Cenozoic Rocks of the 

Santa Rita Mountains, 

Southeast of 

Tucson, Arizona 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 746 
Cenozoic Rocks of the 

Santa Rita Mountains, 

Southeast of 

Tucson, Arizona 

By HARALD DREWES 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 746 

A systematic description) augmented by 19 

radiometric age determinations) of one of 

the most complete sequences of Cenozoic 

rocks in southeastern Arizona 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. ll. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 72-60015 8 

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

Washington, D.C. 20402 - Price $1.50 (paper cover) 
CONTENTS 

Page Page 

Abstract______ __________ _____ ______ ___ __ _______ ___ ___________________________ ___ _________ ____ 1 Oligocene rocks- Continued 

Introduction_________________________________ __________ __ __________________ __________ ___ __ 1 Igneous complex of the southern Santa Rita MounObjectives________________________________________ __________ ___________ __ ___________ 2 tains- Continued 

Acknowledgments____________ _____ __ __ __ ______________________________________ 4 Stratigraphy and petrography- Continued 

Geologic setting__________________________________________ _______________________ 4 Grosvenor Hills Volcanics- Continued 

Paleocene and Paleocene ( ?) rocks________________________________________ 6 Rhyodacite member.................................. 32 

Gringo Gulch Volcanics_________________________________________________ __ 6 Dikes and laccoliths in the Grosvenor Hills 

Stratigraphy and petrography______________________________ 6 area................................................................. 36 

Lower member___ ___ ________________________________ ________ ____ 7 Dikes and stock of the San Cayetano 

Upper member________ __________________________________________ 10 Mountains...................................................... 39 

Age .. ·--------------·------------------------------------------------------------ 11 Chemistry of the igneous rocks.............................. 41 

Environment of deposition______________________________________ 12 Fauna and age__________________________________________________________ 41 

Volcanics of Red Mountain_________________________________ ____________ 12 Structure ............... ....... -- -------------------------------------------- 43 

Intrusive rocks____________________________________ ____________________ ___ _______ 14 Origin ................... ---------------------------------------------------·-- 45 

Plugs of Gringo Gulch area_______________________________ ____ _ 14 Rhyolite intrusives of the northern Santa Rita 

Dacite porphyry________________________________________________ 14 Mountains .................. ------------------------------------ 47 

Microgranodiorite_____________________________________________ 14 Petrography______________________________________________________________ 49 

Cottonwood Canyon dike swarm_____________________ _______ 16 Chemistry................................................................... 49 

Helvetia stocks.·---------·-·---··--------·---------·-------------------- 17 Age and origin ............ ---------------------------------------------- 50 

Granodiorite______________________________________________________ 19 Miocene ( ?) to Hoiocene rocks______________________________________________ __ 50 

Quartz diorite_______________________________________________ _____ 20 Gravel of Nogales............................................................. 52 

Greaterville intrusives________________________ __ ______ __________ __ 21 Basin-fill graveL.............................................................. 53 

Paleocene to Oligocene rocks____________________ __ ____________________________ 25 Piedmont facies_________________________________________________________ 54 

Andesitic dikes.·---------------------------------------------------------------· 25 River facies ..... ---------------------------------------------------------- 55 

Olivine andesite plugs .......... ---------------------------·----------- ----· 25 Tuff beds.------------------------------------------------------------------- 56 

Rhyolitic volcanics............................................................ 26 Age and correlation_____________________________________________ __ __ 57 

Quartz vein swarm___________________________________________________________ 26 High-level pediment deposits.......................................... 57 

Oligocene rocks_________________________________________________________________________ 28 Intermediate-level pediment and terrace deposits...... 59 

Igneous complex of the southern Santa Rita Moun- Low-level terrace deposits............................................... 60 

tains........................................................................ 28 Gravel facies.................................... ......................... 62 

Stratigraphy and petrography______________________________ 28 Silt facies................................................................... 62 

Grosvenor Hills Volcanics.............................. 28 Youngest graveL. .............. ------------------------------------------------ 62 

Gravel and silt member___________________________ 29 References............................. .................................................... 62 

Rhyolite member....................................... 29 Index ................ --------------------------------------------------------------------------- 65 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing location of the Sahuarita and Mount Wrightson quadrangles and the Santa Rita 

Mountains ...................... ----------------------------------···-------···········--------------------------------------------------------------------------------------------- 2 

2. Map showing the distribution of the Gringo Gulch Volcanics near Patagonia...................................................... 8 

3. Photograph showing the Gringo Gulch Volcanics ........ ------------------------------------------------------- -------------------------------------------- 11 

4. Map showing location of Cottonwood Canyon dike swarm and distribution of coarsely porphyritic and 

finely porphyritic dikes, southern Santa Rita Mountains .............................. ------------------------------------------------------- 16 

5. Photograph of quartz latite porphyry of the Cottonwood Canyon dike swarm.................................................... 17 

6. Map showing the distribution of Helvetia stoeks and Greaterville intrusives, ore porphyry, in the northern 

Santa Rita Mountains .......................................................... ----------------------------------------------------------------------------------------- 18 

7. Photograph of granodiorite from the Helvetia stocks__________________________________________________________________________________________________ 19 

8. Photomicrograph of granodiorite of the Helve~tia stocks --------- ------------------------------------------------------------------------------------ 19 

9. Photomicrograph of quartz diorite of the Helvetia stocks............................................................................................. 21 

10. Photograph of quartz latite porphyry, ore 1-'('rphyry, from the Greaterville intrusives........................................ 22 

11. Photomicrograph of quartz latite porphyry, 01re porphyry, of the Greaterville intrusives.................................... 22 

12. Map showing the distribution of the quartz vein swarm in the southern Santa Rita Mountains...................... 27 

13. Generalized geologic map of the Grosvenor Hllls and San Cayetano Mountains.................................................... 30 

14. Photograph of typical agglomerate of the rhyodacite member of the Grosvenor Hills Volcanics........................ 33 

III 
IV CONTENTS 

Page 

FIGURE 15. Photograph of a vitrophyre block from the rhyodacite member of the Grosvenor Hills Volcanics__________ __________ 33 

16. Photomicrographs of welded tuff from the rhyodacite member of the Grosvenor Hills Volcanics_____ ____ __ ____ __ ___ 35 

17. Photomicrograph of vitrophyre from a flow in the rhyodacite member of the Grosvenor Hills Volcanics___ _ 36 

18. Photomicrograph of quartz xenocrysts rimmed by augite, from the rhyodacite member of the Grosvenor 

Hills Volcanics __ __ _____ _____ ______ ___ ______ __ ___________ ___ __ ___ _______ ____ ____ ____________ _________ ___ _____________ ___ ______ _____ ___________ __ ___ _____________ ~- --- ---- 36 

19. Photograph of the northeastern margin of the laccolith northeast of Tejano Spring_________ ______________ _____ _____ _____ 37 

20. Diagrams illustrating two kinds of laccoliths and an extrusive dome___ _______ ___ __ _____ __ ______ ____ _____ ___ ___________ ___ ___ ____ _____ __ 38 

21. Photomicrograph of rhyodacite vitrophyre from the southeastern laccolith intruding the Grosvenor Hills 

V olea nics_ _______ _____ ___ ___ ____________ __________ _____ ___ _________ ________ _ _ _ _ _ __ _ _ __ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ _ __ _ ___ _ __ _ _ _ _ __ _ _ ___ _ __ __ _ _ _ __ _ __ _ _ __ _ _ _ _ ___ _ __ _ _ _ __ _ _ _ __ __ _ _ ___ _ __ _ _ _ 3 9 

22. Photomicrograph of a compound xenocryst of biotite and hornblende from a rhyodacite· vitrophyre dike 

cutting the Grosvenor Hills Volcanics ____ ____ ______ ___ ___ ____ __ ______ ___ __ _________ __ __________ ___ ___ ____ _______ :___ __________ _____ ________________ ___ ___ 40 

23. Schematic structure section west and northeast of Sheuy Well, Grosvenor Hills and San Cayetano Mountains·----·-··-·---·--··-------·-·--···-·--·-···---··--·--·--·--····-··--·--··----------------·--------------------------·--·-----------------------··-------·---------··--··------·- 44 

24. Graphs showing the minor changes in chemical composition with depth of emplacement of rocks of the 

magmatic complex of the southern Santa Rita Mountains __ ·-- ------·-·--···-·-·--------------·------·--···-----··--·--·--·-··----·---- 46 

25. Map showing the distribution of Oligocene rhyolite intrusives of the northern Santa Rita Mountains, and 

their relations to some faults_·-·-·------------··----------··--·-- ··--·---··--·-·-·---·--·-·--· ··--··--·--··--·--·--··-·--·-·-·-··--·-·-------·-·-'--·-·---------- 48 

26. Schematic sections showing the relations among the surficial deposits on the flanks of the Santa Rita 

Mountains_··-··-·--· ·---·--·--··--·--·····-·--·--·-·-·------··-·········-·····-····-······--·····-·-·····--···-·-···-·--·-···--·--·-·-·-·····--·--·------------ --····--·------- 51 

27-31. Photographs: 

27. Typical gravel of Nogales cropping out north of Cottonwood Canyon on the southwest flank of the 

Santa Rita Mountains·--------------··--------···--·-----··--···--·-----·-·--·---·-··--··--·---- --·--·-··--·--·------·--·---·-·-------·-··--·---·-·-- 52 

28. Basin-fill gravel overlain by terrace deposits of intermediate height, in a cut along Josephine 

Canyon at the north end of the San Cayetano Mountains .. ·-·--·-----··--·--·-··--··--·--·--·--·--------·--·-·--·--·-·-· 54 

29: Channeled, well-sorted, and subrounded river facies deposits of the basin-fill gravel, a mile north 

of ContinentaL·---·-··-·--·--····-·----···-·--··--··-··--·--··--·--·-·--·-------··-·-·----·-·--·-------··--·--·-----··--·-----··--·-----··----------·-----·-- 56 

30. Tuffaceous sandstone and overlying massive nearly white rhyolite tuff beds intercalated in the 

lower part of the basin-fill gravel south of Alum Canyon·-··---·--·--··--·--·--·------·--·--·-----·--------·-·-·-------- 57 

31. Typical poorly sorted angular gravel of the intermediate-level terrace deposit, from high on the 

fan (pediment?) east of ContinentaL .............. ------·--···-··--·----·-····-·--·--·-··----··-------------------·-------------------·- 60 

TABLES 

Page 

TABLE 1. Summary of Cenozoic rocks of the Santa Rita Mountains·---------------· ···--·---·--··---···--··-·---·-·-·--------·-·------·--·-------·--·--·· 3 

2. Summary of radiometric dates of Cenozoic rocks of the Santa Rita Mountains and vicinitY---·--·--··--·-------------- 13 

3. Modes of dacite porphyry and of microgranodiorite from the plugs of the Gringo Gulch area_ _______________________ 15 

4. Chemical and spectrographic analyses and CIPW norms of two specimens from the plugs of the Gringo 

Gulch area·-··--······-····-·-·--···-······---····--·--·--------·--·--·--·---------------------------------------------·--------------------------------------------------------·· 15 

5. Modes of quartz latite porphyry of the Cottonwood Canyon dike swarm·-···-·--·---·--··-·--·-···-··-··-----··-·-·-·····-··········- 17 

6. Modes of representative specimens of granodiorite and quartz diorite from the Helvetia stocks_____ ________ _____ ______ 20 

7. Chemical and spectrographic analyses and CIPW norm of a representative specimen of grandiorite from 

one of the Helvetia stocks ____ ·--·····--······--·-····----·-·-·········-·---··--··--·--··-·--·-·--·--···--·---·--···-·--·--·--··--·--·--·---------·--·-------·------ 20 

8. Modes of quartz latite porphyry, ore porphyry, of the Greaterville intrusives·-··--··-·--·--··--····-·····--··-·--·--··--·-·-··-· 23 

9. Chemical and spectrographic analyses and CIPW norms of six specimens of quartz latite porphyry, ore 

porphyry, compared with Nockolds' average analyses ................. ---··--·-·-·--·-········-···--···--·-·--·· ··--·--·------·-···--·····-·-· 24 

10. Modes of some rocks of the rhyolite member of the Grosvenor Hills Volcanics.---·--·--·-----·-···--·--··--·-·--·-------------·-· 32 

11. Modes of rocks of the rhyodacite member of the Grosvenor Hills Volcanics........................................................ 35 

12. Modes of rhyodacite intrusives into the Grosvenor Hills Volcanics .... ----·······-·····-·····-··--·················-·--··--··-·--·-·-·-- 40 

13. Modes of some dikes and a stock from the San Cayetano Mountains·--··--·--·---------·--·--·-·----------·--·-·------·----··-·--·--·- 41 

14. Chemical and spectrographic analyses and CIPW norms of the Grosvenor Hills Volcanics and related intrusive rocks in the Grosvenor Hills area compared with N ockolds' average analyses_______________ ____ ___ __ __ 42 

15; , Modes of rhyolite dikes of Oligocene age from the northern Santa Rita Mountains.·---·--·--·-·----·--·-----·---·····--·- 49 

16. Chemical and spectrographic analyses and CIPW norms of rhyolite from the dike swarms in the northern 

Santa Rita Mountains·--·-·-------·-··-·--·--·-----------·--·--·-·-·-··------·--··---·--·--·-·--·-·-··--·---·--·-·--------·------·------·--·------------·-------·--·---- 50 
CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, 

SOUTHEAST OF TUCSON, ARIZONA 

By HARALD DREWES 

ABSTRACT 

The Santa Rita Mountains of southeastern Arizona are 

underlain, in part, by volcanic and sedimentary rocks and by 

many small intrusives, of Cenozoic age. These rocks provide 

a more complete geologic record than that of other ranges 

in the region, and consequently the Santa Rita Mountains 

are a useful reference area from which to develop the Cenozoic geologic history of the region. Many isotopic ages provide 

the basis for dating key units and add to the confidence in 

the interpretations of the geologic record of the rocks themselves. The rocks of the greatest potential economic interest 

are the Greaterville intrusives of the Helvetia and Greaterville mining districts and the quartz vein swarm of the 

Tyndall and Wrightson mining districts. 

The Gringo Gulch Volcanics, deposited in the southeastern 

part of the mountains, consist largely of rhyolitic to dacitic 

pyroclastic rocks and some intercalated lava flows sandstone 

and conglomerate. These are of Paleocene ( ?) 'age, lyin~ unconformably upon plutonic rock as young as 65 m.y. 

(million years) old and probably intruded by a plug dated 

as 55-60 m.y. old. The Gringo Gulch Volcanics probably are 

correlative with the much altered and mineralized rocks 

here termed volcanics of Red Mountain, which are abundant 

in the adjacent part of the Patagonia Mountains to the 

southeast. 

Several kinds of late Paleocene small intrusives are dated 

as about 55 m.y. old. The plugs of the Gringo Gulch area, 

to the southeast, are dacite porphyry to microgranodiorite in 

composition. The Cottonwood Canyon dike swarm, intruding 

mainly the west flank of the mountains, is a GOarsely porphyritic quartz latite. The Helvetia stocks, granodiorite and 

quartz diorite in composition, and the Greaterville intrusives 

of quartz latite porphyry, occur in the northern part of th~ 

mountains. The Greaterville intrusives, locally called the 

ore porphyry, are associated spatially with base-metal and 

noble-metal mineralization and may be related to it genetically, at least to the extent that magmas and mineralizing fluids 

used the same ducts. 

Small intrusive bodies and veins also were emplaced during 

the Paleocene to Oligocene interval, possibly mainly during 

the Eocene. Andesitic dikes and plugs intruded rocks as 

young as the Helvetia stocks, and small plugs of olivine 

andesite intruded late ( ?) Paleocene faults, in the northern 

part of the mountains. A large swarm of quartz veins contains base metals and silver in the southern part of the 

range. 

Two groups of rocks were deposited and intruded into the 

area during the Oligocene. One group, on the southwestern 

flank of the mountains, is made up of an igneous complex of 

abundant rhyolite and rhyodacite volcanics and subordinate 

epiclastic rocks (all of the Grosvenor Hills Volcanics), rhyodacite vitrophyre laccoliths and dikes emplaced near the 

surface, a swarm of rhyodacite dikes emplaced at depths of 

a few thousand feet, and the top of a granodiorite stock. 

A genetic association among the rocks of this complex is 

demonstrated by the similarity of their chemical, petrographic, structural, and isotopic analyses. The other group, in 

the central and northern parts of the mountains, consists of 

rhyolite porphyry of one plug and of two northeast-trending 

dike swarms. 

Gravels of late Tertiary and Quaternary age fill the basins 

around the mountains and lap across narrow pediments on 

many of their flanks. The oldest of these is Miocene ( ?) and 

Pliocene gravel of Nogales, which is commonly tilted and 

faulted. The gravel of Nogales is overlain by a widespread 

basin-fill Pliocene and Pleistocene gravel which is only 

locally tilted and faulted. The basin-fill gravel is overlain by 

a sequence of three soil-capped terrace and pediment units 

of Pleistocene age, none of which are tilted and only the older 

two of which are locally faulted, and by Holocene gravel. 

INTRODUCTION 

Sedimentary and volcanic rocks and related intrusive rocks, of Cenozoic age, underlie much of the 

flanks of the Santa Rita Mountains. These rocks 

constitute most of the geologic record of the time 

between the main events of the Laramide orogeny, 

a time of major mountain-making activity that 

peaked 70-75 m.y. (million years) ago, and the 

present. During this time the rocks of the region 

were frequently deformed but less severely than 

during the Laramide, parts of the region were mineralized, basins containing the chief water supplies 

were formed, and the present topography was developed. The Santa Rita Mountains provide one of the 

most extensive geologic records of Cenozoic events 

in southeastern Arizona and thus serve as a reference area from which to develop a regional geologic 

history. 

The Santa Rita Mountains are the first range 

southeast of Tucson. They extend more than 25 

miles southward from Pantano Wash, along which 

the main railroad and highway east of Tucson lie, 

to Sonoita Creek, about 12 miles from the Mexican 

border (fig. 1). The mountains commonly reach 

heights of 6,000 to 7,000 feet, and the highest 

mountain, Mount Wrightson, is 9,453 feet high. The 

1 
2 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

0 10 20 MILES 

0 10 20 KILOMETERS 

FIGURE 1.-Index map showing location of the Sahuarita and Mount Wrightson quadrangles and the Santa Rita Mountains. Positions of detailed maps used in text are shown by text figure numbers. J, Johnson Ranch and Sycamore 

Canyon; C, Cottonwood Canyon; G, Gardner Canyon; R, Red Mountain. 

broad valley to the east lies at an elevation of about 

4,500 feet, whereas that to the west is only at 3,000 

feet. The mountains thus are sufficiently high to 

receive enough moisture to support extensive, largely scrubby forests, which contrast markedly with the 

grasslands to the east and the Sonoran Desert vegetation to the west. 

The Santa Rita Mountains are nearly surrounded 

by an apron of coalescing alluvial fans; in many 

places the fans end abruptly against the foot of 

steep mountains, but in other places they end gradually in a transitional belt of rolling foothills. Along 

the northwest flank, for instance, the mountains 

terminate abruptly and the fans lap directly against 

steep slopes that rise many hundreds of feet. The 

knob called Elephant Head, 8 miles southeast of 

Continental, rises 2,000 feet above the flanking alluvial fan in about half a mile. Along much of the 

east flank and the south end of the mountains, the 

fans have been dissected to form gentle hills with 

a local relief of a few hundred feet. Some bedrock 

hills project above the highest alluvial apron, whereas some have been exhumed from beneath it. Broad 

pediments are common adjacent to the gentle mountain flanks, and narrow or discontinuous pediments 

lie off parts of the steeper mountain flanks. 

OBJECTIVES 

Description of the stratigraphy and petrography 

of the Cenozoic rocks of the Santa Rita Mountains 

and interpretation of their environments of deposition or emplacement are the main objectives of this 

report. The area of concern is the Mount Wrightson 

and Sahuarita 15-minute quadrangles, which includes in addition to the Santa Rita M·ountairis the 

relatively small areas of the San Cayetano Mountains, the Grosvenor Hills, and low unnamed hills 
INTRODUCTION 3 

which lie between the Santa Rita Mountains and the 

Patagonia Mountains to the southeast (fig. 1). A 

small part of the northeast end of the Santa Rita 

Mountains extends outside the Sahuarita quadrangle, 

however, and is not described here. 

some of the intrusives of group 3 are probably associated with the magmatic events of the Helvetian 

phase, or late phase, of the Laramide orogeny. They 

are briefly described in this report, to preserve the 

continuity of the discussion of geologic events, but 

inasmuch as they are ancillary to the major plutonic 

masses of the Laramide, they will be described more 

fully in another report I am preparing on the 

plutonic rocks. 

The geologic un-its described here are all younger 

than the Piman phase (Drewes, 1969), or main 

phase, of the Laramide orogeny. They consist of 

(table 1) (1) some volcanic and clastic rocks deposited between the two local phases of the Laramide, during Paleocene ( ?) time, (2) several dike 

swarms and associated plugs, of Paleocene age, (3) 

stocks of granitoid rocks and intrusives of quartz 

latite porphyry, also of Paleocene age, ( 4) quartz 

veins, volcanics, plugs, and dikes, of Oligocene to 

Paleocene age, ( 5) volcanics, laccoliths, and dikes, 

of late ( ?) Oligocene age, and (6) assorted gravel 

deposits and soils flanking the mountains. At least 

The geologic investigation of the Santa Rita Mountains is part of a larger program of the U.S. Geological Survey to map and interpret the geologic 

history of all the ranges roughly between Tucson 

and Bisbee (fig. 1). Many geologists are involved 

in this program, and most of their field studies are 

completed. Cooper (1970) has mapped the Sierrita 

Mountains; Finnell (1971}, the Empire Mountains; 

Creasey (1967), the Whetstone Mountains; R. B. 

TABLE 1. - Summary of Cenozoic 1·ocks of the Santa Rita Mountains 

[Broken lines indicate that the adjacent formations are contemporaneous or that their relations are uncertain] 

Age Rock unit Description Thickness (feet) 

Youngest gravel Gravel and intercalated sand on floors of modern watercourses. 0-100± 

Holocene Gravel facies Cobble to pebble gravel and intercalated sand; ~apped by infantile 0-200± Low-level terrace gray soil. 

deposits - - ---- Silt and loam of the Santa Cruz River terrace; capped by infantile Silt facies 0-200± gray soil. 

C1l 

5::: Intermediate-level pediment and Boulder to pebble gravel, grit, and intercalated sand; capped by C1l Late 0-400± <.> terrace deposits weakly developed thin reddish-brown soil. 

~ -~ Middle High-level pediment deposits Boulder to pebble gravel and intercalated sand; locally cemented; 0-50± p:; capped and overlapped by well-developed thick red soil. 

Pleistocene Piedmont facies Cobble to pebble gravel, grit, and intercalated sand and silt; com- 0-2,000± 

and Basin-fill gravel r-- - ---- monly pinkish gray, slightly indurated, and locally well cemented. 

Pliocene River facies Well-sorted cobble to pebble gravel and sand, locally cemented. 0-300± r- --- - - Tuff beds Very light gray rhyolitic tuff and tuffaceous sandstone. 0-20 

Pliocene Cobble to pebble gravel, which is rich in volcanic clasts, and tuffa- and Gravel of Nogales ceous sand and silt, which is pale red to brownish gray, poorly 0-1,000± Miocene sorted, and slightly indurated. 

Rhyolite intrusives of the northern Rhyolite porphyry of a plug and of two dike swarms; locally a 

-- Santa Rita Mountains brown vitrophyre . C1l .---- - - - - - - -- Dikes and stock of the Rhyodacite porphyry dike swarm and moderately coarse grained 'il,f5 San Cayetano Mountains light-gray granodiorite . ..... -~ 

0~ r-- - - - - - --- Dikes and laccoliths in the 

Late ( ?) >< ol"' Grosvenor Hills area Rhyodacite vitrophyre intrusives, medium-gray to light-gray. Ill+> I:: r-- ---- - --- Oglicene - ~::·~ 

s~~ Rhyodacite 150-2,200 member Rhyodacite lava flows, agglomerate, tuff, and welded tuff. 

3 E g Grosvenor Rhyolite ~1i~ Hills Rhyolite tuff and a little welded tuff and lava. 500+ o+> Volcanics member <lJ ;:l 

I:: 0 Gravel and Pale-red silt and gravel with well-rounded pebbles; includes some ~r.n 0-200 silt member shale and fresh-water limestone. 

Quartz vein swarm Mineralized quartz veins of the southern Santa Rita Mountains. Oligocene Rhyolitic volcanics Rhyolitic tuff and lava ( ?) of Wasp Canyon. 0-100 to Paleocene Olivine andesite plugs Vesicular and amygdaloidal olivine andesite of Deering Spring. 

Andesitic dikes Andesite, dacite, and diorite dikes, sills, and some small pipes. 

Quartz latite porphyry dikes and plugs, light-gray to grayishGreaterville intrusives orange-pink; contains stubby bipyramidal quartz phenocrysts; 

associated with base- and noble-metals enrichment; plugs are 

ameboid shaped. 

Paleocene Helvetia Quartz diorite Medium-dark-gray medium-coarse-grained quartz diorite. -- --

stocks Granodiorite Light-gray medium-coarse-grained stocks of granodiorite to quartz monzonite composition; elliptical shaped. 

Cottonwood Canyon dike swarm Quartz Iatite porphyry, finely porphyritic to coarsely porphyritic. 

Plugs of Gringo Microgranodiorite Light-brownish-gray very fine grained granodiorite in core of a 

Gulch area ----- plug. 

Dacite porphyry Hornblende dacite, medium-dark-gray, locally coarsely porphyritic. 

Volcanics of Red Mountain Intensely altered rhyolitic and andesitic pyroclastic rocks. - - --- --l-- --- - Paleocene(?) Gringo Upper member Rhyolitic to dacitic tuff, welded tuff, tuffaceous sandstone, and a 900+ Gulch . capping andesite lava flow. 

Volcanics Lower member Rhyolitic and dacitic pyroclastic rocks and some flows ; contains 700+ intercalated epiclastic rocks. 
4 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

Raup (unpub. data, 1971), the Canelo Hills; Simons 

(1971a, b), the Patagonia Mountains; Hayes, the 

Huachuca Mountains, and Raup, the Mustang Mountains (Hayes and Raup, 1968) ; and Hayes and 

Landis (1964), the Mule Mountains. 

ACKNOWLEDGMENTS 

The collateral work produced through a larger 

program of the U.S. Geological Survey has provided 

both supporting and restraining data for the interpretations of the Cenozoic geology of the Santa Rita 

Mountains. However, the Cenozoic record is less 

complete outside the Santa Rita Mountains than 

within them, so the validity of the interpretations 

on correlation and environments of deposition has 

generally not been widely tested. Nevertheless, some 

of these interpretations were developed jointly with 

my colleagues, whose assistance is gratefully acknowledged, and will also be applied to geologic 

situations in adjacent areas. The interpretations 

here are thus not necessarily the results solely of my 

efforts; yet the validity of their local application 

remains my responsibility. 

The success of the field investigation was facilitated by many people. Invaluable assistance in mapping and sampling was provided by G. C. Cone, 

Bruce Hansen, C. W. Norton, F. W. Plut, J. R. 

Riele, R. A. Rohrbacker, Albert Sutheimer, and W. 

M. Swartz between 1963 and 1968. Discussions in 

the field with R. E. Wallace, M. D. Crittenden, Jr., 

J. H. Courtright, and P. H. Pickard helped to clarify 

some problems and to raise others for which I am 

particularly grateful. The courtesies of George 

Bradt, Roy Green, Dewey Keith, and George Yakobian, residents near the Santa Rita Mountains, and 

of Profs. John Anthony, D. L. Bryant, Evans Mayo, 

and S. R. Titley, at the University of Arizona, and 

of E. S. Davidson, M. E. Cooley, Fred Pashley, and 

S. C. Martin, of various government offices, are 

also acknowledged. 

Invaluable support was obtained from many laboratories. Some radiometric dates were obtained 

from Prof. P. E. Damon before their publication, 

and many other dates were provided by R. F. Marvin, T. S. Stern, S. C. Creasey, and their colleagues. 

The numerous chemists and spectrographers who 

have contributed are acknowledged in the tables 

included in this report. G. C. Cone also ably assisted 

in many phases of the general preparatory work. 

GEOLOGIC SETTING 

The geologic record of the development of the 

Santa Rita Mountains during the Cenozoic Era 

follows a fragmentary record of the Precambrian 

history and a more extensive record of the Paleozoic 

and Mesozoic history. The basic data appear on the 

geologic maps, first open-filed in 1966 and 1968 and 

now published (Drewes, 1971a, c), which will be 

referred to frequently. Some collateral reports on 

geochemistry of the area and on stratigraphy are 

now or will soon be available (Drewes, 1967, 1968, 

1971b; 1972a, b; Hayes and Drewes, 1968; Simons 

and others, 1966). · 

The oldest known rocks of southeastern Arizona 

are the Pinal Schist, which generally includes some 

gneiss and migmatite. The schist is most extensively exposed and best studied in the Little Dragoon 

Mountains, where Cooper and Silver (1964, p. 11-

23) described it in considerable detail as a sequence 

at least 20,000 feet thick of metamorphosed conglomerate, sandstone, siltstone, shale, and rhyolite 

and basalt lava flows. They further described the 

major aspect of this sequence as a "cyclic graywacke-slate lithology," which contains "graded bedding, the intercalated lava flows, and the abundant 

volcanic debris." L. T. Silver (oral commun., 1968) 

dated a rhyolite flow as 1,715±10 m.y. old using the 

U-Pb isotopic method on a suite of zircon samples. 

The thickness and lithology of the Pinal Schist were 

interpreted by Cooper and Silver ( 1964) as indicative of geosynclinal deposits. Some aspects of this 

geosyncline were reviewed by Anderson (1951, 

p. 1345). 

In the Santa Rita Mountains, schist and gneiss 

that have been correlated with the Pinal Schist form 

inclusions, roof pendants, and remnants of wallrock of large granitoid plutons of Precambrian and 

younger ages. The schist and gneiss are incons~­

quential as detrital components of the Cenozoic 

clastic deposits of the mountains. 

Extensive masses of coarse-grained and porphyritic alkali granite, quartz monzonite, or granodiorite occur in many of the ranges of southeastern 

Arizona in which the Pinal is exposed. The batholiths and stocks intruded the Pinal and commonly 

metamorphosed the schist along their contacts. 

Several of these granitoid masses have been dated 

by the U -Pb, Ph-alpha, and Rb-Sr methods. Based 

on these dates and on geologic relations currently 

being studied, intrusives in southeastern Arizona 

are probably representative of two older (1,650-

1,760 m.y.) magmatic episodes and a younger 

(1,430-1,460 1n.y.) magmatic episode (Silver, 1969). 

In the Santa Rita Mountains much of the basement rock is the Continental Granodiorite, a 

coarse-grained porphyritic granodiorite and quartz 

monzonite with a radiogenic date of 1,450 m.y. but 

possibly older. One sample of this rock was dated 

by Rb-Sr and by Ph-alpha methods as Precambrian, 
INTRODUCTION 5 

but another sample, dated by the K-Ar method, 

records only a Laramide thermal event (Drewes, 

1968, p. C5). This rock characteristically disaggregates upon weathering to form grus and gritty 

detritus that is a widespread component of the 

younger deposits in the northern part of the 

mountains. 

Rocks of the Apache Group of Precambrian age, 

although absent in the Santa Rita Mountains, are 

present in areas a few miles to the northeast, and 

they shed light on the geology of the Santa Rita 

Mountains at that time. The Apache Group unconformably overlies the Pinal Schist and the 

granodiorite masses, and it is intruded by diabase 

of Precambrian age. The stratigraphic sequence of 

this group is probably most complete in the Little 

Dragoon area, where it was described in detail by 

Cooper and Silver (1964, p. 36-41). But even there 

the upper of the three formations comprising the 

group, as present farther to the north, is absent. 

The total absence of the Apache Group in the central 

and southern parts of southeastern Arizona, including the Santa Rita Mountains, and the absence 

of the upper third of that group in the northern 

part indicate either that during the Precambrian 

these areas remained sufficiently high so as not to 

receive sediments during all of Apache time or, 

more likely, that they were uplifted and the top of 

the Apache deposits was eroded. 

The Precambrian geologic history of the area 

thus includes an early time of sedimentation, orogeny 

and attendant metamorphism and batholithic intrusion of at least one episode, further sedimentation, and diabase intrusion and minor crustal 

disturbance. 

The Precambrian rocks of the Santa Rita Mountains are unconformably overlain by a sequence of 

Paleozoic shallow marine deposits 5,000-6,000 feet 

thick. The sequence extends from the Bolsa Quartzite of Middle Cambrian age to the Rainvalley Formation of late Early Permian age. Limestone and 

dolomite are the most common rocks of the sequence, 

but clastic rocks are fairly abundant near its bottom 

and top. The continuity of the sequence is broken 

by several disconformities, of which the largest 

represents at least most of Ordovician time and all 

of Silurian time. 

The Paleozoic geologic history of the area is one 

of alternating marine transgressions and regressions, reflecting successive epeirogenic movements. 

Toward the middle of the Permian Period the area 

was uplifted more strongly, and continental conditions became prevalent. 

Three thick sequences of continental clastic and 

472-410 0- 72 - 2 

volcanic rocks, of Mesozoic age, lie unconformably 

upon the Paleozoic and Precambrian rocks of the 

Santa Rita Mountains (Drewes, 1971b). The lowest 

sequence, of Triassic rocks, consists of rhyodacite 

volcanics and some eolian sandstone and conglomerate of the Mount Wrightson Formation and of 

red beds and some volcanics and conglomerate of the 

Gardner Canyon Formation. The middle sequence, 

of Early Cretaceous age, consists of rhyolitic to 

andesitic volcanics and of coarse conglomerate of 

the Temporal and Bathtub Formations, overlain by 

arkose and conglomerate of the Bisbee Group; the 

Apache Canyon Formation of the Bisbee Group 

contains also a little intercalated marine limestone. 

The highest sequence consists of the Fort Crittenden 

Formation of Late Cretaceous age, which also contains arkose and conglomerate, and the overlying 

Salero Formation, which is made up of coarse conglomerate and abundant dacitic and rhyodacitic 

volcanic rocks. The Santa Rita Mountains were 

intruded by monzonitic stocks of Triassic age, by 

a large granite stock, the Squaw Gulch Granite, of 

Jurassic age, and by several stocks of quartz diorite 

to quartz monzonite of late Late Cretaceous age. 

The Mesozoic geologic history of the area is more 

complex than the pre-Mesozoic geologic history. Soon 

after the beginning of continental conditions, the 

area was strongly faulted, and volcanics and red 

beds filled some large basins. Late during the Triassic, the rocks were faulted again and were intruded 

by Piper Gulch Monzonite. These events are possibly 

distant and slightly delayed effects of the Sonoran 

Orogeny, proposed by Fries (1962). Further volcanism was followed, near the middle of the Jurassic, 

by the intrusion of a large granite stock and by 

strong uplift. During the Early Cretaceous, volcanism was resumed locally, along with the deposition of conglomerate at the foot of block-faulted 

mountains. The finer clastic rocks deposited later 

during this time indicate a waning local relief and 

possibly some subsidence. An arm of a continental 

sea briefly encroached upon the area and deposited 

the limestone beds. 

A final major geologic event preceded the deposition of Cenozoic rocks and began during the Late 

Cretaceous, perhaps as long as 90 m.y. ago (Drewes 

and Finnell, 1968, p. 315; Drewes, 1969). Initial 

tectonic activity was recorded by a coarsening of 

the epiclastic deposits, a resurgence of volcanism, 

and the appearance of unconformities in the stratigraphic column. Before the close of the Cretaceous 

these rocks and older rocks were strongly thrust 

faulted and folded, and then they were invaded by 

a group of granitoid stocks. These stocks are (1) 
6 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

a large stock of Josephine Canyon Diorite, mainly 

a medium-coarse-grained rock of diorite to quartz 

diorite composition but also containing a late quartz 

monzonitic phase; (2) smaller stocks of coarsegrained Elephant Head Quartz Monzonite; and (3) 

a composite stock of coarse-grained Madera Canyon 

Granodiorite. I have briefly described these rocks 

previously (Drewes, 1968). Collectively, these events 

are referred to as the Piman phase of the Laramide 

orogeny, which is the earlier and stronger of the 

two phases recorded southeast of Tucson. 

The sedimentary and volcanic rocks deposited 

after the Piman phase, and described in this report, 

are summarized in table 1. The oldest of these 

rocks was intruded by both plutonic and hypabyssal 

masses and was deformed during the Helvetian 

phase, the younger and weaker subdivision of the 

Laramide. 

PALEOCENE AND PALEOCENE(?) ROCKS 

Two groups of rocks of the Santa Rita Mountains 

are largely or wholly of Paleocene age. The older 

group consists of a sequence of volcanics and some 

intercalated epiclastic rocks assigned to the Gringo 

Gulch Volcanics, of Paleocene ( ?) age, and a small 

mass of highly altered volcanic rock assigned to an 

unnamed unit, here referred to as the volcanics of 

Red Mountain, also of Paleocene ( ?) age. The younger group of rocks consists of small stocks, plugs, 

and dikes, which commonly are of porphyritic texture and of latitic to dacitic composition. Some of the 

intrusives have associated mineralized rocks. Radiometric dating of some of these intrusives indicates 

that they are Paleocene. 

GRINGO GULCH VOLCANICS 

The Gringo Gulch Volcanics (Drewes, 1968, p. 

C14) are a sequence largely of rhyolitic to dacitic 

tuffs 1,500-2,000 feet thick that unconformably 

overlies rocks as young as the Salero Formation and 

the Josephine Canyon Diorite, both of late Late 

Cretaceous age. The scanty available geologic data, 

described later, suggest that these volcanics were 

deposited before about 55 m.y. ago, at which time 

they were intruded, altered, and in places mineralized. 

The volcanics underlie an area of at least 6 square 

miles in the southeast corner of the Mount Wrightson quadrangle near Patagonia (fig. 2), and they 

probably underlie an additional 4 square miles 

beneath a cover of gravel within the quadrangle. 

The volcanics also extend southward into the Nogales quadrangle (fig. 1), and their probable correlatives, the volcanics of Red Mountain, which are 

described under the section "Volcanics of Red 

Mountain," are extensively exposed in the Elgin 

and Lochiel quadrangles to the east and southeast. 

In outcrops the Gringo Gulch Volcanics are typically composed of light-colored and flat-lying or 

gently inclined massive units. Although most of the 

tuffaceous rocks are only moderately well indurated 

and rather friable, a few bedded units are strongly 

indurated and form small cliffs, ledges, and buttes 

that contrast markedly with the gently rolling hills 

characteristic of the weakly indurated tuffs. In addition to the indurated bedded units, many pods of 

silicified rock near the Narrows along Sonoita Creek, 

3-4 miles southwest of Patagonia (fig. 2, loc. D), 

form knobs and east-west alined rock ribs. Most of 

the tuffs are very light gray to very pale orange, 

some are darker gray or bluish gray, and a few are 

variegated in pale pink and green. The few andesitic 

lava flows of the formation are brownish gray, and 

the dacitic and andesitic epiclastic rocks are greenish gray or bluish medium gray. 

STRATIGRAPHY AND PETROGRAPHY 

The Gringo Gulch Volcanics consist of many 

lenticular units which are divided on a stratigraphic 

basis into the lower and upper members. Both 

members are composed largely of rhyolitic tuffs and 

tuff breccias; the upper member is distinguished by 

a basal bluish-gray partly welded tuff and some andesitic flows, whereas the lower member contains 

some dacitic flows and epiclastic rocks derived from 

dacitic volcanics. 

The formation overlies an unconformity of moderate relief. To the north, along Gringo Gulch (fig. 

2), the two members lap unconformably over rocks 

of the Lower Cretaceous Bathtub Formation 

(Drewes, 1971b), which is one of the formations 

mapped as older rocks (Mesozoic) in figure 2. To 

the south, between Sanford Butte and locality A in 

figure 2, rocks of the lower member lap against the 

Josephine Canyon Diorite and some older rocks, 

such as the Salero Formation (Drewes, 1968, and 

unpub. data). Although the actual contact of the 

volcanics with the diorite is everywhere covered 

because of the friability of the volcanic rocks, that 

contact is believed to be depositional rather than 

intrusive because the volcanics are neither contact 

metamorphosed nor included in the diorite, as is the 

slightly older Salero Formation. Furthermore, in 

a few places conglomerate lenses in the lower 

member of the Gringo Gulch Volcanics contain dioritic clasts that could have been derived from the 

Josephine Canyon Diorite, although sparse alternative sources are also available. 
PALEOCENE AND PALEOCENE(?) ROCKS, GRINGO GULCH VOLCANICS 7 

LOWER :\fE\-IBER 

The lower member of the Gringo Gulch Volcanics 

is about 700 feet thick and is made up of a lower 

unit containing dacitic and rhyolitic rocks and an 

upper unit containing only rhyolitic tuff. The lower 

unit is exposed in low topographic positions south 

and west of the lower reaches of Temporal Gulch; 

the upper unit caps all but the highest of the hills 

in the same area and extends farther to the northeast. 

The lower unit of the lower member consists of 

a few dacitic flows, many lenses of dacitic sandstone and conglomerate, a rhyolite flow, and some 

rhyolitic tuff and tuff breccia. 

Dacitic lava of the lower unit is medium gray 

on unweathered surfaces and olive gray to pale 

yellowish brown on weathered ones. It commonly 

is finely porphyritic, and along Squaw Gulch it is 

slightly vesicular and amygdaloidal. In thin section, 

the phenocrysts are seen to make up 10-20 percent 

of the rock and to be set in a felty to pilotaxitic 

groundmass. In a typical sample, plagioclase phenocrysts make up 2-5 percent of the rock and have a 

sodic labradorite (An,, 3 ) composition. Quartz is 

sparse and probably secondary. Pyroxene makes up 

4-8 percent of the rock; either orthopyroxene or 

clinopyroxene is present, but they have not been 

found together. One specimen contains relicts of 

amphibole and biotite. Magnetite and apatite accessory minerals are commonly present, and interstitial altered glass is a rare constituent. The rocks 

are commonly argillized and propylitized, but generally to a much lesser degree than are the Cretaceous and older rocks. The plagioclase is usually 

albitized as well, and the glass is devitrified. 

The rhyolite flow of the lower unit is the only 

rock of its kind in the Gringo Gulch Volcanics, and 

inasmuch as it lies in the lowest stratigraphic position, southwest of Sanford Butte, its assignment to 

the Gringo Gulch is provisional. This flow differs 

from the next older known stratigraphic unit, the 

Salero Formation, whose lava flows commonly contain less quartz and more biotite. For convenience, 

then, the rhyolite is accepted as part of the younger 

formation. 

The rhyolite flow contains about 35 percent phenocrysts, as much as 4 mm (millimeters) long, and it 

also contains traces of xenoliths. As seen under 

the microscope, the phenocrysts comprise 15 percent 

quartz, 10 percent each of intensely argillized potassium feldspar and of albitized ( ?) plagioclase, and 

traces of chloritized biotite, magnetite, and zircon. 

Rhyolitic tuff and tuff breccia also occur in the 

lower unit of the lower member of the Gringo Gulch 

Volcanics, mainly west of Squaw Gulch (fig. 2), 

where they interfinger with the dacitic rocks. These 

tuffaceous rocks so closely resemble the more abundant tuff of the upper unit that they are not described separately here. 

Dacitic rocks overlie the rhyolite flow and are 

fairly widespread along canyon bottoms. Dacitic 

epiclastic rocks are commonly greenish gray to 

bluish gray and are poorly bedded and poorly sorted. 

Dacitic sandstone and grit are more abundant than 

conglomerate, and few of the clasts in the conglomerate are larger than pebbles. Most clasts in the 

conglomerate were derived from nearby dacitic lavas, 

but some may have been supplied by the more 

distant volcanics of the Salero Formation. Along 

Squaw Gulch north of the Salero Road, the conglomerate beds in this unit contain some clasts of 

strongly indurated layered volcanics derived from 

the Triassic Mount Wrightson Formation (Drewes, 

1968, 1971b) exposed to the north, some of the 

Jurassic Squaw Gulch Granite (Drewes, 1968, and 

unpub. data) probably derived from the north, and 

some of a diorite whose source may have been the 

nearby Upper Cretaceous Josephine Canyon Diorite 

but which may alternatively have been derived from 

inclusions of lamprophyre dikes found in the Squaw 

Gulch Granite 2 miles to the north. 

The upper unit of the lower member consists of 

a sheet of rhyolitic tuff breccia about 300 feet thick 

which lies over the dacitic rocks of the lower unit. 

This sheet extends continuously from Sanford Butte 

northeastward to the mouth of Temporal Gulch and 

extends discontinuously northward to Gringo Gulch. 

The lower part of this tuff, as much as 80 feet thick, 

is poorly indurated and resembles the rhyolitic tuff 

of the lower unit to such an extent that the two 

tuffs are commonly indistinguishable in the field 

where the intervening dacitic rocks pinch out. Some 

of the tuff breccia above the dacitic rocks contains 

fragments of dacite, suggesting that part of the 

lenticularity of the dacite is the result of erosion. 

The poorly indurated tuff thins out east of locality 

A in figure 2 and is not distinguished from the overlying rocks southeast of Sonoita Creek. 

The upper part of the upper unit is a well-indurated and strongly jointed very pale orange to pinkish-gray rhyolitic tuff- rarely a welded tuffabout 200 feet thick. In some places the rock forms 

a strongly pinnacled or castellated topography, of 

which a modest sample appears in the foreground of 

figure 3; in other places large surfaces are stripped 

of cover; and elsewhere joint blocks of indurated 

tuff have slumped from their source cliffs over the 

gentle slopes on the underlying weaker tuff. The 
8 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS. ARIZONA 

+ + + + + 0+ + + + + + + + + + 

0 

+ + + +Kj + + + + 

+ + + + + 

+++~ ++++ 

+ + + ~ + 

+ a. 

+ 

2 MILES 

0 2 KILOMETERS 

QTg 

110"45' 

31.0 

35' 

FIGURE 2. - Distribution of the Gringo Gulch Volcanics near Patagonia. Base from U.S. Geological 

Survey: Mount Wrightson quadrangle, 1: 62,500, 1958. 
PALEOCENE AND PALEOCENE(?) ROCKS, GRINGO GULCH VOLCANICS 

Volcanics of 

Red Mountain 

Trr, rhyolitic rocks 

Trd, dacitic rocks 

EXPLANATION 

[§] 

Gravel 

Intrusive rocks 

T d, dacite porphyry 

Tm, microgra7Wdiorite 

rn 

-.> .ia 

ell 

-.> 

0 

> 

...c:: 

-.> 

"5 

C!l 

0 

bll 

s:: 

·;:::: 

C!l 

1-o 

~ 

..0 

s 

~ 

s 

1-o 

~ 0 

.....:l 

Upper member 

Tgua, andesitic lava 

Tgus, andesitic sandstone 

Tgut, rhyolitic tuff and 

tuffaceous sandstone 

Tguw, bluish-gray tuff 

Upper 

Tgli, indurated rhyolitic tuff 

Tglu, rhyolitic tuff breccia 

Stipple, silicified tuff 

Lower unit 

Tgld, dacitic clastic rocks 

Tglf, dacitic lava flows 

Tglt, rhyolitic tuff and tuff breccia 

Tglr, rhyolite flow 

Josephine 

~ 

~ 

Canyon Diorite 

::-~{<<l Older rocks (Mesozoic) 

Contact 

Dashed where approximately located 

Fault 

Dotted where concealed. Bar and 

ball on downthrown side 

5 ____[__ 

Strike and dip of beds 

xA 

Locality referred to in text 

1::,.257 

Collection site of 

chemically analyzed specimen 

_.281 

Collection site of 

radiometrically dated specimen 

0>-

Zo::: 

:~ o:::O::: 

<(W 

-if-<( 

O::::::J 

~a 

>-

0::: 

<( 

f0::: 

w 

f9 
10 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

tuff in the upper part of the upper unit decreases 

rapidly eastward in degree of induration and 

merges with the tuff in the lower part of the upper 

unit. In a few places, as near the unnamed spring 

along lower Goat Canyon, near locality B in figure 

2, and also in the cliffs southeast of. State Highway 

82 at the Narrows, locality D in figure 2, pumiceous 

fragments in the tuff are flattened, presumably a 

result of incipient welding. 

Under the microscope, specimens of the tuff and 

indurated tuff are seen to contain crystal and lithic 

fragments in equal abundance, neither of which 

exceeds 10 percent of the rock. The tuff groundmass 

is cryptocrystalline, is faintly fragmental, and contains abundant clay minerals and some sericite and 

secondary ( ?) quartz. Quartz xenocrysts make up 

at least 5 percent of the crystals, many of which are 

partly resorbed. Plagioclase makes up 21/2 percent 

of the rock; it is albite, and presumably it is a.lbitized 

from a more calcic mineral. Potassium feldspar, in 

one specimen a sanidine, also makes up 21/2 percent. 

Most rocks contain no pseudomorphs of ferromagnesian minerals, but remnants of altered biotite are 

preserved in a thin indurated tuff breccia 11/2 miles 

northwest of Patagonia. The ubiquitous accessories 

-magnetite, apatite, and zircon- occur in minor 

amounts, and a trace of leucoxene enclosed in sanidine was seen in one specimen. The welded tuff in 

Goat Canyon contains an inclusion of glass in 

quartz, and it also contains several unidentified 

secondary minerals. Alteration of the rock was 

widespread, and though relatively mild, it affected 

most of the original minerals sufficiently to leave 

them unsuitable for isotopic dating. 

Between Sanford Butte and the Flux Canyon road, 

the tuff breccia contains many east-trending silicified pods, which form bold outcrops stained with 

oxides of iron and manganese. The pods are commonly 100-200 feet wide and 500-1,000 feet long; 

those exposed along the deeper canyons have a 

greater height than width. Near Sanford Butte the 

pods blend into the intensely indurated sheet of tuff, 

but a few pods seem to be superposed on the indurated rock and to · cross its basal contact. Under 

the microscope, specimens from the pods are seen 

to contain abundant secondary quartz in small disseminated crystals and in patches of small crystals. 

Clay minerals are abundant, and X-ray diffraction 

tests indicate that alunite is commonly present. 

UPPER MEMBER 

The upper meinber of the Gringo Gulch Volcanics 

is probably more than 900 feet thick and is made up 

of a lithologic assortment much like that of the 

lower member except for a distinctive basal bluishgray tuff and for the presence of andesitic rather 

than dacitic rocks. The basal tuff is welded west of 

Temporal Gulch but is unwelded to the east and 

north. Rocks of the upper member cap the higher 

hills between locality A (fig. 2) and lower Temporal 

Gulch, and they underlie numerous areas between 

the Temporal Gulch-Gringo Gulch area and Patagonia. In sequence above the bluish-gray welded 

to unwelded tuff are a light-colored rhyolitic tuff 

and tuffaceous sandstone unit, a brownish-gray 

volcanic sandstone unit, and a medium-gray andesitic 

lava unit. Even within the limited area of exposure 

of these four units, the lithologies change laterally, 

but at least the andesitic lava extends east of 

Patagonia. 

The basal bluish-gray tuff commonly is about 50 

feet thick, but near locality B (fig. 2) it is as much 

as 150 feet thick, and farther south and east it is 

as little as 15 feet thick. The thicker tuff forms small 

ledges and cliffs (fig. 3) ; the thinner tuff underlies 

gentler slopes strewn with blocky detritus. Where it 

is welded, the tuff has lenticular, flattened inclusions 

of light-gray altered glass, some small xenoliths, 

and phenocrysts of altered biotite and plagioclase. 

In the unwelded facies, the flattened inclusions are 

absent,· and the xenocrysts and phenocrysts are 

scarcer than in the welded facies. In thin section, 

the welded tuff is seen to have a coarsely microcrystalline groundmass in which the flattened pods, 

elongated to 15 times their thickness, are of altered 

glass having a still finer grain size (less than 0.005 

mm). Oxybiotite phenocrysts make up 7 percent 

of the rock and appear only as relict cores in pseudomorphs of iron oxide and sericite. Albitized plagioclase phenocrysts form 3 percent of the rock and 

are strongly argillized. Magnetite and apatite are 

present in trace amounts. Quartz forms many small 

patches of anhedral crystals in the groundmass, 

as well as replacements of some phenocrysts, and 

so it is at least in part secondary. 

The second unit, of light-colored rhyolitic tuff 

and tuffaceous sandstone, is about 300 feet thick. In 

most places this rock is only moderately well indurated, but 11j2 miles west of Patagonia it contains 

a medial, thin, well-indurated tuff marker bed. Locally, at hill 4,335 (loc. C in fig. 2) and also south 

of Sonoita Creek, the rock is so intensely altered 

that the distinction between the two lower units 

of the upper member is unclear; at locality C at 

least, the abundance of coarse breccia in the tuffs 

suggests a proximity to a vent area. Along Gringo 

Gulch the unit consists of a friable pale-pink and 

pale-green tuff overlain by, and locally interfingering 
PALEOCENE AND PALEOCENE(?) ROCKS, GRINGO GULCH VOLCANICS 11 

FIGURE 3.- Gringo Gulch Volcanics; view toward Patagonia Mountains to the southeast. Pinnacled outcrops in foreground are the top of the lower member of the volcanics; ledgy outcrops on low ridge to left are the basal welded 

tuff of the upper member. 

with, a white tuff. About 4 percent of the tuff is 

made up of crystal fragments, of which half are 

oxybiotite and a quarter each are albite and potassium feldspar. Traces of magnetite, apatite, and 

zircon are present, and in one specimen a small vug 

contains quartz crystals. Iron oxide and clay minerals are dispersed in the largely cryptocrystalline 

groundmass. 

The third unit, of brownish-gray volcanic sandstone, is 200-400 feet thick and lies concordantly and 

probably disconformably on the rhyolitic tuff. West 

of Patagonia the unit is thickest and consists of 

reworked white tuff with admixtures of andesitic 

or dacitic fragments. Half a mile north of Patagonia 

the unit consists of poorly sorted and poorly indurated, thin- to thick-bedded, coarse-grained sandstone. 

Along Gringo Gulch, where the unit is thinnest, it 

consists of andesitic or dacitic conglomerate and 

sandstone. 

The top unit of the upper member, composed of 

medium-gray andesitic lava, is probably more than 

200 feet thick. It is sporadically exposed on small 

somber-colored knolls within about a mile of Patagonia. The lava of the knolls is finely porphyritic, 

but phenocrysts differ slightly in kind from place 

to place; hence, the rocks probably represent more 

than one flow. The phenocrysts are about 1 mm 

long and make up 5-10 percent of the rock. The 

groundmass is felty to hyalopilitic, its crystal laths 

are about 0.1 mm long, and its granular crystals are 

0.01-0.05 mm across. A clinopyroxene, possibly augite, is the most abundant phenocryst type. The 

augite ( ?) of one specimen contains cores of hypersthene, and the same specimen also contains plagioclase crystals whose cores are partly resorbed. 

Magnetite and apatite are the accessory minerals. 

Ferromagnesian minerals make up about 10 percent 

of the groundmass, ~nd pl~gioclase (An 5u-;:;o) most 

of the remainder. Quartz and potassium feldspar ( ?) 

probably form some of the interstitial granular 

grains in the specimens that are free of hypersthene. 

Calcite, epidote, chlorite, _ iron oxide, clay minerals, 

and perhaps some quartz are the secondary minerals. 

AGE 

The Gringo Gulch Volcanics, as suggested by their 

geologic relations, probably are of Paleocene age. 

The volcanics unconformably overlie the upper 

Upper Cretaceous Salero Formation and are inferred 

to lie unconformably upon the Upper Cretaceous 

Josephine Canyon Diorite, whose youngest phase· is 

dated as about 65 m.y. old (Drewes 1968, p. C12). 
12 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

The time lapse between the emplacement of the diorite pluton and the deposition of the volcanics, recorded by the erosion of the roof of the pluton, is 

not believed to have been large, for the abundance of 

inclusions or pendants of the Salero in the diorite 

indicates that the top of the pluton is exposed, and 

the relatively fine grain size of the diorite suggests 

a rapid rate of cooling, also compatible with a shallow intrusion level. 

The volcanics are further believed to be· older 

than a small hornblende dacite plug near the Patagonia town dump. On the basis of petrologic similarity, this small plug is probably correlative with a 

larger plug a few miles to the north, which is isotopically dated as about 55-60 m.y. old (table 2). 

The Gringo Gulch Volcanics thus are dated as about 

60 m.y. old, or as Paleocene ( ?) . This age seems 

especially reasonable in that it explains why these 

volcanics, and more particularly the correlative 

volcanics of Red Mountain, are altered and mineralized in the manner of rocks near many ore porphyries of late Laramide (Paleocene) age but are 

not hornfelsed or deformed to the extent that many 

of the upper Upper Cretaceous rocks are. 

ENVIRONMENT OF DEPOSITION 

The Gringo Gulch Volcanics were deposited largely by airfall and fluvial processes. The few lava 

flows and welded tuff sheets occur mostly in the 

southern and western parts of the area, suggesting 

that their source lay in those directions. Other 

volcanics may have come from the Red Mountain 

area. The irregularities along the basal contact show 

that the surface on which the lowest rocks were 

deposited was hilly, yet the relative sparseness of 

clasts of older rocks in the intercalated epiclastic 

beds does not require strong local relief. The low 

relief and the absence of coarse-textured igneous 

rocks suggest a relatively inactive Gringo Gulch 

time between times of greater tectonic and plutonic 

activity more typical of the Laramide. 

VOLCANICS OF RED MOUNTAIN 

Intensely altered rocks, mainly of rhyolitic to 

dacitic or andesitic composition, underlie the flank 

of Red Mountain in the southeast corner of the 

Mount Wrightson quadrangle and are here referred 

to as the volcanics of Red Mountain. They are only 

briefly described in this report, inasmuch as they 

extend widely into the adjacent quadrangles, where 

they have been more thoroughly studied by R. B. 

Raup and F. S. Simons ( unpub. data) . The volcanics 

are poorly exposed on steep slopes of the mountain, 

but they are well exposed along gully bottoms near 

the mouth of Alum Gulch (fig. 2). 

In the limited area in the Mount Wrightson quadrangle that is underlain by the volcanics of Red 

Mountain, only a few outcrops provide any definitive 

information bearing on the composition, structure, 

or origin of the rocks because the rocks are flooded 

with iron oxide and are strongly argillized and 

partly silicified. However, in some places relict porphyritic and pyroclastic textures indicate that the 

rocks probably are volcanic. Medium-gray rocks 

probably were altered from volcanics of dacitic or 

andesitic composition, and light-gray ones may have 

had a rhyolitic affinity. In many places along the 

canyon bottoms the volcanics contain disseminated 

sulfides, whose oxidation produces the pronounced 

reddish-brown color characteristic of much of Red 

Mountain. The rocks that are grayish brown rather 

than reddish brown are silicified. 

A specimen of relatively unaltered dacitic or andesitic rock is made up of about 20 percent phenocrysts as much as 3 mm long scattered in a felty 

altered groundmass in which grain size is about 

0.05 mm. Partly albitized sodic andesine is the most 

abundant of these phenocrysts. Amphibole and pyroxene ( ?) are less abundant, and accessory magnetite and sphene least abundant. Iron oxide, epidote, 

quartz, montmorillonite ( ?) , kaolinite ( ?) , chlorite, 

and leucoxene are the chief alteration minerals. 

The lighter colored rocks crop out northwest of 

the dacitic and andesitic ones. In some places they 

contain relict textures of sandstone, and in a few 

places they are conglomerate or breccia. Fragments 

of the coarser clastic rocks include granite, probably 

derived from a nearby stock of Jurassic age, and 

quartzite and laminated siliceous volcanics of the 

Mount Wrightson Formation of Triassic age, present 

in the Flux Canyon area to the southwest. The 

conglomerate or breccia forms a steeply northwestward-inclined lenticular body bounded by faults 

along, and subparallel to, the range front. 

Typical rhyolitic rock of this area contains 10 

percent phenocrysts 0.5-1.5 mm long set in a non~ 

descript granular groundmass of possible tuffaceous 

origin having a grain size of 0.01-0.03 mm. Most 

phenocrysts are of albitized and strongly kaolinized 

plagioclase. Traces of titaniferous magnetite and 

zircon are present. Secondary minerals are clay 

minerals, quartz, sericite, iron oxide, and chlorite. 

The volcanics of Red Mountain are provisionally 

correlated with some or all of the Gringo Gulch 

Volcanics of Paleocene ( ?) age. However, inasmuch 

as the volcanics of Red Mountain are part of a complexly altered sequence and are separated by a 

major fault from the less altered Gringo Gulch 

sequence, the actual age assignment, correlation, and 
0 

TABLE 2.- Summary of radiometric dates of Cenozoic rocks of the Santa Rita Mountains and vicinity 

Specimen Field 

No. No. 

70D1133 

65D687 

63D315 

4 64D581 

5 66D710 

6 63D338 7 65D899 

Rock General locationl Material dated Method used Structure Composition 

Pliocene and Miocene rocks 

6 mi. southwest of San Cayetano Mountains. Lava flow ......... Olivine basalt ............. ..... Groundmass ......... ...... __ K-Ar 

Pyroxene ........................ K-Ar 

Upper(?) Oligocene rocks 

San Cayetano Mountains ..... Stock. ................ Granodiorite ...................... B iotite ............. --------------· K-Ar 

Grosvenor Hills ( SE.) --------- Laccolith .......... Rhyodacite vitrophyre __ Hornblende ....... K-Ar 

Zircon ........ --- -·····-----------Ph-alpha Grosvenor Hills (NW.) ..... . ... do....... . ......... do .... Hornblende ................... K-Ar 

Grosvenor Hills (NE.) ......... Lava flow ... . ....... do ....... --- ------------------- Plagioclase3 . ...... . ......... .. K-Ar 

Gardner Canyon ____________________ _ Dike ................. Rhyolite porphyry ___________ Zircon ............................. Ph-alpha 

Greaterville ..... -----------· ·-·----------------do ................ Rhyolite vitrophyre ......... Sanidine ...................... __ K-Ar 

8 65D893 --~----~~~~----~--~-- ------------~~----------=---~~~---- r(?) Paleocene rocks Greaterville .............................. Plug.. Quartz latite porphyry ... Zircon........ .. Ph-alpha 9 68D1472 .......... do ............................................ do.. __ ______ ___ ___ _ do ............................... Biotite........... . ........ K-Ar 

10 66D1185 H elvetia-Rosemont _________ ____ _ ........ do ..... . .. .......... do ............ -- ---------------- .......... do .......................... K-Ar 

11 67Dl245 Helvetia ............ ------------------------·------do ........... do .............................. .......... do ........................... K-Ar 12 66D1051 ..... do ...................................... Stock -·-·--·-- ..... Quartz monzonite ...................... do ............................ K-Ar 13 70D1612 ..... do ....... -- --------------·-- ------- ......... do.... . ......... do ........ _______ ____ ___________ .......... do ... -------------·---------- K-Ar 

Lower(?) Paleocene rocks 

14 63D281 Gringo Gulch ...... --------------------Plug, core'. ___ __ Microgranodiorite.. Biotite ............................. K-Ar Zircon _______________ ______ ________ Ph-alpha 

15 64D660 ..... do ................. -----------------·--Plug, rim4 ••••.•• Dacite porphyry . Hornblende ..................... K-Ar 

16 63D379 Temporal Gulch ....................... Dike.......... . ..... Quartz latite .......... do ........................... K-Ar porphyry. Zircon .... ---------------·---------Ph-alpha 

Calculated age, in m.y.2 

12.6±0.8 

13.8±13.5 

27.6±3% 

27.3± ,...._, 5% 40± 10 (40) 27.8±2.8 

27.6±1.6 

43±10 (40) 

25.9±0.8 

(180± ,_, 20%) 

55.7±1.9 

55.8±1.7 

56.3±1.7 

53.5±2.0 

53.9±2.0 

60.3± ,_, 5% 47.7± (50) 60.4±6.0 

67.5± ,_, 5% 61.2±20 (60) 

1Specific location of specimens 1, 8, and 9 given in this report; that of other specimens given on maps listed .in reference column of this table. 2Reported Ph-alpha ages shown parenthetically. ~K20 content used in calculation based on average of 10 replicate analyses. 

'Plug referred to as Gringo Gulch pluton on map (Drewes, 1971c). 

Author(s) and year of original 

report 

(all are written commun.) 

R. F. Marvin, H. H. Mehnert, and Violet Merritt, 1970. R. F. Marvin, H. H. Mehnert, and W. T. Henderson, 1970. 

Reference 

R. F. Marvin, H. H. Mehnert, Drewes ( 1971c). and Wayne Mountjoy, 1965. S. C. Creasey, 1964..................... Do. T. W. Stern, 1964....... Do. H. H. Mehnert, R. F. Marvin, Do. and Wayne Mountjoy, 1965. R. F. Marvin, H. H. Mehnert, Do . and Violet Merritt, 1968. 

T. W. Stern, 1965 ......... ------------ Do. R. F. Marvin, H . H. Mehnert, Drewes (1971a). and Violet Merritt, 1967. 

T. W. Stern, 1968 ........ ---------------------- R. F. Marvin, H. H. Mehnert, and Violet Merritt, 1968. R. F. Marvin, H. H. Mehnert, Drewes (1971a) . and Violet Merritt, 1967. .......... do......................................... Do. 

.......... do ................ --------------- ----·····-------- Do. H. H. Mehnert, R. F. Marvin, Drewes (1972a) . and Violet Merritt, 1970. 

S. C. Creasey, 1964 .......................... Drewes ( 197lc). 

T. W. Stern, 1964 ........ -------------------- Do. H. H. Mehnert, R. F. Marvin, Do. and Wayne Mountjoy, 1965. 

S. C. Creasey, 1964.------------------------- Do . T. W. Stern, 1965 ...... ---------------------- Do. 

-·...:> 

-
14 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

interpretation of their origin is deferred to the 

reports of adjacent areas by other authors. 

INTRUSIVE ROCKS 

Dikes and plugs of porphyry and small stocks of 

granitoid rocks, of Paleocene age, are scattered 

through the Santa Rita Mountains. Their composition is in the quartz latite (or quartz monzonite) to 

dacite (or granodiorite) range. Plugs are defined 

here as intrusives less than half a square mile in 

extent; stocks, as more than half a square mile. 

Most of the intrusive rocks of this age fall into 

one of four groups, of which the plugs of the Gringo 

Gulch area, referred to on the Mount Wrightson 

quadrangle map (Drewes, 1971c) as Gringo Gulch 

pluton, and the Cottonwood Canyon dike swarm are 

the two oldest, and the Helvetia stocks and the 

Greaterville intrusives are the two youngest. The 

remaining intrusives are small, widely scattered, 

poorly dated, and scantily studied. 

PLUGS OF GRINGO GULCH AREA 

Two plugs, largely of dacite porphyry, intrude 

Paleocene ( ?) and older volcanic and sedimentary 

rocks near Patagonia (fig. 2). The larger plug is 

exposed in an elliptical area of about half a square 

mile lying 3 miles north-northwest of the town and 

slightly west of Gringo Gulch. The plug has a subdued topographic expression both in the hills to the 

north and in the gullies to the south. Toward the 

north the plug walls appear to dip gently outward; to 

the south, however, they are about vertical, perhaps 

reflecting the general attitude of the plug as exposed 

at a deeper level. In the north half of this plug are 

two bodies of microgranodiorite believed to be latephase intrusives. Several large blocks of altered 

volcanics form inclusions in the southern part of 

the plug. 

The smaller plug underlies a low hill a mile southwest of Patagonia. Its contacts, fairly accurately 

located though nowhere actually exposed, seem to be 

very irregular. The plug probably intrudes rhyolitic 

tuff, tuffaceous sandstone, and andesitic sandstone 

exposed on the flanks of the hill ; all are of the 

upper member of the Gringo Gulch Volcanics, and 

they contain no clasts of the dacite porphyry. 

DACITE PORPHYRY 

Rocks of the dacite porphyry phase of the plugs 

are medium light gray to medium dark gray and 

weather to small brownish-gray rounded blocks that 

resemble the weathered Josephine Canyon Diorite. 

Phenocrysts commonly make up 5-10 percent of the 

rock and consist of relatively inconspicuous plagioclase laths and conspicuous hornblende laths. Phenocrysts are absent in the southern end of the larger 

plug, but they make .up as much as 40 percent of the 

rock in the northwestern part of that plug, where 

they are as much as 1 centimeter long. No other 

intrusives of the Santa Rita Mountains have such 

abundant and large hornblende phenocrysts. 

The phenocrysts are seen under the microscope 

to be scattered in a blocky or felty to intergranular 

groundmass, whose crystals are 0.01-0.15 mm long. 

Modes of three rocks are presented in table 3; the 

groundmass of only one specimen of these rocks is 

sufficiently coarse for modal analysis. Specimens 1 

and 2 are from the southern plug and were collected 

in the center of SWl)t sec. 12, T. 22 S., R. 15 E.; 

specimens 3 and 4 are from the northern plug, respectively from the center of the NWl)tNEt;4 sec. 

26, T. 21 S., R. 15 E., and from the NEl)tNEl)t sec. 

26, T. 21 S., R. 15 E. Plagioclase from the northern 

plug has an anorthite content which commonly 

ranges from 35 to 60 percent but which ranges from 

0 to 85 percent in some of the crystals that are 

strongly zoned. Plagioclase from the southern plug 

is partly albitized sodic andesine. Hornblende is 

mostly euhedral, but some is rounded and sheathed 

by granular iron oxide. Some hornblende surrounds 

quartz xenocrysts. Biotite is present in about half the 

specimens. Quartz is likewise present in only half 

the specimens, in which it forms interstitial anhedra, 

crystal clusters, and, rarely, xenocrysts. Orthoclase 

is finely disseminated in the groundmass and is 

strongly kaolinized. Accessory titaniferous magnetite is abundant, and apatite and sphene ( ?) are 

scarce. Alteration minerals are chlorite (rarely 

penninite), kaolinite, sericite, calcite, uralite ( ?) , 

leU:coxene, and iron oxide. 

Chemical and spectrographic analyses and the 

CIPW (Cross, Iddings, Pirsson, and Washington) 

norm of a specimen of dacite porphyry from the 

northern plug are presented in table 4. 

MICROGRANODIORITE 

Two small elliptical bodies of microgranodiorite 

lie within the north half of the northern plug of 

dacite porphyry. The contacts of these bodies are 

unexposed; but inasmuch as the grain size of the 

microgranodiorite in the larger body decreases 

toward the contact and that of the adjacent dacite 

porphyry does not, the microgranodiorite is believed 

to have intruded the dacite porphyry. Microgranodiorite outcrops are small because the rock is closely 

fractured and disaggregates readily. The rock is 

best exposed along a small gully a few hundred feet 

east of an earthen stock tank in the NEl)tNEl)t sec. 

26, T. 21 S., R. 15 E. Unweathered microgranodiorite 

is light olive gray to light brownish gray, and weathered surfaces are a darker brownish gray. In hand 
PALEOCENE AND PALEOCENE(?) ROCKS, INTRUSIVE 15 

specimen the rock is seen to contain fine-grained 

plagioclase and quartz and some slightly . coarser 

biotite and amphibole, as well as scattered small 

light-gray fine-grained segregations or inclusions. 

In thin section the microgranodiorite is seen to be 

slightly porphyritic and to be made up of equal 

amounts of anhedral crystals 0.03-0.1 mm across 

and subhedral crystals 1-4 mm long. The mode of 

a specimen obtained from the gully a few hundred 

feet east of the stock tank is given in table 3. 

TABLE 3. - Modes (in percent) of dacite porphyry and of 

microgranodiorite from the plugs of the Gringo Gulch area 

(Specimen 1 from southern plug; specimens 2-4 from northern plug] 

Rock type ........ _ Dacite porphyry Micrograno- diorite 

Component.·--------··········· Phenocrysts Whole rock 

Specimen No ................ 13 13 4 

Field No ......................... 63D257 64D661 63D199 63D199 63D281 

Plagioclase ..................... 10.8 21.8 25.0 73.5 52.1 

H ornblende .................. ,. 5.8 5.3 3.9 3.3 5.4 

Pyroxene ........................ .5 .3 .5 5.0 .0 

Biotite ................... .......... .0 1.0 .0 .9 4.7 

Quartz ........................... .0 .8 .0 1.3 19.8 

Orthoclase ...................... .0 .0 .0 9.9 14.9 

Magnetite ....................... 1.4 2.6 1.2 5.5 2.5 

Apatite ...... , .................... .0 .0 .0 .6 .4 

Zircon ............................. .0 .0 .0 .0 .2 

Sphene ( ?) ..................... .0 .0 .0 .0 Trace 

Allanite .......................... .0 .0 .0 .0 Trace 

Groundmass ................... 81.5 68.2 89.4 .0 .0 

TotaL ................... 100.0 100.0 100.0 100.0 100.0 

1Phenocryst mode and whole-rock mode were both obtained from specimen 

3, permitting a better evaluation of the phenocryst modes of specimens 1 and 2. 2Sizes of plagioclase in groundmass and phenocrysts are gradational, so this figure is of marginal significance. 

Quartz and strongly kaolinized orthoclase form 

anhedral grains in the groundmass. Plagioclase appears as subhedral phenocrysts and as anhedral 

groundmass grains, and it is moderately altered to 

kaolinite. Generally the plagioclase has an anorthite 

content of about 35 percent (sodic andesine) ; but 

a few crystals are calcic andesine, and other crystals 

and rims of normally zoned andesine grains are 

calcic oligoclase. Biotite forms subhedral phenocrysts 

and groundmass grains that are pleochroic in pale 

yellowish brown to moderate yellowish brown and 

that are slightly altered to chlorite and penninitic 

chlorite. The hornblende forms long subhedral grains 

which are sheathed in granular iron oxide and which 

are strongly altered to chlorite. The accessory minerals are titaniferous magnetite, apatite, sphene ( ?) , 

zircon, and allanite. Kaolinite is the most abundant 

secondary mineral; chlorite and sericite are less 

abundant. 

Chemical and spectrographic analyses and the 

CIPW norm of a specimen of microgranodiorite are 

given in table 4. 

The chemical differences between the microgranoTABLE 4. - Chemical and spectrographic analyses and 

CIPW norms, in percent, of two specimens from 

the plugs of the Gringo Gulch area 

Rock type .................................... Dacite porphyry 

Specimen No............................... 3 Field No....................................... 63D199 

Chemical analyses1 

Microgranodiorite 4 

63D281 

[P. L. D. Elmore, S. D. Botts, Lowell Artis, Gillison Chloe, and H. Smith, analysts] 

Si02.................................. 59.0 66.9 

Al203.............. .................. 16.3 15.5 

Fe203................................ 3.4 2.7 

FeO................................... 2.8 1.4 

MgO.................................. 2.6 1.4 

CaO................................... 4.7 2.8 

Na2 0................................. 4.4 4.0 

K20................................... 2.4 3.2 

H20-............................... 1.1 .36 

H20+............................... 1.4 .90 

Ti02.................................. .98 .56 

P20c;.................................. .21 .18 

MnO................................. .14 .06 

C02 ................................... ___ ._12 ______ .1_0 __ 

TotaL .................. . 100 100 

Semiquantitative spectrographic analyses2 

[J. C. Hamilton, analyst] 

Ba .................................... . Be ..................................... . Co ..................................... . 

Cr .................................... .. Cu .................................... . Ga .................................... . 

La .................................... . Nb .................................... . Ni ..................................... . Pb ..................................... . Sc ..................................... . Sr ..................................... . V ...................................... . 

Y ..................................... .. Yb .................................... . Zr ..................................... . 

0.05 

.0002 

0 

0 

.001 

.002 

0 

.003 

0 

.002 

0 

.005 

.001 

.002 

(3) 

.01 

CIPW norms 

0.15 

0 

.0007 

.001 

.01 

.003 

.005 

0 

.0007 

.007 

.0007 

.07 

.01 

.002 

.0002 

.007 

Q..................................... .. 11.541 24.104 

C........................................ 0 1.032 or...................................... 14.179 18.906 

ab...................................... 37.212 33.829 an ..................................... 17.646 12.080 

di {~~ -- _: ~:m g 

hy{~~---·_·_-_-_·_·_·_·_-_-_ _·_·_-_-_-_-_·_·_·_-_-_-_·_·_·_·_ ·_ ·_ _ 5:~~~ g.485 

mt...................................... 4.930 3.085 

hm..................................... 0 .572 

il....... ............................... .. 1.861 1.064 

ap...................................... .497 .426 CC.... ......................... .... ...... .273 .227 

TotaL ................... --9=-=7::--.0::--6:---------,9=-=8::--.8=-1=----

1Rapid rock analyses. 2Results are reported to the nearest number in the series 1, 0.7, 

0.5, 0.3, 0.2, 0.15, 0.1, and so on, which represent approximate 

midpoints of group data on a geometric scale. The assigned group for the semiquantitative results will include the quantitative value about 30 percent of the time. 3Not looked for. 

diorite and the dacite porphyry suggest that the 

porphyry is not just a border-chilled phase of the 

microgranodiorite, even though their close physical 

association and similar ages seem to indicate that 

they are genetically related. The silica content of the 

porphyry, for example, is only 59 percent, whereas 

that of the microgranodiorite is nearly 67 percent. 

The chemical variation between these two rocks 
16 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

resembles that of another rock pair in the Santa 

Rita Mountains, the Cretaceous Josephine Canyon 

Diorite and Madera Canyon Granodiorite (Drewes, 

1968, and unpub. data), which were emplaced next 

to each other virtually synchronously but which 

differ in composition. The intrusives of the Gringo 

Gulch area, however, seem to be 5-10 m.y. younger 

than the other rock pair. 

The age of the larger plug has been determined 

by three radiometric determinations on specimens 

3 and 4 of table 4 (specimens 14 and 15 of table 2). 

The petrographic similarity of the larger and smaller plugs, both of which are unique among the intrusives of the Santa Ritas, indicates that the two 

plugs are similar in age. In the Tertiary age range, 

Ph-alpha determinations are frequently less accurate 

than are K-Ar counterparts (R. F. Marvin, oral 

commun., 1970). Thus, the ages of the dacite por0 2 

phyry and the microgranodiorite are probably in 

the 55- to 60-m.y. age range. 

COTTONWOOD CANYON DIKE SWARM 

A swarm of more than 60 dikes of quartz latite 

porphyry, which generally look alike but which 

range from finely to . coarsely porphyritic, extends 

discontinuously across 'the Santa Rita Mountains 

from near the junction of Mansfield Canyon and 

Temporal Gulch, in the southeast, to near Yoas 

Mountain, in the northwest (fig. 4). The swarm 

strikes roughly N. 60° W.; and although many 

individual dikes parallel the trend of the swarm, 

some strike slightly more westerly, and a few strike 

northerly and northeasterly. Large dikes, as much 

as 2 miles long and 40 feet thick, are concentrated 

in the Cottonwood Canyon area; smaller dikes also 

are present there, but only small dikes appear in 

the Temporal Gulch and Agua Caliente Canyon 

3 

EXPLANATION 

~~ ~ ~ { D ~ c 

~ ~ § Sedimentary and volcanic rocks ~ ""-4 » 

~{~ ~{ IYtyl ~ g ~ Quartz latite porphyry 

~ ~ ~ a, c?arsely porph1p-:itic 

~ .S .-o b,jtnely porphyntw 

8 -

4 KILOMETE RS 

Plutonic rocks Other rocks 

Include some small Include some small 

bodi es of other bodies of plutonic 

rocks. Large plu- rocks 

tonic mass chiefly 

of Upper Cretaceous 

rock 

Contact 

.&379 

Collection site of radiometrically 

dated specimen 

>-

0::: 

<x: 

z 

0::: 

UJ 

1-

<x: >-=:~ 

~0 

1-

0::: 

UJ 

1-

:r: 

(!) 

=> 

0 

0:::(f) 

I=> 

1-0 

zw 

<x:U _.:X: 

o:::l-

[l)UJ 

::;:0::: 

<x:u 

u 

UJ 

0::: 

0.. 

FIGURE 4.- Location of Cottonwood Canyon dike swarm and distribution of coarsely porphyritic and finely porphyritic dikes, 

southern Santa Rita Mountains. Base from U.S. Geological Survey: Mount Wrightson quadrange, 1: 62,500, 1958. 
PALEOCENE AND PALEOCENE(?) ROCKS, INTRUSIVE 17 

areas. A few of the large dikes bifurcate, and the 

branches completely or partly surround large lenses 

of host rock. Most dikes intrude volcanic and sedimentary rocks of Cretaceous or Paleozoic ages, but 

a few of them intrude plutonic rocks as young as the 

Elephant Head Quartz Monzonite of late Late Cretaceous age, 68-69 m.y. old (Drewes, 1968, 1971c). 

In outcrop, the dikes usually are pale yellowish 

brown to light brown, much like their host rocks, 

and only the larger ones weather out in places as 

low ribs of outcrops. Some of the smaller dikes are 

so poorly exposed that they are marked chiefly by a 

train of weathered blocky debris of distinctively 

porphyritic lithology. Fresh rock surfaces are medium grayish brown to pale greenish gray. The 

phenocrysts of all the larger dikes and of only a 

few of the smaller ones are coarse, those of othoclase 

being 1j 2-1 inch long (fig. 5). Phenocrysts of most of 

the smaller dikes are only about a quarter of an 

inch long, but those of a few smaller dikes are of 

intermediate size. 

The rock contains 20-50 percent phenocrysts 

FIGURE 5. - Quartz latite porphyry of the Cottonwood 

Canyon dike swarm. The specimen, 63D456, is from a 

large dike in the upper basin of Cottonwood Canyon in 

the southern Santa Rita Mountains. 

scattered in a groundmass whose texture is granular 

to microcrystalline or, less commonly, hyalopilitic, 

with laths about 0.05 mm long. Modes of three specimens are given in table 5. 

TABLE 5.- Modes (in percent) of quartz latite porphyry of 

the Cottonwood Canyon dike swarm 

Rock type .................................... __ F_in_el-'- y -=- po_r.=....ph--'-yr--:i,-ti_c Coarsely porphyritic 

Specimen No.............................. 1 2 3 Field No....................................... 63D379 63D646 63D456 

Plagioclase...................... 14.7 27.6 20.4 

Orthoclase ( ?) ................. 3.8 2.8 10.9 

Quartz.............................. 3.8 2.3 0 

Biotite.............................. 11.2 4.5 5.9 

Hornblende...................... 2.4 3.7 Trace 

Magnetite........................ .3 .6 .2 

Apatite............................. .3 Trace .5 

Sphene.............................. Trace Trace .5 

Zircon............................... Trace Trace Trace 

Groundmass..................... 63.5 158.5 61.6 

Total. .................... -.....,.1....,...o-o.-o--....,...10,...o=-.o~----:1-=-o-=-o.-=-o---

11ncludes 1.3 percent quartz. 

Phenocrysts provide most of the petrographic 

data on the quartz latite porphyry dike swarm. 

Most of the feldspar phenocrysts are strongly kaolinized and sericitized. The plagioclase of some specimens is albitized from a calcic andesine. Quartz is 

strongly resorbed and presumably was once present 

even in those rocks of the Cottonwood Canyon dike 

swarm that are now without quartz. Biotite of the 

few hyalopilitic rocks is oxybiotite; that of the 

other rocks is altered to chlorite, epidote, calcite, 

iron oxide, and leucoxene. Hornblende is also largely altered like the biotite, but some crystals contain 

relicts of fresh hornblende, which have an olivebrown to yellowish-brown pleochroism or, in the 

hyalopilitic rocks, the reddish-brown pleochroism of 

oxyhornblende. Accessory magnetite, apatite, sphene, 

and zircon are also commonly present. 

A single specimen from the Temporal Gulch area 

(fig. 4 and table 2, specimen 16) has been dated 

using the K-Ar method on hornblende and the Phalpha method on zircon. The two ages, respectively 

67.5 and 61.2 m.y., are compatible in view of their 

overlapping age ranges (the assigned plus-or-minus 

values of the dates shown in table 2), so this specimen can be accepted as about 65 m.y. old. However, 

the younger age is favored because these dikes cut 

plutons whose main phases are dated as about 68 

m.y. and whose latest phase is 65 m.y. old. Also, 

the northwest trend of this swarm is more characteristic of Laramide structures, than of early postLaramide structures (Drewes, 1972a). 

HELVETIA STOCKS 

Granitic-texhfred rocks of Paleocene age form 

six stocks and other smaller intrusive bodies between 

the Helvetia mining district and the north end of 

the Santa Rita Mountains. These rocks will be 
18 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

described in detail elsewhere as part of the plutonic 

rocks of the region and are mentioned only briefly 

in the present report to preserve the continuity of 

the discussion of the geologic events of the Cenozoic, 

most of which involves hypabyssal rocks. The coarseEXPLANATION 

D 

Quaternary sedimentary rocks 

l::j 

Quartz latite porphyry 

Greaterville intrusives 

1 { Granodiorite 

~ Includes quartz 

Quartz diorite 

Includes diabase 

intrusives 

Q:l rrwnzonite and 

::X:: aplite intrusives 

Older ~ rocks 

Mainly sedimentary and plutonic rocks; in places thrust 

over granodiorite 

Contact 

Dotted where concealed 

Fault 

Dotted where concealed 

xB 

Locality referred to in text 

... 1183 

Collection site of radiometrically 

dated specimen 

61185 

Collection site of chemically 

analyzed specimen 

grained intrusive rocks range in composition from 

granodiorite to quartz monzonite, hereafter in this 

section of the report simply called granodiorite; 

however, one stock of quartz diorite is shown separately in figure 6. 

0 2 MILES 

0 ~ KILOMETERS 

FIGURE 6.- Distribution of Helvetia stocks and quartz latite porphyry of the Greate:t;"ville intrusives in the northern Santa Rita Mountains. Base from U.S. Geological Survey: Sahuarita quadrangle, 1 < 62,500, 1958. 
PALEOCENE AND PALEOCENE ( ?) ROCKS, INTRUSIVE 19 

At the present level of exposure the stocks are 

subelliptical steep-walled bodies which contain only 

a few small dikes of aplite and lamprophyre and 

only a few inclusions. The host rocks also contain 

only a few small apophyses and are not obviously 

more metamorphosed near individual stocks than 

away from them. The quartz diorite stock north 

of Johnson Ranch contains some inclusions of granodiorite, the smaller ones of which are hybridized 

by the quartz diorite; however, in rarer instances, 

diorite masses are included in the adjacent granodiorite. Thus the two rocks apparently were emplaced almost simultaneously, the quartz diorite 

perhaps being slightly younger than most of the 

granodiorite. 

GRANODIORITE 

Typical granodiorite of the Helvetia stocks is a 

light-gray rock that weathers light brownish gray; 

the quartz monzonite variety is grayish orange pink 

on both fresh and weathered surfaces. The rock 

disaggregates readily to form grus, friable massive 

outcrops, and scattered residual boulders. A blockier 

FIGURE 7.- Granodiorite from the Helvetia stocks. The 

specimen, 67D1392b, is from the stock south of Johnson 

Ranch, northern Santa Rita Mountains. 

rubble weathers from the aplitic bodies. Tongues of 

the pediment flanking this part of the Santa Rita 

Mountains extend more widely across the weathered 

stocks than they do across their host rocks, and 

unweathered granodiorite usually crops out only 

along the bottoms of sharply incised gullies. The 

rock only superficially resembles the Precambrian 

Continental Granodiorite (Drewes, 1968), which it 

intrudes near Helvetia. The younger granodiorite 

contains no large porphyroblasts, and its biotite (or 

chlorite) forms discrete books rather than a meshwork of fine crystal aggregates (fig. 7). 

The granodiorite of the Helvetia stocks has a 

hypidiomorphic-granular texture and a grain size 

of about 5 mm (fig. 8). Although porphyritic textures are absent from the normal rock types, the 

crystal sizes of some rocks are bimodally distributed 

in the 2- to 3-mm and the 5~ to 7 -mm sizes. 

The major modal constituents of these plutons 

vary widely. Plagioclase makes up 31-54 percent of 

the rock, potassium feldspar 13-32 percent, and 

FIGURE 8. - Granodiorite of the Helvetia stocks. The 

specimen, 66D1163, from the stock south of Helvetia, 

shows: A, apatite; 8, biotite; M, magnetite; P, plagioclase; pm, perthitic microcline; Q, quartz; S, sphene; 

Z, zircon. Crossed nicols. 
20 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

quartz 19-38 percent. These ranges, of course, are 

smaller within the more typical granodiorite stocks, 

but even within individual stocks the compositions 

is far from uniform. A mode of a representative 

specimen from the first stock south of the ranch is 

shown as specimen 1 in table 6. Most plagioclase is 

TABLE 6.-Modes (in percent) of representative 

specimens of granodiorite and quartz diorite from 

the Helvetia stocks 

Rock type..................................... Granodiorite Quartz diorite Specimen No............................... 1 2 Field No ...................................... : 67D1375 67D1402 

Quartz.............................. 29.8 9.1 

Plagioclase...................... 45.7 52.1 

Microcline........................ 13.0 .8 

Biotite.............................. 10.1 0 

Amphibole....................... 0 34.4 

Magnetite........................ 1.1 1.8 

Apatite............................. .2 .5 

Sphene.............................. 0 1.4 

Zircon............................... .1 .05 

Total ..................... ---1-0-0-.0----1-0-0.-1-5-

calcic oligoclase, whose outer rim is zoned to albite. 

Specimens with niicrocline are as abundant as those 

with orthoclase; both minerals commonly are finely 

but not abundantly perthitic, as seen in figure 8. 

Biotite ranges in abundance from 1 to 10 percent, 

and the range even within a single stock is large. 

One specimen from near the quartz diorite stock 

contains a little hornblende in addition to ubiquitous 

biotite. Accessory magnetite and apatite are everywhere present, zircon and sphene are common, and 

allanite is very sparse. Secondary minerals are 

generally sparse away from weathered rock and include some clay minerals, sericite, epidote, chlorite, 

and leucoxene. 

Chemical and spectrographic analyses and the 

CIPW norm of a granodiorite specimen from one of 

the Helvetia stocks are given in table 7. These 

analyses, like the mode, are of a representative rock 

from a suite which will be described in greater detail in a collateral paper on plutonic rocks (Drewes, 

unpub. data). 

QUARTZ DIORITE 

The quartz diorite is a medium-dark-gray to 

slightly greenish-gray rock that weathers slightly 

brownish gray. The slopes it underlies are more 

somber colored than the adjacent slopes of granodiorite or of Mesozoic and Paleozoic wallrock and 

inclusions. The rock also forms fewer outcrops than 

the host rock, and these outcrops are commonly 

most extensive in small gullies. 

The quartz diorite has a hypidiomorphic-granular 

to faintly felty texture with a trace of poikiloblastic 

and granophyric intergrowths (fig. 9) . A mode of 

a representative specimen, obtained from the wash 

east of the road and half a mile north of Johnson 

TABLE 7.- Chemical and spectrographic analyses and CIPW norm, in percent, of a representative specimen, 67D1375, of 

granodiorite from one of the Helvetia stocks 

Chemical analysis (rapid rock method) 

[P. L. D. Elmore, Lowell Artis, G. W. Chloe, H . Smith, J. Kelsey, and John Glenn, analysts] 

Si02·································· 71.2 

Ah03-················-----·-····-··· 14.3 

Fe20 3··--·-----·····--·-···········- 1.0 

FeO ................. ·-··············· 1.5 

MgO.................................. 1.0 

CaO·-·····-··--·-···--··············· 3.1 

N a 20.................. ............... 3.0 

K 20..................... .............. 3.2 

H 20-............................... 0.20 

H 20+ ............................... .78 

Ti02·································· .35 

P 20 5--·-······························ .18 

MnO................... ............... .12 

C02··································· <.05 

Total.. ..... _ ............ 100 

Semiquantitative spectrographic analysisl 

[J. L. Harris, analyst] 

Ag ................................. <0.0001. 

Ba................................. .07 

Be ... ·-···········----·-··········· .0002 

Cu................................. .001 

Ga................................. .001 

La.................................. .007 

Nb................................. .001 

Pb ................................. . 

Sc ................................. . 

Sr ................................. . 

·v ................................... . Y ................................... . 

Yb ..................... ........... . Zr ................................. . 

0.001 

.0003 

.02 

.002 

.003 

.0003 

.01 

CIPW norm 

Q ......................... ............. 33.305 mt ... ·-················-············· 1.450 

C..................................... .812 il...................................... .665 or .................................... 18.913 ap.................................... .426 

ab .................................... 25.390 cc..................................... .114 

au .................................... 13.890 

hy{~~-·-· ·_ _·_·_-_-_·_·_·_ ·_·_·_·_·::::::: :: ::: : i:~~! Total.. ................. 99.030 

1Results are reported to the n earest number in the series 1, 0.7, 0.5, 0.3, 0.2, 

0.15, and 0.1, which represent approximate midpoints of interval data on a geometric scale. The assigned interval for semiquantitative results will include the quantitative value about 30 p ercent of the time. Elements looked for but not found: As, Au, B, Bi, Cd, Co, Cr, Eu, Ge, Hf, In, Li, Mo, Nd, Ni, 

Pd, Pr, Pt, Re, Sb, Sm, Sn, Ta, Te, Th, Tl, U, W, and Zn. 

Ranch, is listed as specimen 2 in table 6. The plagioclase is a strongly sericitized sodic labradorite. Hornblende composition varies slightly, as indicated by 

the small differences from specimen to specimen in 

the range of its pleochroic colors of bright green to 

pale yellow brown, and of blue green to olive green. 

Biotite is present in some rocks, although absent 

in specimen 2. The accessories resemble those of 

the granodiorite, except that allanite was not seen. 

Alteration minerals are also like those in the granodiorite but are more abundant and include a greater 

proportion of chlorite and epidote to clay minerals. 

Two small intrusive bodies in the Rosemont area 

(fig. 6) are here provisionally associated with the 

quartz diorite because of their general similarity 

of composition and age. These rocks are dark greenish gray, intensely altered, and poorly exposed., The 

core of the larger intrusive is diabase; the margin of 

that intrusive and the entire smaller intrusive are 

andesite, some of which is amygdaloidal or vesicular. 

These intrusives are emplaced in strongly altered 

clastic rocks; the rocks around the larger intrusive 

are mainly arkose and arkosic conglomerate whose 

clastic texture is not entirely obliterated, and therefore the contact position is readily determined. 
PALEOCENE AND PALEOCENE(?) ROCKS, INTRUSIVE 21 

FIGURE 9.- Quartz diorite of the Helvetia stocks. The 

specimen, 67D406b, from the stock half a mile north 

of the Johnson Ranch, northern Santa Rita Mountains, 

shows: 8, biotite; H, hornblende; M, magnetite; P, 

plagioclase; Q, quartz; S, sphene. Crossed nicols. 

However, the clastic rock around the smaller intrusive is mainly hornfels, some of which is porphyroblastic and so closely resembles the andesite 

that the contact position is relatively uncertain. 

Under the microscope, a specimen of the diabase 

shows a strongly developed network of feldspar laths 

which contain intersertal and some intergranular 

amphibole which probably replaced pyroxene. The 

pseudomorphous amphibole has the pale-olive-green 

to yellowish-brown pleochroism of actinolite. Aside 

from some remnants of primary magnetite, the rest 

of the rock consists of abundant clay minerals, 

chlorite, epidote, iron oxide, and a little secondary ( ?) 

quartz. 

The Helvetia stocks are younger than structures 

of the Piman phase of the Laramide orogeny of Late 

Cretaceous age and are older than the quartz latite 

porphyry of the Greaterville plugs, of late ( ?) Paleocene age (Drewes, 197la). Within this interval, the 

stocks are penecontemporaneous with faults of the 

Helvetian phase of the orogeny of Paleocene age. 

472-410 0 - 72 - 4 

The stock south of Helvetia is dated as 53.5±2.0 

m.y. (table 2, specimen 12). This age, which supersedes that of 52.2 m.y. reported earlier by Drewes 

and Finnell (1968, p. 323), was determined by the 

K-Ar method on a biotite concentrate obtained from 

the specimen (collected at site 1051 in fig. 6). The 

dated rock is quartz monzonite, although most specimens of the stock are granodiorite; the rock shows 

no signs of recrystallization, and it is virtually free 

of secondary minerals. A second specimen (table 2, 

specimen 13) , from a new road cut in the first stock 

north of Helvetia townsite (NW1,4NW1,4NW1,4 sec. 

14, T. 18 S., R. 15 E.), gives a K-Ar age of 53.9± 

2.0 m.y. The calculated ages probably are very slightly too young with respect to those obtained for the 

nearby quartz latite porphyry plugs (table 2 and 

fig. 6), as explained in the section on the plugs, 

but they fall within the age range of the estimated 

analytical error. 

GREATERVILLE INTRUSIVES 

At least six plugs and many associated dikes of 

quartz latite porphyry, of Paleocene age, intrude 

the rocks of the northern Santa Rita Mountains. 

The porphyry is typically a closely fractured lightgray to grayish-orange-pink rock. It weathers to 

yellowish-brown or grayish-brown chips and forms 

outcrops less prominent than those of Paleozoic 

limestone host rock but more prominent than those 

of Cretaceous arkose host rocks. The porphyry is 

characterized by its sparse small biotite crystals 

and conspicuous stubby bipyramidal quartz crystals; 

much of the rock contains disseminated sulfides. 

Because the host rocks of these intrusives are mostly contact metamorphosed and are enriched in base 

and noble metals, the porphyry is locally referred 

to as the ore porphyry, and it is so termed in this 

report. 

The ore porphyry forms a small group of plugs, 

dikes, and sills in the Greaterville district (fig. 1; 

Drewes, 1970, fig. 2), two plugs in the Rosemont 

district, one on the crest of the range between Rosemont and Helvetia (fig. 6; Creasey and Quick, 1955, 

pl. 28; Drewes, 1971a), another plug at Helvetia, 

and two in the Sycamore Canyon area. Porphyry 

dikes extend east from the plug on the range crest 

and southeast from the plug south of Sycamore 

Canyon. Another dike extends northwest from the 

south Sycamore Canyon plug but is not contiguous 

with it. A group of dikes strikes west and southwest 

of the plugs at Rosemont. Another group of porphyry 

dikes assigned to these rocks crops out near Box 

Canyon ; of these dikes, the largest strikes east 

(Drewes, 1971a). A third group of porphyry dikes 

lies near Fish Canyon (Drewes, 1971a, c). 
22 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

Many of the Greaterville plugs are irregularly 

shaped in plan, which distinguishes them from the 

elliptically shaped Helvetia stocks. Tongues of porphyry extend outward from the plugs ;:tlong many 

faults, and I assume the plugs have this habit at 

depth (Drewes, 1970, p. A10 and pl. 1) ; some of 

the tongues of porphyry are long enough to be 

dikes. Most of these dikes are less than half a mile 

long, in contrast to the much longer ones of the 

younger dike swarms, but a few dikes are several 

miles long. A typical shorter dike is about. 10-15 

feet thick and 500 feet long, and it pinches and 

swells and changes its trend from place to place. 

In general, though, the dikes trend east. 

Phenocrysts of the dikes are relatively small and 

sparse, but those of the plugs are more abundant 

and consist of conspicuously stubby bypyramidal 

quartz (fig. 10), fairly abundant feldspar, and some 

biotite or chloritized biotite. The groundmass is 

commonly sugary textured and fine, but in a few 

FIGURE 10.- Quartz latite porphyry, or ore porphyry, 

from the Greaterville intrusives. The specimen, 

66D1185, is from the plug on the ridge between 

Helvetia and Rosemont. 

rocks it is granitoid. The subtle variations in kind 

and abundance of phenocrysts in the dikes of the 

Greaterville area suggest that the dikes may have 

a more complex magmatic history than the one outlined here. 

Under the microscope, quartz latite porphyry typically is seen to have a porphyritic idiomorphicgranular texture (fig. 11). Phenocrysts make up 

20-40 percent of the rock of the plugs; in most rocks 

they are less than 4 mm long, but in a few rocks 

they are as much as 8 mm long. Groundmass grains 

range in size from less than 0.01 mm (virtually 

cryptocrystalline) to about 0.2 mm. Granophyric 

intergrowths of quartz and feldspar appear in the 

groundmass of some samples of the main plug at 

Greaterville and of the plug south of Sycamore Canyon. Grain size of the dike rocks is commonly finer 

than that of the plugs. Two of the more southerly 

dikes contain interstitial altered glass. The ore 

porphyry thus has the typical texture of hypabyssal 

intrusives, and the plugs have shapes suggesting an 

emplacement from a fairly fluid magma. The rock 

FIGURE 11.- Quartz latite porphyry, or ore porphyry, of 

the Greaterville intrusives. The specimen, 66D1185, from 

the plug between Helvetia and Rosemont, contains: · B, 

biotite; M, magnetite; 0, orthoclase; P, plagioclase;- Q, 

quartz. 
PALEOCENE AND PALEOCENE(?) ROCKS, INTRUSIVE 23 

texture appears, however, to grade northward, presumably with increasing depth, into granitic texture and southward with decreasing depth into 

vitrophyric texture. 

Modes of the quartz latite porphyry are given in 

table 8. These modes are more uniform than modes 

of the Helvetia stocks summarized in table 6. 

A few mineralogic features of the quartz latite 

porphyry are also distinctive, but many of them 

resemble those of other hypabyssal rocks of the 

Santa Rita Mountains. Generally, the stubby and 

commonly bipyramidal quartz phenocrysts, unlike 

the idiomorphic quartz crystals of the groundmass, 

are partly resorbed. In some porphyry, quartz forms 

loosely clustered crystals rimmed by other small 

interlocking quartz crystals. Undulatory extinction 

of the quartz is slight or absent. Feldspars occur as 

tabular euhedral phenocrysts or crystal clusters, 

as well as hypidiomorphic to idiomorphic groundmass crystals, all of which generally are strongly 

kaolinitized and sericitized. The composition of 

plagioclase, usually determined only on phenocrysts, 

ranges from sodic andesine to sodic oligoclase. Plagioclase of the groundmass, especially in dikes, may 

range in composition from andesine to albite, but 

the fine grain size and intense alteration hinder 

reliable measurements. The potassium feldspar of 

most samples is sanidine or -is suspected to be 

sanidine, but in the coarse-grained rocks it is orthoclase and microcline. A small amount of fine perthitic 

intergrowths appears in both the orthoclase and the 

sanidine. Most of the biotite is chloritized; the 

unaltered remnants are commonly pleochroic in yel .. 

lowish-brown to dark-brown, and rarely in reddishbrown, colors. Accessory minerals are titaniferous 

magnetite, apatite, sphene, zircon, and sparse allanite. Pale-brown devitrified glass constitutes about 

25 percent of a specimen from a flow-laminated part 

of the long dike south of the Greaterville plugs, 

and a trace of opaline material appears in a veinlet 

of that specimen. Secondary minerals usually consist of abundant clay minerals, sericite, chlorite, 

and iron oxide, of less abundant epidote and tremolite ( ?) , and of a trace of tourmaline. 

The quartz latite porphyry is a chemically uniform 

rock. Analyses of four samples, a pair from each -of 

two plugs, are nearly identical. Both pairs of 

analyses are also closely similar to analyses of single 

samples from other plugs, shown in table 9. In the 

last two columns of the table the average of the 

six analyzed specimens is compared with Nockolds' 

(1954) averages. As a whole, the quartz latite 

porphyry resembles his average adamellite (quartz 

monzonite) most closely, but the total iron and the 

magnesium contents resemble more closely those of 

his average calc-alkali granite. 

The spectrographic analyses show that the ore 

porphyry contains anomalous amounts of several 

base metals and especially of copper. As this topic 

is discussed in a collateral paper (Drewes, 1971b), 

only a summary of the pertinent observations is 

given here. The copper content of the six specimens 

in table 9 and that of three other specimens from 

other Greaterville plugs average about 0.2 percent. 

That of 10 specimens of granodiorite of the Helvetia stocks, on the other hand, averages only about 

0.0007 percent, or 21j2 orders of magnitude less than 

the ore porphyry. At least some of the copper in the 

ore porphyry is contained in the biotite, whose concentrates have been shown by Lovering, Cooper, 

Drewes, and Cone (1970) to contain anomalous 

amounts of copper. Their preliminary findings suggest that the copper content of biotite may be a 

practical guide to recognizing ore-associated igneous 

rocks where other direct indications are absent or 

are inconclusive. 

Biotite concentrates from samples of three plugs 

(table 2, specimens 9, 10, and 11) give radiometric 

TABLE 8.- Modes (in percent) of quartz latite porphyry, or ore porphyry, of the Greaterville intrusives 

Location of plug ............. ·--·················----··-- South 

Sycamore 

Canyon 

[Phenocryst modes given in parentheses] 

Helvetia Range crest between Helvetia and Rosemont Greaterville, 

largest plug 

Rosemont, 

larger plug 

Specimen No ........................ ·--········--·········---1- 2 3 4 4 5 6 7 8 9 Field No ......... --............................................. 66D1224 67D1244 66D1245 66D1183 66Dl183 66D1185 65D893 66D921 66D1129 66D1098 

Quartz ......... ·--················--····························· 25.2 30.6 24.1 26.1 ( 4.4) 27.9 22.9 24.4 (22.3) ( 5.4) 

Plagioclase __ ··----·--·--·------··-- ----------·----··--:_______ 45.3 43.4 44.9 37.0 (2.6) 41.9 45.2 56.1 (14.3) (31.2) Potassium feldspar ............. , .......... ,, ..................................................................................................................................................... --······--···--··················································· 

Sanidine----·----·------·----·----------···----------··--·-- 22.4 27.4 Sanidine ( ? ) ---------------------------------------·--·-- 31.8 (19.2) 26.3 (2.1) (.5) 

Orthoclase and microcline __ ··----··--···--··· 26.2 --··----···--····----··--·--------·--·----·----·--··--····------·--····--·--···--·--···--·······------· 26.3 13.6 

Average 

1-7 

25.9 

44.8 

24.9 

Biotite.·----··-----------·----·-------------··--------···----·---- 2.9 2.2 2.7 3.3 ( 3.3) 3.0 3.9 4.2 (2.3) (1.9) 3.2 Magnetite--------------··----···------·----·------------------· .1 1.0 .3 1.3 ( .4) .7 1.1 1.4 ( .1) (Trace) .8 Apatite ____ ·------·--··----···------------·----···----·------·--· .2 .1 .4 .2 ( .2) .2 .3 .2 (Trace) (Trace) .2 

Sphene ... ·--···--------···----··---- --------····--------··----··· .1 .3 .1 .2 (.2) Trace .2 .1 (.1) (Trace) .2 Zircon ______________ ·--------····---- ------··--------------------· 0 Trace .05 .1 (.1) Trace .05 Trace (Trace) (0) Trace Allanite ________ __ ______________________________________________ .. 0 0 .05 0 (0) 0 .05 0 (0) (0) Trace 

Groundmass------·---- --------------------··----··--------·-- 0 0 0 0 169.6 0 0 0 58.8 61.0 0 TotaL. ______ ·-----· ···--··--·----··--·--·----· ________ -----,..1-::-:oo:-:.oc-------,1-=-oo::-. o=---1,..,.. o-=-o.o-,---1.,...,.o..:....o . ...,...o __ 1....::. 0.:....:0 ...:....o --1_,0..:....0 _....,.. o ----,1-=- oo-=-.o=----=-1 o-=-=o~.o,----::-:1 o=o-:.o- ---=1=o-::- o .-::- o ----=1-::-:oo::--:.o::--

1Not included in average. 
24 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

TABLE 9.- Chemical and spectrographic analyses and CIPW norms, in percent, of six specimens of quartz latite porphyTy, or ore porphyry, compared with Nockolds' average analyses 

Location of plug ................................................... Greaterville, Range crest between largest plug Helvetia and Rosemont 

Sample No.............................................................. 62 

Field No.................................................................. 65D893 4 

66D1183 5 66D1185 

Helvetia 

10 3 66D1236 67D1245 

South 

Sycamore 

Canyon 

1 

66D1224 

A verag.e Average of of 6 Nockolds (1954, 

S~~%m~~~ ---=P_-_1_01_2..:...., ...,..10--,1,-4..:....) --,---

Adamellite Calc-alkali Mountains granite 

Chemical analyses1 

[P. L. D. Elmore, Lowell Artis, G. W. Chloe, H. Smith, J. Kelsey, and John Glenn~, a_n_a_:ly_s_ts-::-]:-:::------::-::-::--=-----=-=--=-=---

Si02.......................................................................... 69.4 72.2 72.5 72.6 72.9 70:9 71.8 69.15 72.08 AhOa......................................................................... 16.0 14.7 14.4 14.4 14.2 15.0 14.8 14.63 13.87 Fe20a........................................................................ 1.8 1.2 1.2 .94 .86 .78 1.13 1.22 .86 FeO........................................................................... .82 .52 .36 .60 .76 .84 .65 2.27 1.67 MgO.......................................................................... .51 .46 .46 .47 .49 .58 .50 .99 .52 CaO........................................................................... 2.3 1.2 1.7 1.6 1.4 2.3 1.8 2.45 1.33 Na20......................................................................... 4.2 3.5 3.8 4.2 4.2 3.7 3.9 3.35 3.08 K20........................................................................... 3.5 4.6 4.4 3.9 4.0 3.8 4.0 4.58 5.46 H20-....................................................................... .21 .08 .17 .05 .04 .22 .13 H20+....................................................................... .89 .74 .67 .51 .51 .98 .72 .54 .53 Ti02.......................................................................... .39 .17 .14 .15 .15 .16 .19 .56 .37 P20s.......................................................................... .14 .08 .07 .06 .08 .11 .09 .20 .18 MnO.......................................................................... .06 .07 .02 .04 .03 .16 .06 .06 .06 C02............................................................................ <.05 <.05 <.05 <.05 <.05 .28 

TotaL........................................................... 100 100 100 100 100 100 

Semiquantitative spectrographic analysesa 

[J. L. Harris, analyst] 

0 <0.0001 0 <0.0001 0 <0.0001 0.1 .1 0.07 .07 0.07 .07 

Ag ............................................................................ . Ba ............................................................................. . Be ............................................................................. . .0001 .0003 .0003 .0002 .0003 .0002 Ce ............................................................................. . .01 0 .03 .03 .02 .015 Cr ............................................................................. . 0 0 .0003 0 .002 0 Cu ............................................................................. . .0003 .15 .05 .07 .015 .0003 Ga ............................................................................. . .0015 .0015 .0015 .0015 .0015 .001 La ............................................................................. . .005 0 .01 .01 .007 .007 Mo ............................................................................ . .0003 0 .0003 0 0 .0003 'Nb ............................................................................ . .001 .001 .001 .001 .001 0 Pb ............................................................................. . .0003 .0007 .0007 .0003 .0005 .001 Sc ............................................................................. . .0005 .0003 .001 .0003 .0007 .0003 Sr ............................................................................. .. .05 .03 .03 .03 .03 .015 V ........................................................................ : ..... . .003 .0015 .0015 .0015 .0015 .0015 Y ............................................................................... . .002 .0015 .002 .002 .0015 .002 Yb ............................................................................ .. .0002 .00015 .0002 .0002 .00015 .0002 Zr ............................................................................. . .015 .01 .007 .007 .01 .01 

CIPW norms 

Q ................ , .............................................................. . 26.348 31.479 29.582 29.480 29.668 29.751 C ............................................................................... . 1.567 2.098 .579 .622 .725 1.534 20.626 27.300 26.016 23.145 23.715 22.498 35.443 29.744 32.173 35.692 35.657 31.368 

or .............................................................................. . ab ............................................................................. . 

10.153 5.137 7.665 7.261 6.128 8.938 1.267 1.150 1.146 1.176 1.224 1.447 0 0 0 .152 .495 .933 

an ............................................................................ .. 

hy { f~~~ ~ ~ ~ ~ ~::::::: :::::::: ~:::::: ~ ~::: :::::::::::::::::::::::::::::::::::::: mt ............................................................................. . 1.703 1.418 .820 1.369 1.251 1.133 .620 .227 .635 0 0 0 .739 .324 .266 .286 .286 .304 

hm ...................................................................... . il ............................................................................... . ap ............................................................................. . .331 .190 .166 .143 .190 .261 cc .............................................................................. . .113 .114 .114 .114 .114 .638 

TotaL ........................................................... . 98.91 99.18 99.16 99.44 99.45 98.81 

1Rapid rock analyses. 2Average of 2 replicate analyses. 3

Results are reported to the nearest number in the series 1, 0.7, 0.5, 0.3, 0.2, 0.15, and 0.1, which represent approximate midpoints of interval data on a geometric scale. The assigned interval for semiquantitative results will include the quantitative value about 30 percent of the time. Elements looked for but not found: As, Au, B, Bi, Cd, Co, Eu, Ge, Hf, In, Li, Nd, Ni, Pd, Pr, Pt, Re, Sb, Sm, Sn, Ta, Te, Th, Tl, U, W, and Zn. 

ages, based on the K-Ar method, of 55.7-56.3 m.y. 

Specimen 9, which is not shown on the Sahuarita 

quadrangle map (Drewes, 1971a), was. collected 

from the south end of the dike in a saddle area on 

the ridge between Ophir Gulch and Enzenberg Canyon (NW1;4NW%,SE1;4 sec. 23, T. 19 S., R. 15 E.). 

The consistency of these ages and the absence of 

signs of recrystallization of the porphyry suggest 

that an age of 56 m.y. is essentially a true age of 

the crystallization of the rock. This age provides 

an important reference point in interpreting the 

geologic development of the area, specifically in 

dating faults such as the late-phase faults of the 

Laramide orogeny, with which the porphyry has a 

close temporal and spatial relation (Drewes, 1971a, 

1972a). 

A zircon concentrate from one plug (table 2, specimen 8), collected on a nose at elevation 5,880 feet 

(SW1;4NE1;4NW1;4 sec. 25, T. 19 S., R. 15 E.) is 

dated as 180 m.y., an age that is older than that of 

the rocks it intrudes. Apparently some of this zircon 

is unrecognized xenocrystic material derived from 

the underlying Precambrian rocks. Provisionally 

this isotopic age is considered to be enigmatic and 

is not used in dating the plugs. 

A comparison of the isotopic ages of the Greaterville plugs and the Helvetia stocks with their geologic 

relations indicates a dating problem which makes 

me suspect that the ages that date two of the stocks 

are slightly too young. In the field, the plugs are 

seen to be younger than the stocks, because they 

intrude faults that cut the stocks. But the isotopic 

ages from two stocks are slightly younger than the 

three ages from the plugs. This amount, however, is 

less than the combined range of analytical error 

given for the ages of the two rock types, so it may 
PALEOCENE TO OLIGOCENE ROCKS 25 

not be significant. With the data presently available, 

it is most likely that, for unknown reasons, the ages 

on the stocks are a few million years too young, 

although it is also possible either that the span of 

time between emplacement and cooling of the stocks 

is significantly greater than that of the plugs, or that 

the emplacement of the stocks or development of 

the poststock and preplug faults are not Jully 

synchronous. 

PALEOCENE TO OLIGOCENE ROCKS 

A few igneous rocks, whose geologic ages are inferred to postdate some volcanics and intrusives of 

Paleocene age and to predate other volcanics and 

intrusives of late ( ?) Oligocene age, underlie small 

areas of the Santa Rita Mountains. They include 

a group of andesitic dikes, olivine andesite plugs, 

rhyolitic volcanics, and a swarm of quartz veins. 

The relations among these rocks and generally also 

the relations to older and younger rocks are incompletely known. As a result, the Paleocene to 

Oligocene rocks remain a fairly enigmatic part of 

the local geologic record. 

ANDESITIC DIKES 

Small intrusive bodies of andesite, dacite, and 

diorite occur in widely scattered parts of the Santa 

Rita Mountains. These bodies are typically dikes, 

sills, and intrusive pipes which will here all be referred to as dikes. Only those dikes are described 

here which appear to be sufficiently younger than 

the nearest plutonic body to cast doubt on their 

genetic association with the plutons. 

The andesitic dikes of Sycamore Canyon are ·greenish- to brownish-gray fine- to medium-grained rocks. 

Two northwest-trending dikes at locality A (fig. 6) 

are about 10 feet wide and a mile long, and they 

weather to form rounded blocks. A dike at locality 

B forms a string of pods that follows the folds in 

the steeply dipping beds of the Lower Cretaceous 

Apache Canyon Formation, shown with the older 

rocks in figure 6 and described by Drewes (1971b). 

The dike at locality C is only 2 feet wide and is 

very inconspicuous because it weathers like its host 

rock, which is also steeply dipping Apache Canyon 

Formation. The few other andesitic dikes shown in 

the Sycamore Canyon ·area (Drewes, 1972a, pl. 

5) resemble those at localities A-C and are not 

described separately. 

Under the microscope, the andesitic rocks are 

seen to be nonporphyritic to sparsely porphyritic 

ophitic rocks with grains 0.5-3 mm long. Amphibole 

forms acicular crystals between which the other 

minerals form subhedral to anhedral grains. Unaltered remnants of amphibole have pale pleochroic 

colors like those of actinolite, but most of the amphibole is chloritized. The rocks contain much chlorite, 

epidote, kaolinite, and sericite. Plagioclase is typically albitized, but one specimen contains remnants 

of calcic andesine. Magnetite and apatite are ubiquitous, and one specimen also contains traces of 

sphene, quartz, and possible biotite. 

OLIVINE ANDESITE PLUGS 

Two plugs of vesicular olivine andesite intrude 

metamorphosed Lower Cretaceous Glance Conglomerate (Drewes, 1971b) less than a mile north of the 

east end of Box Canyon (Drewes, 1971a; 1972a, pl. 

· 5). The Glance here dips moderately to the west 

and overlies a thrust fault. The plugs are roughly 

elliptical, and they are separated from each other by 

a septum of conglomerate. One plug cuts the thrust 

fault, and the other is faulted against gravel of 

Pliocene and Pleistocene age. 

The olivine andesite makes only a few inconspicuous outcrops because in most places it is strongly 

weathered; however, relatively unweathered rock 

crops out at the dry falls along the canyon northwest of Deering Spring. The rock is medium gray 

and fine grained, and much of it contains abundant 

large vesicles or calcite amyg9-ules. 

In thin section, the andesite is seen to have a 

strongly felty texture of plagioclase laths with 

interstitial granular ferromagnesian minerals. Plagioclase makes up 65-80 percent of the rock; it is 

albitized and is strongly altered to clay minerals 

and sericite. Iddingsite ( ?) , apparently formed from 

olivine, makes up about 10 percent of the rock. Some 

of the interstitial material may have been an amphibole. Magnetite makes up 2-4 percent of the rock, 

and apatite a trace. The rock also contains calcite 

and an isotropic iron-poor pearly lustered chlorite 

(R. E. Wilcox, oral commun., 1968) in amygdules 

and interstitial · pockets. Iron oxide occurs as rims 

and .disseminated grains. Epidote is widely scattered 

in the rock, as are a few veinlets of chalcedony, 

quartz, and calcite. 

The olivine andesite is provisionally dated as early 

Tertiary because it postdates a thrust fault whose 

latest movement may be as young as Paleocene 

(Drewes, 1972a) and because it lacks the glassy 

groundmass and fresh appearance typical of volcanics of late ( ?) Oligocene or younger age. The 

abundance of vesicular structure suggests that the 

rock was emplaced near the surface, a situation most 

compatible with conditions well after the Laramide 

orogeny. The andesite may have been emplaced 

about the time of extrusion of dacitic flows 6 miles 

to the northeast (Finnell, 1971). 
26 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

RHYOLITIC VOLCANICS 

Rhyolitic lava ( ?) and tuff underlie a small area 

along the east edge of the Sahuarita quadrangle 

about 2 miles north of the east end of Box Canyon 

(Drewes, 1971a; 1972a, pl. 5). These rocks are 

much less altered than the underlying rocks of 

the Cretaceous Bisbee Group, and they are unconformably overlain by terrace gravel. They are a 

light-gray to pale-reddish-purple coarsely fractured 

rock that contains lithic fragments, chiefly 'felsite, 

less than an inch across. Approximately 50 percent 

of the rock consists of fragmental and partly resorbed phenocrysts about 2 mm across, some of 

which are sericitized and kaolinitized plagioclase 

laths. A little biotite, 'replaced by .a hydromica, and 

a trace of magnetite and zircon ( ?) are also present. 

The groundmass consists of brown partly devitrified 

flow-laminated glass, and of clay minerals. 

The absence from the rhyolitic rock of the strong 

mineralization of Paleocene age (Drewes, 1971b) 

that is so widespread in the adjacent and underlying 

Cretaceous rock indicates that the rock probably 

is no older than Paleocene. T. L. Finnell (oral comniun., 1968) indicated that about 3 miles northeast 

of the outcrop area of the rhyolite, where the rocks 

are more extensively exposed, the volcanics unconformably underlie the older part of the Pantano 

'Formation, probably of Oligocene age, which does 

not appear in the Sahuarita quadrangle. The rhyolitic rocks are therefore likely of Paleocene to Oligo-

·cene age. 

Dating by Damon and Bikerman (1964) of tuff 

low in the Pantano Formation near the junction of 

Pantano Wash and Davidson Canyon (Finnell, 

i970), 15 miles northeast of the Wasp Canyon area, 

gives K-Ar ages of 32.8±2.7 m.y. (biotite) and 

36.7±1.1 m.y. (sanidine). Another tuff higher in 

the sequence near Pantano Wash is dated as 29.0± 

0.9 m.y. (biotite) and 29.4±0.9 m.y. (biotite). A 

ttiff from the lower part of the Pantano Formation 

near the Babocomari Ranch, near the intersection 

of Babocomari River and the Pima-Cochise County 

line, 23 miles southeast of the Wasp Canyon area, 

gives a K-Ar age on biotite of 38.9±1.3 m.y. (R. F. 

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

commun., 1969). 

QUARTZ VEIN SWARM 

Veins largely or wholly of quartz are scattered 

throughout the Santa Rita Mountains. In the central 

and northern parts of the mountains the veins are 

generally sparse and fairly small and irregular in 

trend and thickness, whereas in the southern part of 

the mountains the veins are abundant, are at least 

a few feet thick and a few hundred feet long, and 

are regular in trend and thickness. Veins commonly 

occur singly or in small clusters; however, in the 

Alto Gulch and Mansfield Canyon areas (fig. 12) 

there is a swarm of more than 300 quartz veins, 

which is alined roughly east-west and which fans out 

eastward. 

Typical quartz veins of this swarm, here referred 

to as the Alto veins, are about 5 feet wide and 2,000 

feet long, are regularly tabular, and .are inclined 

between 70 ° N. and 70° S. The veins cut rocks as 

young as the Gringo Gulch Volcanics and are unconformably covered by gravel of Pliocene or Pleistocene age. They are more abundant in the volcanic 

and sedimentary ·rocks of the flanks of the mountains than in the · plutons along the crest of the 

mountains, and they are more abundant in the 

Josephine Canyon Diorite than in the other plutonic 

rocks. The veins intrude both altered and unaltered 

r·ock, and with one exception described in a collateral 

report (Drewes, 1972b), alteration does not increase near the veins. Thus the veins seem to be 

genetically unrelated to the alteration, and they are 

probably younger than that alteration. The veins 

commonly weather out as low ribs of nearly white 

rock, many of whose fractures are coated with brown 

iron oxides. A few veins follow normal faults. Only 

the large northwest-trending vein near the Salero 

Ranch is brecciated by younger faulting and is 

recrystallized; younger· movement of silica has 

changed part of the vein into a broad silicified zone. 

The Alto veins are of particular interest because 

of their mineralization, which was des~ribed by 

Schrader (1915) and by Drewes (1972b). Briefly, 

the veins carry sulfides of copper-, lead, zinc, silver, 

and iron. As early as the 18th century the Spaniards 

mined silver from some of the larger veins in the 

Alto and Salero areas. The last widespread mining 

activity ceased with the Indian uprisings toward the 

end of the 19th century, but sporadic interest in 

exploration and development work in the area 

continues. 

The structural significance of the Alto veins is of 

additional interest (Drewes, 1972a). The veins make 

an intriguing pattern mainly in the structural blocks 

between two large northwest-trending faults (to .the 

southwest and to the east) and the zone of weakness followed by the dike swarm at Gardner Canyon 

(to the north). In essence, I believe the vein swarm 

lies in a tension fra_cture system that developed 

after the relaxation of the compressive stresses of 

the Laramide orogeny. The further development of 

the tension fracture system gave rise to the injection 

of the dikes of Gardner Canyon and Box Canyon 

(fig. 25), the injection of the feeder dikes of the 
X 

Josephine 

Peak 

sv 

::::::: :{)y::::::::::::::::: . • • • • • • • • " ••••••• 0 •• 0 •• 0 •••••• 

••••• 0 ••• 0 •• 0 ••••••••••••• 

0 

0 

PALEOCENE TO OLIGOCENE ROCKS 

7Q-&-

/)/00 

sv 

Qy\1 : : : : : : : : : : : 3 1° .......... 40' 

EXPLANATION 

Younger rocks 

(Pleistocene or Pliocene) 

27 

Rhyolite porphyry dike showing dip 

(Oligocene) 

3 1° 

~~~~~---- ------------------- --~-=~30' 

2 3 4 MILES 

2 3 4 KILOMETE RS 

Quartz 

~ 

~ 

vein, showing dip 

(Eocene or Paleocene) 

s, silicified pod 

Older rocks (Mesozoic) 

d, Josephine Canyon Diorite 

(Cretaceous) 

g, Squaw Gulch Granite (Jurassic) and other plutonic rocks 

sv, sedimentary and volcanic 

rocks 

Contact 

Fault 

Dotted where concealed 

FIGURE 12.- Distribution of the quartz vein swarm in the southern Santa Rita Mountains. Base from U.S. Geological Survey: Mount Wrightson quadrangle, 1: 62,500, 1958. 
28 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

Grosvenor Hills Volcanics, and the beginning of the 

northeast-trending range-front faults. 

The age of the Alto veins falls in the range of 

Paleocene to Oligocene. The veins postdate some 

rocks of Paleocene age and appear to postdate the 

metamorphism and alteration, which probably are 

also as young as Paleocene. The veins are older than 

the Grosvenor Hills Volcanics, whose basal gravel 

contains pebbles of vein quartz, and they are also 

probably older than the dike swarms in Gardner 

Canyon and in Box Canyon- which are isotopically 

dated as about 26 m.y. (table 2) -because sulfide 

mineralization is strongly associated with Paleocene 

intrusives and because sulfides are virtually absent 

from the Oligocene intrusives and extrusives. Yet 

the veins were emplaced sufficiently later than the 

Paleocene intrusives to permit the compressional 

stresses of the Laramide to give way to the later 

tensional stresses. Thus the veins may be Eocene. 

The area of east-trending silicified pods in the 

Gringo Gulch Volcanics does not overlap the area of 

east-trending quartz veins. Nevertheless their common orientation suggests that at least a uniform 

structural control existed in both areas. The mineralized rock of the veins, as seen in the field, is similar 

to that of the silicified pods, as determined spectrographically. Many veins contain sulfides of lead, 

zinc, and copper. A little molybdenite was seen, 

and silver ore has been mined from several veins. 

Semiquantitative spectrographic analyses (made by 

E. L. Mosier, written comrnun., 1964) of samples 

from six silicified pods follow (N.D., not detected; 

threshold of detectability, 200 ppm) : 

Concentration Frequency 

Metal (ppm) (of six samples) 

Ag ___ _______ ______ ______ _________ . 1-2 2 

Cu___________ __ __ _______ ___ ___ ___ _ 150 2 

Mo__________ _____ __ __ __ ___ ____ __ _ 1-2 2 

Pb_________ __ _____ __ ______ ____ ____ 50- 100 4 

Zn_____ ________ ___ __ __ __ __ ____ ___ _ N.D. 0 

The concentrations of at least silver, copper, molybdenum, and lead are three or more times their background values. In addition, alunite, indicative of 

hydrothermal alteration, is present in several of the 

pods. 

The similarity in the orientation and mineralization of pods and veins and the restriction of pods 

and veins to adjacent though separate areas suggest 

that the pods are genetically as well as structurally 

related to veins. Possibly the pods are relatively 

near surface expressions of veins. They thus may 

represent the deposits formed as surface leakage of 

veins, presumably in a hot-spring environment, and 

perhaps mineralized veins lie at depth beneath 

them. Since the emplacement of the veins and pods, 

the southern Santa Rita Mountains have been tilted 

gently southward to expose progressively deeper 

levels of the vein and pod system toward the north. 

Southward tilting during the late Tertiary is supported by other structural evidence (Drewes, 1972a). 

OLIGOCENE ROCKS 

Volcanic, intrusive, and minor sedimentary rocks, 

of late ( ?) Oligocene age, occur in scattered parts of 

the Santa Rita Mountains. The southern part of the 

mountains contains an igneous complex of volcanic, 

hypabyssal, and plutonic rocks, which probably are 

genetically related. The northern part of the mountains contains only rhyolite intrusives. 

IGNEOUS COMPLEX OF . THE SOUTHERN 

SANTA RITA MOUNT A INS 

Abundant rhyolite and rhyodacite of the Grosvenor Hills Volcanics, many rhyodacite dikes, some 

rhyodacite laccoliths, and a small granodiorite stock 

occur in the Grosvenor Hills and the adjacent San 

Cayetano Mountains, two small hilly areas on the 

southern and southwestern flanks of the Santa Rita 

Mountains. The relatively deep seated rocks in the 

San Cayetano Mountains and the volcanics and 

shallow intrusives in the Grosvenor Hills are brought 

together by large vertical movement on a normal 

fault of mid-Tertiary to late Tertiary age. The petrography and stratigraphy of the rocks of this 

igneous complex are described separately; their 

chemistry, age, and structure, however, are described together in order to facilitate the comparisons that indicate the rocks are genetically related. 

In brief, these rocks probably represent parts of (1) 

a deep parent-magma reservoir, .(2) shallow magma 

chambers, (3) connecting conduits between the deep 

and shallow bodies, and ( 4) lava spilled out at the 

surface above, and in part adjacent to, the internal 

"plumbing system." 

STRATIGRAPHY AND PET ROGRAPHY 

GROSVENOR HILLS VOLCAN ICS 

Rhyolite and rhyodacite lava and pyroclastic rocks 

and subordinate amounts of sedimentary rocks, together about 2,000 feet thick, have been named the 

Grosvenor Hills Volcanics (Drewes, 1968, p. C14-

C15). This formation underlies an area of about 

20 square miles, chiefly in the Grosvenor Hills but 

extending southward beyond the Mount Wrightson 

quadrangle and across Sonoita Creek (fig. 13) as 

well as northwestward to the pediment along Montosa Canyon in the northwestern part of the Mount 

Wrightson quadrangle (Drewes, 1971c). It lies unconformably upon Jurassic and Cretaceous plutonic 

and volcanic rocks and is unconformably overlain by 

Miocene and Pliocene gravel of Nogales. The volcanics are commonly gently inclined to the south or 
OLIGOCENE ROCKS, IGNEOUS COMPLEX, GROSVENOR HILLS VOLCANICS 29 

west; steeper dips occur only next to some faults and 

intrusives and locally within some tuff sheets that 

were deformed penecontemporaneously with their 

deposition. 

The basal unconformity is best exposed along the 

northeastern edge of the volcanics and around the 

two small inliers within the volcanic pile (fig. 13). 

It forms a nearly flat and gently southwestward-inclined surface into which several small channels have 

been cut. The channels are filled with fairly thick 

lenses of the basal gravel and silt of the overlying 

formation. The gentle topography and the typically 

thin veneer of basal clastic deposits suggest that 

the prevolcanic surface was a pediment. 

The Grosvenor Hills Volcanics are divided into a 

thin basal gravel and silt member, a moderately 

thick medial rhyolite member, and a thick capping 

rhyodacite member. The two volcanic members are 

made up of lava flows, tuff (some of it welded), and 

agglomerate. Still further subdivisions, such as individual welded tuff sheets, were mapped; they cannot be shown at the scale of figure 13, but they are 

shown on the quadrangle map (Drewes, 1971c). 

GRAVEL A!'\D SILT MEMBER 

The gravel and silt member typically occurs as a 

discontinuous thin sheet along the base of the formation, but some lenses are as much as 200 feet thick. 

The member is well exposed along the banks of the 

larger gullies half a mile southwest of the Salero 

Ranch and along the canyon about 1,000 feet north 

of the northernmost laccolith (fig. 13). Small slices 

of the member are also brought up along the margins 

of the laccoliths, and one slice is dragged up along 

the San Cayetano fault. 

The member consists chiefly of a distinctive palered to pale-reddish-brown very weakly consolidated 

silt and intercalated thin beds of gravel. However, 

a mile south of Cinigita Tank the member is made 

up of bedded siltstone, shale, and sublithographic 

limestone, and at locality D in figure 13 it includes 

some olive-gray shale. The pebbles of the gravel 

are distinctly more rounded than those of the younger gravels of the area. They consist of detritus of 

the Squaw Gulch Granite (Jurassic), vein quartz, 

Mount Wrightson Formation (Triassic), and volcanics of the Salero Formation (Cretaceous). The 

limestone and the olive-gray shale contain fossil 

plant material. 

The small size and round shape of the pebbles are 

noteworthy, inasmuch as a likely source of the 

pebbles occurs close by in the adjacent Santa Rita 

Mountains but apparently in few other adjacent 

areas (Drewes, 1971b). A small pebble size suggests 

a source area of low relief. The proximity of the 

472- 4 10 0 - 72 - 5 

apparent source of the pebbles seems to contradict 

the amount of transport implied by the degree of 

rounding, especially of such hard rocks as the volcanics of the Mount Wrightson Formation. Either 

the pebbles were recycled or they were deposited 

by streams in a climate different from the present 

one. For instance, a more moist climate in which 

rainfall was more equitably distributed than it is 

today might have favored more extensive scouring 

of the hard coarser pebbles by the bypassing finer 

particles, whereas during infrequent but heavy 

downpours, typical of the present climate, bypassing is not favored. 

RHY-OLITE MEMBER 

The rhyolite member forms a sheet at least 500 

feet thick that overlies the basal gravel and silt 

member and laps over the Mesozoic rocks (fig. 13). 

It is best exposed along the south, east, and north 

margins of the volcanic pile. The member is chiefly 

tuff and tuff breccia, but it also contains about 5 

percent each of tuffaceous sandstone, agglomerate, 

lava, and welded tuff, and a trace of coarse breccia. 

A small body of rhyolite breccia, possibly an intrusive, lies near the base of the member south of Cinigita Tank. The pyroclastic rocks commonly underlie 

lowlands, in which small outcrops of white to very 

pale yellowish brown massive rock are abundant 

and in which ledges of similarly colored bedded tuffaceous rock are widely scattered. The rhyolite flows 

of this member, which occur between Josephine 

Canyon and Tejano Spring to the north and in the 

lower reaches of Coal Mine Canyon and the upper 

part of Fresno Canyon to the south, are moderateorange-pink - to pale-yellowish-brown or light-red, 

conspicuously color-laminated, finely porphyritic 

bodies. 

In thin section, the tuffaceous rocks are seen to 

contain fragments of vitrophyre and pumice. In a 

few rocks, such as those along the lower part of 

Hangmans Canyon, some of the pumice fragments 

are flattened, so they may be incipiently welded. 

Most lithic fragments, however, resemble the lava 

flows of the member. Some opaline material is also 

present. 

The tuff commonly is altered only by pervasive 

argillic material. However, some tuff 1-2 miles 

southwest of the Salero Ranch is altered to a greenish-gray rock, whose color probably is due to the 

presence of a mineral with X-ray characteristics 

that resemble those of clinoptilolite. 

Rhyolite lava consists of a cryptocrystalline to devitrified groundmass, abundant microlites, and 

phenocrysts. Phenocrysts, 10-20 percent of the rock, 

are, in decreasing order of abundance, plagioclase, 
30 

• ' , • .' • 4 ~ . . . . . ' .. ~ .. : .. ' :- ~ .. ; · . · . 

. . ~ : . ~ : ... 

;'I •• •• : • ••• 

·.·· . 

.... /:.::: .. :·.·: :·. : .. · .. :.:-:.·:_.: .... . . . · .: . . ·: . . .. 

·.·.·.· .. ·· .·. · ... · .. · : : · .. '. 

: ' .· . · .. ~ : ' 

' . ~ . . . 

:··.· : . 

. . . · . .. ·. · ·. · 

CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

• ... . ,•· 

FIGURE 13. - Slightly generalized geologic map of the Grosvenor Hills and San Cayetano Mountains on the southwest flank 
OLIGOCENE ROCKS, IGNEOUS COMPLEX, GROSVENOR HILLS VOLCANICS 

0 

EXPLANATION 

Rhyodacite vitrophyre Intrusive rocks of the 

0 

In laccoliths and San Cayetano Mountains 

associated dikes Ts, granodiorite stock 

r 

\ 

Rhyolite plug 

and dike 

Trp, rhyodacite porphyry 

dike swarm 

SCALE 1:62 500 

2 3 MILES 

2 3 Kl LOM ETERS 

L 

<· 9~~ ..: :I 

Gravel 

(Quaternary and Tertiary) 

Gravel of Nogales 

(Pliocene and Miocene) 

UNCONFORMITY 

Rhyodacite member 

Tgdf,jlows 

gdt 

Tgda, agglomerate; includes 

some tuffaceous rocks 

T gdw, welded tuff 

Tgdt, unwelded tuff 

Rhyolite member 

Tgrf,jlows 

T grt, tuff; includes tuff 

breccia and tuffaceous 

sandstone 

Tgrw, welded tuff 

[:)./!'~:~\~}] Gravel and silt member 

UNCONFORMITY 

[:<:;;~9:<<] Sedimentary, volcanic, and 

plutonic rocks 

(Mesozoic) 

65t 

Contact, showing dip 

Dotted where concealed 

! tBO 

Fault, showing dip 

Dotted where covered or intruded. 

Bar and ball on downthrown side 

25 

_j__ 

Strike and dip of beds 

Breccia 

xA 

Locality referred to in text 

0 3ss 

Collection site of 

chemically dated specimen 

.315 

Collection site of 

radiometrically analyzed specimen 

>-

0:: 

<( 

f0:: 

w 

f31 

of the Santa Rita Mountains. Base from U.S. Geological Survey :Mount Wrightson and Nogales quadrangles 1:62,500, 1958. 
32 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

quartz, oxybiotite, oxyhornblende, and accessory 

magnetite, apatite, and zircon. Most plagioclase is 

sodic oligoclase to sodic andesine, but in some specimens the plagioclase is albite. 

Modal analyses of two specimens of lava, one of 

the intrusive breccia, and one of a lithic fragment 

in tuff breccia are presented in table 10. The analyses 

TABLE 10. - Modes (in percent) of some rocks of the rhyolite 

member of the Grosvenor Hills Volcanics 

Rock represented....................... Lava flow Instrusive Fragment in breccia tuff breccia 

Specimen No.1... .. ... ..... . ........... ... 1 2 12 13 Field No....................................... 63D443 64D624 63D290 63D286a 

Quartz.......................................... 3.7 6.6 7.8 1.9 Potassium f eldspar.................... 0 0 7.6 0 Plagioclase................................... 4.7 12.4 6.6 7.6 

Biotite........................................... 2.0 1.4 .8 3.2 Hornblende.................................. 0 0 0 .6 Magnetite..................................... .2 .2 .2 .2 Apatite......................................... 0 Trace .1 Trace Zircon........................................... 0 0 0 Trace Groundmass ................................. __ 89_.4 __ .:..:.79.:..:..4_:__ _ _:___:__ 76.9 _ _:__:..::..:.:... 86.5__ 

TotaL............................... 100 100 100 100 

Anorthite content of ============ == plagioclase............................... 19-23 30-35 0 18-28 

1Specimens 1 and 2 are keyed to table 14. 

probably are fairly representative of the tuff, too, 

because the tuff contains abundant lithic fragments 

that are like the lava (compare modes of specimens 

1 and 2 with that of specimen 13). Specimens 1 and 

2 were also chemically analyzed (table 14). Specimen 1 is a porphyritic rhyolite from a fragmental 

flow, collected in the SW1;4NW1;4 sec. 28, T. 21 S., R. 

14 E. Specimen 2 is also a porphyritic rhyolite, from 

a flow in the SW1;4NE1;4 sec. 12, T. 18 S., R. 13 E. 

The source of the volcanics of the rhyolite member 

remains conjectural, in part because of the extensive 

cover of younger volcanics. Evidence of a single 

major volcanic center is absent. Indeed, some circumstantial evidence points to sources at several 

small local vents. The breccia body 11/2 miles south 

of Cinigita Tank (fig. 13) may indicate the site of 

one sue~ vent. The widely scattered small bodies of 

lava and welded tuff may have issued from separate 

vents. The group of rhyolite dikes and the plug at 

the south end of the San Cayetano Mountains 

(mapped by F. S. Simons, written commun., 1966) 

may have been feeders of still other vents of the 

rhyolite member. 

RHYODACITE MEMBER 

The rhyodacite member, which caps the Grosvenor 

Hills Volcanics, is commonly at least 800 feet thick 

and consists of abundant agglomerate and lava flows, 

some welded tuff, and a little unwelded tuff. With the 

exception of the nearly white unwelded tuff, these 

rocks are dark gray to dark brown, and they form 

prominent scarps or steep slopes above the palecolored rocks of the rhyolite member. Most of the 

individual units of the rhyodacite member are lenticular or wedge shaped, and therefore the stratigraphic sequence varies from place to place. In 

general, the thicker parts of the units lie to the 

west. Some stratigraphic units also change facies 

laterally; lava is generally most abundant in the 

western part of the pile, and tuff is most abundant in 

the east and northeast parts. 

The thickness of the member increases westward, 

but to the west the full thickness is unknown because either the top of the member is eroded or the 

bottom is concealed. The rhyodacite along lower Ash 

Canyon to the southeast, where both top and bottom 

of the member are present, is 150-250 feet thick. 

Near the center of the pile, in the highest part of 

the Grosvenor Hills (fig. 13), the member is at least 

800 feet thick, and near Sheuy Well it is at least 

1,000 feet thick. A composite of the major subdivisions, probably nowhere actually present, is more 

than 2,200 feet thick. 

The basal contact of the rhyodacite member is 

generally gradational, and locally the member interfingers on a small scale with the underlying rhyolite; 

but in places the contact is unconformable. 

The lowest rocks of the member are commonly 

gray rhyodacite agglomerate, at whose base lies a 

transitional sheet a few tens of feet thick of tuffaceous agglomerate consistir.~.g of scattered rather 

than of abundant rhyodacite blocks in a tuffaceous 

matrix. Such a transitional basal sheet of tuffaceous 

agglomerate is 100 feet thick along parts of Fresno 

Canyon. West of lower Fresno Canyon the lowest 

mapped unit of the rhyodacite member is a welded 

tuff, but it, too, rests on a sheet of tuffaceous agglomerate commonly too thin to be mapped. This 

agglomerate contains many blocks of petrified wood, 

which suggests, according to Damon and Miller 

(1963) , that a forest growing on the tuff was overwhelmed by a rhyodacite block flow. Their interpretation implies that locally the base of the rhyodacite 

member is placed beneath the tuffaceous agglomerates. Only rarely do the rhyodacite fragments so 

decrease in size as well as in abundance as to create 

a gradation between the agglomerate and .the underlying rhyolite tuff. Less commonly gray rhyodacite 

lava flows or welded tuff form the lowest unit of 

the member. Their basal contacts are relatively 

sharp, but most of them also have a thin basal sheet 

of fragmental rocks of mixed types. Less commonly, 

too, tuffaceous agglomerate or tuff breccia of the 

rhyodacite member rests on, · or interfingers with, 

tuff breccia, apparently of the rhyolite member. 

Thus, in a few places, as at the head of Cinigita 

Canyon and nearby to the north and west of that 

canyon head, the contact between the members is 
OLIGOCENE ROCKS, IGNEOUS COMPLEX, GROSVENOR HILLS VOLCANICS 33 

gradational, and locally its mapped position is 

tenuous. 

Agglomerate is the most abundant rock type of 

the rhyodacite member, and it commonly is the 

basal unit. On the flanks of the high hills a mile 

east of Sheuy Well, a lower wedge of agglomerate 

underlying a welded tuff is as much as 1,000 feet 

thick, and an upper wedge of agglomerate near the 

crest of the hills is about 440 feet thick. The lower 

agglomerate wedge thins from the Sheuy Well area 

toward the north, east, and south, and in places it 

seems to pinch out entirely. Similarly, the upper 

agglomerate wedge thins eastward, but it is restricted to the graben between the Sheuy and George 

Wise faults. 

The agglomerate consists of abundant rhyodacite 

blocks, commonly 1-3 feet across (fig. 14) but as 

much as 20 feet across, set in a relatively sparse 

tuffaceous matrix. The rock typically underlies moderately steep slopes that are irregularly broken by 

cliffs and numerous small outcrops. The weathered 

blocks are brownish black, and the matrix is pale 

FIGURE 14.- Typical agglomerate of the rhyodacite member 

of the Grosvenor Hills Volcanics. 

yellowish brown to reddish brown. The blocks are 

more resistant to weathering than is the friable 

tuff and tend to project above the rest of the surface 

and to form a colluvial veneer. Where the proportion 

of matrix to blocks increases, usually toward the 

base or thin margi~s of a wedge of agglomerate, 

the rock grades into tuff breccia. North of Cinigita 

Canyon a little of the relatively thin part of the 

lower wedge of agglomerate is welded, as a result 

of which vitrophyre blocks as much as 2 feet across 

are flattened to 3-4 inches, and the pumiceous tuff 

matrix is indurated and strongly jointed. West of 

peak 5,469 (fig. 13) the clasts of the upper wedge 

of agglomerate include a few blocks of granodioritic 

rock that resembles the granodiorite of the San 

Cayetano Mountains. In hand specimen the vitrophyre is seen to contain moderately large phenocrysts of plagioclase and at least two kinds of 

ferromagnesian minerals (fig. 15). 

Lava flows are next in abundance to agglomerate 

in the rhyodacite member. They form prominent 

cliffs, and the rocks typically contain strongly deFIGURE 15.- Vitrophyre block from the rhyodacite member of the Grosvenor Hills Volcanics. The specimen, 

63D411, is from the agglomerate sheet that caps the 

ridge near hill 5,228. 
34 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

veloped fracture sets and a subhorizontal sheeting. A 

thick sheet of lava, with a few intercalated lenses of 

agglomerate, lies between Sheuy Well and the west 

edge of the volcanic pile. Thinner sheets of lava 

spread northward, eastward, and southward from 

the Sheuy Well area, and they typically overlie agglomerate but in places overlap it to lie upon thP. 

tuff breccia of the rhyolite member. A single rhyodacite flow occurs to the north along Josephine Canyon, 

arid another one crops out to the southwest between 

the lower reaches of Coal Mine and Hangmans Canyons. Two rhyodacite flows, however, lie northeast of 

the Sheuy fault; the upper of these flows forms the 

highest stratigraphic unit of the member. 

The rock is medium gray and contains moderately 

abundant coarse phenocrysts of plagioclase and of 

two ferromagnesian minerals, like those in the agglomerate blocks. Only the flow near lower Coal 

Mine Canyon is different in that it is light gray 

and contains fewer phenocrysts; thus it is somewhat transitional in appearance between the flow 

rocks typical of the rhyolite member and those 

typical of the rhyodacite member. 

Welded tuff makes up less than 10 percent of the 

rhyodacite member. At least five sheets of welded 

tuff, separated by sheets of lapilli tuff breccia, form 

a single sequence between the two wedges of agglomerate in a graben between the Sheuy and George 

Wise faults. These sheets of welded tuff, each 60-

120 feet thick, and intercalated tuff breccia, each 

commonly 10-20 feet thick, have a combined maximum thickness of at least 500 feet along the western 

part of the north margin of the graben. These sheets 

thin rapidly southward across the graben to pinch 

out in places north of the George Wise fault, and 

they thin gradually eastward along the graben. This 

thinning is distributed throughout most of the individual beds and is not a progressive onlapping of 

the base of the unit or a truncation of the top. In 

another downfaulted area, along Fresno Canyon, 

there is commonly only one sheet of welded tuff 

and it is about 100 feet thick. It lies between agglomerate beds toward the northwest but laps across the 

underlying agglomerate sheet and rests on the rhyolite member to the south. The welded tuff of Fresno 

Canyon is roughly contemporaneous with the welded 

tuffs in the graben although none are continuous 

across the intervening area. Apparently, separate 

hot-ash flows were confined to the depressions and 

avoided the intervening high block. 

Welded tuff weathers to form dark-brown cliffs 

having an irregular polygonal jointing that resembles the jointing of the cliffs held up by the lava 

flows. The welded tuff consists of small very dark 

gray lenticular flattened clots of vitrophyre set in a 

pale-brown relatively friable matrix (fig. 16). Both 

the flattening of the clasts and the development of 

the polygonal joints diminish gradually toward the 

intercalated tuff breccia sheets. The phenocryst content of the vitric clots resembles that of the flows 

and agglomerate blocks. 

Unwelded tuff is the least abundant rock type of 

the rhyodacite member. In outcrop and hand specimen it resembles the tuff of the rhyolite member, 

but unlike the underlying tuff, at least some of the 

unit contains no rhyolite fragments, and part of the 

unit contains small vitric fragments that resemble 

the rhyodacite. 

The flows, the blocks of the agglomerate, and the 

clots in the welded tuff all have a glassy to cryptocrystalline groundmass enclosing phenocrysts and 

xenocrysts, which make up 30-40 percent of the rock. 

The glass has an index of refraction of n= 1.505-

1.506. Phenocrysts and xenocrysts, as much as 7 

mm long, are chiefly plagioclase, in part quartz, 

hornblende, biotite, augite, and hypersthene, and 

rarely magnetite, apatite, and zircon (fig. 17). 

Plagioclase commonly has the composition of sodic 

andesine, but it ranges from calcic oligoclase to 

sodic labrado'rite. Most plagioclase is clouded, and 

some crystals are partly resorbed. Some plagioclase 

from · the agglomerate of lower Ash and Hangmans 

Canyons has reverse compositional zoning. Most 

quartz of the rhyodacite is also partly resorbed. 

Hornblende and biotite are somewhat erratically 

distributed in that they occur in some samples of 

the flows north of upper Cinigita Canyon but not 

in others. Hornblende and biotite in about half the 

specimens are oxidized, and many crystals are partly resorbed. Where unoxidized, the hornblende is 

pleochroic in shades of olive brown. Augite occurs 

as euhedra and also as reaction rims around hornblende or biotite, or less commonly, around hypersthene. A nonpleochroic orthopyroxene, possibly 

enstatite, appears in place of hypersthene in the 

agglomerate of Ash and Hangmans Canyons and in 

the flow that caps the central part of the pile. 

Modal analyses of seven rocks of the rhyodacite 

member are listed in table 11. Specimens 3 and 4 are 

hypersthene augite rhyodacite vitrophyres. Specimen 4 contains xenocrystic hornblende, biotite, and 

plagioclase. This plagioclase has reverse zoning, the 

outermost zone of the plagioclase phenocrysts having 

the more calcic composition. Rarely, plagioclase and 

quartz xenocrysts are rimmed by augite (fig. 18). 

Specimen 3 is from a flow in the SW1;4SW1;4 sec. 

22, T. 21 S., R. 14 E., and specimen 4 is from a 

block of the agglomerate sheet in the NE1;4SE1;4 
OLIGOCENE ROCKS, IGNEOUS COMPLEX, GROSVENOR HILLS VOLCANICS 35 

A B 

FIGURE 16.- Welded tuff from the rhyodacite member of the Grosvenor Hills Volcanics. The specimen, 63D414, shows: 

au, augite; G, glass; H, hornblende; hy, hypersthene; P, plagioclase; sh, shards of flattened vesicles. A, At low 

magnification: dark-gray tuffaceous groundmass, light-gray rhyodacite vitrophyre xenocryst, and, across the left side, 

banded xenocryst of flattened rhyodacite pumice or tuff. B, At high magnification: textural details of rhyodacite 

pumice or tuff xenocryst, showing remnants of flattened shards or vesicles. 

TABLE 11.- Modes (in percent) of rocks of the rhyodacite member of the Grosvenor Hills Volcanics 

Rock represented ................................................................ __ ::-----::-L:-a_v_a_f_lo_w---:::-=------=-=---

Specimen No.1 .......................................... ................... ......... 3 14 15 16 

Agglomerate 

4 17 18 Average 

Field No................................................................................ 63D382 63D426 65D710 65D718 63D425 63D386 64D601a 

Quartz............................................................. 0 0 0 Trace 0 0 0 0 

Plagioclase...................................................... 26.2 19.7 26.5 19.2 22.1 36.2 21.5 24.5 

Biotite.............................................................. 0 1.4 .2 0 1.1 .3 0 .4 

Hornblende...................................................... 0 0 .8 3.7 3.3 0 2.4 1.5 

Augite.............................................................. 4.7 0 4.5 3.1 5.5 3.8 1.8 3.3 

Hypersthene................................................... 4.9 0 3.1 1.2 1.9 3.9 .1 2.2 

Magnetite........................................................ 1.4 1.0 1.6 .7 1.7 1.2 .7 1.2 

Apatite............................................................. Trace .1 .2 .2 .2 Trace .2 .1 

Zircon............................................................... 0 Trace 0 0 0 0 0 0 

Groundmass.................................................... 62.8 77.8 63.1 71.8 64.2 54.6 73.3 66.8 

Total. 100.0 100.0 100.0 ................................. ................... - 1-:-:o:-:o-,.o=----1--o-o-.o,-----1-o-o-.o---9-9-.9-----,--,-----,--,--,--,---....,...-:-c:-::------

Anorthite content of plagioclase: 

Phenocrysts................................................. 44-50 27-30 

Xenocrysts .................................................. . 

1Specimens 3 and 4 are keyed to table 14. 

sec. 15, T. 22 S., R. 14 E. (unsurveyed). Of the 

rocks with modal analyses, only these two were 

chemically analyzed (table 14). 

Most rocks of the rhyodacite member- especially 

the vitric rocks, the flows, the blocks of the agglomerate, and the clots in the welded tuff- are only 

40-45 

33 

49-55 45-49 

32-44 

36-43 45-47 

mildly altered to clay minerals. However, along the 

west edge of the volcanic pile, where the flows and 

agglomerates thicken, the rocks are strongly montmorillonitized. 

The site of the vent of the rhyodacite volcanics is 

unknown, as is true for that of the volcanics of the 
36 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

FIGURE 17.- Vitrophyre from a flow in the rhyodacite 

member of the Grosvenor Hills Volcanics. The speeimen, 

65D710, from the top of the lower flow on the ridge 

almost 2 miles south of the Salero Ranch, shows: au, 

augite; hy, hypersthene; oh, oxyhornblende; M, magnetite; P, plagioclase. 

rhyolite member. Unlike the rhyolite volcanics, however, the rhyodacite volcanics may have come from 

a single vent, whose location probably lay west of 

Sheuy Well and either just east of the San Cayetano 

fault or perhaps well within the San Cayetano fault 

block. Such a location is suggested by the thickening 

of many units toward that area and by the gradation 

of some flows into agglomerates or block flows away 

from that area. The montmorillonitic alteration of 

the rocks at the west edge of the pile and the inclusion of granodiorite resembling that of the stock 

in the San Cayetano Mountains are compatible with 

a vent area in or near those mountains. 

DIKES AND LACCOLITHS IN THE GROSVENOR HILLS AREA . 

A swarm of dikes and five bulbous laccoliths, of 

medium-gray to light-gray rhyodacite vitrophyre, 

intrude the Grosvenor Hills Volcanics (fig. 13) . 

With the exception of the southernmost laccolith, 

these intrusive bodies lie in an arcuate zone centered 

FIGURE 18.- Quartz xenocrysts rimmed by augite, from 

the rhyodacite member of the Grosvenor Hills Volcanics. 

The specimen, 63D421, from a lava flow about a mile 

south of George Wise Spring, contains: au, augite; 

dg, devitrified glass ( ?) ; oa, oxidized amphibole, probably 

hornblende; P, plagioclase; qx, quartz xenocryst. 

roughly in the middle of the San Cayetano Mountains. This zone is peripheral to the postulated site 

of the rhyodacite vent. 

The dikes occur either singly or in small clusters, 

spaced at intervals of about a mile. Dikes are commonly more than 50 feet thick, and several of them 

flare out into bodies many hundreds of feet thick. 

Most dikes trend N. 70 ° W., but the northernmost 

dike strikes N. 60° E. One small cluster of dikes is 

continuous with a laccolith, and another dike merges 

with or abuts a tabular intrusive resembling the 

margin of a sill or laccolith. The wider dikes typically form rock ribs, whereas the narrower ones are 

less resistant to weathering and underlie relatively 

low belts. 

Individual laccoliths are as much as. 11/2 miles 

long, almost 1 mile wide, and about 900 feet thick. 

They are shaped like conventional doorknobs (fig. 

20C) that have slightly flattened elliptical cross 

sections, rather than the lenticular cross sections of 
OLIGOCENE ROCKS, IGNEOUS COMPLEX, GROSVENOR HILLS AREA 37 

the classical laccoliths (fig. 20A) of the Henry 

Mountains (Hunt and others, 1953). In plan, the 

laccoliths are also roughly elliptical and are elongated parallel to most of the dikes. Only a few 

minor bulges mar the symmetry of their outlines. 

The laccoliths commonly intrude the tuffaceous 

rhyolite member of the Grosvenor Hills Volcanics, 

but locally the beds adjacent to this tuff are also 

intruded. Laccoliths, because they resist weathering 

better than their tuffaceous host, typically underlie 

buttes or hilly areas rimmed by cliffs. Coarse reddish-gray residual blocks of rhyodacite vitrophyre 

are .strewn over the gentler slopes. The cores of the 

laccoliths contain one or two sets of relatively 

widely spaced fractures more or less transverse to 

the elongation of the laccoliths, and the marginal 

zones display more closely spaced radial fractures 

(fig. 19) and a peripheral sheeting. In many places 

the marginal zones are stripped of most of their 

cover because of the ease of erosion of the host rock; 

however, a thin veneer of talus or rock-fall debris 

commonly covers the contact itself. In such places 

the peripheral sheeting still reliably conveys the 

local attitude of the contact. And nowhere does the 

tuff that surrounds the massive rhyodacite bodies 

contain detritus derived from them. 

The floors of the laccoliths are virtually fiat and 

are roughly parallel to, and at or slightly above, 

the base of the Grosvenor Hills Volcanics. The 

marginal contacts themselves generally ·steepen 

toward the floors, and in places they dip inward, as 

shown in the foreground of figure 19. Consequently, 

small segments of the contacts are actually beneath 

the overhanging cliffs of the outward-bulging, higher 

parts of the laccolith. Flat basal contacts are seen 

where canyons cut deeply into some laccolith margins, such as occurs half a mile northeast of Tejano 

Spring. A gently inward-dipping base of the east 

end of the laccolith at George Wise Spring is also 

suggested at the strong reentrants at both ends of 

the narrows of Hangmans Canyon. 

Some indirect evidence also supports the interpretation· that the intrusives are virtually fiat floored. 

Many small slices of the gravel and silt member of 

the Grosvenor Hills Volcanics lie along the margins 

of the laccoliths, but rocks of pre-Grosvenor Hills 

age are not found along the margins. If the dominant movement direction along the intrusive margins 

was upward, then slices of the older rocks would 

be expected, along with those of the silt and gravel 

member. However, if the dominant movement of 

intrusion was horizontal, such movement not only 

FIGURE 19.- Northeast margin of the laccolith northeast of Tejano Spring. The radially jointed margin makes a cliff about 

100 feet high, as shown in the center of the photograph, and overhangs the contact with the tuff of the rhyolite member 

of the Grosvenor Hills Volcanics, largely covered by the talus to the lower right. 
38 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

would have been favored by the thick sheets of unconsolidated silt and rhyolite tuff but also might be 

expected to have pushed aside the weak rocks without dragging any of the underlying indurated rocks. 

Such horizontal movement implies flat floors. 

Evidence indicating that the laccoliths had flat 

tops is also available. The peripheral sheeting of 

many laccoliths flattens upward and inward from 

vertical margins toward the roofs of the laccoliths, 

which suggests that the margins also flatten upward. 

A flat-lying cap of dark-gray rhyodacite vitrophyre 

directly overlies part of the laccolith northeast of 

Tejano Spring, around locality A in figure 13. This 

vitrophyre resembles the narrow zones of chilled 

border rocks of lava flows and of the rhyodacite 

dikes. Presumably it is the chilled border at the 

top of the laccolith, which thus shows that the upper 

margin of this laccolith was flat lying and which 

further gives a basis for estimating a 900-foot thickness of the doorknob-shaped laccolith. 

The inference that these rhyodacite hodies were 

intruded into the volcanics rather than extruded, 

as domes, onto them is based in part on the interpretation of the caprock on the body northeast of Tejano 

Spring as a chilled border phase. Their intrusive 

origin is also based in part on the absence of a 

breccia border, on the absence of detritus from the 

rhyodacite bodies in the adjacent tuff, and on their 

shape. Extrusive domes, such as the rhyolitic domes 

at Mono Craters, Calif., described by Putnam 

(1938), are sheathed in rubble (fig. 20B), a feature 

which is lacking around the laccoliths. Under 

normal circumstances of surface weathering and 

deposition, some detritus derived from a topographically high extrusive dome would be expected 

to be incorporated into surrounding onlapping tuff, 

which is not the case in the laccoliths. Putnam 

described the shape of the extrusive domes as 

subrounded on their tops but abruptly angular 

toward their bases, whereas the laccolithic bodies 

are rounded toward both their tops and bases. 

Furthermore, the sheeting and radial fractures of 

the rhyodacite bodies seem more likely to have 

formed in a somewhat slower cooling intrusive body 

than in a very rapidly cooling extrusive one. Finally, 

the overall regularity of the ellipsoidal bodies suggests, but more tenuously than the preceding evidence, that they were confined during emplacement, 

and their moderately flat shape also suggests that 

they were emplaced beneath a cover. 

Some structures at the junctures of several of the 

laccoliths also give the impression that the laccoliths 

swelled outward rather than that they were vertically emplaced cylindrical intrusives. Three of the four 

B 

c 

FIGURE 20. -Two kinds of laccoliths and an extrusive dome. 

A, Classical laccolith of Henry Mountains, Utah, modified 

from Hunt, Averitt, and Miller (1953); a, radially oriented 

joints along chilled margins. B , Extrusive dome of Mono 

Craters, Calif., modified from Putnam (1938); b, rubblesheathed margin; c, surface at time of emplacement. C, 

Bulbous laccolith of report area. 

northern laccoliths near Tejano Spring are separated 

from each other by narrow septa of volcanics, mainly 

tuff of the rhyolite member of the Grosvenor Hills 

Volcanics. A mile south of the spring, two of the 

laccoliths actually abut, the intervening tuff septum 

having wedged out completely over a distance of 

about 700 feet. The peripheral sheeting in the laccoliths parallels the margins of the laccoliths against 

the septa and continues adjacent to the contact between the two juxtaposed laccoliths. Clearly the 
OLIGOCENE ROCKS, IGNEOUS COMPLEX, SAN CAYETANO MOUNTAINS 39 

magma did not flow laterally between the laccoliths. 

Furthermore, no evidence suggests that one of these 

laccoliths cuts the other; the intrusive masses seem 

merely to flatten slightly against each other. The 

preservation between the intrusives of these septa 

and especially of the thin wedges of weak tuff seems 

incompatible with a field of vertically moving magma 

cylin_ders emplaced near the surface. Their preservation appears to be more reasonable, on the other 

hand, in a field of horizontally swelling magma 

blisters. 

The intrusive rocks are petrographically much like 

the rocks of the rhyodacite member of the Grosvenor 

Hills Volcanics, particularly the rhyodacite flows 

north of the upper reaches of Cinigita Canyon. The 

groundmass of the intrusive rocks is vitric or cryptocrystalline, and it surrounds abundant phenocrysts 

of andesine and sparse phenocrysts or xenocrysts of 

quartz, hornblende, biotite, augite, and hypersthene. 

The hornblende of most intrusive rocks is bright 

green rather than olive brown as in the volcanics. 

Non pleochroic orthopyroxene, possibly enstatite, is 

sparse, and oxybiotite and oxyhornblende are also 

fairly sparse. Most specimens from the interiors of 

laccoliths contain phenocrysts of andesine, hornblende, and biotite (fig. 21), but two specimens from 

the vitric cap of the laccolith at Tejano Spring 

contain andesine, augite, and hypersthene. One of 

the Tejano Spring specimens is from the chilled 

margin of the narrow dike three-fourths of a mile 

east-southeast of the juxtaposed laccoliths, and the 

other is from the 15-inch-wide chilled margin of the 

feeder dike to the southeastern laccolith. 

The change from hornblende and biotite in the 

cores to augite and hypersthene in chilled margins is 

apparently due to subtle differences in the composition of the magma during emplacement. Such 

changes in magma composition are also indicated by 

the reaction rims around some ferromagnesian minerals of the rhyodacite member of the Grosvenor 

Hills Volcanics, by the reverse zoning of some plagioclase, and perhaps even by the sporadic association 

of quartz and hypersthene and the frequent occurrence of xenocrysts in general. A compound xenocryst of biotite and hornblende, rimmed by augite 

and plagioclase, is illustrated in figure 22. 

Modal analyses of eight specimens of rhyodacite 

from the laccoliths and related dikes are shown in 

table 12. Five of the eight specimens have been 

chemically analyzed (table 14), and their collection 

localities on the Mount Wrightson quadrangle map 

(Drewes, 1971c) are as follows: Specimen 5 is a 

hornblende biotite rhyodacite from near the base 

of the laccolith in the NW1,4SE1,4 sec. 18, T. 22 S., 

FIGURE 21.- Rhyodacite vitrophyre from the southeastern 

laccolith intruding the Grosvenor Hills Volcanics. The 

specimen, 63D315, from locality shown in figure 13, 

contains: B, biotite; H, hornblende; M, magnetite; P, 

plagioclase. 

R. 15 E. (unsurveyed). Specimen 6 is also a hornblende biotite rhyodacite, collected about 75 feet 

from the edge of a laccolith in the NE1,4NE1,4 sec. 

20, T. 21 S., R. 14 E. Specimen 7 is a hornblende 

biotite rhyodacite from near the core of a laccolith 

in the SE1,4NE1,4 sec. 28, T. 21 S., R. 14 E. Specimen 8 is a hypersthene augite rhyodacite vitrophyre 

from the probable chilled cap of the same laccolith 

as specimen 7 at the NW1,4NW1,4 sec. 27, T. 21 S., R. 

14 E. Specimen 9 is a quartz-bearing hypersthene 

augite rhyodacite vitrophyre from the chilled margin 

of the thick dike ( ?) in the SE1,4SW1,4 sec. 32, T. 

21 S., R. 14 E. 

DIKES AND STOCK OF THE SAN CAYETANO MOUNTAINS 

A swarm of rhyodacite porphyry dikes and a 

granodiorite stock cut the rocks of the San Cayetano 

Mountains (fig. 13). The host rocks are chiefly 

Josephine Canyon Diorite and gently southwarddipping volcanic and sedimentary rocks of the Salero 

Formation, both of Late Cretaceous age. Other dikes 
40 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

and a plug, of porphyritic rhyolite, at the south end 

of the mountains were mapped by F. S. Simons 

(written commun., 1966). The rocks are unconformFIGURE 22. - Compound xenocryst of biotite and hornblende from a rhyodacite vitrophyre dike cutting the 

Grosvenor Hills Volcanics. The specimen, 63D518, from 

the intrusive three-fourths of a mile north of Sheuy 

Well, contains: au, augite; bx, biotite xenocryst; G, 

glass: hx, hornblende xenocryst; hy, hypersthene; M, 

magnetite; P, plagioclase. 

ably overlain by gravel of Pliocene and Pleistocene 

age. The swarm is made up of more than 60 nearly 

vertical dikes; most of these dikes strike N. 60°-

700 W., but a few to the north strike N. 60°-70° E. 

The dikes of the swarm are clustered at intervals 

of about a mile. Each cluster commonly has one or 

two larger dikes, at least 50 feet wide and a mile 

long, and many narrower, shorter dikes. 

Seen at a distance, the dikes form very light 

gray bands along which outcrops are scarce. However, low on the west flank of the range, near the 

granodiorite stock, the dikes form low ribs and 

alined low knobs. Locally on ridge crests and along 

gullies the dikes form bold outcrops. 

The dike rock generally contains abundant phenocrysts, as much as 5 mm long, of feldspar, quartz, 

biotite, and amphibole, but near the stock, phenocrysts are scarcer. The phenocrysts are scattered in 

a groundmass whose grain size is commonly 0.02-

0.1 mm and is rarely as much as 0.5 mm. Much of 

the groundmass consists of finely interlocking quartz 

and feldspar, and in some specimens the gro~ndmass 

has a crudely granophyric texture that is considerably obscured by intensive kaolinization. 

Modal analyses of two rhyodacite porphyry dikes 

are shown in table 13. Specimen 10 is from a dike 

in the SW1;4SvV1;4 sec. 13, T. 22 S., R. 13 E. (unsurveyed). Its chemical analysis is given in table 

14. The groundmass of both dikes contains a large 

proportion of orthoclase, and that of specimen 10 

contains about 7.5 percent quartz. 

Granodiorite underlies a semicircular area low on 

the west flank of the San Cayetano Mountains, 

where its west edge is buried beneath gravel. The 

covered part of the stock is presumed to be no larger 

than the exposed part, so the stock is a nearly circular body about a mile in diameter. The contact 

TABLE 12. -Modes (in percent) of rhyodacite intrusives into the Grosvenor Hills Volcanics 

Intrusive body ............................................. Laccoliths Dikes 

Specimen No.1 ....... ....•. .. ................ ... .......... . 5 6 7 8 9 19 20 21 Average 

Field No. (63D-) ....................................... 315 498 385 386 518 420 422 410a 

Quartz ........................................... 0 0 0 0 Trace 4.4 0.6 1.0 0.8 

Plagioclase .................................... 38.8 26.0 23.0 28.4 35.5 18.8 25.2 23.8 27.4 

Biotite ............................................ 2.2 .8 Trace 0 .1 2.9 .5 1.5 1.1 

Hornblende ____ ______ __ __ ___________ _____ _____ 3.4 10.1 9.1 0 .3 10.2 4.3 1.4 4.9 Augite _______ ____________ ____ ____ ______ ___ __ ___ __ 0 0 0 2.4 4.2 0 0 1.5 1.0 

Hypersthene ................................. 0 0 0 3.1 0 0 0 3.9 } .6 Enstatite ....................................... 0 0 0 ·o 1.9 0 0 0 r ;:n~!~ ~~- --_-_-_ _-_-_- _- -_-_-_ _-_ -_-_-_-_-_-_-: :_-_-_-_-_-_-_-_-_-_-_-_-_- 1.5 1.3 .7 .5 1.6 1.6 .8 .5 1.1 

.2 .2 Trace Trace .1 .3 .1 .1 .2 

Zircon ............................................. .1 .2 0 0 0 .2 .2 .1 .1 

Groundmass __ __ __ ____ _____ _____ ____ ________ __ 53.9 61.4 67.3 65.7 56.3 61.6 68.3 66.3 62.6 

TotaL ................................. 100.1 100.0 100.1 100.1 100.0 100.0 100.0 100.1 

Anorthite content of 

plagioclase: Phenocrysts __ _____ __ __________ ___ ________ 40-44 35-40 36-43 33-44 31-42 32-35 35-46 45-53 

Xenocrysts ................................ 32-33 

lSpecimens 5-9 are keyed to table 14. 
OLIGOCENE ROCKS, IGNEOUS COMPLEX 41 

TABLE 13. -Modes (in per cent) of some dikes and a stock 

from the San Cayetano Mountains 

Rock t yp e and Rhyodacite porphyry, intrusive body........................ dikes 

Specimen N o.

1............ .. ... ... .... .... 10 22 

Field No. (64D-)...................... 564 558 

Quartz.......................................... 8.9 6.1 

Plag ioclase................................... 35.7 36.4 

Orthoclase.................................... 0 0 

Biotite........................................... 2.2 3.4 Hornblende.................................. 2.2 5.2 

Magnetite..................................... . 7 .8 

Apatite......................................... .2 .1 

Zircon........................................... .1 Trace 

Sphene.......................................... .2 Trace 

Granodiorite, stock 

11 23 570 571 

23.2 22.4 

50.6 47.7 

15.4 20.7 5.2 5.3 4.5 3.2 .8 .4 

.2 .2 Trace Trace 

.1 .1 

48.0 

100.0 100.0 100.0 

Groundmass ................................. --,-._:..:49..:..:. ~::-'-:------,:-::-::--::---·---::-::-::-:--

TotaL................................ 100.0 ___________ _ 

lSpecimen 10 is keyed to table 14. 

of the granodiorite with its host, the Josephine 

Canyon Diorite, is mostly covered, and in the few 

gullies where the contact is exposed, it is steep and 

without apophyses, chilled margins, or contact metamorphic features. The granodiorite, however, is a 

massive moderately coarse grained light-gray rock 

that, because it is distinctly less chloritized and 

epidotized than the diorite, is presumed to be 

younger. 

The stock cuts some of the rhyodacite porphyry 

dikes of the large swarm ,and is cut by others. 

These relations suggest that dikes and stock were 

emplaced penecontemporaneously. 

The microscopic features of the granodiorite are 

typical of granitoid stocks. The rock has a hypidiomorphic-granular texture and a 4-mm grain size. 

Of the two specimens whose modes are shown in 

table 13, No. 11 was collected about 1,000 feet from 

the margin of the stock, and No. 23 about 50 feet 

from the margin; both are from the SE1;4SW1)t 

sec. 2, T. 22 S., R. 15 E. (unsurveyed). A chemical 

analysis of a typical specimen is given in table 14. 

Anhedral quartz and orthoclase grains are interstitial to subhedral plagioclase. The orthoclase contains a little lace perthite. The anorthite content of 

the plagioclase is 32-35 percent. Biotite forms subhedral grains that are only very slightly altered to 

chlorite and leucoxene. Amphibole grains, pleochroic in shades of pale olive green, are subhedral 

to euhedral. The magnetite is titaniferous. Traces 

of sericite, epidote, kaolinite, and iron oxides are 

also present. 

CHEMISTRY OF THE IGNEOUS ROCKS 

Chemically, the several groups of igneous rocks 

of Oligocene age of the Grosvenor Hills area have 

much in common with each other, and some of them 

are much like those of the San Cayetano Mountains; 

but in detail there are some differences among them 

(table 14). The chemical similarity is greatest between the volcanic rocks of the rhyodacite member 

of the Grosvenor Hills Volcanics and the intrusive 

rhyodacites in the Grosvenor Hills area, as illustrated by comparison of the analyses of specimens 3 and 

4 with those of 5-9. The combined average of these 

seven analyzed rocks is nearly identical with Nockolds' (1954, p. 1014) average rhyodacite. Likewise, 

the analyses of a representative dike and a stock of 

the San Cayetano Mountains, specimens 10 and 11, 

are much alike, and these rocks closely resemble 

Nockolds' (1954, p. 1014) average hornblende-biotite granodiorite. 

The average of the analyses of the vi trophyric 

rocks in the Grosvenor Hills area (specimens 3-9) 

differs slightly from the analysis of the nonvitric 

dikes of the San Cayetano Mountains (specimen 

10). The vitrophyres contain less Si02 and NazO, 

for example, and a little more Fe20 3, MgO, and CaO. 

Chemically, the San Cayetano dikes are intermediate 

between the rhyolites and the rhyodacite volcanics, 

with a greater similarity to the rhyodacite volcanics. 

An assumed mixture of one part of rhyolite to four 

parts of rhyodacite volcanics would be chemically 

closest to the composition of the granodiorite of the 

stock. These chemical variations may be related to 

the origin of the rocks, described in a following 

section. 

The abundance of 17 trace elements (table 14) 

also shows a strong similarity between the rhyodacite vitrophyres and the intrusives of the San Cayetano Mountains. In general, the sulfophile and siderophile elements are less abundant in the rhyolites 

than in the other rocks. The high copper content 

in the dike from the San Cayetano Mountains with 

respect to the otherwise very similar analysis of the 

granodiorite (compare specimens 10 and 11) is the 

main apparent anomaly. 

Specimen 2 of table 14 was collected from a site 

north of the area of figure 13, in the SWl)tNEl)t 

sec. 12, T. 21 S., R. 13 E. All other collection localities are shown in figure 13. 

FAUNA AND AGE 

A few of the sedimentary rocks intercalated in 

the Grosvenor Hills Volcanics are fossiliferous. 

Damon and Miller (1963) described remnants of 

wood, possibly of Pinus and apparently in place, in 

the tuff of the rhyolite member. The wood was 

buried by, and was partly incorporated into, a thin 

sheet of agglomerate at the base of a welded tuff 

of the rhyodacite member east of the lower end of 

Fresno Canyon (locality B, fig. 13). They indicated 

that the plant material is insufficiently diagnostic to 

date the rock, but they judged its ageto be Miocene. 

A small lens of olive-gray poorly indurated shale 

that is part of the gravel and silt member exposed 

in a fault block at localty C (fig. 13), east of the 
TABLE 14.- Chernical and spectrographic analyses and CIPW norrns, in percent, of the Grosvenor Hills Volcanics and related intrusive rocks in the Grosvenor 

Hills area corn pared with N ockolds' average analyses 

[Modes of the specimens and the locations of collection sites are given in the text sections on the respective units] 

Grosvenor Hills Volcanics Dikes and laccoliths in the Grosvenor Hills area Dikes and stock Average of the San Average Rhyolite member Rhyodacite member rhyodacite Average Cayetano Mountains hornblendeBase of Margin of Core of Cap of member rhyodacite biotite Average Dike and Flow Flow rhyolite Flow Block in laccolith laccolith laccolith laccolith dikes and of Nockolds granodiorite breccia (specimens agglomerate laccoliths (1954, Dike Stock of Nockolds p. 1014) (1954, 1, 2) (specimens p. 1014) Specimen No ......................... 1 2 3 4 5 6 7 8 9 3-9) 10 11 Field No .................................. 63D443 64D624 63D382 63D425 63D315 63D498 63D385 63D386 63D518 64D564 64D570 

Chemical analyses1 

[P. L. D. Elmore, S. D. Botts, Gillison Chloe, Lowell Artis, J. L. Harris, and H. Smith, analysts] 

Si02 .......................................... 76.5 74.2 75.4 63.6 64.1 66.8 66.7 65.5 68.1 65.4 65.7 66.27 68.4 69.5 65.50 

AbOa ........................................ 11.9 13.2 12.6 16.3 15.0 15.9 15.7 16.2 16.0 15.8 15.8 15.39 15.5 15.2 15.65 

Fe20a ........................................ .76 .75 .76 2.0 3.6 2.5 2.4 2.6 1.3 1.7 2.3 2.14 1.4 1.0 1.63 

FeO .......................................... .08 .18 .13 2.2 .72 1.0 .68 .60 1.5 2.0 1.2 2.23 1.2 1.1 2.79 MgO ......................................... .1 .3 .2 2.5 2.6 1.5 1.8 2.1 1.2 1.8 1.9 1.57 1.2 1.4 1.86 

CaO .......................................... .47 .95 .71 4.0 3.8 3.4 3.2 3.4 2.9 3.4 3.4 3.68 2.9 3.2 4.10 Na20 ........................................ 3.0 3.3 3.2 3.9 4.1 4.0 4.2 4.4 3.9 4.0 4.1 4.13 4.5 4.4 3.84 K20 .......................................... 4.4 3.0 3.7 2.7 2.8 2.9 2.9 2.3 3.0 2.4 2.7 3.01 2.8 2.8 3.01 H20 - ...................................... .34 .68 .5 .25 1.2 .66 .88 .29 .31 .47 .6 .20 .27 

H20+-------······························ .60 1.2 .9 1.5 1.0 .74 .72 1.8 1.6 1.7 1.3 .68 1.2 .58 .69 Ti02 .......................................... 1.8 1.8 1.8 .68 .61 .54 .52 .50 .47 .55 .6 .66 .58 .31 .61 P20s .......................................... .02 .18 .1 .22 .16 .16 .19 .22 .26 .13 .2 .17 .14 .14 .23 MnO ......................................... .02 .04 .03 .10 .05 .03 .07 .02 .05 .05 .05 .07 .05 .04 .09 C02 ........................................... .05 .05 .05 .09 .05 .05 .08 .05 <.05 .05 .06 ····---- <.05 <.05 -------- TotaL ........................... 100 100 100 100 100 100 100 100 100 99 -------- 100 100 -------- Spectrographic analyses2 

[A. L. Sutton, Jr., J. C. Hamilton, Barbara Tobin, and W. B. Crandell, analysts] 

B ............................................... 0.003 0.003 0.003 0.002 0 0 0 0.0015 0.003 0.003 0.0015 -------- 0 0 -------- Ba ............................................. .05 .05 .05 .1 .1 .07 .1 .1 .1 .1 .1 -------- .1 .1 -------- Be ............................................. .0001 .0001 .0001 .0003 .0002 0 .0002 .0002 .0002 0 .0002 ········ 0 .0002 ........ Co ............................................. 0 0 0 .002 .005 .0007 .001 .001 .001 .001 .002 -------- .0005 .001 -------- Cr ............................................. 0 .0003 .00015 .01 .015 .005 .01 .01 .005 .007 .01 ........ .002 .007 ........ Cu ............................................. .0003 .0003 .0003 .002 .005 .005 .002 .002 .0015 .003 .003 ........ .01 .0005 -------- Ga ............................................. .0015 .0015 .0015 .003 .005 .002 .002 .002 .002 .005 .003 ·-------- .0015 .002 -------- La ............................................. 0 0 0 .005 .003 0 .002 .003 .002 .003 .003 ------·· .003 .003 ----···· Mo ..................................... : ...... 0 0 0 .0015 0 0 0 0 0 0 .0002 ·····-·· 0 0 -------- NL ........................................... 0 0 0 .007 .01 .003 .007 .007 .003 .003 .005 .003 .005 ·······- Ph ............................................. .0015 .002 .002 .0015 .002 .0015 .002 .0015 .002 .0015 .0015 -------- .002 .0015 ----···· Sc .............................................. 0 0 0 .0015 .0015 .001 .0015 .001 .001 .001 .001 -------- .0007 .001 ........ Sr .............................................. .007 .015 .01 .07 .15 .07 .07 .07 .07 .1 .1 .05 .07 ........ V ............................................... 0 0 0 .01 .01 .01 .01 .01 .01 .007 .01 .005 .007 --·-···· Y ............................................... .001 .001 .001 .002 .0015 .0015 .0015 .0015 .0015 .0015 .0015 ·····--- .001 .002 -------- Yb .......... ................................ . .0001 .0001 .0001 .0003 .00015 .0002 .00015 .0002 .0002 .00015 .0002 -------- .0001 .00015 ·······- Zr ............................................. .007 .007 .007 .015 .007 .007 .001 .01 .01 .01 .01 -------- .015 .01 --------

CIPW norms 

Q ............................................... 41.129 41.566 18.514 18.758 23.388 22.410 21.471 26.233 22.708 -------- 20.8 23.927 24.645 20.0 C ............................................... 1.399 3.231 ........ .429 0 .387 .478 .938 1.691 .756 0 .245 0 0 or .............................................. 25.995 17.724 ········ 15.952 16.543 17.133 17.133 13.589 17.724 14.179 -----·-- 17.8 16.526 16.543 17.8 ab .............................................. 25.372 27.909 32.983 34.674 33.829 35.520 37.212 32.983 33.829 35.1 38.032 37.212 32.5 -----··· ·------- an ............................................. 2.200 3.536 -------- 17.833 14.264 15.818 14.125 15.110 12.685 16.014 -------- 14.5 13.141 13.464 16.4 

r ······································ 0 0 ........ 0 1.477 0 0 0 0 0 1.3 0 .623 .9 di en ....................................... 0 0 0 1.277 0 0 0 0 0 ······-- (3) 0 .462 (4) 

fs ........................................ 0 0 0 0 0 0 0 0 0 0 .100 ----·--- --------

hy ~~::::::::::::: :::: :::::::::::::::: ::::: .249 .747 ·---·-·· 6.224 5.196 3.734 4.481 5.228 2.987 4.481 ·-·----- 2.9.85 3.023 ·····-·· 0 0 ------·· 1.450 0 0 0 0 .997 1.453 -------· .182 .656 mt ............................................. 0 0 2.900 .716 1.756 .913 .550 1.885 2.465 3.0 2.027 1.450 2.3 hm .......... : ................................. .760 .750 -------- 0 3.106 1.289 1.770 2.221 0 0 0 0 0 0 il ............................................... .212 .466 ······-- 1.291 1.159 1.026 .988 .950 .893 1.045 ··-··--- 1.4 1.100 .589 1.2 1.689 1.555 ----···· 0 0 0 0 0 0 0 ---·---· 0 0 0 0 .047 .426 ----···· .521 .379 .379 .450 .521 .616 • . 308 -------- .3 .331 .332 .6 0 0 .205 0 0 .182 .114 0 0 -------- 0 .113 0 0 

Total. ........................... 99.05 97.91 ·-······ 98.30 97.55 98.74 98.45 97.90 98.69 97.24 ----···- 98.61 99.10 

1Rapid rock analyses. 2

Semiquantitative spectrographic analyses. Elements looked for but not detected: Ag, As, Au, Bi, Cd, Ce, Ge, Hf, Hg, In, Li, Nb, Pd, Pt, Re, Sb, Sn, Ta, Te, Th, Tl, U, W, and Zn. Results are reported to the nearest number in the series 1, 0.7, 0.5, 0.3, 0.2, 0.15, 0.1, and so on, which represent approximate midpoints of group data on a geometric scale. The assigned group for semiquantita- tive results will include the quantitative value about 30 percent of the time. 3Reported total en = 3.9 and total fs = 1.3. 4Reported total en = 4.6 and total fs = 2.9. 

~ 

1:'\:) 

C1 

M 

z 

0 

N 

0 

>-1 

C1 

~ 

0 

C1 

~ 

w. 

0 

"%j 

~ 

::r: 

tr:l 

w. 

> z 

~ 

> 

~ >-1 

~ 

> 

~ 

0 

d 

z 

~ 

> >-1 

z 

w. 

~ 

> 

~ >-1 

N 

0 

z 

> 
OLIGOCENE ROCKS, IGNEOUS COMPLEX 43 

north end of the San Cayetano Mountains, contains plant fragments. E. B. Leopold (written commun., 1964) has identified pollen of Pinus from this 

shale and suggests a Late Cretaceous or Cenozoic 

age for these long-ranging fossils. 

Undiagnostic plant ( ?) remains also occur in the 

limestone beds intercalated in a lens of the gravel 

and silt member a mile south of Cinigita Tank. 

Radiometric ages provide much more definite data 

on the age of the igneous rocks of and intruding the 

formation than do the fossils from sediments intercalated in the formation. Altogether, five ages were 

determined from four samples of the igneous rocks. 

Potassium-argon methods on hornblende, biotite, or 

plagioclase were used on each sample, and the Phalpha method was also used on one of them. Sample 

localities are shown in figure 13. The four radiometric ages obtained using the K-Ar method are 

virtually identical at 27 m.y., or late ( ?) Oligocene. 

The date obtained by the Pb-alpha method is 40 

m.y., a typical Pb-alpha age for rocks having a 

25- to 30-m.y. K-Ar age (R. F. Marvin, oral commun., 1970). 

STRUCTURE 

The genesis of the igneous complex of the southern Santa Rita Mountains is closely associated with 

the structural development of the area. The structural development therefore is summarized in the following paragraphs, although it is described in more 

detail in a companion report (Drewes, 1972a). 

Figure 23 illustrates some pertinent structural features of the Grosvenor Hills and the San Cayetano 

Mountains in vertical section, although that section 

does not intersect the fault at the east margin of 

the volcanic field. 

The Grosvenor Hills-San Cayetano Mountains 

area consists of two main structural blocks separated 

from each other by the large north-trending San 

Cayetano normal fault. The Grosvenor Hills block 

forms a broad grabenlike area between this large 

fault along the San Cayetano Mountains to the 

west and a s1naller north-trending fault, the Hang7 

mans fault, to the east. Movement on the large fault 

has been recurrent and diverse, but the volcanics 

appear simply to have dropped as much as 500 feet 

on the fault during or after the late Oligocene 

volcanism. In general the volcanics of the Grosvenor 

Hills block dip regularly and very gently southwestward, but in the western part of the block slightly 

steeper westward dips are more common, and near 

the San Cayetano fault along the west margin of 

the block the rocks dip steeply eastward. The Grosvenor Hills block has dropped an estimated 1,000-

2,500 feet along the southern part of the San Cayetano fault, adjacent to the main part of the San 

Cayetano block. This movement is the cumulative 

result of faulting during or shortly after late Oligocene volcanism and of faulting after the deposition 

of the gravel of Nogales upon the upper ( ?) Oligocene volcanics. Additional, prevolcanism displacements are also inferred along the San Cayetano 

fault (Drewes, 1972a). The amount and direction 

of displacement along the northern part of the San 

Cayetano fault, beyond the area of figure 13, differ 

from those along the segment in the San Cayetano 

Mountains and are of little consequence in the genesis of the igneous complex. 

Many lesser faults cut the volcanics in the Grosvenor Hills block. The largest set of faults trends 

N. 70° W., parallel to the orientation of the intrusives, and another strongly developed set trends 

northward. In those places shown in figure 13 where 

small faults are especially abundant, local marker 

beds are also abundant, which permitted recognition 

of the faults. Similar faults probably remain undetected near the intrusives where such beds are 

lacking. The small faults among the cluster of laccoliths tr.end randomly, separating numerous small 

triangular structural blocks; many of the faults 

merge with the margins of the laccoliths. A few of 

the faults, such as those in the southwest corner of 

the northern laccolith, however, may be truncated 

by the laccolith. This group of faults appears to 

have formed by collapse as the volcanics settled 

between the growing laccoliths; a few of them may 

have formed adjacent to laccoliths during an early 

stage of laccolith emplacement, only to be truncated 

by the laccoliths during their subsequent expansion. 

A graben between the Sheuy and George Wise 

faults trends almost entirely across the Grosvenor 

Hills block. The graben is tilted gently northwestward along its axis and is tilted more strongly northeastward across its axis, as recorded by the attitudes 

of alternating welded and unwelded rhyodacite tuff 

layers which are restricted to the graben and which 

diminish in number and thickness southeastward. 

The wedging out of individual sheets of welded tuff 

across the graben suggests that some fault movement and tilting may have been contemporaneous 

with the outpouring of the hot tuff. A breccia body 

at the northeast margin of the graben, along the 

central part of the Sheuy fault (fig. 13), consists of 

unsheared and loosely packed rubble that resembles 

talus rather than a fault breccia. Welded tuff was 

deposited against, or upon, this talus, which suggests that some movement on the Sheuy fault also 

preceded the outpouring of the hot tuff. Indeed, the 

restriction of five sheets of welded tuff to the graben 
44 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

and the southeastward decrease in the number of 

welded tuff sheets suggest that the sheets flowed 

southeastward along a valley formed along the 

graben during the time of, and perhaps as a result 

of, active volcanism. 

The faults bounding the graben are also penecontemporaneous with the emplacement of the laccoliths, for the Sheuy fault cuts some intrusives, 

whereas the George Wise fault is intruded by a 

feeder dike of the laccolith at George Wise Spring. 

Conceivably an eastward shift of magma, out of 

line with the arcuate zone in which the other in7000' 

6000' 

5000' 

4000' 

3000' 

2000' 

SAN CAYETANO BLOCK 

H M 

Rhyodacite vitrophyre 

laccolith and dike 

1-

_j 

:J 

<( 

l.L 

GROSVENOR HILLS BLOCK 

EXPLANATION 

D 

Gravel 

UNCONFORMITY 

Rhyodacite ~ porphyry dike 

and granodiorite plug 

UNCONFORMITY 

D 

Josephine Canyon Diorite 

Sedimentary ~ and 

volcanic rock 

g 

Welded tuff 

UNCONFORMITY 

(/) 

::::> 

0 

LLJ 

u 

<:( 

1-

I.JJ 

0:: 

u 

u 

Squaw ~ Gulch Granite }~ ~ 

Contact 

Dashed where approximately 

located; dotted where projected 

Fault 

Dashed where approximately 

located; dotted where p¥ojected. A rows indicate direction of relative displacement 

~ Flows Agglomerate ~ 

Rhyodacite member 

Rhyolite 

F-:l 

~ 

member 

D 

Gravel and silt member 

Cl>-

Zc:: 

:~ c::c:: 

c(i.JJ 

-lI-e( 

0::::::> 

~0 

>-

0:: 

<:( 

1-

c:: 

LLJ 

1-

NORTHEAST 

7000' 

6000' 

5000' 

4000' 

3000' 

2000' 

FIGURE 23. - Schematic structure section west and northeast of Sheuy Well, Grosvenor Hills and San Cayetano Mountains. 
OLIGOCENE ROCKS, IGNEOUS COMPLEX 45 

trusives lie and into the laccolith at George Wise 

Spring, occurred together with the formation of the 

tilted graben and its welded tuffs. 

The geology of the Grosvenor Hills block thus 

suggests that the magmatic center of the Oligocene 

volcanics lay along or just beyond the present west 

edge of the volcanic pile, that volcanism was genetically associated with intrusion of the laccoliths, and 

that volcanism and intrusion were closely related 

to faulting. 

The internal structure of the San Cayetano block, 

in or near which the volcanic center of the rhyodacites is believed to lie, contrasts markedly with the 

internal structure of the Grosvenor Hills block to 

the east. Faults are virtually absent within the San 

Cayetano Mountains, and the few faults at the north 

end of the mountains seem to be younger than the 

volcanics, inasmuch as rocks as young as the gravels 

of Tertiary and Quaternary age are involved 

(Drewes, 197lc). If the volcanics once covered. the 

San Cayetano block, as seems likely, any small faults 

were probably restricted to the volcanics (Drewes, 

1972a) in the manner typical of the small faults in 

the Grosvenor Hills block. 

The orientation and spacing of the rhyodacite 

porphyry dikes in the San Cayetano Mountains resemble those of the rhyodacite vitrophyre dikes in 

the Grosvenor Hills, even to the presence of the 

northeast-trending dikes in the north end of both 

blocks. The dikes of the San Cayetano Mountains 

differ from those in the Grosvenor Hills chiefly in 

that they are narrower and more regular and are 

grouped in larger clusters. The structural similarity 

of the dikes of the two blocks suggests that the 

dikes were emplaced under similar stress conditions; 

their differences may be explained as the result of 

different depths of emplacement - the narrower but 

more continuous dikes of the San Cayetano block 

were emplaced under greater confining pressure. 

ORIGIN 

The Grosvenor Hills Volcanics, the dikes and 

laccoliths intruding the volcanics, the dikes of the 

San Cayetano Mountains, and the stock of the San 

Cayetano Mountains probably were emplaced at 

different levels of one magmatic complex. The granodiorite stock is seen as representing a cupola of a 

magma chamber from whose lower levels the hypabyssal intrusives and volcanics were derived. The 

stock was emplaced probably no less than 3,000 feet 

beneath the surface, as estimated from the minimum 

vertical displacement on the San Cayetano fault 

plus the vertical movement of the San Cayetano 

block due to eastward tilting. 

The rhyodacite porphyry dikes are low-level feeder dikes of part of the volcanic pile and were emplaced at an estimated depth of 2,000-3,000 feet. 

Their close spatial and chemical relation to the stock 

supports the idea of their association with deeper 

parts of the igneous complex. Their more general 

chemical similarity to the rhyodacite vitrophyre 

dikes and the structural similarity between the dikes 

of both blocks permit their genetic association with 

higher parts of the igneous complex. However, they 

lack direct physical continuity with the rhyodacite 

vitrophyre dikes, not only through the intervening 

San Cayetano fault, but also through a relatively 

dike-free zone adjacent to that fault. In order to 

propose a direct genetic connection between the lowlevel dikes of the San Cayetano Mountains and the 

high-level dikes in the Grosvenor Hills, it may be 

necessary to assume a large component of lateral 

movement of the magma in the dike system, for 

which there is only tenuous support through the 

relations between the dikes and laccoliths along the 

George Wise fault. Perhaps the igneous complex 

had additional magmatic centers not yet uncovered 

by erosion; if so, the need to assume lateral movement of the magma is eliminated. 

Dikes and laccoliths in the Grosvenor Hills are 

physically connected, and these intrusives and the 

rhyodacite~ volcanics are genetically related, as 

indicated by their chemical and mineralogical similarity. In addition, the structural observations summarized in the preceding section suggest that 

volcanism, faulting, and the emplacement of the 

laccolith at George Wise Spring east of the main 

zone of intrusives are genetically intertwined. 

The hypothesis that the plutonic, hypabyssal, and 

volcanic rocks are all part of one igneous complex 

gains additional support from the fact that they 

are of nearly identical age. That such similar rocks 

would form so close together at one time without 

being genetically related would be fortuitous at best. 

Chemical similarity among the plutonic, hypabyssal, and volcanic rocks supports their genetic relation. This similarity is summarized in figure 24, 

in which the chemical composition of the rock is 

plotted against the estimated average depth of 

emplacement of the four depth-controlled groups of 

rocks of the igneous complex. The forms of these 

curves remain virtually unchanged with any reasonable modification of the estimated depths. 

In the discussion of the chemical similarities of 

the four groups of rocks in figure 24, however, it was 

pointed out, first, that the resemblance between the 

granodiorite and the rhyodacite volcanics was not 

as impressive as that between the hypabyssal rocks 
46 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

1- Rhyodacite w 

w 

LL. 

vitrophyre 

volcanics 

~ Rhyodacite 

t--=" vitrophyre 

~ laccoliths 

~ and dikes 

u 

~ 

...J 

a. 

::2: 

w 

LL. 

0 

I t Rhyodacite 

~ porphyry 

Cl 

w 

1-

~ 

::2: 

dikes 

i= Granodiorite 3000 ~ stock L___L__j_____[___L___L____j___j L-L__j L_L_____j L-L__j L-L__j L-L__j L_____i_____J I I I I I I I I I I I ~ L___l_L---'---'--__l_L_.LJ 

65 7014160 51 32 43 5 2 40 20 0.5 1.00.1 0.20.04 0.08 

SiOz Alz03 2:FeO+Fez03 MgO CaO Na20 K2 0 2:Hz0 TiOz PzOo MnO 

AVERAGE COMPOSITION, IN PERCENT 

FIGURE 24.- Minor changes in chemical composition with depth of emplacement of rocks of the magmatic complex of the 

southern Santa Rita Mountains. 

and the volcanics and, second, that the composition 

of the granodiorite lies between that o~ the rhyodacite and that of the rhyolite volcanics. Apparently, 

a single-magma igneous system is too simple a 

hypothesis to cover the available facts. The mineralogical differences between the granodiorite- with 

its biotite, hornblende, and fairly sodic plagioclase 

-and the rhyodacite vitrophyre- with its pyroxenes and fairly calcic plagioclase - also present 

problems to a single-magma hypothesis. In order to 

reconcile these problems, it seems necessary to tinker 

with the parent magma before it reached the surface. 

For example, a granodiorite parent magma may be 

postulated to have formed two fractions, which 

reached the surface at slightly diff~rent times. Such 

a postulation not only would explain the chemical 

and mineralogical variations, but also would explain the presence of some slightly older granodiorite 

xenoliths in a slightly younger rhyodacite agglomerate, as well as the various reaction rims and the 

reverse zoning seen in the xenocrysts of the rhyodacite vitrophyre. 

In summary, the igneous rocks of Oligocene age of 

this area probably were emplaced during one magmatic episode. The recorded geologic events began 

with upward movement of granodiorite magma late 

during the Oligocene Epoch. For unknown reasons, 

part of the parent magma separated into a slightly 

more acidic fraction and a slightly more basic fraction. The granodiorite stock of the San Cayetano 

Mountains either is the top of the parent magma 

chamber or is a cupola on a larger mass. The lighter 

weight, more acidic, and perhaps less abundant fraction reached the surface first, to form the rhyolite 

volcanics. Being the lighter fraction, it contained 

most of the volatiles, and most of the earlier volcanics to reach the surface thus are tuffaceous. The 

rhyolite tuff spread over a pediment which supported 

a vegetation that contained pines. 

During a brief pause after these initial eruptions, 

another pine forest developed on at least some of 

the ash-inundated area, only to be overwhelmed 

by the flood of volcanics of the second magma fraction. The second fraction, of rhyodacite composition, 

contained xenocrysts of the earlier, more acid fraction. Reaction rims and reversely zoned composition 

rims were formed around some of the xenocrysts. 

The transfer of a large volume of magma to the 

surface was accompanied and followed by the gradual collapse and tilting of parts of the roof area of 

the parent magma chamber, which is perhaps roughly coextensive with the Gro~venor Hills volcanic pile. 

Secondary faulting formed parallel to the trend of 

the feeder dikes, and subsidiary areas collapsed 

between some of the secondary faults. The dikes, 

which were abundant but narrow at depths of a few 

thousand feet, form fewer but thicker bodies nearer 

to the surface, suggesting that the host rocks near 

the magma chamber were more fractured than those 

well above it. Some of the upwelling magma, upon 

passing upward from the relative confinement of 

the indurated Mesozoic rocks to the relative freedom of the unconsolidated silt and rhyolite ash, 

spread laterally into thickened dikes or incipient 

laccoliths. Other dikes continued to spread laterally 

into bulbous laccoliths, alongside and between which 

the volcanics settled to form a terrane cut by many 

shallow faults. One of the 'laccoliths may have been 
OLIGOCENE ROCKS, RHYOLITE INTRUSIVES 47 

emplaced somewhat laterally southeast of its feeder 

dikes and away from the more deeply dropped end 

of a subsidiary graben. Tuffs deposited from successive nuees ardente that flowed down the graben 

during its subsidence were welded. Following the 

magmatic episode, the San Cayetano block was 

strongly uplifted, and a deeper part of the igneous 

complex was exposed by erosion. 

RHYOLITE INTRUSIVES OF THE NORTHERN 

SANTA RITA MOUNTAINS 

Two dike swarms and a plug, largely of rhyolite 

but locally of rhyolite vitrophyre, intrude the rocks 

of the northern Santa Rita Mountains (fig. 25). The 

intrusives are typically light-gray to very pale 

orange flow-laminated rocks that contain sparse 

phenocrysts of quartz and feldspar. The dikes range 

in width from 3 to 50 feet, but most are 10 to 20 

feet wide. Most of the wider dikes form ribs which 

are especially con_spicuous along the lower part of 

the Box Canyon road. Dikes of both swarms trend 

N. 50°-60° E., dip steeply, and are of regular width. 

On the geologic quadrangle maps (Drewes, 1971a, 

c) the dikes to the northwest are referred to as the 

Box Canyon swarm, and those to the southeast, as 

the Gardner Canyon swarm (fig. 25). 

The Box Canyon dike swarm consists of about 

50 dikes which are spread over a fairly wide zone 

that extends between the Yoas Mountain area in the 

southwest and the Helvetia mining district in the 

northeast. Over much of its 15-mile length, the 

swarm lies close to the northeast-trending segment 

of the r ange front. Southwest of Madera Canyon, 

where the host consists of Cretaceous granitic rocks, 

the dikes are few and short. Northeast of the canyon and especially northeast of Sawmill Canyon, 

where the host is either Precambrian granitic rock 

or Mesozoic sedimentary and volcanic rock, the dikes 

are more numerous and longer. Small segments of 

the entire width of two of these dikes, one from 

which specimen 65D899 was collected and the other 

a dike northeast of the first one, are a dark-yellowish-brown vitrophyre with a waxy luster, unlike the 

rocks typical of these dike swarms. 

In the Sawmill Canyon-Box Canyon area the dikes 

intersect northwest-trending faults, whose last recorded movement was Paleocene, or late Laramide 

(Drewes, 1972a). Some dikes abut individual faults 

of this system, other dikes cross the faults without 

a break, and the continuity of still others is broken 

along the faults. The relation shown by the unbroken dikes that cross faults probably is the most 

critical one, inasmuch as it shows the dikes to be 

the younger feature. The other relations probably 

are the result of the development of fractures at 

different sites across the fault, rather than the result of postdike offset. 

The Gardner Canyon dike swarm contains more 

than 30 dikes, most of which are closely bunched 

between the upper reaches of Josephine Canyon to 

the southwest and Cave Creek to the northeast (fig. 

25) . The dikes are longer in the Mesozoic volcanic 

and sedimentary rocks east of Mount Wrightson 

than in the Cretaceous granitic rocks south of that 

mountain. A few short dikes near the east end of 

this swarm trend in an atypical direction, northwest; 

they are mostly dikes (or sills) lying subparallel to 

steeply inclined bedded rocks. The continuity of this 

swarm is briefly interrupted south-southeast of 

Mount Wrightson. Inasmuch as this gap falls along 

the high crest of the mountains, the dikes in this 

area may not extend above an elevation of about 

7,000 feet. 

Two dikes of rhyolite porphyry, resembling those 

of the dike swarms of the northern part of the 

mountains, crop out in the N1j2 sec. 1, T. 21 S., R. 

13 E., along lower Cottonwood Canyon (south of 

Montosa Canyon). They are considered part of the 

southeastern swarm with which they are alined 

even though they lie 5 miles west of the other dikes 

of that swarm. The two dikes cut rocks as young 

as Cretaceous ( ?) diorite and are overlapped by 

the gravel of Nogales of Miocene ( ?) and Pliocene 

age. They lie within half a mile of the nearest outcrops of the Grosvenor Hills Volcanics, but the geologic relation of the dikes to the Grosvenor Hills 

Volcanics is not known. 

The rhyolite plug forms an elliptical body near the 

east end of the southeastern dike swarm, between 

Cave Creek and Gardner Canyon. It crops out 

abundantly in low knobs of closely fractured and 

iron oxide stained rock. This staining of the rock 

along fractures is strongest toward the southwest 

margin of the plug, where fresh rhyolite probably 

contains disseminated pyrite. Two dikes flare out 

toward the margins -of the plug, and they probably 

join it rather than abut it. The host rocks of the 

plug are folded arkose and siltstone of the Upper 

Cretaceous Fort Crittel).den Formation (Drewes, 

1971b), which are mildly metamorphosed near' the 

contact only at the north end of the plug. The crest 

of an anticline, which is open to the southeast but 

which tightens to the northwest, runs roughly down 

the axis of the plug. Contacts of the plug dip outward, gently to the south but more steeply to the 

north, and thus are largely concordant to the adjacent beds. The sheeting along the margins of the 

plug also parallels the contact. These contact rela-
48 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

EXPLANATION 

Quaternary 

~ and 

Tertiary gravel 

17raa 

Tertiary rhyolite plug 

and dike (Oligocene), showing dip 

Cenozoic to Precambrian • igneous, 

sedimentary, and metamorphic rocks 

Contact 

1 +ss --·- Fault, showing dip 

Dashed where approximately 

located ; dotted where concealed. Bar and ball on downthrown side 

... 899 

Collection site of radiometrically 

dated specimen 

LJ.l 

Collection site of chemically 

analyzed specimen 

l 

If 

" 

. . . __ I ~d.~.,___· •• ~ • ..n ~\, 

\-···-<: . .. 

0 

0 

TO SAHUARITA 

QTg 

2 4 MILES 

2 3 4 Kl LOM ETERS 

FIGURE 25. -Distribution of Oligocene rhyolite intrusives of the northern Santa Rita Mountains and the relations 

of the intrusives to some faults. Base from U.S. Geological Survey: Sahuarita and Mount Wrightson quadrangles, 1: 62,500, 1958. 
OLIGOCENE ROCKS, RHYOLITE INTRUSIVES 49 

tions suggest that the top of the plug lay not far 

above the present level of exposure of the rhyolite 

mass. Inasmuch as the base of the mass is unexposed, 

the term "plug" is applied tenuously; the mass could 

equally well be the top of a laccolith or phacolith. 

PETROGRAPHY 

The rhyolite of the Oligocene intrusives of the 

northern Santa Rita Mountains commonly contains 

about 10 percent phenocrysts, of 1-4 mm maximum 

size, set in an altered cryptocrystalline groundmass. 

A mosaic of indistinctly crystalline material in 

grains 0.01-0.1 mm across, probably of devitrified 

glass, makes up the groundmass. Some specimens 

from the northwestern dikes of that segment of the 

Box Canyon swarm between Sawmill and Box Canyons have a microgranophyric groundmass. Two 

specimens from the southeastern dikes of the same 

segment of the Box Canyon swarm have a brown 

glass groundmass in which microlites are flow 

ali ned. 

The modes of eight specimens, giving essentially 

the phenocryst abundance, are shown in table 15. 

Quartz is everywhere present as euhedral grains, and 

commonly a few grains of each specimen are partly 

resorbed. Normally both plagioclase and potassium 

feldspar are present, but in some rocks only one or 

the other is present The plagioclase is mostly albitized, from an oligoclase or andesine, as indicated 

by the relicts in one specimen. Potassi urn feldspar 

is sanidine more commonly than orthoclase, but in 

some specimens it is indeterminate. The finely granophyric rocks contain some perthite. About half the 

rocks contain pseudomorphs of chlorite or of a 

hydromica after biotite, of which a very few unaltered relicts form inclusions in quartz or feldspar. 

In a few rocks amphibole, possibly hornblende, occurs along with, or in place of, the biotite. Accessory 

magnetite is commonly present, and apatite, zircon, 

and sphene occur less frequently. Among the alteration products, clay minerals are abundant, and 

sericite, chlorite, calcite, iron oxides, and epidote 

are sparse. 

TABLE 15.- Modes (in percent) of rhyolite dikes of Oligocene age from the northern Santa Rita Mountains 

Dike swarm .................................................... . Southeastern, near Gardner Canyon Northwestern, or Box Canyon 

Location in swarm ........................................ . East Middle West Middle East 

Specimen No.l 1 3 ................................................. ---=-------=--- 4 5 2 6 7 8 

Field No ........................................................... . 62Dl 63D340 62Dl9 65D745 65D899 66Dl002 66Dl029 66Dl036 

Quartz ............................................. . 9.8 2.6 3.1 2.0 1.7 0.5 9.3 14.8 

Plagioclase ..................................... . 0 3.2 12.4 1.2 0 0 13.0 .7 

Potassium feldspar ....................... . 1.7 Trace 27.3 33.7 23.2 25.6 23.9 36.1 

Biotite ....................... -----······----------·· Trace 0 .8 Trace 0 0 .1 .1 

Hornblende ( ?) ------------------------------- 0 0 1.7 0 .1 .1 0 0 

Magnetite ....................................... . Trace Trace 1.7 Trace Trace .1 Trace Trace 

Apatite ............................................ . Trace Trace .3 0 Trace 0 0 0 

Sphene ............................................. . 0 0 .1 0 Trace Trace 0 0 Zircon __________________ ______ _____________ ___ ______ _ Trace 0 0 Trace Trace 0 0 0 

Groundmass ................................... . 88.5 94.2 72.6 93.1 95.0 93.7 73.7 78.3 

Total .. ___ ....... _________________________ ---=-=-=-=------:--=--=--=-----=-=--=-=----~::-::---=--~--:-::-:=-=-----:-:::-:-::--------:::-::-:--::-- --::-::-::-::--- 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 

1Specimens 1 and 2 are keyed to table 16. 2Identified as sanidine. 

3Identified as orthoclase. 

CHEMISTRY 

Chemical analyses of two rhyolite specimens are 

listed in table 16. Specimen 1 is from a short northwest-trending rhyolite porphyry dike south of the 

plug and was collected in the SW1,4SW1,4 sec. 12, 

T. 20 S., R. 15 E. (unsurveyed). Specimen 2 is from 

the rhyolite vitrophyre at collection site 899 (fig. 

25), in the SW1,4SE1;4 sec. 22, T. 19 S., R. 15 E. 

The analyses differ considerably from each other, 

although the difference between some components 

decreases upon removing the excess water from the 

vitrophyre analysis and recalculating the oxides to 

100 percent. Thus reapportioned, the silica content 

increases to about 7 4 percent, a more appropriate 

amount for a rhyolite. However, the reapportionment does not significantly increase the K20 content 

of the vitrophyre, nor, of course, does it change the 

ratio between CaO and the alkalis, whose abundances 

are not typical of rhyolites nor even of rhyodacites 

(Nockolds, 1954, p. 1013-1014). 

The difference in chemistry between a vitrophyre 

and an altered devitrified rock that may be genetically related is difficult to interpret. The devitrified 

rock contains less CaO and Na20 and more Si02 and 

K2 0 than the vitrophyre. If these rocks were 

crystallized from the same parent magma - as may 

be inferred from the similarity of their modes, 

phenocryst mineralogy, and place of occurrence -

then during or after devitrification and hydration 

the rock must have become so enriched in Si02 and 

K2 0 as to resemble a normal rhyolite. 

A further problem affecting the interpretation of 

the genetic relation of the rocks is that the few dike 

rocks having granophyric texture occur in the same 
50 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

TABLE 16.-Chemical and spectrographic analyses 

and CIPW norms, in percent, of rhyolite from the 

dike swarms in the northern Santa Rita Mountains 

Chemical analysesl 

Vitrophyre 

Box Canyon 

2 

65D899 

[P. L. D. Elmore, S.D. Botts, Lowell Artis, H. Smith, John Glenn and Gillison Chloe, analysts] ' 

Si02---------------------------------- 77.6 69.0 

AhOa................................ 11.9 12.3 

Fe20a................................ .38 .53 

FeO................................... .12 .32 

MgO.................................. .12 .11 

CaO................................... .32 2.8 

Na20................................ .96 2.8 

K20................................... 5.5 1.9 

H20+............................... .71 2.9 

H20-............................... 1.7 6.7 

Ti02---------------------------------- .05 .09 

P205---------------------------------- 0 0 MnO____________ ______________________ .02 .21 

co2-----------------------------------· __ <.....:::._·o_5 ____ <...:::.·_o_5 __ 

TotaL................... 99.3 100. 

Spectrographic analyses2 

[W. B. Crandell and J. C. Hamilton, analysts] 

Ba .................................... . Be ..................................... . Co ..................................... . Cr __________ __ ____ ________________ _____ _ 

Cu ..................................... . Ga ..................................... . 

La ..................................... . Mo .................................... . Nb .................................... . Ni. ____________________________________ _ 

Pb ..................................... . Sc ..................................... . Sr ..................................... . y _______________________________________ . 

Y ....................................... . 

Yb ............ -------------------------· Zr ..................................... . 

0.1 

0 

.002 

.0015 

.005 

.003 

.003 

.0007 

0 

.0015 

.0015 

.001 

.1 

.015 

.0015 

(3) 

.01 

CIPW Norms 

0.015 

.001 

0 

0 

.00007 

.002 

.005 

.0003 

.005 

0 

.002 

0 

0 

.2 

.003 

.0003 

.015 

Q....................................... 50.114 39.351 c _______________________________________ . 3.787 .664 

or....................................... 32.494 11.260 ab_________________ __ _______________ ____ 8.119 23.761 

an...................................... 1.587 13.614 

hy [~~~--~~-- -~~----~ -------~~~~~~~~~~~ ~~~~~ 0.299 :~~~ mt...................................... .307 .771 

hm .... --------------------------------- .168 0 

iL...................................... .095 .171 cc....................................... 0 .114 ------------- TotaL................... 96.97 90.37 

1Rapid rock analyses. 2Semiquantitative analyses. Results are reported to the nearest number in the series 1, 0.7, 0.5, 0.3, 0.2, 0.15, and 0.1, which rep- resent the approximate midpoints of group data on a geometric 

scale. The assigned group for semiquantitative results will include the quantitative value about 30 percent of the time. 3Not looked for. 

structural block as those having a glassy texture. 

These textures suggest differences in crystallization 

habit commonly presumed to be depth 3.ssociated. 

Perhaps the chemical and textural problems are 

related. 

AGE AND ORIGIN 

The dikes were intruded during late ( ?) Oligocene 

time, as determined by isotopic date-s on two dikes, 

one from each swarm (table 2). Sanidine concentrated from the vitrophyre of specimen 65D899 

(fig. 25 and table 16) gives a K-Ar age of about 

25.9±0.8 m.y. (R. F. Marvin, written commun., 

1967). Zircon concentrated from the rhyolite porphyry of specimen 63D338 gives a Ph-alpha age of 

about 40 m.y. (T. W. Stern, written commun., 1965). 

As stated before, a 40-m.y. Ph-alpha age is typically 

compatible with a 25- to 30-m.y. K-Ar age (R. F. 

Marvin, oral commun., 1970). The collection site of 

specimen 65D899 is given in the preceding section 

on chemistry, and that of 63D340 is in the SE1,4 

SW1,4 sec. 1, T. 20 S., R. 15 E.; both localities are 

shown in figure 25. 

The dike swarms and plug were intruded in the 

northern part of the Santa Rita Mountains at about 

the same time as the rhyolitic rocks of. the igneous 

complex in the southern part of the mountains were 

emplaced. Not only are their ages similar, but their 

chemistry and phenocryst mineralogy are roughly 

similar, and they were emplaced in similar structural 

environments associated with normal faults. 

The dike swarms probably intruded post-Laramide tension fractures. Rocks deformed and intruded 

during the Helvetian phase of the Laramide orogeny 

(late Paleocene) were exposed by an amount of 

erosion that implies much uplift during post-Paleocene time. At least some of this uplift occurred on 

the northeast-striking faults along the west front 

of the mountains, parallel to the dike swarms. These 

faults are normal faults, formed as a result of 

tensional stresses which postdate the last compressive stresses of the Laramide. Apparently, some 

of these tensional fractures tapped a magma body 

that lay beneath the northern part of the uplifted 

mass. 

MIOCENE(?) TO HOLOCENE ROCKS 

Several sequences of conglomerate, gravel, and 

silt overlie rocks as young as the Grosvenor Hills 

Volcanics and their associated intrusives. Some of 

these sequences form thick prisms of alluvium 

deposited in fans along the flanks of the present 

mountains, and they presumably fill the intermontane basins. Other sequences form thin sheets of 

alluvium and include relatively continuous pediment 

deposits, as well as discontinuous terrace deposits. 

A few sequences form alluvial deposits restricted to 

the valleys draining the intermontane basins. 

The Miocene ( ?) to Holocene rocks are divided 

into nine map units (Drewes, 1971a, c) which constitute six stratigraphic units that are separated 

from each other and from the underlying rocks by 

unconformities. As shown in figure 26, the six 

stratigraphic units consist, successively, of the gravel 

of Nogales, basin-fill gravel, high-level pediment 
MIOCENE ( ?) TO HOLOCENE ROCKS 51 

NW EAST SIDE SE 

··" •• ·<1 .... <1 •••• •• .j ••• 

• • • •.4 ••• 

NW WEST SIDE SE 

EXPLANATION 

Youngest gravel 

Low-level terrace deposits 

53' very weak brown sml 

s, silt facies 

g, gravel facies 

Intermediate-level 

~ 

~ pediment 

and terrace deposits 

s 2 , weak red soil 

High-level pediment deposits 

s 1, strong red soil 

>-

0:: 

<I: 

z 

0:: 

w 

1--

<t 

::::l 

0 

Basin-fill gravel 

r, river facies 

p, piedmont facies; increasi11{1 

density of pattern shows increasing coarseness of gravel 

D:·•· .. <:·.:· 

Gravel of Nogales 

Contact 

Fault 

Arrows show relative 

direction of movement 

0>-

Zo:: 

<I:: <I: 

>-Z 

0::0:: 

<tW 

-1--

1--<t 

O::::::J 

~0 

\ >- 1 0:: 

J~ 0:: 

w 

1--

FIGURE 26. - Schematic sections showing the relations among the surficial deposits on the flanks .of the Santa Rita 

Mountains. 

deposits, intermediate-level pediment and terrace 

deposits, low-level terrace deposits, and the youngest 

gravel. The three units referred to as "deposits" include soils that are generally distinguishable from 

each other by their different degrees of development. 

Commonly the soils are formed on the pediment or 

terrace gravels, which in many places are distinguished from each other mainly by their soil, for 

the gravels closely resemble each other. In the few 

places where soils appear to have formed directly 

on the basin-fill gravel, they are mapped with the 

appropriate younger deposit but are distinguished 

from that deposit by a queried symbol on the quadrangle maps. Of the six stratigraphic units, the 

basin-fill gravel and the low-level terrace deposits 

each have distinctive facies along the Santa Cruz 

River which are of sufficient extent and economic 

significance to be mapped separately. Analogous 

facies of some other units along the Santa Cruz 

River are of too limited extent to be distinguished on 

the map or may occur only in the subsurface. The 

basin-fill gravel also contains a few small lenses of 

tuff, which are mapped separately; and in the subsurface along the Santa Cruz River 3-6 miles north 

of Sahuarita, according to E. S. Davidson (written 

commun., 1969), the gravel contains abundant gypsum and anhydrite. 

Some of the lithologic distinctions of the six 

sequential units are fairly widespread; others are 

local. The older units, for instance, are generally 
52 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

redder, better indurated, less well sorted, and more 

deformed than the younger deposits in the same 

area. Considered throughout the area, the ·range of 

the lithologic characteristics of several units is large, 

and some of the characteristics overlap those of the 

adjacent units, but in each locality at least some of 

the differences persist. Therefore, in places the 

lithologic criteria used to distinguish some units 

are augmented by the geomorphic position of the 

units and by soil development. 

GRAVEL OF NOGALES 

An extensive body of tilted moderately well indurated tuffaceous gravel and sand containing abundant detritus derived from the upper ( ?) Oligocene 

volcanics crops out around Nogales (fig. 1). Rocks 

having these features extend into the Mount Wrightson quadrangle as far north as Montosa Canyon. 

They overlap several members of the Grosvenor 

Hills Volcanics near Sonoita Creek to the south 

(fig. 13) and overlap rocks as young as the dike 

swarm of late ( ?) Oligocene age near Cottonwood 

Canyon to the north. Elsewhere they are faulted 

against older rocks. The gravel of Nogales is unconformably overlain by basin-fill gravel, as is illustrated along the 120-foot-high steep south bank of 

the lower part of Cottonwood Canyon in the Nlj2 

sees. 1 and 2, T. 21 S., R. 13 E. 

The gr~vel of Nogales southwest of the Santa 

Rita Mountains consists of moderately well indurated tuffaceous sand, silt, grit and pebble gravel, and 

a little cobble and boulder gravel. A typical outcrop 

of this map unit (fig. 27), at an unknown distance 

above its base near Cottonwood Canyon, contains 50 

percent sandy grit to sandy gravel whose pebbles 

are less than an inch across, 35 percent silty to sandy 

grit, and 15 percent coarser gravel. Sorting is poor, 

and bedding is moderately well developed. The gravel 

is commonly pale red but ranges from moderate 

orange red to grayish orange pink, and it is much 

less commonly brown or light gray. 

The pebbles are overwhelmingly of rhyolite and 

rhyodacite derived from the Grosvenor Hills Volcanics and the related vitric intrusives. Only in the 

northernmost outcrops do the. clasts include some 

Paleozoic limestone, some granitic rock, and some 

altered volcanics, possibly of the Salero Formation. 

The sandy and tuffaceous component presumably is 

also derived from the pyroclastic rocks of the Oligocene volcanics. The clasts are subangular to angular; 

only those of moderately friable material are subrounded, apparently due to disaggregation as much 

FIGURE 27.- Typical gravel of Nogales cropping out north of Cottonwood Canyon on the southwest flank of the Santa Rita 

Mountains. 
MIOCENE ( ?) TO HOLOCENE ROCKS 53 

as to abrasion. Although sorting and bedding are 

generally poor, those of some sand and silt beds 

north of Josephine Canyon are good. A westward 

direction of transport is indicated by the imbrication 

direction of the pebbles in some gravel lentils in the 

unit near . Cottonwood Canyon. The clastic components are commonly indurated by compacted silt and 

clay, but locally they are also cemented by silica and 

lime. A thick basal sheet of coarse conglomerate near 

Sonoita Creek is limier than most other beds, apparently because ground-water movement and 

consequent deposition of carbonates were concentrated above the relatively impervious underlying 

volcanics. 

The gravel is generally tilted 10°-25° toward the 

Santa Rita or the San Cayetano Mountains, from 

which it is separated by faults. Where the tilted beds 

strike into the faults, southerly dips are more common than northerly ones. Many of the faults are 

truncated by the basin-fill gravel. 

The gravel of Nogales probably was deposited by 

streams on and next to volcanic hills whose relief 

was much less than that of the present Santa Rita 

Mountains and probably was even less than that of 

the Grosvenor Hills. The preponderance of angular 

and volcanic detritus indicates that the gravel was 

locally derived, and the small pebble size and the 

well-developed bedding, compared with that of the 

youngest deposits in the same area, suggest that the 

old relief was less than the present relief. The preponderance of volcanic detritus also suggests that 

the upper ( ?) Oligocene volcanics, or at least their 

tuffs, mantled much of the core of the mountains. 

The few pre-Oligocene pebbles that do occur in the 

formation appear only in the northern outcrops, 

which are farthest from the inferred source of the 

Oligocene volcanics, and presumably the source area 

of the pebbles was only thinly veneered by ash deposits. That this ash veneer was sufficiently breached 

by erosion to expose the underlying limestone and 

granite by Pliocene time is entirely reasonable, for 

since then the veneer has been largely removed. 

ThP- gravel of Nogales is estimated to be mostly 

of Pliocene age, but it also includes some rocks of 

Miocene age. Fossils have not been found, and the 

only radiometrically datable rocks (table 2, specimen 1) were obtained from an olivine basalt flow 

intercalated low in the gravel about 4 miles southwest of the southwest corner of the Mount Wrightson quadrangle (500 ft east of the SW. cor. sec. 

6, T. 23 S., R. 13 E.). A K-Ar determination on 

the groundmass of the basalt gives an age of 12.6± 

0.8 m.y. (R. F. Marvin, H. H. Mehnert, and Violet 

Merritt, written commun., 1970) and another determination on pyroxene phenocrysts gives an age 

of 13.8±13.5 m.y. (Marvin, Mehnert, and W. T. 

Henderson, written commun., 1970). Marvin stated 

that the large analytical error for the pyroxene is 

due to the presence of 15 percent radiogenic Ar40 ; 

the young age and very low potassium content of 

the pyroxene resulted in only a little argon to 

analyze. Nevertheless, the agreement between the 

two ages is good, and the ages indicate that the 

gravel that contains the basalt flow lies near the 

Miocene-Pliocene boundary. 

Gravel resembling that of Nogales occurs in several other basins near the Santa Rita Mountains. 

In the southern part of the Sierrita Mountains (fig. 

1) J. R. Cooper (oral commun., 1968) mapped a 

unit of Tinaja Peak, whose lower pa:t;t consists of 

vitric volcanics and whose upper part is a gravel 

derived from those volcanics. The volcanic gravel, 

judged from its lithologic similarity to the gravel 

of Nogales, may be correlative with part of the 

gravel of Nogales, whose nearest exposures are only 

15 miles away. South of the Mustang Mountains, 

22 miles east of the Grosvenor Hills, the basal part 

of a poorly indurated tuffaceous sandstone and finegrained angular-pebbled conglomerate sequence contains intercalated biotite-bearing tuff; mentioned 

previously in this report and dated as about 39 m.y. 

The upper part of the sequence overlying the tuff 

may be correlative with the gravel of Nogales. 

BASIN-FILL GRAVEL 

A pinkish-gray gravel of Pliocene and Pleistocene 

age, which commonly is at least several hundred 

feet thick and which may be several thousand feet 

thick in the centers of the intermontane basins, unconformably overlies the gravel of Nogales and is 

overlain by terrace and pediment deposits. The 

gravel consists of subangular clasts that are moderately well sorted and bedded. Commonly the rock is 

weakly indurated. In the Tucson basin, where intensive studies have been made, gravel of this lithology and stratigraphic position is called the basinfill gravel by E. S. Davidson, M. E. Cooley, and 

E. F. Pashley, Jr., (oral communs., 1963-68). This 

informal name is here retained for the extensive 

gravel around the Santa Rita Mountains, even 

though locally it caps a pediment and contains other 

rock types, such as the tuffaceous rocks described 

under "Tuff Beds." 

The basin-fill gravel around the Santa Rita Mountains consists of three mapped units- a widespread 

unit referred to as the piedmont facies, a relatively 

local unit called the river facies, and tuff beds 

(Drewes, 1971a, c). Despite its limited extent in the 
54 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

Santa Cruz River valley, the river facies is mapped 

separately because of its potential commercial importance. The tuff beds have attracted attention as a 

possible means of dating the adjacent gravel by 

radiometric methods. Toward the mountains the 

gravel grades into coarser detrital facies including 

colluvium, talus, and landslide blocks, all generally 

unmapped. 

PIEDMONT FACIES 

The piedmont facies of the basin-fill gravel is at 

least 500 feet thick along Davidson Canyon east of 

the northern part of the Santa Rita Mountains, and 

it is probably much thicker in the basin centers, 

especially along the Santa Cruz River. In many 

places the unit laps directly against Cretaceous or 

older rocks on a steeply dipping unconformity~ but 

along parts of the west side of the range- such as 

between Josephine and Montosa Canyons, at the 

head of Madera Canyon fan, and north of Helvetia 

-it laps across a narrow and gently westwarddipping pediment. In a few places- such as along 

the east side of the Santa Rita Mountains at Box 

Canyon, at the foot of the Patagonia Mountains 

south of Patagonia, and at the north end of the 

San Cayetano Mountains - it is faulted against 

older rocks. 

The piedmont facies consists mainly of light-gray 

to grayish-orange-pink beds of cobble, pebble, or 

boulder gravel, with intercalated beds of sand and 

silt (fig. 28). These beds are typically a few inches 

to a few feet thick and are poorly sorted to moderately well sorted. Channel-fill lenses and intraformational unconformities are abundant. The gravel is 

commonly sufficiently indurated to stand in nearly 

vertical cliffs where undercut by streams, but the 

cliffs weather readily in other circumstances. Outcroppings of gravel offer considerable resistance to 

the penetration of a geologic pick but can easily 

be disaggregated by hand. A little lime cement is 

normally present, but thoroughly cemented or caliche-rich beds are mostly restricted to deposits derived from carbonate source rocks. The clasts of 

the gravel were derived from rocks in the mountains 

near which they were deposited, although In some 

places, described in the following paragraphs, they 

were not derived from the nearest part of the mountains. The clasts are typically Sl,lbangular; in places 

near the mountain fronts, however~ the clasts are 

conspicuously angular, and in beds near the river 

facies they are somewhat more roul)ded. 

The general description of the ·piedmont facies, 

given in the preceding paragr.a,phs, is typical of 

FIGURE 28. - Basin-fill gravel overlain by terrace deposits of intermediate height, in a cut along Josephine CanYo n at 

the north end of the San Cayetano Mountains. 
MIOCENE(?) TO HOLOCENE ROCKS 55 

deposits that fill the basins near the Santa Rita 

Mountains, but a few of the lithologic variations 

and exceptions provide some insight into local conditions. Some of the lithologic variations probably reflect changes in stratigraphic position within the 

facies; others are simply facies changes that reflect 

changes in source material or rate of deposition from 

fan to fan and from the head of a fan to the toe. 

These facies changes are unmapped because of the 

difficulty in maintaining stratigraphic control within the facies, especially where the gravel is only 

sparingly exposed. A few of the other local lithologic 

variations are described in the following paragraphs. 

Gravel in the upper reaches of Box Canyon drainage, 2-3 miles north of Greaterville, was deposited 

by streams that flowed northeastward, rather than 

westward as they do at present. The gravel contains abundant granodiorite and some gneiss clasts 

derived from Precambrian formations, such as 

underlie the hills to the southwest, and it contains 

very few clasts of sedimentary. rocks of 1\tiesozoic 

and Paleozoic formations, such as lie immediately 

west of the gravel. When the basin-fili gravel was 

deposited, the local streams apparently flowed northeastward. Since that time, and probably during the 

Pleistocene, the Box Canyon drainage must have 

captured the northeastward-flowing streams. 

East of the Santa Rita Mountains the piedmont 

facies contains abundant intercalated silt beds 3-4 

miles north of Patagonia and along Gardner Canyon 

north of Sonoita (a few miles east of the Mount 

Wrightson quadrangle), and west of the mountains 

silt is abundant northwest of Helvetia and along the 

lower reaches of Cottonwood Canyon. Melton (1965, 

fig. 2) considered the fine-grained deposits east of 

the mountains to be the southwestern · limit of his 

"lake beds," correlative with the thick silt sequence 

along the San Pedro River, some 25 miles to the 

east. No evidence has been found to suggest that 

the silt beds east of the mountains are lacustrine, 

nor is there any support for the idea that they are 

different in lithology or origin from the silt beds 

west of the mountains.· Most likely these are local 

subaerial deposits on flood plains or at the toes of 

fans, such as those in the lowest stream terraces 

near Sonoita and along the Santa Cruz River. 

The piedmont facies deposits near the Patagonia 

Mountains are atypical in a few respects. In general 

the piedmont facies forms a prism of gravel that 

thickens toward the rugged Patagonia Mountains to 

the southeast. North of Alum Gulch (fig. 2) the 

gravel near the foot of the mountains is a typical 

coarse fanglomerate, and the dips are mostly away 

from ·the mountains. However, south of the gulch 

the deposits are mainly of sand and grit, with some 

scattered small lenses of pebbles and cobbles and 

only a few boulders, and they dip gently toward the 

mountains. These finer grained deposits are believed 

to be abnormal alongside the rugged mountains, and 

they suggest that here a range-front fault separates 

the piedmont facies from the mountains (Drewes, 

1972a). 

Deposits of the piedmont facies on the Madera 

Canyon fan, an alluvial fan with a radius of 10 

miles, are typical of this facies in most ways, but the 

particular assortment of rock types of which it is 

composed has one noteworthy feature. The fan is 

almost wholly made up of detritus of very hard 

siliceous volcanics derived from the Mount Wrightson Formation and of friable granitoid rocks derived 

from the Madera Canyon Granodiorite, Josephine 

Canyon Diorite, and Elephant Head Quartz Monzonite. The · proportion of relatively large clasts of 

durable volcanic rock to weaker granitoid rock increases downfan. In fact, many granitoid cobbles 

simply disaggregate where they were initially deposited on the fan, whereas the volcanics are recycled with only a gradual decrease in size and 

increase in roundness. As a result, abundant detritus 

of the Mount Wrightson Formation reaches the 

Santa Cruz River from this fan. This concentration 

of distinctive volcanic clasts should provide a useful 

marker for subsurface studies of the deposits in the 

Tucson basin. 

In contrast to the piedmont facies deposits on 

the Madera Canyon fan, the deposits north of the 

fan, along the Box Canyon drainage, are much finer 

across the entire stretch of alluvial apron (rather 

than alluvial fan). The change in grain size and 

depositional morphology probably reflects, to a large 

extent, the · change from a less friable source rock 

in the south to a more friable source rock in the 

north and is typical of lateral facies variations in 

the piedmont facies. 

RIVER FACIES 

Deposits of the river facies of the basin-fill gravel 

are of unknown but probably considerable thickness; 

they are mapped along the low bluffs east of the 

Santa Cruz River roughly between Continental .and 

Sahuarita but are probably more extensive in the 

subsurface. The river-facies gravel consists of wellsorted and well-bedded deposits of subrounded to 

rounded cobbles and pebbles alternating with a nearly silt-free and clay-free sand. The bedding is distinctly channeled, as shown in figure 29. The clasts 

include much volcanic rock, some quartzite and 

limestone, and a little granitoid rock. In a few places, 
56 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

FIGURE 29.- Channeled, well-sorted, and subrounded river facies deposits of the basin-fill gravel, a mile north of Continental. 

as on part of the low ridge in the fields 11/2 miles 

southeast of Sahuarita, the gravel is well cemented 

by caliche, but generally it is uncemented and poorly indurated. 

Toward the east and south the river-facies gravel 

interfingers with the piedmont-facies gravel. In the 

transition zone between the two facies, gravel beds 

that contain subrounded pebbles alternate with beds 

that contain subangular pebbles. Away from typical 

river-facies gravel the beds with the subrounded 

pebbles gradually lose their identity through an increasing admixture of more angular locally derived 

detritus. Similar transition beds also occur in the 

bluffs east of the Santa Cruz River in the southwest 

corner of the Mount Wrightson quadrangle; although here these beds are mapped with the piedmont-facies gravel, they suggest that the river-facies 

deposits may be present nearby. 

The river-facies gravel is of potential economic 

importance for two reasons. Its well-sorted condition 

should make it better suited for construction material than the gravel of the piedmont facies, and 

especially better than material rich in weathered 

detritus derived from a granitic terrane. The location of the river-facies gravel near the Santa Cruz 

River makes it accessible from the main roads and 

also reduces the problem of obtaining ground water 

for washing the gravel. 

The river-facies gravel may also be of potential 

economic importance as an aquifer. In the subsurface 

it probably forms a thick midvalley zone of limited 

width and irregular lateral extent but of considerable length and down-valley regularity. Its exceptional sorting should give it excellent permeability, 

provided that caliche is not more abundant elsewhere than it is in the available outcrops. Locating 

the subsurface extension of this facies should be a 

consideration in developing the ground water of the 

valley. 

TUFF BEDS 

A few tuff and tuffaceous sandstone beds are intercalated in the basin fill near Patagonia (Drewes, 

1971c). A bed of tuffaceous sandstone, too small to 

be shown on the map, lies along the base of the 

basin-fill gravel of Adobe Canyon, . north of Patagonia. Another bed, east of the mouth of Alum Canyon (fig. 2), is half a mile long and as much as 4 

feet thick. One of two beds of tuff south of .Alum 

Canyon is as much as 20 feet thick and is more than 

a mile long. Tuffaceous sandstone commonly overlies the tuff beds, and it also underlies the thickest 

of them. All of these beds are low in the basin-fill 

gravel, although not necessarily at the same horizon. 

The tuff is very light gray, and where thickest, it 

forms bold outcrops, in part bedded and in part 

massive (fig. 30). The bedded tuff contains frag-
MIOCENE ( ?) TO HOLOCENE ROCKS 57 

FIGURE 30.- Tuffaceous sandstone and overlying massive nearly white rhyolite tuff beds intercalated in the lower part of 

the basin-fill gravel south of Alum Canyon, low on the flank of the Patagonia Mountains. 

ments of well-sorted pumiceous rhyolite and gritty 

tuffaceous sandstone. The massive tuff contains 

scattered small chips of pumiceous rhyolite and biotite-plagioclase rhyolite, as well as small biotite 

flakes. The tuff beds probably are entirely of epiclastic origin, so an isotopic age of the biotite it contains will not date the enclosing gravel. The interpretation that the tuff is epiclastic is based on its limited 

distribution, its restriction to the margins of the 

Grosvenor Hills volcanic pile where Oligocene tuff 

is extensively exposed, and the occurrence of tuffaceous sandstone immediately beneath some massive 

tuff, as shown in figure 30. 

AGE AND CORRELATION 

The basin-fill gravel in the Mount Wrightson and 

Sahuarita quadrangles probably correlates with 

gravel in nearby areas that has been dated as midPliocene to mid-Pleistocene. A well 21j2 miles north 

of Sonoita penetrated 35 feet of gravel, which is 

probably basin-fill gravel. Stirton (1940) reported 

that two teeth of Neohipparion were recovered from 

the well, and Lance (1960) dated these fossils as 

probably of mid-Pliocene age. The basin-fill gravel 

flanking the Santa Rita Mountains probably is largely correlative with a fossiliferous sequence near 

Benson, along the San Pedro Valley (fig. 1). This 

sequence includes beds ranging from about middle 

Pliocene to middle Pleistocene (Lance, 1960; Wood, 

1960). 

HIGH·LEVEL PEDIMENT DEPOSITS 

At many places around the flanks of the Santa 

Rita Mountains the basin-fill gravel is overlain by a 

thin sheet of gravel that typically is browner, coarser, less well sorted and bedded, and less well indurated. In most places this gravel sheet caps flat 

interfluves on the highest, or all but the highest, 

parts of the high alluvial fans and fan aprons, but 

in some places it descends to the levels of younger 

gravel deposits, which lap across them. The gravel 

caps a pediment that is cut chiefly on basin-fill 

gravel. Commonly the pediment gravel carries a 

mature thick red soil, which in places laps beyond 

the sheet of gravel directly onto the pediment surface. Together, the gravel and the soil are mapped 

as the high-level pediment deposits. 

Along the Santa Cruz River valley, high-level 

pediment deposits form extensive caps on the basinfill gravel of the Madera Canyon and Sycamore 

Canyon fans in the Sahuarita quadrangle (Drewes, 

1971a), and they underlie parts of the fans at 

Montosa, Cottonwood, and Josephine Canyons in the 

Mount Wrightson quadrangle (Drewes, 1971c). They 
58 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAI NS, ARIZONA 

also occur locally on the flanks of the San Cayetano 

Mountains and east of the Santa Rita Mountains, 

where they are outliers of a broad sheet that extends around Sonoita Creek valley to the foot of 

the Whetstone, Mustang, and Huachuca Mountains 

to the east. 

The sheet of gravel of the high-level pediment deposit is commonly 10-20 feet thick, but locally, as at 

the north side of Madera Canyon fan, it is as much 

as 40 feet thick. In places the sheet appears to lap 

out or is eroded out so that it is much thinner. 

Indeed, some of the fiat interfiuves of the Sycamore 

Canyon fan seem to be exhumed pediments from 

which the high-level pediment deposits have barely 

been stripped, leaving only remnants of the base of 

a mature soil. 

The contact at the base of the pediment gravel 

is poorly exposed along the edges of fiat interfiuves 

of the dissected fans. In most places the contact is 

too far back from the adjacent watercourses to be 

undercut, and the gravel sheet is so poorly consolidated that it slumps across all but a few small 

and fairly uninformative segments of the contact. 

Most likely, the deposits rest disconformably upon 

slightly eroded basin-fill gravel. That erosion occurred along the surface below the contact is suggested by the close lithologic and morphologic 

similarity between the high-level pediment gravel 

and the terrace gravels and by the fact that erosion 

cut the terraces upon which the terrace gravels were 

deposited. Locally the gravels may be gradational, 

because of reworking of the older deposits. 

High-level pediment deposits are offset by faults 

that cut across Madera Canyon and Sycamore Canyon fans. These faults have low scarps, across which 

the red soil is displaced as much as 15 feet. 

The high-level pediment gravel consists of boulders, cobbles, and pebbles in a brownish-gray sandy 

to gritty matrix. With very few exceptions, the pediment gravel is coarser than the underlying gravel, 

upfan as well as downfan (that is, the high-level 

pediment gravel is coarser than - the underlying 

gravel in the same part of the fan, although it may 

be finer downfan than the underlying gravel is upfan). At midfan positions, the underlying gravel is 

commonly sandy pebble gravel with some cobbles, 

whereas the pediment deposit tends to be gravel 

with cobbles and boulders, much like the two deposits shown in figure 28. In general the grain-size 

distinction is greater downfan because the range in 

grain size of the detritus in both deposits is smaller 

there and size ranges overlap less. The high-level 

pediment gravel is more poorly sorted and has better 

rounded clasts than most of the basin-fill gravel, and 

locally the entire sheet appears to form a single 

massive bed. Features of the capping gravel suggest 

that it is not a lag concentrate; rather it appears 

to be a fluvial deposit largely carried down from the 

mountains at the heads of the fans. No lag concentrate, for example, would include the considerable 

amount of coarse granitic detritus such as that of 

capping gravel high on the Madera Canyon fan, 

because similar granitic detritus of the underlying 

gravel is seen to decompose so rapidly in place that 

it could not be reworked into a lag deposit. Highlevel pediment gravel is sparsely cemented by caliche 

on only those fans, such as the Sycamore Canyon 

fan, that contain limestone detritus. Normally the 

gravel is unconsolidated except for the clay-enriched 

part of the soil that is developed on it. 

The top surface of the high-level pediment deposits 

is the smoothest of the area. The surface dips gently 

toward the intermontane valleys, and along the 

Santa Cruz River valley it also has a component of 

dip toward the interfan drainages. On several fans, 

such as those of Madera and Sycamore Canyons, 

an old surface slopes more steeply toward the valley 

than do the younger terrace surfaces, although the 

head of the Cottonwood Canyon fan shows the 

reverse relation. The top surface of the high-level 

deposits generally is found only on the upper parts 

of the fans; downfan the top generally is buried, 

but in the northern part of the Sycamore Canyon 

fan it is eroded. In many places, the top surface is 

·crudely paved with a thin layer of grit, chips, and 

sand. Probably the only reason that a well-developed desert pavement is absent is the fact that 

the rainfall is high enough to support a considerable 

vegetation density and rodent population, thereby 

leading to periodic overturning of the lag concentrate of chips and pebbles. 

The soil that has formed on the high-level pediment gravel and on some of the adjacent parts of 

the pediment that are uncapped by gravel is commonly 2-4 feet thick and is brownish pale red 

to moderate reddish brown. In some places this soil, 

or more properly, paleosol, is buried beneath younger gravel deposits, and in others it is partly truncated by erosion. Even where nearly complete, it 

shows the uneven development of zones characteristic of soils in a semiarid environment. An A 

horizon with abundant humic material is virtually 

absent. However, the B horizon with its clay enrichment is so pronounced that it constitutes most 

of the soil. The B horizon consists of clayey sand 

in which there are scattered pebbles and cobbles of 

unweathered parent gravel; their size and abundance 

depend chiefly on their lithology and position on the 
MIOCENE ( ?) TO HOLOCENE ROCKS 59 

fan. Lime concentrates occur locally in the C horizon near the base of the soil, but they are nowhere 

well developed, except where all the gravels contain 

limestone clasts and caliche zones. Thus the degree 

of redness and the thickness of the red zone are 

the most distinctive characteristics of the oldest 

paleosol of the area ; younger paleosols are browner 

and thinner. 

The vegetation on the high-level pediment deposits 

tends to be distinct from that on adjacent gravel. 

High on the fans along the Santa Cruz River valley 

the high-level pediment deposit generally is densely 

grass covered, a situation that is enhanced artificially through a mesquite control program. At lower 

elevations scrubby mesquite is more abundant on the 

pediment gravel than on the basin-fill gravel, apparently because the pediment gravel holds more 

water. In many places ocotillo grows most densely 

along the edges of the pediment gravel, probably 

because there a perched water table beneath the 

pediment gravel reaches the surface. At still lower 

levels, at about an elevation of 3,000 feet, greasewood favors the better cemented and probably limier 

basin-fill gravel to the pediment gravel sheet, except 

where that sheet also contains caliche zones. 

The age of the high-level pediment deposits has 

not been determined, and I can only tenuously propose that the mature red soil capping the pediment 

gravel is mid-Pleistocene. The gravel itself, of 

course, is older, and it probably is sufficiently older 

to have allowed a long period of intensive chemical 

weathering in order to develop the red soil. I favor 

a mid-Pleistocene age of the older red soil of the 

area because I do not believe that the Wisconsin 

time was climatically suitable or the Holocene sufficiently long for any red soil to have formed, and 

because I am tentatively assigning a Sangamon age 

to the younger red soil. The high-level pediment and 

its overlying deposits probably are correlative with 

similar features around the Huachuca Mountains 

which Haynes (1968, p. 79) correlated with the 

Tombstone surface and its red soil, of Bryant (Gilluly, 1956, p. 121). Haynes further correlated the 

red soil overlying the high-level Tombstone surface 

with a red soil in the Safford area, 80 miles northeast of the Santa Rita Mountains. 

The proposed mid-Pleistocene age of the older red 

soil of the Santa Rita Mountains is not supported 

by work by Melton (1965, p. 15), who correlated a 

red soil on the Madera Canyon fan (the only red 

soil recognized by him on that fan) with the l'i..!d 

soil at Safford. He assigned this soil a Sangamon 

age through a further correlation between the lowest beds of the fan at Safford, on which the red soil 

at Safford lies, and the apparently equivalent highest 

beds of "lake" deposits in Safford Valley, which, ac .. 

cording to Lance (1960), contain fossils of late 

Blancan tolrvingtonian (Kansan) age. The above 

correlations seem to need the support of detailed 

field mapping, without which it may be difficult to 

recognize significant facies changes in the gravels or 

to correctly identify the several red soils. 

Haynes (1968, p. 79) extended the Tombstone, or 

higher surface of Bryant (Gilluly, 1956, p. 121), 

with its mature red soil, to the flanks of the Huachuca Mountains. He also correlated this soil with 

Melton's soil in the Safford area. Very likely the 

high-level pediment deposits of the Santa Rita Mountains underlie a further extension of the Tombstone 

surface, and the red soil of the high-level pediment 

deposits of the Santa Ritas is the equivalent of the 

red soil on the flanks of the Huachuca Mountains_. 

INTERMEDIATE-LEVEL PEDIMENT AND TERRACE DEPOSITS 

A sheet of coarse subrounded poorly sorted alluvium· caps terraces of intermediate height along 

both sides of the Santa Rita Mountains. East and 

south of the mountains remnants of this terrace are 

relatively small, widely separated, and 30-100 feet 

above the level of the lowest flood-plain terraces. 

West of the mountains the terrace remnants are 

larger, closer together, and commonly only 20-50 

feet above the flood-plain terraces. Most valleys contain only one intermediate-level terrace (and alluvial sheet), but along lower Josephine Canyon 

there are three terraces of intermediate level, one 

of which is clearly more strongly developed than 

the others. Possibly some of the widely scattered 

terrace . gravels in other valleys are also remnants 

of several terrace levels, which are here combined 

because they seem to be more closely related physiographically to each other than to the high-level or 

the low-level . deposits. 

Along the Santa Cruz River and its tributaries an 

intermediate terrace forms a narrow, fairly continuous bench capped by gravel on which is a weakly developed red soil. Only small remnants of the 

intermediate-level Santa Cruz terrace occur in the 

Mount Wrightson quadrangle, for the river is outside the quadrangle to the west. However, the river 

flows through the western part of the Sahuarita 

quadrangle, and there, large terrace remnants are 

preserved. The terrace remnants merge and become 

broader near Continental and in turn merge with an 

extensive pediment. Along and near the north edge 

of the Sahuarita quadrangle, the sheet of gravel, 

sand, and silt on the pediment laps against hills of 

older gravel. 
60 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

The intermediate-level pediment and terrace deposits contain gravel, sand, clay, and the soil formed 

on them. These generally are coextensive, . but in 

places the soil laps beyond the edge of the gravel, 

and elsewhere the gravel is without a soil cover. 

Locally a soil also occurs within the gravel, to indicate again that this gravel probably consists of a 

closely related complex of deposits rather than a 

single sheet. The deposits typically form a sheet a 

few tens of feet thick lying disconformably on a cut 

surface, as shown in figure 28. Where the deposits 

spread out and overlie what appears to be a pediment, east of Continental, they may thicken to at 

least 100 feet; or, upon changing lithology there, 

they blend with the underlying deposits, making the 

position of their base uncertain. 

Gravel of intermediate terraces, like that of highlevel deposits, is browner, coarser, and better rounded but less well sorted than adjacent basin-fill gravel. 

At midslope it is typically poorly bedded cobble 

gravel which has a brownish-gray sandy gritty matrix and which -is only locally and sparsely cemented 

by caliche. The clasts were probably derived largely 

from the mountains at the heads of the fans. They 

were deposited by streams on the floor of the Santa 

Cruz River valley and its tributaries after their 

incision, in most places well below the level of highlevel pediment deposits. In many places this gravel 

is distinguished from other terrace gravels mainly 

by its soil and its position. 

Intermediate terrace deposits east of the mountains occur as small remnants on the north walls of 

the tributary valleys. Near these valleys the deposits 

consist of the usual kind of alluvial sheet capped 

by a thin red soil. A way from the valleys they grade 

into a locally derived colluvial sheet, also capped 

by or incorporating a red soil, which may be partly 

reworked from the older red soil of the nearby highlevel deposits. 

Gold occurs in the gravel of the intermediate 

terraces near Greaterville, but few remnants there 

are large enough to be shown at a map scale of 

1:48,000. Placer mining was very active at Greaterville in about 1878 (Schrader, 1915). 

The terrace gravel along State Highway 82 near 

the mouth of Alum Canyon is relatively well indurated and is irregularly impregnated with a black 

material that presumably is rich in iron and manganese oxides. The black material contains anomalous amounts of base metals (Drewes, 1972b). This 

mineral matter was probably introduced by surface 

and ground waters that were charged with metals 

derived from mineralized terrane of the nearby 

Patagonia Mountains, as is the present water in 

Alum Canyon. 

East of Continental and roughly a mile north of 

the road to Box Canyon and Madera Canyon, gravel 

of the intermediate terrace becomes gradually sandier, containing increasing amounts of grus derived 

from the Precambrian Continental Granodiorite 

north of Sawmill Canyon, and decreasing amounts 

of volcanic rocks from Madera and· Florida Canyons. Upslope on the ped!ment ( ?) the gravel is 

chiefly grit but has about to percent chips and blocks 

-largely of aplite, quartz monzonite, and granodiorite- occurring in crudely sorted lentils and as 

scattered fragments (fig. 31). At midslope the deposit is a silty grit with about 5 percent chips and 

blocks, almost devoid of granodiorite'. Low on the 

slope and west of marker line b on the Sahuarita 

quadrangle map (Drewes, 1971a), the grain size has 

further decreased so that silt is dominant over grit, 

chips are scarce, and clay is relatively abundant. 

In this area, too, massive beds of silt alternate with 

better bedded lentils of grit and sand. 

The gravel sheet on intermediate terraces is relatively smoothly surfaced and crudely paved, much 

like that of high-level deposits.· Scrubby mesquite 

prefers these deposits to the basin-fill gravel. However, where the coarser terrace gravel gives way to 

the finer silt, grit, and chip deposits (sandy deposits 

that contain scattered small angular pebbles), the 

surface is less smooth, and the control of vegetation 

is more obscure. The surface is typically very gently 

FIGURE 31.- Typical poorly sorted angular gravel of the 

intermediate-level terrace deposit, high on the fan (pediment?) east of Continental. 
MIOCENE ( ?) TO HOLOCENE ROCKS 61 

rolling, and its local relief is commonly no more than 

a few feet. Few gullies are long, and at midslope they 

are rarely incised more than 6 feet. Low on the slope, 

where the silt and clay content increases, there are 

a few vertical-walled gullies that are as much as 

6 feet deep. Along the lower reaches of the broad 

part of the terrace in the northern part of the 

Sahuarita quadrangle (Drewes, 1971a, west of 

marker line b), the surface contains numerous silt 

pans, which are barren flats resembling playa floors 

that are laced by widely spaced shallow gullies. 

A weakly developed reddish-brown soil, typically 

6-9 inches thick, caps the intermediate-level terrace 

gravel. The soil is somewhat thicker where it seems 

to contain fine material washed in from nearby areas 

that are under lain by the older red soil. On the 

bluffs east of the Santa Cruz River valley (Drewes, 

1971a, west of marker line a), the soil is as much as 

4 feet thick, possibly reflecting in part the inwashing 

of red soil from upslope and in part an abundance 

of pinkish-gray silt in the parent basin-fill gravel. 

On the other hand, soil is virtually absent on the silt, 

grit, and chip sheet on the pediment ( ?) (fig. 31) ; 

in part the soil grades out northward along with the 

lithologic transition, and in part a thin red soil is 

buried beneath a few feet of grit low on the slope. 

Thin reddish-brown soil exposed in deep cuts west 

and northwest of Sahuarita is probably correlative 

with that on the terraces of intermediate height. 

The weakly developed reddish-brown soil ranges 

from pale reddish brown to grayish orange pink 

and to moderate orange pink. Redder hues are most 

conspicuous on dry soil (those normally recorded), 

and browner hues, on moist soil. In the transition 

zone between terrace gravel and pediment grit, the 

soil color gradually becomes paler northward until 

it resembles the color of the parent material. 

The soil on the terrace gravel is zoned, like the 

older red soil, with a weak to nonexistent A horizon 

and a rather strong B horizon. Locally, the thicker 

body of soil west of line a contains an A horizon as 

much as 15 inches thick, and in many places the lower half contains abundant small caliche nodules of a 

weak C horizon. In many places this soil is modified 

by cultivation. In several places it appears to consist of several soils, one of them somewhat browner 

than the others. To the east, near line a, the soil is 

buried beneath a few feet of sand and silt that are 

probably part of the deposits of the intermediatelevel complex. To the north a broad, low mound of 

silt of suspected eolian origin lies between ( ?) two 

soils of this complex, and it contains some debris 

from occupation sites of Indians. 

The grit, chip, and silt deposits of the pediment ( ?) 

areas were probably transported largely by sheet 

wash, rather than chiefly by streams, as was the 

terrace gravel. This difference in mode of transport 

undoubtedly reflects the difference in mechanical 

weathering properties of the dominant granodiorite 

parent rock; the piedmont slope was flooded with 

porous grus. The grus was readily transported by 

streams issuing from the canyons, and because of its 

fineness it was transported on a fairly gentle gradient. Rapid water loss, due to the highly porous 

character of the grus and augmented by the gentle 

gradient on the slope, resulted in a system of watercourses in which water gathered locally and spilled 

over the banks in local distributaries. With the 

gradual loss of an integrated drainage system on the 

slope, surface wash transport became increasingly 

effective relative to stream transport. The virtual 

absence of a soil in the grit area suggests that the 

grains were turned over too frequently for products 

of chemical weathering to form and accumulate as 

a soil. Some of the small amounts of soil that may 

have formed locally appears to have been washed 

down slope and to have accumulated in the silt area 

northeast of Continental. 

The age of the intermediate-level terrace deposits 

can only be estimated, for no fossils have been 

obtained from them. Because of the extensive development of reddish-brown soil on the gravel, I 

tentatively assume the soil to be of Sangamon age. 

The gravel on which it is formed, then, is Sangamon 

or older, but it postdates the valley cutting in the 

high-level pediment deposits, whose soil is of presumed mid-Pleistocene age. The surface on the 

deposits may be correlative with the Whetstone 

surface, of intermediate level, along the San Pedro 

River, which was described by Gilluly (1956, p. 121). 

LOW-LEVEL TERRACE DEPOSITS 

A thin sheet of gravel under lies the lowest terraces 

and fills many valley bottoms on the flanks of the 

Santa Rita Mountains; similarly, sand and silt form 

a broad, low bench along the Santa Cruz River (fig. 

26). These low terraces are commonly 2-10 feet 

above the level at which the youngest alluvium is 

being deposited, and locally they are still part of 

the flood plain of the watercourses. Where the watercourses are small, only the dominant deposit was 

mapped, either the low-level terrace deposit or the 

youngest gravel, although both generally are present. 

West of the Santa Cruz River and north of Green 

Valley (fig. 1), the low-level terrace deposits spread 

out as a sheet of slope-wash deposits derived 

from grus. 

Two facies of low terrace deposits are shown on 

the Sahuarita quadrangle map (Drewes, 1971a) : a 
62 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA 

gravel facies of the piedmont and a silt facies of 

the Santa Cruz River valley. Although not shown 

separately on the Mount Wrightson quadrangle map 

(Drewes, 1971c), the silt facies underlies the southwest corner of the area as well as much of the flats 

along Sonoita Creek southwest of Patagonia. 

GRAVEL FACIES 

Deposits of the gravel facies consist largely of 

unconsolidated cobble, boulder, and pebble gravel 

with sand in the matrix and in intercalated lenses. 

The gravel in some of the higher of these terraces 

consists of a coarse, thin, capping deposit and a finer 

underlying unit, as is common in some of the higher 

terrace and pediment gravels, but in other low-level 

terraces this dual character is not apparent. Sorting, 

bedding, and rounding of the gravel are highly variable from place to place but commonly are. poor. 

Small channels are exposed in cutbanks. The terrace 

surface retains vestiges of such channels, as well as 

of cobble trains and subsidiary terrace ledges, which 

leave this surface rougher than that on the older, 

higher surfaces. Vegetation is markedly denser on 

the low-level terrace deposits than on the higher 

ones, and the mesquite is rarely scrubby. The deposits which form a sheet west of the Santa Cruz 

River consist of grayish-yellow-brown grit, sand, 

and silt, with scattered pebbles and lentils of pebbles, much like the deposits on the· intermediate 

terraces east of the river. 

A very immature gray or brown soil is developed 

on much of the low-level terrace gravel. It consists 

of a slight concentration of humic material at grass 

roots level and a slight accumulation of clay in the 

underlying few inches of alluvium. 

SILT FACIES 

Terrace deposits along the Santa Cruz River are 

predominantly of silt and loam. Gravel of the lowlevel terrace deposits of tributaries grades laterally 

into the river silt deposits, and in places some of the 

youngest gravel is washed across the silt terrace to 

form small tributary fans. In places, as west of 

Sahuarita, the youngest grit, sand, and silt of the 

low-level terrace deposits extends at least half a 

mile across the river silt and loam. Some founda-1 

tion problems around the Sahuarita High School may 

be due to subsurface flowage, under heavy loads and 

excessive irrigation, of material at the ·stratigraphic 

level of the river silt and loam. 

The silt facies consists of a mixture of lightbrownish-gray silt and loam that contains lentils 

of sand ·and gravel and, less commonly, humic material and black sand. Locally the deposits contain 

considerable clay and are darker gray, perhaps owing to accumulations of cienega (humic swale) soil. 

In many places the silt facies stands. in vertical 

banks that are maintained, or worn back rather 

than down, during floods. 

A dusky-brown (moist) to pale-yellowish-brown 

(dry) soil caps the silt terraces. It is weakly developed and reflects the high humus-producing environment and fine grain size of its parent material 

rather than an extended period of weathering. It 

has an A horizon several inches thick, a very thin 

B horizon, and no G horizon. 

The low terrace surface was farmed by Indians 

and Spaniards, who used simple irrigation methods 

and the surface waters of the Santa Cruz River, a 

procedure no longer practical because of the depth to 

which the river has cut its bed below the surface 

of the silt deposits. Today, the low river terrace is. 

intensively cultivated and irrigated from wells. During preparation of fields, the terrace surface is 

considerably regraded, and tributary channels are 

commonly enlarged, straightened, and bordered by 

dikes. Ground water is obt.ained at fairly shallow 

depths of 100-200 feet. 

The low terrace deposits are estimated to be 

largely or entirely of Holocene age, and the age of 

the youngest of the deposits probably ranges into 

historic time. If the association of the older terrace 

and pediment deposits with interglacial times is 

valid and if the youngest red . soil is accepted as 

Sangamon, then the low-level terrace deposits postdate the Wisconsin. Carbonaceous deposits from 

similarly low terraces along Cienega Creek, about 12 

miles east of the northern Santa Rita Mountains, 

were reported by Melton (1965, p. 31) to be dated 

by the radiocarbon method as about 1,800-2,500 

years old. 

YOUNGEST GRAVEL 

Unconsolidated gravel and some sand, virtually 

barren of perennial plants, cover the floors of most 

modern watercourses, where they are transported 

by each passing flood. The material is locally largely 

derived from the other surficial deposits. Inasmuch 

as the youngest gravel is restricted to present-day 

channels, which are incised as much as 10 feet, it 

postdates the channeling of the bottom lands. This 

channeling is generally attributed to changes in 

climate or land use that occurred between 1880 and 

1890 (Bryan, 1925). 

REFERENCES 

Anderson, C. A., 1951, Older Precambrian structure in Arizona: Geol. Soc. America Bull., v. 62, no. 11, p. 1331-1346. 

Bryan, Kirk, 1925, Date of channel trenching (arroyo cutting) in the arid Southwest: Science, new ser., v. 62, 

p. 338-344. 
REFERENCES 63 

Cooper, J. R., 1970, Preliminary geologic map of the Twin 

Buttes quadrangle, Pima County, Arizona: U.S. Geol. 

Survey open-file map. 

Cooper, J. R. , and Silver, L. T., 1964, Geology and ore 

deposits of the Dragoon quadrangle, Cochise County, 

Arizona: U.S. Geol. Survey Prof. Paper 416, 196 p. 

Creasey, S. C., 1967, Geologic map of the Benson quadrangle, 

Cochise and Pima Counties, Arizona: U.S. Geol. Survey 

Misc. Geol. lnv. Map 1-470 with 11 p. of text. 

Creasey, S. C., and Quick, G. L., 1955, Copper deposits of 

part of Helvetia mining district, Pima County, Arizona: 

U.S. Geol. Survey Bull. 1027-F, p. 301-323 [1956]. 

Damon, P. E., and Bikerman, Michael, 1964, Potassiumargon dating of post-Laramide plutonic and volcanic 

rocks within the Basin and Range province of southeastern Arizona and adjacent areas: Arizona Geol. Soc. 

Digest, v. 7, p. 63-78. 

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, 1967, A geochemical anomaly of base metals 

and silver in the southern Santa Rita Mountains, Santa 

Cruz County, Arizona, in Geological Survey research 

1967: U.S. Geol. Survey Prof. Paper 575-D, p. D176-

D182. 

____ 1968, New and revised stratigraphic names in the 

Santa Rita Mountains of southeastern Arizona: U.S. 

Geol. Survey Bull. 1274-C, 15 p. 

____ 1969, The Laramide orogeny of Arizona southeast of 

Tucson, in Abstracts for 1968: Geol. Soc. America Spec. 

Paper 121, p. 501-502. 

_ _ __ 1970, Structure control of geochemical anomalies in 

the Greaterville mining district, southeast of Tucson, 

Arizona: U.S. Geol. Survey Bull. 1312-A, 49 p. 

____ 1971a, Geologic map of the Sahuarita quadrangle, 

southeast of Tucson, Pima County, Arizona: U.S. Geol. 

Survey Misc. Geol. lnv. Map 1-613. 

___ 197lb, Mesozoic stratigraphy of the Santa Rita 

Mountains, southeast of Tucson, Arizona: U.S. Geol. 

Survey Prof. Paper 658-C, 81 p. 

____ 197lc, Geologic map of the Mount Wrightson quadrangle, Santa Cruz and Pima Counties, Arizona: U.S. 

Geol. Survey Misc. Geol. lnv. Map 1-614. 

____ 1972a, Structural geology of the Santa Rita Mountains, southeast of Tucson, Arizona: U.S. Geol. Survey 

Prof. Paper 748 (in press). 

____ 1972b, Geochemical reconnaissance of the Santa Rita 

Mountains, southeast of Tucson, Arizona: U.S. Geol. Survey Bull. 1365 (in press). 

Drewes, Harald, and Finnell, T. L., 1968, Mesozoic stratigraphy and Laramide tectonics of part of the Santa Rita 

and Empire Mountains, southeast of Tucson, Arizona, in 

Titley, S. R., ed., Arizona Geol. Soc. Southern Arizona 

Guidebook III: p. 315-324. 

Finnell, T. L., 1970, Pantano Formation, in Cohee, G. V., 

Bates, R. G., and Wright, W. B., Changes in stratigraphic 

nomenclature by the U.S. Geological Survey, 1968: U.S. 

Geol. Survey Bull. 1294-A, p. A35-A36. 

____ 1971, Preliminary geologic map of the Empire 

Mountains quadrangle, Pima County, Arizona: U.S. 

Geol. Survey open-file map. 

Fries, Carl, Jr., 1962, Resefi.a de la Geologia del Estado de 

Sonora con Enfasis en el Paleozoico: Bol. Assoc. Mexicana Ge6logos Petroleros, v. 14, no. 11-12, p. 257-273. 

Gilluly, James, 1956, General geology of central Cochise 

County, Arizona, with sections on age and correlation, 

by A. R. Palmer, J. S. Williams, and J. B. Reeside, Jr.: 

U. S. Geol. Survey Prof. Paper 281, 169 p. 

Hayes, P. T., and Drewes, Harald, 1968, Mesozoic sedimentary and volcanic rocks of southeastern Arizona, in Titley, S. R., ed., Arizona Geol. Soc. Southern Arizona 

Guidebook III: p. 49-58. 

Hayes, P. T., and Landis, E. R., 1964, Geologic map of the 

southern part of the Mule Mountains, Cochise County, 

Arizona: U.S. Geol. Survey Misc. Geol. Inv. Map I-418. 

Hayes, P. T., and Raup, R. B., 1968, Geologic map of the 

Huachuca and Mustang Mountains, southeastern Arizona: U.S. Geol. Survey Misc. Geol. Inv. Map I-509. 

Haynes, C. V., 1968, Preliminary report on the late Quaternary geology of the San Pedro Valley, Arizona, in Titley, 

S. R., ed., Arizona Geol. Soc. Southern Arizona Guidebook III: p. 79-96. 

Hunt, C. B., Averitt, Paul, and Miller,· R. L., 1953, Geology 

and geography of the Henry Mountains region, Utah: 

U.S. Geol. Survey Prof. Paper 228, 234 p. [1954]. 

Lance, J. F., 1960, Stratigraphic and structural position of 

Cenozoic fossil localities in Arizona: Arizona Geol. Soc. 

Digest, v. 3, p. 155-159. 

Lovering, T. G., Cooper, J. R., Drewes, Harald, and Cone, 

G. C., 1970, Copper in biotite from igneous rocks in 

southern Arizona as an ore indicator, in Geological Survey research 1970: U.S. Geol. Survey Prof. Paper 700-B, 

p. B1-B8. 

Melton, M. A., 1965, The geomorphic and paleoclimatic significance of alluvial deposits in southern Arizona: Jour. 

Geology, v. 73, no. 1, p. 1-38. 

N ockolds, S. R., 1954, Average chemical compositions of some 

igneous rocks: Geol. Soc. America Bull., v. 65, no. 10, 

p. 1007-1032. 

Putnam, W. C., 1938, The Mono Craters, California: Geog. 

Rev., v. 28, no. 1, p. 68-82; abs., in Geol. Soc. America 

Proc. 1936, p. 333, 1937. 

Schrader, F. C., 1915, Mineral deposits of the Santa Rita 

and Patagonia Mountains, Arizona, with contributions by 

J. M. Hill: U.S. Geol. Survey Bull. 582, 373 p. 

Silver, L. T., 1969, Precambrian batholiths of Arizona, in 

Abstracts for 1968: Geol. Soc. America Spec. Paper 

121, p. 558-559. 

Simons, F. S., 1971a, Geologic map of the Nogales quadrangle, Santa Cruz County, Arizona: U.S. Geol. Survey 

open-file map. 

_ ___ 1971b, Geologic map of the Lochiel quadrangle, 

Santa Cruz County, Arizona: U.S. Geol. Survey openfile map. 

Simons, F. S., Raup, R. B., Hayes, P. T., and Drewes, Harald, 

1966, Exotic blocks and coarse breccias in Mesozoic volcanic rocks of southeastern Arizona, in Geological Survey 

research 1966: U.S. Geol. Survey Prof. Paper 550-D, 

p. D12-D22. 

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California Univ., Dept. Geol. Sci. Bull., v. 25, no. 4, 

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Ranch area: Arizona Geol. Soc. Digest, v. 3, p. 141-143. 
A 

Abstract-------------------------------·····-----·---················· 

Adobe Canyon.·-·--------··········---------···---············· 

Age, Alto veins·-------------·······------·----················ 

basin-fill graveL. .................................... . 

Box Canyon-Gardner Canyon intruPage 

56 

28 

57 

sives............................................. 50 

Greaterville intrusives............................ 24 

Gringo Gulch Volcanics.......................... 11 

Grosvenor Hills Volcanics...................... 41 

Helvetia stocks ........................................ 21, 24 

high-level pediment deposits................ 59 

intrusives in Pinal Schist...................... 4 

northern rhyolite intrusives.................. 50 

olivine andesite plugs.............................. 25 

ore porphyrY--------····----···-····--·----·---··-·-···· 24 

quartz vein swarm.................................. 28 

rhyolitic volcanics·--···-----····------------·-······· 26 

soils .. ·-·----------------------------··---------------·--------···· 59, 61 

terrace deposits ........................................ 61, 62 

See also Radiometric dates. 

Alluvial aprons, Box Canyon........................ 55 

Alluvial fans .. ----·---··-···---·-------------··-··-·······----·--- 2 

Madera Canyon ............................ 55, 57, 58, 59 

Safford........................................................ 59 

Sycamore Canyon ......•............................. 57, 58 

Alteration of volcanic rocks .......................... 12, 35 

Alto veins---····--·-----·--------····----··-··-------·---······-··· 26 

Alum Gulch ..... -----------····----···········-·····--·--···-······ 12 

Analyses, Box Canyon-Gardner Canyon 

dikes ............. ----····----------·---··-······ 50 

Greaterville intrusives............................ 24 

Grosvenor Hills Volcanics.................... 42 

Helvetia stocks ....................... :.................. 20 

intrusives in Grosvenor Hills area.... 42 

intrusives in San Cayetano Mountains 42 

ore porphyry............................................ 24 

plugs of Gringo Gulch area.................. 15 

See also Modes. 

Apache GrouP----·-···--······---·······-·-··--·-·-··-·------·· 4 

Artis, Lowell, analyst ................ 15, 20, 24, 42, 50 

Ash Canyon ................... ·························-·------·· 32 

B, C 

Basement rocks................................................ 4 

Bathtub Formation ......................................... . 

Botts, S. D., analyst------········---···---------·----·15, 42, 50 

Box Canyon·--------·---·-··-···-···-··········--··---·-···--···· 54, 55 

Box Canyon dike swarm ................................ 26, 47 

Box Canyon-Gardner Canyon intrusives.. 47 

Caliche·-··-···--··-···----·····---·····-···-········-------·---· 56, 59, 61 

Chemistry, Box Canyon-Gardner Canyon 

dikes---·····----···-------·---··-----·····---···· 49 

Greaterville intrusives............................ 23 

northern rhyolite intrusives.................. 49 

plugs of Gringo Gulch area.................. 15 

southern igneous complex .................... 41, 45 

Chloe, Gillison, analyst ................ 15, 20, 24, 42, 50 

Cinigita Canyon·----···---·-···----·-------··--··-···--··---· 32 

Cienega Creek·-----··-----------------------·--··---·····----·· 62 

Coal Mine Canyon·---------------······----·--------·-·····-· 29 

Continental deposits ............ -----·-·--------·---·--·---- 5 

Continental Granodiorite ............ ------------·-·-·· 4 

Cottonwood Canyon .................................. 16, 47, 55 

Cottonwood Canyon dike swarm................ 16 

Crandell, W. B., analyst .............................. 42, 50 

INDEX 

[Italic page numbers indicate major references] 

D,E Page 

Davidson Canyon ................... ___ ··--------·-·-········ 54 

Dikes. See Intrusive rocks. 

Domes, extrusive·---------·······-----------------·----·-····· 38 

Economic geologY----·--------····---·---·-··---- 26, 28, 56, 60 

Elmore, P. L. D., analyst ............ 15, 20, 24, 42, 50 

Environment of deposition, basin-fill 

graveL _______________ ------------·---·-------· 55 

gravel of Nogales.................................... 53 

Gringo Gulch Volcanics.......................... 12 

Grosvenor Hills Volcanics...................... 29 

pediment deposits-------·-------·------··-------·---· 58, 61 

Environment of emplacement, laccoliths .... 38, 46 

northern rhyolite intrusives.................. 50 

·southern igneous complex.................... 45 

Eocene rocks·-----·-------------·---···-----------------··-----· 26 

F 

Fault blocks, Grosvenor Hills...................... 43 

San Cayetano·--------------···------------------·----··· 45 

Faults, George Wise........................................ 44 

Grosvenor Hills block.............................. 43 

Hangmans-------·----------··--··------·····---·····--····· 43 

late Laramide .......................................... 24, 47 

Paleocene·--·--------------··------······-··················· 47 

relation to dikes ...................................... 47, 50 

relation to gravel of Nogales................ 53 

relation to Greaterville intrusives...... 24 

relation to high-level pediment deposits........................................... 58 

relation to laccoliths ... ·------------·----·---·---·-· 43, 44 

relation to quartz veins.......................... 26 

San Cayetano ...... ------··--···-·--·----------·····-···· 43 

San Cayetano block................................ 45 

Sheuy·-······-----··--·---------·------····------···---····-··· 43, 44 

Fossils, basin-fill gravel.................................. 67 

Grosvenor Hills Volcanics, gravel and 

silt member ................................ 29, 41 

Grosvenor Hills Volcanics, rhyodacite 

member ....................................... 32, 41 

Safford ValleY------··----··············--------··--···· 59 

Fresno Canyon ... ·---·--··-------············----·-----· 29, 32, 34 

G 

Gardner Canyon.............................................. 55 

Gardner Canyon dike swarm .... ---------···--··--·· 26, 47 

Geologic setting·-------·-----··---·······-----··-····-···-·--·· 4 

George Wise fault---------------·····-·-----··---············ 44 

George Wise Spring·--·-------···-·------------------··-·-· 37, 44 

Geosynclinal deposits...................................... 4 

Glance Conglomerate...................................... 25 

Glenn, John, analyst ................................ 20, 24, 50 

Greaterville district........................................ 21 

Greaterville intrusives.................................... 21 

Gringo Gulch ........... -----····--···---·--·--··--··----···-·-·· 6, 10 

Gringo Gulch area, plugs.............................. 14 

Gringo Gulch Volcanics................................ 6 

age............................................................... 11 

basal unconformity.................................. 6 

environment of deposition.................... 12 

lower member............................................ 7 

silicified pods------------······-·····--·····-·····---·-·· 10, 28 

source·-------··-------------·---·-···-·----······--·----------· 12 

stratigraphY---------··--------·--- --- ---··----·-·-···-···· 6 

upper member.......................................... 10 

Page 

Grosvenor Hills .... ---·----··-----··········-···-·-······----·· 28, 43 

Grosvenor Hills fault block.......................... 43 

Grosvenor Hills Volcanics ............................ 28, 52 

age·--·-----···----···--····-------····---· ·--··-------·-··--······ 41 

analyses....................................................... 42 

basal unconformity.................................. 29 

environment of deposition.................... 29 

fossils·--·-·-·--·--------·--··---·-····-··--··---·------·· 29, 32, 41 

gravel and silt member .................... 29, 37, 41 

modes----·-··---- -·-------··--------·-··---------···········--· 32, 35 

origin ................ ---·----····-·-·-···---·--·····-------·--·· 45 

rhyodacite member ... ·-···--·---------·------- 32, 39, 41 

rhyolite member ...................................... 29, 38 

source ......... ------·-·--········--··-···---·---·-······· 32, 35, 45 

stratigraphy............................................... 28 

H 

Hamilton, J. C., analyst.. .......................... 15, 42, 50 

Hangmans Canyon·--·-·-·----········-·------··--··-------· 29, 37 

Hangmans fault.............................................. 43 

Harris, J. L., analyst .............................. 20, 24, 42 

Helvetia.............................................................. 21 

Helvetia stocks.................................................. 17 

age ............................................................... 21, 24 

analyses ................................... ---····-------·---· 20 

granodiorite·----------·····--··-····-·····-------·---·--·· 19 

modes.·-·----·--·----····---·-·········----·----------------···· 20 

quartz diorite ... -------------------·---····----·-··-····· 20 

radiometric dates .................................... 21, 24 

Helvetian phase, Laramide orogeny ........ 6, 21, 50 

Holocene rocks .... -------·---·-------·--···-------·············· 61, 62 

Huachuca Mountains...................................... 59 

Igneous complex, southern............................ 28 

Igneous rocks. See Intrusive rocks; Volcanic rocks. 

Intrusive rocks, dacite porphyry................ 14 

dikes, andesitic........................................ 25 

Box Canyon swarm ........................ 26, 47 

Cottonwood Canyon swarm.......... 16 

Gardner Canyon swarm ................ 26, 47 

Grosvenor Hills area .......... 36, 41, 42, 45 

Oligocene ......................... ···---·--·---·---- 36, 39 

Paleocene ...... ----------------······-···-·-····---- 16, 21 

Paleocene to Oligocene.................... 25 

relation to faults .............................. 47, 50 

San Cayetano Mountains .. 39, 41, 42, 45 

granodiorite ...... -----·---------··--········---··------··· 19, 40 

Greaterville ... ----·---····-···-------···············------· 21 

Grosvenor Hills area .................. 36, 41, 42, 45 

laccoliths .... _______ ...... --·-- __ .... ·---·-· ............ -----· 3 6 

analyses............................................... 42 

floors ... -- -- --·-·-···----------·--··----········-------· 37 

margins ..... ----------·----···--·---------------·---· 3 7 

origin ................ ------········----·----·-----····· 38, 46 

relation to faults ............................ 43, 44 

roofs.·--·-----··-----·---···-·······-------········-··-· 38 

microgranodiorite..................................... 14 

northern .......... --------------·-----········--········-····· 47 

Oligocene ............ ---·----------···---··------·····-----·· 25, 36 

ore porphyry .... ---····----·--·--······----·····---·---· 

Paleocene ................................................... . 

Pinal Schist ............................................. . 

21 

14 

4 

plugs, Gringo Gulch area...................... 14 

near Gringo Gulch.......................... 12 

Oligocene ..... --------------------··--·--····---·--·· 47 

65 
66 

Instrusive rocks - Continued Page 

Plugs - Continued 

olivine andesite................................ 25 

Paleocene ............................................ 14 , 21 

Paleocene to Oligocene.................. 25 

rhyolite................................................ 47 

quartz diorite .................................... :....... 20 . 

quartz latite porphyry ............................ 16, 21 

rhyodacite porphyry................................ 39 

rhyodacite vitrophyre ...................... 36, 39, 41 

rhyolite........................................................ 47 

San Cayetano Mountains .......... 39, 41, 42, 45 

stocks, Helvetia........................................ 17 

San Cayetano Mountains ........ 40, 42, 45 

veins............................................................. 26 

Isotopic dates. See Radiometric dates. 

J, K, L 

Josephine Canyon ............................................ 34, 59 

Josephine Canyon Diorite .............. 6, 7, 11, 16, 26 

Kelsey, J ., analyst............................................ 20, 24 

Laccoliths. See Intrusive rocks. 

Laramide orogeny ....................................... 1, 21, 26 

Helvetian phase .................................... 6, 21. 50 

Piman phase ............................................. . 

Lithology, basin-fill graveL .................... 54, 55, 57 

Cottonwood Canyon dike swarm.......... 17 

gravel of Nogales.................................... 52 

Greaterville intrusives............................ 22 

Gringo Gulch Volcanics.......................... 7, 10 

Grosvenor Hills Volcanics ........ 29, 32, 33, 34 

Miocene to Holocene sedimentary 

rocks............................................ 51 

ore porphyry............................................. 22 

pediment deposits .................................... 58, 60 

plugs of Gringo Gulch area.................... 14 

terrace deposits ........................................ 60, 62 

See also Petrography. 

Little Dragoon Mountains............................ 4, 5 

M, 0 

Madera Canyon fan ............................ 55, 57, 58, 59 

Madera Canyon Granodiorite........................ 16 

Marine deposits ............................................... . 

Mesozoic geology.............................................. 5 

Metamorphic rocks.......................................... 4, 21 

Mineralization, Alto veins ............................ 26, 28 

Greaterville intrusives ............................ 21, 23 

terrace deposits........................................ 60 

Mining .......................................................... 26, 28, 60 

Miocene rocks.................................................... 52 

Miocene to Holocene rocks............................ 50 

Modes, Cottonwood Canyon dike swarm.... 17 

Greaterville intrusives............................ 23 

Grosvenor Hills Volcanics, rhyodacite 

member....................................... 35 

rhyolite member................................ 32 

Helvetia stocks........................................ 20 

intrusives in Grosvenor Hills area...... 40 

intrusives in San Cayetano Mountains 41 

northern rhyolite intrusives.................. 49 

ore porphyry............................................. 23 

plugs of Gringo Gulch area.................... 15 

See also Analyses. 

Mount Wrightson Formation...................... 7, 55 

Oligocene rocks ................................................ 25, 28 

Ore porphyry.................................................... 21 

p 

Paleocene rocks................................................ 6 

Paleocene to Oligocene rocks........................ 25 

INDEX 

Page 

Paleosols. See Soils. 

Paleozoic geology ........................................... . 5 

Pantano. Formation........................... ............. 26 

Patagonia Mountains ...................................... 54, 55 

Petrography, andesitic dikes........................ 25 

Box Canyon-Gardner Canyon intrusives............................................. 49 

Cottonwood Canyon dike swarm.......... 17 

dikes in San Cayetano Mountains........ 40 

Greaterville intrusives ............................ 22, 23 

Gringo Gulch Volcanics.......................... 7, 10 

Grosvenor Hills Volcanics ...................... 29, 34 

Helvetia stocks ................................... 19, 20, 21 

intrusives in Grosvenor .Hills area...... 39 

northern rhyolite intrusives.................. 49 

olivine andesite plugs ............ ._................ 25 

plugs of Gringo Gulch area .................... 14, 15 

stock in San Cayetano Mountains:....... 41 

volcanics of Red Mountain.................... 12 

See also Lithology. 

Piman phase, Laramide orogeny ............... . 

Pinal Schist........................................................ 4 

Pleistocene rocks ...................................... 53, 57, 59 

Pliocene rocks .................................................. 52, 53 

Plugs. See Intrusive rocks. 

Pods, silicified .................................................. 10, 28 

Precambrian geology...................................... 4 

R 

Radiometric dates............................................ 13 

basalt in gravel of Nogales.................. 53 

Box Canyon-Gardner Canyon dikes.... 50 

Continental Granodiorite...................... 4 

Cottonwood Canyo~ dike swarm.......... 17 

Greaterville intrusives ............................ 23, 24 

Helvetia stocks .......................................... 21, 24 

intrusives in Pinal Schist...................... 4 

Josephine Canyon Diorite...................... 11 

northern dike swarms............................ 50 

ore porphyry ............................................ 23, 24 

Pantano Formation................................ 26 

plug near Gringo Gulch........................ 12 

plugs of Gringo Gulch area.................. 16 

southern igneous complex...................... 43 

terrace deposits........................................ 62 

See also Age. 

Rosemont district.. .......................................... 20, 21 

s 

Safford alluvial fan.......................................... 59 

Safford Valley................................................... 59 

Salero Formation .......................................... 6, 7, 11 

San Cayetano fault.......................................... 43 

San Cayetano fault block.............................. 45 

San Cayetano Mountains ........................ 39, 43, 45 

San Pedro River ......................... : ............ , ....... 55, 61 

San Pedro Valley............................................ 57 

Santa Cruz River ................................ 54, 57, 59, 61 

Sedimentary rocks, basin-fill graveL.......... 53 

basin-fill gravel, age................................ 57 

environment of deposition............ 55 

fossils................................................... 57 

piedmont facies ................................ 54, 56 

river facies........................................ 55 

tuff beds.............................................. 56 

gravel of Nogales.................................... 52 

Gringo Gulch Volcanics ........................ 10, 11 

Grosvenor Hills Volcanics .............. 29, 37, 41 

Miocene to Holocene................................ 50 

pediment deposits, high-leveL........... 57 

intermediate-level............................. 59 

Page 

Sedimentary rocks, piedmont deposits ...... 54, 56 

terrace deposits, intermediate-leveL.. 59 

low-leveL............................................ 61 

youngest graveL...................................... 62 

Sheuy fault ........................................................ 43, 44 

Sierrita Mountains.......................................... 53 

Silicified pods .................................................... 10, 28 

Smith, H., analyst.. ...................... 15, 20, 24, 42, 50 

Soils..................................................................... 51 

age ............................................................... 59, 61 

pediment deposits .............................. 57, 58, 60 

Safford area.............................................. 59 

terrace deposits .................................. 60, 61, 62 

Tombstone surface.................................. 59 

Whetstone surface................................... 61 

Squaw Gulch..................................................... 7 

Squaw Gulch Granite...................................... 7 

Stocks. See Intrusive rocks. 

Stratigraphy, Gringo Gulch Volcanics...... 6 

Grosvenor Hills Volcanics.................... 28 

Miocene to Holocene sedimentary 

rocks............................................ 50 

Structural g eology, anticline........................ 47 

faults. See Faults. 

Grosvenor Hills-San Cayetano Mountains area ............................ ~: ... . 43 

Sheuy-George Wise graben .................. 34, 43 

tension fracture system.......................... 26 

Summary............................................................ 3 

Sutton, A. L., analyst...................................... 42 

Sycamore Canyon............................................ 25 

Sycamore Canyon fan .................................... 57, 58 

T, U 

Tejano Spring.................................................. 39 

Tinaja Peak...................................................... 53 

Tobin, Barbara, analyst.................................. 42 

To~bstone surface.......................................... 59 

Tucson basin .................................................... 53, 55 

Unconformities, base of Gringo Gulch 

Volcanics.................................... 6 

base of Grosvenor Hills Volcanics...... 29 

base of high-level pediment deposits.. 58 

v , w 

Vegetation .............................................. 2, 59, 60, 62 

Veins................................................................... 26 

Volcanic rocks, agglom.erate.......................... 33 

alteration ................................................... 12, 35 

epiclastic..................................................... 7 

Gringo Gulch. See Gringo Gulch Volcanics. 

Grosvenor Hills. See Grosvenor Hills 

Volcanics. 

lava, andesitic.......................................... 11 

dacitic.................................................. 7 

rhyodacite........................................... 33 

rhyolite............................................... 7 

Oligocene .................................................... 28, 45 

Paleocene.................................................... 6 

Paleocene to Oligocene.......................... 26 

rhyodacitic................................................. 32 

rhyolitic ...................................................... 26, 29 

tuff .......................................................... 6, 10, 34 

basin-fill graveL.............................. 56 

rhyolitic .......................................... 7, 10, 29 

welded ................................................. 10, 34 

tuff breccia................................................ 7, 29 

Volcanics of Red Mountain.......................... 12 

Water resources.............................................. 56, 62 

Whetstone surface........................................... 61 

U.S. GOVERNMENT PRINTING OFFICE : 1972 0 - 472-410