Palaeomagnetism of the Pico Formation, Santa Paula Creek, Ventura Basin, California

Abstract

SUMMARY In the western Ventura Basin in southern California, marine turbidites in the Pico Formation are exposed along Santa Paula Creek in a southeast-dipping homocline. The section is about 3 km thick and contains normal and reverse polarity that was acquired before the development of the homocline. Although the sediment contains a secondary component of normal polarity that cannot be removed entirely by thermal or alternating-field demagnetization, the mean declination for 26 sites (12 normal polarity, 14 reverse polarity) shows a clockwise rotation of 19.8d± 9.0d that is similar to the rotation recorded in the Saugus Formation, a coeval, non-marine deposit in the eastern Ventura Basin that was studied by Levi et al. (1986). The partial overprint of the secondary component of normal polarity results in a mean inclination for each polarity that does not match the inclination of an axial dipole field (the inclination is too steep in normal polarity and too shallow in reverse polarity); still, the inclination for the combined sites is close to the expected inclination (52.0d versus 53.9d). Overall, turbidites in the Pico Formation are suitable for a magnetostratigraphic investigation, but caution is required if the palaeomagnetic data are to be used in a tectonic study.