Quaternary System of Okinawa and Miyako Gunto, Ryukyu Islands

The "Ryukyu Limestone" is divided into the Ryukyu Limestone Group and terrace-forming limetones on the basis of the investigations of the Okinawa-, Miyako-, Kume-, Aguni- and Izena-jima in the Ryukyu Islands. The Ryukyu Limestone Group representing the principal series of the "Ryukyu Limestone" is also classified to the Itokazu Limestone Formation and "Alternated" Limestone Formation in southern Okinawa-jima. The Itokazu Formation is mainly composed of hard and compact calcareous elastics containing a "marked bed" which includes exclusively abundant Cycloclypeus fossils. The Formation can be correlated to the Yajagama Limestone Formation of Kume-jima and the Itokinabaru Limestone Formation of Aguni-jima. The "Alternated" Limestone Formation is characterized by the alternation of compact and loose limestone beds, though the Formation also reveals to be the calcareous elastics. The thickness of each bed ranges 30 to 50 cm. Miyako-jima Limestone Formation encrusting Miyako-jima, can be correlated to this Formation. The "Alternated" Limestone Formation is considered to be younger than the Itokazu Formation from various reason, though the direct relationship has not been found yet. However, the possibility of the contemporaneous heterotopic fades to the Itokazu limestome Formation in some places is still open to question. These Formations resulting from the sediments originally of extensively large basins in the early Quaternary age were tilted and displaced by faulting and afterwards were eroded. Therefore, it is clear now that the geomorphological correlation is not suitable for the Ryukyu Limestone Group. The terrace-forming limestones are represented by the Yomitan Limestone and "Awaishi" Limestone of Okinawa-jima, and the Yanoshita-jima Limestone of Izena-jima. The "Awaishi" Limestone show a typical forminiferal biosparudite which signifies calcareous shallow sea sediment, while the other terrace-forming limestones are biolithites including bioherms and patch reefs in some places. The limestones distributed sporadically throughout the islands, are generally thin formations with less than 10 meters approximately. It is significant that the terrace-forming limestones fringed as the coral reefs the islands which had been formed owing to the tectonic movement occuring after the Ryukyu Limestone Group had constructed. Therefore, the limestones can be correlated to the gravel beds of the non-calcareous terrace deposits. The terrace composed of non-calcareous deposits are typically developed in the northern part of Okinawa-jima, and can be divided into the Higher Terrace (100-180 m a. s. 1.), the Middle Terrace (30-70 m a. s. l.) and the Lower Terrace (15-20 m a. s. l.). Besides them, the post-glacial benchs of less than 10 m above sea level are seen in some islands. The Yomitan Limestone and the "Awaishi" Limestone are correlated to the Middle Terrace deposits and the Yanoshita-jima Limestone is to the Lower Terrace deposits. The Iribishi Limestone formed at the post-glacial optimum stage is typically represented by the limestone bench of the north-western shore of Kumejima.