琉球列島喜界島の完新世海成段丘

書誌事項

タイトル別名
  • HOLOCENE RAISED CORAL REEFS OF KIKAI-JIMA (RYUKYU ISLANDS)
  • リュウキュウ レットウ キカイガシマ ノ カンシンセイ カイセイ ダンキュウ
  • AN APPROACH TO HOLOCENE SEA LEVEL STUDY
  • 完新世海面変化研究へのアプローチ

この論文をさがす

抄録

The raised Holocene sediments on Kikai-jima of the Central Ryukyus form four, topogra-phically well defined, marine terraces around the entire coast of the island. They consist principally of reefoid limestones resting unconformably on wave-cut benches and other erosoinal surfaces of either Pleistocene limestones (Araki Limestone and Ryukyu Limestone) or Pliocene mudstone (Somachi Formation). The limestone varies in lithology from coralalgal boundstones to foraminiferal-algal grainstones with other minor facies.<br> About 30 radiocarbon dates of corals indicate that on each terrace the topographically higher limestones are in general older than the more seaward, lower limestones. Coral dates within a given terrace suggest that a lateral seaward accretion was more likely than a vertical growth pattern for the reefs. Radiometric dating, extensive aerial photo-interpretation and detailed topographic pro-filing with the use of an autolevel along twenty transects perpendicular to the shoreline have established the existence of four ancient strand lines between 6, 800 y. B. P. and 1, 500 y. B. P. All of the strand lines appear to represent an interval of relative high still stand of sea level. The highest and oldest shoreline that is confirmed at the local development of notches attains at least 13m above the present mean sea level. The present altitudes of this as well as other abandoned shorelines are as follows: +9 to +13m for Terrace I (6, 000 to 6, 800 y. B. P.), +5 to +7m for Terrace II (3, 500 to 5, 200 y. B. P.), +2.5 to +5m for Terrace III (3, 000 to 3, 500 y. B. P.) and +1.5 to +2m for Terrace IV (1, 500 to 2, 500 y. B. P.), respectively. The present elevation of the four strand lines are attributed to local uplift due to the island-arc neotectonism which has an average rate of 1.5 to 2.0mm/year for the last 130, 000 years.<br> The steady rate of uplift suggests that original elevation of the strand lines was fairly close to the present sea level. No evidence has been found to substantiate the view that any of these still stands was appreciably above the level of the present sea. The oxygen isotope measurement of fossil molluscs associated with the dated corals seems to be in accordance with this contention.<br> If a fluctuation of sea level should be sought, a relative drop in the magnitude of 2 to 3m below the present sea level may possibly be postulated twice, one at 5, 500_??_6, 000 y. B. P. and the other at 1, 500_??_2, 500 y. B. P. Two maxima of sea level rise can also be inferred, one with +1 to +3m above the present sea level between 6, 000 y. B. P. and 7, 000 y. B. P., and the other of +1m between 3, 500 y. B. P. and 5, 000 y. B. P.<br> At Kikai, formation of the reefs was initiated during the interval represented by the oldest terrace (Terrace I), but it was not as active as elsewhere in the Pacific then, even though the time was possibly the culmination of sea level change and may well be correlated with that of “Climatic Optimum”. Instead, the reef growth around Kikai was the greatest between 5, 000 y. B. P. and 3, 500 y. B. P. This is shown by the next younger terrace (Terrace II), which has the largest and best preserved intact fringing reef among the Holocene sediments on the island. Both of the two youngest terraces (Terraces III and IV) may indicate rather brief duration of relative high stand when the reef growth was similar to that of the present-day.

収録刊行物

  • 地理学評論

    地理学評論 51 (2), 109-130, 1978

    公益社団法人 日本地理学会

被引用文献 (24)*注記

もっと見る

詳細情報

問題の指摘

ページトップへ