U-Th dating of quaternary formations in a seismically active area: an example the French Rhine Valley (Alsace).

C.Innocent, F.Lemeille, M. Cushing, B. Nivière et T. Winter European Geophysical Society XXVII General Assembly - Nice, 21-26 avril 2002.

One approach for the prediction of seismic hazards is to date precisely geologic objects and /or formations that are affected by earthquake-derived faulting. This has been achieved for the Alsace area, where 3 Würmian formations have been investigated for U-Th radiochronology, using the TIMS technique. Gastropod shells have been sampled in the Achenheim site (northern Alsace), and in an Older Würmian terrace from the Sundgau area (southern Alsace). In addition, in this latter area, pebbles covered with calcium carbonate concretions, collected at 3 différent sites (Older Würmian tenace, in which an indurated carbonate crust bas been evidenced, and 2 distinct "Mid"-Würmian terraces, where carbonate-covered pebbles are scattered in the sediment) have been analyzed. U-Th isochron dating bas been carried out on 5 shells from Achenheim , and on 4 shells from the Sundgau area, respectively. All ages are in full agreement with field, stratigraphic observations, but, for both datasets, age discrepancies are evidenced between the 2 U-Th isochron diagrams that have been used. Ages of 83 kyr and 71 kyr have been obtained for thé Sundgau shells, and ages of 68 kyr and 60 kyr have been derived from Achenheim samples. These age discrepancies result probably from posterior, slight U mobilization. The significance and reliability of these ages are discussed. The three carbonate concretions plot more or less on a straight line in the same isochron diagrams, giving ages of 38 kyr and 42 kyr. In this latter case, considering the low number of analyses, and also the fact that these concretions come from différent sites, these two ages are comparable. This may suggest a single carbonate cristallization event in the Sundgau region, during the Würmian ice age (and thus that these concretions could be considered as stratigraphic markers).