熊本県長洲町付近の第四系

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タイトル別名
  • Quaternary System of the Nagasu Area, Kumamoto Prefecture
  • クマモトケン ナガスマチ フキン ノ ダイ4ケイ

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In the Nagasu area the Quaternary sediments are extensively distributed, resting on the basements of granites and the Paleogene formations. They are divided into six formations in ascending order: 1) Fumoto Formation (Higher terrace, 60 m in height), made up of gravel and sand beds. This is characterized by highly weathered gravels, the so-called Kusari-reki (corroded gravels), and partly intercalated with thin clay and silt beds. Gravels mainly consist of the granitic breccias or subangular gravels derived from the hinterland. Fumoto Formation is 10-20 m in thickness. 2) Nagasu Formation, (not forming the terrace), consisting of gray or dark gray siltstone; lower part contains shell remains of the inner bay environment, such as Theora lata, Raeta pulchella, and Barnea japonica, etc., and the plant fossils such as Pinus, Fagus and Quercus. The sediments become yellowish-brown in the upper part. Its thickness is about 5-10 m. 3) Akata Formation (Middle I terrace, its height 30-60 m), consisting of sand and gravel beds mainly composed of the cobbles of andesite, sandstone, crystalline schist and granite, intercalating pumice, silt and clay beds. The uppermost of the formation is covered with red soil. 4) Yame-nendo Formation (Middle II terrace, its height 20-30 m), consisting of clayey pumice tuff; its upper part is pink, lower part is gray or grayish-white and becomes more pumiceous and coarser. The composition of heavy minerals is hyperthene (Hyp)>hornblende (Hn)>augite (Au)>magnetite (Mg). Hyperthene is nearly equal to hornblende in volume. This formation is the fill top of the younger welded-tuff of Aso volcano. Its thickness is 10-20 m. 5) Daimei Formation (Lower terrace, its height 10-20 m), consisting of the alternation of sand bed with remarkable cross-lamina, and gravel bed composed of andestic pebbles. This formation is rich in iron-sand. The composition of heavy minerals changes as follows, in ascending order: Mag>Hn>Hyp>Au>Biotite (Bt), Mg>Hn>Bt>Hyp>Au, Mg>Hn>Bt >Zircon>Hyp>Au. Daimei Formation is 5-10 m in thickness. 6) Reddish brown Loam, slightly clayey, and contains many quartz grains. This is secondary loam. The composition of heavy minerals is Mg>Hn>Hyp>Bt>Au, and is quite similar to that of the upper loam of Yame district. This loam is about 40 cm in thickness.

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