Facies and basin architecture of the Late Carboniferous Salvan-Dorénaz continental basin (Western Alps, Switzerland/France)



The Salvan-Dorénaz Basin formed during the Late Palaeozoic within the Aiguilles-Rouges crystalline basement (Western Alps) as an asymmetric, intramontane graben elongated in a NE–SW direction and bounded by active faults. At least 1700 m of fluvial, alluvial fan and volcanic deposits provide evidence for a strong tectonic influence on deposition with long-term, average subsidence rates of > 0·2 mm yr−1. The early basin fill was associated with coarse-grained alluvial fans that were dominated by braided channels (unit I). These issued from the south-western margin of the basin. The fans then retreated to a marginal position and were overlain by muddy floodplain deposits of an anastomosed fluvial system (unit II) that drained towards the NE. Deposition of thick muds resulted from a reduction in the axial fluvial gradient caused by accelerated tectonic subsidence. Overlying sand-rich meandering river deposits (unit III) document a reversal in the drainage direction from the NE to the SW caused by synsedimentary tectonism, reflecting large-scale topographic reorganization in this part of the Variscides with subsidence now preferentially in the W and SW and uplift in the E and NE. Coarse-grained alluvial fan deposits (unit IV) repeatedly prograded into, and retreated from, the basin as documented by coarsening-upward cycles tens of metres thick reflecting smaller scale tectonic cycles. Volcanism was active throughout the evolution of the basin, and U/Pb isotopic dating of the volcanic deposits restricts the time of basin development to the Late Carboniferous (308–295 Ma). 40Ar/39Ar ages of detrital white mica indicate rapid tectonic movements and exhumation of the nearby basement. In unit I, youngest ages are close to that of the host sediment, but the age spectrum is wide. In unit II, high subsidence and/or sedimentation rates coincide with very narrow age spectra, indicating small, homogeneous catchment areas. In unit III, age spectra became wider again and indicate growing catchment areas.