Les dépots de la Grotte de Lorette (Rochefort)

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Les dépots de la Grotte de Lorette (Rochefort)

TitreLes dépots de la Grotte de Lorette (Rochefort)
Type de publicationBook Chapter
Year of Publication2011
AuteursQuinif, Y, Kaufmann, O, Sagot, D
Book TitleInternational symposium "Karst research challenges for the XXIst century" - Brussels, 30 September & Rochefort, 1st October 2011. The karst network of the Lomme river, Rochefort region : guide book
Series TitleGeological Survey of Belgium Professional Paper
Volume309
Pagination055 - 063
PublisherInstitut royal des Sciences naturelles de Belgiques, Service géologique de Belgique
VilleRue Jenner 13, 1000 Bruxelles
ISBN Number0378-0902
Résumé

Sedimentary deposits in the Lorette cave (Rochefort)The Lorette cave in Rochefort (Belgium) is part of an underground karstic system associated with the Lomme River. This cave shows a complex geometry with large rooms, galleries, and fallen blocks areas. Sediments of various types are encountered in the cave and their location within the complex cave map is discussed. Fallen blocks are abundant in large collapse rooms such as Salle du Sabbat, Val d’Enfer, Marie Sac‘Attrape. Large blocks are also present in tectonically deformed galleries or passages (e.g. Fontaine-Bagdad gallery and Galerie du Robinet ) where some blocks and walls still show the rounded and smooth surfaces of former galleries. A diamictite composed of sandstone pebbles in a clayey matrix without sorting occurs in the Fontaine-Bagdad gallery. These sediments indicate underground flooding by a debris flow. This event led to the filling of the cavity up to a level well preserved in the Fontaine-Bagdad gallery but also visible from place to place in the passages leading to Val d’Enfer. The West gallery is filled by fine grained detrital river sediments. This fill is sealed by speleothems in several places. A trench showed a stratified sequence that fills a large proportion of the gallery’s section. Finally, in a remote part of the cavity near the large Salle des Siphons room where sumps are present, a circular pit, La Fosse, was formed by local subsidence. Its three meters high walls show various sediments in a complex sequence.From these observations, three main sedimentary units have been identified: A- the diamictite of the Fontaine-Bagdad gallery; B- the stratified sequence of the West gallery and C- the complex sequence of La Fosse. The depositional sequence of these sediments is discussed with respect to cave morphology, sediment granulometry and speleothems U-Th dating. This led to order these units from the most recent to the oldest 1- West gallery; 2- complex sequence of La Fosse; 3- the diamictite of the Fontaine-Bagdad gallery.Even if U/Th dating provides some information, the temporal relations between rock falls and the deposition of these units are still unclear. However, speleothems datings point out that, in the Galerie du Robinet passage, speleothems are older than 350 000 to 400 000 years BP (limit of the method). As these speleothems lie at almost the altitude of the actual Lomme River talweg, one important conclusion of this work is thus that the Lomme river and the Lorette cave have remained at almost the same relative altitude for the last 350 000 to 400 000 years.

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