ملخص: The Messinian diatomite-bearing formation from Sig (northwestern Algeria) which is stratigraphically equivalent to the Tripoli Formation present in most of the Mediterranean domain, provides a continuous record of the evolution of Messinian preevaporitic environments. The study is mainly based on both the analysis of the diatom assemblages and SEM observations. The information obtained about the other groups of organisms has also been considered. The diatomite-bearing formation which includes diatomites, marls and clayey limestones has been subdivided into 3 lithological units, or members. The variations of the microflora composition (63 species of diatoms and 3 species of silicoflagellates) make it possible to distinguish three main groups which correspond to the lithological units defined by lithostratigraphical and sedimentological criteria. The first two members correspond to a sedimentation under relatively deep, normal marine conditions. Neritic-littoral and neritic-oceanic planktonic diatoms predominate. A number of cold water species indicate the occurrence of upwelling. The third member was deposited under somewhat restricted conditions and in shallower environments, with warmer surface waters, and some fresh-water input. Benthic warm-water diatoms are more numerous, together with marinebrackish and brackish forms. The Sig diatomite-bearing series thus shows an upward progressive shallowing of the water depth and an increase in fresh water influences which pre-date the transition toward a terrigenous sedimentation including thin gypsum layers.

لغة: فرنسية