Mise en évidence d’une dynamique de fleuves de glace durant la glaciation ordovicienne par l’enregistrement géomorphologique

Abstract

On Contemporary Ice streams the most important factor controlling polar ice-sheet sheet stability and ice flowing occurs in ice streams. On Pleistocene Ice Sheet, Ice stream can be located using several geomorphologic criteria predict from known characteristic of contemporary ice streams, these criteria defining the conceptual Ice Stream land system model. But nowadays, during fossil glaciations, ice streams characterization is still poorly documented and only few studies illustrate fast ice velocity. In this work, we characterize geomorphological markers of ice stream activity in order to define the Ordovician ice stream glacial landsystem. The study area is located in Sahara from Niger to Algeria. In this area, Ordovician glacial landforms have been investigated using satellite images and field investigations. According to these observations, Ordovician paleo ice stream are characterize by their size (20 to 200 km), by the increase of the length or ratio L/l of sub-glacial lineations changing from ice flute to mega scale glacial lineation at the margin of the ice sheet, by the evolution of sub glacial valley morphology ranging from areal scouring to glacial linear incision, and by sub glacial deformation. Finally, marine outlet of ice stream show ice contact deltas deposit. Individually, these criteria may not necessarily be exclusive to ice stream activity, but collectively they are thought to discriminate the characteristic Ordovician ice stream landsystem.