biostratigraphic analysis of eocene clay deposits in henry county ...

interpret the history of the deposition of
lh('s(' clay lens('s. However. the
nwgafossils combinf>d with othf>r floristic
rea turt's and sedimentary features provide
data that can be applied to the history of
deposition of the clay lens under consideration.
We suggest a model for this
deposition here.
The depositional model presented is
based primarily upon the vertical relation
of the clay lenses to one another. It also
provides an explanation for the vertical
variation observed in the pollen profiles of
these clay lenses. There are some changes
in floristic composition, leaf morphology,
and cuticular anatomy observed between
deposits. Also the depositional history of
each clay lens can be traced through
phases, which resulted in similar sedimentary
sequences or parts of sequences for
most clay lenses studied. Support for this
model of the ancient flood-plain sediments
in western Tennessee is also provided by
evolutionary trends of select leaf types.
INTRODUCTION
Clay lenses are commonly distributed
along the sedimentary beds of the
Mississippi Embayment that are exposed
in a narrow zone running northeast by
southwest through western Kentucky and
Tennessee. The age of these sediments
ranges from Upper Cretaceous to middle
Eocene. Following the Embayment axis
(northeast-southwestf the Cretaceous
sediments are exposed along the extreme
eastern margin of this zone and the
youngest sediments lie to the west. This
report is concerned with those clay lenses
in western Kentucky and Tennessee which
are considered to be part of the Claiborne
Formation deposited during the middle
Eocene (Figure 8.l1.
We have examined the sediments
from numerous clay pill; in the area shown
in Figure 8.2. Nearly 2S clay pits have
yielded fossil leaves, fruits, seeds, wood,
flowers. and pollen. Each clay body
212
represents an isolated depositional event
in space; and the question to be addressed
is, do these clay deposits also represent
isolated events in time? And, if these
depositional events do represent limited
and different segments of middle Eocene
time, can some orderly arrangement of
their sequence of deposition be
discovered? This paper is an attempt to
arrange these clay pits in a stratigraphic
sequence. The evidence available is the
preserved palynomorphs, megafossil
material, and the geomorphology of the
clay bodies. The work presented here is an
attempt to establish a preliminary sequence
as a working model to be refined
and affirmed or corrected by further and
more detailed study. The importance of
establishing such a sequence is that it
allows the use of the Mississippi Embayment
sediments as a biostratigraphic tool
to finely divide and sort out in
chronological order vegetational changes
of plant ecosystems and evolutionary
modifications of plant fonn in the structurally
preserved fruits, seeds, leaves,
flowers, and wood common in these
sediments.
Many leaves, fruits, seeds, and
flowers are restricted to particular clay
pits; a few fossil forms are common to
nearly all pits. Palynomorphs, as well,
may be restricted to particular clay pits or
even to particular zones within a clay pit
(Potter, 1976). A careful comparative
study of the pollen types of various clay
pits has not been done, but it would yield a
great deal of useful data. Isolated pollen
samples have been analyzed and
numerous megafossils have been collected
from these clay pits. The data available
from these studies are used in this report.
Recent pollen studies of the
Mississippi Embayment region have been
summarized by Elsik (1974); Elsik and
Dilcher, (19741. Fairchild and Elsik
(19691, Frederickson. (19691, Potter
(19761 and Tschudy (l973a, b, 19751.

These investigators recognize the
usefulness and importance of the pollen
profiles of these isolated clay deposits and
some establish particular zones based on
the presence or absence and the abundance
of various pollen types. Basic pollen
types have been recognized for the Eocene
sediments of the embayment area and
Fairchild and Eisik (1969); Tschudy
0973bi: and Elsik (974). present index
palynomorphs for the Gull Coast (based
on presence or absence and abundance of
pollen types) that allow an investigator to
discriminate among lower, middle, and
upper Eocene sediments. Tschudy charts
the occurrences f the palynomorphs in
the Mississippi Embayment indicating
limited stratigraphic ranges of several
within the middle Eocene; Elsik also plots
relative abundances of palynomorphs for
the Claiborne sediments of the Texas Gulf
Coast. These ata suggest that
palynomorphs can be used to place these
isolated clay lenses in the Wilcox,
Claiborne, or Jackson (lower, middle, upper
Eocene) formations and possibly also
provide precise information as to subdivision
of member age equivalent for middle
Eocene time.
The ability to subdivide Claiborne
time using palynomorphs (Tschudy.
1973b gave some hope that each of the
numerous isolated clay lenses in estern
Tennessee and Kentucky could be placed
stratigraphically if pollen samples were
analyzed. Before such a general project of
sampling was undertaken. however, we
felt that it was critical to examine the
nature of a single clay lens in some detail.
Therefore, the palynomorphs of the Miller
clay pit were examined from precisely
located samples lvertically at 10-cm intervals)
from the base up to the top of a
lOom-thick seam of clay-lignite-clay
exposed in open pit mining. As a result of
this work. Potter (1976) found that the
relative abundance of pollen types present
varied as the lithology of the sediments
214
varied, indicating a strong environmental
control influencing this aspect of the
pollen profile. Potter questioned the
stratigraphic usefulness of the abundance
of palynomorphs and indicated that some
pollen types may be more important as environmental
indicators than as indicators
of the subdivision of Claiborne time.
Detailed pollen profiles of several clay pits
open in this area should be completed.
Such studies will provide important data
to test the model proposed here of the
depositional history of these lenses.
Early paleobotanical reporting was
done by Berry (1916, 1930 Ion the floras of
over 200 Pal ogene plant-bearing
localities (many of which were from clay
deposits) in southeastern North America.
He suggested a paleoenvironment for
deposition of such plant-bearing
sediments generalized from a locality he
examined in Texas. He concluded that
these clay deposits represent near-marine
paleoenvironments with strand vegetation
while some others represent drowned river
valleys Berry (1916, 1930).
Dilcher (1971) considered the origin
of the clay lenses only in western Kentucky
and Tennessee. The information applied
to this question was derived from
outline maps of known clay deposits, the
form of their cross-section, and the
nature of sediments underlying the clays.
Potter (19761, concentrating upon a single
clay lens, carried this analysis further.
The results of these studies demonstrate
that the small clay lenses typica of Henry
County, Tennessee, and nearby areas are
narrow, Linear-elongated, most often
curved, river-channel shaped in
cross-section, surrounded on the sides
and undersides by cross-bedded sands
(occasionally fine gravels), and consist of
uniform fine clays with an occasional
Lignitic seam or cap. Primarily upon this
information, we proposed that these
smaU,curved, elongated clay lenses represent
clay plugs in oxbow lakes on an an­

personal communication 1 an upper
Claiborne (Cockfield Fm., and perhaps
Cook Mt. Fm. or lower Cockfield Fm.1
age for them.
Local
The marine transgressions of the
Eocene (Figure 8.3l can not be identified
in the Henry County area of western Tennessee
(Parks, 19711. The more extensive
transgressions such as those separating the
Wilcox, Claiborne, and Jackson groups
can be identified only in the southwestern
corner of Tennessee (Stearns, 1957; Murray,
19611. The Claiborne Formation in
Henry County, Tennessee, consists mainly
of non-marine sands and isolated disjunct
lenses of clay. As already discussed,
there are numerous plant fossil-bearing
lenses of clay in this area. Data from a
regional consideration of the pollen flora,
where it can be tied into marine sequences.
indicates that these lenses are
best considered to be late-middle Eocene
in age ICockfield Formation of the
Claiborne Group) \Elsik. ]974; Elsik and
Dilcher. 1974; Tschudy, personal communicationl.
Differences in the floras of
particular clay pits in Henry County, Tenn
.ssee. ano near-by areas are obvious by
a comparison of the composition of the
floras from each pit and by the specific
nature of the epidermal cells, trichomes,
and aspects of the overall leaf form of
several taxa. Two explanations for these
differences are: ecological variability or
evolutionary variability. It is important to
attempt to reconstruct the paleoenvironments
and relative ages of these clay
pits in order to understand the cause of the
variations observed in the fossil record
between these sites.
The ecology of most deposits seems to
have repeated a relatively similar history
for each clay lens. As discussed earlier in
this paper and by Dilcher (19711 and Potter
(1976), the nature of these sediments
indicates a typical oxbow-lake cleposi­
21/
tional system. Each clay lens repeats a
similar environmental history with some
variation in the extent of associated lignite
and relative abundance of plant fos"ik
Thus environmental factors most probably
do not account for the differences
observed in the plant fossils from clay pit
to clay pit.
The most logical explanation for the
observed differences of the types, relative
abundance, form, and anatomy of the
plants at the various localities is that each
locality represents a slightly dilferent segment
of time. The time interval of evolu­
,ion preserved in each clay pit is very short
1500 to 1500 yearsl because oxbow lakes
on a flood plain are recognized as shortterm
events when viewed against
geological time (Crisman and Whitehead,
1975). The tools available at the present,
such as palynology and knowledge of the
megafossils are probably not sufficient to
rank order these individual clay pits
within Cockfield Formation time. The
data provided below is presented in an attempt
to establish a rank order for these
clay pits within late-middle Eocene time.
DEPOSITION OF FOSSIL·
BEARING SEDIMENTS
The low-lying features of the
Mississippi Embayment allowed for easy
migration of river s}'stems across broad
flood plains; as sediments brought in by
the rivers accumulated. Deposition of lhe
flood-plain sediments. discussed in lhis
paper. resulted from the meandering of
the"Appalachian" River system entering
the embayment in Kentucky and building
a poorly defined delta in North Central
Mississippi (Grim, 19361.
Based upon field examination; the
clay pits can be differentiated into two
types. One lype ha!l relatively massive
deposits of clay and some associated
lignites. Often these lignite seams overlie
the clays, although in some localities the
lignite is interspersed with the clays. No

good megafossils have been found in these
clays. The lignites arc sufficiently decomposed
to obscure plant structure and in
some localities numerous root channels extend
from the lignite into the underlying
clay. These features suggest a low-lying
area of shallow water receiving fine-grain
sediments interspersed with some swamp
environments with accumulation of
organic material.
The second type of clay pit consists of
small isolated clay lenses, often accompanied
by overlying lignite seams.
Generally, these lenses have 5 to 10 m of
light tan clay lacking micro-and
megafossils, overlain by dark brown to
gray clay rich in plant fossils. The overlying
lignites consist of compacted "leaves,
wood, fruits, and seeds; and occasionally
above the lignites are scattered layers of
light tan, gray, to pink clays or sands. The
clay deposits are underlain by crossbedded
channel sands (as seen at the
Puryear site) and unsorted pea-size
stream gravels (the Warman sitel; and, in
outline and cross-section. they are
characteristic of abandoned channels apparently
representing meander cutoff
sloughs, oxbow lakes, and stream reaches
(Figure 8.41. Field examination indicates
a common depositional history for most of
the lenses of clay. The fossil-bearing clays
are a product of fine sediments, which filled
the oxbow lakes on the ancient flood
plain. The lignites accumulated as the
lake was filled with clay, forming a wet
swampy environment. These events probably
took about 500 to 1500 years.
Several clay lenses contain fossil
assemblages representing both of the
above paleoenvironments, some lack any
evidence of lignite accumulation, but all
represent short individual segments of
time.
Sufficient sites are present in western
Tennessee to permit comparison of vertical
position as determined from
topographic maps and overburden ex­
218
posures observed in the field (Figure 8.41.
Although not all sites align precisely, an
initial eastward meandering-channel
shift is suggested. Once reaching the
eastern margin of the embayment. continued
deposition shifted the channel back
toward the basin axis. This resulted in a
sequence of channel deposits ascending in
age, oldest near the base of the series along
the west edge of the study area, the
youngest also at the western edge but at a
higher elevation (Figure 8.51. The eastern
pits. e.g., Puryear, are of intermediate
age. Thus the sequencing of the relative
ages of the pits is based upon their relative
geomorphic position in the flood plain.
The whole series of clay lenses represents a
geologically short time span, the last
sweep of the river having occurred during
late-middle Eocene (Claiborne Group)
deposition.
The lithology of the two pit types provides
supporting evidence, the basal clay
and lignite sites are believed to represent
an almost flat lowland-swamp
flood-plain environment sloping approximately
three feet per mile southward
(Figure 8.6a), These lowland, swampy
areas were dissected and buried by the
sediments of the shifting river system that
produced the ascending series of clay
plugs (Figure 8.6bl. The amount of silt
and clay accumulation and organic content
of these plugs depended on the
degrees of isolation from the main channel
as the river returned to its original course
(Figure 8.6c).
This model provides a straightforward
explanation for the data presently
available regarding the nature of the fossil
plant record and sedimentology of these
clay lenses. The fossil assemblages can be
examined for evolutionary changes and
environmental influences independently of
their use to establish the time controls.
Selected megafossil distribution
among the clay deposits is presented in
Figure 8.7. This figure is presented to

--------
FIGURE 8.6 Meander sweep of the Appalachian River system during Late-Middle
Claiborne time. a, initial position; b, eastern extent of migration; c, end of migration cycle.
22J

show that more data than is presently
available is required in order to use
megafossils as reliable indicators for arranging
these clay deposits in an ordered
sequence. Much more work is needed to
sort out ecological versus stratigraphical
control over the occurence of megafossils.
To relate the clays to the regional
stratigraphy of the embayment, the most
reliable and useful fossil material in these
terrestrial clay deposits is pollen. The
megafossil material reflects local paleoenvironments.
The variability observed in
angiosperm leaf cuticles provides
recognizable differences between elements
of the floras that otherwise are difficult to
distinguish. Pollen from paleoenvironmentally
equivalent samples of the
clay deposits discussed here have not been
studied. Perhaps if this were done, pollen
would be as useful for local as well as
regional stratigraphy.
SEQUENCE OF DEPOSITION
The suggested sequence of deposition
presented in Table 8.1 is primarily derived
from the information provided in Figure
8.5. The vertical distributions of these clay
deposits in western Tennessee all appear
to be restricted to the upper-middle
Eocene (Cook Mt. and Cockfield formations,
Figure 8.3). Their preservation probably
results from their burial by subsequent
sediments laid down in the rather
extensive transgression marking the boundary
between the Claiborne and the
Jackson groups (Figure 8.3). These clays
deposited in a flood-plain environment
avoided destruction until their final
burial. Many of these clay deposits are
similar in age, while those deposited
deeper into the flood-plain sediments are
most probably older. This depositional sequence
is the first approximation of such
events in the Mississippi Embayment and
is proposed as a model for subsequent consideration
.
223
POST DEPOSITIONAL HISTOR Y
Individual clay lenses often consist of
clays that range in color from dark gray.
brown, tan, cream, white, pink to red.
Previously, some field geologists held the
idea that the color of this clay could be used
as a field guide to differentiate
sediments of the Wilcox Group from those
of the Claiborne Group. The palynology
and the megafossils in the clays
demonstrate that each deposit is a single
unit of a single age. The differences in color
are of no value to mark one group or
formation from another in these deposits.
The variations in color represent differences
of postdepositional history that
each clay lens and each part of a clay lens
has undergrone.
The gray and brown colors are the
result of unoxidized organic material; the
lighter colors indicate either a total lack of
organic materials or some degree of oxidation.
Subsequent coloring by iron is common,
producing pink or red days. Once
deposited, the organic matter in a clay lens
will not oxidize unless the water table is
lowered. If only the upper portion of the
clay lens extends above the water table.
only the upper portion will oxidize. If the
entire lens is above the water table it will
oxidize from the top, bottom, and sides,
slowly discoloring the organic-rich clays.
Oxidation of such a lens of clay will proceed
unevenly and probably take a great
deal of time to be completed. Often a wide
(4-10 feet) band of gray or brown clay may
zxtend through the central portion of a
lens surrounded by tan and cream colored
clays as a result of depressed water tables.
Changes in the water table surrounding
the clay lenses in Tennessee are
possibly an ancient phenomenon and may
have been a seasonal evenl. Some oxidation
of these clay lenses may be the result
of Recent water-table fluxuations in
Henry County, Tennessee.

CONCLUSIONS
The leaf floras, fruits and seeds,
dispersed cuticle, and palynology of the
clay deposits in Henry County, Tennessee
indicate a similar (but not identical) age
and paleoecology for these sediments.
These same fossils also demonstrate some
unique differences among the individual
clay deposits. Many of these differences
can be related to the slightly different ages
of the individual clay lenses. But very few
of these differences are related to the
paleoecology of individual lenses when
similar sediments of two deposits are compared.
Thus, while these differences can
be recognized for each deposit, it is extremely
difficult to use this data to order
rank the clay deposits from oidest to
youngest. In order to circumvent this problem
and establish a working model of the
relative ages of these clay deposits their
ages have been correlated with their
relative elevation of deposition upon a
common ancient flood plain.
The following conclusions have been
discussed above.
1. Many of the clay pits in Henry County,
Tennessee, represent clay plugs in oxbow
lakes on an ancient flood plain.
2. Probably the clay pits represent a last
major sweep of the .. Appalachian"
River system through its flood plain
near the end of the middle Eocene.
3. The basin continued to subside
throughout middle and post middle
Eocene time, burying the river-channel
features by later transgressions of
the Embayment and thus effectively
preventing their loss by further floodplain
activity.
4. The extent of the entire depositional
events discussed here was confined to a
moderately short span of geologic time.
of sufficient length however, to provide
an excellent model for detailed evolutionary
studies between individual
localities.
5. Each individual clay pit represents a
224
very-short-term event lasting approximately
500 to 1500 years.
6. The fossil plants preserved within the
clay lenses in Henry County, Tennessee,
contain a record of
predominantly flood-plain vegetation.
7. Many clay pits represent more than one
paleoenvironment (clay sediments
represent open oxbow lakes and lignites
closed swampy environments).
8. The postdepositional history of these
clay lenses reflects the influence of
changes in the water table allowing oxidation
of those clays that contain
organic material. The oxidized clays
are light in color; those rich in organic
material are darker.
9. Some floristic variations observed may
be ecological while the variations
observed in the leaf form and anatomy
of the cuticle of similar plant fossils
found in several clay pits probably
represent evolutionary changes, illustrating
differences in age of individual
pits.
REFERENCES
Berry, E. W. 1916. The Lower Eocene
floras of southeastern North America.
U.S. Geol. Surv. Prof. Paper 91, 481p.
Berry, E. W. 1930. Revision of the Lower
Eocene Wilcox flora of the southeastern
United States. U.S. Geol. Surv. Prof.
Paper 156, 196p.
Crisman. T.L., and D. R. Whitehead.
1975. Environmental history of Hovey
Lake, Southwestern Indiana. Am.
Mid. Nat. 93: 198-205.
Dilcher, D. L. 1971. A revision of the
Eocene flora of southeastern North
America. Palaeobotanist 20:7-18.
Elsik, W. C. 1974. Characteristic Eocene
palynomorphs in the Gulf Coast,
U.S.A. Palaeotographica, B
149:90-111.
Elsik, W. C., and D. L. Dilcher, 1974.
Palynology and age of clays exposed in
Lawrence clay pit, Henry County, Ten­