Abstract
The Ryukyu Group, composed of Pleistocene reef complex deposits laterally passing into terrigenous sediments, extensively crops out on Yoron-jima, Ryukyu Islands, southwestern Japan. Our investigation leads to a major revision of the previous stratigraphic scheme. This paper aims to provide a formal stratigraphic description of the Group on Yoron-jima. The Pleistocene sequences comprise three groups of strata. The lowest Ugachi and Mugiya Formations consist of partly-karstified coral limestone ; their surface exposures are confined in the type localities. The stratigraphic relationship between them remains unknown. The Yoronjima Formation which unconformably overlies the Ugachi and Mugiya Formations reaches 55 m in thickness and is exposed extensively, covering most of the island. It is divisible into three units, each consisting of proximal coral limestone and distal rhodolith, Cycloclypeus-Operculina, and detrital limestone. The Chichizaki and Tomori Formations unconformably resting on the Yoronjima Formation are composed mainly of thin (<10 m) coral limestone. They crop out on the southern and eastern peripheries of the island at elevations less than ca. 20 m. The relation between them is uncertain. On stratigraphic position and known age-diagnostic nannofossils, the Yoronjima Formation may be correlative to the main body of the Ryukyu Group on Okierabu-jima (lower and upper units) and Toku-no-shima (units 1, 2 and 3), ranging in age from 390 to 890 ka, while the ages of the other formations are left open.
