Pliocene sequence stratigraphy, climatic trends and sapropel formation in the Northern Apennines (Italy)

The Pliocene stratigraphy of the Romagna Northern Apennines foredeep has been reconstructed through mapping and measuring the different lithostratigraphic units. The physically correlated sedimentary bodies have been chronologically calibrated through biostratigraphic analyses. Analysis of facies and fossil content allowed reconstructing the main depositional systems and their evolution. The co-occurrence of shallow- to deep-water environments and carbonate to siliciclastic systems allowed understanding fully the role of eustasy and climate vs. continuous tectonic deformation controlling the accommodation and basin morphology. Following a limited sea-level lowering at 4.2 Ma, associated with the first signal of cooling in the Early Pliocene, a fast growing subtropical type carbonate platform developed, in response to warm and oligotrophic surface waters. Furthermore, a significant drop in sea-level at 3.75 Ma points to an important Pliocene continental glaciation in the Northern Hemisphere. During the subsequent transgression, warm and possibly humid climate favoured the deposition of the first Pliocene sapropel around 3.3 Ma. The growth of a second generation of carbonate platform between 3.3 and 3.042 Ma suggests a renewed warm period and limited runoff. At the top of this highstand, thick sapropels developed, which can easily be correlated across the Mediterranean. The platform abruptly drowned, was overwhelmed by bio-erosion and was covered by iron–phosphate crusts; whereas in the adjacent basins numerous sapropel layers occurred, rich of biosiliceous forms, documenting the shift from Ccarb to Corg deposition. Strong oscillations of climate, due to precessionally controlled seasonality, and increased precipitation and runoff are considered as the main causes of the increase of surface water fertility. After the 2.87 Ma sea-level drop, again very significant, the sapropel rapidly disappears, in conjunction to the development of the lowstand wedge. After this cold period, which ended around 2.6 Ma, carbonate platforms never recovered due to the enduring cooler surface waters and the onset of an active drainage on the Apenninic range. This study documents the role of the eustatic variations on the Pliocene deposition of the actively deforming Northern Apennines foredeep. The abrupt lithostratigraphic changes appear also controlled by palaeoproductivity variations associated with climate and runoff.