Stratigrafia magnetica ad alta risoluzione del limite Eocene/Oligocene nella successione Umbro-Marchigiana

High-resolution magnetostratigraphy of the Eocene-Oligocene boundary in the Umbro-Mar- chean Appenine. High-resolution magnetostratigraphy across the Eocene-Oligocene boundary has been employed in a detailed investigation of the nature of low-amplitude, short-wavelength oceanic magnetic anomalies. A core, 39.4 m long and 10 cm in diameter, was drilled through the Eocene-Oligocene boundary near to the Massignano Quarry stratotype section near Ancona, Italy. The stratigraphy of the core, which traverses the Scaglia Variegata and Scaglia Cinerea formations, was correlated precisely to the quarry section by linear regression of the depths of identifiable biotite-rich layers. The good recovery of intact material allowed an average sampling interval of about 12 cm, which is closer than in preceding magnetostratigraphic studies of Umbrian-Marche sequences. The characteristic remanent magnetization was obtained by both progressive alternating field and thermal demagnetizations. The stable component of the natural remanent magnetization could be isolated by thermal demagnetization at temperatures of 300-540 ◦ C or by alternating field demagnetization in fields higher than 20 mT. It is probably carried by magnetite in the Scaglia Cinerea marls, while some amount of hematite is present in the underlying Scaglia Variegata. A stratigraphic plot of the ChRM directions shows well-defined magnetozones and the resulting polarity sequence correlates well with polarity chrons C12r to C16n-2. A few single-sample normal magnetozones that do not correspond to the geomagnetic polarity timescale are found within chron 16n.1-r. The magnetozones corresponding to chrons C12r or C13r do not exhibit short subchrons that might account for the low-amplitude and short-wavelength magnetic anomalies reported in this part of the marine magnetic record. In investigation of relative paleointensity fluctuations has been carried out in this part of the core, which embraces the Scaglia Cinerea formation. Anhysteretic remanent magnetization (ARM) has been used to normalize the natural remanent magnetization (NRM), compensating variations in sedimentary input. The ensuing NRM/ARM ratio is taken to be a proxy for relative variation of paleomagnetic field intensity. The paleointensity fluctuates systematically and has minimum values close to the reported positions of low-amplitude, short-wavelength magnetic anomalies in the marine record.