Magnetic intensity variations in red beds of the Lodève Basin (southern France) and their bearing on the magnetization acquisition process

SUMMARY An investigation of small-scale variations of magnetization of a red bed sequence in the Permian Lodbve Basin (southern France) has been carried out by means of 512 disc-shaped samples, each 25 mm in diameter and about 6 mm thick. It is found that magnetic intensity variations reaching an order of magnitude can occur over stratigraphic distances of only a few centimetres. These fluctuations show a clear lithostratigraphic control with intensity peaks systematically located at bed tops. Rock magnetic experiments and petrographic observations indicate that intensity variations are not due to changes in the amount or properties of the magnetic material. External causes, such as geomagnetic field fluctuations or superposition of reversed and normal components, can be ruled out. Petrographical evidence suggests that the magnetization is carried by detrital haematite grains and it is proposed that the controlling factor of intensity variations is the efficiency of alignment of these grains along the Earth’s magnetic field. The characteristic intensity patterns with peaks near bed tops can be explained by post-depositional alignment of the grains, with the highest efficiency near the exposed surface. Grain mobility can be enhanced by mechanical perturbations (bioturbations, hydraulic phenomena), increased water content and reduced load. All these factors tend to see their effect diminish away from the surface and could therefore produce the observed magnetization pattern. The proposed model points to an early acquisition of the remanence in the Lodbve red beds.

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