The upper member of the Xiaganchaigou Formation in the Yingxi Area of the Southwestern Qaidam Basin is a set of evaporite-bearing lacustrine series. In this study, petrographic analyses of rock thin sections, cores, drillings, logging as well as geochemistry were used to study the characteristics of the sedimentary cycles of the series. Concurrently, this study adopts a transgressive–regressive (T–R) cycle sequence theory, based on lithology, logging curves, the stacking pattern of the subsequences, and unconformity and seismic reflection terminal relations. This approach uses evaporites as the primary marker layer for identifying the initial lake flood surface and dividing the sequence of the evaporite-bearing series in the upper member of the Xiaganchaigou Formation. Results show a zonal distribution of evaporites transitioning laterally from the center to the margin of the basin to mixed carbonate rocks and then to clastic rocks. Vertically, there are two third-order sedimentary cycles and six fourth-order sedimentary cycles. Five types of evaporite lithofacies combinations were identified. Evaporites were formed by evaporative crystallization at a particular depth beneath lake water during a lacustrine regression during an arid paleo-climate and rapid subsidence of the basin basement. It has been established based on the asymmetric sedimentary cycle development model that during the lacustrine transgression stage, the mixed carbonate rocks were transformed into mudstone upward, and during the lacustrine regression stage, the mixed carbonate rocks transformed into gypsum rocks, glauberite, and halite, upward, while a delta and reef flat limestones developed at the edge of the basin. There is a one-to-one correspondence between the T–R cycle sequence division and the sedimentary cycle division.

中文翻译：

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柴达木盆地西南部英西地区下干柴沟组上段蒸发岩沉积旋回及其在层序划分中的应用

柴达木盆地西南部英西地区下干柴沟组上段为一套含蒸发岩的湖相系列。在这项研究中，利用岩石薄片、岩心、钻孔、测井和地球化学的岩相分析来研究该系列的沉积旋回特征。同时，本研究基于岩性、测井曲线、层序叠加模式、不整合面和地震反射终端关系，采用海进-回归(T-R)旋回层序理论。该方法以蒸发岩为主要标志层，用于识别下干柴沟组上段初始湖泛面和划分含蒸发岩层序。结果表明蒸发岩呈带状分布，从盆地中心到边缘向混合碳酸盐岩和碎屑岩过渡。纵向有2个三级沉积旋回和6个四级沉积旋回。确定了五种类型的蒸发岩相组合。蒸发岩是在干旱的古气候和盆地基底快速下沉期间湖水退缩过程中在湖水下特定深度蒸发结晶形成的。基于不对称沉积旋回发育模型建立了在湖侵阶段，混合碳酸盐岩向上转变为泥岩，在湖退阶段，混合碳酸盐岩转变为石膏岩、钙芒硝、岩盐，向上，而盆地边缘则发育三角洲和礁坪石灰岩。T-R旋回层序划分与沉积旋回划分一一对应。