四川盆地五峰组—龙马溪组页岩岩相类型与沉积环境

Lithofacies types and sedimentary environment of shale in Wufeng-Longmaxi Formation,Sichuan Basin

沉积物的搬运过程和环境条件控制着富有机质页岩的形成。根据矿物学、岩石组构、生物组成及沉积构造的不同,五峰组—龙马溪组页岩识别出7种岩相类型:硅质页岩、粉砂质页岩、黏土质页岩、灰质页岩、含介壳灰岩泥质/泥灰岩、粉砂岩—细砂岩和斑脱岩。五峰组—龙马溪组页岩并不是一直在低能的水动力条件下以悬浮的形式沉积,大量硅质生物化石、具粒序特征的生物碎屑、粉砂纹层、透镜状层理及沙纹交错层理等沉积构造表明,还存在生物沉积、风暴沉积和底流沉积作用。微量元素分析表明五峰组和龙马溪组下部硅质页岩形成于贫氧/缺氧环境,富含有机质;随后海平面逐渐下降,同时受到底流的影响,岩相类型向粉砂质页岩和黏土质页岩转变,贫氧/缺氧环境受到破坏,有机质的保存条件逐渐变差。

T he form ation of organic-rich shale was controlled by sedim ent transport and deposition processes as well as sedim entaryenvironm ent conditions. Based on m ineralogy as well as different rock fabrics, biotic com ponents and sedim entary textu res, seventypes of shale lithofacies w ere identified in W ufeng-Longm axi Form ation; siliceous shale, silty shale, argillaceous shale, calcareousshale,shell-bearing lime m u dsto ne/m arl,siltstone-fine sandstone and bentonite. T he shale in W ufeng-Longm axi Form ation was notalw ays able to deposit in a suspending form under the low-energy hydrodynam ic conditions. A large num ber of siliceous biologicalfo ssils, biological detritus w ith grading characteristics, silt lam inations, lenticular bedding, ripple cross-bedding and other sedim entarytextures indicate that biogenic deposition, storm deposition and bottom current deposition also existed. T race elem ent analysissuggests that the siliceous shale in the low er part of W ufeng-Longm axi Form ation was form ed in the dysoxic/anoxic environm ent,which was rich in organic m atters. T hen, sea level gradually declined. M eanw hile, influenced by bottom cu rren t, lithofacies typesw ere changed and show n as silty shale and argillaceous shale, while dysoxic/anoxic environm ent was dam aged〉 resulting in the graduallyw orsening preservation conditions of organic m atter.