Uranium-series dates of fossil hermatypic corals in Ishigaki-jima Island, Okinawa Prefecture, and their significance

This study provides new uranium-series dates of the Ryukyu Group in Ishigaki-jima Island, South Ryukyu Arc (SRA). Two fossil hermatypic coral samples taken from coral limestone in the northern and southern parts of the island were dated as 118.5±2.0 ka and 119.9±2.0 ka, respectively, which correlates to oxygen isotope stage 5.5. This fact suggests that a reef complex was formed during isotope stage 5.5 in Ishigaki-jima Island, just like in Hateruma-jima and Yonaguni-jima Islands, SRA. The dated coral in the northern part was collected at an elevation of 1.3 m. It is, therefore, inferred that the northern part of the island was tectonically stable since isotope stage 5.5. On the other hand, the dated corals in the southern part was collected at an elevation of 15 m, which indicated that the southern part of Ishigaki-jima has been uplifted at rate of about 0.10 m/ky since isotope stage 5.5. Our data clearly show that the western part of SRA has been situated in the tectonic setting of uprift for the last 125 ky.