新潟平野北東部における完新世中期の地形変化

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The Niigata Plain in located on the western coast of central Honshu island and consists of alluvial plains of the Shinano River in the southwestern part and the Agano River and other small rivers in the northeastern part. In this paper stratigraphy and sedimentary environments of the Holocene sediments in the northeastern part of the plain were studied on the basis of the analysis of boringlogs, AMS ^<14>C dating and diatom analysis. Coastal sand dune ridges are classified into three groups by the discontinuous pattern. They are considered to have been formed discontinuously in the middle and late Holocene, judging from the stratigraphy of the Numazawanuma Pumice, the ^<14>C date and the distribution of archaeological remains. The geomorphic change during the middle Holocene can be summarized as follows ; 1) In the culmination of the Post-glacial transgression, about 6,000 yrBP, brackish lagoons spread out behind coastal barriers. 2) Between about 5,400 yrBP and about 4,800 yrBP, sandy sediments rapidly covered the inland region and two or three sand dune 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 and by the regression of the relative sea level.
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