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.
|Translated title of the contribution||Karstic system of Sprimont (Belgium). Holotype of a speleogenesis by ghost-rock karstification|
|Journal||Bulletin de la Societe Geologique de France|
|State||Published - 2018|
Bibliographical notePublisher Copyright:
© N. Dupont et al., Published by EDP Sciences 2018.
- Karstic system
- Underground sediments