UPPER ORDOVICIAN STRATIGRAPHY OF
THE SOUTHERN PENINSULA OF MICHIGAN

Thesis for the Degree of M. S.
MICHIGAN STATE UNIVERSITY
ROY DAVID NURMI
1972

LIBRA?.Y

7 Leong —
‘UPPLEVENTAR
MATERIAL
IN BACK OF BOOK

ABSTRACT

UPPER ORDOVICIAN STRATIGRAPHY OF THE SOUTHERN PENINSULA OF MICHICAN
By

“

Roy David Nurmi

The Southern Peninsula of Michigan during the Late Ordovician was
the location of a shallow, slowly subsiding tasin centered in the
Ogemaw County area. The deposition of an eastwardly thickening, wedge
of dark gray to black shale, the Utica Shale, was followed by inter-
bedded limestones and shales. Lenses of red silty shales, Queenston
Formation tongues, interfingered with carbonates and gray shales of the
Michigan Basin. Maximum (Jpper Ordovician depths were reached during the
deposition of a dark, organic basinal limestone (Unit Four) which was
deposited slightly below wave base. Subsequent to the deposition of this
basinal limestone was a regressive cycle which culminated in erosion
along the southern margins of the basin and in supratida]. evaporite
deposition in the Upper Peninsula of Michigan. The close of the Late

Ordovician in Michigan was marked by a very shallow restricted basin.
UPPER ORDOVICIAN STRATIGRAPHY OF THE SOUTHERN PENINSULA OF MICHIGAN

By

Roy David Nurmi

A THESIS

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

MASTER OF SCIENCE

Department of Geology

1972
w
ee

I dedicate this work to Carol Ann Nurmi, my life, my wife.

ii
ACKNOWLEDGMENTS

The writer expresses appreciation to Dr. J. H. Fisher for sugsest-
ing this thesis problem and for aiding in its completion. Thanks are
extended to the other committee members, Dr. Sam B. Upchurch and Dr.

R. L. Anstey. Also, thanks to Mr. G. D. Ells and the Michigan Geological
Survey staff for discussions and for the accessibility of invaluable log
and sample data. The author is grateful to P, A, Catacosinos for discus-
sions and subsurface data relating to this investigation. Thanks are
expressed to Union 01] Company of California for furnishing well logs

from Ohio.

Lii
TABLE OF CONTENTS

Pare
INTRODUCTION , . . . . . . . . . . . 1
PURPOSE AND SCOPE . . . . . . . . . 1
ENVIRONMENTAL CONS] DERATIONS . . . . . . 3
METHODS e e e e e e e e e e e e u
LOCATION e e . . 5
PREVIOUS INVESTIGATIONS WITHIN THE MICHIGAN BASIN . 5
CORRELATION OF THE UPPER ORDOVICIAN WITH SCUTHERN
ONTARIO e e e e e e e e e 9
NORTHEAST TLILINOTS CORRELATIVES . . . . . . 11
UPPER PENINSULA CORRELATIVES . . . . . 12
MICHIGAN-CINCINNATI AREA CORRELATIONS . . . . 15
NORTHWEST ONTARIO CORRELATIVES . . . . . . 16
UPPER ORDOVICTAN ROCK-STRATIGRAPHIC UNITS . . . . 18
UNIT ONE . . . . . . : . . . . 18
UNTT TWO e e e e ° a e iJ e e e 20
UNIT THREE . : . . . . . . . . . 20
UNTT FOUR eo e e es e e e . s s e a1
UNIT FIVE, . . ; . . . . . . . 22
UNIT SIX e e e e e e e e s s e 26
UPPER ORDOVICIAN TECTONICS . . . . . . . . 28
DEPOSITIONAL ENVIRONMENTS . . . . : . . . 31
ORDOVICIAN ENVIRONMENTAL SETTING . . . . . 31
DEPOSITIONAL ENVIRONMENT OF UNTT ONE . . . . 33
DEPOSITIONAL ENVIRONMENT OF UNIT TWO . . . . 3a
DEPOSITIONAL ENVIRONMENT OF UNIT THRER . . . . 35
DEPOSITIONAL ENVIRONMENT OF UNIT FOUR. . . : 37
DEPOSITIONAL ENVIRONMENT OF UNIT FIVE. . . . 38
DEPOSITIONAL ENVIRONMENT OF UNIT SIX . . . . 38
CONCLUSIONS . : . ° . . . : . . : : 43
BIBLIOGRAPHY . . . . . : . : . . . . 45

iv
LIST OF FIGURES

Figure
1.- A representative Upper Crdovician sutsurface section
2.7 Location map e . e e e e e e ° e

3.- Southern Peninsula of Michigan Upper Crdovician
subdivisions ‘ . . . . . ° e °

4.- Regional Upper Ordovician correlations . . .

Page

3)

10
LIST Ck PLATES

Plate (In Pocket)
1.- Isopach Unit 1.
2.- Isopach Unit 2,
»- Isopach Unit 3.
4.~- Isopach Unit 4,
e~ Isopach Unit 5,
6,- Isopach Unit 6,
-~ Isopach Total Upper Ordovician.
8,- Cross section NW to SE Michigan.
9,- Cross section SW to basin center,

1N,- Cross section basin center to SE.

vi
INTRODUCTION

The Upper Ordovician shales and carbonates of the Michigan Basin
are in close stratigraphic proximity to the prolific oil producing
Trenton Formation and the gas producing Niagaran carbonates, but have
received little attention from the geologic community. The isopach of
the Upper Ordovician rocks (Plate 7) indicates that the Michigan Basin,
as we know it today, existed during the Upper Ordovician. The basin
seems to have been a rather shallow, stable basin centered slightly
northwest of Saginaw Bay. Following the deposition of the eastwardly
thickening wedge of dark gray to black shale, the Utica Shale (Unit One,
Plate 1), was a transgressive-regressive cycle which ended with erosion
on many of the arches which bounded the basin and with supratidal
evaporite deposition in the Upper Peninsula of Michigan. The Upper
Ordovician of the Michigan Basin has been subdivided into six region-

ally persistent rock-stratigraphic units (Figure 1).

PURPOSE AND SCOPE

Abundant subsurface data in the form of geophysical well logs,
sample logs, and a smal] number of well cores are available for the
lower half of the Southern Peninsula of Michigan. The northern edge of
the dense control roughly corresponds to township fourteen north, or
the southern shore of Saginaw Bay. North of this line only a small

number of wells have been drilled to Ordovician age rocks.
PURE #1 EMERY
MIDLAND COUNTY
SEC. 21, T 13N, R 1W

MANITOULIN FORMATION
j (SILURIAN)

(ORDOVICIAN)
UNIT SIX

UNIT FIVE

“ah bie
iG

t
i.
'

UNIT FOUR

wi lip abit petit

UNIT THREE

al

UNIT TWO

]

UNIT ONE (UTICA SHALE)

Ty

Lis

a
tot

rt matt a iw 7
te $ $
tt tte
to: i" : t i ae :
- - at. Sak aaa ot
¥. a ee
: : Pyar . ar erenaenas
bh —e rs —e ene. ee eee sige * . fe
a \ : » to:
~-:

et

| TRENTON FORMATION

Fig. 1.- A representative Upper Ordovician subsurface section.
3
The primary purpose of this study is to determine persistent,

regional rock~stratigraphic boundaries which may be used to subdivide
the Upper Ordovician and to evaluate the vertical and lateral relation-
ships of these units. The ultimate goals of this thesis are: 1), to
relate the rock-stratigraphic units of the Michigan Basin to adjacent
areas; 2), to determine the nature of the contacts of this group in the
Michigan Basin; 3), to discern environments of deposition and areas of
erosion during the Late Ordovician; and 4), to provide data with which
we might better interpret the structural history of the Michigan Basin

from Middle Ordovician to Early Silurian time.

ENVIRONMENTAL CONSIDERATIONS

Since the outlining of an environmental pattern should be the
ultimate objective of a stratigraphic analysis, a very cursory discus-
sion of the possible depositional environments of the Upper Ordovician
is contained in this study. Only a gross macro-environmental sequence is
considered. It is hoped that the general descriptions will encourage the
study of this much ignored group of rocks in the Michigan Basin.

The stratigraphic environmental interpretations of this study are
based on two important assumptions. The first is that there are no major
erosional breaks within the Upper Ordovician section between the upper-
most Utica shales and a pre-Manitoulin Formation unconformity. The
second, according to Walter's Law, is that “the pattern of juxtaposed
depositional environments of a geologic ‘instant* is reflected in the
stratigraphic column as a vertical sequence of rocks representing these
various environments" (Johnson and Friedman, page 462, 1969). It is

recognized that this is only an idealized model and that there will be
4
minor variations, of a local nature, to the broad pattern exhibited.

The placement of the purlished Upper Ordovician fossils and deposi-~
tional features into a sedimentary environment was accomplished using
those models described by Laporte (1969), Walker and Laporte (1970), and
Bretsky (1970). The publication by Walker and Laporte (1970) was em-
Ployed to a greater degree than any other because the formations they
studied were predominantly carbonates as are the Upper Ordovician strata

examined in this study,

METHODS

The greatest source of information for the study of the Upper
Ordovician rocks is the thousands of geophysical logs available for the
Southern Peninsula of Michigan. Virtually all of the geophysical logs of
wells which penetrated the Ordovician were studied and correlated during
the course of this investigation. Well sample descriptions were examined,
and samples were also checked for critical wells. Through the use of
samples and geophysical logs, the Upper Ordovician Group was subdivided
into six rock-stratigraphic units whose boundaries are identifiable
using both logs and samples. Further subdivision was accomplished for
local areas, but these units did not have the regional persistence of
the aforementioned six,

Initial studies were carried out in the Albion-Scipio Trend Area to
determine the existence of any persistent lithostratigraphic units, The
hundreds of wells drilled along “the Trend" made possible the correla-
tion of five persistent lithostratigraphic units for a distance of
approximately forty miles in a northwest-southeast direction. The dis-

tance between each well never exceeded one mile, and it was possible,
5
when necessary, to utilize wells spaced at distances of one thousand

feet or less,

LOCATION

The area of study is the Southern Peninsula of Michigan (Figure 2),
approximately 60,000 square miles. This area represents the central
portion of the present-day Michigan Basin, a major structural feature
of the North American Craton. The Michigan Basin, which has an area of
122,000 square miles (Ells, 1969), is bounded on the southern circun-
ference by a series of arches and on the northern circumference by

features of the Canadian Shield.

PREVIOUS INVESTIGATIONS WITHIN THE MICHIGAN BASIN

The only published investigations of the Michigan Basin Upper
Ordovician Group are those of Cohee (1947, 1948, 1948). These studies
were conducted as part of the United States Geological Survey's program
of oil and gas investigations, Cohee recognized three lithostratigraphic
units in southeastern Michigan, all three of which he characterized as
shales. Cohee (1948) states that he was unable to carry the boundaries
of these shales out of the southeast corner of the Michigan Basin, and
hence, subdivision of the Upper Ordovician was not completed for the
Major portion of the basin.

Regional studies of the Upper Ordovician Group have been completed
in many areas adjacent to the Michigan Basin: the Eastern Interior
region (Gutstadt, 1958), southwestern Ontario (Sanford, 1961), north-
eastern Illinois (Buschbach, 1964), and Iowa (Parker, 1971). Recent

conodont collections from the Upper Peninsula of Michigan should aid in
OF MICHIGAN

UPPER PENINSULA

UPPER ORDOVICIAN ony
DEPOSITIONAL CENTER
(s)
we
o
- *
‘oh
MICHIGAN at
(2) <
one J o

Fig. 2.- Location map.
7
the proper location of the Ordovician-Silurian boundary. The identifica-

tion of Eden age graptolites (Berry, 1970) from the Utica-Trenton
transitional beds indicates a more complete Upper Ordovician Series
than postulated by Hussey (1950, 1952) in his study of outcrops in the
Upper Peninsula. A core in Schoolcraft County (Ehlers et al., 1967)
records the only complete rock section of the Upper Ordovician available
in the Upper Peninsula, since only a small portion of the Upper Ordovi-
cian is exposed at outcrops. Three well cores which cut small portions
of the Upper Ordovician Group in the Southern Peninsula of Michigan have
also been described (Fauser, 1951; Coupal, 1954; and Wilson, 1955). In
1969 the Stratigraphic Committee of the Michigan Basin Geological Society
published a set of cross sections for the Southern Peninsula accompanied
by a publication describing many of the subsurface formations, The
Stratigraphic Committee (1969) listed a two-fold division of the Upper
Ordovician. The lower portion being the Utica Shale, a lithostratigraph-
ic unit, and an upper portion of undifferentiated shales and limestones
between the Utica Shale and the Manitoulin Formation. The outcrops of
the Upper Ordovician rocks on Manitoulin Island, Cntario, and the Bruce
Peninsula area of Ontario have been described by Liberty (1968, 1971
respectively). The Upper Ordovician section in the Cincinnati area has
been divided into rock-stratigraphic units by Brown and Lineback (1966).
Unfortunately, there are no published subsurface studies which help
bridge the area between the outcrops and the Michigan Basin.
Formational units in the Southern Peninsula of Michigan were
originally recognized by a mixture of biostratigraphic and lithostrati-
@raphic criteria. The term “Utica” was introduced in the Great Lakes

area by Ulrich (1888) who considered the Eden Shale, or Kope Formation,
MICH. BASIN GEOL. SOC.

COHEE, 1949 STRAT. COMMITTEE, 1969 THIS STUDY
S, EB. MICHIGAN
SOUTHERN PEN. OF MICH,
UNIT SIX
UNIT FIVE
UNDIFFERENTIATED
UPPER CINCINNATIAN UNIT FOUR
QUEENSTON SHALE
UNIT THREE
UNIT TWO

LORRAINE SHALE

UTICA SHALE

UTICA SHALE

UNIT ONE (UTICA SHALE)

Fig. 3.- Southern Peninsula of Michigan Upper Ordovician subdivisions.

9
to be identical to the New York Utica Shale. Outside of New York,

"Utica" as a formation name is no longer used, with the exception of the
Southern Peninsula of Michigan. Although Cohee (1948) maintained that
the Utica Shale was mainly “Utica” in age, he defined the unit, as well
as the other two shales comprising the Upper Ordovician, on the basis

of their lithologic characteristics. The Utica Shale was defined as a
dark gray to black shale which could not be separated from the overlying
Lorraine Shale west of Washtenaw and Lenawee Counties. Cohee believed
that the unit continues across the basin to the Upper Peninsula where it
is ten to twenty feet thick in the subsurface. Since Cohee‘'s work, the
upper boundary of the Utica Shale has been placed somewhat higher in the
shale section (M.B.G.S, Stratigraphic Committee, 1969) and includes
portions of Cohee’s (1948) Lorraine Shale. It is the Utica Formation top
defined by the M.B.G.S, Stratigraphic Committee (1969) which is used as
the upper contact of Unit One. The contact of Unit One with the underly-
ing Trenton is “one of the most clear-cut and reliable markers in the
basin on both logs and samples" (M.B.G.S, Stratigraphic Committee, 1969)
with the exception of the northern portion of the Southern Peninsula,

where the Utica Shale interfingers with the Trenton Formation.

CORRELATION OF THE UPPER ORDOVICIAN WITH SOUTHERN ONTARIO

Excellent subsurface correlations into southern Ontario, Canada,
are possible using Beards’ (1967) and Sanford's (1961) subsurface
stratigraphic studies. Beards (1967) tied his regional cross sections
into the Consumer's Power Company, Brine Disposal Well # 1 in St. Clair
County, Michigan. The subsurface Meaford-Dundas Formation plus the

Collingwood Formation are rock-stratigraphic equivalents of Unit One,
10

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11
or the Utica Shale. The upper contact of Unit One in this well is at

3,270 feet, and the lower contact at 3,547 feet; thus the unit has a
thickness of 277 feet. The section from 3,270 feet to the base of the
Manitoulin at 2,987 feet is considered to be the Queenston Formation by
Beards (1967). This section would include Units Two, Three, and a very
thin Unit Four, Units Five and Six thin to zero in a southwesterly
direction in southern St. Clair County. Unit Four, one of the best
developed carbonate units of the Upper Ordovician, thins to less than
ten feet in southern St. Clair County and grades into shale. The thin-
ning is due to a facies relationship with the Queenston shales rather
than an erosional thinning which is postulated for the absence of Units
Five and Six. The carbonates of Units Two, and especially Three, also
demonstrate a facies relationship, becoming more argillaceous and slight-
ly silty approaching Ontario. The thinning of Unit Four coincides with
an increase in red shale and a thickening of Unit Three towards Ontario.
Liberty (1971) describes the interfingering of the Queenston Forma-
tion with marine carbonates in the northern area of the Bruce Peninsula
of Ontario, The Queenston Formation is absent on Manitoulin Island. On
Manitoulin Island the Silurian Manitoulin Formation overlies the Upper

Ordovician biostromal and dolomite Kongawong Formation.

NORTHEAST ILLINOIS CORRELATIVES

The Maquoketa Group of Illinois, Wisconsin, and Iowa has been
divided into four formations (Bushbach, 1964) which can be correlated
with the Upper Ordovician of the southwest corner of Michigan. The
Scales Shale is the lithostratigraphic equivalent of Unit One, the Utica

Shale. The Fort Atkinson Dolomite is correlative with the carbonate
12
member of Unit Two. The Brainard Shale overlies the Fort Atkinson

Dolomite in northeast Illinois and is correlative with at least a por-
tion of the interbedded dolomites and green-gray shales of Unit Three
and the shale member of Unit Two. The Neda Formation consists of a few
feet of red, hematitic oolite-bearing shale. The lithology of the Neda
Formation is identical to that of Unit Six. This unit may have been
continuous with the Neda Formation across the southwest corner of
Michigan and later removed by Pre-Silurian erosion. The absence of Unit
Four in a southwesterly direction, if due to a facies relationship,
would make the division of Units Three and Five impossible to the west.
The Brainard Shale appears to be equivalent to Unit Three, Unit Five,
and the shale member of Unit Two. Both Units Three and Five consist of

interbedded, argillaceous dolomites and silty, greenish-~gray shales.

UPPER PENINSULA CORRELATIVES

The Upper Ordovician section of the Upper Peninsula of Michigan is
only partially exposed, and the age relationships of some formations
have recently been redefined. Eden age graptolites have been found in
the lowest shales of the Upper Ordovician (Berry, 1970). Ordovician

_conodonts (Pollock et al., 1970) were found in a shale identified by
Ehlers (1957) as the "Cabot Head Formation." If one accepts the above-
mentioned age determinations of Berry and Pollock et al., the Upper
Ordovician of Michigan would include a more complete Ordovician succes-
sion than Hussey (1952) recognized. Strata of Eden or Maysville age were
not noted by Hussey, He suggested that rocks of these ages might never
have been deposited in the Upper Peninsula or that they could have been

stripped away by erosion, Eden age biostratigraphic determinations of
13
the lowermost shales in the Upper Peninsula indicate that at least part

of the interval between the Trenton age and the Richmond is present.
Hussey's search for a major Upper Ordovician unconformity may be unnec-
essary.

The middle portion of the Bill's Creek Formation exposed at Delta
County outcrops is described as unfossiliferous (Hussey, 1950, 1952).
There are no descriptions of sedimentary features which might be of help
in the interpretation of its depositional environment, The linguloid
brachiopod, Leptobolus, has been recovered from the base of this unit
and also in the upper portion. An increasing water depth is indicated
by both fossil communities and sedimentological criteria for the transi-
tion from Unit One shales, the Bill's Creek Formation, into Unit Three
carbonates, the Stonington Formation. A high intertidal environment is
postulated for the upper portion of the Bill's Creek Formation,

The fossil communities of the Stonington Formation are analogous to

the Zygospira-Hebertella Community of the Upper Ordovician of the

Appalachians (Bretsky, 1970). As concluded by Bretsky, this community
represented inner and outer sublittoral environments on a broad, gently
sloping shelf. A low intertidal environment is recognized by Walker and
Laporte (1970) for some of these genera. An even deeper water environ-
ment is intimated by the communities of the carbonates of the Big Hill
Formation above the Stonington Formation. The fossil assemblage is
comparable to the subtidal communities described by Walker and Laporte
(1970) for the Middle Ordovician, Black River Group in New York. The
shales, lying stratigraphically higher than the Big Hill Formation, were
called the “Cabot Head Formation" by Ehlers and Kesling (1957). They

represent a return to very shallow water depths and, eventually,
14
supratidal and erosional conditions. The author is placing Ehlers’

"Cabot Head Formation” in the Upper Ordovician as a result of the strat-
igraphic relations reported in this investigation and the Ordovician
conodonts found in this unit by Pollock et al. (1970).

Ehlers and Kesling (1957) identified Leperditia in these shales as
well as described pebbles of gypsum and bedded gypsum. Berdan (1968)
maintained that Leperditia morphology suggests ostracod adaptations to
temporary subaerial exposure. The over-all evidence indicates shallower
water conditions continuing to supratidal evaporites. The evaporites are
overlain by the dolomitic Moss Lake Formation (Ehlers and Kesling, 1957)
which is the stratigraphic position of the Manitoulin Formation,

The Schoolcraft County core contains a similar sequence of deepen-
ing water depths upward from Unit One to a section containing lobate and
digitate bryozoan growth forms, indicating a neritic below wave base
environment. A return to shallower water conditions is postulated for a
section with corals, becoming shallower upward. The uppermost Ordovician
was deposited in a sabkha environment, which is typified by supratidal
evaporites containing Leperditia. Ehlers et al. (1967) described a
marine carbonate section above the supratidal unit which contains chert
nodules and reworked gypsum from the unit below. The Silurian-Ordovician
conodont boundary was placed within a few feet of this erosional inter-
face through conodont dating (Pollock et al., 1969).

The Bill's Creek Formation is considered correlative with Unit One
of this investigation. Unit Two is not clearly recognizable from
Hussey’s (1950, 1952) outerop descriptions. It is possible that the
persistent carbonate stringer at the top of the Bill*s Creek Formation

is the carbonate member of Unit Two, The erosional surface on the upper
15

surface of the persistent limestone unit may explain the absence of the
shale member of Unit Two, which denotes a deeper water environment. The
incompletely exposed Stonington Formation above the Bill's Creek is
correlative with Unit Three, The lobate to digitate bryozoan section
described by Ehlers et al. (1967) in the Schoolcraft County core, but
not described in Hussey's outcrop studies, is roughly correlative with
Unit Four. The section above this bryozoan unit is considered correla-
tive with Unit Five and would include the coralline section just above
the bryozoan unit and the remainder of the core including the supratidal
evaporites. The Big Hill Formation (Hussey, 1950) and the "Cabot Head
Formation" of Ehlers and Kesling (1957) are also correlative with Unit

Five. Unit Six is absent due to erosion or nondéposition,

MICHIGAN~CINCINNATI AREA CORRELATIONS

A regional cross section from Lenawee County, Michigan, to the
Cincinnati area was constructed to relate the outcrop studies of that
area to this investigation. The use of the Cincinnati area formations
in Michigan is impractical for the same reasons which caused Michigan
geologists to drop the formation criteria utilized by Cohee (1948). The
carbonate stringers dividing the Kope and Dillsboro Formations thin to
zero in a northward direction. Unit One is correlative to the Kope
Formation ("Eden") and the lower third of the Dillsboro Formation. The
base of the "Corryville” limestone is correlative with the limestone
member of Unit Two. A Dillsboro-Kope ("Maysville-Eden") contact can be
carried from the Cincinnati area to southern Monroe County. North of
Monroe County the shales equivalent to the Dillsboro and Kope ("Eden")

are no longer divided by a recognizable limestone. The carbonates of the
16

Whitewater Formation correlate approximately with the interbedded car-

bonates and shales of Unit Four in southcentral Michigan.

NORTHWEST ONTARIO CORRELATIVES

Upper Ordovician outcrops on Manitoulin Island, Ontario, are more
complete than those of the Upper Peninsula of Michigan. These outcrops
have been studied by geologists since the end of the nineteenth century.
Recently, Liberty (1968) divided the Upper Ordovician of Manitoulin
Island into lithostratigraphiec units which can in part be related to the
units of this investigation.

The basal shales of the Whitby Formation overlap the Trenton Forma-
tion correlatives and lie disconformably on the Precambrian (Caley, 1936;
Sproule, 1936). Erosion is indicated by the presence of breccias and
conglomerates of Precambrian quartzite and Trenton-equivalent limestone
in the black shales at the base of the Whitby Formation. The Whitby
Formation and the lower member of the Georgian Bay Formation are correl-
ative with Unit One and probably Unit Two also. The lowest 22 feet of
the Whitby contains a fossil assemblage similar to that found in the
basal and upper portions of the Bill's Creek Formation, the Unit One
correlative on the Upper Peninsula. The fossil assemblage includes:

species of Lingula, Leptobolus, Primitia, and two species of Serpulites.

The shales above this lower fossiliferous portion are described as
“sparsely fossiliferous, great thicknesses appearing to be barren"
(Liberty, 1954, p. 11). The more important species reported by Liberty
imcludes brachiopods found in the subtidal to intertidal range in Walker
and Laporte’s (1970) studies in New York. Bretsky (1970) placed these

brachiopods and the gastropod Lophospira abbreviata in the sublittoral

17
environment. These unfossiliferous shales are in turn overlain by a
shale section in which limestone stringers are present in its middle
portion. This interbedded shale and limestone sequence is the lower
member of the Georgian Bay Formation and correlates with the upper part
of Unit One in Presque Isle County, Michigan. The reported fossils of
the lower member of the Georgian Bay Formation include genera which have
been placed in the intertidal environment by Walker and Laporte (1970)
and Bretsky (1970) and also include genera placed in the subtidal envi-
ronment by these geologists. From the published reports, even a very
sketchy environmental sequence cannot be presented for the transition
from Unit One shales to Unit Three carbonate correlatives in the Mani-
toulin Island area. The upper member of the Georgian Bay Formation is in
part correlative with Unit Three. The fossils described from this forma-
tion are those of a subtidal environment. Correlation of biostromes with
units in the Southern Peninsula will have to await a detailed subsurface
analysis of the Manitoulin Island formations in a direction towards
northeastern Michigan, These biostromes may relate to Unit Four, but a
possible correlation with the transgressive carbonates of Unit Three
should be considered. To the south in St, Clair County, both of these

carbonate sections persist as tongues into the Queenston Formation,
UPPER ORDOVICIAN ROCK-STRATIGRAPHIC UNITS

UNIT ONE

In the area of study the lithology of Unit One is almost entirely
that of shale. The unit is thinnest along the western margin of the
Southern Peninsula, thickening in an eastward direction above St. Clair
County and in a southeastward direction below St. Clair County, forming
a clastic wedge (Plate 8). Unit One continues to thicken eastwardly
across Ontario (Sanford, 1961; Beards, 1967). The region of maximum
thickness in the Southern Peninsula is in Lenawee County (Plate 10)
where this unit is slightly thicker than four hundred feet. This thick-
ening persists into central Ohio. As this unit approaches a thickness of
four hundred feet in southeast Michigan, limestone stringers become
prominent in the middle of the unit.

Typically, the unit darkens in color from a gray or dark gray, with
occasional greenish tints in the upper portion, to a dark gray to black
in its lowest portion, This generalization holds true whether the unit
is two hundred or four hundred feet thick. In the area of Lenawee and
Counties, the entire unit becomes browner in color, but the generaliza-
tion of darkening still persists.

Anomalous areas of thinning and thickening of this unit (Plate 1)
appear to be closely related to structures which have been mapped using
the Trenton top as the structural datum, The thinning in the Howell

Region of northern Livingston County coincides with a structural feature

18
19
identified by Ells (1969). The Northville Anticline, a Trenton oil field

with structural closure (Checkley, 1968), also coincides with an anoma-
lously thin Unit One section, It is located in southwestern Oakland
County and in the adjacent areas of Washtenaw and Wayne Counties, Areas
of anomalously thick and thin Unit One sections in Washtenaw, Monroe,
and Lenawee Counties could also be explained by tectonic influences in
these areas (oral communication Fisher, 1972). "The Trend" area, evalu-
ated by Ells (1962), is an area of anomalous thickening of the Unit One
section which, due to the scale of the Unit One isopach map (Plate 1),
is not demonstrated. Ells (oral communication, 1972) believes that the
thickening was due to tectonic activity along "the Trend” rather than
the solution erosional feature described by Rooney (1966). The apparent
similarity of Ells (1962) Utica isopach map and the Trenton structure
contour map to an erosional valley could be reconsidered in light of a
Mariner 9 photo taken on January 12, 1972. This photo can be found on
the cover of the February issue of "Geotimes." The feature photographed
is described as “a vast chasm with branching canyons eroded in the
adjacent plateau lands appears in this view ...." "The resemblance to
dendritic stream patterns of Earth is probably superficial, for many of
_the ‘tributary’ canyons are closed depressions. Subsidence along lines
of weakness in the crust, and possibly deflation by wind, may have
produced the pattern." This writer refers to this martian feature not
for an analogy to "the Trend", but to point out that features of totally
different origins may look very similar, especially those studied only
in the subsurface.

The thickening of the Unit One shale in Schoolcraft County (Plate 8)

is probably best accounted for by a facies relationship with either the
20

Trenton below or the carbonates above.

UNIT TWO

Unit Two is a thin lithostratigraphic unit directly above Unit
One, and has an argillaceous limestone at the base and a gray to dark
gray shale above. The limestone unit thins towards the center of the
basin, and for this reason, it would be possible to miscorrelate the
top of the Unit One shale with the upper shale of Unit Two.

| In the Northville Anticline area there is a distinct thinning of
Unit Two towards the structure. The thinning is restricted to the shale
portion of this unit and appears to be due to a facies change of shale
to limestone.

The embayment of Unit Two in the Saginaw Bay area may be an indi-
cation of deeper water and the beginning of basinal development for the
Upper Ordovician. The fact that this unit is not thicker in the center
of the structural basin does not rule out the existence of deeper water
conditions in that area. The model of a sediment starved basin described
by Adams et al. (1951) should be considered for any environmental inter-
pretation of the Upper Ordovician shales. Starvation within the basin is
caused by deposition of the sediments shoreward in such a manner that

little deposition takes place in the basin center.

UNIT THREE

The lithostratigraphy of Unit Three represents a complex pattern of
lithologies which in future studies of a more local nature should be
subdivided. An acceptable regional subdivision was not feasible during

this investigation, but with a detailed environmental analysis, it is
2i
probable that the subdivision of this unit is possible. A very general

discussion of the lithologies and possible environments of deposition
is presented in this study.

Unit Three in the Monroe and Lenawee Counties area is predominant-
ly a gray to greenish-gray shale section with interbedded, gray argilla-
ceous limestones. The limestone stringers thicken northwardly into the
Michigan Basin (Plate 10),

In a southeastward direction away from the center of the basin and
towards St. Clair County, the limestones interfinger with red and
greenish-gray shales of the Queenston Formation of Ontario (Plate 8).
The reddish shales become siltier and thicken in a southeastward direc-
tion, |

In a southwestward direction the carbonates of Unit Three become
dolomitic, and there is also an increase in shale content (Plate 9). The
shale changes from gray shales at the basin center and Lenawee County
area to greenish-gray shales which become dolomitic in a westward direc-
tion, The dolomitization in the southwestern area of Michigan may repre-
sent either an early dolomitization, indicating shallower conditions,
and/or a later stage of dolomitization. It was not possible to determine

which is the case at this time,

UNIT FOUR

Unit Four is a massive, slightly argillaceous, dark brown limestone
in the basin center. By Folk's (1959) limestone classification, Unit
Four is a biomicrite, with bryozoans being the predominant fossil allo-
chem, In the center of the basin the unit is bounded by the fossilifer-

ous units Three and Five. In the Brazos-Sun-Superior, State-Foster # 1
22

well in Ogemaw County, the fossiliferous section below Unit Four
includes coral material, Associated with Unit Four are thin, black, very
carbonaceous fissile shales. Unit Four thins uniformly in all directions
(Plate 4) except south and southwest from the area of maximum thickness
at the center of the structural basin. A broad area of constant thick-
ness exists in the Allegan-Barry County area. This area is also anoma-
lous on isopachous maps of Units Five and Six (Plate 5, Plate 6
respectively). The greatest anomaly to the uniform thinning of Unit Four
is a region centered in western Lenawee County, In this region Unit Four
thickens to over ninety feet. The limestones are less massive, gray, and
much lighter than the dark brown limestones of Unit Four in the basin
center, This unit with a few carbonate stringers above persists well
into the red Queenston shales of Saniliac County and northern St. Clair

County (Plate 8).

UNIT FIVE

Unit Five is a sequence of interbedded carbonates and shales, The
carbonates are argillaceous limestones in the center of the basin and
grade into dolomites both vertically and laterally. The shales tend to
be a gray to gray-green color in the center of the basin and grade
laterally and vertically into green and red shales containing gypsum.
This unit thickens to a maximum of 248 feet in Ogemaw County and thins
to zero in southern St. Clair County and southwest Berrien County
(Plate 5). Approaching southern Ontario from northern St. Clair County
(Plate 8), there is a facies change from thin, gray carbonates and
shales to a lighter gray, slightly siltier, occasionally red shale se-

quence. The thinning may be a consequence of nondeposition to the east,
23
but the rapid thinning may be due in part to Late Ordovician or Early

Silurian erosion.

In three Southern Peninsula counties this unit has been cored; two
of these cores have been described (Coupal, 1954; Wilson, 1955). The
fossils identified by Coupal (1954) from the Sun 011 Company, Horning
# 1 well drilled in Washtenaw County appear to be of little help in
establishing a Silurian-Ordovician boundary. Unit Six is absent in this
area So we can expect the transition to be from the Manitoulin Formation
into Unit Five. By the log and sample criteria, the contact should be
placed at 3,193 feet. From the core description at this depth, there is
a change from light gray dolomite to dark greenish-gray shale with one
inch of black shale near the top. Brachiopods and bryozoans are locally
abundant in the 25 feet of greenish-gray shale below the Manitoulin
dolomite. The genera identified are common to both the Ordovician and
Lower Silurian. The dolomite stringers below this fossiliferous shale
become argillaceous and darker, ranging from dark brown to dark gray.
Within this section there is an increase in gypsum along with nodular
dolomite, an intraformational conglomerate, and occasional pebbles of
shale. Fossils are rare to poorly preserved in this lower shale sequence
from 3,218 to 3,240.5 feet.

Although a Silurian-Ordovician boundary would be picked at 3,193
feet by log and sample analysis, the regional consideration of Unit Five
might best be explained by an Ordovician or Unit Five top placed below
the fossiliferous shale and just above the more supratidal, and perhaps
erosional, section at 3,218 feet. A systemic boundary could be verified
by microfossil analysis as done in Indiana (Shaver, 1964) and various

Upper Peninsula areas (Pollock et al., 1970).
24

A core from Oakland County was of greater assistance in placing a

Silurian-Ordovician boundary by macrofossils, Wilson (1955) described
this core and the fossils present in it. Wilson located the boundary at
a sharp lithologic change from clean limestones and dolomites of the
Manitoulin Formation to the argillaceous limestones and dolomites of the
Upper Ordovician. Marking this boundary are green shale partings which

contain a specimen of Hebertella sinuata (Hall), an index fossil of the

Upper Ordovician. The 51.5 foot thickness of the Manitoulin Formation
fits in well with the thickness of the formation in this area. Unfortu-
nately, a geophysical log was not run on this well. All of the brachio-
pods collected below the Manitoulin Formation and identified by Wilson
have also been observed in the Richmond age rocks of Ontario.

The lack of any shallow water criteria, especially gypsym, is
understandable in that the isopach line convergence may indicate erosion
in the area of the well core (Plate 5). The truncation of the upper
portion of Unit Five represents rapid thinning; hence, a well which is
within this zone could be expected to have an incomplete sedimentary
cycle.

Other evidence of erosion of this unit, and Unit Six above, can be
found in the southwestern area of the Lower Peninsula. A pattern of thin
and thick areas mapped for this unit strongly suggests erosion (Plate 6).
The anomalous thicknesses of Unit Six correspond with thick zones of
Unit Five (Plate 5). Similarly, the thinning of the two units is related.
There is no evidence that the thickness variability is caused by struc-~
tural features below. Unit Four maintains a fairly uniform thickness
throughout the area in question, supporting the idea that the thinning

is related to a post-Unit Four event. Thinning by erosion subsequent to
25
the deposition of Unit Six explains the isopach patterns under consider-

ation, This conclusion is reinforced by studies of the Upper Ordovician
of northeast Illinois (Workman, 19503 Willman, 1971) which indicate
uplift and erosion following Upper Ordovician deposition.

Unit Six is highly radioactive on gamma-ray curves, The upper con-
tact is sharp and easily placed in areas where Unit Six is present. As
one nears the margins of the basin, where Unit Six is commonly absent,
Unit Five is overlain by the dolomites of the Manitoulin Formation. The
carbonate stringers of Unit Five in these marginal areas are usually
dolomite, which causes a problem in placing the Ordovician-Stlurian
boundary. It was noted during the course of this study that the basal
portion of the Manitoulin Formation often contains chert, which, if
regionally persistent, could be a valuable stratigraphic marker.

Tiny black gastropods (steinkerns) are frequently found in well
cuttings from Unit Five. Min Din (1950), in an examination of Ordovician
microfossils from four wells located in the Southern Peninsula, described
the steinkern genera Cyclora in two of these. He also identified the

gastropods Hormotoma gracilis and Maclurites magnus in the lower portion

of Unit Five. Ells (oral communication, 1972) of the Michigan Geological
Survey has found steinkerns in three additional wells, and this writer
has noted steinkerns in samples from thirteen more wells in the Southern
Peninsula. They are invariably found in Unit Five, although in a few
cases, they have also been reported from lower units. In the Upper
Peninsula outcrops, Hussey (1950) recovered Cyclora from all of the
Upper Ordovician formations with the exception of the coral biostrome,
the Bill‘s Hill Formation.

In the Treatise on Invertebrate Paleontology (Knight, 1960), the
26
geologic range for Cyclora is cited as the Upper Ordovician, Cyclora is

described as a “naticiform, very minute, probably based on steinkerns of
protoconchs of some other genus, possibly Cyclonema, locally very abun-
dant" (p. 1309). The range of Cyclonema is from the Middle Ordovician to
the Upper Silurian, The abundance of the steinkerns in Unit Five makes
them a potentially valuable stratigraphic marker for the Upper Ordovi-

cian, especially in areas where Unit Six is absent,

UNTT SIX

Unit Six has the smallest areal distribution of the Upper Ordovi-
cian units, This unit may have been physically continuous with the Neda
Formation of northeast Illinois and eastern Wisconsin but later removed
from marginal areas of the Michigan Basin, It attains its maximum thick-
ness in excess of 150 feet in Ogemaw County. This unit is dominantly one
of red shale in the southern portion of the basin with dolomite string-
ers increasing in a northward direction. In Newaygo, Allegan, and Mason
Counties, oolitic hematite has been recovered from Unit Six. The Turner ©
Petroleum Corporation, Bradley # 4 well in Newaygo County, cored
approximately two feet of Unit Six just below the Manitoulin Formation
limestones and dolomites, The Unit Six section in this core is a shale:
dark red, hematitic and very hard. It contained many small, flattened,
spheroidal shaped bodies, producing a texture referred to as flax seed
iron ore. Fauser (1951) identified three genera of Conularida in the
core and states that "Dr. G. W. Sinclair (personal communication)
informed the writer that he has never found the genera of Conularida
present in the core in any epoch above the Upper Ordovician" (p. 25).

Oolitic hematite was also reported in the samples of Unit Six from the
27
Superior 011 Company, Sippy # 17 well in Mason County and in a well from

Allegan County.
UPPER ORDOVICIAN TECTONICS

The total Upper Ordovician isopach map (Map 7) demonstrates a
Michigan Basin centered in Ogemaw County and displays a remarkable simi-
larity to the Trenton structure map. The Canadian Shield to the north
Was probably stable throughout the Upper Ordovician and provided little
terrigenous clastic sediment to the Michigan Basin. This stability is
particularly indicated by the uppermost portion of the Ordovician which
has a nondetrital facies for both Units Five and Six in the northern
half of the Southern Peninsula.

The basin is separated from Ontario by an area of thinning which
roughly parallels the center of Lake Huron (Plate 7) and which crosses
the western portion of Manitoulin Island in a northward direction. South
of Lake Huron this thin area is coincident with the St. Clair River and
Lake St. Clair, and it continues southwardly through the western tip of
Ontario, This arch is not coincident with the Algonquin Arch which has
been defined as a structural feature from Trenton structural studies in
Ontario. Although Cohee (1948) also mapped the thinning along the Lake
Huron median, he applied no name to it. This arch did not develop until
the period of deposition of Units Four, Five and Six. Erosion, or non-
deposition, is suggested by the rapid thinning of Units Five and Six in
the direction of Ontario. It should be noted that the Precambrian high
on Manitoulin Island is along this general isopach thin trend. The

incomplete Upper Ordovician sedimentary depositional sequence is present

28
29
on Manitoulin Island. The thinning may be related to a lack of deposi-
tion, a pre-Manitoulin Formation period of erosion, or a combination of
these two alternatives, Whichever the case, a feature affecting the
sedimentation is evidenced,

There is another area of Upper Ordovician thinning between the
Michigan Basin and a northwestern extension of the Appalachian Basin in
Lenawee and Hilisdale Counties. This feature indicates nondeposition
and/or erosion during the time of deposition of Unit Five through the
deposition of the Manitoulin Formation. This region of thinning between
the two basins has been described by Ells (1969) as the Washtenaw anti-
clinorium., He asserts that no significant amount of thinning occurred in
the Trenton-Black River rocks over the higher parts of the structures
located in this anticlinoriun.

Thinning of Units Three, Four and Five towards Lake Erie from the
southeast corner of Michigan could be a facies relationship to shallower
water sediments towards the east. Without any subsurface information for
the Upper Ordovician in northern Ohio, the thinning cannot alone be used
to identify the structural Findlay Arch, The thinning, however, is
towards the area where the Findlay Arch has been described. To the south
in the Cincinnati Arch area, erosion of the Upper Ordovician has been
shown by many studies, including a recent Silurian investigation by
O*Donnell (1972).

Erosion is suggested for the southwestern corner of the Southern
Peninsula approaching the Kankakee Arch. Studies by geologists in north-
eastern Illinois, which have been cited earlier, give additional proof
of erosion along the Kankakee Arch following the deposition of the Upper

Ordovician. Thinning of the units below the erosional surface has also
30
been demonstrated, indicating that this feature affected sedimentation

during the Upper Ordovician,

Over each of the above arches adjacent to the Michigan Basin,
erosion, or at least nondeposition, is shown to have taken place during
the time of Units Five and Six. Unit Four thins towards each of these
arches from the center of the basin, suggesting that the arches were
affecting sedimentation at the time of Unit Four deposition. The marked
thickening of Unit Four and the units above in the center of the basin
intimates that subsidence may have been more pronounced towards the end
of the Upper Ordovician. The water depth at the center of the basin was
probably never below wave base with the possible exception of Unit Four

which may represent low energy carbonate deposition just below wave base,
DEPOSITIONAL ENVIRONMENTS

ORDOVICIAN ENVIRONMENTAL SETTING

If paleomagnetic data are correct, the area which is now the north-
central United States was close to the equator during the Upper Ordovi-
cian (Dott and Batten, 1970). The Southern Peninsula of Michigan has
been placed on the western margin of the Upper Ordovician Appalachian
Mobile Belt which was filled by clastic sediment derived from an eastern
source area. The Upper Ordovician clastic sediment becomes finer in
grain size in a westerly and northerly direction from central Pennsylvan-
ia. During Richmondian time, a delta or a clastic wedge prograded in a
westward direction. These clastic sediments interfinger with marine
carbonates in the central Appalachian area (Thompson, 1969) and in Ontar-
io (Liberty, 1971). It is recognized that the Queenston Formation clas-
tics continue into Michigan in St. Clair, Macomb, Wayne, Sanilac, Lapeer,
and parts of Tuscola Counties. Although silt is common, very little sand
size detritus is present in the Queenston Formation of Ontario or in the
Clastic tongues in eastern Michigan. Since the strata evaluated in this
study includes the section between the Trenton Limestone and the Mani-
toulin Formation, a stratigraphic problem is encountered in central Ohio
and central Ontario. In these areas the Whirlpool Sandstone lies below
the Manitoulin-Brassfield Formation and above the Queenston Formation.
Although the Whirlpool Sandstone is considered Silurian in age, this

writer finds it plausible that this sandstone represents a final clastic

31
32
stage of deposition of the Queenston delta-clastic wedge. Knight (1969)

considered this sand unit to be a strand plain deposit in central Ohio,
and Martini (1971) also considered these sands to be littoral in north-
western New York and in the Niagara Peninsula.

The lower portion of the Upper Ordovician, Unit One (Utica Shale),
needs more study if the environmental history of the Ordovician in
Michigan is to be understood. The interbedded limestone stringers, which
are common in the southeastern counties of Michigan, in northern Ohio,
end in the Cincinnati area, pinchout northwardly and westwardly leaving
a thick, undifferentiated shale section. It is possible that this shale
section ranges from supratidal to subtidal below wave base as does the
predominantly carbonate section above, but due to reducing geochemical
conditions, suitable fossil criteria may be very sparse, If the Unit One
age sea was poorly oxygenated, perhaps as a result of decomposition of
vegetation, the sedimentary depositional and erosional criteria associ-
ated with varying kinetic energy zones may also be very different from
those applied in most geological studies today.

The contact of the Unit One shale with the carbonate section below
may be conformable in some areas, such as in the Cincinnati area (Anstey
and Fowler, 1969). The erosional surface recognized by Rooney (1966) at
the top of the Trenton Formation in Indiana may represent intertidal
erosion (Weiss, 1958) rather than a prolonged period of erosion, For a
long time it has been generally accepted that there are no Eden age
equivalents present to the north in the Upper Peninsula or to the west
in Towa, but recent paleontological work has shown this assumption to be
false (Berry, 1970; Herr, 1972). Graptolite finds, together with cono-

dont studies in the Upper Peninsula, wil] have to be evaluated very
33
carefully as they indicate the need for reconsideration of the present

Upper Ordovician geological history of the central United States.

DEPOSITIONAL ENVIRONMENT OF UNIT ONE

Unit One can be correlated with the Bill's Creek Shale of the Upper
Peninsula described by Hussey (1950, 1952). The lower portion of the
Bill's Creek Shale is unfossiliferous, with fossil content increasing
upward in the section. Unfortunately, nowhere on the Upper Peninsula is
there a complete section of the Bill's Creek Shale exposed. From the
faunal assemblage described, the exposed upper portion contains an
intertidal community ranging from high intertidal to low intertidal. The

most abundant fossils of this section are: Lingula and Leptobolus brachi-

opods; ostracode genera, including Tetradella and Primitia; the pelecy-

pod genus Cleidophorus; the annelid genus Cornulites; and a bryozoan,

Arthropora shafferi. The occasional occurrence of small dessication

features, "numerous water-worn specimens of Cleidophorus"” (Hussey, p. 45,

1952), and numerous mud lumps add to the interpretation of an intertidal
to near-shore environment for the upper portion of the shale section.

The top of the Bill's Creek Shale is marked by a persistent, fine-grained
argillaceous limestone member having a distinct erosional upper surface.
Immediately above this unconformity, there is a coarsely crystalline
argillaceous limestone with a basal conglomeratic unit. Hussey (1950)
states that innumerable water-worn fragments of Isotelus are found at

the base of the limestone above the unconformity. Although the section
above the Bill's Creek Shale is not completely exposed, the fossils
described by Hussey are of deeper water communities ranging perhaps from

the low intertidal community to high subtidal, The biota contains some
Be
anthozoans and numerous genera of bryozoan and brachiopods. The irregu-
lar surface and conglomeratic zone can be formed in littoral to sublit-
toral environments, so the "disconformity" need not be due to subaerial
erosion.

Ehlers et al. (1967) described a core from a well drilled approxi-
mately twenty miles from the type locality of the Bill's Creek Shale in
Schoolcraft County which is the only continuous rock record of the
Trenton-Utica transition in northern Michigan. Immediately above a 72
foot section of Trenton limestone, there is a conglomerate one and one
half inches thick overlain by a dolomite. On top of the dolomite is a
calcareous shale which in turn is overlain by a nine foot limestone.
Above this nine foot, fossiliferous limestone lies Unit One. The
linguloid brachiopod, Leptobolus, was recovered from the lower four feet
of this shale.

In placing this shale unit into a very shallow environment, the
stratigraphy of Manitoulin Island bears mentioning at this point. Unit
One lithostratigraphical equivalent shales rest locally on the Precan-
brian quartzites (Liberty, 1954). The absence of Trenton-like carbonates
4s most simply explained by regarding the shale to be of a shallower
depositional environment than the carbonates below. The fauna described
for the lowermost sequence of shale is similar to that of the uprer
portion of the Bill's Creek Shale. The fossils described include species

of Lingula, Leptobolus, the ostracode genus Primitia, and two species of

the annelid genus Serpulites.

DEPOSITIONAL ENVIRONMENT OF UNIT TWO

Throughout the Southern Peninsula of Michigan Unit Two is a
35
limestone-shale couplet, with the limestone being the lower member, In
the Northville Anticline areas the limestone percentage in this unit
increases while the shale content correspondingly decreases, The thick-
ness of Unit One below undergoes a marked decrease in the Northville
Anticline (Plate 1). Towards the center of the Michigan Basin the shale
content of the Unit Two couplet increases to the point that the correla-
tion of this unit becomes difficult. The shale content increases towards
the basin center, and its decrease over structures seems to indicate
that the shale represents deposition in a slightly deeper environment
than the limestone associated with it. A deeper environment for the
shale is supported by the fact that the shale of this couplet is also

lost in marginal directions.

DEPOSITIONAL ENVIRONMENT OF UNIT THREE

Unit Three represents a complex of depositional environments which
might be better understood if the unit was subdivided. In the center of
the basin the unit is predominantly one of fossiliferous limestones with
minor shale breaks. The shale content increases in most marginal areas
of the basin. In an eastward direction in St. Clair County, there is a
facies relationship between the more basinal limestones and gray shales
and the red and greenish-gray silty shales. The reddish terrigenous
section is part of the Queenston Formation of southwestern Ontario.
These reddish shales become siltier and more highly oxidized in a south-
eastern direction. Unit Three also thickens southeastwardly in this area
(Plate 3). These shales may represent prodelta terrigenous clastics of
the delta complex which prograded westward towards the Michigan Basin.

The limestone beds at the center of Unit Three extend well into the
36

eastern shales. They may be related to a rise in sea level rather than
subsidence of the Michigan Basin at this time. These transgressive
carbonates hold a cyclic position similar to the “Arnheim-Waynesville”™
of the Cincinnati area (Anstey and Fowler, 1969). In a southward direc-
tion towards the Cincinnati area, Unit Three becomes progressively
Shalier. In the Lenawee and Monroe Counties area, the unit is substan-
tially one of gray shales with only thin stringers of argillaceous
limestone,

Unit Three correlates with the Stonington Formation of the Upper
Peninsula. The fossil assemblage of this formation includes many of the

genera of Bretsky'’s (1970) Zygospira-Hebertella Community and Walker and

Laporte's (1970) low intertidal communities from the Black River Group
of New York. Bretsky concluded that this community represented inner and
outer sublittoral environments on a broad, gently sloping shelf. The
relative fossil abundance of Unit Three in the basin, as noted during
this investigation, would he consistent with a low intertidal through
high subtidal environment in consideration of the criteria of taxonomic
diversity utilized by Walker and Laporte (1970). Corals, brachiopods,
bryozoans, gastropods and ostracodes have all been identified in sample
descriptions of this unit. In the well samples examined for microfossils
by Din (1950), this unit has the greatest number of genera as well as
the greatest number of individuals. Most of the microfossils were ostra-
codes, which have received little environmental attention in studies of
the Ordovician. Laporte (1969) found ostracodes to be abundant in the
subtidal carbonate beds of the Devonian rocks of New York.

In Presque Isle County there is little increase in shale in Unit

Three aS compared to most marginal areas. This fact could be an
37
indication that Presque Isle County does not represent a depositionally

Marginal area, or that this area was receiving less terrigenous clastic
materia] than most marginal areas. The latter would be consistent with

environmental interpretations of younger Upper Ordovician units.

DEPOSITIONAL ENVIRONMENT OF UNIT FOUR

A rise in sea level, possibly due to subsidence of the structural
Michigan Basin, is considered for the depositional period of Unit Four.
There is much substantiating evidence for this conclusion. Perhaps the
most important evidence is the fact that the lateral carbonate equiva-
lents of this basinal limestone carry into the Queenston red shales.
Also, the dark coloring of the Unit Four limestones and the presence of
carbonaceous units may point to chemical conditions of a more reducing
nature during Unit Four deposition than for the deposition of the
associated carbonates. Good basinal geometry is displayed by the iso-
pachous map of this unit (Plate 4). The environment of deposition could
be viewed as shallow neritic below mean wave base. The correlation of
Unit Four from the wells drilled on Beaver Island, Charlevoix County, to
the core from Schoolcraft County of the Upper Peninsula also supports
the postulated sublittoral environment (Plate 8). Lobate and digitate
bryozoan growth forms in the correlative section are described by Ehlers
et al. (1967). Although Schopf (1969) states that the evidence is not
conclusive, he does indicate a neritic below wave base environment for
these growth forms. Lagaaij and Gautier (1965), however, found that the
zoarial form of ectoprocts in the Rhoéne delta area depends more on the

sedimentation rate and water turbulence than simply on water depth.
38

DEPOSITIONAL ENVIRONMENT OF UNIT FIVE

Unit Five in the center of the basin appears to be the reverse
sequence of Unit Three. This unit tends to be more fossiliferous than
Unit Four below. Like Unit Three it contains a variety of lithologies
deposited in environments ranging from subtidal to intertidal. Oolites
have been described for this unit from a well drilled in Ostego County.
The Unit Five correlative section in the Schoolcraft County core ranges
from a shallow subtidal coral section upward into supratidal evaporites.
A similar regressive sequence is considered for Unit Five through Unit
Six towards the center of the basin. Minor variations are probably
present throughout this sequence. In the northern half of the basin
there is less shale in the upper portion of Unit Five, which perhaps is
related to the distance from the source areas. Further discussion of the
depositional environment of Unit Five is included in the following

section,

DEPOSITIONAL ENVIRONMENT OF UNIT SIX

Hematitic oolites have been reported in the uppermost Ordovician
rocks in Wisconsin and northeast Illinois (Cohee, 1948; Workman, 1950)
and in Towa (Parker, 1971). In these areas the oolitic hematite is in
the Neda Formation of the Maquoketa Group. Parker (1971) states that the
Neda Formation appears to be highly weathered. Workman (1950) noted that
the Neda Formation occurs only in localities in northeast Illinois where
the Maquoketa Group is at, or near its maximum thickness, Willman (1971)
explains that the irregular thinning of the Maquoketa Group and the
areas of Neda Formation absence is the result of post-Neda valleys which

were cut as much as 150 feet deep into the Maquoketa Group. These
39
valleys were filled with early Silurian sediments. He asserts that
“between the valleys there is only slight evidence of unconformity"
(p. 15). The northernmost reports of oolitic hematite in Wisconsin are
in the position where a facies boundary between the highly clastic red
clays of the lower half of the Southern Peninsula and the interbedded
carbonates to the north may be drawn, The similarity between the Neda
Formation and Unit Six of this study suggests that the depositional
environments are analogous. However, this similarity need not imply any
time equivalence, but merely a repetition of the depositional conditions
necessary for the formation of oolitic hematitic material associated
with the red and greenish-gray shales. There is general agreement in the
literature that these hematitic oolites represent a primary chemical
precipitation followed by mechanical transport and sorting of the
ooliths. The water depth is described as shallow to intertidal for most
oolitic hematite deposits in the geologic record (James, 1966). Plak-
senko (1959) comments that hematite-rich sediments near shore with
shales more seaward are to be expected during a regressive cycle; it is
expected because of the clastic material contributed from the land,
especially if the clastic material is clayey.

Since the Neda Formation is identified in parts of Wisconsin, South
Dakota, Towa, Illinois, Kansas, and Missouri, a western source for the
clays does not seem as probable as the well established eastern Queen-
ston Delta complex. A northern source of terrigenous clays seems improb-
able considering that the northern portion of the Southern Peninsula is
an area containing little terrigenous sediment.

The irregular thickness of Units Five and Six mentioned in the

discussion of Unit Five may be significant not only in considering
40

the Unit Six post-depositional history in the southwest corner of the
state, but also with regard to the depositional history in the center of
the basin, The irregular thick and thin areas of Unit Six in Allegan and
Barry Counties correspond closely to those of Unit Five below, The
oolitic hematite which is found in Allegan County is in that area where
Unit Six reaches its maximum thickness. The Manitoulin Formation thick-
ens in the regions where the Upper Ordovician Units Five and Six thin
as if it were filling topographically low areas. It would seem that Unit
Six was deposited and followed by erosion. These conclusions are the
same as those of Workman (1950) and Willman (1971) in their studies of
the Upper Ordovician of northeast Illinois, If the thinning which has
been mapped in this study is due to stream erosion, as suggested by
Willman (1971) for northeast Illinois, the direction of flow and the
ultimate area of deposition should be taken into account. The most
logical area for the deposition of this youngest of Ordovician clastic
Material would be the Southern Peninsula depositional center of Unit
Six. If this hypothesis is correct, the Michigan Basin depositional
center contains some of the youngest Ordovician and/or oldest Silurian
rocks in the Michigan area. Units Five and Six appear to be of shallower
water depths than the carbonate Unit Four below. This upper sequence
thus represents a regressive and a shallower, more restricted environ-
ment upwards. Din (1950) recorded a decrease in the abundance and number
of ostracod genera upward from the lower portion of Unit Five into the
upper portion of Five, with ostracodes being rare to absent in Unit Six.

If the reason tor their abundance could be established, the stein-
kerns in Unit Five could be a valuable environmental indicator as well

as a stratigraphic marker. Possible explanations for this type of
44

occurrence are summarized by Tasch (1953). Two of the possibilities are:

juveniles that met catastrophic death due to toxicity, either salinity
or iron content; or a pebble (fastropod-protoconch) necrocoenosis,
indicating sedimentary processes (sorting). The environment of deposi-
tion postulated for Units Five and Six would make possible the accept-
ance of either of the above reasons for the steinkern abundance. The
biological causes would point to the beginning of a hostile environment
which may have continued throughout Unit Six's deposition. Din (1950)
indicated that ostracodes were rare in Unit Six. Bretsky and Bermingham
(1970) stated that "under the most restrictive environmental conditions
only bivalves and gastropods would survive, but in a stunted condition,"
Tn his studies of the Upper Ordovician of Iowa, Agnew (1955) noted a
depauperative horizon at the top of the Maquoketa Group "where the Neda
zone (Unit Six equivalent) is lacking, or just below that zone where it
is present" (p. 1718). The highly oxidizing environment, which is
indicated by the oolitic hematite, might mean that conditions for the
preservation of organisms may not have been very good, a factor that
could handicap an environmental approach,

A difference in the sediment types from the northern mapped area
compared to the southern area has been described earlier for Unit Six
and was also recognized for Unit Five. Carbonate units of Unit live, as
well as those below, are calcareous near the center of the basin and
grade into dolomites towards the margins of the basin. This lateral
limestone to dolomite facies has been obscured by a later dolomitization
that is indicated hy the dolomitization of most of the Upper Ordovician
rocks in the Schoolcraft County core, including the fossils.

It is possible that Unit Six represents either deposition in an
42

isolated basin during the Late Ordovician, or is a western extension of
very shallow marine Queenston shales, The thickening of Unit Six towards
the Ogemeaw County region may be more an indication of structural subsid-
ence than an indication of equivalent water depth prior to Unit Six
deposition. In considering the age of Unit Six, it is possible to infer
that the center of the basin contains the youngest Ordovician and/or

oldest Silurian rocks of the Great Lakes region,
CONCLUSIONS

The isopach map (Plate 7) of the Upper Ordovician of the Southern
Peninsula of Michigan indicates that the Michigan Basin, as we know it
today, existed during at least part of the Upper Ordovician, The Mich-
igan Rasin during Late Ordovician time seems to have been a rather
shallow, stable basin centered around Ogemaw County. It was bounded by
the Kankakee Arch to the southwest, possibly a Findlay Arch to the
southeast, and an unnamed arch between Ontario and Michigan trending
north~south along the international border through Lake Huron, The
northern and northwestern margins appear to be broad, stable areas with
no indication of erosion or significant periods of nondeposition. The
thick Upper Ordovician rock section in the area of western Lenawee
County, which continues to thicken southeastwardly, suggests that the
area may best be considered a northwestern extension of the Appalachian
Basin,

The basinal geometry and lithofacies of Units Four, Five and Six
indicate slow subsidence for the Michigan Basin during the Late Ordovi-
cian, The sequence is a regressive pattern beginning with the subtidal
below wave base carbonates of Unit Four and terminating with erosion in
the southern arch areas and with supratidal evaporites in the more
stable Upper Peninsula area. Unit Six, where present in the southern
half of the basin, is highly radioactive on gamma-ray logs and is typi-

cally a nonfossiliferous red shale, occasionally containing oolitic

43
dy

hematite, The units below the regressive sequence described above do not
demonstrate either a good basinal geometry or clearly defineable basinal
patterns of the recognizable lithofacies. Unit One, the Utica Shale, is
a wedge of argillaceous material which thickens eastwardly into Ontario
and southeastwardly into Ohio. The isopach map (Plate 1) of Unit One
does not of itself indicate a basin at that time. There is some sugges-
tion that Unit One is in some places isochronous with the stratigraphi-
cally lower Trenton Formation. To determine whether a depositional basin
existed during the Utica deposition, further studies must include the

Trenton Formation carbonates.
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BIBLIOGRAPHY

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ROY D. NURMI

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TRENTON

McClure | Pease

Turtle

Sec8TIS RI4W

>< —_—<— 12 mi

Sec 10 TIS RIZ2W

| Rumsey

Continental

10
15 mi 21 mi
Sun | Afman
Secl2 T3N RIOW
| Simpson

SeclOT2N RI2ZW

2a

Ambassador
| Burtle

Sec4 TGN RBW

41 mi <

MeClure | Fox

Sec6 T7N RIW

PLATE 9.-

CROSS SECTION

SW TO BASIN CENTER

420
SEF

“THY

Plate 7

“iil ns

Wii)

943

ALE ang

ROY D. NURMI

SUPPLEMENTARY
_____ MATERIAL

33 mi

62 mi

Pure | Emery

Sec2!l TI3ZN RIW

SILURIAN

UNIT 6

NE.

Amoco 1| Hunt

Sec 26 T23N RSE

Pure

Sec 21 TI3Z3N RIW

| Emery

33 mi

2 i6 mi

3

Lee
Sec5 TSN R2E

McClure 1! Fox

Sec 21 T7N RIW

| Ferris

12 mi 4

White
Sec28 T4N R3E

13 mi 5

1 Scepka

Brazos | Kizer

Secl4 T2N R4E

6 21 mi ‘4

22
We

eee.

PLATE -..10.*

CROSS SECTION

Rousek | Grau

Good & Good Drig

22 mi

Great Lakes Drig

“Til STATE UNIVERSIT

ROY D. NURMI

BASIN CENTER TO SE

Sec8 T3S R4E Sees 155 RSE | Fisher
Sec3l T8S RGE
SILURIAN SILURIAN SILURIAN
Fe Ti a ee 2 ———— Pe a oe eas F = a al ss Snete nae
UNIT FIVE
UNIT SIX UNIT FIVE UNIT FOUR
4 cee
wes hy.
| UNIT FOUR
UNIT FIVE UNIT THREE
oy
UNIT FOUR UNIT THREE
UNIT TWO
re ale
at ee
UNIT THRE Mt : UNIT TWO
es
UNIT TWO
UNIT ONE UNIT ONE
UNIT ONE
ee ee
aa f TRENTON ,
TRENTON f | TRENTON

iii”

SUPPLEMENTARY

A4ATEDIAI

Te

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