Rock-magnetic properties of multicomponent natural remanent magnetization in alluvial red beds (NE Spain)

Abstract

An earlier study of the cyclic Miocene red bed sequence of La Gloria (Spain) by Krijgsman et al. indicated complicated behaviour of the natural remanent magnetization (NRM) in parts of the section, particularly close to reversal boundaries. We resampled part of the section with high resolution and used extensive rock-magnetic analyses to characterize the magnetic remanence carriers. Below a conspicuous hydromorphous layer, the magnetic contributions of haematite and magnetite are approximately equal, while in the brown layers on top of the hydromorphous layer the contribution of magnetite is drastically reduced. This is probably related to a change in hydrological conditions. The NRM is characterized by: (1) a low-temperature (200–360 °C) overprint of unknown age, (2) a medium-temperature (360–580 °C) component, interpreted as the characteristic remanent magnetization (ChRM); and (3) a high-temperature haematite component (600– 680 °C), carrying the same directions as the medium-temperature component. There is no perceivable delay in NRM acquisition between the medium- and the high-temperature components; they are both regarded as primary. The behaviour of the NRM seems to be similar in the cyclic lithologies. The acquisition of NRM thus seems to be independent of lithology in the cyclic part of the section. The higher sampling resolution yielded the detection of a new polarity zone, which probably represents a geomagnetic feature according to rock-magnetic properties. However, the more detailed magnetostratigraphy of the resampled part of the section indicates that the earlier correlation to the geomagnetic polarity timescale is no longer tenable. Therefore, the La Gloria section should no longer be used as a magnetostratigraphic dating tool of mammal biochronology.