Arid versus wet climatic evidence in the “middle Cretaceous” calcareous successions of the Southern Apennines (Italy)

Abstract

An upper Albian–lower Turonian shallow-water carbonate succession cropping out near the village of Monteforte Cilento (Campania Apennines, southern Italy) was analyzed in detail within the framework of studies showing contrasting climatic evidence from the “middle” Cretaceous peri-Mediterranean carbonate successions. The full succession covers a time span in which there was a major global transgression, superimposed by fluctuating sea levels, coupled with a significant increase in temperature. There is also evidence of repeated climatic shifts from humid to dry with seasonal contrasting conditions. Restricted peritidal facies dominate the section and demonstrate the general characteristics associated with restricted brackish/schizohaline up to hypersaline depositional settings in which mesotrophic to mildly eutrophic conditions largely prevail. However, an opening and/or deepening trend of the depositional areas is recorded upward in the succession by deeply bioturbated plankton-rich strata at the Cenomanian–Turonian transition. This trend stopped with the inception of new shallow-water depositional settings which radiolitid rudists repopulated or from which shallow-water skeletal remains were supplied. This transient drowning event was anticipated and then marked by the occurrence of sediments bearing low-oxygen tolerant planktonic assemblages adapted to mesotrophic conditions. This suggests ecological conditions that were unfavourable to the main carbonate-producing shallow-water assemblages at the Cenomanian/Turonian boundary. Conditions of stress in the reconstructed shallow-water settings correlate with the deterioration of the water mass that culminated in deep-water domains with the well-known OAE2 anoxic crisis. The uppermost Albian–lower Cenomanian interval is characterized by intertidal to supratidal, partially dolomitized limestone with abundant silica pseudomorphs after evaporites. The silicified evaporite nodules and layers are evidence of episodes of highly evaporitic conditions in very shallow subtidal to supratidal settings in which salty to hypersaline ponds and areas of sabkha occurred. The occurrence of xerophytic megafloral remains also suggests that an arid, evaporite-promoting climate obtained in the depositional areas of the Albian–Cenomanian succession studied. Within the framework of the climatic evolution of “middle” Cretaceous time, the evaporite cycles of the Monteforte Cilento section are of particular interest. The different peri-Mediterranean carbonate successions record widespread karst phenomena and bauxite, which are expressions of hot/humid climates. Accurate biostratigraphic analyses and published geochemical data constrain the evaporitic episodes recorded in the Monteforte Cilento section. The related arid/semiarid climatic conditions fall within a time interval in which different coeval Apennine successions show shallow-water deposits (from stromatolitic/loferitic peritidal cycles of restricted inner shelf areas to more open, subtidal deposits of an open shelf) sandwiched between two tectonically uplifted surfaces marked by humid climate-related mature soils (bauxite) and/or karst phenomena. This suggests that dry climatic episodes post-dated the hot/wet intervals in which the bauxite and karst systems developed but, in turn, predated more recent hot/wet climatic conditions that resulted in the karst systems which occur at the top of the upper Cenomanian limestone.