Updating the alluvial mapping of the middle Loire catchment: Predictive geomorphology approach and mineralogical characterization of the deposits

This study aims to provide a full synthesis of recent work on the fluvial terraces of the middle section of the Loire River, between its exit from the “Massif Central” montane area and its entry into the Touraine region, with the aim of better understanding of the evolution of the river during the Pleistocene period.New fieldwork and topographic analyses (including a predictive geomorphology approach) permit updating the spatial extent and geometry of the different Quaternary alluvial sedimentary formations in this area. In addition, new analytical data were acquired both by X-ray diffraction (XRD) on heavy minerals and by gamma spectrometry measurements on alluvial sandy sediments. These analyses allow us to characterize and distinguish the alluvial formations.This work has identified three distinct periods in the history of the Loire: the oldest, corresponding to the deposition of sediments characterised by an abundance of metamorphic minerals, originating from the Massif Central; an intermediate period, corresponding to the deposition of the high terraces of the Loire system, not characterised by mineralogical analyses but by the approach of the predicted geomorphology showing different terrace geometries; lastly, a recent period comprising the low and medium altitude terraces, characterised by an abundance of volcanic minerals also originating from the Massif Central.