Geomorphic Development of the Northeastern Part of the Niigata Plain, Central Japan, during the Holocene

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  • 完新世における新潟平野北東部の地形発達史
  • カンシンセイ ニ オケル ニイガタ ヘイヤ ホクトウブ ノ チケイ ハッタツシ

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Abstract

The Niigata Plain, which one of the largest plains in Japan, is located on the western coast of central Honshu island, extending about 70 km with various widths from the northeastern end to the south-western end.<br> In this paper, the author studied the stratigraphy and the sedimentary environments of the Holocene sediments in the northeastern part of the plain, northward from the Agano River, on the basis of the analysis of boring data, AMS radiocarbon dating, and diatom analysis to reconstruct the geomorphic development of the region during the Holocene.<br> The Holocene sediments behind coastal sand dune ridges are divided into middle silt and clay (MC), upper sand (US), and top alluvial deposits (T). The middle silt and clay layers are considered to have been deposited in brackish lagoons, judging from the diatom assemblage which is composed of marine-brackish species, brackish-fresh species, and fresh-water species. The upper sand layer is mainly composed of sand and silty sand, mingled with fluvial gravel near the rivers. It is considered to have been formed as a foreset bed, because fresh-water diatoms are dominant in the layer. The top alluvial deposits are composed of sand, silt, and peat, and are considered to have been deposited in sedimentary environments of back marshes or ponds, based on the diatom assemblage.<br> Coastal sand dune ridges can also be classified into three groups. They are considered to have been formed discontinuously in the middle and late Holocene, judging from the stratigraphy of the Numazawanuma Pumice, the radiocarbon dates, and the distribution of archaeological remains.<br> The geomorphic development of the region during the Holocene can be summarized as follows:<br> 1) In the early Holocene, small brackish lagoons were formed behind coastal barriers.<br> 2) In the culmination of the Post-glacial transgression, about 6, 000 yr B. P., brackish lagoons spread out behind coastal barriers. They were relatively high in salinity because of the influx of seawater through the tidal inlets.<br> 3) Between about 5, 400 yr B. P., and about 4, 800 yr B.P., sandy sediments rapidly covered the inland region and two or three beach ridges prograded seaward. It is considered that the sedimentation of sand has been influenced by the increase of coarse sediments from the upper reaches of the rivers.<br> 4) After about 4, 800 yr B. P., seawater flowed into the inland region again.<br> 5) Between about 3, 400 yr B. P. and about 2, 400 yr B. P., peat bogs developed in the inland region and beach ridges approached the present shore.<br> 6) After about 2, 400 yr B. P., back marshes and ponds developed intricately in the inland region and sand dunes developed near the current shore.

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