| 重庆涪陵二龙口剖面P—T界线附近黏土矿物特征及其古环境意义 |
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| 引用本文： | 李佳昱,梁婷,朱小二,郭玉鑫.重庆涪陵二龙口剖面P—T界线附近黏土矿物特征及其古环境意义[J].地质学报,2021,95(3):868-882. |
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| 作者姓名： | 李佳昱 梁婷 朱小二 郭玉鑫 |
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| 作者单位： | 中国石油大学(北京)油气资源与探测国家重点实验室,北京,102249;中国石油大学(北京)地球科学学院,北京,102249 |
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| 基金项目： | 本文为国家自然科学基金项目 (编号41602108)，中国石油大学(北京)科研启动基金项目(编号2462015YJRC019)及中国石油大学(北京)油气资源与探测国家重点实验室基金 (编号PRP/indep- 4- 1517)资助的成果。 |
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| 摘 要： | 在二叠系—三叠系界线附近地层中,利用大气暴露面上沉积物恢复古环境的研究至今尚未展开.四川盆地东部涪陵地区的二龙口剖面在二叠纪末位于扬子碳酸盐岩台地内,为礁后开阔台地相,在上二叠统—下三叠统界线附近沉积了较为完整的海相碳酸盐岩.该地层从下到上可分为1～31层.其中,上二叠统长兴组(1～6层)顶面为侵蚀不整合面,下伏于厚3～10cm(平均厚度约5cm)的古风化壳(7层).相较于1～6层,飞仙关组7～19层中碳酸盐矿物含量降低,黏土矿物含量增高.而20～31层以泥晶灰岩为主,偶尔伴有薄层黏土岩夹层.二叠系—三叠系界线位于19～20层之间.岩石学和矿物学特征显示,风化壳以黏土矿物为主,主要为规则伊蒙混层(I/S)和少量伊利石.在扬子台地范围内不同的沉积相带中,该黏土矿物组合具有较好的地层可对比性,说明研究区长兴组之上风化壳中黏土矿物形成的主控因素可能是区域性的,如火山活动带来的外源沉积.结合其下伏围岩(1～6层)岩石学和稳定同位素组成均缺乏淡水改造特征,可以判断该风化壳形成于加积模式而非改造模式.值得注意的是,此种黏土矿物以规则伊蒙混层和伊利石为主的组合方式,与二叠纪末出现的碱性、高温、缺氧的沉积环境一致.自7层向上,黏土矿物中伊蒙混层相对含量逐渐降低,且混层比逐渐减小,而伊利石、绿泥石、绿蒙混层相对含量逐渐增加,则与逐渐加剧的碱性-高温-缺氧的极端沉积环境有关.因此,二叠系—三叠系界线附近黏土矿物对扬子台地二叠纪末古环境突变有较好的记录能力.
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| 关 键 词： | 古风化壳 P—T界线 黏土矿物 二龙口剖面 古环境 |
| 收稿时间： | 2020/6/12 0:00:00 |
| 修稿时间： | 2020/8/4 0:00:00 |
Palaeoenvironmental implications of clay mineral characteristics in the Permian- Triassic transitional succession in the Erlongkou section, Fuling, Chongqing |
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| LI Jiayu,LIANG Ting,ZHU Xiaoer,GUO Yuxin.Palaeoenvironmental implications of clay mineral characteristics in the Permian- Triassic transitional succession in the Erlongkou section, Fuling, Chongqing[J].Acta Geologica Sinica,2021,95(3):868-882. |
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| Authors: | LI Jiayu LIANG Ting ZHU Xiaoer GUO Yuxin |
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| Abstract: | Palaeoclimate investigations on exposures of Permian- Triassic transitional strata are rare.The Erlongkou section, from the Fuling region in the eastern Sichuan Basin, is located in the Yangtze carbonate platform during the Late Permian. It exposed a set of relatively complete marine carbonate strata from the Late Permian to the Early Triassic, which were deposited in a backreef environment. The succession has been divided into layers 1 to 31 in an ascending order, with the boundary between Changxing Formation and Feixianguan Formation placed between layers 6 and 7, and the Permian- Triassic boundary placed between layers 19 and 20. The boundary between layers 6 and 7 is a subaerial exposure, and the overlying weathered layer 7 is a 3~10cm- thick (averaged 5cm thick). The layers 7~19 have less carbonate minerals, and more clay minerals than the layers 1~6, whereas the layers 20~31 are dominated by mudstones, with lesser thinly bedded claystones.The petrography and mineral compositions show that the weathering crust (layer 7) is largely composed of clay minerals, which are dominated by illite/smectite mixed- layers (I/S) with minor illite. Such assemblage of clay minerals can be stratigraphically correlated with many other sections in different sedimentary facies throughout the Yangtze platform. This indicates that regional geological processes, such as exogenous deposition caused by volcanic activity, may be the major factor controlling the involvement of clays in the weathering crust. As such, the accretion model instead of the alteration model must have formed the weathering crust. This suggestion can be further supported by the petrographic and stable isotopic evidences, which show that meteoric alterrations in the underlying host rock (layers 1~6) are absent. Additionally, the abundant illite/smectite mixed- layer (I/S) minerals and illite in the weathering crust indicates an alkaline, xerothermic and anoxic depositional environment probably initiated at the end of Permian. From layer 7 to layer 31, the abundance of illite/smectite mixed- layer minerals and their I/S ratios decrease, whereas the abundance of chlorite increases. Such a change in clay mineral is consistent with the notion that the alkaline, xerothermic and anoxic conditions were probably exacerbated during the Permian/Triassic transitional period. Therefore, clay minerals at the Permian- Triassic boundary intervals provide a robust archive for paleoenvironmental changes in the Yangtze platform. |
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| Keywords: | paleo- weathering crust Permian- Triassic boundary clay minerals Erlongkou section paleoenvironment |
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