Crowded Rosselia ichnofabric in estuarine settings recording early transgressions in lowermost Permian post-glacial Gondwana (Rio Bonito Formation, Paraná Basin, S Brazil)

Ichnology has widely been used in paleoecological, paleoenvironmental, and evolutionary analysis. Rosselia is a dwelling burrow of infaunal, detritus-feeding terebellid polychaetes and is common in Phanerozoic shallow-marine deposits. The crowded Rosselia ichnofabric (CRI, i.e., a dense occurrence of Rosselia) provides itself in this study as a valuable guide to recognizing high-energy and high-frequency shoreface settings but is poorly recorded in estuarine settings. This study aims (1) to interpret the occurrence of Rosselia and the accompanying ichnofauna in the early Permian postglacial estuarine deposits of the Rio Bonito Formation (Paraná Basin) in southern Brazil and (2) to evaluate the potential of the CRI as an ichnomarker of transgressive events in estuarine settings. The study area presents a glacially incised paleovalley (the Capané paleovalley) near Caçapava do Sul town in the south of Brazil and exhibits 8 facies associations that tell the depositional history of the paleovalley-fill sediments. The succession indicates a confined and retrogradational stacking pattern and constitutes two high-frequency depositional sequences. The CRI is recurrent in three distinct intervals of Sequence 1; it occurs in both the sandy to clayey heterolithic facies and the sandstone facies. The CRI indicates relatively high-energy sedimentation that is likely associated with the storm events influencing the paleovalley during a marine transgression. The Rosselia specimens are either vertical or inclined to the bedding plane. The causative tubes vary from 5 to 10 mm in diameter and the spindle diameter ranges from 20 to 90 mm. Most of the specimens are preserved as funnel-shaped burrows; however, spindle-shaped burrows also occur. The density of burrows is high (60–80 per m2), representing a crowded Rosselia ichnofabric (CRI). The CRI reflects the high energy and the high frequency of the storm surges that impacted the Capané paleovalley estuary during the Early Permian. This scenario inhibited the presence of other tracemakers that colonized the surrounding subenvironments of the paleovalley, resulting in the preservation of a monotypic R. socialis assemblage in the storm-related deposits. These results provide new insights about the distribution of CRI in shallow marine environments and reinforce its value as a stratigraphic ichnomarker of high-energy, high-frequency storm deposits also in estuaries impacted by large-scale episodic marine incursions.