Signification paléoécologique des assemblages de diatomées du Messinien du Dahra sud-occidental (bassin du Chélif, Algérie nord-occidentale)

As one of the main sedimentary Messinian components of the Chelif basin with the gypsum deposits, the diatomites were studied in detail along four sections from the southwestern border of the Dahra.The studied sections show significant vertical and lateral lithological changes, allowing to distinguish two units (the lower member and the upper member). The lower member is dominated by the neritic-oceanic and oceanic planktonic assemblage (Coscinodiscus marginatus, C. oculus-irridis, C. radiatus, Nitzschia reinholdii, N. fossilis, Thalassiosira eccentrica, T. miocenica, Thalassiothrix longissima, etc.). Whereas, the littoral planktonic species (Actinoptychus senarius, Thalassionema nitzschioides, Actinocyclus octonarius, A. tenellus, Paralia sulcata, etc.) and benthic forms characterized the upper member, indicating a general decrease in bathymetry over the Messinian diatoms. This assemblage is also dominated by cold water diatoms characteristic of boreal area (Coscinodiscus marginatus) in the lower member and Actinocyclus curvatulus in the upper member), which correspond to the defined lithological units. The paleoecological data from diatoms, silicoflagellates and radiolarians indicate that the diatomites were deposited in open marine environment submitted to the inputs of Atlantic cold water, until last levels before gypsum deposits. Moreover, the abundance of Thalassionema nitzschioides, constantly present from the base to the top of the section and often associated with Thalassiothrix longissima, is indicative of a high productivity, which can be characteristic of an upwelling regime, responsible of the development of the diatom microflora.