Facies Analysis and Diagenesis of the Lower Engadine Group and the Manistique Group in Manistee, Mason and Oceana Counties, Michigan

Western Michigan University ScholarWorks at WMU

Master's Theses Graduate College

6-1991

Karen S. Mater

Follow this and additional works at: https://scholarworks.wmich.edu/masters_theses

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Recommended Citation Mater, Karen S., "Facies Analysis and Diagenesis of the Lower Engadine Group and the Manistique Group in Manistee, Mason and Oceana Counties, Michigan" (1991). Master's Theses. 981. https://scholarworks.wmich.edu/masters_theses/981

This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. FACIES ANALYSIS AND DIAGENESIS OF THE LOWER ENGADINE GROUP AND THE MANISTIQUE GROUP IN MANISTEE, MASON AND OCEANA COUNTIES, MICHIGAN

Karen S. Mater

A Thesis Submitted to the Faculty of The Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Geology

Western Michigan University Kalamazoo, Michigan June 1991

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FACIES ANALYSIS AND DIAGENESIS OF THE LOWER ENGADINE GROUP AND THE MANISTIQUE GROUP IN MANISTEE, MASON AND OCEANA COUNTIES, MICHIGAN

Karen S. Mater, M.S.

Western Michigan University, 1991

Three lithofacies in the Lower Engadine Group, and

four lithofacies in the Manistique Group were identified.

Facies patterns suggest a transgression during deposition

of the Manistique Group and Lower Engadine Group.

Environments of deposition were determined by analysis

of facies found in the Lower Engadine and Manistique Groups

using available cores, petrophysical logs, and well

cuttings. The diagenetic sequence of events was identified

using thin sections.

Diagenesis has greatly altered original fabric and

porosity. Hydrocarbon accumulation in these units is

dependent on facies distribution, diagenesis, and

structural features.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

I wish to acknowledge and thank my advisor and com

mittee chairperson, Dr. William Harrison, III, for his

encouragement, assistance, advice, and support throughout

my course of study, and to my committee members, Dr. John

Grace and Dr. David Barnes, for their advice and assis

tance .

I must also thank those who provided financial

assistance. They include Miller Oil Corporation, Traverse

City, Michigan? Miller Energy Company, Kalamazoo, Michigan;

American Association of Petroleum Geologists, Tulsa, Okla

homa; Desk and Derrick Educational Trust, Tulsa, Oklahoma?

Society of Petroleum Engineers, Troy, Michigan? and

Michigan Basin Geological Society, East Lansing, Michigan.

Miller Oil Corporation provided cores and petro

physical logs for use in this project. Mobil E&P, Oklahoma

City, Oklahoma provided the database of wells in the study

area.

I could never have completed my thesis without the

love, encouragement, and support of my husband, Wayne, and

my parents.

Karen S. Mater

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Facies analysis and diagenesis of the Lower Engadine Group and the Manistique Group in Manistee, Mason and Oceana counties, Michigan

Mater, Karen S., M.S.

Western Michigan University, 1991

UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106

ACKNOWLEDGEMENTS i i

LIST OF TABLES...... viii

LIST OF FIGURES...... ix

CHAPTER

I. INTRODUCTION...... 1

Statement of the Problem...... 1

Purpose of Study...... 2

Method of Study...... 6

Previous Work...... 10

II. STRATIGRAPHY...... 13

Silurian Period...... 13

Alexandrian Series...... 14

Niagaran Series...... 14

Recommended Nomenclature...... 21

III. ENVIRONMENT OF DEPOSITION...... 26

Preliminary Setting...... 26

Manistique Group...... 29

Lithofacies 1 ...... 31

Lithofacies H ...... 37

Lithofacies H3 ...... 39

Lithofacies H2 ...... 39

Vadose Model...... 43

iii

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Burrow Model...... 44

Differential Compaction Model...... 45

Gravity Flow Model...... 46

Current Model...... 47

Lithofacies HI ...... 49

Lower Engadine Group...... 51

Lithofacies G ...... 52

Lithofacies E ...... 54

Lithofacies F ...... 56

Vadose Model...... 60

Burrow Model...... 60

Differential Compaction Model...... 62

Gravity Flow Model...... 63

Current Model...... 63

Upper Engadine Group...... 66

Lithofacies R ...... 67

Lithofacies D ...... 67

Lithofacies B2...... 69

Lithofacies B1...... 70

Depositional Setting...... 72

IV. DIAGENESIS...... 75

Manistique Group...... 75

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Dolomitization 1 ...... 75

Sabkha Model...... 78

Normal Sea Water Model...... 79

"Dorag" Mixing Model...... 79

Schizohaline Model...... 81

Burial Dolomitization Model...... 82

Chert...... 85

Silica Crystal Structure and Morphology...... 85

Sources of Silica...... 89

Silica Replacement of Dolomite...... 92

Organic Matter Oxidation Model...... 93

Mixing Model...... 95

Hydrogen Sulfide Oxidation Model...... 97

Force of Crystallization Model...... 98

Dissolution of Low-Mg Calcite...... 104

Compaction...... 105

Dolomitization II...... 113

Hydrothermal Event...... 116

Dissolution of Chert...... 117

Dolomitization III...... 119

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Hydrocarbons...... 121

MVT Mineralization...... 122

Salt...... 124

Lower Engadine Group...... 125

Dolomitization I ...... 126

Burial Dolomitization Model...... 127

"Dorag" Mixing Model...... 128

Schizohaline Model...... 129

Dissolution of Low-Mg Calcite...... 131

Compaction...... 132

Dolomitization II...... 135

Hydrothermal Event...... 137

Precipitation of Silica...... 139

Dolomitization III...... 140

Hydrocarbons...... 142

Precipitation of Pyrite and Anhydrite...... 144

Salt...... 146

V. STRUCTURAL AND PALEOGEOGRAPHIC INTERPRETATION...... 148

Overview...... 148

Manistique Group...... 154

Lower Engadine Group...... 158

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VI. CONCLUSION...... 160

BIBLIOGRAPHY...... 164

vii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

1. Cores Used in This Study...... 7

2. Diagenetic Sequence of Events for the Manistique Group...... 76

3. Diagenetic Sequence of Events for the Lower Engadine...... 126

viii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

1. Stratigraphy of the Early and Middle Silurian Period...... 3

2. Location Map of the Study Area...... 5

3. Lithofacies of the Niagaran Series in Manistee, Mason and Oceana Counties...... 20

4. Paleogeographic Map of the Michigan Basin During the Niagaran...... 27

5. Cross-section A-l Extending From the North to the South of the Study Area...... 28

6. Lithofacies 1 ...... 32

7. W.S.C.C. #1-27, Dark Brown Peloids in Dolomite in Lithofacies 1 ...... 33

8. Composite Stratigraphic Section of Lower Silurian Formations in Northern Michigan With Accompanying Sea-level Curves...... 36

9. Lateral Variations of Lithofacies H Within the Study Area...... 38

10. Lithofacies H2 ...... 40

11. Cross-section A-2 Extending From East to West Including Cored Wells...... 41

12. Idealized Carbonate Slope Facies Relationship Produced by Downslope Decreasing Bottom Action...... 48

13. Lithofacies G ...... 53

14. Lithofacies E ...... 55

15. Angular Quartzose Silt in Lithofacies E ...... 57

16. Lithofacies F ...... 58

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17. Isopach Map of the Manistique Group...... 73

18. Large Cloudy Centered, Clear Rimmed (CCCR) Dolomite Crystal...... 77

19. Fibrous Quartz Varieties and the Orientation of Their Crystallographic C-Axes...... 87

20. Scanning Electron Microscope View of a Sponge Spicule...... 92

21. Classification of Pressure Solution Seams...... 106

22. Microstyolite With Perpendicular Fractures Extending out From the Seam...... 109

23. Fractures in the Chert, Some of Which Extend a Short Distance Into the Dolomite...... Ill

24. Sucrosic Texture of Recrystallized Dolomite...... 115

25. Megaquartz Filled Fossil Mold...... 140

26. Fossil Mold Filled With Saddle Dolomite Showing Undulose Extinction...... 141

27. Hydrocarbon Fluid Inclusions in Salt Which Fills a Dolomite and Pyrite Lined Vug...... 143

28. Large Masses of Pyrite and Small Pyrite Crystals Line a Microstyolite...... 144

29. Anhydrite Filled Void...... 145

30. Burnt Bluff Group Structure Map...... 149

31. Manistique Group Structure Map...... 150

32. Lower Engadine Structure Map...... 151

33. Upper Engadine Structure Map ...... 152

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I

INTRODUCTION

Statement of the Problem

The study of the Lower Engadine and Manistique Groups

in Manistee, Mason and Oceana Counties required that three

problems be addressed. The first problem to resolve is the

stratigraphic nomenclature. The stratigraphic nomenclature

of the portion of the geologic column under investigation

is not well understood, and it needs to be clarified and

formalized for future reference. The second problem

needing study is an interpretation of the depositional

environments present, and their distribution through the

study area. The lithofacies identified in the Lower

Engadine and Manistique Groups will be used to identify the

depositional environments. Diagenetic interpretation from

petrographic studies is the third problem to be resolved,

and will allow the paragenetic sequence to be determined.

The last problem focused on in this study is the identi

fication of factors which control the development of

potential hydrocarbon reservoirs.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2

Purpose of Study

The Niagaran Series has been intensively studied in

the Michigan Basin subsurface since the first hydrocarbon

reservoirs were discovered in reefs in Ontario in the 1950s

(Mantek, 1973). Exploration of the reef trend expanded

into southeastern Michigan in the 1960s, and into northern

Michigan, south-central Michigan and southwestern Michigan

in the 1970s. The ensuing exploration led to a better

understanding of environments of deposition in the upper

part of the Engadine Group, but virtually no work was done

on the strata immediately below the reefs.

Stratigraphy of the Niagaran Series had been studied

in detail in outcrops in the northern peninsula of

Michigan, in Ontario, and in Wisconsin (Ehlers and Kesling;

1957, Johnson and Campbell, 1980; Liberty and Sheldon,

1968; Newcombe, 1933; Sanford, 1978). Correlation of the

Niagaran Series in the subsurface of the Michigan Basin

with outcrops in the northern peninsula of Michigan has

only recently begun (Harrison, 1985). Most researchers

have tried to correlate the subsurface stratigraphy with

stratigraphy of New York (Briggs, Gill, Briggs and Elmore,

1980; Jodry, 1969; Mesolella, Robinson, McCormick and

Ormiston, 1974), although Huh, Briggs and Gill (1977) used

the New York stratigraphy only for the Engadine Group

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. equivalent. Kilgour (1972) discusses the difficulties in

correlating the New York stratigraphy with the Ontario

stratigraphy. The stratigraphy of the Niagaran Series in

the subsurface of the Michigan Basin needs to be studied

and correlated to the outcrop nomenclature of the northern

peninsula of Michigan.

The Niagaran Series is divided into three groups

(Figure 1) . In descending order they are the Engadine

W.O0AL CHRONO INFORMAL ITHATIORAPHlC OUTCROP 9UBSURFACE U R Itl GROUP USAGE A - l CARB. A - l CARB A * I C A M . EVA*-l EVAR A - l E V A *A- CAYU0AN PRESENT A - l EVAR A -0 CARB A -0 CARB. BUSH BAY UPPER OOLOSTOKE LOCKPORT1AN

RAPSON CREEK GROUP DOL03TONE

ROCK VIEW

WENLOCKIAN CORDELL noi o u u t CLINTON MANISTIQUE CLINTON GROUP GROUP BURNT V*!Ui4i BURNT BLUFF BURNT BLUFF BLUFF sinw n f Y * O U 1 T F GROUP GROUP H i CABOT HEAD CABOT HEAD CABOT HEAD CABOT HEAD SHALE SHALE SHALE SHALE CATARACT MANITOULIN MANITOUUN OOLOUITE DOLOMITE

Figure 1. Stratigraphy of the Early and Middle Silurian Period. Modified from Shaver (1984).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Group, the Manistique Group, and the Burnt Bluff Group.

Environments of deposition of the Engadine Group (Figure

1) have been studied, especially as applied to exploration

for hydrocarbon bearing reefs (Droste and Shaver, 1985; Huh

et al., 1977; Mesolella et al., 1974). Porcher (1985a and

1985b) determined the environments of deposition of the

interreef portion of the Engadine Group in the subsurface.

Harrison (1985) determined the environments of deposition

of the Burnt Bluff Group in the subsurface of the Michigan

Basin, but no work has been done in the subsurface to

determine the environments of deposition of the Manistique

Group in Manistee, Mason and Oceana Counties (Figure 2).

There are few studies of the diagenesis of the

Niagaran Series in the subsurface of the Michigan Basin.

Diagenesis has been studied (Cercone and Lohman, 1985;

1987; Textoris and Carozzi, 1964) in the reefal facies of

the Upper Engadine Group (Figure 3), but not in the

underlying units. Diagenesis has greatly affected Niagaran

Series rocks, especially the distribution of porosity.

This study will attempt to determine the stratigraphy,

environments of deposition, and the parag'enesis of the

Manistique Group and the lower Engadine Group.

The three county study.area (Figure 2) was chosen because

of the availability of cores and petrophysical logs which

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5

UUCS COflHlOA ' CANADA

TON

Tim

OHIO ILLINOIS 0 ■”!

Figure 2. Location Map of the Study Area.

penetrate the Manistique Group.

Small hydrocarbon shows have been observed in the

Manistique Group, and in the lower Engadine Group in Amber

and Victory townships of Mason County. Production from

intervals in the Manistique Group and the lower Engadine

Group has occurred in at least two wells in the study area,

but these zones have not been extensively evaluated. These

hydrocarbon shows suggest that potentially economic hydro

carbon reservoirs may exist within the study area. Knowl

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. edge of structural and stratigraphic controls on

hydrocarbon accumulation, and the reservoir quality present

in the Manistique Group and lower Engadine Group in the

study area, are necessary for future hydrocarbon exploi

tation.

Method of Study

Six hundred fifty feet of core from five wells were

analyzed for this study (Table 1). Three of the wells

contain cored intervals from the Lower A-l Evaporite and

the A-0 Carbonate of the Salina Group, the entire Engadine

Group, and the upper section of the Manistique Group

(Figure 1). One core has the A-l Evaporite and the A-0

Carbonate of the Salina Group, the upper Engadine Group,

and forty one feet of the lower Engadine Group cored.

These four wells are in Mason County. The fifth well is in

Osceola County, which is out of the study area, but cut the

lower section of the Manistique Group, and the Burnt Bluff

Group, insuring that the entire section under study had

been observed in core.

The cores were slabbed and a portion stained with

Alizarin red S., to differentiate between calcite and

dolomite. The entire cored interval from each well was

photographed. Well cuttings of the Manistique Group in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 Table 1

Cores Used in This Study

Permit Lease Location Core Formations Footage! Cut

37731 Miller Sec. 6, 208 ' A-0 Carbonate Weinert 2-6 T18N R17W Upper Engadine NE NE NW Lower Engadine Mason Co. Manistique

31859 Shell Sec 26, 60' A-l Evaporite Hedrick 1-26 T19N R17W A-0 Carbonate NW SE NW Upper Engadine Mason Co. Lower Engadine

37699 Miller Sec. 27, 160' A-l Evaporite W.S.C.C. 1-32 T19N R17W A-0 Carbonate NE NE SE Upper Engadine Mason Co. Lower Engadine Manistique

38128 Miller Sec. 32, 177 ' A-l Evaporite Mazur 1- 32 T19N R17W A-0 Carbonate SW SW NE Upper Engadine Mason Co. Lower Engadine Manistique

36110 Willmet Sec. 36, 106 ' Manistique Thompson 3-36 T17N R9W Burnt Bluff NE NW SE Osceola Co

Oceana County were analyzed to determine the lateral

variations of the lithofacies.

Two trips were made to outcrops in the northern

peninsula of Michigan. The Byron Dolomite and the

Hendricks Dolomite of the Burnt Bluff Group, the School

craft Dolomite and the Cordell Dolomite of the Manistique

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Group, and the Rockview Dolostone, the Rapson Creek Dolo-

stone, and the Bush Bay Dolostone of the Engadine Group

were studied in twelve different localities. Samples were

collected to insure complete coverage of the section in

outcrops.

One hundred thin sections were made from the five

cores and samples collected from outcrops. Thin sections

from each lithofacies were made. Each thin section was

impregnated with a blue epoxy so porosity could be iden

tified. A portion of each thin section was stained with

Alizarin red S. and Potassium ferricyanide (Dickson, 1966),

to differentiate between calcite and dolomite, and to iden

tify ferroan calcite, ferroan dolomite and ankerite, if

present. Three thin sections were specially prepared by

grinding with oil instead of water to preserve salt.

Each thin section was examined in detail with a

petrographic microscope to identify the mineral content,

bioclastic content, and microfeatures preserved in the

sediment. Photographs were taken of the thin sections to

document important features. Point counts of three hundred

points per thin section were done on each thin section to

quantify the mineral assemblage of each lithofacies. These

data were analyzed using Lotus 1-2-3, version 2.01 (Lotus

Development Corporation, 1988).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Six thin sections were analyzed using fluorescence

microscopy in an attempt to identify diagenetic fabrics,

microporosity and depositional facies not recognizable with

a normal petrographic microscope (Dravis and Yurewicz,

1985). X-ray diffraction was done to identify miner-

alogical components of each lithofacies.

Petrophysical logs and well completion records from

113 wells in Lake, Manistee, Mason, Newaygo, and Oceana

Counties were used. The interval from the A-l Carbonate to

the Cabot Head Shale or any portion encompassed by these

units were used for all the wells. The characteristics

observable in cores and well cuttings were correlated to

the logs, so that lithofacies correlations could be done in

wells with no rock data. Mudlogs were also used t'o help

correlation and evaluation of the Manistique Group and the

lower Engadine Group.

Representative wells which penetrated to the Cabot

Head Shale were used for cross sections. These cross

sections were made with the help of the TERRASCIENCES

workstation. A cross section of the cored wells in Mason

County was created by the same technique.

Structural and isopach maps were made from the

available data using the TERRASCIENCES workstation and a

FoxBase Plus database (Fox Software, Inc., 1988).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10

Previous Work

Very little work has been done on the Lower Engadine

Group or the Manistique Group in the subsurface of the

Michigan Basin. Balakrishna (1972) studied the Silurian

rocks in the subsurface of Grand Traverse County including

the Manistique, Lower Engadine and Upper Engadine Groups.

The data was obtained from cores from one well, and focused

on the petrography of the interval. The work done by

Porcher (1985a and 1985b) deals with the Lower Engadine

Group and the non-reefal facies of the Upper Engadine Group

in the subsurface, concentrating on Kalkaska County. This

is the most comprehensive study of the Lower Engadine in

the subsurface to date.

Considerable work has been done in the Manistique and

Engadine Group outcrop areas. The Manistique Group was de

fined by Smith (1915). Newcombe (1933) discussed both the

Manistique Group and the Engadine Group. Ehlers and

Kesling (1957) described lithologic details of the Manis

tique and Engadine Groups at several outcrops (including

type-sections) in the northern peninsula of Michigan.

Shelden (1963) studied equivalents of the Manistique

and Engadine Groups in outcrops on Manitoulin Island,

Ontario, Canada. He determined that there are no iden

tifiable unconformities in the Niagaran Series. His work

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11

provided detailed environments of deposition for the rocks

on Manitoulin Island and determined facies relationships

between the lithofacies. Liberty and Sheldon (1968) also

studied the Niagaran Series on Manitoulin Island. They

correlated the sections on Manitoulin Island, with the

sections in the northern peninsula of Michigan. Faunal

assemblages were also identified and environments of

deposition determined.

Sanford (1972) attempted to correlate the stratigraphy

of outcrops in the northern peninsula of Michigan, Bruce

Peninsula, Manitoulin Island, Ontario and the Niagara Falls

area with the subsurface in Grand Traverse County

Michigan, Southeastern Michigan, and southwestern Ontario.

The paleogeography and tectonics of the Silurian across the

area was discussed. Kilgour (1972) discussed the Silurian

of Ontario and western New York, and attempted to correlate

the nomenclature of New York with that used in Ontario.

Janssens (1977) discussed the stratigraphy of the

Silurian rocks across Ohio. He discussed the stratigraphic

position of the "Clinton" as identified in Ohio, allowing

for comparisons of the stratigraphic position of the

"Clinton" of Ohio with the informally used "Clinton" in

Michigan.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Copper (1978) and Sanford (1978) identified the

Silurian paleoenvironments and faunal assemblages present

in the Manitoulin Island area. Johnson, Kesling,

Lilienthal and Sorensen (1979) studied the Engadine in

outcrops in the northern peninsula of Michigan. They

identified and described the formations in the Engadine

Group. The stratigraphy and paleogeographic setting of the

Engadine Group, especially of the upper part of the

Engadine Group, was detailed by Briggs, et al. (1980).

Johnson (1981) used relative sea level curves to refine the

correlations of the Lower Silurian, including the

Manistique Group and Lower Engadine Group, in the northern

peninsula of Michigan and Manitoulin Island, Ontario. He

also applied the standard European chronostratigraphic

series to the Great Lakes area Silurian section.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER II

STRATIGRAPHY •

Silurian Period

The Silurian Period is represented in Michigan by a

thick sequence of carbonates with shale near the base and

shales, carbonates and evaporites at the top (Figure 1).

Early, Middle and Late Silurian times are well represented

in the sedimentary section in the Michigan Basin. The

Silurian has been divided into three Series, the

Alexandrian Series, the Niagaran Series, and the Cayugan

Series, respectively.

The stratigraphy of the Middle Silurian is reasonably

well known in the outcrop area of the basin margins, but

less well understood in the subsurface. The application of

stratigraphic names from New York and Ohio have been ap

plied to the subsurface, disregarding the nomenclature used

in the outcrop area of the Michigan Basin. The nomen

clature in the sub-surface of the Michigan Basin was

oversimplified on the Stratigraphic Succession in Michigan

chart (Ells, Kelley, Hardenberg, Johnson and Sorensen,

1964). The Niagaran Series was simply called the Niagara

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and remained undifferentiated. All three Series in the

Silurian are represented in both the Michigan basin outcrop

and subsurface.

Alexandrian Series

The Alexandrian Series underlies the Niagaran Series

(Figure 1). The Cataract Group encompasses the entire

Alexandrian and is subdivided into two formations. The

Manitoulin Dolomite is the basal unit and is overlain by

the Cabot Head Shale. These two formations are widespread.

They are present in the northern peninsula of Michigan, in

the subsurface of the southern peninsula of Michigan, in

Indiana, Ohio, and in Ontario, Canada. In eastern Ohio,

the entire Cataract Group consists of interbedded sandstone

and shale. According to Janssens (1977), where the

Cataract Group is composed of sandstone and shale it is

informally called the Clinton Formation.

Niagaran Series

The Niagaran Series has been subdivided into three

stages (Figure 1). In ascending order, they are the

Clintonian Stage, the Cliftonian Stage, and the Lockportian

Stage (Shaver, 1984). The Clintonian Stage is subdivided

into two groups (Shaver, 1984). The lowermost group is the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Burnt Bluff Group. The Burnt Bluff Group crops out in the

northern peninsula of Michigan, in eastern Wisconsin, and

on Manitoulin Island, Ontario. In Ontario, the nomen

clature for the Burnt Bluff Group is different than that

used in Michigan and Wisconsin. The Burnt Bluff Group was

first recognized in the subsurface of the Michigan Basin by

Harrison (1985). In outcrop the Burnt Bluff Group has been

divided into three formations. These are in ascending or

der the Lime Island Dolomite, the Byron Dolomite, and the

Hendricks Dolomite. Ehlers and Kesling (1957) thoroughly

described the Burnt Bluff Group from outcrops in the north

ern peninsula of Michigan. Comparison of the Burnt Bluff

Group in outcrops and in cores from Alpena County, reveal

that the Burnt Bluff can be broken into its respective

formations in the subsurface. The Burnt Bluff Group thins

south of Alpena County. In northern Manistee County the

log signature of the Burnt Bluff Group is very similar to

that seen in Alpena County and all three formations are

identifiable. In Southern Manistee, Mason, and Oceana

counties, the wireline log signature of the entire Burnt

Bluff is very similar to the wireline signature of the Lime

Island Dolomite in Alpena County. Cores of the Burnt Bluff

Group from Osceola County reveal that the rocks have very

different lithofacies characteristics from those in Alpena

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16

County. This suggests that in Osceola County, the Burnt

Bluff Group is represented by a single formation which is

different from the three formations which occur in Alpena

County. The Burnt Bluff Group is sometimes referred to as

the Clinton, but it is not the same formation as the

Clinton of New York and Ohio, thus should not be applied in

Michigan.

Overlying the Burnt Bluff Group is the Manistique

Group. This group is the uppermost group of the Clintonian

Stage (Shaver, 1984). In the outcrop areas of the northern

peninsula of Michigan, the Manistique is subdivided into

two formations, the Schoolcraft Dolomite, and the overlying

Cordell Dolomite. Ehlers and Kesling (1957) described

these formations in detail. Where the Manistique Group

crops out, index fossils and the abundance of chert in the

Cordell Dolomite are criteria for distinguishing between

the two formations (Ehlers and Kesling, 1957; Liberty and

Sheldon, 1968). An attempt was made to identify these two

formations in cores in Mason County. As with the rocks in

outcrop, the lithology in the cores vary considerably.

General characteristics such as the amount of chert present

can be used to differentiate the two formations, but chert

is not present and index fossils were not recognized

throughout the study area. Another difficulty in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. correlating the Manistique, is that in the southern part of

the Michigan basin, the wireline signature suggests that a

facies change occurs and the entire Manistique Group be

comes argillaceous, and further south, turns into a shale.

Within the three county area under study, the uppermost

portion of the Manistique changes from all dolomite in the

northern part of Manistee County, to dolomite and chert in

southern Manistee County and northern Mason County, and an

argillaceous dolomite and shale in southern Mason County

and Oceana County. This suggests that in part of the study

area the Cordell Dolomite and the Schoolcraft Dolomite may

be identified, but a large portion of the study area such

distinction is not possible.

The term Clinton is informally used to identify the

Manistique Group. This term has also been used to identify

the Burnt Bluff Group, and is often used for the entire

Clintonian Stage. The correlation of the Clinton Group in

Mew York, and the Clintonian Stage in Michigan, and on

Manitoulin Island, Ontario cannot be conclusively estab

lished (Kilgour, 1972). According to Kesling and Ehlers

(1957), the Michigan Basin was probably separated from the

Ohio and Appalachian Basins, because faunal assemblages are

different. Janssens (1977) used the term Clinton Formation

in Ohio for sandstones and shales which comprise the entire

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Cataract Group in eastern Ohio. The rocks above the Burnt

Bluff Group, and below the Engadine Group will be referred

to as the Manistique Group in this study area and cor

related with the type area in northern Michigan.

The Engadine Group overlies The Manistique Group and

encompasses the Cliftonian Stage and the younger

Lockportian Stage (Shaver, 1984; see Figure 1). Hydro

carbon production from reefal rocks in the Lockportian

Stage of the Niagaran Series resulted in informal attempts

to divide the Niagaran Series in the subsurface of the

Michigan Basin. Widespread use of the informal terms

"Brown Niagaran," "Gray Niagaran," and the lesser use of

"White Niagaran" for the subsurface stratigraphic equiv

alents of the Engadine Group in the outcrop area of the

northern peninsula of Michigan, are now deeply entrenched.

The "Gray Niagaran" Formation encompasses the

Cliftonian Stage (Johnson et al., 1979; Shaver, 1984). The

Rockview Dolomite and the Rapson Creek Dolomite (Johnson et

al., 1979) are the stratigraphic equivalents in the out

crops in the northern peninsula of Michigan, but there is

insufficient core data to correlate these units into the

subsurface. The "Brown Niagaran" Formation is the upper

most unit of the Engadine Group, and encompasses the

Lockportian Stage (Johnson et al., 1979; Shaver, 1984). It

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is in the Brown Niagaran Formation that most of the pro

duction of hydrocarbons from reefs has occurred in the

Michigan Basin. Most previously published research has

focused on this unit. Reefs are not exclusive to the

Lockportian. Reefs initially developed in the Michigan

basin in the Alexandrian during deposition of the

Manitoulin Dolomite (Dorr and Eschman, 1970). Reef

development continued throughout the Middle Silurian with

reefs identified in the Burnt Bluff and Manistique groups

in addition to the better known Engadine group reefs (Dorr

and Eschman, 1970; Droste and Shaver, 1985).

The top of the Upper Engadine Group in the study area

is identified by an ash bed which appears basin wide in

non-reefal environments. Above this ash bed is the Salina

Group of the Cayugan Series, which consists of interbedded

carbonates and evaporites with some interspersed shale.

For this study, which is concerned mainly with the

Engadine and Manistique Groups in the subsurface of

Manistee, Mason and Oceana Counties, these groups have been

divided into Lithofacies (Figure 3), modified from Porcher

(1985b). These lithofacies will be described in detail,

and related to the stratigraphic nomenclature as described

above.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to o

ASH ASH BED REEFAL FACIES SILICEOUS NODULAR BIODOLOMICRITE

CRINOIDAL DOLOMICRITE PELLOIDAL DOLOMICRITE -,I -,I LAMINATED DOLOMICRITE 1 MAT CHIP OOLOMICRITE a i /\ : t 1/<=>A\ U 7J U LITHOFACIES I / A / 1 V~/ 32 = = = B S U H G G = HI HI = l~ - - = g | SHALE ond ARGILLACEOUS OOLOMITE MANISTIQUE GROUP H2 H3 : l / c a / | NODULAR DOLOMICRITE F F s l / o / J NODULAR DOLOMICRITE B2 = i z a z D j = BIOTURBATED DOLOMICRITE r = LQWEfi-ENGADINE GROUP E = I ( = / • UPPER UPPER ENGADINE GROUP b i /°y..a. I / « /. / « /. y S&S. A*?!*/**? A A*?!*/**? Modified from Porcher (1985b), p.29. Manistee, Mason and Oceana Counties.

><=>> /o/ ><=>> y<>/. *y«y y y °/Vo/ /o/ >oV 7°/ £ ^ / /-/2 Iz l.7 :/ V-v; 7 /7 Figure 3. Lithofacies of the Niagaran Series in A-1 CARBONATE A- 0 CARBONATE LOWER ENGADINE GROUP A- I EVAPORITEI A- BURNT BLUFF GROUP UPPER ENGADINE GROUP MANISTIQUE GROUP

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Recommended Nomenclature

The stratigraphic nomenclature used in this report is

based on previous literature, studies of stratigraphy and

lithofacies in outcrop and cores from the subsurface.

There are two stratigraphic groups in this study, the

Manistique and the Engadine. The lowermost is the

Manistique Group. It overlies the Burnt Bluff Group. The

Manistique Group has been subdivided into two lithofacies,

a lower Lithofacies I and an upper Lithofacies H. These

lithofacies may correlate to the Schoolcraft Dolomite and

Cordell Dolomite, respectively, as identified in outcrops,

but fossil assemblages were not studied in enough detail to

correlate the outcrops to the subsurface.

From cores, Lithofacies I is a pelloidal dolomicrite

throughout the study area. Lithofacies H varies laterally

across the study area, and is subdivided into three sub

facies. These subfacies include a nodular dolomicrite in

the northern portion of the study area, a siliceous nodular

biodolomicrite in the center of the study area, and shale

and argillaceous dolomite in the southern portion of the

study area.

The Engadine Group overlies the Manistique Group. It

is divided into a Lower Engadine Group and an Upper

Engadine Group for the purposes of this report. The Lower

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Engadine Group is the equivalent of the informal Gray

Niagara, and the Upper Engadine Group is the equivalent of

the informal Brown Niagara (Figure 2). This study concen

trates on the lower Engadine Group, although references are

made to the upper Engadine Group, and some analyses were

done on the non-reefal upper Engadine Group. For this

report, the Engadine Group was further subdivided into

lithofacies (Figure 3). The lithofacies were first sug

gested by Porcher (1985b). The lithofacies as described

by Porcher (1985b) were modified slightly to correspond to

the rocks in Manistee, Mason and Oceana counties.

The Lower Engadine Group is divided into three

lithofacies. The lowermost lithofacies, Lithofacies G, is

not widespread. Lithofacies G is a laminated dolomicrite

containing abundant organic material as seen in thin sec

tions. Overlying Lithofacies G or the Manistique Group

where Lithofacies G is not present are Lithofacies F and

Lithofacies E. These two lithofacies are interbedded with

one another. Lithofacies F is a nodular micrite, and

Lithofacies E is a crinoidal micrite. The light color and

crinoid debris in Lithofacies E contrasts with the dark

brown and tan nodular texture of Lithofacies F. The log

characteristics of each lithofacies are also different,

Lithofacies F consistently has higher porosity than does

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Lithofacies E.

The Upper Engadine Group has been subdivided into four

lithofacies (Porcher, 1985b), of which only three have been

identified in the area under investigation. Lithofacies D

is described as a mat chip dolomicrite. It is charac

terized by very fine short dark linear patches scattered

throughout the lithofacies. These patches are straight or

slightly curved, and not connected to each other. They

appear to resemble desiccated algal mat chips as seen in a

sabkha environment, hence the term "mat chip" dolomicrite.

Lithofacies C is described as an intraclastic dolomicrite

and has not been identified in the cored sections available

in the study area. Lithofacies B2 is a bioturbated dolo

micrite and is present in the study area. Lithofacies B2

and lithofacies D are interbedded with one another.

Lithofacies B1 is the uppermost lithofacies in the

Engadine Group. It is an ash bed with very distinctive

characteristics both in samples and on logs. It appears

to be widespread across the Michigan basin, although it is

not present over reefs. This lithofacies has been noted in

the literature (Briggs et al., 1980; Mesolella et al.,

1974) but usually referred to as a shale, and much effort

has gone into explaining the origin of this "shale" in a

carbonate environment. The study of thin sections from

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this unit reveal the presence of biotite, potassium feld

spar, polycrystalline quartz shaped like glass shards with

a felted texture, and zircon in a clay matrix. This min

eral assemblage is very unusual in carbonate environments,

but is common in volcanic assemblages (Moorhouse, 1959) and

indicate that Lithofacies B1 is an ash bed.

Overlying Lithofacies B1 is the Cayugan Series (Figure

1). The basal unit was identified as the A-0 Carbonate by

Gill (1977). This unit was identified as Lithofacies A by

Porcher (1985b) and identified as an algal laminated dolo

micrite. It is not part of the Niagaran Group, thus not

part of this study, nevertheless it is important to look at

it because it is quite similar in appearance and compo

sition to Lithofacies G. Lithofacies A is more widespread

than Lithofacies G, and it has been reported in the liter

ature (Gill, 1977; Porcher, 1985a), so perhaps an under

standing of Lithofacies A can help make better inter

pretations of the depositional environment of Lithofacies

G. Lithofacies A consists of very finely laminated dark

brown to gray dolomite with some interbeds of anhydrite and

salt. It was probably deposited in a highly saline envi

ronment. The lack of fossils in Lithofacies A also sug

gests that the environment was not suitable for most

organisms. The undisturbed laminations indicate that the

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conditions were to severe for burrowing organisms, and the

currents were not strong enough to disrupt the deposited

sediment. This suggests that Lithofacies A was deposited

in a restricted environment with higher than normal salin

ity and very little circulation (Porcher, 1985a).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER III

ENVIRONMENT OF DEPOSITION

Preliminary Setting

The three county study area is situated along the

western edge of the Michigan basin (Figure 2). The basin

was subsiding during the Middle Silurian and developed into

the classic bowl shape by the end of the Niagaran (Briggs

et al., 1980). During the Lower Clintonian, while the

Burnt Bluff Group was deposited, the basin was decidedly

one sided, with a thick carbonate platform in the north,

but not in the south. This is due to the Clinton Inlet in

the southeast which allowed enough clastic sediment to

enter to prevent the carbonate deposition. (Sellwood, 1986)

which occurred elsewhere in the basin. Figure 4 shows the

location of the Clinton Inlet. The location of the inlet

remained stationary throughout the Niagaran, however, the

amount of water flowing through was later reduced, and a

southern extension of the carbonate platform developed

during the Cliftonian (Figure 4). On the Lower Clintonian

platform, three distinct facies developed as described by

Harrison (1985). These facies correlate to the three for-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. mations described in outcrops. The three formations (Fig

ure 1) are the lowermost Lime Island dolomite, the

Byron dolomite, and the uppermost Hendricks dolomite

(Ehlers and Kesling, 1957). These three formations or

facies can be recognized on logs in northern Manistee Coun

ty (Figure 5). Only one facies, which has the same log

signature of the Lime Island Dolomite, can be identified

in southern Manistee County, Mason County and Oceana

County. The zone where the identification of three sep-

8ASIN

BASIN 'CLINTON X: . • INLET

Figure 4. Paleogeographic Map of the Michigan Basin During the Niagaran. The Clinton Inlet was a main source of water and clastic sediment. Modified From Briggs et al. (1980).

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o +J £ -p p o 2 0 si +j e 0 p ph Iff £ S' 1 X 3 'U-l* MOWS H +j M I

W a) u to a) p 3 U> •H TnrmBRiss^is" fa

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29

arate facies in the Burnt Bluff Group becomes impossible

marks the southernmost extent of the carbonate bank on the

west edge of the basin.

In the deeper water of the carbonate platform, con

ditions were not as conducive to survival of carbonate

secreting organisms. Much of the sediment deposited off of

the carbonate bank was debris washed down from the platform

during storms. The amount of this periplatform sediment

and ooze deposited in the deeper water was much less than

what was deposited in the shallows of a typical of car

bonate platform (Wilson, 1974).

Manistique Group

The depositional environment of the Manistique Group

was influenced by the topography of the Burnt Bluff surface

and by the subsidence of the basin. The Manistique Group

has been studied and described where it is exposed at out

crops and in quarries (Ehlers and Kesling, 1957; Johnson,

1981; Liberty and Sheldon, 1968; Sanford, 1978). In the

northern peninsula of Michigan the Manistique Group has

been divided into two formations. The Schoolcraft Dolomite

overlies the Burnt Bluff Group, and the Cordell Dolomite

overlies the Schoolcraft Dolomite (Figure 1).

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Samples from the outcrop were compared to samples from

the cores in Mason County (Table 1). The samples are sim

ilar in mineralogy, texture and allochems. It is suggested

that they are correlatable. There are only two studies

(Balakrishna, 1972? Harrison, 1985) which have attempted to

relate outcrop studies of Niagaran (Figure 1) rocks to the

subsurface. Harrison (1985) determined that the Burnt

Bluff Group in the subsurface in Alpena County correlated

with the Burnt Bluff Group in outcrops. Balakrishna (1972)

determined that the Manistique Group and Engadine Groups

in the subsurface in Grand Traverse County correlated to

the Manistique and Engadine Groups from outcrops in

Ontario, Canada.

In the study area, the Manistique has been subdivided

into two lithofacies based on studies of cores (Figure 3).

The lowermost Lithofacies I overlies the Burnt Bluff Group

across the entire study area. Lithofacies H overlies Litho

facies I and is further subdivided into Lithofacies HI,

Lithofacies H2 and Lithofacies H3. These subdivisions of

Lithofacies H are based on log characteristics present in

the study area. Lithofacies HI is a shale or shaly dolo

mite, Lithofacies H2 is a nodular biodolomicrite with

scattered chert nodules.. Lithofacies H3 is a dolomite which

contains no chert, but may have characteristics similar to

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Lithofacies H2. No cores have been cut in Lithofacies Hi

and H3, so all information concerning them is inferred from

logs.

Lithofacies I

Lithofacies I is a pelloidal dolomicrite (Figure 6).

Lithofacies I is interbedded with lithofacies H in the mid

dle of the Manistique, but predominates in the lower sec

tion. It is relatively homogeneous, with occasional faint

traces of bedding planes preserved. The lack of well pre

served depositional structures is probably due to intensive

bioturbation (Enos, 1983). Large whole fossils, especially

brachiopods and crinoids are scattered and infrequent, but

fossil fragments occur, especially echinoderm, trilobite

and brachiopod fragments. The fossil fragments are not

equally distributed through the unit, but are often lo

calized. They were probably transported before deposition,

the mechanical abrasion during transportation breaking the

shells into fragments. The fossil fragments are not dis

tributed evenly because the currents which moved the frag

ments were not continuous. Lithofacies I is almost entire

ly devoid of chert, although some fossils have been re

placed by chert. Detrital quartzose silt is present and

represents an average of 5.5% of the rock. The detrital

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32

Figure 6. Lithofacies I.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. quartzose silt is also not distributed equally, it varies

from 0% to 15% of the total rock in Lithofacies I. Much of

lithofacies I is pelloidal (Figure 7). The average size of

pelloids in lithofacies I is .15mm. Most of the matrix of

lithofacies I is composed of micrite. The micrite has been

dolomitized, so its original texture cannot be ascertained.

Figure 7. W.S.C.C. #1-27, 4962.5 Feet. Plane Light, x40 Magnification. Dark Brown Peloids in Dolomite in Lithofacies I.

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The environment of deposition of lithofacies I was

probably a restricted shelf. The presence of pelloids,

bioturbation and the scarcity of fossils are diagnostic of

a restricted shelf (Enos, 1983). It should be noted on the

cross-section (Figure 5) that the thickness of Lithofacies

I is relatively uniform throughout the study area.

The uniform thickness of Lithofacies I is in contrast

to the Burnt Bluff Group, which had a well developed thick

carbonate platform in the north, and thin carbonate muds

south of the platform. This suggests that the Burnt Bluff

Group platform was drowned at the onset of the Manistique

deposition. A platform is drowned when carbonate accu

mulation can not keep up the pace of the rise in sea level

(Schlager, 1981). A rapid pulse of sea level rise in the

Michigan Basin may have been due to increased subsidence of

the basin (Read, 1985). There is limited core coverage of

the Burnt Bluff Group-Manistique Group contact making it

difficult to check for hardgrounds which are commonly found

separating neritic and deeper water deposits (Schlager,

1981). Nevertheless, a completely drowned platform is

suggested by covering the shelf with deeper water sediments

(Read, 1985). This is exactly what has happened in the

study area? the Burnt Bluff Group shelf is overlain by the

deeper water Lithofacies I in the Manistique Group. Within

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the Burnt Bluff Group, the middle formation, the Byron, is

the shallowest water facies present, the overlying

Hendricks Formation is deeper water, suggesting that the

transgression began during the deposition of the Burnt

Bluff Group (Harrison, 1985; Harrison, pers. com, 1990).

The Schoolcraft Formation in northern Michigan had two

transgressions as recorded in sea level curves (Johnson and

Campbell, 1980; see Figure 8). It is possible that the

margins of the basin experienced two transgressions sepa

rated by a regression as suggested in Figure 8. Into the

basin the regression is not observed because of the more

distal conditions. The two transgressions which are present

at the margin of the basin are one longer transgression in

the basin center.

The sediments deposited in the study area were char

acteristic of a restricted shelf, suggesting that the

transgression was not sufficient to connect the Michigan

Basin to open water. The increase in sea level essentially

stopped the production of carbonate on the distal portions

of the platform. The sediment which was deposited on the

platform and on the foreslope was carbonate debris, washed

in from the distal portions of the basin.

Storms had a great influence on the deposition of

Lithofacies I. Storms created winds of sufficient strength

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Formolion/Bods m tltri Community St*' s*' Environment * ENGADINE 0 e c p ~ Shallow 3 S I3H N o -I z Upper Coral-Algal 16.5 ^<3 Coral . Algal ut - i / ? _i aUJ E z O Penlomeroides 11.75 00 Ponlomtrid C6 O / X Lower Coral-Algal 9.0 Coral-Algol o " S 8.0 Fucoid |— Upper Laminated t— u. Uoper Pentamerus 3.0 QO Pentamorid C l < cc N. o o Lower Laminated 9.0 Fucoid Cm o z X < o 2 z O -- 3.5 Pentamerid < Lower Pentamerus 0 0 b. -S' E C| Upper Coral-Algol 9.5 3 Coral-Algal o CO _ i tarn z o V) < E Fucold- e o Plectatrypo 19.5 cc Ui z Ottacode UJ > LU / o X s: o z Lower Coral-Algal 3.0 Coral-Algal

Z Bl-3 < 5 Q BYRON 26.5 **!■& F u co id ) LIME ISLAND 8.5 0 0 P c n la m e rid X

on • • Ottraeade MORMON CREEK a HIR. ASH

Figure 8. Composite Stratigraphic Section ^ower Silurian Formations m Northern Michigan Wit Accompanying Sea-lavel Curve. From Johnson and Campbell (1980), p.1052.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37

to transport detrital quartzose silt aerially from a dis

tant land source. As the winds decreased, the silt was

dropped into the water column. It settled through the

water column to be deposited with the carbonate sediment.

The storms also increased the current strength in the water

so that fossils were reworked from shallower to the deeper

environments.

Lithofacies H

Lithofacies H overlies lithofacies I. The contact

between the two lithofacies is gradational and the two

lithofacies interfinger with one another. Lithofacies H

is not laterally continuous throughout the study area as is

lithofacies I. Lithofacies H has been subdivided into

lithofacies HI, H2, and H3 based on log characteristics

(Figure 5). These three subdivisions of lithofacies grade

laterally from north to south across the study area (Figure

9). The thickness of lithofacies H does not vary much

across the study area, it does not noticeably vary in

thickness across the contacts of the three sub-1 ithofacies.

Lithofacies H2 is the only sublithofacies of Lithofacies H

from which there are cores. Using the data obtained from

the cores, from logs, from mud logs, and from available dry

cuts, the environments of deposition of these units have

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R18W

LITHOFACIES

T18N

*TT4N

Figure 9. Lateral Variations of Lithofacies H Within the Study Area.

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been deduced.

Lithofacies H3

In the northern three townships of Manistee county

(Figure 5 and Figure 9), lithofacies H3 occurs. There are

no cores which cut lithofacies H3, and cuttings were not

available for study. The log signature of lithofacies H3

is very similar to the log signatures from rocks in litho

facies H2 which are nodular dolomicrites. The log signa

ture of lithofacies H3 is similar to lithofacies F and to

lithofacies H2, both of which are nodular micrites. It is

possible that in the northern three townships of Manistee

County the transition from the Lower Engadine and the

Manistique is very gradual, and lithofacies H3 and litho

facies F may if fact be identical.

Lithofacies H2

Lithofacies H2 is a tan, dark brown, and dark gray

dolomite with inclusions of light grey chert. Fossils are

abundant in lithofacies H2, especially where they are

preserved in chert (Figure 10). Lithofacies H2 is the

best understood unit of lithofacies H. It is present in

the southern townships of Manistee County, in the northern

two townships of Mason County, and in the northern half of

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Figure 10. Weinert #2-6, 4642.5 Feet. Lithofacies H2. Light Gray is Chert.

Township 18 in Mason County (Figure 9). Four cores were

available for study in lithofacies H2 (Table 1). These

cores and the logs of the interval are the basis for the

interpretation of the environment of deposition. Figure 11

includes all the cored wells and shows the cored interval

in each well.

Lithofacies H2 is distinguished from the other units

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

AN UPPER UPPER ENGAOtNE 4 0 5 6 5139 4 0 5 6 I PH. S 7 flM n p*tum p*tum Including Cored Wells. 4 0 5 6 4 0 5 6 Figure 11. Cross-Section A-2 Extending From East to West 4 0 5 6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. by the presence of chert. The chert is easily identifiable

in cores and on density logs. Lithofacies H2 closely

resembles the Cordell Dolomite in the northern peninsula of

Michigan (Figure 1). The chert in this lithofacies is

diagenetic, thus is not in itself useful as a criteria for

determining the environment of deposition. The chert does

replace dolomite in such a way as to suggest that even

where the rock is now chert, it originally had the nodular

texture characteristic of this lithofacies. The replace

ment of dolomite by chert is extensive in some intervals,

and rare in others. The distribution of the chert may be

due to the deposition of organisms which had siliceous

tests or parts such as sponge spicules. The occurrence of

chert only in lithofacies H2 must therefore suggest that

this unit was deposited in an environment which was favor

able to the growth of silica secreting organisms.

Lithofacies H2 is a nodular biodolomicrite (Figure

10). The contact with lithofacies I is gradational, with

interfingering of the two lithofacies. The contacts be

tween lithofacies H2, and lithofacies HI and H3 are fairly

sharp, and appear to occur through a very short distance

(Figure 5). Lithofacies H2 is nodular. The nodules are

coarse-grained and the sediment between the nodules is fine

grained.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43

The origin of the nodular texture of carbonates has

been studied by many (Bathurst, 1975; Bissel and Barker,

1977; Hopkins, 1977; Mullins, Neumann, Wilbur and Boardman,

1980; Wanless, 1979). Several different models have been

proposed to explain the origin of nodular carbonates.

These include the Vadose Model, the Burrow Model, the

Differential Compaction Model, the Gravity Flow Model and

the Current Model.

Much of the literature cites a deeper water envi

ronment (30 to 130 feet) for the origin of nodular car

bonates (Harrison, 1985, Mullins et al., 1980, Read, 1985).

Nodule formation in the deeper waters will differ from the

nodules formed in shallower waters. Several ideas have

been presented to explain the formation of nodules in

pelagic deeper waters (Bissel and Barker, 1977; Hopkins,

1977; Savrda and Bottjer, 1988; Scholle, Arthur and Ekdale,

1983). Some of these models suggest that glauconite and

phosphate should be present (Bathurst, 1975; Jenkyns, 1986)

in pelagic deep water sediments. No glauconite or phos

phate has been found in the Manistique Group.

Vadose Model

A nodular texture is known to develop in the vadose

zone in arid climates (Moore, 1989). Evidence for a vadose

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zone origin for the nodular texture of lithofacies H2 such

as the development of a hardpan and a pisolitic chalky

zone, is not present.

Burrow Model

Two models (Savrda and Bottjer, 1988? Scholle et al.,

1983) propose that the nodules are due to biogenic activity

by burrowing. Scholle et al. (1983) suggest that nodules

form when burrows become selectively cemented. Where the

currents are strong enough to winnow away fine sediment,

the burrows are filled with coarser sediment. If the de

position is slow, the sediment is exposed at the sed-

iment-water interface for a long period of time and ce

mented at the sediment surface, creating calcite nodules.

If deposition of sediment is renewed soon after the nodules

are formed, the nodular texture is preserved. If currents

are of sufficient strength to prevent the deposition of new

sediment, the nodules may coalesce to form hardgrounds.

Savrda and Bottjer (1988) suggest that in anoxic sed

iments, concretions develop in the less compacted crests

of wave forms. The sediment then undergoes a shift in

redox conditions, oxygenating the sediment. The sediment

is subsequently burrowed extensively except where the early

concretions were formed. Compaction of the sediment fur-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 ther enhances the development of the nodular texture.

Burrows have not been identified in sufficient numbers

to suggest that either of these two models explain the

mechanism by which nodules developed in lithofacies H2, but

portions of both models are used in other models.

Differential Compaction Model

Another model suggests that a nodular texture develops

by differential compaction between the nodules and the

matrix (McCrossan, 1958; Porcher, 1985a; 1985b). In this

model for nodule formation, interbedded lime and clay are

initially deposited. The lime becomes lithified before the

clay, and due to differential loading and plastic defor

mation of the clay, carbonate nodules form. Characteristics

typical of this type of origin as defined by McCrossan

(1958) include coarser-grained pinch and swell features

termed sedimentary boudinage, fine cracks due to tensional

forces, and gradations of carbonate distribution ranging

from undeformed laminations to scattered isolated nodules.

This process was suggested by Porcher (1985a, 1985b) to

have been the mechanism by which lithofacies F developed.

It is felt that the same mechanism of nodule formation

occurred in lithofacies F and lithofacies H2. Some of the

features described by McCrossan (1958), such as sedimentary

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boudinage and fine cracks, are observable in the cores in

the study area. The apparent laminar orientation of most

of the nodules also support this model. A major drawback

to this hypothesis is that it requires the deposition of

continuous clay layers, and there is no evidence in the

rocks of this type of deposition.

Gravity Flow Model

Another model to create carbonate nodules was proposed

by Hopkins (1977). This model is very similar to the one

presented by Mullins et al. (1980). It suggests that

nodules form by the submarine cementation of thinly bedded

carbonates at the sediment-water interface. The thin

cemented beds were separated by uncemented carbonate.

Displacement of the thinly bedded carbonate down the slope

due to a sediment gravity flow would break up the cemented

beds, and mixing of the interclasts with the uncemented

sediment would create a breccia with a nodular texture.

Lithofacies H2 has no indications of displacement by

gravity flow, and the nodules formed in this manner would

be more angular than the nodules which occur in the studied

cores.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47

Current Model

Another model of nodular carbonate formation has been

presented by Mullins et al. (1980) based on studies of

recent sediments. This model relies on currents just as

does the model suggested by Scholle et al. (1983). In this

model, pelagic sediment is deposited in a deeper water

environment. Bottom currents winnow away finer sediment

leaving a grain supported sand size sediment. This cleaner

sediment is cemented by amorphous or pelloidal high-Mg

cement. This cemented sediment is then broken up by strong

bottom currents. Bioturbation also helps to break up the

sediment and prevent hardgrounds from forming. A decrease

in the strength of the currents allows fine sediment to

filter in around the cemented interclasts. Continued

bioturbation mixes the sediment and interclasts.

Cementation of interclasts by pelloidal to sparry

high-Mg calcite creates the nodules during episodes of

higher current velocity, and fine mud filters between the

nodules to create a matrix during times of slow current

velocities. This nodular texture occurs deeper on the

carbonate platform where currents don't winnow away all the

fine sediment and produce hardgrounds, but they are strong

enough to winnow away much of the fine grained material and

prevent the development of soft oozes (Figure 12).

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STRONG b o t t o m c u r r e n t s .

600 • 1000 m

Figure 12. Idealized Carbonate Slope Facies Relationship Produced by Downslope Decreasing Bottom Current Action. From Mullins et al. (1980), p.128.

In the Manistique, any features that indicate this

mode of formation have been obliterated during diagenesis,

but a faintly pelloidal texture is visible in thin section.

The nodules are made up of coarser grained carbonate than

is the surrounding matrix and this may be due to the win

nowing away of fine sediment which occurred during the

formation of them.

The model of nodular carbonate formation presented by

Mullins et al. (1980) best explains the features observable

in lithofacies H2. This model suggests that the water

depths were probably deeper than the water depths in litho-

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facies I, thus a transgression was still occurring during

the Deposition of lithofacies H.

Lithofacies HI

Lithofacies HI occurs in Mason County in the south

half of Township 18 North and in Township 17 North. It is

found throughout Oceana County, and on the western edge of

Newaygo County (Figure 9). The Gamma Ray curve of logs of

lithofacies HI suggest that it is argillaceous. No cores

are available which cut lithofacies HI, but from mud log

descriptions and dry cuts, the composition of lithofacies

HI was ascertained. Lithofacies HI consists of interbedded

argillaceous dark cryptocrystalline dolomites and red soft

shales. Few depositional criteria can be ascertained from

logs and cuttings, but a generalized environment of depo

sition for lithofacies HI can be deduced.

It has been documented that in deep pelagic envi

ronments below the carbonate compensation depth (the depth

below which calcite is not preserved), red clays with

manganese nodules are slowly deposited (Scholle et al.,

1983). Droste and Shaver (1985) suggested that a shallow

epeiric sea covered the area depositing carbonate and

terrigenous mud. If their interpretation is correct, the

water was not deep enough to have deposition below the

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carbonate compensation depth. The presence of shale

probably suggests that an influx of clastic sediment from

the south prevented the buildup of carbonate. The water

was probably deeper than it was to the north, and very

little carbonate was deposited. Lithofacies HI is slightly

thinner than are lithofacies H2 or H3. This may be due to

compaction of the finer sediment, or it may suggest a much

slower rate of accumulation.

The three sub-lithofacies which make up lithofacies H

occur in three distinct bands across the study area (Figure

9). The environments of deposition of lithofacies H sug

gest that the overall water depth was deeper than it was

during the deposition of lithofacies I. This suggests a

continuance of the transgression which started either in

the Burnt Bluff or at the onset of the deposition of the

Manistique. Lithofacies I is of uniform composition

throughout the study area, suggesting a drowned platform as

previously described. Lithofacies H is subdivided into

three separate facies which suggests a deepening of the

water southward. During the deposition of lithofacies H,

sea level rise still exceeded the rate at which carbonate

could build up into a platform. The rate of sea level rise

was slower than during the deposition of lithofacies I, so

the carbonates in the northern half of the study area re

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mained in the euphotic zone. This has been identified as

incipient drowning. Nodular and argillaceous carbonates

are common on incipiently drowned platforms (Read, 1985).

Lower Engadine Group

The Lower Engadine Group rests conformably on the

Manistique Group. The Lower Engadine Group has been

studied in outcrops in the northern peninsula of Michigan,

and is extensively quarried (Ehlers and Kesling, 1957;

Johnson et al., 1979? Newcombe, 1933? Sanford, 1972;

Sheldon, 1963). The Lower Engadine Group has not been as

extensively studied in the subsurface. The work by Porcher

(1985a, 1985b) is the most comprehensive study of the Lower

Engadine Group in the subsurface to date. This work has

been used and expanded upon to better understand the en

vironments of deposition present in the three county study

area.

The Lower Engadine Group has been subdivided into

three lithofacies based on studies of the cores in Mason

County (Figure 3). The lowermost lithofacies present is

lithofacies G. It is overlain by lithofacies E and litho

facies F. Lithofacies E and lithofacies F are interbedded,

but lithofacies E predominates at the base of the Lower

Engadine Group, and lithofacies F is more prevalent near

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the top. Lithofacies E may be correlative to the Rockview

Dolostone (Figure 1) in the northern peninsula of Michigan

(Johnson et al., 1979). Likewise, lithofacies F may be

correlative to the Rapson Creek Dolostone (Johnson et al.,

1979). Further study and comparison of the Engadine in the

subsurface, and in outcrops is needed before these cor

relations can be made with a degree of certainty.

Lithofacies G

Lithofacies G is a dark finely laminated dolomite

(Figure 13). It contains abundant organic material and

contains some authigenic chert. Detrital Quartz is present

in minor amounts. There are very few fossils in litho

facies G. Interbedded with the dark finely laminated

dolomite are intervals of nonlaminated highly bioturbated

rocks (Figure 13). Lithofacies G is not laterally con

tinuous over the study area. It occurs in small, isolated

patches, and is not easy to identify on logs. Where litho

facies G is present, it directly overlies the Manistique

Group.

Lithofacies G represents a restricted environment.

The preservation of abundant organic matter suggests that

Eh conditions were reducing. The lack of fossils or bio

turbation within the laminar beds suggests that the

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MILLER BROTHERS | . MAZUR* h£2 .y.. J ^|cC^Ti9NRi7^?-'

Figure 13- Lithofacies G. Light Gray Rock in the Laminated Portion is Chert.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. environment was not conducive for the proliferation of

life. The interbedded bioturbated zones suggest that minor

fluctuations occurred in which the conditions went from

reducing to oxygenated. It is probable that some sort of

barrier occurred basin-ward from where lithofacies G was

deposited, restricting the flow of water, and producing

locally reducing conditions. Minor fluctuations of the sea

level would affect the deposition of lithofacies G. When

the sea level increased it would create intervals when the

barrier was breached, and the sediment was oxygenated.

During periods of slightly lowered sea levels, the barrier

would affect the depositional environment behind it and

reducing conditions would develop.

Lithofacies E

Lithofacies E occurs above lithofacies G. It is the

predominant lithofacies at the base of the Lower Engadine

in the absence of lithofacies G. It is interbedded with

lithofacies F in the upper portion of the Lower Engadine.

Lithofacies E is a gray to tan, irregularly laminated

micrite. Within lithofacies E are zones of abundant

crinoid fragments (Figure 14). These crinoidal zones occur

within a matrix of highly bioturbated sediment which also

contains other scattered fossil debris. Some of the fossil

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SHELL OIL COMPANY

H E D R I C K # 1-26

MASON CO., Ml

Figure 14. Lithofacies E. Crinoidal Packstone Zone Interbedded With Irregularly Laminated Micrite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56

fragments have been bored before final deposition. Angular

quartzose silt grains are scattered throughout lithofacies

E (Figure 15). Point count data indicates that the largest

percentage of detrital quartz in the Lower Engadine is

found in lithofacies E.

Porcher (1985b) suggested that lithofacies E rep

resents storm deposits. Shell and organic debris are

washed in from shallower waters during these storms. Shell

material is fragmented as it is transported into the deeper

water. The increased wave energy generated during the

storm events moves large amount of carbonate debris down

the slope into the deeper waters.

Storm generated winds pick up fine siliciclastic

sediment from distant land masses and carry it out over the

sea. The detrital sediment is deposited when the wind

energy decreases and is no longer able to carry the sed

iment. The quartzose silt is deposited with the carbonate

micrite and crinoidal debris by gravitational settling

after it drops into the water.

Lithofacies F

Lithofacies F is a nodular dolomicrite (Figure 16).

Lithofacies F is interbedded with lithofacies E, but be

comes the most common lithofacies at the top of the Lower

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Figure 15. W.S.C.C. #1-27. Crossed Nichols, x250 Magnification. Angular Quartzose Detrital Silt in Lithofacies E.

Engadine. The nodules are coarse grained and generally

light brown or tan. The sediment between the nodules is

much finer grained. It is tan and dark gray dolomicrite

with abundant styolites. The nodules vary in size and

shape, but they are generally elongate parallel to bedding.

X-ray diffraction data indicates that the main mineral-

ogical difference between the nodules and the surrounding

matrix is the presence of salt in the nodules. The salt is

diagenetic, not indicative of the environment of depo

sition. It may suggest that the nodules are more porous

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 16. Weinert # 2— 6 , 4498 Feet. Lithofacies F.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59

than the matrix. Insoluble sediment is visible along

styolites. Insoluble sediment was not as evident in the

nodules, but x-ray diffraction does not indicate that there

is any difference in the amount of insoluble sediment

between the matrix and the nodules. This may be due to

sampling bias, or it may be that the insoluble sediment is

present in the nodules, but more widely dispersed.

The origin of the nodular texture of carbonates has

been studied by many (Bathurst, 1975? Bissel and Barker;

1977; Hopkins, 1977; Mullins et al., 1980; Wanless, 1979).

There are several different origins of nodular carbonates.

They include a vadose zone origin, and a deep water origin.

Several models have been proposed to explain the origin of

the nodular texture in carbonates.

Most of the literature cites a deeper water envi

ronment (30 to 130 feet) for the origin of nodular car

bonates (Harrison, 1985; Mullins et al., 1980; Read, 1985).

Nodules formed in the deepest waters will differ from the

nodules formed in shallower waters. Several ideas have

been presented to explain the formation of nodules in

pelagic deeper waters (Bissel and Barker, 1977; Hopkins,

1977; Savrda and Bottjer, 1988; Scholle et al., 1983).

Some of these models suggest that glauconite and phosphate

should be present (Bathurst, 1975; Jenkyns, 1986) in

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pelagic deep water sediments. No glauconite or phosphate

has been found in the Lower Engadine. A deep water envi

ronment has been cited for the formation of nodular texture

of lithofacies F (Mesolella et al., 1974? Porcher, 1985b).

The models to be discussed are the Vadose Zone Model, the

Burrow Model, the Differential Compaction Model, the

Gravity Flow Model and the Current Model.

Vadose Model

A nodular texture develops in the vadose zone in arid

climates (Moore, 1989). There are many depositional tex

tures which are diagnostic of a vadose zone origin. They

include the development of a hardpan, and the presence of

a pisolitic chalky zone. These distinctive textures are

not present in lithofacies F. The vadose zone model for

the formation of nodules is therefore inappropriate for

lithofacies F.

Burrow Model

Two models (Savrda and Bottjer, 1988; and Scholle et

al., 1983) propose that the nodules are due to biogenic

activity by burrowing. Scholle et al. (1983) suggest that

nodules form when a burrowed sediment fails to develop into

a hardground. Where the currents are strong enough to win

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. now away fine sediment, the burrows are filled with calc-

arenite. If the deposition is slow, the sediment is ex

posed at the sediment-water interface for a long period of

time and calcarenite is formed at the surface, creating

calcite nodules. If deposition of sediment is renewed soon

after the nodules are formed, the nodular texture is pre

served. If the currents are of sufficient strength to

prevent the renewed deposition of sediment, the nodules may

coalesce to form hardgrounds.

Savrda and Bottjer (1988) suggest that in anoxic sed

iments, concretions develop in the less compacted crests

of wave forms. The sediment then undergoes a shift in re

dox conditions, oxygenating the sediment. The sediment is

subsequently burrowed extensively except where the early

concretions were formed. Compaction .of the sediment

further enhances the development of the nodular texture.

Burrows have not been recognized in lithofacies F.

This precludes the use of the models suggested by Savrda

and Bottjer (1988) and by Scholle et al. (1983), but por

tions of both models are used in other models which will

be discussed.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62

Differential Compaction Model

Another model suggests that a nodular texture develops

by differential compaction between the nodules and the

matrix (McCrossan, 1958; Porcher, 1985a, 1985b). Com

paction was also suggested in the model previously

discussed by Savrda and Bottjer (1988). In the dif

ferential compaction model of nodule formation, interbedded

lime and clay are initially deposited. The lime becomes

lithified before the clay, and due to differential loading

and plastic deformation of the clay, carbonate nodules

form. Characteristics typical of this type of origin as

defined by McCrossan (1958) include coarser grained pinch

and swell features termed sedimentary boudinage, fine

cracks due to tensional forces, and gradations of carbonate

distribution ranging from undeformed laminations to scat

tered isolated nodules. This process was suggested by

Porcher (1985a and 1985b) to have been the mechanism by

which lithofacies F developed. Some of the features

described by McCrossan (1958), such as sedimentary

boudinage and fine cracks, are observable in the cores in

the study area. The apparent laminar orientation of most

of the nodules also support this model. A major setback

for this hypothesis is that it requires the deposition of

continuous clay layers, and there is no evidence in the

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rocks of this type of deposition.

Gravity Flow Model

A model to create carbonate nodules was proposed by

Hopkins (1977). This model is very similar to the one

presented by Mullins et al. (1980). It suggests that

nodules formed by the submarine cementation of thinly

bedded carbonates at the sediment-water interface. The

thin cemented beds were separated by uncemented carbonate.

Displacement of the thinly bedded carbonate down the slope

due to a sediment gravity flow would break up the cemented

beds, and mixing of the interclasts with the uncemented

sediment would create a breccia with a nodular texture.

Lithofacies F has no indications of displacement by gravity

flow, so this model is not the appropriate model to use for

the origin of nodules in the Lower Engadine.

Current Model

Another model of nodular carbonate formation has been

presented by Mullins et al. (1980) based on studies of

recent sediments. This theory relies on currents just as

does the model suggested by Scholle et al. (1983). In this

model, pelagic sediment is deposited in a deeper water en

vironment. Bottom currents winnow away finer sediment

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leaving a grain supported sand size sediment. This cleaner

sediment is cemented by amorphous or pelloidal high-Mg

cement. This cemented sediment is then broken up by strong

bottom currents. Bioturbation also helps to break up the

sediment and prevent hardgrounds from forming. A decrease

in the strength of the currents allows fine sediment to

filter in around the cemented interclasts. Continued bio

turbation mixes the sediment and interclasts.

Cementation of interclasts by pelloidal to sparry

high-Mg calcite creates the nodules during episodes of

higher current velocity, and fine mud filters between the

nodules to create a matrix during times of slow current

velocities. This nodular texture occurs deeper on the

carbonate platform where currents don't winnow away all the

fine sediment and produce hardgrounds, but they are strong

enough to winnow away much of the fine grained material and

prevent the development of soft oozes (Figure 12).

Many features needed to indicate this mode of for

mation have been obliterated during diagenesis, but a

faintly pelloidal texture is visible in thin section. The

nodules are made up of coarser grained carbonate than is

the surrounding matrix and may suggest that winnowing away

of fine sediment occurred during the formation of them.

The model of nodular carbonate formation presented by

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Mullins et al. (1980) best explains the features observable

in lithofacies F. This model suggests that the water

depths were probably deeper than the water depths in which

lithofacies E was deposited, thus a transgression was still

in effect during the Deposition of lithofacies F.

Lithofacies E is interbedded with lithofacies F

through most of the Lower Engadine. The interbedded nature

of lithofacies E and F suggest a quiet setting below normal

wave base with periodic large storms generating debris

which is washed down the carbonate slope and mixed with

detrital quartz to form debris lags. The nature of these

storm deposits vary across the platform. Their occurrence

and depositional structure depends on the topography of the

platform, the type of organic growth updip from the depo

sitional site, and the proximity to the land mass from

which the siliciclastic material is sourced, and the

strength of the storm event. Lithofacies E is prevalent at

the base of the Lower Engadine, which suggests that water

depths were increasing throughout the Lower Engadine,

requiring increasingly stronger storms to deposit the storm

debris. Lithofacies E and F are distinguishable in core,

and from petrophysical logs. Lithofacies F tends to have

a lower density than lithofacies E. This is probably due

to differences in porosity. These changes in density are

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readily distinguishable on litho-density logs. The gamma

ray log may be used to identify the fine wind blown silici

clastic sediment present in lithofacies E.

Upper Engadine Group

The upper Engadine Group is the most intensely studied

unit of the Niagaran System due to the presence of a reefal

facies from which large accumulations of hydrocarbons have

been extracted. Most research has centered on the reefs

(Droste and Shaver, 1977? Droste and Shaver, 1985; Jodry,

1969; Mantek, 1973; Mesolella et al., 1974). Some work has

been done on the interreef portions of the upper Engadine

(Porcher, 1985a, 1985b) so as to better understand the de

positional environment in which the reefs formed. The

upper Engadine Group is not the focus of this report, but

the environments of deposition were determined from core

and thin section data.

The upper Engadine Group has been subdivided into four

lithofacies which occur in the study area (Figure 3).

Lithofacies R is the reefal facies. Lithofacies D is the

lowermost interreef lithofacies. Lithofacies B2 overlies

lithofacies D, and lithofacies B1 is the uppermost litho

facies in the upper Engadine Group where it is present.

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Lithofacies R

Reefs and biohems occur throughout the Silurian in

the Michigan Basin (Fay and Copper, 1982; Sheldon, 1963?

Soderman and Carozzi, 1963), but the most studied in the

subsurface are those that occur in the upper Engadine.

These reefs have been designated as lithofacies R. Much

controversy surrounds the initiation and development of

lithofacies R, but many researchers suggest that litho

facies R development began at the onset of the deposition

of the upper Engadine (Mantek, 1973; Mesolella et al.,

1974). Lithofacies R is not the focus of this study, but

the presence of this facies may have influenced the depo

sition of other lithofacies. The presence of bioherms and

reefs suggests that water depths were becoming shallower,

allowing for the proliferation of reef dwelling organisms.

Lithofacies D

Lithofacies D has been termed a Mat chip dolomicrite

by Porcher (1985b). It is a Tan and light to medium gray

dolomite. It is very finely laminated, the laminations

caused by interbedding of fine grained and coarse grained

dolomite. Burrows are present, but there are no other

fossils. Detrital quartz and some detrital muscovite are

present. The most distinctive characteristic of litho-

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facies D is the presence of discrete thin bands of a dark

brown material which resembles organic material. These

bands are very short, and many are concave upward. These

features were suggested to be the remnants of mat chips

(Briggs et al., 1980; Porcher, 1985b.) and suggest shallow

water.

The environment of deposition of lithofacies D is of

the supratidal zone of a tidal flat (Shinn, 1983). Algae

proliferated during times of exposure forming mats. During

these periods of exposure the algal mats dried out and

shrinkage cracks developed. The edges of the curled up

ward, giving them a concave up shape. During storm events,

water washed in carbonate sediment which filled in the

shrinkage cracks, and stuck to the algal mats, covering

them. As the water receded, the algae grew over the

sediment, once again forming an algal mat, and the process

started over again. The laminations of lithofacies D are

very thin, suggesting that they were deposited at the

seaward edge of the supratidal zone (Shinn, 1983). The

presence of detrital quartz and muscovite suggests that

siliciclastic sediment was present on the landmass adjacent

to the tidal flat, and winds blew the siliciclastic sed

iment seaward.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69

The presence of lithofacies D immediately above the

deeper water lithofacies F suggest that a rapid regression

occurred at the end of deposition of lithofacies F. The

cause of this regression is unknown, but it may have been

due to a decrease in the subsidence rate of the Michigan

basin and an increase in the rate of carbonate platform

buildup.

Lithofacies B2

Lithofacies B2 is a bioturbated dolomicrite. It

directly overlies lithofacies D in the study area.

Lithofacies B2 is either not laminated, or it has very

indistinct laminations. Lithofacies B2 is highly bio

turbated. It has fossil fragments present. Lithofacies

B2 has birds eye fenestrae and vertical styolites. Birds

eye fenestrae are characteristic of supratidal environments

lacking algal or sedimentary laminations (Shinn, 1983).

Vertical styolites can form with the compaction of vertical

voids (Shinn, 1983) in supratidal and intertidal zones.

The presence of fossils suggest that this lithofacies was

deposited below water. These criteria suggest that litho

facies B2 was deposited in a nearshore intertidal envi

ronment. Variations in the characteristics of the litho

facies are due to fluctuations in tides. At times the area

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was completely inundated, and organisms were washed in. At

other times the area was dry, but most of the time the

environment was wave washed, so the most common organisms

to inhabit the environment was burrowing organisms. This

suggests that the water had gotten very slightly deeper

than it was during the deposition of lithofacies D.

Lithofacies B1

Lithofacies B1 is an ash bed. It overlies Lithofacies

B2 and is the uppermost lithofacies in the upper Engadine.

It is overlain by the A-0 Carbonate of the Salina Group.

Lithofacies B1 is a very thin green and brown shale. It is

only one or two inches thick, but it has a very distinctive

gamma ray signature. The distinctive gamma ray peak is

present in the entire study area, and perhaps across the

entire basin, except over reefs (Harrison, 1990) .

Thin sections of lithofacies B1 reveal that biotite,

quartz, zircon, and chlorite are present. The quartz is

often elongate and angular, with a felted texture. X-ray

diffraction of lithofacies B1 was done by Porcher (1985b),

and it revealed that quartz, potassium feldspar, Illite,

kaolinite and chlorite are present (Porcher, 1985b).

Lithofacies B1 is mentioned in the published liter

ature (Briggs et al., 1980; Mesolella et al., 1974), but

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has never been investigated. It was suggested that it

represented a break in carbonate deposition, possibly in

dicating exposure, but the source of the shale was never

explained. Based on the petrology of the unit, it was

determined that lithofacies B1 is a volcanic ash.

The source of the volcanic ash is not known at the

present time. The windblown ash was deposited across the

Michigan basin. In portions of the basin which were below

water, the ash settled to the sediment surface. It was

reworked by organisms to a small degree. In areas of high

energy levels such as would be found on a reef, the ash

would be reworked and washed away, preventing its depo

sition on the crests of the reefs. The period of time in

which lithofacies B1 was deposited was geologically instan

taneous, giving a good chronostratigraphic marker in the

Silurian of the Michigan Basin.

It was previously postulated that during the depo

sition of the Upper Engadine, the waters of the Michigan

basin were regressing. The presence of the ash bed may

suggest that the regression was in part due to global

tectonic events, and not entirely due to regional events.

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Depositional Setting

The Michigan Basin was a well developed basin during

the Silurian. The cratonic sea was located near the trop

ics (Droste and Shaver, 1985), and ideally situated for the

proliferation of carbonate secreting organisms. The inlet

for the waters in the Michigan Basin was in the southern

part of the basin (Figure 4). The waters brought in clas

tic sediment which was deposited in the southern portion

of the basin, precluding the deposition of carbonates. The

northern portion of the basin was essentially clastic free

and a carbonate bank developed during the deposition of the

Burnt Bluff Group.

The data obtained from cores, well cuttings, geo

physical logs, and outcrop studies suggest that by the

onset of the deposition of the Manistique Group, the seas

were transgressing over the Michigan Basin. The trans

gression initially drowned the carbonate platform, depo

siting a uniform blanket of deeper water carbonates across

the basin. Figure 17 shows the relatively uniform thick

ness of the Manistique Group. The margins of the basin

experienced several minor transgressions/regressions as sea

level fluctuated within the basin. These minor changes of

sea level did not affect the deposition of the sediment in

the study area. During the deposition of the Lower

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73

1— ------K u o raL tuaoa * o c u u co a ra a

ISOPACH MAP OF THE T24N t MANISTIQUE GROUP

BITK:1IVO T23N 6MLES 0 6MILES

LAKE MICHIGAN T22N

T21N R 18W /R .17 VR16W R15W*RV.WRi3w

T19N

T18N

T 17N

+ T15N

Figure 17. Isopach map of the Manistique Group.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Engadine, the seas were still transgressing, but at a

slower pace, so carbonate production kept pace with rising

sea levels. The carbonate bank which initially developed

during the Burnt Bluff Group deposition was reactivated.

The carbonate bank built up in the northern part of the

basin again, and clastic input had decreased so the bank

developed in the southern part of the basin as well.

The seas began to regress during the Upper Engadine.

Reefs developed when the waters shallowed up, and inter

tidal and supratidal deposits occurred as the shoreline

receded into the basin. Before the restriction of the

basin was great enough to allow for the Salina Group

evaporites to precipitate, a volcanic eruption spread a

layer of ash across the Michigan Basin. This ash bed was

preserved across the basin except over the tops of reefs,

and it is now a useful geochronostratigraphic marker bed.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV

DIAGENESIS

Manistique Group

The diagenetic sequence of events which occurred in

the Manistique affected both Lithofacies H and Lithofacies

I equally. Most differences in response to the diagenetic

sequence of events was due to differences in original depo-

sitional fabrics and constituents.

The diagenetic sequence of events which occurred in

the Manistique Group can be separated into ten events

(Table 2). A significant component in the diagenetic se

quence is a hydrothermal event which significantly in

fluenced the diagenetic history. The emplacement of salt

is the only diagenetic event to occur after the influence

of the hydrothermal event ceased. A detailed description

of each diagenetic event will give insight into the geo

logic history of the group.

Dolomitization I

The early diagenetic events occurred after the initial

deposition of the sediment, and before it was buried.

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These events have already been discussed in the depo-

sitional environments of the Manistique Group (Chapter

III).

Burial diagenesis includes all physical and chemical

events which have occurred since the sediment or rock was

buried to a great enough depth that surficial alteration

was no longer possible.

Table 2

Diagenetic Sequence of Events for the Manistique Group

1. Dolomitization of fine mud, aragonite and high-Mg calcite. 2. Formation of chert. 3. Dissolution of low-Mg calcite creating porosity. 4. Compaction of section forming styolites and fractures. 5. Dolomitization which enlarged porosity along fractures and styolites, and enlarged molds.

HYDROTHERMAL EVENT 6. Dissolution of chert creating additional porosity, and it's subsequent recrystallization as megaquartz and chalcedony. 7. Dolomitization forming large crystals of saddle dolomite. 8. Emplacement of hydrocarbons. 9. Precipitation of pyrite, anhydrite, and minor amounts of sphalerite.

10. Emplacement of salt effectively plugging any remaining porosity.

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Figure 18. W.S.C.C. #1-27. Plane Light, xlOO Magnification. Large Cloudy Centered, Clear Rimmed (CCCR) Dolomite Crystal.

The first of these events was the dolomitization of

the micritic portion of the sediment, any aragonite, and

high-Mg calcite. These sediments are more susceptible to

dolomitization (Sibley, 1982) than are coarser sediments or

those containing low-Mg calcite. There is no aragonite or

high-Mg calcite now present in the Manistique in the study

area, but their presence may be inferred by the presence of

cloudy-centered, clear—rimmed (CCCR) dolomite crystals

(Figure 18). This results from a concentration of in

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elusions, often calcite, in the center of the dolomite

crystals (Sibley, 1982).

The mechanism by which the sediment was dolomitized

is not readily identifiable in the Manistique Group.

Models of dolomitization that have been suggested for the

Michigan basin, are the Sabkha Model, the Normal Sea Water

Model, the "Dorag" Mixing Model, the Schizohaline Model,

and the Burial Dolomitization Model. The Sabkha model and

the Normal Sea Water Model do not adequately explain the

dolomitization of the Manistique Group.

Sabkha Model

The depositional environments of both Lithofacies H

and I were in deeper waters (Chapter III), along the basin-

ward edge of the shelf which rimmed the Michigan Basin, so

it is unlikely that they were ever exposed to the atmo

sphere. This depositional environment excludes the use of

the sabkha model to explain the dolomitization process.

It may be significant that the Manistique Group is dolo

mitized along the edges of the Michigan. Basin as seen in

the cores in Mason County, and in the outcrops, but remains

limestone toward the center of the basin (Willmet Thompson

#3-36 core in Osceola County).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79

Normal Sea Water Model

It has been suggested by Givens and Wilkinson (1987)

that dolomitization may occur simply from the movement of

large amounts of normal seawater through the sediment.

Seawater is the largest source of magnesium on the earth,

and may be the only fluid capable of large scale dolo

mitization (Givens and Wilkinson, 1987). The passage of

1000 pore volumes of normal seawater will supply enough

magnesium for dolomitization (Givens and Wilkinson, 1987).

Circulation of waters through the sediment in the basin

would dolomitize the sediment in a relatively short time.

If the Normal Seawater Model was the mechanism by which the

Manistique Group were dolomitized, limestone would not be

expected to be found in the Manistique Group. Any lime

stone that was present would be expected to be at the mar

gins of the basin, because the margins would have less sea

water flowing through the pore spaces of the sediment.

Limestone is present in the Manistique group, and it occurs

in the center of the basin, which does not conform to the

Normal Seawater Model as proposed by Givens and Wilkinson

(1987).

"Doraa11 Mixing Model

The "Dorag" mixing model (Badiozamani, 1973) and the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Seepage refluxion model (Adams and Rhodes, 1960) were

suggested by Sears and Lucia (1980) to be responsible for

the dolomitization of shelfward Upper Engadine Group pin

nacle reefs. The seepage refluxion model, or reflux model

as it is commonly called, occurs when hypersaline fluids

sink into the underlying sediment. Sears and Lucia (1980)

use the presence of evaporates above the Engadine Group as

evidence that conditions were arid and the necessary hyper

saline fluids could form. The Manistique Group has no beds

of evaporites overlying it, so dolomitization by reflux is

probably not a realistic model. The mixing model is pos

sible because mixing of seawater and fresh water along

basin margins is ubiquitous (Land, 1983). As suggested by

Sears and Lucia (1980), a lowering of the sea level oc

curred after deposition of the Engadine Group. As sea

level dropped, it would have exposed the tops of pinnacle

reefs. The shelf surrounding the Michigan Basin would also

have been exposed with the drop in sea level. Fresh water

would accumulate on the freshly exposed surfaces, forming

fresh water lenses. As sea level continued to drop, these

fresh water lenses would migrate down the pinnacle reefs,

and basin-ward along the shelf. The mixing zone between

the fresh water lens and the sea water would be brackish.

This water would tend to dissolve unstable phases and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reprecipitate stable phases of carbonate, especially

dolomite. In this brackish water, dolomite would form

because of the lower Mg/Ca ratio needed in brackish waters

(Folk and Land, 1975) for the formation of dolomite.

Hardie (1987) suggested that the amount of brackish

water present which has the chemical composition needed for

dolomitization is very small, so the Mixing Model may not

be a good model to explain wide-spread dolomitization.

Although there is no evidence of emergence of the

Manistique Group, dolomite is pervasive along the margins

of the basin, suggesting that fresh water lenses may have

occurred on land masses adjacent to the basin, and the

Mixing Model may be applicable to the Manistique Group in

the Michigan Basin.

Schizohaline Model

Folk and Land (1975) have suggested that a dilution

of sea water could occur in shallow waters due to influx

of fresh rain water from storms in what is called a

schizohaline environment. This environment is ideal for

dolomite to form because in a somewhat restricted envi

ronment, evaporation would make the normal seawater hyper

saline, with high Mg/Ca ratios. The fresh water from a

storm or hurricane would dilute the water without reducing

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the Mg/Ca ratio (Folk and Land, 1975). This would create

conditions ideal for the formation of dolomite which are

very similar to those suggested by the mixing model, but no

drop in sea level is necessary. The dolomite that would

form would be limpid euhedral rhombs. Dolomite of this

type is present in the Manistique Group, suggesting that

the schizohaline model may be responsible for the formation

of some of the dolomite. The entire Michigan Basin was

restricted, and may have had slightly hypersaline con

ditions. On the shelf, the waters would be somewhat shal

lower than in the basin center and so were more restricted,

and probably more saline. The shelf would also be more

susceptible to changes in salinity by the input of storm

waters than would be the deeper portions of the basin.

This explains why dolomite forms along the margin of the

basin in the Manistique Group, and limestone occurs in the

interior of the basin.

Burial Dolomitization Model

Another possible mechanism by which dolomite formed

in the Manistique is by burial dolomitization with a mass

transfer process such as the one suggested by Hardie

(1987). At 60°C, dolomite can form quickly (Hardie, 1987).

Fluid inclusion studies from Silurian pinnacle reefs reveal

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that temperatures in the Michigan Basin reached at least

80°C (Cercone and Lohmann, 1987) by the Jurassic.

In addition to obtaining the proper burial conditions

for dolomitization, magnesium must be supplied. The

magnesium needed can be obtained in a mass transfer process

whereby magnesium-rich basinal brines are circulated up

from the basin center through the carbonate bank by a

topographically induced hydraulic head (Hardie, 1987). The

magnesium is extracted from the brine as it passes through

the carbonate bank and combines with the limestone to form

dolomite. The increased burial temperatures and water

circulation pattern which brings the necessary magnesium

into the system would easily explain the dolomitization of

the Manistique Group in the study area. The deeper basin

sediment was not dolomitized because the hydraulic gradient

was not great enough in the basin center to circulate the

amount of fluid needed to obtain the magnesium needed for

dolomitization.

The most probable model of dolomitization of the

Manistique Group is the mixing model or the schizohaline

model. There is sufficient evidence to indicate that this

initial dolomitization occurred early in the diagenetic

sequence of events. Evidence for the timing of the dolo

mitization is the presence of euhedral dolomite crystals

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within the chert. This indicates that dolomite was present

before the formation of the chert. If calcite had still

been present during the formation of chert, and dolomitized

later, the crystals preserved would not have been dolomite

crystals. , There is no preservation of calcite crystals in

the chert in the four cores available in the study area.

The dolomitization process altered the micrite, ara

gonite, and high-Mg components of the carbonate sediment as

suggested by Sibley (1982) producing a variable crystal

size mosaic of dolomite. The high surface area of micrite

provides more nucleation sites than does coarser grained

sediments, thus allowing them to be preferentially dolo

mitized (Sibley and Gregg, 1987). Low-Mg calcite was not

affected, so fossils and other carbonate grains composed of

Low-Mg calcite were not altered to any great extent. The

abundance of fossils observed in chert and in the Willmet-

Thompson #3-36 core in which the Manistique Group was not

dolomitized, suggests that there were more fossils orig

inally deposited throughout the Manistique Group than are

now preserved.

The texture of the dolomite varied depending on the

initial characteristics of the sediment. Nonplanar uni-

modal dolomite (Sibley and Gregg, 1987) has commonly

replaced the lime mud. Planar-s and planar-e dolomite

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Sibley and Gregg, 1987) are both present. The planar-s

is often cloudy-centered, clear-rimmed. Planar-e dolomite

often occurs with abundant intercrystalline porosity, and

often occurs floating in chert, suggesting that it formed

before the formation of chert.

Chert

The second event in the paragenetic sequence was the

formation of chert (Table 2). Chert is a common mineral

associated with carbonates and has been described as such

since 1851 (Sellwood, 1986). In carbonates, chert is usu

ally identified as either nodular chert, or bedded chert.

These terms define the geometric appearance of the chert.

It has been suggested by Maliva and Siever (1989) that

nodular chert has formed by the replacement of carbonates,

and that bedded chert formed by the recrystallization of

siliceous oozes.

Silica Crystal Structure and Morphology

Silica occurs in many different phases and fabrics,

and can be identified by its crystal habit. It is sub

divided into biogenic silica, equant quartz, fibrous quartz

and microflamboyant quartz.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Biogenic silica is amorphous silica which is commonly

referred to as opal-A (Hein and Obradovid, 1989). Opal-

CT is composed of disordered cristobalite and tridymite

(Greenwood, 1979; Hein and Obradovid, 1989). These two

types of silica are found in recent sediments, and Opal-CT

is also found in the geologic record (Franks and Swineford,

1959).

Equant quartz comes in two varieties, microcrystalline

quartz and megaquartz. Microcrystalline quartz or micro

quartz, is chemically precipitated with individual crystals

less than 20 microns (Folk and Pittman, 1979). Cherts that

are Cretaceous or older are predominately composed of

microcrystalline quartz (Greenwood, 1979). If the crystal

size of the microquartz is below the resolution of an

optical microscope, it is referred to as cryptocrystalline

quartz (Hesse, 1989). Megaquartz is defined as quartz with

crystals larger than 20 microns (Hesse, 1989).

Fibrous quartz has been subdivided into chalcedony,

quartzine, lutecite and zebraic chalcedony (Figure 19).

All types of fibrous quartz are sometimes referred to

generically as chalcedony. More specifically, chalcedony

is the term used for the length fast variety of fibrous

quartz. It has an optically parallel extinction because

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FIBER ORIENTATION C-AXIS QUARTZINE

^ LUTECITE

■------CHALCEDONY

ZEBRAIC CHALCEDONY

Figure 19. Fibrous Quartz Varieties and the Orientation of Their Crystallographic C-Axes. From Hesse (1989), p.260.

the c axis is perpendicular to the fibers. It is the most

common type of fibrous quartz. It is sometimes also called

chalcedonite or length-fast chalcedony (LFC) to differ

entiate it from other types of fibrous quartz (Hattori,

1989). In this report, chalcedony shall be used to iden

tify the length-fast variety of fibrous quartz. The

differences in physical properties between chalcedony and

equant quartz can be attributed to the presence of crypto

fluid inclusions in the chalcedony. Chalcedony is usually

only a minor constituent of cherts, filling cavities that

are present (Greenwood, 1979).

Quartzine is a variety of fibrous quartz where the c

axis is parallel with the fibers. It is length slow and

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has been identified by Folk and Pittman (1979) to have

formed by replacing evaporites. Lutecite is a form of

fibrous quartz where the c axis is at 30° from the fibers

long axis. Zebraic chalcedony occurs in fibrous quartz

when the c axis rotates about the long axis of the quartz

fiber in a helix (Figure 19).

Microflamboyant quartz is a form of quartz which falls

between equant quartz and fibrous quartz (Hesse, 1989).

It is identified by an "undulose extinction caused by com

posite, fanning crystals whose individual crystal bound

aries are not clearly recognizable" (Hesse, 1989, p.260).

Silica in sediments may be detrital or authigenic.

All the above varieties of silica are authigenic. Only a

few of the types of authigenic quartz described are present

in the Manistique Group. Both varieties of equant quartz,

microcrystalline quartz and megaquartz are present. Micro

quartz and the variety identified as cryptocrystalline

quartz is the most common siliceous component in the

Manistique Group. The chert consists of mostly microquartz

with small percentages of other quartz types. Chalcedony

is the variety of fibrous quartz most common in the

Manistique. Quartzine was not identified in any of the

samples studied. Lutecite was observed, but only in very

small quantities. Zebraic chalcedony and microflamboyant

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quartz were not recognized in any of the samples from the

Manistique Group in the study area.

Sources of Silica

There are several possible sources of silica. These

include the direct precipitation of silica, volcaniclastic

sources and clastic sources, but the main source of silica

in Phanerozoic carbonates is presumed to be biogenic

(Hesse, 1989; Weis and Wasserburg, 1987).

Direct precipitation of silica may have been possible

during the Silurian because the oceanic concentration of

silica was probably much higher before the evolution of

diatoms (Noble and van Stempvoort, 1989). This direct pre

cipitation of silica would create beds of chert, not the

nodular texture seen in the Manistique Group. The presence

of fossils in the chert of the Manistique Group indicates

that direct precipitation was not the origin of the silica.

If it were suggested that the included fossils had fallen

into the siliceous gel as it precipitated, most of the fos

sils should still be composed of calci.te. In fact, most

of the fossils in the chert have been replaced by quartz,

and the few fossils that are still carbonate are composed

of dolomite, not calcite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Volcaniclastic sediments are not observed in the

Manistique Group. The only evidence, of volcaniclastic

sediment in the Silurian in the Michigan Basin are the

occurrence of distinct ash beds within the Burnt Bluff

Group and at the top of the Engadine Group (See Chapter

III, Lithofacies Bl). This does suggest that volca-

niclastics could have contributed to the silica within the

Manistique Group if devitrification of any available vol

canic glass had formed silica. In the known Silurian ash

beds biotite is present, and the presence of biotite to

indicate a former ash bed has not been identified in the

Manistique Group.

A clastic source for the silica in the Manistique

Group must be considered. The alteration of clay minerals

will release silica (Noble and van Stempvoort, 1989;

Siever, 1962). The amount of clay present in the Man

istique Group as determined from thin sections and x-ray

diffraction, is considered insufficient to be a major

source of silica. Detrital quartzose silt is present in

the Manistique Group. It is a possible source of silica.

Detrital quartz, however, is most abundant in Lithofacies

I, not Lithofacies H2 which contains the largest percentage

of the chert. The detrital quartz present in Lithofacies

H2 is present in both the dolomite and the chert. In

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. neither lithology does the detrital quartz show any evi

dence of chemical dissolution as would be expected if this

were a major source of silica. The solubility of equant

quartz is the lowest of all types of silica, whereas bio

genic silica is the most soluble (Hein and Obradovid,

1989). It is therefore suggested that detrital quartz is

a minor donor of silica.

The origin of nodular chert in carbonates is usually

postulated as the redistribution of biogenic silica during

diagenesis, with sponge spicules often cited as the source

of silica (Hesse, 1989; Maliva and Siever, 1989; Siever,

1962). Other silica secreting organisms such as diatoms,

radiolarians, and silicoflagellates (Hein and Obradovi*,

1989) contribute to the biogenic silica forming today. In

the Silurian, sponge spicules are the most commonly cited

source of biogenic silica. There may have been other sil

ica secreting organisms which filled the environmental

niches now occupied by any of the above mentioned modern

siliceous organisms. Sponge spicules have been identified

in the chert in the Manistique, with a normal optical

microscope and with a Scanning Electron Microscope (SEM)

(Figure 20). It is thus assumed that in the Manistique

Group, sponge spicules are the main source of silica from

which chert nodules formed.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92

Figure 20. Weinert #2-6, 4598 Feet. Lithofacies H2. Scanning Electron Microscope View of a Sponge Spicule.

Silica Replacement of Dolomite

The mechanisms by which the silica replaced the do

lomite must explain why the replacement was not uniform,

but irregular, with nodules of chert separated from other

chert nodules by the dolomite matrix. It must explain why

fossils are preserved, often in great detail, in the chert,

and it must explain the presence of dolomite crystals found

in the chert. Various mechanisms have been proposed in an

attempt to explain the origin of chert in carbonates

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93

(Dietrich, Hobbs, and Lowry, 1979; Knauth, 1979? Noble and

van Stempvoort, 1989; Siever, 1962).

Four models from the literature can be examined for a

possible explanation of silica-replaced carbonate in the

Manistique Group. The four models are: (1) the Organic

Matter Oxidation model (Siever, 1962), (2) the Mixing model

(Knauth, 1979), (3) the Hydrogen Sulfide Oxidation model

(Clayton, 1986? as discussed in Hesse, 1989, and Maliva and

Siever, 1989), and (4) the Force of Crystallization model

(Biggs, 1957; Maliva and Siever, 1988, 1989).

Orcranic Matter Oxidation Model. The organic matter

oxidation model was proposed by Siever (1962). He proposed

that the formation of chert is associated with organic

decay. Organic material is not uniformly scattered

throughout the sediment, but may be concentrated in

"clumps" or nodules. The decaying of the organic material

lowers the pH in its vicinity and begins to dissolve the

dolomite. The silica adsorbs on the organic matter,

lowering the solubility of silica and leaving the inter-

sticial waters near the organic matter deficient in silica.

In the sediment where there is no organic matter, the

waters are saturated with respect to silica. The dif

ferences in silica concentration between the different

zones causes a migration of silica from the saturated areas

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 94

to the areas undersaturated with respect to silica where

the silica precipitates as the dolomite dissolves (Siever,

1962). This process continues until all the organic matter

is oxidized, bound with silica, or the silica is depleted.

Siever (1962) suggested that this model would only

work in unconsolidated materials because it requires the

bacterial decay of the organic matter to lower the pH.

This requirement makes it difficult to utilize his model

of chert formation to explain the occurrence of chert in

the Manistique Group, because the Manistique Group was

lithified (as previously mentioned, the sediment was

already dolomitized) at the time of chert formation, hence

there could no longer be bacterial decay.

Thermal maturation of organic matter is known to

create organic acids, which would lower the pH, and allow

the dissolution of the dolomite (Surdam, Boese, and

Crossey, 1984) in the same manner as would bacterial decay.

The organic matter oxidation model by Siever (1962), may

then be used to explain the chert formation in the

Manistique Group by replacing the bacterial decay with

thermal maturation of the organic matter. This model

accounts for the discontinuous, non-uniform shape of the

chert because the chert will only form where there is

organic matter. The fossils are better preserved in the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. chert than they are in the dolomite because the dolomite is

more affected by further diagenesis than is the chert.

According to Biggs (1957), euhedral carbonate crystals

resist the silicification process more than do the anhedral

crystals, explaining the presence of euhedral dolomite

crystals in the chert. The main problem with this model is

timing. Thermal maturation of the organic matter to form

hydrocarbons probably did not occur until after the for

mation of chert, and petrographic evidence presented later

suggests that the formation of hydrocarbons did not occur

until late in the diagenetic history of the Manistique

Group.

Mixing Model. A second model for the formation of

chert is the mixing zone model. Knauth (1979) and Knauth

and Epstein (1976) have proposed the mixing zone model of

chert formation which is similar to the mixing zone model

for dolomite formation (Chapter III). In the studies of

isotopic ratios of microcrystalline chert of various ages

throughout the central and western United States it was

determined that much of the chert had meteoric water

signatures (Knauth and Epstein, 1976). Knauth (1979) used

this data to suggest a mixing model for cherts. He sug

gests that in the mixing zone, the waters are under

saturated with respect to calcite and aragonite, and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. supersaturated with respect to silica. When this condition

occurs, the silica will replace the limestone. Knauth

(1979) further suggests that the replacement of limestone

with silica occurs simultaneously with the replacement of

limestone with dolomite. This could produce dolomite crys

tals within the chert, if the dolomite is resistant to re

placement by silica. The mixing model as applied to cherts

cannot be fully assessed here because no isotope data was

available for comparison with the published observations of

Knauth (1979).

There are some features within the chert that are not

adequately explained with this model. The dolomite crys

tals in the chert are usually euhedral with corroded edges,

and they are most abundant near the chert-dolomite contact.

The mixing model would predict a random scattering of dolo

mite crystals, and if the dolomite is resistant to silica

replacement, the edges should not be corroded. There

should also be equal numbers of dolomite crystals in the

chert that are euhedral and anhedral, since the crystal

shape and size of the dolomite is dependant of the calcite

shape and size of the original sediment, and is independent

of the silica replacement. The chert contains mostly eu

hedral larger dolomite crystals, with very little anhedral

small dolomite crystals present. In addition, the mixing

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 97

model may explain the replacement of chert in beds, but it

does not account for the nodular texture of the chert in

the Manistique Group, nor does it explain the detailed

preservation of fossils and their internal structures.

Hydrogen Sulfide Oxidation Model. The third potential

model for the formation of chert in the Manistique Group is

the Hydrogen Sulfide Oxidation model proposed by Clayton

(1986). This model allows for the replacement of carbonate

by chert in marine waters, as opposed to the mixing model

which requires brackish waters. According to Clayton

(1986) pH is the determining factor in the precipitation of

carbonate, dissolution of carbonate, and the precipitation

of silica. PH changes needed for this model result from

bacterial sulfate reduction and the associated iron sulfide

precipitation (Hesse, 1989). Hydrogen sulfide is produced

by anaerobic bacterial sulfate reduction in sediments below

the oxic-anoxic boundary. The hydrogen sulfide combines

with iron to form iron sulfide (pyrite) which raises the

pH, and under the higher pH conditions, carbonate pre

cipitates. After the anaerobic bacteria had reduced all

available iron, the precipitation of iron sulfide and

carbonate would cease. A buildup of HS'and S2' would then

ensue, which would lower the pH of the system. The lower

pH would then allow the dissolution of the carbonate. The

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liberation of carbonate ions would then induce precip

itation of silica which was in solution (Maliva and Siever,

1989). This model can be used to explain the origin of

both chert nodules and bedded cherts. The Manistique Group

does have pyrite in its mineral assemblage, but petro

logical evidence suggests that the pyrite precipitated

after the precipitation of chert, as shown in Table 2. If

the carbonate was initially dissolved, then silica precip

itated into the voids, fossils would not be expected in the

chert, yet they are common. These inconsistencies with the

Hydrogen Sulfide Oxidation model suggests that it is not a

good model for the Manistique Group.

Force of Crystallization Model. The fourth model of

silica precipitation is the Force of Crystallization model.

This model has been proposed by Biggs (1957), and Maliva

and Siever (1988, 1989) for silica replacement of car

bonates. Maliva and Siever (1988) theorized that dia-

genetic replacement of dolomite by chert could be explained

by a controlled replacement force of crystallization

mechanism.

Biggs (1957) defined the force of crystallization as

the additional pressure which must be applied to a crystal

in order for it to be in equilibrium with its super

saturated surroundings. The definition of force of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. crystallization is " the expansive force of a crystal that

is forming within a solid medium. The force varies ac

cording to crystallographic direction." (Bates and Jackson,

1980, p.152). The force of crystallization may also be

defined as the pressure exerted by a crystal growing in a

supersaturated solution upon its surroundings (Maliva and

Siever, 1988). A developing crystal can dissolve and

replace another crystal if it has a greater force of

crystallization, and there are enough available ions for

the new crystal to continue growing (Biggs, 1957). The

force of crystallization of minerals generally increases

with increases of density, hardness and strength, and with

decreasing solubilities (Wardlaw, 1979). Quartz has a

greater force of crystallization than does dolomite, so

quartz replacement of dolomite is much more common than

dolomite replacement of quartz (Biggs, 1957). In the

Manistique Group, petrographic evidence strongly supports

the chert replacement of dolomite. There is no indication

of dolomite-replaced chert.

The controlled replacement by force of crystal

lization explains how authigenic minerals develop in

lithified sediments. Force of crystallization does not

require that the bulk pore fluid become supersaturated with

respect to silica and undersaturated with respect to dolo

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mite. Force of crystallization instead suggests that thin

solution films, probably only a few nanometers in thickness

(Maliva and Siever, 1988), occur between crystals. The

solution films are supersaturated with respect to silica

because of the dissolution of biogenic silica, most of

which is probably sponge spicules. If the sediment were

not lithified, the precipitating silica would push away

carbonate sediment instead of replacing it. In the

Manistique Group, the sediment had already been dolo-

mitized, thus was already lithified. A solution film

occurred between crystals in the lithified carbonate. The

silica began to crystallize at nucleation sites due to the

supersaturated state of the solution film. Nucleation

sites occur where the force of crystallization is the

highest. This occurs in kinks on irregular surfaces of the

substrate (Sibley, 1982). The silica precipitates at

nucleation sites more readily than it would precipitate

elsewhere. The number of nucleation sites increases with

increasing supersaturation, decreasing surface energy, and

decreasing crystal size of the substrate (Sibley, 1982).

The abundant number of quartz crystals in chert suggests

that nucleation sites were abundant in the sediment which

was replaced by chert.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Three controls of nucleation sites for chert in nod

ules were suggested by Maliva and Siever (1989). They are

the organic matter content, the porosity and permeability

of the sediment, and the biogenic silica content. Siever

(1962) suggested that organic matter is not uniformly

distributed throughout the sediment, but is concentrated in

"clumps" or nodules. The organic matter may have enhanced

the development of nucleation sites because of- the ir

regular surfaces which provide many kinks, and the tendency

for silica to adsorb to organic matter (Siever, 1962). The

numerous nucleation sites near the organic matter would

create ideal sites for the precipitation of silica, and the

"clumped" distribution of the organic matter would explain

the nodular texture seen in the cherts in the Manistique

Group. Maliva and Siever (1989) suggest that chert nodules

form in sediment with more permeability and porosity than

found in surrounding sediment. This is because the move

ment of the solution film probably occurs through molecular

diffusion, and diffusion would occur faster in sediment

which was more porous and permeable. The environment of

deposition suggested for Lithofacies H2 fits with this

control of silicification. Lithofacies H2 was suggested to

consist of porous carbonate nodules in a finer less porous

mud matrix. Indeed, the chert nodules appear to replace

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carbonate nodules (Figure 11), and these carbonate nodules

would have had the porosity and permeability to allow for

migration of the solution film. The third control on nu

cleation sites was the biogenic silica content. Maliva

(pers. comm., 1989) stated that chert is most abundant in

carbonates "deposited in low energy, aerobic, subtidal

marine environments that were presumably ecologically

preferred by siliceous sponges." Based on the inferred

depositional environment, sites of nucleation were probably

plentiful in Lithofacies H2.

At a nucleation site, once silica has begun to crys

tallize, it will continue to precipitate until all avail

able silica is used up, or until it contacts a surface with

a force of crystallization equal or greater than its own,

such as another quartz crystal which began at a different

nucleation site. The growth of a quartz crystal will apply

a stress on a dolomite crystal which is greater than the

force of crystallization of dolomite, causing the dolomite

crystal to dissolve (Maliva and Siever, 1988). The nodular

texture of the chert in the Manistique Group is due to the

non-uniform distribution of nucleation sites, and the

amount of dissolved silica available. Preservation of

features in the dolomite are possible because of this

crystal by crystal dissolution and precipitation. The

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great detail preserved in silicified fossil fragments,

including the preservation of internal structures, that are

present in Lithofacies H2 (Figure 11), are possible by a

force of crystallization process, but are not possible in

the other processes.

Several textural criteria have been given by Maliva

and Siever (1988) which help identify force of crystal

lization controlled replacement. The first is that the

material immediately adjacent to replaced zones should have

no evidence of dissolution and should look identical to

material away from the diagenetic replacement. In the

Manistique, the dolomite adjacent to the chert shows no

evidence of replacement (Figure 11). If the replacement of

dolomite by silica was due to bulk pore waters under

saturated with respect to dolomite, the dolomite would

dissolve, and the resulting voids would be filled in with

chert. The dolomite in the matrix adjacent to the area

where dolomite was replaced by silica would be expected to

show signs of dissolution. No such signs of dissolution

have been observed in any of the cores and thin sections

studied. The second textural criterion cited, is the

presence of ghosts of microstructural features formed in

the original matrix within the authigenic chert. This

indicates that dissolution and precipitation occurred

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104

simultaneously along thin solution films (Maliva and

Siever, 1988). The great detail preserved in the fossil

fragments (Figure 11) suggests that microtextural features

formed in the original sediment are preserved in the authi

genic silica. The third criterion suggested by Maliva and

Siever (1988) to suggest a force of crystallization re

placement is the presence of euhedral silica crystals in

planar contact with unreplaced dolomite crystals. This

criterion was not found with an optical microscope due to

the small size of the quartz crystals in the chert.

Scanning electron microscopy (SEM) was used to try to

identify this criterion. The size of the crystals of both

the carbonate and the quartz is so small however, that at

the magnifications needed to attempt to observe this

criterion, the difference in relief of the two minerals was

so great, that it was not possible to study interlocking

dolomite and quartz crystals at the contact to observe

their relationship with one another. It appears that the

force of crystallization model may best explain the pat

terns of dolomite and chert in the Manistique Group.

Dissolution of Low-Ma Calcite

The third event to occur in the diagenetic sequence

of events was the dissolution of low-Mg calcite (Table 2).

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Fossils composed of low-Mg calcite were resistant to the

dolomitization event and subsequent chert formation. The

chemical makeup of the interstitial fluids was eventually

altered sufficiently so the low-Mg calcite was no longer

stable. The primary low-Mg component of the carbonate was

fossils. As the low-Mg calcite dissolved, moldic porosity

was created. The low-Mg calcite fossils occurred both in

the dolomite, and in the chert, so the moldic porosity

enhanced porosity for both lithologies.

Compaction

The Manistique was buried deeper as younger sediment

was deposited over it. Increased pressure from the sed

iment being deposited on top of the Manistique Group,

increased temperature due to burial, and increased fluid

pressure caused the rock to respond to the stress. The

lithology and composition of the rock determined how it

would respond to the stresses exerted on it, so the re

sponses were varied. The two main responses to stress in

the Manistique was the development of styolites and frac

tures (Table 2).

Styolites in carbonates have been studied by several

researchers in an attempt to use them to understand the

paragenesis (Mossop, 1972; Nelson, 1981; Wanless, 1979).

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Styolites occur in carbonates which have non-carbonate

insoluble material, often clay or organic matter. A

styolite is a seam in which this insoluble material is

concentrated. The formation of styolites is a pressure-

solution phenomenon (Choquette and James, 1987). When the

stress applied to a dolomite crystal becomes greater than

the crystal can resist, it will begin to dissolve, because

the solubility increases with increasing pressure. The

dissolution occurs much in the same way as occurs with

force of crystallization, (i.e. along a very thin solution

film). The force exerted is not by a growing crystal, but

by a directed stress; in this case the overburden pressure

(Mossop, 1972). As dolomite crystals continue to dissolve,

STYOLITE

MICR0STY0UTE

Figure 21. Classification of Pressure Solution Seams (Modified From Mossop, 1972, p.265).

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insoluble residue which occurs in the crystals and between

the crystals are concentrated into seams.

Three basic seam types have been identified (Figure

21). The first is the styolite, or sutured seam. The

styolite is defined an insoluble seam between two inter

locking or interpenetrating units. The seam can be very

irregular with both large and small 'zig-zags' occurring

along the same seam. Styolites usually form in units which

structurally resist stress, and in which small amounts of

platy insoluble material occurs (Wanless, 1979). The

second seam type is the microstyolite or residual seam.

These seams are also called non-sutured seam and clay

seams. These occur in carbonates with abundant clay, other

platy insoluble material, and organic matter.

Microstyolites occur in both resistant and non-resistant

carbonates. As the seam develops, some stress is relieved

by slippage along the microstyolite on the platy clays and

other insoluble residues, thus inhibiting the development

of sutured seams (Wanless, 1979). The third type of seam

identified is the "horsetail" group or microstyolite swarm.

The "horsetail" group is defined as an aggregate of micro

styolites which converge into one thick residual seam

(Mossop, 1972). "Horsetail" groups develop in fine grained

carbonates in which large fossil fragments or intraclasts

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. which would have better stress resistance than the matrix

occur. In general, each different type of styolite seam

occurs in a distinct lithotype.

All three seam types are present in the Manistique

Group, but the microstyolites are the predominant type.

Wanless (1979) suggested that the nodular texture of

carbonates was due to dissolution along the non-sutured

seams, but it is more probable that the non-sutured seams

or microstyolites developed in response to the presence of

the nodules. As previously discussed, the nodules were

relatively free of insoluble residue from reworking, so

most insoluble residue occurred in the material surrounding

the nodules. Compaction probably enhanced the nodular

texture by selectively dissolving the less resistant

carbonate encircling the nodules, forming the micro

styolites which also emphasize the nodular texture.

Some of the microstyolites were observed to have

displacement along them, forming microfaults, and some were

observed to have fractures perpendicular to them (Figure

22). The fractures appeared to originate at the styolite

and occur on both sides of the styolite. They are thicker

nearest the styolite, and narrow away from the styolite.

The displacement or microfaults observed along some

microstyolites is readily explainable by the formation of

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Figure 22. Hedrick #1-26, 4786 Feet. Plane Light, x40 Magnification. Microstyolite With Perpendicular Fractures Extending out From the

microstyolites as discussed earlier. The slippage was a

response to the stress caused by overburden pressure along

platy insoluble materials which had accumulated along the

seam as dolomite was dissolved. These microfaults occurred

simultaneously with the development of the styolites and do

not represent later tectonic movement. The fractures are

also associated with the formation of the microstyolites.

These fractures are tension gashes (Nelson, 1981) and

develop as a response to the same stresses which caused the

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formation of the microstyolites. They parallel the maximum

stress direction during formation (Nelson, 1981). This

indicator of stress verifies that the stress was due to

overburden pressure because only horizontal styolites have

these tension gashes associated with them. Vertical and

angled styolites do occur in the dolomite, but they never

have associated tension gashes. The vertical and inclined

styolites are a result of tectonic origin (Mossop, 1979)

and may have occurred due to the increased rate of down-

warping of the Michigan Basin or horizontal stresses

associated with the Appalachian orogeny in the Devonian.

The styolites occur in the dolomite but not in the

chert. Styolites sometimes occur along the chert-dolomite

contact. In only one sample was a styolite observed which

appeared to cut through a chert nodule. Upon closer obser

vation, it was apparent that the styolite did not cut the

chert nodule, instead, the dissolution of the dolomite had

been extensive enough that all the dolomite between two

separate nodules had been removed, leaving only the re

sidual seam between the two nodules. In the same manner,

it can be postulated that microstyolites rim some chert

nodules because as the stress built up and dissolved away

dolomite, the seam came in contact with a chert nodule.

The silica reaction to the stress was different from the

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dolomite and it did not dissolve. The residual seam in

stead wrapped around the nodule dissolving the dolomite

around the nodule.

The main way the chert responded to the stresses

resulting from the overburden pressure, was to fracture

(Figure 23). The chert was more brittle and had a higher

resistance to pressure-solution than did the dolomite, thus

miller brothers

WEINERT # 2 -6

MASON CO., Ml

Figure 23. Fractures in the Chert, Some of Which Extend a Short Distance Into the Dolomite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. it did not deform plastically or dissolve. Most of the

fractures in the chert are vertical, and by inference with

the tension gashes in the dolomite, can be suggested to

have formed due to overburden stresses, not tectonic

stresses. The fractures are generally straight and smooth,

and sometimes extend into the dolomite (Figure 23). Eu-

hedral crystals line the fractures, indicating that they

are natural fractures. The fracture shape suggests that

each fracture occurred instantaneously in geologic time.

If the fractures had developed slowly, they would have

become irregular, shifting slightly as compaction in the

dolomite caused slight shifting in the sediment, which

would have affected the chert nodules. If the fractures

had developed slowly, it is improbable that they would have

extended into the dolomite, for as already seen, the dolo

mite reacted by pressure-solution and the subsequent dis

solution. The pressure must have built up within the chert

until a threshold was reached, at which point the chert

could no longer absorb the stress and it deformed in a

brittle manner; a fracture developed. The release of

stress was great enough that the fracture sometimes

extended into the dolomite.

Some chert nodules are not fractured, whereas others

are extensively fractured, suggesting that the pressure

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. exerted on the nodules varied, or that inhomogeneities

within the chert caused it to behave differently at dif

ferent locations. The absorption of stress within the

dolomite by dissolution varied with crystal size and per

centage of impurities (Moore, 1989). The chert would

absorb any stress not absorbed by the dolomite, therefore

small differences in pressure may have occurred between

chert nodules. The chert is not homogeneous. The dis

tribution of silicified fossils, moldic porosity, fluid

inclusions, dolomite crystals and possibly organic material

occur heterogeneously within the chert. These inhomo

geneities would alter the mechanical response to the

applied stress (Hobbs, Means and Williams, 1976), frac

turing in some places, and not in others.

Dolomitization II

A second stage of dolomitization occurred after or

possibly in conjunction with compaction. This event may

have been related to the dissolution of dolomite during

compaction. Neomorphic recrystallization occurred in the

dolomite, destroying much of the original fabric. The

dolomite matrix was neomorphosed to nonplanar and planar-s

dolomite (Sibley and Gregg, 1987). This texture may rep

resent the neomorphic recrystallization of dolomite at

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temperatures in excess of 50°C (Gregg and Sibley, 1984).

This temperature is within the burial temperature range for

the Manistique Group (Cercone and Lohmann, 1987). Fossils

were neomorphosed so that they can now only be recognized

by the clear, inclusion free dolomite which replaced them.

It is possible that the fossils were dissolved and the

voids then filled with dolomite, or that the dolomite was

neomorphosed. The method of replacement cannot be deter

mined, but the result is that fewer fossils are preserved

in the dolomite than in the chert.

The neomorphic recrystallization of the dolomite was

also responsible for creating the sucrosic textured dolo

mite. This sucrosic dolomite also completely destroys the

remaining original fabric of the rock, and the fabric cre

ated by the original dolomitization (Figure 24). This su

crosic texture is referred to as equigranular xenotopic

dolomite (Sibley, 1982), and Planar-e dolomite (Sibley and

Gregg, 1987). It has been suggested that the sucrosic tex

ture occurs as a replacement of low-Mg calcite (Sibley,

1982). The low-Mg calcite that was still present was neo

morphosed into sucrosic dolomite.

This dolomitization event created intercrys-

talline, vugular, and channel porosity. The intercrys

talline porosity formed between euhedral dolomite crystals

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Figure 24. W.S.C.C. #1-27, 5017 Feet. Plane Light, xlOO Magnification. Sucrosic Texture of Recrys tallized Dolomite.

in the patches of sucrosic dolomite (Figure 24). Fossil

molds were enlarged to form vugular porosity. The vugular

porosity is concentrated in Lithofacies H. Fluid migrated

along fractures, enlarging the fractures and creating chan

nel porosity. Styolites served as barriers for vertical

fluid migration. Fluids reaching a styolite traveled along

it creating channel porosity.

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Hydrothermal Event

At this point in the diagenetic sequence of events, a

hydrothermal event occurred in which the intersticial

fluids were heated to temperatures above predicted burial

temperatures. It is possible that the origin of the heated

fluid was from deeper in the basin. Subsurface fluids are

generally not static, they are flowing continuously through

the basin in response to hydrostatic gradients which are

dependant on temperatures, relief, and compaction (Anderson

and Macqueen, 1982). A gravity driven hydrologic fluid

flow of subsurface waters would force the waters from the

deepest parts of the basin up to the margins of the basin

(Garvin, 1985). The expulsion of waters from the deeper

portions of the basin carry the warmer water to the margins

(Cathles and Smith, 1983). As the Michigan Basin subsided,

the center of the basin was deeply buried under a thicker

package of sediment than was deposited at the margins.

Basin-centered subsidence place the Manistique Group in the

center of the basin at greater depths than it was in the

study area. The thick sequence of evaporites in the Salina

may have acted as an impermeable seal to subsurface waters

creating a hydraulic path in which the fluids flowed updip

within the Middle Silurian rocks from the basin center with

little vertical migration through the evaporites.

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Studies of the Upper Engadine Group pinnacle reefs

suggest that temperatures reached 80°C sometime between the

Mississippian and the Jurassic (Cercone and Lohmann, 1987).

The Manistique Group is at least 150 feet deeper than the

Upper Engadine Group, implying temperatures in excess of

80°C. This temperature is within the range of Mississippi

Valley-Type (MVT) mineralization (Anderson and Macqueen,

1982). MVT mineralization is related to the migration of

heated saline waters on the flanks of intracratonic basins

mostly through dolomites (Mazzullo, 1985). MVT mineral

ization fluids are probably the source of this hydrothermal

event.

The diagenesis related to the hydrothermal event are

listed in Table 2. The five events are given in para-

genetic order. The timing of these events is relative,

with emplacement of different mineral phases occurring

separately or together. Each event will be discussed in

the order in which they appear on Table 1.

Dissolution of Chert

The initial event to occur as the heated fluids

migrated through the Manistique Group was the dissolution

of chert. The solubility of silica rises with increased

temperature and pH (Siever, 1962). The pH of the heated

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waters may have been higher than the pH of the intersticial

fluid in place (Anderson, 1975). Just a small increase of

pH may have been enough to enhance the solubility of silica

when combined with the higher temperatures. Only a small

proportion of the total chert dissolved, leaving a tri-

politic texture, and greatly enhancing the intercrystalline

porosity of the chert. The dissolved silica migrated small

distances vertically and horizontally, precipitating as

megaquartz, chalcedony and small amounts of lutecite (Folk

and Pittman, 1979), in open fractures, in vugs and fossil

molds within Lithofacies H2 and Lithofacies I. Some minor

amounts of silica even migrated into the lower Engadine

Group. Megaquartz predominately occurs in fractures,

whereas molds and vugs are filled with all three varieties

of silica.

It is not fully understood why megaquartz precipitates

at one location, and chalcedony or lutecite at another, nor

why megaquartz preferentially precipitates in fractures,

but it may be due to the availability and size of nucle-

ation sites. Sibley (1982) has suggested that dolomite

nucleation is controlled by the abundance of nucleation

sites, the degree of supersaturation of the fluid, the

surface energy of the substrate, the crystal size of the

substrate, and the state of saturation of the replaced

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mineral. These same variables may be important in deter

mining the crystallographic form of silica. A larger the

number of nucleation sites will produce finer crystalline

silica. This explains why the pores are not filled with

microquartz or cryptocrystalline quartz? there are not

sufficient numbers of nucleation sites available.

Dolomitization III

The next diagenetic event to occur was a volumet-

rically minor precipitation of large sparry dolomite

crystals and saddle dolomite. The precipitation of large

sparry dolomite is a common occurrence in Mississippi

Valley-Type (MVT) settings. This late-stage dolomite

coprecipitates with other MVT minerals (Beales, 1975;

Mazzullo, 1986) and often, but not always, forms as saddle

dolomite.

Saddle dolomite is identified in hand samples by its

white, pearly, coarsely-crystalline habit (Figure 16),

often with distinctively warped faces. In thin section it

is most often identified by its undulose extinction. Sad

dle dolomite has been identified in small quantities,

lining vugs in the Manistique Group, from cores and thin

sections.

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Saddle dolomite is indicative of higher temperatures

and thermochemical sulfate reduction processes (Machel,

1987; Radke and Mathis, 1980), such as those associated

with MVT mineralization. Saddle dolomite often occurs as

void filling cements (Radke and Mathis, 1980). It is often

associated with pyrite, sphalerite, barite, celestite,

hydrocarbons, anhydrite and fluorite (Radke and Mathis,

1980). Many of these associated minerals have been

observed in the Manistique Group. Pyrite and anhydrite are

the most common, with minor amounts of sphalerite, celes

tite, hydrocarbon residue and fluorite also present. This

precipitation of dolomite is volumetrically insignificant

when compared to the previous dolomitization events, but is

important as an indicator of the passage of higher temper

ature fluids through the rocks. It is also useful as

evidence of MVT mineralization.

The sparry dolomite and saddle dolomite occludes

porosity, reducing the availability of pores to be filled

with hydrocarbons. Machel (1987) suggests that much of the

saddle dolomite which precipitates in hydrocarbon bearing

formations may actually occur below the oil-water contact,

and so may not be a significant factor in lost porosity in

hydrocarbon bearing zones.

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Hydrocarbons

The emplacement of hydrocarbons took place throughout

the hydrothermal event, but probably most of the hydro

carbon passed through the study area after the precip

itation of sparry dolomite and saddle dolomite. None of

the evidence such as seen by Machel (1987) which would

suggest that the saddle dolomite precipitated after hydro

carbon emplacement were observed. Most of the hydrocarbon

residual found in the Manistique cores occurs in Litho

facies H2, and is found in vugs. The hydrocarbon coats the

sparry dolomite and saddle dolomite crystals which line the

vugs. Some of the hydrocarbon occurs as fluid inclusions

within late void filling salt. The hydrocarbon probably

occurred as small scattered drops immersed in the hydro-

thermal fluid. As these drops passed through the rock,

they were trapped in structural traps, stratigraphic traps,

and chemically trapped by precipitation of salt, and

absorption. The accumulation of these hydrocarbon traps

into hydrocarbon reservoirs would occur if the trap was

efficient, and if the amount of hydrocarbon trapped was

sufficient. In most parts of the study area, the trapping

mechanisms was inefficient or to small to trap and hold

much oil or gas. It may be that there was no seal, or the

seal was not impermeable. All cores of Lithofacies H2

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had indications of hydrocarbons, so it is suggested that

enough hydrocarbons have passed through the study area to

accumulate into reservoirs where a trapping mechanism were

in place. The trapping mechanisms present in the

Manistique Group will be discussed in Chapter V of this

report.

MVT Mineralization

The precipitation of pyrite, anhydrite, and very small

amounts of sphalerite, celestite, and fluorite occurred

next. The amounts of these minerals were not sufficient to

classify them as MVT ore deposits. Their occurrence may be

a normal part of basin evolution (Anderson and Macqueen,

1982), and may be an integral part of the hydrodynamic

evolution of the basin (Mazzullo, 1986). In the Michigan

basin there was no major mechanism in which to concentrate

the metalliferous fluids in one part of the basin margin

(Cathles and Smith, 1983) to form a significant ore de

posit, thus small amounts of these minerals may occur along

the entire perimeter of the basin.

Pyrite is ubiquitous in the Manistique Group. It

occurs as finely disseminated particles throughout the

Manistique Group. Finely disseminated pyrite may have pre

cipitated early in the diagenetic history. Pyrite also

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occurs as large masses or large euhedral crystals, and

often line styolites. Pyrite crystals also line some open

vugs, and occurs as a crust lining anhydrite filled vugs.

The pyrite crystals in the dolomite often contain inclu

sions of dolomite, and pyrite crystals in chert have chert

inclusions, indicating that the pyrite is a replacive

mineral, not just a pore filling mineral. Large pyrite

crystals are known to be associated with MVT mineralization

deposits (Mazzullo, 1986).

Anhydrite is also present. It is a void filling and

replacive. It fills voids lined with large dolomite crys

tals and pyrite, indicating that it precipitated after they

did. Anhydrite often forms as radiating crystals which

penetrate the dolomite. This characteristic is due to

anhydrites displacive precipitation growth mechanism

(Maliva and Siever, 1988). The mechanism by which dis

placive precipitation occurs is that the growing anhydrite

"pushes" the dolomite aside instead of directly replacing

it. Anhydrite is not as common in chert as it is in dolo

mite, and when it does occur, it is exclusively void

filling.

Very small amounts of sphalerite, celestite, and

fluorite are also present in the Manistique Group. Their

presence helps to confirm the migration of heated fluids

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 124

and MVT mineralization in the Manistique Group. A few of

the sphalerite crystals were large enough to see in hand

samples. They grew in vugs in the chert. They were

banded, with the darker, more iron-rich zone precipitating

last. This banding of sphalerite suggests that the

sphalerite precipitated in distinct intervals, each

progressively more iron-rich, but with each interval

containing uniform chemical conditions (McLimans, Barnes

and Ohmoto, 1980). This interpretation of the banding

observed in the sphalerite crystals enforces the suggestion

that all diagenetic events associated with the hydrothermal

event occurred throughout the event. The celestite and

fluorite only were observed in such minute quantities that

their distribution cannot be determined.

Salt

The emplacement of salt was the last event in the

paragenesis of the Manistique (Table 2). The salt precip

itated into pores, effectively plugging most of the re

maining porosity. The reasons for the precipitation of

salt are unknown, but may have occurred due to the concen

tration of the salt in the brines becoming high enough that

precipitation occurred. It may also have been due to a

cooling of the fluid which would force the super-saturated

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brine to precipitate some of the salt. Fluid inclusions

are common in the salt, mostly inclusions of brine. There

are some hydrocarbon fluid inclusions, which suggests that

either hydrocarbon was still migrating through the rock

when the salt precipitated, or that ,the pore fluids had a

very low hydrocarbon saturation. In areas where hydro

carbon was trapped, the salt would not have precipitated,

as the pores were already filled. It is uncertain why the

salt didn't fill all the pore space, because there are

still some open pores. It may be that salt precipitation

is still occurring, that the saturation of the brines

decreased enough to prevent the precipitation of more salt.

It may be that all available pore space was filled with

salt, but a later dissolution event removed salt from some

pores.

Lower Engadine Group

The diagenesis of the Lower Engadine Group is very

similar to the diagenesis of the Manistique Group. Table

3 shows the diagenetic sequence of events which occurred

in the lower Engadine Group. The main differences in the

paragenesis of the Manistique Group and the Lower Engadine

Group is controlled by differences in the sediment which

had been deposited. This similarity in paragenesis sug-

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Table 3

Diagenetic Sequence of Events for the Lower Engadine

1. Dolomitization of fine mud, aragonite and high-Mg calcite. 2. Dissolution of low-Mg calcite creating porosity. 3. Compaction of section forming styolites. 4. Dolomitization which enlarged porosity along fractures and styolites, and enlarged molds.

HYDROTHERMAL EVENT 5. Precipitation of authigenic quartz as megaquartz and chalcedony in moldic pores and vugs. 6. Dolomitization forming large crystals of saddle dolomite. 7. Emplacement of hydrocarbons. 8. Precipitation of pyrite and anhydrite.

9. Emplacement of salt effectively plugging any remaining porosity.

gests that some of the diagenetic steps may have occurred

simultaneously.

Dolomitization I

Dolomitization was the earliest event in the para

genesis of the Lower Engadine Group. The dolomitization

occurred after the sediment was buried to depths at which

surficial processes such as bioturbation and reworking by

wave action had no affect. These surficial processes have

already been discussed in Chapter III. The process by

which the Lower Engadine Group was dolomitized is just as

uncertain as was the case for the Manistique Group. The

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mixing model (Badiozamani, 1973), the schizohaline model

(Folk and Land, 1975), or the burial dolomitization model

as presented by Hardie (1987) are the suggested potential

models for the early dolomitization event.

Burial Dolomitization Model

Hardie (1987) suggested that a burial dolomitization

model with a mass transfer process is an effective method

for dolomitizing thick sequences of limestone. At 60°C,

dolomite can form quickly (Hardie, 1987). Fluid inclusion

studies from Silurian pinnacle reefs reveal that temper

atures in the Michigan Basin reached at least 80°C (Cercone

and Lohmann, 1987) by the Jurassic.

Magnesium is needed for dolomitization. The magnesium

needed can be obtained in a mass transfer process whereby

magnesium-rich basinal brines are circulated up from the

basin center through the carbonate bank by a topograph

ically induced hydraulic head (Garvin, 1985, Hardie, 1987).

The magnesium is extracted from the brine as it passes

through the carbonate bank and combines with the limestone

to form dolomite.

The increased burial temperatures and water circu

lation pattern which brings the necessary magnesium into

the system would easily explain the dolomitization of the

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Lower Engadine Group in the study area. The deeper basin

sediment was not dolomitized because the hydraulic gradient

was not great enough in the basin center to circulate the

amount of fluid needed to obtain the magnesium needed for

dolomitization.

"Doraq” Mixing Model

The mixing model (Badiozamani, 1973) and the reflux

model (Adams and Rhodes, 1960) were suggested by Sears and

Lucia (1980) to be responsible for the dolomitization of

shelfward pinnacle reefs. The reflux model occurs when

hypersaline fluids sink into the underlying sediment.

Sears and Lucia (1980) use the presence of evaporates above

the Engadine Group as evidence that conditions were arid

and the necessary hypersaline fluids could form. They

found two different types of dolomite which suggested to

them that the dolomite must have formed by two mechanisms.

The Upper Engadine Group in interreef areas averages 10

feet thick, so presumably the reflux of hypersaline brines

could extend down into the Lower Engadine Group. The only

differences in the characteristics of the dolomite in the

Lower Engadine Group are due to the different lithofacies,

not depth from the top of the unit, so dolomitization by

reflux is probably not a viable model.

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The "Dorag" mixing model is possible for the Lower

Engadine Group. As suggested by Sears and Lucia (1980), a

lowering of the sea level occurred after deposition of the

Engadine Group. As sea level dropped, it would have ex

posed the tops of pinnacle reefs. The shelf surrounding

the Michigan Basin would also have been exposed with the

drop in sea level. Fresh water would accumulate on the

freshly exposed surfaces, forming fresh water lenses. As

sea level continued to drop, these fresh water lenses would

migrate down the pinnacle reefs, and basin-ward along the

shelf. The mixing zone between the fresh water lens and

the sea water would be brackish. This water would tend to

dissolve unstable phases and reprecipitate stable phases of

carbonate, especially dolomite. In this brackish water,

dolomite would form because lower Mg/Ca ratios are needed

in brackish waters (Folk and Land, 1975) for dolo

mitization.

Schizohaline Model

A problem exists with the '•Dorag" Mixing model when

attempting to use it to explain the dolomitization of the

Lower Engadine Group in the study area. There is no evi

dence of emergence of the lower Engadine Group, and yet the

dolomite is pervasive along the margins of the Basin. Folk

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and Land (1975) have suggested that a dilution of sea water

could occur in shallow waters due to influx of fresh rain

water from storms in what they call a schizohaline envi

ronment. This environment is ideal for dolomite to form

because in a somewhat restricted environment, evaporation

would make the normal seawater hyper saline, with high Mg/Ca

ratios. The fresh water from a storm or hurricane would

dilute the water without reducing the Mg/Ca ratio (Folk and

Land, 1975). This would create conditions ideal for the

formation of dolomite which are very similar to those sug

gested by the mixing model, but emergence of the shelf is

not necessary. The dolomite that would form would be

limpid euhedral rhombs. Dolomite of this type is present

in the Lower Engadine Group, suggesting that the mixing

model-schizohaline model may be possible. The Michigan

Basin was restricted during the deposition of the Engadine

Group, and may have had slightly hypersaline conditions.

On the shelf, the waters would be somewhat shallower than

in the basin center and so were more restricted. This

would have resulted in greater variability in the salinity

of the waters. The shelf would also be more susceptible to

changes in salinity by the input of storm waters than would

be the deeper portions of the basin. This explains why

dolomite forms along the margin of the basin in the

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Engadine Group, and limestone occurs in the interior of the

basin. The dolomitization process varied within the sed

iment depending on the original mineralogy and texture.

According to Sibley (1982), a carbonate sediment composed

of fine mud is more susceptible to dolomitization than are

coarser grained sediments. Aragonite and high-Mg calcite

is also more susceptible to dolomitization than is low-Mg

calcite.

The nodular carbonate of Lithofacies F may have been

originally cemented by a pelloidal high-Mg calcite (Mullins

et al., 1980). This cement would have been more easily

dolomitized. The intraclasts cemented by the pelloidal

high-Mg cement were coarse grained and could resist dolo

mitization. Lithofacies E contained fine lime mud admixed

with the fossil fragments and detrital silica, so dolo

mitization would proceed with relative ease due to higher

permeabilities.

Dissolution of Low-Mcr Calcite

The second event of the diagenetic sequence of events,

was the dissolution of low-Mg calcite (Table 3). Fossils

composed of low-Mg calcite were resistant to dolomi

tization. The chemical makeup of the intersticial fluids

had been sufficiently altered by the dolomitization

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process so the low-Mg calcite was no longer stable. As the

low-Mg calcite dissolved, moldic porosity was created.

Compaction

The lower Engadine Group was buried deeper as the

basin subsided and younger sediment was deposited. The

overburden pressure from the sediment deposited over the

lower Engadine Group created stress. The lithology and

composition of each lithofacies determined how it would

respond to the stresses exerted on it, so the responses

were varied. The main response to stress in the lower

Engadine Group was the development of styolites (Table 3).

Styolites in carbonates have been studied by re

searchers in an attempt to use them to understand the

paragenesis which occurred (Mossop, 1972; Nelson, 1981;

Wanless, 1979). Styolites occur in carbonates which have

non-carbonate insoluble material, often clay or organic

matter. A styolite is a seam in which this insoluble

material is concentrated. The formation of styolites is a

pressure-solution phenomenon (Choquette and James, 1987).

When the stress applied to a dolomite crystal becomes

greater than the crystal can resist, it will begin to

dissolve, because the solubility increases with increasing

pressure. The dissolution occurs along a very thin solu

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tion film (Mossop, 1972). As dolomite crystals continue to

dissolve, insoluble residue which occurs in the crystals

and between the crystals are concentrated into seams.

Three basic seam types have been identified (Figure

21). The first is the styolite, or sutured seam. The

styolite is defined an insoluble seam between two inter

locking or interpenetrating units. The seam can be very

irregular with both large and small "zig-zags" occurring

along the same seam. Styolites usually form in units which

are better able structurally, to resist stress, and in

which small amounts of platy insoluble material occurs

(Wanless, 1979). The second seam type is the microstyolite

or residual seam. These seams are also called non-sutured

seam and clay seams. These occur in carbonates with

abundant clay, other platy insoluble material, and organic

matter. Microstyolites occur in both resistant and non-

resistant carbonates. As the seam develops, some stress is

relieved by slippage along the microstyolite on the platy

clays and other insoluble residues, thus inhibiting the

development of sutured seams (Wanless, 1979). The third

type of seam identified is the horsetail group or micro

styolite swarm. The horsetail group is defined as an

aggregate of microstyolites which converge into one thick

residual seam (Mossop, 1972). Horsetail groups develop in

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fine grained carbonates in which large fossil fragments or

intraclasts which would have better stress resistance than

the matrix, occur. In general, each type of seam occurs in

distinct lithotypes.

All three seam types are present in the lower Engadine

Group, but the microstyolites are the predominant type.

Wanless (1979) suggested that the nodular texture of car

bonates was due to dissolution along the non-sutured seams,

but it is more probable that the non-sutured seams or

microstyolites developed in response to the presence of the

nodules. As previously discussed, the nodules were rela

tively free of insoluble residue from reworking, so most

insoluble residue occurred in the material surrounding the

nodules. Compaction probably enhanced the nodular texture

by selectively dissolving the less resistant carbonate

encircling the nodules, forming the microstyolites which

also emphasize the nodular texture.

Some of the microstyolites were observed to have

displacement along them, and some were observed to have

fractures perpendicular to them. The fractures appeared

to originate at the styolite and occur on both sides of the

styolite. They are thicker nearest the styolite, and nar

row away from the styolite. The displacement observed is

readily explainable by the formation of microstyolites as

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discussed earlier. The slippage was a response to the

stress caused by overburden pressure along platy insoluble

materials which had accumulated along the seam as dolomite

was dissolved. These microfaults occurred simultaneously

with the development of the styolites and do not represent

later tectonic movement. The fractures are also associated

with the formation of the microstyolites. These fractures

are tension gashes (Nelson, 1981) and develop as a response

to the same stresses which caused the formation of the

microstyolites. They parallel the maximum stress direction

during formation (Nelson, 1981). This indicator of stress

verifies that the stress was due to overburden pressure

because only horizontal styolites have these tension gashes

associated with them. Vertical and angled styolites do

occur in the dolomite, but they never have associated

tension gashes. The vertical and inclined styolites are a

result of tectonic origin (Mossop, 1979) and may have

occurred due to the increased rate of downwarping of the

Michigan Basin in the Devonian.

Dolomitization II

A second stage of dolomitization occurred after or

possibly in conjunction with the compaction of the section.

This event may have been related to the dissolution of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dolomite during compaction. The coarse grained calcite

still present in the nodules of Lithofacies F were dolo-

miti2ed with sucrosic and cloudy centered, clear rimmed

(CCCR) dolomite. This sucrosic and CCCR dolomite com

pletely destroyed the remaining original fabric of the

rock. This sucrosic texture is also referred to as equi-

granular xenotopic dolomite (Sibley, 1982), and Planar-e

dolomite (Sibley and Gregg, 1987). It has been suggested

that the sucrosic texture occurs as a replacement of low-

Mg calcite (Sibley, 1982). The biogenic low-Mg calcite was

already removed by this time, however, it is possible that

the nodules in Lithofacies F were still composed partially

of low-Mg calcite. Neomorphic recrystallization occurred

in the dolomite, destroying much of the preserved original

fabric. The dolomite matrix was neomorphosed to nonplanar

and planar-s dolomite (Sibley and Gregg, 1987). This tex

ture was suggested by Gregg and Sibley (1984) to represent

the neomorphic recrystallization of dolomite at temper

atures in excess of 50°C. This temperature is well within

possible boundaries for the lower Engadine in the sub

surface at the time of this event (Cercone and Lohmann,

1987). Fossils were neomorphosed so that they can now only

be recognized by the clear, inclusion free dolomite which

replaced them.

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The dolomitization event which occurred at this time

also enhanced porosity by creating intercrystalline poros

ity, vugular porosity, and creating channel porosity along

styolites. The intercrystalline porosity was formed be

tween euhedral dolomite crystals in the patches of sucrosic

dolomite which occur as noted above. In addition to this

porosity, fossil molds were enlarged to form vugular poros

ity (Figure 16). The vugular porosity is concentrated in

Lithofacies F. Styolites served as barriers for vertical

fluid migration, so when fluids reached a styolite, it

traveled along the styolite, creating channel porosity

along them. These channels don/t exclusively follow the

styolites, often they break away from the styolites,

cutting across the matrix.

Hydrothermal Event

At this point in the diagenetic sequence of events, a

hydrothermal event occurred in which the intersticial

fluids were heated to temperatures above what was ex

pected. The hydrothermal event is probably the same event

which affected the Manistique Group. It is possible that

the origin of the heated fluid was from deeper in the

basin. Subsurface fluids are generally not static, they

are flowing continuously through the basin in response to

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. hydrostatic gradients which are dependant on temperatures,

relief, and compaction (Anderson and Macqueen, 1982). A

gravity driven hydrologic fluid flow of subsurface waters

would force the waters from the deepest parts of the basin

up to the margins of the basin (Garvin, 1985). The expul

sion of waters from the deeper portions of the basin carry

the warmer water to the margins (Cathles and Smith, 1983).

As the Michigan Basin subsided, and the carbonate shelf no

longer built up, the center of the basin was deeply buried

under a thicker package of sediment than was deposited at

the margins. The subsidence of the Michigan Basin placed

the lower Engadine in the center of the basin at greater

depths than it was in the study area. The thick sequence

of evaporites in the Salina may have acted as an imper

meable seal to subsurface waters creating a hydraulic path

in which the fluids flowed updip within the Middle Silurian

rocks from which they originated with little vertical mi

gration through the evaporites.

Cercone and Lohmann (1987) did studies of the pinnacle

reefs of the upper Engadine Group and determined that the

temperatures reached 80°C sometime between the Mississippian

and the Jurassic. The temperature of the lower Engadine

Group during the hydrothermal event probably exceeded 80°C,

but was less than the temperatures achieved in the Man-

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istique Group.

The diagenetic events related to the hydrothermal

event are listed in Table 3. The five events are given in

a chronological order in which they appear to have oc

curred. It is suggested that these diagenetic events were

somewhat simultaneous, as one event took place, the other

events were also going on at a much smaller scale. Each

event will be discussed in the order in which they appear

on Table 3, but it must be remembered that they are not

discrete events, but part of the larger hydrologic event.

The hydrothermal event did not affect the lower Engadine

Group to as great an extent as it did the Manistique Group,

hence MVT mineralization did not occur.

Precipitation of Silica

Minor amounts of authigenic silica occurs in the lower

Engadine Group. It fills fossil molds, and occurs mostly

as chalcedony and Megaquartz (Figure 25). The detrital

quartz silt does not appear to have been dissolved to any

extent, so it is probably not the source for the silica.

The occurrence of the authigenic quartz is equally dis

tributed between Lithofacies E, F and G, suggesting that

the silica did not come from an internal source. It is

felt that the authigenic silica came from the Manistique

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 140

Figure 25. Weinert #2-6, 4587 Feet. Crossed Nichols, x40 Magnification. Megaquartz Filled Fossil Mold.

Group. During this hydrothermal event, some of the silica

which was dissolved from the chert in Lithofacies H2 mi

grated upwards into the lower Engadine Group, and precip

itated as megaquartz and chalcedony.

Dolomitization III

The next diagenetic event to occur was a minor dolo

mitization event in which large sparry dolomite crystals

and saddle dolomite precipitated. Saddle dolomite is iden

tified in hand samples by its white, pearly coarsely crys

talline habit, often with distinctively warped faces. In

thin section it is most often identified by its undulose

extinction (Figure 26). Saddle dolomite has been iden

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tified in small quantities, lining vugs in the lower

Engadine Group.

Saddle dolomite is a late stage dolomitization event

indicative of higher temperatures and thermochemical

sulfate reduction processes (Machel, 1987; Radke and

Figure 26. Hedrick #1-26, 4771 Feet. Crossed Nichols, x40 Magnification. Fossil Mold Filled in With Sad dle Dolomite Showing Undulose Extinction.

Mathis, 1980). Saddle dolomite often occurs as void

filling cements (Radke and Mathis, 1980) as seen in the

lower Engadine Group.

The precipitation of saddle dolomite and sparry dolo

mite is very minor when compared to the previous dolo

mitization events, but is important as another indicator of

the passage of higher temperature fluids through the rocks.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 142 The sparry dolomite and saddle dolomite occludes

porosity, reducing the availability of pores to be filled

with hydrocarbons. Machel (1987) suggests that much of the

saddle dolomite which precipitates in hydrocarbon bearing

formations may actually occur below the oil-water contact,

and so may not be a significant factor in lost porosity in

hydrocarbon bearing zones.

Hydrocarbons

The emplacement of hydrocarbons took place throughout

the hydrothermal event, but probably most of the hydro

carbon passed through the study area after the precip

itation of sparry dolomite and saddle dolomite. Most of

the hydrocarbon residual found in the lower Engadine Group

occurs in Lithofacies F. It is found in vugs and in inter

crystalline porosity between coarse CCCR dolomite. The

hydrocarbon coats the sparry dolomite and saddle dolomite

crystals which line the vugs. Some of the hydrocarbon

occurs as fluid inclusions within the salt (Figure 27),

suggesting that the hydrocarbon emplacement was occurring

while salt was precipitating. The hydrocarbon probably

occurred as small scattered drops immersed in the hydro-

thermal fluid. As these drops passed through the rock,

they were physically trapped in the pore spaces, and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 143

chemically trapped by precipitation of salt. The hydro

carbon would accumulate in structural traps and strati-

graphic traps. The accumulation of these hydrocarbon

traps into hydrocarbon reservoirs would occur if the trap

Figure 27. Weinert #2-6, 4498 Feet. Hydrocarbon Fluid Inclusions in Salt Which Fills a Dolomite and Pyrite lined vug.

was efficient, and if the amount of hydrocarbon trapped was

sufficient. In most parts of the study area, the trapping

mechanisms was inefficient and to small to trap much oil or

gas. All cores of Lithofacies F had indications of hydro

carbons, so it is suggested that enough hydrocarbons have

passed through the study area to accumulate into reservoirs

if the trapping mechanism were in place. The trapping

mechanisms present in the lower Engadine Group will be

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discussed in detail later in Chapter V.

Precipitation of Pyrite and Anhydrite

The precipitation of pyrite and anhydrite occurred

next. Their occurrence may be a normal part of basin

evolution (Anderson and Macqueen, 1982), and may be an

integral part of the hydrodynamic evolution of the basin

(Mazzullo, 1986). Pyrite is ubiquitous in the lower

Engadine Group. It occurs as finely disseminated particles

throughout Lithofacies E, F and G. It also occurs as large

masses or large euhedral crystals, and often line styolites

(Figure 28). Pyrite crystals also line some open vugs, and

k r

: ’*'1

V* ' „ K / >* : " . -’i*. i ’> & rt , 3 v . . A. -v -b ,- * f i ■ ■ - J-,s. * % _«■

. V V 'w,' .;. y:' t, ’- h \£r i:- ■*ilA *

Figure 28. Weinert #2-6, 4539 Feet. Crossed Nichols and Reflected Light, x40 Magnification. Large Masses of Pyrite and Small Pyrite Crystals Line Microstyolite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 145

occurs as a crust lining anhydrite filled vugs. The pyrite

crystals often contain inclusions of dolomite, indicating

that the pyrite is a replacive mineral, not just a pore

filling mineral.

Anhydrite is also present. It is a void filling and

replacive. It fills voids lined with large dolomite crys

tals and pyrite (Figure 29), indicating that it precip

itated after they did. Anhydrite often forms as radiating

Figure 29. Weinert #2-6, 4451 Feet. Crossed Nichols, x40 Magnification. Anhydrite Filled Void. Euhe- dral dolomite crystals line void, which is then filled with laths of anhydrite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 146

crystals (Figure 29) which seem to pierce the dolomite.

This characteristic is due to anhydrites displacive pre

cipitation growth mechanism (Maliva and Siever, 1988). The

mechanism by which displacive precipitation occurs is that

the growing anhydrite "pushes" the dolomite aside instead

of directly replacing it.

Salt

The emplacement of salt was the last event to affect

the lower Engadine Group (Table 3). The salt precipitated

into pores, effectively plugging most of the remaining

porosity. The reasons for the precipitation of salt are

unknown, but may have occurred due to the concentration of

the salt in the brines becoming so high that precipitation

occurred. It may also have been due to a cooling of the

fluid which would force the super-saturated brine to

precipitate out some of the salt.

Fluid inclusions are common in the salt, mostly

inclusions of a brine. There are some hydrocarbon fluid

inclusions, which suggests that hydrocarbon was still

migrating through the rock when the salt precipitated

Figure 27). In areas where hydrocarbon was trapped, the

salt would not have precipitated, as the pores were already

filled.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The salt did not fill all the pore space, because

there are open pores adjacent to salt filled pores. It may

be that salt precipitation is still occurring or that the

saturation of the brines decreased enough to prevent the

precipitation of more salt. It is also possible that

fluid flow was fast enough in the pores which remain open

to prevent the precipitation of the salt.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V

STRUCTURAL AND PALEOGEOGRAPHIC INTERPRETATION

OVERVIEW

The three county study area lies on the western edge

of the Michigan Basin (Figure 2). All of the structural

features in the study area developed in response to the

subsidence of the basin and horizontal forces due to the

tectonic events occurring along the eastern coast of the

United States. Subsidence of the Michigan Basin was

accelerated during the Silurian (Dorr and Eschman, 1970).

The sediment accumulation in the center of the basin during

the early and middle Silurian was much less than the accu

mulation around the margins because of the carbonate bank

development. Structure maps, isopach maps and cross-

sections across the study area suggest that the basin

margins are a complex of carbonate bank facies.

Structure maps were made of the Burnt Bluff Group,

Manistique Group, Lower Engadine Group and the Upper

Engadine Group (Figures 30 through 33). The basinward dip

averages 60'per mile for all units mapped. Slopes are

essentially equal for all the mapped units. This suggests

148

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 149

BURNT BLUFF GROUP T24N STRUCTURE MAP

T23N

ANI T22N

LAKE ■ MICHIGAN T21N R18W/R-17w 3W

T20N'

T19N

T18N

T17N

T16N

TUN

T-13N

Figure 30. Burnt Bluff Group Structure Map.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 150

SCALE >Ul£S C.i.noo'

T22N LAKE AN I ST'

MICHIGAN T21N R18W/R.17'

T20N

T19N

T18N

T17N

T16N

715N

TUN

T-13N

Figure 31. Manistique Group Structure map.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 151

s c ale T23N 6 miles 6 I U 3

LAKE

MICHIGAN

R18W,

T20N

T19N

T18N

T1 7N

T16N

T15N

'% gT14N

T-13N ,9°o

Figure 32. Lower Engadine Structure map.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 152

6WIES

MICHIGAN

Figure 33. Upper Engadine Structure map.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that post-depositional subsidence was relatively uniform.

Small local fluctuations in the dip are apparent on

all the structure maps. The Burnt Bluff Group structure

map Figure 30) is the smoothest because it contains the

least number of data points, and so reflects mostly the

regional dip. The upper Engadine Group structure map

(Figure 33) conversely contains the largest numbers of

anomalous dips and structures, because more data points are

available to locate the local, smaller structures. There

are not enough data points on the deeper structure maps to

determine if the Upper Engadine structures, especially the

reefs, are located on deeper anomalies.

The cross-section using the wells which penetrate the

Cabot Head Shale show that the depositional structures were

masked by subsequent deposition and subsidence (Figure 5).

The cross-section indicates that carbonate bank growth was

not uniform throughout the study area. During the depo

sition of the Burnt Bluff Group, a carbonate bank developed

in Manistee County which was 200 feet thicker than in

Oceana County.

Small patch reefs may have developed in the Burnt

Bluff Group along the carbonate bank in the study area,

much as they did in the outcrop area, but none have yet

been reported, although cores from Alpena and Otsego

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 154

counties suggest the presence of reefs. Small reefs in the

upper Engadine Group occur in Oceana County. The depo-

sitional conditions of Oceana County in the Burnt Bluff

Group may have been similar to the depositional conditions

in the Upper Engadine Group. During deposition of both the

Burnt Bluff Group and the Upper Engadine Group, a carbonate

bank was well developed (Figure 5), and biohermal devel

opment occurred. The differences in the size and shape of

the reefs which developed in the Burnt Bluff and Upper

Engadine Groups may have been due to differences in subsi

dence rate, climate, or suitability of the environment.

Reefs may occur in the Burnt Bluff Group in Oceana County,

although none have been identified from either samples or

petrophysical logs. The Manistique Group in Oceana County

is characterized by shale, which would create a seal over

any Burnt Bluff Group reefs which may have developed.

Manistique Group

The overlying Manistique Group blankets the Burnt

Bluff Group with a package of sediment which has an average

thickness of 120 feet (Figure 18). This relatively uniform

blanket of sediment suggests that the localized rapid

growth of the Burnt Bluff Group carbonate bank ceased. The

cessation of carbonate bank development was probably due to

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 155

rising seawater, which drowned the platform as discussed in

Chapter III.

Reefs are known to have developed in the Manistique

Group in the northern peninsula of Michigan (Ehlers and

Kesling, 1957; Sheldon, 1963). In the study area, the

water depths were probably too deep during the deposition

of the Manistique Group for reefs to develop. If they had

developed, they would have probably developed in Manistee

County, because the water depths would have been shallower

over the carbonate bank than anywhere else in the study

area. In the southern portion of the study area, the

clastic input would also have inhibited the growth of

reefs.

The isopach of the Manistique Group (Figure 18) in

dicates a prominent thinning of the Manistique Group in

Mason County. This thinning occurs at the contact between

lithofacies HI and lithofacies H2. This is due to depo-

sitional environment and compaction. At the contact of

lithofacies HI and lithofacies H2, the clastic source was

too distant to supply a sufficient amount of sediment, so

the shale deposition was slow and inconsistent. The water

depth during the deposition of the Manistique Group was

great enough to inhibit the growth of carbonate secreting

organisms, reducing the amount of carbonate which was de

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 156

posited. Although there are few data points in the saddle,

the contact between Lithofacies HI and Lithofacies H2

probably interfingers here. Compaction along the contact

could thin the zone even further. The saddle in Mason

county is a local feature which follows the contact of the

shaly lithofacies HI, and the cherty lithofacies H2. The

chert in the Manistique Group is limited in extent. There

is enough porosity in Lithofacies H2 to suggest that hydro

carbon reservoirs may be present. A structural or stra-

tigraphic trap would be needed to contain the hydrocarbons.

The structure map of the Manistique Group (Figure 31) sug

gests that a prominent high occurs in T19N of Mason County.

This structure is evident in part by the presence of many

data points, but it suggests that structures do occur in

the Manistique Group which could contain hydrocarbons. The

contact between Lithofacies HI and H2 forms a good strati-

graphic trap, especially if the contact is interfingered

tongues of each lithofacies.

The fracturing of the chert in the Manistique occurred

because of the brittle response to stress (Chapter IV).

Many of the fractures are vertical and several feet in

length. These larger fractures cut through the chert and

the dolomite, and often have mineralization along them.

Most of them are not completely cemented, creating conduits

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 157

for fluid migration. These large fractures may be related

to local faults. Seismic data was not available for study

to determine if such faults exist.

It has been suggested that the Michigan Basin de

veloped over, and in response to an ancient rift system

(Davies and Esch, 1988? Howell, 1988? Miall, 1984). Re

activation of movement along faults which developed during

the PreCambrian rift system would most easily relieve

stress buildup. This rift system lies just east of the

study area, so it is probable that reactivated older faults

cut through the Silurian in the study area. Reactivation

of these faults through the geologic history of the area

may have helped to define the boundary of the carbonate

bank buildup. The abrupt change in slope over a relatively

short distance (Figure 5) may be due to a fault in which

the carbonate bank grew on the upthrown side. They could

also have directly or indirectly caused part of the exten

sive fracturing seen in the Manistique Group cherts. Most

of the fractures in the Manistique Group cherts are ver

tical, and many have mineralization along their length.

These faults may serve as conduits for fluid migration, and

enhance reservoir quality by dissolution or dolomitization.

Seismic data could identify any such faults which occur in

the study area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 158

Lower Engadine Group

Growth of the carbonate bank in Manistee County re

sumed during the deposition of the Lower Engadine Group.

The thickness of the sediment in the carbonate bank is 175

feet thicker than it is in Oceana County. Very little ad

ditional regional build-up of the carbonate bank occurred

during the Upper Engadine Group. Most variations in thick

ness of the Upper Engadine Group are due to the presence of

pinnacle reefs which locally are much thicker than the sur

rounding sediment.

Figure 5 suggests that the carbonate bank was actively

growing during deposition of the Lower Engadine Group.

South of this bank, the debris accumulation may have been

great enough to create thick packages of Lithofacies F,

which could become hydrocarbon reservoirs if a trapping

mechanism was present. Lithofacies F has porosity devel

opment which increases its potential as a hydrocarbon res

ervoir. Large scale structures have not been identified

in the study area, but much of the area lacks the well

control needed to identify these structures.

Much of the porosity development which occurs in the

Manistique and Engadine Groups is due to environments of

deposition, but structural factors were important in cre

ating ideal conditions for the development of the different

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 159

environments, as well as enhancing and adding to the po

rosity with fractures. This suggests that the structural

features cannot be separated from the other components when

trying to understand the history of the Manistique and

Engadine Groups.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER VI

CONCLUSION

Middle Silurian stratigraphic nomenclature used in the

outcrop area of the northern peninsula of Michigan should

be used as the formal terminology for the rocks in the sub

surface of the Michigan Basin (Figure 1). Compared to ad

jacent states, the rocks in the northern peninsula of Mich

igan most closely resemble rocks in the basin subsurface.

The environments of deposition of the Manistique and

Lower Engadine Groups in Manistee, Mason and Oceana Coun

ties, suggest that during deposition, the sea which covered

the Michigan Basin was transgressing. The transgression

started during or at the end of the deposition of the un

derlying Burnt Bluff Group. Successively younger rocks

were deposited in deeper water environments because of the

transgression. The Manistique Group was deposited on the

drowned Burnt Bluff platform. The Lower Engadine Group was

deposited under conditions very similar to those of the

Manistique, however, the platform resumed its growth.

The digenesis of the Manistique Group and the Lower

Engadine Group are very similar. Minor differences in the

paragenesis are due to differences in original lithology.

160

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. . - 161

The features present in the Manistique and Lower Engadine

Groups such as compactional features, and hydrothermal

minerals such as sphalerite suggest that much of the dia-

genetic sequence of events occurred after burial. This

suggests that much of the digenesis occurred after the

deposition of the lower Engadine Group. The diagenetic

sequence of events have been summarized in Tables 2 and 3.

Digenesis has greatly altered the appearance of the

rocks, and affected the porosity. Porosity has been oblit

erated by cement filling of voids, and enhanced by disso

lution, dolomitization, and fracturing. A hydrothermal

event occurred, where super heated fluids passed through

the rocks. The hydrothermal event was probably responsible

for the emplacement of hydrocarbons. The petrographic data

cited in Chapter IV suggests that hydrocarbon emplacement

occurred coincident with the deposition of hydrothermally

deposited minerals.

Structural and stratigraphic traps may occur in the

three county study area in which hydrocarbon accumulations

could occur. Reefs are known to occur in the Upper Enga

dine Group in Manistee, Mason and Oceana Counties.

Bioherms occur in the Manistique Group in Ontario and the

northern peninsula of Michigan (Dorr and Eschman, 1970),

and they may also occur in the study area. Stratigraphic

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traps may have developed in the Manistique Group along the

lateral facies changes in lithofacies H. Cores in the

Manistique Group and the Lower Engadine Group indicate that

there is porosity development in the study area in litho

facies H2 and lithofacies F.

Hydrocarbon shows seen in the study area indicate that

there is potential for hydrocarbon accumulations which

could be exploited in Manistee, Mason and Oceana Counties.

Potential hydrocarbon accumulations could be expected in

lithofacies H2, possibly lithofacies H3, and in lithofacies

F. Specific conditions would have to occur to accumulate

hydrocarbons. Structural or stratigraphic traps are re

quired to trap hydrocarbons. A seal is required to retain

hydrocarbon in the trap. A pore system must be developed

in which the porosity and permeability of the rocks is

conducive to the accumulation and subsequent extraction of

an economically viable accumulation of hydrocarbons.

Lithofacies G, E, D, and B2 are sufficiently dense to

provide needed seals. Stratigraphic trapping may occur in

lithofacies H due to the lateral changes of lithology. The

data base is not large enough to determine if there are any

structural traps in lithofacies H or F. Porosity develop

ment is seen in lithofacies H2 and F in the cores and on

petrophysical logs. Permeability was not studied, but the

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nodular nature of lithofacies F suggests that permeability

between layers of porous nodules may be poor. Fracturing

of chert creates conduits between nodular beds in litho

facies H2, enhancing permeability.

There is still work which could be done to enhance the

understanding of the depositional environments, digenesis,

and hydrocarbon potential of the Manistique and Lower

Engadine Groups. Isotopic studies of fluid inclusions

would increase the knowledge of paleotemperatures and

paleoenvironments. Incorporating geophysical data into the

study would greatly improve the understanding of the struc

ture of the study area. Cathodoluminescence studies would

improve the understanding of the digenesis, and paleon-

tologic studies would improve correlations.

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