奄美大島の地質,とくに重力滑動と崩壊による地質の構成について

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The pre-Neogene sedimentary rocks of Amami Oshima are classified into the Upper Jurassic to Lower Cretaceous Yuwan Formation, Cenomanian to Turonian Naze Formation, Turonian Odana and Ogachi Formations, and Eocene Wano Formation. General strike is northeast and regional dip is northwest. In the eastern to central part of the main island of Amami Oshima, they are distributed principally in zones of, from southeast to northwest, theNaze, Wano, Naze, Ogachi, Naze, Odana and Yuwan Formations. Repeated occurrence of the formations is due to thrust and gravity sliding. In the western part including Kakeroma-jima, Uke-jima and Yoro-jima, local features of gravity transportation modify the zonal distribution greatly; the distribution of the formations is concentric, though general strike and regional dip are similar to those in the eastern part. Granite of 50-60 Ma in K-Ar age intrudes the Naze and Odana Formations in six small areas. The Yuwan Formation consists of siliceous slate, sandstone, chert and basalt, and contains allochthones of bedded chert, basalt and limestone. Most of the allochthones are of Permian and Triassic, but some of them contain again allochthones of Carboniferous and Permian limestone. The Permian allochthones contain the allochthonous Permian limestone, and the Triassic allochthones contain the allochthonous Carboniferous limestone. In the autochthonous part of the Yuwan Formation, the Upper Jurassic sediments contain allochthonous Triassic rocks and the Lower Cretaceous sediments contain allochthonous Permian rocks. Gravity transportation of the Triassic and Permianal lochthones from the northwest took place successively in the Late Jurassic to Early Cretaceous. Albitization was recognized in the allochthonous basalt. The Naze Formation consists of slate, phyllite, sandstone and basalt. The Odana Formation consists mostly of sandstone and slate, but contains allochthonous masses of Aptian slate and basalt, which were derived from strata originally overlain the Yuwan Formation. The Ogachi Formation consists of sandstone and sandy slate, which are heteropic with the sandstone and slate of the Odana Formation. The Wano Formation consists of sandstone, slate and conglomerate. Intraformational deformation is common in these formations. In the west ern part, the Yuwan Formation contains huge masses of allochthone, which have basin structure and are in discordant contact with autochthonous layers. The older Yuwan Formation as a whole overlies the younger Odana Formation; the contact between them is a listric surface of southeastward gravity sliding. The Odana Formation slumped a short distance from its original site southeastwards on to the Naze Formation. In the central part, theNaze Formation thrust southeastwards over the Ogachi Formation, which is in turn in slip-plane contact with the Naze Formation on the east side. The Wano Formation forms an assymmetrical syncline in a trough between the blocks of the Naze Formation on both sides in the eastern part of the island.
日本地質学会の論文
1983-03-30

奄美大島の地質,とくに重力滑動と崩壊による地質の構成について

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