The evolution of narrow reef flats at high-latitude in the Ryukyu Islands

This paper describes a Holocene reef structure observed from a cutting in a modern reef on Okierabu Island in the Ryukyu Islands and proposes a process for the initiation and formation of narrow, lagoon-less fringing reef flats. Reef formation began around 7050 yBP at 11 m below present sea level. The reef was constructed by a uniform facies of in situ tabular corals and kept up with sea level rise. Geomorphological zonation has been restricted by the lack of a cross-reef energy gradient during reef formation. Sediment trapped by limestone caves abutting the shore has contributed to this characteristic. The slope break around-10 m and steep scarp of the shore create a narrow substrate that is responsible for the development of an equally narrow reef flat.