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＜departmental bulletin paper＞
Paleoenvironment of the Late Pleistocen in the Chikushi Plain, West Japan
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| Abstract | The Late Pleistocene Series in the Chikushi Plain consists of the Nakabaru Formation, Takagise Formation, Aso-3 and Aso-4 Pyroclastic Deposits and Mitagawa Formation. In order to clarify the tephra st...ratigraphy, the changes in paleo-environments and the origin of finegrained soil on the Lower Terrace, geological survey and analyses of pollen, tephra, isotopic ratios of strontium and oxygen were carried out. The results obtained are summarized as follows: 1. The Takagise Formation is marine deposit formed at the maximum high-stand period of sea-level during the Last Interglasial Epoch (5e of the oxygen isotopic stage) and is correlated to the level between Aso-3 and Aso-4. 2. Pollen analysis shows prevailing of a sub-boreal forest in the Chikushi Plain when the Aso-4 Tephras were erupted at Mt. Aso and indicates that it was already in the Last Glacial Epoch. Remarkable changes in the vegetation on and under the Aso-4 Deposits strongly indicate the destruction of the forest by the Aso-4. 3. 18^O/16^O ratios of fine-grained guartz and 87^Sr/86^Sr ratios of soil samples show that they resemble particles in mud-rain deposits in northern Kyushu. The fine soil materials are derived from aeolian dusts from the Asian continent. Aeolian dusts are also included in the fine part of the ancient forest soil on both overlying and underlying the Aso-4 Pyroclastic Deposits. 4. Volcanic glass particles of AT and K-Ah are identified in size-interval between 63 and 125 μm separated from the fine soil materials. The rate of deposition of the fine-grained soil was 0.052 m/1000 years. The formation of the main part of the fine-grained soil occurred during the past 40,000 years. 5. Lack of the soil materials older than about 41,000y. B. P. on the gentle slope and fluvial terrace in north Kyushu indicates that the a catastrophic flow of the Aso-4 pyroclastics exerted erosion intense enough for destruction of forests and for scraping soils and beds from the gentle slope and the top of the terrace.show more |
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| Created Date | 2021.10.15 |
| Modified Date | 2022.06.20 |