ASSESSING THE SUITABILITY OF A BURDIGALIAN GSSP IN THE MIOCENE BISCIARO FORMATION, CONTESSA VALLEY, ITALY WITH U-PB ZIRCON GEOCHRONOLOGY

The stratigraphic sections along the Contessa Valley near Gubbio, Italy have yielded numerous insights into Cenozoic Earth history. Several bio-, chemo-, and magnetostratigraphic studies have been performed on these sections, many calibrated in age by astronomical tuning. A studied section in an active quarry of the Bisciaro Formation, representing the lower Miocene in the Contessa succession, was put forward as a candidate for the Aquitanian-Burdigalian GSSP (Global Stratotype Section and Point) based on its documented first occurrence of Helicosphera ampliaperta (Fabbrini et al., 2019). However, the Bisciaro Fm. has not been the subject of a recent high-precision geochronological study, which would be beneficial for a new GSSP, and the new quarry section seems unsuitable for astronomical tuning because of a number of discontinuities and hiatuses. Nevertheless, the Bisciaro Fm. is noteworthy for its quantity of volcaniclastic layers interbedded with pelagic carbonates. These layers contain zircons amenable to dating through CA-ID-TIMS U-Pb geochronology, with analytical precision of ~0.1%. Here we present 4 new U-Pb ages for the Bisciaro Fm. from the original highway roadcut section of Montanari et al. (1997), which is correlated with the nearby GSSP-proposed quarry section. Our samples range in age from ~22.3-20.2 Ma, compared to 22-18 Ma from the original study of Montanari et al. (1997) based on bio-magnetostratigraphic assessments and ⁴⁰Ar/³⁹Ar geochronology on feldspar from two volcanosedimentary layers. The previous overestimate of the amount of time in the section likely is due to poor foraminiferal preservation and unreliable carbonate paleomagnetic data, and the Aquitanian-Burdigalian boundary in this section potentially falls during a suspected hiatus. The proposed GSSP section’s age model, calibrated on bio- and magnetostratigraphy susceptible to the same concerns, presents numerous discrepancies with our radioisotopic age model, including with the age of the Raffaello regional marker bed, which we find is 22.302±0.040 Ma, compared to a suggested age of ~21.08 Ma. Our work shows the importance of obtaining high-precision radioisotopic ages in addition to astronomical tuning as a check on bio- and magnetostratigraphic age calibration, especially for sections treated as possible GSSPs.