GEOMORPHOLOGICAL QUATERNARY EVOLUTION OF THE ANTI-ATLAS EDGE OF THE CENTRAL PART OF THE SOUSS PLAIN (MOROCCO)

The middle part of the Souss plain extends over 40 km between the High Atlas in the north and Anti-Atlas in the south. Its morphology between Taroudannt and the Admine Forest shows a dominance of inherited forms, both in proximal regions of piedmont and in their distal parts. Relics of at least three main generations of alluvial fans remain in proximal regions, which are vaster and more flattened on the anti-atlasic piedmont than on the atlasic one; the first two ones being preserved owing to the incrusted limestones and cementations. The third generation of alluvial fans - adventitious cones - essentially composed of sandy loam in surface, occupies both the median and distal parts. The major morphogenic system is inherited from the Plio-Pleistocene (or Plio-Villafranchian) transition. From that period, the plain has kept incrusted cone apical parts as well as a carbonated bedrock in its central part, between El Freija and Ouled Teima, witnesses of the existence of palaeolakes. This system, already consolidated, is also tectonised. In distal parts, the current Souss Valley hardly cuts thick carbonated accumulations representing remains of at least two generations of contemporaneous palaeoplayas of the former alluvial fans. Since the Pleistocene-Holocene transition, adventitious cones and the major bed of the Souss wadi have given birth to a particularly active fluvio-eolian surface relief.