CO-OPERATIVE STUDY OF TERTIARY SILICEOUS SHALE FORMATIONS

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Neogene siliceous shale formations in northern Japan are a part of the Tertiary siliceous deposits of the North Pacific region. Lithostratigraphy and microbiostratigraphy (diatom, Radiolaria, calcareous nannofossils, foraminifera, and pollen) of the siliceous shale formations of Aomori, Northeast Honshu have been established. The siliceous shale formations cover from Middle Miocene to Pliocene, of which the Middle Miocene is most widespread. Glauconite bed deposited on a bank exists at the base of the Middle Miocene Ohdoji Formation in the Ajigasawa area of western Aomori; the 7.3 m-thick glauconite bed on the Kurosakigawa route represents 330 m.y. with an average accumulation rate of about 45 m.y./m or 2.2 cm/m.y. In the Ajigasawa area, the average sedimentation rate of biogenic silica had been nearly constant, 7.3 - 9.3 g/cm^2kry, from Middle Miocene to Pliocene, although the average sedimentation rates of the siliceous shale formations increase from Middle Miocene to Pliocene. There … More fore, the variable sedimentation rates of the siliceous shale formations are not due to the difference in the productivity of siliceous organisms, in particular diatom, but due to the inflow of terrigenous material. REE. patterns of the siliceous shale formations show normal or positive Ce anomalies, suggesting strong terrestrial influence. The uranium content is 1 - 18 ppm, largely 10 ppm in the west of Aomosri and less than 5 ppm in the east; these values are far less than those in siliceous shale of the Monterey Formation of California. Neither phosphorite beds nor phosphatic rocks have not been recognized in the siliceous shale formations of Aomori. So far as known, some phosophatic nodule of a late diagenesis origin were reported from Upper Miocene black mudstone of the Funakawa Formation of Akita on the south of Aomori and guano-type phophorite was mined in the Noto Peninsula. The Middle Miocene diatomaceous and lithified siliceous deposits of the Northeast Honshu Arc and Noto Peninsula were accumulated in an elongated, upper to middle bathyal basin which was formed along a rift zone on the west coast of Japan. The east side of the basin was bordered by the small landmasses of the Kitakami and Abukuma Plateaus, whereas its west side was limited by a submarine rise or landmass separating the basin from the Paleo-Yamato Basin of the newly-born Sea of Japan. The width of the basin filling the Middle Miocene siliceous shale formations would be 200 - 250 km. Less