A soil sequence (Oxisol → Ultisol → Alfisol) was studied on the border of a plateau, which is the oldest regional surface near the Piracicaba river valley (southeastern Brazil). Seven pits were identified, sampled and morphologically described under the different segments of the sequence (end of summit, shoulder, backslope and footslope). A bi-dimensional sketch of the soil horizons was drawn. Undeformed soil samples were taken for micromorphological studies, and mineralogical, chemical and granulometric analyses were performed. The very clayey Oxisols of the flat summit are developed on a neo-cenozoic cover and the Ultisols resulted from a lateral pedologic Latosol transformation gradually receiving at the end of the backslope a contribution from fresh siltites (Tatuí formation, Permian). The genesis of the top Oxisol microaggregates is complex, but two processes became evident: the geochemical microstructure ant the zoogenetics. The Bw → Bt transition was attributed to the subsurface densification of the microaggregates due to the levelling and convexity of the original surface and to the lateral water flow's growing influence downstream. Further mechanisms of argilluviation and clay destruction of the Bt horizon top by temporary hydromorphism will complete the transformation of Bw into Bt and of the latter into E, respectively.
microaggregates; natural aggregate densification; argillic horizon degradation; soil geomorphology; soil micromorphology