MARLS-GULLYING - CANONICAL-ANALYSIS OF EROSION AND SEDIMENTATION SEQUENCES (BARONNIES-FRANCE)

The gullying action on Jurassic and Creta ceous marls in the Baronnies is observed, warm season after cold season, thanks to a network of me asuring stakes arranged in 12 gullies. The climate, characterized by s ummer heath, rather cold but luminous winters, and quite heavy rain, c an be classified as mountainous-mediterranean. Seasonal ablation/sedim entation measures have been carried out upon 119 stakes five times wit hin two years (768 days): two in automn, two in early spring (april), and one at end-of-spring. Ablation and sedimentation sequences are des cribed among the stakes population by Correspondance Factor Analysis ( CFA), and their covariations in space and time are explained by Canoni cal Correspondance Analysis (CCA) according to biophysical environment variables. Axis 1 of the CFA opposes ground level variations accordin g to their amplitude, high or moderate, regardless of the direction of variation and of measurement date. Axis 2 presents a temporal variati on, opposing, on one side, sedimentation sequences at post-winter term s and ablation sequences at post-summer terms, to their converse on th e other side. The CCA indicates that talwegs in their medium and lower parrs are more likely to undergo hard gullying or strong accumulation , especially on sleep, or conversely Slat, vegetation-free slopes of o xfordian marls; on the contrary, interfluve ridges and Gonson moderate gentle to moderate slopes on cenomanian marls, possibly with some veg etation are less dynamic (canonical axis 1). This result allows to ext end former observations on rythmic, contrasted variations of gullies b ottoms, well different of interfluves and gonson, progressive, regular , ones. Owing to their varied reactions to seasonal erosive agents (cr yoclasty, concentrated run-off), upper parts of talwegs and lower part s of gully slopes and interfluves (canonical axis 2) originate seasona l contrasts revealed by CFA's axis 2. However, to insure this point of view, more sampling is needed.