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Un âge crétacé inférieur probable pour les paléoaltérations latéritiques du graben de Saint-Maixent-l'École (seuil du Poitou) d'après l'étude isotopique (Rb-Sr, δ18O-δD, U-Th) de pisolites ferrugineux
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Un âge crétacé inférieur probable pour les paléoaltérations latéritiques du graben de Saint-Maixent-l'École (seuil du Poitou) d'après l'étude isotopique (Rb-Sr, δ18O-δD, U-Th) de pisolites ferrugineux
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Un âge crétacé inférieur probable pour les paléoaltérations latéritiques du graben de Saint-Maixent-l'École (seuil du Poitou) d'après l'étude isotopique (Rb-Sr, δ18O-δD, U-Th) de pisolites ferrugineux
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Journal Article
Un âge crétacé inférieur probable pour les paléoaltérations latéritiques du graben de Saint-Maixent-l'École (seuil du Poitou) d'après l'étude isotopique (Rb-Sr, δ18O-δD, U-Th) de pisolites ferrugineux
2019
Overview
It has been previously established that Western Europe underwent an important episode of lateritic weathering during early Cretaceous times. In this context, the red clays and ferrugineous pisoids deposits occurring in the graben of Saint-Maixent-l'Ecole (Poitou saddle) may represent remnants of lateritic formations associated with this paleoweathering episode. However, their exact age is unknown, and since they occur on top of the Oligocene limestones filling the graben, it cannot be ruled out that these lateritic soils formed more recently. In order to better constrain the age of these lateritic formations, goethite pisoids were sampled in the Fouilloux forest and investigated for Rb-Sr, U-Th and δO-δD isotopic systems. Rb and Sr have been measured on bulk pisoids, and also on leachates and residues from leaching experiments carried out in the attempt to isolate a radiogenic component. No reliable Rb-Sr isochron age could be derived, although the isotopic data would favor an early Cretaceous age for these lateritic formations. δ18O-δD measurements have been done on bulk goethite pisoids, and also on sub-fragments. The results indicate that the pisoids likely formed under a warm and/or arid climate. No argument for a late of post-Oligocene age can be found from the oxygen and hydrogen isotope data. Finally, U-Th analyses have been carried on bulk samples of goethites, and on sub-fragments as well. 234U/238U activity ratios are typically slightly higher than the equilibrium value, which indicates that goethites have recently evolved as open systems (if not still open). Hence, pedogenesis processes are still active from a geochemical point of view in these lateritic soils. This study has not allowed to derive a reliable age for these lateritic soils. However, when the results from the different isotopic systems investigated here are considered together, the best interpretation is that these lateritic soils developed most probably over the Kimmeridgian limestones along the edge of the Saint-Maixent-l'Ecole graben, and were reworked and redeposited at the bottom of the graben later on.
