Younger Precambrian 

Geology in 

Southern Arizona

GEOLOGICAL SURVEY PROFESSIONAL PAPER 566
Younger Precambrian 

Geology in 

Southern Arizona

By ANDREW F. SHRIDE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 566

Stratigraphic^ lithologic^ and structural 

features of younger Precambrian rocks 

of southern Arizona as a basis for 

understanding their paleogeography and 

establishing their correlation

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog-card No. OS 67-238

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

Washington, D.C. 20402 - Price 65 cents (paper cover)
CONTENTS

Abstract.__________________________________________

Introduction. ______________________________________

Older Precambrian basement----_----___--___________

Pre-Apache unconformity._----_-_-_-________________

Thickness and distribution of younger Precambrian rocks. 

Apache Group____________________________________

Pioneer Shale__________________________________

Thickness and general character-_____________

Lithology and stratigraphic relations-_________

Scanlan Conglomerate Member...________

Tuffaceous mudstone..__________________

Arkose._______________________________

Lateral variations.._________________________

Dripping Spring Quartzite___-_--___--_________

Barnes Conglomerate Member________________

Arkose member___________________________

Siltstone member.__________________________

Lithology._____________________________

Weathering modifications._______________

Bedding and secondary structures... ______

Features that distinguish principal units. _ _ 

Pre-Mescal unconformity-_______________________

Mescal Limestone.._____________________________

Lower member.____________________________

Lithology and stratigraphy_______________

Structural and textural features._________

Halite molds in chert______________

Sheaflike impressions._______________

Minor slump features._________.-__.

Intraf ormational conglomerates__ _ _ _

Founder breccia.___________________

Algal member._____________________________

Argillite member..__________________________

Distribution and stratigraphic relations. 

Lithology-_____________________________

Precambrian weathering and silicification______

Solution features_______-___-___.._____.

Late-formed cherts____________________

Karst and related breccias.______________

Hematite concentrations._-____-_--_-____

Metamorphic limestones.-_--__--_-__-_-_____

Variations in thickness._____________________

Stratigraphic nomenclature._________________

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Apache Group Continued

Basalt flows.___________________________________

General features and distribution.____________

Petrology.___--_______-_____-_--__________.

Criteria for distinguishing basalt from diabase.. 

Pre-Troy unconformity.______---_-___________-.____.

Troy Quartzite.____________________________________

Definition and subdivision.______________________

Arkose member_________________________________

Distribution and thickness.__________________

Stratigraphy and lithology___________________

Chediski Sandstone Member.____________________

Thickness__ ______________________________

Lithology and stratigraphy_._________________

Lateral variations__._________--__-___-_-_.

Quartzite member______________________________

Stratigraphy. _________________-_____---_-_.

Lithology--___-______--_____-------___--__.

Distribution and topographic expression.______

Distinctive features of members____________----_.

Diabase and related rocks..__________________________

Form and dimensions of intrusions._______________

Stratigraphic distribution._______________________

Pe trology_ _____________________________________

Diabase.___.___-____________----_--------.

Feldspathic differentiates..____________--_-..

Diabase pegmatite._______________----..

Granophyre.._ _____________-______---_.

Aplite. __________________--__---_-----.

Mode of formation..______________----_.

Metamorphism associated with diabase__________----_.

Late Precambrian deformation._____________-__-----.

Prediabase deformation.-.._________--_____-----.

Deformation associated with diabase intrusion __.

Relations of Paleozoic formations to Precambrian rocks..

Rocks adjacent to the pre-Bolsa unconformity _.

Pre-Middle Devonian unconformity.________-_--_.

Age of diabase and associated rocks._____--_----------

Geologic history.________--_-_------_---------------

Interregion correlations.-__-__--_----__--------------

References cited._________-___-_-_--_---------------

Index.______________---_---_----------------------

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ILLUSTRATIONS

FIGURES 1-4. Maps:

1. Outcrops of younger Precambrian rocks in Arizona._______________-____--_-------------------

2. Outcrops of older Precambrian terrane, showing types of rocks that were sources of elastics during 

late Precambrian time._____________________________-___-------_----------------------

3. Outcrops of younger Precambrian strata and coextensive diabase intrusions in southeastern Arizona.

4. Some paleogeographic features of younger Precambrian formations in southeastern Arizona .____

m

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IV CONTENTS

Page 

FIGURE 5. Photograph showing delicate cross-lamination in tuffaceous siltstone of the Pioneer Shale._______________ 15

6. Photomicrograph of tuffaceous siltstone of the Pioneer Shale.________________________________________ 15

7-9. Photographs:

7. Typical lamination and irregular bedding in siltstone member of Dripping Spring Quartzite______ 22

8. Scour-and-fill and compaction features in the siltstone member of the Dripping Spring Quartzite-__ 23

9. Structures typical of the stromatolite beds of the algal member of the Mescal Limestone.---______ 33

10-14. Diagrams:

10. Schematic sections showing relations of multiple injections of diabase to plates of host rock isolated

between intrusions._________________________________________________________________ 55

11. North-south geologic section after the Troy Quartzite was deposited,__________________________ 64

12. North-south geologic section at the end of Devonian time._---__-______-__---_________-______ 64

13. Effects of sill intrusion on displacement along a preexisting fault-__-___--__-_-____-____-_-____ 67

14. Idealized geologic section showing relations of faults to diabase intrusions and to pre-Paleozoic

unconformities_ _ __________________________________________________________________ 67

TABLES

Page 

TABLE 1. Pre-Devonian formations of southeastern Arizona.-_________________________________________________ 8

2. Criteria for distinguishing clastic strata of the Bolsa Quartzite and Abrigo Formation from those of the Troy

Quartzite-_________________________________________________________________________________ 74
YOUNGER PRECAMBR1AN GEOLOGY IN SOUTHERN ARIZONA

By ANDREW F. SHEIDE

ABSTRACT

Younger Precambrian rocks of southeastern Arizona consist of the Apache Group, the overlying Troy Quartzite, and 

large diabase bodies intrusive into and coextensive with the 

sedimentary formations. These rocks are widely exposed 

and little deformed in northern Gila County and in the southwest corner of Navajo County areas that are structurally 

part of the Colorado Plateau. In the Basin and Range province farther south, outcrops are scattered and less extensive, 

and structural relations with younger formations are commonly obscure.

The Apache Group ranges in thickness from 1,250 to 1,600 

feet, and includes, in ascending order, the Pioneer Shale, the 

Dripping Spring Quartzite, the Mescal Limestone, and unnamed basalt flows. The Scanlan and Barnes Conglomerate 

Members, formerly designated as separate formations, are 

considered basal conglomerate members of the Pioneer Shale 

and Dripping Spring Quartzite, respectively. The upper part 

of the Dripping Spring is divided into a massive arkose member and an upper thin-bedded member of siltstone and arkose. 

The Mescal Limestone comprises three members: a lower 

thin- to medium-bedded member of cherty dolomite or of calcitic limestone that is the metamorphic equivalent, a middle 

partly massive algal member of similar carbonate, and an 

upper argillite member.

The Troy Quartzite is locally at least 1,200 feet thick and 

is made up of three members. The low'est, a remnant only 

in northwestern Gila County, is well-sorted arkose characterized by very large scale crossbedding. The middle, or Chediski Sandstone Member, is poorly sorted, partly pebbly, sericitic or feldspathic sandstone, and is the only member that 

extends into the southern part of the region. The upper 

member is well-sorted evenly bedded clean quartzite.

All sedimentary formations are shallow shelf deposits; 

individual members reflect a variety of environments, each of 

which was notably uniform over as much as 15,000 square 

miles. Two volcanic episodes are apparent: (1) the Pioneer 

Shale is dominantly of water-laid rhyolitic(?) tuff; (2) basalt 

flows that locally underlie the argillite member of the Mescal 

and those that extensively overlie the formation are kindred 

and probably were not extruded at widely separate times. 

Evaporites and, incongruously, chert were deposited in the 

early sea of Mescal time; apparently during later carbonate 

deposition the salts were leached away, the result being 

a founder breccia of regional extent at the base of the formation. The uniform environments of subsequent dolomite deposition are shown especially in the intertidal deposits that 

make up the biostrome of the algal member. One and perhaps two aspects of the Troy suggest eolian activity. Even 

in areas of lapout, strata of the Pioneer and Dripping Spring 

generally do not vary greatly; the Troy, however, does exhibit

changes where it laps onto older terrane. The recognition of 

these aspects of the Apache and Troy sequence provides new 

bases for speculation on interregional correlations of younger 

Precambrian rocks.

At least two major marine transgressions, sharply marked 

by the unconformities at the bases of the Pioneer and the 

Dripping Spring, occurred during Apache time; the Chediski 

Sandstone Member of the Troy was deposited during another 

transgression. The erosional unconformity between the Mescal and the Dripping Spring and the unconformity at the 

top of the algal member of the Mescal could also represent 

appreciable time intervals. The unconformities that separate 

the argillite member from the overlying and underlying basalt 

are apparently effects of brief subaerial exposures during recurrent broad warping which preceded uplift that terminated 

Apache sedimentation. Apache strata were variously eroded 

and locally stripped completely away before the Troy was 

deposited. Conceivably, unconformities also separate the 

members of the Troy.

Before and during the pre-Troy erosion, solution caverns 

were formed in the dolomite of the Mescal, and in a few 

areas a karst topography was formed. During this period 

the dolomite was extensively silicified and locally was converted to a massive rubble of chert. Hematite deposits of ore 

grade accumulated locally in the karst areas.

After lithification of the Troy, the upper Precambrian strata 

were intensely folded and faulted along narrow widely spaced 

north-trending belts. Between these belts strata remained 

virtually undeformed until they were extensively intruded 

and displaced by diabase sills. Many faults caused by diabase 

inflation have been visualized as post-Paleozoic in age, but 

all these rocks were deeply eroded and the structures truncated before Cambrian formations were deposited.

The diabase was emplaced largely as sills, some of which 

exceed 1,000 feet in thickness. In some areas their aggregate 

volume is as much as the strata they intrude. Sills occur 

throughout the Apache sequence but were intruded along 

certain stratigraphic horizons in preference to others; they 

are only locally conspicuous in the Troy. Commonly two or 

more were intruded along a given horizon, but in some areas 

multiple injections are rare. Dikes are insignificant, but some 

dikelike connections between concordant sheets at different 

horizons are regionally conspicuous. Because the sills are 

ubiquitous in the younger Precambrian terrane, thermal metamorphism related to them is widespread, and for the carbonate rocks it is a regional characteristic.

Olivine diabase is dominant; albite diabase is subordinate 

in these intrusions. Both are in part unusually coarse 

grained, a feature which sets those bodies apart from minor 

intrusions of dark-colored rock of later age. Gravitational 

settling of olivine is apparent only in the thicker sills. Exten1
YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

sive tabular bodies of diabase pegmatite occur just in the 

upper parts of thick sills at or near discordant parts of the 

intrusions. Quartzose differentiates, mainly granophyre and 

a highly quartzose aplite, occur in similar geometric habit 

but apparently are hybrid rocks formed by interaction between 

feldspathic xenoliths and late magmatic fluids.

The recognition of certain features in the basal formations 

of the Paleozoic is critical to unraveling the younger Precambrian history. In southernmost Arizona the Cambrian 

sequence consists of the Bolsa Quartzite and the overlying 

Abrigo Limestone, which is dominated by carbonate strata. 

Traced from south to north this sequence laps from a smooth 

surface on older Precambrian rocks, onto an irregular surface 

on the Apache Group, and ultimately onto the Troy Quartzite. 

In the same interval the Bolsa thins and in some areas laps 

out, and the Abrigo Formation becomes a sequence dominated 

by sandstone and mudstone. Because the Bolsa and Troy 

Quartzites seemed homotaxial equivalents and because in 

places fossiliferous sandstone here recognized as Abrigo 

strata seemed gradationally transitional into the underlying 

Troy, early workers correlated the Bolsa and Troy. In the 

northern part of the region, in a similar way, channel-fill sandstone at the base of the Martin Limestone (Devonian) has 

been included with the Troy. Criteria for recognizing the 

various sandstone and quartzite units as separate entities are 

given. Isotopic data indicate that a hiatus of at least 500 

million years is represented in the pre-Bolsa unconformity.

INTRODUCTION

Younger Precambrian formations crop out in two 

parts of Arizona. In northern Arizona the Grand 

Canyon Series is exposed along the depths of the 

Grand Canyon in outcrops that aggregate less than 

100 square miles in area. In southeastern Arizona 

outcrops of equivalent formations are much more 

widespread and occur mainly in a belt 20-60 miles 

wide that extends from the foot of the Mogollon Kim 

southward 170 miles to about the latitude of Tucson 

(fig. 1). Scattered outcrops also occur in an area 

50-65 miles south of Phoenix.

The younger Precambrian rocks in southeastern 

Arizona consist of the Apache Group, the Troy 

Quartzite, and coextensive intrusions of diabase. Outcrop areas of these rocks are shown in figure 3. Figure 4, which covers the same area, shows the principal 

paleogeographic features that influence our understanding of the younger Precambrian terrane.

Kansome (1916, p. 133-135; 1923, p. 1) divided the 

State into three physiographic regions the plateau 

region to the northeast, the desert region to the southwest, and the mountain region between the two (fig. 1). 

Apache and Troy outcrops are almost wholly in the 

mountain belt. It is commonly assumed, erroneously, 

that these geographic subdivisions are applicable in 

discussing structural disposition of these strata. If 

we apply the structural criteria commonly accepted 

for distinguishing the Colorado Plateau from the 

Basin and Range province, we find that the boundary

between the geologic terranes is well within the mountain region and does not coincide with the Mogollon 

Eim as commonly assumed. Throughout this report, 

reference is made to this structural definition rather 

than to a physiographic delineation of provinces. In 

the report area the boundary is sharply defined and 

extends northwest through Gila County and is 40-50 

miles south of the Mogollon Eim (fig. 3).

From this boundary northward to the Mogollon 

Rim, in the Colorado Plateau, the younger Precambrian formations are approximately horizontal and 

are laterally little interrupted by structure. The mesacanyon topography of the plateau country, which has 

a relief of 2,500-4,500 feet, provides excellent exposures for the observation of stratigraphic detail of the 

sedimentary formations, the petrologic detail of the 

diabase, and the structural relations between formations. Conclusions drawn from this area can be more 

fully substantiated than those drawn from observations made elsewhere in the region of younger Precambrian outcrops.

In contrast, in the Basin and Range structural 

province most outcrops of younger Precambrian formations are in narrow mountain ranges, which are 

separated by broader deeply alluviated basins that are 

mainly floored on older Precambrian rocks. Huge 

mosaics of tilted fault blocks characterize many of 

the ranges. Thus, outcrops of the individual formations may be widely separated (fig. 3) and structural 

relations to younger and older formations obscured. 

Because the Troy Quartzite and formations of the 

Apache Group were first studied in and near the great 

copper-mining districts of Ray, Globe-Miami, and 

Superior, they were defined in this part of the region, 

and, until recently, conclusions derived in this area 

have dominated our understanding of younger Precambrian geology.

I first became aware that there were unreconciled 

descriptions and interpretations of the younger Precambrian terrane while examining an iron-ore deposit 

near the head of Canyon Creek during the winter of 

1941-42. Prime stratigraphic details of the upper 

formations of the Apache Group and characteristics 

of the diabase intrusions, as well as the regional aspects of the metamorphism associated with them, were 

recognized incidental to the study of asbestos deposits 

in the Mescal Limestone between October 1942 and 

May 1944 and during the summer of 1946. These observations were confined mainly to the Sierra Ancha 

and to the canyon entrenched by the Salt River east 

of its junction with Canyon Creek. The basic data 

on the stratigraphy of the lower formations of the 

Apache Group and the Troy Quartzite and on regional structures of Precambrian age were collected
INTRODUCTION

112 110°

V M O \H

i V

\ \/u Jerome °^ Tl

V'RaoviaH' /-N ^Prescott V"- !

I Y Ay A P A I

32

50

I

100 MILES 

_I

FIGURE 1. Outcrops of younger Precambrian rocks in Arizona in relation to the three principal physiographic regions. 

Apache Group and associated formation's (solid) crop out in shaded area; Grand Canyon Series (solid), in northern 

Coconino County. Area of figures 2, 3, and 4 shown by shading. Physiographic regions based on Ransome (1919, 

p. 27; 1923, fig. 2).
YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

during detailed mapping of parts of the McFadden 

Peak quadrangle between July 1954 and June 1956. 

During the spring of 1957, a strip 10-25 miles wide, 

from north of the Salt Eiver south almost to Globe 

along U.S. Highway 60, was mapped with C. T. 

Wrucke; this reconnaissance mapping aided in understanding details seen elsewhere. Premises thus evolved 

were tested, during June 1960, by the examination 

of Apache rocks as far south as the Little Dragoon 

Mountains in Cochise County, as far west as the Vekol 

Mountains in southwestern Pinal County, and as far 

east as Mount Turnbull in Graham County. At least 

a few observations of the younger Precambrian rocks 

have been made in almost every area of known outcrop. The large area of outcrop west of the lllth 

meridian and north of Theodore Eoosevelt Lake has 

not been studied. Observations in this area might 

add to our knowledge of Pioneer and Dripping Spring 

strata, but probably would not provide significant 

data on the higher formations.

This report presents such reinterpretations of the 

regional geology of younger Precambrian rocks as can 

now be made, and provides background of stratigraphic and petrographic data so that these reinterpretations can be applied to deciphering the geologic 

settings of several mineral-bearing districts in southeastern Arizona. Eock descriptions in this report employ the "Eock-Color Chart" (Goddard and others, 

1948) and the Wentworth grain-size scale (Wentworth, 1922). Bedding and splitting features of 

stratified rocks.are described by the quantitative terms 

proposed by McKee and Weir (1953).

E. D. Wilson, geologist of the Arizona Bureau of 

Mines, has provided me with much information from 

his exceptional store of knowledge of the regional 

geology. In particular, Wilson guided me to areas 

in northern Gila County where relations at the base 

of the Apache section are especially well exposed and 

to three localities in the Mescal Mountains where 

Cambrian fossils had been collected. During the 

1940's B. S. Butler, of the University of Arizona, 

offered much helpful comment. N. P. Peterson has 

advised me of many aspects of the geology of the 

Globe-Miami mineral belt. During 1954-56 H. C. 

Granger and E. B. Eaup, of the U.S. Geological 

Survey, studied uranium deposits in the Dripping 

Spring Quartzite, and this work furnished many details on the stratigraphy. C. T. Wrucke examined 

many diabases petrographically that were collected 

during various studies and contributed careful observations and stimulating discussions. Other colleagues 

in the U.S. Geological Survey who contributed greatly 

to my understanding of the area include J. E. Cooper,

S. C. Creasey, M. H. Krieger, C. E. Willden, D. W. 

Peterson, and A. E. Palmer.

OLDER PRECAMBRIAN BASEMENT

Eocks of the older Precambrian terrane that underlies the Apache Group are of interest here only as 

sources for the clastic materials of the Apache and 

Troy. Outcrops of the older formations, differentiated as to rock types recognized as detritus in the 

Troy and Apache, are shown in figure 2. Only the 

larger occurrences of the older Precambrian quartzites 

are shown, but additional thin units are fairly common in some of the pregranite formations north of the 

Salt River. Outside the region shown in figure 2, 

the outcrops of schistose rocks that have been studied 

are largely devoid of quartzite. Resistant rhyolites 

that have been recognized in detritus have been included with the nonresistant rocks. The proportions 

of rock types seen in present outcrops are very likely 

representative of those exposed in source areas just 

before Apache sedimentation began. The map indicates the dominance of granitic rocks and the sparsity 

of quartzites.

The Pinal Schist, dominantly a quartz-muscovite 

or quartz-muscovite-chlorite rock (Peterson, 1954; 

Gilluly, 1956, p. 10-11), underlies the Apache Group 

in many areas south of Globe. The Pinal consists 

mainly of metamorphosed sedimentary rocks, but also 

includes some intercalations of volcanic rocks (Eansome, 1919, p. 35-37; Gilluly, 1956, p. 11). Amphibolites and intrusive and extrusive rhyolitic units have 

been noted (Anderson, 1951, p. 1334-1335).

Slightly schistose and nonfoliated sedimentary and 

volcanic rocks, in part probably equivalent to the 

Pinal Schist, underlie the Apache Group north and 

west of the Sierra Ancha and crop out widely in the 

Mazatzal Mountains farther west (Wilson, 1939; 

Gastil, 1958). The sedimentary rocks are mainly 

shale, thin- to thick-bedded quartzite, and fine-grained 

to conglomeratic sedimentary units composed largely 

of volcanic debris. Many of the quartzites were thoroughly sheared or sheeted, then were firmly recemented, and are fine grained and highly vitreous. The 

relict sheeting effects as well as the dark shades of 

brown, red, or purple distinguish these rocks from 

later quartzite. The volcanic rocks are dominated by 

rhyolite flows, tuffs, and agglomerates, but basalt flows 

and agglomerates are also common. The southernmost large remnant of such rocks, within the area of 

Apache exposures, crops out along the south side of 

the Salt Eiver southeast of the Sierra Ancha (fig. 2).

Much farther northwest, in the vicinity of Jerome, 

Prescott, and Bagdad, the Yavapai Series is made up 

of sedimentary and volcanic formations that are simi-
112°

OLD PRECAMBRIAN BASEMENT

111° 110°

N A V A J 0

GRAHAM

A _ _,_ _ ___ 

EXPLANATION

Granitoid rocks 

Mostly quartz monzonite and granodiorite

 -,C 0 C H I

Only larger outcrops shown

P I M A

Nonresistant rocks 

Mostly schistose metasedimentary and

10 0 10 20 30 40 MILES

32°-

FIGURE 2. Outoropsi of older Precambrian terrane, showing types of rocks that were sources of elastics during late

Precambrian time.
6 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

lar to those in the Mazatzal Mountains, except that 

quartzites and conglomerates are sparse (Anderson, 

1951; Anderson and others, 1955, p. 7-12; Anderson 

and Creasey, 1958, p. 8-^5). Small bodies of pyroxenite, gabbro, diorite, and rhyolite were intruded into 

all these rocks prior to invasion by granitic magmas.

All these rocks were strongly deformed, then invaded by granitic masses of batholith dimensions, and 

deeply eroded prior to deposition of the Apache 

Group. The granitoid masses in the area of Apache 

outcrop are dominantly of quartz monzonite or granodiorite (Peterson, 1938, p. 8-9; Peterson, 1954; Cooper 

and Silver, 1964).

Much of the quartz monzonite is a rather distinctive 

coarse-grained rock. In northern Gila County an 

abundance of large grayish-orange-pink to pinkishgray microcline phenocrysts, which enclose many 

small grains of plagioclase and biotite, make the texture seem coarser on casual observation than it actually 

is. These poikilitic subhedral phenocrysts, ordinarily 

Vij-l inch long but sometimes as long as 2 inches, 

are conspicuously scattered through a coarse (1-8 mm) 

hypidiomorphic groundmass of pinkish potash feldspar, almost clear quartz, pale-greenish-yellow or 

yellowish-gray distinctly zoned chalky plagioclase, and 

thick books of black biotite. Sparse crystals of 

brownish-black sphene commonly can be noted megascopically. Apatite and minute grains of hematite(?) 

are accessories observed under the microscope. Orangish-pink aplite dikes seem to be the most common of 

minor associated facies. Quartz monzonite of this 

general description is termed Euin Granite in the 

vicinity of Globe and is the dominant basement rock 

as far southeast as the Gila Eiver; from Eay southward along the Tortilla Mountains a similar rock is 

said to make up most of the older terrane (Eansome, 

1919, p. 37). The Oracle Granite of Peterson (1938), 

which crops out widely in and north of the Santa 

Catalina Mountains, is also similar.

Granitic detritus is abundant in the Pioneer, Dripping Spring, and Troy Formations of late Precambrian age. The coarser debris is readily recognized 

as petrographically similar to the Euin Granite of 

the Globe-Miami area and the Oracle Granite of the 

Santa Catalina Mountains. Without doubt, these and 

similar monzonitic masses in adjacent regions furnished arkosic and quartzose detritus through most 

of late Precambrian time.

The Pinal Schist and lithologically equivalent formations depicted mostly as nonresistant rocks in 

figure 2 seem to have furnished little detritus other 

than quartz and quartzite to the upper Precambrian 

formations. Some of the argillaceous materials of the

several siltstone units in the Apache Group might 

have been derived from such sources. Eeddish-brown 

jasper pebbles, reworked from the coarser fractions 

of the older quartzites, and pebbles from some of the 

denser rhyolites, are conspicuous but sparsely distributed in the conglomerates of the younger Precambrian. The bulk of the gravels in the conglomeratic 

units of the Apache Group and the Troy Quartzite 

are petrographically similar to the older quartzites. 

Granules and pebbles of quartz, which abundantly 

veins the older Precambrian rocks in some places, are 

locally abundant in the later conglomerates and are 

the principal coarse constituent of the Chediski Sandstone Member of the Troy.

During accumulation of most of the younger Precambrian sediments, the older Precambrian quartzites 

and resistant rhyolites now known only within the 

area of figure 2 were buried by the basal units of 

the Apache Group and could not have been sources of 

the debris found in the succeeding formations. Therefore, a lithologic terrane similar to that shown in 

figure 2 must have been exposed somewhere outside 

the region of present Apache outcrops.

PRE-APACHE UNCONFORMITY

The mountains that were formed during the orogeny that terminated deposition in older Precambrian 

time in southern Arizona were leveled to a plain 

during the long episode of erosion that preceded 

Apache sedimentation. In degree of smoothness, this 

unconformity is comparable to the equivalent surface 

in the Grand Canyon, which has been interpreted as 

a peneplain formed by subaerial erosion and slightly 

modified by marine erosion (Sharp, 1940).

For most of the region the pre-Apache peneplain 

exhibits local relief of only a few feet at most. From 

the Sierra Ancha southward, variations in the thickness of the Pioneer Shale the basal formation of the 

Apache Group suggest that the surface may have 

had broad low hills rising slightly above the general 

plain or broad basins cut below it.

North of the Sierra Ancha the Pioneer Shale and 

at least the lower half of the Dripping Spring Quartzite lap out against a high on the basement surface 

(fig. 4). It seems more than a coincidence that this 

lapout occurs where sedimentary and volcanic rocks 

generally the nonresistant basement formations include small but numerous resistant units of quartzite 

and dense rhyolite. The line of lapout apparently 

trends east-northeast (fig. 4). It crudely parallels 

the northeast structural grain of the basement rocks 

and the boundary between these rocks and the less 

resistant granite the prevalent basement rock as far 

south as Globe (fig. 2).
THICKNESS AND DISTRIBUTION OF YOUNGER PRECAMBRIAN ROCKS

The northern belt of metasedimentary and metavolcanic rocks is the only area in which a prominent 

deviation from the pre-Apache peneplain has been 

observed. This is probably because no other extensive 

schistose zones include abundant ribs of resistant rock. 

Outcrops of the Apache formations are found overlying high parts of this northern belt, and undoubtedly 

Apache strata once extended much farther northwestward but were stripped away before the basal Paleozoic formations were deposited. Whether the area of 

pre-Apache lapout represents a large elongate monadnock rising above the general pre-Apache surface 

or a broad high of regional extent is not known.

The surficial granitic rocks disintegrated prior to 

deposition of the Apache Group. As remnant now, 

this zone of disintegration ranges from a few inches 

to a few tens of feet in thickness. In places the 

granitic debris, generally reddened by iron oxides, 

shows ill-developed bedding structures, but generally 

it represents residual material in virtually original 

position. Even where the basement rocks are not 

granitoid, the coarse fraction of the matrix of the 

overlying conglomerate consists largely of this disintegrated granite.

Where the unconformity truncates metasedimentary or metavolcanic rocks, the foliation or other 

planar structures may be somewhat obscured immediately adjacent to the erosion surface. In places a 

rubble of schistose fragments a few inches thick separates the basal conglomerate of the Apache Group 

from undisturbed schistose basement rock. Where 

such a regolith exhibits crude bedding, it commonly 

includes abundant angular to subrounded fragments 

of vein quartz, but is a unit lithologically different 

from the overlying conglomerate, which ordinarily 

consists mostly of debris from a distant source.

THICKNESS AND DISTRIBUTION OF YOUNGER 

PRECAMBRIAN ROCKS

Sections of individual formations of the Apache 

Group and of the Troy Quartzite are most fully represented in the Sierra Ancha. Farther south, owing 

to unconformities within and at the top of the sequence, sections generally are less complete.

Within the region between the Gila and Salt Eivers, 

the Troy Quartzite was everywhere thinned, and locally the Mescal Limestone and even the upper part 

of the Dripping Spring Quartzite were removed by 

pre-Paleozoic erosion. South of the Gila Eiver the 

upper part of the Dripping Spring and all the overlying formations with their diabase intrusions were 

removed from large areas prior to Cambrian sedimentation ; locally, however, most of the Mescal Limestone and moderately thick sections of the Troy Quartzite were preserved in blocks downfaulted prior to 

post-Troy erosion.

Thin to thick diabase sills are coextensive with the 

Troy Quartzite and the Apache Group, and they invaded the Apache in almost every known exposure; 

many sections of the Apache are displaced by sills at 

several horizons. Consequently, formations must often 

be pieced together from two or more partial sections 

found between sills. Actually, there are only a few 

places where the entire Apache section can be viewed 

from bottom to top, and these sections are not completely free of diabase sills.

Post-Paleozoic erosion of the younger Precambrian 

section was more extensive south of the Colorado 

Plateau than to the north. Also, these rocks to the 

south were much faulted during Tertiary time, and 

were partly covered by Cenozoic rocks. In the southern part of the region, therefore, thicknesses of the 

Troy Quartzite and the Apache Group are difficult to 

ascertain.

The thickness of the Troy Quartzite in any given 

area is in large part dependent on the degree of prePaleozoic faulting and the depth of pre-Paleozoic erosion. Therefore the only meaningful thicknesses of 

Troy, and those cited in table 1 and elsewhere 

throughout the report, are those remnant in exposures beneath a cover of Paleozoic rocks.

In the Colorado Plateau, where sections have been 

pieced together with considerable confidence at several 

localities, the Apache Group ranges in thickness from 

1,250 to 1,600 feet. North of the Sierra Ancha, of 

course, these thicknesses are too great by roughly a 

factor of two because the Pioneer Shale and locally 

at least the lower half of the Dripping Spring Quartzite are lapped out. Throughout the Basin and Eange 

area some sections of the Apache Group are as much 

as 1,600 feet thick, but other sections appear to be as 

little as 1,100 feet thick. The thinner sections are 

those reduced largely by (1) thinning of the Pioneer 

Shale, (2) erosion of the Mescal Limestone and the 

overlying basalt prior to deposition of the Troy 

Quartzite, or (3) a combination of these factors.

In the highest parts of the Sierra Ancha, the Troy 

is at least 1,200 feet thick and once was probably at 

least 800 feet thick over the entire region. Away 

from the Sierra Ancha as far south as the Gila River, 

remnants of the Troy generally range in thickness 

from 300 to 600 feet, but locally the formation is 

entirely eroded away. Farther south the formation 

is missing over large areas.

The younger Precambrian sedimentary sequences are 

thickest in the central Sierra Ancha, where the combined Troy and Apache total 2,650-2,800 feet. The 

thinnest sections are in the southern part of the region,
TABLE 1. Pre-Devonian formations of southeastern Arizona

Age Group, formation, and member Thickness (feet) Lithology a

Late and 

Middle (?) 

Devonian

Martin Limestone

 Erosional unconformity

30-400

Paleozoic

Late and

Middle

Cambrian

Abrigo Formation 0-840

Middle 

Cambrian

Bolsa Quartzite 0-480

Unconformity

Diabase

Intrusive contact

Quartzite member 0-500

Interbedded limestones, d

stones. Carbonate strata 

from Globe, overlies Prec

formity at base becomes 

basal sandstone, mostly l

thickens to more than 250 

fill paleochannels as much 

South of Santa Catalina M

nate lower third; thin-bed

third; and sandy brown 

comes increasingly sandy 

North of Santa Catalina 

irregularly bedded muds

stones of lower part mer

and quartzites; sandy dol

are the only conspicuou

northern occurrences re

rocks. Abrigo and Bols

Globe owing to pre-Mart

Thin- to thick-bedded gr

sorted fine-grained cros

stone. Basal conglomera

ward into Abrigo Form

Mountains, from older P

irregular surface on youn

North of Santa Catalina 

completely, and north o

widely absent owing to la

Mainly olivine diabase in s

thick, and in narrow dik

are small differentiated 

granophyre, and aplite. 

in all older formations.

Light-colored medium- to c

bedded tangentially cros

ally free of pebbles and 

well sorted.

Friable to firmly cemented 

stone of well-rounded fro

sericite matrix; basal c

Convolute lamination ch
Younger Precambrian

Troy Quartzite

Apache 

Group

Mescal

be

Erosi( 

Erosi

jal 

;one

Chediski Sandstone 

Member

Arkose member

Basalt flows

Argillite member

-Erosional unconformityBasalt flow (local)

-Erosional unconformityAlgal member

Lower member

0-700

0-450

0-100

0-110

40-130

150-270

0-1,200

0-375

250-420

1,250-1,600 (where all formations arepresent)

middle part, and medium-

thin conglomerate layers 

North of Salt River, is 

color, massive outcrops, 

Farther south, is less c

irregularly bedded in p

locally quartzitic. Is ba

region.

Mostly fine-grained well-so

cross stratified on very 

northwestern Gila County

One to four flows of very he

conspicuously porphyritic 

on algal member of M

Chrysotile.

Flaggy- to massive-croppin

siliceous argillite and sub

includes thin lenses of sili

thin unit of chert conglom

Member is best develope

rangle and is absent south 

Basalt flow like those abo

locally, in two small area

rangle and at Theodore R

Consists of two units, of abo

thick-bedded stromatoliti

thin-bedded and regularly 

structures. Both are dol

Generally chert is abund

upper unit. Upper unit c

pre-Troy erosion.

Thin- to thick-bedded che

some areas dolomite th

formation of karst brecci

metamorphosed to calcit

silicate minerals. In mo

feet is a coarse breccia of 
10 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

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Thin- thick-bedded massive-cropping to firmarkosend 

feldspathic Crossbedding quartzite. characteristic Member but obscure. laps Gila innorthern out

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Mainly pebbles quartzite in arkosic matrix.

Mostly grayish-red tuffaceous siltstone mud-silty or 

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PIONEER SHALE 11

where only Apache formations underlie the Paleozoic. 

In the southeastern part of the region, pre-Bolsa 

erosion was the principal cause of thinning and south 

of the Little Dragoon Mountains caused complete 

removal of the younger Precambrian rocks. If the 

diabase sills are included, the aggregate thickness of 

younger Precambrian layered rocks, in many areas 

north of the Gila River, are roughly twice those 

noted above, that is, as much as 5,000 feet.

The Apache and Troy rocks once extended over a 

15,000-square-mile area (fig. 1) and undoubtedly once 

even had a much wider distribution. Individual stratigraphic units, whether formations, members, or 

even lesser units, are remarkably consistent in lithology 

and other features throughout this broad region and 

thus reflect a common site of deposition on a wide 

shallow shelf. The principal variations that do exist 

are due to postdepositional processes, which also affected large parts of the region.

Discussion of the origins of the younger Precambrian 

formations is largely deferred until all have been described, because they have much in common. Similarly, age and correlation are discussed in better perspective after description of relations to the diabase 

and to the Paleozoic formations.

APACHE GROUP

PIONEER SHALE

THICKNESS ANI> GENERAL CHARACTER

On the basis of the original descriptions, the Pioneer Shale has generally been supposed to range in 

thickness from 100 to 250 feet; actually the regional 

range is considerably greater. Near the west end of 

San Carlos Lake, the formation is thin or missing (fig. 

3); and in exposures along the valley of the San Pedro 

Kiver north of Mammoth, it is generally less than 30 

feet thick and locally is absent. As far as is known 

the Pioneer does not exist east of this belt, and as 

already noted it definitely laps out north of the Sierra 

Ancha (fig. 4). In the canyon of Cherry Creek and 

along the southern front of the Sierra Ancha, the 

formation ranges in thickness from 250 to about 500 

feet. Although as little as 150 feet of Pioneer Shale 

exists in many areas south of the Salt Eiver such as 

along the south escarpment of the Natanes Plateau and 

for several miles farther south and in places in the Ray, 

quadrangle (Ransome, 1916, p. 136) no pattern of 

thinning that would indicate other areas of lapout lias 

yet been discerned. Indeed, as far south as the 

Apache Group extends, the Pioneer has maximum 

thicknesses of 300-500 feet. For example, Cooper and 

Silver (1964) measured a 306-foot section in the northwestern part of the Dragoon quadrangle, S. C. Creasey (written-commun., Apr. 1961) measured thicknesses 

of 450-500 feet in the southwestern part of the Mammoth quadrangle, and Carpenter (1947) estimated a 

thickness of 400 feet in the Vekol Mountains.

The Pioneer Shale includes at its base the thin 

Scanlan Conglomerate Member; in many areas the 

remainder of the formation is mostly thin-bedded 

tuffaceous mudstone or siltstone. Where the Pioneer 

is more than 150 feet thick, mudstone ordinarily constitutes the upper one-half to two-thirds of the formation. The lower part is of fine- to medium-grained 

arkose or feldspathic sandstone intercalated with mudstone like that higher in the formation. These coarser beds may make up most or only a small part of the 

lower half of the Pioneer. In a few areas, as in the 

Dragoon quadrangle (J. R. Cooper, oral commun., 

1958) and in a few places in the McFadden Peak 

quadrangle, similar arkose makes up as much as 50 

feet of the uppermost part of the formation.

Outcrops of the Pioneer and encompassed diabase 

sills disintegrate readily and are commonly mantled 

in their own debris and the more resistant debris from 

overlying formations. Therefore it is frequently difficult to determine the proportions of mudstone, sandstone, and even diabase in a typical exposure. Along 

steep canyon walls the most firmly indurated siltstones 

crop out in thin ledges, and the sandstones form thin 

to thick ledges.

The contact between the Pioneer and the basal 

conglomerate of the overlying Dripping Spring 

Quartzite is everywhere sharp but in many places is 

also slightly undulatory. Rarely it is slightly discordant, or the conglomerate fills channels 1-2 feet 

deep in the top of the Pioneer. In some areas the 

upper 6 inches-3 feet of the Pioneer, normally grayish 

to blackish red, is bleached greenish gray or almost 

white. The bleached interval may represent a zone of 

regolithic material. No fragments of the Pioneer have 

been noted in the Dripping Spring. Only in the 

northernmost areas of outcrop does the lower part of 

the Dripping Spring have a purplish cast that might 

be attributed to fine material derived from the Pioneer. 

Because the upper part of the Pioneer is almost everywhere a monotonous poorly exposed sequence of mudstones lacking in distinctive marker beds, a regional 

angular unconformity would not be readily recognized. 

In general, the Dripping Spring and the Pioneer 

seem concordant. However, the few features that suggest post-Pioneer erosion and, in particular, the abrupt 

change in lithology indicate an unconformity, of regional extent separating the formations.

HTHOLOGY AND STRATIGRAPHIC RELATIONS

SCANLAN CONGLOMERATE MEMBER

The Scanlan Conglomerate Member is merely a 

coarse-textured thin basal facies of the Pioneer. The
12

112°

YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

111° 110°

. R ~A H A

P I NXA L

EXPLANATION

Quadrangles in which younger Precambrian I \COCHI

Diabase formations have been or are being studied

Principal outcrops only

DB, Diamond Butte 

McF, McFadden Peak 

BH, Blue House Mountain 

RM, Rockinstraw Mountain 

HC, Haunted Canyon 

I, Inspiration 

G, Globe

S, Superior 

R, Ray 

C.Christmas 

HJ, Holy Joe Peak 

M, Mammoth 

D, Dragoon

Troy Quartzite and 

Apache Group

FIGURE 3. Outcrops of younger Precambrian strata and coextensive diabase intrusions in southeastern Arizona, 

from county geologic maps published by Arizona Bureau of Mines, 1958-60.

Modified
112°

PIONEER SHALE

ill 0

13

P I N \A\

EXPLANATION

Principal area of karst in Mescal Limestone

Location of reference section described in text C\0 (\ H I S

CM, Center Mountain section of Troy Quartzite 

PM, Pendleton Mesa section of Mescal Limestone 

DC, Deep Creek section of Dripping Spring Quartzite

Note: Formations are present on side of 

32° line with wording

CONTOUR INTERVAL 2500 FEET

FIGURE 4. Some paleogeographic feature® of the younger Precambrian formations in southeastern Arizona.

248-121 O - 67 - 2
14 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

conglomerate ordinarily crops out as narrow ledges or 

small cliffs and, except on dip slopes or in other unusual exposures, cannot be separately depicted even on 

large-scale maps. Therefore it should not be afforded 

formation status as in the past.

Typically the Scanlan Conglomerate Member consists of subangular to well-rounded pebbles or cobbles 

in a fine- to coarse-grained matrix and it is 1-8 feet 

thick. In many places the conglomerate is represented 

by only a few inches of granitic debris in which granules or small pebbles are sparsely distributed. Elsewhere, as at Roosevelt Dam, the conglomerate is as 

much as 30 feet thick, consists mainly of closelypacked cobbles, and comprises two or more beds. In a 

common variation the gravels of the Scanlan may include boulders as much as 1 foot in diameter and consist mainly of cobbles 4 6 inches in diameter; within 

half a mile along the outcrop, the gravels may be 

largely pebbles 1-2 inches in diameter. Similarly, 

along such a length of outcrop, the thickness of the 

conglomerate ranges from a few inches to in a few 

places 20 feet; and in one locality the conglomerate 

consists of well-rounded gravels mostly of one lithology tout in an adjacent area includes subangular gravels 

of the same or different rock types. In many localities 

pebbles and cobbles are disk shaped, and in some they 

are disposed in imbricate patterns. No consistent direction of imbrication has been noted. The regional 

variations in gravel sizes, composition, disposition, or 

thickness, which might suggest distance or direction 

from sources, seem to be no greater than the variations 

noted in relatively small areas.

Most of the gravels are of rock types foreign to the 

sites of deposition, but the matrix of the Scanlan 

Conglomerate Member reflects to some degree the composition of the underlying rocks. In most areas the 

pebbles and cobbles are mainly of light- to dark-gray 

and grayish-red quartzite and are closely packed in 

arkosic debris derived almost entirely from older 

Precambrian granite. White quartz pebbles are also 

common, and in some places white quartz constitutes 

most of the granules. Granules and small pebbles of 

reddish-brown jasper are sparse but conspicuous in 

many outcrops; the few pebbles of volcanic and dense 

schistose rocks, similar to those of the older Precambrian terrane in northwestern Gila County, are easily 

overlooked. Where the conglomerate overlies the more 

friable Pinal, it commonly includes abundant schist 

fragments, and the matrix may consist in large part 

of dark-colored shaly debris from the schist. Even 

here, however, angular grains of feldspar and quartz 

derived from granite are abundant in the matrix. 

Nearly everywhere the finer material of the matrix is

hematitic, similar to the overlying mudstone or sandstone. In places, lenses of sandstone or tuffaceous mudstone are included in the conglomerate. Ordinarily 

the contact between conglomerate and overlying beds 

is sharp; however, transitional intervals are found 

which are a few inches to a few feet in thickness. 

Sporadic thin lenses of conglomerate or conglomeratic 

sandstone exist as far as 40 feet above the basal conglomerate.

TUFFACEOUS MUDSTONE

The so-called shale that dominates most Pioneer 

sections is in reality tuffaceous mudstone or tuffaceous 

siltstone that includes abundant grains of fine sand. 

The rock is minutely laminated or cross laminated (fig. 

5) in beds i/g-S feet thick. Sparse small asymmetrical 

ripple marks occur in some sections. The rock is firmly indurated but closely jointed, both normal and parallel to bedding, so that it erodes to receding slopes. 

The mudstone is friable only in the sense that diskshaped flakes, y^-^-Vz inches across, occur in all stages 

from initial spalling on the outcrop to trains of chips 

that cover the slopes. The grayish-red mudstone is 

dotted with yellowish-gray to light-brown bleached 

spots, as much as 1 inch, but generally 0.1-0.2 inch, in 

diameter (fig. 5). Minute cubic grains of limonite, 

obviously derived from pyrite, can be seen in the 

centers of some of the smaller spots. In some localities, bluntly terminated lenticular bleached zones, as 

much as half an inch thick and several inches in diameter, lie parallel to either the bedding or, less commonly, the high-angle jointing. These bleached 

zones seemingly are most prevalent in the more quartzose beds.

The mudstone is composed of angular feldspar and 

quartz grains that are silt size to fine-sand size and of 

slightly larger irregularly lenticular devitrified glass 

shards all of which are set in a matrix of smaller 

shards and clay-size material (fig. 6). Quartz and sodium and potassium feldspar grains typically make up 

about a quarter of the rock. In some beds feldspar, 

mostly plagioclase, is dominant in this fraction. Minute flakes of muscovite spangle freshly fractured 

specimens of some units. Thin sections from specimens in which mica is not particularly noticeable 

to the unaided eye contain sparse to fairly abundant 

shreds of muscovite. The delicate lamination is defined by concentrations of platy grains muscovite 

plates, shards, and tabular cleavage fragments of feldspar all of which tend to be alined parallel to bedding. But the bedding is more obviously marked by 

the hematite that is thickly dusted throughout the rock 

and that opaques the finest grained parts of the 

matrix.
PIONEEE SHALE 15

FIGURE 5. Delicate cross-lamination in tuffaceous siltstone of 

the Pioneer Shale. Black specks in centers of bleached spots 

are limonite pseudomorphous after pyrite.

FIGURE 6. Photomicrograph of tuffaceous siltstone of the Pioneer Shale. Wispy, irregular, or curved grains are relicts 

after glass shards; clear (white) nearly equidimensional 

grains are quartz; similar, but clouded, grains are feldspar; 

dark areas are heavily dusted with hematite. From same 

specimen as that of figure 5.

On the basis of X-ray studies, Gastil (1954) suggested that the devitrified shards consist largely of 

finely divided muscovite and very fine grained chalcedony. Some of the fine-grained matrix material appears to be optically similar to the shards, and probably much of it is of like mineralogic composition.

Gastil also postulated that the glass was rhyolitic in 

composition. Eobert L. Smith, who has studied tuffaceous rocks extensively, stated (oral commun., Aug. 

1956) that the rodlike and bifurcated cross sections 

and other features of the shards suggest a composition 

no more mafic than andesite.

In some localities, hard tuffaceous beds, which look 

like very fine grained impure quartzite, are interlayered with the usual grayish-red mudstone. These beds 

generally are dusky red purple or blackish red. Planar 

lamination but no cross-lamination has been observed. 

Muscovite flakes are especially conspicuous in these 

beds and in the adjacent mudstones. Quartz and feldspar grains are slightly larger than those in the 

softer mudstone but still are mostly of fine-sand size. 

Quartz composes more of the silt-size and larger 

grains, clay-size materials make up less of the matrix, 

and there are fewer shards than in the usual mudstone. Beds of quartzitic mudstone have been observed throughout the region,, but they seem to be 

more prevalent south of the Apache Mountains than 

north and are particularly noteworthy in the southernmost occurrences of the Pioneer.

ARKOSE

Arkose occurs mostly as thin- to thick-bedded crossstratified layers intercalated with tuffaceous mudstone. 

Some sections include similarly bedded feldspathic 

sandstone in small amounts. These arkosic beds are 

grayish red; most are fine to medium grained, but some 

are coarse grained. Individual beds are fairly well 

sorted, but adjacent beds may differ in grain size. In 

a few areas the lower 40-90 feet of the formation is 

composed almost entirely of thick-bedded massivecropping arkose that is medium grained and firmly 

cemented. Even in thin section these beds are not notably different from the flesh-colored sandstone of the 

arkose member of the Dripping Spring Quartzite. 

Isolated outcrops have been mistaken for Dripping 

Spring. In still fewer localities one or two beds of 

light-gray or pinkish-gray medium-grained feldspathic 

sandstone (more like parts of the Chediski Sandstone 

Member of the Troy than any other unit) also occur in 

this part of the formation.

Coarse-grained and very coarse grained arkose is 

usually confined to the basal 25^0 feet of sections that 

are at least 250 feet thick. This arkose is thick to very 

thick bedded and generally grayish red. Locally it 

contains thin beds of light-gray coarse feldspathic 

sandstone or of feldspathic conglomerate made up 

largely of quartz granules. In a few localities very 

coarse grained arkose that is mostly light colored, very 

poorly sorted, partly pebbly, poorly bedded, and obviously derived from granitoid rocks of the immediate
16 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

vicinity constitutes the lower part of the Pioneer. A 

noteworthy outcrop, in which detritus of the basal 45- 

50 feet above the Scanlan Conglomerate Member is 

from the underlying Madera Diorite, is on the north 

flank of Pioneer Mountain, in the Ray quadrangle near 

the type locality. These various coarse strata are not 

similar to any other unit in the younger Precambrian 

sequence.

Generally, the abundant hematitic clay in the matrix 

of the arkose of the Pioneer imparts a reddish cast 

that distinguishes it from all other arkoses in the 

Apache Group. Furthermore, although individual 

beds of the Pioneer may be well sorted, probably no 

unit comprising several arkose beds is as well sorted as 

an equivalent thickness in the arkose member of the 

Dripping Spring. In sections of medium-grained 

arkose, where these features may not suffice for recognition, thin seams of brownish-black or blackish-red, 

dense, and in places exceedingly tough tuffaceous siltstone invariably separates some of the beds. Even in 

sequences where these seams are discontinuous and 

sparse, a few are as much as 8 inches thick.

LATERAL VARIATIONS

Significant lateral variations in the Pioneer Shale 

have been noted only in the northwestern part of the 

region and are restricted to the belt of lapout. There 

the Pioneer becomes appreciably coarser and particularly in its lower part varies greatly in distances of a 

mile or two. In the northwest corner of the McFadden Peak quadrangle, for example, the lowest 60-90 

feet of the Pioneer is very coarse grained crossbedded 

feldspathic sandstone, which includes beds of granule 

conglomerate and many lenses of pebble conglomerate. 

The remainder of the 200-foot section is of medium- to 

coarse-grained gray arkose that, except for dusky-red 

weathered surfaces and a tendency to be more friable 

in parting, is similar to the overlying basal arkose of 

the Dripping Spring. To the northwest the lower half 

of the section is even more conglomeratic. In the 

nearest exposures to the southeast and northeast, 3-5 

miles distant, Pioneer sections dominantly are tuffaceous mudstone, as are most sections farther south. 

This aberrant coarse facies of the Pioneer apparently 

marks areas where the pre-Pioneer surface not only 

was high but also had appreciable local relief. If mudstone is lacking or subordinate in a moderately thick 

section, the Pioneer thins abruptly nearby or is missing because of lapout.

This relation to local relief is even more strikingly 

exhibited in the Scanlan Conglomerate Member. Variations are seen in tracing the lower strata of the 

Apache north 6 miles from the northwest corner of the 

McFadden Peak quadrangle to Potato Butte, which is

4 miles west of Young. At the south end of this interval the Pioneer is about 200 feet thick, but it laps out 

about 3 miles to the north. Where the pre-Apache 

surface exhibits little local relief, the conglomerate is 

fairly typical; in places it is represented by a few 

inches to 3 feet of granitic debris which contains only 

scattered gravels of quartzite, and in nearby areas 3-8 

feet of cobble conglomerate mark the base of the 

Pioneer. Northward from about the place where the 

Pioneer thins out completely, however, the basal conglomerate begins to thicken and become coarser; at 

Potato Butte where possibly as much as 100 feet of 

the Dripping Spring Quartzite is missing because of 

lapout the conglomerate is 85-110 feet thick. Here 

the conglomerate is mostly closely packed well-rounded 

cobbles 3-6 inches in diameter, but the basal 50 feet 

includes abundant larger cobbles and scattered boulders as much as 18 inches in diameter. This unusually 

thick conglomerate overlies an irregular surface cut on 

granite; local relief of as much as 35 feet exists along 

an outcrop length of 200 feet. Here, and in similar 

examples, the granite surface was swept clean of the 

arkosic debris usually seen below the conglomerate.

To the southwest 2-3 miles, where hills of the 

pre-Apache surface project up through about 200 feet 

of Pioneer and into the Dripping Spring, the Scanlan 

is as much as 75 feet thick, and the remainder of the 

basal two-thirds of the Pioneer is unusually coarse 

grained and ill sorted and contains many conglomerate 

beds. The beds are lenticular, and medium- to largescale cross-stratification is conspicuous. Mudstone occurs only in the upper third of these sections. In contrast, east and north of Young, where the pre-Apache 

surface is high but virtually planar, the basal conglomerate generally is only 6-8 feet thick and in places 

is only a few inches thick. In this area the Pioneer is 

thin or missing; where present, in sections only 25- 

100 feet thick, mudstone beds make up at least onethird of the formation.

Some of the basal conglomerate that is continuous 

with the Scanlan but directly underlies the Dripping 

Spring Quartzite exhibits poor sorting and rounding. 

The most accessible exposure is 9^/2 miles due north of 

Young along the north wall of the canyon of Haigler 

Creek. (Specifically, this exposure is in the W^2 sec. 

6, T. 10 N., E, 14 E., at and immediately west of the 

hairpin turn of the Chamberlain Trail road, which 

there turns north to follow the crest of the ridge that 

lies east of Gordon Canyon.) At this locality the 

older Precambrian quartzite dips 55°-60° ESE., and 

the siltstone member of the Dripping Spring that here 

overlaps this quartzite dips about 15° E. The preApache surface exposed in the roadcut along the can-
DRIPPING SPRING QUARTZITE 17

yon has relief of several feet. Above the unconformity 

is a breccia, 20 feet thick, composed of angular blocks 

of the older Precambrian quartzite in a matrix of medium- to coarse-grained sandstone. Some of the blocks, 

which are as much as 2 feet in diameter, traveled only 

a few feet and can be matched back into the irregularities on the surface from which they were plucked. 

A few hundred feet to the west, the breccia gives way 

to a conglomerate of poorly rounded boulders, some as 

much as 3 feet in diameter; at 600 feet west of the 

breccia exposure, this conglomerate is 40 feet thick. 

The matrix of this conglomerate and the beds several 

feet above the conglomerate are arkose, similar to the 

sandstone that ordinarily makes up the arkose member 

of the Dripping Spring. Though rare, such coarse 

breccia and conglomerate are not abnormal where 

basal units of the Apache Group lap onto very resistant older Precambrian rocks along a surface of some 

relief. Wilson (1939, p. 1151-1152) described other 

examples in this vicinity.

Where the Pioneer is thin along the San Pedro 

River valley (fig. 4), in contrast with the northwestern 

area of thinning, the basal part of the section shows no 

coarsening. The Scanlan Conglomerate Member is generally very thin or isr represented only by scattered 

pebbles in the basal few inches of the formation, and 

the overlying strata are mostly the hard quartzitic 

mudstone. Such sections may have been deposited on 

a high of very slight local relief, or they may represent sections in which most of the Pioneer was eroded 

away prior to deposition of the Dripping Spring 

Quartzite.

The medium- to coarse-grained sandstone that occurs 

at the base of locally thick sections, away from the lapout area of northwest Gila County, probably has no 

regional paleogeographic significance. The differences 

in lithology apparently reflect local derivation of 

detritus.

DBIPPING SPBING QUABTZITE

In a discussion of Dripping Spring stratigraphy 

published since this report was prepared, Granger 

and Raup (1964) presented the three-member subdivision that is used here. They, too, regarded exposures 

in Bull Canyon see reference section as among the 

most representative of the formation. In presenting 

detailed descriptions of sections at several other localities in Gila County, they elaborated on some lithologic 

details that are given little consideration here. Thus, 

although the following may supplement, it does not 

take the place of their descriptions, which are easily 

the most definitive since the early definitions of Ransome (1915; 1916).

The Dripping Spring Quartzite, which was redefined to include at its base the Barnes Conglomerate 

Member (Granger and Raup, 1964), is generally about 

600 feet thick, and it consists fundamentally of two 

elements of approximately equal thickness that contrast in composition, texture, bedding structures, and 

forms of outcrops. The lower half of the formation 

includes subordinate units of light-colored feldspathic 

quartzite but is mostly massive-cropping flesh-colored 

arkose. (In this report, a quartzose sandstone that 

includes 10-25 percent feldspar is termed feldspathic; 

one that contains more than 25 percent feldspar is 

described as an arkose.) The lower element, excluding 

the thin basal conglomerate, is herein termed the arkose 

member. Somewhat more than half of the upper element is dark-colored siltstone, which is interbedded 

with fine-grained arkose and feldspathic quartzite. In 

contrast with the arkose member, all strata are thin 

bedded, and outcrops are conspicuously flaggy or 

slabby; and overall they are considerably darker. The 

siltstone layers so strongly influence overall aspects of 

the upper element that it is designated the siltstone 

member. The middle and upper members of Granger 

and Raup (1964) are the arkose and siltstone members, 

respectively, of this discussion.

Although the Barnes has previously been designated 

a formation separating the Dripping Spring and Pioneer, mapping of the Apache Group has demonstrated 

repeatedly that separate delineation of the unit is 

impracticable, even at map scales as large as 1:12,000. 

The Barnes has obvious lithologic and genetic affinities 

with the Dripping Spring Quartzite. Put into proper 

perspective, this conglomerate is analogous to the basal 

sandstone of the Mescal Limestone, the basal conglomerate of the Troy Quartzite, and the conglomerate at 

the base of the Cambrian System. It merges with the 

Scanlan Conglomerate Member where the Pioneer laps 

out, and it is basal to the siltstone member where the 

Dripping Spring laps out; because of these features 

and especially because it is usually thin and locally 

absent, the Barnes is best visualized simply as the basal 

unit of the Dripping Spring, perhaps without even 

member status. In deference to its historical designation as a singular unit, however, and inasmuch as it 

does furnish conspicuous outcrops or float that serve 

to emphasize the unconformable contact between the 

Dripping Spring and Pioneer, it is termed the Barnes 

Conglomerate Member.

Gross aspects of the Dripping Spring Quartzite are 

apparent at the type locality (Ransome, 1903, p. 30- 

32) but, owing to faults, alteration, and cover, the 

piecing together of stratigraphic details at that locality is tenuous. The following section in Deep Creek
18 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

canyon, measured 2,1/2 miles west of Cherry Creek and 

near the south boundary of the McFadden Peak quadrangle,'is representative of complete sequences and is 

considered the reference section.

Reference section of Dripping Spring Quartzite

[Measured on west wall of Deep Creek canyon, 3,000 ft north of junction of Deep 

Creek and Bull Canyon, in SWJ£SWJ£ sec. 18 (unsurveyed), T. 5 N., R. 15 E.J

Thickness Mescal Limestone: (feet)

Dolomite breccia, in sandy dolomite matrix._______ 20+

Arkose, pale-yellowish-brown; mostly medium 

grained but contains well-rounded very coarse 

quartz grains; feldspar content about 40 percent, 

dolomitic; firmly cemented; beds tabular, 6-24 in. 

thick, internally cross stratified mostly on large 

scale; unit crops out as massive locally vuggy ledge 

with poorly defined partings at >£-3-ft intervals; 

weathers moderate yellowish brown with lustrous 

black coating between grains. In adjacent areas, 

it is 4-9 ft thick and forms ledge that is prominent 

below slope-forming dolomite breccia; lower contact 

everywhere sharp.____---------_______________ 11

Unconformity. 

Dripping Spring Quartzite: 

Siltstone member:

9. Arkose, pale-yellowish-brown to grayishorange-pink, fine- to medium-grained, thinbedded (2-36 in.); includes some beds of 

feldspathic quartzite; medium-scale planar 

cross-stratification conspicuous; weathers 

pale to dark yellowish brown, black surface 

coatings and grayish-red stains common; 

mud cracks and asymmetric ripple marks 

common. Lower half thicker bedded (1-3 

ft), coarser grained, and less feldspathic 

than upper half and is cliff former; upper 

half crops out as ledges on slope; top 15 ft 

includes thin beds of siltstone like those of 

unit 8_----__--------____-_-____-_-__ 99

8. Siltstone, dusky-yellow-green to very dark 

gray, very feldspathic; laminated, thinly 

laminated, and cross laminated; stylolites 

abundant, mud cracks common; undulant 

or irregular partings, 0.1-6 in. apart, 

characterize outcrops; unit weathers pale 

yellowish brown to grayish red; weathering 

obscures original texture; pyrite or minute 

yellow-green spots from which pyrite has 

been leached are abundant. Lowest 33 ft 

includes interbeds of arkose like those of 

unit 7 and crops out as small cliff; siltstone 

above exposed.in a slope, which steepens so 

that uppermost 40 ft crops out as a cliff or 

steep ledgy slope.______________________ 117

7. Arkose, pale- to dark-yellowish-brown; weathers slightly darker, and black coating common; very fine grained; low-angle mediumscale planar cross-lamination etches out 

poorly; outcrops part conspicuously at 2- 

in.-6-ft intervals; "pockmarks," J^-3 in. 

in diameter, on weathered surfaces are accentuated by black coating. Crops out as 

vertical face in recess below cliff ________ 14

Dripping Spring Quartzite Continued

Siitstone member Continued (feet)** 

6. Siltstone, like that of unit 8, with subordinate 

thin beds and scour fills of arkose, like that 

of unit 7; sparsely micaceous, thinly laminated to thin bedded; many beds cross 

stratified; scour-and-fill channels, 3-24 in. 

wide, are abundant; slabby undulatory 

partings at 2-10-in. intervals prominently 

defined by shaly seams; shaly parting common in siltstone beds; stylolites abundant; 

weathered rock assumes porcelaneous texture. Slabs, weathered grayish red, thickly 

cover moderate slope, which steepens 

upward to almost vertical cliff locally. _ _. . 54 

5. Arkose, like that of unit 7, with minor beds of 

siltstone like that of unit 8, except moderately micaceous. Basal 6 ft of dusky-redweathering especially micaceous thinbedded arkose grades upward into shaly 

siltstone that dominates interval 6-14 ft 

above base. Interval 14-23 ft up is hematite-flecked arkose in beds 6-30 in. thick. 

Upper 27 ft is arkose, in beds 2-8 in. thick, 

and subordinate thinner beds of siltstone; 

stylolites numerous; this part forms prominent hacky fractured ledge at top of 

slope-forming unit. Contact with arkose 

member sharp and planar.______________ 50

Thickness of siltstone member (units 

5-9)__------_----------_------_--- 334

Arkose member:

4. Quartzite, feldspathic, grayish-orange-pink to 

pale-yellowish-brown, medium-grained; 

weathers pale yellowish brown; crops out 

as massive rim-forming cliff; slightly 

etched face suggests large-scale low-angle 

thin cross-stratification; poorly defined 

partings at intervals of 2-15 ft (average 

6 ft) suggest tabular beds; otherwise bedding structures obscure. Outcrops of basal 

10 ft and uppermost 20 ft pock marked. 

Bottom 8 ft and top 12 ft less feldspathic. 

White quartz pebbles and irregular disks 

of white-weathering chert, as much as 1 in. 

in diameter, sparse in interval 20-30 ft 

above base; in adjacent areas, pebbles exist 

through thicker interval_______________ 65

3. Arkose, light-brown to pale-reddish-brown, 

fine-grained; in thinly laminated sets of 

beds 4-6 in. thick; breaks with quartzitic 

fracture; composition like that of unit 2, 

but appears more vitreous on fresh fracture; 

crops out everywhere as steep to vertical 

cliff that exhibits partings at intervals of 

1 in.-15 ft, mostly at intervals greater than 

6 ft; weathers pale red to pale reddish 

brown; on fresh break, weathered arkose 

exhibits minute patches of yellowish-orange 

clay (?). Cross-stratification, medium-scale 

and low-angle, apparent only in top 8 ft 81
DRIPPING SPRING QUARTZITE 19

Dripping Spring Quartzite Continued

Arkose member Ccntinued (feet) 

2. Arkose, moderate-reddish-orange, fine- to 

medium-grained, thin- to thick-bedded 

(1-4 ft); small- to medium-scale straight 

and concave planar cross-stratification 

etched out locally; concave crossbedding 

dominant in upper 20 ft, straight crossbedding dominant in lower 50 ft; asymmetric ripple marks sparse. Crops out on 

steep slope as bare rounded ledges with 

conspicuous partings at 51, 116, and 134 

(top) ft above base of unit; in lower 50 ft 

of outcrop, obscure partings occur at intervals of 2 in.-4 ft (mostly latter); obscure 

partings of upper 80 ft at intervals of 1-8 ft 

(typically at 4-6 ft). Composed of angular 

to subrounded grains of clear quartz and 

reddish-orange potassium feldspar (30-40 

percent of rock); scattered minute aggregates of hematite common; minute patches 

of dark-yellowish-orange limonite or limonitic clay characterize matrix of freshly 

broken weathered rock__________________ 134

Thickness of arkose member (units 

2-4)____________________________ 298

Barnes Conglomerate Member:

1. Conglomerate, well-rounded pebbles and 

cobbles J^-8 in. in diameter (average 3 in.) 

in matrix of coarse-grained to very coarse 

grained light-brown to grayish-red arkose. 

Gravel mainly of fine-grained vitreous 

quartzite; pebbles of white quartz common; 

pebbles of reddish-brown jasper and andesite(?) scarce. Contains sparse lenses of 

arkose. Crops out as one massive ledge; 

to east across canyon, unit is 1-8 ft thick. 

Contact with Pioneer Shale sharp and 

slightly undulant.--__-_______---__--___ 18

Thickness of Dripping Spring Quartzite______-_---__-____-_________ 632

Unconformity.

Pioneer Shale:

Siltstone, grayish-red; has yellowish-gray to lightbrown bleached spots; abundantly tuffaceous; 

forms steep slope; top 2 ft shaly and bleached light 

greenish gray_______________________________ 30 +

Ordinarily a twofold subdivision, in which the 

arkose and conglomerate members are combined as one 

unit, proves most practical for general discussion and 

for detailed delineation of units as an aid in deciphering structurally complex areas.

Only small parts of the Dripping Spring can be 

described correctly as quartzite. Strata throughout 

the formation are so firmly cemented, however, that 

the rock fractures across component grains and many 

units form resistant outcrops comparable to those

eroded from quartzites. In view of these aspects, 

rather than its having a highly quartzose composition, 

the formation was originally termed a quartzite, and 

this designation has unfortunately delayed recognition 

of the true makeup of the formation. Nonetheless the 

quartzite look is such a useful characteristic for distinguishing the Dripping Spring from adjacent formations that retention of the term "quartzite" in the 

formation name is deemed worthwhile.

The arkose member is 200-350 feet thick, the siltstone member is 200-370 feet thick, and the combined 

thickness of these subdivisions in sequences normally 

underlain by the Pioneer and overlain by the Mescal 

is apparently everywhere at least 550 feet. The Barnes 

Conglomerate Member adds negligibly to the thickness. 

Sections in excess of 700 feet have been reported, but 

those checked by me were suspect because of faulting 

or inflation by diabase intrusions.

From the latitude of Young south as far as the Gila 

River, the Dripping Spring is generally overlain by 

the Mescal Limestone, but in a few localities south of 

the Salt Eiver, part of the siltstone member of the 

Dripping Spring has been stripped away during one 

of the intervals of erosion that preceded deposition of 

the Troy Quartzite, the Bolsa Quartzite, or the Martin 

Limestone.

South of the Gila Eiver in the area presently 

drained by the San Pedro Eiver, the arkose member 

of the Dripping Spring was locally exposed prior to 

deposition of the Troy, and in large areas the formation was again eroded before the Bolsa Quartzite was 

deposited. Fairly thick and presumably complete sections exist, farther west, in the Vekol Mountains and 

adjacent ranges 50-60 miles south of Phoenix (L. A. 

Heindl, oral commun., 1959).

The arkose member of the Dripping Spring laps 

out north of the Sierra Ancha, so that in a few localities the siltstone member rests directly on the older 

Precambrian terrane. Locally, as along the headwaters of Canyon Creek, the formation was thinned additionally by erosion that preceded deposition of the 

Martin Limestone.

BARNES CONGLOMERATE MEMBER

The Barnes Conglomerate Member, except for a different matrix and perhaps a tendency to be more persistent and uniform in texture, is similar to the Scanlan Conglomerate Member of the Pioneer. The Barnes 

generally consists of well-rounded granules, pebbles, 

and cobbles moderately well to firmly cemented in a 

matrix of arkose; locally the matrix is feldspathic 

sandstone. The gravels are mainly of light- to darkgray and grayish-orange to grayish-red vitreous
20 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

quartzite; granules and pebbles of white quartz are 

common; granules and small pebbles of jasper and 

pebbles of volcanic rock, largely rhyolite, are sparse. 

The gravels are mostly spheroidal or ellipsoidal, but in 

some localities many are discoidal. Imbrication is uncommon and shows no dominant direction. The matrix 

ranges from very fine grained to very coarse grained, 

and is ill sorted; in composition it resembles the arkose 

of the overlying beds, but it is generally much coarser.

The conglomerate ordinarily ranges in thickness 

from 5 to 30 feet. Locally, however, it is represented 

only by scattered granules and small pebbles along the 

bottom of the basal bed of the arkose member; at the 

other extreme it is as much as 40 feet thick. The 

thicker sections commonly consist of two or more beds 

of conglomerate, partly separated by lenses of pebblefree arkose. Variations in thickness of the conglomerate and variations in composition, size, shape, rounding, and sorting of the gravels seem to be no greater 

regionally than those that can be observed in a small 

area.

The transition zone from conglomerate to the overlying arkose is usually only a few inches thick, but transition zones of pebbly arkose several feet thick exist 

in some localities. A few feet or even a few tens of 

feet of the arkose immediately above the conglomerate 

is ordinarily slightly coarser grained, however, than 

the overlying beds.

ARKOSE MEMBER

The arkose member is so firmly cemented that details 

of bedding are obscure, and the member forms massive 

outcrops. The upper half of the member commonly 

forms a cliff, and the lower half, a series of steep 

ledges. In most areas three to five partings consistent 

in stratigraphic position are etched out strongly; other 

bedding separations are seen only on close examination. Only in a few areas, where the section was 

deeply weathered prior to the present cycle of canyon 

cutting, do all bedding structures etch out to refute 

the general impression that the arkose member is made 

up of very thick bedded units having little or no internal bedding structure. Usually the Barnes Member 

forms a separate prominent ledge, although in areas of 

extreme relief the two members may form one cliff.

The lower two-thirds to three-fourths of the member 

is composed of arkose, and the upper one-quarter to 

one-third is feldspathic quartzite. The arkose is pale 

brown to pale reddish brown or reddish orange, fine to 

medium grained, thinly laminated to thick bedded, 

cemented as firmly as a quartzite, and grayish orange 

pink to moderate brown when weathered. A unit of 

several beds of arkose may be uniformly fine grained

or uniformly medium grained; little vertical gradation 

of sand sizes exists within a bed or even within a unit 

of several beds. The feldspathic quartzite is light gray 

to pale red and generally medium grained. Scattered 

small pebbles of light-gray chert and white quartz, 

not seen in every bed or outcrop, characterize this 

quartzite. Otherwise, in texture and bedding the 

quartzite is similar to the arkose. Outcrops of the 

quartzite tend to be pock marked, owing to the weathering out of less firmly cemented aggregates of sand.

In general the member consists entirely of tabular 

beds, most of which are internally cross stratified. 

Some thin beds are thinly laminated. Many of the 

beds are only 4-6 inches thick, but those that make up 

most of the member exceed 4 feet and in some areas 

are 12-20 feet thick. The crossbedding is of the 

straight or concave (tangential) type, of low to moderate angle, and of small to moderate scale (McKee 

and Weir, 1953).

Appreciable departures from the usual lithology of 

the arkose member have been noted only north of the 

Sierra Ancha in or near the area where the Dripping 

Spring thins because of lapout. Bedding features etch 

out more conspicuously in those areas than farther 

south: cross-stratification of medium to large scale (5- 

40 ft between planes of truncation) is obvious in 

wedge-shaped sets of strata that constitute the thicker 

bedding units. Locally the dips of such cross-strata 

exceed 20°, but generally dips are less than 10°. 

Wedge-shaped bedding units have been noted only 

rarely farther south but may have been overlooked in 

the quartzitic outcrops. In some sections of the northern part of the region, the arkose is mostly medium to 

coarse grained, rather than fine to medium grained, 

but is still fairly well sorted.

The lack of local variation in the textural or bedding features in the northern area is well illustrated 

in exposures along Bryant Canyon, 7 miles southwest 

of Young. There, a moderately thick section of Pioneer and most of the arkose member of the Dripping 

Spring lap out against a small steep-sided hill of 

older Precambrian quartzite and volcanic rock. A 

zone of cemented rubble 4-12 feet thick, consisting of 

angular quartzite blocks, mantles this "fossil" hill. 

Thin layers of such blocks occur along certain bedding 

planes of the Dripping Spring for distances of a few 

tens to a few hundreds of feet from the hill, and 

sparse lone blocks are found even farther away. Otherwise, the member is uniformly of the medium- to 

coarse-grained, fairly well sorted arkose that is usual 

within a radius of several miles of this locality. Bedding structures immediately adjacent to the hill are
DRIPPING SPRING QUARTZITE 21

no different from those in the same stratigraphic units 

a mile or two distant.

Near the hill the Barnes consists of one and in 

places two 6- to 10-inch layers of pebbles; locally 

there is no basal conglomerate. White quartz pebbles, 

unusual only in that they are mostly angular to subrounded, locally dominate the gravel. The quartzite 

and rhyolite pebbles, however, are well rounded. 

Although not representative of the average Barnes, 

these exposures illustrate that the Barnes in the area 

of lapout is not notably different from many outcrops 

farther south.

SII/TSTONB MEMBER

The thin-parting upper member of the Dripping 

Spring Quartzite, in marked contrast to the topographic expression of the massive arkose member, forms 

slopes broken intermittently by ragged ledges or 

small cliffs. The boundary between the members is 

sharp 'and is commonly marked by a bench a few feet 

to a few tens of feet wide cut into the less resistant 

basal beds of the siltstone member.

The five units delineated separately in the reference section are easily recognized in most exposures. 

Three units are conspicuous either because of color 

or topographic expression. These are an uppermost 

cliff-forming arkose unit, which makes up the upper 

quarter to third of the member, and two fairly thick 

units of dark-colored siltstone, which make up much 

 of the lower two-thirds of the member. The two 

siltstone units are separated by a relatively thin unit 

of arkose, and the lower siltstone is also underlain by, 

a thin arkose unit. Thin arkose beds, singly or in 

sets, are interlayered with the parts of the member 

that are mostly siltstone; similarly thin beds and 

seams of siltstone are numerous in parts that are 

mainly arkose. The siltstone causes the entire member to be fairly dark and to contrast strongly with the 

flesh-colored strata of the lower member.

The five units seemingly vary appreciably in thickness and in volume of arkose beds from one outcrop 

to another. These variations may be more apparent 

than real, however, because the boundaries between 

siltstone and arkose units are difficult to define consistently from one locality to another. This difficulty 

exists primarily because weathering obscures textural 

differences between siltstone and arkose. Secondarily, 

the rocks have been modified by metamorphism adjacent to the numerous diabase intrusions. As criteria 

for recognition of original lithology become better 

defined, future studies may prove the units to be more 

consistent in thickness than is now demonstrable.

LITHOLOGY

Strata that crop out and fracture like quartzites 

are mostly very fine grained or fine-grained arkoses; 

a few are medium or even coarse grained. On fresh 

fracture these rocks are grayish orange pink to dark 

yellowish brown. Silt- and clay-size material makes 

up 30 percent or more of the rock. Only in the 

coarsest grained sandstone that includes the least fine 

matrix can the feldspar content be accurately estimated. Such rocks are mostly arkose and contain 

from 30 to 50 percent clastic feldspar grains; a few 

beds are feldspathic quartzite with a feldspar content 

approaching 25 percent.

The siltstone is highly feldspathic and slightly to 

moderately micaceous. As noted by Granger and 

Eaup (1959, p. 425, 439), the potassium content is 

abnormally high for a clastic sedimentary rock. Six 

specimens from widely separated outcrops of the lower 

siltstone (unit 6, reference section) contained 10.7- 

14.6 percent K2O, the average of the six specimens 

being 12.3 percent.

The entire siltstone member is unusually radioactive, 

and in some areas the finer grained strata are notably 

uraniferous. During 1950-56 the member was intensively prospected, especially in northern Gila County, 

for the local concentrations of uraninite and secondary 

uranium minerals that approach ore grade. According to Granger and Eaup (1959, p. 438-439), an appreciable part of the radioactivity is attributable to the 

high potassium content of the siltstone units.

Fine-grained pyrite is disseminated in the siltstone 

and is the main cause of the medium- to dark-gray 

color characteristic below the zone of weathering. 

Small amounts of carbonaceous material are also disseminated in the rock, and in some strata thin seams 

of fine-grained graphite have been noted (Granger 

and Eaup, 1959, p. 442-443). Pyrite 'occurs in smaller 

amounts in the arkose, principally along joints, fractures, and stylolites.

WEATHERING MODIFICATIONS

Much of the siltstone in surface exposures is very 

dark, but leaching of pyrite causes it to weather 

yellowish gray to moderate brown, much like the arkose. Additionally, this process causes both the finer 

grained interbedded arkose and the siltstone to assume 

a coarsely porcelaneous texture. As a consequence 

the two rock types cannot be readily distinguished, 

and lateral correlation of individual beds or sets of 

beds is difficult. Weathering effects extend from a 

few feet to a few tens of feet below the outcrop, and 

in some areas the dark color of the siltstone can be 

seen only in deep excavations or mine workings. Ad-
22 YOUNGER PRECAMBEIAN GEOLOGY IN SOUTHERN ARIZONA

jacent to fractures, joints, and other partings, the 

bleached strata commonly are strongly impregnated 

with, a grayish-red limonite stain; such stains are 

much more abundant in the siltstone units than in 

the arkose units.

In many areas, such as along the canyon of the 

Salt Kiver, outcrops of the siltstone member are lightly 

coated or even crusted with reddish-orange to darkreddish-brown limonite. This rusty coloration, coupled with the thin bedding, distinguishes these outcrops at a distance from all others in the younger 

Precambrian sequence. Because of slight differences 

in porosity, texture, composition, and exposure, the 

coatings produce a conspicuous surficial color banding, 

which parallels the gross bedding features.

Conspicuous color banding is characteristic of only 

the siltstone member. It is not an aid to recognition 

of the entire formation, as generally has been extrapolated from E'ansome's (1903, p. 138) early, and apparently inadvertent, reference to color stripping in 

the arkose member. Only very locally, mostly in the 

southern part of the region, does a poorly comparable 

thin color stripping mark outcrops of the arkose 

member.

BEDDING AND SECONDARY STRUCTURES

The siltstone and arkose of the siltstone member 

occur in thin tabular beds and in irregularly pinching 

and swelling beds (fig. V), which range in thickness 

from -a fraction of an inch to 3 feet. The arkose is 

somewhat thicker bedded than the siltstone, and the 

lower 50 feet of the uppermost arkose unit is generally 

the thickest bedded. The beds are internally laminated and cross laminated (fig. V). Flaggy to slabby 

partings characterize all outcrops. Partings along 

the internal laminae tend to be as conspicuous as those 

along planes between beds, causing some units especially the siltstone to appear thinner bedded than is 

actually true.

Mud cracks and ripple marks are abundant locally 

and are more prevalent in the siltstone than in the 

arkose.

Stylolites everywhere characterize the siltstone 

member and are especially abundant in the upper 

arkose unit. The stylolites ordinarily have an amplitude of less than a fourth of an inch, but amplitudes 

exceeding an inch have been noted. In unbleached 

siltstone many are filled with pyrite, and some include carbonaceous material. Those in bleached rock 

invariably are filled with limonite.

Large channels, filled with strata similar to those 

they were cut into, have been noted in a few localities 

north of the Salt Kiver. Most range in depth from

FIGURE 7. Typical lamination and irregular bedding in siltstone member of Dripping Spring Quartzite. Siltstone of 

unit 8, measured section, page 18. Photograph by C. T. 

Wrucke.

20 to 30 feet and in width from 200 to 250 feet, and 

they are generally restricted in stratigraphic position 

to the lower siltstone unit and to the arkose unit that 

separates the principal siltstone units. Granger and 

Raup (1959, p. 424) noted a channel, about 50 feet 

deep and VOO feet wide at the outcrop, that apparently was cut through the basal strata of the siltstone 

member and into the arkose member.

In the lower siltstone unit small subparallel cigarlike masses, of silt- to fine-sand-size arkose, are embedded in thinly laminated finer grained siltstone 

(fig. 8). These features, here interpreted as scourand-fill phenomena, have been described briefly by 

Granger and Eaup (1959, p. 43Y-438 :and fig. 55) as 

"pseudochannels." They are unusual in trend and 

distribution and therefore warrant particular note as 

possible factors in interpreting the environment of 

sedimentation.

The scour-land-fill channels are 1 inch to as much 

as 8 feet in width, but most are from 6 inches to 2^ 

feet. The depth of many of the smaller channels is 

roughly the same as the width, but the depth of the 

larger channels is commonly one-third to threefourths the width, and some of the wider channel fills 

are even lenticular or platelike in cross section rather

FIGURE 8. Scour-and-fill and compaction features in the siltstone member of the Dripping Spring Quartzite. Canyon of 

Cherry Creek in NE% sec. 10, T. 7 N., R. 14 E. (unsurveyed). 

A, Cross-sectional view showing compaction of shaly-parting 

siltstone around cores of slightly coarser siltstone. B, Common longitudinal view in which cigarlike outlines of the 

cores and compaction of host siltstone are not apparent; 

head and pick and end of handle are against filled channels.
DRIPPING SPRING QUARTZITE 23

B

than crudely elliptical. Figure SA illustrates a variety of the cross-sectional forms displayed by the 

fills and a typical range of sizes in a given outcrop. 

Longitudinal sections and cross sections commonly are 

not well exposed in the same outcrop, so that the ratio 

of length to diameter of the cigar-shaped fills is not 

well documented. At two localities, where fills ranged 

in diameter from 6 inches to 2!/2 feet, their lengths 

were determined to be from 20 to 30. times their widths.

The lower surfaces of the fills are mostly rounded 

 and troughlike in cross section; the uuper surfaces 

arch gently upward or are almost flat. The fills are 

lighter colored and slightly coarser grained than the 

enclosing rock. The arkose fill is dense and may not 

show perceptible bedding structures, but where perceptible, fine laminae are undisturbed and horizontal.

The siltstone surrounding the cores shows marked 

compaction (fig. 8A). The laminated host strata are 

truncated and commonly warp abruptly downward 

where they terminate under and against fill material. 

The thin laminae are sometimes so intricately folded 

where they impinge against the fills that details of 

the minute folds are difficult to trace. Laminae of 

the siltstone host arch gently over the cores and are 

not truncated against them. The effects of compaction 

may be seen for several inches or even a few feet 

above and below the fills. Secondary shaly partings, 

developed in weathered siltstone, emphasize these 

effects.

In the fraction of an inch of siltstone that lies immediately over the sandstone fills, shrinkage cracks 

ordinarily are abundant. These cracks are somewhat 

irregular in outline, but all are alined approximately 

normal to the axes of the fills. Possibly these cracks 

also are the effects of compaction.

Where the scour-and-fill features are particularly 

numerous, the cigarlike cores impinge one on another. 

An earlier core was partly scoured away before the deposition of a second, and both may have been modified 

by the formation of a third. Invariably, such multiple fills are separated by thin seams >of siltstone. In 

some beds, also, separate cores may be joined along 

the bedding by thin layers of sandstone. The lower 

surfaces of such connecting layers are commonly undulating in cross section.

In longitudinal sections the scour-and-fill structures 

are inconspicuous and may be overlooked (fig. 8Z?). 

Because joints ordinarily trend parallel to the fill, longitudinal sections are more commonly exposed than 

cross sections. Scour-and-fill structures exposed in a 

face that is neither normal nor parallel to their trend 

go unrecognized or are mistaken for undulant bed-
24 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

ding, such as that depicted in figure 7. Eelatively 

little of the undulant bedding seems related in origin.

A unique feature of the cigarlike fills is their nearparallel alinement. In a given outcrop the fills commonly show a variation in trend of only 10°-15°. In 

the McFadden Peak quadrangle, an area of about 250 

square miles, they trend N. 5°-25° E., and at the latitude of Globe they trend from N. 15° W. to N. 10° E.

The scour-and-fill structures occur through stratigraphic intervals of 20-100 feet and may characterize 

the whole siltstone unit. They are especially abundant 

in outcrops of the northern half of the McFadden 

Peak quadrangle; southward to the latitude of Superior they are sporadic in occurrence. Their occurrence farther south remains to be determined. These 

structures have not been recognized in the upper siltstone unit.

FEATURES THAT DISTINGUISH PRINCIPAL UNITS

The lowest arkose unit of the siltstone member 

makes up the basal 20-50 feet of most sections. This 

unit is very fine grained, notably micaceous, and has a 

greater tendency to weather to reddish hues than has 

the overlying arkose. Individual beds are thin to very 

thin, and many are separated by seams of siltstone. 

The basal several feet of the unit commonly includes 

as much siltstone as sandstone. Consequently the 

upper part, having fewer siltstone layers, crops out as 

a ragged ledge or small cliff that overhangs the slopeforming lower part.

The sandstone unit that separates the two siltstone 

units, about one-third of the way up in the member, 

ranges in thickness from 10 to 35 feet and consists 

mostly of very fine grained to medium-grained arkose 

and feldspathic sandstone. Many thin beds of similar 

composition are interleaved in the basal 20-40 feet of 

the upper siltstone unit. These interbedded strata and 

the underlying arkose unit form one cliff in canyon exposures. In places other than canyons the arkose unit 

generally forms a ledge, and it may be the only exposure in the middle part of the member. Abundant 

pockmarks or pits, \.-A inches across, mark outcrops of 

the coarser grained 'beds in this unit more commonly 

than they mark similar <beds in other units.

In many areas the two siltstone units are mantled 

with flaggy and slabby rubble, but in much of northern Gila County both units crop out as smooth dark 

slopes slightly convex upward that are almost free 

of rock debris and vegetation. Apparently, where considerable sulfate is being formed by weathering of the 

highly pyritic siltstone, vegetation cannot flourish. 

Such barren slopes occur where the strata have been 

metamorphosed adjacent to a thick diabase sill. In

many places in northern Gila County the separate 

units of the siltstone member are readily distinguished 

from distances of 1-3 miles because of the conspicuous 

parallel rock scars that mark the two siltstone units. 

Farther south, particularly at lower elevations where 

all outcrops support less vegetation, the siltstone outcrops are not so conspicuous, but they still are crudely 

marked by a thinner cover of shrubs and trees.

Thin-bedded fine-grained arkose dominates the 

upper 100-130 feet, or roughly the upper one-third 

(unit 9, reference section) of the member. In some 

places the unit is seemingly no more than 60 feet thick. 

Freshly broken rock ranges from grayish orange pink 

to light brownish gray or pale yellowish brown, and 

the rock weathers to rusty yellowish brown. Thus, in 

hand specimen or from a distance the uppermost unit 

appears lighter in color than the arkose units between 

and below the siltstone units. The lower 50 feet of 

the uppermost arkose unit includes many mediumgrained beds, contains fewer siltstone beds, and is 

thicker bedded and less feldspathic than the upper 

part. In many localities the upper 5-30 feet includes 

numerous siltstone layers or is mostly siltstone. The 

lower half of the unit forms cliffs or a steep series of 

ledges; the upper half forms thin ledges on a moderate 

to steep slope. Where exposed near the bottom of a 

canyon, the entire unit is a cliff former.

PRE-MESCAL UNCONFORMITY

An unconformity, only recently recognized (Granger and Raup, 1964, p. 44), exists between the Mescal 

and the Dripping Spring. According to Ransome 

(1916, p. 138), upward in the Dripping Spring "the 

beds become thin, flaggy, and rusty, with a tendency 

to grade into the Mescal." On the basis of this early 

description, the boundary between the formations generally has been accepted as gradational. Actually the 

Dripping Spring is separated everywhere north of 

the Gila River, at least from stratigraphically higher 

limestone or dolomite by a distinctive poorly sorted 

sandstone unit a few feet thick. The contact between 

this basal sandstone of the Mescal and the underlying 

beds is sharp. In many places, but only for short distances, the sandstone overlies a breccia, 6-30 inches 

thick, of angular chips obviously derived from the 

uppermost beds of the Dripping Spring. This breccia 

has a matrix of dolomitic sandstone. In very few 

places, angular fragments from the Dripping Spring 

also occur in the sandstone unit or in the overlying 

carbonate rocks.

In places the thin-bedded siltstone at the top of the 

uppermost arkose unit of the Dripping Spring is thin 

or missing. Perhaps, therefore, appreciable erosion
MESCAL LIMESTONE 25

did occur after the Dripping Spring was lithified and 

before the Mescal was deposited. In some critical 

areas where the section seems thin, however, outcrops 

are not adequate to furnish unequivocal evidence of 

significant erosion. Furthermore, some apparent thinning may be attributable to variations in sedimentation. Pending better understanding of the stratigraphy of the upper member of the Dripping Spring, 

the unconformity probably should not be visualized as 

representing a profound hiatus 'but the possibility 

must be admitted.

MESCAL LIMESTONE

The Mescal Limestone includes internal structures 

and lithologic and stratigraphic elements that previously have not been recognized as prime characteristics 

of the formation. The Mescal was described by E-ansome (1919; p. 42-43) as "composed of thin beds that 

have a varied range of color but are persistently 

cherty. The siliceous segregations as a rule form 

irregular layers parallel with the bedding planes, and 

on weathered surfaces stand out in relief and give the 

limestone the rough, gnarled banding that is its most 

characteristic feature." He further described the formation as including limestones and dolomitic limestones with or without chert. Darton (1932) indicated 

that the formation includes "a large amount of algal 

material," and Wilson (1928, p. 30) noted that "the 

upper portion of most sections contains a massive, 

algal member that is an important horizon marker." 

In most descriptions, however, Kansome's descriptions 

have been paraphrased and the formation characterized as a hard thin-bedded cherty dolomitic limestone. 

Such descriptions do not typify complete sections and 

are wholly inadequate to identify certain ordinary 

f acies of the Mescal.

The Mescal Limestone is divisible into three readily 

recognized members: a thin- to thick-bedded lower 

member, 150-270 feet thick; a stromatolite-bearing, in 

part massive, middle algal member, 40-130 feet thick; 

and an upper argillite member as much as 100 feet 

thick. The lower and algal members are dominantly 

carbonate and are present wherever the Mescal is 

found, though over much of the region a part of the 

middle member is missing owing to pre-Troy erosion. 

The argillite member was recognized first as a distinctive stratigraphic element overlying the Mescal by 

Hinds (1935, p. 32), but was not thereafter widely 

noted as a part of the Apache sequence because it is 

absent south of the latitude of Theodore Eoosevelt 

Dam. The argillite and the two carbonate units have 

been enumerated as members of the Mescal (Bromfield

and Shride, 1956, p. 622), but details of stratigraphy 

and regional relations of the individual members have 

not been delineated previously.

In different areas the carbonate (lower and algal) 

members have three distinctly different lithologies, two 

of which represent changes that took place after the 

strata were lithified. The original carbonate beds 

were cherty dolomites. During deep leaching, before 

and during deposition of the Troy, the dolomite was 

silicified completely in some areas and silicified at least 

to a small degree elsewhere throughout the region. 

The last and most widespread modification, also in 

Precambrian time, was the metamorphism adjacent to 

the diabase sills that converted the cherty dolomite 

almost wholly into silicate-bearing calcitic limestone. 

These metamorphic limestones generally lack chert and 

are so different from the unmetamorphosed dolomites 

that they are described separately (p. 39^1).

There are few extensive occurrences that have not 

been modified either by silicification or thermal metamorphism. However, cherty dolomites do occur throughout an area of about 30 square miles north and west 

of Sombrero Butte along the east side of Cherry Creek 

(McFadden Peak quadrangle). Much smaller areas 

exist at the south end of the Sierra Ancha (4 miles 

west of Cherry Creek along the south border of the 

McFadden Peak quadrangle), in the central part of 

the Sierra Ancha immediately north of McFadden 

Peak, along the upper reaches of Haigler Creek 10 

miles north of Young, in the vicinity of Theodore 

Roosevelt Dam, and in a narrow belt that extends 

about 12 miles southeast from U.S. Highway 60 along 

the south rim of the Natanes Plateau. Farther south, 

areas of cherty dolomite are apparently smaller and 

even more sparse. Those known to me are on the 

southeast flank of the Apache Mountains, at widely 

scattered localities in the Mescal Mountains, and along 

the east flank of the Vekol Mountains. E-ansome's descriptions and map of the Dripping Spring Mountains 

suggest a few scattered outcrops in that range. Most 

other southern exposures of the Mescal are partly or 

entirely metamorphic limestone.

The sequence of units and the lithologies shown in 

the following stratigraphic section on Pendleton Mesa, 

here proposed as the reference section for the Mescal, 

are representative of the unmetamorphosed sequence. 

The lower member is possibly the thickest seen to date; 

the middle member is thicker than most occurrences, 

although thicker sections do exist. The section is 

atypical only in that it was measured in one of the 

three small areas where a basalt flow separates the 

algal and argillite members.
26 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

Reference section of unmetamorphosed Mescal Limestone 

[Measured on west slope of Pendleton Mesa, 0.6 mile north of Horse Tank Creek, in 

NWJi SWJi sec. 11, T. 7 N., R. 14 E. (unsurveyed)]

Thickness

(feet) 

Troy Quartzite: Conglomerate overlain by pale-red pebbly

arkose_______________________________________ 20 +

Unconformity. 

Mescal Limestone: 

Argillite member:

12. Siliceous argillite, grayish-orange-pink to 

moderate-yellowish-brown; poorly exposed 

on gentle detritus-covered slope________ 44

Basalt flow: Basalt, hematitic and vesicular. Fossil soil 

1-3 ft thick at top._____________________________ 52

Unconformity.

Mescal Limestone Continued 

Algal member:

11. Dolomite, pale-red or yellowish-brown to 

medium-light-gray, finely crystalline, thinbedded (^-2 ft); stromatolites of unit 10 

disappear in basal 10 ft of this unit. Gray 

chert, in thin layers and lenses, increases 

in abundance upward; closely crowded 

hematite-flecked chert nodules occur locally along bedding in irregularly lenticular 

zones 2-4 ft thick. Several bedding partings marked by thin layers of grayishorange argillite. Top-most bed very cherty; 

contact with basalt slightly irregular._ _ _ 28 

10. Stromatolitic dolomite, pale red to grayish 

red at base grading to medium light gray 

and yellowish brown at top, finely crystalline, beds K-6 ft thick, mostly about 4 ft. 

Unidentified stromatolites, of inverted 

cone form, mark basal 4 ft; the stromatolite 

Collenia frequens throughout rest of unit. 

Very irregular pinkish-gray to mediumgray chert lenses, %-3 in. across, sparse 

throughout but most common toward top. 

Very light gray claystone, which weathers 

grayish orange and is in lenticular layers 

1-15 in. thick, caps beds at 4, 13, 50, 60, 

and 67 ft above base. Dolomite weathers 

brownish to yellowish gray, with extremely 

rough, locally fluted surfaces; bottom 60 

feet crops out as cliff. Contact with unit 

9 is sharp____________________________ 77

Thickness of algal member (units

105

Lower member:

9. Dolomite with sparse chert: Dolomite, pale 

yellowish brown, weathers grayish brown 

with rough, fluted surfaces on which silty 

laminae etch out. Chert, light to medium 

gray, dense; weathers grayish orange; in 

thin layers and lenses (J4-4 in. thick). 

Thin bedded (8-24 in.), with slabby parting; crops out as thin ledges on partly 

covered slope. Contrasts with lower units 

in that it contains little chert ____________ 27

8. Dolomite interbedded sparsely wth cherty 

dolomite. Dolomite, pale yellowish 

brown, dense to finely crystalline, weathers

Thicknsss 

(feet)

Mescal Limestone Continued 

Lower member Continued

pale yellowish brown to brown with silty 

surface. Chert, mostly light gray, 

weathers yellowish to brownish gray with 

brownish-black coating locally conspicuous. Beds 1-5 ft thick, slabby to massive 

parting, crop out as ledges on steep slope. 

Chert is mainly of the secondary variety, 

occurs as lenses that cap individual beds, 

and is sparse in lower 34 ft but becomes 

abundant in upper part of unit. Vuggyweathering 3-ft zones, at 38, 49, and 58 ft 

above base of unit, consist of irregular 

nodules that contain fragmented earlyformed chert. In places, 3-in.-2-ft-thick 

lenses of gritty sandstone border these 

zones or cut dikelike through chert and 

separating dolomite beds. Lenticular 

layers, as much as 8 in. thick, of reddishorange argillite separate a few beds in 

upper half__-___-_---_---------------- 68

7. Interbedded dolomite and cherty dolomite: 

Dolomite, grayish orange, yellowish brown 

and pale red brown, dense to very finely 

crystalline; in beds 2-4% ft thick; includes 

clay and fine silt; weathers yellowish 

orange to yellowish gray and smooth. 

Chert, mostly medium gray, in coalescing 

layers %-l in. thick that make up 10-80 

percent of individual beds. The layers 

etch out conspicuously, have black coatings, and appear finely spongy to vesicular, 

reflecting poorly formed cubic molds after 

halite. Hopper-shaped molds as much as 

1 in. across, occur in chert layer 13 ft 

below top of unit. Unit forms darkcolored cliff________________---_-.__- 47

6. Dolomite, grayish-red, dense; contains much 

clay and fine silt; weathers reddish to 

yellowish brown with smooth surface and 

inconspicuous splitting planes at 2-8-ft 

intervals; breaks conchoidally; bottom 6 

ft forms ledge, top 13 ft smooth cliff with 

flat slope between. Top 6 in. includes 

small poorly rounded to angular pebbles 

of light-gray chert; interval 4-5}£ ft above 

base includes similar granules and small 

pebbles of chert and sparse coarse grains 

of vitreous quartz.________-__--------- 22

5. Dolomite, grayish-red, slightly silty; weathers to very rough surface mottled grayish 

orange to grayish pink. Single bed-__-_ 3

4. Calcareous dolomite alternating with cherty 

dolomite: Dolomite, grayish red, very 

finely crystalline, some beds silty and 

cross laminated; weathers pale brown to 

pale red with silty surface; thin to thick 

bedded (1-5H ft); five beds include sheaflike impressions. Chert, brownish gray 

to white, in thin irregular layers that 

weather brownish black and etch out 

conspicuously; some layers include abundant small halite molds. Massive beds
MESCAL LIMESTONE LOWER MEMBER 27

Thickness 

(feet)

Mescal Limestone Continued 

Lower member Continued

of top half grade from chert-free dolomite 

at bottom to very cherty dolomite at top 

of bed, and crop out as ledges on otherwise steep slope.___________------__-_- 29

3. Cherty dolomite: Dolomite, pale red to 

grayish red, thick bedded, massive parting, weathers brown to grayish brown; 

chert, brownish gray to black in very 

irregular vesicular layers, 1-4 in. thick, 

makes up 25-50 percent of each bed; 

chert juts out in very rough ledges that 

form steep slopes. Beds locally show 

slump and brecciation features, and many 

chert layers are seen as trains of angular 

fragments. Chert layers of basal 4 ft include abundant hopper-shaped molds 

after halite--------------------------- 44}^

2. Dolomite breccia, of fragments of cherty 

dolomite like that of unit 3, in matrix 

of grayish-orange, silty and locally calcareous dolomite. Weathers pale to yellowish brown and to silty or gritty surface 

having rough blocky black-mottled projections; exposed on partly covered slope. 

Fragments range from minute chips to 

blocks 1X3X3 ft; large sizes most abundant in upper part. Bottom 3 ft includes 

abundant coarse sand grains as in unit 1; 

these diminish upward.__-_______-_-.._- 23

1. Arkose, grayish-orange-pink to pale-brown, 

fine-grained; has dolomitic, clayey matrix; 

breaks with quartzitic fracture; includes 

coarse to very coarse grains of clear 

vitreous quartz. Single massive bed 

forms prominent ledge abundantly veined 

by limonite. Contact with Dripping 

Spring Quartzite planar._____-_.__-.___ 5H

Thickness of lower member (units 1-9) _ 269

Thickness of Mescal Limestone (units 

1-12)____________________________ 418

Unconformity. 

Dripping Spring Quartzite: 

Siltstone member:

Arkose and siltstone, pale-red to pale-yellowishbrown, thinly cross laminated to thin-bedded 

(as much as 30 in.), very firmly cemented, 

slabby-parting (J-.-15 in.); stylolites abundant. 

Top 10 ft mostly siltstone_-----_-________ 32 +

LOWER MEMBER

LECHOLOGY AND STRATIGRAPHY

The lower member is mostly thin- to thick-bedded 

dolomite, in which chert is sparse to abundant. The 

dolomite is dense almost lithographic to finely crystalline; most beds include very little clastic material. 

The small amounts of fine sand, silt, and clay that are 

included cause minor bedding structures to etch out 

on weathered surfaces. The dolomite ranges from yellowish brown through pale red to grayish red. Brown 

hues, which are characteristic of sections that have 

had a minimum of postlithification solution and silicification, dominate in most sections. Red hues characterize 

strata deeply leached after lithification. The chert 

ranges from white through brownish black to black. 

The darker chert occurs mostly as irregular discontinuous laminae and thin layers that parallel bedding. 

These layers etch out conspicuously upon weathering 

and thereby produce the very rough gnarly banding 

supposedly typical of Mescal outcrops but actually 

absent from much of the formation. In most sections 

the dark cherts characterize only the lower one-half to 

two-thirds of the member. Upward in the lower member the chert is mostly light gray to medium and 

brownish gray and more massive. It commonly occurs 

in fairly thick irregular layers and as nodules or lenticular aggregates of nodules that only grossly follow 

bedding. The dark cherts formed nearly contemporaneously with the dolomite, whereas much of the lightcolored chert formed later, contemporaneously with 

the development of karst topography in the Mescal. 

Although considerable lithologic modification accompanied the formation of the later chert, certain sequences of beds and bedding features are distinctive 

of individual units throughout the region; consequently the stratigraphic position of a given outcrop can 

be determined within narrow limits.

The basal sandstone of the Mescal (unit 1, reference 

section) probably averages 5-6 feet in thickness; in a 

few places it is only a few inches thick or is missing; 

in other places it is as much as 15 feet thick. Generally it crops out as a conspicuous massive ledge. Where 

the sandstone is especially thin, however, as along the 

canyon of the Salt River east of Canyon Creek, it may 

not be recognized readily because of a cover of debris. 

In places the unit is thinly crossbedded, in other places 

slump structures are common, and in still other areas 

the unit is massive and in effect structureless. Commonly the unit is a moderately well cemented arkose, 

made up at least in part of detritus from the Dripping 

Spring, but in places it is a quartzite virtually free 

of feldspar. The rock is slightly dolomitic or, if 

metamorphosed adjacent to diabase, calcitic. Reddishbrown limonite mottles and veins weathered surfaces. 

Abundant coarse to very coarse well-rounded grains 

of clear quartz and smoky opalescent chert, set in a 

matrix that is generally medium grained, characterize 

the sandstone. These distinctive grains, the carbonate 

cement, and poor sorting differentiate this sandstone 

from any in the Dripping Spring.

Immediately above and in sharp contact with the 

basal sandstone of the Mescal is a breccia of dolomite,
28 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

which is generally 15-40 feet thick (unit 2, reference 

section). In and near the Sierra Ancha the breccia in 

a few places is as much as 100 feet thick. In the 

canyon of the Salt River, between the mouth of Canyon 

Creek and U.S. Highway 60, it is thin or missing, and 

in at least one locality in the Vekol Mountains there is 

no breccia. Everywhere else the breccia has been 

recognized. The overlying unit of cherty dolomite 

(unit 3, reference section) is relatively thin where the 

breccia is thick and vice versa. In the McFadden Peak 

quadrangle these two units make up roughly the lower 

fourth of the member.

The breccia is composed of fragments of cherty 

dolomite, ranging from small chips to blocks 5 X 10 X 

20 feet, in a structureless matrix of pale-red or grayishred to grayish-orange silty dolomite. The blocks may 

be sparsely distributed in the matrix or may be closely 

spaced in comparatively little matrix. Small fragments are most abundant in the basal part, where the 

matrix makes up a large proportion of the rock. 

Large blocks may be found anywhere in the breccia, 

but are most numerous near the top. No voids exist 

in the breccia. The matrix of the basal few feet includes abundant coarse grains of quartz and chert like 

those that characterize the basal sandstone, and in 

many places such grains are sparsely distributed upward through the matrix.

The breccia weathers to a smooth gentle slope, which 

collects much debris. Because exposures are poor and 

because weathering tends to obscure the outlines of 

the fragments and to emphasize the silty and sandy 

aspect of the matrix, in most exposures the breccia 

simulates a dolomitic mudstone. For this reason, the 

fine-grained upper strata of the Dripping Spring 

were formerly interpreted as grading into the Mescal.

The breccia is gradational upward into flat-bedded 

massive thin- to thick-bedded (1-5 ft) dolomite that 

is especially cherty (unit 3, reference section). Very 

irregular layers of brownish-gray to black chert, %0-4 

inches thick, make up 20-60 percent of each bed. In 

most places this unit crops out as a steep slope; in 

steep-walled canyons it forms cliffs.

The next overlying unit is 20-30 feet thick and consists of dolomite beds 1-6 feet thick that alternate with 

beds of cherty dolomite of comparable thicknesses 

(unit 4, reference section). Chert is concentrated only 

in the upper third or half of some beds; otherwise the 

cherty beds are similar to those of the underlying unit. 

Parting planes of a few of the chert-free beds are 

characterized by peculiar sheaflike markings. Locally, 

asymmetric ripple marks are abundant; ripple marks 

have been noted elsewhere in the lower member, but 

this is the only unit in which they are common. In

all unmetamorphosed sections viewed, this unit is a 

striking marker because of the alternation of cherty 

and chert-free layers. However, this alternation is 

not everywhere an aid to recognition of stratigraphic 

units: in some areas of metamorphic limestone, the 

distribution of silicate minerals suggests that all beds 

of the interval were locally cherty. This unit occurs 

within the interval 50-85 feet above the base of the 

Mescal.

The fifth distinctive stratigraphic interval (units 6 

and 7, reference section) crops out mostly as a cliff 

the most prominent outcrop of the lower member 

and makes up most of the middle third of the member. 

The basal 15-25 feet (unit 6, reference section) is 

dense virtually structureless clayey and silty dolomite, 

which may form ledges but as a rule is covered by 

debris from the cliff. This dolomite is free of chert 

except for detrital granules and small pebbles. The 

cliff-forming part (unit 7, reference section) is mostly 

thin bedded but does include beds as much as 5 feet 

thick. The beds are alternately very cherty dolomite 

(as much as 80 percent chert) and moderately cherty 

dolomite (10-25 percent chert). Where weathered, the 

dark chert etches out as wispy or coalescing pitted or 

spongy bands; most are less than an inch thick, and 

many are less than a quarter of an inch.. Black coatings characterize the weathered surface of the chert 

and cause the cliff exposures to be darker than those 

of adjacent strata. This unit is generally the highest 

in which the early formed chert is prevalent.

Above the cliff-forming strata is a unit composed of 

1-5-foot beds of dolomite, erratically variable in chert 

content (unit 8, reference section). This unit, 50-70 

feet thick, forms prominent ledges on a slope. It 

differs from lower units in that the dolomite tends to 

be in light shades of brown rather than in dark shades 

of brown or red, and the chert is mostly light gray. 

Generally chert is sparse in the lower third to half of 

the unit. In some areas only a few of the higher beds 

are cherty; in others all the beds include chert and in 

places two or three thin layers of chert a few feet 

apart coalesce laterally to one zone as much as 10 feet 

thick. Chert occurs in some beds as thin wavy parallel 

layers, but it more commonly occurs as lenses and 

layers, 2-10 inches thick, that cap individual beds 

and project irregularly 1-8 inches down into .the 

dolomite. Nodules, a fraction of an inch to 3 feet 

in largest dimension, are abundant within the beds 

in some of the most cherty sections. Lenses and dikelike masses of sandstone, which fill solution openings, 

are more common in this unit than in any other.

The uppermost unit, unit 9 of the reference section, 

is 20-30 feet thick and is similar to the one just de-
ME9CAL LIMESTONE LQWElR MEMBER 29

scribed except that it is wholly thin bedded, parts 

conspicuously at intervals of a few inches, and forms 

gentler slopes. Layers and lenses of light-colored 

chert range from paper thin to 6 inches thick, but 

generally this unit contrasts with all lower units in 

containing very little chert.

Although the upper two units vary laterally in 

thickness and lithology, because of solution and late 

silicification, the sequence of beds within them is remarkably uniform from the Vekol Mountains to the 

Mogollon Rim, a distance of 135 miles.

The contact between the algal and lower members 

is sharp and readily recognized; the flat-bedded slabby 

dolomite below the contact contrasts strongly with 

the massive outcrops above in which algal structures 

are conspicuous.

STRUCTURAL AND TEXTURAL FEATURES

Abundant halite molds and small-scale sedimentary 

structures indicate that much of the dolomite was deposited in very shallow highly saline waters and that 

the dark cherts were formed early. Interrelations of 

such features show that the basal dolomite breccia and 

minor related breccias are not original sedimentation 

phenomena but are founder breccias formed before the 

upper part of the member was deposited. The origin 

of certain peculiar structures is not understood, but 

their description here may aid in future interpretations of the Mescal.

HALITE MOLDS IN CHEET

Many layers of the dark-gray to black chert include 

innumerable molds interpreted as representing halite 

crystals. In most cherts the molds are equidimensional 

pits, 1-5 millimeters across, filled with brown dolomite. 

These are sparse in some layers and constitute more 

than half the volume of others. On weathered outcrops, in which the fillings have been leached out, the 

highly pitted chert bands resemble vesicular basalt. 

Although most pits are rounded or have cubic outlines 

that are distorted and so are not diagnostic of halite, 

a few cubic forms can be found in most layers, and in 

some they are abundant. More diagnostic, however, 

are larger sharply outlined molds found in very few 

layers. These molds, which range from 5 to 30 mm 

in diameter, have the hopper-shaped form of skeletal 

halite crystals common in salt deposits (Shrock, 1948, 

p. 146-148; Dellwig, 1955, p. 89-95; Brooks, 1955).

Halite molds are particularly abundant in the cherts 

of the blocks and slabs in the founder breccia at the 

base of the member, and they characterize most chert 

layers in the cherty dolomite (unit 3, reference section) 

that overlies the breccia. Fewer of the chert layers 

in the unit of alternating beds of dolomite and cherty

dolomite include the molds, and only sparse layers in 

the cliff-forming unit show them. They have not been 

preserved, if they existed, in the dolomite between 

the chert layers.

The association of halite and chert is an anomaly, 

and the molds in chert are almost unique. Hoppershaped halite crystals apparently form in brine at the 

surface of hypersaline bodies of water and settle to 

the bottom when the surface of the water is disturbed. 

After settling, the fragile incipient cubes are subject 

to destruction by solution or modification by overgrowth (Dellwig, 1955, p. 88-92). Most of the 

large sharply defined hopper imprints are on the 

upper surface of chert layers, and apexes of the 

hollow pyramidal forms point down in the position 

of initial formation. Thus, these particular salt crystals were little disturbed by wave or current action, 

and little dissolved before leaving their imprints. 

Probably the halite crystals were impressed directly 

into silica gels, which accumulated on bottom muds 

before these sediments were diagenetically much 

modified.

During deposition of the lower 100 feet of the Mescal, circumstances favorable for the formation and 

preservation of halite crystals alternated innumerable 

times with those favorable for the deposition of silicafree carbonate muds. Possibly the latter were initially 

dolomite, because delicate laminae and crosslaminae 

outlined by clastic grains of carbonate are preserved. 

Anhedral molds of halite are ubiquitous in individual 

beds; it therefore seems unlikely that salt crystallized 

only during the desiccation of scattered pools, as has 

been reported where halite imprints are preserved as 

casts rather than molds (Brooks, 1955). Rather, the 

lower part of the Mescal probably accumulated in a 

broad uniformly shallow quiet body of water, such as 

a lagoon or other almost landlocked setting.

SHEAFLIKE IMPEESSIONS

Peculiar and distinctive markings characterize several chert-free beds in the unit of alternating dolomite 

and cherty dolomite (unit 4, reference section). On 

bedding planes these markings look like clusters of 

reeds or blades, which lie parallel to the bedding and 

radiate out from a center. They do not spread through 

360° of arc, however, but subtend two opposite 20°-90° 

arcs, like a 'sheaf of straw gathered tightly at the middle. The long dimension of the markings mostly is 

from 2 to 8 inches but in places is as much as 2 feet. 

In a given bed and area the individual sheaves are 

fairly uniform in size, but in >a different bed in the 

same area the markings may be uniformly of a different size. Everywhere they are so abundant that the

248-121 O - 67 - 3
30 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

sheaves overlap each other. In cross sections the features commonly are defined by discontinuous and 

slightly irregular films of silt arranged subparallel to 

the bedding. In a few outcrops, bedding edges show 

an anastomosing pattern of closely spaced silt films, 

parts of which diverge as much as 30° from the general plane of bedding.

These markings occur throughout beds or sets of 

beds, ranging in thickness from 1 to 6 feet, that are 

separated by beds comparable in thickness but free 

of such features. The markings are not obvious in 

the unmetamorphosed dolomite, but they weather out 

conspicuously in beds that have been converted to 

calcitic limestones. The metamorphic limestones tend 

to part readily, or even to be shaly, and if pulled 

apart they show these imprints on every plane of easy 

separation. The sheaflike markings commonly are 

confined to the lower 10-15 feet of the unit but have 

been seen throughout. Wherever this stratigraphic 

interval has been examined carefully north of Globe, 

the markings have been found. Farther south no 

search has been made for them.

These markings have not been explained. At first 

they were noted only on weathered outcrops of silicated limestone beds, and it was thought that they 

might be impressions of some silicate mineral. Recognition of the impressions on fresh parting planes, as 

well as in unaltered dolomite, made this idea untenable. The apparent occurrence of the sheaflike markings in certain beds throughout an area of at least 

3,500 square miles suggests an environmental control. 

Some consideration has been given to the possibility 

that they represent imprints of ice crystals, which 

might have formed on exposed mudflats, or imprints 

of evaporite crystals now leached away, but no crystal 

forms like these markings have come to my attention. 

Ripple marks in adjacent and intervening beds indicate an environment in which agitation could cause 

mud-chip breccia (edgewise conglomerate), but the 

thinness of the layers and invariable sheaflike aggregation is unlike most such conglomerate. The uniformity in size of the sheaves in a given bed and their 

different but uniform size in another bed might be 

explainable if they are impressions of organic remains, 

probably plants. Such organisms might have been 

sensitive to environmental factors, such as water salinity or temperature, so that at a given place or time 

they developed only to certain sizes. The stromatolites of the algal member, undoubtedly of organic 

origin, exhibit analogous size variations that must reflect environment. Paleontologists apparently have 

not described organisms that would leave the sheaflike imprints.

MINOR SLUMP FEATURES

The regularly bedded dolomite (units 3 and 4, reference section), between the basal founder breccia and 

the cliff-forming unit, locally exhibits small slump 

features or slight brecciation. Bedding laminae may 

be slightly distorted or actually disrupted, and bedding details are blurred as though slumping occurred 

before the carbonate mud was completely lithified. 

The commonest manifestation of such disruption is 

seen in the dark chert layers, which appear to be 

pulled apart in the form of angular blocks; the narrow 

spaces between the blocks are filled with dolomite. In 

cross section the band <of chert appears as a train of 

small blocks that are only slightly reoriented. Commonly these small-scale slump and breccia features 

are confined to one bed and are not found in the beds 

immediately above and below, though these adjacent 

beds may include similar features in an outcrop not 

far away. Where slump structures are widely spaced, 

they appear to be phenomena formed independently 

of similar structures in adjacent beds.

INTRAFORMATIONAL CONGLOMERATES

In a few areas the top of each disrupted bed is 

marked locally by a lense of conglomerate, generally 

only a few inches thick, that consists of subangular 

granules or small pebbles of chert. Many chert fragments show no abrasion and obviously were derived 

from the underlying bed. Sparse pebbles of dolomite 

are mostly well rounded and disk shaped. In a very 

few places slabs of cherty dolomite as much as 1 foot 

thick and 5 feet across make up a part of the intraformational debris. In such places the conglomerate 

may be as much as 6 feet thick, and many of the chert 

fragments are fairly well rounded. Delicate details 

of bedding are preserved in dolomite pebbles and in 

the reworked dolomite sand that makes up the matrix 

of the conglomerate. Thus, again, there is evidence 

that the beds initially, or almost initially, were composed of dolomite and that the dark cherts were 

formed early.

Lenses of conglomerate have been noted only in a 

few areas, all in northern Gila County; where they 

are especially numerous, however, local slump stru^- 

tures are also abundant through a stratigraphic interval of 30-50 feet, and therefore these features may 

be genetically related. Individual beds, immediately 

after or perhaps during lithification, apparently 

slumped to produce irregularities in the sea floor. 

Periodically, while certain areas were above wave or 

current base, parts of the last deposited bed or beds 

were truncated, and the eroded materials were redeposited in adjacent low areas as lenses of chert gravels
MESICAL LIMESTONE. LOWE1R MEMBEiR 31

with a clastic dolomite matrix. Undisturbed layers of 

conglomerate separate deformed layers 'of carbonate; 

again the indication is that slumping of the dolomites 

occurred bed by bed as the strata accumulated.

FOUNDER BEECCIA

The coarse breccia that makes up the basal part 

of the carbonate section is a stratigraphic entity 

throughout an area of at least 4,000 square miles. It 

represents the disruption of strata on a much larger 

scale than is apparent in the above described features. 

It is not a tectonic breccia, because there are no traces 

of related tectonic structures higher or lower in the 

Apache sequence.

The configuration of the breccia mass and the distribution of the coarse fragments within it indicate that 

the breccia is not of sedimentary origin. The base of 

the unit is planar; the top is irregular. Along the 

upper margins slabs of cherty dolomite, surrounded by 

typical matrix material, have moved only a few 

inches from their original position. Many slabs in the 

upper 15 feet of the unit have the orientations and 

positions relative to each other that might be expected 

if they had spalled from an overhanging cliff but had 

been arrested after falling only a few feet. Still 

lower in the breccia the slabs cannot be matched directly with the sites from which they spalled. Where 

the unit is thick, however, many of these blocks can 

be correlated by pecularities in individual chert 

layers or by a sequence of chert layers or bedding features with flat-lying beds that overlie a thin section 

of breccia in adjacent areas. The coarse breccia might 

reasonably be interpreted as sedimentary only if all 

these features were inverted in their order of stratigraphic succession.

All features of the breccia suggest the collapse of 

consolidated and almost consolidated cherty dolomite 

beds into an underlying mud a mud so incompetent 

that it flowed readily to fill all voids. Such a soft 

base, mostly dolomite, that would persist while higher 

dolomite strata were lithified is difficult to conceive. 

The occurrence of halite suggests, however, an alternative process that could have caused foundering. The 

founder breccia, in typical 15-40-foot thicknesses, is 

largely confined to the interval in which halite was 

especially abundant. In this interval, also, slump features are conspicuous in the larger fragmental blocks 

of cherty dolomite; in adjacent areas where the breccia 

is thin the equivalent cherty dolomite is slightly folded. Intercalated with these dolomite beds are silty 

and clayey dolomitic layers that pinch and swell and 

are somewhat contorted giving the appearance of

having flowed. Thus these silty but otherwise aphanitic layers suggest dolomitic residuum left after the 

removal of gypsum from carbonate sections; no other 

indications of gypsum or anhydrite have been seen in 

the Mescal. It is here postulated that the basal few 

tens of feet of the section once included a much greater 

amount and perhaps variety of evaporite salts than 

is now apparent and that these salts were leached away, 

the residue being a carbonate mush into which higher 

strata collapsed. Locally, at least 100 feet of section 

foundered.

The founder breccia formed while the lower member was being deposited. It is not related in origin 

to the karst breccias that formed after the entire 

dolomite section was lithified, uplifted, and exposed 

to a still later leaching process. In a few places, 

where the founder breccia is about 100 feet thick, the 

broken strata are truncated and overlain locally by 

intraformational conglomerates. Minor slump features, noted in beds that overlie the founder breccia, 

occur in beds that apparently were in an environment 

of alternately high and low salinity, which would allow 

deposition and then leaching of evaporite salts. The 

minor slump structures and breccias probably were 

formed as already noted independently of those 

formed in earlier beds and distinctly later than the 

main founder breccia. Where the founder breccia is 

thick and minor slump features are numerous in higher beds, however, it cannot be demonstrated with confidence that the structures of the higher beds were 

formed wholly independently of the founder breccia. 

In any case, the relations of local unconformities to 

deformed beds indicate that all slumping and brecciation occurred before deposition of the cherty dolomite 

that makes up the cliff-forming unit (unit Y, reference 

section).

The mechanism of the leaching is an enigma. The 

distribution of halite-bearing chert layers and the decrease in halite molds in strata above the founder 

breccia suggest that lagoonal waters were alternately 

very saline and then only moderately saline; but overall, as higher carbonate strata were deposited, the 

salinity gradually decreased. Ultimately, late sea 

waters of lower salinity could have caused the solution of salts in the basal strata. But how did such 

waters gain access to these salts ? Perhaps the hydration of anhydrite to gypsum caused initial deformation 

that allowed less saline waters to percolate downward. 

Once started, leaching of the salts could cause brecciation that would continue until all the salts were 

removed.
32 YOUNGER PEECAMBEIAN GEOLOGY IN SOUTHERN ARIZONA

ALGAL MEMBER

The algal member, generally 50-100 feet thick, consists of two units: a lower thick-bedded cliff-forming 

unit characterized by stromatolites and an upper thinbedded slope-forming unit. The algal structures grade 

out in the basal 10 feet of the upper unit. The stromatolites are not in mounded or reeflike masses (bioherms) but instead are in a regularly bedded blanketlike deposit (biostrome) (Cumings, 1932; Link, 1950). 

In most areas north of Globe the biostrome is 50-60 

feet thick and makes up the lower one-half to twothirds of the member. Farther south similar thicknesses exist, but in most places the biostrome is only 

30-35 feet thick. Throughout the region, because of 

erosion that preceded deposition of the Troy, the 

upper unit is locally only a few feet thick or is missing. The contact with overlying rock units is everywhere sharp.

In recent years the biostrome of the Mescal has 

been widely recognized as a distinctive unit by those 

who prospect for and mine asbestos, and it is popularly 

termed the algal limestone. Generally, the upper unit 

has not been recognized because (1) in isolated outcrop it is quite similar to the upper parts of the lower 

member, (2) outcrops are commonly thin or not conspicuous, or (3) it is separated from the biostrome 

unit by a diabase sill. Because the biostrome dominates the middle member, the two units are here designated the algal member.

The member is composed of grayish-red to yellowishbrown dolomite. The brown dolomite weathers brownish to yellowish gray; the color of the red dolomite is 

little modified by weathering. From the Salt Eiver 

south as far as Globe, most beds within the massive 

lower unit are more than 6 feet thick. In other areas, 

bedding planes are 6 inches-6 feet apart and typically 

are about 4 feet apart. The thinnest beds are near 

the top of the biostrome. The upper unit is thin 

bedded throughout.

Light-gray to gray chert, in many places tinted red 

by minute flecks of hematite, is generally sparse 

throughout most of the member. In each unit the 

chert content increases upward. In the biostrome, 

irregular lenses of chert, a fraction of an inch to a 

few inches thick, are typical. Toward the top of the 

upper unit, closely spaced chert nodules locally coalesce to form zones 1-4 feet thick, and the top few 

inches or feet is almost completely silicified.

A tabular chert zone, 8-30 inches thick, is a distinctive marker high in the upper unit because it displays 

conspicuous forms similar to cone-in-cone structures 

(Pettijohn, 1949, p. 155-156). Throughout several 

areas, each at least 20 square miles, this unusual zone

is the effect of secondary silicification of one bed or a 

group of beds. It appears likely that one stratigraphic interval was especially susceptible to the formation 

of cone-in-cone structures throughout the region. In 

most of the area between the Salt Eiver and the abandoned mining village of Chrysotile, this zone was a 

base that resisted erosion which preceded later sedimentation in Precambrian time. Elsewhere this chert 

zone is as much as 15 feet below the top of the member.

The algaloid structures are distinguished in cross 

section by laminae that have a wavelike pattern (fig. 

95) and are marked by chert or silt that etches out 

conspicuously when weathered. The structures appear 

in plan as flattened hemispheres (fig. 9J.) or inverted 

saucerlike disks outlined by concentric rings. Exfoliated outcrops, such as the outcrop shown in figure 9J.. 

are common only where the individual colonies are 

large. Outcrops showing concentric rings are more 

typical of smaller colonies. The structures are 2-30 

inches in diameter, and the amplitude of the waves 

ranges from 1/4 inch to 5 inches. Most structures are 

5-10 inches across and 1-11/2 inches in amplitude. In 

one bed in a given outcrop, the size is fairly uniform.

Such fossil forms preferably are termed stromatolites rather than algae. According to Eezak (195Y, p. 

129), "Fossil algae preserve recognizable organic microstructures that enable the examiner to determine 

their true biologic relationships." But all that generally remains of these algae in stromatolites are large 

headlike masses or the gross outlines of the original 

colonies. Johnson (1946, p. 1089,1095), among others, 

has stated that each form-species, typified by a certain 

habit of growth and shape of colony, may have resulted from the life processes of several species or even 

several genera of algae that lived in constant association. Because binomial names applied to stromatolites have little or no biological significance, there is 

considerable difference of opinion as to the desirability 

of classifying stromatolites (Cloud, 1942, p. 364-366). 

Eezak (1957) has suggested the use of names of formgenera and form-species in a classification that is admittedly artificial but convenient in cataloging specimens with wide geographic and geologic ranges.

Eezak (oral commun., Apr. 1956) has identified the 

principal stromatolites in the Mescal Limestone as 

Collenia frequens Walcott. This form is also common 

in the younger Precambrian Belt Series of northwestern Montana (Eezak, 1957, p. 133, 136-140).

The basal bed, 4-6 feet thick, of the algal member 

contains a different stromatolite. In cross section the 

individual colonies are crudely conical. The apexes 

of the cones point downward, and the axes are inclined 

at 40°-80° to bedding surfaces. Laminae within each
MESCAL LIMESTONE ARGILLITE MEMBER 33

A B

FIGURE 9. Structures typical of the stromatolite beds of the algal member of the Mescal Limestone. Stromatolites identified 

as Collenia frequem Walcott. From cliff above Regal mine, 3% milesi east of the junction of Canyon Creek; and the Salt 

River. A, View of upper surface. B, Cross-sectional view.

cone are convex upward. The base of the cones is 

2^2-4 inches across. This stromatolite has not been 

identified.

The lack of variation in the biostrome of the Mescal 

warrants particular note. Throughout the region the 

laminated 'algaloid structures occur with lateral continuity comparable to that illustrated in figure 9, and 

they are interrupted vertically only by bedding planes 

that represent hiatuses in sedimentation. Sequences of 

stromatolite-bearing beds in the various exposures are 

remarkably similar even at lateral intervals of as 

much as 10 miles. There are few exceptions to the 50- 

60-foot thickness typical through about 3,000 square 

miles in the northern part of the region. Similar 

stromatolites of other regions, studied in some detail 

as potential aids for stratigraphic correlation (Kezak,

1957), seemingly have not been as consistent. Elsewhere single stromatolite-bearing beds are generally 

discontinuous. Individual biostromes also differ considerably in thickness, in lateral makeup of bedding 

units, and in areal extent. Apparently biostromes of 

the makeup noted in a given exposure of the Mescal 

do not generally extend more than a few miles. Thus 

the continuity and uniformity that is a prime characteristic of the Mescal occurrence, bed by bed and overall, is very unusual.

ARGlLIiITE MEMBER

DISTRIBUTION AND STRATIGRAPHIC RELATIONS

Through much of northern Gila County an upper 

member, mostly argillite, unconformably overlies the 

algal member. The member generally is 50-80 feet
34 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

and locally as much as 100 feet thick. It is separated 

from the overlying Troy Quartzite by flows of basalt. 

South of the latitude of Theodore Roosevelt Dam, this 

member apparently was eroded away before the overlying flows were laid down or before the Troy was 

deposited. (See fig. 4 for area of outcrop.) North 

of the 34th parallel, the Troy Quartzite generally rests 

on the algal member or on the lower member of the 

Mescal. In three small areas remnants of basalt flows, 

like those that overlie the Mescal, intervene between 

the algal and argillite members. South of the Salt 

River most sections are disrupted by diabase intrusions, and full sequences are difficult to piece together 

from widely separated outcrops.

The unconformity at the base of the argillite member represents more than one episode of erosion and 

probably represents an appreciable hiatus. Through 

most areas the argillite member seemingly is concordant with underlying strata; the only indication of 

unconformity is the silicified rock at the top of the 

algal member. Such concentrations of chert below contacts have been recognized as evidence of erosional 

unconformity (Leith, 1925). The chert zone continues 

beneath the basalts that locally separate the members. 

Angular truncation of the algal member occurs in a 

few places. Pebbles and angular fragments from the 

topmost silicified beds of the algal member are incorporated sparsely in the basal strata of the argillite 

member or in the bottom part of intervening basalt 

flows. Where flows occur, fossil soils cap the basalt 

and are included locally in the basal bed of the argillite member. The configuration and significance 

of the erosion surface at the top of the algal member 

is considered in more detail in the next section.

LITHOLOGY

The basal unit of the argillite member is made up 

of chert and generally is a few inches to 10 feet thick. 

Most commonly this unit is a thinly bedded breccia 

of small angular gray chert fragments in a matrix 

of pink or brown chert ; in places it is a conglomerate 

of well-rounded chert pebbles in a matrix of silty 

chert, and in other places it is made up of thin beds of 

laminated chert. In a few small areas the conglomerate facies is as much as 40 feet thick. The breccia 

and conglomerates apparently are mostly intraf ormational derived from the bedded chert, rather than 

from the underlying member. The chert beds form 

a prominent marker ledge. The unit is absent in few 

places north of the Salt River, but elsewhere is commonly absent.

The argillites that make up most of the member 

are yellowish brown, minutely laminated (but flaggy

to massive splitting), siliceous, and very hard and 

dense. Subordinate medium-gray to black argillite 

beds are somewhat less siliceous and commonly have 

shaly partings. The original sediments apparently 

were entirely of clay-size material; however, because 

the upper and lower boundaries of the member were 

particularly favorable horizons for diabase intrusions, 

virtually everywhere the rocks are somewhat recrystallized, and their original mineralogy iand texture is 

obscured. The argillite is abundantly spotted with 

aggregates, 0.2-3.0 mm in diameter, of finely divided 

mica or amphibole; smaller aggregates, even more 

abundant, are too fine grained for microscopic identification. Immediately adjacent to diabase intrusions 

certain argillite layers may be coarsely mottled with 

such aggregates.

East of Canyon Creek and south of the Salt River, 

the lowest 20-30 feet of the member tends to be shaly 

or crumbly greenish-gray to dark-gray mudstone, 

which is mostly concealed by debris from the overlying argillite. Calcareous or siliceous concretions, as 

much as 3 inches in diameter, are locally abundant in 

the mudstone.

In a few places in the Sierra Ancha, one or two 

units of thin-bedded lenticular limestone occur in the 

section. Included aggregates of silicate minerals indicate that this limestone was silty and moderately 

cherty dolomite before metamorphism. Isolated outcrops are readily mistaken for the metamorphic 

limestone that occurs below the member. A wellexposed lens, 2-6 feet thick, occurs 20 feet above the 

base of the member in the vicinity of Asbestos Point 

at the south end of the range. (See McFadden Peak 

quadrangle map for locations.) Occurrences near 

Workman Creek Falls 2 miles northwest of Aztec 

Peak and along the west wall of Cherry Creek canyon 

3 miles southeast of McFadden Peak are unusual in 

that thin limestone beds dominate the upper 15-20 feet 

of the 60-foot sections. None of the limestone occurrences appears to be more than one-half square mile 

in area.

Radioactivity of the argillite member, determined 

in and near the Sierra Ancha by an airborne scintillometer, is higher than that of any other unit in the 

Apache Group except the upper member of the Dripping Spring Quartzite, which has about the s.ame 

radioactivity (Magleby and Mead, 1955, p. 9). This 

characteristic led to considerable prospecting during 

1954-55; however, only ia few insignificant showings 

of uranium-bearing rock were discovered. One typical 

specimen of argillite, analyzed by semiquantitative 

spectrographic methods, contained more than 10 percent potassium (Neuerburg and Granger, 1960, p. 766).
MESCAL LIMESTONE PRECAMBRIAN WEATHERING AND SILICIFICATION 35

If the one sample is representative of the member, the 

unusually high potassium content may account in large 

part for the radioactivity.

PRECAMBRIAN WEATHERING AND SELICtPICATION

Weathering, which occurred in several episodes after 

the algal member was lithified and before the Troy was 

deposited, caused extensive modifications of the dolomites of the Mescal. In at least two large areas ( outlined approx. in fig. 4), and in several small areas, the 

algal and lower members of the Mescal were almost 

entirely converted to chert. In several areas of a 

square mile or less, all the algal member and the upper 

part of the lower member were similarly silicified. 

Silicification on such a scale apparently occurred 

through much less than 10 percent of the region. Individual beds of the two members were at least partially silicified, however, throughout the region. In 

certain areas, where solution was especially pervasive, 

a karst topography was formed on the Mescal, and 

in parts of this terrane the entire formation was converted to a massive collapse breccia. In certain of the 

karst areas, lateritic iron deposits accumulated.

The most extensive and complete areas of solution 

and silicification are those in which the argillite member und the basalt flows were removed by pre-Troy 

erosion. With few exceptions they are areas where 

the Chediski Member of the Troy, rather than the 

arkose member, rests directly on the algal member or 

lower member of the Mescal. Carbonate rocks displaying such relations to the Troy are not, however, 

everywhere strongly modified. The effects of solution 

and silicification became more pervasive with each 

successive episode of erosion before the Troy was deposited. Therefore, the degree of modification depends on whether or not the Mescal of >a particular 

area was subjected to several periods of weathering.

SOLUTION FEATURES

The only solution-related features that are both 

wide-spread and conspicuous are massive bedlike layers of sandstone or quartzite, which occur locally in 

the carbonate section. These sandstone bodies are 

common in some areas but absent from most. They 

occur mainly in the upper third of the lower member 

of the Mescal but are not everywhere at the same 

stratigraphic horizons. The bodies are entirely devoid 

of bedding features, and all are apparently lenticular. 

Because they occur mostly as isolated bodies in the 

limestone terranes, where even their gross aspects are 

obscured by metamorphism and faulting, their origin 

was not understood at first. Eecently the concordant 

layers and associated dikes have been observed in wellexposed outcrops of dolomite, where they have been

recognized as solution cavities filled with sandstone. 

The evidence for fillings is especially clear in the central part of the McFadden Peak quadrangle. There, 

sinkholes and solution enlargements of joints are 

abundant in the algal member, and some sandstonefilled cavities have been traced downward almost to 

the bottom of the lower member. Even more numerous are the solution openings along bedding planes; 

the sandstone layers that fill these spaces pinch and 

swell but commonly are continuous for distances of 

several hundred yards from individual sinkholes. 

Some of these sandstone layers are as much as 15 feet 

thick for distances of at least 500 feet.

Where sinkholes are numerous and tend to coalesce, 

as near the headwaters of Canyon Creek, strata between sinkholes exhibit a variety of solution effects. 

Some beds are remnant only as layers of clayey and 

silty residues. Beds that were least affected by solution folded irregularly or fragmented into breccias to 

fill voids. In some places these beds drape and thin 

over great void masses of breccia or sandstone. Sandstone bodies in these settings tend to be very irregular 

in outline and are not of great lateral extent.

Solution of the dolomite began during the forming 

of the erosion surface that separates the algal and 

argillite members of the Mescal and continued while 

the argillite was being deposited. Sparse detrital 

chert fragments in the basal beds of the argillite member were derived from underlying beds that had already been somewhat thinned and silicified. In places 

the bedding of the basal 10-40 feet of the argillite 

member is undulatory and in cross section crudely approximates the configuration of underlying dolomite 

beds that subsided locally as a consequence of partial 

leaching. These undulations and angular relations between argillite beds indicate that subsidence occurred 

while some argillaceous sediments were still plastic and before later strata were deposited. In many 

sections several bedding planes well down in the carbonate section are conspicuously marked by thin seams 

of reddish-orange argillite. (See description of units 

8 and 11, reference section.) These seams represent 

voids filled while the argillite member was being deposited. In a few sinkholes the matrix of the collapse 

breccia is similar argillite. The thinning of dolomite 

sections represented in all these features is minor compared to the thinning effected by still later leaching.

Collapse and probably additional solution continued 

during extrusion of the basalt flows that overlie the 

argillite member; such processes certainly continued 

during deposition of the two lower members of the 

Troy Quartzite. Angular blocks of argillite partly 

fill some sinkholes and in a few places are in a matrix
36 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

of basalt that flowed into the cavities. A very few 

apophyses of basalt flowed downward and partly fill 

tabular voids in the carbonate rocks. A few cavities 

are filled with sandstone of the arkose (lower) member of the Troy. The prevalent filling is sandstone 

identical with that of the Chediski Member of the 

Troy. In some examples those best exposed are 1-2 

miles southwest of Gunsight Butte in the north-central part of the McFadden Peak quadrangle the 

roofs of caverns collapsed, allowing plug-shaped 

masses of the two lower members of the Troy to drop 

and fill large sinks. Within the plugs of the Chediski 

Sandstone Member, there are almost no vestiges of 

bedding, and brecciation is not apparent. Therefore, 

the collapse must have occurred while these sediments 

were unconsolidated. No fillings equivalent to the 

quartzite (upper) member of the Troy have been recognized, and strata of this member apparently were 

not disrupted by collapse. Thus solution and silicifioation may have ceased before deposition of the 

quartzite member. Quartzitic lenses do exist, but these 

represent fillings of the earlier sandstone that were indurated adjacent to diabase intrusions.

LATE-FORMED CHERTS

Cherts that formed after the dolomites were lithified and first eroded differ in texture, form, and distribution from the dense dark chert that formed penecontemporaneously with the dolomites. These late 

cherts are mostly light in color, are flecked with 

hematite, and generally occur in thicker and more irregular masses than the older chert. In sections that 

are still mostly dolomite, the late cherts occur in greatest volume immediately adjacent to and out from the 

larger solution cavities. Elsewhere they seemingly 

occur only in dolomite that exhibits indications of 

considerable solution. In most of the region evidence 

of leaching is subtle and can be seen only on close 

inspection of the textures in the cherts themselves.

In sections that are mostly dolomite, the late-formed 

cherts are largely confined to the algal member and 

the upper third of the lower member, and certain beds 

obviously were more susceptible than others to leaching and silicification. Some beds contain only small 

amounts of secondary chert as discontinuous laminae ; 

other beds contain numerous discrete ellipsoidal nodules, and in still others chert nodules coalesce to form 

layers or lenses that make up much of the bed. The 

nodules, or aggregates of nodules, are concentrated 

mostly in the upper parts of individual beds, and the 

upper surfaces are fairly smooth, whereas the lower 

surfaces of the chert masses may project very irregularly into the dolomite. On outcrops the unreplaced

dolomite within the nodules and between closely 

spaced nodules has been partly leached, so that the 

nodules appear to be spongy and in a vuggy matrix.

A sequence of beds that includes much secondary 

chert may show striking lateral variations in thickness. In an ordinary section 20 feet thick, a sequence 

of sparsely cherty beds may be overlain and underlain by 1-foot zones of nodular chert. If traced a 

mile in one direction, the nodular zones of chert virtually pinch out, and the equivalent sequence may be 

22-24 feet thick. In the opposite direction, however, 

the dolomite thins because of solution and the chert 

content may increase considerably; thus in a distance 

of a mile, the two chert zones may coalesce, and the 

equivalent stratigraphic interval consisting almost 

entirely of chert and lacking in well-defined bedding 

may be only 6-10 feet thick. In some places such modifications take place in a lateral distance of a few 

hundred feet. In these localities brecciation is obvious, 

and voids in and between secondary chert zones are 

commonly filled with sandstone.

Without doubt a considerable part of the secondary 

chert represents remnants of the first-formed chert 

mechanically concentrated by the removal of dolomite. 

Fragments of older chert cemented in the masses of 

secondary chert can be recognized in many places, although recognition can be difficult because much of the 

older chert was bleached during weathering. The 

spatial affinities with solution features suggests also 

that the silica which makes up the cementing chert 

was locally derived and locally reprecipitated. Completely silicified beds are made up of fragments of 

several generations of secondary chert. Apparently 

on initial solution only certain thin layers in the bed 

were silicified; with recurrent solution these collapsed, 

and additional layers were silicified; this process was 

repeated until the whole bed became chert. In a notable occurrence of such beds along the southeast flank 

of Shell Mountain, 8 miles east of Young, the sequence 

of thick and thin beds in the upper half of the lower 

member and in the basal 30 feet of the algal member is so perfectly mimicked that detailed etratigraphic 

correlations can readily be made. The sequence and 

most of the individual beds, however, are telescoped to 

only about one-third the thicknesses of equivalent intervals nearby where little silicification took place.

As a corollary to this interpretation, the spatial relations of different chert masses to other fillings in 

the solution openings indicate several stages of silicification. For example, argillite fillings along bedding 

planes may include fragments of chert, and, if the 

lithified argillite was then fragmented, it is cemented 

by similar chert. Also, dikelike fillings of Chediski
MESCAL LIMESTONE PRECAMBRIAN WEATHERING AND SILICIFICATION 37

Sandstone Member cut across layers of secondary 

chert, but grains of this sandstone are included in 

other chert nodules or in zones that border the sandstone bodies. Spatial relations indicate that more than 

one episode of sandstone filling occurred.

KARST AND EELATED BRECCIAS

The fragmented cherts described above are but one 

aspect of the chert breccia formed concomitantly with 

the forming of karst surfaces on the Mescal. Most 

fragments in these breccias are less than an inch 

across; some relict beds include domino-shaped blocks 

a few inches across. The fragments and the cementing matrix may not be greatly different in texture or 

color. At the other extreme are the heterogeneous breccias that fill caverns whose roofs collapsed about the 

time the first Troy sediments were deposited. These 

breccias may include angular blocks of silicified Mescal that are several feet on a side, partly disintegrated 

boulders of basalt, and cobbles of older Precambrian 

quartzite from the basal conglomerate of the Troy; 

the matrix may be coarse sandstone or ferruginous 

siltstone.

In a few areas, which have mature karst features, 

outcrops are lithologically so different from the wellbedded carbonate rocks that they are hardly recognizable as parts of the Mescal. The algal member in 

particular was susceptible to thorough brecciation during the forming of solution cavities. The remnant 

unit is massive-outcropping coarse silica-cemented 

chert rubble, in which large blocks of dolomite can occasionally be found. The best examples of gradational 

relations between karst breccias and little modified 

carbonate terrane have been seen in the central and 

northwest parts of the McFadden Peak quadrangle. 

There a zone of rubble, containing recognizable but 

scrambled relicts from the algal member, may be as 

little as 20 feet thick; but half a mile away an equivalent dolomite section may be as much as 100 feet 

thick. Where the algal member is represented by a 

mixed rubble, the upper 20-30 feet of the lower member is similarly brecciated, and the breccias merge to 

obscure the usual sharp contact between members. 

Bedlike fillings of sandstone are more abundant in the 

dolomite section directly below such rubble zones than 

in other settings.

Completely silicified sections in which the entity of 

individual beds is preserved also grade laterally into 

extensive zones of heterogeneously mixed breccia. The 

mixed breccia is similar to the fillings seen in individual sinkholes and was probably formed by the coalescence of sinks, which initially were widely spaced 

through the dolomite terrane. The vestigial strata

were preserved in the areas between belts of active 

sink formation, where the beds were gradually thinned 

and recemented by chert in such a way that large-scale 

collapse, characteristic of the sinkholes, did not occur. 

Thus, locally within karst areas the stratigraphic sequence of whole units is still preserved such as at 

Shell Mountain.

Where dolomite units were appreciably thinned by 

the mechanical erosion that preceded Troy sedimentation, layered karst breccias were formed intermediate 

in character relative to the breccias just described. 

In these breccias original gross bedding features can 

be followed in trains as seen in cross section of 

slightly reoriented blocks of chert, individually isolated in a matrix of hematitic siltstone or sandstone. 

Several parallel layers of blocks may make up the 

thicker layer that was originally a dolomite bed. In 

some relict beds the chert blocks are separated horizontally and vertically by little matrix; in other beds 

individual blocks and layers are separated by 2-4 

inches of matrix material. Commonly, alternate relict 

beds are characterized by blocks iof different size, separation- or orientation. These distinctive breccias make 

up the upper 10-70 feet of the Mescal section that 

remained after erosion.

Delicate bedding structures, which are aids in the 

recognition of dolomitic equivalents, are preserved 

only in the individual chert blocks, where they are 

outlined by films of silt or pseudomorphed in porous 

laminae outlined by minute aggregates of specular 

hematite. In blocks relict from the stromatolite-bearing unit, the curvature of the stromatolites as seen in 

cross section is commonly greatly accentuated.

Chert blocks low in these breccias generally have 

reddish-brown rims; many blocks high in the breccias 

are stained reddish brown or grayish red throughout 

and could be termed jasper rather than chert. Near 

the bottom of these breccias, the chert blocks have 

sharp corners, are domino shaped, and may be more 

than a foot long. Upward in the sections their dimensions decrease, the edges are rounded, and generally 

the blocks are more widely separated. However, in 

many places, layers that represent the original beds 

remain little disturbed right up to the contact with the 

Troy. Therefore the rounding of the blocks and 

their wider separation must be solution phenomena and 

not the result of mechanical erosion. Apparently the 

higher cherts were more thoroughly impregnated 

with iron .oxides during the solution process.

The largest area of outcrop in which the dolomite 

strata were thoroughly silicified and brecciated is in 

the Canyon Creek drainage basin north of the 34th 

parallel. This karst belt extends northwest as far as
38 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

the head of Haigler Creek. Farther along Haigler 

Creek to the west, considerable thicknesses of the Mescal are locally silicified. Some parts of the main karst 

belt do include extensive remnants of dolomite, but 

most of these are interrupted by obvious solution 

features, such as local masses of sinkhole breccia. Dolomite is abundantly remnant only in the basal part 

of the section, where the older founder breccia lacked 

the bedding planes that were favored channelways 

for solution.

Another large karst area, only partly exposed in a 

belt of outcrops 1 to 3 miles wide, extends from U.S. 

Highway 60 southeast about 13 miles along the southern margin of the Natanes Plateau. In this area the 

Mescal was considerably eroded in pre-Troy time, and 

locally the Troy rests on the Dripping Spring Quartzite. Where the Mescal is very thin, even the founder 

breccia was converted to ferruginous karst breccia and 

was much silicified.

Just within the northeast corner of the Globe quadrangle and immediately to the east, in an area possibly 

not exceeding 6 square miles, a somewhat different 

aspect of the silicified facies exists. The Mescal consists almost entirely of chert, which occurs as thin 

layers and lenses separated by thin seams of silty 

material. The effects of repeated small-scale brecciation and cementation are obvious in many layers, but 

in wholly as many layers such effects are not apparent. 

Large-scale jumbling of breccia blocks is seen only 

very locally. Apparently the early-formed chert settled with a minimum of brecciation as the dolomite 

was removed; secondary chert cement exists in smaller 

amounts here than in most of the karst occurrences, 

and little extraneous sand, silt, or argillaceous material was introduced. This belt contains little hematite, 

whereas large parts of the two karst belts noted above 

contain abundant hematite. Chert sections representing the entire lower member and part of the algal 

member are 60-120 feet thick; 1-2 miles to the east, 

where solution phenomena are few, equivalent dolomite sections are 300 to almost 400 feet thick. In 

several other small areas where brecciation is inconspicuous, similar chert sections grade laterally into 

highly ferruginous obviously brecciated sequences.

HEMATITE CONCENTRATIONS

As a consequence of Precambrian weathering, hematite may be sparse or abundant in the upper part of 

the Mescal. Some concentrations are extensive and 

thick enough to be potential sources of iron. These 

have been prospected sporadically in the past (Burchard, 1931; Stewart, 1947), and since 1958 have been 

actively explored.

The larger iron deposits are in the upper parts of 

karst breccias in which layers of chert nodules mimic 

the bedding of original dolomites. Earthy but dense 

hematite and unctuous specular hematite, in massive 

layers, constitute most of the ore-grade material. Earthy hematite is dominant in the lower grade ores. 

Small botryoidal masses of hematite do not make up 

a great volume but are widely distributed. Micaceous 

specular hematite commonly lines small vugs and 

forms veins in the hematite and in hematitic rock 

that is interlensed with the ore. Hematite, chert, and 

detrital quartz grains generally make up 90-98 percent 

of material that assays more than 25 percent iron.

The best and largest known deposit (in 1961) is 

partly exposed along the walls of Canyon Creek just 

above its junction with Swamp Creek, a minor tributary 6 miles south of the Mogollon Kim. There, layers 

of hematite are intercalated with, and gradational 

into, highly hematitic layers of argillite, siltstone, silicified sandstone, and light-colored to dusky-red chert 

breccia in a zone that dips gently south and is 20-YO 

feet thick. The zone crops out almost continuously for 

a distance of 10,000 feet along the east wall of the canyon, is discontinuously exposed on the west wall, and 

is known from drilling to continue southward below 

stream level for at least 1,500 feet (A. P. Butler, Jr., 

written commun., 1959). Before erosion the zone undoubtedly was continuous beneath an area of at least 

1 square mile. An interval 5-25 feet thick in the middle of the zone has the highest content of iron. Discontinuous crudely lenticular bodies 2-20 feet thick 

that contain more than 50 percent iron make up 

roughly two-thirds of this interval. An exceptional 

lens may foe as much as 40 feet thick. Above the interval much of the zone is made up of thin layers of 

highly hematitic sandstone, and layered chert breccia 

dominates the part of the zone below the high-grade 

interval.

Lenses of hematite, too thin and small to constitute 

potential ore, occur locally in silicified strata 20-YO 

feet below the main iron-bearing zone. The basal few 

feet of the overlying Chediski Sandstone Member and 

thin lenses higher in the member also include local 

concentrations of hematite, some of which approach 

ore grade.

Several other iron deposits, petrographically similar 

to the one at Swamp Creek, occur in the belt of silicified Mescal that crops out along the upper reaches of 

Canyon Creek. Some are possibly more extensive than 

the deposit at Swamp Creek, but none has been found 

to contain as thick or continuous high-grade lenses.

Thinner zones of ferruginous rock occur at the top 

of the Mescal in some areas south of the 34th parallel,
MESCAL LIMESTONE METAMORPHIC LIMESTONES 39

but widespread lenses of almost pure hematite are 

sparse. Instead, thin layers of chert or of slightly reoriented chert fragments occur in a hematitic matrix, 

which in many places consists of coarsely micaceous 

hematite. The karst belt along the south rim of the 

Natanes Plateau includes local concentrations of such 

material; some are several feet thick over several acres. 

This is the southernmost known locality of such concentrations. Commonly, thin lenses of hematite, no 

more than 6 inches thick, occur wherever an appreciable thickness of the Mescal is silicified, and in a few 

places, thin lenses even occur very sparsely in dolomite 

beds immediately below the silicified zone. Everywhere, regardless of the degree of silicification lower 

in the section, hematite flecks the chert that immediately underlies the unconformity at the top of the 

algal member.

In past speculations on origin, it has been erroneously concluded that the iron was deposited along the 

Troy-Mescal unconformity from hydrothermal solutions, which had a source in the diabase intrusions. 

However, well-rounded waterworn pebbles of cherty 

hematite are common in the basal beds of the Troy 

Quartzite and occur sporadically in conglomerate lenses tens of feet higher in the formation. These distinctive pebbles undoubtedly were derived from the 

Mescal, as was silt- and sand-sized hematitic material 

that locally forms lenses almost of ore grade in the 

basal Troy. Even without the evidence afforded by 

the pebbles, spatial relations between diabase bodies 

and the iron-bearing zones clearly indicate that the 

intrusions postdate the hematite deposits.

The hematite deposits are here regarded as fossil 

laterites, firmly cemented by silica released during 

laterization. The source rocks for the residual iron 

were either the dolomites an unlikely possibility or 

the basalt flows, which once everywhere overlay the 

Mescal.

The red ferruginous dolomite, which dominates the 

Mescal in certain areas, either contains considerable 

secondary chert or is vuggy or spongelike and 

obviously has been leached. Thick sections of red 

dolomite occur only near conspicuous solution features, 

such as sinkholes. Brown dolomite, which contains 

relatively little iron, is much more abundant and is 

characteristic of sections far removed from sinkholes. 

The spatial relations of the two types of dolomite suggest that the red dolomite is hematitic as a consequence 

of laterization and is not the source of great amounts 

of iron.

Basalt flows probably furnished most of the iron. 

Even the least altered basalt is dusted throughout with 

hematite, and specular hematite is abundant in vesicules and voids. Fossil soils on the basalt are almost 

indistinguishable from the hematitic argillite and siltstone that make up the matrix of many chert breccias 

or the basal parts of some iron-ore zones.

The iron-rich zones are here interpreted as layered 

karst breccia with a matrix of lateritic residues from 

basalt. These residues filtered down into the innumerable and in places large spaces left by the solution of 

dolomite. Some reworking and transport of these 

hematitic sediments occurred later, during the advance 

of the Troy sea, as shown by the makeup of narrow 

conglomeratic sandstone dikes in the tops of some 

iron zones. These dikes contain gravels mostly of 

foreign derivation in a matrix of locally derived hematite mud; a few also include well-rounded hematite 

pebbles. Some of the thicker layers of hematite and 

associated siltstone and sandstone in the upper parts 

of the hematite zones are discrete bodies with layering 

suggestive of sedimentation processes. However, some 

of these are between layered breccia units in which 

the chert fragments show little reorientation; these 

bodies must represent filled voids between beds, similar 

to the sandstone lenses seen in sections where relations 

are not obscured by hematite. Most tabular hematite 

bodies are probably fill features. In any event, silica 

was available as a cement during the initial accumulation of the iron, and the principal iron layers were 

lithified before encroachment of the sea in which the 

Chediski Sandstone Member of the Troy was laid 

down.

METAMORPHIC LIMESTONES

Adjacent to diabase intrusions, the dolomite of the 

lower and algal members was converted to calcitic 

limestone, and most of the chert was altered to silicate 

minerals. These effects of thermal metamorphism 

might be expected to be local, and in a few places 

where only one thin intrusion was emplaced, the effects 

are not widespread. Typically, however, several extensive diabase sills intrude the two members; consequently the metamorphic effects are regional in extent, 

and more than half of all Mescal exposures are silicated limestones. Indeed these limestones are so prevalent, and the included silicates are commonly so 

inconspicuous, that a sedimentary origin has been 

tacitly assumed. In the following paragraphs some 

of the physical rather than mineralogical effects are 

stressed, so that metamorphic and premetamorphic 

features of stratigraphic equivalents can be compared.

Mineralogic changes in the carbonate sections were 

brought about largely through a recombination of the 

sedimentary constituents. Silica and small amounts 

of alumina combined with the magnesia and lime of
40 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

the dolomite to form silicates. The calcite remaining 

after this dedolomitization makes up the bulk of the 

reconstituted strata. With few exceptions these calcitic limestones are very fine grained and sugary. The 

silicates, with calcite, occur as mixed aggregates 

crudely pseudomorphic after masses of chert. They 

are mostly light colored, weather white, and do not 

etch out conspicuously, as does chert; therefore their 

presence in the metamorphic limestones may not be 

evident to the casual observer. Where zones of secondary chert were thick, the zones of silicate minerals 

include relicts of unreplaced chert; otherwise chert is 

largely absent from the metamorphic limestones. 

Highly siliceous equivalents of the dolomite were 

sparsely intruded 'by diabase, so that thermal metamorphic effects in these rocks are uncommon. The 

metamorphic minerals are described further under 

"Metamorphism associated with diabase."

The physical changes caused by metamorphism are 

striking. Dolomite that was silty or that contained 

chert in layers less than an inch thick was metamorphosed to shaly limestone. Unless viewed microscopically, silicate minerals seem to be lacking in these 

rocks. The partings simulate shaly bedding but are 

actually cleavages formed during metamorphism. At 

the other extreme, some of the thicker silt-free dolomite 

beds that were separated by massive irregular layers of 

secondary chert were literally welded together by silicate zones and formed layers more massive than their 

dolomite counterparts. In many areas, however, some 

silty material had been concentrated along solution 

planes between beds, and as the result of metamorphism the bordering carbonate rock became shaly; 

therefore such units appear thinner bedded than the 

original dolomites. Consequently, limestone and dolomite sections weather differently units that as dolomites are ledge formers may as limestones form gentle 

slopes. The limestones are mostly gray to white, and 

from a distance shaly units are chalky in appearance.

Although mineralogic changes indicate the same degree of metamorphism throughout the Mescal, the various stratigraphic units differ in the physical modifications, caused by metamorphism. The basal sandstone 

of the Mescal appears little changed the matrix contains calcite rather than dolomite, and limonite stains 

are more common. The argilliceous and sandy matrix 

of the overlying founder breccia is altered to a greenish-gray massive rock. The dolomite of the breccia 

blocks was largely converted to calcite, but chert layers 

within blocks more than a foot thick are surprisingly 

little changed. The borders of the blocks grade into 

the silicated matrix. The remainder of the lower twothirds of the lower member was converted to limestone

with partings at intervals ranging from paper thin to 

30 inches. The thicker pseudobeds are separated by 

a fraction of an inch to several inches of shaly limestone and are themselves very friable where moderately 

weathered.

These limestone layers ledge and cliff makers as 

dolomite form moderate to gentle slopes, are subject 

to slope creep, and commonly are covered by resistant debris from higher in the section. Thus in the 

metamorphic rocks it is difficult to distinguish between 

slumping that occurred when the founder breccia was 

formed and slumping that represents recent creep. 

Where the founder breccia was especially thin, as 

along the Salt Eiver east of Canyon Creek, a few 

thick ledges of limestone, comparable with those in 

the upper third of the lower member, do crop out 40- 

50 feet above the base of the Mesoal. Otherwise, resistant limestone beds are generally absent in the lower 

two-thirds of the lower member.

The upper third of the lower member (units 8 and 9, 

reference section) was modified to thin- to thick-parting units of white to medium-gray limestone separated by thin units of shaly limestone. Details of 

original bedding are preserved more faithfully here 

than in lower strata. In the thicker layers, metamorphic welding of beds is common. The resistant 

thick layers tend to form rounded ledges, accentuated 

by the etching out of the shaly seams. In general, 

metamorphic sequences of this interval crop out on 

steeper slopes than their dolomite equivalents.

Gray metamorphic limestone of the lower unit of 

the algal member retains original details of bedding 

and stromatolite structures, and the unit remains a 

massive cliff former. The upper unit of the algal 

member was converted to limestone much like that of 

the upper 50 feet of the lower member. Concentrations of secondary chert at the top of the algal member 

are represented by partially serpentinized remnants; 

otherwise relicts of chert are sparse.

It has not previously ibeen recognized that most 

sections of the Mescal are entirely one lithologic type 

or the other that is, entirely thin- to thick-bedded 

brown or red cherty dolomite or entirely silicate-bearing white-weathering shaly metamorphic limestone. 

Along the fringes of a metamorphic belt, pinkish-gray 

or brown dolomitic limestones partially dedolomitized do make up transitional zones, and cherty dolomites or cherty limestones are locally intercalated with 

silicate-bearing limestone, but such transition zones 

are narrow and readily overlooked. Descriptions that 

characterize the formation as consisting of dolomitic 

limestone interbedded with nonmagnesian limestone 

and descriptions that fail to correctly indicate that
MESCAL LIMESTONE VARIATIONS IN THICKNESS 41

chert is almost ubiquitous probably were based on 

observations of metamorphosed sections in which the 

white silicates were not recognized. Certainly the 

stressing of thin-splitting characteristics, as though 

they were original, reflects such observations.

VARIATIONS IN THICKNESS

Six events or processes caused thinning of the Mescal before Paleozoic sedimentation was complete. 

These were (1) intraforrnational erosion before the 

argillite member was deposited, (2) pre-Troy erosion, 

(3) solution, (4) metamorphism, (5) pre-Bolsa erosion, and (6) pre-Martin erosion.

The thickness of the argillite member depends 

mainly on the amount of slumping due to solution of 

underlying strata and the amount of erosion prior to 

deposition of the Troy Quartzite. Over the large 

areas in which basalt flows cap the argillite member, 

thicknesses are fairly uniform except where basins 

were formed during deposition by slumping of the 

dolomite. In these basins, a square mile or less in area, 

argillaceous strata are abnormally thick. Beneath the 

basalt cover in the McFadden Peak quadrangle, the 

argillite member is typically 45-60 feet thick; locally, 

however, it is 100 feet thick. Toward the boundaries 

of the area of outcrop outlined in figure 4, the argillite 

member thins rapidly, and locally it is missing within 

that area, owing to pre-Troy erosion.

Thickness variations of the algal member mainly 

reflect four episodes of Precambrian erosion. The 

two erosional events that preceded deposition of the 

argillite member caused as much variation as later 

episodes. Along the north edge of the McFadden 

Peak quadrangle the upper flat-bedded unit is mostly 

missing, and the member is 50-60 feet thick; 3 miles 

to the south, thicknesses of as much as 105 feet have 

been measured. Along the canyon of the Salt River 

east of Canyon Creek few sections are less than 60 feet 

and most are 85-100 feet thick. One section of 130 

feet, the thickest known, was measured near the Regal 

mine on the south rim of the canyon 5^ miles west of 

U.S. Highway 60. From the village of Chrysotile as 

far south as Globe, the upper strata of the member 

were generally truncated by erosion surfaces at the 

base of the basalt flows or at the base of the Troy; the 

upper unit is thin or missing, and the thickest remnants of the member are 50-70 feet thick.

In a few small areas north of Globe, such as along 

the south rim of the Natanes Plateau south of Sawmill, the Mescal Limestone is entirely eroded away, 

and the Troy rests directly on the Dripping Spring 

Quartzite. Further south, as at the southeast end of 

the Mescal Mountains and in the Holy Joe Peak quadrangle, the Mescal is missing over large areas owing to 

pre-Troy erosion.

Thinning on a large scale by solution is a local 

phenomenon, restricted as far as is known to the karst 

areas north of the latitude of Globe. In several of 

these areas, solution caused the combined thickness of 

the lower and algal members to be less than 100 feet. 

Throughout the region of outcrop, however, subtle 

thinning was caused by small amounts of solution 

along many bedding planes. This thinning, probably 

on the order of 10-50 feet, was largely obscured by 

later metamorphic effects.

Where metamorphosed and unmetamorphosed sections can be compared directly in immediately adjacent areas, the thickness of the lower and algal 

members seems to decrease toward the area of metamorphism. The cherty dolomite in the lower member 

of the reference section, for example, is 269 feet thick; 

but seemingly equivalent metamorphosed sections 

within a 1-mile radius northwest, north, and northeast 

of this section are about 220 feet thick. Also, in comparing widely separate sections apparently similar 

before metamorphism, no limestone section has been 

seen that is as thick as the equivalent dolomite section. 

Limestone or dolomite sections of the lower member in 

the Sierra Ancha and east to Canyon Creek range in 

thickness from 200 to 270 feet. Southeast, in the 

canyon of the Salt River and south as far as Chrysotile, the member ranges in thickness from 150 feet to 

slightly more than 200 feet. Only the thinnest of the 

western sections are reconstituted throughout to the 

same degree as the sections of the Salt River-Chrysotile area. Farther south comparable sections exceed 

180 feet, and those that show the least metamorphism 

are 220-250 feet thick.

Because other processes contributed widely to thinning of the algal member, similar comparisons of 

metamorphic effects are not practicable for that member.

Although exact equivalence is difficult to determine, 

because of earlier solution thinning, seemingly comparable samples of the lower member suggest that 

metamorphosed sections are 15-20 percent thinner than 

cherty dolomite sections. A similar loss of volume on 

dedolomitization has recently been noted in a comparable metamorphic setting (Cooper, 1957, p. 582-588), 

and the probability of such shrinkage certainly should 

be considered for all the limestone of the Mescal.

The sequence of beds is the same wherever the Mescal is exposed, and partial sections similarly modified 

 whether by solution or metamorphism are about 

equal in thickness; thus dolomite sections throughout 

the region were probably once uniform in thickness.
42 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

This thickness, if a few sections are representative, 

was about 400 feet for the lower and algal members 

combined. Because the original extent of the argillite 

member is not known, it cannot be included in a total 

that is applicable regionally.

Apache and Troy strata were differentially displaced 

by the intrusion of diabase sills, and later, during two 

episodes of erosion, the most elevated parts were 

stripped away (fig. 12 ). In places from Globe south 

to the Gila River, part of the Mescal remains under 

a cover of Bolsa Quartzite (Cambrian), but farther 

south, where most of the Mescal previously had been 

stripped away by pre-Troy erosion, the last remnants 

of large areas were removed by pre-Bolsa erosion. 

North of-the latitude of Gldbe, pre-Bolsa erosion had 

little effect on the Mescal; but as far north as the 

Natanes Plateau, the Mescal was thinned or completely 

removed by an erosion cycle that predated the deposition of the Martin Limestone (Devonian). Farther 

north pre-Martin erosion breached the Troy cover 

locally, but only in areas of a few square miles.

To sum up, the Mescal is generally thickest (350-420 

ft) in the northern part of the region, and southward 

progressively larger amounts of the Mescal are missing; even so the most complete local remnants in the 

south are only moderately thinner (250-300 ft) than 

those to the north. In the northern area, the Mescal 

was thinned locally to less than 100 feet during the 

development of karst features.

STRATIORAPmC NOMENCLATURE

The lower and algal members of the Mescal Limestone are distinctive units that logically can be 

grouped as a formation. However, in view of the 

lithologic differences and the erosional unconformity 

between the algal member and the overlying argillite 

unit, many geologists would be inclined to consider 

the argillite a separate formation. Such formational 

designation would "be well within the principles stated 

by the American Commission on Stratigraphic Nomenclature (1961, p. 648-654). Indeed, in the only previous discussion of the argillite, formational status was 

proposed by Hinds (1935, p. 32), who said "At and 

near [Theodore] Roosevelt Dam, there are present 

locally above the vesicular basalt a maximum of 30 

feet of chert and dark reddish and purplish siliceous 

shales which I include in the Apache group * * * and 

propose to call the Roosevelt member." Hinds here 

used the term "member" in a formational sense. The 

section described by him is incomplete, and because of 

deep weathering it is not typical of sections elsewhere; 

nor was it possible to determine that the principal 

basalt flows overlie the argillite.

The argillite member is preferably included as the 

uppermost member of the Mescal Limestone because 

(1) it is thin, (2) it crops out mainly in cliffs too 

narrow to depict except on large-scale maps, and (3) 

it almost certainly will be recognized and separately 

mapped only in an area of about 800 square miles in 

northern Gila County. Lithologic similarities between 

limestone of the argillite member and that of the 

underlying members indicate the recurrence of a similar environment of sedimentation and suggest a genetic, but not compelling, reason for including the 

argillite member in the Mescal. North and south of 

the area of outcrop shown in figure 4, the argillite 

member is virtually absent; west of Theodore Roosevelt Dam, where additional outcrops may yet be 

found, few remnants of the Apache Group exist; and 

east of the area the Apache Group is deeply buried 

by younger formations. In view of these facts, formational status would serve little practical purpose.

Although the Mescal originally was mostly dolomite, and although it now is locally a chert formation 

and includes an argillite member, limestone of metamorphic origin makes up most exposures. Therefore 

the name Mescal Limestone has been retained.

BASALT FLOWS 

GENERAL FEATURES AND DISTRIBUTION

Basalt flows occur at two Stratigraphic positions 

high in the Apache sequence. Throughout the region 

the principal occurrences separate the Troy Quartzite 

from the Mescal Limestone; .flows petrographically 

similar but restricted in lateral extent locally separate 

the argillite and algal members of the Mescal. The 

two basalt sequences in their juxtaposition with the 

argillite member of the Mescal are best exposed in 

cliff faces in the southern part of the Sierra Ancha.

The basalt flows immediately beneath the Troy 

Quartzite generally aggregate 40-90 feet in thickness, 

and these apparently once extended in much greater 

thickness over the entire region. Variations in present thickness are the consequence of pre-Troy erosion. 

In the Sierra Ancha and east to Canyon Creek, this 

basalt ranges in thickness from 20 to 100 feet. North 

of the 34th parallel and also along the canyon of the 

Salt River east of Canyon Creek it is largely absent. 

In the vicinity of the village of Chrysotile, 5 miles 

south of the canyon, occurrences are slightly more than 

100 feet thick. Southeast from Chrysotile along the 

Natanes Plateau, the basalt is generally absent; but 

in a small area 16-20 miles to the southeast, one flow 

at least 110 feet thick is locally remnant. Farther 

south the basalt is absent from large areas; where 

present, it directly overlies the algal member of the
BASALT FLOWS 43

Mescal. In the Apache Mountains the basalt is entirely missing; yet along the low chain of hills that 

extends 12 miles southeast from these peaks, it is commonly more than 250 feet thick, and in one place, 

10 miles east of Globe, it is 375 feet thick. Along 

the mountain front 1-3 miles south of Superior, the 

basalt locally exceeds 200 feet in thickness. Elsewhere south of the Natanes Plateau most sections are 

50-75 feet thick. South of the Gila Kiver remnants 

are sparse but do exist as far as the Vekol Mountains.

Basalt is known to separate the argillite and algal 

members of the Mescal in only three localities: (1) 

along the southern front of the Sierra Ancha in an 

area of at least 9 square miles, (2) along Cherry 

Creek 6 miles northeast of McFadden Peak, over an 

area of about 1 square mile, and (3) in the vicinity 

of Theodore Roosevelt Dam. Basalt at this horizon 

is as much as 50 feet thick in the southern Sierra 

Ancha and at Theodore Roosevelt Dam and as much 

as 110 feet thick at the Cherry Creek locality.

The flows in the vicinity of Globe, Superior, and 

Ray, in the Mescal and Dripping Spring Mountains, 

and as far southwest as the Vekol Mountains could 

correlate with the basalt that underlies the argillite 

member of the Mescal in the Sierra Ancha, but this 

correlation cannot be made with any assurance. 

Obviously the upper (or principal) basalt overlies 

the argillite member where it thins along its southern 

extremities and could also be the basalt unit that 

directly overlies the algal member farther south.

The boundaries between flows are commonly obscure. 

In most sections only one flow is seen, but unusually 

thick sections contain as many as four flows and in 

some a fifth may be present. Individual flows are 

50-125 feet thick.

The basalt formed as subaerial flows, which were 

very fluid. Individual flows poured out in uniform 

thickness over extensive areas. In many areas the flow 

breccias at the top of a flow were eroded away, and 

a planar erosion surface largely free of residual soils 

was formed before extrusion of the next flow. About 

half a mile south of Chrysotile two flows are separated 

by 10-12 feet of silty limestone, and between Cherry 

and Canyon Creeks a few inches of laminated argillite 

locally separates flows. These sedimentary layers and 

the planar surfaces suggest inundation of the region 

between outpourings of lavas. No feeder dikes for 

these lavas have been found.

PETROLOGY

The basalt is grayish red to blackish red or brown; 

a few outcrops weather pale brown to dark yellowish 

brown. Hematite is so abundantly disseminated

through the rock that blackish red is the dominant 

color of fresh and weathered exposures. Most specimens show an intersertal or intergranular texture, defined by laths of plagioclase 0.1-0.3 mm long. Needlelike crystals of apatite are abundant in some of the 

basalt. The rock has been altered to such a degree that 

identification of the original plagioclase has not been 

possible. Under the microscope the alteration minerals seen are albite, calcite, serpentine, quartz, and 

chlorite; all are dusted with hematite and so aggregated that the original texture is not as apparent as 

in a hand specimen. In some altered specimens, magnetite, epidote, and an amphibole (hornblende (?)) are 

common.

Large tabular phenocrysts of plagioclase are sparse 

to abundant in most flows. Some vertical sections of a 

given flow are porphyritic throughout; only parts of 

other sections are porphyritic. The vesicular parts of 

some flows include the greatest number of phenocrysts. 

They typically range in thickness from 0.5 to 2 mm 

and in diameter from 5 to 20 mm. Plates as much as 

40 mm across have been seen, and in a few places 

phenocrysts make up as much as 40 percent of the rock. 

In many outcrops the phenocrysts weather almost 

white and are very striking. In especially hematitic 

basalts they are moderate red to dusky red and are inconspicuous. Regardless of the size or abundance of 

the phenocrysts, the groundmass varies little in texture or grain size. Except that no porphyritic varieties have yet been observed in it, the basalt below the 

argillite member resembles that above the member.

Vesicles and amygdules are especially abundant in 

the tops and bottoms of flows; ropy flow breccia is 

conspicuous in places. Partly filled elongate vertical 

vesicles (pipe vesicles) occur locally and abundantly 

in the bottoms of flows. Uncommonly the basal breccia of the lowest flow of the upper sequence includes 

fragments derived from the argillite member of the 

Mescal. Variolitic basalt, a few inches to a few feet 

thick, makes up the basal part of the lowest flow in 

a few localities. Voids and vesicles are filled by calcite, quartz, specular hematite, and an unidentified 

grayish-green material. In a few localities blue-green 

copper minerals coat weathered joint and fracture 

surfaces.

CRITERIA, FOR DISTINGUISHING BASALT 

FROM DIABASE

Typically the basalt is readily recognized as conspicuous blackish ledges or cliffs just below the Troy 

Quartzite. In places, however, the distinctive outcrops 

are obscured by talus; and in other areas they do not 

exist, because the basalt that was altered adjacent to
44 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

diabase intrusions disintegrated on weathering. The 

basalt has been mistaken for diabase where it is not 

exposed in cliffs.

The two rock types can usually 'be differentiated by 

noting the following characteristics: (1) The plagioclase phenocrysts of the basalt have no counterparts in 

the diabase. Some coarse facies of the diabase contain 

plagioclase crystals even longer than the phenocrysts 

of the basalt; however, they are lath shaped and intergrown with equally large grains of other minerals, 

whereas the phenocrysts of the basalt are tabular and 

set in a fine-grained groundmass. (2) Amygdules of 

quartz, quartz-hematite, or hematite are characteristic 

of the basalt. (3) The basalt decomposes to a fine 

powdery dark-reddish-brown soil, which contains abundant residual amygdules even in areas where amygdules are not obvious in adjacent outcrops. The soil 

typically derived from diabase is granular, contains 

abundant fragments of plagioclase laths in a yellowish-brown clayey matrix, and tends to be adobelike. 

Surfaces of some diabase soils are powdery, but the 

typical granular detritus can be found by digging a 

few inches. (4) The finer grained diabase commonly 

breaks down into a rubble of small ellipsoidal pebbles 

which represent the pyroxene host crystals in the 

ophitic-textured rock. The small plagioclase laths in 

these pebbles tend to weather chalky white and are 

conspicuous. In contrast, basalt soils include only 

scattered pebbles and fragments of irregular form, and 

the feldspar laths, if distinguishable, almost invariably 

weather reddish orange to yellowish brown. The 

basalt pebbles where freshly broken do not show the 

flashing cleavage surfaces of pyroxene characteristic 

of the diabase pebbles.

PRE-TROY UNCONFORMITY

The angular relations of the unconformity that separates the Troy Quartzite from the Apache Group are 

apparent only on a regional scale. Apache strata were 

broadly warped, then eroded before Troy deposition, 

as shown diagrammatically in figure 11. Except for 

channels that occur in a few areas and that locally exceed 100 feet in depth, the pre-Troy surface was virtually planar. In the northwestern part of Gila County, this surface was downwarped to form a structural 

basin a few hundred feet lower than the general surface. Basal strata of the Troy lap out against the 

margins of this basin, but elsewhere in the northern 

part of the region they are practically concordant with 

underlying strata. In the southern part of the region, 

the surface truncates the underlying strata; therefore, 

at localities several miles apart, the Troy rests on different formations of the Apache Group.

TROY QUARTZITE 

DEFINITION AND SUBDIVISION

On redefinition of the Troy Quartzite as Precambrian rather than Cambrian and on delineation of three 

distinctive members, two of which are regional in extent, it becomes obvious that the Troy encompasses a 

greater thickness and variety of strata than is usually 

conceived. Nonetheless, there can be no doubt that 

Ransome (1915, p. 384-385; 1916, p. 139-141, 154) included all these strata in his earliest descriptions of the 

Troy. In the Ray quadrangle, where it was originally 

defined as separate from the Dripping Spring and was 

first mapped (Ransome, 1919, p. 44-45; 1923), the Troy 

consists mainly of strata that are here assigned to the 

Chediski Sandstone Member. The first section Ransome 

described, 9^ miles southeast of the type locality on 

Troy Mountain in the Ray quadrangle, is 362 feet 

thick. From reconnaissance outside of the quadrangle, 

Ransome realized that the Troy occurs in much more 

extensive outcrops to the north and noted that it is 

especially thick in the Sierra Ancha. He therefore 

described, in generalities, a 900-foot section on Baker 

Mountain, which is 47 miles north of the type locality. 

The Baker Mountain section is 2^ miles southwest of 

the Center Mountain section designated as a reference 

section in this report. The two sections are similar, 

but the Baker Mountain section is not so well exposed 

and lacks, owing to pre-Devonian erosion, the upper 

200 feet or more of the sequence remnant in the Center 

Mountain section. Ransome also recorded, apparently 

with some misgivings, a third section at Theodore 

Roosevelt Dam. That section is only 160 feet thick 

and is partly or perhaps wholly of Devonian sandstone. It is now apparent that none of the southern 

sequences of the Troy are so complete as several remnants that are in the high part of the Sierra Ancha, 

within a few miles of the reference section described 

here.

From his most detailed observations, Ransome considered 400-foot thicknesses representative of the Troy 

in the Ray quadrangle and preferred not to give credence to sections seemingly much thicker at and near 

the type locality. Local abrupt changes in thickness 

are readily reconciled, however, when one recognizes 

that the Troy was differentially uplifted along faults, 

then variously eroded before deposition of Cambrian 

strata and, in the northern part of the region, again 

deeply eroded before Devonian strata were deposited. 

As a consequence adjacent Troy sections separated by 

a Precambrian fault and overlain by an unfaulted capping of Paleozoic strata may vary several hundred feet 

in thickness. In the northern part of the region, most 

sections are more than 400 feet thick, and a few are at
TROY QUARTZITE DEFINITION AND SUBDIVISION 45

least 1,200 feet thick; in much of the southern part, the 

Troy is absent, but where remnant it ranges in thickness from 300 to 700 feet.

In a few places small amounts of rusty-weathering 

thin-bedded sandstone, now recognized as Cambrian, 

were included in the top of the unit first designated 

as Troy Quartzite. The 362-foot section described by 

Eansome includes 50 feet of Cambrian sandstone. North 

of the Eay quadrangle, except in a few small areas between Globe and Superior, Cambrian strata are entirely missing from outcrops designated as Troy by 

Eansome and later workers. Therefore, published 

maps that show the Troy in the northern areas require only slight modification. However, quartzitic 

strata called Troy in the area between the latitudes of 

Globe and Tucson must now be reassessed. Some of 

the outcrops are Troy; many others are entirely 

the Cambrian Bolsa Quartzite or are wholly sandstone f acies of the overlying Abrigo Formation.

In the Sierra Ancha the Troy can be readily subdivided into three members: the lowest member is a 

reddish well-sorted arkose, the middle member is a 

white sericitic and pebbly sandstone, and the uppermost is a gray clean quartzite. The following section, measured along the east face of Center Mountain 

in the highest part of the range, is one of the most 

complete yet found.

Reference section of Troy Quartzite

[Measured along east face of Center Mountain; arkose member described largely from 

exposures in NEH sec. 16, T. 6 N., R. 14 E.; Chediski Sandstone Member measured 

in NWJ^SWJi sec. 15 and NEJ^SEJi sec. 16, T. 6 N., R. 14 E.; quartzite member 

descriptions from SEJi sec. 16, NEJi sec. 21, and NWJi sec. 22, T. 6 N., R. 14 E.]

Troy Quartzite:

Quartzite member:

Top eroded. Probably one of the thickest 

remnants of the member.

11. Quartzite, dominantly medium light gray 

but many beds pale red to grayish-red 

purple; mostly medium-grained with 

sparse content of coarse grains; a few 

beds coarse grained; in tabular beds 

3-12 in. (mostly 4^8 in.) thick, most 

cross stratified on small to medium 

scale; flaggy to slabby parting is everywhere conspicuous. Lower part crops 

out in steep slope partly covered with 

talus; slope merges upward into cliffs 

as much as 150 ft high_._____________ 250

10. Quartzite, very light gray to pale-redpurple, medium-grained; in tabular 

beds 4 in.-3 ft (mostly 1-2 ft) thick; 

more vitreous than unit 11, and internal stratification obscure; on weathering, outcrops stain slightly to pale or 

moderate reddish brown; joints may be 

thinly coated by limonite. Forms cliff 

with narrow to broad bench at top; 

upper 80 ft not as resistant as lower 

part.___________------____---_____- 185

Thickness 

(feet)

Troy Quartzite Continued

Quartzite member Continued

9. Quartzite, pale-red to grayish-red-purple, 

coarse-grained; voids between rounded 

grains imperfectly filled by quartz 

overgrowths. Otherwise like unit 10, 

and crops out as steep ledgy slope or 

cliff continuous with that of unit 10_ _ _ 60

Thickness of quartzite member

(units 9-11) remnant._________ 495

Chediski Sandstone Member:

8. Sandstone, very light gray to pinkishgray; mostly coarse grained, some beds 

very coarse grained; sand-grain and 

matrix characteristics as in unit 6; 

tabular beds 3-8 ft thick cross stratified 

on medium scale. Well-rounded pebbles, mostly less than 1 in. in diameter, 

of white to pink quartz and very sparse 

pebbles of reddish-brown jasper are 

scattered through a few beds and concentrated in upper 1-2 in. of many 

beds. Pebbles most abundant in upper 

30 ft, and topmost bed conspicuously 

conglomeratic. Lower 44 ft friable; 

upper 30 ft tends to be quartzitic. 

Crops out as massive rounded ledges, 

in which bedding structures are 

obscure.___________________________ 74

7. Sandstone, light-gray to pinkish-gray;

not pebbly; otherwise similar to unit 6_ 80

6. Sandstone, pinkish-gray to grayish-redpurple; progressively darker and more 

mottled by reduction spots upward, 

fine- to coarse-grained (mostly mediumgrained), poorly sorted, friable; of 

well-rounded minutely pitted quartz 

grains in matrix of sericite and clay. 

Convolute lamination conspicuous 

throughout massive layers, mostly 10- 

30 ft thick; layers display little continuity of bedding. A few thin (2 ft 

maximum) tabular beds, with mediumto large-scale tangential cross-stratification, separate these layers. Unit 

contains scant pebbles like those in 

unit 5. Forms gentle to steep slope, 

locally surmounte d by hoodoos - - - - 67

5. Conglomeratic sandstone, light-brownishgray, poorly to firmly cemented; bedding obscure but dominantly horizontal; 

mostly coarse grained, but part very 

coarse grained; includes lenses of 

granule-size gravels. Sand grains well 

rounded, minutely pitted, mostly of 

quartz; very sparse granules of orangepink feldspar conspicuous. Pebbles 

mostly of white to moderate-red quartz; 

about 20 percent are of pale-yellowishbrown to grayish-red quartzite; a few of 

feldspathic quartzite, reddish-brown

248-121 O - 67 - 4
46 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

Troy Quartzite Continued

Chediski Sandstone Member Continued

jasper, or rhyolite. Most pebbles well 

rounded and less than % in. in diameter; 

maximum diameter, 3 in.; some of 

larger pebbles are ventifacts. Crops 

out as massive ledges or small cliff___

Thickness 

(feet)

39

Thickness of Chediski Sandstone

Member (units 5-8)___________ 260

Arkose member:

4. Arkose, light-brown to pale-brown or 

pale-red (pale red dominant), mediumgrained, firmly cemented; in tabular 

beds 2-8 ft thick that exhibit largescale cross-stratification; some beds 

wedge shaped; unit forms cliffs. Crossstratification truncated at sharp slightly 

irregular contact with unit 5_---______ 110

3. Arkose, pale-brown to grayish-red, finegrained, firmly cemented; cross stratified on very large scale; planes separating tabular bedding sets arc mostly 

tens of feet apart; slabby parting; 

crops out only onnarrow hoodoo-studded 

spurs, talus covered elsewhere_________ 165

2. Arkose, like unit 1 except in tabular 

beds 4 in.-3 ft thick. Some beds 

separated by silty seams that preserve 

mud cracks and, uncommonly, ripple 

marks. Crops out as receding cliff of 

massive ledges._____________________ 60

1. Arkose, pale-brown, fine-grained, very firmly 

cemented; mostly in wedge-shaped bedding sets (l}_-8 ft thick at thickest part) 

cross stratified on medium to large scale. 

Lower 15 ft in cross-stratified tabular sets 

4-18 in. thick. Basal 6-18 in. includes 

abundant fragments from underlying 

basalt. Forms ragged cliff_____________ 108

Thickness of arkose member (units

443

Thickness of Troy Quartzite------ 1, 198

Unconformity.

Basalt flow: Basalt, blackish-red, fine-grained, porphyritic, amygdaloidal; crops out as steep slope or cliff-- 40 +

As indicated in the reference section, the three members have distinctive lithologies and bedding structures. These distinctions are conspicuous north of the 

Salt Eiver, but they are subdued in most areas to the 

south owing in part to lithologic changes and in part 

to the incompleteness of sections. The arkose member 

is restricted to the pre-Troy basin of northwestern Gila 

County; the Chediski Sandstone Member is represented throughout the region; the quartzite member 

thins drastically southward and probably is missing 

from all areas south of the Mescal Mountains.

ARKOSE MEMBER 

DISTRIBUTION AND THICKNESS

The arkose member is exposed in an area of only a 

few hui^dred square miles, and outcrops are numerous 

only within the McFadden Peak quadrangle. In the 

highest parts of the Sierra Ancha, within 2-4 miles 

of the Center Mountain section, the member is 400-450 

feet thick; to the north, east, and south it thins against 

successively higher parts of the pre-Troy surface; 

within 8-15 miles it laps out completely, and the 

Chediski Sandstone Member becomes the basal subdivision of the Troy (fig. 11). The arkose member 

can be traced into areas in which it is less than 100 

feet thick, but it is now missing owing to Cenozoic 

erosion from areas where it thinned further and ultimately pinched out. Nevertheless, the extrapolated 

limit of the member shown in figure 4 is narrowly restricted except along the southern segment, where the 

first remnants of the Chediski that rest directly on 

Apache strata are as much as 10 miles south of the 

southernmost remnants of the arkose member. Judged 

from the pattern of thickening, the arkose member 

might be as thick or even thicker in the western Sierra 

Ancha than in the high parts of the range; but west 

of the 111° meridian, erosional remnants of the Troy 

are so few, thin, and widely scattered that this has not 

been confirmed.

STRATIGRAPHY AND LITHOLOGY

Where the member is thin, or where it rests mainly 

on the argillite member of the Mescal, a conglomerate 

or conglomeratic sandstone 5-20 feet thick lies either 

at the base of the arkose or is 10-20 feet above the 

base. The pebbles and cobbles mostly are well-rounded 

quartzite and rhyolite from the older Precambrian 

terrane and are in a matrix similar to the overlying 

arkose. Where thickest, the member generally rests 

on the upper basalt of the Apache Group, and the 

basal part is not conglomeratic. In a few areas a 

thick residual soil mantled the basalt; this debris was 

incorporated in the basal few feet of the member and 

made it dark, earthy or friable, and from a distance 

difficult to distinguish from the basalt.

The arkose above the conglomerate is fine to medium 

grained, firmly cemented, and distinguished from other 

Precambrian sandstone by striking bedding features. 

This arkose is mainly pale red to grayish red and 

secondarily light to pale brown on both fresh and 

weathered surfaces. Individual beds are uniformly 

sorted. Most are fine grained, tabular, and more than 

10 feet thick; beds as much as 40 feet thick are common. In areas of low relief, the arkose erodes to 

gentle smooth slopes and, in areas of high relief, to
TROY QUARTZITE CHEDISKI SANDSTONE MEMBER 47

cliffs or to steep slopes studded with rounded to very 

angular ledges. Eegardless of outcrop form, largescale or very large scale cross-stratification within the 

tabular beds is conspicuous. Sets of cross-strata are 

a few tens to several hundreds of feet in length. All 

examined by me dip eastward; most dip 18°-20°, but 

dips as low as 7° and as high as 22° have been noted.

Such bedding features characterize all except the 

lowest parts of most sections. Where the member is 

more than 400 feet thick, the lower 80-175 feet may 

be made up of tabular beds 3 inches-10 feet thick, 

rather than tens of feet thick; these beds are cross 

stratified on a medium to small scale. In some basal 

sections wedge-shaped beds are abundant. Where the 

member is less than 250 feet thick, wedge-shaped beds 

and small-scale cross-strata are seen only in the lowest few feet of the section. In a few exposures along 

the canyon of Cherry Creek, particularly in the north 

half of the McFadden Peak quadrangle, the lowest 

arkose is notably different. In that area, channels, 

generally broad and shallow but in places narrow, 

steep-walled, and deep, were eroded into the basalt 

and in places several tens of feet below it. In most 

channels the arkose is poorly sorted, sparsely pebbly, 

and devoid of well-defined bedding structures. Locally these conglomeratic arkoses are 100 feet thick.

This arkose member has been mistaken for the arkose member of the Dripping Spring Quartzite; in 

hand specimen the two are somewhat alike, but in 

outcrop they are entirely different. Actually, specimens of the Dripping Spring arkose differ in being 

orange pink or reddish orange, in being somewhat less 

uniform in texture, and in breaking with a quartzitic 

fracture. Owing to erosional etching or well-defined 

partings along the inclined planes, the large-scale 

cross-stratification of the Troy arkose is everywhere 

conspicuous, whereas that of the Dripping Spring 

arkose is small to medium scale and obscure. The basal 

conglomerate differs from the Barnes Conglomerate 

Member mainly in that it contains some fragments of 

Mescal Limestone and basalt.

CHEDISKI SANDSTONE MEMBER

A white sericitic sandstone, which forms rounded 

slopes surmounted by knobs and hoodoos bizarrely 

sculptured by erosion, was recognized by Burchard 

(1931, p. 54, 56-57) as a distinctive unit that separates 

the principal iron deposits of the Mescal and quartzites 

of the Troy. Without additional definition, he termed 

this unit the Chediski White Sandstone Member of the 

Troy Quartzite. The type locality was designated as 

Chediski Mountain, a high flat-topped ridge on the 

west side of Canyon Creek, 23 miles north of the Salt

Eiver. The member forms an almost horizontal characteristically conspicuous outcrop around the east end 

of this ridge. (The member derives its name from this 

outcrop: "chediski" as used locally by the Apache 

means long white rock.) South of the Salt Eiver the 

member is locally quartzitic and is neither so distinctively white nor so spectacular in its erosional forms; 

it therefore has not been distinguished previously as 

a separate member. Less obvious features do set it 

apart everywhere, however, and under the designation 

Chediski Sandstone Member it is here recognized as 

the most widespread subdivision of the Troy. It is 

the basal member, as Burchard specified, except where 

it is underlain by the arkose member in northwestern 

Gila County.

THICKNESS

In a general way the Chediski Sandstone Member 

thickens from north to south. North of Young it thins 

by lapping out; and in its northernmost exposures 

along and near the Naegelin Eim, it is only 75-100 

feet thick. Between the latitude of Young and that of 

Globe, it is about 250 feet thick. Deviations reflect 

irregularities in the pre-Troy surface: where the surface truncates uparched Apache strata along the south 

rim of the Natanes Plateau northwest of Sawmill, 

for example, the member is less than 200 feet thick. 

South of Globe the member commonly is directly overlain by Paleozoic strata, and many sections are obviously incomplete; nevertheless, thicknesses of 250- 

300 feet are common, and several occurrences are much 

thicker. Sections 2-4 miles east of Eay are at least 

500 feet thick, and in the Holy Joe Peak quadrangle 

some sections may exceed 700 feet.

L.ITHOLOGY AND STRATIGRAPHY

The features described below are typical of outcrops 

north of the Salt Eiver; the less distinctive southern 

f acies are described later.

Three subdivisions are characteristic of the Chediski Sandstone Member. Although variable in thickness they are easily recognized by differences in bedding features. More subtle are the lithologic differences, which reflect composition and manner of occurrence of the coarsest constituents. The upper unit 

crops out in well-defined ledges and cliffs in which 

cross-stratification is obvious, and it is consistently 

characterized by thin layers or lenses of small pebble 

conglomerate. The middle unit is largely free of granules and pebbles and is characterized throughout by 

convolute lamination. In areas of high relief this is 

the part of the member that eroded to fluted cliffs and 

hoodoos; in areas of low relief rounded massive out-
48 YOUNGER PEECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

crops are typical. The lower unit is also massive but 

differs from higher strata in being sparsely and erratically pebbly and devoid of well-defined bedding structures. Generally there is a ledge-forming conglomerate 

layer at the base of the lower unit, and the rest of 

the unit forms a gentle concave slope partly hidden 

in debris. Where the Chediski exceeds 200 feet in 

thickness, the crossbedded unit makes up the upper 

third of the member, and the lower unit is 25-60 feet 

thick. Everywhere in northern occurrences the middle unit is the thickest of the three. In sections appreciably thinner than 200 feet, it commonly comprises 

all except the uppermost and lowermost 10-15 feet of 

the member.

Distinctive colors also cause the member to contrast 

with overlying and underlying strata. In hand specimens the sandstone is mostly pinkish gray; viewed 

from a distance on a bright day, outcrops of this hue 

have so white a glare that the casual observer tends 

to overlook lesser details essential to regional recognition of the member. Commonly the lowest 10-80 feet 

of the member that is, part, or all, of the lower unit 

and sometimes part of the middle unit is grayish red 

to grayish reddish purple. Parts of the red sandstone 

are mottled or streaked by reduction spots. In some 

sections the mottled parts are light brownish gray to 

light greenish gray, but in most they are almost white. 

The coarser grained strata or the boundaries between 

layers of different grain size are mottled in preference 

to the rest of the rock. In several areas the lower unit 

is largely devoid of red-purple coloration, and the lower part of the middle unit retains that color. Thus 

an irregular but conspicuous band of color 20-40 feet 

thick may roughly mark the boundary between the two 

units.

The member consists largely of very friable to 

moderately well cemented very poorly sorted porous 

sericitic sandstone. Subrounded to well-rounded pitted and frosted clear quartz grains, ranging in size 

from fine sand to granule, make up most of the rock. 

The matrix consists of sericite and a whitish clay; the 

sericite is usually the more abundant. Cleavage fragments of orange-pink microcline or orthoclase are very 

sparse; clay aggregates that have the outlines of such 

cleavage fragments occur somewhat more abundantly. 

The feldspar grains are generally only a fraction of 

the size of the quartz grains, but in the coarsest grained beds where feldspar grains are the most numerous 

some may be granule size. Although much of the 

rock appears devoid of feldspar, except under the microscope, chemically the sericitic sandstone is probably 

equivalent to a moderately feldspathic sandstone.

In general as the grain size of the sand increases,

the clay or sericitic matrix decreases, and the sandstone 

is more firmly cemented by quartz. The crossbedded 

upper unit is generally the only part of the member 

that is firmly cemented; but commonly even the lower 

part of this unit is very friable. In some sections a 

bed or two of quartzite can be found near the top of 

the unit. In places the voids in a coarse-grained lens 

in the middle unit are completely filled with quartz. 

Where the matrix is sparse, clusters of minute needlelike quartz crystals visible only under a microscope 

form meshworks between grains; these needles are not 

attached to the clastic quartz grains. Indeed, except 

in the minor occurrences of quartzite and in the firmly 

cemented sandstones just noted, individual sand grains 

do not exhibit overgrowths of quartz.

Lithologic details, considered in stratigraphic sequence, provide additional criteria for delineation of 

the three units and are especially useful in regional 

correlation.

Where the Chediski is the lowest member of the 

Troy, the basal 2-40 feet is pebble to cobble conglomerate or conglomeratic sandstone. Generally the conglomerate is less than 20 feet thick and is a sharply 

defined ledge-forming layer that does not grade into 

the overlying pebbly sandstone. The gravels of these 

conglomerates are mostly quartzite and quartz derived from the older Precambrian rocks; pebbles of 

basalt and chert from the Mescal and angular fragments from the argillite member of the Mescal are 

sparse. Locally the gravels are almost monolithic, 

especially where the conglomerate overlies a terrane 

of thoroughly silicified hematitic Mescal. Where the 

Chediski overlies the arkose member, the basal few 

feet may be conglomerate or erratically pebbly sandstone, or may be practically free of pebbles. Detritus from Apache strata is virtually absent from these 

occurrences. The contact between the two members is 

sharp, and though small channels and truncated crossstrata in the arkose member obviously indicate erosion, 

debris from the lower member has not been recognized in the Chediski.

The sandstone of that part of the lower unit immediately above the basal conglomerate is generally 

sparsely pebbly, but lenses of granule, pebble, or in a 

few places even cobble conglomerate do occur in this 

interval. Unit 5 of the Center Mountain section, which 

happens to be an occurrence lacking the basal conglomerate, is typical of many such intervals. The 

vague bedding features are defined almost entirely by 

thin discontinuous lenses of very coarse sandstone or 

conglomerate, which suggest an irregular horizontal 

stratification. Lenticular beds of cross-stratified sandstone do occur sparsely. The pebbles and cobbles
TROY QUARTZITE CHEDISKI SANDSTONE MEMBER 49

mostly are fine-grained vitreous quartzite. In some 

sections quartz pebbles predominate; part of these 

differ from quartz pebbles higher in the section in 

being shades of red or brown, rather than white. Some 

gravels, and especially the largest, show polished 

facets and the forms characteristic of windworn pebbles or ventifacts (Bryan, 1931, p. 29-36). In some 

sections these ventifacts make up 5 percent of the 

pebbles more than 1 inch in diameter; in most they do 

not approach this abundance.

Because soft-rock deformation caused the mixing 

of layers of different grain size and obliterated textural 

characteristics, it is difficult to be specific about the 

degree of sorting represented in the middle unit. Perhaps sands of certain layers were once moderately well 

sized. As a unit it is not well sorted, but overall it is 

much better sorted than the adjacent units.

Apparently thin to thick beds, individually dominated by either fine or medium sand, made up most 

of the massive sets of strata as each was deposited. 

Many thin lenses or tabular layers of coarse sand, not 

as well sorted as the finer sands, were intercalated in 

these sets in most localities. There are occurrences, 

however, in which such layers are few. The upper 

half to three-fourths of the middle unit is free of 

gravels, but th^ lower part contains pebbles and cob 

bles distributed as erratics without apparent relation 

to original bedding features. In one outcrop small 

pebbles are scattered at intervals of 2-5 feet but nearby, 

in outcrops of equivalent beds, only cobbles 15-50 

feet apart are seen. The sandstone of this unit differs 

considerably from that above and below in lacking 

layers or lenses of very coarse sand and granule conglomerate and in including relict masses suggestive of 

layers that were once moderately well sorted.

Internal structures of the forms collectively termed 

convolute lamination or convolute bedding (Ten Haaf, 

1956) mark all massive tabular layers (sets(?) of 

beds), which make up most of the middle unit. The 

layers of contorted sandstone, mostly more than 10 

feet thick, are defined by inconspicuous horizontal 

planes; a few are separated by cross-stratified beds, 1- 

2 feet thick, that are tabular and sharply defined top 

and bottom. Convolute lamination is absent only from 

the well-defined thin beds. The convolute lamination 

is obscure except where outlined by layers of contrasting grain size. In a small proportion of outcrops, the 

structures are everywhere conspicuous, because textural differences in adjacent layers are accentuated by differential cementation or by color mottling if the sandstones are red. The most abundant structural forms 

are contorted wispy laminae or swirl-form mixtures of 

sands that occur as though separate layers had been

gently stirred together. The coarse sand layers that 

survived plastic deformation as crude lenses are more 

striking, because they project prominently from outcrops and are the most continuous features. Many 

such lenses are saucer- or bowl-shaped in cross section; others are intricately thickened and thinned and 

contorted. Where such marker layers were displaced 

no more than a foot or two, saucer-shaped lenses thinned to form sharp narrow "anticlines" where their 

"rims" meet, and these may be alined laterally to give 

a scalloped effect in cross section. Less common are 

isoclinally folded laminae that occupy the full thickness of tabular layers, some as much as 8 feet thick. 

Folds of this and the scalloped type persist uninterrupted as far as the enclosing layer can be traced in 

some places, several hundred feet.

The convoluted structures were formed mainly while 

the individual massive layers were being deposited. 

Truncation of the internal convolutions at the planar 

surfaces shows that a sand layer was deposited, deformed, and then eroded before the next similarly 

marked layer was laid down. If the internal corrugations had formed as a consequence of submarine sliding on a depositional slope, irregular thinning or thickening of the layers or the piling up of disrupted strata 

within wedge-shaped masses might be expected. No 

features suggestive of this or any other sort of extensive lateral slump have been noted. The separating 

planes are continuous for considerable distances, the 

indication being that after a set of beds crumpled, 

there generally was no additional deformation. Locally and at widely separate places, the thin crossstratified beds and separating planes are obliterated by 

slumping. Thus, in one outcrop, layers of contorted 

sandstone may be separated by planes 10-20 feet apart, 

whereas in an adjacent outcrop of the same interval, 

well-defined planes are as much as 50 feet apart. Such 

relations are uncommon, but they do suggest composite causes for some apparent slump structures. Perhaps gross slump effects were superimposed on smaller 

scale convolutions that were entirely the effects of 

earlier intrastratal crumpling and stretching of very 

plastic sand layers.

Bounded outcrops, in which several layers differ 

little in texture and in which the slump structures are 

inconspicuous, may be additionally marked by peculiar 

patterns of V-shaped furrows. These furrows or 

etched-out surficial joints in some places penetrate the 

outcrop to a depth of 4 inches. The furrows generally 

are in two sets with about equal spacing; together the 

sets define a reticulate pattern, so that the outcrop simulates a rock wall built of crudely squared but tightly
50 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

matching sandstone blocks. One outcrop may have 

furrows 6 inches-1 foot apart; another may have furrows 2-3 feet apart. In a few outcrops the blocks have 

five or more sides, rather than four.

The boundary between the middle unit of contorted 

sandstone and the lower unit is poorly defined; that 

between the contorted sandstone and the upper unit is 

sharp and apparently represents a surface of intraformational erosion.

The upper unit of the Chediski Sandstone Member is 

readily recognized in most localities by tangential 

cross-stratification that etches out in bold relief. The 

beds are grossly tabular, but if traced far enough many 

prove to be wedge shaped or lenticular. In some areas 

the beds are 3-10 feet, in others 10-20 feet thick. Dips 

of cross-strata exceed 10° and commonly approach or 

slightly exceed 20°. No dominant direction of dip 

has yet been noted.

Where crossbedding is not conspicuous in the upper 

unit, as in unit 8 of the Center Mountain section, the 

composition and disposition of the small gravels serve 

to distinguish it from all other Troy strata. Granules 

are possibly as voluminous as pebbles. Most pebbles 

are less than three-quarters of an inch in diameter; 

some are as much as 2 inches across. The gravels are 

well rounded and are mostly of white or pink quartz, 

with a few being reddish-brown jasper. The beds are 

coarse to very coarse grained, and perhaps half of 

them are sparsely pebbly throughout. Gravels become 

more abundant and coarser upward both in individual 

beds and in the section. In the top 25-35 feet of the 

upper unit, the top 2-6 inches of almost every bed is 

conglomerate. The uppermost 1-6 feet of the member 

is everywhere a conglomerate or a conglomeratic sandstone. The positions of the pebbles and their increase in size and number upward indicate that the 

finer constituents in the top parts of each bed were 

winnowed away leaving behind only the coarser fractions of what was originally a much thicker layer 

and that such winnowing was increasingly more effective and prevalent as successive beds were deposited.

LATKKAL, VARIATIONS

North of the Salt River, lateral variations are mainly 

in thickness; lithologic details do not differ enough 

to hinder immediate recognition of the Chediski Sandstone Member or of its three units. South of the 

river, lithologic variations, which are mainly manifestations of slight changes in the makeup of matrix materials and in the degree of cementation, overshadow 

and obscure the stratigraphic entities so apparent farther north. Thus, detailed regional correlations have 

not been attempted previously. A quartzitic facies,

generally considered the facies typical of the entire 

Troy, actually is not important regionally. The lateral variations in occurrence and composition of the 

gravel-size constituents and variations in bedding 

structures are the significant modifications to be interpreted in making regional correlations. The tripartite subdivision of the member is not as sharply 

delineated but is still valid.

Compositionally the sandstone of the southern part 

of the region is akin to that of the northern part. The 

matrix is dominated by sericite and clay, but in a general way these constituents decrease in amount southward, especially in the upper half of the member. In 

some localities coarse muscovite, which is uncommon 

in the northern facies, fills interstices between grains 

and is abundant in a few beds. Orange-pink feldspar 

is more abundant in the southern facies, and in the 

coarse grained beds compound fragments of feldspar 

and quartz, obviously derived from granitoid rocks 

similar to those that underlie the Apache Group, are 

locally conspicuous.

As the content of clay decreases southward, the sandstone becomes more firmly cemented by quartz, and 

the Chediski becomes almost entirely quartzite over 

areas of a few square miles and partly quartzitic over 

even broader areas. Nevertheless, if emphasis had not 

been placed on occurrences in the vicinity of the 

abandoned mining camp of Troy, near Kansome's type 

locality, and in and near the northeastern part of the 

Globe quadrangle, where Kansome apparently first 

noted extensive outcrops of the Troy, the Troy might 

have been designated a sandstone. The northernmost 

exposures in which the whole member is quartzitic extend from Cibecue Creek east along Salt River canyon 

and southeast across the western part of the Natanes 

Plateau. There the three units of the member are obscure and large parts of the section are crossbedded 

massive red quartzite comparable with that previously 

emphasized as typical of most of the formation (Ransome, 1916, p. 139-141 and pi. 27^4.). Large parts of 

the member are vitreously quartzitic and except for 

feldspar and pebble contents could be mistaken for the 

overlying quartzite member. Bedding structures distinctive of the Chediski, such as convolute lamination, 

are almost wholly obliterated by silicification. In perhaps 50 percent of all exposures farther south, the distinctions between members are similarly obscure because the crossbedded upper unit of the Chediski is 

cemented almost as firmly as the overlying member. 

The two lower units, however, are generally sandstone, 

and parts of most sections are loosely cemented and 

friable.
TROY QUARTZITE QUARTZITE MEMBER 51

The southern sandstone and quartzite are mostly 

yellowish gray or light brownish gray to medium darkgray or pale red, and in these drab hues they do not 

contrast with the adjacent stratigraphic units as do the 

white-hued outcrops of northern Gila County. Certain medium- to dark-gray beds do, however, contrast 

with interlayered beds that are brown or red hued, and 

where they occur these gray beds are distinctive of the 

Chediski Member. The gray sandstone owes its color 

to abundant minute aggregates of black hematite that 

partly fill voids between quartz grains. Similar gray 

sandstone does not occur in the northern area, although the red-hued basal sandstone of that area does 

owe its color to hematite. In the widespread exposures in the range of hills southeast of the Apache 

Mountains, the basal 50-75 feet of the member is grayish red or grayish red purple, a darker red than generally seen farther north. Such hues characterize as 

much as 120 feet of the member in a very few localities, but in most southern areas dark sandstone is 

subordinate. In only a few localities, such as the 

Hayes Mountains, are white outcrops as conspicuous 

as in the northern area.

In the northern part of the region, about the same 

degree of poor sorting,characterizes stratigraphic intervals 30-100 feet thick, whereas in the southern part, 

adjacent beds contrast because of differences in sorting. Sands of coarse-grain to granule size, in particular, tend to be segregated as lenses and beds in the 

southern sections.

This contrast in sorting is emphasized because gravels like those found only in the basal conglomerate 

north of the Salt River, as well as the ubiquitous quartz 

and quartzite gravels, are seen throughout. Angular 

to well-rounded fragiments derived from resistant 

strata of the Apache Group are found through the 

lower half of the member. Lenses of gravels may 

also include sparse fragments of granite and Final 

Schist, which are absent from the northern sandstone. 

In places, such as near the mouth of Putnam Wash and 

farther east in the Holy Joe Peak quadrangle, many 

beds of the lowest 50-70 feet of the Chediski contain 

abundant angular fragments, as much as iy% inches 

across, of orange-pink feldspar derived from the preApache granite and of reddish-orange siltstone from 

the Dripping Spring Quartzite. Typical polished ventifacts are extremely sparse; however, some faceted 

pebbles have rounded edges and were possibly windworn, then modified by subaqueous abrasion. In northern parts of the region the upper half of the contorted 

sandstone unit is virtually pebble free; in southern 

areas pebbles are fewest in the upper half, and

throughout the unit they are mainly confined to layers 

or lenses that separate pebble-free beds. In the northern outcrops nearly all the pebbles are quartz in the 

cross-stratified upper unit; southward, quartz is dominant, but small pebbles of other composition are also 

common.

Southward from the Natanes Plateau individual 

beds become increasing well defined. The lower 100- 

150 feet of the member is mainly of tabular beds ^-4 

feet thick. Some beds are tangentially cross stratified 

on small to medium scale; others are horizontally laminated. The tops of many were channeled before deposition of the overlying bed, and lenses of conglomerate or conglomeratic sandstone, y^-^/z feet thick, 

may fill the channels. South of Globe a large proportion of the undulant partings between the beds are 

marked by gravels. Where lenses of conglomerate are 

absent, granule-size gravels thickly coat some 'bedding surfaces, small pebbles sparsely litter other surfaces, and in extreme examples cobbles as much as 8 

inches in diameter are scattered very sparsely along 

the bedding planes. Some beds wedge out laterally, 

and the gravel layers coalesce. Sharply defined layers 

and lenses of gravels, laterally continuous though 

anastomosing as viewed in cross section, do not characterize northern exposures of the member.

In many areas south of Globe, the individual layers 

of convoluted sandstone, 8-15 feet thick, are sharply 

separated by gravel layers or by thin sets of undeformed beds. The middle unit does not crop out as 

massively, and if convolute lamination is inconspicuous, as in quartzitic sections, the casual observer may 

include beds of this unit with those of the lower unit 

and assume that the contorted sandstone interval is 

very thin or missing. Actually, in southern areas 

the stratigraphic interval marked by convolute lamination does vary considerably in thickness from 50 to at 

least 125 feet.

Bedding features of the cross-stratified upper unit 

are not notably different from those of equivalent 

strata in the northern part of the region, but because 

this unit is commonly quartzitic, they are not as 

conspicuous. Perhaps fewer beds are capped by concentrations of winnowed pebbles, but such pebble layers are still common enough to characterize the unit.

QUARTZITE MEMBER

The upper member is a fairly clean well-bedded 

quartzite, which lacks the distinctive bedding features 

that characterize the two lower members. The section measured at Center Mountain is one of the thickest in northern Gila County and is typical.
52 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

STRATIGRAPHY

The quartzite member can be subdivided into three 

units, although the differences are slight. Coarsegrained quartzites in which individual grains appear 

angular because the interstitial voids are only partly 

filled by overgrowths characterize the lowest unit; a 

few beds are very coarse grained. This subdivision 

(unit 9, reference section) is 50-80 feet thick and 

grades upward into the middle subdivision (unit 10, 

reference section), which is 150-200 feet thick and 

is medium grained and highly vitreous. Both units 

are made up of tabular beds, which range in thickness 

from 3 inches to 6 feet but are mostly 1-2 feet. Tangential cross-stratification on small to medium scale 

apparently is characteristic, but in most outcrops internal bedding details and partings between beds are 

obscure. The uppermost subdivision (unit 11, reference section) is thinly bedded, bedding structures etch 

out distinctly, and flaggy to slabby partings are everywhere present. It differs more subtly in not being as 

uniformly sorted as the lower units; some beds are 

coarse grained, and a scattering of coarse grains is 

common in the medium-grained beds. The beds are 

tabular and range in thickness from 2 to 30 inches; 

most are less than 12 inches thick. Some are horizontally stratified, most are cross stratified. In some outcrops the more conspicuous splitting surfaces follow 

cross-strata rather than planes between beds and give 

a false impression of undulatory bedding.

LITHOL.OGY

The quartzite member differs from the Chediski and 

arkose members by being remarkably free of detrital 

grains other than clear quartz. Thin sections show 

well-rounded quartz grains locked together by overgrowths that, except in the lowest beds, fill the voids 

between original grains. A few beds include random 

grains of orange-pink feldspar. Granules of quartz 

are extremely sparse. South of Globe, feldspar and 

quartz granules, the quartz at places in thin lenses that 

include a few small pebbles, occur in the lowest quartzite unit; otherwise such constituents are practically 

nonexistent.

The quartzites are mostly grayish pink to pale red, 

although light gray or yellowish gray dominates in 

some localities and beds of dark grayish red or grayish 

red purple are conspicuous in other places. The red is 

due to a thin dust of hematite (?) on the original detrital grains and to minute aggregates of hematite along 

boundaries between overgrowths. The coarse-grained 

lowest unit is generally grayish red or grayish red 

purple, but in some localities it is very light colored;

light-gray or yellowish-gray quartzite seems to be 

characteristic in the southern part of the region.

Weathering accentuates the red. Where the quartzites are deeply weathered, detached blocks may be 

stained throughout. Such extremes of discoloration 

particularly characterize the slabby upper unit of the 

member where it is exposed on benches or mesia tops. 

Detached slabs are stained moderate brown to very 

dusky red throughout, exhibit knobby or pitted surfaces, and lie in a thin red soil of earthy sand.

Generally the grayish quartzite is unmodified by 

weathering, ~ / but in some localities a few beds are pit- * 

ted where partially cemented aggregates of sand 

weathered free. The pockmarks, commonly not more 

than an inch across, may show thin limonite stains, 

coatings of black oxides, or lichen growths and are 

very conspicuous against a background of light-colored 

quartzite. In different localities pock-marked layers 

occur in different parts of the member, so that they 

have little use as stratigraphic markers.

DISTRIBUTION ANI> TOPOGRAPHIC EXPRESSION

Thick remnants of the quartzite member are common north of the Salt River but not south, and in a 

general way the proportion that is remnant below 

Paleozoic formations can be judged from the topography. The two lower units commonly form a steep 

ledge-studded slope or a single cliff. The contact between the middle unit and the slabby upper unit is 

marked by a broad topographic bench. Because the 

upper 30^0 feet of the middle unit is somewhat less 

resistant than the underlying quartzite, in many areas 

there is a steplike transition zone of ledges between 

the top of the cliff and the bench. The lower part of 

the upper unit forms a steep slope that merges downward into the bench and upward into ragged cliffs. 

The lower 25 feet is largely covered by flaggy and 

slabby talus from higher beds. Where the upper unit 

is 150-200 feet thick, as in the higher parts of the 

Sierra Ancha, the quartzite member is readily recognized at a distance of 2-5 miles by its distinctive profile of two prominent cliffs separated by a steep slope 

and bench. Elsewhere north of Globe the slabby unit 

is thin or absent, and only the lower cliff typifies the 

member. Farther south, Paleozoic formations commonly rest directly on the Chediski Member; if thin 

remnants of the lower unit of the quartzite member do 

intervene, they do not crop out distinctively from the 

Chediski.

DISTINCTIVE FEATURES OF MEMBERS

In most areas north of Globe, the boundary between 

the Chediski and the quartzite member is a sharp 

easily recognized contact between sandstone and quartz-
DIABASE AND RELATED ROCKS FORM AND DIMENSIONS OF INTRUSIONS 53

ite. Where strata below this contact are quartzites, as 

in many of the southern occurrences, the contact may 

seem rather vague. Although the large-scale crossstratification of the upper beds of the Chediski does 

distinguish them from the overlying quartzites, in sections of vitreous quartzite or sections much shattered 

by faulting, bedding structures are not easily discerned; 

even convolute structures of much lower strata may be 

overlooked. Differences in sorting and composition do 

serve, nonetheless, to distinguish the members. The 

upper part of the Chediski, even where it is a vitreous 

quartzite, includes some clay or sericite between grains 

or includes appreciable feldspar, whereas the lower 

unit of the overlying member is free of such constituents, except for sparse feldspar in a very few beds. 

The upper beds of the Chediski are very poorly sorted, 

and most beds are pebbly. Comparatively, the coarsegrained lower unit of the quartzite member is much 

better sorted. A conspicuous conglomerate or conglomeratic quartzite or sandstone is the uppermost bed 

of the Chediski; no comparable bed occurs in the overlying member. Where the conglomerate zone locally 

is absent, in the southern part of the region, one of 

the lower, thinner, and less persistent zones might be 

mistaken for it. Even so, the highest fairly persistent 

pebbly layer in :a sequence of quartzite probably can 

be taken as the contact between members, without being 

in error more than 20-30 feet stratigraphically.

DIABASE AND RELATED ROCKS 

FORM AND DIMENSIONS OF INTRUSIONS

Through much of the region, diabase sills and dikes 

were emplaced in the Apache Group almost to the 

exclusion of other formations, but in places extensive 

bodies also intruded the older Precambrian formations 

and the Troy Quartzite. Paleozoic formations rest 

unconformably on the diabase. Figure 3 serves well 

to show that the larger intrusions of diabase are singularly coextensive with the younger Precambrian sedimentary rocks. Small intrusions and even sills hundreds of feet thick, but narrow in outcrop, are omitted; 

so the map does not show how virtually every sedimentary outcrop is somewhere interrupted by diabase 

bodies. The bodies that are shown separate from 

Apache and Troy exposures were mostly intruded into 

older Precambrian formations only slightly below the 

Apache; some are exposed in windows eroded through 

younger formations. None show structural or other 

features that would cause them to be categorized as 

different in age, and all show petrographic details 

peculiar to the intrusions in the Apache rocks.

The sills, which are virtually horizontal in the 

Colorado Plateau and which generally dip less than

25° in the Basin and Range region, tend to erode into 

broad concave slopes, whereas the sedimentary host 

rocks tend to form cliffs. Therefore, outcrops of diabase are conspicuous in most landscapes where younger 

Precambrian formations are exposed. Indeed they may 

dwarf outcrops of the cliff-forming rocks, as in areas 

where the aggregate thickness of diabase sills is as 

great as the combined thickness of Apache and Troy 

strata.

Sills predominate in volume, and range in thickness 

from a few inches to 1,200 feet. Thicker sills have 

been reported, but these are almost certainly sheets 

that comprise two or more separately injected sills. 

The sills are remarkably persistent. Some only a foot 

thick extend laterally more than 1 mile, and several a 

few hundred feet thick extend at least 20 miles.

Dikes are numerous but insignificant in volume. Perhaps some are as much as 100 feet wide, but most 

range in width from a few inches to a few feet. A 

few dikes 10-30 feet wide can be traced several miles.

Though sills by definition are tabular concordant 

intrusive bodies, all the Arizona sills are locally discordant, and this is especially apparent in the principal 

diabase masses (fig. 14). Concordant sheets at different horizons are connected by discordant tabular bodies, 

which are as much as 1,000 feet wide and can be 

traced for several miles. These connecting elements 

generally do not extend above the upper contact of 

the higher concordant body or below the bottom of 

the lower body, and they should be visualized as the 

discordant parts of a stepped sill rather than as dikes. 

(See feature C, fig. 14.) Many sills change horizon by 

high-angle abrupt steps up or down across the host 

strata; less obvious is the gently undulant contact that 

transects the sedimentary layers of the host formations 

at a very low angle. Most discordant parts of sills 

are not simple tabular bodies but are stairlike masses 

bounded in one place by high-angle discordant contacts and in another by low-angle or concordant contacts. Depending on the attitudes of the high-angle 

"riser" and of the low-angle "tread," some steps are 

consistent in height for miles along the strike, whereas 

the height of others varies considerably in a short 

distance. Lest reiteration of discordant features be 

misleading, it should be emphasized that only a small 

proportion of the diabase of each sill is encompassed 

between discordant contacts.

Because a cover of younger formations and postPaleozoic deformation obscured intrusive relations in 

mining districts of the Basin and Range structural 

province, some misconceptions exist concerning the 

forms of the diabase bodies. There has been a tendency to visualize the sills as masses that bulge and pinch
54 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

irregularly and have bulbous or irregularly ragged 

terminations. Such outlines have also been considered 

typical of discordant sections of sills. The structural 

features seen in extensive cliff and canyon exposures 

in the Colorado Plateau and in the better exposures 

farther south wholly refute such interpretations. 

Actually, the diabase bodies are so regular in outline 

that they can be projected in cross sections with almost 

as much confidence as can the layered sedimentary 

rocks.

Most contacts of sills, including those that bound 

the stepped discordances, are virtually planar and 

parallel; they circumscribe masses of diabase that are 

distinctly tabular. (See fig. 14.) Even the terminations tend to be planar, because many sills end abruptly with chilled contacts against high-angle faults. Such 

terminations have been noted for a few sills that are 

as much as 800 feet thick and are common for sills 

less than 200 feet thick. High-angle connections between concordant parts of sills were also emplaced 

along faults. Most discordant and concordant boundaries join abruptly in an angle, rather than along a 

rounded or irregular surface.

Because the separate concordant parts of a given 

intrusion emplaced at horizons hundreds of feet apart 

are not exposed in some areas and are entirely eroded 

in other areas, the extent and the shape in ground 

plan of individual thick sills is largely indeterminate. 

Thick sills do not seem to thin significantly by wedging. The parts at different horizons may differ in 

thickness, and sills may thin by steps, as shown between points B and C of figure 14; but each concordant 

element of a sill is very uniform in thickness through 

areas of as much as a few tens of square miles. Some 

thin sills that are stepped apophyses from still thicker 

sills do wedge out, but the wedging is gradual. Near 

the edges of these apophyses, the contacts tend to become undulant, and concomitantly the sills thin rapidly to less than 100 feet, and many then terminate 

abruptly in a narrow discontinuous dike that marks 

a high-angle fault. From the habit of such minor 

elements and the planar terminations of some thick 

elements, it seems likely that large parts of sill perimeters are marked by abrupt terminations. Such apparent or actual terminations are linear in plan, and 

some do not deviate for several miles, except for small 

steplike irregularities.

Large tabular plates of sedimentary rock, common 

in many areas and seemingly isolated in the diabase, 

provide insight into the mode of intrusion. In the 

exceptional exposures of the Salt River canyon, the 

relations of representative plates of Mescal Limestone 

and Dripping Spring Quartzite can be deciphered unequivocally. These plates are 5-300 feet thick, and 

some are at least 2 miles long and V2 mile wide. They 

are separated from roof or floor of the main host by 

layers of diabase a few feet to several hundred feet 

thick. Few of the larger plates can be demonstrated 

to be entirely separate, but many a few hundred yards 

across are definitely isolated in diabase. Some plates 

wedge out, whereas others end abruptly. The bedding within most is parallel to the strata above and 

below the diabase. Where a plate is terminated discordantly, the truncated beds match perfectly in thickness and sequence the truncated beds on the opposite 

wall of the discordant mass (fig. 10). The complete 

lack of even small-scale assimilation by the diabase 

can be demonstrated in many examples. In some 

canyon walls only one plate is seen; in other localities 

several plates are separated by layers of diabase.

The diabase masses over and under the individual 

plates were injected at different times. As shown 

diagrammatically in figure 10, a simple sill first intruded the section. A second intrusion followed the 

same general path but locally deviated and pried 

away a plate or plates of sedimentary rock, which 

therefore appear suspended in the composite layer. 

The sequence of intrusions is generally recognized only 

from the interrelations of chilled borders; in places an 

early sill is obviously crosscut or displaced by a later 

sill. In places tabular segments of an intrusion intermediate in age occur between sills in the same manner as plates of sedimentary rock. Individual intrusions invariably completely crystallized before the next 

one was injected. Excellent exposures of multiple intrusions along a single irregular plane and of chilled 

contacts between sills are readily inspected along the 

part of U.S. Highway 60 that traverses the north wall 

of the Salt Eiver canyon. Actually, it is uncommon 

that one chilled border will be welded tight to the 

border of an adjacent sill. Instead a thin seam of 

hornfelsed sedimentary rock, easily overlooked, separates adjacent sills. Where the diabases are altered 

and chill zones are obscure, the recognition of these 

seams can aid in deciphering the geometry of the sills.

The general lack of warping, or of wedging phenomena, even in the thinner plates of incompetent strata 

and the lack, along discordant contacts, of breccias or 

folds that would suggest intrusive drag indicate that 

the magma was introduced passively. Certain noteworthy small-scale faults and folds, considered in the 

section on structure, did originate with the inflationary process. These in no way invalidate the above conclusion; indeed, the occurrence of delicate tenuous 

tongues of diabase along some of the structures enhances it.
DIABASE STRATIGRAPHIC DISTRIBUTION

FIGURE 10. Relations of multiple injections of diabase to 

plates of host rock isolated between intrusions. A, First 

locally discordant diabase sill. B, Second sill intruded generally along same plane as first but locally discordant, so 

thiat large plates of strata remain between sills.

The large plates can in no way be confused with true 

xenoliths. Furthermore, the average sill can be traced 

for miles without xenoliths being seen. In the few 

localities where xenoliths do occur, they are confined to 

the margins of intrusions, and most of them are measureable in inches. The largest that has come to my 

attention is 4 feet thick and 15 feet long. The large 

plates are so different and so consistent in habit that 

they can be considered everywhere as proof of multiple 

intrusions.

Kecognition that the intrusions are typically multiple has practical aspects for geologists concerned 

with the geometry of sills in the mining areas. Certain 

diabase bodies and related features in the Magma mine 

at Superior, for example, have long been an enigma 

(Short and others, 1943, p. 37; Sell, 1960). Of two 

sills exposed by the mine workings, the upper one is 

said to be 2,000 feet thick, and the two are reported to 

have a combined thickness of more than 3,000 feet. 

Described as xenoliths within the 2,000-foot sill "are 

numerous isolated bodies of quartzite, which range 

from a few feet to 300 in thickness. * * * one of them 

extends * * * for a distance of 3,000 feet." It is stressed 

that these xenoliths "have the same dip and strike as 

the main mass of quartzite from which they are separated" and, that, aside from other considerations, 

"this attitude is difficult to understand if the blocks 

sank into a molten magma." Descriptions of the extent and attitude of the xenoliths, considered together 

with the known relations of such sedimentary blocks 

elsewhere and the complete absence of sizeable xenoliths within any single diabase intrusion, provide the 

basis for an explanation. Almost certainly the larger 

bodies of quartzite exposed in the supposedly simple

thick sill of the Magma mine are blocks or plates that 

locally separate two or more sills intruded at different 

times. The lack of any particularly thick sill in nearby surface exposures, which must represent the same 

complex of diabase intrusions seen underground, tends 

to support this interpretation. Because formations of 

the area are intricately broken by Tertiary faults, 

many of the smaller so-called xenoliths could be fault 

segments offset from the principal inclusions between 

separate sills.

If this 3,000-foot sequence of diabase is a composite 

unit, it is the thickest one known in Arizona. In review of this sequence and of other thick sheets, however, no single intrusion thicker than the 1,200-foot 

sill beneath Aztec Peak in the Sierra Ancha has been 

unequivocally confirmed.

In one area the overall habit of the diabase intrusions may be considerably different from that in another. The extremes of habit are fairly well represented by intrusions of the Sierra Ancha and by intrusions in and near the Salt Kiver canyon: (1) 

Multiple injections are characteristic of the canyon 

but are almost nonexistent in the Sierra Ancha, (2) 

the ratio of diabase to sedimentary rock is about 1:1 in 

the canyon, or perhaps 2-4 times that of the Sierra 

Ancha, (3) the representative sill of the Sierra Ancha 

is probably much thicker than the individual sill of the 

Salt Kiver area but is not greatly different from the 

composite sheets of the latter. Apparently corollary, 

(4) in the Salt Kiver canyon the formations especially 

susceptible to diabase intrusion were inflated much 

more intricately and at more horizons, (5) although a 

few steps of several hundred feet have been observed, 

most sills do not deviate more than a few tens of feet 

from a given stratigraphic horizon'in outcrop intervals of 5-10 miles, and (6) the many dikelike bodies 

that connect concordant parts of sills are mostly thin 

and of no great extent, whereas several connecting 

bodies in the Sierra Ancha are more than 500 feet 

wide and 2-10 miles long and traverse 500-1,500 feet 

of section. Comparable contrasts in habit have been 

noted farther south in the region.

STRATIGRAPHIC DISTRIBUTION

Through some understanding of the characteristics 

of strata most commonly split by sills, supplemented 

by knowledge of the horizons preferentially inflated 

 perhaps because of some local factor in immediately adjacent areas, moderate success is possible in predicting where a discordant step in a sill will feed into 

a concordant segment. Particular application is afforded in predicting sites of unexposed asbestos deposits, since they are contact metamorphic phenomena
56 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

adjacent to diabase. Such details could be used more 

widely to interpret displacements of the sedimentary 

rocks.

Sills are commonest and most consistent along horizons that separate layers of markedly different competencies. The extensive sills in the Mescal, for example, are at or near the contact between the massive 

algal member and the thinner bedded less competent 

lower member. Intrusions that are not at the contact 

between competent and incompetent strata are mostly 

within a few feet of the contact and in the incompetent unit. Sills in the Dripping Spring split the siltstone units of the siltstone member within a few feet 

of the more competent quartzitic arkose units or are 

along a horizon not far above the contact between the 

siltstone member and the massive arkose member. As 

a corollary, sills also are common along all unconformities in the Apache Group except the one between 

the Mescal and the Dripping Spring. The stratigraphic units that were particularly susceptible to intrusion are also those that contain the thickest intrusions.

Massive units of the younger Precambrian sequence 

are in few places hosts for concordant sills. These 

units are the arkose member of the Dripping Spring, 

the founder breccia of the lower part of the Mescal 

Limestone, the karst breccia that makes up the Meecal 

in some areas, and the arkose member and Chediski 

Sandstone Member of the Troy. Dikes and discordant parts of sills do cut across these units but are not 

so numerous as in the units that are hosts for concordant intrusions.

In most areas, the Mescal Limestone was intruded 

along more horizons and contains a greater amount of 

diabase than do other formations. The sills may be 

thick or thin. The Pioneer Shale was intruded by one 

or two sills, which are thin but persistent for long distances. Locally the siltstone member of the Dripping 

Spring is host to sills, but these are not so extensive 

as in the Mescal or Pioneer; over many areas of several tens of square miles, no intrusions occur in the 

formation. Through much of the Sierra Ancha southeast of McFadden Peak, one sill, 500-1,000 feet thick, 

splits the siltstone member. This is atypical; elsewhere in northern Gila County one or two sills, which 

are less than 200 feet thick, inflate the member; south 

of the Apache Mountains, sills that are 200-500 feet 

thick are common.

Sills are almost ubiquitous along or adjacent to the 

unconformity between the Troy Quartzite and the 

Apache Group; their configuration depends on the 

character of the rocks beneath the unconformity. 

Where the Troy directly overlay the massive unit of

the algal member of the Mescal or was separated from 

it only by basalt, one thick sill was typically intruded; 

where the argillite member intervened and the thinbedded upper unit of the algal member was remnant, 

two to five thin sills intruded the upper part of the 

Mescal.

Because even discordant intrusions are very sparse 

in the Chediski Sandstone Member, which makes up 

most of all Troy exposures, there has been argument 

as to the existence of diabase in the Troy. Actually, 

sills as much as 200 feet thick are common in the 

quartzite member, and the thick sill at Aztec Peak 

happens to be in the upper slabby-splitting unit of the 

member. Sills would be seen more widely in the Troy 

if this unit were extensively remnant.

Sheets of diabase 100-400 feet thick are common, 

and in some areas widespread, in the upper part of 

the older Precambrian terrane. In the granites these 

sheets are fairly regular in thickness and were emplaced along joints that had formed parallel to the 

pre-Apache surface. In the layered and schistose rocks 

of the older Precambrian, the diabase intrusions tend 

toward the same habit but are more irregular in outline. At depths of more than 500 feet below the base 

of the Apache Group, extensive diabase intrusions are 

practically nonexistent. Therefore, where broad exposures of granitoid or schistose formations do not 

include diabase, it seems reasonable to postulate that 

the Apache plus a considerable thickness of the older 

formations has been eroded and that remnants of the 

Apache Group are not likely to be found in the vicinity.

PETROLOGY

The normal diabase is a fine- to coarse-grained 

medium- to dark-gray holocrystalline rock of ophitic 

or subophitic texture. The aspect that distinguishes 

this rock from other dark-gray intrusive rocks of southeastern Arizona and causes it to be notably different 

from most diabases is its coarse-grained facies. Though 

definitely subordinate in volume to the medium-grained 

rock, the coarse facies makes up conspicuous parts of 

most bodies and, in places, whole sills. These coarse 

elements might well be termed ga'bbro if they were 

not prevailingly subophitic in texture. Feldspathic 

differentiates or the so-called red-rock types diabase 

pegmatite, aplite, and granophyre make up only a 

minute part of the diabase bodies but are conspicuous 

because of their light color and distinctive textures.

The diabase weathers light olive gray to moderate 

yellowish brown and can be distinguished at a distance 

by the characteristic greenish-gray or olive-gray hue 

of barren slopes. The larger poikilitic grains of pyroxene characteristically protrude on weathered sur-
DIABASE PETROLOGY 57

faces, which therefore appear knobby or warty. Spheroidal weathering to rounded boulders is common. The 

rock disintegrates to a light-brown or yellowish-brown 

granular soil that commonly includes abundant kernels 

that also represent the ophitic aggregates.

Much of the diabase disintegrates rapidly. Some 

newly blasted outcrops, first observed as tough and 

very firm rock faces, crumbled to sand during periods 

ranging from 3 to 10 years. The tough knobby outcrops that disintegrate very slowly are indicative of 

diabase which has had a minimum of deuteric alteration. In many localities the only resistant parts of the 

outcrop are conspicuous light or dark ribs, mostly less 

than a quarter of an inch in width, which project as 

much as 2 inches above the weathered surface of the 

diabase. The light, commonly pink ribs are concentrations of albite along joints, or minute fractures, in 

the diabase; the dark ribs are concentrations of hornblende, biotite, and chlorite. These ribs mark diabase 

that has been much altered deuterically.

Very fine grained chilled borders are characteristic 

of the normal diabase. These selvages are in few 

places more than 3 feet thick, and most are 6-18 

inches. The effects of chilling on grain size are commonly reflected through an additional 15-30 feet of 

sill borders. Very dense borders, in which only scattered microlites of plagioclase and magnetite can be 

recognized, are restricted to intrusions less than 2 feet 

thick.

Most of the normal diabase is composed largely of 

labradorite and clinopyroxene; olivine may be absent, 

but it generally occurs in sufficient amount that the 

rock can be characterized as olivine diabase. A texturally similar but subordinate facies consists mainly 

of albite and either pyroxene or amphibole. All variations between olivine diabase and albite diabase have 

been seen in one sill.

DIABASE

Thin sections of fresh olivine diabase show that 

labradorite (An55-An70 ) composes 45-65 percent of 

the rock; clinopyroxene, 10-25 percent; olivine, 5-20 

percent; biotite and hornblende, each 1-5 percent; and 

ilmenite-magnetite, apatite, and sparse sphene, a total 

of 2-8 percent. Subhedral plagioclase laths range in 

length from 1 to 4 mm. Commonly the outer third of 

the crystals is normally zoned. Grains of colorless or 

slightly brown or pink augite, apparently the only 

pyroxene in much of the rock, range in diameter from 

a fraction of a millimeter to 40 mm and are commonly 

5-^20 mm. A small part of the clinopyroxene in some 

thin sections is clear pigeonite (2V <30°). A little 

hypersthene has been observed in some specimens.

Olivine occurs as small rounded grains or clusters of 

rounded grains. A composition of about Fo60Fa4o is 

indicated from refractive indices of a very few specimens by the method of Deer and Wager (1939, p. 21). 

Strongly pleochroic yellowish-brown to reddish-fbrown 

biotite is widely distributed, but in few places abundant. Pyrite is sparsely distributed through most of 

the normal diabase, and in some specimens chalcopyrite can be identified.

Much of the olivine diabase shows late magmatic or 

deuteric alteration. Olivine is replaced by chlorite or 

by serpentine that includes the usual fine granules of 

magnetite. Olivine was also converted to talc, iddingsite, or bowlingite. The pyroxenes were replaced mostly by green hornblende and chloritic minerals; locally 

stilpnomelane or tremolite are abundant. The hornblende appears dark green or black in hand specimens. 

The reddish-brown biotite, seemingly an early mineral 

in the fresh rock, at least in part is a replacement of 

both pyroxene and olivine in the altered rock. Green 

biotite is abundant in diabase so thoroughly altered 

that the primary mafic minerals are difficult to identify. 

Epidote occurs only in a few localities, where it replaces both mafic minerals and plagioclase. Cores of 

labradorite laths commonly were converted to finegrained aggregates of sericite, clay minerals, and chlorite and in places were converted to prehnite. Rims became turbid; in some specimens these turbid rims are 

mostly albite. Ilmenite-magnetite is especially conspicuous in the coarser facies because it is there partly altered to leucoxene.

Albite diabase consists essentially of albite, augite, 

and hornblende. The dominant mafic mineral may be 

augite only moderately altered to hornblende or may 

be a fibrous gray-green hornblende. The fibrous hornblende is plumose in habit and strongly pleochroic 

from yellowish green to blue green, and thus is very 

different from the darker hornblende which occurs 

more sparsely in the olivine diabase. Olivine is absent. The reddish-brown variety is the dominant biotite, but the green variety is more abundant than in 

the olivine diabase.

In most of the albite diabase, the albite is grayish 

pink to orange pink. The turbid laths or turbid rims 

of the grayish-pink mineral are, in a small proportion 

of occurrences, microperthite. Commonly, alteration 

products completely obscure the twinning of the albite. Prehnite is a more abundant alteration product 

of the plagioclase in this rock than in the olivine diabase. In some intrusions the albite is mostly white 

to gray; where the diagnostic pink color is lacking, 

albite can be surmised in hand specimen from the
58 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

clouded rims and from the vague serrate boundaries 

which are characteristic of the albite laths.

Albite and quartz, as graphic intergrowths on a 

microscopic scale, occupy interstices between plagioclase laths in some of the diabase. This micropegmatite has been observed in small amounts in little-altered olivine diabase but occurs more abundantly in the 

albite diabase. Textures appear normal in hand specimen, but micropegmatite can be surmised in the field 

where exposures are lighter in color than most of the 

diabase. Ilmenite-magnetite occurs in extraordinary 

amounts with micropegmatite in some of the albite 

diabase, however, and causes these rocks to be almost 

black.

Much of the rock in which hornblende is conspicuous and several of the separate sills of albite diabase 

are coarser grained than the average diabase. Some 

is very coarse grained and includes labradorite or albite laths, 5-25 mm in length, and augite crystals, 

which may be slightly to largely altered to gray-green 

hornblende and which are as much as 50 mm in length. 

In some of this rock the augite tends to fracture as 

curved blades; many of these augite grains are 

twinned. Where the hornblende is obvious it is megascopically fibrous. Biotite tends to be more abundant 

in such rocks than in the average olivine diabase. 

Apatite needles are larger, seemingly more abundant. 

These needles and skeletal crystals of ilmenite-magnetite are readily seen by the unaided eye. Where the 

diabase is coarser, the ophitic texture may not be apparent in outcrops but is readily seen under the microscope. It is generally true, however, that ophitic texture is not so well formed in the coarser albite diabase 

and that it is lacking in some layers in which the 

plagioclase content is uncommonly high.

Gravity differentiation is subtly apparent in some 

sills of the Sierra Ancha that are more than 500 feet 

thick. Certain layers are more feldspathic and others 

are more olivine rich than the average diabase. Olivine, however, is not largely restricted to a zone low 

in the sill, as in the classic Palisade sill of New Jersey 

(Walker, 1940, p. 1067), but is dispersed through 

three-fourths of the body. The olivine concentrations 

nowhere appear high enough to account for the low 

silica content (43-49 percent) that seems to be general 

in these diabases. Olivine occurs in greatest amounts 

through an interval 200-300 feet thick, which is slightly below the middle of a sill. The olivine of certain 

layers is coarser than that in the rest of the interval. 

These layers and zones are not sharply defined but 

are gradational into the rock above and below. Nevertheless, the zone in some places forms a resistant bench 

that can be mistaken for a separate intrusion.

FEUD SPATHIC D-IFFUBENTJATES

Outcrops of the feldspathic differentiates contrast 

strongly with outcrops of diabase because they have 

different textures, orange to pink colors, and general 

staining by limonite. Furthermore, the transition between the differentiates and the diabase of normal 

texture is rather abrupt. Commonly it is a zone not 

more than a few feet wide; in many places the transition to the larger masses of granophyre or pegmatite 

is only a few inches wide, and some dikes of quartzfree aplite have knife-edge boundaries with diabase.

DIABASE PEGMATITE

Diabase pegmatite is the most voluminous of the 

feldspathic differentiates. It occurs in two habits. As 

lenses or crudely lenticular dikes, a few inches to a 

few feet in width, or less commonly as irregular 

masses, a few inches to several feet in diameter, it 

may be found in any part of a sill, and such masses 

occur in many sills throughout the region. As roughly 

tabular masses, a few feet to tens of feet thick and 

tens to several hundreds or even a few thousands of 

feet across, the pegmatite roughly parallels and is not 

far below the upper boundary of a sill. The large 

tabular masses have been seen only in a few areas in 

the Sierra Ancha.

The pegmatites consist of irregular or blotchy aggregates of very coarse grained clusters of light and dark 

minerals. Plagioclase laths commonly are 3-6 mm 

wide and 1^-4 centimeters long, and some as long as 

9 cm have been seen. Augite grains are 1-5 cm across 

and may be as long as 10 cm. Skeletal plates of 

ilmenite-magnetite tend to be concentrated with augite 

crystals. The turbid light-gray to grayish-orangepink plagioclase is apparently all albite. Sphene is 

a conspicuous accessory in some occurrences. Pyrite 

occurs only sporadically.

The pegmatite ranges from a dark rock which contains abundant augite and ilmenite-magnetite, through 

a pinkisli-gray rock which contains dark-greenish-gray 

fibrous hornblende but little other essential mafic mineral, to a yellowish- or pinkish-gray rock which is 95 

percent plagioclase. A particular mass may include 

this entire range or be nearly uniform in mineralogy. 

The darkest type apparently approximates the mineral 

composition of the diabase that includes no olivine 

and no hornblende except as an alteration of the pyroxene. The variety that is probably the most plentiful 

is composed almost entirely of albite and hornblende.

GRANOPHYRE

Granophyre occurs mostly as irregular tabular bodies 

which are a few feet to a few tens of feet thick and 

a few tens to a few hundreds of feet in horizontal di-
DIABASE PETROLOGY 59

mensions. These bodies closely parallel the upper 

boundaries of sills, and most are contiguous to a discordant contact. Most outcrops are pervasively decomposed owing to leaching of ubiquituous pyrite. Some of 

the tabular bodies of pegmatite include small masses of 

granophyre.

Equidimensional grains of orange-pink feldspar, intergrown graphically with quartz, give the granophyre 

a predominantly orange hue. Euhedral or subhedral 

quartz grains can be observed with the aid of a hand 

lens. Under the microscope, the feldspar is so clouded 

with hematite and limonite that identification is difficult; a large part is albite, but potash feldspar may 

also be present. The feldspar grains range in diameter from 1 to 10 mm. Some occurrences of granophyre are very rich in quartz and almost lacking in 

mafic minerals; others include 10-25 percent of a 

rustic-weathering fibrous gray-green amphibole and 

abundant thin plates of ilmenite-magnetite. Transitional zones between granophyre and diabase contain

augite.

APLITE

The diabase aplites comprise two varieties, one 

quartz free and one quartz bearing. The two varieties 

are texturally similar but differ considerably in mode 

of occurrence.

The quartz-free aplite occurs mainly as veins and 

subordinately as dikes, which are insignificant in 

volume but :are nonetheless conspicuous because they 

are equigranular and pink against the dark diabase 

that is everywhere their host. The veins are a fraction of an inch to 6 inches in width and are characterized by gradational borders which are from onetenth of an inch to a few inches in width. The less 

common dikes are sharp walled, and few are more 

than 12 inches wide. The quartz-free aplite bodies 

occur sparsely in most sills and are not restricted to 

any particular part of a sill; they are most abundant 

in the diabase adjacent to a discordant contact. Where 

the veins are especially numerous, the host diabase 

commonly is albitic.

Quartz-free aplite is composed mainly of pink feldspar, which constitutes 80-95 percent of the rock. The 

anhedral equant feldspar grains typically range in 

diameter from 0.05 mm to 2 mm. In thin section, the 

feldspar is clouded with claylike alteration products. 

Most of the feldspar is untwinned albite, but potash 

feldspar may also exist. Typically the aplite is mottled with gray-green hornblende, and commonly with 

epidote, both interstitial to albite. Sparse small miarolitic cavities are lined with hairlike needles of hornblende. Sphene is sparse but ubiquitous. In the 

borders between aplite veins and diabase, the aplitic

texture appears to be superimposed on the intergranular texture of the diabase, the result being a saltand-pepper effect.

The quartzose aplites occur mostly as irregular crudely lenticular bodies, a few inches to several feet thick, 

that invariably are adjacent to a feldspathic host rock. 

Most bodies lie parallel to contacts between diabase 

and the siltstone member of the Dripping Spring 

Quartzite, and these may have outcrop lengths of as 

much as several hundred feet. Similar smaller lenses 

are along contacts between the argillite member of the 

Mescal and the diabase or are along contacts between 

basalt flows and diabase. Some quartz-aplite layers 

form a vague-boundaried transitional zone between 

hornfelsed host rock and other feldspathic differentiates. To a lesser extent, the aplite occurs as dikes, a 

few inches to 5 feet wide, that either are in diabase 

sills but close to their borders or are in the host rock 

and adjacent to a sill. All quartzose aplite occurrences 

noted to date have been in or near the Sierra Ancha.

Quartz micrographically intergrown with pink to 

gray feldspar makes up a few percent to more than 50 

percent of this rock. In many specimens, minute 

rounded grains of diopside are scattered through the 

rock. Biotite and chlorite have been seen in some thin 

sections. Pyrite, absent from much of the quartz-free 

aplite, is minutely disseminated in patches in parts of 

virtually every body of quartz aplite. It occurs throughout some bodies and causes their outcrops to be conspicuously stained or even crusted with limonite.

MODE OF FORMATION

The spatial relations of the feldspathic differentiates indicate that they do not represent later and independent intrusions but that they can be visualized as 

products that evolved in two ways from volatile-rich 

residues fractionated from nearby parts of the host 

intrusion. Once accumulated, the residues either (1) 

crystallized to form the quartz-free differentiates or 

(2) reacted with certain wallrock materials to form 

the quartz-bearing differentiates.

Most of the small randomly oriented pegmatite 

bodies are narrow and elongate and were obviously 

emplaced along joints and faults which opened after 

the diabase had crystallized enough to sustain fractures. That they could not have been effects of assimilation is indicated by the form of these pegmatite 

bodies, of the narrow dikes and veins of quartz-free 

aplite, and of the albite veinlets that are small-scale 

manifestations of the aplites, as well as by their distribution through sills. Apparently volatile-rich materials migrated locally to fill the fracture openings and 

to crystallize in and near them.
60 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

The positions of concentrations of quartz-free differentiates suggest the paths of migration and a reason 

for large accumulations of the late fluids. The small 

bodies are especially numerous, and somewhat more 

persistent, in the diabase adjacent to hanging walls of 

some steep-dipping discordant bodies and thus mark 

the sites favored as channelways. Extensive bodies of 

pegmatite exist only in the upper parts though not 

necessarily at the very margin of thick sills. The 

thickest parts of these layers occur where the discordant contact of a tabular feeder body joins the concordant roof of the sill. The layers have their maximum lateral dimension along this intersection and their 

minimum dimension exceptionally as much as a mile 

away from it. As the high point and terminus of the 

cliannelway, this is the logical site, and empirically 

the only one, for large amounts of residual materials 

to accumulate.

Quartzose differentiates only occur contiguous to 

tops of sills, they commonly border flat-dipping discordancies, and the thicker and more extensive bodies 

invariably contain feldspathic xenoliths. The only 

extensive masses are in sills that intrude the siltstone 

member of the Dripping Spring. Lesser bodies do 

exist along walls of sills intruded into the argillite 

member of the Mescal and into the basalt flows. These 

associations set them apart somewhat from the tabular 

masses of diabase pegmatite; nevertheless their details 

provide cause to speculate on a metasomatic origin for 

large masses of all the differentiates.

Details are best seen in the tabular masses of quartzose aplite that separate siltstone of the Dripping 

Spring from diabase or one of the other differentiates. 

In a few of these masses the relict bedding in xenoliths seems without exception to parallel the bedding 

of the adjacent siltstone. Apophyses from aplite 

masses commonly extend as narrow dikes into the host 

rock. From many such dikes thin tongues of similar 

aplite extend a few inches to several feet along the 

bedding of the hornfelsed siltstone. The tongues merge 

gradationally into the siltstone, and within the aplite, minute details of the delicately laminated strata 

are preserved. Furthermore, where the parent dike 

crosses strata that include several aplite tongues, the 

dike may be wider than elsewhere. These features 

suggest that the quartzose aplite is of metasomatic 

origin.

In some occurrences, certain dikes of quartz aplite 

appear definitely intrusive. These dikes cut cleanly 

across and dilate coarser differentiates, normal diabase, the host sedimentary rocks, and even vagueboundaried xenoliths dispersed in the main tabular 

mass of aplite. Where these dikes do vary slightly in

width, the variations do not seem to reflect the rock 

type traversed, as is common in dikelike bodies that 

are replacement phenomena. Furthermore, though 

most of the xenoliths in the main layer of aplite have 

vague, boundaries, appreciable numbers have sharp 

boundaries, and in some occurrences all are randomly 

oriented.

An elongate pod-shaped mass of differentiates exposed on the north wall of the canyon of Reynolds 

Creek 2^ miles south-southeast of McFadden Peak 

(in the SW^4 sec. 7, T. 6 N., R. 14 E., McFadden 

Peak quadrangle) shows relations to host rocks particularly well. Here an extraordinarily thick layered 

mass, exposed continuously for a mile along the strike 

of a locally discordant step in the sill contact, separates normal diabase from overlying sedimentary formations. The upper part of the differentiate body consists of quartzose aplite as much as 60 feet" thick. In 

places the top of the aplite layer is against limestone 

15 feet above the Mescal-Dripping Spring contact; in 

other places it is against siltstone or arkose 35-40 feet 

feet below this contact. Everywhere the contact between aplite and hornfelsed sedimentary rock is vague. 

Dikes of the aplite occur in both the overlying sedimentary strata and the underlying granite. Downward the aplite layer merges, through a transition zone 

a few inches to a few feet thick, with a light-gray uniformly coarse grained rock termed hornblende granite 

in the field. This granite, generally 30-50 feet thick 

but locally about 80 feet thick, merges abruptly downward with a coarse-grained gabbroic rock. This gabbroic rock grades downward through an interval of 

5-20 feet into ordinary medium-grained diabase. The 

body of layered differentiates is locally as much as 150 

feet thick where its contact with the host rock is 

markedly discordant. As the contact becomes concordant downdip, the body pinches out, and normal 

diabase is in contact with the sedimentary formations.

Xenoliths apparently settled differentially into this 

mass, perhaps partly as a function of size (Lovering, 

1938) and partly as a function .of impedance offered 

by different strata of the crystallizing magma. Small 

angular fragments of siltstone and arkose, heterogeneously oriented, occur in great numbers above the granite. Many are so thoroughly reconstituted that they 

are difficult to distinguish from the aplitic matrix. Two 

small rounded blocks of fine-grained diabase, perhaps 

remnants of a chilled selvage of normal diabase, were 

seen a few feet below the top of the aplite. Xenoliths 

are sparse through most of the granitic layer, but they 

increase in number downward and can be seen every 

few feet along the basal 10 feet of the layer. Those 

20 feet or more above the bottom of the granite range
METAMORPHISM ASSOCIATED WITH DIABASE 61

in largest dimension from 6 inches to 2 feet and are 

randomly oriented, whereas those in the basal 20 feet 

of the granitic layer are larger, as much as 15 feet long 

and 1-4 feet thick, and many are oriented parallel to 

the bottom of the granite layer. Some of these large 

xenoliths are bordered by a reaction zone 1-6 inches 

thick that is texturally like some of the highly feldspathic diabase pegmatites. Many, however, have fairly sharp contacts with the granitoid host. Xenoliths 

are uncommon in the underlying gabbroic facies; the 

few seen were small and had narrow pegmatitic rims.

About a mile to the southeast, in a similar setting 

and probably along the same contact, a layer of granophyre 10-30 feet thick is separated from the siltstone 

by only a few feet of quartzose aplite. The layers of 

granophyre and aplite are ill defined; the aplite includes moderate numbers of xenoliths, but the granophyre includes only a few. This occurrence of granophyre and aplite is more representative of the composite bodies of differentiate generally seen.

Seemingly the extensive pegmatite bodies are the 

differentiates that formed where xenoliths did not contaminate the accumulating residuum. Some bodies, as 

much as 20 feet thick, border the upper contact of a 

sill. Sporadic patches of granophyre that might represent assimilated xenoliths occur in these bodies, and 

some pegmatite layers merge upward or laterally with 

bodies of granophyre that are in turn against the host 

rock. In a few places one or two 10-20-foot pegmatite 

layers also occur separately within the interval 60-90 

feet below the top of a sill. These layers are commonly uniform in thickness along several hundred feet of 

outcrop; but in places they pinch and swell, and where 

one layer thins the adjacent layer may thicken. Some 

layers have subparallel apophyses, separated from the 

main mass by thin wedges of diabase. Small irregular 

lenses of pegmatite may be abundant in the diabase 

between the principal pegmatite layers. The geometry 

of all except the border layer would seem to preclude 

an origin that involved reaction with xenoliths. Only 

the border layer contains granophyre, and peculiar 

coincidences of xenolith distribution would be required 

to account for the interrelations between the other 

layers and their apophyses.

The quartzose differentiates occur not only in the 

part of a thick intrusion where residual fluids accumulated in volume, but also where xenoliths dropping into 

a mush of crystaffizing material triggered the formation of rock different from the pegmatite. The only 

effective contaminants were those with at least a modest feldspar content. Xenoliths of quartzite or limestone lack any features that would suggest assimilation 

and nowhere have caused similar effects. Possibly a

considerable volume of contaminating materials was 

required for this reaction; the quartzose differentiates 

generally occur where diabase magma was insinuated 

along preexisting shatter zones that could furnish 

great volumes of material. Further, some hint exists 

in the Reynolds Creek examples that xenoliths in different sizes, numbers, and surface areas in different 

parts of the residual magma may have had a considerable effect on the type of rock that finally crystallized. 

The quartzose aplite in part formed in place by the 

soaking of feldspathic rocks with magmatic fluids. 

But in part it must also represent a melt modified by 

reaction and injected as discrete dikes into a variety 

of earlier formed rocks.

METAMORPHISM ASSOCIATED WITH DIABASE

Except for thermal metamorphic effects induced at 

the time of intrusion of the diabase, the Apache and 

Troy strata generally are metamorphosed no more 

than the Paleozoic formations that overlie them. However, because diabase intrusions are virtually ubiquitous in Apache strata, the effects of thermal metamorphism associated with the diabase intrusions are 

of regional scale. The heat was probably transferred 

mainly by solutions emanating from the diabase, but 

carbon dioxide, released as a result of the dissociation 

of dolomite, must have been an important transporting 

agent in the Mescal Limestone. Intrusion of diabase 

caused small-scale fracturing, particularly in carbonate rocks, and this fracturing was an additional factor that caused wider distribution of metamorphic 

products in the carbonate rocks than in other rock 

types. The mineralogic changes were brought about 

largely by reconstitution of the original rock-forming 

minerals. Some constituents were added from the diabase, but whether or not the additive process had an 

appreciable role in the metamorphism is uncertain.

Where the metamorphic effects are inconspicuous, as 

in many weathered outcrops, the original lithology 

may be misinterpreted as generally has happened 

with the lower and algal members of the Mescal. Except in isolated small outcrops, however, the metamorphic effects are not such as to cause misidentification of a formation.

Quartzose rocks and the coarser grained more massive feldspathic rocks were the least susceptible to 

alteration. Quartzitic parts of the Troy in few places 

exhibit effects of metamorphism; immediately adjacent 

to a diabase intrusion the quartzite member locally becomes more dense and vitreous. Locally, the porous 

sandstone of the Chediski Member was converted to 

quartzite through widths of at least 100 feet. Some 

wedges of the arkose member of the Troy and of the

248-121 O - 67 - 5
62 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

lower member of the Dripping Spring Quartzite, 

which are engulfed in diabase, were recrystallized, 

through thicknesses of as much as 100 feet, to a hornfels in which discrete grains of quartz appear to be 

embedded in a dense matrix of potash feldspar. The 

original bedding structures were largely obliterated, 

and in isolated outcrops the hornfels of the Troy arkose is difficult to distinguish from that of the Dripping Spring. In other settings the two arkoses were 

little altered.

Generally the pre-Apache rocks are little modified 

adjacent to diabase intrusions. Weathered outcrops of 

quartz monzonite adjacent to diabase tend to be more 

abundantly stained by limonite than those that are not 

intruded, but textural modifications are generally lacking. Thoroughly sheared quartz monzonite or the 

granitic regolith that underlies the Pioneer Shale may 

be more strongly metamorphosed. One modification 

consists of abundant egg-shaped relicts of partly absorbed microcline phenocrysts in a fine-grained olivegray or dark-greenish-gray matrix of feldspar, quartz, 

dark micas, and other dark minerals. From a distance 

this rock would seem to be an "orbicular" diabase. In 

other occurrences, the phenocrysts have been completely destroyed, and their places are marked by 

elongate blebs of bluish-gray opalescent quartz in a 

similar matrix; the quartz blebs are as much as 1 cm 

in diameter and in places have coalesced as vermicular 

aggregates. In some of the less altered regolithic material, broken fragments of feldspar remain angular, 

but the quartz grains are rounded. Certain regoliths, 

in which only granules of quartz and feldspar remained after the fines had been winnowed, were reconstituted to an orange-pink rock in which abundant 

coarse blebs of quartz lie in an almost textureless 

matrix of feldspar.

Because strata of the siltstone member of the Dripping Spring were intruded more extensively than the 

more massive sandstone, they exhibit metamorphic 

effects more widely. The siltstone units were more 

susceptible than the arkose units of the member. Strata 

a few feet to a few tens of feet from diabase are better 

indurated, extremely tough, and where weathered are 

more abundantly stained by limonite; but to the unaided eye most of the hornfels otherwise is little different from the original siltstone or arkose. Hornfels 

coarse enough to contrast texturally occurs in only a 

few areas adjacent to some of the thicker sills. Where 

a thick intrusion discordantly invaded strata previously shattered by faulting, zones of hornfels as much as 

100 feet thick exist, and the coarse hornfels may be 

conspicuous in a zone 5-30 feet thick. The coarser 

hornfels is pinkish gray to medium gray and occurs as

thin layers or lenses that crudely parallel bedding or 

as veinlike zones or irregular masses that follow joints 

or fractures across- the bedding. All are gradational 

into the dark fine-grained hornfels. Some of the more 

extensive examples have been considered on pages 59-60, 

in discussion of the quartzose aplites of metasomatic 

origin.

As seen under the microscope, the potassium feldspar of the hornfels is coarsened in texture, and in 

places is replaced by albite. In the coarsest hornfels, 

quartz and feldspar commonly exist as micropegmatitic intergrowths, and the mafic minerals are clear 

anhedral grains of pyroxene and an amphibole. Biotite may be common. Locally, sphene is an abundant 

accessory. In freshly broken hornfels thickly disseminated patches of fine-grained pyrite may be apparent 

megascopically.

The tuffaceous mudstone and siltstone of the Pioneer 

Shale generally are metamorphosed for a few feet adjacent to thin sills, and the entire formation may be recrystallized to some degree where it is intruded by 

thick sills. Outlines of the microscopic shards are 

obliterated wherever there has been the slightest reconstitution of the rock. The dusky red strata were 

metamorphosed to hard dense grayish-orange to grayish-green hornfels, in which bedding structures are 

largely obliterated. The hornfels is mottled throughout with gray patches, mostly less than 1 mm in diameter, of finely divided dark minerals. In partially 

recrystallized strata the hornfels retains a pink or red 

tinge.

The claystone and siltstone strata that originally 

made up the upper member of the Mescal were converted to argillite virtually everywhere. The sparse 

dolomite beds that were interlayered in a few localities were invariably converted to silicate-bearing limestones. Adjacent to thick sills the argillite is a reddish 

orange to almost black very dense novaculitic rock, 

which contains abundant conspicuous metacrysts or 

knots of metamorphic minerals. In some beds the knots 

are elliptical, are as much as 3 mm in diameter, and 

are mostly aggregates of fine-grained dark mica. Other 

beds are irregularly mottled with light-brown to darkyellowish-brown aggregates, which consist of a very 

fine grained mineral or minerals and which do not exhibit cleavage, or are thoroughly flecked with lathlike 

metacrysts, 1-10 mm long, of greenish-gray amphibole. 

Parting planes of the rock containing the amphiboles 

resemble surfaces covered with miniature bird tracks.

The basalt flows adjacent to some sills were thoroughly albitized and locally veined by epidote. Where 

recrystallized the basalt texturally resembles the diabase aplite but is mottled greenish gray to reddish
METAMORPHISM ASSOCIATED WITH DIABASE 63

orange. The gray patches are mostly chlorite, biotite, and fibrous amphiboles(?), whereas the orange 

rock consists mainly of very clouded feldspar, probably mostly albite. In a few localities both argillite 

and basalt were converted to an orange-pink rock which 

cannot be distinguished in hand specimen from the 

diabase aplites. This rock forms irregular masses, a 

few inches to 30 feet across, which gradationally interfinger with less altered rock along bedding in the 

argillite and along planes of fracture in the basalt.

The carbonate members of the Mescal are the units 

most widely and pervasively metamorphosed by diabase. The original strata of dolomite and cherty dolomite were dedolomitized and converted to calcitic 

limestone that contains concentrations of magnesian 

silicate minerals. Where a concordant sill at least 50 

feet thick intruded the carbonate section, the entire 

section was generally dedolomitized. Where wide dikelike parts of diabase sills cut across the carbonate 

strata, the dedolomitized zones extend as much as 

2,000 feet along the bedding. Metamorphic halos a few 

tens to a few hundred feet wide border dikes that are 

not more than 25 feet wide. The thin-bedded lower 

member of the Mescal tends to be more completely converted to limestone thari the massive algal member.

The cherty dolomite yielded a variety of products 

that largely reflect the original composition. It was 

most commonly reconstituted to mixtures of tremolite, 

diopside, talc, serpentine, and calcite in a host of finegrained sugary-textured calcite limestone. Diopside, 

tremolite, and talc were formed first, and they occur 

as irregular layers, lenses, and nodules that mimic the 

form of the chert concentrations. Where chert was 

moderate in amount, it was entirely replaced, and 

relicts are nowhere abundant, except in strata that 

once contained extraordinarily large concentrations of 

secondary chert. Dolomite virtually free of chert was 

altered to limestone in which the sparse silicate concentrations commonly are almost entirely serpentine. 

Serpentine is by far the most abundant silicate in tho 

limestone and tends to obscure the interrelations of the 

earlier formed minerals and chert. It is also the most 

widely distributed of the silicate minerals and is found 

in parts of the dedolomitized zones most distant from 

diabase intrusions, where the other silicates apparently 

are absent.

Serpentine may be in paper-thin wispy discontinuous layers, in tabular zones locally as much as 8 feet 

thick, in microscopic blebs, or in elliptical nodules 3 

feet in maximum diameter. Most of the serpentine replaced the early silicates or chert, and grossly outlines the original chert zones or chert nodules. Considerable amounts of serpentine, however, replaced the

carbonate rock along points and faults, and some of the 

last-formed serpentine is in sharp-walled veins that 

cut across earlier concentrations of all minerals.

In hand specimens, masses dominantly of diopside 

are light gray to greenish gray. Diopside occurs as 

minute anhedral grains or aggregates of such grains. 

Tremolite occurs as very fine needles or radiating aggregates of needles. Aggregates of both minerals are 

tough and difficult to break. Small amounts of finegrained talc are associated with diopside and tremolite; 

its paragenetic relations are not clear, owing to envelopment in the ubiquitous serpentine. All the silicates 

weather white and are much less conspicuous than the 

chert, which etches out boldly where remnant.

The dense serpentine is soft, exhibits a waxy or 

greasy luster on freshly exposed surfaces, and has a 

wide range of colors in hues of yellow, green, brown, 

and black. Microscopic inclusions of other silicates, 

calcite, and magnetite commonly cause variations in 

luster, color, and hardness. The late-formed vein serpentine is mostly grayish orange pink and in a very 

few places pale green. The dense serpentines are fibrous, but the fibers generally are too small to be resolved with the optical microscope. Selected specimens from the various occurrences studied by optical 

microscope, differential thermal analysis, electron microscope, and X-ray diffraction have all been identified as chrysotile (E. J. Young, written commun., 

1959). Many serpentine layers contain sharp-walled 

veins of megascopically fibrous chrysotile. Locally 

these occur in volume and constitute the asbestos deposits that have been prospected or mined at more 

than 150 sites in Gila County.

The silicate minerals just described were formed 

from dolomite that contained very little impurities 

other than chert; the metamorphic products derived 

from clayey or silty dolomites have been given little 

attention. The silty or sandy dolomite matrix of the 

basal breccia of the Mescal has been converted to a 

dark-greenish-gray soft rock, apparently a very fine 

grained mixture of serpentine, chlorite(?), and calcite. 

In some areas much argilliceous material was concentrated near the top of the algal member during preTroy weathering. As a result of metamorphism, this 

clayey and siliceous dolomite yielded large amounts of 

gray-green micaceous minerals, which occur in thick 

hexagonal books as much as 5 mm in diameter. These 

metacrysts are in a matrix of calcite, serpentine, and 

other silicate minerals too fine grained for microscopic identification. They consist of at least two mineral species, one apparently a chlorite, the other unidentified. Locally, beds of apparently similar original lithology are now composed of greenish-gray
64 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

tremolite as randomly oriented blades which are as 

much as ^ inch wide and 3 inches long. In places the 

tremolite occurs as irregular veins of woodlike cross 

fibers. Small amounts of still other silicates occur 

sparsely; an example is dark-yellowish-tbrown garnet, 

which has been noted in only three localities.

Magnetite is present in many localities but is not 

as prevalent a contact-metamorphic mineral as in many 

other areas where carbonate rocks are intruded by 

diabase. Locally magnetite is sparsely to abundantly 

disseminated through 20-100-foot thicknesses of limestone; a few such occurrences have an areal extent of 

several acres. The woody tremolite and coarse micaceous minerals occur in the same or adjacent beds. This 

magnetite was probably reconstituted from dolomites 

that were highly hematitic and argillaceous as a consequence of weathering. In many places, massive 

magnetite replaced limestone, which was much fractured within a few feet of a discordant diabase intrusion. Some of this magnetite is disseminated in the 

limestone, but most of it forms discrete narrow veins or 

small pods; a few veins are 1/2-3 feet thick and several 

tens to a few hundred feet long. These occurrences 

may represent iron expelled from the diabase magma 

or iron picked up from the dolomite and concentrated

along shear zones by solutions emanating from the 

diabase.

LATE PRECAMBRIAN DEFORMATION

The Apache Group and the Troy Quartzite were 

folded and faulted prior to intrusion of the Precambrian diabase. However, the effects of this deformation 

have been obscured by the intrusion of diabase and, in 

the southern part of the region, by deformation of 

post-Paleozoic age, so that this early period of deformation has not previously been recognized. Details of 

prediabase structures have been studied mainly in the 

Colorado Plateau.

PREDIABASE DEFORMATION

The earliest deformation that affected the younger 

Precambrian rocks were the episodes of gentle warping 

that elevated the Mescal for subaerial erosion before 

the basalt flows were laid down and again before the 

Troy was deposited. Downwarping in northwestern 

Gila County formed the 'basin in which the arkose 

member of the Troy was deposited (fig. 11).

After deposition and lithification of the Troy, but 

before diabase intrusion, younger Precambrian strata 

were strongly folded and faulted along widely spaced 

narrow belts; between these belts the formations reDragoon Little Dragoon 

Mountains Mountains

Holy Joe 

Peak Winkelman 

I

Quartzite member of Troy y;;-:^:/;v 

skV Sandstone Member'of Troy

Dripping Spring Quartzite

.Mescal Limestone ;'^^v;^:->;;^r:^-:^^:-'A'Basalt;^^

^.^-^ M'T.' ' ' ' ' ' ' ' . ' ' ' ' t Ja'l .J__g>*B

oder'Precambrianformations10 10I

20 30 MILES

Dragoon Little Dragoon 

Mountains Mountains

HORIZONTAL SCALE

FIQUBE 11. North-south geologic section after the Troy Quartzite was deposited. Vertical

Contacts dashed where there are no

Winkelman 

I

10 10 20 30 MILES

HORIZONTAL SCALE

FIGURE 12. North-south geologic section at the end of Devonian time. Structures are diagrammatic, but the positions of the
PREiDIABAtSEi DEFORMATION 65

mained horizontal. Some of these belts can be followed 20^40 miles before they disappear beneath a 

cover of younger rocks. The belts commonly have a 

sinuous trace, but all have a general north or northnorthwest trend. Because they were loci for deep erosion during late Cenozoic time, some are readily seen 

along major north-south tributaries to the Salt Biver, 

such as Cherry Creek and Canyon Creek.

The canyon of Cherry Creek in its southerly course 

through the McFadden Peak quadrangle exposes a 

greater variety of structural details than is generally 

found along one belt. A single nearly vertical fault, 

or in places a zone of steep subparallel faults 1,000- 

5,000 feet wide, has been traced along the west wall of 

the canyon from a locality 2 miles south of the quadrangle to a locality 2 miles north of it; the belt probably extends much farther to the north and south. In 

the southern 10 miles of this exposure, most of the 

many faults in a zone about a mile wide strike N. 5°- 

10° W. In the central part of the quadrangle, for a 

distance of about 8 miles, the zone of faults is narrow or is represented by one fault that strikes N. 30° 

W. Along the northernmost 5 miles, a single fault, intruded by a wide dikelike apophysis from a diabase 

sill, strikes N. 10° E. Thus, along a 23-mile length,

Mescal 

Mountains

Apache

Globe Mountains 

I I

major segments of this fault zone differ in strike by as 

much as 40°. A fault of greater throw than the rest 

generally defines the west edge of the zone; the lesser 

faults that strike obliquely to the local trend of the 

belt terminate against this principal fault.

Along or immediately east of Cherry Creek proper, 

the formations were folded into a westward-dipping 

monocline aibout a mile wide that persists the length of 

the structural belt. From the horizontal strata east 

of the belt, there is a gradual transition westward to 

dips of 3°-10° W., which are characteristic of most of 

the monocline. Near Cherry Creek the dip increases 

rapidly, and where the monocline terminates against 

the principal fault west of Cherry Creek, dips of 20°- 

30° are common; in a few places near the fault, beds 

are almost vertical. Along parts of the canyon, some 

displacement downward to the west is due to faulting 

as well as monoclinal folding, and thus in places the 

Cherry Creek structural belt resembles half of a graben. The faults are intricately intruded by diabase 

dikes or dikelike discordant parts of sills along most 

of the length of the belt, and the monocline generally 

terminates against an intrusion.

Immediately west of the principal fault of the structural belt, sedimentary formations generally dip steepChristopher Mogollon 

Mountain. Rim

'.-!-.-:.: '--';'-: :.vChediski Sandstone Member of Trby;.V'.- :.--'' : -V/' ::-:

/ \' \ "_' " " /N'/V ! "" i 1" \~x X N ^>, ,L\-^\ 

Older Precambrian formations /\ I 

'-T/r/- --/^'-r^v'^

0 3000 FEET 

I , . . . I . i .. I , , , i I 

VERTICAL SCALE

exaggeration overemphasizes pre-Troy folding and angularity between Troy and Apache strata, 

remnants to confirm interpretation.

Mescal 

Mountains Globe

I

Aztec 

Peak

Christopher Mogollon 

Young Mountain Rim 

I ^^-^ \

Martin Limestone and overlying formation

3000 FEET 

i i i I i i i i I i i i i I

VERTICAL SCALE

Troy Quartzite and the Apache Group relative to the pre-Bolsa and pre-Martin unconformities are probably representative.
66 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

ly west. For considerable distances along the fault, 

the formations are vertical or nearly vertical, and in 

places the strata within a few hundred feet of the fault 

are overturned to the east, in a few places dipping less 

than 50°. This intense folding affects only a narrow 

zone; 500-2,000 feet west of the principal fault, the formations are virtually horizontal and continue so westward into the Sierra Ancha. The Troy Quartzite, the 

Apache Group, and the underlying granite are involved in this fold. Because some of the formations 

are very competent and because the stratigraphic 

thickness (about 3,000 ft exposed) was too great to be 

accommodated in such a tight fold, the folded section is 

thinned by shearing. In places incompetent parts of 

formations are missing, and in a few places competent 

parts are duplicated along a shear zone roughly parallel to the steep fault that bounds the fold on the east.

Displacements caused by faulting and folding within the Cherry Creek structural belt are noteworthy, but 

corresponding horizontal strata on both sides of the 

canyon outside the deformed belt are if one ignores 

local displacements caused by later diabase inflation 

at about the same elevations. Within the belt, stratigraphic displacement locally exceeds 1,500 feet; the 

Troy Quartzite, for example, is locally in juxtaposition with pre-Apache granite.

Including all elements, the Cherry Creek fold-fault 

belt ranges in width from 2 to 4 miles. Other structural belts are similar in their gross details. Few, 

however, show such intense folding, and most are 1-2 

miles wide.

In general, the belts are several miles apart; between them the formations, though locally displaced 

by high-angle faults of later date, are practically horizontal. Canyon Creek, which is 10-12 miles east of 

Cherry Creek, also follows a north-northwest fault 

zone downstream from a locality 23 miles north of its 

junction with Salt Eiver; south of the Salt Eiver this 

belt can be traced an additional 19 miles before it is 

covered by Cenozoic gravels. West of Cherry Creek 

4-10 miles, another narrow belt of north-northeast 

trend has been traced from the southwest corner of 

the McFadden Peak quadrangle northward to within 

4 miles of Young a distance of 20 miles. A fault is 

continuous the length of this belt, except perhaps 

across the east flank of McFadden Peak where an eastdipping monocline defines the structure. Along much 

of its length north of McFadden Peak, the fault is 

dilated by a diabase dike(?), which is 300-1,000 feet 

wide. Narrow zones of sharply folded strata border 

the dike. South of McFadden Peak the position of the 

fault is marked by the discordant part of a thick diabase sill. The belts just described are the principal

ones in the McFadden Peak and Blue House Mountain 

quadrangles. Other north- or north-northwest-trending structural belts of considerable length apparently 

exist east and west of these quadrangles.

The narrow but persistent zones of intensely folded 

strata that border some structural belts are not everywhere readily identified as drag phenomena adjacent 

to major faults. These zones are so intimately associated with dikelike bodies of diabase that the folds 

might be interpreted to be the result of forceful injection of diabase magma. As shown earlier, the diabase 

magma was a very fluid material that insinuated its 

way along splitting planes or fractures, rather than a 

viscous mass that punched its way along planes of 

weakness and aggressively folded the wallrocks. Therefore, except for minor superimposed details, it is 

wholly unlikely that any part of the fold structures 

can be explained in this way. This conclusion is reinforced by spatial relations of diabase in the belts. In 

some localities diabase intrusions do not crop out in 

juxtaposition with the fold, and the complete lack of 

thermal metamorphism, especially in rocks particularily susceptible to reconstitution, indicates that diabase 

must be remote if present at all. In some places the 

fold adjacent to a fault is discordantly split by the 

sill; such a split indicates that the fold predated the 

intrusion. (See horizon y, fig. 135.)

Along much of a belt, the displacement now apparent in the bordering fault does not reflect the movement that caused the sharp folds. Where drag shown 

by a fold is opposite that which is expected from apparent displacement, the sense of the original movement 

was reversed by later faulting caused by diabase inflation (fig. 135). In other places, inflation caused 

additional displacement in the same direction as the 

original throw (fig. 13tf). Whatever the sense of 

later movements the folds generated in the early episode of faulting were not notably modified.

The dikes along the north-trending structural belts 

may represent the principal feeder conduits for the 

sills of the region. These are the only thick dikelike 

bodies that can be inferred to extend from far down 

in older Precambrian rocks up through entire sections of younger Precambrian strata. Elsewhere, 

most thick tabular crosscutting masses merely connect 

concordant parts on one sill.

The areal distribution of sills that join with the possible feeder dikes is particularly noteworthy, because 

they occur in patterns that may aid in the recognition 

of prediabase structural belts farther south. Thick or 

extensive sills on one side of a structural belt perhaps 

the west side commonly terminate abruptly against 

the principal fault or merge with the dike that occu-
DEFORMATION ASSOCIATED WITH DIABASE INTRUSION 67

 x- -

- -X -

FIGTJEE 13. Effects of sill inflation on displacement along a 

preexisting fault. Arrows indicate direction of relative 

displacement. A, Prediabase fault bordered by sharply 

folded strata. Lettered horizons are those shown in B and 

G. B, Sill terminating at fault of A; displacement along

the upper part of the fault has been reversed by inflation. 

Note that fold is cut discordantly by sill. C, Sill terminating against fault occupied by dike; displacement along 

upper part of the fault is increased.

pies the fault, and then may be insignificant or missing 

on the opposite side of the fault. Elsewhere along the 

belt, thick sills may inflate strata on the east side of 

the fault but be absent on the west side. Finally, sills 

on one side of a structural belt commonly were intruded along horizons that were different from those on the 

opposite side. Perhaps such spatial relations of sills 

can be used as clues to the existence of such belts. 

Whether prediabase structures are also dominantly of 

northerly trend in the southern part of the region is 

not now known. It is of interest that the Cherry Creek 

belt, if projected south, could extend into the northeast corner of the Globe quadrangle, where the fault 

and intrusion patterns mapped by Peterson (1954) are 

similar to those along the east wall of the canyon of 

Cherry Creek.

DEFORMATION ASSOCIATED WITH DIABASE 

INTRUSION

High-angle normal or reverse faults, which displace 

younger Precambrian strata but not Paleozoic formations, are common throughout the region. Most of 

these faults owe their dislocation entirely to diabase 

inflation. Some interrelations between faults and diabase are shown in figure 14, which is idealized from 

clear-cut examples in the Sierra Ancha and the Salt 

River canyon.

For most of these faults the stratigraphic throw 

closely approximates the thickness of the inflating 

sills. Some faults obviously terminate either at the 

top of a sill (see loc. A, fig. 14) or the bottom (loc. D, 

fig. 14). The throw of such faults does equal the 

thickness of the sill. (Compare intervals a? and a?',

Basal Paleozoic formations Bolsa Quartzite and Abrigo Formation (Cambrian) in southern part of region; Martin Limestone (Devonian) in northern part

Mescal Limestone and overlying basalt flows

1000-

2000-1

2 3 MILES 

J_______I

FIGURE 14. Idealized geologic section showing relations of 

faults to diabase intrusions and to pre-Paleozoic unconformities. Note vertical exaggeration. Diagram is greatly simplified; typically two or more sills inflate the Apache 

sequence, and several minor steps exist along sill boundaries. Letters A-D indicate features discussed in text.
68 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

fig. 14.) Faults of this type occur where sills step 

discordantly from one horizon to another but may not 

be recognized if they are occupied by a thick dikelike 

body of diabase (loc. C, fig. 14). The footwall contact of a sill may be concordant, but a discordant step 

and fault may offset concordant parts of the hanging 

wall (loc. B, fig. 14). The throw of the fault in such 

an example is not the thickness of either adjacent part 

of the sill but the height of the step (the difference 

between y and yf, fig. 14). As a consequence of inflation at different horizons, a fault may be displaced in 

one sense along its upper part and in the opposite sense 

along a lower part. (See loc. A, fig. 14.) Such reversals in throw can also be observed at the junction 

along the strike of two sill-related faults of opposite 

throw. Where more than one sill inflated a sedimentary 

section or where successive sills intruded along one 

horizon displaced one another, the relations between 

intrusions and faults can be more complex.

In the Colorado Plateau the trends of diabase-related features were influenced by two conjugate systems of joints. The joints of one conjugate system 

strike N. 10°-25° E. and N. 70°-85° W.; those of the 

other system strike N. 5°-30° W. and N. 45°-60° E. 

The principal inflationary faults, dikes, and discordant 

parts of sills of a given area generally parallel one of 

the joint sets. Subordinate faults and intrusions mostly parallel the complementary set of that system, but 

may instead follow one of the other sets. As seen in 

plan, discordant intrusions and faults of the dominant 

trend deviate locally along joints of the secondary 

trend. The exceptions to these generalizations occur 

in and near the belts of prediabase structure; there 

the diabase-related features follow one or the other of 

the north-trending sets of joints.

Sill-related faults of various trends differ in lateral 

persistence and displacement. Those of northerly trend 

are the most numerous, and most of the individual 

faults of appreciable throw and length fall in this 

group. But those of west-northwest and northeast 

strike are conspicuous because they cut across the prevailing structural and topographic grain of the area. 

A few of west-northwest trend are extensive or show 

considerable displacement, and are particularly noteworthy because they are entirely inflationary in origin. 

An outstanding example is a fault of N. 70° W. strike 

that has been traced westward from the northwestern 

corner of the Blue House Mountain quadrangle to a 

point on Cherry Creek 2 miles north of the McFadden 

Peak quadrangle; the fault probably continues farther 

west. Throughout its known length of 11 miles, it is 

followed by the discordant part of a diabase sill.

Another example, only about 3 miles long but of greater throw, extends from a point about half a mile south 

of McFadden Peak east to Cherry Creek canyon. This 

fault strikes about N. 85° W., is downthrown on the 

north about 1,000 feet, and terminates a diabase sill 

in exactly the manner shown at locality D of figure 14. 

Eastward, this fault ends abruptly against the principal fault of the north-trending Cherry Creek structural belt; south of McFadden Peak, the throw of the 

fault decreases abruptly at its intersection with a northeast-trending discordancy, and not far to the west, the 

fault probably dies out. Faults of N. 30°-60° E. 

strike are numerous, but most are short and of small 

throw. They perhaps seem more prevalent than comparisons warrant because discordant bodies of diabase 

were commonly intruded along them and in certain 

areas many asbestos occurrences were opened adjacent 

to these intrusions.

In the southern part of the region, where later deformation has obscured them, the Precambrian structural features presumably are similar and equally as 

abundant. The prediabase faults are probably deep 

seated, but those of inflationary origin are literally 

without "roots". The latter would not likely be reopened, and they do not extend to such depths that 

they would likely tap sources of metal-bearing solutions. Thus most of the younger Precambrian faults 

were probably not significant in the localization of 

metal deposits, and in the metal-mining districts they 

should be distinguished from later faults that were 

effective as channelways for ore-bearing solutions.

Throughout the region small-scale monoclines, synclines, anticlines and domes locally occur along or near 

discordant parts of intrusions. Many folds are so 

subtle that they are recognized only by detailed mapping on mine scales. Small bedding-plane faults, 

thrust faults, and near-vertical strike-slip faults occur 

directly adjacent to discordant contacts and are especially abundant in the vicinity of the more strongly 

defined small folds. These faults and folds represent 

mild structural adjustments that occurred during and 

immediately following the emplacement of diabase, as 

is apparent from their geometric relations to successive 

intrusions of diabase. They occur mainly in the least 

competent units of the stratigraphic sequence the lower member of the Mescal Limestone, the lower twothirds of the siltstone member of the Dripping Spring 

Quartzite, and the tuffaceous siltstones and mudstones 

of the Pioneer Shale. Those in the Mescal are particularly noteworthy as the prime structural factors in 

the localization of the chrysotile-asbestos deposits.
(BOCKS ADJACENT TO THE PRE-BOLSA UNOONFO'RMITY. 69

RELATIONS OF PALEOZOIC FORMATIONS TO 

PRECAMBRIAN ROCKS

The base of the Paleozoic sequence in southeastern 

Arizona is defined everywhere by an erosional unconformity that truncates diabase or diabase-related structures. There are two of these erosion surfaces, and 

each in different areas separates the basal Paleozoic 

strata from older Precambrian rocks. Therefore a 

variety of relations must exist between the Paleozoic 

and the younger Precambrian rocks. South of a line 

that extends from the vicinity of San Carlos Lake 

northwest almost to Theodore Eoosevelt Lake (fig. 4), 

the younger Precambrian formations are commonly 

overlain by the Bolsa and Abrigo Formations >of Cambrian age. As drawn, this line represents the present 

northern limits of Cambrian outcrops, and it approximates their northern limits in Early Devonian time. 

A few miles to the north, the Martin Limestone of 

Devonian age is everywhere the basal formation of the 

Paleozoic sequence.

Figure 12 is an idealized reconstruction of the northsouth geologic section that must have existed at the 

end of Devonian time. The significance of certain features depicted in this figure has not been appreciated 

until recently. The pfe-Troy and pre-Bolsa unconformities have been assumed to be the same surfaces; 

furthermore it has not been generally recognized that 

sandstones of the Abrigo Formation can rest directly 

on Apache strata, as do the Bolsa and Troy. Consequently the Bolsa Quartzite, certain sandstone f acies 

of the Abrigo Formation, and in a few areas the locally thick basal sandstone of the Martin Limestone have 

been correlated erroneously with the Troy Quartzite. 

In the following section, emphasis is placed on the 

structural relations of the various basal units of 

Paleozoic sandstone to the Precambrian rocks, and on 

criteria for distinguishing between formations previously presumed to be lithologically similar.

BOCKS ADJACENT TO THE PRE-BOLSA 

UNCONFORMITY

As originally defined in the Bisbee area by Eansome (1904, p. 28-30) and as described for several 

localities northward to the Little Dragoon Mountains 

by Gilluly (1956, p. 14-15, 24) and by Cooper and 

Silver (1964, p. 45^7), the Bolsa Quartzite is a 

white to grayish-red rusty-weathering quartzite which 

ranges in thickness from 390 to 480 feet. This quartzite occurs in regular internally cross stratified beds 

that range in thickness from a few inches to 10 feet 

but perhaps average 2-4 feet (Gilluly, 1956, p. 15). A 

basal conglomerate a few inches to a few feet thick is 

characteristic. The overlying 100-200 feet of the formation is made up of medium- to coarse-grained beds 

that have a considerable content of white quartz granules and, in places, of small quartz pebbles. These are 

the eye-catching "gritty" strata frequently stressed, out 

of proportion, as definitive of the whole formation. 

Actually this unit is transitional upward into much 

better sorted medium-grained strata and ultimately into fine-grained and very fine grained beds that make up 

the upper third of the formation. The Bolsa is 

transitional upward, by an increase in sandy and 

micaceous mudstone layers, into the Abrigo Limestone. 

Gilluly described this transition zone as generally about 

50 feet thick and noted that at least in one locality it 

is almost 100 feet.

Eansome (1904, p. 30) originally considered the Bolsa Quartzite to be Middle Cambrian in age because of 

its comformable relation with the overlying Abrigo, in 

which Middle Cambrian fossils had been found. No 

diagnostic fossils have yet been found in the Bolsa, 

but in recognition of the transitional relations between 

the two formations, the original designation is generally accepted.

Between Bisbee and the Little Dragoon Mountains, 

the Abrigo Limestone ranges in thickness from TOO to 

844 feet. Green-tinted claystones, siltstones, and sandy 

mudstones, and sparse units of limestone ribboned by 

mudstone, make up the lower third. Limestone ribboned by thin layers of similar mudstone dominate 

the middle third. These strata give way to yellowishor reddish-brown dolomite, in which quartz sand becomes coarser and increasingly conspicuous toward 

the top of the formation. The Abrigo is distinguished 

by thin bedding, beds marked by edgewise conglomerates, numerous units that exhibit irregular partings, 

and irregularly anastomosing layers of silty and sandy 

material that separate lenticular layers of sandy carbonate. Worm trails and fucoidal markings are abundant. Beds of massive limestone form a few ledges, 

and beds of quartzite, sandstone, or sandy shale occur 

sporadically throughout the sequence. Northward 

from the type locality near Bisbee, the amount of sand 

increases (A. E. Palmer, in Gilluly, 1956, p. 20).

Fossils of both Middle and Late Cambrian age have 

long been recognized in the Abrigo (Stoyanow, 1936, 

p. 466^81). The list of recognized forms has recently 

been expanded from collections made by Gilluly and 

associates, and the implications of the several faunal 

zones have been concisely summarized by A. E. Palmer 

(in Gilluly, 1956, p. 20-24).

Sequences of the sort seen in the principal outcrops 

of western Cochise County, visualized by many as 

typical of the Cambrian elsewhere in southern Arizona, 

are clearly separated from and lithologically different
70 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

from the underlying rocks. For the most part they rest 

on a planar surface cut on older Precambrian rocks; 

in the Dragoon quadrangle they rest on the Pioneer 

Shale, on Dripping Spring Quartzite, and on diabase 

sills which intrude into these formations. Although 

they exhibit many features in common, the western 

Cochise County sequences are hardly typical of the 

sequences farther north, which are more arenaceous 

and locally seem transitional into underlying strata.

The first of the aberrant sequences demonstrated in 

some detail to be temporally equivalent to the type 

Abrigo was described by Stoyanow (1936, p. 476-477), 

from outcrops in the northern part of the Santa Catalina Mountains, near the head of Peppersauce Wash, 

in the southwest corner of the Mammoth quadrangle. 

This section, 750 feet thick, includes considerably more 

sandstone and quartzite than sections farther south. 

The lower 400 feet, separately designated the Santa 

Catalina Formation by Stoyanow, consists largely of 

rusty-weathering thin-bedded and irregularly bedded 

micaceous mudstones and quartzitic sandstones, in 

which are intercalated thin layers of dolomite. This 

unit is capped by a conspicuous ledge-forming unit of 

clean quartzite, which Stoyanow termed the Southern 

Belle Quartzite. The Southern Belle is 25-30 feet 

thick along the upper canyon of Peppersauce Wash 

but occurs only locally elsewhere in the northern Santa 

Catalina Mountains (S. C. Creasey, oral commun., 

1960). Overlying the Southern Belle is almost 300 

feet of thin-bedded sandy limestone and intercalated 

thin sandstone beds, and between this unit and the 

Martin Limestone is 20-25 feet of sandstone and 

quartzite.

Stoyanow (1936, p. 465-481) recognized several 

faunal zones during his study of the Cambrian strata 

in the Santa Catalina Mountains and elsewhere in 

southeastern Arizona, and consequently proposed that 

Kansome's terminology should everywhere be revised. 

He restricted the designation Abrigo Formation to the 

limestone at the top of the section and to paleontologically equivalent sections farther south. The overlying 

sandstone-quartzite unit he designated as a separate 

formation, the Peppersauce Canyon Sandstone. Other 

formational designations were also proposed for southern limestone equivalents of the Peppersauce Canyon 

and Santa Catalina. The proposed subdivisions make 

up an apparently unbroken sedimentary sequence and, 

for reasons enumerated by Palmer (in Gilluly, 1956, 

p. 24), cannot be consistently defined over broad areas. 

Therefore Kansome's original usage, which applied the 

term Abrigo to all Cambrian strata between the Bolsa 

Quartzite and the overlying Martin Limestone, is preferred.

The units of the Peppersauce Wash locality undoubtedly grade southward with a decrease of clastic constituents into dominantly carbonate 'beds. As an example, the Santa Catalina Formation and Southern 

Belle (Juartzite of Stoyanow can be readily correlated 

with the-, lowest of the three members of the Abrigo 

Formation recognized by Cooper and Silver (1964, 

p. 47-50). In the northwestern part of the Dragoon 

quadrangle, 35-40 miles southeast of Peppersauce 

Wash, this lowest member has similar bedding characteristics and contains mudstones and sandstones like 

those of Stoyanow's Santa Catalina Formation, but 

includes considerably more brown-weathering dolomite. 

It is capped by a thin unit of quartzite, which pinches 

out to the south. On the basis of distinctive faunal 

zones, Stoyanow defined the Santa Catalina and Southern Belle units as Middle Cambrian, and the overlying 

strata as Late Cambrian. In the Dragoon quadrangle 

the boundary between the lower and middle members 

of the Abrigo is shown by fossils to be the boundary between the Middle and Upper Cambrian (A. E. Palmer, 

oral commun., Nov. 1960).

Stoyanow's work in the northern Santa Catalina 

Mountains provides the background for recognizing 

the more northern Abrigo facies, which include an 

even greater content of coarser elastics. Briefly stated, 

a thin- to medium-bedded quartzitic sandstone unit in 

the relative position of the Southern Belle Quartzite of 

Stoyanow thickens northward, at the expense of thinbedded finer grained strata like those of Stoyanow's 

Santa Catalina unit. At Zapata Mountain, 11/2 miles 

northwest of Holy Joe Peak, for example, regularly bedded typical Bolsa Quartzite, 160 feet thick, 

is overlain by 170 feet of beds which resemble the 

Santa Catalina, and this 170 feet in turn is overlain 

by 110 feet of cliff-forming quartzitic sandstone beds 

(M. H. Krieger, written commun., 1960). Except that 

Scolithus conspicuously marks beds in the upper and 

lower part of this cliff-forming unit, it might easily 

be mistaken for the Bolsa Quartzite of the same area. 

Above the unit, similar quartzitic beds form ledges in 

an otherwise friable sandstone that is transitional, 

through an interval of about 100 feet, into the brown 

dolomite and dolomitic limestone unit that makes up 

the upper 150 feet or more of the Abrigo at this and 

other northern localities. The Peppersauce Canyon 

Sandstone of Stoyanow is not a recognizable element 

of this or any other northern section. Generally several dolomitic sandstone beds occur in the carbonate 

unit, and many of the dolomite beds contain an abundance of coarse quartz sand, which etches out to outline the cross-stratification distinctive of this unit. 

Many beds are conspicuously glauconitic. In these
'ROCKS ADJACENT TO THE PRE-BOL'SA UNCONFORMITY 71

and other aspects, this is the only stratigraphic unit in 

the northern part of the region that has a direct lithologic counterpart in the type section at Bisbee and in 

the thickest sections of western Cochise County.

In areas north of the 32d parallel, where it rests 

on Troy and Apache strata, the Cambrian sequence 

was deposited on an irregular erosion surface; furthermore the Bolsa is thinner there than to the south, and 

in places it has lapped out. As much as 300 feet of 

local relief is recorded for the northeastern part of the 

Little Dragoon Mountains. There the surface is generally plateaulike, reflecting the stripping of flat-lying 

Dripping Spring strata, but it is surmounted by hills 

as much as 100 feet high and is cut by valleys 150-200 

feet deep. In this area the Bolsa ranges in thickness 

from 14 to 335 feet (Cooper and Silver, 1964, p. 43- 

46). The influence of resistant formations on the preBolsa surface is readily seen in the area between Holy 

Joe Peak and Brandenburg Mountain, in the east-central part of the Holy Joe Peak quadrangle. The lowest part of the surface is on diabase, and there the 

Bolsa is as much as 150 feet thick. At Brandenburg 

Mountain the Bolsa laps out, and at least 350 feet of the 

Abrigo is absent where the formations pass from diabase onto higher surfaces formed on the Dripping 

Spring and Troy Quartzites. In several exposures 

farther north in the quadrangle, the Bolsa is less 

than 100 feet thick; and through much of the Dripping Spring and Mescal Mountains, sandy strata of the 

Abrigo directly overlie the younger Precambrian formations.

Though the Bolsa Quartzite is notably thinner in 

northern localities than in areas farther south, it is 

not significantly different in lithology. In areas of 

obvious local relief, the basal conglomerate contains 

boulders and varies locally in thickness and composition. Even in sections less than 100 feet thick, the 

Bolsa exhibits the usual gradation from gritty strata 

above the basal conglomerate to finer grained beds at 

the top. It is not as well cemented as in many southern 

outcrops, and in many places sandstone would be a 

better designation for it than quartzite.

Where the Bolsa Quartzite thins drastically the 

mudstone that intervenes between it and the 100-200- 

foot sandstone tongue of the middle part of the Abrigo 

Formation varies locally in texture and composition. 

In places fine-grained micaceous sandstone dominates 

the interval; elsewhere clayey mudstone alternates with 

sandstone. Dolomite is easily overlooked but exists as 

a cement or as nodules or thin lenses in sandy mudstone 

of the basal 20-100 feet of the Abrigo. Kegardless of 

composition, these strata retain the character of the 

fine-grained claystone and mudstone of southern sections by being thinly and irregularly bedded, drab in 

color, and rusty weathering. The relatively clean resistant sandstone of the Bolsa generally gives way 

rather abruptly to the dirty friable sandstone of the 

Abrigo, so that the contact between formations is 

readily identified in most areas.

Fossils diagnostic 'of key faunal zones in southern 

sections bracket the northern sandstone tongue of the 

Abrigo above and below, and confirm correlations 

based on the recognition of comparable bedding features in lateral f acies that are different in composition. 

Trilobites have been noted in the brown-weathering 

sandy mudstone of the lower part of the Abrigo at 

widely scattered localities from the Santa Catalina 

Mountains north into the Hayes Mountains, mostly 

as fragments that permit only the generalization that 

the rocks are Cambrian in age (A. K. Palmer, oral 

commun., Nov. 1960). In collections of trilobites made 

by C. E. Willden at Poverty Flat, 5 miles southwest 

of Coolidge Dam in the southeastern part of the Mescal Mountains, however, an association of bolaspidellid 

and marjumiid forms generally characteristic of the 

upper Middle Cambrian has been recognized (A. R. 

Palmer, written commun. to C. K. Willden, Aug. 5, 

1960). These fossils and compatible occurrences confirm the earlier suggestion by Darton (1925, p. 255) 

that "brown to greenish-gray shales and soft earthy 

sandstone" of this and other northern localities should 

be correlated with the Abrigo Limestone. Kansome 

(1919, p. 44) considered such "layers of fine-grained 

unevenly colored brown, pink, and green quartzite, an 

inch or two thick, separated by films of olive-gray 

shale whose cleavage faces are ridged and knotted 

with numerous worm casts" to be the "most characteristic material" of the upper few feet of the Troy in 

the Kay quadrangle. The sandstone of the outcrops 

which Ransome discussed is seemingly in gradational 

contact with the underlying Chediski Sandstone Member of the Troy. It is more to the point here, therefore, to recognize that the northern occurrences of Middle Cambrian fossils show that the supposed transitional strata at the top of the Troy are correlative 

with the Middle Cambrian mudstone member that is 

at the base of the Abrigo in the Dragoon quadrangle 

 and farther south. Specimens of Billingsella, a brachiopod of Late Cambrian (Franconia) age that characterizes the uppermost parts of the Abrigo Limestone 

in southern sections, have been found in most northern 

occurrences of the glauconitic sandy dolomite unit at 

the top of the Abrigo.

In a recent review of the data of earlier workers, 

Lochman-Balk (1956, p. 538-549) rightly concluded 

that the northern sandy f acies that is here considered
72 YOUNGER PREGAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

Abrigo "should be kept distinct" from the underlying 

Precambrian sandstone of the Troy, but she also proposed that the northern facies should be interpreted 

as lithogenic equivalents of the Bolsa. In making 

this interpretation she did not have advantage of the 

following facts: (1) The Bolsa-Abrigo transition is 

everywhere fairly abrupt, tand northern occurrences 

of the contact are strictly comparable with the contact 

at the type locality. (2) Sandy elements at the top 

and bottom of northern Abrigo sequences really are 

not greatly different in lithology from units of equivalent age in the type section. (3) Where1 the Abrigo 

is most fully remnant in northern outcrops, it is 600- 

650 feet thick or almost as thick as the classic southern sections. (4) Further, the stratigraphic interval 

between the Middle Cambrian and Upper Cambrian 

faunal zones, cited in the preceding paragraph, is 

about the same in northern sequences as in southern 

sequences. Therefore, the Bolsa-Abrigo contact is not 

notably time-transgressive from south to north.

Where nondiagnostic fossils occur in abundance, 

they can be considered lithologic aspects that distinguish the Abrigo Formation from the Bolsa Quartzite. 

Throughout the region of Abrigo outcrops north of 

the Santa Catalina Mountains, primitive phosphatic 

brachiopods of Cambrian aspect are locally abundant. 

These brachiopods are particularly numerous in sandstone in the transition interval at the very base of the 

Abrigo, in similar friable sandstone within and at the 

top of the mudstone unit, and in the transitional sandstone above the cliff-forming sandstone tongue of the 

Abrigo. None have been seen in sandstone certainly 

identified as Bolsa. Rod-shaped fucoids of the size 

of cigars parallel the bedding and, with Scolithus, 

are locally very abundant and conspicuous in the 

coarser and cleaner sandstone of the Abrigo. Scolithus 

has been reported in a very few places in the Bolsa; 

apparently it occurs only sparsely and would be easily 

overlooked even in careful search of that formation. 

Narrow worm( ?) or trilobite( ?) trails and other smaller scale markings of possible organic origin characterize bedding surfaces throughout the lower two-thirds 

of the northern Abrigo sections. These markings have 

not been seen in the Bolsa.

Recognition of such features is especially helpful 

in identifying certain of the more northern outcrops 

of the thick sandstone tongue of the Abrigo. In these 

occurrences the tongue is made of light-gray almost 

white, fine- to medium-grained sandstone that does not 

necessarily weather rusty to make dark outcrops. The 

thin bedding may be obscure, and the unit commonly 

forms massive ledges or cliffs, parts of which have 

gnarly surfaces easily taken for convolute structures

when seen from a distance. Thus, misidentification 

with the Chediski Sandstone Member is possible where 

outcrops are light colored, and with Bolsa Quartzite 

where they are dark, especially where the unit rests 

directly on Precambrian rocks. On close inspection 

the gnarled surfaces prove to be sites of abundant 

/Scolithus or fucoids. Where these fossil features were 

not made obvious by differential silicification, they can 

generally be found in a brief search.

The reasons for previous correlations of Bolsa and 

Troy and for inclusion of certain occurrences of the 

Abrigo with the Troy are now apparent. The increase in sandy strata, particularly quartzite, and the 

decrease in carbonate content northward from the 

least clastic occurrences in western Cochise County, 

which are generally considered representative of the 

Abrigo, cause incomplete sequences of the Abrigo to 

appear grossly similar to parts of the Troy and Bolsa 

Quartzites. In many places the Bolsa or sandstone 

units of the Abrigo rest on the Troy in seeming concordance, and the contact between formations is obscure. In other areas the Troy is entirely missing, 

and the combined Bolsa and Abrigo sections, which occupy the same relative stratigraphic position, are approximately the thickness of the Troy noted in other 

southern occurrences. In places, however, the Bolsa 

Quartzite or the Abrigo Formation rests in angular 

or sharp erosional unconformity on the Troy Quartzite, 

on formations of the Apache Group, and on diabase. 

In these places, criteria for distinguishing sandstone 

and quartzite of Precambrian age from those of Cambrian age can be unequivocally defined. These diagnostic features are summarized in table 2, which also 

highlights some features not mentioned in the preceding generalized descriptions. Details of relative 

positions of quartzites and sandstones in the sequences, 

differences in textures of the sandstones, and the common occurrence of fossil features in sandstones of the 

Abrigo are particularly noteworthy. By the use of 

such criteria, for example, the so-called Troy Quartzite of the northern Santa Catalina Mountains (Bansome, 1916, p. 144; Stoyanow, 1936, p. 473-477) clearly 

should be redesignated Bolsa Quartzite.

Where other criteria for identifying the pre-Bolsa 

unconformity are not readily applicable, the recognition of regolithic debris from the diabase can be helpful. Beneath Cambrian strata the diabase is decomposed through thicknesses generally ranging from 2 

to 25 feet. In this interval the diabase is platy or 

shaly parting and generally is grayish red. Very 

little of the remaining regolith was reworked and 

transported. In a few places the decomposed diabase 

is greenish gray; in such outcrops, in particular, the
PRE-MIDDLE DEVONIAN UNCONFORMITY 73

original ophitic texture may be relict up to the contact 

with the Cambrian strata. Although diabase soil incorporated in the basal part of the Bolsa or the Abrigo 

commonly causes several feet of strata to be dusky red, 

fragments of diabase in the basal conglomerate are 

almost nonexistent. In a few localities the diabase 

debris below the contact is bedded. In an unusual 

example 2 miles southeast of Superior, grayish-green 

diabase debris, mixed with a considerable amount of 

well-rounded quartz sand, is conspicuously cross stratified through a vertical interval of 25-30 feet. Here 

the zone of platy-parting diabase underlies the reworked material. In some areas Cambrian sandstone 

is separated from older strata by a few inches to a 

few feet of transported reddish-weathering diabase 

debris.

PRE-MIDIXLE DEVONIAN UNCONFORMITY

The Martin Limestone generally overlies Cambrian 

formations in the southern part of the region and 

younger Precambrian formations in the northern part. 

The hiatus between the Cambrian and Devonian is 

represented in the southeasternmost Arizona by a disconformity, along which even minor channeling is not 

apparent (Oilluly, 1956, p. 25-26). As the disconformity is traced northward from Cochise County, 

however, erosional relief gradually becomes marked, 

and in parts of northern Gila County relief of a few 

hundred feet is apparent. The characteristic form 

of the erosion surface is seen along the Salt Eiver 

canyon, where the unconformity can be traced many 

miles without interruption. Along the canyon, the 

pre-Devonian surface is a planar surface incised at 

wide intervals by steep-walled channels of considerable 

extent. Generally hills do not rise above this plateaulike surface, but in the area between Canyon Creek 

and Payson there are hills of moderate relief. The 

400-foot carbonate section that makes up most of the 

Martin practically laps out against some of these hills 

of Precambrian rock.

In most places south of the Pinal Mountains, Cambrian strata probably intervene between the Martin 

and the Precambrian formations, but at least locally, 

such as near the abandoned settlement of Troy in the 

Dripping Spring Mountains, the Martin rests on Troy 

Quartzite or on the Apache Group.

The Martin Limestone generally overlies the Troy 

Quartzite in the area north of the line that defines 

the limit of Cambrian outcrops (fig. 4). In some areas 

of as much as a few square miles, however, the Martin 

Limestone rests 011 the Mescal Limestone or on the 

Dripping Spring Quartzite, or on diabase sills intruded into these formations. Northwest from Canyon

Creek along the foot of the Mogollon Eim, where 

the Apache Group was already thin as a consequence 

of lapout on a pre-Apache high, the pre-Martin unconformity progressively truncates strata lower in 

the Apache Group. Ultimately at the west end of 

Christopher Mountain, northwest of Young, the Martin rests on older Precambrian formations (fig. 12). 

Farther west the Apache and Troy strata were everywhere removed during the pre-Martin cycle or an 

earlier erosion cycle.

The pre-Martin unconformity truncates the younger 

Precambrian formations and structures in the same 

manner as the pre-Bolsa surface. Angular discordances of as much as 30° exist, but in most areas the 

Martin appears to be concordant with the older strata. 

Except that it is generally thinner, the regolith zone 

on diabase below the Martin is comparable to that 

below the Cambrian formations. Basal strata of Devonian age contain even less regolithic debris than do 

their Cambrian counterparts.

The Martin sequence that surmounts the usual 

plateaulike surface is mostly limestone and dolomite 

but does include beds of shale and sandstone; regionally the formation is somewhat variable in lithology. 

(See Huddle and Dobrovolny, 1952, p. 73-76, 83-85; 

Gilluly, 1956, p. 26-29.) In the northern part of the 

region a dolomitic sandstone, a few inches to 20 feet 

thick, occurs everywhere at the base of this sequence. 

As thus delineated the Martin has generally been considered Late Devonian in age, but recently Teichert 

and Schopf (1958, p. 213-215) suggested that plant 

remains, collected a few feet above the base of the 

formation, indicate that the lower part of the Martin 

in northern Gila County is probably not younger than 

Middle Devonian and do "not rule out the possibility of 

assignment to the Early Devonian."

Of particular concern here is the correlation of sandstone that fills the widely separated channels below the 

general level of these dated carbonate strata. Previously, the sandstone has been considered part of the 

Troy. The channels range in depth from a few feet 

to at least 300 feet and in width from a few hundred 

feet to as much as half a mile. At least two exceed 5 

miles in length. The channels everywhere transect the 

latest rocks and structures of the younger Precambrian terrane and merge with the higher pre-Martin 

surface; therefore the sandstone fillings are almost certainly Paleozoic rather than Precambrian in age. Because the carbonate section has been stripped from 

some of the prime examples and the contacts of other 

sandstone bodies with overlying strata are not definitivea it can be speculated that some and perhaps most
TABLE 2. Criteria for distinguishing clastic strata of the Bolsa Quartzite and Abrigo Formation from those of the Troy Q

[These descriptions apply only to the part of the region where Bolsa and Abrigo Formations overlie Troy Quartzite]

Strata of younger Precambrian age

Quartzite member of Troy Chediski Sandstone Member of Troy

Strata of Middle and Late Camb

Bolsa Quartzite Sand

Position in 

sequence

At top of Troy. If remnant, is 

underlain by Chediski Member.

Basal part of Troy; generally 

makes up most if not all of 

Troy where Cambrian strata 

are overlying.

At base of Cambrian sequence; 

transitional by intercalations of 

sandy mudstone into Abrigo. 

Contact with Precambrian 

rocks generally marked by 

basal conglomerate.

Sandston

of Abr

by fiss

mudst

lap dir

rocks, 

has b

basal 

Lithology

Well-sorted clean quartzite, 

practically free of feldspar and 

pebbles. Basal unit coarse 

grained; rest mostly medium 

grained.

Poorly sorted pebbly sandstone 

of well-rounded frosted quartz 

grains in sericite-clay matrix; 

weakly cemented to quartzitic. 

Coarse feldspar grains may be 

conspicuous. Locally includes 

coarse muscovite.

Quartzite or firmly cemented 

sandstone; basal part feldspathic and gritty, with sparse 

pebbly beds; grades up into 

medium-grained and ultimately 

into fine-grained moderately 

well sorted strata.

Tongue 

moder

quartz

stone 

comm

shaly-

ceous 

sparse

thin s

or lens

Bedding and 

related 

features

Tabular beds thin to thick; 

cross-stratification may not be 

conspicuous.

Irregularly bedded; tops of many 

beds channeled and marked by 

scattered pebbles or by thin 

layers or lenses of conglomerate. 

Convolute lamination in middle 

part of section. Uppermost 

beds cross stratified on large 

scale. Where quartzitic, bedding structures are obscure.

Tabular beds, J/y-4 ft thick; well 

defined in outcrop by partings 

that etch out much more conspicuously than those of Troy. 

Internal small- to mediumscale cross-stratification is 

common.

Sandston

beddin

may b

sive w

Undu

beddin

mudst

Color

Light-colored. Generally pinkish 

gray to light gray; may be 

light brownish gray to medium 

gray.

Mostly yellowish gray or light 

brownish gray, but locally includes medium-dark-gray beds. 

Basal strata locally pale to 

grayish red. Limonite stains 

largely restricted to fracture 

faces in quartzite outcrops of 

mineralized areas.

Very light gray to grayish red; 

weathered surfaces generally 

stained reddish brown. Minimal rusty weathering in vitreous quartzites, which may display conspicuous color banding. 

Color banding more common in 

Bolsa but also occurs in other 

quartzites.

Sandston

yellow

especi

rusty 

pale-y

dish-b

stones 

yellow

gray, 

weath
None.

Fossils

None. Scolithus sparse and rarely recognized; apparently can ignore 

for practical purposes. Transitional mudstones immediately 

above Bolsa commonly marked 

by phosphatic linguloid 

brachiopods.

Scolith

to ab

tong

trans

unde

brach

very 

worm

trails 

finer 

fragm

Remarks

Generally more vitreous and 

massive cropping than Cambrian quartzites. Bedding 

comparable to basal part of 

Bolsa, but strata are not 

gritty.

Where quartzitic, is distinguished 

by massive outcrops, poor 

sorting, pebbles, and sparse 

coarse grains of orange-pink 

feldspar.

Commonly cliff former. Gritty 

part lacks undulant bedding 

planes and lenticular layers of 

pebbles that characterize lower 

coarse-grained strata of 

Chediski.

Mudsto

mica

unkn

Ligh

pebb
76 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

of these fillings are the outlying remnants of Cambrian 

sandstone that once extended over the area.

The various channel fills differ in that mediumgrained sandstone dominates some and coarse-grained 

or very coarse grained sandstone dominates others. 

These differences apparently reflect variations in 

amounts of locally derived sands. Where a channel 

cut through the especially friable parts of the Chediski Sandstone Member, for example, the fill may be 

compositionally very like the Chediski; features seen 

along the margins may be the only aspects readily of 

use in distinguishing host from fill. Some fills are 

bottomed by as much as 40 feet of cobble or boulder 

conglomerate, well rounded in one occurrence and angular in another; other fills lack a conspicuous basal 

conglomerate.

The fills are similar in that all contain many beds of 

very poorly sorted sandstone and in that scattered 

small pebbles and lenses of granule conglomerate are 

typical. In some places the sandstone ranges in color 

from almost white to dusky red, but generally it is yellowish brown to reddish brown. In perhaps four out 

of five occurrences, certain layers in the upper parts of 

the fills are conspicuously marked by abundant hematite-cemented concretions. The sandstone is generally 

very friable. Most is in tabular beds, a few inches to 

10 feet thick, in which small- to large-scale cross-stratification etches out conspicuously. Shaly-parting thin 

layers of sandy siltstone locally separate sandstone 

beds. A few beds are highly feldspathic, and a few 

beds are cemented by dolomite or calcite.

This sandstone is wholly different from any sandstone of the Troy, Bolsa, or Abrigo Formations. Features common to most fills suggest that all the sandstone is of one age. These features in the more southern occurrences stand in obvious contrast to the diagnostic features of nearby outcrops of Cambrian strata. 

It is therefore unlikely that the fills can be correlated 

with the Bolsa or Abrigo. In general these sandstone 

beds merge upward into the sandstone that is basal to 

the Martin, and in a few places sandstone high along 

the margins of channels seemingly grades horizontally 

into limestone within the channels. Thus these are 

almost certainly local basal units of the Martin.

From Christopher Mountain west to the vicinity of 

Payson, an arkosic sandstone about 50 feet thick separates carbonate strata of the Martin from older Precambrian rocks. Certainly this sandstone, usually 

speculated to be Cambrian in age, does postdate the 

Troy Quartzite. Whether it in part or in entirety can 

be correlated with the channel sandstone remains to 

be determined.

AGE OF DIABASE AND ASSOCIATED ROCKS

In the southern part of the region, the Troy Quartzite, the Apache Group, and the diabase are separated 

from Middle Cambrian strata by an unconformity so 

profound that their assignment to the Precambrian 

is quite reasonable. In the northern part of the region, though similar age designations can be made 

by analogy (Shride, 1958), stratigraphic relations 

directly prove only that the diabase is pre-Devonian; 

radiometric data that prove Precambrian age have 

recently become available for this area.

Because there has been much question concerning 

whether the diabase intrusions are all of the same age, 

a brief review of the critical relations is worthwhile. 

The age of the extensive intrusions has been variously 

inferred to be: (1) Precambrian, (2) Precambrian 

and Cambrian, (3) post-Cambrian but pre-Devonian, 

(4) post-Pennsylvanian and probably early Mesozoic 

or late Paleozoic, or (5) Late Cretaceous or early Tertiary.

A Late Cretaceous or early Tertiary age, based on 

inferred relations in the structurally complex mineralbearing belts in and near southern Gila County, where 

exposures of contacts commonly are poor, is the one 

most widely accepted for the diabase. In these belts 

large masses of Paleozoic strata have been interpreted 

as masses that foundered into some of the larger sills. 

Furthermore, large diabase bodies that underlie Paleozoic strata locally show steplike contacts with the 

Paleozoic formations, and faults alined with these 

steps suggest offsets attributable to intrusions by diabase. These discordant contacts are either obvious 

fault contacts or are so poorly exposed that their 

nature cannot be directly determined. Nowhere are 

diabase sills both overlain and underlain by Paleozoic 

formations; that is, none of the supposedly engulfed 

blocks contain sills. In contrast, wherever Apache 

or Troy strata adjoin or are engulfed by large masses 

of diabase, they are intruded by sills.

The Martin Limestone is the Paleozoic formation 

most commonly supposed to be intruded by diabase. 

The Martin consists partly of cherty dolomite beds, 

which are comparable to the cherty dolomite of the 

Mescal and should be converted to silicate-bearing 

limestone if intruded by diabase. They have proved 

quite amenable to such metamorphism adjacent to 

quartz monzonite of Tertiary (?) age (Cooper, 1957, 

p. 582-587). These rocks, even where exposed within a 

few inches of diabase, are completely lacking in contactmetamorphic minerals. Where fully exposed contacts are found in adjacent areas, they prove that the 

Martin rests in sedimentary contact on the diabase 

body that supposedly intrudes it. I have not found
GEOLOGIC HISTORY 77

a single example of a chilled selvage in diabase 

against Paleozoic rocks, although selvages are readily 

seen along contacts of the diabase body with Apache 

rocks in the same area. All the supposedly engulfed 

blocks of Paleozoic strata are blocks that have been 

faulted against diabase rather than torn loose from 

their original position by diabase intrusion.

Dark dikes of diorite (?) and andesite basalt that occur very sparsely in Paleozoic rocks have been equated 

erroneously with the diabase. Ransome (1919, p. 53, 56) 

clearly stated that small dikes of diabase cut into 

Paleozoic formations only in a few places and that in 

assigning a post-Paleozoic age to the diabase he "supposed" these dikes "to represent parts of the same 

magma that solidified in the larger masses." Darton 

(1925, p. 254) confirmed Ransome's observations for 

at least one locality, but he took exception to Ransome's correlation of these dikes with the large diabase 

sills and suggested that the small dikes were feeders 

for some of the Cenozoic basalt flows of the region. 

I have made a particular effort to confirm the reported examples and to find others, and have yet to 

find in a Paleozoic host rock a dike that compositionally or texturally resembles the diaJbase.

It is now very clear that the diabases are petrographically somewhat distinctive, that all large masses 

are kindred, and that none postdate any of the Paleozoic rocks. Abundant stratigraphic evidence shows 

that they antedate the Paleozoic formations throughout the region.

The Precambrian age of the diabases in the northern 

part of the region has recently been confirmed by various isotope determinations. In 1955 H. C. Granger 

and others of the U.S. Geological Survey collected 

specimens of uraninite and galena for isotopic analyses 

from four localities in the Sierra Ancha. The uraninite and galena occur mainly as discontinuous veinlets 

in the Dripping Spring Quartzite, but some of the 

sampled veinlets transect quartzose aplite dikes which 

are associated with the principal diabase sill of the 

Sierra Ancha. This sill transects the highest parts of 

the Troy Quartzite and is overlain by Devonian strata. 

On the basis of Pb207/Pb206 ratios, determined from 

both galena and uraninite, L. R. Stieff (written commun. to H. C. Granger, Jan. 26, 1956; Neuerburg and 

Granger, 1960, p. 775-776) estimated the age to be 

1,100 m.y. (million years). Determinations by Silver 

(1960; oral commun., Nov. 1960) of Pb207/Pb206 ratios 

in uraninitite collected from Workman Creek (3^ 

miles northwest of Aztec Peak, McFadden Peak quadrangle) and in zircons collected from the granitic differentiate on Reynolds Creek give a minimum age of 

1,075±50 m.y.; Silver extrapolated a probable age of

at least 1,200 m.y. From potassium-argon ratios determined for biotite in very slightly altered diabase of 

the same sill, an age of 1,140+4:0 m.y. has been determined (P. E. Damon, written commun., Dec. 1961; 

Damon and others, 1962). Thus, throughout southern 

Arizona the large sills of diabase considerably predate 

the Cambrian, and the Troy and older formations may 

be considerably older than 1,200 m.y.

A maximum age for the sedimentary formations 

can be stated only relative to granitoid rocks that unconf ormably underlie the Apache Group. In the Dragoon quadrangle an age of 1,660 m.y. has been estimated for the underlying Johnny Lyon Granodiorite 

(Silver and Deutsch, 1961), and a minimum age of 

1,500 m.y. has been determined for Ruin Granite at 

the south end of the Sierra Ancha (Damon and others, 

1962).

GEOLOGIC HISTORY

The following summary of younger Precambrian 

events emphasizes some features that persist laterally 

and some that vary. Where possible, the emphasis is 

on features indicative of an environment or process, 

in the hope that recogition of like or related geologic 

phenomena may aid in correlating the Apache and 

Troy strata with those of younger Precambrian age 

in surrounding regions.

Throughout the outcrop area of the Apache Group, 

most of the individual members of the Precambrian 

formations show remarkably little lateral variation 

in composition, texture, and bedding features or other 

internal structures. The bedding structures of the 

Chediski Member of the Troy Quartzite do change 

considerably from north to south, but no marked 

variation in the overall lithology is apparent in the 

same interval. Lateral differences in thicknesses are 

largely the effects of erosion before a succeeding unit 

was deposited. Certain units, the Pioneer Shale and 

the arkose member of the Troy for example, vary in 

thickness because of the configuration of the basin of 

accumulation. Some lateral lithologic changes, such 

as from dolomite to ferruginous chert or from cherty 

dolomite to silicated limestone, as observed in the 

Mescal, are postdepositional but are related to a significant tectonic event. Other variations known to 

exist seem of little paleogeographic significance.

Prior to deposition of the Apache formations, the 

older Precambrian rocks were deeply eroded to a surface of generally low relief which was largely on 

granitoid rocks. In northwestern Gila County, this 

surface stood above the rest of the region and exhibited noteworthy local relief. The surface was a 

peneplain, additionally smoothed and swept clean of 

most of its residual debris by marine abrasion.

248-121 O - 67 - 6
78 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

As the first sea of Apache time encroached across 

the region, wave and current action must have been 

vigorous. In places the Scanlan Member of the Pioneer is composed of subrounded or angular gravels, 

almost entirely of local derivation, but the bulk of the 

Scanlan contains well-rounded pebbles and cobbles 

of highly resistant quartzite that were deposited several tens of miles from any quartzite source. A moderately thick blanket deposit of arkose might be expected to have accumulated on a granitic terrane. 

Arkose does make up the lower part of the Pioneer 

in many areas, but even the cleanest beds are separated 

by thin layers of tuffaceous siltstone; in many areas 

fine-grained tuffaceous strata immediately overlie the 

basal conglomerate and make up most of the formation. Small-scale lamination and cross lamination in 

these beds, and lack of abrasion of the fragile glass 

shards, suggest that currents were incompetent to 

transport great amounts of arkosic debris during much 

of Pioneer time. Voluminous and extensive falls of 

ash, from a source as yet unrecognized, must have 

literally flooded out the arkosic strand deposits of an 

encroaching sea that was placid during most of Pioneer time. The arkoses found in places at the top of 

the Pioneer may be remnants of coarser sands laid 

down during regression of the sea.

An erosional episode must have intervened between 

the deposition of the Pioneer and the deposition of 

the Dripping Spring. Except that the contact between the Pioneer Shale and the overlying Barnes 

Member is sharp and the conglomerate does fill a few 

shallow channels, there is no direct indication of erosion. The conglomerate and overlying arkose are 

entirely lacking in tuffaceous material, and thus must 

mark a break in sedimentation and herald new environments at the source of sediments and at sites of 

deposition. The thin Barnes Conglomerate Member 

is not readily explained except as a deposit of a transgressive sea. Local sources for the typical gravels 

were buried by the Pioneer. Thus the well-rounded 

and in part very coarse gravel of the Barnes must 

have been transported long distances during a second 

episode of vigorous marine erosion at the start of 

Dripping Spring time. The source terrane, perhaps 

not now exposed to any appreciable degree, was dominantly granitoid but contained a considerable amount 

of quartzite of the sort that ribs the older terrane 

in northwestern Gila County.

The composition and bedding features of the arkose 

member of the Dripping Spring Quartzite suggest 

vigorous mechanical erosion of an elevated granitic 

terrane and rapid transport into a shallow sea. 

Kather uniform grain size through thick sets of crossstratified tabular beds implies frequently repeated 

winnowing by tidal or wave action that was competent 

to sort and spread the sand widely in thick sheets. 

Toward the top of the member, feldspar content decreases, the grain size increases slightly, and scattered 

small pebbles are included in the beds. Perhaps these 

features hint the termination of this period of shallow 

marine sedimentation.

The significance of the rather abrupt change from 

well-sorted sand of the arfcose member to ill-sorted mud 

of the siltstone member is not understood. The environment of the source area at the beginning of late 

Dripping Spring time may have been modified abruptly, or there may be a hiatus in deposition for which 

the evidence is not clear.

The thin and irregular bedding of the siltstone member, the many scour-and-fill structures, the few largescale channels, and the abundant ripple marks and 

mud cracks collectively indicate deposition in very 

shallow water. No feasible explanation for the consistent northerly alinement of the scour-and-fill features can be suggested. Subaerial exposure and reworking of the muds occurred repeatedly for some 

parts of the section and provided opportunity fpr oxidation. The effects of aeration must have been largely 

nullified by rapid burial; otherwise the sulfur now 

represented in the ubiquitous pyrite and the carbonaceous matter would not have been preserved. The 

muds probably accumulated on broad tidal flats of a 

shallow sea that sporadically advanced onto a granitic 

terrane of low relief.

The Dripping Spring sediments were indurated and 

eroded before the Mescal Limestone was deposited. 

The thin basal sandstone of the Mescal Limestone was 

reworked in large part from the upper part of the 

Dripping Spring Quartzite, but some coarse sand of 

distinctive type was derived from another source. 

Similar coarse sand also was supplied during early 

stages of carbonate sedimentation in the Mescal sea, 

but otherwise little clastic debris was contributed.

During the early stages of carbonate deposition, 

circulation in the Mescal sea must have been restricted 

to permit the high salinity necessary for the precipitation of halite. Probably other evaporites were deposited. Such an environment would also favor the 

deposition of dolomite or the penecontemporaneous 

conversion of calcareous mud to dolomite. The formation of chert in such sediments is an enigma. The 

sea floor was so shallow that wave action was repeatedly effective in reworking mud and partly lithified cherty carbonate strata. After about a third of 

the lower member had accumulated, the sea waters 

freshened, and perhaps the freshening was the cause
GEOLOGIC HISTORY 79

of leaching of the evaporite section. This leaching 

caused the peculiar founder breccia that makes up 

the lower part of the Mescal. The sea remained sufficiently saline thereafter, however, to favor the continued formation of dolomite. An almost invariable 

sequence of clastic-free beds suggests that conditions 

for sedimentation were uniform and sea-floor abrasion 

less vigorous during deposition of the upper third 

of the lower member.

During the deposition of the algal member of the 

Mescal, the sea floor also must have been remarkably 

stable and uniform in depth; otherwise the stromatolites would probably occur in bioherms rather than in 

biostromes (Link, 1950). This environment was all the 

more remarkable in that the stromatolites are probably 

intertidal accumulations and in that these tidal flats 

were persistently hospitable to algal growth for some 

time over an area of at least 15,000 square miles.

The carbonate members of the Mescal were elevated 

above sea level, at least briefly, and eroded before 

additional rocks were laid down. At this time solution cavities began to form in the dolomite. Basalt 

was extruded onto this erosional surface and in turn 

was mostly eroded away. At least the northern part 

of the region then subsided, and very fine grained 

siliceous mud and subordinate carbonate mud of the 

argillite member of the Mescal accumulated in a quiet 

body of water. Following another episode of erosion, 

several thin basalt flows were extruded.

During the period of instability that followed the 

deposition of the carbonate members of the Mescal, 

the Apache strata were broadly warped; consequently 

the extensive subaerial planation that preceded deposition of the Troy Quartzite locally exposed formations low in the Apache Group, and southeast of 

the Mescal Mountains all the Apache was removed. 

The enlargement of solution cavities in the Mescal 

had continued with each episode of erosion, and solution became particularly effective during the pre-Troy 

episode. Few, if any, of the dolomite sections completely escaped effects of solution and related silicification. In some areas a karst topography was formed 

and part of or all the dolomite section was converted 

to a massive rubble of chert. Concomitantly, hematite, probably a result of laterization of the basalt 

flows, was concentrated in certain karst areas. Solution and collapse of the dolomite continued during 

deposition of the Troy.

In the northwestern part of the present area of outcrops, a structural basin a few hundred feet deep 

existed during early stages of Troy sedimentation, 

and arkosic sand was literally poured into this basin. 

The prevalent eastward dip of crossbeds indicates

transport from the west, but because the arkose that 

is visible today in the local basin probably represents 

only a small part of a much larger deposit, these attitudes may not be significant. Good sorting, fine 

grain, and very large scale steep-dipping cross-stratification suggest eolian deposition, but these deposits 

might as likely be a local phase of a rapidly growing 

delta.

The sharp contact between the arkose member and 

the Chediski Member of the Troy may reflect a significant erosional hiatus or may merely indicate a 

minor break in sedimentation. Through most of the 

region the base of the Chediski Member is marked by 

conglomerate comparable to the Barnes Conglomerate Member and, like the Barnes, was necessarily derived from a distant source and was spread widely 

by a vigorously transgressing sea. The Chediski 

Member represents an episode in sedimentation quite 

different from that of the arkose member and therefore was perhaps separated from it by an appreciable 

length of time.

The sands of the Chediski Member are coarser or 

otherwise entirely different from any sands deposited 

previously in the younger Precambrian sequence, and 

no part of the member, except small amounts of gravel, 

could have been derived from these older strata. The 

sands were derived, however, from granitic terranes 

similar to those that furnished the arkoses that are 

lower in the sequence. Angular pebbles and cobbles 

of Final Schist, Kuin Granite, Dripping Spring 

Quartzite, and Mescal Limestone, which in places are 

in abundance in sections south of Globe, indicate vigorous erosion of Apache strata in nearby areas during 

Chediski time.

Certain features suggest an eolian origin for sand 

in the Chediski Member, but other features oppose 

this interpretation. Typical ventifacts at the base 

of the member do confirm an eolian stage in the formation of the sand; the excellent rounding and characteristic pitting of quartz grains throughout the 

member and the moderate to high-angle cross stratification in the upper third of the member also suggest 

eolian deposition. Modern dune sand tends to be fine 

grained and well sorted (PettiJohn, 1949, p. 232-236), 

and in this respect the coarse fractions of the upper 

and lower units of the Chediski are definitely atypical. 

Perhaps the convoluted strata are sorted adequately 

enough to be of eolian deposition, but they are more 

reasonably explained as individual beds hydroplastically deformed in place as each was being deposited 

(Ten Haaf, 1956), rather than as dune sands that 

slumped when saturated by water of an encroaching 

sea. In the southern part of the region, the Chediski
80 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

Member lacks virtually all features suggestive of 

eolian origin, except for the degree of rounding and 

the abundant pitting of the grains. Irregular bedding, channeling, and lenses of conglomerate between 

beds might suggest fluviatile deposits. However, many 

of the conglomerate lenses seem to be lag gravel 

concentrated by the winnowing of pebbly sands. This 

gravel tends to form broad sheets rather than local 

channel fills, and thus suggests concentration on a sea 

bottom.

All features considered, the Chediski Sandstone 

Member is possibly best interpreted as a deposit laid 

down in a sea which transgressed the area from south 

to north or from southwest to northeast and which was 

so abundantly supplied with coarse quartz-feldspar 

sand that only crude sorting could be accomplished. 

Great volumes of sand and subordinate amounts of 

quartz gravel, previously rounded, mechanically 

etched, and partially sorted by wind, must have been 

available for deposition into this sea. The sea bottom 

must have been a site of considerable agitation for 

formation of blanket deposits as extensive as those of 

the Chediski Member.

The quartzite member of the Troy calls for a different paleogeographic interpretation. The quartzite 

member may well have had a source of elastics in 

common with the Chediski Member; if so, these elastics were subjected to considerable transport and sorting, and accumulated in a marine environment as thin 

layers of well-rounded well-sorted quartz sand, virtually free of other minerals or pebbles. Its uniformly 

sharp contact with the Chediski Member suggests a 

hiatus rather than an abrupt change in environment.

Undoubtedly the Troy was once deeply buried by 

additional Precambrian strata; otherwise the diabase 

now seen in the uppermost parts of the thickest sections of Troy could not have been emplaced as thick 

sills. We do not have a sound basis for estimating 

the additional thicknesses of younger Precambrian 

strata once in southern Arizona. The diabase in the 

top of the Troy is petrographically identical to that 

emplaced at least 3,000 feet lower and might indicate 

an equivalent minimum cover on the Troy. If the 

Apache Group and Troy are correlative with the Unkar Group of the Grand Canyon, as Darton (1925, 

p. 36) surmised, additional thicknesses of at least 10,- 

000 feet may have been deposited and removed in 

Precambrian time.

All strata of the Troy Quartzite and the Apache 

Group were deposited on a shallow stable continental 

shelf. Formations and even members of the sequence 

are individually very different from each other; the 

differences suggest abrupt changes in the environment

of deposition, in the mode of transport, or in the environment at the source, but not in the kind of materials. Some of the abrupt changes are marked by unconformities, and additional unconformities may yet 

be recognized. The sequence is analogous to the sequence of Paleozoic formations in the same area, and 

I believe, even without consideration of the missing 

strata, that it may well represent as long a time interval.

After lithification of the Troy, the younger Precambrian strata were first locally faulted and folded, 

then extensively intruded and displaced by diabase 

intrusions, and finally uplifted regionally and deeply 

eroded, all in Precambian time. Despite this long 

history of deformation, most of the younger Precambrian strata were virtually flat lying when Cambrian 

seas encroached across the region. Northeast and 

southeast of the area of present outcrops, these formations were completely removed prior to Cambrian sedimentation, and elsewhere they were further eroded before Devonian sedimentation. The remnants were 

then deeply covered by Paleozoic formations and everywhere remained almost horizontal at least into Mesozoic time. During Tertiary time the younger Precambrian formations were extensively faulted and tilted 

throughout the Basin and Range region. Only since 

this orogeny have the Troy Quartzite and Apache 

Group been again widely exposed to erosion. Farther 

north, in the Colorado Plateau, these strata have remained almost horizontal to the present time.

INTERREGION CORRELATIONS

Our knowledge of younger Precambrian rocks elsewhere in the Southwestern United States is still so 

scant that interregion correlation is largely impracticable ; but past speculations can be improved. There 

is now reason to postulate that the sequence of environmental features and unconformities recognized in 

the Apache and Troy section may be partly duplicated 

in the Grand Canyon Series. A search for similar 

indicators of sequence in other shelf deposits of the 

Southwest may provide basis for additional correlations. Aside from being a potential source of material 

for isotope dating, diabase should be viewed with the 

thought that the singular petrography of the southern 

Arizona intrusions may be common to the diabase of 

each of the widely separate terranes; further, the 

diabase may mark a tectonic event common to- all.

Darton (1925, p. 36; 1932) noted that formations 

of the Apache Group have some similarity to the 

younger Precambrian rocks of the Franklin Mountains and Van Horn areas of western Texas, and was 

particularly impressed by their similarity to parts of
GEOLOGIC HISTORY 81

the Grand Canyon Series of northern Arizona. The 

Grand Canyon Series has been divided into two groups 

of formations: the Unkar, or lower group, about 7,000 

feet thick, and the Chuar, or upper group, about 5,000 

feet thick (Walcott, 1894, p. 508-516; Van Gundy, 

1946, p. 1902). Although Darton suggested that the 

basalt flows of the Apache Group might have counterparts in the basalt flows found in the upper part of 

the Unkar Group, he speculated in particular that the 

Troy Quartzite and parts of the Apache Group might 

correlate with the lower part of the Unkar. Stoyanow 

(1936, p. 473-474), who preferred a provisional correlation with the Chuar Group, and Hinds (1935, p. 30; 

see 1937 supp., p. 1994-2000, of Stoyanow, 1936) have 

taken exception to Darton's suggestions.

Formations of the Apache Group crop out through 

a north-south interval of 160 miles, and features distinctive of the individual units are remarkably consistent throughout this interval. The southernmost 

exposure of the Grand Canyon Series is only about 

135 miles distant from the northernmost outcrop of 

the Apache Group (fig. 1); therefore, some of the 

distinctive characteristics of the Apache Group and 

Troy Quartzite might yell be duplicated in the Grand 

Canyon Series.

The descriptions of the Grand Canyon Series by 

Noble (1910; 1914, p. 37-60, 80-83) and Walcott 

(1894, p. 508-518), supplemented by the general statements of Hinds (1935; 1936) and Van Gundy (1934; 

1951), do suggest that comparable features can be 

found. Near the bottom of the Bass Limestone, which 

is 200-330 feet thick and is the lowest formation of 

the Unkar Group if the underlying thin Hotauta 

Conglomerate is excepted, Noble noted distinctive 

chert layers "dotted with small cubic depressions" that 

"strongly resemble salt hoppers." Perhaps these layers are counterparts of vesicular chert layers in the 

lower part of the Mescal that contain halite molds. 

Massive irregular chert nodules, jasper, intercalated 

red or purple shale and thin arkosic sandstone, undulatory banded cherty limestone, and their interrelations and metamorphic equivalents in higher parts 

of the Bass Limestone certainly suggest the solution 

phenomena and related secondary features common 

in the upper parts of the Mescal dolomite and limestone sections. Beds of stromatolitic limestone are 

noted and illustrated for the Bass Limestone, but 

their relative stratigraphic position has not been 

stated. It may be significant that where sections of 

the Bass are thin and solution (?) features suggest 

erosion of the top of the formation, no mention of 

algal structures has been made. According to E. D. 

McKee (oral commun., Aug. 1959) the algal structures in the Bass Limestone are of Collenia aspect like 

those of the Mescal. To the best of his knowledge, 

the stromatolites in the thin limestone units of the 

Chuar Group are all "large biscuit-forms," entirely 

different from those found in the Mescal. Indeed, 

the scant descriptions of the limestone of the Chuar 

hint that they have few features in common.

The Hakatai Shale, which overlies the Bass Limestone and ranges in thickness from 500 to 580 feet, 

is collectively characterized as comprising argillaceous 

shale, dense, hard, and varicolored jasper, blue slate, 

and fine-grained pink or red sandstone, quartzite, or 

mudstone a description that could fit the argillite 

member of the Mescal very well. Neuerburg and 

Granger (1960, p. 766) have presented semiquantitative speotrographic analyses of one specimen of the 

Hakatai Shale and one specimen from the argillite 

member of the Mescal. Both specimens were unusual 

in their high content of potassium about 10 percent 

in the Hakatai Shale and appreciably more than 10 

percent in the Mescal argillite (H. C. Granger, oral 

commun., Oct. 1960). The two specimens show remarkable agreement in trace-element content, and for 

no element except sodium (Mescal argillite, 0.7 percent; Hakatai Shale, 3 percent) were the analyses 

notably different. The two specimens, chosen only as 

seemingly representative of their respective units, 

are virtually indistinguishable megascopically. However, they are very different from siltstone of the 

upper member of the Dripping Spring, which also 

contains unusually large amounts of potassium. If 

such data are representative, singular chemical characteristics may be an aid in correlating younger Precambrian formations of different parts of the Southwest.

In certain aspects the Shinumo Quartzite, which 

overlies the Hakatai Shale and which is about 1,500 

feet thick, resembles the Troy Quartzite. Although 

Noble (1914, p. 51-53) did note lenses of conglomerate, he characterized the sandstone and quartzite of 

the Shinumo as composed of well-rounded well-sorted 

fine grains x of quartz. One of Noble's statements 

implies that the lowest 400 feet of the Shinumo section 

is arkosic. Higher parts of the section are at least 

in part massive and are white, purple, or red. Walcott (1894, p. 511) and Noble made special comment 

on "curiously twisted and gnarled" layers of white 

or purple sandstone which are suggestive of the convoluted sandstone of the Chediski Sandstone Member. 

None of the descriptions indicates that quartzite like

1 From the context of Noble's descriptions, his fine grained designation may be equivalent in part to medium-grained or even coarsegrained designations of the Wentworth scale.
82 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

that of the upper member of the Troy constitutes a 

distinct separate unit of the Unkar section. However, 

E. D. McKee (oral commun., Aug. 1959) confirmed 

that arkose and quartzite like those of the lower and 

upper members of the Troy do occur and stated that 

white, purple and mottled sandstone texturally similar 

to the Chediski Sandstone Member is conspicuous in 

the Shinumo Quartzite. The large-scale cross-stratification so conspicuous in the arkose member of the 

Troy was not noted by McKee, but he did see convolute 

structures very like those in the Chediski.

The Dox Sandstone, which overlies the Shinumo 

Quartzite and is 2,300-3,000 feet thick, is apparently 

highly micaceous, is commonly friable, and has shaly 

partings; except for its basal beds, it seemingly has 

no counterpart in the southern Arizona sections. The 

basalts of the Apache Group are thin compared to 

the 800-1,000 feet of basalt flows noted near the top 

of the Unkar Group. The differences in thickness 

can be overlooked because the southern basalts were 

greatly thinned by pre-Troy erosion; if the Troy and 

Shinumo Quartzites are correlative, however, the basalt sequences can hardly be stratigraphic equivalents. 

The sedimentary formations, especially those of the 

Chuar Group, that overlie the Unkar basalts mostly 

are thin-bedded nonresistant strata unlike any of the 

Apache formations.

Correlatives of the Dripping Spring Quartzite and 

Pioneer Shale have not been suggested in this resume, 

but it may be reasonably surmised that, owing to lapout, they were not deposited at the base of the Grand 

Canyon sequence. Past descriptions have not hinted 

of unconformities that should be recognized in the 

Unkar Group if the ideas presented have merit. It 

would seem that collection of a modest amount of 

additional information might, despite the differences, 

permit firm correlation of the Apache and Troy rocks 

with those of the lowest 1,500 feet of the Unkar Group.

Not far west of the Nevada-California boundary 

and astraddle the Inyo County-San Bernardino 

County line in California, a younger Precambrian sequence of formations known as the Pahrump Series 

(Hewett, 1940) crops out. The belt of Pahrump outcrops, according to Noble (1941, p. 949), does not exceed 25 miles in width and extends northwest from 

the Kingston and Shadow Mountains 75 miles into 

the ranges that border the southern part of Death 

Valley. The Pahrump belt is 200-250 miles west of 

the westernmost outcrop of the Grand Canyon Series 

and about 350 miles northwest of the Apache outcrop 

area.

The Pahrump Series, 5,500-7,000 feet thick where 

described in some detail (Hewett, 1956, p. 25-28;

Wright, 1952, p. 7-15), has been subdivided into three 

formations in ascending order the Crystal Spring 

Formation, Beck Spring Dolomite, and Kingston Peak 

Formation. The Crystal Spring Formation, 1,600- 

2,300 feet thick, includes units of feldspathic quartzite 

and siltstone, red siltstone or shale, quartzite, and 

dolomite that in places is associated with massive 

chert units. The Beck Spring Dolomite, 1,000-1,300 

feet thick, is largely a monotonous sequence of massive-bedded dolomite but includes subordinate beds 

of quartzite and shale. Most of the Kingston Peak 

Formation, 1,000-2,000 feet thick, is conglomerate or 

conglomeratic quartzite, but the upper and lower parts 

are shaly-parting sandstone.

Noble (1934, p. 174), who has worked extensively 

in both the Grand Canyon and Death Valley regions, 

has commented without giving details that parts of 

the Pahrump Series are strikingly similar to the 

Grand Canyon Series. Apparently the Crystal Spring 

Formation, in particular, includes units comparable 

with those in the Apache Group. In May 1952, I 

viewed briefly some of these strata, in the vicinity 

of Tecopa and the Saratoga Hills, under the guidance 

of L. A. Wright of the California Division of Mines 

and Geology. The Saratoga Hills area, 35 miles 

northwest of Baker, Calif., is the locality of the most 

detailed stratigraphic descriptions of the series yet 

published (Wright, 1952). A thick feldspathic quartzite unit makes up the lowest part of the Crystal 

Spring Formation. Overlying this quartzite, and midway in tLe section, is a purple shale or siltstone unit, 

which appears in outcrop remarkably like the siltstone of the Pioneer Shale. Overlying this shale in 

the Saratoga Hills is a brown-weathering dark finegrained quartzite unit which somewhat resembles the 

siltstone member of the Dripping Spring Quartzite. 

The dolomite and chert that make up much of the 

upper part of the Crystal Spring Formation have 

many features, including stromatolite beds, in common 

with the Mescal Limestone. It would be presumptuous, however, to suggest any correlation of the Apache 

Group and Pahrump Series at this time. As Hewett 

(1956, p. 26-28) has pointed out, lateral variations 

of the Pahrump are known, and problems exist in 

correlating units of the series from place to place 

within the Death Valley region.

In the Franklin Mountains, immediately north of 

El Paso, Tex., a Precambrian sequence that is structurally little disturbed was recognized by Richardson 

(1909). The uppermost formation of Precambrian age 

is extrusive rhyolite, as much as 1,400 feet thick, 

which unconformably underlies the Bliss Sandstone of 

Cambrian age. The rhyolite in turn unconformably
GEOLOGIC HISTORY 83

overlies a sequence dominated by beds of quartzite and 

sandstone, 2,600 feet thick, which Richardson termed 

the Lanoria Quartzite. These formations, probably 

owing to the lack of deformation, have been provisionally regarded as occupying the same place in the 

Precambrian sequence as the Apache Group and the 

Grand Canyon Series (Darton, 1932). Recently, 

Harbour (1960) recognized that the Lanoria Quartzite unconformably overlies an indurated rubble of 

basaltic debris, which he termed the Mundy Breccia. 

The Mundy Breccia, as much as 250 feet thick, in 

turn rests on the channeled top of a limestone formation, which Harbour named the Castner Limestone. 

Excluding diabase sills from the type section Harbour 

measured, the Castner Limestone is exposed through 

a thickness of almost 800 feet; its base is intruded by 

granite, and the section is therefore not complete. The 

stromatolite form-species Collenia frequens characterizes a 4-foot bed near the bottom of the Castner section, and in other aspects much of the formation 

resembles the Mescal Limestone. The Mundy Breccia 

might, of course, have a counterpart in the basalt 

flows of the Apache Group. The Lanoria Quartzite 

seemingly is not particularly like the Troy Quartzite.

Strata of younger Precamibrian aspect also are exposed near Van Horn, Tex., about 100 miles southeast 

of El Paso. These strata were tentatively correlated 

with those of the Franklin Mountains by King (in 

King and Flawn, 1953, p. 125-131) after a careful 

consideration of the pitfalls in equating the formations. King also considered the problems of correlating the Precambrian strata of west Texas with those 

of Arizona. Harbour (1960) took exception to King's 

correlations of formations between the Franklin 

Mountains and Van Horn areas.

The information bearing on the problems of correlation of the west Texas formations is given in full 

in the cited publications. Without going into detail, 

I would suggest that if limestone formations were not 

common to both the Texas and Arizona sections, 

there would be little inclination to equate the sections. 

If structural and plutonic events were given more 

emphasis in comparisons, one might be inclined rather 

to correlate the west Texas strata with those underlying the Apache Group. Until additional bases for 

correlation are available, little weight should be given 

to lithologic similarities of the Precambrian formations in the two regions.

Diabase intrusions of Precambian age are common 

in all the Precambrian terranes mentioned in the preceding discussion. However, if the relatively thin 

Apache and Troy terrane is excepted, the intrusions

are restricted stratigraphically to only certain parts 

of each sequence. In the Grand Canyon, diabase 

occurs in great volume only in the Bass Limestone 

and the Hakatai Shale, although thin sills do exist 

in the Shinumo Quartzite and the Dox Sandstone 

(Noble, 1914, p. 55 and pi. 1). Diabase has not been 

reported in the Chuar strata, which by analogy should 

be favorable host types for diabase inflation. The 

inconspicuous unconformity that marks the boundary 

between the Unkar and Chuar Groups (Walcott, 1895, 

p. 325), or one of the similar unconformities stratigraphically not far distant from that contact (Van 

Gundy, 1951, p. 954-957), may therefore have more 

significance than is now appreciated. In the Death 

Valley region, diabase intrusions have been recognized 

only in the Crystal Spring Formation. According to 

Wright (1)52, p. 15) the Beck Spring Dolomite's lack 

of diabase intrusions and the Kingston Peak Formation's abundant diabase detritus, almost certainly derived from the known sills, strongly suggest that "the 

intrusion ] ad taken place before the basal Beck Spring 

dolomite beds were deposited." In the Van Horn 

area of Texas, greenstone bodies that possibly represent diabase sills seemingly are restricted to the Allamoore Formation of the Precambrian sequence (King 

and Flawn, 1953, p. 78-79). In the Franklin Mountains, several thin diabase sills intruded the Castner 

Limestone; these may predate the Lanoria Quartzite 

and definitely are older than the Precambdan granite 

that intruded the Castner Limestone (Harbour, 1960). 

If diabase intrusions have any significance for correlation, the suggestion might be made that the Apache 

and Troy strata correspond to the oldest of the 

younger Precambrian formations in the Southwest.

Feldspathic differentiates, which might provide zircons suitable for isotope dating, have been noted in 

the thickest sill in the Unkar strata. Such differentiates, if t ley exist in the diabase of the Death Valley 

region, have not come to my attention. Descriptions 

indicate that some of the thicker diabase bodies of 

that region are multiple thin sills in which granitoid 

differentiates would not be expected. Sills of the 

Franklin Mountains are also too thin. Greenstone 

of the Var. Horn area is not a likely source of suitable 

material. Owing to pervasive alteration, biotite suitable for >otassium-argon dating would be difficult 

to find in any of the occurrences, except possibly those 

of the Grand Canyon. There are hints in the descriptions that diabases of the canyon are petrographically

data, this 

terranes.

kindred td> those of southern Arizona. For lack of

speculation cannot be extended to the other
84 YOUNGER PRECAMBRIAN GEOLOGY IN SOUTHERN ARIZONA

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Geol. Soc. America Bull., v. 64, no. 4, p. 381-389.

Magleby, D. N., and Mead, W. E., 1955, Airborne reconnaissance project, Dripping Spring quartzite, Arizona: U.S. 

Atomic Energy Comm. RME-2023, issued by Tech. Inf. 

Service Dept. Commerce, Oak Ridge, Term., 17 p.

Neuerburg, G. J., and Granger, H. C., 1960, A geochemical 

test of diabase as an ore source for the uranium deposits 

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Mineralogie Abh., v. 94 (Festband Ramdohr), p. 759-797.

Noble, L. F., 1910, The geology of the Shinumo area, Part 2 

of Contributions to the geology of the Grand Canyon, 

Arizona: Am. Jour. Sci., 4th ser., v. 29, p. 497-528.

1914, The Shinumo quadrangle, Grand Canyon district,

Arizona: U.S. Geol. Survey Bull. 549, 100 p.
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Noble, L. F., 1934, Rock formations of Death Valley, California: 

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 1941, Structural features of the Virgin Spring area,

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no. 7, p. 941-999. 

Peterson, N. P., 1938, Geology and ore deposits of the Mammoth

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 1954, Geology of the Globe quadrangle, Arizona: U.S.

Gecl. Survey Geol. Quad. Map GQ-41. 

Pettijohn, F. J., 1949, Sedimentary rocks: New York, Harper

and Bros., 526 p. 

Ransome, F. L., 1903, Geology of the Globe copper district,

Arizona: U.S. Geol. Survey Prof. Paper 12, 168 p.

 1904, The geology and ore deposits of the Bisbee quadrangle, Arizona: U.S. Geol. Survey Prof. Paper 21, 168 p.

 1915, The Paleozoic section of the Ray quadrangle, 

Arizona: Washington Acad. Sci. Jour., v. 5, no. 11, p. 

380-3C8.

 1916, Some Paleozoic sections of Arizona and their 

correlation: U.S. Geol. Survey Prof. Paper 98-K, p. 133-166.

 1919, The copper deposits of Ray and Miami, Arizona: 

U.S. Geol. Survey Prof. Paper 115, 192 p.

 1923, Description of the Ray quadrangle, Arizona:

U.S. Geol. Survey Geol. Atlas, Folio 217, 24 p. 

Rezak, Richard, 1957, Stromatolites of the Belt Series in Glacier

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Prof. Paper 294-D, p. 127-154. 

Richardson, G. B., 1909, Description of the El Paso district

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Sell, J. D., 1960, Diabase at the Magma mine, Superior, Arizona:

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Sharp, R. P., 1940, Ep-Archean and Ep-Algonkian erosion

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Short, M. N., Galbraith, F. W., Harshman, E. N., Kuhn, T. H.,

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151, geol. ser. 16, 159 p. 

Shride, A. F., 1958, Younger Precambrian geology in southeastern

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Shrock, R. R., 1948, Sequence in layered rocks [1st ed.]: New

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1974.

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 1946, Faulting in east part of Grand Canyon of Arizona: 

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 1951, Nankoweap group of the Grand Canyon Algonkian 

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 1895, Algonkian rocks of the Grand Canyon of the

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1059-1105. 

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 1939, Pre-Cambrian Mazatzal revolution in central

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1163. 

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22 p.

INDEX

A Page

Abris?. Formation._.. __...___ 70,71,72 

contact with Precambrian rocks_____ 69, 71 

description. __......__._____ 69,70

Abrigo Limestone, thickness_________ 69

See also Abrigo Formation.

Acknowledgments..______________ 4 

Age, Apache Group. ......_ ..... 77

diabase .._______________ 76 

founder breccia______________ 31 

iron deposits ......_________ 39

Martin Limestone..___________ 73 

sandstone channel fill__________ 73 

sedimentary formations__________ 77 

Troy Quartzite______________ 44,77 

Albite diabase .________............ 57

Algal member, Mescal Limestone... ____ 25

Mescal Limestone, late-formed chert__ 36

lithology........___.._......... 32

retention of structures in metamorphic

facies_______________ 40 

American Commission on Stratigraphic Nomenclature. ___________ 42 

Amygdules, characteristic of basalt _____ 44 

Apache Group, basalt flows...________ 48 

compared with Crystal Spring Formation. 88 

conglomerates.______________ 6 

correlation ...._____________ 81,S3

deformation._______________ 64 

diabase sills... ______________ 56 

distribution.............................. 7,11

formations.-.._______________ 8 

late Precambrian deformation. _____ 64 

lateral variations in lithology______ 77 

paleogeography- . . 77 

Pioneer Shale.... ... ......._ 11

radioactivity.... ___._____ 34

Tertiary deformation___________ 80

thickness.. . ..___ 7

Apache time, sea encroachment..______ 78

Apatite, in basalt ...._._.. 43

Aplite, feldspathic differentiates_______ 59 

Argillite, void ruling..__.................... 35

Argillite member, Mescal Limestone.. __ 25, S3, 41 

Mescal Limestone, compared with Hakatai Shale --.-. .-...-. ...-. 81 

karst features in_____. _____ 35 

lithology-..-._____________ 34 

metamorphism. . _._______ 34,62 

potassium content__________ 34,81 

radioactivity... ..........__.. 34

thickness...._____________ 41

Arkose, in Dripping Spring Quartzite--___ 15 

in Dripping Spring Quartzite and Troy

Quartzite, compared.... __. 47

in Pioneer Shale_____________ 11, 15

metamorphism.-____________ 62

Arkose member, Dripping Spring Quartzite-- 17,

47,56 

Dripping Spring Quartzite, lithology-.... 18

thickness ______________ 19 

Troy Quartzite-_.____....__ 45,40,47

Asbestos_.____________________ 63,68

[Italic page numbers indicate major references]

B

Barnes Conglomerate Member, Dripping

Spring Quartzite, contact with

arkose member __________

Diipping Spring Quartzite, lithology. . ...

mergence with Scanlan Conglomerate

member. ___________ . _

source and depositional environment.

thickness ____________ . __

Basalt, alteration.. _ . ___ ..... _ . ___

metamorphism ______________ 

soils . ... . 

variolitic.. _ . .....

Basalt flows. 

relation to karst features . .....

source of iron. ________ . _____

Basin and Range structural province, Precambrian outcrops __ .. __

Bass Limestone, correlation... ________

Beck Spring Dolomite, correlation... . 

Biostrome ______ .. ______ . ___ . 

Bolsa Quartzite, contact with Abrigo Formation..................... ..

lithology.... ... .- . 

Breccias, related to karst feature _______ 

See also Founder breccia.

80

17, 19

17 

78

19, SO 

43 

62 

44 

43 

42 

35 

39

2

81

82

32,33

71,72

69

37

Cambrian System, regional relation 69 

sandstone and quartzite.... . . 72

See also Abrigo Formation; Bolsa Quartzite; Peppersauce Canyon Sandstone; Santa Catalina Formation; 

Southern Belle Quartzite.

Castner Limestone, correlation. 83

Channels, in Dripping Spring Quartzite 22

Chediski Sandstone Member, Troy Quartzite- 44

Troy Quartzite, as basal member 47

bedding structures ... 50,51

color..._____._____ ..... . 48,51

compared with Abrigo Formation and

Bolsa Quartzite. 72

composition_______ 48,50 

conglomeratic unit_____._ .. 50 

contact with arkose member. 48 

contact with quartzite member 53 

convolute lamination. 47, 49, 51 

criteria for regional recognition 48 

crossbedding.... 50

description-_____._._.... 47

detritals from Apache Group. __ . 51 

feldspar content. __ 50 

furrowed outcrops___ 49 

lithology- - . __..... . 45,50

metamorphism_____ . . . 61 

northern occurrences... . . 47 

origin_________________ 80 

pebble content-. ___...... 49

quartzite facies. __________ 50 

relation to iron deposits...__ ... 38 

sedimentation_____________ 79

Chediski Sandstone Member Continued 

Tioy Quartzite Continued

soft-rock deformation.. _ ... _ . _ ..

southern occurrences... _ _ .. _ .

stratigraphy 

texture. .. ______ - 

type locality. . 

ventifacts . . 

Chert, in argillite member. . ... 

in karst breccias 

in Mescal Limestone -. ... 27,

in metamorphic limestone. . ____ . _ ..

mechanical concentration 

metamorphism . 

penecontemporaneous. 

See also Late-formed chert.

Chrysotile, asbestos.... .. . 

Classification of bedding and splitting features.

Colorado Plateau, outcrops and structural

disposition of younger Precambrian rocks 

Compaction features, in siltstone member of

Dripping Spring Quartzite - ... 

Cone-in-cone structures, algal member, Mescal

Limestone 

Conglomerate. See Intraformational conglomerates.

Convolute lamination, Chediski Sandstone

Member, Troy Quartzite. ..

origin . 

Correlations, interregion 

Crystal Spring Formation, correlation . 

D

Darton, N. H., quoted 

Dedolomitization . 

Deformation, age. 

Apache Group 

Cherry Creek structural belt... 

faults and folds 

predlabase 6*4,

structural belt, relation to diabase . - -

Deposition environment, Dripping Spring

Quartzite ... 

Diabase, age 

alteration 

composition and color. .. 

correlation 

distinguished from basalt 

feeder conduitsin Crystal Spring Formation... 

in older Precambrian rocks . - 

in Precambrian of west Texas. .- 

intrusions, contacts and forms. 

deformation.. 

metamorphism caused by. .. - 

mistakenly correlated with diabase. . .

multiple. 

related small-scale structures.. 

xenoliths in 

magma, passive injection 

87

51

48

51

47

49

34

37

32,38

40

36

63

30

63

4

23

32

47

49

80

82

25

40

64

64,79

66

65,67

66,67

78

?8

57

57

80,83

83

53, 56

83

53,54

67

61

77

64

68

55

54
88 INDEX

Page 

Diabase Continued

olivlne-bearlng---_____________ 57 

regolith, unconformity__________ 7S 

relation to Paleozoic rocks. ________ 76 

sills ...___....________ 53, 55,56

concentrations of olivlne _____ 58 

distribution relative to structural

belts___.__________... 66 

Dripping Springs Quartzite_____ 56 

feldspathic differentiates..____... 60 

soils._____..__________ 44 

See also Albite diabase; Feldspathic differentiates; Petrology; Weathering.

Diabase pegmatite, distribution In sills___ 58,61 

emplacement___________.....__ 59

feldspathic differentiates______.__ 58 

mineralogy________________ 58 

origin __________________.. 61 

See also Feldspathic differentiates. 

Dikes. See Diabase. 

Distribution, Apache Group ..-.._..... 7

basalt flows_.______________ 42,43 

Precambrian rocks.-.-__________ 7 

Troy Quartzite.__. .__....___ 7,52

Dolomite, Mescal Limestone, color and Iron

content______________ 39

metamorphism______________ 63

transition to metamorphic limestone__ 40

Dox Sandstone, correlation__________ 82

D ripping Spring Quartzite, arkose member... 17,

M.47,56

basal conglomerate . __________ 17 

deposition environment----.__-___ 78

division...________________ n 

erosion..__________________ 19 

granitic detritus- _____________ 6 

lithology----.______.___-__... 19

Mescal contact, unconformity-.____. 24

metamorphism______________ 62

northern lapout-..____________ 6

potassium content and pyrite______ SI

reference section______________ 18

siltstone member__. __________ 17

description.-_____________ 2J

diabase sills in____________ 56

lithology---.__--------_-_..... 18

recognition of units__________ 24

topographic expression._______ 21, 24

stratigraphy.___.___________ 17, 18

tidal flat deposits-_____....._..... 78

underlain by Scanlan Conglomerate___ 16 

uranium content.-.-_ ._____.. SI 

See also Diabase; Radioactivity; Weathering.

E

Erosion, intraformational effects in Mescal

Limestone____._______ 41

late Cenozoic_______._______ 65

post-Paleozoic_______________ 7

pre-Bolsa._________________ 42

pre-Martln_______________... 42

pre-Troy------.--- - ._..__.. 41

Evaporites. leaching____________.._ 31

Mescal Limestone__._._...____ 31

F

Faults, inflationary origin.._._._.__.. 68

relations to diabase Intrustions.... ___ 54

See also Deformation. 

Feldspathic differentiates___________ 58

See also Aplite; Diabase pegmatite; Granophyre; Xenoliths. 

Feldspathic sandstone, in Pioneer Shale...... 15

Feldspathic siltstone, metamorphism_. __ 62

Folds. See Deformation.

Fossils... ....__.___....... 32,53,69,70, 71,72

See also Stromatolites.

Page

Fossil laterites, iron deposits as________ 39

Founder breccia_. ______________ 31

Mescal Limestone, description______ 28,79

metamorphism-___________. 40

Fucoids, Abrigo Formation. .._._.__ 72

Q

Geologic history............ .........

Grand Canyon Series____ 

Granite, diabase sills in___.____. 

Granophyre, feldspathic differentiates.. 

Gravity differentiation, In diabase sills.

H

Hakatai Shale.... 

Halite, association with chertfounder breccia, relation-- 

molds, In Bass Limestone. 

in chert___. _ 

in Mescal LimestoneHematite, earthy____

in basalt.._ _ 

In Mescal Limestone......

specular_______.....

Hematitic clay...______.

 .. 81

 ... 29

 ...... 31

 81

 ...- 29

.... 29

 . 38

 - 43

... ... 58

......... 38

 . 16

Hinds, N. E. A., quoted _-______- 42

Hotauta Conglomerate..----- - 81

Intraformational conglomerates...._____ SO 

Intrusive rocks, diabase sills and dikes 63 

Iron deposits-....____ 38 

See also Chediski Sandstone Member, 

Troy Quartzite; Mescal Limestone; Swamp Creek iron deposits.

Joints, relation to fault deformation ........ 68

K 

Karst breccias________... -...- 57

Karst features.___._. . . 55,38 

See also Basalt flows; Mescal Limestone;

Troy Quartzite. 

Kingston Peak Formation, correlation ... 82

L

Lanoria Quartzite, correlation. ... 83 

Late-formed chert, remnants of first-formed

chert... --__.--....__... 36

Leaching of evaporites_... . 31

Limestone, calcitic, origin..________.. 59 

in argillite member, Mescal Limestone... 34 

metamorphic____----- 39

Lithology, Dripping Spring Quartzite, Bames

Conglomerate Member...... . 17

Dripping Spring Qnartzite, siltstone member.-.. . .. 21

Mescal Limestone, algal member...___ 26,32

argillite member___________ 26, 84

Pioneer Shale, arkose... -------__.... 16

Scanlan Conglomerate Member...... 11

pre-Devonian formations.. . 8

Troy Quartzite, arkose member... __. 46

Chediski Sandstone Member.-___ 46,47

Quartzite member . 46,61

Location, area of study.___ ... 2

McKee, E. D., classification of bedding and

splitting features...-.___._._. 4 

Magnetite___________________ 64 

Martin Limestone____________.__ 73,76

Mescal Limestone, algal member Si 

argillite member.._ ..... 55,42

karst features in. _ ___ 55 

metamorphism-. . 62 

Basalt flows overlying- 45 

biostrome..____ . . 33 

breccia In . 24 

chert In...... ..... . - 27,28

comparison with Bass Limestone. .... 81

description.-...-__ . 25 

diabase sills...- .- 56 

evaporite environment- ... _.____ 29 

founder breccia 79 

halitemolds In.-... 29 

intraformational conglomerates In - - - __ 50 

iron deposits __ 58 

karst features ... 35,37 

lithology, carbonate members..... 25

unmetamorphosed sections. _.... 27

lower member...... 27

metamorphism-__ .... . 25,59.62,63

pre-Troy erosion_.. . _.. 38 

reference section 2tf 

saline deposit...__ 78 

sandstone in_... 27 

sandstone-filled cavities..... . 35

sheaflike impressions... -- 29 

solution features.... 55

stratigraphic nomenclature. 42 

thickness 41,42

thinning, causes of 41 

Metamorphic limestone--- - 25,40 

Metamorphic minerals in carbonate rocks... - 63,64 

Metamorphism, areal extent - 61 

chert and dolomite.. 63 

founder breccia . 40 

Precambrian rocks, adjacent to diabase... 62 

See also Arkose; Basalt, Diabase Intrusions; Dripping Spring Quartzite; 

Feldspathic siltstone; Mescal 

Limestone; Pioneer Shale; Troy 

Quartzite. 

Mud cracks, siltstone member, Dripping

Spring Quartzite------ .. 22

Mundy Breccia, correlation-..- 83

N 

Noble, L. F., quoted. 81

O 

Olivlno diabase 57,58

Oracle Granite...... - - -------- 6

Pahrump Series, correlation. 82

Peneplain, pre-Apache... 77

Peppersauce Canyon Sandstone of Stoyanow. 70

Petrology, basalt flows.-. 43

diabase and related rocks 56

granophyre____ 59

Final Schist-. _ 4

Pioneer Shale, along San Pedro River valley. 17 

arkose 11,16,16

ash falls-...__ .. -- 78

bedding, tuffaceous mudstone. 14

bleached zones . - 14

deposition environment-.. 78

distribution.--- . 11

granitic detritus.. 6

lateral variations.. 16

metamorphism- 62

northern lapout---- 6,16

pyrite. U

Scanlan Conglomerate Member.... 11

shard content 14

sills.... 56

stratigraphy..__ - 11 

tuffaceous mudstone 11, 14, 15

unconformity 19
INDEX 89

Plagioclase phenocrysts, In basalt flows___

Potassium content, argillite member, Mescal

Limestone..___________

Dripping Spring Quartzite----...........

Hakatai Shale __._....____....

Pre-Apaehe peneplain, Arizona...............

Precambrian rocks.------ _ .. .

deformational history............

distribution...... ........

in Franklin Mountains, Texas... 

in northern Arizona...___....

paleogeographic features..._... 

relations to Paleozoic formations, 

stratigraphy ... 

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

thinning, cause of . ........

Prediabase deformation..__.......

Page 

43

34

SI 

81

77

80

2,7

82

81

Q

Quartzmonzonite. _._.. ...._ 6,62 

Quartzite-___________________ 4, 72 

Quartzite member, Troy Quartzite..... .. 45,51

Radioactivity, argillite member, Mescal Limestone. . _. __.____ 34

Dripping Spring Quartzite-----_____ 81 

Ransome, F. L., quoted_______ 2,24,25,71,77 

Reference section, Dripping Spring Quartzite. 18

Mescal Limestone_____________ $6

Troy Quartzite . . _ 45 

Refractive indices, diabase. ...______ 57 

Regolith, granitic, pre-Apache.._______ 7

underlies Pioneer Shale.... .... 62 

Rezak, Richard, quoted..__.. . ... 32 

Ripple marks, related to sheaflike markings.. 30

siltstone member, Dripping Spring Quartzite . 22

Rock-color chart- _-----_ - 4

Ruin Granite._________________ 6

Salt River, thickness of Troy Quartzite south

of..... - ... 7

Sandstone channel fill. age. . .. ... 73

Sandstone-filled cavities, in Mescal Limestone. 35

relation to Troy Quartzite- .. 36

Santa Catalina Formation....__.____.. 70

Scanlan Conglomerate Member, Pioneer

Shale...... . 11, 14

Pioneer Shale, composition___...__ 78 

simlliarity to Barnes Conglomerate 

Member of Dripping Spring Quartzite-.. . ........ 19

source and depositional environment. 78

thickness___ _________ 16

Schopf, J. M., quoted-..._______.__ 73

Scour-and-fill features, distribution.__..._ 24

in siltstone member of Dripping Spring

Quartzite - .... .__... 22

Sedimentary rock units _ ________ 4,77 

Serpentine-__________________ 63 

Shards, in Pioneer Shale___________ IS 

Sheaflike markings...__..____.___ 28,30

Page

Shinumo Quartzite, correlation._______ 81,82 

Short, M. N., quoted . . 55 

Shrinkage cracks, in siltstone member of Dripping Spring Quartzite-..____ 23 

Sierra Ancha, thickness of Troy Quartzite in. 7 

Silicification.................... .......... 35

Sills. See Diabase.

Siltstoue member, Dripping Spring Quartzite- 17

Dripping Spring Quartzite, bedding___ 88

diabase sills.... ................. 56

secondary structures. _______ 28

thickness ___..___ _... 19

Slump features, in cherty dolomite...-. 31

in Mescal Limestone.-. _______ . 30

minor..___...____ -__ _ 31

Solution, effects..--.- . . 35

Solution features, relation to late-formed chert. 36

Mescal Limestone.____________ 35

Southern Belle Quartzite 70

Stratigraphy, Dripping Spring Quartzite--_ 17, 18

Mescal Limestone_____________ 85

lower member .........____.. 27

Pioneer Shale.. .. .. . 11

preservation in karst areas . . 36

Troy Quartzite- . ... 45

arkose member. . 46

Chediski Sandstone Member.__.... 47,48

quartzite member....________. 58

Stromatolites______ __..... . 83

biostrome.--____________-__ 33

Chuar Group . 81

descriptions.. _____.. . 32

in algal member, M escal Limestone... 32

in Bass Limestone.______.____ 81

intertidal accumulations._^____._ 79

Stylolites, in Dripping Spring Quartzite-.. _ 22

Swamp C reek iron deposits, d escribed.. ___ 38

T

Teichert, Curt, quoted...__._-__-__ 73

Thickness, Abrigo Limestone _______ 69

basalt flows. - ........ ...... 42,45

Bolsa Quartzite..... .......... ...... 69,71

diabase sills... 53,55

Dripping Spring Quartzite-..._____ 19 

arkose member.. ..... 19

Mescal Limestone_____________ 41,42

Pioneer Shale. 11

Scanlon Conglomerate Member... _ 14,16

Precambrian rocks.....__.. ___.. 7

Troy Quartzite. ._ . 44 

arkose member ____________ 46 

Chediski Sandstone Member____. 4? 

Topographic expression, basalt flows .___ 43 

diabase sills... ______________ 53 

Dripping Spring Quartzite, arkose member.. .. 20,21,24

metamorphic limestone. _ _____ 40 

Troy Quartzite, Chediski Sandstone

Member....___________ 47

quartzite member...... _______ 52

Troy Quartzite, arkose member... ... 47 

comparison with Shinumo Quartzite.__ 81 

conglomerates- 6,45,48 

criteria for distinguishing from Cambrian

sandstones____________ 72,74

Page 

Troy Quartzite Continued

definition..... . .... ....... . 44

depth of cover in Precambrian time... _ 80 

diabase sills . 66 

distinctive features of members,._._.. 68 

distribution.... 7,11

division--. ~_.. 45 

eolian aspects _______.__ 79 

erroneous correlation with Cambrian

formations... ........... 72

granitic detritus...___.________ 6 

lateral variations--. . . .... 77

metamorphism ... __.... 61

overlain by Martin Limestone. ____ 73 

Precambrian deformation._______.. 64 

quartzite member... __.... 51

reference section . 45 

relation to basal Paleozoic sandstone near

Payson . ~_.. . 76 

relation to karst features..________ 35 

significance of contacts between members. 79,80 

stratigraphy- ...... . 45

thickness-____ ... 7,44,45 

Tuffaceous mudstone, Pioneer Shale ..... 14

U

Unconformity, along diabase sills.______ 56 

at top of algal member,Mescal Limestone. 34 

diabase regolith __ _._. . 78 

Pioneer Shale . ._... . . 15 

pre-Apache_ 6, 16,77 

pre-Bolsa__... . .. . _._ 69 

pre-Bolsa surface, configuration._____ 71 

pre-Dripping Spring .. . . 11,78 

pre-Martin_...... . .. - 69

pre-MescaL- - 84, 78 

pro-Middle Devonian.... . . 73

pre-Troy... - 44.79 

relation of pre-Bolsa and pre-Martin erosions surface ------- 69

relations to breccias 31 

relief on pre-Martin surface_______ 73

Unkar Group. . - 81

Ventifacts, in Chediski Sandstone Member_ 49, 79 

Volcanic rocks_____ 4

W

Weathering, diabase _ ... . 57 

Dripping Spring Quartzite...... ... 81

Precambrian.. .. 35 

Troy Quartzite, quartzite member. __ 52 

Weir, G. W., classification of bedding and

splitting features.....__.___ 4

Wentworth, C, K., grain-size scale by.___ 4 

Wilson, E. D., quoted. 25

Xenoliths..-. . .. 55,60,61

Yavapal Series.

U.S. GOVERNMENT PRINTING OFFICE : 1967 O - 248-121