Palaeoenvironmental interpretations based on molluscs from mid-Holocene lacustrine limestones, Mato Grosso do Sul, Brazil

In central Brazil there are continental carbonate rocks in three different geological contexts - the Pleistocene wackestones in the Corumbá city (Xaraiés Formation), the mid-Holocene limestones lenses in the Pantanal Plain (Pantanal do Miranda) and old and modern tufas in the Serra da Bodoquena (Serra da Bodoquena Formation). We present here a systematic and taphonomic study of the molluscs found in the carbonate deposits in the Serra da Bodoquena and the limestones lenses of the Pantanal do Miranda, which provide information concerning the carbonate palaeoenvironments of these two distinct areas. Seven molluscan species were found: the gastropods Marisa sp., Pomacea canaliculata, an unidentified hydrobiid, Idiopyrgus sp., Biomphalaria sp. and Megalobulimus sp., and the bivalve Pisidium sp. The gastropods include species still living in the area today, which supports a Holocene age for the limestones, as indicated by published 14C dates. Overall, the data obtained from the Serra da Bodoquena carbonates suggest a freshwater environment with minimal current-wave action, in an area probably fed by springs, close to a terrestrial environment, in view of the assemblage composed of Pomacea canaliculata, the unidentified hydrobiid, Idiopyrgus sp., Biomphalaria sp. and Megalobulimus sp. The lacustrine palaeoenvironment in the Pantanal do Miranda area was different, as indicated by the presence of Marisa sp. and absence of shallow-water species such as Biomphalaria sp. The gastropods from Serra da Bodoquena and Pantanal do Miranda are predominantly aquatic and amphibious; they are probably younger than the terrestrial gastropods reported from fine-grained limestones in the Corumbá area.