The Ordovician glacial pavements of the Tassili n’Ajjer

The Tassili n’Ajjer is an extensive sandstone plateau gently plunging northeastward into the Illizi (Algeria) and Murzuq (Libya) basins. In its western part wadis have deeply dissected the plateau, and to the east Cenozoic erosion has beautifully exposed stratal surfaces corresponding to paleo-landscapes. From 26°30’N, glacial pavements are identified along the Algerian-Libyan border, as far south as Djado in northern Niger (~700 km). Several surfaces are demonstratively exposed west and south of the town of Ghat, where they constitute 1-10 km-scale corrugations superimposed by subglacial streamlined bedforms best identified on satellite imagery (Moreau et al., 2005). Shear bands are observed in outcrop. They developed in soft sediments —mainly sand—, the substrate of the Ordovician glaciers. Striated and grooved surfaces formed either as intraformational shear planes within the subglacial sand, and/or at the ice-sand interface. Sheath folds, sigmoidal fabrics, fluid-escape structures, hydro-schistosity and extensional step fractures (Riedel shears) are additional features linked to subglacial shearing that related to complex sequences of superimposed stages of deformation (Denis et al., 2010). Deformation processes and glacial geomorphology allow us to interpret these streamlined bedforms as the imprints of Ordovician ice streams (Deschamps et al., 2013; Le Heron et al., 2022).