Holocene Sea Level and Tectonic History Related to the Formation of Coral Terraces at Kikai Island, Northern Ryukyu Islands

摘要

Kikai Island in the northern Ryukyu chains, 80 km from the Ryukyu Trench, has the high- est uplift rate in Japan, 1.8 m/ka since isotope stage 5e. Four subdivided Holocene terraces, from the highest terrace I to the lowest IV, fringe Kikai Island and record the Holocene uplift. These terraces were previously described and considered as evidence for coseismic uplift and sea level fluctuation since the culmination of the Postglacial transgres- sion by Nakata et al. (1978) and Ota et al. (1978), on the basis of surface observation. Here, in addition to the surface data, we use subsurface data to infer the history of formation and emergence of Holocene coral reef terraces. We used five drilling cores at the Shidooke coast, northeastern Kikai Island. These drill- ings reached the base of the Holocene sedi- ment, with maximum thickness of 26m. The basal topography beneath the Holocene sedi- ment was reconstructed. Holocene sediment is classified into five lithofacies : A (coral lime- stone), B and C (detrital limestone), D and E (detrital limestone with terrestrial fragments). Forty in situ corals are dated by the a- spectrometric ~230Th/~234U method. Isochron for Holocene sediments indicates upward reef growth from ca. 10 ka to ca. 6.5 ka, followed by seaward growth since that time. The highest Terrace I is mostly underlain by non-coral sediment of Facies C. In contrast, Terrace II, the widest and with moat and crest system, is underlain by Facies A, typical coral limestone. Thus, Terrace II is a constructional terrace of coral limestone which emerged at 5. 3 ka. Narrow terraces