Les archives sédimentaires quaternaires de la grotte sous les Sangles (Bas-Bugey, Jura méridional, France). Indices paléo-climatiques et sismo-tectoniques

Résumé En Fr

Quaternary sedimentary archives of the sous les sangles cave (Bas-Bugey, Jura Méridional, France) ; paleo-cumatic and sismo-tectonic evidences. The " Sous les Sangles " Cave is located in southern part of Jura mountain at the front part of the north western alpine tectonically active massifs. This region was covered by alpine and jurassian glaciers during the Last Glacial Maximum. An old gallery contains stratified fluvial and moraine injection, covered by a 3.5 meters thick deposit of finely laminated silty carbonate and clays. Sedimentological investigation reveals several periods of different water circulations depending on glacial and inter glacial periods. The upper finely laminated sediments correspond to the end of the last glacial maximum according to the exokarstic equivalent of the Cerin lake and the U/Th ages obtained with speleothems. Spectral analysis (using Fourier methods and pass-band mapping technics) on grey-level raw data have been used on the "Sous les Sangles" sediment. The main result shows evidence of a cyclic deposition according to the three main periodicities recognised through the 1.5 m top sequence. The laminated material is affected by plastic and brittle deformations. The entire deposit is characterised by (1) a vertical faulting without apparent dominant relative movement which can be interpreted as tension faults ; (2) an associated soft and brittle deformation similar to thin skin tectonic at centimetre scale affecting the base of the deposit and testified to gravity reworking which could correspond to discrete sismotectonic activity ; (3) brittle deformations associated with fluid escape patterns occurring at two specific levels along the vertical faults, emphasising the earth tremor existence according to several broken speleothems. These observations are highly supported by the geodynamic and tectonic frame of this part of Jura massif which reveal an actual uplift of several millimetre/year, especially in this part of the " Cluse des Hôpitaux" cross valley. Numerous historical earthquakes have been documented in this area. The microtectonic study of the cave shows dominant inverse and strike-slip structures correlated to the general tectonic frame.

Située au front des Alpes nord occidentales à la limite entre les parties externe et interne du Jura méridional, la grotte sous les Sangles contient une riche séquence de dépôts détritiques (dépôts glaciaires et fluviatiles, formation laminée et rythmée). L'étude morphologique et sédimentologique de la cavité montre que le fonctionnement hydrologique n'est pas le même en périodes glaciaires et interglaciaires. L'analyse à haute résolution des sédiments souterrains laminés, comparée à un équivalent exokarstique du lac de Cerin, souligne la complémentarité entre les varves glacio-lacustres s.s. et les rythmites endokarstiques. Ces deux formations correspondraient de façon synchrone à la déglaciation pléniglaciaire en accord avec les âges radio-chronologiques obtenus à partir des spéléothèmes. Par ailleurs, l'étude micro-tectonique et morphologique des galeries souterraines et des remplissages rythmés permettent de mettre en évidence une notable activité tectonique dans cette région de l'avant-pays alpin depuis le dernier Pléniglaciaire.