Juvenile continental crust of the Arabian-Nubian Shield (ANS) formed within a Neoproterozoic supercontinent cycle. Subsequent late Neoproterozoic deposition overlapped a series of dramatic climatic events that are unparalleled in subsequent Phanerozoic time, as proposed by the "Snowball Earth" hypothesis. In particular, extreme negative δ 13 C excursions coincident with glacial diamictite and cap carbonate sequences imply that profound carbon flux changes accompanied widespread glacial transitions (Snowball Events). Such a succession appears to be partially preserved in metasediments of the diamictite-bearing Tambien Group of northern Ethiopia (Negash syncline). Here, a pronounced negative carbon excursion from values of +7 to −2 per mil accompanies the transition to diamictite deposition. New zircon evaporation dates from granites intruding the base of the sequence but post-dating early deformation of the entire sequence suggest that the Tambien Group is older than 613 Ma, and therefore diamictite genesis is incompatible with younger Marinoan, Varanger, or "Ediacaran" glacial intervals. Syn-tectonic granites elsewhere within the region date between 750 and 800 Ma and provide an older age constraint for the Tambien Group. The limestone sequence underlying the diamictite has an average 87 Sr/ 86 Sr composition of 0.7066, which supports either a middle (ca. 750-740 Ma) or late (ca. 725-720 Ma) Sturtian age. Combined Sr and C isotopic compositions of the limestones and the upper age constraint from the granite ages provide a compelling argument that the Tambien Group sediments are ∼720-750 Ma or a little older and strongly support the prospect that the Negash diamictite is the product of Sturtian glaciation. If so, these are the first Sturtian Snowball Earth sequences identified in the ANS.

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Présenté par Ghislain de Marsily RésuméLa présence de plusieurs sources à teneur élevée en chlorures, appauvries en isotopes stables, sans tritium et avec une faible activité en carbone 14 est bien connue dans les Alpes franco-italiennes. L'origine des anomalies chimiques a toujours été au centre des discussions scientifiques. Les études réalisées récemment en vue de la définition du tunnel transalpin de l'axe ferroviaire Lyon-Turin ont mis en évidence la présence de couches évaporitiques qui pourraient expliquer les faciès chlorurés sodiques des eaux thermales circumalpines.