Contribution des données du nord de la France à l’étude du rôle du thermokarst dans les phases d’activités pléistocènes des talwegs élémentaires

• Main Author: DESCHODT, Laurent
• Format: Article in Journal/Newspaper
• Language: French
• Published: Groupe français de géomorphologie 2022
• Subjects: Bassin versant; Escaut; vallée sèche; paléotalweg; ravine; réseaux de coins de glace; thermokarst; thermo-érosion; morphogenèse; Pléistocène; Pléniglaciaire weichselien; River bassin; dry valley; palaeo-talweg; gully; ice wedges network; morphogenesis; Weichselian Pléniglacial; geo; archeo; Greenland; Ice; permafrost; Thermokarst; wedge*
• Online Access: http://journals.openedition.org/geomorphologie/17174

The network of elementary talwegs on slopes and interfluviums is considered to be a Pleistocene periglacial inheritance. However, the sedimentary deposits of the dry valleys are generally sparse and poorly informative. Eighteen indices of fluvial activity on slopes are reviewed in the French Scheldt basin and its margins. Most of them are infilled gullies. Several gully phases occurred during the Weichselian Pleniglacial (about 65 to 15 ka) in small basins (a few tens of thousands of m²), on interfluviums and upper slopes (six cases recorded) or in the plains (four cases). They are frequently associated with the remnants of ice wedges. Present-day examples of gully incision along ice wedges (thermo-erosion) in a context of permafrost degradation appear to be suitable analogues to explain the Pleistocene data. Stratigraphic arguments, some geochronological data and comparisons with neighbouring regions suggest that during the last cycle a rapid degradation of ice-rich permafrost occurred in the early Greenland Interstadial GI-15, -8, -4 and possibly in the early GI-2. The repetition of the phenomenon during the Pleistocene probably played a role in the genesis of the talwegs network on the interfluviums. The Pleistocene sediment budget of the collecting valleys is generally poorly developed, which implies an efficient downstream transit of the solid load during these episodes. Only three cases of thicker deposits are recorded in small valleys that were later completely or partially filled in by the covering loess of the late Weichselian Pleniglacial. A strong later activity is perceived on loessic lower slopes (2 cases). It is attributed to slope runoff during the Recent Dryas, but this time without evidence of permafrost. Le réseau de talwegs élémentaires sur versants et interfluves est réputé être un héritage périglaciaire pléistocène. Cependant les dépôts sédimentaires des vallons secs sont en général lacunaires et peu informatifs. Dix-huit indices d’activités fluviatiles sur versant sont passés en revue ...