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The Chad Basin is the largest endorheic drainage basin in Africa, centered on Lake Chad. It has no outlet to the sea and contains large areas of desert or semi-arid savanna. The drainage basin is roughly coterminous with the sedimentary basin of the same name, but extends further to the northeast and east. The basin spans seven countries, including most of Chad and a large part of Niger. It has an ethnically diverse population of about 30 million people as of 2011, growing rapidly. A combination of dams, increased irrigation, and reduced rainfall are causing shortages of water. Lake Chad continues to shrink. ==Geology== 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 around the same time as the Benue Trough. It covers an area of about . 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 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 deep, lying over continental sediments 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 waterbodies, 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 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 peak about 2,300 years ago. The remains of fish and molluscs from this period are found in what are now desert regions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chad Basin」の詳細全文を読む スポンサード リンク
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