Silica diagenesis of Neogene diatomaceous and volcaniclastic sediments in northern Japan*

Abstract

ABSTRACTMarine diatomaceous siliceous sediments in Neogene sections of northern Japan contrast with the Monterey Shale of California in containing many intercalations of acidic volcaniclastic sediments. Diagenesis of these sediments from deep boreholes and surface sections was investigated. Three diagenetic zones—biogenic opal, opal‐CT and quartz zones—are recognized in siliceous sediments, corresponding roughly to amorphous silica, low cristobalite and quartz zones in acidic vitric volcaniclastic sediments. Opal‐CT consists almost exclusively of silica and water, while low cristobalite contains appreciable amounts of A1, Ca, Na and K.In subsurface sections, values of d(101) spacing of opal‐CT decrease progressively with increasing burial depth. The progressive ordering is not associated with additional silica cementation. In surface sections, the behaviour of d(101) spacing is complicated owing to the modification of the progressive ordering developed during burial diagenesis by later silica cementation during uplift. The cementing opal‐CT is probably precipitated from percolating groundwater which dissolves siliceous skeletons in porous diatomaceous mudstones overlying the opal‐CT porcellanite. Opaline cherts that form during burial diagenesis are designated as early opaline chert, while those which form during uplift are later opaline chert. The later opaline chert contains two groups of opal‐CT; one is progressively ordered opal‐CT and the other is additionally cemented opal‐CT with higher d(101) spacing than that in the host porcellanite. In diatomaceous siliceous sediments, early opaline chert is scarce. Most, if not all, opaline cherts in surface sections are of later origin.