江汉盆地潜江凹陷上始新统含盐岩系准原生白云岩的沉积学特征与形成机理

江汉盆地是我国东部独具特色的古近纪盐湖盆地，其潜江凹陷上始新统含盐岩系中的泥晶白云岩由泥晶白云石组成。泥晶白云石在阴极发光下呈红色、橙红色，与其伴生的矿物除原生钙芒硝、石盐、陆源泥质外，方解石没有或极少； X衍射分析结果表明，泥晶白云石的CaCO3摩尔含量平均为57.30％，有序度平均为0.40，具高富钙、低有序的特征；与钙芒硝岩的紧密共生，说明泥晶白云岩沉积环境的水介质为高盐度的咸水或淡卤水；根据氧同位素数据计算的沉积环境的水体温度平均为30.5℃，与泥晶白云石伴生的原生钙芒硝晶体中的包裹体均一温度最低值为44℃，这2组温度数据基本接近，表明泥晶白云石沉淀时环境水体的温度大致在30.5～44℃之间。由此推断，潜江凹陷上始新统含盐岩系产出的泥晶白云岩是在具较高温度和盐度、高Mg/Ca比值、常年性分层古盐湖水体中，由化学沉淀作用直接形成的准原生白云岩。

Sedimentary characteristics and formation mechanism of peneprimary dolostone in the Upper Eocene salt-bearing interval in Qianjiang Sag,Jianghan Basin

The Jianghan Basin is a typical Paleogene salt lake basin in eastern China. Micro- crystalline dolostone in the Upper Eocene salt-bearing series in the Qianjiang sag, Jianghan Basin is composed of micro-crystalline dolomite, and the dolomite is red and orange-red in cathodoluminescence images. Associated minerals are primary glauberite, halite and terrigenous mud; while calcite is absent or rare. The X-ray diffraction analysis shows that the micro-crystalline dolostone has an average CaCO3 mole content of 57.30% and an average degree of order of 0.40, revealing the characteristics of higher calcium content and lower degree of order. It is closely intergrown with glauberite, indicating that the aqueous medium in the depositional environment of micro-crystalline dolostone was hypersaline water or dilute brine. The temperature of water in the depositional environment calculated according to the oxygen isotope data averaged 30.5℃ and the minimum homogenization temperature of inclusions in primary glauberite crystals associated with micro-crystalline dolomite was 44℃. The two groups of temperature data are close, indicating that the temperature of the water in the paleolake environment during the deposition of micro-crystalline dolostone ranged from 30.5℃ to 44℃. Thus it may be inferred that the micro-crystalline dolostone in the Upper Eocene salt-bearing interval in the Qianjiang sag is peneprimary dolostone formed directly by chemical precipitation in a high-temperature, high-salinity, high-Mg/Ca ratio and perennially meromictic salt lake.