1.0 Location
The study area is located in Enugu state, Nigeria, and lies between latitudes 6o
23’N and 7o
23’N; and longitude 7o
23’E and 7o
30’E.It is situated on the edge of the escarpment and has an area extent of approximately 167.7km2
.It covers Enugu-Ngwo,Ngwo-Uno, 9th
MileConer, Ameke-Ngwo,Etiti-Ngwo,Okwojo-Ngwo,Amachala-Ngwo, Ukaka-Ngwo,Iva Valley,Uboji-Ngwo,Enugu town, Ogbete, Akwuke, parts of Nsude,Eke,Abor and Ebe.
1.1 Accessibility
The major routes that enhance accessibility to the area are the old Enugu-Ngwo – Enugu road, ENUGU – Onitsha expressway, old Nsukka road, road to Okpara and Onyeama Coal mine, and road to Ameke brewery. Several other secondary roads and footpaths created by villagers which make movement easier also exist. Figure show the accessible map of the study area.
1.2 Objectives and Scope of Study
The objective of this thesis is to study the detailed geology of the study area with respect to the litology, structural features, stratigraphy, and assessment of viable economic deposits. It embraces the study of sedimentological and petrographic characteristics of the rocks and the hydrogeology of the study area. The objectives also include the production of detailed geologic map of the area. The geography (climate, vegetation, topography) and the associate environmental hazards are evaluated and suggestions are made to effect hazard control.
The method of investigation involves four stages, namely: desk study, reconnaissance survey, detailed geologic survey and laboratory analysis.
1.3.1 Desk Study
This involves the consultation of available journals, bulletin, reports and relevant materials about the study area. This exercise was done in the geological science department library, Nnamdi Azikiwe Univerity, Awka, Enugu State University and University of Nigeria, Nsukka.The topographic map of the area was also studied. All these provided hints on the location, boundary, extent and access to the area.
A reconnaissance survey of the area was embarked few days to the detailed study, .During this exercise, a formal notification letter was given to the traditional rulers of the surrounding towns. Also, an insight about the accessibility, lithology, drainage pattern and topography of the area was gained.
1.4.3 Detailed Geologic Survey
A topography map 1: 25000 of the study area was used for the detailed geologic survey. During the study, lithologic units and boundaries were delineated based on changes in vegetation, topograghy, soil colour, and rock type. Strike and dip of beds and structures were measured on the outcrops, while rock samples were collected from the field for laboratory analyses. Each of the samples collected from the outcrop locations was appropriately labeled and named. Detailed measurement of outcrops as well as detailed description and sketches of these outcrops and structures were made. Photograph of important geologic features were taken for reference purposes.
The field instruments used during the study include: brunton compass, geologic harmer, tapes, sample bags, camera, field notebook, writing materials and ruler.
1.3.4 Laboratory Studies
The laboratory analysis carried out include: grain size analysis, hydrogeochemical analysis, plastic and liquid limit test. The grain size analysis was carried out on four samples collected from Mamu Formation and Ajali Sandstone. Two samples from each of the formations were analyzed. From the grain size analysis, sorting, skewnes.kurtosis, grain size distribution and environment of deposition were determined. Hydrochemical analysis was carried out on some of the water samples collected from surface water of the area at WATSAN Awka, Anambra state. Plastic and liquid limit test were carried out the shale collected from the Mamu Formation to determine the plasticity and liquid limit.
The task of moving across thick vegetation and crossing streams and rivers in order to measure the dip, strike and thickness of beds was problematic during the study. The prevalent weather condition during the period of the study also imposed big problem as the work was often interrupted by rainfall. The problem of tracing the lithologic boundaries due to thick vegetation was also encountered. The scarcity of outcrops of Enugu Shale made difficult the measurement of dip and strike and laboratory study of the formation.
Care was taken to ensure that the samples (rocks and waters) collected from different locations of the study area were not mixed up by rebelling them properly at the point of collection. The dip and strike amount and direction of the studied outcrops were also recorded on site. The boundary between the formations were traced far inside the bush to guard against confusion between the road construction materials and the true lithology. In both field and laboratory study, sensitive instruments were used. Thus, the good working condition of instruments such as brunton compass clinometer, weighing balance and sieve sets were confirmed before use. The samples were also guarded against contamination before the laboratory analysis was carried out.
1.6 Literature Review
The geological survey of the area and indeed, the whole of southern Nigeria started in 1903 by the mineral survey of Nigeria. Simpson (1954), Wilson (1925) and Bain (1924) geologically investigated the sedimentary rocks of southern Nigeria and established the geology of the eastern Nigeria. This was after the discovery of coal at River Ofan in Udi in 1909, which prompted the geological survey of Nigeria to embark on a broader mapping of the southern Nigeria in1922 where a lot of emphases were based on the Nigerian coal field, with particular detail on the geology of the escarpment area, west of Enugu.
Shell Petroleum Development Company of Nigeria Limited documented important information on geological and geophysical survey of the Southeastern Nigeria while exploring for crude oil between 1938 – 1941 and 1946 – 1951.The stratigraphic succession in the southern sedimentary basin was developed by Shell-Bp in 1956 and was modified by De-Swardt and Cassey (1963).
A biostratigraphic correlation of the southern Nigerian sedimentary basin was attempted by Reyment (1965).He used the abundance of fossils such as foraminifera,ostracods,pelecypods and ammonites to date the rock units such as Ezeaku Shale,Nkporo Formation which is laterally equivalent to Enugu Shale,Mamu Formation,Ajali Sandstone and Nsukka Formation. Ladipo (1989) showed the influence of tide in the depositional environment of Ajali Sandstone. The depositional environment was termed deeper marine shelf facies. Three major depositional cycles in southern Nigerian sedimentary basin was identified by Short and Stauble (1967). The first cycle( Albian to Santonian) was confined to the Benue – Abakaliki trough. The second cycle (Campanian – Cretaceous) filled the Anambra basin and the Afikpo syncline. The last cycle led to the development of the modern Niger Delta. In the Albian to Santonian sedimentary phase of the tectonic cycle, the southern Benue trough came to an end with the folding, faulting and uplifting. Erosion of the Coniacian, Turonian, and the Albian deposits occurred with the uplifting of the Abakaliki anticlinorium.Subsidence in the area began a new marine transgression which led to Campanian – Maastritchian age of Nkporo Shale as well as its lateral equivalents (Enugu Shale and Owelli Sandstones) as described by Reyment (1965).The Campanian began with a short marine transgression followed by a regression in southeastern Nigeria. The Nkporo Shale constitutes the basal beds of the Campanian. The broad sea gradually become shallower and the paralic sequence of the lower coal measure was deposited. This formation was underlain by the continental sequence of the Ajali Formation followed by a return to partial condition with the deposition of the upper coal measure.
Hoque and Ezepue (1977) studied the petrology and paleocurrent direction of Ajali Sandstone and proposed the Cameroon granite as the provenance of the sandstone. The comparative analysis of transmissivity and hydraulic conductivity values within the Ajali Sandstone and Mamu Formation aquifer was shown by Egboka and Uma (1984, 1985). They established the most reliable of all statistical method of obtaining the aquifer parameters and noted that the Ajali and Mamu aquifers are the source of acid water flooding in the Enugu coal mine.
A noticeable feature in the Nigerian geological literature is the frequent change in the naming of formations. Thus, what is now known as the Mamu Formation was originally called Coal Measure by Bain (1924). It was changed to Lower Coal Measure by Tattam (1944) who first observed that the Coal Measure is actually made up of two coal bearing formations namely: the Lower Coal Measure and the Upper Coal Measure separated by a thick series of sandstone. The Lower Coal Measure was later called Enugu Coal Measure by Tattam. There was later a return to the name Lower Coal Measure by Simpson (1954). The current name Mamu Formation was given by Reyment(1965) after the Mamu River. The Mamu River is in the neighborhood of Enugu. The Upper Coal measure was also renamed Nsukka Formation by the same Reyment. The name Enugu Shale was given by Groove,(1951) and has been in use since that time.