Geomorphic Development of Modern Fringing Reefs of Yoron Island, Ryukyu Arc, Japan.

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  • 琉球列島与論島の現成裾礁の地形発達
  • 琉球列島与論島の現成裾礁の地形発達〔英文〕
  • リュウキュウ レットウ ヨロントウ ノ ゲン セイ キョショウ ノ チケイ ハ

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Modern fringing reefs of Yoron Island, Ryukyu Arc, Japan, have been investigated for the purpose of understanding the geomorphic development of the surface features and shallow sedimentological structure of the coral reef. Field investigations have been conducted by means of geomorphological observations, scuba divings, bathymetrical soundings and multiple shallow drillings on a modern reef at the northeastern coast of the island. Modern fringing reefs of Yoron Island form a distinctive zonation of landforms, which are composed of sandy or rocky shores, a moat, a reef crest, a reef front with spurs and grooves, and outer slopes from the shore to the reef margin. The cored samples from the modern coral reef are divided into the following four facies: reef crest, reef pavement, bioclasts and moat. A rigid framework of the reef crest has been formed upon the unconsolidated bioclasts facies. The reef crest of Yoron Island started to grow upward from 3m below the sea level to near the sea surface for the period from 5, 260 to 3, 230yrs BP. The reef pavement behind the crest has extended toward the moat along with the reef crest formation. After 3, 230yrs BP, the crest is also extending its width by forming spurs and grooves toward the ocean. These results indicate that the geomorphic zonation of the modern fringing reef of Yoron Island has been formed in the recent 5, 000 years.

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