Sclerobiont assemblage of oyster-dominated concentrations from the Tortonian (Upper Miocene) deposits of the Beni Chougrane Mountains, northwestern Algeria

Abstract

Bioerosion traces (endobionts) from the Upper Miocene (Tortonian) deposits exposed along the southern margin of the Lower Chelif Basin, northwestern Algeria, are described and interpreted. Fifteen ichnotaxa, belonging to nine ichnogenera have been identified. These include traces produced by clionid sponges (Entobia), endolithic bivalves (Gastrochaenolites), polychaete annelids (Caulostrepsis, Maeandropolydora, Trypanites), cirriped barnacles (Anellusichnus, Rogerella), carnivorous gastropods (Oichnus), and phoronid worms (Talpina). Epibionts are rare and represented only by balanoid barnacles and juvenile oysters. The ichnoassemblage is assigned to the Entobia ichnofacies, a recurrent association typical of hard substrates such as boulders and oyster shells across a wide range of temporal and spatial settings. Taphonomic attributes and bioerosion patterns indicate that the oysters inhabited a shallow, nearshore, high-energy marine environment characterised by low sedimentation rates. Following death, the shells were likely transported downslope by storm-induced currents to a deeper shelf setting. The borings appear to have formed during multiple bioerosional phases, most of which occurred post-mortem and prior to final burial during the Tortonian. This bioerosional record provides valuable insights into the palaeoenvironmental conditions of the Tortonian Blues Marls Formation in the Lower Chelif Basin.