Geology and Depositional Environment of Clastic Sediments at Ozara Ukwu and Its Environs in the Southern Benue Trough, Nigeria

ABSTRACT

Ozara Ukwu and its environs (the study Area) is located within the Abakaliki Basin and the Afikpo sub Basin, respectively within the Afikpo Area. Detailed geologic mapping was carried out in other todetermine the various stratigraphic lithologic units, depositional environment, provenance, age and kerogene characterization, respectively using the basic field and laboratory procedures. Based on field observations, two lithostratigraphic units of the Ezeaku Group and Nkporo Group were identified based on the various diagnostic tools such as, dip of beds and microfossil assemblages. The older Ezeaku group sediments is made up of the basal older Unit of the map area consisting of alternative sequence of indurated fissile shale, calcareous rock (limestone?) and medium to coarse grained, poorly sorted, positively to near symmetrical, leptokurtic sandstone lithofacies, respectively, representing the Amasiri sandstone. It is overlain by narrowindurated Amasiri shale and the Ozara Ukwu sandstone facies respectively. The Ozara Ukwu sandstone unit is made alternating sequence of indurated gray shale, and medium to coarse grained, poorly sorted, positively to near symmetrical leptokurtic cross bedded sandstone lithofacies. These sediments of theEzeaku group are highly indurated, jointed, and highly dipping in the southeast direction with maximum angle of about 500, probably suggesting influence of tectonism that affected the group during the Santonian times. laboratory evidence suggest that depositional environment probably estuarine environment..The younger nkporo shale sediment represented by late campanian in age following results from an outcrop from the map area..This unit represented by dark fissile shale located at the Amangu area. Also, the presence of gastropods (Turittela species)is an indication and confirmation that the sediments were deposited in a marginal marine environment. The paleoenvironmental inference therefore, is that the Amasiri sandstone, the Ozara ukwu sandstons and the nkporo formation were deposited in environments varying from marginal marine to normal marine. Therefore the map area has provided a diagnostic evidence  of the various  depositional environment within the southeastern basins of the Lower Benue Trough