Paleomagnetism of 

Some Lake Superior 

Keweenawan Rocks 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 760 

Paleomagnetism of 

Some Lake Superior 

Keweenawan Rocks 

By KENNETH G. BOOKS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 760 

Remanent magnetization directions for lower and 

middle Keweenawan rocks fall into three groups 

that have their polarities alternately normal) 

reversed) and normal 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1972 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 72-600117 

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

Washington, D.C. 20402 - Price 50 cents (paper cover) 

Stock Number 2401-2180 
CONTENTS 

Abstract ___________________________________________ _ 

Introduction ________________________________________ _ 

Acknowledgments ___________________________________ _ 

Field and laboratory techniques _______________________ _ 

Collection and measurement ______________________ _ 

Sources of error _________________________________ _ 

Magnetic cleaning _______________________________ _ 

General geology _____________________________________ _ 

E:eweenawPeninsula ________________________________ _ 

Geologic setting _________________________________ _ 

Description of results ____________________________ _ 

Portage Lake Lava Series ____________________ _ 

Rhyolite flows north of Lake Gogebic, Mich ____ _ 

Quartz porphyry north of Lake Gogebic, Mich __ _ 

Rhyolite intrusion in sec. 4, T. 56 N ., R. 32 W ___ _ 

Rhyolite intrusion in sees. 24 and 25, T. 56 N ., R. 34 W _________________________________ _ 

Page 

1 

1 

3 

3 

3 

3 

4 

4 

5 

5 

6 

6 

6 

12 

12 

12 

Ironwood-Mellen area _______________________________ _ 

Geologic setting _________________________________ _ 

Description of results ____________________________ _ 

Lowermost E:eweenawan lava flows ____________ _ 

Portage Lake Lava Series ____________________ _ 

North Shore of Lake Superior ________________________ _ 

Geologic setting _________________________________ _ 

Description of results ____________________________ _ 

North Shore Volcanic Group __________________ _ 

Isle Royale lava flows _______________________ _ 

Discussion of results _________________________________ _ 

Lower Keweenawan _____________________________ _ 

Middle E:eweenawan ____________________________ _ 

Portage Lake and North Shore sequences ______ _ 

Quartz porphyry north of Lake Gogebic, Mich __ _ Upper Keweenawan _____________________________ _ 

Rhyolite intrusions __________________________ _ 

Summary and conclusions ____________________________ _ 

References cited _____________________________________ _ 

ILLUSTRATIONS 

Page 

12 

12 

16 

16 

16 

16 

16 

16 

16 

18 

18 

18 

20 

22 

22 

24 

24 

27 

40 

Page 

FIGURE 1. Generalized geologic map of the Lake Superior region__________________________________________________ 2 

2. Simplified columnar section for the Portage Lake Lava Series near E:earsarge, Mich _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 7 

3-15. Equal-area diagram showing directions of remanent magnetization for3. Flows of the Portage Lake Lava Series below conglomerate No. 14 near E:earsarge, Mich_____________ 8 

4. Flows of the Portage Lake Lava Series between conglomerate No. 14 and conglomerate No. 15 near 

Kearsarge, Mich_________________________________________________________________________ 9 

5. Flows of the Portage Lake Lava Series above conglomerate No. 15 near Kearsarge, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 

6. Flows of the Portage Lake Lava Series northeast of those near E:earsarge, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 13 

7. Middle E:eweenawan rhyolite flows north of Lake Gogebic, Mich__________________________________ 15 

8. Quartz porphyry north of Lake Gogebic, Mich________________________________________________ 17 

9. Rhyolite intrusion in sec. 4, T. 56 N., R. 32 W________________________________________________ 19 

10. Rhyolite intrusion in sees. 24 and 25, T. 56 N., R. 34 W _ _ _ __ _ _ _ _ __ _ _ __ _ _ ___ _ ____ _ _ __ _ _ _ ___ ______ _ 21 

11. Basal quartzite and lowermost 400 feet of South Trap Range lava flows near Ironwood, Mich_ _ _ _ _ _ _ _ 23 

12. Middle E:eweenawan flows from the Chippewa Hill quarry, Michigan_____________________________ 25 

13. Middle Keweenawan flows on the Black River near Algonquin Falls, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 26 

14. North Shore Volcanic Group, Minnesota_______________________________________________________ 28 

15. Middle E:eweenawan lava flows on Isle Royale, Mich___________________________________________ 30 

16-18. Equal-area diagram showing16. Summary of mean directions of remanent magnetization for lower(?) E:eweenawan rock units________ 32 

17. Directions of remanent magnetization for pre-E:eweenawan units near Ironwood, Mich., and Grand 

Portage, Minn__________________________________________________________________________ 33 

18. Summary of mean directions of remanent magnetization for middle E:eweenawan rock units__________ 34 

19. Columnar sections showing comparison of chronological movements in inclination of the magnetic fields of the 

Portage Lake Lava Series and North Shore Volcanic Group________________________________________ 35 

20. Columnar sections showing comparison of chronological movements in declination of the magnetic fields of the 

Portage Lake Lava Series and North Shore Volcanic Group________________________________________ 36 

III 
IV CONTENTS 

Page 

FIGURES 21-23. Equal-area diagram showing21. Comparison of mean directions of remanent magnetization for lava flows near Kearsarge and on Isle 

Royale_________________________________________________________________________________ 37 

22. Alternative directions of remanent magnetization for the quartz porphyry north of Lake Gogebic, Mich_ _ 38 

23. Directions of remanent magnetization for rhyolite intrusions in T. 56 N., R. 32 W., and T. 56 N., R. 

34 W ., near Kearsarge, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 39 

TABLES 

Page 

TABLE 1. Partial demagnetization data for four sites on the Portage Lake Lava Series________________________________ 4 

2. Paleomagnetic results for Portage Lake Lava Series near Kearsarge, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ 11 

3. Paleomagnetic results for miscellaneous flows of the Portage Lake Lava Series northeast of Kearsarge, Mich____ 14 

4. Comparison of declination and strike along the Ashbed Flow in the Portage Lake Lava Series________________ 14 

5-141

• Paleomagnetic results for5. Middle Keweenawan rhyolite flows north of Lake Gogebic, Mich___________________________________ 16 

6. Quartz porphyry north of Lake Gogebic, Mich___________________________________________________ 18 

7. Rhyolite intrusion in sec. 4, T. 56 N., R. 32 W., east of Kearsarge, Mich____________________________ 20 

8. Rhyolite intrusion in sees. 24 and 25, T. 56 N., R. 34 W., southwest of Kearsarge, Mich______________ 22 

9. Basal quartzite and lowermost 400 feet of South Trap Range flows near Ironwood, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ 24 

10. Portage Lake lava equivalents at the Chippewa Hill quarry, Michigan______________________________ 27 

11. Portage Lake lava equivalents on the Black River near Algonquin Falls, Mich_______________________ 27 

12. North Shore Volcanic Group in Minnesota______________________________________________________ 29 

13. Middle Keweenawan lavas on Isle Royale, Mich_________________________________________________ 31 

14. Pre-Keweenawan rocks near Ironwood, Mich., and Grand Portage, Minn____________________________ 31 

15. Summary of new paleomagnetic data for rock units in the Lake Superior region_____________________________ 31 

16. Tentative correlation chart of Lake Superior Keweenawan units based on magnetic polarity sequences_________ 40 
PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

By KENNETH G. BooKs 

ABSTRACT 

The averaged directions of remanent magnetization for the 

Keweenawan rock units investigated fall into three general groupings that have their polarities alternately normal, reversed, and 

normal. The older normal polarity and subsequent reversed 

polarity groups are found in lower Keweenawan rocks and have 

significantly different directions from the younger group with 

normal polarity which characterizes middle Keweenawan rocks. 

From all av..ailable paleomagnetic data for this area, it seems that 

magnetization directions among various rock units in the lower 

Keweenawan around Lake Superior are characteristically scattered, which may indicate long periods between volcanic events, 

a rapid rate of change in magnetic field directions, or changes due 

to metamorphism. Less scatter of magnetization directions among 

middle Keweenawan rock units may indicate a shorter time interval between volcanic events or a slower rate of change in magnetic field direction. Upper Keweenawan magnetization directions, 

though not included in this study, are again more scattered among 

rock units. 

Beginning with known basal Keweenawan, the three magnetic 

polarity groups are represented in rocks on all sides of Lake 

Superior. The older normal polarization is found in the basal rocks 

near Ironwood, Mich., and in the lower part of the lower Keweenawan Sibley Series on Sibley Peninsula, Ontario. The reversely 

polarized group is a distinctive representation of a time interval 

that is present at many locations around the lake. Reverse polarization is found in some 6,700 feet of South Trap Range lava 

flows near Ironwood, Mich., lava flows near Grand Portage, 

Minn., gabbro in Cook County, Minn., sedimentary rocks of the 

Sibley Series in Ontario, and lava flows at Alona Bay on the 

eastern end of the lake. 

The younger normal polarization is also widespread and is 

represented in the great bulk of the extrusive and intrusive rocks 

around Lake Superior that are classified as middle Keweenawan 

in age. This group includes the Portage Lake Lava Series on the 

Keweenaw Peninsula and most of the North Shore Volcanic 

Group of Goldich and others (1961) in Minnesota. 

With the completion of this study, paleomagnetic field directions are now available for all major geologic units of the Keweenawan Series in the Lake Superior region. 

INTRODUCTION 

The area of investigation lies in parts of the States 

of Michigan, Wisconsin, and Minnesota adjacent to 

Lake Superior (fig. 1) and is underlain mainly by 

igneous rocks of late Precambrian age. 

Because of its economic significance and structural 

complexities, the Precambrian of the Lake Superior 

region has been the object of geologic investigations 

for more than a century, and the section of lava flows 

of Keweenawan age near the lake is probably as well 

known as any such succession in North America. 

Nevertheless, the positioning of some nonfossiliferous 

rocks in the geologic column has been hard to assess, 

because of inherent difficulties in determining time 

relationships between extrusive rocks at some distance 

from each other, and even more difficulty in relating 

different intrusive rocks of the same age. 

Before radiometric dating methods were developed, 

most Precambrian rocks were correlated by comparison 

of facies and petrographic similarities, unconformities, 

and degree of metamorphism. Goldich and others (1961) 

built a time scale of radiometric dates for the Precambrian of Minnesota, into which geologic units and 

events in the Lake Superior region can be fitted. 

Paleomagnetism can also be used to correlate rock 

units within the Precambrian. In recent years much 

progress has been made in measurement and interpretation of natural remanent magnetization of rocks. 

Paleomagnetic studies are based on the knowledge that 

rocks can acquire a stable magnetization in the direction of the earth's magnetic field during formation, 

and this direction can be measured by sensitive instruments in the laboratory. Therefore, by collecting many 

oriented samples from a particular rock unit, the 

direction of the earth's magnetic field at the time the 

rock was formed can be determined, provided .that the 

magnetization has been preserved. By collecting and 

averaging magnetization directions of samples from 

various rock units, it is possible to establish a sequence 

of paleomagnetic field directions to which data from 

rocks of all types may be compared. DuBois (1962) 

has attempted to show how paleomagnetism studies 

can aid and supplement geological interpretations in 

the Lake Superior region and has laid the groundwork 

for such studies in the area. The present investigation 

continues this type of study by detailed analysis of 

the paleomagnetic field directions in the Keweenawan 

lava flows and related rocks on both the southeast 

and northwest shores of Lake Superior. 

1 
49° 

48° 

92° 

Cambrian 

~ 

~ 

rocks 

Upper 

~ 

~ 

Keweenawan 

Sedimentary and m a('ic 

extrusive ·rocks . 

D 

Middle Keweenawan 

intrusive rocks 

91° 

EXPLANATION 

0 

Middle Keweenawan 

extrusive rocks 

May include some lower 

Keweenawan rocks 

m 

Lower Keweenawan 

Sedimentary rocks includes 

the Puckwunge Formation 

( pu), and the Sibley sedimentary rocks (S) 

s c 

goo 

D 

Pre-Keweenawan rocks 

Contact 

Fault 

Dashed where approximately 

located 

J.;~l{B 

0 

.... Marenisco --...._._ 

0 N S I N--..._._ 

87° 

N T A R 

0 SLATE ISLANDS 

sllrtitroit 

Keweenaw Point 

'" 

Lower Keweenwan 

diabase dikes 

20 0 20 40 60 MILES 

0 

'" 86° 85° 

COLLECTION AREAS 

CURRENT COLLECTIONS 

1. Upper Keweenawan intrusive rocks 

2. Portage Lake and related middle Keweenawan lava flows 

3. North Shore Volcanic Group 

4. Middle Keweenawan on Isle Royale 

5. Lowermost Keweenawan lava flows 

6. Marquette Range Supergroup 

PREVIOUS COLLECTIONS 

7. Gabbro at Mellen, Wis. 

8. Lowermost Keweenawan volcanic rocks 

BECK AND LINDSLEY <1969> DATA 

9. Northern tongue, Duluth Gabbro Complex 

DUBOIS '1962 > DATA 

10. Logan sills 

11. Alona Bay lavas 

12. Sibley Series 

13. Baraga County dikes 

PALMER <1970> DATA 

14. Osier Series volcanic rocks 

15. Middle Keweenawan lava flows on Michipicoten Island 

16. Keweenawan lava flows on Gargantua Point 

17. Keweenawan lava flows on Mamainse Point 

MICHIPICOTEN 

~~ISLAND 

Gargantua 

Point 

Mamainse Point 

FIGURE !.-Generalized geologic map of the Lake Superior region. Modified from Leith, Lund, and Leith (1935, pl. 1), White (1966a, fig. 1), and Halls (1966, figs. 4, 5). 

~ 

1-d 

> t"4 

t?:1 

0 

a= 

> C) 

z 

t?:1 

1-3 

1-1 

rn 

a= 

0 

l'%j 

rn 

0 

a= 

t?:1 

t"4 

> 

~ 

t?:1 

rn 

0 

l;] 

~ 1-1 

0 

~ 

~ 

t?:1 

~ 

t?:1 

t?:1 

z 

~ > z 

~ 

0 

(") 

~ 

rn 
FIELD AND LkBORATORY TECHNIQUES 3 

ACKNOWLEDGMENTS 

For geologic background material, this report draws 

on many published sources, which are listed under 

references. For paleomagnetic background material, 

the publication by DuBois (1962) has been invaluable. 

I wish to acknowledge the guidance of Walter S. 

White, U.S. Geological Survey, on matters geologic 

in the Lake Superior region, for field and office help 

on stratigraphy and structure and geologic background material relating to the Keweenaw Peninsula 

collections, and for critical reviews and suggestions on 

this and previous manuscripts. I further wish to 

acknowledge help and direction from Myrl E. Beck, Jr., 

who was with the U.S. Geological Survey in the initial 

stages of the project; also, Harold A. Hubbard, U.S. 

Geological Survey, supplied me with geologic data 

and advice on the Ironwood-Mellen area. Finally, 

I wish to thank N. King Huber, U.S. Geological Survey, 

for stratigraphic locations on Isle Royale and William 

E. Huff, Jr., for field and laboratory help. 

FIELD AND LABORATORY TECHNIQUES 

COLLECTION AND MEASUREMENT 

The Keweenawan rock samples were obtained with 

portable coring equipment (Doell and Cox, 1965, p. 

A22-A24) that yields short cores approximately 2.49 

em (centimeters) in diameter. These cores were oriented 

in place relative to the horizontal and to geographic 

north, and in the laboratory they were cut into two 

2.54-cm lengths on dual nonmagnetic diamond circular 

saws. 

Natural remanent magnetization down to 10-4 

emu/cm3 (electromagnetic units per cubic centimeter) 

was measured on the U.S. Geological Survey motordriven spinner magnetometer. Both cores from each 

sample were measured for remanence, and one was 

subjected to magnetic cleaning in a-c (alternatingcurrent) fields. The coring rig, orientation techniques, 

and description of the U.S. Geological Survey magnetometer are explained in a paper by Doell and Cox 

(1965, p. A1-A25). 

Samples with natural remanent magnetizations less 

than 10-4 emujcm3 amounted to less than 5 percent of 

the total number collected and were from intrusive 

rhyolites, quartz porphyries, and some andesite lava 

flows. These samples were measured on a high-sensitivity, motor-driven, rock generator (spinner) magnetometer designed by Phillips and Kuckes (1967). This 

instrument uses a commercially made lock-in amplifier 

and low-noise, high-gain preamplifier. The main design 

features of the pickup coil transducing system, with 

a high signal-to-noise ratio, include the use of a compensated dual-coil array and aluminum eddy current 

shielding. The instrument has a claimed sensitivity 

of 6 x1o-s emu/cm3 for 10-cm3 cores, and we have 

had good results down to 3X10-7 emujcm3 in the U.S. 

Geological Survey laboratory. Over a period of 2 

years of use, comparative measurements within the 

limits of the U.S. Geological Survey magnetometer for 

the same samples indicated a difference in results 

between the two magnetometers that is well within 

the claimed measurement error of 5° for direction and 

10 percent for intensity for the high-sensitivity instrument. 

All data in this paper were reduced with computer 

programs and are presented in terms of declination 

(D), measured in degrees east of geographic north, 

and inclination (J), measured in degrees below the 

horizontal. All directions are corrected for present 

geologic dip of the rock unit unless otherwise noted. 

Radii for circles of confidence ( a95) have been determined by a Fisher (1953) statistical computer program at the 95 percent level; K is the precision parameter. Unless otherwise noted, N refers to number of 

individual samples. In the summary table, N is the 

number of sites for which Fisher (1953) analyses have 

been computed. In the various figures, remanence 

results are plotted on the lower hemisphere of an equalarea net to permit direct comparison of directions on a 

single plane, regardless of polarity. Solid circles represent polarization north-seeking down and open circles 

represent polarization south-seeking down. 

Due to local declination anomalies that were commonly several degrees and occasionally near 10°, all 

magnetic compass readings at the collecting site had 

to be verified with a sun dial compass. 

Routine procedure included calibration of the sun 

dial compass with watch time for the particular latitude and in the particular area of site locations. Calibration stations were established at Kearsarge and 

Ironwood, Mich., at Mellen, Wis., and at Grand 

Marais, Minn. These stations were reoccupied whenever 

the sample collection period exceeded several weeks. 

A comparison of the magnetic compass reading of 

true north with that determined by the sun dial compass was made for each core. With the sun dial compass 

as base, the differential was applied as a correction to 

all magnetic compass readings for each core. The sun 

dial compass marks are placed at 5-minute intervals so 

that the instrument can be set up to within 2 72 minutes 

of time. This is equivalent to about 1 o in azimuthal 

readings on the magnetic compass, and the precision 

of declination readings for each core sample is probably near 1 o. 

SOURCES OF ERROR 

The total core collection and orientation errors in 

the field and in the laboratory are most probably less 
4 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

than 3°. The prime source of error in computing paleomagnetic field directions is probably due to uncertainty 

in the geologic dip component of the attitude for the 

lava flows, as their orientations are not easily measured 

in the field. Geologic dip measurements in the Kearsarge, Mich., area where drill-hole data are plentiful 

are probably accurate to 1 o (W. S. White, written 

commun., 1970), but dip values may be as much as 5° 

inaccurate in other places on the Keweenaw Peninsula. 

Dip values outside the peninsula are less certain. The 

North Shore Volcanic Group in Minnesota and the 

Isle Royale lavas dip less than 20°; consequently, the 

possibility of large errors in dip measurements is 

lessened, and geologic dip values are probably accurate 

to 5°. In the Ironwood, Mich., and Mellen, Wis., areas, 

the rocks commonly dip as much as 80°, and dip 

measurement errors are proportionately greater; dip 

values may be 5° to 10° inaccurate on individual flows 

and could be greater. 

MAGNETIC CLEANING 

Many groups of samples from igneous bodies collected for paleomagnetic research have scattered 

remanent magnetization directions, and the average 

direction may not represent the direction of the earth's 

magnetic field at the time the rocks cooled. Such a 

scatter can often be attributed to secondary components 

of magnetization superimposed upon the original 

magnetization after cooling. One of the most common 

secondary components encountered is viscous magnetization, which may be acquired over a long period of 

time. The intensity of this magnetization is proportional to the logarithm of time (Rimbert, 1959), and 

the direction is parallel to that of the magnetic field 

in which it is acquired. 

Other secondary components which arise from lightning, chemical alteration, and stress effects may also 

be superimposed on the original magnetization. The 

components due to lightning effects tend to be scattered, 

as observed between samples from the same site. Both 

viscous magnetization and magnetization due to lightning may be removed by alternating (ac) field demagnetization. The components due to chemical alteration and stress effects may be stable and will be in 

the direction of the earth's magnetic field at the time 

of the remagnetization. 

The partial demagnetization technique used in this 

investigation is that utilized by U.S. Geological Survey 

laboratories. In this method an alternating magnetic 

field (fi) is slowly decreased as the sample orientation 

is changed by simultaneous rotation about three 

orthogonal axes. 

All samples reported in this paper were subjected to 

a-c partial demagnetization in order to eliminate any 

unstable secondary components. The optimum peak 

demagnetizing field selected was based on an empirical 

procedure utilized by others (Cox 1961; Irving and 

others, 1961). This procedure involves selecting a few 

samples from one collecting site and studying the 

dispersion of magnetic directions after each demagnetization step. The a-c demagnetization that is found 

necessary to produce minimum dispersion is selected 

and applied to all samples from the site. Table 1 

shows examples of partial demagnetization data in 

steps of 100, 200, and 300 Oe (Oersteds) for some of 

the sites in the Portage Lake Lava Series. Though 

small, the within-site scatter increases beyond the 

100-0e demagnetization level (as shown by a95 values), 

which indicates that 100 Oe is the optimum peak field 

for Portage Lake lavas, using this procedure. The 

100-0e demagnetization level for the Portage Lake 

lavas was also generally applicable for other igneous 

rock samples from the Lake Superior region, though 

some of the sites on the older and lithologically distinct 

flows required 150-0e levels, and Beck and Lindsley 

(1969, table 1) found 75-0e levels to be optimal for 

rocks of the Beaver Bay Complex of Grout and 

Schwartz (1939). 

TABLE 1.-Partial demagnetization data for four sites on the Portage 

Lake Lava Series, Keweenaw Peninsula, Mich. 

[N is number of within-site samples. Il is peak alternating magnetic field (in 

Oersteds) used in magnetic cleaning. Other data for these samples are found 

in tables 2 and 3] 

Mean direction of 

magnetization at collecting site Precision Radius of 

Site No. N parameter, confidence 

Declination, Inclination, K circle, ag5 

D I 

(degrees) (degrees) 

Natural remanent magnetization 

PL4 __________ 6 266.9 42.7 85.5 7.3 8 __________ 5 304.9 50.6 30.0 14.2 335 ________ 5 296.9 33.7 616.7 3.7 

345 ________ 5 275.0 41.3 22.7 19.7 

H= 100 Oe 

PL4 __________ 6 261.0 43.2 204.6 4.7 8 __________ 5 297.7 44.2 423.2 3.7 

335 ________ 5 296.1 33.7 762.4 3.3 

345 ________ 5 281.2 38.3 104.7 7.5 

H=200 Oe 

PL4 __________ 6 263.8 43.8 197.2 4.8 8 __________ 5 297.9 44.1 374.1 4.0 335 ________ 5 294.9 34.8 303.5 4.4 

345 ________ 5 286.4 35.6 34.2 13.3 

H=300 Oe 

PL4 ____ - _____ 6 264.4 41.8 109.9 6.4 8 __________ 5 297.1 44.6 410.6 3.8 335 ________ 5 294.7 35.0 382.1 3.9 

345 ________ 5 289.4 31.1 9.4 26.4 

GENERAL GEOLOGY 

The Keweenawan Series of the Lake Superior region 

is characterized by great thicknesses of mafic volcanic 

rocks exposed around the margin of the lake (fig. 1). 

Structurally the series forms a large syncline with 
KEWEENAW PENINSULA 5 

dips ranging from near zero to vertical. On the western 

end of the lake, the syncline trends northeast and has 

low-angle dips on the north limb and steep dips on 

the south limb. Gravity and magnetic highs and well 

data (Lyons, 1959; Thiel, 1956; Craddock and others, 

1963) indicate that the Keweenawan igneous sequence extends southwest across the midcontinent 

toward Kansas. The Keweenawan rocks have been 

separated into three subdivisions: 

1. The term lower Keweenawan has long been 

applied to sedimentary rocks that overlie the Animikie 

Series (now called Marquette Range Supergroup) 1 and 

are comformable with overlying Keweenawan lava 

flows. The maximum exposures of these rocks are on 

the Sibley Peninsula of northwest Ontario and consist 

principally of red, fine-grained sandstones and siltstones. In Minnesota, lower Keweenawan rocks are 

represented by the Puckwunge Formation (Grout and 

others, 1951, p. 1051-1053) that consists of conglomerate and sandstone; near Grand Portage the Puckwunge overlies the Rove Slate of the Animikie Group 

with no visible discordance, and near Duluth it rests 

on the Thomson Formation as used by Schwartz (1942) 

with marked angular unconformity. Lower Keweenawan rocks also have been reported (Aldrich, 1929, 

p. 109-110) in northern Wisconsin overlying Animikie 

rocks. The Sioux Formation in Minnesota and Barron 

Quartzite of Winchell (1895) in Wisconsin are also 

possibly lower Keweenawan (Goldich and others, 

1961, p. 149). 

It now seems desirable locally, if not regionally, to 

place the boundary between lower and middle Keweenawan rocks several thousand feet above the lava flows 

that immediately overlie these lower Keweenawan 

sedimentary rocks, thereby including a thick sequence 

of mafic lava flows within the lower Keweenawan 

(Hubbard, 1967; Books, 1968). Hubbard suggested 

that at least one unconformity separates the rocks of 

the so-called South Trap Range in the IronwoodMellen area (described in a later section) from the 

overlying Portage Lake Lava Series, which, in its 

type area, makes up the whole of the middle Keweenawan. 

2. The middle Keweenawan is composed mainly of 

a thick sequence of basaltic andesite lava flows with 

subordinate beds of sedimentary rock and rhyolite 

conglomerate. Rhyolite flows account for perhaps 10 

l The Animikie Group of Ontario and Minnesota is only partly equivalent to the 

Animikie Series as previously used in Michigan and Wisconsin (James, 1958) 

In that area, the Animikie Series included the Chocolay, Menominee, Baraga, and 

Paint River Groups, and, according to the stratigraphic code, should have supergroup rank. To avoid the confusion inherent in an Animikie Group and an Animikie 

Supergroup, Cannon and Gair (1970) abandoned the term Animikie Series and 

replaced it with the term Marquette Range Supergroup. In this paper, the term 

Animikie Series is used only when used by the author cited; otherwise the term 

Marquette Range Supergroup is used. The term Animikie Group remains in good 

usage in Minnesota and Ontario. 

471-715 0-72-2 

percent of the total thickness of flows on the north 

shore in Minnesota (Grout and others, 1951, p. 1054) 

but are less well represented on the south shore. 

These lavas are exposed around the periphery of 

Lake Superior in Michigan, Wisconsin, Minnesota, 

and in Ontario on the north and east shores. Michipicoten Island and most of Isle Royale are also underlain by middle Keweenawan lava flows. Grout, Sharp, 

and Schwartz (1959) gave a thickness of 25,000 feet 

for the lava flows exposed along the southern half of 

the Minnesota coast, and a greater thickness (White, 

1966b) is suggested by the breadth of outcrop in the 

Keweenawan lavas in northwestern Wisconsin. White 

(1966b, p. 30) summarized by suggesting that parts 

of the section may be repeated in Minnesota and 

Wisconsin, and that thicknesses of 20,000 to 30,000 

feet or more exist in parts of the Lake Superior basin. 

Middle Keweenawan intrusive rocks are abundant 

in the Lake Superior region, especially in Minnesota 

and along the northwest shore. Sills, dikes, and more 

irregular bodies of basaltic rocks intrude the gently 

dipping Keweenawan and older rocks. With the exception of the Duluth Gabbro Complex and the Beaver 

Bay Complex of Grout and Schwartz (1939), these 

bodies are known generally as the Logan intrusions. 

Paleomagnetic pole directions (DuBois, 1962, p. 59) 

indicate that some of these intrusive rocks in the 

Nipigon, Ontario, and Baraga County, Mich., areas 

are probably lower Keweenawan. 

The Duluth Gabbro Complex is a large sill-like mass 

intruded below the middle Keweenawan lava flows 

and is, by far, the largest Keweenawan intrusion in the 

region. It extends northeast into Cook County, Minn., 

where it divides and extends approximately another 

40 miles eastward as two separate sills (Grout and 

others, 1959, p. 40-41). Elsewhere around Lake Superior, Keweenawan dikes occur locally in large numbers, 

and one other large sill-like mass akin to the Duluth 

Gabbro Complex intrudes Keweenawan lava flows near 

Mellen, Wis. 

3. The upper Keweenawan sequence consists largely 

of fine-grained sandstones and shales. On the Keweenawan Peninsula and west into Wisconsin, these sedimentary rocks overlie a thick conglomerate (the Copper 

Harbor Conglomerate) interbedded with a few lava 

flows. Upper Keweenawan rocks are exposed mainly in 

northern Michigan and Wisconsin but are also found 

in Minnesota southwest of Duluth, and on Isle Royale. 

KEWEENAW PENINSULA 

GEOLOGIC SETTING 

On the Upper Peninsula of Michigan the Keweenawan rocks form 'part of the southern limb of the Lake 
6 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

Superior syncline so that at most localities they are 

inclined northwest toward the lake. Dips range from 

near horizontal to near vertical, and generally the lower 

beds dip more steeply than the upper beds. Strike of 

the rocks is generally northeast, but it turns from 

N. 33° E. near Kearsarge to S. 79° E. on Keweenaw 

Point. 

The Portage Lake Lava Series is more than 15,000 

feet thick in the Delaware 7Y2-minute quadrangle, 

Michigan (Cornwall, 1954). This sequence is terminated 

at the base by the Keweenaw fault which has thrust the 

flows over the adjacent Jacobsville Sandstone to the 

south, cutting out an unknown thickness of lava flows. 

The Portage Lake lavas are composed of basalt and 

andesite flows with a few thin interbedded rhyolite 

conglomerates. Some of the flows are fine grained, 

but most flows increase in grain size from both top and 

bottom toward the center. Capping each flow is a 

layer of amygdaloidallava generally 5 to 10 feet thick. 

Conglomerate and sandstone beds within the lava 

series are mostly rhyolitic material. The bulk of the 

clastic particles are less than 6 inches in diameter, and 

boulders more than 1 foot in diameter are uncommon 

(White and others, 1953). The Copper Harbor Conglomerate (Lane, 1911, p. 37-40) comformably overlies 

the Portage Lake Lava Series and consists mainly of 

pebble to boulder conglomerate and lesser amounts of 

sandstone. Lava flows interstratified with the conglomerate are typically fine-grained andesite. 

Intrusive rhyolites are exposed at a number of locations on the Keweenaw Peninsula and are typically 

pale reddish brown on fresh surfaces and lighter on 

weathered surfaces. Most of the rhyolites intrude middle Keweenawan lava flows, but one intrudes rocks at 

least 10,000 feet above the Portage Lake Lava Series. 

These intrusions are believed to be of middle and upper 

Keweenawan age (White and others, 1953). 

DESCRIPTION OF RESULTS 

PORTAGE LAKE LAVA SERIES 

A total of 380 samples from 76 sites were collected 

from flows of the Portage Lake Lava Series for remanent 

magnetization measurements. Of these, 35 samples from 

seven sites were not utilized because of the large 

scatter of directions after partial demagnetization in 

a-c fields as great as 300 Oe. 

Of the remaining 69 sites, 17 are scattered along the 

peninsula northeast of Kearsarge, Mich., and 52 represent a rather detailed sample collection across the lava 

flow sequence in the vicinity of Kearsarge in the 

Ahmeek 7Y2-minute quadrangle, Michigan (White and 

others, 1953). The bulk of the collections around 

Kearsarge represent the central parts of flows, which 

are probably least affected by later metamorphic 

events. Figure 2 is a stratigraphic column (simplified 

from White and others, 1953) indicating the horizons 

at which the samples were collected. 

Remanence data from the Kearsarge section are 

subdivided into three parts as follows: (1) all samples 

below conglomerate No. 14, (2) all samples between 

conglomerate Nos. 14 and 15, and (3) all samples 

above conglomerate No. 15. The separation into three 

parts is based on a significant difference in the direction of magnetization for the samples between conglomerate Nos. 14 and 15 and the directions for samples both below conglomerate No. 14 and above conglomerate No. 15. Results are shown in figures 3-5 

and table 2. 

The paleomagnetic results for the 17 sites in the 

Portage Lake Lava Series northeast of Kearsarge are 

shown in figure 6 and table 3. These flows have an 

average remanence direction that is not significantly 

different from directions of magnetization in the intervals below conglomerate No. 14 and above conglomerate No. 15 near Kearsarge, Mich. 

In order to investigate the possibility of rotation of 

the lava flows around vertical axes during the anticlinal bending in the northern part of the Keweenaw 

Peninsula, samples from the Ash bed Flow were collected 

at points where the strike ranged from N. 38° E. to 

N. 78° E. This component of structural movement 

(rotation about vertical axes) is not diminished by 

geologic-dip corrections, and, if present, it would remain in the data and would be reflected as differences 

in the declination of the paleomagnetic field directions. 

In table 4, a comparison of the variations in strike 

with the variations in paleomagnetic declination indicates no systematic relationships, and hence there is no 

evidence for rotation about vertical axes in this region. 

RHYOLITE FLOWS NORTH OF LAKE GOGEBIC, MICHIGAN 

Thirteen samples from three separate sites represent 

rhyolite lava flows from the top of the lava series along 

the Bergland fire tower road north of Bergland, Mich. 

Paleomagnetic results are given in figure 7 and table 5. 

W. S. White (written commun., 1970) suggested that 

these flows belong to a group that locally overlies the 

Portage Lake Lava Series in this area. Whether this 

group underlies the Copper Harbor Conglomerate or 

grades laterally into it is not yet known. The proximity 

of the mean direction of magnetization for the rhyolite 

flows to those for the Portage Lake Lava Series and the 

Copper Harbor Conglomerate makes paleomagnetic 

measurements of little use in distinguishing this local 

group of flows from the formations above and below it. 
KEWEENAW PENINSULA 

FEET FEET ~PL33 FEET FEET 11,000- Green- f- PL44 stone Flow f-PL34 

Conglomerate No. 15 

1--PL35 

1--PL7 

f--PL4 f-- PL324 

f-PL5 

t-PL6 

1- PL3 Kearsarge 

Flow 

~ ~t~~5 

Conglomerate No.9 

t- PU 

Conglomerate No. 14 I- PL55 

1000- f-PL61 

4000- I- PL56 

~PL150 

I- PL333 7000-

I--PL49 

10,000- f-- PL326 

Iroquois 

Flow I--PL50 

onglomerate No. 17 

f--PL329 Copper f- PL343 'Ash bed PL43 City 

Flow Flow 

Conglomerate No. 13 

Old Colony Sandstone f- PL331 

0 Conglomerate No.6 

f- PL344 

3000-

1Conglomerate No. 16 

6000 

-PL27 Osceola 

9000- Flow I--PL51 

f- PL26 

f- PL25 

f- PL24 1- PL330 

f-- PL327 

f- PL22 f-- PL57 

f- PL52 

I- PL58 

t- PL20 Conglomerate No. 12 

1-PL19 Scales 

Creek 

Flow I- PL59 

f-PL14 I- PL323 2000-

I-PL13 I- PL60 

I-PL12 1- PL53 

f-PL11 5000- I- PL325 

~=~t§O 

f-PL8 

8000- Greenstone Flow I-PL32 

FIGURE 2.-Simplified columnar section for the Portage Lake Lava Series near Kearsarge, Mich., showing stratigraphic 

position of samples studied. Section from White, Cornwall, and Swanson (1953). Labels next to points in this 

figure and succeeding figures refer to sampling sites. 

7 
8 

w 

PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

• Site-mean direction of magnetization. 

North-seeking polarization 

s 

EXPLANATION 

... Mean of site-mean directions of magnetij[ation, 

with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

E 

FIGURE 3.-Directions of remanent magnetization for flows of the Portage Lake Lava Series below conglomerate No. 14 (Houghton 

Conglomerate) near Kearsage, Mich. Data corrected for geologic dip. Equal-area projection, lower hemisphere. See table 2 and 

figure 2 for sample locations. 
w 

Mean of site-mean 

directions 

Figure 5 + 

Figure 3-+-

• 

KEWEENAW PENINSULA 

N 

• 

+ 

s 

EXPLANATION 

.. Site-mean direction of magnetization. Mean of site-mean directions of magnetization, 

North-seeking polarization with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

E 

FIGURE 4.-Directions of remanent magnetization for flows of the Portage Lake Lava Series between conglomerate No. 14 

(Houghton) and conglomerate No. 15 (Allouez) near Kearsarge, Mich. Data corrected for geologic dip. Equal-area projection, 

lower hemisphere. See table 2 and figure 2 for sample locations. 

9 
10 

w 

PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

• Site-mean direction of magnetization. 

North-seeking polarization 

N 

• 

+ 

s 

EXPLANATION 

-+-

Mean of site-mean directions of magnetization, 

with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

FIGURE 5.-Directions of remanent magnetization for flows of the Portage Lake Lava Series above conglomerate No. 15 (Allouez 

Conglomerate) near Kearsarge, Mich. Data corrected for geologic dip. Equal-area projection, lower hemisphere. See table 2 

and figure 2 for sample locations. 
Site No. 

PL150 ____________ 

50 _____________ 

331_ ___________ 

344 ____________ 

51 _____________ 

330 ____________ 

52 _____________ 

323 ____________ 

53 _____________ 

324 ____________ 

335 ____________ 

54 _____________ 

55 _____________ 

56_---- -- - - -- - -

326 ____________ 

327 ____________ 

57 _____________ 

58 _____________ 

59 _____________ 

60 _____________ 

325 ____________ 

61 _____________ 

343 ____________ 

PL35 _____________ 

7 ______________ 

6 ______________ 

5 ____ ----------

4 ______________ 

3 ______________ 

1 ______________ 

PL44 _____________ 

333 ____________ 

KEWEENAW PENINSULA 11 

TABLE 2.-Paleomagnetic results for Portage Lake Lava Series near Kearsarge, Mich. 

[Paleomagnetic data are corrected for geologic dip. Stratigraphic locations of samples are shown in fig. 2] 

Mean direction of magnetization Number of at collecting site Remanent samples intensity Location and attitude of lava flow 

(N) Declination, D Inclination, I (emu/cmax 10-6) 

(degrees) (degrees) 

Samples from below conglomerate No. 14 

4 284.4 33.0 4,110 700ft north, 1,100 ft west of SE. cor. sec. 31, T. 57 N., R. 32 

w. Strike N. 37° E., dip 38.5° NW. 

5 284.2 19.9 900 Iroquois Flow. 2,050 ft south, 2,700 ft west of SE. cor. sec. 6, 

T. 56 N., R. 32 W. Strike N. 35° E., dip 40° NW. 

5 283.3 35.3 1,450 1,950 ft south, 2,350 ft west of NE. cor. sec. 6, T. 56 N., R. 32 

W. Strike N. 38° E., dip 40° NW. 

3 294.7 38.2 450 350ft south, 950ft east of NW. cor. sec. 7, T. 57 N., R. 32 W. 

3 290.9 24.8 480 

Strike N. 38° E., dip 40° NW. 

Osceola Flow. 850 ft south, 100 ft west of NE. cor. sec. 6, T. 

56 N., R. 32 W. Strike N. 40° E. dip 41 o NW. 

5 286.8 21.0 431 725ft north, 2,500 ft west of SE. cor. sec. 6, T. 56 N., R. 32 W. 

Strike N. 33° E., dip 40° NW. 

5 287.4 32.9 1,580 1,300 ft south, 1,700 ft east of NW. cor. sec. 7, T. 56 N., R. 32 

W. Strike N. 32° E., dip 40° NW. 

4 289.6 30.6 235 2,150 ft south, 1,850 ft east of NW. cor. sec. 7. T. 56 N., R. 32 

W. Strike N. 34° E., dip 40° NW. 

5 300.8 24.6 694 1,500 ft south, 2,100 ft east of NW. cor. sec. 7, T. 56 N., R. 32 

W. Strike N. 32° E., dip 40° NW. 

5 293.0 35.0 2,600 1,900 ft south, 2,150 ft west of NE. cor. sec. 7, T. 56 N., R. 32 

w. Strike N. 31 o E., dip 40° NW. 

4 296.1 33.7 249 Kearsarge Flow. 2,250 ft north, 1,900 ft east of SW. cor. sec. 27, T. 57 N., R. 32 W. Strike N. 43° E., dip 38° NW. 

5 285.6 32.0 320 Kearsarge Flow. 1,350 ft south, 1,500 ft west of NE. cor. sec. 7, T. 56 N., R. 32 W. Strike N. 31 o E., dip 41 o NW. 

5 291.1 48.8 148 1,850 ft south, 1,300 ft west of NE. cor. sec. 7, T. 56 N ., R. 

32 w. Strike N. 31 o E., dip 41.5° NW. 

5 285.6 51.5 125 1,800 ft south, 1,350 ft west of NE. cor. sec. 7, T. 56 N., R. 32 

W. Strike N. 31 o E., dip 41.5° NW. 

5 279.8 50.0 197 2,500 ft north, 200ft east of SW. cor. sec. 8, T. 56 N., R. 32 

W. Strike N. 27.5° E., dip 40° NW. 

4 283.8 41.5 132 2,350 ft north, 600ft east of SW. cor. sec. 8, T. 56 N., R. 35 

W. Strike N. 27.5° E., dip 40° NW. 

4 283.6 39.6 720 2,750 ft south, 800 ft east of NW. cor. sec. 8, T. 56 N., R. 32 

W. Strike N. 26° E., dip 45° NW. 

4 290.9 30.2 3,200 2,760 ft south, 800 ft east of NW. cor. sec. 8, T. 56 N., R. 32 

W. Strike N. 26° E., dip 45° NW. 

4 290.3 32.5 1,280 Scales Creek Flow. 2,600 ft south, 1,200 ft e~st of NW. cor. sec. 8, T. 56 N., R. 32 W. Strike N. 27° E., dip 46° NW. 

4 288.5 40.0 353 2,750 ft south, 1,300 ft east of NW. cor. sec. 8, T. 56 N., R. 

32 W. Strike N. 27° E., dip 46° NW. 

5 293.2 33.6 307 1,300 ft south, 100ft west of NE. cor. sec. 7, T. 56 N., R. 32 

W. Strike N. 25° E., dip 44 o NW. 

4 294.2 31.7 114 3,450 ft south, 2,350 ft east of NW. cor. sec. 8, T. 56 N., R. 

32 W. Strike N. 22° E., dip 49° NW. 

5 285.7 37.1 2,030 Copper City Flow. 250ft south, 2,350 ft east of NW. cor. sec. 17, T. 56 N., R. 32 W. Strike N. 17.5° E., dip 52° NW. 

Samples from between conglomerate Nos. 14 and 15 

5 264.9 45.6 554 2,150 ft north, 1,175 ft west of SE. cor. sec. 31, T. 57 N., R. 32 

W. Strike N. 38° E., dip 38° NW. 

4 294.6 45.2 747 1,350 ft north, 1,850 ft west of SE. cor. sec. 31, T. 57 N., R. 32 

W. Strike N. 37.5° E., dip 38° NW. 

4 274.7 45.2 117 2,000 ft north, 1,150 ft west of SE. cor. sec. 31, T. 57 N., R. 32 

W. Strike N. 38° E., dip 37.5° NW. 

5 280.8 49.7 330 2,100 ft north, 1,200 ft west of SE. cor. sec. 31, T. 57 N., R. 32 

W. Strike N. 38° E., dip 37.5° NW. 

6 261.0 43.2 107 750ft north, 2,350 ft west of SE. cor. sec. 31, T. 57 N., R. 32 

W. Strike N. 37° E., dip 38° NW. 

5 279.5 36.6 1,020 700ft north, 2,200 ft west of SE. cor. sec. 31, T. 57 N., R. 32 

W. Strike N. 37° E., dip 38° NW. 

5 265.6 51.6 2,200 0 ft north, 2,200 ft west of SE. cor. sec. 31, T. 57 N., R. 32 W. 

Strike N. 37° E., dip 38.5° NW. 

Samples from above conglomerate No. 15 

3 287.9 29.6 1,800 Section line, 1,250 ft south of NW. cor. sec. 1, T. 56 N., R. 33 

W. Strike N. 37° E., dip 34.5° NW. 

5 281.8 28.8 2,380 2,450 ft north, 900 ft west of SE. cor. sec. 36, T. 57 N ., R. 33 

W. Strike N. 34° E., dip 33.5° NW. 
12 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE 2.-Paleomagnetic results for Portage Lake Lava Series near Kearsarge, M ich.-Continued 

Mean direction of magnetization Number of at collecting site Remanent Site No. samples intensity Location and attitude of lava flow 

(N) Declination, D Inclination, I (emu/cmax IQ-6) 

(degrees) (degrees) 

Samples from above conglomerate No. 15--Continued 

PL49 _____________ 5 290.1 27.5 

329 ____________ 3 286.1 24.4 

43 _____________ 4 293.7 38.1 

27 _____________ 4 282.8 41.5 

26 _____________ 5 285.9 33.6 

25 _____________ 5 279.3 49.7 

24 _____________ 4 280.2 35.7 

22 _____________ 6 291.4 43.6 

20 _____________ 5 295.3 40.2 

19 _____________ 3 299.0 31.7 

14 _____________ 5 294.7 32.0 

13 _____________ 5 296.9 28.2 

12 _____________ 5 297.8 33.6 

11 _____________ 5 293.6 30.3 

10 _____________ 5 293.6 33.7 

9 ______________ 4 297.6 40.8 

8 ______________ 5 297.1 45.5 

32 _____________ 5 296.4 32.0 

33 _____________ 5 291.0 37.2 

34 _____________ 4 293.7 39.1 

QUARTZ PORPHYRY NORTH OF LAKE GOGEBIC, 

MICHIGAN 

1,720 

188 

1,410 

2,640 

1,450 

298 

2,260 

802 

880 

1,150 

713 

634 

1,300 

826 

423 

837 

526 

431 

3,330 

767 

Thirteen samples from three sites represent the 

quartz porphyry outcrops along the Bergland-White 

Pine road about 4 miles north of Lake Gogebic, Mich. 

This geologic unit lies between typical middle Keweenawan lava flows and is intruded by a basalt dike in this 

area. Dip of the lower contact is approximately 35° 

NW. (Brooks and Garbutt, 1969), and data in figure 

8 and table 6 have been corrected for this dip. 

RHYOLITE INTRUSION IN SEC. 4, T. 56 N., R. 32 W. 

Paleomagnetic data for eight samples are shown in 

figure 9 and table 7. The samples are from a poorly 

exposed rhyolite body that intrudes the Portage Lake 

Lava Series east of Kearsarge, Mich. 

3,350 ft north, 2,200 ft west of SE. cor. sec. 36, T. 56 N., R· 

33 W. Strike N. 35° E., dip 35° NW. 

Ashbed Flow. 1,400 ft north, 2,250 ft west of SE. cor. sec. 20, 

T. 57 N., R. 32 W. Strike N. 39° E., dip 27.5° NW. 

Ashbed Flow. 2,050 ft north, 450ft west of SE. cor. sec. 36, T. 

57 N., R. 33 W. Strike N. 38° E., dip 36° NW. 

2, 750ft north, 2,950 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 W. Strike N. 37° E., dip 35.5° NW. 

2, 900ft north, 3,350 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 W. Strike N. 37° E. dip 36° NW. 

2,650 ft north, 3,300 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 W. Strike N. 37° E., dip 36.5° NW. 

2,550 ft north, 3,250 ft west of SE. cor. sec. 31, T. 57 N ., R. 

32 w. Strike N. 37° E., dip 36.5° NW. 

2, 200 ft north, 3,200 ft west of SE. cor. sec. 31, T. 57 N ., R. 

32 W. Strike N. 37.5° E., dip 37° NW. 

2,150 ft north, 3,000 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 W. Strike N. 37.5° E., dip 37° NW. 

1,950 ft north, 2,975 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 W. Strike N. 37.5° E., dip 37° NW. 

1,275 ft north, 2,950 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 W. Strike N. 37° E., dip 37.5° NW. 

1,250 ft north, 2,900 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 w. Strike N. 38° E., dip 37.5° NW. 

1,125 ft north, 2,875 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 w. Strike N. 37.5° E., dip 37.5° NW. 

1,110 ft north, 2,850 ft west of SE. cor. sec. 31, T. 57 N ., R. 

32 w. Strike N. 38° E., dip 38° NW. 

1,100 ft north, 2,825 ft west of SE. cor. sec. 31, T. 57 N ., R. 

32W. Strike N. 38° E., dip 38° NW. 

1,200 ft north, 2,800 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 w. Strike N. 37.5° E., dip 38° NW. 

1,050 ft north, 2,800 ft west of SE. cor. sec. 31, T. 57 N., R. 

32 w. Strike N. 38° E., dip 38° NW. 

Greenstone Flow. 600ft north, 3,700 ft west of SE. cor. sec. 29, 

T. 57 N., R. 32 W. Strike N. 46° E., dip 35.5° NW. 

500 ft north, 3,650 ft west of SE. cor. sec. 29, T. 57 N ., R. 32 

w. Strike N. 46° E., dip 35.5° NW. 

350ft north, 3,600 ft west of SE. cor. sec. 29, T. 57 N., R. 32 

w. Strike N. 46° E., dip 35.5° NW. 

RHYOLITE INTRUSION IN SECS. 24 AND 25, 

T. 56 N., R. 34 W. 

Paleomagnetic data for six samples from one site are 

shown in figure 10 and table 8. These samples are from 

a rhyolite body that intrudes the Freda Sandstone in 

sees. 24 and 25, T. 56 N., R. 34 W. southwest of 

Kearsarge, Mich. No other intrusions are known to 

cut rocks so high in the stratigraphic column. Dip of 

the Freda in this area is 22° NW., and figure 10 and 

table 8 show the direction of magnetization corrected 

for this dip. 

IRONWOOD-MELLEN AREA 

GEOLOGIC SETTING 

Formations of the Keweenaw Peninsula in Michigan 

can be traced southwest toward the Wisconsin border. 
• 338 

340 

• 

IRONWOOD-MELLON AREA 

N 

.348 

.347 

339 • 

• .154 .349 

+ Ji..57 

s 

EXPLANATION 

• Site-mean direction of magnetization. 

North-seeking polarization 

... Site-mean direction of magnetization for 

locations in the anomalous conglomerate Nos 14-15 interval. North seeking polarization 

Mean of site-mean directions • of magnetization, 

with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

13 

E 

FIGURE 6.-Directions of remanent magnetization for miscellaneous flows of the Portage Lake Lava Series northeast of the Kearsarge, Mich., area. Data corrected for geologic dip. Equal-area projection, lower hemisphere. See table 3 for sample locations, 

471-715 0- 72- 3 
14 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE 3.-Paleomagnetic results for miscellaneous flows of the Portage Lake Lava Series northeast of Kearsarge, Mich. 

[Paleomagnetic data are corrected for geologic dip] 

Mean direction of magnetization Number of at collecting site Remanent Site No. samples 

Declination, D intensity Location and attitude of lava flow 

(N) Inclination, I (emu/cma X 10-6) 

(degrees) (degrees) 

PL340 ____________ 5 284.8 27.9 197 Ashbed Flow. 650ft north, 2,700 ft west of SE. cor. sec. 8, T. 

338 ____________ 4 286.9 19.7 2,240 

339 ____________ 4 294.6 29.8 1,810 

342 ____________ 5 284.5 36.8 2,370 

349 ____________ 4 274.6 49.1 154 

348 ___ !_ ________ 4 315.6 33.6 2,980 

347 ____________ 4 301.0 33.4 752 

346 ____________ 4 288.7 30.9 338 

345 ____ -------- 5 281.2 38.3 308 

158 1----- -- - - - - 5 283.0 38.9 639 

160 1 ___________ 4 283.0 44.3 293 

159 1

----- - ----- 4 284.3 48.8 498 

157 1 _____ - ----- 5 254.7 40.4 2,660 

34L ___________ 5 287.9 34.9 138 

154 ____________ 5 274.2 33.1 256 

153 ____________ 6 286.2 41.0 115 

42 _____________ 4 326.7 19.5 376 

1 Site is in the anomalous interval between conglomerate Nos. 14 and 15. 

TABLE 4.-Comparison of declination and strike along the Ashbed 

Flow in the Portage Lake Lava Series at sites in the northern part 

of the Keweenaw Peninsula 

[Site locations are given in tables 2 and 3] 

Site No. 

PL43 ______________ _ 329 _____________ _ 338 _____________ _ 339 _____________ _ 340 _____________ _ 342 _____________ _ 

Strike 

N. 38° E. 

N. 39° E. 

N. 61° E. 

N. 70° E. 

N. 76° E. 

N. 78° E. 

Declination, D 

(degrees) 

293.7 

286.1 

286.9 

294.6 

284.8 

284.5 

58 N., R. 30 W. Strike N. 76° E., dip 26° NW. 

Ashbed Flow. 100ft south, 2,750 ft west of NE. cor. sec. 30, T. 

58 N., R. 31 W. Strike N. 61 o E., dip 28° NW. 

Ashbed Flow. 1,700 ft south, 950ft east of NW. cor. sec. 21, T. 

58 N., R. 31 W. Strike N. 70° E., dip 27.5° NW. 

Ashbed Flow. 600ft south, 250ft east of NW. cor. sec. 12, T. 

58 N., R. 29 W. Strike N. 77.5° E., dip 22° NW. 

Greenstone Flow. 1,500 ft south, 100 ft west of NE. cor. sec. 30, T. 58 N., R. 31 W. Strike N. 77° E., dip 24° NW. 

Greenstone Flow. 600ft south, 4,150 ft west of NE. cor. sec. 16, T. 58 N., R. 30 W. Strike N. 77o E., dip 24° NW. 

Greenstone Flow. 600 ft south, 2,500 ft west of NE. cor sec. 16, T. 58 N., R. 30 W. Strike N. 77° E., dip 24° NW. 

Greenstone Flow. 1,000 ft south,~ 000 ft west of NE. cor. sec. 16, T. 58 N., R. 30 W. Strike . 77° E., dip 24° NW. 

Greenstone Flow. 2,100 ft south, 160 ft west of NE. cor. sec. 16, T. 58 N., R. 30 W. Strike N. 77o E., dip 24° NW. 

2,250 ft north, 550ft west of SE. cor. sec. 16, T. 58 N., R. 30 

W. Strike N. 79° E., dip 24° NW. 

2,150 ft north, 650ft west of SE. cor. sec. 16, T. 58 N., R. 30 

W. Strike N. 79° E., dip 25° NW. 

2,000 ft north, 500ft west of SE. cor. sec. 16, T. 58 N., R. 30 

W. Strike N. 79° E., dip 25° NW. 

1,350 ft north, 300ft east of SW. cor. sec. 15, T. 58 N., R. 30 

W. Strike N. 79° E., dip 25° NW. 

Scales Creek Flow. 1,100 ft south, 400ft west of NE. cor. sec. 25, T. 58 N., R. 30 W. Strike N. 74.5° E., dip 42° NW. 

2,400 ft north, 2,400 ft east of SW. cor. sec. 30, T. 58 N., R. 

29 W. Strike N. 70° E., dip 56° NW. 

800ft north, 1,950 ft east of SW. cor. sec. 30, T. 58 N., R. 29 

W. Strike N. 71.5° E., dip 59° NW. 

1,000 ft south, 2,050 ft east of NW. cor. sec. 32, T. 58 N., R. 

29 W. Strike N. 79° E., dip 63° N. 

basalts, but they are lithologically distinct (Hubbard, 

1968, p. 35) from the middle Keweenawan basalts 

farther north. In the Marenisco, Mich., area the South 

Trap Range basalts lie unconformably upon the 

Animikie (Van Hise and Leith, 1911, p. 234; Fritts, 

1965) and near Ironwood they dip 60° to 85° northward. 

Near Lake Gogebic the flat-lying Jacobsville Sandstone 

overlaps the South Trap Range lavas onto the Animikie 

(Van Hise and Leith, 1911; Fritts, 1965) and again 

farther east the South Trap Range lavas are overlapped and buried by the Jacobsville Sandstone. 

Westward from the Ironwood area the South Trap 

Range lavas dip steeply northward and appear to be 

conformable on (Marquette Range Supergroup) rocks 

of the Gogebic Iron Range. According to Aldrich (1929, 

p. 108-109) proof of the unconformity in this area 

rests entirely upon broad field relations; however, 

suggestions of an unconformable relationship have been 

observed (Van Hise and Leith, 1911, p. 234; Aldrich, 

1929, p. 108-109) west of Mellen, Wis. 

In the Ironwood, Mich., area they are joined by lavas 

of the so-called South Trap Range; these are Keweenawan volcanic rocks distinct from those of the main 

outcrop belt of the Portage Lake Lava Series that can 

be traced eastward past the south end of Lake Gogebic, 

Mich. Two isolated exposures, one at Silver Mountain 

in Michigan (fig. 1) and one 3 miles southeast at 

Sturgeon Falls, are 25 miles northeast of the most 

easterly exposure of the main belt of South Trap Range 

rocks. Lavas of the South Trap Range are mostly 

At Mellen and westward a gabbro-granophyre complex has been intruded along zones of weakness in 
IRONWOOD-MELLON AREA 

Mean of site -mean directions of magnetization 

for Copper Harbor Conglomerate, Portage 

Lake lava flows of figure 5, and Portage Lake 

lava flows of figure 3 

• 

• 

+ 

s 

EXPLANATION 

.. 

Site-mean direction of magnetization. Mean of site-mean directions of magnetization, 

North-seeking polarization with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

15 

FIGURE 7.-Directions of remanent magnetization for middle Keweenawan rhyolite flows north of Lake Gogebic, Mich. Data 

corrected for geologic dip. Equal-area projection, lower hemisphere. See table 5 for site locations. Data for Copper Harbor 

Conglomerate from DuBois (1962). 
16 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE 5.-Paleomagnetic results for middle Keweenawan rhyolite flows north of Lake Gogebic, Mich. 

[Sites are in T. 49 N., R. 42 W. Data are corrected for geologic dip] 

Mean direction of magnetization Number of at collecting site Remanent 

Site No. samples intensity Location and attitude of lava flow 

(N) Declination, D 

(degrees) Inclination, I (emu/cm3X lQ-6) 

(degrees) 

ER66 ______ ---- _- 5 281.1 24.8 44.6 

67 ____________ 4 290.3 32.7 15.8 

68- - - - - - - - -- - - 4 290.4 26.4 24.5 

middle Keweenawan lava flows. Leighton (1954, p. 406) 

has shown that layering in the gabbro dips about 20° 

less than that in the enclosing lavas. His interpretation 

of the relation suggests the flows were inclined before 

intrusion of the gabbro and that further tilting occurred 

after intrusion. Paleomagnetic evidence (Books and 

others, 1966, p. D123) supports this interpretation. 

DESCRIPTION OF RESULTS 

LOWERMOST KEWEENAWAN LAVA FLOWS 

In the Ironwood area, the South Trap Range rocks 

crop out in two parallel east-west ridges that have a 

total width of about 1.6 miles; these ridges are separated from the main belt of the Portage Lake Lava 

Series to the north by an area of low relief 2 miles wide 

in which the nature of the bedrock is not known. 

Northeast of Ironwood the lowest South Trap Range 

flows are underlain by quartzite in sec. 12, T. 47 N., 

R. 47W. 

Samples were collected at various sites across the 

South Trap Range outcrop belt and represent about 

8,000 feet of lava flows. Results representing the upper 

6,700 feet of flows have already been reported (Books, 

1968). Those data show south-seeking polarization 

similar to that found in the upper part of the lower 

Keweenawan Sibley Series of the Nipigon basin by 

DuBois (1962, p. 42-45). Results representing the 

lowermost 400 feet of lava flows in the Ironwood area 

as well as the basal quartzite northeast of Ironwood are 

presented in figure 11 and table 9. These data show a 

magnetization that is northseeking down. 

PORTAGE LAKE LAVA SERIES 

Portage Lake lavas north of Ironwood trend approximately N. 80° E., across the Black River. Collections were made from four sites in the quarry at 

Chippewa Hill, Mich., in sec. 32, T. 49 N., R. 46 W., 

and from five sites 1 mile north near Algonquin Falls 

on the Black River. Figures 12 and 13 and tables 10 

and 11 show the results. 

1,000 ft south, 1,050 ft west of NE. cor. sec. 5. Strike N. 

64 o E., dip 23° NW. 

1,100 ft south, 1,100 ft west of NE. cor. sec. 5. Strike N. 

64° E., dip 23° NW. 

2,000 ft north-northeast of Bergland fire tower. 800 ft north, 

0 ft west of SE. cor. sec. 32. Strike N. 65° E., dip 23° 

NW. 

NORTH SHORE OF LAKE SUPERIOR 

GEOLOGIC SETTING 

The Puckwunge Formation of sandstone, quartz 

conglomerate, and quartzite forms the base of the 

Keweenawan Series in Minnesota, whereas to the 

northeast, in the Thunder Bay district, Ontario, the 

lower Keweenawan sedimentary rocks are represented 

by the Sibley Series. In both areas the sedimentary 

rocks are overlain by thick sequences of volcanic rocks, 

known in Minnesota as the North Shore Volcanic 

Group (Goldich and others, 1961) and in the Thunder 

Bay district, as the Osler Series. On Isle Royale, 

Mich., the middle Keweenawan lavas are overlain by 

upper Keweenawan clastic sedimentary rocks. 

These Keweenawan rocks form the northwest flank of 

the Lake Superior syncline and dip toward the lake at 

shallow angles that rarely exceed 20°. In general, the 

traces of individual stratigraphic units form a broad 

arc that is almost parallel to the shoreline, but near 

Duluth and Grand Portage, Minn., the units curve 

lakeward and intersect the shoreline at sharp angles. 

Grout, Sharp, and Schwartz (1959, p. 37-38) have 

made a detailed study of the volcanic succession between Grand Portage and Tofte, Minn. A total of 94 

flows were found in this interval-the lowest is at 

Grand Portage and the highest, at Tofte. Total estimated thickness along the shoreline (less interflow sedimentary rocks, numerous diabase intrusions, and the 

Duluth Gabbro at Hovland, Minn.) is 17,500 feet. 

Thickness of the complete section could be as much as 

29,000 feet. 

DESCRIPTION OF RESULTS 

NORTH SHORE VOLCANIC GROUP 

Sample collections were made from 52 of the total of 

94 flows (Grout and others, 1959, p. 37-38) found 

between Grand Portage and Tofte, Minn. The collection 

sites are on or close to the shoreline and extend from 

flow No.2 to flow No. 94 of Grout, Sharp, and Schwartz 

(1959). Paleomagnetic results for 11 site locations on 
• 

NORTH SHORE OF LAKE SUPERIOR 

N 

• 

---1-

s 

EXPLANATION 

-+- Site-mean direction of magnetization. Mean of site-mean directions of magnetization, 

North-seeking polarization with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

17 

E 

FIGURE 8.-Directions of remanent magnetization for quartz porphyry north of Lake Gogebic, Mich. Data corrected for geologic 

dip. Equal-area projection, lower hemisphere. See table 6 for site locations. 
18 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE 6.-Paleomagnetic results for quartz porphyry north of Lake 

Gogebic, Mich. 

[Sites are in sec. 17, T. 49 N., R. 42 W. Quartz porphyry strikes N. 68° E. and 

dips 35° NW. Data are corrected for geologic dip] 

Mean direction of 

Number magnetization at Remanent of collecting site intensity Site No. samples (emu/cma Location 

(N) Declina- lnclina- X10-&) 

tion, D tion,J 

(degrees) (degrees) 

QP69 ________ 4 310.5 25.0 11.7 3,200 ft south, 600 ft east of NW. cor. 71_ _______ 4 289.1 20.9 7.2 3,700 ft south, 750ft east of NW. cor. 73 ________ 5 305.8 44.8 13.7 se~~n }isw~·g2r~ ft 

the lowermost 20 flows (representing 4,550 feet of 

strata) include a reversal of the paleomagnetic field 

direction and have been described by Books (1968). 

Paleomagnetic data for the remaining overlying flows, 

less five site locations where the data were too scattered 

to utilize, are shown in figure 14 and table 12. 

ISLE ROYALE LA VA FLOWS 

Nine site locations, represented by 44 samples, are 

in the vicinity of Lane's (1911, pl. 1) Greenstone Flow 

in the Washington Harbor area of Isle Royale. Resulting 

directions of magnetization fall into two distinct groups 

as shown in figure 15. Site locations are given in table 13. 

DISCUSSION OF RESULTS 

LOWER KEWEENAWAN 

The Sibley Series of Tanton (1927) is lower Keweenawan according to the generally accepted classification 

of Keweenawan rocks (Leith and others, 1935). DuBois 

(1962, p. 42-45) collected at several different locations 

on the Sibley Peninsula and on the Black Sturgeon 

River (fig. 1). He cited (DuBois, 1962, p. 44-45) two 

reliable groupings that have opposing magnetic field 

directions. The group with the north pole on the lower 

hemisphere (fig. 16, point JI) was collected not far 

above the Animikie rocks on the Sibley Peninsula. The 

group with the south pole on the lower hemisphere 

(fig. 16, point JII) was collected from the upper part 

of the Sibley Series (DuBois, 1962, p. 44). He also indicated that the Logan sills of Lawson (1893) (fig. 16, 

point F), the Alona Bay lavas (fig. 16, point G), and 

the Baraga County dikes (Graham, 1953) (fig. 16, 

point H) have the south pole on the lower hemisphere, 

and he included them in the lower Keweenawan along 

with the Sibley Series. 

Palmer (1970) collected at various sites on the north 

shore of Lake Superior. He reported that the Logan 

sills in Ontario, the section of volcanic rocks at Alona 

Bay, Ontario, and the Osler Series near Thunder Bay, 

Ontario, have a reversed polarity; the Michipicoten 

Island volcanics have a positive polarity; and the North 

Shore Volcanic Group of Minnesota, the lavas at 

Gargantua Point, Ontario, and the section at Mamainse 

Point, Ontario, have both normal and reversed polarity 

sequences. 

Books (1968) reported on 14 sites in the South Trap 

Range (fig. 16, point C) near Ironwood, Mich., and on 

11 sites in the lowermost flows (fig. 16, point E) near 

Grand Portage, Minn. Beck and Lindsley (1969, p. 

2011) reported on seven sites in the northern tongue of 

the Duluth Gabbro (fig. 16, point 11) extension into 

Cook County, Minn. All these sites have paleomagnetic 

field directions that are south-seeking below the horizontal. 

In addition, the author has collected samples from 

the lower 400 feet of lava flows near Ironwood, Mich., 

and from sedimentary units below the basal Keweenawan flows; these samples represent new data for 

which results are shown separately in figure 11. From a 

summary of the new paleomagnetic data and previously 

published data (fig. 16), it is apparent that the directions of magnetization fall into two groups with opposing directions of magnetization. The two north-seeking 

magnetization directions (fig. 16, group A) are representative of rocks that lie above and are apparently 

structurally conformable with units believed to be 

of Marquette Range Supergroup age. At Ironwood, 

this direction (fig. 16, point 10) includes the basal 

quartzite and overlying lower 400 feet of basal Keweenawan lava flows. On the Sibley Peninsula, Ontario, this 

direction includes DuBois' (1962, fig. 20) Group I, 

replotted on an equal-area projection (fig. 16, point JI). 

The south-seeking magnetization directions (fig. 16, 

group B) are representative of rock units at many locations around Lake Superior. At Ironwood, this direction of magnetization (fig. 16, point C) appears in 

volcanic rocks that are stratigraphically above and 

are conformable with volcanic rocks (fig. 16, point 10) 

lower down in the same section that have the nearly 

opposite direction of magnetization. On the Sibley 

Peninsula and in the Nipigon basin, the south-seeking 

directions of magnetization include DuBois' (1962, fig. 

20) Group II, replotted on an equal-area projection 

(fig. 16, point JII). This direction probably represents 

rocks in a higher stratigraphic position than the Sibley 

Series rocks represented at point JI because most 

samples for this data were colleCted from the red 

dolomite unit on the Black Sturgeon River (DuBois, 

1962, table XII). Wilson (1910, p. 67-69) believed 

that these rocks represent the upper part of the Sibley 

in the Nipigon basin. , 

Evidently, then, similarities exist between magnetic 

field directions and between magnetic field direction 

changes in the Ironwood, Mich., and Sibley Peninsula, 
w 

• 

DISCUSSION OF RESULTS 

N 

s 

EXPLANATION 

Direction of magnetization for individual samples Site-mean direction of • magnetization, with circle 

(site R36). North-seeking polarization of confidence. North-seeking polarization 

Direction of axial • dipole component 

of present geomagnetic field 

19 

E 

FIGURE 9.-Directions of remanent magnetization for samples from the rhyolite intrusion in sec. 4, T. 56 N., R. 32 W., east of 

Kearsarge, Mich. Data uncorrected for geologic dip. Equal-area projection, lower hemisphere. See table 7 for sample locations. 
20 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE 7 .-Paleomagnetic results for rhyolite intrusion in sec. 4, 

T. 56 N., R. 32 W., east of Kearsarge, Mich. 

Sample No. 

R36-o _________ ~ _ 1 __________ _ 

2 __________ _ 3 __________ _ 4 __________ _ 

5 __________ _ 6 __________ _ 

7-----------

[Data are uncorrected for geologic dip] 

Direction of magnetization at collecting site 

Declination, D 

(degrees) 

241.8 

278.3 

251.5 

233.1 

276.5 

285.5 

246.2 

239.9 

Inclination, I 

(degrees) 

60.0 

70.3 

32.2 

54.0 

11.2 

45.5 

2.2 

36.0 

Remanent 

intensity 

(emu/cma 

X10-6) 

3.51 

4.20 

3.74 

3.29 

4.21 

3.64 

4.33 

3.62 

Location from southeast corner of section 

870 ft north, 2,540 ft west. 870 ft north, 2,530 ft west. 800 ft north, 2,450 ft west. 880 ft north, 2,500 ft west. 850 ft north, 2,500 ft west. 890 ft north, 2,570 ft west. 860 ft north, 2,510 ft west. 860 ft north, 2,520 ft west. 

Ontario, areas. The paleomagnetic sequence of events 

during lower(?) Keweenawan time for these two areas 

as well as for other lower(?) Keweenawan locations 

throughout the Lake Superior region appears to be as 

follows: 

1. First, the earth's magnetic field had a northseeking polarization during the time interval in which 

the sedimentary and igneous rock units represented in 

figure 16, group A, were formed. In the Ironwood, 

Mich., area the upper limit of this interval can be 

located rather closely because the polarity change 

occurs somewhere between 400 feet and 1,200 feet 

above the base of the Keweenawan flows. The lower 

limit of this interval falls somewhere below the lowest 

Keweenawan rocks of the region. Figure 17 and table 

14 show paleomagnetic results for one site from a 

pre-Keweenawan (Animikie) Marquette Range Supergroup sedimentary rock unit east of Ironwood, Mich., 

that not only appears to conformably underlie nearby 

basal Keweenawan rocks, but also has a similar direction of magnetization. Thus, the time interval represented by the direction of magnetization in the basal 

Keweenawan quartzite and lower 400 feet of flows in 

the Ironwood area may have its lower limit in rocks 

classified by Irving and Van Hise (1892), Van Hise and 

Leith (1911), Allen and Barrett (1915), and Atwater 

(1938) as of Animikie age. Such a possibility is supported by similar directions of magnetization for one 

site from the Rove Slate of the Animikie Group in the 

Grand Portage area, Minnesota (fig. 17, table 14). 

An alternative explanation for the similar directions 

in the lowermost Keweenawan rocks and the Marquette 

Range Supergroup sedimentary rocks would require 

that the original magnetization directions were subsequently changed to those presently found. This interpretation does not seem impossible for the Ironwood 

area, because the basal Keweenawan there has been 

sufficiently metamorphosed to produce actinolite. However, this interpretation does not explain the near 

reversal in magnetic directions that. begins some 1,200 

feet above the basal Keweenawan flows in the Ironwood 

area (fig. 16, point C); the later, middle Keweenawan 

magnetic field directions, which are significantly different (fig. 17, Portage Lake Lava Series and North 

Shore Volcanic Group); and the similar direction 

found in the Rove Formation (fig. 17) near Grand 

Portage, Minn. As the reversed field directions for 

units above the basal flows-both near Ironwood and 

near Grand Portage-are believed to be original magnetization (Books, 1968, p. 251-253), any change in 

original magnetic directions for the basal flows must 

have occurred before the overlying reversely magnetized units were formed. In addition, if any phenomenon 

had effected such a magnetic direction change in the 

basal Keweenawan flows and Marquette Range Supergroup rocks it must have been sufficiently widespread 

to include the Ironwood and Grand Portage areas. 

2. Second, following a magnetic reversal, the earth's 

magnetic field in the Lake Superior region had a southseeking polarization during the time interval in which 

the sedimentary and igneous rock units represented 

in figure 16, group B, were formed. This time interval 

was of sufficient length for some 6, 700 feet of lava 

flows to be extruded in the Ironwood, Mich., area as 

well as for the formation of various other sedimentary 

and igneous rock units around the periphery of Lake 

Superior. 

The northern tongue of the Duluth Gabbro extension 

(fig. 16, point 11) in northern Cook County, Minn., 

is included in this time interval, indicating that this 

gabbro complex must have begun to form very early 

in the history of volcanism in the Lake Superior region. 

In addition, Palmer's work (1970) on the north and 

east shores of Lake Superior indicates that the Osler 

Series near Thunder Bay, some of the lavas at Gargantua Point, and some of the lavas at Mamainse 

Point, Ontario, may also belong in this same general 

time interval, as these units, in part, have a southseeking polarization. 

MIDDLE KEWEENAWAN 

The middle Keweenawan is represented by the 

Portage Lake Lava Series of the Upper Peninsula of 

Michigan, their equivalents to the southwest in Wisconsin, the major exposures of lava flows in Minnesota 

along the northwest shore of Lake Superior from Grand 

Portage to Tofte, and the lava flows on Isle Royale; 

also included are the major intrusions in the region, 

most of th~ Duluth Gabbro Complex, the Beaver Bay 

Complex, the Logan intrusions, and various rhyolitic 

and granitic rocks. Although the Copper Harbor Conglomerate has been arbitrarily classed as the lowermost formation of the upper Keweenawan (White and 

others, 1953), it is included with the middle Keweenawan formations because, as DuBois has pointed out 

(1962, p. 61-62), there is little change in the direction 

of magnetization between the Copper Harbor and the 
w 

DISCUSSION OF RESULTS 

N 

.2 

4 

0 

.6 

.5 

• 

• 

+ 

s 

EXPLANATION 

Direction of magnetization tor individual samples Site-mean direction of • magnetization, with circle 

(site R62). North-seeking polarization of confidence. North-seeking polarization 

Direction of axial • dipole component 

of present geomagnetic field 

21 

E 

FIGURE 10.-Directions of remanent magnetization for samples from the rhyolite intrusion in sees. 24 and 25, T. 56 N., R. 34 W., 

southwest of Kearsarge, Mich. Data are corrected for dip of the Freda Sandstone, which the rhyolite intrudes here. Equal-area 

projection, lower hemisphere. See table 8 for sample locations. 
22 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE B.-Paleomagnetic results for rhyolite intrusion in sees. 

24 and 25, T. 56 N., R. 34 W., southwest of Kearsarge, Mich. 

[Data corrected for dip of Freda Sandstone, which the rhyolite intrudes in this area. Freda Sandstone strikes N. 25° E. and dips 22° NW] 

Direction of 

magnetization at Remanent collecting site intensity Location from SW. 

Sample No. (emu/cma cor. sec. 24 

Declina- Inclina- X10-&) 

tion, D tion, I 

(degrees) (degrees) 

R62-L _________ 2 __________ 296.1 15.1 696 2,550 ft north, 3,350 ft east. 

3 __________ 305.3 18.5 700 2,600 ft north, 3,330 ft east. 

4 __________ 297.8 27.9 608 2,600 ft north, 3,300 ft east. 

5 __________ 301.6 18.0 455 2,650 ft north, 3,260 ft east. 

6 __________ 283.9 33.5 538 2, 700 ft north, 3,200 ft east. 285.5 18.2 705 2,800 ft north, 3,100 ft east. 

underlying Portage Lake Lava Series, but there is a 

large change at the conglomerate's boundary with 

the overlying Nonesuch Shale. 

The data presented herein complete determination of 

directions of magnetization for most of the larger 

middle Keweenawan geologic units in this region. 

Previous paleomagnetic data are from DuBois (1962, 

table XVIII) for the Portage Lake Lava Series, the 

Duluth Gabbro, and the Copper Harbor Conglomerate; 

from Beck and Lindsley (1969, table 1) for the Beaver 

Bay Complex; from Beck (1970, table 2) for the 

Duluth Gabbro Complex; from Palmer (1970, tables 

3-6) for the North Shore Volcanic Group and lava 

flows on Gargantua Point, Michipicoten Island, and 

Mamainse Point; and from Books, White, and Beck 

(1966, p. D120) for the gabbro near Mellen, Wis. The 

new data include directions of magnetization for the 

Portage Lake Lava Series and equivalents north of 

Ironwood, Mich., the North Shore Volcanic Group, 

the lava flows on Isle Royale, and the quartz porphyry 

north of Lake Gogebic, Mich. The results are shown in 

figure 18. 

PORTAGE LAKE AND NORTH SHORE SEQUENCES 

As noted in a previous section, the data for the 

Portage Lake Lava Series were divided into three 

parts, mainly because the middle division, between 

conglomerate Nos. 14 and 15, had a direction of magnetization significantly different from directions in the 

flows below and above. No further subdivisions in 

magnetic field direction data for Portage Lake lavas 

are shown because none are obvious. Similarly, no 

subdivisions in paleomagnetic data are shown for the 

North Shore Volcanic Group as no significant shift in 

the direction of the earth's magnetic field is discernible 

in these rocks. Figures 19 and 20 represent an attempt 

to correlate changes in the paleomagnetic field between 

the Portage Lake Lava Series and the North Shore 

Volcanic Group by comparing chronological movements in inclination and declination of the magnetic 

field for the two areas. Although the scatter in directions between sites may be attributed to secular variations, no obvious paleomagnetic patterns or correlations between the two sequences are found. If the flows 

in the two areas are of equivalent ages, either (1) the 

sampling was not sufficiently detailed to document 

equivalent variations in the earth's paleomagnetic field 

on both sides of the lake, or (2) an insufficient number 

of lava flows of equivalent extrusion times are present 

to provide similar paleomagnetic patterns in both areas. 

If the two groups of lavas do not both represent 

the same time interval but overlap in part, the best 

fit for the paleomagnetic data would make the Scales 

Creek Flow (figs. 2, 19, and 20) of the Portage Lake 

Lava Series similar in age to a flow near flow No. 65 

(figs. 19 and 20) of the North Shore Volcanic Group. 

Where equivalent groups of lava flows are present at 

some distance from each other there is some indication that magnetic field direction similarities can be 

found. Figure 4 shows that the flows between conglomerate Nos. 14 and 15 of the Portage Lake Lava 

Series near Kearsarge, Mich., had a direction of magnetization significantly different from that which prevailed before (fig. 3) and after (fig. 5) their extrusion. 

The same departure in magnetization direction can be 

found in rocks representing the same stratigraphic 

interval in the Washington Harbor area of Isle Royale; 

this supports the generally accepted correlation first 

noted by Lane (1898). In figure 21, point 4B represents 

lava flows on Isle Royale that would be stratigraphically 

just below the probable position of the Greenstone Flow 

as recent mapping has shown that flow to be slightly 

south of where it is shown on Lane's map (N. K. Huber, 

written commun., 1969). Point 4A (fig. 21) represents 

the next lower group of lava flows. The site-mean directions of the comparable flows near Kearsarge, Mich., 

presented in figures 4 and 3, respectively, are plotted 

as points 6 and 7 in figure 21 for comparison. The 

flows above the aberrant interval near Kearsarge, 

Mich., represented by figure 5 and by point 5 in figure 

21, are covered by water in the sampling area on Isle 

Royale. 

QUARTZ PORPHYRY NORTH OF LAKE GOGEBIC, 

MICHIGAN 

A large body of quartz porphyry within the lava 

series north of Lake Gogebic, Mich. (fig. 1), has long 

been regarded as an intrusion, but Brooks and Garbutt 

(1969) have recently presented evidence that the body 

is extrusive and separated from overlying mafic lava 

flows by an unconformity. Figure 22 shows the mean 

direction for this quartz porphyry before (A) and 

after (A") a 35° correction for dip of the underlying 

lava flows .. The proximity of A" to the average direction for the Portage Lake Lava Series and Copper 
w 18306 

• 

1875 

• 

18301 

• 

• 18303 

• 

DISCUSSION OF RESULTS 

N 

• 

+ 

s 

EXPLANATION 

Site-mean direction of magnetization. Mean of site-mean directions • of magnetization, 

North-seeking polarization with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

23 

E 

FIGURE 11.-Directions of remanent magnetization for the basal quartzite (site IB322) and lowermost 400 feet of South Trap Range 

lava flows near Ironwood, Mich. Data corrected for geologic dip. Equal-area projection, lower hemisphere. See table 9 for site 

locations. 
24 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE 9.-Paleomagnetic results for the basal quartzite and lowermost 400 feet of the South Trap Range flows near Ironwood, Mich. 

[All sites are in T. 47 N., R. 47 W. Data corrected for geologic dip] 

Mean direction of magnetization 

Number of at collecting site Remanent Site No. samples intensity Location and attitude 

(N) Declination, D Inclination, I (emu/cm3X 10-6) 

(degrees) (degrees) 

IB74 _____________ 6 260.8 29.3 1,150 600ft north, 2,150 ft west of SE. cor. sec. 12. Strike N. 78° 

E., dip 69° NW. 75 _____________ 4 257.6 21.7 10,400 650ft north, 2,150 ft west of SE. cor. sec. 12. Strike N. 78° 

E., dip 69° NW. 76 _____________ 6 258.1 44.4 5,640 750ft north, 2,150 ft west of SE. cor. sec. 12. Strike N. 78° 

E., dip 69° NW. 301 ____________ 5 274.7 53.8 1,420 1,800 ft south, 2,000 ft west of NE. cor. sec. 10. Strike N. 

82° E., dip 65° NW. 303 ____________ 6 258.2 52.6 6,460 1,850 ft south, 2,050 ft west of NE. cor. sec. 10. Strike N. 

82° E., dip 65° NW. 306 ____________ 4 268.1 17.9 550 700ft north, 2,200 ft west of SE. cor. sec. 12. Strike N. 78° 

E., dip 69° NW. 

322 1

- - - - - - - - - - - 5 261.3 38.1 904 590ft north, 2,150 ft west of SE. cor. sec. 12. Strike N. 78° 

E., dip 69° NW. 

1 Site IB322 represents the basal quartzite. 

Harbor Conglomerate is consistent with the interpretation of Brooks and Garbutt (1969), but not inconsistent with intrusion during Portage Lake or Copper 

Harbor time before much tilting had occurred. The 

paleomagnetic data are not, however, consistent with 

the Rb-Sr (rubidium-strontium) age of 978±40 ;m.y. 

obtained by Chaudhuri and Faure (1967, p. 1020); if 

the quartz porphyry were younger than the Nonesuch 

Shale (see following section), as this Rb-Sr age suggests, the direction of magnetization should be closer 

to that of the Freda Sandstone and Nonesuch Shale. 

(See fig. 22, point F-N.) 

UPPER KEWEENAWAN 

Immediately above the Copper Harbor Conglomerate 

are the fine-grained siltstones of the upper Keweenawan Nonesuch Shale, which in turn grade upward 

into the Freda Sandstone; 5,000 feet of the Freda is 

exposed on the Keweenaw Peninsula (Lane, 1911, p. 

40, 604), and 12,000 feet of Freda is estimated in Wisconsin (Tyler and others, 1940, p. 1479). DuBois (1962, 

table XVIII) calculated a magnetic field direction for 

the Freda-Nonesuch units (see fig. 22, point F-N), as 

well as for some of the succeeding sedimentary rock 

formations. 

RHYOLITE INTRUSIONS 

The two rhyolite intrusions whose magnetic field 

directions are shown in figure 23 are included in the 

upper Keweenawan more on the basis of geologic information than on precise paleomagnetic data, which 

does not disagree with the geologic interpretation but 

adds little to it. 

The rhyolite intrusion in sees. 24 and 25, T. 56 N., 

R. 34 W., near Kearsarge, Mich., cuts the Freda Sandstone, which is the youngest formation known to be 

intruded by igneous rocks in this part of the Lake 

Superior region. Dip of the Freda near the intrusion is 

estimated to be about 22° NW., and the paleomagnetic 

data have been corrected for this dip. The divergence 

of point a (fig. 23) from the upper Keweenawan magnetic field represented by the Freda direction (point 

F-N) can be explained by secular variation because 

the samples collected (6) were of limited extent. 

Placement of the rhyolite intrusion in sec. 4, T. 56 

N., R. 32 W., near Kearsarge, Mich., in the upper 

Keweenawan on the basis of paleomagnetic data is 

somewhat tenuous and requires a more detailed explanation. The rhyolite intrudes middle Keweenawan 

lava flows whose present dip is near 62° NW., so that 

in the strictest sense, the paleomagnetic data indicate 

only that the time of intrusion could have ranged from 

after structural movement was completed (no dip correction at all-point b, fig. 23) to before structural 

deformation of the enclosing flows (dip correction of 

62° NW.-point c, fig. 23). The geologic evidence, 

however, suggests that the rhyolite is upper Keweenawan. White (1968, p. 313) stated that "5° to 10° of 

the total present dip of the top of the Portage Lake 

Lava Series and about 20° of the dip of the bottom 

antedates the deposition of the Nonesuch Shale." 

White believes (written commun., 1970) that the rocks 

which now enclose the rhyolite intrusion were dipping 

15° to 20° during early Freda time. If these dips were 

subtracted from the present 62° dip of the enclosing 

flows, the paleomagnetic data would plot at points d 

and e and would indicate a possible range for the direction of magnetization at the time of intrusion. Note in 

figure 23 that points d and e are not significantly 

different from the Freda-Nonesuch unit direction at 

point F-N. 
w 

M K217 ___.,.,_ Portage Lake lava flows MK21®_,.- (t;g"" 5) 

MK216 e MK219 

• 

DISCUSSION OF RESULTS 

• 

+ 

s 

EXPLANATION 

+ Site-mean direction of magnetization. Mean of site-mean directions of magnetization, 

North-seeking polarization with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

25 

E 

FIGURE 12.-Directions of remanent magnetization for middle Keweenawan lava flows (Portage Lake equivalents) from the 

Chippewa Hill quarry, Michigan, in sec. 32, T. 49 N., R. 46 W. Data corrected for geologic dip. Equal-area projection, lower 

hemisphere. See table 10 for site locations. 
26 

w 

PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

Portage Lake lava flows 

(figure 5) 

• MK316 

• 

Site-mean direction of magnetization. 

North-seeking polarization 

Direction of axial • dipole component 

of present. geomagnetic field 

N 

• 

+ 

s 

EXPLANATION 

Mean of site-mean directions • of magnetization, 

with circle of confidence. North-seeking 

polarization 

... Direction of magnetization for rhyolite 

flows north of Lake Gogebic (figure 

7) which may be stratigraphically 

similar 

FIGURE 13.-Directions of remanent magnetization for middle Keweenawan lava flows (Portage Lake equivalents) on the Black 

River near Algonquin Falls, Mich. Data corrected for geologic dip. Equal-area projection, lower hemisphere. See table 11 for 

site locations. 
SUMMARY AND CONCLUSIONS 27 

TABLE 10.-Paleomagnetic results for Portage Lake lava equivalents 

at the Chippewa Hill quarry north of Ironwood, Mich. 

[All sites are in sec. 32, T. 49 N., R. 46 W. Flows strike N. 85° E. and dip 40° NW. at all sites. Data are corrected for geologic dip] 

Mean direction 

Number of magnetization at Remanent of c:ollecting site intensity Location from 

Site No. samples (emu/cma northwest corner (N) Declina- Inclina- X10-6) of section 

tion, D tion, I 

(degrees) (degrees) 

MK216 ______ 5 28:~.6 30.9 698 2,600 ft south, 1,300 

ft east. 217 ______ 5 288.0 28.1 825 2,620 ft south, 1,320 

ft east. 218 ______ 5 2813.3 29.4 586 2,640 ft south, 1,320 

ft east. 219 ______ 5 27~9.4 35.0 732 2,680 ft south, 1,350 

ft east. 

TABLE 11.-PaleomagneUc results for Portage Lake lava equivalents 

on the Black R-iver near Algonquin Falls, Mich. 

[All sites are in sec. 29, T. 49 N., R. 46 W. Lava flows strike N. 80° E. and dip 30° 

NW. at all siteE:. Data are corrected for geologic dip] 

Mean direction 

Number of magnetization at Remanent of collecting site intensity Location from 

Site No. samples (emu/cma northeast corner (N) Declina- Inclina- Xl0- 8) of section 

tion, D tion, I 

(degrees) (degrees) 

MK316 ______ 5 28'7.8 39.5 836 1,200 ft south, 1,200 

ft west. 317 ______ 4 929.9 28.5 1,110 1,500 ft south, 900 

ft west. 318 ______ 4 282.0 32.6 1,130 1,750 ft south, 850 

ft west. 319 ______ 4 275.0 29.1 305 2,200 ft south, 1,100 

ft west. 320 ______ 4 278.4 31.9 471 2,200 ft south, 1,300 

ft west. 

SUMMARY AND CONCLUSIONS 

The directions of magnetization in all the Keweenawan units sampled by the author (table 15) and others 

fall into three signifieantly different groups. The groups 

have polarities that are alternately normal, reversed, 

and normal. 

In the oldest norrnal polarity group are sedimentary 

and igneous rocks in the Ironwood, Mich., area and 

some of the lower sedimentary rocks of the Sibley 

Series on Sibley Peninsula in Ontario. In the Ironwood 

area, this group includes the basal quartzite unit and 

approximately 400 feet of immediately overlying lava 

flows in the lowermost exposures of South Trap Range 

rocks. 

In the reversed polarity group are some 6, 700 feet 

of lava flows near Ironwood, Mich., some 4,550 feet of 

lava flows near Grand Portage, Minn., the northern 

tongue of the Duluth Gabbro in Cook County, Minn., 

part of the Sibley Series, part of the lava flows at 

Gargantua Point and Mamainse Point, Ontario, the 

lavas at Alo~a Bay, Ontario, part of the Logan intrusions, the Baraga. County dikes, and the Keweenawan Osler Series on the north shore of Lake Superior. 

In the youngest normal polarity group are intrusive 

and extrusive rocks usually considered middle Keweenawan in age including the Portage Lake Lava Series 

and equivalents to the southwest in Wisconsin, most of 

the North Shore Volcanic Group, part of the Logan 

intru~ions, the major part of the Duluth and similar 

gabbros, and many lesser sills and dikes cutting middle 

Keweenawan and earlier rocks. Also included in this 

group according to DuBois' (1962, p. 61) results is the 

upper Keweenawan Copper Harbor Conglomerate. 

The paleomagnetic data indicate that many different 

rock units in the Lake Superior region may have 

formed within the same magnetic epoch as did the 

sedimentary Sibley Series (Tanton, 1931) in the 

Thunder Bay district, Ontario. Tanton (1931, p. 56, 

58) found evidence for pre-Osier Series lavas in the 

red volcanic debris that makes up the greater part of 

the Sibley Series and the red porphyry pebbles in the 

basal conglomerate of the Osler Series of Ontario. 

DuBois (1962) related the Logan sills in Ontario and 

the lavas at Alona Bay, Ontario, to the lower Keweenawan and suggested a period of intrusive and extrusive 

activity in the Lake Superior region in early Keweenawan time. 

Thus, major igneous activity probably began very 

early in Keweenawan history and volcanism and sedimentation probably were concurrent in the Lake 

Superior region. While the basal quartzite and succeeding lava flows were being formed in the Ironwood area, 

the early Sibley sediments were being deposited in the 

Sibley Peninsula area. While the Sibley continued to be 

deposited during the reversal of the earth's magnetic 

field, volcanism was dominant elsewhere. This reversepolarization interval probably marked the initial 

stages of formation of the Duluth Gabbro Complex as 

well as the Logan intrusions; intrusion took place in 

such widely separated areas as Baraga County, Mich., 

and Nipigon, Ontario. Widespread extrusion at this 

time is represented by some 6,700 feet of lava flows 

near Ironwood and by lesser amounts across Lake 

Superior near Grand Portage, Minn., Alona Bay, 

Ontario, and Gargantua Point and Mamainse Point, 

Ontario. 

By middle Keweenawan time when the earth's 

magnetic field was again normal, the igneous activity 

had almost replaced sedimentation, resulting in the 

bulk of the flows and intrusions found in the Lake 

Superior region today. During this normal-polarization interval some 20,000 to 30,000 feet of lava flows 

poured out, and the Duluth Gabbro Complex and 

major parts of the Logan intrusions were emplaced. 

Sedimentation again became dominant in Copper 

Harbor time; only minor igneous activity occurred 

thereafter. 

A tentative correlation chart based on magnetic 

polarity changes and including all available paleomag-
28 

w 

PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

226 .169 

• 376 ,171 . 

377 ;J37• 369 227 229 

170 362 255J', 380 • ~-·2~ 263 

256.382 254 381 • 

:::a 72. 3;: -;;; 371 265 • 

269 • 

.. 261 370 •• 367 

260. • 

379 

• 

N 

• 

+ 

s 

EXPLANATION 

-+- Site-mean directiol"! o.f magnetization. Mean of site-mean directions of magnetization, 

North-seeking polarization with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

FIGURE 14.-Directions of remanent magnetization for North Shore Volcanic Group from flow No. 33 to flow No. 94 off.G;rout, 

Sharp, and Schwartz (1959), between Grand Portage and Tofte, Minn. Data corrected for geologic dip. Equal-area projection, 

lower hemisphere. See table 12 for site locations. 
SUMMARY AND CONCLUSIONS 29 

TABLE 12.-Paleomagnetic results for North Shore Volcanic Group, between Grand Portage and Tofte, Minn. 

[Flow numbers are from Grout and others (1959). Data are corrected for geologic dip] 

Mean direction of magnetization Number of at collecting site Remanent Site No. samples intensity Flow No. Location and attitude of flow 

(N) Declination, D Inclination, I (emujcm3Xl0-6) 

(degrees) (degrees) 

NS254 ____________ <.:• 

... 291.1 46.4 103 94 2,800 ft south, 850 ft east of NW. cor. sec. 28, T. 

59 N., R. 4 W. Strike N. 43° E., dip 14° SE. 255 ____________ 4: 293.6 47.8 629 91? 500 ft south, 3,300 ft east of NW. cor. sec. 28, T. 

59 N., R. 4 W. Strike N. 41 o E., dip 14° SE. 256 ____________ ~· d 286.6 32.4 800 89 1,650 ft south, 2,850 ft east of NW. cor. sec. 22, 

T. 59 N., R. 4 W. Strike N. 55° E., dip 12° SE. 263 ____________ 4, 298.3 52.1 2,810 85? 650 ft south, 50 ft west of NE. cor. sec. 14, T. 59 

N., R.4W. Strike N. 69°, dip 17° SE. 38L ___________ E• 296.0 51.1 226 82? 750 ft south, 50 ft west of NE. cor. sec. 31, T. 59 

N.,R.4W. Strike N. 59° E., dip 12° SE. 370 ____________ ~· 

... 285.0 54.2 2,890 81? 2,250 ft south, 1,350 ft west of NE. cor. sec. 12, T. 

59 N., R. 4 W. Strike N. 59° E., dip 18° SE. 382 ____________ 3 287.8 37.4 401 80 1,000 ft south, 100 ft west of NE. cor. sec. 31, T. 

59 N., R. 4 W. Strike N. 52° E., dip 12° SE. 372 ____________ 4: 288.8 43.3 236 80 1,850 ft south, 1,600 ft west of NE. cor. sec. 12, T. 

59 N ., R. 4 W. Strike N. 59° E., dip 18° SE. 379 ____________ 8 279.2 51.9 381 79? 400 ft south, 700 ft west of NE. cor. sec. 31, T. 59, 

N.,R.4W. Strike N. 52° E., dip 12° SE. 371 ____________ 4: 285.2 41.6 1,600 79 2,100 ft south, 1,400 ft west of NE. cor. sec. 12. T. 

59 N., R. 4 W. Strike N. 59° E., dip 18° SE. 380 ____________ 3; 299.4 57.3 566 78? 10 ft south, 856 ft west of NE. cor. sec. 31, T. 59 

N.,R.4W. Strike N. 52° E., dip 12° SE. 262 ____________ E· 286.6 40.1 985 78 150 ft south, 1,300 ft west of NE. cor. sec. 34, T. 

60 N., R. 3 W. Strike N. 61 o E., dip 15° SE. 261_ ___________ ~· 

... 282.4 43.4 939 76 2,000 ft south, 1,050 ft west of NE. cor. sec. 19, T. 

260 ____________ 60 N., R. 2 W. Strike N. 76° E., dip 10° SE. 

E· 279.0 44.8 649 74 4,650 ft south, 1,750 ft west of NE. cor. sec. 17, T. 

60 N., R. 2 W. Strike N. 75° E., dip 10° SE. 259 ____________ ~· 

... 292.8 52.8 437 72 3,700 ft south, 250ft west of NE. cor. sec. 17, T. 

60 N., R. 2 W. Strike N. 65° E., dip 10° SE. 258 ____________ ~· d 289.1 45.7 10,300 71 2,600 ft south, 1,350 ft east of NW. cor. sec. 16, T. 

60 N., R. 2 W. Strike N. 69° E., dip 10° SE. 264 ____________ 4: 289.0 43.9 2,450 68 800 ft south, 2,250 ft east of NW. cor. sec. 11, T. 

60N., R.1 W. Strike N. 63° E., dip 10° SE. 257 ____________ ~· 292.4 43.6 985 67 1,600 ft south, 2,050 ft east of NW. cor. sec. 6, T. ·' 60 N., R.1 W. Strike N. 70° E., dip 12° SE. 377 ____________ ~· ,) 291.8 32.1 3,230 64 4,850 ft south, 1,050 ft east of NW. cor. sec. 32, T. 

61 N., R. 1 W. StrikeS. 87° E., dip 13° SW. 171 ____________ 4 297.4 37.5 1,220 64 450 ft north, 1,950 ft east of SW. cor. sec. 32, T. 

61 N., R.1 W. Strike S. 85° E., dip 12.5° SW. 170 ____________ I' ,) 291.1 34.1 1,420 63 450 ft north, 2,000 ft east of SW. cor. sec. 32, T. 

61 N., R.1 W. StrikeS. 85° E., dip 13° SW. 169 ____________ 4 301.3 34.7 2,480 63 500 ft north, 2,000 ft east of SW. cor. sec. 32, T. 

61, N., R. 1 W. StrikeS. 85° E., dip 13° SW. 376 ____________ I' ,) 296.9 36.4 3,480 62 4,150 ft south, 3,050 ft east of NW. cor. sec. 32, T. 

61 N., R. 1 W. StrikeS. 80° E., dip 13° SW. 374 ____________ I" ,) 294.3 35.2 772 60 3,300 ft south, 2,500 ft west of NE. cor. sec. 33, T. 

369 ____________ 61 N., R. 1 W. StrikeS. 88° E., dip 14° SW. I" ,) 294.0 38.5 558 60 3,150 ft south, 1,500 ft west of NE. cor. sec. 33, T. 

368 ____________ 61 N., R. 1 W. Strike N. 87° E., dip 14° SE. 

·:I: 291.7 34.9 627 59 2,000 ft south, 900ft east of NW. cor. sec. 34, T. 

61 N., R. 1 W. Strike N. 76° E., dip 15° SE. 265 ____________ a 286.0 47.2 2,150 58 600ft north, 2,700 ft east of SW. cor. sec. 27, T. 

61 N ., R. 1 E. Strike S. 77° E., dip 11 o SW. 367 ____________ 5 285.5 56.0 244 58? 3,800 ft south, 100 ft east of NW. cor. sec. 26, T. 

61 N., R. 1 W. Strike N. 76° E., dip 15° SE. 365 ____________ 4: 285.2 38.7 237 57? 3,000 ft south, 1,200 ft east of NW. cor. sec. 16. T. 

362 ____________ 61 N ., R. 1 E. Strike N. 87° E., dip 14 o SE. 

5 292.9 43.5 2,620 54 2,400 ft south, 2,000 ft east of NW. cor. sec. 13, T. 

61 N., R. 1 E. StrikeS. 61 o E., dip 11 o SW. 226 ____________ 3 295.5 25.7 2,330 50 4,250 ft south, 2,850 ft west of NE. cor. sec. 9, T. 

61 N., R. 2 E. Strike N. 86° E., dip 10° SE. 375 ____________ 5 287.9 53.8 407 49 4,800 ft south, 1,100 ft west of NE. cor. sec. 3, T. 

229 ____________ 61 N., R. 2 E. Strike N. 84° E., dip 10° SE. 

3 298.4 46.9 534 47 2,300 ft south, 1,900 ft west of NE. cor. sec. 1, T. 

227 ____________ 61 N ., R. 2 E. Strike S. 71 o E., dip 10° SW. 

5 296.1 43.6 3,660 45 1,300 ft south, 250 ft west of NE. cor. sec. 6, T. 

61 N., R. 3 E. StrikeS. 77° E., dip 11 o SW. 378 ____________ 5 290.6 47.9 356 42 4,000 ft south, 2,450 ft west of NE. cor. sec. 32, T. 

62 N., R. 3 E. Strike N. 90° E., dip 10° S. 269 ____________ 3 264.5 38.2 257 33 1,950 ft north, 50 ft west of SE. cor. sec. 28, T. 

62 N., R. 3 E. StrikeS. 87° E., dip 20° SW. 
30 

w 

PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

MK25GMKA~ 

tMK248 

• MK 249 

• 

MK246 

• 

Site-mean direction of magnetization. 

North-seeking polarization 

N 

• 

+ 

s 

EXPLANATION 

.. 

Mean of site-mean directions of magnetization, 

with circle of confidence. North-seeking 

polarization 

Direction of axial • dipole component 

of present geomagnetic field 

E 

FIGURE 15.-Directions of remanent magnetization for middle Keweenawan lava flows on Isle Royale showing two significantly 

different groupings (A and B) for adjacent series of flows (see fig. 20 for comparison with the same stratigraphic position in 

the Portage Lake Lava Series). Data corrected for geologic dip. Equal-area projection, lower hemisphere. See table 13 for site 

locations. 
SUMMARY AND CONCLUSIONS 

TABLE 13.-Paleomagnetic results for middle Keweenawan lavas on Isle Royale, Mich. 

[All sites are located in T. 63 N., R. 39 W. Flows strike N. 60° E. and dip 12.5° SE. at all sites. Paleomagnetic data are corrected for the geologic dip] 

Mean direction of magnetization 

Number of at collecting site Remanent Site No. samples intensity Location 

(N) Declination, D Inclination, I (emu/cm3X10-6) 

(degrees) (degrees) 

~1{243 ___________ 4 264.4 47.3 2,080 3,100 ft south, 2,600 ft west of NE cor. sec. 1. 244 ___________ ;· 

,) 274.1 46.4 7,320 3,100 ft south, 3,000 ft west of NE. cor. sec. 1. 

24 5----- - - - - - - ,. 

,) 272.9 49.3 8,450 3,200 ft south, 3,350 ft west of NE. cor. sec. 1. 246 ___________ 4 290.4 67.1 2,240 3,250 ft south, 3,800 ft west of NE. cor. sec. 1. 247 ___________ ;· 

,) 263.7 56.7 4,520 3,300 ft south, 4,200 ft west of NE. cor. sec. 1. 

248 ___ -------- r ,) 285.2 25.2 1, 710 2,700 ft south, 150ft west of NE. cor. sec. 1. 

249------ ----- ;· ,) 281.9 24.7 3,970 2,400 ft south, 250 ft west of NE. cor. sec. 2. 250 ___________ r ,) 289.2 14.1 863 1,200 ft south, 2,400 ft west of NE. cor. sec. 1. 25L __________ f) 292.9 29.5 3,240 Section line, 1,700 ft west of NE. cor. sec. 1. 

TABLE 14.--Paleomagnetic results for pre-Keweenawan rocks near Ironwood, Mich., and Grand Portage, Minn. 

[Data are corrected for geologic dip] 

Direction of magnetization 

Sample No. at collecting site Remanent 

intensity Location and attitude of bedding 

Declination, D Inclination, I (emu/cmaX10-6) 

(degrees) (degrees) 

31 

~arquette Range Super~:roup rocks near Ironwood, ~ich.: 1{3111 ______________________________ _ 

2-------------------------------

264.0 

267.1 

260.2 

266.3 

46.5 

48.1 

57.2 

40.7 

14.5 

2.64 

12.1 

In the Black River. SE%'sec. 12, T. 47 N., 

R. 46 W. Strike N. 85° E., dip 55° NW. 4 ______________________________ _ 6 ______________________________ _ 

Rove Slate near Grand Portage, ~inn.: 1{1661 ______________________________ _ 2 ______________________________ _ 3 ______________________________ _ 

4 ______________________________ _ 5 ______________________________ _ 

222.9 

246.0 

187.4 

264.9 

266.2 

60.1 

66.0 

54.1 

52.0 

40.9 

1.28 

52.0 

37.0 

41.0 

47.0 

44.0 

Roadcut on U.S. Highway 61. Center sec. 34, 

T. 64 N., R. 6 E. Strike S. 66° W., dip 

20° SE. 

TABLE 15.-Summary of new paleomagnetic data for rock units in the Lake Superior region 

[N is the number of sites for which Fisher (1953) analyses have been computed, unless otherwise noted. Magnetization directions are corrected for geologic dip, unless otherwise noted. Om and Op are semiaxes of the confidence oval about a virtual pole and are respectively perpendicular and parallel to the virtual paleomeridian] 

Mean of site-mean 

Locadon of rock unit Number directions of magnetization Precision Radius of Pole position 

Rock unit of Om op 

North lat West long sites Declination, 

(degre·es) (degrees) (N) D 

(degrees) 

l_ ------------- 47 .:~ 88.6 16 295.0 2 ______________ 47 .:~ 88.4 18 2 275.1 o ______________ 46.G 89.6 3 298.6 4 ______________ 46.'7 90.1 5 287.7 

5_- ------------ 46.G 90.1 4 284.2 6 ______________ 46.'7 89.6 3 287.2 7 ______________ 47.:3 88.4 22 291.0 8 _______________ 47.:3 88.4 7 274.5 9 ______________ 47.:3 88.4 23 289.3 10 _____________ 47A 88.1 17 288.4 11 _____________ 47 .!~ 89.2 5 271.8 12 _____________ 47 .!~ 89.2 4 287.3 13 _____________ 47.'7 90.4 36 291.2 14 _____________ 46.6 90.1 7 262.5 15 _____________ 46.5 90.0 14 261.2 16 _____________ 48.0 89.7 15 240.9 

1 Indicates number of sampl4ls rather than number of sites. 2 Data uncorrected for geolo1;tic dip. 

1. Rhyolite intrusion in sec. 24, T. 56 N., R. 34 W., near Kearsarge, Mich. 

2. Rhyolite intrusion in sec. 4, T. 56 N., R. 32 W., near Kearsarge, Mich. 

3. Quartz porphyry north of Lake Gogebic, Mich. 

4. Portage Lake lavas near A.gonquin Falls, Mich. 

5. Portage Lake lava equivalHnts from the Chippewa Hill quarry, Mich. 6. Rhyolite flows north of Lake Gogebic, Mich. 

parameter confidence 

Inclination, (K) circle North lat West long 

I (a96) (degrees) (degrees) 

(degrees) 

22.2 58.1 8.9 25.4 189.3 9.4 5.0 2 23.2 8.8 19.8 12.3 173.8 21.1 11.2 

36.0 25.4 24.9 33.9 186.0 29.0 16.8 

26.2 51.5 10.8 22.1 183.2 11.7 6.3 

30.9 332.7 5.0 21.7 178.4 5.6 3.1 

28.0 164.3 9.7 22.5 181.5 10.6 5.8 

34.9 105.5 3.1 28.2 180.0 3.6 2.1 

45.8 72.9 7.1 22.6 162.1 9.1 5.8 

35.1 77.1 3.5 27.1 178.6 4.0 2.3 

36.4 27.2 7.0 27.1 177.1 8.1 4.7 

53.8 60.8 9.9 25.9 155.7 13.8 9.7 

23.4 106.7 8.9 20.6 183.7 9.5 5.1 

43.9 46.2 3.6 32.6 176.9 4.4 2.8 

37.0 28.8 11.4 9.8 160.4 13.5 7.8 

45.0 88.4 9.8 13.3 155.3 12.4 7.9 

58.5 14.8 20.6 12.6 133.6 32.2 20.6 

11. Upper lavas on Isle Royale, Mich. 

12. Lower lavas on Isle Royale, Mich. 

13. North Shore lava flows (Nos. 33 to 94) between Grand Portage and Tofte, 

Minn. 

14. Basal quartzite and lower 400 feet of South Trap Range lavas near Ironwood, 

Mich. 7. Portage Lake lavas above eonglomerate No. 15 near Kearsarge, Mich. 

8. Portage Lake lavas between conglomerate Nos. 14 and 15 near Kearsarge, Mich. 

9. Portage Lake lavas below eonglomerate No. 14 near Kearsarge, Mich. 

15. Marquette Range Supergroup sedimentary rocks in sec. 12, T. 47 N., R. 46 W., near Ironwood, Mich. 

16. Rove Slate in sec. 34, T. 64 N., R. 6 E., near Grand Portage, Minn. 

10. Portage Lake lavas northeast of Kearsarge, Mich. 
32 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

N 

E 

s 

EXPLANATION 

Mean direction of magnetization, • with circle 

of confidence. North-seeking polarization 

FIGURE 16.-Summary of mean directions of remanent magnetization for lower(?) Keweenawan igneous rock units 

in the Lake Superior area, showing two groups (A and B) 

with opposing magnetization. The rock units are: 10, 

basal quartzite and lower South Trap Range lava flows 

near Ironwood, Mich.; 11, northern tongue of Duluth 

Gabbro extension in Cook County, Minn.; C, lower sequence of Keweenawan lava flows at Ironwood, Mich.; 

E, lower sequence of Keweenawan lava flows at Grand 

Portage, Minn.; F, Logan sills of Lawson (1893); G, 

lavas at Alona Bay, Ontario; H, mafic dikes in Baraga 

Mean direction of magnetization, with 

circle of confidence. South-seeking 

polarization 

County, Mich., circle of confidence not shown; and J (I 

and II), the Sibley Series of Tanton (1927). The data 

for point 10, the quartzite and succeeding flows, are new 

(from fig. 11); data for point 11 are from Beck and Lindsley 

(1969, table 3); data for points F, G, H, JI, and JII are 

from DuBois (1962, table 18); data for C and E are from 

Books (1968, fig. 3); and data for points 12, 13, and 14 

are from Palmer (1970, tables 2, 4, and 6). Data are corrected for geologic dip. Equal-area projection, lower 

hemisphere. 
SUMMARY AND CONCLUSIONS 33 

w 

Portage Lake Lava Series 

@@North ShO<o Vol"olo Gm"p 

( Lower 400' of lava 

flows at Ironwood. 

\ • Rove Slate near 

Grand Portage 

N 

+ 

S· 

EXPLANATION 

• Site-mean direction of magnetization, with circle 

of confidence. North-seeking polarization 

FIGURE 17.-Directions of remanent magnetization for preKeweenawan units near Ironwood, Mich., and Grand 

Portage, Mi~n., compared to directions of magnetization 

in lower Keweenawan units. Rock units are: Rove Slate 

of the Animikie Group near Grand Portage, Minn.; 

Marquette Ran)~e Supergroup sedimentary unit near 

Ironwood, Mich.; and lower 400 feet of lava flows near 

Mean of site-mean directions • of magnetization, 

with circle of confidence. North-seeking 

polarization 

Ironwood, Mich. Directions of remanent magnetization 

for the North Shore Volcanic Group and Portage Lake 

Lava Series are also shown. Data corrected for geologic dip. 

Equal-area projection, lower hemisphere. Site locations for 

Rove Slate and Marquette Range Supergroup unit are 

given in table 14. 
34 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

N 

w + E 

s 

EXPLANATION 

Mean of site-mean directions • of magnetization, with circle 

of confidence. North-seeking polarization 

FIGURE 18.-Summary of mean directions of remanent magnetization for middle Keweenawan rock units in the Lake 

Superior region. The units are: 1, lava flow sequence near 

Algonquin Falls, Mich; 2, lava flow sequence in the 

Chippewa Hill quarry, Michigan; 3, rhyolite flows north 

of Lake Gogebic, Mich.; 4A and 4B, lava flow sequences 

on Isle Royale; 5, lava flow sequence above conglomerate 

No. 15 of Portage Lake Lava Series at Kearsarge, Mich.; 

6, lava flows between conglomerate Nos. 14 and 15 at 

Kearsarge, Mich.; 7, lava flow sequence below conglomerate No. 14 at Kearsarge, Mich.; 8, North Shore Volcanic 

Group; and 9, quartz porphyry north of Lake Gogebic, 

Mich. Data corrected for geologic dip. Equal-area projection, lower hemisphere. 
SUMMARY AND CONCLUSIONS 

10 20 30 40 Copper Harbor Conglomerate FEET11,000· 

10,000 

8000 

Greenstone 

Flow 

Conglomerate I No. 15 

ConglomeratE. 

No. 14 

7000 

Kearsarge 

Flow 

Scales 

Creel< 

Flow 

2000 

1000 

CoppE!r 

CityFlow 

() 

Keweenaw fault 

PORTAGE LAKE LAVA SERIES 

50 60 

INCLINATION, IN DEGREES 

Underwater 10 

FEET 

11.000 

20 

70 

andstone 

and 

shale 

Diabase and gabbro 

30 40 50 

NORTH SHORE VOLCANIC GROUP 

60 

FIGURE 19.-Comparison of chronological movements in inclination of the paleomagnetic fields of the 

Portage Lake Lava Series and North Shore Volcanic Group. Bars indicate confidence circle radii for a 

Fisher :analysis (Fisher, 1953) of no less than three samples from a single site. Data corrected for geologic 

dip. Numbers within column are lava flows of Grout, Sharp, and Schwartz (1959). 

35 
36 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

8000 

Greenstone 

Flow 

Conglomerate 

No. 15 

Conglomerate 

No. 14 

7000 

250 260 270 280 290 

er Harbor Conglomerate 

Keweenaw fault 

DECLINATION, IN DEGREES 

300 310 Underwater 260 270 

FEET 

11,000 

9000 

3000 

2000 

1000 

0 

Diabase and gabbro 

280 290 300 310 

PORTAGE LAKE LAVA SERIES NORTH SHORE VOLCANIC GROUP 

FIGURE 20.-Comparison of chronological movements in declination of the paleomagnetic fields of the Portage Lake 

Lava Series and North Shore Volcanic Group. Bars indicate confidence circle radii for a Fisher analysis (Fisher, 

1953) of no less than three samples from a single site. Data corrected for geologic dip. Numbers within column 

are lava flows of Grout, Sharp, and Schwartz (1959). 
SUMMARY AND CONCLUSIONS 37 

N 

w + 

s 

EXPLANATION 

-+-

Mean of site-mean directions of magnetization, with circle 

of confidence. North-seeking polarization 

FIGURE 21.-CompariBon of mean directions of remanent 

magnetization for Keweenawan lava flow sequences near 

Kearsarge and on Isle Royale, Mich. Rock units are: 4A, 

lava flow sequenee in the Washington Harbor-Grace 

Harbor area, Isle Royale; 4B, lava flow sequence above 

4A and near Lane's (1911, pl. I) "Greenstone" horizon; 

5, lava flow sequence above conglomerate No. 15 near 

Kearsarge; 6, lava flow sequence between conglomerate 

Nos. 14 and 15; and 7, lava flow sequence below conglomerate No. 14 near Kearsarge. Data corrected for 

geologic dip. Equal-area projection, lower hemisphere. 
38 

w 

PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

\ 

\ 

\ 

... A 

N 

+ 

s 

EXPLANATION 

• Mean direction of magnetization, with circle Mean of site-mean directions • of magnetization, 

of confidence. North-seeking polarization with circle of confidence. North-seeking 

polarization 

E 

FIGURE 22.-Alternative directions of remanent magnetization for the quartz porphyry north of Lake Gogebic, Mich., before (A) 

and after (A") correction for 35° dig of underlying lavas. Other points represent data for: PL, Portage Lake Lava Series; CH, 

Copper Harbor Conglomerate; F-N, Freda Sandstone-Nonsuch Shale unit. The Copper Harbor and Freda-Nonesuch data 

are from DuBois (1962, table XVIII); the other data are new. Equal-area projection, lower hemisphere. 
SUMMARY AND CONCLUSIONS 

N 

w + 

s 

EXPLANATION 

• Mean direction of magnetization, with circlt~s 

of confidence. North-seeking polarization 

FIGURE 23.-Directions of remanent magnetization for the 

rhyolite intrusion::; in T. 56 N., R. 32 W., and T. 56 N., 

R. 34 W., near Kearsarge, Mich. Directions shown are: a, 

rhyolite intrusion in sees. 24 and 25, T. 56 N., R. 34 W., 

after correction for dip of the enclosing Freda Sandstone; 

and b, c, d, and e, alternative directions based on dip corrections for the rhyolite intrusion in sec. 4, T. 56 N., R. 

32 W. Dip corrections for the latter intrusion are: b, no 

dip correc1:ion; e, 42° dip correction; d, 47° dip correction; 

and c, 62° dip correction. The Freda Sandstone-Nonesuch 

Shale unit (F-N) direction is from DuBois (1962, table 

XVIII); the other data are new. Equal-area projection, 

lower hemisphere. 

39 
40 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS 

TABLE 16.-Tentative correlation chart of Lake Superior Keweenawan units based on magnetic polarity sequences 

[Based on data from this report and from DuBois (1962, table XIX), Books and others (1966, table 1), Books (1968, table 1), Palmer (1970, tables 2, 4, and 6), and 

Beck and Lindsley (1969, tables 2, 3)] 

Polarity Upper Peninsula Ironwood, Mich., Mellen, Northwest shore North shore Eastern shore 

sequences of Michigan Wis., and south shore (western Ontario) 

Chequamegon Sandstone 

Later Jacobsville Sandstone Orienta Sandstone Jacobsville Sandstone 

Keweenawan 

polarizations Eileen Sandstone 

Rhyolites Freda Sandstone and Nonesuch Shale 

Copper Harbor Conglomerate Beaver 

Bay 

Complex of 

Grout 

and Major Upper Schwartz part of Logan Logan normal Gabbro at (1939) Duluth intrus- intruspolarizations Mellen Gabbro ions, in ions, in 

Portage Lake Quartz Com- part part Lava Series porphyry plex '--------------- North 

Keweenaw fault Keweenaw shore 

fault(?) lavas, 

upper Gargantua and Mamainse 

flows Point lavas, in part 

North Osler Volcanic Gargantua and Mamainse 

shore Series Point lavas, in part 

lavas, 

lower North20 em flows tongue Logan Logan Reversed Upper 6, 700 feet of of intrus- intruspolarizations South Trap Range lavas Duluth ions, in ions, in 

Gabbro part part 

Complex 

Baraga County Upper Sibley Alona Bay lavas 

diabase dikes Series 

Lower 400 feet of Lower Sibley 

South Trap Range lavas Series(?) 

Lower 

normal 

polarizations 

Basal quartzite 

Upper part of Marquette Rove State (Animikie 

Range Supergroup 

netic data for Keweenawan rocks is presented in table 

16. Placement of the geologic units within the several 

polarity divisions on the chart is somewhat arbitrary 

because deposition times for the intrusive and extrusive 

rocks may overlap. It is suggested (Books, 1968, p. 

D253, and Hubbard, 1968, p. 35) that the top of the 

reversed polarization sequence be taken as the top of 

the lower Keweenawan and following DuBois (1962, p. 

62), that the base of the Nonesuch Shale be taken as 

the base of the upper Keweenawan. 

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