Cenomanian carbonate depositional settings: a case history from the central-southern Apennines (Italy).

During the late Aptian-late Turonian time interval global changes triggered widespread crisis events that resulted, among others, in world-wide anoxic episodes and drastic variations of biogenic assemblages and lithofacies in the carbonate factories. The related carbonate depositional systems changed as well. In this context, the Cenomanian represents one of the keys to understanding the turnover of facies and biota observed in Cretaceous carbonate platforms. In particular, in this time interval, both the eustatic oscillations and the tectonic events of the periadratic region produced localized emersions and deep physiographic alterations of the Apenninic carbonate platforms. Emersive phases that affected the Periadriatic carbonate platforms resulted in reduced and discontinuous Cenomanian records in central-southern Italy. The Cenomanian strata generally show restricted peritidal deposits and more open lagoon sediments, colonized by radiolitids, nerineids and ostreids, which laterally pass into a large and complex mosaic of mollusc bioclastic shoals, closely associated with small Caprinid mounds. Despite the difficulty of interpreting the spatial relationships in the studied sections pertaining to the Apennine chain, outcrops of Cenomanian rudist-bearing limestones have been selected in the Central-Southern Apennines in order to show differences in the sedimentological/paleontological characterization. Particular attention was given to the evolution and interrelationships of the different depositional areas that were characterized by marginal/exposed conditions vs. areas which suffered restricted circulation. The lithofacies architecture indicates a complex zonation corresponding to transitions from shelf-margin rudist patches and their related debris to semi-restricted areas and tidal flats. Articulated sand shoal-related deposits also occur. These sediments, arranged in a complex way, built up small “sand cays”. A transitional zone was characterised by the intercalation of reworked sands with silty-muddy deposits. From the high energy marginal belt, skeletal sands periodically extended backward into the inner shelf migrating across the colonized areas. Waves, currents and/or storm-related events controlled the dispersion of the skeletal grains along active current pathways and/or tide-related small channel networks. In their evolution, the Cenomanian depositional settings had a strong pre-Cenomanian and syn-(sin)sedimentary tectonic control. Tectonics controlled the relationships between different areas and the creation of the related accommodation space. In addition, the presence of a tectonically-driven topographically irregular bedrock resulted in the co-existence of areas with strongly differentiated hydrodynamic conditions allowing colonization by different organic communities and complex distribution patterns of the skeletal debris.