仙台付近の古赤色土

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Geomorphological and stratigraphical studies have hitherto been made on the red weathering crusts occurring on the Tertiary tuffaceous sediments near Sendai City. However the chemical and mineraological studies of the red weathering crusts of the area have not yet been made. From the field investigation and the reports already issued, the author considered as follows. 1) These red weathering crusts have generated on the Tertiary tuffaceous sediments regardless to the kind and nature of parent materials, and run parallel to the unduration of present land surface. The red weathering crusts are, without exception, covered thinnly with a diluvial volcanic aeh (Medeshima volcanic ash). According to Dr. Nakagawa and others, Medeshima volcanic ash is reported to have fallen before the formation of Kamimachi Terrace (lower one next to Dainohara Terrace). Therefore, these red weathering crusts are regarded as relics of ancient red soil which had developed under warmer climate during the period of Dainohara Terrace formation or middle Pleistocene. 2) From the chemical and mineralogical studies of Tomegaya profile, it was made clear that there are apparent differences between upper A and B horizons which are influenced by volcanic ash and lower horizons ((C_1 (PB_<21>), C_2 (PB_<22>), C_3 (PBC) and C_4 (PC) horizon)) which are red weathering crust and its starting material. As for the mineralogical compositions of fine sand fractions, the ratios of bipyramidal quartz (considered as volcanic origins) and heavy minerals are higher in the upper horizons than in the lower horizons. As a whole, the quantity of heavy minerals and volcanic glass are very small throughout the profile. All minerals except quartz and opaque minerals have received severe weathering and corrosion. The clay contents are relatively high in the lower horizons, while coarse sand shows decrease in the lower horizons. SiO_2/Al_2O_3 ratios are higher in the upper horizons, which may be due to the comparative richness of quartz horizons. Within the lower horizons, SiO_2/Al_2O_3 ratios are nearly constant. From the identification of clay minerals, the C_4 (past C) horizon is abundant in montmorillonite mineral and poor in kaolinite mineral, whereas in the horizons of the red weathering crust [(C_1 (past B_<21>) and C_2 (past B_<22>)] kaolinite mineral is predominant. The author thought that kaolinite formation had proceeded from the tuffaceous, montmorillonite rich parent materials under the past warmer climate during the aforesaid period.
日本ペドロジー学会の論文
1965-12-30

著者

岩佐安
農技研

仙台付近の古赤色土

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