Geochronoloy of sedimentary deposits in caves from São Desiderio region: implications for speleogenesis and geomorphological evolution of West Bahia State

Abstract

This proposal aims the geochronology and stratigraphy of detritic sediments from caves of the São Desidério municipality, west of Bahia state, in order to get burial ages from quartz grains using cosmogenic nuclide dating techniques. The ages of burial and erosional phases of clastic sediments, registered inside the João Rodrigues and Sopradeira - Manoel Lopes cave systems, in the São Desidério watershed, will be correlated with events of widening and morphological modification of cave conduits (speleogenesis), as well as higher and lower precipitation levels in the region and consequent variations of hydraulic recharge to the cave systems. Minimum ages of the initiation and enlargement processes of the subterranean drainage systems are intended to be obtained, as well as incision rates from subterranean fluvial canyons, through the dating of fluvial deposits, preserved in different levels of those passages. The results will also contribute to the study of the effects of climatic events in the Quaternary (e.g. millennium Younger Drias events, Heinrich and the last glacial maximum, LGM) over the dynamics of the karst system. Those discharge and water level fluctuations in subterranean rivers will be compared and integrated with paleo-pluviosity variations, indicated by records already obtained from ´18O variations in carbonatic speleothems in the region. The U/Th dating of calcitic deposits interbeded with part of the clastic record might bring ages from low flow periods in the rivers in consequence of less pluvial phases. The occurrence of fluvial sands with ages that extends 600 ka (actual limit for the U/Th method) have been proved in the studied caves, allowing the use of radiometric cosmogenic isotope ages obtained from quartz sands to quantify cave river incision processes in the region of the João Rodrigues and Sopradeira - Manoel Lopes systems, bringing new data for uplift rates in cratonic areas of Brazil during the Quaternary and contributing to quantitative studies in recent geomorphology and tectonics. This proposal represents the continuity of the paleo environmental studies of the Quaternary and karst system dynamics, initiated through the FAPESP research supports, procs. 99/10351-6, 06/02834-2, 06/06761-0 and 2010/20560-2, developed by integrants of the present research team. (AU)