Tectonics of the 

Keweenawan Basin, Western 

Lake Superior Region 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 524-E 

Tectonics of the 

Keweenawan Basin, Western 

Lake Superior Region 

By WALTER S. WHITE 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 524-E 

Analysis of geological and geophysical data indicates 

divergence between trends of ancient Keweenawan 

troughs and the trend of the present structural 

depression 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1966 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

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

Washington, D.C. 20402 - Price 25 cents (paper cover) 
CONTENTS 

Page 

Abstract------------------------------------------- E1 Subsurface structure of Keweenawan rocks ____________ _ 

Situation A: Gravity anomalies due entirely to Introduction _____________ _ ..: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 Keweenawan rocks ____ -------------------_----

Acknowledgments __ ---------- ______________________ : 2 

Analysis of geophysical data ___________ ~_-_-_ 

Step 1--------------------------------- Step2 ________________________________ _ 

Geologic data ______________ ----------_______________ 2 Step3 ________________________________ _ 

Middle and upper Keweenawan stratified rocks_____ 2 Discussion of interpretation derived from 

Intrusive rocks ____________ ~-___________________ 4 geophysical data _____ ------ _____ ------

Relation of Keweenawan to pre-Keweenawan rocks__ 5 Geologic evidence for interpretation __________ _ 

Relation of Keweenawan to Paleozoic rocks________ 5 Negative gravity anomaly-- ____ -- ___ ------ __ 

Regional structure__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 5 Positive gravity anomaly over Duluth Gabbro 

CompleX---------------------------------

Geophysical data___________________________________ 5 

Gravity measurements ______________ ------------ 5 

Conclusions ________________ --- _______ ------

Situation B: Gravity anomalies due in part to preKeweenawan rocks ___ ------------------------- Aeromagnetic investigations __________ ----_------- 7 Summary of tectonic features------------------------

Paleomagnetic measurements ____ ----------------- 12 References cited _________________ ---_--- ______ ------

ILLUSTRATIONS 

Page 

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12 

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18 

18 

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22 

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FIGURE 1. Generalized geologic map __ ----- _______ ------------ _______________________________________ - _________ -- E3 

2. Bouguer gravity anomaly map _________ -------------------- _____________ ----- __ ----------------------- 6 

3. Aeromagnetic profiles and generalized geologic sections _______ --- _____ ------------------------------------ 8 

4. Generalized aeromagnetic map __________________________________________________________ - _____ -------- 10 

5. Gravity anomaly map with inferred gravitational component due to upper Keweenawan sedimentary rock separated from that of other rock types_--------- _________________________________________ --- __ ---------- 13 

6. Gravity and aeromagnetic profiles, north of Bayfield Peninsula-------------------------------------------- 17 

7. Major tectonic elements __________ ------------------------ ____________________________ -_---_---------- 21 

m 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

TECTONICS OF THE KEWEENAWAN BASIN, WESTERN LAKE SUPERIOR REGION 

By WALTERs. WHITE 

ABSTRACT 

The subsurface structure of the western Lake Superior region 

iJ3 analyzed by combining information on surface geology with 

fl,eromagnetic, gravity, and paleomagnetic data. Surface attitudes and the map pattern suggest that the upper Keweenawan 

sedimentary rocks (Bayfield and Oronto Groups) have the general form of a lens, thickening southeastward from a featheredge close to the Minnesota shore of Lake Superior. 

1 The gravitational effect of such a lens can be calculated if 

various assumptions are made about its maximum thickness. 

If the component of gravity that is due to the sedimentary rocks 

iJs subtracted from the Bouguer anomaly, the residual anomaly 

reflects, for the most part, the thickness of the underlying mid-

<J.le Keweenawan extrusive and intrusive mafic rocks. This 

subtraction can be performed graphically if the calculated gravi-

~ational effect of the sedimentary lens is expressed by contours 

on a map at the same scale as the Bouguer anomaly map; the 

resulting contour map of the residual anomaly gives the general 

configuration of the anomaly that is due to the older rocks, primarily middle Keweenawan mafic rocks. If residual anomaly 

J!llaps for various assumed thicknesses of the sedimentary rocks 

~re compared with the aeromagnetic map, the patterns almost 

eoincide when the assumed thickness of sedimentary rocks under 

the Bayfield Peninsula is at least 25,000 feet. 

The residual maps show a conspicuous gravity low crossing 

the present synclinal structure of the Keweenawan rocks almost at right angles. This gravity low is interpreted as a positive area in which the middle Keweenawan lavas are thin; ' 

the low separates basins to the east and west in which the lavas 

are thick. The positive area coincides with places where the 

lava series, as exposed at the surface, seems to be exceptionally 

thin; the thickness and lithofacies of two upper Keweenawan 

formations likewise suggest a positive area. 

If the Duluth Gabbro Complex is a sheetlike intrusion of 

fairly uniform thickness dipping to the southeast under Lake 

Superior, the combined thickness of gabbro plus lava should 

attain a maximum somewhere under the lake. The gravity maximum is actually about 10 miles northwest of the Minnesota 

shore of the lake, and its position indicates that the gabbro 

pinches out beneath the lava series somewhere near the shore. 

Three stages in the tectonic evolution are (1) accumulation 

during middle Keweenawan time of a thick series of lava :flows 

and mafic intrusives in two basins or troughs separated by a 

positive area trending almost north-south, (2) evolution of the 

present Lake Superior basin, having an axis trending northeast, 

during late Keweenawan time, and (3) evolution of the Ashland syncline and the major faults (Douglas, Keweenaw, and 

Lake Owen) of the region still later in Keweenawan time. 

INTRODUCTION 

The Keweenawan rocks that give rise to the midcontinent gravity high and flanking lows are covered 

by Paleozoic rocks southeast of the general vicinity of 

Minneapolis-St. Paul, Minn. In this area of Paleozoic 

cover, interpretation of the geologic structure and 

stratigraphy of the Keweenawan rock sequence is based 

on geophysical data, a miniscule amount of drill-hole 

information, and extrapolation from the Lake Superior 

region where locally, at least, the relationships of the 

anomaly-generating rocks can be studied at the surface. 

Even in the Lake Superior region, however, enormous 

areas are covered by the lake itself, by surficial deposits, 

or by flat-lying sandstone of lU~te Keweenawan age, and 

geophysical data are of great value in filling out the 

rather skeletal picture obtained from surface and nearsurface observations alone. In the Lake Superior region the surface observations help considerably by 

narrowing the choice of interpretations that geophysical 

measurements permit. 

This paper is a review of geologic features that seem 

to have a significant bearing on geophysical interpretations, and it includes examples of some of the geologic 

questions that geophysical data pose and can help answer. It is primarily a synthesis from existing information rather than a presentation of new facts. 

Craddock, Thiel, and Gross ( 1963) provided a similar 

review of information on the area immediately to the 

southwest. 

The exposures of Keweenawan rocks in Michigan and 

the Canadian shore of Lake Superior represent less than 

20 percent of .the area of the central and eastern Lake 

Superior region that is probably underlain by 

Keweenawan rocks; the remainder of the area is covered 

by water. These exposures are relatively narrow belts 

of lava and sedimentary rock that generally dip toward 

the central axis of the lake. The belts suggest, according to the simplest possible rnterpretation, that the axis 

of the lake generally follows the axis of a broad syncline 

and has a configuration something like that portrayed 

El 
E2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

by Irving (1883, pl. 28). This simple picture may well 

be illusory, however, because of scanty information, and 

the more complicated geologic structure revealed by the 

better exposures and more abundant geophysical data at 

the western end of the Lake Superior region probably is 

a far more reliable indication of the kinds of geologic 

structure and stratigraphic relation to be found along 

the buried midcontinent gravity high farther southwest. 

The geologic structure of the western Lake Superior region may also give clues to the structure beneath Lake 

Superior farther east. 

ACKNOWLEDGMENTS 

I am particularly indebted to my colleagues Isidore 

Zietz, Myrl Beck, and W. B. Joyner, U.S. Geological 

Survey, for discussion and suggestions concerning the 

geophysical interpretations, and to J. C. Wright and 

R. B. Taylor, U.S. Geological Survey, for their thoughtful comments· on the geological interpretations. The 

generosity of the Bear Creek Mining Co. in permitting 

the use of data from their exploration of the upper 

Keweenawan rocks of the region is gratefully acknowledged. Finally, I wish to thank Gerald L. Zandle, U.S. 

Geological Survey, for his conversion of the aeromagnetic profiles to the proportions used herein. 

GEOLOGIC DATA 

Figure 1 is a highly generalized geologic map of the 

Keweenawan and contiguous rocks in the western Lake 

Superior region. Gross surface distribution of these 

rocks is based on outcrops and has been known for many 

years. Published sources used for the compilation include Irving ( 1883) ; Leith, Lund, and Leith ( 1935) ; 

Thwaites (1912, 1935) ; Tyler, Marsden, Grout, and 

Thiel (1940); .Aldrich (1929); Leighton (1954); Grant 

(1901); Taylor (1964); Sandberg (1938); Grout and 

Schwartz (1939); and Grout, Sharp, and Schwartz 

( 1959). For parts of northern Wisconsin, geologists 

of the Bear Creek Mining Co. have added to this fund 

of information, in part by restudying known outcrop 

areas using unpublished open-file maps of the Wisconsin Geological Survey and in part by ground magnetic 

surveys and diamond drilling (Moerlein, 1963). Elements of the map compiled by the author from unpublished information made available by the Bear Creek 

Mining Co. include ( 1) modifications of earlier maps in 

the vicinity of Mellen, Wis., (2) revisions of the upper 

boundary of the lava series on both sides of the syncline 

extending southwest from Ashland, Wis., (3) the position of the dashed trend line labeled con the map (base 

of Nonesuch Shale) and the dips of sedimentary rocks 

in the vicinity of this line, all of which are based on 

projections from drill-hole informa:tion, and ( 4) dips 

of the sedimentary rocks south and southeast of Ashland, Wis. (Prof. W. F. Read, unpub. data). 

The only other significant difference from earlier 

geologic maps is in an area of poor exposures east of 

Mellen. The location of the Keweenaw fault(~) and 

the belt of sedimentary rock just south of the fault is 

based primarily on a continuous· aeromagnetic low, 

similar to the one that marks the belt of sedimentary 

rock south of Aldrich's ( 1929) Lake Owen fault west 

of Mellen. The Bouguer gravity anomaly over the 

belt east of Mellen, likewise, is appreciably lower than 

would be expected unless the belt contains abundant 

lighter-than-average rock. 

A fault shown within the mafic lava series 20 miles 

south to southeast of Duluth is based solely on an interpretaJtion of the gravity data in the vicinity. 

MIDDLE AND UPPER KEWEENAWAN STRATIFIED 

ROCKS 

Geologic units differentiated on the map include lava 

flows of middle Keweenawan age, overlain by sedimentary rocks, predominantly sandstone, of late Keweenawan age. The typical flows are :Hood-basalt sheets 

that in Michigan average about 43 feet in thickness. 

The total thickness of the lava series seems to be as 

much as 30,000 feet in some parts of the Lake Superior 

region; so locally, at least, there may be as many as 

700 individual sheets. In composition, most of the 

flows are basalt and andesite. The amygdaloidal tops 

of the flows tend to be moderately altered and contain 

secondary minerals of low-grade metamorphic origin 

(Stoiber and Davidson, 1959), but generally the lava 

of the flows is remarkably unaltered for their age. 

Rhyolite flows are fairly common in some areas and 

scarce or absent in others. They make up about 10 

percent of the section of lava flows and diabase sills 

measured by Sandberg (1938, p. 806) along the Minnesota shore northeast of Duluth but are far less abundant in most areas that have been studied in detail in 

Michigan. A few of the individual flows are separated 

by beds of sandstone or rhyolite conglomerate; these 

sedimentary layers generally make up 1-5 percent of the 

total stratigraphic sequence of middle Keweenawan 

rocks, and only locally make up as much as 10 percent. 

It has long been recognized that the upper Keweenawan sedimentary rocks fall naturally into lower and 

upper groups, which Thwait.es (1912) called the Oronto 

and Bayfield Groups, respectively. The formations of 

the Oronto Group are far more arkosic and contain 

more abundant conglomerate and shale than those of 

the overlying Bayfield Group, which are generally 

more quartzose and mature (Thwaites, 1912; Tyler and 

others, 1940; Hamblin, 1961, p. 2-8). The lowermost 
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EXPLANATION 

LAYERED ROCKS 

S t Croixan Series 

Bayfield Group 

Oronto 

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Lava flows and 

diabase sills 

Metamorphic rocks undivided 

INTRUSIVE ROCKS 

Granite 

Gabbro-granophyre complex 

Includes Duluth Gabbro Complex 

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FIGURE !.-Generalized geologic map showing locations of geologic sections shown in figure 3. Thin dashed lines are reference horizons within map units and 

are identified by letter symbols as follows: a, a sequence of ophitic flows, persistent along strike (Sandberg, 1938, :fig. 4); b, base of Devils Island Sandstone (Thwaites, 1912) of Bayfield Group; c, base of Nonesuch Shale (Bear Creek Mining Co., unpub. data) of Oxonto Group; d, base of Nonesuch Shale 

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(Irving, 1883). Southern part of area is underlain by mafic lava flows that in Minnesota contain about 10 percent rhyolite flows and 17 percent diabase tr:1 

sills with associated granophyre (Sandberg, 1938, p. 806) ; proportions not known farther north. O.j 
E4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

forma,tion of the Oronto Group, the Copper Harbor 

Conglomerate, interfingers with the underlying lava 

series and seems to be conformable with it (White and 

Wright, 1960). In contrast, rocks that are probably 

equivalent to the Bayfield Group farther east in Michigan overlie an irregular erosional surface cut on the 

lavas (Hamblin, 1958, p. 46-49). Although there is no 

direct evidence of angular unconformity between the 

Bayfield and Oronto Groups, the differing regional 

stratigraphic relations just noted and the striking difference in heavy-mineral suites described by Tyler, 

Marsden, Grout, and Thiel (1940, p. 1469- 1483) suggest a significant change in the pattern of sedimentation (see also Hamblin, 1961, p. 7). It is not 

unreasonable, therefore, to postulate the existence of an 

angular unconformity between the Bayfield Group and 

the older rocks anywhere in the region of the midcontinent gravity high should geophysical or other evidence 

suggest it. 

The gross distribution of these two groups as shown 

in figure 1 is well established for much, but not all, of 

the area. The rocks in the trough of the Ashland syncline belong exclusively to the Oronto Group, at least 

as far east as long 91 °22' W. The rocks immediately 

overlying the lava series in the southeastern part of the 

area of figure !likewise belong to this group. The area 

north of the Douglas fault and the area of the Bayfield 

Peninsula are underlain by rocks of the Bayfield Group 

that gently dip except where they are dragged up along 

the north side of the Douglas fault. At one point, this 

drag brings rocks of the underlying Oronto Group to 

the surface ( 17 miles southeast of Duluth; see Thwaites, 

1912, p. 66- 69; Tyler and others, 1940, p. 1478- 1479). 

This exposure is important because it shows tha,t rocks 

of the Oronto Group do occur beneath the Bayfield 

Group north of the Douglas fault. 

The separation of the two groups on the map in the 

vicinity of Ashland is arbitrary and is based on the assumption, following Thwaites (1935), that east of the 

point where the lava series ceases to form the south 

side of the Douglas fault, the fault continues as the 

boundary between rocks of the Oronto and Bayfield 

Groups. Most outcrops of the sedimentary rocks south 

of the fault, as shown in figure 1, belong to the Oronto 

Group as redefined by Tyler, Marsden, Grout, and Thiel 

(1940, p. 147(}-1479). The outcrops in an area 6 miles 

southwest of Ashland are an exception; although they 

lie a mile or so south of the fault as shown, their heavymineral suite is typical of the Bayfield Group (Tyler 

and others, 1940, p. 1470-1471). The mean orientation 

of their remanent magnetism differs appreciably from 

that of either the upper part of the Oronto Group or 

the lower part of the Bayfield (DuBois, 1962, p. 3(}-37, 

ta:ble 18, and fig. 29). In the field the relations of the 

outcrops in this vicinity permit several interpretations 

of the local structure (see Tyler and others, 1940, fig. 5), 

and no attempt has been made in figure 1 to portray the 

local situation. 

The eastward extrapolation of the Douglas fault in 

figure 1 is supported by the known distribution of outcrops (except as noted) , by the occurrence of steep 

dips suggesting drag on a fault in the area 4-6 miles 

southwest of Ashland, and by a local steepening of the 

gravity gradient where a detailed gravity profile crosses 

the inferred position of the Douglas fault (Thiel, 1956, 

pl. 3, profile A) . It should be emphasized, however, 

that outcrops are extremely sparse in this region. Even 

if the position of the Douglas fault is correctly shown, 

there is ample room for large inliers of Bayfield Group 

rocks in the area south and southeast of Ashland. The 

wide range of attitudes displayed by the strike and dip 

symbols of figure 1 show that major folds and possibly 

faults are present. 

INTRUSIVE ROCKS 

Intrusive rocks of Keweenawan age include diabase 

and gabbro-granophyre complexes and granite. The 

Duluth Gabbro Complex (Grout, 1918; Schwartz, 1949; 

Taylor, 1964; Goldich and others, 1961, p. 82-90, 98-99) 

is a sheetlike or lenticular multiple intrusion that seems 

to dip southeast beneath the middle Keweenawan lava 

series of the Minnesota shore of Lake Superior. The 

Beaver Bay complex (Grout and Schwartz, 1939, p. 32-

63; Goldich and others, 1961, p. 91) is a network of 

dikelike and sill-like bodies in which diabase predominates. The gabbro-granophyre complex near Mellen, 

Wis. (Aldrich, 1929; Leighton, 1954), is apparently 

similar to the Beaver Bay complex. These mafic intrusives commonly have granophyre ("red-rock") facies 

along their upper contacts. The mafic intrusives have 

metamorphosed the lava with which they are in contact and, with the possible exception of the Duluth 

Gabbro Complex, can be no older than late middle 

Keweenawan. The gabbro near Mellen cuts conglomerate above the lavas if Aldrich's (1929, p. 125-

126) interpretation of the structure and stratigraphy 

west of Mellen is correct. Leighton (1954, p. 410- 411) 

presented evidence suggesting that the gabbro-granophyre complex west of Mellen was intruded after some 

faulting and tilting of the lava flows, but before tilting was completed. On the other hand, in Michigan, 

granophyre pebbles resembling the siliceous facies of 

the gabbro-granophyre complexes are abundant in conglomerate beds in the middle Keweenawan lava series, 

as well as in the overlying Copper Harbor Conglomerate; therefore, considerable intrusion and partial 

unroofing of one or more gabbro bodies actually took 

place well before the end of middle Keweenawan time. 
TECTONICS OF THE KEWEENAWAN BASIN, WESTERN LAKE SUPERIOR REGION E5 

These apparently conflicting relationships can only 

be reconciled by an explanation involving intrusion of 

gabbro at intervals throughout the general time span 

that is represented by the mafic lava flows, namely, all 

of middle Keweenawan time and the beginning of upper 

Keweenawan time. Paleomagnetic data should ultimately be helpful in solving this and other problems 

concerning the intrusive rocks. 

Many similar sill-like bodies of diabase, not distinguished in figure 1 from the lava flows they intrude, are 

exposed along the Minnesota shore between the Beaver 

Bay complex and Duluth. 

The granite at Mellen contains numerous inclusions 

of gabbro as well as of mafic lava, so it is presumably 

the youngest intrusive rock of the area. Its contact 

relations with the upper Keweenawan sedimentary rocks 

are not known. Similar granite that has intruded the 

pre-Keweenawan rocks south of Mellen (Tyler and 

others, 1940, p. 1468-1469) may be of Keweenawan age. 

RELATION OF KEWEENAW AN TO PREKEWEENAWAN ROCKS 

In the southeastern part of the. area of figure 1, the 

Keweenawan lava flows overlie the older Precambrian 

rocks with slight angular unconformity. A sandstone 

or conglomerate formation 100 feet or more in thickness, called lower Keweenawan by Van Rise and Leith 

(1911, p. 393-394) and Aldrich (1929, p. 109-110), 

separates the two groups of rocks. A similar formation, the Puckwunge Conglomerate of Winchell ( 1897), 

overlies a major angular unconformity between the 

Keweenawan and older rocks in Minnesota (Grout and 

others, 1951; Goldich and others, 1961, p. 79-81, 96-97). 

Upper Keweenawan sandstones directly overlie the 

pre-Keweenawan basement at many places in Michigan, 

Wisconsin, and Minnesota. 

RELATION OF KEWEENAWAN TO PALEOZOIC ROCKS 

Paleozoic rocks occur locally in the southwestern part 

of the area of figure 1. Hamblin (19611 p. 8-9) summarized the unconformable relationship of the Upper 

Cambrian to the Keweenawan and older rocks of the 

Lake Superior region. 

Thwaites' (1940, fig. 1) contour map of the buried 

Precambrian of Wisconsin indicates that the rock units 

of Paleozoic age are thin in the area of figure 1, but 

thicken westward. He showed this westward-dipping 

wedge terminating abruptly at a line on the projected 

strike of the Lake Owen fault. The relations shown 

by Thwaites strongly suggest minor movement on the 

Lake Owen fault in Cambrian or post-Cambrian time. 

7~7-873--66----2 

REGIONAL STRUCTURE 

Some elements of the regional structure, such as the 

Ashland syncline and the Douglas fault, can be 

described in general terms on the basis of surface geologic evidence but other elements are obscure or completely hidden. Inferences concerning these cryptic 

structural elements depend partly or wholly on geophysical evidence; for this reason, a brief discussion of 

the geophysical data used herein precedes the section 

on the major structural features of the region. 

GEOPHYSICAL DATA 

GRAVITY MEASUREMENTS 

Figure 2 is a generalized Bouguer gravity map of the 

western Lake Superior region based on the data of the 

Bouguer gravity map of the United States (Am. 

Geophys. Union, Spec. Comm. for Geophys. and Geol. 

Study Continents, 1964) and of Thiel (1956). Though 

the density of the station network permits some flexibility in contouring, particularly for the contours interpolated in the area under Lake Superior, the main elements of the map are well defined. These elements 

include gravity highs in all the areas where lava or 

gabbro is exposed at the surface and an extraordinarily 

large low under the Bayfield Peninsula. In the ·area 

covered by Lake Superior, for which there are no observations, the configuration of gravity contours was 

chosen to provide a fit with the aeromagnetic data. 

Some of the conclusions of this paper are based on 

the results of matching the observed values for gravity 

with values computed for various geologic models as 

viewed in geologic profiles. Where profiles cross the 

strike of formations, gravity changes in a direction 

normal to the profiles are typically gradual, and calculations for two-dimensional models should approximate 

those for three dimensions. Measurements on the profiles were made by use of a template divided into small 

compartments, each representing a vertical gravitational attraction of 1 milligal for a density difference 

of 1.0. 

The calculated values are based on the following 

assumed densities (Thiel, 1956, p.1086-1090, and fig. 4) : 

mafic lava flows and gabbro, 2.90; Bayfield Group, 

2.30; Oronto Group, 2.44; pre-Keweenawan rocks, 2.67. 

The average density of the mafic lava should, ideally, be 

reduced to compensate for the rhyolite flows and sedimentary rocks locally interbedded with the basalt flows. 

Within the area of figure 1, however, the ratios of rhyolite and sedimentary rock to mafic lava are only known 

along the Minnesota shoreline; so no meaningful correc-
E6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

tion can be made for most of the area. Along the Minnesota shoreline, there are about 2,500 feet of rhyolite 

and 250 feet of sandstone and conglomerate (Sandberg, 

1938, p. 806). Assigning a density of 2.90 to these 

lighter rocks introduces an error into the thickness of 

the complete lava sequence (including rhyolite and 

sandstone as calculated from the gravity data; the calculated thickness is probably about 3,000 feet less than 

10 0 

the actual thickness. This may be an extreme error for 

the region as a whole, because the deficiency for most 

of the Michigan copper district, which lies east of the 

area of figure 1, would he less than 1,000 feet if calculated on the same basis. 

The datum assumed for matching calculated with 

observed Bouguer values is -40 milligals .(Thiel, 1956, 

p. 1088). 

10 20 MILES 

FIGURE 2.-Bouguer gravity anomaly map of western Lake Superior region (modified from Am. Geophys. Union, 

Spec. CoiDJ:q. for Geophys. and Geol. Study Continents, 1964, and Thiel, 1956). Contour interval is 10 milligals. 
TECTONICS OF THE KEWEENAWAN BASIN, WESTERN LAKE SUPERIOR REGION E7 

AEROMAGNETIC INVESTIGATIONS 

The U.S. Geological Survey has made 11 aeromagnetic profiles across the central and southern parts of the 

area of Keweenawan rocks (Kirby and Petty, 1966). 

The profiles, much reduced (fig. 3), are located on figure 

4, which is an aeromagnetic map generalized from the 

data of the profiles. The magnetic contours for St. 

Louis County and the southernmost townships of Lake 

County, Minn., are based on north-south aeromagnetic 

traverses spaced at 1-mile intervals (Henderson and 

Meuschke, 1952a, b; Bath and others, 1964). Similar 

aeromagnetic surveys of the Michigan copper district 

(Balsley and others, 1963) have provided useful guidance in the interpretation of the magnetic data from 

the western Lake Superior region. 

Comparison of the generalized aeromagnetic and geologic maps (figs. 1, 4) shows a close correlation between 

narrow elongate magnetic highs and the northwarddipping lava flows northeast and southwest of Mellen. 

The southward-dipping lava flows on the northwest 

limb of the Ashland syncline are expressed by the same 

type of narrow elongate magnetic anomalies, but here 

they are negative rather than positive. The granite 

north of Mellen is magnetically low; the elongate magnetic high just south of the lavas south and east of Mellen is caused by the magnetic iron-formation and associated rocks of the Gogebic iron range. 

Bath (1960) analyzed the magnetic anomalies along 

two profiles that correspond roughly to the profiles 

I J-J' and K-K' of figure 3. Calculated and observed 

values for the magnetic profiles correspond only if the 

remanent magnetism is assumed to be greater than the 

induced. The remanent magnetism in the lava was imposed as cooling progressed, so where the lava flows 

have been tilted, the present direction of the remanent 

magnetism differs from its original orientation. If the 

present attitude of the flows is known, the original 

orientation of the remanent magnetism can be easily 

determined by calculating the effect of tilting the flows 

back to a horizontal attitude. Dubois (1962, p. 23-27, 

and table 18) found that the average direction of remanent magnetism in the Keweenawan lava flows of Michigan, after corrections for the present dip of the layering, 

is inclined at 44° below horizontal in a direction N. 78° 

W. Using a similar value for the original direction of 

the remanent magnetism, and assuming that remanent 

mao-netism is five times as intense as induced, Bath 0 

(1960, p. B213) obtained good correspondence between 

calculated and measured magnetic profiles. Because 

the present direction of remanent magnetism is ne~rly 

horizontal on the north limb of the Ashland synclme, 

owing to the southward dip of that limb, the south pole 

of the remanent magnetism exerts a strong influence 

and causes negative anomalies on the north side of the 

lava mass. 

The magnitude and sign of the magnetic anomalies, 

therefore are notably affected by the amount of dip of 

' 1 . the lava flows; the anomalies range from strong y positive where the flows dip moderately to steeply northwestward to strono-ly negative where the dips are ' b • steeply southeastward. Empirically, the magnetic 

highs tend to lie in the range of 2,500-3,000 gammas 

above the arbitrary d3Jtum of figure 2 where the flows 

dip more than 10° mv., and in the range of 2,000-2,500 

gammas where the flows dip 10°-20° SE.; values less 

than 1,500 gammas are common where southeasterly 

dips exceed 40°. These correlations apply to profiles 

flown at heights roughly 1,000 feet above the flows. A 

thick cover of nonmagnetic rock overlying the flows 

would increase the distance between anomaly source and 

magnetometer and tend to subdue both positive and 

negative magnetic anomalies. . . 

The aeromagnetic profiles are too widely spaced m 

Lake and Cook Counties, Minn., to make reliable contouring in this area possible. Where the traverses are 

spaced at 1-mile intervals in St. Louis County, the 

published aeromagnetic maps (Henderson and Meuschke, 1952a, b; B3ith and others, 1964) and even the generalized data of figure 4 show that both the Duluth 

Gabbro Complex and the gently dipping middle Keweenawan lava flows generate a very hummocky magnetic "topography." This "topography" has round or 

irregular highs and lows rather than the. c~ntinuous 

magnetic crests and troughs so char.actenstiC. o~ the 

more steeply dipping lavas in Wisconsm and MIChigan. 

Contours have been drawn to connect highs and lows, 

respectively, in adjoining aeromagnetic profil~s in Lake 

County, but these connections may not be vahd. . 

Although it is not known whether the magnetic 

"topoo-raphy" in Lake County is hummocky, like that 

farth:r southwest or composed of magnetic ridges and 

troughs, as in wisconsin, it is clear that t!lls area is 

characterized by positive rather than negative anomalies. A small part of this change from dominantly 

negative anomalies near Duluth to positive anomalies 

farther northeast may be due to a gradual northeasterly 

decrease in the dip of the rocks, combined with a change 

in strike from roughly north at Duluth to northeast in 

Cook County. R. B. Taylor (oral commun., Feb.1965) 

suggested the possibility that the change is related to 

the higher iron content of the Beaver Bay complex. 

A broad aeromagnetic anomaly, U-shaped in plan, 

crosses the Bayfield Peninsula north-northwest of Ashland and curves to the northeast where it extends northward into the area covered by Lake Superior. Some 

of the conclusions reached in this paper depend on the 
E8 

w 

(f) 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 
NW SE 

F 

G 

I 

IJ I' 

J 

I 

~ ... ;;! 

~~ ~ 

EXPLANATION 

LAYERED ROCKS 

Bayfield Group 

Oronto 

-

Group 

.., 

~ ~~ ~ :;l., 

::!::1 ~ 

.~ 

Lava flows and 

diabase sills 

~;;! ~ ~{ ~ ~ Metamorphic rocks 

;;;--

'i 

~~ 

~ ~ "' tl ., 

~~ ~ 

~ 

INTRUSIVE ROCKS 

Granite 

Gabbro-granophyre complex 

Includes Duluth Gabbro Complez 

• 

Point at which calculated value of 

gravity anomaly is approximately 

equal to values read from 

Bouguer gravity map. 

GAMMAS 

2000 

1000 

Vertical scale (gam mas) for aeromagnetic profi les. Ticks along sides 

of diagram are 2000 gammas above 

0 arbitrary datum 

10 0 10 20 MILES 

FIGURE a.-Aeromagnetic profiles and generalized geologic sections. Location of aeromagnetic profiles shown on figure 4; for geologic sections, fig. 1. Geologic 

sections along profiles A-A', 0-0', E-JJJ', H-H', and J-J' have been constructed to be consistent with the gravity values of fig. 5. These sections are reduced 

from originals at 1:250,000 on which separate gravity calculations were made for sedimentary and mafic rocks. Values calculated at each of the points indicated by black dots agree with the values read from figs. 2 and 5 to within about 3 milligals. 

8 

t;j 

~ 

..... 

~ 0 

rJl 

0 

"':1 

t;j 

~ 

~ 

~ t;j 

~ t:l:j 

> rJl 

~!Z 

~ 

~ ~ 

z 

t" 

e 

~ ~ ::0 

~ 

t;j 

~ 

~ 
ElO SHORTER CONTRIBUTIONS· TO GENERAL GEOLOGY 

w 

10 0 10 20 MILES 

EXPLANATION 

Contour interval (solid 

lines) is 500 gammas. 

Supplementary half-

-interval contours, wher~ 

shown, are dashed. 

Datum arbitrary 

Less than 1500 

FIGURE 4.-Generalized aeromagnetic map. Line X-X' is line of cross section in figure 6. Contours in area of 

flight lines generalized from Kirby and Petty (1966). Contours in St. Louis County and adjacent parts of Lake 

County, Minn., from Henderson and Meuschke (1952a, b), Bath, Schwartz, and Gilbert (1964), and unpublished 

U.S. Geol. Survey maps. 

following inferences about the source of this magnetic 

anomaly. 

In the vicinity of the Bayfield Peninsula, the magnetic anomaly trends virtually at right angles to : the 

strike of formations of the upper Keweenawan Bayfield Group exposed at the surface. The anomaly is 

clearly due, therefore, to rocks that underlie the 

Bayfield Group. 

The width and amplitude of the anomaly are relatively uniform within the area of figure 1; this fact suggests that the source is a single geologic body. Only a 

most remarkable coincidence could lead to abutment of 

two or more discrete geologic bodies in such a way as to 

leave no obvious junction where the magnetic expression 

of one leaves off and that of the other begins; therefore, 

I shall not consider alternatives to the assumption that 

a single geologic body is involved. 

The elongate character of the anomaly suggests the 
TECTONICS OF THE KEWEENAWAN BASIN, WESTE-RN LAKE SUPERIO:R REGION 

trace of a sheetlike mass, and the U-shape suggests that 

the sheet is involved in a fold. The northwestern part 

(>f the anomaly is approximately parallel to the strike 

bf the southeastward-dipping middle Keweenawan 

rocks in Minnesota, and the part of the anomaly extending east from Ashland is approximately parallel to 

the northward-dipping middle Keweenawan rocks south 

pf Ashland. The fold suggested by the U-shape, therefore, seems to be the Lake Superior syncline, and the 

anomaly follows the trace of a magnetic formation that 

wraps around the eastward-plunging nose of this 

syncline. 

. It is considered unlikely that the magnetic formation 

is everywhere separated from the ·exposed middle 

Keweenawan lava series by an arcuate fault or synclinal 

axis; therefore, the geometry of the syncline requires 

that this magnetic formation overlie the exposed middle 

Keweenawan lava flows. As noted previously, the magnetic formation must underlie the rocks of the upper 

Keweenawan Bayfield Group. These various geologic 

relationships could be satisfied by one of the following 

possibilities : 

1. The magnetic formation is a unit at the top of the 

middle Keweenawan lava series. It either lenses 

out along a line parallel to the outer edge of the 

U-shaped anomaly or is cut off along this line by 

I 

an unconformity at the base of the upper 

Keweenawan rocks. 

~· The magnetic formation is a unit within the Oronto 

Group. It either lenses out or is cut off by an 

unconformity at the base of the Bayfield Group. 

The problem of discriminating between these possibilities cannot be settled with the available data. 

Inasmuch as the basic conclusions of this paper are not 

materially affected by the comparatively minor differences between these alternatives, from a geophysical 

point of view, the problem need not be pursued, except 

to call attention to the choice implied by the geologic 

sections of figure 3. The outer edge of the area underlain by the magnetic formation is the outer margin of 

the U-shaped anomaly; so the magnetic formation 

causing the high wedges out or is cut off at the northwest 

side of the magnetic high just southeast of the Minnesota shore. The base of the magnetic formation lies 

about 10,000 feet above the base of the mafic lava series 

as drawn in the northwestern parts of sections A-.A' 

and 0-0'. Section E-E' lies along the outer margin 

of the U-shaped anomaly (fig. 4) and, therefore, repreiSents a line along which the magnetic formation is abisent, or at most very thin. The sections of figure 3, 

rtherefore, imply that the magnetic formation is part 

of the middle Keweenawan lava series. If it were part 

of the Oronto Group, the base of the Oronto would have 

to be shown as lying about 10,000 feet above the base of 

the middle Keweenawan lava series in the northwestern 

parts of sections A-.A' and 0-0', instead of much higher 

as it is now shown. 

The cross sections do not show bedding planes and 

therefore beg the question of whether the magnetic 

formation lenses out or wedges out against an unconformity. 

The magnetic expression of the magnetic formation 

indicates that the bedding in this formation dips very 

gently, probably less than 15° throughout most of the 

area. Assuming a direction of remanent magnetism 

similar to that of the exposed middle and upper Keweenawan lavas (DuBois, 1962, p. 57), steep dips would 

cause negative magnetic anomalies on the west and 

north side of the U similar to the negative anomalies 

along the north west flank of the Ashland syncline. 

Gentle dips of bedding are consistent with the location 

of the magnetic formation near the central, flatter part 

of the Lake Superior syncline. 

The preceding paragraphs discuss the stratigraphic 

position and physical location of the magnetic formation but not its lithologic character. The size of the 

anomaly indicates that the rocks causing the anomaly 

are highly magnetic. Mafic igneous rock is the principal constituent of the middle Keweenawan lava series 

and is locally abundant in the lowermost part. of the 

upper l{eweenawan section. The only evidence for 

igneous activity after deposition of the Copper Harbor 

Conglomerate is a single small plug of alkalic rhyolite 

near Hancock (Cornwall and Wright, 1956). Other 

types of highly magnetic rock, such as magnetic slate 

or iron-formation, are not known to be present in the 

Keweenawan rocks. Is seems most reasonable to assume, therefore, that the U-shaped anomaly is caused 

by mafic rock. According to the interpretation discussed above, the base of the magnetic formation lies 

some 10,000 feet above the base of the lava series near 

the Minnesota shore along the lines of sections A -.A' and 

0-0', figure 3. This interpretation implies a significant 

difference in the average intensity of magnetism in the 

upper and lower parts, respectively, of the lava series in 

that area. Noteworthy contrasts, in the range of 500-

1,000 gammas, can be seen on aeromagnetic maps of the 

Michigan copper district (Balsley and others, 1963) in 

areas where the stratigraphic section is known, from 

drilling, to consist almost entirely of mafic lava; therefore, a difference between the magnetic intensity of different groups of flows is not an unreasonable assumption. It should be noted, however, that such a contrast 

between upper and lower parts of the lava series is not 

apparent in the western part of the area shown in figure 

1, and it is necessary to assume that the U -shaped anom-
El2 SRORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

aly outlines the principal area in which this relationship exists. 

PALEOMAGNETIC MEASUREMENTS 

The pioneer paleomagnetic work of DuBois (1962) 

illustrated the large potential value of such studies to 

problems of correlating Keweenawan geologic units. 

More recently, Books, Beck, and White (unpub. data) 

used the attitude of the direction of remanent magnetism to set limits to the time at which the gabbro bodies 

at Duluth and Mellen were emplaced. The directions of 

remanent magnetism and their secular changes are not 

yet known with sufficient precision and detail to solve 

this type of problem unequivocally, but the spans of time 

in which intrusion might conceivably have taken place 

are very much narrowed by the sma;ll amount of paleomagnetic data already published. Books, Beck, and 

White ( unpub. data) conclude that the gabbro at Duluth 

was intruded during the time interval represented by the 

middle Keweenawan lava flows and the Copper Harbor 

Conglomerate and probably before tilting of the underlying lava flows was completed; available data do not 

permit a more precise chronology. The gabbro near 

Mellen was probably intruded during Copper Harbor 

time, when the lava flows into which it was intruded 

already had dips close to 50° ; this sequence of events 

implies a more profound unconformity within the Oronto Group, at the top of the Copper Harbor Conglomerate, than is suggested by any known field relationship. 

SUBSURFACE STRUCTURE OF KEWEENAWAN ROCKS 

The following paragraphs review the present status 

of knowledge and inference concerning the subsurface 

structure of the Keweenawan rocks in the western Lake 

Superior region. 

The structure of the Ashland syncline in the southwestern ninth of the area of figure 1 seems fairly simple. 

Profiles across the syncline constructed by projection 

of surface attitudes (fig. 3) suggest that near the west 

edge of the area of figure 1 the lava mass is roughly 

40,000 feet thick and that in the center of the syncline, 

farther northeast, the rocks of the Oronto Group are 

7,000-9,000 feet thick. These thicknesses are consistent 

with the gravity data (fig. 2). 

Lava does not reach the surface on the north limb of 

the Ashland syncline east of long. 91 °22' W. Because 

of poor exposures, the eastward continuation of the 

Douglas fault and the configuration of the Ashland 

syncline south and southeast of Ashland are not known. 

Near the Wisconsin-Michigan State line, all the exposed upper Keweenawan strata dip northward, for the 

most part steeply, for a distance of at least 7 miles away 

from the top of the underlying lava series. These 

strata could represent a single unbroken homoclinal 

sequence 35,000 feet· or more in thickness, or a thinner 

sequence that includes one or more folds or faults. Sections A-A', 0-0', and E-E' (fig. 3) suggest, following 

Thwaites (1912, map), that the section has been folded, 

but a variety of other interpretations is possible. 

The interpretation of subsurface geology in the remainder of the western Lake Superior region, north of 

the Douglas fault, depends heavily on geophysical data 

because so much of the area is covered either by Lake 

Superior or by the gently dipping sandstone of the Bayfield Group, a sandstone that may overlap the older 

formations unconformably. We may consider two general situations: (A) the pre-Keweenawan basement 

rock has a uniform density, and all the large gravity 

anomalies within the area of the Keweenawan rocks are 

caused by the Keweenawan rocks themselves, or (B) 

some of the large gravity anomalies are due to the character of the pre-Keweenawan rocks. 

SITUATION A: GRAVITY ANOMALIES DUE ENTIRELY 

TO KEWEENAWAN ROCKS 

ANALYSIS OF GEOPHYSICAL DATA 

The assumption that gravity anomalies are due entirely to Keweenawan rocks is most readily explored by 

working from the gravity map (fig. 2). In general, the 

value of the Bouguer gravity anomaly at any point can 

be explained by a range of paired values for the gravitational effect of mafic rock and of overlying sedimentary 

rock, respectively. Because the density of mafic rock is 

greater and that of sedimentary rock is smaller than 

the value assumed for the pre-Keweenawan rocks ( 2.67) 

in the analysis of a given value of the Bouguer anomaly, 

the larger the thickness assumed for the sedimentary 

rock, the larger the thickness of mafic rock required to 

compensate it. Where both rock types are present, 

therefore, any value read from 1figure 2 can be thought 

of as the algebraic sum of two components, one positive, 

caused by the mafic rock, and the other negative, caused 

by the overlying sedimentary rock. Separate maps of 

these two components reveal the shapes and rude dimensions of the geologic bodies and are a device for comparing alternative geologic structures. Figure 5 is a 

map exemplifying the separation of these two components of gravity ; the solid -line contours represent the 

inferred negative gravity effect due to an upper prism 

of sedimentary rock-derivation of the shape of this 

prism is described in the following paragraphs'-and 

the dashed-line contours represent the complementary 

positive gravity effect due to the underlying mafic rock; 

algebraic addition of these two components at any point 

will yield the value shown in figure 2. The following 

steps were taken in analyzing the subsurface structure 

by means of diagrams similar to figure 5. 
TECTONICS OF THE KEWEENAWAt~ BASIN, WESTERN LAKE SUPERIOR REGION El3 

"'( 

I 

"'( 

"'( 

I 

10 0 10 

0 .," ,. 

Mellen __ .J. - - _ - ::..--"' 

I ------ I ---- / ) t -- ..... / _.8----L--""l~..,-~~ IRON/ -------50 / ,"' ~-T- I / ; 1'... ,.._, ~...,...,.,., ,..,... ," 

y ,-------, -,.. .,. 

>-" ~SHLAND -- 1'" ",.. _ ... _ ........ / ... - L-,., / .,.,.,. ......... , ....... ___ __../ "".,. ,., I 

-- ..,-"'/ ,"" ----

20 MILES 

FIGURE 5.-Gravity map with inferred gravitational component due to upper Keweenawan sedimentary rock separated from that of other rocks. Solid lines represent inferred negative anomaly due to upper Keweenawan 

sedimentary rock. Subtracting this negative anomaly from the Bouguer anomaly (fig. 2) yields the anomaly 

represented by the dashed lines. Contour intervallO milligals. 

STEP 1 

The first step is to estimate the configuration of the 

base of the sedimentary sequence using all available 

geologic data. As shown in figure 1, the Bayfield 

Group north of the Douglas fault dips very gently 

southeast. The base of the Devils Island Sandstone 

(line b, fig. 1) as mapped by Thwaites (1912, map) is 

approximately parallel to the Minnesota shoreline 25 

miles to the northwest. The regularity of that shoreline 

has long invited speculation that it is due to a fault. 

Thiel (1956, p. 1096-1098) presented a brief history of 

this idea together with geophysical evidence against it. 

Where the "fault" should cross the shoreline at the west 

end of Lake Superior, just south of Duluth, Thiel found 
E14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

no inflection in the gravity profile such as would be 

caused by a thick section of sandstone faulted against 

lava. Schwartz (1949, p. 69) pointed out that a fault 

trace parallel to the shoreline would not pass between 

lava and sandstone outcrops. The northwestern 

boundary of the upper Keweenawan sandstone at 

Duluth, therefore, appears to be the trace of an angular 

unconformity that cuts across the beveled edges of the 

flows and intrusions that underlie the sandstone (fig. 1). 

Because sandstone of the Bayfield Group overlies preKeweenawan rocks at many places in Michigan, Wisconsin, and Minnesota, there is ample reason to postulate 

the existence of such an unconformity. An extrapolation is made from the relation at Duluth, therefore, and 

the regularity of the Minnesota shoreline as a whole is 

here attributed to exhumation of an angular unconformity of relatively low relief cut on a surface of middle Keweenawan lavas and associated rocks and overlain 

by upper Keweenawan sandstone or conglomerate. 

Comparatively rapid erosion and removal of the softer 

rocks above the unconformity and less effective dissection of the harder lavas below would tend to make the 

shoreline fairly close to the beveled surface of the unconformity; the shoreline would, therefore, be fairly 

straight, overall, where the unconformity is not 

deformed. 

This interpretation of the Minnesota shoreline gives 

the approximate location of the contact surface between 

the sedimentary rock sequence and the lava series at 

lake level. We need to know, next, th~ dip of this 

contact surface. The general parallelism of the Minnesota shoreline and the base of the Devils Island Sandstone on the Bayfield Peninsula (line b, fig. 1) suggests 

that the sedimentary sequence above the unconformity 

contains no marked discordance and that the typical 

dips of 3°-6° on the north shore of Bayfield Peninsula 

(Thwaites, 1912, map) give the order of magnitude of 

the dip of the unconformity in that area. The topography of the Minnesota coast provides an additional 

clue. If we draw a smoothly arcuate line skirting the 

promontories of the Minnesota shore and measure the 

vertical angle to points on the skyline as they might be 

measured with a theodolite from a small boat on the 

line, we find that the maximum vertical angle is commonly about 8%0 • Actual measurements were made on 

large-scale topographic maps, and the points selected 

were 1-2 miles inland from the shore. Sculpture of the 

present topography of this coastal strip involves so 

many factors, some tending to steepen and some to lower 

gradients, that no great significance can be attached to 

the precise value of 8%0 for the angle, but this physiographic evidence does suggest, as a reasonable starting 

point, the inference that an unconformity between sandstone and lava has a lakeward dip of 8°-10°. 

This model of a sedimentary wedge or prism overlying an angular unconformity cut on the lava is at 

least qualitatively supported by the aeromagnetic data 

(figs. 3, 4) . The aeromagnetic anomalies are far more 

subdued than they would be if the lava cropped out on 

the lake bottom a few miles out from the Minnesota 

shore; this fact strongly suggests that the lava is overlain by an appreciable thickness of nonmagnetic rock as 

well as water. The anomalies on the profiles of figure 3 

are not suitable for very reliable estimates of the depth 

to lava, but estimates based on width of the anomalies at 

half maximum are at least consistent with a dip of 

10° or less for the base of sedimentary rock overlying 

the lava. 

These fragments of geologic information- the attitude of bedding on Bayfield Peninsula, the regularity 

of the Minnesota shoreline and the slope of the hillsides facing it, and the general parallelism of the Minnesota shoreline with the strike of the Devils Island 

Sandstone-all suggest that the sedimentary rock is 

a prism-shaped mass thickening southeastward from a 

featheredge at the Minnesota shoreline. Dips in the 

range of 8°-10° are reasonable for the attitude of the 

unconformity at this northwestern edge, and gentler 

dips are likely under the Bayfield Peninsula. 

The shape assumed for the prism is very similar to 

the figure that would be represented by the solid lines 

of figure 5 if these lines were structure contours instead of gravity contours. The prism shape was derived by drawing generalized structure sections of the 

sedimentary rocks in the conventional manner, using 

all available surface dips. The dips are flattest in the 

vicinity of the bay northeast of Ashland; therefore, this 

location was assumed to be the deepest part of the 

trough. 

STEP 2 

The second step in the analysis of subsurface structure was to compute very roughly the gravity deficiency 

due to the sedimentary rocks alone for three different 

assumptions concerning the maximum thickness of a 

sedimentary prism having the general configuration 

suggested by the solid lines of figure 5. The gravity 

deficiency is based on the difference between the densities of the sedimentary rock and the value of 2.67 

assumed for the pre-Keweenawan rocks. For purposes 

of computation, the proportion and distribution of Bayfield and Oronto Group rocks, respectively, was assumed 

to be that shown in figure 3, sections A-A' and 0-0', 

for the northeastern part of the area shown in figure 5, 

where greatest doubt exists. The assumed angular unconformity at the base of the sedimentary rocks at the 
TECTONICS OF THE KEWEENAWAN BASIN, WESTERN LAKE SUPERIOR REGION E15 

innesota shore is most compatible with known relaonships elsewhere if the strata above the unconformity 

long to the Bayfield Group, and this overlap requires 

al northwestward pinchout of the Oronto Group. Actually, raising or lowering the Bayfield-Oronto boundary as much as 5,000 feet has only a small effect on the 

ihterpretations that follow. 

I The negative gravitational effect of the sedimentary 

rocks was calculated for numerous points, and the values 

vfiere then contoured. The solid -line contours of figure 

~ give the gravity deficiency for the sedimentary rocks 

allone if the dip at the Minnesota shore is assumed to 

be 9° and the maximum thickness of the sedimentary 

~rism is 25,000 feet. 

i The gravity map for each possible model of the sedi-

$entary rock has a complementary map that represents 

tpe gravitational effect of the underlying mafic rock. 

these gravity maps for the mafic rock are very simply 

constructed by graphic subtraction of the values on the 

gravity maps of the sedimentary rocks from the values 

$the Bouguer anomaly map (fig. 2). The dashedline contours of figure 5 represent the anomaly that re-

$ains when the values represented by the solid-line 

cpntours are subtracted algebraically from the values 

~epresented by the Bouguer anomaly map (fig. 2). 

I Pertinent data for each of the three models are given 

ih the following table : 

I Model 

A B c 

Dip of unconformity, Minnesota lakeshore ______________________ 

Maximum thickness of sedimentary 6~0 go 13° 

· prism _________________________ 15,000 25,000 40,000 

4-verage thickness of mafic rock be- 1 low deepest part of sedimentary 

1 prism where Bouguer anomaly ex- ceeds -80 milligals _____________ 10,000 20,000 30,000 

, The situation described in column B of the table is 

represented by figure 5. A dip of 4° gives a thickness of 

10,000 feet for the sedimentary rock sequence, and a 

thickness of lava close to zero; these thickness relations 

astablish 4° as a minimum dip. 

' The various assumptions made for the area north of 

·the Douglas fault do not affect the southwestern quarter 

df figure 1, where the structure is fairly well established, 

but they do change the gravity gradients in the area 

douth and east of Ashland, where, as noted previously~ 

the surface geologic structure is most uncertain because 

of poor exposures. 

The gravity rna ps of the mafic rock obtained by subtraction prima.rily represent the thickness of the mafic 

rock. Some minor gravity irregularities due to structure are apparent, as along the Douglas fault and in the 

vicinity of the Michigan-Wisconsin State line, where 

the surfaces separating light and heavy rocks dip 

steeply at shallow depth; the largest departures from 

infinite-sheet models are to be expected in these places. 

Elsewhere, the positive gravity anomaly is roughly proportional to the thickness of mafic rock; for sheets of 

infinite extent, 30 milligals is roughly equivalent to 

10,000 feet of lava. This correspondence is not close 

enough to permit an isopach map to be drawn directly 

from a gravity map, but it is close enough to show 

the shapes and general size of large bodies of rock 

with considerable fidelity. Although sharp ridges or 

troughs appear more subdued on the gravity map than 

.they would on a corresponding isopach map, this modification is not important where position and general 

shape are the principal concern, as described in the next 

step. 

STEP 3 

The third step in the analysis is a comparison of 

the various configurations for the mafic rock obtained 

by step 2 to see which are geologically reasonable and 

agree best with the aeromagnetic data. Before exploring the effect of assuming larger or smaller thicknesses 

for the sandstone, however, it is desirable to consider 

the geologic meaning of the residual gravity anomalies 

represented by the dashed -line contours of figure 5. 

The large positive gravity anomalies in Wisconsin and 

Minnesota correlate perfectly with areas where abundant mafic rock is known to exist below the surface. 

The magnitudes of these anomalies correspond, generally, to what can be inferred from the dip of strata with 

respect to the thickness of the mafic rocks in these areas. 

The gravity high off the tip of the Bayfield Peninsula 

is to be expected if the lavas that rim the Lake Superior 

basin have any continuity under the lake. The most 

striking and puzzling feature of the map is the gravity 

trough, extending west-northwest across Bayfield 

County; this trough seems to separate the high off the 

tip of the Bayfield Peninsula from the high in the 

western half of the map area. The northward extension of this gravity low across Lake Superior, as shown 

in figures 2 and 5, is hypothetical. It was drawn to 

make the gravity anomaly due to the mafic rocks parallel the magnetic anomalies under the lake (fig. 4) and to 

connect the gravity trough of the Ashland area (fig. 

5) with the gravity saddle at long 91 oW. in Minnesota. 

If actual Bouguer anomalies in the area of the lake 

are found to correspond to the values inferred in figure 2, they will tend to confirm the structure derived 

here. In accordance with the present assumption that 

all the gravity anomalies within the area of Keweenawan rocks are due to those rocks, the gravity trough 
E16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

represents an area where the lavas are thin or absent. 

Structure contours on the base of the Keweenawan lava 

series would roughly follow the gravity contours to 

outline a ridge or anticline of pre-Keweenawan rocks 

trending west-northwest, cutting directly across the 

projected strike of the Wisconsin part of the midcontinent gravity high. 

The effect of increasing or decreasing the assumed 

thickness of sandstone can now be explored with these 

implications of the gravity trough in mind, and using 

figure 5 as the point of departure. As might be expected, changing the thickness of the sedimentary rock 

changes the amplitude of the gravity anomaly representing the sedimentary prism but does not change the 

general shape or position of that anomaly. Changing 

the assumed thickness -of the sedimentary rock, however, has an exceedingly important effect on the shape 

and position, as well as amplitude, of the gravity anomalies representing the mafic rock. Decreasing the assumed thickness of the sandstone brings the gravity 

contours for the mafic rock closer to congruency with 

the contours of the Bouguer anomaly map (fig. 2). 

The gravity trough becomes wider, deeper, and more 

rounded, and its axis is displaced toward the northeast. 

The changes neither increase nor decrease the geologic 

credibility of the structure of the mafic rock implied by 

figure 5. The changes do, however, affect profoundly 

the degree of concordance between aeromagnetic and 

gravity anomalies. 

The large positive magnetic anomaly, U-shaped in 

plan, that trends generally northwestward across the 

Bayfield Peninsula and curves around to the northeast 

under Lake Superior (fig. 4) crosses the center of the 

large negative Bouguer anomaly north of Ashland (fig. 

2). The positive character, size, geographic position, 

and shape suggest that this high is due to mafic rocks 

in the upper part of the middle Keweenawan lava series, as described earlier, which means that there should 

be some correspondence between the aeromagnetic map 

and a map showing the gravitational effect of the mafic 

rock. Such a correspondence does exist between the 

anomalies of figures 4 and 5 ; so the thickness of the sedimentary rock assumed in the construction of figure 6 is 

reasonably consistent with the aeromagnetic data. If 

a smaller thickness of sedimentary rock is assumed, 

however, the gravity trough becomes more rounded and 

its axis is displaced northeastward, as noted previously, 

and the correlation of magnetic and gravity contours is 

destroyed. The thickness of sedimentary rock assumed 

in figure 5 is about the minimum thickness that will 

produce a fit between aeromagnetic and gravity data. 

Concordance between aeromagnetic and gravity data, 

therefore, requires that the prism of sedimentary rock 

have a minimum thickness of roughly 25,000 feet where 

it is presumed to be thickest under the bay northeast of 

Ashland. 

The dimensions used for figure 5 were actually determined by reversing the procedure described in step 2. 

Gravity contours for the mafic rock were dra.wn more or 

less parallel to the aeromagnetic contours, and these 

values, when graphically subtracted from the Bouguer 

anomaly (fig. 2) gave an approximate figure for the 

gravity gradient and, indirectly, for the maximum 

negative anomaly due to the sedimentary rock. 

Increasing the assumed thickness of the sedimentary 

prism above the amount for figure 5 increases the gravity gradient on the east side of the gravity trough, but 

even an increase to 40,000 feet of sedimentary rock 

under the bay northeast of Ashland does not materially 

affect the configuration and location of the trough, or 

the concordance between aeromagnetic and gravity 

map. The test of geologic reasonableness, therefore, 

does not suggest any well-defined limit for the maxiInurn thickness for the sedimentary prism, and it would 

be difficult to prove, with available evidence, that 40,000 

feet is more or less probable than 25,000 feet. 

DISCUSSION OF INTERPRETATION DERIVED FROM 

GEOPHYSICAL DATA 

A section parallel to the north shore of the Bayfield 

Peninsula (X -X', fig. 6) is helpful for visualizing what 

the manipulations actually mean, both geologically and 

mechanically. This seotion intersects each of the sections of figure 3, and the thickness of the various .geologic units at each intersection was therefore taken from 

figure 3 to construct figure 6. Curve M is an aeromagnetic profile, and curves R, S, and Tare gravity profiles. R shows values of the Bouguer anomaly, and S 

is a profile of the gravity surface defined by the dashed 

lines of figure 5. T shows the effect of assuming a 

·thicker prism of sedimentary rock than is assumed in 

the construction of figure 5 ; the construction represented by curve T is based on an assumed thickness of 

40,000 feet for the maximum thickness of the sedimentary prism northeast of Ashland. 

The difference between curves R and S represents the 

gravity deficiency due to the sedimentary rock. The 

difference between 8 and T shows how an increase 

in the assumed thickness of the sedimentary sequence 

tends to narrow the gravity trough and shift its axis 

toward the west. 

The geologic section, X-X', is governed by the 

gravi~ty profile S-values of gravity are the basis for 

estimating the thickness of the mafic rocks-so the base 

of the layer of mafic rock is virtually a mirror image 

of gravity profileS. The high~ and lows of the aero-
TECTONICS OF THE KEWEENAWAN BASIN, WESTERN LAKE SUPERIOR REGION E17 

w E 

.... T 

~-- /;;;?s 

MILLIGALS 

+50 

0 

J-J' H-H' E-E' 

10 0 10M ILES 

C-C' 

-50 

GAMMAS 

2500 

2000 

1500 

A-A' 

FIGURE 6.-Gravity and aeromagnetic profiles, north of Bayfield Peninsula. For locution of section 

X-X', see figures 1, 4, or 5. Lithologic symbols are the same as those in :figure 3. Aeromagnetic profile, M, is drawn from :figure 4, and does not represent an actual flight line. Similarly, 

gravity profile R, representing values of the Bouguer anomaly, is drawn from :figure 2, and gravity 

profile S is drawn from the values represented by dashed lines in figure 5. Profile T is derived 

in the same way as profile S, but represents a situation in which the upper Keweenawan sedimentary sequence is assumed to be thicker. 

:p1agnetic profile do not show a one-to-one correlation 

with those of the gravity profile S, although the correspondence is better than with the original Bouguer 

profile R. It is desirable, therefore, to see if the features of the aeromagnetic profile are compatible with 

the relations shown in the geologic section. 

The eastern aeromagnetic high, representing the 

U-shaped anomaly of figure 4, lies above the area where 

the layer of mafic rock thins rapidly on the flank of the 

fidge of pre-Keweenawan rock. The western slope of 

the high, therefore, appears to be due to change in 

thickness of the mafic layer and the more gentle eastern 

~lope to change in the depth to the top of the mafic 

layer. As suggested earlier, the location of the crest of 

this U-shaped anomaly seems to indicate that the rocks 

/Well above the base of the mafic layer have a greater 

p1agnetic effect than those near the base. The proximrty of the western magnetic high to the center of the 

western lava trough suggests the possibility of a similar relation west of the ridge of pre-l(eweenawan rock. 

Some elements of the aeromagnetic profile may be a 

response to the attitude of the rocks. As discussed 

earlier, areas in which the middle Keweenawan lava 

flows dip steeply toward the east or southeast are characterized by negative anomalies, due, presumably, to a 

nearly horizontal attitude of the direction of remanent 

magnetization in these places. This dip is suggested as 

the explanation for the negative character of the west 

end of the profile, near Duluth. Along the rest of the 

profile, neither the actual attitude of the mafic rocks 

nor their composition (basalt flows versus gabbro intrusions) is known ; so no strong inference can be 

drawn. 

Thus, although the aeromagnetic highs and lows do 

not show a simple one-to-one correlation with individual 

features of the geologic profile of figure 6, which was 

derived from the gravity data, there is a general correspondence, and departures are capable of reasonable explanation. Comparison of profiles S and T (fig. 6) 

shows that the degree of correspondence between aero-
El8 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

magnetic and gravity profiles is only slightly improved 

by very large increases in the thickness assumed for the 

sedimentary rocks. 

GEOLOGIC EVIDENCE FOR INTERPRETATION 

The existence of a positive area has been deduced from 

evidence that is primarily geophysical. In addition, 

there is some independent geologic evidence to support 

this conclusion. 

Although the unusual narrowing of the outcrop belts 

of the lava in the vicinity of Mellen may be attributable 

to faults (the Lake Owen and Keweenaw(~) faults) 

that cut out more and more of the stratigraphic section 

as they approach Mellen, the coincidence between the 

gravity low and the pinching down of the lava is remarkable. The comparable narrowing of the width of 

the lava belt on the north limb of the Ashland syncline is 

likewise accompanied by a decrease in the gravity, suggesting that here, too, the narrowing of the outcrop 

belt represents real thinning of the whole lava section 

rather than the nearsurface thinning of a fault slice cut 

from a thicker sheet at depth. If one calculates the 

thickness of the lavas directly from breadth of outcrop 

and connects points of equal thickness on the north 

and south limbs of the Ashland syncline, the isopachs 

are almost parallel to the Bouguer gravity contours of 

figure 2. 

Other supporting evidence comes from work on the 

upper Keweenawan sedimentary rocks. The Bear 

Creek Mining Co. drilled a number of holes in the upper 

Keweenawan rocks of the Ashland syncline in an area 

ranging from 15 miles south to 60 miles southwest of 

Ashland. These holes revealed that the Copper Harbor 

Conglomerate, immediately above the lava flows, grades 

from a coarse conglomerate in the more northerly and 

northeasterly holes to fine-grained sandstone, siltstone, 

and shale to the southwest and that theN onesuch Shale, 

immediately above the Copper Harbor (base of this 

formation shown by dashed line o on fig. 1), thickens 

southward (Moerlein, 1963). East of Mellen, northeastward thickening of the Nonesuch Shale was first 

recognized by Irving ( 1883), as was the conglomeratic 

character of the Copper Harbor Conglomerate in that 

area. Farther east in Michigan, the Nonesuch Shale 

thickens in a direction away from its ancient shorelines 

and in the same direction the Copper Harbor Conglomer~te changes rapidly from conglomerate to sandstone. 

Geologic observations in the Ashland-Mellen region, 

therefore, confirm a positive area in the general vicinity 

of the present gravity low at the beginning of hite 

Keweenawan time. These observations thus, indirectly, support the assumption that the gravity features 

are due primarily to the Keweenawan rocks themselves. 

NEGATIVE GRA. VITY ANOMALY 

This analysis points to a double rather than a single 

cause for the large nega,tive Bouguer anomaly north of 

Ashland: the anomaly marks the high-angle intersection of the axis of a thick prism of upper Keweenawan 

sedimentary rock with a ridge of pre-Keweenawan 

rock-an area in which the middle Keweenawan lavas 

are thin or absent. This dualistic hypothesis may seem 

more complex and therefore inherently less attractive 

than some of the single causes .that might be suggested; 

therefore, the adequacy of possible single causes needs 

to be at least briefly reviewed. 

The most likely single causes are an exceptionally 

thick prism of sedimentary rock (see Thiel, 1956, p. 

1090-1093), exceptional abundance of rhyolite within 

the normally mafic lava sequence, and low-density rocks 

in the pre-I{eweenawan basement. The last possibility 

is discussed in the next section and is not considered 

here. 

A thick sedimentary sequence alone will not explain 

the negative. anomaly north of Ashland. Southwest of 

the anomaly, the value of Bouguer gravity decreases 

steadily from a high of more than + 30 milligals to a 

low exceeding - 80 milligals over a distance of 25-30 

miles. Lava is at or near the surface for approximately 

two-thirds of this distance on the north flank of the 

Ashland syncline; therefore, in the area of this long 

gradient, an appreciable fraction of the large negative 

Bouguer anomaly, perhaps as much as half, is due to 

something other than sedimentary cover. 

If a thick sedimentary sequence is the sole cause of the 

negative Bouguer anomaly, the shape of the anomaly 

suggests that the sedimentary mass has the general form 

of a basin. Surface trends, however, do not lend any 

support to this interpretation of the structure. The 

base of the Devils Island Sandstone (line b, fig. 1) 

crops out on a line that cuts directly across the anomaly. 

Aeromagnetic trends, representing rocks deeper in the 

stratigraphic section, lend equally little support to the 

interpretation; the large U-shaped aeromagnetic high 

of figure 4 crosses the lowest part of the negative gravity 

anomaly. 

The possibility that the middle Keweenawan lava 

series contains exceptionally large amounts of rhyolite 

in the vicinity of the large gravity low is less readily 

dismissed than the preceding hypothesis. Rhyolite 

flows make up about 10 percent of the section measured 

by Sandburg (1938, p. 806) near Duluth, but are extremely rare in the Michigan copper district. They do 

not appear to be a conspicuous element of the lava 

sequence in the outcrop areas close to the gravity low 

(Aldrich, 1929; Grant, 1901; Moerlein, 1963). In addition, the Bouguer gravity anomaly maP· of the United 
TECTONICS OF THE KEWEENAWAN BASIN, WESTERN LAKE SUPERIOR REGION E19 

States (Am. Geophys. Union, Spec. Comm. for Geophys. 

~d Geol. Study Continents, 1964) shows almost no 

expression of a large volume of rhyolite in the Porcupine 

Mountains and adjoining areas, 7-25 miles east of the 

area of fig. 1. Rhyolite flows cannot be ruled out as a 

possible cause of at least part of the large gravity low, 

but there is no positive reason to suggest that they are 

the sole cause. 

No single simple cause, therefore, seems wholly adequate to explain all the features associated with the 

large negative Bouguer anomaly north of Ashland, and 

a dual cause involving both a thick prism of sedimentary rock and a ridge in the pre-Keweenawan basement 

rock is favored. 

;E"OSITIVE GRAVITY ANOMALY OVER DULUTH GABBRO 

COMPLEX 

Up to this point, the analysis of geophysical and geologic data has been concerned primarily with the subsurface structure of the Wisconsin part of the western 

Lake Superior region. In the Minnesota part of the 

region, the shape of the positive gravity anomaly over 

the Duluth Gabbro Complex has important implications if the gravity anomalies within the area of Ke-

;weenawan rocks are due primarily to the distribution 

bf these rocks. The crest of this anomaly lies about 10 

miles northwest of La:me Superior (figs. 2, 5). The 

size of the anomaly suggests that the gabbro is 20,000-

30,000 feet thick in the area of the maximum anomaly. 

The mafic lavas overlying the gabbro appear to have 

a homoelinal dip toward the southeast. The actual 

thickness of the lava series is not known. Sandberg 

(1938, p. 806) gave a thickness of more than 20,000 feet 

for the sequence of mafic and rhyolite flows exposed 

along the shore northeast of Duluth, and Grout, Sharp, 

and Schwartz (1959, p. 35-40) obtained a thickness of 

17,500-25,000 feet for the flows exposed along the shore 

in Cook County. These measurements, made by adding 

thicknesses in traverses almost parallel to the strike of 

the rocks are subject to large error-it would be easy, 

for example, to overlook faults at right angles to the 

shore and theneby fail to recognize repetitions within 

the section. Despite this possibility, the large area underlain by southeastward-dipping lava in Minnesota is 

evidence for a large volume of lava. The thickness of 

this lava increases southeastward and probably reaches 

a maximum southeast of the shoreline ·at most places. 

The thickness of lava to be added to the thickness of 

gabbro, therefore, increases lakeward from the crest of 

the gravity anomaly and sh~uld make an increasingly 

large positive contribution to the value for the gravity. 

The fact that the value of gravity decreases in this direction is clearly anomalous if, as many geologists have 

assumed, the gabbro is a sheetlike mass extending downdip to the southeast for some distance. 

Two alternative explanations of the observed decrease · 

in gravity are geologically reasonable : ( 1) the gabbro 

thinning southeastward faster than the lava series 

thickens, pinches out close to the lakeshore, or ( 2) a 

thick section of sandstone is downfaulted against the 

lava beneath the lake just offshore, and the decrease in 

gravity toward the lake is due to the mass deficiency 

caused by this sandstone. This latter effect is quantitatively inadequate to explain the observed gravity profile 

if the downfaulting is taken as an alternative to thinning of the gabbro (that is, if the gabbro is not thinned 

drastically). If it is assumed that the combined thick-

. ness of lava and· gabbro on both sides of the fault is on 

the order of 25,000 feet (which, in itself, implies appreciable lake ward thinning of either gabbro or lava), 

rough calculations indicate that the observed values for 

gravity at the lakeshore would require vertical displacements on the fault (and sandstone thicknesses) of the 

order of 10 miles or more. These difficulties concerning 

the fault hypothesis can be reduced or eliminated by 

assuming that the fault dips gently northwest and that 

the lavas are thrust far out over sandstone; however, 

such a fault would not be consistent with what is known 

of geologic structure anywhere else in the Lake Superior 

region. 

For these reasons, the better explanation for the lakeward decrease of gravity along the Minnesota shore 

appears to require that the gabbro thin rapidly downdip 

beneath the lava flows, pinching out entirely somewhere 

near the line of the lakeshore. 

CONCLUSIONS 

In summary, the following principal conclusions result from the assumption that the gravity anomalies 

within the area of l{eweenawan rocks are due primarily 

to the distribution of mass within these rocks: 

1. A rna p showing the general configuration of the 

masses of Keweenawan mafic rocks can be prepared 

by subtracting the estimated gravitational effect of 

overlying sedimentary rock from the Bouguer 

anomaly map. · 

2. Mafic rocks are thin or absent over a ridge or upwarp 

of pre-Keweenawan rock that crosses the Bayfield 

Peninsula southwest of Ashland. This upwarp 

may connect with a gravity saddle at long N. 91° 

W. on the Minnesota shore to effect a separation 

of major lava basins-the thick section of mafic 

lavas in the Ashland syncline continues north into 

Minnesota, and thins in the vicinity of the Beaver 

Bay complex (fig. 1); the thick section of lavas 

above the Duluth Gabbro Complex northeast of 

the Beaver Bay complex is probably part of an-
E20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

other lava basin extending eastward beneath Lake 

Superior. It may be significant that the Beaver 

Bay complex itself adjoins the ridge of preKeweenawan rock, as do the intrusions near Mellen. 

3. Distributions of sedimentary and mafic rock that 

produce maximum compatibility of surface geology, gravity data, and magnetic data suggest a 

thickness of 25,000-30,000 feet for the thickest 

part of the sedimentary wedge, under the bay 

northeast of Ashland. 

4. The Duluth Gabbro Complex thins downdip beneath 

the middle Keweenawan lavas and may pinch out 

entirely in the vicinity of the Minnesota shore of 

Lake Superior. 

SITUATION B: GRAVITY ANOMALIES DUE IN PART TO 

PRE-KEWEENAWAN ROCKS 

In Situation A the differences in the Bouguer anomaly 

from place to place within the area of l{eweena wan 

rocks were assumed to be due entirely to the distribution of Keweenawan mafic and sedimentary rocks. 

Large positive Bouguer anomalies coincide with areas 

where the thickness of mafic rock is known to be large 

and prove that the assumption is rut least partly correct; however, the interpretation of some gravity features, particularly the negative anomalies, might be 

quite different if the pre-Keweenawan basement rock is 

not assumed to have a uniform density of 2.t>7. In general, removing the constraint of a uniform basement 

density opens the door to such a wide range of speculative possibilities that discussion is not constructive 

without more information than is now available. It is 

worthwhile, however, to consider briefly the suggestion 

that the major negative anomaly of the area is due, in 

part, to differences of gravity in the pre-Keweenawan 

basement complex. 

A large negative gravity anomaly, elongate eastnortheast, lies within the a.rea of pre-l{eweenawan rocks 

in the southeastern part of the area of figure 2. This 

gravity low is clearly unrelated to anything in the Keweenawan sequence, except possibly Keweenawan granite, and proves the existence of important density contrasts in the basement rocks. Casual inspection of figure 2 suggests the possibility that this gravity low and 

the one north of Ashland are caused by the same large 

mass of low-density rock in the pre-Keweenawan basement; in other words, the two anomalies are parts of the 

same large anomaly, and the gravity high that separates 

them near Mellen on figure 2 is due solely to the local 

gravitational effect of the Keweenawan mafic rocks that 

reach the surface in that area. The station density of 

figure 2 is not high enough to rule out this possibility 

completely, but the details of the shape of the anomaly 

southeast of Mellen do not support it. Specifically, the 

axis of the negative anomaly southeast of Mellen should 

lie much closer to the steeply dipping southeastern margin of the Keweenawan rocks; it is at least 9-10 miles 

south of that margin. The gentle gradient southeast 

of Mellen suggests that the rocks immediately below 

the Keweenawan rocks do not have exceptionally low 

density, so that even after subtraction of the positive 

influence of the l{eweenawan rocks, a positive area. 

would remain in the vicinity of Mellen separating the 

gravity low in the pre-l{eweenawan basement from the 

low north of Ashland. A gravity profile of the negative anomaly southeast of Mellen cannot be duplicated 

or even approximated by a gravity profile calculated 

using the following restrictions: All the observed gravity effects are due to the distribution of rocks within 

60,000 feet of the surface; the boundary between the 

Keweenawan and older rocks dips steeply northward; 

the rocks underlying the bottom of the gravity low have 

the same density as the rocks between the bottom of the 

low and the base of the Keweenawan rocks; this last· 

condition is necessary if the negative anomaly southeast of Mellen is part of a single broad anomaly that 

also includes the low north of Ashland. 

For the foregoing reasons, the low southeast of Mellen is useful as an indication that important gravity 

contrasts do exist in the basement, but it does not provide direct evidence for a connection between lowdensity basement rocks and the anomaly north . of 

Ashland. The correlation of density contrasts in the 

pre-l{eweenawan basement with specific Bouguer anomalies, therefore, must remain in the realm of speculation 

until other types of geophysical ·and geologic evidence 

specifically support it. For the present, it is clear that 

most of the gravity features of the area are readily explained by gravity contrasts within the I\::eweenawan 

sequence, and there is no positive reason for looking to 

a more complex origin involving the pre-l{eweenawan 

basement. 

SUMMARY OF TECTONIC FEATURES 

Figure 7 is a sketch showing the main tectonic features of the western Lake Superior region as derived 

above. These features belong to three genera.tions, and 

the high-angle intersections of some of them are a principal cause of structural complexity in the region. 

Features of the youngest generation dominate the 

geologic map. They include the Ashland syncline and 

the faults parallel to it (Douglas fault, including its 

eastward projection, and the Lake Owen-Keweenaw( n 

fault). Because the faults dip toward the axis of the 

syncline, the block containing the syncline has the form 

of a keystone. In the latest movement, at least, the 

block has risen with respect to adjacent blocks. Craddock, Thiel, and Gross (1963, p. 6030) have called this 
TECTONICS OF THE KEWEENAWAN BASIN, WESTERN LAKE SUPERIOR REGION E21 

0 

r:r 

m 

m 

<( 

(!) 

STLOUIS 

E S 0 T 

10 0 10 20 MILES 

LAVAS THICK? 

1 0 

I IRON L __ _, I 

ASHLAND I 

I 

I 

I 

FIGURE 7.-Major tectonic elements. Bedrock of shaded areas is middle Keweenawan lava. Hachured-dashed 

lines suggest general trend of isopachs of middle Keweenawan lavas; hachures point toward areas in which lava 

series is thick. 

particular block the "St. Croix horst." From its synclinal structure and the fact that the Keweenawan 

rocks within the block are depressed with respect to the 

older Precambrian rocks outside the Keweenawan basin, one would expect the inward-dipping boundary 

faults of this "horst'' to have the movement sense of a 

graben. If these faults came into being as the boundary faults of grabens or rift valleys in which the Keweenawan lava flows accumulated, the latest movements on them were opposite to the principal sense. As 

is shown by the following, however, these youngest 

faults locally lie athwart the trend of the troughs in 

which the lava series accumulated; therefore, it is more 

likely that they do not necessarily coincide with structural features that existed in middle Keweenawan 

time. 
E22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

The second generation of tectonic features is represented by a second axis, more or less parallel to the 

Ashland syncline. This axis, which is shown northeast of Ashland on figure 7, represents the line along 

which the section of upper Keweenawan sedimentary 

rocks, both Bayfield and Oronto Groups, is assumed to 

be thickest. This definition would permit one to extend 

the line westward along the trough of the syncline 

formed by drag along the Douglas fault, but as envisaged here, the line represents the axis of a sedimentary trough of late Keweenawan time, before the 

Douglas fault was formed. The direction of sediment 

transport indicated by cross-stratification in the rocks of 

the Bayfield Group on the Bayfield Peninsula is northeast to east-northeast (Hamblin, 1961, fig. 3); this 

direction suggests that the Bayfield Peninsula area is 

on the axis of this late Keweenawan trough or near its 

western end, or both. 

Tectonic features of the oldest generation are the 

belts or areas in which the lava sequence is thick or 

thin, representing zones of subsidence and positive 

areas, respectively, in middle Keweenawan and the beginning of late Keweenawan time. These features are 

cut at a high angle by the Ashland syncline and its associated reverse faults. They are strikingly parallel. 

however, to some of the more conspicuous cross faults 

shown on figure 1, notably the large one cuUing the 

north limb of the Ashland syncline. This large fault, 

especially, and perhaps some of the others seem to mark 

places where the thickness of the lava changes abruptly. 

The Beaver Bay complex and the gabbro bodies east 

and west of Mellen occupy a similar position. This relationship suggests that the middle Keweenawan 

troughs may be, in part, at least, fault troughs, not unlike the Triassic troughs of eastern North America, 

which they resemble in many other respects (Pettijohn, 

1957, p. 629). It is necessary to assume that these older 

cross faults were reactivated during or following the 

movements that formed the Douglas and related faults, 

as the latter appear to be offset, but this is a reasonable 

expectation. Abrupt east-west changes in thickness 

have been observed in the Ashland-Mellen area and 

these changes ·are very simply explained if some of the 

cross faults that cut the Keweenawan rocks were present, perhaps as scarps, during the accumulation of the 

lava and sediments. The best example is the abrupt 

thinning of the Copper Harbor Conglomerate (shown 

by the interval between line a, fig. 1, and the middle 

Keweenawan lava) on the north side of the Ashland 

syncline in the general vicinity of the large cross faults 

mentioned above. Field relations near Mellen suggest 

other areas of abrupt thinning (see Aldrich, 1929, 

pl. 1). 

The principal value of this tectonic analysis may lie 

in demonstrating the possibility of major differences 

between the ancient Keweenawan troughs of accumulation and the present structural depressions. There is 

evidence in the Michigan copper district for minor departures from coincidence of the two (White, 1957, p. 

6-8) , but nothing there is comparable to the differences 

suggested in figure 7. 

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Wisconsin: Wisconsin Geol. and Nat. History Survey Bull. 

71,279p. 

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Aeromagnetic map of the Phoenix quadrangle, Keweenaw 

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TECTONICS OF THE KEWEENAWAN BASIN, WESTE.RN LAKE SUPERIOR REGION E23 

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---1952b, Total intensity aeromagnetic and geologic map of 

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i

eighton, M. W., 19. 54, Petrogenesis of a gabbro-granophyre 

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795-830. 

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1 

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