北海道の中新世岩石穿孔性二枚貝による生物侵食構造
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概要
- 論文の詳細を見る
北海道羽幌地域の中新統三毛別層から産出する岩石穿孔性二枚貝化石とその生痕化石の詳細な検討に基づいて，古環境変遷を復元した．調査地点の羽幌地域曙では，始新統逆川層と中新統三毛別層の明瞭な不整合が見られる．大型生痕化石Type I は棍棒型をなし，所属未定のグループに相当する．この生痕化石中には，二枚貝化石Platyodon nipponicaが含まれていた．一方，小型生痕化石Type II はフラスコ型で，Gastrochaenolite turbinatusに同定される．この生痕化石中には，二枚貝化石Penitella kotakaeが含まれていた．不整合におけるこのような産状は，いずれも明らかに自生的な産状を示すものである． これらは以下のような堆積過程を経て，形成されたと推察した．初めに軟質岩の波食棚が広くPlatyodonの穿孔を受け，所属未定の生痕化石を形成した．次に堆積物による充填を経て次第に基盤が固結した．さらに新たな海進が生じて，硬質岩の波食棚がPenitellaの穿孔を受け，G. turbinatusを形成した．最後に波食棚は，海進の進行に伴って外浜環境へと移行し，そこではストーム起源による貝化石層が形成されていた．このような穿孔性二枚貝化石による生物侵食構造の解析は，海進初期の特徴，非堆積期間の程度，古水深の推定等の有効な古環境情報を提供するものである．
著者
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Suzuki Akihiko
Department Of Brewing And Fermentation Tokyo University Of Agriculture
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Suzuki Akihiko
Department of Earth Science, Sapporo Campus, Hokkaido University of Education
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Hiranaka Nobuhide
Sapporo Touun Senior High School
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- 北海道の中新世岩石穿孔性二枚貝による生物侵食構造