海南岛西北部海相地层的沉积时代及其古环境演化记录研究

On the coast of northwestern Hainan Island, a set of marine strata is exposed near the Shenjian light tower. These strata are by now the only outcrop of Pleistocene marine sediments discovered in coastal areas of China except Taiwan. They are excellent materials for understanding the late Neozoic paleoceanography and are greatly better than drill cores in reconstruction of geological history. However, only a few research reports on the strata can be reached, which were most published in 1980s and 1990s. Because of the limitation of techniques and methods at that time, age of the strata remains unsettled, and, as a result, exact information of geological events recorded by these marine sequences is still unknown. In this project, we try to obtain the reliable and accurate ages for the uplifted marine strata by using new dating methods including quartz Ti-center ESR and quartz SAR-OSL, in combination with magnetostratigraphy. Study of Oxygen and Carbon isotope variations and surface textural analysis of quartz grains by scanning electron microscopy are also carried out. Results will give great insights into the evolution of sedimentary environment, which was mainly effected by sea level changes and volcanic activity. In addition, stratigraphic correlation of these Pleistocene marine strata with those uplifted in Taiwan will operated. Results of the project not only benefit our learning of the paleoceanography and tectonic evolution of the northern South China Sea, but also provide scientific data for the estimation of geologic hazard in Yangpu economic development zone.

海南岛西北部出露的海相地层，是目前我国除台湾省外在沿海和岛屿所见的惟一的更新世海相地层，为晚新生代古环境研究中不可多得的材料。作为地层露头，其在地质历史重建中的作用也是钻孔岩心无法比拟的。但是，有关这套地层的研究主要是在上个世纪八九十年代，由于当时可用的测试手段有限，导致地层的时代存在多种说法，对其所记录的地质演化信息也揭露得不够深入。随着测试技术的进步，有必要利用新的方法对这套特殊的地层开展深入研究。本项目主要利用近几年新发展起来的石英Ti心ESR法，结合石英SAR-OSL测年法和磁性地层学，测定地层的“绝对”年龄，解决其年代争议问题；通过野外调查，结合碳氧同位素研究及石英砂扫描电镜分析，揭示海相地层在海平面变化和火山活动作用下的沉积演化过程，并与台湾海相地层开展对比研究。研究成果将为揭示南海北部古海洋环境与构造演化做出学术贡献，为洋浦经济开发区的地质灾害调查与评价提供科学依据。

沉积地层是人们认知地史时期环境变迁及构造演化的重要材料。雷琼地区第四纪火山活动使海南岛西北部出露了一套更新世海相地层。该地层含有丰富的海相生物化石，呈现多种沉积结构和多个沉积旋回，并发生过强烈的构造变形，因此是南海北部第四纪古海洋环境和雷琼地区构造演化的重要信息载体。然而，过去有关这套地层的研究报道并不多，而且相关的研究工作主要开展于上个世纪八九十年代。由于当时可用的测试手段比较有限，对于该套地层所记录的古环境演化信息也就揭露得不够深入，有关其沉积时代也存在多种说法。本研究通过野外考察和钻探取样，结合地形地貌，初步查明了该套地层的分布特征和各露头的层序关系。海南岛西北部被抬升的更新世海相地层基本以莲花山为穹隆轴部，往东北和西南侧地层变年轻。结合钻孔揭露的海相地层厚度和莲花山周缘露头可判断，该地层厚度可达百米。表征风暴沉积的特征层中首次发现的珊瑚礁，表明海南岛在更新世已出现珊瑚生长。利用石英ESR法测定了多处不同位置的地层年龄，结果显示海南岛西北部出露海相地层的年龄介于（33±6.6）~（2567±256）ka之间，反映了该地层的沉积时代基本属于更新世，这可结合后续的古地磁测试结果和玄武岩40Ar-39Ar年龄进一步确认。覆盖于海相地层之上的火山凝灰岩中，其锆石的U-Pb同位素年龄主要介于100~3000Ma之间，表明这些锆石均为碎屑锆石，可能来源于火山喷发时的围岩或沉积时的非火山沉积物，可用于验证该地层是否在德义岭火山喷发前由自东南向西北方向入海的河流带来的大量长石石英质砂堆积而成。这一研究解决了该套特殊地层过去一直未明确的“绝对”年龄问题，研究结果将促进我国低纬度沿海地区早、中第四纪古环境、古气候及构造等问题的深入探讨，并有利于该地层与邻区及国际第四纪地层的对比。