Karst et fantômisation dans la dolomie de la Valle Imagna (Préalpes de Bergame, Italie)

Ghost-rock karstification in the dolomites of Valle Imagna (Lombardy). In the Lombardian Alps, the "Dolomia Principale" (DP) is a carbonated dolomitised shelf showing many faciès characterized by porosity differences. In the more porous fades, such as the conglomeratic dolomites of shelf slopes, pinnacles are common and can occupy large surfaces along the slopes. There are in dense groups or isolated. These features are the result of deep ghost-rock weathering of the dolomite, as it is seen in quarries with buried pinnacles in deep alterite pockets. In Valle Imagna, the entire slope is within DP, from an altitude of 1000 m down to 500 m, and has pinnacles. Stratigraphically, DP was overlain by shales which can favour the ghost-rock weathering. Without shales, the thickness of the weathered part can reach several ten meters. One can see the deep roots of the ghost-rock because of roadcuts in several altitudes. Numerous blind cavities, several ten meters deep, result from piping of ghost-rocks. Very large and long caves open in the slopes at various altitudes, independant of evolution of the baselevel. Narrow and deep channel-like canyons cut the slopes, but they have neither catchment are nor water. The Valle Imagna existed during the Middle and Upper Miocene with a different morphology. It has been deepened during the messinian, and the sea invaded the valley during the Lower Pliocene. There was no glacier during the Quaternary period. The ghost-rock weathering dates at least back to the Miocene, and a first stage of emptying of the alterite is of Messianian age. During the rhexistasy, corresponding to the glaciations, the emptying of the alterite was almost complete, and large caves substituted the alteritic sediments.Carsismo e fantomizzazione nella dolomia della Valle Imagna (bergamo, Italia). La Dolomia Principale è uno dei litotipi più diffusi delle prealpi Lombarde ed è co stituita da una piattaforma carbonatica dolomitizzata caratterizzata da numerose fades a diversa porosità. Nelle fades più porose, come le dolomie conglomeratiche del pendio della piattaforma, è molto comune il rinvenimento di pinnacoli che possono coprire a m pie superfici dei versanti con una discreta densità oppure essere isolati. Queste forme derivano da una profonda fantomizzazione della dolomia come si è potuto osservare in cava oppure perché, tra i pinnacoli ancora semisepolti, si osservano profonde tasche di alterazione ancora in posto. In Valle Imagna tutti i versanti in Dolomia Principale da 1 000 m sino a 500 m di quota sono coperti da pinnacoli. Sulla Dolomia Principale erano presenti argilliti che possono aver favorito la fantomizzazione. Non tenendo in conto le argilliti, lo spessore delle alteriti doveva essere di qualche decina di metri. Le parti più profonde dei fantôme, lungo le fratture, sono ben visibili a quote variabili in corrispondenza delle strade più recenti. Sono presenti anche numerose cavità a cul di sacco, profonde al massimo qualche decina di metri, dovute alio svuotamento dei fantôme. Grotte sorgenti di grandi dimension i e sviluppo sono presenti sui versanti, a quote diverse completamente indipendenti dall'evoluzione del livello di base. Alio stesso modo corridoi stretti e profondi, simili a canyon, incidono il versante, ma sono privi di bacino di alimentazione e nessun corso d'acqua ha mai percorso il versante. La Valle Imagna era già présente, anche se con una différente morfologia, durante il Miocene medio-superiore ; è stata sovraescavata durante il Messiniano e, durante il Pliocene inferiore, il mare è entrato profondamente nella valle. Durante il Quaternario la valle non è mai stata occupata da ghiacciai. La fantomizzazione è quindi, almeno, di età miocenica, mentre una prima fase di asportazione delle alteriti con svuotamento dei fantôme è di età messiniana. Durante le fasi di resistasia in corrispondenza delle glaciazioni l'asportazione delle alteriti è stata quasi compléta e grandi cavità hanno sostituito i fantôme.Dans les Préalpes lombardes, la «Dolomia Principale » (DP) est une plate-forme carbonatée dolomitisée montrant de nombreux faciès caractérisés par des différences de porosité. Les faciès plus poreux, comme les dolomies conglomératiques de pente de plate-forme, se caractérisent par la présence en surface de nombreux pinacles qui peuvent occuper de larges secteurs sur les versants, en groupe dense ou isolés. Ces formes dérivent d'une profonde fantômisation de la dolomie comme on peut le constater en carrière où, parmi les pinacles encore ensevelis, on trouve en place des poches très profondes d'altérites. En Valle Imagna, la totalité du versant en DP, depuis 1 000 m jusqu'à 500 m d'altitude, est couvert de pinacles. Stratigraphiquement, la DP était recouverte par des argilites qui peuvent avoir favorisé la fantômisation. Sans compter les argilites, l'épaisseur des altérites pouvait être de quelques dizaines de mètres. Les parties plus profondes des fantômes, le long des fractures, sont bien visibles à différentes altitudes là où des routes récentes les ont recoupées. On trouve aussi de nombreuses cavités en cul-de-sac profondes de quelques dizaines de mètres au maximum dues à la vidange des fantômes. Des grottes de grandes dimensions et de développement appréciable s'ouvrent sur le versant, à des altitudes variables complètement indépendantes de l'évolution du niveau de base. De même, des couloirs étroits et profonds sous forme de canyons sillonnent les versants, alors que le bassin versant n'existe pas et qu'aucun cours d'eau n'a jamais parcouru le versant. La Valle Imagna était déjà présente, même si sa morphologie était différente, pendant le Miocène moyen et supérieur. Elle a été surcreusée pendant le Messinien et, durant le Pliocène inférieur, la mer a envahi la vallée lors de l'ingression post-messinienne. Durant le Quaternaire, la Valle Imagna a toujours été libre de glacier. La fantômisation est donc, au moins, d'âge miocène, tandis qu'une première exportation des altérites avec vidange des fantômes est d'âge messinien. Pendant les phases de rhexistasie que sont les glaciations, l'exportation des altérites a été presque complète et des grandes cavités se sont substituées à l'altérite résiduelle des fantômes.Bini Alfredo, Zuccoli Luisa, Quinif Yves. Karst et fantômisation dans la dolomie de la Valle Imagna (Préalpes de Bergame, Italie). In: Karstologia : revue de karstologie et de spéléologie physique, n°60, 2e semestre 2012. Dynamique sédimentaire dans le siphon de Chevaline. pp. 1-10

Rapport entre karst et glaciers durant les glaciations dans les vallées préalpines du Sud des Alpes

Karst and glaciations in the Southern pre-alpine valleys. At least 13 glaciations occurred during the last 2.6 Ma in the Southern prealpine valleys. The glaciers scouring alpine and pre-alpine valleys had all the same feature, being valley temperated glaciers. Their tracks and feeding areas were always the same, just like the penological contents of their deposits. Contrary to previous assumptions until a few years ago, the origin of these valleys and of the lakes occupying the floor of some of them (Orta, Maggiore, Como, lseo, Garda Lakes ) is due to fluvial erosion related to Messinian marine regression. The valley slopes modelling is Messinian in age, too, while most caves are older. As a general rule, glaciers worked on valley slopes just as a remodelling agent, while their effects were greater on valley floors. The karstic evolution began as soon as the area was lifted above sea level (upper Oligocene -lower Miocene), in a palaeogeographical environment quite different from the present one, although the main valley floors were already working as a base level. During Messinian age, the excavation of deep canyons along pre-existing valleys caused a dramatic lowering of the base level, followed by a complete re-arrangement of the karstic networks, which got deeper and deeper. The Pliocene marine transgression caused a new re-arrangement, the karst network getting mostly drowned under sea level. During these periods, the climate was hot-wet tropical, characterized by a great amount of water circulating during the wet season. At the same time, tectonic upliftings were at work, causing breaking up of the karst networks and a continuous rearrangement of the underground drainage system. In any case, karstic networks were already well developed long before the beginning of Plio-Quaternary glaciations. During glaciations, karst systems in pre-alpine valleys could have been submitted to different drainage conditions, being : a) isolated, without any glacial water flowing ; b) flooded, connected to the glacier water-filled zone ; c) active, scoured by a stream sinking at glacier sides or in a sub-glacial position. The stream could flow to the flooded zone (b), or scour all the unflooded system long down to the resurgence zone, the latter being generally located in a sub-glacier position. The glacier/karst system is a very dynamic one : it could get active, flooded or isolated depending on endo-and sub-glacial drainage variations. Furthermore, glaciers show different influences on karstic networks, thus working with a different effect during their advance, fluctuations, covering and recession phases. Many authors believe, or believed, the development of most surface and underground karst in the Alps is due to glaciations, with the last one held to be mostly responsible for this. Whatever the role of glaciers on karstic systems, in pre-alpine valleys caves, we do not have evidence either of development of new caves or of remarkable changes in their features during glaciations. It is of course possible some pits or galleries could have developed during Plio-Quaternary glaciations, but as a general rule glaciers do not seem to have affected karstic systems in the Southern pre-alpine valleys with any remarkable speleogenetic effects : the glaciers effects on them is generally restricted to the transport of great amounts of debris and sediments into caves. The spotting of boulders and pebbles trapped between roof stalactites shows that several phases of in-and out-filling of galleries occurred with no remarkable changing in pre-dating features , including cave decorations. The presence of suspended karst systems does not prove a glacial origin of the valleys, since most of them pre-date any Plio-Quaternary glaciation, as shown by calcite cave deposits older than 1,5 Ma. The sediments driven into caves might have caused a partial or total occlusion of most galleries, with a remarkable re-arrangement of the underground drainage system. In caves submitted to periglacial conditions all glaciations long, we can find deposits coming from weathered surface sediments, sharp-edged gelifractbn debris and, more rarely, alluvial deposits whose origin is not related to the circulation of the glacial meltwater. In caves lower than or close to the glaciers limit we generally find large amounts of glacier-related deposits, often partly or totally occluding cave galleries. These sediments may be directly related to glaciers, i.e. carried into caves by glacial meltwaters, resulting from surface glacial deposit erosion. They generally show 3 dominant fades : A) lacustrine deposits B) alluvial deposits and C) debris flow deposits fades. The only way of testing the soundness of the forementioned hypothesis is to study the main characters and spreading of cave sediments, since they are the only real data on connection of glaciers to endokarst networks.Bini Alfredo, Tognini Paola, Zuccoli Luisa. Rapport entre karst et glaciers durant les glaciations dans les vallées préalpines du Sud des Alpes. In: Karstologia : revue de karstologie et de spéléologie physique, n°32, 2e semestre 1998. Grottes ornées de Bornéo (E Kalimantan) et Grotte de Blue Lagoon (Afrique du Sud) pp. 7-26

Dynamique sedimentaire et paleoenvironnements durant la transition Weichselien - Holocene a partir des depots endokarstiques de la Grotte de Han-sur-Lesse (Belgique)

The cave sediments in the galleries near the entrance of the Han-sur-Lesse cave constitute a sedimentary record of the palaeoenvironmental evolution during the end of the Pleistocene, tardi-glacial transition and the Holocene. Lithostratigraphic study and granulometric analysis of three sections (the section of the 'Galeries des Potirons', the section of the 'Cave a Vin', the section of the 'Galerie des Petites Fontaines') enable to reconstruct the hydrological evolution of those galleries at the end of the last glacial period. This reconstruction is dated by uranium-series disequilibrium datings on speleothems and 14C datings on charcoals.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Un karst sous la glace de l'Antarctide ?

Are there karstic landforms under Antarctic icecap ? A new bedrock map of the Dome C area based on all radar data collected during Italian Antarctic Expeditions in 1995, 1997, 1999 and 2001 is presented. The map can clearly distinguish the Dome C plateau, along with some valleys and ridges develop. The plateau develops at three different altimetric levels and its morphology is characterised by hills and closed depressions. There are no visible features which can be ascribed to glacial erosion or deposition. The major valley is 15 km wide and 500 m deep ; its axis is parallel to that of other valleys and ridges in the plateau. The valley bottom is not flat, but contains a saddle in its centre. The morphology of the major valley could be considered as a relict one which was not modified by the overlying ice cap. Two big ridges, characterised by hills, saddles and depressions, lie near the boundaries of the area. The hill and depression landscape may be the result of two different processes : the weathering of granitic rocks, with the development of a "demi-oranges" and inselberg landscape, or the karstification of limestones, and development of a cone karst. The karstic hypothesis should be the more suitable, but it is impossible to exclude the granitic rock wheathering. Both proposed genetic hypotheses call for a warm, humid climate and a long period of stability in a continental environment. Consequently, the ice cap did not largely modify the landscape.L'article présente une nouvelle carte de la topographie sous-glaciaire dans le secteur de Dôme C (Antarctique) levée sur la base de données radar collectées par des chercheurs italiens et français lors des campagnes menées en 1995, 1997, 1999, 2001. Sur la carte, il est possible de distinguer nettement la surface du Dôme C parcourue par des vallées et des alignements de reliefs. Le plateau se développe à trois niveaux altitudinaux, et sa morphologie présente de nombreuses collines et dépressions fermées. Les formes visibles ne peuvent être mises en relation avec des mécanismes d'érosion ou de dépôts glaciaires. La vallée principale est large de 15 kilomètres et profonde de 500 mètres. Son axe est parallèle à celui des autres chaînes et vallées qui sillonnent le plateau. Le fond de cette vallée n'est pas plat mais s'articule de part et d'autre d'un col. Sa morphologie laisse penser qu'il s'agit d'une relique qui a été peu retouchée par la calotte glaciaire sus-jacente. L'aire étudiée est bordée par deux chaînes importantes, caractérisées par la présence de collines, d'ensellements et de dépressions. Le "paysage" de collines et de dépressions peut résulter de deux types de processus : soit de l'altération de roches granitiques, avec le développement de demi-oranges et d'inselbergs, soit de la karstification de calcaires aboutissant à un karst à cônes. L'hypothèse karstique paraît la plus vraisemblable, même si l'on ne peut exclure l'autre hypothèse. Toutes deux supposent, pour aboutir à de pareilles formes, un climat chaud et humide et une longue période de stabilité dans un environnement continental. Après quoi la calotte glaciaire n'aurait finalement que peu modifié le "paysage".Bini Alfredo, Forieri A., Rémy F., Tabacco I.E., Zirizotti A., Zuccoli Luisa. Un karst sous la glace de l'Antarctide ?. In: Karstologia : revue de karstologie et de spéléologie physique, n°42, 2e semestre 2003. Les cavités glaciaires sous le regard des spéléologues. pp. 45-49