Holocene Lake Mega-Chad palaeoshorelines from space

During the Holocene a giant lake, known as Lake Mega-Chad (LMC), extended over more than 350,000 km2 in southern Sahara. Morphodynamic features of sedimentary systems outlining the LMC palaeoshorelines have been identified by the joint analyses of new topographic images (Digital Elevation Model) acquired by radar interferometry and Landsat Thematic Mapper images. Here, we characterize for the first time at the scale of the Chad Basin a wave-dominated sedimentary system including river deltas, longshore sandridges, beach ridges, spits and a wave-ravinement surface. They provide new evidence of the environmental impact of Quaternary climate changes in the sahelo-saharan area. Continental trade winds controlled the longshore drift in the northern part of the palaeolake. Two distinct LMC episodes dated as lower and middle Holocene are clearly identified, contemporaneous with the two phases of wetter conditions usually recognized in central and northern Africa.