MACROSCOPIC AND MICROSCOPIC ANALYSIS OF SEDIMENTARY STRUCTURES: A BASIS FOR THE PALEOHYDROLOGIC INTERPRETATION OF FOOT SLOPE DEPOSITS

Preliminary results based on macroscopic and microscopic analysis of quaternary sedimentary structures and paleosoils preserved on foot slope deposits are presented. Together with radiocarbon dates for the main pedo-stratigraphic units, this work put forward an evolutionary pattern for geomorphic surfaces situated at the highlands of the States of Paraná and Santa Catarina, in Southern Brazil. The association of that pattern with global environmental changes is postulated.

Based on sedimentologic and geomorphologic evidences, paleohydrologic interpretations of valley heads suggest the predominance of relative wet local environments at the end of the Pleistocene. Indeed, radiocarbon ages estimated for paleosoils in these areas suggest the development of organic and hydromorphic soils during the coldest periods of isotopic stages 3 and 2. As peat horizons need anaerobic conditions to fully develop, we have to reconsider the predominant role attributed to lower precipitation as a cause of geomorphic and vegetation changes during the Quaternary in the tropics and subtropics. Indeed, together with lower precipitation, lower temperatures and, most important, an excess of precipitation over evaporation should be considered as the same importance, at least for highland tropical areas.

In so far, the existence of waterlogged soils, even during the Last Glacial Maximum, well explains the pattern of evolution of geomorphic surfaces and related pedo-stratigraphic features in the area. Indeed, taking into account the calibrated isotopic curves from Antarctic ice cores for the last 40 ka, we conclude that, at the studied foot slopes, soil development and colluvial building by diffuse processes prevailed during Stadials, while pulses of erosive incision punctuated Insterstadials.