Fluvial sedimentation in a semi-arid region: the fan and interfan system of the Middle Souss Valley, Morocco

The Souss Valley is a structural basin located between the High Atlas Range and the Anti-Atlas Range in southern Morocco. Quaternary deposits in this valley are stacked as torrential piedmont fans, in which coarse deposits are dominant, and as flat gently sloping interfans, in which silty deposits are dominant. The visible part of the deposits, referred to as the Taroudannt Group, is composed of a lower silty unit (S0), three sequences (SI, S2 and S3), and a discontinuous cover of Rharbian (Late Weichselian—Holocene) deposits. Each sequence begins with an erosional disconformity and contains three superimposed members: a torrential cobble unit, lenses of highly calcareous sand, and a silt unit related to piedmont silt fans composed of aeolian deposits reworked by diffuse rain-wash and sheet-wash. A climatic origin is ascribed to this repeated superposition of facies. Calcareous epigenesis has modified all these deposits. Percentages of kaolinite, palygorskite and hornblende decrease from the base to the top of the sections, neogenic palygorskite and silica being observed by scanning electron microscope on quartz grains from the two lower silt units. The upper sequence (Maader Formation) is correlated with the upper sequence of the Atlantic coast of Morocco and represents the interglacial—glacial Ouljian—Soltanian (Eemian—Weichselian) climatic cycle. The older units may range from the Early to the Middle Pleistocene.