SEM and magnetic fabric study of a compacting sediment

Abstract

Scanning electron microscopy (SEM), anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence (AAR) studies were conducted on kaolinite/magnetite samples compacted to pressures between O and 0.2 MPa to determine if clay and magnetic fabric development mimic a change in compaction behavior marked by a break in slope in the compaction curve. This break in slope indicates that the rate of void ratio decrease with increasing pressure abruptly slows down at pressures of about 0.04 MPa. About one‐half of a compaction‐caused inclination decrease occurs at pressures less than 0.04 MPa. SEM photomicrographs show that horizontal clay fabric develops inhomogeneousIy within a sample at pressures below the break in slope. Since pore spaces are larger than magnetite grains at low pressures below the break in slope the observed pattern of inclination decrease during compaction may be due to magnetite grains being attached to clay particles and constrained to follow clay fabric development. This interpretation is supported by the observation that AAR minimum axes become vertical at pressures above the break in slope. This may indicate that magnetic fabric is developing at low pressures and that most of the magnetite particles are constrained to lie subhorizontally. The AMS maximum axes tend to lie 90° from a strong remanence in these samples suggesting single domain magnetic behavior. Measurement of AMS after alternating field demagnetization yields maximum axes which cluster near to the AAR maximum axes suggesting multidomain behavior. This apparent contradiction aay be due to magnetic conditions prior to AMS aeasurement controlling domain wall nucleation.