Mesozoic Stratigraphy of the 

Patagonia Mountains and 

Adjoining Areas, 

Santa Cruz County, Arizona

GEOLOGICAL SURVEY PROFESSIONAL PAPER 658-E
Mesozoic Stratigraphy of the 

Patagonia Mountains and 

Adjoining Areas, 

Santa Cruz County, Arizona

By FRANK S. SIMONS

MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

GEOLOGICAL SURVEY PROFESSIONAL PAPER 658-E

Descriptive stratigraphy of Triassic, Jurassic, 

and Cretaceous rocks that are mainly rhyolites 

but that include some sedimentary rocks and 

intermediate volcanic rocks

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

ROGERS G. B. MORTON, Secretary

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director

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

Washington, D.C. 20402 - Price 40 cents (paper cover)

Stock Number 2401-1205
CONTENTS

Abstract 

Introduction 

Triassic and Jurassic rocks... ... ... ... 

Canelo Hills Volcanics. ... ...

Triassic or Jurassic rocks ._- .

Volcanic rocks in the southern Patagonia

Mountains ... __ __ . ....

UX Ranch block .

Duquesne block.........._ ..... .. ......

Corral Canyon block..... _ . ..

Volcaniclastic sequence. .._ .

Volcanic sequence.. .._.

American Mine block. .._ 

Thunder Mine block _ 

Chemical composition... ....... .......

Age and correlation.. 

Volcanic and sedimentary rocks 

in the Flux mine area..___....._..._ .

Page Page

El Cretaceous rocks . E13

1 Bisbee Formation . . ..

2 Fossils and age. _....... 

2 Volcanic rocks of lower Alum Gulch 

3 Volcanic rocks of Dove Canyon.. 

Trachyandesite of Meadow Valley 

3 Tuff and shale.... ... . .....

3 Thin lava flows 

3 Thick lava flows 

6 Chemical composition 

6 Alteration of trachyandesitic lavas. ... 

7 Age .. - - 

8 Cretaceous or Tertiary rocks ... .........

9 Volcanic rocks of the Humboldt

9 mine-Trench Camp area ... ... 

10 Volcanic rocks of Red Mountain 

References cited.. -.. . . _ 

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ILLUSTRATIONS

Page

FIGURE 1. Index map of part of southeastern Arizona . - ... E2

2. Generalized geologic map and sections of part of the Patagonia Mountains and Canelo Hills.. .... 4

3. Sketch of a partial section of volcanic and sedimentary rocks of the UX Ranch block north of the

Kansas mine ... _.__ . . -_ __ - -- - - -. - - --- - -- 6

4. Sketch of a partial section of sedimentary and epiclastic volcanic rocks in the lower part of the Bisbee

Formation west of the UX Ranch. . . . -__. .._. . .. 

5. Sketch of a geologic section of volcanic and sedimentary rocks of the trachyandesite of Meadow Valley

along a gully northeast of the Little Outfit Ranch._ . ..._ 

14

19

TABLES

TABLE 1. Comparison of chemical analyses, norms, and semiquantitative spectrographic analyses of Mesozoic

volcanic rocks, Patagonia Mountains, with those of associated igneous and Nockolds' average rocks._-

2. Average chemical analyses of rhyelites and granites, Patagonia Mountains 

3. Comparison of chemical analyses, norms, and semiquantitative spectrographic analyses of trachyandesite 

and rhyodacite of Meadow Valley with those of Nockolds' average rocks 

Page

Ell 

12

21

III
MESOZOIG STRATIGRAPHY IN SOUTHEASTERN ARIZONA

MESOZOIG STRATIGRAPHY OF THE PATAGONIA MOUNTAINS 

AND ADJOINING AREAS, SANTA GRUZ COUNTY, ARIZONA

By FRANK S. SIMONS

ABSTRACT

The Patagonia Mountains and Canelo Hills in the Lochiel 

and Nogales 15-minute quadrangles of southeastern Arizona 

consist in large part of volcanic and sedimentary rocks of 

Mesozoic age. These rocks are in fault contact with Precambrian plutonic igneous rocks and overlie or are faulted 

against Paleozoic sedimentary rocks ranging in age from 

Cambrian to Permian. They have been intruded by granodiorite of early Tertiary age and are overlain by sedimentary 

and volcanic rocks of middle Tertiary to Holocene age. At 

least 75 percent of the Mesozoic stratigraphic column is volcanic, mostly silicic volcanics; the remainder is sedimentary 

with some interbedded volcanic rocks. The entire section is 

not exposed in any one area, and correlations between areas 

and structural blocks are uncertain. The aggregate total 

thickness is unknown but is at least 10,000 feet and probably 

is 14,000 to 15,000 feet.

Silicic volcanic rocks of probable Triassic or Jurassic age 

make up several structural blocks in the southern half of 

the Patagonia Mountains. Correlation between blocks is 

uncertain, but the thickness of the volcanic sequence is estimated to be at least 6,000 to 7,000 feet and probably is considerably more. Rocks are mostly rhyolitic lava, welded tuff, 

and tuff interlayered with minor sandstone, conglomerate, 

and less-silicic volcanic rocks. Most of the Canelo Hills consist of rhyolitic lava and welded tuff very similar to those 

in the Patagonia Mountains; the lava is at least 1,000 feet 

thick, and the overlying welded tuff is at least 6,000 feet thick.

Cretaceous rocks consist of several thousand feet of dominantly fine grained sedimentary rocks assigned to the Bisbee 

Formation of Early Cretaceous age in the central Patagonia 

Mountains; several thousand feet of dominantly trachyandesitic lava in the northern Patagonia Mountains, upper 

San Rafael Valley, and Canelo Hills; and at least 2,000 'feet 

of heterogeneous volcanic and sedimentary rocks on the 

southwest flank of the Canelo Hills. The Bisbee Formation 

comprises mostly brown siltstone and mudstone with numerous thin limestone beds and some sandstone, conglomerate, 

and tuffaceous rocks; it rests disconformably on Triassic or 

Jurassic silicic volcanic rocks. The trachyandesite is perhaps 

3,000 feet thick in the Patagonia Mountains, where it lies 

unconformably either on Triassic or Jurassic volcanic rocks 

or on about 1,500 feet of rhyolitic to latitic lava and tuff, 

which in turn rest unconformably on the older volcanics. 

In the Canelo Hills, where the trachyandesite is much thinner, it is underlain conformably by 1,000 feet or more of 

tuff, shale, and conglomerate, which are faulted against

older volcanic rocks. The volcanic-sedimentary sequence southwest of the Canelo Hills is mainly epiclastic volcanic conglomerate of andesitic composition with some more-silicic 

tuff and tuffaceous sandstone; some of these rocks resemble 

those underlying the trachyandesite farther north.

The youngest rocks, of Late Cretaceous or early Tertiary 

age, are coarse silicic tuff, tuff breccia, and lava west of 

Trench Camp, where they are at least 800 feet thick, and 

massive silicic volcanic rocks of uncertain origin on Red 

Mountain, where the thickness is perhaps 1,500 feet. These 

rocks lie unconformably on rocks of Late Cretaceous and 

of Triassic or Jurassic age.

INTRODUCTION

The Patagonia Mountains, a north-northwesterly 

trending range lying east of Nogales and south of 

Patagonia in southeastern Arizona (fig. 1), consist 

in large part of Mesozoic volcanic and sedimentary 

rocks. This report describes these rocks, together 

with some of similar age in nearby parts of the 

Canelo Hills to the east and in the intervening area 

at the head of the San Rafael Valley. Figure 2 is a 

generalized geologic map showing the distribution 

of these rocks, which range in age from Triassic 

to Late Cretaceous or early Tertiary. Plutonic igneous rocks of Mesozoic age are also widely represented 

in the same region but are noted only briefly herein.

The layered rocks comprise six cartographic units 

of known or strongly inferred Mesozoic age and two 

units of uncertain but probable Late Cretaceous 

or early Tertiary age. Some pairs of units may be 

stratigraphic equivalents but either do not crop 

out in the same area or are separated by major 

faults and therefore are described separately. Only 

partial sections are available for all but one unit, 

and correlations are uncertain. The aggregate total 

thickness is unknown; in the Patagonia Mountains 

this thickness is at least 10,000 feet and probably 

is as much as 14,000 or 15,000 feet, and in the 

Canelo Hills it may be 10,000 feet or more. Volcanic

El
E2 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

rocks of silicic to intermediate composition make 

up 75 percent or more of the Mesozoic stratigraphic 

column; the remainder is sedimentary rocks and 

some interlayered volcanic beds.

Fieldwork was done over a period of about 7 

months during 1960-63 as part of the geologic mapping of the Lochiel and Nogales 15-minute quadrangles. Office work included microscopic study of 

about 275 thin sections and X-ray diffractometer 

analysis of 30 altered rocks. Chemical analyses were 

made by rapid rock analysis methods (Shapiro and 

Brannock, 1962) on 14 volcanic rocks and two granites. Various sections of the report have been 

strengthened by discussions with my colleagues 

John R. Cooper, Harald Drewes, Philip T. Hayes, 

and Robert B. Raup, all of whom were working 

concurrently in adjacent or nearby areas; none, 

however, should be held responsible for any conclusions reached herein.

TRIASSIC AND JURASSIC ROCKS 

CANELO HILLS VOLCANICS

The Canelo Hills Volcanics (fig. 2) were named 

and described by Hayes, Simons, and Raup (1965) 

and were further described by Hayes (1970a, p.

A3-A9) ; hence, they will not be discussed here in 

detail. The formation is divided into three informal 

members: a lower, volcaniclastic and volcanic member as much as 2,000 feet thick; a middle, rhyolite 

member at least 1,000 feet thick; and an upper, 

rhyolitic welded tuff member at least 6,000 feet 

thick. It is assigned a Late Triassic and Early 

Jurassic age on the basis of field relations with 

paleontologically dated rocks of Permian age and 

of Early Cretaceous age, and on the basis of radiometric ages of 173±7 m.y. (million years) determined by the potassium-argon method on biotite 

from the welded tuff member and 144±4 m.y. determined by the same method on biotite from the lower 

member. The lower member was correlated by Hayes 

(1970a, p. A8-A9) with the Gardner Canyon Formation of the Santa Rita Mountains (Drewes, 1968, 

p. C7), for which a radiometric age of 190±20 m.y. 

(Triassic) was obtained by the lead-alpha method 

on zircon from a dacite. In the area under consideration, the Canelo Hills Volcanics are in fault 

contact with the trachyandesite of Meadow Valley 

and the volcanic rocks of Dove Canyon, both of Late 

Cretaceous age, and are overlain by late Tertiary 

and Quaternary alluvium.

113° 112° 111° 110°

San Manuel \fe I GRAHAM '\°l

-25

I

50 

I

75

I

100 MILES

FIGURE 1. Index map of part of southeastern Arizona, showing Patagonia Mountains 

and other features mentioned in text. Area of figure 2 shaded. Quadrangles: D, 

Dragoon; K, Klondyke; L, Lochiel; N, Nogales; TB, Twin Buttes.
PATAGONIA MOUNTAINS AND ADJOINING AREAS E3

TRIASSIC OR JURASSIC ROCKS

VOLCANIC ROCKS IN THE SOUTHERN PATAGONIA 

MOUNTAINS

A sequence of Mesozoic rocks at least 10,000 feet 

thick occupies an area of about 15 square miles in 

the south half of the Patagonia Mountains, where 

they crop out in several blocks separated by faults, 

some of which are of great magnitude (fig. 2). Only 

very brief descriptions of these rocks have been 

published heretofore (Schrader, 1915; Stoyanow, 

1949, p. 30-31), although the rocks are discussed 

at length in an unpublished thesis by Baker (1961).

The UX Ranch block of volcanic and sedimentary 

rocks (the UX Ranch is named the Flying R Ranch 

on the Lochiel quadrangle map) underlies much of 

the north half of the mapped area in the Patagonia 

Mountains (fig. 2). To the northeast the UX Ranch 

block is faulted against Precambrian, Paleozoic, and 

Mesozoic rocks along the Harshaw Creek fault, on 

which the stratigraphic separation is at least 2 miles 

and the apparent left-lateral slip may be as'much 

as 4 or 5 miles. The northwest boundary of the block 

is the Bluenose fault, a major structure of undetermined but surely large displacement. At its south 

end the block is separated from the Duquesne block 

of silicic volcanic rocks by an upfaulted irregular 

body of Paleozoic sedimentary rocks that underlie 

the Washington Camp-Duquesne mining area, between the Kansas and Bonanza faults.

Northeast of the UX Ranch block, and separated 

from it by the Harshaw Creek fault, are the smaller 

American Mine and Corral Canyon blocks, mostly 

of volcanic rocks. These blocks are separated from 

each other by the American fault, a fault zone poorly defined for most of its length, but along which 

the stratigraphic separation appears to be at least 

4,000 feet. On its south side, the Corral Canyon 

block is in contact with Paleozoic rocks along the 

Corral Canyon fault, which has a displacement of 

at least a few hundred feet, perhaps much more.

Northwest of the UX Ranch block is the Thunder 

Mine block, composed of highly altered silicic volcanic and plutonic rocks and of some sedimentary 

rocks. The mutual boundary of the two blocks is 

the Bluenose fault.

UX RANCH BLOCK

The southeastern part of the UX Ranch block 

consists of volcanic rocks dominantly silicic and 

alkalic lavas and tuffs and minor amounts of sandstone, siltstone, and conglomerate. These rocks dip 

moderately northwest, but reliable attitudes can 

rarely be measured. The base of this sequence is

not exposed, as it is cut out along the Kansas fault, 

which forms the contact between the volcanics and 

the Paleozoic rocks of the Washington Camp-Duquesne area; but the thickness northwest of the fault 

is estimated at 6,000-7,000 feet. The volcanic rocks 

are overlain disconformably by several thousand 

feet of fine-grained clastic sedimentary rocks of the 

Bisbee Formation (Lower Cretaceous).

The volcanic rocks of the UX Ranch block are 

mostly light-colored gray to to light-gray, pink, or 

brownish-gray fine-grained to sparsely porphyritic 

lavas and tuffs. In most of the block, tuffs appear 

to be appreciably more abundant than flows; but 

in the western and southern parts of the block, 

metamorphism by Tertiary granodiorite has made 

it difficult or impossible to distinguish between them. 

Among 60 rocks studied microscopically, about 30 

were identified as tuff or welded tuff and 20 as lavas; 

the other 10 could not be identified with certainty 

because of metamorphism.

The lithology of a partial section about half a 

mile north of the Kansas mine is typical of the bulk 

of the sequence (fig. 3).

Sandstone and quartz-rich tuffaceous sandstone 

are interlayered with volcanic rocks throughout the 

sequence and also form three thick lenses that are 

shown in figure 2. Sandstones probably comprise 

less than 5 percent of the sequence. The stubby lens 

of sandstone 2,000 feet south of the UX Ranch is 

about 2,000 feet long and has a maximum thickness 

of about 600 feet; to the northeast it is cut off by the 

Harshaw Creek fault. The sandstone is a poorly 

sorted and obscurely bedded pale-red rock which 

contains a,few pebbly layers made up of fragments 

of volcanic rocks. It consists mainly of subrounded 

quartz grains in a very fine grained matrix of clay 

and hydromica(?). Minor components include lithic 

fragments, iron ore, and blue-gray tourmaline. About 

65 percent of the rock is quartz grains, 10 percent 

is other mineral or rock fragments, and 25 percent 

is matrix.

The sandstone lenses near the middle and near the 

head of Finley and Adams Canyon are even-grained 

quartzitic rocks which are composed of quartz and 

minor tourmaline, feldspar, and rock fragments, in 

a sparse mica-rich matrix.

Grayish-red tuffaceous siltstone is a minor rock in 

the middle and upper parts of the volcanic sequence. 

It consists largely of very dusty devitrified glass, 

quartz, and feldspar.

DUQUESNE BLOCK

The Duquesne block consists largely of light-colored sparsely porphyritic rhyolite that has conspicu-
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Upper Cretaceous

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 11

Meadow Valley

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PRECAMBRIAN PALEOZOIC TRIASSIC OR JURASSIC CRETACEOUS CRETACEOUS 

OR 

TERTIARY 

LOCH1EL QUADRANGLE
PATAGONIA MOUNTAINS AND ADJOINING AREAS E5

31°20'

1 

i I

4 MILES

J

part of the Patagonia Mountains and Canelo Hills.
E6 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

WNW ESE

/Jiff

1000 

I

2000 FEET

FIGURE 3. Partial section of Jurassic or Triassic volcanic 

and sedimentary rocks of the UX Ranch block north of the 

Kansas mine. Vertical scale equals horizontal scale. From 

youngest to oldest:

Tg Tertiary granodiorite.

J Tr v Jurassic or Triassic volcanic and sedimentary rocks of the UX Ranch 

block:

A, Dominantly silicic tuffs; some lavas.

B, Yellowish-gray fine-grained sandstone. Mostly interlocked grains of 

quartz 0.1-0.4 mm across. Sparse interstitial sericite and potassium 

feldspar and a little zircon and iron ore.

C, Light-greenish-gray fine-grained porphyritic rhyolite. Phenocrysts 

0.2-0.5 mm across of dusty perthitic potassium feldspar and albite. 

Groundmass is fine-grained mosaic of quartz and potassium feldspar 

which contains lenses of granular quartz.

D, Medium-light-gray finely streaked rhyolite (table 1, col. 10). Almost 

entirely devitrifled glass; a little sericite and tourmaline. Quartz 

forms lenses along flow layering.

E, Grayish-purple to dark-gray prominently streaked porphyritic rhyolite 

(table 1, col. 4). Phenocrysts of sericitized and epidotized antiperthitic albite in a devitrified glassy groundmass. Very sparse 

grains of potassium feldspar and pale-green hornblende.

F, Light-gray to white lava with prominent flow layering. Phenocrysts 

of sericitized albite in a dusty groundmass of devitrified glass containing numerous lenses of coarse quartz and fine-grained potassium 

feldspar.

G, Medium-dark-gray sparsely crystallic quartz latitic(?) welded tuff. 

Resembles unit F but has more distinct vitroclastic texture.

H, Medium-gray quartz latitic vitrie-crystal welded (?) tuff (table 1, col. 

1). Sparse fragments of very sodic plagioclase and perthitic potassium feldspar embedded in a vaguely vitroclastic matrix of 

devitrified pumice and shards. Secondary minerals are abundant 

pyrite and sericite, and a little tourmaline.

/, Medium-dark-gray coarsely porphyritic hornblende trachyandesite. 

Phenocrysts 1 cm long of dusty sericitized oligoclase and smaller 

crystals of green hornblende in a matrix of oligoclase microlites, 

hornblende, potassium feldspar, and iron oxides.

J, Light-gray very fine grained tuff(?). Small grains of sparse quartz, 

a little sericite, and abundant pyrite in a very dusty matrix of 

quartz and potassium feldspar.

K, Pale-yellow rhyolitic welded tuff (table 1, col. 6). Sparse small grains 

of sodic plagioclase and quartz in groundmass of devitrified glass 

showing faint eutaxitic texture. Sparse muscovite and opaque 

minerals.

L, Dark-gray porphyritic latite(?). Phenocrysts 1-2 mm long of very 

dusty much altered sodic plagioclase in a matrix of devitrified glass.

M, Medium-gray poorly sorted sandstone. Interlocked grains of quartz 

0.05-0.3 mm across; some zircon. Interstitial sericite, biotite, clay, 

and alunite. Abundant secondary tourmaline and pyrite. 

Pz s Paleozoic sedimentary rocks.

ous flow layering and streaks and lenses of granular 

quartz. Spherulitic lavas are common. Tuff and 

epiclastic volcanic conglomerate are minor components, and the north tip of the block is a small intrusive body of hornblende andesite. Much of the 

block has been converted to hornfels by Tertiary 

granodiorite. The scarcity of volcaniclastic or sedimentary interbeds that might aid in determining the 

attitude of the silicic flows precludes any but a very 

rough estimate of thickness; the rocks in general 

appear to dip moderately to steeply eastward, and 

the thickness may be several thousand feet. Neither 

the base nor the top of the volcanics is present; to 

the north the block is delimited by the Bonanza fault 

and to the west, by granodiorite; to the east it is 

overlain by alluvium. The rocks closely resemble

those of the lower part of the UX Ranch block, the 

principal difference being the lack of large sandstone 

bodies in the Duquesne block, and the two sequences 

almost surely are correlative.

A typical lava in this block is a light- to mediumgray or pale-purple rock with sparse small phenocrysts of antiperthitic albite in a groundmass of 

glass that has devitrified to patches of fine-grained 

potassium feldspar and quartz. Abundant lenses of 

granular quartz with irregular selvages of finegrained potassium feldspar lie parallel to the flow 

layering. Where converted to hornfels, the lavas 

have a fine-grained sugary groundmass of quartz, 

potassium feldspar and a little sericite, epidote, piedmontite, and tourmaline.

The hornblende andesite at the north tip of the 

Duquesne block is a dark-gray rock which contains 

abundant millimeter-sized or smaller phenocrysts of 

andesine set in an intergranular groundmass of 

andesine microlites, strongly pleochroic blue-green 

hornblende, and iron ore. Dikes of similar rock cut 

volcanic rocks in both the Duquesne and UX Ranch 

blocks but were not seen in any younger rocks; therefore, the hornblende andesite is shown on the geologic map as part of the volcanic sequence of the 

Duquesne block.

CORRAL CANYON BLOCK

The Corral Canyon block consists of a variety of 

volcaniclastic and volcanic rocks. The total thickness 

of these rocks is unknown inasmuch as the base is 

not exposed and reliable structural data are few in 

the upper part of the sequence, but it is probably at 

least 5,000 feet and may be considerably more. Rocks 

within the block comprise a lower, dominantly volcaniclastic sequence at least 2,000 feet thick and an 

upper, dominantly volcanic sequence at least 3,000 

feet thick.

VOLCANICLASTIC SEQUENCE

The volcaniclastic sequence is made up of epiclastic volcanic conglomerate, tuffaceous sandstone 

and siltstone, and silicic to intermediate tuff, welded 

tuff, and lava flows. The thickest section of the 

sequence is along the west side of the block, and the 

following descriptions apply to that area. Because 

many units in the sequence are lenticular, the section 

on the east side of the block differs in many details 

from that on the west side, but the same rock types 

are present and therefore will not be further described. The volcaniclastic rocks are similar to those 

in the upper part of the volcanic sequence of the 

UX Ranch block; in particular, beds of red tuffaceous siltstone and sandstone are common to both 

sequences.
PATAGONIA MOUNTAINS AND ADJOINING AREAS E7

The lowest beds in Corral Canyon are red to gray 

epiclastic volcanic conglomerate and interbedded red 

tuffaceous sandstone, pebbly tuff, and silty tuff. Conglomerate forms beds as much as 6 feet thick and 

consists mainly of rounded to subanglar pebbles and 

cobbles of silicic volcanic rocks in a grit or sand 

matrix containing abundant rock fragments. The 

conglomerate clasts include brown to purple silicic 

lava, welded tuff, and silty to sandy tuff as well as 

some limestone, chert, and quartzite. This conglomeratic sequence is several hundred feet thick and is 

overlain by about 100 feet of crossbedded and channeled maroon tuffaceous siltstone and sandstone 

containing abundant carbonate grains.

Resting on the maroon beds is about 200 feet of 

purple or red tuff, tuffaceous sandstone, volcanic 

pebble conglomerate, and, at the top, about 15 feet 

of purplish-gray porphyritic andesite(?). Beds of 

fine-grained and finely laminated maroon tuff 0.5-4 

feet thick are conspicuous. A typical grayish-redpurple feldspathic tuff consists of fragments of 

altered sodic plagioclase, fragments of mafic minerals completely altered to carbonate and iron oxide, 

and dark lithic fragments in a very dusty matrix 

containing much iron oxide.

Overlying the dominantly red basal part of the 

volcaniclastic unit is a heterogeneous sequence 900- 

1,000 feet thick that is mostly gray tuff in the lower 

part and gray to green lava and flow breccia in the 

upper part. The basal 30 feet or so of the tuffaceous 

part is a light-greenish-gray massive vitric tuff of 

latitic or dacitic composition. Locally the tuff is separated from the underlying red rocks by a thin flow 

of purplish-gray porphyritic andesite. The tuff consists of phenocrysts 1-3 mm (millimeters) across of 

altered plagioclase, pink sericitized potassium feldspar, biotite largely converted to iron oxide, and a 

few of quartz, in a matrix of devitrified glass shards. 

The overlying tuffs are similar but contain more 

lithic fragments. Near the west end of the outcrop 

area the tuff contains a thin flow of very fine grained 

violet silicic lava. The upper part consists mainly of 

grayish-green to green porphyritic andesitic or trachyandesitic lava and flow breccia with minor moresilicic and conspicuously flow-layered lavas. The andesitic lavas enclose lenses and thin beds of brown 

slightly feldspathic sandstone. All the andesites are 

altered; plagioclase is saussuritized, and pyroxene is 

converted to serpentine minerals.

The upper part of the volcaniclastic sequence is 

another heterogeneous group of rocks 600-700 feet 

thick that comprises varicolored silicic welded tuff, 

tuff, and pebbly tuff, maroon sandstone and siltstone,

and red to gray lavas and flow breccias of intermediate and silicic composition. The lowest rock of 

this group is a light-olive-gray latitic (?) welded 

tuff as much as 60-70 feet thick that forms a prominent outcrop along the high hill north of Corral Canyon. This tuff is made up of sodic plagioclase phenocrysts, pumice lentils, and a few lithic fragments in 

a dense matrix of moderately welded and devitrified 

potassium-rich glass shards. The upper rocks of the 

group are, from bottom to top, a bed 40 feet thick of 

hard maroon sandstone containing sparse flecks of 

carbonate, grayish-red fine-grained porphyritic andesite, dark-gray very fine grained conspicuously 

streaked rhyolite or latite, fine-grained platy red 

vitric rhyolitic tuff, and, at the top, a prominent flow 

of biotite trachyte (?). The trachyte (?) is a mediumgray porphyry that contains 1- to 2-mm phenocrysts 

of sodic plagioclase and smaller crystals of highly 

altered biotite in a very fine grained groundmass of 

plagioclase microlites and interstitial potassium feldspar and a little quartz.

VOLCANIC SEQUENCE

The volcanic sequence comprises a lower part 650- 

750 feet thick of intermediate to silicic lavas and an 

upper part of highly silicic lavas and tuffs of uncertain but appreciable thickness, perhaps 2,000-2,500 

feet. Stratigraphic relations between these rocks and 

those in other structural blocks are not known; some 

lithologic similarities between lavas in the Corral 

Canyon and American Mine blocks, however, suggest a possible correlation of at least parts of those 

blocks.

The lower part of the volcanic sequence consists 

of conspicuously flow-layered porphyritic andesite or 

trachyandesite, with some very platy and streaked 

porphyritic rhyolite or quartz latite toward the base 

and silicic welded tuff and a little feldspathic sandstone at the top. The andesitic lavas are commonly 

red or purple and have abundant phenocrysts of 

sodic plagioclase and much-altered mafic minerals, 

together with sparse potassium feldspar phenocrysts, 

in a very fine grained conspicuously flow-layered 

groundmass. The silicic lavas are gray to reddishpurple very fine grained and finely laminated rocks 

that contain abundant lenses and streaks of quartz. 

They contain sparse phenocrysts of sodic plagioclase, potassium feldspar, and highly altered mafic 

minerals. The quartz lenses are coarsely granular 

and have selvages of tiny crystals of potassium feldspar. Welded tuff at the top of the lava pile is 45-50 

feet thick. It is a moderate-red to yellowish-brown 

rock which contains sparse phenocrysts 1-2 mm
E8 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

across of sodic plagioclase, potassium feldspar, biotite, and quartz, and a few lithic fragments in a 

completely devitrified matrix of shards and pumice.

The upper part of the volcanic sequence is a 

monotonous pile of silicic lava and flow breccia, tuff, 

welded tuff, and a few thin beds of quartzite. Lavas 

appear clearly to predominate over tuffs, although 

no quantitative estimate of proportions was attempted. The rocks appear in general to dip 25°-45° 

N., but structural data are so scanty that no accurate 

estimate of thickness is possible.

The lavas of the upper part are typically light 

colored, very fine grained, and sparsely porphyritic 

to nonporphyritic, and they display conspicuous 

much-contorted 'flow layering. The groundmass is 

everywhere devitrified, and many lavas are highly 

spherulitic; one flow 50 feet thick consists of about 75 

percent spherulites 1-3 inches in diameter. Another 

flow contains lithophysae as much as 1 foot across. A 

typical rhyolite from a locality 1,500 feet southeast 

of the Hardshell mine consists of phenocrysts as 

much as 5 mm across of albite, potassium feldspar, 

and possibly biotite in a coarsely devitrified groundmass that is mostly potassium feldspar and quartz. 

A common rock along upper Hermosa Canyon is a 

pale-brown to pale-red abundantly porphyritic quartz 

latite or dacite. It contains 1- to 2-mm phenocrysts 

of albite and perthitic potassium feldspar, and a 

few of cloudy and altered biotite, in a groundmass 

of glass now devitrified to a fine-grained aggregate 

of quartz and feldspar.

Welded tuffs occur throughout the upper lava 

sequence. All appear to be rhyolitic and potassic. 

Light-gray welded tuff at the Hardshell mine contains phenocrysts of potassium feldspar, and a few 

of quartz, in a moderately welded coarse matrix of 

devitrified glass shards and pumice fragments. Another similar welded tuff is underlain by a bed as 

much as 20 feet thick of quartzite that crops- out 

for more than 1,500 feet along the strike. The quartzite has about 70 percent subrounded quartz grains 

0.1-0.3 mm in diameter and 30 percent chalcedonic 

groundmass containing small amounts of feldspar 

and rock fragments.

A small body of limestone conglomerate is enclosed in volcanics about 1,000 feet south of the 

Hardshell mine (fig. 2). The conglomerate is 40-45 

feet thick. It consists entirely of jumbled blocks of 

limestone (most of which appear to be Concha Limestone of Permian age) in various shapes and in 

sizes as large as 3 feet across.

The stratigraphically highest rocks in the Corral 

Canyon block, immediately underlying the trachyandesite of Meadow Valley, consist of purplish-gray 

rhyolitic flow breccia, gray finely laminated silicic 

tuff, and some volcanic conglomerate.

AMERICAN MINE BLOCK

The American Mine block is a faulted wedge of 

Paleozoic sedimentary rocks and Mesozoic volcanic 

rocks lying between the UX Ranch block to the west 

and the Corral Canyon block to the east. The contact 

between Paleozoic and Mesozoic rocks is very poorly 

exposed but is interpreted as an unconformity; if 

such an interpretation is correct this is the only 

place in the Patagonia Mountains where the contact 

is not a fault. The volcanic rocks appear to dip moderately northward; but, as in the upper part of the 

Corral Canyon block, almost nowhere can an attitude 

be determined with confidence.

The rocks of the American Mine block are silicic 

lavas and flow breccias with subordinate silicic tuffs 

and a large lens and several smaller bodies of limestone conglomerate (Simons and others, 1966, p. 

D17-D19). The volcanic rocks are similar to those 

of the other Patagonia Mountains blocks, especially 

the upper part of the Corral Canyon block. Limestone conglomerate is much more abundant than it 

is in other blocks.

The lavas are varicolored but mainly light gray 

to pale red. They are porphyritic and nonporphyritic 

and commonly have conspicuous contorted flow layering and quartzose streaks and lenses. The porphyritic rocks contain 30-40 percent phenocrysts, most 

of sodic plagioclase and quartz but some of potassium feldspar and completely altered mafic minerals, 

in a matrix of feldspar microlites and devitrified 

glass. The nonporphyritic lavas are made up almost 

wholly of devitrified, and in some flows, spherulitic, 

glass that encloses irregular streaks and lenses of 

quartz, or of quartz with a selvage of tiny potassium 

feldspar crystals, or of a granular aggregate of the 

two minerals.

The base of the volcanic rocks at the east edge 

of the block is marked locally by silicified rock that 

could possibly indicate a fault or a fault zone. The 

lowest volcanic rocks are a sequence of light-colored 

hard well-bedded fine-grained vitric tuff and sandy 

tuff about 50 feet thick. These rocks are about parallel to the underlying Paleozoic limestone. A typical 

tuff here consists almost entirely of devitrified glass 

shards and dust and includes a few small quartz 

grains.

Rocks in the uppermost part of the block, immediately beneath the trachyandesite of Meadow 

Valley, are light-gray to reddish-purple porphyritic 

flow breccias and lavas that are conspicuously flow
PATAGONIA MOUNTAINS AND ADJOINING AREAS E9

layered. Phenocrysts are albite and completely altered mafic minerals, and the groundmass is devitrified glass. Just southwest of the Hardshell mine the 

stratigraphically highest rock is a light-gray welded 

tuff. It consists of millimeter-sized phenocrysts of 

potassium feldspar and sparse quartz, and lithic 

fragments, in a matrix of glass shards whose texture has been almost completely obliterated by devitrification.

The large lens of limestone conglomerate near the 

north end of the block (fig. 2) is a chaotic aggregate 

of irregular subangular to angular blocks, primarily 

of brecciated limestone but including many of quartzite and some of dolomite and silty or sandy limestone, 

in a sparse matrix of silicic volcanic debris. The 

maximum thickness is estimated to be several hundred feet, but it is very uncertain because exposures 

are poor. Eastward the conglomerate thins abruptly 

and pinches out about 3,500 feet east of Harshaw 

Creek; to the west it is cut off by the Harshaw Creek 

fault (Simons and others, 1966, p. D18, fig. 4A). 

Conglomerate clasts generally range in size from 

sand to blocks 5 feet or more across, and one block 

is 100 feet or more across. Most blocks have been 

pervasively shattered into fragments less than an 

inch across and then healed. The clasts are mainly 

Concha Limestone (Permian) ; the quartzite is probably from the Scherrer Formation, and the dolomite 

and impure limestone may be from the Epitaph 

Dolomite, both also of Permian age. Locally the base 

of the conglomerate contains much angular volcanic 

debris, some fragments of which are as much as 1 

foot across. The conglomerate lens is as much as 

1,000 feet stratigraphically above the base of the 

volcanics, and the nearest outcrops of possible source 

rocks are about 1,000 feet south.

THUNDER MINE BLOCK

The Thunder Mine block is the most northwestern 

of the Patagonia Mountains Mesozoic blocks. It is 

bounded on the east by the Bluenose and Harshaw 

Creek faults and on the west by plutons of Tertiary 

and Jurassic age. The block is composed of sheared 

and much-altered silicic volcanic rocks, some quartzite, and highly altered monzonite and other intrusive 

igneous rocks. No direct evidence of the age of the 

volcanic and sedimentary rocks is available; but they 

are surely of Mesozoic age, and their considerable 

degree of alteration and deformation suggests an 

early Mesozoic age.

The thickness of the volcanic-sedimentary sequence in the Thunder Mine block is very uncertain. 

Almost no reliable data on the attitude of the rocks 

could be obtained, and the combination of shearing

and alteration makes it difficult, and in places impossible, to distinguish between extrusive and intrusive igneous rocks. The general impression is of 

north-northwesterly strike and steep dips, and it 

seems likely that at least 1,000 feet, and perhaps 

2,000 or even 2,500 feet, of mixed volcanic and sedimentary rocks is present.

The rocks of the Thunder Mine block are mainly 

light-colored light-gray to grayish-red, pale-red, 

and lavender porphyritic to aphanitic silicic lavas, 

flow breccias, and tuffs with intercalated fine- to 

medium-grained quartzose sedimentary rocks. Porphyritic lavas have phenocrysts 1-5 mm long of 

sodic plagioclase (commonly nearly pure albite), 

and a few of quartz, embedded in a fine-grained 

quartz-feldspar aggregate that probably is devitrified glass. Flow (?) breccia near the Sunnyside mine 

is a light-gray to lavender rock consisting of abundant angular fragments of volcanic rock mostly less 

than 2 inches across in a fine-grained matrix of porphyritic rhyolite. Many rocks identified as volcanic 

in the field proved upon microscopic examination to 

be so completely converted to various combinations 

of quartz, sericite, alunite, and kaolinite that their 

original nature could not be determined; presumably, some of these were originally fine-grained 

silicic and glassy volcanics.

Sedimentary rocks interbedded with the volcanics 

are also altered but have retained more of their 

original features. They include nearly pure quartz 

sandstone and silty or tuffaceous sandstone. A bed 

of quartzite that forms a prominent rib along the 

east side of the block (fig. 2) ranges from a few feet 

to at least 100 feet in thickness. It is a light-gray to 

pale-red rock made up of partly rounded and somewhat interlocked or sutured grains of quartz 0.05- 

0.5 mm across and sparse interstitial sericite. Quartz 

grains 1-2 mm in diameter are scattered through 

the finer grained material. Very sparse zircon and 

feldspar are the only other detrital minerals in the 

rock. Locally the quartzite has some thin beds of 

pebble conglomerate or breccia, and siltsone. Near 

the Flux mine (fig. 2), the quartzite is brecciated 

and silicified.

Silty sandstones consist of angular to subrounded 

quartz grains in an abundant silty or muddy matrix 

now altered to sericite. In some rocks the matrix 

is a fine-grained quartz-sericite-iron oxide aggregate, possibly altered tuffaceous material. As in the 

sandstones, the only detrital minerals are quartz 

and very sparse zircon and feldspar.

CHEMICAL COMPOSITION

Chemical analyses, norms, and semiquantitative
E10 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

spectrographic analyses of nine volcanic rocks from 

the UX Ranch and Duquesne blocks are given in 

table 1. Also tabulated for comparison are analyses 

and norms of rhyolite and rhyolitic welded tuff from 

the Canelo Hills Volcanics, two Jurassic granites 

from the Patagonia Mountains, and Nockolds' 

(1954) average alkali rhyolite and average dellenite 

[= quartz latite].

The rocks from the UX Ranch and Duquesne 

blocks are, with one exception, rhyolites with high 

alkali contents (8-9.5 percent), high to very high 

ratios of K2 0 to Na2 0 (1.3:1 to 3.1:1; in one rock 

the ratio is less than 1:1), and very low CaO (<0.5 

percent) ; they correspond closely to average alkali 

rhyolite of Nockolds (1954, p. 1012). The exception, 

a quartz latitic welded (?) tuff, has 1.3 percent CaO 

and somewhat less Si02 than the rhyolites but is 

otherwise similar. Rhyolites from the Canelo Hills 

Volcanics are more silicic and show extremely high 

ratios of K2 0 to Na2 0 (8:1 and 43:1). Alkali-granite 

that underlies much of the west-central part of the 

Patagonia Mountains (not shown in fig. 2) and 

intrudes silicic volcanic rocks of the Thunder Mine 

block is chemically almost identical with the rhyolites 

in both major and trace element composition; the 

similarity is evident in the individual analyses and 

is particularly striking in averages of chemical analyses of the eight alkali rhyolites and the two 

granites of table 1, as shown in table 2.

Consanguinity of the rhyolites and granites suggested by the chemical analyses is supported by 

similarities in probable ages. Geologic evidence suggests that the volcanics are Triassic or Jurassic in 

age, and zircon from the analyzed granite of table 

1, column 14, has been dated by the lead-alpha 

method at 160±20 m.y., or Jurassic (T. W. Stern, 

written commun., 1965). Evidence is accumulating 

from the Patagonia Mountains as well as elsewhere 

in southern Arizona that the time interval within 

10-20 m.y. of the Triassic-Jurassic boundary (about 

190-195 m.y.) was a period of widespread alkalic 

magmatism.

AGE AND CORRELATION

The assignment of the silicic volcanic rocks of the 

southern Patagonia Mountains to the Triassic or 

Jurassic is based on geologic relations within and 

among the several structural blocks, on radiometric 

dating of a granite that intrudes the volcanic rocks, 

and on correlation with lithologically similar rocks 

in nearby areas that have been dated by various 

radiometric methods. No identifiable indigenous fossils have been found in any of the volcanics and 

probably none will be found, as the rocks are mostly

welded tuff or lava and thus were unfavorable for 

the incorporation or preservation of fossils.

The volcanic rocks of the American Mine block 

rest unconformably on Concha Limestone and Scherrer Formation of Early Permian age and contain 

conglomerate with clasts of undoubted Concha Limestone; the volcanics in this block are therefore no 

older than Early Permian. Similar volcanic rocks 

in the UX Ranch block are overlain disconformably 

by the Bisbee Formation of Early Cretaceous age 

and therefore are no younger than Early Cretaqeous. 

No other evidence on the age of the volcanics based 

on paleontologically dated rocks is available. Rocks 

of both the American Mine and the Corral Canyon 

blocks are overlain unconformably by the trachyandesite of Meadow Valley, which has been dated 

radiometrically as Late Cretaceous (p. E21).

Volcanic rocks of the Thunder Mine block are 

intruded by a granite that is believed to be the same 

as a granite in Comoro Canyon on the west-central 

slope of the Patagonia Mountains. The granite of 

Comoro Canyon has been dated as Jurassic (160±20 

m.y.) by the lead-alpha method on zircon. Although 

the two granites are separated by a screen a mile 

or so wide of plutonic and metavolcanic rocks, they 

are correlated with considerable confidence on the 

basis of similar geologic relations and chemistry 

(table 1, cols. 14 and 15). It seems likely, therefore, 

that the volcanics are no younger than Jurassic. The 

rocks are lithologically similar to those of the Mount 

Wrightson Formation of Triassic age in the Santa 

Rita Mountains (Drewes, 1968, p. C6-C7), and, as 

noted previously, their considerable degree of deformation and alteration also suggests an early Mesozoic age.

The volcanic rocks of UX Ranch block, especially 

the lower 5,000 feet or so, and those of the Duquesne 

block are lithologically and chemically very similar 

to those of both the Mount Wrightson Formation 

and the Canelo Hills Volcanics. In particular, they 

resemble the middle member of the Mount Wrightson, which is dominantly rhyolitic and latitic lava, 

welded tuff, and breccia enclosing lenses of sandstone 

and quartzite, and a few of conglomerate.

The volcaniclastic sequence of the Corral Canyon 

block resembles both the Gardner Canyon Formation 

and the lower member of the Canelo Hills Volcanics 

in containing epiclastic volcanic conglomerate, red 

siltstone, sandstone and tuffaceous sedimentary 

rocks and silicic lava and tuff. Although no thin 

beds of limestone such as those in the Gardner Canyon Formation were found, some of the red siltstones 

have carbonate-rich layers, and other sedimentary
PATAGONIA MOUNTAINS AND ADJOINING AREAS Ell

TABLE 1. Comparison of chemical analyses, norms, and semiquantitative spectrographic analyses of Mesozoic volcanic rocks, 

Patagonia Mountains, with those of associated igneous and Nockolds' (1954) average rocks

[Leaders (---) in figure columns indicate no data]

Quartz latite

Patagonia 

Mountains

-------- 1

-------- 163091

Nock- olds' 

(1954) 

average

2 3 

. __ ... 163089

Alkali rhyolite and rhyolitic tuff

Patagonia Mountains

456 

163092 163090 163094

7 8 

165055 165053

Nock- olds' 

(1954) 

average

9 10 11 

165054 163093 ......_.

Canelo Hills 

Volcanics

12 

159243

13 

163553

Granite

Comoro 

Canyon

14 

163087

Flux 

Canyon

15 

160480

Chemical analyses (percent)

(Analysts: Nos. 1, 3, 4, 5, 6, 10, 14, 15, by P. L. D. Elmore, S. D. Botts, G. W. Chloe, Lowell Artis, and H. Smith; 7, 8, 9, 13, by P. L. D. 

Elmore, S. D. Botts, and Lowell Artis; 12, by P. L. D. Elmore, S. D. Botts, and G. W. Chloe)

SiO 2 .------------_.

AlzOs-.---.------.

FezOs.- _ -.----..

FeO---_--------_..

MgO --------.

CaO-------------_.

Na«O----------..

KjO. ---.----...

H^O .............

HjO+___ -. ..-.

TiOs.--. ..........

P^Os... ...........

MnO.------- ___ .

C0i._ __. .._.

.-----.-- 66.7

.----.--- 15.9

.------.- 2.8

.. ___ .. 1.3

.-------. .90

. ___ ... 1.3

. -..--- 3.0

6.1

.-------. .26..

.74

.---..-.. .67

..... __ .18

.-----.-. .05

.-.-----. <.05-_

70.15

14.41

1.68

1.55

.63

2.15

3.65

4.50

.68

.42

.12

.06

70.0

15.4

1.9

.62

1.1

.31

3.9

5.0

.12

.85

.53

.05

.12

<.05

71.4

14.8

1.8

.22

.50

.44

3.3

6.2

.15

.60

.39

.05

.05

<.05

72.4

14.0

2.7

.20

.10

.28

4.0

5.4

.08

.37

.41

.09

.06

<.05

74.0

13.9

2.1

.10

.15

.38

4.6

3.4

.12

.71

.42

.03

.01

<.05

75.4

12.8

1.4

.10

2

.47

2.8

5.2

.17

.63

.20

.00

.04

.22

75.6

12.5

1.2

.12

.1

.16

2.2

6.8

.19

.53

.17

.00

.02

<.05

75.8

12.4

1.1

.26

.1

.23

2.7

6.0

.20

.45

.18

.02

.03

.11

76.1

12.6

.79

.16

.52

.21

2.2

6.2

.36..

.74

.11

.00

.02

<.05-_

74.57

12.58

1.30

1.02

.11

.61

4.13

4.73

.66

.17

.07

.05

78.1

11.0

.75

.08

.2

.07

.92

7.4

.19

1.0

.15

0

.04

<.05

79.7

10.0

.96

.04

.48

.18

.13

5.6

.69

1.3

.17

.51

.09

<.05

73.6

13.4

1.0

.40

.15

.53

3.3

6.1

.19

.53

.22

.00

.04

<.05

- 75.9

12.8

1.0

.15

.07

.03

3.1

5.6

.28

.74

.20

.00

.02

<.05

Sum 100 100 100 100 100 100 100 100 100 99.9 100 100 100

Norms (percent)

Q .............

ab ............

C- ------------

wo

fS- ------------

il-.. ... . _ .-..

. - - -_ 22.3

---------- 36.0

---------- 25.4

---------- 5.3

---------- 2.4 -.

---------- 2.2

2 4

-.- ___ .- 1.1 -.

1.3

---------- .43

83.7

26.1 

26.7 

30.9 

9.5

.2

1.6 

.8 -

2.5

.8 

.3

83.7

26.0 

29.5 

33.0 

1.2 

3.1

2.7

.85--

1.3 

1.0 

.12

88.6

27.0 

36.6 

27.9 

1.9 

2.0

1.2

1.8 

.57 

.12 

.09 

91.5

28.0 

31.9 

33.8 

.8 

1.3

.25

2.7- 

.55 

.21 

.12 

93.7

33.3 

20.1 

38.9 

1.7 

2.0

.37

2.1 

.23 

.07..

.30 

92.3

38.8 

30.6 

23.6 

1.1 

2.1

.50

1.4 

.15

.16 

93.0

36.6 

40.1 

18.3 

1.1 

1.2

.20

1.3 

.15

.16_-

95.0

36.4 

35.6 

23.1 

.56 

1.2

.20

.23 

.96 

.46

95.1

38.4 

36.6 

18.6 

1.0 

1.9 ..

1.3

.26

.61 ..

.21

93.6

31.1 

27.8 

35.1 

2.0

.1 -

.3 

.6 .

1.9 -

.3 

.2 .

94.0

44.0 

43.7 

7.8 

.35..

1.4

.50

.75 

.26

.02 

95.5

56.8 

33.4 

1.1

3.7

1.2

.96 

.15 

1.3 ..

91.3

29.7 

36.0 

27.9 

2.6 

.41

.37

.78--

.46 

.42

93.7

36.3 

33.1 

26.2 

.15 

1.6

.17

1.0 

.36

.01 

95.6

Semiqnantitative spectrographic analyses (parts per million)

(Analysts: Nos. 1, 3, 4, 5, 6, 10, 14, by Barbara Tobin and J. C. Hamilton; 7, 8, 9, by W. B. Crandell; 12, N. M. Conklin; 15, by J. C. Hamilton. 0, looked for but not detected. Looked for but not found: Ag, As, Au, Bi, Cd, Co, Eu, Ge, Hf, Hg, In, Li, Mo, Ni, Pd, Pr, Pt, Re, Sb, 

Sm, Sn, Ta, Te, Tb, Tl, U, W, Zn.)

B_ ... _ .--..

Ba ----.

Be ---------..

Ce ...........

Cr. ___ ... __ .

Cu._ ...........

Ga. -------....

La ----------

Nb.. ...........

Nd-.---------.

Ph.. . __ ....

Sc ______ . _ .

Sr... __ .......

V _ __ ----..

Y._. _.__---. 

Yb.. -.-.-.-. ...

Zr.... ..........

... ... ------ 0.015 ..-.

.---.-.-- - .15 --..

. ____ ..... .0002 ....

.... __ _-.. 0 ---..

. __ ....... .0001 -.--

. ___ . __ . .0005 ---..

.-.---.--- .003 _ ..

.---.- ... .005 ....

....... __ . 0 __ .

----------- 0 .....

.-.---..--. .003 .-.-

.... __ . _ .0015 ....

----------- .03 .....

. _______ . .002 __ .

..... __ ... .005 ....

...... ____ .0005 ....

.------.---- .03 ....

--.. 0

.... .2

.. .0002

.... 0

.... .00015

.... .00015

.... .005

-.-- .005

. __ .0015

---. 0

 .. .005

.... .001

. _ .03

__ .0015

---. .005

 .0005

-.-. .03

0

.07

.0003

0

0

.00015

.003

.005

.0015

0

.005

.001

.015

0

.003

.0005

.015

0

.15

0

0

.0005

.015

.0015

.005

0

0

.003

.001

.015

.0015

.005

.0005

.03

0.005

.05

0

0

.00015

.007

.0015

.003

0

0

.0015

.0005

.007

0

.002

.0002

.02

0

.03

.0001

.01

0

.0001

.001

.007

.001

0

.001

.0005

.005

.0007

.003

.0003

.02

0.003

.03

.0002

.01

0

.0003

.001

.005

.001

0

.005

.0005

.007

.0005

.003

.0003

.02 '

0

.03

.0002

.01

0

.0002

.001

.007

.001

0

.0015

.0005

.007

.0003

.003

.0003

.02

0 .._-

.015 _ .

0 ....

0 --..

.0002 ....

.0005 ---.

.002 __ .

0 __..

0 ....

0 -._.

.002 ___.

.0007 ....

.01 ....

0 ....

.003 ....

.0005 ____

.015 ---.

.... 0 -..-

.--- .03 _...

--.. .0003 ---.

.._. 0 __ .

.. .0003 ....

..__ .0007 ....

.... .002 ....

.... 0 ....

.... .002 ....

.... .007 .....

-.-- .003 ....

.. .002 ..

.-.. .1 .. .

.... .02 .....

.... .005 ....

.--- .0005 _...

-.._ .03 ..__

.... 0

.... .05

.._. 0

.... .015

.... .0002

.__. .0005

---. .003

. .01

0

--.. .007

-_-- .002

--. .0007

..-- .01

..-- .0015

. _ .005

-.-. .0005

---. .015

0

.03

.0002

0

0

.0005

.002

.003

.002

0

.0015

.0005

.003

0

.005

.0005

.01

differentiation index.

SPECIMEN DESCRIPTIONS

1. Quartz latitic welded(?) tuff, 1,200 ft north of Kansas mine. Sparse fragments of sodic plagioclase and potassium feldspar in matrix of devitrified pumice and glass shards.

2. Average dellenite ( =quartz latite) plus dellenite-obsidian (Nockolds 1954, p. 1014, table 2, col. 2).

3. Alkali rhyolitic tuff(?), 1.5 miles west of UX Ranch. Sparse phenocrysts of sodic plagioclase, potassium feldspar, and quartz in hornfelsic matrix of quartz, potassium 

feldspar, chlorite, and muscovite.

4. Alkali rhyolite, 1,600 ft northwest of Kansas mine. Phenocrysts of altered antiperthitic sodic plagioclase in matrix of devitrified glass.

5. Alkali rhyolite, 1 mile south-southeast of UX Ranch. Abundant phenocrysts and crystal fragments of albite in groundmass of small densely packed spherulites.

6. Alkali rhyolitic welded tuff, readout 600 ft northwest of Kansas mine. Sparse grains of sodic plagioclase and quartz in matrix of devitrified glass shards.

7. Alkali rhyolite, 1 mile southeast of Duquesne. Sparse phenocrysts of sodic plagioclase in streaked-appearing groundmass of devitrified glass.

8. Alkali rhyolite, 1 mile southeast of Duquesne. Sparse phenocrysts of antiperthitic sodic plagioclase in matrix of coarsely devitrified glass enclosing lenses of coarsely 

granular aggregates of quartz and potassium feldspar.

9. Alkali rhyolite, 0.9 mile southeast of Duquesne. Sparse phenocrysts of perthite in streaked-appearing matrix of devitrified glass.

10. Alkali rhyolite, 1,800 ft northwest of Kansas mine. Almost entirely devitrified glass enclosing streaks of granular quartz. Traces of sericite and tourmaline.

11. Average alkali rhyolite plus rhyohte-obsidian (Nockolds, 1954, p. 1012, table 1, col. 4).

12. Alkali rhyolitic welded tuff, Canelo Hills Volcanics. Biotite from this rock has been dated by the potassium-argon method at 173 + 7 m.y. (Hayes and others, 1965, 

p. M6-M7).

13. Alkali rhyolite, Canelo Hills Volcanics (P. T. Hayes, written commun., 1969).

14. Granite of Comoro Canyon, west-central Patagonia Mountains, Nogales quadrangle. Zircon from this rock has been dated by the lead-alpha method at 160±20 

m.y. (T. W. Stern, written commun., 1965).

15. Granite of Flux Canyon, northwest flank of Patagonia Mountains, Nogales quadrangle.
E12 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

TABLE 2. Average chemical analysis of rhyolites and granites, 

Patagonia Mountains

[Results in percent]

SiO 2 -----------------.

A1 2O 3 ----------------.

Fe 2O 3 ---------------.

FeO____ _____ _.__...

MgO---_-. _.._.......

CaO--__. ---___ ..__._.

Na tO_. ---------------

K,0.._...._..__......

HtO ________________

H,0+. ._....._.......

TiO ,..........._......

Average of 8 

alkali rhyolites

.-_---_-_-_ 73.8

 __- ___ 13.5

.-----._-.-_ 1.6

.-----....-_ .22

.__.._ .__ .34

..----..-.-- .31

._____- _ 3.2

..--.-_-..._ 5.5

..._.. .._ .17

..-_----__ .61

._--_.--_.._ .30

Average of 2 

alkali granites

74.7

13.1

1.0

.27

.11

.28

3.2

5.8

.23

.63

.21

Sum, not including very small 

amounts of P 2O 5, MnO, and 

COt-_-~ - - -- 99.55 99.53

or tuffaceous sedimentary rocks contain abundant 

irregularly dispersed carbonate, some of which may 

be redistributed primary material. As stated previously (p. E4), the lower members of the Canelo Hills 

Volcanics was correlated by Hayes (197Qa, p. A8- 

A9) with the Gardner Canyon Formation.

In summary, then, the silicic volcanic rocks in the 

American Mine, Corral Canyon, Duquesne, Thunder 

Mine, and UX Ranch blocks of the southern Patagonia Mountains constitute sequences that probably 

are correlative in part with the Mount Wrightson 

and Gardner Canyon Formations (Triassic) of the 

Santa Rita Mountains and the Canelo Hills Volcanics 

(Late Triassic and Early Jurassic) of the Canelo 

Hills. Some of the silicic volcanics may be younger 

Late Jurassic or early Early Cretaceous but none is 

known to be younger than the Bisbee Formation of 

late Early Cretaceous age.

VOLCANIC AND SEDIMENTARY ROCKS IN THE. 

FLUX MINE AREA

A small north-northwesterly trending body of volcanic and sedimentary rocks probably of Triassic 

or Jurassic age is centered roughly on the Flux mine 

(fig. 2). The outcrop area is about 2.5 miles long and 

less than 2,000 feet wide. To the west these rocks 

are in contact along an inferred extension of the 

Harshaw Creek fault with rocks of the Thunder Mine 

block. To the east they are probably overlain unconformably by volcanic rocks of presumed Cretaceous 

age; the nature and location of this contact, however, is very uncertain. In most places the attitude 

of the rocks is unclear, but in the northern part of 

the outcrop area, sedimentary beds dip moderately

west; consideration of regional geologic relations 

suggests that these beds are overturned.

The rocks are predominantly light-colored silicic 

tuff breccia and tuff, welded tuff, and minor lava 

flows. Sedimentary rocks, which are mainly in the 

western (lower?) part of the section, consist of 

coarse- to fine-grained quartzite, sandstone, arkose 

and shale. A large block of Paleozoic limestone at 

the Flux mine, and other large blocks or accumulations of blocks of fossiliferous Paleozoic limestone 

south of the mine, probably were emplaced in the 

volcanic sequence by landsliding.

Rock along much of Flux Canyon south of the 

Flux mine is a green silicified tuff breccia composed 

of angular to rounded clasts of volcanic rocks, and 

a few of limestone, in a dense tuffaceous or gritty 

matrix. Breccia fragments are as large as 3 feet 

across, but most are smaller than 2 inches. Volcanic 

clasts are gray, purple, or red fine-grained to porphyritic lavas which show conspicuous flow layering. 

Interlayered with the breccia is some dark-greenishgray fine-grained porphyritic latite(?). All the phenocrystic minerals plagioclase, biotite, and pyroxene (?) are much altered, and the glassy flow-layered groundmass is completely devitrified.

Tuff breccias elsewhere in the Flux mine area 

are finer grained but have abundant rock fragments 

(mainly silicic volcanics) and variable amounts of 

feldspar and quartz grains in a devitrified matrix 

of glass shards that has a well-preserved vitroclastic 

texture. A welded tuff breccia 1,200 feet north of 

the Chief mine has conspicuous secondary flowage 

structures that resemble those of silicic lavas; abundant phenocrysts of potassium feldspar, and a few 

of quartz and biotite, are embedded in a devitrified 

matrix of compressed and bent shards and pumice 

fragments.

Gray, green, and brown shale and some sandstone 

exposed northwest of the Flux mine are the westernmost rocks of the sequence. Thickness ranges from 

a few feet to somewhat less than 200 feet. These 

beds are much faulted and folded, and the rocks 

weather to small chips. A typical sandstone is made 

up of about 40 percent angular feldspar grains 0.1- 

0.3 mm across, 10 percent quartz, 10 percent veinlets and patches of epidote and carbonate, and 40 

percent fine-grained micaceous matrix.

The limestone block at the Flux mine is a lens 

about 1,000 feet long and as much as 75 feet thick 

(Simons and others, 1966, p. D18-D19, fig. 4#). The 

rock is a thoroughly brecciated and healed obscurely 

bedded gray to dark-gray somewhat cherty limestone 

with subordinate light-gray dolomitic limestone. The
PATAGONIA MOUNTAINS AND ADJOINING AREAS E13

lens dips steeply west and is overlain by several tens 

of feet of grit and sandstone and underlain by a 

highly altered green diabasic rock and pebbly volcanic sandstone containing sparse fragments of limestone. To the south the limestone block passes into 

a thin lens of limestone conglomerate. The rocks to 

the west are made up of coarse arkose, fine-grained 

sericitic sandstone, and quartzite, all replaced by 

variable amounts of epidote and tourmaline. No fossils were found in the limestone, so its age is not 

definitely known but is surely Paleozoic; possibly 

the limestone is Concha or Colina Limestone (Permian) or, less likely, Escabrosa Limestone (Mississippian).

-Another body of limestone, a lens-shaped jumble 

of large blocks rather than a single block, is 1,000 

feet northwest of the Chief mine._It4§JxQQ-600 feet 

long and as much as 100 feet wide, andrtTconsists 

of fossiliferous limestone and silty and sandy limestone, either Horquilla Limestone (Pennsylvanian) 

or Earp Formation (Pennsylvanian and Permian). 

Another lens of fossiliferous cherty limestone 700 

feet farther northwest is about 300 feet long and is 

probably Horquilla Limestone. These limestone bodies are entirely enclosed in volcanic rocks containing 

ai little interbedded sandstone and shale.

CRETACEOUS ROCKS

Cretaceous sedimentary rocks were first recognized among Mesozoic rocks in the Patagonia Mountains by Schrader ^(i 915, p. 53-54) during a reconnaissance study of the^Santa Rita and JPatagpnia 

Mountains. Fossils found near the Mowry mine were 

assigned by T. W. Stanton (in Schrader, 1915, p. 

53) to the Comanche Series of the Cretaceous. 

Schrader recognized the possibility that some of the 

rocks he referred to the Mesozoic might be of preCretaceous age, as has since proved to be true.

Apparently no further work on these rocks was 

published until Stoyanow (1937) described briefly 

a Cretaceous sequence in the Patagonia Mountains 

and named the Patagonia Group, the upper part of 

which he designated the Molly Gibson Formation. 

The rocks and their ammonite fauna were described 

in more detail by Stoyanow in 1949 (p. 30-31, 40- 

41, 53, 128-129). He believed that the Molly Gibson 

is of latest Early Cretaceous (latest Washita) age 

and (1949, p. 31, 41) that the Patagonia Group 

might be equivalent to the entire Fredericksburg 

and Washita Groups (late Early Cretaceous). The 

Washita Group is now considered to be of Early 

and Late Cretaceous age (Imlay, 1944).

Stoyanow's descriptions of the Patagonia Group

and Molly Gibson Formation and his age assignments are unsatisfactory because (1 }^the Jjthology 

as given in his 1949 paper (p. 30-31) differs grossly 

from that determined both by Baker (1961) and 

by me; (2) the type localities are only vaguely 

identified, and the name Molly Gibson does not appear on any published map of the region; and (3) 

the ammonites on which the age assignment is 

based are poorly preserved, no sutures are shown 

by any of the specimens (Stoyanow, 1949, p. 128), 

and no other diagnostic fossils are reported. For 

these reasons, Stoyanow's rock-stratigraphic names 

are not used in this report.

BISBEE FORMATION

Siltstone and mudstone with intercalation^ of limestone, sandstone, epiclastic volcanic sandstone and 

siltstone, and conglomerate in a triangular area of 

about 3 square miles in the northwest half of the 

UX Ranch block (fig. 2) were correlated by Hayes 

(1970b) with the Bisbee Formation of late Early 

Cretaceous age. These beds rest disconformably on 

silicic volcanic rocks which probably are of Triassic 

or Jurassic age. The total thickness of the sequence 

is not known inasmuch as the top is not present, 

outcrops are generally poor, and structural data are 

hard to obtain. A partial section nearly 1,400 feet 

thick has been measured, and a thickness of more 

than 3,000 feet is likely. The fine-grained rocks originally were probably brown, gray, or red but are 

now largely dark gray to black owing to contact 

metamorphism by Tertiary granodiorite.

The base of the Bisbee Formation is placed at 

the bottom of a bed of light-gray to gray or pinkishgray conglomerate that ranges in thickness from a 

few tens of feet to 300 feet or more. Just west of 

the UX Ranch the conglomerate rests on a conspicuous bed of grayish-red fine-grained silicic tuff and 

tuffaceous siltstone. The conglomerate is made up 

of subangular to rounded clasts as much as a foot 

across of silicic volcanic rocks in a sand or grit 

matrix. Most of the clasts are less than 2 inches 

across. In places the rock has a very sparse matrix 

and is a closely packed aggregate of pebbles and 

cobbles. Bedding is usually made evident by thin 

layers or pods of gray sandstone or purple to red 

sandy siltstone. The most abundant clasts are of 

light-gray to gray porphyritic rhyolite containing 

phenocrysts of quartz or feldspar or both minerals.

Thabasal conglomerate is overlain by dominantly 

fine-grained sedimentary rocks that are gray to 

grayish red in the eastern part of the outcrop area 

but grade to dark gray to nearly black in the western
E14 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

part, where they are intruded by granodiorite. Interlayered with these siltstones or mudstones are some 

gray, greenish-gray, or purplish-gray poorly sorted 

sandy or gritty beds. All these rocks commonly contain abundant epiclastic volcanic material, mainly 

fine-grained to devitrified glassy rocks. Except for 

the lower 100 feet or so, the rocks are poorly exposed, and the thickness is uncertain but is estimated 

to be 600-700 feet. The basal part of the sequence 

is well exposed just west of the UX Ranch, and a 

generalized section of this part is shown in figure 4. 

Northwest of the UX Ranch this sequence is overlain by a lenticular bed of gray epiclastic volcanic 

conglomerate possibly as much as 100 feet thick. 

This conglomerate resembles the basal conglomerate 

in consisting of subrounded to rounded pebbles, cobbles, and boulders as much as a foot in diameter of 

silicic volcanic rocks in a coarse arkose matrix. 

Porphyritic rhyolite is the most abundant constituent; but fine-grained silicic tuff and lava are also 

common, and quartzite occurs sparingly.

NNW SSE

A .-- B -C. D

200 FEET

FIGURE 4. Partial section of sedimentary and epiclastic volcanic rocks in the lower part of the Bisbee Formation west 

of the UX Ranch. Vertical scale equals horizontal scale. 

From youngest to oldest:

A, Gray pebble conglomerate.

B, Interbedded gray or light-purplish-gray sandstone and purplish-gray 

siltstone. Sandstone consists of angular to subrounded millimetersized grains of quartz, albite, and potassium feldspar, and a few 

of biotite, and larger grains of fine-grained to glassy volcanic rock, 

all set in a silty matrix. Siltstone is similar mineralogically but 

apparently contains much more volcanic glass debris.

C, Purplish-gray to grayish-red siltstone.

D, Gray, purplish-gray, or greenish-gray sandstone and grit with interbeds of dark-gray to purplish-gray siltstone and gray pebble conglomerate.

E, Gray pebble to cobble conglomerate.

The conglomerate is overlain by 800-900 feet .of 

very poorly exposed beds that seem to be mainly 

dark-gray to black fine-grained clastic rocks with 

minor light-gray to gray sandstone and arkose. At 

the top of this sequence is another bed of gray conglomerate as much as 20 feet thick. This conglomerate contains angular to rounded pebbles mostly 

less than one-half inch across of light-gray to darkgray dense fine-grained rocks of uncertain origin 

in a matrix of quartz sand and mud. Toward the 

east side of the outcrop area, the conglomerate 

grades to a light-brown coarse quartzite.

The only partial section of the Bisbee Formation 

suitable for measurement starts stratigraphically 

near probably just above this conglomerate. Inasmuch as this section contains rock types representative of the whole formation, a detailed description 

is given. Because of structural complications and 

lack of outcrops, it was not possible to measure and 

include beds higher in the formation, but it is doubtful that any great additional thickness of beds is 

present.

Measured partial section of Bisbee Formation

[Section begins near junction of the Patagonia-Washington Camp road and 

the old road to the Morning Glory mine; it ends about 4,000 ft N. 65° W. 

of the Mowry mine. Location of section is shown in fig. 2]

Thickness

Bisbee Formation: (feet)

1. Sandstone, tuffaceous, gray to dark-gray, fine 

grained, thick bedded, pyritic; weathers brown, 

reddish brown, or light gray. Blocky outcrops. 

Bed 6 in. thick of highly fossiliferous gray 

limestone 12 ft above base......_ __ ___... 20

2. No outcrops; canyon and old road to Morning

Glory mine . _.. _.___ . _ ____.__ 48

3. Sandstone; like unit 1. Sparse thin beds of gray

mudstone in lower part.._........___.._.._...._ 25

4. No outcrops __ ...___._____.________ 35

5. Mudstone, dark-gray, hard, pyritic; weathers 

greenish gray to brown. Breaks down to very 

small rounded flakes __~_-_ _._ 11

6. Sandstone, white, light-gray, some brown; weathers to conspicuous mottled white and dark 

brown. Thick bedded (5 ft) with sparse silty 

partings. Breaks down to fine rubble. Bed 6 

in. thick of fossiliferous dark-gray limestone 

20 ft above base _________________ 51

7. Sandstone, limy, dark-gray; weathers grayish

brown ... . _.._.. _ _ _ 1

8. Mainly brown, light-gray, or white epiclastic 

volcanic sandstone; weathers brown. Poorly 

exposed -_ -_ ......_.._ . 22

9. Sandstone, probably tuffaceous, light-gray to 

gray, fine-grained and equigranular, pyritic; 

weathers brown. Beds 1-2 ft thick, distinct. 

Highly jointed, breaks down to fine angular 

rubble; fragments typically have gray core 

enclosed in thin alternating white and brown 

rinds -_.__.__________--_ _ 32

10. Mudstone hornfels, dark-gray, pyritic, some 

slightly calcitic; weathers brown, reddish 

brown, or yellowish brown. Some very hard 

dense beds as much as 18 in. thick. Highly 

jointed, blocky outcrops break down to angular chips. Bed 1 ft thick of dark-gray fossiliferous limestone 43 ft above base . 91

11. Limestone, dark-gray, massive; weathers gray. 

Irregular patches of silicic material stand out 

on weathered surfaces. Practically a coquina.. 5

12. Mudstone, brown, purple, greenish-gray, poorly 

exposed. Some flaky mudstone, one bed 1 ft 

thick of crossbedded greenish-gray sandy 

limestone, and several thin beds of shelly 

limestone. Becomes hard dark-gray to black 

hornfels toward top. At top, 3 ft of hard limy 

and pyritic dark-gray siltstone hornfels 50

13. Limestone, sandy, gray, hard; weathers cavernous. Irregular patches of pink siliceous limestone . ..___-._ 7
PATAGONIA MOUNTAINS AND ADJOINING AREAS E15

Bisbee Formation Continued

Thickness 

(feet)

Limestone unit 13 traced north to gully where section continues.

14. Mostly brown or purple soft siltstone and shale.

Poorly exposed __ _________ ______ _ _ ._-__________ 69

15. Limestone, light-gray, coarse-grained, thinbedded. Interlayered light-brown limy sandstone that weathers light gray to tan. Lenticular; maximum thickness given. Abundant 

pelecypod fragments 8

16. Sandstone, light-brown, soft, impure; weathers 

light brown. Upper part is very limy sandstone that weathers lavender... . ________ _ 8

17. Siltstone, light-gray; becomes shaly toward top.... 43

18. Limestone, light-gray, coarse-grained, recrystallized; weathers gray. Abundant poorly preserved pelecypods ~ _ 4

19. Siltstone, slightly limy, light-gray, hard, distinctly bedded, pyritic _______________________ 43

Covered area along gully; assumed to be along 

strike.

20. Siltstone; same as unit 19. Bed 10 ft thick* of

purple mudstone 140 ft above base _ _. 205

21. Sandstone, arkosic, light-brown; weathers rusty 14

22. Siltstone, limy, gray, hard, pyritic; weathers

rusty _.____.._._..____.__________ __ 11

23. Siltstone, light-gray. Abundant irregular pods 

of gray slightly silty limestone that weathers 

very light gray ______________-___ _________ __ _ 15

24. Siltstone, light-gray, pyritic; weathers rusty. 

Obscure bedding, highly fractured. Sparse 

thin lenses of gray limestone that weathers 

light gray ________ __________ _ _ ___ 55

25. Limestone, light-gray, fine-grained ____._ ________ 1

26. Siltstone, light-gray, massive, pyritic, weathers

rusty. Poorly exposed 59

27. Limestone conglomerate; pebbles of silty limestone mostly less than 1 in. across 2

28. Poorly exposed. Abundant chips of limestone and

rusty-colored siltstone _______ 8

29. Limestone, silty, purple, fine-grained, lenticular; 

weathers mottled purple and gray; breaks 

down to small angular fragments _ ______.___ 2

30. Siltstone, light-gray, very poorly exposed; breaks 

down to abundant rusty-weathering chips. Bed

2 ft thick of coarse sandstone 45 ft above 

base, overlain by brown-weathering obscurely 

bedded siltstone that contains thin beds of 

gray-weathering limy siltstone. Bed 5 ft thick 

of purple limy siltstone 95 ft above base. Bed

3 ft thick of gray arkose 105 ft above base. 

Bed 5 ft thick of light-greenish-gray silty 

limestone 120 ft above base 148

31. Siltstone, limy, and shale, both purple and much 

fractured. Interlayered light-purplish-gray 

limy sandstone and purple silty limestone. 

Unit weathers mottled purplish red and white 

or gray. Some beds of hard fine-grained lightbrownish-gray sandstone or siltstone with 

films of epidote along fracture surfaces 45

Bisbee Formation Continued

Thickness 

(feet)

32. Light-gray fine-grained tuffaceous(?) rock that 

weathers variegated rusty color. Poorly exposed. Few thin beds of light-gray arkose that 

weathers brownish gray- _.___ 80

33. Siltstone, limy, and shale; like unit 31. Thin lenticular beds of purple fine-grained silty limestone with abundant small angular fragments 

of siltstone that stand out in relief on weathered surfaces _______ 55

34. Limestone, silty, brown; weathers red and rusty 

brown. Some light-gray slightly sandy lime, 

stone that weathers brownish gray _________.. 11

35. Siltstone, limy; like unit 31. Cut by calcite seams 79

Total measured partial section 

of Bisbee Formation .1,363

Fault contact.

Bolsa Quartzite (Cambrian).

In the Patagonia Mountains the Bisbee Formation 

appears to contain considerably more volcanic material than it does elsewhere. Numerous rocks were 

identified in the field as having an appreciable volcanic component, or even as being volcanic ash, and 

more than half of the clastic rocks examined in thin 

section were feldspathic and had abundant finegrained matrix material resembling devitrified volcanic glass. However, few of these volcaniclastic 

rocks are believed to be of primary volcanic origin; 

rather, they are interpreted on the basis of texture 

and mineral composition as epiclastic and as derived 

from a predominantly volcanic terrane.

Very few limestone beds of the Bisbee are more 

than 10 feet thick, and most are traceable for only 

a few tens of feet. One bed, however, was traced 

with fair confidence for 3,000 feet. Commonly the 

limestone is dark gray and fine grained and contains 

abundant fragmental fossils; locally it has wavy thin 

silty or sandy layers.

In the southwest part of the outcrop area the Bisbee has been metamorphosed to hornfels by Tertiary 

granodiorite. A common feature of hornfelsed siltstone or mudstone is the presence of geodelike bodies 

as much as 8 inches in diameter that consist of an 

outer shell of light-green bleached-appearing hornfels, an intermediate shell of epidote and gypsum (?), 

and a tiny core of pyrite. The core and intermediate 

shell are frequently eroded, and only a green-rimmed 

depression is left on outcrop surfaces. These green 

spheroids are most abundant near the contact with 

granodiorite and become less abundant and finally 

disappear with distance from that contact.
E16 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

FOSSILS AND AGE

The limestone beds of the Bisbee Formation, and 

some of the siltstones as well, contain abundant fossils, but most of the fossils are highly fragmented 

and poorly preserved. No fossils clearly diagnostic 

of a more closely denned age than late Early Cretaceous have been reported previously, and none 

were found during this study.

Schrader (1915, p. 53) reported that fossils found 

"about half a mile northwest of Mowry" were identified by T. W. Stanton as Ostrea sp., a fragment of 

an ammonite probably belonging to the genus Acanthoceras, and fragments of bivalves, some of which 

appeared to belong to the family Chamidae. This 

fauna was assigned by Stanton to the Comanche 

Series, and the rocks were correlated by Schrader 

with the Bisbee Group of the Bisbee quadrangle, 

some 40 miles east.

Stoyanow (1949, p. 128-129) described three new 

species of ammonite from the upper part of the 

Molly Gibson Formation and placed them in the 

genus Stoliczkaia. On the basis of these fossils he 

correlated the Molly Gibson with the Del Rio Clay 

of Texas, which he considered to be of late Early 

Cretaceous age. However, as previously noted (p. 

E13), the fossils are poorly preserved and show no 

sutures, so their identity is uncertain.

A small collection (USGS colln. 29206) of pelecypods and gastropods from thin limestone beds near 

the junction of the Patagonia-Washington Camp 

road and the old road to the Morning Glory mine 

(SW^ sec. 16 (unsurveyed), T. 23 S., R. 16 E., 

Lochiel quadrangle) was reported by N. F. Sohl 

and R. W. Imlay to contain Arctical sp., Gryphaeal 

sp., Trigonia cf. T. mearnsi Stoyanow, and Turritella sp. They stated (written commun., Sept. 13, 

1965) :

The above-listed fossil assemblage cannot be definitively 

dated. The Trigonia is too poorly preserved to be positively 

identified but has coarsely noded ribs similar to T.meamsi 

described by Stoyanow (1949, p. 78-79) from the Perilla Member of the Lowell Formation [of Stoyanow; in Bisbee Groujv 

of late Early Cretaceous age]. The most abundant fossils in 

the collection are high^sp&edrgastrppflds^of the genus 2Vfr&- 

tella. All are rather smatt=an3^itherjnay represent a new 

species or may be immature forms of T, seriatimgranulata, 

a long-ranging species in Texas. At least they belong to this 

species group. Although Stoyanow figured no gastropods, he 

did mention that gastropods of this species group are numerous in the basal and topmost parts of the Lowell Formation. 

These remarks are meant to indicate only the possibility that 

your collections are pre-Washita, and better preserved material will be needed before we can do more.

Another collection (USGS Mesozoic loc. D3773) 

from dark-gray fine-grained limestone 3,400 feet

south of American Peak (NW^4 sec. 16 (unsurveyed) , T. 23 S., R. 16 E., Lochiel quadrangle) contained only SerptUal sp., on which W. A. Cobban 

commented (written commun., June 19,1962): "The 

fossils seem to be calcareous worm tubes, but the 

preservation is not good. They indicate a marine 

environment but tell nothing of the age." These 

sparse faunal data indicate only a late Early Cretaceous age for the Bisbee Formation in the Patagonia Mountains. It seems likely, however, that the 

rocks are approximately equivalent to the Mural 

Limestone of the Bisbee Group in the Mule and 

Huachuca Mountains to the east and that they are 

of late Trinity to Fredericksburg age (Hayes, 1970a, 

p. A25).

VOLCANIC ROCKS OF LOWER ALUM GULCH

A sequence of dominantly light-colored silicic tuffs 

and lavas is exposed along lower Alum Gulch in the 

northwest corner of the Lochiel quadrangle and the 

northeast corner of the Nogales quadrangle (fig. 2). 

These rocks underlie an area about 2.5 miles long 

and as much as three-quarters of a mile wide. They 

probably overlie unconformably the volcanic and 

sedimentary rocks of the Flux mine area and are 

overlain unconformably or disconformably by the 

trachyandesite of Meadow Valley. The thickness of 

the Alum Gulch sequence is probably about 1,500 

feet at most. Although the age is uncertain, the 

rocks are approximately parallel to the trachyandesite of Meadow Valley, of Late Cretaceous age, 

whereas they rest with angular unconformity on 

rocks of probaBte Triassie^or^Jurassje^ger^hey are 

therefore thoughtjxxJbe ^retaceoiis^rathei^ahjolder.

The rocks in theTower^partoftn^ieqSence are 

somewhat altered but retain enough original features to be identifiable as fragmental volcanic rocks 

and as volcanic conglomerate. Some rocks in the 

upper part are also little altered, but others, especially toward the northwest end of the outcrop area, 

are so thoroughly silicified, sericitized, or otherwise 

altered that their identity is doubtful. 

^ The; lowest rocks, between the Flux and Chief 

mtnes,^are light gray, greenish-gray, or brownishgray silicic ^ttff breccia, tuff, welded tuff, volcanic 

conglomerate, and minor lava. The tuffs consist of 

phenocrysts 0.5-4 mm long of potassium feldspar, 

sodic plagioclase, quartz, and biotite and of abundant volcanic fragments, all enclosed in a finegrained base of mineral fragments and devitrified, 

locally spherulitic, glass shards and pumice. Vitroclastic textures are well preserved, and alteration 

to sericite, carbonate, epidote, and chlorite is slight. 

Lava interlayered with the volcaniclastic rocks is a
PATAGONIA MOUNTAINS AND ADJOINING AREAS E17

greenish-gray porphyry composed of phenocrysts 

1-3 mm long of pink potassium feldspar, albite, and 

chloritized biotite in a groundmass of devitrified 

and spherulitic glass that shows conspicuous flowage 

structure. This rock makes up at least one lava flow, 

and it also forms several small irregular intrusive 

bodies in the underlying older volcanic sequence. 

Volcanic conglomerate consists entirely of angular 

to rounded clasts as much as a foot across of silicic 

volcanic rocks in a generally sparse matrix.

The middle part of the Alum Gulch sequence is a 

lens-shaped mass of light-gray fine-grained porphyritic biotite latite (?) with conspicuous contorted 

flow layering. It forms very prominent rounded outcrops along Alum Gulch near the Worlds Fair mine. 

The latite body is about 1.5 miles long and as much 

as 1,000 feet thick and is interpreted as an extrusive 

lava dome. It consists of sparse small phenocrysts 

of sericitized plagioclase, now albite, and biotite 

completely altered to sericite and iron oxide, in a 

very dusty fine-grained groundmass of feldspar laths 

and devitrified glass.

The upper part of the sequence is made up of 

light-colored locally well-bedded tuff and less abundant silicic lava and flow breccia. These rocks consist mainly of devitrified glass with only sparse 

phenocrysts of feldspar. At one place where the upper part of the sequence directly overlies rocks of 

the Flux mine sequence, the basal rock is a coarse 

breccia that consists almost entirely of blocks as 

much as 5 feet across of pink porphyritic aplite and 

granite probably derived from Jurassic granitic 

rocks. In lower Alum Gulch the rocks are massive 

and sparingly porphyritic, and they have been 

thoroughly silicified, sericitized, epidotized, and pyritized. Many are finely brecciated. Most appear to 

be tuffs that consist largely of glass fragments devitrified to a very fine grained aggregate of feldspar 

and quartz. Some may be lavas but none was definitely identified as such.

The uppermost rocks, immediately underlying the 

trachyandesite of Meadow Valley, are white to lightgray vitric tuffs and subordinate lithic tuff.

VOLCANIC ROCKS OF DOVE CANYON

A heterogeneous sequence of dominantly volcanic 

and epiclastic volcanic rocks is exposed southwest 

of the Canelo Hills in upper Dove Canyon and in an 

unnamed canyon to the south (fig. 2). These rocks 

were referred to by Hayes and Raup (1968) as volcanic and sedimentary rocks of Jones Mesa but were 

not described. Hayes (1970b, p. B20) discussed the 

sequence briefly. The volcanics of Dove Canyon do 

not closely resemble any others in the report area,

but they are about the same age, Late Cretaceous, 

as the trachyandesite of Meadow Valley and the 

Salero Formation of the Santa Rita Mountains to 

the north (Drewes, 1968, p. C11-C12).

The volcanics of Dove Canyon are exposed mainly 

along gully bottoms and have a very small outcrop 

area. Neither the base nor the top of the sequence 

is exposed, nor do exposures permit measurement 

of any continuous partial section. However, a thickness of at least 2,000 feet is likely. The Dove Canyon 

rocks ^re in fault contact with the Canelo Hills Volcanics. No other Mesozoic rocks are found nearby.

The volcanics of Dove Canyon comprise a varied 

group of volcanic conglomerate and mudflow, tuff 

and tuffaceous sandstone, andesitic lava flows, and 

agglomerate.

Epiclastic volcanic conglomerate is the most abundant rock type. It is made up largely or entirely of 

rocks other than those now exposed in the Canelo 

Hills; in particular, neither the widespread welded 

tuff unit of the Canelo Hills Volcanics nor its associated and easily recognized intrusive rhyolite porphyry is represented. Volcanic rocks are by far the 

most abundant components of the conglomerate, and 

among them andesitic rocks greatly predominate 

over more silicic types, and lavas predominate 

over tuffs. Clasts of sedimentary rocks sandstone, 

quartzite, limestone, and dolomitic limestone are 

sparse. Conglomerate boulders are rounded to subangular and are as much as 3 feet across. The conglomerate matrix is very heterogeneous and includes 

much volcanic glass and numerous lithic fragments 

together with feldspar, pyroxene, amphibole, biotite, 

quartz, and iron ore. Individual grains are of sand 

size or granule size and are somewhat rounded. 

Locally, pebbly tuff and tuffaceous sandstone are 

interbedded in the conglomerate.

Tuff and tuffaceous sandstone are second in abundance to volcanic conglomerate. The tuff varies considerably in appearance from place to place but 

commonly is a gray to pink or pale-red crystal-lithic 

variety, perhaps latitic or dacitic. Well-bedded waterlaid tuff is abundant. Most of the lithic components 

are silicic volcanic rocks, largely tuff and welded 

tuff, but some are white or gray quartzite.

A conspicuous north-trending bed of light-gray 

to buff coarse biotite latitic tuff crops out intermittently for about a mile north of Paloma Spring. The 

tuff dips 25°-35° W. and ranges in thickness from 

perhaps 125 feet at its northernmost exposure to 

at least 500 feet near Paloma Spring. It is separated 

from the underlying volcanic conglomerate by 5-30 

feet of green or greenish-gray tuffaceous sandstone.
E18 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

East of Paloma Spring the lower part of the tuff 

contains abundant distinctive fragments of orange 

pumice. A modal analysis gives plagioclase (An30 ), 

28 percent; biotite, 5 percent; pumice fragments, 

8 percent; accessory minerals (quartz, potassium 

feldspar, sphene, and iron ore), 2 percent; and matrix, 57 percent. The matrix consists of moderately 

welded and completely devitrified glass shards.

Lava flows are uncommon and make up only a 

few small outcrops. Those examined microscopically 

are gray to grayish-red augite or augite-hypersthene 

andesites.

Agglomerate was noted in only one place. It consists of completely unsorted partly rounded blocks 

of purple to grayish-green fine-grained to porphyritic andesite in a sparse matrix of greenish-gray 

glassy tuff. Most of the blocks are less than 3 feet 

across, but some are as much as 12 feet across.

Biotite from partly welded biotite latitic tuff at 

Paloma Spring has been dated radiometrically by 

the potassium-argon method at 72.0 m.y., or Late 

Cretaceous (S. C. Creasey, written communs., 1963, 

1964). This age is the same as the potassium-argon 

age of biotite from welded tuff in the trachyandesite 

of Meadow Valley at Vaca Ranch, 7 miles to the 

northwest. Moreover, a tuff very similar lithologically to the dated rock from Dove Canyon underlies 

trachyandesite south of Vaca Ranch. The volcanics 

of Dove Canyon and the trachyandesite of Meadow 

Valley may thus be the lower and upper parts, respectively, of a single thick volcanic sequence.

TRACHYANDESITE OF MEADOW VALLEY

A belt of trachyandesite 1 and associated rocks 

several miles wide extends from the northeast part 

of the Patagonia Mountains eastward across Meadow Valley to the Canelo Hills, a distance of about 

11 miles (fig. 2). These rocks are named informally 

for their extensive exposures in Meadow Valley. 

Most of the rocks are not very resistant to erosion, 

but some thick massive flows erode to steep slopes 

that have a local relief of 500-600 feet.

The trachyandesite ranges in thickness from a 

few feet at the west end of the outcrop area to possibly as much as 3,000 feet north and west of Harshaw. No section was found that would permit accurate measurement of the maximum thickness.

The trachyandesite rests unconformably on volcanic and sedimentary rocks of Cretaceous age and 

on silicic volcanic rocks of probable Triassic or 

Jurassic age. It is overlain unconformably by rhyolitic volcanic rocks of Cretaceous or Tertiary age, 

silicic tuff of middle Tertiary age (not shown on

map), and alluvium of Tertiary and Quaternary age. 

About halfway between the Patagonia Mountains 

and the Canelo Hills, the trachyandesite is intruded 

by a stock of porphyritic monzonite of similar mineralogical composition. Both trachyandesite and 

monzonite are cut by numerous rhyolite necks and 

dikes; the conspicuous peak of Saddle Mountain is 

one of these necks.

The dominant rocks of the trachyandesite of Meadow Valley are gray to grayish-purple porphyritic 

lavas. Individual flows are a few tens of feet thick 

in the eastern and northern parts of the outcrop 

area but may attain thicknesses of 300-400 feet or 

more in the southwestern part in the Harshaw area. 

Tuff, tuffaceous standstone, welded tuff, rhyolite (?) 

and shale are minor components. For purposes of 

description, the trachyandesite is divided into three 

lithologic groups: tuff and shale of the Vaca RanchLittle Outfit Ranch area, thin lava flows in and west 

of Meadow Valley, and thick lava flows of the Harshaw-Trench Camp area.

TUFF AND SHALE

Between the Vaca Ranch and the Little Outfit 

Ranch the lowest part of the trachyandesite unit is 

a sequence of chloritic tuff, tuff, tuffaceous shale, 

shale, siltstone, and volcanic conglomerate at least 

1,000 feet thick. Outcrops of these rocks are limited 

largely to gully floors, and therefore the stratigraphy 

and structural relations of the rocks are not entirely 

clear.

In the Little Outfit Ranch area the sequence appears to comprise a lower unit 100-150 feet thick 

of greenish-gray to gray medium- to coarse-grained 

tuff and sandy tuff, the base of which is not exposed; 

a middle unit 700-800 feet thick consisting mainly 

of gray to greenish-gray or yellow shale and siltstone, commonly tuffaceous; and an upper unit 350- 

400 feet thick of grayish-green to light-brown finegrained tuff that contains some brown shale and 

tuffaceous shale and locally very abundant plant 

remains. Figure 5 is a sketch of a geologic section 

along a gully northeast of the Little Outfit Ranch, 

summarizing the lithology in this area; the section 

trends east-northeast. No lava flows are exposed 

along this traverse, but presumably the lowest flows 

would lie a few feet or, at most, a few tens of feet 

stratigraphically above the highest beds shown.

Southwest of the Little Outfit Ranch, rocks of the

1 Trachyandesite, as used in this report, denotes a volcanic rock with the 

potassium feldspar-plagioclase ratio of granodiorite (between 1:10 and 

4:10) and with less than 10 percent quartz. It is thus intermediate between 

latite and andesite, and it is referred to by Nockolds (1954, p. 1008) as 

doreite; the name doreite, however, has not been widely used, and trachy-. 

andesite seems preferable.
PATAGONIA MOUNTAINS AND ADJOINING AREAS E19

wsw

Little Outfit Ranch 

/ A B C

i - <^ /J;

x^-.:--i-: : ' :- '; ^xv-ri":..^;-: ;:. 

D ^ F G H I j K L

'MW^^K^^i&^\ \

^.- "' i-^-v:'/.': ' ' ;. ; ' : -'=. : '/V:;::;;; :|^;::?-V: :;;:.^;:j )

M ^ 0

\ \ N

1 11

P$ R S T

9\\ \

V:; A \ \

< El>

IN"

\^\

200 FEET

_I

FIGURE 5. Geologic section of volcanic and sedimentary rocks of the trachyandesite of Meadow Valley along a gully northeast 

of the Little Outfit Ranch. Units that do not crop out are shaded. From youngest to oldest:

A, Green medium- to fine-grained chloritic tuff.

B, No outcrop.

C, Green medium- to fine-grained chloritic tuff.

D, No outcrop.

E, Massive green tuff.

F. No outcrop.

G, Gray vitric biotite rhyolitic tuff and light-brown lithic rhyolitic tuff.

U, No outcrop.

I, Gray siltstone containing plant remains; light-brownish-green 

sandy tuff.

J, No outcrop.

K, Olive siltstone and interbedded olive to gray siltstone and sandstone 

in beds 1-2 inches thick; abundant white to rusty plant remains; grades downward into light-gray siliceous shale and 

interbedded olive sandy tuff.

L, Olive siltstone and mudstone with hackly fracture; carbonaceous 

plant remains.

M, Olive shale and siltstone.

tuff-shale unit consist, from bottom to top, of wellbedded poorly sorted volcanic conglomerate with 

subrounded blocks as much as a foot across of purple porphyritic andesite in a matrix of green altered 

vitric tuff (base not exposed), green vaguely bedded 

palagonitic tuff with scattered blocks of purple andesite as much as 2 feet across, and light-gray to pinkish-gray massive vitric biotite rhyolitic tuff. Northeast of Vaca Ranch the unit includes a 1-foot-thick 

layer of red and green chert, and south of Vaca 

Ranch it contains a distinctive bed 10-15 feet thick 

of grayish-orange vitric rhyolitic tuff with abundant 

fragments of orange pumice; this tuff closely resembles a tuff in the volcanics of Dove Canyon near 

Paloma Spring (p. E17-E18).

THIN LAVA FLOWS

An extensive area in and west of Meadow Valley 

is underlain by a pile of thin flows of porphyritic 

trachyandesite and olivine andesite and subordinate 

flow breccia, welded tuff, rhyolitic (?) lava, and mudflow. The aggregate thickness of these flows is unknown but probably is several thousand feet. Most 

of the trachyandesites have phenocrysts of andesine 

and augite, but some have only feldspar phenocrysts. 

All are altered to some extent, and the pyroxene in 

most is extensively replaced by chlorite and carbonate. Olivine is invariably altered to iddingsite.

The basal flow of the sequence north of Little Outfit Ranch is a blackish-red very fine grained rock 

that is conspicuously flow layered. It contains sparse 

tiny phenocrysts of plagioclase almost entirely replaced by carbonate, as well as scattered grains of 

an altered green mafic mineral. The groundmass is

N. Massive olive vitric-crystal rhyolitic tuff.

O, Light-gray to light-olive finely laminated siltstone and shale and 

sparse beds of olive sandy tuff; indistinct graded bedding.

P, No outcrop.

Q, Cream to pale-yellow hard very fine grained tuffaceous mudstone 

in prominent bed 5 feet thick.

R, Olive to gray finely laminated siliceous shale and 2- to 4-inch-thick 

beds of olive siltstone containing abundant carbonaceous plant 

remains; grades downward through few feet of gray to purple 

thin-bedded siltstone and sandstone to unit S.

S, Gray to greenish-gray sandy or silty tuff and some granule tuff; 

5 feet of green sandy palagonitic tuff at base.

T, Greenish-gray obscurely bedded sandy tuff containing numerous 

small fragments of fine-grained purple andesite; lighter colored, 

coarser, and more massive downward; ends at fault contact 

with unit U.

U, Biotite latitic welded tuff (probably part of the Triassic and Jurassic sequence of the Canelo Hills).

a very dusty pale green mixture of glass and tiny 

plagioclase microlites that seem to be about Ani7 in 

composition.

In the Vaca Ranch area the lavas are very dusky 

red purple to olive-gray porphyritic pyroxene-olivine 

andesite. Some contain large vesicles lined with salmon-colored stilbite and clear heulandite. A typical 

rock has phenocrysts 3-4 mm across of andesine and 

augite, together with olivine phenocrysts less than 

0.5 mm across, in an intersertal or trachytic matrix.

Some of the rocks are more silicic than trachyandesite. Southwest of Vaca Ranch, a low knob is 

capped by light-brownish-gray to pale-red porphyritic biotite-bearing lava, perhaps a latite, with highly 

contorted flow layering. The only phenocrysts are 

andesine and dusty biotite. The groundmass contains 

potassium feldspar, iron oxide, and devitrified glass. 

Just north of Vaca Ranch, a sheet of biotite latitic 

welded tuff is interlayered with lava flows. The 

welded tuff is 25-30 feet thick and is exposed for 

about 900 feet along the strike. It is a pale-red 

streaked rock that contains crystals of white feldspar and brown biotite, together with abundant 

lithic fragments (some of them porphyritic andesite) as much as 2 cm across, set in a stony matrix 

whose texture is distinctly eutaxitic. In thin section 

the rock is seen to contain plagioclase (about An20 ), 

biotite, a little potassium feldspar, and partly collapsed pumice lentils in a matrix of partly devitrified glass shards that are only moderately welded.

Rocks that probably are silicic lavas, perhaps 

rhyolite or trachyte, form prominent outcrops over 

an area of about 1.5 square miles centered on 

a point about 1 mile west of Saddle Mountain. These
E20 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

rocks appear to be intercalated in, to rest on, and 

to intrude the trachyandesite, but contact relations 

are very obscure in most places, and indeed some 

silicic lavas may instead be highly altered trachyandesite. The rocks are varicolored white, light 

gray, violet, or brown and fine grained to sparsely 

porphyritic, and they weather to yellow, brown, or 

rust-colored cavernous outcrops. They generally are 

massive and featureless, and evidence for their origin is meager. Flow layering, breccia texture, and 

platy structure observed in a few places suggest 

that they are mostly lavas and flow breccias. Phenocrysts are small and invariably are completely 

altered to chalky aggregates whose shapes indicate 

that they were formerly feldspar. No other original 

mineral can be identified. The groundmass commonly is a patchy intergrowth of quartz and potassium 

feldspar that probably is devitrified glass. Alteration 

products include quartz, kaolin, alunite, sericite, 

iron oxides, and opal.

THICK LAVA FLOWS

In the area between Harshaw and Trench Camp, 

the lower part of the trachyandesite of Meadow Valley is made up of very thick massive coarse-grained 

lava flows that only here and there give any evidence 

of their origin. These flows closely resemble the 

monzonite north and northwest of Meadow Valley 

that is interpreted as being intrusive into trachyandesite, and indeed some of this monzonite may 

also be of extrusive origin.

Some of the flows appear to be 300-400 feet thick, 

and one flow in the area west of the Hardshell mine 

may be at least twice that thick. Contacts between 

individual flows are obscure, however, and some 

contacts may have been overlooked. In the area 

southwest of Harshaw, flows are separated by variable thicknesses, commonly only a few feet, of purple or red vesicular lava that contains fewer phenocrysts than the overlying or underlying rock. Northeast of Trench Camp, thin layers of well-bedded red 

or green tuff are intercalated between flows. A very 

thick flow west-southwest of Harshaw is massive 

in its lower part and gradually becomes vesicular 

and amygdaloidal toward its top, the quartz-lined 

vesicles increasing upward in both number and size 

and attaining diameters of as much as 5 mm. The 

transition from massive to amygdaloidal takes place 

through a thickness of about 70 feet.

No flow breccia was seen at the base of the sequence of thick trachyandesitic flows, and none is 

likely to be present inasmuch as the basal contact 

is fairly well exposed for about 1.5 miles between 

Hermosa Canyon and Harshaw Creek. No evidence

of chilling of the trachyandesite or of alteration of 

the underlying rocks was found along this contact. 

As far as could be determined, the thick flows have 

about the same attitude as the underlying silicic volcanic rocks.

Typical coarse-grained trachyandesite 1,200 feet 

southwest of the Hardshell mine is a brownish-gray 

to grayish-red-purple rock that contains closely 

packed small phenocrysts of plagioclase and pyroxene. In thin section the plagioclase is seen to be very 

cloudy and near An20 in composition. Dark minerals 

are augite and aggregates of chlorite and iron oxide, 

probably pseudomorphs after biotite. Potassium 

feldspar and abundant apatite are interstitial to the 

phenocrysts.

CHEMICAL COMPOSITION

Chemical analyses, norms, and semiquantitative 

spectrographic analyses of four lavas believed to be 

representative of the trachyandesite unit are shown 

in table 3, together with the analyses and norms of 

Nockolds' (1954) average doreite [ trachyandesite] and average rhyodacite.

The trachyandesites chemically resemble typical 

andesites except that they contain less lime and 

more alkalis and have a much higher ratio of K20 

to Na^O. Rocks of similar composition and of Late 

Cretaceous or Tertiary age are apparently widespread in southern Arizona. They include the Childs 

Latite of the A jo region in western Pima County 

(Gilluly, 1946, p. 44-45), lavas in the Cloudburst 

Formation near San Manuel in Pinal County (Creasey, 1965, p. 14-15), "turkey track" porphyry in 

the Twin Buttes quadrangle in Pima County and the 

Dragoon quadrangle in Cochise County (Cooper, 

1961), lavas of the Demetrie Volcanics (Thorns, 

1967) in the Twin Buttes quadrangle (J. R. Cooper, 

written commun., 1962), and lavas in the lower part 

of the Galiuro Volcanics in the Klondyke quadrangle 

(Simons, 1964, p. 71-72). Petrographically similar 

rocks have been noted in many other places in the 

region, and additional chemical analyses undoubtedly 

will reveal that some are trachyandesites.

ALTERATION OF TRACHYANDESITIC LAVAS

The trachyandesitic lavas of Meadow Valley are 

variably altered locally in a belt about 1 mile wide 

and 6 miles long along the west half of the north 

edge of the Lochiel quadrangle. Within this belt the 

lavas are pyritized and bleached, and they weather 

to light-gray to yellowish outcrops. Porphyritic texture may be faintly preserved or completely obliterated. Alteration appears to have occurred principally 

along steep-dipping fractures trending approximate-
PATAGONIA MOUNTAINS AND ADJOINING AREAS E21

TABLE 3. Comparison of chemical analyses, norms, and semiquantitative spectrographic analyses of trachyandesite and 

rhyodacite of Meadow Valley with those of Nockolds' (1954) 

average rocks

[Leaders (---) in figure columns indicate no data]

Trachyandesite Rhyodacite

Meadow Valley

Nockolds'

(1954)

average

Nockolds' 

Meadow (1954) 

Valley average

Sample No.. 12345 

Lab. No._- 161448 D110285W 161447 _.-..-_- 159244

Chemical analyses (percent)

(Analysts: Nos. 1, 2, 3, by P. L. D. Elmore, S. D. Botts, G. W. Chloe, Lowell 

Artis, and H. Smith; 5, by P. L. D. Elmore, S. D. Botts, and G. W. Chloe)

SiO2_

FesOsFeO._- 

MgO___ 

CaO.-- 

NazO.- 

K 2O__. 

H 2O - 

H2O+- 

TiOj-.

P2O5--

MnO_. 

COs.-_

56.7 

18.9 

4.6

.96 

2.6 

6.3 

3.8 

3.0 

.27 

.77 

.98 

.48 

.22 

<.05

54.8

16.8

6.6

2.0

3.6

6.0

3.6

3.0

.33

1.9

1.2

.36

.14

.14

55.6

15.9

4.4

4.0

4.1

5.1

3.5

3.4 

.28-

2.0

1.2 

.35 

.16 

.09-

56.00

16.81

3.74

4.36

3.39

6.87

3.56

2.60

.92

1.29

.33

.13

62.7 

17.3 

4.8

.65 

1.1 

2.4 

4.5 

2.2 

1.0 . 

2.1 

1.1 

.37 

.06 

<.05.

66.27

15.39

2.14

2.23

1.57

3.68

4.13

3.01

.68 

.66 

.17 

.07

Sum. 

Powder 

density.--

100 100 100 100 . 

2.73.

Norms (percent)

Q

or-_ 

ab 

an. 

C

WOen.. 

fs 

mt 

hm7.7

17.7

32.1

25.7

6.8

17.7

30.4

20.8

6.6

20.1

29.6

17.6

7.2

15.6

29.9

22.2

1.0 

6.5

2.4 

9.0

ap1.0 

3.9 

1.9 

1.1

3.4

4.2

2.3

.9

2.0

10.2

2.0

6.4

22.6

128

38.3

9.2

4.2

20.8

17.8

35.1

14.5

4.1 

8.5 

3.0 

5.3

2.7

2.3 2.4

4.8

1.5

1.0

.3

1.3 

3.9 

1.3 

3.0

1.4 

.3

DIi. 57.5

.3 

54.9

.2 

56.3 no data 73.7 no data

Semiquantitative spectrographic analyses (parts per million

0.15 

.0002

0

.002 

.005 

.02 

.003 

.007

0 

.001

0

.005 

.005 

.0015 

.15 

.03 

.003 N 

.02

0.1

0

0

.002

.003

.02

.003

.005

0

.0015

0

.0015

.0015

.003

.07

.03

.003

.0005

.015

0.2

.00015 

0

.002

.003

.007

.003

.005 

0

.001 

0

.002

.01

.003

.07

.03

.005 

N

.015

0.1 

.0002 

.01 

.0015

0

.0015 

.005 

.007 

<.001 

.001 

.007 

.0007 

.002 

.002 

.1 

.02 

.005 

.0005 

.05

1 Differentiation index.

SPECIMEN DESCRIPTIONS

1. Trachyandesite. Thin flow of porphyritic lava, 1.1 mile S. 23° W. of Saddle 

Mountain. Mode: Plagioclase (An 4 5), 33 percent; clinopyroxene, 4 percent; 

biotite, 4 percent; iron ore, 1 percent; other, 1 percent; groundmass, 57 

percent.

2. Trachyandesite. Thia flow of porphyritic lava 3,000 ft west-northwest of 

Harshaw. Rock consists of sodic plagioclase and clinopyroxene with minor 

potassium feldspar, iron ore, quartz, and apatite.

3. Trachyandesite. Thick holocrystalline lava flow 1 mile southwest of Harshaw. 

Rock consists of sodic plagioclase, cliaopyroxene, potassium feldspar, chlorite, 

and sparse quartz.

4. Average doreite ( = trachyandesite). Nockolds (1954, p. 1018, table 5, col. 2).

5. Rhyodacite or dacite. Thin fine-grained lava flow 1,000 ft north-northwest of 

Little Outfit Ranch.

6. Average rhyodacite plus rhyodacite-obsidian. Nockolds (1954, p. 1014, table 2, 

col. 4).

ly east but also follows fractures of other orientations. Contacts between altered and unaltered rock 

are irregular and sharp, and dark unaltered remnants commonly are preserved within lighter colored 

altered rock, so the nature of the original rock is 

seldom in doubt. In the partially altered rock, plagioclase is wholly replaced by sericite and kaolinite, and 

the devitrified glass of the groundmass is very dusty 

and nearly opaque. In more strongly altered rock, 

phenocrysts are entirely converted to a light-brown 

dusty clay mineral, perhaps montmorillonite, or to 

gypsum; in some rocks only vague outlines of phenocrysts remain. The most intensely altered rocks are 

white, and they weather rust colored; apparently 

they consist almost entirely of sericite and silica.

AGE

Biotite from biotite latitic welded tuff intercalated 

in trachyandesitic lava just north of Vaca Ranch 

was dated radiometrically by the potassium-argon 

method at 72.1 ±2.2 m.y. (R. F. Marvin, H. H. Mehnert, and Viola Merritt, written commun., 1966). 

No field evidence was found to indicate any appreciable age differences among the rocks composing the 

trachyandesite of Meadow Valley, and therefore the 

unit is assigned a Late Cretaceous age.

A nearly identical age (72.5±2.2 m.y.) has been 

reported for biotite from the welded tuff member 

of the Salero Formation in the southern Santa Rita 

Mountains (Drewes, 1968, p. C11-C12). The two 

formations thus appear to be approximate temporal 

equivalents, but they differ markedly in lithology 

and clearly represent separate volcanic-sedimentary 

episodes.

CRETACEOUS OR TERTIARY ROCKS

VOLCANIC ROCKS OF THE HUMBOLDT MINE-TRENCH

CAMP AREA

An area of a little more than 1 square mile lying 

west of the Humboldt mine and Trench Camp is 

underlain by silicic volcanic rocks, mostly thickbedded coarse tuff. The rocks are very resistant to 

erosion and form a rugged terrain with steep slopes 

and abundant conspicuous knobby outcrops. The 

relations of these rocks with the surrounding rocks 

are very complex. The northwest, west, and southwest boundaries are probably faults or silicified fault 

zones. The contact on the east and southeast is an 

unconformity along which the silicic tuffs overlie the 

trachyandesite of Meadow Valley and older rocks of 

the American Mine, UX Ranch, and Thunder Mine 

blocks of the southern Patagonia Mountains. The 

Humboldt mine-Trench Camp volcanic rocks are at 

least 800 feet thick, and the maximum thickness is
E22 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA

probably not much greater. No direct evidence of age 

is available, but the rocks rest unconformably on the 

trachyandesite of Meadow Valley, of Late Cretaceous 

age, and are altered and mineralized. Inasmuch as 

mineralization in southeastern Arizona is generally 

considered to be of Late Cretaceous or early Tertiary 

age, the rocks are probably no younger; therefore, 

they are tentatively assigned a Cretaceous or Tertiary age.

The rocks consist of both thick-bedded crudely 

layered subaerial lapilli tuff, tuff breccia, and block 

breccia and well-bedded and locally channeled waterlaid tuff and pebbly tuff, as well as a little silicic 

lava. They are cut by several irregularly shaped 

breccia masses, one about 1,200 feet long, that probably are volcanic necks, and they are traversed by 

numerous highly silicified zones, including a very 

large one that forms the southwest border for about 

2,000 feet. The rocks everywhere are sericitized to 

some extent, and in the southern one-third of the 

outcrop area they are considerably alunitized.

The tuffs and lapilli tuffs are light-gray rocks that 

contain phenocrysts of one or more of the minerals 

quartz, feldspar, and biotite in a devitrified and very 

dusty matrix that commonly shows only a vague 

vitroclastic texture. Northeast of the Chief mine, 

the tuffs have conspicuous phenocrysts of quartz as 

much as 3 mm across, together with phenocrysts of 

potassium feldspar and completely sericitized biotite. 

In the Humboldt mine area the tuffs have only highly 

sericitized phenocrysts of feldspar and biotite.

The tuff breccias are massive rocks which consist 

of various-sized volcanic fragments in a matrix that 

in some rocks is recognizably vitroclastic but in 

others is merely a fine-grained irresolvable aggregate, probably mostly devitrified glass. Some of these 

rocks are obscurely bedded, but most show no evidence of bedding. Northwest and west of Trench 

Camp the breccias consist of fragments 1-6 inches 

across in a very dusty matrix in which the only 

recognizable mineral grains are quartz. Other breccias have grains of feldspar as well as quartz, and 

rarely they contain fragments of quartzite.

A prominent vulcanian block breccia on the east 

side of the outcrop area is a chaotic jumble of partly 

rounded red to grayish-red blocks of silicic lava, 

lapilli tuff, and sandy tuff as much as 3 feet across. 

The sparse matrix material is darker red and fragmental.

Lavas appear to be mainly on lower slopes along 

Alum Gulch. They are light-gray rocks that contain 

small phenocrysts of quartz and a little sericitized 

biotite, together with sparse lithic fragments, in a

very fine grained groundmass of quartz and sericite 

that shows no evidence of original vitroclastic texture and presumably is massive devitrified glass.

The abundance of breccias and coarse tuffs suggests a nearby source for the volcanic rocks, as do 

also the several volcanic necks within the outcrop 

area and the rhyolitic dikes that cut both the volcanics and the trachyandesite of Meadow Valley 

immediately to the east.

VOLCANIC ROCKS OF RED MOUNTAIN

Red Mountain is a prominent irregular mountain 

mass about 3 miles southeast of Patagonia. The 

south flank of this mountain, the only part in the 

report area, appears to be made up of massive silicic 

volcanic rocks. These rocks are very resistant to 

erosion and form steep, in places cliffy, slopes. They 

overlie the trachyandesite of Meadow Valley; although the contact is nowhere clearly exposed, it 13 

probably an unconformity. The attitude of the volcanics is uncertain, but the basal contact, although 

irregular, is roughly horizontal, and the few structural data available also suggest near horizontality. 

The thickness is estimated at about 1,500 feet and 

may be somewhat more; the original thickness has 

been reduced an unknown amount by erosion. The 

volcanic rocks of Red Mountain are all highly altered 

and locally mineralized and are considered to be 

about the same age as the volcanic rocks of the 

Humboldt mine-Trench Camp area that is, Cretataceous or Tertiary.

The volcanics of Red Mountain are white, lightgray, or pinkish-gray to pale-red thick-bedded very 

fine grained and, in general, rather featureless rocks 

that weather to various hues of brown and red. Only 

rarely do they provide any clues to their origin. 

Although both lava flows and tuffs were tentatively 

identified in the field, microscopic study of seven 

rocks confirmed the identity of only one, a lava; 

original textures in the rest have been completely 

destroyed by devitrification or other alteration. The 

rocks commonly contain sparse completely altered 

phenocrysts of feldspar and biotite, but they may 

also be nonporphyritic. Flow layering was recognized in only a few places, and possible bedding was 

found at only one place.

The source area of the volcanics of Red Mountain 

is not known. Several dikes of similar lithology half 

a mile or more long cut the trachyandesite of Meadow Valley south of Red Mountain, and they or 

similar dikes beneath Red Mountain may have been 

feeders. One possible volcanic vent was recognized 

in the volcanics near their south edge.
PATAGONIA MOUNTAINS AND ADJOINING AREAS E23

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