Low- to high-amplitude oscillations and secular variation in a 1.2 km late Miocene inclination record

Abstract

A Miocene section of 2 km thickness was continuously cored near Szombathely, NW Hungary. A detailed magnetostratigraphic study has been integrated with results of lithologic, sedimentologic, and paleontologic studies. Progressive alternating field (a.f.) and thermal demagnetization, and rock magnetic and mineralogic studies indicate that the natural remanent magnetization (NRM) resides in magnetite and the strata contain only minor secondary magnetizations. Major polarity zones have been defined by inclinations and correlated with the geomagnetic polarity time scale for the interval 10-9 Ma. Inclinations for samples collected at 1 2 m intervals display fine-scale oscillations representing secular variation with period times of 400–700 years. Fluctuating amplitudes of oscillation range from low (less than 10° peak-to-peak) to high (more than 40°), reflecting apparently varying stabilities of the geomagnetic field. Additionally, amplitudes of oscillation progress from low to high, and return to low, forming oscillation cycles with a periodicity of 6.2 ± 1.8 kyr. The oscillations and boundaries of oscillation cycles are generally unrelated to lithology and stratigraphy. Many oscillation cycles appear to be incomplete and ‘interrupted’ by high-level oscillations. Some incomplete cycles appear to arise from brief interruptions to the depositional record, whereas other interruptions may irregularly arise from a separate component of the geomagnetic field.