Quaternary colluvial-eluvial covers of the Eastern Parana Basin, Southeastern Brazil

Colluvial-eluvial sandy-clayey deposits are widespread Quaternary covers in the interior of the State of Sao Paulo, Southeastern Brazil, and neighboring areas in the Parana Basin. These sediments were formerly designated as the Santa Rita do Passa Quatro Formation (Qsr). The formation is thin (5-20 m thick) and contains about 20% detrital kaolinitic clay enveloping rounded quartz grains, producing a porous and fragile framework that causes hazardous geomorphic processes such as surface erosion, piping and collapse phenomena. Charcoal of Holocene age is common in the colluvial-eluvial covers. Its distribution suggests colluvial and eluvial/illuvial processes, together with biological turnover of the regolith. The presence of sandstones of the Parana Basin as source rocks and the geomorphological setting control the occurrence of the colluvial-eluvial covers. These are common over sandy substrata, and are absent only upon the youngest among five planation surfaces recognized in the area, suggesting, that there was not enough time for accumulating the covers. The colluvial-eluvial covers are regarded as the product of long-term surface accumulation of weathering materials that began at the end of Tertiary. However, the formative processes are still active nowadays. They comprise weathering of unstable minerals, eluviation/illuviation and leaching, authigenesis of clay minerals, concentration of quartz, water erosion, colluviation of regolith, and biological turnover of weathered friable materials. (C) 2003 Elsevier Science Ltd and INQUA. All rights reserved.