1995 Fiscal Year Final Research Report Summary
Studies on the Paliogine tephron in Northwest Kyushu and West Chugol
| Project/Area Number |
06804026
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
Geology
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| Research Institution | KYUSHU UNIVERSITY |
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Principal Investigator |
MIYACHI Mutsuai Post-graduate school of Social and Cultural Stadia Kyushu University, 大学院・比較社会文化研究科, 教授 (70038470)
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| Co-Investigator(Kenkyū-buntansha) |
SAKAI Harutaka Post-graduate school of Social and Cultural Stadia Kyushu University, 大学院・比較社会分化研究科, 助教授 (90183045)
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| Project Period (FY) |
1994 – 1995
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| Keywords | Tsuyazaki Formation / pyroclastic flow deposits / Oligocene / fusinite / phreatomagmatic oruption / zircon / fission-track age / volcanic front |
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| Research Abstract |
The Koinoura Tuff Member of the Early Oligocene non-marine Tsuyazaki Formation consists of three non-welded rhyolitic and dacitic pyroclastic flow beds (KP-A, -C and -D), and possible pyroclastic surge and phreatoplinian ash fall beds (KP-B), two fluvial interbeds, and a volcanic debris flow bed at the top. The pumice flow beds of KP-A and -D and the ash flow beds of KP-C contain charred and silicified fossil tree trunks which show a typical fusinite exture. The KP-D has degassing pipes in the upper part and a ground layr in the basal part. The volcanic debris flow bed, referable to a lahar, is deposited in a channel dccply croded in the KP-D and contains a great number of fossil tree trunks whose reflectances are very mauch varied.
The KP-B comprises thin-bedded fine-grained tuff in the lower part and conformably succeeding 1.6m thick accretionary lapilli tuff in the upper part. The former contains abundant carbonized wood-trunks which are often coated with ash-spray. The latter compri
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ses densely packed coarse accretionary lapilli, and show no lithologic differences throughout the thickness. These features indicate that the lower part of the KP-B is referable to surge deposits and that the upper part formed by phreatoplinian ash-fall.
Reflectance of the charred trees in the pumice and ash flow beds of the KP-A and -D indicate that the temperature of the beds at the time of deposition was about 550ﾟC, and that of the surge of the KP-B was about 330ﾟC.
Flow directions of pyroclastic flow and surge obtained from the preferred orientation of the contained fossil tree trunks are W to E and NW to SE.They are concordant with flow directions of the fluvial interbeds. These data suggest that the source of the pyroclastic flows was situated to the west of the Tsushima-Goto Islands, in consequence of the paleogeographic adjustment of Kyushu and the Tsushima Islands in Late Eocene and Early Oligocene times. The volcanoes there constituted a volcanic chain that is connected to the San'in district. It is very likely that global lowering of sea level during Late Eocene and Early Oligocene time caused a major regression, called the Onga regression. As a result of expansion of fluvial land area and narrowing of the sea between the Tsushima Islands and mainland Kyushu during the regression, torrential pyroclastic flows could have rushed over these land and sea areas, and reached to North Kyushu. The pyroclastic flows and co-ignimbrite ash falls were spread over the whole Paleogene coalfield basin in North Kyushu and West Chugoku, and formed a thick sequence of tuff beds. Less
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