Magnetic Fabric in Two Sedimentary Rock-Types from the Southern Pyrenees.

Abstract

The anisotropy of magnetic susceptibility (AMS) of some red sandstones, grey marly limestones and fine sandstones in various structural settings from the Southern Pyrenees has been analyzed. Red beds have been studied in a slightly deformed Eocene foreland sequence and in a meter-scale fold involved in a thrust sheet. Marly limestones of the same age as the redbeds were sampled in both thrust sheets included in the Upper Thrust sheets with no cleavage developed and units which belong to the cleaved Lower Thrust sheets (Gavarnie Thrust sheet). In every case the K1 (maximum) susceptibility axis lies in the axis of intersection of the plane of bedding and the plane of tectonic flattening or cleavage. The K3 (minimum) susceptibility axis tends to be normal to the bedding plane if cleavage is weak and normal to the cleavage plane if cleavage is well developed. In these cases the AMS ellipsoid is oblate. Where both bedding and cleavage compete to determine the AMS ellipsoid, the K3 axis lies in a girdle between the normals to the two planes, the K2 (intermediate) axis is automatically forced to lie on the same great circle, and the AMS ellipsoid may be prolate. The measurement of AMS of both marly limestones and sandstones with the same degree of deformation, reveals that the AMS of the first group is sensitive to the imprint of a tectonic fabric, whereas that of the second group reflects the sedimentary fabric. Circumstantial evidence suggests that phyllosilicate grains are more important than iron oxides in determining the AMS of the studied rocks.