Six informal palynomorph assemblage zones, labeled A–F as their zone codes, were established based on the first and last occurrences of two or more sporomorph species (Fig. 4). The palynozones were correlated and compared with existing biozones in the subsurface, down-dip series of the Niger Delta Basin, and with the pantropical palynozones of Germeraad et al. (1968), in the tropical areas (Fig. 5). The zonal species are presented in Figs. 8, 9, 10.
5.1 Zone A: Scabratriporites simpliformis–Bombacidites annae zone, middle Paleocene
The palynological zone A has middle Paleocene index species such as Scabratriporites simpliformis, Bombacidites annae and Echitriporites trianguliformis. Some species of the pollen group such as Bombacidites (similar to that of extant Bombax and relatives) are characteristic of middle Paleocene–Eocene (Jaramillo and Dilcher 2000), and several of their Paleocene species have regional extinctions at the end of the Paleocene (e.g. Bombacidites annae). Here, Bombacidites annae was recorded only in this zone and was absent in the subsequent zones up the stratigraphic sequence.
The first abundance peak of the peridiniacean species, Apectodinium homomorphum (38%), also associated with this zone. Samples L2/01 to L2/04 fall within this zone and were recognized in the carbonaceous dark shale at Loc 2 in the Imo Formation. The absence of Scabratriporites simpliformis, Bombacidites annae and Echitriporites trianguliformis in the section above sample L2/04, marks the top of this zone (Fig. 4).
5.2 Zone B: Foveotricolporites crassiexinus–Mauritidiites crassiexinus zone, late Paleocene
Samples of zone B were found from L8/01 to L8/04 (Fig. 4). The base was defined by the disappearance of the middle Paleocene sporomorphs assemblage Scabratriporites simpliformis, Bombacidites annae and Echitriporites trianguliformis of zone A, and the appearance of the late Paleocene Foveotricolporites crassiexinus and Mauritidiites crassiexinus. The dinoflagellate cyst assemblage from this zone includes Kallosphaeridium brevibarbatum, Apectodinium spp., Homotryblium abbreviatum, Adnatosphaeridium multispinosum, and Oligosphaeridium complex. Other rare to occasional species, which are also confined in this zone include, Cylonephelium spinetum, Glaphyrocysta exuberans, Ifecysta spp., and Muratodinium fimbriatum.
Awad and Oboh-Ikuenobe (2016), referring to Masure et al. (1998), used the species of Apectodinium spp. and Adnatosphaeridium multispinosum to determine a late Paleocene age at the Depth of 828.70 m of ODP Hole 959D, in the CÔte d’Ivoire-Ghana Transform Margin. They also held that the occurrence of Ifecysta pachyderma supports an age designation of late Paleocene, which was confirmed by earlier researchers in West Africa (Jan du Chêne and Adediran 1985; Antolinez 2006; Antolinez and Oboh-Ikuenobe 2007; Bankole et al. 2007). The top of this zone was marked by the disappearance of the late Paleocene index species, Foveotricolporites crassiexinus and Mauritidiites crassiexinus.
5.3 Zone C: Striatopollis catatumbus–Momipites africanus zone, early Eocene
The base of zone C was defined by disappearance of the late Paleocene species, Foveotricolporites crassiexinus and Mauritidiites crassiexinus, and the appearance of early Eocene index species, Striatopollis catatumbus and Momipites africanus. Samples L8/05 to L16 fall within this zone (Fig. 4). Striatopollis catatumbus first occurred in early Eocene deposits in Colombia (González Guzmán 1967), Nigeria (Takahashi and Jux 1989), Venezuela (Colmenares and Teran 1993), and Qatar (El Beialy 1998). González Guzmán (1967) further remarked that the taxon also occurs in the Caribbean, and at the base of the Retibrevitricolpites triangulatus zone in Nigeria.
Towards the base of this zone, sample L8/05 marks the topmost Paleocene strata and the beginning of early Eocene deposition, i.e., the Paleocene–Eocene boundary. At the Paleocene–Eocene boundary, in sample L8/05, Proxapertites operculatus reaches its abundance peak (11%); the Paleocene index species assemblage of Scabratriporites simpliformis, Bombacidites annae, Foveotricolporites crassiexinus, and Mauritidiites crassiexinus disappeared and early Eocene markers, Striatopollis catatumbus and Momipites africanus first appeared. Moreover, the percentage abundance of the dinoflagellate cyst species, Apectodinium homomorphum, remained high (15%) across the P/E boundary. A characteristic assemblage of gonyaulacacean, Spiniferites ramosus, was commonly developed and reached its abundance peak at the top of this zone. Thus, the disappearance of the early Eocene index species in sample L17/01 above sample L16, marks the top of this zone (Fig. 4).
5.4 Zone D: Margocolporites umuahiaensis–Gemmastephanocolporites brevicolpites zone, middle Eocene
The base of zone D was marked by the disappearance of early Eocene index species and the appearance of middle Eocene taxa of Margocolporites umuahiaensis and Gemmastephanocolporites brevicolpites. Samples L22 to L23/02 fall within this zone (Fig. 4), in which shows the abundance peak of Psilatricolporites crassus (20%). The dinoflagellate cyst, Glaphyrocysta ordinata, also reached its abundance peak while the foraminifer linings become common. Margocolporites umuahiaensis and Gemmastephanocolporites brevicolpites were first described from middle Eocene sediments around Oba, Nnewi and Orlu areas, southeastern Nigeria (Jan du Chêne et al. 1978). The related species, Margocolporites vanwijhei, was reported by Germeraad et al. (1968) as having an Eocene–Oligocene age range in West Africa, Caribbean and Borneo. The top of this zone was marked by the disappearance of Margocolporites umuahiaensis and Gemmastephanocolporites brevicolpites species (Fig. 4).
5.5 Zone E: Cicatricosisporites dorogensis–Perfotricolpites nigerianus zone, late Eocene
Samples L25/01 to L25/03 were recognized in the fossiliferous mudstone section of the Ameki Formation in Loc 25 at Isiadu-Ameke town, and fall within zone E (Fig. 4). The base of this zone was defined by the disappearance of Margocolporites umuahiaensis and Gemmastephanocolporites brevicolpites species, and the appearance of late Eocene Cicatricosisporites dorogensis and Perfotricolpites nigerianus species.
Perfotricolpites nigerianus was first reported from Middle Tertiary lacustrine deposits in the Jos Plateau (Takahashi and Jux 1989). The related species, Perfotricolpites digitatus, was first recognized from early–middle Eocene strata of Columbia (González Guzmán 1967), and then from Oligocene strata of New Zealand (Pocknall 1982). This species was also reported by Germeraad et al. (1968) from the Caribbean and Nigeria, occurring for the first time approximately at the base of the Verrucatosporites usmensis zone (late middle Eocene–late Eocene). This taxon also has a fossil record extending into the Late Neogene of the West Coast of Africa (Partridge 1978). Moreover, Germeraad et al. (1968) held that the species of Cicatricosisporites dorogensis had initial worldwide distribution in Early Cretaceous sediments but contracted in Late Cretaceous time; and in the middle Eocene, its second expansion took place in the Caribbean and Nigeria, but declined at the end of the Oligocene Epoch.
Also associated with this zone are the abundance peaks of Monoporites annulatus (11%) and Verrucatosporites usmensis (11–18%). The freshwater ferns, Pediastrum botryanum and P. simplex, are common. The dinoflagellate cyst, Achilleodinium biformoides, also reached its abundance peak. This zone was also characterized by the abundance and diversity of macrofaunae such as gastropod and bivalve shell fragments. The disappearance of the late Eocene Cicatricosisporites dorogensis and Perfotricolpites nigerianus, in sample L33/01 above sample L25/03, marked the top of this zone (Fig. 4).
5.6 Zone F: Verrucatosporites usmensis–Magnastriatites howardii zone, Oligocene–early Miocene
Samples L33/01 to L33/03 were recognized in coal and the overlying grey shale at Loc 33 in the Ogwashi Formation, and fall within zone F (Fig. 4). The base of this zone was defined by the disappearance of late Eocene Cicatricosisporites dorogensis and Perfotricolpites nigerianus, and the first appearance of Oligocene–early Miocene assemblage, Zonocostites ramonae, Retibrevitricolporites protrudens, and Magnastriatites howardii, with the abundance peak of Verrucatosporites usmensis (25%). The rhizophora pollen, Zonocostites ramonae, was reported by Germeraad et al. (1968) to have made its first appearance during the Oligo-Miocene in the coastal and offshore marine sediments. Also, the Ceratopteris pollen (Magnastriatites howardii) made its first appearance in the Oligo-Miocene (Germeraad et al. 1968).
This zone comprises the abundance peaks of freshwater fern spore, Laevigatosporites ovatus (26%), and fungal spore, Fusiformisporites pseudocrabbi (14%). Other species found in this zone include Striamonocolpites undatostriatus, Psilastephanocolporites minor, Psilastephanocolporites laevigatus, Arecipites crassimuratus, Chenepodipollis dispersus, and Periporopollenites spp. This zone shows the initial appearance of dinoflagellate species, Distatodinium ellipticum. The top of this zone, however, was not encountered in this study.