Diagenetic process of bedded chert

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Bedded cherts of Japanese Paleozoic and Mesozoic systems mainly consist of radiolarian remains and siliceous sponge spicules with subordinate claymaterial of aeolian origin and Fe-Mn minerals precipitated from sea water. By using siliceous skeletons as a starting material, experimental investi-gation under hydrothermal condition (hydrostatic pressure of 100 to 300 kg/cm2, about 100 to 300゜C )were carried out, in particular with relation to textural change of siliceous sediments during diagenesis. The results after 130 or more hours shows that interspaces among siliceous skeletons were filled with secondary preci-pitated silica, while some skeletons were dissolved. A dissolution-redeposition process is involved in the reaction, suggesting that neoformation of matrix silica and cementation progress. The experiments treated under high-temperature condition, corresponding to those of contact metamorphism, show that all siliceous skeletons of amorphous silica were trans-formed into equigranular aggregates of quartz by recrystallization. Evidences from texture of the bedded chert and the experimental products support the following diagentic process: highly porous siliceous sediments (amorphous silica) compaction and change of intersti-cial water due to silica solution decrease of porosity due to precipitation of silica from the intersticial water as increas of burial depth and temperature (opal CT) hard and compact chert completely filled with secondarily redeposited silica (cryptocrystalline quartz). The transformation from opal CT to quartz is probably a solid-state reaction because original sekeleton shapes are preserved even in quartz chert. The bedded cherts of orogenic belts were formed under deep-sea floor conditon and after diagenesis were accret-ed in subduction zones.