The karst of the chalk
The carbonate of calcium the chalk of which is established is a soluble mineral in the water. In a liter of pure fresh water in 16°C, approximately 13 mg can pass in solution, which is obviously little. If on the other hand we add of the CO2 in this water, the solubility becomes clearly stronger. The atmosphere contains approximately 370 ppm of CO2. Because of the breath of the roots of plants, activity of bacteria and decomposition of the organic matter, the gases of the ground contain approximately 2 % of CO2. The rainwater, by crossing the ground, grows rich in CO2 and acidifies, according to the following reactions:
1) Dissolution of the CO2 in the water:
CO2 + Hour2O? Hour2CO3
2) Aqueous dissociation of the carbon dioxide:
Hour2CO3 + Hour2O? Hour3O+ + HCO3-
There is an acid attack of carbonates, according to the reaction:
Hour3O+ + CaCO3 ? That2 + + HCO3- + Hour2O
The general balance sheet can spell:
CO2 + Hour2O + CaCO3 ? That2 + + 2 HCO3-
He leads to the production of 2 ions hydrogénocarbonate. In each of these ions, an atom of carbon results from some limestone and other one of the carbon dioxide. CO2 Is trapped in this ion.
The dissolution of limestones engenders cavities which develop:
- From the surface, it is the exokarst or karst superficial;
- In depth, from flooded cracks, it is the endokarst or karst deep.
The deep karst
The process of training of the karst (= the karstification) is fast. An underground network is set up from pre-existent cracks and widens "by settling comfortably" on the lowest level of limestones in the landscape: it is the base level, where appears the source. When a crack is widened by the dissolution, its debit increases at the expense of the nearby cracks and it becomes gradually a karstic conduit.
In lands, the base level of the subterranean flows is imposed by the the lowest point on the outcrop of the carbonated rocks. Near the sea, it is the marine level which settles the position of the karstic networks.
In every case, if the base level falls, a new network is set up in a quotation lower than that of the previous karstic network who is dénoyé and who is not functional any more.
We indicate by karst of introduction the upstream zone of the karstic drainage, where waters penetrate into the massif and participate in the collection and in the organization of waters become subterranean. On the contrary, the karst of return corresponds to the zone approval of the drainage, where subterranean waters, concentrated and organized in a network of treated on a hierarchical basis drains, get ready to emerge.
In Country of Callus, the Holocene recent endokarst is flooded, under pressure and impenetrable (except it dive), only the former Pleistocene karst can be penetrated only in caving. Openings are arranged by the recoil of cliffs, on the coast or in the valley of the Seine
The superficial karst
Since his emersion at the end of the Cretaceous, the Norman chalk was covered again by the sea only locally and during short geological durations (Paléocène-Yprésien and Stampien). Thus his superior fringe underwent a continental evolution under a relatively warm climate until the Pleistocene glaciations. A coat of change established collecting the insoluble residues of the chalk or, later, more Eocene. The most frequent facies of this coat is called Clay with FlintThus. The chalk was practically never in nude or then on tectonic convexities. The evolution of the exokarst has of be made under a screed of superficial materials which, according to their properties, slowed down her or accelerated.
Under a waterproof cover (clays sparnaciennes, for example), the water does not reach the chalk, the dissolution is blocked and upper forehead of the chalk does not move and "fossilizes".
Under a permeable cover, the water reaches the chalk. If his capacity of retention is raised, the superficial training plays the role of a "compress" which soaks the chalk permanently and the upper forehead of the chalk sinks under the influence of the dissolution. This mode of evolution was described by In. Bonte (1971). The heterogeneousnesses of permeability concentrate preferentially the water in certain points of the surface of the chalk at the level of which the dissolution is stronger. There is itself a cone of dissolution which drains the flow as the funnel and amplifies the phenomenon. The cone deforms downward by raising edges and becomes cylindrical. The coat of change so fills up in its base of roots.
The sinking in its base of the root leads to a racking of the elements of the coat of change, propagating up to surface to create a depression where come to concentrate superficial waters.
The ultimate stage is the one where the root approaches or meets the endokarst and made collapse the ceiling creating a sort of goufre, called Bétoire In Pays-de-Caux, similar to sinkholes evolving aven there.