SEDIMENTOLOGICAL CHARACTERISTICS OF NKPORO FORMATION AT OKIGWE, LOWER BENUE TROUGH, SOUTHEASTERN NIGERIA.

Abstract

The Lower Benue Trough has three main structural units; Anambra basin, Abakaliki Anticlinorium and Afikpo Syncline. Four significant outcrop sections occur within the basinal area. The lithology and biostratigraphy of selected facies from the outcrops were studied to establish their stratigraphic correlations Based on the foraminiferal recovered from the shale facies of the outcrops, all the sections are Turonian-Maastrichtian in age and interpreted as Nkporo Formation. The analyses of the stratigraphic sections in the area of study distinguished four lithofacies namely; fine to medium grained sandstone, ferruginised fine grained sandstone, fissile and laminated shales, and indurated shales. The petrographical analysis of the sandstone shows that the predominant detrital minerals of the sandstones consist of sub-angular to sub-rounded quartz occurring as frame work grains while feldspar, lithic or rock fragments, cement and matrix are absent, the heavy mineral present are Tourmaline and Zircon, accessory minerals present are biotite and muscovite. The micropaleontological analysis of the shale samples yielded rare to common abundance of forams assemblages, which include Ammobaculites bauchensis, Ammobaculites benuensis, Ammobaculites jessensis, Ammobaculites spp., Miliammina pindigensis, Miliammina telemaquensis, Reophax spp., Spiroplectammina hausorium, Trochammina dutsuna., Trochammina taylorana, Ammobaculites coprolithiformis, Ammotium nkalagum, Ammotium nwalium, Reophax guineana, Haplophragmoides spp., Textulariopsis fulani, and Miliammina telemaquensis, thereby deducing Turonian-Maastrichtian age (Period/Epoch) of geologic time scale with Ammobaculites zone established from the assemblages mentioned. The diversity of the benthonic Foraminiferal assemblage in the shale of the Nkporo Formation is low, with high abundance of Genus; Ammobaculites. The high percentages of Ammobaculites indicate brackish-water.