Abstract
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.
Highlights
Rhodolith or maërl deposits consist of either alive or dead aggregations of coralline algae, which blanket wide coastal zones in the present-day oceans [1–3] and represent shared facies in carbonate platform settings
The Ischia Bank is characterized by an active carbonate factory dominated by coralline algae, which have colonized an area where suitable environmental conditions have been established for the deposition of native and living rhodalgal deposits
In the Ischia offshore the investigated deposits were found in a similar bathymetric range, these deposits have shown how in different geomorphological and hydrological environment the coralline algae facies have different structures
Summary
Rhodolith or maërl deposits consist of either alive or dead aggregations of coralline algae, which blanket wide coastal zones in the present-day oceans [1–3] and represent shared facies in carbonate platform settings. The rhodoliths are the main components of the rhodalgal skeletal assemblage that characterizes the carbonate production in the oligophotic zone of Cenozoic and modern carbonate platforms [4, 6–10]. The zonation of benthic assemblages of the Mediterranean sea performed by Peres and Picard [11] has improved the knowledge on lithology and facies interpretation of rhodolith layers (Figure 1). In the Mediterranean sea the bioclastic deposits occur at water depths ranging between – 40 m and – 100 m (“Détritique Cotier” of Peres and Picard; Figure 1) [11]. The main components of the “Détritique Cotier” are composed of the reworking and deposition of benthic communities on both mobile sea bottoms (biocoenosis of the “Détritique Cotier”) and on hard sea bottoms (biocoenosis of the “Détritique Du Large”), more than the assemblages of Posidonia oceanica meadows and maërl deposits. As a consequence of the Holocene sea level rise the deep seafloor was covered by relict and drowned sediments (“Détritique Du Large”; Figure 1), characterized by low rates of sedimentation and by the occurrence of glauconite
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