SEDIMENTARY PROCESSES AND BIOFACIES OF LATE CRETACEOUS LOW-ENERGY CARBONATE RAMP SYSTEMS (SOUTHERN ITALY)

This paper characterizes a low-energy, open-shelf setting by focusing on the sedimentary processes and biofacies of a Late Cretaceous, temperate-type open shelf in the southern Apennine nappes of Italy. The facies evolution of the related rudist-rich limestones takes into account the macrofossil characteristics and the microfaunal assemblages.The open-shelf depositional setting is interpreted by facies analysis. The facies are gradually transitional and the facies belts are broad, but wave-resistant biogenic frameworks are absent. Presumably the sea floor was characterized by a low bathymetric slope on which the wave energy was dissipated across a wide shelf, resulting in low-energy environments. Storm waves were damped by the wide, shallow bottom, and the fine fraction was not winnowed. Many rudists are in growth position, and those reworked are well preserved and commonly articulated. Different bathymetries and hydrodynamic conditions in two study areas, the Sorrento Peninsula and the northern Cilento, resulted in different biofacies and stratal geometries. In the Cilento area, high-energy currents such as tidal currents created a network of small channel-like depressions. Different rudist species colonized these channels and their margins. Rudists in growth position are rare, although reworked bivalves are commonly conjoined and the shells are pristine. In this subtidal facies key foraminifer species vary in abundance. The presence of marine open-shelf agglutinate forms supports the interpretation of a marine, high-energy shelf.In the Sorrento area, ephemeral subaerial exposure surfaces and pedogenic deposits indicate intertidal conditions and rudist facies indicate subtidal sandy shelf. The rudists formed monospecific to paucispecific assemblages. The foraminiferal assemblages are much more diversified than those in the Cilento successions, indicating small but meaningful oscillations in sub-environments.The rudist shell beds exhibit great taphonomic variability. The foraminiferal assemblages are characterized by distinct taxa such as abundant conical and discoidal agglutinate forms (Cuneolina and Dicyclina), thick-shelled evoluted Soritidae (Murgella, Scandonea, and Pseudorhapydionina), and larger, thicker ovoidal or subspherical Rotaliidae that indicate well-lit depositional environments with open water circulation and normal salinity, characterized by medium values of the hydrodynamic energy with scarce or no evidence of traction currents.