Rudist lithosomes related to current pathways in Upper Cretaceous, temperate-type, inner shelves: a case study from the Cilento area, southern Italy

Shallow-water foramol limestones have been studied from a locality in the southern Apennines in which outcrop conditions show an excellent overview of the lateral and vertical evolution of rudist bodies and allow their geometry and the dynamic aspects to be reconstructed. The lithofacies suggest open depositional settings characterized by peloidal silty-muddy sediments. Rudists inhabited well-defined sectors of these shelves, giving rise to wide biostromal bodies, and supplied most of the skeletal debris via bioerosion and minor physical breakdown. In particular, the characteristics of rudist lithosomes document the existence of a complex network of channel-like depressions. In such a depositional context, the evolution of rudist lithosomes seems to have been strongly controlled by the hydrodynamic conditions of the environment. The resulting composite rudist assemblages are characterized by individuals quite often found toppled and oriented parallel to bedding, suggesting continuous sediment movement and/or removal between the organisms. The good preservation of the shells and the common articulation of the valves, however, point to an absence of sustained transport but rather a slight sediment destabilization. The gross lenticular geometry of the shell beds could be related to the above-mentioned patterns of weak, maybe channelized, currents and/or pathways. In most cases, lithosomes may be considered as a multistorey growth in channel-like systems in a persistently subtidal setting. The depositional setting may be considered as a low-energy system with low tidal range, such that facies transition is very gradual and facies belts are broad, while distinct shoal/barrier deposits are absent. Such a depositional system implies a very low bathymetric slope on which wave energy was minimal, dampened by friction along a broad expanse of shallow water hundreds of kilometres wide. The resulting sea bottoms, dominated by biogenic carbonate sedimentation, were separated by troughs and/or tidal passes. In such a depositional context, rudist colonization on channel margins assumes particular importance as it documents the rudist ability to exploit a wide array of environments, comparable to that of oysters in Recent seas, and reflecting the probable opportunistic nature of rudists.