The chad basin: An active intra-continental basin

Abstract

The Chad basin, lying well within the African continent, has accumulated about 0.5 km of sediment over an area roughly 500 km in diameter during the Neogene and Quaternary. When the climate is moist, Lake Megachad fills the basin and spills over into the Benue Valley at Bongor. Thick Neogene and Quaternary sediments (>200 m in thickness) occur only within the shorelines of Megachad indicating that the thickening may be attributable to repeated episodes of sediment and water loading. The Chad basin has apparently originated in response to peripheral uplifts that began to develop about 25 m.y. ago. An annular pediment, averaging about 500 km wide, separates the basin from eleven discrete uplifts (up to 3 km ASL) forming a crude ellipse 1000 km × 1500 km across around the lake. If sea level rose enough (doubled spreading rates for 70 m.y. would suffice), salt water would flood the Chad basin and a structure rather like the Palaeozoic Michigan basin filled with limestone, sandstone and salts could develop. When in the future the uplifts peripheral to the Chad basin cease to be dynamically supported, they will be eroded away in a few million years and both the basin and the sites of the uplift may be buried under later sediment. It will not be possible to locate the former uplifts accurately because the only evidence of their sites will be an absence of the Chad basin deposits and sporadic dikes and volcanic plugs. It seems likely that old intracontinental basins, for example the Michigan basin, may have developed, like the Chad basin, in response to peripheral uplift rather than by thermal recovery from a preceding doming event.