壱岐島の地質 : とくに中新統壱岐層群について

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The Iki Island is situated about 20 km northwest of Kyushu and lies in Genkai-(nada) sea, which is less than 100 m in depth. This island consists exclusively of the Cenozoic sediments and volcanics (MATSUI, 1958). The geological successions are summarized in Table 1. This paper is mainly described to the stratigraphy of the volcanics and sediments in the Iki Group. The Katsumoto Group is composed of the alternations of sandstone and shale. They are gently [Table] folded. The shales yield marine molluscs and echinoderms. This group is correlated to the Oligo-Miocene Taishu Group in Tsushima lslands. The Iki Group is divided to the four Formations, the Wakamatsu, the Kuki, the Monobe and the Haze in ascending order. The Wakamatsu Formation is distributed at the southern coast, the south of Kuki and the eastern coast, the Chojabaru. It consists of andesites, andesitic tuff breccias, mudstone and diatomite. The volcanic rocks are altered and contain zeolite group such as levyne, erionite, chabazite, phillipsite and thomsonite. The Chojabaru diatomite yields the early to middle Miocene plant fossils as Keteleeria ezoana, Glyptostrobus europaeus, Quercus (Cyclobalanopsis) nathorstil, Podogonium knorrii, Koelreuteria miointegrifolia, Carpolithes sp. Ishida (1970), etc. This also yields fish and insect fossils, Iqiuus nipponicus (JORDAN, 1919) and Terpandrus ? ikiensis, Sphaenogster (Deromyrma) avita, Plecia kanetakii, etc. (FUJIYAMA, 1970). The diatomite consists of a rather few species of diatom. The lowerpast is characterized by Coscinodiscus rothii, and the upper part by Melosira. They suggests a gradual change from lagoon to a fresh water lake (ISHIDA et al., 1970). The Kuki Formation consists of ash flow deposits, tuffs, conglomerate, basalt, andesites and rhyolite. The Monobe Formation is distributed at lowland in the central part of the island. It consists mainly of tuffaceous sediments intercalating basalt, andesite and rhyolite lavas. The Haze Formation consisting of rhyolite and pumice flow deposits is correlated to the upper part of the Monobe Formation. The Ashibe Group represented by basalts widely cover the Iki island and form a plateau, 174.8 m in highest altitide. Their maximum thickness is about 110 m. The basal or intercalated sediments yielded Stegodon and the Pliocene-Pleistocene plant fossils, Glyptostrobus, Metasequoia and Liquidambar etc. The Gonoura Group consists of basalt scorias and lavas, which make the cinder cones at Takeno-tsuji (212.9 m), Shikanotsuji (101.1 m), Tsunokami-yama (133.7 m), south of Tokami-yama (about 120 m), Ondake (147.2 m) and Medake (about 140 m). The volcanic rocks of the Wakamatsu and Kuki Formations are characterized by pigeonitic or hypersthenic rock series. The olivine basalts of the Monobe Formation and the Ashibe and Gonoura Groups belong to alkali rock series.
日本地質学会の論文
1973-07-01

壱岐島の地質 : とくに中新統壱岐層群について

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