Ichnology of Permian marginal-marine to shallow-marine coal-bearing successions: Rio Bonito and Palermo Formations, Parana basin, Brazil

The Lower Permian Rio Bonito and Palermo Formations represent part of the infill of the Paraná Basin, southern Brazil. Integrated analysis of cores, outcrops and well logs from coal fields in Rio Grande do Sul allows sedimentologic, ichnologic, and sequence-stratigraphic characterization of these successions. The Río Bonito Formation has been typically interpreted as fluvio-deltaic. However, the transgressive nature of the succession, the vertical increase in ichnodiversity and bioturbation intensity, and the vertical passage from brackish-water ichnofaunas to fully marine assemblages argue against a prograding delta and suggest deposition in fluvio-estuarine settings. The lowstand fluvial deposits are unbioturbated. Estuarine deposits of the transgressive systems tract comprise tidal channel, point bar, coal-bearing marsh, and estuary mouth deposits. Estuarine ichnofaunas are characterized by simple tiering structures, low degrees of bioturbation, low diversity, and dominance of simple burrows produced by inferred trophic generalists. The top of the Rio Bonito Formation is represented by a shoreface unit consisting of high-energy, storm-dominated, lower to middle shoreface sandstones, laterally grading into moderate-energy shoreface deposits. High-energy shoreface deposits typically lack bioturbation, but deep burrows may be present locally. Moderate-energy shoreface deposits show alternations of opportunistic and climax suites. The transitional interval between the Rio Bonito and Palermo formations displays features indicative of deposition in a restricted, brackish-water lagoon. The Palermo Formation mostly represents transgressive deposition in open marine environments. A wave ravinement surface separates the underlying marginal-marine deposits from the overlying open marine interval. Open marine strata form regional parasequences. Offshore transition to upper and lower offshore deposits are punctuated by transgressive surfaces of erosion, demarcated by suites attributable to the Glossifungites Ichnofacies. Upper offshore to offshore-transition softground trace fossil assemblages are the most diverse. Degrees of bioturbation are high and tiering structures are relatively complex. Lower offshore deposits are highly variable in their degrees of bioturbation. Shelf deposits are unbioturbated, probably reflecting oxygen-depleted conditions, and delineate maximum flooding of the basin. Comparison with other ichnofaunas suggests that Permian brackish-water assemblages are more similar to Mesozoic ichnofaunas than to earlier Paleozoic ones. However, Permian brackish-water deposits are less pervasively bioturbated and contain less diverse trace fossil suites than their Cretaceous equivalen