High-precision U–Pb zircon geochronology of the Miocene Bisciaro Formation, Contessa Section, Italy: A case study for requisite radioisotopic calibration of bio- and magnetostratigraphy

The stratigraphic sections along the Contessa Valley near Gubbio, Italy, have yielded numerous insights into Late Mesozoic and Cenozoic Earth history from integrated biostratigraphic, magnetostratigraphic, and chemostratigraphic studies of the carbonates comprising the sections. However, the quality of microfossil preservation, faithfulness of magnetic mineralogy, and presence of hiatuses may hinder the development of accurate and precise age models for the critical climatic events documented by these rocks. The deposition of volcaniclastic horizons containing zircon in the uppermost, Miocene portion of the section allows for the acquisition of absolute, high-precision ages through U-–Pb zircon geochronology. Here, we present four U-–Pb ages from the Bisciaro Formation in the Contessa – Il Testimone section, which revise prior ⁴⁰Ar/³⁹Ar geochronology that was less precise and potentially affected by alteration. We find that the section was deposited between ~22.3 and 20.1 Ma, with a hiatus lasting ~1 Myr in the Lower Bisciaro. Our results suggest that biostratigraphic data may be skewed by an underestimation of the true stratigraphic range of foraminifera, as a result of poor preservation, and that prior magnetostratigraphy overestimated the number of magnetic field reversals. Given these inconsistencies and the presence of hiatuses in the section, we reject the recent suggestion that the Contessa section could serve as the Burdigalian Global Stratigraphic Section and Point (GSSP). We suggest that wherever possible, absolute geochronology should be used to calibrate sedimentary sections, especially candidate GSSPs, as a way to verify the accuracy of age models based on biostratigraphy, magnetostratigraphy, and astronomical tuning.