A CHEMICAL AND PETROGRAPHIC IAN'ALYSIst ’17 ‘_
‘ WOODEN No, 6 WELL: MIDDLE ORDOVICIAN FROM;
' HILLSDALE COUNTY MICHIGAN ,,,,, 21:5 ‘

Thesis for 'the Degree of M._ S
MICHIGAN STATE UNIVERSITY

L Olaylnka Asseez
1964

mass

LIBRARY

Michigan State
University

 

 

 

 

 

   

A CHEMICAL AND PETROGRAPHIC ANALYSIS
OF WOODEN No.6 WELL; MIDDLE ORDOVICIAN FROM

HILLSDALE COUNTY, MICHIGAN.

By

LL Olayinka Asseez

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Geology

1964

3

»-b.f
1’97'

ABSTRACT.
A CHEMICAL AND PETROGRAPHIC ANALYSIS OF WOODEN No.6 WELL» MIDDLE
0RDOVICIAN FROM HILLSDALE.COUNTY, MICHIGAN.

By L. Olayinka Asseez

Oil and gas showings and production in the Ordovician limestones of
Michigan are restricted to areas where the Trenton-sBlack River sequence
is partially or completely dolomite“ In 1957 Tinklepaugh observed that
there is a correlation between structure and epigenetic dolomitization=
the highest degree of dolomitization being attained in the anticlinal

crests .

In an attempt to separate diagenetic from epigenetic dolomite in
the Ordovician system in the Albion-Scipio field areass Dr° C.E.Prouty
started the first part of a series of studies in 1961. On the first well
core, the PetersoncHoward No.1, Hamil (1961) studied the vertical Ca/Mg
variation, Kirschke (1961) the petrographic analysis and Zaitzeff (1962)
the microfossil analysis.

In this second part of the investigation, the writer studied the
Wooden No.6 well, Hillsdale County, MichiganaThis study consisted of:

1. Detailed chemical analysis using Versenate methodo
2. Insoluble residue analysis.

3. Staining, using potassium ferricyanide and

4. Thin sections. of Trenton limestone.

An attempt was made to correlate the Mg/Ca ratios and the results
of the insoluble residues with the radioactive logs. The common associ=

ation of abundant dolomite with shale was noted and four reasons were

11

suggested for this common occurrence.

The writer believes that there are two periods of dolomitization-early
and late. These two types occur together although one type may be more
dominant than the other in any particular horizon. The writer feels that
future work would not! be able: tarseparate them completely sinceuthere. is

.—-u’

often no clear-cut stratigraphic boundary between then.

iii

 

TABLE OF CONTENTS

Page
I. INTRODUCTION ...................................................1

II. GENERAL STRATIGRAPHY..........................................-
Lower Ordovician.......................................
Middle Ordovician......................................5
Upper Ordovician........................................

Sampling...............................................
Chemical Analysis......................................
Digestion.....................................5
Determination of Ca1cium......................9
Determination of Magnesium....................9
Titration.....................................J0
Calculations..................................Jl
Standardization with Calcium Solution.........Jh
Insoluble Residues.....................................Jh
Staining...............................................J6
Preparation of Thin Sections...........................J8

5
7
III.LABORATORY TECHNIQUES..........................................B
8
8

IV. OBSERVATION AND RESULTS......................................18
Classification of Limestones based on Ca/Mg ratios.....J9

Chemical Analyses......................................2h
Dolomitization.........................................25

Megascopic Descriptions................................26

Columnar Section of Wooden Well...............h3

Microscopic Descriptions...............................56

v. SUMMARY AND CONCLUSIONS........................................6Q

VI. BIBILIOGRAPIIY.000.000.000.00...0000000°00OOIOOOOOOOOOODOOOOOCDO6O

iv

U h,

ILLUSTRATIONS

Page
Figures
1. Location of Wooden Well....................... . ........... ....3.

2. Lithology of Middle Ordovician Rocks.............................4.

3. Diagram for Computing Magnesium /Ca1cium Ratios using volume
of Versenate Solution used in titration. ....... . . . . . . . . . . . . . . . . . 4;;

4. Diagram for Correcting the Calculated Ratios.....................13.

U!
0

Comparison of the Mg/Ca,Insoluble Residue Curves with the
Radioactive IOgSOOIOOOOIIIIODOOOOOCOOOOOOOO.OOOOOOOCBOOIOOOCIOOO~23-‘

Tables

1. Generalized Stratigraphic Section of the CambrianwOrdovician
Systems of Michigan.............................................6.

2. Results of Chemical Analyses.... ...... .... ........ .............21.

 

ACKNOWLEDGEMENTS

The writer is deeply indebted to Dr. C.E. Prouty under whose directi=
on the problem was undertaken and without whose many suggestion this work
could not have attained its present standard,

He is also grateful to Dr. J.W.Trow and Dr. H.B.Stonehouse, both mem-
bers of the guidance committee, for their helpful support.

Dr. B.G. Ellis of the Soil Science Department, James Lammons and
Daniel Robbins of the Geology Department were also very helpful with the
many unsolved problems of the differential thermal analysis machine.The
writer is very grateful to them all.

The Davis Drilling Company Inc. deserve the writer's unqualified gra-

titude for supplying the well core which made this study possible.
It is hoped the company will always support the department in this and

many other ways.

INTRODUCTION

The Trenton limestone normally has low effective porosity except where
dolomitization has led to increased porosity and permeability. Hence some
oil and gas prospecting has been based on the lateral variation in the cal=
cium /magnesium ratio, with a h0pe of determining the limit of the Albionu
Scipio structures. However little attempt has been made to determine how
much of this variation is stratigraphic.

This led Dr. C.E.Prouty to the feeling that if the vertical variation
of calcium/magnesium ratio was determined, and the paleontologic and petrm
ographic characters of several wells were known, we might be able to
separate epigenetic from stratigraphic dolomitization. The first three
parts of this problem were conducted on the Petersonnfloward No.1 Well
( Hamil 1961, Kirschke 1962 and Zaitzeff 1962 ) located in Pulaski Town»
ship, Jackson County ( section 17, T 4 S, R 3 W)

In an attempt to contribute to the solution of this very important
problem, the writer undertook the chemical, staining, petrographic and
thermal analyses of the second core, the WOoden No. 6, located in Hills»
dale County just southwest of the structural trend of the Albion-Scipio
.field ( section 4, T 5 89 R 3 W, Fig. l.) The top of the Trenton was
intersected at 3562’ but coring was not started until 3765, and was
stopped just short of the Black River at 3915".

The writer had haped to be able to co-ordinate all the aspects of
his studies and especially to compare the thermal and chemical results.
However the thermal program was reluctantly abandoned after he had failed

to make the differential thermal analysis machine work.

It is hoped that more wells will be available and that other interested

students will carry this problem to its logical conclusion.

 

MICHGAN

DEPARTMENT OF CONSERVATION

Location of Wooden No.6 Well.

Fig.1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

§ Entirely Dolomite
J Limestone and Dolomite

Em irely Limestone

,/ ’Boundary of Trenton and Black River
at surface.
'* Location of Wooden No.6 well.

 

 

Fig.2 Lithology of Middle Ordovician Rocks

( modified after Cohee , G.V.l9h7)

 

 

5
GENERAL STRATIGRAPHY
Lower Ordovician

The lower Ordovician rocks of Michigan consist of the Oneota dolomite,
the Richmond sandstone and the ShakOpee dolomite ( Table 1). The Oneota is
a buff or brown dolomite with little green shale and may be cherty, sandy
and oolitic in places. In the southwestern parts of Midhigan where it is
over 300 feet thick, it is largely sandstone with surbordinate dolomite.
The New Richmond sandstone is thin and underlies the buff, brown or gray
Shakopee dolomite. This dolomite has thin beds of shale and is cherty or
sandy in places.

Middle Ordovicianr

The Middle Ordovician rocks consist of the St.Peter sandstone,the
Glenwood shale, the Black River limestone and the Trenton limestone. The
basal St. Peter sandstone is white to brown, very pure, friable and has
chert fragme ts and pyrite crystals in places.It is well sorted, well
rounded ,frosted ( Horowitz 1961) and unconformably overlies the lower
Ordovician rocks in the southeastern parts where the Prairie du Chien
has been eroded.

The Glenwood shale consists chiefly of fine grained sandstone,
shaly dolomite and is 10°100 feet thick. In the southeast of MiChigan,

a green ,brown, pyritic sandy shale underlies the Black River where the
St. Peter has been eroded. This shale is lithologically similar to the

Glenwood formation although it may not be the exact time equivalent of

the Glenwood farther to the west ( Cohee, 1948 ).

The Black River and the Trenton lhmestones are generally brown,gray
crystalline limestone and dolomite in varying prOportions ( Fig.2 ).

Dolomite is predominant in the eastern parts of the Michigan Basin,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6
F . s
E, 8 3 Series Group Formation Member
. a g
m Queenston
3 CIICINNATIAN RICHMOND
Utica
__, C olliggflggd
TRENTON Groos Quarry
z 0 z Chanlefl
4 g 4 mHAwKIAN er a
H E H BIACK RIVER Bony Falls
U —-i O ’-
H H
p p CHAZYAN Glenwood
o o
a: a: o
o '3. o shakOpee dol.
3 PRAIRIE DU
3‘3 CANADIAN New Richmond
CHIEN
Oneota dol
L—_u
Jordan Ss.
Trempealeau Ipdi
IAKE St.‘I.Bwrence
3 3T- CRCIXAN Franconia 38.
II 3 z . .
4 4 SUPERIOR Mumsmg Dresbach
H H Eau Claire
a: a:
m m - Mount Simon
)2 a) x:
H
a: '0 <2
0 3" o
i. Jacobsville
3? 35.
x.
L: a :_—_l
Table l. Generalized Section of the Cambrian and Ordovician

Systems of Michigan. ( modified after Mich. Geol.Survey,

Chart 1, 1961:)

 

7
limestone in most parts of central Michigan, and limestone and dolomite
in northeastern Illinois, northwestern Indiana and along the Kankakee
and Findlay arches. The occurrence of dolomite along major anticlinal
axes is strongly suggestive of a secondary origin, possibly related to
folding ( Cohee 1948 ).

The base of the Trenton limestone in southwestern Ontario consists
of argillaceous limestone and shale about 400 feet below the Trenton and
is easily traced into Michigan , except in Berrien County where it is
largely dolomite ( Cohee, 1948 ). The base is distincly identifiable
on the neutron log at 3920 feet.

The maximum thickness of the Black River and Trenton limestone is
attained in eastern Mishigan and the adjoining areas of Ontario where
they are over 900 feet. The structural thickening in the Chatham sag has
led Cohee (1948 ) to suggest that it was a structurally low area that
served as the link between the Mishigan Basin and the seas on the east
in Black River time. The Black River and Trenton thin very considerably
over the adjoining arches.

Upper Ordovician

The upper Ordovician formations attain a maximum thickness of 800
feet in the southeast 0 close to the area of greatest thickness of Middle
Ordovician formations. Largely they consist of dark gray shale with minor
amounts of-dolomite and limestone, and.oomprise shiefly @meUtica, Lorraine
and Queenson shales.

The Utica is a dark gray to black shale lSOeZOO feet thick and is
overlain by the Lorraine,a dark gray calcareous shale, with thin beds of
limestone and dolomite, and is 290~390feet thick. The Queenston shale is
predominantly red with interbedded limestone or dolomite and is up to

200 feet thick in places.

LABORATORY TECHNIQUES

Sampling

Using a diamond rock saw, a relatively thin slice was made from each
piece of core whose tap had been carefully marked. It was necessary to
number each of the pieces,(the tOp being No. l) in each core box. This
made it easy to align several pieces together for comparative purposes.

After all the slices had been made, they were bagged at two-foot
intervals and each bag was carefully labelled. Each sample was then pulve~
rized in a Denver jaw crusher and later ground finer in a steel motar. It
was then split into several parts ( Krumbein & Pettijohn 1938 )until about
one eighth of the sample was left. This was finally pulverized in a steel
mortar, sieved in a 0.417 mm Tyler sieve, bottled and then labelled.It was
believed that by preparing the samples this way, each would be represen=

tative of the two-foot interval whiCh Rones (1955) has shown to be Optimum.

Chemical Analysis.
Digestion:
l. The sample was washed, dried and one gram of it weighed into a 250 ml.
flask, followed by 10 m1. of perchloric acid.
2. It was heated gently until the solution turned colorless after which
it was evaporated to dryness.
3. The cooled residue was dissolved in 3 m1. of 50% hydrochloric acid and
10 ml.of distilled water. The residue was filtered off and the solution
was diluted to 250 ml.

It is essential that all work be done under the hood and as cautio-

usly as possible since perchloric acid is very explosive and also has very

9
poisonous fumes. It is also very important that distilled water be used
to make solutions and to rinse apparatus after washing owing to the
high amount of dissolved inorganic materials in tap water.
Determination of Calcium:

The analytical method used herein is that preposed by K.L.Cheng,
et al ( 1952 ).

Versenate Solution. Dissolve 4 grams of the disodium salt of ethyl-
ene -dinitrilo tetra-acetic acid in one liter of water. Standardize the
solution against a standard calcium solution as described below.

Standard calcium solution. Dissolve 2.5 grams of reagent grade of
calcium carbonate in about 5 ml. of 50% hydrochloric acid and dilute
exactly to one liter with water. This solution contains one milligram
of calcium per milliliter.

Potassium hydroxide. Use a 20% aqueous solution.

Calcium indicator powder. Mix thoroughly 40 grams of powdered pota-
ssium sulfate and 0.2 gram of murexide.

Titratio n:

l. Pipette a 10 m1. aliquot of the solution to be analysed into a 200 m1.
porcelain dish; add approximately 20 mloof water, one m1. of potassium
.hydroxide and a tiny sc00p of of the prepared calcium indicator powder.
2. Stir and titrate with the standard Versenate until the color of the
solution changes from pink to violet.

Determination of Magnesium:

Buffer sclution. Dissolve 60 grams of ammonium chloride in about 200 nfl..
of water, and add 570 m1. of concentrated ammonium hydroxide. Dilute to

1 liter with water.

Potassium cyanide. Prepare a 10% solution.

10

F°241 indicator. Dissolve 0.15 gram of Erichrome black T ( F~241 ) and
0.5rgram of sodium borate in 25 ml of metanol.
Titration:
1. Pipette a 10 m1 aliquot of the solution to be analysed into a 200 ml
porcelain dish, add 25 m1.of water, 2 to 3 m1.of buffer solution, a few
drops of potassium cyanide solution and about 8 drops of F=24l indicator.
2. Stir and titrate with Versenate solution until the solution changes
from wine red to clear blue.

For faster and more consistent results the above solutions were mixed
before titration as follows;
For magnesium combine the buffer solution, water and potassium cyanide in
the ratio of 3: 26: 1, and use 30 m1 of this solution for titration. For
calCimm, mix a certain quantity of potassium hydroxide solution with
twenty times its volume of water and use 20 m1 of this solution for titra»
tion.

Each sample was titrated three times and the average of the readings
was used in calculation.
Calculation:

Since 1 gram of the sample is made upto 250 ml.solution and 10 m1.of
this aliquot is used for titration, we have:

A x 25 x 1.4 x B x 100 =percent calcium oxide.
1000

C x 25 x 1.66 x ( D-B )

 

x 100 = percent magnesium oxide.

1000
A = milligrams of calcium per m1.of Versenate solution.
C =“milligrams of magnesium per ml. of Versenate solution.
B = milliliters of Versenate solution used in titration for calcium using

murexide as indicator.

D = milliliters of Versenate used in titration for magnesium using Fb24l

11

as indicator.
Standardization with standard calcium solution:

As prepared the calcium solution contains 1 milligram of calcium per
mluof solution; 25.5 ml. of Versenate was needed to titrate 10 ml of this
solution to its end point.

10 milligrams of calcium ( solution ) 25.5 ml Versenate
Versenate = 10 mg Ca / 25.5 ml. =0.40 mg ("A" of Equation )

To compute the milligrams of magnesium per ml. of Versenate we use

the figure as follows:

Atomic weight of Mg = 24.32 = 0 607
Atomic weight of Ca 40.08 °

Versenate = 0.40 x 0.607 = 0.242 mg /ml. Versenate ( 0C0 of Equation.)

This basic formula permits computations without using exactly 1 gm.
of the sample , and without standardizing the Versenate solution for
either calcium or magnesium, although these were done in this investigation.
Recent work ( R.L.Jodry 1961 ) in the Research and Deve10pment Division of
the Sun Oil Company has produced two curves - Figs. 3 & 4. Figure 3 may
be used to compute the magnesium /calcium ratio by dividing the milliliters
of Versenate used to titrate for calcium by the milliliters used to titra~
te for magnesium, and locating this point on the vertical axis and then
reading down to obtain the ratio on the horizontal scale at the bottom of
the curve. This solves the equation graphically.

It has also been shown that the ratio calculated from the basic form»
ula needs to be corrected for a slight error in the formula due to an
inherent error in the chemical reaction- the inability to differentiate
exclusively between calcium and magnesium. The calculated ratio is located
on the vertical scale of Fig.4 and the true ratio is read off on the

horizontal scale. The basic formula corrected in this way gives the same

 

 

900

750

160

300

150

 

13'

 

A1331)“ muéfin 160 600
Cilia/CALCIUM OXIDE RATIO 1:
or Jodry, R.L. 1961)

103356

l4

ratio as that calculated from Fig.3, but the writer used. the formula
corrected by Fig. 4 since he was also seeking the percentage composition
of calcium and magnesium.

The writer has found it easier to work with Mg/Ca ratio since small
changes in magnesium become accentuated. It also produces a ratio varying
from 0.00 for limestones to 0.6 for dolomites and minimizes changes in
predominantly calcitic rocks. 0n the other hand, variation in the Ca/Mg
ratio is from0.6 for dolomites to infinity for limestones , thus making
graphical representation impractical. It also emphasises small changes
in predominantly calcitic rocks. The writer has included this ratio in
Table 2 because it might be useful for comparative purposes with Hamil

( 1961 ) who used it in his work.

Insoluble Residues.
1. 1 gram of the sample prepared for the chemical analysis was put in a
weighed test-tube and three or four drops of 4% hydrochloric acid were
added. A few more drOps were added (three drops at a time ) after the
effervescence had stopped.
The samples were left overnight after which they were centrifuged and the
clear liquid on top was decanted.
2 . MOre acid was added and the process repeated until there was no react~
ion when fresh drops of acid were added.Each sample was left in the acid
for two to three hours where it was not necessary to leave it overnight.
3. The residue was washed three times, oven dried, cooled and weighed. The

difference in weight represents the weight of the insoluble materials.

Staining.

The process of staining employed in this investigation was that deve-

10ped by Steidman (1917). This method stains dolomite blue when the

15

sample is immersed in a solution of potassium ferricyanide and depends
on the presence of ferrous oxide in the dolomite. This ferrous oxide
reacts with the ferricyanide to give the blue color.Ca1cium remains
unaffected.

The sliced surface of each piece of core was immersed in a 5%
solution of hydrochloric acid to which a few drOps of concentrated
solution of potassium ferricyanide had been added.Each piece was
supported on two small balls of wire gauze so that the submerged surface
did not rest on the bottom of the container.

Each sample was immersed in the solution for two to three minutes
at first, but the time was increased as the solution became weaker.The
solution was changed after the fourth set of samples had been
stained. This process produced excellent relief in the unstained parts
and hence identification was relatively easy.

The stained surfaces were then observed under the binocular micro-
scope with a view of determining:

1. Distribution, nature, and amount of dolomite.

2. Distribution, nature and amount of insoluble residues.
3. Fessils.

4. Texture.

5. Diagenetic changes.

6. Relation of insolubles and dolomite to bedding.

In addition to these records, a diagram of the stained surface was
made in order to give a better picture of the stained surface to the
reader. It was not possible to use conventional lithologic symbols
because each lithologic type was drawn as close as possibe to its
true position. In a section one inch thick and using the same scale as

in this investigation, ( 1" = 1' ) it was not feasible to use these

 

16

symbols. With the approach used in this investigation it was much easier.
Preparation of Thin Sections

While the megascopic descriptions were being made, portions of the
core from which thin sections would be made were marked with a marking
pencil and numbered. The position of each marked section was located on the
columnar section with the appropriate number. This portion was made so that
the long axis was perpendicular to bedding, and the top was marked by an
arrow.

When all the selections had been made, thin slabs about 1/16 of an
inch thick were cut. These were stored until all the slabs had been cut.
Each slab was then ground for a few minutes on a lap wheel with 600 carbo-
rundum to provide a flat smooth surface for mounting. This surface was then
stained in a strong solution of potassium ferricyanide to which a few drOps

of 5% hydrochloric acid had been added.

The slab and the glass slide, which had first been frosted with No.
180 grit, were laid on a heated metal plate with the clean side of the
slab and the frosted side of the slide turned up. The flame of the bunsen
burner was adjusted so that the aroclor did not melt too rapidly when
applied to the heated slide. When most of the bubbles had been 'cooked'
out, the glass slide and the slab were removed. The slab was then gently
placed, one side first, on the slide and the excess aroclor gradually work-

ed out as the slab settled into place. By gently pressing down with the
eraser of a pencil, most of the bubbles and excess aroclor were worked out.
The slide was then labelled by means of a diamond pencil.

Each of the mounted slabs was then ground on a lap wheel using No.

180 carborundum to about 0.03 mm thickness. Grinding was then transferred

successively to other lap wheels using Nos. 11000, 6000 and 8000. Final grind—

 

17
ing was done by hand on a glass plate with No. 800 powder until the
desired thinness was attained.

The finished sections were then immersed in a strong solution of
potassium ferricyanide to which a few drops of 3% hydrochloric acid had
been added. The slides were taken out of the staining solution after about
six hours. They were then studied for fabric, composition, Occurrence
and nature of dolomitization.

Although this test involves the recognition of the presence of
iron, it is recognized that dolomitization and iron content are
related, and so this is a recognized staining test for dolomite, The
good correlation (pp.26-42) of chemical analyses and stain observation
in this work justifies its application. A notable exception occurs
between 3801.5 and 3805.3, where the magnesium content observed from
chemical analyses is very much lower than would be expected from the

observation of the stained surface.

 

 

18
OBSERVATIONS AND RESULTS
Classification of limestones based on Ca/Mg Ratio:
The following classification was proposed by G.V. Chilingar (1957)
and is based on Ca/Mg ratios. The writer has adepted the same c1assific=

ation in order to make room for easy comparison of all past and future

investigations in this program. oa/yg Ratio
Magnesflmt dolomite 1.0 e 1.5
Dolomite 1.5 .. 1.7
Slightly calcareous dolomite 1.7 m 2.0
Calcareous dolomite 2.0 e 3.5
Highly dolomitic limestone 3.5 a 16.0
Dolomitic limestone 16.0» 60.0
Slightly dolomitic limestone 60.09 105.0
Calcitic limestone Over 105.0

Magnesian dolomite. A pure dolomite has a ratio of 1.648 2 l Ca/Mg
but dolomites formed at elevated temperatures contain an excess of
magnesite. We can place some hydrothermal dolomites in this group.

Calcareous dolomite. The selection of the upper limit here is based
on the fact that the ratio of 3.44 z 1 is the lowest known in the skelee
tal structure of organisms. Many dolomites fall in this group and might
have resulted from excess precipitation of calcium over magnesium. or may
be due to incomplete dolomitization.

Highly dolomitic 1imestone.Several limestones composed almost entiw

 

rely of skeletal parts of organisms have ratios over 1621, even though
the ratio in the skeletal Parts themselves may be as low as 3.44 2 l.
Diagenetic and epigenetic dolomitization may play a significant role in

their origin.

l9

Dolomitic limestones. These are mostly due to the accumulation of
skeletal structures of organisms. The fine grained limestone paste may
originate by direct precipitation, by comminution of skeletal structu=
res or by diagenetic or epigenetic dolomitization.

Slightly dolomitic limestones. The upper limit of 105 selected here
is due to the fact that 105.2 is the highest ratio known for calcitic
organisms ( oysters ) and the limestone probably resulted from the
accumulation of organisms. Their finer grained limestone paste is often
of chemical origin.

Calcitic limestones. These may have their origin in the accumulation
of aragonitic skeletal structures, by direct precipitation or from
subsequent loss of magnesite. The high ratio is probably due to the low
magnesium content of the cementing limestone paste precipitated from
sea water.

Chemical Analysis

The results of the chemical analyses herein could not be adequately
compared with those of Hamil ( 1961 ) inasmuch as his work was based on
the Black River section while this was entirely Trenton. However the
range of magnesium content in the two investigations was comparable.

Throughout the Trenton section dealt with in this investigation,the
magnesium/calcium ratio varies from about 0.02 to a maximum of about
0.22- with the majority being about 0.05 ( table 2 ). Inasmuch as most
of the ratios fall within 0.035 and 0.075, it may be arbitrarily assumed
that this represents the end points for stratigraphic dolomite. 0n the
other hand those regions in which the ratio is significantly above or
below these end points may represent epigenetic dolomite. It cannot be
over-emphasized that these end points are arbitrary and are subject to

change in the light of future results. It is interesting to note that

20

some of the regions with high ratios do, in fact, have positive evidee
nce for post-lithification dolomite. For example at 3807“=38090 and about
3785' there are fractures which have been filled by dolomite and calcite.

An attempt has been made to correlate this ratio with the radioactive
’logs ( Fig;5). Apart from a few places where high magnesium/calcium rat-
ios correspond with negative kicks on the neutron log and vice versa. no
correlation was apparent. The apparent correspondence may be only fortu=
itous and so no significant reliance should be placed on it. However it
is important to note the difficulty of comparing a bar graph with a
smooth curve. The writer did not think a smooth curve would adequately
represent the ratio or the insoluble residues,because such a curve would
place the peak at a point rather than over the whole interval, and hence
the picture would be very misleading.

0n the whole there was a greater correlation between the results of

the.insolub1e residue and the calcium and magnesium analyses.MOSt of the
high and low points of the two agree although not in direct proportions.
Mbst students of dolomitization agree that there is a significant assoc=
iation of dolomite and shaly partings and so one would expect a good
correlation between the two in a chemical analysis. The lack of a perfect
correlation (Fig.5) may be attributed to three major reasons:
1. Inasmuch as the whole Trenton section is dominantly calcitic. high
calcium content necessarily means low insoluble residue and vice versa,
especially since the absolute magnesium content was generally less than
10% and would average about 3.14% ( Table 2).Hence the magnesium content
could not exert any significant control on the amount of insoluble residue.
2. The insoluble residues are made up not only of carbonaceous, shaly
materials which are known to be richer in dolomite ( Carozzi.l960 ) but

also include other siliceous materials especially cherty bands and lenses.

 

 

 

 

 

 

Table 2. Results of Chemical Analyses

Insoluble Calcium/Magnesimm
Residue % Ratio.

S a m p l e Calcium Magnesium
No. D e p t h Oxide % Oxide Z

 

1. 3765-67
2. 3767-69
3. 3769-71
4. 3771~73
5. 3773.75
6. 3775477
7..3777=79
8. 3779-81
9. 3781o83
10.3783~85
11.3785-87
12.3787-89
13.3789a91
14.3791e93
15.3793a95
16.3795~97
17.3797~99

18.3799-3801~

19.3801403
20.3803=05
21.3805407
22.3807~09
23.3809=11
24.3811-13
25.3813-15
26.3815al7
27.3817419
28.3819eZl
29.3821423
30.3823=25
31.3825427
32.3827o29
33.3829=31
34.3931~33
35.3834e35
36.3835637
37.3837~39
38.3839o41
39.3841443
40.3843445
41.3845-47
42.3847-49
43.3849=51
44.3851~53
45.3853~55
46.3855a57
47.3857459
48.3859a61

51.68
51.70
47.54
50.54
50.13
45.69
47.08
48.72
44.39
50.87
49.66
43.86
48.87
52.83
49:07
52.78
46.92
50.99
50.24
54.01
50.30
45.70
53.46
49.49
49.67
43.97
79.68
88.04
49.38
45.95
50.72
46.22
46.40
47.30
44.62
47.17
46.14
44.92
45.37
47.17
51.47
49.33
47.23
47.23
41.43
47.37
49.37
49.71

2.959
2.70

2.862
2.917
2.371
3.756
2.568
2.865
9.788
3.945
5.228
3.65

3.698
3.333
2.320
4.635

5.285 '

3.828
4.132
4.132
4.33

3.529
3.149
6.198
2.617
3.120
6.017
3.193
2.815
1.788
2.426
2.752
3.106
2.957
2.936
2.661
2.294
3.716
2.614
1.835
2.202
2.361
1.927
1.835
1.927
2.431
1.697
2.730

mucosa99wMmoowrdowedmmMHNNwMHHMHO-bmn—i(brew NMMWF‘WLOWHMNNNM
UINLnOOOOOOOOVOwmeMGwOMwOOwNMOQNOOOmCNMNMQOOOOMOOHMOOOOQ:

17.47
22.78
16.62
17.33
21.13'
12.16
18.38
17.00
4.62
12.89
9.50
12.01
13.21
15.81
21.14
13.07
8.88
16.76
12.15
13.07
11.61
12.95
16.97
7.98
18.94
17.34
13.24
25.21
17.54
24.69
20.90-
16.80
14.60
16.0
15.19
17.73
20.11
12.09
17.35
25.68
21.42
21.80
25.61
25.73
21.51
19.48
18.09
18.21

Magnesium/Calcium Ratio
Calculated

0.05724
0.04387
0.06016
0.05771
0.04733
0.08224
0.05441
0.05882
0.22610
0.07757
0.1053

0.08322
0.07568
0.06300
0.04729
0.07649
0.1127

0.05967
0.08224
0.07650
0.08592
0.07721
0.05892
0.1252‘
0.05279
0.05768
0.07553
0.05133
0.05527
0.04051
0.04785
0.05926
0.06848
0.06250
0.06581
0.5638

0.04973
0.08271
0.05761
0 .03894
0.04669
0.04586
0.03904
0.03887
0.04649
0.05134
0:05529
0.05491

Corrected

0.0512
0.0390
0.0540
0.0515
0.0418
0.0764
0.0490
0.0530
0.2170
0.0718
0.0990
0.0770
0.0692
0.0570
0.0418
0.0705

.0.1065

0.0540
0.0764
0.0705
0.0800
0.0713
0.0530
0.1185
0.0465
0.0520
0.0695

‘ 0.0453

0.0493
0.0345
0.0419
0.0509
0.0627
0.0569
0.0600
0.0505
0.0436
0.0770
0.0514
0.0339
0.0405
0.0400
0.0340
0.0339
0.0405
0.0450
0.0493
0.049

Classification

( After G.V. Chilingar 1957 )

Dolomitic limestone
Dolomitic limestone
Dolomitic limestone

Dolomitic
Dolomitic

limestone
limestone

Highly dolomitic limestone
Dolomitic limestone
Dolomitic limestone

Highly
Highly
Highly
Highly
Highly
Highly

dolomitic
dolomitic
dolomitic
dolomitic
dolomitic
dolomitic

limestone
limestone
limestone
limestone
limestone
limestone

Dolomitic limestone
Highly dolomitic limestone
Highly dolomitic limestone
Dolomitic limestone

Highly
Highly
Highly
Highly
Highly
Highly

dolomitic
dolomitic
dolomitic
dolomitic
dolomitic
dolomitic

limestone
limestone
limestone
limestone
limestone
limestone

Dolomitic limestone
Dolomitic limestone
Highly dolomitic limestone

Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic

limestone
limestone
limestone
limestone
limestone

Highly dolomitic limestone
Highly dolomitic limestone
Highly dolomitic limestone
Dolomitic limestone

Dolomitic limestone

Highly dolomitic limestone
Highly dolomitic limestone

Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic

limestone
limestone
limestone
limestone
limestone
limestone
limestone
limestone
limestone

 

 

 

m
. D

p l e Calcium Magnesium Insoluble Calcium/Magnesium
e p t h Oxide Z Oxide Z Residue Z Ratio
. 3861-63 44.48 1.697 5.9 26.19
. 3863-65 42.70 1.661 10.0 25.40
. 3865-67 50.00 2.755 8.0 18.16
3867-69 49.15 4.182 5.5 11.75
. 3869-71 46.24 1.63 7.1 28.36
. 3871-73 52.49 2.509 5.3 20.92
. 3873-75 53.04 2.313 3.6 22.93
. 3875-77 48.37 1.422 4.6 34.03
. 3877-79 51.13 1.763 2.2 31.08
. 3879-81 43.37 2.963 11.0 14.64
. 3881-83 51.68 3.493 3 l 14.78
. 3883-85 50.82 2.479 3.4 20.50
. 3885-87 51 26 2.657 2.7 19.31
. 3887-89 53.25 2.657 2.1 20.04
. 3889-91 47.33 2.568 2.6 18.43
. 3891-93 52.94 2.529 1.9 20.94
. 3893-95 No Sample
. 3895-97 No Sample
. 3897-99 52.90 2 60 2.1 20.34
. 3899-3901 50.37 3.099 5.0 16.38
. 3901-03 44.20 4.940 3.7 8.962
. 3903-05 46.30 4.803 4.2 9 640
. 3905-07 42.95 4 347 10.7 9.88
3907-09 46.50 4.034 10.5 11.53
. 3909-11 39.85 2.936 9.8 13.57
. 3911-13 46.40 3.106 6.8 14.95
. 3913-15 36.82 2.174 19.5 16.93

Magnesium/Calcium Ratio
Calculated

0.03819
0.03938
0.05507
0.08511
0.03526
0.04780
0.04361
0.02939
0.03218
006829

0.06770
0.04877
0.05178
0.04987
0.05426
0.04776

0.04915
0.06103
0.1116
0.1037
0.1037
0.08674
0.07370
0.06689
0.05906

Corrected

0.0332
0.0343
0.0491
0.0793
0.0308
0.0420
0.0380
0.019

0.027

0.0629
0.0618
0.0436
0.0463
0.0449
0.0480
0.0419

0.043

0.0552
0.1051
0.0975
0.0971
0.0809
0.0676
0.0609
0.0538

No
No

22

Classification
( After G.V. Chilingar )

Dolomitic limestone
Dolomitic limestone
Dolomitic limestone
Highly dolomitic limestone

Dolomitic

limestone

Dolomitic limestone

Dolomitic
Dolomitic
Dolomitic

limestone
limestone
limestone

Highly dolomitic limestone
Highly dolomitic limestone
Dolomitic limestone
Dolomitic limestone
Dolomitic limestone

Dolomitic
Dolomitic

a m.p l e
a m p l e

Dolomitic
Dolomitic

Highly
Highly
Highly
Highly
Highly
Highly

dolomitic
dolomitic
dolomitic
dolomitic
dolomitic
dolomitic

1miestone
1miestone

limestone
limestone

limestone
limestone
limestone
limestone
limestone
limestone

Dolomitic limestone

 

 

 

24

These cherty bodies were generally devoid of dolomite and hence would
mean lower Mg/Ca and a higher percentage of insoluble residue in the
section where they abound ( see columnar section pp.30-47 ).

3. In order to make calculations easy and make results as comparable as
possible with the chemical analyses for calcium and magnesium, only one
gram of the prepared sample was used to determine the amount of insol-
uble materials. This required extreme precaution to prevent any loss

of part of the sample. Unfortunately weighing the residues could be done
only to two decimal places of'a'gram,”and since a reSidue of étfséahé
0.02ggramractual weight, each recorded figure-( Table 2') should be.
regarded as ‘1 1%; but this would make only A slight change in the gene-

ral picture of the graph. f;?., - r'”
Dolomitization

The application of staining techniques permitted certain observat-
ions concerning dolomitization. Although there were variations in the
nature of dolomitization throughout the section, five principal types
were the most abundant:

1. Abundant dolomite crystals inishaly partings.

2. Irregular spotty or patchy distribution in limestone,being richer in
calcarenites and calcilutites.

3. Irregular bodies of dolomite resulting fom the dolomitization of the
groundmass while the larger crystals were left unreplaced or partially
replaced.

4.Partial replacement of fossil fragments, nodules, pisolites and
concretions.

5. Insignificant spots in recemented fractures and chert bands and lenses.

In most cases replacement of calcite rhombs was complete and there

25

was abundant evidence of recrystallization. However two types of incomp-
lete replacement were noticed throughout the section. In one case each
rhomb was made up of a nucleus of pure calcite rimmed by a layer of dolo-
mitic calcite and finally a third layer of dolomite.In the second type
the arrangement is reversed. It could be concluded from this type of
arrangement that in the first case dolomitization progressed into the
original calcite crystal and that the presence of a nucleus of calcite
is probably due to the inhibition of dolomitization before_ complete
replacement. The reverse would be true for the second case.

There were cases of greater concentration of dolomite at the shale-
limestone contacts from where it spread out into the shale while dying
out into the limestone. There were cases of tongues of dolomite penetra-
ting pure limestone. These tongues were invariably located in the finer
grained limestone.

The irregular bodies of dolomite referred to above (3) were
observed to be mostly due to differential replacement of the micritic
groundmass, while the coarse grained crystals remained relatively unalte-
red.There were cases of dolomite transecting primary structures. Associ-
ation with stylolitic surfaces is strongly indicative of post-consolida-

tion origin of some of the dolomite.

Megascopic description:

The following is a megasc0pic description of the stained surface.
Initial description was done at one foot intervals but this was later
rewritten so that similar sections could be described together and so
avoid repetition.

All descriptive terms used herein retain their conventional connota-
tions. The same is true of the colors used on the columnar section, where

limestone is represented by blue,purp1e for dolomite and green for shale.

 

5765'

5766‘

5767'

5768'

5769'

5770'

5771'

3772'

5775'

 

Section of Wooden Well 3236, 5165 t973915 feet depth.

 

 

Limestone,light brown, arenaceous, banded. Top band
uniforrdy speckled with doloaite, nodular, and bears
coarse-grained calcite crystals. Bottom band shaly and
d010mitic.

Limestone, brown, shaly , fossiliferous, capabially
brachiopods, bryozoans, corals and.algae.;argillaceous
and almost uniformly dolomitized.Dolomitization is in
form of uniformly speckled and patchy forms , with
occasional concentration around ooncretions, pisolites
and fossils, and stringers parallel to shale partings.
Shale partings bear rudaceous crystals of calcite, with
the calcarenite progressively getting finer towards
shale partings. Two vertical and one horizontal recom-
ented fractures occur- the vertical ones being partia-
lly dolomitized.

ninestone, brown, banded and fossiliferous.Top band
of arenaceous calcirudite bearing euhedral crystals of
calcite, bioclastic,allochemical. Thin stringers and
partings of shale with tongues and lenses of coarse-
grained limestone with a micritic matrix largely repla-
ced by dolomite. The underlying 18" is composed of
alternating bands of limestone; with arms of calcareous
shale stringers and bands;calcareous shale with tOngues
and lenses of limestone; and a limestone-shale layer.
Texture of linestone'and dolomite is semi-uniform
lutaceous calcarenite grading into a rudaceous calcare-
nite lower down. Dolomitization is in irregular bands
at shale-limestone contacts as well as patches.Repla-
cement phenomenon is seen in dolomitized shells, nuclei
of pisolites and crinoid stems.

Limestone, light brown, bioelastic, lutaceous above
to arenaceous below, allochemical, with irregular layers
08 shaly partings and brachiopod shells (coquinoid).
Abundant brachiopods, bryozoans and corals throughout.
Dolomitization is speckled with greater concentration
near shale layers. stylolitic . Dolomite is more
common on the lower parts of the stylolitic surfaces.
Several calcite euhedra and occasional quartz grains
are rimmed by dolomite.

26

 

5775'

3774'

5775'

3776'

3777'

5778'

5779’

5780'

 

27

Limestone, light brown, fossiliferous, arenaceous-rud-
aceous, crinoidal with irregular bands of calcareous
shale and argillaceous limestone bands. Dolomitizatiou
is speckled or patchy in areas but concentrated in or
around the shale bands where the groundmass has been
preferentially replaced,leaving.scattered rudaceous
crystals of euhedral or pseudohedrel calcite, rimmed
with dolomite.

Limestone, grayish-brown, stylolitic, arenaceous,
with euhedral crystals of calcite, more fossiliferous
towards Lhe bottom. sanded with argillaceous limestone
and calcareous shales, rimmed with dolOmitegthe lower
portion being richer in dolomite than the upper.Irre§u-
1ar lenses of limestone rimmed with dolomite.

Limestone, light brown, siliceous, argillaceous,
carbonaceous in parts with dispersed euhedral granules
of calcite, crinoid stems; more dolomitic near bottom
with two bands of euhedral, arenaceous-rudaceous

allochemical limestone.

Linestone, light brown, allochemical, bioclastic,
polletal, pisolitic, with arenaceous shale bands rimmed
with dolOmite and with COquinoid bands.Dolomitization
is patchy, speckled or associated with the shale bands
and is nore concentrated near the lower bands and
encloses arenaceous crysrels of calcite.

 

 

5782'

3785'

3704'

3785‘

3786'

5787'

5759'

3739'

5790'

5790'

 

28

Limestone,5rayish-brown, allochemical, bioclastic
pisolitic, stylolitic, ecumenly fractured. flicritic
limestone common in and around shale layers with occas-
ional branules of calcite. Limestone is coarse-grained
outside these layers, with the coase grains being made
up of pellets, pisolites, oolites and euhedral calcite
rhombs in a matrix of carbonaceous mud. This coarse-
grained limestone commonly bears recemented fractures-
nowhere dolomitized.Dolomitization is commonly patchy
or speckled in the arenaceous limestone or as rims of
shale partings where they tend to fan out into the fin—
er-drained limestone. They also show a vein type effect
at limestone-shale contacts, and coarse-grained and
finer-grained limestoue contacts. Limestone nodules
and pellets are enclosed in dolOmite in places. There
is a tendency towards preferential replacement of the
groundmass of limestone by dolomite, leaving rudaceous-
arenaceous crystals of calcite. Veiiets of calcite cut
into,but not across concretionary features.

Limestone, light brown, arenaccous, rudaceous in parts
pisolitic ,nodular, carbonaceous in parts, fossilifero—

s, with bands of calcareous shale and arfiillaceous
limestone enclosing lenses,ton5ues and other irregular
hodies of arenaceous limestone. The shales bear
dispersei crystals of calcite. Dolomitization is
speckled and patchy throughout the section; with great—
er COncentration around shaly bodies and arenaceous-
rudaceous crystals of calci e, pisolites, nodules an
concretions, because these are set in a natrix of
ndcrite. Interior of nest shales are dolOmite poor.
speckled dolomites enclose 'bhosts' of calcite. A
Y-shaged fracture has dolomite SJOtS in the cement;and
cuts across stylolitic boundaries and clay partinbs-
evidently post-consolidatiOn. from 3789-{0' the lime-
stone is more coarsely crystalline, lens dolomitic and

hm;thhuer:flalc_mrtiks.

Limestone, light brown, IudsceOus-arenaceous,nodu-
lar, banned with irregular bodies of carbonaceous and
calcareous shale. The six major bands of limestone
are coarse-grained, predo.inantly a calcarenite, often
ranping to calcirudite in places. About 3791.7 there
is a limestone concretion about 2“ in diameter. Banding

 

 

5723' a,

5794'

37:5

3796'

5797‘

5798'

5759'

 

 

29

is very distinct near the bottOm. The shale bands
host euhedral calcite crystals. A shale band, about
6" wide. at 3791' is made up of several stringers and
veinlets in a patchy background of calcilutite,and
occasional rudaceous crystals, pisolites and nodules.
Dolomitization is predowinant near some shale bands,
elsewhere it is patchy,apotty or occurs as stringers
at limestone-shcle contacts. Dolomite is lacking in
the calcarenite, but is more concentrated in the
limestone tongues interspersed in the shale layers.

Limestone,brown, oolitic, pisolitic, bioclastic,
nodular, with variously oriented calcite rhombs. Text-
ure varies progressively from lutaceos-arenaceous on
top to rndaceous at the bottom.There are numerous
stringers of calcareous shale ani carbonaceous shale
partings with numerous calcite rhombs scattered all
through. Dolomitization presents a speckled and powdery
appearance with no preferential concentration around
shale bodies, except in some micritic limestone.Repla-
cement features are seen in 'chosts‘ of calcite in dol-
omite rhombs; and both intra-crystal and extra-crystal
origins are indicated.

Limestone, brown, allochenical, pisolitic, banded,
fossiliferous, stylolitic, the top 5' is predominantly
arenaceous while the lower portion is made Hy of aren-
naceous crystals of calcite in a micritic groundmass;
partially silicified towards the bottoa. Shale stringers,
bands aid partings are insignificant at the upper part
out very rtrOsQ at j 99.5—3800.6'.Ihis zone is almost
uniformly argillaceous with lutaceous limestone lenses
and tongues, fossils, pisolites, nodules and pellets
scattered throughout . Dolomiti ation is mottled and
patchy in places and presents strong evidence for late
stage replacement. SOme pure calcite crystals are rim-
med by d010mite and then by calcite. cs il shells,
although not replaced, are filled with calcite, 50ml of
which have been replaced by dolomite. There is no

 

5800'

5801'

5802'

3803'

5804'

5805'

5606'

5807'

3809'

 

30

significant concentration of dolouite anywhere except
around some fossils, pisolites and nodules. There
appears to be a preferential dolomitization of the
groundmass of coarse-grained calcite in the shale areas.

Limestone, brown, strongly argillaceOus, fossiliferous
nodular, and dolouitic.lt is so greatly argillaceous
as to appear as a uniform shale band. The argillaceOus
limestone bodies enclOse lenses and tongues of arenace-
ous limestone. lost siLnificant dolomitization
consists of stringers, lenses and tongues , mostly
composed of euhedral rhombs enclosing calcite.

Limestone, light brown, fossiliferous, pisolitic,
nodular, allochemical, arbillaceous and micritic in
places. The top is chiefly coralline while the lower
portions are richer in brachiOpods an‘ bryozoans. Ihe
limestone varies IrOm an aphanitic band on top, to
arenaeeous-rudaceous lower down, uith'euhedral calcite
crystals. Local and preferential silicification has

led to chert blobs and silicified brachiopod shells.
Calcareous shale partings very few and stylolitic.
Dolomitization Occurs both in shales and as irregular
bodies in the interstices of coarse-grained calcite
crystals, it is patchy and speckled all through, and
as stringers in fossiliferous layers. A number of
orachiopod shells are filled with calcite and doloaite
rhOnbs some of which havc'qhosts' of calcite.Vertica1
fractures are COmen and all these are filled with
calcite an“ dolOmite. Stylolites are cenmon in this
section but in no case is dolOnite associated with
them.

 

5814'

 

3815"

 

3816' _

 

 

31

Limestone, brown, rudaceous to arenaceous, occurring
as lenses, tonbues and sausage-shaped bodies
surrounded by argillaceous bodies. Carbonaceous shale
bodies of irregular geometry are dominant and enclose
nodules, crinoids and a few other fossils, and have
rudaceous calcite crystals scattered through them.
Dolomitization is as above.

Limestone, nodular, stylolitic, arenaceous,alloche-
mical, argillaceous with shale bands, most of which
have stylolitic COntacts with limestone. Dolowitiza-
tion is as above but stronger towards the top.

Linestone, brown, crinoidal, pisolitic, arbillaceous
fossiliferous, arenaceous-rudaceous , and banded. The
lower sectiOn is more fossiliferous while the top is
mostly allochemical and stylolitic. The balcareous-
carbonaceous shale bodies occur as stringers, parti-
ngs, and bands with tongues and lenses of arenaceous
limestone whose groundmass consist of lutaceous-are-
naceous d010mite rhombs. Dolomitization in the shale-
limestone contacts is alnmst uniform except for
SP°t5 which represent coarse-grained calcite crystals
that have not been replaced .Elsewhere d010mite

 

3818'

5819'

5820'

5821'

5822'

3825'

3824'

3825'

3826'

3827'

 

52

occurs as patches , spots and irregular bodies in
limestone.

Limestone, light brown, banded, fossiliferous,

rudaceous with euhedral calcite crystals. Alternat-
ing sub-equal bands of limestone and calcareous shale.
The limestone bands lower down are more fossiliferous
nodular and pelletal. The caleareous- carbonaceous
shale bands are of irregular shape and enclose lenses
and stringers of limestone which are invariably
arenaoeous-rudaceous and also have euhedral calcite
crystals scattered through them. The shale bands are
fossiliferous especially near the bottom of the
section. Predominant dolowitization is confined to the
shale-limestone contacts from where it fans out into
the shale.0ther modes of occurrence are as rims of
nodules, fossils and rudaceous calcite crystals. The
limestone bands in the middle section are poor in
doloaite but the others are speckled with it.

Limestone, brown, allochemieal, nodular; bryozoan-
~bearing, pisolitie,arenaceous calcirudite with two
thin bands of shale . Patchy dolOmitization.

Limestone, light brown, lutaeeous-arenaceous,
lentieular-sausage-shaped. Irregular stringers of
calcareous shale enclose lenses of boudin-like
limestone bodies which are arenaceous generally.
DolOmitization spotty.

Limestone, generally as above but more arenaeeOus-

 

 

 

 

3827'

5828' »

5829‘

5852'

3834'

5856'

 

 

 

 

 

53

rudaceous , with more nodules, fossils, pisolites and
pellets. The limestone and shale show a better banded
appearance and dolomitization is in form of stringers
at contacts of the bands and also as rims of nodules,
fossils and calcite rhombs. Obviously a replacement
feature. Local silicifieation seen in chert needles
and silicified shells.

Limestone, light brown, banded, arenaeeous, cherty
and interbedded with calcareous shale and shale.
DolOmitization is almost regular in the shaly bands,
spotty in limestone.

Limestone, grayish-brown, arenaceous, nodular,
pelletal and partially silieified. The shale bands
are as in other parts and enc10se limestone bodies
of various outlines.Silicification is partial and tends
to be restricted to brachiopod shells most of which
are convex upwards. Dolomitization is poor towards
the bottom where it is speckled, but elsewhere
it appears asua powdery coating of shales and also as
stringers at the limestone -shale contacts where
they fan out into the shale .Irregular patches of

 

 

5858'.' I

 

 

5839

 

5840'

3841',,, ‘T"

.f"
5842' f

3843'

3844'rfef" “

 

 

 

 

 

 

54

dolomite occur in places enclosing euhedral rudaceous
calcite crystals while the finer groundmass of
d010mite enclose nuclei of unreplaced calcite crystals.

Limestone, light brown, uniformly arenaceous with
scattered nodules , crinoid stems, brachiopod shells,
chert lenses and irregular and discontinuous shale
bodies enclosing lenticular and sausage-shaped
limestone bodies. Dolomitization is patchy or
speckled . Limestone is rudaceous towards the bottom.

Limestone,light brown , consisting of alternating
bands and lenses of rudaceous and lutaceous calcarenite
and calcirudite, interbanded with calcareous shale bands
and stringers. Fossiliferous, cherty and nodular.
Dolomitization is in form of irregular stringers in
shale as well as rims of shales. It occurs as patches
and spots in other places , while it has an irregular
outline where it has replaced the micritic groundmass
of arenaceous-rudaceous limestone lenses and tongues.
Calcite'ghosts' are common in dolomite areas.

 

 

5845' in

3846' ."

5847'

3848';I

5849'?*’

3850'

5851'

5852' a

3853'

 

55

LimestOne, dark brown, irregularly banded, arenaceous
nodular with thin irregular bands of shale.
Dolomitization is mottled or patchy and is more
dominant in the micritic groundmass.

Limestone, grayish-brown, arenaceous-rudaceous,
fossiliferous, ( chiefly brachiopods and ostracods ),
cherty, stylolitic with shale bodies of irregular
shape and outline which are mostly fossiliferous
especially around 5849.5' where a planispiral gastro-
pod shell bears some dots of dolomite . The chert
lenses are oriented parallel to bedding. Dolomitiza-
tion is most prominent around 3848.8' but occurras
stringers elsewhere in shale reginns and as patches,

spots and other irregular bodies in limestone.
It encloses rudaceous crystals of calcite locally.
There are occasional quartz grains.

Limestone, light brown,alternating bands of arenace-
ous calcilutite and rudaceous calcarenite, fossilifer-
ous, pisolitic-oolitic, nodular, cherty and with
occasional quartz grains. the shale bodies vary in
shape and outline, but occur mainly as bands ,stringlrs
and partings with stylolitic boundaries with limestone;
while at the same time enclosing variously shaped
bodies of limestone invariably ofr coarser crystals
than those outside the shale. Dolomitization is as
above but also occurs as dots in chert lenses, and
hence younger than silicification. Preferential
replacement of nodules and foasil shells are common
and occur at limestone-shale contacts , at the same
time presentinb a powdery appearance in shale.

 

3854'

3855'

3856'

3857'

5858'

3859'

3860'

5861'

3862'

3365'

 

 

 

56

Limestonc,5rayish brown, cherty, pelletal, stylolitic
fossiliferous, pisolitic , arenaceous-rudaceous with
irregular stringers and bands of calcareous shale
enCIOSing limestone lenses and tongues.30me fossil
shells are concentrated at the limestone-shale
contacts giving a coquinoid appearance ,while others
form thin layers in limestone. ihe randomly scattered
shells have been mostly silicified. host of the chert
lenses are oriented parallel to bedding . Dolomitizat-
ion is more prevalent in the shale section where
it occurs as stringers and laminae of variable
geOmetry. In the limestone areas it occurs as patches,
spots and other irregular bodies.A fracture occurs
around 3856.3' which is associated with two chert
lenses, and has been filled with calcite and dolomite
rhombs.

Limestone, grayish-brown, banded, lenticular,
argillaceous, fossiliferous, cherty, stylolitic,
pisolitic-oolitic and lutaceous calcarenite, with thin
bands ,stringers and threads of shale. The chert lenses
are very abundant especially at 3861-62' and the bot-
tom 2'. Host of these chert lenses are replacements
of biachiopod shells oriented parallel to bedding. "he
irregular shale bodies enclose limestone lenses and
tongues and also have euhedral calcite crystals
scattered through them. Fossil shells are especially

 

 

37

abundant at the limestone-shale contacts. DolOmitiza-
tion is generally mottled or patchy except in shales
where it forms rims and then fans out into them,
while dying out into ‘ the limestOnI. Some of the
chert lenses have spots of dolomite and hence
younger than chert. Replacements of nuclei of pisolites,
oolites and fillings of silicified shells are very
common.

Calcareous chert, grayish brown, made up of two bands
on either side of an arehaceous limestone band about
2%" thick. Dolomitization spotty.

Alternating bands of shales and limestone. The limes-
tone bands are arenaceous-rudaceous, fossiliferous,
stylolitic and carbonaceous towards the top. Bands at
5868' consist of two fossiliferous calcirudite bands
and one of calcarenite between them. In other places
the relation is not so well defined- all bands being
rudaceous calcarenites. Two geodes at 3871.5'. Fossils
are abundant all over the section- especially brachiop-
ods, ostracods, bryozoans and corals. However, they
form some coquinoid layers in limestone and shales.
Some of these shells are replaced by chert. Chert lenses
are numerous and they are oriented parallel to bedd-
ing. Two calcareous chert bands and a tongue occur in
the lower half of the section. The shale bodies are of
irregular geometry- the thicker ones being made up of
numerous stringers, They enclose several lenses and
tongues of limestone,and c0mmonly have stylolitic
boundaries with limestone. Dolomitization is conspic-
uously spotty, with local concentration in shales,
around nodules, fossils, limestone lenses and tongues,
and as fillings of some fossils. The uniformly coarse-
grained limestones ( calcarenites and calcirudites) are
conspicuously lacking in dolomite, while those which
have diversified textures ( lutaceous calcarenites,etc)
tend to have their matrix replaced and recrystallized.
Inc0mplete replacement of calcite by dolomite shows
evidence for both inward and outward growth of dolomite.

 

 

   

5872'

3873'

5874'

5875'

3876'

5877'

3878'

3879'

3880'

5881'

 

 

 

 

 

 

38

Limestone,light brown, fossiliferous, siliceous,
cherty, stylolitic, arenaceoue-rudaceous calcilutite;
with occasional bands of calcarenite. There are fewer
fossils than above, mainly brachiopods and bryozoans,
and they have been mostly silicified. Shale bodies are
irregular but stringers are more common. One chert band
at 5886'. Dolomitization is speckled, patchy and
irregular.Argillaceous limestones are rich in dolomite.
Siliceous limestones on the other hand are dolomitic
only where they are bounded by shale partings. Some
shale bodies enclose pockets of d010mite.

LimestOne,light brown, banded, stylolitic, locally
silicified, only slightly fossiliferous, lOCaIly carbn-
naceous, arenaceous to rudaceous.Locally lutaceous.
shale stringers and bands are carbonaceous and have a
coarsely arenaceous background. Silicification is in
form of chert lenses, bands and tongues of calcareOus
chert. DOIOmitization is patchy as well as dots in fra-
cture fillings and chert bands.

Limestone, brown,rudaceous at top becoming lutaceous

 

3881'

3882'

5883'

3884'

3885'

5886'

5887'

5888'

5889'

3890'

 

 

 

 

 

 

 

 

 

59

lower down; fractured,locally carbonaceous, fossilif-
erous, with thicker carbonaceous shale bands.Locally
silicified with patchy dolomitization.

Limestone, light brown, locally siliceous, increasi-
inbly fossiliferous towards the bottom, stylolitic,
arenaceous with two chert bands and a few shale
stringers. Dolomitization occurs as patches and
occasional lenticular bodies with sharp tongues penet-
rating the arenaceous limestone.Scattered quartz grains.

Limestone, grayish brown, medium grained calcarenite,
argillaceous, siliceous, fossiliferous, coquinoid in
parts with silicified geodes. A band of brachiopod
shells and some corals and bryozoans occurs at 3882.c'.
Strinyers and threads of shales spotted with dolomite.
untire limestone section bears patches of dolomite.

Limestone, light brown, fossiliferous, cherty,rudace-

 

3890'.i

3891 I

 

5893' a

3894'

5897'

5898'

5899'

5900' ,

3901'

 

 

 

 

 

 

 

 

 

3

40

one arenaceous, stylolitic with shale stringers and
irregular patches, stringers and speckles of dolowite.

Limestone, brown, fossiliferous, arenaceous-rudace-
ous, with local bands of micrite; stylolitic, banded
with irregular bodies of dolomitic shale. Dolomite
patches elsewhere. Silicified fossils near bottom and
some silicified patches have some dolomite spots.

Limestone, grayish brown, finely arenaceous, stylol-
itic, banded, cherty, micritic in places, fossiliferous
with irregular stringers and bands of shale locally
speckled with dolOmite. Dolomite occurs as coarsely
arenaceous patches in limestone and fillings of fossils.

Limestone, grayish brown, finely-moderately arenace-

 

 

5901'

3902'

3905'

5904'

5905'

3906!

5907‘

5908'

3909'

3910'

 

 

 

 

 

 

 

 

 

41

ous, micritic in parts, with occasional coarsely arena-
ceous patches and lenses, stylolitic, cherty, with str-
ingers and partings of calcareous shale.Two lenses

of calcareous chert. DolOmite occurs as rims of shale
as well as patches and specks in limestone - with
pronounced evidence for replacement and recrystalliza-
tion. '

Limestone,moderately arenaceous, with irreiular bands
of shale, silicified fossils, and patchy, mottled
dolomitization. hodules and pisolites partially repla-
ced or transected by d010mite.

Limestone, light brown, finely arenaceous-lutaceous,
semi-lithographic in parts, siliceous - generally an
arenaceous calcilutite. Chert concretion at top, a band
towards the bottOm and lenses of silicified fossils.
The chert band has a stylolitic upper contact with
limestone and a vertical fracture which has been
filled with dolomite and calcite. Dolomitization is
dominantly patchy but also occurs as rims of shale.

Limestone, grayish brown, fessiliferous -generally
an arenaceous calcilutite. Stylolitic, pelletal,
carbonaceous, semi-lithoaraphic, pisolitic in parts,
argillaceous , ferruginous locally, nodular and silice-
ous with irregular bands of dolomitic shale.Dolomitiz-
ation occurs as recrystallized patches in limestone,

 

3910'

3911'

3912‘

5915'

3914'

5915

 

 

 

 

 

 

42

as well as rims of limestone lenses and shale bands.

Limestone, grayish brown, pelletal, siliceous, nodu-
lar, argillaceous, cherty towards the bottom, semi-lit-
hographic locally but generally an arenaceous calcilu-
tite; calcareous shale stringers enclose chert or
limestone lenses and tongues, and are rimmed with dol-
omite. Cherty bands prominent near the bottom and are
obviously pre-dolomitization, since fractures both in
and outside the chert bands are partially filled with
dolomite. Nodules and fossils are silicified.
DolOmitization patchy and speckled.

No core.

Lolozite

IaiLJJLILUYIe

 

4mg.nxons of Thin sections

 

43
Microscopic Description

Unlike Kirschke ( 1961 ) who selected samples for thin sectioning at
two feet intervals, the writer attempted to select his samples in such a
way that all lithologic variations were represented.

Inasmuch as the nature of dolomitization is the principal objective
of this study, the writer has laid a greater emphasis on the variation
in texture and structure of dolomitized areas of the thin sections. The
description accompanying each photograph represents the observation.made
on the whole slide and not just the area covered by the photograph.

The writer has strictly adhered to conventional connotations of
descriptive terms. For example arenaceous and lutaceous retain their
textural implications. Except in two cases (see photographs) the magnifi~

cation for all the photographs is 75 times the true size.

fli

44

1. 3771.7' Fossiliferous,loca11y

2.3772'

pelletal, lutaceous in
parts but generally a
calcarenite, bearing a few
patches of dolomite, and
irregular stringers of
carbonaceous shale.

 

 

Calcarenite with some
interlocking coarse calcite
crystals are set in a matrix
of argillaceous micrite.
Dolomitization speckled near
the top.

 

3. 3772.5' Finely arenaceous limestone

locally lutaceous. Dolomi-
tization is in form of
inc0mplete replacement of
calcite, most predominant
in shale. Fossiliferous,
stylolitic.

Lee A

 

4. 3773.8

5.3774.2'

6.3777.8'

45‘

Coarsely-crystalline
calcarenite with scattered
quartz grains and irregular
bodies of chert and opaline
silica. Dolomite occurs on
one side mostly as few
scattered rhombs enclosing
calcite. Fossiliferous and
with minor shale stringers
near the top. Recrystalli-
sed with moderate sorting.

 

Moderately fossiliferous,
locally shaly rudite and
calcarenite, speckled with
dolomite crystals enclos-
ing ghosts of calcite.
Recrystallized.

 

Calcarenite admixed with
lutite and rudite.

Locally argillaceOus, spar
sely dolomitized.Fossilif-
erous.

 

7.3779'

8.3782.5'

9.3783.7'

46

Interlocking coarsely-
arenaceous crystals of
calcite held together

by a maze of carbonaceous
shale stringers near the
top. Elsewhere shale is
insignificant.Scattered
rhombs of dolomite and
quartz grains. Small
tongues of crypto-crys-
talline silica-— mostly
chert.

Arenaceous limestone,
fossiliferous. Calcite
crystals are mostly pseud-
ohedral .Dolomite occurs
as incomplete replacement
of calcite rhombs and 15
most abundant in shaly
regions. Outside the shale
region and towards the
bottom, the limestone is
arenaceous and is entirely
dolomite free.

Fossiliferous, argillaceous
calcarenite and some rudite,
highly dolomitic towards the
bottom. The limestone becomes
lutaceous near the bottom and
is speckled with dolomite,
most of which occurs as an

interlocking mosaic of coarse-

ly-lutaceous rhombs enclosing
calcite rhombs or quartz
grains.Pisolite partly rimmed
by dolomite rhombs.

 

 

(
w
‘
\

x
‘

 

 

 

10.3794.2'

11.3766.7'

12.3801.6'

47

Lutite and calcarenite.
rudaceous towards the
bottom.Locally cherty.
Dolomite crystals and
patches are most abundant
in a band of argillaceous
limestone of micritic
texture.

Qalcarenite,locally fossi-
liferous and argillaceous.
Dolomitization consists of
several arenaceous crystals
of dolomite associated
with shale.

Pelletal, dolomitic and
strongly argillaceous
limestone.Sha1e consists
of a cluster of stringers
with interlocking crystals
of dolomite and some
calcite.

 

 

 

 

 

 

 

 

.... .mq W} ._-.

EM

 

13.3820'

14.3828.l'

15.3835.4'

48

Fossiliferous calcarenite
Pelletal towards the
bottom and shaly near the
top with scattered euhedral
calcite rhombs and quartz
grains. Undolomitized.

Generally a lutaceous lime-
stone with a few stringers
of micritic limestone and
shale. A.few scattered dol-
omite crystals occur near
the bottom of one corner.
Four lenses of chert occur
in this area.

Finely arenaceous limes-
tone towards the bottom.
Locally fossiliferous,
cherty and pelletal with
a few quartz grains.

Near the top is a band of
argillaceous material
consisting of a network
of shale stringers which
enclose lenses and
tongues of calcite and
d010mite rhombs.0ne quartz
grain rimmed by dolomite
near the middle.

 

i

 

 

 

 

 

 

16.3847.4' Arenaceous limestone,loca-

17.3849.6 Fossiliferous calcarenite,

l8.3852.l

49

11y fossiliferous and
argillaceous. There are
three major areas - two
near the middle and the
third near the bottom.
They consist of an inter-
locking mosaic of coarse
dolomite rhombs embedded
in round bodies of argill-
aceous limestone mud.

locally pisolitic.Argill-
aceous near the top .
Largely undolomitized.

Well sorted, pelletal,

fossiliferous and lutace-
ous limestone.Largely undolo-
mitic except for two patches
of dolomite near the bottom.

 

 

 

 

 

19.3854.1

20.3856.3

21.3864.6

 

50

Arenaceous calcite crystals
in a lutaceous matrix.There
are a few irregular stringers
of shale and fossils;
undolomitic.

 

 

 

Lutaceous to finely
arenaceous limestone,with
bands of shale.The shale
encloses moderately lutace-
ous rhombs of dolomite and
finer crystals of calcite.
Dolomite occurs as speckles
throughout the remaining
parts of the area.

 

 

 

Lutaceous limestone with

a few arenaceous crystals
of calcite and some

chert lenses. Anhedral,
moderately arenaceous
rhombs of dolomite are
scattered throughout the
field. The shale bodies
enclose lenticular bodies
of micrite.

 

 

 

 

22.3865.5

23.3867;2

24.3856.2

Calcarenite with a pocket
of carbonaceous shale
bearing scattered calcite
crystals.

Moderately well sorted
calcarenite, pelletal,
pisolitic and fossiliferous.

Calcareous shale. The shale
consists of a complex net-

work of stringers enclos-

ing lutaceous crystals of

calcite and quartz grains;

fossiliferous.

 

 

 

 

 

 

52

25.3872.5 Arenaceous limestone,foss-
iliferous and with shale
stringers enclosing calcite
and dolomite rhombs. Two
patches of dolomite occur
near the top.

 

 

26,3873.9 Dominantly a calcilutite
with local arenaceous
texture. Patchy to speckled
dolomitization.Irregular
bodies of shale.

 

27.3878.1 Moderately we11 sorted
calcarenite,pelleta1
O pisolitic and fossiliferous.

 

 

 

28.3879'

29.3888.6

30.3887.4

53

Uniformly fine-grained
limestone, essentially

a calcilutite. It is
stylolitic and dolomitiza-
tion is spotty.

Very fossiliferous,arenace-
ous limestone,with quartz
grains scattered here and
there.The arenaceous
crystals are set in a
groundmass of micrite.
Shale consists of irregu-
lar stringers and
carbonaceous patches-
largely undolomitic.

Lutaceous to semilithogr-
aphic limestone with disn-
ersed crystals of arenace-
ous calcite; and bearing
irregular patches of calcitic
dolomite, with few stringers
of shale. There are some
fossils and fractures bearing
some dolomite.

 

 

 

 

 

54

31.3894.3' Lutaceous micrite with two
argillaceous bands filled
with coarser-grained
dolomite rhombs;pe11eta1
near the bottom.

 

32.3892.4 Dolomitic calcilutite,
locally finely arenaceous.
Dolomite occurs as irregu-
lar patches. Local bands of
micrite.

 

 

33.3898.4 Finely arenaceous,stylolitic
and fossiliferous limestone,
with bands of argillaceous
calcarenite.Some of the bands
bear lutaceous crystals of
dolomite, but on the whole,
dolomite is insignificant.

 

 

34.3902.4

35.3908.8

36.3913.1.

55

Calcilutite enclosing a
tongue of cherty micrite.
The cherty micrite bears
numerous rhombs of dolo-
mite.Dolomite in the
lutite is also speckled
but it is very dense and
gives an almost uniformly
blue stained appearance
with a definite contact
with the cherty micrite.

Lutaceous micrite with pet»
ches of dolomitic limestone.
It bears several parallel
shale stringers with calcite
veinlets and is locally
arenaceous at the bottom.

Calcarenite, divided into
parts by a band of argill-
aceous limestone.The argil-
laceous limestone consists
of a maze of shale stringers
enclosing anhedral,lutaceous
crystals of calcite and quartz
Above the shale band, the
limestone is speckled with
dolomite crystals which are
larger than the calcite
groundmass.The top is
fossiliferous.

 

 

 

 

56
SUMMARY AND CONCLUSIONS

In one of the first parts of the problem, Kirschke (1962) suggested
that'the lack of appreciable pin point or vuggy porosity would infer
replacement dolomitization, possibly early diagenetic.‘ The writer has
recorded a number of features ( see columnar section) which suggest very
strongly that dolomitization here is a replacement phenomenon. Most impor-
tant of these are:

1. Great range in chemical composition ( Table 2).

2. Dolomitization of fossils, fossil fillings and oolites.

3. Speckled nature.

4. Calcite 'ghosts' in some dolomite rhombs.

5. Nests of dolomite in limestone, mostly due to preferrential replace-
ment of the groundmass of coarsely-crystalline limestone.

6. Irregular contacts of limestone and dolomite.

7. Pseudostratification effects.

Any direct precipitation theory would have a lot of trouble explaining
these features as well as the great abundance of fossils. They are however
easier to explain by invoking a replacement origin.

Hamil, (1961) on the other hand , recognized two important zones-
4232'-4258' and 429l'-4311' depth in the Peterson Howard well core, and
suggested that the zones "are more likely to have a stratigraphic origin
due to the consistently high magnesium content, through a large thickness.
It will, of course, take similar studies of other wells to show which,if
any of these zones are stratigraphic." The writer has also noted two simi-
lar zones of consistently high magnesium.content - 3797'-3805' and 3899'-
3913' depth. However the writer could not suggest a possible origin of the
dolomite in these zones without reservation.The writer is not suggesting

correlation of these zones in the two wells, as the core studied herein is

57

of younger age ( Trenton ) than in Hamil's study ( Black River ).

In the present investigation the writer does not feel that it is
possible to separate diagenetic (elsewhere referred to as ' stratigraphic')
dolomite from the epigenetic dolOmite. This is because the evidences whidh
indicate each origin are not restricted to any particular depth zone,
although they may be more strongly indicative of one than the other.

The following three properties were observed in the petrographic
aspects of the investigation while the fourth is inferred from the
results of the chemical analyses:

1. Tendency for dolomitization to follow stratification, shale partings
and limestone-shale contacts.

2. Spreading of dolomite in all directions.

3. Occurrence of perfect rhombs of dolomite in incompletely altered limes-
tone.

4. Semi-uniform chemical composition over a considerable thickness sugg-
ests that the dolomitizing agent Operated over a long period of time.
All these evidences are clearly indicative of early alteration and so
the dolomite may be regarded as being diagenetic. On the other hand the
following additional features indicate an epigenetic origin.

1. Dolomite spots on upper surfaces of stylolites.

2. Dolomite tongues penetrating limestone beds.

3. Partial dolomitization of fossils and fossil fillings.

4. Dolomite spots in fracture fillings.

Inasmuch as there is no stratigraphic boundary between these two types
of dolomite, we may assume that the magnesium content of the stratigraphic
dolomite would be fairly uniform ( Table 2). This is the basis of the
assumption that conspicuosly low magnesium/calcium ratios probably repre-

sent areas that were not altered during the first phase of dolomitization,

58

and hence the dolomite is epigenetic. In the same way conspicuously high
figures could be attributed to an epigenetic enrichment of the earlier
diagenetic dolomite. Since most of the samples have magnesium/calcium -
ratios falling within 0.035 and 0.075, they have been arbitrarily
selected as end points for areas where the stratigraphic type is more
significant than the epigenetic dolomite.

As pointed out earlier the most significant occurrence of dolomite
is that associated with shale partings. Several reasons may account
for this common occurrence but only four will be suggested herein.

1. Shale partings were probably the dominant channels through which magn-
esian solutions migrated.

2. Inasmuch as most of the shales are carbonaceous,they might have had a
greater concentration of magnesium compounds than the surrounding areas.
These magnesium compounds could start off the process of dolomitization
even when the magnesium content in the invading solutions is not suffi-
cient to start the same process in the adjacent limestone areas.

3. If the original clay was montmorilonite ( or illite ) the original
magnesium content might be helpful in creating the same condition as (2).
4. The shale-dolomite association may also be related to the amount of
humic acids permeating the rocks.

It is not known which of these is the most significant, but it is not
impossible that they could all have been contributory.

Patchy distribution of dolomite was probably a result of selective
replacement in and around the shale partings. Outside the shale partings
permeability might have been the most important factor influencing
replacement. Consequently dolomite patches are larger, more abundant and

more clearly defined in areas where shale partings are thin or few . The

59

reverse is in cases of thick or more numerous partings. In the first case
solutions are slowed down and so increase the opportunity' for replacement
( Carozzi, 1960 ).

In the coarse-grained limestone the matrix is replaced first, and
thereafter the coarser grains and fossils. This is because the finer the
material the greater the surface area exposed to the dolomitizing fluids,
and hence they are more susceptible.

The oolitic-pisolitic textures that were very conspicuous in the
megasc0pic observations were not readily apparent under the microsc0pe
owing to alteration.They were represented by structureless , rounded and
darker areas.

The relation between calcareous chert bands,chert lenses,and dolomite
is not altogether clear. In some cases they were spots of dolomite in
chert, but most of the chert bodies were devoid of it. The earliereformed
chert may be due to lower alkalinity in the environment ( Carozzi 1960 ).
Silica was precipitated to the exclusion of dolomite which formed at higher
alkalinity . There were at least two periods of silicification and it is
believed that each of these periods was earlier than the corresponding
period of dolomitization ( see columnar section ).

The writer does not believe that future work would be able to draw
a line onclusively between diagenetic and epigenetic dolomitization beli-
eved to be present in the Ordovician limestones of Midhigan. Such work
would only be able to locate stratigraphic horizons where diagenetic

dolomite is more significant than that of epigenetic origin.

10.

11.

12.

13.

Carozzi, A.V° 1960

Cheng, K.L.(et a1)
1952

Chilingar, G.V.,l957,

Cohee, G.V. _ 1947,

Hamil, D.F. 1961,

Horowitz, M. 1961,

Jodry,R.L. 1954,

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