Plant macrofossil assemblages from the upper Pliocene to the lower half of the lower Pleistocene of the Osaka Group in the southern part of the Osaka basin are described. 221 taxa including 48 extinct plants are identified. Seven assemblage zones, OS-I to OS-VII in ascending order, are established. Zones OS-I, OS-III, OS-V, and OS-VII are characterized by the occurrence of subtropical elements, suggesting warmer climatic conditions than those of zones OS-II, OS-IV, and OS-VI. During the deposition of zone OS-I, about 3 million yrs BP, the climate was the warmest in the Plio-Pleistocene, and many extinct plants flourished. During zone OS-II, the climate became colder. From zone OS-II to OS-III, in combination with climatic deterioration, a change in topography resulted in the extinction of plants including Keteleeria sp., Liquidambar sp., Melliodendron sp., and Sequoia sp., and a increased occurrence of conifers including Abies, Tsuga, and Cryptomeria. Most of the plant species had become extinct by the end of zone OS-III, about 2.5 million yrs BP.

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Many active faults are distributed in and around the Kumamoto Plain (Watanabe et al., 1979; Watanabe, 1984, 1987 and others). In this study, more than 50 drilling cores were re-examined, and some valuable key beds for correlation and age determination were recognized. They are the Akahoya Ash, which originated from the Kikai caldera about 6, 300 years BP, and Aso-1 to Aso-4 pyroclastic flow deposits, which were erupted from the Aso caldera in the past 300, 000 years. From their radiometric ages and differences in depths between the drilling cores, the subsidence rate of the Quaternary deposits in the Kumamoto Plain has been estimated. The average subsidence rate is about 0.2-0.5mm/year in the southern part of the Kumamoto Plain, which corresponds to the Kiyama-Kashima graben (Watanabe et al., 1979) and westward. In the western part of the Kumamoto Plain, the subsidence rate has been calculated at 0.2-0.3mm/year. The subsidence of these Quaternary deposits was probably caused by the movement of some active faults below the ground surface.

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The Red-Yellow soil which is commonly developed on gentle slopes in northern Kyushu has been thought to form in situ from weathered rocks underneath the subsurface soil materials. Recent pedological and geochemical studies indicate that aeolian dust derived from the Asian continent plays an important role as a parent material of the soil. This paper aims to clarify the sources and ages of fine Red-Yellow soil. Twenty-eight soil samples from two locations were collected. One sample site is Kokonoebaru, in the northeastern part of Dazaifu city, and the other is Nanatsugama, in the northwestern part of Karatsu city. Fine soil materials from the former cover the middle terrace deposits which mainly consist of granitic rock gravels. Samples from the latter site cover a basaltic plateau which was dated at 3Ma.
The soil samples were analyzed for the followings:
1) Particle size distribution of quartz grain.
2) Measurement of ¹⁸O/¹⁶O ratio of quartz grain.
3) Measurement of ⁸⁷Sr/⁸⁶Sr ratio of whole soil materials.
4) Percentage of volcanic glass particles in a number of sizes (63 to 125μm in diameter).
5) Measurement of refractive index of the volcanic glass.
The main results obtained by the analyses are summarized as follows:
1) Quartz particle size distribution of soil samples from the two locations is nearly identical. In this tendency they resemble dust particles in mud rain deposits in Fukuoka.
2) The values of the ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr ratios show that they resemble aeolian dusts rather than those from basement rocks.
3) Volcanic glass particles of AT and K-Ah ashes are identified in size-separates from both locations. Aso-4 glass from Aso-4 ash or pyroclastic flow deposit was not identified.
These results lead to the following conclusions;
1) Fine soil materials are not derived from local parent materials but are derived from aeolian dusts from the Asian continent.
2) The formation of the main part of the fine-grained soils occurred during the past 56, 000 years. The rate of deposition of the fine-grained soil was 0.027m/1, 000 years.
3) Although the underlying substrates of those soils are far older than 56, 000yrs BP, the older soils are not identified.
It is probable that the landscape surface of northern Kyushu is continuously regenerated by repeated truncation on a time scale of ten thousand years.

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[in Japanese]

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