宮崎平野の完新世地形発達史

抄録

The Miyazaki Plain in southern Kyushu, Japan, is characterized with well-developed Holocene terraces. On the basis of the distribution and altitude of the terraces, they are subdivided into four groups: Shimotajima I, Shimotajima II, Shimotajima III, and Shimotajima IV terraces.
This paper discusses the evolution of terraces, sea level change, associated environmental change and tectonic movements during the Holocene in the study area, based upon the field observation of outcrops and cored samples, ¹⁴C datings, archaeological remains, and paleontological analysis of mollusks. The results are as follows:
1) Before 5, 000y.B.P.
The sea level had risen rapidly since at least 9, 000y.B.P., and reached +4 to +5m (a.s.l.). Thick sandy or silty sediments beneath the Akahoya ash(K-Ah) which erupted from the Kikai caldera about 6, 300y.B.P. were deposited during the transgression. The shallow marine mollusks such as Tegillarca in the deposits indicate that tidal flats and bays were widely formed in the Miyazaki Plain. The Akahoya ash is covered with sandy marine sediments 2-3m in thickness, which might indicate that the sea level rose slightly after the ash fall. At the maximum transgression, the sea level rose to 7.0-7.5m above the present sea level around the Ishizaki River, judging from the height of the buried benches and the upper limits of the sandy marine deposits. The sandy deposits on the Akahoya ash include many mollusks such as Tellinemactra edentura, one of the typical tropical species which lived on the sandy bottom beneath the open sea. Their presence suggests that the bays were buried by sandy material which was carried by the river and the long-shore current and changed into the open sea with a sandy beach.
2) 5, 000-3, 000y.B.P.
The regression started about 5, 000y.B.P. While the shoreline retreated, many sandy ridges emerged on the Shimotajima I surface. Lagoons behind the sandy ridges gradually changed into swamps. After the sea level lowered to 5-4m (a.s.l.), it became stable or rose slightly, and the Shimotajima II surface was formed.
3) After 3, 000y.B.P.
Many sandy ridges on the Shimotajima II surfaces emerged as the regression began about 3, 000y.B.P. After the regression, the sea level became relatively stable, and the Shimotajima III surfaces were formed. The sea level was declining again around 1, 600y.B.P., and became adjacent to the present sea level, forming the Shimotajima IV surfaces.
The altitudes of the Shimotajima I and II terraces show that the Miyazaki Plain has been uplifted, tilting northward during the Holocene. The uplift might be due to aseismic crustal movement, because there have been no historical earthquakes which caused emergence of the Miyazaki plain, and we can find no submarine active faults which could uplift the Miyazaki plain. The aseismic uplift would be related to the buoyancy of the Kyushu-Palau Ridge which is subducting under southern Kyushu.