Geochemistry and palinology of calc tufas of the bonito region (Ms): Environmental implications

Abstract

Tufas are the product of calcium carbonate precipitation related to fresh water at ambient temperature, containing micro and macrophyte remains, invertebrates and bacteria. The tufas from Bonito region, Brazil, occur mostly along rivers and creeks in a karstic system of the Corumba Group. These deposits are included in the Serra da Bodoquena Formation. This work aims: 1) understand the physical and chemical conditions of the fluid; 2) identify the depositional environment; 3) correlate isotopic and palynological data to paleoclimatic studies. Petrographic descriptions, XRD, XRF and stable isotopes analyzes were carried out, besides palynological studies. The samples were systematically collected in an outcrop at Mimosa site, Rio Formoso Member. Three lithofacies were defined at Bonito tufas: I) phytoherm, formed by the accumulation of leaves, branch fragments and bryophytes cushions; ii) shrubs composed by radii-fibers structures; and, iii) stromatolites, laminated forms. All the facies were correlated to pools, barriers and cascade or waterfalls, as the depositional context. The negative trend of the isotopic data revealed a meteoric fluid and biological influence on the precipitation. The presence of fungi and hyphae, pteridophytes spores and angiosperms pollen indicate a humid ambient with large vegetation, typical of tropical conditions.