Origin of a silicified wood-bearing bed in the nonmarine early Oligocene Tsuyazaki Formation, North Kyushu

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  • 北部九州非海成漸新統津屋崎層中の珪化木層の起源
  • ホクブ キュウシュウ ヒ カイセイ ゼンシントウ ツヤザキソウチュウ ノ

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An impressive silicified wood-bearing bed occurs in the upper part of the nonmarine early Oligocene Tsuyazaki Formation, north of Fukuoka, North Kyushu. This paper describes the internal structures, wood trunk fabric and vitrinite reflectance (Ro)of the bed. The depositional fades, fades distribution, paleocurrent patterns of the under-and overlying successions are also examined to confirm the origin and depositional environment of the bed. The wood bed includes volcano-elastic sediments abundantly and shows disorganized pebbly sandstone fades with inversely graded and scaly cleaved pebbly siltstone at the base, massive pebbly sandstone at the middle and the upper part, and flow folds at the top. Wood trunks seem to be accumulated in the lower middle part of the bed, and vary in size and frequency both vertically and laterally. They show a collisonal fabric pattern nearly parallel to the paleocurrent, judging from cross-stratifications of the water-laid sediments below and above the bed. A variety of Ro frequency relationship among vitrinite, semifusinite and fusinite contained, may imply a hybrid origin for the coaliferous materials in the bed. Those lines of evidence indicate clearly that the bed is comparable to a non-channelized, lobe-like debris flow deposits, which were emplaced on the flood-plain originally.

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