Palaeocological anatomy of shallow-water Plio-Pleistocene biocalcarenites (Northern Apennines, Italy)

Shell-rich biodetrital carbonate lithosomes punctuate the Plio-Pleistocene marine shallow-water successions of the Mediterranean basins. These m-thick sedimentary packages often display large-scale clinostratified geometry and show a remarkable rhythmic alternation with mudstone lithosomes. Our palaeoecological analysis defines the depositional settings and hints to the factors involved in the development of these peculiar bio-detrital carbonate units. Six distinct biofacies, have been identified through a two-way cluster analysis based on the macrofossil content, mostly molluscs, and matched with sedimentary facies. These are Aequipecten gr. opercularis and Bittium reticulatum biofacies, Anomia ephippium biofacies, Ditrupa arietina and Tritia semistriata biofacies, Corbula gibba and Saccella commutata biofacies, Timoclea ovata and Corbula gibba biofacies, Timoclea ovata and Anomia ephippium biofacies. Quantitative analyses suggest that these bio-detrital deposits have a multiphase history, and that, along with their bracketing marine mudstones, are not nearshore or shoreface deposits but developed in mid to outer shelf settings. Our results suggest that sea-level change was neither the only nor the main factor controlling the internal stacking pattern of individual biocalcarenites. They provide instead arguments in favour of periodic, high-amplitude climatic/oceanographic oscillations determining significant modifications of the source-to-sink dynamics of the basin and affecting the trophic structure, the supply of terrigenous sediments, as well as the energy and pattern of bottom currents of shelfal areas.