In the coastal region of the northern end of Hokkaido, thick gravel deposits called the Keihoku formation unconformably covers the Neogene Tertiary deposits. The bottom of this formation is especially sunken in the subsurface of the Sarobetsu coastal Plain. It is situated at approximately 60m below sea level. While the highest outcrop of this formation is found at the hill immediately north of this plain. It is about 60m above sea level. The formation is observed little tilting or folding. Accordingly it is estimated to be about 120m in thickness in all, if it was not largely eroded away in the past. In the Keihoku formation, peat deposits are rather widely traceable especially in the lower horizon, in which dominant seeds of Menyanthes, elytra of beetles, and small branches of willow and birch trees are generally found. According to the result of pollen analysis made by N. Fuji, the climatic condition of this deposits is indicated to be cooler than the present days. The Keihoku formation is unconformably covered by the pumice fall deposits, called the Wankonosawa pumiceous deposits, which simultaneously cover the gravel beds of the lower terrace. The age of the pumiceous deposits is estimated to be the latest Pleistocene. These pumiceous deposits are also covered by the another ash called the Hotoku volcanic ash. The age of the ash may be at least the Pre-Subboreal age, because the ash is locally highly disturbed by cryoturbation. The extension and isopach of these two volcanic products, shows their origin to be the Rishiri volcano, now situated off the Sarobetsu coast. The age of the Keihoku formation is unknown for us at the present, but is undoubtedly older than the age of the Middle terrace, because it is cut by the terracing of that age. In all probability, it may be correlated to the Nopporo formation or the Obihiro formation composed of heavy-beded gravel beds, and may eventually represent the early Pleistocene.
Two examples of microtopography of fluvial terrace plains are described from the River Shinano, central Japan (Figs. 2 and 3). The topography consists of narrow, shallow depression beneath the backward scarp, and of low swells in front of the depression. They are regarded as abandoned channels and old meander-channel bars, respectively. Another example of the meander-channel bars, just growing into terraces with the similar topography is given (Fig. 4a and 4b). From the observation of these examples, a generalized mechanism of formation of a type of fluvial terraces are reduced (Fig. 5). Wider terraces will be formed from braided river floors which are considered to be aggregates of this kind of channel bars. They belong to non-cyclic erosional terraces which do not necessarily represent discontinuous uplift of the area.
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.
The Mabechi River drains the northwestern part of Iwate Prefecture and the southeastern part of Aomori Prefecture, Northeast Honshu, Japan. This paper describes the terraces and volcanic ash layers distributed along the middle and lower courses of the trunk stream (Fig. 1), and their chronological interrelation (Figs. 2 and 3). The Hachinohe Volcanic Ash covers the surface of the Tamonoki and the older terraces and comprises ash-flow tuff which builds a flat-top topography at the foot of the valley walls. Along the middle stream, the main part of the Fukuoka Terrace consists of this ash-flow tuff. Some features of the ash-flow tuff of the Hachinohe Ash are reported. Decision of the ages of the terraces is based on the radio-carbon date of the natural charcoal from the Hachinohe Ash, 12, 700±270yr. B. P.(GaK-205), and the standard sequence of development of the terraces along the Pacific coast of Japan by Nakagawa (1961b). The Takadate Terrace is dated to the last high sea-level stage of the late Pleistocene and the Nejo and Tamonoki Terraces are dated to have developed during the oscillations of the low sea-level stage of the latest Pleistocene. This latter date corresponds to the Last Glacial stage.