陕西镇安石炭系—二叠系界线剖面层序地层和相对海平面变化研究

Sequence stratigraphy and relative sea-level change in the Carboniferous-Permian boundary section at Xikou, Zhen`an County, Shaanxi

在陕西镇安西口上石炭统逍遥阶至下二叠统隆林阶的剖面上,区分出含生物碎屑灰泥灰岩、生物碎屑粒泥灰岩、生物碎屑泥粒灰岩、生物碎屑颗粒灰岩、团块颗粒灰岩和核形石颗粒灰岩6种主要的碳酸盐岩微相类型。通过详细的微相特征分析,确定了碳酸盐岩沉积过程中的水动力状况和相对水深变化趋势,识别出低能潮下型碳酸盐旋回A1、A2及高能浅滩型碳酸盐旋回B1、B2两类4种高频旋回。根据这些高频旋回在剖面上的有序叠置型式,将镇安西口地区的上石炭统逍遥阶至下二叠统隆林阶划分出12个四级层序和5个II型三级层序地层单元,反映了这一地区晚石炭世逍遥期至早二叠世隆林期曾发生过12次四级相对海平面波动和5次明显的三级相对海平面升降。

In the section from the Upper Carboniferous Xiaoyao Stage to Lower Permian Longlin Stage at Xikou, Zhen`an, Shaanxi Province, from below upward, 6 main types of carbonate microfacies are identified. They are bioclastic mudstone, bioclastic wackstone, bioclastic packstone, bioclastic grainstone, lump grainstone, and oncolite grainstone. Based on the detailed analysis of these microfacies, the hydrodynamic conditions and the trend of relative change in water depth during deposition of carbonate are determined, and four types of high-frequency cycles, types A1 and A2 in the low-energy subtidal zone and types B1 and B2 in high-energy shoals, are distinguished. According to the ordered stacking patterns of these high-frequency cycles in the vertical section, 12 4th-order sequences and 5 3rd-order sequences are recognized. This suggests that 12 4th-order relative sea-level fluctuations and 5 3rd-order pronounced relative sea-level changes took place from the Late Carboniferous Xiaoyao Age to Early Permian Longlin Age in this area.