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L'âge du modelé de roubines dans les Préalpes du Sud : l'exemple de la région de Digne/The age of the badlands in the French Southern Prealps: the example of the Digne region

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In the region ofDraix, north of Digne (French southern Prealps), the present slopes are characterized by an extreme development of badlands cut in very thick black marls. In the context of the DM2E research program of the European community, we tried to determine the timing of the appearance of the badlands. The interfluves between the small torrential watersheds (from some tens to some hundreds of hectares) correspond to Middle Pleistocene (?) or older glacis, alluvial terraces and debris flows. In the watersheds, an alluvial terrace exists locally, especially downstream where it merges with the probably Upper Pleistocene terrace of the Bouinenc and the Bléone rivers (Haccard et ai, 1989). Below this terrace, a silty grey formation is generally observed. It represents the main Holocène formation. Two-to-ten metre thick it is not bedded, except in the lower reach of the Peyrouret torrent. It has fossilized some Pinus vertical stumps radiocarbon dated 8860 ±70 B.P., 8010 ± 80 B.P. and 7950 ± 160 B.P. This accumulation is considered as the local fades of the Main Holocène Fill (Jorda, 1985). It fills depressions cut into the black marls. These depressions have everywhere gentle slopes. In the Laval watershed, the depression is a paleo- valley whose longitudinal slope is 1 % less than the present one. Thus, the badlands did not exist during the Lower Holocène (Preboreal to Atlantic) period. Then the Laval river entrenched its bed with a mean incision rate of about 3 mm/year in its middle course. This rate is in agreement with the mean incision rate of rivers in the plains and middle mountains of western mediterranean watersheds (Ballais, 1995). Younger formations are very rare, thus suggesting that incision was a general trend. They subsist as very narrow and discontinuous remnants, for example four-to-two metre above the

* Institut de Géographie et CAGEP-URA 903 CNRS, Université de Provence, 29 avenue Robert Schuman, 13621 Aix en Provence Cedex

Géomorphologie : Relief, processus, environnement, 1996, n° 4 pp. 61-68

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