INDEPENDENT GEOLOGIC FIELD MAPPING OF THE IGARRA AXIS OF THE BASEMENT COMPLEX
1.1 BACKGROUND OF THE STUDY
Detailed Geological field mapping is an indispensable exercise in geology as a result of the fact that geology as a discipline is field-oriented. It is the aspect that completes and gives life to the study of geology. An independent Geologic field mapping of the Somorika-Aiyetoro-Aiyegunle-Ogugu-Imiegba-Fugar-Okpekpe-Bawa Hill-Auchi area and their environs in parts of Etsako East local government area in the Northern part of Edo state, Nigeria, was undertaking by the 400 Level Geophysics students and 300 level students of the Department of Earth Sciences, Federal University of Petroleum Resources Effurun (FUPRE), Warri Delta state, Nigeria in partial fulfillment of the requirement for the award of Bachelor of science (B.Sc. Honours) degree in Geophysics. The field mapping exercise lasted for thirteen days (13 days). The Imiegba-Fugar-AuchiBawa Hill area are made up of sedimentary rocks and lies within the Benin flank of the Anambra basin.
The Somorika-Aiyetoro-Ayegunnle-Ogugu axis is made up of basically Igneous and
Metamorphic rocks (schist belt). The Igneous rock is of the Pan – African Orogeny (600±150 mya) (Turner, 1989). The rocks found here are not the oldest igneous rocks in the African region. Interesting geologic features was seen such as contact metamorphism, Batholith, Xenolith, Vein Quartz amidst others.
On the other hand, the Okpekpe-Imiegba-fugar-bawa hill consists entirely of sedimentary rocks, specifically ferruginous sandstone which was seen around Imiegba and it underlain the Okpekpe sandstone/shale sequence, the Mamu Formation which contains carbonaceous shale which can serve as a good source rock as well as a good reservoir rock because it is well sorted (Brume, 2014). These together make up part of the Benin flank which is a western extension of the Anambra Basin. It is a CampanianMaastrichtian continental basin located in south central part of Nigeria with a total approximate sediment thickness of about 9km and covers a surface area of about 40,000 sq.km and is a roughly triangular shaped basin that is bounded to the North by the Bida basin and Northern Nigerian massif, to the East by Benue trough, to the West by West African Massif, and to the south by Niger Delta complex.
The Council for Mining and Geosciences (COMEG) stipulates a minimum of about 100 – 180 days of field experience to be graduated from Geology and/or other allied departments of Nigerian universities. This is deservedly so in view of the common saying that the greatest geologist is he who has seen the most number of rocks.
Field geology basically seeks to describe and explain the surface features and underground structure of the lithosphere (Lahee, 2002). The knowledge of field geology prepares the student for key future operations such as exploration and mining. Report writing from geologic field mapping also helps to prepare the student for the important tasks of writing seminar papers, articles for publication as well as project (research thesis) writing.
1.2 AIM OF THE STUDY:
Basically, field works or geological field mappings are carried out with the aim of developing practical skills and knowledge in students necessary for them to adequately fit into real world of industrial and/or societal challenges. Specifically, the objectives of the Geologic Field Mapping include: include:
I To imbibe in students, the scientific attributes of keen observation and making necessary inferences.
II To train students to be able to recognize the various structural, textural and other geological features existing in rock.
III To further demonstrate to students, what had been studied in the classroom.
IV To train students on how to use the various geological structures and features seen in the field to interpret the geological history of a rock formation, bearing in mind, the various geological principles.
V To train students in the act of logging and production of geological maps.
VI To develop team work among students.
GEOGRAPHIC LOCATION AND AREA
The study area lies within longitude 0060 04.00’ and 0060 12.989’ of the Greenwich Meridian and latitude 070 05.881’and 070 23.022’of the Equator in Edo State region. The areas visited includes Somorika village, Aiyegunle village, Aiyetoro village, Imiegba village, Imiegba-okpekpe road, Okpekpe village, fugar sandstone and the Bawa hill. The area has good road network but no railway line was found. Most of the villages were not easily accessible but there were footpath leading to them.
Generally, the study areas were accessible by roads (major and minor), and footpaths.
CLIMATE AND VEGETATION
The climatic condition of Igarra and its environs fall within the warm-horrid tropical climate region where the wet and dry seasons are noticed prominently in the area. The rainy season last for about seven months (May to October) and the dry season last for about five months (November to April). Rainfall is moderate between the months of March and May and heaviest between June and September with average rainfall between 1000mm and 1500mm and temperature as high as 36.7° especially within the hottest period of February to April.
The study area lies within the Guinea Savannah vegetation belt characterized by short trees and tall grasses. The vegetation here is prominently made up of sparsely distributed trees, herbs, shrubs, and grasses. The trees (mango and orange) help check the activities of erosion. Trees in this area are mostly concentrated along fracture zones within the plutonic bodies and on the Quartzite ridges were adequate soil cover has resulted and there is adequate groundwater retention.
The vegetation in this area is mostly secondary i.e. the natural vegetation is being altered and such agricultural crops such as Maize, Yam, Cocoa, Cassava, Pineapple, Cashew, Mango, and Sugar cane are grown.
SETTLEMENT AND LAND-USE
The major occupation of the inhabitants of Igarra and its environs is mainly subsistence farming and the major crops produced are yams, cassava and pineapples, others are maize and cocoa. Most of these farming activities are carried out in the valleys which in most cases have loamy soils and also within a region that has a high water table. The process of bush burning is followed by hunting of bush animals by the indigenes. Some of the farmers produce palm oil in small quantities from the palm trees.
WEATHERING AND EROSION
The study area and most part of southwestern Nigeria are currently being devastated by soil and gully erosion at alarming rates and magnitudes. Several workers have attributed the prevalence of gully erosion to climatic and anthropogenic factors (Ofomata, 1973). Nwajide (1980), recognized biologic and hydro geotechnical characteristics of the gully areas as important factors in the gullying process.
The high incidence of gullying and land sliding result from the susceptibility of the sandy units to erosion under the influence of meteoric and anthropogenic factors. Landslides are common along many of the Nigerian highways particularly those that traverse the sedimentary areas of the country.
REVIEW OF PREVIOUS WORK
The only detailed published work in the area was by Geological Survey of Nigeria (GSN) in 1965. Auchi sheet is what cover Auchi, Igarra and environs.
Works of individual workers was also published in journals of Mining and Geosciences (NMGS) in 1985. Most of the work in the area was unpublished and mostly done by students in Universities notably those of University of Benin. Students from Obafemi Awolowo University and Federal University of Petroleum Resources also mapped the area.
1.3 SCOPE OF STUDY
The scope of this field work borders on:
- Detailed independent geological field mapping of rock outcrops in the study area.
- Collection of samples from the various lithological units of all the outcrops mapped in the field.
- Plotting of field data on a field map.
The interpretation of the sedimentological features obtained from the sedimentological log drawn from the field and from the laboratory results of the samples obtained from outcrops.