GEOMORPHIC DEVELOPMENT OF THE ALLUVIAL LOWLANDS IN THE CENTRAL PART OF THE KANTO PLAIN, JAPAN

Abstract

In the central part of the Kanto plain, alluvial lowlands develop along the Tone, the Arakawa and the Watarase rivers. The alluvial lowland of River Tone is divided into Menuma, Kazo and Nakagawa lowlands. In the Kazo lowland and the northern part of the Naka gawa lowland, (1) microrelief of the flood plain, (2) stratigraphy of the Recent formation and (3) buried topographies were investigated. On the basis of the investigation results, the geomorphic development of the alluvial lowlands was discussed in relation to buried topo- graphies under the Recent formation (the “pre-landforms” of the alluvial lowlands). In the northern part of the Nakagawa lowland, the deposition of the Recent formation was taken place in a wide and deep valley, which is called the “Nakagawa buried valley”. This valley was formed by River Watarase at the maximum of the Lastglacial period (before at least about 18, 000 y. B. P. in 14C age). The Postglacial transgression reached to the inner part of this valley at about 4, 000 y. B. P. in 14C age. Marine deposits are found at almost all the locations in the Nakagawa lowland. After about 3, 000 y. B. P. in 14C age, additionally to River Watarase, River Tone and a distributary of River Arakawa flowed down in this wide and gentle valley floor, and deposited thick (more than lOm) fluviatile deposits. The natu-ral levees in this lowland developed along the many former river courses with free meander-ing. In the Kazo lowland, Obaradai - Musashino (late Pleistocene) terraces exsist which are thinly covered with the Recent formation. These Pleistocene terraces were dissected by narrow valleys (called the “Kazo buried valleys”) at Tachikawa period (latest Pleistocene). At the maximum of the Lastglacial period, no big rivers dissected the Pleistocene terraces in this area and the Pleistocene terraces with only narrow valleys remained. At maximum of the Postglacial transgression, the marine area extended only to the narrow valleys which dissected the Pleistocene terraces. After the period of peat deposition (about 4, 0003, 000 y. B. P.), River Tone and a distributary of River Arakawa began to flow in the Kazo buried valleys. At the early stage of the fluviatile deposition, the sedimentation occurred only in the narrow valleys and the Pleistocene terraces still remained widly in this area. When the narrow valleys were filled up, sedimentary area of fluviatile deposits spread over on the Pleistocene terraces. Due to (1) the extention of this sedimentary area and (2) the subsidence continued since late Pleistocene in this area, the Pleistocene terraces were rapidly covered with the alluvium. As a result, this neighboring area was changed into an “alluvial lowland” (the Kazo lowland). In this lowland, the thickness of the Recent formation is very thin (less than 5m). The river courses are located almost above the Kazo buried valleys, so that the width of each meander belt or that of the natural levee becomes narrower than that in the Nakagawa lowland. Many slightly high mounds of the Pleistocene terraces thinly covered with the fluviatile deposits are found on the present flood plain. It is concluded that the “pre-landform” of the alluvial lowland is one of the important factors which affect the depositional processes of each alluvium and the microrelief of the present flood plain.