Variations hydrologiques et extension des lacs holocenes du Desert Danakil

Abstract

During the Holocene period, lake basins existed in some parts of the Danakil Desert. These basins were supplied either by the Awash River or by ground waters flowing through highly fissured basaltic flows. It is possible to reconstruct the areas and fluctuations of lake levels because of the preservation of old shore lines. Calcareous and diatomaceous bottom deposits give information about lacustrine paleo-environments (isotopic analyses, diatoms). It is suggested that the Holocene period started with an extensive lacustrine episode between 11,000 and 7,000 years B.P. At least one other transgressive stage, of lesser importance, took place between about 2,000 and 1,000 years B.P. The lakes mainly supplied with ground waters (Afrera) exhibit evidence of chemical and biochemical sedimentation. The depostion continued after 7,000 years B.P., and indicates a buffering effect owing to the aquifers. Hence, these basins do not have the simple behaviour of a rain gauge recording the local climate fluctuations of the Danakil Desert. The Dobi Graben can be partly related to this kind of basin. Because of the strong turbidity of the flood waters, the lakes with both shallow and underground supplies are quickly filled up in the head of the alluvial plain of the Awash River. The lakes at the downward part of the plain (Gamari and terminal Lake Abhé) have been preserved from the solid supply during the two humid episodes. During the intermediate stage, the lacustrine regression allowed the transport of silts and muds as far as Lake Abhé. The Awash River has not ceased flowing and the progression of the alluvial deposits isolates some area of retention on the margins of the plain (lacustrine relicts at about 4,000 and 5,000 years B.P.). After subtracting the local hydrological influences, by means of comparison with the recent phenomena, this succession fits into the general pattern of the Holocene climatic events of the southern margin of the Sahara.