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Diagenesis, sediment strength, and pore collapse in sediment approaching the Nankai Trough subduction zone

Glenn A. Spinelli^,1, Peter S. Mozley¹, Harold J. Tobin^,1, Michael B. Underwood², Nicole W. Hoffman³ and Glen M. Bellew⁴

¹ Earth & Environmental Science Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
² Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA
³ Earth & Environmental Science Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
⁴ Department of Civil & Environmental Engineering, University of Missouri, Columbia, Missouri 65211, USA

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.

Key Words: diagenesis • opal • cement • subduction • Ocean Drilling Program