Coralgal buildups associated with the Bolca Fossil-Lagerstätten: new evidence from the Ypresian of Monte Postale (NE Italy)

The Monte Postale and Pesciara di Bolca sites are well-known Eocene Fossil-Lagerstätten. The origin of these deposits has been previously related to a lagoon environment, protected from the open sea by a barrier formed by some kind of reef. However, no detailed description of a bioconstructed system has ever been reported in the area. This study provides the first detailed characterization of the upper Ypresian Monte Postale reef, made of in situ corals, calcareous algae, and encrusting foraminifera. Three main facies associations have been recognized: (1) a coralgal rim, roughly E/W oriented, (2) lagoon deposits in the south and (3) fore-reef sediments towards the north. The coralgal rim is represented by a wave-resistant framework made of heavily calcified organisms associated to Alveolina accumulations; the adaptation of most of these organisms to the euphotic zone promoted their growth in proximity to the sea level, allowing this structure to act as an effective barrier. In particular, during relative sea-level lowstands or highstands, the coralgal rim could have limited the water exchange between the lagoon and the open sea, reducing the oxygenation in the lagoon and allowing the formation of the Fossil-Lagerstätten. During relative sea-level transgressions, the water could have risen well above the margin, establishing normal oxygenation and hampering the fossils’ preservation. The Monte Postale succession represents one of the few examples of lower Eocene bioconstructions. The description of this biotic assemblage provides new data for the characterization of reef communities during a crucial climatic phase, near the end of the early Eocene climatic optimum global warming.