First record of chitinozoans from the Ordovician-Silurian boundary beds of Mauritania; palaeogeogrphic implications

In the Hodh area (SE Mauritania), siltstones and shales overlying glacial deposits yield numerous chitinozoans. They provide a high resolution biostratigraphy for these Upper Hirnantian and Lower Rhuddnian deposits. Glacial evidence disappears in the Upper Hirnantian along with the development of the first marine faunas indicative of an opening toward the Rheic Ocean. This opening appears to be younger in the Hodh than in the northern Sahara Basins. The exceptional development of the Hirnantian and of part of the Rhuddanian in the Hodh area, compared with the highly condensed sequences representing the contemporaneous deposits in the northern Sahara Basins, suggests an end-glacial transgressive event followed by a northward progradational event. (C) Academic des sciences/Elsevier, Paris.