Aspect of Biostratigraphic and Petrographic Studies of the Late Palaeocene – Eocene sequence of the Eastern Dahomey Embayment, Southwestern Nigeria

Abstract

Petrographic characterization and biostratigraphic distribution of the lithofacies exposed at Ibese and Sagamu were carried out in order to determine the chronology and depositional environment of the associated formations and different microfacies identified with the associated rock units in the embayment. The methods employed include detailed lithostratigraphic description carried out in the field while the carbonate petrography, nannopalae-ontological, palynological and micropalaeontological analyses were carried out in the laboratory. The lithostratigraphy in the Ibese quarry has a thickness of 29.5 m which consist of lower to middle limestone with intercalation of marl with an upper shale and silty sand overburden approximating 2m while the Sagamu quarry has a thickness 30m comprising of lower to middle limestone with an upper shale and lateritic overburden of about 5 m. The limestone intervals assigned to the Ewekoro Formation are dark grayish, fossiliferous and calcitic in some parts, while the shale intervals of the Oshosun Formation are dark and highly fossiliferous. The biostratigraphic results indicate the presence of palynomorphs, calcareous nannofossils and other microfossilsas recovered from the samples in different diversity and abundance. Abundant fossils forms are recorded in some of the samples while others have few and in some cases barren. Cibicides spp. are the only foraminifera recovered. Toweius callosus and Coccolithus formosus are among the calcareous nannofossils retrieved from the samples, while Monoporites annulatusis the most abundant of the palynomorph recovered. Petrographic results show two microfacies units. The biomicrite facies is the most re-occurring while, the biosparite facies is sparse. The environment of each microfacies is determined from available data. The presence of Cibicides spp. and Monoporites annulatus show environments of deposition of the rock units to be open circulated shallow water. The occurrence of Cruciplacolithus formosus and Toweius callosus dated the Akinbo shale as exposed in Sagamu quarry to be Early Eocene while the presence of Cruciplacolithus frequens, Cruciplacolithus tenuis and Toweius eminens dated the Akinbo shale as exposed in Ibese quarry to be Late Palaeocene to early Eocene. The abundance of gastropods, pelecypods, scaphopods and diverse microfacies identified suggest quiet shallow marine environment of deposition for the locations. The presence of Toweius callosus and Coccolithus formosus indicated the age of the Oshosun Formation to be Ypresian (55.8+0.2 Ma) while the Ewekoro Formation age range from Thanetian (58.7+0.2 Ma) toYpresian (55.8+0.2 Ma).