Depositional and environmental controlling factors on the genesis of Quaternary tufa deposits from Bonito region, Central-West Brazil

Abstract

The Pleistocene and Holocene tufas of the Serra da Bodoquena (Bonito region, Brazilian Midwest) consist of several deposits related to a karstic system developed in the carbonatic rocks of Corumbá Group. Tufas currently forming in the Serra da Bodoquena area (Mimoso and Formoso rivers) were investigated for a better understanding of which environmental changes control the deposition of different types of tufa facies. The Mimoso River displays waterfalls, cascades, barriers and large pools as morphological elements. The topographic breaks on the bedrock of this river control the distribution of these main morphological elements. Five facies were identified: (i) stromatolitic boundstones, characterized by an alternation of dense and porous laminae; (ii) phytohermal boundstones, corresponding to bryophyte cushions; (iii) phytoclastic rudstones, formed by the accumulation of leaves and plant fragments; (iv) oncoidal rudstones, composed by coated grains with concentric laminae (oncoids); and (v) unconsolidated detrital sediment, formed by bioclasts and tufa fragments. SEM images indicated the presence of microorganisms such as Oocardium stratum Nägeli 1849 and Phormidium incrustatum Gomont ex Gomont 1892, as well as diatoms and fungi filaments colonizing tufa samples. On the other hand, the Formoso River is more sinuous, lacking significative topographic breaks and presents flooded sub-environments, such as inactive barriers and other shallow fluvial areas, containing unconsolidated detrital sediment, lime mud and oncoidal rudstones. No microbial remains have been observed. The Bonito region has a humid warm climate with a rainy summer season and a dry winter season, and temperatures barely varying through year. The seasonality of rainy and dry seasons influences both the physicochemical changes of water and, indirectly, the microbial communities of tufa. However, the very different facies displayed by each river, Mimoso and Formoso, are not directly linked to climate. The stepped profile of the Mimoso River allows fast flowing waters producing fast CO2 outgassing that leads to high calcite precipitation rates. The gentle slope of the Formoso River and its slow flowing waters leads to slow passive CO2 outgassing and photosynthesis controlling calcite precipitation at lower rates than in the Mimoso River.