Abstract
The northern coast of Amakusa Islands, situated in the southwest of the “Nagasaki Dreiecke” named by F. v. Richthofen, preserves terrace topography relatively well. The writer tried to clarify the geomorphic history of the area in relation to the formation of Pa-leo-Ariake Bay.
Terrace surfaces in the area are classified into six levels, namely H1, H2, M1, M2, Ap and L surfaces in descending order.
1) H1 Surface
The H1 surface 80 to 108m in height is erosional into Paleogene system and lower Pleisto-cene Saitsu Formation. The surface is thought to have been formed after the deposition of Saitsu Formation.
2) H2 Surface
The H2 surface is widely distributed at 40 to 78m in elevation in the study area, which is erosional into both Paleogene system and Saitsu Formation.
3) M1 Surface
The M1 surface is most widely distributed and very well continued. It is marine in origin, i. e., mostly the former wave-cut platform cutting into Paleogene system and Saitsu Formation. At the Shigi Trough located in the northwest of the study area, the M1 surface is depositional of thick Kama Formation, mainly consisted of the alternation of silt and subangular pebble of black shale. At Shirakio, in the center of Shigi Trough, Kama For-mation consists of silt and silty sand containing marine mollusca and sand pipes. The former shoreline of the M1 surface is 25 to 35 m in altitude. The M1 surface is considered to have been formed during the last major interglacial stage of Pleistocene period. Paleo-Ariake Bay was formed during this interglacial stage.
4) M2 Surface
Since the formation of M1 surface, the sea-level began lowering. The M2 surface 8 to 20m in height was formed during the earlier period of this regression.
5) Ap Surface
The Ap surface was made up of Aso pyroclastic flow deposit. Its age is estimated 33, 000 +3, 000 -2000 years BP by the radiocarbon dating. The distribution of this surface is restricted to the area of Saitsu Formation.
6) L Surface
The L surface is the lowest terrace of the area and it was formed during the maximum of the last glacial stage. The surface is submerged by alluvium at the coast and it continues to the submarine flat plane 10 to 20m in depth.
Saitsu Formation, the lower part of Kuchinotsu Group, is correlated to Takio Formation, the lower part of Oita Group. In the early Pleistocene period, Oita and Kuchinotsu sedi-mentary basins about 150km apart east to west with each other, both of which are fault angle depressions bordered by the Oita-Kumamoto Tectonic Line at the south, represent the final main activity of the Tectonic Line with the axis of E-W trend called Southwest Japan Arc direction. After the M1 surface was formed, the crustal movement with E-W trend was active again in the western part of Amakusa Kami Shima (Island). The dis-location of a dissected fan correlated to the M1 surface was resulted from the above crustal movement.
Another crustal movement with NE-SW trend called Ryukyu Arc direction may have been active in the Shigi Trough during or after the M1 surface was formed.
