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＜departmental bulletin paper＞
Akasaki formation in Amakusa, western Kyushu, Japan
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| Abstract | The sediments forming the Akasaki formation, the lowest member of the Paleogene sequence in Amakusa, are for the most part "purple" in color accompanying sometimes drab mottlings providing striking co...ntrasts with those of the under-and overlying formations. It is the purpose of the present paper to provide a reasoned interpretation of both color genesis and depositional environment of this characteristic formation. Results obtained from field survey, and laboratory works such as microscopic observation and heavy mineral-, chemical-, X-ray-, EPMA-analyses are summarized as follows. Geochemical and petrographical studies support a presumption that the source materials forming the formation were derived chiefly from metamorphic rocks such as hematite-bcaring schists and green schists, which are considered to have been exposed near the depositional basin. The pigmenting agents responsible for "purple" color are infinitesimal hematite and limonite occurring mainly interstitially in the matrix. Limonite is probably postdepositional in origin, but it is impossible to decide at present, whether hematite was supplied from schists as detrital one or from ground water as hydroxide that aged to hematite through diagenetical dehydration. It is safely concluded that the "purple" color observed today reflects the duplication of some causes which occurred syn-and epigenetically. Judging from a fanglomeratic appearance of the basal conglomerate, remarkable lateral change of litho-facies, and high quantities of hematite and limonite in spite of absence of siderite and pyrite, the depositional environment of the Akasaki formation is imagined as of river flood plain which transformed gradually to coal swamp corresponding to increasing of the depth. I. はじめに II. 赤崎層の地理的，層位的分布 III. 赤崎層について IV．紫赤色岩について V. 考察 VI. 考察show more |
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| Created Date | 2021.11.10 |
| Modified Date | 2023.12.07 |