Geometry of glaciofluvial deposits and dynamics of the Lyonnais lobe ice front during the last glacial period (France, Northern Alps)

Thibault Roattino,Jean-Francois Buoncristiani,Hélène Tissoux,Christian Crouzet

doi:10.1051/bsgf/2021012

Abstract

Previous studies in the foreland of the French Western Alps, based on the analysis of geomorphological criteria for the internal moraine complex, show several stages of retreat or stagnation of the Lyonnais ice lobe during marine isotopic stages 4 and 2. Based on chronological data of the literature, the age of the Würmian maximum extension of the Lyon ice lobe must have occurred during MIS 4 but this result is still debated. At the Last Glacial Maximum, in the western part of the Lyonnais ice lobe, glaciofluvial corridors were active during flash floods draining glacial meltwater. Today, these corridors are dead valleys and display a series of terraces. In this paper, we analyse the sedimentary geometries and dynamics of three glaciofluvial corridors (Moidieu, Septeme and Heyrieux) located at the front of the internal moraine complex of the Lyonnais ice lobe. Upstream, the Moidieu corridor splits into three branches called North Moidieu, Central Moidieu and South Moidieu. Glaciofluvial deposits in the corridors are composed of pebbles and gravels in a sandy matrix. Sedimentary structures show mass flow events and the migration of river bars in braided channels which are characteristic of proximal glaciofluvial rivers in a proglacial environment. According to a new geomorphological map built using a high-resolution digital elevation model and an isopach map of the Quaternary deposits created from a compilation of the borehole data, we suggest that these corridors correspond to “tunnel valleys” built during the most extensive Riss glaciation. Then during the Würm maximum glacial extension, these “tunnel valleys” show complex infilling by glacio-fluvial sediments during the Würmian maximal extent. In the three corridors, the number of river terraces can be better defined by using new geomorphological analyses. A total of three Würm terraces can be observed: two in the north and three in the south. This difference between the south and north is probably a result of climatic and tectonic forcing.

Highlights

Quaternary glacial extent in the French Alps has been studied since the 19th century
A new geomorphological map of the glaciofluvial corridors is established using a high-resolution digital elevation model (DEM) RGE ALTI® 5 m published by the French National Institute of Geographic and Forest Information (IGN®)
They cannot be geomorphologically connected to any moraines of the internal morainic complex. They cannot be linked with the external morainic complex as this latter is far to the west. These FgR terraces are probably built during retreat phase of the penultimate glaciation and they lie above the erosional surface of the Rissian glacier

Summary

Introduction

Quaternary glacial extent in the French Alps has been studied since the 19th century. These studies have primarily focused on the identification of erratic boulders and the mapping of moraine structures (Falsan and Chantre, 1879). Very large ice lobes are located north of the Alps, which are from the west to east the Lyonnais, Rhône, Reuss, Rhine and Isar-Loisach ice lobes. These ice lobes are much smaller in the southern part of the Alps, e.g. the Ivrea, Ticino, Oglio, Garda and Tagliamento ice lobes (Ehlers and Gibbard, 2004) (Fig. 1)

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