A minor amount of opal cement inhibits consolidation of sediment approaching the Nankai Trough subduction zone at Ocean Drilling Program Sites 1173 and 1177. Secondary and backscattered electron images of sediments from Site 1173 reveal a low-density, silica phase (opal-CT) coating grain contacts. The grain-coating cement is more widespread in the upper Shikoku Basin facies than in the lower Shikoku Basin facies. Numerical models of opal-CT content display increases with depth through the cemented upper Shikoku Basin section. Once temperature increases above ∼55 °C, the rate of opal-CT dissolution outpaces precipitation, the cement can no longer support the overburden, and the open framework of the sediment begins to collapse. Cementation followed by cement failure is consistent with observed anomalies in porosity, seismic velocities, and shear rigidity. Porosity is anomalously high and nearly constant near the base of the upper Shikoku Basin facies, whereas seismic velocity increases with depth in the same interval. Across the boundary between the upper Shikoku Basin facies and the lower Shikoku Basin facies, there are step decreases in porosity from ∼60% to ∼45%, P-wave velocity from ∼1800 m/s to ∼1650 m/s, and S-wave velocity from ∼550 m/s to ∼300 m/s. Similar cementation and porosity collapse may be important in other locations where heating of hemipelagic deposits, with minor amounts of opal, is sufficient to trigger opal diagenesis.

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