Lithostratigraphy and palaeoenvironments of Messinian deposits of the north eastern termination of Ouled Ali Mounts (Lower Chelif basin, NW Algeria).- Lithostratigraphic description and palaeoenvironmental study of Messinian outcrops located on the southern edge of the Lower Chelif basin (NW Algeria) are described in the present paper. The study is based on the analysis of geological sections situated on the south and the northernmost terminations of Djebel Aoud Sma in Sig area. In this study, three main lithostratigraphic formations are identified and interpreted on the environmental plan : (i) Greyish marls, attributed to the Messinian and deposited in an outer shelf ; (ii) Diatomites, characterized by pure and marly diatomites deposited in a shallow marine setting ; (iii) Limestones, corresponding to reefal and peri-reefal limestones, subdivided into five distinct facies. Also, Halimeda-rich facies has been recorded in the Haoudh el Matmar. This facies is well known in the Messinian west Mediterranean carbonate platforms, and indicates important environmental changes. It is considered as an index level which allows correlating the Tessala-Beni Chougrane platform with the other Messinian carbonate platforms of the Mediterranean.