相模川沖積低地の地形と沖積層

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タイトル別名
  • GEOMORPHOLOGY AND SUBSURFACE GEOLOGY OF THE ALLUVIAL PLAIN OF THE LOWER SAGAMI RIVER, CENTRAL JAPAN
  • サガミガワ チュウセキ テイチ ノ チケイ ト チュウセキソウ

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抄録

The Sagami River, 110 km long, flows from Mt. Fuji down to the Sagami Bay through the Tanzawa Mountains. The lower course of this river makes an alluvial plain, which is surrounded by Pleistocene hills and terraces (fig. 1). The main subjects and the results obtained in this paper are as follows:<br> 1) To classify the landform of the alluvial plain, and also to disclose the subsurface geology of the alluvial fills by using bore holes: The landform of the alluvial plain is classified as in fig. 1. The basal landform under the alluvial fills is shown in fig. 2, where exist buried river terraces (UT, MT, LT, etc.), buried abrasion platforms (TW and AW), and buried valleys (NV, TV, KaV, etc.). The main buried valley made by the previous Sagami River (SV) cut down to 90 m below sea-level at the mouth of the present Sagami River. The alluvial fills above the basal landform is named “Recent deposits” according to the usage in the Mississippi River plain.<br> The facies of the Recent deposits are illustrated in the geologic sections of fig. 3-8, the section lines of which are seen in fig. 1. Depending upon these sections, lithostrotigraphic units of the Recent deposits are classified in descending order as the following:<br> 2) To correlate the Pleistocene subaerial river terraces, which inclined under the alluvial surface, with the subsurface buried terraces or valley floors under the Recent deposits ; then to explain the late Quaternary succession of the Sagami River: From the longitudinal section along the Sagami River (fig. 3), it may be said that the steepest river terrace, designated as Mm, is correlative with the Sagami buried valley floor, and also with the submarine shelf of 110-120 m below sea-level. The stratigraphic horizon of the Mm and other subaerial terraces are given within tephra of this region, which were derived mainly from Fuji Volcano situated 60 km west of this region (fig. 9). Using previously measured C-14 ages of the tephra layers concerned, the age of the Mm terraces is counted between 25, 000 and 17, 000 years B. P. Therefore, it seems from fig. 3 that the Recent deposits have been accumulated after the maximum drop of sea-level in the Würm glacial age. Thus it is supposed that the facies change in the Recent deposits represents the change of depositioal environment caused mainly by the ecstatic change of sea-level and also land uplift. The rate of land uplift seems to be rather uniform, and the value of uplift during the last 20, 000 years is estimated as much as 30 meters.<br> 3) To explain a discrepancy between the morphology and deposits of the river, which have natures of ordinary river beds in an alluvial fan, and the surrounding flood plain morphology and deposits, which is of natural levee-backswamp type: It has been said that alluvial plain morphology in Japan is classified into three main types, i. e., alluvial fan, natural levee-backswamp, and delta. Moreover, it is generally accepted that in each type of plain, morphology and deposits of a river bed have characteristic natures corresponding with the natures of morphology and deposits in the surrounding flood plain of the river. That is, a river on an alluvial fan has braided channels and gravelly deposits on the bed, while on a natural levee-backswamp type plain, a river has meandering channels with sandy deposits on the bed. However, the alluvial plain of the lower Sagami River, in its central part, being of a natural levee-backswamp type, nevertheless has gravelly braided channels like on alluvial fans. Why this discrepancy was caused may be explained as follows: in case of this plain, the river flowed into a lagoon or narrow inlet enclosed by the sand barrier of the upper sand (US-f), therefore, the transported muddy materials accumulated in the closed water.

収録刊行物

  • 地理学評論

    地理学評論 42 (2), 85-105, 1969

    公益社団法人 日本地理学会

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