Palaeomagnetic location of the Jaramillo Subchron and the Matuyama-Brunhes transition in the Castlecliffian stratotype section, Wanganui Basin, New Zealand

Abstract

Magnetostratigraphy is described for the lower part of the Castlecliff section in Wanganui Basin, New Zealand, based on analysis of multiple samples from 37 horizons. The Castlecliff section is the stratotype of the middle Pleistocene Castlecliffian Stage and comprises eleven unconformity-bound sequences which accumulated at inner shelf depths, during interglacials. A range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition, in the stable single domain or pseudo single domain size range. However, at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral, chiefly within the superparamagnetic-single domain size range, resulting in hard secondary components of magnetization. Nevertheless, at most sites the primary component of remanence was clearly identified by stepwise demagnetization. Four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints. The section sampled spans the interval from below the Jaramillo Subchron to above the Matuyama-Brunhes boundary. The Matuyama-Brunhes transition extends over 10.5 m of section within the Upper Westmere Siltstone. Virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path. These represent the first published data on the occurrence of the Matuyama-Brunhes boundary on-land in New Zealand.