Properties of a detrital remanence carried by haematite from study of modern river deposits and laboratory redeposition experiments

Abstract

Summary. Although detrital haematite is often observed in red sedimentary rocks, its contribution to the magnetization is usually a matter of debate. Part of the problem is that the properties of magnetic remanence carried by detrital haematite are not well known. Studies on both naturally and experimentally deposited modern river sediments whose remanence is carried by detrital haematite lead to the following observations: (1) The declinations of river-laid sediments deposited under known field conditions average to that of the Earth's field. (2) A substantial inclination error is observed in both river-laid and experimentally deposited sediments which varies as: tan (Io) =f. tan (If) where Io and If are the remanent and applied inclinations respectively and f is about 0.55 in these experiments. (3) The intensity of remanence is a function of both the magnitude and the orientation of the applied magnetic field, increasing with field strength and decreasing with field inclination. This observation is consistent with models involving contributions to the remanence by plates (constrained to lie nearly horizontally) and spheres (aligned with the applied field). (4) Sediments deposited in zero field and then subjected to an applied field acquired a p-DRM by grain rotation. The intensity of p-DRM increased with time according to a power law, P-DRM is acquired parallel to the applied field but, unless the sediment is disturbed, has an intensity an order of magnitude lower than the DRM acquired in the same field. (5) If generally valid, the inclination error for a haematite DRM presents the paradox that while both the age and the polarity of the DRM may be determined, the direction of the DRM magnetization will tend to underestimate palaeolatitude and give palaeopole positions that are far-sided.