The lower Triassic is defined by argillite and sandy fluvial, river sequences, overlain by a carbonate unit with palynomorphs indicating a Ladinian, Middle Triassic age. The Upper Triassic is marked by the Messaoudi dolomite, evidence of marine transgression. Lower and Middle Jurassic units are mainly calcareous and marl sediments, although some from the Late Jurassic preserve a deep ocean environment with the fossils of radiolarians.