Storm-related taphofacies in estuarine settings: An integrated analysis on the Early Permian deposits of the Rio Bonito Formation (Parana Basin, S Brazil)

摘要

In the Gondwanan Paraná Basin, two fossil assemblages with affinity to the Eurydesma Fauna are well known in the Early Permian post-glacial deposits of the Rio Bonito Formation. Although the assemblages display similar invertebrate fossil content, the fossil concentrations were potentially accumulated in different paleo-bathymetric zones represented by the shallow marine and marginal marine settings. This paper provides a taphonomic analysis of the body-fossil concentration and the ichnological signatures of the fossiliferous deposits of the Rio Bonito Formation at the southern border of the Paraná Basin and also discusses their paleo-ecological and paleo-environmental significance. Three distinct taphofacies representing the fair-weather deposits, storm-wave deposits in the inner part of estuary, and time-averaged and storm-influenced deposits in the outer part of the estuary were recognized in the studied succession. They characterize the depositional processes that filled a shallow, marginal marine paleovalley formed during the Late Paleozoic Ice Age. The taphonomic signatures of the fossil concentrations suggest deposition under the transgressive and highstand phases of the paleo-valley infill. These new data reinforce the previous interpretation of the paleo-valley infill as estuarine depositional setting and further contribute to refining the stratigraphic framework of the post-glacial deposits at the southern margin of the Paraná Basin.