Analysis of bioerosion in clasts from a Miocene rocky-shore, Concepción Formation, Veracruz, México

Study of bioerosion traces in the geologic record of rocky shores is can establish the location of ancient shorelines and aid understanding of the evolution of marine communities in marginal environments. As an example, the ichnologic record associated with cobbles of the conglomerate that crops out along the east side of Neovolcanic Ridge, near Plan del Río village in Veracruz State, south-central Mexico is described and explained. The conglomerate corresponds to the upper part of the Concepción Formation, deposited during the Miocene. The bivalves and gastropods included in the sandy matrix, and the presence of oysters and bryozoans on the surfaces of clasts, allowed interpretation of this conglomerate as an ancient rocky shore. The present work deals with bioerosion recorded in the clasts. Six ichnogenera and ten ichnospecies were determined, corresponding to the activity of polychaetes and/or sipunculids (Caulostrepsis taeniola, Maeandropolydora sulcans and Trypanites weisei), echinoids (Circolites isp.), clionaid sponges (Entobia cateniformis, E. geometrica and E. ovula), and boring bivalves (Gastrochaenolites cluniformis, G. lapidicus and G. torpedo). The relative abundance of boring organisms allows these biota to be placed within the Entobia ichnofacies, which is characteristic of hard substrates with deep bioerosion. The abundance and diversity of Gastrochaenolites and Entobia in the ichnoassociations studied indicate a shallow marine environment with a low sedimentation rate, including episodes of removal or rolling of clasts. This process is inferred from clast roundness and the presence of bioerosion traces on most clast surfaces. On the other hand, the presence of complete and eroded Gastrochaenolites borings in the same clast shows different episodes of bioerosion. Results of this work allow for an understanding of the genetic history of the conglomerate, confirming its deposition in a rocky shore palaeoenvironment and indicating the location of an ancient shoreline on the west margin of the Gulf of Mexico during the Miocene.