Trace fossils on and above the transgressive surface: substrate consistency and phosphogenesis (Lower Ordovician, St. Petersburg region, Russia)

The basal layers of the Leetse Formation (Lower Ordovician, St Petersburg region, Russia) are characterized ichnologically by: 1 – local occurrence of the ichnogenera Conichnus, Bergaueria and ?Thalassinoides at the base. These traces originated by burrowing in consolidated substrate of the underlying black, clayey Dictyonema Shale and were buried by the basal bed of the Leetse Formation; 2 – the layer with camerate burrows (Amphorichnus div. isp.), filled with phosphatic substrates, ca 15 cm above the base. Considering the position of the localities of the phosphatized burrows in the basin, we presume that burrows on tops of tectonically uplifted blocks were filled by cryptocrystalline phosphorite matter. Rapid phosphate mineralization points to conditions of shallow burial, high concentration of dissolved pore water phosphate, and suitable redox interval caused by decomposition of organic matter of burrow dweller tissue.