抄録
The Ryukyu Islands consist of many islands which have coral reefs. Such coral reefs are classified into fringing reefs, patch reefs, and table reefs. Barrier reefs are scarecely developed (Mezaki et al. 1977; Mezaki 1980).
In the Kume Island, its west coast forms emerged fringing coral reefs with a shallow moat (max. -2m) and barriers with a interposing lagoon (max. -20m) develop along the east coast (Fig. 1, 2, and 3). On the west coast, the present living coral reefs are found on the outer reef flats, while on the east coast, they are found on the inner and outer reef flats. The purpose of this investigation is to examine the differences of these two types of carbonate sand beach sediments. In order to know the depositional features of these beach materials, the two types of coral reef sediments with bioclastic carbonate grains were analyzed using the grain-size parameters and the component propotions of sediments.
There is a distinct difference in the grain-size parameters of sediments between the west coast and the east coast. Sands from the west coast are finer and more poorly sorted than those of the east coast (Fig. 4, Table 1). In general, grain-size parameters of the east coast sediments have variable values in comparison with the west coast sediments (Fig. 5). Concerning the components, foraminifera (Baculogypsina sphaerulate and Calcarina spengleri) are abundant on the west coast, while coral fragments and shell fragments occupy about 85% on the east coast (Fig. 6).
An increase in foraminifera suggests a decrease in mean grain size and poor sorting, and an increase in coral and shell fragments suggests an increase in mean grain size and well sorting. This relationship implies that the depositional enviroment at a given locality in the Kume Island is greatly influenced by the differences (1) in biota between the two types of coral reefs and (2) in the coral reef landforms themselves.
