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No.27 (1979/03) >

 
Title :Quaternary Geologic History of the Ryukyu Islands
Authors :Furukawa, Hiroyasu
Authors alternative :古川, 博恭
Issue Date :Mar-1979
Abstract :In this paper the Quaternary geologic history of the Ryukyu Islands has been made clear on the grounds of the field data of regional stratigraphy and also the results of the geochronology by ^<14>C measurements. The Quaternary System in the Ryukyu Islands is divided in ascending order into the Upper Shinzato Formation and Chinen Sandstone Member of the Shimajiri Group, Ryukyu Limestone and Terrace Deposits of the Ryukyu Group in the Pleistocene, Alluvial Deposits, Younger Dune Sands and Recent Coral Reef Sediments in the Holocene. The correlation of the Quaternary strata among various islands of Ryukyu Arc is shown in Table 1. The Shimajiri Group was deposited during the transgression from Upper Miocene to Lower Pleistocene, consisting mainly of thick mudstone with some intercalated sandstones and tuffs. The Ryukyu Limestone was deposited with a distinct unconformity on the Shimajiri Group and other basement rocks. Sand and gravel beds are found locally to replace part of the limestone. The Ryukyu Limestone is a continuous series of strata in which no great unconformity intervenes. It is generally 40 to 60 m thick and scarcely exceeds 100 m at the maximum. Faulting was remarkable after the deposition of the Ryukyu Limestone, cutting perpendicularly the island arc with accompanied uprise of the basement rocks.The terraces are generally divided into higher, middle, and lower groups, and the sediments forming these terraces comprise mainly sand and gravel and partly silt and limestone. In the Latest Pleistocene the land-area was much enlarged, owing to the lowering of ancient sea level and reddish brown soil was distributed on the subaerial flat place during that time, which now lies mostly under the sea. The sediments accompanied by the transgression during the period from the Latest Pleistocene to Holocene are distributed on the reddish brown soil with an unconformity, wheras the reef sediments were deposited around these islands. This redords a slight uprise of sea level, at least one meter higher than the present position at the peak phase of the transgression during at about 5,000 to 4,000 Y.B.P. There was the lowering of sea level down to two meters lower than the present position during the period of 3,000 to 2,000 Y.B.P. Accompanied by it Younger Dune Sands were formed along the present coast lines. The correlation of the geologic history from the latest Pleistocene to Holocene between Ryukyu and Kyushu is shown in Tables 3 and 4..
Type Local :紀要論文
Publisher :琉球大学理工学部
URI :http://hdl.handle.net/20.500.12000/23768
Citation :琉球大学理工学部紀要. 理学編 = Bulletin of Science & Engineering Division, University of Ryukyus. Mathematics & natural sciences no.27 p.99 -161
Appears in Collections:No.27 (1979/03)

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