a'U'U‘ W III. m'v--—--—--.

WlSCONSlN

Thesis {or the Degree of M. S.

MTCHIGAN STATE UNIVERSITY

Charles Howard Murrish
196.6

 

 

 

ABSTRACT

AN INTEZEATEI GEO LGGIC— GE OPHYSICS STUDY OF
THE LJ‘UEHEALE AEE., TJGCD COUNTY,
'1-7 TC.”- ‘TC‘T‘T
”.va A‘W

VV 1 Tw” . . i ‘-
r‘ .4 T" Van . 'VI'V‘F "r‘
Q llOyva- q i'LUll E;

Y"

An area located in north—central WlSCOHSin, centered
around fiuouiriale Township in Wood County, was selected for

a gravity and nefhetie survey.. The purpose of the geo—
physical survey is to ascertain the value of a gravity and
magnetic survey in mappinr geology in an area of Precambrian
bedrock where geologic control is limited. The Precambrian
metamorphosed igneous and sedimentary rocks are masked by a
mantle of Pleistocene glacial drift.

As a result of eeoloeic field investigations and
petrograonic exafiinaticn, the rock hes pr resent are iden-
tified as: gneissic iuartz diorite, greenstone, hornblendite,

feldspath ic quartzite, magnetiferous "quartzite,”

met agraywacke, granite, and granite—doiorite contact rocks.
The ,espn sital survey consists of 215 gravity and 277
magnetic stations including tho e of detailed profiles. The

residual gravitv anomalies gen nerallv rane e from +2 .5 to +5.0

)
\
(fir
(
l
I?
UT
.5
7.
31
f—
u)

and from —c.: The vertical magnetic intensity
anomalies range up to +5500 :ammas, but most are less than

Lve anomalies averae e less

Charles Howard Murrish

than —lO gammas. The gravity and magnetic anomalies trend
westerly and northwesterly and are generally correlated
with each other.

Densities and magnetic susceptibilities were measured
for each rock type found in outcrop. These physical prop—
erties are used as the criteria for identification of rock
types from the ge:ihysical anomalies.

Analytical te:hniques were employed to determine a
geologic cross section from the residual gravity anomalies.
Gravity calculations were performed along a section where
some geologic c:n:rol is available to establish limits to
the interpretation. inis cross section, the geophysical
anomaly maps, and the physical properties of the rock
types, formed the basis for the construction of the geologic
map of the area.

The value of a gravity and magnetic survey in mapping
Precambrian bedrock geology in an area of limited geologic

control is proven.

.—.—.-v—

“4

V) A '1 K-‘

II *‘—’

l

I
.~

39

0. .4

Lo

4"
.L.

A
\4 \_,l

A.

'3' V0 .;. o
,2 "

e E

ii

e

.
V1
..

(‘fl'r'|:§'rw'r:
Q‘J_IL.‘_A$‘\JA_¢

r‘Y—W
.2. m
AL V‘-

H

.—.—.v——-
>

\

T.”

(T1
A’AFAV Lg

ACKNOWLEDGMENlS

The writer wishes to recognize and sincerely thank the

following for their aid to the study:

The United States Steel Corporation for the use of
their Worden gravimeter, Askania magnetometer, altimeter,
and continuous recording micro—barograph, and especially
thank Mr. Cedric Iverson and Mr. George Durfee of the
U. S. Steel Corporation for their suggestions and
assistance during the early phases of the field work.

The Department of Geology, Michigan State University,
for the use of their Worden gravimeter, Askania magneto—
meter, and altimeter, and, also for making possible the
use of the CDC. 3600 computer.

Dr. James W. Trow and Dr. Justin Zinn for their
critical reading of the manuscript and suggestions
concerning the petrography.

And, as special recognition and in sincere appreciation
to Dr. William J. Hinze for his genuine interest and

continued guidance during the course of the study.

ii

ACKHOHLED3ZETTS. .

LIST OF TABLES .

LIST OF FIGURES. . . . . .

INTRODUCTION.
Purpose. .
Geographic

Se
General Geology . . .
Previous Investi;: ations . .

GEOLOGICAL FIELD I VESTICATIO.S
PETEOGRAP} IC STUDY.

Method Employed . . . . .

1 F"

Descriptions of HoCk -ypes .
GEOPHYSICAL FIELD INVESTIGA C"S

Gravity Survey . . . .
Magnetic Survey . . .

REDUCTION OF DATA

Gravity. .
Magnetics . . . . .

ACCURACY OF GEOPHYSICAL SURVEYS

Gr ravity. . . . . . .
Magnetics . . .

ISOLATION OF ANCfeLIl .

Gravity.
Magnetics . .

FJ
H.
FJ

*0
W

t i‘)

IE»)

I“ LU +4 ed

M. .

k, u g t) to . ‘1
‘ KL) ON C)‘\

{1.)

Lklkli

1::

EESUL TS OF THE GEOPEl

Gravity Survey
Mag netic Survey

INTE RPRETAT IOI OE

E‘f

General S a
Physical Prope
Regional G olo:'

)

f7 1‘“

LA)

C:

T‘-’v-:fjr——"

( 4
(I)

If

H
m

}
D

H

(‘3

I;

(I)
.3.

y

D

(‘1‘ ’5
“x! “JR

Interp retati n
Anomalies. .

CONCLUSIONS . .

BIBLIOGRAPHY . .

""3 O H- 1+

:0 m (/2

F3.
ore
$3
9‘ 0‘3
F4

CE ‘\

[—4
H
m
+ 3
0
Fri
H
:I>
[D
t 4
m
(I)

Table Page
1. Stratigraphic column . . . . . . . . . 5
2. Estimations of relati ive mineral percentages of

the gneissic quartz diorite . . . . . . 12

3. Estimations of relative mineral percentages of
the greenstone. . . . . . . . . . . 1M

4. Estimations of relative mineral percentares of
the hornblendite . . . . . . . . . . lo

5. Estimations of relative mineral percentages of
the metaqraywacke. . . . . . . . . . l7

6. Estimations of relative mineral percentages of
the feldspathic quartzite . . . . . . . 19

7. Estimations of relative mineral percentages of
the magnetiferous ”quartzite”. . . . . . 20

8. Estimations of relative mineral percenta age s of

the granite and pegWatite . . . . . . . 23
9. Estimatio ns of relative mir lera al percentages of

the 3ranit e— diorite contact rocks . 23
l0. Mean rock type densities and "ainetic

susceptibilities . . . . . . QC

Figure
1.

2.

L‘“'
H
(/3
F—]
0
*TJ
F1i]
H
c. 2
G
:U
[11
(0

Area of Study.

Outcrop Location Map

Barometric Pressure Change Curve as Expressed in
Changes of Altimeter Reading for Known
Elevations

Bouguer Gravity Anomaly Map

Relative Vertical Magnetic Intensity Map.

Residual Gravity Map

Regional Gravity Map

Comparison of Gravity and Magnetic Profiles
Over a Proposed Geologic Cross Section.

Interpretative Geologic Map

vi

Pa”
LJ
O

10

33
35
39
140

51
54

INTRODUCTION

 

The objective of this study is to ascertain the value
of a gravity and magnetic survey in mapping Precambrian

- . 4

igneous and metamorphic geology in an area of
logic control. Relationships between known geology and the
geophysical data should make possible the interpolation of

rock types through areas where bedrock is hidden beneath

glacial sediments. The geophysical—geological relationships

'~_./ by

K

can be determined from outcrop mapping and petrologic exami—
nation of the rock types combined with measurements of the
density and magnetic susceptibility of the rocks. Utilizing

empirical and analytical methods of examining the geophysical

Q

eololic map

I; 3"»?

maps, the construction of an interpretative

should be possible.

CD
(D
(3
41*;
"i
Q)
'0
I3“
}1
(3
(f)
(D
(f
(f
l,_l
3

 

The area of study shown in Pi ure 1 is located in north—

UH

as a

central Wisconsin and centers around the town Oi Auburndal:

Fr‘
’12
I’“
i
l
{’1
(f
$1)
(1‘
F"
k)
F
1']:

(f

in T. 25 N., R. U E. The to.a arga :: 'ths is

H
(1
2.
U)
«’1‘
#1
’4

approximately 95 square miles. This includes a nar'

E
Li)
H;
d
L\
(L
tr

of the southern edge of Marathon County) Wlth the rel
lying in the north—central port on of LQud County.

1

 

 

 

 

WISCONSIN
JEN—m _ ___‘
Mum hr‘o -. |
J,
foo-X]
\ \
\ \
\
\l
T25N.. R 45

 

 

 

T I
FIGURE l. AREA OF STUDY

The landscape is typified by gently rolling topography.
The maximum elevation difference is of the order of 140 feet;
this relief is expressed over a distance of nearly five miles.
There are numerous small streams, but no great relief is
associated with these features. The area is accessible by
a network of roads which are, in most cases, the section
lines. Only a minor portion of the area does not have roads

defining the section boundaries.
General Geology

The bedrock is Precambrian in age and consists of
igneous and metamorphic rocks. Rock types present include
gneissic quartz diorite, greenstone, hornblendite, granite,
granite-diorite contact rocks, quartzites, and metagraywackes.

Locally within the area, Upper Cambrian sandstone of
St. Croixan age rests uncomformably on the Precambrian
crystallines. The sandstone is found erratically throughout
the area but seems to occur predominately near the western
edge of the area of study.

Covering the basement rocks and sandstone is a thin
veneer of glacial drift. The drift ranges in thickness from
zero to at least 67 feet, concealing the majority of the
underlying rocks.

OutcrOppings are found where bedrock protrudes through
the drift or where erosion has cut through the drift. Large

angular boulders, possibly termed "rubble outcrops," also

are prevalent. Often these are found near true outcrop.
The geology previously has been mapped only in a general
manner because of the masking effects of the glacial drift
and lack of economic deposits.

The stratigraphic column of the north—central
Wisconsin region is presented in Table l. The rocks en—
countered in the area of study are correlated only roughly
to those shown in the column. The gneissic quartz diorite
and greenstone may be representatives of the Laurentian and
Keewatin, the metasediments, most likely, are Huronian or
Animikian as is the granite. The sandstone is, undoubtedly,

Upper Cambrian, and the glacial drift is Pleistocene.

Previous Investigations

 

The first detailed account of the geology of this area
is presented in a report of the Wisconsin Geological Survey
found in Bulletin No. XVI, published in 1907; Samuel
Weidman, the author, described the geology, including meg—
ascopic and microscopic descriptions of the main lithologies
represented. Also described are the geomorphology and the
natural resources of the north-central Wisconsin region.

In 1918 another study was conducted in this vicinity.
This survey consisted of a geological investigation and was
accompanied by a dip needle survey. Maps showing the
locations of anomalous dip needle readings and locations

and types of outrops and ”rubble outcrops" were products of

Table l.——Stratigraphic Column (Weidman, 1907).

 

 

__.. _. .
a fi 3 _. -

Wisconsin Drift

Third Drift
Pleistocene Second Drift

First Drift

Alluvial deposits contemporary

with

UNCONPORMITY
Paleozoic . . . Potsdam sandstone (Upper

UNCONFORMITY

Upper Sedimentary Series
(Middle Huronian?)

UNCONFORMITY

Precambrian . . . Igneous Intrusives

UNCONPOLMITY

Lower Sedimentary Series
(Lower Huronian?)

UNCONFOPMITY

Basal Group
(Laurentian or Keewatin?)

drift

Cambrain)

North Mound con-
glomerate and
quartzite

Arpin conglomerate
and quartzite

Mosinee conglomerate
Marshall Hill con—

glomerate

Marathon conglomerate

3. Granite,
Nepheline Syenite
Series

2. Gabbro Diorite
Series

1. Rhyolite Series

Rib Hill quartzite

Powers Bluff
quartzite

Hamburg slate

hausau graywacke

Gneisses and
schists

 

this survey.
geomorphology,

resources .

O\

Unedited reports on the area

geologv dip needle

to J 9

GEOLOGICAL FIELD INVESTIGATIONS

Outcrops and ”rubble outcrops” located by the 1918
survey of the Wisconsin Geological Survey were visited. In
some areas the land had been cleared of all loose rock in
order to render the soil tillable. In other locations, out—
crops were not found where shown by the available maps.

The maps, however, proved invaluable in the selection of
areas of possible outcrops.

Due to the large area involved in this study, the
majority of exposures which were sampled were proximal to
roads. However, locations were visited in the interior of
sections where the maps indicated much outcrOp or along
streams suspected of revealing bedrock. The outcrops were

.q‘
A

sampled taking care to avoid weathered specimens. In are

n)

1

where no outcrops were found, stone piles were examined.
These, in many cases, contained one dominate rock type.
Obviously, this only could be used in providing a clue to
the underlying rock type.

The rock types identified on the maps of the 1918
survey and those of the 1907 report presented a nomenclature
problem. The rocks appear to have been given general names

because of the reconnaissance nature of the 1918 survey.

f

The problems occur in the identification of dioril

(

(I)
n:
b

granite, both in the 1907 report and in the 1918 survey.

7

The diorite of the older surveys referred to greenstone; the
diorite of this study refers to an intermediate basic,
phaneritic rock. The diorite of this study is believed to
be what was named in the older reports as gneissic granite.
Also, some of the rock types shown on the maps of the 1918
survey were not encountered by the writer. For example,
rhyolite which was reported to have been found in 20 of the
36 sections in Auburndale Township (Eidemiller, 1918), was
not observed in the field. Possibly this rhyolite corre—

Sponds to the fine grained granite mapped in this study.

Method Employed

 

Several thin sections were prepared of each rock type

that was collected in the field. The mineralogy was deter—
mined by petrographic study. Estimaticns were made of the

relative percentages of the mineral constituents. The

purpose of the petrographic study was mainly to ascertain a
working nomenclature for the various rock types encountered.
A map showing outcrop locations and thin section numbers of
samples is found in Figure 2. The following are generalized

descriptions and relationships which were observed mega-

sc0pically and microscopically.

P1

Descriptions of Rock lype

 

Gneissic Quartz Diorite

 

Megascopic.-—The rock is distinctly foliated, the

 

U)

oriented constituents being mo tly biotite. The strike of

T

the foliation in outcrop ranges from M. 5C0 N. to N. 85° W.
and the dip is nearly vertical. At the surface the rock is
weathered to a light tan revealing considerable quartz. The
felsic and mafic portion give a white and black effect.

Other'minerals:Ldentified are plarioclase, epidote, and in

some samples hornblende.

lO

 

 

Zrm

 

 

. - 1.. £02058

T (.0550 mg
.3. Goodies _ union

mug 30:03 2.x... 0» zwkuc a.» v

as). ZO_.r<OO.._ momohbo

II

 

 

..:...G

/./

.N manor...

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

wnm

mva

 

 

 

35 ES 5 n!
I
L H L\\
a o L“ p a. . o. n o u n. .
_ .: ... 1.3.3:! L ..
_\..|\.rll\ _ _ .
_ L
It’ll # i . a. I
L l l . Ilinl '. vl! \ll.||| i..|l .I l l. . . i ll
_ $35.... 4
silo-I:-
_ c a c u mum“. v n u . -
1.3.1. -
L !.!'~,ll. .. ”.w
I L z n o. .n . .— .n 2 a on no
_
L L
i . l L .1 1L 1:- NT 1.1. . - L.
L L
A L L
. c. on as L ox H as . c. on on on .-
_ L
L L ..+ .1 + . . I | . .. . I l.‘ | l
. _ .......... _
L _ _ L
L L _ ,
L . Sal.“ . _ izmuzc/ u _ h o- -. cu .-
i I. I \c N I'mh‘nn; _
...a ‘ 1r , an .I A .. ‘7 \\ L z AWN P
_ 3Q E5] til
_ .3 . s \ l .r.
if I E- .L. ‘ v ‘ ‘1
. 7.23.5 ...fl‘. , T . ‘~ 4
_ If: ..‘l 5,5. to §\ (N111- ...I- L
I". L
_ \ L L
c \Q a. n L o. a. a. o
. . I _ L _
I. . i. 3. L _
L _ s .. LL L
...n- «I‘ll \ .. x ‘1 l 1 l .1 TI 1
- - - L - 4 ‘1 L I ..
wrrli. 1/
3.5;: v.3“... _ i E. I.
.. . lion!
L . . s r a _ o . c . ~ _. v
)\ _
. Kr on! flu... .1 Ital-“Ii
L . s '5'!
L h ,1 g ' I 1’ L I I In!
.| r, :‘2 ,
. I&)\ - (r . ‘- . .Il . .01
z. I. a.
, sill
L L Iii
_ L L
~ g . _ n c .
.... L
.333
L :33.
H nu I..\ L .l’u.°!
4 i
, e s e u .. ~.
g . : . .. 3L3. V .. 1/
t'h'a L
L flak!!- Eii i
» u a w o» r. v. a a .u A a u. on
L L . _
L E091 :- no .
In. ,
1. - \ liltiL . -/.7..:.: 11 L
L L L L 4. L

wnx

 

 

ll

A curious feature occurs in an outcrop in the north—

DJ

east corner of section 8, T. 5 2., R. 5 E. Here exists

a good exposure, and in the mafic ”bands" occur large "augens"
measuring almost a foot in cross section. Petrographic

examination indicates these features are primarily biotite

N
(D

and epidote pods. The si of the pods gives them a

xenolithic appearance, but the relationships in outcrop indi—
'cate that they were formed bv metamorphic segregation.

V

Microscopic.——The major constituent in this gneissic

 

rock is plagioclase which is found in the oligoclase—
andesine range. The plagioclase is highly fractured and
altered. In many instances the lathe of plagioclase are
fractured and offset, even rotated by shearing. In these

zones recrystallization of the plagioclase and introduction

N

of quartz have occurred. The quart appears fresh and in

every instance has sutured contacts.

The mafic fraction is mostly biotite. In the majority
of the thin sections studied, the biotite did not show
remanent forms of an amphibole or pyroxene. However, in a
few slides, hornblende is found with the biotite; the
hornblende is believed to be of metamorphic origin. The
biotite is found predominately in sheared zones and in areas
surrounding the plagioclase phenocnysts.

Epidote occurs associated with plagioclase, biotite,
and hornblende. Epidote is identified in all the slides

of this rock type as a minor constituent. Another alteration

12

product is calcite. The calcite probably was formed as a
result of recrystallization of the plagioclase. Paragonite
is observed as a plagioclase alteration product.

Accessory minerals are apatite, sphene, chlorite,
pyrite, ilmenite, leucoxene, and magnetite. The relative
mineral percentages and the slides studies are shown in

Table 2.

Table 2.——Estimations of relative mineral percentages of
the gneissic quartz diorite.

 

 

Slide No.'s . . . base 1 A 5 NEB 38 aver. %
Minerals: Percentages:
Andesine—Olig. 50 35 A5 35 30 30' 39%
Quartz 15 25 20 15 10 13 16%
Biotite 20 2 20 25 20 20 22%
Calcite 5 5 v 5 l5 - 5%
Epidote 5 tr 5 10 10 10 7%
Paragonite A 10 9 9 A 2 6%
Others: 1 l l l l 25 5%
Apatite
Magnetite
Ilmenite
Leucoxene
Sphene 5
Chlorite
Hornblende 20

Pyrite

 

13

On the basis of the amount of fracturing and recrys—
tallization of the plagioclase and other constituents, the
rock is believed to have been originally more basic, possibly
a gabbro. The majority of the quartz is thought to have

been introduced.

Greenstone

 

Megascopic.——The rock is very dense and fine grained.

 

On the fresh surface the rock is a very dark green—black;

the weathered surface is slightly lighter in color. In

some locations a distinct foliation can be seen but, else-

where, is not as apparent. Strike and dip of the foliation

is almost the same as that of the gneissic quartz diorite.
The mineral composition cannot be determined in hand

specimen because of the fine texture. Some iron—staining

is observed, and a pencil magnet is strongly drawn to these

areas of staining. This would indicate the presence of

magnetite. Small quartz veins are observed cutting the rock.

Microscopic.-—In thin section the rock is seen to

 

consist mostly of a very fine hornblende. Interstitially
plagioclase and minor quartz occur. The plagioclase was not
seen in a good section and identification could not be made.
The plagioclaseand quartz never exceed 30—40%. Other
minerals present are sphene, calcite, epidote, and magnetic.
The magnetite is found disseminated throughout the rock and
ranges in percentages from less than one per cent to four

to six per cent. The majority of the magnetite is believed

14

to have formed from the alteration of the original mafic
constituents. The position of the magnetite seems to have
controlled to a degree by the shearing. In Table 3 the

relative mineral percentages are shown.

Table 3.—-Estimations of the relative mineral percentages of
the greenstone.

 

W m. «1*:—

 

 

Slide No.'s . . . 11 13 29 U5 aver. %
Minerals: Percentages:
Hornblende 80 85 60 70 7U%
Quartz and
plagioclase l5 13 32 20 20%
Magnetite =4 2 3 l 2%
Others: 1 5 9 4%
Epidote
Calcite 9
Sphene
Hematite

 

The original rock is thought to have been a fine

grained basic extrusive, very possibly a basaltic flow.

Hornblendite

 

Megascopic.——The rock is fine to medium grained and

 

is very dark, almost black in color. The majority of the
rock appears to be composed of hornblende. Slight foliation
can be seen in the rock. Pelsic grains are interstitial to

the amphibole. In sample no. 39 a zone of epidote is

l5

concentrated at one end of the specimen. In the epidote
zone are found much pyrite and magnetite. The thin section
was cut from the amphibole area.

Microscopic.-—In sample no. 39, plagioclase identified

 

as oligoclase slightly exceeds the amount of hornblende.

The oligoclase is highly altered, fractured, and granulated.
It appears to have been recrystallized forming a mosaic of
oligoclase grains displaying sutured grain contacts. The
hornblende seems to have formed in the mosaic of plagioclase
and is slightly oriented. There also is some minor paragonite
which has altered from the plagioclase.

The thin section of sample no. 16 shows the amount of
hornblende exceeding the plagioclase by a ratio of 2:1. The
hornblende, again, has a preferred orientation. The inter-
stices between the hornblende crystals are filled mostly
with a mosaic of twinned and untwinned andesine grains.

Also present in minor amounts are stringers of sphene which
transect the rock intermittently.

The rock is believed to have formed as a result of
thermal metamorphism of greenstone. The hornblendite is
considered to be a hornfels facies product. In Table A the

relative mineral percentages are shown.

16

Table A.-—Estimations of relative mineral percentages of
the hornblendite.

 

 

 

 

Slide No.'s . . . 16 39 aver. %
Minerals: Percentages:
Hornblende 65 A3 5A%
Andesine-Olig. 30 55 42%
Epidote and
paragonite 2 2 2%
Magnetite <1 <<l <l%
Sphene 3 2%
Metagraywacke
Megascopic.-—The hand specimens reveal a fine grained

 

and coarsely foliated rock. There are mafic bands, dark
gray to black, which measure nearly 3/4 of an inch in width
and consist mostly of biotite. The remainder of the rock

is light tan to light gray in color and has a texture
looking much like a quartzite. One sample displays bands
which have been highly contorted. Also present are feldSpar
"augens" whose dimensions range up to 8 x 12 mm. These

seem to be prevalent only in one zone of the hand specimens.
The groundmass seems to be fine quartz, either quartz grains
or granulated quartz. Pyrite and chalcopyrite are observed
in minor amounts along fracture surfaces.

Microscopic.——Thin sections show this rock to be in

 

a highly altered state. The majority of the feldspars have

17

been altered to paragonite and/or sericite. The major
constituents are quartz, plagioclase, orthoclase,
antiperthite, biotite, and paragonite and/or sericite. The
quartz is found in mosaic bands, and the grain contacts are
highly sutured. The texture closely resembles that of a
quartzite. No original grain boundaries could be dis—
tinguished. In the banded samples, shear zones are composed
of biotite. These bands or zones are in some areas highly
contorted. Found with the biotite are epidote, chlorite,
apatite, magnetite, and minor hematite. Calcite also is
present in minor amounts in the interstices between the
quartz and feldspar. The relative mineral percentages are
shown in Table 5.

Table 5.—-Estimations of the relative mineral percentages
of the metagraywacke.

 

Slide No.'s . . . l5 l7 l8 aver. %
Minerals: Percentages:
Quartz 25 A0 35 33%
Feldspars 22 35 3A 30%
Sericite and/or
paragonite 50 — 31 27%
Biotite 2 15 — 6%
Epidote and
calcite l 9 — 3%
Others: 1%
Chlorite
Apatite
Magnetite
Hematite
Pyrite

Chalcopyrite

 

 

The original rock may have been a graywacke. However,
no rock fragm en ts are identified in the thin section study
of this rock type. A statement concerning the original

identity of the rock is difficult because of the altered

state in which the rook is found.

Feldspathic Quartzite

 

Megascopic.——ihe

4
:‘T
ILJ
(I)
<1
(D
H
\.
t—t)
}_J
*3
(D

(“3
W
:5
(D
L1.
9)
:5
CL
"S
(D
L1.
L1.
P-
ET

I

 

pink on the fresh surface. The weathe ed surface is a pale
yellowish—tan and is soft enough to be scratched With a

knife. The weathering would indicate the presence of

feldspar(s). The majority of the weathered surface, however,
is not feldspar, but fine quartz. On close inspection of

the fresh surface, the rock resembles a quartzite. Occa—
sionally observed are "metaclasts" of plagioclase whose

dimensions range up to approximately 1 x 1.5 mm. No bedding

(/1
(f
}L
53
1
C
}.J«
U)
“T
(D
5.1.

features could be di

Microscopic.——lhe thin section shows the rock to be

 

composed mostly of quartz grains ranging from cherty

appearing to 0.5 x 0.4 mm, the majority being less than 0.1 x

H.
$1)
FJ
mi
’. J:
(-1”
(D
Q)
3
LL

osed o‘

0.1 mm. About 20% of the rock is co

E?
*(j

microcline. The albite attains the largest dimensions seen
in thin sectio , namely 0.9 x 0.4 mm. The grain contacts
are all sutured, and the grain “oundarie s are not rounded,

but irregular. lie irregularity is mainly due to recrystal—

lization. The relative mineral perce entages are shown in

 

”S

centages of

 

Table 6.—eEstimations of relat
the feldspathic
Slide N . . .l4

Minerals:

1°C

O

V

nt

o-x

d.

ges:

 

Quar z
Albite

T\-’Y' “‘14 n.
micrOcllne and

rthoclase

O

T

.5.

luscovite

‘— / 4
Others: \mincr)
Magnetite

Hematite

Pyrite

\fl

}_ J
\ f]
:)(l

\‘v

\fl k)".

‘53

BS

 

The origin

sandstone or a siltstone w.

 

 

 

Negasoo'io.——These
color and are very fine grai
are found irregilar, elongat
are set in a dark tan backgr
represent areas where ma;net
pencil magnet is drawn strci
tan areas apparently are p
weathering cf the lTQ'. The
appearance of vs n quartz, t

"5

C)

(I)

iiCh Ul’l»

erl‘Jent
:4 V‘:_l ”$’.
of) tile

U)

1“

.__.
I .5 A K..-
v‘. .3 o r' c:
A A .A. J k, x-
.— - V- V
L/ v: I ‘11:.

al rock could have been an argillaceous

recrystallization.

V 1
t3»! 8

a

(‘f

n
\J

thered surface

h—black areas which

A ::
\Aq‘

..iri

1

(2

W

n

1 \
\

(1‘)
5-3

A ~"~‘
‘4 L l

}__J
(1‘

Q")

(n

’l

U

L4

()

recrystallization. A parallelism of the dark constituents

can be distinguished. The dark constituents form band-

like features which are less than l mm apart.

7 F '
' f". V, A
V L n}

}J¢
()

I

I
}4
:3
Cf
{1.
F3
(r
(I)
(I\
C

Q n Ah
&. .1’ v ‘_.-

A

, ‘1
.
a a ‘—

“tion the rock is seen to

consist primarily of quartz and with a varying minor amount

of magnetite. The m:gnetite content ranges from zero to
almost ten per cent. The grain sizes of the quartz are

"a.

approximately 0.1 x 0.2 mm. lne ;

sutured and a definite orientation of the quartz grains

exists. Much recrystallization must have occurred. The

wrain contacts are highly

quartz in some areas ofthe slide appears chert-like. The

magnetite seems to favor zones parallel to the direction of

elongation of the quartz grains. However, the magnetite
appears, both as irregular blobs, and as parallel bands

consisting of elongated masses of m"
magnetite is euhedral. Relative mineral percentages are

shown in Table 7.

 

 

netite. Some of the

 

 

 

 

Table 7.——Estimations of relative rineral percentages of
the magnet ferous "quartzite."
- . .: "‘ _‘ . .-~ J7
Slide Ho.'s . . . :3 37 4/ aver. m
Minerals E:‘ot 1: es
Quartz ;: 9) )0 9Q%
Magnetite o l l 0%
Hematite <l <l <l <l%

 

21

Because of the lack of original grain boundaries, the
rock is believed to have undergone much recrystallization.
Initially the rock may have been a chert and was later
granulated and recrystallized to the present quartzite
appearance. The magnetite may have been introduced via
hydrothermal media. Or, the rock may have been a highly
siliceous iron formation, the quartz being recrystallized

chert and the magnetite being originally present.

Granite and Pegmatite

 

Megascopic.--The granite is medium grained and pink

 

in color. Potash feldSpar phenocrysts range in size to
nearly 2 x 4 mm. An occasional twinned plagioclase also can
be seen. Much quartz appears throughout the rock and
appears to be interstitial to the feldspar. Small booklets
of muscovite and biotite also are present.

The pegmatite is reddish—pink and is very coarse
grained; phenocrysts of feldspar are over 5 cm in length,
the majority being about 6 x 10 mm. Quartz occurs in
crystals whose dimensions are 10 x l3 mm, but most are of
the size 4 x 6 mm. Finely twinned plagioclase is very
abundant and is frequently cut by quartz or other plagio-
clase crystals. Muscovite plates are observed and range
up to 2 mm in diameter and are usually enclosed within a
feldspar phenocryst. Much limonite staining is seen on

most of the surfaces of the larger feldspar phenocrysts.

h)
f‘d

Microscopic.——The granite in thin section is observed

 

to be composed of quartz, microcline, albit , and much
antiperthite. The crystal boundaries of the quartz and
the feldspars are sutured. There appears to be at lecst
two compositions of feldspar; one is high y altered to mica
and epidote, and the other, for the most part, is fresh
appearing. The latter is mostly m crocline and antiperthite.
There are many instances where quartz has intergrown with
the feldspar and where feldspar has grown into feldSpar
(micropegmatitc texture). Also present are biotite,
chlorite, and hematite as minor constituents. Pleochroic
halos are numerous in the biotite, possibly around zircon.
The pegmatite in thin section is found to consist
mostly of antiperthite, plagioclase (albite to oligoclase
in composition), quartz, and microcline. The quartz in some
areas is in optical continuity and also is seen as inter—

growths into feldspars. Again, there has been much recrys—

)
(‘1‘

(D

of the mineral

m

C

L
(I)

tallization evidenced by sutured cwnt

present. The antiperthites illustrate that there must have
been a varying composition of the feldspars. Muscovite,

chlorite, and biotite are primarily found in areas of shears
and adjacent to crystal edges. Epidote and h matite are
present in minor amounts. Mineral percentages are presented

in Table 8.

 

23

Table 8.——Estimations of relative mineral percentages of
granite and pegmatite.

J V—Ih _'

 

 

Slide No.'s . . . 19g 19p 40 aver.%
Minerals: Percentages:
Quartz 30 25 15 23%
Albite—Olig. 10 20 40 23%
Microcline 20 15 10 15%
Antiperthite 2 25 10 18%
Muscovite l0 10 23 lU%
Others: 9 N 2 5%
Calcite and
epidote H
Biotite 2
Chlorite H u
Hematite
Apatite

 

Granite—diorite Contact Rocks

 

Megascopic.-—The rock types found occurring where

 

the granite intrudes the gneissic quartz diorite range in
color from a light tan to a dark grayish—tan. The phenocryst
sizes are primarily l x l mm, however, some the feldspar
laths attain a size of 2 x 3 mm. The rock varies from non—
foliated to foliated. The tan variety appears to be a fine
grained granite which has undergone alteration and possible
granulation. There is an abundance of feldspar and quartz
and a development of fine muscovite along shears which

appear infrequently. The mafics are minor constituents.

One sample, no. 3, is noticeably foliated, the aligned

2H

mineral being biotite. There also is a subparallelism of
the plagioclase laths. The major constituents in this
sample are feldspar, quartz, and biotite. Sample no. H6

is thought to be more closely related to the gneissic quartz
diorite than to the granite, although there is not gross

orientation of the mafics as seen in the diorite.

Microscopic.——The thin section observations support

 

the idea that sample no. A6 is more closely related to the

 

diorite than to the granite. Oligoclase—andesine, the major
constituent, is highly fractured and has been altered ex—
tensively to paragonite. The interstices have been filled
with quartz, calcite, and biotite. Areas of calcite exceed
1 x 2 mm. The opaque minerals are commonly associated with
biotite.

Sample no.'s three and eight represent rocks which
were apparently closer to the granite. The major constituent
is oligoclase. Again, the plagioclase laths are highly
fractured and recrystallized, possibly downgrading the
plagioclase. Quartz appears to have been a late addition.
Biotite and calcite seem to have been formed in zones of
weakness and are adjacent to the plagioclase laths.
Paragonite is another common alteration product as is epi—
dote. Sample no. 12 is believed to represent rocks located
very close to the granite—diorite contact. The potassic
feldspars greatly exceed the plagioclase in abundance.

Quartz appears, in part, as intergrowths into the feldspars.

25

Sample no. 12 has been sheared creating an alignment of the
muscovite. Also present is much antiperthite. The relative

percentages of the minerals identified are shown in Table 9.

Table 9.--Estimations of relative mineral percentages of the
granite-diorite contact rocks.

 

 

 

 

Slide No.'s . . . 3 6 7 12 8H 46 aver. %
Minerals: Percentages:
Oligoclase- ,
andesine MO 37 3O 20 35 H0 34%
Biotite 10 13 20 — l 5 8%
Quartz 38 10 2O 25 28 10 22%
Epidote 8 15 5 — - - u%
Calcite 2 2O 2O - 10 25 13%
Sericite and/or
paragonite 2 3 A 10 25 15 10%

Microcline,
orthoclase,
and antiperthites AM u 8%

Others: 1%
Pyrite
Ilmenite
Luecoxene
Hematite
Magnetite
Apatite

 

The abundance of the quartz occurring in the contact
zone, as well as in the gneissic quartz diorite, is believed
to have been added from the intruding granite. These
silicifying solutions would have, most likely, been potassic-

sodic.

 

GEOPHYSICAL FIELD INVESTIGATIONS

Gravity Survey

 

A total of 215 gravity stations were occupied, in-
cluding those along two detailed profiles, with Worden
Gravimeters no. 99 and no. 262. Over most of the area,
stations were spaced at one mile intervals. In areas where
detail was desired on the basis of known geology or where
steep gradients occurred, the spacing was tightened to one-
half mile intervals. The locations of the stations were
determined by automobile odometer and with the aid of topo—
graphic and planimetric maps. The base station was cen-
trally located within the area of study, 500 feet north of
the south-west corner of section 1“, T. 25 N., R. A E.
adjacent to a bench mark. The base was tied gravimetrically
to a station located on a bench mark at the railroad station
in Babcock, Wisconsin. The station in Babcock was es-
tablished by the United States Steel Corporation through a
tie to an absolute gravity station in Winona, Wisconsin.

Elevations of road intersections are given on U.S.G.S.
topographic quandrangles for two—thirds of the area. Plani—
metric maps are available for the remainder of the area.

The elevations of stations along detailed profiles were ob—

tained by leveling and tying to a point of known elevation.

26

27

The elevations of stations located between road inter—
sections or in the area of planimetric map coverage were
determined through the use of barometric altimeters. Two
altimeters were used: a Wallace and Tiernan instrument and
a Surveying Micro—altimeter made by American Paulin Systems.
The American Paulin Systems' instrument and Micro—surveying
Barograph, Model MB5, were used for the majority of these
elevations. The Micro—survey Barograph was centrally
located in the area. This instrument continuously records
the change of barometric pressure. The change of pressure
effects the readings of the altimeter producing error in
the measured elevations. The barograph allowed the appli—
cation of a barometric correction to the altimeter readings
increasing the accuracy of elevations. The Wallace and
Tiernan altimeter was used without the aid of a continuous
recording barograph. To decrease error, barometric pressure
changes were determined by periodic occupation of stations
of known elevations at intervals not exceeding 20 minutes.
This procedure resulted in correction curves such as shown
in Figure 3. The curves reflect the barometric pressure
changes as a function of time. The correction for each
station was taken from this type of curve.

The stations were determined from topographic and
planimetric maps and, also, from mileage obtained from
automobile odometers. The stations were then located on

maps scaled at 1:2“,000. An east—west line was extended

28

.mzo_.—.<>m4w Z3073. mou— mwza<mm
mmewghfix m0 mmoz<zo z. ommmmmmxm m4 ”.5ch mez<ro wmnmwmmn— OEPMZOKAB .n mmDmu—n.

 

 

mu...32=z Z. NEE.

1.33:! NI SNOILVAE'IB 308i W083 SEONEUBJJIO

29

through the gravity base and was established as the latitude
datum. Measurements of each station'svariation from the

datum were taken from these maps.

Magnetic Survey

 

The magnetic observations were made with Askania
Torison-type magnetometers which measure variations in the
vertical component of the magnetic field. A total of 277
stations were occupied including those of detailed profiles.
The stations were spaced at one mile and one—half mile
intervals except where highly anomalous observations neces—
sitated closer intervals to define the areal extent of the
anomalies. The base for the magnetic survey was not co-
incident with the gravity base because of the presence of
metallic fencing. The magnetic base was located approxi—
mately 600 feet west of the southeast corner of section 15,
T. 25 N., R. A E.

The magnetic stations were located in the same manner

as the gravity stations.

REDUCTION OF DATA

Gravity

Many corrections must be applied to the observed
readings before the data can be considered meaningful. ,
Gravimeter drift is the first correction which was applied
to the readings; this drift is the change of meter readings
for a station as a function of time. This factor incor—
porates the effects of variation in the elastic components
of the gravimeter as well as environmental and tidal effects.
Drift corrections were obtained by periodically reoccupying
the gravity base. The difference in meter readings between
two successive base checks was distributed as a function of
time among the stations observed between the base checks.

The readings in scale divisions were multiplied by
the constant of the meter. This converted the readings into
milligals. The resulting values in milligals, because of
their relativity to the base, can be properly termed rela-
tive observed gravity.

A latitudinal variation of the acceleration of gravity
between the equator and the poles exists because of the
polar flattening of the earth and the centrifugal force due
to the earth's rotation. A latitude correction of 1.307
mgals/mile was applied according to each station's varia-
tion in the north-south direction from the datum. All the

30

31

stations were related to the arbitrary east—west datum ex-
tending through the base.

The free-air correction also was applied. The free—
air correction takes into account the vertical gradient of
gravity and corrects for varying station elevations with
respect to the horizontal datum. Sea level was used as
the datum in this survey.

Consideration also must be made for the mass between
a station and the horizontal datum, namely sea level. For
the purpose of calculations,the earth's surface was assumed
to approximate an infinite horizontal slab. A density of
2.67 gm/cc was selected for the rock material of the infinite
horizontal slab. The density of 2.67 gm/cc is the commonest
value assumed for the mass or Bouguer correction in gravity
surveys in Precambrian terranes.

A terrain correction takes into account the gravi—
tational effect due to variations from the infinite hori-
zontal slab which is assumed in the Bouguer correction. In
the area of study the topography is very gentle, hence,
the terrain correction was not applied to any station
occupied in this survey.

The Bouguer gravity anomaly is employed to study the
geological effects causing the gravitational variations.
This anomaly is defined as the difference between the
observed gravity and the calculated theoretical gravity

based on the preceding corrections. Utilizing the

32

International Gravity Formula and applying the elevation

and mass corrections, a theoretical gravity value was
obtained for the base. The base's Bouguer gravity anomaly
value was found to be —63.5 mgals. Because the gravity
values of all the stations were related to the base, a
simple constant was applied to the relative observed gravity
to obtain an absolute observed gravity value. The Bouger

Gravity Anomaly Map is shown in Figure A.

Magnetics

 

The reduction of the magnetic data was much simpler
than the reduction of the gravity data. Time variations
or drift also is a factor in magnetics and is caused by
the diurnal variation of the earth's magnetic field. The
magnetic drift was removed from the meter readings by the
same method employed in the gravity reductions. The
magnetometer readings in scale divisions then were multiplied
by the constant of the meter. This converted the scale
divisions to observed magnetic values in gammas. The value
for the magnetic base was arbitrarily given the value in
gammas of that obtained in the first observation at the
base.

The size of the area of study is sufficiently small
so that a correction was unnecessary for the regional change

in the earth's magnetic field. The variation of the earth's

33

 

 

  

 

 

'. in. o in.
g
33 33 o .83 .83
.3. ooo.~.coc. _ Hmien
.33.... 9 3:5»! S328
.96.. com ”6290 Nod :23... £350
262083 dig—30h min—#594

n32 >._<EOZ< >t><mo mmanom

 

 

 

 

 

 

 

 

\i A \a

_\ .\ \_ ,\\\\\

 

4» $59....

 

 

 

 

 

 

 

Qulu u «r

1
IBQK».MU‘

 

 

 

 

Zn-

 

 

 

 

mm:

 

 

 

34

magnetic field was found to of the order of three to five
gammas from the northern to the southern extremities of
the area.

The Relative Vertical Magnetic Intensity Map is shown

in Figure 5.

35

 

 

 

 

3.1. I.- o ..:-u
"IE. .- I-
097v .025 o .a....ru.~ .06....
3: ooo.~ . 55. _ 33m

3.5.30 coo; m 355% 00. 52:32. «20-20...

ZEZOQWE .n._1wZ>>O._. m4dn2m_)_m34

n32 >tmzme2. OCNZOdS ..dOFKmS m>_.r<:_mm

 

.m mmDoC

 

 

 

 

 

 

 

 

ACCURACY OF GEOPHYSICAL SURVEYS

Gravity

The accuracy of the gravity data is dependent upon
the accuracy of the gravimeters, the meter readings, the
drift corrections, the density assumed for the mass
correction, station location, and the elevation of each
station. The accuracy of the station elevation is the most
significant factor in producing error. Collectively con—
sidered, the other factors normally do not exceed 0.20 mgals.
A five foot error in elevation would create an error of
0.30 mgals. The elevations of this survey are believed to
be within five feet of the true elevations. Therefore, the
relative error of the gravity survey is thought to be

approximately 0.50 mgals and definitely less than 1.0 mgal.

Magnetics

 

The causes of possible error in the magnetic values
are the accuracy of the magnetometers, the accuracy of the
meter readings, the effects of non-lithologic magnetic
materials, and the accuracy of the drift corrections. The
first two factors are believed to be minimal. Areas observed
or suspected of having non—lithologic magnetic materials

present were avoided; therefore, the error from this factor

36

37

aljs<3 is thought to be minimal. The change in the rate of

(iri_ft in magnetics can be sporadic and sometimes can go
Lnidetected even with frequent base checks. This is arbi-
‘trarily assigned a value of 25 gammas at a maximum. The
maximum total error of the magnetic survey is believed to

not exceed 50 gammas.

ISOLATION OF ANOMALIES

Gravity

The primary purpose of this survey is the mapping of
the rock types close to and/or at the surface. The varying
rock types should be reflected in rather abrupt changes in
the acceleration of gravity as compared to the gradual
changes of large magnitude resulting from deep seated
crustal phenomena. The latter is termed the regional anomaly.
To isolate the gravity effects of the surface rocks, the
regional anomaly must be removed. The deviation of the
Bouguer gravity anomaly from the regional anomaly is termed
the residual gravity anomaly.

The residual gravity anomaly values for each station
were determined by the cross—profiling method. The area
was gridded by north—south and east—west profiles. Each of
these profiles was separated by approximately one mile. The
Bouguer gravity anomaly values were plotted for each of the
profiles. A regional anomaly was then ”smoothed-in" and
adjusted so that the regional was the same at the inter-
section points for the east-west and north—south profiles.
The Residual Gravity Map is shown in Figure 6. The
regional anomaly which was determined by the cross-profiling
method is shown in Figure 7. The residual map was utilized

in the analysis of the anomalous features.

38

39

 

 

 

 

II. Ila. o I».
g

3:05.;

..: ooo.~.fic. _ @436
38...... nd 445w»! E653
.264 3m E33 .5.~ 8.8.. 5.23
2620093 digs mgd

n32 >t><mo 4485mm

 

 

 

 

 

 

 

.m HEDGE

 

 

lamb

 

 

 

 

AO

 

 

 

 

 

«Emu-Hulda”

iii-g;

So. ooo.~.fil _ .313
18!... 9 51in 5958
.25.. 8m .638 .B.~ :28» 2.2.8
zfizoofl? .afmzssbb 3493403

n35. >._._><mo n_<zo_omm

 

 

N mmDoE

 

 

 

 

 

 

 

Di

 

 

 

 

 

 

 

 

 

 

 

 

 

wnm

 

 

Al
Magnetics

Examination of the Relative Vertical Magnetic Inten-
sity Map revealed that the anomalous features have a common
background of approximately 3500 gammas. No regional

magnetic anomaly is apparent on the map, therefore, a

residual map was not constructed.

ravity Survey

 

1 ‘ up
Anomaly

..'
o .

The Bouguer Gravity
depicts a strong general positive w

+»_.
‘AI' (.1 '

northeasterly from the

the north—central portion of the ma

flanked either ide l

(I)

b” gravitv
t, f v—fi .. LI ‘4‘"

T853710

,_2

present are dis this

shown in Figure 7 correlate

(

anomaly

0

gradient determined by Mack 1 57)

\

h
+ .

4.

,- x'; .g
u 5'
ID ..‘1 ..Ln

J}

O

Q
A.)

O

I1

(f)

C

(

gravity anomal
The residual anomalies which

map in Figure 6. The magnitude of

residual anomalies are primarily in
+5.0 mgals. There is a noticeable

J

the anomalies, either westerly or n

largest magnitude anomalies are app

. 1 \«fi 4-
miles in extent.

and two by five

NVESTIGATIONS

ap shan in Figure A

hich trends north—
corner of the map to

p. This trend is
ows. The anomalies
nal trend. The regional

s with the regional
in of the regional
remained are shown on the
the major positive

h.

t r

SD

nge from +2.5 to

(

fl

tren t strike of

e
orthwesterly. The

roximately one by five

The other positive

anomalies are more limited in extent. The gravity highs are
separated by gravity minima, mostly of the magnitude of

—2.5 mgals, but one major negative anomaly attains —A.5 mgals.
Many of the negative anomalies are incomplete“y defined
because they ‘ie near or at the edge of the area of study.

“3

Magnetic Survey

 

The Relative Vertical Magnetic Intensity Map shown in
Figure 5 discloses a common magnetic background level of
about 3500 gammas. If the 3500 gammas is accepted as the
background level, the anomalies can be differentiated into
relative highs and lows. As with the residual gravity
anomalies, the trend of the strike of most of the magnetic
highs and lows is westerly or northwesterly. The magnetic
positives are commonly 300 gammas above background, but
locally some anomalies attain higher magnitudes, up to
5500 gammas above the background. Very local positive
features exceed a relative +600 to +1A00 gammas. Re—
lative magnetic lows are found between the positive anomalies.
These lows are commonly only 100 gammas below the background,
but in a limited area are as great as a —A00 gammas. The

magnetic anomalies generally correlate with the residual

gravity anomalies.

INTERPRETATION OF THE RESULTS OF THE
INVESTIGATIONS

General Statement

 

The purpose of the gravity and magnetic survey is to
aid in mapping the Precambrian geology. This requires that
the geology be interpreted from the geophysical anomalies by
empirical and/or analytical methods. However, the inter—
pretations of the anomalies, gravity and magnetic, are not
unique for this survey because of the limited geologic
control for the area of study. Geologic control is necessary
because various combinations of parameters for hypothetical
geologic bodies can result in the same or very similar
anomalies.

There are five main variables of geologic bodies which
affect gravity anomalies: volume, density contrast, depth,
shape, and isolation (Romberg, 1958). The magnitude of an
anomaly varies directly with the product of the volume and
the density contrast. Sharpness is a function of depth of
the disturbing body and decreases with depth one power faster
than the magnitude. If more than one anomalous body is
present, the disturbing bodies must be sufficiently isolated
to produce distinct anomalies. The shape of the body is
critical in that a large magnitude anomaly will not be
produced unless the body is concentrated, even though, the

AA

A5

depth is shallow and the volume is great. These variables
must be considered in order to present a reasonable inter—
pretation. Other than density contrast, all the factors
mentioned above, plus magnetic susceptibility, affect the
vertical magnetic intensity anomalies.

The paucity of geologic control in the area of study
makes possible many interpretations from the gravity and
magnetic maps. But, through careful analysis of the
geophysical data, coupled with the available geological in—
formation, the various possible interpretations should be
reduced to yield a reasonable solution to the geology.

Even though the gravity and magnetic anomaly positions
correlate quite well, the gravity data will be more heavily
weighted in the geological interpretation. The magnetic
data generally are more difficult to interpret than the
gravity data. The magnetic data are complicated by the
dipolar effect, by the possibility of effects of natural
remanent magnetization, and by the variance of the magnetite
content within a single rock type. Grant and West (1965)
state, ”. . . magnetic prOperties of rocks are subjected

to sudden and violent fluctuations over short distances
magnetic properties depend, in the main, upon trace rather

than on bulk constituents."

Physical Properties of the Rock Types

 

The physical properties of interest are density and

magnetic susceptibility. The density for each rock type

A

A6

was determined in the laboratory by the volume displacement
method. The densities of the samples of each rock type were
averaged and are shown in Table 10. The magnetic suscep—
tibilites of the rock types also were measured in the labora-
tory using a magnetic susceptibility bridge. The measured
values also are shown in Table 10. Even though there would
be more variance in the magnetic susceptibility of a rock
type than in the density, the susceptibilities measured can
be utilized to predict the general order of magnitude of a
magnetic anomaly which may be expected from a particular

rock type assuming a constant source.

Table 10.——Mean rock type densities and magnetic

 

 

 

susceptibilities.
No. of Density No. of MagL6Suscept.

Rock Type: Samples: g/cc: Samples: X10 c.g.s °
Greenstone 5 3.10 A 135 to 3965*
Hornblendite 2 3.01 1 8A
Magnetiferous
"quartzite" 5 2.79 2 10A to 6055*
Gneissic quartz
diorite 9 2.77 3 56
Metagraywacke 3 2.71 2 78
Granite—diorite
contact rocks 8 2.69 1 AA
Feldspathic
quartzite 2 2.6A 1 11
Granite and
pegmatite 2 2.6A 1 9

 

* Where such a range exists, an averaged magnetic suscep—
tibility would be meaningless.

£47

From examination of Table 10 generalizations can be
made concerning gravity and magnetic anomalies which may be
expected from the various rock types. Gravity anomalies of
the greatest magnitude would be expected from the greenstone;
the magnetic susceptibility suggests that moderate to high
magnetic anomalies would be expected. The hornblendite has
a density which would be expected to result in gravity
anomalies of second order and possibly accompanying low to
moderate magnetic anomalies. The magnetiferous ”quartzite”
would possibly be represented by intermediate gravity
anomalies and moderate to very high, first order, magnetic
anomalies. The gneissic quartz diorite also would be ex-
pected to produce intermediate gravity anomalies and rela-
tively low magnetic anomalies. The metagraywacke would be
expected to yield low to intermediate gravity anomalies
associated with low to moderate magnetic anomalies. The
granite—diorite contact rocks would be expected to be
associated with gravity lows and low value magnetic
anomalies. The granite, pegmatite, and feldspathic quartzite,
geOphysically speaking, could not be distinguished; all
would be expected to be associated with gravity and magnetic

minima anomalies.

Regional Geologic Setting

 

Geological field studies indicate that granite occurs

throughout the region surrounding the area of investigation

as

and strongly suggest granite is or underlying the Precambrian
basement rocks. A core of granite and pegmatite was ob—
tained from a water well in the east l/A sec. 10, T. 25 N.,
R. 3 E. lear the center of sec. 19, T. 25 N., R. 5 E., a
profusion of vitreous, white quartz fragments were found,
some nearly two feet in diameter; these were not in outcrop
and would be classed as ”rubble outcrop.” Aplite diklets
were found in a very good exposure in the northeast corner
of sec, 8, T. 25 N., R. 5 E.; the host rock is the gneissic
quartz diorite. In the north l/A sec. 13, T. 2A N., R. 3 E.
decomposed granite was observed as ”rubble outcrop." A
weathered, sheared granite was found in the east l/A sec. 5,
T. 25 N., R. A E. and also in the northeast corner of sec. 6,
T. 25 N., R. A E. In the southeast corner of sec. 31,
T. 26 N., R. A E., large angular boulders of granitic
pegmatite were found scattered abundantly on a hillside.

The regional gravity anomaly removed by the cross—
profiling method does not contradict the interpretation
that granite underlies the area. The regional anomaly shown
in Figure 7 is an arched—shaped gravity high trending nolth—
northeasterly. The regional can be explained as a thinning
of the granite and a thickening of the denser layer below.
This denser layer is commonly given a density of 2.8A gm/cc.
Mack's (1957) regional gravity anomaly in this area is

interpreted by him to have a similar origin.

 

 

 

AA

/

Interpretation of Residual Gravity and
Magnetic Anomalies

 

 

Analytical Interpretation

 

A quantitative evaluation is desirable for the inter—
pretation of the gravity data to establish a geologic cross
section. If the geology of the cross section is related to
the residual gravity, the relationships can be utilized to
extrapolate the geology from areas of geologic control.

The profile selected for quantitative evaluation had
to meet the following conditions: geologic control must be
available; the gravity and magnetic coverage must clearly
define the anomalies present; and, the section must be
perpendicular to the strike of the anomalies. The section
selected (section A—B in Figure 9) extends north from the
southeast corner of sec. 27, T. 25 N., R. A E., through the
west portions of sections ll and 2 and midway into sec. 35,
T. 26 N., R. A E.

The causative geologic bodies can be assumed to be
two—dimensional for the purposes of calculation of gravity
because of the linear nature of the majority of the anomalies.
A two—dimensional gravity method outlined by Talwani, Worzel,
and Landisman (l959) was used in these computations uti-
lizing the CDC. 3600 computer.

In these calculations the observation surface is
assumed to be a horizontal plane and the upper boundary of

the Precambrian bedrock. Neither of the assumptions is

50

correct, but the error imposed upon the results is negligible
considering the magnitude of the anomalies and the inherent
ambiguity of the results.

Geologic contacts were selected by examination of the
residual gravity and magnetic profiles. The geologic bodies
were first approximated by rectangular forms. The density
contrasts were held constant through the course of calcu-
lations. Ultimately a close fit was obtained between the
calculated gravity and the residual gravity profile by varying
the parameters of volume, depth, and shape of the geologic
bodies. The resulting geologic bodies and accompanying pro-
files are shown in Figure 8.

The rock types associated with the anomalies shown in
Figure 8 are as anticipated from the anomalies. The first
order magnitude gravity anomalies are attributed to green—
stone and are associated with a high magnitude magnetic
anomaly. Referring to Figure 8, the surface dimension of
the greenstone is 2800 feet in width and has a vertical
extent of 2500 feet. The second order magnitude gravity
anomalies are attributed to a hornblendite which has a wedge—
shape. At the surface the exposed width is only 1000 feet,
and the depth of the lower boundary varies from 2800 to
1000 feet. The gneissic quartz diorite lies below the
slopes of the higher order magnitude gravity anomalies and
is associated with an intermediate magnitude gravity anomaly.

The lower extent of the gneissic quartz diorite varies from

51

.ZO_._.omw mmomo UGOJOwo ommoaomm < mm>0 mmdmoma o_._.mzw<s_ 024 >._._><mo mo zom_m<n_200 .w wKDmV—u

 

 

 

 

 

 

 

 

 

.8wa .83 w
boo?! I l l I "I IIIII
lllllllllll JI/ ) \ \ L
.OOONJ \u\\lll|) _IIIIh l/l/ \\\ / \\\|\\ "
/// " //\ / mtz<mo \ \ \\.\ . utmoa
.ooo. . mtmoa .Npo .zo / / mzoSzmmmo _ utmoa .NS .zo / llllll l \ .\ mtozmamzmo: “ .Npc .zo
/. | l _ _f I
.u _ !!!!!!! _
p p IIIII _
m 2.85 Nbfikmsm. 2:53 Jm
a T
I nl
T NI 1
I _I \fokbiobi inc/o .
w .
m 80.. i
n
9
V
_.|
S
oooma
.1 9
. m
.. + H 30.25 0.52qu ooow 1w
. S
o u mzoEfim $345
. m +||ll+ “3:25 C6232. 0.5232 48.5% cock -
olllo "magma E36 45.56%
”Sign. C545 82.535 008 1
80m -

 

52

3300 to 1000 feet below the surface. The entire cross
section is underlain by granite. Gravity minima occur where
the granite is closest to the surface. A sharp boundary is
drawn between the granite and the other rock types, but in
actuality the boundary probably would be gradational.

The magnetic profile provides limited information.
However, the greenstone is clearly revealed by a relative
positive magnetic anomaly of approximately 5300 gammas. The
magnetic high located over the gravity minimum is not related
to any feature shown in Figure 8 but is an end effect of a
magnetic high to the west which is shown in Figure 5. A
magnetic low is shown over the hornblendite; the low also is
an end effect of a magnetic low lying to the east.

All the rock types found in the area of study are not
represented in the proposed geologic bodies in Figure 8. Also,
the "rubble outcrop” evidence of granite—diorite contact
rocks is not shown in Figure 8. These rocks could represent
an apOphysis of the granite below. The geophysical data do
not indicate its presence and, therefore, the apophysis is
not included among the proposed geologic bodies. Hornblendite
is not found in outcrop along section A-B, but hornblendite
is found to the south and is associated with a second order
magnitude gravity anomaly. Hence, hornblendite is proposed
to be the causative body of the positive gravity anomaly at

the southern end of the section.

53

There is a significant difference between the calculated
gravity and the residual gravity at the southern end of the
section. This was unavoidable because the hornblendite
situated two miles south of the profile was not accounted for
in the calculations. The gravity effects of the southern
hornblendite cause the residual gravity values at the southern
end of the profile to be higher than the calculated values.

The calculated gravity profile illustrates that the
proposed geologic bodies can produce gravity anomalies which
very closely approximate the residual gravity anomalies.

The proposed geologic bodies are believed to represent a

reasonable interpretation of the geophysical data.

Empirical methods

 

Having proposed a reasonable geologic interpretation
for the geOphysical anomalies shown in Figure 8, the geology
is extrapolated from and interpolated between areas having
geologic control. The Residual Gravity Map shown in Figure 6
and the magnetic map shown in Figure 5 were carefully
examined as was the Outcrop Location Map shown in Figure 2.

A geologic map was drawn using the Outcrop Location Map

for geologic control and utilizing the profiles over proposed
geologic bodies as an interpretational guide. The Interpre-
tative Geologic Map is shown in Figure 9.

Reference is made to the residual gravity and magnetic
maps, Figures 6 and 5, respectively, in the following

discussion. The greenstone is mapped in areas of first order

 

5A

 

 

'- ‘C. O -..-

E

giogg

3:83.65. ”33»

252893 £523» Wan-2594 .m mmaei
a4: 0.00485 w>_._.<._.mmmmmkz_

 

 

 

 

  

‘‘‘‘‘‘
.......

 

X t x... _ ..x...

 

 

55

positive gravity anomalies and very strong magnetic anomalies;
these areas also contain numerous outcrops and "rubble
outcrops" of greenstone.

Hornblendite is considered to be a more highly meta—
morphosed equivalent of greenstone. Therefore, the magne—
tite content should vary as it does in the greenstone. For
this reason the hornblendite is mapped in areas of magnetite
anomalies of varying magnitude and areas having second order
positive gravity anomalies.

An outcrop of the magnetiferous "quartzite" is coinci-
dent with the maximum magnetic value recorded in the survey.
The outcrop is situated in the northeast corner of sec. 19,
T. 25 N., R. 5 E. The outcrop is positioned along the
southern flank of the positive gravity anomaly resulting
from the gravity effect of the greenstone to the north. The
geophysical data and geological evidence indicate the
”quartzite" is of limited extent.

The gneissic quartz diorite is mapped in areas on the
flanks of the first and second order positive gravity
anomalies and where the magnetics are of low magnitude and
where indicated by geologic evidence.

The metagraywacke, because of its physical properties,
is expected to be expressed geophysically similarly to the
gneissic quartz diorite. The metagraywacke is mapped where
low to moderate magnitude gravity and low magnitude magnetic

anomalies occur and on the basis of outcrops. The low

56

magnitude positive gravity anomaly trending northwesterly
from the outcrop evidence in sec. 3, T. 2A N., R. A E., is
arbitrarily defined as the westward manifestation of the
'graywacke.

The graniterdiorite contact rocks are mapped only in
the immediate area surrounding the gravity minimum in
sec. 2A, T. 25 N., R. A E. Contact rocks also must be
present in zones where the granite is in contact with other
rock types. The area around the gravity minimum, mentioned
above, is the only area where the granite—diorite contact
rocks were found in outcrop. A possible explanation for the
lack of extensive contact rocks in other areas is that the
granite formed vertical contacts with the country rocks. In
the zone where the granite—diorite contact rocks are present,
the contact may be gently inclined resulting in a thin cap
of the gneissic quartz diorite over the granite. This would
produce an extensive exposure of the contact rocks.

The feldspathic quartzite has nearly identical density
and magnetic susceptibility as the granite. There is geologic
evidence for the granite and the feldspathic quartzite at
the southern edge of the area of study. Here, the gravity
and magnetic anomalies are of very low magnitude, and there
is nothing to indicate the location of the contact between
the two rock types. The contact is mapped midway between
the exposures of each rock type. Granite is, elsewhere,

mapped where gravity minima and magnetic low occur.

57

A feature of the magnetic map not included in the
previous discussions is the magnetic high of very limited
extent found in the northwest corner of sec. 31, T. 25 N.,
R. 5 E. This high is speculated to be caused by local
mineralization of a fracture or small fault in the gneissic
quartz diorite.

Identification of the relative ages of the rock types
is not possible on the basis of field evidence. No rock
contacts were found as a result of the field observations.
However, from examination of the literature of the older
surveys (Weidman, 1907, and Eidemiller, 1918), some of the
age relationships may be inferred. The gneissic quartz
diorite and greenstone apparently are the rock types shown
in the stratigraphic column in Table l which belong to the
Basal Group. The Basal Group rocks are of Laurentian and
Keewatin age. The magnetiferous "quartzite" also may be of
the Basal Group. The "quartzite,” which possibly may be
recrystallized iron formation, may be associated with the
greenstone as the Soudan iron formation is associated with
the Ely greenstone in Minnesota. Or, the "quartzite" may
be recrystallized chert and of the Lower Sedimentary Series
of the stratigraphic column. The nature of the ”grains"
of the quartz in thin section is very similar to the
description given by Weidman (1907) for the Powers Bluff
quartzite. The magnetite may have been introduced by

hydrothermal solutions. The latter explanation is preferred.

58

The feldspathic quartzite and the matagraywacke are
tenuously correlated with the Lower Sedimentary Series.

The granite is thought to be the youngest crystalline
rock type in the area. The granite is probably a portion
of the widely spread granite, nepheline syenite series
shown in Table 1. According to Weidman, the granite consti—
tutes one-half to two—thirds of the surface rock in the
north—central Wisconsin region.

The hornblendite is considered to be the result of
thermal metamorphism of the greenstone. The granite could
have possibly been the main source of heat. Therefore, the
hornblendite may have metamorphically developed during the
emplacement of the granite.

One can only Speculate on the interpretation of the
structural geology. If the relative ages of the rocks
presented are correct, the gneissic quartz diroite and
greenstone may represent the remains of anticlinal features
and where the metasediments occur, synclinal features.
Contacts between rock types derived from the study may, in
large part, represent fault contacts. However, it is
difficult, if not impossible, to distinguish the type of
contact on the basis of geophysical results.

The geophysical maps of the area of study, no doubt,
can be interpreted differently from the interpretation
presented. The interpretation of some areas, namely the

central, west central, and the southwest central, is almost

59

solely based on the gravity and magnetic data. Neverthe-
less, the interpretational reasoning for these areas, as
well as the entire area of study, is believed to be valid.

The gravity and magnetic survey resulted in the
definition of the general limits of the rock types, the
regional geologic picture, and approximations of the lower
limits for some of the rock types.

Not only did the gravity and magnetic survey prove
of value in mapping Precambrian bedrock in an area of
limited geologic control, but also provided information
which would not be possible from geologic field mapping

techniques.

CONCLUSIONS

A gravity and magnetic survey integrated with a
geological investigation, and utilizing the measured physical
properties of the rock types, proved the importance of a
gravity and magnetic survey in mapping geology in a
Precambrian terrane of limited geologic control.

The resulting geologic map shows the areal distri-
bution of rock types and increases the knowledge of the
Precambrian geology of this area. Metamorphosed igneous
and sedimentary rocks are found associated with, and sur-
rounded by, granite which in part may be intrusive into these
rocks.

Calculations performed on the gravity anomalies
suggest that the lower limit of the metamorphosed rocks is

at approximately 3300 feet below the surface.

BIBLIOGRAPHY

61

BIBLIOGRAPHY OF

CITED AND RELATED REFERENCES

Bean, Robert J., 1953, Relation of Gravity Anomalies to the
Geology of Central Vermont and New Hampshire: Geol.
Soc. America Bull., Vol. 6A, p. 509—538.

Eidemiller, H. N., 1918, chief geologist, unedited report
on Auburndale Township, Wisconsin: Misconsin
Geological Survey.

Grant, F. S., and West, G. F., 1965, Interpretation Theory
in Applied Geophysics: New York, Harper an Brothers,
p. 205.

 

 

Hinze, w. J., 1959, A Gravity Investigation of the Baraboo
Syncline Region: Journal of Geology, Vol. 67, p. A17-
AA6.

Mack, John N., 1957, A Regional Gravity Study of Crustal
Structure in Wisconsin: Master of Science Thesis,
University of Wisconsin.

Mooney, H. M. and Bleisfuss, R., 1953, Magnetic Susceptibility
Measurements in Minnesota, Part II Analysis of Field
Results: Geophysics, Vol.18, p. 383—393.

Nettleton, L. L. and Elkins, T. A., 19AA, Association of
Magnetic and Density Contrasts with Igneous Rock
Classification: Geophysics, Vol. 9, p. 60—78.

Oldham, C. H. G., 195A, The Correlation Between Precambrian
Rock Densities and Bouguer Gravity Anomalies Near
Parry Sound, Ontario: Geophysics, Vol. 19, p. 76—88.

Romberg, F. E., 1958, Key Variables of Gravity: Geophysics,
Vol. 23, p. 68A—700.

Steenland, N. C. and Woollard, G. P., 1952, Gravity and
Magnetic Investigation of the Structure of the Cortland
Complex, New York: Geol. Soc. America Bull., Vol. 63,
p. 1075-110A.

Talwani, M., Norzel, J. and Landisman, M., 1959, Rapid Gravity
Computations for Two—Dimensional Bodies with Application
to the Mendocino Submarine Fracture Zone: Journal of
Geophysical Research, Vol. 6A, p. A9—59.

62

63

Weidman, Samuel, 1907, Geology of North Central Wisconsin:
Wisconsin Geological Survey Bulletin No. XVI.

111i" 8'

I

“@6111“

 

IIIIIIHIII

310013