L'ouverture du gouffre de Belvaux et la capture souterraine de la Lesse : chronologie et impacts géomorphologiques et archéologiques

Abstract

The karst site of Han-sur-Lesse, located in the Calestienne region, exhibits a unique geomorphological configuration where the Lesse River disappears into the karst system at the Chasm of Belvaux, the largest karst swallow hole in Western Europe. This phenomenon has profoundly reshaped the hydrological and sedimentary organization of the Chavee Valley, located immediately downstream from swallow hole. However, the precise timing of the opening of the Chasm of Belvaux, as well as the associated impacts, remains uncertain. This study aims to better understand the dynamics of the swallow hole’s opening and its consequences on sedimentation processes, surface water flow, and human occupation of the valley. To this end, a multidisciplinary approach was employed, combining the study of sediment cores, bank section survey, tomographic profiles, and topographic analyses. These were complemented by granulometric and sedimentological analyses, as well as the study of recent flood events. Radiocarbon dating of organic materials found within the deposits enabled the chronological reconstitution of the system’s main phases of evolution. The results reveal a series of incision events in the alluvial plain, indicative of successive stream captures. Altimetric analysis of gravel levels suggests that the "Trou d’Enfaule", another sinkhole located downstream from the Belvaux swallow hole, was active prior to the opening of the Belvaux feature. This indicates a regressive opening dynamic of the various sinkholes present on the site. Drone imagery acquired during a recent flood event enabled the mapping of the flood extent and confirmed the presence of a lacustrine area at the Belvaux swallow hole, associated with a natural dam formed by a low terrace covered with colluvium from the Chession meander. Radiocarbon dates from previous studies, combined with the sedimentary results obtained in this work, suggest that the karst system was already partially functional before 3000 BP. However, two radiocarbon dates indicate that the most recent significant phase of opening of the Belvaux swallow hole occurred between 2000 BP and 3000 BP. The interpretation of past dynamics remains complex due to the interweaving of alluvial, colluvial, and karstic processes, which makes it difficult to distinguish between active phases and periods of temporary blockage of the swallow hole. This study highlights the complexity of the evolutionary history of the Han-sur-Lesse karst system. It illustrates the value of an interdisciplinary approach combining physical geography, archaeology, and geochronology to better understand surface-subsurface interactions.