陸奥湾底質の自動粒度分析機による解析
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概要
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A grain-size analysis was made on the bottom sediments of Mutsu Bay, northern Honshu, Japan, with an automatic analyser designed by one of the authors (Niitsuma, 1971). One hundred and twenty-four samples were taken by a Fhleger's Bottom Core Sampler. One centimeter thick surficial parts of the cored bottcm sediments were analysed by the Autcmatic Grain-size Analyser. The sediments studied were classified into three major types namely sands, sandy muds and muds, and were further subdivided into 17 groups according to the characteristics of their frequency curves obtained by the analyser (Fig. 2). The distribution of the sediment types is shown in Fig. 3. The sands were classified into 6 types according to their frequency curves as shown in Fig. 2. The distinction between sandy muds and muds was based upon the minimal around 4.0 φ on the frequency curves and upon the agent of transportaticn which is called Agent I in this paper. The grains around 4.0φin diameter were removed from the distribution area of the sandy muds. The boundary between the areas of sandy muds and muds follows an irregular pattern. The Agent I was thought to have worked in the direction of the black arrows in Fig. 4. The criteria for further subdivisions within the sandy muds are based on the main maximals on the frequency curve, which are shown in qp by the arabic numbers in Fig. 3. The main maximal of the sandy muds changes from finer to coarser in the bay in the direction from the shore towards the central part off Higashitazawa and Yokohama as shown in Fig. 3. The small maximals in the frequency curves of sandy muds and muds are represented by the grain-sizes ranging from 3.4 to 4.Oφas shown in Fig. 5 and the distribution of the largest grain-sizes among these maximals is shown in Fig. 6. In this distribution, the line of 3.4φ projects towards the central part of the bay off Sumichigai and Yokohama. The agent which is called Agent II in this paper, could be estimated to have brought the grains of 3.4 to 4.0φin the direction of the open arrows in Fig. 6. The distribution of the sandy muds with the largest main maximal in the central part of the bay is explained by the coexistence of the independent Agents I and II. The central part of the bay is at the southern limit of the area affected by Agent II. The coarser grains were unable to extend farther southwards than the central part of the bay. Agent I independently effects the two routes extendingjfrom the south off Higashitazawa and Yokohama towards the central part of the bay. Although it could not be decided that the Agents I and II were the prime agents in Mutsu Bay from the result of grain-size analysis alone, they could be assumed to be the currents in Mutsu Bay as shown in Fig. 7. Recently, measurements of the currents (direction and velocity) [by a current meter were carried out in Mutsu Bay by the Hydrographic Office, M. S. B., Japan. According to the results shown in Fig. 8, the measured current directions agree well with those estimated I independently by the grain-size distribution of the sediments with the exception of a reversed current direction estimated along the route from the central part of the bay to Bentenjima.
- 東北大学の論文
- 1971-10-05
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