Nicolas Dupont

Le système karstique de Sprimont (Belgique). Holotype d’une spéléogenèse par fantômisation

Karstic system of Sprimont (Belgium). Holotype of a speleogenesis by ghost-rock karstification. Karstic system of Sprimont shows varied karstic phenomena in a very well defined geological context: a syncline of Carboniferous carbonates enclosed at the north, east and south by impermeable Famennian formations. The axis of the syncline has evolved like a small valley. At the west, Ourthe River cut perpendicularly this syncline in a typical Appalachian relief and constitutes the karstic basis level. Little rivers flow from the Famennian formations to the valley of Sprimont which follows the axis of the syncline. They disappear in swallow holes named “chantoirs” in the local language as soon as they arrive in contact with the carbonates. Tracing tests prove that these rivers merge underground to resurge at the “Trou Bleu”, which constitutes the resurgence of the system, near the Ourthe River. Many caves have been discovered, often of modest dimensions except the recent discovery of the cave “Noû Bleû”. On the other hand, quarries have opened karstic ghost-rock phenomena. Ghost-rock karstogenesis consists in an in situ weathering of the limestone with a phase separation. The soluble part, essentially calcium, magnesium, bicarbonate and colloidal silica, goes out of the system. The residual alterite is constituted by sparitic calcite, dolomite pro parte and insoluble part like quartz, clay and organic matter. This material, the “ghost-rock”, is very porous and mechanically fragile. In a first stage, this evolution takes place with the same initial volume. This first stage of karstification happens during a geological period where there is no hydrodynamical potential. The underground waters flow in the phreatic zone very slowly. They don’t erode the residual alterite and only the soluble elements go out of the system. This ghost-rock can be studied in the walls of the Coreux quarry. During a second phase, if a new hydrodynamics potential appears due to a surrection with incision of the rivers, the underground waters can erode mechanically the residual alterite by subterranean rivers circulation. A “speleological” cave is created with a “normal” karstic evolution. In Sprimont, the Coreux quarry have also opened cavities outcome from initial ghost-rock whose a part of the residual alterite has been eroded to form a cave. In particular, the “Nico” cave was cut by the quarry in which a geological section shows the residual alterite capped above a discordance by river deposits and speleothems. Thus, this erosion is due to underground river circulation. The target of this presentation is to establish the lithostratigraphy of this geological section and make the granulometry with morphology and mineralogy of the grains. The lithostratigraphy analysis shows that the “Nico” cave is the result of the coalescence between two superposed caves. The wall between these two caves has been weathered in ghost-rock. A great part of the residual alterite has been eroded by an underground river which has brought river sediments. The granulometry and mineralogy prove this origin. We find removed grains of alterite like crinoids. The grains are constituted by calcite, dolomite and a little part of insoluble minerals like clays. The sedimentary series are capped by a flowstone which has been dated: 53.851 ± 2.493 and 61.542 ± 1.235 yr B.P. This situates the erosion of the alterite and the hollowing of the cave from a recent period. Those phenomena bring very important informations about the genesis and evolution of the karst in Upper Belgium. The recent evolution of the karst of Sprimont is characterized by two types of phenomena from the ghostrocks. First, partial regressive erosion erodes mechanically the residual alterite from the basic level. At downstream, we have a great cave near the resurgence: the cave of “Noû Bleû”. In this case, the process of alterite erosion is the best succeeded. Second, at upstream of the system, the compaction of the alterite in the vadose zone generates some voids which can favor the birth of swallow holes. Those lost of rivers participate also to the erosion of the alterite. Our present observations in the “Nico” cave result from this phenomenon. After the first phase of karstification with the formation of a ghost-rock, the apparition of a potential, due to the surrection of the Ardenne massif with the incision of rivers, permits to the underground waters to erode mechanically the residual alterite. Thus, we have here a holotype of the speleogenesis in the frame of an Appalachian relief. The cave “Nico” and the associated phenomena permit to extract a model of evolution of the karsts of Upper Belgium. This example shows too that we can make the difference between the term of karstogenesis which consists in the ghost-rock karstification and the speleogenesis which is the opening of caves by erosion of the residual alterite. Keyword: karstogenesis / ghost-rock / karstic system / speleogenesis / swallow-holes-resurgence / underground sediments

GPR Positioning and Imaging of Buried Remnants of a Gallo-Roman Villa in Matagne-la-Petite, Belgium

Ground-penetrating radar (GPR) was applied to locate and image buried remnants such as walls or foundations of a Gallo-Roman villa near Matagne-la-Petite (SW Belgium). Archaeological excavations were conducted on the site in 1982-83 and the base of walls the villa was exposed but rapidly backfilled. Rather scarce information about the dig of the villa was published, especially the accurate positioning of the building. Visual inspections permitted to reduce the area of investigation, thanks to the presence of rubbles and tile fragments in the field. The GPR slice-map very clearly reveals the presence of the base of walls recognized during the archeological excavations, with some discrepancies noticed in the central part of the building. Rooms corresponding to hot and cold baths are characterized by strong signals at depth over their whole surface. Several aerial photographs corresponding to different periods had been examined in order to pinpoint the buried structure without success. However, based on the accurate positioning resulting from the GPR survey, a new examination was carried out and we were able to very clearly identify the buried structure on a photo taken in spring 2007.

Advances in the Assessment of the Deep Geothermal Reservoirs of Hainaut, Belgium

In the urbanised area of Mons-Borinage (Hainaut, SW Belgium), deep geothermal potential is known since the late seventies. Since 1986, the resource is exploited by two wells. Current projects aim at developing this resource near Mons city for heat production, but also using a suspected deeper reservoir for electricity production. The development of deep geothermal projects in Hainaut requires a better knowledge of the structure of the reservoirs. For this purpose, a new geophysical survey (2D reflection seismic lines) was conducted in 2012, and data from old surveys were reprocessed. Distinctive discontinuities were expected, according with the regional tectonic framework. Based on our interpretation of the final seismic sections, main geological horizons were picked (in TWT) and integrated in a 3D geological model.

Detection of a Fracture Network Near a Cluster of Sinkholes using 3D Electrical Resistivity Tomography

Over the last two decades, 2D electrical resistivity tomography (ERT) surveys have been conducted to detect and map karstic features as the expected electrical resistivity contrast is high between sound limestone and weathering residue as well as between sound limestone and cavities filled with clayey or sandy sediments. However, karstic features often show complex 3D geometries which are not well reconstructed through 2D surveys. In this paper, the efficiency of 3D ERT in the identification of a fracture network located at the top of limestone bedrock is assessed through a field experiment. The results of the 3D ERT interpretations were confirmed by visual observations of an outcrop located 100 meters away. Moreover, former sinkholes which occurred near the investigated area are aligned following the directions pointed out with the 3D ERT survey. This indicates that local karstification probably affect fractures located at the surface of the bedrock leading to the formation of a cryptolapiaz.

Assessing the Contribution of ERT Monitoring in Karstic Risks Management by Numerical Simulations and Lab Experiments

Accurate methodologies are required to manage risks linked to land-use planning in covered kart terrains, especially in densely urbanized areas. The main risk lies in the occurrence of a collapse at the base of buildings or infrastructures. These sinkholes often occur suddenly with no warning. The induced damages range from temporary loss of serviceability during repair works to complete destruction and loss of human lives. ERT (electrical resistivity tomography) used in time-lapse mode has shown great promise in monitoring rapid dynamic subsurface processes as the electrical conductivity of rocks and soils depends on their water content, ionic concentrations and clay content. The efficiency of ERT monitoring in detecting cavities rising to the surface is assessed through numerical simulations and laboratory experiments. The aim of the study is to propose an automated detection strategy to manage these risks based on the comparison of the inverted resistivity of each model block to the range of resistivities expected when nothing is changing of every model blocks. These resistivity ranges will be computed from Monte-Carlo simulations to take into account measurement and inversion errors.