Revision of Precambrian 

Stratigraphy in the 

Prescott-Jerome Area, 

Yavapai County, Arizona

By C. A. ANDERSON, P. M. BLACET, L. T. SILVER, and T. W. STERN

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1324-C

Prepared in cooperation with the 

California Institute of Technology
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog card No. 71-610541

U.S. GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1971

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

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract. _________________________________________________________ Cl

Introduction. _____________________________________________________ 1

Ash Creek Group.________________________________________________ 3

Big Bug Group_-__------_---_------_-_-_-------------_--_-_---__- 7

Chaparral Volcanics-__________________________________________ 8

Green Gulch Volcanics.________________________________________ 8

Texas Gulch Formation.___________________________________________ 10

Brady Butte Granodiorite_-___---______--___-___--__--__--___--_-_- 11

Isotopic studies.__________________________________________________ 12

Abandonment of name Alder Group in this area ______________________ 14

Yavapai Series..___-_-___-_--_-______-_-__--__-___--______----_-_- 15

References _______________________________________________________ 16

ILLUSTRATIONS

FIGURE 1. Index map of Arizona showing location of area discussed.____ C2

2. Geologic map of the Prescott, Mingus Mountain, Mount

Union, and Mayer quadrangles.___________-_--_----__-- 4

3. Generalized sections of the Ash Creek and Big Bug Groups and

of the Texas Gulch Formation._________________________ 7

4. Concordia diagram of Pb2°e/U238-Pb207/U235 systematics in 

cogenetic zircon suites from Precambrian volcanic and 

plutonic rocks________________________________________ 13

in

CONTRIBUTIONS TO STRATIGRAPHY

REVISION OF PRECAMBRIAN

STRATIGRAPHY IN THE PRESCOTTJEROME AREA, YAVAPAI COUNTY,

ARIZONA

By C. A. ANDERSON, P. M. BLACET, L. T. SILVER, and T. W. STERN

ABSTRACT

The metamorphosed volcanic, volcaniclastic, and sedimentary rocks that predate the oldest plutonic rocks in the Prescott-Jerome area (1,770±10 m.y.) 

are placed in the Yavapai Series, a provincial time-stratigraphic term denned 

as the 'time interval from 1,770±10 to 1,820+m.y. Two rock-stratigraphic 

groups are recognized; the Ash Creek Group is1 separated from the Big Bug 

Group by a major fault and a large pluton of quartz diorite. Isotopic dating now 

indicates that the Ash Creek Group is older. The name Big Bug Group replaces 

the term Alder Group, used in earlier publications on the Prescott-Jerome area. 

Within the Big Bug Group the Indian Hills' Volcanics and Green Gulch Volcanites 

have beem shown to be the same formation, and the latter term is retained. 

Where first described the Chaparral Volcanics was bounded by faults, but subsequent mapping has demonstrated that it occupies the upper part of the Spud 

Mountain Volcanics; so the term Chaparral Volcanics is abandoned. The Texas 

Gulch Formation is removed from the Yavapai Series because it is separated 

from the rocks of the Big Bug Group by a period of plutonism, represented by 

the Brady Butte Granodiorite, and subsequent erosion.

INTRODUCTION

Precamibrian metamorphic rocks are exposed in four 15-minute 

quadrangles in the Prescott-Jerome area (figs. 1 and 2). The purpose 

of this report is to revise some of the stratigraphic nomenclature 

given in reports by Anderson and Creasey (1958) and Krieger (1965) 

that cover the Mingus Mountain and Prescott quadrangles (fig. 2). 

Anderson and Blacet completed the Mount Union and Mayer quadrangles in 1968, and the evidence requiring this revision is based on 

the additional mapping and on geochronological studies made by Silver

Cl
C2 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 1. Location of area discussed.

and Stern of U-Pb isotope systems in zircons from the Precambrian 

rocks.

The metamorphic rocks are largely of volcanic origin, and sufficient 

relict textures and structures are exposed to determine the character 

of the rocks before metamorphism; therefore, the prefix "meta" is not 

used in our stratigraphic nomenclature. Most of the Precambrian strat-
PRECAMBRIAN STRATIGRAPHY, PRESCOTT-JEROME AREA C3

ified rocks have been metamorphosed to the greenschist facies, but 

adjacent to younger intrusive rocks, higher grades of metamorphism 

were developed.

In the Mingus Mountain quadrangle, Anderson and Creasey (1958) 

distinguished two groups, the Ash Creek and Alder, separated by the 

Shylock fault, and no evidence was found to indicate the age relations 

of these two groups. Isotopic dating now reveals that the Ash Creek 

Group is the older. New evidence requires that Big Bug Group be 

substituted for Alder Group in the Prescott-Jerome area. Proposed 

revisions of formational status are limited to the Big Bug Group.

Acknowledgments. The isotopic studies of uranium-lead systems 

described by Silver in this report are part of a program of research in 

the application of basic principles of U-Th-Pb isotope geochemistry to 

probbms of geology. This aspect of research is supported by the U.S. 

Atomic Energy Commission under Project Agreement 1 under Contract AT (04-3)-767, for which this contribution is CALT-767-57, 

and by the California Institute of Technology, for which this is Division of Geological Sciences Contribution 1739.

ASH CREEK GROUP

The Ash Creek Group is exposed in the northern half of the Mingus 

Mountain quadrangle (fig. 2). The Shylock fault is the western boundary and the Verde fault is the eastern boundary. A pluton of quartz 

diorite intrudes the Ash Creek to the south. Paleozoic sedimentary 

rocks and Tertiary volcanic rocks cover the Ash Creek Group to the 

north and east.

The Ash Creek Group is approximately 20,000 feet thick, and 

neither the bottom nor the top of this sequence is exposed; the Ash 

Creek consists of nonfoliated basalts, rhyolitic tuffs, breccias, and 

flows, dacitic flows and domes, andesitic flows and breccias, and mixed 

andesitic and silicic bedded tuffs. Of the formations in the Ash Creek, 

Gaddes Basalt is the oldest formation, consisting largely of pillow 

lavas, and the Grapevine Gulch Formation is the youngest, consisting largely of bedded tuffs and breccias. The general character of 

each formation and its stratigraphic position are shown in figure 3.
C4 CONTRIBUTIONS TO STRATIGRAPHY
r* gV + ?S&J'.:'i:S* + + + + + +-V ' ; r * + K'5S-. (f;^'J"A+ + + + + + +\ + ^J L. +^txfe^^»] + + Av+ + + * ^.j***?" -"' - > '-;A+ + +gy + + + ' +

112°30 112°00'

FIGURE 2. Geologic map of the Prescott, Mingus Mountain, Mount Union, and Mayer quadrangles. (See map explanation on

following page.)
C6 CONTRIBUTIONS TO STRATIGRAPHY

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PRECAMBRIAN STRATIGRAPHY, PRESCOTT-JEROME AREA C7

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FIGTJBE 3. Generalized sections of the Ash Creek and Big Bug Groups and of 

the Texas Gulch Formation.

BIG BUG GROUP

The Big Bug Group is exposed more or less continuously southward from the Prescott and Mingus Mountain quadrangles into the 

Mount Union quadrangle and western margin of the Mayer quadrangle (fig. 2). It is also exposed east of the Shy lock fault zone and 

south of the younger pluton of quartz diorite in the Mayer quadrangle. The Big Bug Group is here named for its exposures along Big 

Bug Creek, originating west of Big Bug Mesa in the Mount Union 

quadrangle and joining the Agua Fria River in the Mayer quadrangle 

(fig. 2). This type locality cuts across a complete section of the Spud 

Mountain Volcanics and Iron King Volcanics. In the eastern half of 

the Mount Union quadrangle and the western part of the Mayer 

quadrangle, the Big Bug Group is isoclinally folded and, in general, 

well foliated. East of the Shylock fault zone and northwest of the
C8 CONTRIBUTIONS TO STRATIGRAPHY

Chaparral fault, the rocks are less foliated and isoclinal folding is 

limited. The group is divided into three formations the older Green 

Gulch Volcanics and the younger Iron King Volcanics, which contain appreciable pillow and amygdaloidal basalts, and the middle 

formation, the Spud Mountain Volcanics, which consists of an intertonguing complex of andesitic and rhyolitic pyroclastic rocks and 

amygdalodal basaltic flows (fig. 3).

In this report, data are given to document the need of substituting 

the name Big Bug Group for Alder Group, which Anderson and 

Creasey (1958, p. 20) described as containing six formations, divided 

into two classes. One class comprised three volcanic formations of 

probable known stratigraphic succession. From oldest to youngest, 

they were the Indian Hills Volcanics, Spud Mountain Volcanics, and 

Iron King Volcanics. The second class included three formations of 

unknown stratigraphic succession: the Chaparral Volcanics, Green 

Gulch Volcanics, and Texas Gulch Formation.

CHAPARRAL VOLCANICS

The Chaparral Volcanics, as originally defined, consists of a relatively narrow band of intertonguing rhyolitic and andesitic tuffaceous 

rocks that are strongly foliated and isoclinally folded. It occurs southeast of the Chaparral fault in the northeast corner of the Mount 

Union quadrangle and southeast corner of the Prescott quadrangle. 

The southeastern boundary is a fault, which separates the Chaparral 

Volcanics from the Spud Mountain Volcanics. Subsequent mapping 

by Anderson southward into the Mount Union quadrangle has shown 

that this fault dies out and that the Chaparral Volcanics is the upper 

part of the Spud Mountain Volcanics. Therefore, the name Chaparral Volcanics is abandoned.

GREEN GULCH VOLCANICS

The Green Gulch Volcanics is a sequence of flows and tuffaceous 

beds well exposed in Green Gulch northwest of the Chaparral fault in 

the southeastern corner of the Prescott quadrangle. Anderson and 

Creasey (1958, p. 31) defined this formation in their report on the 

geology and ore deposits of the Jerome area which included part of 

the Prescott quadrangle in the area surrounding the Iron King mine. 

The geologic mapping was done by Krieger (1965), who demonstrated 

that the formation consists of an eastern unit of basaltic flows and a 

western unit of dominantly rhyolitic flows and admixed rhyolitic and 

andesitic tuffaceous rocks.
PRECAMBRIAN STRATIGRAPHY, PRESCOTT-JEROME AREA C9

In the area west of the Chaparral fault in the Prescott quadrangle, 

the tops of basaltic flows face either east or west, but the prevalence 

of west-facing flows suggests that the rhyolitic rocks to the west overlie the basaltic unit. Krieger (1965, p. 17) estimated that the Green 

Gulch Volcanics in that particular area is more than 5,000 feet thick. 

Recent mapping in the northwestern part of the Mount Union quadrangle (fig. 2) suggests a greater thickness, and 7,000 feet is more 

likely the minimum.

Unfortunately, in the southern part of the Prescott quadrangle 

and in the northern part of the Mount Union quadrangle (fig. 2), the 

Green Gulch Volcanics is intruded by several diverse igneous rocks 

so that correlations between the widely spaced stratified rocks are 

difficult. Krieger (1965) cautiously avoided assigning specific formation names to some of the volcanic rocks in the southern part of the 

Prescott quadrangle and to the west of the exposures of the type 

Green Gulch Volcanics; these rocks were referred to as "unnamed 

volcanic rocks of the Alder (?) Group."

Recent mapping of the southern extension of these unnamed volcanic rocks in the northern part of the Mount Union quadrangle has 

shown that these volcanic rocks can be assigned with assurance to the 

Green Gulch Volcanics. At the center of the northern margin of the 

Mount Union quadrangle (fig. 2), an anticline is exposed with a core 

of slate beds conformably overlain by Green Gulch basaltic flows. 

Excellent top determinations indicate west-facing flows west of the 

slate core and east-facing flows to the east. The slate beds are now 

considered the basal unit of the Green Gulch Volcanics, and the probability that they are the products of subareal weathering of older 

rocks suggests that an unconformity may exist at the base of the Big 

Bug Group.

The Indian Hills Volcanics was named by Anderson and Creasey 

(1958, p. 21) for a sequence of basaltic and rhyolitic flows exposed 

in the Indian Hills that protrude through Cenozoic rocks in the western 

part of the Mingus Mountain quadrangle. Sufficient geologic mapping 

by Anderson and Blacet has been done in the Mayer and Mount Union 

quadrangles to document that the lower part of the Spud Mountain 

Volcanics consists dominantly of andesitic breccias, whereas the upper 

part consists dominantly of thinly bedded tuffaceous rocks (fig. 3). 

In the Indian Hills (Mingus Mountain quadrangle), Spud Mountain 

andesitic breccias are conformable with the Indian Hills Volcamcs that 

crops out west of the breccia. Exposures of the younger Spud Mountain tuffaceous beds to the east of the andesitic breccia (Anderson and 

Creasey, 1958, pi. 1), prove that the Spud Mountain Volcanics faces 

east in that area and that the Indian Hills Volcanics is older.
CIO CONTRIBUTIONS TO STRATIGRAPHY

The small area of Spud Mountain Volcanics northwest of the 

Chaparral fault is exposed between east-facing Green Gulch Volcanics 

(on the east side of the anticline, northeastern part of the Mount Union 

quadrangle, fig. 2) and west-facing Green Gulch Volcanics in the type 

locality (west of the Chaparral fault, Prescott quadrangle). This mass 

of Spud Mountain Volcanics consists of the lower breccia facies, indicating that it occurs near the keel of a syncline. Unfortunately the 

younger intrusive rocks have destroyed much critical evidence, but 

the general .relations strongly suggests that here Spud Mountain 

breccia is younger than the Green Gulch Volcanics.

The lithologic similarity of the Green Gulch and Indian Hills Volcanics, plus the strong evidence that both underlie the Spud Mountain 

breccia, essentially prove that the Green Gulch and Indian Hills Volcanics, consisting largely of basaltic and rhyolitic rocks, are the same 

formation. The name Green Gulch Volcanics was selected as the stratigraphic term to be retained, and the term Indian Hills Volcanics was 

dropped (Anderson and Creasey, 1967).

TEXAS GULCH FORMATION

The Texas Gulch Formation, as defined by Anderson and Creasey 

(1958, p. 28-30), is composed of alternating belts of bedded rhyolitic 

tuff and purple to gray slate, cropping out in a band about 2,500 feet 

wide in the Shylock fault zone in the Mingus Mountain quadrangle. 

The formation at this type locality is so intensely deformed that the 

abrupt termination of a particular slate or tuff band cannot be distinguished as a fold, a lens, or a gross boudin. Both the east and the west 

boundaries of the formation are faults, and internally most of the contacts between tuff and slate are probably faults. Anderson (1967) has 

defined this intensely sheared band as part of the Shylock fault zone in 

which the Texas Gulch Formation terminates abruptly in the northwest corner of the Mayer quadrangle (fig. 2).

Geologic mapping by Anderson and Blacet in the Mount Union - 

quadrangle has revealed a similar formation with a maximum outcrop width or more than 2 miles. The eastern margin of this band of 

Texas Gulch Formation is in fault contact with the Iron King Volcanics, and the western margin is in fault contact with the Spud Mountain Volcanics. This band of Texas Gulch Formation extends northeastward to the northeast corner of the Mount Union quadrangle, 

where it has an outcrop width of about 300 feet ( shown only as a fault 

in fig. 2). Gray slate is more common than purple slate in this belt of 

Texas Gulch Formation, and more arkosic beds are present than in 

the Shylock fault zone. Rhyolitic tuff is more common to the north 

than to the south, but the general lithologic similarity to the Texas
PRECAMBRIAN STRATIGRAPHY, PRESCOTT-JEROME AREA Cll

Gulch Formation in its type locality supports correlation of these 

two belts of slate and rhyolitic tuff.

Blacet (1966) found that the wide outcrop of Texas Gulch Formation is in depositional contact with the underlying Brady Butte Granodiorite (fig. 2). Angular blocks of the granodiorite appear in a basal 

conglomerate that is dominated by rounded cobbles and boulders of 

granophyre. Several hundred feet of arkose and feldspathic sandstone 

grades upward into bedded rhyolitic tuff and gray slate, with minor 

interbeds of pebble conglomerate. Anderson (1968, p. 14) concluded 

that this uncomformable relationship to the Brady Butte Granodiorite 

proved that the Texas Gulch Formation is the oldest formation in the 

Alder Group. This interpretation was based on the assumption that the 

Texas Gulch Formation is the correlative of the slate sequence in the 

core of the anticline in the Green Gulch Volca,nics that is exposed near 

the north-central margin of the Mount Union quadrangle (fig. 2). This 

same lithologic correlation was made in the Prescott quadrangle by 

Krieger (1965) on the advice of Anderson during the period when 

mapping was in progress in the Prescott and Mingus Mountain 

quadrangles.

The isotopic data and the field relations indicate that the Texas 

Gulch Formation is not the oldest formation in the Alder Group; 

rather, the Texas Gulch Formation in this area must be younger than 

the Big Bug Group.

BRADY BUTTE GRANODIORITE

The Brady Butte Granodiorite is exposed in the core of a major 

faulted anticline that plunges gently northward (fig. 2). The granodiorite is somewhat gneissic, with local zones of mylonite (Blacet, 

1966). The southwestern part of the Brady Butte Granodiorite is 

coarsely crystalline to porphyritic granodiorite (loc. 4, fig. 2) and is 

clearly intrusive into the Spud Mountain Volcanics. Our first interpretation was that the porphyritic f acies is younger than the Brady Butte 

Granodiorite, in harmony with the conclusion that the Texas Gulch 

Formation is the oldest unit in the Big Bug Group; however, isotopic 

measurements made by two authors, Silver and Stern, have shown 

that the Brady Butte Granodiorite and the southerly porphyritic 

granodiorite are essentially the same age. Mapping by Blacet failed 

to reveal clearcut intrusive relations between the porphyritic and 

nonporphyritic granodiorites, and chemically these two f acies are indistinguishable. These data indicate that the Brady Butte Granodiorite at its type locality, where it uncomformably underlies the Texas 

Gulch Formation, is also younger than the Spud Mountain Volcanics, 

the middle formation of the Big Bug Group (fig. 3).
C12 CONTRIBUTIONS TO STRATIGRAPHY

ISOTOPIC STUDIES

Uranium-lead isotope systematics in cogenetic zircon families (Silver and Deutsch, 1963) have been studied from samples of many different igneous rock units in the local stratigraphic column. The 

analytical data are too extensive and complex for effective presentation 

here and will have to be published separately.

There are a number of significant age relations apparent in the 

isotope data which bear directly on the problems of the Precambrian 

stratigraphy. They are presented in a summary form in figure 4, a 

Pb206/U238-Pb207/U235 concordia diagram (Wetherill, 1956). Two or 

more fractions of zircon from each rock unit, generally from a single 

sample and differing distinctly from each other in uranium concentrations, have been analyzed. In general, the individual fractions of each 

family differ significantly from each other in their daughter-parent 

isotope ratios, indicating some isotopic disturbance. On the concordia 

diagram, the maximum extent and direction of the observed differences 

between the zircon fractions are represented by lines (chords). From 

the extrapolation of these lines to upper intersections with concordia, 

the time of initiation of the isotope system (zircon crystallization) is 

inferred. Where the zircon families appear to be very similar in age 

and in degree of isotopic disturbance (discordance), lateral displacement of plotted positions on the diagram is an indicator of slight differences in ages. From these systematic relations, it is possible to develop interpretations of apparent ages which are quite precise and 

hence capable of remarkable resolution. The general basis for this 

approach has been established by Silver and co-workers elsewhere 

(Silver, 1963a, b,and unpub. data).

The general pattern of isotope relations in zircons from two volcanic 

units and three plutons (fig. 4) indicates that all samples are discordant and accordingly have been disturbed. The degree of discordance 

varies from about 5 to 50 percent. Despite the discordance, all the family lines, each representing a rock unit, extrapolate to an apparent 

age interval of between 1,760 and 1,820 m.y. (million years). Stratigraphically significant age resolution can be made within this interval.

The zircons obtained from the Spud Mountain Volcanics and Deception Ehyolite can be attributed to the crystallization episodes of 

each of the host rocks, by virtue of their morphology, trace-element 

characteristics, and certain isotope systematics. Zircons from a quartz 

porphyry facies in the upper part of the Deception Khyolite of the 

Ash Creek Group near Jerome yield an apparent age of 1,820±10 m.y. 

Zircons from a rhyolite flow in the upper part of the Spud Mountain 

Volcanics north of Big Bug Mesa are apparently 1,775 ±10 m.y. old.
$0.20

2.5 3.0 

pb207 /lj235

FIGURE 4. Concordia diagram of pb^VU^-Pb207/!!235 sy sterna tics in coge

volcanic and plutonic rocks.
C14 CONTRIBUTIONS TO STRATIGRAPHY

These ages suggest that a large part of the Big Bug Group is distinctly 

younger than one of the younger formations in the Ash Creek Group. 

Provisionally (lacking adequate field relations to test the assignment 

properly), the entire Big Bug Group is considered to be younger than 

the Ash Creek Group.

The Brady Butte Granodiorite has been sampled at two localities, 

and the two chords representing the zircon families yield similar ages of 

1,770±10 m.y. The porphyritic phase of the Brady Butte Granodiorite (fig. 2, loc. 4) has intruded the Spud Mountain Vblcanics. The 

ages of the Brady Butte Granodiorite and the Spud Mountain rhyolite 

zircons are so similar that they overlap in apparent age assignment, 

and this indicates the essential synchroneity of plutonism and extrusive volcanism at the time of accumulation of the Big Bug Group.

Isotope relations are indicated for two other areally significant plutons in figure 4. The quartz diorite which intruded the Ash Creek 

Group in the southern part of the Mingus Mountain quadrangle has 

an apparent age of 1,760 ±15 m.y. This provides a minimum age for 

the Ash Creek Group. The Government Canyon Granodiorite intrudes 

the Green Gulch Volcanics of the Big Bug Group and yields an apparent age of 1,770±15 m.y. The granodiorite is a late synkinematic 

or postkinematic intrusive which suggest that the major part of the 

deformation of the Big Bug Group had been completed prior to the 

emplacement of the granodiorite.

All three plutons are remarkably similar in the ages interpreted 

from the isotope relations and indicate that synchronous plutonic activity occurred in the area discussed in this report. Late volcanism was 

closely related in time and space to the major regional plutonic episode.

ABANDONMENT OF NAME ALDER GROUP IN THIS AREA

Wilson (1939, p. 1121 and 1158) applied the term Alder Series to 

the northern exposures of Precambrian volcanic and sedimentary 

rocks west of the Shylock fault (fig. 2) because of their lithologic 

similarity to his Alder Series in its type section in the Mazatzal Mountains about 60 miles southeast of the Mingus Mountain quadrangle 

(fig. 1). Anderson and Creasey (1958, p. 20) noted the lithologic similarity of their Texas Gulch Formation to the Alder Series in the Mazatzal Mountains, but they pointed out that the map by Wilson (1939, p. 

1158) of the Mingus Mountain area included their Spud Mountain 

Volcanics in his Alder Series. They concluded that it was desirable 

to retain Wilson's term Alder for all the sedimentary and volcanic 

rocks exposed west of the Shylock fault or south of the pluton of 

quartz diorite (,fig. 2). The term Alder Group was then used because 

of its subdivision into six formations.
PRECAMBRIAN STRATIGRAPHY, PRESCOTT-JEROME AREA C15

The lithologic correlation of the Texas Gulch Formation with Wilson's type Alder Series may be justified but future field and isotopic 

studies may prove the contrary. It is desirable to discontinue use of 

the term Alder Group in the Prescott-Jerome area because the Texas 

Gulch Formation is separated from the Green Gulch Volcanics, Spud 

Mountain Volcanics, and Iron King Volcanics by a period of plutonism 

and subsequent erosion. The name Big Bug Group is proposed as a 

substitute for Alder Group in this area.

YAVAPAI SERIES

Jaggar and Palache (1905) first designated the metamorphic rocks 

in the Prescott-Jerome area the Yavapai schist, and Wilson (1939, p. 

1119) extended this term to the Mazatzal Mountains, modifying it to 

Yavapai Group because schistosity is only local. Anderson and Creasey 

(1958, p. 9) used the term Yavapai Series in keeping with the rules of 

stratigraphic nomenclature at that time, as two groups of rocks were 

involved. Eecently, Anderson (1968) used the term Yavapai Supergroup in order to follow the Code of Stratigraphic Nomenclature 

(Am. Comm. on Strat. Nomenclature, 1961); the term was used as a 

rock-stratigraphic unit because no evidence was available to place the 

two groups in stratigraphic succession.

With the geochronological data now available, it is possible to use 

the term Yavapai Series as a time-stratigraphic term in a provincial 

sense to indicate the length of time represented by the Ash Creek and 

Big Bug Groups. The Ash Creek Group yields an age of 1,820 ± 10 

m.y. based on zircon dating of quartz porphyry in the upper part of 

the Deception Ehyolite (fig. 2, loc. 1). The data from a rhyolitic rock 

in the Big Bug Group (fig. 2, loc. 2) indicates an ageof 1,775 ± 10 m.y. 

The isotopic dates of approximately 1,770 ± 10 m.y. obtained from 

the Brady Butte Granodiorite and associated porphyritic granodiorite 

(fig. 2, Iocs. 3 and 4) give the younger time limit for the age of the 

Big Bug Group. Thus, the Yavapai Series can be defined as the time 

interval from 1,770 ± 10 to 1,820 + m.y.

With the restriction of the Yavapai Series to rocks no younger than 

1,770 ± 10 m.y., the Texas Gulch Formation is not included in the 

Yavapai Series. The Alder Series in the Mazatzal Mountains should 

not be included in the Yavapai Series at this time because no isotopic 

dates are available. Silver (1965) has dated the Ked Kock Ehyolite 

in the Mazatzal Mountains as 1,715 ± 15 m.y., well below the suggested 

younger limit for the time interval of the Yavapai Series. Silver 

(oral presentation at International Conference "Geochronology of 

Precambrian stratified rocks," Edmonton, Alberta, June 12-14, 1967) 

has suggested, provisionally, a major time distinction between the 

older stratified rocks of northern and southern Arizona based upon
C16 CONTRIBUTIONS TO STRATIGRAPHY

TJ-Pb isotope relations in zircons. The central Arizona relations described here provide an important element in the construction of 

regional Precambrian stratigraphic relations.

Butler and Wilson (1938, p. 11) divided the Precambrian rocks of 

Arizona into older and younger Precambrian. The younger Precambrian rocks include the unmetamorphosed Grand Canyon Series and 

Apache Group whereas the older Precambrian rocks include the metamorphosed rocks below. The terms "older" and "younger" were used 

in a provincial sense to indicate that for a particular area, rocks are 

early or late in the Precambrian sequence in Arizona. With the isotopic information now available for the Precambrian rocks in Arizona, the distinction between "older" and "younger" is not needed. In 

the Prescott-Jerome area, the isotopic dates of the Yavapai Series 

and the younger plutons are preferable substitutes for "older 

Precambrian."

REFERENCES

American Commission on Stratigraphic Nomenclature, 1961, Code of stratigraphic 

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