Posted by on 1/29/2003 9:10:39 AM
Post Comment PHYSICAL SETTING Nigeria

Geology And Mineral Resources:

Anambra State lies in the Anambra Basin, the first region where intensive oil exploration was carried out in Nigeria. The

Premier Breweries Factory,Onitsha
Premier Breweries Factory,Onitsha

Anambra basin has about 6,000 m of sedimentary rocks.The sedimentary rocks comprise ancient Cretaceous deltas, somewhat similar to the Niger Delta, with the Nkporo Shale, the Mamu Formation, the Ajali sandstone and the Nsukka Formation as the main deposits. On the surface the dominant sedimentary rocks are the Imo Shale a sequence of grey shales, occasional clay iron stones and Sandstone beds.

The Imo Shale underlies the eastern part of the state, particularly in Ayamelum, Awka North, and Oruma North LGAs. Next in the geological sequence, is the Ameke Formation, which includes Nanka Sands, laid down in the Eocene. Its rock types are sandstone, cal careous shale, and shellylimestone in thin bands. Outcrops of the sandstone occur at various places on the higher cuesta, such as at Abagana and Nsugbe, where they are quarried for construction purposes. Nanka sands out crop mainly at Nanka and Oko in Orumba North LGA.

Lignite was deposited in the Oligocene to Miocene; and it alternates with gritty clays in places. Outcrops of lignite occur in Onitsha and Nnewi. The latest of the tour geological formations is the Benin Formation or the coastal plain sands deposited from Miocene to pleistocene. The Benin Formation consists of yellow and white sands. The formation underlies much of lhiala LGA. Thick deposits of alluvium were laid down in the western parts of the state, south and north of Onitsha in the Niger and Anambra river floodplains.

Landforms And Drainage:

Anambra State falls into two main landform regions: a highland region of moderate elevation that covers much of the state south of the Anambra River, and low plains to the west, north, and east of the highlands. The highland region is a low asymmetrical ridge or cuesta in the northern portion of the AwkaOrlu Uplands, which trend roughly southeast to north west, in line with the geological formations that underlie it.

It is highest in the southeast, about 410m above mean sealevel, and gradually decreases in height to only 33m in the northwest on the banks of the Anambra River and the Niger. At Onitsha and Otuocha, the cuesta provides well drained low land, very close to the river, thereby enabling settlements to extend to the banks of the river.

The cuesta has confined the wide and braided channel of the Niger to a comparatively narro' valley bed at the southern part of Onitsha, making an appropriate location for the construction of bridge across the river. The highlands consist two cuestas, a lower and a higher one, each with a eastfacing escarpment. The two cuestas merg south of Nanka.

The lower cuesta, formed by the more resistar sandstone rocks of the Imo Shale, rises to onl 150m above mean sealevel at Umuawulu an decreases in height northwestward to only 100m < Achalla. Its escarpment faces the Mamu Rive plain and has a local relief of between 80 and 30n West of it, is the higher cuesta, formed by the sane stones of the Ameke Formation. Its height is abov 400m in the south-east at lgbolkwu and lsuofii decreasing northwestward to less than 300m ; Agbana, and to only 100m at Aguleri.

There are only of moderate height, they provide elevated, welldrained and attractive settlernei sites, hence, they are closely settled even up I their crests. Agulu, Agbana, Awkuzu, Nteje an Aguleri are some of the settlements on the crest the higher cuesta, and lfiteAwka, Mgbakwi Amanuke and Achalla are some of those on th crest of the lower cuesta. The dip slope of the h er cuesta extends westwards for over 30km and is heavily settled.

The plains lie west and north of the highland; it The River Niger plain, south of Onitsha, about 9km wide, and the NigerAnambra River plain north of Onitsha, which stretches for over 36km east of th an Niger, are really low plains, well below 30m abov ge mean sealevel, and are liable to flood. They ar underlain by recent alluvium; and, east of the Anambra River, by the Imo Shale formation.

The plains are almost featureless, except for sporad broad undulations, rising above the flood plaini at forming sites for the farming and fishing settlement 'er in the area. Such settlements include Nzan m. Nmiata, and Anam in Anambra West LGA, an id Atani, Odekpe, and Oshita in Ogbaru LGA. East ve the Anambra River, a narrow and elongated san ia, stone ridge, projecting about 30m above the level at the plain, formed settlement sites for Anaki lgbakwu, lfute, and Umueje inAyamelum LGA.

The Mamu River plain, east of the cuesta landscape, int a little higher than the other two plains. It lie to between 30 and 70m above sealevel in the are nd underlain by the Imo Shale, rising higher souti of wards. East of the Mamu River are found the mor iu, resistant sandstone ridge, at some 50 m above th he level of the plains. The extension of this ridg )h southward is settled by the people of Ufuma, Aj'al is lsuUlo, Ezira, and Urnunze.

the main drainage system in the state. The Anambra River rises on the Gala Plateau near Ankpa in Kogi State and, for its over 85km course in Anambra State, flows through the northern low plain where it, as well as its right bank tributaries, meander heavily, developing oxbow lakes and abandoned meander channels. Its largest left bank tributary is the Mamu River, which drains the east ern low plain on the Imo Shale Formation.

The higher cuesta forms the watershed separating the numerous eastflowing tributaries of the Mamu River from the westflowing rivers, the Idemili, the Nkisi, and the Oyis, which drain the dip slope of the cuesta. All but one of the main rivers in Anambra state empty into the River Niger, which forms the western boundary of the state and constitutes the local baselevel for the rivers.

The exception is the Ulasi River, which rises near Dikenafai in Imo State, flows northward to Ozubulu in Anambra State and then turns round in a wide loop and heads for the Atlantic ocean. The dip slope of the higher cuesta between Nsugbe, Onitsha, Ogbunike and Urnunya is dissected by the numerous tributary streams of the MamuAnambra into a rolling landscape.

Vegetation And Soils:

Although annual rainfall is high in Anambra State, ranging from 1,400mm in the north to 2,500mm in the south, it is concentrat ed in one season , with about four months of dry ness, November to February. Consequently, the natural vegetation in the greater part of Anambra State is tropical dry or deciduous forest, which, in its original form, comprised tall trees with thick under growth and numerous climbers.

The typical trees (silk cotton, Iroko and oil bean) are deciduous, shedding their leaves in the dry season. Only in the southern parts of the state, where the annual rain fall is higher and the dry season shorter, is the nat ural vegetation marginally the tropical rainforest type. Because of the high population density in the state, most of the forests have been cleared for set tlement and cultivation.

What exists now is secondary regrowth , or a forestsavannah mosaic, where the oil palm is predominant, together with selectively preserved economic trees. Relics of the original vegetation may, however, be found in some "juju" shrines or some inaccessible areas.

Three soil types can be recognised in Anambra State. They are: (i) alluvial soils, (ii) hydromorphic soils, and (iii) ferallitic soils. The alluvial soils are palebrown loamy soils. They are found in the tow plain south of Onitsha in Ogbaru and in the Niger Anambra low plain north of Onitsha. They differ from the hydromorphic soils in being relatively immature, having no welldeveloped horizons.

They, however, sustain continuous cropping longer than the other two types. Hydromorphic soils are developed on the Mamu plain east of the cuesta, extending northward into the eastern part of Anambra River floodplain, where the underlying impervious clayey shales cause waterlogging of the soils during the rainy season. The soils are fine loamy, with lower layers

faintly mottled; while the subsoil layers are strongly mottled and spotted, containing stiff grey clay. The soils are good for yam, cassava and maize, and for rice in the more heavily waterlogged areas. The cuestas and other elevated areas under lain by sandstones and shales of the Ameke Formation and the Nanka Sands are regions of fer rallictic soils. The soils are deep, red to reddish brown loamy sands, often referred to as "redearth" or acid sands because of low fertility. They are eas ily eroded into gullies.

Ecological Hazards:

The main ecological hazards in the state are accelerated gully erosion and flooding. Extensive forest clearing, often by bush burning, and continuous cropping with little or no replenishment of soil nutrients, resulted in the dis ruption of the ecological equilibrium of the natural forest ecosystem. Such a situation in a region of loosely consolidated friable soils is prone to ero sion, giving rise to extensive gully formation.

In the Agulu, Nanka and Oko areas, which are underlain by the Nanka Sands, the gullies have attained spectacular and alarming proportions, turning the area into real "bad lands." Many of the gullies are at the head streams of the rivers that flow down the cuestas. The head streams carve their valleys deep into the deeply weathered red earth, develop ing dendritic patterns of gullies.

Such gullies are also found in Nnobi, Alor and Ideani, along the course of the Idemili River. Besides, the greater part of the state is prone to severe sheet erosion. In the low plains of the Niger and Mamu Rivers, heavy rains often result in excessive flooding, such that the undulations occupied by settlements are marooned for some months. The people resort to the use of canoes for movement and transportation. Orba Ofemili and Ugbenu on the plains of the Mamu River are sometimes, in the rainy season, cut off from others as their roads remain flooded kneedeep for many weeks. The floods also cause serious damage to crops.

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