Inception and crisis of a Miocene temperate-type carbonate shelf in a compressive setting (northern Apennines)

A detailed study including stratigraphy and composition has been performed on shallow-water carbonates of early-middle Miocene age deposited in a wedge-top basin in the northern Apennines. In the representative outcrop of Torriana (Val Marecchia valley, Romagna Apennines) more than 140 samples were collected in order to identify factors controlling the inception and crisis of the shelf. The succession unconformably rests on the allochthonous Ligurian units and is constituted by up to 100 m thick carbonates and mixed siliciclastic-carbonate rocks. The basal portion consists of rhodalgal rudstones and grainstones dominated by echinoids, bryozoans, coralline algae and benthic foraminifera. It gradually passes into mixed carbonate-siliciclastic shallow water-facies, characterized by an increase in terrigenous components and planktonic and benthic taxa. The occurrence of glaucony-rich packstones with abundant planktonic foraminifera marks the transition to the upper fine-grained sediments. Results of a detailed compositional study evidence four main phases in the platform evolution, and a progressive decreasing of the carbonate productivity, gradually replaced by detrital sedimentation. Regional factors linked to the Apenninic tectonics seem to primarily support the eu- to mesotrophic conditions in the examined carbonate shelf. However this time interval (late Burdigallian - Serravallian) is characterized by the global fertility event, the Monterey Event, recorded also in the Mediterranean. The stable isotope study shows that this global event also influenced the carbonate production. The inception and demise of these temperate rhodalgal-foramol carbonate sediments located in a wedge-top basin is primarily controlled by synsedimentary tectonics related to the thrust migration, through the combined effect of two processes: increasing the subsidence of the basin, and triggering the terrigenous discharge from the erosion of the uplifted Apenninic areas.