Mesozoic Stratigraphy of 

the Sierrita Mountains, 

Pima County, Arizona

GEOLOGICAL SURVEY PROFESSIONAL PAPER 658-D
Mesozoic Stratigraphy of 

the Sierrita Mountains, 

Pima County, Arizona

By JOHN R. COOPER

MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

GEOLOGICAL SURVEY PROFESSIONAL PAPER 658-D

Descriptive stratigraphy of more than 20,000 

feet of Triassic, Jurassic, and Cretaceous 

terrestrial volcanic and sedimentary rocks

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1971
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS G. B. MORTON, Secretary

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director

Library of Congress catalog-card No. 70-171033

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 - Price SO cents (paper cover)

Stock Number 2401-1123
CONTENTS

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

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

Previous work ........................................................................

Triassic rocks ..........................................................................

Ox Frame Volcanics ....................................................

Name and type locality ........................................

General description, distribution, and thickness 

Lithology ..................................................................

Members in the Twin Buttes quadrangle .. 

Members in the Palo Alto Ranch 

quadrangle ..................................................

Contacts ....................................................................

Age and correlation ..............................................

Tascuela Redbeds ..........................................................

Name, type section, and general description ....

Age and correlation ..............................................

Rodolfo Formation ........................................................

Name and type locality ........................................

General description, distribution, and 

thickness ..............................................................

Lithology ..................................................................

Basal conglomeratic member ......................

Middle siltstone member ..............................

Upper volcanic member ................................

Contacts ....................................................................

Age and correlation ..............................................

Upper Triassie or Lower Jurassic rocks ..........................

Stevens Mountain Rhyolite ..........................................

Name, type locality, and general description .. 

Age and correlation ..............................................

Lower Cretaceous (?) rocks ................................................

Whitcomb Quartzite ......................................................

Name and type area ..............................................

General description, distribution, and 

thickness ..............................................................

Page

Dl

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4

6

6

6

8

8

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13

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19

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23

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24

24

24

24

24

Lower

^

Lower

A

Upper

D

Refen

Index

Lower Cretaceous (?) rocks Continued 

Whitcomb Quartzite Continued

Lithology ................................................................

Contacts ................................................................

Age and correlation ............................................

Lower Cretaceous rocks ....................................................

Angelica Arkose ..........................................................

Name, type locality, and general description 

Lithology ...............................................................

Basal conglomerate member ......................

Middle arkose and siltstone member ......

Upper arkosic grit and conglomerate 

member ......................................................

Contacts ................................................................

Age and correlation ............................................

Upper Cretaceous rocks ....................................................

Demetrie Volcanics ....................................................

Name and type locality ......................................

General description, distribution, and 

thickness ............................................................

Lithology ................................................................

Conglomerate ................................................

Andesite and dacite ....................................

Rhyolitic tuff members ................................

Contacts ................................................................

Age and correlation ............................................

Red Boy Rhyolite ........................................................

Name, type locality, and general description 

Lithology ................................................................

Contacts ................................................................

Age and correlation ............................................

Page

D24 

25

25

26

26

26

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32

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ILLUSTRATIONS

Page

FIGURE 1. Map of southeastern Arizona and southwestern New Mexico showing location of study region ...................... D2

2. Index map of the Sierrita Mountains region showing areas of figures 4-6 .............................................................. 3

3. Diagrammatic cross section through the Sierrita Mountains showing Mesozoic sedimentary and volcanic

formations and their approximate maximum thickness ........................................................................................ 3

4. Geologic sketch map of the Ox Frame Canyon area .................................................................................... ........... 7

5. Geologic sketch map of the Helmet Peak area ............................................................................................................... 17

6. Geologic sketch map of the Esperanza mine area............................................................................. ............................ 31

7. Cross section through Red Boy Peak and Lobo Peak ................................................................................................ 35

8. Field sketch showing andesite breccia of the Demetrie Volcanics intruded by tuff of the Red Boy Rhyolite .. 36

ill
MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

MESOZOIG STRATIGRAPHY OF THE SIERRITA MOUNTAINS,

PIMA COUNTY, ARIZONA

By JOHN R. COOPER

ABSTRACT

Mesozoic rocks in the Sierrita Mountains, Pima County, 

Ariz., comprise more than 20,000 feet of terrestrial sedimentary and volcanic rocks and are divided into eight formations. 

Three formations are newly named in this report. These 

eight formations are not all found at any one place, however, 

and none has yielded closely datable fossils. Local geologic 

relations, radiometric age determinations, and regional correlations suggest that three of the formations are Triassic, 

one is Upper Triassic or Lower Jurassic, one is Lower 

Cretaceous (?), one is Lower Cretaceous, and two are Upper 

Cretaceous.

The lower Mesozoic formations are the Ox Frame Volcanics, Tascuela Redbeds, Rodolfo Formation, and Stevens 

Mountain Rhyolite. The oldest formation, the Ox Frame Volcanics, is at least 5,000 feet thick and is divisible into three 

members: a lower silicic member, which consists predominantly of massive rhyolitic flows in the core of the mountains 

and rhyolitic and rhyodacitic tuffs and tuff-breccias in the 

western foothills; a middle andesitic member, which consists 

predominantly of flows of dark porphyritic andesite; and an 

upper silicic member, which is characterized by rhyolitic 

welded tuffs and flows containing intercalated lenses of 

quartzite and, particularly in the western foothills, volcanic 

sandstone and conglomerate. Sedimentary interbeds are present locally but are scarce in the middle and lower members 

except in a unit of rhyolitic tuff, conglomerate, and slate that 

probably is the basal unit of the formation. This unit is 

found only within a fault block in the western foothills, 

where it lies unconformably on Paleozoic carbonate rocks. 

The upper contact of the formation in the western foothills 

is a disconformity at the base of the Tascuela Redbeds. Elsewhere the Ox Frame is bounded by faults, by intrusive 

igneous rocks that range in radiometric age from 190 to 56 

million years, and by unconformably overlying Upper 

Cretaceous and Cenozoic rocks.

The Tascuela Redbeds and Stevens Mountain Rhyolite are 

exposed in the western foothills of the mountains and are 

each as much as 2,000 feet thick. The Tascuela lies disconformably on the Ox Frame Volcanics and generally has a 

basal conglomerate, 0-200 feet thick, composed of pebbles and 

cobbles of the Ox Frame Volcanics and Paleozoic limestone,

quartzite, and chert in a matrix of red silty to sandy mudstone. The conglomerate grades upward into a sequence, 

1,600-1,800 feet thick, composed of red shale, argillite, and 

sparse interbeds of quartzite, sandstone, and sandy limestone. 

The Stevens Mountain Rhyolite lies with apparent conformity 

on the Tascuela Redbeds and consists of a lower volcanic 

conglomerate unit tens to hundreds of feet thick overlain by 

rhyolitic crystal tuffs, welded tuffs, and flows which contain 

a few lenticular interbeds of sandstone and quartzite. The 

Stevens Mountain Rhyolite is overlain with major unconformity by the Demetrie Volcanics.

The Rodolfo Formation (new), a sequence of red beds and 

volcanic rocks, overlies Paleozoic rocks and underlies the 

Whitcomb Quartzite in the pediment east of the Sierrita 

Mountains. The formation is at least 2,400 feet thick and is 

divisible into three members. The basal member, at most 

about 70 feet thick, lies unconformably on the Rainvalley 

Formation of Permian age and consists of coarse-grained 

grayish-orange sandstone and conglomerate composed largely 

of fragments of Paleozoic chert, quartzite, and limestone. The 

middle member, at least 1,300 feet thick, is made up of siltstone, a small amount of sandstone, and conglomerate all 

grayish red except in metamorphosed or hydrothermally altered areas where they are mostly medium gray. The conglomerate and sandstone in the lower part of this member 

are generally clean silica-cemented rocks containing fragments of the Ox Frame Volcanics, whereas rocks near the top 

are generally andesitic graywacke and conglomerate of intraformational source. The upper member, 200 to nearly 1,100 

feet thick, has an interfingering lower contact with the upper 

part of the middle member and consists of a thick lower unit 

of andesitic breccia, which locally contains two thin beds of 

much-altered rhyolitic tuff, and an upper unit of red beds 

and andesitic flows. The lower and middle members of the 

Rodolfo Formation are correlated with the Tascuela Redbeds; the upper member may be correlative with the Stevens 

Mountain Rhyolite.

The Whitcomb Quartzite (new) lies disconformably on the 

Rodolfo Formation west of Helmet Peak and consists of 300- 

600 feet of yellowish-gray fine- to medium-grained orthoquartzite which contains lenses of light-gray, well-indurated 

rhyolitic tuff in the upper part. The Whitcomb is overlain

Dl
D2 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

disconformably by the Angelica Arkose near the type localities of the two formations, but the contact is gradational 

and conformable in an isolated exposure about 5 miles to the 

east. Because of the locally gradational contact and also the 

presence in the Angelica Arkose of quartzite beds similar in 

lithology to the Whitcomb, the Whitcomb Quartzite is assigned to the Lower Cretaceous (?) with recognition of the 

possibility that the formations in the isolated outcrop have 

been misidentified and that the Whitcomb could be a preCretaceous equivalent of the Stevens Mountain Rhyolite.

The Angelica Arkose (new) overlies the Whitcomb Quartzite and is a thick Lower Cretaceous sequence of interbedded 

feldspathic sandstone, siltstone, conglomerate, and a little 

quartzite and limestone. A basal conglomerate unit, locally 

more than 500 feet thick, contains pebbles and cobbles of 

various Mesozoic and Paleozoic formations. The succeeding 

unit, as much as 2,000 feet thick, consists of interbedded siltstone and well-sorted arkosic sandstone in sharply defined 

beds mostly less than 2 feet thick. Overlying this unit with 

sharp but interfingering contact is a thick-bedded unit as 

much as 3,000 feet thick that is characterized by coarsegrained ill-sorted arkosic grit and pebble conglomerate containing recognizable fragments of Jurassic granite. Locally 

the grit unit contains a thick quartzite bed and sparse lenticular beds of limestone. It is overlain with angular unconformity by the Demetrie Volcanics.

Upper Cretaceous rocks comprise the Demetrie Volcanics

and the Red Boy Rhyolite. The Demetrie Volcanics, a sequence 

of andesitic and dacitic breccias and flows having a local 

basal conglomerate and two intercalated rhyolitic tuff units, 

lies with angular unconformity on formations as old as the Ox 

Frame Volcanics and as young as the Angelica Arkose. The 

Demetrie is apparently about 8,000 feet thick south of the 

Sierrita Mountains, but it wedges out beneath the Red Boy 

Rhyolite in the core of the mountains. The Red Boy Rhyolite 

is 700 to perhaps 1,000 feet thick and consists of nonwelded 

and welded rhyolitic tuffs and tuff-breccias and minor rhyolitic flows. The formation lies unconformably on the Demetrie 

Volcanics, on Jurassic and Triassic(?) intrusive rocks, and 

on the Ox Frame Volcanics. South of the mountains the Red 

Boy is overlain unconformably by gravels and volcanic rocks 

of middle Tertiary age.

INTRODUCTION

The Sierrita Mountains are about 25 miles southwest of Tucson, Ariz., within the Twin Buttes and 

Palo Alto Ranch quadrangles. (See fig. 1.) An index 

map of the Sierrita Mountains region is shown in 

figure 2. This report deals principally with the Twin 

Buttes quadrangle, where I started fieldwork in 

1957, but it also contains information obtained 

during my reconnaissance mapping of the Palo Alto

111° 110° 109°

50 

I

100 MILES 

_I

FIGURE 1. Map of southeastern Arizona and southwestern New Mexico showing location of study region.
SIERRITA MOUNTAINS, PIMA COUNTY D3

TO TUCSONi

32-00'

31°45'

R. 9 E

FIGURE 2. Index map of the Sierrita Mountains region showing areas of figures 4-6. Contour interval 1,000 feet.

SOUTHWEST

Western pediment 

and foothills

Core of 

Sierrita Mountains

NORTHEAST

Helmet Peak 

area

Red Boy Rhyolite (1000)

Mesozoic and early Tertiary plutons 

apparently intruded in zone of 

repeated Mesozoic tectonism

FIGURE 3. Diagrammatic cross section through the Sierrita Mountains showing Mesozoic sedimentary and volcanic formations and their approximate maximum thickness, in feet. Exposed stratigraphic relations shown in solid lines, inferred 

relations in dashed lines. Not to scale.

Ranch quadrangle during 1960 as well as information from the work of Thorns (1966). J. C. Wright, 

J. H. Stewart, A. R. Conroy, and J. L. Gualtieri 

assisted in the Twin Buttes work at various times. 

Mesozoic rocks in the Sierrita Mountains comprise more than 20,000 feet of terrestrial sedimentary and volcanic rocks which are herein divided 

into eight formations. These formations, their 

stratigraphic relations, and their probable ages are 

shown diagrammatically in figure 3. Note that the
D4 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

oldest Mesozoic formation, the Ox Frame Volcanics, 

is absent northeast of the mountains; that the 

youngest Mesozoic formation, the Red Boy Rhyolite, 

lies directly on the Ox Frame Volcanics in the core 

of the mountains; and that the intervening formations are largely different on the two sides. Evidently there was repeated Mesozoic tectonism that 

localized basins of deposition and areas of erosion 

during Mesozoic time. A major tectonic line active 

throughout the Mesozoic and now occupied by a 

large Paleocene pluton trends northwest between 

the core of the range and the Helmet Peak area. The 

Laramide orogeny started after deposition of the 

Lower Cretaceous rocks and ended after deposition 

of the Upper Cretaceous rocks (and apparently 

before emplacement of the Paleocene pluton).

PREVIOUS WORK

Although copper, zinc, and lead-silver deposits in 

the Sierrita Mountains have been known for considerably more than 100 years, the geology was first 

described by Ransome (1922), who briefly examined 

the eastern and western pediment and foothills in 

1920. He concluded that the range consisted principally of an intrusive granitic core flanked by 

sedimentary and volcanic rocks which have been 

metamorphosed to various degrees. He noted that 

the intruded rocks on the east side, near Helmet 

Peak and Twin Buttes, included volcanic and clastic 

sedimentary rocks of probable Mesozoic age as well 

as Paleozoic limestone and probable Precambrian 

granite. On the west side, he noted a sequence of 

uncertain age consisting of highly sheared and 

schistose volcanic and clastic sedimentary rocks 

containing limestone lenses which he interpreted as 

lenticular beds.

Barton (1925, p. 284) found a probable Camerotoechia of Devonian age in the limestone on the 

west slope of the range and depicted the limestone 

as Paleozoic on the old geologic map of Arizona 

(Barton and others, 1924); the enclosing volcanic 

and clastic sedimentary rocks, together with Ransome's Mesozoic (?) beds on the east side of the 

range, were shown as Early Cretaceous, the only 

well-dated Mesozoic rocks then known in southeastern Arizona. Later investigations support Barton's general interpretation, although the Mesozoic 

stratigraphy is more complex then he supposed.

Between 1922 and 1949, mineralized parts of the 

eastern pediment and foothills were described in 

seven unpublished theses by students at the University of Arizona (Gordon, 1922; Brown, R. L., 

1926; Park, 1929; Eckel, 1930; Mayuga, 1942; Whitcomb, 1948; Houser, 1949). Work on the stratigraphy was hampered by the complex structure, 

metamorphism, and rather small discontinuous outcrops. Nevertheless, it was shown that the Cambrian 

through Permian formations then recognized in the 

Bisbee-Tucson region were present and were for the 

most part entirely normal in lithology. Mayuga 

(1942) recognized that the Mesozoic strata of Ransome (1922) in the Helmet Peak area were similar 

in a general way to the Cretaceous rocks of W. H. 

Brown (1939) in the Tucson Mountains and suggested that their ages and sequences might be the 

same undifferentiated volcanic and sedimentary 

beds at the bottom overlain in turn by the Recreation Redbeds of W. H. Brown (1939) and arkose 

containing shale and a little limestone. The state of 

knowledge of stratigraphy gleaned during this 

period was concisely summarized by Wilson (1950, 

p. 40-41).

In 1950 the Pima copper deposit was discovered 

beneath 200 feet of gravel east of Mineral Hill 

(Thurmond and Storms, 1958), and much exploratory work by many mining companies followed. 

Other large low-grade copper deposits, listed in the 

following table, have been found and developed, and 

a tremendous amount of geological, geophysical, 

geochemical, and drill-core data has been added to 

company files.

Major new open-pit mines in production or development in the 

Sierrita Mountains at the end of 1968

Mine

Year Rated capacity

Company production (tons of ore

started per day)

Pima .... 

Mission

Pima Mining Co ..............

American Smelting and

Refining Co. 

..........do................................

Esperanza ...........

West Esperanza 

Sierrita ...............

Twin Buttes .......

Duval Corp .................

..........do ..........................

Duval Sierrita Corp 

The Anaconda Co .....

1956

1961

1967

1959;

1964 i

30,000

27,000

15,000

72,000

30,000

1 An unnamed open pit on San Xavier Indian Reservation about 2 miles 

northwest of Mission mine.

2 A little copper ore was mined during 1967; production was expected to 

increase in succeeding years. 

8 Production was expected to start during late 1969.

Geologists with the American Smelting and Refining Co. were the first to complete a geologic 

survey of the entire Sierrita Mountains. Courtright 

(1958) and Richard and Courtright (1960) pointed 

out that the distinctive part of rocks assigned in this 

paper to the Bemetrie Volcanics is much like a unit 

they called the Silver Bell Formation in the Silver 

Bell Mountains and other units in southeastern 

Arizona and southwestern New Mexico that commonly have unconformities at their bases and locally 

overlie fossiliferous Upper Cretaceous (Colorado)
SIERRITA MOUNTAINS, PIMA COUNTY D5

beds. A. G. Blucher (unpub. data) distinguished the 

principal stratigraphic and plutonic units in the central, southern, and western parts of the range. In 

1959 he collected silicified brachiopods from one of 

the limestone lenses in the highly schistose volcanic 

and sedimentary rocks on the west slope (SW1/4 

sec. 23, T. 17 S., R. 10 E.). E. D. McKee and R. J. 

Ross, Jr., examined these fossils and stated (written commun., May 22, 1961): "The brachiopods 

in these samples are poorly preserved but are 

certainly Paleozoic types. That they are of late 

Paleozoic age (perhaps Pennsylvanian) is suggested. However, far more extensive collections 

would be required to determine unquestionably the 

system to which the enclosing rock should be assigned." Although to my knowledge no other fossils 

have been found in the lenses, their lithology suggests that parts of several Paleozoic formations are 

represented, including the distinctive quartzite of 

the Scherrer Formation of Permian age.

Lacy (1959) assigned post-Paleozoic rocks east 

of the range to the Cretaceous and Tertiary. He 

agreed with earlier workers that these rocks contain 

units that resemble the Recreation Redbeds and 

Amole Arkose of W. H. Brown (1939) in the 

Tucson Mountains, but he believed that too little 

work had been done to permit correlation. A large 

low-angle fault, long known in the Helmet PeakMineral Hill area, was named the San Xavier thrust 

and interpreted as a premineralization fault that 

underwent major postmineralization movement.

My preliminary report (Cooper, 1960) on the 

pediment and foothills northeast of the range dealt 

primarily with the postmineralization (middle 

Tertiary) stratigraphy and structure. It was shown 

that the San Xavier thrust is a postmineralization 

fault and that the upper plate contains the Pima and 

Mission ore deposits. The proposed hypothesis that 

the upper plate moved about 6.5 miles north-northwest required that the roots of these deposits be 

concealed beneath the alluvium near Twin Buttes, 

where the Twin Buttes (Anaconda) ore deposit was 

later found. The great complexity of the Mesozoic 

stratigraphy was recognized, but no attempt was 

made to decipher the sequence, which was mapped 

as the Cretaceous (?) complex and crudely subdivided on a lithologic basis. Fossils collected from a 

limestone unit in arkose consisted of unidentifiable 

gastropods and pelecypods and fresh-water ostracodes of a type common in Upper Jurassic through 

Holocene sediments (Cooper, 1960, p. 69).

During reconnaissance mapping of the central 

and western parts of the Sierrita Mountains in 

early 1960, I found two groups of post-Paleozoic

plutonic rocks that differ from one another in 

petrography and geologic relations: (1) An older 

group of monzonite, quartz monzonite, and alkali 

granite which cuts the Ox Frame Volcanics and 

which is overlain unconformably by the Demetrie 

Volcanics and the Red Boy Rhyolite and (2) a 

younger group of diorite, granodiorite, and quartz 

monzonite that cuts the Demetrie Volcanics. The 

two episodes of plutonism are of great importance 

in the proper interpretation of the stratigraphy, but 

the date of neither episode was firmly established in 

1960. On the geologic map of Pima and Santa Cruz 

Counties (Wilson and others, 1960), the younger 

intrusives are assigned to the Laramide interval 

(Late Cretaceous and early Tertiary), the Red Boy 

Rhyolite of this report to the Tertiary, and the 

Demetrie of this report and older Mesozoic rocks to 

the Cretaceous. The recently published geologic map 

of Arizona (Wilson and others, 1969) is only partly 

updated to the age assignments herein made, but it 

does indicate that the Demetrie, Red Boy, and 

younger plutonic rocks are all Laramide and that 

the Ox Frame Volcanics are Mesozoic.

K-Ar (potassium-argon) age determinations 

(Creasey and Kistler, 1962, p. Dl; Damon and 

Mauger, 1966, p. 103; Mauger, 1966, p. 11-12) indicate that the Laramide plutonic rocks, the socalled biotite rhyolite southeast of Helmet Peak, 

and the mineralization are all of Paleocene age, 

obviously a minimum age for all the older strata. 

The pre-Laramide rocks are too metamorphosed 

and altered to date meaningfully by the K-Ar 

method. R. L. Mauger (in Damon, 1965, p. 28-31; 

1966, p. 22-23) dated several of these rocks by the 

Rb-Sr (rubidium-strontium) method, but his date 

of 93 m.y. (million years) for the rocks now called 

Ox Frame Volcanics from the rim of the Esperanza 

open-pit mine is incompatible with his dates of 130 

and 140 m.y. (Jurassic-Cretaceous boundary) for 

post-Ox Frame plutonic rocks. Pb-alpha (leadalpha) analyses of zircons (Drewes, 1968, p. C7; 

this report, p. D13) suggests that the dates for 

plutonic rocks are probably their approximate dates 

of crystallization and that the age given for the 

Ox Frame is too young, perhaps because of potassium and rubidium metasomatism at the time of 

mineralization.

Lutton (1961, p. 44) recognized the angular unconformity that separates the volcanics (Ox Frame, 

this report) and Silver Bell Formation (Demetrie, 

this report) near the Esperanza mine. He also recognized that the Ox Frame (this report) consists of 

rhyolitic pyroclastic rocks and subordinate crossbedded quartzite and that most of the rhyolitic rock
D6 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

is welded tuff whose flattened and coarsely devitrified pumice fragments have the mineral composition 

of true rhyolite.

Lootens (1965, 1966), in a study of the core of 

the Sierrita Mountains, recognized this ascending 

stratigraphic sequence: Ox Frame Formation, Silver Bell Formation (Demetrie Volcanics, this report) , and younger rhyolite (Red Boy Rhyolite, this 

report). However, he did not recognize that an 

interval of plutonism and deep erosion followed 

deposition of the Ox Frame and preceded deposition 

of the Demetrie. He believed that the plutonic rocks 

were differentiation products of a single source 

magma and that they were emplaced during an interval of about 5 m.y. in early Tertiary and perhaps 

latest Cretaceous time. He assigned all the stratified 

rocks to the Cretaceous.

Thorns (1966) gave a detailed description of 

about 25 square miles on the west flank of the 

Sierrita Mountains in the vicinity of Tascuela Wash. 

He proposed the following stratigraphic sequence 

for this area:

Stratigraphic sequence in the Tascuela area 

excerpted from Thorns (1966, fig. 4)

Age

Quaternary

Late Tertiary

Early Tertiary

Early TertiaryLate Cretaceous

Late Cretaceous

Early Cretaceous ( ? )

Nevadan( ?)

Paleozoic

Formation

Alluvial deposits

Olivine basalt flows

Fresnal Quartz Monzonite; dike rocks 

t Plutonism 3]

Red Boy Rhyolite

Demetrie Formation

Albite granite; quartz monzonite, monzonite, and quartz syenite; leucogranite 

[Plutonism 2]

Stevens Mountain Rhyolite

Tascuela Redbeds

Ox Frame Volcanics

Sunshine Formation

Porphyritic granodiorite; diorite 

[Plutonism 1]

Metasedimentary rocks including marble, quartzite, and calc-silicate hornfels

Thickness 

(feet)

Not given.

Not given.

Not given.

2,000 +

2,000

2,000

5,100 +

700

2,000 +

Thorns' sequence of sedimentary and volcanic 

formations is well established by superposition, but 

his age assignments are based entirely on the inferred age of intrusive rocks. His Fresnal Quartz

Monzonite, correlated for good reasons with radiometrically dated lower Tertiary intrusive rocks on 

the east side of the mountains, establishes only a 

minimum age for older formations. Intrusives that 

Thorns assigned to the Late Cretaceous include those 

dated as Jurassic and earliest Cretaceous by the 

Rb-Sr method (Damon, 1966). Because these intrusives are confined by field evidence to the interval 

between Ox Frame and Demetrie times, the Ox 

Frame Volcanics and probably the Tascuela Redbeds 

and Stevens Mountain Rhyolite are pre-Cretaceous. 

Evidence that Thorns' Nevadan(?) intrusives represent a separate plutonic episode is largely negatiye 

and not entirely convincing. These rocks intrude the 

Paleozoic rocks but were not found intruding 

younger formations; the only cited positive evidence 

that intrusion predated all the Mesozoic beds is in 

the following statement by Thorns (1966, p. 45): "A 

few pebbles and cobbles lithologically similar to 

the granodiorite occur in conglomerate of the 

Cretaceous (?) sequence . . . ."

TRIASSIC ROCKS 

OX FRAME VOLCANICS

NAME AND TYPE LOCALITY

The name Ox Frame Volcanics was applied by 

Lootens (1966) to a thick rhyolitic to andesitic 

volcanic sequence exposed in Ox Frame Canyon in 

the Sierrita Mountains (fig. 4). This name is 

adopted in this report, and Ox Frame Canyon is 

here designated as the type locality. The bottom of 

the section there is the large Cowboy fault in the 

NE1/4 sec. 22, T. 18 S., R. 11 E., and the top is the 

parallel Ash Creek fault in the Nl/2 sec. 3 of 

the same township and range. Between these 

boundaries, the formation is concealed at one place 

by an unconformable capping of the Red Boy 

Rhyolite. The type outcrops are cut by plugs and 

dikes of siliceous porphyry and are more metamorphosed than those in the western foothills of the 

mountains (Thorns, 1966). The sequence in the 

western foothills along Tascuela Canyon in sec. 18, 

T. 18 S., R. 11 E., where the bottom is defined by 

intrusive monzonite and the top by unconformably 

overlying conglomerate of the Tascuela Redbeds, is 

here designated as a reference section.

GENERAL DESCRIPTION, DISTRIBUTION, AND THICKNESS

Volcanic rocks of the Ox Frame consist of generally light-colored silicic types that range in composition from rhyolite to rhyodacite and darker 

colored andesite and dacite. These rocks occur as 

flows, flow breccias, welded and nonwelded tuffs, 

and tuff-breccias. Lenticular beds of sandstone and
SIERRITA MOUNTAINS, PIMA COUNTY D7

EXPLANATION

I

-"Rou

-"Sou

Sedimentary rocks

Holocene alluvium and Tertiary

Intrusive rocks

Quartz monzonite, granodiorite,

diorite, andesite, and rhyolite; in

part possibly of Late Cretaceous

Red Boy Rhyolite

Rhyolitic tuff and subordinate

Demetrie Volcanics

Andesitic and dacitic breccia;

conglomerate at base

Intrusive rocks

Monzonite, quartz monzonite,

granite, and granophyre

Rodolfo(?) Formation

Hornfelsed siltstone, sandstone, and

Ox Frame Volcanics

"Rou, upper member: rhyolitic flows 

and welded tuff

q, quartzite

"Rom, middle member: andesitic to 

rhyolitic flows and tuffs, volcanic sandstone, and conglomerate

"Rol, lower member: rhyolitic flows

50

Contact

Dotted where concealed

Fault 

Dotted where concealed

Strike and dip of beds

crui >-

o

2 MILES

FIGURE 4. Geologic sketch map of the Ox Frame Canyon area. Section A-A' shown in figure 7. Geology mapped, 1957-68; 

base from U.S. Geological Survey Twin Buttes 15-minute quadrangle, 1957.
D8 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

conglomerate are intercalated with the volcanic 

rocks and make up a minor part of the formation.

The type locality is in Ox Frame Canyon, where 

the formation is about 4,000 feet thick. However, 

neither the top nor the bottom is exposed, as both 

contacts are faults. The lower third is characterized 

by light-gray to pale-red rhyolitic flows, the middle 

third by medium-dark-gray to dark-greenish-gray 

andesite and dacite flows and a few flow breccias, 

and the upper third by light-gray to medium-darkgray rhyolitic welded tuffs and flows that contain 

interbeds of quartzitic sandstone. These members 

have interfingering contacts, and each contains some 

units lithologically characteristic of the adjacent 

member. The sequence is cut by penecontemporaneous rhyolitic dikes that are similar in lithology to the 

extrusive rock and by Triassic(?) and Jurassic 

quartz monzonite and granophyre. Both the Ox 

Frame and the quartz monzonite are overlain with 

angular unconformity by the Red Boy Rhyolite.

Rocks that resemble the middle and upper members are faulted against the lower member immediately south of the type locality (fig. 4), and to the 

east they crop out in a patch several square miles in 

extent in and around the Esperanza mine (fig. 6). 

The middle andesitic member is only partly exposed 

near the Esperanza mine, but the upper rhyolitic 

member, here predominantly rhyolitic welded tuff, 

appears to be at least 2,000 feet thick. These rocks 

are cut by Triassic(?) and Jurassic quartz monzonite, and near the Esperanza mine they are overlain with angular unconformity by the Demetrie 

Volcanics.

The Ox Frame also crops out in a separate belt 

about 7 miles long and less than a mile wide along 

the west side of the Sierrita Mountains within the 

Palo Alto Ranch quadrangle. Thorns (1966, p. 55) 

stated that here the formation is more than 5,100 

feet thick and consists of four members which he 

summarized "from oldest to youngest" as follows:

(1) tuff breccia member, 1,200 to 1,500 feet thick, rhyodacitic 

breccias (1,000 feet) of probable flow origin overlain by finegrained rhyolitic tuffs, welded tuffs, and interbedded thin 

arenites; (2) andesite porphyry member, 1,200 feet thick, 

flows and minor flow breccias; (3) coarse tuff member, 1,500 

feet thick, rhyodacitic to quartz latitic to rhyolitic coarse 

tuffs, minor welded tuffs, and a few lenticular arenites; (4) 

rhyolite member, more than 1,200 feet thick, mainly welded 

tuffs, interbedded thin flows, lenticular conglomerates and 

arenites.

North of Ash Creek the top unit of the rhyolite

member is a distinctive amygdaloidal andesite flow.

The reference locality, the most complete and least

disturbed continuous section, is about 4,000 feet

thick and differs from the outcrop described by 

Thorns in the summary just quoted in that the 

rhyolite member lies directly on the andesite 

porphyry member and the coarse tuff member is 

represented, according to Thorns (1966, p. 63), by a 

conformable lens of rhyodacitic tuff near the middle 

of the andesite porphyry member. This relation and 

others shown on Thorns' map suggest that, despite 

its varied lithology, the entire coarse tuff member 

is a lateral time equivalent of the andesite porphyry 

rather than a younger unit.

A narrow faulted belt of interbedded pyroclastic 

and clastic rocks that crops out in the western foothills and extends northwestward from the SE1/4 

sec. 12, T. 18 S., R. 10 E., was described by Thorns 

(1966, p. 48-54; 1967). According to him, the 

sequence is at least 700 feet thick and consists of 

schistose latitic to rhyolitic tuff, stretched conglomerate, sandstone, and slate. The sequence overlies 

Paleozoic marble with slightly angular unconformity and contains stretched pebbles and cobbles of 

Paleozoic quartzite and marble. Other boundaries 

are fault or intrusive contacts. Thorns called the 

sequence the Sunshine Formation and assigned it a 

Cretaceous (?) age but recognized that it might be 

part of the Ox Frame Volcanics. Except for being 

somewhat sheared, the rocks are lithologically 

similar to the lower member of the Ox Frame Volcanics of this report and are here considered to be 

part of that formation.

In general, the Ox Frame sections in the western 

foothills and in the core of the range resemble each 

other in total thickness, lithology, and stratigraphic 

succession silicic rocks, andesitic rocks, silicic 

rocks. The rocks in the two areas are regarded as 

general correlatives but are described separately inasmuch as they differ in details of lithology and are 

everywhere separated by younger formations.

LITHOLOGY 

MEMBERS IN THE TWIN BUTTES QUADRANGLE

The Ox Frame Volcanics in the Twin Buttes 

quadrangle comprises metamorphosed and altered 

rhyolitic to andesitic volcanic rocks and a little 

quartzite, volcanic sandstone, and conglomerate. 

The dip and strike of these rocks, which indicate 

stratigraphic succession, are not well displayed, inasmuch as flow and pyroclastic units commonly lack 

discernible bedding or other primary planar structure and thus look alike in outcrop. My mapping 

and observations of the attitude of sedimentary interbeds, flow \banding in lavas, and eutaxitic structur§ in welded tuffs indicate that strata at the type 

locality along Ox Frame Canyon north of the Cow-
SIERRITA MOUNTAINS, PI MA COUNTY D9

boy fault have a generally northward dip and that 

therefore the oldest Ox Frame units are exposed at 

the south and the youngest at the north (fig. 4). 

This succession is opposite to that of Lootens (1965, 

1966), whose inference of a southward dip seems to 

have been based on the attitude of the Red Boy 

Rhyolite, part of which he included in the Ox Frame 

Volcanics.

The lower member in Ox Frame Canyon consists 

predominantly of light-gray to pale-red rhyolitic 

flows that are massive and only rarely flow banded. 

Some of the rock is entirely aphanitic, but most of 

it contains 2-20 percent phenocrysts of feldspar 1-3 

mm (millimeters) long. Some hand specimens also 

show tiny grains of quartz, very sparse aggregates 

of chlorite and epidote, specks of iron ore, and small 

aggregates of black tourmaline. The chlorite-epidote 

aggregates seem to have replaced primary biotite 

and perhaps pyroxene. The tourmaline probably was 

introduced at the time of emplacement of Triassic(?) and Jurassic granophyre dikes and plugs, 

which are common in this area and are generally 

tourmaline bearing. Other dikes are very similar to 

the flows in appearance and age and are difficult to 

distinguish from the extrusive rock.

In thin section, the feldspar phenocrysts in the 

flows are seen to be potassium feldspar having 

moderate to large optic angle and sericitic albite 

commonly rimmed by potassium feldspar. The 

groundmass is cryptocrystalline to microcrystalline 

felsic material in which very fine grained micrographic and sieve textures are locally perceptible. 

On the basis of cobaltinitrite-stained thin sections 

and rock slabs (Bailey and Stevens, 1960), the 

groundmass feldspar is judged to be mostly a potassic variety, and the presence of abundant quartz 

is demonstrated by poikiloblasts in optical continuity with adjacent quartz phenocrysts. Subparallel fluidal orientation of phenocrysts and 

microlites is discernible in some thin sections.

The lower member along the bottom of Ox Frame 

Canyon contains a few thin flows or sills of andesite 

but no clastic rocks, so far as is known. East and 

west of the canyon, the member contains lenses of 

rhyolitic tuff, tuff-breccia, and tuffaceous sandstone. 

The pyroclastic rock tends to be medium gray on 

fresh fractures and yellowish gray on weathered 

surfaces. Some of it contains conspicuous phenocrysts of quartz and feldspar.

The middle member of the Ox Frame Volcanics, 

as exposed in Ox Frame Canyon, has an interfingering lower contact and consists of andesite 

flows that contain some lenticular rhyolite flows like 

those in the lower member. Hand specimens of

typical andesite show 15-35 percent phenocrysts of 

plagioclase 1-5 mm long in a dark-gray to greenishgray aphanitic groundmass. The upper part of some 

flows contains sparse small amygdules of quartz 

and epidote and irregular pods, lenses, and streaks 

of dense red andesite.

All the andesite examined in thin section is much 

altered. The plagioclase phenocrysts are albite that 

contains epidote, sericite, and locally clay minerals. 

Scarcer and somewhat smaller pseudomorphs of 

actinolite and epidote have the stubby form of 

pyroxene, and a few contain unreplaced cores of 

augite. The groundmass shows microlites of albite, 

actinolite, and magnetite, and interstitial cryptofelsite strewn with epidote, sericite, and commonly 

chlorite, calcite, and a little tourmaline. Flow 

orientation of the plagioclase microlites is commonly conspicuous. The cryptocrystalline interstitial 

matter is rich in potassium feldspar, as shown by 

cobaltinitrite staining of thin sections and rock 

slabs; therefore the rock as a whole is probably a 

potassic andesite.

The upper part of the middle member is exposed 

northwest of the younger mass of Red Boy Rhyolite 

and is partly exposed in faulted areas east and south 

of Ox Frame Canyon. Both east and south of the 

canyon, the andesite flows in the upper part of the 

member are intercalated with flows of dark dacite 

that contain small phenocrysts of quartz. Locally 

the member also has lenses of andesitic tuff, tuffbreccia, volcanic conglomerate, and crossbedded 

andesitic sandstone. The tuff-breccia and conglomerate generally contain fragments of rhyolite and andesite in a clastic andesitic matrix. Light-weathering 

rhyolite occurs as lenticular flows and welded 

tuff units and, in the ridge on the east side of Ox 

Frame Canyon, as numerous dikes oblique to the 

bedding. Inasmuch as the rhyolite is very resistant 

to erosion, one can easily get an exaggerated 

impression of its abundance.

Andesitic rock having the texture of microdiorite 

occurs where the middle member is exposed l 1^ 

miles west of the Esperanza mine. Hand specimens 

are medium dark gray and generally similar in appearance to flows already described, but commonly 

they are less distinctly porphyritic and contain discernible patches of dark mafic silicates as well as 

plagioclase crystals. In thin section, subhedral 

crystals of saussuritized plagioclase and actinolite, 

mostly a millimeter or less in size, are seen to be 

fairly closely packed, and the interstices are filled 

with subhedral orthoclase, magnetite, apatite, and 

poikiloblastic quartz. Epidote and secondary leafy 

biotite are abundant throughout the rock. In places,
D10 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

biotite flooding has destroyed the actinolite and 

masked the primary texture. A few amygdules of 

quartz and epidote have been found in the rock, but 

no flow boundaries or intercalated clastic beds have 

been recognized. Thus, the rock could be intrusive 

or extrusive in origin. It is assigned to the Ox 

Frame Volcanics inasmuch as it is cut by 

Triassic(?) and Jurassic quartz monzonite and related aplite dikes and is overlain with apparent 

conformity by quartzite and rhyolite of the upper 

member.

The upper member of the Ox Frame Volcanics 

in the Twin Buttes quadrangle consists of rhyolitic 

extrusive rocks that contain sparse intercalated 

lenses of quartzite. The extrusives include lava 

flows, common in the lower part of the member in 

Ox Frame Canyon, and welded and nonwelded tuff. 

The rock is light gray to medium dark gray and is 

locally flow banded. Hand specimens show 5-10 

percent feldspar crystals 0.5-2 mm long in a hard 

flinty groundmass. The feldspar is subhedral and in 

thin section is seen to be dusty perthitic orthoclase 

that in most specimens is accompanied by sericitic 

albite and a little phenocrystic quartz. Small books 

of biotite much altered to muscovite, chlorite, and 

opaque material were found in one specimen studied. 

These minerals, together with accessory magnetite, 

leucoxene, and rarely zircon, are embedded in microcrystalline to cryptocrystalline felsic material that 

locally contains sparse flow-alined alkali feldspar 

microlites. The groundmass is commonly a sievetextured mosaic of anhedral quartz grains 0.25-1 

mm in diameter loaded with tiny unoriented grains 

of alkali feldspar and opaque specks. The flows are 

similar in composition to those in the lower member 

but, so far as is known, lack micrographic groundmasses.

Welded rhyolitic tuff is associated with the flows 

of the upper member of the Ox Frame and is the 

predominant rock type near the Esperanza mine. 

In the Ox Frame Canyon area the tuff is generally 

medium gray, and near the Esperanza mine it varies 

in color from dark gray to yellowish gray, brownish 

gray, and pale red. Some of the tuff is distinguishable in the field by planar eutaxitic structure resulting from flattened pumice fragments and by the 

texture of compressed glass shards that are discernible with a hand lens on freshly broken and 

moistened surfaces. The welded tuff origin of these 

rocks and of some rocks indistinguishable from 

flows in the field is shown in thin section by the 

microeutaxitic structure, the texture of compressed 

shards, the relict axiolitic devitrification structure, 

and the broken condition of many of the phenocrysts. At many places the primary features of the 

once-glassy material have been erased by recrystallization, and the broken phenocrysts are the only 

clue to the origin.

Lithic tuff and tuff-breccia occur as sparse units 

10 to about 100 feet thick in the upper member of 

the Ox Frame Volcanics in the Twin Buttes quadrangle. These units are distinctive lithologically but 

seem to be highly lenticular and hence of little use 

as stratigraphic markers. They are well indurated 

and consist of poorly sorted angular and subangular 

fragments, as much as several inches in diameter, 

of light and dark volcanic rock and scarce quartzite 

in a matrix of light-gray to grayish-pink flinty volcanic ash. Hand specimens show flow-banded fragments and other fragments that lack this structure 

but differ from one another in color, groundmass 

texture, and the presence or absence of phenocrysts. 

In cobaltinitrite-stained thin sections, most fragments are seen to apparently range in composition 

from rhyolite to trachyte and include spherulitic 

flow-banded types and sieve-textured and granophyric-textured types resembling flows in other 

parts of the formation. Fragments of latite, andesite, and quartzite are scarce. The matrix is cryptocrystalline to microcrystalline felsic material that 

contains scattered chips of alkali feldspar and 

quartz and locally shows obscure markings whose 

form suggests pumice and shards. Neither axiolitic 

structure nor indications of compaction have been 

found. Apparently the rock was formed by explosive 

eruptions of the same magma that gave rise to the 

associated flows and welded tuffs the juvenile constituents being contaminated by accessory and accidental fragments torn from the walls of the vent.

A small amount of volcanic conglomerate and 

sandstone is associated with the tuffs just described, 

but the most abundant and characteristic sedimentary rock in the upper member is fine- to mediumgrained quartzite, which occurs as sporadic lenses a 

few feet to several hundred feet thick and a few tens 

of feet to several thousand feet long. The largest 

lenses are shown in figure 4. The quartzite locally 

shows relict sandstone texture and alluvial bedding 

and crossbedding. The thin sections examined are 

about 90 percent quartz, in part having mosaic texture and in part occurring as detrital grains that are 

well rounded, well sorted, closely packed, and cemented by fine-grained sericitic material. The 

quartzite must represent unusually pure quartz 

sand, presumably derived from pre-Ox Frame rocks 

outside the basin of deposition and deposited by 

streams during intervals of volcanic quiescence.

Some rock that superficially resembles the
SIERRITA MOUNTAINS, PIMA COUNTY Dll

quartzite has been formed by silicification of welded 

tuff. Exposures near the Esperanza mine indicate 

that silicification started at scattered centers several 

inches to about a foot apart and gradually spread 

outward; it first formed fuzzy leopard-type spots of 

granular quartz in which all primary features of 

the rock are erased, and finally it replaced the entire rock. The end product is a fine- to mediumgrained quartz rock distinguishable from the 

sedimentary quartzite only by the lack of bedding 

and relict sedimentary texture. Only sedimentary 

quartzite is shown in figure 4, although some silicified tuff may have been included inadvertently.

MEMBERS IN THE PALO ALTO RANCH QUADRANGLE

Rocks in the Palo Alto Ranch quadrangle considered herein to be the lower member of the Ox 

Frame Volcanics were described by Thorns (1966). 

The lower part is bluish-gray schistose tuff consisting of tiny grains of magnetite and partly albitized 

sodic andesine in an abundant foliated matrix of 

microcrystalline quartz, sericite, feldspar, biotite, 

chlorite, epidote, and calcite. A thin basal zone contains small lenses of marble that probably represent 

flattened pebbles and cobbles of the subjacent 

Paleozoic rock. Overlying the bluish-gray tuff is 

sheared greenish-gray rhyolitic tuff that contains 

thin beds of stretched volcanic conglomerate. The 

rhyolitic tuff interfingers laterally with an overlying 

unit of maroon to purple slate that contains subordinate interbedded tuff, sandstone, and conglomerate. A few cobbles of marble occur in one 

conglomerate bed.

In fault contact with the basal member is a 

diversified assemblage of andesitic to rhyolitic 

extrusive rocks and subordinate interbeds of sandstone and conglomerate. The sequence, best displayed near Tascuela Canyon, was described in 

detail by Thorns (1966). A brief summary of his 

units and lithologic description follows, beginning 

at the base.

The tuff-breccia member, whose lower contact is 

with intrusive monzonite, comprises about 1,000 

feet of rhyodacitic tuff-breccia overlain by 200-500 

feet of lighter colored rhyolitic rock that contains 

thin interbeds of sandstone and conglomerate. The 

rhyodacitic tuff-breccia is pale red purple to grayish 

red purple. Angular fragments of fine-grained 

andesitic to rhyolitic rock, typically less than 3 cm 

(centimeters) in diameter, make up 25-50 percent 

of the rock. The matrix is fine-grained tuff that 

contains sparse crystals of quartz, sanidine, and 

sodic andesine (An32 ); a few collapsed devitrified 

pumice fragments were noted in thin section. A

dark-grayish-blue quartzite bed 4-8 feet thick and 

about a mile long divides the tuff-breccia into two 

parts that are slightly different in color and texture. 

The top several hundred feet of the tuff-breccia 

member includes, in ascending order: (1) grayishorange-pink rhyolitic tuff that contains sparse rock 

fragments and faint eutaxitic structure, (2) duskyred welded vitric tuff only 4 feet thick, (3) palered rhyolitic crystal tuff, (4) pale-reddish-brown 

lithic graywacke that has volcanic pebble conglomerate at the base, (5) light-brownish-gray aphanjte 

that contains sparse indistinct white phenocrysts, 

and (6) grayish-red arenite. The last two units are 

absent locally.

The andesite porphyry member is about 1,200 feet 

thick and consists of andesitic flows, minor flow 

breccias, and, near the top, a few lenticular units of 

crossbedded sandstone. These rocks lie on the tuff 

breccia member and locally contain inclusions of it 

in the bottom few feet. On the ridgecrest south of 

Tascuela Canyon, they are in nearly continuous outcrop to the contact with the overlying rhyolite member. Elsewhere lenses of rhyodacitic tuff that Thorns 

regarded as part of the coarse tuff member are 

intercalated in the andesite porphyry member and 

separate it into a lower and an upper part.

The andesitic flows and flow breccias are grayish 

red purple on fresh fracture and grayish red to 

brownish gray on weathered surfaces. Most are 

porphyritic and show well-developed flow structure 

by the parallel alinement of plagioclase laths 2-10 

mm long. Other constituents discernible in hand 

specimen are sparse tiny grains of magnetite and 

dusky-green pyroxene. According to Thorns (1966, 

p. 59-60), the rock consists of 26.0 percent sodic 

andesine as partly saussuritized, serpentinized, and 

sericitized subhedral laths 2-10 mm long; 8.6 percent partly altered pyroxene; 2.8 percent magnetite; 

0.6 percent rock fragments; and 62.0 percent 

cryptocrystalline to microcrystalline groundmass 

that contains disseminated red iron oxide dust and 

minute grains of magnetite and leucoxene. The 

flows and flow breccias have gradational contacts. 

Vesicular structure was observed at only one place, 

and other distinctive top-and-bottom structural and 

textural features are so obscure that individual flow 

boundaries are virtually unknown except at the few 

places where there is sandstone between the flows.

The coarse tuff member is represented near 

Tascuela Canyon by two lenticular units of rhyodacitic tuff, locally as much as 500 feet thick, near the 

middle of the andesite porphyry member. The tuff 

contains crystals and crystal fragments, as much as 

3 mm in diameter, of quartz, sodic andesine, ortho-
D12 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

clase, and altered biotite. The crystals are set in a 

cryptocrystalline to microcrystalline matrix with 

axiolitic structures resulting from the collapse and 

devitrification of pumice fragments.

Also included in the coarse tuff member is a 

rhyolitic sequence, apparently at least 1,500 feet 

thick, exposed 0.5-1.2 miles southeast of Tascuela 

Canyon on strike with the rhyodacite tuff and 

andesite but separated from them by a transverse 

fault. The lower contact of the sequence is concealed 

by alluvium and down-faulted conglomerate of the 

Demetrie Volcanics. The upper contact is conformable with the rhyolite member or with a thin intervening andesite unit correlated with the upper part 

of the andesite porphyry member (Thorns, 1966, p. 

63). This part of the coarse tuff member includes 

crystal and crystal-lithic tuff, small amounts of 

welded vitric tuff, and several intercalated lenses of 

quartzite. The volcanic units contain variable proportions of crystals, crystal fragments, and rock 

fragments in a microcrystalline matrix that invariably makes up a dominant fraction of the rock. 

Quartz, sanidine, and orthoclase, in grains 0.5-3.0 

mm in diameter, are the most abundant constituents 

discernible in hand specimen; plagioclase and biotite 

are less common and are much less abundant than 

in the rhyodacitic tuff. In thin section, the matrix 

was seen to be dusty-looking microlitic or felted material that contains tiny crystal fragments and 

locally shows microtextures indicating derivation 

from compacted pumice and glass shards. The 

quartzite lenses are grayish orange, medium 

grained, and well bedded. They consist of 70-80 

percent quartz as well-rounded grains and secondary overgrowths, 5-10 percent rock fragments, and 

10-15 percent fine-grained interstitial sericitic 

material.

The upper member of the Ox Frame Volcanics 

in the Palo Alto Ranch quadrangle, the rhyolite 

member (Thorns, 1966), has a maximum thickness 

in excess of 1,300 feet and consists of rhyolitic 

welded tuffs, subordinate flows, volcanic sandstones, 

and conglomerates. The lithologic sequence varies 

markedly along the strike owing to the lenticularity 

of units. Part of the reference locality in Tascuela 

Canyon, which is modified slightly from Thorns 

(1966, p. 73), shows most of the rock types found 

in the upper member.

Section of the rhyolite member of the Ox Frame Volcanics in 

Tascuela Canyon (sec. 18, T. 18 S., R. 11 E.) as interpreted 

by Thomas (1966)

Feet

Tascuela Redbeds. 

Ox Frame Volcanics: 

Rhyolite member:

12. Conglomerate, grayish-yellow; felsite and 

milky quartz pebbles and cobbles; lenticular .................................................................. 100

11. Rhyolite flow, grayish-yellow to grayishpurple; sparse phenocrysts of quartz and 

sanidine in distinctly flow banded aphanitic groundmass .............................................. 60

10. Welded tuff, grayish-red to purple, very

fine grained; has eutaxitic structure ........ 50

9. Welded crystal tuff and tuff-breccia, 

pinkish-gray to yellowish-gray, rhyolitic 

to trachytic; vitroclastic matrix (not apparent in hand specimen) ; bottom 25 ft is 

finer textured and has eutaxitic structure 175

8. Volcanic arenite, red to maroon, coarsegrained; in part conglomeratic; well 

bedded ............................................................ 50

7. Welded crystal tuff; light gray in upper 

part, pink to pale reddish brown in lower 

part; rhyolitic to quartz latitic; subordinate fine-grained matrix has eutaxitic 

structure ........................................................ 550

6. Quartz latite porphyry (flow?), yellowishgray to pale-red-purple; small phenocrysts in aphanitic groundmass ................ 75

5. Felsite, grayish-yellow, dense ........................ 35

4. Quartzite, white, medium-grained; quartz 

grains well rounded; fragments from 

underlying unit near base ........................ 30

3. Volcanic arenite, dark-bluish-gray, mediumto coarse-grained .......................................... 10

2. Felsite, white (very finely crystalline rhyolite) .................................................................. 50

1. Volcanic arenite, bluish-gray, mediumgrained; fragments of andesite porphyry 

near base ........................................................ 15

Total ..........................................................1,200

Andesite porphyry member.

The sedimentary rocks in the lower part of this 

section (units 1, 3, and 4) have little lateral extent, 

whereas the thick welded crystal tuff (unit 7), 

which makes up almost half the member in Tascuela 

Canyon, extends at least a mile along the strike. 

This tuff generally consists of close-packed crystals 

and crystal fragments, as much as 3 mm in diameter, of plagioclase, potassium feldspar, and quartz 

in a subordinate microcrystalline matrix that shows 

inconspicuous small-scale eutaxitic structure in 

hand specimen and deformed axiolites in thin sec-
SIERRITA MOUNTAINS, PIMA COUNTY D13

tion. Northwest of Tascuela Canyon in the vicinity 

of Ash Creek and Stevens Mountain, the rhyolitic 

flows and tuffs above the thick welded tuff unit contain considerable interbedded volcanic conglomerate; a little volcanic sandstone and maroon slate; 

and two basaltic andesite flows, each about 50 feet 

thick. The upper andesite flow has remarkable 

lateral extension for such a thin unit. It is mostly 

aphanitic and grayish purple and contains conspicuous white amygdules. In thin section, its texture is trachytic. Nearly parallel subhedral laths of 

andesine (An33.37 ) make up more than 60 percent of 

the rock. The length of these crystals is as much as 

2 mm but averages less than 1 mm. The interstices 

in the feldspar framework are filled with very fine 

grained leucoxene, red iron oxide, and magnetite. 

The amygdules are composed of quartz, calcite, and 

locally epidote. This distinctive amygdaloidal flow 

is the top unit of the Ox Frame Volcanics for about 

1.5 miles.

CONTACTS

The lower contact of the Ox Frame Volcanics is 

a fault contact or an intrusive contact at most 

places. On the west side of the Sierrita Mountains, 

however, the interbedded pyroclastic and sedimentary rock unit which I regard as the lower 

member of the Ox Frame lies with slight angular 

unconformity on Paleozoic sedimentary rocks.

The upper contact of the Ox Frame Volcanics is a 

disconformity at the base of the Tascuela Redbeds. 

At most places the Tascuela has a basal conglomerate that contains pebbles, cobbles, and boulders 

from two sources: many kinds of volcanic rocks 

found in the Ox Frame; and quartzite, limestone, 

and chert that may well have come from the 

Paleozoic rocks east of the range, where the Ox

Frame is missing. The coarse texture of this conglomerate and the lithologic diversity of its constituents indicate that the disconformity was a surface of much greater topographic and structural relief than is apparent in outcrop.

AGE AND CORRELATION

Dating of these rocks must be based on local 

geologic relations, radiometric age determinations, 

and correlation of units with dated formations in 

other areas. Superposition of formations indicates 

that the Ox Frame postdates Paleozoic rocks and 

predates the Tascuela Redbeds and the Triassic(?) 

and Jurassic quartz monzonite.

Geologic mapping by Thorns (1966) and by the 

U.S. Geological Survey shows that the Ox Frame 

was faulted, intruded by monzonite, quartz monzonite, and alkali granite, and then deeply eroded 

before deposition of the Demetrie Volcanics and 

Red Boy Rhyolite. Following another period of deformation, diorite, granodiorite, and quartz monzonite were emplaced. The older intrusive suite was 

not recognized by Lootens (1966), who thought that 

all intrusive rocks of the area were of early Tertiary 

age and were "emplaced over a very short span of 

time (approximately 5 million years)." The 

radiometric age of the younger intrusive rocks and 

associated mineralization (Creasey and Kistler, 

1962; Damon and Mauger, 1966) is about 60 m.y. 

and supports Lootens' generalization; but the 

radiometric age of the older intrusives is much 

greater as would be expected from the complex 

intervening geologic record indicated by field evidence. Obviously the age of the older intrusives is 

a critical minimum age for the Ox Frame Volcanics. 

(See following table.)

Radiometric ages of Mesozoic igneous rocks that cut the Ox Frame Volcanics

Sample Field No. Rock type Locality

Apparent age, in millions of years, as 

determined by indicated method

K-Ar 

on biotite

Rb-Sr Pb-alpha on 

zircon

1 T169 Alkali granite ..................................................................................SW% sec. 35, 1 55 ............ 2 150±20

T. 17 S., R. 11 E.

2 RM-2-65 ................ do ......................................................................................Very near sample 1 locality.............. ............ 3 140±14 ............

3 RM-10-63 Monzonite border phase of quartz monzonite porphyry SE% sec. 8, ............ * 130±6 ............

intrusion. T. 18 S., R. 11 E.

4 T400 Quartz monzonite of Harris Ranch ........................................NE% sec. 13, ............ ............ 2 190±20

T. 18 S., R. 11 E.

1 S. C. Creasey (written commun., 1964). 

2 T. W. Stern (written commun., 1965, 1966).

3 Whole-rock analysis, from Damon (1966, p. 23).

4 Isochron age based on analyses of whole rock, potassium feldspar, and plagioclase; from Damon (1966, p. 23).
D14 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

The disparity in apparent age of the alkali granite 

of sample 1 is undoubtedly due to Laramide 

metamorphism. The biotite in this rock is in recrystallized leafy form, and hence its K-Ar age should 

and probably does indicate the time of metamorphism.

As the Pb-alpha and Rb-Sr clocks are less readily 

reset by metamorphic events, the older intrusives 

are here assigned to the Jurassic (samples 1-3) and 

Triassic(?) (sample 4). The Ox Frame, which predates all these rocks, is assigned to the Triassic.

One attempt has been made to date the Ox Frame 

Volcanics directly, but the result is misleading. R. L. 

Mauger (in Damon, 1965, p. 31) obtained a wholerock Rb-Sr age of 93±29 m.y. on rhyolitic welded 

tuff of the Ox Frame from the rim of Esperanza 

open-pit mine. This age is incompatible with Rb-Sr 

and Pb-alpha ages of post-Ox Frame intrusive rocks 

just discussed in that, about a mile west of the 

mine, the tuff is cut by quartz monzonite whose Pbalpha age is 190 m.y. The tuff sample analyzed came 

from a mineralized area in which potassium 

metasomatism has taken place (Lynch, 1966). As 

rubidium accompanies potassium geochemically, one 

would expect the Rb-Sr age to be somewhere between the true age of the rock and the age of 

mineralization (dated radiometrically at about 60 

m.y.).

The Ox Frame is correlated with the Mount 

Wrightson Formation in the Santa Rita Mountains. 

Both formations are thick sequences of rhyolitic to 

andesitic flows and pyroclastic rocks that contain 

intercalated beds of conglomerate, sandstone, and 

quartzite. Both are cut by intrusive rocks dated 

radiometrically as Jurassic (by several methods) 

and Late Triassic (by the Pb-alpha method). The 

Pb-alpha age of 220 m.y. reported by Drewes (1968, 

p. C7) for the middle member of the Mount Wrightson Formation is compatible with all other geologic 

and radiometric evidence. However, the Pb-alpha 

method is not very precise, and I regard this date as 

only a first approximation of the true age of the Ox 

Frame Volcanics.

TASCUELA REDBEDS

NAME, TYPE SECTION, AND GENERAL DESCRIPTION

The name Tascuela Redbeds, of Thorns (1966, 

1967), is here adopted for the thick sequence of 

sedimentary rocks that overlies the Ox Frame Volcanics and underlies the Stevens Mountain Rhyolite 

in the western foothills of the Sierrita Mountains. 

The type section is along Tascuela Canyon, mostly 

in the Wfa sec. 13, T. 18 S., R. 10 E., but extending 

a short distance into sec. 18, T. 18 S., R. 11 E.

In the type section the formation reaches a maximum thickness of about 2,000 feet and comprises 

two main units. A coarse basal conglomerate, as 

much as 200 feet thick but absent locally, is overlain 

by 1,600-1,800 feet of maroon shale and argillite 

that contains sparse beds of sandstone, quartzite, 

and sandy limestone.

The basal conglomerate consists of granules, 

pebbles, cobbles, and boulders in a maroon silty to 

sandy mudstone matrix. The larger fragments include a wide variety of volcanic rocks, evidently derived from the underlying Ox Frame Volcanics, and 

also pieces of quartzite, chert, and limestone of 

Paleozoic aspect. Upward in the basal unit, the conglomerate interbeds with maroon shale, and the 

particles decrease in grain size, until the unit becomes a monotonous sequence of maroon shale and 

argillite that contains a few blue-gray to green beds. 

The lower part of the sequence contains a few 

ledge-forming beds of yellowish-gray fine- to 

medium-grained quartzite, and the upper part contains a few thin beds of grayish-purple to grayishred medium-grained volcanic graywacke. The 

graywacke consists of angular to rounded grains of 

quartz, feldspar, and rock fragments in a clay 

matrix. About 15 percent of the rock is matrix and 

at least 25 percent is feldspar and microcrystalline 

volcanic rock fragments.

The type section of the Tascuela Redbeds is near 

the south end of the known outcrops of the formation. The area of outcrop trends northwest and is 

about 61/2 miles long and as much as half a mile 

wide. The formation passes beneath overlapping 

Demetrie Volcanics to the southeast and overlapping 

pediment gravel to the northwest. There is no 

evidence of pinching out in either direction. Owing 

to increasing metamorphism toward the north, the 

characteristic red color is gradually lost and the 

northern outcrops are gray slate, phyllite, and finegrained mica schist. Thorns (1966, p. 100-101) 

reported conglomerate beds that contain elongate 

granitic cobbles and boulders near the top of the 

formation in the metamorphosed area. Except for 

the quartzite beds already mentioned, all parts of 

the formation are nonresistant to erosion and are 

inconspicuous topographically.

The Tascuela overlies the Ox Frame Volcanics 

with disconformable contact, as described in the 

section on the Ox Frame (p. D13), and it is overlain 

with apparent conformity by the Stevens Mountain 

Rhyolite. According to Thorns (1966, p. 101-102),

The lowest, distinct unit in the Stevens Mountain Rhyolite is 

a grayish-pink to light-red conglomerate comprised of wellrounded pebbles and cobbles of light-colored volcanic rocks in
SIERRITA MOUNTAINS, PIMA COUNTY D15

a tuffaceous matrix. The shales, argillites and slates of the 

Tascuela red beds grade upward into this conglomerate 

through an interval which ranges from a few feet to a few 

tens of feet in thickness. The interval usually contains at 

least one recognizable coarse tuff or sandy tuff bed. To complete the transition, lenticular shales, one or two feet thick, 

are interbedded in the lowest part of the conglomerate.

At some places, the Stevens Mountain Rhyolite is 

missing and the Tascuela is overlain with angular 

unconformity by the Demetrie Volcanics. PreDemetrie erosion removed all but remnants of the 

Stevens Mountain Rhyolite and cut deeply into and 

even through the Tascuela Redbeds. Red shale fragments are common in the basal conglomerate of the 

Demetrie where it lies directly on the Tascuela Redbeds.

AGE AND CORRELATION

Field evidence fixes the Tascuela Redbeds as postOx Frame Volcanics and pre-Stevens Mountain 

Rhyolite. Its age with respect to the Triassic(?) and 

Jurassic intrusive rocks in the Sierrita Mountains 

is not directly determinable, as the Tascuela is 

nowhere in contact with these rocks. Thus, direct 

geologic evidence provides only broad limits within 

which age assignment can be made.

The general lithology of the Tascuela resembles 

that of the lower part of the Bisbee Group in the 

latter's type area. The limestone-bearing conglomerate suggests the Glance Conglomerate, and the 

overlying red predominantly fine-grained detrital 

beds suggests the Morita Formation. Correlation 

with these formations is permitted by local field 

evidence but raises a number of problems when 

regional stratigraphic relations are considered. P'or 

instance, much evidence shows that the type Bisbee 

changes facies toward the northwest and grades 

into different-appearing arkosic rocks such as the 

Amole Arkose of W. H. Brown (1939) in the 

Tucson Mountains and the Angelica Arkose in the 

Helmet Peak area. Close to and west of the arkosic 

facies, the Tascuela resembles typical Bisbee; for 

them to be equivalent, however, would seem to require major fault displacement of the Bisbee, which 

would be entirely hypothetical in the present state 

of knowledge. Furthermore, the beds of volcanic 

graywacke and conglomerate that contain granite 

boulders in the upper part of the Tascuela are unlike typical Bisbee. Of more significance is the apparent upward gradation of the Tascuela into the 

Stevens Mountain Rhyolite. Elsewhere in southeastern Arizona, Cretaceous volcanic rocks of postBisbee age are separated from the Bisbee by a 

conspicuous unconformity. This unconformity is unquestionably represented in the Sierrita Mountains

by the unconformity at the base of the Demetrie 

Volcanics a surface of considerable relief cut in a 

wide variety of formations, including the Stevens 

Mountain Rhyolite, Tascuela Redbeds, and Angelica 

Arkose (which is a Bisbee equivalent). Either the 

Stevens Mountain Rhyolite represents Early 

Cretaceous volcanic activity not evident elsewhere 

in Arizona, or the Tascuela is not a Bisbee 

equivalent.

The Tascuela could be equivalent to the lower 

part of the Canelo Hills Volcanics whose basal unit, 

according to Hayes, Simons, and Raup (1965), is 

as much as 2,000 feet thick and consists of interbedded red siltstone, shale, sandstone, conglomerate, 

and rhyolitic to latitic volcanic rocks. The unit 

shows much lateral variation, but sedimentary rock 

is generally predominant in it, particularly in the 

lower part. Although no volcanic rocks have been 

found in the Tascuela, the presence of graywacke 

beds containing volcanic detritus in the upper part 

of the formation suggests contemporaneous volcanic 

activity in the vicinity. If the Tascuela is correlated 

with the basal unit of the Canelo Hills Volcanics, 

there is no problem with the distribution of 

Cretaceous facies and the Stevens Mountain Rhyolite simply represents the upper rhyolitic part of the 

Canelo Hills Volcanics.

Other red-bed units which resemble the Tascuela 

in lithology but which generally contain volcanic 

rocks are the Recreation Redbeds of W. H. Brown 

(1939) in the Tucson Mountains, the Gardner 

Canyon Formation in the Santa Rita Mountains, 

and the Rodolfo Formation in the eastern pediment 

of the Sierrita Mountains. The Recreation, long considered Cretaceous, is evidently Jurassic or older 

inasmuch as it is cut by porphyry whose K-Ar age 

is 150±5 m.y. (Damon, 1967, p. 69-70). Zircon 

from a lava flow in the Gardner Canyon Formation 

has a Triassic Pb-alpha age (Drewes, 1968, p. C7), 

and the Rodolfo Formation is Triassic as pointed 

out on page D22. Available geologic and radiometric 

age data suggest that all these formations are correlative. Therefore, the Tascuela Redbeds are here 

assigned to the Triassic.

RODOLFO FORMATION

NAME AND TYPE LOCALITY

The name Rodolfo Formation is here applied to 

the sequence of red beds and intercalated volcanic 

rocks that overlies Paleozoic rocks on the northeast 

side of the Sierrita Mountains. The composite type 

locality of the formation is east, southeast, and west 

of Helmet Peak (fig. 5). The most complete section 

is just south of Rodolfo Wash along the east side of
DIG MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

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D18 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

Helmet Peak where 2,400 feet of beds crops out but 

where neither the base nor the top is exposed. To 

the west, the lowest beds are in fault contact with 

Permian limestone and quartzite, and to the south 

the formation is overlain by lower Tertiary siliceous 

tuff known locally as the biotite rhyolite; this upper 

contact is discordant and is either an angular unconformity (Cooper, 1960, p. 76) or an intrusive 

contact (Kinnison, 1966, p. 281).

Rocks which I interpret to be the basal unit of 

the Rodolfo Formation are exposed at the south end 

of the Helmet Peak west of the fault between 

Paleozoic rocks and Rodolfo Formation. Here the 

Permian Rainvalley Formation is overlain unconformably by conglomerate and sandstone made up 

mostly of chert, limestone, and quartzite fragments 

from the underlying rocks. All rocks are in a very 

tight diapiric anticline, and no angular discordances 

in bedding are apparent (fig. 5). The'conglomeratic 

unit is separated, by a short covered interval, from 

outcrops of red beds typical of the Rodolfo and is 

evidently the basal unit of that formation.

In support of this interpretation, the same relationship of Rainvalley Formation, conglomeratic 

unit, covered interval, and Rodolfo red beds is found 

at several places in a small faulted area half a mile 

north-northwest of Helmet Peak. Of course, faulting in the covered intervals has possibly cut out all 

but the basal conglomerate of some unknown unit 

and juxtaposed the conglomerate and the Rodolfo 

Formation at places nearly a mile apart. Faults 

with the location and magnitude required are unsupported by direct field evidence, and therefore 

the simplest interpretation is accepted that the 

conglomerate is indeed the basal conglomerate of 

the Rodolfo Formation.

The top of the Rodolfo Formation is exposed about 

4,000 feet west of Helmet Peak. Here a slightly 

metamorphosed gray facies of the Rodolfo is overlain by the Whitcomb Quartzite for about threefourths mile. There is little angular discordance 

between the formations but, on the other hand, no 

indication of a gradation between them. The Whitcomb Quartzite lies with sharp gently undulating 

contact on an andesite unit and at one place on a 

sedimentary unit of the Rodolfo.

GENERAL DESCRIPTION, DISTRIBUTION, AND THICKNESS

The Rodolfo Formation in the composite type 

locality is at least 2,400 feet thick and comprises 

three members as follows: (1) a basal conglomeratic member, as much as 70 feet thick, which lies 

unconformably on Paleozoic rocks and consists 

mostly of fragments derived from these rocks; (2)

a middle member of siltstone, less abundant sandstone, and conglomerate, at least 1,300 feet thick, 

and (3) an upper volcanic member, ranging from 

200 to nearly 1,100 feet in thickness, which is in 

interfingering contact with the middle member and 

consists mostly of andesitic breccia but contains 

some flows and a thick red-bed unit. The upper 

member is overlain with sharp disconformable contact by the Whitecomb Quartzite.

The formation is nonresistant to erosion and 

crops out discontinuously on pediment surfaces and 

on the lower slopes of hills supported by more resistant rocks. In the exposures east of Helmet Peak, 

the siltstone member and much of the volcanic member are grayish red and distinctive, even from a 

distance. Elsewhere the andesitic units are medium 

gray to dark greenish gray, and the sedimentary 

units are mostly medium gray and commonly have 

a green or brown hue. The change in color of the 

sedimentary units from red to gray is strictly 

gradational at places and evidently is due to thermal 

metamorphism. Elsewhere hydrothermal processes 

were involved, as shown by the fact that the change 

in color begins along fractures and finally pervades 

the whole rock.

Most exposures of the Rodolfo are in the vicinity 

of Helmet Peak. Within about a mile to the west 

and northwest of the peak, parts of the formation 

crop out in fault blocks and as the predominant 

material in a large breccia pipe that cuts the Whitcomb Quartzite and Angelica Arkose (fig. 5).

To the northeast, the formation underlies Quaternary alluvium for a long distance, as shown by 

drilling. Siltstone and volcanic pebble conglomerate 

at the Mission mine were named the Papago Formation and the Kino Formation, respectively, by 

Kinnison (1966, p. 283) and were assigned an early 

Tertiary age on the basis of district and regional 

geologic considerations. These rocks are lithologically similar to the middle siltstone member of the 

Rodolfo Formation, and they immediately overlie 

altered Paleozoic rocks; I believe, therefore, that 

they represent part of the Rodolfo. Recently what 

seems to be part of the same member has been exposed beneath alluvium in the Twin Buttes open-pit 

mine 5.5 miles southeast of Helmet Peak (fig. 2).

West of the Twin Buttes mine, within the contact metamorphic aureole of a large granodiorite 

intrusive of Paleocene age, metamorphosed Rodolfo 

Formation overlies Permian tactite, marble, and 

quartzite in a small much-faulted area near the old 

Senator Morgan mine (fig. 2). The Rodolfo here 

consists of foliated and hornfelsed siltstone and 

minor sandstone and conglomerate. The conglomer-
SIERRITA MOUNTAINS, PIMA COUNTY D19

ate contains pebbles of calc-silicate hornfels, recrystallized chert, and quartzite, which evidently 

represent pieces of Paleozoic rock recrystallized 

and silicated in Paleocene time.

Also assigned to the Rodolfo (?) is about 2,000 

feet of beds exposed over an area of one-fourth 

square mile at the head of Ash Creek in the Sierrita 

Mountains (fig. 4). These rocks are mostly finegrained epidotic hornfels derived from argillaceous 

siltstone and fine-grained sandstone. There are also 

beds of medium-grained sandstone and, in the upper part, some conglomerate and breccia composed 

of volcanic material. Correlation with Rodolfo is 

based entirely on lithology inasmuch as the unit is 

everywhere bounded by fault and intrusive contacts. 

The intrusive rocks that cut the unit are those with 

radiometric ages between 140 and 190 m.y. (p. D13, 

table, Nos. 1, 2, 4).

LITHOLOGY 

BASAL CONGLOMERATIC MEMBER

The basal member of the Rodolfo Formation near 

Helmet Peak comprises as much as 70 feet of conglomerate and sandstone lying unconformably on 

the Rainvalley Formation of Permian age. The 

principal outcrops are shown in generalized form 

in figure 5. Field relations already discussed indicate that these rocks are overlain by the siltstone 

member of the Rodolfo although the actual contact 

is concealed by Quaternary alluvial deposits.

The lower few feet to several tens of feet of the 

member is conglomerate consisting of ill-sorted 

pebbles and cobbles of chert, quartzite, and limestone in a light-gray to grayish-orange sandy matrix. All the pebbles and cobbles probably came from 

the underlying Paleozoic formations; most of them 

are well rounded, but some are subangular. Fragments as much as 8 inches in diameter are present, 

but those of pebble size are most abundant. The 

matrix commonly makes up about half the rock 

and consists of granules and coarse sand cemented 

by calcite. The matrix grains consist of quartz, 

quartzite, chert, and sparse nearly white argillized 

grains that are altered beyond recognition.

Coarse-grained sandstone, lithologically similar 

to the matrix material, occurs as lenses in the conglomerate and as overlying beds. In the thickest 

exposed section of the member, about 3,000 feet 

northwest of Helmet Peak, the conglomerate is 25 

feet thick and is overlain by about 45 feet of 

grayish-orange coarse-grained clean sandstone and 

medium-grained sandstone having an abundant silty 

matrix. The sequence indicates an upward gradation from conglomerate toward rock resembling the 

overlying siltstone member in texture.

MIDDLE SILTSTONE MEMBER

The middle member of the Rodolfo Formation 

consists of siltstone having widely dispersed but 

generally subordinate interbeds of mudstone, sandstone, and conglomerate and at least one thin bed 

of impure limestone. In the type locality east of 

Helmet Peak, these rocks are red and show wellpreserved sedimentary textures. Elsewhere, they 

are generally gray, and their sedimentary textures 

are masked, but rarely obliterated, by metamorphism 

and hydrothermal alteration. The total thickness of 

the member is not known inasmuch as there are 

no complete sections and no persistent stratigraphic 

markers, by which the total section could be pieced 

together from separated areas. The incomplete section on the east side of Helmet Peak is about 1,300 

feet thick and contains much volcanic detritus. A 

thick underlying sequence that contains less volcanic detritus has been found by drilling in the 

alluvium-covered area to the northeast. Therefore, 

the total thickness of the member is probably several thousand feet. It is the only part of the formation found at most places.

Indistinctly bedded, nonfissile siltstone predominates in the middle member of the Rodolfo Formation. The siltstone and associated mudstone near 

| Helmet Peak are grayish-red soft rocks in which 

I sparse sand-size grains and tiny glistening flakes 

of mica are commonly discernible with a hand lens. 

Detrital constituents identified in thin section include quartz, muscovite, potassium feldspar, plagioclase, and minute rock fragments. The recognizable 

grains lie in an abundant argillaceous matrix which 

is stained with red iron oxide and which locally 

contains a little calcite. There is an upward gradation from normal siltstone rich in recognizable 

quartz to volcanic siltstone in which grains of plagioclase and andesite are almost the only recognizable 

constituents and make up as much as half the rock. 

The highly volcanic facies is found in the upper 

part of the member, which interfingers with extrusive andesite breccia.

With metamorphism and hydrothermal alteration 

the siltstone was changed from red to gray and commonly became harder. The chief difference apparent in thin section is the presence of varying 

amounts of very fine grained chlorite, biotite, sericite, and feldspar instead of the hematite-rich matrix material. Near the Mission and Pima mines, the 

siltstone is represented by medium-dark-gray to 

light-greenish-gray or brownish-gray hornfels. Thin
D20 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

sections of these rocks, including many sections 

kindly lent me by J. H. Courtright of the American 

Smelting and Refining Co., show that the darker 

colors are due to relatively abundant biotite and 

that the rocks are generally richer in quartz than 

those near Helmet Peak. The biotite is an alteration 

product, whereas most of the quartz seems to be 

primary detrital grains. Because detrital quartz increases in abundance downward in the siltstones 

east of Helmet Peak, I believe that a stratigraphically lower part of the same sequence occurs in the 

Mission mine area.

Lenses of pebble conglomerate a few inches to a 

few feet thick are widely distributed in the siltstone 

member. Those in the lower part of the member 

near Helmet Peak are pale red to grayish red and 

consist of subrounded to well-rounded granules and 

pebbles of chert and rhyolite in a matrix of coarse 

sand cemented with silica. Paleozoic rocks are an 

obvious source of the chert pebbles. The Ox Frame 

Volcanics is the probable source of the rhyolite pebbles, which comprise welded tuff, nonwelded mixed 

tuff, and lava having the sieve-textured groundmass 

found in that formation. An unusually thick and 

persistent conglomerate of this type near the bottom 

of the member wraps around the anticlinal axis 

several thousand feet southeast of Helmet Peak (fig. 

5). Unmapped conglomerates of the same type crop 

out north and northwest of the peak.

Pebbles of chert and rhyolite become increasingly 

scarce in the conglomerates stratigraphically higher 

in the member, and pebbles and cobbles of andesite 

become increasingly abundant. The andesitic conglomerate lenses one of which, 1,200 feet eastsoutheast of Helmet Peak, is shown in figure 5 

tend to be coarser, thicker, and more abundant than 

the lenses lower in the section. The andesitic conglomerates are evidently of intraformational derivation and are as much as 30 feet thick where the 

siltstone member interfingers with the upper volcanic member of the formation.

Pebble conglomerate also occurs near the Mission 

and Pima mines in the altered gray part of the siltstone member referred to by Kinnison (1966) as 

the Papago and Kino Formations. The conglomeratic texture here is commonly obscure in outcrop 

but is readily apparent in moistened drill cores, 

polished surfaces, and thin sections. Pebbles consist 

of recrystallized chert, volcanic rock, and apparently 

intraformational siltstone and sandstone. Pebbles 

of rhyolitic rock with relict vitroclastic texture occur in both the Papago and Kino Formations of 

Kinnison (1966).

Sandstone is more abundant than conglomerate

in the siltstone member but is analogous to it in 

lithology, mode of occurrence, and distribution. In 

the lower part of the section east of Helmet Peak, 

the sandstone is grayish red, fine to coarse grained, 

and looks like arkose in hand specimen. Many 

grains that resemble argillized feldspar macroscopically are seen in thin section to be devitrified rhyolitic rocks like those in the associated conglomerate. 

Other grains include quartz, sodic plagioclase, potassium feldspar, chert, siltstone, and sandstone. Most of 

the sandstone beds in the lower part of the member 

are quite clean the grains being coated with finegrained hematite, and the interstices being filled 

with chemically deposited quartz. Sandstone beds 

in the upper part of the sequence are generally of 

the graywacke type, and locally grade upward into 

siltstone. Grains of andesite and plagioclase predominate, and very fine grained slightly calcareous 

hematite-rich matrix makes up a large part of the 

rock. Modal analyses illustrating the two kinds of 

sandstone follow.

Percentage composition of two kinds of sandstone

Rhyolitic arenite 

(specimen T307)

Andesitic graywacke 

(specimen T310)

Chemical cement ........................

.................. 21.7

.................. 5.0

.................. 4.5

.................. .3

.................. 49.3

.................. 19.2

0.3

16.5

.9

40.0

42.3

Total...................................... ..............100.0 100.0

Thin sandstone interbeds in the siltstone member 

near the Mission and Pima mines, though gray and 

altered, were probably similar in original lithology 

to the sandstones just described. Most were of the 

graywacke type but were not andesitic. Indeed these 

rocks generally contain more detrital quartz than 

do the sandstones in the Helmet Peak area.

The only limestone found in the middle siltstone 

member of the Rodolfo Formation is a lens about 

a foot thick near the bottom of the member 2,400 

feet southeast of Helmet Peak. The lens can be 

traced 150 feet along the strike and is faulted off 

at its thickest point. The limestone is brown and 

contains much dispersed argillaceous and silty material. Alteration of this readily metamorphosed 

impure limestone could cause localized patches of 

calc-silicate rock.

UPPER VOLCANIC MEMBER

The upper volcanic member southeast of Helmet 

Peak is about 1,100 feet thick and consists of a thick 

lower unit of andesite breccia followed in turn by
SIERRITA MOUNTAINS, PIMA COUNTY D21

about 250 feet of red beds, 75 feet of laharic breccia 

and conglomerate, and 100 feet of andesite flows 

and intercalated red beds (fig. 5). These units are 

overlain with angular unconformity by lower Tertiary rhyodacitic tuff known locally as the biotite 

rhyolite; they are overlapped by the tuff a short 

distance to the northeast beneath the alluvial cover, 

as shown by drill-hole data. West of Helmet Peak, 

the volcanic member of the Rodolfo is less than 600 

feet thick and is composed entirely of andesite breccia. It is overlain unconformably by the Whitcomb 

Quartzite and is faulted off at both ends. This 

locality and the one southeast of Helmet Peak are 

the only places where the volcanic member is now 

known.

The lower andesite breccia unit has an interfingering contact with the underlying siltstone member 

but is not perceptibly bedded or layered. It consists 

of unsorted subangular fragments of andesite as 

much as several inches in diameter in a fine-grained 

clastic matrix compositionally like the fragments. 

The least altered parts are dark gray to dark grayish 

red on fresh fracture; more altered parts are lighter, 

and the most altered parts are yellowish gray and 

felsic looking. The rock west of Helmet Peak is gray 

and strongly magnetic, whereas that southeast of 

the peak is red and virtually nonmagnetic because 

magnetite has been converted to hematite by oxidation.

In hand specimen, the andesite fragments in the 

breccia are characterized by phenocrysts about 1 

mm long of plagioclase and mafic minerals in an 

abundant aphanitic groundmass; the matrix looks 

like the fragments except that many phenocrysts 

in it are broken. Thin sections examined are much 

propylitized but indicate the following initial composition: 25-35 percent plagioclase phenocrysts, 

about 10 percent mafic phenocrysts, 2-3 percent 

accessory magnetite and apatite, and 50-60 percent 

cryptocrystalline groundmass that shows tiny microlites of plagioclase locally. The plagioclase is mostly 

albitized but contains scarce remnants of unreplaced 

andesine. The primary mafic silicates are hornblende 

and less abundant augite occurring as kernels in the 

hornblende and as discrete grains. The igneous texture and minerals are largely destroyed in the 

much-altered yellowish-gray facies, which consists 

of euhedral to subhedral crystals of dusty albite in 

a microcrystalline groundmass of anhedral epidote, 

quartz, sericite, and opaque grains. Though quartz 

makes up nearly 20 percent of these rocks, a rough 

computation based on modal analysis indicates an 

overall silica content near 56 percent, about the same 

as Nockolds' (1954) average andesite or latite.

A small amount of probably extrusive dacitic and 

rhyolitic rock has been noted in the andesite breccia 

member. In the uppermost part of the siltstone 

member east of Helmet Peak, there is a small unmapped tongue of rock which superficially suggests 

loosely packed andesite conglomerate but which has 

a matrix of pale-red porphyry containing phenocrysts of quartz, feldspar, and altered biotite. In thin 

section, the porphyry shows saussuritized plagioclase, resorbed quartz, and scarce pseudomorphs of 

epidote, hematite, muscovite, and quartz after bent 

biotite books, in a cryptocrystalline felsic groundmass. As this interstitial porphyry is petrographically different from the fragments, the rock is 

interpreted as a dacitic flow loaded with andesite inclusions. A few hundred feet higher in the section, 

in the lower part of the andesite breccia unit, there 

are two poorly exposed thin layers of pale-red 

tuff aceous-looking rock in which scattered rock chips 

and round quartz grains are discernible macroscopically. As seen in thin section, the quartz grains 

have the form of resorbed crystals and angular 

crystal fragments indicating that the rock is indeed a silicic tuff. The rock fragments include sandstone, siltstone, and cryptofelsite. The remainder of 

the rock is a confused aggregate of secondary clays, 

calcite, epidote, and quartz. Large blocks of very 

similar rocks containing only part argillized and 

calcitized sanidine occur in andesite breccia in the 

breccia pipe just west of Helmet Peak.

About 250 feet of grayish-red sandstone containing at least one thin bed of gray limestone overlies 

the andesite breccia unit southeast of Helmet Peak. 

The sandstone is fine to medium grained, well sorted, 

and locally laminated and cross laminated. The limestone is nowhere actually exposed, but it is found as 

abundant float and as tabular pieces mixed with red 

sandstone fragments in the dump of an old prospect 

pit now virtually filled by sheet wash. The limestone 

bed, apparently 4 inches or less in thickness, undoubtedly represents a local pond deposit intercalated with the sandstone. It is light olive gray and 

silty looking in hand specimen. In thin section, the 

main impurities are seen to be glass shards and 

pumice fragments containing tiny crystals and 

crystal fragments of andesine and clinopyroxene. 

Evidently volcanic ash fell into the accumulating 

lime mud and was protected from the alteration that 

affected other parts of the formation. The index of 

refraction of the glass (1.521-1.539) suggests a 

silica content near 60 percent, midway between the 

average andesite and average rhyodacite of Nockolds (1954).
D22 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

Overlying the red sandstone unit just described 

is about 75 feet of andesitic conglomerate and laharic breccia. The breccia typically consists of unsorted and unbedded material ranging in size from 

particles of mud to subangular cobbles, whereas the 

conglomerate is fairly well sorted and distinctly 

bedded pebble and cobble conglomerate having a 

sandy matrix. The constituents are mostly of intraformational source but include some pebbles and 

cobbles of grayish-red andesitic porphyry and white 

plagioclase phenocrysts as much as 15 mm in diameter a distinctive rock not found elsewhere in the 

Rodolfo Formation.

The top 100 feet of the Rodolfo Formation southeast of Helmet Peak consists of at least two andesite flows separated by a few feet of red beds. The 

flows are grayish-red propylitized rocks similar in 

lithology to fragments in the andesite breccia unit 

at the base of the volcanic member. The flows are 

not brecciated, however, and at least locally have 

chilled amygdaloidal tops. The amygdules, which are 

circular to elliptical in outline and as much as 10 

mm in maximum diameter, are composed of white 

quartz, sericite, and calcite. Like other red rocks 

in the Rodolfo Formation, the color of the flows is 

due to disseminated fine-grained hematite.

CONTACTS

The basal contact of the Rodolfo Formation is an 

unconformity that, in small exposures near Helmet 

Peak, truncates tens of feet of beds in the underlying Rainvalley Formation of Permian age. The contact is easy to recognize in outcrop inasmuch as the 

Rodolfo Formation has a basal conglomerate composed of fragments of Paleozoic rock overlain by 

red beds whose lithology is different from that of 

any underlying formation. Except in the small outcrops near Helmet Peak (fig. 5), exposed contacts 

of the Rodolfo Formation with Paleozoic rocks are 

faults, many of which have been healed and obscured by alteration.

The Rodolfo Formation is overlain by the Whitcomb Quartzite. The contact, where exposed, is an 

undulating surface that apparently was eroded down 

to andesite breccia near the bottom of the upper 

volcanic member of the Rodolfo. Along the strike 

the contact bevels an overlying tongue of siltstone 

and graywacke of the Rodolfo, as shown by geologic 

mapping and exploratory drilling records. Because 

the exposed contact is slightly sheared and no basal 

conglomerate or recognizable fragments of Rodolfo 

have been found in the Whitcomb, the contact could 

be interpreted as a fault contact. However, the

shearing is no greater than that commonly evident 

between units of contrasting lithology in this area, 

and so the contact is best interpreted as a disconformity along which trivial slippage has taken place.

AGE AND CORRELATION

The Rodolfo Formation obviously postdates the 

Permian Rainvalley Formation, which it overlies 

unconformably, and also the Ox Frame Volcanics of 

the Sierriia Mountains and the Mount Wrightson 

Formation of the Santa Rita Mountains, which 

yielded the rhyolite fragments in conglomerates and 

sandstones in the lower part of the Rodolfo. The 

Rodolfo predates the Whitcomb Quartzite and Angelica Arkose, which overlie it. If the hornfels at 

the head of Ash Creek (fig. 4) is truly Rodolfo, the 

formation must also predate the Jurassic and Triassic(?) intrusive rocks. Thus, field evidence indicates that the Rodolfo Formation is of Triassic age 

but is younger than the Ox Frame Volcanics.

Red beds and volcanic rocks correlated with the 

Rodolfo Formation on the basis of lithology and 

stratigraphic position consist of the Tascuela Redbeds, the Recreation Redbeds and overlying volcanic 

rocks of W. H. Brown (1939) in the Tucson Mountains, the Gardner Canyon Formation in the Santa 

Rita Mountains, the lower part of the Canelo Hills 

Volcanics, and the Walnut Gap Volcanics near the 

Little Dragoon Mountains. The upper volcanic member of the Rodolfo may be correlative with the 

Stevens Mountain Rhyolite.

Radiometric data bearing on the age of the 

Rodolfo Formation and its correlatives are consistent with each other and with the geologic field 

relations. The K-Ar age of 150 m.y. obtained by 

Damon (1967, p. 69) for an intrusive porphyry cutting the Recreation Redbeds indicates that the Recreation is Jurassic or older. The K-Ar age of 173 

m.y. reported by Hayes, Simons, and Raup (1965, 

p. M7) for a rhyolite tuff unit high in the Canelo 

Hills Volcanics indicates that the lower part of the 

formation correlated with the Rodolfo cannot be 

younger than Early Jurassic. The Pb-alpha age of 

192 m.y. reported by Drewes (1968, p. C7) for a 

dacite flow in the Gardner Canyon Formation is a 

Triassic age. The oldest intrusive cutting the Rodolfo (?) Formation in Ash Creek, the quartz monzonite of Harris Ranch, has a Pb-alpha age 

calculated as 187 m.y. and presented «by T: W. Stern 

(written commun., 1966) as 190±20 m.y. Although 

the Pb-alpha method is not very precise, the Rodolfo Formation is here assigned to the Triassic.
SIERRITA MOUNTAINS, PIMA COUNTY D23

UPPER TRIASSIC OR LOWER JURASSIC ROCKS

STEVENS MOUNTAIN RHYOLITE

NAME, TYPE LOCALITY, AND GENERAL DESCRIPTION

The name Stevens Mountain Rhyolite as defined 

by Thorns (1966, 1967) is here adopted for the 

rhyolitic volcanic sequence that overlies the Tascuela 

Redbeds and underlies the Demetrie Volcanics on 

the west side of the Sierrita Mountains. The type 

locality is on Stevens Mountain in sec. 34, T. 17 S., 

R. 10 E. Known exposures are in a belt as much as 

half a mile wide and about 5 miles long that extends 

northwest and southeast of this locality.

The Stevens Mountain Rhyolite consists of a basal 

volcanic conglomerate and overlying rhyolitic crystal tuffs, welded tuffs, and flows. Weathered surfaces 

of these rocks are generally pinkish or yellowish 

gray and contrast with the darker colors of the 

Tascuela Redbeds and Demetrie Volcanics. Lenticular interbeds of shale, siltstone, sandstone, and 

orthoquartzite are scarce. The formation as a whole 

is resistant to erosion and forms the crest of Stevens Mountain and some lower hills and ridges. Its 

contacts commonly are expressed by breaks in 

topography and by the contrasting colors of the 

formations in outcrops. The thickness of the Stevens 

Mountain, owing to pre-Demetrie erosion, ranges 

from 0 to about 2,000 feet.

The basal conglomerate in the type* locality is 

light gray to pale red and is composed of wellrounded pebbles and cobbles of felsic volcanic rock 

in a tuffaceous matrix. Many of the larger clasts, 

though well rounded, lithologically resemble overlying extrusive rocks. The matrix consists mostly of 

grains of quartz and feldspar commonly 1-3 mm in 

diameter, but it contains scattered angular fragments of rhyolitic rock as much as 1 cm in maximum 

dimension. Thorns (1966, p. 105-106) reported that 

the small rock fragments have nondefinitive microcrystalline texture in thin section and that the principal matrix constituents are embayed grains and 

angular chips of quartz; euhedral crystals and crystal fragments of anorthoclase, orthoclase, and moderately sericitized plagioclase; and subordinate 

interstitial very fine grained sericitic material. The 

conglomerate grades upward into overlying crystal 

tuff that is practically identical with the conglomerate matrix in lithology.

The basal conglomerate unit of the Stevens Mountain Rhyolite is at most about 100 feet thick at the 

type locality but, according to Thorns (1966, p. 109), 

is about 1,000 feet thick in outcrops a mile or so to 

the southeast. Here the lower 200 feet of conglomerate is dark blue or maroon and contains a few 

thin beds of maroon shale and dark-purple siltstone.

The upper part is light gray or pink and is like the 

upper part of the basal conglomerate in the type 

locality except for some interbeds of shale, tuff, and 

eutaxitic felsite and a capping maroon sandstone 

bed 6-10 feet thick. Detailed stratigraphic relations 

between the two localities are indeterminate because of intervening Demetrie Volcanics that fill an 

ancient valley.

The rhyolitic tuffs and flows that overlie the conglomerate and make up most of the formation are 

well-indurated light-colored rocks. Weathered surfaces are generally grayish pink, grayish orange, or 

yellowish gray; fresh- surfaces range from pale red 

through pale red purple to grayish blue. Phenocrysts, 

generally 1-3 mm in diameter of orange anorthoclase, vitreous gray quartz, chalky plagioclase, and 

locally clear orthoclase and altered biotite are discernible in an aphanitic felsic groundmass. Phenocrysts are generally sparse in units which lack discernible pyroclastic texture and which are regarded 

as flows, but they make up about 25 percent of some 

eutaxitic welded tuffs and an even higher percentage 

of some crystal tuffs. Individual volcanic units are 

commonly 100-500 feet thick.

Lenticular interbeds of quartzite a few tens of feet 

thick have been noted in the volcanic rocks in the 

type locality and elsewhere. A typical specimen was 

described by Thorns (1966, p. 113) as a fine- to 

medium-grained nearly equigranular rock that consists almost entirely of subangular to subrounded 

grains of quartz. A few grains of microcline, anorthoclase, and plagioclase are also present. Sericite 

and microcrystalline quartz form the scant matrix. 

Although the rock is well indurated, individual grain 

outlines are maintained, and sutured or intergrown 

contacts are lacking. The quartzite is unquestionably 

of sedimentary origin.

In addition to the quartzites, Thorns (1966, p. 

112) reported two lenticular maroon sandstones 

near the bottom of the volcanic sequence southeast 

of the type locality. These units, 50 and 200 feet 

thick, are in the area where the basal conglomerate 

is very thick and contains volcanic units in the 

upper part.

The contact of the Stevens Mountain Rhyolite 

with the underlying Tascuela Redbeds is gradational and is described in the section on the Tascuela. 

Though such a contact could possibly result from 

reworking of subjacent material along an unconformity, no positive evidence of unconformity has 

been found and the contact is believed to be conformable.

The top of the Stevens Mountain is a sharp angular unconformity at the base of the Demetrie Vol-
D24 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

canics, which fills channels cut into and through the 

Stevens Mountain. Fragments from the Stevens 

Mountain are abundant in the basal conglomerate 

of the Demetrie.

AGE AND CORRELATION

Local geologic relations prove that the Stevens 

Mountain Rhyolite postdates the Tascuela Redbeds 

and predates the Demetrie Volcanics. Geologic relations also show that the Stevens Mountain is in 

gradational conformable contact with the underlying Tascuela. For reasons already pointed out (p. 

D15), both these formations are correlated with the 

Canelo Hills Volcanics. According to this interpretation the Stevens Mountain Rhyolite is equivalent 

to only part of the rhyolitic rocks of the Canelo Hills 

Volc.anics. The uppermost very thick and highly 

potassic welded tuff of the Canelo Hills, which has a 

K-Ar age of 173±7 m.y. (Hayes and others, 1965, 

p. M7), appears to be unrepresented inasmuch as 

the Stevens Mountain extrusives contain albitic 

plagioclase and anorthoclase and hence are sodiumrich rocks. Assuming that the Stevens Mountain 

Rhyolite represents the middle rhyolitic unit of the 

Canelo Hills Volcanics, the age of the Stevens Mountain is Early Jurassic or Late Triassic.

LOWER CRETACEOUS(?) ROCKS 

WHITCOMB QUARTZITE

NAME AND TYPE AREA

Quartzite containing lenses of rhyolitic tuff in 

the upper part overlies the Rodolfo Formation west 

of Helmet Peak, from Whitcomb Hill (fig. 5) northward for about three-quarters of a mile. This formation, which is faulted off at both ends, is here 

named the Whitcomb Quartzite, and the exposures at 

Whitcomb Hill are designated the type area.

GENERAL DESCRIPTION, DISTRIBUTION, AND THICKNESS

The Whitcomb Quartzite is very light gray wellsorted fine- to medium-grained indistinctly bedded 

orthoquartzite that contains lenses of medium-lightgray flinty rhyolitic tuff in the upper part. About 

three-fourths of the formation is quartzite, which 

is resistant to erosi,on and forms a low ridge whose 

highest point is Whitcomb Hill. Small generally 

sill-like but locally discordant bodies of andesite 

cut the formation.

The Whitcomb overlies the Rodolfo Formation 

and underlies the Angelica Arkose. Both contacts 

are disconformities in the type area. The lower contact is described in the discussion of the Rodolfo 

Formation (p. D22). The upper contact is well 

exposed at one place and is clearly depositional. The

Angelica Arkose here lies with no apparent angular 

discordance on a rhyolite tuff unit of the Whitcomb 

and contains a few pebbles of the tuff. Along the 

strike, the Angelica truncates a small tongue of 

quartzite and another lens of tuff, indicating that 

the contact is a disconformity.

The total thickness of the Whitcomb ranges from 

about 300 feet to nearly 600 feet in a distance of 

less than a mile between the faults that bound its 

type outcrops. The variation is due mostly to lateral 

thinning and thickening of the lower tuff-free quartzite that seems to have resulted from the undulatory 

surface on which the formation was deposited. Some 

variation is attributable to undulation in the upper 

contact. There is no indication that the formation 

is wedging out in either direction.

About 450 feet of quartzite, here assigned to the 

Whitcomb, crops out on a small hill in the alluvial 

plain 41/2 miles east of Helmet Peak in sec. 10, T. 

17 S., R. 13 E. The base of the quartzite is not 

exposed. Its top is a gradational contact with conglomerate which, as pointed out later (p. D26), 

seems best interpreted as a local basal conglomerate 

of the Angelica Arkose.

LITHOLOGY

Quartzite makes up 65-85 percent of the type 

Whitcomb. The remainder is rhyolitic tuff, occurring in lenticular beds 10 to perhaps as much as 

150 feet thick in the upper half of the formation. 

The uppermost unit is tuff at most places.

The quartzite is well-sorted fine- to mediumgrained quartz sandstone cemented by silica. Freshly 

broken surfaces have a rough subconchoidal fracture and are very light gray to grayish pink. 

Weathered surfaces are yellowish gray, grayish 

orange, or light brown and are locally coated with 

black desert varnish. Bedding planes are obscure. 

However, some weathered surfaces show a distinct 

lamination because of more desert varnish on alternate thin layers. The lamination, which is not apparent on fresh fracture, has almost the same dip 

as the formation as a whole but is oblique in strike 

and evidently represents crossbedding.

Constituents of the quartzite discernible with a 

hand lens are vitreous quartz, sparse disseminated 

specks of black iron ore, and grains of chalky white 

feldspathic material. Thin sections show rounded 

and subrounded detrital grains of quartz, 0.2-0.3 

mm in diameter, whose interstices are filled with 

quartz in optical continuity with the worn grains. 

A film of dust generally separates worn grain from 

overgrowth. Quartz makes up 90 percent or more 

of the rock. The remainder consists of feldspathic
SIERRITA MOUNTAINS, PIMA COUNTY D25

grains sericitized and argillized beyond recognition, 

zircon, tourmaline, iron ores, and local wisps and 

pockets of sericite between the grains. The rock is 

a well-sorted orthoquartzite.

The rhyolitic tuff is a medium-light-gray to 

pinkish-gray, hard, compact rock in which phenocrysts of feldspar and quartz 0.5-3 mm long are 

easily discernible in a flinty aphanitic groundmass. 

Outcrops generally lack evident bedding or planar 

structure of any kind and could easily be mistaken 

for intrusive porphyry. At a few places on freshly 

broken and moistened surfaces, however, relict 

vitroclastic texture in the groundmass has been detected with a hand lens.

In thin section, vitroclastic texture is generally 

conspicuous despite complete devitrification. Pumice 

fragments and shards, which have axiolitic structure in the best preserved specimens, are moderately 

compacted and molded under and over phenocrysts 

and small rock fragments. Phenocrysts make up 15- 

20 percent of the rock and consist of sodic plagioclase (An5.10 ), microperthite, quartz, and sparse 

pseudomorphs of muscovite and iron oxides after 

biotite. The rock fragments are mostly rhyolitic rock 

but include some accidental fragments of thoroughly 

recrystallized quartzite. Devitrified glass, which 

makes up most of the rock, is readily and strongly 

stained by cobaltinitrite solution and appears to be 

almost wholly potassium feldspar and quartz. On 

the basis of these properties, the rock is classed as 

rhyolitic welded tuff.

The quartzite east of Helmet Peak, assigned to 

the Whitcomb, is mostly well-sorted fine- to mediumgrained nearly pure quartzite that weathers medium 

light gray with a brownish to purplish cast. Some 

beds contain disseminated coarse well-rounded 

frosted grains of quartz and medium to coarse subangular grains of fresh alkali feldspars and felsic 

volcanic rock. According to the terminology of 

McKee and Weir (1953), the unit is thin to thick 

bedded, commonly thinly laminated, and locally 

crossbedded and cross laminated on a medium to 

large scale. It contains a little detrital tourmaline 

like the type Whitcomb but on the whole is less pure 

and less well sorted. Tuff beds are lacking, but the 

presence of unweathered volcanic detritus indicates 

a dual source for the unit and suggests that there 

may have been nearby volcanic activity at the time 

of deposition.

CONTACTS

The contact of the Whitcomb Quartzite with the 

underlying Rodolfo Formation is a disconformity 

which is evident in outcrop because of the abrupt

and major change in lithology. From a distance it 

can be approximately located along the west flank 

of a low ridge formed by the quartzite.

The contact of the Whitcomb with the overlying 

Angelica Arkose is a disconformity in the type area 

west of Helmet Peak. The disconformity is not 

clearly marked topographically but is easily recognized in outcrop. Locally the contact is gradational, 

if my interpretation of outcrops east of Helmet 

Peak is correct. Here the contact is placed where 

pebble and cobble conglomerate at the base of the 

Angelica becomes dominant. The contact defined in 

this way is actually rather sharp although each 

formation contains sparse thin interbeds characteristic of the other in a zone a few tens of feet thick.

AGE AND CORRELATION

The problem of age and correlation of the Whitcomb Quartzite is difficult and as yet not satisfactorily resolved. Two possible interpretations are 

as follows:

1. The Whitcomb correlates with the Stevens Mountain Rhyolite and therefore is pre-Cretaceous 

in age (Early Jurassic or Late Triassic). Both 

these formations display the unusual assemblage of rhyolite tuff and orthoquartzite that 

obviously represents a mature sediment, and 

both lie on correlative Triassic formations. 

Thus, lithology and stratigraphic position support a correlation. However, the two formations have a very different proportion of 

quartzite and rhyolite; also, the underlying 

formations are somewhat different lithologically. The Stevens Mountain has a gradational 

basal contact whereas the Whitcomb has a 

disconformity at the base. Correlation of the 

two formations would require lateral variations in lithology and local disconformity, 

which are certainly possible in a basin filled 

with sedimentary and volcanic rocks.

2. The Whitcomb is a general correlative of volcanic 

units assigned to the basal part of the Lower 

Cretaceous sequence in mountain ranges to the 

east (Gilluly, 1956, p. 68-70; Hayes and Raup, 

1968; Drewes, 1968). The stratigraphic position of the Whitcomb is compatible with this 

interpretation, and a lithologically similar 

quartzite unit occurs in the overlying Angelica 

Arkose. Though the type Whitcomb has a disconformable upper contact, the quartzite assigned to the Whitcomb east of Helmet Peak 

has a gradational upper contact with conglomerate that I think must be of Cretaceous
D26 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

age and could well represent the Glance Conglomerate. The preponderance of evidence 

favors this interpretation, and the Whitcomb is 

therefore assigned to the Early Cretaceous(?).

LOWER CRETACEOUS ROCKS

ANGELICA ARKOSE

NAME, TYPE LOCALITY, AND GENERAL DESCRIPTION

The name Angelica Arkose is here applied to a 

sequence of drab arkosic rocks about 5,000 feet 

thick that overlies the Whitcomb Quartzite with 

local disconformity and underlies the Demetrie 

Volcanics with angular unconformity. The formation crops out in several places, covering a total area 

of about 2 square miles, in the geologically complex 

outer part of the pediment east and northeast of the 

Sierrita Mountains. The chief outcrop area lies 

south and west of Helmet Peak (fig. 5). Other outcrops are in two small hills 4.5 miles' east of Helmet 

Peak and also in low hills 5.5-6.8 miles to the south 

in the area west, south, and southeast of the Senator 

Morgan mine (fig. 2).

The formation, as pieced together from these 

isolated exposures, consists of a basal conglomerate 

member, a middle arkose and siltstone member, and 

an upper arkosic grit and conglomerate member. 

The basal conglomerate is a highly lenticular pebble 

and cobble conglomerate which is absent in some 

places but more than 500 feet thick in others. The 

middle member is a thin-bedded sequence, as much 

as 2,000 feet thick, of well-sorted light-olive-gray 

arkosic sandstone and greenish-gray to grayish-red 

siltstone. The upper member is a thick-bedded 

sequence, several thousand feet thick, of poorly 

sorted light-gray to light-brown arkosic grit and 

pebble conglomerate containing some gray siltstone 

and locally a little light-gray limestone and quartzite.

The Angelica Arkose is named for its exposures 

along Angelica Wash. The composite type locality is 

in this wash in sees. 10 and 11, T. 17 S., R. 12 E. 

(fig. 5), where the middle part of the formation is 

exposed; 4.5 miles east of Helmet Peak in sec. 10, 

T. 17 S., R. 13 E. (fig. 2), where the basal member 

is exposed; and just west of the Angelica Wash 

locality and southwest of the No. 6 thrust fault (fig. 

5) in the Sy2 sees. 10 and 11 and the SWVi sec. 12, 

T. 17 S., R. 12 E., where the upper part of the 

formation crops out.

In the Angelica Wash locality, the basal contact is 

a disconformity separating the middle sandstone 

and siltstone member from the underlying Whitcomb Quartzite. A few small pebbles of the Whitcomb occur in the lowermost sandstone beds. In the

locality east of Helmet Peak, the basal conglomerate 

member is in conformable contact with the underlying quartzite. Thus, if my identification of formations in the locality east of Helmet Peak is correct, 

deposition was continuous east of Helmet Peak while 

the Whitcomb was being eroded in the Angelica 

Wash area. Presumably some deformation followed 

deposition of the Whitcomb and caused the basal 

contact of the Angelica to be conformable in some 

places and disconformable in others.

Identification of rocks in the locality east of Helmet Peak as Whitcomb Quartzite and Angelica 

Arkose, on which this interpretation depends, is 

based on lithology and on the presence of conglomerate at the base of the Angelica at one other place. 

South of the Senator Morgan mine near the Duval 

Road, conglomerate at least 500 feet thick that has 

been metamorphosed to hornfels underlies lithologically typical Angelica Arkose with poorly exposed 

but apparently conformable contact. The base of 

this conglomerate unit is concealed. The conglomerate east of Helmet Peak is stretched and foliated 

but not metamorphosed to hornfels. It overlies the 

quartzite with gradational contact, and on a nearby 

hill it underlies a flow or sill of distinctive amygdaloidal andesite virtually identical in hand specimen 

and thin section with andesite found by Harald 

Drewes (oral commun., 1968) in the Bisbee Group 

in the Santa Rita Mountains and interpreted by him 

as intrusive. Above the andesite is about 300 feet 

of brownish-gray arkosic sandstone and pebble conglomerate, whose lithology suggests a transitional 

zone between the conglomerate and the arkosic sandstone and siltstone in the lower part of the Angelica 

in Angelica Wash. A fault may be concealed under 

alluvial deposits between Helmet Peak and the 

nearby hill, but none is required by the geometric 

relations; and a large fault in this position is unlikely because the attitudes, both of the beds and of 

the distinct foliation, and the stretching at a large 

angle to the attitudes are the same in the two hills. 

The steep dip of the beds and of the pervasive foliation strongly indicates that the beds predate at least 

part of the Laramide orogeny. Among the known 

stratigraphic sequences in the region that fulfill this 

requirement, the Whitcomb and Angelica sequence 

provides the closest lithologic match.

LITHOLOGY 

BASAL CONGLOMERATE MEMBER

The basal conglomerate is characterized by illsorted subrounded pebbles and cobbles tightly bound 

in a matrix of ill-sorted coarse-grained sandstone, 

which is generally medium gray on fresh fracture
SIERRITA MOUNTAINS, PIMA COUNTY D27

and brownish gray on weathered surface. The clasts 

are mostly silicic and intermediate volcanic rocks 

but also include some quartzite, graywacke, chert, 

limestone, felsic porphyry, aplite, and vein quartz. 

All the clasts probably came from pre-Angelica 

Mesozoic and Paleozoic rocks now exposed in the 

region. The sandstone matrix makes up 20-50 percent of the typical conglomerate and grades laterally 

and vertically into small sandstone lenses within the 

unit. Minor interbeds of fine-grained fairly well 

sorted sandstone and shale are scarce.

The effects of metamorphism are evident in all 

exposures of the unit. The original lithology of the 

clasts has been considerably obscured by thermal 

metamorphism and hydrothermal processes in the 

outcrops, west and south of the Senator Morgan 

mine, which are next to a large Laramide pluton. 

East of Helmet Peak the unit is stretched and 

foliated at a large angle to the plane of the beds but 

has not been thermally metamorphosed.

MIDDLE ARKOSE AND SILTSTONE MEMBER

At the type locality, near Angelica Wash, the middle member comprises about 2,000 feet of arkosic 

sandstone and siltstone in sharply defined beds generally less than 2 feet thick. The sandstone ranges 

from fine to coarse grained and is predominantly 

well sorted. In most exposures the rocks are light 

olive gray to medium gray on fresh fracture, but 

locally the siltstone and fine-grained sandstone are 

pale red to grayish red.

The sandstone consists of subangular grains of 

quartz (generally 40-45 percent), feldspar (25-35 

percent), rock fragments (1-5 percent) and accessory heavy minerals tightly cemented in several 

different ways. In the predominant olive-gray types, 

the interstitial material is mostly epidote, presumably formed by metamorphism of a calcareous 

matrix. Overgrowths of quartz on the worn quartz 

grains are generally present and locally make up 

much of the cement. Sericite and chlorite, representing argillaceous matrix material, make up a 

small percentage of the rock generally and as much 

as 20-25 percent of some of the fine-grained 

grayish-red sandstones. According to Pettijohn's 

(1957, p. 291) classification, the sandstone is mostly 

arkose but grades locally to feldspathic graywacke.

In the sand fraction, albitic plagioclase is as much 

as three times as abundant as potassium feldspar. 

The rock grains consist of rhyolite from tuff beds 

in the Whitcomb Quartzite and cryptofelsite and 

andesite from the older Mesozoic formations.

Most of the sandstone beds are less than a foot 

thick, but a few coarse-grained beds are as much as

10 feet thick. Lamination due to slight differences 

in grain size or color is common. Some individual 

beds have normal grading, others reversed grading, 

and still others a combination of the two. As a 

result, textural variations have proved of little use 

in distinguishing tops from bottoms of beds. Many 

beds are lenticular and fill scours in underlying beds. 

The resulting channel structures and sparse crossbedding are the best indication of the top direction.

Siltstone, though less abundant than sandstone, 

forms many interbeds generally less than a foot 

thick. The rock is massive to weakly fissile argillaceous siltstone. It locally contains disseminations 

or laminae of fine sand having the same mineralogy 

and mineral proportions as the associated sandstone 

beds. The siltstone interbeds in much-epidotized 

sandstone are generally light greenish gray. Where 

epidotization was less intense, they are generally 

grayish red and slightly calcareous.

The middle member of the Angelica varies 

laterally in thickness and lithology. In the hills west 

and south of the Senator Morgan mine, where the 

member has normal lithology, it is less than 1,000 

feet thick. An apparent pinching out in the southeastern part of this area is probably due to shearing 

out of beds. In the reference section 4.5 miles east of 

Helmet Peak, only the lower 300 feet of the middle 

member is exposed above the basal conglomerate. 

The middle member here consists of brownish-gray 

arkosic sandstone and pebble conglomerate in 

lenticular beds 2 inches to 10 feet thick the proportion of conglomerate decreasing from about 70 percent at the bottom to about 10 percent at the top. 

The sandstone in the upper part is generally well 

sorted and locally laminated like the middle-member 

sandstone near Angelica Wash. Apparently these 

beds are a transitional facies between the basal conglomerate and the sandstone-siltstone facies at 

Angelica Wash.

UPPER ARKOSIC GRIT AND CONGLOMERATE MEMBER

Along Angelica Wash the upper member of the 

Angelica Arkose consists of about 1,600 feet of 

light-gray to light-brown arkosic grit and pebble 

conglomerate containing many interbeds of siltstone. These rocks lie with sharp but interfingering 

contact on the finer grained, better sorted, and 

thinner bedded rocks of the middle member, and 

they are cut off at the top by faults. As the result of 

the interfingering relationship, the upper member 

thickens several hundred feet toward the northeast 

at the expense of the lower member in a distance of 

less than half a mile. The coarse detritus thus did 

not come from the southwest; local evidence pro-
D28 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

vides no basis for appraising other possible source 

directions.

Grit, here denned as coarse-grained ill-sorted 

sandstone containing disseminated granules and 

small pebbles, is the most abundant rock in the 

upper member. This grit is light gray to medium 

light gray on fresh fracture. Its constituent particles are mostly subangular and commonly range in 

size from granules to silt within the same hand 

specimen. Two specimens examined in thin section 

consist of 45-55 percent quartz, 15-25 percent feldspar, 5-10 percent rock fragments, and 15-25 percent calcareous silty matrix. In these specimens, 

grains of potassium feldspar and of plagioclase are 

about equally abundant, unlike the lower unit where 

plagioclase is predominant.

The grit contains lenses of conglomerate consisting of poorly sorted pebbles, granules, and sand 

cemented by calcite. Well-rounded pebbles of quartzite and less rounded ones of coarse-grained leucogranite are conspicuous in the rock. Other clasts are 

quartz, feldspar, and fine-grained igneous and sedimentary rocks. The granite pebbles are lithologically 

identical with much of the Jurassic granite now 

exposed in the Sierrita Mountains and undoubtedly 

came from that formation.

Interbeds of siltstone and fine- to medium-grained 

sandstone are common in the grit and conglomerate. 

These finer grained rocks are light olive gray to 

brownish gray, fairly well sorted, feldspathic, and 

calcareous. The ratio of potassium feldspar to 

plagioclase grains is about 1:1, as in the associated 

grit.

Bedding in the upper member is distinct. The 

grit and conglomerate units are 15-50 feet thick. 

They are not graded but commonly show crossbedding and scour-and-fill structure. The finer 

grained units are as much as 25 feet thick but are 

mostly 1-15 feet thick.

At the locality southwest of the No. 6 thrust fault 

(fig. 5), about 3,000 feet of beds consists mostly of 

light-gray feldspathic grit generally similar to beds 

in the upper part of the section along Angelica 

Wash. However, this grit is less distinctly bedded 

and commonly contains more fine-grained detrital 

matrix. Lenses of pebble conglomerate are fairly 

common, particularly in the lower part. There are 

also beds of light-gray fine-grained graywacke and 

dark-gray siltstone and mudstone that have been 

metamorphosed to hornfels. A conspicuous ridgeforming quartzite unit several hundred feet thick 

occurs in the lower part of the sequence, and a few 

lenticular units of locally fossiliferous limestone occur in the middle and upper parts. Thin-bedded

grayish-red siltstone, shale, and fine-grained arkose 

are found in the top few hundred feet.

The sequence is assigned to the upper member of 

the Angelica Arkose because of general lithologic 

similarity of the principal rock types, but it does 

not correlate in detail with beds at the Angelica 

Wash locality. Either the sequence is younger than 

any beds exposed along Angelica Wash or it is a 

different facies of partly or wholly time-equivalent 

beds juxtaposed by the No. 6 thrust fault. In either 

case, the rocks clearly predate the major unconformity at the base of the Demetrie Volcanics 

(fig. 5).

Conglomerate, composed of subangular to rounded 

pebbles and granules in a matrix of light-gray illsorted sandstone,, makes up 5-10 percent of the 

lower part of the sequence as lenses a few inches to 

a few feet thick. The clasts consist of silicic and 

sparse andesitic volcanic rocks, quartzite, feldspathic sandstone and siltstone, granite, micropegmatite, and silicic porphyry. All the clasts appear 

to have come from Triassic and Jurassic formations 

now exposed in the Sierrita Mountains. The sandstone matrix is invariably feldspathic and commonly 

of the subgraywacke type. The percentage of the 

matrix in the conglomerate is highly variable, and 

the lenses grade vertically and laterally into grit.

Feldspathie sandstone of several types is present 

in the sequence. The predominant type is coarsegrained ill-sorted feldspathic grit having the same 

provenance as the conglomerate. The rock resembles 

the grit in the upper part of the section along 

Angelica Wash, but the beds are thicker and are not 

crossbedded. No calcite or other chemical cement 

has been detected in thin section, its place being 

taken by fine-grained sericite, quartz, potassium 

feldspar, and a little chlorite, apparently derived 

from argillaceous silt. Potassium feldspar is slightly 

more abundant than plagioclase in the thin sections 

examined. The grit and conglomerate in the lower 

part of the sequence are associated with some fairly 

thick units of medium- to fine-grained indistinctly 

bedded graywacke. This rock is light gray with yellow and olive hues. It is distinguished from the grit 

by its finer texture and more abundant sericitic 

matrix. Very different in lithology are relatively 

thin bedded fine- to medium-grained feldspathic 

sandstone and siltstone beds that interfinger with 

and overlie a limestone unit at the top of the sequence. These rocks are well sorted and commonly 

pale red to grayish red. The sandstone contains little 

argillaceous material and is characteristically cemented by calcite. These beds at the very top of the
SIERRITA MOUNTAINS, PIMA COUNTY D29

sequence resemble parts of the lower member along 

Angelica Wash.

The quartzite, which occurs in a single thick unit 

low in the sequence (fig. 5), is fine grained and not 

distinctly bedded. It is light gray on fresh fracture 

and yellowish gray to dark yellowish orange and 

light brown on weathered surfaces. In thin section 

the sand grains are seen to be subangular to 

rounded, fairly well sorted, and closely packed. 

Quartz makes up about 95 percent of the grains, and 

the remaining grains are potassium feldspar, 

sericitized plagioclase(?), iron ores, silicic volcanic 

rock, and siltstone. The cementing material, which 

makes up about 20 percent of the rock, is mostly 

microcrystalline sericite and quartz. Obviously the 

quartzite is a much more mature sedimentary rock 

than the associated grit, conglomerate, and graywacke. Its contacts with these rocks are not exposed 

and therefore could be fault contacts or disconformities. However, no direct evidence of either faulting 

or disconformity could be found, and the quartzite is 

assumed to be part of the same sedimentary sequence because the lithology of underlying and overlying rocks is the same and because the bedding and 

contacts of the quartzite appear to be parallel to 

bedding in the adjacent rocks.

Siltstone has been metamorphosed to hornfels and 

occurs in beds a few feet thick between some of the 

grit units; it is a minor and generally inconspicuous 

component of the sequence. The rock is medium gray 

to dark gray and homogeneous looking except for 

scarce soft white spots about 0.5 mm in diameter. In 

thin section some silt and very fine sand composed 

of quartz, alkali feldspars, and an opaque mineral 

may be seen in an abundant matrix of very finely 

crystalline and weakly foliated material in which 

sericite and green biotite are the only recognizable 

constituents. The white spots visible in some hand 

specimens are altered anhedral andalusite or cordierite loaded with inclusions of the matrix material. The rock is evidently metamorphosed argillaceous siltstone.

Units consisting of thin-bedded limestone, sandstone, and siltstone as much as 200 feet thick occur 

locally in the middle and upper parts of the sequence 

(fig. 5). The principal occurrence is about half a 

mile south of Helmet Peak, and another smaller occurrence, considerably lower stratigraphically, is 

about a quarter of a mile southwest of Whitcomb 

Hill. The limestone beds finger out laterally in 

distances of hundreds to several thousands of feet 

and, therefore, they have proved of limited use as 

stratigraphic markers, either individually or collectively. Only a very few thin beds of limestone and 

calc-silicate hornfels have been found outside the 

two areas mentioned.

The limestone is in lenticular beds a few inches to 

several feet thick. It is finely to coarsely crystalline 

and medium gray to light gray on fresh fracture. 

Sand and silt are common as disseminations or in 

laminae and cross laminae. South of Helmet Peak, 

the rock contains recrystallized pelecypod and 

gastropod shells, concentric algal (?) heads as much 

as 6 inches in diameter, and brown-weathering 

silicified ostracodes. Where ostracodes and detrital 

material are abundant, weathered surfaces are light 

olive gray, yellowish brown, and even brownish 

black; elsewhere they are medium gray to light 

gray, like the fresh rock.

CONTACTS

The disconformable and probable gradational 

contacts of the Angelica Arkose with the underlying 

Whitcomb Quartzite are described in the section on 

the Whitcomb Quartzite (p. D25). The contact of 

the Angelica with the overlying Demetrie Volcanics 

is an angular unconformity. This unconformity is 

not expressed topographically but is easily recognized on the ground, where the steep-dipping lightcolored sedimentary beds of the Angelica are 

truncated at a large angle by dark-colored andesitic 

breccia of the Demetrie.

AGE AND CORRELATION

Fossils found in the Angelica Arkose are very 

poorly preserved and not diagnostic. The pelecypods 

and gastropods are indeterminable as to type (J. B. 

Reeside, written commun., 1958), and the ostracodes 

are of a fresh-water type common in Upper Jurassic 

and younger sedimentary rocks (I. G. Sohn, in 

Cooper, 1960, p. 69). The minimum age of the

I Angelica is fixed geologically by the unconformably 

overlying Demetrie Volcanics of Late Cretaceous

! age.

The Angelica Arkose and the Amole Arkose of 

W. H. Brown (1939) in the Tucson Mountains are 

very similar in lithology and are undoubtedly general correlatives. Both resemble the Bisbee Group, 

in the Whetstone, Empire, and northern Santa Rita 

Mountains, which is subdivided into five formations 

(Finnell, 1970). These five formations become less 

distinct and more like the Amole toward the west 

and have not been recognized west of the Santa Cruz 

River. The Angelica Arkose is here correlated with 

this sequence and is assigned to the Early Cretaceous.
D30 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

UPPER CRETACEOUS ROCKS 

DEMETRIE VOLCANICS

NAME AND TYPE LOCALITY

A thick sequence of andesitic and dacitic breccias 

and flows that locally contains conglomerate and 

rhyolite tuff is exposed along and to the west of 

Demetrie Wash (fig. 6) and was named for that 

geographic feature (Thorns, 1967). The type locality 

is west of the wash, in a belt that extends from the 

NW*4 sec. 15 to the SW. cor. sec. 21, T. 18 S., 

R. 12 E.

As first pointed out by Courtright (1958), parts 

of the Demetrie Volcanics are very similar to a unit 

designated the Silver Bell Formation in the Silver 

Bell Mountains 45 miles to the northwest; and later, 

Lynch (1966) and Lootens (1966) used the name 

Silver Bell for the rocks in the Sierrita Mountains. 

As these rocks include types not reported in the 

Silver Bell Mountains, the name Demetrie Volcanics 

is preferable and therefore adopted.

GENERAL DESCRIPTION, DISTRIBUTION, AND THICKNESS

The Demetrie lies with angular unconformity on 

rocks as young as the Angelica Arkose and as old as 

the Ox Frame Volcanics (figs. 5, 6). It locally has a 

basal conglomerate, tens to hundreds of feet thick, 

composed of debris from the underlying rocks. The 

local conglomerate units finger out laterally into 

unbedded andesitic breccia. Where conglomerate is 

missing, the breccia lies directly on the older rocks 

and contains at most only sparse fragments of them. 

The breccia and sporadic unbrecciated flows of 

andesite and dark dacite make up most of the formation and are the only rocks at many places. In the 

type locality, however, the massive ridge-forming 

lower rhyolitic tuff member averages about 750 feet 

in thickness and occurs about 1,000 feet above the 

base of the formation; and the upper rhyolitic tuff 

member, a thinner unit, occurs near the top (fig. 6). 

The lower rhyolitic member is largely welded tuff, 

whereas the upper one is bedded ash-fall material associated with conglomerate. The upper contact of 

the Demetrie Volcanics comprises several unconformities, the oldest of which is at the base of the 

Red Boy Rhyolite (fig. 4).

The Demetrie is the most widely distributed 

Mesozoic formation in the Sierrita Mountains; its 

outcrops are found southwest of Helmet Peak and 

in a belt about 18 miles long and as much as several 

miles wide around the south and west sides of the 

range. The formation tends to be recessive topographically and is generally poorly exposed in low 

hills, pediments, and slopes supported by more resistant rocks.

The thickness and lithology of the Demetrie Volcanics are highly variable laterally. This variability 

is unquestionably due in part to the considerable 

relief of the prevolcanic surface, in part to the 

local sources of the materials that make up the 

formation, in part to erosion of the formation in 

pre-Red Boy time, and perhaps in part to deformation during the period of Demetrie volcanism.

On the basis of outcrop width, fairly uniform 

strike, and steep dip evident at a few widely spaced 

places, the thickness in the type locality is thought 

to be about 8,000 feet. The true thickness may be 

less inasmuch as stratigraphic markers are few and 

parts of the formation could be repeated by unrecognized folds and faults. With due allowance for such 

possible repetition, the formation must be thousands 

of feet thick in this area. It thins and pinches out 

beneath the Red Boy Rhyolite toward the core of the 

range, which appears to have been the site of a 

topographic high at the time of deposition.

LITHOLOGY 

CONGLOMERATE

The local basal conglomerates of the Demetrie 

vary in lithology both vertically and laterally. In 

general, the rock is characterized by poorly rounded 

pebbles, cobbles, and boulders of one or several of 

the locally subjacent formations in a matrix of finegrained poorly sorted material apparently from the 

same source. Thus, in the type locality, the detritus 

is very largely from the Ox Frame Volcanics and 

Mesozoic intrusive rocks, whereas on the west side 

of the range, detritus from the Stevens Mountain 

Rhyolite and Tascuela Redbeds is commonly abundant, and a unit in the intervening area is composed 

entirely of detritus from Jurassic granite. The 

conglomerate grades upward and laterally into 

andesitic breccia, largely by interbedding and interfingering and, in part, by the appearance and upward increase in abundance of intraformational 

andesitic fragments.

The matrix of the basal conglomerate is characteristically ill sorted like the larger fragments. Near 

Demetrie Wash it is generally light-gray to 

greenish-gray graywacke. On the west side of the 

range, it is commonly red mudstone, whose color 

and texture are probably due to a substantial increment from the Tascuela Redbeds.

Interbeds of graywacke, siltstone, and mudstone 

are common in the thicker conglomerate units. 

These beds are generally like the matrix of the conglomerate in color, texture, and composition.

The basal conglomerate in the type locality could 

be part of a tongue of sedimentary rock of regional
EXPLANATION

Sedimentary and volcanic rocks

Conglomerate and andesite and

rhyodacite flows and pyroclastics

Intrusive rocks

Andesite, diorite, granodiorite, and 

Quartz monzonite porphyry

<

=3 Z

O

Demetrie Volcanics

Andesitic to dacitic breccias and

flows 

Kdur, upper rhyolitic tuff and

conglomerate 

Kdlr, lower rhyolitic tuff Kdc, basal conglomeratic units

Quartz monzonite

Ox Frame Volcanics 

"fiou, upper member: rhyolitic

welded tuff and guartzite 

"Rom, middle member: andesite,

possibly intrusive

Intrusive breccia 

Fragments mostly from Ox Frame 

Volcanics and lower Tertiary 

quartz monzonite porphyry

u

>!

a:

Contact

Dotted where

concealed

Fault

Dotted where 

concealed

Strike and dip 

of beds

Od2

JB

^

o

od

OCO

FIGURE 6. Geologic sketch map of the Esperanza mine area. Geology mapped, 1957-68; base from U.S. Geological Survey Twin Buttes 15-minute

quadrangle, 1957.
D32 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

extent. The unit fingers out to the west but thickens 

to the east, and near Demetrie Wash it occupies the 

entire stratigraphic interval beneath the lower 

rhyolitic tuff member (fig. 6). The lower part of the 

conglomerate, like basal units elsewhere, is characterized by poorly rounded, locally derived clasts. 

Toward the east this rock is overlain by interbedded 

siltstone, pebbly and cobbly graywacke, and conglomerate consisting of better rounded clasts which 

are lithologically more diverse. The clasts, which 

range in size from granules to boulders as much as 5 

feet in diameter, consist of fine- to coarse-grained 

arkose, possibly from the Angelica Arkose, biotite 

granite, and diorite of uncertain source. The granite 

and diorite fragments suggest parts of the local 

Precambrian complex, but fragments of Paleozoic 

rock have not been recognized. In general, the 

heterogeneous conglomeratic unit lacks intraformational clasts. The unit is bounded on the east by 

intrusive Paleocene granodiorite and Quaternary alluvial deposits.

ANDESITE AND DACITE

Well-indurated andesitic breccia is the most widespread rock in the formation. The breccia consists of 

angular to subrounded, granule- to boulder-sized 

fragments of andesitic rock in a clastic andesitic 

matrix. Where least altered, the rock is dark greenish gray to grayish purple on fresh fracture and 

weathers to brown hues. Over extensive areas it is 

fairly light greenish gray as a result of intense 

propylitization, and in some smaller patches it is 

nearly white as a result of later alteration to pyrite 

and colorless silicates, apparently largely sericite, 

clay, and quartz. The primary texture of the rock is 

gradually obscured with increasing alteration.

The breccia is poorly sorted and only very rarely 

bedded. Except for alteration effects, it is homogeneous looking in outcrop. The constituent fragments 

ordinarily have 15-35 percent phenocrysts, 0.5-3.0 

mm long, of white feldspar and less abundant dark 

pseudomorphs after mafic minerals in an aphanitic 

groundmass. Secondary minerals such as epidote and 

chlorite are commonly discernible in hand specimen 

but macroscopic quartz is very rare. In thin section 

the feldspar phenocrysts are locally identifiable as 

normally zoned andesine, largely or wholly altered 

to an aggregate of albite, epidote, calcite, and other 

minerals. The mafic pseudomorphs consist of variable proportions of iron oxides, chlorite, and epidote; they locally have the form and habit of 

hornblende and biotite. The groundmass is commonly pilotaxitic and consists of microlites of 

plagioclase, tiny acicular mafic pseudomorphs,

specks of iron ore, and interstitial cryptocrystalline 

material. Quartz is perceptible in some specimens 

as irregular blebs in the groundmass and as a constituent of vesicle fillings. The potassium feldspar 

content of the cryptocrystalline material is highly 

variable, as the staining of thin sections and rock 

slabs with cobaltinitrite solution indicates. Thus, in 

spite of its generally homogeneous appearance, the 

rock probably ranges in composition from andesite 

to trachyandesite and dacite.

The matrix of the breccia is clastic material similar in composition to the fragments. Thorns (1966, 

p. 147-150) regarded it as tuff and the rock as tuffbreccia. In the hand specimens and thin sections of 

the predominant unbedded breccia that I have 

examined, however, the matrix lacks discernible 

pumice, shards, or other recognizable airborne ejecta 

and looks like the ill-sorted detrital matrix of volcanic mudflow or the crushed matrix of flow breccia 

formed in viscous lava flows.

Flow breccia, volcanic mudflow, and tuff-breccia 

are probably all present, but are not everywhere 

distinguishable from each other. Rocks believed to 

be flow breccia have clasts of petrographically uniform andesite in a clastic matrix of the same material, whereas those believed to be mudflow commonly 

have fragments of several kinds of andesite in a 

matrix that contains sparse grains of quartz and 

other minerals not found in the fragments. This 

suggests an epiclastic origin, but the lack of bedding 

and sorting is unlike a normal water-laid deposit. 

At some places, particularly on the southwest side 

of the range, beds of andesitic conglomerate are 

intercalated with this kind of breccia. A lens of 

dacitic tuff and tuff-breccia about 100 feet thick 

underlies an unusual brown dacitic flow along 

Esperanza Wash 1.6 miles south of the Esperanza 

mine. The pyroclastic rock has the same composition 

as the flow, which is described in the following 

paragraph. The tuff and tuff-breccia are well bedded, 

unlike most of the breccia in the Demetrie Volcanics.

Lava flows, apparently of little lateral extent, are 

associated with breccia several miles west of 

Demetrie Wash, southwest of Helmet Peak, and perhaps elsewhere. The flows resemble the associated 

breccia petrographically but are massive. A common 

type is greenish-gray to grayish-purple andesitic 

rock with pilotaxitic texture. West of Demetrie 

Wash there are also pale-brown to brownish-gray 

flows containing more silica in which phenocrysts of 

quartz, plagioclase, and mafic minerals are readily 

discernible with a hand lens in an aphanitic flinty 

groundmass. The phenocrysts are 0.5-3.0 mm long 

and make up about half the rock. In thin section,
SIERRITA MOUNTAINS, PIMA COUNTY D33

they are seen to include resorbed quartz, largely 

saussuritized andesine (about An35 ), locally a little 

anorthoclase, olive-green hornblende, chloritized 

biotite, and the common accessories magnetite, 

apatite, and zircon. The groundmass is dusty-brown 

cryptocrystalline material having an aggregate index of refraction less than that of Canada balsam. 

This rock and the associated tuff and tuff-breccia 

are certainly as felsic as dacite.

RHYOLITIC TUFF MEMBERS

The two rhyolitic tuff members are conspicuous 

stratigraphic markers within the Demetrie Volcanics. They are generally white to yellowish gray 

on weathered surfaces in contrast with the darker 

colors of the rest of the formation. The members are 

readily traced for 1-2 miles in the type locality, 

where they are cut off laterally by faults, a diorite 

intrusion, and an unconformity. Probably the lower 

member laps out laterally against the topographic 

high that appears to have been present in the 

Sierrita Mountains area in Demetrie time.

The lower rhyolitic member, which averages about 

750 feet in thickness, consists of ash-flow tuffs and 

subordinate ash-fall tuffs. The tuffs vary somewhat 

in lithology because of differences in the mode of 

deposition and in the abundance and nature of included rock chips, but as a whole they are very 

similar in appearance. They are all well-indurated 

rocks, generally medium gray to medium light gray 

on fresh fracture and yellowish gray on weathered 

surfaces. Hand specimens of all but scarce thin 

crystal tuff layers show sparse tiny phenocrysts and 

angular fragments of quartz and feldspar in an 

aphanitic flinty matrix; the phenocrysts are 1 mm 

or less in longest dimension and rarely make up as 

much as 5 percent of the rock.

In thin section, the phenocrysts are seen to include quartz, plagioclase, sanidine, and locally 

anorthoclase. The quartz is largely in angular fragments, but it also occurs as bipyramidal and resorbed crystals. The plagioclase consists of albite 

and oligoclase, both of which appear quite fresh in 

some of the sections examined. Sanidine and 

anorthoclase are generally less abundant than 

plagioclase. Small books of brown biotite that is almost completely altered to muscovite, sphene, and 

opaque material occur locally. Accidental grains of 

orthoclase and microcline perthite, probably derived 

from a granitic rock, are found in some specimens ; 

there are also abundant lithic fragments that range 

in size from fine ash to very small blocks. Most of 

the fragments are devitrified rhyolite and are 

similar in composition to the rest of the rock but

display flow banding, vitroclastic and eutaxitic 

structure, and spherulitic and other distinctive devitrification textures. These fragments are evidently 

accessory; that is, they represent previously consolidated volcanic rock torn from the volcanic vent 

by the same eruption that yielded the juvenile constituents of the tuff. Some accidental fragments of 

granite, aplite, and andesite are also present.

The matrix of the tuff consists of devitrified 

shards, pumice fragments, and dust. The devitrification products are coarser than those in many tuffs 

and appear to be almost entirely quartz and alkali 

feldspar. As this material is predominant, the rock 

as a whole is classified as rhyolite.

The rock is cut by tiny quartz-pyrite veinlets and 

commonly contains a little disseminated pyrite and 

microscopic streaks and patches of disseminated 

sericite. Presumably these features are due to the 

widespread early Tertiary alteration and mineralization in the area.

The best exposed section of the lower rhyolitic 

member is along a dry wash three-quarters of a mile 

southeast of the Esperanza mine. Here the tuff lies 

on a lens of conglomerate that contains fragments 

of the Ox Frame Volcanics and Jurassic granite and 

aplite. The upper part of the conglomerate has a 

tuffaceous matrix and contains at least one bed of 

rhyolite tuff. Overlying the conglomerate is about 

90 feet of fine-grained indistinctly bedded vitric 

and crystal-lithic tuff that contains a few lapillibearing beds. Evidently this material is of ash-fall 

origin. Above it is about 120 feet of massive, apparently structureless lapilli-bearing tuff which contains, at the base, a 1-foot-thick much-compacted 

eutaxitic vitric tuff. Above another thin unit of finegrained indistinctly bedded tuff is another massive 

lapilli-bearing tuff, much like the one below, which 

makes up most of the member. Eutaxitic structure 

is weakly expressed on weathered surfaces by narrow lenticular depressions, presumably due to flattened pieces of pumice that have weathered out. The 

shards and tiny pieces of pumice are little compacted in the single thin section examined, however. 

Apparently at this locality the member consists of 

two ash flows, each underlain by ash-fall material.

A few feet of conglomerate consisting of reworked debris from the tuff here separates the lower 

member from the overlying andesitic breccia. This 

conglomerate is evidently intraformational and is 

not present everywhere along the contact.

The upper rhyolitic member is fine-grained white 

argillized tuff. Parts are well bedded and evidently 

of ash-fall origin. Hand specimens show sparse 

small grains of quartz, altered feldspar, and rarely
D34 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

tiny hexagonal plates of golden altered biotite in a 

soft white matrix. The predominant constituent to 

be seen in thin section is argillized vitric ash, which 

occurs both as sand-sized grains that locally display 

relict spherulitic and vitroclastic structure and'as 

fine-grained interstitial dust and poorly preserved 

shards. Primary minerals other than quartz have 

been completely altered to clays, calcite, fine-grained 

dusty quartz, and other minerals. The unit clearly 

represents f elsic volcanic ash which may or may not 

have the same composition as the lower rhyolitic 

member.

CONTACTS

The basal contact of the Demetrie Volcanics is a 

major unconformity easily recognized in outcrop by 

the basal conglomerate locally present in the formation and, where the conglomerate is absent, by the 

abrupt appearance of andesitic breccia lying with 

structural discordance on commonly lighter volcanic, 

sedimentary, and plutonic rocks. The recessive topographic expression of the Demetrie and its common 

dark hues on fresh and weathered surfaces are of 

some aid in approximately locating the contact from 

a distance.

The contact of the Demetrie with the overlying 

Red Boy Rhyolite is a very rarely exposed unconformity. Where exposed on the southwest side of the 

mountains near Papago Well (fig. 2), the contact 

is almost perpendicular to bedding in the Demetrie. 

As the contact is tight, apparently unfaulted, and 

virtually parallel to layering in the adjacent tuff 

breccia of the Red Boy, it has the characteristics of 

an angular unconformity. However, its dip (about 

55°) is much steeper than that indicated by its trace 

on the topography, and the parallelism of contact 

and layering in the Red Boy is not what would be 

expected from onlap against a local steep slope. 

Possibly a local intrusive phase of the Red Boy 

here occupies an intra-Red Boy fault, as tuff identical with the extrusive material occurs in small 

demonstrably intrusive bodies in the Demetrie Volcanics as pointed out lajter (p. D35). Regardless of 

whether the exposed contact is depositional or intrusive, the Red Boy in this area has the map pattern of a gently dipping sheet that bevels definitely 

steeper dipping Demetrie in places and lies in large 

part on pre-Demetrie rocks.

At most places the Demetrie-Red Boy contact is 

in a narrow concealed belt which, at least locally, is 

underlain by a soft unit of decomposed Demetrie; 

this underlying unit contains blocks and tuffaceous 

matter of Red Boy Rhyolite in its upper part. 

Evidently the Red Boy was deposited unconformably

on a weathered surface, and its early ejecta were 

incorporated with erosion products derived from the 

older rocks. The top of the Demetrie is placed where 

Red Boy material first becomes evident. This contact 

is not sharply defined but can be located within a 

few feet stratigraphically. From a distance, the 

approximate position of the contact is marked in 

most places by an abrupt change in slope due to the 

Red Boy's great resistance to erosion.

AGE AND CORRELATION

The age of the Demetrie Volcanics can be approximated from geologic relations in the Sierrita 

Mountains. The formation is in angular unconformity on rocks as young as the Angelica Arkose 

and hence is not older than Late Cretaceous. It is 

overlain by the Red Boy Rhyolite and is cut by intrusive bodies of diorite, granodiorite, and quartz 

monzonite. The age of the Red Boy is still uncertain, 

but some radiometric ages are available for the intrusive rocks. Biotite from a large granodiorite 

pluton that cuts the Demetrie has yielded concordant 

K-Ar ages that average 60 m.y. (Paleocene), and 

the same age was obtained on biotite in a vein that 

cuts an adjacent but older diorite pluton. Obviously 

the diorite in this mass and presumably in another 

mass that cuts the Demetrie was emplaced before 

the vein, but how long before is not known. The 

radiometric data and geologic relations confine 

deposition of the Demetrie within the Late Cretaceous to Paleocene interval.

Volcanic rocks very similar to the Demetrie in 

lithology and stratigraphic position are common in 

southeastern Arizona and southwestern New Mexico 

as shown by the descriptions and correlations of 

Richard and Courtright (I960), Jones, Hernon, and 

Moore (1967, p. 59-61), and Hayes and Drewes 

(1968). Included are the Silver Bell Formation of 

Richard and Courtright, the lower dacitic breccias 

and flows of the Salero Formation in the Santa Rita 

Mountains, the lower andesitic part of the Bronco 

Volcanics near Tombstone, and mostly unnamed 

andesitic to dacitic breccias and flows at many other 

places at least as far north as Christmas, Ariz., and 

as far east as the vicinity of Silver City, N. Mex. 

Although these rocks may not be strictly contemporaneous or parts of a once-continuous sheet, they are 

undoubtedly general correlatives. Richard and 

Courtright recognized that they were deposited after 

the Laramide orogeny had started and tentatively 

assigned them to the early Tertiary. Recent evidence 

indicates that some and perhaps all of them are of 

Late Cretaceous age.

From the vicinity of Silver City, N. Mex. (Jones
SIERRITA MOUNTAINS, PIMA COUNTY D35

and others, 1967, p. 59-61), westward as far as 

Christmas, Ariz., the Demetrie-like rocks lie unconformably on fossiliferous beds of early Late Cretaceous (Colorado) age; and in western New Mexico 

they are overlain unconformably by subaerial deposits that contain plant fossils dated as Late 

Cretaceous (Elston, 1960). The indicated Late Cretaceous age of the Demetrie-like rocks is supported 

by evidence found southeast of the Sierritas where 

these rocks overlie deposits as young as the Upper 

Cretaceous Fort Crittenden Formation and underlie 

welded tuff dated as 72 m.y. (Drewes, 1968, p. Cll, 

C12). On this indirect basis, the Demetrie Volcanics 

are here assigned to the Late Cretaceous.

RED BOY RHYOLITE

NAME, TYPE LOCALITY, AND GENERAL DESCRIPTION

In accordance with the usage of Thorns (1966, 

1967), the name Red Boy Rhyolite is here adopted 

for a sequence, 700 to perhaps 1,000 feet thick, of 

gray to pale-red rhyolitic flows and pyroclastic rocks 

that unconformably overlies the Demetrie Volcanics 

and unconformably underlies gravel and basaltic 

andesite of middle Tertiary age. The type locality 

is on the south slope of Red Boy Peak (fig. 4), 

where the Demetrie-Red Boy relationship is well 

displayed.

The Red Boy crops out almost continuously in 

a belt more than 9 miles long and as much as 2.5 

miles wide extending from the alluvial cover south 

of Papago Well (fig. 2), northeastward into the 

Sierrita Mountains. Its total area of outcrop is about 

10 square miles. As the formation is very resistant 

to erosion, it forms conspicuous foothills athwart 

the broad pediment south of the mountains, and it 

caps several high peaks, including Red Boy Peak, 

Placer Peak, and Keystone Peak, which is the highest peak in the range (fig. 4). The exposures in the 

high part of the range are in a complex graben 

probably formed in part during deposition of the 

Red Boy.

The formation is characterized by massive wellindurated rhyolitic tuff and tuff-breccia containing 

intercalated rhyolitic flows and locally having a thin 

basal conglomerate. The formation also includes a 

volcanic neck and associated small irregular intrusions litholcgically like the tuff and a few dikes 

lithologically like the flows. The rhyolitic rocks 

grade laterally from an argillized facies south of 

the Sierrita Mountains to a hard metamorphosed 

facies in the core of the range. The argillized rocks 

are generally pinkish gray to pale red and weather 

grayish orange, whereas the metamorphosed rocks 

are medium gray on fresh fracture and lighter gray

on weathered surfaces. The change in color and 

hardness takes place in a broad zone in the approximate latitude of Lobo Peak (fig. 4).

The rhyolite is characterized by phenocrysts of 

quartz and feldspar in an aphanitic matrix, which 

is hard and flintlike in the gray facies but fairly 

soft and commonly somewhat porous looking in the 

argillized facies. Both facies contain units tens to 

hundreds of feet thick of nonbedded and nonsorted 

lapilli tuff and tuff-breccia which contain pieces of 

devitrified pumice and numerous accessory and accidental rock fragments. These units have most of 

the characteristics of ash-flow tuff although compaction, foliation, and eutaxitic structure are 

generally obscure in outcrop and typical axiolitic 

devitrification structure is generally lacking in thin 

section (Ross and Smith, 1961). Fine-grained bedded ash-fall tuff is sparse. Flows, recognized only 

in the gray facies, are locally banded but are mostly 

massive and commonly difficult to distinguish from 

pyroclastic rock in outcrop.

Fine-grained dark andesite, in the form of small 

dikes, plugs, and apparently extrusive bodies, occurs 

in the Red Boy Rhyolite, particularly north of the 

type locality. No andesite is found in the type locality. It is very scarce farther south and is rarely 

well exposed.

A probable volcanic vent filled with rhyolitic tuff 

is found about a mile south of Red Boy Peak. Here 

a mass of tuff 1,200-1,700 feet in diameter forms 

Lobo Peak, a steep necklike hill which rises about 

500 feet above a pediment cut on the Demetrie Volcanics. The contact surrounding the tuff mass is 

not exposed, and the topographic relations permit 

the mass to be interpreted as an erosional remnant 

of the extrusive tuff sheet, which it resembles lithographically (fig. 7). The almost inevitable first imLEVEL

FIGURE 7. Cross section through Red Boy Peak and Lobo 

Peak (line of section shown in fig. 4). QTs, Quaternary and 

Tertiary sedimentary rocks; Kr, Red Boy Rhyolite; Kd, 

Demetrie Volcanics; "Eo, Ox Frame Volcanics.
D36 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

Approximate top of outcrop

5 FEET

FIGURE 8. Field sketch showing andesite breccia of the Demetrie Volcanics (Kd) intruded 

by tuff of the Red Boy Rhyolite (Kr), in 

nearly vertical north wall of Ox Frame Canyon just north of Lobo Peak. Dikelets of 

rhyolite tuff in the Demetrie and blocks of 

andesite breccia in the Red Boy shown schematically.

pression that the mass has a transgressive necklike 

form is probably correct, however, as lithologieally identical tuff is found in small intrusive bodies 

in the Demetrie Volcanics nearby. One of perhaps 

a dozen known intrusions of this kind is illustrated 

in figure 8. Most of the rhyolite in this exposure is 

lapilli tuff containing scarce fragments of the Demetrie and other formations. The irregular part on 

the right is full of lapilli and blocks of the Demetrie, 

and thin irregular dikelets of the tuff are found as 

much as several feet from the main mass. The intrusive tuff is identical in lithology with that on 

Lobo Peak even regarding the presence of xenoliths of granite aplite which must have come from 

below and which are rare in the extrusive tuff now 

preserved. I conclude that the tuff on Lobo Peak was 

emplaced by the same intrusive mechanism and fills 

a vent that opened to the surface upon which the extrusive tuff was deposited. Presumably, some of the 

extrusive rhyolite in the formation came from this 

vent.

LITHOLOGY

The basal conglomerate of the Red Boy Rhyolite 

where present is a few feet to a few tens of feet 

thick. On the south slope of Red Boy Peak, the basal 

unit is poorly indurated and poorly bedded conglomerate that consists of subangular pebbles, cobbles, 

and boulders of rhyolite and andesite apparently 

derived from the Demetrie, Ox Frame, and Red Boy 

formations in an ill-sorted brown matrix containing 

rhyolitic ash. No basal conglomerate was found 

where the Red Boy overlies the Ox Frame Volcanics ; but it is present, at least locally, where the Red 

Boy overlies Jurassic granite and Triassie(?) and 

Jurassic quartz monzonite (sees. 34 and 35, T. 17 S., 

R. 11 E.). Here the Red Boy has an unusually steep 

dip and is largely fault bounded. Locally it consists 

of a basal unit of medium-gray, well-bedded, wellindurated, and commonly foliated pebble conglomerate and graywacke that contain granules and 

pebbles of the subjacent intrusives. Other constituents are fragments of rhyolite and graywacke from 

the Ox Frame Volcanics and rhyolitic material probably from early Red Boy eruptions. The rhyolitic 

rocks of the Red Boy include a considerable variety 

of tuffs and tuff-breccias, a possible lahar, and some 

intercalated lenticular flows and intrusive rocks.

Lava flows, which provide the best indication of 

overall mineral composition, have been identified 

in the Red Boy Rhyolite at a few places west and 

north of Lobo Peak. They locally show planar and 

contorted flow bands and even flow breccia structure, but more commonly they are homogeneous and 

massive. Hand specimens are characterized by as 

much as 25 percent phenocrysts of quartz and feldspar, 1-3 mm in diameter, in an aphanitic chertlike 

groundmass whose color ranges from light to medium gray, light olive gray, and light brownish gray. 

Thin sections from a thick flow near the top of the 

high ridge west of Tank Canyon (fig. 4) show large 

embayed crystals and clots of quartz, albite, and 

sanidine, scarce small books of biotite, tiny crystals of 

magnetite and leucoxene, and traces of apatite and 

zircon in a cryptocrystalline to microcrystalline 

groundmass. The albite is flecked with sericite, the 

sanidine is slightly turbid and has a larger optic 

angle than commonly found in that mineral (2V 

estimated to be 20°-50°, optic sign negative), and 

the biotite is largely altered to muscovite and 

leucoxene. The groundmass, where little altered and 

coarse enough to be resolved under the microscope, 

consists mostly of quartz and potassium feldspar 

and contains dispersed flakes of mica.

Lapilli tuff and tuff-breccia, in nonbedded and 

nonsorted units tens to hundreds of feet thick, are 

very distinctive and widespread rocks in the Red 

Boy Rhyolite. These rocks consist of well-consolidated ash which resembles the flows in composition 

but which contains numerous pieces of devitrified 

pumice and rock fragments as large as blocks
SIERRITA MOUNTAINS, PIMA COUNTY D37

(>32 mm). Most of the rock fragments are gray, 

pink, brown, and red-purple rhyolitic rock and, in 

thin section, have the characteristics of flows, 

welded tuffs, and nonwelded tuffs. Accidental fragments of andesite and other rocks are less abundant. 

The juvenile constituents of the tuff look different 

in the two alteration facies of the formation. South 

of the mountains, the pumice fragments are represented by yellowish-green to white clay and the ash 

fraction is dull-pink, red, or brown argillized material that contains crystals and crystal fragments of 

quartz, albite, and sanidine of the same types as 

those in the flows. North of Lobo Peak, pumice fragments and ash are difficult to distinguish, inasmuch 

as both are represented by hard chertlike material 

that is generally medium gray or greenish gray.

The argillized tuff south of the mountains is 

mostly massive but locally has a weak eutaxitic 

structure due to the parallel arrangement of pumice 

plates. The gray northern facies also appears massive at most places but locally shows a faint foliated 

eutaxitic structure on some weathered surfaces and 

on some freshly broken and moistened surfaces. 

Thus, the tuff has the principal field characteristics 

of ash-flow tuff (Ross and Smith, 1961, p. 18-26), 

which is welded toward the north and nonwelded 

toward the south.

Thin sections of the tuff confirm its pyroclastic 

origin and increasing compaction toward the north. 

Many of the juvenile crystals are broken into 

sharply angular fragments, and the once-glassy material shows poorly preserved vitroclastic texture. 

Very little compaction is evident in thin sections of 

the argillized facies examined, although the internal porosity of the pumice has been obliterated 

by argillization. Compaction foliation in the northern gray facies is much more apparent in thin section than in hand specimen. The pumice fragments 

are deformed into subparallel thin lenses that are 

molded over and under mineral and rock fragments. 

The rock fabric in the northern area is that of 

welded tuff, but only rarely is there even the 

faintest suggestion of axiolitic structure in the 

devitrified pumice and shards. Apparently the ash 

flows were sufficiently hot and rich in volatiles to 

weld locally but not to form normal devitrification 

textures. It is possible of course that original 

devitrification textures have been erased by later 

metamorphism, but this seems unlikely inasmuch as 

the pumice fragments are finely crystalline and the 

relict axiolitic, spherulitic, and plumose devitrification structures are preserved in some of the contained rock fragments. The argillized pumice in the 

southern area lacks the relict structure of any

earlier devitrification products and probably remained in the glassy state until it was argillized, 

presumably by weathering in Quaternary time.

Fine-grained, well-sorted, and locally bedded 

rhyolitic tuff is a minor constituent of the formation. Thorns (1966, p. 163) described well-stratified 

light-gray slightly friable tuff at a locality about 2 

miles southwest of Red Boy Peak that consists 

mostly of crystals and crystal fragments, 0.1-1.0 

mm in size, of quartz and alkali feldspars in a 

matrix of microlitic debris. I have observed finegrained indistinctly bedded tuffs at a few places in 

streambeds southeast and south of this locality and 

other tuffs without apparent bedding within the 

gray facies of the formation in the core of the range.

A possible volcanic mudflow is present in a canyon 

bottom several miles south-southwest of Red Boy 

Peak. This rock consists of subrounded pebbles, 

cobbles, and boulders of rhyolite and more rarely 

andesite in an abundant friable matrix of rhyolitic 

debris. Boulders of andesite breccia from the 

Demetrie Volcanics as much as 4 feet in diameter 

occur in this rock. The unit is near the bottom of the 

formation and could be part of the basal conglomerate. A few lenses of rhyolite conglomerate 

stratigraphically higher in the formation are intercalated with lapilli tuff and tuff-breccia on the high 

peak 0.3 mile north of Red Boy Peak.

The Red Boy Rhyolite is more metamorphosed 

and altered in the center of the Sierrita Mountains 

than it is farther south. The tuffs south of the 

mountains are pervasively argillized but otherwise 

virtually undeformed and unaltered, whereas those 

to the north within the mountains show the effects 

of metamorphism and alteration as well as greater 

primary compaction. The effects of metamorphism 

are generally apparent only in thin section and 

include strain and rupture of phenocrysts, microfaulting, and microveining oblique to earlier compaction foliation. The principal vein and replacement 

minerals are quartz, chlorite, and potassium 

feldspar (?). In Laramide shear zones the rock has 

been converted to well-foliated and commonly 

lineated mylonite containing tiny augen of quartz 

and feldspar. Hand specimens of the mylonite commonly resemble much-compacted welded tuff, 

whereas thin sections are clearly cataclastic. The 

mineral augen show such strain effects as undulatory extinction, mortar structure, and granulated 

rims and tails. Other foliate features are small flat 

lenses of granoblastic quartz and alkali feldspar and 

intervening similar but finer grained material that 

contains oriented tiny crystals of biotite and tourmaline. The potassium feldspar in the augen is
D38 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

largely orthoclase perthite presumably formed by 

annealing of the original sanidine.

The Red Boy is slightly metamorphosed where it 

comes closest to Laramide plutons, and it is mineralized in two widely separated areas. Southwest of 

Red Boy Peak in the SE^ sec. 21, T. 18 S., R. 11 E., 

the formation is cut by narrow quartz veins containing a little galena, sphalerite, chalcopyrite, chalcocite, and oxidized iron and copper minerals; and 

northeast of Red Boy Peak in Ox Frame Canyon 

(near boundary of sees. 3 and 10, T. 18 S., R. 11 

E.), the formation is locally altered and contains 

veinlets and disseminations of pyrite and oxidized 

iron and copper minerals.

The northern gray facies of the Red Boy Rhyolite 

resembles in many respects the rhyolitic rocks in 

the Ox Frame Volcanics. Of some aid in distinguishing the two formations are the common lithic lapilli 

in the Red Boy and the quartzite beds in the Ox 

Frame. In addition, there is a greater abundance 

and variety of intrusive igneous rocks in the Ox 

Frame. However, I believe that quartz phenocrysts 

provide the best basis for making the distinction. 

The Red Boy generally contains quartz phenocrysts 

large enough (1-3 mm) to be readily seen in hand 

specimen, whereas the Ox Frame contains quartz 

phenocrysts that large only in sparse and generally 

thin units. The "quartz-eye" criterion is not infallible, but seems to be valid at least in the larger 

areas of outcrop. It yields a reasonable map pattern 

and assigns to the Red Boy rocks whose stratigraphic relations are compatible with this assignment but are incompatible with assignment to the 

Ox Frame. For example, it reveals the Red Boy in 

angular unconformity on the Ox Frame near the 

head of Ox Frame Canyon (SEi,4 sec. 3, T. 18 S., 

R. 11 E.), and farther north (SE^ sec 34, T. 17 S., 

R. 11 E.) it shows the Red Boy having a basal conglomerate that lies unconformably on post-Ox 

Frame quartz monzonite.

CONTACTS

The basal contact of the Red Boy is an angular 

unconformity cut on the Demetrie Volcanics, Ox 

Frame Volcanics, and Mesozoic intrusive rocks. The 

contact with the Demetrie is easily recognized and 

is described in the section dealing with that formation. The contact with the Ox Frame is generally 

obscure because the Ox Frame is also resistant to 

erosion, is largely rhyolite, and rarely shows distinct 

bedding. The Red Boy has no known basal conglomerate where it overlies the Ox Frame. Criteria 

which help to locate the contact include angular 

discordance and those discussed in the preceding

paragraph. The location of the basal contact is readily apparent where the underlying rock is plutonic 

and is further marked, at least locally, by a basal 

conglomerate containing fragments of the Demetrie, 

the Ox Frame, and other pre-Red Boy formations.

The upper contact of the Red Boy on the south 

flank of the Sierrita Mountains is an easily recognized angular unconformity with overlying unconsolidated Holocene alluvium or with weakly 

consolidated Tertiary gravel and basaltic andesite 

flows of middle Tertiary age. The upper contact at 

most places is the present erosion surface.

AGE AND CORRELATION

The age of the Red Boy Rhyolite must be intermediate between that of the underlying Demetrie 

Volcanics and overlying sedimentary and volcanic 

rocks of middle Tertiary age. Indirect field evidence 

suggests that the formation predates Laramide 

plutonism and mineralization in the area. The Red 

Boy is cut by andesite dikes and plugs petrographically like those of early Tertiary age. Several faults 

and shear zones that cut the Red Boy are intruded 

by unfaulted and unsheared granodiorite that forms 

a large pluton whose K-Ar age is Paleocene (about 

60 m.y.). Rocks much like the Red Boy in lithology 

and apparent stratigraphic position have been dated 

by the K-Ar method in at least five mountain ranges 

in southeastern Arizona. The results are shown in 

the following table:

Stratigraphic unit 

and locality

Mineral 

analyzed

Number Apparent

of age

analyses (m.y.)

Reference

Mount Lord Ignimbrite 

of Watson (1964). 

Silver Bell Mountains ............................Alkali feldspar.... 1

Cat Mountain Rhyolite 

of W. H. Brown 

(1939), Tucson 

Mountains ............................do............. .. 2

60 Mauger (1966).

66-70 Bikerman and

Damon (1966).

Roskruge Rhyolite, 

Roskruge Mountains ...................................... do.................. 1

Do . .................Plagioclase .......... 1

Do ............................Biotite .................. 9

Rhyodacite welded tuft 

in upper part of 

Salero Formation, 

Santa Rita Mountains .... ...............................do.................. 1 73

Volcanics of Jones

Mesa, Canelo Hills..............do.................. 1 72

69 Bikerman (1967).

66 Do.

69-74 Do.

Drewes (1968).

S. C. Creasey 

(written 

commun., 1964),

Biotite, which is an argon-retentive mineral, 

gives the best indication of the true age of the 

formations. The feldspars seem to. have lost argon
SIERRITA MOUNTAINS, PIMA COUNTY D39

and hence yielded younger apparent ages. For example, a single sample from the Roskruge Rhyolite 

yielded ages of 72.6 and 72.2 m.y. for the biotite, 

68.8 m.y. for the alkali feldspar, and 66.3 m.y. for 

the plagioclase (Bikerman, 1967, table 1, sample 8). 

The discrepancies in this sample are almost within 

the limits of analytical error and are not very serious. The apparent age of 60 m.y. for the Mount 

Lord Ignimbrite is misleadingly low, however. This 

rock predates quartz monzonite whose biotite has 

yielded ages of 67.1 m.y. where fresh, 65.5 m.y. 

where mineralized, and 63.4 m.y. where mineralized 

and leached (Mauger, 1966). Clearly the Mount 

Lord is little, if any, younger than the other dated 

units. As the Cretaceous-Tertiary boundary is now 

closely defined at 63 m.y. ago in the K-Ar time 

scale (Folinsbee and others, 1963), the dated units 

and presumably the Red Boy Rhyolite also are of 

Late Cretaceous age.

REFERENCES

Bailey, E. H., and Stevens, R. E., 1960, Selective staining of 

K-feldspar and plagioclase on rock slabs and thin sections: Am. Mineralogist, v. 45, nos. 9 and 10, p. 1020- 

1025.

Bikerman, Michael, 1967, Isotopic studies in the Roskruge 

Mountains, Pima County, Arizona: Geol. Soc. America 

Bull., v. 78, no. 8, p. 1029-1036.

Bikerman, Michael, and Damon, P. E., 1966, K/Ar chronology 

of the Tucson Mountains, Pima County, Arizona: Geol. 

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D40 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

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INDEX

[Italic page numbers indicate major references]

Page 

Age, Angelica Arkose ......................................1)29

Demetrie Volcanics ........................................34

Ox Frame Volcanics......................................JS

Radiometric. See Radiometric age

determinations. 

Red Boy Rhyolite...................... ..................SS

Rodolfo Formation ........................................HZ

Stevens Mountain Rhyolite............................2^

Tascuela Redbeds ............................................ 15

Whitcomb Quartzite ......................................25

Amole Arkose ........_......................................5, 15, 29

Angelica Arkose ........................................15, 18, 26

age .................^........^........................................J19

contacts ...................................... it, 24, 20, 34

Angelica Wash ........................................................26

Ash Creek ... ......... ................8, 13, 19, 22

Ash Creek fault ........................................................6

Biotite rhyolite, contacts ................................18, 21

Bisbee ..........................................................................4

Bisbee Group ..........................................15, 26, 29

Bronco Volcanics ....................................................34

Canelo Hills Volcanics ..............................15, 22, 24

Christmas ..............................................................34

Composition, Angelica Arkose ..........................27

Ox Frame Volcanics, andesite

porphyry member ......................................11

Rodolfo Formation, andesite 

breccia unit ................................................21

Contacts, Angelica Arkose..............22, 24. 29, 34

biotite rhyolite ..........................................18, 21

Demetrie Volcanics ..........5, 8, 15, 23, 26, 20,

£4.38 

Jurassic rocks ..................................................36

Mesozoic intrusive rocks ..............................38

Ox Frame Volcanics......4, 5. 8, 11, IS, 36, 38

middle member ..........................................9

Paleozoic rocks ................................................13

Permian rocks ..................................................18

Quaternary alluvium ......................................18

Rainvalley Formation ....................................22

Red Boy Rhyolite..................4, 6, 8, 30. 34, 38

Rodolfo Formation ....................18. 21, 22. 24

Silver Bell Formation ....................................5

Stevens Mountain Rhyolite................l4, 23, 25

Tascuela Redbeds ......................................13, 23

Tertiary rocks ..................................................35

Whitcomb Quartzite ........18, 2J, 24, 25, 29

Copper ........................................................................4

Cowboy fault ..............................................................8

Cretaceous rocks, Upper ......................................SO

Cretaceous (?) rocks, Lower..........................24, 25

Demetrie Volcanics ..................4, 12, 28, 30, 36

age ......................................................................34

contacts ....................5, 8. 15, 23, 26, 20, 34, 38

Demetrie Wash ..................................................30, 32

Distribution. Demetrie Volcanics-.......................S0

Ox Frame Volcanics ........................................8

Rodolfo Formation ..........................................18

Whitcomb Quartzite ......................................24

Empire Mountain ..29

Page 

Erosion ..........................................................D13, 15

Esperanza mine ................................5, 8, 14, 32, 33

Esperanza Wash ...................................................32

Faulting ...._.. . ............ ....... 5, 13. 18

Fort Crittenden Formation...................................35

Fossils, algal heads ...............................................29

brachiopods .......................................................5

Camerotoechia ..................................................A

gastropods ......................... .......................5, 29

ostracodes ....................................................5. 29

pelecypods ................................................ ..-5, 29

plant .................................................................35

Fresnal Quartz Monzonite .....................................6

Gardner Canyon Formation............................15. 22

Glance Conglomerate .........................................15

Harris Ranch .22

Introduction .............................................................2

Investigations, previous .........................................4

Jurassic rocks ............................................... 23, 36

Keystone Peak ........................................................35

Kino Formation ................................................18, 20

Laramide orogeny ........................................4. 26, 34

Lead ..........................................................................4

Lithology, Angelica Arkose..................................26

Demetrie Volcanics, andesite 

and dacite ......................................................32

conglomerate ............................................30

rhyolitic tuff members ......................33

Ox Frame Volcanics, Palo Alto 

Ranch quadrangle ......................................11

Twin Buttes quadrangle ........................8

Red Boy Rhyolite .. .... ..*«

Rodolfo Formation, basal 

conglomerate member ................................ 19

middle siltstone member ......................19

upper volcanic member ........................20

Whitcomb Quartzite ......................................24

Little Dragoon Mountains ..................................22

Lobo Peak ................................................................35

Location ......................................................................2

McKee, E. D., and Ross, R. J.. Jr., 

written communication .....................................5

Mesozoic intrusive rocks, contacts ...................38

Metamorphism ............................................14 18, 37

Mineral Hill .........................................................4, 5

Mission mine ........................................18, 19, 20

Mission ore deposit ............................................ .5

Morita Formation ..................................................15

Mount Wrightson Formation .......................14, 22

Ox Frame Canyon ...........................................6, 38

Ox Frame Formation ................................_..._.......6

Ox Frame Volcanics........................5, 20, 22 30, 33

Page 

Ox Frame Volcanics Continued

age ....... ............... .. .. . D./S

contacts ............................4, 5, 8, 11, IS, 36, 38'

lower member, Twin Buttes quadrangle......9

middle member. Twin Buttes quadrangle....9

upper member, Palo Alto Ranch 

quadrangle ....................................................12

Twin Buttes quadrangle........................lO

Paleozoic rocks ..........................................................8

Pato Alto Ranch quadrangle........................2, 8,12

Papago Formation ..........................................18, 20

Papago Well ............................................................35

Permian rocks, contacts ......................................18

Pima copper deposit ................................................4

Pima County ..............................................................5

Pima mine ..........................................................19, 20

Placer Peak ..............................................................35

Plutonism ............ . .. .. ... .......v ...5

Quaternary alluvium, contacts ............................18

Radiometric age determinations, 

biotite rhyolite ......................................................5

Canelo Hills Volcanics....... 22, 24

Gardner Canyon Formation....................15, 22

intrusive rocks ..........................................13, 19

Mount Lord Ignimbrite ................................39

Ox Frame Volcanics....................................5, 14

Recreation Redbeds ................................15, 22

Red Boy Rhyolite............................................38

Rodolfo Formation ..........................................22

Roskruge Rhyolite ..........................................39

Rainvalley Formation ....................................18, 22

Recreation Redbeds ................................4, 5, 15, 22

Red Boy Peak ..........................................................35

Red Boy Rhyolite............................................5, 9, 35

age ... ..-.............................................................38

contacts ..................................4, 6, 8, 30, 34, 38

Reeside, J. B., written communication..............29

Rodolfo Formation ................................................15

age ......................................................................22

contacts ..........................................18, 21, 22, 24

Rodolfo Wash ..........................................................15

Ross, R. J., Jr., and McKee, E. D.,

written communication ......................................5

Salero Formation ....................................................34

San Xavier thrust fault.............................-........ 5

Santa Cruz County ..............................................5

Santa Cruz River ....................................................29

Santa Rita Mountains ................14, 22, 26. 29, 34

Scherrer Formation ................................................5

Senator Morgan mine ....................................18, 26

Silver ..........................................................................4

Silver Bell Formation..................................*, 30, 34

Silver Bell Mountains..-..-------.-..-.....*, 30

Silver City, N. Mex.. ..... .. ... ......34

Stern, T. W., written communication..............22

Stevens Mountain ............................................13, 23

Stevens Mountain Rhyolite..................6, 22, 23, 30

age ........ ..... ......- - ..... .24

contacts ................................................14, 23, 25

Sunshine Formation . . . .......8

D41
D42 INDEX

Page 

Tank Canyon ......................................................D86

Tascuela Canyon ..........................................6, 11, 14

Tascuela Redbeda ................................14, 22, US, 30

age ......................................................................15

contacts ...................................................... is, 23

Tascuela Wash ..........................................................6

Tertiary rocks, contacts ......................................35

Thickness, Angelica Arkose..................................26

Demetrie Volcanics ........................................SO

Mesozoic rocks ..................................................3

Ox Frame Voleanies ........................................5

Red Boy Rhyolite ..........................................35

Rodolfo Formation ..........................................18

Stevens Mountain Rhyolite............................23

Page 

Thickness ^Continued

Tascuela Redbeds ........................................D14

Whiteomb Quartzite .....~...............................S4

Thorns, J. A., quoted....................................6, 8, 14

Tombstone ................................................................34

Triassic rocks ......................................................6, is

Tueson ...............................................................4

Tueson Mountains ................................4, 15, 22, 29

Twin Buttes ....................................................4, 9, 10

Twin Buttes mine......................... .......................18

Twin Buttes ore deposit................................. ......5

Twin Buttes quadrangle..........................................2

Type locality, Angelica Arkose...........................Jt6

Demetrie Voleanies ........................................SO

Ox Frame Voleanies....................................6, 8

Page 

Type locality Continued

Red Boy Rhyolite ........................................DM

Rodolfo Formation ...........................^...........15

Stevens Mountain Rhyolite............_.............JM

Taseuela Redbeds ............................................14

Whiteomb Quartzite ......................................t4

Walnut Gap Volcanics..........................................22

Whetstone Mountain .............................................J9

Whiteomb Hill ........................................................29

Whiteomb Quartzite ..................................18, t4, 27

age .....................................................................JtS

contacts ....................................18, tl, 24. SS, 29

Zinc

U.S. GOVERNMENT PRINTING OFFICE: 1971 O «23-S3»