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Formation of an Oxisol-Ultisol Transition in São Paulo, Brazil: II. Lateral Dynamics of Chemical Weathering¹

ABSTRACT

Depending on the mineralogical nature of the parent material and underlying rock, Fe, Mn, Si, and bases are concentrated downslope as a result of lateral- and/or base-water flow action. Dissolution at the rock-saprolite interface is an important supplier of those elements, which are transported by the base-water flow affecting mainly the soils in the footslope.

Chemical weathering does not always proceed in the same direction. The alternate process of desilication-resilication occurs horizontally downslope along the toposequence. This approximately corresponds to Walther's Law of Succession of Facies. The lateral resilication process is important in tropical regions because it counterbalances the allitization process and explains the occurrence of relatively less weathered clay minerals on lower slope positions in the soilscape.

¹ Paper no. 8239 of the Journal Series of the North Carolina Research Service, Raleigh, NC 27650.

² Soil Scientist, Instituto Agronômico, Campinas, Brazil, and Professors, North Carolina State University, respectively.

Received for publication March 16, 1982. Accepted for publication July 7, 1982.