Tectonic Imbrication and Foredeep 

Development in the Penokean Orogen, 

East-Central Minnesota An Interpretation 

Based on Regional Geophysics and 

the Results of Test-Drilling

The Penokean Orogeny in Minnesota and 

Upper Michigan A Comparison of 

Structural Geology

U.S. GEOLOGICAL SURVEY BULLETIN 1904-C, D
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Tectonic Imbrication and Foredeep 

Development in the Penokean Orogen, 

East-Central Minnesota An Interpretation 

Based on Regional Geophysics and 

the Results of Test-Drilling

By D.L SOUTHWICK and G.B. MOREY

The Penokean Orogeny in Minnesota and 

Upper Michigan A Comparison of 

Structural Geology

By TIMOTHY B. HO 1ST

Chapters C and D are issued as a single volume and are not available separately

U.S. CEO LOGICAL SURVEY BULLETIN 1904-C, D

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and LM.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

Southwick, D.L. (David Leroy), 1936-

Tectonic imbrication and foredeep development in the Penokean orogen, 

east-central Minnesota an interpretation based on regional geophysics and 

the results of test-drilling / by D.L. Southwick and G.B. Morey. The Penokean 

orogeny in Minnesota and upper Michigan a comparison of structural 

geology / by Timothy B. Hoist.

p. cm. (Contributions to Precambrian geology of Lake Superior 

region ; ch. C-D) (U.S. Geological Survey bulletin ; 1904-C-D)

"Chapters C and D ... are not available separately."

Includes bibliographical references (p. )

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

1. Geology, Stratigraphic Proterozoic. 2. Geology Minnesota. 

3. Geology, Structural Minnesota. 4. Plate tectonics. 

5. Geology Michigan. 6. Geology, Structural Michigan. I. Morey, 

G.B. II. Hoist, Timothy B. Penokean orogeny in Minnesota and upper 

Michigan a comparison of structural geology. 1991. III. Title. IV. Title: 

Penokean orogeny in Minnesota and upper Michigan a comparison of 

structural geology. V. Series. VI. Series: U.S. Geological Survey bulletin ; 

1904-C-D.

QE75.B9 no. 1904-C-D 

[QE653.5] 

557.3 s dc20

[551.7'15'09776] 90-14023

CIP
Chapter C

Tectonic Imbrication and Foredeep 

Development in the Penokean Orogen, 

East-Central Minnesota An Interpretation 

Based on Regional Geophysics and 

the Results of Test-Drilling

By D.L SOUTHWICK and G.B. MOREY

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and LM.H. CARTER, Editors
CONTENTS

Abstract Cl

Introduction Cl

Acknowledgments Cl

Previous investigations Cl

New work C4

Tectonic conclusions and stratigraphic implications C7

Regional considerations Cll

Possible equivalence of Niagara fault zone and Malmo discontinuity CH 

Correlation of stratigraphic sequences north of Niagara-Malmo zonePossibilities and problems C13 

Summary C13

References cited C14

FIGURES

1. Index map of Lake Superior region, showing Penokean orogen and Midcontinent rift system C3

2. Generalized tectonic map of Penokean orogen showing major subdivisions and 

place names mentioned in text C4

3. Map showing medial zone of Penokean orogen in Minnesota, including part of 

deformed fold-and-thrust belt and part of Animikie basin C5

4. Schematic diagram illustrating evolution of an oceanic trench into a 

foredeep C8

5. Jensen plot of volcanic rock compositions from various parts of fold-and-thrust 

belt C9

6. FeO-MgO-Al2O3 plot showing distribution of basaltic rock compositions from 

fold-and-thrust belt C9

7. Diagram of Zr:Y plotted against Zr, showing distribution of basaltic rock 

compositions from fold-and-thrust belt CIO

8. Chondrite-normalized rare-earth element plots for basaltic rocks of fold-andthrust belt CIO

9. Section of inferred stratigraphic relationships among rock units in Minnesota 

segment of Penokean orogen CH

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Tectonic Imbrication and Foredeep Development in the 

Penokean Orogen, East-Central Minnesota 

An Interpretation Based on Regional 

Geophysics and the Results of Test-Drilling

ByD.L. Southwick1 andG.B. Morey1

Abstract

Regional aeromagnetic and gravity data, substantiated 

by data from follow-up shallow drilling, archived exploration 

drilling, and sparse bedrock outcrop, are the basis for a 

plate-tectonic reinterpretation of the segment of the Early 

Proterozoic Penokean orogen that lies in Minnesota, 

immediately west of the crosscutting Midcontinent rift system 

(Middle Proterozoic). This segment occupies a major 

northwest-facing salient in which structural trends curve from 

east-west near Lake Superior to north-south in central 

Minnesota. Within that broad arc the orogen consists of a 

southerly internal fold-and-thrust belt that contains tectonically shuffled sequences of sedimentary and volcanic rock in 

several terranes of contrasting stratigraphy, structural style, 

and metamorphic grade, together with an external turbidrte 

basin (Animikie basin) that has the attributes of a migrating 

foredeep. The deformed supracrustal rocks in the medial part 

of the fold-and-thrust belt are characterized by mainly 

continental-margin petrogenesis and by low to moderate 

metamorphic grade. The southernmost terrane in the foldand-thrust belt contains amphibolite-grade metamorphic 

rocks and migmatite from deep tectonic levels, as well as 

abundant syntectonic and post-tectonic intrusions.

The Penokean orogen developed along the south 

margin of the Archean Superior craton as the craton margin 

was subducted toward the southeast (present directions). 

Crustal loading by imbricated thrust sheets produced a 

frontal downflexure that migrated cratonward as 

convergence and subduction continued, and the thick 

section of turbidite that makes up the bulk of the Animikie

Manuscript approved for publication June 26, 1990. 

1Minnesota Geological Survey, 2642 University Avenue, St.

Paul, MN 55114.

Group (Early Proterozoic) accumulated therein. Ironformations were deposited in three distinct tectonostratigraphic settings as the Penokean orogen evolved 

through time.

INTRODUCTION

The Penokean orogen is an Early Proterozoic 

deformed belt that trends west-southwest along the south 

margin of the Archean Superior craton (Card and 

Ciesielski, 1986; Hoffman, 1988). It is truncated on the 

east by the Grenville orogen, and on the southwest by the 

Central Plains orogen (Sims and Peterman, 1986) at a 

poorly defined location in the subsurface of Nebraska. 

Sedimentation, volcanism, deformation, and plutonism 

occurred variably and episodically along the Penokean 

trend over a span of time that began perhaps as early as 

2,491 ±5 Ma with extension in the Lake Huron area and 

ended as late as 1,770±10 Ma with the emplacement of 

post-tectonic plutons in southern and east-central 

Minnesota (Andrews and others, 1986; Horan and 

others, 1987). There is no evidence to suggest that any 

one segment of the orogen was affected by the entire 

700-m.y. interval of activity; rather, it appears that tectonism may have started in the east and migrated west, as 

first suggested by Young (1983).

The Minnesota segment of the Penokean orogen 

(fig. 1) is very poorly exposed, and its investigation 

therefore has relied heavily on geophysical techniques 

and drilling. The northwestern, tectonically external part 

of the orogen consists of the Animikie basin and at least 

two remnant outliers to its west and southwest (first 

recognized in the present study) that are analogous to the

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C1
Animikie in their tectonic setting and sedimentary fill 

(figs. 1 and 2). Although the Animikie basin is best 

known for the huge iron ore deposits that occur in the 

Biwabik Iron-formation of the Mesabi iron range 

(Morey, 1983 and references therein), sparse outcrop, 

drilling, and aeromagnetic signature indicate that most 

sedimentary rocks in the basin are dark-colored, 

turbiditic graywacke and argillite in formations that 

stratigraphically overlie the Biwabik (Morey and Ojakangas, 1970; Lucente and Morey, 1983). The sedimentary 

succession along the northwest margin of the Animikie 

basin (the Early Proterozoic Animikie Group) rests with 

erosional unconformity on Archean basement. It consists 

of a basal quartz arenite-siltite unit that was deposited 

under tidal influence (Ojakangas, 1983), the overlying 

Biwabik Iron-formation deposited on a shallow marine 

shelf (Morey, 1983), and the very thick sequence of 

orogenic turbidite above the iron-formation. Folding and 

cleavage are lacking along the Mesabi iron range; broad, 

open folds with incipient slaty cleavage appear about 15 

km south of the range (fig. 2) and folding becomes 

progressively tighter south of there (Southwick, 1987; 

Hoist, 1982, 1984).

The southeastern, tectonically complex part of the 

Penokean orogen in Minnesota contains highly deformed 

sedimentary and volcanic rocks that have been metamorphosed under varying conditions. Overall, the 

intensity of deformation and metamorphism increases 

regionally from northwest to southeast. Prominent 

among the supracrustal rocks in the central part of the 

deformed belt are carbonaceous black slate, ironformation, quartzite, and various mafic to intermediate 

volcanic rocks (now mainly of greenschist rank) that 

collectively constitute the Early Proterozoic Mille Lacs 

Group (Morey, 1978). The southern part of the belt 

contains supracrustal rocks of amphibolite and lower 

granulite grade, various kinds of gneiss (including some 

of documented Archean age; see Goldich and Fischer 

(1986) and Southwick and others (1988) for data and 

discussion), and an assortment of syntectonic to posttectonic intrusions that range in age from about 1,980 to 

1,770 Ma (Koran and others, 1987; Goldich and Fischer, 

1986; Spencer and Hanson, 1984; Morey, 1978).

ACKNOWLEDGMENTS

This truly has been a group effort. We are 

especially grateful to Val W. Chandler, R.J. Ferderer, 

and K.E. Carlson for their assistance with geophysical 

interpretation, to P.L. McSwiggen for his insights into 

some of the complexities of east-central Minnesota, and 

to P.K. Sims and Cedric Iverson for sharing their 

accumulated wisdom on matters Penokean. We also 

thank Warren Beck for permission to quote isotopic data 

from his still-warm Ph. D. dissertation. Technical reviews

by P.K. Sims and T.B. Hoist and an informal critique by 

Paul Hoffman helped us codify our arguments, and are 

much appreciated.

The aeromagnetic mapping and scientific testdrilling programs of the Minnesota Geological Survey 

were funded by the Legislative Commission on 

Minnesota Resources; we thank the members and staff of 

LCMR for their support. Finally, we thank Gail DeShane 

for typing the manuscript and its many revisions, and 

Richard Darling for drafting the original illustrations.

PREVIOUS INVESTIGATIONS

Various workers have attempted to correlate the 

weakly metamorphosed supracrustal rocks in the 

strongly deformed central zone of the orogen with those 

in the less deformed Animikie basin to the north (fig. 3), 

using units of iron-formation present in both terranes as 

the key correlative intervals. A fundamental and 

persistent question has been whether there are several 

units of Early Proterozoic iron-formation in central 

Minnesota or only one. The major iron-formation of the 

Cuyuna iron-mining district in the deformed zone of the 

orogen, the Trommald Formation of Schmidt (1963), 

was thought to be continuous with the Biwabik Ironformation of the Animikie basin by Grout and Wolff 

(1955), Marsden (1972), and Morey (1978), whereas 

Zapffe (1925,1933) had argued earlier that the two were 

separated by a regional unconformity and thus were not 

the same stratigraphic entities. The virtual lack of 

outcrop in the broad area between the Cuyuna and 

Mesabi mining districts (fig. 2) has contributed greatly to 

the uncertainty of these stratigraphic reconstructions. 

Nevertheless, this question has major tectonic 

implications because, if the iron-formations of the 

southern, more deformed internal and the northern, less 

deformed external terranes are physically parts of the 

same stratigraphic entity, then the terranes must have 

been contiguous at the time the iron-formation was 

deposited. If, on the other hand, the iron-formations 

were not once continuous, it is conceivable that the two 

terranes developed separately and were subsequently 

juxtaposed by tectonic processes.

Structural studies by Hoist (1982,1984,1985) and 

Holm and others (1988) have shown conclusively that the 

southeastern part of the Penokean orogen in Minnesota 

has undergone an early phase of deformation that was of 

recumbent style, and that the early, subhorizontal 

structures of that phase were refolded into upright 

second-generation folds. These observations are consistent with the presence of nappe structures in the 

southern terranes, and Hoist (1984) has postulated that 

the brow of a major north-verging nappe lies about 2 km 

south of Carlton, at a position shown by the long-dashed 

line on figure 2. Hoist's work clearly implies that much of

C2 Precambrian Geology of Lake Superior Region
i

o

a<D <D O

I

<D

i 

£D)

100 KILOMETERS

Minnesota segment

^-MINNESOTA S 

I I

Wisconsin-Michigan segment 45° - EXPLANATION

Early 

Proterozoic

Turbiditcs overlying shelf deposits 

Plutonic and volcanic rocks, mainly calc-alkalic 

Complex continental-margin sequences 

Archean rocks, undivided

Contact Dashed where approximately located; 

queried where uncertain

- High-angle fault-* * Thrust fault Sawteeth on overthrust block; queried where uncertain- "^ ^ Shear zone

............. South edge of good outcrops

GLTZ Great Lakes tectonic zone

Figure 1. Lake Superior region, showing Penokean orogen (Early Proterozoic) and Midcontinent rift system (Middle Proterozoic). The latter divides the Penokean belt into a Minnesota segment (west of the rift) and a Wisconsin-Michigan segment (east of the rift). In both segments, a complex zone of tectonically imbricated 

continental-margin rocks lies between a belt of igneous, volcanic, and high-grade metamorphic rocks (on the south) and a series of turbidite basins that overlap Archean 

basement (on the north). Order of Early Proterozoic map units in explanation is not necessarily their deposrtional order. Wisconsin-Michigan geology generalized from 

Sims and others (1989). The term "McGrath-Little Falls terrane " has been modified from our earlier term 'structuralpanel" (Southwick and others, 1988) to conform with the use of an effective term in broader-published work.
GLTZ

EXPLANATION

Diabase dike of Kenora-Kabetogama swarm 

Autochthonous rocks of Mille Lacs (?) Group

Archean rocks

Contact Approximately located

High-angle fault

Thrust fault

Great Lakes tectonic zone

Northern limit of subve tica 

cleavage assoc ted wit

Northern limit of ubho on alY

cleavage associat d w 

recumbent fold

N MROD OUTLIER

McGRATH DOME

McGRATH-LITTLE FALLS TERRANE 

Little Falls

Figure 2. Generalized tectonic map of Penokean orogen in 

east-central Minnesota, showing major tectonic subdivisions 

as well as place names mentioned in text. Southern, more 

complexly deformed (internal) part of orogen consists of four 

named lithostratigraphic terranes, coupled with a fifth 

unnamed belt (mostly graphitic schist) between Cuyuna 

South range and Moose Lake-Glen Township terranes.

the southeastern Penokean terrane in Minnesota may be 

allochthonous, and opens various plate-tectonic 

possibilities for interpreting the Minnesota segment of 

the orogen.

NEW WORK

By 1985 new high-resolution aeromagnetic data 

were available over all the Penokean orogen in

Internal and medial terranes are bounded by structural 

discontinuities. Northern, external part of orogen consists of 

turbidite basins, including main bowl of Animikie basin, 

Nimrod outlier, and Long Prairie basin. The Animikie and 

Nimrod probably were once joined. Long-dashed line 

demarks northern limit of recumbent deformational style as 

mapped by Hoist (1982, 1984) and extended in this study.

Minnesota (Chandler, 1983a, b, c, 1985). About 80 test 

holes were drilled in the Penokean area by the Minnesota 

Geological Survey to substantiate the sources of 

particular magnetic anomalies and anomaly patterns 

(Southwick and others, 1986). This drilling was 

supplemented significantly by the release, in the early 

1980's, of a great amount of drill core and related 

subsurface data that had been acquired by iron mining 

companies formerly active in the Cuyuna district.

C4 Precambrian Geology of Lake Superior Region
,. * f *' + Schist, gneiss, migmatite, 

* + * granitoid intrusions ^ Sedimentary rocks 

 Iron- formation

Sedimentary and volcanic 

rocks, interstratified

Volcanic and 

hypabyssal rocks

Contact 

High-angle fault 

Thrust fault

Figure 3. Medial zone of Penokean orogen in Minnesota, including northern part of the strongly deformed fold-and-thrust 

belt and southern part of the Animikie basin. Dot-dashed line, younger unconformity and approximate basin boundary; 

dashed line, older unconformity.

Although these company data are largely confined to the 

immediate neighborhood of magnetic iron-formations 

that were located decades ago by dip-needle surveys, they 

provide much stratigraphic and structural information in 

heavily drift covered areas where virtually no public data 

were previously available.

The combined approach of regional geophysical 

analysis, follow-up drilling, and integration of data from 

outcrop studies and historic drilling led us at an early 

stage to recognize four major structural discontinuities in 

the southern part of the orogen (fig. 2). These are 

defined primarily by map-view geophysical discordances 

where strike-parallel linear anomaly patterns are 

abruptly truncated by different linear trends, or where 

one characteristic anomaly pattern abruptly gives way to

another. They are most clearly seen in shaded-relief 

maps of aeromagnetic data, but also are indicated on 

contoured aeromagnetic maps, contoured maps of the 

first vertical derivative of the aeromagnetic data, and 

contoured maps of the second vertical derivative of the 

Bouguer gravity anomaly. A published small-scale 

shaded-relief image that includes the Penokean area 

(Chandler and others, 1984) illustrates the quality of the 

aeromagnetic data; the images actually used for 

interpretation are unpublished, variously enhanced 

computer plots at scale 1:250,000 and larger.

The drilling and outcrop data suggest that 

structural discontinuities bound three areas or terranes 

that are distinct from one another in rock type, 

stratigraphic rock proportions, metamorphic grade, and

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C5
structural style. For example, the central two terranes 

delineated on figure 2 (Moose Lake-Glen Township and 

Cuyuna South range, together with an unnamed belt 

between them) contain stratigraphic sequences that 

include abundant volcanic material, as well as a variety of 

sedimentary rocks of pelitic to arenaceous affinity (fig. 

3). These rocks were metamorphosed chiefly under 

greenschist-facies conditions and display two generations 

of cleavage over wide areas. The first cleavage is a 

shallow-dipping continuous cleavage that varies areally in 

its distribution and orientation, and is associated with 

recumbent folds. The second cleavage is most commonly 

a steeply dipping crenulation cleavage that is associated 

with inclined to upright small- and meso-scale folds. In 

contrast, the rocks in the supracrustal sequences in the 

terrane north of the Serpent Lake discontinuity (fig. 2) 

contain very little volcanic material. These rocks possess 

only one slaty cleavage in most areas (two seen locally), 

and the metamorphic grade is barely of greenschist rank. 

The contrast is even greater across the Malmo 

discontinuity (figs. 2,3), where the low-grade terranes on 

the north are in contact with the McGrath-Little Falls 

terrane, of medium- to high-grade schist, gneiss, and 

migmatite that is shot through with granitoid intrusions.

We interpret the observed geophysical discontinuities to be structural breaks that probably involved 

brittle-ductile thrust-faulting at shallow crustal levels and 

the development of mylonite zones at deeper levels. (See 

Holm and others, 1988, for discussion.) The discontinuities are inferred to dip mainly toward the south, 

consistent with the sense of northward overturning of 

folds in the region (Schmidt, 1963; Morey, 1983; Hoist, 

1982, 1984, 1985), although local reversals of dip are 

likely. The Werner deconvolution studies of Ferderer 

(1988a, b) indirectly confirm the general southward (or 

southeastward) dip; south-dipping slabs or lithologic 

contacts give the best calculated fits for most of the 

aeromagnetic profiles analyzed. The Malmo 

discontinuity was independently defined magnetotellurically by Wunderman and Young (1987) and Wunderman 

(1988), who deduced it from a major conductive zone 

that dips southward beneath the McGrath dome (fig. 2). 

The surface trace of the conductive zone corresponds 

closely with a profound aeromagnetic discontinuity, and 

also marks the position of an abrupt change in metamorphic grade of the rocks on either side as previously 

described.

In previous discussion of the southern part of the 

Penokean orogen (Southwick and others, 1988), we used 

the terms "panel" and "structural panel" rather than 

"terrane" for the tectonostratigraphic entities described 

above. Those terms were preferred because the 

dimensions of the entities, the state of knowledge of their 

internal stratigraphy and external stratigraphic 

correlations, and the geologic control on the positions

and characteristics of their bounding discontinuities were 

less than adequate for the definition of "terrane" as we 

understood it. Since then, we have found that the terms 

"terrane" or "sub-terrane" have been used for many 

fault-bounded tectonostratigraphic entities that are as 

small and obscure as those in the Penokean orogen of 

central Minnesota, and that this usage is permitted by the 

definitions (Howell and others, 1985, p. 5). Therefore we 

have changed our terminology to conform with accepted 

practice.

Our work in the northwestern, more external part 

of the orogen has demonstrated the existence of two 

previously unknown turbidite remnants that lie west and 

southwest of the Animikie basin and that appear similar 

to the main bowl of the Animikie basin in terms of lithic 

fill, metamorphic grade, structural pattern, and tectonic 

setting. The Nimrod outlier (fig. 2) is a patch of 

essentially flat lying turbiditic material resting on 

Archean basement, and is analogous to the 

northernmost, undeformed parts of the Animikie basin. 

The Long Prairie basin contains turbiditic rocks in which 

intensity of cleavage development and metamorphic 

recrystallization increases from northwest to southeast 

(Southwick, 1987). These observed structural and metamorphic gradients mimic those that occur across the 

central and southern zones of the main Animikie basin. 

Geophysical patterns along the southeast margin of the 

Long Prairie basin strongly imply that the graywacke and 

slate of the basin fill have been overridden by tectonic 

slices of volcanic and related rocks of the Cuyuna South 

range terrane. In addition, a lithic conglomerate near the 

southeast edge of the Long Prairie basin contains clasts 

of characteristic South range rocks that indicate a 

younger age for the Long Prairie fill that can be 

attributed to a South range source. These observations 

suggest that the southeast margin of the Long Prairie 

basin was a dynamic zone of interspersed deposition and 

thrusting across which northwestward transport brought 

detritus shed from a tectonic hinterland to the southeast.

Geophysical and drilling data suggest that the 

sedimentary rocks at the south edge of the main bowl of 

the Animikie basin rest unconformably above rocks in 

the structural terranes to the south and southeast (fig. 3). 

East-northeast-trending aeromagnetic anomalies 

associated with iron-formations in the Cuyuna North 

range and Cuyuna South range terranes are sharply 

truncated by west-northwest-trending anomalies 

associated with iron-formations that occur in the lower 

part of the Animikie Group in the Emily iron-bearing 

district. Severely damped magnetic signals colinear with 

the truncated Cuyuna district anomalies occur farther 

northeast, however, and these have been interpreted by 

Carlson (1985) as arising from Cuyuna-district ironformations that now are beneath a thick pile of Animikie 

Group strata. A short distance east of the Emily district

C6 Precambrian Geology of Lake Superior Region
(fig. 3), a folded iron-formation of considerable strike 

length in the Cuyuna South range terrane ends abruptly 

on the northeast; intensive exploration drilling northeast 

of the iron-formation terminus encountered gray argillite 

and graywacke typical of the Animikie Group, rather 

than the green chloride phyllite and black carbonaceous 

slate with which the iron-formation is interbedded along 

strike to the southwest. These relationships are 

consistent with unconformable overlapping and burial of 

the iron-formation-bearing sequence by a younger 

graywacke-argillite sequence.

The existence and position of the inferred 

unconformity both become problematical farther east 

toward Carlton (figs. 2 and 3), where the structural 

trends in rocks above and below the postulated erosional 

contact are essentially east-west and parallel (Winchell, 

1899; Schwartz, 1942; Hoist, 1984). Outcrop mapping 

and drilling show that an abrupt transition, if not a sharp 

contact, occurs between contrasting supracrustal rock 

sequences along the inferred unconformity trace, but 

they do not prove an unconformity. Mineralogical and 

geochemical contrasts across the transition zone 

strengthen the case, however (Southwick and others, 

1988), and these data, together with the geophysical 

evidence along strike to the west, provide a 

circumstantial argument for extending the unconformity 

as mapped (fig. 3).

The inferences of a thrust-faulted boundary at the 

southeast edge of the Long Prairie basin (fig. 3) and an 

unconformable boundary at the south edge of the Animikie basin might appear to be incompatible. They can be 

reconciled, however, if the rocks within the Long Prairie 

and Animikie basins are viewed as tectonically equivalent 

but temporally somewhat diachronous, or if the latest 

thrust motion on the Serpent Lake discontinuity was 

confined to the west half of its length. Either of these 

possibilities is quite conceivable in dynamic 

environments associated with convergent tectonism near 

zones of subduction.

TECTONIC CONCLUSIONS AND 

STRATIGRAPHIC IMPLICATIONS

We regard the southern part of the Penokean 

orogen in Minnesota as a fold-and-thrust belt made up of 

four major structural terranes. The McGrath-Little Falls 

terrane contains relatively high grade schist and gneiss, a 

mantled gneiss dome that is cored by multiply deformed 

Archean gneiss (Holm and others, 1988), and several 

late-tectonic to post-tectonic granitoid plutons (fig. 2). 

Taken as a whole, the McGrath-Little Falls terrane has 

the attributes of a deep-seated crustal slice that has been 

elevated by tectonic imbrication. It is interpreted to 

occupy an internal position within the Penokean orogen.

To the north of the McGrath-Little Falls terrane, the 

Moose Lake-Glen Township and Cuyuna South range 

terranes both contain folded volcanic and sedimentary 

rocks of variable but generally low metamorphic grade 

(McSwiggen, 1987; Morey, 1978). These two terranes are 

separated from each other by a long, arcuate zone of 

probable thrusting that is localized by a belt of weak and 

highly deformed graphitic schist. Both terranes contain 

much more mafic to intermediate volcanic and hypabyssal rock than was portrayed on earlier geologic maps 

(for example, compare Morey and others (1981) with 

Southwick and others (1988)). In addition, they contain 

abundant metapelite, metasiltite, and graphitic argillite, 

many thin, lensoidal units of iron-formation (Morey and 

Morey, 1986), and poorly known amounts of quartzite 

and related arenaceous rocks. All these rocks are closely 

folded and cleaved, and locally show evidence of multiple 

fold generations (Hoist, 1982, 1984, 1985). Taken as a 

whole, the Moose Lake-Glen Township and Cuyuna 

South range terranes have the attributes of a medial 

tectonic zone dominated by fold-and-thrust deformation. 

Northwest of these terranes, the Cuyuna North range 

terrane contains weakly metamorphosed, less strongly 

deformed sedimentary rocks. Volcanic rocks are 

volumetrically minor. The main stratigraphic units 

include a thick lower section of metapelite and metasiltite, a medial section dominated by iron-formation units 

that define a complex synclinorium near the center of the 

terrane, and an upper unit that consists of dark-colored, 

graphitic argillite and siltite with local interbeds of ferruginous chert. Taken as a whole, the Cuyuna North range 

terrane has the attributes of a small restricted basin that 

was incorporated tectonically in the more external part of 

a fold-and-thrust belt. Additional details on the geology 

of the terranes in the fold-and-thrust belt are presented 

in Southwick and others (1988).

The fold-and-thrust belt abuts, and is partially 

overlapped by, a complex turbidite-filled sedimentary 

basin lying to the northwest (the Animikie basin and its 

smaller analogs). The Archean basement beyond and 

beneath the northwest flanks of the turbidite basins 

constitutes the cratonic foreland against which 

northwest-directed tectonic imbrication is thought to 

have occurred.

Sedimentary strata of the Animikie Group rest 

with unequivocal unconformity on the Archean craton on 

the northwest side of the Animikie basin, and with 

inferred unconformity on previously deformed rocks of 

the fold-and-thrust belt on the southeast side. Rocks of 

the Animikie Group near the southern flank of the basin 

were folded when the underlying rocks were refolded 

during the later stages of regional compression (Southwick and others, 1988), whereas they were scarcely 

deformed at all on the cratonal, northern flank. 

Sedimentary fill in the basin decreases in total thickness

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C7
NW

RIFTED CONTINENTAL MARGIN

, Volcanic arcs, 

, accreted arcs,

and continental 

, fragments. :

« ^ + Continental crust ^ * 

* * (Superior craton) - *

FOLD-AND-THRUST 

BELT

PERIPHERAL 

BULGE

ACTIVE FOREDEEP

MIGRATING NW 

(MAIN BOWL, ANIMIKIE BASIN) |

Trench and foredeep deposits

EXPLANATION

Initial-rift deposits j Passive-margin deposits

Figure 4. Schematic diagram modified from Hoffman (1987) illustrating the evolution of an oceanic trench (stage 1) into a 

foredeep (stage 2). A, passive-margin deposits accumulating on continental edge; B, location of fold-and-thrust belt involving 

volcanic and sedimentary rocks and their basement.

and in degree of low-grade metamorphism from 

southeast to northwest. Primary sedimentary structures 

in rocks near the rim on the north side of the basin clearly 

indicate a northern (cratonal) source, whereas the sedimentological and geochemical attributes of at least some 

of the lithic graywacke near the southern basin margin 

are consistent with a southern provenance (Southwick 

and others, 1988, p. 7-9 and 16). Taken as a whole, the 

broad features of the Animikie basin and its smaller 

analogs are consistent with those of a migrating foredeep 

produced by tectonic loading and downbowing of 

continental crust during attempted subduction of an 

Archean continental margin. Our conceptual model for 

the formation of the basin accords well with the general 

foredeep model developed by Price (1973), Beaumont 

(1981), Jordan (1981), and Quinlan and Beaumont 

(1984), and as developed more fully for Proterozoic 

orogens in North America by Hoffman (1987). Figure 4 

summarizes our general views for the origin of the 

Animikie basin as a migrating foredeep.

With respect to the Minnesota segment of the 

Penokean orogen, stage 1 of figure 4 represents the rifted 

margin of the Superior craton approaching a southeastdipping subduction zone in Early Proterozoic time. 

Initial-rift deposits, including the volcanic rocks of the 

Mille Lacs Group, are riding on the continental edge; 

passive-margin deposits, possibly preserved as quartzose 

sedimentary rocks of the Mille Lacs Group, are 

accumulating near A. Stage 2 of figure 4 represents active 

subduction of the continental margin. Crustal loading by 

the ensuing fold-and-thrust mass has occurred, 

generating as a flexural response an actively migrating 

foredeep (main bowl of the Animikie basin) on the 

continental side. Various volcanic and sedimentary rocks 

and their stratigraphic basement are involved in foldand-thrust nappes near B, representing elements such as 

island arcs, accretionary wedge deposits, and continental 

fragments that have been swept together tectonically. 

The marginal deposits from stage 1, as well as early 

deposits laid down toward the rear of the foredeep, also

C8 Precambrian Geology of Lake Superior Region
may be incorporated in the deforming mass; these appear 

to be dominant in the Penokean imbricate stack in 

Minnesota as preserved in the region from the Malmo 

discontinuity to the south margin of the Animikie 

basin (fig. 2). No oceanic crust, trench deposits, or 

volcanic rocks of arc affinity have been identified 

positively in Minnesota, although Horan and others 

(1987) suggested that an isolated gabbro body near 

Mora, in the McGrath-Little Falls terrane (fig. 2), has 

oceanic attributes.

Although we are relatively confident that the 

principal collisional event (or events) in the Penokean 

orogen in Minnesota involved subduction of an Archean 

craton beneath a northwest-transported fold-and-thrust 

mass, and the consequent development of a migrating 

foredeep, serious uncertainties exist regarding the 

tectonic setting and age of the supracrustal rocks in the 

fold-and-thrust mass, as well as the timing of subduction.

We first consider evidence that pertains to the 

tectonic setting and age of the supracrustal rocks in the 

fold-and-thrust belt. Despite the stratigraphic 

uncertainties introduced by complex structure, tectonic 

shuffling, and poor outcrop, it does appear that in a 

broad way the lower parts of the Mille Lacs Group are 

mainly quartzose sedimentary rocks, volcanic rocks, 

graphitic slate, and iron-formation, whereas the upper 

parts are mainly pelitic and semipelitic slate and phyllite 

with lenses of iron-formation and volcanic rock (Morey, 

1978, 1983). Collectively these rocks resemble parts of 

the Early Proterozoic Chocolay Group and overlying 

Menominee Group in Upper Michigan (Cannon, 1986, 

and references therein). Rocks of the Chocolay Group 

vary abruptly in thickness and stratigraphic composition 

over short distances, and have been interpreted (Larue, 

1981, 1983) as accumulations in an extensional 

environment, possibly within local rift basins and on the 

interbasin platforms that developed along a foundering 

continental margin. Morey (1978, 1983) has interpreted 

the general stratigraphic relationships within the 

presumably lower parts of the Mille Lacs Group in 

approximately the same way.

The geochemistry of the volcanic rocks in the Mille 

Lacs Group further supports an extensional setting. Both 

tholeiitic and calc-alkalic compositions are represented 

(fig. 5), but the tholeiitic compositions greatly 

predominate. Furthermore, the compositions of volcanic 

rocks largely avoid the "erogenic" field in the A12O3 - 

FeO-MgO discrimination diagram of Pearce and others 

(1977) and the "island-arc basalt" field in the Zr:Y versus 

Zr diagram of Pearce and Norry (1979) (figs. 6 and 7). 

These data suggest that island-arc volcanic assemblages 

probably were not significant constituents of the tectonic 

mass now present in the deeply eroded fold-and-thrust 

belt. Rather, the basalts have a "within-plate" or 

"continental" affinity consistent with a continentalFeO+Fe2O3+TiO2

High-Fe tholeiitic 

lit

KOMATIITE /

v \/ v v

A12 03 MgO

Figure 5. Jensen plot (Jensen, 1976) of volcanic rock 

compositions from various parts of fold-and-thrust belt. 

Includes metamorphosed lavas, hypabyssal intrusions, and 

fragmental volcaniclastic rocks ranging from 45 to 72 weight 

percent SiO2.

MgO

Figure 6. FeO-Mgp-AI2O3 plot of Pearce and others 

(1977) showing distribution of basaltic rock compositions 

from fold-and-thrust belt. FeO refers to total Fe reported as 

FeO. Data in weight percent. Note sparsfty of points in 

orogenic field.

margin genesis. Rare-earth element plots for Mille Lacs 

volcanic rocks (fig. 8) show moderate LREE enrichment 

and no Eu anomaly. Although this REE pattern is not 

diagnostic of tectonic setting, it is consistent with the 

observed patterns for continental tholeiites reported by 

Dupuy and Dostal (1984), and therefore is consistent 

with a rifted-margin genesis.

Beck (1988) obtained a SmrNd isochron age of 

2,197 ±39 Ma for a suite of six basalt and diabase 

samples from the Mille Lacs Group in the Moose LakeGlen Township structural terrane. This isochron age is 

very close to the model age of 2,199 Ma based on an

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C9
15

10

> 5

U

N

WITHIN-PLATE BASALT:

10 50 100 

Zr, IN PARTS PER MILLION

500 700

Figure 7. Zr:Y plotted against Zr, from Pearce and Norry 

(1979), showing distribution of basaltic rock compositions 

from fold-and-thrust belt. Note virtual absence of points from 

island-arc field. MORB, mid-ocean ridge basalt.

200

100

50

10

La Ce Nd Sm Eu Tb

RARE-EARTH ELEMENTS

Yb Lu

Figure 8. Chondrite-normalized rare-earth element plots 

for basaltic rocks of fold-and-thrust belt. Includes 

unpublished data provided by K.J. Schulz, U.S. 

Geological Survey.

initial e Nd value of 2.84±0.26 for the samples and the 

neodymium mantle evolution curve of Albarede and 

Brouxel (1987), and thus the isochron age is probably 

close to the crystallization age of the rocks. RbrSr 

investigations of the same samples yield an errorchron 

age of 1,746±86 Ma (Beck, 1988) that may be related to 

metamorphism and hydrothermal activity associated with 

the latest stages of regional compression and the crustal 

response to tectonic thickening, uplift, cooling, and 

unroofing (Dewey and others, 1970; Haxby and Turcotte, 

1976; Peterman and Sims, 1988). Taken together, the

Sm:Nd and Rb:Sr isotopic data suggest that some of the 

volcanic rocks now in the fold-and-thrust belt were 

extruded at about 2,200 Ma and compressionally reassembled into nappes prior to about 1,740 Ma.

The next question is when, and for how long, 

subduction occurred, and, relatedly, the age of the 

turbiditic rocks in the Animikie basin. Goldich and 

Fischer (1986) reported a U-Pb zircon age of 1,982±5 

Ma for a tectonized granite gneiss within the core 

complex of the McGrath Gneiss of the McGrath dome 

(fig. 2). Goldich and Fischer interpreted the granite 

gneiss as an Early Proterozoic pluton that was emplaced 

into the Archean McGrath Gneiss and later deformed, 

and they regarded the reported age as a crystallization 

age, even though metamorphic disturbance of the U-Pb 

systematics cannot be ruled out. In the same report, 

Goldich and Fischer also gave a U-Pb zircon age of 

1,869±5 Ma for a tonalite gneiss (Bradbury Creek 

Granodiorite of Morey, 1978) that is a syntectonic pluton 

emplaced into polygenetic migmatite (Hillman Migmatite of Morey, 1978) in the west-central part of the 

McGrath-Little Falls structural terrane (fig. 2). The 

reported age is interpreted as a crystallization age (Goldich and Fischer, 1986). It corresponds well with a Rb:Sr 

metamorphic age of 1,850 Ma from strata in the Cuyuna 

North range terrane, and with the crystallization age of 

1,850±30 Ma from several syntectonic Penokean plutons 

in Wisconsin (Van Schmus, 1980). Other syntectonic 

intrusions of tonalite, granodiorite, and granite 

structurally akin to the Bradbury Creek Granodiorite 

were emplaced into the Hillman during the Penokean 

(Morey, 1978), but these have not been studied isotopically.

The post-orogenic Rockville and Reformatory 

Granites of Morey (1978) that occur mainly within the 

McGrath-Little Falls structural terrane and poorly 

mapped terrane farther southwest (fig. 3) yield U-Pb 

crystallization ages of 1,812 ±9 Ma (S.S. Goldich, cited in 

Horan and others, 1987). Other post-orogenic plutons, 

such as the St. Cloud Granite, yield a minimum U-Pb age 

of 1,770 Ma (Spencer and Hanson, 1984). Although most 

of the St. Cloud Granite plutons occur in the southern, 

tectonically deeper parts of the orogen, a few minor 

stocks and plugs petrographically identical to those in the 

southern area do occur in more external zones, where 

they appear to pierce and "pin" the imbricate stack. We 

conclude, therefore, that tectonic stacking had ceased 

prior to about 1,770 Ma, and it may have stopped long 

before that.

Rb:Sr studies yield an errorchron age of 1,741 ±18 

Ma for one of the 1,770-Ma St. Cloud Granite plutons 

(S.S. Goldich, cited in Horan and others, 1987). This age 

accords approximately with Rb:Sr ages in the interval of 

1,750 to 1,740 Ma for rocks in zones of brittle to 

semi-brittle cataclasis within the McGrath Gneiss

C10 Precambrian Geology of Lake Superior Region
(Keighin and others, 1972), an age of 1,730 Ma for 

metamorphic recrystallization in the southern part of the 

Animikie basin (Peterman, 1966, as recalculated by 

Keighin and others, 1972), and one of 1,746 Ma for 

hydrothermal reequilibration in volcanic rocks of the 

Mille Lacs Group (Beck, 1988). As previously stated, we 

consider the Rb:Sr events clustered loosely around 1,740 

Ma to be caused by uplift-related phenomena.

To summarize, the available isotopic data suggest 

that rocks in the southern part of the fold-and-thrust belt 

were undergoing magmatism, and possibly deformation, 

as early as 1,982 Ma. Deformation was under way at 

1,869 Ma, when the syntectonic Bradbury Creek Granodiorite was emplaced, and had ceased by 1,770 Ma, when 

the post-tectonic St. Cloud Granite was emplaced into 

the fold-and-thrust mass. Uplift, erosion, and cooling 

were under way by about 1,740 Ma.

If the foregoing interpretation of geochronologic 

data is correct, subduction-driven thrusting and foredeep 

development could have started any time after the 

eruption of continental-margin volcanic rocks at about 

2,200 Ma, and they probably were well under way by 

1,982 Ma, when the earliest granitoid plutonism was 

occurring. On the other hand, we cannot rule out a later 

onset of foredeep development (say, 1,870 Ma). Foredeep sedimentation may well have continued for an 

extended period of time as basins formed, were partly 

consumed, and were succeeded by other basins in 

response to attempted subduction of the cratonic foreland. Turbidite wedges of several different ages may very 

well constitute the fill of the Animikie basin, and 

turbiditic rocks presently exposed along the south 

margin of the basin are in no way required to be 

time-correlative with those along the north margin. 

On the northwest side of the basin, northwest of the 

Mesabi iron range (fig. 2), basal units of the Animikie 

Group unconformably truncate a swarm of diabase dikes 

in the foreland that are dated at 2,125 ±45 Ma by a 

whole-rock Rb:Sr isochron (Southwick and Day, 1983; 

Southwick and Halls, 1987; Beck, 1988). This dike age is 

a maximum limit for the onset of sedimentation on the 

north rim of the basin, but sheds no light on the actual 

time (or times) during which sedimentation occurred 

thereafter.

The unconformable southern contact of the Animikie basin against previously folded rocks of the Cuyuna 

district precludes correlation of the Biwabik Ironformation in the Animikie basin with the Trommald 

Formation in the Cuyuna North range terrane. 

Furthermore, that the Trommald and other ironformation units in the Cuyuna North range terrane 

correlate with the iron-formation units in the Cuyuna 

South range terrane is by no means certain. Besides 

being separated from each other by the Serpent Lake 

structural discontinuity, the iron-formations of the two

terranes differ substantially in facies, geochemistry, and 

stratigraphic associations. The iron-formation of the 

South range terrane is mainly of sulfide, carbonate, and 

silicate facies and was deposited as lenticular masses and 

thin layers in close stratigraphic proximity to mafic 

volcanic rocks and euxinic black shale (Morey and 

Morey, 1986; Southwick and others, 1988). The ironformations of the Cuyuna North range terrane have more 

blanketlike morphology, are interbedded with darkcolored argillite and siltite, and are predominantly of 

carbonate and oxide facies. They also are enriched 

geochemically in manganese and phosphorus as 

compared to the iron-formations in either the Cuyuna 

South range terrane or the Animikie basin.

Therefore it appears that iron-formations were 

deposited in three distinct geologic environments during 

the evolution of the Penokean orogen. Iron 

sedimentation occurred in close association with mafic 

volcanism under euxinic depositional conditions, 

probably at an early stage in the tectonic evolution of the 

orogen, and the rocks of this episode now reside in the 

folded and thrust-faulted domain of the two central 

structural terranes. Iron-formations of this type 

constitute unnamed units within the Mille Lacs Group of 

Morey (1978). The Trommald and associated ironformation units of the Cuyuna North range terrane were 

deposited in close association with carbonaceous, argillaceous, and locally calcareous muds in a small basin that 

was ancestral to the North range synclinorium. The 

well-established and distinctive stratigraphic sequence on 

the North range, defined informally as the "North range 

group" by Southwick and others (1988), is called the 

North range sequence in subsequent discussion. Finally, 

a third episode of iron sedimentation occurred on the 

cratonic rim of the main bowl of the Animikie basin at a 

later time in the history of the orogen. The ironformations of this generation, units traditionally assigned 

to the Animikie Group, were deposited in association 

with tidalites on a well-oxygenated to moderately 

oxygenated shallow marine shelf (Ojakangas, 1983). 

Units of this type include the Biwabik Iron-formation of 

the Mesabi range and the thin units of equivalent 

stratigraphic position in the Emily district in the 

southwestern part of the basin. Figure 9 illustrates the 

inferred stratigraphic relationships among Early Proterozoic units of group rank in east-central Minnesota.

REGIONAL CONSIDERATIONS

The inference of subduction-driven horizontal tectonism in the Minnesota segment of the Penokean 

orogen fits broadly with recent plate-tectonic 

interpretations in Michigan and Wisconsin (Sims and 

Peterman, 1983; Larue, 1983; Morey, 1983; LaBerge and 

Myers, 1984; Larue and Ueng, 1985; Sims, Peterman,

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C11
NW -<- -PENOKEAN OROGEN

Iro 

anc

rune

Orogenic 

turbidite^

Biwabik 

n-formation 

equivalent-^

-

'S^/'///

ANIMIKIE GROUP

Unconformity on Archean 

rocks of cratonic foreland 

and Proterozoic rocks of 

fold-and-thrust belt

EXTERNAL ZONE

(Cuyuna North range terrane)

Trommald

Formation 

and

equivalent^

^^-^

^222==-

LU

2o

< 2

QC LU

^Bcc LU o w z.

unnamed

iron- . 

formation^

 ^- unconformity - -

Unconformity on Archean 

rocks of cratonic foreland

- FOLD-AND-THRUST BELT -

MEDIAL ZONE 

(Cuyuna South range and 

Moose Lake Glen 

Township terranes)

INTERNAL ZONE 

(McGrath Little Falls terrane)

unnamed 

graphite schist

rtectonic contact- i

Basement rocks unknown

-^- - unconformityUnconformity on Archean 

rocks of McGrath dome

Figure 9. Inferred stratigraphic relationships among rock units in Minnesota segment of the Penokean orogen. Width of 

orogen is approximately 100 km. Modified from Southwick and others (1988).

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

1987; Schulz, 1987). Although the Minnesota 

interpretation put forward here does differ in several 

respects from the described geology to the east, and 

raises questions as to how the two segments of the orogen 

might correlate in detail across the Midcontinent rift, the 

general tectonic framework of both areas is consistent 

. 1).

Possible Equivalence of Niagara Fault 

Zone and Malmo Discontinuity

In Michigan and Wisconsin, the Niagara fault zone 

is a tectonic suture between rocks of cratonic affinity on 

the north and rocks of island-arc affinity on the south 

(Lame, 1983; Larue and Ueng, 1985; Sims, Peterman, 

and Schulz, 1985; Sedlock and Larue, 1985). The 

predominantly igneous suite of rocks south of the

Niagara fault zone, which includes both volcanic and 

intrusive constituents, yields crystallization ages 

clustered near 1,870 Ma (Banks and Rebello, 1969, 

recalculated; Van Schmus, 1976,1980; Van Schmus and 

Bickford, 1981; Afifi and others, 1984; Beck, 1988; Sims 

and others, 1989). In Minnesota, neither a suture zone 

analogous to the Niagara fault zone nor a suite of 

arc-related 1,870-Ma volcanic rocks has yet been 

identified positively. However, the structural terrane 

south of the Malmo discontinuity contains a number of 

calc-alkaline intrusive rocks that are comparable in geochemistry and age to arc-related plutons in the magmatic 

terranes south of the Niagara fault zone in Wisconsin 

(Horan and others, 1987), and therefore the Malmo and 

Niagara zones may correlate across the Midcontinent 

rift. This correlation is further suggested by the magnetotelluric anomalies associated with the Malmo zone in 

Minnesota (Wunderman and Young, 1987) and the 

Niagara zone in northwestern Wisconsin (Flambeau

C12 Precambrian Geology of Lake Superior Region
anomaly of Sternberg and Clay, 1977). Upon removal of 

about 60 km of extension on the intervening Midcontinent rift (Chandler, 1983d), the magnetotelluric features 

become essentially colinear.

The Malmo discontinuity becomes difficult to trace 

with assurance in south-central Minnesota, but it most 

probably swings from east to northeast to an 

approximately north-south orientation (fig. 1). This is 

suggested by the oroclinal bend of structural trends from 

west to southwest in the fold-and-thrust belt and the 

continuing southward deflection of geophysical trends 

into poorly mapped terrane south of the area we have 

studied. Work is currently under way in that region.

Correlation of Stratigraphic Sequences 

North of Niagara-Malmo ZonePossibilities and Problems

The volcanic rocks of cratonic affinity north of the 

Niagara fault zone in Michigan and Wisconsin (Ueng and 

others, 1988; Beck, 1988) are associated with 

sedimentary rocks that have attributes suggesting 

continental-margin and early-rift depositional 

environments (Larue, 1981; Sedlock and Larue, 1985). 

In those respects they resemble volcanic rocks in the 

fold-and-thrust belt of the Penokean orogen in 

Minnesota, but age data do not agree between the two 

areas. Isotopic studies of rocks from the Mille Lacs 

Group in the Moose Lake-Glen Township terrane of 

Minnesota yield ages some 300 m.y. older than the 

1,910±10 Ma age for the geochemically similar metabasalts of the Hemlock Formation in the Menominee 

Group of Upper Michigan (Van Schmus, 1976; Van 

Schmus and Bickford, 1981; Beck, 1988). In terms of 

classic stratigraphy, the interval from 2,197 ±39 Ma to 

1,910±10 Ma may represent the span of deposition for 

strata in the Lake Superior region that are older than the 

Animikie Group in Minnesota.

Although the age disparity and a number of other 

stratigraphic uncertainties must be resolved before 

regional correlations can be entertained, the broad 

similarities between the Iron River-Crystal Falls-Marquette area of Michigan (Cannon, 1986) and the foldand-thrust terrane in east-central Minnesota suggest a 

tectonic linkage. Both are terranes of tectonically 

imbricated "continental" rocks that abut large turbidite 

basins on the north. The turbidite basins in both areas are 

framed by, and lie unconformably upon, Archean 

cratonic basement along their north margins. The 

stratigraphy, sedimentology, and structural styles of the 

Animikie Group in the Animikie basin of Minnesota and 

the Baraga Group in the Baraga basin of Michigan are 

strikingly similar on the regional scale (Morey, 1983, and

references therein) and are compatible in both cases with 

a foredeep depositional setting for the rocks (Hoffman, 

1987).

Because the Penokean foredeeps would have 

developed and filled under dynamic conditions, the 

turbiditic sedimentary rocks they contain should provide 

a record of changing slope conditions, current regimes, 

and provenance as a function of time, especially near 

their south margins. Such a record of depositional 

complexity has been deduced on isotopic grounds for the 

south margin of the Baraga basin (Barovich and others, 

1989). It follows that cryptic unconformities between 

physically similar turbidite wedges of differing age should 

be expected in the Penokean foredeeps (see Robertson, 

1987, for an illustrative Phanerozoic example), but none 

has yet been demonstrated. Although cryptic unconformities will be difficult to demonstrate, owing to the 

lack of marker beds and critical outcrops, the possibility 

of their existence should guide future stratigraphic field 

work in the Animikie and Baraga basins and be factored 

into tectonostratigraphic reconstructions.

In a previous section, we pointed out that Early 

Proterozoic iron-formations in Minnesota occur in 

stratigraphic association with volcanic rocks and black 

shales in the Mille Lacs Group, with laminated siltite and 

pelite in the North range sequence, and with tidally 

deposited arenite and siltite in the basal part of the 

Animikie Group. These lithostratigraphic associations 

are broadly similar to those in the Iron River-Crystal 

Falls area of Michigan where the main iron-formations 

are associated with volcanic rocks of the Menominee 

Group, with various fine-grained argillaceous rocks in 

the Paint River Group (Cannon, 1986, and references 

therein), and with tidalites in the lower part of the Baraga 

Group (R.W. Ojakangas, oral commun., 1989). 

However, the currently defined stratigraphic order in 

Michigan places the Paint River Group as the youngest 

of these, whereas the lithologically analogous North 

range sequence is not the youngest group-rank unit in the 

inferred Minnesota stratigraphic section. This disparity 

may mean that the inferred stratigraphic order is wrong 

in either Michigan (Cambray, 1987) or Minnesota, or 

that the association of iron-formations with argillaceous 

strata was repeated within the stratigraphic section.

Summary

Penokean convergence appears to have been 

driven by south- or southeast-directed subduction of 

Archean craton beneath tectonically imbricated terranes 

built of continental-margin, island-arc, and perhaps some 

back-arc rocks. The imbricate mass in Minnesota 

contained mainly continental-margin rocks along with 

possible back-arc material (Southwick and others, 1988),

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C13
whereas in Michigan and Wisconsin it consisted of both 

continental-margin and island-arc rocks. The imbrication 

process undoubtedly was complex in space and time, and 

led to variations along strike in the composition and 

subsequent history of the accreted mass. No matter what 

the exact character of the load was, however, the tectonic 

imbrication and crustal loading produced migrating foredeeps on downbowed continental crust to the north and 

northwest of the main tectonic zone.

In Minnesota, we infer that deposition of the 

pre-collisional Mille Lacs Group occurred in the interval 

2,197-1,910 Ma. Deformation, metamorphism, and plutonism were under way as early as about 1,982 Ma and 

culminated about 1,870 Ma, when voluminous syntectonic plutons were emplaced. Crustal downflexure 

and foredeep development may have begun prior to the 

onset of extensive plutonism and persevered for a long 

period of time. Early foredeeps (such as the ancestor to 

the Cuyuna North range synclinorium) eventually were 

overridden and incorporated in the imbricate thrust 

stack. Later foredeeps, including the Animikie basin, 

followed in turn as subduction continued and then 

waned. A second regional episode of deformation and 

metamorphism accompanied the emplacement of latetectonic to post-tectonic intrusions in the interval 

1,820(?) to 1,770 Ma; structures primarily of this 

generation developed in rocks of the Animikie Group 

and are overprinted on earlier structures in older rocks. 

Iso static uplift is inferred to have culminated by about 

1,740 Ma.

REFERENCES CITED

Afifi, A., Doe, B.R., Sims, P.K., and Delevaux, M.H., 1984, 

U-Th-Pb isotope chronology of sulfide ores and rocks in 

the Early Proterozoic metavolcanic belt of northern 

Wisconsin: Economic Geology, v. 79, p. 338-353.

Albarede, F., and Brouxel, M, 1987, The Sm/Nd secular 

evolution of the continental crust and the depleted 

mantle: Earth and Planetary Science Letters, v. 82, 

p. 25-35.

Andrews, A.J., Masliwec, A., Morris, W.A., Owsiacki, L., and 

York, D., 1986, The silver deposits of Cobalt and Gowganda, Ontario, II An experiment in age 

determinations employing radiometric and paleomagnetic measurements: Canadian Journal of Earth 

Sciences, v. 23, p. 1507-1518.

Banks, P.O., and Rebello, D.P., 1969, Zircon ages of a Precambrian rhyolite, northeastern Wisconsin: Geological 

Society of America Bulletin, v. 80, p. 907-910.

Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sims, P.K., 

1989, Nd isotopes and the origin of 1.9-1.7 Ga Penokean 

continental crust of the Lake Superior region: Geological 

Society of America Bulletin, v. 101, p. 333-338.

Beaumont, C., 1981, Foreland basins: Geophysical Journal, v. 

65, p. 291-329.

Beck, J.W., 1988, Implications for Early Proterozoic tectonics 

and the origin of continental flood basalts, based on 

combined trace element and neodymium/strontium 

isotopic studies of mafic igneous rocks of the Penokean 

Lake Superior belt, Minnesota, Wisconsin and Michigan: 

Minneapolis, Minn., University of Minnesota Ph. D. 

dissertation, 262 p.

Cambray, F.W., 1987, An alternate stratigraphic correlation 

within the Marquette Range Supergroup, N. Michigan: 

Geological Society of America Abstracts with Programs, 

v. 19, p. 192.

Cannon, W.F., 1986, Bedrock geologic map of the Iron River 

I°x2° quadrangle, Michigan and Wisconsin: U.S. 

Geological Survey Miscellaneous Investigations Map 

I-1360-B, scale 1:250,000.

Card, K.D., and Ciesielski, A., 1986, DNAG #1, Subdivisions 

of the Superior Province of the Canadian Shield: Geoscience Canada, v. 13, p. 5-13.

Carlson, K.E., 1985, A combined analysis of gravity and 

magnetic anomalies in east-central Minnesota: 

Minneapolis, Minn., University of Minnesota M.S. thesis, 

138 p.

Chandler, V.W., 1983a, Aeromagnetic map of Minnesota, St. 

Louis County: Minnesota Geological Survey Aeromagnetic Map Series A-2, scale 1:250,000, 2 pis.

____ 1983b, Aeromagnetic map of Minnesota, Carlton and 

Pine Counties: Minnesota Geological Survey Aeromagnetic Map Series A-3, scale 1:250,000, 2 pis.

____ 1983c, Aeromagnetic map of Minnesota, east-central 

region: Minnesota Geological Survey Aeromagnetic Map 

Series A-4, scale 1:250,000, 2 pis. 

1983d, Correlation of magnetic anomalies in eastcentral Minnesota and northwestern Wisconsin Constraints on magnitude and direction of Keweenawan 

rifting: Geology, v. 11, p. 174-176.

____ 1985, Aeromagnetic map of Minnesota, central region: 

Minnesota Geological Survey Aeromagnetic Map Series 

A-5, scale 1:250,000, 2 pis.

Chandler, V.W., Norstrand, E., and Anderson, S., 1984, 

Shaded-relief aeromagnetic anomaly map of 

northeastern and east-central Minnesota: Minnesota 

Geological Survey Miscellaneous Map M-53, scale 

1:1,000,000.

Dewey, J.F., McKerrow, W.S., and Moorbath, S., 1970, The 

relationship between isotopic ages, uplift, and 

sedimentation during Ordovician times in western 

Ireland: Scottish Journal of Geology, v. 6, p. 133-145.

Dupuy, C., and Dostal, J., 1984, Trace element geochemistry of 

some continental tholeiites: Earth and Planetary Science 

Letters, v. 67, p. 61-69.

Ferderer, R.J., 1988a, Application of Werner deconvolution to 

the Penokean fold-and-thrust belt, east-central 

Minnesota: Institute on Lake Superior Geology, 34th 

Annual Meeting, Marquette, Michigan, Proceedings and 

Abstracts, pt. 1, p. 30-32.

___ 1988b, Werner deconvolution and its application to the 

Penokean orogen, east-central Minnesota: Minneapolis, 

Minn., University of Minnesota Ph. D. dissertation, 

284 p.

C14 Precambrian Geology of Lake Superior Region
Goldich, S.S., and Fischer, L.B., 1986, Air-abrasion 

experiments in U-Pb dating of zircon: Chemical Geology, 

v. 58, p. 195-215.

Grout, F.F., and Wolff, J.F., Sr., 1955, The geology of the 

Cuyuna district, Minnesota, a progress report: Minnesota 

Geological Survey Bulletin 36, 144 p.

Haxby, W.F., and Turcotte, D.L., 1976, Stresses induced by the 

addition or removal of overburden and the associated 

thermal effects: Geology, v. 4, p. 181-184.

Hoffman, P.F., 1987, Early Proterozoic foredeeps, foredeep 

magmatism, and Superior-type iron-formations of the 

Canadian shield, in KrOner, A., ed., Proterozoic lithospheric evolution: American Geophysical Union Geodynamics Series, v. 17, p. 85-98.

____ 1988, United plates of America, the birth of a craton 

Early Proterozoic assembly and growth of Laurentia: 

Annual Review of Earth and Planetary Sciences, v. 16, 

p. 543-603.

Holm, D.K., Hoist, T.B., and Ellis, M., 1988, Oblique subduction, footwall deformation, and imbrication A model 

for the Penokean orogeny in east-central Minnesota: 

Geological Society of America Bulletin, v. 100, 

p. 1811-1818.

Hoist, T.B., 1982, Evidence for multiple deformation during the 

Penokean orogeny in the Middle Precambrian Thomson 

Formation, Minnesota: Canadian Journal of Earth 

Sciences, v. 19, p. 2043-2047.

____ 1984, Evidence for nappe development during the early 

Proterozoic Penokean orogeny, Minnesota: Geology, v. 

12, p. 135-138.

____ 1985, Implications of a large flattening strain for the 

origin of a bedding-parallel foliation in the Early 

Proterozoic Thomson Formation, Minnesota: Journal of 

Structural Geology, v. 7, p. 375-383.

Horan, M.F., Hanson, G.N., and Spencer, K.J., 1987, Pb and 

Nd isotope and trace element constraints on the origin of 

basic rocks in an early Proterozoic igneous complex, 

Minnesota: Precambrian Research, v. 37, p. 323-342.

Howell, D.G., Jones, D.L., and Schermer, E.R., 1985, Tectonostratigraphic terranes of the circum-Pacific region, in 

Howell, D.G., ed., Tectonostratigraphic terranes of the 

circum-Pacific region: Circum-Pacific Council for Energy 

and Mineral Resources Earth Science Series, no. 1, 

p. 3-30.

Jensen, L.S., 1976, A new cation plot for classifying subalkalic 

volcanic rocks: Ontario Geological Survey Miscellaneous 

Paper 66, 22 p.

Jordan, T.E., 1981, Thrust loads and foreland basin evolution, 

Cretaceous, western United States: American 

Association of Petroleum Geologists Bulletin, v. 65, 

p. 2506-2520.

Keighin, C.W., Morey, G.B., and Goldich, S.S., 1972, Eastcentral Minnesota, in Sims, P.K., and Morey, G.B., eds., 

Geology of Minnesota A centennial volume: 

Minnesota Geological Survey, p. 240-255.

LaBerge, G.L., and Myers, P.E., 1984, Two early Proterozoic 

successions in central Wisconsin and their tectonic 

significance: Geological Society of America Bulletin, v. 

95, p. 246-253.

Larue, D.K., 1981, The Chocolay Group, Lake Superior region, 

U.S.A. Sedimen to logic evidence for deposition in 

basinal and platform settings on an early Proterozoic 

craton: Geological Society of America Bulletin, pt I, v. 

92, p. 417-435.

____ 1983, Early Proterozoic tectonics of the Lake Superior 

region Tectonostratigraphic terranes near the 

purported collision zone, in Medaris, L.G., Jr., ed., Early 

Proterozoic geology of the Great Lakes region: 

Geological Society of America Memoir 160, p. 33-47.

Larue, D.K., and Ueng, W.L., 1985, Florence-Niagara terrane An early Proterozoic accretionary complex, Lake 

Superior region, U.S.A.: Geological Society of America 

Bulletin, v. 96, p. 1179-1187.

Lucente, M.E., and Morey, G.B., 1983, Stratigraphy and 

sedimentology of the lower Proterozoic Virginia 

Formation, northern Minnesota: Minnesota Geological 

Survey Report of Investigations 28, 28 p.

Marsden, R.W., 1972, Cuyuna district, in Sims, P.K., and 

Morey, G.B., eds., Geology of Minnesota A centennial 

volume: Minnesota Geological Survey, p. 227-239.

McSwiggen, P.L., 1987, Geology and geophysics of the 

Denham-Mahtowa area, east-central Minnesota: 

Minnesota Geological Survey Miscellaneous Map Series 

M-63, scale 1:48,000.

Morey, G.B., 1978, Lower and Middle Precambrian 

stratigraphic nomenclature for east-central Minnesota: 

Minnesota Geological Survey Report of Investigations 

21, 52 p.

____ 1983, Lower Proterozoic stratified rocks and the 

Penokean orogeny in east-central Minnesota, in Medaris, 

L.G., Jr., ed., Early Proterozoic geology of the Lake 

Superior region: Geological Society of America Memoir 

160, p. 97-122.

Morey, G.B., and Morey, D.D., 1986, Distribution of ironformations in the main Cuyuna range, east-central 

Minnesota: Minnesota Geological Survey Miscellaneous 

Map Series M-60, scale 1:48,000.

Morey, G.B., and Ojakangas, R.W., 1970, Sedimentology of the 

Middle Precambrian Thomson Formation, east-central 

Minnesota: Minnesota Geological Survey Report of 

Investigations 13, 32 p.

Morey, G.B., Olsen, B.M., and Southwick, D.L., 1981, Geologic 

map of Minnesota, east-central Minnesota, bedrock 

geology: Minnesota Geological Survey, scale 1:250,000.

Ojakangas, R.W., 1983, Tidal deposits in the early Proterozoic 

basin of the Lake Superior region The Palms and the 

Pokegama Formations Evidence for subtidal-shelf 

deposition of Superior-type banded iron-formation, in 

Medaris, L.G., Jr., ed., Early Proterozoic geology of the 

Lake Superior region: Geological Society of America 

Memoir 160, p. 49-66.

Pearce, J.A., and Norry, M.J., 1979, Petrogenetic implications 

of Ti, Zr, Y, and Nb variations in volcanic rocks: 

Contributions to Mineralogy and Petrology, v. 69, 

p. 33-47.

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C15
Pearce, T.H., German, B.E., and Birkett, T.C., 1977, The 

relationship between major element chemistry and 

tectonic environment of basic and intermediate volcanic 

rocks: Earth and Planetary Science Letters, v. 36, 

p. 121-132.

Peterman, Z.E., 1966, Rb-Sr dating of middle Precambrian 

metasedimentary rocks of Minnesota: Geological Society 

of America Bulletin, v. 77, p. 1031-1044.

Peterman, Z.E., and Sims, P.K., 1988, The Goodman swell A 

lithospheric flexure caused by crustal loading along the 

Midcontinent rift system: Tectonics, v. 7, p. 1077-1090.

Price, R.A., 1973, Large-scale gravitational flow of supracrustal 

rocks, southern Canadian Rockies, in DeJong, K.A., and 

Scholten, R., eds., Gravity and tectonics: New York, John 

Wiley, p. 491-502.

Quinlan, G.M., and Beaumont, C, 1984, Appalachian 

thrusting, lithospheric flexure, and Paleozoic stratigraphy 

of the eastern interior of North America: Canadian 

Journal of Earth Sciences, v. 21, p. 973-996.

Robertson, A., 1987, The transition from a passive margin to an 

Upper Cretaceous foreland basin related to ophiolite 

emplacement in the Oman Mountains: Geological 

Society of America Bulletin, v. 99, p. 633-653.

Schmidt, R.G., 1963, Geology and ore deposits of Cuyuna 

North range, Minnesota: U.S. Geological Survey 

Professional Paper 407, 96 p.

Schulz, K.J., 1987, Early Proterozoic evolution of a rifted 

continental margin in the Lake Superior region: 

Geological Society of America Abstracts with Programs, 

v. 19, p. 243.

Schwartz, G.M., 1942, Correlation and metamorphism of the 

Thomson Formation, Minnesota: Geological Society of 

America Bulletin, v. 53, p. 1001-1020.

Sedlock, R.L., and Larue, D.K., 1985, Fold axes oblique to the 

regional plunge and Proterozoic terrane accretion in the 

southern Lake Superior region: Precambrian Research, 

v. 30, p. 249-262.

Sims, P.K., and Peterman, Z.E., 1983, Evolution of the 

Penokean foldbelt, Lake Superior region, and its tectonic 

environment, in Medaris, L.G., Jr., ed., Early Proterozoic 

geology of the Lake Superior region: Geological Society 

of America Memoir 160, p. 3-14.

____ 1986, Early Proterozoic Central Plains orogen A 

major buried structure in the north-central United 

States: Geology, v. 14, p. 488-491.

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., 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., 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., 1987, Bundled slaty cleavage in laminated 

argillite, north-central Minnesota: Journal of Structural 

Geology, v. 9, p. 985-993.

Southwick, D.L., and Day, W.C., 1983, Geology and petrology 

of Proterozoic mafic dikes, north-central Minnesota and 

western Ontario: Canadian Journal of Earth Sciences, v. 

20, p. 622-638.

Southwick, D.L., and Halls, H.C., 1987, Compositional 

characteristics of the Kenora-Kabetogama dike swarm 

(Early Proterozoic), Minnesota and Ontario: Canadian 

Journal of Earth Sciences, v. 24, p. 2197-2205.

Southwick, D.L., Meyer, G.N., and Mills, S.J., 1986, Scientific 

core drilling in central Minnesota Summary of lithologic and geochemical results: Minnesota Geological 

Survey Information Circular 23, 186 p.

Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, 

Geologic map (scale 1:250,000) of the Penokean orogen, 

central and eastern Minnesota, and accompanying text: 

Minnesota Geological Survey Report of Investigations 

37, 25 p.

Spencer, K.J., and Hanson, G.N., 1984, Pb, Nd isotope and 

REE constraints on east-central Minnesota Proterozoic 

granites Evidence for newly derived crustal 

components [abs.]: Eos, v. 65, p. 292.

Sternberg, B.K., and Clay, C.S., 1977, Flambeau anomaly A 

high-conductivity anomaly in the southern extension of 

the Canadian Shield: American Geophysical Union 

Monograph 20, p. 501-530.

Ueng, W.C., Fox, T.P., Larue, O.K., and Wilband, J.T., 1988, 

Geochemistry and petrogenesis of the early Proterozoic 

Hemlock volcanic rocks and the Kiernan sills, southern 

Lake Superior region: Canadian Journal of Earth 

Sciences, v. 25, p. 528-546.

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

of the Great Lakes area, North America: Royal Society 

of London Philosophical Transactions, v. 280, Series A, 

p. 605-628.

____ 1980, Chronology of igneous rocks associated with the 

Penokean orogeny in Wisconsin, in Morey, G.B., and 

Hanson, G.N., eds., Selected studies of Archean gneisses 

and lower Proterozoic rocks, southern Canadian Shield: 

Geological Society of America Special Paper 182, 

p. 159-168.

Van Schmus, W.R., and Bickford, M.E., 1981, Proterozoic 

chronology and evolution of the Midcontinent region, 

North America, in Kroner, A., ed., Precambrian plate 

tectonics: New York, Elsevier, p. 261-296.

Winchell, N.H., 1899, The geology of the Carlton plate, in 

Winchell, N.H., and others, Geology of Minnesota; 

volume 4 of the final report, 1896-1898: Minnesota 

Geological and Natural History Survey, p. 550-565.

Wunderman, R.L., 1988, Crustal structure across the exposed 

axis of the Midcontinent Rift and adjacent flanks, based 

on magneto telluric data, central MinnesotaWisconsin A case for crustal inhomogeneity and 

possible reactivation tectonics: Houghton, Mich., 

Michigan Technological University Ph. D. dissertation, 

209 p.

C16 Precambrian Geology of Lake Superior Region
Wunderman, R.L., and Young, C.T., 1987, Evidence for 

widespread basement decollement structures and related 

crustal asymmetry associated with the western limb of the 

Midcontinent Rift: Institute on Lake Superior Geology, 

33rd Annual Meeting, Wawa, Ontario, Proceedings and 

Abstracts, v. 33, pt. 1, p. 85-86.

Young, G.M., 1983, Tectono-sedimentary history of Early 

Proterozoic rocks of the northern Great Lakes region, in

Medaris, L.G., Jr., ed., Early Proterozoic geology of the

Great Lakes region: Geological Society of America

Memoir 160, p. 15-32. 

Zapffe, C, 1925, Stratigraphy and correlation of the Cuyuna

iron ore district, Minnesota: Lake Superior Mining

Institute Proceedings, v. 25, p. 219-227. 

___1933, The Cuyuna iron-ore district: International

Geological Congress, 16th, United States, Guidebook 27,

Excursion C-4, p. 72-78.

Tectonic Imbrication and Foredeep Development, Penokean Orogen, Minnesota C17

Chapter D

The Penokean Orogeny in Minnesota and 

Upper Michigan A Comparison of 

Structural Geology

By TIMOTHY B. HOLST

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and LM.H. CARTER, Editors
CONTENTS

Abstract Dl 

Introduction Dl 

Acknowledgments Dl 

General tectonic setting D3 

Comparison of structural geology D3

Descriptive structural geology in Minnesota terranes D3

Strain analysis in Minnesota terranes D5

Comparative structural geology in Minnesota and Michigan D5 

Discussion D7 

References cited D8

FIGURES

1. Map showing Minnesota and Wisconsin-Upper Michigan segments of Penokean 

orogen D2

2. Logarithmic deformation plot showing results of three-dimensional strain

determinations in the Penokean orogen in Minnesota and Upper Michigan D5

3. Sketches comparing mesoscopic structures in the northern structural terranes in 

Minnesota and Upper Michigan D6

4. Photograph showing F2 folds in hand sample from outcrop of Michigamme 

Formation, along Sturgeon River, Michigan D7

5. Sketches of small-scale refolded F! folds and associated foliations from outcrops 

in the southern structural terrane in Minnesota and Upper Michigan D7

6. Sketch cross sections from Keweenaw Bay and Falls River exposures near L'Anse 

in Upper Michigan D8
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

The Penokean Orogeny in Minnesota and 

Upper Michigan A Comparison of 

Structural Geology

By Timothy B. Hoist1

Abstract

Recent geophysical studies and shallow drilling work in 

Minnesota have led to a reinterpretation of the Penokean 

orogeny. In both the Minnesota and Wisconsin-Michigan 

segments, north of a postulated suture zone, the orogen is 

interpreted to consist of an internal fold-and-thrust belt of 

tectonicatly imbricated rocks and an external (northerly) 

migrating foredeep. Mesoscopic and microscopic structures 

from rocks exposed along two transects, one in Minnesota and 

one in Upper Michigan, across the proposed fold-and-thrust 

belt/foredeep boundary have been examined, revealing a 

remarkable and detailed similarity between the two areas. In 

each region, a marked difference in geologic structure 

delineates the proposed boundary.

INTRODUCTION

The Precambrian craton of North America consists of 

a series of segments of Archean crust, which represent the 

remnants of microcontinents, that were assembled during 

Proterozoic time (Hoffman, 1988). Early Proterozoic collisional orogenies sutured the Archean microcontinents 

together and served to accrete juvenile Proterozoic crust to 

the margins of the growing protocraton (Hoffman, 1989). 

One of these orogenies, the Penokean orogeny, which 

occurred near the close of Early Proterozoic time (1,875- 

1,825 Ma, Van Schmus, 1976, 1980, 1981), deformed and 

metamorphosed rocks in Minnesota, Wisconsin, Upper 

Michigan, and the Superior, Southern, and Grenville provinces in Canada (for example, Hoist, 1982; Maass and

Manuscript approved for publication August 30, 1990. 

'University of Minnesota Duluth, Duluth, MN 55812.

others, 1980; Cannon, 1973; Brocoum and Dalziel, 1974). 

The belt of deformed rock that constitutes the Penokean 

orogen is truncated on the east by the Grenville orogen and 

on the west by the Central Plains orogen (Sims and 

Peterman, 1986); it is comparable in length and width to 

such Phanerozoic belts as the Appalachian orogen (Southwick and others, 1988). In the Lake Superior region, the 

Penokean orogen is cut by younger rocks (=1,100 Ma) of 

the Midcontinent rift system, a thick series of extrusive and 

intrusive igneous rocks and associated sedimentary rocks 

(fig. 1). The rocks of this rift system cut completely across 

the Penokean orogen and isolate the segment in Minnesota 

from the segment in Wisconsin and Upper Michigan 

(fig. 1). The geology of these two segments is similar in 

rough outline. Stratigraphic correlations of Early Proterozoic supracrustal rocks from the two segments are fairly 

well established (for example, Morey, 1973, 1978, 1983; 

Young, 1983; Ojakangas, 1983; Larue, 1981, 1983), and 

tectonic syntheses of the Penokean involving both Minnesota and Wisconsin-Michigan segments have appeared (for 

example, Van Schmus, 1976; Sims, 1976; Sims and others, 

1980; Sims and Peterman, 1983). The purpose of this report 

is to point out some of the striking and detailed similarities 

in mesoscopic and microscopic structural geology that exist 

in a part of the exposed Penokean orogen in east-central 

Minnesota and Upper Michigan, within the context of a 

recent interpretation of the Penokean orogen based on 

geophysics and drilling in Minnesota (Southwick and 

Morey, this volume, ch. C).

ACKNOWLEDGMENTS

This work was supported in part by National Science 

Foundation Grant EAR8420089. Thanks go to F.W. Cambray and J.S. Klasner for helpful comments in review, and

Structure of Penokean Orogeny, Minnesota and Upper Michigan D1
.Area of

Minnesota segment

v\\\\Vx^\\\

"^ 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 overthr

 » * 1_ Thrust fault Sawteet

GLTZ Great Lakes tectonic 

Figure 1. Minnesota and Wisconsin-Upper Michigan segments of Penokean orogen, divided by rocks of Midcontinent rift sy

this volume, ch. C, fig. 1, and Sims and others, 1989). Transects along which structural geology is discussed run approximately 

(D), and from Covington, Mich. (C) to Iron Mountain, Mich. (IM). GLTZ, Great Lakes tectonic zone; LA, L'Anse, Mich.; CF, C
to PJ. Hudleston, D.L. Southwick, and F.W. Cambray for 

discussions of the Penokean orogeny in Minnesota and 

Michigan.

GENERAL TECTONIC SETTING

Early models for the Penokean orogeny in Minnesota 

put forth intracratonic tectonism as a principal mechanism 

(Morey and Sims, 1976; Sims, 1976; Sims and others, 

1980), and emphasized the role of basement rock undergoing "vertical remobilization" (Morey, 1979). In Upper 

Michigan as well, early discussions of the tectonic history 

stressed the importance of vertical movements of basement 

rocks in the structural evolution (Cannon, 1973). In the last 

decade, however, a plate collisional environment has 

become a principal feature of tectonic models proposed for 

the Penokean orogeny, both for the segment in Minnesota 

(Hoist, 1984a, 1984b, 1984c; Morey and Southwick, 1984; 

Southwick and others, 1988; Southwick and Morey, this 

volume, ch. C) and the segment in Wisconsin and Upper 

Michigan (Van Schmus, 1976; Cambray, 1977, 1978; 

Larue, 1983; LaBerge and others, 1984; Schulz, 1983; 

Schulz and others, 1984; Sims and others, 1987; Klasner 

and Attoh, 1986; Attoh and Klasner, 1989).

East of the Midcontinent rift system the Penokean 

orogen consists fundamentally of two assemblages of rocks 

(Cannon, 1973; Sims and Peterman, 1983; Larue, 1983). 

The northern assemblage consists of Early Proterozoic 

turbidites and underlying shelf deposits of the Marquette 

Range Supergroup (Cannon and Gair, 1970), which overlie 

Archean basement and are exposed around and between 

basement highs (fig. 1). These rocks are thought to be 

broadly correlative to the Early Proterozoic supracrustal 

rocks of the Mille Lacs and Animikie Groups in Minnesota 

(Cannon, 1973; Larue, 1981, 1983; Morey, 1983; Ojakangas, 1983; Sims and Peterman, 1983). The southern 

assemblage is composed primarily of plutonic and volcanic 

rocks of the Wisconsin magmatic terranes. Geochemical 

studies have shown these rocks to be of island arc affinity 

(Schulz, 1983; Greenberg and Brown, 1983; Schulz and 

others, 1984; Sims and others, 1989). The boundary 

between these two assemblages is the Niagara fault zone. A 

number of tectonic models identifying plate collision (some, 

multiple collisions) in this area cite the Niagara fault zone as 

the collision boundary or suture zone (Van Schmus, 1976; 

Cambray, 1977, 1978; Larue, 1983; LaBerge and others, 

1984; Sims and others, 1985; Klasner and Attoh, 1986; 

Klasner, Ojakangas, and others, 1988; Klasner, Sims, and 

others, 1988; Attoh and Klasner, 1989).

West of the Midcontinent rift system, surface 

exposure of the Penokean orogen is quite poor, but recent 

geophysical work and shallow drilling have led to a much 

clearer picture (Southwick and others, 1988; Southwick and 

Morey, this volume, ch. C). Southwick and Morey have 

interpreted this segment of the orogen to consist of an

internal fold-and-thrust belt and an external turbidite basin 

(the Animikie basin and its outliers, fig. 1) developed during 

a collisional event entailing southward-dipping subduction. 

Several structural terranes, each with its own stratigraphy, 

structural style, and metamorphic character, make up the 

internal fold-and-thrust belt. Southwick and Morey (this 

volume, ch. C) have presented tectonic and petrogenetic 

interpretations for each of the terranes, and suggested that 

the boundary between two of the terranes, the Malmo 

discontinuity (fig. 1), may be correlative with the Niagara 

fault zone across the Midcontinent rift. The turbidite basin 

(the Animikie and outliers), external to the fold-and-thrust 

belt, is interpreted as a migrating foredeep (Hoffman, 1987), 

or more likely, a series of foredeeps (Southwick and Morey, 

this volume, ch. C), whose sediments became involved in 

the deformation at the margin of the fold-and-thrust belt.

Southwick and Morey (this volume, ch. C) pointed 

out the broad similarities between rocks east of the Midcontinent rift just north of the Niagara fault zone and the 

fold-and-thrust belt in Minnesota, and the similarities in 

stratigraphy, sedimentology, and structural style between 

the Animikie Group in the Animikie basin in Minnesota and 

the rocks of the Early Proterozoic Baraga Group in the 

Baraga basin (fig. 1) in Upper Michigan; they have 

suggested that these similarities fit a foredeep model.

COMPARISON OF STRUCTURAL 

GEOLOGY

Limited rock exposures exist on a roughly northsouth transect across parts of the postulated internal foldand-thrust belt, and on one across the postulated external 

foredeep basins in both Minnesota and Upper Michigan. 

(Approximate locations of the transects are shown by letter 

symbols, fig. 1.) Exposures in Minnesota extend from 

Cloquet on the north roughly to Denham on the south, and 

are in rocks historically mapped as Early Proterozoic 

Denham and Thomson Formations; structure along the 

Minnesota transect, described in detail elsewhere (for 

example, Hoist, 1982, 1984c, 1985a; Holm, 1986a), is 

summarized herein. In Michigan, the exposures extend from 

Covington on the north to just north of Iron Mountain on the 

south, and are in rocks that are mapped as Michigamme 

Formation (Cannon, 1986). Mesoscopic and microscopic 

structures in rocks along these two transects suggest the 

existence in each area of two fundamentally different 

structural terranes, a northern and a southern terrane.

Descriptive Structural Geology in 

Minnesota Terranes

Exposures of Early Proterozoic metasedimentary and 

metavolcanic rocks historically called the Thomson Formation consist of a thick sequence of interbedded slate, slaty 

graywacke and metagraywacke, and some intercalated

Structure of Penokean Orogeny, Minnesota and Upper Michigan D3
volcanic rocks. Some of the southern exposures previously 

mapped as Thomson Formation have recently been assigned 

to other, as yet unnamed units of Early Proterozoic supracrustal rocks, which are older than the type locality of the 

Thomson Formation (Southwick and Morey, 1988; Southwick and others, 1988; Southwick and Morey, this volume, 

ch. C). The southern two-thirds of the region of Early 

Proterozoic supracrustal rocks, here designated the Minnesota southern structural terrane, has a pervasive, nearly 

bedding-parallel foliation (Sj). It ranges from a slaty 

cleavage in the north to a schistosity in the south (Hoist, 

1982). Strain analysis (Hoist, 1985a) has firmly established 

the tectonic nature of this bedding-parallel foliation. Also 

present in the southern structural terrane are isoclinal 

recumbent folds with east-west fold axes (Fj), whose scales 

range from centimeters to kilometers (nappes). The main 

foliation in the southern terrane is axial-planar to these 

recumbent folds and is nearly always subparallel to bedding, 

as the folds are isoclinal and hinge regions are relatively 

rare.

Both the northern and southern structural terranes in 

Minnesota have been affected by later gentle to open 

(locally closed) upright folds. Fold axial surfaces strike 

east-west and dip vertically or steeply to the south (northward vergence). Fold axes have horizontal to gentle plunges 

either east or west, except where folds die out rapidly along 

strike, where axes may plunge up to 60°. In the southern 

structural terrane, these upright folds (F2) refold the earlier 

isoclinal recumbent folds (Fj). In the northern terrane no 

folding took place prior to the upright generation of folds.

A well-developed cleavage, vertical or dipping 

steeply to the south, (axial-planar to the upright folds), is 

present in both the northern and southern structural terranes. 

In the northern structural terrane this cleavage is well 

developed, continuous, and slaty in the finer grained units. 

In the units with graded bedding, and in the units with 

interlaminated fine and coarse layers, the continuous 

cleavage grades into a disjunctive spaced cleavage (terminology of Powell, 1979). The spacing of the cleavage 

domains ranges from continuous up to 1 cm in some of the 

coarser grained units, but it is rarely more than a few 

millimeters. Cleavage domains constitute from 25 percent 

of the rock (in the thick graywacke units) to 100 percent (in 

the slates with a continuous cleavage). Domain shapes range 

from rough to smooth (smooth shapes predominate) with 

some anastomosing shapes. Within the microlithons, a weak 

fabric, at least, is developed everywhere; and commonly the 

fabric is strong to complete (terminology of Powell, 1979). 

In the southern structural terrane, the steep foliation is a 

well-developed crenulation cleavage (S2). This S2 cleavage 

can be discrete but is most commonly transitional to zonal, 

or entirely zonal (Gray, 1977; Powell, 1979). Spacing of the 

cleavage domains is variable. For the most part the spaced 

crenulation cleavage strikes east-west and dips steeply to 

the south or is vertical, and is axial-planar to the F2 folds.

Around some microfolds, however, the spaced crenulation 

cleavage may fan, or be at a constant angle (up to 40°) to the 

axial plane on one limb, and axial-planar on the other limb. 

The intersection of Sj and S2 defines a well-developed 

lineation in the rock, trending east-west with subhorizontal 

plunges; a mineral lineation of the same orientation exists as 

well.

Underlying the southern part of what has historically 

been called the Thomson Formation is the Early Proterozoic 

Denham Formation. The Denham has now been divided 

into lower and upper parts by Southwick and others (1988). 

The lower member comprises quartzite, micaceous quartzite, and pelitic schist, with some local conglomerate and 

mafic to intermediate metavolcanic rocks. The upper 

member is dominantly quartzite, with interbedded dolomite 

marble (Southwick and others, 1988). The rocks of this 

formation have been multiply deformed and metamorphosed in a fashion similar to the overlying rocks (Holm, 

1986a, 1986b) and are part of the same nappe terrane. The 

Denham Formation is a sequence of primarily quartz rich 

metasedimentary rocks (meta-arkose, quartzite, mica schist, 

and garnet-staurolite schist) with minor amounts of marble 

and volcanic rocks. Bedding strikes east-west and dips 

steeply. A nearly bedding-parallel foliation is present 

everywhere in the Denham Formation. The foliation is 

refracted at a higher angle to bedding in the more competent 

arkosic and quartzitic units. The foliation and bedding have 

been folded with the development, locally, of a crenulation 

cleavage. Orientations of axial surfaces to these folds vary 

from horizontal to vertical and strike east-west. The Denham Formation also contains a very well developed, nearly 

horizontal, east-west mineral (extension) lineation and 

crenulation lineation. Chocolate-tablet boudinage of quartz 

veins occurs parallel to bedding throughout the area.

In the Minnesota northern structural terrane, some 

late-stage features deform the steep cleavage in the Thomson Formation. Kink bands are locally well developed, and 

although their orientation is quite variable, poles to more 

than 100 kink bands define a single maximum on an 

equal-area projection (Clark, 1985), with a gentle dip to the 

north. The gentle dip of the kink bands indicates a subvertical finite compression during their formation, estimated to 

be about 5 percent by Clark (1985). In addition, along a few 

isolated outcrops in the northern terrane, the cleavage dips 

very gently (Clark, 1985). No younger cleavage is 

developed, and the rocks appear identical, both mesoscopically and microscopically, to rocks of the surrounding area, 

except that the cleavage is flat, not subvertical as it is in 

virtually the entire northern terrane. The areas of exposure 

of this amomalously oriented cleavage are small, few, and 

isolated from other exposures by thick glacial cover.

In summary, the essential features of the northern 

structural terrane are a single generation of upright folds, 

with an axial-planar cleavage, and some later features such 

as kink bands and isolated areas of flat cleavage. The

D4 Precambrian Geology of Lake Superior Region
essential features of the southern structural terrane are an 

early generation of isoclinal recumbent folds with an 

axial-planar cleavage (subparallel to bedding in all but rare 

hinge regions of the early folds), and a later set of upright 

folds with associated axial-planar crenulation cleavage. The 

rocks of the southern terrane also show a well-developed 

lineation in most areas. Detailed mapping has allowed a 

boundary to be drawn between the area of a single main 

Penokean deformation in the north (the northern structural 

terrane), and the area of two Penokean deformations in the 

south (the southern structural terrane). This boundary was 

interpreted by Hoist (1984c) as a nappe front because of the 

abrupt nature of the change across this terrane boundary, 

and because the refraction pattern in the early foliation just 

south of this boundary suggests that a nappe front must be 

located in the immediate vicinity to the north. This structural boundary lies near the boundary between the internal 

fold-and-thrust belt and the external turbidite basin (foredeep), postulated by Southwick and Morey (this volume, 

ch. C). Some rocks at the south margin of the younger, 

external foredeep are part of the southern structural terrane 

(Southwick and others, 1988; Southwick and Morey, this 

volume, ch. C, fig. 2). For the most part, however, the rocks 

of the inferred foredeep are part of the northern structural 

terrane, and were folded only when the underlying rocks of 

the southern structural terrane were refolded. Also, within 

the northern terrane, the effects of deformation die out to the 

north, so that the rocks on the north margin of the basin (in 

particular, the Mesabi iron range) show little effect of the 

Penokean orogeny.

Strain Analysis in Minnesota Terranes

Finite strain markers, in the form of deformed concretions, mud chips, and conglomerate clasts, have provided 

data for the determination of finite strain at 23 localities in 

the Penokean orogen in Minnesota (Hoist, 1985a), 13 in the 

northern structural terrane, and 10 in the southern structural 

terrane. Details of the analysis are omitted here, but may be 

found in Hoist (1985a). The strain analysis emphasizes the 

difference in structural history of the two structural terranes 

(fig. 2). The northern terrane shows strains typical of slate 

belts (Wood, 1974) with all data points plotting within the 

flattening field. Strains in the rocks of the southern terrane 

reveal a different geometry, reflecting the more complex 

structural history. In the analytical model, Hoist (1985a) 

removed the strains associated with the second deformation 

(northern terrane strain) from the total strain in the rocks of 

the southern terrane in an attempt to infer the strain 

associated with the early, nappe-producing deformation 

(fig. 2). Further deformation inversion modeling by Holm 

and others (1988), taking into account possible volume loss 

and rotational effects, confirms that the strain associated 

with the early deformation produced a large vertical finite 

shortening (in the range -84 percent to -89 percent) with

horizontal extensions of greater than 200 percent in both 

orogen-parallel and orogen-perpendicular directions. Hoist 

(1985b) used strain estimates for the early deformation in 

the southern terrane to produce models of this deformation 

factored into simple shear and pure shear components, or 

simple shear and constant-volume non-pure shear components. The results of that modeling showed that shear 

indeed played a large role in the development of the strain 

during nappe emplacement as suggested in other studies of 

strain within nappes (for example, Ramsay and others, 

1983; Siddans, 1983; Coward and Kirn, 1981).

Comparative Structural Geology in 

Minnesota and Michigan

In Michigan, in the area from Covington (fig. 1) south 

to the Baraga County-Iron County line (a distance of about 

15 km), appear rocks and structures similar to the northern 

terrane in Minnesota. The rocks comprise slate, slaty 

graywacke, and metagraywacke, identical in outcrop 

appearance to the type area of the Thomson Formation in 

Minnesota. A single foliation exists, again a continuous 

slaty cleavage in the fine-grained units and a disjunctive

Figure 2. Logarithmic deformation plot showing results of 

three-dimensional strain determinations in the Penokean orogen in Minnesota and Upper Michigan (modified from Hoist, 

1985a, figs. 6, 8). K=1 is line of plane strain at no volume loss. 

Solid boxes, plotting just into the prolate field for the most part, 

are data from Minnesota southern structural terrane. Plotting in 

the flattening field are data from Minnesota northern structural 

terrane (solid dots) and from Upper Michigan northern 

structural terrane (solid triangles). Open diamond is inferred 

strain of the first, nappe-producing deformation in Minnesota 

southern terrane (see Hoist, 1985a, for details). Three cubes 

are sketched to illustrate the strain ellipsoid represented by the 

group of data in each case. For each cube, the top face is 

horizontal, and the front face is an east-west section, with east 

on the right. The difference in finite strain in northern and 

southern terranes is clear, and reflects a difference in 

deformation history.

Structure of Penokean Orogeny, Minnesota and Upper Michigan D5
THOMSON

Figure 3. Sketches comparing the mesoscopic structures in the northern structural terranes in A, Minnesota, and B, Upper 

Michigan. Length of each section about 70 m. A, Section in Minnesota near type .area at Thomson. B, Covington is at north end 

of transect in Upper Michigan shown in figure 1, and Tracy Creek is about 12 km south of Covington. Shown schematically are 

upright to north-verging folds, axial-planar cleavage, and some of the deformed concretions amenable to strain analysis.

space cleavage in the coarse-grained graywacke beds. In 

thin section, rocks from Minnesota and Michigan appear 

essentially identical. The attitude of the cleavage changes 

somewhat from north to south along this part of the transect, 

dipping as gently as 40°-50° to the south at Covington, and 

dipping nearly vertically 15 km to the south. The cleavage 

is axial-planar to some mesoscopic folds, and normal 

bedding-cleavage vergence relationships (between slate and 

graywacke beds) are observed. The folds are upright in the 

southern part of the transect, but axial surfaces dip as gently 

as 40°-50° south at Covington, showing northward vergence. Where fold axes can be found, they are subhorizontal, and bedding-cleavage intersections along the transect 

are subhorizontal. Late-stage kink bands also exist here, and 

are also gently dipping. In outcrop and in thin section the 

kink bands in Minnesota and Upper Michigan show detailed 

similarity. Hoist and R.G. Clark have detailed work in 

progress concerning these kinks.

Deformed concretions and mud chips, whose appearance is again similar even in detail to strain markers in 

Minnesota, exist in the rocks along this part of the transect 

in Upper Michigan. Strain analysis of rocks in this area is at 

a preliminary stage, but three-dimensional strain determinations have been completed for three outcrops located 

2-15 km south of Covington. Results reveal flattening 

strains similar to those in the Minnesota northern terrane 

(fig. 2), which are in marked contrast to the Minnesota

southern terrane. Similarities between the structural features 

found in the northern structural terranes in Minnesota and 

Michigan are shown in figure 3.

In the southern part of the transect in Upper 

Michigan, rocks were examined at Horse Race Rapids just 

south of Crystal Falls and just north of Iron Mountain at 

Steele Farm and along the Sturgeon River. The structural 

geology of these rocks is distinctly different from that of the 

rocks of the northern terrane in Michigan, in the area just 

south of Covington, and quite similar to the structural 

geology of rocks in the Minnesota southern terrane. A 

ubiquitous feature of the rocks in this area is a welldeveloped foliation that is subparallel to bedding, and the 

rocks are commonly lineated. In this southern terrane in 

Michigan, two phases of folding are also evident. The early 

phase is isoclinal and recumbent, and a well-developed 

foliation is axial-planar to the folds (the ubiquitous foliation 

mentioned above). Fj fold hinges are quite rare, so this 

foliation is usually found to be subparallel to bedding. F2 

folds (fig. 4) are open, upright, and subhorizontal. A spaced 

crenulation cleavage is found in some places, in an attitude 

which is axial-planar to the F2 folds. A lineation is parallel 

to the intersection of the two foliations. Figure 5 shows that 

small-scale refolded Fl folds and associated foliations from 

the southern structural terranes in Minnesota and Upper 

Michigan are remarkably similar. Although no strain 

markers have yet been identified in the southern

D6 Precambrian Geology of Lake Superior Region
structural terrane in Upper Michigan, the similarity of style 

and geometry of folds and foliations to the Minnesota 

southern structural terrane suggests that these areas have 

similar strain histories, each considerably different from the 

known strains in the northern terranes.

DISCUSSION

The presence of nappes and (or) thrust faults has 

recently been reported in the Penokean orogen in Upper 

Michigan (for example, Sims and others, 1987; Klasner, 

Ojakangas, and others, 1988; Maharidge, 1986; Stahl and 

Matty, 1988). In the area near L'Anse (fig. 1), WJ. Gregg 

and students have mapped an east-west-trending thrust, 

named the L'Anse thrust, which is not specifically exposed 

but which is inferred, based on the differences in structural 

style and lithology seen in exposures on the shores of 

Keweenaw Bay of Lake Superior, and those just inland on 

the Falls River (Sikkala and Gregg, 1987; Klasner, Sims, 

and others, 1988). The exposures on Keweenaw Bay show 

a single phase of folding and a single cleavage (fig. 6). 

Along the Falls River, early fold axial surfaces dip 

moderately to the south for the most part; but in places they

Figure 4. F 2 folds in hand sample from outcrop of Michigamme Formation along the Sturgeon River just north of Iron 

Mountain, Mich. (alternate stop 13 of Klasner, Sims, and 

others, 1988). Coin (1.9 cm diameter) sits on top of the hand 

sample on an undulating S, surface. F2 folds with axial-planar 

S2 crenulation cleavage are observable on front face of the 

sample. Lineation seen on the top surface is the intersection of 

the S2 crenulation cleavage and the S, foliation.

B

1 cm

Figure 5. Small-scale refolded F, folds and associated 

foliations from outcrops in the southern structural terrane in A, 

Minnesota, and B, Upper Michigan. The folds are defined in 

each case by metagraywacke embedded in slate or schist 

matrix. S, is axial-planar to the early fold, and S2 (vertical in 

sketch) is axial-planar to F 2 folds. Each fold exhibits Ramsay 

type 3 interference pattern (Ramsay, 1967), and geometries 

and attitudes of folds and axial-planar foliations are nearly 

identical in Minnesota and Michigan. Outcrop in Minnesota 

(A) is near Park Lake, between Cloquet and Denham along the 

traverse shown in figure 1. Outcrop in Michigan (B), same as in 

figure 4.

are nearly horizontal, and in these areas there is a steep 

crenulation cleavage. The crenulation cleavage has been 

interpreted (Sikkala and Gregg, 1987) as large kink-like 

younger folds (fig. 6). The areas of anomalously flat 

cleavage in the northern structural terrane in Minnesota may 

well reflect a later kink-like folding, similar to that seen at 

Falls River, Mich. In Minnesota, however, no second 

cleavage has developed. A number of other thrust faults 

have been interpreted in a transect across the orogen to the 

south of L'Anse (Klasner, Sims, and others, 1988). Most of 

these are also inferred to exist between outcrops because of 

structural and (or) lithologic differences.

North-verging folds, some of them with axial surfaces 

of low to moderate dip, certainly do exist in the northern 

terrane in Upper Michigan, and north-verging folds also 

exist in the Minnesota northern terrane. Although the best 

evidence for nappe-scale folding and overthrust tectonic 

style in Minnesota is in the southern structural terrane 

(Hoist, 1984c, 1985b), with much of the best evidence for 

such features being exposed near the boundary between the 

northern and southern terranes, detachment thrusting may 

well exist at depth in the northern terrane in Minnesota, to

Structure of Penokean Orogeny, Minnesota and Upper Michigan D7
A Y FALLS RIVER

-50 mFigure 6. Sketch cross sections (modified from Sikkala and Cregg, 1987; W.J. Cregg, oral commun., 1989) from 

Keweenaw Bay and Falls River exposures near L'Anse in Upper Michigan (location, fig. 1). Falls River section 

approximately 200 m long; Keweenaw Bay section about 50 m long. These two sections are on opposite sides of 

proposed L'Anse thrust (Sikkala and Cregg, 1987; Klasner, Sims, and others, 1988). North-verging folds are similar to 

those in Minnesota northern structural terrane; younger, kink-like fold at the Falls River section may be represented in 

Minnesota by isolated exposures where cleavage is nearly horizontal. Crest line of fold shown by solid line.

accommodate the shortening associated with the upright 

folds. Indeed the argument can be (and has been) made that 

detachment must lie at some level within all fold belts. 

Detachment thrusting may well also exist in the northern 

terrane in Michigan, but the structural geology indicates that 

the deformation history in the northern terrane is 

fundamentally different from that of the southern terrane. 

Analysis of mesoscopic and microscopic structural 

geology in both Minnesota and Michigan corroborates the 

suggestion by Southwick and Morey (this volume, ch. C) 

that the Penokean orogen consists of an internal foldand-thrust belt with tectonically imbricated sequences of 

supracrustal rocks, and a relatively complex deformation 

history, and a northern external turbidite basin (foredeep) 

with a simpler deformation history.

REFERENCES CITED

Attoh, K., and Klasner, J.S., 1989, Tectonic implications of 

metamorphism and gravity field in the Penokean orogen of 

northern Michigan: Tectonics, v. 8, p. 911-933.

Brocoum, S.J., and Dalziel, I.W.D., 1974, The Sudbury Basin, the 

southern province, the Grenville Front, and the Penokean 

orogeny: Geological Society of America Bulletin, v. 85, 

p. 1571-1580.

Cambray, F.W., 1977, Field guide to the Marquette district, 

Michigan: Michigan Basin Geological Society Annual 

Meeting, 62 p.

____ 1978, Plate tectonics as a model for the environment of 

deposition and deformation of the Early Proterozoic (Precambrian X) of northern Michigan: Geological Society of 

America Abstracts with Programs, v. 10, p. 376.

Cannon, W.F., 1973, The Penokean orogeny in northern Michigan, 

in Young, G.M., ed., Huronian stratigraphy and sedimentation: Geological Association of Canada Special Paper 

12, p. 211-249.

____ 1986, Bedrock geologic map of the Iron River I°x2° 

quadrangle, Michigan and Wisconsin: U.S. Geological 

Survey Miscellaneous Investigations Map I-1360-B, scale 

1:250,000.

Cannon, W.F., and Gair, I.E., 1970, A revision of stratigraphic 

nomenclature for middle Precambrian rocks in northern 

Michigan: Geological Society of America Bulletin, v. 81, 

p. 2843-2846.

Clark, R.G., 1985, The structural geology of the Thomson 

Formation; Cloquet and Esko quadrangles, east-central 

Minnesota: Duluth, Minn., University of Minnesota Duluth 

M.S. thesis, 114 p.

Coward, M.P., and Kim, J.H., 1981, Strain within thrust sheets, in 

McKlay, K.R., and Price, N.J., eds., Thrust and nappe 

tectonics: Geological Society of London Special Publication 

9, p. 275-292.

Gray, D.R., 1977, Morphologic classification of crenulation 

cleavages: Journal of Geology, v. 85, p. 229-235.

Greenberg, J.K., and Brown, B.A., 1983, Lower Proterozoic 

volcanic rocks and their setting in the southern Lake 

Superior district, in Medaris, L.G., ed., Early Proterozoic 

geology of the Great Lakes region: Geological Society of 

America Memoir 160, p. 67-84.

Hoffman, P.P., 1987, Early Proterozoic foredeeps, foredeep 

magmatism, and Superior-type iron formations of the 

Canadian Shield, in Kroner, A., ed., Proterozoic lithosphere 

evolution: American Geophysical Union Geodynamics 

Series, v. 17, p. 85-98.

____ 1988, United plates of America, the birth of a craton 

Early Proterozoic assembly and growth of Laurentia: 

Annual Reviews of Earth and Planetary Sciences, v. 16, 

p. 543-603.

____ 1989, Speculations on Laurentia's first gigayear (2.0 to

1.0 Ga): Geology, v. 17, p. 135-138.

Holm, D.K., 1986a, A structural investigation and tectonic interpretation of the Penokean orogeny East-central Minnesota: Duluth, Minn., University of Minnesota Duluth M.S. 

thesis, 1 14 p.

____ 1986b, An account of multiphase deformation during the 

Penokean orogeny Evidence from analysis of the Archean 

McGrath Gneiss and overlying Early Proterozoic metasedimentary rocks in east-central Minnesota: 32nd Annual 

Institute on Lake Superior Geology, Wisconsin Rapids, 

Wisconsin, Abstracts, v. 32, p. 32-33.

D8 Precambrian Geology of Lake Superior Region
Holm, D.K., Hoist, T.B., and Ellis, M., 1988, Oblique subduction, 

footwall deformation, and imbrication A model for the 

Penokean orogeny in east-central Minnesota: Geological 

Society of America Bulletin, v. 100, p. 1811-1818.

Hoist, T.B., 1982, Evidence for multiple deformation during the 

Penokean orogeny in the Middle Precambrian Thomson 

Formation, Minnesota: Canadian Journal of Earth Sciences, 

v. 19, p. 2043-2047.

____ 1984a, Penokean tectonics Constraints from the structural geology in east-central Minnesota: 30th Annual 

Institute on Lake Superior Geology, Wausau, Wisconsin, 

Abstracts, v. 30, p. 19.

____ 1984b, The Early Proterozoic Penokean orogeny as a 

convergent plate boundary: Geological Association of 

Canada Program with Abstracts, v. 9, p. 75.

____ 1984c, Evidence for nappe development during the early 

Proterozoic Penokean orogeny, Minnesota: Geology, v. 12, 

p. 135-138.

____ 1985a, Implications of a large flattening strain for origin of 

a bedding-parallel foliation in the Early Proterozoic Thomson Formation, Minnesota: Journal of Structural Geology, 

v. 7, p. 375-383.

____ 1985b, Strain within early Proterozoic nappes in the

Penokean erogenic belt, Minnesota: Geological Society of 

America Abstracts with Programs, v. 17, p. 293.

Klasner, J.S., and Attoh, K., 1986, Tectonic implications from 

metamorphic and gravity data at the Peavy and Republic 

nodes, northern Michigan: Geological Society of America 

Abstracts with Programs, v. 18, p. 658.

Klasner, J.S., Ojakangas, R.W., Schulz, K.J., and LaBerge, G.L., 

1988, Evidence for development of an early Proterozoic 

overthrust-nappe system in the Penokean Orogen of 

northern Michigan: 34th Annual Institute on Lake Superior 

Geology, Marquette, Michigan, Abstracts and Proceedings, 

v. 34, p. 56-57.

Klasner, J.S., Sims, P.K., Gregg, W.J., and Gallup, C, 1988, A 

structural traverse across a part of the Penokean orogen 

illustrating early Proterozoic over-thrusting in northern 

Michigan: 34th Annual Institute on Lake Superior Geology, 

Marquette, Michigan, Field Trip Guidebooks, v. 34, pt. 2, 

p. C1-C36.

LaBerge, G.L., Schulz, K.J., and Myers, P.E., 1984, The plate 

tectonic history of north-central Wisconsin: 30th Annual 

Institute on Lake Superior Geology, Wausau, Wisconsin, 

Abstracts, v. 30, p. 25-27.

Larue, O.K., 1981, The Chocolay Group, Lake Superior region, 

U.S.A. Sedimentologic evidence for deposition in basinal 

and platform settings on an early Proterozoic craton: 

Geological Society of America Bulletin, v. 92, pt. 1, 

p. 417-435.

____1983, Early Proterozoic tectonics of the Lake Superior 

region Tectonostratigraphic terranes near the purported 

collision zone, in Medaris, L.G., Jr., ed., Early Proterozoic 

geology of the Great Lakes region: Geological Society of 

America Memoir 160, p. 33 47.

Maass, R.S., Medaris, L.G., Jr., and Van Schmus, W.R., 1980, 

Penokean deformation in central Wisconsin, in Morey, 

G.B., and Hanson, G.N., Selected studies of Archean

gneisses and lower Proterozoic rocks, southern Canadian 

Shield: Geological Society of America Special Paper 182, 

p. 147-157.

Maharidge, A.D., 1986, The structural and tectonic history of a 

portion of the Felch Trough, central Dickinson County, 

Michigan: Bowling Green, Ohio, Bowling Green State 

University M.S. thesis, 67 p.

Morey, G.B., 1973, Stratigraphic framework of middle Precambrian rocks in Minnesota, in Young, G.M., ed., Huronian stratigraphy and sedimentation: Geological Association of Canada Special Paper, v. 12, p. 211-249.

____ 1978, Lower and Middle Precambrian stratigraphic 

nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p.

____ 1979, Field trip guidebook for the Precambrian geology of 

east-central Minnesota: Minnesota Geological Survey, 

Guidebook Series 12, 28 p.

____ 1983, Lower Proterozoic stratified rocks and the Penokean

orogeny in east-central Minnesota, in Medaris, L.G., Jr., ed., 

Early Proterozoic geology of the Great Lakes region: 

Geological Society of America Memoir 160, p. 97-112.

Morey, G.B., and Sims, P.K., 1976, Boundary between two 

Precambrian W terranes in Minnesota and its geologic 

significance: Geological Society of America Bulletin, v. 87, 

p. 141-152.

Morey, G.B., and Southwick, D.K., 1984, Early Proterozoic 

geology of east-central Minnesota A review and reappraisal: 30th Annual Institute on Lake Superior Geology, 

Wausau, Wisconsin, Abstracts, v. 30, p. 35-36.

Ojakangas, R.W., 1983, Tidal deposits in the Early Proterozoic 

basin in the Lake Superior region The Palms and Pokegama Formations Evidence for subtidal-shelf deposition 

of Superior-type banded iron formations, in Medaris, L.G., 

Jr., ed., Early Proterozoic geology of the Great Lakes 

region: Geological Society of America Memoir 160, p. 

49-66.

Powell, C. McA., 1979, A morphological classification of rock 

cleavage: Tectonophysics, v. 58, p. 21-34.

Ramsay, J.G., 1967, Folding and fracturing of rocks: New York, 

McGraw Hill, 568 p.

Ramsay, J.G., Casey, M., and Kligfield, R., 1983, Role of shear in 

development of the Helvetic fold-thrust belt of Switzerland: 

Geology, v. 11, p. 439-442.

Schulz, K.J., 1983, Geochemistry of the volcanic rocks of 

northeastern Wisconsin: 29th Annual Institute on Lake 

Superior Geology, Houghton, Michigan, Abstracts and 

Proceedings, v. 29, p. 39.

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 Program with Abstracts, v. 9, p. 103.

Siddans, A.W.B., 1983, Finite strain patterns in some Alpine 

nappes: Journal of Structural Geology, v. 5, p. 441-448.

Sikkala, K.M., and Gregg, W.J., 1987, A structural analysis of 

Proterozoic metasediments, northern Falls River, Baraga 

County, Michigan: 33rd Annual Institute on Lake Superior 

Geology, Wawa, Ontario, Abstracts and Proceedings, v. 33, 

p. 65-66.

Structure of Penokean Orogeny, Minnesota and Upper Michigan D9
Sims, P.K., 1976, Precambrian tectonics and mineral deposits, 

Lake Superior region: Economic Geology, v. 71, p. 1092- 

1118.

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, v. 91, pt. 1, p. 690-698.

Sims, P.K., and Peterman, Z.E., 1983, Evolution of Penokean 

foldbelt, Lake Superior region, and its tectonic environment, 

in Medaris, L.G., Jr., ed., Early Proterozoic geology of the 

Great Lakes region: Geological Society of America Memoir 

160, p. 3-14.

____ 1986, Early Proterozoic Central Plains orogen A major 

buried structure in the north-central United States: Geology, 

v. 14, p. 488-491.

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., 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., 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., 1988, Tectonic imbrication and 

foredeep development in the Penokean orogen, east-central 

Minnesota: 34th Annual Institute on Lake Superior

Geology, Marquette, Michigan, Abstracts and Proceedings, 

v. 34, p. 106-107.

Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, 

Geologic map (scale 1:250,000) of the Penokean orogen, 

central and eastern Minnesota, and accompanying text: 

Minnesota Geological Survey Report of Investigations 37, 

Saint Paul, Minnesota, 25 p.

Stahl, S.D., and Matty, D.J., 1988, Early Penokean deformation in 

the Peavy Pond area, northern Michigan Evidence from 

the Michigamme Formation and the Peavy Pond Igneous 

Complex: 34th Annual Institute on Lake Superior Geology, 

Marquette, Michigan, Abstracts and Proceedings, v. 34, 

p. 108-109.

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

the Great Lakes area, North America: Philosophical Transactions of the Royal Society of London, v. 280, p. 605-628.

____ 1980, Chronology of igneous rocks associated with the 

Penokean orogeny in Wisconsin: Geological Society of 

America Special Paper 182, p. 159-168.

____ 1981, Possible interpretations of the Penokean orogeny: 

International Proterozoic Symposium, University of Wisconsin, Madison, Abstracts Volume, p. 44.

Wood, D.S., 1974, Current views of the development of slaty 

cleavage: Annual Reviews of Earth and Planetary Science 

Letters, v. 2, p. 1-35.

Young, G.M., 1983, Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes region, in Medaris, 

L.G., ed., Early Proterozoic geology of the Great Lakes 

region: Geological Society of America Memoir 160, 

p. 15-32.

D10 Precambrian Geology of Lake Superior Region

*U.S. GOVERNMENT PRINTING OFFICE. 1991-573-049/26033


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