Lithification of biogenic siliceous sediments: Evidence from Neogene diatomaceous sequences of northeast Japan

Biogenic siliceous deposits are a significant component of Neogene stratigraphic sequences of the northeast Japan arc. Field observations demonstrate that the siliceous rocks are classified into five lithologic types: diatomite, opaline porcelanite, opaline chert, quartzose porcelanite, and black shale. Stratigraphic data suggest that the siliceous formations were deposited continuously from the Middle Miocene to Early Pliocene. Diatomite occurs throughout the sedimentary sequences and changes laterally into chert and porcelanite. Multivariate statistical analysis of geochemical data indicates that lithification and mineralogical transformation of silica-rich deposits was governed not only by compaction and cementation during burial but also by sorting of grains by biological sediment-seawater mixing. Diatomite is composed of frustules without fine ornamentations and delicate structures. Fine and delicate tests were more susceptible to dissolution than coarse and solid ones, and dissolved in the course of biological sediment disturbance. Ubiquitous occurrences of diatomite in the stratigraphic sequences are attributed to residual frustules which were more resistant to the diagenetic dissolution-precipitation reaction of silica. Biogenic silica is the main source of silica to form chert. Diagenetic precipitation of silica in diatomaceous beds through progressive sediment burial formed porcelanitic rocks, with opaline porcelanite being the precursor of quartzose porcelanite. Chert was not subjected to biological sediment reworking, and fragile shells were preserved during accumulation. Silicification of precursor deposits of chert was promoted by intraformational dissolution-precipitation reaction of silica with increasing sediment compaction and dehydration.