Anomalous compaction due to silica diagenesis — Textural and mineralogical evidence from hemipelagic deep-sea sediments of the Japan Sea

Abstract

Although anomalous compaction (dramatic physical property changes) is widely recognised from scientific drilling of sections of marine biosiliceous sediments that have undergone silica diagenesis, the precise mechanisms governing vertically abrupt compaction are poorly understood. To better understand relationships between silica diagenesis and anomalous compaction, the microfabric and composition of hemipelagic sediments at the Ocean Drilling Program Sites 794 and 795 in the Sea of Japan were analysed in detail. Textural and mineralogical examination of core samples from these stations shows that dissolution of opal-A and precipitation of opal-CT are the two major controls on anomalous compaction. Other observed components of the diagenetic history, such as structural ordering of diagenetic opal and precipitation of authigenic phases (clay minerals and pyrite) do not strongly affect physical properties. Above the opal-A to opal-CT transition zone (anomalous compaction interval), opal-A deposits impart significant porosity to the sediments. Because of this high porosity and the relative incompressibility of siliceous tests, normal mechanical compaction of biosiliceous units is retarded and mainly postdates silica diagenesis. Across the transition zone, a sharp reduction in opal-A content under dissolution (from 35 to ~12% in Site 794 and from 45 to 7% in Site 795) leads to a significant reduction in sediment framework stability, which makes it vulnerable to sudden collapse, abrupt reduction in intergranular and intragranular porosity (and increasing bulk density), and a decrease in pore-water content. Although later opal-CT precipitation does influence the petrophysical response, mineralogical analyses suggest a lesser role for opal-CT precipitation on host porosity than precursor opal-A dissolution. This is attributed to the inhibiting effects of high abundance of clay-sized components that have restricted significant precipitation of opal-CT.