A fast way to estimate the clay fabric from shale fragments. Key example from a strained thrust footwall (Pyrenees).

Abstract

This paper presents a new approach based on Anisotropy of Magnetic Susceptibility, that aims to define the spatial evolution of deformation in shales through the analysis of non-oriented fragments. This technique facilitates and speeds up sampling and measurements, and consequently, hundreds of samples can be analyzed in a few weeks of work. The proposed procedure is here applied to a high-resolution study of deformation in the Eocene shales of the South Pyrenean basin. This study shows that it is possible to accurately measure small irregularly shaped fragments of a few grams. Scalar data that are not dependent on the orientation of the sample are studied. The magnetic fabrics indicate a deformation gradient in the foot-wall of a major thrust, for which a five-stage development is proposed. On the other hand, gradient heterogeneities reveal non-previously detected tectonic features, such as blind thrusts. This technique can be applied both to surface geology and cuttings from boreholes, and its sound results indicate that the study of the magnetic fabric is particularly sensitive and accurate for understanding the process and state of deformation in shales.