The Transformation of Sediment Into Rock: Insights From IODP Site U1352, Canterbury Basin, New Zealand

Abstract

Abstract: At Integrated Ocean Drilling Program (IODP) Expedition 317 Site U1352, east of the South Island New Zealand, we continuously cored a 1927-m-thick Holocene-to-Eocene section where we can uniquely document downhole changes in induration and lithification in siliciclastic to calcareous fine-grained sediment using a wide range of petrological, physical-property, and geochemical data sets. Porosity decreases from around 50% at the surface to 5–10% at the base of the deepest hole, with a corresponding increase in density from ∼ 2 to ∼ 2.5 g cm 3 . There are progressive bulk mineral changes with depth, including an increase in carbonate and decrease in quartz and clay content. Grain compaction is first seen in thin section at 347 m below sea floor and intensifies downhole. Pressure solution (chemical compaction) begins at 380 m and is common below 1440 m, with stylolite development below 1600 m, and sediment injection features below 1680 m. Porewater geochemistry and petrographic observations document two active zones of cementation, one shallow (eogenetic) down to ∼ 50 m, as evidenced by micritic nodules and pore-water geochemistry driven by methane oxidation by sulfate, and another burial-related cementation zone (mesogenetic) starting at ∼ 300 m. A transitional zone occurs between 50 and 300 m. Our results quantify downhole diagenetic changes and verify depth estimates for these processes inferred from outcrop studies, and provide an actualistic example of cementation and compaction trends in a slope setting.