岡山県蒜山原層の化石珪藻について
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概要
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1) As the result of analysis for the diatomite contained in the lower part of Middle Pleistocene Hiruzenbara formation, 21 genera and 51 species (including 11 varieties) are identified. All of them belong in the type of fresh water. They are composed of dominant species: Stephanodiscus and Cyclotella, a few characteristic species: Tabellaria, Asterionella, Synedra, etc., and a few companion sdecies : Meridian, Pinnularia, Navicula, and others. 2) The diatomaceous sediments at Hanazono are divided into three assemblages judging from the sedimentary facies and fossil diatoms described in the following. Assemblage A; Stephanodiscus and Cyclotella are dominant. Fructules of Stephanodiscus are small in size. Assemblage B; There exist thin layers of tuff and breccia in the diatomite. Many characteristic and companion fossils are found. Frustules of Stephanodiscus are variable in size. Assemblage C; Stephanodiscus remarkably increases, on the contrary, Cyclotella decreases extremely, but Eutrophic species appear. The size of Frustules of Stephanodiscus are about twice of those in assemblage A. 3) The differences in sizes of Frustules of Stephanodiscus; i) The size of Furustules of Stephanodiscus gradually get larger from the lower to the upper of the geological succession. ii) In the uppermost part (Sample 7) of the upper stratum (Assemblage C) Frustules of Stepha nodiscus are small in size again. iii) Judging from the pollen analysis and the others, he found the tendency that Frustules of Stephanodiscus get larger with the change of climatic condition from the warmer to the colder. 4) The environmental and geohistorical condition which controlled the diatomaceous formation at Hanazono are summarized as follows; i) The lower stratum (Assemblage A) consists of planktonic diatoms which are favorable for still water. He considers that they accumulated in the lake was comparatively pure. ii) In the middle stratum (Assemblage B) benthonic and offshore diatoms get abundant. It shows the environment of eutrophic lake. iii) In the upper stratum (Assemblage C), it may be considered that the temperature of water fell down with cold climate. iv) In the uppermost part of the upper stratum (Assemblage C), the deposition of the diatoms has arrived to an end.
- 地学団体研究会の論文
- 1975-11-20