Natural history of biogenic siliceous sedimentary rocks as a paleoceanographical guide

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Sedimentary rocks characterized by high silica content were geologically investigated in relation to paleoceanographical reconstruction. They occur in the circum-Pacific orogenic belts. In Japanese Islands, late Paleozoic and Mesozoic bedded chert, Tertiary siliceous shale (so-called hard shale), and diatomite are the representatives of the siliceous sedimentary rocks. They consist mostly of such siliceous microfossils as radiolarian remains, siliceous sponge spicules and diatom frustules and contain no detrital coarse-grained clastics. These facts show that the siliceous rocks are of biogenic origin and were formed in pelagic to hemi-pelagic environment. And the contents of biota are most important for estimation of nature of ocean water-mass and for reconstruction of global ocean circulation in geohistory. Triassic and early Jurassic cherts embedded as exotic blocks in the Jurassic accretion complex, Chichibu, Ashio, and Mino-Tanba belts, are regarded as a sign of high productivity of marine siliceous plankton possibly in tropical to subtropical zone of the Panthalassa. Cretaceous chert in the Cretaceous to Tertiary accretion complex, Shimato Belt, is related to the formation of equatorial current as a result of the breakup of supercontinent Pangaea. Tertiary, mainly middle Miocene, siliceous shale and diatomite distributed in Sea of Japan side were accumulated under the influence of cold saline deep water. The cold deep water was related to the formation of Antarctic circumpolar current due to the opening of Drake passage and the isolation of the Antarctic Continet. Uneven distribution of siliceous sedimentary rocks in geohistory is considered to reflects plate reorganization and change in global ocean circulation and in productivity of marine siliceous microbiota.