北部九州の緩斜面上に発達する風成塵起源の細粒質土層

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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.
日本第四紀学会の論文

北部九州の緩斜面上に発達する風成塵起源の細粒質土層

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