Recherche de pièges dans le “biseau sec” du flanc occidental du bassin de Taoudeni, désert de Sarakollé, confins sud-est de la Mauritanie

Abstract

Resistivities above 60,000 Ω·m have bee measured alongside the desert border, under an over-burden of approximately 20 m. Such unusually high resistivities had been previously attributed to the presence of dolerite domes. Considering that there are only two permanent wells in the area surveyed (80 × 100 km²), that dolerites had been found in one well and that their presence is suspected in the other, it was hoped to find perched aquifers in the depressed areas at the top of the domes.

In fact, wide-mesh (10–20 km) electrical soundings have shown that these high resistivities are due to dry sedimentary formations consisting of coarse sand and sandstone. The impermeable bedrock consists of argillites of 200 Ω·m. The dip of such formations (of the order 2 per thousand) is sufficient to produce a “dry wedge” formation in the topographically elevated parts of a 80 km-wide belt along the desert border.

The two wells are located on low-resistivity anomalies within that wedge. Other narrow anomalies were found, some of them on lineaments that are visible on satellite imagery. This suggests that water is trapped by dykes and not in the top of the domes. A borehole drilled to test one of the conductive anomalies encountered weathered dolerites clay, and water was found in fractures of the bedrock.

A resistivity map (AB = 200 m) shows the overall characteristics of the sedimentary basin surveyed, including the conductive anomalies within the dry wedge that may bear water.