Geology of Paleoenviroment of the Afikpo Sandstones

LOCATION
The study area, Afikpo and it environs, is located in the southern part of Ebonyi State. It lies within latitude of 60 501N and 60 551N and longitude of 70 551E and 80 001E. The study areas are made up of many villages namely: Mgbom, Anangbara, Amizu, Ndibe, Ozizza, Anofia Nkanu, and Unwana. Afikpo forms the highest elevation in the study area, being underlain by sandstone ridges and shales.

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ACCESSIBILITY
The study area Afikpo is accessible through roads network from Abakaliki – Afikpo road, Afikpo – Okigwe road, Amasiri – Edda road, Unwana road, Ndibe beach road, Ozizza beach, Unwana beach and some outcrops are trekable few meters into the bush and others are along river channels. The main access road to Afikpo area is the Okigwe – Afikpo road and Abakaliki – Afikpo road.

Fig. 1: Map of Ebonyi State showing the study area.

Fig 2: Showing the accessibility of Afikpo and its environs.

AIMS AND OBJECTIVES OF STUDY
-To map and demarcate different lithologic units within the study area.
-To determine their paleoenvironment.
-To determine the paleontologic content of the rocks.
-To determine the Economic viability of the map area.

THE METHODOLOGY
Field Mapping
The mapping exercise started with a complete desk study and literature review of the area. This was followed by a thorough recognaissance survey of the area to determine the accessible routes and to be familiarized with the area. Compass and Transversing method was adopted in the field mapping. The equipment use in are the base map, magnetic compass, sample bag, hammer, camera, and field note. Detailed field mapping was under taken during which data were collected and contacts of different structures feature and lithology were identified and delineated, samples were collected for analysis, pebble morphometric, sieve analysis and shale.

Sieve Analysis
The sieving is a method of mass measurement used in determining the size frequency distribution of sandstones particles especially grain size ranges. The equipment for sieve analysis includes a set standard sieve, an automatic sieve shaker, a sensitive weighing balance and mechanical sample splitter. 50 g of sandstone was measured from the sample collected at the field from a particular location and poured into the automatic sieve shaker for about 5minutes to shake particles and then weigh each of the sample splitter with a weighing balance to determine the weight of its particles grain sizes.

Pebble Morphometric Anaylsis
Pebble morphometric analysis involves the measurements of the diameter of the three main axes of the particles with a venier caliper, used by (folk, 1980). The diameters of the axes are Long (L), Intermediate (I) and Short (S) axes of the particles. Only pebble formed from rocks with isotropic constitution and high resistance to wear, such as vein quartz, chest, trough and some related types are used (Sames, 1966). Other parameters used are Flatness Ratio, Elongation ration, Maximum Projection Sphericity, Forms and Oblate-Prolate (OP) index are computed from these three dimensions.

Shale Washing
Shale samples were collected to determine their micro fossil contact each samples consist of about 0.5 to 1.0 kg of rock material were collected, soaked with white spirit for 24 hours and latter decanted, the decanted samples were re-soaked with de-ionized water for another 24 hours. The disaggregated samples were washed through a 63 micron sieve until all the mud was washed clear. The washed samples were dried under the sun and taken to the laboratory for viewing under the microscope to locate and pick fossils for naming and for further analysis. Classification of the foraminifera follows the descriptions of Bronninann and others (1992), castelain and others (1962).

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GEOMORPHOLOGY
This are the various feature that are found or associated with the terrain of the basin e.g. topography, drainage pattern, vegetation and climate.

Topography
The Afikpo and it environs are characterized by undulating plan alternating from East-West comprising of as high as 450ft to low land of about 50ft above the sea level. The gap between the highest and the lowest is due to resulting of erosion within Afikpo Sandstone. The plains underlain by shales, while the ridges are underlain by siltstones and sandstone.

Drainage Pattern
The drainage pattern is systematical flow of water, water surface or underground due to geomorphological feature of the area and it environment. It depends on the soil type, vegetation and relief. The Afikpo town is drained by the Ozizza beach, Ndibe beach and Unwana beach which are major tributaries of the Cross-River. The river has a dendritic drainage pattern with tributaries forming trees structure and mainly associated with coastal area and clay siltstone dominated the environment.

Vegetation/Climate
The vegetation of Afikpo falls within the tropical rainforest and savannah belt of South Eastern Nigeria and the vegetation is characterized by trees, shrubs and abundant of palm trees. The climate of Afikpo area is dominantly of 2 seasons the dry and rainy seasons. The rainy season is between late April to October and the rainfall is about 152cm to 203cm. The dry season is between late October to late March and temperature is high as 280 to 340 degree.

CHAPTER TWO
LITERATURE REVIEW
TECTONIC SETTING
The southern Benue Trough is a segment of the failed arm of a triple junction, which accompanied the rifting that gave rise to these separation of South America from Africa, and the opening of South Atlantic Ocean in the Cretaceous times (Burke et al., 1971; Fairhead and Okereke, 1987).

The Benue Trough can be subdivided into Lower, Middle and Upper parts that were affected by Santonian tectonic activity. Regional Santonian compression deformed the “Benue Trough proper” inverting the main depocenter of the Abakaliki Trough and subsequently creating the Anambra and Afikpo Basins to the north-west and south-east respectively (Murat, 1972; Benkhelil and Guiraud, 1980; Benkhelil 2001). The Santonian tectonic differentiated the sedimentary successions into pre- and post Santonian packages. The post Santonian successions are Campanian-Maastrichtian in age (Reyment, 1965); they occur both in the Anambra and Afikpo basins respectively. In the Afikpo basin, the Campanian-Maastrichtian succession comprises the Nkporo, Mamu, Ajali and Nsukka Formations. The Nkporo Formation rests on the post Santonian unconformity plane and consists basically of sandstones, shales and coal which accumulated during the Early Campanian.

The Benue Trough is a linear NE- SW trending intracontinental basin. Structurally it consists of a series of N-E trending transform fault system, anticlines and synclines. In the Afikpo basin, transform faulting was reactivated during late Maastrichtian terminal tectonic event (Odigi and Amajor, 2009c). The basin was modified by sinistral strike-slip activity. The sedimentary fill in the Afikpo basin is divided into three tectonic-stratigraphic mega sequence the Asu River Group, Eze-Aku Group and proto-Niger Delta succession. The Nkporo Formation in the Afikpo syncline area thins towards the NW and SE at the trough margins of the basin. The Mamu Formation lies conformably upon the Nkporo Formation. The Nkporo Formation is the basal formation of the Campanian-Maastrichtian sediments, and is relatively undisturbed but intruded by igneous rocks. High geothermal gradients prevail in this area close to the intrusives. Simple synsedimentary growth faults and tectonic structures such as joints and strike-slip faults are present in the Afikpo subbasin (Odigi, 2007). The igneous activity of dolerite sill was observed at Edobi mine along Afikpo – Amasiri road (Umeji 1984).

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REGIONAL STRATIGRAPHY
(A). Albian
The study area lies within the southeastern part of Nigeria which is the oldest sediments. These sediments are undifferentiated and unnamed and are deposited with “Asu River Group” which is along the type area of the group (Reyment, 1965). The Asu River Group consists of very thick, dark grey shales, which are frequently calcareous, with subordinate fine grained, micaceous sandstones, micaceous siltstones, sandy shales and limestones. Their exposures may occur in ditches and drains (Abakaliki Shale). The sandy 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 radiolarian and echinoids abundant while pelecypods and gastropods are rare (Reyment 1965).

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

(C). Turonian
The Eze-Aku Formation is next and Turonian in age. The formation comprises of hard grey to black shales and siltstone with frequent fancies changes to sandstones or sandy shale. They vary in thickness but may attain 1000m in places (Reyment, 1964). Locally, the outcrops of this sequence are seen at Amasiri “Amasiri Sandstone,” Egedde-Oju (South of Otukpo), Nkalagu, Ezillo with its lateral equivalence 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).

(D). Coniacian – Santonian
These sediments have been assigned to the Awgu Formation (Reyment, 1965). The deposits consist of well bedded shales, occasional intercalations of fine-grained sandstones, shelly limestones. The formation is less thick than that of Turonian. The thickness in about 800 meters and they tend to give an impression of rather quick lateral changes in facies (kogbe, 1989). They are rich in ammonites, mollusics 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 meters (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). A good outcrops exposure of Awgu Shale is the (Aki na Nkwa) junction but not well shown deeply gully erosion at Awgu junction.

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(E). Campanian
The Campanian is the divide that both marine regression and transgression occurred. The sediment belongs to the base of Nkporo Shale group and lateral equivalent which is the Enugu Shale Formation of basal bed of the Campanian. The outcrop of Nkporo shales is at Enugu shale found at Trans Ekulu road near new market at Gariki (Enugu) and Enugu – Port Harcourt expressway road. The Afikpo Sandstone lies unconformably on the Calcareous Amasiri Sandstone of Eze Aku group. The Afikpo Sandstone consists of thick friable and micaceous, poorly sorted, medium to coarse grained, milky-whitish to brownish in colour. They are no typical ammonite found in the formation but they are traces of index fossils pelecypods, gastropod, echinoids and also burrow of Ophimorphia and Skolithos ichnofacies.

(F). Maastrichtian
The Campanian – Maastrichtian sediments of the southeastern Nigerian started with Nkporo Formation. The sediments of Nkporo group are scarce and rest unconformably upon the folded pre-Campanian strata (Awgu Shale). They begin with dark grey, often friable, shales with occasional thin beds of limestone and sandstone (Kogbe, 1989). Owelli Sandstone, Enugu Shale and Asata Shale are lateral equivalent of the Nkporo Formation, which are inner basin depositions of shallow water origin. They are black argillaceous and fossiliferous and pass laterally into coarse pebbly fluviatile Owelli Sandstone in the Agwu Shales (Simpson, 1954). The commencement of the Campanian – Maastrichtian is marked b a short transgression followed by a regression (Short and Stauble, 1967).

The Nkporo Formation is overlain by Mamu Formation formally Lower Coal Measures which consist of marine intercalations composed of ammonitifetous shales (Reyment, 1964), fresh water and low salinity sandstones, mudstones, and sandy shales, coal occur 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, 1954). 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 meters thick of coarse grained sandstone. The Nsukka Formation previously known as “Upper Coal Measures” lies unconformably on the Ajali Formation. The rocks consist of an alternating succession of sandstones, dark shale and sandy shale with thin coal seems at various horizons (Reyment 1965) as series of highly fossiliferous grey – green sandy clays with calcareous concretions. It is Eocene in age. A good exposure outcrop at the valley of Nadu River and on the road-cuts along the Enugu – Onitsha expressway.

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To Place An Order For The Complete Project Material Pay N,5000 To

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Acct. No. - 0127561472

Then Send a text of your names, the topic you paid for, a valid email address to 07036785443

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