四川盆地上三叠统马鞍塘组上部黑色页岩成因机制

Formation Mechanism of Black Shale in the Upper Member of Upper Triassic Ma'antang Formation,Sichuan Basin

通过剖面实测、黄铁矿粒径分析、页岩有机碳同位素、锶同位素、黏土矿物种类和含量以及全岩主、微量元素的测定,综合研究四川盆地马鞍塘组上部黑色页岩成因机制。结果表明:页岩段草莓状黄铁矿直径4μm、U/Th≥0. 75共同揭示其沉积于贫氧—缺氧水体中。化学蚀变指数CIA集中在80～90,层位中部w(87Sr)/w(86Sr)具有最小值0. 707 550,I/S突然增大,均表明源区化学风化作用强烈,板块构造活跃,气候湿热,降雨量激增。有机碳同位素的突然负漂移(最大2‰),表征火山活动造成的大气圈有大量轻碳注入。该套黑色页岩是龙门山前陆盆地发育的早期沉积记录,具有全球一致性的前陆盆地地层沉积建造模式,其形成机制主要受控于前陆盆地导致的构造挠曲沉降,在此基础上叠加了全球性的卡尼期洪水事件CPE这一气候扰动事件。东特提斯域边缘(松潘甘孜海盆)上升洋流、超级季风气候和降雨增加导致的陆源营养物质注入,促进了盆内水体表面初级生产力的提高,水体的分层缺氧条件利于有机质的保存,有机地化数据表明黑色页岩生烃潜量大。马鞍塘组黑色页岩是四川盆地海相页岩气的潜力勘探层位。

The formation mechanism of the black shale in the upper member of the Upper Triassic mahntang Formation,Sichuan Basin is studied by profile measurement and pyrite particle size analysis, the measurement of shale organic carbon isotope and strontium isotope, the determination of the species and content of clay minerals and the analysis of the main and trace elements in wholerock. The results show that ： the framboidal pyrite particle diameter of 4 μm and U/Th not less than 0.75 in the shale section indicate that the black shale deposited in euxinia or dysoxic water. CIA focusing on 80 - 90, the minimum value of 87Sr/86Sr being 0.707 in the middle of the shale section, and kaolinite/illite ratio increasing suddenly all indicate that the chemical weathering is strong, and the plate tectonic is active, the climate is hot and humid, and the rainfall surges in source area. The sudden negative drift of shale organic carbon isotope （ the maximum is 2 ‰） indicates that volcanic activity led to large amounts of light carbon injecting into the atmosphere. Regional distribution of Ma＇antang Formation black shale is an early sedimentary record of Longmenshan foreland basin, it has a globally consistent sedimentary formation pattern of foreland basins. The formation mechanism of the black shale is mainly controlled by the tectonic deflected subsidence caused by foreland basin, superimposing the CPE global climate disturbance events. Upwelling ocean currents, supermonsoon climate and sudden rainfall increasing at the edge of the East Tethys Basin （ Songpan-Ganzi Basin） resulted in the injection of terrestrial nutrient, which promoted the improvement of surface primary productivity of the basin. Layered and anoxic conditions of the water body were conducive to the preservation of organic matter. Geochemical data indicate that the hydrocarbon generation potential of the black shale is large. The black shale of the Ma＇antang Formation is a potential exploration horizon for marine shale gas in Sichuan Basin.