Abstract

Diagenetic changes corresponding to silica-phase transformations (opal-A --> opal-CT --> quartz) were studied. In opal-A diatomite, siliceous tests are well preserved, no cementation is observable, and pores exist as inter- and intragranular micropores (2 degrees 10mu ). During the opal-A to opal-CT transformation, siliceous tests are dissolved to form the 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, which may be due to the destruction of intragranular pores in siliceous tests by dissolution. Ultramicropores (<2mu ) 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 particles of around 1mu in diameter in quartzose porcelanite. In spite of the change in texture, no abrupt decrease in porosity occurs between the opal-CT and quartz zone. Pores exist as intergranular micropores among quartz particles and macropores (>10mu ) as molds and chambers of siliceous tests in quartzose porcelanite. At a depth of 4.5 km and a temperature of 130 degrees 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 other than mechanical compaction, silica-phase transformations and clay alteration. Pressure solution-reprecipitation of quartz is the most probable one.--Modified journal abstract.

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