Late Cretaceous palynostratigraphy in the Gongola Basin (Upper Benue Trough, Nigeria)
Research highlights
► Article provides most complete palynostratigrapy of Late Cretaceous Gongola Basin, Nigeria. ► It amended existing palynostratigraphical zonations of the basin. ► It introduced new Upper Albian local assemblage zone in the basin.
Introduction
Available sub-surface palynostratigraphic data on the Cretaceous sequences of the Upper Benue Trough, Nigeria (Fig. 1) are mainly those of Lawal (1982) and Lawal and Moullade (1986), from three shallow water boreholes. The deepest of the three boreholes is the Mona-2 with a total depth of 272.6 m (Lawal and Moullade, 1986). Most recent preliminary palynostratigraphic study however, on a much deeper (total depth of 1905 m) petroleum exploration well (well Nasara-1), was provided by Abubakar et al. (2006). The present study therefore, is an attempt to provide detail palynostratigraphic account of the well. This is highly needed for correlation purposes in view of the current petroleum exploration campaign in the basin.
The well Nasara-1 is located at longitude 10°54′ and latitude 9°50′ in the Gongola Basin of the Upper Benue Trough, Nigeria (Fig. 1). The well has penetrated all the Cretaceous formations of the basin, except the upper Maastrichtian Gombe Formation (Fig. 2).
Section snippets
Geologic settings and stratigraphy
The Benue Trough is a rift basin in Nigeria that trends SSW–NNE for about 800 km in length and 150 km in width (Fig. 1). Its southern limit is the northern boundary of the Niger Delta, while its northern limit is the southern boundary of the Chad Basin. Its structural origin is related to the opening of the South Atlantic Ocean (Benkhelil, 1989) and is part of series of Cretaceous rift basins that formed the West and Central African Rift System (WCARS) (Guiraud and Maurin, 1992). The Benue Trough
Materials and methods
Materials for this study are borehole cutting samples collected from the well Nasara-1. Generally, the samples were collected at every 9.0 m (30 ft) interval. The sampled cuttings were washed to remove the drilling fluid and other contaminants. Foreign materials which might result from caving were not considered. The information derived from samples description was used to construct the lithologic log of the well.
A total number of 90 samples from the well were palynologically studied
Results
The genera and species of the palynomorphs identified are typical of the African-South American palynoflora (ASA) province of Herngreen and Chlonova (1981) which Nigeria belongs. A total of 400 spores and pollen have been identified and some of the most important species are presented in Fig. 3, Fig. 4, Fig. 5, Fig. 6.
Elater-bearing pollen of the genera Elaterocolpites, Elateroplicites, Elaterosporites and Senegalosporites have been identified. Other important genera include among others
Discussion
The recovered palynomorphs in the well Nasara-1 made possible the identification of five local palynozones. Four of the palynozones were earlier proposed by Lawal and Moullade (1986) whereas the fifth zone is new (Fig. 7). Important zonal species are illustrated on Fig. 3, Fig. 4, Fig. 5, Fig. 6.
Conclusion
The recovered spores and pollen within the well Nasara-1 made possible the identification of five palynologic zones: D. senonicus (Coniacian–Santonian), C. scabratus (Turonian), T. africaensis (late Cenomanian), A. jardinus (early–middle Cenomanian) and E. africaensis–E. Klaszii–E. protensus (late Albian). This confirms the late Cenomanian age of the basal Pindiga Formation, the Cenomanian age of the Yolde Formation and the Albian to perhaps pre-Albian age of the Bima Formation.
Acknowledgements
The German Academic Exchange Service (DAAD) is thankfully acknowledged for providing financial support to the first author that made this work possible. We thank the Institut für Geowissenschaften, Universität Tübingen, Germany for providing the research facilities. Chevron Petroleum Nigeria Limited is also thankfully acknowledged for inviting us to participate in the well-site sample collection and for allowing us to work independently on the samples. The review comments of Dr. Ali Ahmed
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