Submarine morphology of the Island Shelf off the middle and south Ryukyu Islands, southwest Japan

The submarine morphology and distribution of depositional features of marine sediments on the island shelf off the middle and south Ryukyu Islands were examined using echo sounding and seismic profiling data and drilling cores through the modern reef crest and moat. The geomorphic development of the island shelf is discussed in terms of relative sea level change since the Last Glacial Maximum. The distinctive morphological features on the island shelf are classified as the reef slope (RS), inner break (IB), outer break (OB), inner shelf break (ISB), and shelf break (SB), in descending order. The average depth of the IB is about 50-55 m on the investigated part of the island shelf with the exception of Kikai Island which has been uplifting rapidly. Pinnacles are well developed and sediments are expected to be composed mainly of reef materials on the slopes above the IB. On the other hand, pinnacles are poorly developed and detrital deposits accumulate below the IB. Gently seaward-sloping platforms are often observed from the IB to OB. The depth relationship between the IB and relative sea level change suggests that the reef accumulation started to grow actively after 10-11 kyr B.P. The reef accumulation area, however, has been estimated to move landward because the rate of sea level rise exceeded the rate of upward growth of the coral reefs. Erosional processes may have occurred on the seabed from the IB to OB to form some platforms with gently seaward slopes during the periods of stand still or slow rise in sea level after the Last Glacial Maximum.