干旱区生态环境知识资源中心

From morphological and sedimentological observations, in the Niger valley, in the Azawagh basin and in the Air mountains, it is possible to distinguish a succession of different paleoclimatic phases, from Tertiary wet tropical to arid Quaternary climates. They correspond with those already known in other parts of West Africa, from Senegal to Chad. According to the K-Ar datations of basaltic lavas from the Air which cover the paleoformes, we have discovered precise landmarks which pinpoint these different phases: the surface in which the valleys are cut is Pliocene (before 3,8 My B.P.). Erosion was very quick at first, then slower, but always in response to seasonal climates. Wetter and more arid phases have alternated, causing either erosion or pedogenesis. The first arid phase appeared before the end of the Pliocene, but hydrolysing climates still occured at the beginning of lower Pleistocene. The basaltic lavas flows dated at 2 My show slight weathering; those, however, dated at 1 My have remained unchanged. The phenomena of catastrophic erosion, caused by torrential flows, persisted until around 1 MY B.P. Thus, on the whole, the climate has become progressively drier. But this progression is continually marked by irregularities of both wet and dry periods.