Fósseis em micritos quaternários da Serra da Bodoquena, Bonito-MS e sua aplicação em estudos paleoambientais

Abstract

Bonito town and surrounding areas in Mato Grosso do Sul state are tourist attractions related with many quaternary carbonate deposits which form waterfalls, dams and render almost zero turbidity waters. That area is part of the National Park of Bodoquena Range and Pantanal Biosphere Reserve (UNESCO). Calcareous tufa are porous rocks formed by calcium carbonate laid down in freshwaters containg a great variety of fossils. They were presented in form of waterfalls, dams and lacustrine sediments, forming unconsolidated micritic deposits, being part of the Xaraés Formation. Micritic deposits are extensive and thick in whole area. Most of the deposits are weathered, partly eroded and distributed in restricted areas. The present study focuses on paleontological, granulometric, and geochemical (major, minor and trace elements, carbon and oxygen isotopes) data. The micritic deposits of Calcário Xaraés Mine in Bonito (MS) are detailed studied. Samples were obtained by auger drilling, also in outcrops area and one orientated centimetric indeformed block for taphomic study. The research was complemented with field study and sampling in the Bodoquena Range, Pantanal and Corumbá. Calcário Xaraés Mine' micrites are lacustrine deposits. We described through three events: 1: their base does not contain fossils, but bears oncoids, clayminerals. Isotope data point to a wetter period, 2: ostracods, charophytes and gastropods fossils colonized this episode and homogeneity of isotope data suggest a stable event, 3: freshwater gastropods endured scarce water conditions so colonized the area. The isotope data then was varied suggesting alternated periods of evaporation and humidity. Total water lake evaporation resulted in the mass death of the Biomphalaria gastropod and consequent with diffusion of Idiopyrgus gastropod. Absence of sedimentary structures, bioclastic orientation, selection and fragmentation, weak to dispersal packing bioclast and bioclastic alteration features, are indications of long durantion in the water-sediment interface. These features suggest environmental static conditions during the final deposition. These features also suggest groundwater supplies to the lake, leading to less bioclastic alteration, and maintaing chemical water conditions, such as homogeneity of geochemical data. Other studied micritic deposits show almost the same results. There is strong presence of gastropod Biomphalaria and Hydrobiidae Family on these deposits. Field data show calcareous tufa deposits with an extension larger than the studied area, occurring mainly next to the rivers, which dissolve micritic in the rainy period. Bodoquena Range calcareous tufa deposits, mainly micritic sediments, probably were formed in the last 10.000 years, indicating a hotter and drier period than nowadays. The last 2.700 years in this region can be characterized by humidity increase, micritic deposits extinctions and installation of waterfalls and dams tufa depositions.

Microfossils in micrites from Serra da Bodoquena (MS), Brazil: taxonomy and paleoenvironmental implications

“…This part of the section represents a significant period of stability, which was mainly related to the water quality and level, allowing the development of a more complex community. The genus Biomphalaria (gastropod) is found from 2.08 to the top, which is in accordance with the interpretation of shallow water condition for this interval (Utida et al 2007, Utida 2009). …”

“…Ostracods started to decline and only a few carapace fragments can be observed there ( Figure 5). In addition to that, the genus Biomphalaria (macrogastropod), which indicates shallow waters, as well as Pomacea and Idiopyrgus, developed in this interval, with a higher concentration on the top (Utida et al 2007, Utida, 2009). These macrogastropods were probably responsible for the decline of ostracods and Acrobis (Figure 5).…”

“…They are easily found next to river channels due to fluvial erosion, or in areas where there is economic exploitation (Oliveira 2009, Sallun Filho et al 2009a, Utida 2009.…”

Continental freshwater carbonate coated grains: oncoids in Quaternary deposits of the Serra da Bodoquena region, Central-West Brazil

“…Subordinately, the oncoidal floatstones and rudstones were observed in alluvial terraces of the Formoso River. Oncoidal facies were described in a riverine environment at the Formoso River, cemented in a muddy matrix (Oste et al 2021), and in the Fazenda São Geraldo's oncoids underlying unconsolidated micrites (paludal tufas environment) (Utida 2009. We observed oncoids underlying the paludal tufas (massive mudstone Facies), separated by an abrupt and irregular contact.…”

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

“…Segundo estes autores, o gênero Pomacea habita lagos, lagoas e áreas inundadas em geral, o mesmo ocorre com a Biomphalaria, mas exclusivamente em áreas de pouca correnteza. Neste depósito também ocorrem ostracodes, girogonites de algas caráceas e microgastrópodes, caracterizando um ambiente lacustre (Utida et al, 2008).…”