新疆柯坪地区寒武纪-奥陶纪碳酸盐岩沉积旋回的碳氧同位素研究

Sedimentary sequences in Keping area, Xinjiang: Constraints from carbon and oxygen isotope compositions of Cambrian to Ordovician carbonate rocks

野外观测发现,在新疆柯坪地区蓬莱坝剖面,中寒武统阿瓦塔格组(2a)至下奥陶统蓬莱坝组(O1p)的碳酸盐岩在剖面上存在不同尺度的沉积旋回:体系域级、米级旋回级、藻纹层级。对49件样品进行微区取样,共得到98件样品,对其进行了精细的不同尺度沉积旋回碳氧同位素组成的分析研究。结果表明,碳酸盐岩样品的δ13C值(PDB)分布在-1.6‰～1.6‰之间,平均为-0.5‰,δ18O值(PDB)为-12.7‰～-4.3‰,平均-6.2‰,因此,除了明显受到后期成岩蚀变作用(退白云岩化)的藻碳酸盐岩外,其余样品的碳氧同位素组成总体位于正常的海相碳酸盐岩沉积范围内。在体系域尺度上,对于Ⅲ2旋回层序高水位体系域(2a1),海平面变化是控制碳酸盐岩碳氧同位素组成的主要因素;对于Ⅲ3旋回层序(2a2—O1p),伴随海平面的升降变化,岩石沉积微环境复杂多变。在米级旋回尺度上,米级旋回的形成除了受高频海平面变化影响外,还受到古构造、古气候的叠加影响。在藻纹层旋回尺度上,δ13C值比δ18O值更能反映藻纹层的旋回性变化特征,藻纹层的形成受控于有机质沉积速率的季节性变化。

Field observations indicate that three types of sedimentary sequences are developed in the Middle Cambrian to Early Ordovician carbonate rock section from the Keping area, Xinjiang. They include system tract sequence, meter-scale cyclic sequence and laminar alga sequence. In this study, 98 samples were micro-sampled from 49 rock samples for carbon and oxygen isotopic analyses, in order to investigate the isotope compositions of sedimentary sequences. Analytical results show that the δ^13CPDS values of the samples range from - 1.6‰ to 1.6‰ with an average of 0.5‰, and the δ^18CPDS values vary from - 12.7‰ to - 4. 3‰, with an average of -6. 2‰. Thus, the isotopic compositions are generally consistent with ancient seawaters, except the alga carbonate rocks undergone clear post-diagenesis （e. g., de-dolomitization）. As to the isotopes of system tract sequences, variation of sea level is the main controlling factor of HST of Ⅲ2 cyclic sequence （∈2a^1）. In contrast, in the Ⅲ3 cyclic sequence （∈2a^2 -O1p）, the micro-sedimentary environments have big changes. The meter-scale cyclic sequence is formed under mixing effect of highly-frequent sea-level change and paleo-structures and paleo-climate. As to the laminar alga sequences, δ^13CPDS value is better to reflect the cyclic variation of laminar than δ^18 OPDB, and its formation is mainly due to seasonal sedimentation rate of organic matters.