Sedimentology and foraminiferal paleoecology of the exposed Oligocene-Miocene Ogwashi-Asaba Formation in Issele-Uku area, Anambra Basin, southern Nigeria: A paleoenvironmental reconstruction

J.A. Adeoye , V.O. Jolayemi , S.O. Akande

a Department of Geology and Mining, Ibrahim Badamasi Babangida University, Lapai 911101, Niger State,Nigeria

b Department of Geology and Mineral Sciences, University of Ilorin, Ilorin 240101, Kwara State, Nigeria

Abstract Sedimentological and foraminiferal paleoecological studies were carried out for the Oligocene-Miocene Ogwashi-Asaba Formation of the Anambra Basin at the Issele-Uku area in southern Nigeria,to reconstruct paleoenvironments. The pattern of rock successions and textural characteristics suggest the prevalence of fluvio-deltaic setting during the formation of lignites and clastic rocks of Ogwashi-Asaba Formation. Agglutinated benthic foraminiferal species such as Ammotium sp., Textularia sp., Gavelinella sp.,Milliamina sp., Reophax sp., Bolivina sp., Eponides sp., Pseudobolivina sp. and Lenticulina sp. dominate the shale units whereas the planktonic forms are absent.All these species have very low population and diversity in the shales,which could be due to the presence of harsh conditions such as low oxygen level and scarcity of food, leading to poor survival. The assessments of benthic foraminifera's habitat suggest middle-to-inner neritic marine environment for the shales. Granulometric results suggest fluvial environment for the sandstone and deltaic for the lignite.The Ogwashi-Asaba Formation in Issele-Uku area of Anambra Basin,southern Nigeria, therefore, have records of shallow marine, and fluvio-deltaic paleoenvironments.

Keywords Foraminiferal, Sedimentology, Ogwashi-Asaba Formation, Anambra Basin, Paleoecology

1. Introduction

Anambra Basin, located north of Niger Delta Basin,forms a part of the southern Benue Trough of Nigeria.It consists of about 8-km-thick Cretaceous to Paleogene-Neogene sediments, covering an area of about 40,000 km2(Genik,1992).The rocks are characterized by enormous lithologic heterogeneity in both lateral and vertical extension,ranging from Campanian to recent(Akande et al.,2015).The basin has the largest known lignite deposits in Africa with proven reserves exceeding 300 million tons(Orajaka et al.,1990)and has received attention for exploration of commercial hydrocarbon and coal resources. Five of the twenty exploratory wells drilled in the basin encountered hydrocarbons,i.e.,one oil and four gas fields(Nwachukwu,1985).

This study focusses on understanding depositional environments on the petroleum potentials of the outcropping source rocks along river channels in Azagba-Ogwashi (Mgbiligba River Section), and Obomkpa (Iyiodo and Nnem-Agadi sections) in Issele-Uku area, southern Nigeria within latitudes of 5°54＇N-6°30＇N and longitudes of 6°25＇E-7°00＇E(Fig. 1).

2. Geology and stratigraphy of Anambra Basin

Anambra Basin belongs to the southern part of Benue Trough, which formed as a result of rifting events that separated the South American and African plates in the Late Jurassic and Early Cretaceous. Tectonic events responsible for the development of southern Benue Trough has been attributed to the existence of a triple junction and active spreading ridge below the Niger Delta Basin in the Albian to Santonian (Burke et al., 1972). Benue trough which is commonly referred to as the failed arm(or aulacogen)of a rift system related to the development of hot spots (Olade, 1975; Wright, 1976), which pulled apart of the continental plates during its evolution(Benkhelil,1982,1989).The oldest sedimentary rocks in southern Benue Trough comprises the Albian to Santonian.At the base,it is the Albian Asu River Group of micaceous sandstone, siltstone and limestone,succeeded by Cenomanian Odukpani Formation, a shoreline carbonates, followed by a Turonian Eze-Aku Formation, and then overlain by the late Turonian-Coniacian Awgu Shale (Table 1; Reyment,1965). The continuous deposition was interrupted by the widespread Santonian episode of compressional deformation and magmatism, leading to the displacement of depositional axis of the Benue Trough westwards,and resulting in the formation of Anambra Basin since the Campanian. Sediments derived by the erosion of the Abakaliki anticlinorium and the ancestral Niger River filled the newly-formed depocenter in Anambra Basin(Fig. 1).

Fig. 1 Geological and geographical map of the study area within Anambra Basin, southern Nigeria, showing the sample locations (modified after Iserhien-Emekeme, 2014). 1-Mgbiligba River Section; 2-Iyiodo Section; 3-Nnem-Agadi Section.

Table 1 Stratigraphic chart of Cretaceous to Paleogene-Neogene sequence in southeastern Nigeria(modified from Nwajide,1990).

The bulk of the Campanian-Maastrichtian rock in the basin is predominantly deltaic coal measure.Stratigraphy sequence in the basin starts with Upper Campanian transgressive cycle represented by the marine Nkporo and the equivalent paralic Enugu Shale(Table 1) which were succeeded by the Maastrichtian-Danian deltaic coal-bearing Mamu Formation,followed by Maastrichtian fluvial sandstones of Ajali and Nsukka formations. These pass on to the lower-upper Paleocene marine deposits consisting of the Imo Formation and upper Paleocene-Oligocene Nanka and Ameki formations (Murat, 1972). Ogwashi-Asaba Formation comprises alternating coarsegrained sandstone, lignite seams and claystone, suggested as Oligocene-Miocene equivalent of subsurface Agbada Formation in the Niger Delta (Reyment, 1965;Akande et al., 2015). By Eocene, the Anambra Basin was filled and the Niger Delta Basin prograded southwards across shallow Anambra shelf (Bullard et al.,1965; Stoneley, 1966).

3. Methods and materials

Fieldwork was carried out in Azagba-Ogwashi and Obomkpa area (Fig. 1) where rock exposures were found along river valleys and springs. Fresh rocks of various lithologies such as conglomerates, sandstones,shales, siltstones and coals within different beds were sampled, described and properly logged. Five sandstone samples were analysed for their grain sizes,sorting, skewness and kurtosis from individual weight percentages obtained through mechanical dry sieve method. The sandstones were gently loosed and 100 g of each were measured and sieved,using a stacked set of selected sieves (sizes of 4.00 mm, 3.35 mm,2.36 mm, 2.00 mm, 1.70 mm, 1.60 mm, 0.60 mm,0.50 mm, 0.30 mm, 0.25 mm, 0.149 mm, 0.112 mm,and 0.063 mm,respectively)and a receiving pan.Each sample was then poured into the topmost sieves,which have been arranged in the order of largest sizes to the smallest above the receiving pan of Rotap machine.The Rotap machine was switched on for a period of 10 min and revolution speed of 320 rpm,to shake the sample in the sieve depending on their relative grain sizes. The weight of the retained samples in each sieve was determined and recorded appropriately. About 200 g each of eleven shale samples were analyzed for their microfossil contents. Each sample was dried to eliminate the moisture content.The samples were digested using a solution of concentrated hydrogen peroxide(H2O2)which was diluted with distilled water in a ratio of 1:3 (i.e., 100 ml of H2O2 reacting with 300 ml of water). This was left for 24 h and washed thereafter under running water with a 0.063 mm sieve. The samples were left to dry,and the foraminifera were picked out of the residues under a binocular microscope.

4. Results and discussion

4.1. Lithologic description

The rock types of the mapped sections of the Ogwashi-Asaba Formation along the river channels in the study areas comprise of shale, lignite, claystone,siltstone, sandstone and conglomerate.

4.1.1. Mgbiligba River Section

This section, with coordinates of 6°14＇6.6＇＇N,6°37＇52.7＇＇E,is about 8 m thick.It has claystone at the base,which is overlain by a lignite bed.The lignite bed is subsequently covered by a carbonaceous shale, a matrix-supported conglomerate and a coarse-grained sandstone, respectively (Fig. 2). Lignite seam at the bottom part of the section is split by a fault resulting in its lateral and vertical displacement.This normal fault is ca.50 m in width and with a 3 m throw.From bottom to top, (1) the basal lignite is about 1.95 m thick and dark brown in colour;(2)the shale is carbonaceous and coaly,poorly laminated and grey to dark grey in colour;(3) the clayey siltstone is 0.28 thick; and (4) the sandstone is pebbly, coarse to medium grained,weakly- to parallelly-stratified, and is about 2.5 m thick. Occasionally, the pebbly clastic materials form irregular bands on the erosional surface defining a bounding surface. The frequently-interbedded pebbly sandstone and medium-to coarse-grained sandstone in the study area suggest low sinuosity channel bars arising from high discharge, channel switching, and lack of point bar sedimentation (Ojo, 2012).

Fig. 2 Graphic log representing the Ogwashi-Asaba Formation exposed at Mgbiligba River Section, Azagba-Ogwashi, Nigeria. “B”,“T”, and “M” are marked in some samples Nos., which refer to the“Base”, “Top” and “Middle” parts of the certain bed, respectively;the same below.

4.1.2. Iyiodo Section

Iyiodo Section is exposed at the Iyiodo spring with coordinates of 6°24＇59.9＇＇N and 6°29＇56.6＇＇E with a total thickness of about 9.0 m. The 1-m-thick lignite bed (Fig. 3), which is black, massive with an earthy texture occurs within the middle of the section.Beneath it,it develops a 3-m-thick carbonaceous coaly shale bed; while above it, is a 5-m-thick silty carbonaceous coaly shale bed. The whole section is capped by a thick vegetation.

4.1.3. Nnem-Agadi Section

Nnem-Agadi section lies within 6°24＇58.0＇＇N,6°29＇3.6＇＇E, comprising lignites and shales, which in total are about 18.5 m thick (Fig. 4). Two distinct lignite beds are observed at the base and middle parts of the section,with thicknesses of about 2 m and 2.1 m respectively.The lignites are generally brownish-black and massive,woody to earthy texture.A 10.4-m-thick shale bed occurs in between the two lignite beds,which is massive, carbonaceous and coaly. Another 5.5-m-thick shale bed is observed at the upper part of the section, which is less massive, distinctively laminated and carbonaceous.

Fig. 3 Graphic log representing the Ogwashi-Asaba Formation exposed at the Iyiodo spring, Obomkpa, Nigeria.

Fig. 4 Graphic log representing the Ogwashi-Asaba Formation exposed at Nnem-Agadi Section, Obomkpa, Nigeria.

4.2. Grain size analysis

Graphic mean size ranges from Ф 0.62 mm to Ф 1.95 mm (average of Ф 0.97 mm) (Table 2). These mean values suggest predominantly medium- to coarse-grained sandstones which are peculiar products of medium energy of transportation and deposition.Standard deviation (sorting) and skewness values range from 0.95 to 1.83(average of 1.30)and-1.03 to 0.67 (average of -0.16), respectively, indicating that the sandstones are mainly poorly-sorted and strongly coarse skewed. Kurtosis values also suggest they are platykurtic(Table 2).

4.3. Benthic foraminifera

Agglutinated benthic foraminifera were the dominant forams recovered from the shales.Planktic forms are completely absent (Table 3). The representative species are Ammotium sp., Textularia sp.,

Gavelinella sp.,Milliamina sp.,Reophax sp.,Bolivina sp.,Eponides sp.,Pseudobolivina sp.and Lenticulina sp. (Fig. 5). The generally low abundance and poor preservation of foraminifera may be attributed to the high percentage of coaly fragments within the shales,while the high influx of sandy materials into the basin made the shales to be sandy. These factors coupled with their small sizes reflect harsh and unconducive environmental conditions and insufficient organic nutrient to support their large population are not favourable for the foraminiferal species thus constraining their survival.

5. Discussion

5.1. Paleoecology of the benthic foraminifera

Benthic species dwell in a relatively shallow water depth within the photic zone for their metabolism.They are particularly abundant in shallow-marine waters but are less common in the deeper environments.Planktonic foraminifera are stenohaline and typically marine organisms (Phleger, 1960; Bandy, 1967). They are independent of sea bottom conditions and their distribution is influenced primarily by the temperatureof the surface waters. Therefore, foraminifera habitats serve as a good indicator for assessing paleowater depth, paleoclimate, and other paleoenvironmental conditions of the host rocks (Gibson, 1989). The recovered species (Fig. 5) and their paleoecology as stated below reveals the possible prevailing conditions during the deposition of the shales of the Ogwashi-Asaba Formation.

Table 2 Grain size analysis.

Table 3 Benthic foraminifera distribution and number of diversities per sample.

Fig. 5 SEM photographs showing representative benthic foraminifera in the Ogwashi-Asaba Formation at Issele-Uku area, Anambra Basin,southern Nigeria. A) Ammotium sp.; B) Bolivina sp.; C) Eponides sp.; D) Gavelinella sp.; E) Lenticulina sp.; F) Milliamina sp.; G) Pseudobolivina sp.; H) Reophax sp.; I) Textularia sp.

Fig. 6 Grain-size bivariate plot of standard deviation versus skewness showing fields in which beach and river sands plot(modified from Friedman, 1967).

Ammotium sp. is an infaunal deposit feeder restricted to shallow, brackish waters (tidal marshes,brackish lagoons and estuaries, and brackish enclosed shelf seas)(Murray,1991);Bronnimann et al.(1992).It tolerates a broad range of salinities (4%-41%), but is mainly developed in the brackish water (Fig. 6).

Textularia Sagittula sp. inhabits normal marine environments ranging from lagoonal to bathyal. They live epifaunally on hard substrates, muddy silts and sands (Murray, 1991). Some Cenomanian-Turonian textulariids seem to resist reduced salinities(Koutsoukos et al., 1990). Because of its great morphological similarities, the genus Textulariopsis is assumed to have ecological preferences similar to those of Textularia (Fig. 5).

Gavelinella sp. Bernhard (1986) considered small to medium-sized Gavelinella to dysoxic environments.Koutsoukos et al.(1990)showed that Gavelinella is an epifaunal active deposit feeder, which tolerates low oxygen levels. Its co-occurrence with relatively large macro-benthonic taxa indicates slight oxygen depletion during the late Cenomanian - early Turonian in the Sergei Basin of NE Brazil.

Milliamina sp. inhabits a wide range of environments from brackish to hypersaline marshes to upper bathyal depths. It is an infaunal deposit feeder that settles in muddy and silty sediments(Murray,1991).It is restricted to brackish lagoons and is more typical of marshes.

Reophax sp. is an infaunal deposit feeder in muds and sands of lagoons, shelves and bathyal regions(Culver and Buzas, 1981; Murray, 1991). It is mainly a marine genus but has also been reported from brackish lagoons and estuaries(Brazier,1980;several authors in Murray,1991).

Bolivina sp. is probably a detritivore, that lives infaunally or epifaunally in muddy marine saline environments(Murray,1991;Sliter and Baker,1972).It thrives mainly on the shelf but also in lagoons and in the upper bathyal realm (Murray, 1991). The genus belongs to the elongated flattened morph group of Bernhard (1986). According to Koutsoukos et al.(1990), the genus can tolerate oxygen deficiency.

Eponides Antillarum: This species dominates tropical to subtropical waters and it represents shallow marine environments with decreasing oxygen content and low salinity.

Lenticulina sp. is an infaunal species, which prefers cool marine conditions on the outer shelf to bathyal depths (Murray, 1991). They are indicative of high phytodetritus influx and dysoxic bottom water.

5.2. Paleoenvironment

The pattern of rock succession and association in a section corresponds to prevailing depositional condition and environments in the basin during sedimentation. Pebbly sandstones and conglomerates are common channel deposits where energy of transportation is high. Lignite develops from peat that are commonly found in a swamp and vegetated coastal areas around the warm and temperate regions of the world. High plant productivity and well-protected environment against microbial degradation favours lignite formation.Therefore,the association of lignite beds with pebbly sandstones, conglomerates, coarse sandstones and as well claystone and shale in all the sections suggest a wide range of depositional settings described as fluvio-deltaic environments.

Fig. 7 Scatter plot of skewness against kurtosis (Friedman, 1979).

Fig. 8 Typical genera of foraminifera and their paleoecological water depth.

Grain sizes and distribution are product of transportation systems and distance. They are useful in environmental discrimination. Thus, the predominant medium- to coarse-grained texture peculiar to medium energy of transportation coupled with poor sorting and negative skewness suggest fluvial origin of sandstones (Friedman, 1967). The plots of standard deviation against skewness(Fig.6)and kurtosis versus skewness (Fig. 7) further support dominance of river transportation and thereby corroborates the fluviatile origin for the sandstones.

Micropaleontology also offers important information for paleoenvironmental reconstruction through understanding of paleoecology of the organisms. Microfossils(Fig.5)are usually found in fine-grained lessporous sediments like clay,shales and limestones,but are relatively rare in highly-porous sandstone and lignite. Foraminifera is one of the important varieties of microfossils commonly employed in environmental reconstruction due to their ecological peculiarities and significance, e.g., some are bottom dweller (benthics),and other floating species(planktic)at all depth and salinity (Jenkins, 1993). Their abundance, diversity and dominance patterns enable the discrimination of a range of environments(Fig. 8).

Benthic foraminiferal paleoecological distribution,morphology and symbiotic relationships are the attributes commonly considered for the reconstruction of depositional environments. They are controlled by factors such as depth, salinity, temperature, waterenergy, substrate condition, topography, turbidity and nutrient-supply in the basin during and after sedimentation (Kaiho, 1994; Beavington-Penney and Racey, 2004).

5.3. Water depth

Water depth inferences are based on the occurrences and relative abundances of depth-dependent species in term of the upper depth limits where they can survive(Bandy,1960).Agglutinated taxa(Reophax and Textularia) usually represent a range of environments from marginal marine to bathyal depths, but their co-occurrence and low diversity of species suggests marginal marine conditions. With increasing paleowater depth, calcareous taxa (Eponides sp. and Bolivina sp.) are the more prominent components.Lenticulina is usually found on the shelf, while Ammotium is a genus characteristic of a variety of brackish marginal marine environment. Morris (1971)reported slightly higher diversity Ammotium sp. and Reophax sp.from brackish and lagoon environments.

5.4. Salinity

The benthic foraminifera genera in the study area are divided into two groups according to their ecology:brackish and unrestricted. Brackish genera such as Ammotium are restricted to or prefer distinctly reduced salinities less than 32‰. Whereas Milliamina and Reophax(Fig.5),the unrestricted genera inhabit a wide spectrum of salinities and occur in almost all samples. They make up 20%-100% of the fauna and likely represent slightly reduced salinities slightly below 32‰. These ecological characteristics suggest slightly reduced salinities of normal-marine environments for the study area.

5.5. Oxygenation

The degree of oxygenation of the bottom sediment can be deduced from the proportions of epifaunal and infaunal morphotypes, representing oxygen-poor and oxygen-rich environments respectively. Relatively simple subdivisions to categorize oxygen levels, proposed by Bernhard (1986) are: anoxic (<0.1 ml/l), dysoxic(0.1-0.5 ml/l, and oxic (＞0.5 ml/l). Anoxic bottom conditions are characterized by the absence of calcareous benthic foraminifera and the exclusive occurrence of planktics (Koutsoukos et al., 1990; Kaiho, 1994).However,agglutinated benthic foraminifera can survive under dysoxic to anoxic conditions (Kaiho, 1994). Sediments deposited under such conditions are characterized by the total absence of macrofauna and are well laminated. Similar environmental conditions existed during the deposition of the shales in the study area,which are devoid of bioturbation. Typical low-oxygen foraminiferal assemblages show the abundance of small-sized and thin-shelled individuals with flattened or tapered test shapes(Koutsoukos et al.,1990),which help them easily acquire oxygen for body metabolism.Infaunal mud dwellers are adapted to the low-oxygen content of interstitial waters and may dominate oxygen-depleted environments. Thus, the ratio of epifaunal to infaunal genera may reflect the degree of oxygenation.Relating to the investigated sections,low number to rare occurrence of calcareous foraminifera,small-sized individuals and an abundance of infaunal forms all point to low oxygen levels.The dominance of agglutinated foraminifera, small individuals, and an abundance of infaunal forms indicate oxygen depletion in the environment. Anoxic bottom conditions are characterized by the absence of calcareous benthic foraminifera and the planktics(Koutsoukos et al.,1990;Kaiho, 1994). However, agglutinated benthic foraminifera can survive under dysoxic to anoxic conditions(Kaiho,1994).

Low oxygen content may also create difficulties in calcium carbonate secretion and therefore leave agglutinated foraminifera as the only survivors under oxygen-deficient conditions.Assemblages of the Iyiodo and Nnem-Agadi sections are dominated by Ammotium sp. and Reophax sp. with subordinate Lenticulina sp.,Bolivina sp. and Milliamina sp. Percentages of Ammotium sp. (in combination with Reophax sp. and Milliamina sp.)at Iyiodo and Nnem-Agadi sections are usually indicative of brackish conditions. Thus very shallow paleo-water depths are inferred. These assemblages correspond well to those of other Cretaceous oxygen-depleted inner shelf deposits (Olsson and Nyong 1984; Wightman, 1990) and thus, suggest an inner to outer shelf.

6. Conclusions

Rock associations of lignite beds, pebbly sandstone,conglomerate,coarse sandstone,claystone and shale reflect deposition by high to medium energy flows across fluvio-deltaic (floodplain, swamp) and shallow marine settings. The small-sized and thinshelled marine agglutinated benthic foraminifera in the shale, with abundance of infaunal forms and low occurrence of the epifaunal forms typify dysoxic to anoxic situation. The low population, diversity and a display of monotonous components of benthic foraminifera are consistent with harsh, middle to inner neritic marine environmental conditions, which may not be conducive for the survival of organism. Therefore, the Oligocene-Miocene Ogwashi-Asaba Formation, outcropping in the Issele-Uku area of Anambra Basin, is dominantly deposited in a fluvio-deltaic to shallow marine environment.

Author contributions

JAA carried out part of the investigation, drafted the manuscript, contributed Resources, and review.VOJ conceptualized the project, did formal analysis,and contributed funding and resources and validated the manuscript. SOA conceptualized the project, did data curation,methodology,supervision and validated the manuscript.

Conflicts of interest

The authors declare that they have no known conflict of financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors wish to appreciate Pastor (Dr.) & Mrs.M.O Awojobi for funding this research work,Prof.S.O.Akande for his selfless effort, guidance and keen interest in supervision,and Prof.O.A Adekeye for good counsel and encouragement.