Diatom biostratigraphy of the middle to upper Miocene Nomura Formation in the Otonashigawa section, Tsugawa area, Niigata Prefecture, Japan

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  • 新潟県津川地域音無川ルートに分布する中部～上部中新統野村層の珪藻化石層序
  • ニイガタケン ツガワ チイキ オトナシガワ ルート ニ ブンプ スル チュウブ ジョウブ チュウ シントウノムラソウ ノ ケイソウ カセキソウ ジョ

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    The middle to upper Miocene diatom biostratigraphy has been established for the Nomura Formation exposed along the Otonashigawa River in the Tsugawa area, Niigata Prefecture, Japan. Neogene North Pacific diatom zones NPD5B through NPD6A are recognized in the Nomura Formation. Sediment accumulation curve contructed by diatom biochronology indicates that the sedimentation rate remarkably decreased to 0.3-0.15 meters per 1000 years around the base of the Nomura Formation. The last occurrence of Denticulopsis dimorpha var. areolata was recognized between the diatom biohorizons D59 (93 Ma) and D60 (9.2 Ma), and this event can potentially serve as a stratigraphic marker to improve the resolution of late Miocene diatom biostratigraphy. A mass occurrence of Goniothecium rogersii has been found in the lower part of zone NPD 5D between the horizons D56 and D58 where the species composes over 20 % of the total diatom assemblages. This event consists of four peaks of the abundance and persisted from 9.75Ma to 9.45Ma. The event has been recognized widely in several stratigraphic sections of the northern part of the Neogene Niigata sedimentary basin.

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