四川盆地东部开江．梁平海槽东西两侧三叠系飞仙关组碳酸盐岩碳氧同位素组成对比及古海洋学意义

Comparison of carbon and oxygen isotopic composition of Feixianguan carbonates, Early Triassic, between east and west sides of Kaijiang-Liangping trough, Sichuan Basin, and the significance for paleoceanography

海相碳酸盐岩的碳氧同位素组成具有显著的古海洋学意义,其组成具有随时间变化的长期趋势,但不同沉积环境中古海水碳氧同位素组成的异同仍存在争议并需要进一步深入研究。本文测试了四川盆地东部开江-梁平海槽东侧孤立台地内的四川宣汉渡口剖面飞仙关组及相邻地层133件海相碳酸盐岩样品的碳氧同位素组成,对比研究了海槽西侧作为镶边陆棚沉积的华蓥/邻水仰天窝剖面对应地层样品的碳氧同位素组成。研究表明,孤立碳酸盐台地和镶边陆棚飞仙关组具有非常类似的碳同位素组成和演化趋势,并与对应时间全球海水碳同位素演化趋势一致,显示全球事件是海水碳同位素组成的主要控制因素。然而,开江-梁平海槽东侧孤立碳酸盐台地和西侧镶边陆棚飞仙关组的氧同位素组成并不完全一致,存在如下共性和个性:(1)两种台地类型飞仙关组的氧同位素组成都显示出类似的变化样式,即从二叠-三叠纪界线附近δ^(18)O的低值演化至其余时间相对稳定的氧同位素组成,曲线主体氧同位素组成也具有类似的变化幅度;(2)两种台地类型飞仙关组底部地层中都检测到氧同位素组成非常低的样品,它们可能不完全是成岩蚀变的结果,如可能与界线附近广泛的热事件有关,但仍需要数据的进一步积累以及更多剖面高分辨率年代框架内氧同位素的对比研究;(3)孤立台地内碳酸盐岩δ^(18)O显著低于镶边陆棚,差值达到1‰左右,由此换算的古海水温度差值大致在4℃左右,显示孤立台地内部具有更高的碳酸盐形成温度;(4)孤立台地存在一些氧同位素组成较正、且显著偏离演化曲线主体的样品,其高δ^(18)O值的原因除白云石-水和方解石-水具有不同的氧同位素分馏系数以外,同时还反映海槽东侧孤立台地内部飞仙关组沉积时海水具有较高的盐度。开江-梁平海槽东西两侧氧同位素组成的差别表明东侧孤立碳酸盐台地内海水比西侧镶边陆棚内具有更高的温度和盐度,海水循环更受限制,台内海水更具有白云石化作用的潜力,这不仅造成东侧台地内飞仙关组发育更多的与萨布哈模式有关的准同生白云岩,还会通过回流作用或台内热对流作用造成台地边缘滩相颗粒岩的白云石化,这使得开江-梁平海槽东侧的滩相颗粒岩更可能成为质量较好的颗粒白云岩储层。

Carbon and oxygen isotopes of marine carbonates are significant for paleoceanography as they show a general secular trend with geological time. However, whether carbon and oxygen isotopic compositions of carbonates are controlled by sedimentary environment is still disputed, so requiring further study. In this study 133 marine carbonate samples of the Feixianguan Formation and adjacent strata were collected from the XuanhanDukou section, which was deposited in the isolated platform at the east side of the Kaijiang-Liangping trough in Sichuan Basin. We tested the carbon and oxygen isotopic compositions of these samples and compared the data to those for carbonate samples from Huaying/Linshui Yangtianwo section, which was deposited in the environment of rimmed shelf at the west side of the Kaijiang-Liangping trough. The results show that high similarity in carbon isotope composition and evolution tendency for Feixianguan Formation exists between isolated carbonate platform and the rimmed shelf of the two sides of the Kaijiang-Liangping trough. The evolution tendency, which is coincident with that for carbon isotopic composition of global coeval seawater, presents variation from low values for the Permian-Triassic boundary to high values for the top of Feixianguan Formation, and show higher frequency and larger fluctuations in 813C values, indicating that the global events are the main causes for carbon isotopic compositional variations of sea water. However, the oxygen isotopic composition of Feixianguan Formation between the two sides of the Kaijiang-Liangping trough is not completely consistent with the following points： （a） showing a similar variation pattern in terms of δ180 trend of Feixianguan Formation at both sides of the Kaijiang-Liangping trough, which is characterized by a very low δ180 value near the boundary of Permian-Triassic strata and a subsequent relatively stable and gradual variation trend, （b） some samples at the bottom layer of the Feixianguan Formation at the two sides of Kaijiang-Liangping trough showing lower 6180 values, this could not be simply explained by diagenetic alteration, as it is probably caused by globally thermal events, so further research and data accumulation are needed, （c） The 8180 value for carbonates deposited in the isolated platform is lower than that in the rimmed shelf, for an approximate difference of 1‰, which corresponds to a paleoseawater temperature difference of about 4℃, indicating a higher precipitation temperature of carbonate in the isolated platform, （d） some samples with higher 8180 values departing far from the main variation curve trend, this is probably caused by difference in 180/160 fractionation factors between dolomite-water and calcite-water because the samples contain dolomite to some degree, or may indicate a higher salinity of coeval seawater in the isolated platform at the east side of the trough. The difference in oxygen isotope of the carbonates deposited at the two sides of the Kaijiang-Liangping trough suggests that the seawater in the isolated carbonate platform at the east side of Kaijiang-Liangping trough is more restricted due to higher temperature and salinity than that in the rimmed shelf at the west side of the trough, which results in higher dolomitization potential in the isolated platform. As a result, more penecontemporaneous dolomicrite related to the Sabkha environment would be produced and more beach-facies grainstone deposited in the platform margin would be dolomitized by refluxing and/or thermal convection in the platform interior, and the latter could result in formation of dolomite reservoir of higher quality.