琉球列島新生代地史の概要

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The Ryukyu Islands form in an island arc stretching for 1200 km between Kyushu and Taiwan. The Ryukyu Trench on the east and Okinawa Trough on the west are parallel with the islands. Climate is subtropical and coral reefs are growing around the islands. The Cenozoic of the Ryukyu Islands is classified into the Lower Eocene Wano and Kayo Formations, Upper Eocene Miyara Group, Lower Miocene Yaeyama Group, Sonai Conglomerate of unknown age, Middle Miocene Takarajima Group, Pliocene Shimajiri Group, Middle to Upper Pleistocene Ryukyu Group and Holocene coastal and terrestrial deposits. Besides, the Miocene intrusions are on a few islands. The Lower Eocene formations crop out on Amami-Oshima and Okinawa-jima in fault contact with the Mesozoic rocks. They consist of an alternation of slit stone, sandstone and conglomerate, and yielded Nummulites and molluscan fossils. Intraformational folding is common in these formations besides secondary structural features. The Miyara Group overlies the pre-Tertiary rocks on Ishigaki-jima, Kobama-jima and Iriomote-jima in the southwestern part of the Ryukyu Islands. The group comprises two formations. The Miyaragawa Formation consists of conglomerate, sandstone and limestone which are fossiliferous. The Nosoko Formation conformably overlies the Miyaragawa Formation, and consists largely of andesite, dacite and rhyolite, but it includes conglomerate and sandstone intercalations which yielded fossils. The pyroclastic rocks of the Nosoko Formation are slightly altered into green colored. Geologic structure is rather simple. The Yaeyama Group crops out on Kobama-jima, Iriomote-jima and Yonaguni-jima. It overlies the Miyara Group with unconformity, and consists of conglomerate, sandstone and thin layers of limestone, siltstone and coal. Micro fossils from the upper part indicate the transition from the Lower to Middle Miocene. On Iriomote-jima, the Sonai Conglomerate unconformably overlies the Yaeyama Group. The conglomerate is composed largely of cobbles of fossiliferous limestone and sandstone of the Yaeyama Group. The Takarajima Group is exposed on Kume-jima and the north along the Tokara Islands. Rocks are mostly andesite. Green colored rocks are similar in appearance to those of the Nosoko Formation and so-called green tuff of Miocene age in the mainland of Japan. Volcanic rocks distributed on sea bottom between the northern half of the Ryukyu Islands and Okinawa Trough belong probably to the Takarajima Group. No Neogene volcanic rocks are exposed in the southwestern part of the Ryukyu Islands. Granite of 22 Ma in K-Ar age intrudes the pre-Tertiary rocks on Ishigaki-jima and dacite of 12 Ma intrudes the Mesozoic on Okinawa-jima. Many other acidic intrusions are distributed around the northern extention of the Ryukyu Islands onto Kyushu. The Shimajiri Group crops out on Kikai-jima, Okinawa-jima, Kume-jima, Miyako-jima and Hateruma-jima. The rocks are mostly fossiliferous sandstone and marly siltstone, but andesite and basalt exist on Kume-jima and dacitic tuff on Okinawa-jima. The sections of these islands are correlated with the Upper Neogene of Kyushu and Taiwan by micro fossils and paleomagnetism. The basal part of the Shimajiri Group on Okinawa-jima may be the upper most part of the Miocene and the uppermost part is the lowest part of the Pleistocene. The lower part of the group yielded elephant fossils on Miyako-jima. The Ryukyu Group occurs as coastal terrace formations between sea level and 200 m above it on most islands, but in the southwestern part only the younger part is exposed between sea level and 100 m above it. The Ryukyu Group consists largely of biogenic limestone including coral reefs, but it includes terrestial deposits in rather large islands. The limestone yielded fossils of elephant and deers on Okinawa-jima; one of the deers is a Miocene-type. Along the Miocene volcanic belt many volcanoes erupted in the Quaternary, and some of them are still active. In the northern extensi
日本地質学会の論文
1983-03-30

琉球列島新生代地史の概要

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