層状放散虫チャートの起源 : I. 堆積環境および成因
スポンサーリンク
概要
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Radiolarian bedded cherts occur extensively in pre-Tertiary orogenic belts in Japan. Their origin has been debated by many geologists. At present, two different interpretations, (1) offshore or marginal sea origin and (2) oceanic deep sea origin, are considered. The shallow sea origin (1) is demonstrated mainly by chemical geologists, and the deep sea origin (2) by some stratigraphers and sedimentologists. We examined pre-Cretaceous radiolarian bedded cherts of North Kitakami (northeast Japan), Gifu (central Japan) and Amamioshima (southwest Japan), stratigraphically, mineralogically, chemically and sedimentologically. The relationship of SiO_2 with the total other major elements shows that silica in cherts is wholly derived from radiolarian shells and sponge spicules. The ^<18>O/^<16>O ratios in on-shore and DSDP cherts clearly indicate that they completely attained oxygen isotope fractionation with deep seawater after the accumulation of those siliceous remains. Sedimentary structures in cherts infer that the fractionation was progressed not in the burial stage but near the sediment-water interface. Stratigraphy of cherts shows that the sedimentary basin was situated below CCD. These data strongly suggest that radiolarian bedded cherts in the Japanese orogenic belts are of deep sea origin. This paper is divided into two parts, and the origin of cherts is mainly discussed in primary part (Part I). We tested hydrodynamic characters of different siliceous materials in a water tank. Threshold velocity of radiolarian shells was measured to be 6-7cm/sec. It is inferred from this value that pure radiolarian ooze is very erosive and can accumulate under calm water condition. No quick movement of water mass near bottom surface at the time of chert deposition is also suggested by the sedimentary structures. We tried to synthesize chert from artificial siliceous gels in hydrothermal vessels, and after a long experimental run (about 1000 hours at 250℃, 60 bars) we get cherty materials. This reveals that oxygen isotope fractionation of siliceous organic remains with surrounding bottom seawater was attained by colloidization of them and gelatinous silica is the most probable origin of micro-crystalline quartz in cherts. In secondary part (Part II), the sedimentation and mineralization of siliceous shells are mainly discussed.
- 日本古生物学会の論文
- 1984-06-25
著者
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