Magnetostratigraphy of the Eocene/Oligocene boundary in a short drill-core

A 39.4 m long, 10 cm diameter vertical core was drilled through the Eocene/Oligocene boundary close to the Massignano quarry stratotype section near Ancona, Italy. Total recovery was about 95% and 73% of the core was intact. It was oriented by comparing the dip of bedding planes visible on the core surface with bedding measured in the nearby quarry outcrop. The core was correlated to the quarry section by linear regression of the depths of four biotite-rich layers in each section. The fissile condition of the Scaglia cinerea marls made sampling difficult, but 260 samples were obtained at an average separation of 12 cm in this and the underlying Scaglia variegata formation. Progressive alternating field and thermal demagnetization isolated two magnetization components in most samples. A stable component with unblocking temperatures of 300–540°C and coercivities over 20 mT is probably carried by magnetite. A stratigraphic plot of the ChRM directions shows well-defined normal and reversed magnetozones. The polarity sequence correlates well with chrons C12r to C16n-2. A few single-sample normal magnetozones within chron C16n.1-r have no corollary in the geomagnetic polarity timescale. The magnetozones corresponding to chrons C12r or C13r exhibit no short subchrons that might account for low-amplitude, short-wavelength magnetic anomalies reported in this part of the marine magnetic record.