東天北地域の表層地質と成因的土壌型

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The authors tried to elucidate the genetical porblems of the upland soils in the eastern Tempoku region from the viewpoint of the historical geology of the Quaternary Period. The geomorphology of this region consists of low mountainous and hilly lands, coastal terraces, sand-dunes and alluvial plains. Terraces are divided into as follows : Osachinai Plane (erosional flat plane, 90-120 m above the sea-level), Kamisarufutsu Plane (50-80 m), Pon'nitachinai Plane (40-60 m) and Asajino Plane (15-30 m). The latter three planes are depositional ones. These terraces are mainly composed of the Quaternary system, the stratigraphy of which is illustrated and summarized in Fig.1 and Table 1. The upland soils of this region are classified and systematized as shown in Table 2, based on the descending genetical system of the soil classification. The most important genetic soil types are Podzol, Brown forest soil and Pseudogley, among which there may be an intimate mutual relationship as shown in Fig.5. It seems that the soil pattern (Fig.6) reflects approximately the features of the surface geology. By way of example, the genesis and distribution of the Brown forest soil and Pseudogley on the Asajino Plane, could be explained as follows : Though the terrace is mainly composed of sand and gravel, the superficial part consists of sand mixed with volcanic ash on the coastal side, while of lacustrine clay on the inland side. It may be said that such a contrast in facies of surface deposit has been caused by the epirogenetic movement during the latest stage of the deposition of Asajino formation, as shown by the palaeogeography in Fig.7, No.4. Then, in order to elucidate the genesis of Pseudogley, developed mainly in the inland side and derived from clayey materials the following succession of palaeogeographical events should be taken into consideration : Formation of an inland lake→filling up by fine sediments ⇄ formation of marsh land→gleization of marsh soils→upheaval, terracing and lowering of ground-water table→formation of Pseudogley from the former gleyed marsh soils. In the coastal side where the grain-size of the superficial materials is coarser, on the contrary, the well drained hydrological conditions may have been maintained, and have favoured the genesis of Brown forest soils. It may be safely concluded that the genesis and distribution of the genetic soil types in this region should be closely related to the development of landscape since the beginning of the Wurm glacial stage.
地学団体研究会の論文
1964-11-30

東天北地域の表層地質と成因的土壌型

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