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Chad Basin
The Chad Basin is the largest endorheic basin in Africa, centered approximately on Lake Chad. It has no outlet to the sea and contains large areas of semi-arid desert and savanna. The drainage basin is approximately coterminous with the sedimentary basin of the same name, but extends further to the northeast and east.
The basin spans four modern nations, including most of Chad and a large part of Niger, Nigeria and Cameroon. A combination of dams, increased irrigation, climate change, and reduced rainfall are causing water shortages. Lake Chad continues to shrink.
The geological basin, which is smaller than the drainage basin, is a Phanerozoic sedimentary basin formed during the plate divergence that opened the South Atlantic Ocean. The basin lies between the West African Craton and Congo Craton, and formed about the same time as the Benue Trough. It covers an area of about 2,335,000 square kilometres (902,000 sq mi). It merges into the Iullemmeden Basin to the west at the Damergou gap between the Aïr and Zinder massifs. The floor of the basin is made of Precambrian bedrock covered by more than 3,600 metres (11,800 ft) of sedimentary deposits.
The basin may have resulted from the intersection of an "Aïr-Chad Trough" running NW-SE and a "Tibesti-Cameroon Trough" running NE-SW. That is, the two deepest parts are an extension of the Benue Trough that runs northeast to the margin of the basin, and another extension running from below the present lake to below the Ténéré rift structure to the east of the Aïr massif. The southern part of the basin is underlain by another elongated depression. This runs in an ENE direction and extends from the Yola arm of the Benue trough.
At times, parts of the basin were below the sea. In the northeastern part of the Benue Trough where it enters the Chad Basin there are marine sediments from the Late Cretaceous (100.5–66 Ma). These sediments seem to be considerably thicker towards the northeast. Boreholes under Maiduguri have found marine sediments 400 metres (1,300 ft) deep, lying over continental sediments 600 metres (2,000 ft) deep. The sea seems to have retreated from the western part of the basin in the Turonian (93.5–89.3 Ma). In the Maastrichtian (72.1–66 Ma) the west was non-marine, but the southeast probably was still marine. No marine sediments have been found from the Paleocene (66–56 Ma).
For most of the Quaternary, from 2.6 million years ago to the present, the basin seems to have been a huge, well-watered plain, with many rivers and water bodies, probably rich in plant and animal life. Towards the end of this period the climate became drier. Around 20,000-40,000 years ago, eolianite sand dunes began to form in the north of the basin. During the Holocene, from 11,000 years ago until recently, a giant "Lake Mega-Chad" covered an area of more than 350,000 square kilometres (140,000 sq mi) in the basin. It would have drained to the Atlantic Ocean via the Benue River. Stratigraphic records show that "Mega-Chad" varied in size as the climate changed, with a maximum about 2,300 years ago. The remains of fish and molluscs from this period are found in what are now desert regions.
The Chad Basin covers almost 8% of the African continent, with an area of about 2,434,000 square kilometres (940,000 sq mi). It is surrounded by mountains. The Aïr Mountains and the Termit Massif in Niger form the western boundary. To the northwest, in Algeria, are the Tassili n'Ajjer mountains, including the 2,158 metres (7,080 ft) Jebel Azao. The Tibesti Mountains to the north of the basin include Emi Koussi, the highest mountain in the Sahara at 3,415 metres (11,204 ft). The Ennedi Plateau lies to the northeast, rising to 1,450 metres (4,760 ft). The Ouaddaï highlands lies the east. They include the Marrah Mountains in Darfur at up to 3,088 metres (10,131 ft) in height. The Adamawa Plateau, Jos Plateau, Biu Plateau, and Mandara Mountains lie to the south.
To the west the basin is separated by a watershed from the Niger River, and to the south it is separated by a basement dome from the Benue River. Further east, watersheds separate it from the Congo Basin and the river Nile.
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Chad Basin
The Chad Basin is the largest endorheic basin in Africa, centered approximately on Lake Chad. It has no outlet to the sea and contains large areas of semi-arid desert and savanna. The drainage basin is approximately coterminous with the sedimentary basin of the same name, but extends further to the northeast and east.
The basin spans four modern nations, including most of Chad and a large part of Niger, Nigeria and Cameroon. A combination of dams, increased irrigation, climate change, and reduced rainfall are causing water shortages. Lake Chad continues to shrink.
The geological basin, which is smaller than the drainage basin, is a Phanerozoic sedimentary basin formed during the plate divergence that opened the South Atlantic Ocean. The basin lies between the West African Craton and Congo Craton, and formed about the same time as the Benue Trough. It covers an area of about 2,335,000 square kilometres (902,000 sq mi). It merges into the Iullemmeden Basin to the west at the Damergou gap between the Aïr and Zinder massifs. The floor of the basin is made of Precambrian bedrock covered by more than 3,600 metres (11,800 ft) of sedimentary deposits.
The basin may have resulted from the intersection of an "Aïr-Chad Trough" running NW-SE and a "Tibesti-Cameroon Trough" running NE-SW. That is, the two deepest parts are an extension of the Benue Trough that runs northeast to the margin of the basin, and another extension running from below the present lake to below the Ténéré rift structure to the east of the Aïr massif. The southern part of the basin is underlain by another elongated depression. This runs in an ENE direction and extends from the Yola arm of the Benue trough.
At times, parts of the basin were below the sea. In the northeastern part of the Benue Trough where it enters the Chad Basin there are marine sediments from the Late Cretaceous (100.5–66 Ma). These sediments seem to be considerably thicker towards the northeast. Boreholes under Maiduguri have found marine sediments 400 metres (1,300 ft) deep, lying over continental sediments 600 metres (2,000 ft) deep. The sea seems to have retreated from the western part of the basin in the Turonian (93.5–89.3 Ma). In the Maastrichtian (72.1–66 Ma) the west was non-marine, but the southeast probably was still marine. No marine sediments have been found from the Paleocene (66–56 Ma).
For most of the Quaternary, from 2.6 million years ago to the present, the basin seems to have been a huge, well-watered plain, with many rivers and water bodies, probably rich in plant and animal life. Towards the end of this period the climate became drier. Around 20,000-40,000 years ago, eolianite sand dunes began to form in the north of the basin. During the Holocene, from 11,000 years ago until recently, a giant "Lake Mega-Chad" covered an area of more than 350,000 square kilometres (140,000 sq mi) in the basin. It would have drained to the Atlantic Ocean via the Benue River. Stratigraphic records show that "Mega-Chad" varied in size as the climate changed, with a maximum about 2,300 years ago. The remains of fish and molluscs from this period are found in what are now desert regions.
The Chad Basin covers almost 8% of the African continent, with an area of about 2,434,000 square kilometres (940,000 sq mi). It is surrounded by mountains. The Aïr Mountains and the Termit Massif in Niger form the western boundary. To the northwest, in Algeria, are the Tassili n'Ajjer mountains, including the 2,158 metres (7,080 ft) Jebel Azao. The Tibesti Mountains to the north of the basin include Emi Koussi, the highest mountain in the Sahara at 3,415 metres (11,204 ft). The Ennedi Plateau lies to the northeast, rising to 1,450 metres (4,760 ft). The Ouaddaï highlands lies the east. They include the Marrah Mountains in Darfur at up to 3,088 metres (10,131 ft) in height. The Adamawa Plateau, Jos Plateau, Biu Plateau, and Mandara Mountains lie to the south.
To the west the basin is separated by a watershed from the Niger River, and to the south it is separated by a basement dome from the Benue River. Further east, watersheds separate it from the Congo Basin and the river Nile.