Siliceous sediments from the Nauru Basin: diagenetic alteration of biogenic opal and authigenesis of silica and silicates.

Abstract

This study deals with the mineralogical variability of siliceous and zeolitic sediments, porcellanites, and cherts at small intervals in the continuously cored sequence of DSDP Site 462. Skeletal opal is preserved down to a maximum burial depth of 390m (middle Eocene) . Below this level, the tests are totally dissolved or replaced and filled by opal-Ct, quartz, clinoptilolite, and calcite. Etching of opaline tests does not increase continuously with deeper burial. Opal solution accompanied by a conspicuous formation of authigenic clinoptilolite has a local maximum at 150m. A causal relationship with the lower Miocene hiatus at this level is highly probable. Oligocene to Cenomanian sediments represent an intermediate stage of silica diagenesis: the opal-CT/quartz ratios of the silificied rocks are frequently greater than 1. As at other sites, there is a marked discontinuity of the transitions from biogenic opal via opal-CT to quartz with increasing depth of burial. The silica for authigenic clinoptilolite was derived from biogenic opal and decaying volcanic components.-from Author