Mise en évidence d'une erreur de fossilisation du champ magnétique terrestre dans un dépôt actuel

We present a study of the remanent magnetization of the so-called "ocres d'Apt" clayey sediment, resulting from an artificial sifting carried out industrially for obtaining pigments. Our investigation concerned 172 specimens of several hundred of cubic centimetres, corresponding to a sampling distributed over the whole area of four basins (approximately 600 m2) and 10 m along the axis of a channel. We show that the main carrier of the magnetization is hematite and that the primary magnetization is a detrital one. An extremely moderate thermal treatment (<88 °C) erases the intense viscous magnetization acquired since the sampling (10 years) but preserves a weak component of a primary magnetization, a differential analysis of which (comparison between sites), as well as laboratory drying experiments, shows that it was acquired before the complete drying out of the clay and that its direction is not modified, either by currents that existed during some phases of the sedimentation, or by the final drying out. The intrasite mean direction of the primary magnetization is approximately identical for three of the basins and for the channel, with a zero difference in declination with, but with an inclination that is 16 °shallower than, that of the Earth's magnetic field. As for the fourth basin, the declination coincides with, but the inclination is 13 °higher than, that of the field.