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Stratigraphy of the Lower and Middle(?) Triassic 

Union Wash Formation, East-Central California 

By PAUL STONE, CALVIN H. STEVENS, and 

MICHAEL j. ORCHARD 

U.S. GEOLOGICAL SURVEY BULLETIN 1928 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1991 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Stone, Paul. 

Stratigraphy of the Lower and Middle(?) Triassic Union Wash Formation, eastcentral California I by Paul Stone, Calvin H. Stevens, and Michael J. Orchard. 

p. em. - {U.S. Geological Survey bulletin ; 1928) 

tncludes bibliographical references. 

Supt. of Docs. no.: I 19.3:1928 

1. Geology, Stratigraphic-Triassic. 2. Geology--California-lnyo Mountains 

Region. 3. Paleontology-Triassic. 4. Paleontology--California-lnyo Mountains 

Region. 5. Union Wash formation (Calif.) I. Stevens, Calvin H. II. Orchard, M.J. 

Ill. Title. IV. Series. 

QE75.B9 no. 1928 

lQE677) 

557.3 s-dc20 90-13877 

[551.7'62'09794 CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Previous studies 1 

Outcrop distribution 3 

Stratigraphy and contact relations 3 

Union Wash area 3 

Cerro Gordo area 9 

Darwin area 13 

Fossils and age 16 

Union Wash area 16 

Cerro Gordo area 18 

Darwin area 18 

Summary 19 

Depositional environments 19 

Depositional history 20 

References cited 21 

Measured sections 22 

FIGURES 

1. Location map of east-central California 2 

2. Geologic map of Union Wash area, Inyo Mountains, California 4 

3. Type section of the Union Wash Formation in Union Wash area 6 

4. Geologic map of Cerro Gordo area, Inyo Mountains, California 8 

5. Reference section of the Union Wash Formation in Cerro Gordo area 11 

6. Measured section of structurally repeated parts of subunit 4 of upper member of 

the Union Wash Formation on northwest side of Cerro Gordo Road 12 

7. Geologic map of Darwin area, California 14 

8. Reference section of the Union Wash Formation in Darwin area 17 

9. Chart showing age range of the Union Wash Formation 19 

Contents Ill 

Stratigraphy of the Lower and Middle(?) Triassic Union 

Wash Formation, East-Central California 

By Paul Stone, Calvin H. Stevens, and Michael J. Orchard 

ABSTRACT 

Marine sedimentary rocks of Early and Middle(?) Triassic age in the lnyo Mountains and nearby areas of east-central 

California are herein assigned to the Union Wash Formation, 

the type locality of which is at Union Wash on the west side 

of the lnyo Mountains. In the areas studied, the Union Wash 

Formation ranges in thickness from slightly less than 700 m to 

about 850 m and is divided into three informal members. 

The lower member, generally less than 50 m thick, is composed of silty and sandy limestone; the middle member, approximately 250 to 680 m thick, is composed of thin-bedded 

mudstone and micritic limestone; the upper member, approximately 450 m in maximum thickness, is composed of limestone, silty and sandy limestone, shale, quartzose siltstone, 

and very fine grained quartzose sandstone. This stratigraphic 

sequence defines a transgressive-regressive depositional cycle 

on the subsiding continental margin of North America. Conodonts and ammonoids indicate that the Union Wash Formation 

contains rocks of Smithian, Spathian, and possibly earliest 

Anisian age. Deposition of the Union Wash Formation followed a complex episode of basinal marine sedimentation 

and subsequent uplift in the Permian and was followed in 

later Triassic or Jurassic time by the onset of arc magmatism 

that continued through the remainder of the Mesozoic. 

INTRODUCTION 

Marine sedimentary rocks of Early and Middle(?) 

Triassic age crop out widely in the Inyo Mountains and 

nearby areas of east-central California (fig. 1). These rocks, 

the youngest marine rocks in the Inyo Mountains region, 

are composed of sediment deposited on the North American 

continental margin before the onset of subduction and arc 

magmatism in later Triassic or Jurassic time (Dunne and 

others, 1978). 

The best-known and most accessible exposures of these 

Triassic marine rocks are at the west foot of the In yo Mountains at Union Wash (fig. 1), a locality from which J.P. Smith 

Manuscript ~pproved for publication, December 21, 1989. 

described ammonoids early in this century (Smith, 1904, 

1914, 1932). The Triassic marine rocks exposed at this 

locality were named the Union Wash Formation by Mount 

(1971), who designated the type locality on the south side of 

Union Wash near Smith's ammonoid localities. This fonnational name did not become established in the geological 

literature, however, and to our knowledge it has not been used 

since it was first proposed. We herein adopt the name Union 

Wash Formation as defined by Mount (1971) and extend its 

usage throughout the In yo Mountains and southeastward into 

the vicinity of Darwin and the Argus Range. 

In this report we describe the lithology, stratigraphy, 

and contact relations of the Union Wash Formation at its 

type locality and at two additional localities: (1) the Cerro 

Gordo area near the crest of the Inyo Mountains, 25 km 

southeast of Union Wash, and (2) the Darwin area, another 

35 km to the southeast (fig. 1). The descriptions given 

herein are largely based on recent work and are intended to 

supersede previous descriptions of the formation by us and 

our coworkers (Lewis and others, 1983; Stone, 1984; Stone 

and Stevens, 1986). In addition, we list new conodont 

identifications that, together with ammonoids identified by 

previous workers, constrain the age of the Union Wash 

Formation in these three areas to Early and Middle(?) 

Triassic. 

PREVIOUS STUDIES 

The presence of Triassic marine rocks in the Inyo 

Mountains was first noted by J.P. Smith (1898), who subsequently described ammonoids from Triassic marine rocks 

exposed at Union Wash (Smith, 1904, 1914, 1932). The 

Triassic marine rocks of the Inyo Mountains were first 

mapped and described by Knopf (1918) and Kirk (1918). 

More recently, the Triassic marine rocks of the In yo Mountains have been reported on by several workers, including 

Merriam (1963), Ross (1967), Mount (1971), Elayer (1974), 

Stinson (1977), Conley (1978), Lewis and others (1983), 

Obome (1983), Obome and others (1983), Stone (1984), 

Stone and Stevens (1986, 1987, 1988a), and Stone and others 

(1987). Triassic marine rocks in the Darwin area were first 

Previous Studies 
noted by Stone and others (1980) and have since been reported on by Lewis and others (1983), Stone (1984), Stone 

and Stevens (1987, 1988a), and Stone and others (1989). 

Triassic marine rocks in the Argus Range were first noted 

by Lewis and others (1983) and were briefly described by 

Stone (1984). 

\ 

\ 

\ 

\ 

I 

I 

I 

I tP 

\ ~ \ ~ \ ~ 

\ L-

\\{~i:~· ~ash\ ~~)_ ( ~('\ • I I \J' ..,- A~ \.. .,..0 ......_ I 00 

<::.-1- ---...................... I :..>...>-

(dry) 

~ \ !'~) 0~ 

1'~ \; ~0 ~ \ 

Area of figure 4 \.. 0o 

....-'\. ........ 

(/ \ " /~CERRO ~ORDO MINE \ 

-, ) A Conglomerate / \ ;-

\ I 

l 

) 

( 

) 

\ 

0 

........ 't- , ... 

\'o . ./ 

EXPLANATION 

-- Mainroad 

---- Secondaryroad 

- - - - Unimproved road 

~ Outcrops of Union 

Wash Formation 

and equivalent 

rocks 

- \~lr 

Darwin i:, Area of figure 7 

ylll 

Bendire Canyon 

"x 

0 10 20 30 40 KILOMETERS 

Butte Valley 

Fonnatlont 

Figure 1. East-central California, showing outcrops of the Union Wash Formation and the correlative Butte Valley Formation of 

Johnson (1957) and Cole (1986), areas discussed in report (see figs. 2, 4, and 7), and other localities discussed in text. 

2 Stratigraphy of the Lower and Middle(?) Triassic Union Wash Formation, East-Central California 
OUTCROP DISTRIBUTION 

Rocks herein assigned to the Union Wash Formation 

are exposed in the Inyo Mountains, the Darwin area, and 

the Bendire Canyon and Water Canyon areas in the Argus 

Range (fig. 1). Together, these rocks form a discontinuous, 

northwest-trending outcrop belt almost 100 km long. Correlative strata in the southern Panamint Range, 30 km 

southeast of Water Canyon (fig. 1), constitute the Butte 

Valley Formation of Johnson (1957) and Cole (1986). 

In the Inyo Mountains, the Union Wash Formation 

forms a nearly continuous, northwest-striking, southwestdipping outcrop belt approximately 35 km long (fig. 1). 

The type locality of the formation at Union Wash is close 

to the northwest end of this outcrop belt, and the exposures 

near Cerro Gordo are in the central part of the belt. Rocks 

herein assigned to the lowermost part of the Union Wash 

Formation also are present approximately 5 km east of the 

primary outcrop belt at Conglomerate Mesa. These rocks, 

which were referred to as unnamed Triassic rocks by Stone 

(1984) and Stone and others (1989), had not been previously differentiated from the underlying Permian rocks by 

McAllister (1956) and Ross (1967). 

Near Darwin, rocks herein assigned to the Union Wash 

Formation form scattered outcrops within an area of about 

25 km2 (fig. 1). These rocks, which were referred to as 

unnamed Triassic rocks by Stone and others (1980, 1989), 

Lewis and others (1983), and Stone (1984), had nolt been 

previously differentiated from the underlying Permian rocks 

by Hall and MacKevett (1962). 

In the Argus Range, rocks herein assigned to the Union 

Wash Formation are exposed over an area of several square 

kilometers near Bendire Canyon and in a smaller area ncar 

Water Canyon (fig. 1). These rocks, which were referred to 

as unnamed Triassic rocks by Lewis and others (1983) and 

Stone (1984), had not been previously differentiated from 

the underlying Permian rocks by Moore (1976). Our study 

of the Triassic rocks in these areas has not yet been completed, and these rocks are not described in this report. 

STRATIGRAPHY AND CONTACT RELATIONS 

In the three areas under consideration here, the Union 

Wash Formation ranges in thickness from slightly less than 

700 m to about 850 m and is divisible into three informal 

members. The lower member, generally less than 50 m 

thick, is composed of silty and sandy limestone. The middle 

member, which ranges in thickness from approximately 250 

m to approximately 680 m, is composed primarily of thinbedded siliceous mudstone, calcareous mudstone, and micritic limestone. The upper member, which has a maximum 

exposed thickness of approximately 450 m, is composed of 

thin-bedded to massive micritic limestone, silty and sandy 

limestone, shale, quartzose siltstone, and very fine grained 

quartzose sandstone. Except for the resistant limestone of 

the lower member and some resistant limestone beds in the 

middle and upper members, rocks of the Union Wash Formation are generally nonresistant and form smooth slopes. 

The Union Wash Formation is underlain by Permian 

sedimentary rocks that are assigned to the Owens Valley 

Group and the Keeler Canyon Formation, and it is overlain 

by unnamed volcanic and sedimentary rocks of Triassic and 

(or) Jurassic age. The depositional base of the Union Wash 

Formation is exposed throughout the outcrop extent of the 

formation, but the contact with the overlying rocks is exposed only in the Inyo Mountains. Early workers interpreted the contact between the Union Wash Formation and 

the underlying Permian rocks as an unconformity (Kirk, 

1918; Merriam, 1963). Recent work has shown that the 

Union Wash Formation paraconformably overlies a lenticular sequence of Upper Permian rocks, and that an angular 

unconformity of regional extent separates these Upper Permian and Triassic rocks from the underlying Lower Permian rocks (Stone and Stevens, 1987, 1988a; Stone and 

others, 1987). The contact between the Union Wash Formation and the overlying Triassic and (or) Jurassic volcanic 

and sedimentary rocks has been interpreted as conformable 

(Kirk, 1918; Mount, 1971), as apparently conformable but 

possibly unconformable (Merriam, 1963), and as generally 

conformable but locally unconformable (Oborne, 1983; 

Oborne and others, 1983). Our studies indicate that this 

contact is unconformable at Union Wash, where relations 

are well preserved, but that elsewhere, as in the Cerro Gordo 

area, faulting has obscured the original stratigraphic relations. 

Following are descriptions of the stratigraphy and 

contact relations of the Union Wash Formation in each of 

the three areas studied. The text is accompanied by geologic maps and graphic columns of stratigraphic sections 

measured in the three areas. Detailed written descriptions 

of the measured sections follow the main text of the report. 

Union Wash Area 

The Union Wash area is at the west foot of the Inyo 

Mountains in the northeast quarter of the 7 1/2-minute Union 

Wash quadrangle, approximately 15 km north-northeast of 

Lone Pine (figs. 1, 2). This area is accessible by several 

unimproved roads that diverge from the Owenyo-Lone 

Pine Road on the east side of Owens Valley. 

In this area the Union Wash Formation is exposed on 

the ridges that bound Union Wash on the north and south, 

on the lowermost part of a ridge located 2 km north of 

Union Wash, and on Fossil Hill (fig. 2). The formation 

strikes northwest, dips moderately to steeply southwest, lies 

on Permian rocks of the Owens Valley Group, and is overlain by unnamed volcanic and sedimentary rocks of Triassic and (or) Jurassic age. The Owens Valley Group and the 

underlying Keeler Canyon Formation are extensively 

Stratigraphy and Contact Relations 3 
intruded by granitoid rocks and are moderately to intensely 

metamorphosed, but the Union Wash Formation is not extensively intruded and is only slightly metamorphosed. 

118' 03' 118' 02' 

0 0.5 1.0 

Dikes considered to be part of the Jurassic Independence 

dike swarm cut the Union Wash Formation but have not 

caused widespread thermal alteration. 

118' 01' 118'00' 

1.5 2 KILOMETERS 

Figure 2. Geologic map of Union Wash area, lnyo Mountains, California, showing location of type section at type locality of 

the Union Wash Formation. See figure 1 for location of map. Base from U.S. Geological Survey, 1:24,000 scale, Union Wash 

quadrangle, 1982. Contour interval 20 m. Geology mapped in 1981 and 1990. 

4 Stratigraphy of the Lower and Middle(?) Triassic Union Wash Formation, East-Central California 
) 

The Owens Valley Group in this area consists of two 

formations-the thick Lone Pine Formation of Early PerEXPlANATION 

Alluvium and talus (Quaternary) 

Granitoid rocks (Jurassic)-Primarily diorite 

Volcanic and sedimentary rocks (Jurassic and (or) Triassic)-

Thickness greater than 500 m 

Union Wash Formation (friassic)-Divided into: 

Upper member-Divided into: 

Subunit 2-Pink shale and brown-weathering siltstone. 

Measured thickness 40.5 m 

Subunit 1-Dark-gray micritic limestone. Abundant ammo- noids 1-2 m above base represent the Parapopanoceras 

bed of Smith (1904, 1914). Measured thickness 54 m 

Middle member-Medium- to dark-gray, thin-bedded mudstone, siltstone, very fine grained to fine-grained sandstone, and micritic limestone. Meekoceras ammonoid 

bed of Smith (1904, 1932) present at base southeast of 

Union Wash. Measured thickness 679 m 

Lower member-Gray to brown, silty and sandy limestone. 

Maximum thickness approximately 30-40 m 

Owens Valley Group-In this area, consists of: 

Conglomerate Mesa Formation (Permian)-Ught-gray 

sandy limestone and minor shale, siltstone, and sand- stone. Maximum thickness approximately 150 m 

Lone Pine Formation (Permian)-Divided into: 

Reward Conglomerate Member-Massive and thickbedded chert-pebble conglomerate and quartzite; sandy 

marble at base. Thickness approximately 200 m 

Member C-Massive siltstone, quartzite, and minor chertpebble conglomerate. Thickness approximately 120 m 

Member B-Thin-bedded calcareous mudstone and 

siltstone; minor bioclastic limestone. Thickness approximately 180 m 

Member A-Thin-bedded siliceous to calcareous mudstone, 

calcareous siltstone, and silty limestone. Generally weathers 

reddish brown. Thickness approximately 500 m 

I PIPk I Keeler Canyon Formation (Permian and Pennsylvanian)-

Thick-bedded limestone; limestone-boulder conglomerate at top. Thickness approximately 400 m 

Contact-Dotted where concealed 

Fault-Dotted where concealed 

+- Anticline 

+- Syncline 

Strike and dip of beds 

0 Inclined 

-+- Vertical 

$ Horizontal 

~ Dikes of Jurassic Independence dike swarm 

- Type section of Union Wash Formation 

Location and number of conodont collection of Lewis and 

others (1983) 

Figure 2. Continued. 

mian age and the thin, lenticular Conglomerate Mesa Formation of Late Permian age-which are separated from one 

another by an angular unconformity (Stone and Stevens, 

1987, 1988a). In the northwestern part of the area, the 

Union Wash Formation rests paraconformably on the Conglomerate Mesa Formation; in the southeastern part of the 

area, beyond the depositional wedgeout of the Conglomerate Mesa Formation, the Union Wash Formation unconformably overlies the Lone Pine Formation (fig. 2). Contact 

relations between the Lone Pine, Conglomerate Mesa, and 

Union Wash Formations are clearest on the ridge 2 km 

north of Union Wash, where the Reward Conglomerate 

Member and member C of the Lone Pine Formation are 

erosionally truncated beneath the Conglomerate Mesa 

Formation and the concordantly overlying Union Wash 

Formation (fig. 2). 

All three members of the Union Wash Formation are 

present in the area, but the lower member is present only in 

part of the northwestern sector (fig. 2). At its type locality 

on the south side of Union Wash, the formation consists of 

only the middle and upper members. Because Mount (1971) 

designated no type section at the type locality, we herein 

designate a section that we measured on the south side of 

Union Wash as the type section at the type locality of the 

Union Wash Formation. This measured section is located 

in an unsurveyed part ofT. 14 S., R. 36 E., in the northeast 

quarter of the Union Wash quadrangle (fig. 2). The measured section (fig. 3) is 773.5 m thick; the middle member 

is 679 m thick, and the upper member is 94.5 m thick. The 

section dips nearly homoclinally to the southwest except in 

its central part, which is folded into a syncline and an anticline (fig. 2). Continuity of measurement across the folded 

part of the section was maintained by tracing a zone of 

limestone beds around the fold hinges. 

The lower member of the Union Wash Formation 

consists of well-indurated, ledge-forming, gray- to brownweathering silty and sandy limestone. These rocks are characterized by crinkly or wavy lamination and locally contain 

minute black gastropod molds. A sharp, concordant contact 

described by Stone and Stevens (1986, 1987) separates these 

rocks from the underlying Conglomerate Mesa Formation. 

This contact, although sharp, is subtle because of the general 

lithologic similarity between the lower member of the Union 

Wash Formation and the Conglomerate Mesa Formation, 

which in this area also consists mainly of well-indurated silty 

to sandy limestone. The Conglomerate Mesa Formation, 

however, differs from the lower member of the Union Wash 

Formation in containing lenses of chert gravel and shell 

debris and in lacking minute blac~ gastropod molds. A thin 

coquina is present directly beneath the base of the Union 

Wash Formation. Originally, the rocks now assigned to the 

Conglomerate Mesa Formation and to the lower member of 

the Union Wash Formation at this locality were considered to 

comprise a single formation, the Owenyo Limestone, of 

presumed Permian age (Kirk, 1918). Merriam and Hall 

Stratigraphy and Contact Relations 5 
~ .....1 Z(J) Z(J) 

~ ~-:.a: ~~;,ffi AGE ROCK UNIT CH-.....1 LITHO- a: DESCRIPTION u(/)w 

(THICKNESS, IN METERS) zzg LOGY LU -(/)tu 1-'~(J)I- I-

~~::2: Q:CWW 88__. ~ 1-t-Z::iE 

JURASSIC Volcanic and sedimentary Lowermost 7.5 m is conglomerate; overlying 10m is 

AND (OR) vvvv brown volcanic sandstone; overlain by several 17.5+ rocks (part) vvvvv TRIASSIC hundred meters of volcanic rocks 000000 

773.5 

Subunit 2 f-::::.:.-:-=:7=...:.:-: Siltstone, dark-gray (weathers brown), laminated; and r-_------= 49- shale, pink to light-brown, fissile. Scattered 40.5 (40.5 m) r....:..-=..:=:..= 54 r-_------= ammonoids in middle part Upper ""------

Member - 43- Shale, light-brown to pink, and limestone, dark-gray, 24 (94.5m) Subunit 1 48 thin-bedded 

(54m) Limestone, dark-gray, fine-grained, thin-bedded; - 700 

42 abundant ammonoids 1-2m above base represent 30 

the Parapopanoceras bed of Smith (1904, 1914) 

r·-·-·- r=--=--=--=-- r=-...:..-=...:=·- Siltstone and shale, calcareous, mostly gray to 

~-:--===.- 35- light-brown but some pink; and limestone, dark- 83 

~-=--=--=-- 41 gray, micritic, thin-bedded. Dike at base ~-=----=..:- r----- r=-....:...-=...:=·- 600 

34 Limestone, dark-gray, micritic, thin-bedded 9 r- ·- r=:::-=--=::.... =....:.- 33 Siltstone and shale, calcareous, brownish-gray 28.5 ~=·=:.= 

I 32 Limestone, dark-gray, micritic, thin-bedded 31.5 

'"=-·-·-··- 31 Siltstone and shale, calcareous, brownish-gray 12 

UW-5 I I 30 Limestone, dark-gray, micritic, thin-bedded 15 500 

-=-.-7-...=7=:- 29 Siltstone and shale, calcareous, brownish-gray 14.5 

E I 28 21 LO I I Limestone, dark-gray, micritic, thin-bedded 

C'i -=....-7--=7=. 27 Siltstone and shale, calcareous, brownish-gray ..... 13.5 ..... 

c u 0 

·.;::; 26 Limestone, dark-gray, micritic, thin-bedded 37.5 I I U5 «:1 

(/) E 

<( 25 Dike 13.5 0 + + + + + 400 a: LL (2) 24 Limestone, dark-gray, micritic, thin-bedded 14.5 1- .s:::. 

(I) --

~ - - Largely covered; rare outcrops of dark-gray calcareous 

Middle - - 23 46.5 c: :::::c::... mudstone and light- to medium-gray micritic 0 member - - limestone ·c: (679 m) - - ::::> UW-4-

22 Limestone, dark-gray, micritic, thin-bedded 31.5 

I 

~------~ 21 Mudstone, calcareous, dark-gray; weathers 15 - 300 

reddish-brown 

20 15 Limestone, medium-gray, thin-bedded 

r----- 18- Mudstone, weathers dark-gray; partly covered 27 

r=:::-==-== -==- 19 r-----

17 Limestone, medium-gray; and siltstone, brown 13.5 

r----- 13- Mudstone and siltstone, gray to brown; and 33 

f----- 16 limestone, dark-bluish-gray 

- 200 

- - - - - -

--

-- 5- Largely covered. Outcrops and float of medium-gray to 

138.5 ~ 12 black mudstone, tan-weathering siltstone and light- to 

medium-gray, fine-grained limestone 

P= - - - 100 ---

--- r--- --- E ~-==-==-=-- r--- 15 «:1 4 Mudstone, very dark, massive 

s 

E f-·-·-·- Limestone, medium-gray; and siltstone, tan; lower 2-3 36 part mostly covered LL 

Cl) ~ (1) 

~ Limestone, light-gray, massive; contains abundant c: 14.5 1 ammonoids; Meekoceras bed of Smith (1904, 1932) a: 0 Cl) 

c: Member A ~ Largely covered. Outcrops and float of thin-bedded, 0 --- PERMIAN ....I (part; not -·-·- silty limestone, calcareous siltstone, and siliceous to 

measured) ~ calcareous mudstone ---

Figure 3. Type section of the Union Wash Formation in Union Wash area. Location of section shown on figure 2. 

6 Stratigraphy of the Lower and Middle(?) T~iassic Union Wash Formation, East-Central California 
(1957) later revised the Owen yo as a member of the Permian 

Owens Valley Formation. The little-used name Owen yo was 

abandoned by Stone and Stevens (1987) when the Owens 

. Valley was raised to group rank and the Conglomerate Mesa 

Formation was named. 

The lower member of the Union Wash Formation is 

most fully developed on the ridge 2 km north of Union 

Wash (fig. 2), where it is about 40 m thick. Northwest of 

this locality, the lower member extends only as far as the 

southeastern part of Fossil Hill, where it is truncated by 

faulting. To the southeast, the lower member extends approximately to the crest of the ridge on the north side of 

Union Wash, where it wedges out depositionally slightly 

beyond the wedgeout of the underlying Conglomerate Mesa 

Formation (fig. 2). Southeast of these wedgeouts, member 

A of the Lone Pine Formation is directly overlain by rocks 

herein assigned to the middle member of the Union Wash 

Formation. 

The thick, slope-forming middle member of the Union 

Wash Formation consists primarily of nonresistant, mediumto dark-gray, thin-bedded mudstone, siltstone, and micritic 

limestone. Most of the limestone is concentrated in discrete zones 10-30 m thick (fig. 3). In the northwestern part 

of the area, the lower part of the middle member contains 

abundant beds of very fine grained to fine-grained sandstone and appears to grade up from the lower member. 

Sandstone becomes less abundant to the southea'St, however, and none is present in the vicinity of the type section 

of the Union Wash Formation on the south side of Union 

Wash, where the lower member of the formation also is 

absent. Here, the base of the formation, which is also the 

base of the middle member, is at the base of a prominent 

bed of massive limestone that contains abundant ammonoids-the Meekoceras bed of Smith (1904, 1932). This 

bed, which extends continuously from the south side of 

Union Wash to the east edge of the study area (fig. 2) and 

beyond, is 14.5 m thick where measured by us (fig. 3) but 

generally is only about 2-5 m thick. A lithologically similar, possibly equivalent ammonoid bed, 3m thick, is locally 

present a few meters above the base of the middle member 

on the ridge immediately north of Union Wash; the ammonoid assemblage in this bed is identical to that of the 

Meekoceras bed on the south side of the wash (N.J. 

Silberling and E.T. Tozer, oral common., 1988). No 

equivalent bed is exposed farther northwest. · 

Our placement of the base of the Union Wash Formation on the south side of Union Wash at the base of the 

M eekoceras bed is a significant departure from previous 

interpretations. Kirk (1918, p. 45-46) considered the base 

of the then-unnamed Triassic sequence on the south side of 

Union Wash to be in the deep tributary canyon downslope 

to the northeast from the Meekoceras bed (fig. 2), albout 200 

m stratigraphically below that bed; he interpreted the base 

of the sequence to be a fault, an interpretation not supported 

by later studies. Mount (1971) interpreted the base of the 

Union Wash Formation on the south side of Union Wash as 

disconformable and placed this surface "about 300 feet" 

(91 m) stratigraphically below the Meekoceras bed, but he 

did not clearly explain the lithologic basis for this interpretation. In a previous study by two of us (Stone and Stevens, 

1986), we placed the base of the Union Wash Formation on 

the south side of Union Wash at the base· of a 30-m-thick 

interval of fine-grained sandy limestone and calcareous 

sandstone, 105 m stratigraphically below the Meekoceras 

bed. In that study, we considered this interval laterally 

equivalent to the zone of sandy rocks that gradationally 

overlies the lower member of the Union Wash Formation 

on the ridge 2 km north of Union Wash. Mapping done 

during the present study indicates that these previous interpretations are in error and that the actual stratigraphic contact between the Union Wash Formation and the underlying 

member A of the Lone Pine Formation on the south side of 

Union Wash is precisely at the base of the Meekoceras 

bed. This interpretation cannot be firmly demonstrated in 

the immediate vicinity of Union Wash, where the nonresistant beds above and below the Meekoceras bed are poorly 

exposed on a steep, talus-covered slope. The contact relations are clear, however, on a high, relatively level ridge 

1.5 km beyond the east edge of the study area, where the 

beds above and below the Meekoceras bed are not covered. 

At this locality, it is evident that rocks typical of member A 

of the Lone Pine Formation, here consisting of finely laminated, thin-bedded silty limestone, calcareous siltstone, and 

calcareous to siliceous mudstone, extend all the way to the 

base of the Meekoceras bed. The thin-bedded mudstone, 

siltstone, and limestone above the Meekoceras bed are 

grossly similar in lithology to the rocks below, but differ in 

detail. In particular, limestone beds above the M eekoceras 

bed are thicker, darker, and less silty that those below, and 

they are not laminated. The 30-m-thick interval of finegrained sandy rocks that we previously interpreted as the 

basal part of the Union Wash Formation on the south side 

of Union Wash (Stone and Stevens, 1986) is herein 

reinterpreted as part of the Lone Pine Formation. We have 

observed no significant angular discordance between the 

Lone Pine and Union Wash Formations in the area south of 

Union Wash. 

A sharp contact separates the middle member of the 

Union Wash Formation from the upper member, which is 

exposed only south of Union Wash. The upper member 

consists of two subunits (figs. 2, 3). Subunit 1, 54 m thick 

in the type section, is composed primarily of well-indurated, 

dark-gray micritic limestone that forms a conspicuous marker 

horizon. A thin, fossiliferous layer that contains abundant 

ammonoids is present 1-2 m above the base of this subunit. 

This layer, which is best developed several hundred meters 

northwest of the type section, is equivalent to the Parapopanoceras ammonoid bed of Smith (1904, 1914), described 

by him as a bed of impure earthy black limestone about 5 ft 

thick, 150 ft below the top of the Triassic marine section 

Stratigraphy and Contact Relations 7 
38" 

34' 

38" 

33' 

38" 

32' 

38" 

31' 

0 

117" 51' 

0.5 

117" 50' 

1.0 1.5 2 KILOMETERS 

8 Stratigraphy of the Lower and Middle(l) Triassic Union Wash Formation, East-Central California 

117" 49' 
EXPlANATION 

Alluvium and talus (Quaternary) 

Volcanic and sedimentary rocks (Jurassic and (or) Triassic)-

Thickness greater than 500 m 

Union Wash Formation (friassic)-Divided into: 

Upper member-Divided into: 

Subunit 4-Brown to yellowish-brown, thin-bedded siltstone 

and shale; upper part contains limestone and dolomite 

including a thick bed of medium- to dark-gray micritic 

limestone that is structurally repeated several tiimes near Cerro Gordo Road. Measured thickness 234.5 m 

Subunit 3-Dark-gray micritic limestone. Measured thick- ness 92.5 m 

Subunits 1 and 2, undivided-Subunit 1: dark-gt-ay micritic 

limestone; measured thickness 11 m. Subunit 2: yellow 

shale; gray, purplish-gray, brownish:1Jray, and brown siltstone and very fine grained sandstone; and light- to medium-gray limestone. Measured thickness 84 m 

Middle member-Yellow shale and medium-gray, thin-bedded 

micritic limestone. Measured thickness 236.5 m 

Lower member-Gray to brown, silty limestone and cal- careous siltstone. Measured thickness 31.5 m 

Owens Valley Group-In this area, consists of: 

Conglomerate Mesa Formation (Permian)-Ught-gray sandy 

limestone, conglomerate, and gray to yellowish-brown 

sandstone. Maximum thickness approximately 200 m 

Lone Pine Formation (Permian)-L.ower part (member A) 

composed of medium- to dark-gray, thin-bedded cal- careous and dolomitic mudstone, calcareous siltstone to 

very fine grained sandstone, and micritic dolomite. Upper 

part (member B) composed of light-colored mudstone, siltstone, very fine grained sandstone, and locally abundant 

medium- to dark-gray bioclastic limestone. Maximum 

thickness approximately 310 m 

Keeler Canyon Formation (Permian and Pennsylvanian)-

Lower part: gray to brown calcareous siltstone and 

medium-gray bioclastic limestone. Upper part: mediumgray bioclastic limestone and minor gray to pink mudstone and shale. Thickness approximately 1,2!>0 m 

~ Rest Spring Shale (Mississippian)-Dark-brown shale. L::.:.J Thickness approximately 350 m 

-t-

-t-

-+-

__JIO 

fl);jiie;:) 

-

Perdido Formation (Mississippian)-Ught-brown siltstone. 

Thickness approximately 50 m 

Tin Mountain Umestone (Mississippian)-Dark-gray limestone. Thickness approximately 50 m 

Lost Burro Formation (Devonian)-Ught- to medium gray 

limestone and dolomite. Only uppermost part exposed 

Contact 

Fault-Dotted where concealed 

Anticline 

Syncline 

Strike and dip of beds 

Inclined 

Vertical 

Overturned 

Dikes 

Marker beds in Keeler Canyon Formation 

Reference section of Union Wash Formation 

and 800 ft above theM eekoceras bed. By our measurement, 

Smith's Parapopanoceras bed is actually 665 m (2,182 ft) 

above the Meekoceras bed. Subunit 2, 40.5 m thick in the 

type section, is composed of pink fissile shale and brownweathering quartzose siltstone. Rare ammonoids are observed about 20 m above the base of subunit 2 a few hundred 

meters northwest of the type section; this locality probably 

is equivalent to an ammonoid locality described by Silberling 

and Tozer (1968, p. 39) as "several tens of feet" above 

Smith's P arapopanoceras bed. 

The upper member of the Union Wash Formation is 

sharply overlain by 5 to 10m of conglomerate that contains 

clasts of limestone and volcanic rocks, including scattered 

blocks of limestone more than 1 min diameter. This conglomerate marks the base of the unnamed sequence of 

Triassic and (or) Jurassic volcanic and sedimentary rocks. 

As was first pointed out by Obome (1983), this conglomerate lies unconformably on the Union Wash Formation. 

Traced northwestward from the hilltop where the type section ends, subunit 2 of the upper member of the Union 

Wash Formation is gradually truncated beneath the conglomerate, which directly overlies subunit 1 at the base of 

the ridge (fig. 2). Less than 1 km farther northwest, on a 

low hill at the mouth of Union Wash, the basal rocks of the 

volcanic and sedimentary sequence overlie rocks of the 

middle member of the Union Wash Formation. Thus, the 

entire upper member of the formation is erosionally truncated beneath the upper contact within a distance of less 

than 2 km. We interpret the contact as an unconformity 

having slight angular discordance. 

Cerro Gordo Area 

The Cerro Gordo area is located on the west slope of the 

In yo Mountains in the southwest quarter of the 7 1/2-minute 

Cerro Gordo Peak quadrangle, approximately 6 km northeast 

of Keeler and 3 km west of the Cerro Gordo mine (figs. 1, 4 ). 

This area is reached by Cerro Gordo Road, a steep graded 

road that connects Keeler and the Cerro Gordo mine. 

The Union Wash Formation in the Cerro Gordo area 

forms a continuous northwest-striking, generally southwestdipping belt that is complicated by faulting, folding, and 

dikes (fig. 4). The formation lies on Permian rocks of the 

Keeler Canyon Formation and Owens Valley Group and is 

overlain by unnamed volcanic and sedimentary rocks of 

Triassic and (or) Jurassic age. The depositional lower contact 

of the Union Wash Formation is widely exposed, but the 

upper contact is invariably faulted. Despite the structural 

complexity of the area and the presence of intrusive rocks, 

the Union Wash Formation and the bounding units are 

..... Figure 4. Geologic map of Cerro Gordo area, lnyo Mountains, California, showing location of reference section of the 

Union Wash Formation. See figure 1 for location of map. Base from U.S. Geological Survey, 1 :24,000 scale, Cerro Gordo Peak 

quadrangle, provisional edition 1987. Contour interval 40ft. Geology mapped in 1974-75 and 1982. · 

Stratigraphy and Contact Relations 9 
generally well preserved and are not significantly sheared, 

altered, or metamorphosed except in and near fault zones. 

As in the Union Wash area, the Owens Valley Group 

in the Cerro Gordo area is divided into the Lone Pine Formation of Early Permian age and the Conglomerate Mesa 

Formation of Late Permian age. Both of these formations 

thin and wedge out from northwest to southeast within the 

area (fig. 4). In the northwestern part of the area, the Union 

Wash Formation rests paraconformably on the Conglomerate Mesa Formation, which in turn rests unconformably on 

the Lone Pine Formation. In the central part of the area, 

beyond the depositional wedgeout of the Conglomerate Mesa 

Formation, the Union Wash Formation unconformably 

overlies the Lone Pine Formation, which is much thinner 

here than to the northwest. Farther southeast, 1.5 km northwest of Cerro Gordo Road, the Lone Pine Formation wedges 

out, and southeast of this wedgeout the Union Wash Formation unconformably overlies the Keeler Canyon Formation. 

The unconformable contact between the Keeler Canyon and 

Union Wash Formations .has a slight angular discordance; 

several marker beds in the Keeler Canyon Formation are 

truncated beneath this contact (fig. 4). Between the wedgeout of the Lone Pine Formation and Estelle Tunnel at the 

southeast corner of the study area, a distance of 3 km, a 

total of about 230 m of beds in the Keeler Canyon Formation 

is truncated beneath the contact with the Union Wash Formation (Stone and Stevens, 1988a). 

All three members of the Union W a.Sh Formation are 

present in the Cerro Gordo area. The lower and middle 

members extend continuously across the area and are nearly 

homoclinal, but the upper member is intensely faulted and 

folded; it is stratigraphically most complete in the northwestern part of the area (fig. 4). Our measured section of 

the Union Wash Formation crosses the lower member, 

middle member, and subunits 1-3 of the upper member 

approximately 2 km northwest of Cerro Gordo Road; subunit 

4 of the upper member was measured 1 km farther northwest, 

on the southwest flank of an anticline (fig. 4). This measured 

section (fig. 5), herein designated as a reference section of 

the Union Wash Formation, is 690 m thick; the lower member is 31.5 m thick, the middle member is 236.5 m thick, 

and the upper member is 422 m thick. The lower and 

middle members of the Union Wash Formation, as defined 

herein, are equivalent to the "lower, brown-mottled limestone" and the "middle shale-limestone zone," respectively, 

described by Merriam (1963, p. 29). Subunit 4 of the upper 

member is equivalent, in part, to the "upper reefy limestone 

zone" described by Merriam (1963, p. 29). Rocks comprising subunits 1, 2, and 3 of the upper member were not 

described by Merriam (1963). 

The lower member of the Union Wash Formation is 

composed of gray to brown silty limestone and calcareous 

siltstone that form resistant crags and hogbacks. These 

rocks arc characterized by thin, planar to wavy bedding, a 

distinctive nodular texture, and the local presence of minute 

black gastropod molds. The lower member is readily distinguished from all of the units it overlies within the area. 

The uppermost part of the Conglomerate Mesa Formation 

consists of sandstone and conglomerate; the Lone Pine Formation consists of dark-gray, thin-bedded calcareous mudstan~ and siltstone; and the Keeler Canyon Formation 

consists of medium-gray, thick-bedded, bioclastic limestone 

characterized by graded bedding and local concentrations 

of fusulinids. 

The lower member of the Union Wash Formation fines 

upward and grades into the slope-forming middle member, 

which is composed of yellow fissile shale and medium-gray, 

thin-bedded micritic limestone. Limestone beds are planar 

and 1 to 10 em thick. Most parts of the member consist 

primarily of shale and scattered limestone interbeds, but a few 

limestone-dominated intervals 1 to 25 m thick are present. 

Only locally does the limestone contain bioclastic debris 

visible in the field, but petrographic studies reveal the local 

abundance of spherical radiolarians (now replaced by calcite). 

The uppermost shale interval in the member, 44 m thick in the 

measured section, is a distinctive marker zone characterized 

by a bright, yellowish-brown color. 

A sharp contact separates the middle member of the 

Union Wash Formation from the lithologically heterogeneous upper member, which is divided into four subunits. In 

ascending order, these are called subunits 1, 2, 3, and 4; their 

measured thicknesses are 11, 84, 92.5, and 234.5 m, respectively (fig. 5). Subunit 1 consists of a massive ledge of darkgray micritic limestone that stands out in marked contrast to 

the underlying yellow shale. Subunit 2 consists of a 10-m -thick 

basal zone of yellow shale overlain by 74 m of interbedded 

quartzose siltstone, very fine grained quartzose sandstone, 

and limestone. The siltstone and sandstone are gray, purplish 

gray, brownish gray, and light brown, and commonly are 

finely laminated; the limestone is light to medium gray and 

commonly contains thin interbeds of brown siltstone. Subunit 3 is composed primarily of dark-gray, ledge-forming 

micritic limestone. This limestone, despite its massive appearance, forms planar beds 1 to 5 em thick that are separated 

by thin partings of light-brown siltstone or mudstone. Subunit 4 is composed primarily of brown to yellowish-brown, 

thin-bedded quartzose siltstone and shale, most of which is 

cut by a well-developed slaty cleavage. In addition, subunit 4 

contains two prominent zones of carbonate rocks: (1) a 

resistant bed of medium-gray limestone, 4.5 m thick and 123 

m above the base of the subunit, which contains local concentrations of pelecypod shells in its upper part; and (2) a zone of 

dark-gray limestone and light-gray-weathering dolomite, 12 

m thick and 156 m above the base of the subunit. Above this 

upper carbonate zone, the uppermost 66.5 m of the measured 

section consists almost entirely of shale, which in turn is 

overlain by a thick unit of conglomerate comprising the basal 

part of the unnamed Triassic and (or) Jurassic volcanic and 

sedimentary sequence. A 2-m-thick conglomerate bed containing clasts of limestone and volcanic rock is present 29 m 

10 Stratigraphy of the lower and Middle(?) Triassic Union Wash Formation, East-Central California 
below the base of this thick conglomerate. It is not clear 

whether this bed and the overlying shale are in proper stratigraphic sequence or are structurally interleaved in a shear 

zone, although the latter interpretation is preferred. 

to as the "upper reefy limestone zone" of the Triassic sequence. This belt of deformed rocks extends about 2.5 km 

northwest of the road and beyond the boundary of the study 

area to the southeast. The belt is characterized by numerous discontinuous bodies of massive, medium-gray limestone, some as thick as 15 to 20 m, that form high, resistant 

crags and hogbacks above the enclosing shale, siltstone, 

and thin-bedded carbonate rocks. Merriam (1963) counted 

five of these limestone bodies and implied that each was a 

In the vicinity of Cerro Gordo Road, intensely deformed rocks assigned to subunit 4 of the upper member of 

the Union Wash Formation are in fault contact with the 

middle member of the Union Wash (fig. 4). These are the 

rocks of subunit 4 that Merriam (1963) specifically referred 

AGE 

>- 1-

ROCK UNIT 1 ::::i LITH-· 

(THICKNESS, IN METERS) ~ ~ ~ OLOGY 

ggg 

~ > a: 

UJ 

1-

~ 

DESCRIPTION 

Basal conglomerate overlain by several hundred meters Volcanic and sedimentary 

rocks (part; not measured) vv v v v v v" v of volcanic and sedimentary rocks. Contact with 

o o o o o Union Wash Formation faulted 

r-----+--~---.------+-----+=-o=o=-'o==o~c:::=-' -::::: 0+---V'./ Shale, yellow, fissile; conglomerate containing volcanic ,..,.___ _ ____. 690 

~ ~~ ~~ ~ j6- rock clasts at base. Basal contact may be faulted; 

JURASSIC 

AND (OR) 

TRIASSIC 

E 

c: 

0 

·c: 

::J 

Upper 

member 

(422 m) 

Subunit 4 

(234.5 m) 

Subunit 3 

(92.5 m) 

Subunit 2 

(84 m) 

~?o-; ;?S:~ ~hale may be structurally repeated from underlying 

? -=-= ~:-:=- 44_ Intervals 

~ ~= ~~ ~ 45 Shale, calcareous, yellowish-brown, fissile 

-·-·-·-~ 

__ --: -.:___---:-.:___=:_::_ 

383 Dolomite, medium-gray, and limestone, dark-gray 

Siltstone and shale, calcareous, yellowish-brown ·-·--·--~ 

CG-13 _ ··-. ·-·· ·--~ Limestone, medium-gray, fine-grained; uppermost 

~~~-=3.:.'0 "i- meter contains abundant pelecypod shells 

-.:.: ____ .-=-.· .-=.: .. .-:. _ ::-=:·=·:·:: 

-·-·-·-~ 

-

----=--"~ 34 

I 

Siltstone and shale, calcareous, yellowish-brown, 

intensely cleaved 

Dike 

Shale, purplish-gray, calcareous 

Limestone, dark-gray, micritic, forms ledges; beds 1-5 

1--L--.--~ 31- em thick, separated by thin partings of lig,t-brown 

............. 

:::::::::::;: 

33 siltstone or mudstone. One 4.5-m-thick interval of 

grayish-brown, fine-grained calcareous sandstone 

Siltstone and very fine-grained sandstone, light-brown, 

gray, and purplish-gray, laminated and thin-bedded; 

and limestone, light- to medium-gray, fine-grained, 

thin-bedded, silty; basal 10.5 m is bright, yellowishbrown shale. Partly covered 

29 

37.5 

12 

28.5 

4.5 

105 

3 

15 

92.5 

84 

V Subunit 1 ~---F-=-==-=-:=;-~-::..:::-:::r-:-::-+-:-:---:---:--:-----:-:::-:---:-----:--:-------f-:-::---; (

11 m) -=- I _ 18 Limestone, dark-gray, micritic, forms ledge 11 

Middle 

member 

(236.5 m) 

------ ---·-

====== ===:=:-:=-

16-

17 

===.=.~: 

====== 4-

----== 14 

====== 

CG-2 ----·-

====== 

----·- CG-1 

Shale, calcareous, bright yellowish-brown, fissile; 

uppermost 4 m is medium-gray limestone 

Limestone, medium-gray, micritic, thin-bedded; minor 

yellow shale 

Shale, yellow, fissile; minor limestone, medium-gray, 

micritic, in beds 2-10 em thick 

48 

25.5 

163 

600 

500 

400 

300 

200 

100 

Lower member ~ _J.:" 1

_3 Limestone, silty, and siltstone, calcareous, light-gray to 

~---~--l_-~(~31~.5~m~)~--~--~~~~~~-=t-~y~e~ll~o~w~is~h:-~br~o~w~n~;~no~d~u~la~r~----------~ ~- 0 I ~ ~~ Limestone, medium-gray, bioclastic, thick-bedded; minor 

PERMIAN 

Keeler Canyon Formation 

(part; not measured) shale, pinkish- to purplish-gray 

Figure 5. Reference section of the Union Wash Formation in Cerro Gordo area. Location of section shown on figure 4. 

Stratigraphy and Contact Relations 11 
separate bed. Our mapping and stratigraphic studies, however, indicate that only one massive limestone bed, structurally repeated several times by complex folding and 

faulting, is present. The stratigraphy of subunit 4 in this 

[METERS 

Subsection A 

Fault 

(Base of B) 

Subsection B 

Fault 

Siltstone and shale, ochre to 

brown 

-i·~-~-i-~-~-r:=~~~/~~~ Dolomite, cream-colored, micritic; 

contains stromatolitic lamination 

Dolomite, cream-colored, laminated; and limestone, mediumgray, thin-bedded 

Siltstone, tan to gray 

Fault 

Limestone, medium-gray, micritic, 

massive. Contains thin, anastomozing stringers of light-gray to 

ochre mudstone that impart a 

mottled texture. Upper part 

contains thin layers of pelecypod shells 

't::::::::-:-=:-1-=:-=::H Limestone, micritic, light-brown, 

~:::::::~ mottled 

Fault 

(Top of A) 

Siltstone and shale, brown 

Figure 6. Measured section of structurally repeated parts of 

subunit 4 of upper member of the Union Wash Formation on 

northwest side of Cerro Gordo Road. Measured section consists of two subsections (A and B) that are inferred to be 

stratigraphically equivalent to one another and to be separated by inconspicuous bedding-plane fault. Subsection B 

structurally over I ies subsection A. 

belt is revealed by a measured section on the northwest side 

of Cerro Gordo Road (fig. 6), which consists of two very 

similar subsections (A and B) that we interpret as structurally repeated equivalents of one another. Each subsection 

contains the following distinctive stratigraphic sequence, 

listed in ascending order: (1) a bed of light-brown, mottled 

micritic limestone, 1 to 1.5 m thick; (2) a massive bed of 

medium-gray limestone, 9 m thick in subsection A and 16 

m thick in subsection B, that contains local concentrations 

of pelecypod shells in its uppermost part; (3) an interval of 

tan to gray siltstone, 2 to 3 m thick; (4) an interval of thinbedded limestone and dolomite, 7 to 8 m thick, in which 

thin sections reveal coated grains and ooliths; and (5) a bed 

of cream-colored, micritic dolomite, 1 m thick, that contains fine stromatolitic lamination (fig. 6). The lowermost 

bed in this distinctive sequence overlies about 60 m of brown 

siltstone and shale, and the uppermost bed is overlain by 

about 20 m of ochre to brown shale. Both the base and top 

of the measured section are bounded by faults, and the two 

subsections are separated by a narrow, inconspicuous zone 

of recrystallized and brecciated rocks that we interpret as a 

bedding-plane fault. 

We interpret the massive limestone bed in the vicinity of Cerro Gordo Road to be equivalent to the 4.5-m-thick 

limestone bed 123 m above the base of subunit 4 in our 

main measured section of the Union Wash Formation (fig. 

5). That bed is thinner than the bed exposed at Cerro 

Gordo Road, but both beds are similar in lithology and each 

overlies a thick sequence of brown siltstone and shale. The 

overlying thin-bedded limestone and dolomite at Cerro 

Gordo Road probably are equivalent to the 12-m-thick zone 

of lithologically similar carbonate rocks 156 m above the 

base of subunit 4 in our main measured section (fig. 5). 

Throughout the area, the upper member of the Union 

Wash Formation is overlain by thick-bedded, 

brown-weathering conglomerate that marks the base of the 

unnamed Triassic and (or) Jurassic volcanic and sedimentary 

rock sequence. The rocks directly beneath the conglomerate 

commonly form a shear zone as much as 30 m wide in 

which shale, limestone, conglomerate, and volcanic rocks 

are mixed together; in places, tight folds can be mapped 

within this shear zone. On this basis we consider the contact zone to be a fault that obscures the original stratigraphic relation between the Union Wash Formation and the 

volcanic and sedimentary rock sequence. 

We interpret subunits 1 and 2 of the upper member of 

the Union Wash Formation in the Cerro Gordo area to be 

equivalent to the lithologically similar subunits 1 and 2 of the 

upper member in the Union Wash area. If this correlation is 

correct, subunits 3 and 4 of the upper member are not represented in the Union Wash area, presumably because they 

were removed by erosion before the overlying Triassic and 

(or) Jurassic volcanic and sedimentary rocks were deposited. 

Mapping in the area between the Cerro Gordo and Union 

Wash areas is needed_ to confirm this interpretation. 

12 Stratigraphy of the lower and Middle(?) Triassic Union Wash Formation, East-Central California 
Darwin Area 

The Darwin area is located in the central part of the 

7~-minute Darwin quadrangle, approximately 2 km northeast of Darwin (figs. 1, 7). The area extends from the east 

side of the Darwin Hills to Darwin Canyon. Primary access 

to the area is by Darwin Road, which connects Darwin and 

Panamint Springs by way of Darwin Canyon. 

The Union Wash Formation in the Darwin area overlies the Lower Permian Darwin Canyon Formation of the 

Owens Valley Group and is unconformably overlain by 

Cenozoic basalt and alluvium. The original stratigraphic 

top of the formation is not exposed. Exposures of the formation are present both north and south of the Darwin fault, 

the main structural feature in the area, which has an apparent 

left-lateral separation of 1.2 km (Stone and others, 1989). 

The most complete section of the Union Wash Formation 

north of the Darwin fault is in an erosional window beneath 

Cenozoic basalt in the northwestern part of the area (fig. 7); 

two less complete sections are present farther east. All 

three of these sections expose the base of the formation. 

South of the Darwin fault, the Union Wash Formation is 

primarily exposed in a broad, northwest-trending syncline, 

much of which is covered by Cenozoic basalt and alluvium 

(fig. 7). Small additional exposures are present in Darwin 

Canyon. Most parts of the Union Wash Formation are well 

preserved despite the effects of widespread low-grade 

metamorphism related to the intrusion of plutonic rocks in 

and adjacent to the area. 

The contact between the Union Wash Formation and 

the underlying Darwin Canyon Formation is an angular 

u~conformity that has been described by Stone and Stevens 

(1988a). In the eastern part of the area, in the vicinity of 

Darwin Canyon, the Union Wash Formation overlies the 

Panamint Springs Member of the Darwin Canyon Formation, which in turn overlies the Millers Spring Member of 

the Darwin Canyon Formation (fig. 7). The unconformity 

is superbly exposed on the south side of the Darwin fault in 

Darwin Canyon, 1 km north of Millers Spring (fig. 7), where 

the Union Wash Formation overlies the Panamint Springs 

Member with an angular discordance of about 15°. The 

unconformity cuts stratigraphically downward into the 

western part of the area, where the Panamint Springs Member is generally absent and the Union Wash Formation rests 

directly on the Millers Spring Member. Structural relief on 

the unconformity within the study area is at least 500 m, the 

approximate thickness of the Panamint Springs Member in 

the vicinity of Darwin Canyon (Stone and others, 1987). 

All three members of the Union Wash Formation are 

present in the Darwin area, but the discontinuous outcrop 

pattern precludes measurement of the entire formation in a 

single traverse. Our measured section (fig. 8), herein designated as a reference section of the Union Wash Formation, 

consists of three subsections. The lower and middle members were measured north of the Darwin fault in two subsections (2A and 2B) separated from one another by a minor 

fault (fig. 7); the top of subsection 2A was correlated with 

the base of subsection 2B by means of marker beds. The 

upper member was measured south of the Darwin fault in 

subsection 1. The composite section of the Union Wash 

Formation is 853 m thick; . the lower member is 22.5 m 

thick, the middle member is 520.5 m thick, and the upper 

member is 310 m thick. In the geologic map of the Darwin 

area by Stone and others (1989), the lower, middle, and 

upper members of the Union Wash Formation are referred 

to as units 1, 2, and 3-5, respectively, of the unnamed 

Triassic sedimentary rocks unit. 

The lower member of the Union Wash Formation is 

composed primarily of light- to medium-gray, thick-bedded 

silty and sandy mottled limestone that contains even laminations and irregular stringers of brown siltstone. Lewis and 

others (1983) reported the presence of minute gastropod molds 

in these rocks. The largest grains in most beds are of very 

fine sand size, but some beds contain concentrations of coarse 

siliceous sand and granules. On the line of the measured 

section, the lower member contains lenses of massive cobble 

to boulder conglomerate derived from the underlying Permian 

rocks. The lower member is readily distinguished from both 

the Panamint Springs Member, which is composed of thin, 

graded beds of brown siltstone and very fme grained sandstone, 

and the Millers Spring Member, which is composed of thickbedded brown siltstone to very fine grained sandstone and 

thick-bedded, medium-gray bioclastic limestone containing 

corals and fusulinids. 

The middle member of the Union Wash Formation is 

composed primarily of mudstone and micritic limestone. 

The member consists of a dominantly siliceous lower part 

and a calcareous upper part. The most abundant rock type 

in the lower part of the middle member (approximately the 

lower 135 m in the measured section) is dark-gray, laminated, siliceous mudstone that commonly is reddish brown 

on weathered surfaces. The mudstone locally contains 

abundant medium-gray, spherical to subspherical concretions of micritic limestone as much as 10 em in diameter. 

Thin beds of light-gray silty limestone, some of which contain small ammonoids, are present at scattered intervals. 

Lewis and others (1983) noted the presence of radiolarians 

in this part of the section. These rocks grade into the upper 

part of the middle member, which is composed primarily of 

brown-weathering, finely laminated calcareous mudstone and 

light- to medium-gray, thin-bedded micritic limestone. Large 

parts of the section are composed predominantly of calcareous mudstone with little or no interbedded limestone; other 

parts are composed of rhythmically alternating beds of calcareous mudstone and micritic limestone, 1 to 3 em thick. 

A few intervals as much as 15 m thick, particularly in the 

upper part of the member, are composed primarily of 

thin-bedded micritic limestone interbedded with minor calcareous mudstone. These intervals stand out as conspicuous markers. 

Stratigraphy and Contact Relations 13 
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117' 35' 117' 34' 

36" 

17' 

0 0.5 1.0 1.5 2 KILOMETERS 

Figure 7. Geologic map of Darwin area, California, showing location of reference section of the Union Wash Formation. Reference section consists of three 

separate subsections (1, 2A, 28). See figure 1 for location of map. Base from U.S. Geological Survey, 1:24,000 scale, Darwin quadrangle, provisional edition 

1987. Contour interval40 ft. Geology mapped in 1980--81. 
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EXPlANATION 

Alluvium and talus (Quaternary) 

Volcanic rocks (Tertiary) 

Leucogranite (Cretaceous or Jurassic) 

Quartz monzonite (Jurassic) 

Darwin Falls pluton (Jurassic?}-Gabbro, granodiorite, and 

minor quartz monzonite 

Union Wash Formation (Triassic}-Divided into: 

Upper member-Divided into: 

Subunit 3-Light- to medium-gray micritic limestone and 

grayish-orange silty limestone. Measured thickness 64.5 m 

Subunit 2-Brown massive siltstone. Measured thickness 

62.5m 

Subunit 1-Medium- to dark-gray micritic limestone; upper 

part contains marker horizon of greenish-gray to grayishorange shale. Measured thickness 183m 

Middle member-Lower part composed of dark-gray siliceous 

mudstone and minor silty limestone; upper part composed 

of brown-weathering calcareous mudstone, light- to medium-gray, thin-bedded micritic limestone, and minor 

limestone-clast conglomerate. Upper part contains 

abundant soft-sediment deformational features. Measured 

thickness 520.5 m 

Lower member-Light- to medium-gray, silty and sandy limestone; minor conglomerate containing clasts of underlying 

Permian rocks. Measured thickness 22.5 m; maximum 

thickness approximately 50 m 

Figure 7. Continued. 

~ 

I Pdm I 

""'--

----r 

Owens Valley Group-In this area, consists of: 

Darwin Canyon Formation (Permian}-Divided into: 

Panamint Springs Member-Brown, thin-bedded siltstone 

and very fine grained sandstone; medium- to dark-gray 

limestone in lowermost part. Maximum thickness 

approximately 500 m 

Millers Spring Member-Brown, thick-bedded, very fine 

grained sandstone and medium- to dark-gray, thin- to thick-bedded bioclastic to conglomeratic limestone. 

Locally includes fault breccia of Mesozoic or Cenozoic 

age. Thickness approximately 475-650 m 

Contact 

Fault-Dotted where concealed. Arrows show direction of 

relative movement -+- Anticline-Dotted where concealed 

-Y-

-+-

-Y-

_J....E...O 

-+-

_ __J.?O 

2A 

-

5. 

Overturned anticline 

Syncline 

Overturned syncline 

Strike and dip of beds 

Inclined 

Vertical 

Overturned 

Reference section of Union Wash Formation-Numbers refer to segments of measured section 

Location and number of conodont collection of Lewis and 

others (1983) 
Although most rocks in the middle member are evenly 

and continuously bedded, some rocks, particularly in the 

calcareous upper part, show evidence of soft-sediment deformation and resedimentation. Soft-sediment folds, sedimentary boudinage, zones of broken and disrupted beds, 

and matrix-supported limestone conglomerate beds (debrisflow deposits) are common, especially in the intervals 

dominated by limestone. Most clasts in the conglomerate 

beds are lithologically similar to the associated thin-bedded 

micritic limestone and probably were formed by disruption 

and breakage of contemporaneous limestone beds on the 

margins and slopes of the depositional basin. Some clasts 

are irregular in shape and apparently were transported and 

deformed while soft. On the east flank of the Darwin Hills, 

the siliceous lower part of the middle member contains at 

least one limestone slide block several meters thick and 

several tens of meters long. 

The upper member of the Union Wash Formation, 

which sharply overlies the middle member, is divided into 

three subunits (figs. 7, 8). In ascending order, these are 

called subunits 1, 2, and 3, which are equivalent to units 3, 

4, and 5, respectively, of the unnamed Triassic sedimentary rocks unit of Stone and others (1989). The measured thicknesses of subunits 1, 2, and 3 are, respectively, 

183, 62.5, and 64.5 m. Subunit 1 is composed primarily 

of medium- to dark-gray micritic limestone that forms rugged cliffs and ledges. An interval of fissile greenish-gray 

to grayish-orange shale, approximately 20 m thick, is present in the upper part of the subunit. The limestone below 

the shale interval ranges from thin bedded (2 to 5 em) in 

the lower part to medium and thick bedded (20 to 50 em) 

in the upper part; beds commonly are separated by thin 

partings of grayish-orange to maroon shale. Soft-sediment 

folds are common. Bedding in the limestone above the 

shale interval is generally indistinct and disrupted; limestone conglomerate is present locally. Subunit 2 is 

composed almost entirely of massive brown quartzose siltstone. Subunit 3 is composed of medium- to thick-bedded, 

light- to medium-gray micritic limestone and grayishorange silty limestone in which thin partings of brown siltstone are common. Subunit 3 is exposed only in the core 

of the large syncline south of the Darwin fault and is the 

highest part of the Union Wash Formation present in the 

area. 

None of the subunits of the upper member of the 

Union Wash Formation in the Darwin area can be correlated with certainty with any of the subunits distinguished 

in the Union Wash and Cerro Gordo areas. Similar rock 

types are present in all three areas, however, and some of 

the subunits in the Darwin area resemble some of those in 

the other study areas. A particularly close lithologic similarity exists between subunit 1 in the Darwin area and subunit 

3 in the Cerro Gordo area, both of which are composed of 

dark-gray micritic limestone containing irregular shaley or 

silty partings. 

FOSSILS AND AGE 

Rocks of the Union Wash Formation have yielded 

fossils useful for the determination of age in all three of the 

areas studied. The most significant fossils are ammonoids 

from the Union Wash area (Smith, 1904, 1914, 1932; 

Silberling and Tozer, 1968) and conodonts that have been 

identified from all three areas by Orchard during the present study. The conodonts include some specimens originally identified by Lewis and others (1983) and reexamined 

during the present study in addition to those collected from 

our measured sections. The reexamined conodonts are from 

localities 1, 2, 5, and 9 of Lewis and others (1983, fig. 12, 

table 1). Most of these conodonts are here identified as 

different taxa than those listed by Lewis and others (1983, 

table 1), and some of the age determinations are different 

than theirs as well. The stratigraphic positions of the conodont collections are shown in figures 3, 5, and 8; the approximate positions of the collections of Lewis and others 

(1983) were inferred from map localities provided by 

Michael Lewis (written common, 1988). The conodonts 

are stored at the Geological Survey of Canada, Vancouver, 

B.C., and the collections have been assigned Geological 

Survey of Canada (GSC) locality numbers. 

The age range of the Union Wash Formation, as interpreted herein, is shown in figure 9. The Triassic stage 

terminology followed herein is that defined by Silberling 

and Tozer (1968). The known stratigraphic ranges of the 

conodonts identified during this study are shown by 

Collinson and Hasenmueller (1978), Carr and Paull (1983), 

and Dagis (1984). 

Union Wash Area 

The Triassic rocks herein assigned to the Union Wash 

Formation were first dated by Smith (1904, 1914, 1932) on 

the basis of ammonoids from two limestone beds on the 

south side of Union Wash-the Meekoceras bed, at the base 

of the middle member of the Union Wash Formation; and 

the Parapopanoceras bed, 665 m higher, 1-2m above the 

base of subunit 1 of the upper member of the Union Wash 

Formation (fig. 3). Smith (1904, 1914, 1932) considered 

the Meekoceras bed to be Lower Triassic and the Parapopanoceras bed to be lowermost Middle Triassic. In the 

biostratigraphic classification of Silberling and Tozer (1968), 

Smith's Meekoceras bed is considered to represent the lower 

Smithian Meekoceras gracilitatis zone and Smith's 

Parapopanoceras bed is considered to represent the uppermost Spathian Neopopanoceras haugi zone. 

Ammonoids have been found at two other localities within the Union Wash Formation in this area. One 

locality, discovered during the present study, is in a 

limestone bed 3 m thick and a few meters above the 

base of the middle member on the ridge north of Union 

16 Stratigraphy of the lower and Middle(l) Triassic Union Wash Formation, East-Central California 
Wash. According to N.J. Silberling and E.T. Tozer (oral 

commun., 1988), this bed contains many of the same 

ammonoid species as Smith's Meekoceras bed and also 

represents the Meekoceras gracilitatis zone. The other 

Z ~ ;i_ Z I ~C/) 

0 ROCK UNIT 1 :J LITH- > ~- ~ ;;J_ ~O wffi 1 j:: 01-<( a: Q9iWI- I-_Cf)l- ~frl ~ (THICKNESS, IN METERS) aa8 OLOGY ~ DESCRIPTION :di:Jw ~j!:~~ 

~CD~CD~~<C~r---~------~~-------l~o~o~~~==c:~~t-~z~~~==::=-~~::~~-=~::~::::~=:::~~----ti-~Z~~~853---- i Limestone, light gray to grayish orange, micritic, in 

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Subunit 3 

(64.5 m) 

Subunit 2 

(62.5 m) 

42- part silty and nodular; thin beds and partings of 

1 1 44 brown siltstone. Top of section at axis of syncline 

l 

-·-·-·-

~·.-=..·.-.- 39- Siltstone, brown, slightly calcareous, massive 

-·-·-·- 41 

-·-·-·- Upper 

member 

(310m) 

1---------1------+-r--,....,....· -.,...-·---+------1-'v Limestone, medium- to dark-gray, micritic; one bed of 

JL~ 38 limestone conglomerate. Disrupted bedding in 

Subunit 1 

(183m) 

Middle 

member 

(520.5 m) 

(9) 

o o a o o o uppermost part 

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Shale, greenish-gray to grayish-orange, fissile 

Limestone, medium- to dark-gray, micritic; forms cliffs 

and ledges. Beds increase in thickness from 1-5 em 

in lower 50 m to 20-50 em in upper 80 m. Softsediment folds present throughout 

Mudstone, calcareous, light-brown; and limestone, 

light- to medium-gray, micritic; in alternating beds 

1-3 em thick. Poorly exposed 

Limestone, medium-gray, massive; locally nodular 

Mudstone, calcareous, light-brown; minor limestone, 

light- to medium-gray, in beds 1-3 em thick 

Limestone, medium- to dark-gray; disrupted and 

broken beds common. Forms ledge 

Mudstone, calcareous, brown; minor limestone, lightto medium-gray, in beds 1-4 em thick. Few zones 

of limestone and subordinate mudstone 1-7 m thick. 

Rare limestone conglomerate. Soft-sediment folds 

present locally 

9 Limestone, dark-gray, weathers light-gray; contains 

~==== '-----' abundant small ammonoids 

-----

- - 3-5 =====::== 

Mudstone, calcareous, dark-gray; weathers brown to 

reddish-brown; laminated; contains medium-gray 

limestone concretions. Minor limestone, dark-gray 

(weathers light-gray); contains ammonoids 

(5) c_ ___ _ 

Mudstone, siliceous, dark-gray; weathers reddishbrown; upper part contains limestone concretions. 

Lower part contains beds of light-gray silty 

limestone 5-10 em thick, some of which contain 

small ammonoids 

64.5 

800· 

62.5 

30 

700 

23 

130 

600 

49 

500 

12 

54 

15 

400 

228.5 

300 

200 

7.5 

68 

100 

86.5 

Lower member D2A_1 o 1 o 1 1

_2 Limestone, light to medium-gray, silty, nodular. 

L..~~~(~2~2~·~5~m!M)~~~----~E=~~~~~~~--~~~L~e~n~s~of~m~a;ss~iv~e~li;m;es;,-tottn~e~co~n;g~lo~m~e~rabtke~~~-----r- _·_-t 0 Millers Spring Member Limestone, medium-gray, thick-bedded, bioclastic 

(part; not measured) 

Figure 8. Reference section of the Union Wash Formation in Darwin area. Section consists of three separate subsections (1, 2A, 

28), locations of which are shown on figure 7. 

Fossils and Age 17 
locality, originally described by Silberling and Tozer 

(1968, p. 39) as "several tens of feet" above Smith's 

Parapopanoceras bed, is probably equivalent to a thin 

ammonoid-bearing layer we observed about 20 m above 

the base of subunit 2 of the upper member a few hundred meters northwest of our measured section. 

Silberling and Tozer (1968) considered the ammonoids 

from this locality to be similar to Keyserlingites, an 

identification compatible with a latest Spathian age. 

During this study, the following conodonts were identified from the Union Wash Formation on the south side of 

Union Wash: 

GSC Loc. Nos. C-176541 and C-176542 (Field Nos. 

81-ML-5 and 81-ML-6). Equivalent to Loc. 1 of Lewis 

and others (1983). Union Wash Formation, middle member, Meekoceras ammonoid bed; corresponds to 0-14.5 m 

above base of member and formation in the type section 

(figs. 2, 3). 

Neospathodus n. sp. A 

Ellisonia? sp. 

Age: Early Triassic (Smithian) 

Remarks: Neospathodus n. sp. A is identical to conodonts associated with Smithian ammonoids in Oman. 

GSC Loc. No. C-158468 (Field No. UW-4). Type 

section of Union Wash Formation, middle member, 332m 

above base of member and formation (fig. 3). 

Neospathodus homeri (Bender) 

Age: Early Triassic (Spathian) 

GSC Loc. No. C-176543 (Field No. 81-ML-9). 

Equivalent to Loc. 2 of Lewis and others (1983). Union 

Wash Formation, middle member, approximately 400 m 

above base of member and formation (figs. 2, 3). 

Neospathodus homeri (Bender) 

Age: Early Triassic (Spathian) 

GSC Loc. No. C-158469 (Field No. UW-5). Type 

section of Union Wash Formation, middle member, 512 m 

above base of member and formation (fig. 3). 

Neospathodus homeri (Bender) 

Age: Early Triassic (Spathian) 

The middle member in the Union Wash area thus 

ranges in age from Smithian to Spathian, and the upper 

member is considered to be latest Spathian in age. The 

lenticular lower member, from which no fossils have been 

recovered, is undated. 

Cerro Gordo Area 

In a study of the Cerro Gordo mining district, Merriam (1963) reported ammonoids from beds that we include 

in the lower member of the Union Wash Formation and that 

are located about 7 km northwest of the area shown in 

figure 4. Merriam identified these ammonoids as Ussuria 

and interpreted their stratigraphic position to be lower than 

that of the Meekoceras bed at Union Wash. According to 

N.J. Silberling (oral commun., 1976), Merriam's ammonoids 

probably represent either Metussuria or Parussuria and are 

indicative of a Smithian age. 

During this study, the following conodonts were identified from the Union Wash Formation in the reference section northwest of Cerro Gordo Road (fig. 5): 

GSC Loc. No. C-158459 (Field No. CG-1). Union 

Wash Formation, middle member, 13.5 m above base of 

member, 45.0 m above base of formation. 

Neospathodus ex gr. waageni Sweet 

Age: Early Triassic (Smithian) 

Remarks: Basal cavity is strongly expanded in these 

neospathodids, which are relatively short. 

GSC Loc. No. C-158460 (Field No. CG-2). Union 

Wash Formation, middle member, 69.0 m above base of 

member, 100.5 m above base of formation. 

Neospathodus ex gr. waageni Sweet 

Age: Early Triassic (Smithian) 

Remarks: Basal cavity is narrow in these neospathodids, which are relatively long. 

GSC Loc. No. C-158461 (Field No. CG-3). Union 

Wash Formation, middle member, 180.0 m above base of 

member, 211.5 m above base of formation. 

Neospathodus ex gr. homeri (Bender) 

Age: Early Triassic (Spathian?) 

Remarks: Basal cavity is narrow in this neospathodid, 

which has a downturned posterior. 

GSC Loc. No. C-158462 (Field No. CG-13). Union 

Wash Formation, upper member, subunit 4, 127 m above 

base of subunit, 314.5 m above base of member, 582.5 m 

above base of formation. 

Neogondolella? sp. indet. 

Age: Triassic 

Remarks: Fragments only. 

We thus consider the lower member of the Union 

Wash Formation in the Cerro Gordo area to be Smithian 

and the middle member to be Smithian and Spathian(?). 

The age of the upper member cannot be closely 

determined. 

Darwin Area 

During this study, the following conodonts were identified from the Union Wash Formation in the Darwin area: 

18 Stratigraphy of the lower and Middle(?) Triassic Union Wash Formation, East-Central California 
GSC Loc. No. C-158465 (Field No. D-2A-1). Union 

Wash Formation, lower member, 1 m above base of member and formation in reference section (fig. 8). 

Parachirognathus ethingtoni Clark 

Ellisonia sp. 

Age: Early Triassic (Smithian) 

GSC Loc. Nos. C-176539 and C-176540 (Field Nos. 

80-CB-2 and 80-CB-3A). Equivalent to Loc. 5 of Lewis 

and others (1983). Union Wash Formation, middle member, near base of member (figs. 7, 8). 

Neospathodus ex gr. waageni Sweet 

Neogondolella buurensis Dagis 

Neogondolella sp. indet. 

Ellisonia? sp. 

Age: Early Triassic (Smithian) 

Remarks: The neogondolellids most closely resemble 

species described by Dagis (1984) from the Olenekian Stage 

of northern Siberia, where they also occur with Neospathodus 

waageni. 

GSC Loc. No. C-176544 (Field No. D-1575). 

Equivalent to Loc. 9 of Lewis and others (1983). Union 

Wash Formation, upper member, subunit 1, above shale 

horizon in upper part of subunit (fig. 8). Located 0.93 km 

beyond the northern boundary of figure 7 at lat. 36°19 '53", 

long. 117°34 '51". 

Neogondolella cf. N. timorensis (Nogami) 

Age: Early or Middle Triassic (Spathian-Anisian 

boundary?) 

Remarks: The specimens at hand are mostly fragments not adequately preserved for confident assignment. 

Conodonts from the lowest of the three collections 

indicate that the base of the Union Wash Formation in the 

Darwin area is no older than Smithian. Lewis and others 

(1983) assigned a Griesbachian age to rocks in the lowermost part of the formation on the basis of bivalves that they 

identified as Claraia stachei, a common species in Griesbachian rocks. According to recent work by Yin (1985), 

however, the stratigraphic range of Claraia stachei extends 

from the Griesbachian into the Smithian, so its occurrence 

is compatible with that of the Smithian conodonts reported 

herein. Reidentification of the conodonts from locality 5 of 

Lewis and others (1983) indicates a Smithian age for the 

lowermost part of the middle member of the Union Wash 

Formation, rather than a Dienerian age as previously reported. The age of the uppermost collection is questionable, 

both because the conodonts are not adequately preserved 

for confident identification and because the boundary between the Spathian and Anisian Stages is not clearly defined 

on the basis of conodont biostratigraphy. The stratigraphic 

range of Neogondolella timorensis is thought to be upper 

Spathian to lower Anisian (Collinson and Hasenmueller, 

1978; Carr and Paull, 1983); the form in this collection may 

belong to this species and, if so, may indicate an age close 

to the Early-Middle Triassic boundary. 

Summary 

Based on the evidence available, we assign an age of 

Early and Middle(?) Triassic to the Union Wash Formation. 

The lower member is of Smithian age (Early Triassic), the 

middle member is of Smithian and Spathian age (Early Triassic), and the upper member is of latest Spathian and possibly 

Anisian age (Early and Middle(?) Triassic) (fig. 9). Additional 

study is needed to resolve the upper age limit of the upper 

member, from which few fossils are presently known. 

DEPOSITIONAL ENVIRONMENTS 

The Union Wash Formation is of marine origin. The 

formation appears to represent a variety of water depths 

and sedimentary environments, but the observations that 

L 

w 

1-

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>-

(/) 

~ (/) 

(/) 

<( 

a: 

1-

(/) 

w 

a: 

w 

(/) 

STAGE 

Anisian 

Spath ian 

Smithian 

ROCK UNIT 

.Q Upper member 

n; 

E 

0 

ll.. 

..c. 

(/) 

CIS 

:s 

Middle member 

Lower member 

t---------t--,--.--+--...,.---r- ? 2--.--....--....--.---l 

Diener ian 

Griesbachian 

1 Top of Union Wash Formation could be Spathian instead 

of Anisian as shown. 

2 Base of Union Wash Formation could be within the 

Smithian Stage instead of at the Smithian-Dienerian 

boundary as shown. 

Figure 9. Chart showing age of the Union Wash Formation. 

Stages after Silberling and Tozer (1968). 

Depositional Environments 19 
have been made to date allow only general interpretations 

of the depositional setting. 

The silty and sandy limestone that makes up the lower 

member of the Union Wash Formation is characterized by 

wavy, irregular bedding and mottled texture that may have 

resulted from bioturbation. We interpret the sediment that 

forms these rocks to have been deposited at shallow subtidal 

depths in a nearshore environment where currents were strong 

enough to winnow out most sediment finer than coarse silt. 

By contrast, we interpret the middle member of the 

Union Wash Formation to have been deposited in quiet, 

moderately deep water. This interpretation is based on the 

dominance of mudstone and micritic limestone; the paucity 

of silt and sand (except in the lowermost part of the member); the thin-bedded, laminated nature of the rocks; the 

presence of radiolarians in the Cerro Gordo and Darwin 

areas; the absence of megafossils other than ammonoids; 

and the general lack of sedimentary features indicative of 

shallow-water, nearshore deposition. Features reflecting softsediment deformation and resedimentation in the Darwin 

area are suggestive of a slope or base-of-slope environment; such features are absent in the Cerro Gordo and Union 

Wash areas, where the middle member may have accumulated in basinal or outer-shelf environments. 

The upper member of the Union Wash Formation 

differs from the middle member in containing a much smaller 

proportion of mudstone or shale and greater proportions of 

limestone, siltstone, and very fine grained sandstone. This 

general contrast in lithology suggests that the upper member 

accumulated in somewhat shallower water and somewhat 

closer to shore than the middle member. Limestone and 

clastic rocks in the upper member are largely segregated 

from one another in mappable subunits, a pattern that apparently reflects sharp fluctuations in the supply of clastic 

sediment. The limestones hold the best clues to the depositional environments represented by the upper member. The 

lower part of the upper member is dominated by dark-gray, 

thin-bedded micritic limestone that is lithologically similar 

to much of the limestone of the middle member. Such 

limestone forms subunit 1 in the Union Wash area, subunits 

1 and 3 in the Cerro Gordo area, and most of subunit 1 in 

the Darwin area. This limestone, which contains no 

megafossils other than ammonoids, probably accumulated 

in moderately deep water. Soft-sediment folds and slump 

features in subunit 1 in the Darwin area suggest a slope or 

base-of-slope environment for at least some of this limestone; 

elsewhere, outer-shelf environments may be represented. 

By contrast, limestone (and dolomite) in the upper part of 

the upper member probably accumulated in relatively shallow water. In the Cerro Gordo area, carbonate rocks high 

in subunit 4 contain abundant evidence of shallow-water 

deposition, including ooliths, coated grains, and probable 

stromatolites; in the Darwin area, the light-gray, thickbedded, mottled (bioturbated?) limestone of subunit 3 seems 

to reflect shallower water depths than the dark-gray, thinbedded limestone of subunit 1. The carbonate rocks of the 

upper member thus appear to record shallowing of water 

depths through time. 

In summary, we interpret deposition of the Union 

Wash Formation to have begun in shallow water (lower 

member), and then to have moved rapidly into moderately 

deep water (middle member and lower part of upper member) before returning into shallow water (upper part of upper 

member). 

DEPOSITIONAL HISTORY 

Deposition of the Union Wash Formation followed a 

period of active tectonism in the Inyo Mountains, Darwin 

area, and Argus Range that spanned most of Pennsylvanian 

and Permian time. During the Pennsylvanian and Early Permian, these areas were sites of deep-water marine sedimentation in basins interpreted by Stone and Stevens (1988b) to 

have formed by transtensional deformation on a segment of 

the continental margin that was undergoing active transform 

faulting. This sedimentation produced the Tihvipah Limestone 

(Stone and others, 1987, 1989), the Keeler Canyon Formation, 

and the Lower Permian formations of the Owens Valley Group. 

Deep-water sedimentation ended in late Early Permian time 

when the area was rapidly elevated to sea level and above, 

possibly in response to a change from a transtensional to a 

transpressional tectonic regime. During this event, Pennsylvanian and Lower Permian rocks were broadly folded, tilted 

to dips as steep as 30°, and erosionally truncated on the crests 

and flanks of large anticlines (Stone and Stevens, 1988a). 

The resulting landscape had a surface relief of at least 250 m. 

Deposition of the shallow-water marine to nonmarine Conglomerate Mesa Formation during the Late Permian, after 

tectonic activity had ceased, filled the major topographic depressions and left the surface nearly featureless. This was the 

surface upon which the Union Wash Formation was later 

deposited. 

Marine transgression in late Early Triassic time resulted in deposition of the Union Wash Formation, which 

onlapped remaining topographic highs that had not been 

covered by the Conglomerate Mesa Formation. The nearshore strata of the thin lower member of the Union Wash 

Formation may have been deposited primarily in response 

to a moderate rise in eustatic sea level (Haq and others, 

1987), but the ensuing increase in water depth and the accumulation of the thick middle member clearly reflect renewed subsidence of the continental margin. The tectonic 

significance of this late Early Triassic subsidence is not 

known, but it could represent the continued effects of transform faulting or the initial stages of subduction along the 

continental margin. An increased rate of sedimentation, a 

decreased rate of subsidence, or a combination of both in 

latest Early Triassic time led to shoaling, as represented by 

the lithologically heterogeneous upper member of the Union 

20 Stratigraphy of the lower and Middle(?) Triassic Union Wash Formation, East-Central California 
Wash Formation. Deposition of the upper member was 

followed by emergence (possibly accompanied by slight 

tectonic deformation), erosion, and deposition of the overlying Triassic and (or) Jurassic volcanic and sedimentary 

rocks in nonmarine environments. 

REFERENCES CITED 

Carr, T.R., and Paull, R.K., 1983, Early Triassic stratigraphy and 

paleogeography of the Cordilleran miogeocline, in Reynolds, 

M.W., and Dolly, E.D., eds., Mesozoic paleogeography of 

the west-central United States: Denver, Colo., Society of 

Economic Paleontologists and Mineralogists, Rocky Mountain Section, p. 39-55. 

Cole, R.D., 1986, Geology of the Butte Valley and Warm Springs 

Formations, southern Panamint Range, Inyo County, California: Fresno, Calif., California State University, M.S. thesis, 

126 p. 

Collinson, J.W., and Hasenmueller, W.A., 1978, Early Triassic 

paleogeography and biostratigraphy of the Cordilleran miogeosyncline, in Howell, D.G., and McDougall, K.A., eds., 

Mesozoic paleogeography of the western United States: Los 

Angeles, Calif., Society of Economic Paleontologists and 

Mineralogists, Pacific Section, p. 175-187. 

Conley, D.E., 1978, Stratigraphy and depositional history of the 

Owens Valley Formation at the type locality, Inyo County, 

California: San Jose, Calif., San Jose State University, M.S. 

thesis, 63 p. 

Dagis, A.A., 1984, Rannetriasovyye konodonty severa Sredney 

Sibiri [Early Triassic conodonts from north-central Siberia]: 

Trudy lnstituta Geologii i Geofiziki (Novosibirsk), no. 554, 

72p. 

Dunne, G.C., Gulliver, R.M., and Sylvester, A.G., 1978, Mesozoic evolution of rocks of the White, Inyo, Argus, and Slate 

Ranges, eastern California, in Howell, D.G., and McDougall, 

K.A., eds., Mesozoic paleogeography of the western United 

States: Los Angeles, Calif., Society of Economic Paleontologists and Mineralogists, Pacific Section, p. 189-207. 

Elayer, R.W., 1974, Stratigraphy and structure of the southern 

Inyo Mountains, Inyo County, California: San Jose, Calif., 

San Jose State University, M.S. thesis, 121 p. 

Hall, W.E., and MacKevett, E.M., Jr., 1962, Geology and ore 

deposits of the Darwin quadrangle, Inyo County, California: 

U.S. Geological Survey Professional Paper 368, 87 p. 

Haq, B.U., Hardenbol, Jan, and Vail, P.R., 1987, Chronology of 

fluctuating sea levels since the Triassic: Science, v. 235, 

no.4793,p. 1156-1167. 

Johnson, B.K., 1957, Geology of a part of the Manly Peak quadrangle, southern Panamint Range, California: University of 

California Publications in Geological Sciences, v. 30, no. 5, 

p. 353-424. 

Kirk, Edwin, 1918, Stratigraphy of the Inyo Range, in Knopf, 

Adolph, A geological reconnaissance of the Inyo Range and 

the eastern slope of the southern Sierra Nevada, California: 

U.S. Geological Survey Professional Paper 110, 130 p. 

Knopf, Adolph, 1918, A geologic reconnaissance of the Inyo Range 

and the eastern slope of the southern Sierra Nevada, California: U.S. Geological Survey Professional Paper 110, 130 p. 

Lewis, Michael, Wittman, Charles, and Stevens, C.H., 1983, 

Lower Triassic marine sedimentary rocks in east-central 

California, in Marzolf, J.E., and Dunne, G.C., eds., Evolution of early Mesozoic tectonostratigraphic environments-southwestern Colorado Plateau to southern Inyo 

Mountains: Utah Geological and Mineral Survey Special 

Studies 60, p. 50-54. 

McAllister, J.F., 1956, Geology of the Ubehebe Peak quadrangle, 

lnyo County, California: U.S. Geological Survey Geologic 

Quadrangle Map GQ-95, scale 1:62,500. 

Merriam, C.W., 1963, Geology of the Cerro Gordo mining district, Inyo County, California: U.S. Geological Survey Professional Paper 408, 83 p. 

Merriam, C.W., and Hall, W.E., 1957, Pennsylvanian and Permian rocks of the southern Inyo Mountains, California: U.S. 

Geological Survey Bulletin 1061-A, 13 p. 

Moore, S.C., 1976, Geology and thrust fault tectonics of parts of the 

Argus and Slate Ranges, Inyo County, California: Seattle, 

Wash., University of Washington, Ph.D. dissertation, 128 p. 

Mount, J.D., 1971, Stratigraphy and paleontology of the marine 

Triassic, Inyo Mountains, Inyo County, California: Bulletin 

of the Southern California Paleontological Society, v. 5, no. 7, 

p. 1-4 and 9. 

Oborne, M.S., 1983, Stratigraphy of Early to Middle(?) Triassic 

marine-to-continental rocks, southern Inyo Mountains, California: Northridge, Calif., California State University, M.S. 

thesis, 101 p. 

Oborne, Mark, Fritsche, A.E., and Dunne, George, 1983, Stratigraphic analysis of Middle(?) Triassic marine-to-continental 

rocks, southern lnyo Mountains, California, in Marzolf, J.E., 

and Dunne, G.C., eds., Evolution of early Mesozoic 

tectonostratigraphic environments-southwestern Colorado 

Plateau to southern Inyo Mountains: Utah Geological and 

Mineral Survey Special Studies 60, p. 54-59. 

Ross, D.C., 1967, Generalized geologic map of the Inyo Mountains region, California: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-506, scale 1:125,000. 

Silberling, N.J., and Tozer, E.T., 1968, Biostratigraphic classification of the marine Triassic in North America: Geological 

Society of America Special Paper 110, 63 p. 

Smith, J.P., 1898, Geographic relations of the Trias of California: 

Journal of Geology, v. 6, p. 776-786. 

---1904, The comparative stratigraphy of the marine Trias of 

western America: California Academy of Sciences Proceedings, Third Series, v. 1, no. 10, p. 323-430. 

---1914, The Middle Triassic marine invertebrate faunas of 

North America: U.S. Geological Survey Professional 

Paper 83, 254 p. 

---1932, Lower Triassic ammonoids of North America: U.S. 

Geological Survey Professional Paper 167, 199 p. 

Stinson, M.C., 1977, Geologic map and sections of the Keeler 15-

minute quadrangle, Inyo County, California: California Division of Mines and Geology, Map Sheet 38, scale 1:62,500. 

Stone, Paul, 1984, Stratigraphy, depositional history, and paleogeographic significance of Pennsylvanian and Permian rocks 

in the Owens Valley-Death Valley region, California: 

Stanford, Calif., Stanford University, Ph.D. dissertation, 399 p. 

Stone, Paul, Dunne, G.C., Stevens, C.H., and Gulliver, R.M., 1989, 

Geologic map of Paleozoic and Mesozoic rocks in parts of 

the Darwin and adjacent quadrangles, Inyo County, California: 

References Cited 21 
U.S. Geological Survey Miscellaneous Investigations Series 

Map 1-1932, scale 1:31,250. 

Stone, Paul, and Stevens, C.H., 1986, Triassic marine section at 

Union Wash, Inyo Mountains, California, in Dunne, G.C., 

compiler, Mesozoic and Cenozoic structural evolution of selected areas, east-central California: Geological Society of 

America, Cordilleran Section, Field Trip Guidebook, p. 45-

51. 

--1987, Stratigraphy of the Owens Valley Group (Permian), 

southern Inyo Mountains, California: U.S. Geological Survey 

Bulletin 1692, 19 p. 

---1988a, An angular unconformity in the Permian section of 

east-central California: Geological Society of America Bulletin,v. 100,no.4,p.547-551. 

---1988b, Pennsylvanian and Early Permian paleogeography 

of east-central California: Implications for the shape of the 

continental margin and the timing of continental truncation: 

Geology, v. 16, no. 4, p. 330-334. 

Stone, Paul, Stevens, C.H., and Cavit, C.D., 1980, A regional Early 

Permian angular unconformity in eastern California [abs.]: 

Geological Society of America Abstracts with Programs, v. 12, 

no. 3, p. 154. 

Stone, Paul, Stevens, C.H., and Magginetti, R.T., 1987, Pennsylvanian and Permian stratigraphy of the northern Argus Range 

and Darwin Canyon area, California: U.S. Geological Survey 

Bulletin 1691,30 p. 

Yin, H.F., 1985, Bivalves near the Permian-Triassic boundary in 

south China: Journal of Paleontology, v. 59, no. 3, p. 572-

600. 

MEASURED SECTIONS 

Union Wash section (figs. 2, 3). Type section at the type locality of the 

Union Wash Formation. Measured on south side of Union Wash, Inyo 

Mountains. in September 1985. Section consists of middle and upper 

members of the Union Wash Formation; lower member is not present. 

Unnamed volcanic and sedimentary rocks (Triassic 

and (or) Jurassic) (incomplete): 

2. Sandstone, volcaniclastic, brown; overlain by a thick sequence of volcanic 

rocks .................................................... .. 

1. Conglomerate, composed mainly of small 

pebbles of limestone and volcanic rock. 

Scattered limestone blocks more than 

1 m in diameter appear to be enclosed 

by the conglomerate ............................ .. 

Incomplete thickness of unnamed 

volcanic and sedimentary rocks ..... 

Unconformable contact 

Union Wash Formation (Triassic): 

Upper member: 

Subunit 2: 

54. Siltstone, dark-gray, weathers brown; calThickness 

in meters 

10.0 

7.5 

17.5 

careous, plane laminated....................... 15.5 

53. Shale, pink, silty......................................... 4.0 

Union Wash Formation (Triassic }-Continued 

Upper member-Continued 

Subunit2-Continued 

52. Siltstone, as in 54 ..................................... .. 

51. Shale, fissile, pink, soft ............................ .. 

50. Interval occupied by crosscutting dike .... .. 

49. Shale, fissile, light-brown to pink ............ .. 

Total thickness of subunit 2 .............. . 

Subunit 1: 

48. Limestone, as in 42 .................................. .. 

47. Shale, light-brown ..................................... . 

46. Limestone, as in 42 .................................. .. 

45. Shale, pink ................................................. . 

44. Limestone, as in 42, interbedded with 

equal amount of light-brown to pink 

shale ...................................................... . 

43. Siltstone, calcareous, orange ..................... . 

42. Limestone, micritic, dark-gray, in platy beds 

typically less than 25 em thick; interbedded with less abundant light-brown shale 

and siltstone. Fossiliferous, ammonoidbearing zone 1-2 m above base is 

equivalent to Parapopanoceras bed of 

Smith (1904, 1914) ............................... . 

Total thickness of subunit 1 .............. . 

Total thickness of upper member .... .. 

Middle member: 

41. Siltstone, shale, and limestone. Siltstone 

and shale calcareous, mostly gray to 

light brown but some pink. Limestone, 

micritic, dark-gray, in beds less than 

Thickness 

in meters 

3.0 

7.5 

4.5 

6.0 

40.5 

1.0 

4.0 

2.5 

3.0 

9.0 

4.5 

30.0 

54.0 

94.5 

20 em thick. Forms recessive slope ...... 48.0 

40. Dolomite, light-gray to light-brown, faintly 

laminated .. .. ...... .. . .. .. ...... ........ ... .. ... ... .. ... 1.0 

39. Limestone, dark-gray, massive; contains 

recrystallized shell fragments .. .. ...... .. .... 1.0 

38. Siltstone and shale, calcareous, light- 1 

brown; some pink shale......................... 15.0 

37. Limestone, dark-gray, micritic, flaggy ...... 1.0 

36. Siltstone and shale, calcareous, lightbrown..................................................... 11.0 

35. Interval occupied by crosscutting dike ...... 6.0 

34. Limestone, as in 22 .................................... 9.0 

33. Siltstone and shale, as in 27 ....................... 28.5 

32. Limestone, as in 22 ....... ............................. 31.5 

31. Siltstone and shale, as in 27 ....................... 12.0 

30. Limestone, as in 22; some ledges as much 

as 1m thick. Conodont sample UW-5, 

12m above base .................................... 15.0 

29. Siltstone and shale, as in 27 ....................... 14.5 

28. Limestone, as in 22, in beds to 25 em 

thick....................................................... 21.0 

27. Siltstone and shale, calcareous, brownishgray; weathers yellowish brown; plane 

laminated .. . . ... ... .. ... ... .. ... ... .. ... ... .. ... ... ..... 13.5 

26. Limestone, as in 22 .................................... 37.5 

25. Interval occupied by crosscutting dike ...... 13.5 

22 Stratigraphy of the lower and Middle(?) Triassic Union Wash Formation, East-Central California 
Union Wash Formation (friassic )-Continued 

Middle member-Continued 

24. Limestone, as in 22 ................................... . 

23. Interval largely covered; rare outcrops of 

dark-gray mudstone as in 21 and lightto medium-gray micritic limestone in 

beds less than 10 em thick .................... . 

22. Limestone, micritic, dark-gray; weathers 

medium gray; forms even to irregular 

beds 5 to 15 em thick separated by 

grayish-orange silty partings less than 

3 em thick. Conodont sample UW -4, 

25 m above base ................................... . 

21. Mudstone, calcareous, dark-gray; weathers 

reddish brown; forms planar beds 2 to 4 

em thick ................................................ . 

Section offset 200 m to northwest along top of 20 (in 

gully) 

20. Limestone, medium-gray, thin-bedded, and 

tan-weathering mudstone ...................... . 

19. Covered interval ....................................... .. 

18. Mudstone, weathers dark gray .................. . 

17. Limestone, medium-gray, and brownweathering siltstone .............................. . 

16. Mudstone and siltstone, dark-gray ............ . 

15. Limestone, dark-bluish-gray,and tanweathering siltstone .............................. . 

14. Mudstone, weathers medium brown ......... . 

Minor fold in section; top of 13 traced around fold 

hinge 

13. Limestone, dark-bluish-gray, in beds 30 

to 50 em thick; and tan-weathering silt 

stone ...................................................... . 

12. Mudstone, black, poorly exposed ............. . 

11. Covered interval; black mudstone in float . 

10. Limestone, weathers light gray ................. . 

9. Mudstone, weathers medium gray ............ . 

8. Covered interval; light- to medium-gray 

limestone in float .................................. . 

7. Limestone, medium-gray, fine-grained ..... . 

6. Covered interval; medium-gray, finegrained limestone and tan-weathering 

siltstone in float .................................... . 

5. Covered interval; dark mudstone in float .. . 

4. Mudstone, very dark, massive .................. . 

3. Limestone, medium-gray, and tan-weathering siltstone; interbedded in beds as 

thick as 15 em ....................................... . 

2. Interval mostly covered; medium-gray 

silty limestone in float .......................... . 

1. Meekoceras bed. Limestone, calcarenitic, 

light-gray, massive; contains abundant 

ammonoids except in upper 2 m; ammonoids highly concentrated. Measured 

350 m southeast of 2 ............................. . 

Total thickness of middle member ... . 

Thickness 

in meters 

14.5 

46.5 

31.5 

15.0 

15.0 

7.5 

19.5 

13.5 

9.0 

7.5 

9.0 

7.5 

16.5 

41.5 

3.0 

3.0 

10.5 

1.5 

35.5 

27.0 

15.0 

18.0 

18.0 

14.5 

679.0 

Total thickness of Union Wash Formation 773.5 

Unconformable contact 

Lone Pine Formation (Permian) (not measured): 

Largely covered. Float and rare outcrops 

consist of thin-bedded silty limestone, 

calcareous siltstone, very fine grained 

calcareous sandstone, and siliceous 

mudstone. 

Cerro Gordo section (figs. 4, 5). Reference section of Union Wash Formation measured on west slope of Inyo Mountains about 2 km north of 

Cerro Gordo Road in September 1985. Section consists oflower middle 

and upper members of Union Wash Formation. ' ' 

Unnamed volcanic and sedimentary rocks (friassic 

and (or) Jurassic) (not measured): 

Conglomerate composed of volcanic rock clasts 

at base; overlain by a thick sequence of 

predominantly volcanic rocks. 

Faulted contact 

Union Wash Formation (friassic ): 

Upper member: 

Subunit4: 

48. Shale, fissile, maroon ............................... .. 

4 7. Shale, fissile, yellow ................................. . 

46. Conglomerate composed of limestone and 

volcanic rock clasts; may be in fault 

contact with underlying shale ............... . 

45. Shale, calcareous, fissile, yellowishbrown .................................................... . 

44. Siltstone and shale, as in 36 ..................... .. 

43. Limestone, massive, dark-gray ................. . 

42. Interval mostly covered; some outcrops of 

light-gray dolomite as in 40 .................. . 

41. Limestone, massive, dark-gray ................. . 

40. Dolomite, massive, medium-gray; weathers 

very light gray ....................................... . 

39. Limestone, massive, dark-gray ................. . 

38. Siltstone and shale, as in 36 ..................... .. 

37. Limestone (calcarenite), medium-gray, [megrained, partly bioclastic, in planar beds 

30 em to 1 m thick separated by thin 

partings of yellowish-brown siltstone 

or mudstone. Upper 1 m of unit is 

dark gray and mottled and contains 

local concentrations of pelecypod 

shells. Conodont sample CG-13, 4 m 

above base ............................................ . 

36. Siltstone and shale, calcareous, intensely 

cleaved, yellowish-brown. Minor sandstone, brown, fine-grained to very fine 

grained, slightly calcareous, in beds 5 to 

10 em thick .......................................... .. 

35. Interval occupied by a white siliceous 

dike ....................................................... . 

34. Shale, purplish-gray, calcareous, silty, 

recessive ............................................... . 

Total thickness of subunit 4 .............. . 

Thickness 

in meters 

2.0 

25.0 

2.0 

33.0 

4.5 

1.5 

3.0 

3.0 

3.0 

1.5 

28.5 

4.5 

105.0 

3.0 

15.0 

234.5 

Measured Sections 23 

' 
Union Wash Formation (friassic }-Continued 

Upper member-Continued 

Section offset 1 km to northwest along top of 33, 

which was followed around two large fold hinges 

Subunit 3: 

33. Limestone, dark-gray, micritic, ledgeforming, generally in planar beds 1 to 5 

em thick separated by thin partings of 

light-brown siltstone or mudstone. Locally, siltstone or mudstone forms 

irregular, anastomosing patterns giving 

limestone nodular appearance .............. . 

32. Sandstone, calcareous, fine-grained, 

grayish-brown; weathers light brown; 

platy ...................................................... . 

31. Limestone, dark-gray, micritic, generally 

in planar beds 3 to 6 em thick separated by thin partings of pinkish-gray 

mudstone ............................................... . 

Total thickness of subunit 3 .............. . 

Subunit 2: 

30. Sandstone, calcareous, gray; weathers light 

brown; some beds of fme-grained, darkgray limestone 1 to 5 em thick ............. . 

29. Limestone, (calcisiltite), medium-gray, 

massive ................................................. . 

28. Siltstone, purplish-gray, laminated, 

recessive ................................................ . 

27. Limestone (calcisiltite), medium-gray; 

lower part is well bedded and contains 

thin interbeds of siltstone; upper part is 

massive ................................................. . 

26. Sandstone, light-brown, fine-grained, 

laminated .............................................. . 

25. Limestone (calcarenite), light- to mediumgray, fine-grained, silty; dark-brown 

siltstone interbeds 1 to 3 em thick ......... 

24. Sandstone, gray, fine-grained, planelaminated; weathers brownish gray; 

contains ovate calcareous nodules 1 to 2 

em thick. Minor maroon siltstone or 

very fine grained sandstone near top .... . 

23. Covered interval; sandstone scree ............ .. 

22. Limestone, dark-gray, in beds 1 to 5 em 

thick ...................................................... . 

21. Covered interval; sandstone scree ............. . 

20. Limestone, as in 22 ................................... . 

19. Shale, calcareous, fissile, bright yellowishbrown ................................................... .. 

Total thickness of subunit 2 ............. .. 

Subunit 1: 

18. Limestone, dark-gray, micritic, massive 

except for irregular partings of grayishpink mudstone; forms ledges ................ . 

Total thickness of subunit 1 .............. . 

Total thickness of upper member .... .. 

Thickness 

in meters 

72.0 

4.5 

16.0 

92.5 

10.5 

1.5 

4.5 

8.5 

5.0 

2.5 

18.0 

15.0 

1.5 

4.0 

2.5 

10.5 

84.0 

11.0 

11.0 

422.0 

Union Wash Formation (friassic }-Continued 

Middle member: 

17. Limestone, medium-gray, in beds 1 to 4 em 

thick ...................................................... . 

16. Shale, calcareous, fissile, bright yellowishbrown .................................................... . 

Section offset 350 m to northwest along top of 15 

15. Limestone, medium-gray, micritic, in beds 

1 to 3 em thick; minor yellow fissile 

shale. Conodont sample CG-3, 17 m 

above base ............................................ . 

14. Shale, fissile, yellow ................................. . 

13. Limestone and minor shale, as in 15; beds 

1 to 4 em thick ..................................... .. 

12. Shale, fissile, yellow; minor medium-gray, 

thin-bedded micritic limestone ............. . 

11. Limestone and minor shale, as in 15; 

limestone beds 2 to 10 em thick .......... .. 

10. Shale, fissile, yellow; minor medium-gray 

micritic limestone in beds 1 to 3 em 

thick ...................................................... . 

9. Limestone and minor shale, as iit 15; limestone beds 2 to 10 em thick, some containing bioclastic debris ........................ . 

8. Shale, fissile, yellow; minor thin-bedded 

limestone ............................................... . 

7. Limestone and minor shale, as in 15; limestone beds' 2 to 8 em thick. Conodont 

sample CG-2 ....................................... .. 

6. Shale, fissile, yellow ................................. . 

5. Limestone and minor shale, as in 15; limestone beds 2 to 8 em thick ................... .. 

4. Shale, fissile, yellow, and less abundant 

limestone, medium-gray, micritic, in 

beds 1 to 5 em thick. Conodont sample 

CG-1, 13.5 m above base ...................... . 

Total thickness of middle member .. .. 

Lower member: 

3. Limestone, silty, and siltstone, calcareous, 

light-gray to yellowish-brown; similar 

to rocks of unit 2 but somewhat finer 

grained and thinner bedded. Rare beds 

of gray micritic limestone 2 to 5 em 

thick ...................................................... . 

2. Limestone, silty, and siltstone, calcareous. 

Limestone, calcarenitic, light-gray, 

grayish-orange, and light-brown, in planar to gently undulating beds 1 to 2 em 

thick; some beds contain gray limestone 

nodules about 1 em in diameter; tiny 

black gastropod shells locally abundant. 

Siltstone, gray to grayish-orange, platy, 

in beds less than 1 em thick ................. .. 

1. Covered interval; partly occupied by dike 

rock ....................................................... . 

Total thickness of lower member .... .. 

24 Stratigraphy of the lower and Middle(?) Triassic Union Wash Formation, East-Central California 

Thickness 

in meters 

4.0 

44.0 

25.5 

6.0 

5.0 

40.5 

4.5 

10.5 

4.0 

20.5 

3.0 

3.5 

1.0 

64.5 

236.5 

10.5 

18.0 

3.0 

31.5 
Union Wash Formation (friassic )-Continued 

Lower member--Continued 

Total thickness of Union Wash 

Formation ................................ .. 

Unconformable contact 

Keeler Canyon Formation (Pennsylvanian and Permian) (not measured): 

Limestone, calcarenitic and bioclastic, mediumgray, thick-bedded; minor pinkish-gray 

to purplish-gray shale. 

Thickness 

in meters 

690.0 

Darwin section (figs. 7, 8). Reference section of Union Wash Formation 

measured between Darwin Canyon and the Darwin Hills in September 

1985. Consists of three subsections. Subsection 1, south of the Darwin 

fault, traverses the upper member of the Union Wash Formation; subsections 2A and 2B, north of the Darwin fault, traverse the lower and middle 

members of the Union Wash Formation. Subsections 2A and 2B are 

correlated across a minor fault by means of marker beds. 

Subsection 1. 

Top of section at axis of syncline 

Union Wash Formation (friassic ): 

Upper member: 

Subunit3: 

44. Limestone, light-gray, micntJ.c, massive 

except for some closely spaced partings 

of brown siltstone ................................. . 

43. Limestone, light-gray, micritic, in beds 20 

em to 1 m thick, separated by beds 20 

to 50 em thick of grayish-orange silty 

limestone containing light-gray lime 

stone nodules 1 to 2 em across ............ .. 

42. Limestone, light- to medium-gray; forms 

ledges 1 to 2 m thick separated by 

thinner zones of thin-bedded brown 

siltstone ............................................... .. 

Total thickness of subunit 3 .............. . 

Subunit 2: 

41. Siltstone, brown, slightly calcareous, 

massive; upper 3 m more calcareous ..... 

40. Limestone, micritic, light- to medium-gray, 

laminated ............................................. .. 

39. Siltstone, brown, slig!ttly calcareous, 

massive ................................................ .. 

Total thickness of subunit 2 ............. .. 

Subunit 1: 

38. Limestone, medium- to dark-gray, micritic, well bedded in lower 5 m but indistinctly bedded higher. Upper 5 m has 

disrupted bedding and irregular partings 

of pink shale. A bed of limestone conglomerate is present about 10 m above 

base ...................................................... .. 

37. Shale, fissile, greenish-gray to grayishorange, poorly exposed ......................... . 

36. Covered interval (wash bottom) ............... .. 

Thickness 

in meters 

22.5 

37.5 

4.5 

64.5 

54.5 

0.5 

7.5 

62.5 

30.0 

18.0 

5.0 

Union Wash Formation (friassic )-Continued 

Upper member-Continued 

Subunit1--Continued 

35. Limestone, medium- to dark-gray, micritic; 

forms cliffs and ledges. In lower 50 m, 

unlaminated beds 2 to 5 em thick alternate with laminated beds less than 1 em 

to 3 em thick and with thin partings of 

grayish-orange-weathering calcareous 

shale; in upper 80 m, limestone forms 

irregular, unlaminated beds 20 to 50 em 

thick separated by lenticular partings of 

maroon-weathering shale. Soft-sediment 

folds are present throughout. Underlain 

by calcareous mudstone and minor 

limestone conglomerate assigned to the 

middle member ..................................... . 

Total thickness of subunit 1 ............. .. 

Total thickness of upper member ..... . 

Subsection 2B. 

Middle member: 

34. Calcareous mudstone and limestone, as in 

11; beds 1 to 3 em thick; poorly exposed. 

Overlain by medium-gray, thin-bedded 

to massive limestone that forms the 

lowermost part of the upper member ..... 

33. Limestone, massive, medium-gray; contains 

thin, irregular stringers of tan-weathering 

calcareous mudstone; limes tone 

locally nodular ...................................... . 

32. Calcareous mudstone and limestone, as in 

11; beds 1 to 3 em thick ....................... .. 

31. Calcareous mudstone, as in 8 ................... .. 

30. Limestone and less abundant calcareous 

mudstone, as in 20 ............................... .. 

29. Calcareous mudstone, as in 8, poorly 

exposed ................................................ .. 

28. Limestone, medium-gray, and minor 

brown-weathering calcareous mudstone; 

evenly bedded except at top, where 

bedding is disrupted ............................. .. 

27. Calcareous mudstone, as in 8, and minor 

thin-bedded gray limestone .................. . 

26. Limestone, medium- to dark-gray, and 

minor brown-weathering calcareous 

mudstone. Disrupted and broken beds 

common, especially in upper 7 m. 

Forms a prominent marker ledge ......... .. 

Section offset 100 m to northwest along top of 25 

25. Calcareous mudstone and limestone, as in 

11; bedding mostly planar, but softsediment folding is present locally ........ 

24. Limestone and less abundant calcareous 

mudstone, as in 20 ................................ . 

23. Calcareous mudstone, as in 8, and minor 

thin-bedded gray limestone .................. . 

Thickness 

in meters 

130.0 

183.0 

310.0 

49.0 

12.0 

8.5 

15.0 

1.5 

10.0 

4.0 

15.0 

15.0 

20.5 

7.0 

12.0 

Measured Sections 25 
Union Wash Formation (friassic }-Continued Thickness 

Middle Member-Continued in meters 

22. Calcareous mudstone and limestone, as in 

11; well-developed soft-sediment folding 

at base.................................................... 10.0 

21. futerval mostly covered; brown-weathering 

mudstone scree...................................... 10.5 

20. Limestone, medium-gray, micritic, in beds 

1 to 2 em thick, and less abundant brown 

calcareous mudstone in beds less than 1 

em thick .... ............................................. 6.0 

19. Calcareous mudstone and limestone, as in 

11; beds 1 to 4 em thick. A 0.5-m-thick 

limestone bed is present 6 m above base; 

a thin, matrix-supported limestone 

conglomerate is present at top .. ..... ... ..... 27.0 

18. futerval mostly covered; brown-weathering 

mudstone scree ....... ............. ... ..... .......... 9.5 

17. Limestone, medium-gray, micritic, and 

minor calcareous mudstone; beds less 

than 1 em thick...................................... 1.0 

16. Calcareous mudstone and limestone, as 

in 11; limestone in beds 1 to 2 em thick, 

mudstone laminated; bedding mostly 

planar, but soft-sediment folding is 

locally present....................................... 67.0 

15. Limestone, silty or argillaceous, light-gray, 

laminated .......... .......... ........ ...... ........ .. ... 2.5 

14. Calcareous mudstone, as in 8, and minor 

thin-bedded gray micritic limestone; 

some disrupted bedding......................... 15.0 

13. Calcareous mudstone and limestone, as in 

11; disrupted bedding common ....... ... ... 2.0 

12. Limestone, medium-gray, strongly cleaved 1.5 

11. Mudstone, calcareous; weathers light 

brown; laminated; and limestone, micritic, light- to medium-gray, in alternating beds 1 to 3 em thick .. ........ .. ...... .. 4.0 

10. Calcareous mudstone, as in 8. Underlain 

by light-gray-weathering limestone 

correlated with 9 .................................... 33.0 

Subsection 2A. 

9. Limestone, dark-gray; weathers light gray; 

contains abundant ammonoids 1 to 2 em 

in diameter .. .. ... ... .. . . . ... .. ... ... . . ... ... .. . . . ... .. . 7.5 

8. Mudstone, calcareous; weathers brown; 

laminated .............................................. . 

7. Limestone, silty, dark-gray; weathers light 

gray; plane laminated to gently crosslaminated; contains abundant small 

ammonoids ........................................... . 

18.0 

1.0 

Union Wash Formation (friassic }-Continued 

Middle Member-Continued 

6. Mudstone, mostly calcareous except in 

lower 5 m, dark-gray; weathers brown 

to reddish brown; laminated; large, 

spherical to subspherical, medium-gray 

limestone concretions common ............ . 

Section offset 100m along strike to northwest (across 

wash) 

5. Mudstone, siliceous, dark-gray; contains 

scattered limestone concretions up to 5 

em in diameter ...................................... . 

4. Mudstone, siliceous, very dark gray; 

weathers reddish brown; laminated; 

minor light-gray silty limestone in beds 

5 to 15 em thick .................................... . 

3. Limestone, silty, light-gray, in beds 5 to 

10 em thick, and mudstone, siliceous, 

dark-gray, laminated. A few limestone 

beds contain small ammonoids and 

other bioclastic material ....................... . 

Total thickness of middle member ... . 

Section offset 50 m to northwest along top of 2 

Lower member: 

2. Limestone, light- to medium-gray, in beds 

as thick as 1 m separated by thin, even 

laminations of brown-weathering siltstone. Tops of beds strewn with grayishorange-weathering granules .................. . 

1. Limestone, silty, light-gray; contains irregular stringers of brown-weathering 

siltstone, which enclose the limestone 

and impart a nodular appearance. Upper 

3 to 4 m composed of massive limestone 

conglomerate containing subangular 

clasts as long as 1 m and coral fragments reworked from underlying Permian rocks. Conodont sample D--2A-1, 

1 m above base .................................... .. 

Total thickness of lower member ..... . 

Total thickness of Union Wash 

Formation ..................................... . 

Unconformable contact 

Millers Spring Member of Darwin Canyon Formation 

(Permian) (not measured): 

Limestone, medium-gray, thick-bedded; contains abundant corals and other bioclastic material 

26 Stratigraphy of the lower and Middle(?) Triassic Union Wash Formation, East-Central California 

Thickness 

in meters 

49.0 

20.5 

45.0 

21.0 

520.5 

12.0 

10.5 

22.5 

853.0 


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