<国立科博専報>岩手県宮古湾および山田湾の底質の粒度分析

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Miyako Bay is one of the representative notches developed along the rias coast of "Sanriku", the Pactific coast of Northeast Japan. Its bottom rises up from the depth about 70m at the mouth rapidly to the exit of the Port of Miyako, and then gently to the innermost corner of the bay. Distribution of the depth gradients is concordant with the shape of an acute-angled triangle, excluding the west coast south of the mouth of the Hei-gawa, where the bottom is relatively shallow probably due to the exceptional accumulation of sand. The inner part of the bay is almost occupied with muddy material, while the mouth region has sandy bottom. On the other hand, Yamada Bay possesses a rather extraordinal shape among many bays along the "Sanriku" coast; it develops as a nearly circular inlet in the interior of a triangle-shaped bay beyond a narrow canal of ca. 1km width and of ca. 50m depth. The depth gradients spread out in fan shape from this deepest area of the canal toward the inside, except for the zone surrounding two islets located a little west of the center of the inlet. The major part of Yamada Bay is also occupied with muddy sediments but the sandy material develops well along almost all coasts. The outside from the canal is buried with sandy sediments similarly to the case of Miyako Bay. The above-mentioned outlines were made by compiling the data obtained from the charts (Sheet Nos. 1094 and 54 of the Hydrographic Office; summarized in Figs. 2 and 3), and those from the new observations undertaken there. The sediment samples, 24 in total number, were taken from July 5 to 9,1967,using the PHLEGER gravity core sampler which could catch the cores of ca. 3cm diameter and of maximum length of 40cm. Every top 1cm of these cores was analyzed by the EMERY Tube Method for the sandy fraction and, at the same time, by the Pippette Method for the muddy one. The results are summarized in Table 1 and Figs. 4 and 5. Md_φ (median size of grains) of every sample, except Nos. 503 and 601,indicates very fine grained sand to silt size and no tendency of geographical differentiation. On the contrary, skewness (aφ) suggests a rough dividing of sediments of Miyako Bay into two types, the inner part and the outer one, boundary of which appears to coincide with the change in depth decreasing. The sediments of the inner part has greater value of skewness, in other words, greater amount of the coarser grains than those of the outer one, despite that their values of median size are similar to one another. It may have been expected before that the skewness of nearly every sample from Yamada Bay falls in the same category as the inner part of Miyako Bay. Besides, sorting coefficients (σφ) observed in Yamada Bay seem to be slightly greater than those of Miyako Bay as a whole; namely, the muddy sediments of the former were slightly less sorted. This tendency is also expectable because Yamada Bay has a more closed shape.
国立科学博物館の論文
1968-03-30

<国立科博専報>岩手県宮古湾および山田湾の底質の粒度分析

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