The upper Tortonian-lower Messinian at Monte dei Corvi (Northern Apennines, Italy): Completing a Mediterranean reference section for the Tortonian Stage

摘要

A high-resolution integrated stratigraphy including biostratigraphy, magnetostratigraphy, tephrostratigraphy and cyclostratigraphy is presented for the upper Tortonian and lower Messinian (Upper Miocene) at Monte dei Corvi. Numerical age control comes from a combination of magnetobiostratigrapic dating and astronomical tuning, while radioisotope dating of ash layers produced less reliable ages. The deep marine succession is characterized by large-scale non-repetitive stratigraphic changes allowing the discrimination between a Lower, Brownish, Rossini and Euxinic Shale Interval. The sedimentary cycles, which occur superimposed on these large-scale changes, are related to astronomical climate forcing. Astronomical tuning of the sedimentary cycles provides not only absolute ages for biostratigraphic events, reversal boundaries and volcanic ash layers, but also for the observed non-cyclic changes. These can be linked to environmental and geodynamic changes in the Mediterranean region preceding the Messinian Salinity Crisis. The beginning of the Brownish Interval marked by tripartite cycles at 8.017 Ma coincides with tectonic rearrangements in the Mediterranean, such as compression in the Betics and opening of the Rifian corridor. At 7.616 Ma the change to the calcareous Rossini Interval can be linked to tectonic uplift in the northern Apennines and the deepening of the Rifian corridor, while the Betic corridor became restricted coinciding with the end of evaporite deposition in the eastern Betics. The change to euxinic shales at 7.168 Ma reflects basin-wide restriction marked by diatomite and sapropel formation in the Mediterranean basin related to the severely constricted Betic and Rifian corridors. The occurrence of a tripartite construction of cycles in the euxinic shales at 6.719 Ma coincides with diatomite formation in the eastern Mediterranean, uplift and shallowing in the Northern Apennines and further constriction of the Atlantic gateways. We conclude that late Tortonian-early Messinian tectonic and environmental events occur remarkably synchronously in the Mediterranean basin and most likely result from a combination of geodynamic processes and eccentricity-controlled climatic changes. In combination with the upper Serravallian and lower Tortonian interval, which includes the formal Tortonian Global Stratotype Section and Point (GSSP), the Monte dei Corvi is the only complete section in the Mediterranean covering the entire Tortonian Stage. We therefore propose the Monte dei Corvi section as Tortonian reference section, particularly for the Mediterranean region. In addition, it may serve as a unit-stratotype for the Tortonian.