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

Hironobu Kan, Tatsuo Takahashi, Motoharu Koba

Research output: Contribution to journalArticle

14 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)114-131
Number of pages18
JournalGeographical review of Japan, Series B.
Volume64
Issue number2
DOIs
Publication statusPublished - Jan 1 1991
Externally publishedYes

Fingerprint

reef
accretion
drilling
Holocene
sea level
geography
stabilization
wave action
topography
water
wave direction
rate
coral
shallow water
deep water
sea level rise

All Science Journal Classification (ASJC) codes

  • Geography, Planning and Development

Cite this

Morpho-Dynamics on Holocene Reef Accretion : Drilling Results from Nishimezaki Reef, Kume Island, the Central Ryukyus. / Kan, Hironobu; Takahashi, Tatsuo; Koba, Motoharu.

In: Geographical review of Japan, Series B., Vol. 64, No. 2, 01.01.1991, p. 114-131.

Research output: Contribution to journalArticle

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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.",
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