Karstification as geomorphological evidence of river incision : the karst of Cousance and the Marne valley (eastern Paris Basin)
Résumé
The Cousance karst (located between the valleys of the Saulx and Marne) has been studied to determine the temporal records of river incision in the eastern Paris Basin, around the ANDRA experimental nuclear waste repository. Two generations of karst are recognized. The first is a palaeophreatic karst indicative of an old base level, now uplifted +75 m above the underground streams. It is underlain and drained by a second generation of karst with active sinks, which records the vertical evolution following the entrenchment of the River Marne. Ten U/Th dates of speleothems from shafts in the karst show that there were discontinuous growth episodes, mainly during isotopic stages 3 and 5 (between 102.4 ± 1.2 and 49.4 ± 0.4 ka bp) but also during isotopic stage 2 at 16.3 ± 0.1 and 20.9 ± 0.3 ka bp. These dates provide an absolute age limit for the start of vertical development of the karst, at the latest during isotope stage 5c.