西表島ウダラ川河口におけるマングローブ沼の堆積環境

抄録・要旨

Mangrove swamps are commonly developed in tropical and subtropical estuaries. To clarify on the sedimentological environment of mangrove swamp, we investigated at the estuary of the Udara River. The sediment samples are analyzed for the grain size distribution, mud content and carbonate content.
The sediments in the studied area are divided into 4 types as follows; 1) gravel, 2) calcareous coarse sand, 3) non-calcareous fine sand, and 4) mud. The gravel sediments are distributed in the upper part of mangrove swamp within the Udara River. The gravels may be lags of the previous deposit. The calcareous coarse sand is distributed in the lower part of swamp. This sediment is socalled tropical coralline sediment, and is mainly composed of fragments of the coral, foraminifera, mollusks and echinoid. Oxygen and carbon isotope data of foraminiferal test (Calcarina sp.) comprized in the calcareous sediments indicate that the source of coarse sand is previous beach sand. The noncalcareous fine sand is distributed in whole area of swamp. The grain size of the sand is characterized by having a mode at 2.5-3.0φ in diameter. The origin of sand is considered to be Tertiary sediment from the grain size distribution.
In the mangrove swamp, quantity of outflow by the ebb tide is similar to that by the flood (about 2m^3/s). Therefore, the sedimentological environment is controlled by the tidal cycle. The sediments are transported to the upper part of the swamp by inflow tidal current. On the other hand, the transportation to the lower part and to Amitori Bay is carried by the strong ebb current. The recent sedimentologrcal environment of mangrove swamp in the Udara River has a tendency of an erosional one.