Sedimentary facies and environmental evolution of the alluvium in the Iriai lowland, Oga Peninsula

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  • Nishikawa Osamu
    Mineral Industry Museum, Graduate School of International Resources Sciences, Akita University
  • Shimada Chieko
    Department of Geology and Paleontology, National Museum of Nature and Science

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Other Title
  • 男鹿半島五里合低地を埋積する沖積層の層序と環境変遷

Abstract

<p>The burial of a sedimentary basin and Holocene environmental evolution were investigated based on facies analysis, radiocarbon dating, diatom and pollen analyses of six alluvial drilling cores drilled in the Iriai lowland, north coast of the Oga Peninsula, NE Japan. In the Iriai lowland, about 15 m-20 m of alluvium buries a broad valley topography that was formed by scouring the Pleistocene sediments during the last glacial stage. The basement of the basin consists of a sand layer of the upper Pleistocene Katanishi Formation and a mud layer of the lowermost horizon of the Hakoi Formation of approximately 40,000 years ago. The environment of the Iriai lowland changed from an alluvial fan at the last glacial maximum to a floodplain about 10,000 years ago and to a bay about 8,500 years ago due to the sea-level rise caused by postglacial warming. At the height of the Jomon transgression, the area was transformed into a lagoon or fresh water lake which was often influenced by seawater. Subsequently, the delta progradation and the shrinkage of water area gradually occurred from the south as regression, and about 3,000 years ago, the area became a lowland with a hinterland swamp environment.</p>

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