Magnetochronology applied to assess tempo of turbidite deposition: A case study of ponded sheet-like turbidites from the lower Miocene of the northern Apennines (Italy)

This paper investigates the magnetostratigraphy of the ~750 m-thick Costa Grande Member (lower Miocene) from the Castagnola Basin of NW Italy, which represents the turbidite fill of a structurally confined basin where flow ponding resulted in a complete record of deposition from diverse sediment gravity flow types. The magnetostratigraphic profile of the Costa Grande Mb was correlated to the Geomagnetic Polarity Timescale using a well-established statistical method devised to be applied to sedimentary successions with steady accumulation rate. The results of the correlation exercise, validated with the available biostratigraphy, indicate an early Miocene age (from Chron C6AAr.3r to C6Bn.2n) between 21.7 and 22.3 Ma. The obtained age model was then used to calculate accumulation rates and frequencies of small volume and low-density vs. large volume and high-density gravity flows over an estimated 650 kyr time span. Results show that low-density flows contributed at a constant pace of 545 m Myr−1 to sediment accumulation, depositing relatively thin-bedded turbidites with a minimum recurrence time of 1.7 kyr, whereas the high-density flows, which are at least four times less frequent, are clustered in the stratigraphy, and become more abundant up-section. We also show that the minimum sediment volumes discharged by high-density flows suggest triggering by submarine failures whose recurrence and magnitude were probably not random. Lastly, we propose that the statistical method used in this study to correlate the Costa Grande Mb magnetostratigraphy to the GPTS provides best results when the products of episodic but voluminous depositional events are carefully identified and removed.