Formation of limestone shore platforms; a study from Cape Hedo, Okinawa Island

Two morphological types of limestone coasts are observed around the Cape Hedo on the north coast of Okinawa Island: one is a shore platform that has a nearly horizontal surface with a marked drop at the seaward margin, and the other is a plunging cliff that has a precipitous slope passing far below sea level. The Cape Hedo has little regional difference in input offshore wave energy, tidal range and rock resistance; these conditions are favorable for understanding major factors affecting the formation of these contrasting landforms. This study attempted to investigate (1) the relationship between the landform types and the water depth at the seaward edge of platforms or in front of plunging cliffs, and (2) the relationship between the elevation of shore platforms and wave height just in front of the platforms. The two coastal types are clearly distinguished by the frontal water depth measured from H.W.L., called the front depth. Shore platforms develop at sites where the front depth, h, is 1. 9 to 10. 1 m, while plunging cliffs develop where it is deeper (h = 11. 3 - 18. 9 m). This suggests that the difference in the landform types results from the difference in wave assailing force, because the force is closely related to the wave type, breaking and standing waves, which is controlled by nearshore water depth. Shallower front depth brings about breaking waves that produce large erosive force, while larger depth causes standing waves with little assailing force. Examination of the relationship between the elevation of shore platforms and wave assailing force, incorporating the results of previous shore platform experiments without the effects of weathering and bioerosion in cliff material, showed that the platform elevation decreases with increasing the assailing force. The result strongly suggests that the formation of limestone platforms in the study area is primarily associated with the wave-dominant process, not with weathering and biological processes.