GEOMORPHOLOGICAL DEVELOPMENTS GP THE WESTERN SHIMÔSA UPLAND IN CHIBA PREFECTURE, JAPAN

Abstract

This study was made on the geomorphological developments of the western part of the Shimosa Upland in the north of Chiba Prefecture. The results are summarized as follows: (1) Landf orm surfaces of this region can be divided into Shimosa Upper Terraces, Shimosa Lower Terraces, Chiba 1st Terraces, Chiba 2nd Terraces and alluvial surfaces. Land-form surfaces except alluvial surfaces are covered with volcanic ash layers (so-called “Kanto Loam”). (2) Shimosa Upper Terraces are coastal plains formed by marine Pleistocene strata mainly comprising sand and silt. In the Shimosa Lower Terraces, coastal districts of Tokyo Bay are coastal terraces formed by pebbles and sands, while the downstream basins of the Tone River are flood plains consisting of river-made sand. The Chiba 1st Terraces and Chiba 2nd Terraces are river terraces formed by river-made pebbles. (3) Shimosa Upper Terraces are 25 to 35 meters above sea level. Their principal parts extend from northwest to southeast and form the waterhead between the Tokyo Bay drainage system and the Tone River drainage system. The Shimosa Lower Terraces are 10 to 25 meters above sea level. They are distributed along the northeast coast of Tokyo Bay at the edges of the Shimosa Upper Terraces by making gentle inclination toward Tokyo Bay. They can be traced further to the north along both Tone and Kinu Rivers, and also are developed in the region between Lake Inba and Lake Taga. The Chiba 1st Terraces and chiba 2nd Terraces are distributed only partially along the valleys carving the terraces. From the aforementioned status of geomorphological surface distribution, it was clarified how the sea piling up sea layers to form the Shimosa Upper Terraces had retreated. (4) With Kanto Loam as a key bed, a comparison was made with various terraces surrounding it by the so-called tephrochronological method. As a result, it was clarified that the land- form surfaces which have hitherto been divided into two terraces, namely Shimosuyoshi Terraces and Musashino Terraces, should be divided into three terraces, namely S1, S2, and M. Among them, those widely distributed as the principal terraces of the Upland are S1 and S2 Terraces, and the existent Musashino Terraces are distributed only in an extremely limited area.