Magnetobiochronology of ponded turbidites (Castagnola Fm.) from the Tertiary Piedmont Basin of NW Italy: implications for turbidite deposition

This contribution focuses on the ca. 800m-thick Costa Grande Mb. (CG) of the Castagnola Fm. (Tertiary Piedmont Basin, NW Italy), the turbidite fill of a ponded basin (early Miocene), and aims at: i) retrieving a magnetic polarity stratigraphy for CG; ii) defining an age model by tieing the CG magnetostratigraphy to the reference Geomagnetic Polarity Time Scale (GPTS) with the support of biostratigraphic data from a few hemipelagic beds intercalated in CG; iii) calculating depositional rates and frequency of turbidites by parent flow type; iv) look into whether and how (are they cyclic vs. erratic? Is there a trend?) these quantities changes in time. A base-to-top stratigraphic section of CG, recently logged with cm-scale resolution for sedimentary facies (Marini et al., 2016) was sampled with an average stratigraphic spacing of ca. 7m yielding 140 samples that were thermally demagnetized to isolate their characteristic component of natural remanent magnetization (NRM). The correlation of the CG magnetostratigraphy with the Late Oligocene-Early Oligocene GPTS was first evaluated with a statistical approach by assuming thickness as a proxy of time. Sliding the CG magnetostratigraphy along the GPTS resulted in a total of 28 options, 4 of which above pass the student t-test with a 90% confidence threshold. However, only 1 out of these 4 options agrees with the available biostratigraphic data, suggesting CG was deposited during the early Miocene (from Chron C6AAr.3r to C6Bn.2n) between 21.7 and 22.3 Ma. When only the thin-bedded turbidite component of CG is taken into consideration, the reliability of the selected correlation option improves, suggesting the thin beds of CG might reflect a sort of background deposition by low-density flows with a frequency in the range 0.5-1/ky. Conversely, thicker beds tend to locally cluster in the stratigraphy, suggesting larger parent flows have a lower (0.05-0.3/ky) yet highly variable frequency. These results have important consequences on linking turbidite facies of ancient turbidite systems to likely trigger and parent flow types of submarine sediment gravity flows.