Bioerosion pattern in a nearshore setting as a tool to disentangle multiphase transgressive episodes

Abstract

Palaeoichnology is known to be a powerful tool for the characterisation of unconformity surfaces. In a Neogene post-orogenic basin of the Betic domain (Alicante, SE Spain), a complex Miocene transgressive event on an exposed Eocene succession is recorded. The resulting unconformity surface as well as the overlying beds resulting from the establishment of a nearshore environment are ichnologically and palaeoecologically analysed. Different boring structures related to the Entobia ichnofacies were observed on a variety of substrates, including i) the rocky Eocene substrate, ii) Tortonian conglomerates and breccias, iii) oyster and coral patches (mainly represented by Tarbellastraea). Boring structures are mainly represented by Gastrochaenolites, Entobia and, more rarely, Trypanites and Spirolites? Different colonization patterns were identified, including truncated Gastrochaenolites and Entobia that are extensively developed on the Eocene rockground, small-sized Entobia, and superficial bioerosion on conglomerates and breccias, and Gastrochaenolites hospitium on corals; a selective colonization of G. hospitium was observed, with Tarbellastraea much more intensively bioeroded than Porites. Finally, the integration of stratigraphic, palaeoecological and ichnological data allowed for the identification of three different phases in the Miocene transgression: i) slow transgression with the consequent colonization of the Eocene rockground, the production of a wide planar surface and the truncated Gastrochaenolites-Entobia assemblage; ii) the breaking of this uniform coastal setting into areas characterized by different hydrodynamics, with coral development where turbulence was higher; this second phase was of limited temporal extension, with a limited accommodation space; iii) a final phase characterized by a new transgressive pulse leading to the establishment firstly of a restricted inner platform and secondarily of a slightly deeper sandy platform; in this phase, bioerosion was limited to shells, while soft- and loose-ground burrows as Ophiomorpha developed within the substrate.