“s
' ‘w'w .
‘

 

A GRAVITY SURVEY IN THE VI-CINITY OF ;
MELLEN, WISCONSIN

O 421—“—

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

 

 

é€$%3n&m .

   

 

 

 

 

 

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

3

of -.21 gms./cm. 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

ACKNOWLEDGMENTS . . . . . . . . . . . .

LIST OF

LIST OF

Chapter
I.

II.

III.

IV.

VI.

VII.

TABLES I O O O O 0 O O O O O O

FIGURES O O O O O O O O O O O 0

INTRODUCTION . . . . . . . . .
AREA OF STUDY 0 o o o o o 'o o 0

Geography . . . . . . . . . .

Physiography . . . . . . . .>

PREVIOUS INVESTIGATIONS . . . .
REGIONAL GEOLOGY . . . . . . .

Introduction . .
Stratigraphy . .
Intrusives . . .
Faulting . . .

LOCAL GEOLOGY . . . . . . . . .

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

GRAVITY SURVEY . . . . . . .
GRAVITY REDUCTION . . . . . . .

Introduction . . .
Observed Gravity .
Latitude Correction
Free Air Correction
Mass Correction . .
Terrain Correction
Error Analysis . .

iii

0 O O O

O O O O O O O

Page

11

vi

tUOLAJI-J

\ON

Chapter
VIII.

IX.

XI.

XII.

BIBLIOGR
APPENDIX

ROCK DENSITIES . . . . . . . . . . . .
INTERPRETIVE TECHNIQUE . . . . . . . .

Introduction . . , -. .
Least Squares Statistical Technique
Cross- Profile Graphical Technique .
Theoretical Gravity Formula . . . .

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

General Remarks . . .-.

Regional Magnetic Anomaly . . . . . .
Regional Gravity Anomaly . . . . . .
Local Gravity Anomalies . . . . . . .
Interpretation of the Mellen Granite

CONCLUSIONS . . . . . . . . . . . . . .

RECOMMENDATIONS o o o o o o y o o ' o o o o

APHY o o o o_o o o o o 0'. o o o o o o

o o o o o o o o o o o o o o o o o o o ’

iv

0 O O O

O O O O

O O O O O

O O O O O

Page

77
81

LIST OF TABLES

Table Page

1. Density of Rock Samples . . . . . . . . .,. . . A3

Figure

LIST OF FIGURES

Area of Investigation . . . . . . . . . . . . .

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

Generalized Geologic Map of Wisconsin . . . . .
Gravity Station Location Map . . . . . . . . .
Density Sample Location Map . . . . .‘. . . . .
Elements of 3-Dimensional Gravity Calculations
Bouguer Gravity Map . . . . . . . . . . . . . .
Regional Magnetic Map . . . . . . . . . . .
Least Squares Regional O-3'rd Degree . . . . .
Cross Profile Regional . . . . . . . . . .
O-3'rd Degree Residual . . . . . . . . . .
Cross-Profile Residual . . . . . . . . . . . .
A Composite Geologic Map . . . . . . . . . . .
Observed Gravity vs. Computed Profile (N-S)

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

vi

0

Page

11
16
3LI
LI1
LI8
52
5n
56
57
61
62
63
71
73

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.

 

_ $39...

 

 

 

 

zo_Eo_5m>z_ Lo 3%
muz<m
m. z: :6 3m- 2:.
_ I _ i
\_\
\ I . \. III
\ \ I... I (5238 I
\ i\\\oz<axm<
zazoomi \ x I a .//4
T4 7/
V
2. ///N/// II III/9.23:2

 

 

 

w

 

 

3

N¢

m¢

¢¢

n?

wv

NV

POBZmI—a

 

 

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

 

 

 

 

 
  
 

 

 

 

 

 

 

 

 

L E RA I QQQ'TOERT {SEQUENCE lFORMATION I
. VA Bayfield. am. A
UPfiER OroMo 'Otoup
UM
'- A T E awe mm.
HPRECAMBRIAN KEWEENAWA" MIDDLE cm“
. ' amt-I‘M“
Hon
'0 I It and
LOWER 6:363...
ANIMIKIE‘ Tylc 3|.
MIDDLE , V snoop” Itrzu'tm.
C m. ,
Pnecmm AN'M'KEA" . -' 4%
'15": 1.36.721
LOWER ‘1“1'19':
TIMISKAMING mg“,
7 GLOWM‘
ran .0 CI
EARLY (induct
IPRECAMBRIAN
Quinton.“
ONTARIAN KEWATIN Scum

 

 

STRATIGRAPHIC SUCCESSION OF
PRECAMBRIAN ROCKS IN WISCONSIN

(MODIFIED AFTER LEITH LUN
GDLDICH,I96I AN'D

FIGURE

2

D AND LEITH l935 AND
WHITE ET AL.,I§66)

 

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

l6

 

m mmao:

«000. H.513 3:3

Z_mzoom_>> .Z “.0 n_<_>_ 010040.”.111110 QMN_I_<mwzwo

$33.65
3.00.. 85.6536:

 

 

 

 

3.525

I

OSCOLQ

aIPPIW

 

 

965 2:20

‘
.0000...
......I.

uuunuun” Ann
955 30.3.5 M

nnnnnn

 

 

 

 

 

nnnnnn

 

 

 

 

 

 

1

NVMVNBBMBN
-38d

   

01
0.1
.

 

 

 

_.__-

 

 

 

--—_f
_1_———

.—
I _
' /—:
1

 

OOIOOOOIIOIOOOICIOOOIII
OUOCOIOOIOOOOOIIUIDIOII
III-uuao UICOIIICOOIIOCOOOO
00"... IEOIIIIOOIIIIOOOOOI
.CI—OIOIIOIJIO c.0000000000000l1
OOQOOOQOOIODIIIIOIOII o.
0.9 0 ”Olin-COIOIOOOIOOOOCO

.nnxnmmwuunn .mmmm0 0”3”31é1

111;

111111 ..
... m .fl w .. 11111111111111111111 11111.111111111111111111111111 . H H H 1 1.

. 54. . . H H H H .. 1 111.11111111111111111111111 ........... say. 1 H 1 H H H H H H

.........

'
o

..................................................

.....................................

......................

 

 

 

 

NVMVNBBMBM

 

 

 

 

 

 

 

. \/ 1 11u~uu~uuu
‘
\ o 6.. A 11 1 IIIIIIII
.b .,. 11 III/IIII
, $ ._ IIIIIII
.. IIIIIIII
. III/III
Q Q 0 9 == IIIIIII ..
L I p 3 111111 $3.63 . t
T Hnnnnnhn .
. 6 9‘ c =1 IIIIIIII
. III/IIII .
. o I IIIIIIII ..
A} HIIIHII ..
_ II I II .
0 III I¢IIII
¢ IIII IflIIII .
III/I IIII ..
/ IIIII VII/II .nu
. IIIIII IIIII .
XUJQEOU IIIIIII IIII .
III/III IIIII ..
>40 I IIIIIIII IIIII
IIIIIII III/I ...
\ IIIIIIIII IIIII ..
.U><Um IIIIIIIII IIIIII .
I;.1IIIIIIIIII IIIIII .
_?1__._ IIIIIIIIII IIIIII ..
:é III IIIIIIIIII IIIIII ...
III/I. III/III Ian IIIIII
II III/III? I IIIIII .H
I-§_I IIIIIII II IIIIII .
15? a: IIIIII III IIIII ...
i 1 _ 1 III/IAN” III IIIII ...
:II111§1IIIII/flfl IIII IIIII ...
1 _1IIIII IIII IIIII ..
IIIIII III/II IIII IIII .. .

I

I
I
I
I
I
I
I
I

 

         

       

I
I
l
I
I
I
I
/

1
31¢ IIIIIIIIIIIIII
I IIII i0“ III/III
I III/fig IIIIIIIIII
I IIII IIIIIIIIIIIIIII
I III/IIIIIIIIIIIIIIIIIII

           

 

 

 

17

are less apparent. The Keweenaw fault has been traced
westward from Keweenaw Pt. in Michigan into Wisconsin,

but here its exact loci is a matter of controversy.
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
TANN,R6W; however, fmmnthis point on it is difficult to
trace. Leighton (195“, p. A01), studying the area of
THAN,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 TAAN,R3W) and schists to the east. Metavol-
canics are found in TAAN,RAW, but its exact boundary is
unknown. The schists are generally regarded as the oldest,
the gneisses next, and the volcanics of indeterminate

age.

AnimikeanfiIMiddle 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 A0° E and A5° NW were found by
Leighton (195A, p. A05) in TAAN,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 A50 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 TAAN,RAW,
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 (192A, p. 2) extends along the eastern border of
the Granite. This fault may exist further to the east.
Likewise, Olmsted (1966, p. 1A), 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 TA5N,RAW),
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 TAAN,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 TA5N,R2W to zero
ft. at Mineral Lake in TAAN,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
TAAN,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 TA5N,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 (195A, p. A01) have shown two basalt "stringers"
extending into TA5N,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 TA5N,RAW. Leighton (195A, p. A05) 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 TA5N,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 TA5N,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 TA5N,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 (195A, 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 60° E and N A0° E were
found by Leighton (195A, 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, TAAN,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 TA5N,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 (195A, p. A06), 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 1/16 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 TA5N,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

yin 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 980,6A7.0 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 1/2 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

 

 

 

I 4 1
d 4 a a q 00 .0
0: o v o n ... n .
.on .3 tux . inc. . 3cm .
. I11. .1 .1I La
0‘ In. 6V
‘1‘.
n. L
r. .0. 6
C O
3‘, Qt
o
‘N 0 1I.1
\‘
o o :0 a nvo Vs
\H OH ‘v
0
h»
0 O In
Z'VF c C. 2. 3 6. who Z'CF
.... o
«N 1. 3
o o
v~ .2. u t. c
0 ch
NV : PM
“0 5-D 00,, O
o :0 go. o .9
l. o .3 .3,
I1 . 5‘
0 O ‘Qr ‘50 r. 1 O
nu 3 o so 0 «to .6 we 0 E
.011 ...! «I
‘Q0 #"WR J» 8 O O 0 O O 1 I+I \Mu
w o o . 1 1 , I! -
2 Q :\o v. .1 5/: Wm. .9. .2 v0 .1 g! tux v! as ON
I ON NN both 0 r , a . .. \
\ so 01% I! 3.1.. 2:0 5‘: an
o I 3n... .1
Rx O 0 O 0 O O O 0 m1 ‘J. 1..
I9 go 5 a! no. 3. 12 a! 3: x! at... o.
{to a! 55. .2 .. . kw
flsxouxxo o o o 1.1 o r. o
n v! I! 9! .3! Q i 3 I o
L “:0 \0
co 2 I. I 1 1.11 3 o
o o no 2 51% EA. as o cum 0‘0 p30 «firrsr‘ ”3...? a o
\N v 3 10 c 2‘ ._ s.
Ix “won Eu 0 V K O O O RSV w .
o s A .1 3 ...: Ex 9! 1.
x \ ax >5
\\ .Qv \ N tho ”WNW 1t O
. l
l 0' .—. o .0 9w 0 3w 9&0 am 0 c . I no I 0' F
Qfi L3 ‘20 O G V! ... ....»
o o o ‘30 h! s 2‘ 1
3: u! a! o \3
sin”. ~30 n! o k
. 0
o o o o o o c o q ‘o
t x v! a! o 93 9! ix N: .i o
h! bum.
o
.3 %
I o 0
o
n h! \2
I o
o
9% s\\ k“. k.
1. .6
\ x O
O a: CM 0. 1
r .n u 1 2 _ 1 _ 1 1 .n u
0 o
I: \n? .1
90., :O‘
I I n .m
V n H _ Us. 0 :0
o o
o: ‘0‘
o
Obxo Q.\0 8‘
2520023 .ZmJIEE _ l1
. 1 O O O
. . oonw Iq%\ 1! d9 3‘
’ N K 3 n I ’ t K o n O O .0
\Orn \nbfl \Opt h»? \Opn \ p \ b

 

 

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 0.099u5

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:

2

gl=978.0u9(1+0.005288u sin2¢- 0.0000059 sin 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

latitude

23' 6-
II II

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.

3

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

Moreover, a density of 2.67 gms./cm. 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
(w

 

‘0
II

- TARE)

AIR ' H 0

where

density

'0
ll

W = weight in air

2:
I

— weight in water

*3
11>
:U
m
ll

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

2.1

 

 

 

of Mn ...... aft of a.» co 3
. . :2. . 3n... . 3:. . . .
m mew _n_
+ I. i. +
T..." :Imo TWO \O—L
OTm 0
9:9.
ltth «Two 2"...
TB}. $.36
nTmo cum. 0.?
mi .1; 0
time
Elmo
t-m o
mlwno hlwo +
73 IT warm“? #8.»? [T 9.3.? .3 i
Muzimo .-uzmmwuémo WWW”
_ o NYInNIN
~-mo o fwmrnzWMNo syzmmeWoN 73.3.. o me a
7m 0 ...sz ....o. o o o 1.6-8
78.8
29» «l... o swims: «02.3 o . znvh
333.9
:3 IT IT .1— 1T .+. .3 i
00:!
v n a _ lazlo
C. 238 32519.
252009; .Zw._I1_wE _.I L
ob» ....n :2. .03. 32. .@c .0.» it: .nsn .o a...

 

 

A2

base of the granite, that is, the diorite zone. The
3

average value for sixteen granitic rocks was 2.72 gms./cm.

which is a little above the normally accepted value of
3

2.67 gms./cm. .- The densities varied from a low of

3
3

2.65 gms./cm. in the porphyritic phase, to a high of

2.78 gms./cm. 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
(195U, 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
3 3

gms./cm. . A value of 2.62 gms./cm. makes the average

3

density of the conglomerate only .03 gms./cm. 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.

 

Iientiflaation

1 1 ’7
k)a.VTD_Le AIO.

"\
U

r‘

E.

ensity

ms./cm.

#1

Density #2

gms./cm.3

 

Granite

. Forphyritic
Granite

m

r\
.1) "
r I]

SO IIE
36 SN
SQ SE
27 SW 2
23 SW
36 an
3% NW 1
( i

7‘.) [\J R) .'\J M m

I o o o
if» \‘ CD \3 0‘» C7“. "\1 C‘.
1:"fo \] LA.) \f

C

. 9
2 Q U
’1 A
L. Q U

MMNNMMMFU
ONU‘C‘0.0\C3\.G‘ 0‘1
WW (11(wa U\\fl

 

 

 

b. Transition Rock 83 P; 1 2.65 2.68
c. Granodiorite 25 SF 5 2.87 2.80
33 Hi 1 2.76 2.60
a. Dioritc 30 SE 7 2.82 2.83
33 HR 9 2.77 2.76
_3(' :;YI‘IIV :7 2.7:) 2.70
Th as *n 2.59 2.81
Average 2.72 gws,/gm,5
Gabbro F 2.86
a? 2 an
:‘7 ?.E"?
C8 3 ”1
Flt 2.53
l-ll—9 2.88
[-7.73-IJ“? 3.11
5-30-15 2.08
“-l3-3 3. 8
5-; t—j‘ 8.03
r-SK-l 3.01
{—54-0 8.82
Average 2.93 gms./...
Coyper Harbor' lw—Sf.(A) 2.62
Conglereraflo 17—3H (r) 2.01

 

Averarn k.h;

 

 

Average 3.U2

Average 2.78

gms./cm.

SIC
IT;
s):

ng./cm.3

0' o ()1;
up
.“1

LA) LA.)

A

.75

MR)

 

Basalt

Tyler Slate

Average (Tyler

’2 1’) 7-".
...
i

b—JF—J H CH
(Exx‘u.

U) U13

Slate) 2.69

gms./cm.

3

.86

1‘0

.67
.69
.72

M R) K: D.)

.79”

 

Quartzite
Basalt

Granophvre

SIN
811
2.75

2.50
2.99
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

HM

“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:

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

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.

M6

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.v 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

 

 

 

 

 

 

 

 

 

 

 

 

ELEMENTS OF 3-D GRAVITY CALCULATIONS

(after TALWANI and EWING, I960)
FIGURE 6

j

“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 av]

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+v)
into (2) and noting that Pi’ ¢i’ and Wi+l are all constants,

the line integral of any side, say BC, can easily be

solved to give the value of:

Zcose1 Zcosei

- arc sin
(P12 + 22);5 (P12 + Z2)%

 

 

(A) arc sin

The total contribution to V of side BC is

Zcose
(5) kn Wi+l-Wi-arc sin 2
(Pi

Zcosei
(P12+z2)g.

 

 

% ; + arc sin
+2 )2

50

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

n

(6) V = kp iiAEi+ITW1TarC sin

 

 

Zcose Zcos¢i
, +arc sin
(pi2+22>¥ (P12+22)g}

Noting that Pi’ W1, wi+l’ 0086i and cosd).i can all be expressed

terms of xi, yi,tflueco-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:
z 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 TASN,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 THAN 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

52

 

.on .3 In: 3.0 finch: .o a .3 ‘03.:

N mewE
d<$_ >t><mo mmnoaom

...?! . 1T , . / 4: . . ....I J .\\|_H . ..I

    

 

 

 

‘VF I'VF
vb" \ON 1
80'? In'h
r .uu .90 1
.32. m Isiah... .5058 .

. *FZBU 02(413
EmZOUmt.) . ZUAJNE L .
.0.» . .n.» D u: .o.o in: .9. .otn tea 5% .0 on.

 

 

53

found in TASN,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 T45N,RAW, and
(5) broadly spaced uniform contours in the northern part

of the map.

 

54

 

 

 

 

 

 

 

 

 

 

 

 

 

.o n .n n i u c ..o c 325$ .o.... 3 cc .mn .08.
a! :83. m mm3w_h_
3.7.2:-.. .22 952032 ._<zo_oma
T .2... ... i
Z'Vh ZVVF
38 .3 I
29.... on“ \ an:
\\OOW\'\F
......
v n . a . d.“
9213 09» 1:35.32. zaohzou
.kFZDOU 02(412
2520093 .quqwt r
.o.» .9... 3 N: .0... 3n: .9. .0... 3:. a.» be...

 

 

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
TASN,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

56

 

 

 

    

 

 

 

.0qn .0.» t n c .06 in c ...v .0.» 3 2. .mn .0940
. m mmeE
mmmwmo omm-OI4<ZO_0mm mmm<30m Hm<mj
+ 8.3-?
/.M.. .. \
ZVVP Z'Qh
. ..0u .01
ZOVP ZGVP
to» .aun
8:: II
.30... n I455»... .5053
>FZDOU o2<41m< .
2520023 .Zw.._.._wr_ _I
.0.» .....n Bum .0.o 3n: 0... .0.» 3': aha .00.?

 

 

 

57

 

1

d d d d 4 41
.0 0 .0 0 D a c .00 .015 0 .0 0 .0 0 .080

o. “#59...

.._<zo_omm mqioma mmomo

 

 

 

 

 

 

 

 

 

 

1 .0..00 .0. L
2!; 2......
Y.00 .0;
lath lath
v.00 Bu 1

.040: 0|u<>¢up£ .5053 . 00.0.... .
Frzaoo 025:2 /
252009)) .zm._..mt r L
0r0 0 0 tax 0 0 )0: .0ko 0+0 ’3. .0“ .00 0

 

 

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 (195“), 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 T45N,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 —US.OO and -50.00 mgal. contours, is

 

61

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

.0 0 .».» 3 u c .0.0 32.0.0 .0.» .00.. Wk .0940
_ _ HEDGE .
1_<Do_mm_m mmmomo 0mm-O
r .0..00 .0. 1
:00» 200»
:2 ..L
200... z»:
r .3 .00 n
8:0.
0 0 0 . ISI..
.30: _ I253... «00500
>FZDOU QZ<JIm<
z_m200m_>> .ZUJJmE
..0.» .0.» 33. .0.0 30...»... .0.» 30¢ .0.» .00.?

 

 

v .0.

r53

62

...»0

 

 

.00 .00 t u x .00 3090.0 .0 0 .0 0 .00
N_ mewE
|_<Do_mwm w1._.._0mn_ mmomo

fit:

 

 

0. 1
I'VE ZC'h
.2 L
80th 20.5.
.00 .1
“40! . |J(>¢u.—.Z_ SOPZOU
>FZDOU 02(412
23.1.2009; . ZUAJwE
00 00 Du: . 00 fins 00 .00 30¢ 00, .00.;

.P ~b .b sf F s»

 

63

 

0.

n

a:
gaun- ' nun-u! I I I I

:0: 00000 I
50:03:00.0
u .0 ..m 0 :00. .0000. 444
=80... 0.00.000 ll
5!..- 2808 00000.0. .....
sgh‘z‘tx-

IS
I

zvcb

BI

55
:5
i2
is

D

I
NVMVNIIMIN-Ifld

I...“ '.J>.P

 

L

J

3.
J
l .
I I I :
IIMO'I
J

znch

  

j
NVMVNIIMIN

. .4.
I§ngi "5.“:

I

UIOPI-uz"

I ..m.

 

 

...»0

 

0
O?

m _

Bum

00000000

n_<_>_ 0.0040mw mtmodioo <

 

.1: .
|,..1\.| I” .

 

On

 

>FZDOU oZ<.._Im<
z_szom_>> . Zmnhmz

ENC

a . .
3mm .0 0 .0 0 3!... .00 ..0

meoE

 

.2 1

00000
....
000000

00000
0000000

00000000000
0 0 0
000000000000000000

zv¢h

............

nu...

 

gnm at .00 3V! .nnbhn

 

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 TMSN,R2W across THSN,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 TUSN,RHW 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 THSN,RUW 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 UN and ASN,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
TH6N,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 TU6N 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 TUSN,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 T45N,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 T45N,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 THSN,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

3

density contrast of -O.21gms./cm. 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 '
north-south profile 7-7' are shown in Figure 1A.

Since the densities of the Granite grade from north 3%
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

3 was more compatible with the

density of 2.65 gms./cm.
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 2UO0 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.

71

 

 

 

¢_ manor”.

...... 00.00....
0.5.0.... 0050.200 ...... 00.00.... 002.0000

 

 

 

 

 

 

 

 

 

 

 

 

000.. 00:00.. 00.50200 .......
02.02 00.00 J... 00.00.... 002.0000
0000
§N A“
000. ..w
0 n
0000
00.2400
0. J 00.. 00.0.0200 0.: .I o.
m
V D] Iii?! I II D
m //
OI. III... r0
h .h

1.55: thaom

 

 

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 1“. A rectangular slab with an average thickness
of 2&00 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.

73

 

 

m. manna...—
.._l m min—Omn—

. min—own. QMPDQEOO .0> min—0mm ow>mwmm0

 

 

 

 

 

 

 

 

 

0.00. 00.0000 0050200 uuuuuuu
02W. Sam I... 0......000 00>00000
0000 ,
OOON i H
000. M _ . .N.l “\Q. —
p000 ,
0.] 0.
w j
0 T n
1 \\. I
.S ..
0L- .lo
.0 ...

hm<w . . ham;

 

 

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.w 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

7H

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
2400 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 s00pe. 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, '
and heat flow near Calumet, Michigan, Am. J. Sci.,

vol. 252. pp. 1-25, 195A. {7
Dobrin, M. B., 1960, Introduction to Geophysical A
Prospecting. k2

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

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 1467, 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., 1954, Petrogenesis of a gabbro-granophyre
complex in northern Wisconsin:

Geol. Soc. American
Bulletin., vol. 65, p. AOl-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-dimensiona1 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 Bu11., vol. 67.

Tyler, S. A., Marsden, R. W., Grout, F. F., and Thiel,

G. A., 1940, Studies of the Lake Superior precambrian
by accessory—mineral methods:

Geol. Soc. America
Bu11., 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-A1.

White, W. S., 1966, Tectonics of the Keweenawan Basin,

Western Lake Superior Region, Geol. Survey Prof.
Paper 52U-E.

80

White, W. S., and Wright, J. C., 1960, Lithofacies of the

Wold,

Copper Harbor conglomerate, northern Michigan:
U. S. Geol. Survey Prof. Paper MOO-B, pp. 85—88.

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 46.3848 980664.30 1166 —20.08
2 46.3988 980652.55 1143 —34.48
3 46.3708 980661.99 1166 -21.13
4 46.3570 980657.95 1230 —20.08
5 46.3422 980649.00 1350 _20.49
6 46.3423 980652.40 1234 —24.06
7 46.3347 980647.80 1324 -22.58
8 46.3357 980652.05 1222 —24.53
9 46.3423 980651.93 1225 -25.07

10 46.3595 980646.66 1245 -30.69
11 46.3522 980652.22 1159 -29.63
12 46.3588 980653.80 1135 —30.09
13 46.3697 980654.51 1116 -31.51
14 46.3203 980639.34 1381 -26.31
15 46.3277 980642.81 1382 —23.44
16 46.3277 980645.04 1405 -l9.84
17 46.3385 980647.87 1394 —18.65
18 46.4138 980657.65 1201 -27.25
19 46.3853 980658.48 1358 -14.43
20 46.3710 980657.75 1393 -11.76
21 46.3563 980655.88 1372 -13.56
22 46.3292 980645.69 1471 -15.36
23 46.3180 980646.54 1427 -16.14

+7

_F”—*‘

83

 

 

83:32:" 88:31:? 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

 

 

81:22:“. 83:31:? 83:81:;
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 -3l.59

85

 

 

888888“ 8:21:11: 8:21:11:
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

 

 

888888“ 8:21:11? 8:21:11:
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

 

 

8:118? 881881? 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

 

 

88:81? 88::11? 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

 

 

88:88? 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

9O

 

 

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

 

 

1:111? 811111? 81:21::
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

 

"I11111111111111111111111411ES

31293