Rudist-bearing rhodalgal facies in the post-Turonian recovery of peri-Tethyan carbonate systems: a case history from the Nurra region (northwestern Sardinia, Italy)

The presence of non-geniculate coralline red algae and bryozoans (rhodalgal lithofacies) in association with rudists has been mentioned only rarely in the literature. Nevertheless, because of the significance of rhodalgal facies in the characterization of shallow-water carbonate factories, a correct interpretation of the related ecological factors may improve the palaeo-environmental reconstruction of some rudist-bearing carbonate depositional systems. Uppermost Coniacian-Santonian rhodolith-rich rudist-bearing carbonate successions in the Nurra region (northwestern Sardinia, Italy) record several discrete depositional settings, from autochthonous shallow-water mobile skeletal deposits including coralline algal fragments and rhodoliths, to re-mobilized deposits rich in skeletal components with rhodoliths. The rudist-bearing rhodalgal limestone studied is part of an uppermost Turonian-Campanian transgressive sequence that covers a tectonically-modelled Lower Cretaceous substrate. The recovery of the Upper Cretaceous carbonate factories followed an interval of time during which the Lower Cretaceous carbonate systems, dominated by chlorozoan assemblages and non-skeletal grains, had experienced "Mid-Cretaceous" worldwide crises presumed to have been caused by global climatic/oceanographic perturbations.In particular, Early-Middle Turonian times, characterized by the hyper-greenhouse conditions then prevailing, witnessed a significant reduction or even complete demise of highly productive carbonate factories. In carbonate settings, biotic assemblages grew in mesotrophic/eutrophic environments. Cyanobacterial consortia, with variable contributions from rudists, largely prevailed in shallow-water domains. 'Thus far, the latest Turonian-Coniacian recovery of Sardinian carbonate factories with "impoverished chlorozoan assemblages" might be considered as an indication of ameliorated environmental conditions. However, ecological constraints did nor allow the tropical "chlorozoan assemblages" to thrive in the Late Cretaceous low-latitudinal carbonate shelves of Sardinia. A Santonian shift toward foramol/rhodalgal depositional systems occurred with sciaphile- (shadow preferring), and mesotrophic-adapted ("rudist-bearing rhodalgal") assemblages flourishing in the new shallow-water domains. In Santonian times relatively cool and mesotrophic to eutrophic conditions are presumed to have become dominant in the water mass impinging on the marginal sectors of the shelf or distal ramp of the Nurra carbonate system. Deterioration in the quality of water presumably caused the demise of large sectors of the Nurra carbonate factory, which underwent local drowning episodes controlled by tectonic activity