琉球列島中部地域における後氷期の海水位

抄録

In the central part of the Ryukyu Islands, especially at Takara I., Yoron I., and Iheya-lzena Is., the post-glacial sea level is discussed by means of considering the beaches. In the coral reef coasts, the post-glacial sea level higher than at present, may be testified by (1) the formation of red soil in the sediments, (2) terraces and beaches, (3) beach ridges and recent dunes, (4) notches and caves, (5) beach rocks, (6) the limestone filling up the fissures, (7) the reef under the negro heads and (8) the lowest knick point in the streams, etc. If these are considered in relation to C-14 dating, the period of the sea level will be clearer. In this paper terraces and benches were mainly discussed in addition to the reef under negro heads. In the central part of the Ryukyu Islands the post-glacial terraces and benches may be safely classified roughly into (1) Kikai-jima type, (2) Yoron-jima type, and (3) Izena-Yanoshita-jima type. Kikai-jima type has the characteristics of the broad terraces often more than I km in width, of the height of more than 5 - 6 m at the highest terraces, and of being unable to distinguish the benches. But, Takara-jima subtype shows less than 3 m in height, thin sand and gravel bed, and often benches at high tide level or mean tide level. Yoron-jima type has the broad reef at present sea level which is often more than 1 km in width. Former shoreline can be identified by beach rocks and notches, but there are no distinct benches higher than high tide level. Yanoshita-jima type which is the intermediate type between Kikai-jima type and Yoron-jima type, is the characterized by broad and flat benches, and former notches. In the case that the limestone is not sufficiently consolidated, the bedding-plane is weak and torn off like a storm bench. This may fittingly be called Okinoerabu-type. It is estimated by the distribution of beach rocks and double notches that the post-glacial high sea stand in Yoron-jima attained to 2 - 3 m in height. At that time the barrier reef was situated in the inner part where we can see minute pedestals at present. But it must be noticed that the surface of flat topped minute pedestals do not indicate the sea level then, because the benches have been lowered by erosion since the time of formation. 6 m-bench represented at Yoron-jima, showing incomplete distribution and shape, and overlying red soils, seems to be formed before the last glacial period. It cannot be considered also that 3 m bench was formed at the post-glacial period, because the distribution and shape of benches is very incomplete, but it has been deformed to a storm bench. The reef flat covered with the negro heads would have been relieved of the erosion. Accordingly in the most desirable case, it has reserved the initial reef surface exactly. This will Simultaneously show that the raised reef be lowered in parallel with the former surface as time goes on' Therefore the former sea level cannot be identified directly with the present geomorphological surface' The conclusion of this paper is that the post-glacial high Sea stand attained a few meter higher that at present in the central part of Ryukyu Islands.