Lithological control on coastal rock cliffs erosion of Safi, Morocco

Résumé En

The coastline of the Safi region, between the Beddouza Cape (32°54'N - 09°28'W) and Jorf Lihoudi (32°18'N - 09°26'W), is characterised by different forms of coastal cliffs ranging from 5m to 153m high. Three different lithological units have been defined from the bottom to the top: 1) Plio-Quaternary aeolianites and coquina, 2) Cretaceous clays and sandstones, "Clay of Safi" 3) "Limestone of Dridrat" and 4) Jurassic limestones with clay, gypsum and sandstone. The aim of this study is to identify 1/ the spatial and temporal distribution of coastal instabilities with a catalog events developed from aerial photographs using the GIS (1954-2011) and 2/ the lithological controlling factor, with a special focus on Clay of Safi, using a ratio : clay layer thickness/ cliff height × 100. A conceptual model is used to define the processes acting on Safi coastal cliffs. Lithological, geomorphic, climate and anthropic parameters of 15 representative cliff sites are listed in a database. The results of aerial photographs analysis revealed a low number (10 significant erosion hotspots) of events (collapse, rock fall and landslide) that have affected cliffs during the last 60 years. Historical erosion rates range from 7 to 13 cm/yr, where they are subject to erosion. In Safi, the cliff retreat may be controlled by the proportion of clays whereby lower cliff angles are more clay-dominated. Cliffs, with a clay ratio upper than 60%, are more prone to mass movement than the others. This reflects the efficiency of continental processes. However, cliffs with less than 30% of clays ratio have no mass movement and are more subject to marine erosion. Laboratory analysis will be performed to confirm this hypothesis.