Stacked Rosselia socialis from the middle Pleistocene Yokaichiba Formation, Chiba Prefecture, central Japan : Sedimentation and responses of Rosselia animal

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  • 千葉県北東部,中部更新統八日市場層における化石棲管連結ロッセリア : 地層形成と造営動物の反応
  • チバケン ホクトウブ チュウブ コウシントウ ヨウカイチバソウ ニ オケル カセキ セイカン レンケツ ロッセリア チソウ ケイセイ ト ゾウエイ ドウブツ ノ ハンノウ

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Abstract

Based on the study of stacked Rosselia socialis, impregnated in the fine sand of the lower part of the middle Pleistocene Yokaichiba Formation in the northeastern part of Chiba Prefecture, new idea were proposed on the mechanism of the formation for these burrows as follows. Examined specimens are crowed abundantly in fine sand bioturbated heavily intercalating hummocky stratified fine sand layer. These sediments might be originally the hummocky sequence in the inner shelf, however changed into the present facies with tiering structures due to the bioturbation by various animals. As to the shape of segment of stacked burrows, funnel-type and spindle-type had been recognized, however many "cone-type" segments piling upwards with interval of 5cm〜10cm were marked in this study. The difference of these three shapes of segments might be produced by burrowing action of animal in Rosselia socialis affected by the cyclic event of erosion and deposition occurred during the course of sedimentation. Piles of cone-type segments were formed by the repeat of the sedimentation in miniature. The repeat of the sedimentation in miniature resulted the increase of thickness of bioturbated portion, occurring amalgamation of the hummocky sequence.

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