Holocene Geomorphic Development of the Kitakami River Lowland, Northeastern Japan.

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  • ITO Akifumi
    Institute of Geography, Graduate School of Science, Tohoku University

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  • 北上川下流沖積低地の完新世地形発達
  • キタカミガワ カリュウ チュウセキ テイチ ノ カンシンセイ チケイ ハッタツ

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The Kitakami river lowland is an alluvial-coastal plain, located on the Pacific side of northeastern part of Honshu Island, extending about 50km long from north to south and 20km wide from east to west. Most of surface level is less than +10m a. s. l. The lowland is geomorphologically divided into three parts: the northern part composed of many natural levees and abandoned channels, the central part dominated by back marsh, and the coastal part with remarkable beach ridge ranges.<br>In order to reconstruct the geomorphic development of the region, the author clarified the stratigraphy and the sedimentary environments of the Latest Pleistocene and Holocene sediments, on the basis of the analysis of borehole records, and radiocarbon dating, FeS2-S (pyritic sulfer) analysis.<br>The Latest Pleistocene and Holocene sediments beneath the lowland are divided into the following five layers: lower gravel layer (LG), lower sandy layer (LS), middle clayey layer (MC), upper marine sand layer (Ua), and upper terrestrial sandy layer (Ub) in ascending order. LG is fluvial gravel which were deposited during the stage of the lower sea-level in the Last Glacial stage. LS is composed of sandy sediments deposited in the flood plain during the early transgression stage after the Last Glacial Maximum. MC is composed of clayey sediments with many marine mollusca. MC was deposited in the delta and the bay during the Holocene transgression stage. Ua is composed of sandy sediments with shell fragments, and distributed only in the coastal part. Ua was deposited in shallow marine environment. Ub is sandy sediments with many organic material. Ub was deposited formed on the floodplain as marsh deposit in the Holocene regressional stage.<br>The Holocene transgression and regression process was reconstructed based on distribution of marine and terrestrial sediments. There had been three basin previously until the sea-level rose to -7m (ca. 7, 500y. B. P.). The volume of supplied sediment load of the Kitakami river was much more than the other rivers in the regressional stage, about 8, 000y. B. P. and 7, 000y. B. P.

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