Magnetic anisotropy, scanning electron microscopy, and X ray pole figure goniometry study of inclination shallowing in a compacting clay‐rich sediment

Abstract

Anisotropy of anhysteretic remanence (AAR), scanning electron microscopy (SEM), and X ray pole figure goniometry studies of clay‐rich sediments were conducted to delineate the interactions between magnetite and clay particles which cause inclination shallowing during compaction. These studies used synthetic sediments composed of kaolinite or illite and two grain sizes of magnetite, 0.45 μm and 2–3 μm. Natural marine sediments which contained 40–50% clay were also used. The sediments were compacted by pressures as high as 0.157 MPa. The sediments' porosity decrease suggests that the compaction experiments model burial depths up to 400–500 m. The main result of this study is that inclination shallowing and void ratio versus pressure curves show two points where behavior changes. A rapid decrease in inclination and void ratio occurs up to a pressure of 0.02 MPa. This is accompanied by a magnetic intensity decrease of 20–30%. Compaction at pressures between 0.02 MPa and 0.05 MPa causes a further decrease in these parameters at moderate rates, while compaction at pressures above 0.05 MPa causes very little change. Percent magnetic anisotropy and X ray pole figure intensity ratio versus pressure curves show only one point where behavior changes at 0.05 MPa with a rapid increase at lower pressures and little change at higher pressures. A model to explain this behavior, based on the SEM observations as well as the magnetic anisotropy, pole figure, and inclination data, suggests that compaction up to 0.02 MPa causes a decrease in pore volume with little reorientation of clay particles. Some magnetic particles are firmly attached to clay particles, whereas other loosely attached particles are either randomized or subvertical ones are preferentially disturbed causing inclination shallowing and an intensity decrease. At pressures between 0.02 MPa and 0.05 MPa, magnetite particles have become firmly attached to clay particles and start to follow the reorientation of clay particles as the clay fabric develops. At 0.05 MPa there is a major change in the clay microstructure with a horizontal fabric becoming evident. Compaction at higher pressures causes little further volume loss and clay particle reorientation and hence little additional inclination shallowing. The randomization/disturbance process suggested by this model causes slightly more than half of the inclination shallowing observed in the samples studied, whereas magnetite‐clay attachment causes the remainder of the inclination decrease.