Compaction‐induced inclination shallowing in synthetic and natural clay‐rich sediments

Abstract

Experiments using single‐clay slurries consisting of acicular or equidimensional magnetite mixed with either kaolinite, illite, montmorillonite, or chlorite were conducted to determine the effect of compaction on inclination. A natural marine sediment and a “reconstructed” marine sediment, made up of the clays used in our single‐clay experiments, were also studied. A laboratory postdepositional remanence, imparted to the slurries by stirring, had its inclination shallowed by compaction for both the single‐clay samples, as seen previously (Anson and Kodama, 1987), and the natural and reconstructed marine sediments, suggesting that our single‐clay sediments are valid analogues of natural sediments. Magnetic intensity also decreased, by 30–50%, during compaction, the greatest decrease occurring for samples with the steepest initial inclination. Altering the pH of a slurry, so that the magnetite would have a negative surface charge and be repelled by clay particles, did not affect a slurry's inclination‐shallowing behavior, suggesting that electrostatic attraction between magnetite and clay is probably not a major contributor to inclination shallowing. However, most of the inclination shallowing observed in our experiments occurs at low pressures when void ratios are high. Comparison with a clay microfabric/compaction study (McConnachie, 1974) suggests that volume loss at these pressures is accommodated by reorientation of clay particles or clay‐particle aggregates (clay domains). Since pore spaces are large at these low pressures, inclination shallowing is probably caused by magnetite particles either being attached to clay particles, perhaps by van der Waals forces, or having been incorporated into clay domains. Alternating field demagnetization of compacted samples, which contain magnetite with a range in grain sizes, supports this model, indicating that the smallest magnetic grains are slightly more shallowed by compaction than larger grains. Our results suggest that a sediment's clay content may be an important contributor to inclination shallowing.