2.1 Geology and Hydrogeology
Cross River State is situated at the boundary of Nigeria and Cameroon Republic, and lies closer to the Gabon – Congo carton than any other part of Nigeria.
Cross River State is subdivided into four (4) units, namely; (i) The Oban Massif, (ii) The Calabar Flank, (iii) The Mamfe Embayment, and (iv) The Obudu Plateau.
According to Ekwueme (2003), the mapping of the rocks of the Oban Massif revealed rocks that falls within the following groupings of the Nigerian Basement Complex including; (a) Schist belts, (b) the Magmatite – gneiss complex, (c) Charnockitic rocks, (d) Gabbro and doleritic rocks, (e) Older granities with Amphiboles, (f) Syenites, basic and acidic intrusive rocks like granodiorities, pegmatites and quartz veins.
The Calabar Flank represents the part of the southern Nigeria Continental Margin bounded by Oban Massif towards the North and by the Calabar Hinge line delineating the Niger Delta Basin to the west. The Calabar Flank has early Cretaceous deposits followed by the first marine incursion which resulted in the deposition of the platform, the Mfamosing Limestone during the middle Albian (Petters, 1982). In the Cenomanian and Turonian, Subsidence of faulted blocks resulted in widespread deposition of shale with minor calcareous intercalations. The sediments are uncomformably overlain by a dominantly shale lithology with occasional mudstone and thin gypsum beds during the Campanian to Mastrichtian (Nyong and Ramanthan, 1985).
The study area is underlain by Teritary and Quarternary to Recent sediments known as the Benin Formation (Coastal Plain sands) (Amah et al., 2008). The Benin Formation consists of alternating sequences of gravel and sand of various grain size, silt, clay and alluvium (Amah et al., 2008, Ekwueme et al., 1995).
The alternating sediments built up a multi-aquifer system in the area. The Cretaceous sediments include mostly conglomerate, sandstone, limestone, mudstone, shale and marl of Calabar Flank (Ekwueme et al., 1995). The Precambrian basement complex rocks are migmatite, gneisses, schists, phyllite, granodiorite, pegmatite, granite, tonalite, charnokite and many more (Rahman et al., 1981, Ekwueme et al., 1995).
The geology of Cross River is primarily dominated by the basement complex of the Oban Massif. The Oban Massif extends from the Central Region to the Southeastern part and the Obudu Plateau of the Northeast (Akpeke, 2006). The sedimentary terrain in Cross River State consists of rocks of the Lower Benue Trough and Calabar Flank. Cross River State (CRS) has a history of regional metamorphism, tectonism, magmatism that leads to the development of fault, fold and fractures that creates sites for mineralization. CRS is known to have great potentials for economic mineral deposits such as Gold, Manganese, Rutite, Mica, Barite (Akpeke 2006, Adamu et al., 2015). CRS is noted for the occurrence of countless aporadic clusters of dimension stones and pegmatite veins. In addition, Cross River State occupies the southern fringes of Nigeria. The State has an area of approximately 23, 000 Km2 and extends from the Atlantic Ocean (in the southern part).
The basement of southeastern Nigeria is made up of the Oban Hills and Obudu Plateau in Cross River State (Akpeke, 2006). All in the Precambrian basement outcrops in southeastern Nigeria. The Oban hills and Obudu Plateau constitutes the western prolongation of Cameroon Mountains into the Cross River plains (southeastern Nigeria). The Oban – Obudu Massif is flanked on the south by the Cameroon volcanic line and on the Northwest by the Benue Trough which is believed to be complementary structural features (rift. Cretaceous to Tertiary to Sedimentary rocks (Fitton 1987).
There are two (2) main rivers in the Calabar area, namely; (i) the Calabar River in the west, and (ii) the great Kwa River in the east.
The main River runs almost the entire length of the Cross River State is the Calabar River which takes its rise from the Cameroon Mountains to the east (Akpeke 2008). Several and distributaries flow southwards and westwards to the main River.