Shonai Plain, located in the western coast of Northeastern Japan, is fed by the Mogami river and several small rivers, and margined by a large coastal sand dune. This paper intends to clarify sediment yield change during the last 12, 000 years, confirming the relationship between the movement of shoreline and that of alluvial fan's front. In order to confirm this relationship, the palaeogeography was restored based on the analysis of 758 borehole data, radiometric datings and field observations.
In this paper, the following method was adopted to restore the past geomorphological surface according to every sea level. Commonly, the level of alluvial fan's seaward front should take a slight elevation at every past alluvial surface (a m) was obtained as Fig. 9. The gradient of the present surface was adopted as that of line ((2)) which conbined shoreline and the seaward front of alluvial fans. By the above method, the elevation and the location of alluvial fan's seaward front according to each sea-level was estimated. The palaeogeomorphology was restored by 9 periods (Figs. 10-a-i). Shown in Fig. 8, Post-Glacial sea-level change was restored in order to estimate the age of each geomorphological map.
The first period (Fig. 10-a) indicates the geomorphology when the sea-level reached at about -50m (12, 000-11, 000yr B. P.). The lagoonal environment was located at the river mouth of the Mogami river. After that, the sea area (lagoon) was invading inlandward until the level reached at -15m (Fig. 10-a). After the time when the sea-level rose above -15m, the shoreline rapidly moved seaward (Figs. 10-f-h).
On the other hand, these palaeogeomorphological maps indicate an irregularity between the expansion of alluvial fan and the shoreline progradation. That is to say, during the period when the shoreline migrated inlandward, the alluvial fan formed by the Nikko river was expanded and the fans formed by the Akagawa and the Sasagawa rivers were not (Figs. 10-c-e). When the shoreline began to migrate seaward, the alluvial fan formed by the Sasagawa river was expanded and the fans formed by the Nikko and the Akagawa rivers were not (Figs. 10-e-f). After that, though the shoreline continued to migrate seaward, the alluvial fans front moved inlandward (Figs. 10-g-h).
The author assumed that the change of supplied load from each upstream area is rather effective on the expansion of alluvial fan than the shoreline progradation.