Pleistocene fluviatile deposits in the Ourika drainage basin (Marrakech High Atlas, Morocco): indicators of climatic variations associated with base level change
Résumé
Aggradational packages of alluvial sediment are preserved in the Ourika river basin. Field mapping and DEM analysis allowed us to identify a model sequence of river terraces. We focus on sites where the study of sedimentary sections allow us to decipher the sedimentary evolution of the Ourika watershed. Quaternary evolution of the drainage basin is characterized by major phases of sediment accumulation and erosion, forming alluvial fans and cut-fill terraces. More intense rainfall events during the Middle Pleistocene Ourika drainage basin resulted in increased erosion and transport of sediment from the hillslopes into the trunk river. First, the cut-fill terraces near the sub-basins outlets are formed by a large-scale aggradation, followed by a main vertical incision and lateral erosion. Then, the second sedimentation period was probably a result of increased precipitation that caused landsliding in steep sub-basins. Finally, a last stage of incision in the Ourika Valley is linked to a base level lowering due to climatic fluctuations. We suggest that cyclic climatic fluctuations superimposed on a continuous uplift of the High Atlas are responsible for the generation of stepped terraces along the Ourika River. Sub-basins steep affected by erosion processes dominated by landslides rocky shallow were accompanied by debris flows along convex profiles at their downstream end and associated with steep knickpoints. We interpreted erosion of the Pleistocene deposits as the result of an autocyclic negative feedback such as exhaustion of the hillslope sediment stocks and the resulting increase of the relative capacity of the trunk stream to bring and transfer sediment towards the Ourika Valley.