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Entobia is a well-known ichnofacies typical for nearshore environments, differentiated from the Gnatichnus ichnofacies as being related to more stable conditions in high-energy settings and longer times of exposure of the bottoms. Surroundings of the city of Alicante (Villafranqueza section, SE Spain), boring structures related to the Entobia ichnofacies were observed on an Eocene rockground colonized after a Miocene marine ingression. Reefrelated facies are present as coral colonies developed after the transgression and show evidence of bioerosive activity. The relationship existing between the growing coral colony and boring organisms is addressed in order to elucidate ecological interactions and palaeoenvironmental conditions. The studied section is characterized by a very evident angular unconformity dividing the underlying Eocene sandy turbiditic beds from the overlying Miocene coastal limestones. The Entobia ichnofacies is developed on a variety of substrates, represented by the Eocene rockground, resedimented lithoclasts, different bioclasts (mainly thick shelled bivalves and gastropods) and coral bioconstructions. Boring structures are mainly represented by Gastrochaenolites, Entobia (usually preserved as internal mould) and, more rarely, Trypanites. Differences in the relative abundance of the three ichnotaxa were observed according to the substrate: i) on the rockground, Gastrochaenolites and Trypanites are the most abundant ichnotaxa, being Gastrochaenolites represented by different ichnospecies; ii) the three ichnotaxa are present in litho- and bio-clasts; iii) in corals, almost exclusively Gastrochaenolites torpedo is found. On the rockground, deep penetrating borings as Gastrochaenolites are differently preserved, being present complete forms together with partially eroded ones on the same surface. This would suggest that colonization occurred over long times, due to prolonged exposure of the bottoms. Together with possible ecological restrictions, a preservation bias induced by such a prolonged exposure must be considered in order to justify the relative abundance of the deep structures typical for the Entobia ichnofacies with respect to the shallow-tiered ones. Where Gastrochaenolites is associated to corals, bivalves were found inside the bioerosion structures being mainly represented by Lithophaga sp.. In this sense, a bioerosive activity specialism is detected, since Gastrochaenolites is extremely more abundant in the bioconstructions produced by the coral Tarbellastrea than in those where other types of corals are present (e.g. Porites and Siderastraea), thus suggesting some kind of ecological interplay. Moreover, the study of the geometrical and morphological relationship between Tarbellastrea and Gastrochaenolites shows that, in part of the observed specimens, the coral was growing all around the lithophagid borehole. This would suggest that corals and bivalves developed at the same time and that the shelly colonization and the progress of the host patch reef did not occur in different successive phases.

Cite as: Giannetti, A., Falces-Delgado, S., and Baeza-Carratalá, J. F.  (2019): Interaction of boring organisms with corals. Entobia ichnofacies development in a transgressive nearshore scenario. 34th IAS International Meeting of Sedimentology, Rome 10-13 September 2019. “Sedimentology to face societal challenges on risk, resources and record of the past.  Sesion 8.A-660.

The Villafranqueza unconformity in the logo of the Alicante University Departament of Earth Science and Environment.