A DETAILED RECORD OF NORMAL-REVERSED-POLARITY TRANSITION OBTAINED FROM A THICK LOESS SEQUENCE AT JIUZHOUTAI, NEAR LANZHOU, CHINA

A record of normal-reversed-polarity transition has been obtained from a 4 m thickness of loess exposed at a section near Lanzhou, China. Ma gnetostratigraphic studies suggest it may represent a reversal boundin g the onset of a reversed-polarity zone within the Jaramillo Normal Su bchron. The natural remanent magnetization consists of two components: a low-coercivity (less than or equal to 20 mT), low-unblocking-temper ature (less than or equal to 300 degrees C) component of viscous origi n and a high-coercivity (>20 mT), high-unblocking-temperature (250-700 degrees C) component carrying the characteristic remanence. Mineral m agnetic analyses confirmed the presence of magnetite, its low-temperat ure oxidation products and haematite, each contributing to the remanen ce properties. Grain size and concentration showed limited variations and there was little evidence for the presence of the ultrafine magnet ic phase commonly associated with palaeosol formation. Pedogenic proce sses appeared negligible and their effects unimportant, with detrital processes dominating the mineralogy and most probably the acquisition of the characteristic remanence. The reversal record was characterized by the decay and recovery of the geocentric axial dipole term with la rge directional swings occurring during periods of reduced relative pa laeofield intensity. The virtual geomagnetic poles traced a complex pa th exhibiting no particular geographical confinement. Relative palaeof ield intensity determinations were insensitive to the choice of normal ization parameter and showed a distinctive asymmetry. Striking similar ities were observed with the Matuyama-Jaramillo reversal record, obtai ned from the same section (Rolph 1993), and the Steens Mountain revers al record (Prevot et al. 1985), lending further support for the existe nce of unusually high post-transitional held intensities