Graphic correlation and paleoenvironmental investigation of the upper Eocene–lower Oligocene sediments in the Dahomey Basin, southwestern Nigeria: insights from palynomorphs.

Abstract

This study presents the results of a comprehensive qualitative and quantitative palynological analysis of four shallow boreholes (Epe-1, Olokonla-1, Ikoyi-1 and Badore-1) in the lower Eocene–upper Oligocene periods of the Dahomey Basin, southwestern Nigeria. The lithostratigraphic analysis revealed three lithologies (sandy-shales, shaly-sands and sandstones). Two most important palynological zones, the Verrucatosporites usmensis , as well as the Magnastriatites howardi Pan-tropical zones, are delineated. First Appearance Datum (FAD) of Achomosphaera alcicornu marked the late Eocene–early Oligocene boundaries (E/O) within the four boreholes, while FAD of Adnatosphaeridium multispinosum defined the late Eocene–early Oligocene boundary (E/O) in Ikoy-1. Two informal palynological assemblage zones were revealed in the four boreholes. Common occurrences of palms, mangrove, fresh water and brackish water pollen, and pteridophyte spores with spots records of marine elements and algae species during marine invasion suggested fluctuation from fluvio-deltaic/nearshore to marginal marine paleoenvironments. Graphic correlation discloses that for every meter of sedimentary rock accretion within Epe-1, equally amounted to merely 0.31 m, 0.56 m, as well as 0.47 m of sediments, which were put down in Olokonla-1, Ikoyi-1, as well as Badore-1, respectively. These comparative rates of sedimentary rock accretion show with the purpose of either there is fewer attrition occurrence and/or extra accommodation gap within Epe-1 than Olokonla-1, Ikoyi-1, as well as Badore-1. The assessment of the slopes, intercept and correlation equations through graphic correlation procedures enable the identification of four biostratigraphic events in Epe-1, Olokonla-1, Ikoyi-1, as well as Badore-1. The findings in this investigation are necessary for depositional succession and paleoenvironmental understandings, as well as basin investigation, and above all, to show a relationship of broadly separated boreholes.

Keywords

• palynology
• stratigraphy
• graphic correlation
• depositional environment
• eocene–oligocene boundary
• dahomey
• basin