Evolution of clay mineral assemblages and organic matter in the late glacial-Holocene sedimentary infill of Lake Annecy, (northwestern Alps): paleoenvironmental implications
Résumé
The basin fill of Lake Annecy was investigated from a 44 m core which reached down to glacial sediments of the last glaciation (called Würm in the alpine areas). We analyzed three main parameters: sediment texture (optical microscopy and laser microgranulometry), clay mineral assemblages (CMA by XRD), and organic matter (OM by Rock-Eval pyrolysis). Settling of suspended load, under variable hydrodynamic conditions is the main depositional process. Both CMA and OM provenances can be recognized for the different sedimentary and igneous-metamorphic formations (Carboniferous to Quaternary, and older crystalline basement) and corresponding areas, in the surrounding region of Lake Annecy. Oligocene-Miocene molasses, Early Cretaceous marls, and Early-Middle Jurassic marls and shales are the main sedimentary sources. Two distinct processes were operating: destruction of glacial sediments (till sensu largo) and reworking, or direct erosion and run-off from ice-free catchment areas. Clay minerals related to pedogenesis, and non-reworked terrestrial and lacustrine OM, were progessively added to these primary sources during the Late Würmian/Holocene transition to warmer climatic conditions. Rapid modifications of CMA and OM sources during the earlier phase of sedimentary infilling (Unit 2) suggest that valley glaciers connected to the lake basin almost completely disappeared within a few centuries.