Late Holocene Coastal Morphology and Sea-level Records on Three Small Islands, the South Ryukyus, Japan

Abstract

The Holocene coastal morphology of three small islands, Kohama, Kuro and Hateruma, of the southern part of the Ryukyu Islands, Southwest Japan, is discussed in relation to paleo sea-level indicators. Three islands are located between about 24°0′ N and 24°20′ N, and are largely underlain by Pleistocene coral limestones. They are fringed by emergent coastal features such as marine terraces and marine notches. Emergent marine deposits and fossils are also preserved. Present elevations of these landforms and fossils generally range from 1.0 to 1.5 m above mean sea level (AMSL) and C-14 dates for five samples are younger than ca. 4,200 yBP. Sea level was slightly higher than at present, about 1 m AMSL from ca. 3,300 to 800 yBP in Kohama and Hateruma and about 1.5 m AMSL at about 4,200 yBP in Kuro Island. No definite evidence is found to confirm the culmination of postglacial sea-level rise which is usually observed on the Japanese coast at about 6,000 yBP. Age of placement and orientation of large boulders scattered on the terrace up to 22 m high on Hateruma Island probably suggest their tsunami origin, associated with the 1771 earthquake. © 1985, The Association of Japanese Geographers. All rights reserved.