Bioclastic deposits in the NW Gulf of Naples (Southern Tyrrhenian sea, Italy): a focus on new sedimentological and stratigraphic data around the Island of Ischia

Bioclastic deposits in the Gulf of Naples have been studied and compared based on new sedimentological and stratigraphic data, particularly referring to the rhodolith layers. They represent detrital facies deriving mainly from in situ rearrangement processes of organogenic material on rocky sea bottoms. These deposits are composed of medium-coarse-grained sands and bioclastic gravels in a scarce pelitic matrix and crop out at the sea bottom in a portion of the inner shelf located at water depths between - 20 m and - 50 m. Below water depths of - 30 m the bioclastic deposits are rhodolith, characterized by gravels and lithoclastic sands. Rhodolith deposits are often found near the Posidonia oceanica meadows and/or in protected areas near the rocky outcrops.The Ischia Bank represents an excellent natural laboratory for studying the rhodolith layers. On the Ischia Bank, below the Posidonia oceanica meadow, both bioclastic sands immersed in a muddy matrix and volcaniclastic gravels were sampled. Both the Mollusk shells and the volcaniclastic fragments, where the contribution of the silty and sandy fractions is lower than 20%, were colonized by some species of red algae, while in the marine areas with a low gradient a maërl facies was deposited.