Tectonic Setting and Lithology 

of the Winterhaven Formation: 

A New Mesozoic Stratigraphic Unit 

in Southeasternmost California and 

Southwestern Arizona 

By GORDON B. HAXEL, RICHARD M. TOSDAL, and 

JOHN T. DILLON 

U.S. GEOLOGICAL SURVEY BULLETIN 1599 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Catalog No. 85-600629 

UNITED STATES GOVERNMENT PRINTING OFFICE : 1985 

For sale by the 

Distribution Branch, Text Products Section 

U.S. Geological Survey 

604 South Pickett St. 

Alexandria, VA 22304 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 1 

Geologic setting 6 

Stratigraphy and lithology 6 

Distribution in outlying areas of California; similar strata in southwestern Arizona 9 

Metamorphism and deformation 9 

Basal contact 11 

Age 11 

flelations of the Winterhaven Formation to the Orocopia Schist and Chocolate Mountains thrust 11 

Fault contacts between the Winterhaven Formation and the Orocopia Schist 11 

Distinction of the Winterhaven Formation from the protolith of the Orocopia Schist 13 

Tectonic significance of the Winterhaven Formation 13 

Implications for the age of the Orocopia Schist 13 

Tectonic affinities of the Sortan fault 16 

Possible correlation with the McCoy Mountains Formation 11 

References cited 18 

FIGURES 

1. Map showing distribution of the Winterhaven Formation, Orocopia Schist, and Chocolate 

Mountains thrust along the Chocolate Mountains anticlinorium 2 

2. Geologic map of part of the Picacho-Peter Kane Mountains area 4 

3. Geologic map of the area between Little Picacho Wash and White Wash 1 

4. Generalized stratigraphic column of the Winterhaven Formation 8 

5. Photographs of strata of the Winterhaven Formation 8 

6. Lithostratigraphy of the sedimentary and volcanic rocks of Slumgullion and provisional correlations 

with the Winterhaven Formation 10 

7. Geologic map of the area southwest of Little Picacho Peak 12 

8. Geologic map of the Sortan Wash area 13 

9. Schematic cross sections showing tectonic development of the Sortan fault 14 

Ill 

Tectonic Setting and Lithology of the 

Winterhaven Formation: 

A New Mesozoic Stratigraphic Unit in 

Southeasternmost California and Southwestern Arizona 

By Gordon B. Haxel, Richard M. Tosdal, and john T. Dillon1 

Abstract 

A 450-m-thick sequence of distinctive, variably 

metamorphosed Jurassic(?) supracrustal rocks in 

southeasternmost California and southwesternmost 

Arizona is herein named the Winterhaven Formation. 

The formation consists of a basal dacite member, 

which evidently rests depositionally on and interfingers 

with Jurassic rhyodacitic metavolcanic rocks, a medial 

quartz arenite member, and an upper argillitic 

siltstone member. The formation is unconformably 

overlain by Tertiary rocks. Complex relations of the 

Winterhaven Formation to the late Mesozoic Orocopia 

Schist and overlying Chocolate Mountains thrust 

represent the superimposed effects of several late 

Mesozoic to late Tertiary deformational episodes. 

Significant lithologic contrasts indicate that the 

Winterhaven Formation is not related to the protolith 

of the Orocopia Schist. The Winterhaven Formation 

evidently was originally part of the upper plate of the 

Chocolate Mountains thrust and was subsequently 

placed directly over the Orocopia Schist along the 

Sortan fault, a newly recognized late Mesozoic 

low-angle normal fault. Because this juxtaposition and 

later metamorphism and intrusion by granite occurred 

before about 60 m.y. ago, the metamorphic age of the 

Orocopia Schist must be no younger than Late 

Cretaceous, rather than early Tertiary. The 

Winterhaven Formation has several similarities to, and 

may correlate with, the lower part of the Jurassic and 

(or) Cretaceous McCoy Mountains Formation. 

INTRODUCTION 

Geologic mapping and reconnaissance in 

southeastern California and southwestern Arizona 

from 1972 to 1985 have revealed a distinctive and 

widespread but previously unrecognized Mesozoic, 

probably Jurassic, lithostratigraphic unit (fig. 1). This 

sequence of siliciclastic sedimentary rocks, with a 

basal volcanic unit, is here named the Winterhaven 

Formation. The formation is important as part of the 

Mesozoic supracrustal record and because of its 

relations to the enigmatic Orocopia Schist. 

1 Alaska Division of Geological and Geophysical 

Surveys. 

Elucidating the Mesozoic tectonic history of the 

region in the southeast corner of California and 

southwest corner of Arizona is substantially a matter 

of determining the temporal and tectonic relations 

among several major Mesozoic supracrustal 

lithostratigraphic units (Crowell, 1981). These units 

are: (1) The late Mesozoic Orocopia Schist, composed 

of thoroughly metamorphosed graywacke and minor 

basalt, mudstone, chert, and peridotite (Haxel and 

Dillon, 1978); (2) the Jurassic and (or) Cretaceous 

McCoy Mountains Formation of Harding and Coney 

(1985), a 7-km-thick continental clastic sedimentary 

sequence; (3) Jurassic and Jurassic(?) silicic and 

subordinate intermediate volcanic and hypabyssal 

rocks (Tosdal, 1982); and (4) the Jurassic(?) 

Winterhaven Formation and similar strata. 

The purpose of this report is to summarize the 

extent and lithology of the Winterhaven Formation and 

to describe and discuss its relations to other Mesozoic 

lithotectonic units, particularly the Orocopia Schist. 

Most of the information presented here was gathered 

during mapping of the Picacho-Peter Kane Mountain 

area of southeasternmost California (fig. 2) as part of 

a study of the Orocopia Schist and related Chocolate 

Mountains thrust fault (Haxel, 1977), and during later 

mapping of the adjoining area to the north by R.M. 

Tosdal and D.R. Sherrod (unpub. data, 1982-85). The 

stratigraphy and sedimentary petrology of the 

Winterhaven Formation have yet to be studied in 

detail. The formation is named after a small 

California town just north of Yuma, Ariz. 

Previous mention of some of the rocks here 

assigned to the Winterhaven Formation is restricted to 

brief descriptions by Olmsted and others (1973, p. 32) 

and Morton (1977, p. 16); in both of these reports, the 

rocks are tentatively correlated with the McCoy 

Mountains Formation. The name Winterhaven · 

Formation was first used informally by Haxel (1977, 

app. 2). 

ACKNOWLEDGMENTS 

The presence in the Picacho area of supracrustal 

rocks intermediate in metamorphic grade between the 

Orocopia Schist and the middle Tertiary volcanic and 

sedimentary rocks was first pointed out to us by John 

Crowell and Perry Ehlig, both of whom have been of 

considerable help in our efforts to understand the 

Winterhaven Formation. We have also benefited from 

discussions with Bill Bagby, Peter Coney, Bruce 

1 
10 20 KILOMETERS 

100 KILOMETERS 

EXPLANATION 

McCoy 

Mountains 

Formation 

AREA Of MAP 

Figure 1. Distribution of the Winterhaven Formation, the Orocopia Schist, and the Chocolate Mountains thrust 

along the Chocolate Mountains anticlinorium in southeastern California and southwestern Arizona. Strata 

similar to the Winterhaven Formation in southwestern Arizona east of the Trigo Mountains are informally 

designated as the sedimentary and volcanic rocks of Slumgullion. Complex map relations among the Orocopia 

2 
. ( ____, l 

\'\ ( I 

J l~'\ I \ 

I 

\ 

N 

_j I\ " 

/ \ \ ( . ~ . ~ 

J ! ~ I 

~I c: 

~. :l ~ . 

:1---?---~ J 

\ 

-

[ill[]] 

J 

Winterhaven Formation 

(Jurassic?) 

Sedimentary and volcanic rocks 

of Slumgullion (Jurassic?) 

Orocopia Schist (late Mesozoic) 

Contact 

Fault-Dotted where concealed 

J 

EXPLANATION 

~ Chocolate Mountains thrust-Sawteeth on upper plate 

~+ Antiform of Chocolate Mountains anticlinorium-Dashed where 

approximately located; queried where uncertain 

Outline of mountainous area 

Dillon, 1976 

: Parker, 1966; I 1 R.M. Tosdal and 1 G.B. Haxel, M.J. Grubensky,J.T. 

~-, D.R. Sherrod 1 Dillon, and R.D. Koch [1973-85]; 

: \ [1982-85] / Wilson and others, 1969 

I '---,_A -- ;' 1 Haxel, 1 '~ v Crowe, 1973; 

: 1977 i ~Haxel, 1977 

SOURCES OF GEOLOGIC DATA 

Schist, Chocolate Mountains thrust, and Winterhaven Formation cannot be portrayed at this scale (see figs. 2,. 3, 

7, 8). Inset map shows location of map area and of outcrop area of the McCoy Mountains Formation of Harding 

and Coney {1985). Jn "Sources of Geologic Data," dates in brackets refer to unpublished mapping and (or) 

reconnaissance. 

3 
( 

N 

0 

i 

f .. / 

5 KILOMETERS 

( 

-~ ----+--ir ..;..;.;- ..;-:-;.-:~~ ::7:1 

Picacho 6 

Peak ---

\\e ~r·· 

... / 

~} 

Figure 2. Pr'e-middle Tertiary rocks in the area between Indian Pass and Ferguson Lake, and Julian Wash and 

Arrastra Wash areas, southeastern California and southwestern Arizona (see fig. 1 for location). The granite of 

Marcus Wash may locally include some unmapped bodies of rhyodacitic metavolcanic rocks. "Gavilan Hills" is an 

4 
I 

(): 

&I 

;):.; 

~ .. 

~?"7: 

/ 

., 

\ 

\ 

D 

EXPLANATION 

Volcanic, sedimentary, and hypabyssal rocks 

(Quaternary to Oligocene) 

Granite of Marcus Wash (Late Cretaceous 

or Late Jurassic) 

Granodiorite of Trigo Peaks (Jurassic) 

Winterhaven Formation (Jurassic?)-- Divided into: 

Argillitic siltstone member and (or) quartz 

arenite member 

Dacite member 

Rhyodacitic metavolcanic rocks (Jurassic) 

Orocopia Schist (late Mesozoic) 

Gneiss (Mesozoic and Proterozoic?) 

Axial trace of antiform of Chocolate 

Mountains anticlinorium 

Culmination 

Depression. 

Contact 

Fault-Dotted where concealed 

~ • Chocolate Mountains thrust-Sawteeth on 

upper plate. Dotted where concealed 

Parker, 1966; A.M . Tosdal 

and D.R. Sherrod [1982-85] 

Haxel, 1977 

Crowe, 1973; Haxel, 1977 

I 

SOURCES OF GEOLOGIC MAPPING 

informal name used to designate the hills south of the Gatuna fault between Carrizo Wash and Indian Pass. 

Picacho is headquarters of the Picacho State Recreation Area. m "Sources of Geologic Mapping, II dates in 

brackets refer to unpublished mapping. 

5 
Crowe, Bill Dickinson, Eric Frost, Mike Grubensky, 

Lucy Harding, Keith Howard, Carl Jacobson, Richard 

Koch, Donna Martin, Dan May, Frank Olmsted, Gary 

Pelka, Bob Powell, Lee Silver, and Paul Stone; and 

from the cooperation of the personnel of Picacho State 

Recreation Area. Much of this work was supported by 

U.S. National Science Foundation Grants EAR 71-

00498 (Crowell) and EAR 81-15730 (Crowell). 

GEOLOGIC SETTING 

The dominant late Mesozoic tectonic feature of 

the Picacho-Peter Kane Mountain area and adjacent 

southern Chocolate Mountains (Dillon, 1976) is the 

Chocolate Mountains thrust, along which Mesozoic 

(and Proterozoic?) gneissic and granitoid rocks overlie 

the late Mesozoic Orocopia Schist. This regional 

thrust fault is exposed on the flanks, and the Orocopia 

Schist in the core, of the Tertiary Chocolate 

Mountains anticlinorium, which extends some 110 km 

from the central Chocolate Mountains east to Neversweat Ridge (fig. 1). This narrow, complexly faulted 

anticlinorium consists of several aligned, subparallel, 

or echelon antiformal segments. 

Two subparallel east-trending antiforms control 

the distribution of exposures of the Winterhaven 

Formation and the Orocopia Schist within the PicachoPeter Kane Mountain area and adjoining areas. The 

southern antiform extends from the area west of 

Indian Pass eastward toward Ferguson Lake; three 

separate exposures of the Orocopia Schist, overlain by 

segments of the Chocolate Mountains thrust, mark 

three culminations along the antiformal trace (fig. 2). 

The Winterhaven Formation is exposed along the flanks 

of the antiform and on the noses of the culminations. 

The northern antiform, likewise marked by 

culminations exposing the Orocopia Schist and (or) the 

Winterhaven Formation, extends from the easternmost 

southern Chocolate Mountains eastward through Peter 

Kane Mountain and the Julian Wash area to the 

southern Trigo Mountains. This antiform is disrupted 

by numerous high- to low-angle Tertiary faults and is 

less clearly defined than the southern antiform. In 

several areas along these antiforms, the Winterhaven 

Formation is folded into smaller, open to tight 

anticlines and synclines with wavelengths of about 0.5 

to 1 km; both late Mesozoic and middle Tertiary folds 

are present. 

In most areas, the basal contact of the 

Winterhaven Formation is, or evidently was, a newly 

recognized late Mesozoic low-angle normal fault along 

which the Winterhaven Formation overlies both plates 

of the Chocolate Mountains thrust. Field relations in a 

few areas indicate that at its base the Winterhaven 

Formation originally stratigraphically overlaid 

Jurassic(?) rhyodacitic volcanic rocks that form part 

of the upper plate of the Chocolate Mountains thrust. 

In most places, these two types of Mesozoic basal 

contacts of the Winterhaven Formation have been 

modified or excised by Tertiary faults. The 

Winterhaven Formation is intruded by the epizonal 

granite of lVIarcus Wash (fig. 2). This granite is 

younger than the 163-m.y. (Middle or Late Jurassic) 

minimum protolith age and maximum metamorphic age 

6 

of the Orocopia Schist (Mukasa and others, 1984), 

which it also intrudes, and older than earliest Tertiary 

K-Ar minimum ages of about 60 m.y. (Frost and 

Martin, 1983). The granite of Marcus Wash is 

considered to be either Late Jurassic or Late 

Cretaceous because igneous rocks of both these ages 

are common in the southeastern California-southwestern Arizona region. The Winterhaven Formation 

is overlain with angular unconformity by Oligocene to 

Holocene volcanic and sedimentary rocks (Crowe, 

1973; Dillon, 1976; Crowe and others, 1979). 

STRATIGRAPHY AND UTHOLOGY 

The thickest section of known stratigraphic 

position is designated as the type section of the 

Winterhaven Formation (fig. 3). This section, which 

forms the northern limb of an anticline just west of 

Little Picacho Wash (in the southeast corner of the 

Picacho SW 7-1/2-minute quadrangle), consists of 

three members, with a total exposed thickness of 

about 450 m (fig. 4). Except where strongly 

metamorphosed, most of the Winterhaven Formation is 

typically dull purplish gray in outcrop. 

The basal unit of the Winterhaven Formation is a 

dacite member, somewhat more than 80 m thick as 

presently exposed, composed of massive, purple and 

dark-brown, strongly altered, aphanitic to sparsely 

porphyritic rocks of intermediate composition, 

probably dacite and possibly including some andesite. 

Phenocrysts of plagioclase, biotite, and (or) another 

mafic mineral were originally present; the plagioclase 

and biotite are now completely altered to chlorite, 

sericite (that is, fine-grained white mica), and opaque 

minerals. These rather nondescript rocks presumably 

are largely volcanic flows. Rare beds of coarsegrained volcaniclastic graywacke are interlayered with 

the volcanic rocks. Near the top of the dacite 

member, some rocks contain amygdules of quartz or 

calcite, and there are some layers of dark-gray, very 

poorly bedded breccia composed of angular fragments 

(typically approx. 1-3 em across) of strongly altered, 

sparsely porphyritic volcanic rock in a matrix of finegrained sericitic sandstone. Locally this breccia forms 

a distinct layer, as much as 10 m thick, at the top of 

the dacite member. The dacite member also includes 

a few dikes and small, irregular, probably intrusive 

bodies of grayish-purple porphyry containing 

epidotized plagioclase laths, as much as 1 em long, in a 

groundmass similar to that of the dacitic flows. 

The middle unit of the Winterhaven Formation is 

a quartz arenite member (fig. 5), approximately 60 m 

thick, consisting chiefly of brown, tan, or white quartz 

arenite and feldspathic quartz arenite, some of which 

is slightly sericitic and (or) calcareous. The sandstone 

is very well indurated, fine to coarse grained (mostly 

medium grained), and laminated through medium 

bedded to locally massive. Subordinate rock types 

interbedded with the quartz arenite are brown sandy 

limestone, purplish-gray to brown argillitic siltstone, 

and brown to light-gray, thinly laminated, silty 

calcareous argillite, which in some places is converted 

to flaggy phyllite or semischist. At the base of the 

quartz arenite member, as much as a few meters of 
0 1 KILOMETER 

EXPLANATION 

I OT g I Gravel and conglomerate (Quaternary to Miocene) 

~ Volcanic rocks (Miocene and Oligocene) 

0 Sedimentary rocks (Oligocene)-Conglomerate, sandstone, 

and tuff 

~Mixed rocks-Metamorphosed Winterhaven Formation, 

and subordinate Orocopia Schist, intruded and severely 

hydrothermally altered by the granite of Marcus Wash 

b?d I Hornblende diorite dikes (Cretaceous or Jurassic) 

Winterhaven Formation (Jurassic?)-Divided into: E2ill2J Argillitic siltstone member and {or) quartz arenite member 

Dacite member 

Figure 3. Relations between the Winterhaven 

Formation and the Orocopia Schist and Chocolate 

Mountains thrust in the area between Little Picacho 

Wash and White Wash (from Haxel, 1977, app. Z). See 

figure l for location. Fault-bounded sliver labeled 

"mr" (mixed rocks) northwest of center of map consists 

of metasedimentary and metavolcanic rocks of the 

Winterhaven Formation and subordinate Orocopia 

Schist, all intruded and severely hydrothermally 

altered by the granite of Marcus Wash; several faults 

Orocopia Schist {late Mesozoic) 

~ Gneiss (Mesozoic or Proterozoic) 

f--------i Type section of Winterhaven Formation 

~~-Contact 

--1...--. Fault-Dashed where approximately located; dotted 

where concealed . Ball and bar on downthrown side 

~ Chocolate Mountains thrust-Sawteeth on upper plate 

-+-

-+-

55 

___L_ 

-+-

Axial trace of anticline 

Axial trace of syncline 

Strike and dip of bedding 

Inclined 

Vertical 

within this sliver are not shown. Area southeast of 

map center that consists of gneiss strongly intruded by 

the granite of Marcus Wash is shown by superposition 

of patterns for these two units. Some pre-Tertiary 

contacts between the gneiss, the Winterhaven 

Formation, and the granite of Marcus Wash in the 

southeastern part of the map area have been modified 

or overprinted by Tertiary low-angle normal faults. 

Distribution of map units Ts and Tv is largely from 

Crowe 0973). 

7 
LITHOLOGY 

UNIT THICKNESS, 

IN METERS Unconformity 

~--------~ ~~~~---~ -~-~--------~--~ 

>300 

.:::,60 

>80 

-~~~~~-~~~~~~-~~~~ 

._-... : _ _. _...:._ .:_.·_ .. : _ _'...:..._"_:·_:_·_:_:_·.:._-.:....:._:_·.....:.·-__: ·_:_ 

·.:...:.·_.....:. _:_:___:_·_:_·~----=-·....:....-~·:_: .. ~- ....:.,·-.:....:... _:_·...:...:..:...;.-

-:: :-.~ : :_ ":.<·: ·-- ; .:. :: :.-: ·.>:·:·-·:··>· <· ... .-. ~: .... . : ··.:. ': :·.:: 

: ~-:~ .. ~ .. ~-~- ~-.. :·.:;:._:~.::i ~:-. ;_-._.;·:·· 

• ...:..._ .__:_._·;_ .:._-__:. ··--=-·'....:..:.....:...·.....:...·....:....·._:_ -.....:..,_-:_ 

-·~-::. -~·,...:· ;.;. -:_;_. ,:..;_ :,.::<~;..:, ;. ~: .,_>_c_: 

·:LL-i:~.L :·~L~-~ --' ~ 

- - - - - - - - - - - - - - - - · "-'-·~·:..:. ~_:..:.•:...c :..:..:.::.-~·c..:..·:..:..:..:. c;.-...::.·~·:..:.

cc . .:.:-.:_:c... :;.:..:. :~_-:...c.-;c:..,...: '--::_.: .. :,'--' . ,_;:~c: 

'i-t ::: ~~~=~~:=[~~~ A.~ ..,-...:_-'- -~·- . ..::..,-_:_.~_:_.,.....:_.-_:_..,-_:_ ~-...:..-:-...::.-:-.::..-..:..-.:... 

=:l.~=~~5: ~~'::~ i...:::::~ 

:_;~~~t.tt.i5.E~£=1~!±i±E~~=~=~~E~~~ 7"T.·. 

·-·-:.-:::·.- : ::. 7 7 :· o:-. ::. 

·... ·:: .·: ,::.• :.: 7'7,::· 

... . <: ·' :·::-: · .· .. ·.::- :::. 

.:.: -:. "·? 

::. · . . 

FAULT 

EXPLANATION 

(Symbols may be combined) 

Argillitic 

siltstone 

member 

Quartz 

arenite 

member 

Dacite 

member 

j,J' ~"':: ,"1 Dacite (and 

' 

1 

" ' ' andesite?) 

z 

0 

f:: 

<( 

~ 

0::: 

0 

t.L. 

z 

I.Ll 

> <( 

:c 

0::: 

~ z 

§ 

t:··. :<: \{j Sandstone or siltstone 

t=3::===-===J Argillite 

~ Conglomerate 

r:.:.:·::.:.:·::.:.] Breccia (sedimentary 

• ·· · · · · and (or) volcanic) 

~Limestone ~ ~Porphyry 

Figure 4. Generalized stratigraphic column for type 

section of the Winterhaven Formation (see fig. 3 for 

location). Position and thickness of individual 

lithologic layers within each of the three major units is 

schematic only. See text for lithologic descriptions. 

brown and purple pebble conglomerate and 

conglomeratic sandstone are present; in some places 

these beds are absent, owing to minor faulting 

localized along the top of the dacite member. 

8 

A 

B 

Figure 5. Strata of the Winterhaven Formation, on a 

ridge about 250 m north-northeast of south end of type 

section (fig. 3). Both views are approximately 

westward. ~ Upper part of dacite member overlain 

by quartz arenite member, in turn overlain by base of 

argillitic siltstone member. Width of view, about 

120 m. _!!, Upper part of quartz arenite member (right 

side of A); resistant beds of quartz arenite are 

interbedded with less resistant siltstone, argillite, and 

limestone. Dark unit capping sequence is base of the 

argillitic siltstone member, intruded by a hornblende 

diorite dike (fig. 3). 

The quartz arenite member is overlain by an 

argillitic siltstone member, which in the type section 

has a maximum exposed thickness of about 300 m. The 

most common rock type is massive, dark-purple to 

dark-brown, slightly calcareous argillitic siltstone 

(micrograywacke), some of which is slightly sandy and 

some of which contains brown, decimeter-size, discoid 

calcareous concretions(?). Other abundant rock types 

are dark-purple and dark-brown silty argillite and 

brown to gray, medium-grained to very coarse grained, 

locally pebbly, slightly calcareous graywacke. 

Relatively minor rock types are dark-purple argillite, 

dark-brown sandy limestone, tan, slightly argillitic 

sandstone, and granule to pebble conglomerate with a 

graywacke matrix. The member as a whole coarsens 

upward; several rock types coarsen toward the top of 

the exposed section; and the coarser grained rock 

types, notably conglomerate, are more common toward 

the top. 
The most common clasts in the conglomerates of 

the Winterhaven Formation are well-rounded to very 

well rounded pebbles, typically about 0.5 to 3 em in 

diameter, of black, white, brown, and dark-bluish-gray 

chert or very fine grained quartzite; a few of these 

pebbles are faintly laminated. Also common are 

well-rounded clasts, typically 2 to 5 em in diameter 

but locally as much as 10 em in diameter, of 

fine-grained, vitreous, white or light-gray quartzite. 

A few conglomerate beds also contain intraformational 

clasts of purple and brown graywacke, argillite, and 

siltstone; these large pebbles and small cobbles are 

typically subrounded to rounded. Small, pebble-size 11shale chips 11 are locally present along bedding planes. 

Pebbles of dull-red-brown or light-gray, finely 

porphyritic volcanic rocks and fine-grained, 

nondescript granitic rocks are rare. No clasts of 

Orocopia Schist or of gneissic rocks were found, and 

these are believed to be absent. 

All exposures of the Winterhaven Formation 

within the Picacho-Peter Kane Mountain area and the 

adjoining area to the north (fig. 2) are lithologically 

similar to some part of the type section. A 500- or 

600-m-thick homoclinal section along Marcus Wash 

about 4 km southwest of Little Picacho Peak is 

lithologically similar to the argillitic siltstone member 

of the type section but has not been correlated with it 

in detail because of the absence of distinctive marker 

units in either section. In the area between Carrizo 

Wash and Gavilan Wash, the quartz arenite member is 

less than 10 m thick and in most places absent, so that 

the argillitic siltstone member rests directly on the 

dacite member. 

DISTRIBUTION IN OUTLYING AREAS OF 

CALIFORNIA; SIMll..AR STRATA IN SOUTHWESTERN 

ARIZONA 

Northwest of Indian Pass, the Winterhaven 

Formation is in part poorly exposed beneath a cover of 

basalt-boulder talus and has been examined in less 

detail than farther east. In the southern Chocolate 

Mountains (fig. 1), rocks similar to the Winterhaven 

Formation are restricted to a few localized outcrops 

at the southeast end of the range (Dillon, 1976, p. 

203). Strata equivalent to the Winterhaven Formation 

have not been recognized in the main part of the 

Chocolate Mountains (fig. 1) nor west of the San 

Andreas fault. The only other strata similar to the 

Winterhaven Formation known in California occur 

within a fault-bounded block in low hills west of the 

Imperial Dam, 9 km south of Ferguson Lake (fig. 2). 

Strata lithologically similar to the Winterhaven 

Formation are exposed in a large area of the southern 

Castle Dome Mountains, and in smaller areas in the 

Middle Mountains and at the north end of Neversweat 

Ridge (fig. 1). These strata are here referred to as the 

sedimentary and volcanic rocks of Slumgullion (after 

Slumgullion Pass in the Castle Dome Mountains). In 

the southern Castle Dome Mountains, the Slumgullion 

unit is considerably thicker and more diverse than the 

Winterhaven Formation (fig. 6). In particular, the 

Slumgullion unit contains large amounts of rhyodacitic 

volcanic rocks and coarse conglomerate and 

sedimentary breccia, rock types that are absent from 

the Winterhaven Formation. Nonetheless, the 

Slumgullion section includes lithologic units that are 

similar to, and occur in the same stratigraphic order 

as, the three members of the Winterhaven Formation. 

Provisional lithologic correlations between the 

Winterhaven Formation and the Slumgullion unit are 

shown in figure 6; confirmation of these correlations 

awaits completion of studies in progress of the 

Slumgullion unit. 

METAMORPIDSM AND DEFORMATION 

The Winterhaven Formation is incipiently to 

strongly recrystallized, but over much of its outcrop 

area, mainly along the southern (Indian Pass to 

Ferguson Lake) antiform, it is not penetratively 

deformed. Sedimentary rocks in these areas are 

typically quartzitic or· argillitic and show 

well-preserved sand and silt grains in thin section. The 

volcanic rocks recrystallized more readily and in some 

places have been converted to massive, very fine 

grained granofels composed of relict plagioclase laths 

(partially to completely altered to epidote and (or) 

sericite), epidote, chlorite, biotite, opaque minerals, 

calcite, and sericite, with or without sparse quartz or 

actinolite. 

In the area of Arrastra Wash and lower Marcus 

Wash (fig. 2), where the Winterhaven Formation is 

intruded by the granite of Nlarcus Wash, deformation 

and metamorphism are more intense. The dacite 

member has been converted to greenschist-facies 

schist or hornfelsic granofels, both composed of 

chlorite and (or) biotite, actinolite, epidote, albite, and 

quartz; relict plagioclase laths are uncommon. The 

metasedimentary rocks overlying the dacite member 

have been converted to quartzofeldspathic phyllite, 

semischist, and schist containing metamorphic 

muscovite, biotite, epidote, chlorite, and calcite. The 

Winterhaven Formation in the Julian Wash area is 

intermediate in textural and mineralogic metamorphic 

grade between the generally undeformed rocks along 

the southern antiform and the more or less schistose 

rocks of the Arrastra Wash-Nlarcus Wash area. 

In both the Arrastra Wash and Julian Wash areas, 

the Winterhaven Formation is folded into a southfacing, shallowly plunging, asymmetric syncline with a 

steep to slightly overturned northern limb and 

subhorizontal, stratigraphically upright lower limb. 

Deformation increases in intensity toward the hinge 

area of the fold, where primary sedimentary and 

volcanic fabrics are strongly overprinted by cleavage 

axial-planar with respect to upright to recumbent 

mesoscopic folds. 

In the Arrastra Wash and lower Marcus Wash 

areas, metamorphism of the Winterhaven Formation is 

accompanied by remetamorphism of the Orocopia 

Schist, including reorientation of the regional fabric 

elements of the schist and development of a new 

metamorphic fabric with elements parallel to that in 

the Winterhaven Formation. This second fabric also is 

widespread in the granite of Marcus Wash. It is 

9 
...... 

0 

1 KILOMETER SEDIMENTARY AND VOLCANIC ROCKS OF 

SLUMGULLION (JURASSIC?), SOUTHERN 

CASTLE DOME MOUNTAINS, ARIZONA Order of 

magnitude 

of vertical 

scale 

West East 

Quartz arenite 

0 

·wiNTERHAVEN FORMATION 

(JURASSIC?) (AND 

UNDERLYING RHYODACITIC 

METAVOLCANIC ROCKS), 

PICACHO AREA, CALIFORNIA 

=.:.? =-~=? =--:-==.? =-:-

member --r~·~-- ;\ ·. -·. ·.· · ... : ::::::: :::( J 

Rhyodacitic 

metavolcanic 

rocks 

7-7~?~~7'7~-~K~?,~~-~ ,;~?,-/, \,t- f' .... .......... ,,, \TK\1 ~~I~ 

'- t .... '" ~ '- I f ~ ,- '_-... f; \. \ ..._ '\ /'1 ,; - I 

P g? ' • ',>{ ·c· 0 !: t£ii:[0;' ··~ ;.i; 

---

I· :=::15 KM ·I 

Rhyolitic volcanic rocks and sandstone 

-Sandstone, pebble conglomerate, and 

minor siltstone 

Conglomerate to sedimentary breccia, 

sandstone, and rhyolitic volcanic rocks 

Rhyolitic volcanic rocks, conglomerate, 

and minor sandstone 

Quartz arenite, argillitic sandstone, silty 

argillite, minor conglomerate, and 

sandy limestone 

Argillitic sandstone to silty argillite and 

sandy marl 

Rhyolitic volcanic rock~ and intrusions, 

conglomerate, sandy argillite, and 

sandstone 

Intermediate-composition lavas and 

volcanic wacke 

Rhyolitic volcanic rocks and derivative 

sandstone 

Rhyolitic intrusive porphyry 

I· Z45 KM ·I 

Figure 6. Generalized lithostratigraphic column for the sedimentary 

and volcanic rocks of Slumgullion and provisional correlations of the 

Slumgullion unit with the Winterhaven Formation. Arrangement of 

lithologic subunits within the column schematically represents observed 

and inferred facies relations within the Sluingullion unit across the 

15-km width of the southern Castle Dome Mountains (fig. 1). Details 

of stratigraphic relations and thicknesses within these incipiently to 

moderately metamorphosed rocks are considerably more complex and 

uncertain than portrayed here, and stratigraphy of the upper part of 

the Slumgullion unit is highly uncertain, owing to intrusion by Jurassic 

granitoids, faulting, and Neogene volcanic cover. The dacite and 

quartz arenite members of the Winterhaven Formation and their 

counterparts within the Slumgullion unit are sufficiently distinctive, in 

lithology and sequence, that their correlation is straightforward. 

Correlation of the argillitic siltstone member of the Winterhaven 

Formation is less clear; it could correspond to either of two 

Slumgullion subunits. 
unclear how much of the low-grade recrystallization of 

the Winterhaven Formation in the area from upper 

Marcus Wash westward to Indian Pass (fig. 2) and 

farther west (fig. 1) occurred during this metamorphic 

episode and how much occurred earlier, during 

intrusion of the Jurassic granodiorite (see next section) 

and (or) during movement on the Chocolate Mountains 

thrust. 

BASAL CONTACT 

In the Julian Wash and Arrastra Wash areas 

(fig. 2), the base of the Winterhaven Formation is a 

structurally concordant contact along which schistose 

rocks of the basal dacite member overlie schistose, 

rhyodacitic, quartz-phenocrystic metavolcanic rocks 

of presumed Jurassic age. This contact is neither 

obviously tectonic nor obviously depositional, but 

several lines of evidence favor the latter 

interpretation. In the Julian Wash area, rare sandstone 

interbeds within the dacite member of the 

Winterhaven Formation appear to have been derived 

from the underlying rhyodacitic volcanic rocks. In the 

Arrastra Wash area, the dacite member and the 

subjacent rhyodacitic metavolcanic rocks locally are 

inter layered and appear to be interbedded. Finally, in 

the Slumgullion unit of the southern Castle Dome 

Mountains, a lithologic unit that includes intermediate 

volcanic rocks similar to those of the basal member of 

the Winterhaven Formation clearly is interbedded with 

rhyodacitic volcanic to metavolcanic rocks (fig. 6). 

These relations, taken together, strongly suggest that 

the Winterhaven Formation stratigraphically overlies 

and interfingers with the rhyodacitic metavolcanic 

rocks. 

These rhyodacitic metavolcanic rocks are 

intruded by the sphene-bearing hornblende-biotite 

~ranodiorite of Trigo Peaks in the southern Trigo 

Mountains (fig. 2). Both the rhyodacitic volcanic rocks 

and the granodiorite are considered Jurassic because 

of similarities in lithology and stratigraphic position to 

isotopically dated units elsewhere in the region of 

southern Arizona, southeastern California, and 

northern Sonora, Mexico (Dillon, 1976; Anderson and 

Silver, 1978; Crowl, 1979; Haxel and others, 1980, 

Powell, U~81; Hamilton, 1982; Wright and others, 1981; 

Tosdal, 1982; L.T. Silver, oral commun., 1983). 

AGE 

The Winterhaven Formation is older than earliest 

Tertiary K-Ar dates (Frost and Martin, 1983) and is 

lithologically dissimilar to any of the major 

Precambrian or Paleozoic tectonostratigraphic units of 

the southern Cordillera. Therefore, it is almost 

certainly of Mesozoic age. The field relations and 

regional lithologic correlations just described imply a 

Jurassic age. In the absence of direct paleontologic or 

isotopic age determinations, the age of the 

Winterhaven Formation is designated as Jurassic(?). 

RELATIONS OF THE WINTERHAVEN FORMATION 

TO THE OROCOPIA SCHIST AND THE CHOCOLATE 

MOUNTAINS THRUST 

The Orocopia Schist is the structurally lowest 

lithotectonic unit exposed in the region of the 

southeast corner of California and the southwest 

corner of Arizona (Haxel and Dillon, 1978). This schist 

forms the lower plate of the regionally extensive 

Chocolate Mountains thrust, the upper plate of which 

comprises almost all of the Mesozoic (and 

Proterozoic?) gneissic and granitoid rock units of the 

region. Elucidating the tectonic significance of the 

Winterhaven Formation with respect to the Orocopia 

Schist and the Chocolate Mountains thrust (Haxel, 

1977; Crowell, 1981) involves choosing among four 

possibilities: (1) The Winterhaven Formation was 

originally a facies of the protolith of the Orocopia 

Schist, (2) the Winterhaven Formation was deposited 

nonconformably on the schist and upper-plate rocks, 

(3) the Winterhaven Formation and Orocopia Schist 

were originally juxtaposed along a fault younger than 

the Chocolate Mountains thrust, or (4) the Winterhaven 

Formation was originally part of the upper plate of the 

thrust. Evidence presented below indicates that the 

fourth possibility is the correct one. 

Fault Contacts .Between the Winterhaven Formation 

and the Orocopia Schist 

Most present contacts between the Winterhaven 

Formation and the Orocopia Schist or gneiss of the 

upper plate of the Chocolate Mountains thrust are lowto high-angle faults. Most of these faults are largely 

or entirely of middle and (or) late Tertiary age because 

they cut the Oligocene and Miocene volcanic and 

sedimentary sequence or form part of some larger 

fault system that cuts that sequence. Evidence as to 

pre-middle Tertiary relations between the Winterhaven 

Formation and the Orocopia Schist and Chocolate 

Mountains thrust is preserved in only a few places. 

This evidence indicates that the Winterhaven Formation is, or was, separated from the Orocopia Schist by 

two different late Mesozoic low-angle faults-the 

Chocolate Mountains thrust and a younger, low-angle 

normal fault. 

In the Arrastra Wash area, the upper plate of the 

Chocolate Mountains thrust is the Jurassic 

granodiorite that intrudes the rhyolitic metavolcanic 

rocks which are inferred to stratigraphically under lie 

the Winterhaven Formation. hssentially continuous 

exposures of crystalline rocks, extending structurally 

upward from blastomylonitic granodioritic gneiss just 

above the Chocolate Mountains thrust to the basal 

dacite member of the Winterhaven Formation, are 

interrupted only by a couple of northwest-trending 

Tertiary (and Mesozoic?) faults (fig. 2). The vertical 

separation across these faults is probably appreciable, 

at least several kilometers, but the blastomylonitic 

gneiss northeast of the northeastern fault is clearly 

derived from the undeformed granodiorite to the 

southwest. These relations indicate that the 

11 
Winterhaven Formation was originally part of the 

upper plate of the Chocolate Mountains thrust. 

The map pattern in the Arrastra Wash area (fig. 

2) and the low metamorphic grade of the Winterhaven 

Formation compared to the Orocopia ;:)chist and 

gneissic and granitic rocks of the base of the upper 

plate of the Chocolate lVlountains thrust strongly 

suggest that the Winterhaven Formation originally was 

at a high structural level within the upper plate. This 

implies that where the Winterhaven Formation and the 

Orocopia Schist are in direct contact they must have 

been juxtaposed by faulting younger than the 

Chocolate Mountains thrust. In several areas east of 

the longitude of Picacho Peak (fig. 2), the contact 

between the Winterhaven Formation and both plates of 

the Chocolate Mountains thrust is, or was, a late 

Mesozoic low-angle normal fault younger than and 

distinct from the Chocolate Mountains thrust. 

Evidence for the existence of this younger fault 

is found chiefly in three areas. In the first area, the 

antiform culmination oetween Little Picacho Wash and 

White Wash (fig. 2), the Winterhaven Formation 

entirely surrounds and largely faces outward from an 

antiform culmination consisting of Orocopia Schist and 

upper-plate gneiss separated oy a short segment of the 

Chocolate Mountains thrust (fig. 3). The Winterhaven 

Formation, in turn, is surrounded by middle Tertiary 

sedimentary and volcanic rocks that dip outward on 

three sides from this crude dome. On the south side of 

the dome, the Winterhaven Formation and gneiss are 

separated by an intrusive tongue of the granite of 

Marcus Wash. Around most of the rest of the 

circumference of the dome the Winterhaven Formation 

is separated from the schist and gneiss by one of 

several faults that dip steeply to moderately outward 

toward the Winterhaven Formation. The overall 

geometry and tectonic setting of this fault system 

strongly argues against appreciable strike-slip 

movement. The north-dipping fault on the north side 

of the dome continues to the west, where it displaces 

middle Tertiary volcanic and sedimentary rocks 

downward against the Winterhaven Formation. The 

south-dipping fault along the southwest side of the 

dome is paralleled, 400 m to the south, by another 

south-dipping fault along which the middle Tertiary 

strata are faulted down against the Winterhaven 

Formation. The faults separating the Winterhaven 

Formation from the schist and gneiss are thus Tertiary 

dip-slip normal faults along which the Winterhaven 

Formation has been faulted downward against the 

schist and gneiss. This configuration suggests that, 

before Tertiary faulting, the Winterhaven Formation 

overlaid the schist and gneiss. 

This conclusion is confirmed by relations in the 

second area, about 0. 7 km southwest of Little Picacho 

Peak. Here, a small body of volcanic rocks of the 

basal dacite member of the Winterhaven Formation 

subhorizontally overlies upper-plate gneiss and, on its 

southeast side, Orocopia Schist and the Chocolate 

Mountains thrust (fig. 7). The rocks of this area are 

intimately intruded and strongly hydrothermally 

altered by the granite of Marcus Wash. In the few 

places where the actual contact between the volcanic 

rocks and schist or gneiss is exposed, eith~r the two 

rock types are separated by a thin dike of granite, or 

12 

0 500 METERS I I I I I 

EXPLANATION 

D Volcanic and sedimentary rocks (Miocene and Oligocene) 

Winterhaven Formation (Jurassic?)-- ,.,--::--v-..., 

Dacite member 

CJ Orocopia Schist (late Mesozoic) 

~ Gneiss (Mesozoic or Proterozoic) 

f~/J:)i.}/ j Rocks strongly intruded and altered by the granite of 

··· ·· ·· ·· · Marcus Wash (Late Cretaceous or Late Jurassic) 

--- Depositional contact 

--- Sortan bult 

...A......A.... • Chocolate Mountains thrust--Sawteeth on upper plate. 

Dotted where concealed 

Figure 7. Relations between the Winterhaven 

Formation and the Orocopia Schist and Chocolate 

Mountains thrust in a small area southwest of Little 

Picacho Peak (see fig. 2 for location). 

the rocks straddling the contact are so severely 

altered that the contact can be located only to within 

a meter or so and its nature is unclear. 

In the third area, along Sortan Wash, 

metamorphosed rocks of the Winterhaven Formation 

overlie the Orocopia Schist and a short segment of the 

Chocolate Mountains thrust along a low-dipping 

contact (fig. 8). Where locally relatively well exposed, 

this contact is marked by gouge and (or) microbreccia, 

and in several places it truncates the contact between 

the dacite and quartz arenite members of the Winterhaven Formation. This fault is here referred to as the 

Sortan fault. 

The Sortan fault may be in part localized along 

the older Chocolate Mountains thrust (figs. 2, 8), in a 

manner analogous to localization of middle and late 

Tertiary normal faults along Mesozoic thrust faults 

(Haxel and Grubensky, 1984; Tosdal and Sherrod, 

1985). In the area east of lower Marcus Wash and on 

the west side of Arrastra Wash (figs. 2, 8), the granite 

of Marcus Wash forms a small hemilaccolith intruded 

largely along the Sortan fault between the 

Winterhaven Formation and the Orocopia Schist 

(Haxel, 1977, app. 3). 
114°34'30" 

0 500 METERS 

EXPLANATION 

Granite of Marcus Wash (Late Cretaceous or Late 

Jurassic) 

Winterhaven Formation (Jurassic?)-

[)///1 Argillitic siltstone member and (or) quartz arenite 

member 

Dacite member 

Orocopia Schist (late Mesozoic) 

~ Gneiss (Mesozoic or Proterozoic) 

---Contact 

--- Sortan fault 

~ Chocolate Mountains thrust-Sawteeth on upper plate 

--- High-angle fault 

Figure 8. Relations among the Winterhaven 

Formation, the Orocopia Schist, and the granite of 

Marcus Wash in a small area along Sortan Wash (see 

fig. 2 for location). 

The altered subhorizontal contact southwest of 

Little Picacho Peak (fig. 7) evidently is a segment of 

the Sortan fault, and the Sortan fault apparently also 

was the pre-middle Tertiary contact along which the 

Winterhaven Formation overlaid the Orocopia Schist 

and Chocolate Mountains thrust in the area west of 

Little Picacho Wash (fig. 3). Field relations in the 

three small areas where it is best preserved (figs. 3, 7, 

8) unequivocally show that the Sortan fault postdates 

the Chocolate Mountains thrust and predates the 

granite of Marcus Wash; thus, this fault is of late 

Mesozoic age. The Sortan fault or fault system 

probably originally extended at least as far west as the 

Indian Pass area (fig. 2) and at least as far east as the 

southern Castle Dome Mountains (fig. 1). The Sortan 

fault appears to be a major structure in that a 

substantial thickness, probably about 5 to 10 km, of 

rocks of the upper plate of the Chocolate Mountains 

thrust evidently has been excised along the fault (fig. 

9). The tectonic affinities of the Sortan fault are 

discussed below. 

Distinction of the Winterhaven Formation from the 

Protolith of the Orocopia Schist 

A suggestion in connection with the Winterhaven 

Formation is that it might represent a facies of the 

·protolith of the Orocopia Schist. This possibility is not 

likely for several reasons. Although the Winterhaven 

Formation and the Orocopia protolith are both 

dominated by quartzofeldspathic to semipelitic 

sedimentary rocks of continental provenance, the two 

units have several significant lithologic, chemical, and 

tectonic contrasts indicative of deposition in distinct 

environments. The presence within the Orocopia 

protolith of basalt, ferromanganiferous chert, and 

peridotite indicates an oceanic or ensimatic 

environment; the Winterhaven Formation lacks mafic 

or ultramafic rocks, evidently rests on subaerial rhyolitic volcanic rocks (including welded tuff), and is 

inferred to represent an epicontinental environment. 

Quartz arenite and derivative quartzite in the 

Winterhaven Formation are detrital and lack the 

ferromanganiferous composition of the metachert of 

the Orocopia Schist. Psammitic and semipelitic rocks 

in the Orocopia Schist are commonly carbonaceous 

(graphitic), whereas those in the Winterhaven 

Formation are not. Metamorphosed dacitic and (or) 

andesitic volcanic rocks in the Winterhaven Formation 

of the Arrastra Wash-Marcus Wash area locally 

resemble some of the metabasite of the Orocopia 

Schist in outcrop but typically are finer grained, have 

a lower color index, and lack the albitic porphyroblasts 

that are rather common in Orocopia metabasite. 

Finally, the Winterhaven Formation and Orocopia 

Schist belong to the upper and lower plates, 

respectively, of the Chocolate . Mountains thrust, which 

is part of a regionally extensive and presumably fartraveled thrust system in southern California and 

southwestern Arizona (Haxel and Dillon, 1978). Not 

only are the two formations separated by the thrust, 

but the oceanic protolith of the Orocopia Schist was 

deposited where it could subsequently be subducted or 

otherwise deeply tectonically buried, whereas the 

epicontinental Winterhaven Formation has evidently 

remained at relatively shallow crustal levels. 

TECTONIC SIGNIFICANCE OF THE WINTERHAVEN 

FORMATION 

As mentioned in the Introduction, the 

Winterhaven Formation is important for two reasons: 

because it is a part of the Mesozoic sedimentary 

record of the southeastern California-southwestern 

Arizona region and because of its relations to the 

Orocopia Schist. 

Implications for the Age of the Orocopia Schist 

Consideration of the significance of the 

Winter~aven Formation with respect to the Orocopia 

Schist requires some additional evidence from the 

central Transverse Ranges northeast of Los Angeles 

(fig. 1). This terrane was, before Neogene 

displacement along the San Andreas fault system 

13 
14 

I 

I 

I 

0 

I 

I 

I 

I 

I 

5 KILOMETERS 

Order of magnitude 

of horizontal and 

vertical scale 

(Eroded or concealed) 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 
EXPLANATION 

Granite of Marcus Wash (Late Cretaceous or Late Jurassic) 

Winterhaven Formation and underlying volcanic rocks and the 

sedimentary and volcanic rocks of Slumgullion, undivided 

(Jurassic?)-Divided into: 

[(\\>1 Sedimentary and volcanic rocks 

~I nterme d iate-composition volcanic rocks 

~ Rhyodacitic volcanic and hypabyssal rocks 

Granitoid and gneissic rocks (Mesozoic and Proterozoic?)-

Includes Jurassic granodiorite 

Orocopia Schist (late Mesozoic) 

Contact 

Trace of bedding 

~ Sort an fault (late Mesozoic)-Ticks on upper plate. Dashed 

where approximately located; queried where uncertain. 

Arrows indicate direction of relative movement 

.............._ Chocolate Mountains thrust (late Mesozoic)-Sawteeth on 

upper plate. Arrows indicate direction of relative movement 

Figure 9. Schematic tectonic diagrams showing how 

strata of the Winterhaven Formation and the 

sedimentary and volcanic rocks of Slumgullion 

originally part of the upper plate of the Chocolat~ Mountains thrust (A), were displaced downward onto 

the Orocopia Schist (lower plate of the thrust) by 

movement on the Sortan fault (.!!). Development of the 

Sortan fault was followed by intrusion of the granite of 

Marcu~ Wash (C). Figure 9_!! represents upper-crustal 

extensiOn of area of figure 9A· figure 9C is an 

enlargement of part of figure "fs. The Chocolate 

Mountains thrust and the Sortan fault are both late 

Mesozoic-younger than 163 m.y. and older than the 

pre-earliest Tertiary (pre-60 m.y .) granite of Marcus 

Wash. Granitoid and gneissic rocks of the upper plate 

of the Chocolate Mountains thrust include the Jurassic 

granod~orite that intrudes the Slumgullion unit and the 

volcamc rocks underlying the Winterhaven 

Formation. These diagrams are not cross sections 

through. any single area of the crust but, instead, are 

compos1t~ sketches, not to scale, based largely on map 

patterns m the area from the Gavilan Hills eastward to 

Sortan Wash and northward to upper Arrastra Wash 

(figs. 2, 8), and in the southern Castle Dome Mountains 

(fig. 1). 

(Crowell, 1979), adjacent to the southern Chocolate 

Mountains (Dillon, 1976; Ehlig, 1981; Powell,1981; 

Silver, 1982). The Pelona Schist and the 

synmetamorphic Vincent thrust of the central 

Tr~sverse Ranges are considered tectonically 

eqUivalent to the Orocopia Schist and Chocolate 

Mountains thrust of southeastern California. (The 

~a~d Schist of the northern Mojave Desert, though 

s1m1lar to the Orocopia and Pelona Schists, is not 

discussed here because it apparently was never 

contiguous with them, at least at presently exposed 

crustal levels.) 

The role of the Orocopia and Pelona Schists in 

. the tectonic evolution of the southern Cordillera 

remains uncertain (Haxel and Dillon, 1978; Burchfiel 

and Davis, 1981; Crowell, 1981; Ehlig, 1981; Jacobson, 

1983a; Silver, 1983; Vedder and others, 1983). Because 

of the rapid tempo of events during Late Cretaceous 

and early Tertiary time (Dickinson, 1981), the age of 

the Orocopia Schist needs to be determined as precisely as possible. The relations described above 

between the Orocopia Schist and Winterhaven 

Formation are important in this regard. 

The Orocopia and Pelona Schists were pervasively metamorphosed under high-pressure upper 

greenschist- to lower amphibolite-facies conditions 

and have undergone multiple penetrative folding and 

transposition (Graham and England, 1976; Haxel and 

Dillon, 1978; Jacobson, 1983a, b). In contrast, much of 

the Winterhaven Formation is only incipiently or 

partially recrystallized, probably in the lower greenschist facies, and has not been penetratively 

deformed. (Where locally more strongly recrystallized 

and deformed, the Winterhaven Formation was 

affected by a separate and younger metamorphic and 

magmatic event that also remetamorphosed the 

Orocopia Schist.) The Winterhaven Formation clearly 

has not been subjected to the deformational and 

metamorphic event that produced · the Orocopia 

Schist. The Orocopia Schist was metamorphosed at 

considerable depth beneath the upper plate of the 

synmetamorphic Chocolate Mountains thrust (Dillon, 

1976; Haxel, 1977; Ehlig, 1981; Jacobson, 1983c). The 

Winterhaven Formation, originally part of the upper 

plate of the thrust, has not been subject to this deep 

tectonic burial, and it overlies both plates of the 

thrust along the pre-60-m.y. (Frost and Martin, 1983), 

shallow-level Sortan fault. Therefore, the Orocopia 

Schist was returned to upper-crustal levels, and the 

overlying Chocolate Mountains thrust breached, before 

60 m.y. ago. 

This conclusion implies that metamorphism of 

the Orocopia and Pelona Schists was somewhat earlier 

than previously thought. The currently accepted 

metamorphic age of these schists is based largely on 

Rb-Sr mineral-isochron and K-Ar whole-rock minimum 

ages of 52-59 m.y. from the Pelona Schist and the 

Vincent thrust zone (Ehlig, 1981). A sequence of 

several major events-return of the Orocopia Schist to 

the upper crust, juxtaposition against the Winterhaven 

Formation, remetamorphism, and intrusion by the 

granite of Marcus Wash-intervened between the 

metamorphism of the Orocopia Schist and setting of 

K-Ar dates about 60 m.y. ago. Thus, the 52- to 59-

m.y. isotopic dates for the Pelona Schist and Vincent 

thrust are postmetamorphic cooling ages. The primary 

metamorphic age of the Orocopia and Pelona Schists 

must be appreciably older than 60 m.y. and is almost 

certainly pre-Tertiary. 

The 80±10-m.y. U-Pb zircon age (Carter and 

Silver, 1972) of a granodiorite apparently in the upper 

plate of the Vincent thrust has been considered a 

maximum age for the Vincent thrust and 

metamorphism of the Pelona Schist (Ehlig, 198J.). 

15 
However, three factors indicate that this isotopic age 

may not provide a straightforward maximum age for 

the thrust and schist. First, the dated granodiorite 

lithology cannot be unequivocally tracked into the 

Vincent thrust zone (L.T. Silver, oral commun., 1983; 

P.L. Ehlig, oral commun., 1984). Second, the 

Chocolate Mountains thrust has been modified or 

replaced by both the late Mesozoic low-angle Sortan 

fault and numerous middle and late Tertiary low-angle 

faults, and Tertiary low-angle faults in the southern 

Arizona-southeastern California region are not 

uncommonly localized along Mesozoic thrust faults 

(Frost and others, 1982; Haxel and Grubensky, 1984; 

Tosdal and Sherrod, 1985). Third, observations 

presented by Evans (1982) suggest that some segments 

of the Vincent thrust may have been reactivated 

during faulting younger than the original formation of 

the fault zone and metamorphism of the Pelona 

Schist. The second and third of these factors suggest 

that the 80-m.y.-old granodiorite in the central 

Transverse Ranges may be cut not by the Vincent 

thrust but by younger fault(s). 

The principal data bearing on the metamorphic 

age of the Orocopia and Pelona Schists can be 

summarized as follows. A concordant U-Pb age of 163 

m.y. on zircon from a metamorphosed dioritic dike in 

the Orocopia Schist (Mukasa and others, 1984) provides 

a maximum age. As explained above, metamorphism 

must be appreciably earlier than 60 m.y. (K-Ar dates 

of Frost and Martin, 1983). Field relations and 

isotopic ages in the central Transverse Ranges (Carter 

and Silver, 1972; Ehlig, 1981; D.J. May, oral commun., 

1985) suggest, but do not prove, that the 

metamorphism is Late Cretaceous or earlier. The 

probable age of metamorphism of the Orocopia and 

Pelona Schists is thus Late, but not latest, Cretaceous; 

an early Tertiary age is unlikely, and a Late Jurassic 

age cannot be precluded. 

Tectonic Affinities of the Sortan Fault 

A puzzling aspect of the low-dipping late 

Mesozoic Sortan fault is its normal-fault geometry-it 

places the Winterhaven Formation, originally part of 

the upper plate of the Chocolate Mountains thrust, 

over the Orocopia Schist, the lower plate of the thrust 

(figs. 3, 7-9). Pre-Tertiary low-angle normal faults 

have not been widely reported in the southern Cordillera, the late Mesozoic tectonic evolution of which 

was generally characterized by thrust faulting and 

crustal shortening rather than nor mal faulting and 

crustal extension. It is unclear whether the Sortan 

fault represents local extension within the overall 

framework of late Mesozoic shortening or a 

widespread but previously unrecognized late Mesozoic 

extensional episode. 

Field relations in the Sortan Wash-Arrastra Wash 

area (figs. 2, 8, 9c) suggest that movement on the 

Sortan fault was part of the same event as 

metamorphism of the Winterhaven Formation and 

emplacement of the granite of Marcus Wash. If so, 

this composite event may be related to the Late 

Cretaceous and early Tertiary episode of thrust 

16 

faulting, regional metamorphism, and granitic 

plutonism that affected the region from southeastern 

Arizona to southeastern California (Keith and others, 

1980; Reynolds, 1980; Hamilton, 1982; Miller and 

others, 1982; Haxel and others, 1984; Tosdal, 1984b). 

Within this crustal-shortening regime, localized 

extension might have occurred by either of two 

mechanisms. First, extensional faults might have 

formed as a result of gravitational relaxation along a 

topographic front within the thrust belt (Royden and 

Burchfiel, 1985). Second, extension could have taken 

place within the upper plate of a downwardsteepening, thick-skinned thrust fault (Coward, 1983), 

in particular the Mule Mountains thrust; the Sortan 

fault might thus be akin to a keystone fault (Wise, 

1963). 

Two considerations, in addition to the existence 

of the Sortan fault, indicate that late Mesozoic lowangle normal faults could be more common than 

previously suspected. (1) Evidence presented by Silver 

and others (1984) and C.E. Postlethwaite and C.E. 

Jacobson (written commun., 1985) suggests that one of 

the faults overlying the Rand Schist (which is similar 

to the Orocopia Schist) is not the thrust fault beneath 

which the schist was originally metamorphosed but, 

instead, a late Mesozoic or early Tertiary low-angle 

normal(?) fault comparable to the Sortan fault. (2) 

Coney and tlarms (1984) hypothesized that Tertiary 

crustal extension in the Cordillera was caused by 

lateral spreading consequent to Mesozoic crustal 

thickening, and that extension was initiated or 

facilitated by the thermal input accompanying 

Tertiary magmatism. This model suggests that the 

Tertiary extensional episode was not necessarily 

unique. Late Mesozoic crustal extension may have 

occurred, boundary conditions permitting, in regions 

where earlier Mesozoic crustal thickening was 

followed by a separate, late Mesozoic thermal 

episode. In this context, crustal thickening caused by 

emplacement of the upper plate of the Chocolate 

Mountains thrust may have been followed by 

extensional deformation, including development of the 

Sortan fault, at the time of the thermal episode in 

which the granite of Marcus Wash was emplaced and 

the Winterhaven Formation metamorphosed. 

If there was a widespread episode of late 

Mesozoic crustal extension, some or much of the 

postmetamorphic uplift of the tectonically buried 

Orocopia and Pelona Schists may have taken place 

during that episode (C.E. Postlethwaite and C.E. 

Jacobson, written commun., 1985). In southeastern 

California, the original tectonic stratigraphy that 

characterizes the Chocolate Mountains thrust zone at 

the top of and overlying the Orocopia Schist i~ widely 

preserved (figs. 1, 2; Dillon, 1976; Haxel, 1977); only 

locally has the thrust been disrupted by the Sortan 

fault. This observation has two important 

implications. First, the Chocolate Mountains thrust 

and part of its crystalline upper plate were uplifted 

along with the Orocopia Schist. Second, because the 

Chocolate Mountains thrust is extant over a wide area, 

the Sortan fault is not simply a wholesale reactivation 

of the thrust but, instead, a new and independent 

structure. 
Possible Correlation with the McCoy Mountains 

Formation 

The McCoy Mountains Formation is a Jurassic 

and (or) Cretaceous siliciclastic sedimentary and 

metasedimentary sequence, as much as 7 km thick, 

exposed in the region around Blythe, Calif., and 

Quartzsite, Ariz. (fig. 1; Pelka, 1973; Harding and 

Coney, 1985). This formation is definitely older than 

latest Cretaceous K-Ar minimum ages of crosscutting 

plutons, faults, and veins (Pelka, 1973; Reynolds, 1980; 

Tosdal, 1984a; L.B.G. Pickthorn, oral commun., 1985), 

but other data as to the age of the formation are 

equivocal or seemingly contradictory. The basal strata 

of the McCoy Mountains Formation are apparently 

interbedded with volcanic rocks of probable Early and 

(or) Middle Jurassic age (Harding, 1982), and 

paleomagnetic data suggest that the formation, or at 

least its lower part, is older than middle Late Jurassic 

(Harding and others, 1983). However, the upper part 

of the formation contains fossil angiosperm wood of 

probable mid-Cretaceous or younger age. These data, 

if all taken at face value, imply that either the McCoy 

Mountains Formation spans a considerable part of 

Jurassic and Cretaceous time or, more likely, the 

formation consists of two distinct sedimentary units, 

one Early and (or) Middle Jurassic and the other middle 

and (or) later Cretaceous, separated by an unrecognized unconformity (compare with Miller, 1966). 

The southern boundary of the known extent of 

the McCoy Mountains Formation is the south-dipping 

late Mesozoic Mule Mountains thrust (Tosdal, 1982), 

the upper plate of which is composed largely of 

Jurassic granodiorite. This granodiorite extends 

southward to the lithologically identical granodiorite 

of Trigo Peaks that intrudes the metavolcanic rocks 

beneath the Winterhaven Formation (fig. 2). The 

presence of several distinctive lithologic units cut by 

(and thus older than) the Mule \otountains thrust in both 

plates of the thrust indicates that displacement on the 

thrust is relatively small, probably about 1-10 km 

(Tosdal, 1984a, b). 

The Winterhaven Formation and the lower part 

of the McCoy Mountains Formation are both of 

probable Jurassic age, have a broad lithologic 

similarity, are exposed within about 70 km of one 

another, and are separated by a fault of minimal 

displacement. These considerations suggest that the 

two units could be correlative and related. 

The Winterhaven Formation and the lower part 

of the McCoy Mountains Formation have two 

significant stratigraphic similarities. First, both 

formations evidently rest depositionally on and, at 

least locally, interfinger with Jurassic silicic volcanic 

rocks. Second, the quartz arenite member and the 

argillitic siltstone member of the Winterhaven 

Formation appear to correspond to the basal sandstone 

members and the mudstone member, respectively, of 

the McCoy Mountains Formation (stratigraphic 

nomenclature of Harding and Coney, 1985). In 

particular, the quartz arenite member of the 

Winterhaven Formation, the probably correlative unit 

within the sedimentary and volcanic rocks of 

Slumgullion (fig. 6), and basal sandstone member 1 of 

the McCoy Mountains Formation in the area southeast 

of Quartzsite all consist of or include a lithologic 

association of interbedded light-tan or gray quartz 

arenite, dark-purplish or maroonish-gray siltstone or 

mudstone, and minor quartzite-pebble conglomerate 

and brown-weathering arenaceous limestone. This 

distinctive lithologic association suggests that the 

Winterhaven Formation, the Slumgullion unit, and the 

lower part of the McCoy Mountains Formation may all 

record the same depositional episode or sedimentary 

environment. Despite the general similarity of the 

silicic volcanic (and related hypabyssal) rocks beneath 

the Winterhaven and McCoy Mountains Formations, 

the· dacite member of the Winterhaven Formation has 

no counterpart in the McCoy Mountains Formation. If 

correlation of the Winterhaven Formation with the 

lower part of the McCoy Mountains Formation is valid, 

then the dacite member is more limited in extent than 

the underlying silicic volcanic rocks and overlying 

sedimentary rocks, and probably was deposited over a 

smaller area. This restricted distribution may, in part, 

reflect the lesser mobility of intermediate volcanic 

flows compared to silicic ash-flow tuff. In summary, 

correlation of the Winterhaven Formation and the 

lower part of the McCoy Mountains Formation is 

reasonable, even likely, but cannot be proven or 

disproven until additional geochronologic data are 

available. 

The Mule Mountains thrust has been interpreted 

as a tectonostratigraphic terrane boundary separating 

the McCoy Mountains Formation, which has a North 

American provenance, from suspect terranes to the 

southwest (Harding and Coney, 1985). This interpretation is based largely on the apparent absence in the 

region southwest of the McCoy Mountains Formation 

of Proterozoic and Paleozoic rocks of cratonic North 

American affinities. However, the several similarities 

between the McCoy Mountains and Winterhaven 

Formations and their underlying volcanic and 

hypabyssal rocks, in addition to the evidence for 

minimal slip along the Mule Mountains thrust, indicate 

caution in inferring a major late Mesozoic tectonostratigraphic terrane boundary between the McCoy 

Mountains Formation and the Winterhaven 

Formation. Although these stratigraphic and 

structural ties are geometrically imprecise and do not 

necessarily preclude some displacement of the Winterhaven Formation along and (or) perpend~ular to the 

continental margin, they do suggest that the Winterhaven Formation is indigenous to southwestern North 

America. If future research shows that the Winterhaven Formation (and the sedimentary and volcanic 

rocks of Slumgullion) are, indeed, related to the 

McCoy Mountains Formation, then any late Mesozoic 

or Tertiary suture between indigenous and suspect or 

exotic tectonostratigraphic terranes must lie either 

within the upper plate of the Chocolate Mountains 

thrust south of the outcrop area of the Winterhaven 

Formation or structurally below the upper plate of the 

Chocolate Mountains thrust. 

17 
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19 
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