Enregistrements sedimentaires et controle tectonique dans la genese des archives morphosedimentaires quaternaires de la coupe d'alhnayn (Vallee Meridienne Du N'fis, Maroc)

Abstract

To show the response of fluvial and torrential morphodynamics to climatic variations and neotectonics in the semi-arid mountains, the authors present a preliminary geomorphological study of the main geosystem of the Alyhnayn area. This level is located in the upstream part of the intramountainous valley of N'fis river belonging to the northern side of High-Atlas. Its geomorphological analysis, based on the distribution of alluvial deposits and their characterization, reveals an alluvial-fans/terrace system. The latter is formed by a surface modeled on glacis and lacerated by more or less deep ravines. It is arbitrarily attributed to the middle Quatemary age. The geometric and the morpholithostratigraphic analysis of its deposits show a local control of a sedimentation of fluvialtorrential and gravitational types. A complex slop deposit is embedded within the fluvial sequences. It can be explained by a drop of the base level related to neotectonics. The sedimentation process is caused by the reactivation of a preexisting fault in relation with the High-Atlas elevation. The evolution of the Quatemary morphogenesis of the Alhnayn area has been influenced by uplift and climatic changes. Neotectonics controls fluvial and torrential dynamics by the local morphostructural framework. The fluvial and afluvial fans environments responded differently to recorder impacts.