Problème des karsts en roches silicatées... lacunes d’observation et paradigme du karst

Conference (2009, December 05)

Depuis le développement de la karstologie, la grande majorité des cavités prospectées l'ont été dans les roches carbonatées ou apparentées, réputées fort solubles. Partant de ce principe, les autres ... [more ▼]

Depuis le développement de la karstologie, la grande majorité des cavités prospectées l'ont été dans les roches carbonatées ou apparentées, réputées fort solubles. Partant de ce principe, les autres lithologies n'ont été à ce jour que partiellement investiguées puisque réputées peu ou pas solubles. Pourtant, depuis plusieurs siècles, des grottes et autres formes apparentées sont mentionnées notamment dans des grès, des granites ou les quartzites. Très rapidement, elles ont été catégorisées comme pseudokarst en se basant sur deux principes, le premier, la convergence de forme, le second, les processus physico-chimiques qui présideraient à leur formation seraient différents de ceux rencontrés dans les calcaires. L'évolution des recherches et des techniques de ces vingt dernières années ouvre de nouvelles perspectives sur une réalité karstique commune qui transcenderait les lithologies tant carbonatées que non carbonatées. Peu à peu, elle oblige les chercheurs à se repositionner quant à la notion de karst. [less ▲]

EXEMPLOS BRASILEIROS DE CARSTE EM ARENITO: CHAPADA DOS GUIMARÃES (MT) E SERRA DE ITAQUERI (SP)

in Espeleo-Tema - Sociedade Brasileira de Espeleologia (2009), 20(n. 1/2), 7-23

The issue of non-carbonate karst is still controversial in academic circles, there is resistance from classical karst researchers. However, in many studies throughout the world, the karst in non-carbonate ... [more ▼]

The issue of non-carbonate karst is still controversial in academic circles, there is resistance from classical karst researchers. However, in many studies throughout the world, the karst in non-carbonate rocks is a constant. Although their existence is controversial, this is due to the small amount of research conducted in these rocks had not generated a sufficient amount of information that would establish general comparison with the carbonate karst, identifying boundaries and differences. In this article, we attempted to show some of the evolution of the concept through the literature and present two examples of sandstone karst in Brazil. [less ▲]

Karsts in sandstones and quartzites of Minas Gerais, Brazil

in Cadernos do Laboratorio Xeolóxico de Laxe (2008), 33

The state of Minas Gerais (Brazil) is characterized by significant karst regions, which develop in both sandstone and quartzite terrains and display complex suites of underground and surfaceforms. In the ... [more ▼]

The state of Minas Gerais (Brazil) is characterized by significant karst regions, which develop in both sandstone and quartzite terrains and display complex suites of underground and surfaceforms. In the Espinhaço Ridge, Central Minas Gerais, several caves of up to a few hundred metres long, occur in the surroundings of the town of Diamantina. Some of these caves, such as Salitre actually consist of swallow-holes. Other horizontal caves are characterized by corrosion forms generated in the phreatic zone. In some places, such as in the Rio Preto area, these phreatic forms are overprinted by ceiling tubes, suggesting a polyphase karst evolution, prior to the draining in the cave. Remains of paths, with circular cross section up to one metre in diameter, can be found through residual tower-like surface landforms widely present in the landscapes. Their dissection is due to a generalised karstification in the area, resulting in closed canyons, megakarrens and kamenitzas. In Southern Minas Gerais, close to the Mantiqueira Ridge, the caves of the Ibitipoca state park can reach more than 2 km in length. These caves are associated with a very large hanging geological syncline. Several of these caves contain active streams, which flow for hundreds of metres before disappearing in sand-choked passages. Keyhole cross sections characterize steeply descending passages in these caves, indicatinga chan ge from slow phreatic flow towards a faster vadose flow responsible for the vertical incision of the passage. Such change is probably related to base level lowering and/or to turn in the direction of the water flow. Several generations of wall-pockets, from a few centimetres to over a metre long, occur into the caves. These features are good indicators of the initial phase of speleogenesis, generating the initial conduits by their coalescence. This mechanism is also responsible for cut-off meanders. In the area, the main river flows along the syncline axis and cuts through a rock barrier, generating a tunnel-like passage. This cave drains, through resurgences in its walls, part of the water that flows in other caves located in the flank of the syncline. [less ▲]

Karsts des craies et calcarénites de la Montagne Saint-Pierre (Basse Meuse liégeoise)

in Bulletin des Chercheurs de la Wallonie (2007), XLVI

The “Montagne Saint-Pierre” (Sint Pietersberg) is a separate part of the Hesbaye plateau, isolated between the lower valleys of the Geer and the Meuse rivers. It is exploited by a big open-air quarry and ... [more ▼]

The “Montagne Saint-Pierre” (Sint Pietersberg) is a separate part of the Hesbaye plateau, isolated between the lower valleys of the Geer and the Meuse rivers. It is exploited by a big open-air quarry and by numerous underground quarries developing galleries on hundreds of kilometers long. Excavated in Cretaceous chalk and calcarenite, these artificial networks allow an exceptional 3-D observation of karsts inside a very porous and permeable rock, less favourable to a concentrated solution. The most numerous of them are “organ pipes” or “earth pipes”. They are vertical tubular solution pipes that may exceed 60 m in depth. Sponge networks and subhorizontal caves occur, without any visible connection with fracturation. Finally, downwards to at least 20 m below the alluvial plain of the Meuse river, pluridecametric nodes of weathered chalk are found. By their size and rounded morphology, the nodes resemble to the natural caves occurring in the calcarenite and intersected by the underground quarries. All the studied karsts allow us to propose a scenario for the genesis of a polyphase karst system. Independently of surface conditions, caves are generated deeply in the phreatic zone (endokarsts). During the downcutting of theMeuse valley, and related to the fluvial terraces, solution pipes (input karsts) are generated. Due to the valley incision and to the lowering of the aquifer,theses solution pipes progress downward and cut the endokarsts. A concentrated water circulation takes place. In the dewatered upper part of the system, caves cut by solution pipes are rapidly filled by superficial deposits. The high porosity of the calcarenite makes it comparable to a sponge. The rock absorbs quickly the out-flows coming from the surface and causes a rapid deposit of the fine particles transported inside horizontal passages. The sealing of these conduits allows their conservation inside a very crumbly rock. [less ▲]