1.1 General Introduction
Field mapping is an indispensable exercise in geology because geology is a field oriented discipline. It is in this basis that this work was carried out and a report presented on the geology of Ilaka. This field project is an exercise in accordance to the partial fulfillment of the requirement for the award of bachelor of sciences. Geologic mapping exercise was carried out on Ilaka and its environ, to determine and understand the geology of the area. Nigeria lies approximately between latitude 4N and 15N and 3E and 14E, within the pan African mobile belt in between the west African and Congo Cratons in the region of late Precambrian to early Paleozoic orogenesis. The basement complex is made up of Precambrian rocks and consists of schist belts folded in them.
Previous works have been carried out on the basement complex of Nigeria and it has bed shown that this is the most abundant lithology in Nigeria (Rahaman, 1988; Dada, 2006;Odeyemi, 1977) of which the study area (Ilaka-Ife) is not exempted. Geological mapping of rocks around Ilaka-Ife area was carried out to map out the different lithology in the area through detailed examination of the various rocks both in filed and laboratory studies by examining and mineral behavior relationship, micro and mega structure and their textural properties. The main lithologic units in the study area include; granite .gneiss which are massive and quartzite which occur at the south eastern upward to the north eastern parts of the plot. Although considerable work has been done in this study area by Odeyemi (1977), Oyawoye (1964) but new ideas about this area are required to improve and integrate the information about the rocks in this area and to achieve this aim, field work was embarked upon and detailed geologic mapping was done and documented with collections of samples for further analysis to be carried out in the laboratory.
The aim of this geological mapping exercise in the area is to produce a detailed geological map
OBJECTIVES AND SCOPE OF STUDY
The project was carried out in order to achieve the following objectives
· To determine the rock types underlying Ilaka environs.
· To produce a detailed geological map showing the boundaries between the rock types
· To carry out petrographic analysis on the rocks encountered.
1.3 LOCATION AND SIZE
The plot covered by this study lies approximately between 4052.5’ and 4035’E and equally lies between latitude 7010’ and 7012.5’N. The size of the plot is 5km, the plot of study (Ilaka) is about 35km from Akure and it is situated west ward of Akure.
The area studied is typically characterized by a combination of highland and lowland terrain. With reference to the topographic map, the northwestern, North central, northeastern and south central parts of the study area are characterized as lowland environment with the contour lines that are well spread. The relief of the area is relatively rugged, the highest point on the plot is about 1200m. The granite gneiss from the highland. The trend of the ridges is more or less north-south which conform to most other ridges in the southwest, therefore, the characteristics features of the study area are the undulating relief type.
1.4.2 CLIMATE AND VEGETATION
The climate condition of Ilaka plot 12B falls within the tropical humid climate region where the wet and dry seasons are noticed prominently in the area. The prevailing seasons the dry (November- February) while the rainy (April-October). The mean minimum temperature observed ranges from 200c in January to 230c in February, while the mean maximum for the hottest month August is 260c. The vegetation of this area is dense vegetation with thick bushes and tall trees that cannot be easily accessible during the peak of the rainy season
The drainage system of the study includes the streams and rivers using the topographic map as a guide, a major river (Owena river) which flows roughly across the plot from north-south. The numerous tributaries all originates from the hilly areas and they flow north to south to connect with the major river the flow pattern of the drainage is dendritic. The drainage pattern shows the resistance of the underlying basement rock and their structural features, and also tells about the fracturing pattern of the rock since fractures goes along way to impact on development of stream or rivers, the drainage pattern of the mapped area is largely dependent on the topography of the area. The study area is drained by a number of streams and channels which are tributaries of the Owena river.
1.5 ACCESSIBILITY AND HUMAN ACTIVITIES
Both old and new roads were used as stress as access path for the field mapping exercise in Ilaka plot 12B. Major and minor footpaths were also used when necessary. In addition villages and hamlets are interconnected by footpaths. Generally, accessibility by these roads is good and it is further enhanced by the extensive cocoa plantations, those areas that were not easily accessible were cut through using cutlass. The major occupation of the inhabitants of the study area is mainly commercial farming and the major crops produced are; cocoa, oil palm etc. farming activities are carried out in all areas of the plot while oil palm production is restricted to river channels. Tree felling and wood production is also carried out by the villagers.
1.6 LITERATURE PREVIEW
1.6.1 REGIONAL GEOLOGY
The Nigerian Basement Complex is one of the three major lithological components that make up the geology of Nigeria. It forms the southern part of the Trans Saharan mobile belt east of the West African craton and northwest of the Congo craton (Caby, 1989; Ferre et al., 2002; Caby, 2003) and has also explained to be south of the Tuareg shield in Black (1980). This basement complex comprises Archean and Proterozoic rocks which is believed to be the results of three major orogenic cycles of deformation, metamorphism and remobilization, and basement reactivation corresponding to the radiometric ages indicated by Liberian (2700 ± 200 Ma), Eburnean (ca 2000 Ma), Pan-African (ca 600 Ma) which resulted from plate collision between the passive continental margin of the West African craton and the active Pharusian continental margin (Grant, 1970; Oversby, 1975; van Breemen et al., 1977; Fitches et al., 1985; Rahaman, 1988; Dada et al., 1994). Three broad lithological groups (Fig. 5 and 6) are usually distinguished within the Nigerian basement complex (Ajibade et al, 1987). These divisions are (i.) polymetamorphic migmatite-gneiss complex, (ii.) low grade sediment dominated schists and (iii.) syntectonic to late tectonic granitic rocks which cut both the migmatite-gneiss complex and the schist belts.
The polymetamorphic migmatite-gneiss complex is composed largely of migmatites and gneisses of various compositions and amphibolites. Relict of meta-sedimentary rocks represented by medium to-high grade calcareous, pelitic and quartzitic rocks occur within the migmatites and gneisses, and they have been described as “Ancient Metasediments” by Oyawoye, (1972). The migmatite-gneiss complex is considered to be the basement sensu stricto, and isotopic ages varying from Liberian to Pan-African have been obtained from the rocks. The Pan-African ages have been interpreted as due to isotopic re-homogenization in preexisting rocks during the Pan-African orogeny. Low-grade sediment dominated schists form narrow belts in the eastern half of the country have been described as “Newer Metasediments” (Oyawoye, 1972), “Younger Metasediments” and “Unmigmatised to Slightly Migmatised Schists” (Rahaman, 1976). These schist belts are believed to be relicts of a supracrustal cover which was infolded into the migmatite gneiss complex. The schist belts are intruded by Pan-African granitoids. Further attempts on the classification of the Nigerian Basement Complex (e.g. Rahaman, 1988) distinguished four major petro-lithological units, namely:
(i.) The Migmatite – Gneiss- Quartzite Complex
(ii.) The Schist Belt (Metasedimentary and Metavolcanic rocks)
(iii.) Older Granites and associated granitic rocks(The Pan-African Granitoids Undeformed Acid and Basic Dykes or The minor felsic and mafic intrusives