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ABSTRACT

PETROLOGY AND PETROFABRICS
OF THE RANDVILLE DOLOMITE IN THE
FELCH MOUNTAIN TROUGH, DICKINSON COUNTY, MICHIGAN

by Donald A. Dryden

Samples were taken of the Randville Dolomite where it
outcropped in the Felch Mountain trough between Felch and
Randville in Dickinson County, Michigan. The Randville is
of Huronian Age and has been metamorphosed to a dolomitic
marble. Ten outcrops were used from which thirteen speci-
mens were taken. Four specimens were taken from one out-
crop to standardize areal deviation.

Each outcrop, from which a specimen was taken, was
mapped at a scale of l" = 10'. The maps show specimen
locations plus joint attitudes and locations.

Twenty-six thin-sections were prepared from the 13
samples and were studied under a petrographic microscope.
The results showed that dolomite was the major constituent
of the marble with tremolite, diopside and talc as major
accessory minerals.

The thin-sections were also used to do a petrofabric
analysis on the dolomitic marble. Standard orientation

procedures were used.

Donald A. Dryden

Orientation analysis of 9530, C-axes of dolomite grains
from twenty-four thin-sections revealed a weak fabric pat-
tern with maximum orientations lying normal to the regional
structural strike (north-northeast) or paralleling the
direction of the deforming force.

A statistical analysis, using a correlation coefficient
as a measure of preferred orientation, substantiated the

findings and interpretations of the petrofabrics work.

ii

PETROLOGY AND PETROFABRICS
OF THE RANDVILLE DOLOMITE IN THE

FELCH MOUNTAIN TROUGH, DICKINSON COUNTY, MICHIGAN

BY

DONALD A. DRYDEN

A THESIS

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

MASTER OF SCIENCE

Department of Geology

1962

ACKNOWLEDGEMENTS

The author wishes to express his sincere appreciation
to Dr. J. W. Trow for his cooperation and assistance during
the preparation of this paper; also to Drs. J. W. Zinn and
H. B. Stonehouse for their helpful suggestions and con-
structive criticisms.

He also wishes to thank James Olmsted for his help in
taking and developing of the photomicrographs and to his

wife, Genevieve, for her constant encouragement.

iv

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . .
GEOLOGICAL SETTING . . . . . . . . . . . . .

Stratigraphy

Structure
PETROGRAPHY AND PETROLOGY . . . . . . . .

Dolomite

Tremolite

Diopside

Talc

Interpretation
PETROFABRICS . . . . . . . . . . . . . . . .
ANALYSIS OF RESULTS . . . . . . . . . . . .
STATISTICAL ANALYSIS . . . . . . . . . . .
SUMMARY . . . . . . . . . . . . . . . . . .
BIBLIOGRAPHY . . . . . . . . . . . . . . . .

10

ll

12

13

14

26

29

47

51

59

LIST OF TABLES
Table Page

I. Stratigraphic Succession of the Middle Pre-
cambrian in the Felch Mountain District,

Dickinson County, Michigan . . . . . . . . . . 5
II. Modal Analysis of Dolomitic Marble . . . . . . 9
III. Comparison Grain Size Chart of Outcrops . . . 18

IV. Comparison of the Statistical Findings for
Each Point Diagram and the Maximum Contour
(Per Cent) for Each Diagram . . . . . . . . . 50

LIST OF PLATES AND FIGURES
Plate Page
I. Outcrop locations map . . . . . . . . . . . . 2

II. (Figure l) Photomicrograph of dolomite mar-
ble, showing pressure twinning taken from
slide Ah- Crossed nicols, X90 . . . . . . . . 19

(Figure 2) Photomicrograph of dolomite mar-
ble, showing pressure twinning taken from
slide Ah. Plain light, x90 . . . . . . . . . 19

III. (Figure 3) Photomicrograph of dolomite
marble and tremolite, showing tremolite
surrounding dolomite and replacing it.
Crossed nicols, X90 . . . . . . . . . . . . . 20

(Figure 4) Photomicrograph of dolomite mar-

ble and tremolite, showing tremolite sur-

rounding dolomite and replacing it. Plain

light, X90 . . . . . . . . . . . . . . . . . . 20

vi

Plate

IV.

VI.

VII.

VIII.

Page

(Figure 5) Photomicrograph of dolomite mar—

ble, showing clouded appearance from slide

E2v' Plain light, X90. . . . . . . . . . . . 21
(Figure 6) Photomicrograph of dolomite mar-

ble, showing twinned and untwinned grains

under low power objective. Crossed nicols,

X35 . . . . . . . . . . . . . . . . . . . . . 21

(Figure 7) Photomicrograph of tremolite in
diopside, showing long bladed tremolite.
Crossed nicols, X70 . . . . . . . . . . . . . 22

(Figure 8) Photomicrograph of tremolite in
diopside, showing long bladed tremolite.
Plain light, x70 . . . . . . . . . . . . . . . 22

(Figure 9) Photomicrograph of tremolite in
dolomite, showing a basal section of tremo-

lite exhibiting excellent amphibole cleavage.
Crossed nicols, X90 . . . . . . . . . . . . . 23

(Figure 10) Photomicrograph of tremolite in
dolomite, showing a basal section of tremo-

lite exhibiting excellent amphibole cleavage.

Plain light, X90. . . . . . . . . . . . . . . 23

(Figure 11) Photomicrograph of diopside,
showing 90 degree pyroxene cleavage. Crossed
nicols, X90 . . . . . . . . . . . . . . . . . 24

(Figure 12) Photomicrograph of talc in mar-

ble, showing single plate edges of talc

along with fine grained scaly aggregates of

talc which resemble sericite. Crossed nicols,

X90 . . . . . . . . . . . . . . . . . . . . . 24

(Figure 13) Photomicrograph of quartz in
dolomite marble and tremolite, showing
stringers of quartz with dolomite replaced
by tremolite, taken from slide C . Crossed

nicols, X70 . . . . . . . . . .h. . . . . . . 25

vii

Plate Page
(Figure 14) Photomicrograph of quartz in
dolomite marble and tremolite, showing
stringers of quartz with dolomite replaced by
tremolite, taken from slide Ch' Plain light,
X70 . . . . . . 25
Figure
15. 400 dolomite axes from sample A . . 35
16. 400 dolomite axes from sample B . 36
17. 400 dolomite axes from sample C . - . 37
18. 400 dolomite axes from sample D . . 38
19. 400 dolomite axes from sample El . . 39
20. 400 dolomite axes from sample E2 . . 4O
21. 400 dolomite axes from sample E3 . 41
22. 400 dolomite axes from sample E4 42
23. 400 dolomite axes from sample G 43
24. 330 dolomite axes from sample H . . . . 44
25. 400 dolomite axes from sample I . . . . 45
26. 400 dolomite axes from sample J . . . 46
27. A one per cent grid used in counting points
for the computation of "r" . 48
Plate
IX. (Figure 28) Photograph of active quarry at
outcrop "B," showing where oriented specimen
was taken plus joint face (N 46° E, 43° SE) . 54

viii

Plate

XI.

XII.

XIII.

XIV.

(Figure 29) Photograph of outcrop "B," show-
ing joint surface within quarry

(Figure 30) Photograph of inactive quarry at
outcrop "C," taken at oriented specimen and
looking east . . . . . . . . . . . . . . . .

(Figure 31) Photograph of quarry "C," look-
ing across quarry at oriented specimen. Note
parallel fractures . . . . . . . . . . . . . .

(Figure 32) Photograph of outcrop "C," show-
ing joint surface in marble (N 420 E, 770 SE).

(Figure 33) Photograph at outcrop "C," show-
ing joint surface at west end of quarry
(N 15° E, 69° NW) . . . . . . . . . . . . .

(Figure 34) Photograph of Rian Quarry at
outcrop "H," looking across quarry at orient—
ed specimen location . . . . . . . . . . . .

(Figure 35) Photograph of dolomite at Rian
Quarry, showing close-up of oriented specimen
location . . . . . . . . . . . . . . . . . .

(Figure 36) Photograph of wall in Rian Quarry
showing parallel joint surfaces (N 220 W,
72° sw) . . . . . . . . . . . . . . . . . . .

(Figure 37) Photograph of marble at outcrop
"H," showing joint surface in dolomitic mar-

ble (N 600 W, 86° NE) . . . . . . . . . . . .

Map of outcrop "A" showing specimen location
and joint attitudes . . . . . . . . . . . . .

Map of outcrop "D" showing specimen location
and joint attitudes . . . . . . . . . . . . .

ix

Page

54

55

55

56

56

57

57

58

58

pocket

pocket

Plate

XVII.

XVIII.

XIX.

Map
and

Map
and

Map
and

Map
and

Map
and

of outcrop "E" showing
joint attitudes . . .

of ourcrop "F" showing
joint attitudes . . .

of outcrop "G" showing
joint attitudes . . .

of outcrop "I" showing
joint attitudes . .

of outcrop "J" showing
joint attitudes . . .

specimen locations

specimen location

specimen location

specimen location

specimen location

Page

pocket

pocket

pocket

pocket

pocket

INTRODUCTION

The Felch Mountain trough is a fairly narrow structure
running roughly east and west between the villages of Felch
on the east and Randville on the west in Dickinson County,
in the western part of the northern peninsula of Michigan.
Geologically, it is a syncline of Precambrian metamorphosed
sediments and igneous intrusions.

Although the relief is not great, the area is still
rather rugged topographically. This is due in large part
to the glacial debris and alternation of weak and resistant
rocks forming abrupt changes in relief. Furthermore swamps,
heavy forest and thick underbrush hinder field study. The
West Branch of the Sturgeon River, formed between the low
steep-sided knolls, intervening swamps and occasional steep-
sided valleys, meanders nearly the length of the trough.

The area of greatest relief and elevation is in the Felch
Mountain area itself.

This report concerns an area which is over 13 miles long
and includes the southern portion of T. 42N, R.°s 28, 29,
and 30W. The map on page 2, Plate #1, shows the locations

of the outcrops from which samples were taken. The samples

 

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A. Nu

came from ten outcrops already located by the United States
Geologic Survey. Detailed sketches of these outcrops, show—
ing the location of samples and joint attitudes were taken
at a scale of l" = 103 (in pocket). Four oriented samples
were taken from outcrop ”E” which were used to standardize
areal deviation.

The outcrops from which samples were taken consisted
of dolomitic marble, some metamorphosed more than others.
All outcrops were massive so that strike and dip of beds
could not be observed.

The field work on the area was done in late June and
early July of 1960. Petrofabric and petrOgraphic examin—
ations were then made of twenty-six thin sections taken
from thirteen samples. Of primary concern was the orienta-
tion of the dolomite crystals of the marble in relation to
the structural geology and, the correlation of these find-
ings with those of Solbeng (1958), who did a similar study
on rocks exposed in the Metronite quarry at the eastern end
of the trough. Of secondary nature was the study of the

petrography and metamorphism of the dolomite.

GEOLOGIC SETTING

Until recently, little geologic data concerning the
trough was available. In 1961, Professional Paper 310
(Geology of Central Dickinson County, Michigan), was re-
leased. This work, compiled primarily from works of James,
Clark, Lamey and Pettijohn, is partly concerned with the
area studied in this problem.

Earlier works are incomplete or only partially cover
the area. Some of the better works are: 1) Progress report
briefly outlining the regional geology, by C. E. Dutton
(1950), 2) A portion of a paper by H. L. James (1955), on
regional metamorphism in Northern Michigan, 3) An older
work by Van Hise and Leith (1911) on the geology of the
Lake Superior region. Other monographs pertaining to the
area are available. Bailey and Smythe (1899) is perhaps

the best, as it includes previous works in the area.

Stratigraphy

The Felch trough is underlain almost entirely by meta-
morphosed Animikie metasediments. These rocks are of Middle
Precambrian age and in most stratigraphic successions are

included as part of the Huronian series. Table I gives the

4

TABLE 1

(AFTER LAMEY GEOLOGICAL SURVEY PROFESSIONAL PAPER 310)

Middle Precambrian:
Post-Animikie . . . . .

Animikie series:
Baraga group . . .

Unconformity.
Menominee group . .

Unconformity.
Chocolay group . .

Unconformity.
Lower Precambrian rocks:
"Algoman"(?) . . . .

Dickinson group . . . .

Unconformity.
Pre-Dickinson . . . . .

Granite pegmatite.
Massive granite.
Metadiabase and metagabbro.

Badwater greenstone.
Michigamme slate.
Hemlock formation.

Vulcan iron-formation.

Felch formation (includes
schists of at least
four types, believed to
be correlative).

Randville dolomite.

Sturgeon quartzite
(includes schist below
and above vitreous
quartzite).

Fern Creek formation.

Gneissic granite.

Six~mile Lake amphibolite.

Solberg schist and Skunk
Creek Member.

East Branch arkose.

Granite gneiss.

probable stratigraphic succession of the Middle Precambrian
in the Felch Mountain District as interpreted by Lamey
(1961). The sedimentary rocks, as well as the younger meta—

diabase, are cut by granite and granite pegmatite dikes.

Structure

Although the local structure has been interpreted by a
few workers, the best is perhaps the oldest by Van Hise
(1911). It is a very general but accepted report.

"The structure is an east-west synclinorium constitut-
ing a narrow strip nowhere more than 1 1/2 miles and usually
less than a mile wide, which as a whole runs over 13 miles.
Although the general structure is synclinal, a single fold
of simple type has nowhere been found to occupy the whole
cross-section of the Algonkian formation, but usually two
or more synclines occur, separated by anticlines, which may
have different degrees and directions of plunge and different
strikes, complicated besides, both, by subordinate folds and
by faults" (p. 302).

It might be well to note here that all of the local
synclines have a northeast to east trend.

The Randville Dolomite outcrops over a larger part of

the trough's surface than any other member of the succession.
It is probable that the thickness of the formation is not

uniform, but increases from east to west.

PETROGRAPHY AND PET ROLOGY

The major rock unit of study is a dolomitic marble with
the formational name of Randville Dolomite and of lower
Huronian Age. The Randville has certain mineral assem-
blages and characteristics that identify it. Here it will
be called "dolomitic marble" or "Randville formation."

Detailed study of the marble was limited to samples
taken along a fourteen-mile east—west strike at varying
distances from each other. Where there were not enough
outcrops the spacing was as far apart as five and one-half
miles. The dolomite outcrops are identifiable by their
massive knobbish form and the gray to black weathering sur—
face.

Field study supplemented the thin section observation.
There is no visible foliation. Megascopically, dolomite is
the main constituent of the marble, with tremolite occurring
in most samples in varying quantities. Talc, diopside, and
quartz are present in minor amounts in many samples, with
diopside becoming the main constituent in sample "F".

Samples "E” and "H” were taken from outcrops on the
south side of the trough and the rest were taken from out—
crops believed to be on the north side.

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Dolomite.' Texturally, the rock is granoblastic. The
grain size ranges from 0.1 mm. to 0.83 mm. with the average
of all samples being 0.24 mm. Samples from the Felch
district seem to have :much larger crystals than the
samples from the western end of the trough.

Dolomite makes up 86% of the marble in all sections,
except "F" which has completely changed to diopside and
tremolite. In half of the samples it makes up over 90% of
the rock.

The dolomite in thin section shows typical character-
istics, such as high order white interference color and
marked change in relief upon rotation of the stage. The
average index of refraction of Ne and No for the dolomite
is 1.664. To help identify the dolomite, interference
figures were taken on many of the crystals that stayed gray
to dark on 3600 rotation of the stage. There are many of
these in each slide except ones from sample "F." Figures 1
and 2, Plate II show the dolomite with some crystals ex-
hibiting characteristic pressure twinning with crossed
nicols and under plain light.

When using the universal stage and recording the orien-

tation of the optic C-axes, it was noted that the isotropic

10

sections could be distinguished by a lower order interference
color (brighter).

In many cases, crystals of dolomite are enclosed by
tremolite. Plate III, Figures 3 and 4 from sample "E2” show
a good example of this.

In other cases the dolomite seems to be corroded or
clouded by iron and silica (Figure 5, Plate IV). This is
especially true when in contact with amphiboles or if there
is a lot of amphibole in the rock.

Thin section study revealed no foliation.

Tremolite. The tremolite for the most part occurs in
aggregates of subhedral blades, although many individual
blades are present throughout the series of samples. Some
of the blades were as long as 1.00 mm. and 0.15 mm. in
width, the average being much smaller. Sample F revealed
excellent long blades (Figures 7 and 8, Plate V). Although
the tremolite is bladed, no preferred orientation appeared.

The greenish white or gray bladed tremolite was mega-
scopically visible in nearly all sections and obviously had
some iron content. It is easy to recognize on the weathered
surface of the outcrops as it stands out in relief against

the surrounding dolomite which has been weathered away by

11

solution. Diopside, which also stands out, was mistaken
for it occasionally.

It was found after microscopic examination that samples
A, B, E2, H, I and J were fairly high in tremolite content,
especially B and J. As stated before, some of what was
thought to be tremolite when studied megascopically, was
later found to be basal sections of diopside when examined
under the microscope.

The tremolite was identified by its blade-like form,
its 160-20o extinction angle and distinguished by the bi-
axial negative interference figure. From diopside's biaxial
positive figure, high (390) extinction angle and short
stubby crystal habit.

When tremolite was cut across a basal section, it showed
remarkably well formed 560 and 1240 cleavage (Plate VI).
This is in contrast with diopside's nearly 900 cleavage.

Diopside. The diopside is an accessory mineral in most
samples but in sample ”F" it is the main constituent. Its
crystal habit is short and stubby and some very large indi-
viduals appear, especially in the "F" specimen, where one
crystal is 10.00 mm. long and 3.50 mm. wide.

In this particular sample the dolomite is completely

12

replaced by tremolite and diopside, the diopside making up
88.1% of the rock. The identification of the pyroxene has
already been discussed in previous paragraphs.

Megascopically it can be identified in samples "D" and
"F" as very flat basal partings which seem to have a green-
ish core and a more cream colored rim. Some good diopside
(90°) cleavage can be seen in Figure 11, Plate VII, taken
from outcrop ”F."

Iglg, Talc is another accessory mineral which appears
in minor amounts in most sections, especially the "E" series.
Megascopically, the talc can be found in soft small segre-
gations in the rock and can be picked out by a knife blade.
When this is done the talc appears very fibrous (probably
pseudomorphous of tremolite) — (Dana, 1932), but when rolled
between the finger-tips it breaks down readily into a soft
smooth powder.

Microscopically, the talc appears in plates which give
the "birds-eye" effect, a sort of speckled appearance to the
bright green and yellow interference colors. Also many of
the plates seem to bend. It can be identified by its optic
angle (6-30°) and association with magnesium bearing minerals.

In some sections it occurs as a very fine grained, scaly

13

aggregate which looks like sericite. This and some indi-
vidual plates are shown on Plate VII, Figure 12.

Other minerals associated with the dolomite are rare.
Quartz is found in only four samples. It runs in veinlets
in one or two places which would suggest secondary injec-
tions, yet in other places it appears to be native to the
rock before metamorphism, i.e., quartz which has not been
used to change the dolomite to pyroxene. The quartz is
fine grained and is associated with pyroxenes and amphiboles,
as pictured in Figures 13 and 14, Plate VIII.

An opaque mineral, identified as pyrite in all cases,
is found in nearly every thin section. The pyrite appears
to have partly replaced some of the silicates.

Interpretation. Through thin-section examination, it

 

appears that the marble has experienced complete recrystal-
lization, for none of the constituents are present in their
original form. This is indicated by the large grain size,
blades of tremolite, grains of diopside and other minerals
foreign to an original dolomite. Although the original
dolomite was probably impure, containing such elements as

silica and iron as some of its impurities.

l4

It seems in many places as though only the original
constituents were used in the recrystallization. Yet in
others, some quartz and perhaps pyrite appear to have
been introduced by granite pegmatites, as suggested by
stringers of quartz with pyrite in association with it.
This might be the case in outcrop I'I," where granite dikes
appear nearby and free quartz is evident on the weathered
surface of the outcrop (outcrop map in pocket). Other
free quartz may have crystallized later than the general
metamorphism, by hydrothermal action, as suggested by Lamey
(1961).

Frequent and multiple twinning shows that the dolomite
was affected by the folding. Hawkins (1949) reported that
twinning in dolomite is found only where stress and strain
of deformation occur and Fairbairn(l954) concluded that
twins develop only in grains of restricted orientation.

Tremolite is present in varying amounts in every slide.
Tremolite is a metamorphic mineral and is usually restricted
to metamorphosed limestones or limy sediments (Moorehouse,
1959) which proves that there has been recrystallization
due to some type of metamorphism.

In a few cases, some of the tremolite has in turn altered

15

to talc. Here the tremolite has probably been subjected to
hydrothermal alteration. It is a common alteration of non-
aluminous magnesium silicates, like tremolite (Dana, 1932).

In sample "F,' the major constituent is diopside which
is a minor constituent in other samples. The diopside is
believed to be an alteration of the dolomitic marble, but
there is no direct evidence of this as there are not any
remnants of marble left. From observing other slides, such
as "D," it appears that the diopside is a direct alteration
of the dolomite.

Diopside is a common constituent of hornfels and meta-
morphosed dolomite and limestone (Moorehouse, 1959). Since
the outcrop is found on the north side of the trough and
since there are no hornfels or other carbonates found in
the trough, it is safe to assume the outcrop is the altered
Randville.

Why the dolomite is more altered at "F” than at other
points is not hard to explain once the outcrop map is viewed.
It will be noticed at the south end of the main outcrop is
a pegmatite knob. The direction the dike trends is ques-

tionable, but Lamey (1961) has observed that most of the

pegmatite dikes trend nearly northward. From this pegmatite

l6

would come the heat and large amounts of quartz needed to
alter the complete outcrop.

The course grained texture of samples "G" and "H" which
were collected from outcrops near the village limits of
Felch (Table III) is also of interest. None of the other
dolomitic samples even approach the average grain size of
"G" and "H." ”I," which is in the same vicinity of these
two has very large grains also, but these large grains are
surrounded by a matrix of fine grains which reduce the
average grain size of the sample. It is thought that where
metamorphism has progressed further, there is a general
tendency toward an increase in grain size (Spock, 1953).
This indicates that the eastern end of the trough received

a higher degree of metamorphism than the western half.

17

TABLE III

GRAIN SIZE CHART

Sample Average Size
A 24.0

B 22.9

C 30.0

D 11.3
El 20.9
E2 19.0
E3 16.4
E4 21.0

F 142.2

G 43.8

H 44.1

I 18.7

J 16.2
Average of all dolomite samples (excludes F) = 24.0

18

(mm.)

PLATE II

Figure l. Photomicrograph of dolomite marble, showing
pressure twinning taken from slide Ah’ Crossed nicols,

X90.

Figure 2. Photomicrograph of dolomite marble, showing
pressure twinning taken from slide Ah' Plain light,

X90.

 

 

 

 

19

PLATE I I I

Figure 3. Photomicrograph of dolomite marble and tremo—
lite, showing tremolite surrounding dolomite and replacing

it. Crossed nicols, X90.

Figure 4. Photomicrograph of dolomite marble and tremo-
lite, showing tremolite surrounding dolomite and replacing

it. Plain light, x90.

 

 

 

 

20

PLATE IV

Figure 5. Photomicrograph of dolomite marble, showing

clouded appearance from slide E Plain light, X90.

2v°

Figure 6. Photomicrograph of dolomite marble, showing
twinned and untwinned grains under lower power objective.

Crossed nicols, X35.

 

 

 

 

 

21

PLATE V

Figure 7. Photomicrograph of tremolite in diopside, showing

long bladed tremolite. Crossed nicols, X70.

Figure 8. Photomicrograph of tremolite in diopside, showing

long bladed tremolite. Plain light, X70.

 

 

 

22

PLATE VI

Figure 9. Photomicrograph of tremolite in dolomite,
showing a basal section of tremolite exhibiting ex-

cellent amphibole cleavage. Crossed nicols, X90.

Figure 10. Photomicrograph of tremolite in dolomite,
showing a basal section of tremolite exhibiting ex-

cellent amphibole cleavage. Plain light, X90.

 

 

 

 

PLATE VII

Figure 11. Photomicrograph of diopside, showing 90 degree

pyroxene cleavage. Crossed nicqls, X90.

Figure 12. Photomicrograph of talc in marble, showing
single plate edges of talc along with fine grained scaly

aggregates of talc which resemble sericite. Crossed nicols,

X90.

 

 

 

 

24

PLATE VIII

Figure 13. Photomicrograph of quartz in dolomite marble
and tremolite, showing stringers of quartz with dolomite
replaced by tremolite, taken from slide Ch. Crossed
nicols, X70.

Figure 14. Photomicrograph of quartz in dolomite marble
and tremolite, showing stringers of quartz with dolomite
replaced by tremolite, taken from slide Ch’ Plain light,

X70.

 

 

 

 

 

25

PETROFABRI CS

A petrofabric analysis was run on twelve of the thir-
teen rock samples, to determine if there was any significant
preferred fabric orientation. This study was necessary,
because the rocks showed no megascopic or microscopic fabric
orientation. A Brunton compass was used to orient the sam-
ples in the field. Since the magnetic declination in the
area is less than one degree off true north, the samples were
oriented with respect to magnetic north.

Locations of all specimens studied are shown on the
map on page two. The attitude of each specimen was marked
on the rock sample with red lead pencil and later scratched
in with a knife blade. All location and geographic direc-
tions were noted in a field notebook.

Of the thirteen specimens taken, only sample "F" did
not have a petrofabric study made on it. As mentioned
previously, this sample has completely altered to diopside
and amphiboles Which are biaxial. A biaxial mineral must
be measured by a five—axis universal stage. A microscope
to use with the five axis stage was not available in the
Geology Department at Michigan State University; conse-

quently the "F" sample was discarded for petrofabric use.

26

Because the rocks had no obvious megascopic foliation,
a section perpendicular to the trend of the regional struc-
ture was taken. Since the trough runs nearly eastawest, a
vertical north—south section was taken and a horizontal
section was also studied. As twelve samples are being
studied, this gives a total of 24 thin sections prepared
for petrofabric examination.

Two hundred grains of dolomite in each section were
studied, except for H-vertical, in which only 130 grains
could be used. Orientation of the C-crystallographic axis
(the optic axis) of each grain was determined. Because
dolomite is a uniaxial mineral a four-axis universal stage
was attached to an ordinary polarizing microscope and
standard orientation procedures used.

In measuring a uniaxial crystal, either the optic axis
is oriented parallel to the microscope axis (No) or the
plane perpendicular to the optic axis is oriented in a
vertical position parallel to N-S (Ne).

Both attachment and orientation procedures are outlined
in Fairbairn (1954). The universal stage readings were
plotted directly upon measurement on a Schmidt equal-area

net.

27

Because carbonates are highly birefringent, a correc—
tion must be made when plotting them. The corrections are
obtainable from Plate 8 in Emmons (1943), using the index
for the hemisphere and the low index for the carbonate.
The correction is used for plotting Ne only as No is not
affected, since for readings of less than fifty degrees
there is no appreciable error for No' But as the N-S
reading of Ne gets larger, the correction also increases.

The N-S readings were adjusted as follows: 19-23 = 1°,

24-27 = l l/2°, 28-33 = 2°, 34-38 = 2 1/2°, and 39-43 = 39
All corrections were made to decrease the readings. For
example, if a N-S reading was 21, it was adjusted to 20
and plotted on the lower hemisphere of a Schmidt equal-
area net, as were all points.

After the points were plotted, they were rotated to a
horizontal plane and a composite diagram was compiled from
points of the two sections of each sample, giving twelve
composites in all. Concentrations of the C—axis were
determined by standard center and peripheral counters of

1.0 cm. radius. These composites were contoured and then

studied.

28

ANALYSIS OF RESULTS

It will be noticed that in most of the diagrams a
north-south girdle is evident. The girdle consisting of
a supposed direction of movement ("A") and a normal to the
cleavage or beds ("C") (Billings, 1954).

Sample A, Figure 15, shows one 4% maximum bearing N
22° W and plunging 24 degrees northwest, another larger
one striking approximately the same as the first maximum
but lying nearly horizontal, while the last 4% high is
very small and trending N-S horizontal. The attitudes of
these maxima are essentially normal to the regional struc-
ture. A weak "A-C" girdle is present, but not nearly as
strong or evident as in most other diagrams.

Figure 16 pictures 400 contoured C-axes from sample
"B." Notice the stronger "A-C" orientation in this
diagram. Two strong three per cent maxima are seen, both
striking the same direction (N 26° W) but are plunging in
opposite directions. One plunges 21 degrees to the north-
west and the other plunges 20 degrees to the southeast.
Again, as in sample "A," this is normal to the regional

structure.

29

Four hundred oriented optic axes taken from two thin
sections of sample "C" are shown in Figure 17. Once more
a strong "A-C" girdle is evident running north-south, but
a high 5% maximum is found in the northeast quadrant of the
diagram. It bears N 14° E and plunges 18° to northeast.
This is contrary to local structure and might be attributed
to being taken from an outcrop at the western terminal of
the trough.

Figure 18 is a composite diagram of two slides from
sample "D." This diagram has a very strong "A-C" girdle
culminating in a north-south three per cent maximum which
plunges 58 degrees to the south. This finding is in con-
junction with the local geology in that the optic axes
parallel the direction of compression.

As has been stated before, four specimens were taken of
outcrop "E" as a standard of areal deviation. Figure 19,
20, 21, and 22 show composite diagrams of 400 grains each
for specimens E1, E2, E3, and E4 respectively. A marked
resemblance will be seen when the four diagrams are com-
pared. Each has a strong "A-C" girdle and the highs in

each are situated in approximately the same positions. A

two and three per cent maximum can be seen in the N—S,

30

"A-C" girdle; two per cent highs are visible in all north—
east quadrants, all northwest quadrants and all southwest
quadrants.

Specimen "E " has north-south 3 per cent highs, plung-

1.
ing 25 and 55 degrees to the north which is parallel to the
supposed direction of the deforming force.

Figure 21 exhibits two 3 per cent highs, one trending
north-south and plunging 31 degrees to the north and an-
other bearing N 26° W with a plunge of 30 degrees to the
northwest. Again both conform to the regional geology.

Three different highs, all small, are observed in the
E3 specimen. Two strike north-south, but plunge in opposite
directions; one 50 degrees north, the other 56 degrees
south. Since these are found in the "A-C” concentration,
they are parallel to the maximum deforming stress. The
third also coincides with the deforming stress as it bears
N 56° W plunging 60 degrees to the northwest.

For the most part E4 conforms to the local structure
also. As is shown, four 3 per cent highs are recorded,
three of which are normal or nearly normal to the synclinal

axis. One strikes N 22° W with a plunge of 23 degrees to

the southeast. Once more two of the highs trend north-

31

south, one plunges 20 degrees south, and the other is nor-
mal to the "A" axis. The remaining high strikes N 14° E
and plunges 19 degrees to the northeast. This deviation
will be of little concern, as it is over-shadowed by the
significance of the other three maxima.

Overall, the diagrams from outcrop "E" fit the regional
picture of being oriented normal to the axis of the trough.

Figure 23, of sample "G” depicts 400 C-axes contoured
on a composite diagram. The "A-C" girdle is again very
evident with another 3 per cent maximum in the southeast
quadrant. The high bears N 24° W and plunges 25 degrees
to the southeast. Once more, we see the orientation trend-
ing parallel to the folding thrust.

From outcrop "H," 330 C-axes were plotted from two
thin-sections and are shown in Figure 24. The "A-C" girdle
is not so distinct in this diagram as in previous ones and
a 3 per cent high exists in the southwest quadrant. The
other maximum which strikes N 28° W and plunges 54 degrees
northwest, may depend upon regional structures. The first
mentioned high bears N 69° E, plunging 64 degrees to the
southwest and seems independent of local structures, but

may be influenced by either of two faults adjacent to the

32

quarry from which the sample was taken. One fault strikes
WNW and the other bears to the NE.

A dolomite axis diagram of sample "I" (Figure 25) repre—
sents 400 contoured C-axes. Again, an ”A-C" girdle is
evident with a 3 per cent maximum striking N 28° E and
plunging 20 degrees southwest. This northeast bearing and
southwest plunge is probably independent of any regional
structure. Outcrop I is directly west of the Felch Metro-
nite Quarry and is most likely influenced by the granite
dikes mentioned by Solberg (1958). The free quartz
stringers found in the "I" thin—sections help substantiate
this belief.

In sample "J," Figure 26, the "A—C" girdle appears to
disappear with only remnants remaining. Preferred orien-
tation is apparent with a small but strong 5 per cent
maximum that strikes north-south and plunges 21 degrees to
the north. This clearly complies with the regional
structure.

There is a weak but consistant dolomite orientation in
nearly all samples. Assuming the regional east—northeast
strike, the orientation of nearly all diagrams is dependent

upon regional structure; as the maximum orientations trend

33

roughly north-northwest and plunge from 60 to 20 degrees
and average 37 degrees in the same direction. The strong
"A-C" girdle present in every diagram is also north-
trending.

The conclusion reached is that the preferred orienta-
tion is essentially normal to the regional strike. There-
fore, the deforming stress and preferred orientation parallel
one another, which means the deforming force must have come
from the northwest or southeast. It is stated by Turner
(1952), that it is conceivable that recrystallization of
marble under simple compression may yield a fabric with
strong preferred orientation of C-axis parallel to the

direction of maximum compressive stress.

34

 

 

 

 

 

 

Figure 15.

 

 

 

400 dolomite axes from sample A

CODtOUIS‘4—3—2—l percent, plotted
on the lower hemisphere of a Schmidt

equal—area net.
35

 

 

 

 

 

 

 

 

Figure 16. 400 dolonite are: from seflple B

contours 3~“—l percent, plotted
on the lower hemisohere of a Schmidt
equal—area net.

36

r—0.415

 

 

 

 

 

 

 

 

400 dolomite axes from samale C
contours 5—4—3—2—1 percent, plotted

on the lower hemisphere of a Schmidt

equal—area net.
37

 

 

r—0.103

 

Figure 18. 400 dolonite axes from sample D
contours 3e9ml percent, plotted

on the lower hemisphere of a Schmidt
equal—area net. I

38

 

'Z.

 

 

 

 

 

 

 

 

 

 

Figure 19. 400 dolomite axes fron samnle El
contours 3—“—l percent, plotted
on the lower hemisphere of a Schmidt
equal—area net.

39

 

 

 

 

 

 

 

 

 

 

 

Figure 20. 400 dolomite axes from sample EQ

contours 3—2—1 percent, plotted
on thelower hemisnhere of a Schmidt
equal—area net,

40

 

F

 

igure 21.

 

 

 

 

 

 

 

 

 

 

403 dolomite axes from sample E3

contours 3—2—1 percent, plotted
on the lower hemisphere of a Schmidt

equal—area net.
41

 

r—0.3l4

'N

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

403 dolomite axes from sample E4
contoUrs 3—9—1 percent, plotted
on the lower hemisphere 6f 9 Schmidt

equal—area net.
42

 

 

 

 

 

 

 

 

 

 

r—0.302

 

 

 

 

 

 

 

 

 

 

 

Figure 23. 403\dolomite axes from sample G
contours 3—2—1 percent, plotted
on the lower hemisphere of a Schmidt

equal—area net.
43

-g-‘

 

 

 

Figure 24.

ra0.0h3

 

 

 

 

330 dolomite axes from sample H
Contours 3—2.l—l.2 percent, plotted
on the lower hemisphere of a Schmidt
equal-area net.

44

 

400 dolomite exes from sen3le I

contours 3—2-1 percent, plotted

on the lower hemisphere of e
equal—area net.
45

Schmidt

2

 

 

 

 

 

 

O
O.
‘ I
O .
‘ o
I.
U \ ' '
D . v a
. ‘ Q .l I. o
.O ‘ - .
. I . " '
. o
. 9. ll . . . ' o
O . ~ . . .
I . '
.
‘ I

{10.272

0

 

 

.\
~
.' ° . o
I ' O . .
.°. 0'
‘ O
. o
‘
..
.

 

 

Figure 26. 400 dolomite axes from samole J
contours 5—4-3—2-1 percent, PIOIIEd
on the lower henisQNere of a Schmidt
equal-area net.

46

STATISTICAL ANALYSIS

Where orientations are "weak," it sometimes warrants a
statistical test on the petrofabric diagrams to determine
if the attached interpretations are valid. Considering the
petrofabric diagrams done in this problem, we find that nine
have only 3% maxima and only two have 5% maxima.

Therefore, because the majority of the maxima were weak,
a statistical analysis was done on each diagram to help sub—
stantiate their preferred orientations.

In this case, a correlation coefficient will be used as
a measure of preferred orientation. The correlation coef-
ficient measures the degree of preferred orientation of the
individual diagrams. It does not determine the nature of
the fabric, it is used only as a comparison.

The coefficient of correlation (r) measures how the
number of cells or points on one square correlates with the
number of cells on each of the four squares nearest it; if
there is a tendency for squares of equal densities to lie
closer together in some parts of the diagram than in others,
this correlation is significantly positive and interpreted

as evidence of preferred orientation in the parent fabric.

47

In preparing this test a one per cent comparison grid
was made, each square being 1.77 cm. on a side; the result

is an array of 96 squares (Figure 27).

 

Figure 27. One per cent grid used in counting points

for the computation of "r."

Procedures and computations needed to find the coef—
ficient of correlation are found in Fairbairn (1954, pp.
317-321).

A two per cent grid was not used here, because all the
diagrams averaged over 4 cells per square except "H" which
averaged slightly over three.

Fairbairn states that if the average cell content is

48

less than 3 or 4 per square, then it will be best to coarsen
the grid (a two per cent grid).

He also remarks that in the absence of a formal analysis
of the problem, it is suggested that the degrees of freedom
for the test be set at one less than the number of squares
in the grid. The test here involves 272 comparisons made
on the above grid containing 96 squares, hence there W111
be 95 degrees of freedom. Also, it is customary to use the
.05 and .01 significance levels. For 95 degrees of freedom
the .01 level of "r” is 0.26 and the .05 level is 0.20.

Table IV gives the statistical findings for each point
diagram besides each correlation coefficient being placed
on its respective contoured fabric diagram. It may be
noted here that "r" does not in any way indicate the loca-
tion of the plotted maxima.

It is seen by Table IV, that the statistical analysis
seems to validate the point diagrams. That is the diagrams
with the higher per cent maxima have the highest coefficient

of correlation and the ones with the lower per cent contours

have a lower coefficient of r.
"J" which has a 5% maximum and a low "r" seems to con-

tradict this, but when diagram "J" is looked at more closely

49

it is seen that the 5 per cent and 4 per cent maxima are
smaller than the maxima in the other diagrams. So the 3
per cent high is the more valid one for the diagram and
this would more readily coincide with the coefficient of
correlation found for this diagram.

It will also be noticed that nine of the twelve dia-
grams reached the .05 significance level with seven of these

reaching the .01 level.

 

 

TABLE IV
Out- Maximum Coefficient Signi—

o Contour of Correla- ficance
cr p (Per Cent) tion (r) Level
A 4 0.446 .01

B 3 0.377 .01

C 5 0.415 .01

D 3 0.103 *

El 3 0.227 .05
Ez 3 0.114 *

E3 3 0.232 .05
E4 3 0.314 .01

G 3 0.302 .01

H 3 0.068 *

I 3 0.276 .01

J 5 0.272 .01

 

*Did not reach .05 level.

50

SUMMARY

After field and laboratory study, including megascopic

and microscopic work, certain conclusions have been reached:

1. Through thin-section study, it appears the original
dolomite has undergone complete reconstruction. The char-
acter of the accessory minerals indicates that in most cases
the altered rock is a reconstruction of the original impure
dolomite. In some outcrops, elements have been introduced
to the dolomite from pegmatite dikes and hydrothermal

solutions.

2. The Felch Trough area is frequented by pegmatite
dikes and other intrusions, suggesting that they originated
from a deep-seated magmatic mass that may have caused wide-
spread regional metamorphism throughout the area at an
earlier date. To crystallize minerals, such as diopside
and tremolite from a dolomite silica must have been intro-
duced and heat must have been an important agent of meta-
morphism. Due to the abundant pressure twinning, it is
acknowledged that pressure had also played an important

role.'

51

3. Outcrops from the east end of the syncline have
much larger grains. Seeing that temperature and pressure
are of primary importance, these large grains were likely
caused by increased heat and pressure in the eastern sec-
tion of the trough. Heating, squeezing and flowing of the
dolomite might accompany the increased heat and pressure,
suggesting a possible thickening of the marble from east
to west. This theory is not proven here, but is merely

submitted as a possibility.

4. Petrofabrically, the point diagrams show a consis-
tant but weak preferred orientation. This orientation is
normal to the regional structure and parallel to the de-

forming stress, assuming a north-northeast regional trend.

5. Statistical analysis supplements the finding and

interpretations of the composite petrofabric diagrams.

6. As this problem is an extrapolation of Solberg“s
Master of Science thesis, it can be said that the two com-
pare favorably. The Metronite Quarry, where his study was
made, is located east of outcrOp "I" a short distance and is

shown on the locations map (page 2). The mineral assemblages

52

and their respective per cents are in accord with most of
the outcrops used in this thesis. Also Solberg”s fabric
diagrams comply with the regional structure. Altogether
his study, if used as another outcrop, supplements the

other findings of this thesis.

The only place that I would differ with Solberg is in

the identification of some minor minerals.

53

PLATE IX

Figure 28. Photograph of active quarry at outcrop "B,"
showing where oriented specimen was taken plus joint face

(N 46° E, 43° SE).

Figure 29. Photograph of outcrop "B,” showing joint

surface within quarry (N 61° W, 85° SW).

«ca—

39 <

 

2:..— 62¢.

-

 

wa \~

’-

-\ _ A __ -\—"\.M "W‘V’v‘x’-

~-\4\— ~<‘\..r ’ —'\.

4 _,4\ "\

54

PLATE X

Figure 30. Photograph of inactive quarry at outcrop "C,"

taken at oriented specimen and looking east.

Figure 31. Photograph of quarry "C," looking across

quarry at oriented specimen. Note parallel fractures.

_ 2‘ v." ‘53 . A; " \‘, 1‘

, $4

‘-

.,

>13 :5» >20 :5»

 

-~—‘\’\ - \-\, Mr“

-\ . ~/\-

‘\—— ‘5‘ '\,~_.

‘\¥L—‘~ \l'\ ~N,~ V-‘d fi \ A 1V-..

.AN'

\-

55

PLATE XI

Figure 32. Photograph of outcrop "C," showing joint

surface in marble (N 42° E, 77° SE).

Figure 33. Photograph at outcrop "C," showing joint

surface at west end of quarry (N 15° E, 69° NW).

,"f_

>CO :5“

 

>ZO :5»

56

PLATE XII

Figure 34. Photograph of Rian Quarry at outcrop "H,"

looking across quarry at oriented specimen location.

Figure 35. Photograph of dolomite at Rian Quarry,

showing close-up of oriented specimen location.

‘901 011V

396! 011V

 

 

 

m.\__ 3“.“ ,NF \.'M-———‘\ K ”1‘
M

57

PLATE XI I I

Figure 36. Photograph of wall in Rian Quarry, showing

parallel joint surfaces (N 22° W, 72° SW).

Figure 37. Photograph of marble at outcrop "H,” show-

. o 0
ing joint surface in dolomitic marble (N 60 W, 86 NE).

«Q: 32¢.

«2: 3: <

 

58

Turner, F. J., and Verhoogen, J. (1951); Igneous and Meta-
morphic Petrology, McGraw-Hill, Inc., New York.

 

Van Hise, C. L., and Leith, C. K. (1911); The geology of
the Lake Superior region, USGS Mon. 52.

 

60

M

We

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aft

31’;

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Scale —-- l” 110'

 

 

 

 

 

 

 

Sketch

 

 

of outcrop E.

iflliffi XVI

 

 

Scale-l”

 

10'

 

 

 

 

 

 

 

 

Sketch of outcrop D.

i) I- I l‘ E K V

 
    
    

Iomr FACE
mrrws 52‘Nw

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NAS‘E
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Scale —

 

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Sketch of outcrop I.

    

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