Constraints on alluvial clay mineral assemblages in semiarid regions. The Souss Wadi Basin (Morocco, Northwestern Africa)

Illite, chlorite and kaolinite, resulting directly from substratum weathering, dominate the clay assemblages recorded in altered rocks and soils of the Souss Wadi watershed. Nevertheless, the transformation and neoformation of diverse clay minerals, which is mainly controlled by changing climatic, lithological and topographic conditions, enables one to establish three distinctive source areas. High altitude, largely watered zones where chlorite is transformed into vermiculite; middle altitude drier areas where more pronounced seasonal contrast induces the transformation of chlorite into chlorite-smectite mixed layer; finally, semiarid low altitude zones with gentle slopes where the weathering of carbonate-dominated substrata, evaporation and chemical concentration result in smectite and palygorskite neoformation. These minerals, which result from transformation and neoformation, provide suitable environmental indicators and local to zonal tracers of sediment contribution. Nevertheless, the original clay mineral assemblages undergo a significant homogenisation due to the destruction of most of the transformed and neoformed minerals during fluvial transport. As a consequence, inherited clay minerals (illite, chlorite and kaolinite) usually dominate the assemblages recorded in the downstream alluvial sediments. The clay assemblages recorded in different tributary wadi alluvial deposits reflect major sediment contribution from the High-Atlas and the older alluvial plain deposits affected by intensive erosion. On the other side, Anti-Atlas contribution is lesser as a consequence of its wide tableland topography and the widespread outcropping of karstified carbonate rocks, which enhance water infiltration and preclude sediment supply