Treatment of partially stable sedimentary rocks showing planar distribution of directions of magnetization

Viscous magnetization is often imposed on rocks by the earth's magnetic field in geologically recent times so that the directions of magnetization measured in specimens from the same site are often strung out between the stable direction of magnetization and the present earth's field. Such viscous components are an important source of error in paleomagnetic work. This paper shows how, in some Upper Carboniferous sediments, these viscous components may be removed by partial thermal demagnetization so that the initial planar distribution condenses around the direction of the stable magnetization. The demagnetization was carried out by heating in an inert atmosphere in a nonmagnetic oven followed by cooling in zero field. Application of Graham's fold test shows that this stable magnetization is of very great antiquity, certainly pre-Triassic and probably pre-Permian in age.