Karstic system of Sprimont (Belgium). Holotype of a speleogenesis by ghost-rock karstification

摘要

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 "Noll Bleu". 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 su