Lithostratigraphic study of cores in the vicinity of Hassi Messaoud and Oued Mya, Erg el Anngueur, and Fort Miribel (Algeria) has resulted in the description of a type section for the Cambro-Ordovician series in the western Sahara which includes five stratigraphic units. The lower Cambrian is represented by the Hassi Messaoud sandstone group, overlain by the Cambrian-Ordovician Isometric sandstone formation. The Ordovician is represented by the Tremadocian lower argillite formation and the lower Arenig intermediate quartzites. In the Erg el Anngueur area a shale-sandstone complex overlies the lower argillites The Ol.1 sandstone is transgressive on the stage below and is overlain by the upper argillite formation and the terminal sandstone-shale complex which represent undifferentiated Arenigian-Llanvirnian. A brief outline of the paleogeographic aspects of the region is discussed in relation to each unit.