Les auteurs proposent trois itinéraires qui conduisent à la montmorillonite : -la transformation à partir des édifices micacés ; -la néo formation dans les feldspaths et -la néo formation extra-cristalline à partir des ions libérés par hydrolyse et concentrés in situ ou dans les bas de pente.
Summary The geochemical development of laterites, a very common superficial formation in the tropics, is still a matter of debate. To determine the main steps of their formation, and to interpret lateral geochemical variations often observed within laterites, we studied the Kaya ferricrete in northern Burkina Faso by analysing two profiles in contrasting topographic positions. We determined the mineralogy and the composition in major and trace elements of whole rocks and of < 0.2 µm granulometric fractions. The nature and proportion of relictual primary minerals and of secondary clays and Fe‐oxyhydroxides control the distribution of major and soluble trace elements. The distribution patterns of Fe, transition metals, lanthanides, U and Th in the two profiles require (i) an initial accumulation in the top ferruginous horizon during its formation and (ii) a secondary redistribution downwards in the underlying horizons. Lanthanides, Ni and Co were remobilized to a much greater extent than Cu and Sc, whereas Fe, V, Cr and Th accumulated in the ferruginous horizon. The uphill better drained profile showed more intense redistribution than the downhill profile. Uranium in particular is poorly redistributed in the downhill profile, whereas it was redistributed like the lanthanides in the uphill profile. Remobilizations are also more intensely recorded in the fine fractions than in the whole rocks. These results allow us to propose a scenario for the formation of the Kaya laterite that accounts for both vertical and lateral chemical distributions. They also highlight the potential of multimethod geochemical studies to uncover the sequence of evolution of weathering profiles.
In the «forêt de Parroy » (Lorraine), there are peculiar soils, which are developed on Keuper-marls. It is difficult to compare these dark-grey and very argillaceous soils, showing a polyhedral structure, with other soil-types already known. A careful study of two profiles points out that they are «lithomorphous vertisols», which have superficially evolued towards a «leached brown soil » type.
Au pied des inselbergs, en pays tropical à saisons contrastées, des études pédologiques détaillées ont permis de rendre compte, non seulement de l'origine des dépressions annulaires, mais également des relations de ces dépressions avec le façonnement contrasté du pied de versant et des glacis alentour. En pays aride, nombre d'Inselbergs présentent des traces ou reliques des mêmes actions pédologiques qui ont façonné leur pied de versant sous des climats plus humides antérieurs.
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