A bed of Middle Miocene (Serravallian) lagoonal facies with well-developed patch reefs is described from a section at the Siwa Oasis, northern Western Desert of Egypt. It is well-exposed in the middle Siwa Escarpment Member of the Marmarica Formation and displays remarkable bioerosion structures that show abundant ichnofossils. Nine ichnotaxa, belonging to four ichnogenera, were identified: two correspond to the clionaid sponge boring Entobia (E. laquea and E. ovula), five to the bivalve boring Gastrochaenolites (G. lapidicus, G. torpedo, G. cluniformis, G. hospitium and G. cf. orbicularis) and two to the annelid-worm boring Maeandropolydora (M. sulcans) and Trypanites (T. weisei). In addition, traces of the boring polychaete worm Caulostrepsis and the boring acrothoracican barnacle Rogerella were recorded. These ichnoassemblages have been assigned to the Entobia ichnofacies. The organisms bored into a hard, fully lithified carbonate substrate in a low-energy, shallow-marine environment. The ichnotaxa associations indicate water depths of a few metres (<10 m) and a very low sedimentation rate in a lagoonal setting during a Serravallian regressive cycle.
