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FACTORS CONTROLLING DEPOSITION OF THE ARCOLA
MEMBER OF THE MOOREVILLE FORMATION
(UPPER CRETACEOUS) IN EAST-CENTRAL MISSISSIPPI
AND WEST-CENTRAL ALABAMA

BY

Marc D. Florian

A THESIS

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

MASTER OF SCIENCE

Department of Geological Sciences

1984

ABSTRACT
FACTORS CONTROLLING DEPOSITION OF THE ARCOLA MEMBER

OF THE MOOREVILLE FORMATION (UPPER CRETACEOUS)
IN EAST-CENTRAL MISSISSIPPI AND WEST-CENTRAL ALABAMA

BY

Marc D. Florian

The Upper Cretaceous Arcola Limestone Member of the
Mooreville Formation, in Mississippi and Alabama, is a thin
but persistent hardground sequence. The individual lime-
stones are approximately one-foot thick and range from
calcisphere wackestones to calcisphere grainstones.

Examination suggests that synsedimentary lithification
proceeded in a semi-restricted marine environment under
water depths of less than 30 meters. Water depth, restricted
circulation, aragonite solubility, and sediment permeability
were the main factors controlling its distribution.

Hardground development proceeded through a series of
stages, the lithification process being accompanied and
aided by intermittent current activity, possibly through
barrier vorticies, and the flushing action of burrowing
Thalassinidea.

The depth to which cementation progressed beneath the
omission surface was directly related to the permeability of
the sediment which, in turn, was a function of its unique

biogenic components (calcispheres). Mineralization of the

Marc D. Florian
Arcola hardground was probably inhibited by insufficient

water depth and lack of clay minerals.

To my parents and my wife--
With deep appreciation for giving me the opportunity,

support, and encouragement to learn.

ii

ACKNOWLEDGMENTS

I am particularly grateful to Dr. Chilton E. Prouty of
the Department of Geological Sciences, Michigan State
University, for his assistance, criticism, and valuable sug-
gestions during this study. Appreciation is also extended to
Dr. Aureal T. Cross and Dr. James W. Trow, also of the
Department of Geological Sciences, for their comments and
.critical reading of the manuscript.

I would like to thank D. M. Keady of the Department of
Geology at Mississippi State University, C. W. Copeland,
Jr., Director, and E. A. Mancini, State Geologist, of the
Alabama Geological Survey, N. F. Sohl, of the U.S.G.S., and
D. V. Bottjer of the Department of Geological Sciences at
the University of Southern California, for their encourage-
ment, discussion, and suggestions.

Thanks are also due to J. A. Borgensen, Jr., for his
assistance in drafting Appendix figures 1, 2, and 3.
Technical assistance and facilities of the Department of
Geography at Michigan State University are also

acknowledged.

iii

II.

III.

IV.

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . . .
Objectives . . . . . . . . . . .
Method of Study . . . . . . . .
Previous Investigations . . . .
ARCOLA LIMESTONE MEMBER . . . .
Geographic Distribution . . . .
Stratigraphy . . . . . . . . . .
HARDGROUNDS . . . . . . . . . .
Synsedimentary Lithification . .
Morphology . . . . . . . . . . .
Occlusion Hypothesis . . . . . .
Mineralization . . . . . . . . .

Glauconitization . . . . .

Phosphatization . . . .‘. .

Phbsphate Nodules . . . . .

Limonitization .. : . . . .
Terrigenous Materials . . . . .
Depositional Rhythms . . . . . .
FACTORS RESTRICTING CIRCULATION
Barriers . . . . . . . . . . . .

Monroe-Sharkey Platform . .

Jackson Dome . . . . . . .

iv

14
15
23
25
26
29
31
33
35
36
37
39
39
41

42

VI.

VII.

VIII.

IX.

Upwelling Hypothesis

Secondary Features

FAUNA AND FLORA . .

Microfossils

Ecological Implications

Macrofossils

Ecological Implications
Trace Fossils
Pre-Lithification Suite

Post-Lithification Suite

Ecological Implications .

Calcispheres and Dasycladacean

Ecological Implications .

CLIMATIC STUDIES . .

COMPARISON WITH OTHER HARDGROUNDS

(EUROPEAN VS.

SUMMARY

CONCLUSION

APPENDIX

A

REFERENCES

PLATES .

ARCOLA)

44
46
49
49
50
53
54
54
55
62
65
67
68

73

75
77
81
83
86
96

LIST OF FIGURES

FIGURE 2.1: Facies Relationship of Part of the Upper
Cretaceous of Mississippi and Alabama .
FIGURE 2.2: Portion of the Stratigraphic Column

of Mississippi . . . . . . . . . . . .

APPENDIX A
FIGURE 1: Outcrop of the Arcola Member and
Sampled Localities . . . . . . . . . .
FIGURE 2: Subsurface Extent of the Arcola Member.
FIGURE 3: Tectonic Map of the Central Mississippi

Embayment . . . . . . . . . . . . . . .

vi

I. INTRODUCTION

Objectives

The basis of this study involves the Upper Cretaceous
Arcola Limestone Member of the Mooreville Formation in east-
central Mississippi and west-central Alabama. The Arcola is
a thin but persistant limestone unit within a thick section
of chalks and marls and exhibits several characteristics
similar to EurOpean chalk hardgrounds. The individual lime-
stones are approximately one foot thick and range from calc-
isphere wackestones to calcisphere grainstones. It has been
suggested that the calcispheres are reproductive cysts of
Acetabularia-like benthonic algae (Rupp, 1974: Marszalek,
I975). Pelagic coccoliths and forams are the major calcitic
components of the surrounding chalks and marls. The prolific
coccolith productivity has been attributed to conditions
produced by coastal upwelling (Johnson, 1975). The benthonic
algal production of calcispheres became dominant when up-
welling was forced further seaward. It is postulated that
uplift and emergence of areas to the north and west were
responsible for temporary cessations of upwelling in the
region (Johnson, 1975).

It was originally intended that this study should
examine and evaluate the validity of the aforementioned
hypotheses and their influence on the depositional environ-

1

2
ment before, during, and after formation of the Arcola Lime-
stone. As the inquiry progressed, however, the field of
study was changed and the purpose became one of hardgrounds
in general, comparing and contrasting similar sequences in
Europe and the Persian Gulf to the Arcola, a unit which
appears to represent the only extensive EBFEEEE hardgrounds

in North American, Upper Cretaceous chalks.

Method of Study

Examination of an extensive amount of literature
involving subjects related to this inquiry was completed.
Initially literature appertaining specifically to the Arcola
Member was evaluated. The problems of calcisphere produc-
tion, oceanographic upwelling, and submarine lithification
were then surveyed and finally work on other hardground
sequences, mainly in Europe and the Persian Gulf, were
studied together with papers on various special aspects of
hardgrounds including phosphatization and glauconitization.

During field reconnaissance of the exposed section from
north-central Lee County, Mississippi, to eastern Montgomery
County, Alabama, twelve locations were chosen for detailed
measurement and sampling (Appendix Figure 1). Specimens were
collected from stratigraphically sequential sample tra-
verses which originated in the soft chalk of the Mooreville
Formation, below the basal limestone, continued up into the
progressively indurated sediment of the Arcola Member and

intervening marl units, and terminated in the overlying soft

3
chalk of the Demopolis Formation. This scheme was designed
to demonstrate the major changes (both biological and min-
eralogical) which occur during the transition from soft
chalk to indurated limestone.

The bulk of the laboratory analyses consisted of the
preparation and examination of several hundred petrographic
and palynological specimens. These techniques, including
electric log evaluation of wells throughout Mississippi and
southern Alabama, facilitated the study of sedimentation and
diagenisis.

Consultation with several authorities in the Upper
Cretaceous of the Gulf Coast Region and others working in
related fields of research has considerably aided the pro-

gress of this study.-

Previous Investigations,

The Upper Cretaceous Selma Group of northeastern Mis-
sissippi and west-central Alabama consists chiefly of chalks
and marls. Only one unit, the Arcola Limestone Member, con-
tains persistent limestones.

The first reported observation of the Arcola was made
in Alabama by Withers (1833) who described it as a variety
of chalk different from those of surrounding units. Toumey
(1850) and Thornton (1858) briefly referred to the limestone
and its areal extent while Harper (1857) noted its high cal-
cium carbonate content. Harper (1857), Johnson and Smith

(1887), and Smith et a1. (1894) further described the lime-

4
stone as "bored rock," referring to its appearance when the
poorly cemented marl, which fills the burrows, is removed
during weathering.
Stephenson and Monroe (1938) proposed that the thin
unit of limestone at the top of the Mooreville tongue be

1 The

named the Arcola Limestone Member of the Selma Chalk.
type locality was designated as a bluff on the Black Warrior
River at Old Arcola Landing, Hale County, Alabama. They
traced this member westward and northward into the Coffee
Sands of northeastern Mississippi and eastward into the
Blufftown Sands of eastern Alabama. A few feet above the
Arcola a thin phosphate-bearing bed, together with
”reworked” limestone cobbles, was interpreted by these

authors to represent a minor stratigraphic break in the

deposition of the Selma Chalk.

Stephenson and Monroe (1940) later indicated that the
limestone, possibly produced by inorganic precipitation,
accumulated in an environment in which the terrigenous and
organic influx (clay, sand, coccoliths) was almost totally
lacking. They further noted that the preservation of numer-
ous open burrows in the limestones indicated penecontempor-

aneous lithification of the sediments.

 

1In 1945 the Mississippi Geological Survey raised the
term Selma to the rank of group to include all Cretaceous
beds above the Eutaw Formation. At that time the Mooreville
tongue was raised to the rank of formation and the Arcola
was given member rank within it.

5

Monroe (1947) and Newell (1968) correlated the Arcola-
Demopolis contact with the Austin-Taylor contact in Texas
based on foraminiferal and coccolithophorid evidence.
Russell and others (1982) have dated the Arcola as late
Early Campanian in age based on extensive foraminiferal
studies.

Johnson (1975) proposed that upwelling of nutrient-rich
water was responsible for the vast numbers of nannOplankton
which now form the chalks and marls of the Selma. He further
postulated that a barrier (Monroe-Sharkey Platform) caused
temporary cessations in upwelling resulting in deposition of

the calcisphere-rich Arcola Member.

II. ARCOLA LIMESTONE MEMBER

Geographic Distribution

The Arcola Member is exposed in a 300 mile long arcuate
belt extending from the vicinity of Tupelo, Lee County, Mis-
sissippi, southward through east-central Mississippi, where
the strike changes to a more easterly direction into central
Alabama (Appendix Figure 1). The member dips westward at a
rate of 30-40 feet per mile and, on the basis of electric
log characteristics, it can be traced into the subsurface
several miles (Appendix Figure 2).

In contrast to the indurated limestones, the surround-
ing chalks and marls of the Arcola and Selma Group, as a
whole, exhibit little evidence of lithification and as such
they present a lithology not greatly altered from the state
in which they were deposited. Under these conditions the
hardened beds of the Arcola stand out clearly and striking-
ly, usually forming low hills and ridges (Plate 1A). The
Arcola is recognized as an excellent marker bed in an other-
wise generally monotonous sequence of chalks and marls.

The easternmost typical exposure of the limestone is
near Downing in Montgomery County, Alabama (Plate 1B). In
the western part of Bullock County, near Union Springs, the
member merges laterally into the uppermost part of the
Blufftown Formation (Figure 2.1). There, beds of very hard

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8
calcareous quartz sandstone occupy the same stratigraphic
position as the limestone farther west and appear to repre-
sent its eastward facies.

Similarly, a single bed of hard sandy quartz limestone
near Tupelo, Mississippi, appears to represent the north-
western continuation of the Arcola as it merges with the
cross-bedded and glauconitic Coffee Sands (Plate 5A, SB). To
the east and north of here, within the same stratigraphic
position, only calcareous nodules have been reported

(Stephenson and Monroe, 1940).

Stratigraphy

Within the study area the Selma Group is composed of
four formations. In ascending order these are the Moore-
ville, Demopolis, Ripley and Prairie Bluff. The Arcola is
the uppermost unit of the Mooreville Formation (Figure 2.2).

The Arcola consists of one or more beds of nearly pure,
fine-grained calcispheric limestone interbedded with soft,
chalky marl. In Mississippi the member consists of one to
two limestone beds each 0.S'-1.0' in thickness, separated by
a thin bed of marl two to three and one-half feet thick. In
Alabama, however, the member is composed of two to four beds
of the same buff to tan-colored limestone separated by
relatively thin beds of marl (as compared to Mississippi).
The greatest observed thickness was along Hatcher Bluff on
the Alabama River (Dallas Co., Ala.) where the member

exceeds fifteen feet in thickness and is composed of four

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10
limestone beds each approximately 1.5' in thickness, sep-
arated by two foot intervals of marl (Plate 2).

The limestone is riddled with numerous burrows that are
filled with soft sediments of the overlying chalks and
marls. On weathering, this marl is washed out leaving a
highly perforated unit of limestone (Plate 7A, 7B). It is
this characteristic that initially led observers to refer
to the limestone as the ”bored rock.”

The calcium carbonate content of the Arcola Member
averages 90%. The beds of limestone, Specifically, have a
calcium carbonate content of nearly 99% (Dinkins, 1960:
Johnson, 1975). Thin-sections of these units have shown them
to be composed predominantly of calcispheres (Plate BA, BB,
9). These small spheres (40-60 microns) are thought to be
the reproductive spores or cysts of Acetabularia-like
benthonic algae. They are commonly filled with micro-
crystalline calcite (Plate 3B). Terrigenous materials (sand,
silt, clay) comprise a negligible percentage of the lime-
stone except near the facies change with the Coffee Sand and
Blufftown Formations where the quartz sand content is
locally in excess of 25%. The limestone ranges from cal-
cisphere packstone to wackestone and the matrix consists
chiefly of micro-crystalline calcite.

The marl, which separates the limestone units of the
Arcola, has a calcium carbonate content ranging from 52-88%
(Johnson, 1975) and is characteristic of the underlying

Mooreville sediments. In the deep subsurface, to the south

11
and southwest, where electrical characteristics of the lime-
stones are absent, it is not possible to differentiate lith-
ologically the marls of the Arcola from the underlying
Mooreville or overlying Demopolis sediments.

The contacts between the limestones and marl beds are
generally sharp due to change in sediment components (calci-
spheres vs. coccoliths). This change, in addition to the
presence of numerous preserved (open) burrows, has been
interpreted as indicating depositional breaks at the con-
tacts (Stephenson and Monroe, 1940). Early lithification of
the limestones during these breaks is suggested by the pres-
ence of post-lithification borings.

The Mooreville-Arcola contact is sharp, distinct and
apparently conformable based on the nannofossil studies of
Newell (1968: however, see Stephenson and Monroe, 1940, and
O'Quinn, 1961). Massive, blue-buff chalk of the Mooreville
is overlain by dense indurated limestone. The Mooreville
contains a high sand content near its contact with the
Arcola. Phosphate and siderite nodules have been observed
beneath the contact and glauconite is common.

The DemOpolis-Arcola contact is also sharp and dis-
tinct. Massive light blue to buff chalk overlies the
yellowish-tan limestone. At most localities reworked cobbles
of limestone and phosphatic nodules are present in the chalk
immediately above the Arcola and have been interpreted as a
basal conglomerate and representative of a local haitus.

Similar conclusions have been made by Bromley (1965) during

12
studies of pebble beds above hardground sequences in Europe.
He interpreted the pebbles as having been exposed at the
surface of the sea floor for extended periods, indicating
slowness of deposition and slight erosion.

Surrounding the upper contact, the limestones of the
Arcola are bored and the Demopolis is especially glauconitic
and sandy. This contact may represent a slight increase in
water depth and current activity which terminated restric-
tive conditions and prevented deposition of fine-grained
materials. Furthermore, a distinct change in the nannofauna
between the Arcola and a sample collected just above the
base of the Demopolis has been observed by Newell (1968). In
this interval the ranges of three species terminate and nine
species appear for the first time.

The Arcola Limestone Member contains a calcareous
nannoplankton flora nearly identical to that of the upper-
most "typical" Mooreville (Russell et al., 1982). Samples
collected from the indurated, calcisphere-rich limestone
beds of the member show a reduction in both preservation and
diversity, while nannofossil floras from the interbedded
chalky marls are quite rich and well-preserved. Russell and
others (1982) have assigned the Arcola interval to the upper

part of the Calculites ovalis zone of late Early Campanian

 

age based on the presence of Bukryaster hayi and absence of
Tetralithus aculeus and Marthasterites furcatus.
In addition, planktonic foraminifera are especially

abundant and well-preserved in the intervening marl beds of

13
the Arcola. Similarly, no significant faunal differences
have been observed between those of the uppermost Mooreville
and Arcola sediments. Therefore, both units have been
assigned by Russell et al. (1982) to the upper part of the
Globotruncana elevata subzone of late Early to Middle

Campanian age.

III. HARDGROUNDS

An attempt has been made to identify the conditions
leading to the formation of hardgrounds in chalk sequences
by means of a review of many such beds. Although hardgrounds
are typically a common feature of European chalks, the
indurated limestones of the Arcola Member are the only
igfgitu Upper Cretaceous chalk hardgrounds that have been
found in North America (Kennedy, 1975). Based on the pres-
ence of bored lithoclasts in cores Dravis (1980) has sug-
gested that hardgrounds may have existed in Austin chalk
from south Texas and northern Mexico. However, no EEFEEEB
hardgrounds have been observed. The Arcola exhibits many
characteristics found in other well-documented hardgrounds
as well as having several of its own. It therefore forms the
nucleus of this investigation.

The term hardground was first proposed by Murray and
Renard (1891) as a descriptive term for rocky sea floors
encountered during studies aboard the H.M.S. Challenger.
Subsequently, the term has been used for inter- or intra-
formational discontinuity surfaces in marine sediments which
exhibit evidence of exposure as lithified sea floors.
Kennedy and Garrison's (1975) excellent review of the term
reveals that it has been associated with surfaces exhibiting
major unconformities and usually with levels of early dia-

14

15
genetic cementation, commonly associated with an obvious
hiatus (Voigt, 1959: Hallam, 1969) however, frequently lack-
ing a biostratigraphically detectable gap (Jaanusson, 1961).
Most examples of hardgrounds are found in calcareous sedi-
ments: however, similar surfaces in sandstones have been
recorded. Ancient examples occur primarily in shallow water
limestones ranging from Ordovician to Pleistocene
(Jaanusson, 1961: Bromley, 1965, 1974: Purser, 1969; Schloz,
1972: Bathurst, 1975). Many of these occurrences exhibit
similarities which parallel the synsedimentary cemented
crusts described in shallow water carbonates of the present
tropics (Purser, 1969: Shinn, 1969, 1971: Taylor and Illing,
1969: Bathurst, 1975). While literally referring only to the
erosion surface, the term has been used in this thesis to
define the whole bed of early diagenetically hardened sed-

iment.

Synsedimentary Lithification

Synsedimentary sea floor lithification or hardground
formation represents the earliest stage of diagenisis for
most pelagic carbonates. Hardgrounds result from the cemen-
tation of sediments by high magnesium (high-Mg) calcite,
aragonite, and in some cases, glauconite and calcium phos-
phate. Chalk hardgrounds are the end products of a sequence
of depositional and early diagenetic events usually favored
by long contact between the sediment and seawater. Areas of

slow or interrupted sediment accumulation, whether due to

16
current removal of fine-grained material or reduced primary
sediment supply rate, are therefore the most common sites of
hardground formation. These interruptions may vary greatly
in extent and duration resulting in mere surfaces of non-
deposition or levels of actual erosion. In many regions of
paleotopographic highs and restricted straits, sections
exhibit abundant submarine cementation as a consequence of
sediment winnowing, especially at times of lowered sea
level. Such areas commonly show multiple hardgrounds with
significant amounts of time representing each omission sur-
face.

Although the chemistry of the process resulting in
lithification is unknown, most authors generally agree that
it would not require any special conditions other than
carbonate-saturated seawater. The process has been shown by
Shinn (1971) to be operating today in areas of relatively
normal salinity and temperatures in depths ranging down to
30 meters. His studies suggest that the principle physical
factors involved are relatively low rates of sedimentation,
sediment stability, and high initial permeability of the
sediment.

Cementation beneath the Arcola sediment surface common-
ly proceeded to a depth of 12-18 inches. The depth to which
the chalk was affected was not related directly to the
degree of hardening or necessarily to the length of the
period of nondeposition, but rather was a function of sed-

iment permeability. The depth of lithification may therefore

17
be attributed to grain size of the constituent sediments.
Coarse sediments enable the introduction of greater quan-
tities of cement through their increased permeability and
water circulation.

It must be emphasized that porosity and permeability
are both sensitive to the shape, packing, size, and size
distribution or sorting of the constituent particles in non-
indurated sediments. Rounded particles, such as calci-
spheres, will not be able to interdigitate and will there-
fore be less compact, resulting in large values of porosity
and permeability (Davis and DeWiest, 1966, p. 376). The
grain or particle size will determine the relative impor-
tance of surface tension and smaller molecular forces in
retaining water within pores. More importantly, the size
will also determine pore diameters which have a dominant
controlling effect on permeability. Sorting will determine
the extent to which smaller grains can occupy space within
larger pores. Other things being equal, poorly sorted sed-
iments will have lower values of porosity and permeability
and, consequently, will be unable to transfer large quan-
tities of super-saturated water necessary for hardground
develOpment. A

In general, clay contents in excess of a few percent
appear to have inhibited the development of early cements
and lithification in the chalks and marls of the Arcola and
Selma Group in general. Clay and colloidal material may have

formed coatings on larger particles and open networks next

18
to smaller particles, in which case permeabilities would
have been reduced to only a small fraction of their original
values.

Slow or reduced contemporaneous sedimentation does
apparently aid the hardening process. Too rapid an accumu-
lation may isolate the site of initial hardening from the
overlying supply of super-saturated sea water. Once cohesive
carbonate precipitation has been initiated, permeability
would be reduced and there would be a reduction in the cir-
culation of fresh supplies of super-saturated water: con-
sequently, the content of cement in a lithified horizon
decreases downward (Bathurst, 1975).

Aside from permeability inhibition of clay minerals,
another possible reason for the lack of cementation in the
chalks and marls of the Arcola (and Selma Group) is that the
original sediments were so poor in aragonite that there was
an inadequate source of metastable carbonate which might, by
dissolution and reprecipitation, give rise to a cement
(Kennedy and Garrison, 1975). These units (chalks and marls)
are composed predominantly of a stable low-Mg calcite
micrite derived from coccolith and foraminiferal debris with
a sand fraction of calcite skeletal grains; they are lacking
in the remains of aragonite skeletons. Kennedy and Garrison
(1975) suggest that the most densely cemented chalk of a
hardground sequence must initially have had a higher content

of aragonite.

19

It is difficult to see how the chemistry of pore water
in a chalk ooze could have been affected by a period of
reduced sedimentation. Bathurst (1975) has estimated the
actual rate of sedimentation in Upper Crataceous chalk seas
to be 50 cm/1000 yrs. or 1/2 mm/yr. As he points out, a
change in sedimentation rate from this to practically zero
would be hardly dramatic. It is more probable that the chem-
ical situation and the change in sedimentation were both the
result of a shallowing of the chalk sea which led to a dras-
tically reduced rate of sedimentation, a vast lengthening of
the time during which the near-surface sediments were in
effective contact with sea water, and as paleontologic
evidence indicates, a drastic change in the biologic regime
(coccoliths vs. calcispheres).

The lack of detrital terrigenous material within hard-
ground sequences, including the Arcola, indicates that shal-
lowing did not correspond with any marked approach of shore-
lines. This appears to be a characteristic of most hard-
ground sequences (Cayeux, 1941: Bromley, 1965: Kennedy and
Garrison, 1975). Hardgrounds that developed near coastlines,
such as those identified in Ireland by Bromley (1965), show
a considerable quartz content due in those cases to approach
of the shore.

In addition, it is perhaps significant to note that
hardgrounds are most frequently encountered at the tops of
regressive carbonate sequences which by definition coincide

with a change from shallow marine to deeper marine sedimen-

20
tation (initial phases of transgression). Purser (1969)
indicates that towards the tops of French Middle Jurassic
cycles there is a progressive slowing of the rate of car-
bonate sedimentation. He considered this due to cooling of
paleo-climate or decreasing depth of the sedimentary basin.
As carbonate sedimentation slowed, the sea floor tended to
lithify and was then slowly covered by the return of deeper
marine carbonate muds. There was, therefore, a progressive
reduction in the area of carbonate sedimentation and a
gradual replacement by a deeper marine, perhaps colder water
.regime. The two sedimentary phases are separated, both in
time and place, by a diagenetic phase of submarine lith-
ification.

The chalk hardgrounds of the Arcola Member are now com—
posed of low-Mg calcite. Based on the mineralogic nature of
calcisphere-bearing algae (Marszalek, 1975) and analogy with
modern areas of sea floor lithification (Shinn, 1969), it is
proposed that the original cement was most likely aragonite.

The precipitation of calcium carbonate (aragonite) from
sea water would be promoted by carbon dioxide removal by
algae (Acetabularia-like?) and increased water temperatures
(Ginsburg, 1957). Generally it is precipitated as aragonite
from warm, shallow water and super-saturated conditions,
whether by organic or inorganic processes.

In the Arcola, disaggregation of aragonitic algal skel-
eta on the sea floor prior to burial would have resulted in

a clay-grade sediment with individual particles having the

21
great surface area and resultant high solubility necessary
to supply carbonate to pore fluids migrating through a
chiefly calcispheric substrate of higher permeability than
normal chalk oozes. Hancock (1963) concluded that aragonite
sediments of such fine size could be discounted as the
origin of cementing material below a water depth of about 60
meters.

Initially, according to DeGroot (1969), Shinn (1969),
and Taylor and Illing (1969), many grains may be coated with
extremely thin films of carbonate (aragonite) cement. These
coats have been described by Bathurst (1975) as cements pre-
vented from causing cohesion by the vigor of grain movement.
The degree of grain to grain cohesion attained may therefore
depend on both the level of agitation and rate of precipita-
tion. Where agitation is slight enough and precipitation
fast enough, grapestone is known to form (Shinn, 1969). It
has also been shown that precipitation is assisted by the
presence of substantial areas of substrate of the same
material as the precipitate, on which epitaxial growth can
continue.

The critical parameters of water depth, aragonite solu-
bility, and sediment permeability were, therefore, the main
factors controlling the distribution of the Arcola hard-
grounds. Shallowing would raise the sea floor to within the
regime of surface currents. The higher temperature of the
water would increase the super-saturation with respect to

calcium carbonate. The warm, super-saturated water impinging

22
on and permeating through the unconsolidated calcispheric
sediment would precipitate aragonite. This may have been
accompanied and aided by enhanced, possibly intermittant,
current activity and/or flushing action of burrowing
organisms in the top sediments, whose activity may also have
been promoted by shallowing. The depth to which cementation
took place beneath the erosion surface was therefore
directly related to the permeability of the sediment which,
in turn, was a function of its biogenic components (i.e.,
calcispheres).

The lithification process of hardgrounds has been dis-
cussed by several authors (Bromley, 1965: Hallam, 1969:
Shinn, 1969: and others) and can be divided into a series of
progressive stages, all of which have been preserved in
chalk sequences as a result of termination of cementation by
renewed sedimentation. These stages are: (1) formation of an
omission surface (representing a marked pause in deposition
or sediment influx): (2) growth of nodules beginning near
the surface of the sediment and leading to the development
of a nodular chalk, erosion of which produces intraforma-
tional conglomerates; (3) fusion or coalescence of nodules
into a continuous or semicontinuous lithified layer (incip-
ient hardground of Kennedy and Garrison, 1975): (4) current
scour of unconsolidated sediment to reveal the lithified
layer as a true hardground: (5) repetition of the sequence

to produce composite hardgrounds.

23

European and Persian Gulf studies (Bromley, 1965;
Shinn, 1969) further note that the vertical gradation
(nodules, coalescence, true hardground) appears also in a
horizontal transition where a hardground passes laterally,
through nodular lithology, into soft chalk. The environment
in which these hardgrounds deveIOped has therefore been
interpreted as a limited area of sea floor where conditions
which favored hardening are gradational laterally.

Similar observations have been recorded regarding the
Arcola Member, particularly in the northwest and southeast
study regions where thin nodular zones are separated by
several feet of marl. These locations are likewise inter-
. preted as representing areas where the effectiveness of the
lithification process had diminished and conditions favor-
able for the formation (and preservation) of thicker com-

posite sequences were lacking.

Morphology

Pelagic sediments can be winnowed and sorted by cur-
rents, both in shallow and deeper water environments. In
shallow shelf chalk settings, periodic winnowing or erosion
can produce relatively pure calcarenites lacking a fine-
grained fraction or result in areas of nondeposition which
may stabilize the sediment through pervasive submarine
cementation (Wilson, 1975). Sequences of multiple hard-
grounds or sequences in which carbonate beds relatively rich

in clay-size detrital material alternate with beds of lower

24
detrital content (such as the Arcola interval) are probably
produced, in part, by periodic changes in current velocity
related to variations in sea level (Kennedy and Garrison,
1975).

The morphology of chalk hardground surfaces may vary
enormously as a reflection of the interaction of current
activity (physical erosion and abrasion), biological
erosion, and accretion of successive generations of cemented
material. While the sediment was soft a certain amount was
winnowed and further accumulation of fine-grained material
was prevented by bottom currents. Evidence for these cur-
rents in the Arcola sea is seen in the increase in grain
size of bioclastic material (in the form of limestone peb-
bles) resting directly upon the erosion surfaces, the dis-
aggregation and apparent removal of algal skeleta, and the
preferred convex-side-up orientation (Potter and Pettijohn,
1977) of larger bivalve shells (primarily Exogyra ponderosa)
on the surfaces of lithified horizons (Plate SB). The
process of winnowing was terminated or at least reduced by
lithification of the sediments.

The degree of current activity which aided the cessa-
tion of deposition is often registered by the convolution or
flatness of the erosion surface. Strong currents usually
result in flat erosion surfaces while a hardground with a
convoluted erosion surface often develops when increasing
currents bring sedimentation to a gradual stop. The increas-

ing winnowing action upon the soft chalk sediment from the

25
surface of the Arcola hardground has produced an essentially
level surface in a regional sense which is often convoluted

in detail.

Occlusion Hypothesis

A study was not undertaken to determine which of the
two beds, upper or lower, is more persistent in northern
Mississippi, nor was it possible to determine at which point
the two beds in Mississippi and west-central Alabama develop
into the four beds of the Hatcher Bluff area (central
Alabama).

Bromley (1965), however, offers an explanation for this
question suggesting that increased erosion associated with
the higher of these hardgrounds may cut down to the hard-
ground below thus occluding two or more lithified surfaces
and greatly reducing the thickness of the sequence in a
short distance. Furthermore, his observations indicate that
when two of these surfaces converge and meet, the lower is
left relatively intact while the upper one disappears
against it, suggesting that the lower surface was lithified
not only before erosion of the upper one, but more com-
pletely.

The greater number of limestone beds in central Alabama
(Hatcher Bluff) may reflect not so much the more favorable
conditions for their formation, but rather the increased
likelihood of their preservation. Based on the increase in

combined thickness of the Eutaw and Mooreville Formations in

26
central Alabama (750 ft.) versus that of western Alabama and
east-central Mississippi (440 ft.), Monroe (1947) suggests
that central Alabama may have been slowly or intermittently
warped down to provide for deposition of a thicker mass of
sediments than on the east or west.

It may then be postulated that while conditions favor-
able for hardground formation apparently existed throughout
the region, the greater number of limestone beds in the cen-
tral area is the result of an increased (intermittent) rate
of subsidence thus removing them from prolonged effects of
erosion by raising the base level of deposition above the
sea floor and permitting sedimentation (marl beds). In con-
trast, the reduced number of limestones in the eastern and
western regions may reflect a decreased rate of subsidence
and increased exposure time in which erosion may have
occluded larger composite sequences. The limestones in these
regions may therefore be equivalent to the lower two lime-
stones of the Hatcher Bluff region (utilizing Bromley's
occlusion model) and as such as probably representative of
periods of omission greater than or equal to that thicker

composite sequence.

Mineralization

Lithification of hardgrounds is often closely followed
by mineralization. In most instances both glauconitization
and phosphatization are well developed. Studies of Jurassic

hardgrounds, however, suggest this is not always the case.

27
The lack of mineralization is not necessarily a result of
different bathymetries, but rather a result of the drastic
changes in paleogeography and oceanographic conditions (in
particular upwelling: Bromley, 1967) of the continental
shelf situation of European chalk deposition versus the rel-
atively landlocked epeiric seas of the French Jurassic and
North American Upper Cretaceous.

It has been observed that the amount of glauconitiza-
tion and phosphatization generally increases at the hard-
ground surface. This observation together with the fact that
unbored or sparsely bored hardgrounds are usually unmineral-
ized, whereas glauconitized and phosphatized surfaces are
heavily bored, suggests that only those hardgrounds exposed
at the sea floor for extended periods could become mineral-
ized (Kennedy and Garrison, 1975). While small amounts of
secondary glauconite and phosphate are present throughout
the chalks, both European and North American (Selma Group),
indicating that mineralization occurred continuously, only
during genesis of the spectacularly mineralized European
Cretaceous hardgrounds did the process attain rock-forming
proportions (Kennedy and Garrison, 1975).

Prolonged exposure on the sea floor would suggest that
the sediments had access to a continuously renewable supply
of seawater. This would provide for a large reserve of
potassium, iron, andphosphate ions for the genisis of glau-

conite and phosphate. It is therefore not cementation itself

28
that leads to mineralization, but the environment that
accompanies and postdates its initiation.

The petrography of the glauconitization and phosphati-
zation of chalk hardgrounds has been discussed in detail by
Bromley (1965, 1967) and Kennedy and Garrison (1975). In
general, where replacive glauconite and phosphate occur
together the textural evidence suggests that glauconitiza-
tion consistently occurred prior to phosphatization
(Bromley, 1965: Kennedy and Garrison, 1975; Jarvis, 1980).
The change in mineralization appears to be related to depth
and temperature, with glauconite precipitation being typ-
ically characteristic of deeper, cooler water (Porrenga,
1967).

Emery (1960) reports glauconite precipitation in modern
seas between depths of 50-2000 meters, while Bromley (1967)
limits phosphatization to the range of 30-300 meters. Hard-
grounds which exhibit well-developed phosphatization only
may therefore be interpreted as having formed in warm water,
perhaps with relatively rapid "uplift" into the phosphatiz-
ing zone, allowing little glauconite precipitation. Alter-
natively, well glauconitized, but little phosphatized hard-
grounds may reflect insufficient shallowing (deeper oceanic
environment) and cool water. Furthermore, while it has been
observed that the amount of glauconite generally increases
at the hardground surface, suggesting that its formation
occurs where lithified chalk is in prolonged contact with

sea water, phosphatization apparently occurs in areas that

29
have been mantled by sediment-~an idea consistent with the
replacement process being a subsediment/water interface
phenomenon (Jarvis, 1980). Glauconitization will proceed
only in the early stages of development when the hardground
is continually or intermittently exposed on the sea floor,
whereas phosphatization will predominate during the burial

phase.

Glauconitization

Like the Jurassic hardgrounds of northern France, glau-
conitization within the lithified layers of the Arcola is
relatively uncommon: it is usually more abundant in the
marls and marl-filled borings and burrows of the limestones.
The largest concentrations-of glauconite have been recorded
directly beneath and above the Arcola horizon (Plate 4).

In thin sections of both the indurated Arcola lime-
stones and poorly cemented marls, the glauconite is crypto-
crystalline and generally (though often variegated) a pale
greenish-yellow color under plane polarized light and
assumes a yellowish-green color more typical of glauconite
when nichols are crossed. The majority of grains are seen to
be the internal casts of foraminifera tests. They consist
primarily of pure glauconite with no ghost structure to
indicate diagenetic replacement and, therefore, probably
formed in empty foram tests before they could fill with
sediment. In most cases the tests have since been removed,

yet the morphological similarities still exist. This feature

30
is common to many glauconite sediments (Cloud, 1955: Burst,
1958: Ehlmann, et al., 1963; Triplehorn, 1966; Bromley,
1965, 1967; Jarvis, 1980): the explanation requires recog-
nition of the conditions necessary for glauconite precipita-
tion.

It is generally considered that reducing conditions are
necessary for the precipitation of glauconite. The great
activity of burrowing organisms, however, suggests that sea
water was relatively well oxygenated before lithification of
the sea floor in Arcola time.2 Suitable reducing conditions
would have existed in the microenvironments afforded by the
tests of dead foraminifera within the sediment. After decay
of organic matter in the test had ceased the pH would fall
until conditions within the microenvironment became suitable
for the precipitation of glauconite. If the test remained
empty, pure glauconite would form. Rounded grains which
exhibit a lighter pale-green color under white light usually
show ghost structure of sediment which had filled the test
prior to its glauconitization.

Glauconitization of many European hardgrounds may be
associated with the replacement of clay minerals during the
early stages of hardground development while lithified car-
bonate remained in contact with sea water. The diagenetic

replacement of degraded layer silicates (particularly clay

 

2It has been observed that a burrowing infauna can
persist in oxygen-deficient environments even where oxygen
restriction has become too severe to support a shelly
epifauna (Enos, 1983).

31
minerals) is considered by many authors (Burst,1958; Hower,
1961: McRae, 1972; Jarvis, 1980) as the major process of
glauconite formation. It has been postulated that the occur-
rence of pure glauconite within microborings results from
the alteration of an original clay infilling (Jarvis, 1980).
The lack of sufficient clay materials (<2%; Dinkins, 1960)
within the lithified layers of the Arcola together with
excessive shallowing during deposition (<30m) may have

inhibited glauconite formation.

Phosphatization

The relative absence of phosphate within the Arcola
Member is not an unusual characteristic; many well—developed
hardgrounds lack phosphate (Bromley, 1965: Kennedy and
Garrison, 1975). Recognition of the conditions which pre-
vented phosphate formation in the Arcola requires a brief
examination of the conditions under which phosphates are
attributed and known to exist.

Conceptual models of phosphate formation invariably
include the upwelling of cool, nutrient-rich water (usually
from a depth of 100-300 m) as the primary source of phos-
phorous (Bremner, 1980). Phytoplankton abstract phosphorous
and other nutrients from euphotic waters and after death
settle to the sea floor where they fix 1-2% of their orig-
inal amount (Baturin and Bezrukov, 1979) in the sediment.
The majority, partly in the form of orthophosphate ion dis-

solves directly into the water (Bromley, 1965) during its

32
descent. At a depth of 1000 meters nearly all of the phos-
phorous has returned to the water in this manner (Bromley,
1965). Organic matter which does reach the sea floor is
attacked by bacteria and the phosphorous liberated is made
available for recycling. Only by the upwelling of this
deeper water by ascending currents do the phosphorous and
other nutrients return to the euphotic layer to become
available for re-elaboration by plants. In shallow seas
(i.e., the Arcola sea) there may be a temporary or permanent
"lock-up“ of phosphorous, which is thus lost to the cycle
(Ehlers and Blatt, 1982), possibly due to complete and con-
tinual uptake by phytOplankton or lack of substantial
ascending phosphate-rich source waters.

High concentrations of phosphorous have, however, been
observed in interstitial waters of bottom sediments in seas
and basins (Rittenberg et al., 1955). The greatest accumu-
lation occurs in sediments of shallow water seas and may be
attributed to periods of agitation during which fine, non-
phosphatic material is winnowed from the sediment, subse-
quently enriching it with phosphorite (Bromley, 1967). The
alternation of agitating conditions may be due to changes in
depth or currents. It has been suggested that these proces-
ses take place between a depth of 30-200 meters (Bushinski,
1964).

The occasional presence of stromatolites in certain
Chinese and Russian phosphorites led Bushinski (1964) to

indicate depths of 5-50 meters (possibly up to 100 m) for

33
their formation. Bromley (1967), however, points out that
alternations in sea level have also been associated with
formation of these phosphorites, suggesting that stromatol-
ite growth and phosphatization may not have occurred at the
same depth. Hardgrounds in northern Ireland similarly show
remnants of a cover of algal stromatolites on their glaucon-
itized and later phosphatized erosion surface which indicate
shallowing to emersion or near emersion (Reid, 1958). The
stromatolites are, however, unphosphatized and provide
further indication that excessive shallowing may terminate

the process of phosphatization (Bromley, 1967).

Phosphate Nodules

Phosphate concretions or nodules have been recorded at
both the upper and lower Arcola contacts. Studies indicate
that they can be recovered today from a variety of depths.
For example, the Challenger dredged them from depths down to
2700 meters (Murray, 1885). The deeper recoveries were, how-
ever, made from the bottom lepes or banks of shelves sug-
gesting that they probably moved from their place of origin.
Many of the deposits of phosphatic nodules on the sea floor
today may, according to Bromley (1967), be relict and have
little relation to the present depth of the sea.

Bremner (1980) attributes nodule formation to an
increase in the phosphate concentration of interstitial
fluids. Like most models the mechanism requires initial up-

welling of nutrient-rich water along the inner shelf and

34
subsequent extensive growth of phytoplankton in the euphotic
zone. A small portion of the phosphorous associated with
organic matter in the dead cells eventually reaches the bot-
tom sediments where bacteria cause release to the overlying
water column. Before the phosphorous can be recycled, how-
ever, it is rapidly adsorbed by a slow-settling layer of
mica-illite clays which cause its concentration in the
interstitial fluid to be elevated to the point of super-
saturation. Apatite slowly precipitates and diagenisis pro-
motes replacement of phosphatic components, such as fish
debris, with microgranular apatite. Changes in sea level
result in reworking and weathering of nodules as well as
resulting in a change in the location of nodule formation or
its complete cessation.

Studies by Miller and Swineford (1957) indicate that
many phosphate nodules formed in the same locations in which
they are found, and as such are often indicative of a par-
ticular paleoenvironment. In a detailed study of the nodu-
lose zone at the base of the Robbins Shale (U. Penn.) they
concluded that many nodules formed during depositional
breaks in shallow epicontinental marine settings character-
ized by a broad flat sea floor and restricted circulation.
Their model envisions foul bottom conditions (due to inad-
equate circulation) in which organic matter accumulated.
Organic matter is thought to form nuclei for nodule forma—
tion (Miller and Swineford, 1957). Animal remains reaching

the foul bottom area would decay slowly because of generally

35
reducing conditions and remain potential centers of nodule
formation for extended periods of time.

Although siderite nodules, generally associated with
mildly reducing conditions, have been observed in associa-
tion with the phosphate nodules of the Arcola (Johnson,
1975), little faunal or sedimentologic evidence exists with—
in the contact zones to suggest deposition in oxygen-
deficient environments. In fact, Bottjer (in press) has
noted borings and encrusters on Saratoga (Upper Cretaceous)
phosphate nodules, suggesting that during at least some part
of the time in which they were formed, they existed in a
well-oxygenated environment. It is possible, however, that
deposition may have occurred under fluctuating conditions of
oxygen content. Intermittent anoxia may have occurred just
below the sediment/water interface but with the identity of
individual anoxic events being destroyed by bioturbation

during subsequent oxic phases (Jarvis, 1980).

Limonitization

Heavy mineral counts of the Mooreville and lower
Demopolis formations made by Dinkins (1960) show that limon-
ite is found in its greatest concentration at the
Mooreville-Arcola (64%) and Arcola-Demopolis (92%) contacts.
The presence of iron minerals at horizons of slow or non-
deposition in European Cretaceous chalks has been attributed
by Bromley (1965) to represent shallowing of the sea with

attendant increase both of pH and of the activity of the

36

biocoenosis. There appears to be no critical range of water
depth indicated by iron precipitation. Taylor (1964), how-
ever, has associated the presence of limonite in Jurassic
limestones with extremely shallow water conditions.

Foraminiferal tests within the Arcola which exhibit a
limonite infilling probably reflect initial pyrite crystal-
lization and subsequent oxidation, as limonite is rarely
associated with organic materials and is not reported to be
forming in such situations today (Bromley, 1965: Jackson,

1970).

Terrigenous Materials

’ Terrigenous material, primarily clay, is a significant
component of the Arcola chalks and marls. The limestones,
however, contain a negligible amount of terrigenous material
((2%: Dinkins, 1960) throughout the region except near the
facies change with the Coffee Sand in northeast Mississippi
and the Blufftown Formation in east-central Alabama, where
quartz sand is locally abundant. Near Tupelo, Mississippi, a
single lithified bed in the Arcola horizon is composed of
nearly 50 percent coarse, angular quartz sand grains (Plate
6A, 68). Approximately ten miles to the south, however, this
bed exhibits a decrease in both percentage (25%) and grain
size of sand particles.

The high percentage of sand observed in the extremities

of the member together with the relative absence of clays

throughout the limestone beds may represent clay-fraction

37
winnowing by shallow-water currents similar to those repre-
sented by the cross-bedded Coffee Sands of northeasthissis-
sippi (Stephenson and Monroe, 1940; Dickas, 1962), as well
as reflecting shallower depths in general and a major shift
in the drainage pattern flowing off the Appalachian land
mass (from a southwest direction of flow, to the northwest
and southeast) in Selma time. This change is recognized as
having brought about a reduction in the amount of clastics
formerly delivered to the central part of the study region
(Tuscaloosa and Eutaw clastics) and an increase in the
northern and eastern regions (Dickas, 1962). Stephenson and
Monroe (1940) suggest that in the central region, for a dis-
tance of at least 250 miles along the strand line from
northeast Mississippi to central Alabama, no major streams
carrying coarse sediments flowed southwest into the Selma
sea. They additionally suggest that the finer clay-sized
particles which form a significant percentage of the chalks
and marls of the lower Selma may have been delivered in part
by small streams flowing directly into the sea, or trans-

ported from greater distances by longshore currents.

Depositional Rhythms

The apparent rhythmicity of the Arcola and of hard-
grounds in general, is a unique reflection of the overall
cyclic nature of all shelf sea chalk deposits (Kennedy and
Garrison, 1975). Alternations of hardgrounds with beds of

soft chalk and marl are probably a response to the same

38
mechanism as that which produced the alternations of more or
less argillaceous chalks and marls present at other levels
in the Cretaceous Gulf Coast section. Faunal changes across
cycles, together with the presence of burrows mixing sedi-
ment types, according to Kennedy and Garrison (1975), indi-
cate these to be depositional rhythms occasionally modified
by diagenesis.

In most European instances these cycles have been
interpreted as the result of a fluctuating supply of ter-
rigenous material against a background of continuous pelagic
carbonate deposition, or fluctuating carbonate supply
against a background of continuous clay deposition (Kennedy
and Garrison, 1975). The interpretation proposed here, how—
ever, suggests that these cycles in the Selma represent low
to high energy and deeper to shallower water regimes and, as
such, represent varying rates of sedimentation (involving

surface productivity) and restriction.

IV. FACTORS RESTRICTING CIRCULATION

Barriers

The tectonic framework of a region may directly affect
the water circulation along its coasts. Because of the low
overall bottom slopes in epeiric seas, such as that which
existed in Arcola time, topographic irregularities of small
size may have had disproportionately large effects. A relief
of a few tens of feet may have mimiced the restrictive
effects of hundreds of miles of horizontal distance (Shaw,
1964). Almost any elevation of the sea floor may have caused
the development of many features commonly attributed to bar-
riers.

In quiet water areas, behind barriers, or across shal-
low water over wide shelves, restricted circulation and
climatic factors may combine to strongly influence the type
of carbonate sedimentation in a different way. Insufficient
circulation results in restricted environments for most
marine organisms. It also results in more variable and
extreme salinities when it occurs in shallow epeiric seas.

In epeiric seas it is unlikely that movement of water
in great currents of the type characteristic of today's Open
oceans could even have existed (Shaw, 1964, p. 11). The
shallowness of the seas in Arcola time would have dissipated
the energy of the mass through friction on the bottom. The

39

40
very slight depth of water would probably have precluded the
transfer of such large values except at very slow speeds.
This is not to infer that epeiric seas during deposition of
the Arcola were without currents and always stagnant.

Epeiric currents were probably created largely by wind.
Prevailing winds could have moved some waters persistently,
setting up epeiric currents. If such winds blew consistently
from the open ocean onto the shallow epeiric shelf they
could extend the effects of normal marine salinities some-
what beyond the limit of tidal exchange. In this case, how-
ever, the existence of the current would only flatten the
salinity gradient, because as Shaw (1964) points out, once
the water in the current passed beyond the zone of tidal
exchange it would come under the inexorable forces of evap-
oration, and the salinity would increase as the water was
further away from its source of replenishment. Movement does
not prevent evaporation; waters in motion are as much sub-
ject to increased salinities as still waters.

In addition to supporting generally restrictive circu-
latory conditions, preferred structural orientation of pos-
itive features or uplifts within the epeiric sea floor
normal to prevailing wind and current direction may result
in a vortex situation. The resultant constriction may cause
an increase in current velocity through the aperture, which
combined with shallowing, intermittently increases the
energy level of bottom waters in the shelfward region and

leads to periods of erosion and minimal net sedimentation

41
(Bottjer and Bryant, 1980). Subsequent transgression (pos-
sibly due to isostatic adjustment) would cause an increase
in water depth and a widening of the adjacent aperture. This
would then result in a reduced energy level for bottom
waters, which increases the sedimentation rate and leads to
renewed deposition of chalks and marls.

The postulation of such a barrier in Selma time is sup-
ported by subsurface data which indicate thinning and trun-
cation of the Arcola interval over the Monroe-Sharkey Plat-
form (Johnson, 1958: Murray, 1961: Shreveport Geological
Society, 1968). Mellen's (1958) subsurface mapping of the
Arcola-Coffee Sand facies shows that the paleodepositional
strike remained generally east-west across northeast and
north-central Mississippi. As the facies boundary is traced
westward, however, the strike changes to the southwest,
paralleling the Monroe-Sharkey Platform. . .

Monroe-Sharkey Platform: The Monroe Uplift is located in
northeastern Louisiana, southeastern Arkansas, and west-
central Mississippi (Appendix Figure 3). The uplift is bounded
on the east by the Mississippi Structural Trough, on the south-
west by the North Louisiana Syncline, on the northwest by the
possible extension of the Mexia-Luling fault system (Johnson,
1958), and on the northeast by the Desha Basin. It is
recognized as a complexly truncated dome that blends into
regional structure to the north and northwest. Because of its
truncation and regional orientation, the areal extent of the

uplift has been difficult to establish. Johnson (1958),

42
however, defined its limit as the truncated edge of the Annona
Chalk (Gulf. Cret. in age) which reveals a platform structure
approximately 80 miles in diameter.

Further investigation indicates that the region of max-
imum uplift was a local area near the southern limit of the
platform (Johnson, 1958; Murray, 1961) where sediments of
Taylor age (Campanian) are completely lacking. In its
northern portion, however, these deposits are nearly com-
plete and continuous and exhibit no evidence of subaerial
exposure. This region of the platform may have represented a
shallow aperture (between the uplifted area to the south and
the continental margin and uplifted regions to the north,
e.g., south Arkansas Uplift, Pascola,Arch, etc.) which could
have acted as an effective current barrier and/or source of
intermittent vortex flow with fluctuations in sea level.

Jackson Dome: To the southeast and intimately related
to the Monroe-Sharkey Platform is the Jackson Dome. Struc-
tural studies of the region show that the Jackson Dome has a
closure of approximately 2400 feet and an area of at least
216 square miles on the horizon of the Tuscaloosa Formation
(Dickas, 1962). This uplift, like the Monroe, probably
formed a submerged high during all of Tuscaloosa time
(Monroe, 1954; Dickas, 1962). In late Taylor time, however,
both of these regions underwent further uplift to the extent
that they were very near sea level and possibly, for the

first time, were represented as islands.

43

Continued intermittent uplift throughout Navarro and
early Tertiary (Midway) time and subsequent denudation by
wave action and subareal exposure occasionally left the
crests of these features at a shallow water level where con-
ditions were favorable for the formation of the Monroe and
Jackson gas rocks: hydrocarbon-bearing, biogenic limestones
of reef-life origin. Final submergence appears to have taken
place in late Selma, early Midway time (Dickas, 1962).

The eastward nosing of the Monroe-Sharkey Platform has
been termed the Midnight Volcano by the Mississippi Geolog-
ical Survey. This volcanic vent, as well as intrusive and
extrusive rock, and volcanic ash are ample evidence of the
igneous activity associated with the Monroe and Jackson fea-
tures. Radiometric age dates determined from phonolite
recovered from both the Monroe-Sharkey Platform and Jackson
Dome indicate that volcanism associated with these struc-
tures occurred contemporaneously (Merril, 1983). Further-
more, a larger northwest-southeast trend of contemporaneous
volcanism in Mississippi is supported by the Door Point vol-
canic sequence in the vicinity of the present Mississippi
River delta (Braunstein and McMichael, 1976).

McGlothlin (1944) and Dickas (1962) have shown that
volcanism was active in the west-central region of Missis-
sippi throughout late Eutaw time. Their studies show that
volcanic ash has swollen this clastic facies to a thickness
of approximately 750 feet just east of the Monroe-Sharkey

Platform, compared to a regional thickness of 250 feet

44
(Dickas: Plate 6). In addition, recent studies by Merril
(1983) have named the Midnight Volcano and other volcanic
structures associated with the Monroe Uplift as the source
of ash from which bentonite deposits located in the
Tombigbee Sand Member of the Eutaw Formation were derived.
Based on nannofossil correlation, an early Campanian age has
been assigned to these deposits in Monroe County, Missis-
sippi (Russell et al., 1982: Merril, 1983).

Bentonites from the Mooreville Formation record at
least three periods of volcanic activity (Dinkins, 1960).
Other bentonite deposits including a six-inch bed within the
Coffee Sand, stratigraphically a few feet above the Arcola
horizon, and a thin bed within the Ripley Formation have
also been attributed to Monroe/Jackson volcanism, and
together with age dates, indicate that volcanism associated
with these features occurred intermittently throughout late
Eutaw and Selma time (Stephenson and Monroe, 1940;
McGlothlin, 1944: Murray, 1961; Dickas, 1962; Braunstein and
McMichael, 1976; Hunter and Davies, 1979: Russell et al.,
1982: Merril, 1983).

Upwelling Hypothesis: Johnson (1975) suggested that the
prolific coccolith and planktonic foraminiferal productivity
apparent throughout most of Selma time be attributed to con-
ditions involving coastal upwelling. His model emphasizes
the temporary cessation of upwelling in response to uplift
of the Monroe-Sharkey Platform in Arcola time. Although the

purpose of this inquiry was originally intended to examine

45
and evaluate the validity of these hypotheses and their
influence on the depositional environment before, during,
and after formation of the Arcola Limestone, several lines
of evidence rapidly discounted their feasibility.

First and foremost, was activity of the Monroe Uplift.
As previously discussed, evidence indicates uplift of the
Monroe-Sharkey Platform occurred intermittently from late
Eutaw through early Midway time. This was, therefore, a
structural event neither unique to Arcola time nor to its
deposition.

While Johnson (1975) stressed the association of cocco-
lith productivity with upwelling events, studies indicate
that these organisms can reach the high growth potential
suggested at low nutrient concentrations (Berger, 1975).
Furthermore, plankton tows through modern upwelling regions
have shown a substantial increase in the number of foramin-
ifera and little or no increase in the productivity of coc-
coliths (Phleger, 1969: Diester-Haass, 1978).

The conspicuous absence of specific planktonic fora-
minifers invariably associated with both recent and ancient
regions of upwelling (Mancini, pers. communication; Sohl,
pers. communication), including Globigerina bulloides
(Phleger, 1969), together with the abundance of warm, shal-
low water genera such as Globotruncana (Lowenstam and
Epstein, 1954: Russell et al., 1982) further suggest that
upwelling in the Selma sea on the scale suggested by Johnson

(1975) was unlikely at best. Surface productivity may rather

46
have been a function of fluvial-induced fertility and/or
tendency toward open marine conditions.

Secondary Features: This inquiry is by no means
intended to result in a structural study of the Gulf Coast
Region. It is, however, necessary to recognize local fea-
tures which may have significantly affected sedimentation
during deposition of the Arcola. In addition to the Monroe
and Jackson features, there are several less familiar struc-
tures which should be mentioned. Among these are the Pascola
Arch, the South Arkansas Uplift, the LaSalle Arch, the
Hancock County High, the Adams County High, the South Mis-
sissippi Uplift, the Hatchetigbee Anticline and the Desha
Basin.

Throughout post-Tuscaloosa time the Pascola Arch,
located in west-central Tennessee, southeast Missouri, and
northeast Arkansas, continued to be eroded to base level.
The region remained an effective highland in early Selma
time. As this feature was reduced in elevation, Upper
Gulfian seas encroached into western Tennessee and by Middle
Selma time Stearns and Armstrong (1955) indicate all but
extreme northwest Tennessee was submerged.

The south Arkansas Uplift appears to be closely asso-
ciated with the general rise of border areas which occurred
near the end of the Lower Cretaceous, and may constitute
only a local accentuation of this regional upwarp

(Bornhauser, 1958). It affected an area Which was again

47
affected at its southeast margin by the younger Monroe
Uplift.

The LaSalle Arch is a gentle structural arch plunging
to the southeast through east-central Louisiana. It is situ-
ated south-southeast of the Monroe-Sharkey Platform. In the
subsurface, upwarping of the LaSalle Arch has effected thin-
ning of all Tertiary strata: similar but possibly lesser
thinning of Gulfian and Comanchean units is suggested by
sparse data (Murray, 1961). Pre-Tertiary rocks are prin-
cipally calcareous-argillaceous: the pre-Gulfian being some-
what more arenaceous than the Gulfian (Murray, 1961).

The Hancock County High is located in the extreme
southern section of this county in Mississippi. Evidence for
this structure is suggested by Dickas (1962) in isopachous
maps of post-Tuscaloosa sediments.

Evidence for the growth of the Adams County High is
seen throughout Gulfian time and indicates a definite high
in Selma time. Currents over this submarine plateau may be
responsible for the regionally thin cover of Selma strata,
particularly in Adams and adjoining Mississippi counties.

Due to its secondary role in the structural history of
the region, little information is given in the literature
concerning the South Mississippi Uplift. Bornhauser (1947)
discusses and gives only a footnote reference to this Early
Mesozoic and Tertiary submarine plateau located in south-

eastern Mississippi.

48

The Hatchetigbee Anticline extends northwest-southeast
through central Choctaw, northeast Washington, and west-
central Clarke counties in Alabama. This feature is approx-
imately 50 miles long by 18 miles wide. Structural contours
on top of the Selma Group give it a local closure of at
least 400 feet (Dickas, 1962).

In southeast Arkansas and western Mississippi, thick
Gulfian and early Tertiary sediments fill the Desha Basin.
This small triangular feature lies north-northeast of the
Monroe-Sharkey Platform and represents a closed basin within
the larger Mississippi Embayment.

Although the above-mentioned features may have affected
deposition in Selma time, it cannot be shown that they sig-
nificantly affected deposition of the Arcola Member specif-
ically. The generally restrictive conditions which prevailed
during Arcola deposition were most likely a reflection of
the broad, shallow shelf on which it was deposited and not
necessarily the result of barrier influences. The existence
of these features cannot be neglected, however, as they
represent a unique facet of the paleoenvironment which

existed during Selma deposition.

V. FAUNA AND FLORA

Microfossils

Calcareous nannofossils and foraminifera are diverse
and well-preserved within the intervening chalks and marls
of the Arcola Member. Samples collected from the indurated,
calcisphere-rich limestones, however, exhibit great reduc-
tion in both preservation and diversity. Intensive investi-
gation by Russell et al. (1982) indicates a calcareous nan-
noplankton flora nearly identical to that of the uppermost
typical Mooreville, consisting of at least 64 species,
including common to abundant Arkhangelskiella specillata,
Broinsonia parca, Bukryaster hayi, Eiffellithus eximius, E;
turriseiffeli, Gartnerago obliquum, and Micula decussata, as
well as several species of the genera Cretarhabdus,
Cribrosphaerella, Lucianorhabdus, Microrhabdulus, Parhab-
dolithus, Tetralithus, Watzaueria, and Zyggdiscus.

Additional study of the Arcola interval by Russell et
a1. (1982) shows an abundance of well-preserved planktonic
foraminifera. These include Ventilabrella glabrata, Archaeo-
globigerina blowi, A. cretacea, Rugoglobigerina rugosa,
Globotruncana arca, G. Bulloides, G. elevata, G. fornicata,
G. linneiana, G. stephensoni, and G. stuartiformis among
many others.

49

50

Analyses indicate that the chalks and marls of the
Arcola Member may be characterized as friable biomicrites
composed predominantly of whole and fragmentary coccolitho-
phorids and planktonic foraminifera (each consisting of
low-Mg calcite skeleta) with varying proportions of clay,
authigenic minerals and organic debris. These sediments may
be interpreted on the basis of general faunal and sedimen-
tological grounds as having accumulated in water depths of
50-200 meters (Reid, 1968; Kennedy, 1967, 1970; Sohl, 1984).

Ecological Implications: Pelagic organisms (primarily
coccoliths and foraminifera) are often considered ”tracers"
of different types of water and as reflecting average ocean-
ographic conditions. If water mass distribution can be
inferred it is therefore possible to infer the geography and
distribution of sedimentary processes. It is obvious that
analyses of these organisms alone cannot result in a com-
plete environmental interpretation. It is, however, signif-
icant to this inquiry to recognize the ecological implica-
tions which may be derived from them.

The presence and composition of pelagic sediments in
modern oceans are controlled by a variety of factors. First,
the production of pelagic micro—organisms in surface waters
and supply to the sea floor are dependent on ecologic con-
trols such as fertility of surface waters (nutrient supply),
water temperature, light, and salinity. These controls, in
turn, are dependent in part on latitude, regional climate,

and regional and local patterns of circulation. Other

51

physical factors may also be effective in preservation of
microorganism tests (i.e., oxygen deficient layers, etc.)

Coccoliths and planktonic foraminifera are generally
most productive in warm near-surface waters. In abundance,
these organisms are indicative of open-marine conditions or
at least a trend toward these conditions. Both diversity and
specimen abundance have been shown to increase away from
shore with increasing water depth (Bandy and Arnal, 1960;
LeOpold and Pakiser, 1964: Dodd and Stanton, 1981). Although
these organisms are usually concentrated in areas of high
illumination, they generally do not thrive in near-shore,
shallow, shelf waters as these are often characterized by
variability in turbidity, nutrient content, salinity, and
temperature.

‘Because of their small size and planktonic nature coc-
coliths and planktonic foraminifera are easily held in sus-
pension and transported through turbulent environments. Con-
sequently, few of these organisms may be deposited in the
coarse sediments of high energy environments. Furthermore,
increased sediment suspension (primarily clays) in turbulent
zones may inhibit light penetration and subsequently the
ability of coccoliths and diatoms (primary food source of
foraminifera) to photosynthesize. Close association between
clay-sized sediments'and large concentrations of pelagic
organisms reflect greater settling rate and preservation
potential due to decreased sedimentation (coarse grades) and

turbulence (Moore, 1958: McKee et al., 1959: Phleger, 1960).

52

An abundance of pelagic carbonate is a function of pro-
ductivity as well as sedimentation rate. The production of
organic materials is dependent on the supply of plant nutri-
ents in the upper water layers as there is a direct
zooplankton-phytOplankton-nutrient relationship. Today such
regions may be are characterized by upwelling of deeper
nutrient-rich waters along the west coasts of continents,
current boundaries, or in the wake streams of islands
(Tappan and Loeblich, 1973). Little data were available on
coccolith species and productivity reflected in sediments
from upwelling regions. These algae, however, are known to
reach high growth potentials at lower nutrient levels: with
increasing fertility their numbers increase only slightly
and to a lesser degree than planktonic foraminifers (Berger,
1975). Plankton tows through upwelling regions have shown a
substantial increase in the number of foraminifera and
little or no increase in the productivity of coccoliths
(Phleger, 1969: Diester-Haass, 1978).

Small variations in salinity (a few grams per thousand)
have considerable influence on planktonic organisms because
of the accompanying variations in density affecting their
bouyancy. Coccoliths and planktonic foraminifera are gener-
ally confined to normal marine salinities of 30%-40% (Dodd
and Stanton, 1981).

Most calcareous plankton production is in relatively
warm surface waters that, at present, are confined to lower

latitudes (Scholle et al., 1983). As a group, planktonic

53
foraminifera are very tolerant of temperature variations.
Individual species, however, are much more limited in their
distribution. The genus Globotruncana, for example, is rep-
resentative of warm relatively shallow shelf waters
(Lowenstam and Epstein, 1954; Barron and Washington, 1982)
while the genus Globigerina is indicative of cooler waters,
often associated with regions of oceanic upwelling (Phleger,
1969). In the foraminifera, colder temperatures may slow the
rate of maturation so that although the individual grows
more slowly it reaches a larger final size (Dodd and
Stanton, 1981). Test size is influenced by other factors, as
well. Variation from normal salinity has been shown to cause
a reduction in the size of individuals. In addition,
unusually favorable conditions resulting in rapid reproduc-
tion have been correlated with a decrease in foraminiferal

test size (O'Quinn, 1961: Dodd and Stanton, 1981).

Macrofossils

Macrofossils are generally scarce within the Arcola
(presumably a reflection of water depth and restriction).
However, specimens of Exogyra ponderosa, Anomia argentaria,
Paranomia scabra, Ostrea plumosa, Gryphaeostrea vomer, and
others have been identified (Monroe, 1947). Of particular
significance are the large specimens of E. ponderosa affixed
to the upper surface of the hardened limestone bed near
Tupelo, Mississippi (Plate SB). According to Wilson (1975),

this characteristic is often indicative of submarine lithifica—

54
tion. In addition, immediately above the lower limestone bed
in south-central Lowndes County, Mississippi, a rudistid,
apparently in growth position, has been observed by Johnson

(1975).

Ecological Implications

Kauffman (1967) states that Exogyra and Ostrea from the
Upper Cretaceous North American Western Interior are charac-
teristic of a middle shelf environment. The abundance of
Exogyra in association with an indurated bed at the base of
the Saratoga Formation in southwestern Arkansas led Bottjer
(in print) to suggest deposition in water depths of less
than 35 meters.

According to Dodd and Stanton (1981), the geographic
distribution of rudists (a group of inequivalve, sessile,
epifaunal bivalves) in a broad band parallel to the present
equator suggests that they required warm, tropical condi-
tions. Their sedimentary and stratigraphic setting indicates
that they were well adapted to their niche in the reef and

high-energy. shallow water environment.

Trace Fossils

Primary sedimentary structures, if once present in the
limestones and marls of the Arcola Member, were obliterated
by a highly mobile benthonic fauna. Profuse biogenic struc-
tures (burrows) and bioerosion structures (borings) reflect

the activity of this fauna. It is important to distinguish

55
the actions of burrowing and boring, for these two modes of
activity are clearly different. Pre-lithification burrowing
refers to the excavation of unconsolidated sediment, post-
lithification boring to the penetration of hard rock, well
consolidated sediments, and organic skeletal material. Trace
fossils associated with omission surfaces may be further
divided into three categories (Kennedy and Garrisson, 1975):
(1) trace fossils that predate the hiatus are termed pre-
omission suite: (2) those produced during the period of non-
deposition represent the omission suite: (3) and traces
formed after renewal of sedimentation constitute the post-
omission suite.

Pre-Lithification Suite: The pre-lithification, omis-
sion suite of the Arcola Member is characterized by dense
networks of the burrow system Thalassinoides. Upper Creta-
ceous chalk hardgrounds characteristically contain extensive
branching systems of Thalassinoid burrows. This is apparent-
ly the only burrow system consistently associated with chalk
hardgrounds (Bromley, 1968). They were constructed before
the sea floor was cemented and have been attributed to
sediment-dwelling decapod crustacea similar to the living
Callianassa (Bromley, 1968). .

The burrows and burrow networks of the Arcola are very
irregular in size and configuration. Burrows range in diam-
eter from about 0.6 to more than 6 cm and this range may be
found within a single system. Two or more layers of closely

spaced dominantly horizontal burrow systems are often

56
present (esp. at China Bluff, Ala.) and these are usually
interconnected by irregularly inclined smaller burrows
(Plate 7A, 7B). Bulbous and elongate burrow enlargements are
observable in both horizontal and vertical components.
Branches are typically y-shaped and enlarged at the point of
bifurcation. Burrows are cylindrical to elliptical in trans-
verse section and unornamented.

Although the main burrow system described above is
attributed to the work of Thalassinoides, the small burrows
((0.6 cm dia.) can not be specifically identified. Much of
the general disturbance of the sediment may, however, be
attributable to the burrowing of Micraster and Dentalium
(Bromley, 1965).

It is important to recognize the apparent intimate
relationship between the burrowing organism and the harden-
ing process as each clearly influences the other. At its
onset, according to Bromley (1965), the lithification pro-
cess may have only weakly affected the sediment ooze between
the open burrows. Consistent churning of the sediment by the
construction of new burrows would not be conducive to cemen-
tation. In those areas of least-disturbed sediment where
cementation succeeded in taking hold, rounded hard patches
of chalk may have formed which would cease to be available
to the burrowers. As the lithification process progressed,
the activity of the burrowers would be increasingly restric-
ted as the several stages of lithification were completed.

The activity of the burrowers would control the form of

57
cementation, while simultaneously the restriction due to
hardening would produce the irregularity of the paths fol-
lowed by the burrowers (Bromley, 1965).
‘ The presence of actual decapod remains in thalassinoid
burrows has been reported (Kennedy, 1967). However, such
associations are rare. Decapod body parts were not found in
the Arcola, nor were they seen in the English chalks studied
by Bromley (1967). Authors note that dying callianassids
ordinarily desert their burrows and that molts shed inside
them are later carried out (Schafer, 1972). In addition,
Callanassa apparently prefers areas of slow sedimentation
(Bromley, 1965), and its exoskeleton, except for the claw,
is thin and poorly calcified (Schafer, 1972).

Recent members of the Thalassidea occupy diverse
habitats and are potentially valuable in paleoecological
interpretations.

Trace fossils are useful as paleodepth indicators
(Seilacher, 1967). The depth limits for particular ichno-
facies (true fossil assemblages) are variable, however, and
can overlap with those of other ichnofacies.3 The Egg;-
igga, ZOOphycos, and Nercites ichnofacies, which are used to

characterize shallow-shelf, outer-shelf and basinal environ-

 

3Trace fossils may appear in shallower deposits than
predicted by Seilacher's model, but the reverse is less
likely (Kennedy, 1975). Traces such as Thalassinoides, for
example, are not found in true deep-water chalks.

58
ments, respectively, have been recognized in numerous
studies and have become standards in the study of trace
fossils (Seilacher, 1967). In both North America and western
Europe there appear to be two major chalk ichnofacies (Frey,
1970: Kennedy, 1975; Bottjer, 1978). The first is a shallow-

water assemblage dominated by dense Thalassinoides burrows

 

(Seilacher's 'Cruziana Ichnofacies'). The second assemblage
is the Chondrites-Planolites-Zoophycos association.

Although the definition of ichnofacies and distribution
patterns of traces has been primarily related to water
depth, the assemblages are bathymetrically controlled only
in the sense that the sedimentary facies in which they occur
are influenced by water depth. It is probably not depth
alone, but rather water mass characteristics (temp., salin-
ity, nutrients, oxygen content, etc.) and substrate types
(fluid vs. hard bottoms) that control ichnofacies distribu-
tion (Enos, 1983).

Studies involving several living species of Callanassa
indicate a depth range of 10-65 meters (Lutze, 1938: Enos,
1983). Mertin (1941) considered this genus in the Upper
Cretaceous to be limited to near coast environments probably
not exceeding 100 m in depth. In addition, studies conducted
by Frey (1970) on the thalassinoid burrows of the Fort Hayes
Limestone suggest that the organism preferred shallow areas
swept by intermittent, moderately strong currents. According

to Purdy (1964) and Seilacher (1967), suspension-feeding

59
organisms (Thalassinidea) depend upon suspended organic mat-
ter for food, which requires at least a certain amount of
turbulence.

The characteristic presence of thalassinoid burrows in
hardgrounds is due to the appearance of conditions suitable
for their preservation rather than to the sudden appearance
of the burrowers. Observations suggest that burrowing organ-
isms similar to those of the omission suite were active
throughout much of Mooreville and Demopolis deposition.
Frequently, where they contain a fill which contrasts with
the surrounding chalk, the burrows can be traced downwards
below the lower limit of hardening. Different species or
even genera may be involved, but the burrows appear to be
the work of crustacea, and probably members of the Thalas-
sinidea. An increase in individual traces may, however,
indicate either a decrease in depositional rate or a partice
ularly favorable environment and a consequent increase in
the abundance or activity of burrowing crustacea.

In several European hardgrounds, burrows of the pre-
lithification omission suite apparently remained open and
more or less empty during long periods of nondeposition and
their hardened walls became strongly mineralized (Bromley,
1967). These burrows may have initially had organic or
mucus—rich linings which were used to maintain their form in
less cohesive sediments (Kennedy, 1970). The lining may
often act as a potential site for the development of reduc-

ing microenvironments. Subsequent burrow fill and adjacent

60
sediments probably had different porosity, Eh, pH, and
organic content further acting as foci for replacement.
Pyritic films have been observed by Bromley (1967) and
Kennedy (1970) along these burrow fill-burrow wall inter-
faces and in many cases have developed into pyrite cylinders
and nodules. When the burrow system in a hardground remains
empty and preserved as a cavity, however, the walls may be
etched by water migrating through the network and the
original lining (if present) and surface destroyed.

Burrows that are well-preserved in original shape and
outline, showing no indication of having been lined and hav-
ing apparently remained open during life of the inhabitant,
are indicative of a sediment with high cohesiveness and
bearing strength (Dodd and Stanton, 1981), while indistinct,
collapsed and deformed burrows and a general swirled, wispy
bioturbation structure are suggestive of soft and fluid sed-I
iment (Dodd and Stanton, 1981). Deep vertical burrows in
littoral and shallow sub-littoral environments were origin-
ally thought to reflect adaptations by organisms to exploit
currents (Seilacher, 1967); Frey (1970), however, has shown
them to be constructed as a means of further combating
unstable substrates.

Although thalassinoidélike pyrite nodules and burrow-
wall mineralization zones have been reported from English
chalks, neither of these characteristics was found in asso-
ciation with the Thalassinoides of the Arcola Member. The

walls of the burrows were probably not originally reinforced

61

with sufficient mucus or other organic substances to cause
concentration of diagenetic sulfides. The walls were evi-
dently not weak, however. The dominantly horizontal nature
of the burrow system (China Bluff section) together with the
presence of delicate shell fragments found within it and the
absence of any observable pre-diagenetic collapse features
indicates that the walls remained intact during the period
of lithification and subsequent infilling of sediment.

The burrows of the Arcola hardgrounds appear to have
remained inhabited and empty of sediment throughout the
omission period and were apparently filled at the onset of
renewed sedimentation and subsequent burial; the infilling
remained unconsolidated and was protected from compaction
and other processes of diagenesis by the hard surrounding
limestone. In many cases these soft fills contain finely
preserved fossils that were spared reworking, transport, and
exposure on the sea floor.

With increased rate of sedimentation and subsequent
burial, the lithified layers of the Arcola continued to
affect infaunal elements, especially those of the post-
omission suite. When the burrowing crustacea, working in the
newly deposited soft sediment encountered a buried lithified
surface a deflection or imposed horizontality often resul-
ted. This phenomenon is particularly evident at the Hatcher
Bluff (Ala.) exposure where four lithified beds are in close
succession. In some locations in the European section, at

levels of closely spaced hardgrounds, re-excavation of the

62
soft burrow fill of hardground Thalassinoides by post-

omission Thalassinoides produces the effect of a continuous

 

burrow system crossing several omission surfaces without
interruption (Bromley, 1967).

Post-Lithification Suite: It has been shown elsewhere
in this study that hardgrounds vary greatly in morphology.
In some, the lithified surface is planar and substantial
erosion has occurred. Hardgrounds of this type show a pre-
dominance of omission suite borings which post-date lithifi-
cation. In others, including highly irregular or convoluted
hardgrounds, evidence of borers is often subsidiary to that
of burrowing organisms which were contemporaneous with
cementation (Bromley, 1968). Borings in chalk hardgrounds
are fully documented by Bromley (1965, 1968, 1970, 1974).
Groups present include various thallophytes, cirripedes,
sponges, bivalves, and polychate worms.

Since most groups of boring organisms are restricted to
fully hardened substrates and because their walls truncate
grains and carbonate cement binding them (Plate 8), identi-
fication of a bored surface is considered conclusive proof
of sedimentary omission and syn-sedimentary lithification
‘(Plate 73; Bromley, 1965, 1968; Purser, 1969; Shinn, 1959).
Whether the quantity of borings in a hardground provides an
indication of the duration of sedimentary omission, however,
is doubtful (Bromley, 1968). Many hardgrounds apparently
represent the omission of hundreds or thousands of years of

sediment in shallow, oxygenated water (Hofker, 1958:

63
Bathurst, 1975) and yet they are bored to only a slight
degree.

The chief inhibiting factors of boring in hardgrounds
are insufficient hardness of the sediment and burial by new
sediment. Several minor factors may also play an important
role in some hardgrounds. For example, an extremely rich
overgrowth of encrusting organisms (algae) may threaten to
cover the entrances of borers.

If the rate of sediment accumulation is low enough to
allow complete cementation, the rock is likely to be period-
ically uncovered on the sea floor by currents and thus sub-
ject to intensive boring. Conversely, if the rate of accumu-
lation is higher and cementation less complete, or if there
is rapid lithification followed by an increased sedimenta-
tion rate, it is probable that the rock would be permanently
covered with a protective, extensively burrowed layer of
sediment several inches thick, which would prevent the entry
of boring organisms and ”fix" the cavities of previous
borers in a stage of partial destruction. The majority of
borings which are preserved on hardgrounds are, therefore,
those which existed during (and were arrested by) the onset
of periods of sedimentation.

Erosion of the Arcola hardground, either as sediment
scouring or by boring organisms subsequent to lithification,
appears to have truncated the top of the burrow system so

that the entrances to the erosion surface show no special

64
structures. The intervention of scouring at early stages of
lithification is indicated by beds of lag intraclasts.

The erosional rate of organisms boring in rock is often
remarkably high. In many European hardgrounds the sculpture
of the omission surface was completely modified by the
activity of boring organisms. In these hardgrounds, the.‘
activities of clinoids perhaps equalled that of their recent
shallow water counterparts, where rates of biological
erosion of up to 1-4 m per 100 years are known (Neumann,
1966: Kennedy and Garrison, 1975). The result of this activ-
ity is the production of elastic material of all grades.
Fines are usually removed by currents, whereas small to
large, angular intraclasts remain. The angular intraclasts
are products of mechanical breakage of the substrate through
the weakening effect brought about by boring activity.
Broken or crushed grains, often attributed to the effects of
compaction (Scholle, 1974) may rather be the result of
breakdown wrought by boring and burrowing organisms as these
features (fragmentation of skeletal grains) have been
observed in uncompacted chalk hardgrounds (Kennedy and
Garrison, 1975).

In highly convoluted hardgrounds, the prominent bosses
between entrances to the hardground burrows commonly have a
mushroom or bridge-like form. Boring organisms, according to
Bromley (1965) preferentially attacked these structures on
the undersides of overhangs, weakening their bases and caus-

ing them to break off as large irregular intraclasts. The

65
appearance of numerous limestone cobbles on and immediately
above the Arcola hardgrounds may be the result of such
activity.

While the activity of boring organisms may substantial-
ly weaken the rock in their superficial zone of operation,
the thickness of the rock which they facilitate removal of
is not great. Their activity, however, together with current
scour, constitutes the most active agent in eroding the
chalk once it becomes lithified. Perkins (1971) further
observed that the shallower types of borings are destroyed
in erosive succession (due primarily to current scour)
according to their depth of penetration into the substrate,
until in a deeply eroded hardground, only the distal ends of
the deepest borings remain. Intensive boring of a thin hard-
ground may, however, reduce it to an apparent rubble of
clasts of the underlying rock type in a matrix of the over-
lying rock type. As a result, a surface of nondeposition may
appear to be an eroded, unconformable contact, indicative of
a much greater hiatus in the stratigraphic record (Stanton
and Warme, 1971).

Ecological Implications: Ichnological evidence, includ-
ing the abundance of suspension-feeding Thalassinidea, indi-
cates that intermittent currents of low to intermediate
strength were present during Arcola deposition. Burrow sys-
tems are often truncated, the eroded upstanding bosses being
mechanically rolled along the sea floor and frequently

deposited in the Open burrow network. Such currents probably

66
kept the waters moderately well circulated, thus enhancing
the oxygenation of sediments.

Although the abundance of trace fossils has been inter-
preted by Seilacher (1964) as good evidence against highly
euxinic conditions in the depositional environment, Thalas-
sinoid burrows, in abundance, are characteristic, although
not diagnostic, Of restricted subtidal deposits (Enos,
1983). Such burrows, in a sediment of one or a few distinc-
tive types, are indicative of a restricted fauna. It has
been observed that a burrowing infauna can persist in
environments where restriction has become too severe to sup-
port a shelly epifauna (Enos, 1983). Geographic and salinity
restrictions result in simple faunal assemblages with low
diversity, high-member populations. This is due to the abil-
ity Of one or two well-adapted organisms to multiply in a
stress environment with essentially no competition from
other organisms.

The lack or rarity of certain characteristic
bioclasts--brachiopods, bryzoans, crinoids, red algae,
corals, and common molluscs--indicates the bioclastic mater-
ial of the Arcola hardgrounds did not derive from organisms
living in fully open marine conditions, but from shelf
waters, probably somewhat warmer than open marine, nutrient
depleted, and more saline than the Open sea. The physico-
chemical environments which existed from place to place over
the shallow epeiric sea floor in Arcola time would not have

been uniform if for no other reason than the gradually

67
changing salinity. In a previous chapter discussion empha-
sized the significance Of epeiric currents on salinity.
Although these currents obviously existed (truncated bur-
rows, rounded cobbles, etc.), it was shown that they would
only flatten the salinity gradient because once the water in
the current passed beyond the zone of tidal exchange it
would come under the inexorable forces of evaporation, and
the salinity would increase as the water was further and
further away from its source Of normal marine replenishment.
The existence Of hypersaline waters beyond the limits Of
tidal exchange would, therefore, appear to be a normal and
inevitable characteristic of the Arcola and shallow epeiric
seas in general and the primary restrictive factor on what

organisms would survive.

Calcispheres and Dasycladacean Algae

Calcispheres are the major component of the Arcola
limestones (Plate 3A, 3B, 9). They were first described from
Upper Devonian and Mississippian rocks by Williamson (1880).
Subsequently, these small spherical bodies have been
described from a number of Upper Paleozoic and Mesozoic
rocks (Cayeux, 1929: Chapman, 1929: Baxter, 1960; Stanton,
1963; Banner, 1972: Bolli, 1974). All are hollow, having
well-defined outer walls (Often composed of several layers)
and range in diameter from 40-225 pm (Bathurst, 1975; Wray,

1977).

68

Observation Of the Arcola calcispheres reveals an aver-
age diameter of 40-60 pm, although diameters of up to 100 pm
are not uncommon. In thin-section they exhibit a distinct
micritic wall with thicknesses ranging from 3-15 um.
Although multiple-layered walls have been reported (Baxter,
1960: Johnson, 1961), none were Observed in the Arcola. The
internal, originally hollow, cavities are now filled with
microcrystalline calcite or more or less occupied by an
internal sediment of micrite (Plate 3B). Johnson's (1975)
initial S.E.M. observation of these calcispheres indicates a
surface structure consisting of prominant calcite crystals
arranged in rows which radiate from the aperature, which is
represented as a slit or circular opening in the calcisphere
wall. -

The origin of calcispheres has at one time or another
been attributed to a variety of plant and animal taxa.
Stanton (1963) suggested they represent some form of plant
spore or reproductive body based on several similarities
including their tendency to occur clumped together as aggre-
gates. Rupp (1967) and Marszalek (1975) have since recog-
nized the marked similarity Of nonornamented, nonspinose
fossil calcispheres to the reproductive cysts Of living
species of calcareous green dasycladacean algae belonging to
the subfamily Acetabulariae.

Ecological Implications: Since calcispheres are often,
constituents of many carbonate rocks interpreted as having

formed in shallow relatively restricted environments similar

69
to those in which Acetabulariae exist today, it is presently
the concensus that most fossil calcispheres, including those
of the Arcola, owe their origin to dasycladacean algae.
Based on extensive studies of Permian through Early Cenozoic
dasyclad assemblages, Elliott (1968) has further shown that
the ecological requirements of fossil dasyclads were essen-
tially the same as those of living descendents. Thus, when
found in abundance, these algae and the calcispheres attrib-
utable to them, act as excellent paleoenvironmental indica-
tors. In this inquiry it is, therefore, significant to
recognize the conditions under which dasycladacean algae,
particularly those of the subfamily Acetabulariae, are cur-
rently known to exist.

Several studies, including those of Wray (1979) have
related the distribution of various algal groups to paleo-
geographic setting. These distribution patterns are actually
the result of several environmental parameters rather than
being a single independent factor. Wray's idealized distri—
bution of major algal groups shows a predominance of dasy-
cladacean green algae in shallow shelf and lagoonal environ-
ments. Although the spatial distribution of this group may
appear rather broad, the occurrence maxima tends to be
lrestricted to particular environments.

Although the depth range of benthic dasycladacean algae
normally extends from just below low tide to about 30
meters, their maximum abundance is Often confined to depths

of less than 5 meters (Wilson, 1975: Wray, 1977). The depth

70
distribution is largely a function of their adaptation to
growth at specific light levels and their tolerance of
salinity and temperature variation. Dasyclad algae have
their maximum absorption and most efficient photosynthesis
rate in the red section Of the spectrum and therefore flour-
ish primarily at shallow depths (since red is absorbed
first). And although rather small variations in salinity
have considerable influence on planktonic organisms because
Of the accompanying variations in density affecting their
bouyancy, they seem to have little if any influence on the
benthonic algae. As a group the dasycladacean algae prefer
normal marine salinities, but the Acetabulariae, for
example, proliferate in salinities ranging from hypersaline
tO brackish and temperature variations Of 50°F to more than
100°F (Rupp, 1967: Wray, 1977). These characteristics
enable Acetabulariae to grow and flourish in shallow water
Of restricted circulation that is generally inhospitable to
most normal marine biota.

It is therefore apparent that a threshold of dominant
algal production Of calcium carbonate may be reached at very
shallow depths. As Wilson (1975) notes, any geographic situ-
ation resulting in vast areas Of water less than 10 meters
deep may result in several times more calcium carbonate per
unit area than in deeper epeiric seas based solely on
calcareous algal productivity.

The pereneal dasycladacean green alga Acetabularia has

been described by Marszelak (1975) in shallow-water restric-

71
ted environments of the Florida Keys where it is periodical-
ly present in extreme abundances. The hollow calcispheres
from this algae are characteristically 140-185‘pm in
diameter, with a 10-25‘pm thick wall, and like the skeletal
elements in all marine green algae, are composed Of
aragonite. .This alga is not only an important producer of
calcispheres, but an equally important local producer of
aragonite sediments in the less than lo‘pm size range. The
heavily calcified Acetabularia antillana has been shown to
be approximately 75% skeletal carbonate by dry weight,
producing up to 720 gm Of aragonite per standing crop per
m2 of bottom area (Marszelak, 1975).

Periodic blooms have been recorded during which Acetab-
ularia may rapidly become the dominant plant in a local
floral community (Marszelak, 1975). The explosive growth of
Acetabularia in south Florida appears to be limited to
waters with restricted or semi-restricted circulation and
depths not exceeding three meters. There the plants (Acetab-
ularia) attach themselves to all available firm substrates

including sand-sized particles, the thalli of other algae,

shell debris, Thalassia blades and mangrove roots. The

 

growth of Acetabularia is extremely dense and forms a con—
tinuous layer over the bottom which may be several plants
thick and contain individuals in all growth stages
(Marszelak, 1975).

Upon its death, the stalk and disc Of the plant rapidly

disaggregate into aragonite in the less than 10‘pm size

72
range. In addition, indications are that the calcispheres
apparently do not survive transport over any appreciable
distance and may, therefore, require rapid burial or entrap-
ment for their preservation.

Marszelak's (1975) study involving the sedimentary con-
tribution of this alga reveals an accumulation rate of .062
cm/yr. or 1 cm of aragonitic sediment every 16 years. This
rate, however, must be viewed as a conservative estimate
because it was based on the assumption that only one crop of
algae is produced during the year. Certain species (dasy-
cladacean) with tropical affinities, for example, are found
throughout the year around the Mediterranean where they are
Often able to produce several successive overlapping genera-
tions yearly (Feldmann, 1951). In addition, the large number
Of genera and species described from the Mesozoic suggests
that the family reached an acme Of development and produc-
tivity during this time (Wray, 1977). A more realistic
figure may, therefore, be three to five times that presented
by Marszelak (Marszelak, 1975).

Aside from environmental implications, these large
accumulations of skeletal calcareous (aragonite) algae would
obviously be potential sources of soluble meta-stable carbo-
nate materials involved in synsedimentary sea floor lithifi-
cation (hardground formation) and other diagenetic

processes.

VI. CLIMATIC STUDIES

An attempt was made by Urey et al. (1951) to determine
the absolute temperature of the post-Aptian chalk sea using
oxygen isotope methods and belemnite calcite. Elaboration Of
this initial work using other forms Of organic calcite as
well as belemnites (Lowenstam and Epstein, 1954) showed this
to be impossible at present. General trends of climatic
change during the Upper Cretaceous, however, were apparent.

In the southeastern United States a progressive rise in
ocean temperature followed by a general decline was indi-
cated from Cenomanian (Tuscaloosa) to Maestrichtian
(Ripley/Prairie Bluff), reaching maximum temperatures during
Coniacian-Santonian times (Eutaw: Lowenstam and Epstein,
1954). Studies further indicate that during the Coniacian-
Santonian temperature climax, marginal subtropical ocean
temperatures extended northward into the present-day cold-
temperature belt, with this northward displacement decreas-
ing toward Maestrichtian time (Lowenstam and Epstein, 1954).
An increased temperature maximum can therefore be expected
in the Cretaceous epeiric sea in Arcola time.

Although no data are available for stratigraphic zones
nearer the Coniacian-Santonian temperature maximum, includ-
ing the Arcola Member, analyses of upper Selma chalks
(primarily Coon Creek Tongue of the Ripley Formation;

73

74
Maestrichtian) have consistently revealed elevated mean tem-
perature inferences suggestive of a subtropical climate sim-
ilar to that found today in Bermuda (Urey et al., 1951;
Lowenstam and Epstein, 1954). Inoceramus fragments analyzed
by Lowenstam and Epstein (1954) yielded inferred tempera-
tures Of 31.4°C and 29.3°C. Additional specimens from the

same location (near Macon, MS) including Ostreidae and

 

Exogyra cancellata indicated temperature values of 31.4°C
and 29.30C, respectively, while the isotopic abundance
ranges of brachiOpods examined were further suggestive of
temperatures well within the 30°C range (Lowenstam and
Epstein, 1954). Lowenstam and Epstein interpreted these high
values to possibly represent conditions of restricted circu-
lation noting that higher temperatures ranging from 31°-
34.5°C are not entirely unreasonable as they lie within the
range of upper extremes in present-day shelf sea waters of

restricted circulation.

VII. COMPARISON WITH OTHER HARDGROUNDS

European chalk hardgrounds differ from the Arcola hard-
ground: the differences reflecting the deeper water setting
and Open shelf circulatory pattern Of the region in con-
trast to the restricted circulation prevailing during
deposition of the Arcola.

Diagenetic differences reflect differences in original
sediment composition. The abundance of calcispheric arago-
nite in the shallower Arcola is in direct contrast to the
predominantly stable low-Mg calcite skeletons of coccoliths
and planktonic forams comprising the European hardgrounds.

Lithification Of the European sea floor was brought
about by calcitic cement precipitated from sea water upwell-
ing from the deeper neighboring regions. It is considered
that these hardgrounds represent a region of the sea floor
locally raised above the surrounding floor by anticlinal
movement (Bromley, 1965). Currents passing over this area of
the European sea floor may have caused an increase in satur-
ation with respect to phosphate. The upwelling of nutrient-
rich waters would result in increased fertility in the shal-
lower sea over the area of nondeposition, and increased
planktonic growth could be expected. The rain of zooplankton
excrement would tend to enrich the bottom waters with phos-
phate, where it would accumulate over and be incorporated

75

76.
into the hardground surface. The European chalk sea was deep
enough (always greater than 50 m; Bromley, 1965) and warm
enough for sufficient regeneration of orthophosphate to have
taken place.

Phosphate mineralization of the Arcola hardgrounds was
probably inhibited by excessive shallowing. In shallow seas
(less than 30 m) there may be a temporary ”lock-up" of phos-
phorous which is thus lost to the cycle, possibly by com-
plete and continual uptake by phytoplankton or lack of sub-
stantial ascending phosphate-rich source waters (Bromley,
1965; Ehlers and Blatt, 1982).

It is generally considered (Burst, 1958: Hower, 1961:
McRae, 1972) that the diagenetic replacement Of degraded
layer silicates (particularly clays) is the major process of
glauconite formation. The apparent lack Of any appreciable
amount Of clay in the Arcola hardgrounds ((2%: Dinkins,
1960) may, therefore, in addition to insufficient water
depths and elevated temperatures, have inhibited glauconite
precipitation.

The abundance of chert (derived mainly from siliceous
sponges) in European hardgrounds is a further indication of
the diversity of benthic organisms reflecting the more open
connections of the European shelf to the Atlantic and
Tethyan oceans which led to stronger water circulation and
more normal salinity compared tO the semi-restricted Arcola

sea.

VIII. SUMMARY

The study of the lithology and certain aspects of the

fossil record of the Arcola Limestone Member and other chalk

hardgrounds which has occupied the previous chapters Of this

thesis has revealed evidence of the changing conditions

under which these units formed. The various aspects of the

changing environment will be reviewed, concentrating on the

conditions which prevailed during deposition of the Arcola.

1.

Lithification Of the Arcola Member proceeded in a semi-
restricted marine environment under water depths of
less than 30 meters.

The broad expanse Of shallow water effectively dissipa-
ted the energy of large currents and tidal exchange
through friction with the bottom.

Wind-driven epeiric currents apparently existed,
although they served only to flatten the salinity
gradient. As the water in these currents passed beyond
the zOne of tidal exchange, water was permanently lost
through evaporation and, as a consequence, elevated
salinities developed.

Shallowing did not correspond with any marked approach
Of shoreline, the access to dilution effects of signif-
icant terrestrial run-Off being limited to extreme
northern and southeastern regions.

77

78
Salinity and temperature restriction resulted in simple
well-adapted faunal/floral assemblages (Thalassinidea
and Dasycladacean algae) of low diversity and high mem-
ber populations.
The scarcity of coccoliths and planktonic foraminifera
in the Arcola limestones and their diversity in the
intervening marls is attributed to environmental fac-
tors; that this change is related to increased water
depth and a trend toward Open marine conditions is sug-
gested by the Overall lithologic evidence and by modern
environments in which these organisms accumulate.
The lithification process has been divided into a
series of progressive stages: A) formation Of an omis-
sion surface: B) growth of nodules leading to develop-
ment of nodular chalk, erosion of which produces intra-
formational conglomerates; C) fusion or coalescence of
nodules into continuous or semicontinuous lithified
layers (incipient hardground); D) current scour to
expose the lithified layer as a true hardground; E)
repetition of the sequence to produce composite hard—
grounds.
Based On the mineralogic nature of calcisphere-bearing
dasycladacean algae and analogy with modern areas of
sea floor lithification, the original cement was prob-
ably aragonite.
The cementation process was accompanied and aided by

enhanced current activity, possibly through barrier

10.

11.

12.

13.

14.

79
vorticles and the activity of burrowing organisms--
primarily Thalassinidea.
The thickness of the hardened beds is attributed to
increased grain size of the calcispheric sediment:
coarse sediments enabled the introduction Of greater
quantities of cement through increased permeability and
water circulation.
Once lithified and exposed on the sea floor the hard-
ground was modified by current scour and, to a slight
degree, by boring organisms.
Clay content in excess of a few percent may have inhib-
ited the develOpment of lithification in the Arcola
marls; clay and colloidal material tend to form coat-
ings on larger particles and Open networks next to
smaller particles in which the permeabilities are
reduced to only a small fraction Of their original
values.
The chalks and marls of the Arcola (being composed pre-
dominantly Of stable low-Mg calcite coccolith and fora-
minifera debris) lacked an adequate source of meta—
stable carbonate (argonite/calcispheres) which may
have, by its dissolution and precipitation, given rise
to a cement.
The lack of appreciable amounts Of clay minerals within
the limestones Of the Arcola may have inhibited glau-

conite formation.

15.

16.

17.

18.

19.

20.

80
The varigated color and general absence of glauconite
in the hardgrounds of the Arcola suggest a very shallow
water environment (LeOpOld and Pakiser, 1964).
Phosphate mineralization of the Arcola was probably
prevented by excessive shallowing.
The apparent rhythmicity of the Arcola (Selma Group) is
a reflection of the overall cyclic nature of shelf sea
chalk deposits, involving variations in sedimentary
rate with depositional phases separated by phases of
nondeposition: the hardground being the extreme product
of this variation.
The larger composite sequence in central Alabama sug-
gests an increased rate of intérmittant subsidence
which effectively removed the hardgrounds from pro-
longed effects of exposure at the sea floor (erosion)
by raising the base level of deposition above the sea
floor and permitting sedimentation (resulting in a pro-
tective layer Of marl).
European chalk hardgrounds differ from the Arcola: the
differences reflecting their deeper water setting and
Open-shelf circulatory pattern of the region in con-
trast to the restricted circulation prevailing during
Arcola deposition.
The general poverty of the Arcola hardground epifauna
is presumably a reflection Of insufficient water depth,
poor or inadequate water circulation, high water tem-

peratures and excessive salinites.

IX. CONCLUSION

Early lithification of the Arcola Member proceeded in a
semi-restricted marine environment under water depths of
less than 30 meters. Water depth, restricted circulation,
aragonite solubility, and sediment permeability were the
main factors controlling its distribution. Regression of the
lower Selma sea resulted in a broad expanse Of shallow water
which effectively dissipated the energy of large currents
and tidal exchange. Although smaller epeiric currents may
have existed, normal marine salinities could not be main-
tained and faunal/floral assemblages tolerant of the
restrictive, warm hypersaline conditions flourished. Eleva-
ted temperatures further increased the super saturation of
the water with respect to calcium carbonate. This water,
impinging on and permeating through the coarse calcispheric
sediment precipitated aragonite. Hardground development then
proceeded through a series of stages, the lithification pro-
cess being accompanied and aided by intermittent current
activity, possibly through barrier vortices,and the flushing
action of burrowing Thalassinidea. The depth to which cemen-
tation progressed beneath the omission surface was directly
related to the permeability of the sediment which, in turn,
was a function of its unique biogenic components (calci-
spheres). Mineralization of the Arcola hardgrounds was prob-

81

82
ably inhibited by insufficient water depth and lack Of clay

minerals.

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PLATES

 

Plate 1A. Outcrop of Arcola Member at China Bluff

Alabama (32-23N-ZN) .

 

Easter-most typical exposure of the Arcola
ng,. Montgomery County, Alabama (25-14N-18E).

Plate 18.
near Down‘l

 

Plate 2. Composite sequence at Hatcher Bluff,
Dallas County, Alabama (36-16N-IOE).

 

Plate 30. Calcisphere packstone from the Tibbee Creek Section,
Clay County, Mississippi (4-19N-168).

 

Plate 38. Calcispheres with sparry calcite infilling from
Tibbee Creek.

 

Plate 4. Glauconite zone (white light) directly beneath the
Arcola at Tibbee Creek, Clay County, Mississippi (4-19N-16E).

 

Plate 5A. Northernmost exposure of the Arcola near
Tupelo, Lee County, Mississippi (34-9S-6E).

 

Plate 58. Arcola Member near Tupelo, Mississippi;
note Exggyra mnderosa fragments affixed to surface.

 

Plate 6A. Photomicrograph of the Arcola Limestone from
Tupelo, Mississippi location.

 

Plate GB. Photomicrograph of the Arcola Limestone from
Tupelo, Mississippi location (crossed nichols).

 

Plate 7A. Dominantly horizontal thalassinoid burrow
system (indicated by arrows) at China Bluff section
(Alabama).

 

Plate 78. Thalassinoid burrows in fallen boulder at
China Bluff, Alabama: note oblique and transverse
sections of prominent borings at top of pencil and
upper right surface, respectfully.

 

Plate 8. Contact between calcispheric Arcola and bore infilling.

 

Plate 9. Arcola calcispheres from east-central Noxubee County,
Mississippi (29-14N-19E).