 187 74 13.3 0.72 2.10 14.8 7.9 1.10 9.1 3.6 29.8 68W98(2) 1.64 nd 1.14 229 3.6 89 180 72 13.2 0.69 2.00 nd 7.8 1.03 9.4 3.5 26.1 FALOR-2 1.67 280 1.15 212 3.7 91 174 69 13.3 0.69 2.10 13.9 7.8 1.01 9.0 3.6 29.7  Mean ±1 s.d.

1.73 297 1.21 204 3.4 96 186 75 13.8 0.77 2.10 13.9 7.7 1.03 9.3 3.4 28.9 0.06 12 0.04 12 0.3 6 10 5 0.6 0.07 0.08 0.4 0.3 0.04 0.3 0.2 1.3 

Table 10. Instrumental neutron-activation analysis of selected samples of the tuff of Blind Spring Valley, from the Cowan Pumice Mine (TTC-)chemically related tephra layers erupted from the Glass Mountain volcanic source area.[Samples from the Waucoba lake beds of Hopper (1947) (WAC-;W2A, W3A), the Manix Basin (MANIX-), as well as the compositionally contrasting Huckbed, erupted from the Yellowstone National Park area of Wyoming and Idaho, from several selected localities in the Western U.S. Values given are atomper million, except for iron, which is given in atomic weight percent. Analyses were done in the U.S. Geological Survey’s Radiochemistry Laboratories aBudhan, R. Knight, and D. McKown, analysts; and Reston (R): P. Baedecker and J. Rowe, analysts; and at the University of California’s Lawrence BerkeleAsaro, H. Bowman, and H. Michael, analysts. Numbers in parentheses after sample number are replicate analyses of the same sample.]
44 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

graphic data and isotopic age determinations, represent a set of 

chronostratigraphic datum planes that provide age control and 

correlation among several depositional basins within the western conterminous United States for the age interval ~2.85 to 

~0.76 Ma. Within this ~2-m.y. period, correlation is possible 

by means of the tuff of Benton Hot Springs (2.89-2.81 Ma), 

the lower tuffs of the Badlands section (~2.7-~2.6 Ma), the 

upper tuffs of the Badlands section (~2.6-~2.2 Ma); the tuffs 

of Blind Spring Valley (2.22-2.14 Ma), the Huckleberry Ridge 

ash bed (~2.06 Ma), the tuffs of Emigrant Pass (~2.06-~1.92 

Ma), the lower tuffs of Glass Mountain (1.92-1.30 Ma), the 

upper tuffs of Glass Mountain (~1.2-~0.8 Ma), and the Bishop 

ash bed (~0.76 Ma)(fig. 21). Furthermore, presence of the 

tuffs of Blind Spring Valley, the Huckleberry Ridge ash bed, 

and the Bishop ash in both continental and marine sediments 

provides an important chronostratigraphic reference level 

that allows correlation and dating of uppermost Pliocene and 

Pleistocene marine and continental sections (Sarna-Wojcicki 

and others, 1987).

On the basis of their somewhat different glass composition, mineralogy, and significantly older age (2.89 - 2.81 

Ma), we consider the tuffs of Benton Hot Springs to represent 

a different eruptive episode from the tuffs of Blind Spring 

Valley (2.22 to 2.14 Ma), and from the lower tuffs of Glass 

Mountain at the Cowan Pumice Mine (1.92 to 1.86 Ma). The 

tuffs of Benton Hot Springs antedate eruptions of the tuffs 

of Blind Spring Valley by about 0.6 m.y., and eruptions from 

Glass Mountain by about 0.8 m.y., because the oldest K-Ar 

ages obtained on lava flows from the Glass Mountain source 

are about 2.1 Ma (Metz and Mahood, 1985). Judging by their 

coarse grain size, however, the eruptive source of the tuffs of 

Blind Spring Valley must have been close to, or at, the site 

of Glass Mountain; the lava flows and pyroclastic deposits 

exposed at Glass Mountain may cover older pyroclastic and 

flow rocks equivalent to the tuffs of Benton Hot Springs.

The tuffs of Blind Spring Valley are different from the 

underlying tuffs of Benton Hot Springs, and from the overlying lower tuffs of Glass Mountain, both with regard to chemiFigure 20. Tuff of Blind Spring Valley (light layer) in Beaver Basin, Utah (see Izett and others, 1988). About 14 cm thick, this 

layer stratigraphically underlies the Huckleberry Ridge ash bed by about 30 m. This tephra layer most likely correlates with the 

thickest, coarsest unit at the Cowan Pumice Mine in Pit 6, TTC-22, (figs. 5, 6, 7), and also probably with the same unit in Pit 2, 

TTC-18 (fig. 11). The basal, nearly pure white unit, about 1 - 1.5 cm thick, is most likely the initial airfall ash. Overlying this is a 

somewhat less pure, finely layered unit, about 3 cm thick, that represents repeated reworking of the tephra from the highlands 

tributary to the Beaver basin. Above this are additional fine layers and lenses, ~10 cm altogether, of progressively less pure ash 

and ashy sediment, products of further reworking and redeposition of the ash from the adjoining countryside.
AGE

(Ma)

0.758

1.8

2.0

2.2

2.42.6

3.22

3.33

 Magnetostratigraphy Cowan Pumice

Mine, CA

Yellow- jacketCanyon, CA Fish Lake Valley, CA, NV

EmigrantPass, NV

 WillowWash, CA

Waucoba Road, Big Pine, CA

 Death Valley,Confidence Hills, CA

 LakeTecopa, CA

ManixBasin, CA

 Corona,Los Angeles Basin, CA

Ventura Basin, CA

Bea Bas Ut

BRUNHES MATUYAMA GAUSS TUFFS OF GLASS MOUNTAIN (UNDIFFERENTIATED) LOWER TUFFS OF GLASS MOUNTAIN TUFFS OF BLIND SPRING VALLEY TUFFS OF EMIGRANT PASS UPPER TUFFS OF THE BADLANDS LOWER TUFFS OF THE BADLANDS TUFFS OF BENTON HOT SPRINGS BISHOP TUFF, ASH TTC-7 YJC-1 FLV-57-CS, FLV-163-CS FLV-160-CS to FLV-162-CS, FLV-147-CS, FlV-178-EP FLV-124 to 126-WW. FLV-47, 48WW TTC-23, 24 FLV-46 FLV-44 and FLV-189 and 45-WW 190-EP HUCKLEBERRY RIDGE ASH BED 

Altered tuffs CH880012 to CH880021; CH86-WA5, CH86-GA6, CH86-GWA8, CH86-GA9 CH88009,10,11, CH86BT3 CH88007 TTC-4,5,6,9, 14 to 18; TTC-19 to 22 TCH* TECO-5 TECO- 23,24,31 MANIX-2 MANIX 7, 8 TECO-1, 2, 12 MANIX-6 

85-G-54N PICO-162Q PICO-169Q 

PICO-15 FLV-180 to 183, FLV-185 to 188-EP, FLV-211-EP FLV-39-WW to FLV-43-WW; FLV-123-WW WAC-6,7; W2A; DG1,3a,4 CH88004, CH88006, CH86T-1; Altered tuffs CH88004, CH88006, CH86T-1; Altered tuffs FLV-36-WW to FLV-38-WW; FLV-138-WW FLV-26-WW to FLV-29-WW FLV-32-WW to FLV-35-WW; FLV-121-WW; FLV-24-WW, FLV-25-WW YJC-4 YJC-2,3 FLV-22-WW; FLV-23-WW TTC-26A, B

hiatus

Basalt, ~3.5 Ma

FLV-97-WW, NOMLAKI TUFF

FLV-96-WW, PUTAH TUFF

MEMBERS OF THE TEHAMA ANDTUSCAN FORMATIONS (~3.3 Ma)

hiatus

 hiatus

2.8

*Note: TCH - Tuff of Confiedence Hills, 

EXPLANATION:

Magnetostratigraphy Stratigraphy

normalreversed

TUFFS OF 

Sample numbers

Nam

Light-gof closerespecchemic

 Individual tephra laye

BISHOP TUFF, ASH

CH88008,CH86GA4

OLDUVAI REUNION MAMMOTH

Figure 21. Magnetostratigraphy, chronology, and correlation of tuffs of Blind Spring Valley and related tephra layers for selected sites in southwLight-shaded bands represent the presently-known age ranges of specific tuffs or tuff groups. Sample numbers (given by letter and number desigeach tuff or tuff group at each locality where they are identified. Solid horizontal lines mark individual tephra layers with precisely-determined 40A

bed, 0.758±0.002 Ma (Sarna-Wojcicki and Pringle, 1992; 0.774±0.002 Ma, using monitor values of Samson and Alexander, 1987); Huckleberry Ridge (this report). TCH is tuff of Confidence Hills, reported at only one locality in this report. 
46 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

cal composition and isotopic age. Again, the coarse airfall 

pumice of the tuffs of Blind Spring Valley suggests that the 

source was nearby, and may have been the same as that of the 

later flows and tuffs of Glass Mountain, but extensive proximal exposures have not been found at Glass Mountain. Among 

the older rhyolitic lavas of Glass Mountain of Metz and 

Mahood (1991), their undated unit OF (CT75-5-6) may correlate with the tuffs of Blind Spring Valley, and their unit OG 

(200), dated 2.13 Ma, may correlate with the tuffs of Emigrant 

Pass that closely underlie and overlie the ~2.05-Ma Huckleberry Ridge ash bed, as suggested by the lower iron contents 

of these units compared to the other rhyolitic lavas of Glass 

Mountain of Metz and Mahood (1991). The tuffs of Benton 

Hot Springs, the tuffs of Blind Spring Valley, the tuffs of Emigrant Pass, and the tuffs of Glass Mountain all have chemical 

“family” resemblance to each other, but they are sufficiently 

different so that they can be distinguished from each other, 

and with ease from the Huckleberry Ridge ash bed, which 

lies within the tuffs of Emigrant Pass, above the tuffs of Blind 

Spring Valley, and below the older tuffs of Glass Mountian.

The Bishop Tuff at the top of the section at the Cowan 

Pumice Mine can also be distinguished chemically from tephra 

layers of Glass Mountain—although with difficulty and only 

by means of minor- and trace-element analyses—unless other 

criteria such as stratigraphic sequence, magnetostratigraphy, or 

direct isotopic age data, are available. Stratigraphic sequence 

and magnetostratigraphic orientation aid considerably in identification of these tephra layers.

The combined K-Ar ages of lavas erupted from Glass 

Mountain (Metz and Mahood, 1985), together with our 40Ar/39Ar 

laser-fusion ages of tephra layers from the Cowan Pumice Mine, 

appear to define perhaps as many as six eruptive episodes from 

the Glass Mountain area (fig. 22). The tuffs of Benton Hot 

Springs may define the first, and earliest period of activity, about 

2.89 to 2.81 Ma, although the location of their eruptive source is 

not certain. A long period of volcanic activity, represented by the 

chemically related tuffs of the Badlands that are present in the 

sections at Willow Wash (~2.81 to 2.22 Ma), is not represented 

at the Cowan Mine, and we infer that a hiatus representing ~0.6 

m.y. exists at the latter site. Ages of the tuffs of Blind Spring 

Valley from the Cowan Pumice Mine, combined with chemical 

data on the composition of their volcanic glasses, suggest a range 

from about 2.22 to 2.14 Ma, representing an episode of activity 

of about 90 k.y. Judging by their coarse grain size and thickness 

gradients of their beds, these tephra layers were probably derived 

from the vicinity of Glass Mountain.

The four ages on tephra layers overlying the tuffs of 

Blind Spring Valley at Pit 6 at the Cowan Pumic Mine, the 

lower tuffs of Glass Mountain, coincide closely with those 

of the oldest lavas of Glass Mountain of Metz and Mahood 

(1985; 1991); together with the coherent chemical glass 

compositions, these ages define an eruptive episode about 180 

k.y. in length, from 1.92 to 1.74 Ma. A dormant interval of 

about 140 k.y., from ~1.74 to ~1.6 Ma, intervenes between this 

and the next period of activity, although the analytical errors 

associated with the K-Ar ages make its existence and duration 

uncertain. During the long, 400-k.y. period between ~1.6 and 

1.2 Ma, there appears to have been sporadic volcanic activity 

from the Glass Mountain source. Another dormant period of 

~100 k.y. followed, from ~1.2 to ~1.1 Ma; again, its duration 

is uncertain because of the large analytical error of the K-Ar 

ages on the associated flow rocks. This dormant period was 

followed by two episodes of apparently frequent and intense 

activity, the first from ~1.1 to 0.9 Ma, and the second from 

~0.82 to ~0.79 Ma, with an ~80-k.y. dormant period intervening between the two (fig. 22).

The chemical composition of these younger tuffs of Glass 

Mountain is very similar to that of the Bishop Tuff and ash, 

on the one hand, and to the lower tuffs of Glass Mountain, on 

the other. These several groups of tuffs are also all very similar 

petrographically to each other. Detailed stratigraphy, magnetostratigraphy, minor- and trace-element analyses, and isotopic 

ages are required to identify, correlate, and date the tuffs of 

Glass Mountain and the Bishop ash bed with any significant 

degree of confidence.

The chemical similarity of the tuffs of Glass Mountain 

and the Bishop Tuff/ash suggest that they are genetically 

related and have been erupted from the same magma chamber, or at least from a single, continuously evolving magma 

system (Smith, 1979; Metz and Mahood, 1991). The eruption 

of the Bishop Tuff can be considered as an isolated event in 

time (see fig. 22). Its age was determined most recently by the 

laser-fusion 40Ar/39Ar method (Sarna-Wojcicki and Pringle, 

1992; Sarna-Wojcicki and others, 2000), and the associated 

analytical error is small compared to the analytical errors of 

our K-Ar ages and those of Metz and Mahood (1985). If the 

error were comparable to those of the dates on the younger 

Glass Mountain lavas (~0.79 to ~0.82 Ma), the Bishop Tuff 

could reasonably be viewed as the youngest eruption of the 

last eruptive episode from the combined Glass Mountain-Long 

Valley source. Moreover, resurgent volcanism from the Long 

Valley Caldera began shortly after eruption of the Bishop Tuff 

(Bailey and others, 1976); conventional K-Ar dates on the 

oldest post-Bishop Tuff eruptive episode range from 0.73 to 

~0.63 Ma. The entire 1.6-m.y. period, from the beginning of 

volcanic activity with the eruption of the tuffs of Blind Spring 

Valley at about 2.2 Ma, until about 0.6 Ma, can be thus viewed 

as a long period of alternating frequent and sporadic activity, 

interspersed with short periods of dormancy, from the combined Glass Mountain-Long Valley magmatic system. Part of 

the Glass Mountain volcanic field, some near-vent exposures 

of the lower Glass Mountain pyroclastic rocks, the tuffs of 

Blind Spring Valley, and possibly even the tuffs of Benton Hot 

Springs, may have subsided with the floor of the Long Valley 

caldera after eruption of the Bishop Tuff and now lie buried by 

a thick cover of younger volcanic deposits and alluvium.

The volume of volcanic and pyroclastic rocks erupted 

from the Glass Mountain source area during the 2-m.y. period 

from ~2.8 Ma to ~0.8 Ma is probably large but difficult to 

estimate because much of these deposits is buried by younger 

sediments and rocks or has been removed by erosion from 

uplands. The volume of the tuffs of Blind Spring Valley alone 
                    

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4UFFS OF"LIND3PRING6ALLEY

4UFFS OF'LASS-T

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4UFFS OF'LASS-TUPPER	

3TRATIGRAPHICORDERINFERRED

)NSTRATIGRAPHIC

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TUFFS OF%MIGRANT0ASS OR TUFFS OF"LIND

3PRING6ALLEY

'LASS-OU TAINUPPER	'LASS-OU TAINLOWER	

Figure 22. Chronology of tuffs of Blind Spring Valley and related tephra layers, based on ages reported here (top), compared to chronology (botto

Glass Mountain rhyolites, reported in Metz and Mahood (1985). K/Ar is conventional potassium-argon age; LF 40Ar/39Ar is laser-fusion 40Ar/39Ar a
48 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

is estimated to be in the range of ~67 to ~97 km3, using the 

method of Fierstein and Nathenson (1992)(J. E. Fierstein, 

written commun., 2002). The lower end of the above range is 

estimated from the volume of the coarsest and thickest of the 

tuffs of Blind Spring Valley (TTC-22 at Pit 6, Cowan Pumice 

Mine), about 4 m thick at a minimum distance of about 17 km 

from the eruptive source near Glass Mountain and about 1.0 

cm thick at distal sites, including in Beaver Basin, Utah, ~400 

km to the east (figs. 1, 2, 5, 6, 7, 20). The total area covered by 

this unit is estimated to be ~200,000 km2 (fig. 1). The upper 

end of the above estimate is based on an aggregate thickness 

of the tuffs of Blind Spring Valley of ~6.5 m, the ~1.0-cm 

thickness at distal sites, and the approximate ~200,000-km2 

area covered by the tuffs of Blind spring Valley.

An additional volume of proximal rhyolite and ash-flow 

tuffs and proximal and distal tephra fall of the tuffs of Benton Hot Springs and tuffs of Glass Mountain, together with 

an unknown volume of coeval fall- and flow-rocks buried in 

the subsided part of Long Valley Caldera, may account for 

as much as another ~50 km3. The total volume of material 

erupted from this source area in the time interval from ~2.8 

to ~0.8 Ma may thus be as much as ~150 km3. Volume of the 

subsequently erupted Bishop Tuff, 0.76 Ma, is estimated to be 

~500-550 km3 (Bailey and others, 1976).

Laser-fusion 40Ar/39Ar ages on sanidine that are presented 

here, in Sarna-Wojcicki and Pringle (1992), and in SarnaWojcicki and others (2000), are somewhat older than ages on 

the same volcanic units obtained by the conventional K-Ar 

method. This is probably because all radiogenic argon was not 

extracted from sanidine in conventional analyses. In 40Ar/39Ar 

analysis, it is not necessary to extract all the radiogenic argon, 

because only the ratios of Ar isotopes are measured, not the 

absolute amounts. This factor may result in some errors in the 

detailed interpretation of the volcanic history of Glass Mountain and Long Valley, such as durations of episodes of activity 

and dormancy, when K-Ar and 40Ar/39Ar ages are compared, 

but most likely has little effect on the main outline of the chronology discussed here.

The stratigraphy in northeastern Fish Lake Valley and 

at Willow Wash, and the presence of multiple tephra layers 

in both sections that are missing at the Cowan Pumice Mine, 

suggest that hiatuses representing a considerable amount 

of time are present within the latter section (fig. 19). These 

observations also suggest that volcanism was much more 

continuous during the period from about 3.3 to 0.76 Ma than 

is represented by volcanic units at the Cowan Pumice Mine 

and immediate vicinity. The Willow Wash section in particular 

contains many more tephra layers than are present elsewhere, 

which makes it a particularly good reference section for unravelling the late Cenozoic tephrostratigraphy of this region. This 

section may have been near the depositional center of a basin 

for all of Pliocene time, before it was deformed and uplifted 

sometime after ~1.7 Ma. Although this section provides an 

excellent reference for determining the relative stratigraphic 

positions of tephra layers, and though much of the section 

is also suitable for magnetostratigraphy, attempts to date it 

directly by isotopic analysis of feldspars in the tephra layers 

by laser-fusion 40Ar/39Ar have had little success, owing to the 

presence of abundant detrital feldspars within the size range 

required for this type of analysis (appendix 3).

The combined chronostratigraphy developed for the 

stratigraphic section in Confidence Hills of Death Valley, 

based on data presented in Troxel and others (1986), Pluhar 

and others (1992), Beratran and Murray (1992), and in this 

report (fig. 18), provides new age estimates for three Reunion 

magnetostratigraphic events. The interpolated midpoint of 

the oldest of these is 2.22 Ma. The data of Pluhar and others (1992) indicate that this is a minor excursion that did not 

reach negative inclinations. The two younger Reunion events, 

which did reach complete reversal for some period of time, are 

bracketed between 2.18 and 2.17 Ma for the older (Reunion I), 

and between 2.16 and 2.14 Ma for the younger (Reunion II). 

The Huckleberry Ridge ash bed is stratigraphically well above 

these excusions, and must be younger, as proposed in Lanphere and others (2002).

Our laser-fusion 40Ar/39Ar date on sanidine separated 

from the Huckleberry Ridge ash bed is 2.063±0.007 (weighted 

mean of 19 determinations, with error reported as standard 

error of the mean; Sarna-Wojcicki and Pringle, 1992; appendix 

1). The analyses were on a proximal sample of the Huckleberry Ridge ash bed obtained from J.D. Obradovich of the 

U.S. Geological Survey, Denver. The above date is the same, 

within analytical errors, as a combined weighted age of 2.059± 

0.004 Ma obtained by Lanphere and others (2002) on this 

unit using the incremental-heating 40Ar/39Ar method. In figure 

18, for the purpose of comparing our age with the published 

ages of magnetostratigraphic boundaries, we converted the 

Huckleberry Ridge ash bed age to ~2.09 Ma, using the more 

generally accepted, somewhat older values of the Taylor Creek 

Rhyolite (Rennie and others, 1998). The age estimates for the 

Reunion events are based on using the latter set of dates for 

interpolation.

Distal correlative layers of the tuffs of Blind Spring Valley are found in west-central Utah, western Nevada, and in 

California in Owens Valley, Death Valley, the Manix Basin, 

eastern Los Angeles Basin, and the Ventura Basin. As one 

would expect, we see a progressive “simplification” of the 

stratigraphy for the tuffs of Blind Spring Valley and related 

tephra layers with distance from the eruptive source area. 

Although the section at the Cowan Pumice Mine is incomplete 

owing to hiatuses, multiple thick, coarse units are present 

nearest to the source area. For any group of stratigraphically 

closely associated and chemically similar tephra layers in 

the proximal area, such as the tuffs of Blind Spring Valley, 

the thickness, grain size, and number of tephra layers generally decrease at successively more distal localities (figs. 8, 

13, 15, 20). At the most distal sites, such as the Ventura or 

Beaver Basins for example, only one thin, fine-grained tephra 

layer is found that represents the tuffs of Blind Spring Valley. 

One likely explanation for this simplification is that eruptive 

episodes vary in character and magnitude: products of only 

the most voluminous and explosive eruptions are likely to be 
found at distal sites. Alternatively and in addition, multiple 

eruptions spaced closely in time can result in a complex of 

multiple layers deposited near the source, but fewer or only 

one layer at distal sites, owing to homogenization of tephra 

during transport by wind or subsequent reworking near the 

site of deposition, as has been observed for modern eruptions 

such as those from Mount St. Helens in 1980 (Sarna-Wojcicki 

and others, 1981; Shipley, 1983). Furthermore, variable wind 

directions with altitude and with time during an eruption or 

eruptive episode, will also tend to disperse tephra in different 

directions, resulting in a simplified and less complete stratigraphy at any single distal site (Sarna-Wojcicki and others, 1981; 

Shipley, 1983).

Our experience points toward an optimal strategy for 

developing regional chronostratigraphic frameworks using 

tephrochronology, isotopic data, and magnetostratigraphy. 

Depositional centers of long-lived sedimentary basins are the 

sites where the most complete records of sedimentation are 

preserved. It is within such depositional environments, where 

the energy for transport of sediments is at a minimum, that 

relatively complete records of volcanic ash falls from multiple eruptive sources are likely to be preserved. Such basins 

are often at some distance from plate margins or other loci of 

crustal instability such as mantle plumes or hot spots, where 

active tectonic deformation and associated volcanism occur. 

It is at or near volcanic source areas, however, that the best 

materials are available for direct isotopic analysis of tephra 

layers—coarse airfall pumice layers containing large mineral grains, and stratigraphically associated lava flows. Near 

source areas, thick, complete stratigraphic sections are rarely 

preserved because of rapid tectonic deformation and accompanying erosion or burial by both volcaniclastic sediments and 

lavas.

The optimal strategy, then, is to isotopically date tephra 

layers near volcanic sources, and to correlate them by chemical and petrographic criteria to sites where more complete, 

long-term stratigraphic records are available and where 

complementary age control can be obtained through magnetostratigraphy. In this manner, parts of stratigraphic sections can 

be bracketed by correlated ages of tephra layers, and the ages 

of intervening, undated tephra layers and other stratigraphic 

horizons can be estimated or determined by paleomagnetic 

orientation and interpolation.

Recent additional work in the region of Death Valley 

National Park (Knott, 1998; Knott and others, 1999; Machette 

and others, 2001; Sarna-Wojcick and others, 2001), and in 

Owens Lake (Smith and Bischoff, 1997; Sarna-Wojcicki and 

others, 1997), as well as new unpublished work (E. Kirby, 

A. Sarna-Wojcicki, M. Reheis, and D. Burbank, 2002) have 

already enlarged the present temporal/spatial framework 

that we present here. This framework (figs. 19 and 21) will 

undoubtedly be expanded and refined in the future, as new 

sites are found and additional tephra layers are analyzed.

References

Baedecker, P.A., and McKown, D.M., 1987, Instrumental 

neutron activation analysis of geochemical samples, in

Baedecker, P.A. ed., Methods for geochemical analysis, 

U.S. Geological Survey Bulletin 1770, p. H1-H-14.

Bailey, R.A., Dalrymple, G. B., and Lanphere, M. A., 1976, 

Volcanism, structure, and geochronology of Long Valley 

Caldera, Mono County, California: Journal of Geophysical 

Research, v. 81, p. 725-744.

Beratran, K.K., and Murray, Bruce, 1992, Stratigraphy and 

depositional environments, southern Confidence Hills, 

Death Valley, California: San Bernardino County Museum 

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Appendixes 1–3
Cowan Pumice Mine Locality, Blind Spring Valley Pit 6, and underlying section (in stratigraphic order, from youngest at top) Tuffs of Glass Mountain (lower), TTC-23 Irradiation No.: CIV-A-23 J=0.0003835 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0563A TTC-23 (1) sanidine 5 0.000 33.646 92.043 3.043 0.015 0.001 92Z0569A TTC-23 (2) “ 10+ 0.000 32.724 76.679 3.579 0.015 0.003 

Weighted Me

± 1 SEM 

MSWD 

Tuffs of Blind Spring Valley, TTC-22 Irradiation No.: CIV-A-22 J=0.000385 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0572A TTC-22 (1) sanidine 1 0.000 50.261 89.101 3.496 0.001 0.001 92Z0572B TTC-22 (2) “ 4 0.000 130.120 91.937 3.399 0.004 0.001 92Z0572C TTC-22 (3) “ 4 0.000 34.017 84.015 3.872 0.014 0.002 92Z0572D TTC-22 (4) “ ~4 0.000 13.099 82.768 3.773 0.037 0.002 92Z0572E TTC-22 (5) “ 4 0.000 16.854 85.043 3.808 0.029 0.002 92Z0572F TTC-22 (6) “ 4 0.000 15.902 80.262 3.898 0.031 0.003 

Weighted Me

±1 SEM 

MSWD 

Tuffs of Blind Spring Valley, TTC-21 Irradiation No.: CIV-A-19 J=0.0003895 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0589A TTC-21 (1) sanidine ? 0.000 31.482 92.871 3.321 0.016 0.001 92Z0589B TTC-21 (2) “ ? 0.000 20.729 86.718 3.598 0.024 0.002 92Z0571C TTC-21 (3) “ 2 0.000 40.782 90.442 3.467 0.012 0.001 92Z0571D TTC-21 (4) “ 2 0.000 29.079 91.639 3.384 0.017 0.000 92Z0571E TTC-21 (5) “ 2 0.000 52.575 94.293 3.307 0.009 0.001 92Z0571F TTC-21 (6) “ 2 0.000 134.598 90.140 3.509 0.004 0.001 92Z0571G TTc-21 (7) “ 2 0.000 44.831 87.161 3.541 0.011 0.002 

Weighted Me

± 1SEM 

MSWD 

Appendix 1. Analytical data for laser-fusion 40Ar/39Ar age determinations of tephra layers. Individual dates followed by an asterisk (*) are not includ

weighted mean age for the group because of outliers in one or more of the critical parameters: age, standard deviation, 40Ar/39Ar, or 40Ar*(mol). The w

standard deviation (±1r), and weighted standard error of the mean (SEM), are given for those groups where the mean standard weighted deviates (Mless than one. For those groups where the MSWD is greater than one, the weighted standard deviation and SEM are calculated for a MSWD forced Pringle for experiments no. 92Z (1992) and Robert Fleck for experiments 94Z (1994), U.S. Geological Survey, Menlo Park, Ar/Ar Laboratory.
Tuffs of Blind Spring Valley, TTC-20

Irradiation No.: CIV-A-18 J=0.000391Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0568B TTC-20 (2) “ 1 0.000 78.599 91.135 3.385 0.006 0.001 92Z0568C TTC-20 (3) “ 1 0.000 32.913 78.673 3.949 0.015 0.003 92Z0568D TTC-20 (4) “ 3 0.000 29.763 94.590 3.244 0.016 0.001 92Z0568E TTC-20 (5) “ 3 0.000 27.880 85.527 3.628 0.018 0.002 92Z0568F TTC-20 (6) “ 3 0.000 81.525 83.004 3.711 0.006 0.002 

Weighted Me

± 1SEM 

MSWD 

Tuffs of Blind Spring Valley (basal unit in Pit 6)

Irradiation No.: CIV-A-17 J=0.0003925Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0567A TTC-19 (1) sanidine? 2 0.000 2.857 85.450 3.549 0.171 0.002 92Z0567B TTC-19 (2) sanidine 2 0.000 47.718 93.102 3.277 0.010 0.001 92Z0567C TTC-19 (3) “ 2 0.000 24.369 91.074 3.345 0.020 0.001 92Z0567D TTC-19 (4) “ 2 0.000 37.767 85.052 3.609 0.013 0.002 92Z0567E TTC-19 (5) “ 2 0.000 45.777 92.764 3.352 0.011 0.001 92Z0567F TTC-19 (6) “ 2 0.000 31.145 90.799 3.388 0.016 0.001 92Z0590A TTC-19 (7) “ ? 0.000 31.010 92.693 3.344 0.016 0.001 92Z0590B TTC-19 (8) “ ? 0.000 29.524 92.625 3.331 0.017 0.001 

Weighted Me

± 1SEM 

MSWD 

Tuffs of Blind Spring Valley, TTC-25 (tuff in tuffaceous arkosic sediments stratigraphically beneath TTC-19), 

Irradiation No.: CIV-A-24 J=0.000382Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0573A TTC-25 (1) sanidine 1 0.000 77.845 96.256 3.355 0.006 0.000 92Z0573B TTC-25 (1) “ 1 0.000 34.440 92.525 3.494 0.014 0.001 92Z0573C TTC-25 (1) “ 1 0.000 40.293 95.989 3.374 0.012 0.001 92Z0573D TTC-25 (1) “ 3 0.000 29.364 90.565 3.541 0.017 0.001 92Z0573E TTC-25 (1) “ 3 0.000 61.804 93.065 3.471 0.008 0.001 92Z0573F TTC-25 (1) “ 3 0.000 10.050 88.379 3.582 0.049 0.001 92Z0588A TTC-25 (1) “ ? 0.000 20.685 88.717 3.642 0.024 0.001 92Z0588B TTC-25 (1) “ ? 0.000 25.791 93.937 3.427 0.019 0.001 

Weighted Me

± 1SEM 

MSWD 

Appendix 1—Continued
Tuffs of Benton Hot Springs, TTC-26B (tuff in arkosic tuffaceous sediments stratigraphically below TTC-25) Irradiation No.: CIV-A-25 J=0.000381 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0587A TTC-26B (1) plagioclase 10 to 15 0.000 0.407 62.218 6.346 1.204 0.008 92Z0587B TTC-26B (2) “ 8+ 0.000 0.342 53.785 7.521 1.433 0.012 92Z0587C TTC-26B (3) “ ? 0.000 0.369 67.709 6.134 1.326 0.007 92Z0587D TTC-26B (4) “ 1 0.000 0.422 80.235 5.075 1.160 0.004 92Z0574A TTC-26B (5) “ 1 0.000 0.491 73.471 5.718 0.997 0.005 92Z0574B TTC-26B (6) “ 1 0.000 0.417 83.598 5.036 1.174 0.003 92Z0574C TTC-26B (7) “ 1 0.000 0.251 57.858 7.133 1.949 0.011 92Z0574D TTC-26B (8) “ 1 0.000 0.350 77.251 5.707 1.397 0.005 92Z0574E TTC-26B (9) “ 4 0.000 0.310 72.903 5.811 1.584 0.006 92Z0574F TTC-26B (10) “ 4 0.000 0.431 54.578 7.059 1.135 0.011 

Weighted Me

± 1SEM 

MSWD 

Pit 2 (Fault Pit) Tuffs of Blind Spring Valley, TTC-18 Irradiation No.: CIV-A-16 J=0.000394 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0566A TTC-18 (1) sanidine (0.857 mg) 0.000 50.467 84.633 3.370 0.001 0.002 92Z0566B TTC-18 (2) “ (0.890 mg) 0.000 36.207 88.271 3.420 0.014 0.001 92Z0566C TTC-18 (3) “ (1.042 mg) 0.000 18.274 97.413 3.172 0.027 0.000 92Z0566D TTC-18 (4) “ (1.557 mg) 0.000 47.910 81.512 3.684 0.010 0.002 92Z0566E TTC-18 (5) “ (1.522 mg) 0.000 49.546 86.336 3.506 0.001 0.002 92Z0566F TTC-18 (6) “ (2.050 mg) 0.000 44.726 87.611 3.494 0.011 0.001 

Weighted Me

± 1SEM 

MSWD 

Pit 3 (West Pit) Tuffs of Blind Spring Valley, TTC-5 Irradiation No.: CIV-A-7 J=0.000405 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0560A TTC-5 (1) sanidine 1 0.000 51.486 87.390 3.377 0.001 0.001 92Z0560B TTC-5 (2) “ 1 0.000 31.229 92.611 3.147 0.016 0.001 92Z0560C TTC-5 (3) “ 1 0.000 48.394 89.479 3.260 0.010 0.001 92Z0560D TTC-5 (4) “ 1 0.000 37.253 92.605 3.178 0.013 0.001 92Z0560E TTC-5 (4) “ 3 0.000 41.258 90.127 3.250 0.012 0.001 92Z0560F TTC-5 (6) “ 3 0.000 10.009 85.715 3.405 0.049 0.002 

Weighted Me

± 1SEM 

MSWD 

Appendix 1—Continued
Pit 4 and vicinity

Tuffs of Blind Spring Valley, TTC-4

Irradiation No.: CIV-A-6 J=0.000406Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0559C TTC-4 (1) sanidine 3 0.000 9.818 80.750 3.616 0.050 0.002 92Z0559D TTC-4 (2) “ 3 0.000 54.460 84.597 3.472 0.009 0.002 92Z0559E TTC-4 (3) “ 3 0.000 38.013 83.754 3.491 0.013 0.002 92Z0559F TTC-4 (4) “ 3 0.000 50.147 89.585 3.281 0.001 0.001 92Z0559G TTC-4 (5) “ 3 0.000 30.127 86.502 3.383 0.016 0.002 92Z0559H TTC-4 (6) “ 3 0.000 33.239 88.875 3.289 0.015 0.001 Irradiation No.: CIV-A-34 J=0.0003655Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 92Z0586A TTC-4 (7) Sanidine (1.634 mg) 0.000 29.957 86.798 3.706 0.016 0.002 92Z0586C TTC-4 (8) “ (1.993 mg) 0.000 25.660 93.603 3.460 0.019 0.001 92Z0586D TTC-4 (9) “ (2.819 mg) 0.002 41.117 95.049 3.425 0.012 0.001 

Weighted M

± 1SEM 

MSWD 

Tuffs of Blind Spring Valley, TTC-15

Irradiation No.: CIV-A-11 J=0.000400Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0564A TTC-15 (1) qtz-san-plag? 4 0.000 5.089 12.818 16.643 0.096 0.049 92Z0561A TTC-15 (2) sanidine 5 or 6 0.000 37.261 88.916 3.334 0.013 0.001 92Z0561B TTC-15 (3) “ 5 or 6 0.000 36.843 91.569 3.257 0.013 0.001 92Z0561C TTC-15 (4) “ 5 or 6 0.000 12.247 91.098 3.303 0.040 0.000 92Z0561D TTC-15 (5) “ 5 or 6 0.000 23.481 81.732 3.669 0.021 0.002 92Z0561E TTC-15 (6) “ 5 or 6 0.000 17.314 92.591 3.206 0.028 0.001 92Z0561F TTC-15 (7) “ 5 or 6 0.000 26.660 84.296 3.565 0.018 0.002 

Weighted M

± 1SEM 

MSWD 

Appendix 1—Continued
Pit 5 Tuffs of Blind Spring Valley (upper unit in pit), TTC-17 Irradiation No.: CIV-A-13 J=0.000398 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0565A TTC-17 (1) sanidine 1 0.000 43.700 93.734 3.218 0.011 0.001 92Z0565C TTC-17 (2) “ 2 0.000 46.596 92.923 3.236 0.011 0.001 92Z0565D TTC-17 (3) “ 2 0.000 38.794 93.789 3.193 0.013 0.001 92Z0565E TTC-17 (4) “ 2 0.000 45.332 93.461 3.218 0.011 0.001 92Z0565F TTC-17 (5) “ 2 0.000 49.818 95.139 3.199 0.001 0.001 92Z0565G TTC-17 (6) “ 2 0.000 39.463 95.746 3.179 0.012 0.001 92Z0565H TTC-17 (7) “ 2 0.000 47.516 93.643 3.203 0.010 0.001 

Weighted M

± 1SEM 

MSWD 

Tuffs of Blind Spring Valley (lower unit in pit), TTC-16 Irradiation No.: CIV-A-12 J=0.000399 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0562A TTC-16 (1) sanidine 1 0.000 80.842 92.049 3.311 0.006 0.001 92Z0562B TTC-16 (2) “ 2 0.000 45.756 92.955 3.263 0.011 0.001 92Z0562C TTC-16 (3) “ 3 0.000 32.627 98.887 3.076 0.015 0.000 92Z0562D TTC-16 (4) “ 3 0.000 34.947 91.701 3.306 0.014 0.001 92Z0562E TTC-16 (5) “ 3 0.000 67.937 80.799 3.771 0.007 0.002 92Z0562F TTC-16 (6) “ 3 0.000 32.145 89.501 3.364 0.015 0.001 

Weighted M

± 1SEM 

MSWD 

Chalk Hills Locality Tuffs of Glass Mountain (upper), BT-16 (underlies Bishop Tuff, 0.76 Ma) Irradiation No.: CXXI-35 J=4.08632E-04 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

94Z0059A BT-16 (1) sanidine 1 0.000 8.698 42.555 2.755 0.056 0.005 94Z0059B BT-16 (2) “ 1 0.000 8.523 67.193 1.777 0.057 0.002 94Z0059C BT-16 (3) “ 1 0.000 7.261 37.204 2.187 0.067 0.005 94Z0059D BT-16 (4) “ 1 0.000 8.467 52.637 2.265 0.058 0.004 94Z0059F BT-16 (5) “ 1 0.000 5.180 26.503 0.747 0.095 0.007 

Weighted M

± 1SEM 

MSWD 

Appendix 1—Continued
Appendix 1—Continued

Tuffs of Glass Mountain (upper), BT-15 (underlies BT-16; may be contaminated)

Irradiation No.: CXXI-34 J=0.000409Experiment No.: Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

94Z0058A BT-15 (1) sanidine 12 to 15 0.000 6.148 46.202 2.637 0.080 0.005 94Z0058B BT-15 (2) “ 12 to 15 0.000 4.608 67.297 2.503 0.106 0.003 94Z0058C BT-15 (3) “ 10 to 12 0.000 2.684 49.696 5.013 0.183 0.009 94Z0058D BT-15 (4) “ 10 to 12 0.000 6.088 75.214 1.806 0.080 0.002 94Z0058E BT-15 (5) “ 12 to 15 0.000 9.526 54.430 3.334 0.051 0.005 94Z0058F BT-15 (6) “ 12 to 15 0.000 5.622 55.053 2.357 0.087 0.004 

Weighted Me

± 1SEM 

Contaminated? MSWD 

Willow Wash Locality

Tuffs of Benton Hot Springs, FLV-23-WW (see TTC-26B, above)

Irradiation No.: CXXI-30 J= 0.000413Experiment No.: Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

94Z0055A FLV-23-WW 

(1)

plagioclase 7 0.000 0.274 85.892 4.390 1.783 0.003 

94Z0055B FLV-23-WW 

(2)

“ 6 0.000 0.323 86.905 4.438 1.513 0.002 

94Z0055C FLV-23-WW 

(3)

“ 10 0.000 0.299 85.971 4.567 1.636 0.003 

94Z0055D FLV-23-WW 

(4)

“ 7 0.000 0.472 86.738 4.373 1.036 0.002 

94Z0055E FLV-23-WW 

(5)

“ 4 0.000 0.419 73.354 5.246 1.168 0.005 

94Z0055F FLV-23-WW 

(6)

“ 6 0.000 0.372 97.164 4.200 1.315 0.001 

Weighted Me

± 1SEM 

MSWD 
Huckleberry Ridge Ash Bed (sample from locality proximal to Yellowstone National Park source area; J.D. Obradovich, written commun., 1992) Irradiation No.: CIV-A-4 J= 0.000408 Experiment No.: Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 

92Z0547F 68-O-40 (1) sanidine 1 0.000 40.914 77.725 3.572 0.012 0.003 92Z0547E 68-O-40 (2) “ 1 0.000 29.841 76.800 3.645 0.016 0.003 92Z0547D 68-O-40 (3) “ 1 0.000 26.938 87.734 3.191 0.018 0.001 92Z0547C 68-O-40 (4) “ 1 0.000 26.590 88.468 3.235 0.018 0.001 92Z0547B 68-O-40 (4) “ 1 0.000 48.286 82.541 3.301 0.010 0.002 92Z0547A 68-O-40 (6) “ 1 0.000 26.327 90.047 3.110 0.019 0.001 92Z0547F 68-O-40 (7) “ 1 0.000 29.286 77.748 3.591 0.017 0.003 Irradiation No.: CIV-A-4 J=0.000407 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 92Z0547E 68-O-40 (8) “ 1 0.000 15.947 75.974 3.665 0.031 0.003 92Z0547D 68-O-40 (9) “ 1 0.000 21.060 87.445 3.208 0.023 0.001 92Z0547C 68-O-40 (10) “ 1 0.000 20.589 88.140 3.252 0.024 0.001 92Z0547B 68-O-40 (11) “ 1 0.000 29.345 82.242 3.319 0.017 0.002 92Z0547A 68-O-40 (12) “ 1 0.000 21.222 89.757 3.127 0.023 0.001 Irradiation No.: CIV-A-31 J=0.000371 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 92Z0546E 68-O-40 (13) sanidine 1 0.000 18.329 80.895 3.823 0.027 0.002 92Z0546F 68-O-40 (14) “ 1 0.000 24.531 81.116 3.805 0.020 0.002 Irradiation No.: CIV-A-31 J=0.000369 Experiment No. Sample Material No. of grains 40Ar*(mol) K/Ca 40Ar*(%) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 92Z0544B 68-O-40 (15) sanidine 1 0.000 41.949 92.625 3.373 0.012 0.001 92Z0546B 68-O-40 (16) “ 1 0.000 17.151 93.340 3.296 0.029 0.001 92Z0546C 68-O-40 (17) “ 1 0.000 35.930 94.729 3.363 0.014 0.001 92Z0546D 68-O-40 (18) “ 1 0.000 15.436 89.246 3.432 0.032 0.001 92Z0544A 68-O-40 (19) “ 1 0.000 34.855 87.526 3.591 0.014 0.002 

Weighted mean N= 19 

± 1SEM 

MSWD 

Appendix 1—Continued
Sample No. Latitude N Longitude W Quadrangle Rock Type

Material 

Dated Ave. K2

O 40Ar/

R40Ar 40Ar*mol/g 

TTC-24-OB(1) 37 47 00 118 32 00 Glass Mt. 15’ Obsidian glass 4.75 14 0.000 TTC-24-OB(2) 37 47 00 118 32 00 Glass Mt. 15’ Obsidian glass 4.70 25 0.000 

Weighted Mean 

Weighted SEM 

MSWD 

Appendix 2 Analytical data for conventinal K-Ar analysis of obsidian clasts from the lower tephra layers of Glass Mountain at the Cowan Pumice MCalif. John Nakata, analyst, United States Geological Survey, Menlo Park, 1987.
62 Tephra Layers of Blind Spring Valley and Related Upper Pliocene and Pleistocene Tephra Layers, California, Nevada and Utah

Sample # Run # 40Ar* (mol) K/Ca 40Ar* (%) Age (Ma), ±1s Independent age 

estimate (Ma)

Chalk Cliffs Locality, California

BT-14 57A 0.000 2.41 87.2 52.94 ± 0.33 >0.76; <0.87

Sanidine 57B 0.000 9.13 67.9 1.36 ± 0.10

57C 0.000 2.04 90.9 13.32 ± 0.13

57D 0.000 1.27 94.1 48.47 ± 0.37

57E 0.000 10.15 89.7 23.90 ± 0.16

57F 0.000 2.31 81.0 28.40 ± 0.19

Willow Wash Locality, California: In stratigraphic order, from top to base

FLV-125WW 871A 0.000 0.02 62.7 31.70 ± 5.41 >1.79; <1.86

Plagioclase 871B 0.000 0.03 95.0 5.51 ± 2.07

871C 0.000 0.08 97.1 2.62 ± 1.62

871D 0.000 0.01 95.5 6.69 ± 3.05

871E 0.000 0.08 94.2 2.71 ± 1.26

FLV-41WW 870A 0.000 0.06 90.0 29.5 ± 4.6 >2.14; <2.22

Plagioclase 870B 0.000 0.12 95.8 39.4 ± 2.4

870C 0.000 0.05 96.6 38.0 ± 5.4

870D 0.000 0.06 93.2 8.7 ± 4.2

FLV-38WW 869A 0.000 0.16 103.8 2.8 ± 1.5 >2.22: <2.58

Plagioclase 869B 0.000 0.65 97.2 2.6 ± 0.7

869C 0.000 0.27 92.2 4.1 ± 0.9

869D 0.000 0.24 90.0 4.2 ± 1.1

FLV-24WW 56A 0.000 226.16 100.0 89.8 ± 0.5 >2.58; <2.89

Mixed 56B 0.000 3.84 98.6 54.6 ± 0.4

Feldspar 56C 0.000 0.15 95.9 62.9 ± 0.5

56D 0.000 6.21 97.6 13.0 ± 0.1

56E 0.000 2.52 96.3 27.5 ± 0.2

FLV-76WW 868A 0.000 0.13 99.1 21.57 ± 2.08 >2.58; <2.89

Plagioclase 868B 0.000 0.16 97.1 26.88 ± 2.59

868D 0.000 0.08 93.2 19.10 ± 3.22

868E 0.000 0.09 95.6 13.97 ± 7.10

868F 0.000 0.01 97.3 21.67 ± 2.73

FLV-23WW (see appendix 1) 2.89±0.03

FLV-21WW 839B 0.000 0.02 99.1 9.1 ± 13.2 ~3.3

Plagioclase 839C 0.000 0.09 99.0 14.7 ± 2.8

839D 0.000 0.02 98.1 62.9 ± 7.9

839E 0.000 0.04 99.5 65.1 ± 7.4

FLV-96WW 821A 0.000 0.09 92.1 63.56 ± 3.57 ~3.4

Plagioclase 821B 0.000 0.09 94.4 76.52 ± 3.85

821D 0.000 1.93 98.5 18.75 ± 1.38

FLV-119WW 521A 0.000 0.43 95.8 17.99 ± 0.58 >3.4; <5.4

Plagioclase 521B 0.000 0.31 91.1 16.18 ± 0.79

521C 0.000 0.08 84.4 19.60 ± 3.27

521D 0.000 0.36 96.9 15.65 ± 0.69

521E 0.000 0.07 92.5 45.76 ± 2.28

Appendix 3 Analytical data for laser-fusion 40Ar/39Ar analysis of samples that were contaminated with detrital material.
Sample # Run # 40Ar* (mol) K/Ca 40Ar* (%) Age (Ma) Independent age 

estimate (Ma)

FLV-118WW 520A 0.000 0.38 95.1 15.96 ± 0.36 >3.4; <5.4

Plagioclase 520B 0.000 0.50 96.5 19.85 ± 0.56

520C 0.000 0.71 95.4 17.76 ± 0.39

520D 0.000 0.63 98.4 15.93 ± 0.43

FLV-170WW 519A 0.000 0.06 103.6 15.08 ± 0.21 >3.4; <5.4

Plagioclase 519B 0.000 0.03 97.1 31.84 ± 0.99

519C 0.000 0.003 139.3 51.52 ± 0.66

519D 0.000 0.07 104.4 15.21 ± 0.21

519E 0.000 0.04 102.8 17.51 ± 0.76

FLV-169WW 511A 0.000 1.06 95.2 42.06 ± 0.70 >3.4; <5.4

Plagioclase 511C 0.000 0.88 80.4 14.95 ± 2.33

511D 0.000 0.28 99.7 16.34 ± 0.30

511E 0.000 0.00 74.4 20.29 ± 2.14

511F 0.000 0.00 84.6 28.79 ± 2.60

FLV-116WW 518A 0.000 0.01 94.7 26.51 ± 2.97 >3.4; <5.4

Plagioclase 518B 0.000 0.26 98.9 13.90 ± 0.30

518C 0.000 0.25 84.3 60.93 ± 1.22

518D 0.000 0.15 98.0 15.83 ± 0.31

FLV-115WW 514A 0.000 0.002 73.3 33.55 ± 10.5 >3.4; <5.4

FLV-137WW 513A 0.000 0.94 49.9 25.35 ± 2.97 >3.4; <5.4

Plagioclase 513B 0.000 0.03 78.0 93.29 ± 5.19

513C 0.000 0.09 73.6 41.39 ± 1.66

FLV-113WW 511B 0.000 0.07 93.8 24.66 ± 0.35 >3.4; <5.4

Plagioclase 512A 0.000 0.45 95.0 15.71 ± 0.60

512B 0.000 0.17 77.5 14.40 ± 0.36

512C 0.000 0.07 96.3 14.65 ± 0.51

512D 0.000 0.63 88.3 13.65 ± 0.48

Appendix 3—Continued

Appendix 3 63