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 the Ezeaku 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.

CHAPTER ONE

 1.1 INTRODUCTION

The area of study is in the northeastern border of Afikpo North Local Government Area of Ebonyi State. The scope area of work covers geological mapping and aspects of geology like: lithology,sedimentology, stratigraphy, [biostratigraphy], and geomorphology, production of geological map, structural geology, and environmental analysis of samples collected from the study area. In the Southern Benue Trough, the nature of most of the units and the dense vegetation as a result of a wet tropical climate have hindered field studies and created a missing link in the proper explanation of the depositional and provenance framework of the basin. The Afikpo area offers a unique opportunity to study and understand the depositional and provenance processes in the southern Benue Trough as the highly indurated nature of the sediments allow for an abundance of outcrops that is unmatched anywhere else in the region. The fundamental bjectives of this field work is to give detailed description of the geology of Ozara Ukwu and its environs which can be used in interpretation and delineating the depositional environment, areas of environmental hazards and minerals of economic importance in the study area.

1.0 BACKGROUND STUDY

The Benue trough is thought to be as a collection of pull-apart basins, related to transcurrent or strike-slip movement along deep-seated basement shear zones of Pan African origin, reactivated as oceanic transform faults (Benkhelil, 1982 and 1989, Guiraud, 1993).This view is supported by field evidence in the northern Benue Trough where the climate and the nature of the sedimentary units allow for classic geologic study. Geological and geophysical studies have enabled a broad understanding of the geology of the Benue Trough. It was only in the latter part of the 20^thcentury that the structural framework, within the Benue Trough was largely resolved. The Ozara Ukwu area offers a unique opportunity to study and understand the depositional processes, and Paleoenvironmental reconstruction of the Southern Benue Trough. Being highly indurated, the sediments allow for an abundance of outcrops which were studied in details. However, the dense vegetation of the area, which was as a result of wet tropical climate, hindered field studies and created a missing link in the proper explanation of the structural framework of the basin, (Okonkwo, 2014).

 1.1Geographical setting

The study area, Ozara Ukwu and its environs, is located at the northeastern border of Afikpo North Local Government Area of Ebonyi State. It lies within latitude of 05⁰55’N and 05⁰50’N and longitude of 07⁰55’E and 07⁰50’E. Notable towns that situates within the study area include, to the West is Amaseri town, to the south is Amangwu, and Owutu communities, respectively,  etc.

1.1.1       Accessibility

The study area, Ozara Okwu is accessible through road network from Abakaliki–Afikpo road, Afikpo–Okigwe road, Amasiri–Edda road, Unwana road, Ndibe and some outcrops are trekable few meters into the bush and others are along river channels. The main access road to Afikpo area is the Okigwe – Afikpo road and Abakaliki – Afikpo road.