1990 年 29 巻 5 号 p. 405-416
In order to re-examine an earlier theory on the subdivision of the soft sediment which is sometimes called “Alluvium” in enclosed coastal seas, an acoustic survey was carried out in Osaka Bay. Although the tools which were used for the surveys differ, the depths of acoustic reflectors in the old and the new records correspond well. The results of the new acoustic survey and drilling show good correlation between the depths of the reflectors and those of tephra seams and facies boundaries, for example, between those of mud and sand or those of sand and gravel.
The earlier stratigraphic subdivision was based chiefly on differences in reflection patterns; the importance of reflectors as time markers was neglected. Consequently, the boundary between Alluvium A and Alluvium B in the earlier stratigraphy apparently crosses some reflectors. The distribution boundary between Alluvium A and Alluvium B on the sea floor was regarded as corresponding to that between sand and mud in the same area. This is, however, not correct. The combined thickness of Alluvium A and Alluvium B, which was supposed to equal that of total “Alluvium”, only corresponds to that of the upper and middle muddy parts, excluding the channel areas.
On the basis of these facts, stratigraphy in enclosed coastal seas can be better understood, as follows. Unconsolidated muddy sediment, which forms the upper and middle parts of “Alluvium” in Japanese enclosed coastal seas, cannot be stratigraphically subdivided. The sandy part near the channel area and the muddy parts in the central bay and the area far away from the channels (both on the bottom surface) are in the relation of a contemporaneous heterotopic facies. The most obvious reflector in the acoustic record, which had been regarded as the boundary between “Alluvium” and the underlying strata, is better understood as the boundary between the upper and middle muddy “Alluvium” and the underlying “lower Alluvium”. The previous stratigraphy for the subdivision of unconsolidated sediment in other enclosed coastal seas around the Japanese Islands should be corrected in a similar way.