Trace fossils and ichnofabrics across the Volkhov depositional sequence (Ordovician, Arenigian of St. Petersburg Region, Russia)

Ichnofabric (bioturbation and bioerosion) patterns show a conspicuous symmetry across the Volkhov depositional sequence, which suggests a close relationship between the inchnofabric and the sea-level change. Bioclastic limestones interpreted as calcareous tempestites show evidence of relatively deep bioturbation (ca. 15 cm) of high intensity. Ichnogenera Thalassinoides, Chondrites, Paleophycus,? Dolopichnus, Trypanites, Gastrochaenolites and Planolites participate for the most part in the overall substrate disturbance. Intensive deep bioturbation and even bioerosion occurs in the studied area even below the recently re-interpreted Early/Middle Ordovician boundary. This observation suggests that diverse strategies and deep bioturbation, as well as other modern features (e.g., complex tiering, re-visiting of filled burrows), first appeared during the Early Ordovician in this region.