Resumen de Modalités et chronologie de la déglaciation würmienne dans la chaîne jurassienne
- français
La dernière glaciation, attribuée au Würm, est responsable de la mise en place du complexe des moraines internes sur le versant N.W. de la chaîne jurassienne. L'homogénéité morphosédimentaire de ce complexe permet de l'attribuer à une seule avancée glaciaire constituée d'une phase maximale suivie d'un retrait assez rapide comportant cinq phases de stabilisation. A partir d'arguments stratigraphiques, morphologiques, archéologiques et d'un calage radiochronologique cet événement glaciaire se situerait entre — 25 000 et - 15 000 ans.
- English
Two morainic groups have been defined in the western slope of the Jura range : an external moraines group ascribed to the Rissian complex (Middle Pleistocene) and an internal moraines group ascribed to the Wurmian complex (Upper Pleistocene). This one appears as a well preserved morainic vallum In accordance with each valley, this vallum presents 5 or 9 terminal moraines distributed throughout a distance of about 10 km. The morpho-sedimentary homogeneity of this vallum allows to ascribe it to a single glacial state with one maximal stage followed by a progressive recession of the glacial front interrupted for several stabilisation phases. Each stabilisation phase builds a terminal moraine. No dating element is available from the only glacial deposits. However we can give a chronological situation of this group from 4 different methods : — Stratigraphical method : it consists in giving the situation of the youngest deposit according the underlying formation and the oldest overlying formation. In the Jura range, the internal moraines group puts down Jurassic formations. Minima estimates for déglaciation come from palynological sequences deposited in lakes immediately after glacial retreat. The oldest deposits are of older Dryas age and although no 14c dates were available they are estimated to be 15ka old. — Morphological method : the morphological freshness of the internal morainic group is evident. Especially, morainic ridges, glaciolacustrine deltas, proglacial channel, have kept their genetic forms. The meaning of this character is double : first, this is a recent glaciation phase and second no cold period with penglacial environment, occurred here after the last glacial stage. — Pedological method : the comparison of the weathering complex over the different morainic deposits shows the same thickness (0,5 m) and the same pedocomplex (forest brown soil). — Indirect chronostratigraphical methods and relations with other events in penglacial area : — The study of the caves fillings has showed that, at a short distance from the front of the internal moraines, it appears a big sedimentary hiatus or evident gully event between 23ka BP and 14ka BP. — In the proximity of the glacial front, several archeological sites are known. Moustenan civilisation is found in about 20 localities in low Jura and Haute-Saône plateaux. In the same area many Madgaleman sites were excavated and the are 14c dated from about 14ka BP. to Ilka BP However we shave no evidence of Upper Paleolithic civilisations (Aungnacien, Pengourdian, Solutrean) between, Mousterian and Magdaleman. — Between the lower bound of the 14c datation possibilities (about 32ka) and lOka BP. the relative abondance of datations in this area is significant. Before 22-23ka BP. and after 16-1 5ka BP., the results from archeological or nonarcheological sites are numerous. However between these two limits, no datation exists. We explain this dating hiatus by glacial and penglacial conditions during this period, without organic processes or sedimentation. Finally, two main ideas emerge from these features : — The internal moraines group was built by a single glacial stage, followed by 5 (or more) phases of retreat, indicated by 5 terminal moraines. — This glacial event is ascribed to the Upper Wùrmian période, between about 25ka and 15ka BP. Key-words : Glacial deposits, Wûrm, déglaciation, chronology, Jura, France.
