Palynological age and palaeoenvironment of deposition of Mid-Cenozoic sediments around Umuahia, Niger delta basin, southeastern Nigeria

• Outcrop logging and sampling of the section were systematically done from base to top.
• Field samples were analyzed and logged under the microscope.
• Detailed biostratigraphic age/correlations of the sediments was achieved.
• Paleoenvirionments of deposition of the sediments were ascertained using palynomorphs of environmental value.
• Paleosalinity and paleoclimatic conditions of the sediments were reconstructed.

The sediments of Oligocene – Early Miocene Ogwashi Formation of the Niger Delta Basin exposed in the kaolinite quarry by Oyivo stream in Umuahia comprise four lithological units which from base to top are (a) white to grey mudstone over 5.0 m thick; (b) lignite 1.0 m; (c) carbonaceous grey to dark shale 3.0 m; and (d) medium to coarse grained argillaceous sandstone 8.5 m. Samples from lignite and carbonaceous shale yielded rich palynomorph assemblages. Argillaceous sandstone and mudstone were barren. Lignite recorded more spores and less pollen while carbonaceous shale had more pollen and less spores. Index sporomorphs include Verrucatosporites usmensis, Magnastriatites howardii, Psilatricolporites crassus, Echiperiporites icacinoides, Echiperiporites minor, Retibrevitricolporites protrudens, Retibrevitricolpo-rites obodoensis, Retibrevitricolporites ibadanensis, Chenopodipollis dispersus and Retitricolporites irregularis representing Oligocene – Early Miocene Verrucatosporites usmensis/Magnastriatites howardii pantropical palynozones.Palaeoenvironmental indicators include Monoporites annulatus, grass pollen commonly found in open vegetation of savannah or reed swamps within the rain forests; Psilastephanocolporites laevigatus, the tropical lowland evergreen tree; Proxapertites operculatus, Longapertites marginatus and Psilatricolporites crassus the palms of brackish water swamp, along with Schizosporis parvus (Spirogyra) a fresh-water green alga. The sporomorphs indicate palaeoenvironments ranging from fresh water upper to brackish water lower deltaic plain within the tropical rainforest.