The geology of the Pena Blanca and Walker Canyon areas, Santa Cruz County, Arizona
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Authors Nelson, Francis John, 1935-
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/553982 THE GEOLOGY OF THE PENA BLANCA AND WALKER
CANYON AREAS, SANTA CRUZ COUNTY,
ARIZONA
# ^ V by Frank J. Nelson
} ;,
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOLOGY
In Partial Fulfillment of the Requirements : For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
1963 STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of require ments for an advanced degree at The University of Arizona and is de posited in The University Library to be made available to borrowers under rules of the Library,
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholar ship. In all other instances, however, permission must be obtained from the author.
SIGNED: J.
a p p r o v a l b y t h e s is d ir e c t o r
This thesis has been approved on the date shown below:
Date Professor of Geology THE GEOLOGY OF THE PENA BLANCA AND WALKER CANYON AREAS, SANTA CRUZ COUNTY, ARIZONA
by Frank J. Nelson
ABSTRACT
The Pena Blanca and Walker Canyon area is located in the western part of Santa Cruz County, Arizona, about 15 miles west- northwest of the city of Nogales. Covering approximately 18 square miles, this area is characterized by a complex of volcanic and inter- bedded sedimentary rocks which are believed to range in age from
Cretaceous(?) to Recent.
Exposed extrusive rocks, comprised of tuffs, agglomerates,
and lava flows, vary in composition from rhyolite to basalt. Conglom
erates and breccias are the most common sedimentary rocks. A num
ber of minor intrusions occur in the Pajarito Mountains which occupy
most of the southern half of the area.
Four recognized formations occur—the Pajarito Lavas, Oro
Blanco Conglomerate, Montana Peak Formation, and the Atascosa Formation. A fifth unit, the "Pena Blanca Formation, n can be recog
nized. This unit, which contains the youngest rocks in the area, had previously been considered the upper part of the Atascosa Formation,
The area is severely fractured by a number of systems of
faults and joints. These fractures were formed by at least five epi
sodes of recurring tectonic activity since the Cretaceous(?).
Two masses of lithic rhyolite tuff are discordant with the local
stratigraphic section. These two masses, locally known as “Castle
Rock" and "Mouse Rock, " exhibit features characteristic of diatremes.
They are elliptical in outline, complexly fractured, and contain inclu
sions of rocks not found anywhere nearby.
A semicircular chain of cliffs forms an escarpment about the
area locally called "Whitney M esa." The tuff layers that form the cliffs
dip toward a common center, and may be part of a buried collapse caldera.
Silver and lead have been produced from the Pajarito Mining
District. The epithermal ores of the district are localized in a series
of parallel, steeply dipping shear fractures which generally trend north
east.
The portion of the geologic column exposed in this area is sim
ilar to sequences of rocks in the Tucson, Santa Rita, and Patagonia
Mountains. It may also be possible to correlate formations of the Pena Blanca and Walker Canyon area with some sections of north-central
Sonora,
... :
V TABLE OF CONTENTS
...... Page
INTRODUCTION ...... 1
Location and Extent of A rea Topography and Drainage .. Ecology ...... Present Settlement ...... Pena Blanca Lake ...... Purpose of Investigation... Method of Investigation ....
Previous Work in the Area oo-Ni-*a»ao3^co>-» Acknowledgments ...... 10
GEOLOGY OF THE BOUNDARY AREAS...... 11
A tascosa M ountains...... 11 Sierra De Los Pajaritos ...... 11 Eastern Foothills ...... 13
GEOLOGY OF THE PENA BLANCA AND WALKER CANYON A R E A ...... 14
Stratigraphy-G eneral S tatem ent...... 14 Pyroclastic Terminology...... 16 Cretaceous Rocks ...... 18
P ajarito L a v a s ...... 18 Oro Blanco Conglomerate ...... 23 Montana Peak Formation ...... 26
Tertiary Rocks ...... 37
Atascosa Formation ...... 37
Q uaternary Rocks ...... 43
Pena Blanca Formation ...... 43 vi Page
Intrusive Rocks ...... 49 Possible C o rre la tio n s...... 52
P ajarito Lavas ...... 54 . Oro Blanco Conglomerate ...... 55 Montana Peak Formation ...... 55 Atascosa Formation ...... 56 Pena Blanca Formation ...... 56
Structural Geology ...... : ...... 57
Faulting and Jo in tin g ...... 57 F o ld in g ...... 59 Other Structural Features 60
. Castle Rock and Mouse Rock ...... 62 Whitney M esa ...... 66
. : -■ - : : ■ ...... , ... ■ ...... Economic Geology ...... 69
Geology of the Pajarito Mining D istrict...... 69 History of the Pajarito Mining D is tr ic t...... 74
Summary of Geologic History ...... 75
SELECTED BIBLIOGRAPHY ...... 79
LIST OF FIGURES
Figure Page
1. Index map of Pena Blanca and Walker Canyon area ...... 2
2. Sketch map of area, showing prominent geographic features...... 5
3. Idealized section of Montana Peak Formation ...... 29
4. Sketch map of Whitney M esa ...... 68
vii r
Figure Page
5. Polished section of ore—Midnight Mine ...... 71
60 Polished section of ore—St. Patrick Mine ...... 71
7. Polished section of ore—Morning Glory Mine ...... 72
8. Polished section of o re—Sunset No. 1 M in e ...... 72
LIST OF PLATES .
Plate ... . Page
L Geologic map of Pena Blanca and Walker Canyon area ... in pocket
XL C ross sections of a r e a ...... in pocket
in . Pajarito L a v a s...... 20
IV. Photomicrographs of Pajarito Lavas ...... 21
V. Oro Blanco C onglom erate...... 27
VL Montana Peak Form ation ...... 33
VIL Montana Peak Formation ...... i ...... 34
Vm. Photomicrographs of Montana Peak Formation ...... 35
IX. Atascosa Formation ...... 39
X. Photomicrographs of Atascosa Formation ...... 40
XL Pena Blanca Formation ...... 46
XIL Photomicrographs of Pena Blanca Formation ...... 47
XHL Photomicrographs of intrusive r o c k s ...... 53
XIV. Faulted and folded con g lo m erate...... 61
viii Plate Page
XV0 Castle R o c k ...... 63
LIST OF TABLES
Table Page
L Comparison of geologic columns used in a r e a ...... 15
IL Minerals recognized in the Pajarito Mining D istrict...... 73
ix INTRODUCTION
Location and Extent of A rea
The area discussed in this paper is part of the Coronado Na tional Forest and lies about 15 miles west-northwest of the city of
Nogales, Santa Cruz County, Arizona, Approximately 18 square miles were studied and mapped, consisting of portions of T, 23 S«, R, 12 E., and To 24 S,, R« 12 E» The area is bordered on the south by the United
States-Mexico Boundary along a line from a point 1 mile east of Inter national Boundary Monument No. 127 to a point midway between Bound
ary Monuments No. 128 and No. 129. Lines bearing true north from these two points form the eastern and western limits of the area mapped.
The northern border is a line parallel to and 6 miles north of the east-
west-bearing portion of the International Boundary (fig. 1).
State Highway No. 289, better known as the Ruby-Nogales
Road, provides access to the area. This light-duty road begins 6 miles
north of Nogales on U. S. Highway No. 89 and bisects the area covered
in this report. Several unimproved dirt roads lead to the mines and
prospects of the Pajarito Mining District in the center of the area.
Other dirt roads extend for short distances into Pena Blanca Canyon. 2
ARIZONA
SANTA CRUZ COUNTY
Nogales
Fig. I. Location of Pena Blanca and Walker Canyon area. 3
Topography and Drainage
This part of Arizona lies in the Basin and Range province, which is generally characterized by northerly trending mountain ranges.
An exception to this lineation is found in the Pajarito Mountains, part of which form the southern third of the area studied. These mountains have an east-west trend that has been developed by a major fault zone along their northern flank.
Relief in the Pena Blanca and Walker Canyon area has been mainly controlled by local structural history and by the relative resist ance of the rock types exposed. The area has a maximum relief of
1, 630 feet, with the higher elevations near the southern border. The highest point is 5, 330 feet, located at the site of International Boundary
Monument No. 126, and the lowest is 3,700 feet, located in Pena Blanca
Canyon at the northern limit of the area mapped.
The topography is also dependent upon the rock type exposed and upon its structural deformation. The badly faulted, resistant lavas of the Pajarito Mountains have been eroded into a series of symmetrical steep-sided ridges. Youthful V-shaped canyons, largely located along fault zones, separate the ridges. The less resistant conglomerates and tuffs of the area are cut by a complex system of deep, narrow arroyos, many of which are structurally controlled. The more striking topo graphic features, such as Castle Rock and the white cliffs which give 4
Pena Blanca its name, are composed of tuff. The rest of the area is characterized by moderately steep ridges and scattered hills, most of which have been developed on volcanic flows and pyroclastics.
The area is drained by intermittent streams which empty into
. ' Pena Blanca and Walker Canyons. Part of the watersheds of Bellota " . • v > and Calabasas Canyons drains the northwest and southeast corners, re-
;' - ' . spectively. These four canyons are all roughly parallel and generally
' ■ - *' ' - : follow fault and joint zones. Their streams flow to the northeast, even tually draining into the Santa Cruz River.
Distinctive topographic features have been named to facilitate location and description. : Wherever possible local names have been used for landmarks not recognized on official maps (fig. 2).
Ecology
The ecology of the Pena Blanca and Walker Canyon area is ..... • i similar to the Lower Sonoran Life Zone in most respects. Except for a few interesting exceptions, the flora and fauna seen are typical of " . ' types commonly found between 2,000 and 5,000 feet in semiarid cli m ates. . .
The Pajarito Mountains are well known to herpetologists be cause of the existence there of a great variety of snakes (Clifford H.
■■ - Pope, personal communication). Over 15 species of rattlesnakes and over 35 species, of harmless snakes have been found there. Among the R. 12 E. T. 2 3 S. 5 / N
23 ^ J / • /I j A
cOV \ A7 ° \ s ** ( r O yj \ Mouse Tarantula C Z yr*» 25 Rock Butte » / y a ( C
s R uby-flog ales " / ^ < x V V T \ X.
lyJ v Whitney o Castle --M e sa <->/ Rock
/
10 k Pajarito^ Peak a bases
-4- T. 24 S.
> < / / v r 4//v.
Fig. 2. Detail of are a. 127 126 6 population of harmless species are the Rubber Boa (Lichanura roseofusca), the Tropical Vine Snake (Oxybelis aeneus), and a large tree-climbing type, the Mexican Green Snake (Elaphe triaspis), all of which are more common to the tropics than to Arizona. Some rare predatory mammals, such as the Jaguar and Ocelot, still are found in the Pajarito Mountains as they are able to roam at will across the un fenced stretches of the International Boundary.
In the Pena Blanca and Walker Canyon area the climate is typ ically semiarid. The area has a mean annual rainfall between 14 and
15 inches, most of which accumulates during the thunderstorms of July and August.
Present Settlement
Most of the population of the area centers around the Pena
Blanca Lake Resort. A permanent staff of six people live there, oper ating a concession that caters to fishermen using the lake.
Near the junction of Walker Canyon and the Ruby-Nogales Road is the 61 Ranch, owned by Mr. Thomas G. Bell. The only other people living in the area are Mr. and Mrs. Val Cason, who have a home in
Pena Blanca Canyon and own a number of mining claims in the Pajarito
D istrict, 7
Pena Blanca Lake
Pena Blanca Lake was built by the Arizona Game and Fish De partment in 1957e It is the largest man-made lake on the Coronado Na tional Forest, covering 52 acres with 4-1/2 miles of shoreline,
"" >■ •) • • : : ■ ■' - i ■' ■ ■ • ' \ The lake is held back by an earthen dam 70 feet high and 240 feet long across the top, A fishing and recreation area was developed upon completion of the dam.
Purpose of Investigation
The geology of the part of southern Arizona covered by this re port is known only in a very general way. The purpose of this investi gation was to determine the lithology, distribution, and structure of the rocks in this area, which were believed to be typical of rocks overlying a much larger area of southern Arizona and northern Sonora. This area was chosen to illustrate a representative segment of the geologic col umn, which might be useful in correlating some of the geology of these larger areas.
Method of Investigation
The problem was approached through geologic mapping of the area, using U.S. Forest Service aerial photographs as a base. Field work was begun in January 1962 and continued intermittently through 8
May 1963. A topographic base map was prepared by enlarging a portion of the UoS. Geological Survey 15-minute Ruby Quadrangle to the aver age scale of the photographs, 3.75 inches to 1 mile. The geology was then transferred from the photos to the base map using prominent topo graphic features and exact section-corner locations as control points.
Thin sections of the rock types encountered were prepared and studied as the fieldwork progressed. Polished sections of ores from the few small mines in the area were also studied.
Previous Work in the Area
Most of the geologic investigations in the area have been of the reconnaissance type. Some more detailed work has been done in near by regions and can be applied to parts of the Pena Blanca and Walker
Canyon area.
W. H. Emory (1857) passed through the P ajarito Mountains during his survey of the United States-Mexico Boundary and described some of the prominent geologic features.
W. P. Blake (1901) mentioned the Pena Blanca (Pajarito)
Mining District and suggested that it had a great potential.
N. H. Darton (1925) noticed the sim ilarity between the volcanic sequence of the Tumacacori and the Santa Rita Mountains, To Darton and other writers, the Tumacacori Mountains included what is now called the Tumacacori, Atascosa, and Pajarito ranges. 9
G, M, Fowler (1938) studied the geology of the Montana mine in Ruby which has a similar stratigraphic sequence to parts of the area described in this report.
R. Ee King (1939) worked in northern Sonora and described formations west of Nogales which probably correlate with similar rocks in the Pena Blanca and Walker Canyon area,
B. P. Webb and K. C, Coryell (1954) compiled a reconnais sance geologic map of the Ruby Quadrangle for the Atomic Energy Com mission. This preliminary study was the most detailed geologic mapping done in western Santa Cruz County until the present investigation began.
O. J. Taylor (1959) studied the similarities between some vol canic sequences of Santa Cruz County and those of the Tucson Mountains.
By chemical and petrographic methods he demonstrated their probable correlations.
E. D. Wilson and R. T. Moore (1960) mapped m ost of w estern
Santa Cruz County on a 1:375,000 scale. Their work was published as part of the Arizona Bureau of Mines "Geologic Map of Pima and Santa
Cruz Counties, Arizona."
E. D. Wilson (1962) assigned ages to certain rock types of the
Pena Blanca and Walker Canyon area on the basis of their lithology and structure 10
Acknowledgments
I wish to thank Dr. S. R. Titley for his many constructive sug gestions and direction during the course of this work. I am also in debted to Dr. W. C. Lacy and Dr. J. F, Lance for their criticisms and encouragement.
Thanks go to the Casons of Pena Blanca Canyon for providing a base of operations within the area and for sharing their knowledge of the x region with me. . ;
lam also grateful to my wife, Eleanor, for her help in the field work and her contributions toward the preparation and completion of this
report. ' • GEOLOGY OF THE BOUNDARY AREAS
Atascosa Mountains
: The Atascosa Mountains lie to the north and west of the Pena
Blanca and Walker Canyon area. The name Atascosa comes from the
Spanish word "atasco, " meaning "an obstruction to passage,M and was used to describe the abrupt nature of the eastern face of the range
(Barnes, 1960).
Atascosa Peak, at an elevation of 6,400 feet, is one of the highest mountains in western Santa Cruz County. About a mile south of it is Atascosa Lookout Station, the type locality for the Atascosa
Form ation.
This formation is about 800 feet thick, composed of tuffs, conglomerates, and interbedded lavas, and generally dips to the north east at low angles (Webb and Coryell, 1954). One tuffaceous unit near the lookout is 500 feet thick and forms impressive vertical cliffs along the southern and western fronts of the mountains. .. ' i V : . - - Ifv; . y.:. ■. : M
Sierra De Los Pajaritos
In Sonora, to the south of the area covered by this report, is the Sierra de los Pajaritos (Mountains of the Little Birds). Major 12
William H» Emory (1857) noted that this range was part of the Arizone
Mountains and extended north of the proposed International Boundary.
His description of the geology is interesting, as it is the earliest men tion of the area,s complexity:
The country here presents a new aspect. Powerful vol canic irruptions have at some earlier period of the worldIs : history produced great disturbances in this part of the earth. Strata of limestone once horizontal, are now curved and bent by the force of this action, and masses of igneous rock have been upheaved through the fissures opened on the surface. Here you will find granite rocks, and near them beds of trap, and not far from both, limestone; then again aU fused in one conglomeration.
The famous Planchas de Plata silver deposit was discovered at Rancho Arizonac in the Sierra de los Pajaritos in 1736. This deposit, noted for the masses of native silver once found there, is stiU worked be individual prospectors who high-grade the dumps and old workings.
The Baucarit and Nogales Formations have been recognized in the Sierra de los Pajaritos, although their exact distribution and their lithologies are poorly known. In this area the Baucarit Formation is composed of reddish conglomerate and breccia, with interbedded basalt flows (King, 1939). It overlies the Nogales Formation which was first
described by E. Durable in 1901. The Carta Geologica de la Republica
Mexicana, published in 1960, shows this area to be covered by middle
Cenozoic volcanics and upper Cenozoic volcanic and sedimentary rocks. 13
Eastern Foothills
East of the area mapped is a series of long, northeast-trend ing ridges and flat-topped spurs that extend toward the Santa Cruz
Valley. These ridges and spurs have formed on Quaternary alluvium, which consists mainly of conglomerates with a few thin layers of water- lain tuff.
Near the twin cities of Nogales Arizona-Sonora, the alluvium overlies the Nogales Formation. Durable (1901) described this forma tion as being more than 1,000 feet thick, and composed of rhyolitic lavas, agglomerates, and conglomerates, with some inter bedded an desitic lavas and tuffs. He originally assigned a late Tertiary age to the Nogales Formation, although it is now considered to be Cretaceous or Tertiary (Wilmarth, 1938). GEOLOGY OF THE PENA BLANCA AND WALKER CANYON AREA
Stratigraphy-General Statement
The geology of the Pena Blanca and Walker Canyon area is
characterized by a complex of volcanic and interbedded sedimentary
rocks, which are believed to range in age from Upper Cretaceous to
Recent. Volcanic rocks in southeastern Arizona are usually considered
Cretaceous or older if they are badly &ulted and folded. Tertiary if they
are tilted, and Quaternary if they are flat-lying (Taylor, 1959). Since
no dateable strata were found, ages have been assigned to the formations
studied by using these suppositions, as well as the principle of super
position.
, - ' : Extrusive rocks, comprised of tuffs and lava flows, range
from rhyolite to basalt in composition. Conglomerates and breccias
are the most common sedimentary rocks, with some small interbedded
lenses of arkosic sandstone.
There are four recognized formations in the Pena Blanca and
Walker Canyon area (Wilson and others, 1957). Three of these, the
Pajarito Lavas, the Montana Peak Formation, and the Atascosa For
mation, were named by Webb and Coryell in 1954. The fourth
14 LAVAS PAJARITO FORMATION ATASCOSA FORMATION THIS THIS PAPER ORO BLANCO ORO BLANCO FORMATION PENA BLANCA PENA BLANCA MONTANA PEAK PEAK MONTANA CONGLOMERATE WILSON ANDESITE ANDESITE GRANITE SEDIMENTS ANDESITE TERTIARY RHYOLITE SEDIMENTS CRETACEOUS CRETACEOUS CRETACEOUS CRETACEOUS QUATERNARY QUATERNARY 8 LARAMIDE 8 LARAMIDE 8 TERTIARY 8 TERTIARY • TAYLOR LAVAS FORMATION PAJARITO ORO BLANCO BLANCO ORO MONTANA PEAK PEAK MONTANA CONGLOMERATE LAVAS FORMATION ATASCOSA PAJARITO ORO BLANCO ORO BLANCO FORMATION MONTANA PEAK PEAK MONTANA CONGLOMERATE WEBBaCORYELL
Table I. Comparison of geologic columns that have been used in the Pena Blanca and Walker Canyon area. 16 formation, the Oro Blanco Conglomerate, was named by G. Fowler in
1938. On the basis of field evidence discussed later in this report, a fifth formation can be recognized. This unit, which for the purpose, of this report will be called the "Pena Blanca Formation,M contains the youngest rocks found in the area.
Pyroclastic Terminology
The terminology used in this report to describe pyroclastic rocks is based on a paper by C. K. Wentworth (1932). The following is a list of some of these terms and Wentworth^s definitions of them.
Tuff: Indurated pyroclastic rocks of grain generally finer than 4 mm;
i. e., the indurated equivalent of volcanic ash and dust.
Sedimentary tuff: A tuff containing a subordinate amount of sediment
introduced either during or after deposition, e. g., the finer
deposits of volcanic mud flows, or rocks produced by the
erosion and redeposition of pyroclastic ejecta admixed with
nonvolcanic materials, ;
Volcanic conglomerate: Sedimentary, coarse pyroclastic material con
taining an abundance of large, chiefly rounded, water-worn
fragments. In most cases they result from the erosion and
redeposition of old volcanic rocks, but they may also be formed
by volcanic mud flows and by the action of running water on
freshly fallen ejecta. 17
Volcanic breccia: More or less indurated pyroclastic rocks, consisting
chiefly of angular ejecta 32 mm or more in diameter. If the
fine tuff matrix be abundant, the term ’’tuff-breccia” seems
appropriate.
Agglomerate: Contemporaneous pyroclastic rocks containing a pre-
' dominance of rounded or subangular fragments greater than
32 mm in diameter, lying in an ash or tuff matrix and usually
localized within volcanic necks or at a short distance there
from. The form of the fragments is in no way determined by
the action of running water, as in volcanic conglomerate, but
is a primary feature determined during the actual eruption,
Lithic: An adjective applied to any pyroclastic deposit in which the
fragments are composed of previously formed rocks.
Essential: Pyroclastic detritus, whether loose or indurated, which is
of immediate, juvenile, magmatic origin.
Accessory: An adjective denoting pyroclastic materials derived from
previously solidified volcanic rocks of consanguineous origin,
i. e., the debris of earlier lavas and pyroclastic rocks from
the same cone.
Accidental: An adjective used to designate pyroclastic materials de
rived from volcanic rocks, nonconsanguineous with the magma
involved during an eruption, or from other igneous, metamor-
phic or sedimentary rocks through which the vent was developed. 18
: Cretaceous Rocks
Pajarito Lavas
The oldest rocks that crop out in the Pena Blanca and Walker
Canyon area belong to the Upper Cretaceous(?) Pajarito Lavas. Webb and Coryell (1954) assigned these rocks to the Mesozoic on the basis of their badly faulted and jointed character. On the Geologic Map of Pima and Santa Cruz Counties, Arizona (Wilson, et al., 1960), rocks in this area are shown to be Cretaceous and Laramide in age. In ideal, com plete section the Pajarito Lavas underlie the Cretaceous(?) Oro Blanco
Conglomerate. Locally the lavas are unconformably overlain by the
conglomerates and tuffs of the Tertiary (?) Atascosa Formation and the
Quaternary Pena Blanca Formation. v ^
The Pajarito Lavas occupy most of the southern half of the
mapped area, where they form the Pajarito Mountains. This massive
flow probably extends for 10 miles or more south into Sonora, as
mountains with similar appearance to the Pajaritos of Arizona can be
seen from higher points along the International Boundary.
One of the best exposures of this formation occurs on the east
side of Walker Canyon, about half a mile north-northwest of Pajarito
Peak. Here, over 800 feet of the lavas can be seen, outcropping on the
50° slope of the canyonls wall. The formation is also well exposed in
the deeply cut upper parts of Pena Blanca, Walker, and Calabasas 19
Canyons and in most of their higher tributaries.
It is difficult to determine the exact thickness of the Pajarito
Lavas, as no underlying formations have been observed and the whole unit is extremely faulted and broken# At least 1,300 feet are exposed in the Pajarito Mountains, generally dipping to the northeast and in
creasing in thickness toward the southwest.
Newly exposed outcrops have a blocky, angular appearance due to the closely spaced, intersecting joints that cut the rock in many di rections (pi. HI). The lavas are usually deeply weathered, giving a
rounded symmetrical shape to the mountains of the Pajarito range.
The formation is composed of massive, porphyritic quartz
latite, which varies in color from a reddish pink to light gray# Pheno-
crysts of quartz and pink feldspar are abundant, and can readily be
seen with the unaided eye. A few minor intrusions cut the formation,
and they include quartz latite porphyry, quartz monzonite porphyry,
and andesite porphyry. These hypabyssal rocks are discussed later in
the report# Veinlets of quartz transect the Pajarito Lavas in all direc
tions. Some of them are more than 10 inches wide and usually contain
miarolitic cavities. Occasionally pale amethyst crystals occur in these
cavities.
The quartz latite flow that comprises the Pajarito Lavas has a
seriate porphyritic texture with a hemicrystalline groundmass. Flow
structures and spherulites are readily seen in thin sections. P L A T E m PAJARITO LAVAS
Outcrop appearance of the Pajarito Lavas, The blocky, angular nature of the rock is due to
numerous intersecting joints. Plate III 20 PLATE IV PHOTOMICROGRAPHS OF PAJARITO LAVAS
Fig. 1. Porphyritic quartz latite: Large bent ortho- clase phenocryst. Smaller phenocrysts of quartz, oligoclase, and biotite. Matrix of microgranular quartz and orthoclase, Nichols crossed, X 3.5.
Fig. 2. Porphyritic quartz latite: Corroded quartz phenocryst. Orthoclase, oligoclase, and biotite phenocrysts. Matrix of quartz and orthoclase, partially stained by hematite. Nichols crossed,
X 3 .5 .
Fig. 3. Porphyritic quartz latite: Large subhedral orthoclase phenocryst. Smaller quartz, oligoclase, and biotite phenocrysts. Matrix of quartz and ortho clase shows coarser and finer areas. Nichols crossed, X 3.5. Plate IV 22
\ Quartz, orthoclase, plagioclase, and biotite occur as pheno- crysts and together make up about 56 percent of the total volume of the rock. Magnetite, sericite, and clay minerals occur in lesser amounts as alteration products. The phenocrysts have a wide range in grain sizes, the larger ones averaging about 3,0 mm in diameter and the smaller about 0.5 mm.
Modal composition of this rock is: matrix (quartz and ortho clase) 40 percent, quartz phenocrysts 19 percent, orthoclase pheno crysts 18 percent, plagioclase (An 30+) 15 percent, biotite 4 percent, magnetite, sericite, and clay minerals 4 percent.
Phenocrysts. of quartz are most typically euhedral bipyramids, although subhedral and anhedral grains are common. Almost all of these phenocrysts are embayed and have deep inlets of the groundmass. Many grains are shattered and the resulting cracks have been widened by corrosion and filled with matrix. Inclusions of clear glass are abun dant, some of which carry bubbles of vapor. The glass is usually rounded or droplike, except for a scattering of angular shards.
Orthoclase phenocrysts are euhedral to anhedral and are badly altered to sericite and clay minerals. Many of them are broken and have been healed by matrix material. Carlsbad twinning is common.
Glass and rounded inclusions of the groundmass are found within the larger grains. A few micrographic inter growths of orthoclase and quartz occur as phenocrysts. 23
Plagioclase is almost as abundant as potash feldspar and occurs as anhedral to subhedral phenocrysts of oligoclase. They are usually zoned, complexly twinned, and badly altered to sericite. Outer zones of the crystals are the most affected, often enclosing, a clear, unaltered core. /: : ■; ; . r -
Biotite occurs as dark-brown lath-shaped phenocrysts, which are commonly bent and.broken. The edges of the laths are. resorbed and the grains framed in secondary magnetite. Some biotite phenocrysts are entirely altered to muscovite and contain streaky inclusions of mag netite and chlorite.
The matrix predominantly consists of microgranular quartz
and lesser amounts of orthoclase. Minute grains of biotite and mag netite, as well as glass shards, are found throughout the groundmass.
Flow structures in the devitrified and recrystallized matrix can be best
seen in nonpolarized light. The structures consist of layers of coarser,
more concentrated quartz and orthoclase, with some streaks of brown
ish glass. Matrix material is molded around phenocrysts and fills in
all embayments and cracks. Poorly formed spherulites are common,
composed of orthoclase and minor quartz.
Oro Blanco Conglomerate
The Oro Blanco Conglomerate was first named and described
by G. Fowler (1938), who tentatively regarded it as Mesozoic in age. 24
It was named for the Oro Blanco Mining District, in which its type lo
cality, the Montana Mine, is located.
On the Geologic Map of Pima and Santa Cruz Counties, Arizona
(Wilson, et a!,, 1960), these rocks are shown to be of Cretaceous age.
In the Pena Blanca and Walker Canyon area, the Oro Blanco Conglom
erate rests disconformably upon the Cretaceous(?) Pajarito Lavas and
is unconformably overlain by the Cretaceous(?) Montana Peak Forma
tion. An irregular layer of silicified tuff locally separates the Pajarito
Lavas from the Oro Blanco Conglomerate. This tuff layer probably was
deposited upon an old erosion surface of the lavas, as its thickness
varies greatly in different exposures. The tuff is usually apple green
with scattered patches of pink and is cut by many veinlets of vuggy
quartz. ;
Except for a few tilted and deformed blocks, the conglomerate
dips toward the northeast at steep angles. It is a hard, well-indurated
rock, although not generally a cliff former. Outcrops appear as low,
irregular hills and occasionally as steep canyon walls.
The best exposure of the Oro Blanco Conglomerate is found in
the Montana Mine at Ruby, about 15 miles west of the area covered by
this report. It is the host rock for the lead-zinc-silver-gold ores of
the mine and covers a large area in the western part of Santa Cruz
County (Fowler, 1938). This formation is widespread northwest of
Ruby, with isolated outcrops east and south. In the Pena Blanca and 25
Walker Canyon area it is represented by two relatively small outcrops, one in Castle Rock Canyon and another southeast of Castle Rock.
Fowler (1938) noted that this formation occupied depressions in the underlying rock to a depth of "many hundred feet. " In Castle
Rock Canyon the outcrop of Oro Blanco Conglomerate averages approxi
mately 200 feet in thickness.
Color of the conglomerate varies from bright red to dark gray
ish purple. It has a mottled appearance in some places due to included fragments of green, silicified tuff. The formation shows some effect of
hydrothermal alteration, principally the staining of fragments and ce
menting material by iron oxides. This red staining is most pronounced
where the conglomerate is nearest Castle Rock and grades to sporadic
areas of gray-colored rock farther away.
Classification of the Oro Blanco Conglomerate is difficult, as
it combines characteristics of a conglomerate and a breccia. It contains
angular, subangular, and rounded fragments, which range from less than
an inch to over 12 inches in diameter. Interstices are filled with sand
sized particles. „ Lenses of arkosic sandstone are abundant, interbedded
between layers of conglomerate. Webb and Coryell (1954) mention that
the formation locally contains thin limy beds.
Stratification of this formation is pronounced and is primarily
due to concentrations of similar-sized fragments. Differences in the
degree and type of cementation have been brought out by differential 26
weathering (pL V).
The fragments are badly weathered and consist primarily of
igneous rocks, mainly subangular boulders of the Pajarito Lavas,
Quartzite and silicified tuff fragments also occur and are commonly
smaller and more rounded than the lavas. The cementing material is
generally silica and exceptionally calcium carbonate,
Montana Peak Formation
The Montana Peak Formation was named by Webb and Coryell
(1954), who considered it to be late Cenozoic in age. Its type locality
is Montana Peak, a prominent mountain 1-1/2 miles southeast of the
town of Ruby. On their reconnaissance geologic map Webb and Coryell
indicate that some rocks of the Pena Blanca and Walker Canyon area
are within the Montana Peak Formation.
Wilson (1962) noted that these rocks had a lithology common
to the Cretaceous and that they lie with pronounced angular unconformity
beneath strata of supposed Tertiary age. These features have been ob
served in the field in the present study. Therefore, in this report the
Montana Peak Formation is considered to be Upper Cretaceous(?).
In a complete section this formation is underlain by the Cre
taceous^) Oro Blanco Conglomerate and is unconformably overlain by
the Tertiary(?) Atascosa Formation. Locally the Montana Peak Forma
tion overlies the Cretaceous(?) Pajarito Lavas. In a few places it is PLATE V ORO BLANCO CONGLOMERATE
Outcrop of Oro Blanco Conglomerate in Castle Rock
Canyon. The distinct bedding and reddish color is typical of this formation. Fragments are angular to rounded and vary widely in size. P la te V 27 PLATE IV PHOTOMICROGRAPHS OF PAJARITO LAVAS
Fig. I. Porphyritic quartz latite: Large bent ortho- clase phenocryst. Smaller phenocrysts of quartz, oligoclase, and biotite. Matrix of microgranular quartz and orthoclase. Nichols crossed, X 3,5«
Fig. 2. Porphyritic quartz latite: Corroded quartz phenocryst. Orthoclase, oligoclase, and biotite phenocrysts. Matrix of quartz and orthoclase, partiaUy stained by hematite. Nichols crossed,
X 3 .5 .
Fig. 3. Porphyritic quartz latite: Large subhedral
orthoclase phenocryst. Smaller quartz, oligoclase,
and biotite phenocrysts. Matrix of quartz and ortho
clase shows coarser and finer areas. Nichols
crossed, X 3.5. Plate IV 22
\ Quartz, orthoclase, plagioclase, and biotite occur as pheno- crysts and together make up about 56 percent of the total volume of the rock. Magnetite, sericite, and clay minerals occur in lesser amounts as alteration products. The phenocrysts have a wide range in grain sizes, the larger ones averaging about 3.0 mm in diameter and the smaller about 0.5 mm.
Modal composition of this rock is: matrix (quartz and ortho clase) 40 percent, quartz phenocrysts 19 percent, orthoclase pheno crysts 18 percent, plagioclase (An 30+) 15 percent, biotite 4 percent, magnetite, sericite, and clay minerals 4 percent.
Phenocrysts of quartz are most typically euhedral bipyramids, although subhedral and anhedral grains are common. Almost all of these phenocrysts are embayed and have deep inlets of the groundmass. Many grains are shattered and the resulting cracks have been widened by corrosion and filled with matrix. Inclusions of clear glass are abun dant, some of which carry bubbles of vapor. The glass is usually rounded or droplike, except for a scattering of angular shards.
Orthoclase phenocrysts are euhedral to anhedral and are badly altered to sericite and clay minerals. Many of them are broken and have been healed by matrix material. Carlsbad twinning is common.
Glass and rounded inclusions of the groundmass are found within the larger grains. A few micrographic intergrowths of orthoclase and quartz occur as phenocrysts. 23
Plagioclase is almost as abundant as potash feldspar and occurs as anhedral to subhedral phenocrysts of oligoclase. They are usually zoned, complexly twinned, and badly altered to sericite. Outer zones of the crystals are the most affected, often enclosing a clear, unaltered core. : ■: ...... '
Biotite occurs as dark-brown lath-shaped phenocrysts, which are commonly bent and broken. The edges of the laths are. resorbed and the grains framed in secondary magnetite. Some biotite phenocrysts are entirely altered to muscovite and contain streaky inclusions of mag netite and chlorite.
The matrix predominantly consists of microgranular quartz and lesser amounts of orthoclase. Minute grains of biotite and mag netite, as well as glass shards, are found throughout the groundmass.
Flow structures in the devitrified and recrystallized matrix can be best seen in nonpolarized light. : The structures consist of layers, of coarser, more concentrated quartz and orthoclase, with some streaks of brown ish glass. Matrix material is molded around phenocrysts and fills in
all embayments and cracks. Poorly formed spherulites are common,
composed of orthoclase and minor quartz.
Oro Blanco Conglomerate
The Oro Blanco Conglomerate was first named and described
by G. Fowler (1938), who tentatively regarded it as Mesozoic in age. 24
It was named for the Oro Blanco Mining District, in which its type lo cality, the Montana Mine, is located.
On the Geologic Map of Pima and Santa Cruz Counties, Arizona
(Wilson, et ale, 1960), these rocks are shown to be of Cretaceous age.
In the Pena Blanca and Walker Canyon area, the Oro Blanco Conglom
erate rests disconformably upon the Cretaceous(?) Pajarito Lavas and
is unconformably overlain by the Cretaceous(?) Montana Peak Forma
tion. An irregular layer of silicified tuff locally separates the Pajarito
Lavas from the Oro Blanco Conglomerate. This tuff layer probably was
deposited upon an old erosion surface of the lavas, as its thickness
varies greatly in different exposures. The tuff is usually apple green
with scattered patches of pink and is cut by many veinlets of vuggy
quartz. •
Except for a few tilted and deformed blocks, the conglomerate
dips toward the northeast at steep angles. It is a hard, well-indurated
rock, although not generally a cliff former. Outcrops appear as low,
irregular hills and occasionally as steep canyon walls.
The best ejqrosure of the Oro Blanco Conglomerate is found in
the Montana Mine at Ruby, about 15 miles west of the area covered by
this report. It is the host rock for the lead-zinc-silver-gold ores of
the mine and covers a large area in the western part of Santa Cruz
County (Fowler, 1938). This formation is widespread northwest of
Ruby, with isolated outcrops east and south. In the Pena Blanca and 25
Walker Canyon area it is represented by two relatively small outcrops, one in Castle Rock Canyon and another southeast of Castle Rock.
Fowler (1938) noted that this formation occupied depressions in the underlying rock to a depth of "many hundred feet. " In Castle
Rock Canyon the outcrop of Oro Blanco Conglomerate averages approxi mately 200 feet in thickness.
Color of the conglomerate varies from bright red to dark gray ish purple. It has a mottled appearance in some places due to included fragments of green, silicified tuff. The formation shows some effect of hydrothermal alteration, principally the staining of fragments and ce menting material by iron oxides. This red staining is most pronounced where the conglomerate is nearest Castle Rock and grades to sporadic areas of gray-colored rock farther away.
Classification of the Oro Blanco Conglomerate is difficult, as it combines characteristics of a conglomerate and a breccia. It contains angular, subangular, and rounded fragments, which range from less than
an inch to over 12 inches in diameter. Interstices are filled with sand
sized particles. Lenses of arkosic sandstone are abundant, interbedded between layers of conglomerate. Webb and Coryell (1954) mention that
the formation locally contains thin limy beds.
Stratification of this formation is pronounced and is primarily
due to concentrations of similar-sized fragments. Differences in the
degree and type of cementation have been brought out by differential 26 weathering (pi* V).
The fragments are badly weathered and consist primarily of igneous rocks, mainly subangular boulders of the Pajarito Lavas*
Quartzite and silicified tuff fragments also occur and are commonly
smaller and more rounded than the lavas. The cementing material is
generally silica and exceptionally calcium carbonate.
Montana Peak Formation
The Montana Peak Formation was named by Webb and Coryell
(1954), who considered it to be late Cenozoic in age. Its type locality
is Montana Peak, a prominent mountain 1-1/2 miles southeast of the town of Ruby. On their reconnaissance geologic map Webb and Coryell
indicate that some rocks of the Pena Blanca and Walker Canyon area
are within the Montana Peak Formation.
Wilson (1962) noted that these rocks had a lithology common
to the Cretaceous and that they lie with pronounced angular unconformity
beneath strata of supposed Tertiary age. These features have been ob
served in the field in the present study. Therefore, in this report the
Montana Peak Formation is considered to be Upper Cretaceous(?).
In a complete section this formation is underlain by the Cre
taceous^) Oro Blanco Conglomerate and is unconformably overlain by
the Tertiary(?) Atascosa Formation. Locally the Montana Peak Forma
tion overlies the Cretaceous(?) Pajarito Lavas. In a few places it is PLATE V ORO BLANCO CONGLOMERATE
Outcrop of Oro Blanco Conglomerate in Castle Rock
Canyon, The distinct bedding and reddish color is typical of this formation. Fragments are angular to rounded and vary widely in size. P la te V 27 28 covered by the Quaternary Pena Blanca Formation.
This formation is well exposed in Pena Blanca Canyon, par ticularly along the shores of the lake and north of the dam. Around the lake the best exposures are on the east side, directly across the lake from the boat dock, and on the west shore about 600 yards north of the dock. In the canyon north of the dam the Montana Peak Formation forms steep cliffs 50 to 100 feet high. Except for a few deeply cut washes, the formation in the rest of the area is covered by slope wash, with only the more resistant layers exposed.
It is not possible to accurately measure a complete section of the Montana Peak Formation in the Pena Blanca and Walker Canyon
area, due to the formations badly faulted condition.. At Montana Peak
this formation is 800 feet thick and probably thins to about 600 feet in
the area studied. The formation generally dips steeply toward the south,
although faulted blocks can be found at nearly any attitude from horizon
tal to vertical.
The Montana Peak Formation is usually a red, purple, or tan
color, with some gray, buff, brown, and greenish-colored rock locally
exposed. The unit is characterized by the extreme variety of its
layers, which cause it to outcrop as cliffs in some places and as gentle
slopes in others. Figure 3 is an idealized cross section, intended to
illustrate the variable nature of this formation. The drawing combines
a number of widely separated exposures into one hypothetical section. 29
-MASSIVE LITHIC TUFF
AGGLOMERATE
WELL-BEDDED SEDIMENTARY TUFF
LAVA ASH BANDED VITRIC TUFF PORPHYRITIC LAVA ASH • O VOLCANIC CONGLOMERATE o . f P \ 0 O VOLCANIC BRECCIA
AGGLOMERAT E
Vertical scale - l,,= 3 0 1
Fig. 3. Idealized section of Montana Peak Formation.
i 30
The formation consists mainly of andesite agglomerates, breccias, and tuffs, with numerous ash layers and flows of porphyritic lava. Stratification is distinct throughout the unit except for some hills of massive tuff. The agglomerates, breccias, and conglomerates are by far the most important units of the Montana Peak Formation.
The agglomerates are commonly a deep-red or purple color.
They consist of angular to subangular blocks of porphyr itic andesite that vary from less than an inch to over 5 feet in.diameter. The blocks are set in a matrix of red ashy tuff which is quite soft and easily re moved by erosion. As a result, the agglomerate is rarely exposed as cliffs except in recently cut canyons.
Volcanic breccias and conglomerates occur interbedded with the agglomerates, usually distinctly separated, but occasionally mixed in a chaotic manner. Contacts between the beds are often gradational, with agglomerates grading into breccias and both into conglomerates.
Most breccia layers have fragments that average from a few inches to 3 feet in diameter. A few layers are extremely coarse, con taining boulders up to 12 feet in diameter. The fragments consist most commonly of large angular to subangular pieces of porphyr itic andesite and tuff. Porphyritic quartz latite, probably derived from the under lying Pajarito Lavas, is found as fragments throughout the breccias and conglomerates. The breccias usually grade upward into obviously water- lain conglomerates, which contain rounded fragments with the same 31
lithologies as those in the breccias. These rocks are generally red or purple, although some exposures of gray conglomerate occur.
The breccias and conglomerates are hard, well-indurated rocks
and are usually found as steep cliffs. Softer, less resistant fragments
of tuff in the breccias have been largely removed by weathering and
erosion, giving the breccia a rough, almost "vesicular" appearance.
Tuffs are the most variable rock type of the Montana Peak For
mation. They vary from well-indurated, rather pure layers of bright-
red color to masses of tan-colored sedimentary tuffs full of included
fragments. The fragments include both accidental and accessory rocks
in the lowest tuff layers, with an obvious increase in accessory material
higher in the section.
, The purest tuffs are usually less than 10 feet thick, and they
form small cliffs of wide lateral extent. On close observation many of
these layers are seen to be banded, with definite zones of red, tan,
yellow, and gray.
Other tuffs, which were probably water lain, contain a large
amount of fine-grained sediment. Along the east shore of Pena Blanca
Lake there is a good exposure of this type of rock. There the tuffs are
very well stratified, some layers even showing a "shaly" type, of part
ing, From a distance this outcrop has a "barber-pole" appearance, due
to alternating layers of red and white rock. These layers vary from a
few inches to over 30 feet in thickness and probably represent oxidation 32 of the tuffs during a period of sporadic volcanic eruptions (pi. VI).
A mass of reddish-brown tuff about 200 feet thick is exposed a mile north of Castle Rock. This mass, locally called Tarantula Butte, is composed of sedimentary tuff with a high content of sand and gravel sized accidental fragments. The rock is strongly fractured on the west side of the butte. Bedding is almost completely absent, suggesting that this tuff was deposited as one continuous unit.
The lavas of the Montana Peak Formation usually contain abundant fragments similar to those within the breccia and conglomerate layers. Occasionally the lavas occur as aphanitic or porphyritic flows completely devoid of foreign material (pi, VH).
The Montana Peak Formation is cut by numerous veinlets of quartz, chalcedony, opal, and calcite. These minerals are often found in comb structures, indicating open-space filling of pre-existing joints in the rock. Narrow dikes of andesite porphyry transect the formation in many directions. Somewhat more resistant, these dikes are nearly the same composition as the andesite flows. They are not found in any overlying formations, and thus they may represent the latest Cretaceous
(?) igneous activity in this area.
Porphyritic andesites of this formation commonly have a trachytic texture. The groundmass is holocrystalline, composed of
plagioclase microlites which are well aligned and which ’’flow" around
phenocrysts. PLATE VI MONTANA PEAK FORMATION
Part of a sequence of well-bedded andesite tuffs ex posed a fourth of a mile east of Pena Blanca Lake.
The alternating red and white layers are typical of certain tuff members of the Montana Peak Forma tion, Plate VI 33 PLATE VH MONTANA PEAK FORMATION
Porphyritic andesite lava of the Montana Peak Forma
tion„ This flow, which outcrops along the east shore
of Pena Blanca Lake, overlies a thick section of vol
canic conglomerates and breccias and is overlain by banded vitric tuff, Plate VII 34 PLATE Vin PHOTOMICROGRAPHS OF MONTANA PEAK FORMATION
Fig, 1, Porphyritic andesite: Euhedral hornblende
phenocryst, Subhedral phenocrysts of andesine and
biotite. Matrix of plagioclase and opaques, showing
trachytic texture. Nichols crossed, X 3,5.
Fig. 2. Porphyritic andesite: Large zoned pheno
cryst of andesine. Smaller phenocrysts of biotite,
hornblende, and quartz. Matrix of plagioclase and
opaques. Nichols crossed, X3.5.
Fig. 3. Porphyritic andesite: Phenocryst of
andesine. Biotite altering to magnetite. Smaller
phenocrysts of hornblende and orthoclase. Matrix
of plagioclase and opaques. Nichols crossed, X 3.5.
36
Plagioclase, biotite, and hornblende are the most abundant phenocrysts, along with a few scattered grains of quartz and orthoclase.
Alteration products include sericite, chlorite, hematite, and magnetite.
The phenocrysts occur singly, as several inter grown in parallel posi tion, or as irregular clusters. The lath-shaped plagioclase phenocrysts average between 3 and 5 mm in length. Biotite and hornblende average from 2 to 3 mm in diameter.
Modal composition of this rock is: matrix (plagioclase and opaques) 65 percent, plagioclase phenocrysts (An 37+) 25 percent, biotite 6 percent, hornblende 2 percent, orthoclase, quartz, sericite,
chlorite, hematite, and magnetite 2 percent.
Phenocrysts of plagioclase are most typically large, clear
laths of andesine and a few grains of labradorite. The larger grains
are usually zoned. They contain rounded inclusions of brown to red
glass and minute opaque inclusions, probably iron oxides. The pheno
crysts are mostly twinned and all show some degree of altering to
sericite.
Biotite and hornblende appear as euhedral grains or lath
shaped cross sections, all of which are partially or wholly replaced by
magnetite. Chlorite alteration of biotite is common.
A few badly altered phenocrysts of orthoclase are present, as
well as scattered grains of strongly resorbed quartz. Opal, chalcedony,
and calcite occur as amygdaloidal minerals and as fracture fillings. 37
Tertiary Rocks
Atascosa Formation
The Atascosa Formation was named by Webb and Coryell in
1954. They suggested that it is the youngest formation outcropping within the area covered by the Ruby Quadrangle. In the present study the upper half of Webb and CoryelPs Atascosa Formation is called the
"Pena Blanca Formation." It is considered to be of Quaternary age, as it is generally flat lying and has a lithology common to the Quater nary. The lower half, which is tilted and fractured, is called the :
Atascosa Formation in this report and is considered to be of Tertiary age.
This formation unconformably overlies the Cretaceous(?)
Montana Peak Formation. The angular unconformity that separates these two formations can be best seen in Pena Blanca Canyon north of the lake. There the Montana Peak agglomerates and lavas dip steeply toward the south and are covered by the tuffs of the Atascosa Formation that dip at shallower angles toward the northeast. A few pieces of petrified wood have been found at one exposure along the contact between the Montana Peak Formation and the Atascosa Formation. Small tree stumps were found which were most likely in place, as they were all properly oriented with the swollen base down and the trunk vertical.
This petrified wood probably represents part of a forest which once 38 grew on the old erosion surface of the Montana Peak Formation. The forest was killed by the falling ash that formed the tuffs of the Atascosa
Formation and was eventually partially petrified by the high silica con tent of the as ho Locally the tuffs of this formation unconformably over- lie the Cretaceous(?) Pajarito Lavas.
The generally flat-lying Pena Blanca Formation overlies the
Atascosa Formation. Inside the semicircle formed by the "Caldera
Cliffs” (fig. 2) and in a few other places, these formations meet with obvious angular unconformity.
Best exposed in the Atascosa Mountains north of Bellota Can yon, the Atascosa Formation covers approximately 2 square miles of the area covered by the present study. It crops out along the north and northwestern borders of the area mapped, where it appears as cliff- bounded cappings to a number of hills and ridges. The formation also crops out east of Pena Blanca Lake and forms the Caldera Cliffs be tween the Ruby-Nogales Road and Walker Canyon (pL IX). A few iso lated erosional remnants of the Atascosa Formation form stack-shaped buttes along ridges of the Montana Peak Formation.
In the Atascosa Mountains this formation, as mapped by Webb
and Coryell (1954), is about 800 feet thick. In the Pena Blanca and
Walker Canyon area the Atascosa Formation varies in thickness from
100 to 400 feet, probably due to the topography of the old surface of
deposition. This thickness does not include the uppermost 300 feet of PLATE DC ATASCOSA FORMATION
Part of the Caldera Cliffs, view looking north toward
Pena Blanca Lake, The cliffs are formed by an out crop of the Atascosa Formation, and they are com posed of lithic rhyolite tuffs with a few thin layers of interbedded conglomerate. Plate IX 39 PLATE X PHOTOMICROGRAPHS OF ATASCOSA FORMATION
Fig. 1. Lithic rhyolite tuff: Mass of devitrified glass,
crushed pumice lapilli, and numerous small accidental
inclusions. Nichols not crossed, X 3. 5.
Fig. 2. Silicified tuff (inclusion): Fine-grained quartz
and chalcedony cut by a veinlet of coarser, vuggy
quartz. Nichols crossed, X 3.5.
Fig. 3. Devitrified vitric tuff (inclusion): Spherulites
of orthoclase. Subhedral grains of sanidine and oligo-
clase. Spaces between the spherulites filled with fine
grained quartz. Nichols crossed, X 3.5, Plate X
Fig. I
Fig. 2
Fig. 3 41
Webb and CoryelFs Atascosa Formation,as it has been mapped as the
Pena Blanca Formation for this report.
The Atascosa Formation consists mainly of lithic and vitric tuffs with interbedded lenses of conglomerate. The whole formation is rather well indurated and is usually exposed as vertical cliffs of a dis tinctive white color. Bedding is pronounced, and in places the unit is broken by numerous joints and faults. The rock weathers into rounded shapes which look very much like weathered granitic rocks from a dis tance. . In a number of localities the tuff forms terrain that is extreme ly difficult to.cross. The vertical joints have been widened by weather ing and erosion, which created a maze of deep fissures and jumbled boulders. One of the best examples of this terrain is found on the southernmost part of the Caldera Cliffs just north of Walker Canyon.
The formation there is cut by numerous fissures, many more than 50 feet deep and from 5 to 25 feet wide. The bottoms of the fissures are made virtually impassable by large fallen blocks of rock.
Tuffs of the Atascosa Formation are for the most part well bedded and were probably water lain. Most layers are marked by dis tinct zones of well-sorted accidental fragments. The tuffs were prob ably deposited directly in lakes or were washed into bodies of standing water soon after their original deposition on land.
The tuff layers are quite variable in their relative resistance to erosion and in the amount and type of their included fragments. Lithic 42 tuffs full of accidental inclusions are the most common, although many thin vitric tuff layers occur. Ash layers are numerous and are soft and easily eroded. Near the base of the formation the layers have a baked appearance^ These beds are usually very hard and exhibit a
’’shaly” type of parting. Most of the tuff layers are full of blocks of pumice, some more than 12 inches in diameter. One layer about 25
feet thick is composed almost wholly of accretionary lapilli. These
hard spheres of tuff, also called pisolites or mud pellets, exhibit con
centric structures, each formed around a nucleus fragment. They may
represent accretion of wet ash around a fragment as it rolled along the
ground (Wentworth and Williams, 1932).
The most common type of inclusion in the tuffs of the Atascosa
Formation is angular fragments of welded tuff (ignimbrite). These
fragments are usually a purple or gray color, are often spherulitic,
and have the banding and streaking typical of welded tuffs. They prob
ably represent pieces of a welded ash flow that were deposited as one
of the first layers of the Atascosa Formation. Since no layer of
ignimbrite is exposed within the Pena Blanca and Walker Canyon area,
it is also possible that the source of these fragments was outside the
boundaries of the area. Lithic fragments include pieces of all the older
formations discussed in this paper, most typically lavas, silicified tuff,
and quartzite.
The tuffs have a rhyolitic to quartz latitic composition and 43 consist mainly of devitrified glass and pumice lapilli. They are gen erally argillaceous and contain euhedral to anhedral phenocrysts of orthoclase, sanidine, oligoclase, and quartz. Biotite, magnetite, and glass shards occur in lesser amounts. Secondary minerals include sericite, calcite, opal, and chalcedony.
Quaternary Rocks
Pena Blanca Formation
For the purpose of this report, the upper part of Webb and
CoryelFs Atascosa Formation is called the "Pena Blanca Formation,"
It is considered to be of Quaternary age and can be distinguished from the underlying Atascosa Formation by differences in their lithology and structure. A sequence of rocks similar to the Pena Blanca Formation occurs in the Santa Rita Mountains, overlying lake deposits that are thought to be late Pliocene or Pleistocene (Schrader, 1915). Locally the Pena Blanca Formation unconformably overlies the Montana Peak
Formation and the Pajarito Lavas, both of supposed Cretaceous age.
Overlain by unconsolidated alluvial material, the Pena Blanca Forma tion represents the youngest rocks exposed in the area studied,
This formation covers most of the northeastern quarter of the
Pena Blanca and Walker Canyon area. It is particularly well exposed along the Ruby-rNogales Road and in the east branch of Walker Canyon. 44
Along the eastern side of Pena Blanca Canyon north of the lake, this formation occurs as a prominent cliff cresting a fault-line scarp.
The Pena Blanca Formation is approximately 350 feet thick near the western limit of its outcrop area. It dips eastward at very shallow angles and appears to thin in the direction of dip. In a few places the attitude of these rocks is anomalous, due to several strong faults which have tilted large blocks of the formation.
The formation consists of a sequence of distinctly bedded con glomerates, with interbedded layers of tuff and basalt. Heindl (1952) included these rocks within the "Gila Conglomerate, " a term now used in a restricted sense as a synonym for valley fill. Except for the layers of tuff and basalt, these sediments appear to have been deposited under conditions similar to those existing in southern Arizona today. Numer ous deep, narrow ravines cut the rock, most of them formed along a series of joints. A dense growth of lichen covers most of the forma tion, giving it a characteristic greenish color when viewed from a dis tance.
Conglomerate beds of this formation are well consolidated and
stand in vertical to overhanging cliffs. The conglomerate is generally very coarse, although layers of well-sorted, finer grained conglomer
ate and lenses of arkosic sandstone are common. Fragments in the
conglomerate represent all of the older rocks of the area, as well as
some types not ejqposed anywhere nearby. Subangular to rounded 45 boulders of ignimbrite, andesite, and quartz latite are the most abun dant and are accompanied by smaller, rounded fragments of arkose and rhyolite. The conglomerates are tuffaceous and are cemented by both silica and calcium carbonate. Chalcedony is found in numerous vein- lets and as a veneer on many fragments throughout the formation.
There are many distinct layers of tuff within the Pena Blanca
Formation, measuring from a few inches to over 6 feet in thickness
(pi. XI). They vary from thin, almost pure beds of vitric tuff to layers of sedimentary tuff with a high content of accidental fragments.
The tuff layers are usually well bedded and lens shaped, indicat ing that they were probably deposited in ponds. A few short, relatively thick beds have a baked zone along their base. They probably were formed by ash flows that were not hot enough to completely "weld" the glass shards. Other thin, widespread beds of tuff are soft and ashy and often partially altered to greenish bentonite.
Basalt occurs in one short flow near the eastern border of the area mapped. The flow has a thickness of 25 feet and caps a ridge bordering the east branch of Walker Canyon. This narrow, elongated outcrop is ejqjosed for about 400 yards and probably represents a "re versal of topography" situation. The basalt dips toward the north at about 10°, cutting across conglomerates that are nearly flat lying. This
suggests that the source of the basalt may have been somewhere to the
south of the presently exposed flow. PLATE XI PENA BLANCA FORMATION
Layer of lithic rhyolite tuff interbedded with the con glomerates of the Pena Blanca Formation. This ex posure is on the Ruby-Nogales Road a mile east of
Walker Canyon. Plate XI 46 PLATE XH PHOTOMICROGRAPHS OF PENA BLANCA FORMATION
Fig. Te Basalt: Phenocrysts of labradorite. Smaller
dark grains of olivine replaced by iddingsite. Matrix
of labradorite, magnetite, and glass, showing trachytic
texture. Nichols crossed, X 3.5.
Fig. 2. Ignimbrite (inclusion): Fused and flattened
glass shards. Poorly formed spherulites of ortho-
clase and quartz. Nichols not crossed, X 3.5.
Fig. 3. Ignimbrite (inclusion): Fused and flattened
glass shards. Core of the elongate glass pieces is
hyaline and isotropic, rimmed by normally arranged
fibres of orthoclase and quartz. Nichols not crossed,
X 3 .5 . Plate XII 47
Fig. I
Fig. 2
Fig. 3 48
The basalt overlies a thick section of conglomerates. The basalt-conglomerate contact is marked by a layer of ash about 6 inches thick. Rounded boulders of reddish scoria, which may have formed as bombs, lie on top of the black, somewhat vesicular flow and are found weathering out of the overlying sediments.
In thin sections the basalt is seen to be porphyritic, with a pilotaxitic texture. Plagioclase, olivine, and augite appear as pheno-
crysts, as well as a few grains of orthoclase and badly strained quartz.
The matrix is composed of micrelites of plagioclase, small crystals of
magnetite, and interstitial glass. Plagioclase microlites in the ground-
mass are well aligned and "flow" around phenocrysts. v • ■ . ■■ ' -■ ■■ The plagioclase phenocrysts are labradorite (An 54+) and are
normally zoned, twinned, and only slightly altered. They are lath
shaped, average 3 to 4 mm in length, and are usually somewhat resorbed.
Matrix plagioclase is also labradorite (An 50+).
Olivine is abundant and occurs as anhedral grains about 2 mm
in diameter. The olivine is rimmed by bright-red iddingsite, which has
completely replaced some grains. Zoned subhedral phenocrysts of
augite are common. Glass is found as interstitial material within the
matrix and as rounded inclusions in the plagioclase and augite pheno
crysts. 49
Intrusive Rocks
Only three mappable bodies of intrusive rock are exposed in the Pena Blanca and Walker Canyon area. These rocks occur as two short dikes and one sill, cutting the Pajarito Lavas. They are all oriented roughly parallel to a strong northwest-trending system of joints and faults. The two dikes are mineralogically similar to the rocks of the Pajarito Lavas, and the sill is similar to the lavas of the Montana
Peak Formation.
The larger of the dikes outcrops in Pena Blanca Canyon about
2 miles south of the canyonls junction with the Ruby-Nogales Road. This dike is about 55 feet wide and, being more resistant than the surround ing rock, forms a steep waterfall in the streambed of the canyon. Con tacts between the dike and the lavas are sharp and distinct. A finer grained "chilled zone" a few inches wide occurs in the dike just inside the contacts. This dike strikes N. 50° W, and dips northeast at 55,
This dike is a quartz latite porphyry and is characterized by an abundance of very large phenocrysts of sanidine. Other phenocryst minerals include plagioclase, quartz, biotite, and hornblende. Matrix material is extremely fine grained and appears to consist of quartz and orthoclase.
Modal composition of this rock is: matrix 60 percent, sanidine phenocrysts 15 percent, plagioclase (An 38+) 10 percent, quartz 8 50 percent, biotite and hornblende 4 percent, sericite, calcite, apatite, glass, and magnetite 3 percent.
The sanidine phenocrysts often measure between 2 and 3 cm in length, although most of them are less than 1 cm,. They are general ly twinned by the Carlsbad law and have zonally arranged inclusions of plagioclase, biotite, hornblende, quartz, glass, and apatite. The sanidine is clear and almost wholly free of alteration.
Plagioclase phenocrysts are much smaller than the sanidine, averaging a few millimeters in length. They consist of euhedral to sub- hedral laths of andesine, commonly quite altered to sericite and calcite.
Quartz occurs in the groundmass and as resorbed euhedral phenocrysts with inclusions of clear, droplike glass. Biotite and horn blende appear as euhedral phenocrysts altering to magnetite. Small opaque grains, probably magnetite, are abundant in the matrix.
Another dike is exposed in Calabasas Canyon about 1-1/2 miles east of Pajarito Peak. This dike is about 40 feet thick and transects the
Pajarito Lavas just south of the fault contact between the lavas and the
Pena Blanca Formation, The dike strikes N. 75° W, and dips north east at 66°. It is accompanied by a number of smaller parallel dikes of the same composition as the major dike.
It is composed of holocrystalline quartz monzonite porphyry and has a hypidiomorphic granular texture. Orthoclase, microcline, and plagioclase occur as euhedral to subhedral phenocrysts and as the 51 main constituents of the crystalline matrix. Quartz is abundant in the matrix, where it appears as anhedral interstitial material. Dark-brown euhedral biotite grains occur within the groundmass, partially or wholly altered to magnetite and chlorite. Many of the feldspars are altered to sericite and clay minerals.
Modal composition of this rock is: quartz 35 percent, plagio- clase (An 28+) 25 percent, orthoclase 20 percent, microcline 10 per cent, biotite 5 percent, magnetite, chlorite, sericite, and clay minerals
5 percent.
Both dikes discussed so far have a definite mineralogical sim ilarity to their host rock, the Pajarito Lavas. There are some minor variations in composition, such as the occurrence of apatite in the quartz latite porphyry dike and microcline in the quartz monzonite porphyry dike. The major differences between the dikes and their host rock are textural and structural. Because of this similarity and the fact that the dikes are only found cutting the Pajarito Lavas, it is likely that the dikes and the lavas originated from the same source. After the extru sion, cooling, and fracturing of the lavas, a resurgence of igneous activity probably occurred, forcing magma up into the fractures. The textural differences between the dikes are most likely a result of dissimilar rates of cooling.
A sill 5 feet thick is exposed in a branch of Calabasas Canyon half a mile north-northeast of International Boundary Monument No. 52
128. It is less resistant than the Pajarito Lavas that it intrudes and forms the canyonrs streambed for about 50 feet. The sill is concordant with the bedding in the lavas, strikes N. 46° W., and dips 38° NE.
Chilled zones less than an inch wide occur on the sillrs edges.
The heavy, reddish-brown rock of the sill is composed of andesite porphyry and is darkly stained by iron and manganese oxides.
It is a holocrystalline rock, consisting almost wholly of lath-shaped
grains of plagioclase. Magnetite, chlorite, and sericite are the only
other minerals present.
Plagioclase occurs as zoned and twinned subhedral phenocrysts
of oligoclase, and less commonly, andesine. The matrix is mostly
andesine laths and has a pilotaxitic texture. Magnetite is very abundant
as small discrete grains and in numerous veinlets. Biotite may have
been present in the matrix, but only chlorite is now visible. Many
plagioclase grains are completely sericitized, and a few are wholly re
placed by magnetite.
This rock is quite similar mineralogically to some of the lavas
of the Montana Peak Formation. It was intruded into the uppermost ex
posed layers of the Pajarito Lavas and may have been emplaced during
the time the Montana Peak lavas were being extruded.
Possible Correlations
It is difficult to make any definite correlations away from the PLATE Xin PHOTOMICROGRAPHS OF INTRUSIVE ROCKS
Figo I. Quartz latite porphyry: Large sanidine pheno- cryst with zonally arranged inclusions of plagioclase, hornblende, quartz, and glass. Smaller phenocrysts of andesine and biotite. Matrix of quartz, orthoclase, and magnetite. Nichols crossed, X 3.5.
Fig. 2. Quartz monzonite porphyry: Phenocrysts of orthoclase, microcline, and oligoclase. Groundmass of interstitial quartz and euhedral biotite altered to magnetite. Nichols crossed, X 3.5.
Fig. 3. Andesite porphyry: Large phenocryst of zoned oligoclase. Matrix of andesine, magnetite, and chlorite, showing trachytic texture. Nichols crossed, X 3.5. Plate XIII 54 immediate area studied because of the abruptly varying nature of the volcanic rocks. The sedimentary rocks were deposited in lakes of un known extent or as alluvial material, and being devoid of fossils they are difficult to correlate with other units. Some general similarities do exist, however, and may be used to indicate possible regional cor relations. The work necessary to prove such correlations is beyond the scope of this paper, and only the more outstanding possibilities are suggested.
Many of the mountain ranges of Santa Cruz County are con tinuous into northern Sonora, Mexico. It is likely, therefore, that the most rewarding attempts at correlation will be made by doing further work in these two areas.
Taylor (1959) stated that:
An excellent correlation with Santa Cruz County can be found in central Cochise County as mapped by Gilluly (1956). He assigned the oldest volcanic rocks to the Cretaceous but stated that they might be Triassic or Jurassic, The Cre- taceous(?) andesite and overlying sequence appear to cor relate well with the sequences of Santa Cruz County.
Pajarito Lavas
Rocks very similar to the Pajarito Lavas crop out just east of
Nogales, Sonora. These rocks also form the northern tip of the Sierra de los Pajaritos, discussed earlier in this paper. 55
Oro Blanco Conglomerate
The Oro Blanco Conglomerate covers a large area in the west ern part of Santa Cruz County. Fowler (1938) stated that it is probably part of a similar formation that is widespread in south-central
Arizona, from west of the Baboquivari Mountains to the Santa Rita
Mountains, east of the Santa Cruz River. ”
In north-central Sonora the Trincheras Formation has a lithology very similar to the Oro Blanco Conglomerate. It rests upon volcanic rocks much like the Pajarito Lavas and is overlain unconform- ably by andesite lavas and tuffs (Bumble, 1900).
Taylor (1959) suggests a possible correlation between the Oro
Blanco Conglomerate and a conglomerate in the Santa Rita Mountains which was first described by Schrader (1915).
Montana Peak Formation
Rocks similar to the Montana Peak lavas, agglomerates, and tuffs are quite common in south-central Arizona and north-central
Sonora. Taylor (1959) suggests a possible correlation of the Montana
Peak Formation with the Cat Mountain Rhyolite of the Tucson Mountains and with "rhyolite elastics and flows" of the Santa Rita, Patagonia, and
Mustang Mountains.
The Nogales Formation, described by King (1939), includes 56 andesite lavas and tuffs and probably can be correlated with the Montana
Peak Formation. Its type locality is near Nogales, Sonora, just 15 miles east-southeast of the Pena Blanca and Walker Canyon area.
Atascosa Formation
The tuffs of the Atascosa Formation are similar in composition and thickness to the Safford Tuff of the Tucson Mountains (Taylor, 1959).
Another possible correlative unit occurs in the Salero area near the
Grosvenor Hills, Santa Cruz County (Schrader, 1915).
Talliaferro (1933) described a layer of "vitric rhyolite tuff” about 800 feet thick just south of the International Boundary near
Cabullona, Sonora. Its thickness, composition, and stratigraphic rela tionships suggest that it might be related to the Atascosa Formation.
Pena Blanca Formation
As mentioned earlier in this report (p. 44), the rocks included within the Pena Blanca Formation have been considered part of the ”Gila
Conglomerate. ” Similar sequences of conglomerates, tuffs, and basalt flows are common throughout southern Arizona, particularly in the structural valleys of the Gila River and some of its tributaries (Heindl,
1952).
The Baucarit Formation of northern and central Sonora is equivalent in a broad way to the Gila Conglomerate (Pye, 1959). The 57
Baucarit Formation occurs in valleys between mountain ranges and is usually flat lying or dipping at low angles, and it contains the same rock types as the Pena Blanca Formation,
Structural Geology
Faulting and. Jointing
The Pena Blanca and Walker Canyon area is probably located within a zone of recurrent breaking, as all the rocks exposed have suf fered some degree of fracturing and movement. Angular unconformities seem to separate almost all the formations. However, due to the rela tively steep angle of repose commonly observed in lavas, it is often dif ficult to distinguish between angular unconformities and original bedding.
Faults and joints transect the area and appear to have been formed dur ing a number of intermittent events throughout a long span of geologic time. Four prominent directions of faulting exist, each accompanied by numerous parallel joints.
One of the strongest zones of faults and joints in the area bounds the north flank of the Pajarito Mountains. The fractures along this zone
generally trend between N. 70° W. and N. 80° W. They dip at very
steep angles toward the northeast and in some cases are almost vertical.
The formation on the south side of this set of faults has been relatively
upthrown. This west-northwest fracture trend is best developed in the 58 southern half of the area mapped, although it is quite common in the rest of the area.
A strong second direction of faulting and jointing trends gen erally northeast. Fractures along it usually strike between N. 50° E. and N. 70° E. and are vertical or steeply dipping toward the north west. Parts of Walker, Pena Blanca, Old Bextrum, Calabasas, and
Bellota Canyons have been formed along fractures having this northeast trend. Several small mines on the north flank of the Pajarito Mountains follow a number of parallel tension cracks which generally trend north east.
The third regional fracture set has a prominent north-south
' ...... ' • ' ■ ' 1 ■ : . : L . . : trend. It varies in strike from N. 10° E. to N. 10° W. and is char acterized by numerous closely spaced, vertical joints such as those cutting the west side of Tarantula Butte. A line of cliffs east and north of Pena Blanca Lake, as well as the east branch of Walker Canyon, fol low strong fault zones having a north-south trend.
An east-west fracture system is important locally but is not as
' » . _ . : widespread as the other three systems discussed. Joints and faults with this trend strike between N. 80° E. and due east. Parts of Castle
Rock Canyon and numerous small ravines cutting the Montana Peak For mation are formed along fractures with this trend. 59
Folding
The only major folding in the area covered by the Ruby Quad rangle is represented by a broad synclinal structure in the western sec tion of the quadrangle. The axis of this fold trends northwest and plunges gently to the southeast (Webb and Coryell, 1954),
In the Pena Blanca and Walker Canyon area the rocks are either block faulted or flat laying, and no large folds occur, Penecontempora- neous folding, due to slumping of partially consolidated tuff, is common.
One small fold is particularly interesting, as it may give a clue to the origin of Castle Rock,
This fold is exposed on the north side of Castle Rock Canyon half a mile west of the canyon*s junction with the Ruby-Nogales High way, At this exposure there are a number of disconnected layers and lenses of conglomerate imbedded within a cliff of vitric rhyolite tuff.
The conglomerate, which is very similar to the Oro Blanco Conglom erate, has been faulted, tilted, and folded.
Layers of the conglomerate vary from 2 to 6 feet in thickness and are accompanied by lens-shaped pods of conglomerate 2 or 3 feet long and about a foot wide. Along the top contact of the layers is a hard, baked zone a few inches wide. This baked zone also surrounds all the isolated pods of conglomerate.
All the larger layers of conglomerate are tabular and end 60 abruptly along numerous small faults or are isolated and wholly sur rounded by tuff (pi. XTV, fig. 1). The faults follow no particular trend and include both normal and reverse types.
A small anticlinal fold occurs in one of the least disturbed conglomerate layers. The fold is asymmetrical, about 3-1/2 feet high, and has been somewhat separated from the rest of the layer by a small fault (pi. XIV, fig. 2).
This folding, the chaotic orientation of large blocks of con glomerate and tuff, and the baked contacts probably are a result of a single sudden event. The event may have been a landslide of hot masses of tuff that flowed into a depression containing semiconsolidated gravels.
The force of the rushing blocks of tuff and ash would tear loose slabs of the gravel, finally coming to rest with the slabs oriented at various at titudes. The tuffs unevenly distributed weight would cause squeezing and compression of the soft, probably wet gravels beneath, forming small "diapiric” folds.
The baked contacts indicate that the tuffs might have been quite hot when the landslide occurred. This suggests that the source of the tuffs might not have been too far away.
Other Structural Features
There are three features in the Pena Blanca and Walker Canyon area that may be structural in origin, but they are rather difficult to PLATE XIV FAULTED AND FOLDED CONGLOMERATE
Fig, 1. Slabs of faulted conglomerate exposed in the base of a cliff in Castle Rock Canyon. The conglom erate layers are chaotically oriented within a mass of lithic rhyolite tuff.
Fig. 2. Small ’’diapiric” fold exposed in Castle Rock
Canyon 35 feet east of the preceding photo. Plate XIV 61
Fig. I
Fig. 2 62
definitely classify. Two of these, Castle Rock and Mouse Rock, are
masses of tuff that may have formed as diatremes. The third, Whitney
Mesa, is a circular feature that may be the remains of a caldera,
Castle Rock and Mouse Rock
Castle Rock is a mass of lithic rhyolite tuff. Its tuffs are very
similar to the material surrounding the disturbed conglomerate beds
mentioned on page 60. It is roughly elliptical in outline, about 550 feet
tall, and covers approximately 80 acres (pi. XV, fig. 1). It lies within
a strong zone of east-west-trending fractures a few hundred feet south
of the disturbed conglomerates and is broken by numerous joints and
faults.
Formations surrounding Castle Rock include the Pajarito Lavas
on the south, west, and east, the Oro Blanco Conglomerate on the north,
and the Montana Peak Formation on the northeast corner. Parts of
these formations closest to the borders of Castle Rock are stained a
deep-red color. This color, which may have been caused by weathering
or by the hydrothermal addition of iron oxides, extends into the tuffs in
side the periphery of Castle Rock.
' The tuffs are mostly well bedded, with the exception of a "core"
of massive tuff which is best exposed on Castle Rockrs east face. The
core is composed of the same type rock as the bedded layers but is
harder and somewhat more resistant. It extends about 200 feet up the PLATE XV CASTLE ROCK
Fig, 1, Air view of Castle Rock. The Oro Blanco
Conglomerate is the reddish rock on the right side of the photo. The Pajarito Lavas form the hills on the left and top of the picture.
Fig. 2. Northeast face of Castle Rock, "Core" of massive, nonbedded tuff, surrounded by inward dipping layers of tuff. The trees at the base of the cliff are 15 to 25 feet high and may be used for scale Plote XV 63
Fig. 2 64 vertical east face-and terminates in a blunt, rounded end. Layers of bedded tuff dip toward it from either side. These layers dip steeply near the base of the cliff and gradually flatten out higher up. They join above the blunt end of the core and continue to flatten until, near the top, they are flat lying (pi. XV, fig. 2).
Accidental inclusions in the tuff are very abundant and are gen erally poorly sorted. Many boulders more than 3 feet in diameter occur, mixed randomly with gravel and sand-sized fragments. The fragments are angular to well rounded and consist primarily of pieces of ignimbrite and quartz latite. Rhyolite, greenish silicified tuff, granite, and quartz ite also occur in abundance. The granite fragments are unique, as they are not found in any other formation in the area. Some of the quartzite fragments are also of a type not found anywhere nearby. The granite and quartzite inclusions may represent basement rocks brought up from depth by volcanic activity.
Castle Rock does not seem to be part of any formation described
in this paper. It may be an erosional remnant of a layer that was once
much more extensive. It is also possible that it is the remains of an
ash-filled volcanic vent or a diatreme. It is difficult to prove any of
these possibilities without doing quite a bit more work. Detailed map
ping, combined with gravimetric studies, would probably be necessary.
The facts available are far from conclusive but seem to favor
a diatreme origin for Castle Rock. Briefly they are: 65
1. No rocks are exposed that definitely underlie Castle Rock.
2. Accidental fragments are poorly sorted and include pieces
of rock not found anywhere nearby.
3. Massive core, surrounded by dipping beds which look like
they were deposited in a steep depression.
4. Location along a fault zone.
5. Severe fracturing of neighboring rocks.
6. Elliptical shape, having rather good symmetry.
7. Chaotic orientation of nearby conglomerate beds and small
"diapiric" fold.
Regardless of whether Castle Rock is a diatreme or an erosion- al remnant, the pyroclastic activity that formed it probably occurred soon after deposition of the Oro Blanco Conglomerate and before depo sition of the Montana Peak Formation, This relative age is suggested by the lack of Montana Peak type rock fragments in the Castle Rock tuffs. K Castle Rock was the origin of the tuff slides mentioned on page
60, it must have formed after the Oro Blanco gravels were deposited and before they were completely consolidated.
A smaller similar feature occurs about a mile north-northwest of Castle Rock. This structure, locally known as "Mouse Rock, " is an elliptical body of lithic rhyolite tuff about 600 feet long and 230 feet wide. It lies just west of Tarantula Butte in a canyon developed along an east-west fault zone. The canyon cuts through Mouse Rock, forming 66 a steep narrow defile.
Mouse Rock is completely surrounded by the andesitic lavas and pyroclastic s of the Montana Peak Formation. On its southern and western sides it abruptly rises above the surrounding rock, forming vertical cliffs 30 to 40 feet high. The cliffs become lower on the north
and east sides, and in places the tuffs of Mouse Rock are level with the
andesites.
The tuffs are very much like those of Castle Rock and have a
similar suite of inclusions. Most of the tuffs occur as large, well- bedded blocks which generally dip toward the center of the mass. These
blocks enclose a central core of massive, nonbedded tuff and are sur
rounded by a MsheathM of similar material.
Mouse Rock may have formed as a diatreme, the massive tuffs
emplaced by gas escaping from a vent and the blocks mis-oriented by
sinking. It seems unlikely that this feature is an erosional remnant,
as it lies deep in a canyon well below the general level of the upper con
tacts of the Montana Peak Formation.
Whitney Mesa
The Caldera Cliffs enclose part of a large structural feature
of the Pena Blanca and Walker Canyon area that is locally called
"Whitney Mesa. " Although this feature is not actually a mesa, but
rather a curved escarpment, the local name will be used for the purposes 67 of tMs report. The cliffs half encircle a structural depression which may be a partly buried caldera.
About 170° of arc is closed by the cliffs, forming a segment of a circle 1-1/2 miles in diameter. The cliffs are composed of large blocks of the Atascosa Formation and are broken by a number of faults.
The faults, which are mostly radial to the center of the structure, do not generally enter or affect other formations. Other faults are paral lel to the perimeter of Whitney Mesa. Canyons that formed along these faults roughly outline a circular shape and suggest the location of the borders of the structure where it is covered by younger rocks (fig. 4).
Except for a few blocks that have been rotated and tilted by faulting, the tuff layers that comprise the cliffs obviously dip toward a common center. Dips average from 15° to 30°, the steepest dips occurring on the north and northeast corners of the structure (fig. 4).
The eastern half of the supposed circle of tuff is covered by a thick section of the Pena Blanca Formation. The interior of the circle is also occupied by this formation, which extends almost to the edge of the Caldera Cliffs. Near the center, the flat-lying conglomerate of the
Pena Blanca Formation is at least 200 feet thick and thins rapidly to ward the bordering cliffs.
There is not really enough evidence to be certain that Whitney
Mesa actually is a caldera. It does seem obvious, however, that this feature indicates a collapsed area or a basin-shaped downfold. 68
WHITNEY MESA
1/2 MILE
Fig. 4. Sketch map of Whitney Mesa, showing outcrop of Atascosa Formation. 69
A collapse caldera of the Glen Coe type, as described by
Williams (1941), generally has radial and concentric fractures some what similar to those of Whitney Mesa, The inward-dipping beds of tuff also strongly suggest subsidence of the center of the structure.
Since the overlying beds of the Pena Blanca Formation are not dis turbed, this subsidence must have occurred before their deposition.
It might have been caused by withdrawal of magma, which was then de posited as some of the lower tuff beds of the Pena Blanca Formation.
Economic Geology
Geology of the Pajarito Mining District
The Pajarito Mining District is located on the north flank of the
Pajarito Mountains between Walker and Old Bextrum Canyons. It con sists of a group of silver-bearing lead claims, most of which have been inoperative for the past few years. Most of the claims are held today by Val Cason of Pena Blanca Canyon, and a few are held by W. H,
Clarke of Nogales, Arizona.
Host rock for the ores is the porphyritic quartz latite rock of the Pajarito Lavas, The ore primarily consists of massive galena and oxidized minerals of lead. Silver and uranium ores occur to a lesser degree.
Mineralization has been localized in most of the mines by 70 steeply dipping fractures which generally trend northeast. While these northeast shears actually contain the ore, grouping of the mineral de posits indicates strong control by a northwest-trending fault zone.
The ore occurs as scattered pockets within a quartz and car bonate gangue. These pockets are due to irregularities along the frac tures1 walls, which caused a series of "pinches and swells" to develop.
Evidence of epithermal open-space filling, such as comb struc ture and crustification, are found in some of the mines. The mineralogy and textures suggest that the deposits are epithermal.
Figures 5, 6, 7, and 8 are sketches of polished sections of ore from four mines in the district. Probable stages in the development of the ores were determined by study of the polished sections and are sum marized as follows.
After faulting had occurred marcasite was formed in the gouge, filling fractures and replacing feldspars of the quartz latite host. This is suggested by the fact that little ore is found outside the bounding fault planes and no trace of the host rock is left except quartz phenocrysts.
Galena was next in the paragenetic sequence. It followed many
of the original fractures and replaced most of the marcasite. The
galena, which carries some silver, was then partly replaced by anglesite
and cerussite (figs. 5 and 6). Alteration developed along cleavage direc
tions of the galena, eventually isolating and "rounding” the fragments
(figs. 6 and 7). 71
QUARTZ
MARCASITE -ANGLESITE
Fig. 5. Polished section - Midnight Mine
Fig. 6. Polished section — St. Patrick Mine 72
GALENA ANGLESITE
PYRITE
CO VELLITE
Fig. 7. Polished section — Corning Glory Mine
QUARTZ GALENA PYRITE
ANGLESITE
COVELLITE
ARGENTITE MARCASITE
Fig. 8. Polished section — Sunset no I Mine Table n Minerals recognized in the Pajarito Mining District
Primary: Galena
Sphalerite
Acanthite (argentite)
P yrite
Arsenopyrite
Chalcopyrite
Cinnabar
Secondary: Native silver
M arcasite
Pitchblende
Anglesite
C erussite
Covellite
Wulfenite
Vanadinite
M imetite
Gangue: Q uartz
Amethyst
Calcite
Fluorite 74
Covellite and minor amounts of pyrite were the last minerals to form. In the 14 samples studied covellite was found only within the anglesite, usually near the contact with unaltered galena. Copper needed for the covellite may have been tied up in the galena or other pre-existing sulphides and released to form its own sulphides after alteration. The iron for the pyrite probably originated in the replaced marcasite. (See figs. 7 and 8.)
The mine workings are mostly simple horizontal openings that follow the gouge zone of faults. A few crosscuts and stopes occur in the White Oak Mine in Walker Canyon and the St. Patrick Mine in Pena
Blanca Canyon. Shafts were sunk at the Old Bextrum Mine and at two very old, unnamed workings near the western limit of the district.
Shallow veins were developed by a number of open-cut trenches.
History of the Pajarito Mining District
The mines of the Pajarito Mining District have been worked sporadically since about 1885. No production records are available, as ownership of the mines has changed hands many times. Prior to
1900 over $200,000 worth of silver ores was shipped from these claims
(Blake, 1901). Mr. Val Cason, who has mined in the district for more than 30 years, states that a few native silver pockets were found which brought more than $100,000 each to various operators. Most ore mined in the past 30 years was sold in small shipments to Hugo Miller of 75
Nogales. :
Four mines in the district have accounted for most of the lead- silver production. The St. Patrick, Morning Glory, and Big Steve
Mines produced lead ore with small silver values. Sunset No. 1 Mine, where coarse wires of native silver were found, was the best silver producer.
The White Oak Mine is the only deposit known to contain uranium mineralization in the district. A few tons of pitchblende were extracted from a northwest-trending fracture of the mine (Webb and Coryell, 1954).
Summary of Geologic History
The geologic history of the Pena Blanca and Walker Canyon area is characterized by recurring volcanic and tectonic activity. Ex posed formations range from Cretaceous(?) to Recent in age, and they consist of volcanic and interbedded sedimentary rocks.
The Pajarito Lavas are the oldest rocks in the area. They were formed by a number of flows of porphyritic quartz latite which were probably extruded some time during the Cretaceous(?). The flows thicken to the southwest and probably originated somewhere in the Sierra
de los Pajaritos of Sonora. After the rocks cooled, hardened, and frac tured, they were intruded by dikes of quartz latite porphyry and quartz
monzonite porphyry.
An erosion surface developed on the lavas and eventually was 76 partially buried by a layer of ash. This ash developed into the greenish silicified tuff exposed in Castle Rock Canyon and was found as fragments in all the younger rocks. Reworked pieces of the Pajarito Lavas and other rocks accumulated as alluvial material and formed the Cretaceous
(?) Oro Blanco Conglomerate.
Castle Rock probably was formed during the consolidation of the Oro Blanco gravels. Masses of hot tuff slid into depressions, tear ing loose slabs of the gravels and contorting the soft underlying beds.
Faulting and fracturing of the area surrounding Castle Rock probably took place about the same time and tilted the otherwise flat-lying beds of the Oro Blanco Conglomerate.
The andesitic lavas, tuffs, and agglomerates of the Creta- ceous(?) Montana Peak Formation were the next rocks to form. They were deposited on tilted older rocks during a long period of sporadic volcanic activity. Layers of the Montana Peak Formation were eroded, redeposited as breccia and conglomerate beds, and covered again by deposition of lavas and tuffs from repeated eruptions. The andesite porphyry sill in the Pajarito Mountains may have been emplaced some time during this activity. After the rocks of the Montana Peak Forma tion solidified they were subjected to severe fracturing and faulting.
A soil layer developed on the erosion surface of these rocks.
Trees growing on the soil were killed by ash falls, marking the be
ginning of the deposition of the Tertiary (?) Atascosa Formation. r 77
Mouse Rock may have been formed about this time. It is pos-
, sible that it and Castle Rock are diatrernes or ash-filled volcanic vents,
and, if so, they are the first ones recognized in this part of Arizona.
The ash falls continued, building up a layer more than 500 feet
thick. Gravel from higher surrounding areas mixed with the ash or was
washed into depressions and covered by new layers of ejecta. These
ash layers and inter bedded gravels were lithified and formed the tuffs
and conglomerates of the Atascosa Formation.
Tilting and faulting of the rocks took place again, during which
time the subsidence of Whitney Mesa probably occurred. Whitney Mesa,
which may have formed as a collapse caldera, was then partially
covered by the conglomerates of the Quaternary Pena Blanca Forma
tion.
The Pena Blanca Formation was considered as the upper part
of the A tascosa Form ation by Webb and Coryell (1954) but was mapped
as a separate unit for this report. It can be distinguished from the
Atascosa Formation by the angular unconformity which separates the
units and by differences in their lithologies. The Pena Blanca Forma
tion probably was deposited in structural valleys under conditions sim
ilar to those in southern Arizona today. A number of thin beds of tuff
and one basalt flow are interbedded with the conglomerates and repre
sent the most recent volcanic activity in the area.
After the deposition of the Pena Blanca Formation the area was again subjected to tectonic activity. Strong faults and numerous joints were formed which affected all the formations exposed. SELECTED BIBLIOGRAPHY
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PLATE I
R. 12 E.
T. 2 3 S.
EXPLANATION
Silt, sand and gravel Quaternary -
Qpb) Pena Blanca Formation T - l u f f 8 - b a S a l t
? Tertiary Atascosa Formation
Kmp T Montana Peak Formation T — massive tuff
P Cretaceous^ Kob Oro Blanco Conglomerate
Pajarito Lavas
Post Tuff (undifferentiated) Pre
Quartz latite porphyry
Post Kpl Quartz monzonite porphyry
Andesite porphyry
Fault
High angle fault5 U, up and D, down
Joint
Joint set
— Fractures of unknown displacement
>— Portal of adit
E Shaft
'/?&•)/ V < w y . Joint set
Fractures of unknown displacement
)— Portal of adit
P Shaft
;•...... 7; Trench
VABM Horizontal control station*- A vertical angle elevation 5236
BM 4190 vertical control station T. 24 S 5^32 Spot elevation
12 7* International Boundary monument
+ Section corner
Improved light duty rood
Unimproved dirt rood
Trail
■ Building
■ # » Foke, pond, cattle tank, etc.
o Spring
------Intermittent stream GEOLOGIC MAP OF THE PENA BLANCA AND WALKER CANYON AREA, SANTA CRUZ COUNTY, ARIZONA
SCALE I * 16,700 1/2 I I ] MILES
CONTOUR INTERVAL 250 FEET DATUM-SEA LEVEL
Topography based on Ruby Quadrangle, U. S. Geological Survey 15 Minute Serie EXPLANATION T arantulo Ruby- Nogales Castle Rock Old Bextrum Pena Blanca Butte Road Canyon Canyon Canyon - 4500 Silt, sand and gravel - 4250 -4000 Kmp Qpb Pena Blanca Formation T- tuff B - basalt Kmp - _ - Kpl - 3750 To Atascosa Formation CROSS SECTION ALONG LINE A - A Montana Peak Formation T - massive tuff Kob Oro Blanco Conglomerate Kpl Pajarito Lavas Tuff (undifferentiated) Quartz latite porphyry Ruby — Pena Blonca Nogales Caldera Whitney Wal k e r East branch Lake Road Cliffs Mesa Canyon Walker Canyon Fault - 4000 // Joint set CROSS SECTION ALONG LINE B - B Bedding HORIZONTAL SCALE 1/2 MILES VERTICAL SCALE 2 X HORIZONTAL SCALE