Abstract
At the southem side of the Anti-Atlas mountains, the lower section of the Assaka wadi valley joins the Atlantic ocean by crossing an Appalachian basin and range area. Holocene deposits of the lower section of the Assaka wadi show two main sedimentary formations: the upper sheet-fans and the lower terraces. The upper sheet-fans are probably from the lower Rharbian (Holocene) age, they are located above the Soltanian (Wünnian) formations at the outlets of tributaries. They consist of accumulations with silts, aeolian quartz and high palygorskite contents. After this period, a trench cut through the older formations took place between 8 000 years BP and 4 200 years BP. The lower river terraces, 5 to Il metres thick, are inset in the trench cut through the Soltanian formations and show four superimposed and fining upwards (from pebble to silt) cross-bedded units. This study shows many geodynamic and palaeoclimatic conditions. Lower Rharbian fans indicate that the climate was sufficiently arid to denude the slopes, where, during big storms, gullies and run-off supplied the Assaka wadi with silts and sands. Torrential floods eut the Soltanian formations and then created the Middle Rharbian low terrace. Finally, the Historie terrace, superimposed onto the Middle Rharbian one, shows that important flood events took place during the XVIIO and XVIIIo centuries, but nevertbeless with low frequency. However. this sequence indicates wetter conditions during an arid episode. The 14C age is contemporary with the "cold winters" in Western Europe; this fact proves that the synoptic conditions in South Morocco were the same at 280±30 years B.P. as at present.