A deposição de tufas quaternárias no estado de Mato Grosso do Sul: proposta de definição da formação Serra da Bodoquena

Abstract

Quaternary tufa deposits occur widely in the Serra da Bodoquena (state of Mato Grosso do Sul, Brazil), associated with the karst system that has developed on carbonate rocks of the Corumba Group. The deposition of tufa is favored in the Serra da Bodoquena due to the predominance of autogenic waters, which allow for groundwater to be enriched in calcium carbonate. By means of numerous springs, this groundwater supplies the rivers of the area where tufas are deposited. These rivers have small quantities of siliciclastic sediment, making the water very clear, which favors biological activity and, as a result, the precipitation of carbonate. The lithological characteristics, the existence of clear and abrupt lines of contact and the easy mapping of the tufas make it possible to create a new unit, here named the Serra da Bodoquena Formation. Deposition in a distinct river basin and karst system justify separation from the Xaraies Formation, even if their lithologies are partly similar. The Serra da Bodoquena Formation is discontinuous due to the nature of deposits, formed in association with the surface drainage network. The individual outcropping areas never interconnected with each other or with the Xaraies Formation in the Corumba region. The new unit was divided into two members in order to represent catchment and waterfall deposits and micrite deposits named, respectively, Rio Formoso and Fazenda Sao Geraldo members. The older tufa deposits indicate greater deposition from 6530 cal years BP until 2700 cal years BP, when the decline began reaching the present fate. This more intensive deposition represents a period of more humid climate, which changed at around 2700 cal years BP when similar conditions to those found at present became established. The active deposition of tufas in the Rio Formoso Member is still significant, although limited to the courses of perennial or intermittent drainage channels.