Primary and Secondary Sedimentary Processes in Debris-Flow-dominated Alluvial Fan Deposits Within Karstic Setting: an Example from the Middle Atlas-Sais Foreland Basin Transition Zone, Morocco

The transition between the Middle Atlas and the Sais Basin contains a sequence of alluvial fans from the Plio-Quaternary period. These fans are mostly made up of debrites and are found resting unconformably on top of carbonate beds from the Jurassic Period. This location provides a key case study for investigating the primary processes of aggradation, as well as the secondary processes that alter alluvial fan deposits. Alluvial fans are developed through primary processes that occur intermittently over time, resulting in both active and inactive sectors. Internal and external organization of the deposits is, however, partially influenced by secondary processes that can cause mechanical, chemical, and/or biochemical changes, making the comprehension of alluvial fan development processes more complex. In this study, we investigated fan development processes by examining five lithostratigraphic logs and seven sedimentary facies in eight alluvial fans that have merged together. The findings indicate that debris flows were the main driver of fan development, with some contribution from sheet floods and mud-flows. In addition, the study suggests that secondary processes, including mechanical, chemical, biochemical, and diagenetic processes, have had a substantial impact on sediment deposition. The mechanical processes include brittle deformation, incision, reworking, and desiccation. On the other hand, the chemical and/or biochemical processes are evidenced by paleosol and calcrete development, as well as cementation, micritization, dissolution, development of porosity, oxidation, recrystallization, epigenesis, and neoformation. These processes would have been influenced by hydrological, hydrogeological and karstic factors, which are closely linked to the generally semi-arid climatic conditions punctuated by wet phases.