Miocene phosphatic hardgrounds of the Mediterranean and their biotic assemblage: new insights from the "Aturia level" of the Salento Peninsula (S. Italy)

From the late Oligocene to the late Miocene, the central Mediterranean area was interested by an extensive deposition of phosphate-rich sediments. They are usually represented by 10-20 cm thick, sub-horizontal hardgrounds, made of phosphatic sediments arranged into thin layers separated by erosion surfaces. Macrofossils are very abundant, their assemblage composed of colonial and solitary corals, molluscs, echinoderms, brachiopods, bryozoans and fish teeth. In the past decades, the Mediterranean phosphatic deposits have been the subject of several studies, mainly aimed to understand their depositional processes and the mechanisms leading to phosphogenesis. In particular, the formation of these sediments has been recently interpreted as associated to the occurrence of a complex system of upwelling currents, flowing into the central Mediterranean from the deeper, eastern region of the basin. However, most of the contributions mainly relies on stratigraphic, sedimentary and geochemical features, whereas the macrofossil assemblage, despite its abundance and diversity, has never been analyzed in detail. The main goal of this study is thus a thorough investigation, mainly from a palaeontological/palaeoecological perspective, of the phosphatic deposits of the Salento Peninsula (named “Aturia” level). Special attention is given to the coral and mollusc associations, particularly sensitive to changes in the main environmental stressors and representing the most abundant biotic components of the phosphatic deposits. The Salento Aturia level is up to 25 cm in thickness and made of several phosphatic layers separated by erosion surfaces. Microstratigraphic and microfacies analyses allow to identify two distinct facies. A coral rudstone, about 7-10 cm thick, constitutes the base of the hardground, while on the top lies a detrital rudstone, made of the succession of thinner layers mainly composed of phosphatic fragments. Most of the macrofossils are concentrated within the coral rudstone and particularly abundant are corals, associated to molluscs and to a lesser amount to brachiopods, echinoderms, bryozoans, serpulids and fish teeth. The coral assemblage is composed at least by 16 taxa belonging to 4 families, represented in order of