Fossil Diatoms from the Akima Formation distributed in the northern Part of Annaka City, Gumma Prefecture, Central Japan

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  • 群馬県安中市北部に分布する秋間層の化石珪藻
  • グンマケン アンナカシ ホクブ ニ ブンプスル アキマソウ ノ カセキ ケイソ

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The Akima Formation of the Neogene system mainly consists of pyroclastic rocks and it is divided by lithofacies into seven members. Ten samples from the Akima Formation were analyzed into each assemblage of diatoms. The diatom assemblages are composed of 66 specis and 30 genera. Their names and frequencies are shown in Table 2. The dominant and common species in them are Achnanthes lanceolata, Cocconeis placentula, Cyclotella meneghiniana, C. stelligera, Cymbella ventricosa, Diploneis ovalis, Meilosira distans, M. granulata, Synedra ulna, and all 'species show fresh v/ater forms. It may be sure that diatoms of this assemblage had lived in both stagnant and running water. From these results, the sediments containing this assemblage seem to be settling at the bottom of marsh and a river.

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