WEATHERING AND THE FORMATION OF HILLSLOPE DEPOSITS IN THE TROPICAL HIGHLANDS OF ITATIAIA - SOUTHEASTERN BRAZIL

Deposits of cobbles and boulders found hanging on midslopes of the Ita tiaia Plateau reflect tectonic reactivation and a large range of clima te-controlled processes, probably including frost weathering. These pr ocesses, more efficient in detaching debris from freefaces, were repla ced, at the end of the Pleistocene, by colluviation, On the lower slop es of the plateau, less coarse C-I and C-II colluvia evince the rework ing of upslope materials freed in former phases of weathering or erosi on, Interpretation of hillslope processes was based upon the sedimento logical, mineralogical and micromorphological characteristics of collu via. C-I may have originated in mass movements reaching deep into the regolith. C-II colluvia were probably deposited by shallow processes, which reworked initially highly evolved materials, including on the wa y downslope poorly weathered rock fragments. Changes in weathering cha racteristics and hillslope processes suggest variations in environment al conditions of the Itatiaia Plateau since the Late Pleistocene. Coar ser deposits would be associated to the wet and cold conditions of the Last Glacial Maximum, In the end of the Pleistocene, warmer and still humid conditions would have favored weathering and mass movement proc esses triggering the first generation of colluvia (C-I). Discontinuous stone-lines found on mid-slopes would represent a short return to col der and eventually drier conditions, In the last 8,000 years, prevaili ng humid climates, with temperature excursions larger than those obser ved at present, encompassing phases of more intense gelifraction, woul d explain the deposition of C-II sequences, However, such variations w ere not sufficient to interrupt the geochemical weathering trend towar d kaolinitic and gibbsitic materials that persisted after the Last Gla cial Maximum.