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＜departmental bulletin paper＞
Variation of Heavy Mineral Assemblage in the Tertiary Formation of Sakito Coal-Field, Nagasaki Prefecture, Japan
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| Abstract | 91 samples of sedimentary rocks taken from land surface, undersea exposure, and the pit of the Sakito Coal Mine in Sakito coal-field have been studied for heavy mineral analysis. Local variation of he...avy mineral assemblage is investigated and is led to the following results (refer Table 2 regarding the Tertiary sequence of this area): 1) Itanoura formation ……………Average weight percentage of heavy residues becomes smaller towards west; namely, it counts 4. 2% in Oshima, 1. 6% in Kakinourajima, and only 0. 6% in the pit. The last number contains authigenic pyrite. In addition to the fact mentioned above, no remarkable variation is found in heavy mineral assemblage. It is therefore presumed that the pit area might have been more far-off than Oshima from the source area of heavy minerals, and also that one of the source rocks might have been metamorphic rocks exposed in the Nishisonogi peninsula, Nagasaki Prefecture. 2) From the Fukuura formation to the lower member of the Higire formation, and from the upper member of the Higire formation to the Ainoura group……………The heavy mineral assemblage of the former is characterized by that of the SAK-5 zone and the later by that of the SAK-6 zone (OHARA, 1961a). The heavy mineral assemblages in these zones are nearly the same throughout the area surveyed, but in the former, frequency of garnet in the samples from the land area is a few per cent higher than that from the undersea exposures. 3) Frequencies of authigenic minerals such as glauconite, pyrite, and anatase vary among samples taken from different localities in the coal-field. Therefore, formation of these authigenic minerals might have been more or less influenced by local environmental conditions -in the sedimentary basin. Furthermore, alike to the above case, the average frequency of anatase in the SAK-5 zone is higher in the samples taken from the land area than that from the undersea exposures, but in the SAK-6 zone this tendency becomes reverse. 4) 19 sandstone and conglomerate which were taken from the undersea dredge and by divers (A, see Table 1) and from two pits (B and C, see Table 1) in the nearby areas of the same coal-field have been examined. Their stratigraphical horizons have been hitherto uncertain by macroscopical examination, but the heavy mineral analysis suggests that the sandstones from A are referable to the SAK-6 zone and those of B and C to the SAK-5 zone in their heavy mineral assemblages.show more |
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| Created Date | 2021.12.01 |
| Modified Date | 2022.06.20 |