Petrofabrics of Bedded Cherts

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Bedded chert is one of consttituting rocks of accretion complex characterized orogenic belts in convergent plate boundary. It consists mainly of such siliceous skeletons as radiolarians and sponge spicules with subordinate clay material of aeolian origin and Fe-Mn minerals precipitated from sea water, and is therefore considered to be an ancient pelagic sediment. On the basis of petrofabrics of bedded chert, geologic processes in relation with accretion tectonics were studied in particular ranging from sedimentation and diagenesis to accretion and regional metamorphism due to subduction. Original constituent and secondarily precipitated materials are discriminated by methods using an ordinary polarizing microscope, energy dispersion and also microprobe analyzes, and X-ray power diffraction analysis. Various events during formation process of bedded cherts are recorded in their petrofabrics mentioned below, formed by siliceous organic remains, illite and chlorite, and secondary precipitates filling interstices or fractures such as quartz and manganoan chlorite.Petrofabrics indicating sedimentary environments are parallel lamination, load-cast like structure, cut and fill structure and etc. Diagenetic petrofabrics are existence of intersticial filling cryptocrystalline quartz and of stylolitic texture showing sutured contact among constituent grains due to pressure-sulution. Examples of petrofabrics related to accretion process are brecciated or collapse sedimentary textures in trench-fill and injection of argillaceous material into fractures under hyper-pressure condition. Metamorphic petrofabrics are shown as recrystallized textures including metamorphic minerals such as stilpnomelane and riebeckite. Petrofabrics during uplift process are reflected in quartz vein filling tension cleavages or cracks, probably meaning release of hyper-pressure under deeper condition.