Cette étude, pilotée par J.-F. Buoncristiani, revisite les formes glaciaires du plateau de la Dombes laissées par la glaciation rissienne (MIS 6). À partir de données LiDAR à haute résolution traitées par Volumetric Obscurance (VO), plus de 4 000 formes de relief glaciaire ont été identifiées et caractérisées, permettant notamment de distinguer moraines et formes sous-glaciaires. Leur organisation révèle plusieurs générations de formes et au moins trois réorganisations successives des écoulements glaciaires. Le paysage actuel de la Dombes apparaît ainsi comme l’enregistrement composite d’une déglaciation complexe, alternant phases d’écoulement rapide, de stagnation et de retrait du glacier.
Abstract: Glacial landforms of the Dombes Plateau provide a detailed record of the Rissian glaciation (MIS 6). More than 4,000 convex landforms were extracted using high resolution LiDAR data (5 m) processed by Volumetric Obscurance morphometric index (VO index). These were segmented according to their vertical amplitude, allowing the distinction between morainic deposits (> 6 m) and subglacial bedforms (< 6 m). This approach provides the first exhaustive mapping of glacial landforms in this area.
Three types of glacial landforms are identified. The first corresponds to radially organized bedforms aligned along a southeast–northwest axis, showing a continuum from drumlins to megaflute, which reflects variations in ice-flow velocity and basal shear stress gradients. The second consists of several generations of frontal morainic ridges arranged in successive arcs, indicating a s tepwise retreat punctuated by stillstands. The third corresponds to hummocky moraines, interpreted as deposits formed by the downwasting of dead ice within a saturated till.The superposition of glacial landforms from different generations, often in cross-cutting patterns, records at least three reconfi-gurations of ice flow. The Dombes landscape thus represents a composite and diachronous record of multiphase deglaciation of the Dombes glacial lobe during the Rissian MIS6 stage.
This study confirms the efficiency of the VO index in discriminating glacial micro-reliefs in low-amplitude topographic settings. It highlights the instability of the glacier margin during the Middle Pleistocene, characterized by alternating phases of fast flow, stagnation, and retreat. The Dombes Plateau therefore constitutes a key site to document the polyphased dynamics of Alpine ice lobes and their response to climatic and hydrological variability.