Morpho-Dynamics on Holocene Reef Accretion: Drilling Results from Nishimezaki Reef, Kume Island, the Central Ryukyus

Abstract

This paper focuses on the relationships between time, space and geomorphic environments of the Holocene reef-flat formation. The discussion is based on nine core logs drilled in the Holocene emerged reef-flat of Nishimezaki, Kume Island, the Central Ryukyus. The following results are obtained. The apparent sea-level curve during the period from 7,500 to 2,000 Y.B.P. can be divided into three phases on the basis of sea-level rise rates. 1) 7,500 to 6,500 Y.B.P.; Sea-level rose rapidly at an approximated rate of 10m/l,000yrs. 2) 6,500 to 5,000 Y.B.P.; Sea-level rise rate slowed abruptly to less than 3 m/1,000 yrs. Rise rate gradually decreased up to 5,000 Y.B.P. 3) 5,000 to 2,000 Y.B.P.; Stabilization of sea-level. The formational process of the reef-flat topography could be divided into three phases. 1) Initial growth phase; Reef initiation occurred at ca. 7,500 Y.B.P. The differences of growth fabrics in this phase depend on the direction of driving waves and slopes on the pre-Holocene topography, and the depth variation at that time. 2) Reef-crest growth phase; Vigorous reef growth of autochthonous tabular corals coincides with the slowing down of the sea-level rise rate between 6,500 to 6,000 Y.B.P. The spurs or pinnacles located in shallow water and attacked by strong wave action reached the sea-level earlier than those in other locations (e.g. deep water, not under strong wave action). 3) Reef-flat formation phase; This phase coincides with the slow rise and stabilization of sea-level from 6,000 Y.B.P. The reef-crest ridge is the earliest to reach sea-level. Subsequently, the outer part and inner part of the reef-crest reached the sea-level and formed the reef-flat. Where breaks or interruptions are present in the reef-crest ridge, there is rapid growth of the inner part. In contrast to the sea-level rise rate and the wave direction controlling unilaterally the reef accretion processes, topography is both a result of reef accretion and a factor which controls the subsequent reef accretion. © 1991, The Association of Japanese Geographers. All rights reserved.