Evolution of iron crust and clayey Ferralsol in deeply weathered sandstones of Marilia Formation (Western Minas Gerais State, Brazil)

Extensive flat plateaus are typical landforms in the cratonic compartment of tropical regions. Paleo-climate, pediplanation, laterization, and dissection have created complex and distinct geological, geomorphological, and pedological features in these landscapes. In the Brazilian territory, the flat plateau sculpted in sandstone of Marilia Formation (Neocretaceous) belonging to the Sul-Americana surface presents a very clayey and pisolitic Ferralsol (Red and Yellow Latossolo in the Brazilian soil classification). The clayey texture of soil and the pisolites have been considered as weathering products of a Cenozoic detritical formation which is believed to overlay the Marilia Formation sandstones. Using data of petrography (optical microscopy and SEM), mineralogy (RXD), and macroscopic structures (description in the field of the arrangement of horizons and layers), a complete profile of Ferralsol with ferricrete and pisolites was studied. The complex succession of facies is in conformity with a sedimentary structure of Serra da Galga member (uppermost member of Marilia Formation). The hardening hematite concentration appears as layered accretions in the subparallel clayey lenses of sandstone saprolite, preserving its structure. Iron contents varied according to different soil fabrics. Higher concentrations of iron are found in the massive ferricrete or in pisolites in the mottled horizon. Kaolinite is a dominant clay mineral and shows two micro-organizations: (1) massive fabric intrinsic to the sedimentary rock, and (2) reworked in pisolites and illuviated features. The pisolites are relicts of ferricrete in the soft bioturbated topsoil. The continuous sequence of ferricrete from saprolite to the Ferralsol indicates that the regolith is autoch thonous, developed directly from sandstones of Marilia Formation, through a long and intense process of laterization. (C) 2017 Elsevier Ltd. All rights reserved. (AU)