AGE DETERMINATION AND PALEOECOLOGICAL STUDIES OF CAMPANIANMAASTRICHTIAN SEDIMENTS OF AFIKPO SYNCLINE, SOUTHEASTERN NIGERIA

Abstract

The Afikpo Syncline constitutes the eastern flank of the southern Benue Trough of Nigeria. Sediments within the syncline exposed at Nguzu Edda, Ekoli, and Asanga Amangwu were investigated. The investigation aimed at 
assessing the age and prevailing paleoecological conditions of the sediments. Sediments within the areas are composed of sandstones, ironstones, claystones, siltstones, and shales. The shales are generally dark grey and carbonaceous, with thin laminations. Occasionally, they are highly fossiliferous, with streaks of gypsum minerals. These lithological attributes characterize sediments of the Nkporo Formation. The shales were carefully selected and subjected to microfaunal analysis. The results of the analysis revealed a fair yield of foraminifera that are moderately well preserved. A total of two hundred and forty-eight (248) species were recovered. This recovery is predominantly benthonic, with both arenaceous (90.73%) and calcareous forms (5.65%), as well as other faunal species (3.62%) such as gastropods, ostracods, and pelecypods. Age-diagnostic species encountered in the sediments include Afrobolivina afra, Bolivina explicata, and Ammobaculites benuensis. These species signify the Campanian-Maastrichtian age. The extremely high amount of Ammotium genera recovered from Nguzu Edda, together with the fair to good occurrence of Textularia, Praebulimina, Saccammina, Bolivina, and Uvigerina within Ekoli and Asanga, indicate salinity ranging from brackish to normal marine conditions. The rare to complete absence of calcareous benthics and macrofaunal occurrences within the sediments of Ekoli and Uguzu Edda implies that the oxygenation level ranges from anoxic to dysoxic conditions, while the good occurrence of calcareous and macrofaunal species within Asanga Amangwu indicates dysoxic to oxic prevalence. The presence of agglutinated paleo-water depth indicators such as Ammobaculites, Ammoscalaria, Ammotium, Saccammina, Textularia, and Trochammina, together with calcareous genera such as Bolivina, Bulimina, and Uvigerina, suggests brackish lagoon to outer shelf depositional depths for the sediments. The occurrence of gypsiferous beds, the low diversity of the benthic assemblage, and the low frequency of planktonics confirmed that sediments within the study area were deposited in a shallow marine epicontinental sea.