Ghost-rock karst symposium

Different points of view on karstogenesis: Fundamental and applied aspects

Introduction to the subject

In the paleokarsts from Hainaut (Belgium), you can find weathering features similar to soil features: an “in situ” limestone weathering with residual weathered-rock (“ghost-rock”) keeping its initial volume. Geological features like cherts beds, stratifications, joints filled with calcite veins remain although they present higher porosity. The weathering follows the joints and moves laterally by weathering fronts which can isolate intact blocks. We have given the name of “ghost-rock” to those features. The morphological expressions of ghost-rocks consist in corridors situated on vertical joints which have conducted aggressive water in a quiet phreatic zone and pseudoendokarstic features totally inserted in the unweathered bed-rock. This phenomenon is like a biostasy stage, under a very weak hydraulic potential, in Erhart’s biorhexistasy framework.

When the hydraulic head of groundwater lowers the ghost-rock crosses from the phreatic zone to the vadose zone and the apparition of a hydraulic potential causes the apparition of a void at the summit of the pseudoendokarst, under the limestone roof. Dolines can form above corridors and cause swampy and river sedimentation: lignite, sands with cross-over stratification. If a new potential energy appears, ghost-rocks can be reactivated by the mechanical erosion of the remaining weathered rock. This can be caused by tectonic phases, or eustatic lowerings of the basic level. River sedimentation can occurs and a “normal” cave is created. It is the rhexistasy stage. This emphasizes the continuity between the ghost-rock features and a speleological cave. This assertion is very different from the classical theory of the karstification: the paradigm of total removal which is based on progressive widening of joints by chemical dissolution and simultaneous removal of soluble and insoluble products.

Those notions light on the karstogenesis problems with new aspects. It seems that many karstic systems in the world result from a genesis with a first stage of ghost-rock karstification. We need now regional studies. They concern also many applied problems: karstic hazards, hydrogeology, reservoir engineering, mineral deposits, and limestone’s exploitation.

Problems which are faced by the karstologists are various: nature of the weathering, influence of lithological and petrophysical characteristics of the residual weathered rock, relations with the tectonic evolution of the region, geometry of the shapes, transition from the ghost-rock to the speleological cave, detection of the ghost-rocks in relation with karstic hazard, ghost-rocks and mineralizations, a.s.o.

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