Synthetic and sedimentary records of geomagnetic excursions

Abstract

The geomagnetic excursion recorded in the sediments of Lac St.Front (Massif Central, France) is characterized by shallow and negative inclinations followed by a younger steep inclination interval (Vlag et al., 1996). In the corresponding interval of the nearby Lac du Bouchet only steep inclinations are found. Sedimentary records of the Mono Lake excursion show similar inclination patterns; ‘complete’ records of this excursion show a succession of a shallow by a steep inclination interval, while ‘incomplete’ records only show only steep inclinations (Coe and Liddicoat, 1994). Due to a non‐instantaneous acquisition of the remanence, sedimentary records reflect only a smoothed geomagnetic signal. It will be shown that smoothing of a small low‐intensity ‘reversed’ interval embedded in a non‐antipodal normal field interval may result in records of a shallow inclination interval followed by a steep inclination interval, while further smoothing results in only steep inclinations. Realignment of magnetic grains by the stronger normal field can also produce such records and may explain why such an unusual large lock‐in depth is required by the conventional smoothing model. Whatever the mechanism, the similarities between these synthetic records and the excursional records of Lac St. Front‐Lac du Bouchet and Mono Lake suggest that the latter are more or less affected by vector addition of two non‐antipodal directions.