SANDSTONE KARST AND PALAEO-SANDSTONE WAETHERING - ITS PALAEOENVIROMENTAL IMPLICATION AND HOLOCENE IMPACT ON GROUNDWATER FLOW

Many of the Central Saharan sandstone massifs are characterized by karst features, such as caves and dolines, that reach up to 3 vol.%. Some of them are widened by superimposed mechanical erosion, but thin section analyses give proof of mineral – and even quartz – dissolution sensu strictu. With regard to the present morphological position of the karst phenomena and the degree of transformation, the main karst processes are assumed to have taken place before the Quarternary dissection of the older peneplains. So they are contributed to important ground water flow and to warm and humid climate conditions that favorize intense chemical weathering. On the other hand, the very common silcretes – mainly positioned on the top surface of cuestas and hilly regions – are interpreted as correlate silica deposits. This interpretation is supported by comparison to Central Australian silcretes (Lake Eyre Basin), where they figure as reprecipitation of silica that was dissolved during weathering and dissolution of various silicate minerals in the surrounding highlands. The well cristallized Central Saharan silcretes are mainly precipitations out of groundwaters very poor in dissolved load (that implicates a long time of undisturbed solution and reprecipitation), whereas in the Southern Sahara the silica precipitation is more disturbed by the presence of iron-bearing minerals in the dissolution environment. The quarternary dissection and drainage of the karst systems lead to their fossilization in the higher morphological positions. In the lower parts, however, they are parts of the aquifers until today. Water filled karst galleries influence the present groundwater flow. Under the present arid climatic conditions without recharge of the aquifers, karst related high permeability must be considered for any groundwater and sustainability modelling in the Central Sahara.