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See detailKarsts dans les gneiss - alvéoles géantes et micro-alvéoles dans la région de Milagres, état de Bahia, Brésil
Rodet, Joël; Willems, Luc ULg; Auler, Augusto

Conference (2009, December 05)

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See detailProblème des karsts en roches silicatées... lacunes d’observation et paradigme du karst
Willems, Luc ULg; Ek, Camille ULg; Rodet, Joël et al

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 ▲]

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See detailPolyphased karst systems in sandstones and quartzites of Minas Gerais, Brazil
Willems, Luc ULg; Rodet, Joël; Pouclet, André et al

in Proceeding 14th UIS Congress, Athens-Kalamos, 23-28 august 2005, Abstract Book : 71. (2005)

The state of Minas Gerais (Brazil) exhibits several major karst areas located in sandstone and quartzite terrains, that display a complex suite of underground and surface karstic forms. In the Espinhaço ... [more ▼]

The state of Minas Gerais (Brazil) exhibits several major karst areas located in sandstone and quartzite terrains, that display a complex suite of underground and surface karstic forms. In the Espinhaço Ridge, central Minas Gerais, several caves, up to a few hundred metres long, occur in the surroundings of the town of Diamantina. Some of these caves, such as Salitre, represent swallow-holes and show dome pits. Other horizontal caves are characterized by corrosion forms generated into the phreatic zone. In some places, such as in the Rio Preto area, these phreatic forms have been overprinted by ceiling tubes, suggesting a polyphase karst evolution, prior to the draining of the cave. Relicts of passages, with circular cross section up to a metre in diametre, can be found amidst the residual tower-like surface landforms, which constitute a typical scenery in the landscape. 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 state park of Ibitipoca can extent 2 km in length. These caves are associated with a very large hanging geological syncline. Several of these caves contain active streams, that flow for hundreds of metres before disappearing in sand-choked passages. Keyhole cross sections characterize steeply descending passages in these caves, indicating a change 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. The main river in the area, which flows along the syncline axis, 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. The non-carbonate karst features observed in the state of Minas Gerais demonstrate the complex organisation of polyphase karst systems due to the linkage of underground and surface forms not previously connected. As in carbonate areas, these systems may play an important hydrological role. [less ▲]

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See detailKarsts en roches non carbonatées - l'exemple des quartzites et des itabérites de Minas Gerais, Brésil
Willems, Luc ULg; Rodet, Joël; Auler, Augusto et al

in Rodet, Joël (Ed.) Actes des Journées Européennes de l'AFK 2003 (2003)

The state of Minas Gérais, Brazil, contains numerous karsts booth inside carbonated and non carbonated rocks. Différent caves are found in quartzite and Itaberite formations (Banded Iron formation). The ... [more ▼]

The state of Minas Gérais, Brazil, contains numerous karsts booth inside carbonated and non carbonated rocks. Différent caves are found in quartzite and Itaberite formations (Banded Iron formation). The term karst is used in the meaning of "any features of the classical karst morphology (caves, sink-holes, lapies...) where dissolution plays the main genetically action and this in any kind of rocks". In some areas, lapiaz and kamenitza affect wide landscapes (area of Diamantina, Natural Parc of Rio Preto). Several caves are found and some of these are several kilomeetres long (grutta do Bromelias, Natural Parc of Ibitipoca). Inside caves, we found numerous forms like dissolution alveoli, spelothems, .... Many times, the cave sides and solutional forms are covered by crust which solidify a very crumbly quartzite. Différent morphologies show an primary solutional stage of the rocks before a physical erosional stage which destroy the earliest forms (piping, river érosion, collapsing). [less ▲]

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