Biostratigraphy and Palaeoenvironment of Deposition of Nsukka Formation, Anambra Basin, Southeastern Nigeria S.A

Journal of Palaeogeography, 2017, ▪(▪): 1e15

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Biopalaeogeography and palaeoecology Biostratigraphy and palaeoenvironment of deposition of Nsukka Formation, Anambra Basin, southeastern Nigeria S.A. Bankole a,*,A.O.Ola-Buraimob

a Department of Chemical and Geological Sciences, Al-Hikmah University, Ilorin, Nigeria b Department of Geology, Federal University, Birnin Kebbi, Nigeria

Abstract Biostratigraphy investigation of Nsukka Formation was carried out by subjecting well cutting samples obtained from Nzam-1 well to palynological studies with the view towards determining the relative geologic age and palaeoenvironment of deposition of the sediments. Samples were treated in the laboratory through digestion with hydroﬂuoric acid, sieving with 5 mm, oxidation with Nitric acid and mounting of the slides into sections with DPX mountant. A fairly high abundance and diversity of miospores were recovered, while the interval (847e1372 m) contains distinctive and diagnostic palynomorphs which are stratigraphically important. A Spinizonocolpites baculatus assemblage zone was established based on the basal occurrence of S. baculatus, Spinizonocolpites echinatus, Constructipollenites ineffectus, Periretisyncolpites sp. Peri- retisyncolpites giganteus, Monocolpites sp 1, Foveotriletes margaritae, Syncolporites marginatus, and Longapertites marginatus. The upper part of the interval is marked by association of Anacolosidites luidonisis (at top), Mauritiidites crassibaculatus, Retistephanocolpites williamsi, Proteacidites dehaani, Echitriporites trianguliformis, Proxapertites cursus, Retidiporites magdalenensis, S. baculatus, Retitricolpites gigeonetti, F. margaritae and Araucariacites sp. The Nsukka Formation is dated as Late Maastrichtian based on the co- occurrence of recovered index fossils. Importantly, CretaceouseTertiary (K/T) boundary is marked by high fossil content in the Maastrichtian sediments compared to paucity in palynomorph that characterise the overlying Paleocene facies. Palaeoenvironment of the analyzed section varies alternately from marginal marine to continental setting based on the presence of land-derived miospores and dinoﬂagellates. Keywords Biostratigraphy, Palaeoenvironment, Assemblage, Gonyaulacacean/Peridinacean ratio, Index fossil, Anambra Basin

© 2017 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Received 24 October 2015; accepted 4 November 2016; available online xxx

* Corresponding author. E-mail address: [email protected] (S.A. Bankole). Peer review under responsibility of China University of Petroleum (Beijing). http://dx.doi.org/10.1016/j.jop.2016.11.001 2095-3836/© 2017 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

AlbianeSantonian sediments in the eastern half 1. Introduction referred to as Abakaliki depression while the other half proto-Anambra was a platform consisting of post San- The Anambra Basin is an inland intracratonic basin tonian sediments (Benkhelil, 1982; Murat, 1972; located adjacent to Niger Delta, Nigeria (Fig. 1). Its Nwachukwu, 1972; Nwajide and Reijers, 1996, 1997; study continues to attract attention of many geologists Obi, 2000). because of proven hydrocarbon reserves present. The Past research works showed that the northern and Anambra Basin ranks almost next to Niger Delta in western parts of the Anambra Basin mainly consist of terms of richness in hydrocarbon reserves. Despite post Santonian sediments. However it has been proven enormous amount of work done on the geology of the otherwise that the northern and the western parts of basin including the petroleum geology, biostratigraphy the basin in Onitsha and Edo state respectively, contain and sedimentology, there are no much details avail- Middle Cretaceous to Late Tertiary sediments (Ola- able on the biostratigraphy of Nsukka Formation. Buraimo, 2013a, 2013b; Ola-Buraimo and Akaegbobi, Therefore, there is need to further understand the 2012; Ola-Buraimo and Akaegbobi, 2013a). The Santo- stratigraphic stacking pattern especially the biostra- nian period marked the stage when the basin experi- tigraphy of Nsukka Formation in terms of age and enced tectonic event that involved deformation, palaeoenvironment of deposition which could serve as folding, faulting and uplifting of the pre-Santonian a correlation platform for regional stratigraphic study. sediments in the Onitsha area which evolved as Such study would help to optimize hydrocarbon depression to the uplift (Benkhelil, 1987). However, exploration and exploitation in the basin. These pa- this depression was limited in extent excluding the rameters are the main focus of this research work. northern part of the Anambra Basin and southwestern part of the basin (Ola-Buraimo and Akaegbobi, 2013b). The oldest sediment in the Anambra Basin has been 2. Geologic setting erroneously placed to be Nkporo Group (Nwajide, 1990) but recent study shows that apart from southern Onit- The Anambra Basin is a major inland sedimentary sha area, the basin contains Asu-River Group sediments basin in Nigeria (Fig. 1). Its evolution was based on the dated as Albian to Lower Cenomanian (Ola-Buraimo and theory of the separation of the African and South Akaegbobi, 2013b). American Plates during the Middle Mesozoic period The stratigraphic sequence of the Anambra Basin (Burke et al., 1972; Nwachukwu, 1972). The theory has been discussed extensively by several workers ascertains that the Anambra Basin contains including Agagu et al. (1985); Dessauvagie (1975);

Fig. 1 Geological map of the Anambra Basin showing location of study area (From Nton and Bankole, 2013).

Ladipo (1986); Murat (1972); Nton and Bankole (2013); characterized by evidence of sediment reworking and Ola-Buraimo and Akaegbobi (2013b); Reyment (1965); dated Late MioceneePliocene (Ola-Buraimo and Though, sedimentation in the Anambra Basin was Akaegbobi, 2012). The most recent sediment in the based on the long standing assumption that it started basin is the Benin Formation deposited by fluvial pro- and ranged from late Santonian to Eocene but recent cesses and is suggested to belong to Pliocene to recent research work of Ola-Buraimo and Akaegbobi (2012) on in age based on stratigraphic position. The detailed OgwashieAsaba Formation has shown that the forma- stratigraphy of the basin is given below in Table 1. tion in actual fact is of Late Miocene to Pliocene age based on the presence of dinoflagellate cysts. It has also been advanced that the oldest sediment in the 3. Methodology basin like the other known parts is Asu-River Group, dated as Albian to early Cenomanian (Ola-Buraimo and Twenty two ditch cutting samples of Nzam-1 well Akaegbobi, 2013b) through the use of pollen and situated in the Anambra Basin ranging in depth from spores assemblages. 847 m to 1372 m were used for the preparation of the The Asu-River Group has varied lithologic unit, dark palynological slides. The samples were taken for grey to black coloured shale, variously ferruginized, preparation at 24 m interval except where samples are deposited in environment varying from continental not available. The sample preparation was carried out through marginal marine to open marine systems (Ola- in the laboratory following the common practice Buraimo and Akaegbobi, 2013b). This is overlain by (Green, 2001; Wood et al., 1996). Indurated samples Eze-Aku Formation, composed of predominantly dark were crushed with mortar and pestle in order to shale, dated late Cenomanian to Turonian (Ola- enhance maximum recovery of pollen and spores. Buraimo, 2013a). Awgu Shale dated Coniacian (Ola- The crushed samples along with the friable ones of Buraimo, 2013b) overlies the Eze-Aku Formation and about 20 g weight (20 gm) were poured into labelled fi marked the end of the rst phase of sedimentation in plastic beakers which were initially treated with dilute the Anambra Basin before the commencement of hydrochloric acid (HCl, 10%) in order to eliminate tectonic event in the Santonian. The dislocation of the carbonate substance present in them. They were later depocenter into the Anambra platform resulted into soaked in 60% hydrofluoric acid (HF) overnight for the deposition of the Nkporo Shale (Nwajide, 1990). about 20 h and stirred intermittently in order to ach- The Nkporo Group comprises of Nkporo Shale, Owelli ieve near complete digestion of silica and silicates. Sandstone, and Enugu Shale dated late Campanian The samples were not oxidized in order to avoid (Obi, 2000; Reyment, 1965). However, study based on corrosion, but were sieved with 5 mm mesh in order to palynology has dated the Asata/Nkporo Shale to be maximize concentration of miospore grains and in Campanianeearliest Maastrichtian age (Ola-Buraimo and Akaegbobi, 2013b). The Nkporo Shale is overlain by Mamu Formation deposited during early Maastrichtian (Kogbe, 1989; Table 1 Correlation chart for Early Cretaceous strata in Obi, 2000). It is composed of siltstone, shale, coal southeastern Nigeria (After Nwajide, 1990). seams and sandstone (Kogbe, 1989; Nton and Bankole, 2013). The age dating Mamu Formation has been further refined and recently dated to range from early to middle Maastrichtian based on pollen and spores recovered (Ogala et al., 2009). The Ajali Sandstone broadly dated Maastrichtian age overlies the Mamu Formation (Nwajide, 1990; Reyment, 1965). The sandstone is unconsolidated, coarse to fine grained, poorly cemented, with little mudstone and siltstone (Kogbe, 1989). The Ajali Sandstone is overlain by Nsukka Formation dated MaastrichtianeDanian. This is also known as Upper Coal Measures (Obi, 2000; Reyment, 1965). The Imo Shale (Paleocene) overlies the Nsukka Formation (Nwajide, 1990) while it is successively overlain by Ameki Group, dated Eocene (Obi, 2000); Ogwasi-Asaba Formation consists of dark grey shale and sandy shale,

Please cite this article in press as: Bankole, S.A., Ola-Buraimo, A.O., Biostratigraphy and palaeoenvironment of deposition of Nsukka Formation, Anambra Basin, southeastern Nigeria, Journal of Palaeogeography (2017), http://dx.doi.org/10.1016/j.jop.2016.11.001 4 laect hsatcei rs s akl,SA,OaBrio .. isrtgah n aaoniomn fdpsto fNuk Formation, Nsukka of deposition of palaeoenvironment and Biostratigraphy http://dx.doi.org/10.1016/j.jop.2016.11.001 A.O., (2017), Ola-Buraimo, Palaeogeography S.A., of Journal Bankole, Nigeria, as: southeastern press Basin, in Anambra article this cite Please

Table 2 Palynological distribution chart of Nsukka Formation (847e1372 m). ..Bnoe ..Ola-Buraimo A.O. Bankole, S.A. Biostratigraphy and palaeoenvironment, southeastern Nigeria 5

Fig. 2 All magniﬁcation at ×400. 1eMonosulcites sp.; 2eIndeterminate dinocyst; 3eE. trianguliformis Van-Hoeken Klinkenberg, 1964; 4,6,11ePeriretisyncolpites sp. Lawal, 1982;5ePhelodinium bolonienae Riegel, 1974; 7eInaperturopollenites sp.; 8e C. ineffectus Van Hoeken-Klinkenberg, 1964;9eCrassoretitriletes vanraadshooveni Germeraad et al., 1968;10eCtenolophonidites costatus;12eP. giganteus Kieser and Jan du Chehe, 1979..

order to achieve clean slides for easy petrographic rich in abundance and diversity and they are well analysis. Other stages of preparation include heavy preserved as indicated in the Figs. 2e9. Lithofacies liquid separation of the macerals before they were sequence of the interval is mainly dark grey fissile finally mounted on glass slides with Deepex (DPX) shale but interval 1109e1116 m is composed of dark mountant. grey sandy shale with sand/shale ratio of 30.70%, fine to medium in grain size, angular to subangular in shape, and rarely ferruginized in nature. The assem- 4. Result and interpretation blages of the pollen forms in the interval 847e1372 m are characterized mainly by Maastrichtian forms. The Twenty two samples were prepared and analyzed assemblage belongs to S. baculatus assemblage zone petrographically for palynomorph content. Palynolog- compared with the works of Lawal (1982), Lawal and ical result reveals that the samples were rich in pollen, Moullade (1986) for the Lower Benue Trough, Nigeria, ' spore and dinoflagellates. The lower part of the in- Jardine and Magloire (1965) for Senegal and Cote D terval is moderately to highly rich in miospores, the Ivoire sedimentary deposits. middle part is rare to barren, while the upper part is Palynozone: S. baculatus Assemblage Zone. e increasingly highly rich in pollen, spore and dino- Interval: 847 1372 m. flagelates (Table 2). The forms encountered are quite Age: late Maastrichtian.

Fig. 3 All magniﬁcation at ×400. 1eR. magdalenensis Van der Hammen and Garcia, 1965; 2eBatiacasphaera sp. Lawal, 1982;3eL. marginatus Van Hoeken-Klinkenberg, 1964;4eOligosphaeridium sp. Lawal, 1982;5eFlorentinia sp. Lawal, 1982;7e Cyathidites sp.; 8eP. operculatus Van der Hammen, 1956b; 9eOligosphaeridium pulcherrimun (Deﬂandre and Cookson) Davey and Williams, 1966; 10eP. bolonienae Riegel, 1974; 11,15eCyathidites australis;12eP. giganteus Kieser and Jan du Chene, 1979;13eTriporites sp.; 14eOdontochitina sp.; 16eMicroforaminiferal wall lining.

Characteristic: The base of the interval is marked disappearance. Other forms still present at the top of by the co-occurrence of S. baculatus and S. echinatus the interval are increased angiosperm pollen, Reti- in association with C. ineffectus, Periretisyncolpites monocolpites sp, stephanoporate pollen, and Mono- sp, P. giganteus, Monocolpites sp 1 (Fig. 10), reduced sulcites sp. The top of the zone is also placed where abundance of L. marginatus, P. cursus, and Verruca- there is high abundance and diversity of recovered tosporites sp. sporomorphs end; this conforms to the observations of The top of the zone is deﬁned by the co-occurrence Lawal and Moullade (1986), Ola-Buraimo (2012) in of A. luidonisis and M. crassibaculatus (Fig. 2). At the marking the boundary between late Maastrichtian and level (847 m) Periretisyncolpites spp shows ﬁrst Paleocene (K/T boundary) (Table 2).

Fig. 4 All magniﬁcation at ×400. 1eProteacidites sp.; 2eC. australis 3e Tricolpites gigantoreticulatus Jardine and Magloire, 1965;4eL. marginatus Lawal, 1982;5eAuriculiidites sp.; 6eOligosphaeridium sp.; 7ePhelodinium sp.; 8eSenegalinium bicavatum Jain and Millepied, 1975; 9eSubtilisphaera sp.; 10eIndeterminate pollen; 11eP. giganteus (corroded); 12eInaperturopollenites sp. Lawal, 1982;13eFungal spore; 14eCupanieidites reticularis Cookson and Pike, 1954; 15eA. polymorpha Malloy, 1972; 16e Florentinia sp.

Assemblages of miospores recovered in this zone is upper section of the interval include R. williamsi, P. equivalent in part to those of sequences III, II, I for the dehani, E. trianguliformis, C. ineffectus, L. margin- Maastrichtian sediments in Senegal and Cote d’ Ivoire atus, P. cursus, R. magdalenensis, S. baciulatus, Ana- by Jardine and Magloire (1965). The interval is char- colosidites sp, R. gigeonetti, F. margaritae, and acterized by increased angiosperms and decreased Araucariacites sp. This portion of the interval is sug- gymnosperms. The lower part of the zone is marked by gested to belong to late Maastrichtian to Early Paleo- relatively moderate abundance and diversity of miocene. The presence of Anacolocidites sp suggests the spore, the middle part is poor to barren while the Early Paleocene deposit (Germeraad et al., 1968). upper part is most fossiliferous and diverse in nature The lower part (924e1372 m) is less fossiliferous, (Figs. 11 and 12). The forms that characterize the sparse of miospores and rarely barren. The

Fig. 5 All magniﬁcation at ×400. 1eSubtilisphaera sp.; 2eTriporate pollen 3e Stephanocolpites 4e R. magdalenensis Van der Hammen and Garcia, 1965; 5eL. microfoveolatus Jan du Chene and Adegoke, 1978; 6eLongapertites verneendenburgi Germeraad et al.,1968; 7eOligosphaeridium sp.; 8eRetitriporites sp.; 9eSubtilisphaera sp. Lawal, 1982;10eCyathidites sp.; 11,16eL. marginatus Van Hoeken- Klinkenberg, 1964;12eCaningia sp Lawal, 1982;13eBotryococcus braunii;14e Oligosphaeridium sp Lawal, 1982;15eInaperturopollenites sp.

microﬂoral present include P. cursus, Longapertites (1986) for this zone for the Lower Benue Trough sp, E. trianguliformis, F. margaritae, Tetradites sp, sediments are restricted to older sediments (Ola- Mauritiidites crasibaculatus, S. marginatus, Lycopo- Buraimo and Akaegbobi, 2013a). Those forms may dium sp, and Longapertites microfoveolatus. This be contaminants in the sediment analyzed by Lawal part is suggested to belong to late Maastrichtian age. and Moullade (1986). The S. baculatus assemblage zone established in this The assemblages of miospores present in this zone well is similar to palynological content of S. baculatus are similar to the forms described in late Maastrichtian Assemblage Zone 6 erected by Lawal and Moullade or younger age sediments of other places. This was (1986). However, Trichotomosulcites sp 1,andMil- observed by Germeraad et al. (1968), Jan Du Chene fordia jardinei, described by Lawal and Moullade (1977), Jan Du Chene et al. (1978), Jardine and

Fig. 6 All magniﬁcation at ×400. 1eO. pulcherrimun (Deﬂandre and Cookson) Davey and Williams, 1966; 2eMonocolpopollenites sphaer- oidites Jardine and Magloire, 1965;3eCf Subtilisphaera sp.; 4eForma; 5,6eBatiacasphaera sp.; 7,8eProteacidites longispinosus Jardine and Magloire, 1965;9eCf Distaverrusporites sp.; 10,11ePeridinacae sp (Forma T1); 12eMonocolpites sp.; 13,14eA. laevigata Malloy, 1972; 15eR. magdalenensis Van der Hammen and Garcia, 1965; 16eInaperturopollenites sp.

Magloire (1965), Kieser and Jan du Chene (1979), dehaani zone established by Germeraad et al. (1968) Lawal and Moullade (1986), Muller (1968), Ola- for the pantropical regions. The sediment of the zone Buraimo (2012), Salard Cheboldaeff (1979, 1981, is equivalent to Nsukka Formation in this well which 1990), Sole de Porta (1972), Van der Hammen (1954), shares the same sporomorphs with those found in the Van der Hammen and Wijmstra (1964) and Van Hoeken- Iullummeden Basin in the North-West, Nigeria Klinkenberg (1964). (Boudouresque, 1980); upper part of Fika Formation in The analyzed interval is equivalent to the upper Bornu Basin, Nigeria (Ola-Buraimo, 2012). part of Gombe Formation characterized by decrease in It is noted in this zone that some of the ﬂoral gymnosperms and by development of angiosperms observed such as Proxapertites operculatus, M. crassi- such as sulcate, triporate, tricolpates (Salard- baculatus, S. echinatus, tricolpate grains, R. williamsi Cheboldaeff, 1990). This zone is also equivalent to P. were also reported in Kerrikerri Formation in the

Fig. 7 All magniﬁcation at ×400. 1eL. marginatus Van Hoeken-Klinkenberg, 1964;2eMonosulcites sp.; 3eMonocolpites sp.? 4eTricolpites sp.; 5e F. margaritae (Van der Hammen) Germeraad et al., 1968;6eLaevigatosporites sp.; 7eL. marginatus Van Hoeken-Klinkenberg, 1964; 8eR. magdalenensis Van der Hammen and Garcia, 1966; 9,10,11eMonocolpites sp.; 12eMicroforaminiferal wall lining; 13eVerrucatosporites sp.; 14,15eMonocolpites marginatus Van der Hammen, 1954;16eMonosulcites sp Jardine and Magloire, 1965.

northeast. The Gombe Formation in the northwest Therefore, the interval is dated late Maastrichtian age (Iullemmeden Basin), Ewekoro Formation in the south- and lithologically equivalent to the Nsukka Formation of west (Dahomey Embayment) and Ebenebe Formation in the Anambra Basin, southeastern Nigeria. the southeast were assigned Paleocene age. However, the assemblages of miospores described for the Mamu Formation Coal Seam Measure (Ogala et al.,2009) seem 5. Palaeoenvironment of deposition to be similar in part with those observed in this interval, but well correlated with established forms for the Lower Benue Trough, Nigeria which belong to the S. Palaeoenvironment deduction of the analyzed in- baculatus assemblage zone (Lawal and Moullade, 1986). terval was based on the presence of environmentally

Fig. 8 All magniﬁcation at ×400. 1eMonosulcites sp.; 2eC. ineffectus Van Hoeken-Klinkenberg, 1964;3eR. magdalenensis Van der Hammen and Garcia, 1966; 4,5ePeriretisyncolpites sp Lawal, 1982; 6,7eS. marginatus Van Hoeken-Klinkenberg, 1964;8eInaperturopollenites sp.; 9eMonosulcites sp.; 10eRetimonocolpites sp.; 11eTetradites sp Kotova, 1984; 12eE. trianguliformis Van Hoeken-Klinkenberg, 1964; 13eTricolpites punctus Jan Du Chene, 1977;14eTricolpites sp.; 15eTricolporopollenites sp. Jardine and Magloire, 1965.

significant forms. The presence of marine derived capilata, Trichdiniumsp, Andalusiella laevigata, forms such as dinoflagellate and microforaminifera Andalusiella sp, Andalusiella polymorpha, Subtili- wall lining would suggest marine environment while sphaera sp and Thalassiphora sp. Others are Trichodi- preponderance of terrestrially derived miospores nium delicatum, Histrichodinium pulchrum, without recovery of dinoflagellate forms in the Oligosphaeridium complex, Cyclonephedinium dis- assemblage is suggestive of continental deposit. tictum, Cribroperidinium edwardii, senegalidiniu sp Therefore, interval 847e896 m is characterized by and microforaminiferal wall lining. The interval substantially high abundance of dinoflagellates such as (847e896 m) is suggested to belong to marginal marine Batiacasphaera sp, Polysphaeridium sp, Caningia environment. Interval 896e1234 m is defined by

Fig. 9 All magniﬁcation at ×400. 1eGonyaulacysta sp.? 2eSubtilisphaera sp 1 (Forma J); 3eMicroforaminiferal wall lining (Spiral type); 4eCallaiosphaeridinium asymmetrinun (Deﬂandre and Court) Davey and Williams, 1966; 5eAraucariacites sp.; 6eM. marginatus Van der Hammen, 1954;7eC. edwardii (Cookson and Eisenack) Davey, 1969; 8eL. microfoveolatus Jan du Chene and Adegoke, 1978; 9eP. cursus Van Hoeken-Klinkenberg, 1964;10eR. williamsi in Salard-Cheboldaeff, 1990;11e Cribroperidinium sp.1 Lawal, 1982;12ePalaeocystodinium sp.; 13,14e Spinidinium sp.; 15eMonosulcites sp.; 16eTricolpites sp. 2.

paucity of miospores mainly of terrestrially derived associated with continental deposit. The basal interval pollen and spores, thus, a continental environment of 1320e1372 m has combined recovery of miospores and deposition is suggested for the interval. The underlying dinoflagellates. Among the dinocysts present are Oli- interval 1234e1271 m is characterized by association gosphaeridium sp, Senegalinium sp, Polysphaeridium of terrestrially derived forms and marine grains such as sp 1, dinoflagellate cysts and microforaminifera wall Batiacasphaera sp, A. polymorpha and other forms of lining. The interval is suggested to belong to marginal dinoflagellates. The sediments were deposited in marine setting. In a nut shell the analyzed interval marginal marine setting. Interval 1271e1320 m has varies alternately from marginal marine to continental similar characteristic like interval 896e1234 m, environment of deposition.

Fig. 10 Palynolog of marker fossils appearances with depth in interval 847e1372 m.

palaeoenvironment of deposition varies alternately from marginal marine to continental setting.

Acknowledgements

The authors are grateful to Palystrat Limited for her continuous ﬁnancial and material support in mak- ing this research study a reality. We also appreciate the cooperation of Geological Survey Agency, Nigeria for providing ditch cutting samples and granting permis- sion to publish these ﬁndings.

Fig. 11 Depth-abundance plot for the upper part of interval 847e1372 m. References

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