Thick-Skinned, South-Verging Backthrusting in 

the Felch and Calumet Troughs Area of the 

Penokean Orogen, Northern Michigan

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Chapter L

Thick-Skinned, South-Verging Backthrusting in 

the Felch and Calumet Troughs Area of the 

Penokean Orogen, Northern Michigan

By J.S. KLASNER and P.K. SIMS

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and L.M.H. CARTER, Editors
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993

For sale by

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Library of Congress Cataloging-in-Publication Data

Klasner, J.S.

Thick-skinned, south-verging backthrusting in the Felch and Calumet troughs 

area of the Penokean Orogen, northern Michigan / by J.S. Klasner and P.K. Sims, 

p. cm. (Contributions to Precambrian geology of Lake Superior region ; L) 

(U.S. Geological Survey bulletin ; 1904-L) 

Includes bibliographical references. 

1. Thrust faults (Geology) Michigan 

I. Sims, P.K. (Paul Kibler), 1918- . II, 

Geological Survey bulletin ; 1904-L. 

QE75.B9 no. 1904-L 

[QE606.5.U6] 

557.3 s dc20

[551.8'7'09774] 92-23138

CIP

2. Geology, Structural Michigan. 

Title. III. Series. IV. Series: U.S.
CONTENTS

Abstract LI

Introduction LI

Acknowledgments L2

Regional tectonic setting L2

Bedrock geology L2

Regional structure and metamorphism L5

Kinematic data in the structural domains L5

Bush Lake trough L7

Dickinson Group, South Felch, and Toby Lake blocks L7

Felch trough L8

Toby Lake trough L13

Calumet trough L13

Carney Lake block L20

Summary of kinematic analyses L21 

Structural interpretation L23 

Style of Penokean tectonism L24 

References cited L26

FIGURES

1. Regional tectonic map showing location of study area L3

2. Map showing general geology of the Felch and Calumet troughs area, 

Michigan L4

3. Map showing detailed rock attitudes and location of detailed study areas L6

4. Sketch and stereoplot of structure near Bush Lake fault L8

5. Map and photograph illustrating flat-lying foliation in Archean rocks adjacent to 

Felch trough L9

6. Sketch of asymmetric fold in Archean rocks, South Felch block L10

7. Sketch and stereoplot of structure at Rian quarry, south edge of Felch trough Lll

8. Structure map and cross section of Felch trough near Rian quarry L12

9. Sketch and stereoplot of structure in Sturgeon Quartzite at north edge of Felch 

trough L13

10. Structure maps and cross sections of Felch trough near old Highway 69 and 

569 L14

11. Cross section of structure at Groveland mine L15

12. Bouguer gravity profile and geologic section, western Felch trough L15

13. Structure map and cross section of Toby Lake trough L16

14. Stereoplots showing structure in Calumet trough L17

15. Structure map and Stereoplots of outcrop area along west branch Sturgeon 

River L18

16. Stereoplot showing structure in Archean gneiss, Carney Lake block L19

17. Field sketch and Stereoplots showing structure in Sturgeon Quartzite, west side 

Carney Lake block L19

Contents III
18. Map showing orientation of mafic inclusions in gneiss of Carney Lake block L20

19. Map showing structural vergence of rocks in the Felch and Calumet troughs 

area L22

20. Cross sections comparing structure in Felch and Calumet troughs area with that in 

southern Alps L24

TABLE

1. Explanation of alphanumeric symbols used to describe deformation events and structural features in the Felch and Calumet troughs area L5

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Thick-Skinned, South-Verging Backthrusting in the 

Felch and Calumet Troughs Area of the 

Penokean Orogen, Northern Michigan

By J.S. Klasner 1 and P.K. Sims

Abstract

The Felch and Calumet troughs area of northern Michigan is part of the Penokean fold-thrust belt of the continental 

foreland of the Superior craton. The area lies immediately 

north of the Niagara fault zone, the north-verging suture 

between the continental foreland and the Early Proterozoic 

Wisconsin magmatic terranes to the south. Accretion of the 

magmatic terranes to the continental margin -1,850 Ma produced south-verging backthrusting and backfolding in this 

region involving both Archean basement and Early Proterozoic 

supracrustal strata.

Evidence for backthrusting exists throughoutthe Felch and 

Calumet troughs area. The backthrusting is characterized by 

southward-overturned bedding and small-scale, south-verging 

asymmetric folds with a subhorizontal axial-planar foliation. 

The Carney Lake Archean block appears to be a crystalline-core 

nappe wherein the Sturgeon Quartzite of the Chocolay Group 

forms the lower overturned limb. The deformation probably 

started as a north-verging foreland thrust event, but out-ofsequence south-verging backthrusts and backfolds developed to 

accommodate abrupt changes in crustal thickness along the 

continental margin.

The backthrusting in the Penokean orogen resembles that 

in the younger rocks of the southern Alps. Proceeding inward 

from the continental margin, both orogens have accreted oceanic crust, indicated by the presence of ophiolite, that is thrust 

onto the continental margin; a zone of thick-skinned complex 

deformation characterized by backthrusting and backfolding; a 

marginal basement arch; and, inboard of the arch, a fold-thrust 

belt that mainly involves thin-skinned deformation.

Western Illinois University, Macomb, IL 61455, and U.S. Geological Survey.

INTRODUCTION

Geologic mapping and related studies of the Precambrian rocks in northern Michigan, done mainly during and 

shortly after World War II, have provided excellent detailed 

geologic maps that can be reinterpreted in terms of modern 

structural analysis. In this study we have utilized the 

geologic maps of the area comprising the Felch and Calumet 

troughs (Gair and Wier, 1956; Bayley, 1959; James and 

others, 1961; Bayley and others, 1966; Dutton, 1971) to 

reinterpret the structural evolution. These maps show 

outcrops and numerous faults, most of which were 

interpreted originally as high-angle normal faults. Many of 

these faults are here reinterpreted as thrust and reverse faults.

In a recent structural study, Maharidge (1986) suggested that the Early Proterozoic stratigraphy in the Felch 

trough is inverted and possibly represents the lower limb of 

a crystalline, basement-cored nappe. (See also Sims and 

others, 1987.) Maharidge pointed out that the rocks in this 

area have been subjected to at least two phases of 

deformation. The first phase produced a north-verging 

crystalline-cored nappe with subhorizontal foliation; the 

second phase produced upright folds having a steeply 

dipping foliation. Hoist (1982, 1984) has suggested that 

similar north-verging nappes exist in the Early Proterozoic 

continental foreland in east-central Minnesota.

In this report we point out that although the Bush Lake 

fault is a major north-verging structure, consistent with the 

overall sense of northward tectonic transport of the 

Penokean orogen in northern Michigan and adjacent Wisconsin (Sims and others, 1985; Klasner, Ojakangas, and 

others, 1988; Klasner, Sims, and others, 1988; Attoh and 

Klasner, 1989; Klasner and Cannon, 1989; and Klasner and 

others, 1991), the structures in the Felch and Calumet

Felch and Calumet Troughs Area, Northern Michigan LI
troughs area are south-verging (Klasner and Sims, 1989). 

The south-verging structures, which deform Archean basement as well as rocks of the Early Proterozoic Marquette 

Range Supergroup, are interpreted as out-of-sequence backthrusts and backfolds that developed to accommodate abrupt 

changes in crustal thickness along the continental margin 

caused by thrusting. Our study did not confirm the existence 

of an inverted Early Proterozoic stratigraphy throughout the 

area of the Felch and Calumet troughs.

The structural analysis presented in this report differs 

substantially from the earlier conclusions of Ueng and Larue 

(1988) for the same general area. They recognized local flat 

foliations but related them to vertical compression rather 

than thrusting.

ACKNOWLEDGMENTS

We thank several colleagues for their kind help. James 

Trow provided structural data from the Calumet trough, and 

O.K. Larue and J.J. Mancuso provided information on the 

Groveland iron mine. Discussions in the field with W.F. 

Cannon, Z.E. Peterman, and K.J. Schulz provided considerable insight to this study, but we remain responsible for the 

interpretations set forth in this report. F.W. Cambray and 

R.L. Bauer reviewed final versions of the manuscript.

REGIONAL TECTONIC SETTING

The Felch and Calumet troughs lie near the south edge 

of the exposed continental margin of the Archean Superior 

craton (figs. 1 and 2). Rocks of the Wisconsin magmatic terranes (Sims and others, 1989) were accreted to the continental margin along the north-verging Niagara fault (suture) 

zone at about 1,850 Ma, resulting in northward thrusting and 

development of prominent thrust-fold systems (Klasner, 

Sims, and others, 1988) on the continental foreland. The Niagara fault zone contains a dismembered ophiolite comprising 

LREE (light-rare-earth-element)-depleted basalt, sheeted 

dikes, serpentinite, and plagiorhyolite (Schulz, 1987; Sims 

and others, 1989). In Minnesota, Hoist (1984) and Southwick and Morey (1991) have described large-scale nappes 

and thrusts that presumably are related to the same collision.

The Penokean orogen in northern Michigan has been 

divided into two major structural domains (Klasner and 

Cannon, 1989; Klasner, Ojakangas, and others, 1988; 

Klasner, Sims, and others, 1988). The northern domain, in 

the northern part of the Baraga basin (fig. 1), consists of a 

foreland-basin thrust belt in which deformation was largely 

thin skinned (Klasner and others, 1991); that is, it involved 

mainly Early Proterozoic supracrustal rocks, with apparently 

minor deformation of Archean basement rocks. The southern domain, which includes the study area and the Archean

outliers in the Baraga basin just north of the study area, 

consists of a basement arch in which deformation was 

primarily thick skinned, that is, it involved both Early 

Proterozoic supracrustal rocks and Archean basement. The 

Felch and Calumet troughs area is in the southern domain.

BEDROCK GEOLOGY

The Felch and Calumet troughs (fig. 2) are elongate 

east-trending belts of Early Proterozoic sedimentary and 

minor volcanic rocks that unconformably overlie or are in 

fault contact with Archean basement rocks. The Early Proterozoic rocks in the area are assigned to the Marquette Range 

Supergroup, which has been divided into four groups (Cannon and Gair, 1970), from youngest to oldest, the Paint 

River, Baraga, Menominee, and Chocolay. The youngest 

recognized group, the Paint River, lies mostly to the west of 

the area of figure 2, and is not considered in this study. Sims 

(1992) has suggested that the Paint River Group probably is 

not a valid group, but instead contains rocks correlative with 

the Michigamme Formation of the Baraga Group. The 

Baraga Group consists mainly of graywacke and slate of the 

Michigamme Formation. Within the area of figure 2, the 

Menominee Group comprises the Vulcan Iron-formation 

and the underlying schistose rocks of the Felch Formation 

(combined as unit Xme) and two major volcanic units, the 

Badwater Greenstone and the Hemlock Formation. Chemical data (K.J. Schulz, oral commun., 1990) indicate that the 

Badwater and Hemlock are probably correlative. The Chocolay Group consists of the Randville Dolomite, the underlying Sturgeon Quartzite, and the basal Fern Creek Formation 

(not divided in fig. 2). The Fern Creek is exposed only 

locally, and the Sturgeon Quartzite generally is the lowermost Early Proterozoic unit (Bayley and others, 1966).

The study area, which we call the Felch and Calumet 

troughs area, consists of several fault-bounded domains (fig. 

2), each of which has different rock units and structure. The 

Amasa block, to the north, is bounded on the south by the 

Bush Lake fault; it consists of Archean gneiss and the overlying Hemlock and Michigamme Formations, which have a 

northwest structural grain. The north-central part of the 

study area (fig. 2) contains several east-trending troughs of 

supracrustal rocks that are separated from Archean gneiss 

blocks by faults. From north to south, these domains are the 

Bush Lake trough, the Dickinson Group block of Archean 

gneiss and supracrustal metaconglomerate, the Felch trough, 

the South Felch Archean block, the Toby Lake Archean 

block, the Calumet trough and its westward extension into 

the Horserace Rapids area (fig. 2), and the Carney Lake 

Archean block. The Crystal Falls and Florence-Niagara terranes (Sedlock and Larue, 1985; Ueng and Larue, 1988), 

labeled in figure 2 for completeness and context, are not considered further in this study.

L2 Precambrian Geology of Lake Superior Region
Area of

100 KILOMETERS

47° 

46°-

y ^_ Wisconsin-Michigan segment J'

Early 

Proterozoic

EXPLANATION

Turbidites overlying shelf deposits 

Plutonic and volcanic rocks, mainly calc-alkalic Complex continental-margin sequences Archean rocks, undivided

Contact Dashed where approximately locatedHigh-angle fault

Major thrust fault Dashed where uncertain;

sawteeth on overthrust block 

Thrust fault Sawteeth on overthrust block

Figure 1. Map of northern Michigan-Wisconsin and Minnesota segments of the Penokean orogen, divided by rocks of the Midcontinent rift system (modified from Hoist, 

1991). Towns shown for reference include L'Anse, Mich. (LA), Covington, Mich. (C), Iron Mountain, Mich. (IM), Ironwood-Hurley, Mich. and Wis. respectively (IH) and Duluth, Minn. (D).
87°45'

nn 

o_

I

o

46°00'

45°45'

DICKINSON GROUP BLOCK
REGIONAL STRUCTURE AND 

METAMORPHISM

The Early Proterozoic and Archean rocks in the area 

were subjected to multiple deformations (table 1). These 

deformational phases (Dj, D?, D3) are unequivocally Early 

Proterozoic in age because they deform Early Proterozoic 

rocks, and they predate the =1,824 Ma post-tectonic granite 

pluton (Klasner, Ojakangas, and others, 1988; unit Xrg, this 

report) that intrudes deformed Michigamme slate in the 

northwest corner of the map area (fig. 2).

The initial phase of Early Proterozoic deformation 

(Dj) produced a penetrative, subhorizontal to steeply dipping 

foliation (Sj) in most of the Early Proterozoic rocks and a 

penetrative foliation in much of the Archean crystalline 

rocks. The foliation is expressed by aligned biotite, sericite, 

and chlorite in the Proterozoic rocks and by aligned biotite 

and muscovite, which together crosscut an older steep 

gneissic banding in the Archean rocks. Foliation S] is axialplanar to Dj folds that have gently plunging Fj fold axes; D? 

is characterized by a spaced, but locally a penetrative, foliation (Sp) in both Early Proterozoic and Archean rocks. At 

places S? is axial-planar to observed crenulation folds in S^ 

D3 deformation is recognized at a few localities as crenulation folds that affect both Sj and 82; a late phase of dextral 

faulting may correlate with D3 . These late deformational 

phases were not studied in detail.

EXPLANATION

EARLY PROTEROZOIC (1,600-2,500 Ma) 

Red granite 

Peavy Pond Complex 

Marquette Range Supergroup 

Paint River Group 

Menominee Group 

Sedimentary rocks 

Hemlock Formation 

Chocolay Group

Michigamme 

Formation of 

Baraga Group

Badwater 

Greenstone

ARCHEAN (2,500 Ma and older) 

Gneiss and metaconglomerate

Contact

Fault Sense of movement uncertain

Normal fault Hachures on downthrown side

Thrust fault Sawteeth on upper plate; queried where 

extent uncertain

Figure 2 (above and facing page). General geology of the 

Felch and Calumet troughs area. Modified from Sims (1990), 

Cannon (1986), and James and others (1961).

Table 1. Explanation of alphanumeric symbols used to 

describe deformation events and structural features in the Felch 

and Calumet troughs area

Early Proterozoic:

Dj Deformational event that formed Sj foliation, which is 

axial-planar to Dj folds (Fj). Lj lineations formed by intersection of Sj foliation and S0 bedding. Ls, stretch lineations formed during Fj deformation.

D2 Second deformational event, which formed S2 foliation; S2 

is axial-planar to D2 folds (F2). L^ lineations are caused by 

intersection of Sj and S2.

D3 A third deformation event, which formed rare crenulation 

fold lineations (F3 ) on Sj or S2 foliation surfaces.

Archean:

DA

SA 

LA

Multiple and complex deformation that occurred during 

Archean time.

Archean gneissic foliation. 

Archean stretch lineation.

The dominant Archean structure in the area is a gneissic 

foliation (SA) that dips steeply and is associated with steeply 

plunging fold axes (F^) or stretch lineations (L^). The 

Archean foliation generally strikes east to northeast, except in 

the Carney Lake block, where it strikes northwest.

The Peavy Pond Complex (fig. 2, unit Xpc) is the locus 

of high-temperature-high-pressure metamorphism (Attoh and 

Vander Meulen, 1984). Klasner (1972) and Bayley (1959) 

showed that the metamorphism mostly postdates DI deformation, and Klasner (1978) showed that metamorphism peaked 

during D2 deformation. Through combined metamorphic, 

structural, and gravity modeling studies, Attoh and Klasner 

(1989) suggested that the high-grade metamorphism was 

caused by tectonic stacking during thrusting in the Penokean 

orogen.

KINEMATIC DATA IN THE 

STRUCTURAL DOMAINS

In this section, we present kinematic data from specific 

locations that show the sense of movement in each of the 

structural domains. Features such as asymmetry of folds, fold 

and stratigraphic facing, stretch lineations, and regional structural relations informed the kinematic analyses.

The features most useful for determining the sense of 

tectonic transport are facing directions and the vergence of 

minor folds. Facing refers to the direction of stratigraphic 

younging, which is determined from sedimentary structures, 

in a succession of beds or in a fold (Schackleton, 1957; Bell, 

1981). Vergence of asymmetric folds has been defined (Roberts, 1974, p. 123) as "the horizontal direction, within the 

plane of the fold profile (perpendicular to fold axis), towards 

which the upper component of * * * rotation is directed." 

Also, sense of rotation of cleavage to bedding can define sense

Felch and Calumet Troughs Area, Northern Michigan L5
87°45'

1oWISCONSIN 

MAGMATIC 

TERRANES

45°45'
of vergence (Bell, 1981, p. 200). It is important to use both 

vergence, as determined from minor structures, and facing 

together wherever possible. For example, facing and vergence must be used together to define which limb of a major 

fold one is working on, and in which direction the hinge of 

the fold is located.

Bush Lake Trough

The Bush Lake trough is bounded on the north by the 

Bush Lake fault and on the south by the Norway Lake fault 

(fig. 2). The Bush Lake fault is a classic example of a thrust 

fault; it truncates structures in the footwall whereas structures in the hanging wall are parallel to the thrust. It separates 

the Amasa block on the north from the Bush Lake trough on 

the south. Attoh and Vander Meulen (1984) have shown 

from metamorphic studies in the area of the Peavey Pond 

Complex that the southern block is upmrown. Their reconstruction of the geometry of metamorphic isograds suggests 

that the Bush Lake fault dips toward the south, but definitive 

data on the dip of the fault are not available.

Bedrock is poorly exposed in the Bush Lake trough. 

Most outcrops in the trough consist of the Randville Dolomite. One such outcrop, which lies approximately 260 m 

south of the trace of the Bush Lake fault (fig. 3, locality 1), 

exposes a contact between dolomite and a sandy dolomitic 

layer. A scour channel in the dolomite and inclusions of 

dolomite in the sandy layer indicate that bedding is overturned toward the north and dips steeply south (fig. 4A). 

Foliation (Si) strikes northwest and dips gently southwest 

(fig. 45). Cleavage-bedding relationships indicate that an 

overturned anticline exists to the north. Clearly, rocks in

the hanging wall of the Bush Lake fault were subjected to 

northward-verging deformation.

Dickinson Croup, South Felch, and 

Toby Lake Blocks

The Norway Lake fault separates the Dickinson Group 

block from the Bush Lake trough (fig. 2). A regional 

Archean fabric in gneisses just south of the fault (fig. 3, 

locality 2) consists of an east-trending, steeply dipping foliation (8^). Deformed pebbles in the Archean gneiss have 

axes that plunge steeply southeast. In contrast, mylonitic and 

cataclastic structures within the Norway Lake fault zone 

overprint the regional Archean fabric, and therefore are presumed to be Proterozoic in age.

Several private-company drill holes penetrated a 

fault (fig. 3, locality 2A) 1 km south of the Norway Lake 

fault that definitely is a southward-verging thrust. The fault 

dips 45° N. and is marked by several centimeters of mylonite. Hanging-wall gneisses are intensely brecciated and 

chloritized.

The Dickinson Group block and the South Felch block 

are not continuous; they are separated by the Felch trough. 

However, during our study we recognized for the first time 

the existence of a pervasive, subhorizontal foliation that 

overprints the steep Archean gneissic fabric in both these 

blocks. The flat foliation (Si), primarily aligned biotite, 

occurs in the Archean rocks adjacent to the Felch trough 

(fig. 3, area 3) and crosscuts the consistently steeply dipping 

Archean gneissic foliation (SA) (fig- 5A, B). The flat foliation also crosscuts the Early Proterozoic strata of the Felch 

trough, as discussed following. Thus, it postdates deposition 

of the Early Proterozoic strata.

EXPLANATION

EARLY PROTEROZOIC (1,600-2,500 Ma) 

Red granite 

Peavy Pond Complex 

Marquette Range Supergroup 

Paint River Group 

Menominee Group 

Sedimentary rocks 

Hemlock Formation 

Chocolay Group

ARCHEAN (2,500 Ma and older) 

Gneiss and metaconglomerate

Michigamme 

Formation of 

Baraga Group

Badwater 

Greenstone

30-

Strike and dip of beds

Strike and dip of overturned beds

Strike and dip of Sj foliation

Strike and dip of Sj foliation and bearing of 

mineral lineation

Bearing and plunge of asymmetric fold; down 

plunge asymmetry shown

Strike and dip of Archean (S^) foliation

Vertical

Bearing and plunge of Archean deformed clast 

Contact

 Fault Sense of movement uncertain

 Normal fault Hachures on downthrown side

 Thrust fault Sawteeth on upper plate; queried

where extent uncertain 

Groveland iron mine

Figure 3 (above and facing page). Felch and Calumet troughs area, showing detailed rock attitudes and location of 

detailed study areas (circled numbers) discussed in text.

Felch and Calumet Troughs Area, Northern Michigan L7
Scour 

channel

.:: :.--.:.-. .-A :.-.:.- . :. 50

B

Figure 4. Outcrop near Bush Lake fault (fig. 3, locality 1). 

A, Field sketch; note overturned bedding. B, Lower hemisphere stereoplot; open circle, pole to bedding (S0); solid 

dot, ST foliation. Average bedding plane (stippled) and S-, 

foliation plane also shown.

The structural relationships among the relatively low 

dipping foliation (Si), the steep Archean gneissic foliation 

(SA), and a younger, steeply dipping foliation (82) in the 

Archean rocks within the South Felch block are illustrated in 

figure 6 (fig. 3, locality 4). The gneissic foliation (SA) has 

been deformed into a southwest-verging minor fold with an 

axial-planar Si foliation. Si, and presumably also SA, 

although not directly observed, are deformed into small 

crenulation folds having a steeply dipping axial-planar foliation (82). Si foliation is oriented approximately N. 40° W.,

25° NE., and S2 is oriented generally N. 40° W., 50° NE. The 

asymmetric minor fold and Si foliation in Archean gneiss 

suggest south-verging deformation, consistent with senses of 

vergence elsewhere in the area, but stratigraphic facing 

directions are not known at this location.

The Toby Lake block is bounded to the north by a 

north-verging thrust, the Toby Lake fault, and to the south 

by the south-verging North Calumet fault (figs. 2 and 3). 

The North Calumet fault truncates the Toby Lake fault to 

the east. The dominantly steep foliation (S^.) in the gneiss 

is deformed at places by Si foliation. Si foliation is best 

developed adjacent to the North Calumet fault, where it 

typically is a shallowly dipping mylonitic foliation. Mylonite is especially conspicuous in a post-tectonic (Archean) 

granite body in the southwestern part of the block. The foliation dips gently (25° -30° ) north, and probably reflects a 

shallow northward dip of the North Calumet (thrust) fault.

Felch Trough

Rian quarry, which lies on the south edge of the Felch 

trough (fig. 3, locality 5), exposes a =40-m-long and ~7-mhigh quarry face of Randville Dolomite (Chocolay Group of 

Marquette Range Supergroup). Several features in the 

quarry face, diagrammatically sketched in figure 7A, show 

that the rocks here have been rotated toward the south. 

Bedding is vague in the dolomite, but where it can be 

recognized it is deformed into south-verging minor folds, as 

shown at points A, B, and C. A red sandy conglomeratic unit 

between points E and F is nearly vertical. Clasts of the red 

sandy layer in overlying dolomite (near point F) indicate that 

stratigraphic facing is to the south. A stereoplot (fig. IB) 

indicates that bedding dips steeply, subparallel to the sandy 

layer. Foliation (Si) dips moderately toward the north, as 

shown at point D. Bedding-cleavage intersection lineations 

(Li) plunge gently southeast. Several small concretions in 

the dolomite near point E (fig. 7A) have long (stretch) axes 

that plunge steeply north-northeast. All structural data in the 

quarry indicate southward structural vergence.

A geologic map of the Felch trough near the Rian 

quarry (fig. 8; modified from James and others, 1961) shows 

that the Sturgeon Quartzite of the Chocolay Group crops out 

south of the quarry, suggesting (as shown in cross section of 

fig. 8) that a fault exists between the dolomite in the quarry 

and the quartzite. Such fault complications are supported by 

the presence of a roughly 5-m-wide slice of gneiss within the 

dolomite of the quarry. Presumably the gneiss is Archean 

and was tectonically emplaced from the underlying Archean 

basement.

Cross section A A' (fig. 8) shows that the Felch trough 

in the area is south verging; it has a southward-overturned 

north limb and a gently north dipping south limb. Inasmuch 

as bedding dips more steeply than foliation in Rian quarry 

(fig. 7A, B), the quarry is located on the steep limb of a small

L8 Precambrian Geology of Lake Superior Region
EXPLANATION

 Contact

Fault Probably thrust; 

sawteeth on upthrust 

block

n=9

Figure 5. East end of Early Proterozoic Felch trough and 

adjacent Archean (A) rocks. A, Map of Felch trough (stippled), 

showing lower hemisphere stereoplots of poles to steeply 

dipping Archean gneissic foliation (SA) and smaller lower 

hemisphere stereoplots of planes of S^ foliation. Solid dots are 

localities (n=number of measurements) where S-\ was measured in Archean rocks (fig. 3, locality 3). Modified from 

Klasner, Sims, and others (1988). B, Outcrop 10 (in A), 

showing steeply dipping Archean foliation (SA) and 

superimposed gently dipping Early Proterozoic foliation 

(S-]). Outcrop in roadcut on Dickinson County Highway 

569. Hammer is 30 cm long.

Felch and Calumet Troughs Area, Northern Michigan L9
N75°E

=30 CENTIMETERS

Figure 6. Field sketch showing D^ folding of Archean foliation (SA) and D2 folding of S^ foliation, in the South Felch 

block. S2 is axial-planar to F2 folds in S^ Note that folding is 

south verging (fig. 3, locality 4).

parasitic fold such as that diagrammatically shown in figure 

8. Like the minor folds observed on the cliff face of the 

quarry, the trough is most likely a parasitic fold on a much 

larger, but now largely eroded, south-verging fold system. 

The commonality between small-scale and large-scale structures is shown by the fact that the S j foliation is axial-planar 

to both the small folds at Rian quarry and the much larger 

synclinal fold of the Felch trough itself, which is an upwardfacing fold.

A hilltop outcrop of Sturgeon Quartzite at locality 6 in 

figure 3 reveals small scour channels and crossbeds, indicating stratigraphic tops to the south, and minor folds (fig. 9A). 

Bedding dips north and is overturned toward the south. A 

stereoplot (fig. 9B) shows that bedding has been folded about 

east-northeast-trending axes, as indicated by L and by the 

fold axis as determined from the pi diagram. Planar fabrics 

are poorly preserved in the massive quartzite and are difficult 

to measure. We are uncertain of the relative age of the 

foliation and have designated it as S without a subscript on 

figure 9B. Intersection of bedding (So) (not shown in fig. 9B) 

and S produces intersection lineations (L). The foliation (S), 

intersection lineations (L), and fold axes Tif, as determined

from the pi diagram, may reflect D2 deformation; a steep 

foliation is characteristic of 82 elsewhere (see fig. 6). We 

interpret the southward-overturned bedding to represent the 

steep south limb of a minor fold, similar to that at Rian 

quarry.

A small area (fig. 10A) containing outcrops of 

Randville Dolomite, Sturgeon Quartzite, and Archean gneiss 

(fig. 3, area 7A) lies on the north limb of the Felch trough 

and provides data on structural vergence, the relationship 

between Si and 82 foliation, and the sense of movement on 

a fault within the Felch trough. The outcrops depicted in 

figure 10A show a south-facing, inverted stratigraphy, that 

is, Sturgeon Quartzite on the north overlies steeply north 

dipping Randville Dolomite on the south. A parasitic fold is 

present in the dolomite (fig. 10A). Archean foliation (S^.) 

dips steeply, but it is overprinted and crosscut by a more 

gently dipping Early Proterozoic foliation (Si). A shear zone 

occurs in the Randville Dolomite (near the road intersection 

in the northeast corner of area shown in fig. 10A). Quartz 

rods in the shear zone, which are considered to be a DI 

feature, plunge 30° NE.; and slickenlines also plunge 

northeast. The quartz rods and slickenlines give the direction 

of tectonic transport in the shear zone and, together with 

nearby drag folds and sense of rotation of Early Proterozoic 

strata, indicate a southwest sense of vergence.

Figure 105 provides a view of the stratigraphic and 

structural relationships in a larger area than figure 1QA in this 

part of the Felch trough. The minor fold shown in the 

southwest corner of the figure area has been projected along 

strike onto the figure to show the sense of vergence on the 

south side of the Felch trough. The minor fold actually 

occurs to the west (fig. 3, locality 8). The hilltop exposure of 

Sturgeon Quartzite at locality 6 (fig. 3), which lies within the 

map area of figure 10J5, is also shown in cross section B-B'.

The nature of the southward tectonic transport in the 

area is shown on interpretive cross sections A-A' and B-B' 

of figure 10. Section A-A' suggests that a south-verging 

thrust fault of Archean basement emplaced over the Proterozoic rocks caused southward overturning of the Early Proterozoic strata, as illustrated in section B-B'. The Felch 

trough at this location is a syncline overturned toward the 

south. The layer of Archean gneiss (Wg) in the center of the 

Felch trough (fig. 10 A and section A-A') is interpreted as a 

klippe, that is, an allochthonous slice of gneiss emplaced 

above Early Proterozoic strata along the fault. Low-dipping 

Sj foliation was formed during the DI phase of southverging deformation. The hilltop exposure (fig. 3, locality 6) 

of southward-overturned strata of Sturgeon Quartzite 

remains as an erosional remnant in this south-verging thrustfold system. We interpret the undulating nature of the southverging thrust fault to be caused by D2 deformation.

Ueng and Larue (1988) drew a structural cross section 

of the Groveland iron mine, within the Felch trough (fig. 3, 

locality 9), which we have modified herein (fig. 11). The 

section shows subhorizontal thrust faults that were deformed

L10 Precambrian Geology of Lake Superior Region
SOUTH NORTH

Red sandy / Concretion 

laYer\ ^ % 

C D quarry floor B

10 METERS

B

Figure 7. West cliff face of Rian quarry, Felch trough. A, Diagrammatic sketch. Cliff face is oriented N. 75° E., and composed 

of coarsely crystalline phase of Randville Dolomite. Red sandy layer occurs near south end. S0 , bedding; S-|, foliation (fig. 3, 

locality 5). Points A-F are described in text. B, Lower hemisphere stereoplot of structural features at Rian quarry. Open circle, 

pole to bedding (S0); solid dot, pole to S^ foliation; X, FT fold axis or l^ lineation formed by intersection of S0 and S-,; square, long 

stretch axis (Ls) of concretion. Note orientation of S-j foliation plane.

by a later event. Ueng and Larue did not carry out kinematic 

analyses (D.K. Larue, written commun., 1988), but they diagrammatically showed south-verging minor folds in the 

Sturgeon Quartzite below the uppermost thrust fault. JJ. 

Mancuso (written commun., 1988), who also has done 

extensive work in the Groveland mine, recognized several 

low-dipping structures, but he observed numerous steeply 

dipping faults as well. Although we did not have access to 

the open-pit mine because of flooding and dangerous conditions, the data from Ueng and Larue (fig. 11) and Mancuso 

support the concept of a subhorizontal thrust event followed

by a later, more steeply oriented deformation. In fact, Ueng 

and Larue (1988) pointed out that the low-dipping thrust surfaces, which we designate Si, were significantly folded during a later event.

A gravity profile (fig. 12A) across the Felch trough, 

near its west end (fig. 3, locality 10), provides additional 

information on the structure of the trough. Observed gravity 

along the profile varies only slightly from the gently south 

dipping regional gravity gradient in the region. (See Klasner 

and others, 1985, for data on regional gradient.) The Early 

Proterozoic Felch trough has little gravity expression. The

Felch and Calumet Troughs Area, Northern Michigan L11
N 

A

40,32

A^

u

Wg

EXPLANATION

EARLY PROTEROZOIC 

Metadiabase 

Vulcan Iron-formation 

Felch Formation 

Randville Dolomite 

Sturgeon Quartzite

ARCHEAN 

Gneiss

Contact Dashed where inferred; queried

where uncertain 

Strike and dip of beds and plunge of

mineral lineation on bedding 

Strike and dip of Sj foliation and

plunge of mineral lineation on S^ 

Fault U, upthrown block

Thrust fault Sawteeth on upthrown block 

Rian quarry

Figure 8. Felch trough in area of Rian quarry. Modified from James and others (1961) with additions by P.K. Sims. Quarry lies 

near south edge of trough. Cross section A-A' suggests that the southward-rotated Randville Dolomite strata at Rian quarry (fig. 

7) are part of a drag fold and are separated from Sturgeon Quartzite on the south by a fault. S-, foliation is axial-planar to folds 

in bedding at Rian quarry as well as to synclinally folded strata of Felch trough.

south half of the post-kinematic red granite pluton has a 3 

milligal gravity low above it. An interpretive cross section 

(fig. 125), constructed along the gravity profile, is based on 

correlation of the gravity profile with the bedrock geology 

mapped by James and others (1961) and on our structural 

measurements.

The gravity survey was designed to determine whether 

or not the stratigraphy is inverted in the Felch trough, but it 

yielded no evidence that the Early Proterozoic strata in this 

part of the trough are inverted (overturned). If the Early 

Proterozoic strata were overturned to form an antiformal 

syncline, as suggested by Maharidge (1986), a substantial 

body of buried Vulcan Iron-formation should have shown 

up clearly on the gravity survey. The structural data, which

constrain fold symmetry, combined with the gravity data 

suggest that (1) the trough at this location is upward facing 

and asymmetrically inclined toward the south; (2) the steeply 

dipping north limb is slightly overturned toward the south 

and the south limb dips moderately toward the north; and 

(3) the axial plane of the trough dips about 50° N. The latter 

figure coincides with the earlier interpretation of James and 

others (1961) and is consistent with the relatively moderate 

dipping Si foliation found throughout the region, which is 

axial-planar to minor folds in the trough.

In summary, all the structural data observed along the 

length of the Felch trough indicate southward vergence. 

Evidence for a south-verging deformational event (Dj) is 

most notable in the central and eastern parts of the trough.

L12 Precambrian Geology of Lake Superior Region
30

EXPLANATION

Strike and dip of overturned 

bedding (S0)

Figure 9. Sturgeon Quartzite at north edge of Felch trough 

(fig. 3, locality 6). A, Diagrammatic sketch of primary sedimentary structures. Bedding expressed by faint color banding in 

quartzite. Bedding dips northeast. Crossbeds and scour channels in the area (diagrammatically shown) indicate that beds 

stratigraphically face southwest and are overturned to southwest. Scale is approximate. B, Lower hemisphere stereoplot; 

open circle, pole to bedding (S0); solid dot, pole to (S^ foliation; X, bedding-cleavage intersection lineation (L^). Great circle (dashed line) defines fold axis (triangle) that is nearly 

coincident with L-|.

In these areas, a subhorizontal foliation (Si) occurs both in 

Archean gneiss adjacent to the trough and in the Early Proterozoic strata within the trough.

A second coaxial deformation (D2) deformed bedding 

(So) and Si foliation. Resulting 82 foliation dips steeply 

north and strikes generally west-northwest. The D2 deformation appears to have significantly folded Si and DI fault 

planes, such as those at the Groveland mine; accordingly, the 

pre-D2 orientation of Si and the fault planes is not accurately 

known.

Toby Lake Trough

Except for a single exposure of Randville Dolomite 

(Chocolay Group) at the east end of the Toby Lake trough 

(fig. 3, area 11), only Sturgeon Quartzite is exposed within 

the trough; the presence of the Michigamme Formation (fig. 

13) was inferred by James and others (1961) on stratigraphic 

grounds. Primary structures in the quartzite, such as crossbedding and scour channels, unequivocally indicate that 

bedding, which dips steeply southeast, is overturned toward 

the north. A spaced fracture cleavage in the quartzite strikes 

east and dips moderately to the north (fig. 13); it is interpreted as Si- Archean foliation on either side of the Toby 

Lake trough is steeply dipping, but it is overprinted in places 

by a shallower foliation that is interpreted as Si.

The dip and sense of movement on the Toby Lake 

fault at this location (fig. 3, area 11) are not known; the fault 

could dip steeply south, and the Early Proterozoic strata of 

the trough could represent a fold formed by drag along the 

fault, as shown in the section of figure 13. Farther east along 

the Toby Lake fault (fig. 3, locality 11 A), Archean gneissic 

foliation is oriented about N. 75° E. and is nearly vertical. 

The gneissic fabric is overprinted by zones of fine-grained, 

foliated mylonite. Small minor folds in the gneiss, whose 

axes plunge 10°-50° N., 75° E., have a dextral sense of 

movement in the horizontal plane. Similarly, the orientation 

of a minor fold in the vertical section suggests that the fault 

is nearly vertical at this location and the north side is 

upthrown.

The sense of movement from overturned bedding in 

the Toby Lake trough suggests northward vergence on the 

fault with an upthrown southern hanging wall. Thus, the 

fault appears to be a north-verging, high-angle thrust fault at 

this location (fig. 3, area 11).

In contrast, to the east along the Toby Lake fault 

(fig. 3, locality 11 A), structural data such as minor folds 

suggest that the fault is nearly vertical and that the north 

side is upthrown. Still farther east, the Toby Lake fault is 

truncated by the North Calumet fault (fig. 3, locality 15), 

and structural data indicate that this fault is a south-verging, 

high-angle thrust fault.

Calumet Trough

The Calumet trough is bounded on the north by the 

North Calumet fault and on the south by the South Calumet

Felch and Calumet Troughs Area, Northern Michigan L13
A' B

B'

Wg Wg Xs

Locality 6 

Xs

Xs Xr

Cambrian sandstone 

EARLY PROTEROZOIC

Randville Dolomite 

Sturgeon Quartzite 

ARCHEAN 

Gneiss

EXPLANATION

A A A

70 

30

25.^15 

70

Contact 

Outcrop area

Thrust fault Sawteeth on upthrown block

Strike and dip of bedding

Strike and dip of Sj foliation

Bearing and plunge of slickenlines

Strike and dip of SA foliation

Bearing and plunge of quartz rods in Sj shear

Bearing and direction of plunge of asymmetric

F! fold 

Thrust fault Showing relative movement, in

cross section; dashed above ground surface 

Asymmetric fold, in cross section

Figure 10. Geology of an area of Felch trough near intersection of old Highway 69 and Highway 569 (fig. 3, locality 7B)./A, Structure and interpretive cross section A-A' of area on north limb of Felch trough. 8, Structure and interpretive cross section B-B' of 

larger area, inclusive of area of A, and also showing locality 6 of figure 3.

fault (fig. 2); the trough appears to extend west-northwest to 

Horserace Rapids (fig. 3, locality 12).

At Horserace Rapids (fig. 2), on Paint River, graded 

beds of graywacke of the Michigamme Formation are overturned toward the south (fig. 14A). A subhorizontal foliation 

(Si) is oriented approximately N. 10° W., 25° SW. This 

foliation is axial-planar to Dj folds (Fj) whose axes plunge 

gently northwest. In turn, Si is folded about gently plunging 

D2 fold axes, which are shown by L2 intersection lineations 

of Si and 82 (fig. 14A). The 82 foliation dips steeply. The 

southwestward-overturned strata, northwest-oriented fold

axes, and subhorizontal foliation together indicate that the 

Early Proterozoic rocks at Horserace Rapids were deformed 

by a southwest-verging structural event. A second event 

(D2) deformed Si and SQ about northwest-trending fold axes 

(p2) having a steeply dipping 82 foliation.

A structural scenario similar to that at Horserace 

Rapids exists about 20 km to the southeast, at Steele farm 

(fig. 3, locality 13). At this locality, southward-facing 

graywacke beds are oriented N. 80° E., 80° SE. (fig. 145). A 

subhorizontal Si foliation is oriented N. 80° E., 25° SE. Si 

is folded about gently southwest plunging ?2 fold axes. At

L14 Precambrian Geology of Lake Superior Region
SOUTH NORTH

EXPLANATION

Alluvium Sturgeon 

Quartzite

Vulcan Iron-formation Randville 

(Menominee Group) Dolomite

(Chocolay Group)

- Contact

Brecciated 

Randville 

Dolomite

A- Thrust fault Sawteeth on upper plate; dashed where covered

Figure 11 . Structural cross section of Groveland mine (fig. 3, locality 9), looking west. Modified from figure 14 of Ueng and Larue 

(1988). Note low-dipping thrust faults, folding of these thrust planes by a later event, and drag folds diagrammatically shown in 

the Sturgeon Quartzite. Horizontal scale and vertical scale same.

NNW

A

40 -

45 -

SSE

B

FELCH TROUGH

1 -

EXPLANATION

Granite 

Menominee Group

Vulcan Iron-formation 

Chocolay Group

Randville Dolomite

Sturgeon Quartzite 

Archean gneiss

- Contact

Figure 12. West end of Felch trough. A, Bouguer gravity profile (solid line) across Felch trough at traverse labeled 10 (fig. 3). 

Dashed line, regional gravity gradient, from Klasner and others (1985). B, Geologic section of Felch trough incorporating stratigraphic and structural data of the trough near profile of A. Note that trough is asymmetrically overturned toward the south and is 

about 1 km deep. Length of section about 5 lk km.

Felch and Calumet Troughs Area, Northern Michigan L15
Wg FAULT

EXPLANATION

EARLY PROTEROZOIC

Michigamme Formation

Randville Dolomite

Sturgeon Quartzite 

ARCHEAN

Gneiss

Contact

Strike and dip of bedding

Strike and dip of overturned

bedding 

Strike and dip of 5j foliation

Strike and dip of Sy\ foliation

Thrust fault Sawteeth on upper 

plate

Fault Showing relative sense of 

movement (in section)

Figure 13. Geology (area 1 1 , fig. 3) of Toby Lake trough region. Note that Sturgeon Quartzite is overturned toward the northwest. 

Early Proterozoic foliation (S^ dips moderately northwest. Interpretive cross section A-A' suggests that the Early Proterozoic beds 

were overturned toward the north as part of a north-verging drag fold. The southern, hanging-wall block of the Toby Lake fault is 

upthrown. East-west length of Xr and Xs outcrop band is about 8 l/2 km.

one place, Si has a dihedral fold angle of 40°. F2 and Fj fold 

axes are coaxial. A third deformational event (F3) is 

represented by F3 crenulation folds on Si (fig. 145).

Structural data obtained along the North Calumet fault 

near localities 14 and 15 (fig. 3) provide additional evidence 

for the sense of vergence. Archean gneissic foliation near 

the fault dips steeply and strikes northeast (fig. 14C). At 

places, the gneiss is crosscut by zones as much as a meter 

wide of fine-grained, sugary-textured, foliated mylonite. 

The mylonite typically dips at a much lower angle than does 

the Archean foliation. Near the contact with Early Proterozoic strata, the mylonitic fabric in the Archean gneiss strikes 

northeast and dips steeply northwest (fig. 14C). This 

mylonitic foliation is interpreted to be Sj. Other minor folds 

in the Archean gneiss near the North Calumet fault have a 

Z-symmetry, with a dextral sense of movement and nearly 

vertical fold axes (fig. 14C).

Structural data from exposures of Early Proterozoic 

strata adjacent to the North Calumet fault also indicate 

southward vergence. Bedding is typically tightly folded, and

axial-planar foliation (Si) is nearly vertical to gently north 

dipping (fig. 14ZJ). Bedding generally dips steeply north and 

is right way up, but at one locality it is overturned toward the 

south.

Based primarily on drill-hole and underground mine 

data, James and others (1961, pi. 4) showed the North Calumet fault as being vertical to steeply north dipping. They 

noted that the axial planes of folds in this region dip steeply 

south and that bedding also is steep and slightly overturned 

toward the north. These observations differ from ours (fig. 

14D); the only beds that we observed to be overturned are 

overturned to the south. Bedding (So) and Si foliation are 

nearly vertical in this area, and both have been deformed by 

a second (D2) event as shown by F2 fold axes (fig. 14£>).

We conclude from our accumulated data that the 

North Calumet fault is a generally high angle, south-verging 

thrust fault; the thrust possibly dips gently north in its 

western part. James and others (1961, pi. 4) interpreted drillhole data to indicate that the North Calumet fault dips north 

and is upthrown on the north. This is confirmed from our

L16 Precambrian Geology of Lake Superior Region
D

Figure 14. Lower hemisphere stereoplots of structural data 

from the Calumet trough area. A, Structural data at Horserace 

Rapids (fig. 3, locality 12). Solid dot, pole to S-] foliation; open 

circle, pole to bedding (S0). Several graywacke beds stratigraphically face southward (circled area with arrow). Note that the 

great circle defines a fold that plunges gently west-northwest. 

Open triangle, plunge of FT fold axis in bedding S0 ; solid 

triangle, F 2 fold axis; X, plunge of lineation (L-,) formed by 

intersection of bedding (S0) and foliation (S^; solid square, 

plunge of intersection lineation (L2) of spaced S2 foliation and 

ST foliation. S2 is steeply dipping but was not measured. 8, 

Structural data from Steele farm area (fig. 3, locality 13). Solid 

dot, pole to S-| foliation; open circle, pole to bedding S0; 

graywacke beds stratigraphically face south; open square, pole

to S2 foliation; open triangle, plunge of F2 fold axis in Sj; solid 

triangle, plunge of FT fold axis in S0 ; diamond, plunge of F 3 

crenulation fold. C, Structural data from Archean gneiss along 

North Calumet fault (area between localities 14 and 15). Solid 

dot, pole to Si mylonitic foliation. Note orientation of S} from 

outcrops near contact with Early Proterozoic strata. A, pole to 

Archean foliation (SA). Plus (+), plunge axis of a Z-shaped drag 

fold in Archean gneiss near the Calumet fault. D, Structural data 

from Early Proterozoic strata adjacent to North Calumet fault 

(area between localities 14 and 15, fig. 3). Solid dot, pole of S^ 

foliation; open circle, pole to bedding S0 ; most beds stratigraphically face northward; open triangle, F-j fold in S0 ; solid triangle, 

F2 fold axis.

Felch and Calumet Troughs Area, Northern Michigan L17
10

Edge of 

outcrop area

B

EXPLANATION

Zone of quartz veins

Foliation (S^

Plunge of Z-fold, 

looking down plunge

10

/?T Plunge of fold

/V Strike and dip of bedding

Zb

^^ Strike and dip of bedding 

72 Arrow shows facing direction

D

Area of D

Graded beds

Figure 15. Structure of an outcrop area in Calumet trough, along west branch of Sturgeon River. A, Geologic structure at locality 

16 (fig. 3). B, Contoured lower hemisphere stereoplot of poles to ST . Contours at 1, 5, 20, 30 percent. C, Contoured lower hemisphere stereoplot of poles to bedding (S0). Contours at 1, 4, 6 percent. D, Cross section A-B from view A. E, Interpretive diagram 

suggesting that this area is on the upper limb of a recumbent fold.

LI 8 Precambrian Geology of Lake Superior Region
Figure 16. Lower hemisphere stereoplot of Early Proterozoic 

structures in Archean gneiss terrane of Carney Lake block 

(fig. 3, area 17). Solid dot, S-\ foliation; solid square, plunge 

of stretch lineation (LJ on Si.

structural measurements of mylonitic foliation along the 

fault (fig. 14C, D) and from magnetic data of James and 

others (1961, plates 2-east half and 4) in the region. For 

example, positive east-trending magnetic anomalies, which 

lie just south of the fault above the Early Proterozoic rocks 

of the Calumet trough, extend obliquely across the fault into 

the area of Archean gneiss. This suggests that the Archean 

rocks have been thrust southward, above Early Proterozoic 

strata of the Calumet trough, overriding the magnetically 

susceptible rocks of the trough that caused the magnetic 

anomalies.

Schist and phyllite of the Michigamme Formation are 

exposed in the Calumet trough along the west branch of the 

Sturgeon River (fig. 3, locality 16). At this location, Si 

foliation is subhorizontal and forms a gently undulating

Figure 17 (facing column). Structural data from locality 18 

(fig. 3) in Sturgeon Quartzite, west edge of Carney Lake block. 

A, Vertical cross section of north-verging fold in Sturgeon 

Quartzite. Note horizontal S^ Bedding stratigraphically faces 

south in the area, indicating that the quartzite lies on the lower 

limb of a large-scale recumbent fold. B, Lower hemisphere 

stereoplot. Solid triangle, pole to S0 ; open square, pole to $2 

foliation; open circle, measured F 1 fold axis; solid dots 

enclosed by dashed line, poles to measured S-\ foliation. Great 

circle interpreted to represent folded S0 and S^ foliation with 

the F 2 fold axis (X) as shown. S2 foliation is vertical. See L2 

lineations on C. C, Lower hemisphere stereoplot showing 

general orientation of bedding S0, foliation S-|, and foliation S2 . 

Plus (+), L2 lineation due to intersection of S2 and S-|.

Stratigraphic

facing 

direction 0 1 METER 

J

B

Felch and Calumet Troughs Area, Northern Michigan L19
R 29 W R 28 W

Biotite schist

Amphibolite

 - Approximate edge of

outcrop of gneiss 

Contact 

Fault

R30W R29W R28W

Figure 18. Orientation of long axes of mafic inclusions in Carney Lake Gneiss. Modified from Bayley and others (1966). Note 

that the orientation of the inclusions defines a fold with a northwest-trending axial plane. Note parallel orientation of fold axis in 

quartzite.

surface (fig. 15A, B}. Bedding (So) generally strikes northeast and has a wide range of dips (fig. 15C). The bedding has 

been deformed into a series of south-verging minor folds 

with axial-planar Si foliation. At one place, a small scour 

channel shows that the stratigraphic facing in bedding is 

toward the south. Both S\ and SQ have been deformed about 

northeast-trending, gently plunging fold (F2) axes with a 

steep-dipping 82 foliation. Clearly, the Early Proterozoic 

strata in this area were deformed by a south-verging event 

(Di) that formed a subhorizontal foliation in the rocks. 

Cross-section sketches (fig. 15D, £) suggest that this area 

comprises the upper limb of a south-verging recumbent fold.

Carney Lake Block

Bayley and others (1966) described the Carney Lake 

Gneiss (fig. 3, area 17) as consisting largely of granite gneiss 

but also containing minor phases of granodiorite gneiss,

syenite dikes, and inclusions. The Archean gneissic foliation 

(8^) typically trends northwest and dips steeply. We also 

observed a subtle, less steeply dipping foliation (Si) that 

overprints the Archean foliation (fig. 16). Orientation of Si 

is scattered, and dips range from flat to about 60°. The Si 

surface commonly contains a mineral lineation (Ls), which 

plunges moderately east to southeast (fig. 16) and which we 

interpret as a stretch lineation.

The west edge of the Carney Lake Gneiss block is 

bounded by the Sturgeon Quartzite (Chocolay Group), 

which is in unconformable contact with the gneiss (fig. 2) 

and is locally overturned to the southwest. Higgins (1947, 

p. 489) noted that the Early Proterozoic strata were folded 

into "a northwest-trending monocline that probably was 

thrust faulted along its axial plane. During the second 

phase of deformation this structure was displaced at its 

northwestern and southeastern ends by vertical tear faults 

that trend northeast."

L20 Precambrian Geology of Lake Superior Region
Our studies of structures in the Sturgeon Quartzite 

provide additional information on the nature of deformation 

in the region and the sense of vergence. The quartzite (fig. 3, 

locality 18) has been highly deformed. Beds (So) are isoclinally folded (fig. 17A) and have a low-dipping axial-planar 

foliation (Si). Both SQ and Si have been refolded by a second 

event (D2) into upright folds having a steeply dipping axialplanar foliation (S2). The bedding (So) is deformed about 

gently west plunging fold axes (Fj, fig. 11B). Foliation Sj 

strikes northwest and dips gently southwest (fig. 1 1C), but it, 

as well as bedding (So), is folded about a gently plunging 

fold axis (7i2) determined from the great circle in figure 17B. 

An axial-planar S2 foliation strikes northeast and is nearly 

vertical (fig. 17B, C). Minor folds, similar to that shown in 

figure 11 A, have a northerly sense of vergence, but the folds 

face south and lie within beds that are overturned toward the 

south. This pattern indicates that the quartzite lies on the 

lower limb of a south-verging major fold, possibly a nappe.

Orientations of biotite schist and amphibolite inclusions in the Carney Lake Gneiss define a northwest-trending 

fold (fig. 18). Because the axial plane of this fold is nearly 

coincident with the strike of the Sj foliation and the 

northwest plunge of the Si fold in the quartzite at locality 18 

(fig. 3), we suggest that the orientation of the inclusions 

probably resulted from DI deformation. Observations by 

Higgins (1947, fig. 8) support this interpretation. He 

concluded that the foliation in the gneiss was deformed 

("upturned") along the contact with the quartzite by the same 

event that deformed the quartzite.

Summary of Kinematic Analyses

Kinematic analyses to determine the direction of 

Early Proterozoic tectonic transport in each of the domains 

discussed previously take into account several structural 

features. One salient feature is a newly recognized lowdipping to subhorizontal Si foliation that overprints a subvertical Archean gneissic fabric, and which also occurs in 

Early Proterozoic strata; Si is axial-planar to small-scale 

asymmetric folds that yield a sense of structural vergence. 

As noted previously, the symmetry and vergence of isolated minor folds do not in themselves yield a unique sense 

of overall structural vergence, but when used in conjunction with stratigraphic facing directions, such as overturned 

bedding and fold symmetry, they can provide reliable kinematic information. The orientation of stretch lineations and 

fold geometry are among other features that provide pertinent information on the sense of structural vergence.

Structural vergence was determined at several 

localities in the study area (summarized in fig. 19). 

Northward-overturned bedding with south-dipping axialplanar Si foliation indicates that the Bush Lake trough is 

north verging (A, fig. 19). This finding is consistent with 

the observations of Attoh and Vander Meulen (1984) and

Attoh and Klasner (1989) that the Bush Lake fault is a 

reverse fault. The sense of vergence of the Norway Lake 

fault is not known (B, fig. 19), but a small fault nearby at 

BI is a north-dipping, south-verging thrust fault.

The Carney Lake block is a south-verging feature, and 

the Sturgeon Quartzite along the southwest edge of the block 

(fig. 19) is the lower limb of a possible crystalline-core anticline, the axis of which trends northwest. The vergence is 

indicated by a north-verging, Z-shaped fold in the Sturgeon 

Quartzite within southward-overturned strata; also, the Sturgeon Quartzite along the southwest edge of the Carney Lake 

block has been rotated southwestward and is overturned 

toward the southwest (C, fig. 19). The gneissic foliation 

within the crystalline core appears to have been folded about 

a northwest-trending axis during DI deformation. The Carney Lake block is bounded to the north by the South Calumet 

fault, a possible normal fault.

The other Archean crystalline basement blocks (Dickinson Group, South Felch, and Toby Lake) contain an easttrending, nearly vertical Archean gneissic fabric (S^) that is 

overprinted by the sporadic, subhorizontal Early Proterozoic 

Si foliation. Data indicating structural vergence in these 

rocks are sparse; however, in the South Felch block the 

asymmetric FI fold (D, fig. 19) in Archean gneiss containing 

the low-dipping Si foliation indicates southward vergence. 

This sense of vergence is consistent with the kinematic data 

from the Felch trough.

Kinematic data along the Felch trough (E through I, 

fig. 19) indicate that it is a south-verging structure. Diagnostic features include southward-overturned bedding, southverging asymmetric folds having a low-dipping axial-planar 

Si foliation, stretch lineations, thrust faults (Groveland mine 

pit), and the overall structure of the trough, which is asymmetric toward the south. We did not confirm Maharidge's 

(1986) suggestion that the entire stratigraphic succession in 

the Felch trough may be inverted; we did observe, however, 

that bedding is overturned in several localities.

Early Proterozoic strata along the Felch trough display 

a pinch-and-swell pattern (fig. 2). That is, oval basins 

(swells) within the trough are separated from one another by 

narrow zones of sedimentary rocks (pinches). This pinchand-swell pattern probably reflects patterns of original 

sedimentation. Larue and Sloss (1980) suggested that 

sedimentation in the largely fault bounded troughs of 

northern Michigan began in areas of local subsidence, so that 

uniform sedimentation along the length of the trough would 

not be expected.

The Sturgeon Quartzite in the Toby Lake trough is 

overturned toward the north (J, fig. 19), and the sense of drag 

on the Early Proterozoic strata indicates that the south side 

of the Toby Lake fault is upthrown. At location K (fig. 19), 

to the east, however, the Toby Lake fault has a dextral sense 

of movement and the north side appears upthrown.

Structural data from the North Calumet fault suggest 

that it is a high-angle, south-verging thrust fault that has a

Felch and Calumet Troughs Area, Northern Michigan L21
o 

85

f?

era 5' 46°00' - 45°45

WISCONSIN 

MAGMATIC 

TERRANES
dextral component of movement (L and M, fig. 19). This pattern is shown by north-dipping mylonitic shear zones that are 

axial-planar to south-verging drag folds in the Archean 

gneiss, dextral drag folds in Archean gneiss, and isolated 

southward-overturned Early Proterozoic strata of the Calumet trough adjacent to the fault.

All kinematic data from the Early Proterozoic strata in 

the Calumet trough indicate southward vergence. These data 

include southward-overturned beds and southward-verging 

folds in bedding containing low-dipping S^ axial-planar foliation (N, O, P, fig. 19).

The South Calumet fault, at the south edge of the Calumet trough, possibly is a normal fault upthrown on the 

south (fig. 20A). The fault juxtaposes Archean gneiss of the 

Carney Lake block with the lower limb of a syncline preserved in the Calumet trough.

The DI-deformed bedding, Archean gneiss foliation 

(8^), and subhorizontal S] foliation were deformed by a second (D2) phase of Early Proterozoic deformation. D2 deformation folded Si foliation into undulating to closely folded 

surfaces. 82 foliation is upright and strikes generally eastward. Rocks in the region also were deformed a third time 

as shown by 1.3 crenulation lineations at locality 13 (fig. 3). 

Also, the dominantly dextral offset on transcurrent faults that

EXPLANATION

EARLY PROTEROZOIC (1,600-2,500 Ma) 

Red granite 

Peavy Pond Complex 

Marquette Range Supergroup 

Paint River Group 

Menominee Group 

Sedimentary rocks 

Hemlock Formation 

Chocolay Group

Michigamme 

Formation of 

Baraga Group

Badwater 

Greenstone

ARCHEAN (2,500 Ma and older) 

Gneiss and metaconglomerate

Contact

Fault Sense of movement uncertain 

Normal fault Hachures on downthrown side

Thrust fault Sawteeth on upper plate; queried where 

extent uncertain

Direction of structural vergence

Figure 19 (above and facing page). Directions of structural 

vergence in Felch and Calumet troughs area. Except for Bush 

Lake fault and west part of Toby Lake fault, vergence is southward. Letters mark localities discussed in text.

cut the Sturgeon Quartzite on the southwest margin of the 

Carney Lake block (fig. 3) is a late event that postdates D2-

STRUCTURAL INTERPRETATION

Early Proterozoic deformation in the Felch and Calumet troughs area produced south-verging structures in rocks 

of the Penokean orogen, which otherwise has an overall 

northward sense of tectonic transport. Deformation was 

thick skinned inasmuch as it affected both Archean and 

Early Proterozoic structures. The presence of a flat-lying Si 

foliation and recumbent asymmetric folds suggests largescale recumbent folding in the region, and possibly the 

development of nappes.

A hypothetical vertical section (fig. 20A) across the 

area of the Felch and Calumet troughs illustrates one 

interpretation of our data. Except for the Bush Lake trough 

and the Bush Lake fault, nearly all the structural domains 

in the region were subjected to south-verging deformation. 

Although deformation probably started as a north-verging 

event above the Bush Lake fault a possible sole thrust, as 

suggested by structures in the Toby Lake trough it evolved 

into a south-verging deformational event. Some of the faults, 

such as the Toby Lake fault, and, perhaps also the Norway 

Lake fault, may have been inboard (north)-verging faults 

that were backfolded during subsequent deformation. But 

the Toby Lake fault is the only one, other than the Bush 

Lake fault, that has strong evidence for northward structural 

vergence. The Toby Lake and Norway Lake faults in the 

model (fig. 20A) intersect the ground surface at locations 

where they are nearly vertical. Similar upward steepening 

and overturned backthrusts have been described by Tysdal 

(1986) in the Rocky Mountain foreland of Montana. 

Structural data along the North Calumet fault suggest that 

it is a north-dipping, south-verging, high-angle thrust fault 

with a dextral component of movement in the horizontal 

plane. Data along the eastern segment of the Toby Lake 

fault also indicate a dextral component of movement. The 

south-verging Felch trough is an infolded and faulted 

remnant of once-extensive Early Proterozoic strata, as are 

the Toby Lake and Calumet troughs and the remnants of 

Early Proterozoic strata near the Niagara fault zone (fig. 

20A). The Carney Lake block is part of a crystalline-core 

fold, the lower sedimentary limb of which is exposed on 

the southwest. This interpretation is supported by the 

presence of north-verging parasitic folds in southwardoverturned beds in the Sturgeon Quartzite. Mafic inclusions 

in the Archean gneiss define a west-northwest fold (fig. 18). 

The Carney Lake block is bounded on the north by a possible 

normal fault, the South Calumet fault. The whole package 

of south-verging crust is bounded to the south by the Niagara 

fault zone, the suture between the continental margin and 

the Wisconsin magmatic terranes (Sims and others, 1989) 

(fig. 1).

Felch and Calumet Troughs Area, Northern Michigan L23
NORTH SOUTH

BUSH 

LAKE 

FAULT

NORWAY 

LAKE 

FAULT

TOBY NORTH SOUTH

FELCH LAKE CALUMET CALUMET

TROUGH FAULT FAULT FAULT

5 KILOMETERS

EXPLANATION

EARLY PROTEROZOIC 

Marquette Range Supergroup 

Pembine-Wausau arc terrane

ARCHEAN 

Gneiss

Contact Dashed where projected above

surface 

Fault Barbs show direction of relative

movement; dashed where projected

above surface to indicate regional

structure

Figure 20 (above and facing page). Cross sections comparing 

structure in Felch and Calumet troughs area with that in southern Alps. A, Hypothetical generalized cross section across area 

of Felch and Calumet troughs. No vertical scale given. B, 

Interpretive structure in the Felch and Calumet troughs area in 

relation to cross-sectional structure of the entire Penokean

orogen in the continental foreland region of northern Michigan. Cross section extends from L'Anse through Covington to 

Iron Mountain (fig. 1). Modified from Klasner, Sims, and others 

(1988). C, Structural cross section of the southern Alps, looking 

north. From Debelmas and others (1983). B and P, sites of 

backth rusting.

In summary, we propose that the Felch and Calumet 

troughs area was initially deformed by inboard (north)- 

verging deformation during accretion of oceanic arc rocks of 

the Wisconsin magmatic terranes to the south. During the 

accretionary event, the region evolved into a south-verging 

deformation zone.

The second (D2) phase of deformation, characterized 

by upright folds nearly coaxial with F^ folds, significantly 

folded the Sj surfaces. Such deformation could explain the 

southeast-plunging lineations found mostly in the southern 

part of the Carney Lake block. The lineations may have 

plunged northerly prior to F2 deformation. A later deformational event (D3) caused only weak crenulation folding in the 

region. The offset along wrench faults along the west edge of 

the Carney Lake block may have resulted from emplacement 

of the Crystal Falls terrane along the Badwater thrust fault 

(fig- 2).

STYLE OF PENOKEAN TECTONISM

We conclude that deformation in this part of the 

Penokean foldbelt was thick skinned and involved significant

backthrusting. The south-verging structures in the Felch and 

Calumet troughs area are opposite to the overall northverging tectonic transport in the Penokean orogen of 

northern Michigan (Klasner, Sims, and others, 1988; Klasner and Sims, 1989; Klasner, Ojakangas, and others, 1991). 

They define a 35-km-wide zone of backthrusting in the 

overall 100-km-wide zone of deformation in the continental 

foreland region of northern Michigan. Following the suggestion of Morley (1988), backthrusts as out-of-sequence faults 

can form in foreland thrust belts where an abrupt change 

occurs in thickness of the thrust sheet. Morley called upon 

models of Davis and others (1983) and Chappie (1978) to 

show that backthrusts occur where the critical taper of the 

thrust wedge is such that forward propagation of the thrust 

sheet cannot be maintained. Hence, the thrust wedge is 

thickened by backthrusting and internal deformation until a 

critical taper is accomplished and the in-sequence forward 

thrusting and folding can continue. Morley (1988) also noted 

that backthrusts are generally accompanied by vertical uplift 

and the development of high-angle reverse faults, and the 

Norway Lake and Toby Lake faults certainly are high-angle

L24 Precambrian Geology of Lake Superior Region
NORTH ZONE OF BACKTHRUSTING SOUTH

CONTINENTAL FORELAND »H WISCONSIN MAGMATICTERRANES

ARCHEAN

KILOMETERS 

10 i

EXPLANATION

EARLY PROTEROZOIC

Marquette Range 

Supergroup

Contact Dashed where projected above surface

Dunbar Gneiss Volcanic rocks 

and ophiolite

50 KILOMETERS

Fault Showing direction of structural vergence 

where known or inferred; dashed where 

projected above surface to indicate regional 

structure

B

Zone of backthrusting

Valensole 

plateau

Dara Maira massif

r 10

-20

-30

EXPLANATION

CENOZOIC

Sedimentary rocks, undivided 

MESOZOIC

Sedimentary rocks, undivided 

PRE-MESOZOIC

Supracrustal, ultramafic, and 

crystalline rocks; black, ophiolite

Contact Dashed where inferred

Thrust fault Dashed where inferred; barbs 

show relative movement where known or 

inferred

Felch and Calumet Troughs Area, Northern Michigan L25
reverse faults in which the sense of movement was largely 

vertical.

The zone of backthrusting in the Felch and Calumet 

troughs area occurs at the edge of the continental foreland, a 

short distance north of the Niagara suture, which separates 

oceanic crust from continental crust. Here, an abrupt change 

occurs in the thickness of the thrust sheet, and we suggest 

that the thrust sheet was thickened by backthrusting until the 

northward-verging thrust system could propagate northward 

onto the foreland. Such backthrust systems are not uncommon. Morley (1988) cited several examples of backthrusting 

from the Canadian Rockies (Price, 1986), the Alps (Tricart, 

1984), and elsewhere; in these examples, large-scale backthrusting accompanied in-sequence thrusting.

That nappes may exist in the Felch and Calumet 

troughs area is suggested by the pervasive, low-dipping foliation (Sj) in Archean and Proterozoic rocks. Except for the 

Sturgeon Quartzite on the southwest edge of the Carney 

Lake block and in the Toby Lake trough, evidence is lacking 

for an overturned or inverted stratigraphy on a large scale. 

However, this does not preclude the possibility that nappes 

now eroded from the Penokean orogen may have existed at 

higher tectonic levels.

The deeply eroded 1,850 Ma Penokean orogen is similar structurally to the much younger Alpine orogen, as illustrated in figure 20B, C. Debelmas and others (1983), as well 

as Tricart (1984), noted that tectonism in the southern Alps 

comprised at least two major phases of deformation. The 

first phase consisted of westward (inboard) thrusting with 

isoclinal folding and nappe emplacement. The second phase 

was characterized by "backwards thrusting (retrocharriage)," as shown by eastward-verging, overturned folds 

(Debelmas and others, 1983, p. 84). These zones of backthrusting are labeled "B" and "P" in figure 20C. Debelmas 

and others (1983, p. 84) also noted that "evolution * * * 

begins with a westward phase of thrusting * * * followed by 

a classical eastward thrusting." All these motions are linked 

with high-pressure metamorphism. The presence of ophiolites and ultramafic rocks (L, fig. 20C) indicates that this 

structural domain is accreted terrane.

An uplifted zone of basement rocks in the southern 

Alps, the Argentera massif, lies just west of a major fault (I, 

fig. 20C), and the fault extends westward over the massif. 

Farther west, basement rocks were not involved in the deformational picture, and according to Debelmas and others 

(1983, p. 85), "flysch was stripped off and * * * together with 

sub-Brianconnais substratum, was folded and thrust westward * * *."

The crustal profile of the Penokean foldbelt in northern Wisconsin and Michigan (fig. 20fi), although involving 

rocks some 1,820 m.y. older than those of the southern Alps, 

exhibits many similarities to the Alpine profile. The zone of 

backthrusting in the Felch and Calumet troughs area together

with high-pressure metamorphism in the area of the Peavy 

Pond Complex (fig. 2) (Attoh and Vander Meulen, 1984), 

and the existence of accreted oceanic crust, including ophiolite, are associated features also observed by Debelmas and 

others (1983) in the Alps. The Bush Lake fault in the 

Penokean orogen is a master thrust that extends inboard over 

a basement uplift, the Amasa uplift, and appears in much the 

same attitude as fault I in figure 20C, which extends inboard 

over the Argentera massif, a basement uplift. Inboard of the 

Amasa uplift, turbidite deposits flysch derived from the 

Wisconsin magmatic terranes (Barovich and others, 1989) 

were transported onto the continental margin along sole 

thrusts, which apparently did not involve the basement to 

any appreciable extent. This thin-skinned deformation is 

similar to that depicted in the western part of the area of 

figure 20C from the southern Alps.

Despite the similarities we recognize between the 

crustal profiles of the southern Alpine and the Penokean orogens, the substantial age difference in the rocks (1,850 Ma 

versus 30 Ma) causes some difficulty in making direct comparisons. Erosion of the Penokean orogen has removed most 

of the supracrustal strata and their enclosed evidence of tectonism, and has exposed a deeper tectonic level. Thus it is 

not possible to assess the complexity of deformation in these 

higher level rocks, as can be done in the Alps. The Alps, 

retaining such rocks, may provide an insight into the possible 

style of deformation in the supracrustal rocks of the 

Penokean orogen. Possibly these higher level Early Proterozoic supracrustal strata were deformed by an initial northverging phase of deformation into stacked nappes, which 

were subsequently deformed by backthrusting in a manner 

similar to that outlined by Debelmas and others (1983) for 

the southern Alps.

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Society of America Bulletin, v. 96, p. 1101-1112.

Felch and Calumet Troughs Area, Northern Michigan L27
Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 

1989, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean orogen: 

Canadian Journal of Earth Sciences, v. 26, p. 2145-2158.

Southwick, D.L., and Morey, G.B., 1991, Tectonic imbrication and 

foredeep development in the Penokean orogen, east-central 

Minnesota An interpretation based on regional geophysics 

and the results of test-drilling: U.S. Geological Survey Bulletin 1904-C, 17 p.

Tricart, P., 1984, From passive margin to continental collision A 

tectonic scenario for the western Alps: American Journal of 

Science, v. 284, p. 97-120.

Tysdal, R.G., 1986, Thrust faults and backthrusts in Madison 

Range of southwestern Montana foreland: American Association of Petroleum Geologists Bulletin, v. 70, p. 360-376.

Ueng, W.C., and Larue, D.K., 1988, The Early Proterozoic structural and tectonic history of the south-central Lake Superior 

region: Tectonics, v. 7, p. 369-388.

Manuscript approved for publication October 10, 1991 

Published in the Central Region, Denver, Colorado 

Type composed by Marie F. Melone 

Graphics by Mari L. Kauffmann 

Edited by Lorna Carter

L28 Precambrian Geology of Lake Superior Region *U.S. GOVERNMENT PRINTING OFFICE: 1993-774-049/66038
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that maybe cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases which show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from USGS Map Distribution, Box 25286, 

Building 810, Denver Federal Center, Denver, CO 80225. (See latest 

Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a set 

microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a set 

of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)" may be purchased by mail and 

over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing 

a free subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.