Stratigraphie et étude sédimentologique du gisement de Saint-Vallier

Abstract: The geology of Saint-Vallier is dominated by fine detrital deposits intercalated with indurated horizons. The base of the sequence is characterised by coarse detrital sediments (pebbles, gravels, sands) which fine upwards with the addition of silts and fine sands. Micromorphologic study reveals the importance of carbonate epigenesis throughout. The replacement of siliceous elements by calcite is due to phreatic carbonate drift. Epigenesis, guided by plant roots, lay at the origin of many concretions, such as rootlet pipes and either isolated nodules or those linked together in the “bancs durcis” (indurated layers). Bioturbation is also important: it was responsible for sediment homogenization and high porosity following rootlet disappearance; animal traces (burrows, micro-excrements and earth worms biospheroïdes) are common. Pedogenesis is discrete and marked by argilification, decarbonation of sediments and stronger bioturbation in beds overlying the “bancs durcis”. Soils profiles are incomplete, indicating truncation. All geometrical and sedimentological characteristics of the deposits point to a complex environment where channels alternated with riverbank soils. The processes of sedimentation, as well as dissolution, downcutting and reworking of former deposits, lateral transfer of carbonates and pedogenesis were repeated over time. The environment is that of an alluvial fan with a shallow slope, placed at the distal end of the Chambaran surface and connected to the alluvial plain of the Rhone at the start of Plio-Quaternary incision. [Copyright &y& Elsevier]