Geosites of Yonaguni Island Located at the Westernmost Point of Japan

Abstract

Yonaguni Island is located in the westernmost Ryukyu arc, where the geological setting extends to the Taiwan central range and the Luzon volcanic arc, along the subduction zone between the Eurasian and Philippine Sea plates. This paper reports the results of landform analyses based on the 10 m Digital Elevation Model and field observation at outcrops of three geosites. Miocene sandstone （Yaeyama Group） is broadly exposed on the island surface which forms low-altitude mountains and hills （maximum elevation 231 m）．Pleistocene limestone （Ryukyu Group） partly covers the Yaeyama Group and forms uplifted marine terraces composed mainly of coral reef sediments. This paper focuses on three geosites: Tindabana; Kubura-furishi; and Sanninu-dai. Tindabana is a cliff including unconformity of the Yaeyama Group sandstone and Ryukyu Group limestone, producing groundwater discharge, springs, and notches. Kubura-furishi is a rocky coast consisting of a shore platform and cliffs made of the Yaeyama Group sandstone forming remarkable tafoni. Sanninu-dai shows normal faults and many joints derived from horizontal stresses. These geosites indicate geomorphic processes in isolated island environments and suggest landform development in relation to back-arc tectonics. Yonaguni Island presents an interesting field for geotourism and border tourism in collaboration with Taiwan.