Benue Trough

The Benue Trough is a major geological formation underlying a large part of Nigeria and extending about 1,000 km northeast from the Bight of Benin to Lake Chad. It is part of the broader Central African Rift System.

The trough has its southern limit at the northern boundary of the Niger Delta, where it dips down and is overlaid with Tertiary and more recent sediments. It extends in a northeasterly direction to the Chad Basin, and is about 150 km wide. The trough is arbitrarily divided into lower, middle and upper regions, and the upper region is further divided into the Gongola and Yola arms. The Anambra Basin in the west of the lower region is more recent than the rest of the trough, being formed during a later period of compression, but is considered part of the formation.

The Benue Trough was formed by rifting of the central West African basement, beginning at the start of the Cretaceous period. At first, the trough accumulated sediments deposited by rivers and lakes. During the Late Early to Middle Cretaceous, the basin subsided rapidly and was covered by the sea. Sea floor sediment accumulated, especially in the southern Abakaliki Rift, under oxygen-deficient bottom conditions. In the Upper Cretaceous, the Benue Trough probably formed the main link between the Gulf of Guinea and the Tethys Ocean (predecessor of the Mediterranean Sea) via the Chad and Iullemmeden Basins. Towards the end of this period the basin rose above sea level, and extensive coal forming swamps developed, particularly in the Anambra Basin. The trough is estimated to contain 5,000 m of Cretaceous sediments and volcanic rocks.

A common explanation of the trough's formation is that it is an aulacogen, an abandoned arm of a three-armed radial rift system. The other two arms continued to spread during the break-up of Gondwana, as South America separated from Africa. The two continents seem to have started to split apart at what are now their southern tips, with the rift extending up the modern coastlines to the Benue Trough, then later split along what is now the southern coast of West Africa and the north eastern coast of South America. As the continents were wedged apart, the trough opened up. When separation was complete, the southern part of Africa swung back to some extent, with the sediments in the Benue Trough compressed and folded. During the Santonian age, around 84 million years ago, the basin underwent intense compression and folding, forming over 100 anticlines and synclines. The deposits in the Benue Trough were displaced westwards at this time, causing subsidence of the Anambra Basin.

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