Multi-proxy evidence from the exposed strata at Okpekpe, Imiegba, and environs for nomenclature as Mamu Formation, Benin Flank, NW Anambra basin, Nigeria

Abstract

Litho-sections along Okpekpe, Imiegba, and environs have been studied for their textural and microfossil contents to elucidate this recently exposed lithostratigraphic unit in the Benin Flank of the Anambra Basin. Five of the road-cut sections were mapped for their textural characteristics, facies types and associations, and palynostratigraphy to decipher their age, and palaeoenvironment, for proper assignment to a lithostratigraphic unit. Facies analysis reveals four facies types namely: shale, sandstone, mudstone, and coal facies. The mudstone facies is characterised by abundant Thalassinoides ichnogenus that suggest a deposition under littoral environment. Index palynomorphs markers: Trichodinum lasstaneum,  Andahisiella polymorpha, Cingulatisporites ornatus, Dinogymnium sp., Cerodinium diebelii, Buttinea andreevi, Cyclonephelium sp., Laevigatosporites major, Odontochitina operculata, and Longapertites marginatus, recovered from both the shale and mudstone facies revealed Early Maastrichtian age. The sandstone facies is coarse-grained, poorly-sorted, nearly symmetrical, mesokurtic sandstone. Bivariate plots, linear discriminant and multivariant functions revealed deposition in a range of depositional environments, including shallow marine, beach, and fluvial environments. The combination of facies association and the microfossils suggests a transition between non-marine and marine environments for the exposed litho-sections. The facies types and associations, age, and palaeoenvironment revealed by the exposed sections correlate with the Mamu Formation, suggesting that this lithostratigraphic unit is not restricted to the eastern part of the Anambra basin but also in the Benin Flank.