Paleomagnetism applied to lateritic profiles to assess saprolite and duricrust formation processes: the example of Mont Baduel profile (French Guiana)

Based on the fact that a chemical remanent magnetization (CRM) can be acquired during the lateritization, the paleomagnetic signal was used to interpret the succession of horizons, from the weathering front to the top of the duricrust surface of a thick bauxitic profile in French Guiana. The 54-m-thick profile is composed of a saprolite capped by a duricrust comprising successively a mottled zone, a massive facies and a pisolitic facies at the top. At the base of the saprolite the natural remanent magnetization (NRM) is composed of a superimposed magnetite and hematite contribution, the magnetite showing an apparently inherited Proterozoic direction. Within the profile, the main magnetic carrier is hematite; goethite does not provide any significant contribution. Five magnetic reversals have been identified within the saprolite and through comparing the reversal sequence with the reference geomagnetic polarity time scale (GPTS) it has been possible to deduce the saprolitization rate; thus the Bruhnes/Matuyama (780 ka) transition is observed at 8.8 m above the weathering front, giving a 11.3±0.5 m/Ma saprolitization rate. The duricrust is significantly divided into two levels within the massive facies; the base of the duricrust shows paleomagnetic directions that fit with the present-day situation, whereas the upper part records directions of the South American apparent pole wander path (APWP) older than 10 Ma. The study shows that hematite precipitation was not uniform within the duricrust during lateritization, and seems to preserve traces of successive and distinct lateritization phases; thus the lower massive duricrust formed during a recent lateritization period, whereas the upper duricrust has preserved a paleomagnetic signal of an older lateritization phase. Our duricrust paleomagnetic interpretation is supported by SEM observations of changes in hematite morphology.