更新統下総層群上泉層にみられるギルバート型粗粒三角州

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タイトル別名
  • Pleistocene Gilbert-type coarse-grained delta, the Kamiizumi Formation, Shimosa Group, Japan.
  • コウシントウ シモウサソウグン カミイズミソウ ニ ミラレル ギルバートガタ ソリュウ サンカクス

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抄録

The Kamiizumi Formation, Shimosa Group, which is exposed in the Kanto area, Northeast Japan, was deposited in middle Pleistocene Paleo-Tokyo Bay (ca. 250-200 KaBP). Paralic and shallow marine deposits (river, estuary, delta, shoreface-beach) are recognized in the formation. The delta deposits in Hatazawa sand pit, Kisarazu-city, are composed of gravel and sands with abundant molluscan shells. The delta beds occur as a steeply dipping, large-scale foreset with a variety of facies, mainly products of sediment gravity flows on subaqueous slope of a Gilbert-type coarse-grained delta. The foreset beds are divided into two units, I and II in ascending order. The unit I is composed of alternation of two facies, parallel beds with ripple lamination (facies A) and scour-filling backset beds (facies B). The latter is thought to reflect development of chutes and pools with sedimentation related to upstream-migrating hydraulic jumps. In the top of unit I, there are abundant convolute structures and sand volcanoes which might be induced by earthquake. The unit II is composed of finer sands than those of lower unit and has slightly gentle delta slope. Parallel beds, trough and tabular cross-beds in the foreset beds of unit II might be formed by tidal currents. The direction of paleocurrents of unit II differs from that of the lower units. This difference between unit I and II is caused by shoaling upward and effect of the event of the top of unit I.This Gilbert-type coarse-grained delta developed in the margin of Paleo-Tokyo Bay where stable supply of coarse-sediment, relatively steep channel/basin ratio is present.

収録刊行物

  • 地質学雑誌

    地質学雑誌 106 (7), 461-471, 2000

    一般社団法人 日本地質学会

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