The Ksour sandstones are well known on the whole western part of the Saharian Atlas and are even extended on the neighbouring areas. They are essentially formed by thick sandy beds alterning with clayed layers composed of illite. These sandy beds are hard and can be cemented by secondary silica, iron oxides or by carbonates. They are moderatly sorted, rounded to subrounded and medium grained. The surface texture of the quartz grains, the detailed mineralogical study, the nature of the cementing material and the distribution of heavy minerals showed that the Ksour sandstones of Aïn-Séfra area are mature and polycyclic. They correspond to an orthoquartzite. These Ksour sandstones were most probably deposited in a fresh water environment and derived from a southern source far to south.

Mots clés : « Ksour Sandstones » – Upper Jurassic-Lower Cretaceous – Channel structures – Conglomerates – Sandstones – Clay – Mineralogy- Chemistry- Aïn Séfra- Western Saharian Atlas – Algeria.