New and Revised i 

Stratigraphic Names in 

the Santa Rita Mountains 

of Southeastern Arizona

By HARALD DREWES

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1274-C

A Stratigraphic column is presented for the 

Santa Rita Mountains, and formations 

are defined or revised. Their ages are 

supported by radiometric determinations
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

U.S. GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1968

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

Washington, D.C. 20402 - Price 10 cents (paper cover)
CONTENTS

Page

Abstract _________________-___ ______ - Cl

Introduction ____________ __ _ _ _ 1

Precambrian rocks ________________ __ _ _ 3

Continental Granodiorite _ -_ __ _ _ 3

Triassic rocks _______________ _ _ _ 6

Mount Wrightson Formation ______________________. 6

Gardner Canyon Formation _______ ________ __ 7

Piper Gulch Monzonite ___-_ __ _____-_ 7

Jurassic rocks __________________ __ ____ 8

Squaw Gulch Granite ___ ___ ___ 8

Lower Cretaceous rocks _____ _ _____ ____ 8

Temporal Formation __________________________ 8

Bathtub Formation ____________________________ 9

Upper Cretaceous rocks ____~____________________-_ 10

Fort Crittenden Formation _______________________ 10

Salero Formation _________________________ _ 11

Josephine Canyon Diorite _____________________-_ 12

Madera Canyon Granodiorite ______________________ 12

Elephant Head Quartz Monzonite ___________________ 13

Paleocene(?) rocks __ ______ ___ __ _ 14

Gringo Gulch Volcanics _ _ _ __ _ __ 14

Upper(?) Oligocene rocks _____ __________________ 14

Grosvenor Hills Volcanics _ _ __ ________ 14

References cited __________ ____________ __ 15

ILLUSTRATION

Page 

FIGURE 1. Index map showing location of Santa Rita Mountains, Ariz _ C2

TABLE

Page 

TABLE 1. Sequence and ages of rocks in the Santa Rita Mountains

of southeastern Arizona ___ _____ ___ __ C4

in

CONTRIBUTIONS TO STRATIGRAPHY

NEW AND REVISED STRATIGRAPHIC NAMES IN THE 

SANTA RITA MOUNTAINS OF SOUTHEASTERN ARIZONA

By HARALD DREWES

Abstract

Recent work provides the basis for establishing a stratigraphic column 

in the Santa Rita Mountains of Santa Cruz and Pima Counties of southeastern Arizona. Thirteen formations of this column are here defined and 

one is revised. One new formation is a Precambrian granodiorite. Ten 

new formations are Mesozoic units that include plutonic, volcanic, and 

continental sedimentary rocks. The revised formation is the Upper Cretaceous Fort Crittenden Formation of Stoyanow (1949), dominantly a sedimentary unit. Two new formations are volcanic and sedimentary rocks of 

Tertiary age. Many radiometric dates supplement the sparse fossil record 

in corroborating or redefining the ages that had been assigned to many 

units on the basis of their geologic relations and in establishing the ages 

of other units.

INTRODUCTION

Detailed geologic mapping and petrographic studies of the 

rocks in the Santa Rita Mountains since 1962 (Drewes, 1966) 

provide the basis for establishing a stratigraphic sequence in the 

area (fig. 1). The names of the Paleozoic formations and one of 

the Precambrian formations recognized in the Santa Rita Mountains have previously been used over much of southeastern Arizona and they are retained in this report. However, one additional 

Precambrian unit, many Mesozoic units, and two Cenozoic units 

are newly recognized in the report area, are of considerable areal 

extent and geologic importance, and are formally defined. In 

addition, the Upper Cretaceous Fort Crittenden Formation, originally described by Stoyanow (1949), is now seen to be a more 

complex unit that is several times the thickness of the original 

one and is herein revised.

ci
C2 CONTRIBUTIONS TO STRATIGRAPHY

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FIGURE 1. Index map showing location of Santa Rita Mountains, Ariz. 

1, Box Canyon; 2, Mount Wrightson; 3, Gardner Canyon to south and 

Cave Creek canyon to north; 4, Piper Gulch to north and Mansfield 

Canyon to south; 5, Squaw Gulch; 6, Temporal Gulch; 7, ranch; 8, 

Bathtub; 9, El Pilar Tank and Adobe Canyon; 10, bench mark YOUNG 

and Hog Canyon; 11, Salero ranch and mine; 12, Josephine Canyon; 13, 

Madera Canyon; 14, Elephant Head; 15, Gringo Gulch; 16, Cinigita 

Canyon to north and Coal Mine Canyon to south.
STRATIGRAPHIC NAMES, SANTA RITA MOUNTAINS, ARIZ. C3

The Mesozoic and Cenozoic strata consist largely of continental 

sedimentary and volcanic rocks in which lateral facies changes 

are rapid and thicknesses highly variable. Large stocks and other 

bodies both intrude these strata and, in places where the Paleozoic 

rocks are absent, underlie them. Radiometric ages of the intrusives have provided many critical data on this sparsely fossiliferous sequence.

Many of the formations extend into ranges adjacenrt to the 

report area where they commonly are less complete and less well 

dated or are more intensely altered and faulted than in the Santa 

Rita Mountains. Some geologic relations, however, have been 

clarified or substantiated by studies in /these adjacent areas (fig. 

1). These studies include the work in the Patagonia Mountains 

to the south and southeast by F. S. Simons; in the Oanelo Hills 

to the east by R. B. Raup; in the Empire Mountains, also to the 

east, by Tommy Finnell; and in the Sierrita Mountains to the 

northwest by J. R. Cooper. Enough is known of the geology of 

the Rincon and Santa Catalina Mountains to the north, just 

beyond the area shown in figure 1, to indicate that rocks of those 

mountains present essentially a different problem; enough is also 

known of the rocks of the Tumacacori Mountains to the southwest 

to show that mid-Tertiary volcanics cover most of the older rocks, 

which leaves a very sparse and fragmentary record. As a result, 

the general stratigraphic results of the present study will probably not be appreciably augmented by additional work, although 

further refinement of the facies and subdivisions of the units and 

more complete and accurate dating may be possible.

This report serves to introduce the new formations and their 

geologic relations. Their typical lithology and location are described briefly here. Descriptions in greater detail, along with 

regional correlations and tectonic implications, are being prepared as a series of topical reports.

PRECAMBRIAN ROCKS

CONTINENTAL GRANODIORITE

A coarsely porphyritic Precambrian granodiorite underlies much 

of the northern part of the Santa Rita Mountains. It intrudes 

gneissic rocks, contains inclusions of schistose rock probably correlative with the Pinal Schist, is overlain along a fault contact 

by Paleozoic rocks, and lies unconformably beneath the Lower 

Cretaceous Bisbee Formation. The contact with the Paleozoic 

rocks probably was once an unconformity, as it still is in many 

nearby ranges, along which 'the faulting took place. The rock is
C4 CONTRIBUTIONS TO STRATIGRAPHY

TABLE I. Sequence and ages of rocks in the Santa Rita Mountains of

southeastern Arizona

[Names marked by double asterisk are new in this report. Name marked by asterisk is 

redefined and newly accepted for usage by the U.S. Geol. Survey]

Age Stratiyraphic unit and description

Cenozoic :

Recent to Pliocene Fine gravel and silt units, unnamed, thickness

unknown.

Pliocene and Miocene _ _ Gravel of Nogales, more abundant to south. 

Late(?) Oligocene ___ Quartz monzonite stock and rhyodacite dike

swarm and laccoliths of a single genetic

sequence (K-Ar, 26-27 m.y.). 

Grosvenor Hills Volcanics:** rhyolite and

rhyodacite volcanics, also of the above genetic sequence.

Eocene (?) _ _ _ .Quartz vein swarm; mineralized.

Paleocene _________ . Quartz latite porphyry dike swarms and small

stocks; associated with mineralization (KAr, 56 m.y.). 

Quartz monzonite stocks (K-Ar, 64-60 m.y.).

Paleocene(?) _____ ___Gringp Gulch Volcanics:** largely rhyolitic to

dacitic volcanics.

Mesozoic:

Late Cretaceous _____ Elephant Head Quartz Monzonite:** fine- and

coarse-grained quartz monzonite, two large 

stocks (K-Ar, 68-69 m.y.; Pb-alpha, 188

Madera Canyon Granodiorite:** leucocratic 

and melanocratic granodiorite and porphyritic granodiorite (K-Ar, 68 m.y.).

Josephine Canyon Diorite:** medium-coarsegrained diorite and quartz diorite (K-Ar, 

67 m.y.; Pb-alpha, 62-63 m.y.).

Salero Formation:** rhyodacite to dacite volcanics, arkose and other sedimentary rocks 

(K-Ar, 72 m.y.).

Fort Crittenden Formation:* conglomerate, 

. sandstone, and shale; Santonian to Maestrichtian fauna.

Early Cretaceous ____ Bisbee Formation: sandstone, arkose, siltstone,

conglomerate, and limestone; Aptian-Albian

fauna in type area. 

Bathtub Formation:** rhyolitic to andesitic

volcanics and conglomerate. 

Temporal Formation;** rhyolitic to andesitic

volcanics and conglomerate.

Jurassic __ -__ _____ Squaw Gulch Granite:** coarse-grained granite and quartz monzonite (K-Ar, 145 m.y.; 

Pb-alpha, 160-161 m.y.).

Triassic _________ Piper Gulch Monzonite:** very coarse grained

monzonite (Pb-alpha, 184 m.y.). 

Gardner Canyon Formation:** red beds, arkose, and dacite volcanics (Pb-alpha, 192

Mount Wrightson Formation:** rhyolitic to 

dacitic volcanics and eolian sandstone (Pbalpha, 220 m.y.).
STRATIGRAPHIC NAMES, SANTA RITA MOUNTAINS, ARIZ. C5

TABLE 1. Sequence and ages of rocks in the Santa Rita Mountains of 

southeastern Arizona Continued

Age Stratigraphic unit and description

Paleozoic:

Permian __________Rainvalley Formation: limestone, dolomite,

and sandstone.

Concha Limestone: cherty thick-bedded limestone.

Scherrer Formation: quartzite and dolomite. 

Epitaph Dolomite: dolomite, marl, and limestone. 

Colina Limestone: dark-gray moderately thick

bedded limestone. 

Permian and

Pennsylvanian ____Earp Formation: red siltstone and limestone. 

Pennsylvanian _____Horquilla Limestone: light-gray limestone and

siltstone.

Mississippian ______Escabrosa Limestone: massive coarsely crystalline limestone. 

Devonian _______,_Martin Limestone: limestone, dolomite, and

sandstone. 

Cambrian _ ___ _Abrigo Limestone: sandstone, siltstone, and

limestone.

Bolsa Quartzite: quartzite and conglomerate.

Precambrian __________ Continental Granodiorite: * * coarse-grained

porphyritic granodiorite and quartz monzonite (Rb-Sr, 800 m.y.; Pb-alpha, 1,360- 

1,450 m.y.). 

Final Schist: schist and gneiss.

here named the Continental Granodiorite for its extensive exposures in the hills east of the town of Continental. The type area 

for the granodiorite is along Box Canyon (fig. 1, No. 1), a few 

miles east of Continental, where there are excellent exposures 

along both canyon bottom and road.

The Continental Granodiorite ranges in composition from granodiorite to quartz monzonite, and numerous small bodies of leucocratic quartz monzonite and aplite form a separately mappable 

phase of the formation. Granophyric and myrmekitic textures 

are common, and the large crystals, as much as a few centimeters 

long, are porphyroblasts. The approximate mode of the rock, 

given in percent, is: quartz, 10-20; orthoclase or microcline, 10- 

40; plagioclase, 25-55; biotite or chlorite, 10; and amphibole, 

magnetite, sphene, apatite, and zircon in smaller amounts. One 

sample of the rock is radiometrically dated at 800 ±80 m.y. (million years), by means of the Rb-Sr method on whole rock (Zell 

Peterman, written commun., 1966) and at 1,450±160 m.y., by 

the Pb-alpha method on zircon (T. W. Stern, written commun., 

1966). Zircon from an additional sample of the rock was dated, 

also by means of the Pb-alpha method, at 1,360 m.y.±20 percent 

(T.W. Stern, written commun., 1968), and, by means of the K-Ar

307-798 O - 68 - 2
C6 CONTRIBUTIONS TO STRATIGRAPHY

method, biotite from that sample was dated at 56 m.y. (R. F. 

Marvin, H. H. Mehnert, and Violet Merritt, written commun., 

1967). Recrystallization textures in the rock indicate that at the 

collection locality the rock was metamorphosed, presumably by 

the nearby Paleocene intrusives.

TRIASSIC ROCKS

MOUNT WRIGHTSON FORMATION

The name Mount Wrightson Formation is given to a sequence, 

at least 8,500 feet thick, of Triassic volcanic and sedimentary 

rocks that underlie the crest and upper part of the east flank of 

the Santa Rita Mountains. Mount Wrighitson (fig. 1, No. 2), the 

highest peak in the range, is a resistant mass of rhyolitic rock 

that is part of the middle member of the formation. The type area 

of the Mount Wrightson Formation includes a stretch along the 

trail between Madera (fig. 1, No. 13) and Gardner (fig. 1, No. 

3) Canyons where the lower and middle members are well exposed, and the north slope of Cave Creek canyon at the end of the 

road where the upper member is well exposed. The base of the 

formation is unexposed, for plutonic rocks intrude it everywhere; 

however, the change from a sequence of Paleozoic rocks of marine 

origin to a sequence of Mesozoic rocks of continental origin, such 

as occurs in the report area, is presumably marked by an unconformity. Likewise, the top of the Mount Wrightson is everywhere 

a fault contact in the Santa Rita Mountains, but in the south fork 

of Monkey Canyon of the northern Canelo Hills rocks of similar 

lithology lie beneath red beds correlated with the Gardner Canyon 

Formation.

The Mount Wrightson Formation is divided into three members. The lowest member, at least 1,500 feet thick, is made up 

largely of dacitic to andesitic rocks but also includes some rhyolitic rocks and intercalated thin lenses of quartzite and sandstone. Some of the andesitic rocks are amygdaloidal and vesicular; 

others are coarsely porphyritic and resemble the Tertiary rock 

of the Tucson area that is locally referred to as the "turkey track" 

andesite. In the middle member, about 5,000 feet thick, rhyolite 

and latite are dominant over dacitic volcanics; in addition, lenses 

of sandstone and quartzite are abundant, and of conglomerate, 

sparse. The volcanics of this member are mostly well indurated, 

grayish red, finely flow laminated, and finely porphyritic; some, 

however, are nonlaminated or spherulitic. Near the type area, 

flows are most common; but to the north, welded tuff is dominant 

over flows, and to the south, flow breccia and tuff breccia are most 

abundant. The upper member, at least 2,000 feet thick, contains
STRATIGRAPHIC NAMES, SANTA RITA MOUNTAINS, ARIZ. C7

andesitic and rhyolitic volcanics and thick bodies of eolian sandstone. The andesites include both pillow lavas and amygdaloidal 

flows. A zircon concentrate from a lava flow of the middle member 

was radiometrically dated by the Pb-alpha method as 220 it 30 

m.y. (T. W. Stern, written commun., 1966).

GARDNER CANTON FORMATION

Triassic red beds and intercalated conglomerate, limestone, and 

volcanic rocks, totaling at least 1,000 feet in thickness, crop out 

along a major fault zone that strikes northwestward across the 

Santa Rita Mountains. These rocks are named the Gardner 

Canyon Formation for their excellent exposures along Gardner 

Canyon and adjacent parts of Cave Creek (fig. 1, No. 3), which 

are designated as the type area. The formation lies unconformably on various Permian formations, and conglomerate in the 

red beds contains pebbles of the Mount Wrightson Formation. 

In the Santa Rita Mountains the top of the formation is everywhere a fault contact, but to the southeast, in the northern Canelo 

Hills, it is overlain by tuffs of the lower unit of the Canelo Hills 

Volcanics of Triassic and Jurassic age (Hayes and others, 1965, 

p. M3). This contact is now believed by Raup to be an unconformity.

The Gardner Canyon Formation in its type area consists of two 

members. The lower member, about 200 feet thick, is a siltstone 

that contains lenses of well-rounded pebble conglomerate and two 

thin beds of limestone. Plant fragments were collected from a 

coaly pocket in one of the limestone beds but were unidentifiable; 

fossil pollen was looked for but not found. The upper member is 

a red mudstone that contains some beds of sandstone, conglomerate, and dacite flows and tuff, along with some small dacite 

bodies that probably were intruded at shallow depths. A radiometric age calculated as 192 m.y. and presented by T. W. Stern 

(written commun., 1965) as 190±20 m.y. was obtained for a 

zircon concentrate from a dacite flow.

PIPER GULCH MONZONITE

. A very coarse grained dark-gray monzonite of Triassic age 

forms a string of bodies that lie roughly along the crest of the 

southern part of the >Santa Rita Mountains. This rock intrudes 

the Mount Wrightson Formation and is intruded by the Jurassic 

Squaw Gulch Granite and younger plutonic rocks, in which it 

forms large northwest-alined inclusions and septa. The rock is 

named Piper Gulch Monzonite for its exposures near the jeep road 

in Piper Gulch, the type locality (fig. 1, No. 4), a tributary gulch
C8 CONTRIBUTIONS TO STRATIGRAPHY

to Mansfield Canyon. This monzonite seems to be unique in age 

and composition to this part of southeastern Arizona.

The Piper Gulch Monzonite typically has a grain size of 7-15 

mm (millimeters), and it is finely myrmekitic. Its approximate 

modal composition, in percent, is: quartz, 1-10; orthoclase, 10- 

40; plagioclase, 30-55; amphibole, 2-20; pyroxene, 0-7; magnetite 3-6; and spheme, biotite, apatite, and rutile(?), all in small 

amounts. The plagioclase ranges in composition from calcic oligoclase to sodic labradorite. A zircon concentrate from the Piper 

Gulch was radiometrically dated at 184 m.y. by the Pb-alpha 

method (reported as 180±20 m.y., T. W. Stern, written commun., 

1966).

JURASSIC ROCKS

SQUAW GULCH GRANITE

Much of the south-central part of the Santa Rita Mountains 

is underlain by a coarse-grained pink granite that is assigned a 

Jurassic age. Similar granite forms a large stock in the Patagonia 

Mountains and other stocks of this age appear in more distant 

ranges to the east. The rock of the Santa Rita Mountains is here 

named the Squaw Gulch Granite for Squaw Gulch. The type area 

lies along the middle reaches of Squaw Gulch (fig. 1, No. 5) east 

of the Salero mine and is accessible from the Salero road. The 

granite intrudes rocks as young as the Piper Gulch Monzonite 

and is unconf ormably overlain by the Temporal Formation.

The Squaw Gulch Granite is a myrmekitic and in part granophyric rock that ranges in composition from a true granite to 

a sodic quartz monzonite. Its modal composition, in percent, is: 

quartz, 25-35; orthoclase, 25-50; albite plagioclase, 10-35; accessory biotite or chlorite is sparse and magnetite, apatite, and zircon appear in trace amounts. Biotite from the rock has been 

radiometrically dated by the K-Ar method as 145 ±4 m.y. (H. H. 

Mehnert, R. F. Marvin, and Wayne Mountjoy, written commun., 

1965) and by the Pb-alpha method, on zircon concentrates of two 

specimens, as 160±20 m.y. (T. W. Stern, written commun., 

1965) and as 161±20 m.y. (reported as 160±20 m.y., T. W. 

Stern, written commun., 1964). The zircon, whose age is calculated as 160 m.y., was obtained from the same specimen that provided the datable biotite.

LOWER CRETACEOUS ROCKS 

TEMPORAL FORMATION

The unit here designated the Temporal Formation is of Early 

Cretaceous age and crops out low along the southeast flank of the
STRATIGRAPHIC NAMES, SANTA RITA MOUNTAINS, ARIZ. C9

Santa Rita Mountains. It is the lower of two sequences of volcanic 

and sedimentary rocks that rest on the Squaw Gulch Granite and 

underlie the Lower Cretaceous Bisfoee Formation. The Temporal 

is 1,000-2,000 feet thick and overlies the Jurassic and Trias&ic 

rocks on an unconformity that shows a local rugged paleocanyon 

topography. The sequence is named for its excellent exposures 

(fig. 1, No. 6) along a readily accessible part of Temporal Gulch 

between the mouth of Mansfield Canyon and the ranch (fig. 1, 

No. 7) in Temporal Gulch. Some of the lenticular units of the 

formation are absent, however, from the type area. A composite 

section of all the lenticular units of this formation would be about 

3,800 feet thick. The formation is overlain with minor unconformity by the Bathtub Formation.

The Temporal Formation consists of three members. The lowest 

member contains subunits of rhyolitic to andesitic tuff, welded 

tuffs, lava flows, arkosic fanglomerate, and polymictic conglomerate. The middle member also consists of conglomerate and rhyolitic tuff, as well as porphyritic latite flows. Locally the lowest and 

middle members grade laterally into a thick unit of andesite flows 

and volcanic breccia. The uppermost member consists of a thick 

wedge of boulder-and-cobble conglomerate, rhyodacite breccia, 

and small lenses of other volcanic rock. Zircon concentrated from 

a welded tuff at the base of the formation gives a radiometric age 

comparable with that of the underlying granite and may actually 

have been picked up from the weathered granite by the tuff when 

it flowed across the granite. The formation is assigned to the early 

Early Cretaceous, but it may be slightly older.

BATHTUB FORMATION

The upper of the two post-Squaw Gulch and pre-Bisbee sequences on the southeast flank of the mountains also consists of 

volcanic and sedimentary rocks and is 1,500-2,300 feet thick. This 

sequence is named the Bathtub Formation after a small natural 

basin called the Bathtub (fig. 1, No. 8), which is located a mile 

west of Adobe Canyon. Its type area lies in an unnamed valley 

tributary to Temporal Gulch and on the southeast flank of that 

valley immediately southeast of the ranch (fig. 1, No. 7) in Temporal Gulch (sec. 23, T. 21 S., R. 15 E.) and is a locality in which 

much of the formation is relatively accessible. The formation 

rests with general disconformity, but locally with slight angular 

unconformity, on the Temporal Formation and is unconformably 

overlain by the basal conglomerate of the Lower Cretaceous Bisbee Formation.
CIO CONTRIBUTIONS TO STRATIGRAPHY

The Bathtub Formation consists of three members, each of 

mixed lithology. The lowest member contains mappable units of 

a coarse polymictic boulder conglomerate and volcanic sandstone. 

The middle member is a rhyolite tuff breccia in the northern part 

of the area and andesite flows in the southern. The uppermost 

member is largely dacite volcanic breccia but contains some small 

units of rhyolitic tuff and flows. Additional subdivisions of the 

members have been mapped. The formation is assigned an Early 

Cretaceous age, inasmuch as it lies between two units also of that 

age.

UPPER CRETACEOUS ROCKS

FORT CRITTENDEN FORMATION

A sequence of fossiliferous black shale, sandstone, and conglomerate in the Adobe Canyon area low on the east flank of the Santa 

Rita Mountains was originally described by Stoyanow (1949) as 

the Fort Crittenden Formation. This formation, along with an 

underlying Fort Buchanan Formation, comprised his Sonoita 

Group. The Fort Buchanan Formation is now recognized by me 

to be the upper part of the Bisbee Formation; a subtle angular 

unconformity separates those rocks of late Early Cretaceous age 

from the overlying ones of late Late Cretaceous age. Consequently, I propose that the names Sonoita Group and Fort Buchanan Formation be dropped from stratigraphic nomenclature; 

the name Fort Crittenden Formation is retained but is here 

revised to include not only the 1,000-foot sequence originally 

described but also the overlying sequence, about 3,500 feet thick, 

of conglomerate, sandstone, and siltstone. A composite stratigraphic section of the revised formation that lies roughly between 

El Pilar Tank (fig. 1, No. 9) in Adobe Canyon (SE^NWi/4 sec. 

24, T. 20 S., R. 15 E., unsurveyed) and triangulation marker 

YOUNG, VABM 5510, on the east side of Hog Canyon (fig. 1, No. 

10), is here designated the reference section/The revised formation, then, rests unconformably on the Bisbee and, on the west 

flank of the mountains where it has now also been recognized, it 

lies with probable conformity beneath the Salero Formation.

The Fort Crittenden Formation consists of five members, four 

of which are sedimentary rocks and one of which is a lens of 

volcanic rocks intercalated high in the sedimentary sequence. At 

the base is a shale member which contains, in subordinate amount 

to the shale, a thin lenticular basal conglomerate and a medial 

pelecypod-bearing siltstone unit, essentially unit 3 of Stoyanow
STRATIGRAPHIC NAMES, SANTA RITA MOUNTAINS, ARIZ. Cll

(1949, p. 60). A lower red conglomerate member overlies the 

shale and contains some thin beds of red siltstone. This is overlain 

by, and locally interfingers with, a brown conglomerate member, 

which contains some thin units of brown sandstone and red conglomerate and siltstone. The highest member is an upper red 

conglomerate member; it contains only a few brown beds and is 

distinguished by the presence of clasts much coarser and more 

angular than those of the lower red conglomerate member. A tuff 

member forms a thick sheet in the upper red conglomerate on the 

west flank of the mountains, and it appears as thin marker beds 

near the base of that member and near the top of the brown conglomerate member on the east flank of the mountains. The fauna 

of the shale member, described by Stoyanow (1949), Miller (1964 

and 1966), and H. W. Miller, Jr., and K. W. Schwab (written 

commun., 1967), is of late Late Cretaceous age.

SALERO FORMATION

Another sequence of Upper Cretaceous volcanic and sedimentary rocks that have a composite maximum thickness of about 

5,000 feet crops out in much of the southwestern part of the Santa 

Rita Mountains and in some parts of the northern end. Only parts 

of this sequence crop out in any one area because the rocks lie 

subhorizontally and the relief is only moderate. The rocks are 

named the Salero Formation for the Salero ranch and mine (fig. 

1, No. 11), near which three of the five members are fairly accessible. This locality serves as type area and is reached by the 

Salero road. Locally the formation rests conformably on the Fort 

Crittenden Formation, and over extensive areas it lies unconformably on the Squaw Gulch Granite; it is intruded by several plutons 

of latest Cretaceous age.

Four of the members of the Salero Formation form a vertical 

sequence and the other is a strongly developed lateral facies. The 

lowest member consists of dacitic flows and a little tuff breccia. 

This is overlain by an exotic block member, consisting of a dacitic 

volcanic flow breccia that contains a great many blocks, as much 

as 1,000 feet long, of assorted Jurassic and older rocks. A welded 

tuff member of rhyodacitic composition overlies the exotic block 

member, probably with minor unconformity. The arkose member 

consists of arkosic fanglomerate, conglomerate, sandstone, and 

tuff. It forms a lateral facies, largely equivalent to the welded tuff 

member but also in part to slightly older and younger rocks. It 

lies between the welded tuff and the Squaw Gulch Granite, which 

it overlies unconformably. The uppermost member is of mixed
C12 CONTRIBUTIONS TO STRATIGRAPHY

volcanic and sedimentary rocks, including much tuffaceous sandstone, tuff breccia, agglomerate, and conglomerate, and some 

quartzite and red beds. Biotite from the welded tuff has been 

radiometrically dated by the K-Ar method as 72.5 ±2.2 m.y. (H. 

H. Mehnert, R. F. Marvin, and Wayne Mountjoy, written commun., 1965).

JOSEPHINE CANYON DIORITE

The largest body of Upper Cretaceous plutonic rock is a diorite 

that extends from Madera Canyon (fig. 1, No. 13) south to the 

flank of the Patagonia Mountains and reappears in the structural 

block of the San Cayetano Mountains. The diorite is extensively 

exposed along the upper reaches of Josephine Canyon, for which 

it is here named and which serves as its type area (fig. 1, No. 12). 

The terrain underlain by the diorite is gently rolling to moderately steep and is covered with dark-brown soil typically containing residual rounded blocks 4-8 inches across.

The rock intrudes formations as young as the Salero; it is probably intruded by the other large plutons of Late Cretaceous age 

and is cut by dikes of Paleocene( ?) age and by other dikes and 

a pluton of late(?) Oligocene age. It is unconformably overlain 

by the Gringo Gulch Volcanics of Paleocene( ?) age.

The Josephine Canyon Diorite is a dark-gray moderately coarse 

grained subophitic rock ranging in composition from diorite to 

quartz diorite and to some granodiorite. It also contains a separately mapped late phase of fine-grained quartz monzonite. The 

approximate modal composition of the diorite, in percent, is: 

quartz, 3-15 (to 25 in late phase); orthoclase, 5-25; labradorite 

plagioclase, 35-65; hornblende, 5-18; biotite, 2-18; and pyroxene, 0-3; and accessory magnetite, sphene, apatite, and zircon. 

The diorite is more intensely chloritized than the other Laramide 

plutonic rocks. Biotite concentrated from the diorite is dated, by 

means of the K-Ar method, as 67.1 m.y. (8. C. Creasey, written 

commun., 1964). Zircon from the diorite is dated by the Pb-alpha 

method as 63.0 m.y. (reported as 60±10 m.y., T. W. Stern, written commun., 1964) and from the quartz monzonite phase, also 

by the Pb-alpha method, as 61.3 m.y. (reported as 60±20 m.y., 

T. W. Stern, written commun., 1965).

MADERA CANYON GRANODIORITE

Much of the north-central part of the Santa Rita Mountains 

is underlain by a large stock of granodiorite of Late Cretaceous 

age! This rock is named the Madera Canyon Granodiorite for its 

exposures along Madera Canyon, the lower reaches of which serve
STRATIGRAPHIC NAMES, SANTA RITA MOUNTAINS, ARIZ. CIS

as type area (fig. 1, No. 13). The granodiorite locally cuts the 

Josephine Canyon Diorite and is cut by the upper (?) Oligocene 

rhyolite porphyry dikes; its relations to the Elephant Head 

Quartz Monzonite are unclear.

The Madera Canyon Granodiorite is mapped as a light-gray 

or main phase, a dark-gray melanocratic phase, and a porphyritic 

phase. The modal composition of the main and porphyritic phases, 

in percent, is about: quartz, 18; orthoclase, 25; plagioclase, 44; 

hornblende, 5; and biotite, 5. Sphene, magnetite, apatite, and 

zircon are the accessory minerals. The dark minerals of the melanocratic phase make up 15-20 percent of the rock and commonly 

are clustered; the quartz content is only about 10 percent. Phenocrysts in the porphyritic phase are as much as 5 cm (centimeters) 

long and make up 1-5 percent of the rock. They are typically 

smaller than the porphyroblasts in the Continental Granodiorite. 

The rock is very little altered. Biotite from rock here identified 

as the main phase was dated by the K-Ar method as 67.9 ±2.1 m.y. 

(P. E. Damon, written commun., 1964).

The youngest rock of the large stocks of Late Cretaceous age 

is a coarse-grained quartz monzonite that forms much of the westcentral part of the Santa Rita Mountains. The rock is here named 

for the prominent knob of that name, on whose flanks it is typically exposed (fig. 1, No. 14); this area is designated as the type 

area. Much of the terrain underlain by this rock forms rugged 

knobs, serrate ridges, and deep narrow canyons, but some of it 

is gentler and is veneered with grus. The formation probably cuts 

the diorite pluton and is intruded by the upper (?) Oligocene rhyolite porphyry dikes.

The Elephant Head Quartz Monzonite forms two stocks; a 

smaller stock at Elephant Head intrudes the larger one to the 

east. A quartz monzonite mass northeast of Madera Canyon may 

be either a separate pluton or a cupola of the larger stock. A finegrained phase of the larger stock lies along parts of its flanks and 

intrudes its core. The modal composition of the rock, in percent, 

is roughly: quartz, 22; orthoclase, 27; plagioclase, 45; biotite and 

amphibole, each 2; magnetite, 1; and traces of apatite, sphene, 

zircon, and allanite. Two K-Ar radiometric ages on biotite of the 

main stock are 68.2 ±2.4 m.y. (R. F. Marvin, H. H. Mehnert, and 

Wayne Mountjoy, written commun., 1965) and 69.0±2.1 m.y. 

(Marvin, Mehnert, and Violet Merritt, written commun., 1967). 

A Pb-alpha age on zircon concentrated from the same rock whose
C14 CONTRIBUTIONS TO STRATIGRAPHY

biotite is 69 m.y. old is dated as 188 m.y. (reported as 190 m.y. 

±20 percent by T. W. Stern, written commun., 1968). A zircon 

counterpart of the other biotite concentrate is being dated. The 

apparently anomalous zircon age may suggest that this quartz 

monzonite was initially the Jurassic Squaw Gulch Granite, which 

was completely recrystallized during the Late Cretaceous. Chemical and petrographic data permit, but do not require, such a geologic history.

PALEOCENE(?) ROCKS

GRINGO GULCH VOLCANICS

A sequence largely of rhyolitic to dacitic volcanic rocks 1,500- 

2,000 feet thick underlies the southeastern part of the Santa Rita 

Mountains and extends into the Patagonia Mountains to the south 

and east. It also contains a little sedimentary rock derived mainly 

from the volcanics. This sequence is named the Gringo Gulch 

Volcanics for its type area along Gringo Gulch (fig. 1, No. 15), 

2-3 miles north-northwest of Patagonia (Ei/2Wi/2 sec. 25, T. 21 

S., R. 15 E.). The volcanics unconformably overlie rodks as young 

as the Josephine Canyon Diorite and are intruded by upper (?) 

Oligocene rhyolite porphyry dikes, Eocene (?) quartz veins, and 

possibly also by a hornblende dacite porphyry of late Laramide 

(Paleocene) age.

The Gringo Gulch Volcanics consist of two members. The lower 

member contains much dacitic lava, dacitic sandstone, and conglomerate, and a little rhyolite lava and tuff, overlain by a very 

pale orange tuff, an indurated tuff, and a locally welded tuff. A 

thin but conspicuous bluish-gray welded to nonwelded tuff forms 

the base of the upper member and is overlain successively by 

light-colored tuff, tuffaceous sandstone, and a capping andesitic 

flow. No minerals suitable for radiometric dating were found 

because the rocks are slightly altered. Inasmuch as the formation 

postdates upper Upper Cretaceous intrusives and predates some 

or all of the middle and lower Tertiary intrusives, it is assigned 

to the Paleocene(?).

UPPER(?) OLIGOCENE ROCKS

GROSVENOR HILLS VOLCANICS

Vitric volcanics are exposed in an extensive area on the southwest flank of the Santa Rita Mountains and in the Grosvenor 

Hills, where they are about 2,000 feet thick. They are here named 

the Grosvenor Hills Volcanics and their type area is designated 

as the basins of the uppermost parts of Coal Mine and Cinigita 

Canyons (fig. 1, No. 16). On the southwest flank of the basins
STRATIGRAPHIC NAMES, SANTA RITA MOUNTAINS, ARIZ. CIS

a relatively complete and little-faulted section underlies the northeast spur of the highest peak (5,469-ft altitude) in the hills. The 

formation lies unconformably on Cretaceous and older rocks, is 

unconformably overlain by the gravels of Nogales, and is intruded 

by rhyodacite dikes and laccoliths. These intrusive rocks are 

genetically associated with the Grosvenor Hills Volcanics and are 

roughly coeval with them.

The Grosvenor Hills Volcanics consist of a basal gravel and silt 

member, a rhyolite member, and a capping rhyodacite member. 

The basal beds are very lenticular, are rarely more than 100 feet 

thick, and are made up of poorly consolidated^ pale-red silt containing stringers of well-rounded small pebbles. The rhyolite 

member is dominantly tuff and tuffaceous sandstone, but it also 

contains some small pinkish-gray lava flows. The rhyodaoite member is complex, containing several vitrophyre flows, agglomerate 

sheets, and both welded and nonwelded tun*. Fossil pine wood and 

pine pollen have been reported from the formation by Damon and 

Miller (1963) and by E. B. Leopold (written commun., 1964) but 

indicate only a Cretaceous or Cenozoic age. Biotite from a rhyodacite laccolith intruding the formation, however, is radiometrically dated by the K-Ar method as 26.0 m.y., and hornblende from 

a second laccolith is dated by that method as 27.3 m.y. (S. C. 

Creasey, written commun., 1964). Zircon from the second laccolith is also dated as 403ilO m.y. (T. W. Stern, written commun., 

1964); this age determination is considered less reliable than the 

other dates in this young time range. The combined evidence, 

then, suggests that the formation is late( ?) Oligocene.

REFERENCES CITED

Damon, P. E., and Miller, H. W., Jr., 1963, A lava covered Tertiary forest

in southern Arizona: Arizona Acad. Sci. Jour., v. 2, no. 3, p. 117-119. 

Drewes, Harald, 1966, Preliminary geologic map of the Mount Wrightson

quadrangle, Santa Cruz and Pima Counties, Arizona: U.S. Geol. Survey

open-file rept. 

Hayes, P. T., Simons, F. S., and Raup, R. B., 1965, Lower Mesozoic extrusive

rocks in southeastern Arizona the Canelo Hills Volcanics: U.S. Geol.

Survey Bull. 1194-M, p. M1-M9. 

Miller, H. W., Jr., 1964, Cretaceous dinosaurian remains from southern

Arizona: Jour. Paleontology, v. 38, no. 2, p. 378-384. 

_____1966, Microfossils from the Upper Cretaceous of Adobe Canyon:

Arizona Geol. Soc. Digest, v. 8, p. 65-95. 

Stoyanow, A. A., 1949, Lower Cretaceous stratigraphy in southeastern

Arizona: Geol. Soc. America Mem. 38, 169 p.

U. S. GOVERNMENT PRINTING OFFICE : 1968 O - 307 - 798