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/1, 000 yrs. 2) 6, 500 to 5, 000 Y. B. P.; Sea-level rise rate slowed abruptly to less than 3m/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.