Correlation between magnetic anisotropy and fabric for Devonian shales on the Appalachian Plateau

Abstract

The magnetic anisotropy of Devonian black shale samples was measured from two cores drilled in the Appalachian Plateau. The mineralogy of the shales is predominantly clay, with small quantities of quartz and minor amounts of opaques and chlorite. Magnetite is the predominant ferromagnetic mineral present in the samples. The magnetic fabric was measured at both room temperature and liquid-nitrogen temperature and is dominated by a well-defined bedding (vertical) compaction and a lesser defined magnetic lineation. Measurements of the anisotropy of magnetic susceptibility (AMS) at liquid-nitrogen temperature, which enhances the paramagnetic contribution in the rock, showed a strong increase in both the bulk susceptibility and susceptibility differences. This increase suggests that the AMS is controlled by the paramagnetic minerals, particularly the clays and chlorite. Strain was measured from the orientation of basal planes of the chlorite crystals by texture goniometry. Good correlations have been found (1) between the orientation of the magnetic lineation and the long axes of the chlorite crystals, and (2) between the degree of magnetic foliation and the amount of vertical compaction. The magnetic lineation also agrees well with the direction of seismic anisotropy over the Plateau. The anisotropy of the anhysteretic remanence, which expresses the anisotropy due to the ferromagnetic component in the rocks, shows a weaker correlation with the amount of vertical compaction. A weak magnetic lineation suggests that the magnetite grains were aligned during a deformation phase which post-dates the main Alleghanian orogeny. The magnetic anisotropy of the Devonian shales mirrors the compaction and tectonic fabric on the Appalachian Plateau.