Clay mineral alignment during early stages of burial

The formation of crystallographic preferred orientations (CPO/texture) in sediments is often attributed to rigid grain rotation of minerals and aggregates, plastic-brittle deformation and dissolution-precipitation processes. Due to their platy habit, clay minerals have a large shape anisotropy and are expected to develop a CPO most readily under favorable conditions. Here, we present an experimental approach in order to quantitatively explore the influence of particle settling and subsequent compaction in an undisturbed, ideal environment.

For the experiments, a powder of idiomorphic kaolinite grains (texture-forming components) was mixed with a fine-ground, illite aggregates (texture-inhibiting components) in mass proportions of 0, 30, 50, 70 and 100 %. The illite aggregates have compact shapes and are built up of submicron-sized crystallites. The modal particle size of both components is about 5 µm. Particles were mixed with artificial seawater and the resulting sludge was left to settle in 80 cm high acrylic tubes. For each composition three samples were produced: Sedimentation-only and two drained compaction experiments. The latter were carried out in a in a mechanical press, progressively applying a uniaxial load up to 0.4 MPa and 4 - 8 MPa, respectively. Compaction resulted in a volume decrease of 30 and 60 vol-%. The CPO of clay minerals was measured using high energy X-ray diffraction at beamline P07b at Deutsches Elektronen-Synchrotron (DESY) in Hamburg and pole figure data was directly extracted using single peak evaluation.

The results show that sedimentation alone, can yield a strong texture of the clay minerals. In the compaction experiments, texture strength is logarithmically related to the applied load, i.e. increase of texture strength is decreases at higher loads. Texture strength is linearly related to shortening and porosity reduction. In all cases, kaolinite texture strength is inversely correlated with illite aggregate content.

The results indicate that only a fraction of clay minerals is deposited flat on the sediment surface. Further, alignment during settling is hampered in the presence of particles with compact shapes. It is interpreted that rigid body rotation is limited to the very initial stages of settling and compaction. Additional texture strengthening is hampered by the lack of particles prone to rotate. Sediment surface processes as well as rigid body rotation during initial loading in the first few centimeters of burial are thus the most important processes in the formation of a CPO in clay rich sediments. Subsidence history beyond principal porosity elimination does not have a strong impact on the CPO. Therefore, the texture strength of a fine-grained sedimentary rock can be indicative of settling and early compaction conditions. In return, texture-related properties of artificial clays can be regulated by controlling those conditions.