Age of the 

Coso Formation, 

Inyo County, California 

GEOLOGICAL SURVEY BULLETIN 1527 

Age of the 

Coso Forn1ation, 

Inyo County, California 

By CHARLES R. BACON, DENNIS M. GIOV ANNETT!, 

WENDELL A. DUFFIELD, G. BRENT DALRYMPLE, 

and ROBERT E. DRAKE 

GEOLOGICAL SURVEY BULLETIN 1527 

A re-evaluation of the age of the Coso Formation) 

based on potassium-argon ages of the volcanic rocks that 

are intercalated with) intrude) and overlie ihe 

Coso Formation 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1982 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Bacon, CharlesR., 

Age of the Coso Formation, In yo County, California 

(Geological Survey Bulletin 1527) 

Bibliography: p. 18 

Supt. ofDocs. No.: I 19.3:1527 

I. Geology, Stratigraphic-Tertiary. 2. Geology-Califomia-Inyo County. 

I. Bacon, Charles R. II. Series. 

QE75.B9 No. 1527 [QE90.15] 557.3s 81-607973 [551.7'88'0979487) AACR2 

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

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

Abstract --- - - - - -- - --- --- -- - -- -- - --- -- - -- -- - -- -- - - - -- - ---- - - --- - - - --- - - --- - - 1 

Introduction - -- -- ------ - -- --- - - - -- -- --- - - - -- --- -- - -- - - --- - - --- - - --- - - ---- - - 1 

Acknowledgments ------- ----------- -------------- ---- ------ ----- ----------- 3 

Dating techniques - - - - --- -- --- --- -- --- - --- --- -- - --- ---- --- - - --- - - - --- - - --- -- 3 

Stratigraphic relations between potassium-argon-dated volcanic rocks and beds of 

the Coso Formation -------------------------------------------------- 6 

Miocene volcanic rocks --- - - - --. ----- --- -- - --- --- -- ----- ---- - - --- --- --- - 6 

Pliocene basaltic lava flow ---------------------------------------------- 12 

Pliocene rhyolitic pyroclastic deposits ------------------------------------ 13 

Pliocene rhyodacitic pyroclastic deposits --------------------------------- 14 

Pliocene rhyodacite lava flows ------------------------------------- ----- - 16 

Conclusions ---------------------------------------------------- ---- -------- 16 

References cited - --- -- -- - -- -- --- - - --- -- - - --- - --- -- - - - - - --- - -- - - - - - --- - --- - - - 17 

ILLUSTRATIONS 

Page 

FIGURE 1. Distribution of Coso Formation --- -- -- - - - --- ------- - ---- -- -- - - - -- - - - 2 

2. View southwest from near locality 68------------------------------- 7 

3. Geologic map of northernmost exposures of Coso Formation - -- - - --- -- 8 

4. Correlation sections of southeastern Owens Valley and Coso Range rocks 9 

5. Contact of invasive basalt flow with Coso formation ----------------- 10 

6. Intrusive contact between rhyolite plug-dome and Coso rocks -------- 12 

7. Intrusive contact between rhyodacite and Coso Formation rocks -- ---- 13 

TABLES 

Page 

TABLE 1. Analytical data for potassium-argon measurements -------------------- 4 

2. Fission-track data ------------------------------------------------- 6 

3. Composition of feldspars------------------------------------------- 14 

III 

AGE OF THE 

COSO FORMATION, 

INYO COUNTY, CALIFORNIA 

By CHARLES R. BACON, DENNIS M. GIOV ANETII, 

WENDELL A. DUFFIELD, G. BRENT DALRYMPLE, and 

ROBERT E. DRAKE 

ABSTRACT 

The late Tertiary Coso Formation consisting of sedimentary and volcaniclastic rocks 

occurs in the Coso Range and at the south end of Owens Valley, Calif. Potassium-argon 

dating of the volcanic rocks that overlie, are intercalated with, and intrude the Coso indicates that the Coso Formation contains strata at least as old as 6.0 m.y. (million years) 

and no younger than 2.5 m.y. Within the Coso Formation, Blancan fossils from a locality 

on the northern slope of the Coso Range are probably 3.0 m.y. old and a Hemphillian fossil was recovered northeast of there from beds between 3.6 and 5.3 m.y. old. A rhyodacite 

and four rhyolite plug domes that intrude sedimentary rocks of the Coso on the northern 

flank of the Coso Range range in age from 5.7 to 6.0 m.y. 

The late Miocene fluvial and lacustrine sedimentary rocks of the Coso Formation suggest that a shallow basin had formed in southern Owens Valley by about 6 m.y. ago. Airfall tuff and coarse clastic sedimentary rocks of the Coso Formation within the Coso 

Range suggest that block faulting had begun by about 3 m.y. ago. 

INTRODUCTION 

Tertiary sedimentary rocks that predate Quaternary alluvial deposits 

at the south end of Owens Valley, the westernmost basin of the Basin 

and Range province, and in the Coso Range have been mapped by a 

number of workers as the Coso Formation (fig. 1). This formation is of 

interest because it records sedimentation in response to deformation at 

the western margin of the Basin and Range province, and it contains 

vertebrate fossils of Hemphillian and Blancan mammalian ages and 

uranium minerals of potential economic importance. 

Late Cenozoic rocks in the area that are now included in the Coso 

Formation were described by early workers (for example, Knopf, 1918). 

Schultz (1937) defined the Coso Formation to include all Cenozoic 

sedimentary deposits in the area that lie beneath late Cenozoic lava 

flows and described vertebrate fossils from two localities within the 

Coso at the north end of the Coso Range. In light of more recent work, 

1 
2 

15' 

c:: 

0 

·.;:::; 

co 

E 

0 

u.. 

0 

U) 

0 

u 

AGE, COSO FORMATION, INYO COUNTY, CALIF. 

Owens Lake 

(dry) 

m 

)> 

z 

m 

< 

)> 

c 0 s 0 

R A N G E 

Upper 

Cactus 

Flat 

•43 

,., 

EXPLANATION 

~ Pyroclastic deposits 

~Fanglomerate 

I2IT] Arkosic and lacustrine 

deposits 

--Contact 

•

52 K-Ar sample locality 

.A Locality that yielded Coso Hot 

Agriotherium remains Springs A •F Approximate location <.....$48 

(Stinson, 1964) of !' 

fauna described by 

Schultz (1937) OO'L_ 36° __________ L_ ____________________________ L_ __ ~----------~ 

10 15 20 KILOMETERS 

FIGURE !-Distribution of the Coso Formation generalized and modified from Hall and 

MacKevett (1962), Stinson (1977a, b), Duffield and Bacon (1981), and Giovannetti 

(1979a). Numbers refer to sample localities of table 1 and Duffield, Bacon, and 

Roquemore (1979). 
DATING TECHNIQUES 3 

Schultz's definition (1937, p.79) of the Coso should be clarified because 

the lava flows in the Coso Range have a wide age range. We believe that 

the lava flows referred to by Schultz are the rhyodacite flows on the 

Haiwee Ridge (fig. 1) described in the section on Pliocene rhyodacite 

lava flows. 

Parts of the Coso Formation have been described by Hopper (1947), 

Power (1959, 1961), Bacon and Duffield (1978), Duffield, Bacon, and 

Roquemore (1979), Duffield, Bacon, and Dalrymple (1980), and 

Giovannetti (1979a, b). Deposits mapped as the Coso (fig. 1) Occur in 

the Haiwee Reservoir (Stinson, 1977a), Keeler (Stinson, 1977b), and 

Darwin (Hall and MacKevett, 1962) 15-minute quadrangles and have 

been mapped in the Coso volcanic field (Duffield and Bacon, 1981). 

Potassium-argon dating of volcanic rocks intercalated with, and 

overlying, the Coso Formation has been used by Evernden, Savage, 

Curtis, and James (1964), Bacon, Giovannetti, Duffield, and Dalrymple 

(1979), and Duffield, Bacon, and Dalrymple (1980) to constrain the age 

of the Coso. In this report, we review these potassium-argon ages and 

present additional data that provide more accurate limits on the age of 

the Coso Formation. 

ACKNOWLEDGMENTS 

We thank C. A. Repenning for advice on paleontologic aspects of this 

study. G. R. Roquemore contributed valuable observations of field relations of pyroclastic rocks near Sugar Loaf. J. Metz painstakingly 

prepared and analyzed glass fractions of tuffs. C. E. Meyer and M. J. 

Woodward kindly determined fission-track ages of zircons. Part of this 

report is based on the second author's M. S. thesis, which was completed under the guidance of Professor R. L. Hay. Potassi urn-argon age 

determinations at the University of California, Berkeley, were made 

possible through National Science Foundation grant EAR76-21833. 

DATING TECHNIQUES 

Potassium and argon analyses were performed on crushed whole-rock 

samples of obsidian and basalt and on separates of sanidine, plagioclase, and biotite. The analytical data are presented in table 1 and include some data published previously (Duffield and others, 1980) for 

the sake of completeness. Analyses of samples from localities 43-66 

were performed at the U.S Geological Survey, Menlo Park, Calif., as 

described by Duffield, Bacon, and Dalrymple (1980); those for localities 

67-72 were made at the University of California at Berkley by techniques described by Dalrymple and Lanphere (1969). 

Fission-track ages of zircons (table 2) were determined by techniques 

described by Naeser (1976). 
~ 

TABLE !.-Analytical data for potassium-argon measurements on volcanic rocks 

[Samples from localities 43 to 66 analyzed at U.S. Geological Survey, Menlo Park, California; 67-72 analyzed at Department of Geology and Geophysics, University of California, Berkeley, California;. 

43-63 from Duffield, Bacon, and Dalrymple (1980). Analytical constants: AE+AE' = 0.581X 10- 10 yr- 1, >.13 =4.692X10- 10 yr- 1, 4°K/K = 1.167X 10-• moVmol. Errors are estimated standard deviations (Cox and Dalrymple, 1967)) 

> 0 Per- 40Arrad Per- Calcu- Weighted J:rJ Loc- Sample Lat Long Stratigraphic Material centage Weight (X1012 centage latedage mean age ("':) ality No. relations K20 (gm) mol/gm) .. Arrad (million years) (million years) 0 

r:n 

0 43 9--8-11 36°06'04" 117°50'02" Near top of Coso Formation, Biotite --------- 6.48, 6.47 1.151 22.975 2.4 2.46±0.98 '"%j 

Upper Cactus Flat; 0 

air-fall pumice. Plagioclase------ 0.460, 0.457 5.863 2.004 16.0 3.04±0.20 2.99±0.20 ~ 48 13-113-6 36°02'10" 117°44'22" Near top of Coso Formation, Biotite --------- 7.01, 6.98 1.96 33.45 4.1 3.31±0.42 > t-3 East of Coso Hot l-ot 

Springs; air-fall pumice. Plagioclase------ 0.524, 0.513 6.108 2.203 24.4 2.95±0.13 0 

,;Z 44 9-85-2 36°11'56" 117°55'14" Within Coso Formation, Biotite --------- 7.62, 7.67 1.977 41.34 5.9 3.75±0.32} l-ot 

between Cactus Flat 3.14±0.15 z and Haiwee Reservoir; Sanidine---- ---- 12.31, 12.32 3.835 54.88 66.2 3.09±0.09 t< 

0 

pumice in ("':) 

pyroclastic flow. 0 

52 75G317 36°08'02" 117°52'00" Overlies Coso Formation, Biotite --------- 8.63, 8.63 27.8} c:::: {4.711 30.90 

251±0~1 z McCloud Flat; 5.208 31.69 33.6 t-3 

rhyodacite lava flow. 2.52±0.05 ~t< 

61 13-43-13 36°06'00" 117°53'32" Overlies Coso Formation, - -do - --- - -- -- -- 8.58, 8.55 3.357 31.40 24.5 2.54±0.05 ("':) 

> Haiwee Ridge; t""' 

rhyodacite lava flow. ~ 

63 8-191-1 36°14'10" 117°45'20" Within Coso Formation, Basalt _______ ---{0.883, 0.884} {13.406 4.087 11.7} 3.56±0.10 

Upper Centennial Flat; 0.880, 0.886 15.521 4.736 45.9 

lava flow. 

64 13-51-1 36°19'12" 117°52'00" Intrudes Coso Formation, Biotite - -------- 8.83, 8.84 1.628 72.90 47.0 5.72±0.17 

NW slope of Coso Range; 

rhyolite plug. 
65 13-52-2 36°20'16" 117°50'24" Intrudes Coso Formation, Obsidian -------

{

4

·

35• 

4

·

26} f.184 36.88 43.2} 5.99±0.08 

N. end of Coso Range; 4.32, 4.27 3.437 37.44 54.9 

rhyolite plug. 

66 13-52-3 36°20'32" 117°50'22" Intrudes Coso Formation, _-do -----------{4.32,4.28} {3.211 36.58 48.3} 5.98±0.06 

N. end of Coso Range; 4.32, 4.28 3.323 37.37 42.1 

rhyolite plug. 

67 78--G-137 36°22'20" 117°49'44" Invades Coso Formation, Basalt ---- --- --- 1.238 5.038 10.49 38.30 5.88±0.18 

between Coso River 

and lnyo Mountains; 

lava flow. 

68 78-G-133 36°22'36" 117°49'20" Overlain by Coso Formation, 

- -do ---- ----- -- 1.450 4.965 10.98 22.8 5.25±0.23 

between Coso River 

and Inyo Mountains; tj 

> lava flow. :j 

69 78--G-70 36°22'54" 117°48'54" Interbedded with Coso Sanidine--------10.18 3.192 86.55 83.1 5.90±0.12 z 

Formation, between Coso 0 

River and Inyo t-3 

t_:r::j Mountains; rhyolite 0 dome breccia. ::X:: 

z 70 78--G-135L 36°23'12" 117°48'18" Overlain by Coso Formation, Basalt ----- ----- 1.907 5.004 15.40 69.8 5.60±0.17 ...... 

.0 between Coso River c::: and lnyo Mountains; t_:r::j 

lava flow. en 

71 78-G-131 36°23'24" 117°50'42" Overlies Coso Formation, 

- -do -- -- ---- --- 1.299 5.006 10.35 25.1 5.53±0.22 

between Coso River 

and Inyo Mountains; 

lava flow. 

72 78-G-130 36°23'54" 117°50'00" Overlies Coso Formation, 

- -do ---- ------

- 1.282 5.070 11.17 22.9 6.04±0.26 

between Coso River 

and Inyo Mounains; 

lava flow. 

01 
6 AGE, COSO FORMATION, INYO COUNTY, CAUF. 

TABLE 2.-Fission-track data from zircons 

[First determination by C. E. Meyer, second by M. J. Woodward. Analytical constant X :?.7.03xl0- 17 yr- 1

] 

Locality Sample 

43 9-8--11 

43 9-8-11 

1tracks/cm2 • 

2

neutrons/cm2 • 

Spontaneous tracks 

No. counted Density' 

217 7.1X105 

390 5.3X105 

Induced tracks Neutron 

Age 

(million years, No. counted Density' flux2 ±2u) 

2088 1.4X107 9.59X1014 2.97±0.28 

3754 1.1X107 9.59X1014 2.90±0.20 

STRATIGRAPHIC RELATIONS BETWEEN 

POTASSIUM-ARGON-DATED VOLCANIC ROCKS AND BEDS OF 

THE COSO FORMATION 

MIOCENE VOLCANIC ROCKS 

Between Haiwee Reservoir and the northeast side of the Coso Range, 

several potassium-argon dates of between 5.3 and 6.0 m.y. (million 

years) on volcanic rocks (locs. 64-67, 69, 71, 72) indicate that some of 

the strata mapped as Coso Formation by Stinson (1977b) and Giovannetti (1979a) are late Miocene in age. These rocks occur in three centers 

of silicic volcanism. Basalt flows occur near all three silicic eruptive 

centers (fig. 2). 

The first center, the northermost of the three, has been studied in 

detail by Giovannetti (1979a). At locality 69 (fig. 3), a porphyritic 

rhyolite flow-dome and pyroclastic complex are interbedded with 

basalt dated at 6.04±0.26 to 5.25±0.23 m.y. (figs. 3, 4; table 1, locs. 67, 

68, 70, 71, and 72). Sanidine from the porphyritic allanite-biotitealbite-sanidine-quartz rhyolite yields a potassium-argon age of 

5.90±0.10 m.y. (figs. 3, 4; table 1, loc. 69). Compositions of plagioclase 

and sanidine phenocrysts in this rhyolite are given in table 3. The 

rhyolite is overlain by basalt with potassium-argon ages of 5.25±0.23 

m.y. and 5.53±0.22 m.y. (figs. 3, 4; table 1, locs. 68 and 71). 

At locality 67 (figs. 1, 3), basalt dated at 5.88±0.18 m.y. (table 1) intrudes calcareous lacustrine and arkosic fluvial beds of the Coso Formation in the manner of an invasive lava flow (Byerly and Swanson, 1978). 

Disintegrated lava fragments, generally lapilli size, are thoroughly 

mixed with sediment underlying and peripheral to the flows. Similar 

mixtures of sediment and fragmented lava, termed "peperites," have 

been described by Schmincke (1967) as resulting from the invasion of 

unconsolidated wet sediments by molten lava (fig. 5). In addition, the 

basalt is characteristically altered to green and purple mixtures of clay, 
STRATIGRAPHIC RELATIONS 

Sugar loaf 

I 

7 

FIGURE 2.-View looking southwest from near locality 68. Basalt lava flows in foreground. 

Coso Formation underlying 5-m.y.-old basalt lava flow in middle distance; northern 

end of Coso Range in the far distance; Sugar Loaf, the small rounded feature in the 

middle distance, is a porphyritic rhyodacite plug-dome; Sierra Nevada forms right 

skyline. 

carbonate, and hydrated iron oxide minerals. Similar relations are evident between basalt and sedimentary rocks between localities 66 and 67 

(Bacon and Duffield, 1978). 

Near the northernmost rhyolite complex (loc. 69), arkosic sedimentary deposits are intercalated with basalt flows, some of which may be 

invasive. A thick (30m), altered dacite flow appears to be the youngest 

volcanic rock in the area. Relatively extensive alteration of most of 

these volcanic rocks suggests water saturation of the sediments into or 

onto which they flowed. The presence of large euhedral crystals of gypsum and scalenohedrons of calcite in vugs and fractures suggests post 

eruption exposure of the volcanic rocks to high-salinity, alkaline water, 

either from groundwater or shallow lakes. Bedded arkosic fluvial 

sedimentary rocks overlie the entire section of volcanic rocks in this 

area (Giovannetti, 1979a, b). It was from these sedimentary rocks that 

the Agriotherium remains were recovered. The 4.8- to 6.2-m.y. age 

implied for Agriotherium is consistent with the maximum age for these 

rocks as indicated by the 5.3- to 5.9·-m.y. potassium-argon ages of the 

volcanic rocks stratigraphically beneath them (locs. 68-70) and the 
8 

36° 

25' 

36° 

20' 

I 

AGE, COSO FORMATION, INYO COUNTY, CALIF. 

Owens / 

,/ 

/" 

Lake /,/ 

(dry) / 

,,./'-'.," 

) 

D Alluvium 

I 

I 

CE1 Coso Formation 

Rhyolite 

Dacite 

EXPLANATION 

ci:: }w>- rm t-0:: 

<(<( v Basalt 

:JZ 

li EJ Basement rock 

--Contact 

.71 K-Ar sample locality 

.A Locality that yielded 

' I- Agriotherium remains 

Lower 

Centennial 

Flat 

FIGURE 3.-Geologic map of area showing northernmost exposures of Coso Formation, 

modified from Stinson (1977a, b) and Giovannetti (1979a); faults omitted. 
East of 

Haiwee 

Reservoir 

Haiwee Ridge Southeast 

of Coso 

Hot Springs 

c:J=c><::> 25 

~:::'4-~;;t ,;>' -------------- · -o.u 1 c c::l C:::J Cl- .,." •. ·.c • , oo 1 _: _-_. <:::;, <:? 

Northeast 

of Coso 

Hot Springs 

~·.:; 0 C1l 

~ 

u § 

0 I,A .aQ..q u.. ~~~~ 

~~~~ 

~ 

EXPLANATION 

Fanglomerate 

Air-fall pyroclastic 

deposits 

Silicic flows, domes, 

and plugs 

Basalt flows 

~ 

~ 

~ 

Lacustrine and 

fluvial deposits 

Pyroclastic flows 

[ill Dacite flows 

~ ~ Granitic rocks 

5.3 K-Ar age, in millions of years 

3.0. 

~ 

~,·,·,,:,·;·n5.3 

5.6 

_____ ...-. 

{

5.7 

3 plugs 6.0 

6.0 

Upper and 

Lower 

--=:--=----= ~::~-=-= 

FIGURE 4.-Schematic stratigraphic sections of the southeastern Owens Valley and Coso Range showing correlation and age of volcanic and sedimentary rocks. Unit A is rhyolite pumice of locality 44 and compositionally similar, apparently correlative deposits; B is rhyodacite air-fall 

pumice of locations 43 and 48. Stratigraphic relation between A and B northeast of Coso Hot Springs inferred. 

w 

o-3 

~ 

~ 

~ > "d 

::r:: 

(3 

::0 

t,:tj 

t'"' 

~ 0 

z 

w 

co 
10 AGE, COSO FORMATION, INYO COUNTY, CAUF. 

FIGURE 5.-Contact of an invasive basalt flow with sedimentary rocks of the Coso 

Formation near locality 67. Fragments of highly altered columnar basalt are locally 

mixed with clay-rich sediment. Pencil gives scale. 
STRATIGRAPHIC RELATIONS 11 

m1n1mum age defined by the 3.6-m.y .. -old basalt (loc. 63) overlying 

them. 

The second center comprises a group of three aphyric, apparently 

coeval rhyolite plug-domes (figs. 3, 4; locs. 65 and 66) that intrude tuffaceous lacustrine and fluvial sediments mapped as Coso Formation by 

Stinson (1977b). The three plugs consist predominantly of felsite and 

minor perlitic glass; the southern and middle ones contain small 

amoants of obsidian (marekanite) that are chemically identical (C. R. 

Bacon and R. L. Smith, unpub. data, 1977) and yield potassium-argon 

ages of 5.99±0.08 and 5.98±0.06 m.y. (table 1, locs. 65 and 66, respectively). Thin beds of arkosic fluvial and tuffaceous lacustrine sediments 

are nearly flat lying within less than 100m of the plug-domes but have 

been forced into nearly vertical attitudes adjacent to the plug-domes by 

intrusion of the rhyolite (fig. 6). Glassy and devitrified zones appear to 

parallel contacts with sedimentary rocks, although not so clearly as at 

Sugar Loaf (fig. 7). Moderately dipping air-fall and base-surge rhyolitic 

pyroclastic deposits with abundant, well-developed accretionary lapilli 

are exposed in a small drainage adjacent to the northernmost plugdome on its northwest side. These plug-domes are the Olancha domes of 

Friedman and Smith (1958) and Smith and Shaw (1975). 

The third center, Sugar Loaf, is a porphyritic rhyodacite plug-dome. 

This dome has yielded a potassium-argon biotite age of 5. 72±0.17 m.y. 

(figs. 3, 4; table 1, loc. 64). Several lines of evidence here suggest that 

the rhyodacite intrudes the tuffaceous fluvial and lacustrine beds of the 

Coso Formation and the intercalated basalt: (1) the sedimentary 

deposits dip steeply away from the contact with the rhyodacite, 

whereas less than 100 m from the contad, they dip gently; (2) foliation 

in the rhyodacite, as defined by platy jointing and by preferred orientation of phenocrysts, is steep to vertical everywhere near the contact and 

is nearly parallel to the contact (fig. 7); (3) abundant columnar joints, 

10 em in diameter, are oriented perpendicular to the contact (fig. 7); ( 4) 

crystallization of the rhyodacite is concentrically zoned, with a discontinuous band of dark vitrophyre about 1m thick at the margin (fig. 7), 

followed by several meters of gray perlite that grade into a pink, largely 

devitrified core; and (5) the sediments, particularly those rich in clay, 

are altered and appear to be baked near contacts with the rhyodacite. 

Stinson (1977b) also noted the intrusive nature of the contact between 

this rhyodacite and the Coso. Approximately 350 m due west of the 

southern margin of the Sugar Loaf plug-dome, a 3-m-thick bed of 

pumiceous rhyodacite blocks is interbedded with clayey arkosic sandstone. The blocks form an avalanche or pyroclastic-flow deposit from 

Sugar Loaf, indicating that the rhyodacite reached the surface and that 

sedimentation of the Coso was in progress before and at the time of 

emplacement of the plug-dome. 
12 AGE, COSO FORMATION, INYO COUNTY, CALIF. 

FIGURE 6.-Intrusive north contact between middle rhyolite plug-dome (left; middle 

Olancha dome of Smith and Shaw, 1975) and sedimentary rocks of the Coso Formation. Prominent, steeply dipping outcrops are flow-banded vitrophyre just within 

rhyolite from contact. Bedding in Coso sedimentary rocks is virtually parallel to contact and flow banding. Note man (in circle) for scale. 

PLIOCENE BASALTIC LA VA FLOW 

At the north end of Upper Centennial Flat in the Coso Range, basalt 

dated at 3.56±0.10 m.y. (Duffield and others, 1980; fig. 1; table 1, loc. 

63; fig. 4) is overlain by alluvial fan deposits of the Coso Formation. 

These deposits contain a bed of hornblende-biotite-rhyodacite pumice 

lapilli. The pumice in this bed is probably correlative with pumice 

(table 1, locs. 43 and 48) dated at about 3.0 m.y. Thus, these alluvial 

fan deposits are probably in the upper part of the Coso Formation. Five 

kilometers to the north, at Lower Centennial Flat, the same basalt flow 

lies on about 100m of calcareous lacustrine and arkosic fluvial deposits 

(fig. 3) that were assigned to the Coso Formation by Hall and 

MacKevett (1962) and by Stinson (1977b). The arkosic deposits may be 

correlative with similar deposits 10 km to the north-northwest (figs. 1, 

3, loc. A) where a mandibular ramus identified as Agriotherium sp. cf. 

A. sivalensis (C. A. Repenning, written commun., 1979) was found by 

D. W. Andersen. According to Repenning, Agriotherium lived in North 

America in late Hemphillian time, most probably between 4.8 and 6.2 

m.y. ago. 
STRATIGRAPHIC RELATIONS 13 

PLIOCENE RHYOLITIC PYROCLASTIC DEPOSITS 

Rhyolitic air-fall and ash-flow deposits of the Coso Formation occur 

at several localities. All the occurrences described in the following 

paragraphs apparently represent a single eruption, or a series of eruptions closely spaced in time, from vents immediately east of Haiwee 

Reservoir. 

A weighted-mean potassium-argon age of 3.14±0.15 m.y. on sanidine 

and biotite from large pumice lumps in a rhyolite ash-flow tuff of the 

Coso Formation (loc. 44; Duffield and others, 1980) provides age control 

for the upper part of the Coso. This ash-flow tuff, mapped as rhyolite 

tuff breccia by Stinson (1977a), is interbedded with lacustrine deposits 

of the Coso east of the northern part of Haiwee Reservoir (fig. 4). 

Pumice in the ash-flow tuff contains phenocrysts of quartz, oligoclase, 

sanidine, and biotite. Rhyolite pumice identical in appearance to that 

in the ash-flow tuff east of the Haiwee Reservoir occurs northeast of 

Coso Hot Springs in a bed 1 m thick within alluvial deposits, which are 

overlain by a dacite lava flow (fig. 4). 

Similar pumice and ash form base-surge and near-vent explosion 

deposits at the base of the pyroclastic section mapped as Coso FormaFIGURE 7.-Intrusive contact between rhyodacite of Sugar Loaf and sedimentary rocks of 

the Coso Formation. Flow banding and dark vitrophyric zone in rhyodacite are parallel to contact; columnar joints are perpendicular. Sugar Loaf is to right (west) in 

photo. Pick at contact gives scale. 
14 AGE, COSO FORMATION, INYO COUNTY, CALIF. 

tion at the south end of Haiwee Ridge (fig. 4; Duffield and Bacon, 1981; 

Duffield and others, 1980). 

An approximately 30-cm-thick bed of reworked rhyolite pumice 

lapilli is present within sedimentary rocks of the Coso about 1.5 km 

northwest of the vertebrate fossil locality (fig. 1, loc. F) described by 

Stinson (1964). This material apparently came from the same volcanic 

source as the rhyolite ash flow, on the basis of phenocryst assemblage 

and rubidium, strontium, yttrium, and zirconium content (X-ray 

flourescence analyses) of glass seperated from pumice. Microprobe 

analyses show the oligoclase and sanidine to be similar to, but slightly 

more sodic than, the same phases in pumice from the ash-flow tuff 

(table 3). This observation is consistent with the idea that the bed of 

pumice lapilli represents the reworked air-fall component of the same 

eruption that resulted in emplacement of the ash-flow tuff (Hildreth, 

1979). By similar reasoning, it is unlikely that the pumice lapilli are 

related to emplacement of the late Miocene rhyolitic plug-domes 

because feldspars from the only porphyritic rhyolite (table 3, loc. 69) 

are more sodic than would be expected if the reworked pumice lapilli 

were to represent an early phase of emplacement. 

TABLE 3.-Compositions of feldspars in silicic rocks 

[Compositions calculated from average analyses of phenocrysts determined with ARL-EMX electron microprobe. See 

Bacon and Duffield (1981) for operating conditions and data reduction techniques) 

Weighted mean Average composition of feldspar (mole percent) 

Locality Sample K-Arage Type of 

No. of deposit Sanidine Plagioclase deposit 

(million years) An Ab Or An Ab Or 

F 9-90-2 Not present --- 42.4 55.2 2.4 Pumice in water-laid tuff. 

43 9-8-11 3.0 ---do. -------- 41.2 56.5 2.3 Pumice in air-fall tuff. 

48 13-113-6 3.0 ---do. -------- 33.6 63.1 3.3 ---Do.-----------

9-90-1 0.5 30.5 69.0 11.9 82.8 5.3 Pumice in reworked tuff. 

44 9-85-2 3.1 .6 26.5 72.9 15.5 79.4 5.1 Pumice in ash-flow tuff. 

69 77CDG-70 5.9 .6 34.4 65.0 9.0 84.4 6.6 Pumiceous rhyolite from 

dome. 

PLIOCENE RHYODACITIC PYROCLASTIC DEPOSITS 

A thick sequence of porphyritic rhyodacite air-fall, ash-flow, and 

base-surge deposits overlies the rhyolite pumice deposits at the south 

end of Haiwee Ridge (Fig. 4; Duffield and others, 1979, 1980). These 

rhyodacite deposits were mapped as andesite tuff and breccia by 

Stinson (1977a). Biotite and plagioclase from pumice in correlative 

rhyodacite air-fall deposits from two widely separated localities in the 
STRATIGRAPHIC RELATIONS 15 

Coso Range yield a weighted-mean age of 2.99±0.20 m.y. (Duffield and 

others, 1980; locs. 43 and 48). Fission-track ages of zircons from the 

pumice (table 2) are in good agreement with the potassium-argon age. 

This rhyodacite pumice is easily recognized by its biotite-hornblendeandesine phenocryst assemblage and thus provides a means for the correlation of sedimentary rocks throughout much of the Coso Range (fig. 

1, this report; Duffield and Bacon, 1981; Duffield and others, 1980). Airfall deposits of this rhyodacite pumice occur interbedded with coarse alluvial sedimentary deposits mapped as Coso in a series of grabens south 

of lat. 36°15' N (Duffield and others, 1979). 

Pumice studied by Evemden, Savage, Curtis, and James (1964; sample KA 451) from a water-laid tuff reported to be about 30 m 

stratigraphically above the Blanchan vertebrate fossil locality (fig. 1, 

loc. F) described by Schultz (1937; loc. 131) and Stinson (1964) is apparently reworked 3.0-m.y.-old rhyodacite air-fall tephra. We collected 

several rounded pumice lumps 1-5 em in diameter from a bed of waterlaid tuff believed to be that described by Evernden, Savage, Curtis, and 

James (1964). This pumice contains the same phenocryst assemblage as 

does that from Haiwee Ridge; glass separated from one large lump has 

contents of rubidium, strontium, yttrium, and zirconium (determined 

by X-ray flourescence) that are closely similar to glass from pumice of 

the 3.0-m.y .-old rhyodacite air-fall tuff of Haiwee Ridge. Plagioclase 

phenocrysts in all Coso rhyodacitic rocks are complexly zoned; consequently, microprobe analyses of crystal fragments separated from 

pumice lumps are variable. Despite this fact, the average compositions 

determined by analyzing many grains are very similar for the pumice 

lumps from the water-laid tuff and for dated air-fall pumice (table 3), 

and this similarity provides further evidence for correlation. The large, 

rounded pumice lumps were probably derived from air-fall tuff shortly 

after its deposition. The possibility that the pumice was reworked much 

more recently is considered remote but cannot be ignored. Thus, the 

rocks containing the vertebrate fossils at this locality are probably 

about 3.0 m.y. old. Evemden, Savage, Curtis, and James (1964) determined the potassium-argon age of biotite from pumice lumps, apparently from the same outcrop of water-laid tuff. The biotite concentrate analyzed by them (sample KA 451) contained only 4 percent 

radiogenic 40Ar and 3.87 percent K20. When recalculated using current decay constants (table 2), the original analytical data yield an age 

of 2.31 m.y. with a large unevaluated analytical error; we believe that 

this defines only an imprecise minimum age for the pumice because of 

the appparently altered or very impure nature of the biotite concentrate. 

Wilson (1932) described a small rodent, Cosomys primus, from the 

same locality studied by Schultz (1937, loc. 131). He tentatively assigned a Pliocene (Blanchan) age to fossiliferous strata, on the basis of 
16 AGE, COSO FORMATION, INYO COUNTY, CALIF. 

horse material. Neville, Opdyke, Lindsay, and Johnson (1979), investigating the magnetic stratigraphy, radiometric age relationships, 

and fossil mammals of the Glenns Ferry Formation in Idaho, determined an age of 3.75 m.y. for the lowest stratigraphic occurence of 

Cosomys. According to C. A. Repenning (written commun., 1979) 

Cosomys primus from the Coso Formation is about 3.4 m.y. old. The 

pumice-bearing water-laid tuff appears to be no older than 3.0 m.y. 

Consequently, if the relation of the tuff to the fossiliferous strata is correct, a gap in the record may be present or an unusually low sedimentation rate may have pertained to the intervening period. 

PLIOCENE RHYODACITE LAVA FLOWS 

Several thick flows of porphyritic rhyodacite overlie both the rhyolite 

and rhyodacite pumice deposits of the Coso Formation in the vicinity of 

Haiwee Ridge (Duffield and Bacon, 1981; Duffield and others, 1980). 

These flows were described as andesite by Stinson (1977a) and 

Chesterman (1956). All the flows are similar in appearance and composition; the weighted mean of two biotite age determinations is 

2.52±0.05 m.y. (table 1, locs. 1 and 2; Duffield and others, 1980). This 

age is probably consistent with an age of 2.01 m.y. calculated from the 

original data of Evernden, Savage, Curtis, and James (1964, sample KA 

1026) for low-potassium biotite from the northernmost flow. Duffield, 

Bacon, and Dalrymple (1980, loc. 51) were unable to obtain a 

meaningful age for either biotite or hornblende from the same locality 

studied by Evernden and coworkers (sample KA 1026). Some of the 

rhyodacite flows fill canyons in the under lying pyroclastic rocks of the 

Coso Formation (fig. 4; Duffield and others, 1980) and provide a 

minimum age of approximately 2.5 m.y. for the Coso Formation. 

CONCLUSIONS 

The potassium-argon ages of volcanic rocks that overlie, are interbedded with, and intrude the Coso Formation indicate that the formation 

includes strata that range in age from somewhat greater than 2.5 m.y. 

to at least 6.0 m.y. old and that the Coso Formation is Miocene and 

Pliocene in age. The oldest exposed part of the Coso Formation crops 

out from the north end of the Coso Range northeast to the lnyo Mountains. Sedimentary rocks of the Coso Formation northeast of the Coso 

Range contain a vertebrate fossil (Agriotherium) that is not younger 

than 4.8 m.y. old. The 3.0-m.y. age implied for the Blanchan fossilbearing strata of the Coso in the Coso Range (fig. 1, loc F) described by 

Schultz (1937, locs. 131 and 284) is consistent with radiometric age 
REFERENCES CITED 17 

determinations on rocks associated with Blanchan faunas from other 

areas (C. A. Repenning, oral commun., 1978; Repenning, 1980); the 2.3-

m.y. age calculated from the data of Evernden, Savage, Curtis, and 

James (1964) is probably a minimum age. 

The age of the Owens Valley, the westernmost basin of the Basin and 

Range province, is of considerable significance to the history of Basin 

and Range extension. Fluvial and lacustrine deposits exposed northeast 

of the Coso Range indicate that a basin, possibly the tectonic precursor 

to the Owens Valley, began collecting sediment at least 6 m.y. ago. 

Crustal instability in the region is documented by the presence of 

arkosic sedimentary rocks and by the contemporaneous eruption of 

mafic and silicic volcanic rocks. Sedimentary rocks slightly older than 

2.5 m.y. occur in the northwestern Coso Range and coarse 3-m.y.-old 

clastic sedimentary deposits are cut by graben-forming faults in the interior of the range. Volcanism began within the Coso Range approximately 4 m.y. ago (Duffield and other~, 1980). The ages of volcanic and 

sedimentary rocks in southern Owens Valley show a shift from early 

volcanism and sedimentation (5-6 m.y .) on the north flank and 

northeast of the Coso Range to later volcanism, block faulting, and 

sedimentation (2.5-4 m.y .) within the range. 

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t.U.S. GOVERNMENT PRINTING OFFICE: 1982-576-034/129