A study on growth history of reef terraces deduced from the U-series method of dating

以珊瑚的U系列年龄为时间尺度，对形成于几万年前的基开岛(琉球群岛)、Huon半岛(巴布亚新几内亚)、菲律宾群岛和斐济群岛的珊瑚礁阶地的生长历史进行了研究。1.根据U系列测年和地层学研究结果，基开岛的珊瑚灰岩在海拔10~65m左右零星分布于同位素5a阶段以前。这种石灰岩覆盖在被指定为3-5E阶段的前礁沉积之上。这些事实表明，从同位素阶段5E到同位素阶段3，在陆续变浅的海底形成了珊瑚礁。相比之下，α能谱方法也可以用来解释全新世珊瑚礁阶地的生长历史。岛上的全新世珊瑚石灰岩在大约10ka时开始在水中沉积…。海平面上升时，海侵体系域和高位体系域沉积，达到最大高度(约12m)，最高可达7.5ka。随着海平面的降低，陆源物质逐渐供不应求，大约在7ka2左右，边缘的珊瑚礁开始生长。在宫古岛外的岛架外缘下方发现了一处埋藏的珊瑚礁，暗示着末次冰期最大时的海平面为126米深。根据Huon半岛晚更新世珊瑚的U系列测年和氧同位素分析，表明海平面在约140ka的快速上升在约135-130ka被中断，并在一度下降后重新开始。证实了在5E阶段形成的珊瑚礁阶地在庞老、博和和麦滩群岛的分布。巴拉望群岛的西侧也出现了5E级的潮汐凹槽。这些古海岸线指标的海拔高度表明，菲律宾群岛的一些地区没有抬升，尽管它们位于西太平洋地区的活动边缘。较少

The growth history of coral reef terraces at Kikai Island (Ryukyu Islands), Huon Peninsula (Papua New Guinea), Philippine Islands, and Fiji Islands, which were formed during the last several tens of thousands years, has been elucidated by using U-series dates of corals as a time scale. The main results obtained in this study are outlined as follows :1. As the result of U-series dating and stratigraphic study, coral limestone younger than the isotope stage 5a was sporadically distributed at an altitude of ca. 10 to ca. 65 m in the Kikai Island. Such limestone overlies the fore-reef sediments assigned to stage 3 to 5e. These facts suggest that coral reefs have been formed on the sea floor being successively shallowed with the regression from isotope stage 5e to 3. In contrast, alpha-spectrometric ^<230>Th/^<234>U method was also useful for interpreting the growth history of Holocene coral reef terraces. The Holocene coral limestone at the island began to deposit at ca. 10 ka in water dep … More th of 5 to 10 m. Then the transgressive and highstand systems tracts were deposited during the times when sea level raised and attained to the maximum height (ca. 12 m) up to ca. 7.5 ka. As lowering of the sea level, the terriginous materials became gradually in short supply and a fringing reef started to grow around 7 ka.2. A buried coral reef implying that the sea level at the last glacial maximum was 126 m in water depth, was found beneath the outer edge of insular shelf off the Miyako Islands.3. It was explicated, from U-series dating and oxygen isotopic analysis of late Pleistocene corals at Huon Peninsula, that the rapid rise of sea level at ca. 140 ka has been interrupted at ca. 135-130 ka and restarted after it has been once lowered.4. The distribution of coral reef terrace formed during the stage 5e was confirmed at Panglao, Bohol, and Mactan Islands. A tidal notch of the stage 5e also occurred at the west side of Palawan Islands. The altitude of those indicators for paleo-shoreline denotes that some areas in the Philippine Islands have not uplifted, even though they are situated in the active margin in the western Pacific region. Less