The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
Stratigraphy of the Chinen Formation and the Ryukyu Group on islands off Katsuren Peninsula, Okinawa-jima, Ryukyu Islands, Japan
Takashi OshimizuYasufumi Iryu
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Keywords: limestone
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2002 Volume 108 Issue 5 Pages 318-335

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Abstract

The Pleistocene Ryukyu Group composed of reef complex deposits crops out on six islands off Katsuren Peninsula, Okinawa-jima, Ryukyu Islands, southwestern Japan. It conformably overlies Plio-Pleistocene sandy limestone and calcareous mudstone of the Chinen Formation containing abundant bioclasts and rests unconformably on Pliocene siltstone of the Shimajiri Group. We propose a major revision of the previous stratigraphic scheme for the Ryukyu Group and provide a formal stratigraphic description. The Ryukyu Group comprises the Yokatsu and Minatogawa Formations, in ascending order, on these islands. The Yokatsu Formation constitutes the main body of the group exceeding 30m in thickness and is exposed at elevations up to 125m. It is divisible into five units. Each unit comprises coral limestone and overlying rhodolith, Cycloclypeus-Operculina, and detrital limestones, showing a deepening-upward sequence. The Minatogawa Formation rests unconformably on the Yokatsu Formation, reaches 30m in thickness, and consists of well-sorted detrital and coral limestones that are thought to have been deposited in a shallow lagoon (moat);its surface exposure is confined to the areas at elevations less than ca. 40m in Ikei-jima and Tsuken-jima. Geologic ages of the limestones on these islands are unknown at this time.

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