Benue Trough
Encyclopedia
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.
, 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.
era.The Cretaceous lasted from 145.5 – 65.5 Ma, or million years ago.
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 Abakiliki 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 Basin
s.
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 anticline
s and syncline
s. The deposits in the Benue Trough were displaced westwards at this time, causing subsidence of the Anambra basin.
, where abnormal heat leads to melting of the upper mantle, thinning and stretching the crust
, followed by rifting of the weakened crust. This may have been repeated several times, with the Benue Trough deformed between rifting episodes.
The same plume may be responsible for the line of volcanoes in Cameroon
along the Central African Shear Zone
and for the volcanic island of St. Helena in the Atlantic ocean
.
Three periods of magmatic activity (volcanic action) have been identified, 147–106 Ma, 97–81 Ma and 68–49 Ma.
The first is prominent in the north of the trough, and contemporary with magmatism in Brazil, probably occurring during a period of crustal extension
before the Atlantic started to open. The second is found only in the south of the trough, and may belong to a period when the extension of the Atlantic had slowed down, ending with a period of compression. The third and last period is also found only in the south of the trough, and may be related to an isostatic
response to the earlier crustal thinning.
The mantle plume activity was probably limited in its effect, with most of the basins in the trough being created from a combination of extension and strike-slip faults. The faults extend into the ocean with the Chain and Charcot fault zones, and have their counterparts in northeastern Brazil
.
.
The Enugu deposits formed in brackish marshes during the Late Campanian
- early Maastrichtian
ages, around 70 Ma.
During this time, the Anambra basin became silted up with thick vegetation growing in low-lying marshes on a broad delta fan deposited by rivers from the interior. Later, the deep layer of vegetation became buried under coarse sands.
Coal is potentially a source for oil and natural gas.
An exploratory well drilled in the Gongola Basin of the upper region in 2003 found no oil, although there was a narrow layer of coal between 4,710 ft and 4,770 ft.
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...
and extending about 1,000 km northeast from the Bight of Benin
Bight of Benin
The Bight of Benin is a bight on the western African coast that extends eastward for about 400 miles from Cape St. Paul to the Nun outlet of the Niger River. To the east it is continued by the Bight of Bonny . The bight is part of the Gulf of Guinea...
to Lake Chad
Lake Chad
Lake Chad is a historically large, shallow, endorheic lake in Africa, whose size has varied over the centuries. According to the Global Resource Information Database of the United Nations Environment Programme, it shrank as much as 95% from about 1963 to 1998; yet it also states that "the 2007 ...
.
It is part of the broader Central African Rift System.
Location
The trough has its southern limit at the northern boundary of the Niger DeltaNiger Delta
The Niger Delta, the delta of the Niger River in Nigeria, is a densely populated region sometimes called the Oil Rivers because it was once a major producer of palm oil...
, where it dips down and is overlaid with Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
and more recent sediments.
It extends in a northeasterly direction to the Chad Basin
Chad Basin
The Chad Basin, also known as the Lake Chad Basin, is a large lowland area in north-central Africa. In all directions from the center of this area the elevation changes are gentle. The Chad Basin is an endorheic basin - its water does not flow into any ocean...
, 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.
Rifting and sedimentation
The Benue Trough was formed by rifting of the central West African basement, beginning at the start of the CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
era.The Cretaceous lasted from 145.5 – 65.5 Ma, or million years ago.
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 Abakiliki 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
Tethys Ocean
The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...
(predecessor of the Mediterranean sea) via the Chad
Chad Basin
The Chad Basin, also known as the Lake Chad Basin, is a large lowland area in north-central Africa. In all directions from the center of this area the elevation changes are gentle. The Chad Basin is an endorheic basin - its water does not flow into any ocean...
and Iullemmeden Basin
Iullemmeden Basin
The Iullemmeden Basin is a major sub-Saharan inland basin in West Africa, extending about 1000 km north to south and 800 km east to west...
s.
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
Aulacogen
In geology, an aulacogen is a failed arm of a triple junction of a plate tectonics rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops...
, an abandoned arm of a three-armed radial rift system. The other two arms continued to spread during the break-up of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
, 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
Santonian
The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...
age, around 84 million years ago, the basin underwent intense compression and folding, forming over 100 anticline
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
s and syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
s. The deposits in the Benue Trough were displaced westwards at this time, causing subsidence of the Anambra basin.
Mantle plume theory
A refinement to the model involves the rise of a mantle plumeMantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
, where abnormal heat leads to melting of the upper mantle, thinning and stretching the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
, followed by rifting of the weakened crust. This may have been repeated several times, with the Benue Trough deformed between rifting episodes.
The same plume may be responsible for the line of volcanoes in Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...
along the Central African Shear Zone
Central African Shear Zone
The Central African Shear Zone is a wrench fault system extending in an ENE direction from the Gulf of Guinea through Cameroon into Sudan.The structure is not well understood....
and for the volcanic island of St. Helena in the Atlantic ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
.
Three periods of magmatic activity (volcanic action) have been identified, 147–106 Ma, 97–81 Ma and 68–49 Ma.
The first is prominent in the north of the trough, and contemporary with magmatism in Brazil, probably occurring during a period of crustal extension
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
before the Atlantic started to open. The second is found only in the south of the trough, and may belong to a period when the extension of the Atlantic had slowed down, ending with a period of compression. The third and last period is also found only in the south of the trough, and may be related to an isostatic
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
response to the earlier crustal thinning.
The mantle plume activity was probably limited in its effect, with most of the basins in the trough being created from a combination of extension and strike-slip faults. The faults extend into the ocean with the Chain and Charcot fault zones, and have their counterparts in northeastern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
.
Economic importance
Nigeria is rich in coal deposits derived from terrestrial organic matter, most of which lie in the Benue Trough. These are mined in Enugu StateEnugu State
Enugu State is a mainland state in southeastern Nigeria. Its capital is Enugu, from which the state - created in 1991 from the old Anambra State - derives its name. The principal cities in the state are Enugu, Agbani, Awgu, Udi, Oji, and Nsukka....
.
The Enugu deposits formed in brackish marshes during the Late Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
- early Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
ages, around 70 Ma.
During this time, the Anambra basin became silted up with thick vegetation growing in low-lying marshes on a broad delta fan deposited by rivers from the interior. Later, the deep layer of vegetation became buried under coarse sands.
Coal is potentially a source for oil and natural gas.
An exploratory well drilled in the Gongola Basin of the upper region in 2003 found no oil, although there was a narrow layer of coal between 4,710 ft and 4,770 ft.