Jean Pierre Lautridou

Gélifraction expérimentale d'arènes de roches cristallines: bilan d'essais de longue durée

Nineteen samples of grus derived from granite and other igneous rocks from the Massif Central (France) were submitted to 1500 freeze–thaw cycles. The thermal regime was a daily alternation from −12°C to +20°C. Important frost shattering was observed on all samples, mainly at the beginning of the experiment. Grain size curves reveal significant increases (20 or 30%) in granules (1 to 5 mm particles) and fine sand (0.05 to 0.08 mm). Three types of response can be identified: (1) Coarse elements are reduced more rapidly than fine sand and silt. An increase in granules (1 to 4 mm) and stability in the sand and loam particles can be observed. Polymineral grains are substantially affected by comminution, but not quartz. (2) There is a translocation of the cumulative grain size curve, and an increase in particles at every size (granule and sand and silt). Quartz is as strongly affected by comminution as polymineral grains. (3) There is a moderate increase in granules and a strong increase in fine sand and silt. Overall, quartz and polymineral grains are at the origin of these different responses which are the result of the tectonic and structural history of the rocks from which the grus is derived. No mineral has a uniform response to freeze–thaw activity, and it is not possible to define a general resistance scale, although quartz is more susceptible than feldspar. Copyright

L'eau dans les loess de Normandie

Les caracteristiques sedimentologiques et geotechniques moyennes des loess sont maintenant bien connues. Mais, en fait, il existe une grande diversite de loess en raison de la variete des facies et de leur situation geomorphologique presente et passee. On denombre plusieurs types de sequences loessiques qui incluent des discontinuites importantes comme les glacis penglaciaires derosion, les paleosols en place ou remanies; ils conditionnent de ce fait des bilans hydriques varies de la couverture loessique. Globalement, le loess constitue un sediment reservoir ou la vitesse dinfiltration de leau de surface est importante, mais qui tolere une percolation generalement tres limitee vers la nappe phreatique.

Données nouvelles sur le Quaternaire de Normandie

De nombreux progres recents ont ete accomplis dans la connaissance des faunes continentales et marines et des formations mannes pleistocenes du littoral (plages perchees). De plus des correlations avec les sequences definies dans les carottes oceaniques (stades isotopiques) ont ete tentees.