Integrated stratigraphy of the Smirra Core (Umbria-Marche Basin, Apennines, Italy): A new early Paleogene reference section and implications for the geologic time scale

Pelagic sections of the Umbria-Marche Basin, in the Northern Apennines (Italy), have provided key geological archives for studying critical intervals of early Paleogene time. In addition to classical sections, the Smirra Coring project provides a new record of relatively undisturbed sediments (~ 120 m from 4 overlapping holes) of the upper Scaglia Formation (Paleocene – middle Eocene). Here we present a new high-resolution integrated magneto-bio-stratigraphy of the ~ 93-m composite section drilled in Smirra Holes 1 and 2. The correlation of the magnetostratigraphy to published geomagnetic polarity timescales (GPTSs; CK95, GPTS2004, GPTS2012), as constrained by calcareous nannofossil biostratigraphy, reveals that the cores range from Chron C26r to C21n (Thanetian – Lutetian; ~ 60 to ~ 46 Ma). Sedimentation rates range from ~ 0.6 to ~ 1 cm/kyr and are comparable with those from coeval sections in the Umbria-Marche Basin. They point to significant discrepancies between existing time scales, which mainly involve the duration of Chron C23 and subchrons within C23n and C24n. Application of the average spectral misfit method on the magnetic susceptibility record reveals a strong signature of orbital forcing and allows an independent estimate of the sedimentation rates. This analysis offers additional insights into solving time scale issues, which mainly concern the duration of Chron C23n. Our results imply a significantly longer duration for C23n.2n, which would accommodate the presence of an extra third 405-kyr eccentricity related cycle in C23n. The Smirra record thus provides new constraints on the early Paleogene time scale, a key record for further cyclostratigraphic and astrochronological research, and a reference framework for paleo-climatic and -oceanographic studies.