Foraminiferal analysis and palaeoenvironmental interpretation of borehole ET ( BH-ET ) in the Calabar Flank , South-eastern Nigeria

Foraminiferal biostratigraphy and palaeoenvironmental interpretation was carried out on core samples (5 samples between intervals 5-55 m) recovered from borehole ET (BH-ET) located in Etankpini area of the Calabar Flank, south eastern Nigeria. Lithologic description of the borehole samples were carried out and the borehole was sampled at 5 m interval. The samples were later composited at 10 m interval and a total of six samples were subjected to standard micropaleontological preparation procedures. Results from the lithologic analysis show that the basal section of the well (55-60 m) was made up of granite gneiss; overlain by light grey, calcarenite, bioclastic, pisolitic and stylolitic limestones (30-55). Dark grey fissile shales directly overlie the limestone (5-30 m). The foraminiferal recovery was generally low and characterized by planktonic species (Guembelitria harrisi, Hedbergella crassa, Hedbergella delrioensis, Hedbergella simplicissima, Hedbergellid spp., Heterohelix globulosa, Heterohelix planata, Heterohelix reussi, Hedbergella planispira and Hedbergella sigali). The absence of benthonic forms may be an indication of anaerobic bottom conditions. An age of Cenomanian-Early Turonian was assigned to the studied well based on diagnostic foraminiferal species assemblages. The recovered foraminiferal assemblage points to deposition within the Inner Neritic to Middle Neritic depositional setting for the studied well.


INTRODUCTION
The Calabar Flank is one of the coastal sedimentary basins in Nigeria located at the north-east of the Gulf of Guinea.It is separated from other coastal basins (Douala basin) to the southeast by the Cameroon Volcanic Line (CVL).The basin is bounded to the north by the Oban Massif, to the south by the Calabar hinge line and the Niger Delta basin, to the west it is delimited by the Ikpe Platform (Figure 1) (Nyong and Ramanathan, 1985).
Five stratigraphic units constitute the Cretaceous sediments of the Calabar Flank viz: The Awi Formation,  Nyong and Ramanathan, 1985).
Mfamosing Limestone, Ekenkpon Formation, New Netim Marl and Nkporo Shale, with limestones and shales being deposited during marine incursion in the Middle Albian age.This limestone (Mfamosing Limestone) overlies the Awi Formation which constitutes the basal sediments in the basin but in some other places, the Mfamosing Limestone lies directly on the basement as observed in this study well (borehole ET).This research work utilizes the information derived from core samples to highlight the foraminiferal distribution and palaeoenvironment of borehole ET (BH-ET) drilled recently in Etankpini area of the Calabar Flank.

Geologic setting
The Calabar Flank is that part of southern Nigeria sedimentary basins characterized by crustal block faulting trending in the NW-SE direction.The sedimentary basin was controlled by vertical movements of faulted blocks notably the Ituk High and the Ikang Trough and by associated transgressions and regressions (Murat, 1972;Nyong, 1995).
This sedimentary basin contains up to 4000 m of Albian to Maastrichtian marine deposits in outcrop section (Ehinola et al., 2008) sitting on continental (fluvio-deltaic) Awi Formation.Essien et al. (2005) pointed out the uniqueness of the basin such that within a stretch of about 8 km from the basement complex down-dip, the whole sections can be studied.The stratigraphic successions in the Calabar Flank consist of mostly Cretaceous deposits (Figure 2) however, at some locations near Calabar, these sediments are capped by the deposits of the Benin Formation of the Tertiary (Paleogene -Neogene) and younger Niger Delta.The basin is structurally controlled by horsts and graben structures as a result of crustal block faulting (Murat, 1972).The basal Neocomian-Aptian syn-rift fluvial sandstone, the Awi Formation (Adeleye and Fayose, 1978) marks the onset of sedimentation in the basin (Boboye and Okon, 2014).
The Albian and Late Cretaceous marine post-rift Odukpani Group (Petters et al., 2010) which consists of the mid-Albian Mfamosing Limestone (Akpan, 1992), the Late Albian-Turonian Ekenkpon Formation and the Coniacian New Netim Marl which were deposited during a relative rise in sea level directly overlie the Awi Formation.This group (Odukpani Group) is overlain unconformably by the Late Campanian-Maastrichtian Nkporo Shale (Edet and Nyong, 1993).The Tertiary (Paleogene -Neogene) and younger regressive sands and gravel beds of the Benin Formation overlie these Cretaceous successions.
Several research works so far have been carried out on sediments of the Calabar Flank for diverse reasons.Pioneer works include those of Reyment (1955Reyment ( , 1956Reyment ( , 1959Reyment ( , 1965)), Dessauvagie (1965Dessauvagie ( , 1968Dessauvagie ( , 1974) ) and Murat (1972) among others.Akpan (1996) used the faunal evidence of the bivalvia Protocardia spp. to identify areas with exaerobic bottom conditions and this was in agreement with an earlier work by Petters (1978) in the area.Reijers and Petters (1997), studied sequence stratigraphy and characterized the microfacies including stromatolites to enable the recognition of a late phase in the formation of a high stand systems tract and of a flooded surface followed by a low stand system tract in the Mfamosing limestone.Bassey (1991) and Njoh (2008) worked on the Cretaceous biostratigraphy of the Calabar Flank and adjourning areas.Ukpong et al. (2008), carried out foraminiferal biostratigraphic analysis of the upper part of the Type section of the Ekenkpon Formation which revealed dominant occurrence of the planktonic over the benthonic forams.The nature of the benthonic forams indicated fluctuating oxygen concentration which characterizes shallow oxic and anoxic marine settings worldwide during this geologic period.Njoh et al. (2009) characterized the early Paleogene foraminiferal assemblages within the Calabar Flank and suggested that sedimentation extended into the Paleogene.There is however, some controversies about Paleogene marine influence in the Calabar Flank.The petroleum generation potentials of the Calabar Flank deposits have also been studied (Essien et al., 2005, Ehinola et al., 2008, Ekpo et al., 2013;Boboye and Okon, 2014).Their studies show that the basin has promises for gaseous hydrocarbon deposits.

METHODOLOGY
The samples used for this study were recovered from borehole ET within the Calabar Flank (Figure 3).The borehole has a total depth of 60 m and was sampled initially at 5 m interval.Lithologic description was carried out on the borehole and three principal lithologies (granite gneiss, limestone and shale) were identified.The first 5 m was ignored due to overburden and the last 5 m was crystalline basement rock.The samples were later on composited at 10 m interval making a total of six (6) samples prepared for micropaleontological analysis.Standard procedures (Armstrong and Braisier, 2005) were strictly adhered to during preparation of the samples in the laboratory.
Samples were washed to be mud free and then dried.A small quantity (about 20 -30 g) of each composited sample was treated with one teaspoonful of anhydrous sodium carbonate for thorough disaggregation.Enough water was added to cover the sample and they were allowed to stand over-night.The soaked samples were washed using a 63 µ mesh sieve.Drying of samples at a minimum temperature of 20°C was later carried out followed by sieve analysis to separate the samples into coarse, medium and fine fractions.Fine brushes were used for the picking of foraminifera and finally, foraminifera samples were placed in a 30-60-squared cardboard faunal slides and covered with water soluble glue.The slides were examined using the wild Heerburg binocular microscope for the identification of species, abundance counts and diversity.This information were plotted on a foraminifera distribution chart.The age determination of the well was carefully based on the use of last appearance datum and first appearance datum of age diagnostic foraminiferal species for borehole ET.Biofacies analysis (based on association of organisms that represent a particular depositional environment) was also carried out.This analysis was carried out in the micropaleontology laboratory of the South Sea Petroleum Consultants, Nigeria.

Lithostratigraphy
Borehole ET penetrated lithologic units consisting of lateritic overburden (topsoil), shales, limestone and terminated on the granite gneiss.Figure 4 gives a clear description of the various lithologic units encountered downhole.The first interval (0-10 m) was made up of lateritic topsoil.This was followed by another interval (10-30 m) consisting of dark-grey, highly fissile and fossiliferous shale of the Ekenkpon Formation.Next to this is an interval (30-55 m) consisting of light-grey, calcarenitic, bioclastic, pisolitic and stylolitic limestone of the Mfamosing Limestone.The lower part of the Mfamosing Limestone unconformably overlies the crystalline basement rock from a depth of 55 m.
This foraminiferal assemblage is indicative of Cenomanian to Early Turonian age.Guembelitria harrisi is restricted to the Turonian age in the Eze-Aku Formation in the Benue Trough (Petters, 1980).It occurs in the Cenomanian of the U.S Gulf Coast (Pessagno, 1967) but ranges into the Turonian in New Jersey (Petters, 1977).Heterohelix reussi has its first appearance datum (FAD) in the Turonian age in the Benue Trough (Petters, 1980) while Heterohelix moremani, Heterohelix globulosa, Hedbergella planispira and Hedbergella delrioensis were recorded in Cenomanian -Turonian age in the Eze-Aku Formation of the Benue Trough (Petters,1980).Bassey (1991)   Cenomanian-Turonian age in the upper part of the Typesection of the Ekenkpon Formation in the Calabar Flank.
The middle part of the studied borehole (depth below 30 m) is barren of foraminifera and a precise age could not be assigned to the Mfamosing Limestone that underlies the Ekenkpon Formation (Figure 5).Although, Akpan (1992) used the occurrence of an itieriid gastropod Peruviella dolium (Roemer) to assign a middle Albian age to the Mfamosing Limestone, Essien and Ufot (2010) assigned Albian-Early Cenomanian age to the Mfamosing Limestone on the basis of pollen and spores recovered among which are Classopollis jardinel, Lycopodium sporites, etc.

Palaeoenvironmental Interpretation
The palaeoenvironmental interpretation based on foraminifera distribution in borehole ET (BH-ET) indicates that the Cenomanian-Early Turonian sediments were deposited in an Inner neritic -Middle neritic marine environment (Figure 5).Generally, forams are affected by ecological conditions of their environment and they react to various environmental settings (Petters, 1980;Akpan, 1985;Nyong and Ramanathan, 1985).Thus, it could be deduced that the bottom oxygen concentration was too low during certain periods for benthonic organisms to thrive, so the planktonic forms survived during these  periods, with depths not shallower than they could inhabit.The dominant occurrences of planktonic foraminiferal species lend credence to the inferred paleoenvironment.

Conclusion
Planktonic foraminiferal distribution of borehole ET (BH-ET) samples reveals the age of the study well as Cenomanian-Early Turonian while the lithology supports the penetration of two formations (the Mfamosing Limestone and the Ekenkpon Shale) in the sedimentary basin.This stratigraphic relationship indicates that the first marine transgression in the Etankpini area of the Calabar Flank started with the Mfamosing Limestone which unconformably overlies the basement in borehole ET.
Micro faunal and lithologic study indicates Inner-middle neritic environments of deposition for the studied borehole.The result from these data further indicates a sea level rise at 20-30 m interval which conforms to the high micro-faunal abundance and diversity associated with the onset of deep marine transgression which resulted in the deposition of the Ekenkpon Formation.

Figure 1 .
Figure 1.Map showing the structural elements of the Calabar Flank and adjacent areas (Redrawn fromNyong and Ramanathan, 1985).

Figure 3 .
Figure 3. Geologic map of Calabar Flank showing the well location.
also used these species to indicate Cenomanian-Turonian in the subsurface of the Calabar Flank while Ukpong et al. (2008) used a similar assemblage to indicate