1.1. Study Background
Even though a century of geological studies have enabled a broad understanding the geology of the Benue Trough, it was only in the latter part of the 20th century that a picture of the structural framework, within which the Benue trough evolved, began to emerge. The controversies surrounding the tectonic evolution of the Benue Trough have been largely resolved, with the overwhelming evidence leaning towards the interpretation of the so called French school of structural Geologists which sees the Benue trough 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, 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.
The Afikpo area offers a unique opportunity to study and understand the deformational processes and to determine the tectonic stresses active 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.
1. Geographical Setting
1.1.1. Location and Accessibility
The study area is bounded by latitudes N 5° 51´ and N 6° 03´; and longitudes E 7° 51´ and E 8° 06´ (Figure 1) and covers the areas around Afikpo town, Amaseri, Ohaozara, Akpoha and Abomege in Ebonyi State of South Eastern Nigeria. Itigidi, Ediba, Itegeve and Adadama communities in Cross River state also fall within the study area. Access to the area is through the roughly east-west Afikpo-Okigwe road which connects the Okposi-Amaseri-Amoso road at Amaseri. On the outskirts of Afikpo town, this road connects with the Northbound Abakaliki road passing through Akpoha and Abomege. The eastern side of the study area is accessible through the Abomege-Ugep road which passes crosses the Cross River at Itigidi on its way to Ugep and Calabar to the south of the study area. Other minor roads link the smaller interior villages from these major roads. The major roads are tarred while the minor roads are at best graded and may not be accessible in the peak of the rainy season.
The study area can be divided into roughly two regions, northern and southern, with different topographic styles (Figure 2). The northern region has a lower elevation on average (less than 100 m) and is characterized by ridge and swale topography. The ridges are most prominent towards the Western part of the study area around Amaseri and Ibi where they trend NE-SW. The ridge profiles are asymmetrical with a gentle south eastern dip slope and a steeper north western scarp slope. They are related to the indurated sandstones which comprise them.
Towards the eastern part of the study area the trend of the ridges gradually rotate from NE-SW to NW-SE with dip slopes facing southwest around Ameta-Oziza. In the eastern part of the study the ridges are less prominent and can be seen to be folded about a NE-SW axis from digital elevation models (Figure 2).
The southern part of the study area is has a much higher elevation (above 100 m) and is more rugged than the Northern region. These hills are dissected by the Cross River which has cut gorges with moderately steep slopes at Itigidi and Oziza.
The Cross River is the largest river passing through the study area (Figure 1). It originates in Cameroon, where it takes it is called the Manyu River and flows southwards through the study area to the Atlantic ocean (“Cross River (Nigeria),” 2013). The Aboine River is one of the major tributaries of the Cross River. It passes through the study area from north to south where it joins the Cross River. Its path seems to be controlled by NNW-SSE lineaments. The Asu River is a West-East flowing tributary of the Aboine River. Its flow direction is controlled by the alignment of the ridges and the river path is limited to one of the large shale swales through which the river meanders and forms a wide flood plain before joining the Aboine River at Akpoha. These rivers, though perennial, show a large variation between peak flow (usually at the end of the rainy season) and ebb flow (at the end of the dry season) where they are reduced to a bare trickle. Their banks provide in the dry season, very good exposures of the shale units otherwise hidden in other locations. Other minor smaller streams are also controlled by the ridge and swale topography giving a roughly trellis drainage pattern along with the Aboine and Asu rivers.