| 概要 |
The southern part of Ariake Bay, off the northern coast of Amakusa, was investigated by various methods to make clear the geological constitution of the Alluvium. The Sparker survey is useful to know ...the total thickness of the Alluvium and the relief of the underlying basement, whereas the Sonoprobe survey is very helpful to analyse the stratigraphic units of different fades within the Alluvium. A number of borings were also undertaken. The results may be summarized as follows: (1) The submarine topography is classified into three major flat planes, 0-10m, 10-20m, and 20-30m in depth, and valleys cutting the first and the second flat planes. There are submarine platforms ofー10 to ・-20m in the central part and also in a particular area between Yushima and Kuroshima. order: (2) The submarine Alluvium is stratigraphically divided as follows in descending Upper Alluvium (I) : mainly clay Upper Alluvium (II) : mainly sand Upper Alluvium (Ill) : shelly sand and silt Lower Alluvium : clay and silt The underlying basement is primarily the early Pleistocene Hondo Formation (Kuchinotsu Group) and the locally overlying gravel of "Lower Terrace". The present submarine topography is approximately parallel to the relief of the pre-Alluvial erosion surface on the basement. These and other geological relations may be schematically illustrated in a profile of Fig. 14. (3) To explain concisely, the relief of the basement is classified into three eroded flat planes of 0-10m, 10-20m, and 20-30m in depth and deeper valleys (down to -30 to -40m) cutting the flat planes. The eroded flat plane of O to -10m is overlain by 0-5m thick sandy layer of Upper Alluvium (II) and the valley cutting this plane is overlain by 0-2m thick clay of Lower Alluvium and 5-10m sandy deposits of Upper Alluvium (II). The eroded flat plane of -10 to -20m is overlain by 0-5m thick sand of Upper Alluvium (II) and the valley cutting this flat plane is embedded by 0-5m thick clay of Lower Alluvium and 5-10m thick sandy deposits of Upper Alluvium (II). The eroded flat plane of the basement at the level of -20to -30m is overlain by 0-5m sand of Upper Alluvium(II). The valley cutting this flat plane is embedded by several meters thick clay of Lower Alluvium and 5-10m sandy deposits of Upper Alluvium (II). Clay of Upper Alluvium (I) is only locally distributed off the port of Hondo and off the coast of Oyanoshima. (4) In the Yushima-Kuroshima area the eroded flat plane of the basement at the level of -15 to -20m is overlain by 0-5m thick shelly sand and silt of Upper Alluvium (III) and by 2-5m thick sandy layer of Upper Alluvium (II). The shelly sands of this platform and also similar sands at the shoulders of flat planes seem to be a kind of beach sand dune. The buried valley cutting the basement is embedded by 5-lOm thick clay and silt of Lower Alluvium and 5-8m thick Upper Alluvium sand. (5) The iron sand is found in the Upper Alluvium (II) which is extensively distributed in south Ariake Bay. The Upper Alluvium (II) consists of gravel, sand and sandy silt, containing fragments of shells and calcareous algae. The content of iron is highest in the gravel and followed by the sandy part. It is enriched beneath the buried sea cliff, on the buried beach and also inside the curve of a buried valley. The source of the iron minerals can be ascribed to the pyroclastic rocks in the Hondo Formation. (6) Text-figures 1-25 may illustrate clearly the above summarized features. The figures are briefly explained in English as well as in Japanese.続きを見る
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Mendeley出力
