Temporal dynamics of a trace fossil rocky coast assemblage: Insights into the development of a Middle Miocene transgressive episode (SE Spain)

A study of a Middle Miocene series (lower Langhian–Serravalian) in the El Bellical Basin (Almería) has revealed evidence of an ancient rocky coastline. This conclusion is supported by the discover of bioerosive structures on numerous boulders and pebbles from various conglomeratic beds. Four distinct ichnogenera have been identified: Entobia, Gastrochaenolites, Caulostrepsis, and Maeandropolydora, encompassing a total of eleven ichnospecies. These bioerosive structures were formed by clionaid sponges, lithophagous bivalves, and polychaete or spionid annelids, indicating they belong to the Entobia ichnofacies, typical of shallow, high-energy marine environments. A typical colonisation sequence was identified, beginning with clionaid sponges, followed by lithophagous bivalves, and then annelids, either contemporaneously or shortly thereafter. Additionally, some instances showed encrusting epilithozoan organisms. The conglomerate deposits further suggest the accumulation of large boulders and pebbles at the base of cliffs or as part of extensive, shallow, high-energy littoral environments with low sedimentation rates and abundant suspended nutrients. This evidence points to the existence of an ancient shoreline during the Lower Langhian–Serravalian (Middle Miocene) in this region, linked to the Mediterranean Neogene basins formed by the uplift of the Betic Cordillera and related to the extensional tectonic of this area.