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GRAVITY SURVEY IN THE VI-CINITY OF ; 421—“— A O MELLEN, WISCONSIN

Thesis for the Degree of M. S. MICHIGAN STATEUNIVERSITY JONATHAN BORK‘ _ 1.967

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ABSTRACT

A GRAVITY SURVEY IN THE VICINITY OF MELLEN, WISCONSIN

by Jonathan Bork

In June, 1966, a regional gravity survey was run in north-central Wisconsin near the small town of Mellen.

Most of the survey centered over the Mellen Granite, a

3-by-5 mile acid intrusive found on the south limb of the

Lake Superior Basin. Due to glacial cover, complex faulting, and considerable intrusive activity, the geology remained obscure. Therefore 220 gravity stations were occupied within an l8-by-lS mile area to define the structural geometry of the Mellen Granite and the gross geology of the surrounding area.

The Lake Superior Basin is an east-west trending, assymetric, Precambrian trough filled with Keweenawan basalts and sediments. With the center of the structure underlying the lake, the outcrops, including thick gabbro sills, appear predominantly around the edges. In Wisconsin the Keweenawan series rests unconformably on erosional remnants of an older geosyncline. The Mellen Granite lies on the more steeply dipping south limb surrounded by

Keweenawan mafics and sediments.

Gravity stations were observed at intervals from

1/u mile to 2—3 miles depending on their proximity to the

Granite. A total of seven base stations were occupied Jonathan Bork throughout the area and were read at two hour intervals to eliminate drift and tidal variations. All stations were tied to the international network of gravity values.

Stations were located on topographic maps with the aid of an automobile odometer. Station elevations were observed with an aneroid altimeter and tied to points of known elevation. A C. D. C. 3600 digital computer converted all gravity readings to milligals and corrected for variations in elevation, latitude, drift, and near surface mass.

Terrain corrections were ignored.

Rock samples were collected from important sedi- mentary and igneous facies with their densities determined by the water immersion technique. A large density varia- tion was found in the sixteen samples taken from the granite. The granite varied from porphyritic granite

(2.65 gms/cm.3) at the top of the intrusion to diorite

(2.78 gms./cm.3) found near the base. Overall, the average density was 2.72 gms./cm.3

A The Bouguer gravity map revealed a gravity maxima traversing the area in an east-west direction with gravity minimas to the north and south. This maxima, part of the mid-continent gravity high, coincides with high density basalts and gabbros. The Bouguer gravity anomaly was separated into regional and residual components by graph- ical and statistical means. Both yielded the same type of regional map; however, the least squares residual map was inferior to the cross-profile residual. Jonathan Bork

The residual anomaly over the Granite was a steep- sided, flat-bottomed gravity minima of 6-7 milligals. The anomaly did not close, however, due to the lower density sediments found to the north. In general, gravity maximas were found over the high density gabbros, basalts, and iron—formation with minimas revealed over schists, gneisses, Animikean metasediments, granophyre, Keween- awan sediments and the Granite.

Two kinds of faulting were interpreted in this area: thrust faults parallel to the east-west strike, and cross faults which are perpendicular to the strike. One thrust fault runs across the entire north of the area and may connect the Lake Owen thrust fault, found several miles to the west, with the Keweenaw thrust fault traced into the eastern portion of the area.

Two and three dimensional theoretical gravity pro- grams were used to calculate the anomalies of various assumed body shapes. The anomaly of a rectangular- shaped slab, 2NOO feet in depth, with a density contrast of -.21 gms./cm. 3 fits the Granite's anomaly reasonably well. ‘A GRAVITY SURVEY IN THE VICINITY

OF MELLEN, WISCONSIN

By

Jonathan Bork

A THESIS

Submitted to Michigan State University in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Department of Geology

1967

ACKNOWLEDGMENTS

I want to express my gratitude to the following people without whose help this investigation could not have been completed:

To Michael Katzman, who made known to me the details of his investigation, aided in collecting samples, and reviewed the initial manuscript,

To Graham Williams and Michael Spurgat who assisted me in computer programming for the gravity reduction, least squares and the two-dimensional gravity calculation programs,

To Michigan State University for donating the time with the C. D. C. 3600 computer,

To Doctors Trow and Bennett and my colleagues at

Pan-American Petroleum Corporation for evaluating the initial manuscript,

To Doctor Hinze who guided me from the beginning stages of planning through the final writing and inter- pretation, and, lastly,

To my wife, Mary, who performed many of the onerous calculations and typed and proofread the manuscript.

ii TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ...... 11

LIST OF TABLES I O O O O 0 O O O O O O

LIST OF FIGURES O O O O O O O O O O O 0 vi

Chapter

I. INTRODUCTION ......

II. AREA OF STUDY 0 o o o o o 'o o 0

Geography ......

Physiography ...... > tUOLAJI-J

III. PREVIOUS INVESTIGATIONS . . . .

IV. REGIONAL GEOLOGY ...... \ON

Introduction . . O

Stratigraphy . . O Intrusives . . . O Faulting . . . 0

LOCAL GEOLOGY ......

Introduction ...... Stratigraphy ......

VI. GRAVITY SURVEY ......

VII. GRAVITY REDUCTION ......

Introduction . . . O

Observed Gravity . O Latitude Correction O Free Air Correction O

Mass Correction . . O Terrain Correction O

Error Analysis . . O

iii Chapter Page

VIII. ROCK DENSITIES ......

IX. INTERPRETIVE TECHNIQUE ......

Introduction . . , -. .

O

O Least Squares Statistical Technique

O O Cross- Profile Graphical Technique .

O

O Theoretical Gravity Formula . . . .

0 O

INTERPRETATION o o o o o o o o ' o o o 0

General Remarks . . .-. O O Regional Magnetic Anomaly ...... O O Regional Gravity Anomaly ...... O O Local Gravity Anomalies ...... O O Interpretation of the Mellen Granite O O

XI. CONCLUSIONS ......

XII. RECOMMENDATIONS o o o o o o y o o ' o o o o

BIBLIOGR APHY o o o o_o o o o o 0'. o o o o o o 77

APPENDIX o o o o o o o o o o o o o o o o o o o ’ 81

iv LIST OF TABLES

Table Page

1. Density of Rock Samples ...... ,. . . A3 LIST OF FIGURES

Figure Page

Area of Investigation ...... 0

Stratigraphic Succession of Precambrian Rocks in Wisconsin ...... 11

Generalized Geologic Map of Wisconsin . . . . . 16

Gravity Station Location Map ...... 3LI

Density Sample Location Map . . . . .‘. . . . . LI1

Elements of 3-Dimensional Gravity Calculations LI8

Bouguer Gravity Map ...... 52

Regional Magnetic Map ...... 5n

Least Squares Regional O-3'rd Degree . . . . . 56

Cross Profile Regional ...... 57

O-3'rd Degree Residual ...... 61

Cross-Profile Residual ...... 62

A Composite Geologic Map ...... 63

Observed Gravity vs. Computed Profile (N-S) 71

Observed Gravity vs. Computed Profile (E-W) . 73

vi CHAPTER I

INTRODUCTION

In the spring of 1966 a gravity survey under the auspices of Michigan State University's Geology Depart- ment was planned for an area in north-central Wisconsin.

The survey was centered over the Mellen Granite, a

Precambrian intrusion, and was timed to coincide with a simultaneous geological investigation. Michael Katzman, a Ph. D. candidate from Michigan State University, conducted the geological survey of the Granite.

The Mellen Granite lies on the south limb of the

Lake Superior Syncline, a Precambrian trough. Certain features of the granite make it an interesting subject for investigation by the gravity method. First, it is singu- larly located in the midst of mafic intrusives which stretch scores of miles to either side. Second, the geology of this area is particularly difficult due to the extreme faulting and deformation. Third, the northern sector of the granite is covered with glacial drift which makes actual outcrop surveillance impossible.

With these problems in mind, the survey was planned to determine the structural relationship of the Mellen

Granite with the surrounding rocks. More specifically, the object was to determine the areal extent and structural configuration of the Mellen Granite. CHAPTER II

AREA OF STUDY

Geography

Most of the survey was centered about the small town of Mellen, Wisconsin. Mellen, a town of 1000, lies Just to the east of the Mellen Granite in country of moderately rugged topographic relief. Roughly one-half of the survey area is government owned forest land that is thickly overgrown. Much of the remaining land is owned by mining companies. Twenty miles to the north lies Lake Superior and Ashland, Wisconsin, a city of 10,000. The prominent

Bayfield Peninsula lies immediately to the west of Ashland.

The area of the survey includes townships: TAAN,R2W; TAAN,R3w, THAN,Ruw; TASN,R2W; Th5N,R3w; Tusw,Ruw; and the southern halves of TA6N,R3W and TA6N,RAW (Figure l). The entire area lies in Ashland county between latitudes 46° lA'26.3" and A6°27'33.8" and between longitudes 90°33'A.2" and 90°55'33.2". The area approximates an l8-by—15 mile rectangle centering over the Granite with the extreme northeast sector removed. This sector is part of the Bad

River Indian Reservation and was inaccessible.

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Physiography

Within the area of the survey, two physiographic provinces are recognized: the Northern Highland and the

Lake Superior Lowland. Aldrich (1929, pp. 8-10) described the two provinces;

The Northern Highland has been called a peneplain which may be defined as an area which is essentially a plain, in that, despite undu- lations and an occasional hill, extreme relief of an earlier land surface has been reduced essentially to a level. The rocks of the high- land are in a great part crystalline, for example granites, gneisses, and schists.

He went on to describe the lowlands:

The Gogebic Range is a monadnock of the Northern Highland and is situated near the boundary between that province and the Lake Superior Lowland. Between these provinces there is marked contrast. The lowland is not a plain, for in certain areas active erosion by streams has cut the region into an extremely hilly topography. The total altitude range is approximately from 1000 feet above to about 300 feet below sea level (depth of Lake Superior). The highland on the other hand, has an elevation of around 1500 feet along its northern border and above this the Gogebic rises to an elevation of more than 1800 feet above sea level.

The Gogebic Iron Range, also called the Penokee

Range, extends southwesterly from section 1, TANN,R2W to the southwestern edge of TAHN,RAW. A second ridge is clearly defined a few miles to the north and roughly parallels the Iron Range. This is the so-called Copper

Ridge. A broad valley separates the two; Aldrich named this the Tyler Valley after the Tyler slate. North of the

Copper Ridge, the tOpography rapidly falls off toward the shore of Lake Superior. The Gogebic Range ceases to be a topographic high in the vicinity of Mineral Lake, near the eastern border of TAMN,RAW.

The lowest station elevation encountered within the survey's limits is 752 feet above sea level while the highest is 1567 feet.

Marked erosion, as mentioned by Aldrich, is exempli— fied by the Bad River as it winds its way through the

Gogebic Range. The range overlooks the river at Penokee

Gap where the river has gouged a sag of about 100 feet.

The Cap is located in Section 1A of TANN,R3W. The hilly topography of the lowlands is shown by the deep gorge of

Trout Brook, where, at one place, the depth reaches more than 100 feet. CHAPTER III

PREVIOUS INVESTIGATIONS

Many of the older studies of the region were obtained as "by products" in the search for COpper and iron ore because the Keweenawan basalts and Animikean iron forma- tion pass through the southern half of the area.

Regional geologic investigations include Irving (1883), Irving and Van Hise (1892), Van Hise and Leith

(1911), Thwaites (1912), and Leith, Lund and Leith (1935).

Irving's Monograph 5, from the U. S. Geological Survey, was a preliminary study of the Lake Superior region.

Thwaites investigated the Upper Keweenawan sediments which cover the northern portion of this study. All of the remaining publications were refinements and corrections of Irving's work.

Aldrich (1929), Leighton (1954), Olmsted (1965), and

Puffer (1965) performed geologic investigations which either partially or totally fell within the survey area. Although

Aldrich was primarily concerned with the iron formation, his work covered extensive portions of the survey areas.

Leighton, Olmstead, and Puffer did their work on the Keween- awan gabbroic complex. The first two dealt with the western portion of the area, and the latter with the eastern portion. Theil's gravity survey (195A) over the mid-continent

gravity high was the first geophysical work published on

this area. His work bordered on the Mellen Granite.

Mack (1957) added more stations to Thiel's work and made

the regional gravity of the area complete.

In 1966 the U. S. Geological Survey published an

aeromagnetic map of the western Lake Superior region based

on the work of Kirby and Petty. Two traverses of this

survey pass through the area of investigation with another

touching the northeast corner of the area.

The same year White (1966) combined all available

geologic and geophysical data in his paper on the western

Lake Superior Basin. Although regional in scope, White's paper does cover the Mellen Granite. Recent aeromagnetic work by Wold and Osteno (1966) and Patenaude (1966) seems

to bear out most of White's findings.

Many of the authors mentioned above have borrowed

extensively from the geologic maps of the upper Keweenawan as produced by the Bear Creek Mining Company. CHAPTER IV

REGIONAL GEOLOGY

Introduction

The Mellen Granite lies on the south flank of the

Lake Superior Syncline*, an asymmetric, Precambrian basin with an axis that generally parallels the central axis of the Lake. The Basin extends from northern Minnesota on the west to the eastern end of Lake Superior. Actually, the axis follows a broad, gentle arc shifting from a northeasterly direction in Wisconsin to southeasterly near its eastern boundary. Dips along the south flank range from “0—60 degrees with some dips near vertical recorded near large thrust faults. The north flank has much gentler dips.

Several ages of faulting, often at sharp angles to each other, have clouded the structural picture as it. existed in the Keweenawan. White (1966, p. El) discusses the evolution of the Basin. He classified the tectonics into three stages:

*The Lake Superior Syncline has variously been called the Lake Superior Basin, the Lake Superior Synclinorium, the Lake Superior Geosyncline, and the Keweenawan Basin. White subdivided it according to age into the Middle Keweenawan Trough, the Upper Keweenawan Trough, and the Ashland Syncline. lO

1. accumulation during the 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.

The positive area referred to by White extends generally northward from the center of the survey area to the northern margin of the Syncline. White concluded that the basalts were either thinned or totally lacking over this feature and thicken rapidly to the east and west.

Wold and Osteno (1966, p. 90) agree with White's inter— pretation.

Stratigraphy

All of the rocks, with the exception of glacial deposits and possible Cambrian sandstones, are Precambrian

sedimentary and igneous facies. Figure 2 gives the

classification of Precambrian rocks in Wisconsin. The

Precambrian can be broken down into three eras: the

Early Precambrian, the Middle Precambrian, and the Late

Precambrian. The boundary between the Early Precambrian and the Middle Precambrian marks the division between the

Archean and Proterozoic time spans.

Early Precambrian rocks are represented in Wisconsin by Kewatin greenstones and schists and Laurentian gneisses. ll

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12

Little is known of the rocks and they seem to have no relationship with later rocks. A pronounced and easily observed unconformity separates the Early Precambrian from the Animikean rocks.

Animikean rocks usually start with the Sunday quartzite, but in Wisconsin this formation is missing.

Consequently, the first formation in the Animikean sequence is the Bad River dolomite. An unconformity separates the dolomite from the overlying sequence called the Animikie

Group. This Group consists of quartz, slates, iron—forma-. tion and the thick Tyler slate.

On the South flank Animikean and Keweenawan rocks are-often found dipping at close to the same slope toward the north. However, a time span of indeterminate length separates the two areas. Van Hise (1911, p. 23“), on the basis of broad field relations, considered it a major uncon- formity while Aldrich believed the gap was deceivingly short. Aldrich (1929, p. 121) thought the elimination and shortening of the tilted formations was due to the Keweenaw thrust fault. Thrusting could account for the bevelled beds, he reasoned, for as the fault is traced from east to west, it progressively cuts through the older beds until it reaches the Pre-Animikean basement complex. However, if White (1966, p. E3) has correctly placed the Keweenaw fault, with respect to both time and location, Aldrich's idea of fault shortening must be discounted, and the length of 13 the unconformity would have to be considered longer than he imagined.

The pre-Keweenawan unconformity was followed by a thin discontinuous conglomerate, 100 feet thick, which signals, according to Aldrich (1929, p. 110) imminent vulcanism. Following the conglomerate came the tremendous outpouring of Middle Keweenawan amygdaloidal basalts. The basalts, averaging A3 feet per flow in Michigan, have accumulated to an estimated thickness of over 30,000 ft.

Most geologists believe the basalts erupted from deepseated fissures near the central axis. Considering their age, they remain amazingly unaltered.

Resting over the basalts and gabbros are the thick

Upper Keweenawan sediments. Normally, the sediments are considered conformable with the Middle Keweenawan; however,

Katzman (1967) has found pebbles of Middle Keweenawan granite and basalt in the lowermost member of the Upper

Keweenawan. This discovery makes an unconformity seem possible, at least, in Wisconsin. Upper Keweenawan sedi- ments have been grossly separated into the Oronto group, the oldest, and the Bayfield group. The Oronto group is usually coarser, more arkosic, and has abundant conglom- erates and shales while the younger Bayfield group is more quartzose and mature. Under the Bayfield Peninsula they reach combined thicknesses of, perhaps, 25,000 ft. Like the volcanics below them, they gradually decrease in dip toward the central axis. 1“

Tyler, Marsden, Grant, and Thiel (19A0, pp. 1A69-

1A83) have detected a marked difference in the heavy mineral suites of the two groups. Based on their work and logical inferences from the topography, regional gravity, and magnetic maps, White (1966, p. EA) has postu- lated an unconformity between the Oronto and Bayfield groups. With the deposition of the Upper Keweenawan, the deformation of the Lake Superior Basin was complete and a profound unconformity stretches from the Upper Keweenawan to the present.

Intrusives

Huge gabbroic sills have intruded the Middle

Keweenawan basalts on both sides of the trough. These intrusions welled up into the flows and probably followed bedding planes or, perhaps, thrust faults. Subsequent crystal settling and erosion have displayed the extent to which they have been fractionated. Usually ultramafics are found at the bottom with the more acidic granophyres

("red-rock") near the top. Both the Mineral Lake and

Mellen Gabbros found in the area of study exemplify this type of banding. According to Leighton (1954, p. 410) and

Olmsted (1966, p. 19) crystal orientation, often found in these bodies usually parallels the original boundaries, and imply that the intrusives were emplaced close to horizontal.

Most geologists place the mafic intrusives in the late Middle Keweenawan but conflicting time relationships 15

are found throughout the Lake Superior basin. White

(1966, p. E5) therefore, concluded:

The apparently conflicting relationships can only be reconciled by an explanation involving intrusions of gabbro at intervals throughout the general time span that is represented by the mafic lava flows, namely, all of the Middle Keweenawan time and the beginning of Upper Keweenawan time.

The Mellen Granite has numerous basalt and gabbro

inclusions which verify it as the youngest intrusive body

in the area. Its structural relationship with the gabbros

on either side and the conglomerates to the north were

not known prior to this survey. The very existence of a

solitary acid porphry in the midst of huge volumes of

mafic rock suggests that unusual circumstances were in

effect during the genesis of the Mellen Granite.

Faulting

Thrust faulting has long been recognized along the

flanks of the Lake Superior Basin. In fact, the Douglas

fault to the north and the Lake Owen and Keweenaw faults to

the south have defined the limits of the Ashland Syncline.

The large block that separates the two has been thrust up

in a horst structure which contains the center of the

Syncline. Craddock, Campbell, Thiel, and Gross (1963, p. 6030) have called it the "St. Croix" horst.

While the Douglas fault has been fairly well defined

(Figure 3), the traces of the Lake Owen and Keweenaw faults

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Aldrich (1929, p. 120) believed the gabbroic sills on both sides of Mellen were emplaced along the fault trace; he reasoned that the sill-like bodies would naturally follow planes of weakness, namely, the fault trace. Furthermore, a gradual bevelling at the base of the Middle Keweenawan extrusives lead him to conclude: "The bevelling of the

Keweenawan cannot be explained by erosion. Thrusting is the probable cause." WhfiB (Figure 3), on the basis of aeromagnetic data, has placed the Keweenaw fault several miles to the north along what Aldrich called the Bad RiVer thrust.

Similarly, the Lake Owen fault is also difficult to trace. If followed eastward, its location is certain to

TuHN,R6W; however, flmmnthis point on it is difficult to trace. Leighton (195“, p. “01), studying the area of

THHN,R5W, has traced it further eastward where he has tentatively projected it in a northeasterly direction. If this projection is valid, the Lake Owen fault could very likely be found in the area of investigation.

In addition to the east-west thrust faulting, numerous smaller faults have been detected perpendicular to the regional strike; that is, trending somewhat west of north. Aldrich believed the cross faults and their 18

associated cross folds were due to differential basin

settling. West of Mellen both cross folds and cross faults

are apparent. Most of these faults pass from the Animikean

into the Keweenawan lava pile. There seems to be little movement along most of these faults with the exception of

those in the vicinity of Mellen. Here, fault blocks out-

lined by the cross faults appear to have been thrust updip

very strongly. CHAPTER V

LOCAL GEOLOGY

Introduction

The geology in the vicinity of Mellen when compared with the regional geology (Figure 3) reveals several unusual occurrences. First, the formations seem to thin and converge as they approach a "focus" near Mellen.

Secondly, Mellen is the center of the intrusive belt found on the south limb of the Lake Superior Syncline. These gross features indeed suggest anomalous conditions of diastrophism, deposition, structural attitudes and erosion.

Stratigraphy

Pre-Animikean (Early Precambrian)

Bedrock in this area consists of Kewatin greenstone schists, Laurentian gneisses, and metabasalts. Together, they cover the southern one—fifth of the area (Figure 3) and appear to have little structural relationship with any of the surrounding geology. Both the schist and gneiss have secondary structure with the schist's cleavage striking north—south and dipping to the east and the gneissic banding striking east-west and dipping gently to the south.

Generally, the gneisses are found west of Penokee Gap (the

19 2O center of TUNN,R3W) and schists to the east. Metavol- canics are found in T4“N,R4W, but its exact boundary is unknown. The schists are generally regarded as the oldest, the gneisses next, and the volcanics of indeterminate age.

Animikeanfi(Middle Precambrian)

Overlying the Archean basement complex, and repre- senting the first deposition of the Lower Animikean, is the Bad River dolomite. The unconformity between the

Pre-Animikean and the Animikean is clearly defined. The

Sunday quartzite found elsewhere in the region appears to be absent in Wisconsin. The Bad River dolomite occurs intermittently at the base of the series and consists partly of dolomitic cherts and partly of cherty dolomite.

Strikes and dips of N “0° E and “5° NW were found by

Leighton (195“, p. NOS) in THAN,R5W and Aldrich (1929, p. 81) estimated its average thickness was 270 feet.

Stratigraphically above the dolomite lies the Palms quartzite, a highly quartZOSe deposit averaging 450 ft. thick in Wisconsin. The Palms formation is the first formation in the Animikie Group, a classification that takes in both the Middle and Upper Animikean. An uncon- formity occurs between the Bad River dolomite and the

Palms formation. The lowermost portion of the Palms is a conglomerate, succeeded by a second unit of thin bedded quartz and feldspathic slates. Above these two units is 21 a vitreous quartzite, often a greenish color due to regional metamorphism. The Ironwood formation, trending N60°E and averaging 650 feet in thickness, conformably overlies the Palms formation. A typical iron formation, the Ironwood is composed (Df varying proportions of chert and iron minerals. Typically, it is well-banded and extremely dense with two types of secondary structure: (1) cherty iron carbonates (siderite) with smooth, straight, regular bedding planes and (2) ferruginous cherts (jaspillite) with very irregular or wavy bedding planes. The iron minerals consist of hematite, magnetite, and siderite.

Mellen appears to be the center of a marked contrast in the iron formation. To the east the iron formation seems to be quite uniform and relatively uncontorted, whereas to the west many cross folds and cross faults are observed. In the extreme southwest corner of THHN,RUW, the iron formation has been folded into a northward plunging syncline striking N 150E. Just to the west of this area, the syncline gives way to a plunging anticline.

Both structures appear to have expressions further north in the Keweenawan series.

The most notable of the cross faults mentioned above is the Penokee Gap fault which trends generally north- northwest and can be traced into the Mellen Granite. Other faults found in the area are the Loon Lake-Mount Whittlesey 22

fault, the Reservoir fault, the Brunsweiler Mountain

fault, and the English Lake-Mineral Lake fault. Most of

these faults trend northward and often penetrate the over-

lying Keweenawan sequence. Katzman (1966) found no clear

evidence for the Reservoir fault which, according to

Aldrich (192U, p. 2) extends along the eastern border of

the Granite. This fault may exist further to the east.

Likewise, Olmsted (1966, p. 1“), while finding linearity

in the area found no evidence for the English Lake-Mineral

Lake fault. The Brunsweiler Mountain and Loon Lake-Mount

Whittlesey faults are more clearly defined. The Bruns-

weiler Mountain fault (found on the east edge of THSN,RUW),

according to Olmsted, defined the boundary between the

Mineral Lake intrusive on the west and the Mellen Granite

on the east. Aldrich has traced the Loon Lake-Mount

Whittlesey fault (found in THAN,R2W) northward into the

Keweenawan Series. However, Puffer (1965) showed no

evidence for such a fault in the Keweenawan rocks in this

area.

Proceeding westwardly, the gabbroic sills thicken,

and cut progressively deeper into the underlying Animikean

section. Consequently, this portion of the iron formation

is more crystalline, has greater amounts of magnetite, and

Very little residual hematite. This lack of "soft" hematitic

ore is the main reason the western end of the Gogebic Range has a considerably poorer grade of ore. 23

The thick Tyler slate is the last formation in the

Upper Animikean and conformably overlies the Ironwood. It averages 10,000 ft. in thickness, but is often much thinner due to faulting or deep erosional surfaces. To the east the Tyler slate thins from 10,000 ft. in TASN,R2W to zero ft. at Mineral Lake in THAN,RAW. Actually, the Tyler formation does not have slaty cleavage and technically should not be called a slate. In reality, it approximates a metagraywacke but varies from quartzites to thin bedded shales.

Keweenawan (Late Precambrian)

As mentioned earlier, the contact between the

Animikean and the Lower Keweenawan is considered a major unconformity by most geologists. Aldrich, however, con- sidered the gradual bevelling of the underlying Animikean sequence as partial proof for thrust faulting. At any rate, proceeding westwardly the Keweenawan Series rests on progressively older Animikean formatations until, in

TAHN,R5W, they rest on Archean basement rock.

The Lower Keweenawan consists of a series of sedi- ments ranging from conglomerates to arkoses and totals, perhaps, 100 ft. in thickness. This formation seems to be equivalent to the Pungwunge conglomerate in Michigan.

Aldrich extended this intermittent bed as far west as

Mellen. Puffer, however, places the last outcropping of it in the eastern edge of TASN,R2W (Figure 13). 2A

The Middle Keweenawan basalts which followed are difficult to trace due to glacial cover, extreme faulting, and subsequent intrusions. However, they appear to be thinner in the vicinity of Mellen, suggesting either non— deposition or thinning due to the existence of a tOpo- graphic high in Keweenawan time. White (1966, p. E3) and

Leighton (195”, p. AOl) have shown two basalt "stringers" extending into TASN,RAW from the west (Figure 3). The lowermost extrusive is very thin. The upper layer seems to form a broad arc, convex northward, across the northern part of TASN,RAW. Leighton (195A, p. 405) described this northern lava section as being more acidic than the basalts usually found in this area. The intervening area between the flows has been intruded by gabbros and their associated granophyres.

A large wedge of basalts enters the survey area from the northeast corner of TASN,R2W and according to most published maps partially wraps around the northern part of the granite (Figure 3). These basalts are presumably extended around the Granite on the basis of several out- crops located in section 22,TA5N,R3W.

White (1966, p. E21) has carried the Keweenaw fault down the middle of the basalt wedge where it apparently dies out near the eastern end of the Granite (Figure 3).

One lone segment of basalt appears in the lower part of TASN,R2W and is separated from the large basalt 25 wedge by the Mellen Gabbro (Figure 13). Aldrich inferred that the area now covered by the gabbroic sills and the

Mellen Granite was once occupied by the basalts.

The basalts vary widely from highly acidic to ultra— basic, but the basic flows predominate. On the whole, the flows are relatively unchanged since their extrusion except when found near intrusives or shear planes. They are generally conceded to be fissure type extrusives because of their tremendous thickness and lack of volcanic ash. Measurements made in Michigan of the pipe amygdules suggest a source to the north near the central axis of the basin. The basin foundered simultaneously with extrusion as evidenced by the "fanning" of dips as the flows are traversed basinward.

Gabbroic sills were emplaced in the basalts later in

Middle Keweenawan time. The foremost intrusions on the south limb are, of course, the gabbroic sills along the strike on both sides of Mellen. There appear to be two separate gabbroic intrusions to the west of the Granite.

The southernmost gabbro is the Mineral Lake intrusion and another body to the north which is unnamed. Together the bodies cover the northern and southern halves of TAAN,R2W and TASN,R2W respectively. The northern intrusion has not been studied and its exact boundary is unknown. Olmsted (1966, abstract) concluded that the Mineral Lake intrusion was a "single, very thick (16,000 ft.) 26 anorthositic body that has been concordantly emplaced near the base of the Keweenawan lavas." Leighton (195“, p. A01) studied a portion of the overlying gabbro further to the west where it interfingers with the basalts. He concluded that the "gabbro was intruded between flow units along planes of shear that resulted during formation of the geosyncline. Some gabbroic magma followed thrust faults."

Aldrich, of course, concluded that the base of the gabbros is the original trace of the Keweenaw fault.

The Mineral Lake Gabbro, like all of the gabbros, displays an amazing degree of differentiation, grading all the way from ultra mafics to granophyre. The overlying intrusion as well as the Mellen Gabbro seem to be more gabbroic. Plagioclase crystals in the Mineral Lake intru- sion are exceedingly large with well-marked striations.

Fluxion structure is evident in all of the bodies. This structure is regarded by Olmsted (1966) and Leighton

(195A) as parallel to the original walls of the intrusion. Fluxion strikes of N 600 E and N 40° E were found by Leighton (1954, p. ADA) and Olmsted (1966, p. 19) respectively. Both men recorded the fluxion dip as 60° NW.

A narrow band of fine-grained gabbro and diabase, approximately a mile in width, separates the Mellen granite to the north from the Tyler slate on the south. Midway between the Mineral Lake intrusion and Mellen, in section

2, THAN,R3W, a narrow block of Tyler slate separates the 27 diabase into two east-west bodies. Olmsted regarded this band of intrusive as part of the Mineral Lake intrusion.

Olmsted placed the boundary of the Mineral Lake intrusion at the edge of TASN,RAW where the Brunsweiler

Mountain fault separates the Gabbro from the Granite. If this fault is extended farther northward, it could also be the eastern boundary for the gabbro sill to the north.

Based on Aldrich's magnetic compass work and a prominent lineament in the Brunsweiler River, Olmsted (1966, p. 31) concluded that the Brunsweiler Mountain fault was a

"strike-slip fault of dextral nature."

The Mellen Gabbro is considerably narrower than the western gabbros, but progressively increases in width as it approaches Mellen. Here it is approximately 2 miles in width. Puffer (1965) shows an area with an intrusive chill zone and diabase dikes near the western boundary of the Gabbro.

The gabbros are usually placed in the Middle

Keweenawan and classified as post-extrusive in age.

Leighton (195“, p. “06), however, observed the gabbro cutting the so—called Copper Harbor conglomerate, the first deposit in the Upper Keweenawan. Therefore, White (1966, p. E5) concluded that the gabbros were emplaced contin- uously throughout the Middle Keweenawan and, perhaps, as late as the Upper Keweenawan. 28

The Mellen Granite has xenoliths of both gabbro and

basalt, and is definitely younger than either. However,

up to now its relationship with the overlying Upper Keweenawan sediments was unknown. Goldich 33 a1. (1959)

dated the Granite as .99 billion years old. If this date

is valid, this would put the granite in the late Middle

Keweenawan.

The Granite covers a 3-by-5 mile area and seems to

be located at the focus of this region's geological

events (Figure 3). It was not known whether it was a

stock-like structure or, like the bordering plutons, a

sill-like intrusion. Katzman (1966) has detected a

gradation from porhyritic granite to the north to diorite

at the bottom. Quartz-monzonite is also found. This

gradation from top to bottom seems to indicate the

granite is a sill—like structure dipping to the north. If

this is true, then the mode of emplacement suggested by

Aldrich (1929, p. 117) is possible. He suggested strong

updip thrusting of a block bordered by the Loon Lake-

Mount Whittlesey fault and the Brunsweiler Mountain fault.

The relief of pressure in the underlying basement rock

resulted in remelting and emplacement of the Granite.

Explosion breccias, found at various places in the

granite, consist of mafic xenoliths with granitic magma

literally forced between the fragments. This brecciation

seems to testify to the force of the emplacement. However, 29

other brecciation also found in the Granite seems to

indicate post-intrusive faulting.

Huang (1962, pp. 115-116) has summed up the charac-

teristics of epizone (surface to 5 mi.) emplacement.

Among these are: (l) restricted in size and discordant

with the country rock, (2) abundance of roof pendants and

(3) some brecciation due to explosion brecciation and

upward drag of the magma. Generally speaking, these

characteristics can be found in the Mellen Granite.

Hand specimenscfl‘the Granite reveal a gradual change

from north to south. The uppermost zone is composed of

porphyritic granite dominated by large white to light

pink feldspars up to 3/A in. on a side with large amounts

of free quartz. Hornblende crystals are, on the whole,

less than 1/8 in. in diameter. The feldspars are usually

blocky and weather to a light brown or white color.

Biotite crystals are found in some samples.

The porphyry gives way to granodiorite which has a much finer texture and, as a rule, is much darker in

appearance. Large blocks of feldspar are found "floating"

in a finer matrix of granodiorite. Many of these blocks

seem to be altered around the edges. This type of sample

seems to graphically illustrate the crystal settling in

the granite.

At the bottom of the intrusion is a narrow band of very fine—grained diorite. It is darkish in color, 3O presumably due to the plagioclases and hornblendes. The hornblende crystals vary from l/16 to 1/8 in. on a side.

No free quartz is evident.

Near the southwest corner of the Granite, namely sections 29, 30, 31, and 32 of TASN,R3W, a scattering of basalt and granite outcrops allows several interpretations.

According to one interpretation the area is predominantly granite with several, large basalt xenoliths; the other interpretation is the reverse, that is, most of the area has mafic rocks with only a few granite intrusions.

Olmsted (1966) interpreted this area as case one.

The Upper Keweenawan is represented in the survey area by the Oronto Group. The Copper Harbor conglomerate is the first deposit of this Group. Usually this facies has large angular fragments of granophyre or rhyolite cemented by a fine reddish matrix. However, directly above the Granite, Katzman (1966) detected pebbles of granite and basalt included in the conglomerate. This evidence suggests a possible unconformity between the

Middle and Lower Keweenawan. White and Wright (1960) noticed that the Oronto Group in Michigan interfingered with the underlying basalts which suggested to them a conformable relationship.

The conglomerate is succeeded in most areas by the

Nonesuch shale and the Freda sandstone and probably does

.in the survey area as well. However, where these 31 formations begin or end is open to conjecture. The shale and sandstone thicken rapidly going northward. They probably reach a maximum in Wisconsin near the axis of the Ashland Syncline. White (1966, p. E21) put the axis of the Ashland Syncline Just north of the northwest corner of this survey. At this point, the axis changes its generally eastward trend, veers northeasterly, and passes into Lake Superior. This nearly right angle turn seems to coincide with the convergence of and thinning of the formations further south near Mellen, and may be caused by the Keweenawan positive mentioned earlier. CHAPTER VI

GRAVITY SURVEY

A total of 220 stations were read in a gravity survey covering a portion of Ashland County in northern Wisconsin

(Figure l). A Worden #99 gravimeter was used with a dial constant of .09A5 milligals per scale division. Six base stations were established at appropriate points within the area and were "looped" and "tied" to a primary base station. The primary base is located at the railroad terminal in Mellen, Wisconsin, at the foot of a Geological

Survey bench mark. More specifically, it is located at latitude A6° 19.7' and longitude 90° 39.8'. The primary base was tied to the international datum by Mack (1957), and has an observed gravity value of 980,647.0 milligals and a Bouguer gravity value of -27.8 milligals. Base stations were read at two-hour intervals to eliminate instrumental drift and variations due to diurnal earth tides.

Station density was varied from l/A to l/2 mi. near the granite and gradually decreased to an average of 2 mi. at the margins of the survey. Stations were located near cultural landmarks wherever possible and were "picked" from

32 33

Geological Survey base maps. Consequently, most stations were read along roads and at intersections. Where location of a recognizable feature was impossible, an automobile odometer was used to record the distance from a known position.

An aneroid altimeter was used for determining station elevation. Geological Survey bench marks were used for base stations, and all stations were referenced to these known points of elevation. Base stations were read every hour, and certain stations were repeated to assure accuracy.

Figure A reveals the station locations and the position of the base stations. The base stations have a

"B" prefix for an identifier with the primary base called

"BB." Furthermore, this figure illustrates the more detailed work done near the granite and the regional coverage around the margins.

Appendix A gives a complete listing of latitude, observed gravity, elevation, and the final Bouguer gravity value for each station. 3A

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CHAPTER VII

GRAVITY REDUCTION

Introduction

Observed gravity values are read at stations with various elevations, latitudes and terrains. To make the data useful for these varying situations, corrections must be made. After these corrections have been made, the resulting value is called the Bouguer gravity anomaly.

The complete Bouguer gravity anomaly can be cal- culated with a computer according to the following equation:

nga = go—gl+ge-gm+gt where: G = complete Bouguer gravity anomaly

go = observed gravity

gl = latitude correction

ge = free-air correction

g = mass correction

gt = terrain correction

35 36

Observed Gravity

The gravity observations were made by the author in the summer of 1966. All stations were corrected for drift by returning to the base station at two-hour intervals.

Since all of the base stations were tied to the primary base, the observed gravity values were tied to the inter- national datum. The observed gravity readings are con- verted to milligals by simply multiplying the drift- corrected reading by the gravimeter scale constant. The scale constant for the Worden #99 gravimeter was 0.099A5 mgal./scale division.

Latitude Correction

Due to the increase in sea level gravity values from the equator to the north pole, all gravity stations must be corrected for latitude variation. This was accomplished by calculating the theoretical gravity given by the International Gravity Formula:

gl=978.0A9(l+0.005288A sin2¢- 0.0000059 sin 2 20) where 0 is the latitude. This was accomplished with the use of a high-speed computer.

Free-Air Correction

Since the gravitational attraction of the earth's mass varies as the square of the distance to its center, stations observed at varying elevations must be corrected 37 to a common datum or common radial distance from the earth's center. Usually this common datum is mean sea level. This correction is called the free-air correction and was cal- culated to sea level with the aid of a digital computer using the following expression:

ge = (0.0009 cos 20) h

where

II latitude

6- II

23' elevation above mean sea level.

Mass Correction

The free air correction ignores the mass of material between the station elevation and sea level. The gravity effect of this mass must be subtracted from the observed gravity to fully reduce all stations to a common datum.

A density of 2.67 grams/cm. 3 was used for this material and gives a correction constant of 0.03u096 mga1./ft.

Although this density is undoubtedly too high for certain rocks and too low for others, it has the advantage of tying this survey with others using the same density. Moreover, a density of 2.67 gms./cm. 3 seems to be the best average for crystal rocks. 38

Terrain Correction

The mass correction assumes that the topography

around a station is perfectly flat. This, however, is

seldom true, and the gravitational effects of hills and

valleys around a station must be added to the observed

gravity. Although elevations ranged from 700 to 1800 ft.

within the area, care was taken to place stations in

locations where the topography in the immediate vicinity was relatively flat. Since the majority of any possible

terrain error is found in the near vicinity of the

stations and because of the regional nature of this

survey, the terrain correction was omitted.

Error Analysis

Possible sources of error in a gravity survey

include the instrumental constant, the instrument drift,

earth tides, terrain errors, and inaccuracies in elevation

and station location.

The instrumental constant as reported by the manu-

facturer has been checked by the Department of Geology of

Michigan State University. Drift and earth tides were

eliminated by returning to a base station at two-hour

intervals.

The largest possible error results from discrepancies

in elevation. An aneroid barometer is usually considered

to have an accuracy of i5 ft. Assuming a maximum error 39 of five ft. in elevation, an error of $0.3 mgal. results in the Bouguer gravity anomaly.

The station locations were determined from Geological

Survey base maps, and should have accuracies within .05 mi. which would give an error in the latitude correction of

$.05 mgal. Of course, errors in instrument readings are inevitable, but large errors of this sort were removed by reoccupying stations which were inconsistent with neigh- boring stations.

Lastly, a negative error is introduced in the readings near the Gogebic Range. From theoretical calcu- lations and assuming the range is 300 feet high, 650 feet in width, and infinite in length, with a density contrast of 3.u2 gms./cm.3, a curve of error versus distance was obtained. At a distance of 1200 ft. from the iron forma- tion, the gravitational effect of the range was found to be less than 0.10 mgal. Since most of the stations were much farther than this from the range, this error is negligible over most of the survey.

Summing all of the possible errors from elevation, mass, free air and latitute (flfllld result in an error of

:.50 mgals. Such a combination is highly unlikely, however, since the signs of each possible error are likely to differ. CHAPTER VIII

ROCK DENSITIES

The gravitational attraction of rock masses depends upon this density; consequently, rock samples were taken from all significant facies in the area. Their densities were determined by the water immersion method, by first weighing them in air and then in water and using the following formula:

AIR

II ‘0 (w - TARE) AIR ' H 0 where

II density '0

W = weight in air I

2: — weight in water II *3 11> :U

m weight in water of the container holding the rock samples.

Thirty-nine samples were measured with Table 1 sum— marizing the finds and Figure 5 illustrating outcrop location.

Granite samples were subdivided into porphyritic granite, transition rock, granodiorite, and diorite. There appears to be a noticeable change in density toward the

HO Tl ol'oo’ 5 s 40’ ’ I, R4w 5° RSV nzw 3’ . 3° MELLEN, WISCONSIN ASHLAND COUN TY

Ounl 3 4 Hi1“ L+

.lS—SW—ZM) T45N |5-SW~2(8) ° 5‘5 ° 20-NE-4 T45N 0203-554 23-sw-I o 24- sw-m o 5-3 027-SW-QT OZHW-‘IO 0 S—IG 0 29—SE-I 02-25-42 028-5E-5 ° 25595 °s-? 5'2 06—27—l °33-NE-I ° -

3| NE-G 41 07-34-9 °33—N£—9 °3-34-l5 03455-7 oSS—SW—S 20' A o o 036" W"6 5-7 36-5

S-l7 ° S-IB 054 o H l—9 °H 05-6 OS-IS 544.30 0 4-I 3-3 144” °S-l2 OS—IS T44"

«'II’ 4 °S-Il 4— .1_ DENSITY SAMPLE LOCATION MAP FIGURE 5 ROW u‘oo’ s 5 40' '2' 5 0 A1 4 (Raw

A2 base of the granite, that is, the diorite zone. The average value for sixteen granitic rocks was 2.72 gms./cm. 3 which is a little above the normally accepted value of

2.67 gms./cm. 3 .- The densities varied from a low of 3 2.65 gms./cm. in the porphyritic phase, to a high of

2.78 gms./cm. 3 over the dioritic phase.

Twelve samples of gabbro were measured with their density found to be 2.93 gms./cm.3. This also has a higher density than the normally accepted value of 2.90~gms./cm.3.,

The COpper Harbor conglomerate, overlying the granite, has an average density of 2.62 gms./cm.3. This value agrees well with the value obtained by Birch

(195A, p. 8) of 2.65 gms./cm.3. White (1966, p. 35), using a density that made his calculated gravity profiles fit the observed gravity, also used a value of 2.62 gms./cm. 3 . A value of 2.62 gms./cm. 3 makes the average density of the conglomerate only .03 gms./cm. 3 lower than the porphyritic phase of the Granite.

Only one sample of Keweenawan basalt was measured, and consequently, the average value of 2.90 gms./cm.3 determined by Thiel (1956, p. 1087) was used for the gravity interpretation.

The density of the Ironwood formation reveals the degree to which it has been metamorphosed. Large amounts of magnetite have given it the extremely high density of

3.42 gms./cm.3. 143

TABLE l.--Density of Rock Samples.

"\ U #1 Density #2 . 1 ’7 ensity Iientifiaation k)a.VTD_Le AIO.

E. r‘ ms./CE. gms./cm.3

Granite

C‘. m . Forphyritic SO IIE 0‘1

m

o

\f 36 SN

Granite U\\f1

I\)

o "\1

LA.)

C7\ SQ SE .'\J

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\] 27 SW 2

I‘d 0‘»

I

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23 SW [\J C 36 an \J

17.35 (13(wa .'\J . 9

3% NW 1 2 Q U

\‘ "

( i

I] ’) A

if» ONU‘C‘OxO‘xO‘.G‘

r\ .1) r L. Q U MMNNMMMI‘U WW b. Transition Rock 23 P; l 2.65 2.68 c. Granodiorite 25 SF 5 2.87 2.80 33 Hi 1 2.76 2.60 a. Dioritc 3A SE 7 2.82 2.83 33 HR 9 2.77 2.76

_3(' :;“I‘llv :7. 2.7:) 2.70 Th 93 *n 2.50 2.81 Average 2.72 gws,/gm,5

Gabbro F 2.86 a? 2 an

:‘7 ?.E"? C8 2 ”1 Flt 2.93 1-11—9 2.88

[-7.73-IJ“? 3.11 {-EA-ih 2.08 “-l3-3 3. 8 5-; 7—3 8.03 r-SK-l 3.01 {—54-0 2.82 Average 2.93 gns./...

Copper Harbor' lw—Sf.(A) 2.62 Conrlcreraflo 17—3H.(P) 2.01

Averarn k.fl;

£' 0 ()1;up LA.)

.“1 LA)

Average 3.U2 gms./cm.

810 A IT;

s): MR) .75

Average 2.78 ng./cm.3

n

Basalt 7-"; .86 1‘0

1’) Tyler Slate C‘T‘\ P.) .79”

’2

i H

K: .07 U13

R) .69 U) (Exx‘u, I—JF—J M .72 Average (Tyler Slate) 2.69 gms./cm. 3

Quartzite 81A 2.50 Basalt 811 2.99 Granomhvre 2.75 2.70

*This value appears to be in error and was omitted from the density calculations. CHAPTER IX

INTERPRETIVE TECHNIQUES

Introduction

All Bouguer gravity anomaly maps can be resolved into two separate components: the regional effects and the areally smaller residual effects. The regional effects are due to large, deep density variations and are assumed to vary in a smooth, regular fashion. Residuals are the difference between the regional effects and the Bouguer gravity anomalies. They are usually more discrete and somewhat sharper than regional variations. Two separate methods were used to separate the regional from the Bouguer gravity map: (1) the cross profile graphical technique, and (2) the least squares statistical method.

Least Squares Statistical Technique

The least squares technique is a mathematical method of analyzing mappable data. The basic assumption under- lying this technique is that the regional surface can be approximated by a smooth equation called a polynomial expression. Polynomials can approximate anticlines, homo- clines and other simple shapes. .The various kinds of polynomial expressions are termed orders and are determined

AA “5

by their highest power. Usually, the higher orders fit the

data too well and eliminate much of the residual while very

low orders do not sufficiently approximate the regional.

Essentially, this method fits a three-dimensional

surface represented by various order polynomials to a

discrete set of points. In this case, the points are

gravity stations located along an X and Y coordinate system with the gravity values in the Z direction. The various

order polynomials are made to fit the points in such a way

that the sum of the squares of the residuals must be a minimum. The coefficients of the polynomial are calculated to make the equation fit the data with the above require- ment.-

The general polynomial used is:

A0 regional -— a00+alOX + aOlY + . . . aqu p Y q

where

a's = the coefficients of the equation.

ThiseXpression is solved for the coefficients by the use of matrices and is easily done by a digital computer. The

Bouguer gravity value is subtracted from the regional polynomial with the resulting value being the residual.

While being completely unbiased, this technique has a "moderating" effect on the data. That is, it has a tendency to distort the magnitude and extent of the anomalies. A6

Furthermore, this method cannot take advantage of the experience or insight of the trained interpreter. Figure

9 is a map of a 0-3rd degree least squares regional found to be the best polynomial fit.

Cross-Profile Graphical Technique

Briefly, this method consists of profiles of the

Bouguer gravity map which intersect at right angles.

Smooth surfaces are drawn through each profile and are drawn to approximate the regional surface. The regional values at each intersection must be the same. Through a process of trial and error, an approximation of the regional is determined. This method has the advantage of giving a quick and simple look at the residual and allows the interpreter to use its flexibility to incorporate a knowledge of the area into the resulting regional. Its accuracy, however, is limited and if used arbitrarily can give misleading results. Figure 10 is the regional map made using this technique.

Theoretical Gravity Formula

In 1948 M. K. Hubbert demonstrated a theoretical method of calculating the gravitational effects of two- dimensional bodies. In this method he utilized a line integral technique. However, until the advent of the high speed computer, such a method was impractical. Talwani,

Worzel, Lamar and Landisman (1959) utilized Hubbert's “7 findings and rearranged them to work in a computer. Since many bodies are approximated by linear models (such as faults and anticlines), this method has had widespread applications. However, many features occurring in nature do not approach linearity. Consequently, Talwani and Ewing (1960) determined a method to calculate the gravi- tational attraction of three-dimensional bodies of arbi- trary shape.. The following procedure is after them.

The body that is to be calculated must first be contoured. Each contour is replaced by an n-sided polygon which resembles the actual contours as closely as desired.

The gravity is calculated at any external point. The body is split up into thin lamina and the gravitational attraction calculated for each laminae. A curve is plotted showing the gravitational attraction of each laminae versus its height. This curve, then, is inte— grated to give the attraction of the entire body. This entire procedure may be done analytically with a digital computer with the final integration done numerically. In Figure 6, P is the point where the gravity of body

M is to be calculated. This point is also chosen as the origin of a left-handed cartesian coordinate system with the Z axis positive downward. A contour of the body is replaced by the polygonal laminae ABCDEF . . . of thickness dz. The gravitational attraction is termed Ag. Then, (1) Ag = de 48

y-«tXIs

91- 4x1;

Can {our al- dcpf/v Z

j ELEMENTS OF 3-D GRAVITY CALCULATIONS (after TALWANI and EWING, I960) FIGURE 6 “9 where V is the anomaly caused by ABCDEF. . . per unit of thickness. Now V is expressed by a surface integral, the integration being carried over the surface of ABCDEF.

Talwani and Ewing reduced this expression to:

(2) v = kp [¢dv — ¢2// (r2 + 22)$5 dv]

where both integrals are evaluated around the polygon and where

k = the universal constant of gravitation

p = the volume density of the laminae and

z, W, and r = the cylindrical coordinates used to define the boundary of ABCDEF.

By substituting

(3) r = P1 sin(¢i - Wi+l+T) into (2) and noting that Pi’ 01, and Ti+l are all constants, the line integral of any side, say BC, can easily be solved to give the value of:

Zcose1 Zcosei

(A) arc sin - arc sin (P12 + 22);5 (P12 + Z2)7

The total contribution to V of side BC is

Zcose Zcosei

(5) kp 71+1-Ti-arc sin 2 % ; + arc sin (Pi +2 )2 (P12+z2)g. 50

The total gravity expression of the laminae is determined by summing the expression of all n sides of the polygon to obtain

n Zcose Zcos¢i

(6) V = kp iixii+l-Wi-arc sin , +arc sin (pi2+22>¥ (P12+22)g}

Noting that Pi’ W1, wi+l’ cosei and cosd).i can all be expressed terms of xi, yi,tflxeco-ordinates of B, and the Xi+l’

Yi+l,tflxeco-ordinates of C, we see that V can be expressed exclusively in termscflfthe co-ordinates of the vertices of the polygon ABCDEF. .

The gravitational attraction of the body M is inte— grated from top to bottom by:

2 top Ag total = f VdZ. z bottom CHAPTER X

INTERPRETATION

General Remarks

Many features are evident upon examination of the

Bouguer gravity map in Figure 7. These are listed below:

1. the Bouguer gravity surface approximates a saddle shape, with a gravity maxima trending east-west through the center of the map with minimas trending toward the north and south,

2. a rather broad gravity nose with steep gradients along its sides is found in TA5N,R3W,

3. a positive gravity nose is isolated in the southwest corner of the map, A. an undulating pattern in TA5N,RAW,

5. a steep gradient decreasing toward the northwest is found in the northeast corner of the map,

6. a sinusoidal pattern is found in TAAN and TA5N,R2W,

7. a flattening of the steep northward gradient is found near the -50.00 mgal. contour and can be traced across the entire northern portion of the area,

8. a gravity nose outlined by the —57.50 mgal. contour at the northwest corner of the map, and

9. a broadening of the contours in the northern part of TAAN,RAW.

These features were not evident from previous regional surveys with the exception of points 1 and 5. For example, the highest contour on the present map is -15.00 mgal.

51 I, ’ I ' I, a I o so 5: no! so “3' R2. 35 . 0 oo ‘5 go I HELLEN, wuscowsm ASHLAND COUNTY - couroua INTERVAL—Suns.

2“

~26 ‘

'

T45” TQSN

I 52 P20

1 144::

4 Hi , I ‘ ’ . “P . +/’ R 4—. I, , . + \ o + II" BOUGUER GRAVITY MAP; - FIGURE 7 HQ! 00’ s 3' s o’ “1"." .0. I 2 ' ’ 5‘

53

found in TA5N,R2W; previous maps had their largest contours

as -30.00 mgal. However, the general saddle shape remains essentially the same as that recorded by Thiel (1956,

p. 1079).

Regional Magnetic Anomaly

The regional aeromagnetic map (Figure 8) for this

area is after Kirby and Petty (1966). Unfortunately, only two traverses of their much larger map pass through the

survey area with another traverse touching the northeastern

corner. Consequently, this map can only be used for study of the larger features within the area. Contouring between the traverses is open to conjecture. Patenaude

(1966, p. 118) published an aeromagnetic map of the northern portion of the area; however, because it was incomplete this map is not included. Nevertheless, where applicable, his map was utilized in the interpretation.

Several features are of interest in Figure 8: (l) a high amplitude, elongate maxima trending east-west which

correlates with the Ironwood formation, (2) a low flanked by two highs in the extreme northeast corner of the map, (3) a generally defined low centering about TA5N,R3W,

(A) a steep gradient decreasing northward in TA5N,RAW, and

(5) broadly spaced uniform contours in the northern part of the map.

oI'o o‘ 4 I o I I o _ . new 3’ 3°

”ELLEN, WISCONSIN ASHLAND COUNTY~ couroun INTERVAL-L 500 GAMMAS

Oval 8,! 4

T45" 5A

20’-

T44" T44N

blf «Mr -I

REGIONAL MAGNETIC M A P I 0"" WHITE. I966 I

FIGURE 8 7U“? Lu. I ROW omo’ 55 50' 1 40' "2' 35' l l :o’

55

Point 2 was used by White as evidence of the

Keweenaw fault, however, this feature cannot be followed into the area on Figure 8. In Patenaude's (1966, p. 118) work, however, this low can be traced into the center of

TA5N,R2W; presumably, the Keweenaw fault goes at least this far. Points 3 and 5 are due to the Mellen Granite and the thick sedimentary section north of the Granite.

Regional Gravity Anomaly

Several regional maps were prepared for this area using both the least squares (Figure 9) and cross profile

(Figure 10) techniques. The 0-3'rd degree polynomial surface appears to be the best statistical fit of the area; however, this regional is inferior to the cross profile regional.

The main task in fitting the regional in this area is that of determining the magnitude and extent of the mid-continent gravity high. This task is complicated by the fact that the relatively low density Mellen Granite appears very close to the apex of the gravity high, and thereby reduces its magnitude. Consequently, to restore the regional to its proper magnitude, the effects of the

Granite's minima must be added to the Bouguer gravity anomaly. This sort of addition is possible in the cross profile technique, but it is extremely difficult in the statistical method. A comparison of the Figures 9 and 10

oI'o o’ RZW s o’

—I ”ELLEN, WISCONSIN , ' ASHLAND COUNTY couroun INTERVAL— 5 was.

' 0 III I I ' I 4

- mm a -25 25'?

T45" T45" 56 )- 20' :o’~

T44" T44N

‘,,/’

+/ + «'us' « LEAST SQUARES REGIONAL-O-BRD DEGREE FIGURE 9‘ ss’ 5 o’ 415'33' 40’ _ "2' 35'

MI 50 I 45 I an 4O v a R2. ’5 t I '-° '

I— —I HELLEN, WISCONSIN \ _ ASHLAND COUNTY 7500 comoun INTERVAL—5IGL3.

"i' I t€<

T45" T45"

20” 57

T44" 744“

I- II’ «‘u’ 4 CROSS PROFILE REGIONAL a“ FIGURE I0 I 0500 51' ' 010 ' 410 '85! on ' It! '3 - ‘0 ' A

58 graphically illustrates the "moderating" effect of the least squares technique. The peak values over the gravity high in the center of Figure 10 are -30.00 mgal. while the highest contours over the same area in Figure 9 are —35.00 mgals. Despite its shortcomings, the statistical regional still maintains the basic saddle shape of the Bouguer gravity anomaly.

Thiel's (1956) gravity work in northern Wisconsin as well as the regionals of this survey reveal several features of merit: (l) the mid-continent gravity high passes from west to east through the center of the survey, (2) the gravity high is of a reduced order in the vicinity of Mellen but enlarges toward the east and west, and (3) the gravity high seems to correlate with the Keweenawan basalts and gabbros. Thiel (1956, p. 1089) commenting on the mid-continent gravity high said: The extension of the positive anomaly toward the Keweenaw Peninsula is interrupted in the vicinity of Mellen, Wisconsin. The reduced gravitation there is probably caused by the intrusion of a large mass of lower density granite as mapped by Aldrich.

It might be added, however, that a thinner sequence of high density basalts and gabbros could also cause the reduction in the gravity near Mellen. Based on White's interpretation of this area and the cross-profile regional map, it seems probable that the Granite alone could not cause this reduction of the mid-continent gravity 59

high. White (1966, p. E21) and Wold and Osteno (1966, p. 90) have proposed the existence of a Keweenawan positive trending north-south upon which the basalts are considerably

thinner. The southern part of this positive extends into the survey area. Thrust faulting in this area may also be partially responsible for the reduced mafic section.

Gravity minimas are found to the south and north of the mid—continent gravity high. The northern minima is probably due to a combination of the positive area men- tioned above and a thickening sequence of Upper Keweenawan

sedimentary rocks. The cause of the southern minima is unexplained. White (1966, p. E6) shows this minima closing farther to the south.

Local Gravity Anomalies

The interpretation of the geology in the vicinity of

Mellen depends on three interdependent factors: (1) the steep northward dip of the rocks, (2) the intrusions of the gabbro and granite, and (3) extreme thrust faulting which appears to have been controlled by the cross faults in the area. Depending on their age and order of occurrence each of the three have operated on the events following them.

Figure 13 is a composite geologic map of the survey area based on geologic work done by Aldrich (1929), Katzman

(1966), Leighton (195A), Olmsted (1966), and from inter- pretation of the gravity and magnetic maps. 60

White, as mentioned earlier, traced the Keweenaw

fault into the area on the basis of a magnetic low. The

Bouguer gravity map and the cross profile residual map

(Figures 7 and 12) reveal a slight minima at the extreme northeast corner of the survey. This minima, although based on two widely spaced stations, coincides with the magnetic low. However, both the magnetic and gravity expressions of this fault are lost within several miles

of the northeast corner. At this point the thrust fault

is probably terminated against a north-south cross fault.

Abrupt changes in the contours of the residual gravity map (Figure 12) along this fault and a sudden widening of the Mellen Gabbro farther to the south support this inter- pretation. Patenaude's (1966, p. 118) magnetic map also reveals abrupt changes in contours along this line.

West of this cross fault the Keweenaw fault splits into three thrust faults (Figure 13). Two small thrusts are found on either side of the -7.00 mgal. minima in the northern part of TAAN,R2W; and a large continuous thrust can be traced across the entire northern part of TA5N,R3 and A. The -7.00 mgal. minima is a reflection of the low density Tyler slate wedged between the Mellen Gabbro to the north and the Ironwood formation to the south.

The flattening of the gradient in the Bouguer gravity map (Figure 7), as evidenced by the broadening of the contours between the —A5.00 and -50.00 mgal. contours, is

oI'o o' 4'0' Ta RZW 35 3'0’

”ELLEN, WISCONSIN ASHLAND COUNTY CONTOUR INTERVAL— I MGL‘S.

I ~15 25-1

T45" T45" 61 I 20' zo’~

T44" T44"

«'I5' -I

’ 4’1 O-3RD DEGREE RESIDUAL FIGURE II R4W 50’ 41"R3W 40’ 30' l l

V I to .I 00 4To' 50 I

”ELLEN. WISCONSIN ASHLANO COUNTY CONTOUR INTERVAL— IIGLS. _

I- 30‘ 25" l

T45" T45" 62

I 20’ 20’4

T44" T44"

I- Is' CROSS PROFILE RESIDUAL FIGURE IE "4' 1 .

SI‘ I 55 0 ad 40 R4W RZW 3°

F" I'IELLEN , WISCONSIN ASHLAND COUNTY’

I 2 5' 'I ~25 :5

F; DINO-YON-

_

NVMVNIIMIN WAIT! "MINT.

5 ‘v9 4

: :3 1 Y"&,d Tl ‘J' T45"

IIMO'I IIIIII

J

------I

NVMVNIIMIM-Ifld TVLII CLAY. 63 [I I i a g D

voc nnnnnnn o T44N T44N I ...... 00". ::: ...... u can I ''''''' .....

IXPLANATION ----- mu “I.“ um —— film mum 00000 “‘3 "I.“ "It! 46w“-

...... °°° " - MIIEIFIIIJ oooooooooooooooooooooooooo -— CI”: mu - - - - IIEIIEI H «mm "I! FIGURE I 3 R4W RZW

. I ,dnsw O 9| 00 50 4O l J 1 I

6A the gravitational expression for the large thrust. On the cross profile residual map (Figure 12) this fault is expressed as a series of round to elliptical maximas with amplitudes of 0.00 to 1.00 mgal. The fault trace lies just to the south of-these maximas. Although offset repeatedly by cross faults, this fault can be traced from the northeastern corner of TA5N,R2W across TA5N,R3W where it swings southwesterly and exits the area midway between the northern and southern boundaries of TA5N,RAW. This fault probably continues in a southwesterly direction and connects up with the northeasterly projection of the Lake

Owen thrust fault. If this hypothesis is valid, the

Keweenaw and Lake Owen thrust faults may be one and the same, and their combined fault system would stretch from

Keweenaw Point in Michigan to the western part of Wisconsin.

A total of six cross faults have been interpreted in this area; Figure 13 shows their exact locations. These faults were interpreted on the basis of gravity and mag- netic information with as much geologic evidence as possible. The two faults in TA5N,RAW were interpreted on the basis of linear gravity gradients and magnetic infor- mation. The Brunsweiler Mountain fault is easily seen in the gravity residuals (Figures 11 and 12) and from field evidence but the fault to the west has gone undetected in the field. The gravity gradient cm‘ this fault is based on two widely spaced stations but seems to correlate with 65 the magnetic map of Patenaude (1966, p. 118). Both of these faults, as well as several to the east, are ques- tionable in the northern portion of the area.

The Penokee Gap fault traced into the Granite by

Katzman (1966) has an undetectable gravity expression to the south, but the residual map (Figure 12) has a very good expression of it to the north of the Granite.

However, according to the gravity residual, the movement to the north of the intrusion is opposite to the movement south of the Granite. Katzman's (1966) field study yielded the same results; namely, the Penokee Gap fault is a scissors type fault with the fulcrum point somewhere in the center of the Granite.

Three possible cross faults are found to the east of the Granite (Figure 13). The far eastern one was mentioned earlier. The other two are questionable faults interpreted on combined geologic and gravitational evi- dence. Both of these faults have subtle gravitational expressions and are found flanking a well defined thrust fault at the base of the Mellen Gabbro. The one nearest the eastern border Of the Granite coincides with a prominent southward protrusion of the Mellen Gabbro and a subtle gravity lineament. On the eastern side of thrust, a larger cross fault can be detected in the field by the large block of Mellen Gabbro that is isolated on its eastern side.

In the gravity (Figure 12) this fault can be traced 66 northward between two closed 0.00 mgal. contours and possibly further northward where it "pinches" the gravity nose extending northeasterly from the Granite. This fault, like the Penokee Gap fault, may be a scissors type fault.

The undulating pattern in TA5N,RAW seems to be the result of two factors: (1) a high density gabbro intrusive, and (2) two cross faults on either side of the intrusion.

Drag along these faults has given the gabbro a crescent shape configuration which is convex northward. The crescent shape of this body and the minima immediately to the north are based primarily on one station.

The high amplitude gravity nose found in TAAN,RAW is the combined result of the high density portion of the

Mineral Lake intrusive and metamorphosed Ironwood formation.

Although there is little control over the Mineral Lake

Gabbro, the anorthositic phase of this intrusive is reflected by a minima trough with a value of —2.00 mgals. The cross faults bounding the gabbro intrusion above the Mineral Lake gabbro do not appear to extend southward into the Mineral

Lake intrusion. This may indicate that the Mineral Lake gabbro is younger than the overlying gabbro.

The sinusoidal pattern in townships AA and A5N,R2W is the gravity expression of an alternating sequence of high density iron-formation, low density slate, and high density gabbros. All three bodies are expressed very clearly in the gravity map near the Granite, but further 67 toward the east and west, the paucity of stations makes their gravity expressions difficult to trace.

The gravity minimas in the lower portions of

TA6N,R3 and RAW may be due to steep dips of the basalts on the basinward side of the Lake Owen thrust. Another alternative to the above interpretation is the northward extension of the various cross faults through the Lake

Owen fault. The relative highs and lows in TA6N would then be due to areas of greater or less thrusting. The latter interpretation is illustrated in Figure 13, however, the first interpretation could be inserted just as easily.

Interpretation of the Mellen Granite

The residual anomaly of the Granite (Figures 11 and

12) is a 6-7 mgal. gravity minima with a rather flat peak across the central part of the intrusion.

The flanks of the minima are quite nicely defined on the east, west, and south edges of the Granite, however, the anomaly does not close to the north but rather extends northward toward the positive anomaly of the Lake Owen thrust fault. Part of the minimas northward extension noses toward the northeast where it combines with a well defined minima trough that exits the area in the northern part of TA5N,R2W. 68

The Penokee Gap fault mentioned earlier tends to distort the Granite's residual. This fault cuts through the center of the granite and is the reason for the sharp nosing in the —6.00 mgal. contour in the center of TA5N,R3W.

The Brunsweiler Mountain and Reservoir faults, on the other hand, have aided in defining the limits of the Granite.

These two faults have increased the lineations and gradients along the eastern and western edges respectively and have made these boundaries more recognizable.

Two small, but discrete, anomalies appear along the northern edge of the Granite. One anomaly, lying just east of the Brunsweiler Mountain fault, is a circular gravity minima of -8.00 mgals. The other anomaly is an elliptical maxima near the center of TA5N,R3W. This anomaly is around 2 mgals. in magnitude and is encompassed by the

-5.00 mgal. contour.

The composite geologic map of the Granite, utilizing both the geologic findings of Katzman (1966) and the gravity data, is illustrated in Figure 13. There are several changes from previous maps including:

1. the extension of the Penokee Gap fault through the Granite,

2. the elimination of the basalt layer wrapping around the Granite from the northeast, and

3. the approximate solution of the boundary of the granite at the southwest corner. 69

The southwest boundary of the Granite is similar to the findings of Olmsted (1966). He showed this area as predom- inantly granitic with several large basalt xenoliths.

This is essentially the interpretation presented in Figure

13 with the exception that only one xenolith of high density material was detected in this area. This, however, could be due to the relatively coarse station spacing.

The basalt layer usually shown wrapping around the northeastern portion of the Granite has no expression in the gravity. However, the small but discrete maxima, found in the center of TA5N,R3W is probably due to a small isolated block of basalt lying just north of the Granite and east of the Penokee Gap fault. Perhaps, this basalt is an erosional remnant of a basalt layer or a roof pendant in the Mellen Granite. At any rate, the minima gravity nose to the northeast of the Granite precludes the exis- tence, in_this area, of any basalt layer. Instead, the gravity nose and the minima trough lying farther to the northeast is probably the gravitational effect of a secondary syncline filled with Upper Keweenawan sediments.

The -8.00 mgal. gravity minima lying to the north- west of the Granite is probably due to a mass low density granophyre. Katzman (1966) has recorded an outcrOp of granophyre in section 20, TA5N,R3W which verifies this interpretation. 70

Several models were constructed of the Granite using both two and three dimensional computational methods. A density contrast of -O.2l gms./cm. 3 was used between the granite and the gabbros, and a contrast of +.03 gms./cm.3 was used between the Granite and the Copper Harbor conglom- erate. Figure 13 gives the location of two profiles where theoretical anomalies were calculated. The results of the I north-south profile 7-7' are shown in Figure 1A.

Since the densities of the Granite grade from north In to south; two densities were used in the theoretical computations. The average density value of 2.72 gms./cm.3 was used to define the south flank of the anomaly while a density of 2.65 gms./cm. 3 was more compatible with the north flank. A rectangular slab with a thickness of

2A00 ft. satisfied the minima due to the Granite. The very small density contrast (.03 gms./cm.3) between the northern portion of the Granite and the Upper Keweenawan sediments made it very difficult to determine the struc- tural configuration at this point. However, a thin basalt block 500 ft. thick dipping at a 30° angle to the north seemed to satisfy the small plateau in the observed anomaly. Furthermore, the conglomerates in this area have an average dip of 30°N. Consequently, the northern face of the granite was extended at a 30° angle down to a depth of 2A00 ft. A basalt block 5600 ft. wide, 1750 ft. thick, and at a depth of 1500 ft. satisfied the positive anomaly due to the thrust fault.

I0 —I Two DENSITIES FOR L '0 GRANITE

71

OBSERVED PROFILE <1: 80100 Isgoo

------COMPUTED PROFILE FEET

OBSERVED PROFILE vs. COMP’UTED PROFILE PROFILE 7-7 FIGURE I4

72

Figure 15 illustrates the results of the east-west computer profile F-F'. The theoretical model that fits the Observed profile is much the same as the model in

Figure 1A. A rectangular slab with an average thickness of 2A00 ft. and perpendicular sides satisfied the observed anomaly.

The gradual change from porphyritic granite near the top of the intrusion to diorite at the base indicates that the Granite is probably a sill-like intrusion dipping toward the north. The results of profile 7—7', although not conclusive, lend support to this interpretation. The

Granite was probably intruded during a period of thrust faulting. The relief of pressure may have melted part of the basement rock of the Keweenawan positive with the resulting magma intruding into the country rock along thrust faults or cross faults.

WEST ' - EAST 'SWVOW K) I I) \ l IO---I h—IO

, FEET

I .4¢°= -,2| ' l ZEEIOOO 73

{ ,1 l. 3 2000

3000

' OBSERVED PROFILE 0 l_ 8000 J ISOOO ’1

------COMPUTED PROFILE FEET OBSERVED PROFILE vs. COMPUTED PROFILE . PROFILE F- F FIGURE I5

CHAPTER XI

CONCLUSIONS

The structure in the vicinity of Mellen, Wisconsin

is controlled by three interdependent factors:

1. the igneous intrusion, both acidic and mafic,

2. the steep northward dip of the igneous sills and the Precambrian formations, and

3. the strong thrust faulting which appears to have been controlled by numerous north-south cross faults.

Regional gravity maps of this area have supported White's hypothesis that a Keweenawan positive extended into the

survey area. The mid-continent gravity high is of a reduced order in the Mellen area presumably because of a thinner sequence of basalt. The minima effect of the

Mellen Granite is not enough to account for this reduction.

Two kinds of faulting were interpreted in this area; thrust faulting parallel to the strike of the beds and cross faulting perpendicular to the regional strike.. Six cross faults were interpreted on the basis of gravitational and geologic information. A large thrust fault was inter- preted across the northern portion of the area. This fault could very well connect up with the Lake Owen fault found further to the southwest. Furthermore, the Keweenaw fault

7A 75 traced into the northeast corner of the area could also be nothing more than an offset segment of this fault. If so, the Lake Owen and Keweenaw faults could be the same fault or, at least, part of the same fault system.

The Mellen Granite was interpreted to be a northward dipping sill grading from porphyritic granite at the tOp to diorite at the bottom. Theoretical gravity calcula- tions revealed the Granite's approximate thickness is

2A00 ft.

The Mellen Granite is overlain to the north by the

Copper Harbor conglomerate and a small block of remnant basalt. The basalt does not extend around the north- eastern portion of the Granite. CHAPTER XII

RECOMMENDATIONS

To better delineate the lava flows and mafic intru- sives, a detailed magnetic survey over the same general area would yield fruitful results. Furthermore, gravity and magnetic surveys to the west and northeast of this area could possibly link the Lake Owen and the Keweenaw fault systems.

White suggested that paleomagnetic work be done in the area to give better time relationships. This would be particularly helpful over the gabbros to the west of this area.

The gabbroic mass above the Mineral Lake intrusion would be a good target for a detailed gravity and magnetic investigation.

Perhaps a detailed gravity and magnetic survey could be done along the proposed trace of the Douglas fault found on the north side of the Ashland Syncline. The work to date has been limited and regional in sCOpe. A gravity survey of this area would have to be of high precision due to the great depths of the causative bodies; namely, the

Keweenawan basalts.

76 BIBLIOGRAPHY

77 BIBLIOGRAPHY

Aldrich, H. R., 1929, The geology of the Gogebic iron range of Wisconsin: Wisconsin Geol. and Nat. History Survey Bull. 71.

Birch, Francis, Thermal conductivity, climatic variations, I vol.and heat 252. flow pp. near1-25, Calumet, 195A. Michigan, Am. J. Sci., II Dobrin, M. B., 1960, Introduction to Geophysical I Prospecting. k2

Goldrich, S. S., Nier, A. 0., Baadsgaard, Halfdan, Hoffman, J. H., and Krueger, H. W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bull. A1.

Grant, F. S. and West, G. F., 1965, Interpretation Theory in Applied Geophysics. Halls, H. C., 1966, A Review of the Keweenawan Geology of the Lake Superior Region, The Earth Beneath the Continents, American Geophysical Union, Geophysical Monograph 10, Publication 1A67, pp. 3-27. Huang, W. T., 1962, Petrology. Hubbert, M. K., 19A8, A line-integral method of computing the gravimetric effects of 2-dimensional masses, Geophysics. Katzman, M., 1966, (pers. comm.). Katzman, M., 1967, (pers. comm.). Kirby, J. R., and Petty A. J., 1966, Regional aeromagnetic map of western Lake Superior and adjacent parts of Minnesota, Michigan, and Wisconsin: U. S. Geol. Survey GeOphys. Inv. Map.

Leigh, C. K., Lund, R. J., and Leith, A., 1935, Precambrian rocks of the Lake Superior region: U. S. Geol. Survey Prof. Paper 18A.

78 79

Leighton, M. W., 195A, Petrogenesis of a gabbro-granophyre complex in northern Wisconsin: Geol. Soc. American Bulletin., vol. 65, p. A01-AA2.

Mack, J. W., 1957, Regional Gravity of Crustal Structure in Wisconsin, M. S. Thesis, Univ. of Wisconsin.

Olmsted, J., 1966, Petrology of a Differentiated Anotho- sitic Intrusion in Northwestern Wisconsin, Ph. D. Thesis, Michigan State University.

Patenaude, Robert W., 1966, A Regional Aeromagnetic Survey of Wisconsin, The Earth Beneath the Conti— nents, American Geophysical Union, GeOphysical Monograph 10, Publication 1A67, pp. 3-27.

Puffer, J., 1965, A Petrographic Investigation of the Mellen Gabbro, M. S. Thesis, Michigan State University

Talwani, M., Worzel, J. L., and Landisman, M., 1959, Rapid gravity computations for 2-dimensional bodies with application to the Mendocino Submarine fracture zone, Journal of Geophysical Research, vol. 6A.

Thiel, E., 1956, Correlation of gravity anomalies with the Keweenawan geology of Wisconsin and Minnesota: Grol. Soc. American Bull., vol. 67.

Tyler, S. A., Marsden, R. W., Grout, F. F., and Thiel, G. A., 19A0, Studies of the Lake Superior precambrian by accessory—mineral methods: Geol. Soc. America Bull., vol. 51.

Van Hise, C. R., and Leith, C. K., 1911, The geology of the Lake Superior region: U. S. Geol. Survey Mon. 52.

White, W. S., 1966, Geologic Evidence for Crustal Structure in the Western Lake Superior Basin, The Earth Beneath the Continents, American Geophysical Union, Geo- physical Monograph 10, Publication 1A67, pp. 28-Al. White, W. S., 1966, Tectonics of the Keweenawan Basin, Western Lake Superior Region, Geol. Survey Prof. Paper 52A-E. 80

White, W. S., and Wright, J. C., 1960, Lithofacies of the Copper Harbor conglomerate, northern Michigan: U. S. Geol. Survey Prof. Paper AOO-B, pp. 85—88.

Wold, Richard J., and Osteno, Ned A., 1966, Aeromagnetic, Gravity, and Sub-Bottom Profiling Studies in Western Lake Superior, The Earth Beneath the Continents, American Geophysical Union, Geophysical Monograph 10, Publication 1A67, pp. 66-9A. APPENDIX

81 82

31:32:?“ 83:31:? 23:51:;

1 A6.38A8 98066A.30 1166 —20.08

2 A6.3988 980652.55 11A3 —3A.A8 3 u6.3708 980661.99 1166 -21.13

A u6.3570 980657.95 1230 —20.08 +7 5 A6.3u22 9806u9.00 1350 _20.u9 -FF—T‘ 6 A6.3A23 980652.A0 123A —2A.06 7 A6.33u7 9806u7.80 1329 -22.58 8 u6.3357 980652.05 1222 -2u.53

9 A6.3A23 980651.93 1225 -25.07 10 A6.3595 9806A6.66 12A5 -30.69 11 u6.3522 980652.22 1159 -29.63 12 M6.3588 980653.80 1135 —30.09 13 u6.3697 98065u.51 1116 -31.51 1h A6.3203 980639.34 1381 -26.31 15 u6.3277 9806u2.81 1382 —23.uu

16 A6.3277 9806A5.0A 1A05 -19.8A 17 06.3385 9806u7.87 139A —18.65 18 A6.u138 980657.65 1201 -27.25

19 A6.3853 980658.A8 1358 -1A.A3 20 96.3710 980657.75 1393 -11.76

21 A6.3563 980655.88 1372 -13.56 22 u6.3292 9806u5.69 1u71 -15.36 23 u6.3180 9806u6.5u 1u27 -16.1u 83

33:32:" 83:31:? 22:81:;

24 46.2925 980631.33 1554 —21.43 25 46.2847 980631.45 1567 -19.83 26 46.2885 980627.94 1527 —26.08 27 46.2845 980626.50 1490 —29.38 28 46.2713 980616.43 1490 -38.26 29 46.2635 980613.11 1518 -39.20 30 46.2415 980611.54 1510 -39.26

31 46.2708 980616.60 1559 -33.91 32 46.3173 980644.33 1260 -28.31 33 46.3162 980642.63 1250 -30.51 34 46.3100 980636.98 1316 -31.65 35 46.2938 980623.67 1499 -32.51

36 46.2848 980621.07 1430 -38.44

37 46.2703 980614.49 1457 -42.09 38 46.2418 980611.02 1472 -42.09 39 46.2232 980609.43 1459 —42.78 40 46.2422 980613.29 1453 -40.99

41 46.2667 980614.06 1486 —40.45 42 46.2678 980611.39 1524 -40.95

43 46.2703 980611.57 1494 —42.80 44 46.2705 980614.26 1446 —43.00

45 46.3357 980631.98 1407 -33.50 46 46.3390 980636.23 1342 —33.45

84

888888“. 81:81:88 8:28:11:

47 46.3423 980637.06 1317 -34.42

48 46.3058 980633.38 1488 —24.55

49 46.3078 980633.49 1453 -26.71 50 46.2975 980633.13 1456 -25.97 51 46.3018 980638.29 1445 -21.86 52 46.2865 980639.53 1425 —20.43 53 46.0000 980628.36 1431 —27.53 54 46.2582 980626.17 1422 -31.42 55 46.2775 980633.78 1468 -22.79 56 46.2580 980621.65 1412 ~36.52 57 46.0000 980616.07 -103.58 58 46.0000 980627.30 -92.35 59 46.2927 980631.96 1444 -27.42 60 46.2855 980626.74 1411 -33.97

61 46.2888 980638.97 1406 -22.34 62 46.2938 980629.75 1426 -30.82

63 46.2845 980606.05 -139.27

64 46.3057 980627.63 1416 -34.60

65 46.3115 980628.41 1414 -34.47

66 46.3095 980628.54 '1404 -34.76 67 46.3100 980629.82 1374 -35.32

68 46.3253 980644.84 1259 -28.59

69 46.3147 980637.57 1314 -31.59 85

888888“ 83:31"??? 8:28:11:

70 46.3205 980639.51 1358 —27.54 71 46.3270 980635.98 1436 -26.98 72 46.3297 980633.80 1527 -23.93 73 46.3465 980638.24 1267 -36.62 74 46.3497 980639.24 1200 -39.92 75 46.3497 980637.37 1265 -37.89 76 46.3510 980636.76 1275 —38.03

77 46.3497 980637.47 1316 -34.73 78 46.3497 980639.09 1311 -33.41 79 46.3500 980640.94 1325 -30.75 80 46.3435 980646.93 1263 -27.90 81 46.3530 980643.77 1201 -35.64 82 46.3205 980632.60 1457 -28.51 83 46.3338 980635.31 1361 -32.76 84 46.3210 980632.88 1475 -27.20

85 46.3113 980635.10 1462 -24.89 86 46.3230 980636.03 1403 -28.55 87 46.3587 980645.32 1167 -36.64

88 46.3845 980646.64 1076 —43.11

89 46.4142 980655.19 840 -51.40 90 46.3955 980647.48 1009 —47.29 91 46.3847 980644.52 1074 _45.36 92 46.3853 980645.77 1031 —46.76 86

81111? 8:21:11? 81:11::

93 46.3705 980641.55 1172 -41.18

94 46.3563 980641.59 1289 -32.84 95 46.3473 980644.14 1309 -28.28

96 46.4140 980654.26 845 —52.02 97 46.4065 980651.82 913 -49.70 98 46.4287 980651.77 821 -57.27 99 46.4323 ' 980651.59 796 -59.28 100 46.4430 980649.14 907 -56.04 101 46.4572 980651.07 858 —58.32 102 46.4572 980651.33 825 -60.05

103 46.4430 980651.67 844 -57.28 104 46.4575 980651.33 834 -59.54 105 46.4575 980652.34 827 —58.94

106 46.4432 980652.95 834 -56.62 107 46.4220 980655.22 789 -55.14 108 46.4362 980654.78 799 -56.2

109 46.4577 980653.27 831 -57.79

110 46.4362 980655.75 776 -56.67 111 46.4252 980654.38 786 -56.45 112 46.3857 980648.29 982 -47.21

113 46.3855 980648.64 991 -46.30

114 46.3747 980649.48 1137 -35.73 115 46.3747 980631.56 1310 -43.27 87

11:12? 888811? 8:21:11:

116 46.4140 980655.75 798 -53.35 117 46.4070 980657.80 800 —50.55 118 46.4212 980655.19 772 -56.11 119 46.4072 980656.93 830 -49.64 120 46.3998 980649.36 951 —49.28 121 46.4122 980652.10 930 —48.91

122 46.3807 980641.39 1095 —46.88

123 46.3858 980643.04 1085 —46.29 124 46.3855 980646.18 1102 -42.10 125 46.4005 980646.72 1050 —46.04 126 46.3772 980640.52 1128 _45.45

127 46.3772 980638.24 1134 -47.37 128 46.3928 980649.00 966 -48.11

129 46.3855 980647.59 993 -47.24 130 46.3903 980650.21 931 —48.77 131 46.3855 980648.55 945 -49.15 132 46.3530 980639.59 1175 -41.38 133 46.3570 980639.17 1173 —42.28 134 46.3568 980640.10 1146 —42.96 135 46.3567 980640.25 1146 -42.79 136 46.3565 980641.96 1143 -41.25 137 46.3607 980639.19 1132 -45.05 138 46.3640 980637.82 1142 -46.12 88

81:18? 811211? 8:21:11:

139 46.3638 980639.12 1135 -45.22 140 46.3688 980640.24 1120 -45.46 141 46.3775 980641.90 1069 -47.64

142 46.3682 980635.07 -57.77 143 46.3803 980644.29 1033 -47.67 144 46.3722 980639.39 1112 -47.09 145 46.3497 980630.78 1187 —49.17 146 46.3495 980630.32 1190 —39.48 147 46.3422 980636.14 1297 -36.53

148 46.3495 980636.75 1249 -39.46

149 46.3568 980639.97 1165 -41.95 150 46.3640 980634.59 1229 -44.13 151 46.3640 980633.62 1251 -43.78 152 46.3665 980633.44 1243 -44.67

153 46.3690 980634.68 1214 -45.39 154 46.3713 980638.30 1149 -45.89 155 46.3747 980640.46 1107 -46.55 156 46.3743 980642.12 1089 —45.94 157 46.3622 980636.82 1204 -43.24 158 46.3672 980637.65 1193 —43.51 159 46.3712 980638.28 1151 —45.77 160 46.3712 980641.65 1102 —45.34 161 46.3747 980640.29 1114 -46.30 89

81:12? 8:21:11? 8:21:11:

162 46.3780 980640.89 1090 -47.43 163 46.3822 980641.02 1080 —48.28 164 46.3855 980640.71 1080 —48.89 165 46.3888 980645.24 1009 —48.93

166 46.3778 980640.44 1095 -47.57 167 46.3720 980638.79 1138 —46.12 168 46.3655 980642.10 1129 -42.76 169 46.3613 980642.91 1149 —40.37 170 46.3563 980637.45 1221 —41.06 171 46.3573 980635.04 1273 -40.44 172 46.3563 980638.82 1219 -39.81 173 46.3565 980640.68 1203 -38.92 174 46.3565 980641.03 1204 —38.51 175 46.3595 980642.15 1161 —40.25 176 46.3570 980642.32 1183 -38.53 177 46.3523 980640.32 1243 -36.51

178 46.3495 980638.23 1296 -35.16 179 46.3462 980640.95 1273 -33.52 180 46.3422 980638.32 1321 -32.91 181 46 3420 980634.84 1344 —35.00 182 46.3312 980627.71 1430 -35.99 183 46.3312 980629.55 1392 -36.43 184 46.3535 980633.78 1312 -39.01 90

11:11? 8:21:11? 81121::

185 46.3495 980636.38 1274 —38.33

186 46.3220 980646.22 1233 —28.47

187 46.3295 980644.42 1275 -28.42

188 46.3337 980653.51 1262 -29.74

189 46.3203 980640.34 1327 -28.56

190 46.3262 980637.77 1366 -29.31

191 46.3348 980636.33 1390 -30.10

192 46.3478 980643.25 1260 —32.15

193 46.3422 980644.09 1262 —30.68

194 46.3348 980646.23 1271 -27.33

195 46.3285 980637.96 1342 -30.77

196 46.3282 980635.81 1375 -30.91

197 46.3213 980634.19 1419 -29.27

198 46.3283 980631.37 1442 —3l.35

199 46.3280 980631.01 1437 -3l.98

200 46.3278 980630.40 1440 -32.39

201 46.3277 980632.54 1415 -31.74

202 46.3277 980632.92 1412 -31.54

203 46.3277 980631.33 1444 -31.21

204 46.3277 980632.17 1447 -30.18

205 46.3278 980635.90 1399 —29.35

206 46.3423 980630.03 1405 -36.17

207 46.3387 980629.86 1428 -34.62 91

812888“ 822211? 82121::

208 46.3352 980629.26 1452 -33.48 209 46.3315 980630.56 1446 -32.20 210 46.3628 980640.30 1155 -42.76 211 46.3638 980636.22 1227 -42.61 212 46.3603 980641.10 1175 —4o.53 213 46.3205 980632.91 1448 -28.74 B1 46.3637 980654.22 1347 -17.39 B2 46.2982 980640.33 1405 -21.89 B3 46.3045 980635.14 1451 -24.89 B4 46.3280 980627.51 1513 —30.92 B6 46.3567 980638.25 1208 -41.07 B7 46.4072- 980655.09 837 -51.06 BB 1238 -27.55

nI11111|1|||1|4113|1|11|141 11 11111“ 31293