The Significance of Fine Scale Variations in Magnetization in a Sequence of Continental Red-Beds

MAILLOL, JEAN-MICHEL, and MICHAEL E. EVANS, University of Alberta, Edmonton, Alberta, Canada

A sequence of Upper Permian continental red-beds from southern France has been studied using fully oriented samples as well as mining drill core segments. A systematic sampling has been carried out, with stratigraphic intervals ranging from 10 m down to 5 mm. In order to investigate the lateral variability of the magnetization, samples were obtained from several (up to 10) stratigraphically equivalent sections located a few centimeters to 100 m apart, and at four different sandstone and siltstone horizons (25 cm to 2 m thick).

The stratigraphic variations of directional data from distinct sites generally correlate well, even when observed at a scale of a few centimeters, thus indicating that they originate most probably from variations of the geomagnetic field rather than from geologically induced perturbations. In addition, these correlations show that the isochrons of magnetization acquisition are not horizontal but follow the bed thickness variations inherent at continental deposits, which favors an early locking of the remanence in the rock history.

Magnetization intensity data seem to provide lithological information with a much greater resolution than do the more commonly used susceptibility measurements. While in most cases the susceptibility shows little or no variability, the intensity variations are very clear and match lithological changes. Again a very good correlation is observed between different sites at the same stratigraphic level. The intensity variations are cyclic and can reveal similarly cyclic changes in the conditions of deposition.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)