A reference region for the Ordovician-Silurian boundary: the Oued in Djerane, Algerian Sahara.

The melting chronology of the Latest Ordovician inlandsis takes on a partic ular importance in northern Gondwana. A continuous sequence of Diplograptid range zones has been defined from outcrops in the Oued In Djerane, but its comparison with the sequence observed in the stratotype of the Silurian-Or dovician boundary is problematic because of the endemic character of the Sa haran species. Moreover, some difficulties are linked to the definition of the Ordovician-Silurian boundary, which, in the Algerian Sahara, probably l ies in the Pseudorthograptus (?) kiliani zone. A definite regional diachron ism in the melting of the ice-cap has also been confirmed. (C) 2000 Academi e des sciences / Editions scientifiques et medicales Elsevier SAS.