Rosselia ichnofabrics from the Lower Ordovician of the Alborz Mountains (northern Iran): palaeoecology, palaeobiology and sedimentology

The trace fossil Rosselia with its dominant species Rosselia socialis is found with various densities in both open-marine and delta systems of the Lower Ordovician Lashkerak Formation (Alborz, Iran). The trace fossil shows two types of occurrence in these two depositional systems: solitary Rosselia and crowded Rosselia ichnofabric (CRI). On a proximal-to-distal transect, solitary Rosselia occurs during long-term stable conditions with enriched bottom currents in areas above storm-wave base together with a fair-weather community of dwellers and followed equilibrium strategies (K-selected). In shallower parts, in substrates around and below fair-weather wave base with high amount of deposition and erosion, dense populations of Rosselia (CRI) occur together with subordinate trace-fossil suites (such as Diplocraterion, Skolithos and Arenicolites) produced by deep-tier, suspension-feeding animals due to short-term stable conditions with low food flux. Solitary Rosselia, with lifelong activity due to sophisticated feeding strategies, are well developed in storm deposits, and are part of shallow tiers. Forms with concentric muddy laminae indicate long-term occupation and belong to the Cruziana ichnofacies. In contrast, the behavioural style of the studied CRI, which occurs in unstable, high stress, physically controlled substrates, is comparable to the Skolithos ichnofacies in the occurrence of equilibration traces with stacked segments. In terms of sequence stratigraphy, the transgressive system tract (TST) with lower amounts of organic detritus and low accumulation rates favours the development of solitary Rosselia. In this system tract, CRI is developed during times of ravinement when winnowing and erosion led to the early development of unstable substrates. However, most occurrences of CRI are observed within the progradational packages of the highstand system tract (HST). This study suggests that CRI in event beds can be a good signature of low accommodation space and prograding clinoforms.