Microfossiles (Acritarches) du Proterozoique superieur dans les shales de la formation d'Atar (Mauritanie)

Earlier studies of Stromatolites had permitted the dating of the Atar Formation (Mauritania) as late Riphean by comparing these structures with those of the U.S.S.R. Geochronological data (Rb/Sr) obtained subsequently on illite had given ages ranging between 890 ± 36 and 595 ± 43 Ma for the whole Atar Group, the Atar Formation being dated at 890 ± 36 Ma (late Riphean). The microfossils (Acritarchs) studied here come from black shales cored while drilling for water near Atar. These shales are equivalent to the intercalations between the stromatolitic limestones of the Atar Formation. They have yielded 20 species of Acritarchs, six of which are most important as they provide a diagnostic assemblage of the late Riphean (1000 ± 50–650 ± 10 Ma) of the U.S.S.R. and northern Europe. These are: Chuaria circularis, Kildinosphaera chagrinata, K. verrucata, Leiosphaeridia asperata, Stictosphaeridium cf. sinapticuliferum, Trematosphaeridium holtedahli. The late Riphean age of the Atar Formation is thus confirmed by this microfossil assemblage.