八重山群島石垣島・西表島の地質

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Ishigaki-jima and Iriomote-jima are the largest two islands of Yaeyama Gunto in the westernmost part of the Ryukyu Islands. Major startigraphic units on them are pre-Eocene Ishigaki Group, Upper Eocene Miyara Group, Lower Miocene Yaeyama Group, Sonai Conglomerate of unknown age, Pleistocene Ryukyu Group and the recent coastal and fluvial deposits. The Ishigaki Group comprises the Tomuru and Fusaki Formations. The Tomuru Formation occurs in the northeastern, east central and northwestern parts of Ishigaki-jima and in the eastern part of Iriomote-jima. Rocks are glaucophane schist, graphite schist, quartz-mica schist, phyllite, green and black schists and basalt. Maximum thickness is 2100 m. The Fusaki Formation crops out in the west central to southwestern part of Ishigaki-jima and on Taketomi-Jima, a small island to the southwest of Ishigaki-jima. It consists of sandstone, shale, phyllite, chert and limestone. The measured section in the southwestern part of the island is about 400 m. No fossils have been found in the Ishigaki Group. The sedimentary rocks of the Fusaki Formation are bounded on the east and northwest by the metamorphic rocks of the Tomuru Formation, but the relation between them remains uncertain because of poor exposure along the boundary. The estimated boundaries pass through the central part of Ishigaki-jima in north direction and the northwestern part in northeast direction. In the central part of the island, the boundary extends northward to the eastern margin of the granitic intrusion, along which dunite crops out in a few places. The Miyara Group comprises the Miyaragawa and Nosoko Formations. The Miyaragawa Formation unconformably overlies the Ishigaki Group in many places of Ishigaki-jima and in a small area in the eastern part of Iriomote-jima. It consists of conglomerate, an alternation of sandstone and siltstone and limestone. The limestone is biogenic and cotains well preserved fossils of the Late Eocene age. Fossils occur also in the conglomerate, sandstone and siltstone. Thickness of the Miyaragawa Formation is about 80 m. The Nosoko Formation overlies conformably the Miyaragawa Formation and unconformably the Ishigaki Group. The Nosoko Formation is distributed mainly in the northeastern and northwestern parts of Ishigaki-jima and in the eastern part of Iriomote-jima. It consists largely of andesite, dacite and tuff breccia, but includes conglomerate and sandstone, which yielded fossils. The pyroclastic rocks of the Nosoko Formation are altered and colored green ; they closely resemble the so-called green-tuff of the Early to Middle Miocene in the mainland of Japan. However, the fossils from the interbedded conglomerate and sandstone indicate the Late Eocene age. Total thickness of the Nosoko Formation is about 400 m. Granitic rocks intrude the Ishigaki and Miyara Groups in the northwestern part of Ishigaki-jima. The rocks are biotite adamellite and biotite granite which are accompanied by biotite-hornblende-augite quartzdiorite, granodiorite, dacite and rhyolite. In contact with the intrusions, the rocks of the Ishigaki and Miyara Groups are metamorphosed into hornfels. K-Ar age of the biotite adamellite is 21 Ma. The Yaeyama Group includes the Iriomote Formation, which occupies most part of Iriomote-jima. The Iriomote Formation consists of conglomerate, sandstone and siltstone. By the predominant lithofacies, the Iriomote Formation is subdivided into seven beds ; they are called with the letters A to G, in ascending order. Among them, the bed F is characterized by coal layers interbedded with an alternation of siltstone and silty sandstone, and is named the Uchiba-narejima Coal-bearing Member. The conglomerate and sandstone yielded molluscan and other fossils. The sandstone of the bed C and G are partly fossiliferous, and in places they grade into calcarenite and shell-limestone which form small lenses. Total thickness of the Iriomote Formation amounts to 700 m. The Sonai Conglomerate is distributed in several places in the north central to western part of Iriomote-jima. It unconformably overlies the Iriomote Formation. The conglomerate consists mostly of rounded cobbles, but includes pebbles and boulders in palces. Predominant rocks of cobbles are sandstone in the northern part and limestone and sandstone in the western part of the island, most of which are derived from the Iriomote Formation. The limestone cobbles contain larger foraminifera of the Early Miocene age. Maximum thickness of the conglomerate is 80 m. The Ryukyu Group includes the Ohama Formation on Ishigaki-jima and the Sumiyoshi Formation on Iriomote-jima. The Ohama Formation overlies the Nosoko and older formations with unconformity, and the Sumiyoshi Formation overlies the Sonai Conglomerate and older formations with unconformity. Distribution of the Ryukyu Group is limited to the area less than 70 m above sea level. In the inland area of Ishigaki-jima, the Ohama Formation consists of gravel, sand and clayey silt. The main part of the formation in the coastal area consists of detrital, foraminiferal, algal and coral limestones. The Ohama Formation occupies large area on Ishigaki-jima. Depositional surface is preserved between 20 and 70 m above sea level. The Sumiyoshi Formation is distributed in small areas along the coast of Iriomote-jima except the south coast. It consists of detrital, algal and coral limestones. Depositional surface is between 20 and 40 m above sea level. The Ryukyu Group on Ishigaki-jima and Iriomote-jima is correlated with the younger part of the Ryukyu Group in Amami-Okinawa Gunto. The recent deposits include coral reefs, beach sand and gravel, dune sand, fluvial clay, sand and gravel and residual soils. Deposits of cave and fissure in the limestone of the Ohama Formation yielded various remains of land animals.
東北大学の論文
1982-03-29

八重山群島石垣島・西表島の地質

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八重山群島 石垣島・西表島の地質

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八重山群島石垣島・西表島の地質