Quaternary river incision in NE Ardennes (Belgium)–Insights from 10Be/26Al dating of river terraces

Although it constitutes a main tool to unravel the regional recent tectonics, the chronology of the Pleistocene river incision is still poorly constrained within the uplifted Ardennes massif (Western Europe). Here, we use in situ produced cosmogenic 10Be and 26Al concentrations from depth profiles in terrace sediments of several Ardennian rivers (Meuse, Ourthe and Amblève) in order to date the so-called Younger Main Terrace (YMT), a key-level in the drainage network evolution. We present the first absolute dating of the YMT in the lower Meuse valley, where we obtained an age of 725 ± 120 ka for a terrace deposit buried beneath 3 m of Weichselian loess at Romont. This age is consistent with some previously published estimates based on paleomagnetic data and MIS correlations. However, the ages we obtain for the same terrace level in Ardennian tributaries of the Meuse are significantly younger: 390 ± 35 ka in the lower Ourthe, and only 220 ± 31 ka still farther upstream, in the lower Amblève. We thus demonstrate that the post-YMT incision occurred diachronically in NE Ardennes. The ∼0.5 Ma timespan needed by the erosion wave to propagate from the lower Meuse towards the Ardennian headwaters contradicts the long-held statement of a climatically driven incision that would have been synchronous throughout the catchments. Finally, we interpret the strong 10Be enrichment displayed by the lower half of the Belle–Roche (lower Amblève) profile as betraying the long-lasting, slow accumulation of the ∼8 m thick terrace deposit in that place.