The Distribution of Large Reef Blocks and the Effect of Geomorphic Development in Kudaka Island, the Ryukyu Islands, Japan

Holocene coral reefs are formed over equatorial pressure belts, trade wind belts, and subtropical high-pressure belts (between latitude 30° N and 30° S). In equatorial pressure belt and trade wind belt regions, few coral reefs are affected by storm-induced waves. In subtropical high-pressure belts, on the contrary, sediments on coral reefs are transported by catastrophic waves generated by violent storms (typhoons, hurricanes and cyclones). The wind belts could influence the geomorphic processes of coral reefs, resulting in different landforms. Several typhoons hit the Ryukyu Islands located in southwestern Japan every summer, and their coral reefs have sustained successive damages before recovering. Large blocks (more than 1 m in longest diameter) detached from reef slopes are found on reef flats in some coral reefs. They can move during catastrophic disturbance, and may be transported by storm-induced waves. The object of this study is therefore to evaluate the effect of large reef blocks on geomorphic formation in the typhoon area.<br> Around Kudaka Island in the Ryukyu Islands, fringed coral reefs and the distribution of large reef blocks were investigated by aerial photo interpretation. Forty-one large reef blocks located on the reef flat and moat were measured and characterized. Movements of large reef blocks on them were observed from October 1994 to September 1996. Coral clasts, which were randomly collected from the moat, were placed on the reef flat under calm sea conditions and the direction and distance of movements were measured.<br> Typhoon-induced waves swept across the reef edge and large blocks were transported from the reef slope to the reef flat. These blocks are classified as “groove blocks” and “spur blocks” based on their shape. The spur blocks, which originated from the reef framework and dead corals, are angular, while groove blocks are rounded as a result of abrasion by spur blocks. Spurs developed sideways during storms, and reef blocks were detached from spurs. Through aerial photo interpretation and field observations, it was determined that reef blocks on the reef flat were transported at intervals of a few decades. These movements are much quicker and the production of reef blocks much more frequent in the trade wind zone where blocks are transported on a geological time scale.<br> Movements of blocks were observed during the winter seasonal wind and typhoons of 1996. In the summer of 1996, reef blocks were thrown up over 50m from the reef slope on the reef flat by the wave forces of typhoons with wind speed of 20m/s or more and maximum significant wave height of 6 m or more. Only groove blocks and live corals were transported and no movement of spur blocks was observed. Horizontal movements of large blocks were recorded for distances of over 50m and vertical movements of blocks were recorded for distances of over 5m during typhoons. Spur blocks would be transported by waves of typhoons bigger than those that occurred during this investigation. Reef slopes in the Ryukyu Islands are gentle, and therefore it is easy for reef boulders to be thrown up from reef slopes to reef flats.<br> Most of blocks on the reef flat and the moat are characterized by depressions of erosion that suggest the blocks have been exposed to the air. The scattered reef blocks show a heterogeneous distribution in convex reef flats. This suggests that large blocks have been transported by wave processes. Blocks in the moat become live coral basements, and some blocks become cemented to the coral, causing the moat to pile up.