An Integrated Stratigraphic Analysis of the Problem of the Oligocene-Miocene Boundary at Monte Conero, Italy

An integrated stratigraphic analysis of the Bisciaro and Scaglia Cinerea Formations at Monte Conero in central-eastern Italy offers a detailed, nearly continuous record of pelagic deposition influenced by orbitally controlled climate cycles spanning the Oligocene-Miocene boundary. Previous attempts to precisely constrain the location and age of the boundary have been complicated by syndepositional tectonism and marine regression in the Mediterranean. The present study examines a section that was located farther from Alpine and Apennine orogeny than previous sites and was therefore presumably undisturbed by tectonic activity. Petrographic and geochemical analyses of a volcanic ash layer from the Monte Conero core indicate its correlation with the regional marker bed known as the Livello Rafaello. A biostratigraphic analysis places the FO of Paragloborotalia kugleri at 19.95 m, which informs the placement of normal polarity chron C6Cn.2n at 21.2 m. Both datums are used to locate the boundary, with the boundary usually placed at the base of C6Cn.2n. The section appears to include a 1.2 Ma depositional hiatus in the late Oligocene, likely resulting from regional marine regression, but the paleomagnetic record indicates the boundary is preserved intact in the BiCiCore. A cyclostratigraphic analysis of magnetic susceptibility throughout the core yields a record of eccentricity cycles, which is compared to the eccentricity record of Laskar et al. (2011) to determine an astrochronological age for the boundary. Defining the boundary as the base of normal polarity chron C6Cn.2n, the age of the boundary is 23.7 Ma. This is nearly 0.7 Ma older than the currently accepted age of the boundary, of 23.03 Ma, but well within the range of currently debated dates. The robust biological, paleomagnetic, and paleoclimatic data retrieved from the BiCiCore, as well as its correlation with radiometrically dated bentonite layers within the section, makes Monte Conero an excellent locality for defining the location and age of Oligocene-Miocene boundary