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

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