ABSTRACT

Diagenetic changes in textures and mass properties of Neogene marine noncalcareous siliceous rocks corresponding to silica-phase transformations (opal-A opal-CT quartz) were studied in subsurface and surface sections of northern Japan.

In opal-A diatomite, siliceous tests are well preserved, no cementation is observable, and pores exist chiefly as inter- and intragranular micropores (2 10µ). During the opal-A to opal-CT transformation, siliceous tests are extensively dissolved to form fine aggregates of opal-CT in matrix and cement of opaline porcelanite. An abrupt decrease of as much as 15% in porosity occurs between opal-A and opal-CT zones in subsurface sections, a decrease which may be due to the destruction of intragranular pores in siliceous tests by dissolution. Ultramicropores (<2 µ) are dominant in opaline porcelanite. During the opal-CT to quartz transformation, fine aggregates of opal-CT in matrix and cement change to aggregates of quartz p rticles of around 1µ in diameter in quartzose porcelanite. In spite of this obvious change in texture, no abrupt decrease in porosity occurs between the opal-CT and quartz zones in subsurface sections. Pores exist almost exclusively as intergranular micropores among quartz particles and macropores (>10 µ) as molds and chambers of siliceous tests in quartzose porcelanite.

Porosity of Neogene noncalcareous siliceous rocks in subsurface sections of northern Japan gradually decreases with the increase of burial depth, and silica-phase transformations have a relatively small effect on it. This gradual decrease in porosity, which corresponds well to that of Neogene mudstone and clean, fine-grained sandstone, indicates that little additional silica cementation occurs during burial (Iijima and Tada, 1981). At a depth of 4.5 km and a temperature of 130° C, quartzose porcelanite holds porosity of 10-15%. Burial depth of around 10 km might be required to form dense quartzose chert without porosity by burial diagenesis of siliceous rocks if extrapolating from the burial depth-porosity relation. This depth is too large. Some additional mechanisms are required ther than mechanical compaction, silica-phase transformations and clay alteration. Pressure solution-reprecipitation of quartz is the most probable one.