Sedimentary Process of Plant Macrofossil Assemblages in a Meandering Channel

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  • 蛇行河川内での大型植物化石群の堆積過程
  • ダコウ カセンナイ デ ノ オオガタ ショクブツ カセキグン ノ タイセキ カ

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To clarify sedimentary process of plant macrofossil assemblages, we observed cross sections in a meandering channel buried between latest Yayoi and earliest Kofun Periods, Late Holocene, at the Kouseki Site, Chiba Prefecture, central Japan. We compared species compositions and shape and size of fossils with sedimentary structures and grain size distribution for 12 plant macrofossil assemblages and their sediments. Plant macrofossil assemblages were included in several beds composed mainly of fine to very fine sands accreted laterally to the concave bank of the meandering channel. Each bed in the channel fill deposits had washed out the lower beds and composed an upward fining sequence. With the decrease in water current force from the lower to the middle part of each bed, the shape and size of deposited plants changed, as from larger fruits and seeds, and woods to leaves and smaller fruits and seeds, which resulted in the change in species composition of fossil assemblages. Mean and standard deviation of fruit and seed size correlated with mean and standard deviation of sediment particles respectively. Fruits and seeds seemed to have been transported and deposited by surface creeping and/or saltation together with medium sands of ca. 1.5-2.0ø in diameter. On the other hand, leaves were assumed to have been transported and deposited by suspension together with fine to medium sands of ca. 3.1ø in diameter. The species composition of leaves and that of reproductive organs in the whole fossil assemblages were quite different: leaves mainly consisted of evergreen broadleaved trees including Cyclobalanopsis and some conifers, but fruits and seeds included abundant deciduous broadleaved trees and shrubs. This bias may have resulted from the difference in the production of organs between evergreen broadleaved trees and deciduous broadleaved trees or shrubs.

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