Reconstruction of Tertiary palaeovalleys in the South Alpine Foreland Basin of France (Eocene-Oligocene of the Castellane arc)
Résumé
The dynamics of depositional environments and the spatial deformation of drainage networks in foreland basins reflect the tectonic and erosional dynamics associated with the development of mountain belts. The spatial and temporal organization of the Eocene-Oligocene (40-25 Ma) sedimentation in the external part of the South Alpine Foreland Basin of France was reconstructed using an integrated cartographic, sedimentological and petrographic analysis of the Tertiary sedimentary successions. The depositional geometries and variations in facies and thickness of the Palaeogene Nummulitic succession, as well as the observed flow directions in various continental and marine sediments, suggest that the Barreme, Blieux and Taulanne synclines were present as palaeovalleys since the Eocene. The sedimentological analysis of the Nummulitic succession allows the identification of three depositional sequences separated by transgressive surfaces that are recognized in the Barreme, Blieux and Taulanne synclines. Correlation of these sequences between the three synclines suggests that these palaeovalleys were connected by a local valley network that recorded the same sea-level fluctuations during the marine Nummulitic sedimentation. The palaeovalley network was structurally controlled by the east-west axes of the Blieux and Taulanne synclines and the north-south axis of the Barreme syncline formed during the "Pyrenean-Provencal" (Late Cretaceous-Middle Eocene) shortening and the first stage of the Alpine history (Middle Eocene) respectively. Later on, the westward "Alpine" compression (since the Early Oligocene) induced local depocenter migration and reversal in flow direction. However, compared to the modern river pattern, the palaeovalley orientation highlights a geometrical stability since their formation (about 40 Ma), suggesting a long-term stability of the early structures in the foreland basin. This constancy can be explained by the location of the study area in a piggy-back basin transported at the top of the Provencal thrust sheet that facilitated the preservation of the overall axis orientation of the palaeovalleys.