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São Leopoldo, Brazil

Buatois L.A.,University of Saskatchewan | Netto R.G.,PPGeo UNISINOS | Gabriela Mangano M.,University of Saskatchewan | Carmona N.B.,University of Saskatchewan | Carmona N.B.,National University of Rio Negro

The extensive matgrounds in Carboniferous-Permian open-marine deposits of western Argentina constitute an anachronistic facies, because with the onset of penetrative bioturbation during the early Paleozoic microbial mats essentially disappeared from these settings. Abundant microbially induced sedimentary structures in the Argentinean deposits are coincident with the disappearance of trace and body fossils in the succession and with a landward facies shift indicative of transgressive conditions. Deposits of the Late Carboniferous-Early Permian glacial event are well developed in adjacent basins in eastern Argentina, Brazil, South Africa and Antarctica, but do not occur in the western Andean basins of Argentina. However, the deglaciation phase is indirectly recorded in the studied region by a rapid rise in sea level referred to as the Stephanian-Asselian transgression. We suggest that an unusual release of meltwater during the final deglaciation episode of the Gondwana Ice Age may have dramatically freshened peri-Gondwanan seas, impacting negatively on coastal and shallow-marine benthic faunas. Suppression of bioturbation was therefore conducive to a brief re-appearance of matground-dominated ecosystems, reminiscent of those in the precambrian. Bioturbation is essential for ecosystem performance and plays a major role in ocean and sediment geochemistry. Accordingly, the decimation of the mixed layer during deglaciation in the Gondwana basins may have altered ecosystem functioning and geochemical cycling. © 2013 John Wiley & Sons Ltd. Source

The taphonomic signatures of fossil concentrations preserved preferentially in the fine-grained sandstones of the Paraguaçu Member (Rio Bonito Formation, Lower Permian, Paraná Basin) at Taió, Santa Catarina State (southern Brazil) are analyzed in this paper. The fossil concentrations compound a bivalve mollusk-dominated thaphocenosis (Heteropecten catharinae, Astralomya sinuosa, Myonia costata, Schizodus occidentalis, Stutchiburia brasiliensis, Edimondia sp. e Oriocrassatella itajaiensis), with subordinate gastropods (Warthia sp.), echinoderms (Asteroidea and Ophiuroidea), and brachiopods (Orbiculoidea itajaiensis). Ichnofossils also occur, represented by burrows in the matrix bearing bioclasts (Palaeophycus isp., Planolites isp., Rosselia isp.) and bioerosion (Clionolithes isp., Entobia isp., Oichnus ovalis) in Heteropecten catharinae valves. The taphonomic analysis allowed to recognize two types of biofabrics (matrix-supported and bioclast-supported) and four pavement types (loose or dense, with whole bioclasts of normal-size or very little specimens). The taphonomic signatures revealed that the Taió fossil assemblage is parautochthonus, and that the bioclast accumulations represent shell beds formed by high frequency storm events, being quickly buried in the shoreface-offshore transition zone. The diminutive size of the specimens that compose the shell beds preserved on the basal muddy heterolithic deposits, the low diversity of the associated trace fossils and the dominance of simple, tiny burrows are assumed as a response to environmental stress caused mainly by salinity fluctuations and suggest accumulation in marginal-marine settings. © 2014 by the Sociedade Brasileira de Paleontologia. Source

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