南大東島におけるカルスト化過程

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タイトル別名
  • Karstification Processes of Minamidaito Island, in Japan’s Nansei Islands
  • ミナミダイトウジマ ニ オケル カルストカ カテイ

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説明

<p>To clarify karstification processes, the present study was carried out at Minamidaito Island, southwestern Japan. Between the “Lower Daito Layer” and “Upper Daito Layer,” there exists an unconformity, composed of a fossil soil layer, so- called “rainbow stone”. The fossil soil layer provides proof of the existence of karstified limestone during the period when the layer appeared on the surface. The highest limestone wall of this island rises to about 75m, and the soil covered middle basin is at about 8m a. s. l. Solution rates obtained through 10 years of observation were used for estimating the karstification period of the island. Values observed in the atmosphere 1.5m above ground were used for estimating the values on the limestone wall. Values in soil the B2 horizon, were used for estimating the status in the soil covered basin. The estimated age of karstification is about 1.6 Ma under approximately the same climatic conditions as at present. If the Last Glacial Age had a wetter climate than at present, however this estimated age would be less. In the basin area, there are some horizontal caves. One of them, the Point 7 Cave was chosen for dating by the 230Th/234U, speleothem method. Part of the horizontal cave has formed since 110ka to 50ka. The cave developed continuously during the Last Glacial Age as a vertical cave rich with speleothems. Unfortunately, almost all of them exist under the ground water. The abundance of these speleothems might constitute proof of wetter condition in the Last Glacial Age than at present. On the coast, kamenitzas have been formed abundantly on benches at 10-12m and 3m produced by sea water spray and rain water. Interestingly shaped pinnacles called “saboten,” also formed at about 20-25ma.s.l. These pinnacles were formed, not only by the action of rain water, but also of sea spray from high waves which occur frequently.</p>

収録刊行物

  • 地形

    地形 38 (2), 107-128, 2017-04-25

    一般社団法人 日本地形学連合

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