Oceanographic environment and geologic genesis of the Sekisei Coral Reef region, southern Okinawa.

“Sekisei-sho”拥有一个大型泻湖，该泻湖位于琉球岛弧南部的石垣岛和Iriomote-jima之间，面积约15公里（南北方向），约25公里（东西方向）。这是一个典型的发育在活跃海洋边缘的近代珊瑚礁。STD调查和海水化学分析表明，泻湖内的环境条件具有相当的同质性，两条河流的淡水入侵也有明显但实际上有限的影响。对90个沉积物样品的各种分析表明，钙质有机残留物（特别是六组分）支配着泻湖沉积物的特征，而陆源物质则影响泻湖外部沉积物的特征。单杆调查显示，这个近期礁石的底部可能位于水深25米左右，并有向北略微加深的趋势。这种全新世时期的北倾在邻近岛屿的许多出露的海沟和滩岩的海拔分布和年龄分配中表现得更为明显。根据若干气枪地震剖面的地质复查，晚更新世礁岩组（琉球灰岩）也向北倾斜，并伴有关成铺北侧（即冲绳海槽侧）的两条台阶断裂。南侧为前更新世基底组成的陡坡，坡底发育浊积岩，止于咲岛深海阶地。所有这些事实表明，倾斜运动是该岛弧地区全新世构造的特征，是冲绳海槽扩张的一个有意义的方面。

"Sekisei-sho" is provided with a large lagoon which is developed over the area of ca.15km (north-southly) by ca. 25km (east-westerly) between Ishigaki-jima and Iriomote-jima of the southern Ryukyu Island Arc. This is a typical Recent coral reef developed at the active ocean margin.STD survey and chemical analysis of sea water reveal a considerably homogeneity of envrionmental condition within the lagoon and also a distinct but areally restricted influence of fresh water invasion by two rivers. Various analyses of 90 sediment samples indicate that calcareous organic remains (particularly hexacorals) rule over the characters of the lagoon sediments whereas terrigeneous materials affect on those of sediments in the outside of the lagoon.The uni-boom survey suggests that the base of this Recent reef may be located around 25m in water depth with a trend slightly deepening toward the north. This northward tilting during the Holocene preiod is more clearly shown in the altitudinal deistribution and age-assignment of many emerged notchs and beach rocks in the adjacent islands. According to geological re-examination of a number of air-gun seismic profiles, the late Pleistocene reefal formation (Ryukyu Limestone) also tilts northwards in accompanying with two step faults on the northern side (i. e. Okinawa Trough side) of Sekisei-sho. On the other hand, the southern side shows a steep slope composed essentially of the pre-Pleistocene basements and some turbidite aprons are well developed at the slope base which terminates in the Sakishima Deep-sea Terrace. All of these facts indicate that the tilting movement characterizes the Holocene tectonics of this island arc region as a meaningful aspect of the Okinawa Trough spreading.