Subcritical creep compaction of quartz sand at diagenetic conditions: Effects of water and grain size

Compactional creep was measured for aggregates of quartz sand that were subjected to an effective pressure of 34.5 MPa and temperature of 150°C for times up to 6 months. The effects of grain size and water on creep were determined by loading quartz aggregates of 255 ± 60, 130 ±18, and 35 ± 12 μ m grain size under nominally dry, water vapor, static liquid water, and flowing water conditions. All aggregates displayed transient, decelerating creep; volume strain rates as low as 2 × 10 −10 s −1 were achieved. Volume strain rate increases systematically with decrease in grain size and increase in exposure to water at comparable strain or time. Despite the fact that the effective pressure applied was far less than the critical pressures for short‐term cataclastic compaction of the quartz aggregates, grain‐scale microstructures indicate that the underlying mechanism of creep is crack growth. Creep rates are explained by subcritical crack growth, as governed by water–silicate reactions at crack tips, controlled by access of water at dry and vapor conditions, and solute chemistry under static and flowing liquid water conditions. Extrapolation of the experimental results to natural compaction of quartz sand during burial and diagenesis over times of tens of millions of years indicates that porosity loss through subcritical cracking and grain rearrangement of medium‐grained, porous, wet quartz sands can reach ∼10%.