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

Abstract

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.

A detailed record of normal-reversed-polarity transition obtained from a thick loess sequence at Jiuzhoutai, near Lanzhou, China

“…(Re)magnetization associated with periodic water saturation of continental deposits has been recognised by several authors (Burbank & Li 1985;Hus & Geeraerts 1986;Maillol & Evans 1992). Laboratory experiments confirm the validity of this process (Irving & Major 1964;McIntosh 1993).…”

Magnetic anisotropy and paleomagnetic inclination shallowing in red beds: Evidence from the Mississippian Mauch Chunk Formation, Pennsylvania

“…Some paleomagnetic studies have suggested that hematite‐bearing, red sedimentary rocks carry a detrital remanent magnetization (DRM) [ Tauxe et al , 1980; Elston and Purucker , 1979; Steiner , 1983; Maillol and Evans , 1992; Garces et al , 1996; Kruiver et al , 2000]. In this case, red bed remanence may acquire a magnetization significantly shallower than the geomagnetic field present during deposition.…”

Laboratory depositional and compaction-caused inclination errors carried by haematite and their implications in identifying inclination error of natural remanence in red beds

“…Burial mechanical compaction can only affect red bed remanence if it is a DRM or a CRM that was acquired early in the post‐depositional history of the sediment. Some workers have suggested that red bed remanence is acquired chemically over long periods of time (Walker et al 1981; Larson et al 1982); others have suggested that haematite‐bearing sedimentary rocks carry a DRM (Elston & Purucker 1979; Tauxe et al 1980; Steiner 1983; Maillol & Evans 1992; Garces et al 1996). While a CRM red bed remanence could be acquired long after deposition, some palaeomagnetic studies of red beds suggest that a chemical remanence was acquired before burial by only 1 m of overburden (Liebes & Shive 1982).…”