Nature and Style of Deformation in the 

Foreland of the Early Proterozoic 

Penokean Orogen, Northern Michigan

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Chapter K

Nature and Style of Deformation in the 

Foreland of the Early Proterozoic 

Penokean Orogen, Northern Michigan

ByJ.S. KLASNER, R.W. OJAKANGAS, KJ. SCHULZ, 

and G.L LABERGB

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: 1991

For sale by the

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Library of Congress Cataloging-in-Publication Data

Nature and style of deformation in the foreland of the early Proterozoic Penokean 

orogen, northern Michigan / by J.S. Klasner ...[et al.].

p. cm. (U.S. Geological Survey bulletin ; 1904-K) (Contributions to 

Precambrian geology of Lake Superior region ; ch. K)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1904-K

1. Rock deformation Michigan. 2. Geology, Stratigraphic Proterozoic. 

3. Geology, Structural Michigan. I. Klasner, J.S. II. Series. III. Series: 

Contributions to Precambrian geology of Lake Superior region ; ch. K. 

QE75.B9 no. 1904-K 

[QE604]

557.3 s dc20 91-11096 

[551.7'15'09774] CIP
CONTENTS

Abstract Kl 

Introduction Kl 

Acknowledgments K3 

Geologic background K3 

Structure of key areas K4

Falls River area K4

Little Mountain area K5

Taylor mine K8

Plumbago Creek K8

Komtie Lake K10

Canyon Falls K12

Covington profile K14

Ironwood-Hurley area K15 

Geologic interpretation K15

D! event K15

D2 event K16

D3 event K18

Summary K18

Conclusions and tectonic implications K19 

References cited K20

FIGURES

1. Map of northern Michigan, northern Wisconsin, and northern Minnesota 

segments of Penokean orogen K2

2. Map showing location of key structural areas in L'Anse area, Michigan K5

3. Sketches and stereoplots depicting detailed structure in the Falls River area, near 

L'Anse, Michigan K6

4. Sketch and stereoplots depicting structure in Michigamme Formation at Little 

Mountain, northern Michigan K7

5. Geologic cross section and stereoplots at Taylor mine, northern Michigan K8

6. Geologic map of Plumbago Creek area, northern Michigan K9

7. Sketches and stereoplots depicting structure in Plumbago Creek area, northern 

Michigan Kll

8. Geologic map and drill-hole data at Komtie Lake, northern Michigan K12

9. Geologic map and cross sections of Taylor mine, Komtie Lake, Plumbago Creek, 

and Canyon Falls areas, northern Michigan K13

10. Sketch of photograph showing structure at Canyon Falls area, northern 

Michigan K14

11. Sketch section and stereoplots depicting structural data from Michigamme 

Formation along Covington profile, northern Michigan K14

12. Sketch of structure in the Tyler Formation in Iron wood, Michigan-Hurley, 

Wisconsin area K15

Contents III
13. Generalized block diagram and cross section illustrating structural relationships in 

Ironwood, Michigan-Hurley, Wisconsin area K16

TABLES

1. Generalized stratigraphic column from the eastern and western parts of Penokean 

orogen, northern Michigan K3

2. Summary of deformation observed at Falls River, Little Mountain, Taylor mine, 

Plumbago Creek, Komtie Lake, Canyon Falls, Covington profile, and IronwoodHurley area, Michigan and Wisconsin K17

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Nature and Style of Deformation in the Foreland of the 

Early Proterozoic Penokean Orogen, Northern Michigan

By \.S. Klasner, 1 R.W. Ojakangas, 2 KJ. Schulz, 3 and G.I. LaBerge4

Abstract

Evidence for Early Proterozoic north-verging folding and 

thrusting in the continental foreland of the Penokean orogen is 

present in two widely separated areas in northern Michigan. In 

the eastern part of the exposed orogen, asymmetric to overturned folds in the Early Proterozoic Michigamme Formation 

suggest an initial (D,) phase of possibly north verging thinskinned deformation. A second phase (D2) is characterized by 

a more thick skinned deformation consisting of northward 

thrusting of Archean gneiss and overlying Early Proterozoic 

quartzite along ductile thrust faults. Crosscutting shear zones 

indicate a third phase (D3) of deformation involving eastnortheast thrusting of Archean gneiss. Thick-skinned deformation D2 and D3 phases include spaced fracture cleavage, 

kink folds, and lineations in D-, foliation surfaces.

Near the west end of the exposed orogen in Michigan, 

bedding-cleavage relationships in the Early Proterozoic Tyler 

Formation also indicate northward tectonic transport. These 

structures, which are interpreted as D 1r lie along the north 

edge of the Archean Puritan batholith and associated Archean 

greenstone.

The north-verging structures in the northern Michigan 

segment of the Penokean orogen were formed approximately 

1,850 Ma during collision of the Wisconsin magmatic terranes 

with the continental foreland of Michigan, Wisconsin, and 

Minnesota.

Manuscript approved for publication, January 2, 1991.

'Department of Geology, Western Illinois University, and U.S. 

Geological Survey, Macomb, IL 61455.

Department of Geology, University of Minnesota-Duluth, and 

U.S. Geological Survey, Duluth, MN 55812.

3U.S. Geological Survey, Reston, VA 22092.

4Department of Geology, University of Wisconsin-Oshkosh, and 

U.S. Geological Survey, Oshkosh, WI 54901.

INTRODUCTION

The Early Proterozoic Penokean orogen is a major 

tectonic feature of the Lake Superior region (fig. 1). The 

northern (foreland) part of the orogen, in northern Michigan, Wisconsin, and Minnesota, which comprises the area 

of this report, consists of a continental margin assemblage 

of Early Proterozoic sedimentary and volcanic rocks 

overlying an Archean basement (Morey and others, 1982). 

This domain is flanked on the south by rocks of the 

Wisconsin magmatic terranes (Sims and others, 1989), and 

is separated from them by the Niagara fault zone, a major 

terrane boundary. Several authors (Van Schmus, 1976; 

Cambray, 1978; Larue and Sloss, 1980; Greenberg and 

Brown, 1983; Anderson and Black, 1983; Schulz, 1983, 

1984; Schulz and others, 1984; LaBerge and others, 1984; 

Sims and Peterman, 1984; Hoist, 1982, 1984; Holm and 

others, 1988; Klasner and others, 1985; Sims, Peterman, and 

Schulz, 1985; Klasner and Attoh, 1986; Schulz, 1987a, b; 

Sims, Peterman, and others, 1987; Sims, Van Schmus, and 

others, 1989; and Attoh and Klasner, 1989) have proposed 

plate-tectonic scenarios for the tectonic evolution of the 

Penokean orogen in the Lake Superior region. Although the 

scenarios differ in detail, these authors have suggested that 

evolution of the orogen began with a rifting phase 

accompanied by formation of basins and troughs (such as 

the Marquette and Republic troughs), which led to the 

development of a passive margin along the south edge of the 

Archean Superior craton. Subsequently, the arc-related 

volcanic and plutonic rocks were accreted to the Archean 

continental margin (Sims and others, 1989), resulting in 

northward thrusting of the continental margin rocks (Klasner, Ojakangas, and others, 1988; Klasner, Sims, and others, 

1988; Attoh and Klasner, 1989) and at least local development of nappes (Hoist, 1984).

Evidence for north-verging folding and thrusting 

exists in two areas in the foreland of the Penokean orogen in 

northern Michigan and Wisconsin that are separated by a

Deformation in the Penokean Orogen, Northern Michigan K1
.Area of

NIAGARA 

'- WISCONSIN - > -MAGMATIC ' 

Wisconsin-Michigan segment

Early 

Proterozoic

EXPLANATION

Turbidites overlying shelf deposits 

Plutonic and volcanic rocks, mainly calc-alkalic 

Complex continental-margin sequences 

Archean rocks, undivided

 Contact Dashed whe

 High-angle fault

 A A_ Major thrust fault Da

sawteeth on overthru

 A * *_ Thrust fault Sawteet

GLTZ Great Lakes tectonic 

Figure 1 . Map of northern Michigan, northern Wisconsin, and northern Minnesota segments of Penokean orogen, divided by roc

from Southwick and Morey, 1991, fig. 1, and Sims and others, 1989). Towns mentioned in text are L'Anse, Mich. (LA); Covingto

Ironwood, Mich.-Hurley, Wis. (IH); and Duluth, Minn. (D). GLTZ, Great Lakes tectonic zone, from Morey and Sims (1976) in M

Wisconsin and Michigan. Note location of figure 2 and Hurley, Wis. area.
Table 1. Generalized stratigraphic column from the eastern and western parts of Penokean orogen, northern Michigan

[The Paint River Group, which possibly lies stratigraphically above the Baraga Group in the Marquette Range Supergroup, is not shown. Modified 

from Cannon and Gair (1970) and James (1958)]

West East

EARLY PROTEROZOIC

Marquette Range 

Supergroup (part)

Baraga Group

Menominee Group

Chocolay Group 

ARCHEAN

Michigamme, Copps, and Tyler Formations

Metagraywacke, argillite, and siltstone

Michigamme Formation

Metagraywacke, slate, thin-bedded cherty ironformation, mafic to intermediate volcanic rocks

Goodrich Quartzite

Conglomeratic quartzite

Ironwood Iron-formation

Cherty sideritic iron-formation

Emperor Volcanic Complex

Metabasalt

Palms Formation

Argillite, quartzitic siltstone, quartzite

Negaunee Iron-formation

Cherty-banded iron-formation

Siamo Slate

Argillite and siltstone

Ajibik Quartzite

Bad River Dolomite 

Sunday Quartzite

Wewe Slate 

Kona Dolomite 

Mesnard Quartzite

Undifferentiated granitic to mafic gneisses, metavolcanic rocks, granite

distance of more than 150 km. The eastern area is near 

L'Anse, Mich., and consists of the Falls River, Little 

Mountain, Taylor mine, Plumbago Creek, Canyon Falls, 

Covington profile, and the Komtie Lake areas (fig. 2). The 

western area is near Ironwood, Mich., and Hurley, Wis. 

(fig. 1). Although evidence for north-verging folding and 

thrusting in northern Michigan has been discussed (for 

example, Klasner, Ojakangas, and others, 1988; Klasner, 

Sims, and others, 1988), it has not previously been fully 

documented.

Two distinct structural domains have been recognized 

in the continental margin rocks of northern Michigan. 

Klasner, Sims, and others (1988) and Klasner and Cannon 

(1989) suggested that the Penokean orogen of Michigan can 

be divided into (1) an imbricate thrust belt in the south and 

(2) a foreland thrust belt in the north. In the imbricate thrust 

belt, which is part of a broader marginal basement arch, 

deformation was thick skinned and complex involving 

both Archean basement and Early Proterozoic supracrustal 

rocks. In the foreland thrust belt, structures are less complex 

and deformation was largely thin skinned, that is, mainly 

involving Early Proterozoic supracrustal rocks.

In this report we examine the structural evidence for 

folding and thrusting in the foreland thrust belt of the 

Penokean orogen in northern Michigan. A similar study by 

Klasner and P.K. Sims is being carried out in the Felch and 

Calumet troughs region to the south, within the imbricate

part of the thrust belt in northern Michigan. We conclude 

this report with a brief discussion of the tectonic significance of the foreland part of the Penokean orogen.

ACKNOWLEDGMENTS

This paper benefited from critical reviews by W.F. 

Cannon and T.B. Hoist. Connie Fairchild assisted with 

typing. Drill core data at Komtie Lake are from Resource 

Exploration; drill core at Taylor mine is from Ford Motor 

Company. Permission to examine the drill core at Komtie 

Lake was given by W. Bodwell of Resource Exploration. 

The drill hole data at Taylor mine have been published 

previously (Klasner, 1978); permission to publish these data 

was given at that time by Ford Motor Company.

GEOLOGIC BACKGROUND

The Archean rocks in northern Michigan are overlain 

by dominantly stratified rocks of the Early Proterozoic 

Marquette Range Supergroup (Irving and Van Hise, 1892; 

Van Hise and Bayley, 1897; James, 1958; James, Clark, and 

others, 1961; James, Dutton, and others, 1968; Cannon and 

Gair, 1970). The Marquette Range Supergroup (table 1) 

comprises, from oldest to youngest, the Chocolay,

Deformation in the Penokean Orogen, Northern Michigan K3
Menominee, Baraga, and Paint River Groups (Paint River 

Group not shown). Rocks of the Chocolay Group are 

present at the east end of the exposed foldbelt in Michigan 

and in the area near Ironwood to the west, but are absent in 

between.

Previous structural studies (Cannon and Klasner, 

1972; Klasner, 1972; Cannon, 1973; and Klasner, 1978) 

have shown that the Early Proterozoic rocks in the eastern 

area of this report (fig. 2) were subjected to at least three 

periods of deformation. These were designated F]? F2, and 

F3 by these authors; they are designated D]5 D2, and D3 in 

this report for consistency with other reports in this series. 

The first event (Dt) was thin skinned and resulted in 

shortening of Early Proterozoic strata and development of a 

widespread west-northwest-trending structural fabric. The 

second event (D2) was postulated to involve block uplift of 

Archean basement rocks and formation of prominent structural features such as the Marquette and Republic troughs 

and infolding of Early Proterozoic strata into the troughs. 

Larue and Sloss (1980) suggested that the troughs started to 

form early, during deposition of the sediments, and prior to 

Dj deformation. Klasner (1972, 1978) noted a third, latestage, brittle deformational event (D3) and suggested that it 

could have been caused by late block uplift of Archean 

basement. Cannon (1973) divided the regional structures in 

northern Michigan into "first" and "second" order features. 

First-order structures have a wide range of orientations and 

are related to postulated block uplift and doming (Sims and 

others, 1984; Sims and Peterman, 1976) of the Archean 

crust. Second-order structures have a consistent westnorthwest trend and are related to the D^ shortening of Early 

Proterozoic supracrustal strata.

The dichotomy in style of deformation between Early 

Proterozoic rocks and the underlying Archean rocks led 

Cannon and Klasner (1972), Klasner (1972, 1978), and 

Cannon (1973) to suggest that a decollement exists between 

the Proterozoic and Archean. They suggested that shortening of Early Proterozoic strata relative to underlying 

Archean rocks was possibly caused by gravity sliding off an 

ancestral Penokean mountain range to the south. Hoist 

(1982, 1984) proposed an Early Proterozoic nappe structure 

in the same tectonic environment in eastern Minnesota, and 

Maharidge (1986) and Sims and others (1987) have 

suggested that a crystalline-cored nappe structure may exist 

just north of Iron Mountain, near the Niagara fault (fig. 1).

Three regional metamorphic aureoles have been 

defined in northern Michigan (James, 1955). All the key 

areas of this study lie within the chlorite zone (greenschist 

facies) of the aureoles. Klasner (1978) has shown that peak 

metamorphism postdates the initial Dj phase of deformation 

but predates the second phase. Attoh and Vander Meulen 

(1984) and Attoh and Klasner (1989) suggested that the 

nodal metamorphism was the result of tectonic stacking 

caused by Early Proterozoic overthrusting.

STRUCTURE OF KEY AREAS

All the key areas described herein lie near the 

northernmost edge of the exposed Penokean orogen in 

northern Michigan (fig. 1). The wide separation of key 

outcrops along the strike of the orogen reflects the general 

paucity of outcrops in this region. In this discussion, we first 

describe outcrops in the east at the localities clustered near 

L'Anse, where structural data are more abundant; and we 

conclude with the Ironwood-Hurley area in the west, where 

structural data are less abundant.

Falls River Area

Numerous outcrops of graywacke and slate of the 

Michigamme Formation of the Baraga Group are exposed 

for a distance of about 1.8 km along Falls River near the 

village of L'Anse, Mich. (See fig. 2 for location of the 

outcrop areas discussed herein.) In a detailed structural 

study of these rocks, Sikkala (1987) identified three phases 

of deformation. The first phase, herein designated Dl5 

consists of north-verging regional folds (Fj) in bedding (S0) 

with a gentle south-southwest-dipping penetrative foliation 

(Sj). A roughly coaxial second phase of deformation (D2) 

formed generally open chevron folds (F2) in S t , and a 

steeply dipping axial-planar (S2) fracture cleavage. A third 

event (D3 ) locally deformed S0, S l5 and S2 about a 

northeast-trending fold axis (F3), forming spaced S3 

cleavage locally. Sikkala (1987) noted that in general, 

accompanying thrust faults are not exposed, but in places 

where they are exposed, they are cataclastic zones 

containing quartz and carbonate minerals.

Stereoplots and diagrams (fig. 3) from Falls River 

illustrate the nature and sequence of deformation in this 

locality. Similarly to Sikkala (1987), we conclude that 

bedding (S0) has been folded about near-horizontal, westnorthwest-trending fold axes (F^ during the Dl deformational event (fig. 3A). Poles to Sj foliation (fig. 35) show 

that it is axial-planar to the Fj folds, strikes west-northwest, 

and dips about 15° SW. Bedding is overturned toward the 

north, and second-order parasitic folds (fig. 3C, D) lie on the 

overturned lower limb of a north-verging fold system.

The nature of the second deformation (D2) at Falls 

River is illustrated by figure 3B and 3E: S l foliation has 

been folded into a series of open folds (F2); F2 fold axes are 

nearly coaxial with Fj axes and plunge gently westnorthwest; S2 foliation is nearly vertical.

A third deformation (D3 ) reported by Sikkala (1987) 

consists of a spaced fracture cleavage (S3) about which S2 

foliation is reoriented. The S3 foliation strikes east-northeast 

and dips steeply southeast. This fracture cleavage is axialplanar to an open fold represented by the gently southwest 

plunging fold axis (F3) shown in figure 3A. Poles to bedding 

in the F3 fold define a great circle which is coincident with 

the measured F3 fold axis.

K4 Precambrian Geology of Lake Superior Region
Keweenawan

dike

EARLY PROTEROZOIC 

Michigam me 

Formation 

Goodrich

Quartzite 

ARCHEAN 

Gneiss

Figure 2. Localities in the L'Anse, Mich., area discussed in 

the text: Canyon Falls, Falls River, Komtie Lake, Little 

Mountain, Plumbago Creek, Taylor mine, and Covington 

profile. The Ironwood-Hurley outcrop area is located about 

150 km west of the L'Anse area, as shown in figure 1. (See also 

fig. 12.) Geology modified from Cannon (1986).

Little Mountain Area

Little Mountain (fig. 2) is a small bedrock hill that 

stands above the surrounding plain of Pleistocene deposits. 

An easterly trending 1.1 Ga (Keweenawan) diabase dike is 

exposed along the crest of the hill and intrudes deformed 

graded beds of greenschist-facies metagraywacke.

A structural cross section of a part of the east face of 

Little Mountain (fig. 44) shows that the metagraywacke 

beds have been deformed into a north-verging asymmetric

fold system with axial-planar (Sj) foliation. The overall 

structure of the cliff face is that of a syncline. A lower 

hemisphere stereoplot (fig. 45) of poles to bedding (S0) 

shows that S0 is folded about F! fold axes that plunge gently 

N. 75° W. Figure 4C shows that S t foliation is oriented 

approximately N. 60° W., 40° SW. At locations I and II in 

figure 4A, S l foliation is deformed by a series of chevron 

folds. This may explain the spread in S! foliation in figure 

4C, but such deformation is not widespread. The long 

(stretch) axes of constrictively deformed calc-silicate

Deformation in the Penokean Orogen, Northern Michigan K5
B

Figure 3. Diagrammatic sketches from field photographs and 

equal-area lower hemisphere stereoplots, Falls River area, near 

L'Anse, Mich. A, Stereoplot showing 2, 7, and 10 percent 

contours on 27 poles to bedding. Small dots, axes (F,) in folds 

of S0. Solid square, measured axis (F3 ) of an F3 fold. Large solid 

dots and great circle (short dashes) define poles to bedding of 

the F3 fold. B, Stereoplot showing 2, 10, 20, and 30 percent 

contours on 53 poles to S, foliation. Solid dot, F2 fold axis. 

Open square, pole to spaced S2 foliation. Great circle (dashed 

line) shows folded S.,; vertical S2 plane is shown by solid line. 

C and D, Sketches from photos (both looking east), showing

 V. X7": ,i*".V. ^t 

_^_ .. ,»L .,\ . 

 .I 1 - ii -' ii - i 1 '» '.-." / inn 1 ' : ^-- VFracture 

Vail1 -:;- -' . -\ ^.,.'*

./Fracture

NW

relationship of S0 and Sv Country rock is graywacke-slate. 

C, Sketch from photo of vertical face on east side of Falls River 

near old railroad bridge, just south (upstream) from the bridge 

on U.S. Highway 41. Coin is 2.5 cm in diameter. D, Sketch 

from photo of vertical face on east side of river, north 

(downstream) from bridge on U.S. Highway 41. Rule is 15 cm 

long, £, Sketch from photo (looking northwest), showing 

relationship of S2 foliation and F2 fold axes (arrows) to S^ 

foliation. Outcrop is north from bridge on U.S. Highway 41. 

Hammer is 31 cm long.

concretions have a consistent southwesterly plunge, but 

detailed strain analyses have not been made on them and the 

stretch axes are not shown on the stereonet plots.

Location II (fig. 44) is on the top of Little Mountain 

and set back from the main cliff face by a series of benches. 

Here, and elsewhere on the top of Little Mountain, graded 

bedding is overturned toward the north. At location II, 

overturned bedding is juxtaposed to, but not in direct 

contact with, underlying upright beds that dip 55° NNW. 

Slickenlines on the underside of a bedding surface at IIA

(fig. 44) trend N. 50° E. In the region of the overturned 

beds, S t foliation has been deformed, as diagrammatically 

shown by the kink-folds (area I, fig. 4).

Little Mountain consists primarily of asymmetric 

folds with steep short north limbs. Stratigraphic facing is 

readily discernible on the main cliff face, as shown in figure 

4A. In contrast to the main cliff face in which bedding is 

upright, bedding at the top of Little Mountain is overturned 

toward the north, as shown at location II in figure 44. This 

difference in nature of deformation may represent a local

K6 Precambrian Geology of Lake Superior Region
-*-N METERS 

0 5 10

Diagrammatic series of 

benches. Area II is above 

and back from the main 

F ace below.

%i$te%n*ffK t* 1 * O \**. M.-~- . *» I J-t\ O A \vV V- . _-N> ^

Figure 4. Structure in Michigamme Formation at Little 

Mountain, northern Michigan. A, Field sketch of vertical cliff 

face along east end of Little Mountain (looking west). Graywacke beds are graded; slaty cleavage (hachure marks) is 

developed along the tops of the beds; bottoms of beds consist 

of coarser sandy material. S0, bedding; S1; slaty foliation. 

Triangle pattern, talus. Dashed line, edge of talus. Area I, 

location of kink folds that affect S^ Area II, sketch of a small 

east-facing cliff at the top of Little Mountain. It lies above the 

main cliff face sketched below, and illustrates overturned

bedding. Note also deformed S^ Heavy line, thrust fault; 

arrows show sense of movement. B, Stereoplot (equal-area 

lower hemisphere) of bedding (S0) and measured F, fold axes. 

Contours at 2, 6, 12, and 20 percent on 49 poles to bedding 

(S0). Great circle (dashed line) represents orientation of folds in 

bedding that define f^ fold axes. C, Stereoplot (equal-area 

lower hemisphere) of S, foliation that is axial-planar to folds in 

S0. Contours are at 4, 6, and 10 percent on 25 poles to Sv 

General orientation of S, plane shown by solid curved line.

inhomogeneity in folding caused by a slip along the 

bedding. More likely, however, the overturned beds lie 

above a small thrust fault (fig. 4A, location II) that is

subparallel to the gently south dipping beds of the asymmetric folds. The thrust fault may represent late Dl 

deformation, or possibly the second (D2) deformation.

Deformation in the Penokean Orogen, Northern Michigan K7
NORTH Bedrock exposed 

in roadcut

EXPLANATION

Keweenawan diabase 

dike

Cherty iron-formation 

Sandy layer

Black slate with S1 

foliation and bedding

Drill hole Showing dip 

of foliation in hole

100 200 METERS

B

Figure 5. Geologic cross section and equal-area lower 

hemisphere stereoplots from Taylor mine area, northern 

Michigan. Modified from Klasner (1972, 1978). A, Interpretive 

cross section constructed from roadcut and drill cores showing 

north-verging overturned folds. Dip of foliation in drill cores 

is shown in two directions to signify that the drill core is not

Taken together, the southwest-plunging stretch lineations as expressed by long axes of the calc-silicate 

concretions, the north-verging folds, southwest-dipping Sj 

foliation, and the northeasterly oriented slickenlines on 

bedding indicate a northeast sense of tectonic transport at 

Little Mountain.

Taylor Mine

Rocks in the locality of Taylor mine (location, fig. 2) 

consist of deformed phyllite, chert, iron-formation, and 

graphitic slate of greenschist metamorphic grade that were 

deformed by at least three events (Klasner, 1972,1978). The 

first deformation (D^ folded bedding (S0) into a series of 

folds (F!) that have a wavelength of about 250 m and an 

axial-planar (S^ foliation (fig. 5A); F! fold axes plunge 

gently northwest (fig. 5B). Bedding is overturned toward the 

north, and S! cleavage is generally oriented N. 66° W., 55° 

SW., as shown in figure 5C. Deformation D2 deformed $! 

into a series of open chevron folds (F2) with gently 

northwest plunging F2 fold axes that are roughly coaxial

oriented. All exposed rocks in area have south-dipping 

foliation. B, Stereoplot of 204 poles to bedding (S0). Dot, 

orientation of FT fold axis. Contours at 0.5, 1, 2, 6, and 8 

percent. C, Stereoplot of 236 poles to S, foliation and general 

orientation of S, plane of foliation (solid line). Dot, orientation 

of F2 fold axis. Contours at 0.4, 2, 5, and 9 percent.

with F! fold axes. Spaced (S2) fracture cleavage generally 

dips northeast but is scattered in orientation. Faint crenulation lineations on Sj surfaces lie at an angle to F2 and 

provide evidence for a third deformation (D3) at Taylor 

mine.

Plumbago Creek

The Plumbago Creek locality (fig. 2) is south of 

Taylor mine, along the contact between Archean gneiss and 

Early Proterozoic metasedimentary strata. The area comprises Archean quartzofeldspathic gneiss to the south, Early 

Proterozoic graphitic slate (exposed in an open pit graphite 

mine) to the east, Early Proterozoic iron-formation, and an 

undeformed 1.1 Ga (Keweenawan) diabase dike to the north 

(fig. 6). Structural data from the slate exposed in the 

graphite pit indicate at least three stages of deformation (D1? 

D2 , D3). As shown in figure 1A, the most prominent 

foliation (Sj) strikes west-northwest and dips moderately to 

steeply southwest. Axes of folds (F^ in bedding (S0) are 

shown in figure 7'A. The great circle with accompanying F2a 

fold axis indicates that Sj has been folded during D2

K8 Precambrian Geology of Lake Superior Region
deformation. Small (millimeter wavelength) crenulation 

folds (F2) on S l are roughly parallel to the N. 85° W. fold 

axes (see coincident F2 and F2a, fig. 7A) and may represent 

D2 folding. Other crenulation fold axes (not shown) that are 

roughly perpendicular to the strike of S l may represent F3 

fold axes.

Structural data from the Archean gneiss provide 

additional information on deformation at the Plumbago 

Creek locality. The relationship between Archean gneissic 

foliation (SA) and Early Proterozoic foliation (S2) can be 

observed at location 107 in figure 6 and is shown in figure 

IB. Here, Archean gneissic foliation (SA) was folded during 

the second (D2) deformation about a shallow westnorthwest-plunging fold axis (F2 in fig. 1C). Poles to 

Archean gneissic foliation tend to lie along a great circle 

that has a west-northwest-plunging axis (F2a in fig. 7C), and 

most F2 fold axes or S2-SA intersection lineations (L2x) 

plunge gently west-northwest or south-southeast.

Prominent mylonitic foliation occurs along Plumbago 

Creek in the western part of the map area (locations 121 and 

124, fig. 6). Figure ID is a plot of poles to mylonitic 

foliation and shear zones around Plumbago Creek. The 

poles plot in two groups: one group, designated S2, defines 

a moderate to shallow south-dipping foliation, whereas the 

other group (S3) defines a west-southwest-dipping set.

These distinctly different orientations and the crosscutting 

relationship of the north-northwest set (S3) relative to S2 

(fig. 6, also fig. IF) indicate that S3 postdates S2 foliation.

The S2 mylonitic foliation has approximately the 

same strike as the S ] foliation in the graphitic slate (and also 

at Taylor mine, Little Mountain, and Falls River), but it is 

fundamentally different than S^ S l foliation in the Early 

Proterozoic rocks in the L'Anse area in general has been 

deformed, as at Taylor mine and Falls River, by open 

chevron folds having F2 fold axes and S2 fracture cleavage. 

F2 axes are typically coaxial with F! fold axes. In contrast, 

the mylonitic east-northeast foliation in Archean rocks (S2 , 

fig. 6) is not deformed and does not appear to have 

undergone a second major deformation.

Although we cannot prove conclusively from the 

Plumbago Creek area alone that the easterly trending 

mylonitic foliation postdates S t foliation, previous work in 

the general region suggests that it does. Cannon and Klasner 

(1972), Cannon (1973), and Klasner (1978) have shown that 

S l foliation has a consistent west-northwesterly orientation, 

but a similar Early Proterozoic foliation is lacking in 

Archean rocks. For this reason, Cannon and Klasner 

suggested that D! was a thin-skinned deformational event, 

and they suggested that a decollement must detach Early 

Proterozoic D1 -deformed strata from the Archean basement.

Pond

200 400 METERS

MIDDLE PROTEROZOIC

Diabase dike 

EARLY PROTEROZOIC

Iron-formation

Graphitic slate 

ARCHEAN 

Undifferentiated gneiss

Geologic contact

20

- A

25

80

124

EXPLANATION 

Early Proterozoic foliation

Si Showing dip

82 Showing dip; M, 

mylonitic foliation

83 Showing dip 

Archean gneissic foliation

Zone of foliated mylonite,

all in Archean gneiss 

 Plunge of axis of small fold 

in bedding or in gneissic 

foliation

6-4- Plunge of lineation formed by 

intersection of bedding and 

foliation

Inferred D 2 thrust fault

Inferred D 3 thrust fault 

Outcrop area

Location of outcrop area 

mentioned in text

Figure 6. Geologic map in area of Plumbago Creek and graphite pit south of Taylor mine, northern Michigan. Blank areas are 

covered, not mapped. Area of published map of Taylor mine locality (Klasner, 1972, 1978) lies to north.

Deformation in the Penokean Orogen, Northern Michigan K9
Recent studies by Nachatilo and Bauer (1990) in the 

Marquette trough region confirm the absence of Early 

Proterozoic structures in Archean basement rocks of that 

area. Evidence for D2 deformation, however, is commonly 

observed along the contact between Early Proterozoic strata 

and Archean rocks. The D2 deformation is generally 

(though not always) marked by a reverse fault, with 

associated sheared Archean rocks.

As in the Plumbago Creek area, foliated rocks of 

faulted Archean-Proterozoic contacts have not been 

deformed by subsequent deformation, as have the foliated 

Early Proterozoic rocks. Thus, inasmuch as the mylonitic 

rocks at Plumbago Creek were not deformed by a second 

coaxial deformation (D2) as were the Early Proterozoic 

rocks to the north, we conclude that the mylonitic rocks 

along Plumbago Creek were formed during a second (D2) 

deformation.

Rocks within the mylonitized shear zone range from 

protomylonite to mylonite with more than 75 percent 

recrystallized fine-grained matrix. (See Wise and others, 

1984.) The mylonites consist primarily of sericite, quartz, 

and feldspar. S-C fabric and quartz-sericite fish structures 

(fig. IE} at location 124 within the mylonite indicate a 

northward sense of vergence. The mylonitic foliation in the 

Early Proterozoic rocks dips southward beneath Archean 

gneiss.

The north-northwest-trending shear zones (S3) appear 

to be localized along an offset in the general east-west 

structure along Plumbago Creek. They are mainly expressed 

by the north-northwest-trending foliation (S3) at location 

114 in figure 6. A sketch from a photo at outcrop 114 (fig. 

IF) shows the nature of these shears; they strike about N. 

30° W. and dip steeply southwest. The shear zones crosscut 

and deform the S2 shear zones in the Archean rocks. We 

interpret these shears to be caused by the third (D3) 

deformational event. S-C fabric data from near the northnorthwest-trending D3 shear zones at location 112 (fig. 6) 

indicate an east-northeast sense of tectonic transport (see 

fig. 7G).

In summary, rocks in the Plumbago Creek area were 

subjected to at least three stages of deformation: D! formed 

an S! foliation and Pl fold axes in Early Proterozoic slate (as 

exposed in the graphite pit); D] was thin skinned and 

affected only Early Proterozoic rocks. A second event (D2) 

deformed S! and Archean foliation SA, and resulted in S2 

foliation and F2 fold axes. It also formed prominent westnorthwest-trending, south-dipping mylonite fault zones (S2) 

in the Archean gneiss along the contact between Early 

Proterozoic slate and the gneiss. The Archean gneiss block 

was thrust northward during D2 and deformed the S! 

foliation in the Early Proterozoic slates. A third event (D3) 

formed north-northwest-trending, southwest-dipping faults 

in Archean gneiss that crosscut the D2 fault zones. Eastnortheast movement of Archean rocks along these faults 

caused isolated zones of S3 foliation.

Komtie Lake

A drill hole started in Archean gneiss at Komtie Lake 

(fig. 2) provides further data concerning the structural 

evolution of the L'Anse area. Figure 8A is a geologic sketch 

map of the Komtie Lake area, which lies near the northwest 

edge of a block of Archean gneiss (northern complex, fig. 

1). (For more detail on the regional geology, see the 

geologic map of the Iron River I°x2° quadrangle (Cannon, 

1986).) Archean gneiss in scattered outcrops has a variable 

foliation that dips generally to the southwest. Komtie Lake 

is surrounded by a low swampy area; an undeformed 1.1 Ga 

Keweenawan diabase dike forms a prominent ridge north of 

the lake and crosscuts the gneissic foliation.

The 210-m-deep Komtie Lake drill hole (fig. 85) 

penetrated, from the top downward, 30 m of Archean mafic 

to felsic gneiss and 3 m of mylonite, followed by phyllitic 

slate, iron-formation, and weakly deformed quartzite. The 

latter three rock types are similar to Early Proterozoic strata 

at Taylor mine, Canyon Falls, and other nearby areas where 

they stratigraphically overlie the Archean gneiss. We

Figure 7 (facing page). Structure in Plumbago Creek area, 

northern Michigan. All stereoplots are equal-area lower 

hemisphere plots. A, Contoured stereoplot of 20 poles to S^ 

foliation at graphite pit. Contours at 5, 20, and 40 percent. 

Great circle (solid line), general orientation of S^ F, fold axis 

in bedding shown by solid dot (F^. Square, orientation of F2 

crenulation fold axis (F2 ) on S^ foliation planes. Great circle 

(dotted line) shows folded S, surfaces. F 2A, which coincides 

with F2, is fold axis to the S, great circle. B, Cross-sectional 

sketch from a field photograph at location 107 (fig. 6) of an F2 

fold in Archean foliation (SA). Axial-planar foliation shown by 

S2 . Pencil is 16 cm long. C, Stereoplot showing poles to 

foliation (SA) in Archean gneiss. Open square, orientation of F 2 

fold axis (F2 ) in Archean foliation; X, orientation of intersection 

lineation (L2X) of S2 axial-planar cleavage and Archean foliation (SA). F2A is fold axis that corresponds to the great circle. 

D, Stereoplot of mylonitic foliation and shear bands in 

Archean gneiss, which can be divided into two sets: (1) S2 

defines an easterly oriented set (general orientation of S2 plane 

is shown by great circle), and (2) S3 shows north-northeasterly 

oriented set (general orientation of S3 plane shown by great 

circle). E, Sketch of microscopic view (vertical slab) of a 

deformed quartz grain in mylonite at location 124 (fig. 6). 

Lines indicate S2 sericitic foliation. Quartz grain is partly 

recrystallized to a finer grain size (mortar structure), and its 

shape shows S-C fabric orientation and northward sense of 

vergence. Field of view 3.3 mm in diameter. F, Cross-sectional 

sketch of photograph of an S3 shear band at location 114 

(fig. 6). S3 shear band crosscuts S2 foliation and SA (Archean 

gneissic) foliation. Curvature of SA suggests an east-northeast 

sense of tectonic transport during D3 deformation. Hammer 

approximately 45 cm long. C, Sketch of microscopic view 

(vertical slab) of recrystallized quartz grain that has been 

deformed, from location 112 (fig. 6); shows an east-northeast 

sense of tectonic transport. Heavy line, hematite; some with 

sigmoidal patterns that also indicate east-northeast tectonic 

transport. Sericite (light line) is generally parallel to the Cfabric orientation. Field of view 3.3 mm in diameter.

K10 Precambrian Geology of Lake Superior Region
S2

SA

Hematite

D

Quartz

Sericite

Deformation in the Penokean Orogen, Northern Michigan K11
interpret the Archean gneiss as having been thrust over the 

Early Proterozoic strata. Dip of the foliation in the drill core 

is approximately 35° to 40°, about the same as most of the 

south-southwest-dipping foliations in surface exposure at 

Komtie Lake. A fold in foliation (fig. 8A) within a greenstone layer in the gneiss plunges 30° south-southeast, and 

the asymmetry of the fold indicates east-northeast vergence. 

Based on data from the nearby Plumbago Creek area, 

we interpret the mylonitic zone at the base of the gneiss to 

be a D2 or D3 feature. Because the core is not oriented, only 

the magnitude of dip, and not the strike, of the foliation is 

known. The east-northeast-verging structures are interpreted as D3, similar to those at Plumbago Creek. The D3 

event is also interpreted as having caused the shallow, 

south-southeast-plunging fold in the Archean greenstone.

Canyon Falls

A shallow canyon cut into subhorizontal Early Proterozoic quartzite west of the Archean northern complex 

(fig. 2) provides additional information on the structural 

history of the Penokean foreland area. The quartzite lies 

unconformably (fig. 9) on Archean gneiss (Klasner, 1978; 

Cannon, 1986). Cannon (1986) interpreted this quartzite as 

the stratigraphic equivalent of the Goodrich Quartzite, 

which occurs in unconformable contact with Archean basement in much of the western Marquette Range. It is less 

intensely deformed than other Early Proterozoic strata that 

overlie it in the region. Microscopic examination of the 

quartzite shows that it consists of 70 percent quartz grains

that are 1-2 mm in diameter and embedded in a fine-grained 

matrix of sericite and quartz. Only some quartz grains are 

clast supported. The larger grains show moderate undulatory extinction and their long axes tend to have a preferred 

orientation. The less intensely deformed nature of the 

quartzite may indicate that it is separated from younger, 

intensely deformed Early Proterozoic strata (exposed only 

6 km away at Taylor mine) by a decollement.

Structural data from the quartzite reveal at least one 

period of deformation. A thin pelitic layer within the 

thick-bedded quartzite has a distinct S l and (or) S2 foliation 

oriented N. 75° W., 35° SW. (figs. 9 and 10). The thicker 

quartzite layers have a similarly oriented faint foliation. 

Slickenlines on bedding planes near the pelitic layer trend 

N. 5° E. Intersection of S0 bedding and foliation yields a 

fold axis oriented 10°, S. 65° W.

An outcrop at the contact of sheared quartzite on the 

south and metagraywacke (slate unit in fig. 9) on the north 

occurs 2 km north of Canyon Falls. Foliation in the sheared 

graywacke is oriented N. 52° W., 45° SW. Bedding in 

quartzite about 100 m south of this outcrop is subhorizontal, 

similar to that at Canyon Falls. Spaced fracture cleavage in 

the quartzite is oriented N. 60° W., 62° SW., similar to 

cleavage in the graywacke.

Data from Canyon Falls indicate at least one 

deformation that formed a foliation, especially in the thin 

pelitic layers, and a weak foliation in the massive quartzite. 

The precise age of the deformation is not known. No 

evidence exists to suggest that the foliation has been

'"o 7"-

~42

EXPLANATION

Outcrop

£ Keweenawan diabase dike

Orientation of shear zone,

showing dip 

Orientation of gneissic

foliation, showing dip

Strike of gneissic foliation

Plunge of axis of fold in 

gneissic foliation

Plunge of intersection 

lineation of shear zone 

and gneissic foliation

53

SARCHEAN 0

PROTEROZOIC fcj 

KOMTIE LAKE DRILL HOLE 

RESOURCE EXPLORATION DRILL HOLE NO. 1 

LOCATION: SEtt SE'/4 Sec. 11, T. 49 N., R. 33 W. 

ELEVATION: 1,705 feet (519.6 meters)

DEPTH, IN 

OF FEET 

mON LOG (METERS) LITHOLOGY 

n in\

\*

^

S-L 

KX

V^

p&

IIgJL

- 20 (6) 

, . GREENSTONE Foliated with granite 

' ' stringers, partly amphibolitic 

- 60 (18)

- 80 (24) , 

L GNEISS False, mylomtized 100 (30) - 

- 120 (36) PHYLLITE/SLATE Black, pyritic, 

graphitic, crenulated and sheared 

- 140 (42)

160 48 IRON-FORMATION White, cherty, 

' ' \ interbedded brown siderite beds

-180(54) QUARTZITE 

- 200 (60)

- 220 (66) QUARTZITE Sericitic, slightly feldspathic 

-240 (72) 

250 (78) T.D. 251 feet (75.5 meters)

Figure 8. Geologic map and drill-hole data at Komtie Lake area, northern Michigan. A, Geologic sketch map of Komtie Lake 

area showing location of structural measurements and drill hole. B, Log of core from Komtie Lake drill hole (logged by R.W. 

Ojakangas). Dip of foliation in drill core is shown in two directions to signify that drill core is not oriented and true orientation 

of foliation is not known.

K12 Precambrian Geology of Lake Superior Region
4 KILOMETERS

A' B

Komtie Lake drill hole,

projected to plane of 

section

EXPLANATION

Keweenawan diabase dike

20

Early Proterozoic slate, cherty ironformation, graywacke 

Early Proterozoic quartzite

Archean granite, greenstone, gneiss

Geologic contact Dashed where

inferred 

Unconformity

Fault Showing sense of movement;

dashed where inferred 

Thrust fault Queried where uncertain

Strike and dip of bedding

60

$2 kink fold, on cross section 

Strike and dip of foliation: 

1-S1, 2-S2, 3-S3, A-Archean

Plunge of F1 fold

Plunge of F2 fold

Orientation of F2 beddingcleavage intersection lineation 

Komtie Lake drill hole 

Canyon Falls 

Komtie Lake 

Plumbago Creek 

Taylor mine

Figure 9. Geologic sketch map and cross sections of Taylor mine, Komtie Lake, Plumbago Creek, and Canyon Falls 

areas, northern Michigan.

Deformation in the Penokean Orogen, Northern Michigan K13
Figure 10. Cross-sectional sketch from field photograph of 

vertical face at Canyon Falls showing penetrative foliation in 

pelitic layer (lined) intercalated with massive quartzite 

(stippled). The quartzite has a faint south-dipping spaced 

foliation.

refolded, and we therefore tentatively interpret it as S2 , 

formed during D2 ; but it may be Dl in age. The tectonic 

slickenlines represent slip along the bedding of the gently 

dipping quartzite.

Covington Profile

The Michigamme Formation is exposed in several 

roadcuts along U.S. Highway 141 for a distance of about 

16 km south of Covington, Mich. (fig. 2). Outcrops 

comprise graded beds of graywacke and slate at greenschist 

metamorphic grade. These beds have a cleavage that is 

refracted across bedding, and they contain numerous 

deformed calc-silicate concretions. Figure 11 illustrates the 

general style of deformation in the outcrops along the 

profile. Bedding generally strikes west-northwest and dips 

gently to steeply south. Locally, bedding is overturned 

toward the north. Folds in bedding range from open to 

closed, become progressively upright toward the south, and 

have axes that generally plunge gently west-northwest. A 

prominent cleavage dips steeply south and is axial-planar to 

folds in bedding. Some calc-silicate concretions have been 

deformed into ellipsoids with long axes that plunge generally south. A minor, second, west-trending and steeply 

south dipping foliation occurs in places along the profile 

and produces pencil slate with steeply south dipping 

intersection lineations. Kink bands deform the major 

foliation in places. At the south end of the profile (fig. 11), 

foliation is vertical, and it is deformed by a late, northverging fault that has sigmoidally deformed quartz bands 

within the fault zone.

N 16° E N 20° W

Ground 

urface

5 KILOMETERS

J

EXPLANATION

Formlines in bedding Inferred fault showing 

sense of movement

> Dip of bedding

Figure 11 . Structural data for Early Proterozoic strata of Michigamme Formation along the Covington profile, northern Michigan. 

(See fig. 2 for location.) Lower hemisphere stereoplots show orientations of S^ foliation and bedding (S0) and fold axes (solid dot) 

along profile. Faults along profile are inferred at structural breaks. A hypothetical detachment fault separates the Early Proterozoic 

supracrustal rocks from deeper Archean basement. Configuration of ground surface is at true vertical scale, but formline beds and 

faults are not meant to infer true thickness of the Early Proterozoic section, which is unknown in this area.

K14 Precambrian Geology of Lake Superior Region
Structural data indicate a single north-verging fold 

event (fig. 11). The faults shown on the profile are inferred; 

none were observed in the field, and their locations are 

based on discontinuities in structures along the section. 

Apparently, the north-verging faults do not involve Archean 

basement rocks.

The major structures along the Covington profile are 

interpreted as Dl features. The prominent south-dipping 

foliation (Sj) and associated folds (Fj) were moderately 

deformed by younger events. Possibly the second foliation 

that formed the pencil slates is a continuum of the D! event, 

but the kink folds may be D2 in age or possibly D3.

Iron wood-Hurley Area

At the far west end of the area of this study, structural 

data from an outcrop of Early Proterozoic Tyler Formation 

on U.S. Highway 2 on the Michigan-Wisconsin border, 

about 150 km west of the Canyon Falls area (fig. 1), provide 

additional evidence for a north-verging deformational 

event. The Tyler Formation consists of packets of dark-gray 

quartzose graywacke beds containing minor slate interbeds 

intercalated with units of argillaceous siltstone and slate. It 

is generally believed to be the stratigraphic equivalent of the 

Michigamme Formation to the east (James, 1958; this 

report, table 1).

Bedding at the Tyler exposure on Highway 2 is 

oriented N. 75° E., 70° NW.; a prominent more shallowly 

dipping cleavage in the argillite is oriented N. 75° E., 58° 

NW. Flute casts on graywacke soles and graded beds at the 

highway location show that bedding tops northwest and is 

upright (fig. 12). Exposures of Tyler strata along strike have 

similar cleavage-bedding relationships. The regional 

structural setting of the Tyler Formation is shown in Sims, 

Peterman, Zartman, and Benedict (1985, fig. 1).

The Tyler strata are part of a large north-dipping 

homocline (fig. 13A) related to the 1.1 Ga Midcontinent rift 

(Wold and Hinze, 1982). Keweenawan basalt flows above 

the Tyler strata (fig. 13A) dip similarly to bedding in the 

Tyler. Hendrix (1960) and Schmidt and Hubbard (1972) 

attributed the steeply north dipping orientations in the Tyler 

strata to tilting during formation of the Midcontinent rift. 

Likewise, Alwin (1976) noted that the cleavage cannot be 

axial-plane cleavage developed in response to the same 

forces which tilted the Tyler to its present attitude. The 

cleavage must have been formed prior to Keweenawan 

tilting of the Tyler strata.

Reorientation by rotation through 70° around strike of 

the Keweenawan flows restores the Tyler strata to its 

probable pre-Keweenawan, subhorizontal orientation, and 

restores the foliation to a gentle southward dip and strike of 

N. 75° E. (fig. 135). We suggest that this is additional 

evidence for a north-verging structural deformation event.

Recent studies by Cannon and others (1990) provide 

further data on the northward tilting of the Tyler strata.

Graded beds 

of graywacke

Slaty beds

10 METERS

Figure 12. Structural data from the Ironwood, Mich.-Hurley, 

Wis., area. Sketch from field photograph of the Tyler Formation, subvertical roadcut on U.S.Highway 2. Massive sandy 

to silty graywacke beds have intercalated silty and argillaceous 

(slaty) layers. Argillaceous layers have a foliation that dips 

north, less steeply than bedding. Graded bedding has been 

diagrammatically added to indicate that bedding is upright. 

Beds were tilted northwest during Keweenawan rift event.

From studies of rotated Early Proterozoic dikes and 1.1 Ga 

Rb-Sr ages of biotite from Archean and Early Proterozoic 

rocks, Cannon and others have suggested that the Archean 

and Early Proterozoic strata in the region were rotated 

northward along a north-dipping listric fault that formed 

during the 1.1 Ga Keweenawan rift event. They suggested 

that this fault (named the Marenisco fault) lies some 35 km 

south of the Tyler location and south of the Puritan 

batholith. The location of this fault was based largely upon 

appearance of older (pre-1.1 Ga) biotite ages outside (south 

of) the zone of rocks that were rotated and uplifted during a 

late compressional event that affected the Midcontinent rift.

GEOLOGIC INTERPRETATION

Table 2 summarizes the structural data presented in 

preceding sections. At least five and possibly six of the 

localities described were affected by Dj deformation; all 

have evidence for D2 deformation; and four have evidence 

for D3 deformation. We discuss the geologic significance of 

each of the three phases of deformation following.

Dt Event

Deformation Dl is characterized primarily by northverging folding and thrust faulting. Orientation of Fj fold 

axes and strike of Sj foliation are roughly parallel to the

Deformation in the Penokean Orogen, Northern Michigan K15
SOUTH NORTH

Figure 13. Generalized block diagram and cross section 

illustrating structural relationships in Ironwood, Mich.-Hurley, 

Wis., area. Heavy dashed line, fault. A, Block diagram showing relationship of pre-Keweenawan strata to rocks of Midcontinent rift. Note that Sn foliation in Tyler Formation dips less 

steeply than bedding. Modified from Schmidt and Hubbard 

(1972). B, Reconstructed post-Penokean, pre-Keweenawan 

position of the strata shown in A. Cross section is on same 

surface as front of block in A Note gently south dipping Sn 

foliation. Modified from Schmidt and Hubbard (1972).

easterly oriented axis of the Penokean orogen in Michigan. 

In the eastern group of outcrops, this phase of deformation 

seems to affect only Early Proterozoic strata, and we 

suggest that in this area Dl was a thin-skinned deformational event. A detachment fault or decollement must lie 

within the Early Proterozoic strata because Early Proterozoic quartzite at Canyon Falls is only weakly deformed and 

is unconformably attached to underlying Archean rocks. 

Yet stratigraphically younger rocks in the region are 

intensely deformed. The shape of folds, as well as stratigraphic facing directions in folded Early Proterozoic strata 

at Falls River, Little Mountain, and along the Covington 

profile, indicates northward structural vergence.

We tentatively interpret as Djthe gentle south-dipping 

foliation in the Ironwood-Hurley outcrop of the Tyler strata 

(which now has a steep northerly dip due to rotation during

formation of the Midcontinent rift see fig. 13A). Structural 

data in the Ironwood-Hurley area are limited, and the 

relationship of the Tyler strata to underlying rocks of the 

Archean Puritan batholith is not known. However, the 

gentle south-dipping foliation (Sj) and near-horizontal 

bedding (S0; and fig. 13fl) appear to be part of a northverging deformational system, similar to that present in the 

Michigamme Formation at Falls River near L'Anse.

At Newport mine, about 7 km south and east of the 

Tyler outcrop near Ironwood-Hurley, the Palms Formation 

of the Menominee Group (table 1) is in unconformable 

contact with the underlying rocks of the Archean Puritan 

batholith. Although argillaceous and thinly bedded in 

places, the lower two-thirds of the Palms Formation (Ojakangas, 1983) contains no observable evidence of Early 

Proterozoic deformation at this location. The unconformable contact dips steeply (=60°) north, and the bedding in 

the Palms is parallel to the contact; small clasts of igneous 

rock are present in the lower few centimeters of the 

sedimentary rock. However, the Palms and other Early 

Proterozoic strata are somewhat deformed elsewhere in the 

region. North-verging folds with gently plunging fold axes 

and steeply south dipping axial-planar foliation are present 

at Wakefield, Mich., about 18 km east of the Tyler outcrop. 

Also, Schmidt (1980) reported that although evidence for 

folding in the Tyler Formation is meager (observed at only 

a few places underground), the Ironwood Iron-formation of 

the Menominee Group, which lies stratigraphically below 

the Tyler (table 1), has drag folds with gentle northwestplunging fold axes. The sense of vergence on the drag folds 

is not known.

Thus, in this western area the Early Proterozoic 

section consists of undeformed or weakly deformed basal 

units that are overlain by more intensely deformed strata. A 

similar deformation pattern characterizes the quartzite at 

Canyon Falls near L'Anse, which unconformably overlies 

the basement gneiss. It is possible that the Palms Formation 

at the Newport mine may be separated from the overlying, 

slightly more deformed Tyler strata by a decollement, but a 

decollement has not been observed in the region. Hotchkiss 

(1919a, b) reported the presence of a bedding plane fault, 

with minimum eastward offset, near the base of the 

Ironwood Iron-formation in underground mines.

D2 Event

Whereas D1 deformed only Early Proterozoic strata, 

D2 deformed both Early Proterozoic and Archean rocks. 

Structures attributed to D2 occur in pelitic layers in the 

Early Proterozoic strata at Falls River, Little Mountain, 

Taylor mine, Canyon Falls, and the Ironwood-Hurley areas; 

and D2 deformation occurs as a mylonitic fault zone at 

Plumbago Creek and possibly at Komtie Lake (table 2). 

Kinematic data from Archean gneiss at Plumbago Creek 

indicate that Archean rocks were thrust northward along the

K16 Precambrian Geology of Lake Superior Region
Table 2. Summary of deformation observed at Falls River, Little Mountain, Taylor mine, Plumbago Creek, Komtie Lake, Canyon 

Falls, Covington profile, and Iron wood-Hurley area, Michigan and Wisconsin

Deformation D, Deformation D, Deformation D, Remarks

North-verging thrust system, 

which appears to have 

affected only Early Proterozoic strata. Its confinement to 

Early Proterozoic strata suggests that it was a thinskinned deformational event.

Structural elements: 

Bedding S0 

Fold axes Ft 

Foliation or cleavage Sj 

Stretch lineations LQ

North-verging thrusting colinear 

with deformation Dt . Involved both basement rocks and 

overlying Early Proterozoic 

strata.

Structural elements: 

Fold axes

Foliation or cleavage 

Stretch lineations

Local high-angle thrusting toward east-northeast. Involved 

Archean basement and overlying strata.

Structural elements: 

Local crinkle folds on S l F3 

Foliation or cleavage S3 

Fold axes in S, F,

Falls River: 

FR 80,^,8,

Little Mountain: 

LM S0, F15 SL Ls

Taylor mine:

1 1V1 tJr\« JP 1 . O 1 , J_/C

Plumbago Creek: 

PC 8,,8

Komtie Lake: 

KL S0 ?

Canyon Falls: 

CF S0 ?

Covington profile: 

CP SO.F!, S,

Ironwood-Hurley: 

IH S0, S!

F2,S2

F2,S2

F2,S2

F2,S2

F2, S2 , Ls

F2,S2

F2,S2

F3 , S3 , F3

F3 ,S3

F3 , S3 , F3

D, and D2 structural elements 

prominent; D3 occurs at isolated locations.

DI and D2 structural elements 

prominent; D3 not observed.

Dj and D2 structural elements 

prominent; evidence for D3 

weak.

Dt , D2, D3 elements prominent; 

Dt found only in Early 

Proterozoic rocks; D3 occurs 

as localized shear zones.

Data from unoriented drill core 

difficult to distinguish Dl 

from D2 from D3 . S3 and F3 

definitely observed on 

surface.

Massive quartzite, subhorizontal 

bedding. S2 foliation in thin, 

intercalated pelitic layers.

D! structural elements prominent in roadcuts, southdipping Sj and upright Fx 

folds.

D2 elements, especially F2 and 

S2, seen in broader area to 

south. Foliation (S) at Tyler 

outcrop is interpreted as Sj.

mylonitic fault zone. Thick-bedded quartzite, such as at 

Canyon Falls, was rigidly attached to the underlying 

Archean gneiss and was possibly folded during D2. Such 

folding may have caused slip along bedding that induced S2 

foliation in thin pelitic layers and formed northerly oriented 

striations on bedding planes at Canyon Falls.

The D2 deformation, which was coaxial with Dl (that 

is, both Fj and F2 fold axes trend west-northwest), may have 

caused the overturning of bedding at the top of Little 

Mountain. This was accomplished by movement along 

bedding plane faults that overturned the north limbs of the 

earlier north-verging Fj folds and also caused kink folding

Deformation in the Penokean Orogen, Northern Michigan K17
of Sj foliation. Northeasterly oriented slickenlines on 

bedding planes and a northerly overturning of bedding 

indicate a northeasterly sense of vergence in the overturned 

folds at Little Mountain; also, some of the kink folds in Sj 

along the Covington profile are interpreted to be D2.

The D2 deformation was thick skinned and involved 

Archean crystalline basement as well as Early Proterozoic 

rocks. This is most clearly seen at Plumbago Creek, where 

D2 has formed mylonite zones in the Archean gneiss. 

Similar examples of post-Dj, thick-skinned structures are 

found elsewhere in the Marquette trough region. Klasner 

and Cannon (1974) showed that the contact along the south 

margin of the Marquette trough dips steeply south, 

coinciding with a high-angle reverse fault. Also, a highangle reverse fault at a large angle to Sj (see Cannon and 

Klasner, 1976) occurs along the west edge of the southern 

complex (fig. 1). This fault postdates Dj and has been 

interpreted as D2 in age. It demonstrates that the D2 faults 

occur along the contact between Early Proterozoic rocks 

and Archean gneiss at various orientations relative to the 

consistently west-northwest-oriented Sj foliation. The 

mylonitic fault zone along Plumbago Creek is interpreted to 

be a D2 fault, but it dips less steeply (=30°) than do some 

other reverse faults described previously.

The mylonite zones in the drill core at Komtie Lake 

may have formed due to D2 deformation. Because the strike 

of the mylonitic foliation in the drill core is not known, the 

deformation could be either D2 or D3; D3 foliation (S3) and 

fold axes (F3 ) have been observed in outcrops near Komtie 

Lake.

Geologic and geophysical data collected from areas 

south and east of the Tyler outcrop at Ironwood-Hurley 

suggest thick-skinned (D2) deformation in this region also, 

similar to that found 150 km east near L'Anse. For example, 

structural studies in the Watersmeet area of northern 

Michigan (Sims and others, 1984), about 40-60 km east of 

the Ironwood-Hurley location, show that Early Proterozoic 

strata of the Menominee and Baraga Groups have been 

intensely deformed. A structural cross section by Sims and 

others (1984, fig. 3) suggests that the Archean Puritan 

batholith was also involved in the deformational event, but 

the exact nature of this deformation is not known. Recent 

studies by Cannon and others (1990) suggest that the 

south-dipping fold axes and axial-planar foliation were 

rotated clockwise some 30° to 60° during the Keweenawan 

rifting. If the crust in this area were rotated back to its 

pre-Keweenawan orientation, the folds and foliation in and 

adjacent to the south edge of the Puritan batholith would dip 

steeply south.

Gneiss domes in the Watersmeet area (Sims and 

Peterman, 1976) lie just south of the Puritan batholith. Attoh 

and Klasner (1989) suggested that anatectic remobilization 

of the gneiss, as proposed by Sims and Peterman (1976), 

was caused by tectonic stacking during the Penokean 

overthrust event. In this scenario, Dj thin-skinned

deformation deformed rocks that subsequently were eroded. 

Except for the Tyler strata to the north, the only evidence for 

D2 is preserved at the deep tectonic level exposed in the 

region, particularly in the Watersmeet area where structures 

are largely upright. Structures north of Watersmeet would 

also be upright prior to their rotation during the 

Keweenawan rift event. In any case, the remobilized gneiss 

dome and associated upright Early Proterozoic structures in 

Archean rocks at Watersmeet and adjacent areas indicate 

unequivocally that deformation was thick skinned in that 

region.

D3 Event

Although evidence for a third stage of deformation 

exists at several locations, the deformational features (folds 

and faults) are small, isolated, and poorly developed relative 

to D2 and Dl structural features. D3 structures occur as 

north-northeast-trending faults in Archean rocks at Plumbago Creek and Komtie Lake and clearly deform earlier 

structures. Elsewhere evidence for D3 comprises crenulation 

lineations on Sj at Taylor mine and in the graphite pit at 

Plumbago Creek, some(?) of the kink bands along the 

Covington profile, and small isolated foliation and folds at 

Falls Creek. Because of their wide separation from each 

other, it cannot be unequivocally demonstrated that these 

structures are all genetically related.

Summary

The three stages of deformation identified in this 

study are interpreted as follows: Dj represents thin-skinned 

shortening of Early Proterozoic strata along decollements 

that exist at low stratigraphic levels within the Early 

Proterozoic section. Deformation formed north-verging 

structures such as those at Falls River, Little Mountain, 

Taylor mine, and along the Covington profile. Southdipping foliation was also formed in the Early Proterozoic 

rocks at Plumbago Creek.

D2 refolded already-deformed Proterozoic strata and 

the Archean basement rocks. D2 was coaxial with Dl in the 

areas of this study, but elsewhere in northern Michigan it 

was not. Intense anisotropy due to Sj foliation in Early 

Proterozoic supracrustal rocks caused D2 to behave in a 

brittle, kink-fold manner relative to Dj. The thrust fault at 

Plumbago Creek is interpreted as D2. In the west, D2 

deformation may have produced gneiss domes in Archean 

rocks and upright folds in Proterozoic rocks along their 

contact with Archean basement.

D3 represents late-stage faulting along localized 

zones. It caused weak deformation of preexisting 

(especially Sj) structures, such as at Plumbago Creek. 

Deformation may have resulted from late block uplift of 

Archean basement (Cannon and Klasner, 1972).

K18 Precambrian Geology of Lake Superior Region
CONCLUSIONS AND TECTONIC 

IMPLICATIONS

A north-verging fold-thrust system in the Early 

Proterozoic continental foreland of the Superior craton in 

Minnesota, southern Ontario (Zolnai and others, 1984), and 

northern Michigan is well known. However, documentation 

of this deformation in Michigan has previously been 

confined to the eastern part of the exposed Penokean orogen 

in Michigan (Klasner, Sims, and others, 1988). In this 

report, we show that north-verging deformation also exists 

farther west, extending into northern Wisconsin. We have 

documented the nature and sequence of this deformation at 

two widely separated locations (L'Anse and IronwoodHurley) in northern Michigan and Wisconsin.

Previous workers have generally agreed that deformation of the Penokean continental foreland was caused by 

suturing of the Wisconsin magmatic terranes with the 

continental foreland along the Niagara fault at about 1,850 

Ma. The exact relationship between the multiphase deformational sequence described in the preceding section and 

collision of the magmatic terranes with the continental 

foreland is unclear. Data presented herein suggest that some 

Early Proterozoic strata are allochthonous, that is, were 

transported along a sole thrust during D} deformation. 

However, Barovich and others (1989) have shown from Nd 

isotope studies that Early Proterozoic sediments in the 

northeastern part of the orogen, near L'Anse (north of 

Covington), were derived from Archean crust, whereas 

Early Proterozoic sediments farther south in the Penokean 

orogen were foreland deposits derived in large part from the 

Wisconsin magmatic terranes. Thus, Early Proterozoic 

strata in the northern exposures cannot have been tectonically transported from the vicinity of the Wisconsin 

magmatic terranes. Structural evidence indicates that they 

were transported northward, but only a short distance within 

the distal part of the foreland basin.

In the L'Anse area, D2 deformation involved reverse 

faults similar to those along the south edge of the Marquette 

trough in northern Michigan. Similarly, in the IronwoodHurley area to the west, D2 deformation involved dislocation of Archean basement. Thus, the extent of D2 

collision-related deformation is more extensive than previously thought. It now appears that D2 is a continuum of 

collisional tectonics that started with an initial Dl phase of 

thin-skinned deformation.

Attoh and Klasner (1989) suggested that vertical 

remobilization of Archean basement was caused by anatectic melting beneath tectonically stacked Early Proterozoic rocks. They showed that burial (metamorphic pressure) 

was deepest in the west (Watersmeet node), where evidence 

for anatectic remobilization is strong. In the northeast, depth 

of burial was much shallower. The stacked thrust-fold 

sheets are now eroded, but the Tyler Formation outcrop in 

the Ironwood-Hurley region, which was probably deformed

during Dj, most likely represents a remnant of one of these 

thrust sheets. Erosion may have removed most shallow D! 

structures, leaving mostly D2 structures exposed in the 

Watersmeet region.

Hoist (1989) has documented two structural domains 

in east-central Minnesota, and has suggested that they 

extend into northern Michigan. The northern domain 

comprises one major period of deformation characterized by 

open to closed, upright folds with easterly trending subhorizontal fold axes and a single steep-dipping axial-planar 

foliation. These structures occur in the Animikie basin rocks 

of Minnesota and are correlated with the Covington profile, 

Little Mountain, and Falls Creek areas of northern 

Michigan. The southern domain, west and south of Duluth, 

Minn. (fig. 1), comprises nearly isoclinal recumbent folds, 

with subhorizontal easterly trending fold axes, and subhorizontal axial-planar foliation, which has been folded by a 

later D2 event that formed steep upright foliation similar to 

that found in the northern domain. Holm and others (1988) 

proposed a possible tectonic model for the inferred deformational history involving imbricate northwesterly thrusting associated with oblique subduction to the southeast. 

Hoist (1991) has suggested that the region south of Covington in the Horse Race Rapids, Felch, and Calumet 

troughs areas are structurally comparable to the southern 

domain of Minnesota.

Morey and Sims (1976) recognized a difference in 

structural style in the rocks of east-central Minnesota on 

opposite sides of the Archean Great Lakes tectonic zone 

(GLTZ). The GLTZ separates less complexly deformed 

rocks on the north from more complexly deformed rocks to 

the south. Sims and others (1980) extended the GLTZ into 

northern Michigan, as shown in figure 1.

The two structural domains of the Penokean orogen in 

Michigan, that is, the imbricate thrust belt and the foreland 

thrust belt, fit, in a general sense, the two structural domains 

described by Hoist (1989). However, the upright structures 

of the foreland thrust belt found along the Covington profile 

occur south of the GLTZ (as presently located by Sims and 

others, 1980), suggesting either that the GLTZ was not a 

controlling factor in the structural evolution of the Penokean 

orogen in this area, or that the correlation of structural 

domains between Michigan and Minnesota is not valid in 

detail.

Barovich and others (1989) showed that Early Proterozoic sediments in the L'Anse area, north of the northern 

complex (fig. 1), have Archean model ages, whereas those 

along the Covington profile were derived from an Early 

Proterozoic terrane. Therefore, the sedimentary fill in the 

basins north of the northern complex was derived from 

exposed Archean rocks to the north and was multiply 

deformed (D 1? D2, D3) during Penokean deformation. The 

sedimentary rocks south of the northern complex, along the 

Covington profile, characterized by upright north-verging 

structures and a prominent south-dipping cleavage (S^,

Deformation in the Penokean Orogen, Northern Michigan K19
were derived from a migrating tectonic front and deposited 

in a foreland basin such as that proposed by Hoffman (1987) 

and Southwick and Morey (in press) during the Penokean 

orogeny.

Studies such as those of Hoist (1989), comparing the 

structures of the Penokean orogen from widely separated 

regions, should constitute the next step in determining the 

evolution of the foldbelt; we stress the need for further work 

of this nature from southern Ontario to northern Michigan 

and Wisconsin to Minnesota, along the whole length of the 

foldbelt. Likewise, further studies such as those of Cannon 

and others (1990) are needed to assess the effect of the 

Keweenawan rift event on earlier structures in the region.

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Schulz, K.J., LaBerge, G.L., Sims, P.K., Peterman, Z.E., and 

Klasner, J.S., 1984, The volcanic-plutonic terrane of 

northern Wisconsin Implications for Early Proterozoic 

tectonism, Lake Superior region: Geological Association of 

Canada Abstracts with Programs, v. 9, p. 103.

Sikkala, K.M., 1987, A structural analysis of Proterozoic metasediments, northern Falls River, Baraga County, Michigan: 

Houghton, Mich., Michigan Technological University M.S. 

thesis, 103 p.

Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, 

The Great Lakes tectonic zone A major crustal structure in 

central North America: Geological Society of America 

Bulletin, pt. I, v. 91, p. 690-698.

Sims, P.K., and Peterman, Z.E., 1976, Geology and Rb-Sr ages of 

reactivated Precambrian gneisses and granite in the 

Marenisco-Watersmeet area, northern Michigan: U.S. 

Geological Survey Journal of Research, v. 4, p. 405^-14.

Deformation in the Penokean Orogen, Northern Michigan K21
____ 1984, A partisan review of the Early Proterozoic geology 

of Wisconsin and adjacent Michigan: 30th Annual Institute 

on Lake Superior Geology, Wausau, Wisconsin, Abstracts, 

p. 73-76.

Sims, P.K., Peterman, Z.E., Klasner, J.S., Cannon, W.F., and 

Schulz, K.J., 1987, Nappe development and thrust faulting 

in the upper Michigan segment of the Early Proterozoic 

Penokean orogen: Geological Society of America Abstracts 

with Programs, v. 19, p. 246.

Sims, P.K., Peterman, Z.E., Prinz, W.C., and Benedict, F.C., 1984, 

Geology, geochemistry, and age of Archean and Early 

Proterozoic rocks in the Marenisco-Watersmeet area, 

northern Michigan: U.S. Geological Survey Professional 

Paper 1292-A, p. A1-A41.

Sims, P.K., Peterman, Z.E., and Schulz, K.J., 1985, The Dunbar 

gneiss-granitoid dome Implications for Early Proterozoic 

tectonic evolution of northern Wisconsin: Geological 

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

Sims, P.K., Peterman, Z.E., Zartman, R.E., and Benedict, F.C., 

1985, Geology and geochronology of granitoid and metamorphic rocks of Late Archean age in northwestern 

Wisconsin: U.S. Geological Survey Professional Paper 

1292-C, p. C1-C17.

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 results of test drilling: U.S. Geological Survey Bulletin 

1904-C, 17 p.

Van Hise, C.R., and Bayley, W.S., 1897, The Marquette ironbearing district of Michigan: U.S. Geological Survey 

Monograph 28, 608 p. and folio atlas of 39 sheets.

Van Schmus, W.R., 1976, Early and Middle Proterozoic history of 

the Great Lakes area, North America: Royal Society of 

London Philosophical Transactions, ser. A, v. 280, 

no. 1298, p. 605-628.

Wise, D.U., and others, 1984, Fault-related rocks Suggestions 

for terminology: Geology, v. 12, p. 391-394.

Wold, R.J., and Hinze, W.J., 1982, Geology and tectonics of the 

Lake Superior basin: Geological Society of America 

Memoir 156, 280 p.

Zolnai, A.I., Price, R.A., and Helmstaed, H., 1984, Regional cross 

section of the Southern Province adjacent to Lake Huron, 

Ontario Implications for the tectonic significance of the 

Murray fault zone: Canadian Journal of Earth Sciences, 

v. 21, p. 447^-56.

K22 Precambrian Geology of Lake Superior Region

*U.S. GOVERNMENT PRINTING OFFICE: 1991-573-049/46002


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

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