1966 年 5 巻 1 号 p. 19-27
The research on the process of the development of the alluvium forming coastal plain can be effectively combined with the studies of the adjoining submarine sediments. Fortunately the opportunity to make such an effective work has been offered by the sonic method survey of Osaka Bay in 1962. In this paper, part 1, the writers intend to describe and discuss about the latest Quaternary deposits of Amagasaki area, the western part of Osaka coastal plain north of Osaka Bay.
Fig. 2 is the panel diagram showing the final result of compilation of many subsurface data from boring cores. A prominent gravel bed which is named Itami gravel spreads over the whole area. The younger marine and fluvial deposits covering the Itami gravel are treated in this paper. They are so-called “alluvial deposits” and are divided into two parts, the Upper and the Lower, based on the cycle of sedimentation.
The base of the “alluvial deposits” is mapped in Fig. 5. The Itami gravel had been entrenched by older rivers which almost coincide in distribution with the present river system and the older channels were buried by the deposition of the “alluvium”. The older river system can be traced to buried channels detected on the surface of the basement of the “alluvial deposits” in Osaka Bay.
A line of the buried wave-cut cliff or slope can be recognized somewhat to the north of the present shore line, about 20m below the present sea-level. The cliff is inferred to show the shore line when the Lower part of the “alluvial deposits” was formed.
The writers have established the following processes of sedimentation of the “alluvial deposits” of the area.
1) Large-scale regression and the deep entrenchment of rivers on the surface of the Itami gravel, Considering the extension of the river channels to the basement of the “alluvial deposits” in Osaka Bay, this inner bay must have been dried up completely at that time. This stage may be identified with the maximum phase of the Würm Ice Age.
2) Starting of transgression, The Lower part of the “alluvial deposits” was formed burying the river floors of the older river system.
3) Retardation of transgression. The sea-level retarded at about 20m below the present sea-level. This stage is inferred to be identified with the Alleröd oscillation, according to a chronological datum of 14C obtained from the lowest horizon of the Upper part or the top of the Lower part of the “alluvial deposits” in Osaka.
4) Rapid re-transgression. The Upper part of the “alluvial deposits” was formed. Wavecut cliffs at the highest sea-level have been preserved about 5m above the present sea-level.
The processes stated above almost coincide with those traced in and around Tokyo Bay (HATORI & others, 1962). This fact indicates that the formation of the “alluvial deposits” is closely related to the ecstatic changes of sea-level.