Abstract: Sepiolite-containing succession are developed in the cyclothems composed of limestones and argillaceous limestones in the first Member of Middle Permian Maokou Formation in Sichuan Basin. This paper investigates the paleoenvironment and sedimentary patterns of the sepiolite-containing succession in the first Member of Maokou Formation and reveals their sedimentary geological significances based on analyses such as identification of rock thin section, observations of scanning electron microscopy, determination of major/trace elements and carbon/oxygen isotopes. The research results indicate that there are four types of sepiolites in the first Member of Maokou Formation, namely lenticular, stellate, lamellar and bioclastic sepiolites. Among them, lenticular and stellate sepiolites are mainly developed in limestones, lamellar sepiolite is developed in argillaceous limestones, and bioclastic sepiolite is developed in both limestone and argillaceous limestone. During the limestone deposition, the ancient seawater temperature (T₁) restored using oxygen isotopes (δ¹⁸O) is concentrated between 3.71 ℃ and 12.45 ℃, while the ancient seawater temperature (T₂) restored using the Mg/Ca ratio is concentrated between 13.78 ℃ and 14.20 ℃. The Sr/Ba ratios of the limestones are concentrated between 16.57 and 659.18, with the average paleosalinity of 131.97, average Sr/Cu ratio of 2 175.43, average V/(V+Ni) ratio of 0.9044, and average Ni/Co ratio of 14.32, indicating an cool water sedimentary environment with high salinity, drought, and oxygen-deficiency. During the deposition of argillaceous limestone, T₁ is concentrated between 6.98 ℃ and 14.48 ℃, T₂ is in the range of 13.80 ℃ to 15.14 ℃, the Sr/Ba ratio ranges from 77.34 to 819.59, and the average paleosalinity, Sr/Cu ratio, V/(V+Ni) ratio and Ni/Co ratio is 131.76, 1511.73, 0.912 2 and 16.42 respectively, reflecting a cool-water sedimentary environment with relatively low salinity, humidity and oxygen-deficiency. In the cool-water environment (T₁<12 ℃) with poor aluminum (Al) and rich magnesium (Mg), the silicon(Si)-rich fluid connected by faults forms a large number of sepiolite deposits in low subsags under the drive of gravity and concentration potentials. As the burial depth increases, sepiolites undergo different diageneses, forming a mineral combination sequence of sepiolite, talc, dolomite and quartz. The sedimentary pattern of sepiolite-containing succession in the first Member of Maokou Formation is described as follows. During the deposition period of limestone, the low seawater temperature was low, small amount of fresh water and terrestrial inputs, high salinity of seawater and less siliceous fluids resulted in the deposition of lenticular and stellate sepiolites. During the deposition period of argillaceous limestones, the increase of seawater temperature, increase of fresh water and terrigenous inputs, decrease of salinity, frequent volcanic activities and intrusion of siliceous fluids resulted in the deposition of lamellar sepiolites. The thickness distribution characteristics of sepiolite-containing succession reveal that Maokou Formation in the Sichuan Basin has a sedimentary pattern of "two platforms with one sag". The sepiolite-containing succession are thick in C-shaped Tongjiang-Changshou sag.

Key words: Middle Permian, sepiolite-containing succession, paleoenvironment, sedimentary pattern, Tongjiang-Changshou sag, Sichuan Basin

摘要： 四川盆地中二叠统茅口组一段(茅一段)灰岩-泥质灰岩韵律层发育含海泡石层系。基于岩石薄片鉴定、扫描电镜观察、主/微量元素与碳/氧同位素测定等分析方法,对茅一段含海泡石层系的古环境与沉积模式进行研究,揭示其蕴含的沉积地质意义。研究结果表明,中二叠统茅一段的海泡石主要存在4种形态:透镜状、星点状、层状与生物碎屑状。其中,透镜状和星点状海泡石主要发育在灰岩层中,层状海泡石则发育在泥质灰岩层中,生物碎屑状海泡石在灰岩和泥质灰岩中均有发育。灰岩沉积期,利用氧同位素(δ¹⁸O)恢复的古海水温度(T₁)集中分布于3.71~12.45 ℃,利用Mg/Ca恢复的古海水温度(T₂)集中分布于13.78~14.20 ℃;灰岩中的Sr/Ba集中分布于16.57~659.18,古盐度平均为131.97,Sr/Cu平均为2 175.43,V/(V+Ni)平均为0.9044,Ni/Co平均为14.32,指示沉积环境为 高盐度、干旱、贫氧—缺氧的凉水环境;泥质灰岩沉积期的T₁集中分布于6.98~14.48 ℃,T₂分布于13.80~ 15.14 ℃,Sr/Ba集中分布于77.34~819.59,古盐度平均为131.76,Sr/Cu平均为1511.73,V/(V+Ni)平均为0.912 2,Ni/Co平均为16.42,反映沉积环境为相对低盐度、相对湿润、缺氧的凉水环境。在贫Al、富Mg的凉水环境下(T₁＜12 ℃),断裂沟通的富Si流体在重力与浓度势的驱动下在低洼区形成大量海泡石沉积。随着埋深增大,海泡石发生不同程度的成岩转化,形成海泡石-滑石-白云石-石英的矿物组合序列。茅一段含海泡石层系的沉积模式为:灰岩沉积期,海水温度较低,淡水及陆源输入量少,海水盐度高,硅质流体少,由此沉积透镜状、星点状海泡石;泥质灰岩沉积期,海水温度升高,淡水及陆源输入量增加,盐度随之降低,火山活动频发,硅质流体注入,由此沉积层状海泡石。含海泡石层系的厚度分布特征揭示四川盆地茅口组具有“两台一凹”的沉积格局,“C”形通江—长寿凹陷内含海泡石层系的厚度大。

关键词: 中二叠统, 含海泡石层系, 古环境, 沉积模式, 通江—长寿凹陷, 四川盆地