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＜departmental bulletin paper＞
Heavy mineral distributions in the surface sediments of Kagoshima Bay, Kyushu, Japan
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| Abstract | Kagoshima Bay is located at the southern tip of the Kyushu Island, Southwest Japan. Topographically it is divisible into three portions: bay-head, central and bay-mouth. The bay -head area shows a typ...ical caldera feature. Between the bay-head and central bay areas, a very active volcano Sakurajima is situated. The central bay area is ovoid in shape and is a little more than 200 m deep at the center. The bay-mouth area, about 100 m deep on average, constitutes a sill against the open ocean, making Kagoshima Bay as a whole a semi-closed basin. In order to reveal distributional features of heavy minerals in such a semi-closed basin, 100 dredged samples in the bay area and 15 samples from the coastal sediments were analysed for heavy mineral examinations. The result of heavy mineral analysis shows that the bay sediments are composed of magnetite, ilmenite, framboidal pyrite, biotite, hypersthene, clinopyroxene, common hornblende, zircon, apatite, chlorite, olivine, tourmaline, and moissanite in the order of abundance. Of these minerals, biotite, hornblende, zircon, chlorite, framboidal pyrite and moissanite show very characteristic distributions, from which three petroprovinces Bay-head, Central Bay and Bay-mouth are discriminated. The Bay-head Petroprovince is characterized by a suite of framboidal pyrite and moissanite; the Central Bay Petroprovince by predominance of common hornblende, and the Bay-mouth Petroprovince by a suite of biotite-zircon-chlorite. These petroprovinces clearly reflect both the provenance geology and flow systems of depositional sites. The Bay-head Petroprovince is a water-stagnant, closed basin with oxygen -deficient and very low pH values. The Bay-mouth area is under the influence of anti-clockwise flow of longshore currents. Regarding the peculiar occurrence of moissanite, two possibilities have been suggested. One is that moissanite might have been brought into by a meteorite impact, and the other is that it was artificially dispersed after used as abrasive materials. The present data are impossible to give a proper answer to these possibilities.show more |
| Table of Contents | 1. はじめに 2. 鹿児島湾域の地質概説と海況 3. 試料および研究方法 4. 結果 5. 考察 |
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| Created Date | 2021.10.15 |
| Modified Date | 2022.06.20 |