The Tagnana Formation of the Bechar Region, northwestern Algeria, consists of more than 200 m fine-graind, marine, detrital clastics, sandstone-filled channels with erosional bases, and thin limestones. The Tagnana overlies the Djenien Limestone with apparent conformity, although a paleokarst surface is present at most localities, and it is succeeded unconformably by the Hassi Kerma Formation. Several thin limestone beds, within the Lowwer Tagnana, exhibit wide lateral contiguity, and they can be recognized with assurance in widely separated exposures. Generally, these limestone beds yield conodonts and calcareous foraminifers, while the intervening shales produce ammonoids.

Impoverished assemblages of Anthracoceras in the Lower Tagnana succeed the highest occurrences of Cravenoceras and Delepinoceras in the Djenien. They are replaced by Homoceras and Isohomoceras, with Decorites and Vallitesd occurring together slightly higher. This assemblage apparently extends through the Middle Tagnana, but recoveries are poor. The Upper Tagnana contains the appearance of the advanced reticuloceratid Bilinguites, and the early gastrioceratid Cancelloceras, suggesting correlation with the Yeadonian Stage (G1), but intervening assemblages are not known. Typical Westphalian assemblages with Gastrioceras and Gaitherites occur at the base of the lower member of the Hassi Kerma Formation.

The appearance of Homoceras and Isohomoceras corresponds to the replacement of Adetognathus unicornis and Rhachistognathus muricatus by a typically Lower Pennsylvanian conodont assemblage of Adetognathus lautus, Delicognathodus noduliferus, and D. lateralis. In contrast, this level in the Lower Tagnana still yields the foraminifer Asteroarchaediscus and Millerella does not appear until the horizon with Decorites and Vallites.