CORRELATION BETWEEN MAGNETIC-ANISOTROPY AND FABRIC FOR DEVONIAN SHALES ON THE APPALACHIAN PLATEAU

The magnetic anisotropy of Devonian black shale samples was measured f rom two cores drilled in the Appalachian Plateau. The mineralogy of th e shales is predominantly clay, with small quantities of quartz and mi nor amounts of opaques and chlorite. Magnetite is the predominant ferr omagnetic mineral present in the samples. The magnetic fabric was meas ured at both room temperature and liquid-nitrogen temperature and is d ominated 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 t he paramagnetic contribution in the rock, showed a strong increase in both the bulk susceptibility and susceptibility differences. This incr ease suggests that the AMS is controlled by the paramagnetic minerals, particularly the clays and chlorite. Strain was measured from the ori entation of basal planes of the chlorite crystals by texture goniometr y. Good correlations have been found (1) between the orientation of th e 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 directio n of seismic anisotropy over the Plateau. The anisotropy of the anhyst eretic remanence, which expresses the anisotropy due to the ferromagne tic component in the rocks, shows a weaker correlation with the amount of vertical compaction. A weak magnetic lineation suggests that the m agnetite grains were aligned during a deformation phase which post-dat es the main Alleghanian orogeny. The magnetic anisotropy of the Devoni an shales mirrors the compaction and tectonic fabric on the Appalachia n Plateau.