A new trace fossil assemblage from the Middle Permian Broughton Formation, southern Sydney Basin (southeastern Australia): ichnology and palaeoenvironmental significance

The Permian-Triassic sedimentary succession of the southern Sydney Basin in southeastern Australia contains a wealth of well-preserved trace fossils that are important for systematic ichnological, palaeoecological, and palaeoenvironmental interpretations. In this study, a new ichnofossil assemblage comprising Macaronichnus, Palaeophycus, Psammichnites, Protovirgularia, Rosselia, and Teichichnus is documented from the tide-influenced shoreface deposit of the Middle Permian Jamberoo Sandstone Member of the Broughton Formation. Two distinct ichnofabrics are recognized: i) the Psammichnites ichnofabric characteristic of the upper-middle shoreface deposit; and ii) the crowded Rosselia ichnofabric (CRI) representing the lower shoreface. The Psammichnites-dominated ichnofabric is interpreted to have resulted from opportunistic behaviours of unknown trace makers, whereas the CRI is interpreted to represent possible strategic behaviours of stress-tolerant polychaetes (e.g., terebellid, spionid, and cirratulid polychaetes) in a distal shoreface with high sedimentation rate. Thus, the Psammichnites-crowded Rosselia ichnofabric succession can be considered to represent a response to an environmental shift from proximal to distal shoreface. As such, this study highlights the utility of these two ichnofabrics as stressed environmental indicators.