ABSTRACT
Oso Edda and environs is located within the flank of the Afikpo Basin and detailed geologic mapping was done in other to determine the various stratigraphic framework using the basic field and laboratory procedures. Field observations on exposed outcrops indicated that the study area consists of three basic lithologic units based on the general orientation of the rocks, sample identification and structural observations. These three units are all dipping southeastly towards the centre of the basin. The basal older Unit (Unit 1) is an interbedding of a highly indurated slatycleavaged shale, calcareous rock and sand stone respectively, whose depositional environment is believed to be shallow marine based on the micro paleontologic and palynological evidence obtained from the shale bed. The shales are fractured and the careous rock is highly jointing showing an extesional type of deformation. This unit is dipping in a SSE direction whose average dip is about 760. It is thought to be affected by a kind of dynamothermal early metamorphism evidenced by high orientation, cleavage structure and baking, jointing and possibly folding. Unit 2 is generally made up of interbedding of coarse grained, poorly sorted arkosic to sub arkosic sandstones and laminated planar crosss bedded siltstone and mud which has a sallower dipping orientation of about 250. Pebbles were also seen within the unit. The general depositional environment of the unit is fluvia from laboratory studiesl which is river dominated and subtidal environment whose energy of deposition is considered to be relatively low to very low. Unit 3 is the linear sandstone ridge that trends in the NE-SW direction which is thought to be an extention of the Vaious ride sections seenwithin the Afikpo basin belonging to the Amaseri sandstone. The ridgee is characterize by a coarsining upward ssandstone sequence. The base of the sandstone is a silty sandstone deposited in oxic environment while the top of the ridge is a trough cross bedded sandstone being dominated by a stormy environment. Most of the Units are mostly weathered as evidenced by structures associated by weathering like spheroidal weathering, pot holes and isolated large boulders. The age of the study area is that both Unit 1 and Unit 2 Upper Cenomanian-Middle Turonian in age which is general known as the EzeAku Shale based on the palynological studies of the shales and Unit 3 is the Upper Turonian Amaseri Sandstone. They are generally referred to as the Age Group.
CHAPTER ONE
INTRODUCTION
Background study
Stating from the background of study, it has been taught that the Benue trough is 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 (Benkheli, 1982, 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. In the Southern Benue Trough, the fine grained 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 structural framework of the basin, (Okonkwo,2014)Afikpo Basin is located in the southern Benue Trough, between the AbakalikiAnticlinoriumrunning northeast and the Cameroon line in the southeast. It forms part of the lower Benue Trough and the adjacent Anambra basin. Sedimentation took place in the Afikpo basin ranging in age from Cretaceous to Mastrichian. On the other hand, the Ezeaku Formation is believed to represent typical shallow water deposit, consisting mainly of hard grey to black shales and siltstones. Facies changes to sandstone and sandy shales are common. The thickness of this Formation varies and locally may be up to 100m thick and passes laterally into sandstone ridges at Amasiri sandstone, calcareous sandstone, calcareous sandstone and sandy calcareous sandstone.
Geophysical and geological studies have aided with broad understanding of the geology of the Benue trough, only in the latter part of the 20th century that the picture of the structural framework, within which the Benue Trough has been largely resolve.
Aims and objectives
to study the lithostratigraphic of Oso Edda and its environs
to identify mappable rock units and logging
to produce a geologic map showing detailed description of the structures and lithologies present in the mapped area
description of the geologic details of the different lithology that were encountered
to deduce the geologic history and possibly geologic time sequence of the deposition
Location of the study area
The location of the study area is bound within longitudes 7º45ʺE and 7˚50ʹE and latitude 5˚55ˊN and 5˚50ˊN in the Southern Benue Trough within the Afikpo syncline and Abakaliki anticlinorium in south eastern Nigeria which is in Afikpo local government area of Ebonyi state, Nigeria. Various locations in the study area include: Ndi-Obasi, Oso Edda, Oso-Uduma, NduArusi, AmasoNta, IfuObasi, NdiIkpo, NdiUche, Oso Owutu road. Oso is bound to the east by Amasiri, to the south by Amangwu, to the west by Akaeze area Ivo local Government Area in ebonyi state and to the north by Okposi. Accessibility was by road and tracks and interconnected bush paths and other footpaths. The Enugu-Abakaliki highway and the Okigwe-Afikpo federal road provides a good accessibility into the study area and out.
PHYSIOLOGY AND TOPOGRAPHY
Afikpo is about 164 square kilometers in size with an undulating topography and an elevation of about 17-m above mean sea level. Notably, sandstone forms its ridge and the shale forms its valley (Hulume, 1997). The shale unit underlies the bioturbated sandstones. These bioturbated sandstones have high attitude (deeply steeping strikes and dips), this might be because they have less period of exposure to erosion.
DRAINAGE
The shale underlying the sandstone also makes surface runoff possible as it does allow for water percolation. As a result of the landform of the study area, these seems to be surface water runoff.
VEGETATION
The vegetation type of the area is parkland; this is characterized by stunt trees and pockets of derelict woodland and secondary forest consisting of shrubs and large trees but where there is lowland displaying up accumulation of trees you get sparse vegetation but where there is upland or highland you get heavy vegetation because of the sandstone deposits.
WEATHER AND CLIMATE
There are two marked seasons in the study area; the wet and dry season. The wet seasons begins in March and ends in October and the dry seasons begins from October through February. These two seasons are dependent on the two prevailing winds blowing over the country at different times of the year. The dry harmattan wind from the Sahara desert prevails in the dry season ranges from 20ᵒC to 38ᵒC and during the rainy season temperature ranges from 16ᵒC to 28ᵒC. (Hulume, 1997).
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