MEASURING MECHANICAL COMPACTION OF QUARTZ AND ILLITE MIXTURES USING MAGNETIC FABRICS: EXPLORING A PETROFABRIC APPROACH TO UNDERSTANDING COMPACTION IN FINE-GRAINED SEDIMENTS

Mechanical compaction is a key process operating in fine-grained sediments during burial, however, this process is often only documented through porosity loss as predicted by compaction curves. This study documents complex grain kinematics and clay fabric evolution during incipient compaction using anisotropy of magnetic susceptibility (AMS). Two sets of experiments were performed to test how mixtures of illite powder (from Rochester shale, NY) and quartz respond to low vertical stresses. Experiment set A combined illite (<50 μm) with quartz grains (<150 μm) showing sub/well rounded shapes and low to high sphericity. Experiment set B used spherical quartz blast media (<50 μm) mixed with illite. 5 cm³ of these mixtures were poured into cylindrical holders and compacted from 1.24kg to 4.00kg. Once strain-rates reached steady state behavior in each experiment, the AMS of the specimen was measured.

Bulk magnetic susceptibility of the illite powder (mean = 1.25E-07m³/kg) was two orders of magnitude greater than both quartz grain sizes (mean = 1.39E-09m³/kg). This suggests the AMS signal for mixtures used in this study should be controlled by paramagnetic illite.

High quartz to illite ratios (80:20, 60:40) in set A show little vertical axis rotation (<25°), moderate K1 tensors (5-25°) and sub-vertical K3 tensors (60-85°) during compaction. Shape factor (T) and the degree of magnetic anisotropy (P) fluctuate randomly and likely indicate quartz interacting with clays during compaction. In contrast, for set B, the aforementioned ratios show significant vertical axis rotation (25-160°), near vertical K3 tensors and lower K1 inclinations. T values remain in the oblate domain and following the final compaction stage, become strongly oblate.

Low quartz to illite ratios (40:60, 20:80) in both set A and B show similar trends, with fabrics generally becoming more oblate with increasing load. Substantial vertical axis rotation of K1 is observed in many samples. P increases from the uncompacted stage through the first compaction load in these specimens. Following the second compaction stage, P values fluctuate within a narrow margin.

This study can help provide insight regarding the nature of AMS data in ancient clay-rich rocks and address the propensity of such sediments to preserve paleocurrent lineaments.