Depositional and compositional controls on sandstone diagenesis, the Tetori Group (Middle Jurassic–Early Cretaceous), central Japan

The Tetori Group is a representative Middle Jurassic to Lower Cretaceous deposit in the northern central part of Honshu, Japan. The Tetori Group is divided into the Kuzuryu, Itoshiro and Akaiwa Subgroups with decreasing age. It consists mainly of a marine sequence in the lower part (Kuzuryu Subgroup) and non-marine sequence in the upper part (Akaiwa Subgroup) with transitional sequence in the middle part (Itoshiro Subgroup). Diagenesis of Tetori Group sandstones comprises compaction, cementation and replacement. Early diagenetic events include mechanical compaction, and pore-filling cementation of calcite, siderite, dolomite and formation of kaolinite. Late diagenesis includes formation of quartz cement and albitization of feldspar, and formation of illite, chlorite, calcite, ankerite and pyrite. Among the diagenetic minerals, carbonate forms the major cement in Tetori Group sandstones. Calcite of each subgroup is subdivided into two or three types based on cathodoluminescence. Tetori Group sandstones of marine, fresh-water, and mixed depositional environments are characterized by distinct chemical compositions of early diagenetic calcites. Late diagenesis seems to have been mainly influenced by detrital compositions of the sequence, burial temperatures and partially hydrothermal processes. Especially, late diagenetic carbonate and authigenic clay minerals seem to have formed mainly from ions released from clay mineral transformations in the adjacent shales. Based on the illite crystallinity (IC), the Kuzuryu Subgroup exhibits higher diagenetic grades than the overlying Itoshiro and Akaiwa Subgroups, indicating that the diagenesis of the Tetori Group was mainly affected by depth of burial.