Cover-collapse sinkholes in the “Tournaisis” area, southern Belgium

In the Tournaisis area (southern Belgium), cover-collapse sinkholes have been reported since the beginning of this century. In this area, the relatively flat lying carboniferous limestone is overlain by an unconformable mesocenozoı̈c cover. Recent research has shown that these sinkholes developed from reactivated paleokarsts. The intensive pumping for domestic and industrial water supply combined with the dewatering due to deep limestone quarries operations resulted in the lowering of groundwater levels. This triggered the reactivation of paleokarstic systems and thus the occurrence of cover-collapse sinkholes. These sudden and unexpected collapses have already had major impacts in the region. In 1977, about thirty sinkholes opened up along the Escaut River downstream from the city of Tournai. As a consequence, the groundwater was polluted by an extensive loss of river water into the Carboniferous Aquifer. More recently, a house and industrial facilities were severely damaged by such cover-collapse sinkholes. In this context, a geoharzard map would be a valuable tool as far as regional development is concerned. This points at the importance to develop methods to locate and characterize hidden paleokarsts. A study site has been chosen in an area where sinkholes have been opening up for about ten years. A geophysical survey of the site which included electrical resistance mapping of the area has shown “weakness” lineaments. These lineaments are parallel to a main fault affecting the region.