琉球列島宮古群島の地質

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This article deals with geology and depositional history of the Pleistocene coral limestones of Miyakojima and the adjacent islands. Miyakojima is located in the southern part of the Ryukyu Islands, about 300 km southwest of Okinawajima, and 400 km east of Taiwan. Miyakojima is a right-triangle in outline with its hypotenuse along the northeastern coast. The highest point of Miyakojima, which is 115 m above sea-level, is in one of the NW-trending asymmetrical ridges. Major stratigraphic divisions in Miyakojima are the Pliocene Shimajiri Group, Pleistocene Ryukyu Group, and Recent coastal and lowland deposits. The Shimajiri Group crops out chiefly along the northeastern coast of Miyakojima. It strikes generally NE and dips gently SE. The Shimajiri Group comprises, in ascending order, the Oura, Onogoshi, Yonahama, and Minebari Formations which consist mostly of sandstone, mudstone, tuffaceous siltstone, and an alternation of sandstone and siltstone, respectively. The molluscan and solitary coral fossils and lithofacies indicate increase of water depth during deposition of the Shimajiri Group. In biostra-tigraphy, the highest horizon of occurrence of Globigerina nepenthes Todd and the lowest horizon of Globorotalia tosaensis Takayanagi and Saito (planktonic formainifera), and the lowest horizon of Discoaster asymmetricus Gartner, the highest horizons of Reticulofenestra pseudoumbilica (Gartner) Gartner, Sphenolithus spp. and Discoaster tamalis Kamptner (calcareous nannoplankton) are useful for age determination of the Shimajiri Group. Lithology of the Ryukyu Group is limestone composed of calcareous organic materials and their detritus. It overlies the Shimajiri Group unconformably and occurs in most part of Miyakojima and the adjacent islands. The Ryukyu Group comprises the Miyakojima Limestone and Shimojijima Limestone which overlies the former with unconformity. On the basis of fossils contained, the limestone of the Ryukyu Group is classified principally into four types : coral limestone which contains autochtonous corals, algal ball limestone containing algal balls or rhodolites, detrital limestone which consists of detritus of calcareous organic remains, and larger foraminiferal limestone composed of larger foraminifera Cycloclypeus and Operculina. Three sedimentary cycles, each of which begins from larger foraminiferal limestone or detrital lime-stone, and ends in coral limestone, are recognized in the Miyakojima Limestone, and are named the lower, middle and upper parts. Stratigraphy of the Ryukyu Group in Miyakojima and the adjacent islands is summarized in Table 1. The comparison of lithofacies and their distribution between the Recent coral reefs around the islands and limestone of the Ryukyu Group indicates a reef ridge in the southern part of Miyakojima and a back reef region in the northeastern part during deposition of the lower part of Miyakojima Limestone, and migration of the coral reefs to the northern part of Miyakojima during deposition of the middle part. The coastal and lowland deposits include Recent coral reefs, coastal dune, beach deposits, and residual clay. Their distribution is closely related to the topography. The rows of ridges on the limestone, which are remarkable topography in Miyakojima, were constructed by normal faults and subsequent weathering around them. Minor en echelon faults along the ridges were resulted from variety of throw or slumping along the major faults.
東北大学の論文
1982-03-29

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中森亨
東北大学理学部

琉球列島宮古群島の地質

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琉球列島 宮古群島の地質

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琉球列島宮古群島の地質