地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
多摩川流域における武蔵野台地南部の地質 (1)
寿円 晋吾
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ジャーナル フリー

1966 年 75 巻 4 号 p. 185-199

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The south part of the Musashino Diluvial Upland along the River Tama is a region where the Fluvial terrace topography formed by the River Tama dominates. In this region, four terraces from the older to the younger are distinguished, as follows : The Musashino terrace, Tachikawa terrace, Aoyagi terrace and Haijima terrace (Fig. 1). The Musashino terrace is divided into three terraces which constrict each relative height respectively toward the upper stream. In this paper, the third (the lowest) Musashino terrace is called the Nakadai terrace. To the west of Haijima, develops the Alluvial terrace under the Haijima terrace. It is divided into four terraces from the upper to the lower, as follows : The Amagase terrace, Chigase terrace, Kawahara terrace and Tabata terrace. The Tabata terrace develops to the west of Ome. In the east part of this region, The Den'en-chofu-dai, Ebara-dai and Yodobashi-dai, which are higher than the Musashino terrace, are recognized. They are the marine terraces, and are corresponded with the Shimosueyoshi terrace in the most eastern part of the Tama Hill.
The geology of this region consists of two Groups, that is, the Miura Group (Pliocene) and the Narita Group (Pleistocene). The Miura Group dominates to the west of Okura-machi, Setagaya-ku, Tokyo-to, and to the east of it develops the Tokyo bed belonging to the Narita Group (Fig. 3).
The Miura Group in this region can be divided into the following members in ascending order : The Kazumi gravel (150 m+ in thickness), Komiya sandstone (70 m±), Misawa mudstone (30 m±), Renkoji alternation (100 m+), Inaki sandstone (60 m±), Tsurukawa alternation (60 m+), Hirao sandstone (20 m+), Negata tuffaceous sandstone (30 m±), Ozenji alternation (40 m±), Detana sand and gravel (40 m±), Ikuta alternation (50 m±), Iimuro mudstone (50 m ±) and Takatsu alternation (70 m±). In the western half of this region, the Members of the Miura Group develop in ascending order from west to east in the gentle monoclinal structure. The Kazumi gravel is unconformably in contact with the old rocks of the Tamanouchi Palaeozoic zone in the Chichibu System-Middle Carboniferous system or Lower Permian system-and with the Futamatao sandstone and conglomerate zone in the Ohisano Group-Upper Triassic system at Ome in the Tama Valley mouth. These two systems develop extensively in the Kanto Mountain Land. The Komiya sandstone and the Misawa mudstone are in a disconformable relation with each other, but the time gap is short. In the eastern half of this region, the Miura Group forms a synclinal structure under the influence of the Mizonokuchi basin-like synclinal structure (Fig. 3). In this structure, our attention is attracted to the facts that the upper each member is, the farther east each synclinal axis shifts, and that the thickness of the Detana gravel, Ikuta alternation and Iimuro mudstone constrict to the east. The author thinks that these facts definitely show the following points : The formation of this structure was caused by both the gentle tilting to the east in the west of this region and the fairly rapid upwarping that formed the Kamihoshikawa dome-like anticlinal structure in the southeastern part of the Tama Hill, and these movements continued throughout the time of deposition of the Miura Group in the east.
The Hachioji Fault-line along the eastern margin of the Kanto Mountain Land is a faultline formed before the deposition of the Miura Group, and it does not cut the Miura Group. According to my survey, it is not necessary either to conclude the existence of the Tamagawa Fault-line, which used to be presumed once along the north margin of Tama Hill (Fig. 7).

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