Carbonate Platform Development and Sea-Level Variations of Xisha Islands: Based on BIT Index of Well Xike-1
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摘要: 为了探讨南海碳酸盐台地的发育过程及控制因素, 采用有机分子化合物指标方法对其进行了研究.有机分子化合物指标BIT(branched isoprenoid tetraether)是沉积物中源自陆源的细菌膜脂支链甘油双烷基链甘油四醚(branched glycerol dialkyl glycerol tetraethers，简称bGDGTs)与主要来自海洋泉古菌中的类异戊二烯GDGTs(isoprenoid GDGTs，简称iGDGTs)的含量之比，在古环境研究中，用来区分沉积物有机质的来源、判断沉积环境.通过对西科1井的数据研究发现，西科1井BIT指数随深度呈现三段式规律性变化，从下到上呈现高-低-高的变化，反映了中中新世以来南海海平面变化及碳酸盐台地生长发育的过程：在中中新世晚期，受全球及区域海平面变化下降影响，西沙地区碳酸盐台地形成礁-滩交互的沉积地层，由于大气淡水的影响，造成BIT指数呈现高值；到晚中新世至上新世，全球及区域海平面出现持续上升，有利于西沙碳酸盐台地的生长发育，使该井沉积环境以礁内泻湖相为主，造成BIT指数呈现低值；在第四纪冰期，全球及区域海平面出现总体下降趋势，西沙碳酸盐台地又频繁暴露于地表，造成BIT指数又呈现高值.研究表明，西沙地区碳酸盐台地受区域相对海平面变化影响更大，说明南海海平面变化既受全球海平面变化的影响，也受南海区域构造沉降的控制.Abstract: In order to explore the developmental process of carbonate platform in the South China Sea (SCS) and its control factors, the organic molecular compound index method were described in this study. The branched isoprenoid tetraether (BIT) index is the ratio between the branched glycerol dialkyl glycerol tetraethers (bGDGTs) from terrigenous supply and isoprenoid GDGTs (iGDGTs) from marine crenarchaeol comprising the majority of denominator, which serves as a proxy for provenance discrimination of the organic matter and related sedimentary environments in paleoenvironmental research. The BIT curve of well Xike-1 is characterized by a unique "high-low-high" feature up section, indicating the effects of sea-level variations and carbonate platform development since Middle Miocene in the SCS. Strata with reef-beach facies were developed within carbonate platforms of Xisha Islands influenced by decrease in global as well as regional sea-level during the Late Miocene, with increased BIT values due to plenty of precipitation. Between Late Miocene and Pliocene when sea-level was higher, carbonate platforms were better formed in shallow marine environment with low BIT values. Carbonate platforms of Xisha Islands were periodically exposed in Quaternary as a result of sea-level decline over glacial cycles, leading to the second increase in BIT. Our results demonstrate that carbonate platforms of Xisha Islands have been affected by regional sea-level variations, reflecting a coupling effect on the SCS by global sea-level fluctuations and regional tectonic subductions.
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Key words:
- South China Sea /
- Xisha Islands /
- well Xike-1 /
- bioherm /
- organic compound /
- marine geology /
- stratigraphy
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图 1 西科1井碳酸盐样品GDGT液相色谱-质谱关系
Fig. 1. GDGT HPLC-MS relationship of the carbonate sample in the well Xike-1
图 2 西科1井碳酸盐样品古菌液相色谱-质谱关系
Fig. 2. iGDGT HPLC-MS relationship of the carbonate sample in the well Xike-1
图 3 西科1井碳酸盐样品细菌液相色谱-质谱关系
Fig. 3. bGDGT HPLC-MS relationship of the carbonate sample in the well Xike-1
图 5 西科1井部分胶结类型及成岩环境
a.垂悬状胶结大气淡水胶结作用，7.96 m；b.纤维状胶结海水胶结作用，251.48 m；c.镶嵌粒状胶结海水胶结作用，483.91 m；d.悬垂状及镶嵌粒状胶结大气水-海水混合胶结作用，747.67 m
Fig. 5. Cementation types and diagenetic environment of well Xike-1
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