The Geology and Structural Analysis of Asu River Group Shales in Abakaliki and Its Environs, South-eastern, Nigeria

The Geology and Structural Analysis of Asu River Group Shales in Abakaliki and Its Environs, South-eastern, Nigeria

The present study describes the geology and structure analysis of bedding planes joints and vein/vein lets observed in Abakaliki and its environs Southeastern Nigeria.
The research was carried out mainly to determine the structures of the area, and attempt their analysis and interpretation, so as to establish detailed structural information on the area under study.

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The area under study is Abakaliki and its surrounding areas, and part of
Abakaliki metropolis in Ebonyi State, Nigeria (Fig. 1), which lies
within the south eastern Nigeria sedimentary basin, the mapped area includes Nkwagu,Agbaja, Agu Akpu Azuiyiokwu, Ndiechi Igbeagu, Onu Ebonyi and Ekaeru Inyimagu,lying between latitude 6° 151 N and 6°201N and longitude 8° 051 E and S^C^E. The main geological material is Abakaliki shale of Asu River Group of the middle Albian times.

Fig. 1: Map of Abakaliki showing the study area

The area under study is made accessible four major road networks of Ezz – Ogoja Water Works road, Enugu – Abakaiiki Obubra road and Abakaiiki routes. Gateways are provided to the study area while the numerous footpaths provided access to the outcrops. (Fig. 2).

Fig 2: Map of Abakaliki and its environs showing accessibility

This study is aimed at detailed description of the geology and structural features of the Abakaliki shale and its surrounding areas.
The study was carried out mainly to determine the tectonic structures of the area, analyze and interpret them, and obtain detailed structural information on the study area
• Exposing student to geological survey
• For award of degree

The area under study falls within the humid tropical rainforest region of southeastern Nigeria with annual rainfall of between 1750-2500mm pre annum. Raining and drying seasons are typical in the area. The raining season last from March to October, with its peak in July through August and September, accompanied by ‘ dry season from November to February which most often is caused by seasoned water drought. The harmattan wind keeps the humidity low from December to January. The mean annual temperature range of the dry season there is about 28°c and occurs in February and March. During the raining season there is high humidity, heavy flooding, soil leaching and sheet out wash

Fig 3: Map of Southeastern Nigeria showing Climatic Condition (Inyang, 1979)

The people and inhibition of Abakaliki and its environs are hospitable people. Their major occupation is trading is secondary.
Abakaliki people belong to the Igbo-speaking ethnic group, with a prevalence of Abakaliki dialect. The Abakaliki people are engaged in subsistence farming and are popularly known for the production of rice (the Abakaliki Rice). They also engaged in quarrying activities in the mining sizes. Abakaliki people are generally peace loving, industrious and friendly. They practice both African traditional religion and Christianity. 1.7 GEOMORPHOLOGY

The topography of the study area is of vast low-lying plain. The area falls within the area described to have elevation in the range of 100m above mean sea level (Umeji, 2000). Scattered and isolated hills of pyroclastics form the uplifted areas, which are found in the northern parts of the area (see attached map). The hills are as high as 450m hi the northern part, 350m in the Northwestern part and 400m in the Northeastern part against the backdrop of the low-lying plains underlain by the shales.

The study area is drained by Ebonyi River, lyiokwu River, Ochala River, and Isiachi River with their tributaries. Most of the streams within the study area are ephemeral and dry up soon after the rains stop. During the rainy season, the streams overflow its banks and causes flooding and water logging. The drainage pattern is known as dendritic. Though Ebonyi River is the main drainage course of the study area, it traverses the area and runs north-south hi the eastern part of

the area; meandering through its plains. Its tributaries flow northeast, northwest, and southwest. There are a lot of surface runoffs due to the clayey nature of the soil, which does not allow percolation of water, hence the area is water logged in the rainy season. The river and its tributaries exposed the fresh rocks.

Fig. 4: Drainage map of the study area.

Abakaliki and its environs are located within the vegetation belt called Guinea savanna. It is mainly covered by thick vegetation especially along streams and rivers.
The area is characterized by grasses shrubs and fire-tolerant trees, which show that the area is of derived Savanna. The trees are grouped into deciduous and semi-deciduous trees. Dense vegetation is found along river channels.
The moderately high ridge and hilly areas support sparse vegetation with resistant strong deep rooted shrubs which are located randomly. This vegetation is regularly affected by seasonal bush burning.

Soil Type and Land Use
The soil types are functions of the lithology. There are two major types of soil that dominate the area, they are, lateritic and non-lateritic soil. The lateritic soils are dark to light brownish red in colour. The non-lateritic soils are dark in colour. The study area is mainly characterized by clay-loamy soil. They are mostly found in low land areas and causes water logging. The waterlogged area is avoided, while the dry areas are mainly inhabited, though some of the water logged areas are being reclaimed at the moment. The soils are very fertile and support heavy vegetation. They are primarily used for the cultivation of yam, rice, cassava, millet and maize.

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The method involved every plan and principle applied to ensure that the project work was completed successfully, starting from the desk study to the conclusion stage. The procedure involved the following:

i. Locating approximately the position of any outcrop observed on the
Topographic map.
ii. Observing clearly the different rock types in a location. This was done using parameters such as colour, grain size, mineralogical composition
and field relations.
iii. Collection of fresh samples and giving physical description of them,
iv. Labeling and giving a field name to the sample collected
v. Observing structures present in the out crop and taking measurement,
vi. Sketching or taken photographs of the structures observed,
vii. Production of geologic map of the area at the end of the studies.
Simple compass directions and transverses were employed in mapping the study area. The Brunton compass was used. When identifying and describing the rock types, azimuths, measurement of the attitude of bedding planes, and of such tectonic structures as joints and vein/vein were taken in the area.
The outcrop location from which the measurements were taken are presented in the attached map of the study area.

Abakaliki and its environs (mapped area) are of great geological importance due to its strategic position in the Benue Trough and the tectonic activities witnessed in the area. It is underlain by shales of the Abakaliki Formation of the Albian Asu-River Group (Orajaka and Umenwaliri, 1989).
Reyment (1965) described the litho and bio stratigraphy of the Asu River Group with its type locality along Asu River. A middle Albian age was assigned to the group by Reyment (1957) on the basis of ammonites such as Oxytropidoceras hause and Oxytropidoceras manuaniceras Sp. Deposition was

under marine conditions giving to rise to deposits of alternating shales and siltstone with occurrences of sandstones and limestones.
A sequence of mafic lavas, pyroclastic flows, tuffs and agglomerates forms part of the cretaceous succession within the Benue Trough of Nigeria. These volcanic rocks are best exposed in the area around the study area in the southern part of the trough. The petrology and stratigraphy of the Abakaliki pyroclastics in the south Benue Trough are under controversy.
The rocks have been described variously as pyroclastic flows, intrusive breccias, submarine spilities, andestic tuffs, basaltic agglomerates or alkali basalts (McConnel, 1949; Uzuakpunwa, 1974; 1979). The age and stratigraphic position has been said to be:
i. Interstratified with Albian shales by McConnel, (1949).
ii. Post-Albian (Farrington, 1952).
iii. Pre -Albian or Aptian by Uzuakpunwa, (1974).
iv. Pre -Albian ana overlying the Basement Complex by
v. Olade, (1979).

Because of the complex and complicating nature of the Benue Trough, most young and advanced geologist have found it difficult to conclude the theories of its origin.
It was considered geologically, that Benue Trough originated because of direct consequence of the Atlantic Ocean opening, and is the third aim of the triple function centered beneath the present day Niger Delta (Wright, 1976). This elongated trough like basin is continuous with the coastal basin, and in fact, has been correctly described as the long arm of the Nigeria coastal basin, and in fact, has been correctly described as the long of the Nigeria coastal basin. (Reyment, 1965).
In 1970 and with the emergence of the spreading sea floor and plate tectonics hypotheses, Burke, Dessauvagic and Whiteman came up with a new theory for the valley. The authors contended that the Benue rift first opened in the Cretaceous, due to the spreading of a crustal ridge in the region of the present trough. This spreading, according to Burke et al., (1965), seized by late Cretaceous and then was followed by a closing episode of the north Atlantic and south Atlantic African plates. In the Santonian. The resultant differential motion of the two parts of the African plate, in their view resulted in the Santonian folds and gave them their unique parallel and sub-parallel structure along the valley. Nwachukwu (1972), apparently, disagreeing slightly, suggested that while the tectonic evolution of the Benue Trough may be reconstructed in terms of

descriptive and convergent interactions of the two continental plates, under thrusting was not likely and crustal spreading therefore minimal.
However, the continuity of the sediments of the Benue Trough with the Nigerian coastal Basin is not disputed as have been shown (Cratchley and Jones 1965) to the marked by:
1. The series of long narrow folds with East north east-west South west trend linked with the South western folds of the Abakaliki sedimentary area.
2. The continuity of the paleontological zonation with the coastal marine formation.
3. The narrow lead-zinc mineralization beit running from the Abakaliki area to the north east part of the basin stratigraphically, the Benue Trough is subdivided into three regions;

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The upper or northeast region; middle Benue region or the lafia-Muri area; and the lower or Southern area. Benue Trough which is the area south and West of Makurdi (Figure 4).
The south western end of the Benue Trough is concealed by the Niger Delta. The Mamfe basin connected to the lower Benue is a lower Cretaceous structure stretching in a southeasterly direction. On the other edge of the trough, the Niger Basin (Bida Basin) is a northwest trending sedimentary basin of Maastrichtian age. In the northeast direction, the lower Benue Trough connects with the upper Benue Trough. The study area is Abakaliki shales of Asu River Group which falls within the cretaceous lower (Southern) Benue Trough.

Fig. 5: Geological setting and location of the Benue Trough in relation to the Cretaceous Trilete Rift System (Hoqne, 1977).

Cretaceous Lower Benue Trough
The second tectonic stage occurred in the Santonian and compressional movement along the existing Northeast-Southwest trend of the trough and resulted in large scale folding with axis parallel to the trend of the trough.
The southern part of the Benue Trough is separated from the Atlantic Ocean by the important Niger Delta which covers the relationship’between the sedimentary basin, lying in a continental crust, and the Atlantic Oceanic basin. The Knowledge of the geologic history of the southern part of the Benue Trough (figure 5) helps in reconstructing to the opening of the Gulf of Guinea. The bio stratigraphy of the area was established by McConnel (1949) and Farrington (1952). From then onwards, the first synthesis was published Reyment, (1965); Cratchley and Jones (1965); Murat (1972); Whiteman (1982) and models based on the global tectonic concept were proposed (Stoneley, 1966; Wright 1968; Burke et al, 1971). The deformation, the metamorphism and magnetic events in the lower Benue through during creataceous times were given by Benkhelil, (1989).
Investigations in the Southern came up with a conclusion that the Cretaceous lithologic units occur in the east and north of the region forming the western limb of the Okigwe-Abakaliki Anticlinorium. These were seen possessing a low regional dip to the west and south west regardless of the sharp folding exhibited by the shale formations in the core of the structure. There, Cretaceous formations outcrop in a general North-South direction. Close to the axis of the anticlinorium near Okigwe, the regional dip veers to the south west. Approaching the axis of the structure has the highest formations with remarkable variation in lithology is experienced, with the shale giving place to

sandstone as evidenced in Okposi area. Over the axis of the anticlinorium, the outcrop narrows considerably due to part of steeping of the dip, and also due to the marked thinning of the formations.

Fig 6: Geology of the Cretaceous sediments of the Benue Trough (After Cratchley and Jones, 1965).

The Cretaceous succession in the Southern Benue Trough was grouped into: the lower Cretaceous and upper Cretaceous. In the lower Cretaceous, sedimentation was said to have started during the middle Albian and ended about the Maastrichtian. This middle Albian sedimentation was proved to be responsible for the deposition of every thick marine,dark, grey shales, siltstone and subordinate limestone. The shales in most places lie uncomformably on the Basement Complex.
In the upper Cretaceous, there were no clear evidence on its deposition except in the Odukpani area of the southeastern corner of the Nigeria Coastal Basin; though Cenomanian age have been assigned to Muri Sandstone in the middle Benue region (McConnel,1949), and these sandstones were reported to wedge out in the southwest and have not been evidenced in the south. This Cenomanian witnessed mild deformation associated with back stepping of the sea. This resulted in the restriction of deposition in the southeastern portion of the basin around Calabar flank. The stage also witnessed pockets of volcanism resulting in the placement of alkaline to sub-alkaline suites of igneous rocks. Still on environments, the deformed and uplifted trough became a positive element to shed detritus into the newly created basin (Anambra Basin and Afikpo syncline) flanking it (figure 7).

Fig 7: Configuration of Anambra Basin and Afikpo basin second sedimentary
Cycle (Modified from Murat 1972). Arrows indicate sediment disposal system.

The lower Benue Trough is underlain by the basement which is dominated by grante and migmatic rocks as well as rara relies of metasedimentary rock. Pan-African Orogence affe ted all these basement rock around 600 = 150 Ma (McCury, 1971). The Jurassic Younger Granites of Jos Planteau and Tertiary Cameroon volcanic are post-pan-African geological units that are lying near to the lower Benue Trough north and south respectively.
Cessation of sedimentation in the Anambra Basin and Afikpo syncline occurred in the Neogene with the beginning of the miogeosynclinal sedimentation in the delta. The long established northeast-southwest trending depositional strike of sand bodies of the trough gave way to an East-West trending in the delta (Figure).

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Fig. 8: General geology of southern portion of the Benue Trough (After Olade, 1976).

The study area lies within the southeastern part of Nigeria where Albian Shales occur which is the oldest sediments. These sediments are undifferentiated and unnamed and are deposited with “Asu-River Group” which is the type area of the group (Reyment, 1965). The Asu-River Group consists of very thick, grey shales, which are frequently calcareous, with subordinate fine-grained, micaceous sandstones and micaeous siltstones. On studied shales and limestones. Their exposures may occur in ditches and drains (Abakaliki Shale).The study shales are poorly bedded with sandstone and limestone lenses (Reyment 1965). The sediments are folded particularly in the south of Abakaliki with fold axes stretching northeast-southwest. The shales are deeply weathered containing abundant radiolarian and echinoids while pelecypods and gastropods are rare (Reyment 1965).

The beds of Cenomanian age are restricted to the basin around Calabar. They form the Odukpani Formation and consists of arkose,sandstone, limestone and alternation of limestone and shale (Kogbe, 1972). Through Cenomanian age have been assigned to Muri Sandstone in the middle region (McConnel, 1949), the shales are predominally shally in the uppermost part (Reyment, 1965). The type locality is at the village of Odukpani near Calabar (Kogbe, 1989) and these sediments are of shallow water origin and may be about 600 meters thick.

The Eze-Aku Formation is next and Turonian in age. The formation comprises of hard grey to black shales and siltstone with frequent facies changes.

to sandstone or sandy shale. They vary in thickness but may attain lOOOmetres in places (Reyment 1964). Locally, the outcrops of this sequence are seen at Amasiri (as “Amasiri Sandstone”,) Egedde-Oju (South of Otukpo), Nkalagu and Ezillo with its laterals equivalent in Makurdi Formation and Awgu-Ndeabor Shale formation of Senonian age. The fossils consist of mainly vascoceratids, pelecypods, gastropods, echinoids, fish teeth and plant fragments (Reyment, 1965) which indicate a basal Turonian age (Kogbe 1989). figures 9 and 10.

Fig 9: Tectonic map of Southeastern Nigeria during the Campanian to Eocene (After Murat, 1970).

These sediments have been assigned to the Awgu Formation (Reyment 1965). The deposits consist of well bedded shales, occasional intercalations of fine-grained sandstones, and shelly limestones. The formation is less thick than that of Turonian. The thickness in about 800 metres and they tend to give an impression of rather quick lateral changes in facies (Kogbe, 1989). They are rich in ammonites, molluscs and fish teeth (Reyment 1965).
The Albian to Coniacian age in the eastern portion of the Southern Nigeria basin has the total thickness of sediment of about 3,300 metres (Kogbe, 1989).
The Albian -Santonian succession was uplifted and become the topographic provenance (Abakaliki Anticlinorium) which supplied the bulk of the Anambra Basin fill (Hoque, 1977).

Companian- Maastrichtian
The Companian-Maastrichtian sediment of the southeastern Nigeria started with Nkporo Formation. The sediments of Nkporo group are scarce and rest unconformably upon the folded pre-Campanian strata (Agwu shale). They begin with dark grey, often friable shale with occasional thin beds of limestone and sandstone, (Kogbe, 1989). Owelli sandstone, Enugu shale and Asata shale are lateral equivalents of the Nkporo formation, which are inner basin sediment of shallow water origin. They are black argillaceous and fossiliferous and pass laterally into coarse pebbly fluviatile Owelli sandstone in the Awgu area (Simpson, 1955). The commencement of the Campanian-Maastrichtian is marked by a short transgression followed by a regression (short and Stauble, 1967). The Nkporo formation is overlain by Mamu formation by a regression

(formally lower Coal Measures) which consists of marine intercalation composed of ammonitifefous shales (Reyment, 1964), fresh water and low salinity sandstone, mudstones, and sandy shales, Coal occurs at several levels (Simpson 1955). Locally, outcrops of Mamu Formation are seen along the Enugu-Outcrops of Mamu Formation are seen along the Enugu-Onitsha road at the Milliken Hill on the outskirts of Enugu. The lower level is dominated by shales and coal while the upper horizon is dominated by sand (Simpson, 1955). This formation is overlain by Ajali Formation which consists of thick friable,poorly sorted sandstones, typically white in colour but sometimes iron-stained (Hoque and Ezepue, 1977) and (Kogbe, 1989). It consists of subangular to subrounded quartz arenites which is about 330 metres thick of coarse grained sandstone.
The Nsukka formation previously known as “Upper Coal Measures” lies unconformably on the Ajali formation. The rocks consists of an alternating succession of sandstone, dark shale and sandy shale with thin coal seams at various horizons (Reyment 1965) as series of highly fossiliferous grey -green sandy clays with calcareous and Tertiary sediment in southeastern Nigeria including their petrology is shown in table below.

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