Integrated stratigraphy and astronomical tuning of the Serravallian and lower Tortonian at Monte dei Corvi (Middle–Upper Miocene, northern Italy)

Abstract

An integrated stratigraphy (calcareous plankton biostratigraphy, magnetostratigraphy and cyclostratigraphy) is presented for the Serravallian and lower Tortonian part (Middle–Upper Miocene) of the Monte dei Corvi section located in northern Italy. The detailed biostratigraphic analysis showed that both the Discoaster kugleri acme and the first influx of Neogloboquadrina acostaensis are recorded at Monte dei Corvi; these events, which passed unobserved in previous studies, play an important role in delineating the Serravallian–Tortonian boundary. Thermal and alternating field demagnetization revealed a characteristic low-temperature component marked by dual polarities. The resultant magnetostratigraphy for the upper part of the section can be unambiguously calibrated to the GPTS ranging from C5n.2n up to C4r.2r. Unfortunately, the lower part of the section, including the Serravallian–Tortonian boundary interval, did not produce a reliable magnetostratigraphy despite the fact that some short reversed intervals and a single normal interval are recorded. Using sedimentary cycle patterns in combination with the calcareous plankton biostratigraphy the section can be correlated cyclostratigraphically in detail to the partially overlapping and previously tuned section of Monte Gibliscemi on Sicily. The Monte dei Corvi section is dated astronomically by calibrating the basic small-scale sedimentary cycles to the precession and 65°N lat. summer insolation time series of the La93 solution following an initial tuning of larger-scale cycles to eccentricity. An almost perfect fit is found between the cycle patterns and intricate details, especially precession–obliquity interference, in the insolation target between 8.5 and 10 Ma. The tuning to precession remains robust for most intervals back to the base of the section dated at 13.4 Ma and shows that the section is continuous apart from a possible short hiatus in the Tortonian. It provides accurate astronomical ages for all sedimentary cycles, calcareous plankton events, polarity reversals and ash layers and marks a significant improvement of the recently proposed astronomical calibrations of the Monte dei Corvi section and of parallel sections in the Mediterranean. Astronomical ages for the Ancona and Respighi ashbeds are significantly older than previously reported 40Ar/ 39Ar biotite ages, even if the revised older age for the FCT-san dating standard of 28.02 Ma is applied. The astronomical dating of the magnetic reversals in the Monte dei Corvi section results in the completion of the astronomical polarity time scale for the last 13 Myr. The Monte dei Corvi section has recently been proposed as the stratotype section for the Serravallian–Tortonian boundary despite the moderate to poor preservation of the calcareous microfossils and the lack of a reliable magnetostratigraphy across the boundary interval. Finally, our study reveals that a single dominantly precession controlled oscillatory climate system is responsible for late Neogene sapropel formation in the Mediterranean during the last 13.5 million years.