Geology of Ngbo Area

Geology of Ngbo Area

  Introduction

Ngbo and its environs are part of the sediments of Ezeaku formation in southeastern Nigeria. It has four lithologies of laminated black shale, Siltstone, bioturbated sandstone, and limestone units in order of oldest to the youngest. Ngbo and it’s environs was studied using surface geological mapping with the major of identifying of the geological features. Thereby establishing the depositional history and constructing the local stratigraphy of the area.

  • Location and Accessibility of the Area

The study area is Ngbo and its environs in Ohaukwu Local Government Area of Ebonyi State. It is geographically located between latitudes 6o 251N to 6o 301N and longitudes 7o551 E to 8o 001 E covering a total area of about 81 km2. It is located in the southern extension of the Benue Trough and Western limb of Abakaliki anticlinorium. It is within the North West region of Ebonyi State.

The area was accessed through Enugu-Abakaliki Express Road and Ezza – Ngbo and Effium Road through a network of tarred roads. During the fieldwork, vehicles, motorcycles and footpaths were employed to aid accessibility and movement to outcrops located where motor cycle where not able to access (Fig. 1)

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                            Fig. 1:  Accessibility Map of the Study Area.

  • Topography And Relief

The map area is an expanse of land that is low-lying and is part of gently and undulating cross-river plain. It slopes gentle along the general strike from a height of 200ft (about 61m) above sea level in the south to a height of 250ft (about 75.8m) above sea level in the North the study area is drained by Ebonyi Rivers.

The map area is generally indicated by the presence of sandstone hills and a ridge reaching a maximum height of 350ft above the sea level to the north-east. The plains occupy about two-thirds of the mapped area.

The ridges run more or less NE-SW and exhibit a gentle dipping scarp face. The slope decrease south wards towards Afikpo. The northern part of the area is marked by irregular topography, owing to the shaly and calcareous nature of the rocks. The evolution of the present land scope began in the Tertiary when erosion started on the newly uplifted and folded sediments (Simpson, 1955). The cycle has reached a sub-nature stage. The geomorphology is controlled by the lithology of the area. The region is covered by a network of stream that descends to the plains in deeply V – shaped valleys.

1.3 Climate and Vegetation

Climate

The climate of the study area belongs to the third climate region (Igbozuruike, 1975). This region has two distinct seasons; the wet and the dry seasons. The wet season starts from April and ends in October while the dry season starts from November and ends in March (Fig.2). The rain has high intensity of thunderstorms, particularly at the beginning and towards the end of the raining season.

Temperature in the dry seasons ranges from 20°C to 38° C and results in high evapotranspiration. During the rainy season, temperature ranges from 16° C to 28°C with generally low evapotranspiration. The average monthly rainfall ranges from 31mm in January to 270mm in July, with the dry season experiencing much reduced volume of rainfall, unlike the rainy season that has high volume of rainfall. Average annual rainfall varies from 1.500 to 1.650mm.

Fig. 2: Mean annual rainfall of Nigeria for 1951 – 1995 (Adapted from Igbozuruike, 1975)

Vegetation

Vegetation is greater than area covered by sandstone and limestone (Reyment, 1965) due to the nature of vegetation and climatic conditions in the area, the inhabitants through shifting cultivation method at substance level practice intensive farming also vegetation of Ngbo and its environs are luxuriant vegetation of tropical rainforest. Its vegetation is densely populated range of shale outcrops and the shales are either grayish or reddish brown in colour depending on its constant or degree of weathering. The area had 400ft as it highest contour and 100ft as it lowest contour above sea level.

  • Drainage Pattern

The area is drained by intricate, dendritic network of streams. The dendritic drainage pattern is as a result of stream channels cutting across the lithologies to erosion that almost show uniform resistance. However, lithology is the dominant factor controlling the map area as a result of the drainage pattern. Few of the streams are perennial while others are intermittent and dry up during the dry season. They constitute trellised types of drainage pattern (Fig. 3).

Fig. 3: Map of Ngbo Showing the drainage system of the Area.

  • Soil Type
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The soil type is dominantly the sandy soil. This is dependent on the nature of the parent material, usually sandstone. There are presence of silt and clay soils which were formed as a result of increased weathering and erosion. Owing to the swampy nature in some parts of the area, rice cultivation appears to be the main occupation of the natives. Rice, cassava and yam are the cash crops growth in the area.

1.6   Objectives of the Study

The objectives of this study are as follows:

  1. To carry out the geological mapping of the area and understand the characteristics and distribution of the various rock types.
  2. To conduct a detailed study of the lithology, petrology, sedimentary structures and paleontology of the area.
  3. To determine the pale environmental conditions was prevalent in the basin of deposition in conjunction with the economic and hydro geologic potential of the area.
  4. To determine the geotechnical properties of rock derived from the Ngbo sand stone.

1.7 Materials and Methods

1.7.1    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 were undertaken during which contacts of different structural features and lithologies were identified and delineated. Samples were collected with the aid of hammer.

The following materials were used during my field mapping

  1. Base Map: Used to locate an outcrop or physical features of geologic importance. It also serves as a guide in the journey.
  2. Compass clinometers: was used in knowing direction and for measuring the attitudes of structures such as strike, dip direction and dip amount. The compass was also used to orientate maps to the North.
  3. Field notebook, pen and pencil: They were used to take down observations made in the field such as description of outcrops, areas and locations they were found and measurement of structures associated with the outcrops. Pen and pencil were also used as scale on the outcrop while taking photograph of some small structures.
  4. Geologic Hammer: This hammer is made up hard steal with two ends and a handle, one end of the hammer is blunt and the other end is chiseled which is used to scoop out sample from loosed materials and the bunt end for knocking out samples from the hard compact mass of outcrop. It was also used as scale on the outcrop while taking photographs
  5. Sledge Hammer: This is a very big hammer which is also use for knocking out samples from big and harder rock body.
  6. Sample bag: Used for carrying samples collected from the field for further analysis.
  7. Masking tape and marker: Used for labeling samples collected according to locations in the field.
  8. Ruler and measuring tape: Used for measuring thickness of veins. Ruler can also be used as a scale on outcrop before taking a photograph.
  9. Global Positioning System (GPS) used for quantitative determination of the grids and elevations of the outcrop locations.
  10. Camera: Used for taking photographs of structures seen at locations.

1.7.2   Methodology

The study was carried out in four phases, namely:

(1) Desk study

This involved the examination of geologic reports on different parts of the Ezeaku Formation in south eastern Nigeria.

(2) Field Mapping

This was carried out for a period of four days starting from 20th – 23thof November 2015. Exposures of the different rock types occur along river valleys, quarries, road-cuts, as well as on hill sides. Access to the remotest outcrops was gained by walking through the dry to semi-dry river channels.

(3) Laboratory work

This is the petrographic analysis. Thin sections for the petrographic studies were prepared at the Thin section Laboratory of the Department of Geology, Kogi state University, Anyigba, Kogi State. Modern Petrographic microscopes housed in the Department of Geology, Ebonyi state University, Abakaliki were used for the petrographic studies. Photomicrographs were taken by use of the Konica digital camera.

1.7.3 Equipment and Materials for Thin Sections Preparation

  1. Cutting and slicing machines
  2. Fresh rock samples (collected from Afikpo)
  3. Sheet of glass (about 7cm by 2cm with about 0.15mm thickness)
  4. Canada balsam
  5. Emery cloth (sand paper) and carborundum powder
  6. Spirit liquid
  7. Water
  8. Cover slip
  9. Micrometer screw gauge

1.7.4  Preparation of the thin sections

These samples were prepared into thin sections in the Department of earth sciences, Kogi state University, Each fresh sample was cut perfectly to a square shape of the required thickness of about 1.0cm with the cutting machine. The specimen is reduced perfectly flat with emery cloth (sand paper) and carborundum powder to a thin sheet with thickness of about 0.04mm measured with the micrometer screw gauge. Boiled Canada balsam was placed on top of the flattened specimen. The arrangement is covered with a cover slip and left for 24hrs, after which it is washed, cleaned with spirit and later with water. These methods of thin section preparation were used on all of the fresh samples prepared

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1.7.5 Data analysis

This involved the plotting of the data obtained from the Field and Laboratory work and the writing and interpretation of the results presented in this final form.

1.8 Literature Review

Sedimentation within the South eastern Nigeria sedimentary basin began during the Lower Cretaceous with the opening up of the south Atlantic and marine incursion into the Benue trough (Nwachukwu, 1972). Short and Sauble (1967) recognized that the sediments in the basin has been controlled by epirogeny which led to major transgressive and regressive events. The first transgressive phase was in the middle Albian with the deposition of Asu River Group while a regressive phase in Cenomanian led to the deposition of Odukpanian formation in Calabar flank. The second transgressive phase was in lower Turonian and deposited Ezeaku shale. The sediments of the study area belong to this formation. The third phase of transgression regression, continues till date.

Simpson (1954) and Reyment (1965) suggest that the Turonian sea that deposited the sediments of Ezeaku shale Formation in a shallow marine sea and possibly a tidal one. Umeji (1984, 1993), after a detailed study of pyroclastic limestone and sandstones in the study area, came out with evidence of storm deposition. However, Banerjee (1980) Interpreted the sandstone fancies of the eastern flank as sub tidal bars and the limestone (Banerjee, 1981), as both storm – lag deposits on the bar marine and as storm generated debris flows, on a shallow platform.

Fayose and De Klasz (1976) suggested that the salinity of the environment ranged from brackish to normal marine and depth of less than 100m. Hoque (1977), have differentiated the sandstones of the eastern Nigeria sedimentary basin into first and second sedimentary cycle on the basis of their petrography characters. First cycle sandstones are feldspathic while those of second cycle are quartz arenite. However, Umeji (1993) suggested that the sand tone of Ezeaku formation is fields’ pathic arenite after a detailed study of the facies.

Reyment (1965) further suggested that the concentration of ammonite shells in these sediments marked post humously dispersed material derived mainly from cephalopod shells. The geological age dating of the sediments was largely on the Ammanite fauna.

                                                                            Chapter Two

2.0 Regional Geologic Setting of South Eastern Nigeria

The Benue Trough is an elongated basin form as a result of separation of Afro Brazilian plate from the Atlantic, is also a product of a trilate fracture system. The Benue Trough is largely view by other authors as an elongate structure span from 1000km long to about 250km wide. Reyment (1965) suggested that Benue Trough is a composite part of a central Africa rift system, resulted from the crustal stretching of the Africa plate consequently upon the initiation of the proto equatorial Atlantic (Olade 1976) described the trough as a failed arm of the rift system (Aulacogen) that developed by progressive mantle plume activities and accompanied by sedimentation. Using geophysical data, concluded that the evolution of the trough involved asthenospheric up-doming, crustal thinning and block-faulting which allowed the emplacement of igneous bodies The sequence of Benue Trough that gave rise to Benue Trough started from late Jurassic, and the present structure of Benue Trough has been affected slightly by tertiary extensional tectonics and the product of these gave rise to a compressional structure that resulted in wide spread folding and faulting in the cretaceous.

Geographically, Benue Trough is divided into three (3)

  • Northern Benue Trough
  • Central Benue Trough
  • Southern Benue Trough

However, (Obanje et al., 2004) abiteraly divided the Benue Trough in upper, middle, and lower. The southern portion of the Benue Trough includes majorly, Abakaliki anticlinorium, which partially undercover the younger basin that is Anambra basin and Niger basin. On the surface, it extend northeast ward for about 250km between Anambra, Gboko and Ogoja area. To the southeast of Abakaliki anticlinorium are Afikpo syncline and Mamfe embayment. The southern Benue Trough is demarcated in the north by an abitrally line taken from south of Gboko to Otukpo, and southward along the geographical boundary between Awgu and Nkporo. Lithostratigraphically, the four lithofacies are demarcated by an unconformities which angular and nonconformity and the lithofacies are

  • Asu River group
  • Ezeaku Formation
  • Odukpani Formation
  • Awgu Formation.

The Anambra Basin which is a Cretaceous Basin is located in the Southern part of the regionally extensive northeast southwest trending Benue Trough. It is a synclinal structure consisting of more than 5,000ft thick of Upper Cretaceous to Recent sediments, representing the third phase of marine sedimentation in the Benue Trough (Ladipo 1988) and (Ojo et al 2009). The basin evolved consequently to the Late Jurassic to Cretaceous Basement fragmentations block faulting, subsidence, rifting and drifting apart of the South American and African plates and therefore representing part of the West African Rift System (Genik 1992) and (Ojo et al 2009) respectively. Anambra basin is dated Campanian to lowermost Maastrichtian, and Stratigraphic history of the Anambra Basin shows that is made up of Nkporo group, and  the stratigraphy of the basin dated Campanian to lowermost Maastrichtian comprises of Enugu shale, Nkporo formation, Owelli sandstone, Otobi sandstone, Afikpo sandstone, and Lafia sandstone, Mamu formation which is the lower caol measure, Ajali formation, and Nsukka formation which represent upper coal measure, with all these stratigraphy overlying the stratigraphy of southern Benue trough.

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2.1 REGIONAL TECTONIC SETTING

The history of the tectonic setting of the Benue Trough has also created a lot of controversies among researchers, believes in triple junction model, resulting to formation of an aulacogen. Reyment (1965), Kogbe (1989) also believed the splitting of the South American and African plate during the lower cretaceous. Nwajide (2005) proposed a sinisterly wrench fault system for the Benue Trough. Going with the model of Nwajide (2005), he believed that the Benue Trough itself is a part of the very expansive west and central African rift system in which it opened up as an extensive sinistral wrench complex. A reconstruct by Murat (1972) shows the southern part of Benue Trough as longitudinally faulted with eastern half subsiding preferentially to become the Abakaliki depression, an important depocentre relative to the west which remained stable and received only veneer sediments. As a result of the Santonian thermo-tectonic event, resulting in the folding and uplifting of the Abakaliki axis, it was flexurally inverted displacing the depocentre to the southwest, northwest and southward creating the Anambra and Afikpo basin.

2.2 REGIONAL STRATIGRAPHIC SETTING AND BASIN FILL

Many authors have described the stratigraphic successions in the Anambra Basin (Simpson, 1954; Reyment, 1965; Agagu et al., 1985; Amajor, 1989; Nwajide and Reijers, 1996; Nwajide, 1990). The Nkporo Group constitutes the basal lithostratigraphic unit in the Anambra Basin and was deposited during Late Campanian-early Maastrichtian period.

The Nkporo Formation

Nkporo formation consists of dark shales and mudstones with subordinate sandstones, oolitic ironstone and shelly limestone with commonly burrows of Skolithos sp, Ophiomorpha sp and Thalassinoides sp. The Enugu Formation, a lateral equivalent of the Nkporo Formation, consists of grey, blue or dark shale, occasional white sandstones and striped sandy shale beds. The Enugu shale has the same body fossil with Nkporo formation the only difference between both is the organic content that is as Enugu shale is dark in colour, Nkporo shale is less dark in colour.

The Owelli Sandstone comprises of fining upward successions of hard ferruginous sandstones, cross- bedded sandstone, whitish shale, moderately sorted, medium to coarse grain, siltstone, coal, kaolinitic clay, vitrinite and wood fragments. Is the lateral equivalent of Afikpo sandstone and is a Campanian reservoir.

Afikpo sandstone overlies the Nkporo shale in Anambra basin which represents the basal part of the Nkporo group in the Afikpo sub-basin.

Otobi sandstone actually outcrops in Benin at southernmost out sketch of Otukpo area.  The Mamu Formation, the Ajali Sandstone and the lower Nsukka Formation were deposited conformably during the Maastrichtian stage over the Nkporo Group.

Lafia formation is the oldest unit of Anambra Basin, with its exposure in the far northern part of the Basin representing the northern extension of the Nkporo group facies.

Geology of Ngbo Area

 

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