四川盆地东北部镇巴地区二叠系C-O-Sr同位素组成及沉积演化特征

为认识四川盆地东北部镇巴地区二叠系海平面变化及沉积演化特征,选取陕西省镇巴县杨家湾典型剖面,以岩石学特征研究为基础,结合C-O-Sr同位素组成所反映的古海平面、古盐度、古温度变化来综合分析四川盆地东北部镇巴地区二叠系的沉积演化特征。C-O-Sr同位素分析结果显示:δ¹³C值为-1.67‰～5.85‰,平均值为2.76‰; δ¹⁸O值为-7.26‰～-3.44‰,平均值为-6.24‰; ⁸⁷Sr/⁸⁶Sr值为0.707 147～0.708 133。C-O-Sr同位素组成所反映的古海平面、古温度(12.0 ℃～25.2 ℃,平均值约为21.0 ℃)、古盐度(盐度指示剂为120.88～135.90,平均值为129.84)特征表明:研究区处于水体流通较好的温暖—炎热的高盐度区,且全球海平面升降事件对研究区碳酸盐同位素组成起到良好的控制作用; 研究区与全球海平面变化具有一致性,即栖霞初期的快速海侵后进入相对平稳、缓慢下降阶段,茅口初期经历大规模海侵与茅口末期海平面大幅度下降,吴家坪初期经历大规模海侵及中期海退,长兴初期海平面上升。综合分析沉积特征可知,研究区经历了由古海平面变化及构造运动控制的开阔台地→台地边缘→陆棚→缓坡环境的沉积演化过程。

Characteristics of C-O-Sr Isotope Composition of Permian and Their Sedimentary Evolution in Zhenba Area of the Northeastern Sichuan Basin,China

In order to understand the change of sea level and sedimentary evolution characteristics of Permian carbonate rocks in Zhenba area of the northeastern Sichuan Basin, the Yangjiawan section in Zhenba county of Shaanxi province was selected. Based on the field characteristics of rock formations and indoor petrology characteristics, and combined with the changes of paleosea level, paleosalinity and paleotemperature reflected by C-O-Sr isotopic composition, the sedimentary evolution characteristics of Permian in Zhenba area of the northeastern Sichuan Basin were comprehensively analyzed. The analysis results of C-O-Sr isotopes show that δ¹³C is -1.67‰-5.85‰ with an average of 2.76‰; δ¹⁸O is -7.26‰--3.44‰ with an average of -6.24‰; the ratio of ⁸⁷Sr/⁸⁶Sr is 0.707 147-0.708 133. The changes of paleosea level, paleotemperature(12.0 ℃-25.2 ℃ with an average of about 21.0 ℃), and paleosalinity(salinity indicator is 120.88-135.90 with an average of 129.84)reflected by C-O-Sr isotope compostions, indicate that Zhenba area is warm-hot high-salinity with good water circulation, and the global sea level eustatic events have a good control on the composition of carbonate isotopes; the sea level changes in Zhenba area are basically consistent with global sea level changes, that is, entering a relatively stable and slow decline stage after rapid transgression in early Qixia period, experiencing large-scale transgression in the early Maokou period and sea level decline significantly in the late Maokou period, experiencing large-scale transgression in the early Wujiaping period and regression in the middle Wujiaping period, and experiencing sea level rise in early Changxing period. Based on the comprehensive analysis of sedimentary characteristics, the sedimentary evolution process is open platform→platform edge→shelf→gentle slope in Zhenba area of the northeastern Sichuan Basin.