One of the most spectacular signatures of global “Oceanic Anoxic Events” (OAEs) of the Cretaceous was deposited at the Cenomanian–Turonian Boundary. This global oceanic anoxic event is also referred to as Cenomanian–Turonian Boundary Event (CTBE). This event is marked by the deposition of finely laminated organic carbon rich sediments deposited under oxygen depleted conditions. The main goal of the present research is to get a better understanding of the marine biota characterizing the oceanic anoxic event in the Calabar Flank. Core samples obtained from two (2) study wells in the Calabar Flank, southeastern Nigeria were utilized for this study and standard biostratigraphic sample preparation/ separation and analytical approaches were applied in the course of the study. The Cenomanian – Turonian age was assigned based on age diagnostic foraminifera (Hedbergella crassa, Heterohelix moremani, Heterohelix planata, Heterohelix reussi, Hedbergella delrioensis, Hedbergella planispira) and age diagnostic palynomorphs (Steevesipollenites binodosus, Ephedripites sp, Leiotriletes sp, Classopollis sp, Classopollis classoides, Classopollis annulatus, Ephedripites jansonii, Cretacaeiporites mulleri, Cretacaeiporites polygonalis, Galeacornea clavis and Triorites africaensis). The sediments of the study wells were deposited in a range of environments from non-marine to mid neritic and the recovered foraminifera are characterized by the presence of abundant but dwarfed planktic forms and low diversity of dwarfed arenaceous forms at some intervals which strongly support deposition in an oxygen depleted environment.

1. Cenomanian – Turonain Foraminifera and Palynomorphs from the Calabar Flank, South Eastern Nigerian: Implications for Age and Depositional Environment
IJGM
Cenomanian – Turonian Foraminifera and Palynomorphs from
the Calabar Flank, South Eastern Nigeria: Implications for Age
and Depositional Environment
1,2
Ukpong A.J, 2
Ekhalialu O.M*
1
Gombe State University, Gombe, Gombe State, Nigeria (Sabbatical)
2
University of Calabar, Calabar, Cross River State, Nigeria
One of the most spectacular signatures of global “Oceanic Anoxic Events” (OAEs) of the
Cretaceous was deposited at the Cenomanian–Turonian Boundary. This global oceanic anoxic
event is also referred to as Cenomanian–Turonian Boundary Event (CTBE). This event is marked
by the deposition of finely laminated organic carbon rich sediments deposited under oxygen
depleted conditions. The main goal of the present research is to get a better understanding of the
marine biota characterizing the oceanic anoxic event in the Calabar Flank. Core samples obtained
from two (2) study wells in the Calabar Flank, southeastern Nigeria were utilized for this study
and standard biostratigraphic sample preparation/ separation and analytical approaches were
applied in the course of the study. The Cenomanian – Turonian age was assigned based on age
diagnostic foraminifera (Hedbergella crassa, Heterohelix moremani, Heterohelix planata,
Heterohelix reussi, Hedbergella delrioensis, Hedbergella planispira) and age diagnostic
palynomorphs (Steevesipollenites binodosus, Ephedripites sp, Leiotriletes sp, Classopollis sp,
Classopollis classoides, Classopollis annulatus, Ephedripites jansonii, Cretacaeiporites mulleri,
Cretacaeiporites polygonalis, Galeacornea clavis and Triorites africaensis). The sediments of the
study wells were deposited in a range of environments from non-marine to mid neritic and the
recovered foraminifera are characterized by the presence of abundant but dwarfed planktic forms
and low diversity of dwarfed arenaceous forms at some intervals which strongly support
deposition in an oxygen depleted environment.
Keywords: Cenomanian, Turonian, Calabar Flank, Oceanic Anoxia Event.
INTRODUCTION
The earliest study on Oceanic Anoxic Events (OAEs) was
carried out by Schlanger & Jenkyns (1976) and was
defined as a global-scale transient period of marine anoxia
followed by periods of widespread deposition of organic
carbon-rich sediments at the Aptian-Albian and
Cenomanian-Turonian boundaries. The Oceanic Anoxia
Event (OAE 2) also referred to as Cenomanian–Turonian
Boundary Event (CTBE) is characterized by finely
laminated organic carbon-rich sediments deposited under
oxygen depleted conditions. This boundary/transition is
one of the most studied boundaries in the Cretaceous and
it is dated 93.3Ma by International Commission on
Stratigraphy drafted by Cohen et al., (2015). It is
widespread and prominent and occurred during the
warmest interval of the Cretaceous greenhouse when the
bottom-water temperatures were far higher than today; up
to 25°C (Huber et al.,1999, 2002; Friedrich et al., 2008).
Intensive research has been carried out on the
Cenomanian - Turonian boundary interval of the Calabar
Flank in order to understand the Oceanic Anoxic Event and
their associated marine biota (Nyong and
Ramanathan,1985; Petters ,1980; Petters and Ekweozor,
1982; Ukpong and Ekhalialu, 2015; Akpan, 1985, 1992,
1996). It is typically represented by the worldwide
deposition of organic-rich deposits such as the Ekenkpon
Shale in the Calabar Flank. The Cenomanian – Turonain
*Corresponding Author: Ekhalialu Ogie Macaulay,
University of Calabar, Calabar, Cross River State, Nigeria.
E-mail: ansmacaulay@gmail.com
International Journal Geology and Mining
Vol. 4(1), pp. 156-164, April, 2018. © www.premierpublishers.org. ISSN: 3019-8261
Research Article

2. Cenomanian – Turonain Foraminifera and Palynomorphs from the Calabar Flank, South Eastern Nigerian: Implications for Age and Depositional Environment
Ukpong and Ekhalialu 157
Figure 1. Geologic map of the Calabar Flank showing the location of the two wells
age according to the International Commission on
Stratigraphy drafted by Cohen et al, (2015) range from
100.5Ma – 89.8+ 0.3Ma. This interval of time in the
Calabar Flank is characterized by periods of remarkable
low oxygen levels based on foraminiferal studies by Nyong
and Ramanathan (1985), Petters (1980), Petters and
Ekweozor (1982), Ukpong and Ekhalialu (2015) while
Ichnological analyses and macrofaunal evidence by Akpan
(1985, 1992, 1996) support periods of changing oxygen
levels (long anaerobic and short aerobic periods). The
wells (4N/2W and O/2W) under investigation were drilled
at Mbarakpa, Calabar Flank (Figure 1) for limestone
exploration and the samples were obtained in order to
study the foraminifera and palynomorphs assemblages
from the dark grey fissile shale of the Cenomanian –
Turonian boundary interval in the Calabar Flank.
Geologic setting and stratigraphy
The Calabar Flank (Murat, 1972) is an epeirogenic basin
situated in the easternmost part of the sedimentary basins
in Southern Nigeria (Nyong and Ramanathan, 1985). It is
bounded by the Cameroon volcanic trend to the east, the
Ikpe platform to the west, the Oban Massif and Calabar
hinge line to the north and south respectively (fig 2). It was
thought to be the south – eastern extension of the Benue
Trough trending in a Northwest – Southeast (NW-SE) and
lying between latitudes 40 50’ - 50 50’N and longitudes 70
50’ - 80 50’E (Petters, 1982). The origin of the Calabar
Flank is closely associated with the Rift System that
formed the Benue Trough during the final opening of the
South Atlantic from Africa (Petters, 1982, Petters et al,
1995). Sedimentation in the Calabar Flank started with the
deposition of Fluvio-deltaic shale, mudstone and arkosic
sandstone of the Awi Formation dated to be of the Aptian
age (Adeleye and Fayose, 1978). Ekhalialu et al., (2016)
carried out detailed description of the Awi Formation. The
Awi Formation unconformably overlies the Precambrian
Oban Massif (it consists of weathered granite, schists,
migmatite and gniesses). Carbonate platform of the
Mfamosing Limestone directly overlies the Awi Formation
(Petters, 1982) believed to be deposited during the marine
transgression in the Gulf of Guinea in the Mid-Albian times
(Akpan, 1992). The Mfamosing Limestone is overlain by
thick sequence of black to grey shale unit, the Ekenkpon
Formation (Petters and Reijers, 1987). The formation is
characterized by minor intercalation of marls; calcerous
mudstone and oysters beds (Akpan, 1996). This shale unit
was deposited during the Late Cenomanian-Turonian
times. The Ekenkpon Shale is overlain by a thick marl unit;
the New Netim Marl (Petters et al., 1995). This unit is
nodular and shaly at the base and is interbedded with thin
layer of shales in the upper section. Foraminefera age
suggest Early Coniacian age for this marl unit (Nyong and
Ramanathan, 1985). The New Netim Marl is
unconformably overlain by carbonaceous dark grey shale,
the Nkporo Formation which was deposited during the late

3. Cenomanian – Turonain Foraminifera and Palynomorphs from the Calabar Flank, South Eastern Nigerian: Implications for Age and Depositional Environment
Int. J. Geol. Min. 158
Figure 2: The map of Niger Delta showing structural elements of the Calabar Flank in relation to other sedimentary basins
in Nigeria (Nyong and Ramanathan, 1985).
Campanian-Maastrictian times (Petters et al., 1995). The
Nkporo Shale caps the Cretaceous sequence in the
Calabar Flank. The Nkporo Shale sequence is overlain by
a pebbly sandstone unit of the Tertiary Benin Formation
(Reijers and Petters, 1997).
METHODOLOGY
Sample collection
A total of twelve (12) and eleven (11) core samples were
collected from two wells (O/2W and 4N/2W) drilled to a
depth of sixty-five (65) and seventy (70) metres
respectively. Both wells were drilled at Mbarakpa, Calabar
Flank and the samples were collected at 5m interval. The
samples and were later composited at ten (10) metres
interval resulting in six (6) samples each for the two (2)
study wells
Lithologic description
Lithologic description was carried out on the samples
based on their physical characteristics such as colour,
texture, hardness, fissility and rock type. Chemical tests
were also carried out to determine the presence of
calcareous materials using 10% dilute Hydrochloric acid
(HCl).
Biostratigraphic sample preparation
Foraminifera
Each sample was subjected to foraminifera sample
preparation as outlined by Brasier (1980) and Armstrong
and Brasier (2005). The identification of the foraminifera
was done by comparing picked forms with previously
published forms using a binocular microscope.
Quantitative analysis was done using the number of
species count per sample to establish abundance and
diversity of foraminifera species.
Palynology
The core samples obtained from the study wells were also
analyzed for palynomorphs. Samples preparation was
done by acid maceration techniques used by Doher (1980)
and Traverse (1988) for acid insoluble microfossils. The
steps include dissolution of carbonates and silicate, acid
neutralization and dissolution of humic matter.
Concentration of palynomorphs was achieved by sieving
using 200 and 400 mesh nylon screens and pipetting the
organic residue from a watch glass. Slides of temporary
strew mounts using glycerin jelly was made for each of the
samples. Transmitted light microscope was used for
studying the palynomorphs. The palynomorphs were
counted and recorded. Analysis was done by comparison
with published work to identify the various forms of
palynomorphs.

4. Cenomanian – Turonain Foraminifera and Palynomorphs from the Calabar Flank, South Eastern Nigerian: Implications for Age and Depositional Environment
Ukpong and Ekhalialu 159
RESULTS AND DISCUSSION
Lithostratigraphy
Two (2) distinct lithologic units were recognized in
both wells (O/2W and 4N/2W). The top most layers
could be classified as overburden consisting of dark
brownish silty materials. This layer is five metres
(5m) thick in well O/2W and fifteen metres (15m)
thick in well 4N/2W respectively. The first lithologic
unit at the top consists of 50m and 20m of dark grey
fossil shales in wells O/2W and 4N/2W respectively
and belongs to the Ekenkpon Shale. The second
lithologic unit at the basal section of the study wells
consists of 10m and 20m of light grey fine-grained
limestone in wells O/2W and 4N/2W respectively
and belongs to the Mfamosing Limestone.
Biostratigraphic analysis
The core samples from the two (2) wells yielded
some foraminifera and palynomorph that are
paleontological useful for age determination.
Figure 3 and 4 show biostratigraphic charts for well
02/W and 4N/2W respectively.
Figure 3. The biostratigraphic range chart of foraminiferal species and palynomorphs species, depth of occurrence and their counts for well O/2W

5. Cenomanian – Turonain Foraminifera and Palynomorphs from the Calabar Flank, South Eastern Nigerian: Implications for Age and Depositional Environment
Int. J. Geol. Min. 160
Figure 4. The biostratigraphic range chart of foraminiferal species and palynomorphs species, depth of occurrence and their counts for well 2N/2W

6. Cenomanian – Turonain Foraminifera and Palynomorphs from the Calabar Flank, South Eastern Nigerian: Implications for Age and Depositional Environment
Ukpong and Ekhalialu 161
Foraminifera biostratigraphic analysis
Foraminiferal analysis of wells O/2W and 4N/2W is
characterized by diverse assemblages of planktic and
agglutinated foraminifera with the planktic exhibiting higher
diversity and abundance than the agglutinated forms. Only
a species of calcerous benthic form with one (1) count is
present in well 4N/2W while no count of calcerous benthic
foraminifera was recorded in well 2N/2W. The upper and
middle sections of the study well yielded abundance and
diverse foraminiferal species with the lower section in both
wells being barren. The foraminifera and palynologic
biostratigraphic chart shows the depth distribution of the
recovered foraminifera and palynomorphs.
The age determination of the sediments penetrated by the
study wells were based on age diagnostic planktic
assemblages identified and recorded in the samples. The
planktic foraminifera species include Hedbergella crassa,
Heterohelix moremani, Heterohelix planata, Heterohelix
reussi, Hedbergella delrioensis, Hedbergella planispiral
which confirms the Cenomanian - Turonian boundary
interval. The study of Adegbie and Bassey (2007) and
Ukpong and Ekhalialu (2015) used a similar assemblage
of Hedbergellids and Heterohelicids to define this age
interval.
Fayose (1979) used the occurrence of Heterohelix
moremani and Heterohelix reussi to confirm similar age in
the Ituk-2 well. Sediments of Cenomanian - Turonian age
were also recognized on the basis of the occurrence of
Heterohelix reussi, Heterohelix moremani, Heterohelix
simplicissima and Globigerinelloides caseyi. (Petters,
1980). Petters (1982) also recognized these forms from
road cut samples of the Eze-Aku Shale at km 24.8 on the
Calabar-Itu highway. The single occurrence of Ammotium
nkalagum also points to Cenomanian - Turonian age
Palynostratigraphy
Palynological analysis of the Well O2/W yielded forty-one
(41) palynomorphs species. Large numbers of these
assemblages were gotten from interval 15-45m with
practically no palynomorphs recovered from 0-10m
interval. Palynological analysis of well 4N/2W on the other
hand yielded about thirty-eight (38) polynomorps species
and a few associated dinoflagelate cysts. Large numbers
of these assemblages were gotten from interval of 20-50m
with practically no palynomorphs recovered from 60-70m
interval. The co-occurrences of age diagnostic
palynomorphs such as: Steevesipollenites binodosus,
Ephedripites sp, Leiotriletes sp, Classopollis sp,
Classopollis classoides, Classopollis annulatus,
Ephedripites jansonii, Cretacaeiporites mulleri,
Cretacaeiporites polygonalis, Galeacornea clavis and
Triorites africaensis is indicative of a Cenomanain-
Turonain age. Ukpong and Ekhalialu (2015), used a similar
assemblage including Classopollis sp. Triorites africaensis
and Cretaceiporites mulleri to define this age interval.
Ephredipites sp. was reported by Schrank and Ibrahim
(1990) in Egypt and Abubakar et al (2011) in Nigeria. The
occurrences of this palynomorphs gave credence to the
Cenomanian-Turonian age. The Steevesipollenites
binodosus assemblage zone (Cenomanian-Turonian) is
correlated with the Yolde and Gongila Formations also
dated Cenomanian-Turonian by Lawal and Moullade
(1986). Essien and Ufot (2005) also used Classopollis sp.
and Ephedripites sp. to assign Mid Albian – Early
Cenomanian age for the Mfamosing Limestone in the
Calabar Flank.
Paleoenvironmental interpretation
In the study well, the distribution of foraminiferal forms and
polynomorphs taxa were used as a tool for the
interpretation of the paleoenvironmental condition under
which the sediments were deposited. Paleoenvironmental
interpretations using the recovered foraminifera was
based on modern foraminifera ecology which provides
distinct criteria for the reconstruction of marine
environment (Murray, 2006) using their environmental
preferences. For paleoenvironmental interpretation using
the recovered palynomorphs, the knowledge of
palynomorphs production and dispersal, palynomorphs
source and deposition as well as palynomorphs
preservation and the relationship between preserved
palynomorphs and plant communities were considered. It
is pertinent to note that different palynomorphs possesses
different production rates.
• Non Marine Environment
The lower interval (60-70m) of well 4N/2W and the upper
section (0 – 5m) of the well 02/W shows a barren interval
(no occurrence of forms), and this suggests a non - marine
environment.
• Shallow Inner Neritic Environment
Depth interval 20-30m in well 4N/2W is characterized by
shale and shell fragments. There is complete lack of
planktic foraminifera and a presence of very low
abundance of benthic foraminifera, Haplophragmoides sp.
This genus is a probable infuanal detritivore that is
commonly found in muddy to sandy substrate in
environment ranging from marsh hyposaline lagoon and
estuaries to bathyal (Murray, 1991; Bronnimann et al;
1992). There is also high abundance of terrestrial pollen
and spores in this interval such as Classopollis sp,
Ephedripites sp and Leiotriletes sp which further suggest
deposition in the shallow inner neritic environment.
The shallow inner neritic environment was not
encountered in well 02/W.
• Inner Neritic Environment
The depth interval of 30-40m in well 4N/2W and 25-55m in
well 02/W respectively was inferred to be an inner neritic

7. Cenomanian – Turonain Foraminifera and Palynomorphs from the Calabar Flank, South Eastern Nigerian: Implications for Age and Depositional Environment
Int. J. Geol. Min. 162
setting. The lithology of these interval was characterized
by shale with intercalation of mudstone with significant
counts of agglutinated benthic foraminifera taxa
(Ammotium nkalagum, Haplophramoides sp,
Ammobaculites sp) and calcareous planktonic taxa mainly
of the Heterolicids, Hedbergella moremani was also
encountered. The diversity as well as the abundance of
planktic and benthic foraminifera in this section of the well
is low with the exception of the Ammotium nkalagum which
is relatively very abundant. The high percentage of
Ammotium nkalagum indicates brackish water (Murray,
1991). Olaleye (1983), suggested that the low diversity of
the benthonic assemblage and the scarcity of the
planktonic species in the Pindiga Formation reflects
deposition in a shallow marine Epi-continental sea. Low
diversity is also characteristics of modern brackish lagoons
and estuaries (Murray, 1991).
• Inner to Middle Neritic Environment
The inner – middle neritic environment is assigned to the
depth interval 40-50m of well 4N/2W and 10-25m of well
02/W. The criteria for this inference were based increase
in the abundance and diversity of the planktics and benthic
foraminifera within these intervals. A high planktic/benthic
ratio was also observed in this interval. Increase in
percentage of planktonic foraminifera relative to
benthonics is a general index of increasing water depth
(Phleger, 1960). There is also a general decrease in the
pollen and spores assemblages towards this zone
reflecting deeper water.
• Anoxia conditions
The oxygenation conditions in ancient environments can
be determined based on the recovered foraminifera.
Changes in dissolved oxygen concentration at the water
column, sediment-water interface and in-sediment affects
the foraminifera make up of sediments. The study of Miller
et al (1992) determined that low Oxygen, low pH and more
corrosive bottom waters favour the development of
Agglutinating benthic foraminifera and complete absence
of calcareous benthic foraminifera which suggests total
oxygen-deficient bottom conditions during deposition. This
means that the agglutinated foraminifera forms
(Ammobaculites coprolithformis, Ammotium cf nkalagum,
Ammotium cf nwalium, Haplophragmiodes sp etc.) found
in the study wells suggest deposition in an oxygen-
deficient environment as well as low salinities. The
presence of abundant but dwarfed planktic foraminifera
and the presence of low diversity of dwarfed agglutinated
forms at some intervals further support this interpretation.
This interpretation conforms to the study of Nyong and
Ramanathan (1985), Petters (1980), Petters (1982),
Petters and Ekweozor (1982), Ukpong and Ekhalialu
(2015). It is believed by several authors that the oceanic
anoxia effects was triggered by enhanced volcanic activity
that lead to the release of huge amounts of volcanogenic
CO2 and CH4 in the ocean-atmosphere system that drove
the known worldwide Cretaceous maximum warmth
(Jenkyns et al., 1994; Huber et al., 1995, 2002; Clarke and
Jenkyns, 1999 and Friedrich et al., 2012).
SUMMARY AND CONCLUSION
The study wells (02/W and 4N/2W) located in Mbarakpa,
Calabar Flank, Nigeria were studied for its
sedimentological and biostatigraphic
(micropaleontological and palynological) content. The
study wells penetrated a lateritic layer as well as limestone
and shale lithologic units. These lithologic units belong to
Mfamosing Limestone and the Ekenkpon Shale
respectively of the Calabar Flank. Biostratigraphic results
reveal a fairly high abundant and low diversity of
foraminiferal species together with high abundant and
diversity of Palynomorph. The integration of these results
enables the assignment of a Cenomanian to Early-
Turonian age to the study wells. This was made possible
by the presence of the following age diagnostic
foraminifera such as Hedbergella crassa, Heterohelix
moremani, Heterohelix planata, Heterohelix reussi,
Hedbergella delrioensis, Hedbergella planispira,
Hedbergella sigali and age diagnostic palynomorphs such
as Steevesipollenites binodosus, Ephedripites sp,
Leiotriletes sp, Classopollis sp, Classopollis classoides,
Classopollis annulatus, Ephedripites jansonii,
Cretacaeiporites mulleri, Cretacaeiporites polygonalis,
Galeacornea clavis and Triorites africaensis. The assigned
age is known as one of the warmest periods in the past
300 m.y., with tropical sea-surface temperatures at well
over 30 °C (Pucéat et al., 2007; Forster et al., 2007) and
atmospheric CO2 levels much higher than today.
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Accepted 28 December 2017
Citation: Ukpong A.J, Ekhalialu O.M (2018). Cenomanian
– Turonian Foraminifera and Palynomorphs from the
Calabar Flank, South Eastern Nigeria: Implications for Age
and Depositional Environment. International Journal
Geology and Mining 4(1): 156-164.
Copyright: © 2018 Ukpong and Ekhalialu. This is an
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