Holocene Regressive Coral Reef Terraces at the Northern Shidooke Coast of Kikai Island, Central Ryukyus

Abstract

A history of Holocene terrace formation is discussed through detailed topographic and geologic surveys and α-spectrometric ²³⁰Th/²³⁴U dating of fossil corals along the northern Shiooke coast, Kikai Island. A depositional part of the Holocene coral limestone, a wave cut surface of the Holocene coral limestone, and a shore platform cutting the Pleistocene rhodolith limestone compose a Holocene marine terrace sequence at the northern Shidooke coast. ²³⁰Th/²³⁴U ages of corals from depositional and abrasional parts of the Holocene coral limestone demonstrate the following history of terrace development in this area.
Offlapping growth of the Holocene reef tract and the following erosion near the mean hightide level have formed regressive terraces, during intermittent emergence at 4.0-5.1ka and 2.6-2.9ka after the maximum Holocene transgression. Relative sea levels are estimated to stand at altitudes of 4.0-4.8, 2.6-2.9, and 1.1-1.6m (mean low-tide level) from elevations -1.6m (mean low-tide level) from elevation of depositional parts of the Holocene coral limestone and wave-cut terraces of the Pleistocene rhodolith limestone. Shapes of the transgressive systems tract developed during the post-glacial sea level rise control the subsequent formation of two types of regressive reef terraces, deposition-dominated at the Nakaguma and abrasion-dominated at the northern Shidooke coasts on Kikai Island.