Dados da Submissão

Título

MILLION YEAR SCALE EVOLUTION OF A KARST AQUIFER IN NE BRAZIL: FROM INTERSTRATAL INITIATION TO MODERN FLOW DYNAMICS

Texto do resumo

Most of the central and eastern Brazilian karst areas are formed within Neoproterozoic limestone deposits that were impacted by western Gondwana accretion metamorphism with intensity grades from the margins to the core of cratonic domains. Until the Cenozoic, important neotectonic reactivations of Proterozoic faults resulted in a complex structural arrangement that would later guide dissolution and the development of phreatic karst conduit systems. The onset of modern karstification in this context can be traced back to the arrangement of continental scale watersheds during the Cenozoic, but studies concerning detailed geochronological and geomorphological data to anchor the stages of karst aquifer development in time are scarce in the literature. Here we present an example of a multi-method approach to date cave sediments (cosmogenic 26Al and 10Be, OSL and U-Th-Pb series) from the São Desidério karst area in the São Francisco Craton, where the Neoproterozoic Bambuí Group limestones are overlain by the Cretaceous Urucuia Group sandstones. An average fluvial incision rate of 52.5 ± 13.0 m/My over the limestone rock was estimated by dating fluvial cave terraces at different elevations, while an average 17.1 ± 1.4 m/My erosion rate over the sandstone landscape was determined from 10Be concentrations representative of the cave sediment source area. The period of exposure of the limestone rock to the surface and the beginning of the most prominent karst aquifer development phase was estimated from maximum cave deposit ages and downcutting river rates as approximately 2.7 ± 1.3 to 3.03 ± 0.19 My. Prior to this limestone exposure, infiltration through the sandstone cover probably initiated interstratal karst development along the contact with the limestone rock. The minimum age for this initial stage of the covered karst aquifer evolution was estimated by extrapolating fluvial incision rates to the time necessary to consume a measured thickness of sandstone cover and limestone rock, resulting in a 23.3 ± 5.3 My minimum age for the São Desidério river valley. The novelty in this approach was to bring precise geochronological data to estimate the establishment of the Brazilian Chapadão Central plateau landscape, a regional topographic divide between the São Francisco and Tocantins river watersheds, and to provide a reference for the beginning of the modern karstification process. Paleovalley surfaces are present along the limestone-sandstone contact, and so, paleokarst features may also be preserved, representing an ancient phase of karstification prior to the burial of the Bambuí Group during the Paleozoic. Quantitative groundwater flow studies are still ongoing in the attempt to characterize modern flow patterns, connect enclosed depressions in the assumed recharge area to the major discharge karst springs and estimate maximum depth of flow in the phreatic zone.

Palavras Chave

Karst aquifer evolution; fluvial erosion rates; geochronology; São Desidério; conceptual groundwater flow model.