The experiments reported here compare intensity and thermal resistance of two thermal remanent magnetizations (TRM), successively acquired by the same sample under a magnetic field, but at two different cooling speeds. The samples (one with a base of CrO2 powder, and the others derived from oceanic basalts) were representative of the different types of structure domains characteristic of magnetic grains. The experimental procedure established aimed at limiting the various factors of uncertainty: mineralogical and structural modifications related to heating; inhomogeneity of the sample temperature at crucial stages of each experiment; effects of undesirable magnetizations acquired concurrently with the TRM. The main conclusion of this study is that the TRM intensity increases as a function of time of cooling, independently of the types of structure domains carried by the magnetic grains. The greatest effect is with the CrO2 monodomain: the TRM obtained between 130 and 100 °C during a cooling period of 120 h was 19% superior to the one obtained during a passive cooling period of 30 min for the same temperature limits, the measurements having been taken at 50 °C. Both types of oceanic samples analyzed (pseudo-monodomains and multidomains) showed a similar relative variation, but that much weaker. For example, the TRM induced on cooling from 200 to 50 °C (120 h) provided values only 6% higher than the one obtained during passive cooling (periods of 30–40 min). The theoretical implications of these results are discussed. [Journal Translation]

Magnetostratigraphic studies of Pliocene-Pleistocene continental deposits on the Plateau de Perrier and in the Le Puy-en-Velay basin (Massif Central), as well as in the Riez-Valensole basin (Alpes de Haute Provence, Mediterranean South), provide decisive reference dating of several well-known fossil-bearing Villafranchian sites. These are well distributed in the tephrostratigraphy on both sides of the Gauss/Matuyama geomagnetic boundary: the Roca-Neyra fauna falls within the Matuyama chron, while older sediments at Les Etouaires are situated just above the normal-reversed transition. The Vialette section lies below this reversal, within the Gauss chron. In the Riez-Valensole basin, the discovery of the Gauss/Matuyama boundary toward the top of the Puimoisson section leads to an excellent correlation with the Massif Central localities. Rodents of the Seynes zone occur below the reversal, while Equus appears just above. This calibration improves the Mediterranean upper Neogene biochronology and allows a direct correlation with Villafranchian vertebrate sites of the Massif Central.

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Bulletin de l'Association française pour l'étude du quaternaire Nouvelles données chronostratigraphiques concernant les dépôts lacustres de Vialette (Haute-Loire) D. Biquand, C. Cassignol, A. Chambaudet, J. Couthures Résumé Cherchant à préciser la chronologie du dépôt lacustre de Vialette, gisement fossilifère bien connu du Pliocène du Velay, nous avons mis en oeuvre trois méthodes indépendantes dont nous comparons les résultats :-la méthode utilisant le système radioactif potassium-argon, appliquée à la coulée basaltique sous-jacente au dépôt, -la méthode « traces de fission », appliquée au dépôt lui-même (âge obtenu : 3,14 ±0,60 MA), -la méthode paléomagnétique, également appliquée au dépôt (polarité positive

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