ハンモック状ベッドフォームのサイズ分布　　千葉県北東部，下部白亜系銚子層群を例として

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  • Size population of hummocky bedforms. An example from the Lower Cretaceous Choshi Group, northeastern Boso Peninsula, Japan.
  • ハンモックジョウ ベッドフォーム ノ サイズ ブンプ チバケン ホクトウブ カブ ハクアケイ チョウシソウグン オ レイ ト シテ

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The Lower Cretaceous Choshi Group in the northeastern Boso Peninsula of Japan is characterized by shoreface and offshore deposits up to more than 620 m thick and is classified into four depositional sequences. Lithofacies assemblages of the group are interpreted to have been deposited in an open and stormdominated shallow marine environment.Size population of hummocky bedforms of shoreface and offshore deposits, in particular, those of the Inubozaki and lowest Toriakeura Formations in the middle part of the group, was analyzed based upon detailed field measurements of ripple spacing of hummocky bedforms. The ripple spacing of hummocky bedforms shows a log-normal distribution pattern with a mode about 300 cm and this distribution pattern is similar to that of symmetrical ripples, suggesting oscillatory-dominated combined flow condition. The ripple spacing of hummocky bedforms is also inversely proportional to supposed paleo-water depth. This relationship is interpreted that the orbital diameter possibly responsible for determining the ripple spacing of hummocky bedform decreased in response to the increase in paleo-water depth. Bed thickness of hummocky cross-stratified sandstones, in particular, of those of offshore deposits has been supposed to be proportional to the magnitude of storm waves. The ripple spacing of hummocky bedform of offshore sandstones, however, does not show any distinct relationship with bed thickness. In general, thicker sandstone interbeds of offshore deposits contain planar stratification in the lower part and such a sedimentary structure probably developed under an intense storm wave condition superimposed by an offshoredirected current than for hummocky cross-stratification in an offshore environment.

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  • 地質学雑誌

    地質学雑誌 106 (7), 472-481, 2000

    一般社団法人 日本地質学会

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