Precambrian Geology of the United 

States; An Explanatory Text to 

Accompany the Geologic Map of the 

United States 

By PHILIP B. KING 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 902 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1976 
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secreta t)' 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

King, Philip Burke, 1903-

Precambrian geology of the United States. 

(Geological Survey Professional Paper 902) 

Includes bibliographical references. 

Supt. of Docs. No.: I 19.16:902 · 

1. Geology. Stratigraphic- Pre-Cambrian. 2. Geology - United States. I. United States. Geological Survey. Geologic map of the 

United States. II. Title. Ill. Series: United States. Geological Survey. Professional Paper 902. 

QE653.K55 551.7'1'097 75-619035 

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

Washington, D.C. 20402 

Stock Number 024-001-02839-5 
CONTENTS 

Page 

Abstract 1 

Distribution _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ ___ _ _ _ _ _ _ _ ___ _ _ _ _ _ _ _ _ _ _ 1 

Data for correlation _ _ __ _ _ _ _ _ _ __ ___ _ _ _ _ _ ___ __ __ _ __ _ _ _ _ __ _ _ __ 2 

Paleontological data___ ___________ __ ________ ___ ________ _ 2 

Radiometric data -------------------------------------- 3 

Geologic applications of radiometric dating ______________ 6 

Classification of Precambrian rocks ____ _____ _____ ____ ______ 6 

Classification in Minnesota ---------------------------- 6 

Classification in Canada ______ __ __ ____ __ _____ ____ _____ __ 7 

Discussion of Canadian classification _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 9 

Classification on Geologic Map of United States of 1932 __ 10 

Later usage of U.S. Geological Survey _______ ___ __ __ __ __ 10 

Interim classification of 1972 -------------------------- 11 

Representation of Precambrian on Geologic Map of the United 

States --------------- ------------------------------- 12 

Exposed Precambrian rocks ofthe United States _____ _______ 13 

Lake Superior Region---------------------------------- 13 

Precambrian W ______ ~ _ ___ _ _ _____ _ _ __ _ _ _ _ __ _ _ __ _ _ _ _ 22 

Precambrian X --------------------- --------------- 24 

Precambrian of northern Wisconsin _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 26 

Keweenawan Supergroup of Precambrian Y ________ 27 

Precambrian Y rocks older than Keweenawan ______ 28 

Precambrian Z __ ____ ________ __ _____ __ _________ ____ 28 

Adirondack area __ ________ ____ ___ ____ __ _ ____ ____ ______ 29 

Northern Appalachian region __________ __ ____ ____ ______ 29 

Precambrian Y of western part ____________________ 29 

Precambrian Z of eastern part ____ ___ ___ ______ ______ 31 

The Avalonian belt -----------~-------------------- 33 

Central and Southern Appalachian region ______________ 33 

Blue Ridge belt _ _ _ _ __ __ _ ___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ __ 33 

Precambrian Y ------------------------------------ 34 

Precambrian Z _____ _______ __________ ____ ___ _____ __ 34 

Precambrian of Piedmont province _______ __ _________ 39 

South-central United States ---------------------------- 41 

Ozark area _ _ _ _ _ _ __ __ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ __ _ _ _ __ _ 41 

Arbuckle and Wichita Mountains __________________ 41 

Llano uplift __ __ _ _ _ _ _ _ _ _ _ _ __ ___ _ _ _ _ _ _ _ __ __ _ _ _ _ _ _ __ 42 

Regional problems -------------------------------- 42 

Page 

Exposed Precambrian rocks of the United States-Continued 

Cordilleran Region __ ____ __ ____ ______ __ __ _ ___ __________ 44 

Central Rocky Mountains ------------------------------ 45 

Precambrian W _ _ __ _ _ __ __ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ __ _ __ _ _ _ 45 

Precambrian complex of southwestern Montana____ __ 46 

Precambrian X ------------------------------------ 48 

Precambrian Y __ ____ ______ _____ __ _ __________ ______ 50 

Northern Rocky Mountains ---------------------------- 51 

Precambrian Y ------------------------------------ 51 

Precambrian of central Idaho ____ ___ ___ _ ·---------- 52 

Precambrian Z ------------------------------------ 53 

Southern Rocky Mountains -------- -------------------- 54 

Precambrian X gneiss complex ___ __ ___ __ ____ _______ _ 55 

Precambrian X andY granitic rocks __ ___ ____ ___ ___ _ 56 

Precambrian of Needle Mountains ___ _____ _______ ___ 56 

Eastern Great Basin ------------ --------- ------------- 56 

Crystalline basement (Precambrian X) __ __ ____ ___ ___ 58 

Big Cottonwood Formation (Precambrian Y) ________ 59 

Mineral Fork Tillite and Mutual Formation (Precambrian Z) _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ __ _ _ _ ___ __ _ _ _ _ _ _ _ _ __ _ _ __ _ 59 

Uinta Mountain Group (Precambrian Y) ______ ___ ____ 59 

Supracrustal rocks of the allochthon (Precambrian Z) _ _ 61 

Supracrustal rocks of Utah-Nevada border __________ 62 

Southern Basin and Range province ____________________ 62 

Crystalline basement of Arizona (mainly Precambrian 

X) _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ __ _ _ _ _ __ _ _ _ _ __ _ _ __ 63 

Crystalline basement of southern California (mainly 

Precambrian X) _____ __ ___ _____ ______ _____________ 64 

Supracrustal rocks in Arizona (mainly Precambrian Y) 66 

Pahrump Group of eastern California (Precambrian Y 

and Z) ------------------------------------------ 69 

Precambrian of western Texas (mainly Precambrian Y) 

Precambrian Z supracrustal rocks of western Basin and 69 

Range province __ __ _______ __ ________ _____ ________ 72 

Discussion and synthesis ------------ ------------·---------- 74 

Acknowledgments____ __________________ ____ __ __ ___ _________ 79 

References cited _ _ _ _ _ ___ _ _ __ __ _ _ __ _ _ _ _ _ ___ _ _ _ _ _ _ __ _ _ __ _ _ _ _ 79 

ILLUSTRATIONS 

Page 

FIGURES 1-5. Maps of the United States, showing surface distribution of Precambrian rocks as represented on the Geologic Map of the 

United States: 

1. Map units W, X, Y, and Z, and metamorphic complexes of probable Precambrian age -------------------- 4 

2. Rocks of Precambrian W _____ _____ _________ _____ ____ _______ ________ _____ __ _________ _____ _____ _____ - _-- 14 

3. Rocks of Precambrian X _____ .. ______ _____ ________ __ ________ _____ ________ ___ _______ _____ _______ ___ ----- 16 

4. Rocks of Precambrian Y ---------------------------- ----------------------------------- --------------- 18 

5. Rocks of Precambrian Z ---------------------------------------------------------------- -------------- 20 

6. Geologic map of part of the Lake Superior Region ------------------------------------------------------------ 23 

7. Stratigraphic chart showing Precambrian units northwest and southwest of Lake Superior in Minnesota, Michigan, and 

Wisconsin ------·---------------------------------------------------------------------------------------- 25 

lil 
IV CONTENTS 

Page 

FIGURE 8. Map of north-central United States, showing arcuate pattPrn of surface and subsurface upper Precambrian rocks (Y and 

TABLE 

Z)___ _____________________ ________________________ ___ ______ ___________ ___________ ___ ________ ____________ 30 

9. Map of south-central Vermont, showing part of the Green Mountain uplift and the Athens and Chester domes ____ 32 

10. Map of northern part of Blue Ridge uplift in Virginia, Maryland, and Pennsylvania, showing Precambrian Z rocks, 

Precambrian Y basement, and adjacent Phanerozoic rocks ---------------- -------- -- ------------ ---------- 35 

11. Stratigraphic diagram across Blue Ridge uplift in northern Virginia, showing Precambrian Z and Cambrian______ 37 

12-17. Maps of: 

12. Part of Blue Ridge uplift in the border region ofVirginia, North Carolina, and Tennessee, showing Precambrian 

and Paleozoic units ------------------------ -------------------- ---------------------------------- 38 

13. Southwestern end ofBlue Ridge belt in southern North Carolina and Tennessee, and northern Georgia, showing 

Ocoee Supergroup, related units of Precambrian Z, and Precambrian Y basement __ ___ _______________ 40 

14. Llano uplift, central Texas, showing Precambrian and surrounding Phanerozoic rocks ______ ___ ___________ 43 

15. Part of south-central United States, showing subsurface extent of late Precambrian and Early Cambrian 

supracrustal felsic volcanic rocks _ _ _ _ __ ___ _ __ ____ __ __ _ _ _ _ _ __ _ __ __ _ _ _ _ __ __ _ _ _ _ __ __ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ __ 44 

16. Central Rocky Mountains in Wyoming, South Dakota, and Montana, showing outcrops of Precambrian rocks 46 

17. Part of western United States and southern Canada, showing Precambrian W rocks from the Lake Superior 

Region to the Central Rocky Mountains _______ __ ___ __________ __ ____ ________ __ ___________ ___________ 4 7 

18. Map showing Precambrian rocks in the Black Hills, western South Dakota ------------------------------------ 49 

19. Map of northern Medicine Bow Mountains, Wyoming, showing rocks of Precambrian W and X and surrounding 

Phanerozoic rocks ------ ---- ---------------- ---------------------- ---------------- ---------------------- 50 

20. Map of Southern Rocky Mountains in Colorado and New Mexico, showing Precambrian rocks, the late Paleozoic 

geanticlines, and the Colorado Mineral Belt __ __ __ _ _ __ ___ _ __ _ _ __ __ __ _ _ _ _ ___ _ __ _ _ __ __ __ _ _ _ _ __ __ _ _ _ _ __ _ _ __ _ _ 55 

21. Map showing Precambrian X andY units in Needle Mountains, southwestern Colorado_ ____ __________ ____ _______ 57 

22. Synoptic diagram showing relations of units of Precambrian X andY in Needle Mountains and their implications in the 

Precambrian history ofthe area __ ___ ___ _______ ____________ ____ ______ ________ ______ ____________ ___________ 58 

23. Geologic map of northeastern Utah, showing Precambrian X, Y, and Z rocks in eastern Great Basin, and the adjoining 

mountains and plateaus to the east _ _ _ _ _ _ ____ __ _ _ _ _ _ _ __ _ _ __ _ _ _ _ _ _ __ _ _ _ _ __ __ __ _ _ _ _ _ _ __ __ _ _ _ _ __ _____ _ _ _ __ __ 60 

24. Map of central Arizona showing relations of Yavapai Series and other Precambrian X rocks ---------- ---------- 65 

25. Section showing Vishnu Schist and Unkar Group (Precambrian X andY) in the Shinumo area, Grand Canyon, northern 

Arizona, and the truncation oftheir block-faulted structure by Cambrian deposits; section of Butte fault in eastern 

Grand Canyon; idealized section showing disruption and distention of Apache Group and Troy Quartzite by sills and 

dikes of intrusive diabase ___ __ __ ______ _____ __ __________ ______ ___ ____ ___ ________ __ _______ _________________ 67 

26. Map of Van Horn area, west Texas, showing Precambrian rocks and their relations to surrounding Phanerozoic rocks 70 

27. Synoptic section across Precambrian rocks of Van Horn area, west Texas, showing structural relations of the different 

units and their implications in the Precambrian history of the area ---------------------------------------- 72 

28. Stratigraphic diagram showing relations between late Precambrian (Z) and lower Cambrian units exposed in different 

areas of the western Basin and Range province -- -------------------------------- -- ---------------------- 74 

29. Maps of the United States and parts of Canada and Mexico, showing evolution of the North American continent during 

Precambrian time -------------------------------------------------------------- -- ------ ------ ---------- 76 

30. Map of western United States, showing western known extent of Precambrian rocks -- -------------------------- 78 

TABLES 

1. Sequence and classification of Precambrian rocks of Minnesota, 1951-70---------------------------- ------------ 7 

2. Ages of orogenic events in Canadian Shield, as determined by different radiometric methods -------------- ------ 9 

3. Comparison of recent classifications proposed for the Precambrian of North America ---- -- ---------- -- ---------- 11 

4. Precambrian supracrustal rocks of Arizona __ __ __ ___ _ _ _ __ __ _ _ ____ __ __ __ _ __ _ __ ____ _ _ __ __ _ _ ___ _ __ __ _ _ _ _ ______ _ _ 68 

5. Precambrian rocks of western Texas ___ __ ______ ________________ ________ ____ ________ _____ _____________________ _ 71 

6. Precambrian Z-Lower Cambrian formations in western Basin and Range province ------------------ ------------ 73 
PRECAMBRIAN GEOLOGY OF THE UNITED STATES; 

AN EXPLANATORY TEXT TO ACCOMPANY THE 

GEOLOGIC MAP OF THE UNITED STATES 

By PHILIP B. KING 

ABST RACT 

Precambrian rocks are at the surface in about 10 percent of the area 

of the United States, but are more extensive beneath the Phanerozoic 

rocks, especially in the Central Interior Region. Exposures occur in 

southward-projecting parts of the Canadian Shield in the Lake 

Superior Region and Adirondack Mountains, and in smaller inliers 

farther south in the Central Interior. Precambrian rocks emerge in 

the higher uplifts produced by Phanerozoic deformations in the Appalachian and Cordilleran mountain belts to the east and west, but 

are very scantily represented c;lose to the Pacific Coast. 

Radiometric dating indicates that the Precambrian rocks vary 

widely in age, from as much as 3,550 m.y. to about 600 m.y., rocks with 

the latter ages being conformable or nearly so with the succeeding 

Cambrian. The radiometric data, assisted to a minor extent by scanty 

primitive fossils, make possible correlation of the rocks of different 

exposures, and they also permit a subdivision of Precambrian rocks 

and time into named subdivisions. In advance of a worldwide agreement on nomenclature the U.S. Geological Survey uses an interim 

subdivision into Precambrian W, X, Y, and Z, which correspond 

broadly with the Archean, Aphebian, Helikian, and Hadrynian of the 

official Canadian classification. 

The radiometric data indicate peaks of abundance of ages at different levels, which express significant historical events-times of 

orogeny, of orogenic cycles, or of magmatism with or without orogeny. 

The principal events occurred 2,500-2,750, 1,600-1,850, 1,300-1,400, 

and 900-1,100 million years ago, and are named (following Canadian 

usage) the Kenoran, Hudsonian, Elsonian, and Grenvillian events, 

respectively. The events have been recorded at many places throughout the United States, Canada, and Mexico, and occur between or in 

the latter parts of the named subdivisions. 

Different events characterize certain areas, thereby delimiting 

provinces in the Precambrian terrane. The oldest provinces are in 

northern Minnesota (an extension of the Superior province of 

Canada), and in Wyoming and southern Montana; they contain Precambrian W rocks that yield Keno ran and earlier dates. Younger 

provinces are to the south. Precambrian X rocks with Hudsonian 

dates are extensive in the Southern province of the Lake Superior 

Region, and also through much of the southern part of the Cordilleran 

region. A poorly defined province with Elsonian dates is indicated by 

subsurface data in the southern part of the Central Interior Region, 

and plutons with Elsonian ages are widely distributed in the Precambrian X rocks of the southern Cordillera. Crystalline rocks of 

Precambrian Y with Grenvillian dates form a wide belt in the southeastern United States, especially in the Appalachian region. 

By the time of Precambrian Y, however, a large part of the remainder of the North American continent, in the United States and 

elsewhere, had been stabilized into a craton, and received supracrustal sediments and volcanics that were only moderately deformed, or 

remained undeformed during Precambrian time, producing units 

such as the continental Keweenawan Supergroup of the Lake 

Superior Region, the marine Belt Supergroup of the northern Cordillera, and the Grand Canyon Supergroup and others farther south. 

During latest Precambrian time, or Precambrian Z, accumulation 

of supracrustal sediments and volcanics occurred mainly along the 

eastern and western sides of the continent, in the Appalachian and 

Cordilleran regions-in the east on a crystalline basement produced 

by the Grenvillian event, in the west lying with moderate discordance 

on Precambrian Y supracrustal rocks. However, in the coastward part 

of the Appalachian region is the Avalonian belt of Precambrian Z 

rocks, an exotic element which seems to have been joined to the North 

American continent by plate movements during Paleozoic time. It 

includes supracrustal rocks in the Carolina Slate Belt of the southern 

Appalachians, as well as farther northeast in Canada, but in southeastern New England it is represented by extensive granitic plutons 

that are unconformable beneath the Lower Cambrian, with radiometric dates of 570 m.y. 

In most of the United States the Precambrian is separated from the 

Cambrian by a marked unconformity and hiatus; Middle or Upper 

Cambrian rocks overlie Precambrian Y or older rocks. However, in 

the mountain belts to the east and west, supracrustal rocks of both 

Precambrian Z and Lower Cambrian were deposited, and the boundary between the Precambrian and the Phanerozoic is less obvious. The 

problem is most acute in the southwestern part of the Basin and 

Range province where Precambrian and Cambrian are parts of a thick 

conformable sequence of fine-grained sediments, so that there is no 

clear physical or faunal boundary between them. 

In this account, following a statement of general principles, the 

Precambrian rocks of the different areas of exposure are reviewed, 

described, and correlated in turn. The units selected for description 

are in terms of modern morphology, which correspond only broadly 

with the provinces of Precambrian time-the Lake Superior Region, 

the Adirondack Mountains, t he Northern and Southern Appalachian 

regions, the south-central United States in the Interior Lowlands, the 

Central Rocky Mountains, the Northern Rocky Mountains, the 

Southern Rocky Mountaias, the eastern Great Basin, and the southern Basin and Range province. In general it is assumed that the 

descriptions can be understood by reference to the Geologic Map of the 

United States, but to clarify certain subjects, maps on larger scales or 

maps which illustrate special features are included. A final discussion 

and synthesis deals with the larger Precambrian problems, some still 

obscure, including the origin and evolution of the continent during 

Precambrian time, and the possible participation of the continent in 

plate tectonics. 

DISTRIBUTION 

Precambrian rocks underlie all the Central Interior 

Region of the United States and large parts of the mountain belts east and west of it. However, they are covered 

1 
2 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

extensively by Phanerozoic rocks and form the surface 

of only about 10 percent of the country. By contrast, in 

Canada to the north Precambrian rocks form the surface of nearly half the country, mainly in the Canadian 

Shield. 

The largest exposures of the Precambrian in the 

United States are in southern extensions of the Canadian Shield-in the Lake Superior Region of Minnesota, 

Michigan, and Wisconsin, and in the Adirondack Mountains of northern New York State (fig. 1). Older maps 

(such as the Geologic Map of the United States of 1932) 

imply that an even larger area of Precambrian occurs in 

the Appalachian Region to the east and southeast; large 

parts of this supposed Precambrian are now known to be 

of Paleozoic age, although authentic Precambrian does 

emerge in the higher uplifts through much of the length 

of the chain. In the Central Interior, Precambrian is 

exposed only in small, widely spaced areas on the crests 

of a few uplifts; additional knowledge of the Precambrian of this region is afforded by subsurface data. In the 

Cordilleran Region, a large area of Precambrian 

(mostly the supracrustal Belt Supergroup) extends 

across the Northern Rocky Mountains of western Montana and northern Idaho. Farther south in the Rocky 

Mountains the outcrops of Precambrian are smaller, 

but many of them (as in Colorado) are closely spaced. 

Similar small but closely spaced areas of Precambrian 

occur in the southern part of the Basin and Range province in Arizona and adjacent States. No Precambrian is 

known within 200 miles (320 km) or more of the Pacific 

Coast, except in the Transverse Ranges of southern 

California. 

DATA FOR CORRELATION 

Prime requisites for representation of any group of 

rocks on a regional or national geologic map are 

adequate classification and correlation, but these are 

difficult to achieve in the Precambrian. 

Many parts of the Precambrian have been strongly 

deformed, metamorphosed, and injected with plutonic 

rocks; moreover, even where their primary sedimentary 

structures are well preserved, their fossil remains are 

sparse and enigmatic. While it is true that their structural complexity is perhaps no greater than that of 

many Phanerozoic terranes whose sequences and ages 

have been deciphered, the few fossils in Precambrian 

rocks are not of the diagnostic value of those used for 

stratigraphic purposes in younger rocks. 

In the absence of normal criteria for classification and 

correlation, various indirect methods were formerly 

used in deciphering the Precambrian record. The earth 

was assumed to evolve during the Precambrian, from a 

molten, disordered condition CAzoic" or «Archean" 

time) to a better ordered condition when more familiar 

sedimentary and volcanic processes prevailed CProterozoic" or ~~Algonkian" time). Assumptions were made 

as to the nature of Precambrian orogenic processessupposedly universal cycles of deformation, plutonic injection, and peneplanation, applicable throughout a 

shield, or to even larger regions. Where sequences of 

Precambrian rocks could be worked out by conventional 

laws of superposition, they were compared and correlated with other sequences, even far distant, using as 

starting points supposed type areas, such as the Lake 

Superior Region. 1 These early efforts failed to take into 

account various geological factors that are better understood now, such as the actual great length of Precambrian time-at least five times longer than 

Phanerozoic time. They are merely of historical interest 

today. 

Great progress in understanding the Precambrian 

has been made in recent decades. Radiometric dating 

has made it possible to bring together many hitherto 

unrelated items of the larger history, and even to make 

a beginning in stratigraphic correlation. With this assistance, more can now be deduced as to the geochemical 

evolution of the earth, leading to inferences on 

worldwide events, such as a time of iron formation, the 

times of beginning of carbonate and of evaporite 

sedimentation, and times of glaciation. However, with 

the possible exception of the latter, these have only very 

general application to stratigraphic work. More to the 

point, radiometric dating has assisted in understanding 

. the fossil record, such as it is, and to suggest at least 

rudimentary zonation. Moreover, much wider areas of 

Precambrian rocks have been geologically mapped 

which, coupled with radiometric dating, has assisted in 

understanding regional Precambrian history that is no 

longer restricted to a few classical and supposedly typical areas. 

PALEONTOLOGICAL DATA 

The fossil record is influenced by the evolution of life 

on the earth, but during Precambrian time evolution 

was probably very slow at first, and did not accelerate 

until much later. Classification of Precambrian fossils 

is difficult because even major groups of organisms 

must have become extinct during the long timespans 

involved; even in the succeeding Early Cambrian there 

are shelly invertebrate groups that are not assignable 

to any existing phyla (Glaessner, 1968, p. 586). 

'The strongest statements of these propositions were in the textbooks of the time, whose 

authors were eager to generalize the results of the field geologists; statements by the field 

geologists themselves (with the exception of Lawson, 19141 were more qualified. A judicious 

appraisal of the status of Precambrian problems is contained in C. K. Leith's presidential 

address to the Geological Society of America in 1933 (Leith, 19341, and his strictures have 

been well justified by later developments. 
DATA FOR CORRELATION 3 

During the first three-quarters of Precambrian time 

the only remains or traces of life are those of primitive 

bacteria and plants. The most prominent of these remains are the stromatolites, which are biogenic 

sedimentary structures probably produced by algae; 

they include stratiform, nodular, and columnar carbonate structures. All are notoriously variable in form 

and no doubt were much influenced by local environmental conditions. Nevertheless, when specimens of the 

more distinctive columnar forms have been studied 

through sequences long enough, and over areas wide 

enough, they seem to have changed sufficiently with 

time to permit division into zones dated radiometrically 

between 1,600 and 1,350 m.y., 1,350 and 1,000 m.y., and 

700 and 500 m.y. These express very slow evolutionary 

changes-two orders of magnitude slower than in 

Phanerozoic biostratigraphic zones (Glaessner, 1968, 

p. 587). Stromatolite zonation has been most successfully applied in the Soviet Union (Raaben, 1969; Cloud 

and Semikhatov, 1969) where Precambrian stromatolite-bearing rocks can be studied across the whole 

expanse of northern Eurasia, but similar studies are in 

progress in Precambrian areas elsewhere. 

The earliest authentic metazoan fossils occur in 

strata not far beneath the Cambrian with ages of 600 to 

700 m.y.-especially in the Ediacaran of South Australia, the Vendian of northern Eurasia, and a scattering of 

other formations and localities in the Eastern Hemisphere (Glaessner, 1971). The only reported occurrences 

in North America are in southeastern Newfoundland 

(Conception Group) (Misra, 1971, p. 979-980), in North 

Carolina, and in eastern California (Deep Spring 

Formation) (Cloud and Nelson, 1966). Some or most of 

the forms occur at all localities, indicating a wellcharacterized fauna-various primitive coelenterates, 

and forms with less certain affinities that probably belong to extinct phyla (Glaessner, 1961, p. 73-77; 

Sokolov, 1973, p. 209-215). They were soft-bodied animals, whose imprints are preserved on bedding surfaces 

at unusually favorable situations. Although the strata 

in which they occur are clearly older than the Cambrian, there is some philosophical justification for considering them a basal unit of the Paleozoic, younger 

than the Precambrian as formally defined (Cloud, 1968, 

p. 36-37). Hard-shelled fossils, such as archeocyathids 

and trilobites, only appear in the Cambrian itself, for 

reasons that are still debated (Cloud, 1968, p. 42-49). 

RADIOMETRIC DATA 

Dating by radiometric methods has advanced far 

beyond the first few determinations on uranium and 

thorium ore minerals nearly three-quarters of a century 

ago. Aside from suggesting the possibilities of the 

method and the great length of geologic time, these first 

determinations were oflittle geologic use because of the 

rarity of the minerals, and because most kinds of rocks 

do not contain them, hence were as yet undatable. Subsequently, and especially during the last few decades, 

many other methods have been devised, some of them 

applicable to ordinary rocks. At the same time, however, the hazards and pitfalls of the radiometric 

methods of dating have become more apparent. 

The lead-alpha method of dating zircon gives 

generalized results and is useful as a reconnaissance 

tool, but has little value for detailed work. 

The potassium-argon method uses potassium-bearing 

minerals such as biotite, muscovite, and hornblende, 

hence has wide application to common igneous and 

metamorphic rocks. It is therefore useful for sampling 

and appraisal of wide areas of Precambrian rocks (as in 

the Canadian Shield). The results are mostly consistent 

among themselves, and thus indicate the relative ages 

of different units and provinces. However, the ages obtained in the Precambrian are rather consistently less 

than those by the other methods mentioned below, 

owing to gradual loss of argon from the mineral lattices. 

Because of differences in their molecular structure, this 

loss is greatest in biotite, less in muscovite, and least in 

hornblende. 

Also, argon is lost during the cooling that succeeds 

time of igneous injection or of metamorphism, and it 

does not become fixed in the mineral until the temperature descends to a lower level. Thus many dates are 

((cooling dates" that are younger than the actual times 

of injection and metamorphism. These differences are 

least in low-grade metamorphic rocks and greatest in 

high-grade rocks of granulite facies that underwent the 

deepest burial and the greatest subsequent uplift and 

erosion. 

The rubidium-strontium method is not subject to the 

loss of a gas daughter product as in the potassium-argon 

method, and hence yields more reliable dates, but it has 

several of its own problems. Both elements are subject 

to gain or loss during metamorphism, and there is disagreement as to the Rb87 half-life decay constant. Depending on the constant adopted, the dates obtained on 

Precambrian rocks by the rubidium-strontium method 

may differ by 6 percent, or 150 m.y. at 2,500 m.y. ago. 

The most nearly absolute figures for primary crystallization are those obtained from uranium-lead and 

lead-lead methods, but the elements to be analysed are 

rare. The methods were originally applied to uranium 

and thorium ore minerals which did not have wide 

geological application; but uranium and lead also occur 

in minute amounts in the common accessory minerals 

zircon, monazite, apatite, and sphene, for which analytical procedures are very exacting. Although fewer dates 
4 PRECAMBRIAN GEOLOGY OF THE UNITED STAT~S 

'\ 

\., __ -..... 

I 

I 

, ... --.,_ _____ ___ 

----

----

\' 

r if(. 

) · .. ~ , . •. , .. · 

"--, : - -...... \ 

\ 

,---------------------

1 

J _________ _ 

I : 

FIGURE 1.-Map of the United States showing surface distribution of Precambrian rocks as represented on the Geologic Map of the 

United States (map units W, X, Y, and Z). Also shown are metamorphic complexes (map units ms and m 1-m4 ), which probably 

include rocks of Precambrian age. · 
\ 

I 

' 

I 

/ 

~---------------,;,' I l --\_) 

\., 

I 

' \ 

I 

..r-1 

I / r'l_( 

5·- .s- --""·' 

I 

I 

DATA FOR CORRELATION 

)\ 

i \....---\ --;:-_;; I , ,r • ,r-' ,, I 

I ' /' ' :-v !"'-· ~- \ .. ,_r-_,\._ ? 

('' '- J 

~~·,\ __ { 

·----~ ~-=-'---_;----

.. ---------------~ s 

I --, 

} 

;_ _____ -,---------\.--

( i \ ) 

/' _- .... ~ ... 

'I ~ :_-------- -~· 

I ~ 

[ c/ 

\ I 

~ 

c;--------\ __________ .5 

Q 

<:!: 

_ .. .... -

FIGURE 1.-Continued. 

EXPLANATION 

Precambrian 

-

rocks 

Metamorphic 

complexes 

5 
6 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

can be obtained by uranium-lead and lead-lead 

methods, they are useful as controls for the less exact 

results obtained by the other methods. 

GEOLOGICAL APPLICATIONS OF RADIOMETRIC DATING 

These various methods indicate the times of crystallization during igneous intrusion, and of metamorphism-aside from the cooling factor. Dating of Precambrian supracrustal rocks of sedimentary and volcanic origin is more difficult. 

Direct dating from primary minerals in the Precambrian supracrustal rocks in the Canadian Shield is 

largely unsuccessful, and the dates obtained commonly 

express merely the age of the metamorphism 

(Stockwell, 1968, p. 692). Elsewhere, it has been possible in a few places. Glauconite and argillite in the little 

deformed or metamorphosed Belt Supergroup of the 

Northern Rocky Mountains are susceptible of dating by 

potassium-argon and rubidium-strontium methods; 

also, potassium-argon determinations have been made 

on hornblende from the Purcell Lava and associated 

sills interbedded in the Belt sediments (Obradovich and 

Peterman, 1968, p. 739-7 40). Zircons from the late Precambrian felsic lavas of low metamorphic grade in the 

Blue Ridge of the Central Appalachians have been successfully dated by the uranium-lead method (Rankin 

and others, 1969). 

For dating the Precambrian supracrustal rocks of the 

Canadian Shield and other complex areas, recourse 

must generally be had to indirect methods, which 

bracket the times of accumulation between maximum 

and minimum limits. The maximum age of a sequence 

is indicated by the age of the plutonic and metamorphic 

rocks of its basement; the minimum age is indicated by 

the age of its metamorphism, or by the age of the igneous rocks that intrude it. 

CLASSIFICATION OF THE PRECAMBRIAN 

ROCKS 

With the new data available, proposals are being 

made in many parts of the world for reclassification of 

the Precambrian rocks. The subject is under consideration by the Subcommission on Precambrian of the International Commission on Stratigraphy, Kalervo 

Rankama, chairman. The subcommission is working 

toward an agreement on Precambrian classification and 

nomenclature that will meet worldwide acceptance, but 

such an agreement is still a matter for the future. The 

worldwide implications do not concern us here; our interest is in the interim problems of classification of the 

Precambrian in North America, and specifically in the 

United States. 

CLASSIFICATION IN MINNESOTA 

One of the first efforts to make effective use of 

radiometric data to classify the Precambrian was in 

Minnesota (Goldich and others, 1961) that refined and 

revised an earlier classification based largely on conventional geologic criteria (Grout and others, 1951). 

The State of Minnesota includes nearly half of the area 

of Precambrian rocks of the Lake Superior Region in the 

United States. Moreover, its sequence of Precambrian 

rocks is much like that across Lake Superior in northern Michigan and Wisconsin, so that any classification 

arrived at in Minnesota has applications over a wider 

area. 

Table 1 summarizes the various classifications proposed for Minnesota, including that of 1961. From the 

table, it is apparent that basic concepts of the Minnesota 

sequence have changed little through the years, but 

that significant changes have been made in classification and terminology. A notable change in 1961 was the 

transfer of the Animikie Group from Late Precambrian 

to Middle Precambrian and the Knife Lake Group from 

Middle Precambrian to Early Precambrian, as a result 

of dating the Penokean orogeny at 1,700 m.y. and the 

Algoman orogeny at 2,500 m.y. (the column for 1968 

indicates that these dates are actually greater). These 

extreme ages were incompatible with the relative youth 

previously assumed for the two groups. The so-called 

(<Laurentian orogeny," previously considered to divide 

the Early and Middle Precambrian, was now 

downgraded to a minor role in the Early Precambrian. 

Emendations after 1961 include renaming the socalled nGrenville orogeny" of Minnesota the 

((Keweenawan igneous activity"; even though the event 

is broadly correlative with the true Grenvillian orogeny 

farther east, it was essentially anorogenic in Minnesota. Also, in the classification of 1970 and 1972, the 

absolute distinctions between the Knife Lake and 

Keewatin Groups are discarded, as the sediments of the 

one and the volcanics of the other have variable mutual 

relations. With this, the so-called (<Laurentian orogeny" 

and its accompanying epoch of granite intrusion disappears; granites within the Lower Precambrian are now 

interpreted as local phenomena. Nevertheless, as 

shown in 1968, extremely ancient rocks occur in southwestern Minnesota, dated at 3,550 m.y. 

Radiometric dating indicates major events in Minnesota at 2,700-2,750 m.y., 1,850 m.y., and 1,100 m.y., 

designated the Algoman orogeny, Penokean orogeny, 

and Keweenawan igneous activity (= Grenville 

orogeny). We will find these events again in the Canadian Shield in Canada, and elsewhere, and will interpret them as important markers for classifying the Precambrian of North America. 
CLASSIFICATION OF PRECAMBRIAN ROCKS 7 

TABLE 1.-Sequence and classification of Precambrian rocks of Minnesota, 1951-70. 

Grout and others ( 1951 1 Gol dich and others (1961 1 

Cambrian Cambrian 

Unconformity 600 m.y. 

Later Precambrian Late Precambrian 

Keweenawan Group Keewanawan System 

Medial volcanics intruded Medial volcanics intruded by 

by Duluth Gabbro Duluth Complex (Grenville 

orogeny, 1,100 m .y.) 

Unconformity 

Penokean orogeny, 1,700m.y. 

I 

Middle Precambrian 

Granitic intrusives 

Animikie Group Huronian System 

Animikie Group 

Unconformity Algoman orogeny, 

2,500 m.y. 

Medial Precambrian Early Precambrian 

Algoman intrusives Granitic intrusives 

Knife Lake Group Temiskamian System 

Knife Lake Group 

Laurentian orogeny, age? I 

Unconformity Granitic intrusives Earlier Precambrian 

Pre-Knife Lake Ontarian System intrusives Keewatin Group 

Keewatin Volcanics Coutchiching? 

Soudan Iron-Formation Older rocks 

CLASSIFICATION IN CANADA 

A more far-reaching reclassification of the Precambrian rocks on the basis of radiometric dating has been 

made by the Geological Survey of Canada. This deserves lengthy consideration, as it involves our 

neighbor to the north and its geological survey, as well 

as the largest exposure of Precambrian rocks in North 

America. The reclassification was carried out under the 

leadership of Clifford H. Stockwell for use on the new 

Geologic and Tectonic Maps of Canada then in preparation (1969), and was based on an accelerated program of 

mapping the Precambrian rocks of the country and of 

radiometric dating, chiefly by the potassium-argon 

method. 

Outcrops of Precambrian rocks are nearly uninterrupted in the Canadian Shield in the central and eastern part of the country, except for submerged parts such 

as Hudson Bay, and for the area of Phanerozoic cover in 

the Hudson Bay Lowland. This vast Precambrian area 

was once thought to be a homogeneous body, as implied 

on the Geologic Map of North America of 1912, hence 

subject from time to time to universal cycles of orogeny 

and peneplanation. Field studies during the last halfcentury have demonstrated, on the contrary, that it is 

I 

Goldich (19681 Sims !1970 1: 

Si ms and Morey !1972 1 

Cambrian Cambrian 

600 m.y. 

Late Precambrian Upper Precambrian 

Sediments and medial North Keweenawan Series 

Shore Volcanic Group, in- Medial volcanics intruded by 

truded by Duluth Complex Duluth Complex 

(Keeweenawan igneous activity, 1,000-1,200 m.y.) 

Penokean orogeny, Penokean orogeny 

1,600-1,900 m.y.) 

Middle Precambrian Middle Precambrian 

Granitic intrusives Granitic intrusives 

Animikie Group Animikie Group 

Algoman orogeny, Algoman orogeny 

2,400-2,750 m.y. 

Early Precambrian Lower Precambrian 

Granitic intrusives Granitic intrusives 

Knife Lake Group Metasedimentary and metavolcanic rocks, with various mu- tual relations 

Laurentian orogeny, age? Granitic intrusives, older than 

Granitic intrusives part of metasedimentary rocks 

Keewatin Group 

Coutchiching? 

Older rocks, Gneiss and schist, southwestern 

3,300-3,550 m.y. Minnesota 

inhomogeneous, and divisible into provinces with different rocks and histories, that developed independently during Precambrian time. Increasing knowledge 

has heightened the distinctions between the provinces 

and has sharp~ned their boundaries. Many of the 

boundaries are structural lineaments, emphasized 

further by geophysical anomalies; some are stratigraphic, where supracrustal rocks of a younger province 

overlap the basement of an adjoining older province. 

Of the provinces of the Canadian Shield, only a few 

bear directly on Precambrian problems in the United 

States: the Superior province of ancient rocks which 

includes the Lower Precambrian of Minnesota (see 

above); the Southern province of somewhat younger 

rocks, which includes the remainder of the Lake 

Superior Region in the United States and Ontario; and 

the Grenville province farther east, which extends into 

the Adirondack Mountains of New York State. The Precambrian of the United States no doubt includes other 

extensions of the shield provinces, and additional provinces, but they are less apparent at the surface because 

of the interrupted outcrops. 

Radiometric dating has underscored the discreteness 

of the provinces. Each has its own characteristic peak of 
8 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

abundance of dates, well expressed in histograms (for 

example, Stockwell, 1964, fig. 2). A scattering of older 

and younger dates also occurs, the older expressing 

earlier orogenic events nearly overwhelmed by the 

dominant events, the younger being from dike rocks 

and other anorogenic intrusives. 

The dominant sets of dates in the different provinces 

are interpreted as having been produced by orogenies 

(Stockwell, 1961, p. 111-113). Orogeny is defined as a 

period of mountain building, accompanied by folding, 

metamorphism, and granite intrusion, each orogeny 

being followed by a long period of uplift, erosion, and 

cooling before the next set of supracrustal rocks was laid 

down. The scatter of dates in the rocks of each province 

may extend over a span of as much as 300 m.y., but this 

is interpreted as partly the result of analytical error; the 

actual duration of an orogeny is believed to be 100 m.y. 

or less. 

In order to refine further the orogenic times, the 

available dates have been analyzed statistically 

(Stockwell, 1964, p. 4--7), using those from a single province, by a single method (for example, potassiumargon), and of orogenic origin (rather than relicts of 

earlier events, or ofpostorogenic events). The statistical 

analysis for each province yields a mean on the Gaussian or probability curve, and a standard deviation. The 

mean figure is interpreted as representing the probable 

climax of an orogeny, and the mean minus the standard 

deviation the probable end of this orogeny. 

Three principal orogenies are recognized in the 

Canadian Shield, the Kenoran ( = Algoman of Minnesota), the Hudsonian ( = Penokean ofMinnesota), and 

the Grenvillian ( = Keweenawan igneous activity of 

Minnesota). Each orogeny has its ((type region" in one of 

the provinces; ((it is hoped that, eventually, it may be 

possible to select much smaller areas for type regions, 

while still retaining the present geological definitions 

and still containing rocks and minerals that are suitable for dating by a variety of methods on a variety of 

minerals" (Stockwell, 1972, p. 3). The Kenoran has its 

type region in the Superior province, where it has a 

mean age of2,490 m.y.; the Hudsonian its type region in 

the Churchill province, where it has a mean age of1,935 

m.y.; and the Grenvillian its type region in the Grenville province, where it has a mean age of 945 m.y. 

These orogenies may be poorly expressed or absent in 

other provinces. The Grenvillian is unique in the Grenville province, and has no orogenic counterparts 

elsewhere in the shield; the Hudsonian is missing in the 

Superior province, but it recurs in the Southern province, and in some of the far northern provinces. 

Besides these, an additional Elsonian orogeny was 

proposed, based on a scatter of radiometric dates in the 

Nain province of Labrador, with a mean age of 1,370 

m.y. (Stockwell, 1964, p. 2). Later work demonstrates 

that the events represented by these dates are not 

orogenic; instead, they were produced by ad~mellite 

(quartz monzonite) and anorthosite intrusions into 

rocks already consolidated by the Hudsonian orogeny 

(Taylor, 1971, p. 580-582). The Elsonian is more properly termed an ((event" (King, 1969, p. 35; Stockwell, 

1972, p. 3). 

As indicated earlier, the potassium-argon method on 

which these radiometric ages are based has many advantages, but the dates obtained are consistently 

younger than those obtained by other methods. Subsequent to the work summarized here, the orogenic 

periods have been checked by a smaller number of 

uranium-lead and rubidium-strontium determinations, 

all of which indicate older, and probably truer ages 

(Stockwell, 1972), as shown in table 2. 

Besides the major Precambrian orogenic events recognized by Stockwell in the Canadian Shield, lesser 

events late in the Precambrian have been described in 

other parts of Canada, mostly insecurely dated 

radiometrically and not necessarily of the same agethe East Kootenay and Racklan orogenies in the Cordilleran province (Douglas and others, 1970, p. 373) and 

the Avalonian orogeny in the Appalachian province 

(Poole and others, 1970, p. 232--233). Of these, the latter 

is of the greatest interest here because of its probable 

extension into the Eastern United States; the evidence 

will be treated at greater length later (p. 33, 39). 

The radiometric and orogenic data just summarized 

have been used to redefine the sequence of Precambrian 

rocks in Canada. The Precambrian of Canada has traditionally been divided into Archean and Proterozoic 

Eons, and these and their subdivisions are now more 

precisely defined with the aid of the new data: Archean 

prior to the end of the Kenoran orogeny, Lower Proterozoic between the ends of the Kenoran and Hudsonian orogenies, Middle Proterozoic between the ends of 

the Hudsonian and Grenvillian orogenies, and Upper 

Proterozoic between the end of the Grenvillian orogeny 

and the beginning of the Phanerozoic. Each orogenic 

event is thus placed within the preceding time division, 

and the · end of the orogeny is considered to mark the 

upper boundary of the subdivision. 

New names are proposed for the subdivisions of the 

Proterozoic (Stockwell, 1964, p. 7-9): Aphebian for 

Lower Proterozoic, Helikian for Middle Proterozoic, and 

Hadrynian for Upper Proterozoic. The names are derived from Greek roots: Aphebian from ((aphebos," or old 

maturity; Helikian from ((helikia," or maturity; and 

Hadrynian from ((hadrynes," or young maturity. 

Further subdivisions can then be created; for example, 

the Helikian is divided into Paleohelikian and Neohilikian, bounded by the Elsonian event. The new names 
CLASSIFICATION OF PRECAMBRIAN ROCKS 9 

TABLE 2.-Ages of orogenic events in Canadian Shield, as determined 

by different radiometric methods [Based on Stockwell, 1964, 1972] 

End of event in millions of years (= mean minus Event standard dev iation ! 

K/Ar U/Pb Rb/Sr Rb/Sr constant 1.4 7 constant 1.39 

Grenvillian orogeny ____ __ 880 ca. 1,000 ca. 1,010 ca. 1,070 

Elsonian event _ - -- 1,280 ? 1,400 ------------

Hudson ian orogeny 1.640 ca. 1,800 ? 1,750 ? 1,850 

Kenoran 

orogeny _ 2,390 ca. 2,560 ? 2,540 ? 2,690 

make it possible for there to be many subdivisions 

within the Precambrian (or specifically within the Proterozoic), instead of the three descriptive categories of 

((lower," ((middle," and ((upper" that are available in the 

English language, and they avoid such unfortunate expressions as ((lower upper" and ((middle lower" which 

have sometimes been used for smaller subdivisions. 

DISCUSSION OF CANADIAN CLASSIFICATION 

The classification of the Precambrian set forth above 

has been accepted by the Geological Survey of Canada 

for use in its published maps and reports, but it has been 

criticized by other geologists (for example, Goldich, 

1968,p. 722;James, 1972a, p. 1132; 1972b,p. 2085)in 

the following terms: 

(1) The statistical method of defining orogenies and 

subdivisions is questionable, as it depends on the validity of the areal unit selected for analysis, the effectiveness of the sampling, and whether the dates selected 

rather than discarded represent a single population. 

(2) Reliance on the potassium-argon method of dating produces unreliable results for determining the 

ages of the units. 

(3) The wide scatter of dates within each province is 

difficult to reconcile with the assumption that they were 

produced by a single orogeny, rather than by an 

orogenic cycle comprising many successive orogenies 

(King, 1969, p. 33; compare James, 1960, p. 107). 

(4) Orogenies have been discredited as the fundamental basis for stratigraphic classification in the 

Phanerozoic, and their value for this purpose in the 

Precambrian should be no greater. 

(5) Archean has been differently defined as to age 

limits from one country to another, and from one 

geologist to another. 

(6) The new names proposed for subdivisions of the 

Proterozoic are unfamiliar and cumbersome, and do not 

clearly indicate their sequential relations. 

(7) New names for major units of the Precambrian 

should not be proposed unilaterally, but by international agreement. 

The reader can judge for himself between these adverse criticisms and the Canadian viewpoint just summarized. Here, discussion of only one item, the Archean, 

is desirable. 

The term ((Archean" has been widely used for more 

than a century for the oldest visible rocks of the earth, 

which are supposed to have special characters. ((By later 

Precambrian time, the patterns of sedimentation, 

mountain building, and crustal evolution seem to have 

been much the same as they are now. The Archean is 

commonly thought to have been different-a time when 

the atmosphere and oceans were unlike the present, a 

time prior to crustal organization into cratons and 

geosynclines, a time unique in earth history" (Pettijohn, 1972, p. 133). Moreover, significant geochemical 

differences have been discerned between rocks formed 

during the ((Archean" and the ((Proterozoic," or before 

and after about 2,500 m.y. ago (Engel and others, 197 4, 

p. 852). 

One of the original areas in which the Archean was 

recognized is the Canadian Shield, and especially the 

Superior province, a terrane consisting of linear belts or 

islandlike areas of supracrustal rocks, interspersed 

with or surrounded by a more extensive sea of intrusive 

granite. The supracrustal rocks include metavolcanics 

that are mainly andesitic and basaltic greenstones; and 

metasediments which, where best preserved, are 

graywackes and slates with interbedded conglomerate 

and iron formation, and elsewhere are migmatized 

quartz-mica schists and paragneisses. Their extreme 

age is demonstrated in places by unconformable relations of both the supracrustal rocks and granites beneath the middle Precambrian rocks, and by radiometric dating. Similar terranes are recognized in the shield 

areas of other continents (for example, Australia and 

South Africa), and have likewise been called Archean. 

The term Archean has also, of course, been misapplied to any thoroughly metamorphosed basement, 

especially before the period of radiometric dating. Thus, 

the metamorphic basement of the Appalachian region 

was commonly called ((Archean," until radiometric dating demonstrated that it was not consolidated until 

about 1,000 m.y. ago, at the time of the Grenvillian 

orogeny of the Canadian Shield. 

These misapplications aside, a worldwide survey of 

usage indicates much diversity of judgment as to the 

date of termination of the Archean (Rankama, 1970, 

p. 214, 216), with proposed dates from less than 2,000 

m.y. to nearly 3,000 m.y. Proposals for a termination at 

less than 2,000 m.y. seem to have little merit; the main 

problem is regarding diverse proposals for dates between 2,000 and 3,000 m.y. Some of the latter dis-
10 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

crepancies represent differences in field observations 

and analytical methods and can be adjudicated. Other 

discrepancies are probably genuine; perhaps ~~Archean" 

conditions ended at different (but everywhere ancient) 

times from one shield area to another. 

The ancient features of the earth, expressed by the 

rocks and the conditions that these imply, seem to be 

unique and well characterized, whether they be called 

~~Archean" or by some other name. The problem is how 

to translate these concepts into a definition of stratigraphic value. Valid definitions can be proposed in 

specific areas, such as the Canadian Shield, but difficulties arise when they are expanded into a definition of 

worldwide application. It therefore remains to be seen 

whether such a worldwide definition can be worked out, 

or whether the term Archean must be discarded. 

CLASSIFICATION ON GEOLOGIC MAP 

OF UNITED STATES OF 1932 

The Geologic Map of the United States of 1932 was 

compiled before the development of meaningful 

radiometric dating and was the last major publication of 

the U.S: Geological Survey which used the subdivisions 

'~Archean" and ~~Algonkian" that had been standard in 

Survey publications for the preceding half-century. The 

classification used on this map is illustrated by the 

following abstract of its legend: 

Lake Superior Region 

Algonkian 

Keweenawan: sedimentary, Akl; volcanic, Akv 

Huronian: lower, middle, and upper, Ahl, Ahm, Ahu 

Archean 

Keewatin Series, ..LAk 

Precambrian undivided 

Precambrian intrusives, in 

New England and the Adirondacks 

Adirondacks 

Algonkian? 

Adirondack batholith, Ab 

Archean? 

Older igneous rocks, ,LA i 

Grenville Series, ,LAg 

New England 

Algonkian? 

Younger sedimentary schists, As 

Archean? 

Older sedimentary and igneous gneisses, ,LAgn 

Appalachian Region 

Algonkian? (Glenarm Series) 

Wissahickon Schist: 

oligoclase-biotite schist, Awh 

albite-chlorite schist and garnetiferous phyllonite, Awl 

schist with igneous injections, Awl' ., 

Cockeysville Marble and Setters Formation, Acs 

Granite, gabbro, and hornblende gneiss, Agn 

Mylonitized granite gneiss and hornblende gneiss, Agg 

Volcanic rocks, A v 

Archean? 

Older gneiss, ,LA gn 

Midcontinent Region 

Algonkian? 

Gneiss, schist, and quartzite, Agn 

Granite, porphyry, and gabbro, Agr 

Great Plains 

Algonkian? 

Sedimentary schist and quartzite, As 

Intrusive rocks, Ai 

Rocky Mountains 

Algonkian 

Belt Series: undivided, Ab; lower part, Abl; upper part, 

Abu 

Archean 

Archean rocks, ,LA 

Granite, ..LAg 

Pacific Coast Region, Great Basin, and Columbia River Plateau 

Precambrian 

Granite, diabase, and other intrusive rocks, p£g 

Schist, gneiss, and granite, p£ 

LATER USAGE OF U.S. GEOLOGICAL SURVEY 

When first proposed by the U.S. Geological Survey, 

the Archean and Algonkian were conceived to be 

periods or systems in a Proterozoic Era, which were 

time-stratigraphic units comparable in scope and probably in length to the Phanerozoic periods or systems. In 

actual practice in Survey publications, however, they 

were used empirically, Archean for dominant plutonic 

and metamorphic rocks and Algonkian for dominant 

supracrustal rocks. 

By 1933 the results had become so incongruous that 

these subdivisions were abandoned, and the prePhanerozoic rocks were designated by the title Precambrian alone. Any subdivisions made were applied 

informally as lower and upper (early and late) or as 

lower, middle, and upper (early, middle, and late), and 

were used in a relative sense in local areas, without 

respect to any overall classification and correlation; the 

informal terms might thus vary in absolute age from 

one area to another. This procedure was useful in 

studies of particular areas, but was without value for 

regional work. 

This classification was nevertheless followed on the 

U.S. Geological Survey's Geologic Map of North America of 1965, where the Precambrian was divided in 

many areas into lower Precambrian (p€1) and upper 

Precambrian (p€u), with unrealistic and sometimes 

misleading results. 

On the U.S. Geological Survey's Tectonic Map of 

North America of1969 a more detailed interim classification of the Precambrian was used, for purposes of this 

map only. The Precambrian was divided into Archean, 

Lower Proterozoic, Middle Proterozoic, and Upper Proterozoic, following Canadian usage that had prevailed 

up to 1963, to enable effective use to be made of Canadian tectonic data that were being contributed to the map. 
CLASSIFICATION OF PRECAMBRIAN ROCKS 11 

The classification was also extended to Greenland on 

the northeast, and to the United States and Mexico to 

the south on the basis of radiometric data then available. 

INTERIM CLASSIFICATION OF 1972 

By the time compilation of the present Geologic Map 

of the United States began in 1967, it was clear that 

major improvements could be made in the representation of the Precambrian on the Geologic Map of 1932, 

partly resulting from increased knowledge of the local 

Precambrian sequences, partly from correlation of the 

different sequences by radiometric dating. The experience of the Canadian geologists in the Canadian Shield 

indicated the general lines that a revised classification 

of the Precambrian of North America would assume, 

and the experience of compiling the Tectonic Map of 

North America demonstrated that such a classification 

could be extended to the Precambrian of the United 

States. Compilation of the Precambrian for the Geologic 

Map therefore proceeded on this basis. 

In 1970, to verify the results of the compilation, and to 

produce an interim classification of the Precambrian for 

use on the map and in other Survey publications, the 

U.S. Geological Survey appointed a Special Panel consisting ofM. D. Crittenden, Jr., Chairman, J. E. Harrison, and J. C. Reed, Jr., to advise the Geologic Names 

Committee and the Chief Geologist. After Survey approval, their recommendations were published as Note 

40 of the North American Stratigraphic Commission 

(James, 1972a). 

During its deliberations, the panel reviewed the various units and their age assignments that were shown on 

the Geologic Map, enlisting the advice ofZ. E. Peterman 

and C. E. Hedge, geochronologists of the U.S. Geological 

Survey. Various minor corrections and improvements 

were made in the age assignments of various units, but 

the four gross subdivisions shown on the Geologic Map 

were verified. 

The panel therefore recommended an interim adoption of these subdivisions. However, rather than apply 

formal names to them, as in Canada, it was recommended that they be designated informally by the letters W, X, Y, and Z. These letters would be especially 

useful for map symbols, as there was no likelihood of 

their being confused with any other symbol (other possible letter sequences, such as A, B, C, and D, were 

already preempted by map symbols for other systems). 

The letter W was used for the oldest recognized subdivision, thus providing for the possibility that still older 

Precambrian subdivisions might be separated later, 

which could be symbolized by preceding letters of the 

alphabet. 

The boundaries between the subdivisions ~~were 

selected so as to split as few of the known episodes of 

sedimentation, orogeny, or plutonism as possible" 

(James, 1972a, p. 1129), hence were initially based on 

geologic features. Nevertheless, they were not intended 

to correspond to natural events such as orogeny or 

plutonism; once established, they were defined by geochronology alone. 

The basis for the proposed classification thus differs 

from the basis for the Canadian classification, in which 

the boundaries are defined by natural features or events 

whose ages were established by radiometric means. The 

opposing rationales reflect the different geologic conditions in the two countries. In Canada Precambrian 

rocks are exposed nearly continuously over vast expanses of the Canadian Shield, so that regional geological features are an evident and obvious means of classification. In the United States outcrops are relatively 

small and some are so widely spaced that identification 

of regional geological features are necessarily much 

more subjective. Here, the only assured means of classifying the rocks of an outcrop is by age alone. Despite 

these differences, the major subdivisions of the Precambrian in Canada and the United States are much 

the same and are broadly correlative from one country 

to the other. The two classifications, and the earlier one 

in Minnesota, are compared in table 3. 

Like all stratigraphic schemes, the interim classification of the U.S. Geological Survey creates problems 

when applied in detail. 

New radiometric data sometimes improve the dating 

of rocks or events (see Stockwell, 1972). ~~The most significant practical difference between subdivision based 

on geochronology and that based on stratotypes is that 

revision in age of the given body of rock would result in 

TABLE 3.-Comparison of recent classifications proposed for the Precambrian of North America 

[Numbers are ages in millions of years. In the first column, numbers combine the results.of 

various analytical methods; in the second column first number is by K-Ar method, second 

by U/Pb; in the third column numbers are arbitrary] 

Minnesota, 1961 Canada, 1964, 1972 U.S. Geological 

1968, 1970 Survey, 1972 

Upper Hadrynian Precambrian Z 

Keweenawan 800 Igneous 

activity u Grenvillian orogeny 1,000-- 1,200 -

0 880 (1,000 1 

Precambrian N 

0 Helikian Precambrian Y 

Penokean orogeny "' (!) 

1.700 ..., 1,600 

0 

"' 0... H udsonian orogeny 

1,640 (1,8001 

Middle Precambrian Aphebian Precambrian X 

__ Algoman orogeny 2,500 

2,500 

Kenoran orogeny 

2,390 (2,5601 

I 

Lower Precambrian Archean Precambrian W 
12 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

reassignment of the rock unit in the time scale, rather 

than readjustment of the time scale itself' (James, 

1972a, p. 1131). Under the Canadian scheme the 

boundaries of the subdivisions are revised in age; under 

the United States scheme the boundaries remain fixed 

and the specific rock unit is moved from one subdivision 

to another, even though this might result in geologically unnatural groupings. In general, the boundaries 

between the subdivisions were carefully enough chosen 

by the Special Panel so that most such problems will be 

avoided, but some will certainly arise. 

The defined age boundary between Precambrian Y 

and Z remains problematical. An 800-m.y. boundary 

was chosen by the panel on the assumption that it was 

the age of termination of deposition of the Belt Supergroup in the Northern Rocky Mountains. This date is no 

more than an approximation, as the termination is 

merely bracketed between determined dates of 930 and 

760 m.y.; it is suspect because the lower part of the 

Precambrian Z Windermere Group that lies unconformably on the Belt to the west has been dated between 

820 and 900 m.y. (For details, see p. 53.) Further, the 

upper part of the Precambrian Z supracrustal rocks in 

the Central Appalachians has been dated at 820 m.y.; 

these rocks lie on Precambrian Y infracrustal rocks 

with the greatest discordance in the sequence below the 

Triassic. It would be intolerable to place this discordance, along with the infracrustal rocks below and the 

supracrustal rocks above, all in Precambrian Y. The 

proposed boundary at 800 m.y. is therefore ignored on 

both the Geologic Map and in the ensuing text, where 

the most workable boundary is found to be about 100 

m.y. earlier. 

REPRESENTATION OF PRECAMBRIAN ON 

GEOLOGIC MAP OF UNITED STATES 

The interim subdivisions of Precambrian W, X, Y, 

and Z are used on the Geologic Map to classify the units 

in the different sequences, and to correlate these sequences with those in other parts of the country. Different categories of rocks are indicated in the same manner 

as in the Phanerozoic. Each Precambrian subdivision 

thus contains representatives of stratified sedimentary 

rocks, volcanic rocks, plutonic or intrusive rocks, and 

metamorphic rocks, shown in separate columns in the 

legend. However, Precambrian continental and eugeosynclinal deposits are either not separated or not recognized. 

The arrangement of the stratified rocks in the legend 

indicates that in at least some areas the methods used in 

the Phanerozoic can be applied; the Belt Supergroup of 

Precambrian Y can even be subdivided on the Geologic 

Map in parts of northwestern Montana and northern 

Idaho. Because the assignment of strata to one of the 

new subdivisions or another will not be familiar to most 

users, representative units in different areas are listed 

more completely in the legend than for the subdivisions 

of the Phanerozoic rocks. The volcanic rocks, although 

placed in a separate column in the legend, are important components of the stratified sequences in some 

areas, as in Precambrian Wand Y of the Lake Superior 

Region, and Precambrian Z of the Appalachian Region. 

Among the Precambrian plutonic rocks the most extensive are granitic, but mafic categories are separately 

shown in Precambrian W and Y. Assignment of the 

plutonic rocks to one subdivision or another is based 

partly on their geologic relations to the surrounding 

country rocks, but more upon their radiometric dating. 

The ages determined for the granitic rocks indicate that 

many of them formed during the later stages of a subdivision, but in Precambrian Y an earlier suite is extensive; the granites of Precambrian W include both the 

terminal plutonics, and undifferentiated earlier ones. 

The metamorphic rock units, in general, are complexes so greatly altered as to preclude the application 

of normal stratigraphic analysis. The orthogneisses 

originated from plutonic rocks and the paragneisses 

from sedimentary or volcanic rocks. In places, the latter 

include some bodies of rock capable of more detailed 

analysis, but in such small areas that it would be fruitless to separate them on the scale of the present geologic 

map. On the map, the ages assigned to the metamorphic 

r ocks are based primarily on their time of metamorphism, assuming that the original rocks were mostly 

formed during the time of the same subdivision, but in 

places they may include relict rocks formed during earlier subdivisions that have been overwhelmed by the 

later and dominant metamorphic event. 

These results are summarized on the accompanying 

maps (fig. 2-5), which show the surface distribution of 

rocks of the different major subdivisions, as represented 

on the Geologic Map. To give added meaning to the 

figures, the rocks of each subdivision are divided into 

three classes: (1) Sedimentary and volcanic supracrustal rocks (including their metamorphic equivalents in 

the eanlier subdivisions),2 (2) intrusive and plutonic 

rocks (including those of both felsic and mafic composition), and (3) metamorphic rocks (paragneisses and 

orthogneisses). 

2The word "supracrust al" has been defined briefly as referring to "rocks that overlie the 

basement. " In this account, t he term supracrustal is used for Precambrian sedimentary and 

volcanic rocks that were la id down on the surfa ce of the earth, on a basement of rocks that 

have had a more complex metamorphic and plutonic history. Ideally, they are exemplified by 

such units as the little deformed or metamorphosed Keweenawan and Belt Supergroups. 

However, differences between "supracrustal" and "basement" rocks are relative, and distinctions between them become subjective and blurred in places. Thus, this account describes 

many units as "supracrustal" even though they have been deformed and metamorphosed, 

because they are clearly of sedimentary and volcanic origin , and contrast with more enigmatic paragneisses and orthogneisses. 
LAKE SUPERIOR REGION 13 

EXPOSED PRECAMBRIAN ROCKS 

OF THE UNITED STATES3 

The following is a survey of the Precambrian rocks 

exposed at the surface in the United States, to explain 

the representation adopted on the Geologic Map. It expands the explanation of these rocks in the legend. In 

the legend, the rocks are categorized by age and character (sedimentary, volcanic, plutonic, etc.); here, it is 

better to treat all the rocks of each province collectively, 

in order to demonstrate their mutual relations, and the 

reasons for assigning particular rock units to one or 

another of the broad age divisions. 

The exposed Precambrian rocks are only a small part 

of the Precambrian of the United States; much larger 

areas are concealed beneath Phanerozoic rocks, especially in the Central Interior Region, between the Appalachian and Cordilleran mountain belts, where they 

are known from drill data. The concealed Precambrian 

rocks have been extensively investigated, especially 

during a project of Goldich, Muehlberger, Lidiak, and 

Hedge (1966). Here, these concealed rocks will be mentioned only to suggest c~nnections between the rocks of 

the various areas of exposure. 

In this account the results of many fundamental 

pieces of research will be summarized, but these are not 

always credited with a citation. Literature references 

are made primarily: (1) to recent publications that update the earlier records, (2) to summary reviews that 

contain references to earlier publications, and (3) to 

publications which contain information on radiometric 

dating. The account is illustrated in part by maps and 

diagrams, the maps being mostly on scales larger than 

those of the Geologic Map, which show rock units, structures, and the names of localities which could not be 

represented on the Geologic Map itself. Features not 

illustrated by the maps and diagrams in the text are 

believed to be adequately represented on the main 

Geologic Map, to which the reader should refer. 

Extensive use will be made of radiometric data to 

justify the classifications and correlations that are 

made, and specific ages are cited where appropriate. In 

general discussions, however, I believe it is clearer to 

use names rather than numbers for the broad groupings 

of ages within a few hundred million years of each other 

that express orogenic, plutonic, metamorphic, or other 

significant events in the Precambrian history of North 

America. For this purpose the names used in the Canadian Shield are adapted in this text: The Kenoran with 

ages around 2,500 m.y., the Hudsonian with ages 

3Previous official reviews of t he Precambrian of t he United St ates by VanHise (1892 ) and 

Van Hise and Leith (1909) appeared more than half a century ago. They provide inter esting 

comparisons wit h the present review, both in the amounts of data available, a nd in geologic 

concepts. 

around 1,700 m.y., the Elsonian with ages around 1,300 

m.y., and the Grenvillian with ages around 1,000 m.y. It 

is true that in the United States various local names 

have been used for comparable events, some proposed 

earlier, some later; for example, Algoman and Penokean orogenies in the Lake Superior Region, St. Francois igneous activity and Llano orogeny in the South 

Central States, and Black Hills and Mazatzal orogenies 

in the Cordilleran Region. These names add precision to 

local discussions because they can be tied to specific 

dates within the particular area, but in a regional review such as this they obscure the broader relations. 

LAKE SUPERIOR REGION4 

The most extensive outcrops of Precambrian rocks in 

the United States are in the region west and south of 

Lake Superior. Precambrian forms the northern half of 

Minnesota, the western half of the northern peninsula 

of Michigan, and a large part of northern Wisconsin. 

Also properly part of the region are outlying areas to the 

south, such as that of ancient gneisses in the Minnesota 

River valley, of Sioux Quartzite that extends into South 

Dakota, and of Baraboo Quartzite in central Wisconsin. 

The region is a southern extension of the Canadian 

Shield, the northwestern part belonging to its Superior 

province, and the southeastern part to its Southern 

province. 

The Lake Superior Region in the United States and 

adjacent Canada has been one of the longest known and 

most intensively studied Precambrian terranes in 

North America, particularly because of its wealth of 

mineral resources such as the great deposits of iron ore 

north and south of the lake and the copper deposits of 

the Keweenaw Peninsula. Moreover, it contains a long 

record of Precambrian rocks and events, all the major 

divisions (W, X, Y, and Z) being represented in some 

form or another. Their various supracrustal sequences 

total more than 150,000 ft (46,000 m) of strata, and the 

record is further diversified by several times of major or 

minor orogeny, and of plutonic and volcanic activity. 

The Precambrian rocks and structures have remained 

virtually untouched by Phanerozoic distu~tbances , in 

contrast to the Precambrian of most of the other regions 

of the United States which we will consider later. 

For these reasons, there has long been a temptation to 

regard the Precambrian sequence of the Lake Sueprior 

Region as the North American standard, to which the 

Precambrian of other regions is to be compared and 

correlated. This view, however, would fail to take into 

4For a recent compendium of t he geology of t he pa rt of the La ke Superior Region in 

Minnesota , see Sims and Morey (1972, especially p. 27- 455). This includes recent data not 

available when t he present summary was prepared; the more importa nt revisions are included here . 
14 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

\ I 

·, ---...... / 

I 

I 

z, 

\ 

\ 

.-....._~--------------( 

\ 

\ 

\ 

-~ -. 

\ 

\ 

/ 

I 

\ : \J 

\ 

I ~r- -- ----- -- -­

' 

(, 

\ 

'L, 

I 

--- ------1 j I 

I L ____ _ 

---- -----

I 

I 

r---

' 

'-.- -- i ----- ------- -- --< 

,, ----; i1 

I 

,------- ' ). ! ------

-- ~ j -- - -- - ---.L.. 

,---------------------

1 

I 

- -- - I I ---:- - -- -

----

I 

I 

I 

I 

-----

----- --- --·-c:~:=::_--------------

I : 

.L ' .. --", 

--_, '- ---- :r -_.,__ -· 

:-----_~..._-- ---0------- J 

\ 

\ 

FIGURE 2.- Map of the United States, showing surface distribution of rocks of Precambrian W as represented on the Geologic Map 

of the United States. 
_-... ../""'-, 

\ ; 

LAKE SUPERIOR REGION 

,_ I _ _..,. -- _..,-

-• \ I 

'.----- - ------- "--,-._ i!!f;P; / _ _ %~ -~ ' -., - < ., .. 

> 

l 

c;--------\_-------- -,j 

... . .-· 

FIGURE 2.-Continued. 

EXPLANATION 

Metamorphosed 

supracrustal 

-

rocks 

Plutonic 

-

and 

intrusive rocks 

(felsic to mafic) 

Metamorphic rocks 

(paragneiss and 

orthogneiss) 

15 
16 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

\ I 

·, -- ......._ I 

I 

I 

I 

\ 

, _ _...,_ _____ --.. 

;-----------

/ 

\ 

\ 

\ 

\ 

\ 

----( 

/ 

I 

I 

I 

I 

I 

I 

' (_, 

~ : 

,_ 

I 

-----

t ... 

'----~-;'"', r--------- I 

I 

I 

l 

;'"'! 

I 

I 

----1-- ,1 l 

------------------· 

I 

\ 

I 

~ 

~----------------------; 

I '- --· l 

I 

~ ~ I 

I 

I 

I 

.------ til 

; : 

I 

~---------------------

I 

--- --------r~ ~::_-: ~------ ------

L 

' ... ......,...1 

---_, '-"---y·-'1..._,· 

\ 

\ 

FIGURE 3.-Map of the United States, showing surface distribution of rocks of Precambrian X as represented on the Geologic Map 

of the United States. 
LAKE SUPERIOR REGION 

"'-- '- .r- " I_ 

\. 

_____ ___________ \ 

\ 

I 

' 

1 

\ ; c.~--------\ __ _c--------,5 

. _ .. ~. 

FIGURE 3.-Continued. 

EXPLANATION 

Metamorphosed 

supracrustal 

-

rocks 

Plutonic 

-

and 

intrusive rocks 

(felsic to mafic) 

Metamorphic rocks 

(paragneiss and 

orthogneiss) 

17 
18 

I 

I 

1 

PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

r---------

----

:a ,t./ 1 1 I 

------

I 

I 

I 

------------------· 

\ 

I ~~~~~ ,, ' - ~- -1 

/It , ~----- \ 

·: l"' ~ ",r-·-------- : --------------.---? . ..,...._~--.. ------ I - • \ I ------: I 

" I : 

: I I . 

I 

----! 1 

.------ ~ I : 

"--- : -"\ 

: \ 

L_ j . I : 

I --- ~----- ~~ ---.1._ \ r.. - j ----~ 

\ 

\ I ~o ~· ,. .. i··-··- ·- ·-··-··-·· 

\, ' ' ' ;- ._. 14 ' \ : i i 

\ -'- ---- : ,. : I. ._ i' - -- ----------1 \ ! 

~~~\ ~- ,. r-------- -- d/- ------ i - \j ·- i • :· .. -; r~:~::_-:::_- .. -. -.. -. - \ " I t f{ 

J 

'-, ------,.._ 

~ \ 

.. 

' 

I 

I 

I 

/ r------f ----·---- ___ j ---------·' 

\ 

\ 

\ 

l._ 

'-........._-- '-.... 

FIGURE 4.-Map of the United States, showing surface distribution of rocks of Precambrian Y as represented on the Geologic Map 

of the United States 
____________ \ 

\ 

r----

l 

__ \ 

'? 

'-, 

I 

,-----

\ 

----'"' I '-! 

'l 

\ ; 

\ 

\ 

LAKE SUPERIOR REGION 

'\ 

\ //-X 7 

c~ 

c;--------\----------,s 

• .• •-~*, 

FIGURE 4 . -Contmued . . 

EXPLANATION 

Supracrustal 

-

rocks 

D . Plutonic and 

mtrusive rocks 

(felsic to mafic) 

Metamorph' 

( 

lC rocks 

paragneiss and 

orthogneiss) 

19 
20 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

-7/r--r-----

/ / : ----

' I ----- \ ! 

.'-----.... I 

I 

I 

I 

, _________ ___ 

----( 

, 

I 

/,' \ I I 

\ 

\ ,._ " . 

·~ '> -

\ 1'\ ., .I ~ 0 

. ·--;._ ,-\.: 

I 

\ : \J 

"' I ~ 

I 

.f __ 

\ 

r-- - -

~ 

~----------------------~ · .. -----.r'\,,' ----- I l 

----

I 

I 

-.--- -----

-- -------------------

1 

I I 

----f_____ I 

-----

, - - -- ----c ~-:::_-:::_- -- - - --

I 

I 

; ----- _~.._------------- j 

,_/ 

1 : 

\ 

FIGURE 5.-Map of the United States, showing surface distribution of rocks of Precambrian Z as represented on the Geologic Map 

of the United States. 
'-·_r·-L. 

'•' 

_____________ \ 

\ 

-;-------------~ c-' 

__ \_ 'l, 

? 

\., 

I 

I 

~---------

\ 

I 

I 

I ._, 'l 

\ 

\:--------

\ 

~ 

I 

.r- 1 

LAKES UPERIOR · REGION 

______ , __ _ 

J 

~ "' r' '-. ); ,s-. -/" '.' -' 

I ' 

~r-S- , 

I (_~--------·\ .. --------.5 

FIGURE 5 . - C ontmued. . 

/'•.../' ...... _ 

/ \ 

( \ 

f ' 

r'si 

----· \ ---y ) \ 

I ( ' "'\ 

EXPLANATION 

Supracrustal 

-

rocks 

Felsic plutonic 

rocks 

21 
22 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

account the great length of Precambrian time and large 

gaps in the record in even so complete a sequence, as 

well as the quite different tectonic and sedimentary 

regimes in other parts of North America. 

PRECAMBRIAN W. 

The northwestern and western part of the Precambrian area in Minnesota is an extension of the Superior 

province of the Canadian Shield, a domain of the ancient rocks of Precambrian W age ( = Archean of 

Canada). Its rocks are well exposed toward the northeast, as well as westward along the Canadian border as 

far as the Lake of the Woods. Farther southwest outcrops are sparse; there is an extensive cover of thick 

glacial drift and of the thin intervening Cretaceous 

Coleraine Formation (King and Beikman, 1974, fig. 13), 

so that representation of the Precambrian here must be 

largely by subcrop methods, especially by deductions 

from geophysical surveys. 

The Superior province in Minnesota (as in adjoining 

Ontario) is a great body of supracrustal rocks, probably 

more than 50,000 ft (15,000 m) thick in all, partly 

metavolcanics (Wv), partly metasediments (W), and 

equally extensive bodies of granitic plutonic rocks (W g). 

The volcanics, traditionally called Keewatin Group, 

include the Ely Greenstone of northeastern Minnesota 

with the Soudan Iron-formation in its upper part (commercially productive in the Vermillion district). Much 

of the greenstone is basaltic, but intermediate and felsic 

varieties are present also. Pillow structure is ubiquitous, except where obscured by deformation and 

metamorphism, and indicates subaqueous eruptions. 

The superincumbent sediments-the Knife Lake Group 

of northeastern Minnesota and comparable units in Ontario (Temiskaming, etc.)-are dominantly graywacke, 

with local thick lenses of conglomerate and minor slate; 

quartzite and limestone are virtually lacking. Graded 

bedding and related features in the graywackes indicate they they are turbidites, formed subaqueously in a 

tectonic environment (Pettijohn, 1943, p. 966--968). 

The relation of the sediments to the volcanics has 

been variously interpreted ever since A. C. Lawson 

began fieldwork in the Lake of the Woods area in 1883, 

and has given rise to some o~:'the classic controversies of 

North American geology. It is now clear that most of the 

sediments overlie the volcanics, but a prevolcanic terrane (Coutchiching) has been claimed, especially in adjacent Ontario. In places, at least, the superincumbent 

sediments lie unconformably on the volcanics, and some 

granites intrude the volcanics but not the sediments, 

giving rise to the concept of a far-reaching ((Laurentian 

orogeny" between the two. Actually, these problems are 

not fundamental, as volcanic and sedimentary units are 

probably intermingled in various combinations from 

place to place (Pettijohn, 1943, p. 980-981). Unconformities above the volcanics are of local extent, and gradational or interbedded relationships occur in other 

places. 

A case in point is stratigraphic relations in the Vermillion district of north -eastern Minnesota, where 

many of the classic concepts of the Precambrian Wrocks 

originated. Modern mapping (Sims, in Sims and Morey, 

1972, p. 49-62) has indicated greater stratigraphic 

complexity than originally supposed; in essen~e, the Ely 

Greenstone (or local representative of the Keewatin 

Group) is followed by a unit of Knife Lake sediments, 

and this by a second volcanic body of Keewatin type, the 

last two merging into the main mass of Knife Lake 

sediments in the eastern part of the district. The whole 

sequence is conformable, and there is no evidence for 

any major orogenic interruption, as was formerly believed. 

Of the older granites (traditionally but inappropriately called ('Laurentian") the only example that has 

been cited in Minnesota is the Saganaga Granite on the 

International Boundary in the northeastern corner of 

the State (fig. 6). It clearly intrudes the Ely Greenstone, 

and the adjacent Knife Lake sediments lie on its eroded 

surface. However, it intrudes other parts of the Knife 

Lake, and its radiometric age does not differ greatly 

from that of the surrounding rocks. Probably its pluton 

was emplaced at shallow depths, and quickly unroofed 

during the early part of Knife Lake sedimentation 

(Sims, in Sims and Morey, 1972, p. 53). 

The remaining granites (termed Algoman) intrude 

all the supracrustal rocks of the province: The Vermillion Granite forms a body 80 mi (130 km) long east-west 

and 30-40 mi (50-65 km) wide north-south along the 

International Boundary, and the Giants Range Granite 

farther south extends for more than 100 mi (160 km) 

along the northern edge of the Mesabi Range, where it is 

overlain unconformably by the Animikie Group (Precambrian X). 

The Algoman ( = Kenoran) orogeny deformed and 

metamorphosed the supracrustal rocks and emplaced 

the Algoman granites. The orogeny has been dated between 2,400 and 2,750 m.y., on the basis of a variety of 

radiometric methods (Goldich and others, 1961, p. 69-

74). However, there are unexplained discrepancies between uranium-lead, rubidium-strontium whole-rock, 

potassium-argon and rubidium-strontium mineral 

ages. Available radiometric data seem to suggest that 

all of the features in the Precambrian W complex of 

northern Minnesota-accumulation of the volcanics 

and sediments, and their deformation, metamorphism, 

and plutonism-were created during a remarkably 
48 

46 

LAKE SUPERIOR REGION 

200 Ml LES 

0 200 KILOMETRES 

EXPLANATION 

SUPRACRUSTAL ROCKS 

Upper Cretaceous 

Paleozoic 

Jacobsville, Bayfield, Fon du Lac, 

and Hinckley Sandstones 

Keweenawan 

Supergroup 

Animikie Group and Marquette 

Range Supergroup (with iron 

formations in black) 

Keewatin and Knife Lake Groups, 

and related rocks (with minor 

iron formations in black) 

INTRUSIVE ROCKS 

Duluth Complex and 

Mellen Gabbro 

granite 

m X 

X 

Granite 

Granite and granite 

gneiss 

FIGURE 6.-Geologic map of part of the Lake Superior Region, showing localities and map units mentioned 

in the text. Generalized from geologic maps of United States (1974) and Canada (1969). 

23 
24 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

short interval between 2,700 and 2,750 m.y. ago (Goldich, in Sims and Morey, 1972, p. 32-34). 

South of the area just discussed, in southwestern 

Minnesota, Precambrian granites and gneisses (W g, 

W gn) appear along the Minnesota River valley (Goldich 

and others, 1961, p. 123--146). Here, radiometric determinations have yielded a scatter of dates, with some as 

low as 1,850 m.y. (an overprint of the Penokean 

( = Hudsonian) event), and others, by lead-lead methods 

on zircons from 2,870 to 3,280 m.y. A concordia plot 

suggests an original age of 3,550 m.y. (Goldich, 1968, 

p. 718--720), so that these rocks are among the oldest 

recorded in North America. 5 

South of Lake Superior in Michigan and Wisconsin, 

old rocks are exposed beneath the Marquette Range 

Supergroup (Precambrian X) in the higher folds, and 

have been identified as ((Archean" (that is, Precambrian 

W) since the earliest surveys. Most of the rock is granite 

gneiss, probably mainly Algoman, but Keewatin-type 

greenstone occurs to the north in the Marquette district, 

and farther south is the Dickinson Group of arkose, 

schist, and amphibolite (fig. 2); it is in contact not only 

with the Algoman granite, but with an older granite 

gneiss (James, 1958, p. 31-33). This region has been 

more heavily involved in younger Precambrian events 

(such as the Penokean orogeny) than the region northwest of Lake Superior, so that radiometric dating has 

produced varied results. Nevertheless, feldspar 

rubidium-strontium ages and the diffusion age of zircons establish the age of the basement gneisses at near 

2,700 m.y. (Aldrich and others, 1965, p. 462), or about as 

old as the Precambrian W rocks of northwestern Minnesota. 

Gneisses east of the Minnesota River valley (the 

McGrath Gneiss of central Minnesota and the basement 

gneisses of Michigan and northern Wisconsin) are similar petrographically and in metamorphic history to 

those along the Minnesota River but have so far failed to 

yield dates as ancient. Nevertheless, Morey and Sims 

(1976) suggest that they may all be part of the same 

terrane-a sialic protocontinent against which the 

greenstones and graywackes of northern Minnesota 

and elsewhere in the Superior province were built in 

later Precambrian W time. 

PRECAMBRIAN X 

Southeast of the Superior province is the Southern 

province, which forms the remainder of the Lake Superior Region. The boundary between them is in north5The oldest radiometrically dated rocks in North America, and among the oldest in the 

world, are those of the Godthaab area, western Greenland, where quartzo-feldspathic gneisses with some shreds of iron formation have been dated at more than 3,750 m.y. (Moorbath and 

others, 1972). Very ancient Precambrian rocks are suspected from geological evidence in 

parts of the Canadian Shield in Canada, but so far lack radiometric verification. 

ern Minnesota and adjacent Ontario, where rocks of 

Precambrian X lie with right-angle unconformity on 

rocks of Precambrian Wand dip away from them southeastward. Within the Southern province, rocks of Precambrian X are extensive northwest and south of Lake 

Superior, in Minnesota, Wisconsin, and Michigan, 

flanking on each side the Keweenawan rocks (Precambrian Y) that occupy the trough of the Lake Superior 

syncline. They contain all the commercially exploited 

iron deposits of the Lake Superior Region (shown in red 

on the Geologic Map), except those in Precambrian W of 

the Vermillion district: the Gunflint district of Ontario, 

the Mesabi and Cuyuna districts of Minnesota, and the 

Gogebic, Menominee, Marquette, and other districts of 

Wisconsin and Michigan (fig. 1). (The outcrops of iron 

formations in the various districts are commonly referred to as ((ranges," hence such terms as ((Mesabi 

Range.") 

The supracrustal rocks of Precambrian X northwest 

of Lake Superior are the Animikie Group, named long 

ago for the Thunder Bay district in Ontario, whence the 

group can be traced westward with little interruption 

into Minnesota. South of Lake Superior, the obvious 

stratigraphic and lithologic equivalents of the 

Animikie are in the middle of a more comprehensive 

sequence, the Marquette Range Supergroup (Cannon 

and Gair, 1970) (fig. 7). 

The Animikie of the northwestern area begins with a 

discontinuous basal quartzite lying unconformably on 

Precambrian W, followed by a persistent iron formation 

several hundred feet thick (Biwabic of Mesabi district), 

and topped by the Virginia Slate many thousands of feet 

thick. This iron formation (and those of Precambrian X 

elsewhere) is an alternation of ferruginous chert 

( = taconite), slate, and stromatolitic beds, whose 

weathered products were the readily exploited iron deposits of past decades. The Virginia Slate is interbedded 

argillite and graywacke, a turbidite deposit not unlike 

the much older Knife Lake. 

In the Cuyuna district southwest of the Mesabi district the sequence is much the same, but the iron formation is separated from the Precambrian W rocks on the 

west by il poorly exposed, wider stratigraphic interval. 

It may include pre-Animikie Precambrian X rocks comparable to the Chocolay Group south of Lake Superior 

(Marsden, in Sims and Morey, 1972, p. 227-230). 

These lithologic components reappear in the Marquette Range Supergroup south of Lake Superior. Iron 

formations like the Biwabik occur in each of the principal districts (Gogebic, Marquette, Menominee), again 

with basal quartzites and great overlying bodies of 

((slate" (argillite and graywacke). Here, however, the 

sequence is thicker, more diverse, and interrupted by 

unconformities, so that it has been divided into four 
LAKE SUPERIOR REGION 

GOGEBIC-KEWEENAWAN 

DISTRICTS, MICH.-WIS. 

MESABI-VERMILION 

Cc. 

<O:J 

:!;0 

ro.._ CCl 

Ql'-

Q)Ql 

sc. 

Q):J 

:::£(/) c 

<0 Q) 

Q) Q)+' 

.::,{. ·- ~c. 0 ~ ·- :J c .... .~e ~(.!) c(.!) 

<( 

c. 

:J ~e e E·- (.!) 0 c Cle Q) 

.::,{. ~(.!) 

<0 

-' 

Q) 

:t: 

~ 

c. c ::I 

e 

(.!) 

c 

·.;:::; 

<0 

:!; 

Q) 

Q) 

:::£ 

~ e c ·- Q) c .... <0 ::::1'- roc.!) 

-' 

Rocks of volcanic 

origin 

Basalt 

~ 

~ 

Greenstone tuff 

and breccia 

Greenstone, in part with 

preserved pillow structures 

Amphibolite of basaltic 

composition 

LITHOLOGIC SYMBOLS 

Rocks of sedimentary 

origin 

L8 ~ 

Sandstone and 

quartzite 

Conglomerate, conglomeratic 

sandstone, and arkose 

~ 

~ 

Graywacke, shale, argillite, slate, 

and schist 

Iron Formation 

Dolomite 

Schist; probably includes some 

rocks of volcanic origin 

Crystalline rocks, mostly 

of igneous origin 

Gabbro and granite 

Age 1,100 m.y. 

Granitic rocks 

Age 1,600-1,900 m.y. 

Gneissic granite 

Age 2,600 m.y. 

~ 

~ 

Granitic gneiss 

Age 2,600 m.y. or older 

c. 

:J 

0 

~ 

Q; 

c. 

::I 

(/) 

Q) Cl 

c 

<0 

a: 

c. 

::I 

e 

(.!) 

FIGURE 7.-Stratigraphic chart showing Precambrian units northwest and south of Lake Superior in 

Minnesota, Michigan, and Wisconsin. Thicknesses are diagrammatic, and not to scale. After 

Bayley and James p. 936). 

25 
26 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

groups: the Chocolay, Menominee, Baraga, and Paint 

River (James, 1958, p. 30) (fig. 7). The Chocolay at the 

base is quartzite and dolomite. The Menominee above it 

contains the great iron formations, and the Baraga the 

great ((slate" bodies (Tyler, Michigamme); the latter is 

diversified further by thick but impersistent bodies of 

pillow lava. The Paint River at the top, preserved only 

in the deeper downfolds, is again slate and graywacke, 

with its own productive iron formation (Riverton) in the 

Iron River-Crystal Falls district. The whole sequence, 

totaling at least 50,000 ft (15,000 m), exhib1ts an 

upward progression from shelf or miogeosynclinal 

deposits below, with quartzites, dolomites, and iron 

formation; into eugeosynclinal deposits above, with argillites, graywackes, and pillow lavas. 

The Animikie Group and Marquette Range Supergroup are the ((Huronian" of the older classic reports on 

the Lake Superior Region. The original Huronian of 

Logan and later Canadian geologists is in southern 

Ontario north of Lake Huron, and correlation of the 

Lake Superior rocks with it was recommended by the 

International Committee on the Lake Superior Region 

(Adams and others, 1905). Actually, the original Huronian and its supposed equivalent in the Lake Superior 

Region are separated by a 200-mi (320-km) gap covered by younger strata, and have few physical resemblances; for example, the great iron formations of 

the Lake Superior Region are missing from the type 

Huronian. Moreover, recent radiometric work demonstrates that they are not correlative (Van Schmus, 

1972). The type Huronian overlies a 2,700-m.y.-old 

basement and is intruded by the 2,160-m.y.-old Nipissing Diabase, whereas the Marquette Range Supergroup has been dated between 2,050 and 1,900 m.y. 

Both the Huronian and the Lake Superior rocks are 

units within Precambrian X, but the first is older than 

the second, their ages seemingly do not overlap, and 

they may be separated by a minor period of orogeny. 

The Precambrian X rocks of the Lake Superior Region were variably involved in the Penokean ( = 

Hudsonian) orogeny, a pre-Keweenawan (Precambrian 

Y) event which is expressed by potassium-argon dates of 

1,600 to 1,800 m.y. in the plutonic and supracrustal 

rocks, both northwest and south of Lake Superior (Goldich and others, 1961, p. 156-57; Aldrich and others, 

1965, p. 463). Presently available data suggest that it is 

actually older than 1,850 m.y. (Goldich, in Sims and 

Morey, 1972, p. 35). 

In northeastern Minnesota and adjacent Ontario the 

Animikie slopes homoclinally southeastward at an 

angle of only a few degrees beneath the Keweenawan-a relation that prompted Lawson (1914, p. 70) 

to proclaim the Animikie as the true base of the 

Paleozoic. The orogenic effects increase southward and 

southwestward. In the Cuyuna district the iron formation and associated rocks are folded, and west of the 

head of Lake Superior the Thompson Formation, although coextensive with the Virginia Slate, has been so 

strongly deformed that it has been mistaken by some 

geologists for the Knife Lake. In central Minnesota the 

Animikie belt is truncated southwestward by a complex 

of plutonic rocks mapped as Xg, including the McGrath 

Gneiss t o the northeast and the Saint Cloud and other 

granites farther southwest (fig. 6), all of which have 

yielded potassium-argon dates of 1,640 to 1, 760 m.y. 

(Goldich and others, 1961, p. 116-117). Further field 

and radiometric investigations indicate, however, that 

t he McGrath is much older and probably Algoman, with 

a Penokean overprint. The granites farther southwest 

are shown by rubidium-strontium determinations to 

range in age from 1,730 to 1,820 m.y. (Keigan, Morey, 

and Goldich, in Sims and Morey, 1972, p. 252--254). 

South of Lake Superior, rocks of Precambrian X and 

their Precambrian W basement have been thrown into 

wide steep folds and metamorphosed to greenschist or 

amphibolite facies (with sillimanite) (James, 1955, 

p. 1461-1463). In some areas there is only a slight angular discordance between the Marquette Range rocks 

and the Keweenawan rocks north of them, but even 

here the former are metamorphosed and the latter are 

not. Intrusive rocks are widely distributed in Precambrian X south of Lake Superior, but none attain 

batholithic dimensions; the granitic rocks are mostly 

younger than the deformation and metamorphism, but 

none are younger than 1,700 m.y. Contrary to previous 

belief, there is no post-Keweenawan ((Killarney" granite in the area (Goldich and others, 1961, p. 163-164. 

PRECAMBRIAN OF NORTHERN WISCONSIN 

Precambrian rocks are at the surface for 130 mi 

(210 km) south of the area just discussed, along the 

Wisconsin arch, but outcrops are discontinuous because 

of extensive glacial cover and the bedrock geology is 

known only in part. On most of the earlier maps the 

Precambrian of the arch is shown as undivided granites 

and gneisses, for which ages anywhere from early to 

late Precambrian have been proposed. Subsequently, 

Dutton and Bradley (1970) have assembled all the 

available data and have clarified the picture; their map 

was the chief basis for representation on the Geologic 

Map. After compilation and printing of the map, articles 

by Van Schmus and Medaris (1975a) and Van Schmus, 

Thurman, and Peterman (1975b) have appeared that 

add many new data, which would require revision of 

some of the boundaries shown on the map. 

The oldest rocks of the area are Precambrian W granites which adjoin the Marquette Range Supergroup of 
LAKE SUPERIOR REGION 27 

the Gogebic district in the northwestern part of the 

State, but radiometric dates elsewhere are younger 

than Kenoran, and no Precambrian W rocks probably 

exist farther south. 

The main body of rocks in the Wisconsin arch have 

yielded ages of about 1,650 m.y. by rubidium-strontium 

methods and 1,800 m.y. by uranium-lead methods and 

include belts of metasedimentary (X) and metavolcanic 

rocks (X v) probably equivalent to the Marquette Range 

Supergroup, separated by belts of intrusive granite (Xg). 

On the southeast, however, the Wolf River batholith 

occupies an area of3,600 mi2 (9,300 km2) and has been 

dated by rubidium-strontium methods at 1,468 m.y. and 

by uranium-lead methods at 1,510 m.y. On the Geologic 

Map it is classed as Y g1, or Elsonian. The batholith is a 

fresh, anorogenic body which crosscuts all the older 

rocks adjacent to it. On the Geologic Map the northern 

boundary of the Elsonian granites was projected hypothetically westward across the arch, but they actually 

have a well-defined western boundary beyond which, 

except for a smaller syenite body near Was au, only older 

rocks of Precambrian X occur. Near Tigerton the 

batholith encloses an older mass of anorthosite (Ya). 

The WolfRiver batholith is the local representative of 

anorogenic plutonic intrusions ofElsonian or early Precambrian Y age (Y g1) which we will observe again in 

the southern Interior Region, the Southern Rocky 

Mountains, and Arizona. 

KEWEENAWAN S U PERGRO U P OF PRECAMBRIAN Y 1

; 

In the Lake Superior Region, Precambrian Y is 

primarily the Keweenawan Supergroup, a great body of 

continental volcanics (Yv) and sediments (Y), with associated mafic intrusives (Ymi), that fills the trough of 

the Lake Superior syncline. The Keweenawan includes 

the ((Lake Superior sandstone" and ((cupriferous trap" 

that intrigued the early geological explorers, some of 

whom correlated them with the New Red Sandstone of 

Triassic age. In northern Michigan the Keweenawan 

contains large deposits of native copper and copper 

sulfide that have been mined for more than a century. 

The Keweenawan crops out in wide belts along both 

the northwestern and southeastern shores of Lake 

Superior in Minnesota, Ontario, Wisconsin, and Michigan, and projects into the lake in the Keweenaw Peninsula (fig. 6). It (and the overlying sandstones of Precambrian Z) forms all the islands in the lake, and also 

its floor. Beyond the head of the lake the synclinal belt of 

Keweenawan rocks extends southwestward for another 

hundred miles (160 km) until it passes beneath the 

overlapping Cambrian; it is even more extensive in the 

subsurface, as indicated below. 

6

For a more detailed review of the Keweenawa n rocks, see Halls (1966 1. 

The thickness of Keweenawan supracrustal rocks 

and their associated intrusives is well over 50,000 ft 

(15,000 m) in the trough of the syncline but thins outward and may not have extended far beyond its present 

limits (White, 1966, p. 28--32); accumulation of the Keweenawan and down warping of the syncline were contemporaneous. Although the rocks have been gently to 

steeply tilted they have not been folded or metamorphosed. 

The Keweenawan sequence begins with thin basal 

sandstones preserved discontinuously on both the north 

and south flanks of the syncline. They are overlain by a 

great sequence of amygdaloidal basaltic to andesitic 

lavas in persistent thin to thick flows, with minor rhyolites (Portage Lake Group to southeast, North Shore 

Group to northwest). Observed sequences of the lavas 

are 15,000-25,000 ft (4,500--7,600 m) thick, but are incomplete and the total is clearly much greater. Flow 

structures in the lavas demonstrate that they spread 

out in both directions from the axis of the trough, 

against the present slope of its flanks. Evidently the 

rate of buildup of the lavas exceeded the rate of 

downwarping of the trough, and produced an outward 

slope (White, 1960, p. 368--371). Paleomagnetic studies 

indicate that the lower lava flows have reversed polarity and the uper lava flows normal polarity, which 

suggests a possible criterion for stratigraphic subdivision (Craddock, in Sims and Morey, 1972, p. 285-286). 

Succeeding the lavas on the southeast shore are the 

clastic, continental sediments of the Oronto Group, as 

much as 15,000 ft (5,000 m) thick. The first deposits are 

coarse conglomerates made up largely of volcanic clasts 

(Copper Harbor), but the main body (Freda) is red arkosic sandstone and interbeded micaceous siltstone, derived from erosion of surrounding highlands of earlier 

Precambrian crystalline rocks. The thin Nonesuch 

Shale, which separates the lower conglomerates from 

the Freda, contains organic compounds, microfossils, 

and crude oil. Sedirnentary structures in the sandstones 

indicate transport from the highlands toward the axis of 

the trough (Hamblin, 1961, p. 2-6), indicating that, 

unlike the volcanic buildup, the sedimentary buildup 

did not keep pace with the subsidence of the trough. 

The lower part of the Keweenawan is invaded by 

mafic intrusives, the largest being the Duluth Complex 

northwest of Lake Superior, a lopolith 150 mi (240 km) 

long and as much as 50,000 ft (15,000 m) thick near its 

center, injected near the base of the Keweenawan. It is a 

multiply-layered intrusive, mainly gabbro but with anorthositic phases, and a granophyre phase at the top. 

Smaller mafic bodies south of Lake Superior are at 

about the same stratigraphic level, the largest being the 

Mellen Gabbro of the Gogebic district (fig. 6). The intrusives are deep-seated manifestations of the same 
28 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

HKeweenawan igneous activity" that produced the 

lavas. 

Radiometric dates of the Keweenawan rocks have 

been obtained from the felsic differentiates of the mafic 

intrusives and lavas. Felsites from the North Shore and 

Portage Lake Volcanics, the granophyric facies of the 

Duluth Complex and Mellen Gabbro, as well as other 

igneous rocks, have all yielded ages between 1,120 and 

1,140 m.y. by uranium-lead determinations on cogenetic zircons, suggesting a narrow pulse of magmatic activity (Silver and Green, 1963, 1972). Dates by 

potassium-argon and rubidium-strontium methods 

have a greater span, between 1,100 and 1,300 m.y. 

(Goldich and others, 1961, p. 95; Goldich, in Sims and 

Morey, 1972, p. 35--36), but are less reliable. An age of 

1,075 m.y. has been proposed for the Nonesuch Shale of 

the succeeding Oronto Group (Craddock, in Sims and 

Morey, 1972, p. 185). 

The Lake Superior syncline and its Keweenawan 

rocks are merely an exposed segment of a much larger 

tectonic feature (fig. 8). Prominent gravity and magnetic anomalies demonstrate that the trough and its associated mafic igneous rocks extend another 600 mi 

(960 km) southwestward beneath the Paleozoic cover 

into northeastern Kansas (E. R. King and Zietz, 1971), 

and somewhat vaguer geophysical data suggest that the 

trough turns southeastward near the end of Lake 

Superior, to extend for an unknown distance beneath 

the southern peninsula of Michigan (Oray and others, 

1973). The whole structure thus has a curiously arcuate 

form, concave toward the south-a product of crustal 

rifting of subcontinental dimensions late in Precambrian time. 

PRECAMBRIAN Y ROCKS OLDER THAN KEWEENAWAN 

Southwest of the Keweenawan area is the Sioux 

Quartzite of southwestern Minnesota and southeastern 

South Dakota which forms a plateaulike terrain 200 mi 

(320 km) long, partly concealed by glacial drift; the 

similar Barron Quartzite forms a smaller area on the 

west flank of the Wisconsin arch. The Sioux is warped 

gently into an axial trough and has a thickness of about 

3,000 ft (900 m); the Barron is much thinner. Interbedded in the Sioux are layers oC~pipestone" (argillite) 

which have yielded potassium-argon date of 1,200 m.y. 

(Goldich and others, 1961, p. 49), probably related to the 

mild deformation. A well in northwestern Iowa penetrated Sioux Quartzite interbedded with rhyolite 

layers, the latter yielding an apparent rubidiumstrontium age of 1,4 70 m.y. (Austin, in Sims and Morey, 

1972, p. 450); regardless of whether the rhyolite is intrusive or extrusive, this date suggests a minimum age 

for the formation. 

The Sioux and Barron Quartzites are approximately 

correlative with the Sibley Group north of the Lake 

Superior syncline in Ontario, which lies stratigraphically between the Animikie and the Keweenawan. All 

three units formed after the Penokean orogeny, but 

before the accumulation of the Keweenawan Supergroup, during early Precambrian Y. 

Farther south, in the Baraboo area of central Wisconsin, Precambrian rocks project through the surrounding 

lower Paleozoic strata in a partly exhumed monadnock, 

and have long been classic for student work because of 

their proximity to many Middlewestern universities. 

(My own first field experience was at Baraboo during a 

summer field course of the State University of Iowa.) 

The rocks of the sequence at Baraboo much resemble 

those of the lower part of the Marquette Range Supergroup (Precambrian X) in the Lake Superior Region to 

the north-a thick lower quartzite, followed by slate, 

iron formation, and dolomite-and were called ~~Huronian" in the older reports. Nevertheless, these rocks overlie felsic volcanics with rubidium-strontium age of 

about 1,600 m.y. (Dalziel and Dott, 1970, p. 8--10), so 

that they are younger than those with which they have 

been compared. Evidently they formed during the early 

part of Precambrian Y, like the Sioux and Barron 

Quartzites. 

PRECAMBRIAN Z 

Near the axis of the Lake Superior syncline, between 

the main body of the Keweenawan and the overlapping 

Upper Cambrian, is another body of sandstones, known 

as the Jacobsville in Michigan, the Bayfield in Wisconsin, and the Fon du Lac and Hinckley in Minnesota. At 

one time or another, geologists have assigned these 

sandstones to the Keweenawan or to the Cambrian, but 

they are unconformable with both and are probably part 

of neither. 

Observed sequences of the sandstones are as much as 

5,000 ft (1,500 m) thick, and geophysical surveys 

suggest that they may be 7,000 ft (2,100 m) thick on the 

southeast side of the Keweenaw Peninsula. They are 

red sediments like the underlying Keweenawan, but 

they are more cleanly washed, being quartzites rather 

than arkoses, with a less varied heavy mineral assemblage. Their sediment transport was again toward 

the axis of the Lake Superior syncline (whereas that of 

the Cambrian is mainly southward) (Hamblin, 1961, 

p. 6--13), so that subsidence of the trough continued, but 

dips of the sandstones are much lower than those of the 

Keweenawan. 

These sandstones are probably the representatives of 

Precambrian Z in the Lake Superior Region, and they 

are so indicated on the Geologic Map, although definite 

radiometric proof is not available. 
NORTHERN APPALACHIAN REGION 29 

ADIRONDACK AREA 

The Adirondack area of Precambrian rocks of northern New York State is a domical uplift 120 mi (195 km) 

across, nearly encircled by Paleozoic rocks, but connected northwestward along the Frontenac axis with 

the Grenville province of the Canadian Shield, of which 

it is an extension. The Precambrian area includes two 

contrasting parts: a northwestern lowland 40 mi 

(65 km) broad, dominantly of medium-grade 

metasedimentary rocks, and the Adirondack Mountains to the southeast of high-grade gneisses and extensive plutonic rocks; the two parts are juxtaposed along 

the Highland Boundary fault, downthrown toward the 

lowlands. 

The Grenville Group of the lowlands (Y) is a metasedimentary sequence about 15,000 ft (4,500 m) thick 

(Engel and Engel, 1954, p. 1018), more than half of 

which is calcite or dolomite marble, and the remainder 

quartz-feldspar gneiss and minor quartzite. The rocks 

have been plastically folded and refolded, and metamorphosed to amphibolite grade (with sillimanite). 

They contain many concordant lenses and pods of 

hornblende granite (Y g2), now with phacolithic structure but probably intruded' before or during the folding 

(Buddington, 1939, p. 152--158). 

The rocks of the mountains are a complex of paragneiss (Ym), orthogneiss (Ygn), syenite (Ys), and anorthosite (Ya), metamorphosed to granulite facies, probably at a deep level in the crust. The most prominent 

component is the anorthosite, covering 14 percent of the 

area and forming mountainous massifs, the largest of 

which is the Mount Marcy body 50 mi (80 km) across. 

The syenite (mangerite), in smaller areas, may be 

genetically related. The orthogneisses include both 

granitic and charnockitic varieties. The paragneisses 

were the host rocks of the others and have been correlated with the Grenville Group to the northwest, although they contain less marble. 

The origin and sequence of the plutonic rocks has long 

been debated, and many views have been expressed. 

Buddington (1939, p. 197-235) believed that they were 

introduced as magmas, the anorthosite, syenite, and 

charnockite successively before the deformation, the 

granite during the major orogeny and metamorphism. 

At the opposite end of the spectrum is a proposal that all 

the plutonic components were remobilized from a deeper level, or basement, the mobilities ranging from slight 

in the anorthosite to a maximum in the granite; the 

more mobile the component the more transgressive the 

rock, hence the younger its apparent age (Walton and de 

Waard, 1963). 

The Adirondack Precambrian rocks, like those of the 

rest of the Grenville province, yield characteristic 

Grenvillian radiometric dates of 1,000--1,200 m.y., and 

large parts of them in the province have been called 

Grenville Series in a broad sense. 7 The Geologic Map 

follows present Canadian usage (Emslie, 1970, p. 124--

125) in restricting the Grenville Group to the recognizable metasedimentary rocks of the original Grenville 

area in southern Quebec and Ontario and the adjacent 

lowland of New York State. Igneous and metamorphic 

events in the lowlands have ages of 1,160--1,200 m.y. 

(Silver, 1963); structural evidence in Canada suggests 

that the group itself is Paleohelikian (=early Precambrian Y) (Emslie, 1970, p. 125). The other Precambrian 

rocks of the Adirondack area are likewise classed on the 

map as Precambrian Y, but not as Grenville. 

The Adirondack anorthosite contains zircons which 

have been dated between 1,020 and 1,100 m.y. by 

uranium-lead methods; similar ages, but none older, 

have been found in the associated orthogneisses and 

pegmatites (Silver, 1968, p. 250). These dates record the 

time of granulite metamorphism, but it is claimed from 

the characteristics of the zircons that this is the age of 

the magmatic crystallization as well. 

Nevertheless, the Adirondack anorthosites are part 

of a chain of massifs that extends 1,000 mi (1,600 km) 

north-northeastward, diagonally across the Grenville 

province, into the N ain province of eastern Labrador. 

Those of the latter province, outside the region of Grenvillian influence, yield Elsonian ages of about 1,400 

m.y., and it has been suggested that the other massifs, 

many with apparently younger dates, have been reworked during the Grenvillian event (Stockwell, 1964, 

p. 3). Be that as it may, emplacement of the anorthosite 

massifs appears to have been a unique event in earth 

history; all known massifs, both in eastern North 

America and elsewhere, are datable within a span of a 

few hundred million years of middle Precambrian time, 

as we shall see when considering the anorthosite bodies 

farther west in the United States (p.50, 66). All are 

shown on the Geologic Map as Y a. 

NORTHERN APPALACHIAN REGION 

PRECAMBRIAN Y OF WESTERN PART 

To the southeast and south of the Adirondack area 

in the western part of the Northern Appalachians: 

Precambrian rocks with Grenvillian ages emerge in 

the higher uplifts, where they have been reworked 

during the various Paleozoic orogenies. They form 

7The name "Grenville" has been extended from the original rocks ofthe Grenville Group to 

include a more comprehensive Grenville Series, a province and its northwestern tectonic 

front, and an orogeny. These extensions have been condemned by Gilluly (1966, p. 104--1081, 

but in the absence of any acceptable substitute, common usage must prevail-provided care is 

taken by the geological author to specify clearly which of the several "Grenvilles" he is 

referring to. 
30 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

the basement of the Green Mountains of Vermont, 

the Berkshire Hills of Massachusetts, the Hudson 

Highlands of New York State, and the Reading 

Prong of New Jersey and Pennsylvania. The uplifts 

are links in a chain that extends from the Long 

Range in Newfoundland to the south end of the Blue 

Ridge in Georgia. The basement is overlain by Lower 

----)~.,.. _ _j \ .' ·-' 

' - \ ,,'' -~ ............ 

Cambrian and younger Paleozoic geosynclinal 

rocks-basal miogeosynclinal quartzites. on the west 

(£q, Cheshire and Poughquag), and more varied 

eugeosynclinal clastics and volcanics on the east 

(£e). 

The uplifts are vergent westward or northwestward, and become increasingly allochthonous southOLDER PRECAMBRIAN 

I , \r 

NORTH :\( MINNESOTA ''_.-,---:"":-;;~~~~ DAKOTA '-_'i: 

/I OLDER PRECAMBRIAN 

~){ 

'~ WISCONSIN ~:,1 

..... _., ,_., .. -, r- I,,.,. ,,..) ,'' .. ' .. 

/,/.~_ .... ,_) :,_, 

,-

~----- 1 

' ) ,. ... 

) 

·. --~ --------,( ILLINOIS 

,.. 

I ' \ MISSOURI 

0 

I 

I 

\ 

I' 

I 

0 400 KILOMETRES 

, :::::: 

I 

1.:• :MICHIGAN 

1: 

\ 

I \ ......... .. . 

\: ••• : :•:[ ~~~~j~!:~1!T : \ : :::::::::::: :::::::: o/ 

--~~~1 ': \ 

INDIANA 

__ ;-,~--~.- KENTUCKY 

400MILES 

FIGURE 8.-Map of north-central United States, showing the arcuate pattern of surface and subsurface upper Precambrian rocks (Y and 

Z). (Based mainly on Craddock, in Sims and Morey, 1972, p. 283.) 
NORTHERN APPALACHIAN REGION 31 

ward. The Green Mountains and Berkshire Hills are 

anticlinoria, the first with a steep west flank, the 

second overthrust westward. The Hudson Highlands 

and Reading Prong have commonly been interpreted 

as fault-bounded horsts, but modern work indicates 

that the Precambrian of the Reading Prong, at least, 

is part of a floored nappe with roots farther southeast 

(Drake, 1970, p. 286-289). The smaller Precambrian 

bodies east and southeast of the main chain of uplifts 

are even more complexly involved in the Appalachian 

deformations. Those in the cores of the Chester 

and Athens domes in the Connecticut Valley of 

southeastern Vermont have risen diapirically into a 

thick pile of eugeosynclinal strata. Those south of the 

Hudson Highlands (Fordham and Yonkers Gneisses) 

have been plasticly folded and refolded with the lower 

Paleozoic rocks of the New York City Group (Hall, 1968, 

p. 124). 

The Precambrian rocks are dominantly paragneisses, 

EXPLANATION 

Probable Precambrian Y and z 

rocks in southern Michiga·n 

subsurface 

PRECAMBRIAN Z 

0 . 

. 

Bayfield Group and related 

sandstone units 

PRECAMBRIAN Y 

Oronto Group 

Middle Keweenawan 

mafic intrusives 

Middle Keweenawan 

basaltic lavas 

Sioux Quartzite and 

related units 

PRECAMBRIAN X and W 

Older Precambrian 

metamorphic rocks 

Edge of Phanerozoic 

rocks 

FIGURE B.-Continued. 

with interbedded quartzite and marble units and 

minor intrusive orthogneisses. Many details of the 

subdivision and pattern of the gneisses are shown on 

the modern State Maps on a scale of 1:250,000, but 

this is impractical on the much smaller scale of the 

Geologic Map of the United States, where they are 

indicated merely as paragneiss (Y m). The pattern of 

the units in the Green Mountains uplift, as shown on 

the Vermont Map (fig. 9), is discordant to its elongation and crosses it nearly at right angles, although 

somewhat curved as a result of the Paleozoic uplift. 

The rocks underwent a Precambrian metamorphism 

to high amphibolite grade in the Green Mountains 

and granulite grade in the Reading Prong, but they 

were metamorphosed again and retrograded during 

the Appalachian orogenies. 

As would be expected, the radiometric data reflect 

this complex metamorphic history. Relict Grenvillian 

dates of 900--1,100 m.y. have been obtained from the 

Green Mountains and Hudson Highlands by 

uranium-lead and related methods (Tilton and 

others, 1960, p. 4175; Faul and others, 1963, p. 3, 7). 

Determinations by rubidium-strontium and 

potassium-argon methods on rocks in the uplifts and 

southeastward yield mainly ages of about 360 m.y. 

that express the time of Paleozoic metamorphism, 

but there is a scatter of intermediate dates that express either genuine events, or a resetting of original 

Grenvillian ages by the later metamorphism (Long 

and Kulp, 1962, p. 984-987). 

PRECAMBRIAN Z OF EASTERN PART 

None of the Precambrian Y basement is found east 

of the Connecticut Valley, but younger Precambrian 

is mapped in widely separated areas in eastern New 

England. In western Maine the oldest rocks of the 

Boundary Mountains anticlinorium form the Chain 

Lakes massif, and are largely highly metamorphosed 

paragneiss, quartzite, and amphibolite. They are certainly pre-Ordovician and might be Cambrian, but a 

Precambrian? age has been suggested for them 

(Boone and others, 1970, p. 11); on the Geologic Map 

they are indicated as Z with a metamorphic overprint. Farther southeast in Maine, near Islesboro on 

an island in Penobscot Bay, metamorphic rocks in a 

small horst have yielded a 900 m.y. date by 

rubidium-strontium methods and are cut by 600 

m.y.-old pegmatites (Stewart, 197 4, p. 89-90); they are 

likewise mapped as Precambrian Z. 

In Rhode Island and southeastern Massachusetts, 

adjoining the Pennsylvanian Narragansett basin, is a 

much larger area of late Precambrian rocks. It includes on the east the Dedham Granodiorite and on 
32 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

0 20 Ml LES 

0 20 Kl LOMETRES 

EXPLANATION 

ITlJE8] 

Undifferentiated 

Paleozoic 

Undifferentiated ~ 

Cambrian 

P:L~~}d 

Cheshire 

Quartzite 

Cavendish 

~ 

Formation 

Granite 

Quartzite 

~ 

and marble 

Paragneiss 

FIGURE 9.-Map showing part of the Green Mountain uplift in south-central Vermont, and the Athens and Chester domes east 

of it, showing superposition of north-south Paleozoic ( = Appalachian) trends on east-west Precambrian trends (mainly 

Grenvillian). Generalized from Geologic Map of Vermont (1961). 

the west various granitic orthogneisses (Milford, 

Northbridge, Scituate, etc.). A key locality for 

stratigraphic relations is Hoppin Hill, Mass., near 

the northeastern corner of Rhode Island, where fossiliferous Lower Cambrian strata lie on the eroded 

surface of granodiorite (Dowse, 1950); however, the 

old rocks of the hill are separated from the rest by 

Pennsylvanian cover. Radiometric determinations by 

the rubidium-strontium method on the Dedham: 

Granodiorite and Northbridge Gneiss yield ages of 

591 and 569 m.y. respectively; the granodiorite at Hoppin Hill yields an age of514 m.y. but this may have been 

downgraded during the pre-Paleozoic weathering. The 

true age of all the granitic rocks in the area may be near 
CENTRAL AND SOUTHERN APPALACHIAN REGION 33 

570 m.y. (Fairbairn and others, 1967, p. 324); they are 

represented on the Geologic Map as Zg. 

Large enclaves in the orthogneisses west of the N arragansett basin are an earlier supracrustal sequence, 

the Blackstone Series which is 15,000 ft (4,500 m) or 

more of schist, quartzite, and greenstone (Quinn, 1971, 

p. 8-14); like the plutonic rocks, the supracrustal rocks 

are included in Precambrian Z. 

THE AVALONIAN BELT 

The Precambrian rocks of southeastern New England are an extension of those of the Avalonian belt 

( = Avalon platform) of the Appalachian province in 

Canada-a domain of late Precambrian (Z) supracrustal and magmatic rocks and events different 

from those farther northwest-typified in the A val on 

Peninsula of southeastern Newfoundland, but represented also in Cape Breton Island and southeastern 

New Brunswick (Poole and others, 1970, p. 231-235; 

Rodgers, 1972, p. 512-514). In Newfoundland the belt 

includes basal volcanics intruded by the Holyrood 

Granite, followed by a thick sequence of clastic deposits, the whole overlain unconformably by the 

Lower Cambrian; the granite has been dated at 575 

m.y. (later recalculated at 610 m.y.). An ((Avalonian 

orogeny" has been postulated between the granite 

and volcanics and the succeeding clastic deposits 

(Poole and others, 1970, p. 232-233), but relations 

have been plausibly reinterpreted as a product of 

volcanic and depositional events, punctuated by local 

disturbances, that do not express an ((orogeny" in the 

usual sense (Hughes, 1970; Hughes and Bruckner, 

1971). 

Nevertheless, the term ((Avalonian" is appealing 

and is widely used, in the same manner as the term 

((Grenvillian" discussed earlier (footnote 7). It can 

appropriately be applied to a terrane of wellcharacterized rocks and structures of late Precambrian and early Paleozoic age in eastern Canada and 

the United States, whether or not this involves a 

narrowly defined ((Avalonian orogeny." In the northwestern part of the Appalachian region, Lower 

Cambrian strata with an Olenellus fauna lie on a 

1,100-m.y.-old Grenvillian basement. By contrast, in 

the Avalonian belt to the southeast, Lower Cambrian 

strata with a Paradoxides fauna lie on a 600-m.y.-old 

Avalonian basement (Wilson, 1969, p. 282). The 

Cambrian of the Avalonian belt is much more akin 

to the Cambrian of the southern British Isles and 

western Europe than to the Cambrian of the remainder of North America (Palmer, 1967, p. 143--144), 

suggesting that the belt may be an extension of its 

trans-Atlantic counterparts which was joined to 

North America by plate collision during Paleozoic 

time. 

South of New England the Avalonian belt seemingly extends into the metamorphic rocks of the 

Piedmont province and their buried extensions beneath the Atlantic Coastal Plain (p. 39). 

CENTRAL AND SOUTHERN APPALACHIAN REGIONs 

In the Central and Southern Appalachians, the 

principal occurrence of identifiable Precambrian 

rocks is in the Blue Ridge province, a mountainous 

belt that lies between the Valley and Ridge province 

and the Piedmont province from southern Pennsylvania to northern Georgia. No Precambrian rocks are 

exposed in the Valley and Ridge province, but dated 

Precambrian emerges in some of the higher uplifts of 

the Piedmont province, and Precambrian is probably 

also included in the undeciphered metamorphic complex (m) of the inner Piedmont and the less-metamorphosed strata of the Carolina Slate Belt. 

Compared to the Canadian Shield, all the Precambrian of the Central and Southern Appalachians 

is rather young. Even its crystalline basement yields 

dates no earlier than Grenvillian, and is accordingly 

classed as Precambrian Y. The great body of supracrustal rocks above it is therefore Precambrian Z and 

is, in fact, the gTeatest development of this div,ision 

in the United States, even exceeding that in the 

western Cordillera (fig. 5). 

In the Central and Southern Appalachians, as in 

the Northern, the Precambrian is heavily involved in 

the Paleozoic orogenies. The basement, which 

underwent deformation during the Grenvillian event, 

was reworked and its metamorphic fabric retrograded. By contrast, the Precambrian supracrustal 

rocks were not significantly deformed during Precambrian time, and owe all their present structural 

and metamorphic complexities to deformations during the Paleozoic. 

BLUE RIDGE BELT 

The northern segment of the Blue Ridge is an 

anticlinorium, vergent westward, about 15 mi (25 

km) broad near the Potomac River, but widening 

southward. In central Virginia low-angle thrusts appear along its northwestern border, and the belt becomes increasingly allochthonous. The extent of 

transport on the thrusts in the Tennessee-North 

Carolina segment is suggested by windows southeast 

of their leading edges, notably the Grandfather 

Mountain window on the southeastern side of the 

belt. From North Carolina to the edge of the Coastal 

Plain in Alabama the southeastern tectonic boundary 

of the Blue Ridge belt is the Brevard zone of highangle faults. 

The Precambrian of the Blue Ridge is overlain by 

Paleozoic geosynclinal deposits, the belt marking the 

8For details available through 1966, see King (1970, p. 17-54); the present account includes later observations and revisions 
34 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

approximate boundary between their miogeosynclinal 

and eugeosynclinal parts. Along the northwestern 

flank the basal miogeosynclinal deposits are the 

Lower Cambrian quartzites and clastics of the 

Chilhowee Group (£q), whose mature sediments contrast with the immature sediments of the Precambrian supracrustal sequence (Z). In places the 

Chilhowee transgresses across them onto the basement (Y), perhaps because this flank of the Blue 

Ridge was near the original northwestern limit of 

the Precambrian supracrustal rocks. The contrast 

fades on the southeastern flank of the Blue Ridge, 

where the Precambrian supracrustal rocks and the 

Paleozoic eugeosynclinal rocks (£e) are more alike 

and more accordant (fig. 10). 

PRECAMBRIAN Y 

In the Maryland-Virginia segment of the Blue 

Ridge the basement of the anticlinorium is a plutonic 

complex (Y gn) of granodioritic and granitic orthogneisses and migmatites, with one small body of anorthosite (Ya). Traces of earlier host rocks of undetermined age occur in places, nearly destroyed by 

granitization. All the complex is hypersthene-bearing 

and charnockitic, and underwent metamorphism to 

granulite grade during the Grenvillian event. The 

plutonic basement extends southwestward into the 

Tennessee-North Carolina segment to form the 

Cranberry, Max Patch, and similar gneisses. 

Radiometric determinations on rocks of the complex in northern Virginia yield dates by uraniumlead and related methods of 1,070-1,150 m.y., and by 

rubidium-strontium and potassium-argon methods of 

880 and 800 m.y., respectively; similar results have 

been obtained in the Tennessee-North Carolina segment (Tilton and others, 1960, p. 4175--4176). In the 

latter segment rubidium-strontium whole-rock determinations on many of the basement units yield 

ages between 1,025 and 1,250 m.y. (Fullagar and 

Odom, 1973, p. 3076-3077); it is suggested that the 

basement is a 1,200-1,300-m.y.-old crust that was 

remobilized during the Grenvillian event 1,050 m.y. 

ago, without the addition of new material. 

Distinctions between the basement and its cover 

become blurred farther southwest, in the border region of North Carolina and Georgia (fig. 7). The 

rocks have passed into the high amphibolite (sillimanite) phase of Paleozoic metamorphism (Hadley 

and Nelson, 1971), and are thrown into large-scale 

recumbent folds or nappes (Hatcher, 1971, p. 41-42; 

1973, p. 683). On the Geologic Map, considerable 

areas are shown as Precambrian Y (paragneiss and 

schist, Ym), based on the best information available 

in 1971, but this will require revision on the basis of 

later work, in part still in progress (for example, 

Hatcher, 1973.) Much of the Ym unit on the map is 

biotite gneiss and interbedded amphibolite, which is 

probably a lower unit of Precambrian Z. True basement is probably represented by the Whiteside Granite and related rocks, but even these are in the cores of 

nappes, and rootless in part. 

On the northwest edge of the Blue Ridge belt near 

Cartersville, Georgia, the Corbin Granite (gneiss) 

has sometimes been interpreted as a Paleozoic intrusive, but uranium-lead determinations on zircons 

show that it has an age of 1,100 m.y. (Odom and 

others, 1973); it and probably the adjacent Salem 

Church Granite are therefore basement to the surrounding Precambrian Z Ocoee Supergroup. Farther 

southwest, near the edge of the Coastal Plain in 

Alabama is the Kowaliga Gneiss ( = ((biotite augen 

gneiss" of the Alabama State Map of 1926), which 

was proposed as basement rock (Bentley and Neathery, 1970, p. 19-20) and is so represented on the 

Geologic Map; however, radiometric determinations 

yield ages no greater than 550 m.y., so this assignment is suspect. 

PRECAMBRIAN Z 

The supracrustal rocks of the Blue Ridge belt lie 

unconformably on the deeply eroded surface of the 

basement-the greatest structural break in all the 

Appalachian stratified sequence below the base of the 

Triassic. Although this relation is fundamental to 

the Precambrian geology of the region, it was curiously misapprehended for a long period; in the 

northern Blue Ridge the plutonic rocks were thought 

to intrude the supracrustal rocks, and were so represented on the Geologic Map of the United States of 

1932. It was not until much later that Jonas and 

Stose (1939) deduced the true relation, a deduction 

abundantly confirmed by subsequent investigations. 

The supracrustal rocks are an extensive and varied 

suite, broadly of the same age, although not all their 

mutual relations have been determined with certainty. Volcanic rocks are common in the northwest 

and north, but most of the remainder are immature 

clastic sedimentary rocks. 

In Maryland and northern Virginia, the dominant 

supracrustal unit on the northwesterl} flank of the 

Blue Ridge is the Catoctin Greenstone (Zv), a body of 

mafic lava as much as 5,000 ft (1,500 m) thick, originally basaltic, into which felsic lavas interfinger 

northward. The mafic lavas were spread out in flows 

several hundred feet thick, under terrestrial conditions; many of them are amygdaloidal and some of 

them contain well-preserved columnar jointing (Reed, 

1969, p. 21-32). Between the lavas and the eroded 

surface of the basement is commonly a thin sedimentary layer (Swift Run Formation) (fig. 10). 
CENTRAL AND SOUTHERN APPALACHIAN REGION 

. . . .. . . . . · . . . 

38 0

:•{: ;: Staunt? if •• i ···. . .. .. .... . ·.·:: :: .. 

0 

0 

·. ·. :· . 0 : - . · . ... . • .. 0 

MESOZOIC AND CENOZOIC 

~l?ifJ [\·::~ j Cretaceous and Tertiary Triassic 

Coastal Plain deposits Newark Group and mafic 

intrusives 

L:::_j 

~ 

Sedimentary rocks 

In Valley and Ridge 

province 

PALEOZOIC 

Basal ~ Cam brian 

clastic deposits 

Chilhowee group 

PRECAMBRIAN Z 

Metamorphic and 

plutonic rocks 

In Piedmont province 

Catoctin Greenstone Swift Run 

Metarhyolite included Formation 

to north 

Lynchburg 

Formation 

Old Rag 

Granite 

PRECAMBRIAN Y 

~ Granitic 

orthogneiss 

100 Kl LOMETRES 

Roseland 

Anorthosite 

100 MILES 

FIGURE 10.-Map of northern part of Blue Ridge uplift in Virginia, Maryland, and Pennsylvania, showing relations of Precambrian 

Z rocks to underlying Precambrian Y basement, and to adjacent Phanerozoic rocks. Compiled from state geologic maps, and 

other sources. 

35 
36 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

Across the anticlinorium to the southeast, the lower 

sedimentary unit expands into the Lynchburg Formation (Z), a mass of medium- to coarse-grained turbidites at least 10,000 ft (3,000 m) thick, with lenses 

of bouldery conglomerate in the lower part derived 

from the plutonic basement (Rockfish Member). The 

Catoctin lavas thin out above the Lynchburg, but are 

the principal marker for separating it from the similar and apparently conformable Cambrian eugeosynclinal deposits (£e, Evington Group) (fig. 11). Relations on the southeastern flank of the anticlinorium 

are obscured by the higher grade of Paleozoic 

metamorphism (amphibolite grade) and many of the 

rocks are schistose or gneissic. 

Farther southwest in the Blue Ridge, near the 

Virginia-North Carolina boundary, volcanic rocks are 

again prominent on the northwestern flank (fig. 12). 

Thick bodies of rhyolite form the middle part of the 

Mount Rogers Formation, and both rhyolite and 

basalt occur in the Grandfather Mountain Formation 

(in the Grandfather Mountain window, hence also of 

northwestern facies). Congeneric with the volcanics 

is the Crossnore Plutonic Group, which includes 

many moderate-sized granitic plutons (Zg) that are 

embedded in the adjacent basement rocks. These 

southwestern volcanics are associated with greater 

volumes of clastic sediments than are those of the 

Catoctin. The upper sedimentary unit of the Mount 

Rogers Formation includes red, rhythmically bedded 

siltstone, and coarse diamictite formed of boulders of 

the basement plutonics, very much like the late Precambrian diamictites in several parts of the Cordillera to which have been ascribed a glacial origin 

(Rankin, 1970, p. 232). 

On the southeastern flank of the Blue Ridge, the 

broad band of metasedimentary supracrustal rocks 

continues from the Lynchburg area through southern 

Virginia into North Carolina. In the latter segment 

it is the Ashe Formation (Rankin, 1970, p. 232-233), 

a thick mass of fine- to medium-grained biotitemuscovite gneiss; with interbedded amphibolite (Zv), 

especially in the lower part, that originated from 

mafic volcanic rocks. In the Spruce Pine pegmatite 

district, North Carolina, west of the Grandfather 

Mountain window, the Ashe lies in a southwestplunging synclinorium, in the keel of which, on the 

heights of Mount Mitchell, it is overlain by rocks 

lithically like the Great Smoky Group of the Ocoee 

Supergroup (Hadley, 1970, p. 249). 

The Ocoee Supergroup (Z) dominates the southwestern segment of the Blue Ridge belt, extending 

along its strike for more than 175 mi (280 km), from 

Asheville, North Carolina, to Cartersville, Georgia, 

and across it for 40 mi (65 km) or more; it projects in 

high ranges, such as the Great Smoky Mountains 

(King and others, 1968, p. 3-9) (fig. 13). The-Ocoee is 

a great mass of nonvolcanic clastic sedimentary 

rocks; partial sequences as much as 25,000 ft (7 ,600 

m) thick have been observed but the total is undetermined. Along its southeastern side the Ocoee lies 

unconformably on the- basement orthogneisses and 

paragneisses (Y gn, Y m); on its northwestern side it 

is succeeded disconformably by the Chilhowee Group 

(£q); also, a belt of synclinally infolded younger 

rocks near its center (12>) includes the Murphy Marble that contains sparse lower Paleozoic fossils 

(McLaughlin and Hathaway, 1973). 

The Ocoee has been divided into the contrasting 

Walden Creek, Snowbird, and Great Smoky Groups 

(not differentiated on the Geologic Map but shown in 

fig. 13), which evidently formed in different parts of 

the original sedimentary basin, but they have been 

so telescoped by Paleozoic thrusting that most of 

their original relations to each other are now lost. In 

places one of the groups can be found in sequence 

with another, but it is likely that all of them were 

extensively intergradational. The varied clastics of 

the Walden Creek probably formed on an unstable 

shelf along the northwestern margin of the basin; the 

Snowbird is an intermediate facies; and the Great 

Smoky is a deepwater continental rise deposit. Much 

of the Great Smoky is a medium- to coarse-grained 

quartz-feldspar turbidite with prominent graded bedding, with which thin to thick units of dark sulfidic 

argillaceous rocks are interbedded. 

South of the main Ocoee area, along the southeastern edge of the Blue Ridge belt, strips of paraschist 

and paragneiss altered from rocks like the Great 

Smoky extend to the Coastal Plain border in 

Alabama, where -they are the Heard Group of 

Bentley and Neathery (1970, p. 14--18). Associated 

with them, and mainly underlying them, are biotite 

gneisses and interbedded amphibolites, sometimes 

called Precambrian Y basement (p. 31), but more 

likely comparable to the volcanic rocks in the lower 

part of the Ashe Formation farther northeast. 

A"Q. exceptional feature in northeastern Georgia is 

-the Tallulah Falls dome, exposing a quartzite formed 

of nearly pure siliceous sand; following a suggestion 

of Burchfiel and Livingston (1967, p. 252) we have 

speculatively correlated it on the Geologic Map with 

the Lower Cambrian Chilhowee Group (£q). However, Hatcher (1973, p. 683) interprets the dome as a 

culmination in a much broader recumbent nappe, 

and considers the quartzite to be a unit near the 

middle of the Precambrian Z sequence. 

Although the general position of the Precambrian · 

Z supracrustal rocks of the Central and Southern 
NORTHWEST 

Rockfish 

Conglomerate 

Member 

SOUTHEAST 

,_ .. _ .. _ .. _ .. _ .. _··::. ::::· _ .. _ .. _ .. _ .. ···:: _ :_ .. _ .. _ ... 

••• •: • :EYIN~i<:JN •. $F.!()l1r, •: 

·· ··• ··•~im ••·• ••·••·· 

FIGURE 11.-Stratigraphic diagram extending transversely across the Blue Ridge uplift in northern Virginia, showing relation of units of Precambrian Z and Cambrian. Letter symbols are the same as those in figure 10. Based on Bloomer and Werner (1955, p. 588), but with many modifications. 

0 

trj 

z 

~ 

::0 

~ 

t""< 

~ 

z 

d 

ifJ. 

0 

c: 

~ 

~ 

tr:l 

~ ~ 

'1:1 

'1:1 

~ 

t""< 

~ 

0 

~ 

:; 

z 

::0 

trj 

Q 

0 

z 

UJ 

-:] 
38 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

~cp..l}f:::: .: ~ :•••••••••••••••••••••••••••••••••::::::::: . ......-·u.-_..._..- ::::::::: ::::::::::::::::: ::: :::::: :: :·· .: ·. . ...................... ························· .......

· ·· ·····. ··· . 

'· ········ ············· ....... . . 

. . . .· .: . : : : : ~ : ~ : : : : : : : : : : . . . . : : : . : . : : : : : : : : : : : : : : : : : : : . . . . . . . . . . . . . . . . . . . . . . . . .. 

. . ::::::::::::::::::.:::::: .•••• . :. :::::::: ~: ~::: .: •• : ~z~ :::::::: . .. .... :::::: 

·x~~~~· ········· ··· ··· ·•·4.; ~~~~s·:··· ····· ··········· ······· ........... . .. . ······•••••· ····•••••••••••••••••••::.. ••:<::T6Wip st~p~s~t~iti ........... .:: :::· > :::::::::::: :::::=::•• 

j I • • ••• c~~~ll~A "" ;i; i ::: 

• • •: • • • • • •• • • ·: • • :::·:::: · · · · ······ · · · · · ·· · :::::: :: ::::: :::::: ::: · · · · .•• · • • · • • ::::::::::::::::::: :::::::::::::::::::::::: :: • ·· to ·cfM iLe:s: · · · · · · · ·· · · :::::::::::::: :· :: ·:: :: :: : 

•• 0 

· ······ ················ · ..... · ·· ······ ·········· · ·· ··· ······ · ···· · ··· ····················· ····· ··· · · ·· 

100 Kl LOMETRES 

EXPLANATION 

MESOZOIC 

r{: \/21 Triassic 

Newark Group 

PALEOZOIC 

1: =~== :1 

Lower and Middle 

Paleozoic granite 

Includes mafic intrusives 

near Martinsville, Va. 

Sedimentary 

~ 

~ rocks 

In Valley and Ridge 

province 

rrmmJ 

Basal Cambrian 

clastic deposits 

Chilhowee Group 

Metamorphic 

~ 

~ and 

plutonic rocks 

In Piedmont province 

1~\~:~ .. ~:(:;::j Evington Group 

(in north) and 

Alligator Back 

Formation (in 

south) 

PRECAMBRIAN Z 

-

Crossnore granitic Catoctin ~ Greenstone 

rocks In north 

Mount Rogers and 

Grandfather MounLynchburg Formation 

(in north) and Ashe 

tain Formations Formation (in 

south) 

PRECAMBRIAN Y 

~ 

~ 

Granitic 

orthogneiss 

............... ~~ 

Faults 

Thrusts, normal, and strike-slip 

FIGURE 12.-Map of part of Blue Ridge uplift in the border region of Virginia, North Carolina, and Tennessee, showing Precambrian 

Y and Z, and Paleozoic units. Compiled from Rankin, Espenshade, and Shaw (1973, p. 6, 8), Conley and Henika (1973, p. 41, pl. 4), 

and other sources. 
CENTRAL AND SOUTHERN APPALACHIAN REG ION 39 

Appalachians between their Precambrian Y (Grenvillian) basement and the Lower Cambrian is plain, the 

precise stratigraphic positions and ages of the different parts are as yet uncertain. The sediments of the 

Ocoee Supergroup, for example, contain detrital zircons with lead-alpha dates of 820--1,000 m.y. (Carroll 

and others, 1957, p. 18&-188), which express merely 

the age of the basement from which they were derived; other dates from the Ocoee are around 350 

m.y. and record the age of their Paleozoic 

metamorphism (Kulp and Eckelmann, 1961, p. 410--

413). Even fewer data are available for the other 

sedimentary components of Precambrian Z, such as 

the Lynchburg and Ashe Formations. 

The most specific information on the age of the supracrustal sequence has been obtained from 

uranium-lead determinations on zircons from the felsic volcanic rocks, which indicate an original age of 

820 m.y. and an episodic lead loss at 240 m.y. (the 

latter probably at the time of the Appalachian 

orogeny) (Rankin and others, 1969). Dated specimens 

were obtained from felsic volcanics associated with 

the Catoctin Greenstone in southern Pennsylvania, 

from the Mount Rogers Formation in Virginia, and 

from the Grandfather Mountain Formation in North 

Carolina. No determinations are possible by this 

method on the mafic volcanics, but by extrapolation 

the 820 m.y. age probably applies to the Catoctin 

Greenstone as well; as indicated earlier, the Catoctin 

overlies the sedimentary part of the supracrustal 

sequence (Swift Run and Lynchburg) in the northern 

Blue Ridge, and thus sets a terminal date for Precambrian Z in this segment. The similar ages from 

the Mount Rogers and Grandfather Mountain Formations farther southwest are less decisive, as their felsic volcanics lie farther down in the local sequences. 

PRECAMBRIAN OF PIEDMONT PROVINCE 

The extensive Piedmont province southeast of the 

Blue Ridge belt is a domain of crystalline rocks that 

were mobilized and subsequently consoldiated during 

the orogenies of Paleozoic time. In this respect it resembles the crystalline area of New England in the Northern Appalachians, but whereas the stratigraphic sequence in New England is now fairly well known, much 

of that in the Piedmont is still poorly understood. Representation of the Piedmont province on the Geologic 

Map was assembled from the best data available in 

1971, but investigations are actively in progress which 

will modify this representation in many places. 

Precambrian basement rocks, the Baltimore Gneiss 

(Y gn), form the cores of half a dozen mantled gneiss 

domes in eastern Maryland and adjacent Pennsylvania 

that have risen steeply into the supracrustal eugeosynclinal rocks of the Glenarm Series (£e). Radiometric 

determinations on zircons from the gneiss by uraniumlead and related methods yield ages of1,000-1,100 m.y., 

whereas biotite from the gneiss yields rubidiumstrontium and potassium-argon ages of 300--400 m.y., 

expressing the time of Paleozoic metamorphism (Tilton 

and others, 1958). 

At the northern edge of North Carolina, north of 

Winston-Salem (fig. 12), is the Sauratown anticlinorium, more than 50 mi (80 km) long and 15 mi (25 

km) broad, whose core exposes biotite gneiss and schist, 

and minor granitic gneiss, which are flanked by Precambrian Z Ashe Formation. The granitic rocks of the 

core have yielded an age of 1,192 m.y. by lead-lead 

determinations on zircons (Rankin and others, 1973, 

p. 19). 

The only proved basement rocks farther southwest in 

the Piedmont province are the Woodland Gneiss and 

Jeff Davis Granite near Warm Springs, western Georgia, which have yielded uranium-lead ages of1,000 m.y. 

(Odom and others, 1973; Sandrock and Penley, 1974). 

They lie beneath, but may intrude a metasedimentary 

sequence shown as Z and lH on the Geologic Map. All 

these are components of the Wacoochee belt which is 

bordered on both north and south by major faults, so 

that their relations to the adjacent Piedmont rocks on 

each side is undetermined. 

Most of the country rock of the Piedmont province 

(aside from the abundant plutons) is shown on the 

Geologic Map as unclassified metamorphic complex (m) 

and as Cambrian eugeosynclinal deposits (£e, £v). In 

North and South Carolina and adjacent States the 

eugeosynclinal deposits are the low-grade metamorphic 

sedimentary and volcanic rocks of the Carolina Slate 

Belt. The metamorphic complex, distinguished by its 

higher metamorphic grade, is partly equivalent, but 

may probably be partly older. The rocks of the Slate Belt 

are shown as Cambrian on the map mainly on the basis 

of the occurrence of Middle Cambrian Paradoxides in 

southern North Carolina, but the sequence evidently 

contains older components. Farther north, Lynn Glover 

III and his associates have found Ediacaran ( = Vendian) type fossils at a locality on the Little River 12 

miles north of Durham, N.C. They are the imprints of 

soft-bodied wormlike animals, preserved on bedding 

surfaces of the volcaniclastic strata. Comparable fossils 

occur in the Precambrian Z Conception Slate of the 

A val on Peninsula, southeastern Newfoundland. At the 

north end of the Slate Belt in southern Virginia the 

Slate Belt rocks, the adjacent gneisses, and the associated intrusives have yielded an array of dates by 

uranium-lead methods ranging from 575 to 620 m.y., 
40 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

suggesting an event of supracrustal accumulation, 

magmatic activity, and mild deformation near or a little 

before the beginning of the Cambrian ( = Virgilina deformation of Glover and Sinha, 1973, p. 247). 

These features have little resemblance to the late 

Precambrian-Early Cambrian features to the northwest in the Blue Ridge province, but the precise limits of 

the two terranes are still uncertain. In part of North 

Carolina they are juxtaposed along the Brevard zone, 

but in other places, as noted above, Grenvillian base-

. . . . . . · .. . . . . · . .. . . . . . . . 

ment and its supracrustal cover extends into the northwestern part of the Piedmont. Be that as it may, the late 

Precambrian-Early Cambrian rocks and events in the 

Piedmont most closely resemble those of the Avalonian 

belt farther northeast in the Appalachians (p. 33; Rodgers, 1972, p. 514--516). Like them, the Piedmont rocks 

may have formed in a realm far away from the northwestern belts of the Appalachians and were brought 

against them by plate .collision during Paleozoic time 

(Odom and Fullagar, 1973, p. 140-146) . 

··.. .···· ....··..······ ..... ·.··..·..... ····· 

. . . . . . . . . . . . . . . . . . . . . . . ........................ ... ... .... . .. : . : : : : : : : ~ ~ : : : ; : : : : : : ; : : : : : : : : : : : : : : : : : : : : ; . .. 

. . . . • • • • • .: •• :: •• :. :::~~~~~~~~~11~~~j~j~ ::::::~:~=: ::: .. ·· ··· ········· ········· 

· ····· · .: > ·~ m< :::::.: ::::::::::=: 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... ....... . . 

/][ I 1!\\lll!//1\il!j[iililliill 

. . . . . . . . . . . . . . . . . . . ....... ..... .. 

.. ............... .. 

/~il~ ~ l~ llrW~~H/jYT H : : : : : : : : : : : : : : : : : : ~ : .. 

...................... ...... .. ... . ....... .... ....... . .. ......... . : : : ; ; : : : : : ~ : : : .. 

·: 0 50 

.: ::·::::::::::·:: ::::::::::::::::·· 

: : : : : : : : : : : . : : . : : : : : : : : : : : : : : ~: ::::: : .. ······················· · ··· ···· ··· ·· ......... . ..... ················· 

100 Kl LOMETRES ............. ... :· ::::: :::::::: :::::: :;::: :::· ··:·::: ::::::::::::::::::::::::: ::::: : 

FIGURE 13.-Map of southwestern end of Blue Ridge belt in southern North Carolina and Tennessee, and northern Georgia, showing 

Ocoee Supergroup, its subdivisions, and related rocks of Precambrian Z, as well as their Precambrian Y basement. Compiled 

from State geologic maps, Hadley and Nelson (1971), Hurst (1973), Hatcher (1973), and other sources. 
SOUTH-CENTRAL UNITED STATES 41 

SOUTH-CENTRAL UNITED ST ATES9 

The wide Interior Region of the United States, between the Appalachian and Cordilleran orogenic belts, 

is a domain of little deformed Phanerozoic rocks a few 

hundred to many thousands of feet thick, through which 

their basement emerges only in small, widely separated 

areas. In the southern part of the region the principal 

basement exposures are in the Ozark uplift of Missouri, 

the Arbuckle and Wichita Mountain uplifts of southern 

Oklahoma, and the Llano uplift of central Texas. 

9For a summary of information available through 1966, see Flawn and Muehl berger (1970, 

p. 73-143); additional data on geochronology and subsurface relations are given by 

Muehlberger, Hedge, Denison, and Marvin (19661. 

EXPLANATION 

Sedimentary rocks 

In Valley and Ridge 

province 

Basal Cambrian 

clastic deposits 

Chilhowee Group 

PALEOZOIC 

~ji ii::~:Ei ijijl 

Cataclastic rocks 

In Brevard fault 

zone 

PRECAMBRIAN Z 

OCOEE SUPERGROUP 

Walden Creek 

Group 

Great Smoky 

Group 

Snowbird 

Group 

G 

Crossnore granitic 

rocks 

Ashe Formation 

PRECAMBRIAN Y 

I :~~ rn I 

Metamorphic and 

plutonic rocks 

In Piedmont province 

Alas kite 

In Spruce Pine 

district 

Mafic and 

ultramafic intrusives 

Muscovite gneiss 

and schist 

Biotite gneiss 

and amphibolite 

Granitic 

orthogneiss 

Max Patch and 

Cranberry Gneiss 

Layered gneiss 

and migmatite 

__ __L_ 

Sillimanite isograd 

FIGURE 13.-Continued. 

Elsewhere, especially in Kansas, Oklahoma, and Texas, 

drilling has penetrated the basement in many places, 

and affords information on the extent of the various 

units beyond their outcrops. Stratigraphic relations and 

radiometric dating indicate a Precambrian age for all 

the basement rocks, except those in the Wichita Mountain area, which are Early Cambrian (£g). 

OZARK AREA 

On the crest of the Ozark dome, in the St. Francois 

Mountains of southeastern Missouri, Precambrian 

rocks are exposed in an area of about 600 mi2 (1,550 

km2). At the beginning of the Paleozoic transgression 

they projected in a rough terrain with as rnuch as 2,000 

ft (600 m) of relief, which was buried and variously 

overlapped by Upper Cambrian and Lower Ordovician 

deposits; present outcrops result from partial exhumation of this terrain.lO 

The southwestern and larger part of the St. Francois 

Mountains is formed of stratified rhyolite and other 

felsic volcanic rocks (Yv), mainly flows but with interbedded tuff and breccia, dipping at low angles in various 

directions. The northeastern part of the mountains consists of several varieties of granite (Y g1). The granites 

intrude the volcanics, probably in thick sills at shallow 

depths in the crust, but both granites and volcanics are 

compositionally much alike, and both yield ages of 

about 1,500 m.y. by uranium-lead methods (Bickford, 

1972). They thus express a closely related set of events, 

the ~~st. Francois igneous activity" (Muehlberger and 

others, 1966, p. 5313). Rubidium-strontium ages are 

consistently lower, and may record a minor later event 

at about 1,300 m.y. 

On the western and southwestern edges of the Ozark 

uplift are some smaller outcrops of granitic rocks, also of 

nearly the same age. Those at Spaviflaw, Okla., are on 

the exhumed tops of hills of the Precambrian erosion 

surface, but those at Decaturville, Mo., and Rose, Kans. 

(the latter included inTi on Geologic Map) are rootless 

bodies brought to the surface by Phanerozoic disturbances. 

ARBUCKLE AND WICHITA MOUNTAINS 

The Arbuckle and Wichita Mountains of southern 

Oklahoma (fig. 15) are exposed parts of an intracratonic 

orogenic belt of Paleozoic age, composed of horstlike 

faulted uplifts, separated by deep troughs containing 

strongly deformed Paleozoic strata. 

The Arbuckle Mountains are underlain by Precambrian granitic basement, which emerges in a horst at 

10An alternative proposal is worth mentioning-that most of the Precambrian outcrops in 

the St. Francois Mountains are klippen of a former overthrust sheet that had been transported 230 mi (370 km) northward from the Ouachita orogenic belt in Arkansas (Wheeler, 

1965). 
42 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

the eastern end. Westward, the basement ofthe horst is 

overlapped by Upper Cambrian; eastward, it passes beneath Cretaceous Coastal Plain deposits but continues 

in subsurface 45 mi (72 km) farther, to the front of the 

Ouachita orogenic belt. The principal unit is the coarse, 

porphyritic Tishomingo Granite, but there is also a finer 

grained Troy Granite, as well as minor younger diorites 

and dike rocks. Both the Tishomingo and Troy have 

yielded ages in the range of 1,320-1,400 m.y. by 

rubidium-strontium and other methods (Ham and 

others, 1964, p. 12&-140). 

The basement of the Wichita Mountains (and their 

largely buried extension to the east-southeast) is different and younger. It is a varied assemblage of floored 

felsic and mafic plutons embedded in supracrustal volcanics and sediments, all with ages of about 525 m.y., 

hence early Cambrian (Ham and others, 1964, p. 35-

37); because of their small surface extent they are 

grouped on the Geologic Map as £g (Cambrian granitic 

rocks). Basement of Wichita type extends into the Timbered Hills uplift at the west end of the Arbuckle Mountains. 

LLANO UPLIFT 

In central Texas, south of the Arbuckle-Wichita 

orogenic belt, Precambrian rocks are exposed in an area 

of 2,000 mi2 (5,200 km2) on the crest of the Llano uplift 

(fig. 14). The uplift is a structural high at the edge of the 

North American craton, little disturbed by Phanerozoic 

movements except for high-angle block faulting. Cambrian and younger Paleozoic rocks slope northward and 

westward away from the Precambrian into the craton, 

and all of them are overlapped by Cretaceous deposits 

that dip southeastward beneath the Gulf Coastal Plain. 

A short distance southeast of the edge of the Cretaceous 

overlap the rocks of the Llano uplift adjoin in subcrop 

the much more deformed Paleozoic rocks of the 

Ouachita orogenic belt. 

The country rocks of the Precambrian basement are 

the felsic Valley Spring Gneiss and the mafic Packsaddle Schist (Y m), folded along northwest-trending axes 

and derived from an original supracrustal sequence no 

less than 20,000 ft (6,000 m) thick. In these are embedded granitic rocks (Y g2), which form more than a third 

of the exposed area, as well as minor granite porphyry 

and pegmatite dikes, anc. a single ultramafic body (urn). 

The granites were emplaced in three plutonic series, of 

which the youngest (Town Mountain) is the most extensive; it includes more than half a dozen nearly circular 

plutons 10 mi (16 km) or more across. 

Efforts to obtain radiometric ages from the Llano 

Precambrian extend back three-quarters of a century to 

calculations from the rare-earth minerals in the Barringer Hill pegmatite (Becker, 1908, p. 134). These results are only of historical interest, and reliable dates 

were not obtained until much later. The granites of the 

three plutonic series yield ages of 1,030 m.y. by 

rubidium-strontium and potassium-argon methods 

(Zartman, 1964), and 990-1,070 m.y. by uranium-lead 

methods on zircon; ages from the Valley Spring Gneiss 

by rubidium-strontium methods are 1,120 m.y. 

(Zartman, 1965). The cycle of metamorphism and intrusion had a span of about 100 m.y. and is a Grenvillian 

event, termed for local purposes the ((Llano orogeny" 

(Muehlberger and 'Others, 1966, p. 4522); the rocks involved in it are classed as Precambrian Y. 

REGIONAL PROBLEMS 

It will be observed from the outcrop data just presented that there are three general ages of basement 

rocks in the south-central United States-approximately 1,000 m.y. in the Llano area, 500 m.y. in the 

Wichita area, and 1,200-1,400 m.y. in the Arbuckle and 

Ozark areas, each of which has also been recorded in 

buried basement rocks near the outcrops. The 1,000-

m.y. ages mark a Grenvillian province that probably 

connects with the surface and subsurface Grenville 

province east ofthe Mississippi River, although there is 

a wide intervening gap where basement rocks have not 

been reached by drilling. The 500 m.y. ages represent a 

Cambrian basement province unique in the North 

American interior. The 1,200-1,400 m.y. ages north of it 

recall the Elsonian event in the eastern Canadian 

Shield, and have been recorded in buried rocks over a 

wide expanse of the Interior Province, northward to the 

Wisconsin arch in the Lake Superior region, and eastward to the buried front of the Grenville belt in Ohio 

and Kentucky. 

The regional meaning of the 1,200-1,400 m.y. set of 

dates is not clear. Do they express an age province like 

those in the Canadian Shield, with its own complex of 

metamorphic and plutonic rocks and with well-defined 

strucural boundaries against other provinces? Or does 

it result from extensive overprinting of later events on 

an earlier province? Available evidence is not decisive, 

because so much of it has been obtained from drill data, 

and so little from outcrops, but the second possibility 

seems more likely: 

(1) The boundaries of the region are poorly defined, 

the Elsonian dates being mingled on the north with 

Hudsonian dates, and on the south with Grenvillian 

dates. 

(2) Many of the dates recorded in subsurface are from 

plutonic bodies that might be younger than the complex 

in which they are embedded. 

(3) This situation is true in the few outcrop areas. In 

the Nain province of the Canadian Shield, the Wisconsin arch of the Lake Superior Region, and the Precambrian of the Southern Rocky Mountains, plutons with 
SOUTH-CENTRAL UNITED STATES 43 

Elsonian ages (Y g1) are thickly spaced in metamorphic 

and plutonic complexes with Hudsonian ages (Xm, Xg). 

To some extent, this later plutonism has updated the 

ages in the surrounding complexes. 

(4) Many of the Elsonian ages in the south-central 

States are from volcanic and other supracrustal rocks. 

An impressive feature of the concealed basement of 

this region is the wide extent of little-deformed felsic 

volcanics with Elsonian and younger ages, that presumably overlie earlier complexes (fig. 15). In Missouri 

: .·.·.·.: ·.· .. :· :. 

• • 0 • • • 

0 

0 

they have yielded ages of 1,200-1,350 m.y. (as in the St. 

Francois Mountains), in northeastern Oklahoma ages 

of 1,150-1,300 m.y., in the Texas Panhandle ages of 

1,100-1,200 m.y., and in the Wichita belt ages of 525 

m.y. (Muehlberger and others, 1966, p. 5422 and fig. 3). 

Associated with the Panhandle volcanics is a very extensive stratiform body of intrusive gabbro of somewhat 

younger age. 

On the Geologic Map, we have provided for the 

plutonic rocks with 1,200-1,400 m.y. ages in unitY g1 

•• 0 • • ••••• 

. .. . . · . .... . 

. . . . . . . . 

: .. : ': ·._: : .. _:. ·· .: ·_ ·:. : ... ~: .. :._ : __ :: ·_. . 

• -:::"······ ··· ·· · .. .. ·.· . .-:. ·.·. ·. • . .. · ... 

. , . . ·. · . . ·.· .. 

: : ·.·. . . . . . 

50 MILES 

50 KILOMETRES 

EXPLANATION 

Cambrian 

Valley Spring Gneiss 

(felsic paragneiss) 

~ 

~ 

Ordovician to 

Pennsylvanian 

Cretaceous 

Packsaddle Schist 

(mafic paraschist) 

~~~;~~/] 

Granite 

(dashed lines indicate 

subsurface extent) 

Ultramafic rocks 

FIGURE 14.-Map of Llano uplift, central Texas, showing Precambrian Y metamorphic and plutonic rocks, and their relation to surrounding Phanerozoic rocks. Compiled from Geologic Map of Texas (1937), Flawn and Muehlberger (1970, p. 78), and other sources. 
44 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

Colder Precambrian Y granitic rocks"); metamorphic 

and supracrustal rocks with these ages are not distinguished separately from the remainder of Precambrian Y. 

CORDILLERAN REGION 

The outcrops of Precambrian rocks in the western 

United States are in the Cordilleran Region, a domain 

of later Phanerozoic orogenies which have raised the 

Precambrian to the surface in the higher uplifts. These 

outcrops are separated by 500 mi (800 km) or more of 

.. NEW 

Front 

Range 

COLORADO 

Van Horn area 

~-., 

' \ 

) 

0 

0 

KANSAS 

TEXAS 

Llan~. 

· (( 

.0 

Phanerozoic cover from those in the Lake Superior Region and elsewhere in the central United States, but 

much has been learned about the connections between 

them from drilling in the intervening plains. 

The Precambrian provinces and structures are 

mostly transverse to the Phanerozoic structures and 

landforms of the Cordillera, and are only grossly related 

to them. Nevertheless, it will be useful to describe the 

Precambrian in terms of the modern morphology. 

Under the first heading we will therefore deal with the 

ancient Precambrian crystalline rocks (mainly PrecamMISSOURI St. 

1200-1350 

200MILES 

200 Kl LOMETRES 

EXPLANATION 

Iilli 

Subsurface extent of 

supracrustal felsic volcanic rocks (approximate ages in millions 

of years) 

1~ -1 Outcrops of Early Cambrian 

basement rocks 

Outcrops ~ of Precambrian 

rocks 

FIGURE 15.-Map of part of south-central United States, showing subsurface extent of late Precambrian and Early Cambrian supracrustal felsic volcanic rocks. Based on Muehlberger, Hedge, Denison, and Marvin (1966, p. 5422), and Bayley and Muehlberger 

(1968). 
CENTRAL ROCKY MOUNTAINS 45 

brian W) of the Central Rocky Mountains in Wyoming 

and southern Montana; and following this the later 

Precambrian supracrustal rocks (Precambrian Y and Z) 

of the Northern Rocky Mountains in western Montana 

and adjacent Idaho. In a like manner, we will deal with 

the somewhat younger Precambrian crystalline rocks 

(mainly Precambrian X) of the Southern Rocky Mountains in Colorado and New Mexico; followed by the 

supracrustal rocks (Precambrian Y and Z) of the eastern 

Great Basin in Utah and adjacent States. In a final 

section, we will describe the varied Precambrian rocks 

of the southern Basin and Range province in Arizona 

and adjacent States. 

CENTRAL ROCKY MOUNTAINS 

For purposes of this account, the Central Rocky 

Mountains are the ranges of Wyoming and southern 

Montana, and the Black Hills of South Dakota. They are 

irregularly disposed, widely spaced, broad-backed 

mountain uplifts, many of which expose large areas of 

Precambrian rocks in the,ir cores; they are separated by 

even broader areas of downwarped Phanerozoic rocks, 

whose plains and plateaus are more or less confluent 

with the Great Plains to the east. On the west and 

northwest they are bordered by the more closely 

crowded ranges of the main Cordilleran thrust belt. The 

Precambrian cores of many of the ranges project a mile 

or more above their surroundings, and some peaks attain altitudes of as much as 13,000 ft (4,000 m). 

The larger areas of Precambrian in the Central Rocky 

Mountains are in the Black Hills of western South 

Dakota, well to the east of the others; in the Laramie 

and Medicine Bow Ranges of southern Wyoming; in the 

Wind River Mountains farther west in Wyoming and 

the Bighorn Mountains farther north; and in the Beartooth Mountains which straddle the boundary between 

northwestern Wyoming and southern Montana (fig. 16). 

Smaller Precambrian areas in some of the intervening 

ranges and to the north west provide partial connections 

between the larger areas. 

The dominant structures of the Central Rocky Mountains are a product of late Cretaceous-early Tertiary 

(Laramide) orogeny, in which the Precambrian basement participated. Although the Phanerozoic rocks are 

steeply tilted or faulted at the edges of the uplifts, their 

Precambrian cores were raised mainly as rigid blocks. 

As a result, the Precambrian rocks and their structures 

have been so little modified by Laramide and other 

Phanerozoic deformations that the effects can be disregarded. 

The Precambrian of the Central Rocky Mountains is 

an extension of that of the Superior province of the 

Canadian Shield (fig. 1 7), and most of it is ancient crystalline rocks with Kenoran or even earlier dates (Precambrian W); however, younger dates are reported in 

places in the subsurface of the intervening area (Goldich, Lidiak, Hedge, and Walthall, 1966, p. 5400, fig. 1), 

and there are important areas of outcrop of younger 

supracrustal rocks (Precambrian X) to the southeast 

and east. The southeastern boundary with the younger 

crystalline rocks of the Southern Rocky Mountains is a 

major structural discontinuity (Mullen Creek-Nash 

Fork shear zone) that crosses the Medicine Bow and 

Laramie Ranges, and can be traced in subsurface more 

than 200 mi (320 km) farther northeastward beneath 

the Great Plains. The northwestern boundary is the 

stratigraphic overlap of the Belt Supergroup (Precambrian Y) in central Montana. 

PRECAMBRIAN W 

The rocks in nearly all the ranges of the Central 

Rocky Mountains in Wyoming are gneiss (Wgn) and 

granite (W g), and share a complex history that has only 

partly been deciphered. They have been studied in 

greatest detail in the Beartooth Mountains during a 

project under the direction of the late Prof. Arie Poldervaart (Eckelmann and Poldevaart, 1957; and later reports). Here and elsewhere, the oldest rocks are paragneisses, originally a thick supracrustal sequence of 

dominantly pelitic sediments and minor volcanics, that 

have been plastically folded and refolded, regionally 

metamorphosed to amphibolite grade, and partly converted to migmatite and granite; in addition, there are 

some postkinematic granite plutons, and the whole 

complex is crisscrossed by diabase dikes, formed during 

a late tensional phase. 

Radiometric determinations on the rocks of all the 

ranges by potassium-argon, rubidium-strontium, and 

uranium-lead methods have yielded rather consistent 

Kenoran ages of about 2,750 m.y., but this seems to 

express merely the later orogenic events. Zircons of 

detrital origin from the gneisses of the Beartooth Mountains have yielded ages in excess of 3,100 m.y. and 

express an earlier event upon which the Kenoran event 

was superposed (Catanzaro, 1966, p. 9--11; Butler, 1966, 

p. 61). In the other ranges an earlier event of this kind 

can be inferred from the structural relations, but this 

has not been confirmed by radiometric dating. 

A unique feature of the northwestern Beartooth 

Mountains is the Stillwater Complex (Wmi), a body of 

layered chromite-bearing mafic and ultramafic rocks 

with an exposed length of30 mi (48 km) and a preserved 

thickness of18,000 ft (5,500 m) (Jones and others, 1960, 

p. 283--286). It intrudes and overlies the prevailing 

gneisses and dips steeply away from them, under the 

unconformably overlying Cambrian on the flank of the 
46 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

range. The complex is younger than the 3,100-m.y.-old 

gneisses which it invades, and older than a 2,700-m.y.-

old quartz monzonite which truncates its eastern end. 

Potassium-argon and rubidium-strontium dating of the 

complex itself yields conflicting results (Kistler and 

others, 1968; Fenton and Faure, 1969), but it was probably emplaced nearer the later limiting date than the 

earlier. · 

Some of the ranges farther south in Wyoming expose 

downfolded belts of supracrustal rocks (W) much like 

those in the Superior province of northern Minnesota. 

In the South Pass (Atlantic City) district at the south 

end of the Wind River Mountains one of these belts 

contains 15,000 ft (5,000 m) or more of strata, beginning 

with basal iron formation and quartzite, followed by a 

thick body of turbidites, and greenstones with pillow 

structure (Bayley, 1968, p. 502-598). These are older 

than the Louis Lake Granodiorite to the north with an 

age of 2,690 m.y. 

PRECAMBRIAN COMPLEX OF SOUTHWESTERN MONT ANA 

Precambrian crystalline rocks are exposed in southwestern Montana between the Beartooth Mountains 

and the Cordilleran thrust front on the west, in the 

Madison, Jefferson, Tobacco Root, Ruby, and other 

ranges. On the Geologic Map they are represented as 

Precambrian W like those in Wyoming, but they are 

so mew hat more varied, their ages are less certain, and 

they are more involved with Phanerozoic features, such 

as Laramide plutons (Kg3), Cenozoic volcanism, and 

block faulting. 

Three general rock types recur in the different 

I 

I 

\~ ,o 

I~ 

"'""""""'•'-/..t.... ____ -~- ____________ ~_9_N_T_A~- --__ --i~ 

~ WYOMING > 

Owl Creek -~ 

~ Mo~;n, ~ 

----------

0 200 Ml LES 

0 200 Kl LOMETRES 

I~ 

lo 

F 

rf<J 

\' \ 

I 

I 

t---

FIGURE 16.-Map of Central Rocky Mountains in Wyoming, South Dakota, and Montana, showing outcrops of Precambrian rocks 

and localities mentioned in text. 
CENTRAL ROCKY MOUNTAINS 47 

ranges, called the Dillon (a granitic orthogneiss), the 

Pony (a mafic paragneiss), and the Cherry Creek (a 

sequence of metasediments and metavolcanics) 

(Scholten and others, 1955, p. 350-352; Reid, 1963); the 

first two are rna pped as W gn and the third as W. Their 

structure is complex and their mutual relations are still 

® 

ALBERTA 

I 

I ® 

_ _ _J SASKATCHEWAN 

------- -----

I 

, MONTANA ® 

I 

I 

I 

I 

0 

0 

debated, but the components of the Cherry Creek are 

sufficiently distinctive to suggest that it may be a valid 

stratigraphic unit (H. L. James, oral commun., 1973). In 

the type Cherry Creek area in the Jefferson Range it 

includes mica schist, pillow lava, iron formation, 

quartzite, and dolomite marble (Hadley, 1969a, b); al-

® 

• 

• 

® 

® 

NORTH 

, 

----

• 

DAKOTA 

® ® 

® 

• 

• 

/ SOUTH DAKOTA 

/ 

® 

® 

® 

NEBRASKA ® 

200 Ml LES 

200 KILOMETRES 

EXPLANATION 

L:.:.::_:_j 

~ 

Outcrops of rocks of Precambrian W Outcrops of rocks of PrecambrianW Outcrops of younger Precambrian 

with known age greater than (with ages of 2,500 m.y. or greater) rocks (Precambrian X, Y, and Z) 

3,200 m.y. 

Surface and subsurface extent of 

supracrustal rocks of Precambrian Y and Z 

Wells which have yielded dated 

specimens of Precambrian rocks 

(solid circles with ages of 2,400-

2,700 m.y., open circles with 

ages of 1,600-1,800 m.y.) 

FIGURE 17.-Map of part of western United States and southern Canada, showing surface and subsurface extent of Precambrian 

W rocks in the Superior province of the Lake Superior Region, and westward to the Central Rocky Mountains. Compiled from 

geologic maps of United States (1974) and Canada (1969), and Goldich, Lidiak, Hedge, and Walthall (1966, p. 5390). 
48 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

though here classed as Precambrian W, the prominent 

bodies of quartzite and marble are more characteristic 

of younger parts of the Precambrian in other regions. 

Radiometric dating by potassium-argon and 

rubidium-strontium methods provides equivocal results. The rocks of the ranges toward the southeast yield 

dates in excess of2,600 m.y., but identical rocks farther 

northwest have dates in the range of 1,600--1,800 m.y.; 

in addition, in the northernmost exposures are 175 m.y. 

dates produced by proximity to Laramide plutons 

(Giletti, 1966, p. 4031-4035). Apparently a Precambrian W terrane with original Kenoran dates has been 

downgraded toward the northwest by Hudsonian 

events. The westernmost granitic rocks of Dillon type 

yield rather consistent 1,600 m.y. ages and are therefore 

mapped as Xg; they may represent a younger pluton 

that is at least partly responsible for the mixing of 

Kenoran and Hudsonian dates. 

Farther north, Precambrian crystalline rocks reappear in the core of the Little Belt Mountains uplift, 

where they form the basement of the Belt Supergroup. 

The varied rocks include paragneiss, migmatite, granite gneiss, and diorite. Radiometric determinations by a 

variety of methods yield dominant ages of about 1,900 

m.y., but zircons from the paragneiss and migmatite 

have ages as great as 2,450 m.y. (Catanzaro, 1966, 

p. 13--15). Here, as in southwestern Montana, Hudsonian dates are mingled with Kenoran dates, and the 

Little Belt crystalline rocks are accordingly mapped as 

Wgn. 

The mingling of Kenoran and Hudsonian dates in 

southwestern Montana and the Little Belt Mountains 

seems to represent a gradational boundary between two 

major Precambrian provinces, analogous to the 

Superior and Churchill provinces of the Canadian 

Shield. Here, however, in contrast to conditions in the 

Shield, there seems to be no sharply marked structural 

or stratigraphic boundary between the two provinces. 

A final comment should be made on the crystalline 

rocks in the core of the mantled gneiss domes of the 

Albion Range in southern Idaho and northwestern 

Utah, for which rubidium-strontium whole.:rock isochron yields an age of 2,460 m.y. (Armstrong and Hills, 

1967, p. 118-120). This occurrence is 200 mi (320 km) 

west of the Precambrian W rocks in the Central Rocky 

Mountains, and represents the farthest known extension of the Superior province in the United States. 

PRECAMBRIAN X 

In the eastern and southeastern part of the Central 

Rocky Mountains, as here delimited, younger Precambrian supracrustal rocks (X) are emplanted in the prevailing ancient crystalline terrane (Wgn, Wg). They 

form most of the exposed Precambrian in the Black 

Hills, and smaller areas in the Hartville uplift and 

Medicine Bow Mountains to the southwest. All of them 

have yielded Hudsonian dates and are classed as Precambrian X, but considerable differences in lithology 

and sequence among the several areas preclude more 

exact correlations. 

In the Black Hills of western South Dakota, Precambrian rocks are exposed in a northward-elongated oval 

area of about 900 mi2 (2,300 km2) on the crest of the 

dome (fig. 18). All the Precambrian was surveyed in 

reconnaissance during the early part of the century by 

Sidney Paige (in Darton and Paige, 1925). Economic 

work was done later near the Homestake gold mine at 

the north end of the area (Noble and Harder, 1948; 

Noble and others, 1949) and in the pegmatite district in 

the southern part (Page and others, 1953), but comprehensive regional mapping is of rather recent date 

(Redden, 1963, 1968; Ratte and Wayland, 1969; Bayley, 

1970, 1972a, b, c). 

The rocks are a sequence of metamorphosed sediments and minor volcanics more than 40,000 ft 

(12,000 m) thick, steeply or isoclinally folded along 

northerly axes, and in places curiously refolded. In the 

northern half they form a gross synclinorium plunging 

toward the south. Here, the lower part of the sequence is 

in the Nemo district on the eastern side (Runner, 1934), 

which adjoins basement granite on Little Elk Creek 

(Wgn) with a Kenoran age of 2,500 m.y. (Zartman and 

Stern, 1967). A thick basal quartzite is succeeded unconformably by an equally thick conglomerate with associated beds of iron formation, schist, and limestone. 

The upper part of the sequence, which forms the rest of 

the exposure to the west, is a eugeosynclinal deposit 

originally laid down as graywacke, slate, graphitic 

slate, chert, and pillow lava. It contains several thin but 

very persistent formations of ferruginous cherty rock, 

one of which (the Homestake Formation) contains the 

gold ore at the Homestake Mine. 

Less is known of the overall stratigraphic sequence in 

the southern hills. Basement rocks with a Kenoran date 

(Wgn) project in a mantled gneiss dome at Bear Mountain on the western side. Farther east, at Harney Peak 

and elsewhere, large granitic plutons (Xg) have domed 

the already folded and faulted supracrustal rocks (Runner, 1943) and are surrounded by swarms ofpegmatites. 

The granites and pegmatites have been dated by 

potassium-argon and uranium-lead methods at 1,620--

1,680 m.y., and were intruded during late kinematic or 

postkinematic phases of the ((Black Hills" ( = Hudsonian) orogeny (Goldich, Lidiak, Hedge, and Walthall, 

1966, p. 5401) 

At the crest of the northern Medicine Bow Mountains 

of southern Wyoming is another great sequence of supracrustal rocks, that was deciphered years ago by 
CENTRAL ROCKY MOUNTAINS 49 

Blackwelder (1926). It lies on basement gneisses (W gn) 

on the north, and dips steeply and homoclinally southward to the Mullen Creek-Nash Fork shear zone, which 

.. . 

.... ... . 

I I I I · : I 1111. i 11 .1 ~ ~ ~ = ~ j j : : : : : : = : : : : .. 

···· ········· 

...•• !' I i :m m ' .. .. . . . . ..l '\"'\~u-.; :.: :t;;.E''" 

~;;; [j ~ ~ j [iiHHH ~~::: :· . ..... .. 

............ .... . . ············· · 

··········· .... ·::::: :: : : :.:::: 

:::::·. ::::::::::::::::::::::::::·:.:·· 

II iiJ: ! ..... '"::::::::::: :: :::·· 

·::::::::::::::: .... 

0 

separates it from the crystalline complex of the Central 

Rocky Mountains (Xm, etc.) (fig. 19). At the base of the 

sequence, which totals 35,000 ft (11,000 m) in all, is a 

~~pid CityD 

20 Ml LES 

EXPLANATION 

Tertiary intrusive rocks 

Paleozoic 

(with Upper Cambrian 

at base) 

PRECAMBRIAN X 

Harney Peak Granite 

1,620-1,680 m.y. 

EUGEOSYNCLINAL 

SEQUENCE 

Schist and slate, with thin to 

thick bodies of graywacke and 

quartzite (stippled) 

~ I 

~ 

Pillow basalt, iron formation, 

and slate or schist 

~ 

Slate; and 

~ 

quartzite (stipple) 

with Homestake Ironformation at base in north 

MIOGEOSYNCLINAL 

SEQUENCE 

Quartzite, conglomerate, ironformation, and limestone 

PRECAMBRIAN W 

Little Elk Granite (in northeast) 

and granite gneiss at Bear 

Mountains (in southwest) 

2,500 m.y. 

Contacts within map units 

Precambrian faults 

0 20 Kl LOMETRES 

Phanerozoic faults 

FIGURE 18.-Map showing Precambrian rocks in the Black Hills, western South Dakota. Compiled from many sources, including Paige (in 

Darton and Paige, 1925), Noble and Harder (1948), Noble and others (1949), Redden (1963, 1968), Ratte and Wayland (1969), and Bayley 

1970, 1972a, b, c). 
50 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

thick body of quartzite (Deep Lake Formation), followed 

unconformably by a more varied set of formations 

(Libby Group), including probable tillite in the lower 

part, several prominent quartzite units, and higher up, 

slates, greenstone volcanics, and carbonates with 

abundant, well-preserved stromatolites (Houston and 

others, 1968, p. 15-38). The general aspect of the sequence is miogeosynclinal, in contrast to the eugeosynclinal aspect of most of the Black Hills sequence. 

Rubidium-strontium whole-rock isochrons indicate 

that the Medicine Bow sequence is older than 1,550 m.y. 

and younger than the basement gneiss at 2,350-2,400 

m.y. (Hills and others, 1968, p. 1776). 

PRECAMBRIAN Y 

In the Laramie Range east of the Medicine Bow 

Mountains, the position of the Mullen Creek-Nash 

Fork shear zone is occupied by an anorthosite body (Y a) 

EXPLANATION 

~ 

~ 

Quaternary 

(glacial deposits and alluvium) 

Tertiary 

(Paleocene to Miocene) 

~ 

~ 

Mesozoic and Paleozoic 

Sherman Granite 

(1,335 m.y.) 

Gneissic granite 

(1,410 m.y.) 

Paragneiss 

(1,715 m.y.) 

Libby Group 

Deep Lake 

Formation 

Q) 

u~ 

c: . Q) ':' 

5-E 

GlO 

"'Ill 

-<O Ill • 

t;;'7 

2o 

()ll) 

«<ll) 

.... 

c..-

~- (/) 

Orthogneiss and 

paragneiss (2,400 m.y .) 20 Kl LOMETRES 

Mafic intrusives 

(various Precambrian ages) 

Cataclastic rocks in 

shear zone 

FIGURE 19.-Map of northern Medicine Bow Mountains, Wyo. , showing rocks of Precambrian Wand X, and their relation to surrounding 

Phanerozoic rocks. The Mullen Creek-Nash Fork shear zone separates the Precambrian provinces of the Central and Southern 

Rocky Mountains. Generalized from Houston and others (1968) and Hills and others (1968). 
NORTHERN ROCKY MOUNTAINS 51 

with an area of about 300 mi2 (780 km2) (Newhouse and 

Hagner, 1957); it intrudes the ancient gneisses (Wgn) 

which form the northern part of the range, and it is 

intruded in turn by the Sherman Granite (Y g1) which 

forms the southern part. The anorthosite has a 

minimum age of 1,510 m.y. by potassium-argon 

methods on hornblende (Hills and Armstrong, 1971), 

and the granite has been dated at 1,410 m.y. 

NORTHERN ROCKY MOUNTAINS 

The Northern Rocky Mountains in this account are 

the mountainous terrain of western Montana and 

northern Idaho. Most of the southwestern half of the 

mountains is occupied by the great plutonic mass of the 

Mesozoic Idaho batholith (Kg, Kgn), but a large part of 

the remainder, northward across the International 

Boundary into Canada, is formed of supracrustal rocks 

of later Precambrian age-mainly the Belt Supergroup 

(Y), but including the less extensive younger Windermere Group (Z) on the northwest. 

PRECAMBRI AJ\' Y11 

The Belt Supergroup, or ~~Beltian" ( = Purcell Supergroup in Canada), is exposed nearly continuously across 

an area of about 30,000 mi2 (78,000 km2) in the United 

States and an additional 10,000 mi2 (26,000 km2) in 

Canada-the greatest expanse of well-preserved Precambrian supracrustal rocks in the country. Throughout much of this expanse the Belt is merely tilted or 

warped, broken into coarse-textured fault blocks, and 

subjected only to the lower grades of metamorphism-a 

truly remarkable preservation of rocks so ancient 

through the 900 m.y. of succeeding Precambrian and 

Phanerozoic time. The Belt remained little disturbed 

until the Mesozoic and early Tertiary Cordilleran 

orogenies (Laramide and earlier). 

As a result of these orogenies, much of the Belt is 

allochthonous, having been transported scores of miles 

eastward along the Lewis and other low-angle thrusts, 

across the Paleozoic and Mesozoic rocks of the Cordilleran miogeosyncline and foreland. Southwestward, 

also, it is invaded and disrupted by the Idaho batholith 

and other Mesozoic plutons, near which it is regionally 

metarnorphosed to amphibolite grade. In this western 

area there is, besides, a gneissic terrane (Ym) which 

exceeds the adjacent Belt in its metamorphic complexity, but which may have been converted from Beltian 

rocks by the plutonic activity (Clark, 1973). 

Through a large part of its extent, the Belt is the 

' ' For a summa ry of data on t he Belt Supergroup a vailable up to 1963, see Ross (1963, 1970 l; 

later informa tion and developments for t he part in Canada are given by Price (1964) and 

Gabrielse (1972 , p. 522--528 ), a nd fo r the part in the United States by Harrison (1972l a nd 

Harrison a nd others ( 197 4 ). 

youngest rock preserved, but outliers of Paleozoic occur 

on it in places, and it passes beneath Paleozoic and 

younger rocks at the edges. The next overlying unit is 

generally the Middle Cambrian Flathead Quartzite, 

which is separated from the strata beneath by little or 

no structural discordance. This relation has created a 

persistent misconception that the Belt must be very late 

Precambrian, if not Early Cambrian; Daly (1912, 

p. 174--190) even proposed an elaborate correlation of 

most of the Belt with the Lower and Middle Cambrian 

formations farther north in the Rocky Mountains. Much 

later, Deiss (1935) demonstrated the extensive regional 

truncation of the Belt beneath the Middle Cambrian 

deposits (although his results are somewhat vitiated by 

an assumption of constant thickness of the different 

Belt formations). Moreover, a decade earlier Walker 

(1926, p. 13-20) had discovered the later Precambrian 

Windermere Group unconformable over the Belt on the 

northwest, thus proving that the Belt itself was much 

earlier Precambrian than anyone had hitherto suspected. (See p. 53.) 

Along its southeastern side, in the Little Belt Mountains and the Three Forks region of southwestern Montana, the Belt lies on older Precambrian crystalline 

basement (W gn) with Kenoran and Hudsonian dates 

(p. 46-48). Near Three Forks the Belt abuts southward 

against a rough and partly upfaulted highland of the 

crystalline rocks, near which it assumes a coarse bouldery facies (LaHood Formation), quite unlike the normal fine-grained Belt sediments (McMannis, 1963); this 

is a local feature. An eastward wedging out of the Belt 

deposits on their basement must also exist north of the 

Little Belt Mountains, where the line of overlap is now 

concealed beneath the Lewis thrust plate. Farther west 

in the expanse of Belt rocks their basement nowhere 

reaches the surface; moreover, all the rocks west of the 

Belt area are younger, so that there is no indication of 

any western borderland. 

Along the eastern edge of its exposure, as in the Belt 

Mountains and Glacier National Park, the Belt is about 

20,000 ft (6,100 m) thick, and is readily divisible into 

half a dozen or a dozen distinctive formations, including 

two prominent carbonate units (Newland = Altyn below, Helena= Siyeh above), several units of bright red 

argillite, and (near the International Boundary) the 

Purcell Lava, the only volcanic rock in this sequence, or 

elsewhere in the Belt. 

This facies is marginal to the main area of Belt deposits to the west. Near the probable center of its original depositional basin, observed partial sequences of 

the Belt are 50,000 ft (15,000 m) thick, and the whole 

thickness probably exceeds 65,000 ft (20,000 m). Here, 

contrasts in the deposits have faded and formation 

boundaries are blurred. Most of the deposits are 
52 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

siltstones, grading on the one hand into argillites and 

on the other into fine-grained quartzites. Red colors 

have turned to drab; the carbonates become calcareous 

siltstones with a few thin limestone interbeds. A remarkable feature of the sequence is the indication 

nearly throughout of deposition in shallow water, as 

shown by mud cracks, cut-and-fill, casts of salt crystals, 

and other sedimentary structures. All indications are 

that this great mass of fine sediment was derived from 

cratonic areas to the east and southeast. 

As will be seen presently, radiometric data indicate 

that accumulation of the Belt sediments occupied a span 

of nearly 500 m.y.-a length equal to most of 

Phanerozoic time since the beginning of the Ordovician. 

Even the great known total thickness of the Belt seems 

inadequate the account for this time span on the assumption of continuous sedimentation. This has led to a 

proposal that the Belt sequence must contain several 

hidden unconformities, expressing lengthy time gaps 

(Obradovich and Peterman, 1968, p. 7 46). It is true that 

some unconformities have been observed from place to 

place, but they seem to be minor and local. The best 

guess may be that there was ((long semicontinuous deposition of sediments, interrupted by many hiatuses" 

(Harrison, 1972, p. 1237). 

Many different local names have been given to subdivisions of the Belt Supergroup from one district to 

another, both in the United States and Canada, and 

correlations between them have been much debated. 

Increasing knowledge in recent decades has clarified 

most of the relations and has led to recognition of gross 

regional subdivisions. Within the main area of Belt 

outcrops their pattern is sufficiently coarse textured 

that they can be distinguished on the Geologic Map. On 

the map, it is therefore feasible to represent a unitY 1, of 

siltstones, argillites, and quartzites, that includes the 

Prichard Formation and Ravalli Group of the southwestern area; a unit Y 2, of carbonates and calcareous 

siltstones, that includes the Wallace Formation on the 

west and the Helena and Siyeh Limestones farther 

east; and a unit Y3, again siltstones, argillites, and 

quartzites, that comprises the l\1issoula Group. Similar 

gross subdivisions are also recognizable in some of the 

smaller outlying areas, especially to the southeast, but 

cannot be shown on the scale of the map; these areas are 

indicated merely as undifferentiated Y. 

The fresh appearance of the Belt rocks and their 

well-preserved sedimentary structures have impelled 

geologists since the days ofW alcott (1899, p. 235-239) to 

search for the remains of fossils. The search has revealed abundant and well preserved stromatolites in 

the carbonate rocks at many levels, which seem to be 

capable of at least local zonation (Rezak, 1957); and 

other probable organisms such as bacteria. Traces of 

more advanced, metazoan forms of life have also been 

claimed, such as burrows, trails, and shell fragments, 

but these are questionable and some of them are clearly 

inorganic sedimentary structures. A supposed 

brachiopod, Obolella montana Fenton and Fenton, is 

evidently an algal stromatolite (Cloud, 1968, p. 27-29). 

Radiometric data on the age of the Belt are rather 

extensive for a Precambrian stratified sequence, but 

some of the evidence that they afford is indirect, and 

some of it is equivocal and conflicting (Obradovich and 

Peterman, 1968; Harrison, 1972, p. 1234-1238). Absolute age limits of the Belt sequence are set by the 1,700 

m.y. dates from its crystalline basement and the 760 

m.y. age of a vein cutting its upper part (Garnet Range 

Formation ofMissoula Group). From the Belt sediments 

themselves, dates have been obtained at nearly a dozen 

levels from base to top on glauconite, biotite, and argillite by rubidium-strontium and potassium-argon 

methods, which range from more than 1,300 m.y. to less 

than 900 m.y.; also, the Purcell Lava and associated 

intrusives near the base of the Missoula Group (Y3) 

have yielded a potassium-argon date of about 1,100 m.y. 

In addition, dates of about 1,500 m.y. have been obtained from gneisses (Y m) probably derived from lower 

Belt rocks, but their significance is questionable. Available evidence thus suggests that Belt sedimentation 

took place over at least 400 m.y., and perhaps as much 

as 500 m.y. Further evidence regarding the time of 

termination of Belt sedimentation is afforded by dates 

from the succeeding Windermere Group. 

PRECAMBRIAN OF CENTRAL IDAHO 

In east-central Idaho, southeast of the Idaho 

batholith and north of the Snake River Plain, the 

Paleozoic in the various ranges is underlain by a thick 

sequence of clastic sedimentary rocks which are commonly considered to be equivalents of the Belt Supergroup and are accordingly shown on the Geologic Map 

as Precambrian Y. Their character and sequence have 

recently been summarized by Ruppel (1975). 

Their total thickness probably exceeds 30,000 ft 

(9,1501m), but their structure is complex, largely owing 

to Mesozoic deformation, and the whole sequence is not 

exposed in any one area. They have been subjected to 

low-grade regional metamorphism in the chlorite and 

biotite zones, and additional metamorphism has been 

superposed to the northwest near the batholith. The 

sequence is divided into the Yellowjacket Formation 

below, followed by the Lemhi Group of five formations, 

and the Swager Formation. 

The sedimentary rocks of the area resemble those of 

the typical Belt in being a very thick sequence of clastic 

sediments, lying stratigraphically between the older 
NORTHERN ROCKY MOUNTAINS 53 

Precambrian crystalline rocks to the east and the overlying Paleozoic. In detail, however, the rocks of the two 

areas have surprisingly little in common. In contrast to 

the dominantly silty rocks of the Belt, those of central 

Idaho are .mainly fine- to medium-grained feldspathic 

sandstones. Limestone and dolomite are virtually lacking, and stromatolites are rare. The sandstones show 

none of the shallow-water sedimentary structures of the 

Belt rocks. Observers have been unable to find any 

points of resemblance between details of the two sequences which would suggest correlations, and a proposed correlation based on general sequence (Ruppel, 

1975, p. 18) is tenuous indeed. One reason for the differences seems to be that the central Idaho sequence of 

Precambrian sediments and the overlying Paleozoic 

have been transported a long distance eastward from 

their original site of deposition by Mesozoic thrusting; 

accumulating evidence indicates that the distance of 

transport was more than 100 mi (160 km). Two Precambrian sedimentary basins with different environments of deposition have evidently been brought into 

juxtaposition. 

Additional data on the'Precambrian of central Idaho 

have been presented by Armstrong (1975). He finds that 

granitic gneisses near Salmon, Idaho, shown on the 

Geologic Map as an eastern lobe of the Idaho batholith, 

have yielded ages of about 1,500 m.y. by rubidiumstrontium methods, hence are early Precambrian Y. 

This has many implications. It suggests that the rocks 

that extend across and nearly bisect the middle of the 

batholith are of early Precambrian age. On the Geologic 

Map, indeed, an extremely metamorphosed part of them 

is represented as Xm, but Armstrong believes that the 

remainder, shown on the map as metamorphosed Belt 

(Y) or as ((border phase of the Idaho batholith" (Kgn) 

deserve the same classification also. Furthermore, the 

1,500 m.y.-old gneisses apparently intrude the central 

Idaho Precambrian sedimentary sequence just discussed, which would imply that it is of pre-Belt ageeither early Precambrian Y or Precambrian X. The 

observations so far made on these interesting problems 

are as yet insufficient to provide positive answers, and 

they deserve much further investigation. 

PRECAMBRIAN Z 

Overlying the Belt Supergroup on its northwestern 

border is the Windermere Group, a younger Precambrian supracrustal sequence. Its typical development is 

in the Purcell and Selkirk Mountains of southeastern 

British Golumbia, whence it extends northward 

through most of the length of the Canadian part of the 

Cordillera (Gabrielse, 1972, p. 529-531). It also projects 

southward into the northeastern corner of Washington 

State, as in the Metaline district (Park and Cannon, 

1943, p. 7-13), but is preserved here only in small disconnected remnants (Z). Parts of the Windermere have 

been known for many years; the part along the International Boundary was the ((Summit Series" of Daly 

(1912, p. 141-159), who thought it was equivalent to the 

Belt farther east. Recognition of these different parts as 

a new and hitherto unknown entity first came with 

Walker (1926, p. 13-20), whose discovery was one of the 

major contributions to North American Precambrian 

geology of this century. Nevertheless, the Windermere 

has remained strangely unknown, ignored, or misinterpreted by most geologists in the United States, even 

until today. 

The Windermere lies unconformably on the Belt and 

marks the beginning of a new cycle of sedimentation. 

Local discordances between the two sequences are 

slight, but regionally the Windermere lies on different 

units of the upper Belt; its basal beds contain abundant 

clasts of the Beltian rocks, a few of which have been 

metamorphosed. The unconformity expresses an event 

called the ((East Kootenay orogeny" which involved 

epeirogenic uplift in the Purcell Mountains, and local 

plutonic intrusions (Gabrielse, 1972, p. 528--529). 

Within the main geosynclinal area on the west, the 

Windermere sedimentary cycle continued nearly unbroken into the Paleozoic. (In fact, the higher units of 

Walker's original Windermere, the Hamill, Lardeau, 

etc., have since yielded Lower Cambrian fossils, and are 

now excluded.) Eastward and marginally, a disconformity develops at the top; in the Banff-J as per segment of 

the Rocky Mountains the Windermere (Miette Group) is 

unconformable below the Lower Cambrian quartzites. 

No fossils have been reported in the restricted Windermere, except for Chuaria in the Miette Group (Gussow, 

1973), like that in the Chuar Group of the Grand Canyon (p. 68). 

The Windermere in southeastern British Columbia 

and adjacent Washington is as much as 15,000-20,000 

ft (4,600--6,100 m) thick-modest compared with the 

preceding Belt deposits, but impressive nevertheless. 

Near the International Boundary the sequence comprises (from below upward) the Toby ( = Shedroof) Conglomerate, the Irene ( = Leola) Volcanics, and the 

Horsethief Creek ( = Monk) Formation, the latter followed by the Hamill ( = Gypsy) Quartzite of Lower 

Cambrian age. (Farther south in Washington the first 

two units are combined as the Huckleberry Formation). 

The Irene is a mafic pillow lava and interbedded tuff 

that wedges out a short distance north of the boundary. 

The Toby is a regional deposit of variable thickness; 

much of it is coarse diamictite, formed of clasts of all 

sizes, largely of Beltian rocks, but including a few of 

granite and gneiss from the craton farther east. Diamic-
54 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

tites recur in the lower part of the Monk (probably 

connecting with the main body of the Toby farther 

north), but most of it is a heterogeneous mixture of 

phyllite, carbonate rocks, and quartzite. The Toby (and 

the basal Monk) was probably a glacial marine deposit, 

fed by ice on adjoining lands to the east, seemingly the 

local expression of a worldwide epoch of refrigeration 

that occurred late in Precambrian time (Aalto, 1971, 

p. 778-784). 

The time of beginning of Windermere sedimentation 

(and by implication the end of Belt sedimentation) is an 

important level in the Precambrian evolution of the 

Cordilleran region, but is indicated by only sparse 

radiometric data. Determinations on the granitic stocks 

in the Purcell area that are thought to have originated 

during the ((East Kootenay orogeny" yield equivocal 

results-potassium-argon dates of 705-770 m.y. and a 

rubidium-strontium isochron of 1,260 m.y. (Gabrielse, 

1972, p. 528); if the latter is near the true age, the 

intrusives must have been emplaced during deposition 

of the later Belt sediments. The volcanics in the lower 

part of the Windermere in Washington State (Irene 

equivalent) have recently yielded potassium-argon 

dates on whole rocks and mineral separates of 829--918 

m.y. (Miller and others, 1973), which suggests that 

Windermere sedimentation probably began about 300 

m.y. before the beginning of the Cambrian. 

SOUTHERN ROCKY MOUNT AINS12 

The Southern Rocky Mountains are the ranges that 

extend southward from Wyoming, through the center of 

Colorado, into northern New Mexico. These ranges, like 

those of the Central Rocky Mountains, are broadbacked uplifts that expose large areas of Precambrian 

rocks in their cores, but they differ from those farther 

north in being closely crowded together rather than 

dispersed, so that their intervening lowlands are much 

narrower. Eastward, the Southern Rocky Mountains 

front abruptly on the Great Plains, whereas westward 

they merge with the Colorado Plateau through intermediate ridges and plateaus (fig. 20). 

Facing the Great Plains is the Front Range, a massive 

upland 250 mi (400 km) long and 30-60 mi (50-95 km) 

broad; it branches northward in Wyoming into the 

Laramie and Medicine Bow Ranges, and terminates in 

southern Colorado in the appendage of the Wet Mountains. South of this termination the Sangre de Cristo 

Mountains rise from behind and form the frontal ridge 

southward into New Mexico, to their own termination 

near Santa Fe. West of the Front Range is the equally 

12For a useful summary of Precambria n rocks and events in the Southern Rocky Mountains, and their rela tion t o Phanerozoic rocks and events, see Tweto, 1968, p. 555-571. 

lengthy but narrower Park Range, and beyond that the 

short and massive Sawatch Range, 90 mi (145 km) long 

and 40 mi (65 km) broad, which culminates in the highest summit of the Rocky Mountains (Mount Elbert, 

14,431 ft, 4,399 m). Beyond the Sawatch Range are 

lower uplifts, still roofed over by Phanerozoic strata, in 

which Precambrian is revealed in the deeper cuts and 

canyons: the White River Plateau to the northwest, the 

Uncompahgre Plateau to the west, and nearer at hand, 

the uplift along the Gunnison River, whose Black Canyon exposes Precambrian rocks in sheer walls 2,300 ft 

(700 m) high. Southwest of the Sa watch Range is the 

broad Cenozoic volcanic field of the San Juan Mountains, at the southwestern edge of which the Precambrian projects again in the domical Needle Mountains. 

The gross surface features of the Southern Rocky 

Mountains, as farther north, are a product of late 

Cretaceous-early Tertiary (Laramide) orogeny, but 

here plutonic and volcanic activity was greater. One 

result of the activity is the Colorado mineral belt that 

extends d!agonally northeastward across all the ranges 

(Tweto and Sims, 1963, p. 993--996), containing most of 

the prolific mineral deposits of Colorado and marked by 

numerous faults, veins, and intrusive stocks, as well as 

the large Mount Princeton pluton in the southern 

Sawatch Range (Kg3, Ti). 

Relations in the Southern Rocky Mountains are complicated further by Phanerozoic orogenic events earlier 

than the Laramide, especially during the later Paleozoic, when geanticlines and troughs were created that 

had a somewhat different pattern from the Laramide 

structures-a Front Range geanticline on the sites of 

the Front Range and northern Park Range, and an 

Uncompahgre-San Luis geanticline on the sites of the 

Uncompahgre Plateau and San Juan Mountains (fig. 

20) (Mallory, 1972). 

In the Southern Rocky Mountains the gross patterns 

of the Precambrian rocks and structures are plainer 

than farther north because of the close proximity of the 

ranges, but they are confused in detail because of the 

more complex Phanerozoic events. Confusion is greatest in the Colorado mineral belt, where the Laramide 

plutonism, faulting, and mineralization are superposed 

on ancestral shear zones that originated during Precambrian time. Erosion and sedimentation resulting 

from the Paleozoic orogenies produced contrasts between the strata that lie on the Precambrian from place 

to place: lower Paleozoic shelf deposits in the troughs (as 

in the Sawatch Range), upper Paleozoic clastic deposits 

on the flanks of the geanticlines (as in the Front Range 

and Sangre de Cristo Mountains), and Mesozoic strata 

on the crests of the geanticlines (as in the Uncompahgr~ 

Plateau). 
SOUTHERN ROCKY MOUNTAINS 55 

PRECAMBRIAN X GNEISS COMPLEX a complex of paragneisses, in which are embedded 

numerous small to large granitic plutons. South of the 

The Precambrian of the Southern Rocky Mountains is Mullen Creek-Nash Fork discontinuity in southern 

-------

I 

I 

lo l:l~ .:t:lcG 

t;l~ lu 

I 

I 

I 

I 

WYOMING 

----COLORADO-----

= 

______________ j______ COLORADO 

I -~EW-MEXICO ___ _ 

EXPLANATION I 

~ I 

Colorado mineral belt 

f.@ t.\&tf\l\{@\l 

Geanticlines of late 

Paleozoic time 

~ "',.__ ""_, ~-j 

c~u~:=J 

Outcrops of Precambrian 

rocks 

0 

0 

I 

I 

I 

0 I 

I 

0 I 

100 

100 200 KILOMETRES 

200 Ml LES 

FIGURE 20.-Map of Southern Rocky Mountains in Colorado and New Mexico, showing outcrops of Precambrian rocks, outlines of the 

late Paleozoic geanticlines and the Colorado Mineral Belt, and localities mentioned in the text. Outlines of Colorado Mineral Belt 

and Paleozoic geanticlines after Tweto and Sims (1963, p. 997, 1007) and Mallory (1972, p. 132). 
56 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

Wyoming all the Precambrian rocks yield Hudsonian 

and later dates and no Kenoran dates are known. All 

the rocks of the Southern Rocky Mountains are therefore Precambrian X or younger and no rocks of Precambrian Ware identifiable, if indeed they ever existed. 

The dominant paragneiss (Idaho Springs Formation 

of Front Range) is a biotitic quartzo-feldspathic gneiss 

derived from an original thick geosynclinal sequence of 

shale and graywacke, in which are numerous lenses and 

interbeds of amphibolite (Swandyke Gneiss of Front 

Range), derived from original volcanic rocks. The only 

prominent variant is a thick synclinal mass of quartzite 

at the mountain front northwest of Denver (Wells and 

others, 1964). The gneisses have been plastically deformed into steep folds along northwest to westnorthwest axes, and metamorphosed to almandineamphibolite grade. Relicts of an earlier, more open folding of about the same trend can be detected in places, 

and superimposed on both sets of structures is a later 

cataclastic deformation that produced northeasttrending shear zones, especially in the Colorado mineral belt about midway along the Front Range and in 

the northern Sawatch Range (Tweto and Sims, 1963, p. 

996-1005). 

The main deformation of the paragneisses has been 

dated by rubidium-strontium methods on whole-rock 

and feldspar samples at 1,750 m.y.; the earlier deformation and the original accumulation of the sediments 

could have been no more than 100 m.y. earlier (Hedge 

and others, 1967, p. 555); the gneisses are accordingly 

classed as Xm on the Geologic Map. They may be the 

eugeosynclinal equivalent of the miogeosynclinal Precambrian X rocks of the northern Medicine Bow Mountains (Hills and others, 1968, p. 1777). 

PRECAMBRIAN X AND Y GRANITIC ROCKS 

Embedded in the paragneisses are granitic rocks 

which form nearly half the area of Precambrian exposure. In the Front Range and elsewhere they are divisible into three groups of different ages, each younger 

group emplaced at progressively shallower levels in the 

crust (Peterman and Hedge, 1968, p. 753-754). 

The oldest group (Xg), exemplified by the Boulder 

Creek Granite of the Front Range, forms concordant 

plutons in the paragneisses and is synorogenic to their 

principal deformation, with Hudsonian ages of about 

1,700 m.y. The much more extensive middle group 

(Y g1), exemplified by the Sherman Granite of the 

Laramie Range and the Silver Plume Granite of the 

Front Range farther south, is broadly contemporaneous 

with the final cataclastic deformation of the gneisses, 

and yields Elsonian ages of 1,390-1,450 m.y. The 

youngest group (Y g2), or Pikes Peak Granite, occurs 

only in the southern part of the Front Range, where it 

forms a great pluton with an exposed area of 1,200 mi2 

(3,100 km2), and several satellite bodies; it is postorogenic and has a Grenvillian age of about 1,040 m.y. 

(Hedge, 1970). 

PRECAMBRIAN OF NEEDLE MOUNTAINS 

The Precambrian of the Needle Mountains, at the 

edge of the San Juan volcanic field in southwestern 

Colorado, is more varied than elsewhere in the Southern Rocky Mountains, hence has long intrigued geologists, but relations have only been clarified recently by 

detailed mapping and by radiometric dating (Barker, 

1969; Bickford and others, 1967, p. 1660-1661) (fig. 21). 

An older metamorphic complex (Xm) consists of the 

Vallecito Conglomerate with clasts derived from still 

older terranes, followed by the Irving Formation of amphibolite, paragneiss, and metagraywacke. The complex was steeply folded along northerly to northeasterly 

axes, metamorphosed to amphibolite grade, and invaded by the synkinematic Twilight Granite and the 

postkinematic Tenmile Granite, with rubidium-strontium ages of 1,780 m.y. and 1,700-1,720 m.y., respectively. 

Lying with right-angled unconformity on the deeply 

eroded edges of the metamorphic and plutonic rocks is 

theUncompahgre Formation (Y), a supracrustal sequence of quartzite and interbedded slate more than 

8,000 ft (2,400 m) thick; it was steeply folded along 

west-northwest axes before intrusion of the Eolus 

Granite with an age of 1,460 m.y. (Y g1). Still younger 

minor granites with ages of1,350 m.y. intrude the rocks 

of the older complex in places. 

The time of accumulation and deformation of the Uncompahgre Formation is bracketed between the age of 

the youngest preceding granite (1,700 m.y.- and the age 

of the oldest succeeding granite (1,460 m.y.) (fig. 22). 

The formation is therefore early Precambrian Y 

(Paleohelikian of the Canadian classification), and thus 

probably largely older than the Precambrian Y Belt 

Supergroup of the Northern Rocky Mountains. Its deformation is an Elsonian event (the ((Uncompahgre 

orogeny" of local terminology)-a deformation of which 

there is little indication elsewhere in the western 

United States. 

EASTERN GREAT BASIN 

The eastern Great Basin in the western half of Utah 

and the eastern edge of Nevada is a region of interior 

drainage leading mainly into Great Salt Lake; it is a 

terrain of isolated or nearly isolated ranges that project 

from broad expanses of lowland floored by late Cenozoic 

deposits. Along its eastern border more cohesive 

plateaus and ranges face it in prominent escarpments, 
EASTERN GREAT BASIN 57 

of which the most notable are the Wasatch Mountains 

in the north, whose summits stand 7,000 ft (2,100 m) 

above the basin floor. Also included in this account are 

the Uinta Mountains, actually an outpost of the Southern Rocky Mountains, which extend 150 mi (250 km) 

eastward from the Wasatch Mountains at midlength. 

t ~tv: :v1 

Tertiary volcanic rocks 

Uncompahgre Formation 

(quartzite with argillite 

layers) 

0 

0 

Mesozoic and Paleozoic 

Tenmile Granite 1,700-1 ,720m.y. 

(and Bakers Bridge Granite) 

20 Kl LOMETRES 

EXPLANATION 

Trimble Granite 

1,350m.y. 

Twilight Granite Gneiss 

1,780m.y. 

20MILES 

Eolus Granite 

1,460 m.y. 

Irving Formation 

Diorite and gabbro 

Vallecito 

-

Conglomerate 

FIGURE 21.-Map showing Precambrian X andY units in Needle Mountains, southwestern Colorado. After Barker (1969), with Phanerozoic 

rocks added from other sources. 
58 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

FIGURE 22.-Synoptic diagram showing relations of units of Precambrian X andY in Needle Mountains, southwestern Colorado, and 

their implications in the Precambrian history of the area. Letter 

symbols the same as in fig. 21. After Barker (1969, p. AS). 

The gross forms of the region are of younger origin 

than those in the Rocky Mountains to the east and 

north-products of a late Cenozoic disruption that outlined the ranges and lowlands of the Great Basin by 

block-faulting, and separated it from the Wasatch 

Mountains and other uplands on the east. The late 

Cenozoic disruption is superposed on an earlier Cordilleran fabric, largely a product of Mesozoic orogenies, 

and especially of a mid-Cretaceous (Sevier) orogeny; 

this, in turn, is superposed on a preceding Phanerozoic 

miogeosynclinal regime. 

The late Cenozoic boundary between the disrupted 

region on the west and the more stable region on the 

east follows a persistent zone of weakness (Wasatch 

line), which had previously been the front of the Sevier 

orogenic belt, and the edge of the preceding 

miogeosyncline. The traces of the frontal thrusts of the 

orogenic belt are close to the block-faulted late Cenozoic 

boundary, lying west of it in the central Wasatch Mountains and south of the Wasatch Mountains, and east of it 

in the northern and southern segments of the Wasatch 

Mountains. About 100 mi (160 km) west of the frontal 

thrusts, near the Utah-Nevada border, a zone of decollement in the lower p3.rt of the miogeosynclinal sequence emerges in th1~ cores of the ranges which is 

probably genetically rdated to the frontal thrusts to the 

east. Beneath it is an infrastructure that was highly 

disturbed and metamorphosed during the Mesozoic 

orogenies. 

Within the region here considered, most of the exposed bedrock is Phanerozoic, but the underlying Precambrian emerges in small areas on the lower slopes of 

some of the ranges-mainly Precambrian Y and Z supracrustal rocks, but including some Precambrian X 

crystalline basement toward the east (fig. 23). As the 

frontal thrusts of the Mesozoic orogenic belt involve 

more than 30 mi (50 km) of eastward transport, great 

contrasts exist between the Phanerozoic and Precambrian rocks in the autochthon beneath the thrusts, and 

in the allochthon of the upper plates, contrasts which 

are especially evident in the Wasatch Mountains. 

Autochthonous Precambrian rocks occur in the 60--mi 

(95 km) middle segment of the Wasatch Mountains and 

in the Uinta Mountains to the east. Allochthonous Precambrian rocks lie above them in the upper plate of the 

frontal thrust (Willard thrust) in the northern segment 

of the Wasatch Mountains, but do not come to the surface at the front of the upper plate in the southern 

segment. In the discussion which follows, the autochthonous Precambrian will be treated first, after 

which the allochthonous Precambrian of the northern 

Wasatch Mountains and farther north and west will be 

considered. 

CRYSTALLINE BASEMENT (PRECAMBRIAN X) 

Crystalline basement (Xm) of the autochthon is exposed in the central segment of the Wasatch Mountains, 

in one of the islands in Great Salt Lake immediately to 

the west (fig. 23), and on the northern edge of the Uinta 

Mountains, 150 mi (250 km) to the east. 

The Farmington Canyon Complex (Eardley and 

Hatch, 1940a) forms the frontal ridge of the Wasatch 

Mountains for 25 mi (40 km) between Ogden and Salt 

Lake City. It is a body offelsic paraschist of amphibolite 

grade, derived from an original sedimentary sequence 

more than 10,000 ft (3,000 m) thick, that has been 

thoroughly permeated by granitic and pegmatitic 

material. About 15 mi (25 km) farther south the Little 

Willow Formation forms a small outcrop between the 

mouths of Big and Little Cottonwood Canyons (not 

shown on Geologic Map, but see fig. 23); it is likewise a 

paraschist but lacks injected material, in this respect 

resembling the Red Creek Quartzite of the Uinta Mountains (see below). The Little Willow is succeeded by 

Precambrian Y supracrustal rocks, but these wedge 

out northward, and the Farmington Canyon is overlain 

directly by the basal Cambrian Tintic Quartzite. 

The Farmington Canyon has yielded Hudsonian dates 

of 1,640--1,700 m.y. by potassium-argon methods 

on hornblende, and somewhat younger dates by 

rubidium-strontium methods on muscovite (Whelan, 

1970, p. 15-17). No Precambrian ages have been obtained from the Little Willow, and only dates between 

27-29 m.y. that were produced by the nearby Tertiary 

plutons. 

Near the east end of the Uinta Mountains on their 

north side the Red Creek Quartzite forms a small inlier 
EASTERN GREAT BASIN 59 

at the base of the Precambrian Y Uinta Mountain 

Group (Hansen, 1965, p. 22-32). Determinations on 

muscovite from the Red Creek by rubidium-strontium 

and potassium-argon methods have yielded ages of 

2,320 m.y. and 1,520 m.y., respectively. The larger 

figure is probably near the minimum age of the formation, which would place its accumulation in the early 

part of Precambrian X, or even in Precambrian W. 

Nevertheless, its general aspect is much like the Precambrian X rocks to the east and west, and it is so 

represented on the Geologic Map. 

BIG COTTONWOOD FORMATION (PRECAMBRI A N Y) 

In the Cottonwood area southeast of Salt Lake City, 

the Little Willow crystalline basement is followed unconformably by the Big Cottonwood Formation (Y), a 

16,000-ft (5,000 m) sequence of quartzites and interbedded variegated shales (Eardley and Hatch, 1940b, 

p. 819-820; Crittenden and others, 1952, p. 3-4). Its 

rocks are not metamorphosed except near the Tertiary 

plutons, and they dip gently eastward beneath the Precambrian Z and Cambrian supracrustal rocks farther 

back in the mountains. Ripple marks, crossbedding, and 

mud-flake conglomerates are well preserved, indicating 

deposition in shallow water. Like the other Precambrian supracrustal rocks of the eastern Great Basin, no 

radiometric data are available on the age of the Big 

Cottonwood, but on the basis of relations to the rocks 

above and below, it is commonly believed to be equivalent to some part of the Precambrian Y Belt Supergroup 

of the Northern Rocky Mountains. 

rvllNERAL FORK TILLITE A :'\D MUTU AL FORMATION 

(PRECAMBRIAN/.) 

In the upper reaches of Big Cottonwood and adjacent 

canyons, two higher Precambrian units intervene between the Big Cottonwood Formation and the Cambrian Tintic Quartzite-the Mineral Fork Tillite and 

Mutual Formation (Z) (fig. 23). 

The Mineral Fork Tillite, or diamictite, is a massive 

graywacke in which are embedded numerous clasts of 

all sizes up to large boulders, with interbedded layers of 

quartzite and laminated argillite. Some of the clasts are 

striated, and all are of Precambrian crystalline basement, such as granite gneiss, quartzite, and dolomite. 

The deposit thickens and thins over the eroded surface 

of the Big Cottonwood Formation, reaching more than 

1,000 ft (300m) in broad, smooth-bottomed basins, and 

nearly disappearing in the intervening areas. A glacial 

origin for the deposit was proposed by various early 

geologists, such as Blackwelder (1932, p. 301-303), and 

has been reaffirmed by some modern observers (Crittenden and others, 1952, p. 4--6), but questioned by 

others (Condie, 1967, p. 1,341-1,342), who compare it 

with subaqueous mudflows and turbidites of other regions. Such features could, of course, be one of the manifestations of a general glacial episode, and the reality of 

such an episode is strongly suggested by the regional 

occurrence of the Mineral Fork and correlative diamictites throughout the eastern Great Basin (Crittenden 

and others, 1972). 

The Mutual Formation is a body of red-purple 

quartzites and red to green shales as much as 1,200 ft 

(360 m) thick, which lies on the eroded surface of the 

Mineral Fork, and is truncated in turn by the Tintic 

Quartzite. 

The position of the Mineral Fork and Mutual Formations between the Big Cottonwood Formation and the 

Tintic Quartzite implies a late Precambrian, and probably a Precambrian Z age, comparable to that of the 

Windermere Group in the Northern Rocky Mountains. 

Lll\TA MOL;l\TAIJ\ GROUP (PRECAMBRIAN Y) 

The Uinta Mountains, like the other ranges of the 

Central and Southern Rocky Mountains, are a broadbacked anticlinal uplift, in part faulted on the flanks, 

with a large area of Precambrian exposed in the core. 

The core rocks are, however, not a crystalline basement, 

but a thick supracrustal clastic sequence, the Uinta 

Mountain Group (Y). 

The Uinta Mountain Group lies on the crystalline 

basement of the Red Creek Quartzite, exposed near the 

eastern end of the range, and is moderately unconformable beneath the Paleozoic strata on the flanks, which 

include discontinuous thin Cambrian units at the 

base-Middle Cambrian to the west, Upper Cambrian 

to the east. The group is broadly arched, in conformity 

with the general uplift of the range, and is virtually 

unmetamorphosed. In the eastern exposures a complete 

sequence between the Red Creek Quartzite and the 

Paleozoic is 25,000 ft (7 ,600 m) thick (Hansen, 1965, 

p. 33); farther west, where the basement does not crop 

out, no more than about 10,000 (3,000 m) is exposed 

(Wallace and Crittenden, 1969, p. 129). 

The Uinta Mountain Group contains various mappable subdivisions, but the only formally named unit is 

the Red Pine Shale, at the top in the western half of the 

range, a body of dark shale, siltstone, and minor 

quartzite as much as 5,000 ft (1,500 m) thick. The underlying main body of the group is dominantly quartzite 

and arkose, with shale only as thin interbeds. Three 

different facies are recognizable, representing contrasting sedimentary environments: deltaic-fluvial, 

fluvial-flood plain, and paralic-neritic (Wallace and 

Crittenden, 1969, p. 134--137). Sediments were derived 

from bordering lands of crystalline basement to the 
60 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

north and northeast, and were transported westward 

along the axis of the depositional trough. Quartzites 

and arkoses are especially coarse and massive in the 

eastern exposures, where they contain thick wedges of 

pebble and cobble conglomerate formed of rounded 

quartz and quartzite clasts (Hansen, 1965, p. 36-37). 

Both the present extent of the Uinta Mountain Group 

beyond its outcrops, and its original extent, are difficult 

41 

to determine because the Uinta Mountains are flanked 

north and south by the Green River and Uinta basins, 

filled by Phanerozoic sediments so thick that their base 

has not been reached by drilling. Nevertheless, the 

group seems to be an unusual eastward extension of 

Precambrian supracrustal rocks into a domain otherwise formed of crystalline basement. Such crystalline 

rocks (with Kenoran dates) are known some distance to 

0 0 0 

o oo ocO 

<:) 0 0 ° 0 0 

0 0 0 0 f) 0 0 0 0 

'U 0 0 0 0 0 

0 0 

0 0 0 

0 

0 0 

0 0 

FIGURE 23.-Geologic map of northeastern Utah, showing Precambrian X, Y, and Z rocks in eastern Great Basin, and the adjoining 

mountains and plateaus to the east, which are parts of the allochthon and autochthon of the Cretaceous Sevier orogenic belt. Com- • 

piled from Geologic Map of Utah (1963), with additions from published and manuscript data of M. D. Crittenden, Jr. 
EASTERN GREAT BASIN 61 

the north, near the Wind River Mountains, and others 

(with Hudsonian dates) are known from drilling along 

the south edge of the Uinta basin (Muehlberger and 

others, 1966, p. 5425). These occurrences define vaguely 

an east-trending belt of Precambrian supracrustal 

rocks that is probably a primary feature, as attested by 

the sedimentary facies in the group itself. The belt is 

comparable to the aulacogens of the Soviet geologists, or 

early sedimentary troughs which extend transversely 

into the craton. 

The westward prolongation of the Uinta Mountain 

trough is in the Cottonwood area of the autochthonous 

central segment of the Wasatch Mountains, with its 

sequence of Precambrian Y and Z supracrustal rocks, 

already described. Its northern flank is in the northern 

part of the autochthonous segment, where the supracrustal rocks are missing, and the basal Cambrian lies 

directly on the earlier crystalline basement of the 

Farmington Canyon Complex (fig. 23). 

The age of the Uinta Mountain Group has aroused 

speculation since the first geological explorations of the 

Uinta Mountains a century ago. Former proposals that 

it is of early or even late Paleozoic age are obsolete, as its 

unconformable position beneath Cambrian strata is 

now established. Modern speculation centers around its 

precise correlation with the Precambrian supracrustal 

units in the autochthonous segment of the Wasatch 

Mountains to the west. Is it equivalent, wholly or in 

part, to Precambrian Y Big Cottonwood Formation, or 

EXPLANATION 

~ 

~ 

to the younger Precambrian Z Mineral Fork and 

Mutual Formations? No tillites (diamictites) like those 

in the Mineral Fork occur in the Uinta Mountains, yet a 

considerable part of the Uinta Mounta,in Group beneath 

the Red Pine Shale is lithically much like the Mutual 

Formation, and has been so correlated. Nevertheless, 

the marked variations in sedimentary facies within the 

Uinta Mountain Group itself warn of the dangers of 

correlations on lithology alone, and it might be an onshore phase of the Big Cottonwood Formation. The best 

present judgment on paleogeographic grounds seems 

to be that the Uinta Mountain Group is Precambrian Y 

(Crittenden and others, 1972, p. 337), a decision adopted 

on the Geologic Map. Recently a whole rock isochron by 

rubidium-strontium methods of 950 m.y. has been obtained from the Red Pine Shale at the top of the sequence (Zell Peterman, written commun. 197 4), which 

confirms this inference. 

SUPRACRUSTAL ROCKS OF THE ALLOCHTHON 

(PRECAMBRIAN Z) 

Along the eastern edge of the Great Basin, Precambrian rocks are exposed in widely separated ranges of 

300 mi (480 km), from southeastern Idaho to southcentral Utah. In Idaho, they occur near Pocatello, in 

ranges immediately south of the Snake River Plain. In 

Utah, they are exposed in the northern segment of the 

Wasatch Mountains, in the Promontory Range west of 

Quaternary 

intermontane deposits Tertiary postorogenic deposits 

(includes some Upper Cretaceous) 

Tertiary intrusive 

rocks 

FIGURE 23.-Continued. 

Precambrian Z 

supracrustal rocks 

Precambrian Z 

supracrustal rocks 

Farmington Canyon 

Complex 

ROCKS OF THE ALLOCHTHON 

Cambrian 

ROCKS 0 F THE AUTOCHTHON 

Cambrian 

lxw-fj 

Little Willow 

FOrmation 

Paleozoic 

Paleozoic and Mesozoic 

Precambrian Y 

supracrustal rocks 
62 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

it, and in the Sheeprock, Dugway, Canyon, and Beaver 

Ranges further south (fig. 23). Except for rocks on one 

island in Great Salt Lake west of the autochthonous 

segment, those of all these exposures are allochthonous, 

and in the upper plates of thrusts of the Sevier orogenic 

belt. All the rocks are supracrustal and part of Precambrian Z. No Precambrian Y supracrustal rocks are 

visible, and with one minor exception, no crystalline 

basement; the extent of the older Precambrian rocks in 

the area, if they exist, is indeterminate. 

Equivalents of the two relatively thin, unconformity-bounded Precambrian Z units of the autochthon 

(Mineral Fork and Mutual) occur in the allochthonous 

rocks, but here they are widely separated in a much 

thicker conformable sequence. In the well-known outcrops on the upper plate of the Willard thrust in the 

northern Wasatch Mountains, the sequence beneath 

the Cambrian is 13,000 ft (4,000 m) thick, but it reaches 

20,000 ft (6,100 m) in the structurally more complex 

sequence near Pocatello, as deciphered by D. E. Trimble 

(Crittenden, Schaefer, Trimble, and Woodward, 1971, p. 

582-594). Partial sequences preserved in the ranges 

farther south are thinner. 

In the upper plate of the Willard thrust in the 

Wasatch Mountains the supracrustal sequence bottoms 

on a thin wedge of crystalline basement (not on Geologic 

Map, but see fig. 23), which has been dated by 

rubidium-strontium methods on muscovites as between 

1,600 and 1,800 m.y. (Crittenden, McKee, and Peterman, 1971), or approximately correlative with the autochthonous Farmington Canyon Complex exposed to 

the south. 

Tillite (diamictite) like that in the Mineral Fork occurs low in nearly all the sequences, and in the Pocatello 

sequence contains an interbedded member of 

greenstone flows and tuffs, reminiscent of the Irene 

Volcanics intercalated in the diamictites of the Windermere Group farther north. A unit lithically identical 

with the Mutual Formation of the autochthon occurs 

much higher, the intervening strata being shale and 

siltstone with one or more thick quartzite units and in 

places a thin limestone layer. The rocks above the 

Mutual equivalent are n1ainly quartzites, traditionally 

called Brigham, Tintic, or Prospect Mountain depend-

. ing on locality and considered to be basal Cambrian. 

However, only their upper parts can be proved paleontologically to be of Cambrian age, and the lower parts 

may be Precambrian Z; these lower parts are now 

mostly given other formational names. A volcanic breccia in the Browns Hole Formation, between the Mutual 

and the upper quartzites, has yielded an argon-argon 

date on hornblende of 570 m.y. (Crittenden and Wallace, 1973, p. 128), which indicates that it lies close to the 

Precambrian-Cambrian boundary. 

SUPRACRUSTAL ROCKS OF UTAH-NEVADA BORDER 

To the west, near the border between Utah and 

Nevada, Precambrian Z supracrustal rocks reappear in 

various mountains, from the Pilot Range 150 mi 

(250 km) southward to the southern Snake Range, and 

westward for 50 mi (80 km) into Nevada. They much 

resemble the supracrustal rocks just described, but are 

separated from them by a broad gap, mostly occupied by 

the Great Salt Lake Desert, and their structural setting 

differs. All ofthem lie in an infrastructure beneath the 

regional decollement mentioned earlier, and have been 

subjected to low- to medium-grade metamorphism during the Mesozoic orogenies. 

The rocks are termed the McCoy Creek Group from a 

locality in the Schell Creek Mountains, Nevada, where 

there is a 9,000-ft (2,700 m) sequence beneath the 

Cambrian Prospect Mountain Quartzite (Misch and 

Hazzard, 1962, p. 307-320). The group is nearly as thick 

in the Deep Creek Range, and 3,600 ft (1,100 m) of the 

upper part is preserved in the southern Snake Range. 

The rocks include several persistent quartzite units 

separated by units of argillite and siltstone, with a few 

minor layers of marble. The sequence in the Deep Creek 

Range, farther east than the rest, includes several horizons of ((tillitic schist," originally a diamictite of sand 

and silt with widely dispersed dropstones up to boulder 

size, of granite, gneiss, and quartzite; it is the water-laid 

distal end of the tillites (diamictites) farther east in the 

Great Basin. 

The lowest rocks exposed in the sequences are commonly the most strongly metamorphosed, but are conformable with those above, and no earlier basement 

emerges south of the Albion and Raft River Ranges 

(p. 48). The upper beds have a sharp boundary with the 

overlying Prospect Mountain Quartzite, but the succession is seemingly conformable from one to the other. 

SOUTHERN BASIN AND RANGE PROVINCE 

In the final section of this review the exposed Precambrian of the southwestern United States will be 

considered, from southern California 800 mi (1,300 km) 

eastward to western Texas, and 700 mi (1,100 km) or 

more north from the Mexican border. Although this 

large region is diverse in terms of modern morphology 

and Phanerozoic structure, its Precambrian rocks have 

a certain homogeneity that facilitates description. 

Moreover, the largest area of Precambrian outcrop and 

the most significant sequences are in Arizona, and these 

furnish standards with which the remainder can be 

compared. 

The southern Basin and Range province is a terrain of 

block-faulted mountains and intervening lowlands and 

deserts, much like that of the eastern Great Basin 
SOUTHERN BASIN AND RANGE PROVINCE 63 

treated in the previous section. Here, however, most of 

the drainage is exterior, leading into the Colorado River 

and its tributaries. In the lower ((desert" region of 

southwestern Arizona and southeastern California, 

many of the block ranges have been so eroded that their 

original structural forms have been lost, and they stand 

as islands in a much broader sea of lowlands. On the 

other hand, in the ((mountain" region of Arizona farther 

north, along the edge of the Colorado Plateau, the block 

ranges coalesce so that outcrops of Precambrian rocks 

are nearly continuous, except for outliers and 

downfaulted strips of Phanerozoic rocks. Included in 

this account are also the classic, long-known Precambrian rocks that form the lower walls of the Grand 

Canyon within the Colorado Plateau, and the Precambrian rocks of the Transverse Ranges of southern 

California, both lying in Phanerozoic morphological 

and tectonic settings different from the rest. 

The most extensively exposed Precambrian rocks in 

much of the region, and especially in Arizona, are the 

crystalline basement of Precambrian X-paraschists, 

paragneisses, orthogneisses, and granites-in which 

are embedded a few younger plutons of Precambrian Y. 

Lying on their deeply eroded surface, and preserved in 

smaller areas, are little-deformed supracrustal rocks of 

Precambrian Y, including such units as the Grand Canyon Supergroup and the Apache Group of Arizona. 

Younger supracrustal rocks of Precambrian Z form still 

smaller areas, mainly in southwestern Nevada and 

eastern California. As in the Southern Rocky Mountains and eastern Great Basin, no rocks of Precambrian 

W have been identified in the region, if indeed they ever 

existed. 

CRYSTALLIN£ BASEMENT OF ARIZONA 

(MAINLY PRECAMBRIAN X) 

In Arizona (and elsewhere in the southwestern 

United States) the crystalline basement is divided on 

the Geologic Map into metasedimentary rocks (X), orthogneiss and paragneiss (Xm), and granitic rocks (Xg, 

Y g1, and Y g2). 

These units are modified from those of the Geologic 

Map of Arizona of 1969 as follows: Our unit X includes 

schist, greenstone, rhyolite, and Mazatzal Quartzite 

(units p-€sc, p£gs, p£ry, and p£m of the Arizona Map); 

because of the small scale of the United States Map the 

metavolcanics are grouped with the metasediments. 

Our unit Xm includes Precambrian gneisses (p£gn), as 

well as so-called ((Mesozoic" and ((Cretaceous-Tertiary" 

gneisses ( 1\kgn, TKgn), which are largely Precambrian 

in origin, but were reworked during Phanerozoic 

orogenies. Our units Xg, Y g1, and Y g2 are the granite, 

quartz monzonite, and quartz diorite of the Arizona 

Map (p£gr), which we have subdivided according to 

their radiometric ages. The Precambrian diorite and 

pyroxenite (p£di, p£py) shown on the State Map are too 

small to be shown on the United States Map. 

The metamorphic rocks of Arizona traditionally have 

been called the Vishnu Schist to the north in the Grand 

Canyon, the Yavapai Schist in the central region, and 

the Pinal Schist in the southeastern region, all supposedly more or less correlative. More information is 

available now on all these units, although the schists 

and gneisses of the desert region to the southwest remain poorly understood. Modern radiometric work indicates that all these metamorphic rocks and most of the 

granites which intrude them have Hudsonian ages between 1,650 and 1,850 m.y., thus placing them in the 

later part of Precambrian X, but they are not necessarily correlative, and some of them are clearly younger 

than others. 

The Vishnu in the Grand Canyon includes the 

quartzose, micaceous Vishnu Schist (restricted) and the 

mafic Brahma Schist, the first derived from sediments, 

the second from volcanics; the whole forming an original sequence tens of thousands of feet thick. This was 

steeply folded along northeast axes, metamorphosed, 

and pervasively injected by the Zoroaster Granite 

(Maxson, 1961) (not separated on the map). The Zoroaster has yielded an age of 1,725 m.y. by uranium-lead 

methods on zircons, which indicates the minimum age 

of the whole assemblage (Pasteels and Silver, 1966), 

although dates as low as 1,390 m.y. have been obtained 

from it and the adjoining schists by rubidium-strontium 

methods (Giletti and Damon, 1961, p. 640). 

The Pinal Schist of southeastern Arizona, as represented in the Dragoon quadrangle, is a similarly thick 

body, derived from original graywacke and slate with 

interbedded felsic and mafic volcanics, steeply folded 

along northeast axes, metamorphosed to greenschist or 

amphibolite grade, and intruded by granodiorite and 

granite (Cooper and Silver, 1964, p. 11-34). The rhyolites have yielded an age of1,715 m.y. and the intrusive 

granodiorite an age of 1,615-1,630 m.y. by uraniumlead methods on zircons, suggesting that these rocks are 

somewhat younger than those in the Grand Canyon. 

The Yavapai Schist and associated metamorphic 

rocks of central Arizona, exposed in broader, more continuous areas than the Vishnu and Pinal, are more 

varied. They have been deformed along northeasttrending axes and subjected to greenschist or low amphibolite grades of metamorphism, but original 

sedimentary and volcanic structures are commonly well 

preserved. Wilson (1939, p. 1117-1129) was one ofthe 

first to demonstrate that the Yavapai rocks are divisible 

into distinctive, mappable formations of which he 

named nearly a dozen that he correlated between dis-
64 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

tricts. His original classification has been amplified and 

emended by further mapping, and by radiometric dating that has shown more clearly the relative ages of the 

units. 

The type Yavapai area is in the center of the State 

near Prescott and Jerome, where the sequence includes 

the Ash Creek and Big Bug Groups, each about 20,000 ft 

(6,100 m) thick, both composed of felsic and mafic volcanic and volcaniclastic rocks, and interbedded sediments (Anderson, 1968, p. 14-17) (fig. 24). They are 

separated from each other by granitic plutons and by a 

major north-south strike-slip fault, but radiometric dating by uranium-lead methods on zircons indicates that 

the Ash Creek is the older, the collective ,age of the two 

groups being between 1,775 and 1,820 m.y. (Anderson 

and others, 1971). On this basis the two are considered 

to represent a time-stratigraphic unit, and are called 

Yavapai Series. In separate fault blocks in the same 

area the Texas Gulch Formation lies with basal conglomerate on the Brady Butte Granodiorite. Formerly 

the Texas Gulch was supposed to represent the base of 

the Yavapai sequence, but the granodiorite has an age 

of 1,770 m.y., so that the Texas Gulch is the youngest 

rock in the district, and is excluded from the Yavapai 

Series as now defined. 

In the Mazatzal Mountains farther east, which were 

studied in most detail by Wilson, the Yavapai rocks 

include several units of greenstone, rhyolite, and volcaniclastic sediments which are so complexly faulted 

that their original sequence is conjectural. Only the Red 

Rock Rhyolite is in stratigraphic continuity with the 

uppermost rocks, the prominent, well-bedded Mazatzal 

Quartzite and the minor underlying Maverick Shale 

and Deadman Quartzite (Wilson, 1939, p. 1134-1137). 

The Red Rock has been dated by uranium-lead methods 

at 1,715 m.y. (Silver, 1965), suggesting that a considerable part of the rocks in the Mazatzal Mountains is 

younger than the Yavapai Series as now restricted. 

In the Diamond Butte area a little farther east, on 

the north flank of the Sierra Ancha, Gastil (1958) has 

mapped in detail rocks like those in the Mazatzal Mountains, forming a 20,000-ft (6,100 m) sequence without 

top or base, divided into eight named formations, 

mainly volcanic or volcaniclastic. The apparent equivalent of the Mazatzal Quartzite (Houden Formation) is 

near the middle, and is followed by younger volcanics 

apparently unrepresented farther west. 

Other areas of Yavapai-type rocks in central Arizona 

could be mentioned (see, for example, Livingston and 

Damon, 1968, p. 765-769), but the above are sufficient 

to indicate their features, their complexities, and their 

problems in correlation; further studies are needed before the rocks of the whole area can be integrated into a 

single picture. 

The metamorphic rocks of central Arizona are extensively invaded by synkinematic and postkinematic 

granitic plutons (Xg). In the Prescott-Jerome area and 

farther west these have uranium-lead ages between 

1,760 and 1,775 m.y., suggesting that their intrusion 

was partly contemporaneous with the later volcanism of 

the Yavapai Series. Farther east and southeast, as far 

as the Pinal Schist area mentioned earlier, similar 

granites are slightly younger, with ages of about 1,660 

m.y. A later suite of granitic rocks (Y g1), typified by the 

Ruin and Oracle Granites of the districts between the 

Sierra Ancha and Tucson, has been dated as between 

1,420 and 1,460 m.y. by various methods, and a single 

body in Weaver Mountain south of Prescott (Y g2) has 

yielded a uranium-lead age of 1,000 m.y. 

The deformation, metamorphism, and plutonism of 

the crystalline basement of central Arizona (and 

elsewhere in the southern Basin and Range province) 

preceded the accumulation of the Precambrian Y supracrustal rocks described below, which lie on its truncated, deeply eroded surface. This represents the 

((Mazatzal Revolution" (orogeny) of Wilson (1939, 

p. 1161). For a time this orogeny was thought to have an 

Elsonian age of 1,350-1,550 m.y. (Giletti and Damon, 

1961, p. 642), but this interpretation was based on insufficient sampling, mainly of postorogenic plutons; its 

true Hudsonian date is shown by more complete studies 

to lie between 1,660 and 1,715 m.y. (Wasserburg and 

Lanphere, 1965, p. 736; Silver, 1965). 

CRYSTALLINE BASEMENT OF SOUTHERN CALIFORNIA 

(MAINLY PRECAMBRIAN X) 

Crystalline basement like that in Arizona, and again 

dominated by Hudsonian dates, is exposed in many of 

the ranges in southern California, from Death Valley on 

the north, southward through the Mojave Desert, into 

the Transverse Ranges. In this region, the basement 

rocks have been more involved in and overprinted by 

the effects of Phanerozoic orogenies than those farther 

east. 

In southern Death Valley Precambrian crystalline 

basement forms the prominent, rugged part of the Black 

Mountains on the eastern side, and inliers in the later 

Precambrian supracrustal rocks (Y and Z) of the 

Panamint Range on the western side. In the Panamint 

area, earlier paragneisses and orthogneisses are cut by 

intrusives which have yielded Hudsonian dates of 

1,720-1,780 m.y. by uranium-lead methods on zircons, 

indicating the minimum age of the whole complex 

(Silver and others, 1962). Rubidium-strontium and 

potassium-argon ratios in the rocks have been so disturbed by Mesozoic metamorphism that they give unreliable results (Wasserburg and others, 1964, p. 4400). 

Smaller, more dispersed outcrops of basement occur 
... . . 

~·· · . · 

· ... . : 

Quaternary 

intermontane deposits 

0 

Texas Gulch Formation 

Unconformable on granodiorite 

SOUTHERN BASIN AND RANGE PROVINCE 

. ·.Q 

20 Ml LES 

20 Kl LOMETRES 

EXPLANATION 

Tertiary sediments Tertiary volcanic rocks 

Granodiorite and quartz 

diorite 1,760-1,770 m.y. ~~ 

YAVAPAI SERIES 

1,770-1,820 m.y. 

Paleozoic 

Big Bug Group Ash Creek Group 

Phanerozoic faults Precambrian faults 

FIGURE 24.-Map of Prescott-Jerome area, central Arizona, showing relations of Yavapai Series and other 

Precambrian X rocks. After Anderson and others (1971, p. C4-C6), with Phanerozoic rocks added from 

other sources. 

65 
66 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

farther south in the Mojave Desert. Dating of these 

rocks by potassium-argon and rubidium-strontium 

methods indicates Hudsonian metamorphism of the 

country rock about 1,650 m.y. ago, but a few of the 

plutons (as in the Marble Mountains) have an Elsonian 

age of about 1,400 m.y. (Lanphere, 1964, p. 39~397); 

these have not been separated from the Precambrian X 

rocks on the Geologic Map. 

Of greater interest than these are the Precambrian 

rocks of the Transverse Ranges to the southwest, which 

are nearer the Pacific Coast than any others in the 

United States. They are part of the crystalline complex 

of the rugged San Gabriel and San Bernardino Mountains, whose peaks project to altitudes of10,000 ft (3,000 

m) or more. The mountains are upthrust blocks within 

the network of strike-slip faults of coastal California, 

and lie on opposite sides of the master San Andreas fault 

(Dibblee, 1968). 

The crystalline complexes of the two ranges exhibit a 

remarkable array of metamorphic and plutonic rocks 

diverse ages, including upper Mesozoic eugeosynclinal 

rocks (Pelona Schist, u 1\ke), Paleozoic miogeosynclinal 

quartzites and marbles (ul?), Mesozoic granitic plutons 

with ages of75-90 m.y. and 160-170 m.y. (Kg), and the 

upper Paleozoic Mount Lowe Granodiorite with an age 

of 220 m.y. ( l?g3). These lie in a Precambrian matrix, 

now preserved only in shreds and patches, or orthogneiss and paragneiss of granulite facies (Xm), into 

which a large body of anorthosite (Ya) has been intruded in the western part of the San Gabriel Mountains (Crowell and Walker, 1962, p. 242-261). 

South of the Tranverse Ranges, across the Los 

Angeles Basin and San Gorgonio Pass are other crystalline massifs of the Peninsular Ranges, but their rocks 

are curiously different, and are all Paleozoic and 

Mesozoic in origin. 

In the Transverse Ranges, geologic studies and 

radiometric dating of the Precambrian rocks (especially 

by uranium-lead methods on zircons) indicate a complex structural and metamorphic history, during both 

Precambrian time and later (Silver and others, 1963; 

Silver, 1971). Supracrustal sedimentary and volcanic 

rocks accumulated between 1,680-1,750 m.y. ago, and 

were deformed and me·camorphosed to amphibolite 

grade. They were invaded by granodiorite and quartz 

monzonite 1,650-1,680 m.y. ago, and the whole was 

subjected to a major orogeny 1,425-1,450 m.y. ago that 

refolded the rocks and raised them to granulite 

metamorphic grade. At about 1,220 m.y. ago the body of 

anorthosite and associated gabbro and syenite was intruded into the complex; there were no further Precambrian events, but the anorthosite was greatly disturbed and sheared during the Phanerozoic (Carter and 

Silver, 1971). 

The diverse Precambrian events in the crystalline 

rocks of the Transverse Ranges contrast markedly with 

the simpler events in the regions to the east and north, 

where there was merely a Hudsonian metamorphism 

and plutonism, and a rare Elsonian plutonism. The 

contrast lends credence to the interpretation derived 

from geological evidence that the rocks of these ranges 

are far away from their original positions, whence they 

have been transported by strike-slip movements along 

the San Andreas and related faults, into the foreign 

environment of the Pacific border region. Traces of similar rocks occur in the Orocopia Mountains and nearby 

ranges northeast of the Salton Sea (Crowell and 

Walker, 1962, p. 222-242), but even these are merely in 

wedges in the broader fault network. The original sites 

of all these rocks are still farther southeast, in some 

region as yet unidentified. 

SUPRACRUSTAL ROCKS IN ARIZONA 

(MAINLY PRECAMBRIAN Y) 

Lying on the truncated and deeply eroded edges of the 

crystalline basement just described, especially northeastward toward the cratonic Colorado Plateau, are 

unmetamorphosed and only lightly deformed supracrustal sedimentary rocks, with minor interbedded 

lavas and intrusive diabase. These are shown on the 

Geologic Map of Arizona of 1969 as Grand Canyon 

Series, p£g; Apache Group, p£a; Troy Quartzite, p£t; 

and diabase, p£db. On the Geologic Map of the United 

States all these rocks, including the diabase but excluding the Chuar Group at the eastern end of the Grand 

Canyon, are grouped together as unitY. The Chuar, for 

reasons indicated later, is labeled Z. 

The supracrustal rocks are exemplified especially by 

the well-known Grand Canyon Supergroup exposed in 

the depths of the Grand Canyon, but the Apache Group 

and Troy Quartzite farther south in Arizona are very 

much like it and probably correlative in part. As with 

the Belt Supergroup of the Northern Rocky Mountains, 

their fresh appearance belies their ancient age, leading 

to a first impression that they are early Paleozoic-an 

impression dispelled early in the Grand Canyon by 

Walcott (1895, p. 313-314), but which persisted much 

later in central Arizona, until disproved by Darton 

(1925, p. 34-36). 

In the frequently visited, prominent exposures in the 

main segment of the Grand Canyon, the lower part of 

the Grand Canyon supergroup (Unkar Group) is tilted, 

block-faulted, and truncated by the flat-lying Middle 

Cambrian Tapeats Sandstone (fig. 25). The upper part 

(Chuar Group) in the seldom visited eastern alcoves of 

the canyon is less faulted and synclinally downwarped. 

Some of the faults were displaced only during the Precambrian, but others were reactivated later and offset 

the Paleozoic rocks by varying amounts. The most spectacular example is the Butte fault in the eastern part of 
(/) 

w 

a: 

1-

w 

~ 

0 

0 

0 

1-

w 

w 

u. 

0 

0 ·· .. 0 

N 0 

0 

If) 

0 ~ 

0 

0 

0 0 

0 

0 

SOUTHERN BASIN AND RANGE PROVINCE 

.... 

(I) .. . ;::... . ----- -------------------------------------------

-~--

0 

0 

Kaibab Limestone 

(P) 

2 MILES 

3 KILOMETRES 

Chuar Butte 

3000 FEET EAST 

~::::::: ~- ~ ~ \\ ,, ,, 'l 'l- \\ ,, 

Dripping Spring Quartzite 

1000 METRES 

B 

----=:=----------=-------_---------==--==-Pioneer Shale-==-_-----==-----==---_----====----_-

0 4 MILES 

0 5 Kl LOMETRES 

67 

NORTH 

A 

c 

FIGURE 25.- A, Section showing Vishnu Schist and Unkar Group (Precambrian X andY) in the Shinumo area, Grand Canyon, northern 

Arizona, and the truncation of their block-faulted structure by Cambrian deposits. Note, however, that the major fault on the right 

underwent recurrent reversed displacement after Paleozoic time . After Noble (1914, section B-B'). B, Section of Butte fault in eastern 

Grand Canyon, Ariz., showing Precambrian downthrow to the left and post-Paleozoic downthrow to the right, each accompanied by 

steep dragging of the beds. After Walcott (1889, p. 53). C, Idealized section, showing disruption and distention of Apache Group and 

Troy Quartzite by sills and dikes of intrusive diabase. Based on outcrops in the Sierra Ancha and nearby localities, central Arizona. 

After Shride (1967, p. 67). 
68 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

the, canyon, which was downthrown 5,000 ft (1,500 m) to 

the west in Precambrian time and 2, 700ft (820 m) to the 

east after Paleozoic time, each displacement being accompanied by steep dragging of the beds (Walcott, 1889) 

(fig. 25). The Apache Group and Troy Quartzite were 

little disturbed during Precambrian time, except for 

profuse injection of diabase sills and connecting dikes 

that have much disrupted and greatly distended the 

sequence (fig. 25). Along the Colorado Plateau margin, 

as in the Sierra Ancha, they are almost as little disturbed by Phanerozoic movements as in the Grand 

Canyon, but farther southwest they share the complex 

block faulting of the succeeding Paleozoic strata. 

The supracrustal rocks in the Grand Canyon and 

central Arizona are divisible into persistent, distinctive 

formations, which are listed in table 4. The Apache 

Group and Troy Quartzite are obvious equivalents of 

the Unkar Group and contain identical rocks, but the 

order of the lithic units is strangely different in the two 

areas. The Bass Limestone is nearly at the base of the 

U nkar sequence and the Mescal Limestone is a 

thousand feet (300 m) or more above the base of the 

Apache. The red Hakatai Shale is above the Bass and 

the_ red Pioneer Shale is below the Mescal. The Troy 

Quartzite is at the top of the central Arizona sequence 

and the Shinumo Quartzite is beneath thick higher 

formations of the Unkar Group. Each set of formations 

persists within its own area, and the reasons for the 

reversals from one area to the other are not apparent. 

The differences in thickness of the sequences in the two 

areas are also of interest; the Apache and Troy are less 

than half as thick as the Unkar Group. The first two 

units may be a shelf or platform facies, farther away 

from the center of the depositional basin than the 

Unkar. 

The Troy Quartzite has been poorly understood until 

recently (Shride, 1967, p. 44-45); its full thickness and 

subdivisions could only be deciphered from detailed 

work, which involved untangling the structure produced by the many diabase sills (fig. 21). Even after the 

Precambrian age of the underlying Apache Group was 

established, the Troy was long considered to be partly or 

wholly of Cambrian age, and equivalent to the Middle 

Cambrian Bolsa Quartzite. Actually, the Troy is overlain unconformably by the Bolsa, or by sandy phases of 

the succeeding Cambrian Abrigo Limestone and Devonian Martin Limestone (Krieger, 1968). Even though 

the Precambrian quartzites are everywhere overlain by 

Paleozoic sandstones and quartzites, the Troy is intruded by diabase and the higher strata are not; their 

basal beds frequently contain diabase debris, including 

cobbles and boulders in a few places. 

Both the Mescal Limestone and Bass Limestone contain stromatolites at several levels; those in the Mescal 

TABLE 4.-Precambrian supracrustal rocks of Arizona 

Grand Canyon Centra l Arizona 

(Walcott, 1895; Noble, 1914; (Darton , 1925; Shride, 1967 l 

Ford a nd Breed, 19731 

Cambrian Cambrian or Devonian 

M a j o r unconformity M a j or unconformity 

Grand Canyon Supergroup 

Chuar Group, 6,000 ft (2,000 ml 

Sixty Mile Formation 

Kwagunt Formation 

Galeros Formation 

D i s c 0 n f 0 r m i t y Troy Quartzite, 1,200 ft (360 ml 

maximum 

D i s c 0 n f 0 r m i t y Unkar Group, 5,500 ft (1,700 m l 

Nankoweap Formation Apache Group, 1,250-1,600 ft (380-490 

Rama (=Cardenas) Basalt ml 

Dox Sandstone Mescal Limestone 

Shinumo Quartzite (basalt flow in upper part) 

Hakatai Shale Dripping Spring Quartzite 

Bass Limestone (with Barnes Conglomerate member) 

Hotauta Conglomerate Pioneer Shale 

M a j o r unconformity (with Scanlan Conglomerate Member) 

M a j o r unconformity 

Crystalline basement Crystalline basement 

are comparable to a lower Middle Riphean form and to a 

Middle Riphean to Vendian form of the sequences in the 

Soviet Union (Cloud and Semikhatov, 1969, p. 1031). 

Other fossils have been reported in the Arizona supracrustal rocks, but nearly all of them are inorganic 

sedimentary structures. 

Diabase sills in the Apache Group and Troy Quartzite 

of the Sierra Ancha have been dated by uranium-lead 

and potassium-argon methods at 1,150-1,200 m.y. 

(Silver, 1960; Livingston and Damon, 1968, p. 769). The 

Apache and Troy are older than the diabase and 

younger than the 1,420-1,460-m.y.-old granitic rocks 

(Y g1) in the underlying basement (p. 64). Both the 

diabase sills in the Unkar Group and the Rama ( = 

Cardenas) lavas near the top of the group yield 

rubidium-strontium ages of about 1,100 m.y.; 

potassium-argon ages from the same rocks of 800-900 

m.y. suggest a later heating event (McKee and Noble, 

1974). 

The Chuar Group, or upper unit of the Grand Canyon 

Supergroup, is rather different from the supracrustal 

rocks sq far considered. It is a thick body of varicolored 

argillites, with thin stromatolite-bearing limestones at 

a dozen or so levels, and occasional beds of chert, oolite, 

and sandstone (Ford and Breed, 1973). Shales in the 

upper part of the group contain the small circular carbonaceous structures Chuaria, once thought to be 

primitive brachiopods, but now interpreted as crushed 

spheres of microplanktonic algae (Ford and Breed, 

1972). The dating of the preceding Rama ( = Cardenas) 

lava shows that the Chuar is younger than 1,100 m.y., 

so that it is either very late Precambrian Y, or even a 

part of Precambrian Z. On the Geologic Map it is 
SOUTHERN BASIN AND RANGE PROVINCE 69 

speculatively indicated as Z, although this is by no 

means proved. 

PAHRUMP GROUP OF EASTERN CALIFORNIA 

(PRECAMBRIAN Y AND Z) 

Supracrustal rocks, in part like those in Arizona, 

reappear in the southern part of the Death Valley area 

of eastern California, where they form the Pahrump 

Group (labeled Y on the Geologic Map, although the 

upper part probably includes rocks of Precambrian Z, as 

indicated below). The group is preserved in a belt extending 80 mi (130 km) northwestward from the Kingston Range east of Death Valley to the Panamint Range 

west of it, northeast and southwest of which younger 

strata lie directly on the crystalline basement (Xm) 

(Wright and Troxel, 1967, p. 938-939). 

The group is a package of supracrustal rocks partitioned by unconformities from the older and younger 

Precambrian below and above, but inhomogeneous internally, and with considerable lateral variation. It is 

divisible into the Crystal Spring Formation, Beck 

Spring Dolomite, and Kingston Peak Formation, which 

total5,000 ft (1,500 m) thick in the Kingston Range, but 

reach up to 7,000--8,000 ft (2,100--2,440 m) farther west 

(Wright, 1968, p. 9-10). 

The Crystal Spring Formation lies on the basement, 

is 3,000--4,000 ft (900--1,200 m) thick, and is formed of 

lithic units much like those in the Unkar and Apache 

Groups of Arizona, including quartzites and shales 

below and above, and medial limestones or dolomites 

with associated chert. It is extensively invaded by 

diabase sills, one of which has widely altered the medial 

carbonates to commercial grades of talc. The Beck 

Spring Dolomite is a massive body that attains 1,000 ft 

(300m) in the east, but which wedges out westward and 

southwestward. 

The upper unit of the group, or Kingston Peak Formation, differs from any of the supracrustal rocks to the 

east in Arizona. It is a body 1,000--2,500 ft (300--760 m) 

thick of conglomerate or diamictite and associated 

shaly or sandy layers, some of which contain widely 

dispersed dropstones. The diamictites contain small to 

large clasts of crystalline basement, Crystal Spring and 

Beck Spring sediments, and diabase like that intruding 

the Crystal Spring Formation. Within the area of exposure the Kingston Peak is slightly unconformable on 

the underlying parts of the group, but they must have 

been sharply eroded elsewhere to provide the clasts in 

the diamictites. The formation is angularly truncated 

northeastward by the Noonday Dolomite at the base of 

the main Precambrian Z sequence, but elsewhere the 

discordance is slight or nonevident. 

Stromatolites occur in both the Crystal Spring and 

Beck Spring carbonates; those in the former are comparable to forms in the Middle Riphean to lower Upper 

Riphean of the Soviet Union. Stromatolites in the Beck 

Spring are associated with eucaryotic nannofossils, indicating the very early existence here of precursors of 

the metazoans (Cloud and others, 1969). No reliable 

radiometric dates have been obtained on the rocks of the 

Pahrump Group or the diabase intrusives in the Crystal 

Spring, but the two lower formations are quite comparable to the Unkar Group, the Apache Group, and the 

Troy Quartzite in Arizona, and like them may have an 

age of about 1,100--1,420 m.y. 

The diamictites of the Kingston Peak Formation have 

much the same character as the diamictites farther 

north in the Cordilleran province (Mineral Fork, Toby, 

etc.), and likewise may be of direct or indirect glacial 

derivation (Johnson, 1957, p. 368-369; Crittenden and 

others, 1972, p. 339). Like the comparable deposits 

farther north, they are probably to be assigned to the 

early part of Precambrian Z; here, however, they are 

unconformable with the main body of Precambrian Z 

above. 

PRECAMBRIAN OF WESTERN TEXAS 

(MAINLY PRECAMBRIAN Y) 

In the Basin and Range province east of southern 

Arizona, in southwestern New Mexico, and western 

Texas, small outcrops of Precambrian rocks occur in the 

structurally higher parts of the ranges, in a terrain 

otherwise dominated by Phanerozoic rocks. Those in 

New Mexico are mainly Precambrian X metamorphic 

and plutonic rocks, but those in Texas are more varied 

and of younger ages, including supracrustal rocks of 

Precambrian Y that are 250 mi ( 400 km) or more east of 

those in Arizona. 

In Texas, Precambrian is exposed in the Franklin 

Mountains north of El Paso at the extreme western end 

of the State, in the Van Horn area 100 mi (160 km) 

farther southeast, and in two small patches in the intervening area. Near Van Horn, Precambrian rocks 

(shown on the Geologic Map as X, Y, and Z) emerge in 

several fault blocks in an area of about 225 mi2 (580 

km2) sometimes rather inappropriately called the ((Van 

Horn dome" (fig. 26). In the Franklin Mountains they 

are almost as varied as at Van Horn (although marked 

only as Y gz on the Geologic Map), but are exposed only 

in a narrow 14-mi (23 km) strip along the east face of the 

range. The rock sequences in the two areas are shown in 

table 5. 

In western Texas and southeastern New Mexico the 

next youngest unit above the Precambrian is the Bliss 

Sandstone of latest Cambrian or earliest Ordovician 

age, but this is not preserved everywhere, and 

elsewhere the Precambrian is followed directly by 

upper Paleozoic or even Cretaceous strata. In the 
70 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

Franklin Mountains the Precambrian supracrustal I sive, yet the inclination of their strata conforms closely 

rocks are disrupted by the Precambrian granitic intru- to that of the overlying Paleozoic. In the Van Horn area 

.. .. . ·.· ·:: : . . 

. · .. .. . . . . . . . . . . . ~ . . 

0 

0 

w . 

. 

Quaternary intermontane 

deposits 

EXPLANATION 

Cretaceous sedimentary rocks, in 

places overlain by Tertiary 

volcanic rocks 

PRECAMBRIAN 

.. ... ': ': ... ~:. : . . ·. . . . 

20 Ml LES 

20 KILOMETRES 

Permian ·and Ordovician carbonate 

rocks. Basal Ordovician Bliss 

Sandstone shown where 

preserved 

Van Horn Sandstone Hazel Formation Allamoore Formation Rhyolite 

Mountain 

intrusive 

Formation 

into Carrizo Carrizo Mountain 

-

furmation 

Northern limit of strong 

deformation in Precambrian Y rocks 

Low-angle fault of Precambrian age 

High-angle faults, mainly of 

Cenozoic age 

FIGURE 26.-Map of Van Horn area, west Texas, showing Precambrian rocks, and their relations to surrounding Phanerozoic rocks. 

Compiled from King and Flawn (1953), and other sources. 
SOUTHERN BASIN AND RANGE PROVINCE 71 

TABLE 5.-Precambrian rocks of western T exas [Symbols on left are those used on Geologic Map of Un ited States] 

Van Horn area 

!King a nd Flawn , 1953: Flawn and 

Muehlberger, 1970, p. 89--107 1 

l Pz Bliss Sandstone (Lower Ordovician I l Pz 

Franklin Mountains 

!Harbour, 1960; p. 11- 12; 

Harbour, 1972 1 

Bliss Sandstone (Lower Ordovician and 

Uppe r Cambri an! 

-+---Structural unconformitY--t--+---Structural unconformity __ 

Z Van Horn Sandstone, 800ft (260 ml 

-+---Structural unconformity_ 

Hazel Formation, 5,000? ft 11,500? m 1 

(red sand stone, with conglomPrate below I 

~g1 Granite 

--+----Unconformity ___ --! f--t----lntr usive contact __ _ 

y 

Rhyol ite extrusives, 1,800 ft (600 m l 

Lanoria Quartzite, 2,600 ft (790 m l 

0. 

~ r-------------

Allamoore Formation , 3,000? ft 1900? ·~ m 1 1l1mestone, ,·olcamclast1c sed1- 0 

Mundy Breccia , 0-190 ft (0.63 ml 

!basalt agglomerat e! 

ments, lavas, a nd diabase intrusives! 0 r-------------

5 Castner Limestone, 1,100 ft (350 m l 

~ (with di abase sills! 

--+----Sequence broken -----j ~ 

Carri zo Mounta in Formation, 19,000 ft o 

15,800 m l minimum !clastic metased- Z 

X iments, intruded by sills of 

metarhyolite and metagabbro! 

Base not exposed 

Base not ex posed 

all the Precambrian supracrustal rocks except the Van 

Horn Sandstone have been orogenically deformed in 

what has been termed the ((Van Horn mobile belt" 

(Flawn, 1956, p. 32)-in contrast to those in the 

Franklin Mountains and those farther west that have 

been discussed earlier. 

Within the mobile belt the Carrizo Mountain Formation is to the south and is followed successively northward by the Allamoore and Hazel Formations; the Van 

Horn Sandstone is a postorogenic deposit that lies indiscriminately on the rest. However, the Carrizo Mountain 

metasediments are not in contact with the Allamoore, 

but are separated from it by large intrusive bodies of 

metarhyolite which adjoin the Allamoore along a major 

low-angle fault, the Streeruwitz thrust. For about 3 mi 

( 5 km) north of the thrust trace the Allamoore and 

Hazel are strongly folded and thrust, but the deformation decreases rapidly beyond, and the Hazel in its 

northern exposures is nearly horizontal (fig. 27). 

Metamorphism also decreases northward. The Carrizo Mountain Formation is of amphibolite grade in its 

southern exposures and contains much pegmatite; in its 

northern exposures it is of greenschist grade but it has 

been retrograded near the Streeruwitz thrust, and the 

rhyolite along the thrust has been converted to mylonite with conspicuous south-plunging lineation. The 

Allamoore Formation has been hydrothermally altered 

to jasperoid close to the thrust, and some of the limestone layers farther north have been selectively converted to talc by the same process; blue alkali amphibole and white asbestiform amphibole (richterite) 

occur in places (Rohrbacher, 1973, p. 6-13). Elsewhere 

in the disturbed belt neither the Allamoore nor the 

Hazel Formation are much metamorphosed, although 

some of their weaker layers show marked slaty cleavage. 

Traditionally, the Carrizo Mountain Formation has 

been considered the oldest unit in the sequence, and on 

the Geologic Map this presumed age has been expressed 

speculatively by classifying it as Precambrian X. However, there is little confirmation of this in the known 

geologic and radiometric data; alternatively, the Carrizo Mountain may originally have been a conformable 

downward sequence beneath the Allamoore, or it might 

have been a more internal, eugeosynclinal facies of the 

Allamoore (Flawn and Muehlberger, 1970, p. 105--106). 

The Streeruwitz thrust might even have been a major 

suture in the Precambrian terrane that juxtaposed contrasting sequences which were originally far apart, but 

exposures are too limited for proof of this possibility. 

The limestone of the Allamoore is identical with that 

of the Castner in the Franklin Mountains, and both 

contain stromatolitic layers. Both, in turn, strikingly 

resemble the limestones of the Mescal and Bass in 

Arizona and the Crystal Spring in California. The talc 

deposits in the Allamoore, like those in the Crystal 

Spring, are commercially productive, and are being 

mined on a large scale (Rohrbacher, 1973, p. 1). 

The Hazel and Allamoore Formations are intricately 

folded together in the deformed belt north of the 

Streeruwitz thrust, but the two are mostly separated by 

zones of shearing and thrusting, so that their original 

contact is seldom preserved. It must have been unconformable, because the lower part of the Hazel is a conglomerate composed largely of clasts derived from the 

Allamoore: limestones (including a few marmorized 

pieces), and the lavas and mafic intrusives. Besides 

these, the conglomerate contains a few clasts of red 

granite and rhyolite porphyry like those in the Franklin 

Mountains and elsewhere north of the Van Horn area, 

implying that the Hazel is not only younger than these, 

but younger than all the supracrustal formations in the 

Franklin Mountains. 

The Hazel Formation is a very thick deposit of two 

contrasting facies: coarse, poorly sorted, poorly rounded 

conglomerates below, and fine-grained, almost silty, 

thinly laminated red sandstones above. Passage from 

one facies to the other is by interbedding, yet they are 

seldom intergradational-few of the conglomerates 

have a red sandy matrix, and few of the sandstones are 

pebbly. It is tempting to compare these conglomerates 

with the diamictites of the Kingston Peak Formation of 
72 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

/ 

BEACH MTN 

CARRIZO MTS 

S . / treeruWitz /thrust ~/ 

/ 

pz 

/ 

X 

0 

I NORTH 

0 5 Kl LOMETRES 

FIGURE 27.-Synoptic section across Precambrian rocks ofVan Horn area, west Texas, showing structural relations of the different units 

and their implications in the Precambrian history of the area. Letter symbols are the same as those on fig. 26; black lenses in unit Yr 

are mafic intrusives. Mter King (in King and Flawn, 1953, p. 104). 

California, but verification requires further field 

review. 

Whatever the relations between the Allamoore and 

Hazel may have been, the climactic orogeny in the Van 

Horn mobile belt came later, after the deposition of the 

Hazel Formation. This orogeny resulted in the northward emplacement of the Carrizo Mountain Formation 

and its intrusive rhyolites along the Streeruwitz thrust, 

their retrograde metamorphism, and the deformation of 

the Allamoore and Hazel immediately to the north. By 

this deformation the Allamoore was thrown into northfacing recumbent folds and thrust over the Hazel. Besides these fold and thrust structures there are some 

curious patches farther north of highly crumpled Allamoore resting on nearly flat-lying Hazel that may 

have been emplaced during the orogeny as detached 

gravity slides. 

Radiometric data on the west Texas Precambrian are 

incomplete, but partly clarify some of its geologic and 

orogenic problems (W asserburg and others, 1962, p. 

4023-4031). Radiometric determinations by 

potassium-argon, rubidium-strontium, and a few by 

uranium-lead methods have been made on the granites 

of the Franklin Mountains and the nearby Hueco 

Mountains, on rhyolites from the Pump Station Hills 

north of the Van Horn area, and on metarhyolites and 

pegmatites in the Carrizo Mountain Formation; all 

yield dates of about 1,100 m.y. 

The dates define a widespread igneous event that is 

younger than any of the supracrustal rocks of the 

Franklin Mountains, and by implication younger than 

the Allamoore Formation of the Van Horn area. On the 

other hand, the event must have been older than the 

Hazel Formation, which contains a few clasts of the 

felsic igneous rocks, and it is therefore also earlier than 

the climactic orogeny of the Van Horn mobile belt. 

The 1,100 m.y. dates are comparable to the dates 

determined on mafic intrusives in the Precambrian Y 

supracrustal rocks of Arizona, and they are also comparable to the dates obtained on the infracrustal 

metamorphic and plutonic rocks of the Llano uplift, 300 

mi (480 km) to the east in central Texas (p. 42); similar 

dates have been obtained even nearer at hand from 

basement rocks of the ttTexas craton" that have been 

drilled into west of the Llano uplift (W asserburg and 

others, 1962, p. 4035-4036). The west Texas region thus 

marks the closest approach in the western United 

States of Precambrian Y supracrustal rocks to infracrustal rocks of the Grenvillian orogenic belt. The Van 

Horn mobile belt exposed in the Van Horn area is a 

tantalizingly small segment of what must be a major 

tectonic feature of the Precambrian in this part ofNorth 

America, but one whose further extent and trend are 

unknown. 

The succeeding Van Horn Sandstone is postorogenic, 

and lies with right-angled unconformity on all the earlier Precambrian formations; it is a red, arkosic, coarse, 

conglomeratic, continental deposit, probably laid down 

on compound alluvial fans that were largely fed from 

highlands to the north (McGowen and Groat, 1971). Its 

conglomerates contain clasts of the Allamoore and 

Hazel and of the mylonitized rhyolites from the upper 

plate of the Streeruwitz thrust to the south. However, 

the most prominent components are rounded cobbles 

and boulders of red granite and rhyolite porphyry like 

those ~xposed in the Precambrian areas to the northwest. The Van Horn is tilted at low angles in various 

directions rather than folded, and it was block-faulted 

and beveled before the basal Ordovician Bliss 

Sandstone was deposited on it. In older reports the formation was classed as Cambrian, but it is quite unlike 

any Cambrian elsewhere in the Southwestern States, 

and is almost certainly late Precambrian; on the 

Geologic Map it is marked as Precambrian Z. 

PRECAMBRIAN Z SUPRACRUSTAL ROCKS 

OF WESTERN BASIN AND RA GE PROVINCE 

Besides the Precambrian supracrustal rocks so far 

considered, another great sedimentary body in south-
SOUTHERN BASIN AND RANGE PROVINCE 73 

ern Nevada and eastern California extends conformably through Precambrian Z and the Lower Cambrian. 

It is exposed in many of the ranges from the Spring 

Mountains near Las Vegas westward beyond Death 

Valley, where it is 13,000 ft (4,000 m) thick between the 

Pahrump Group and crystalline basement below, and 

the Middle Cambrian above (Wright and Troxel, 1966). 

Farther northwest it is exposed in the lnyo and White 

Mountains of California and adjacent Esmerelda 

County, Nevada, where it is as much as 21,000 ft 

(6,400 m) thick without visible base (Nelson, 1962); 

this includes Walcott's type Waucoban Series (=Lower 

Cambrian Series). Approximately the upper third of the 

sequence contains diagnostic Lower Cambrian fossils; 

traces of fossils occur in beds lower down, but most of 

them are barren; the proper level of the PrecambrianCambrian boundary in the sequence is problematical 

(see below). 

A comprehensive review has been made by Stewart 

(1970) of the stratigraphy of the units in this rock body, 

with results summarized in table 6. 

As indicated by the table, the recognizable formations 

in the sequence fall naturally into three belts from east 

to west (or southeast to northwest), in each of which is a 

set of widely recognizable rock units, that cannot be 

traced directly into the units of the other belts because 

of disconnected exposures. Hence there are some uncertainties as to correlation, although fairly satisfactory 

results can be obtained by matching successive measured stratigraphic sections. 

On the Geologic Map of the United States the lower 

part of the sequence is indicated as Z and the upper part 

is included in unit£. Compilation of the map was completed before the results of Stewart's survey became 

available, and was based on different assumptions. The 

Noonday Dolomite and Stirling Quartzite of the central 

belt were thought to be correlative with the lithically 

similar Reed Dolomite and Campi to Formation ( = 

Sandstone) of the much thicker western sequence, 

whereas Stewart places both of the last two at a higher 

stratigraphic level. Moreover, it was assumed that the 

bases of the Stirling and Campi to were a ((natural" base 

of the Cambrian; whereas Stewart places the base of the 

Cambrian higher up, showing not only that the boundaries in the two areas are not correlative, but that no 

((natural" boundary exists in a conformable sequence of 

this kind. These discrepancies, while seemingly fundamental, actually do not greatly distort the representation on the small scale of the Geologic Map. 

The Precambrian Z-Lower Cambrian supracrustal 

body of the western Basin and Range province is a great 

sedimentary wedge that was built along the western 

edge of the North American continent in much the same 

manner as the Precambrian Y supracrustal Belt deposits were built farther north in the Cordillera several 

TABLE 6.-Precambrian Z-Lower Cambrian formations in western 

Basin Range Province. 

[Based on Stewart (1970, p. 6 1. Double line is base of Cambrian on U.S. Map; dashed line from 

Stewart] 

------------r----------.--------

Western belt 

Middle Cambrian 

Mule Spring Limestone 

Saline Valley Formation 

Harkless Formation 

Poleta Formation 

Campi to 

Formation 

Deep Spring Formation 

Reed Dolomite 

Wyman Formation 

Base not exposed 

Central belt 

Middle Cambrian 

Carrara Formation 

Zabriskie Quartzite 

Wood Canyon 

Formation 

Stirling Quartzite 

Johnnie Formation 

Noonday Dolomite 

Unconformity 

Pahrump Group and 

crystalline basement 

Eastern belt 

Middle Cambrian 

Bright Angel Shale 

Tapeats Sandstone 

Unconformity 

Hiatus 

Crystalline 

basement 

hundred million years earlier. Like the Belt deposits, it 

was derived from sedimentary waste derived from the 

craton, which accumulated to great thickness in a tectonically quiet regime (Stewart, 1970, p. 64-66). The 

wedge thickens from a few hundred feet in the Grand 

Canyon and elsewhere along the edge of the Colorado 

Plateau to more than 21,000 ft (6,400 m) in the western 

belt 175 mi (280 km) distant. In the central belt are 

thick units of quartzite and fine conglomerate that persist for long distances north-south along the stratigraphic strike, but which fade in the western belt, in the 

thickest part of the wedge, into fine-grained sandstone, 

intertongued with siltstone, shaly siltstone, and carbonate rocks (fig. 28). 

The problem of the Precambrian-Cambrian boundary 

in this deposit is more acute than in any other part of the 

United States. Fossil control disappears downward in a 

conformable sequence, in which no ((natural" sedimentary separation exists. In most of the country there is no 

problem, as Precambrian and Phanerozoic rocks are 

separated by prominent unconformities and large 

hiatuses. Even on the opposite side of the continent, in 

the Southern Appalachians, where both Precambrian Z 

and Lower Cambrian are again represented, there is in 

most places a rather obvious ((natural" boundary at the 

base of the Chilhowee Group. 

In the deposits in the western Basin and Range province, olenellid trilobites, archeocyathids, and other 
74 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

WESTERN BELT CENTRAL BELT EASTERN BELT 

MIDDLE Death Valley Nopah Range Las Vegas Colorado Plateau 

CAMBRIAN Mule Spring 

a: ~ .• •.• ~::tl.o t.:•>••< •·::• •••<:>.· ....... .. ~~~ · ~ i 

9 

:: :~~ 7:{:i. ~~rr:ar~ · ~ ~ ~a r:i~(~s5:_ f6rfl1ation_ ·: _. . ·. · · · o ~ :::i3.o.l.et:a.· .· ·f.9fmatioii<••······· ... • .. ···::•• _-~·:::-:: ·· .. r<<· •~ :::• 

-' :3, ~~G\icccn N 

Muav Limestone 

Bright Angel Shale 

Tapeats Sandstone 

METRES 

FEET 5000 

15,000 

Base 

not 

exposed LITHOLOGIES 

(Greatly generalized) 

~ 

~ 

Dolomite and 

limestone 

Shale and minor 

carbonate beds 

Sandstone and 

siltstone 

Quartzite Lowest occurrence 

of diagnostic 

Cambrian fossils 

FIGURE 28.-Stratigraphic diagram showing relations between late Precambrian (Z) and Lower Cambrian units exposed in different 

areas northwestward across the western Basin and Range province, from the edge of the Colorado Plateau east of Las Vegas, 

Nevada, to the Inyo Mountains, California. Compiled from Stewart (1970, pl. 2-3). Length of area about 240 mi (400 km). 

diagnostic fossils of the Lower Cambrian are fairly 

abundant in the upper part, down to the middle of the 

Wood Canyon Formation in the central belt and the 

middle of the Campi to Formation in the western belt, 

possibly at nearly the same stratigraphic level. This 

level is used by Stewart (1970, p. 7) to define the base of 

the Cambrian, and this may be the best practical solution in a situation of this kind. 

Nevertheless, indications of metazoan life extend 

some distance lower. The lower halfofthe Wood Canyon 

in the central belt contains fossil tracks and worm borings. The middle part of the Deep Spring Formation in 

the western belt contains Rusophycus and Cruziana, 

which are sitz-marks and crawl-tracks formed by trilobites and other arthropods, that resemble markings in 

proved Cambrian strata (Cloud and Nelson, 1966, 

p. 766--768). About 350ft (105m) lower in the formation 

is a ribbed shell like the problematical genus Pteridinium ( =Plagiogomus) wh:,ch occurs in the Ediacaran, 

Vendian, and related latest Precambrian units of the 

Eastern Hemisphere. Near the boundary between the 

Deep Spring and the Reed Dolomite, 600 ft (180 m) 

beneath, is the mollusklike shell Wyattia, resembling 

globorilids found in Cambrian rocks. 

Below the strata in which these remains occur, valid 

fossil control vanishes; tubular structures of probable 

algal origin occur in the Noonday Dolomite (Stewart, 

1970, p. 15), and the presence of eucaryotic nannofossils 

in the Beck Spring Dolomite of the Pahrump Group has 

already been noted; but both of these can be confidently 

relegated to the Precambrian. 

In summary, part of the sequence under discussion is 

clearly Precambrian Z and part is clearly Lower Cambrian, but there is no obvious boundary between them. 

Whatever boundary or boundaries might be selected 

depend less on the data afforded by the rocks themselves 

than on the predilections of individual stratigraphers. 

DISCUSSION AND SYNTHESIS 

The purpose of the preceding review has been to outline the regional features of the Precambrian rocks of 

the United States, insofar as they relate to representation of their outcrops on th~ Geologic Map of the United 

States. By its very nature the review is thus not a 

philosophical or speculative treatise on the Precambrian rocks or the history that they imply. Nevertheless, some generalizations emerge that can be summarized here. 

It is apparent from the review that the Precambrian 

of North America (and specifically the Precambrian of 

Canada and the United States) is not an indecipherable 

complex of rocks older than the earliest stratified and 

fossiliferous Phanerozoic rocks. Nor is it an ((Archean" 

complex of crystalline rocks and a ((Proterozoic" or ((Algonkian" body of less deformed and metamorphosed 
DISCUSSION AND SYNTHESIS 75 

stratified rocks-or, in other terms, an ((early" and a 

((late" Precambrian. Radiometric dating, whatever its 

defects and pitfalls in detail, has greatly amplified and 

refined the picture, which will continue to be improved 

in the future. Using this and other criteria, the Precambrian can now be subdivided and correlated from 

one region to another, and the results can be represented on regional geologic maps, such as those of 

Canada (1969) and the United States. 

Radiometric dating underscores the great length of 

Precambrian time-from more than 4,000 m.y. ago to 

about 600 m.y. ago, or about seven times the length of 

Phanerozoic time. During this vast interval the earth 

evolved from its primitive state to one more like that of 

modern times, with changes in the crust, the hydrosphere, and the atmosphere that influenced the nature 

of geologic processes (Cloud, 1968, p. 48-51). Nevertheless, the basic laws of matter and energy existed 

throughout, so that uniformitarian principles apply, at 

least in modified form. 

Thus, as during the Phanerozoic, processes of deformation and plutonism operated in orogenic belts at the 

same times as cratonic conditions existed elsewhere, 

and there were no universal Precambrian orogenies, as 

was formerly believed. Also, if processes of plate tectonics operated during Phanerozoic time, they must 

have existed during Precambrian time as well, although the obscurity of the record in these ancient rocks 

precludes the nature of these processes from being more 

than speculative. 

Rates of volcanic and sedimentary accumulation 

could not have been drastically different from those of 

Phanerozoic time. It follows that sequences of Precambrian supracrustal rocks, although voluminous in many 

areas, can only record small samples of the inordinately 

long span of Precambrian time. The Precambrian sequences in supposedly typical areas, such as the Lake 

Superior Region, must contain many gaps that are 

probably represented by volcanism and sedimentation 

in other areas. 

Radiometric dating of Precambrian rocks indicates 

that there are peaks of abundance of dates during spans 

of several hundred million years, between which there 

are spans as long or longer with few or no dates. The 

times of abundance express the Kenoran, Hudsonian, 

Elsonian, Grenvillian, and Avalonian events of Canada 

and the United States. These events have been interpreted as orogenies, but most of them more likely represent orogenic eras or cycles, like the Appalachian and 

Cordilleran orogenic cycles during Phanerozoic time. 

As during the Phanerozoic, the effects of the cycles are 

concentrated in provinces or belts, where the dates are 

mainly the products ofinfracrustal metamorphism and 

plutonism. Comparable dates, if present outside these 

belts, express merely anorogenic or cratonic processes, 

such as volcanism, sedimentation, and stray intrusive 

activity. 

Various maps showing radiometric age provinces of 

parts or all of North America have been compiled (for 

example, Gastil, 1960, p. 10; Engel, 1963, p. 146; Goldich and others, 1966, p. 5386; King, 1969, p. 38-39). 

Outside the shield, where exposures are less continuous 

and more reliance must be placed on subsurface data, 

these maps are sometimes misleading in detail, because 

they fail to discriminate between dates of orogenic and 

anorogenic origin. More expressive, although much 

more subjective, are sequential maps showing inferred 

conditions during different parts of Precambrian time 

(fig. 29). 

The maps contribute some evidence, but only partial 

answers to the question of the evolution of the North 

American continent. How it was originally created and 

how it grew has been debated. Some of its continental 

crust must be very ancient CPrecambrian V" or 

((Katarchean"); rocks older than 3,200 m.y. have been 

dated radiometrically in southwestern Minnesota, 

along the Montana-Wyoming border, and in southwestern Greenland (marked by black triangles in fig. 29A ) .. 

Other areas of very ancient rocks are suspected 

elsewhere from geologic evidence but as yet lack 

radiometric proof. Elsewhere, the main body of Precambrian rocks is younger, Precambrian W C(Archean") 

or later. One proposed model of the ((Katarchean" and 

((Archean" rocks is that they were ((components of 

emerging proto-cratons and interspersed, subparallel, 

relatively simatic orogenic belts, presumably involving 

oceanic spreading centers, arcs, and interarc basins, 

and subduction zones. By 2,500 m.y. B.P., however, the 

more (granitic' proto-cratons converged, telescoping 

many oceanic, arc-interarc, and borderland environments into subparallel series of synclinoidal 

(greenstone' belts" (Engel and others, 1974, p. 843; see 

also Engel, 1963, p. 146-149; Goodwin, 1974). 

By the end of Precambrian W time, cratons had been 

stabilized by the processes referred to in the Superior 

province in the center of the continent, and in the Slave 

and Wyoming provinces to the northwest and southeast 

(fig. 29A). Precambrian W rocks have also been recognized in the Churchill and Grenville provinces of the 

Canadian Shield, but they were reworked by subsequent orogenies and not stabilized until later. Following the Kenoran event at the end of Precambrian W 

time, progressively larger areas of the continent were 

converted into craton. Stabilization of a province is indicated not only by its internal plutonic and metamorphic history, but also by unconformable overlaps of 

younger deposits along its edges-for example, the 

overlap of Precambrian X rocks around the edges of the 
76 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

Superior province (fig. 29B). The last part of the Precambrian continent to attain stability was the Grenvillian belt on the southeastern margin, whose metamor500 1000 K ILOMETRES 

500 1000 KILOMETRES 

phic and plutonic activity occurred at a time when the 

remainder of the continent was craton. The contrast is 

dramatic between the Grenville orogenic belt and the 

500 1000 KILOMETRES 

500 1000 KILOMETRES 

FIGURE 29.-Maps of the United States and parts of Canada and Mexico, showing evolution of the North American continent during 

Precambrian time: A, At close of Precambrian W (following Kenoran event, about 2,500 m.y. ago). B, At close of Precambrian X 

(following Hudsonian event, about 1,600 m.y. ago). C, Near middle of Precambrian Y (following Elsonian event, about 1,350 m.y. 

ago). D, Near close of Precambrian Y (following Grenvillian event, about 900 m.y. ago). E, At end of Precambrian (about 600 m.y. 

ago). No provisions have been made for possible later tectonic distortions. The maps are similar to those of Muehl berger and others 

(1967, p. 2374-2377), but have been greatly modified from later data, and from predilections of the present author. 
DISCUSSION AND SYNTHESIS 77 

little-disturbed great sedimentary embankment of the 

Belt Supergroup along the opposite western margin of 

the continent (fig. 29D). 

No Kenoran dates are known in the southern part of 

the continent, south of Wisconsin and Wyoming (fig. 

29A ), where all the dates on the crystalline rocks are 

500 1000 MILES 

500 I 000 KILOMETRES 

EXPL · ANATION 

~ 

~ 

Outlines of Precambrian outcrops 

Outer known liniits of Precambrian rocks (and approximate 

edge of· continent at end of 

Precambrian time) 

Orogenic areas (that were mobile 

during the event shown on map ; 

trends of folding indicated in 

part) 

Other areas probably formed of 

Precambrian rocks that are older 

than the event shown on the 

map 

Cratonic areas (stabilized during 

preceding events) 

Supracrustal 

-

sedimentary and volcanic deposits (continental 

deposits stippled) 

~{ 

{ 

{ 

~{ 

Unconformable overlap of supracrustal deposits on older cratons (dip symbols show homoclinal structure 

Faults (that were probably active 

during the event shown on map 

• • Rocks older than 3,200m.y .(where 

proved by radiometric dating) 

Areas affected by 1,350 m.y. 

(Elsonian) event 

Granite and anorthosite plutons 

Metamorphic geosynclinal rocks 

in Grenville orogenic belt 

• • Occurences of diamictites 

(tillites at least in part) 

FIGURE 29.-Continued. 

younger (fig. 29B). There is a strong possibility that no 

rocks of Precambrian W CArchean") ever existed in 

much of the southern area, suggesting that this part 

was added to the continent after the Kenoran event. 

Similarly, there is a notable absence, in surface or 

subsurface, of any Precambrian rocks in a large area in 

the western United States, west of the line shown in 

figure 30, and here there is much evidence that the crust 

was oceanic during Precambrian time, and was not 

made into continent until Paleozoic time or later. Even 

the westward projection of known Precambrian rocks 

nearly to the Pacific Coast in southern California probably reached its present position by shifts of crustal 

blocks late in Phanerozoic time. 

In the rna ps of figure 29 this line is shown as the 

approximate western edge of the North American continent at the end of Precambrian time. Similar lines are 

shown on the maps along the southern and southeastern sides of the continent. The boundary on the south 

indicates the margin of the Paleozoic Ouachita orogenic 

belt, where no Precambrian basement has been proved; 

possibly an original continental crust in this area has 

been removed by drift during Phanerozoic time to a 

position south of the Gulf of Mexico. The boundary on 

the southeast indicates the outer known limit of rocks of 

the Grenvillian belt; it is true that Precambrian rocks of 

younger ages in the Avalonian belt lie beyond this 

through much of the length of the Appalachian chain, 

but these were probably added to the continent by ·plate 

collision during the Phanerozoic. 

Available evidence indicates that after the Kenoran 

event the North American continent was a cohesive 

body, gradually enlarging by accretion-whatever its 

movements or its relations in space may have been to 

other continental plates. The only clear indication of an 

addition to the continent by plate collision is that of the 

Avalonian belt just referred to. 

Final Precambrian time (Precambrian Z or ((Hadrynian") has been poorly appreciated because of its scanty 

representation in the Central Interior-at most 

perhaps by continental deposits like the Bayfield Group 

of the Lake Superior Region, and by part of the volcanic 

and clastic rocks in the Wichita trough farther south 

(fig. 29E). Major depositional events had now shifted to 

the eastern and western margins of the continent, in the 

Appalachian and Cordilleran belts, where marine sediments and minor volcanics accumulated, forming 

sequences quite as impressive as those of earlier Precambrian times, that lead upward with only slight interruption into the Paleozoic geosynclinal deposits. 

Precambrian Z deposits on the east are less mature than 

those of the succeeding Paleozoic, indicating accumulation under conditions of some tectonic disturbance. 

Those on the west, especially those now preserved in the 
78 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

WASHINGTON 

OREGON 

PRECAMBRIAN UNKNOWN 

I 

I 

I 

CALIFORNIA 

0 

0 

NEVADA 

200 Ml LES 

200 Kl LOMETRES 

·' I 

I 

EXPLANATION 

Outcrops of supracrustal rocks 

(Precambrian Y and Z) 

Outcrops of metamorphic and 

plutonic rocks 

(Precambrian W and X) 

Western known limit of 

Precambrian rocks 

FIGURE 30.-Map of western United States, showing western known extent of Precambrian rocks. 
REFERENCES CITED 79 

western Basin and Range province, formed under conditions of crustal stability quite the equal of those accompanying the earlier Belt sedimentation in the same 

region. 

Diamictites occur near the base of the Precambrian Z 

deposits throughout much of their extent in the Cordilleran belt on the west, and at two localities in the Appalachian belt on the east. It is tempting to correlate 

these with the extensive glacial deposits of late Precambrian time that have been proved in many of the 

other continents, and to consider all of them as a possible time marker. Absolute proof of glacial origin is not 

available for all the diamictites in the United States 

and Canada, and the known pole positions of the time do 

not accord well with the supposed refrigeration. 

Nevertheless, the regional extent of the deposits and 

the variety of their component clasts point to controlling conditions quite different from mere mudslides or 

other locally triggered deposits. 

The much debated question of the boundary between 

the Precambrian and the Cambrian need not concern us 

greatly here. Throughout the Central Interior and the 

eastern part of the Cordilleran belt rocks younger than 

earliest Cambrian lie unconformably on Precambrian 

rocks, which are partly supracrustal, but in more places 

a crystalline infracrustal basement. The question of the 

boundary only arises in the interiors of the Appalachian 

and Cordilleran belts, where the latest Precambrian (Z) 

and the Cambrian are present in the same sequences. 

Here, the rocks were orogenically deformed during 

Phanerozoic time and the outcrop bands of the debated 

rocks are very narrow, so that for purposes of the 

Geologic Map of the United States the question can be 

disregarded. A significant point is that Precambrian 

and Cambrian are not necessarily unconformable (as 

they are in the craton), and that no ttLipalian" or lost 

interval separates them. 

ACKNOWLEDGMENTS 

This review could not have been made without the 

information, aid, and counsel of those of my colleagues 

on the staff of the Geological Survey who are also interested in the Precambrian rocks of the United States. 

Among the many who have so contributed are Charles 

A. Anderson, Helen M. Beikman, Max D. Crittenden, 

Jr., Harold L. James, Zell E. Peterman, Gershon D. 

Robinson, and John H. Stewart, some of whom have 

reviewed parts or all of the manuscript during its various stages of evolution. Needless to say, however, the 

report is neither a collective statement of their opinions, 

nor a definitive official statement of the U.S. Geological 

Survey; I assume sole responsibility for the views expressed. 

I am also deeply grateful for the advice and inspiration I have received, during or before the preparation of 

the report, from Preston E. Cloud, Jr., of the University 

of California, Santa Barbara, and Clifford H. Stockwell 

of the Geological Survey of Canada, whose larger insights into Precambrian problems have done much to 

sharpen my own perceptions. 

REFERENCES CITED 

Aalto, K. R., 1971, Glacial marine sedimentation and stratigraphy of 

the Toby Conglomerate (Upper Proterozoic), southeastern 

British Columbia, northwestern Idaho, and northeastern 

WasHngton: Canadian Jour. Earth Sci., v. 8, no. 7, p. 753-787. 

Adams, F D., Bell, Robert, Lane, A. C., Leith, C. K., Miller, W. G., and 

Van [ise, R. R., 1905, Report of the Special Committee on the 

Lakt:: Superior Region: Jour. Geology, v. 13, no. 2, p. 89-104. 

Aldrich, L. T., Davis, G. L., and James, H. L., 1965, Ages of minerals 

from metamorphic and igneous rocks near Iron Mountain, Michigan: Jour. Petrology, v. 6, pt. 3, p. 445-472. 

Anderson, C. A., 1968, Metamorphosed Precambrian silicic volcanic 

rocks in central Arizona, in Coats, R. R., Hay, R. L., and Anderson, C. A., eds., Studies in volcanology: Geol. Soc. America Mem. 

116, p. 9-44. 

Anderson, C. A., Blacet, P.M., Silver, L. T., and Stern, T. W., 1971, 

Revision of Precambrian stratigraphy in the Prescott-Jerome 

area, Yavapai County, Arizona: U.S. Geol. Survey Bull. 1324-C, 

p. C1-C16. 

Armstrong, R. L., 1975, Precambrian (1,500 m.y. old) rocks of central 

Idaho; the Salmon River arch and its role in Cordilleran sedimentation and tectonics: Am. Jour. Sci., v. 275-A, p. 437-467. 

Armstrong, R. L., and Hills, F. A., 1967, Rb/Sr and K/Ar geochronologic studies of mantled gneiss domes, Albion Range, 

southern Idaho, U.S.A.: Earth and Planetary Sci. Letters, v. 3, no. 

2, p. 114-124. 

Barker, Fred, 1969, Precambrian geology of the Needle Mountains, 

southwestern Colorado: U.S. Geol. Survey Prof. Paper 644-A, 

35 p. 

Bayley, R. W., 1968, Ore deposits of the Atlantic City district, Fremont County, Wyoming, in Ridge, J.D., ed., Ore deposits of the 

United States, 1933-67 (Graton-Sales Volume): Am. Inst. Mining, Metall., and Petroleum Engineers, v. 1, p. 568-604. 

--1970, Structure and mineralization of Precambrian rocks in 

the Galena-Roubaix district, Black Hills, South Dakota: U.S. 

Geol. Survey Bull. 1312-E, p. E1-E15. 

--1972a, A preliminary report on the geology and gold deposits of 

the Rochford district, Black Hills, South Dakota: U.S. Geol. Survey Bull. 1332-A, p. A3-A24. 

--1972b, Preliminary geologic map of the Nemo district, Black 

Hills, South Dakota: U.S. Geol. Survey Misc. Geol. Inves. Map 

I-712, scale 1:24,000. 

--1972c, Geologic field compilation map of the northern Black 

Hills, South Dakota: U.S. Geol. Survey open-file report, scale 

1:48,000. 

Bayley, R. W., and James, H. L., 1973, Precambrian iron formations in 

the United States, in James, H. L., and Sims, P. K., eds., Precambrian iron formationsofthe world: Econ. Geology, v. 68, no. 7, 

p. 934-959. 

Bayley, R. W., and Muehlberger, W. R., 1968, Basement rock map of 

the United States, exclusive of Alaska and Hawaii: U.S. Geol. 

Survey, scale 1:2,500,000. 

Becker, G. F., 1908, Relations of radioactivity to cosmology and geology: Geol. Soc. America Bull., v. 19, no. 2, p. 113-146. 
80 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

Bentley, R. D., and Neathery, T. L., 1970, Geology of the Brevard fault 

zone and related rocks of the inner Piedmont of Alabama: 

Alabama Geol. Society 8th Ann. Field Trip Guidebook (Dec. 4--5, 

1970), 119 p. 

Bickford, M. E., 1972, Chronology of igneous events in the Precambrian of the St. Francois Mountains, southeast Missouri; U/Pb 

ages of zircons and Rb/Sr ages of whole rocks and mineral separates [abs.]: Geol. Soc. America Abs. with Programs, v. 4, no. 7 

(Ann. Mtg., Minneapolis), p. 451-452. 

Bickford, M. E., Wetherill, G. W. , Barker, Fred, and Lee-Hu, ChinNan, 1967, Precambrian Rb/Sr chronology in the Needle Mountains, southwestern Colorado: Jour. Geophys. Research, v. 7 4, no. 

6; p. 1660-1676. 

Blackwelder, Eliot, 1926, Precambrian geology of the Medicine Bow 

Mountains: Geol. Soc. America Bull., v. 37, no. 4, p. 615--658. 

---1932, An ancient glacial formation in Utah: Jour. Geology, v. 

40, no. 4, p. 289-304. 

Bloomer, R. 0., and Werner, H. J., 1955, Geology ofthe Blue Ridge 

region in central Virginia: Geol. Soc. America Bull., v. 66, no. 5, 

p. 579-606. 

Boone, G. M., Boudette, E. L. , and Moench, R. H., 1970, Bedrock 

geology of the Rangeley Lakes-Dead River basin, western Maine, 

in Boone, G. M. , ed., Guidebook for field trips in the Rangeley 

Lakes-Dead River basin region, western Maine: New England 

Intercollegiate Geol. Conf., 63rd Ann. Mtg. , p. 1-24. 

Buddington, A. F. , 1939, Adirondack igneous rocks and their 

metamorphism: Geol. Soc. America Mem. 7, 354 p. 

Burchfiel, B. C., and Livingston, J. L., 1967, Brevard zone compared to 

Alpine root zones: Am. Jour. Sci., v. 265, no. 4, p. 241-256. 

Butler, J. R., 1966, Geologic evolution of the Beartooth Mountains, 

Montana and Wyoming, pt. 6, Cathedral Peak area, Montana: 

Geol. Soc. America Bull., v. 77, no. 1, p. 45--64. 

Cannon, W. F. , and Gair, J . E. , 1970, A revision of stratigraphic 

nomenclature for the middle Precambrian rocks in northern 

Michigan: Geol. Soc. America Bull., v. 81, no. 9, p. 2843-2846. 

Carroll, Dorothy, Neuman, R. B., and Jaffe, H. W., 1957, Heavy 

minerals in arenaceous beds in part of the Ocoee Series, Great 

Smoky Mountains, Tennessee: Am. Jour. Sci., v. 255, no. 3, 

p. 175--193. 

Carter, Bruce, and Silver, L. T., 1971, Post-emplacement structural 

history of the San Gabriel anorthosite complex [abs.]: Geol. Soc. 

America Abstracts with Programs, v. 3, no. 2 (Cordilleran Section 

64th Ann. Mtg.), p. 193-194. 

Catanzaro, E. J., 1966, Correlation of some Precambrian rocks and 

metamorphic events in parts of Wyoming and Montana: Mountain Geologist, v. 4, no. 1, p. 9-21. 

Clark, S. H. B., 1973, Interpretation of a high-grade Precambrian 

terrane in northern Idaho: Geol. Soc. America Bull., v. 84, no. 6, 

p. 1999-2004. 

Cloud, P. E. Jr., 1968, Pre-metazoan evolution and the origins of the 

Metazoa, in Drake, E. T., ed., Evolution and environment: Yale 

Univ. Press, New Haven, p. 1-72. 

Cloud, P. E., Jr., Licari, G. R., Wright, L.A., and Troxel, B. W., 1969, 

Proterozoic eucaryotes from eastern California: N atl. Acad. Sci. 

Proc., v. 62, no. 3, p. 623-630. 

Cloud, P. E., Jr., and Nelson, C. A., 1966, Paleozoic-Cryptozoic and 

related transitions; new evidence: Science, v. 154, no. 3750, 

p. 765--769. 

Cloud, P. E., Jr., and Semikhatov, M. A., 1969, Proterozoic stromatolite zonation: Am. Jour. Sci., v. 267, no. 9, p. 1017-1061. 

Condie, K. C., 1967, Petrology of the late Precambrian tillite(?) association in northern Utah: Geol. Soc. America Bull., v. 78, no. 11, 

p. 1317-1344. 

Conley, J. F., and Henika, W. S., 1973, Geology of the Snow Creek, 

Martinsville East, Price, and Spray quadrangles, Virginia: Virginia Div. Mineral Resources Rept. Inv. 33, 71 p. 

Cooper, J . R., and Silver, L. T., 1964, Geology and ore deposits of the 

Dragoon quadrangle, Cochise County, Arizona: U.S. Geol. Survey 

Prof. Paper 416, 196 p. 

Crittenden, M. D., Jr., McKee, E. H., and Peterman, Z. E., 1971, 

1.5-billion-year-old rocks in the Willard thrust sheet, Utah [abs.]: 

Geol. Soc. America Abstracts with Programs, v. 3, no. 2 (Cordilleran Section 67th Ann. Mtg.), p. 105--106. 

Crittenden, M. D., Jr., Schaeffer, F. E., Trimble, D. E., and Woodward, 

L. A., 1971, Nomenclature and correlation of some upper Precambrian and basal Cambrian sequences in western Utah and 

southeastern Idaho: Geol. Soc. America Bull., v. 82, no. 3, 

p. 581-602. 

Crittenden, M.D., Jr., Sharp, B. J ., and Calkins, F. C., 1952, Parleys 

Canyon to Tranverse Ranges, in Geology ofthe central Wasatch 

Mountains, Utah: Utah Geol. Society Guidebook to the Geology of 

Utah, no. 8, p. 1-37. 

Crittenden, M. D., Jr., Stewart, J. H. , and Wallace, C. A., 1972, 

Regional correlation of upper Precambrian strata in western 

North America: Internat. Geol. Cong. 24th (Canada), sec. 1, p. 

334--361. 

Crittenden, M.D., Jr., and Wallace, C. A. , 1973, Probable equivalents 

of the Belt Supergroup in Utah, in Belt symposium, 1973, v. 1: 

Idaho Bur. Mines and Geol., p. 116-138. 

Crowell, J. C., and Walker, J . W. R. , 1962, Anorthosite and related 

rocks along the San Andreas fault, southern California: California Univ. Pubs. Geol. Sci., v. 40, p. 219-288. 

Daly, R. A., 1912, Geology of the North American Cordillera at the 

Forty-ninth Parallel: Canada Geol. Survey Mem. 38, pt. 1, 546 p. 

Dalziel, I. W. D., and Dott, R. H., Jr., 1970, Geology of the Baraboo 

district, Wisconsin: Wisconsin Geol. and Nat. History Survey Inf. 

Circ. 14, 164 p. 

Darton, N. H., 1925, A resume of Arizona geology: Arizona Bur. Mines 

Bull. 119, 208 p. 

Darton, N. H. , and Paige, Sidney, 1925, Description of the central 

Black Hills, South Dakota: U.S. Geol. Survey Geol. Atlas, Folio 

219, 34 p. 

Deiss, Charles, 1935, Cambrian-Algonkian unconformity in western 

Montana&Geol. Soc. America Bull., v. 46, no. 1, p. 95--124. 

Dibblee, T. W., Jr., 1968, Displacements on the San Andreas fault 

system in the San Gabriel, San Bernardino, and San Jacinto 

Mountains, southern California, in Dickinson, W. R., and Grantz, 

Arthur, eds., Proceedings of conference on geologic problems of 

San Andreas fault system: Stanford Univ. Pubs. Geol. Sci., v. 11, 

p. 260-277. 

Douglas, R. J. ·w., Gabrielse, H., Wheeler, J. 0 ., Stott, D. F., and 

Belyea, H. R., 1970, Geology of western Canada, in Douglas, R. J. 

W., ed., Geology and economic minerals of Canada: Canada Geol. 

Survey Econ. Geol. Rept. 1, 5th ed., p. 366-488. 

Dowse, A.M., 1950, New evidence on the Cambrian contact at Hoppin 

Hill, ,North Attleboro, Massachusetts: Am. Jour. Sci., v. 268, no. 

2, p. '95--99. 

Drake, A. A., Jr., 1970, Structural geology of the Reading Prong, in 

Fisher, G. W. , Pettijohn, F. J., Reed, J. C., Jr., and Weaver, K. N., 

eds., Studies of Appalachian geology; Central and Southern: Interscience Publishers, New York, p. 271-291. 

Dutton, C. E., and Bradley, R. E., 1970, Lithologic, geophysical, and 

mineral commodity maps of Precambrian rocks in Wisconsin: 

U.S. Geol. Survey Misc. Geol. Inv. Map I-631, scale 1:500,000. 

Eardley, A. J., and Hatch, R. A., 1940a, Precambrian crystalline rocks 

of north-central Utah: Jour. Geology, v. 48, no. 1, p. 58-72. 

---1940b, Proterozoic(?) rocks in Utah: Geol. Soc. America Bull., v. 

51, no. 6, p. 795--844. 
REFERENCES CITED 81 

Eckelmann, F. D. , and Poldervaart, Arie, 1957, Geologic evolution of 

the Beartooth Mountains, Montana and Wyoming, pt. 1, Archean 

history of the Quad Creek area: Geol. Soc. America Bull., v. 68, 

no. 10, p. 1225-1262. 

Emslie, R. F., 1970, Grenville province, in Douglas, R. J . W., ed., 

Geology and economic minerals of Canada: Canada Geol. Survey 

Econ. Geol. Rept. 1, 5th ed., p. 121-145. 

Engel, A. E. J., 1963, Geologic evolution of North America: Science, v. 

140, no. 2563, p. 143-152. 

Engel, A. E. J., and Engel, C. G., 1954, Grenville Series in the northwest Adirondack Mountains, New York; part 1, General features 

of the Grenville Series: Geol. Soc. America Bull. v. 64, no. 9, p. 

1013-1047. 

Engel, A. E. J., Itson, S. P., Engel, C. G., Stickney, D. M. and Cray, E. 

J ., Jr., 1974, Crustal evolution and global tectonics; a petrogenic 

view: Geol. Soc. America Bull., v. 85, p. 843-848. 

Fairbairn, H. W. , Moorbath, S., Ramo, A. 0 ., Pinson, W. H., Jr., and 

Hurley, P.M., 1967, Rb/Sr age of granitic rocks of southeastern 

Massachusetts and the age of the Lower Cambrian at Hoppin 

Hill: Earth and Planetary Sci. Letters, v. 2, p. 321- 328. 

Faul, Henry, Stern, T. W., Thomas, H . H. , and Elmore, P. L. D., 1963, 

Ages of intrusion and metamorphism in the Northern Appalachians: Am. Jour. Sci., v. 261, no. 1, p. 1-19. 

Fenton, M.D., and Faure, G., 1969, Another age determination of the 

Stillwater Complex, Montana, by the total-rock Rb/Sr method 

[abs.], in Abstracts for 1968: Geol. Soc. America Spec. Paper 121, 

p. 656-657. 

Flawn, P. T., 1956, Baseme~t rocks of Texas and southeast New 

Mexico: Texas Univ. Bur. Econ. Geol. Pub. 5605, 261 p. 

Flawn, P. T., and Muehlberger, W. R. , 1970, The Precambrian ofthe 

United States of America; south-central United States, in Rankama, Kalervo, ed., The Precambrian, v. 4: Interscience Publishers, New York, p. 73-143. 

Ford, T. D., and Breed, W. J., 1972, The problematical Precambrian 

fossil Chuaria: Internat. Geol. Cong. 24th (Canada), sec. 1, p. 

11- 18. 

---1973, Late Precambrian Chuar Group, Grand Canyon, 

Arizona: Geol. Soc. America Bull., v. 84, no. 4, p. 1243-1260. 

Fullagar, P. D., and Odom, A. L., 1973, Geochronology ofPrecambrian 

gneisses in the Blue Ridge province of northwestern North 

Carolina and adjacent parts of Virginia and Tennessee: Geol. Soc. 

America Bull. , v. 84, no. 9, p. 2065-3080. 

Gabrielse, Hugh, 1972, Younger Precambrian of the Canadian Cordillera: Am. Jour. Sci., v. 272, no. 6, p. 521-536. 

Gastil, Gordon, 1958, Older Precambrian rocks ofthe Diamond Butte 

quadrangle, Gila County, Arizona: Geol. Soc. America Bull., v. 

69, no. 12, p. 1495-1514. 

---1960, The distribution of mineral dates in time and space: Am. 

Jour. Sci., v. 258, no. 1, p. 1-35. 

Giletti, B. J., 1966, Isotopic ages from southwestern Montana: Jour. 

Geophys. Research, v. 71 , no. 16, p. 4029-4036. 

Giletti, B. J., and Damon, P . E., 1961, Rubidium-strontium ages of 

some basement rocks from Arizona and northwestern New 

Mexico: Geol. Soc. America Bull., v. 72, p. 639-644. 

Gilluly, James, 1966, Orogeny and geochronology: Am. Jour. Sci., v. 

264, no. 2, p. 97-111. 

Glaessner, M. F., 1961, Pre-Cambrian animals: Scientific American, 

v. 204, no. 3, p. 72--78. 

---1968, Biological events and the Precambrian time scale: Can. 

Jour. Earth Sci., v. 5, no. 3, p. 585-590. 

---1971, Geographic distribution and time range of the Ediacara 

Precambrian fauna: Geol. Soc. America Bull., v. 82, no. 2, p. 

509-514. 

Glover, Lynn, III, and Sinha, A. K., 1973, The Virgilina deformation, 

a late Precambrian to early Cambrian(?) orogenic event in the 

central Piedmont ofVirginia and North Carolina: Am. Jour. Sci., 

v. 273-A (Cooper volume), p. 234-251. 

Goldich, S. S., 1968, Geochronology in the Lake Superior Region: 

Canadian Jour. Earth Sci., v. 5, no. 3, p. 715-724. 

Goldich, S. S., Lidiak, E. G., Hedge, C. E. , and Walthall, F. G., 1966, 

Geochronology of the Midcontinent Region, United States; pt. 2, 

Northern area: Jour. Geophys. Research, v. 71, no. 22, p. 5389-

5406. 

Goldich, S. S., Muehlberger, W. R., Lidiak, E. G., and Hedge, C. E., 

1966, Geochronology of the Midcontinent Region, United States; 

pt. 1, Scope, methods, and principles: Jour. Geophys. Research, v. 

71, no. 22, p. 5375-5388. 

Goldich, S. S., Nier, A. 0., Baadsgaard, Haltdam, Hoffman, J. H., and 

Krueger, H . W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geol. Survey Bull. 41, 193 p. 

Goodwin, A. M., 1974, Precambrian belts, plumes, and shield development: Am. Jour. Sci., v. 274, p. 987- 1028. 

Grout, F. F., Gruner, J. W., Schwartz, G. M., and Thiel, G. A., 1951, 

Precambrian stratigraphy of Minnesota: Geol. Soc. America 

Bull., v. 62, no. 9, p. 1017-1078. 

Gussow, W. C., 1973, Chuaria cf. C. circularis Walcott from the Precambrian Hector Formation, Banff National Park, Alberta: Jour. 

Peleontology, v. 47, p. 1108-1112. 

Hadley, J. B., 1969a, Geologic map of the Cameron quadrangle, Madison County, Montana: U.S. Geol. Survey Geol. Quad. Map GQ--

813, scale 1:62,500. 

---1969b, Geologic map of the Varney quadrangle, Madison 

County, Montana: U.S. Geol. Survey Geol. Quad. Map GQ--814. 

Scale 1:62,500. 

---1970, The Ocoee Series and its possible correlatives, in Fisher, 

G. W., Pettijohn, F. J., Reed, J . C., Jr., and Weaver, K. N., eds. 

Studies of Appalachian geology; Central and Southern: Interscience Publishers, New York, p. 227-245. 

Hadley, J . B., and Nelson, A. E., 1971, Geologic map of the Knoxville 

quadrangle, North Carolina, Tennessee, and South Carolina: 

U.S. Geol. Survey Misc. Geol. Inv. Map I-654, scale 1:250,000. 

Hall, L. M., 1968, Times and origin of deformation of bedrock in the 

Manhattan prong, in Zen, E-an, White, W. S., Hadley, J. B., and 

Thompson, J. B., Jr., eds., Studies of Appalachian geology; Northern and Maritime: Interscience Publishers, New York, p. 117-

127. 

Halls, H. C., 1966, A review of the Keweenawan geology of the Lake 

Superior Region, in Steinhart, J. S., and Smith, T. J ., eds., The 

earth beneath the continents: Am. Geophys. Union Geophys. 

Mon. 10, p. 3-27. 

Ham, W. E., Denison, R. E., and Merritt, C. A. , 1964, Basement rocks 

and structural evolution of southern Oklahoma: Oklahoma Geol. 

Survey Bull. 95, 302 p. 

Hamblin, W. K. , 1961, Paleogeographic evolution of the Lake 

Superior Region from late Keweenawan to late Cambrian time: 

Geol. Soc. America Bull., v. 72, no. 1, p. 1-18. 

Hansen, W. R., 1965, Geology of the Flaming Gorge area, UtahColorado-Wyoming: U.S. Geol. Survey Prof. Paper 490, 196 p. 

Harbour, R. L., 1960, Precambrian rocks at North Franklin Mountain, Texas: Am. Assoc. Petroleum Geologists Bull., v. 44, no. 11, 

p. 1785-1792. 

---1972, Geology of the northern Franklin Mountains, Texas and 

New Mexico: U.S. Geol. Survey Bull. 1289, 129 p. 

Harrison, J. E., 1972, Precambrian Belt basin of northwestern United 

States; its geometry, sedimentation, and copper occurrences: 

Geol. Soc. American Bull., v. 83, no. 5, p. 1215-1240. 

Harrison, J. E., Griggs, A. B., and Wells, J.D., 1974, Tectonicfeatures 

of the Precambrian Belt basin and their influence on post-Belt 
82 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

structures: U.S. Geol. Survey Prof. Paper 866, 15 p. 

Hatcher, R. D., Jr., 1971, Geology ofRabun and Habersham Counties, 

Georgia: Georgia Geol. Survey Bull. 83, 48 p. 

---1973, Basement versus cover rocks in the Blue Ridge of northeast Georgia, northwestern South Carolina, and adjacent North 

Carolina: Am. Jour. Sci., v. 273, no. 8, p. 671-685. 

Hedge, C. E., 1970, Whole-rock Rb/Sr age of Pikes Peak batholith, 

Colorado, in Geological Survey research 1970: U.S. Geol. Survey 

Prof. Paper 700-B, p. B86-B89. 

Hedge, C. E., Peterman, Z. E., and Braddock, W. A., 1967, Age ofthe 

major Precambrian regional metamorphism in the northern 

Front Range, Colorado: Geol. Soc. America Bull., v. 78, no. 4, p. 

551-558. 

Hills, F. A., and Armstrong, R. L. 1971, Rb/Sr and K/Ar geochronology 

of the Laramie Range, southern Wyoming [abs.]: Geol. Soc. 

America Abstracts with Programs, v. 3, no. 7, 1971 Ann. Mtg. , p. 

599-600. 

Hills, F. A., Gast, P. W., Houston, R. S., and Swainbank, I. G., 1968, 

Precambrian geochronology of the Medicine Bow Mountains, 

southeastern Wyoming: Geol. Soc. American Bull., v. 79, no. 12, 

p. 1757-1784. 

Houston, R. S., and others, 1968, A regional study of rocks of Precambrian age in that part of the Medicine Bow Mountains lying in 

southeastern Wyoming, with a chapter on the relationship between Precambrian and Laramide structure: Wyoming Geol. 

Survey Mem. 1, 167 p. 

Hughes, C. J ., 1970, The late Precambrian Avalonian orogeny in 

Avalon, southeast Newfoundland: Am. Jour. Sci., v. 269, no. 2, p. 

183-190. 

Hughes, C. J ., and Bruckner, W. D., 1971, Late Precambrian rocks of 

eastern Avalon Peninsula, Newfoundland; a volcanic island complex: Canadian Jour. Earth Sci., v. 8, p. 899-915. 

Hurst, V. J., 1973, Geology of the southern Blue Ridge belt: Am. Jour. 

Sci., v. 273, no. 8, p. 643-670. 

James, H. L. , 1955, Zones of regional metamorphism in the Precambrian of northern Michigan: Geol. Soc. America Bull., v. 66, no. 

12, p. 1455-1488. 

---1958, Stratigraphy of pre-Keweenawan rocks in parts of northern Michigan: U .S. Geol. Survey Prof. Paper 314-C, p. 27-44. 

---1960, Problems of stratigraphy and correlation of Precambrian 

with particular reference to the Lake Superior Region: Am. Jour. 

Sci., v. 258-A (Bradley volume), p. 104-114. 

---1972a, Stratigraphic Commission note 40; Subdivisions of Precambrian, an interim scheme to be used by the U .S. Geological 

Survey: Am. Assoc. Petroleum Geologists Bull., v. 56, no. 6, p. 

1128-1133. 

---1972b, Subdivision of Precambrian; reply: Am. Assoc. Petroleum Geologists Bull., v. 56, no. 10, p. 2084-2086. 

Johnson, B. K., 1957, Geology of a part of the Manly Peak quadrangle, 

southern Panamint Range, California: California Univ. Pubs. 

Geol. Sci., v. 30, no. 5, p. 363-424. 

Jonas, A. I., and Stose, G. W., Hl39, Age relation ofthe Precambrian 

rocks in the Catoctin Mou·1tain-Blue Ridge and Mount Rogers 

anticlinoria in Virginia: Am. Jour. Sci., v. 237, no. 8, p. 575-593. 

Jones, W. R., Peoples, J. W., and Howland, A. L., 1960, Igneous and 

tectonic structures of the Stillwater Complex, Montana: U.S. 

Geol. Survey Bull. 1071-H, p. H281-H340. 

King, E. R. , and Zietz, Isadore, 1971, Aeromagnetic study of the 

Midcontinent gravity high of central United States: Geol. Soc. 

America Bull., v. 82, no. 8, p. 2187-2208. 

King, P. B., 1969, The tectonics of North America; a discussion to 

accompany the Tectonic Map of North America, scale 

1:5,000,000: U.S. Geol. Survey Prof. Paper 628, 95 p. 

---1970, The Precambrian of the United States of America; southeastern United States; in Rankama, Kalervo, ed., The Precambrian, v. 4: Interscience Publishers, New York, p. 1-71. 

King, P. B., and Beikman, H. M., 1974, An explanatory test to accompany the geologic map of the United States: U.S. Geol. Survey 

Prof. Paper 901, 44 p. (1975). 

King, P. B., and Flawn, P. T., 1953, Geology and mineral deposits of 

the Precambrian rocks ofthe Van Horn area, Texas: Texas Univ. 

Bur. Econ. Geol. Pub. 5301, 218 p .. 

King, P. B., Neuman, R. B., and Hadley, J. B., 1968, Geology of the 

Great Smoky Mountains, Tennessee and North Carolina: U .S. 

Geol. Survey Prof. Paper 587, 23 p. 

Kistler, R. W., Obradovich, J. D., and Jackson, E. D., 1968, Isotopic 

ages of rocks and minerals from the Stillwater Complex, Montana 

[abs.], in Abstracts for 1967: Geol. Soc. America Spec. Paper 115, 

p. 120-121. 

Krieger, M. H., 1968, Stratigraphic relations of the Troy Quartzite 

(younger Precambrian) and the Cambrian formations in southeastern Arizona, in Titley, S. R., ed., Southern Arizona guidebook 

III: Arizona Geol. Society, Geol. Soc. America Cordilleran Sec., 

64th Ann. Mtg, p. 23-32. 

Kulp, J. L., and Eckelmann, F. D., 1961, Potassium-argon ages on 

micas from the Southern Appalachians, in Geochronology of rock 

systems: New York Acad. Sci. Annals, v. 69, art. 2, p. 408-419. 

Lanphere, M.A. , 1964, Geochronologic studies in the eastern Mojave 

Desert, California: Jour. Geology, v. 72, no. 4, p. 381-419. 

Lawson, A. C., 1914, A standard scale for the Pre-cambrian rocks of 

North America: Internat. Geol. Cong., 12th (Canada), Compte 

Rendu, p. 349-370. 

Leith, C. K., 1934, The Precambrian: Geol. Soc. America Proc. for 

1933, p. 151-180. 

Livingston, D. E., and Damon, P. E., 1968, The ages of stratified 

Precambrian rock sequences in central Arizona and northern 

Sonora: Canadian Jour. Earth Sci., v. 5, no. 3, pt. 2, p. 763-772. 

Long, L. E., and Kulp, J. L., 1962, Isotopic age study ofthe metamorphic history of the Manhattan and Reading Prongs: Geol. Soc. 

America Bull., v. 73, no. 8, p. 969-996. 

McGowen, J. H., and Groat, C. J., 1971, Van Horn Sandstone, west 

Texas; an alluvial fan model for mineral exploration: Texas Bur. 

Econ. Geology Rept. Inv. 72, 57 p. 

McKee, E. H., and Noble, D. C., 1974, Radiometric ages of diabase sills 

and basaltic lava flows in the Unkar Group, Grand Canyon [abs.]: 

Geol. Soc. America Abstracts with Programs, v. 6, no. 5 (Rocky 

Mountain Section, 27th Ann. Mtg.), p. 458. 

McLaughlin, R. E., and Hathaway, R. E. , 1973, Fossils in the Murphy 

Marble [abs.]: Geol. Soc. America Abs. with Programs, v. 5, no. 5 

(Southeastern Sec. 22d Ann. Mtg.), p. 418-419. 

McMannis, W. J., 1963, La Hood Formation, a coarse facies of the Belt 

Series in southwestern Montana: Geol. Soc. America Bull., v. 74, 

no. 4, p. 407-436. 

Mallory, W. W., 1972, Pennsylvanian arkose and the Ancestral Rocky 

Mountains, in Mallory, W. W., ed., Geologic Atlas of the Rocky 

Mountain Region, United States of America: Rocky Mountain 

Assoc. Geol., Denver, Colo., p. 121-132. 

Maxson, J. H., 1961, Geologic map of the Bright Angel quadrangle, 

Grand Canyon National Park, Arizona: Grand Canyon Nat. Hist. 

Assoc. (Scale 1:24,000.) 

Miller, F. K., McKee, E. H., and Yates, R. G., 1973, Age and correlation of the Windermere Group in northeastern Washington: Geol. 

Soc. America Bull., v. 84, no. 11, p. 2723-2730. 

Misch, Peter, and Hazzard, J. C., 1962, Stratigraphy and metamorphism oflate Precambrian rocks in central northeastern Nevada 

and adjacent Utah: Am. Assoc. Petroleum Geol. Bull., v. 46, no. 3, 

p. 289-343. 

Misra, S. B., 1971, Stratigraphy and depositional history oflate Precambrian coelenterate-bearing rocks, southeastern Newfoundland: Geol. Soc. America Bull., v. 82, no. 4, p. 979-988. 

Moorbath, S., O'Nions, R. K., Pankhurst, R. J., Gale, N. H., and 

McGregor, V. R., 1972, Further rubidium-strontium age deter-
REFERENCES CITED 83 

minations on the very early Precambrian rocks of the Godthaab 

district, west Greenland: Nature (Physical Sciences), v. 240, Nov. 

27' p. 78--82. 

Morey, G. B., and Sims, P. K., 1976, Boundary between two Precambrian W terranes in Minnesota and its geologic significance: Geol. 

Soc. America Bull., v. 87, no. 1, p. 141- 152. 

Muehlberger, W. R., Denison, R. E., andLidiak, E. G., 1967, Basement 

rocks ofthe continental interior of the United States: Am. Assoc. 

Petroleum Geologists Bull., v. 51, no. 12, p. 2351- 2380. 

Muehlberger, W. R., Hedge, C. E., Denison, R. E., and Marvin, R. F., 

1966, Geochronology of the Midcontinent Region, United States; 

part 3, Southern area: Jour. Geophys. Research, v. 71, no. 22, p. 

5409-5426. 

Nelson, C. A., 1962, Lower Cambrian-Precambrian succession, 

White-Inyo Mountains, California: Geol. Soc. America Bull., v. 

73, no. 1, p. 139-144. 

Newhouse, W. H., and Hagner, A. F., 1957, Geologic map of the 

anorthosite areas, southern part of Laramie Range, Wyoming: 

U.S. Geol. Survey Mineral Inv. Field Studies Map MF-119. scale 

1:63,360. 

Noble, J. A., and Harder, J. 0., 1948, Stratigraphy and metamorphism in a part of the northern Black Hills and the Homestake 

Mine, South Dakota: Geol. Soc. America Bull., v. 59, no. 9, p. 

941- 976. 

Noble, J. A., Harder, J. P., and Slaughter, A. L., 1949, Structure of 

part of the northern Black Hills and the Homestake Mine, South 

Dakota: Geol. Soc. America Bull., v. 60, no. 2, p. 321-352. 

Noble, L. F., 1914, The Shinumo quadrangle, Grand Canyon district, 

Arizona: U.S. Geol. Survey Bull. 549, 100 p. 

Obradovich, J. D., and Peterman, Z. E., 1968, Geochronology of the 

Belt Series, Montana: Can. Jour. Earth Sci., v. 5, no. 3, p. 737-

747. 

Odom, A. L., and Fullagar, P. D., 1973, Geochronologic and tectonic 

relationships between the inner Piedmont, Brevard zone, and 

Blue Ridge belts, North Carolina: Am. Jour. Sci., v. 273-A 

(Cooper volume), p. 133-149. 

Odom, A. L., Kisch, S. A., and Leggo, P. J ., 1973, Extension of the 

Grenville basement to the southern extremity of the Appalachians; U/Pb ages of zircons [abs.]: Geol. Soc. America Abstracts 

with Programs, v. 5, no. 5 (Southeastern Sec., 22d Ann. Mtg.), p. 

425. 

Oray, Erogan, Hinze, W. J., and O'Hara, Norbert, 1973, Gravity and 

magnetic evidence for the eastern termination of the Lake 

Superior syncline: Geol. Soc. America Bull., v. 84, no. 8, p. 2763-

2780. 

Page, L. R., and others, 1953, Pegmatite investigations, 1942--45, 

Black Hills, South Dakota: U.S. Geol. Survey Prof. Paper 227, 228 

p. 

Palmer, A. R., 1967, Cambrian trilobite distributions in North America and their bearing on Cambrian paleogeography in Newfoundland, in Kay, Marshall, ed., North Atlantic geology and 

continental drift: Am. Assoc. Petroleum Geologists Mem. 12, p. 

135--144. 

Park, C. F., Jr., and Cannon, R. S., Jr., 1943, Geology and ore deposits 

ofthe Metaline quadrangle, Washington: U.S. Geol. Survey Prof. 

Paper 202, 81 p. 

Pasteels, Paul, and Silver, L. T., 1966, Geochronologic investigations 

in the crystalline rocks of the Grand Canyon, Arizona [abs.], in 

Abstracts for 1965: Geol. Soc. America Spec. Paper 87, p. 124. 

Peterman, Z. E., and Hedge, C. E., 1968, Chronology ofPrecambrian 

events in the Front Range, Colorado: Can. Jour. Earth Sci., v. 5, 

no. 3, p. 749-756. 

Pettijohn, F. J., 1943, Archean sedimentation: Geol. Soc. America 

Bull., v. 54, no. 7, p. 925--972. 

---1972, The Archean of the Canadian Shield, a resume, in Doe, B. 

R., and Smith, D. K., eds., Studies in mineralogy and Precambrian geology: Geol. Soc. America Mem. 135, p. 131- 149. 

Poole, W. H., and others, 1970, Geology of southeastern Canada, in 

Douglas, R. J . W., ed., Geology and economic minerals of Canada: 

Canada Geol. Survey Econ. Geology Rept. 1, 5th ed., p. 228--304. 

Price, R. A., 1964, The Precambrian Purcell System in the Rocky 

Mountains of southern Alberta and British Columbia: Canadian 

Petroleum Geologists Bull., v. 12, Field Conf. Guidebook Issue, 

p. 399- 426. 

Quinn, A. W., 1971, Bedrock geology of Rhode Island: U.S. Geol. 

Survey Bull. 1295, 68 p. 

Raaben, M. E., 1969, Columnar stromatolites and later Precambrian 

stratigraphy: Am. Jour. Sci., v. 267, no. 1, p. 1-18. 

Rankama, Kalervo, 1970, Proterozoic, Archean, and other weeds in 

the Precambrian rock garden: Geol. Soc. Finland Bull. 42, 

p. 211-222. 

Rankin, D. W., 1970, Stratigraphy and structure of Precambrian 

rocks in northwestern North Carolina, in Fisher, G. W., Pettijohn, F. J., Reed, J. C., Jr., and Weaver, K. N., eds., Studies of 

Appalachian geology; Central and Southern: Interscience Publishers, New York, p. 227-245. 

Rankin, D. W., Espenshade, G. B., and Shaw, K. W., 1973, Stratigraphy and structure of the metamorphic belt in northwestern 

North Carolina and southwestern Virginia; a study from the Blue 

Ridge across the Brevard zone to the Sauratown Mountains anticlinorium: Am. Jour. Sci., v. 273-A (Cooper volume), p. 1- 40. 

Rankin, D. W., Stern, T. W., Reed, J. C., Jr., and Newell, M. F. , 1969, 

Zircon ages of felsic volcanic rocks in the upper Precambrian of 

the Blue Ridge, Appalachian Mountains: Science, v. 166, Nov. 7, 

p. 741- 744. 

Ratte, J. C., and Wayland, R. G., 1969, Geology of the Hill City 

quadrangle, South Dakota; a preliminary report: U.S. Geol. Survey Bull. 1271-B, p. B1- B14. 

Redden, J. A., 1963, Geology and pegmatites of the Fourmile quadrangle, Black Hills, South Dakota: U .S. Geol. Survey Prof. Paper 

297- D, p. 199-291. 

- --1968, Geology of the Berne quadrangle, Black Hills, South 

Dakota: U.S. Geol. Survey Prof. Paper 297- F, p. 343-408. 

Reed, J. C., Jr., 1969, Ancient lavas in Shenandoah National Park 

area near Luray, Virginia: U.S. Geol. Survey Bull. 1265, 43 p. 

Reid, R. R., 1963, Metamorphic rocks of the northern Tobacco Root 

Mountains, Madison County, Montana: Geol. Soc. America Bull., 

v. 74, no. 3, p. 293-306. 

Rezak, Richard, 1957, Stromatolites of the Belt Series in Glacier 

National Park, Montana: U.S. Geol. Survey Prof. Paper 294-D, 

p. 127-154. 

Rodgers, John, 1972, Latest Precambrian (post-Grenville) rocks ofthe 

Appalachian region: Am. Jour. Sci., v. 272, p. 507- 520. 

Rohrbacher, R. G., 1973, Asbestos in the Allamoore talc district, 

Hudspeth and Culberson Counties, Texas: Texas Univ. Bur. 

Econ. Geology Circ. 73-1, 17 p. 

Ross, C. P., 1963, The Belt Series in Montana; with a geologic map 

compiled by Betty A. L. Skipp, and a section on paleontologic 

criteria by Richard Rezak: U.S. Geol. Survey Prof. Paper 346, 

122 p. 

---1970, The Precambrian of the United States of America; 

northwestern United States-the Belt Series, in Rankama, 

Kalervo, ed. , The Precambrian, v. 4: Interscience Publishers, 

New York, p. 145--251. 

Runner, J . J., 1934, Precambrian geology of the Nemo district, Black 

Hills, South Dakota: Am. Jour. Sci., 5th ser., v. 28, no. 167, 

p. 353-372. 

---1943, Structure and origin of Black Hills granite domes: Jour. 

Geology, v. 51, no. 7, p. 431- 457. 

Ruppel, E. T., 1975, Precambrian Y sedimentary rocks in east-central 

Idaho, in Precambrian and Lower Ordovician rocks in eastcentral Idaho: U.S. Geol. Survey Prof. Paper 889, p. 23. 
84 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 

Sandrock, G. S., and Penley, H. M., 1974, Geologic map of Pine 

Mountain series and adjacent areas in the southwest Georgia 

Piedmont [abs.]: Geol. Soc. America Abs. with Programs, v. 6, no. 

4 (Southeastern Sec. 23d Ann. Mtg.), p. 395. 

Scholten, Robert, Keenmon, R. A., and Kupsch, W. 0., 1955, Geology 

of the Lima region, southwestern Montana and adjacent Idaho: 

Geol. Soc. America Bull., v. 66, no. 4, p. 345--404. 

Shride, A. F., 1967, Younger Precambrian geology in southern 

Arizona: U.S. Geol. Survey Prof. Paper 566, 89 p. 

Silver, L. T., 1960, Age determinations on Precambrian diabase differentiates in the Sierra Ancha, Gila County, Arizona[abs.]: Geol. 

Soc. America Bull., v. 71, no. 12, p. 1973-1974. 

---1963, Isotopic investigations of zircons in Precambrian igneous 

rocks of the Adirondack Mountains [abs.], in Abstracts for 1962: 

Geol. Soc. America Spec. Paper 96, p. 150-151. 

---1965, Mazatzal orogeny and tectonic episodicity [abs.], in 

Abstracts for 1964: Geol. Soc. America Spec. Paper 82, p. 185--186. 

---1968, A geochronologic investigation of the anorthosite complex, Adirondack Mountains, New York, in Isachsen, Y. W., ed., 

Origin of anorthosite and related rocks: New York State Mus. and 

Sci. Service Mem. 18, p. 233-251. 

---1971, Problems of crystalline rocks in the Transverse Ranges 

[abs.]: Geol. Soc. America Abstracts with Programs, v. 3, no. 1 

(Cordilleran Section 64th Ann. Mtg.), p. 193-194. 

Silver, L. T., and Green, J . C., 1963, Zircon ages for middle Keweenawan rocks of the Lake Superior Region [abs.]: Am. Geophys. 

Union Trans., v. 44, no. 1, p. 107. 

---1972, Time constants for Keweenawan igneous activity[abs.]: 

Geol. Soc. America Abstracts with Programs, v. 4, no. 7 (Ann. 

Mtg. Minneapolis), p. 665--666. 

Silver, L. T., McKinney, C. R., Deutsch, S., and Bolinger, J., 1963, 

Precambrian age determinations in the western San Gabriel 

Mountains, California: Jour. Geology, v. 71, no. 2, p. 196-214. 

Silver, L. T., McKinney, C. R., and Wright, L. A., 1962, Some Precambrian ages in the Panamint Range, Death Valley, California 

[abs.], in Abstracts for 1961: Geol. Soc. America Spec. Paper 95, p. 

55. 

Sims, P. K., 1970, Geologic map of Minnesota; bedrock geology: Minnesota Geol. Survey Misc. Map M-14. Scale 1:1,100,000. 

Sims, P. K., and Morey, G. B., ed., 1972, Geology of Minnesota; a 

centennial volume in honor of George M. Schwartz: Minnesota 

Geol. Survey, 632 p. 

Sokolov, B.S., 1973, Vendian of northern Eurasia, in Pitcher, M.G., 

ed., Arctic geology: Am. Assoc. Petroleum Geol. Mem. 19, 

p. 204-218. 

Stewart, D. B., 1974, Precambrian rocks of Seven Hundred Acre 

Island and development of cleavage in the Islesboro Formation, in 

Osberg, P. H., ed., Guidebook for field trips in east-central and 

north-central Maine: New England Intercollegiate Geol. Conf., 

64th Ann. Mtg., p. 86-92. 

Stewart, J. H., 1970, Upper Precambrian and Lower Cambrian strata 

in the southern Great Basin, California and Nevada: U.S. Geol. 

Survey Prof. Paper 620, 206 p. 

Stockwell, C. H., 1961, Structural provinces, orogenies, and time 

classification of rocks of the Canadian Precambrian Shield, in 

Lowdon, J. A., compiler, Age determinations by the Geological 

Survey of Canada, rept. 2, Isotopic ages: Canada Geol. Survey 

Paper 61-17, p. 108-118. 

---1964, Fourth report on structural provinces, orogenies, and 

time classification of rocks of the Canadian Precambrian Shield, 

in Age determinations and geological studies: Canada Geol. Survey Paper 64-17, pt. 1, p. 1-21. 

---1968, Geochronology of stratified rocks of the Canadian Shield: 

Canadian Jour. Earth Sci., v. 5, no. 3, p. 693-698. 

---1972, Revised Precambrian time scale for the Canadian Shield: 

Canada Geol. Survey Paper 72-52, 4 p. 

Taylor, F. C., 1971, A revision of Precambrian structural provinces in 

northeastern Quebec and northern Labrador: Canadian Jour. 

Earth Sci., v. 8, no. 5, p. 579--584. 

Tilton, G. R., Wetherill, G. W., Davis, G. L., and Bass, M. N., 1960, 

1000-million-year-old minerals from the United States and 

Canada: Jour., Geophys. Research, v. 65, no. 12, p. 4173-4179. 

Tilton, G. R., Wetherill, G. W., Davis, G. L., and Hopson,C. A., 1958, 

Ages of minerals from the Baltimore Gneiss near Baltimore, 

Maryland: Geol. Soc. America Bull., v. 69, no. 11, p. 1469--1474. 

Tweto, Ogden, 1968, Geologic setting and interrelationships of mineral deposits in the mountain province of Colorado and southcentral Wyoming, in Ridge, J. D., ed., Ore deposits ofthe United 

States 1933-67 (Graton-Sales Volume): Am. Inst. Mining, 

Metall., and Petroleum Engineers, v. 1, p. 551-558. 

Tweto, Ogden, and Sims, P. K., 1963, Precambrian ancestry of the 

Colorado mineral belt: Geol. Soc. America Bull., v. 70, no. 8, 

p. 991-1014. 

Van Rise, C. R., 1892, Correlation papers; Archean and Algonkian: 

U.S. Geol. Survey Bull. 86, 594 p. 

Van Rise, C. R., and Leith, C. K. , 1909, Precambrian geology of North 

America: U.S. Geol. Survey Bull. 360, 959 p. 

Van Schmus, W. R., 1972, Geochronology ofPrecambrian rocks in the 

Penokean foldbelt subprovince of the Canadian Shield [abs.]: 18th 

Ann. Inst. on Lake Superior Geology, pt. 1, paper 32, Houghton, 

Mich. 

Van Schmus, W. R., Medaris, L. G., Jr., and Banks, P.O., 1975a, 

Geology and age of the Wolf River batholith, Wisconsin: Geol. 

Soc. America Bull. v. 86, no. 7, p. 907-914. 

Van Schmus, W. R., Thurman, E. M., and Peterman, Z. E., 1975 b, 

Geology and Rb/Sr geochronology of middle Precambrian rocks in 

eastern and central Wisconsin: Geol. Soc. America Bull., v. 86, 

no. 9, p. 1255--1265. 

Walcott, C. D., 1889, Study of a line of displacement in the Grand 

Canyon in northern Arizona: Geol. Soc. America Bull., v. 1, no. 1, 

p. 49--64. 

---1895, Algonkian rocks of the Grand Canyon of the Colorado: 

Jour. Geology, v. 3, no. 3, p. 312-330. 

---1899, Precambrian fossiliferous formations: Geol. Soc. America 

Bull., v. 10, no. 4, p. 199--244. 

Walker, J. F., 1926, Geology and mineral deposits ofthe Windermere 

map area, British Columbia: Canada Geol. Survey Mem. 148, 

69 p. ...... 

Wallace, C. A., and Crittenden, M. D., Jr., 1969, The stratigraphy, 

depositional environment, and correlation of the Precambrian 

Uinta Mountain Group, western Uinta Mountains, Utah, in 

Lindsay, J. B., ed., Geologic guidebook of the Uinta Mountains, 

Utah's maverick range: Intermountain Assoc. Geol. 16th Ann. 

Field Conf., p. 126-141. 

Walton, M. S., and de Waard, D., 1963, Orogenic evolution of the 

Precambrian in the Adirondack highlands; a new synthesis: 

Koninkl. Nederlands Akad. Weternsch. Proc. B. 66, p. 98-106. 

Wasserburg, G. J ., Albee, A. L., and Lanphere, M.A., 1964, Migration 

of radiogenic strontium during metamorphism: Jour. Geophys. 

Research, v. 69, no. 20, p. 4395--4401. 

Wasserburg, G. J., and Lanphere, M.A., 1965, Age determinations in 

the Precambrian of Arizona and Nevada: Geol. Soc. America 

Bull., v. 50, no. 7, p. 1113-1164. 

Wasserburg, G. J., Wetherill, G. W., Silver, L. T., and Flawn, P. T., 

1962, A study of the ages of the Precambrian of Texas: Jour. 

Geophys. Research, v. 67, no. 10, p. 4021-4047. 

Wells, J.D., Sheridan, D. M., and Albee, A. L., 1964, Relationship of 

Precambrian quartzite-schist sequence along Coal Creek to Idaho 

Springs Formation, Front Range, Colorado: U.S. Geol. Survey 

Prof. Paper 454-0, p. 01-025. 

Wheeler, H. E., 1965, Ozark Precambrian-Paleozoic relations: Am. 

Assoc. Petroleum Geologists Bull., v. 49, no. 10, p. 1647-1665. 
REFERENCES CITED 85 

Whelan, J. A., compiler, 1970, Radioactive and isotopic age determinations of Utah rocks: Utah Geol. and Mineralog. Survey Bull. 

81, 25 p. 

White, W. S., 1960, The Keweenawan lavas of Lake Superior; an 

example of flood basalts: Am. Jour. Sci., v. 258A (Bradley vol.), 

p. 367-374. 

- - -1966, Geologic evidence for crustal structure in the western 

Lake Superior basin, in Steinhart, J. S., and Smith, T. J., eds., The 

earth beneath the continents: Am. Geophys. Union Geophys. 

Mon. 10, p. 28--41. 

Wilson, E. D., 1939, Precambrian Mazatzal revolution in central 

Arizona: Geol. Soc. America Bull., v. 50, no. 7, p. 1113-1164. 

Wilson, J . Tuzo, 1969, Aspects of the different mechanics of ocean 

floors and continents: Tectonophysics, v. 8, no. 4-6, p. 281-289. 

Wright, L. A., 1968, Talc deposits of the southern Death ValleyKingston Range region, California: California Div. Mines and 

*U.S. GOVERNMENT PRINTING OFFICE: 1976- 789-025/20 

Geology Spec. Rept. 95, 79 p. 

Wright, L. A., and Troxel, B. W., 1966, Strata of late PrecambrianCambrian age, Death Valley region, California-Nevada: Am. 

Assoc. Petroleum Geologists Bull., v. 50, no. 5, p. 846-857. 

- - -1967, Limitations of right-lateral strike-slip displacement, 

Death Valley and Furnace Creek fault zones, California: Geol. 

Soc. America Bull., v. 78, no. 8, p. 933-950. 

Zartman, R. E., 1964, A geochronologic study of the Lone Grove pluton 

from the Llano uplift, Texas: Jour. Petrology, v. 5, pt. 2, p. 359-

408. 

- - -1965, Rubidium-strontium age of some metamorphic rocks 

from the Llano uplift, Texas: Jour. Petrology, v. 6, pt. 1, p. 28--36. 

Zartman, R. E., and Stern, T. W., 1967, Isotopic age and geologic 

relationships of the Little Elk Granite, northern Black Hills, 

South Dakota, in Geological Survey research 1967: U.S. Geol. 

Survey Prof. Paper 575-D, p. D157-D163.