Geology of Abakaliki and Mapping of Pyroclastic Deposits Using Electrical Methods

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Geology of Abakaliki and Mapping of Pyroclastic Deposits Using Electrical Methods

This research work studied the rock units underlying the Abakaliki urban and also applied both vertical electrical sounding (VES) and constant separation traversing (CST) methods of resistivity survey in determining the lateral and vertical extent of a known pyrocrastic deposit within the area.

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The study area is Abakaliki and its environs, geographically it is located between latitude 60 15’ N and 60 20’ N and longitude 080 05’ E and 080 10’ E.
The area is accessible through network of tarred roads which includes Enugu-Abakaliki road, Abakaliki-Afikpo road, water works road, Abakaliki-Ikwo road and Abakaliki-Ugeb road. Foot paths also aided me access to the outcrops. Long distances were covered with the aid of motorcycles. The accessibility is shown in the figure 1.

Fig. 1: Accessibility Map of Abakaliki and its environs

This study is primarily aimed at mapping of pyroclastic deposit in the study area using electrical resistivity method as well as produces its geologic map. The specific objectives include:
To study the rock units underlying the area in their outcrop locations
To make a detailed description of the rock units in hand specimen
To produce a geologic map of the study area
Mapping of a known pyroclastic deposit in the area and determine its depositional contact using electrical resistivity method.


The study area has generally a flat topography with few conical hills formed by the outcrop of the volcanic rocks which are popular in the area. The predominant shale has favored the low erodability of the lithology, resulting in absence or near absence of deep cut valleys and erosion channels.

The major river that drains the area is the Ebonyi River and its tributaries; Iyiudene and Iyiokwu River (fig 2). Both tributaries are perennial and usually overflow their banks at the peak of the rains.

Fig 2: Drainage map of Abakaliki and its environs

Stunted trees and pockets of dreadlock woodland exist where the lithology has undergone high degree of laterization. Elsewhere, typical characteristics of the tropical rainforest are displayed; multitude of evergreen trees, climbing plants, parasitic plants that live on the other plants and creepers. The vegetation of the study area is rain forest (Igbozuluike, 1975) as shown in fig 3.

Fig 3: Vegetation of the Study Area (Modified after Igbozuluike, 1975)

Two main seasons exist in the Abakaliki area, the dry season which lasts from November to March and rainy season which begins in April and ends in October with a short period of reduced rain in August commonly referred to as “August break” temperature in the dry season ranges from 20 – 380C, and results in high evapotranspiration, while during the rainy season temperature ranges from 16-280C, with generally lower evapotranspiration. The average monthly rainfall ranges from 31mm in January to 270mm in July, with the dry season experiencing much reduced volume of rainfall. Average annual rainfall varies from 1500 – 1650mm. these climate conditions is responsible for the development of thick lateritic soil in the area.

Field Mapping
The mapping exercise started with a complete desk study and literature review of the area. This was followed by a thorough reconnaissance survey of the area to determine the accessible routes and to be familiarized with the area. Detailed geologic mapping was undertaken during which contacts of different structural features and lithology were identified and delineated. Samples were collected with the aid of hammer.
The following materials were used during my field mapping
(a) Compass
(b) Geologic hammer
(c) Global positioning system (GPS)
(d) Field Map
(e) Meter rule
(f) Field note, pen and pencil
(g) Camera
(h) Hand lens

Geophysical Survey
During the geophysical survey, the following materials were used, ABEM SAS 1000 Terrameter, two pairs of electrode, measuring tape, cables (wires), compass, cutlass, rechargeable battery and hammer. The following steps were taking to carry out the survey:

A: Reconnaissance Survey:
In this stage, preliminary survey was done in order to establish a ground for the data acquisition. Channel way was created by cleaning the grasses for easy survey.

B: Data Acquisition
This was carried out to establish a lateral thickness of a pyroclastic body by running two constant separation transversing (CST) using wenner electrode arrangement with 5m electrode spacing and one vertical electrode sounding (VES) to delineate the vertical thickness of the pyroclastic body using schlumberger electrode arrangement (fig.3). The data of the resultant resistivity were collected and recorded.

Fig.4: Schlumberger electrode arrangement

C: Data Processing and Interpretation
The data collected was processed using interpex 10 software. The apparent resistivity values were plotted against the electrode spacing and resultant curve produced.
Apparent resistivity is given by
ρa = K x R

The curves were interpreted by placing its type and interpreting the different layers of the curves with its thickness and finally infer the lithology of different layers.

The electrical resistivity method is one of the geophysical methods that are widely employed in numerous activities relating to environment and engineering. Geophysical methods are used to obtain more accurate information about subsurface conditions such as type and depth of materials variation to bedrock, depth of weathered or fractured zone, and depth to groundwater and lateral thickness of hard/volcanic rock deposits as well as it vertical thickness (Telford, 1990).
Geophysical exploration techniques are available which gives on insight into the nature of subsurface. These include geoelectric, electromagnetic, seismic and geophysical borehole logging. The choice of a particular method is governed by the nature of the terrain and cost consideration (Emenike, 2001). But for this work, geoelectric method was used using schlumberger configuration method for vertical electrical sounding (VES) which is used to determine the overburden and vertical thickness of the pyroclastic body and Wenner configuration method for constant separation Transversing (CST) which is used to determine the lateral thickness of the pyroclastic deposit. These electrical methods have been applied effectively for geoelectric assessment (Oseji et al., 2006).

Geophysical investigation method like electrical resistivity method is used to access the average number of geoelectrical layers, vertical and horizontal changes in the resistivity of the subsurface layers and possible rock types and productive potentials of suitable aquifers and their lithological characteristics. This method is based on the fact that certain physical properties of rock change considerably depending on their water content, thereby creating a physical boundary between the host rock and the water bearing strata. The superiority of this technique over others to predict or assess the geoelectric layer was confirmed by the work of (Selemo, et al., 1995; Olanyinka and Mbachi, 1992) .The application of geophysical methods especially electrical method have been the most effective methods in providing information on the geological structure, soil properties and hydrological conditions of the subsurface (Nnokwe et al., 2014).
Evidence has shown that geophysical methods are the most reliable and the most accurate means of all surveying method of subsurface structural investigations and rock variation (Carruthers, 1985 and Emenike, 2001).
The resistivity method is the most effective for locating productive well since the Vertical Electrical Sounding (VES) method can provide information on the vertical variation in the resistivity of the ground with depth and the Constant Separation Traversing (CST) provides a means of determining interval variation in the resistivity of the ground (Olayinka and Mbachi, 1992, Olorunniwo and Olorunfemi 1987, Ariyo et al, 2003).

The resistivity method has been used successfully in investigating groundwater potential (Oseji et al 2005). The method was successfully used by (Oseji and Ujuanbi, 2009) to investigate the groundwater potential in Emu Kingdom, Ndukwa land of Delta State, Nigeria. (Ariyo and Banjo, 2008) used the method to detect depth to bedrock and soil profile in Ilara-Remo, southwestern Nigeria. (Jatan, et al, 2013) used this method for subsurface geophysical investigation in around Bomo area, Kaduna State, Nigeria. This electrical method have been used to determine electrical conductivity of soil and rocks (Mc Neil, 1990), also for measurng hydrologic charateristics of jointed formation (Leonard-Mayer, et al, 1984)
This method has been successfully used by (Eyankware, et al, 2014) to determine reserve estimation of a dolerite deposit in Obiagu Lekwesi area of Abia State, southeastern Nigeria.

Murat (1972) proposed three major tectonic phase which took place in Albian, Santonian and late Eocene or early Oligocene times. Those major tectonic phases resulted in the formation and subsequent remodeling of the Benue Trough. Murat (1972) attributed the control of the geologic history of southern Nigeria to three major tectonic phases during which the axis of the sedimentary basin shifted.
These three phases were.
(a) The Abakaliki – Benue phase (Aptian-Santonian)
(b) The Anambra – Benin (Campanian-Mid Eocene) and
(c) The Niger Delta phase (Late Eocene-Pliocene)

More than 300m of rocks comprising the Asu River group, odukpani and the Eze-Aku and Agwu formation, were deposited during the first phase in the Abakaliki-Benue Basin, the Benue valley and the Calabar flank.
The second sedimentary phase resulted from the Santonian folding and uplift of the abakaliki region and distribution of the depocenter into the Anambra platform and Afikpo region. The resulting succession comprises the Nkporo Group, Mamu formation, Ajali sandstone, Nsukka formation, Imo formation and Ameki Group.
The third sedimentary phases is credited for the formation of the petroliferous Niger Delta commenced in the hits Eocene as a result of a major earth movement that structurally inverted the Abakaliki region and displaced the depositional axis further to the south of Anambra Basin.
These are folds which consist of series of anticlinorium and synclinorium suggesting that there was a deformational episode in the trough. These folds, coupled with the identification of the igneous rock such as undersides in the Abakaliki area which lead some workers to propose a compressional (surbduction) rather than an externsional tectonic setting for the Benue through (Farrington, 1952). The floor of the basins is irregular and sediments thickness varies from place to place. This occurs as a result of extensive block faulting initiated when the trough begon to develop.
The breaking of the south American continent from the African continent in the Nigerian sedimentary basin which marked the beginning of tectonism started in the early Cretaceous (Murat, 1972). Numerous lines of structural, stratigraphic, geomorphologic and paleontologic evidences have been presented to back up a rift model (Burke, 1972).

The oldest sediments in southern Nigeria are around Abakaliki in southeastern Nigeria. These sediments are unnamed and undifferentiated. They constitute of the “Asu River Group”. The type area of the group is along Asu River (Reyment, 1965). The sediments consist of rather poorly bedded sandy shales known as the Abakaliki shales with sandstone and sandy limstonelenses. The limestone beds can attain a thickness of 30m. Paleontologically, the shale is mainly characterized by species of mortoniceras and elobiceras. The shales are deeply weathered and contain radiolarian, sehinoids and some palecypods and gastropids-sediments of Asu River Group are folded particularly in the south of Abakaliki and the folds axis stretch NE-SE (Reyment, 1965).

The Albian sediments are overlain by Cenomanian sediments which are restricted to the Odukpani formation of the southeastern corner of the Nigerian coastal Basin around Calabar, though cenomanian age has been assigned to Muri sandstone in the middle Benue region. The deposit consists of arkosic sandstone, limestone and alternating limestones and shales which become gradually more predominantly shally in its uppermost parts (Reyment, 1965). These sediments are of shallow water origin. The type locality is at the village of Odukpani near Calabar (Kogbe, 1989).
The Turonian deposits mainly belong to the Eze-Aku formation (Eze-Aku shale, Simpson, 1955). The type locality is the Eze-Aku river valley in southeastern Nigeria. The formation comprises of hard grey to black shales and siltstones with frequent facies changes, to sandstones or sandy shale. The bulk textural character of Eze-Aku sandstone comprises a coarsing upward grain size gradient which ranges from fine to very coarse sandstone. Locally, at Amasiri, the Eze-Aku formation posses laterally into the “Amasiri sandstone” facies. The Eze-Aku formation is structurally deformed by the Santonian tectonic event (Nwachukwu, 1972).

Sediments of Coniacian-Santonian age are generally less thick than turonian and they tend to gives an impression of rather quick lateral changes in facies (Kogbe, 1989). The sediments have been assigned to the Awgu formation (Awgu shale of Reyment, 1965). The formation is about 800m thick and consists of marine fossiliferous grey-blue shales associated with subordinate limestones and calcareous sandstones. The santonian is a regressive substrate in Nigeria and sediments of this age have not been found in southern Nigeria (Kogbe, 1989). A doubtful santonian locality in the Awgu shale in Igumale area has yielded ostracods possibly referable to this substage (Reyment, 1965).
The Campanian-Maastrichtian in southeastern Nigeria begins with dark grey, often friable, shale with occasional thin beds of limestone and sandstone. This part of the sequence belongs to the Nkporo formation. The Owelli sandstone, Enugu shale and Asata shale are lateral equivalents of the Nkporo formation. These inner-basin sediments are all of shallow water origin and these are frequent sharp facies changes.

The basal part of the coal measure sequence previously known as the “lower coal measures” but now known as the Mamu formation contains shales (Reyment, 1965). The coal bearing part of the sequence consists of predominantly fresh water and low salinity sandstone, shales, mudstones and sandy shales, coal seams occur at several levels (Simpson, 1955). Mamu formation is well exposed along the Enugu-Onitsha road at the Milliken Hill just on the outskirt of Enugu. The Mamu formation is overlain by the Ajali formation (false bedded sandstone) with its type locality along the valley of the Ajali River near Enugu. This formation consists of thick friable poorly sorted sandstone typically white in colour but sometimes iron stained (Kogbe, 1989). The Ajali formation is exposed along the road cut between 9th mile (from Enugu) to the foot of the Milliken Hill.
Ajali sandstone is subsequently covered by Nsukka formation (upper coal measures). This formation is marked by the deposition of carbonaceous shales, sandstones and some thin coal seams. The upper-most unit has been recorded as containing Paleocene fauna in southern Nigeria (Cratchley and Jones, 1965). Outcrops of this formation can be observed in the valley of the Nadu River and on road cuts along Enugu-Onitsha road.

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