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Volume 42 Issue 5
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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(5): 1784-1798

CHEN WeiZhen, TIAN JingChun, LIN XiaoBing, LIANG QingShao, YANG YanRu, WANG Xing. Geochemical Characteristics and Paleoenvironmental Significance of Lower Cambrian Maidiping and Qiongzhusi Formations in Southwestern Sichuan Basin: A case study of well JS1[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1784-1798. doi: 10.14027/j.issn.1000-0550.2022.101
Citation: CHEN WeiZhen, TIAN JingChun, LIN XiaoBing, LIANG QingShao, YANG YanRu, WANG Xing. Geochemical Characteristics and Paleoenvironmental Significance of Lower Cambrian Maidiping and Qiongzhusi Formations in Southwestern Sichuan Basin: A case study of well JS1[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1784-1798. doi: 10.14027/j.issn.1000-0550.2022.101
shu

Geochemical Characteristics and Paleoenvironmental Significance of Lower Cambrian Maidiping and Qiongzhusi Formations in Southwestern Sichuan Basin: A case study of well JS1

doi: 10.14027/j.issn.1000-0550.2022.101
  • 1.

    College of Earth Science, Chengdu University of Technology, Chengdu 610059, China

  • 2.

    State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China

Funds:

National Science and Technology Major Project 2016ZX05007004-002

SINOPEC Southwest Oil and Gas Branch Project 34450000-19-ZC0607-0017

Chengdu University of Technology Sedimentary Geology Research and Innovation Team Project KYTD201703

  • Received Date: 2022-02-21
  • Accepted Date: 2022-11-01
  • Rev Recd Date: 2022-08-11
    • Available Online: 2022-11-01
  • Publish Date: 2024-10-10
  • Objective The Ediacaran-Cambrian, as a key transition period in geological history, saw a dramatic transformation of the Earth system in terms of tectonic evolution, climate fluctuations, marine environment and biological community evolution. It is of great significance to study in depth the sediment sources, paleo-water depth, paleosalinity and redox conditions during this period. Methods Samples from the Maidiping Formation and the Qiongzhusi Formation of the well JS1 in the southwestern part of the Sichuan Basin were selected and analyzed in detail for total organic carbon （TOC）, major elements, trace elements and rare earth elements （REE）. [Results and Conclusions] The material sources of the Maidiping and Qiongzhusi Formations were found to be relatively consistent. The parent rock primarily consists of sedimentary rock and granite, and the material source area was created in the environment found at the continental margin. The Maidiping Formation contains more light rare earth elements （LREE） than heavy rare earth elements （HREE） （（La/Yb）N=6.05）, slightly to the right, and the average δEu is slightly <1, indicating normal seawater deposition in a shallow water body that was not significantly affected by hydrothermal action. The Qiongzhusi Formation has obvious LREE differentiation （（La/Yb）N=7.69）, LREE enrichment, HREE depletion and an obvious right dip, with weak negative δCe and δEu anomalies, possibly related to intermittent hydrothermal activity. In addition, comparative analyses for different wells have shown that the water body during the sedimentary period of the Maidiping Formation was in an oxidizing environment, and the sedimentary system in the region ranges from tidal-flat to shallow-shelf facies. The Qiongzhusi Formation as a whole belongs to an anaerobic environment, with a reducing environment its base and the degree of anoxia decreasing upwards. The regional develop-ment was a shallow- to deepwater shelf sedimentary system.
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    Shi Chunhua, Shan Shujiao, Hao Jing, et al. Inorganic geochemical characteristics and evaluation of Sinian-Cambrian post-mature source rocks in Sichuan Basin[J]. Petroleum Geology and Experiment, 2022, 44（3）： 505-514.
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    Fan Haijing, Deng Hucheng, Fu Meiyan, et al. Sedimentary characteristics of the lower Cambrian Qiongzhusi Formation in the Sichuan Basin and its response to construction[J]. Acta Sedimentologica Sinica, 2021, 39（4）： 1004-1019.
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  • Received:  2022-02-21
  • Revised:  2022-08-11
  • Accepted:  2022-11-01
  • Published:  2024-10-10

Geochemical Characteristics and Paleoenvironmental Significance of Lower Cambrian Maidiping and Qiongzhusi Formations in Southwestern Sichuan Basin: A case study of well JS1

doi: 10.14027/j.issn.1000-0550.2022.101
  • 1. College of Earth Science, Chengdu University of Technology, Chengdu 610059, China
  • 2. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
Funds:

National Science and Technology Major Project 2016ZX05007004-002

SINOPEC Southwest Oil and Gas Branch Project 34450000-19-ZC0607-0017

Chengdu University of Technology Sedimentary Geology Research and Innovation Team Project KYTD201703

  • Received Date: 2022-02-21
  • Accepted Date: 2022-11-01
  • Rev Recd Date: 2022-08-11
  • Available Online: 2022-11-01
  • Publish Date: 2024-10-10

Abstract: Objective The Ediacaran-Cambrian, as a key transition period in geological history, saw a dramatic transformation of the Earth system in terms of tectonic evolution, climate fluctuations, marine environment and biological community evolution. It is of great significance to study in depth the sediment sources, paleo-water depth, paleosalinity and redox conditions during this period. Methods Samples from the Maidiping Formation and the Qiongzhusi Formation of the well JS1 in the southwestern part of the Sichuan Basin were selected and analyzed in detail for total organic carbon （TOC）, major elements, trace elements and rare earth elements （REE）. [Results and Conclusions] The material sources of the Maidiping and Qiongzhusi Formations were found to be relatively consistent. The parent rock primarily consists of sedimentary rock and granite, and the material source area was created in the environment found at the continental margin. The Maidiping Formation contains more light rare earth elements （LREE） than heavy rare earth elements （HREE） （（La/Yb）N=6.05）, slightly to the right, and the average δEu is slightly <1, indicating normal seawater deposition in a shallow water body that was not significantly affected by hydrothermal action. The Qiongzhusi Formation has obvious LREE differentiation （（La/Yb）N=7.69）, LREE enrichment, HREE depletion and an obvious right dip, with weak negative δCe and δEu anomalies, possibly related to intermittent hydrothermal activity. In addition, comparative analyses for different wells have shown that the water body during the sedimentary period of the Maidiping Formation was in an oxidizing environment, and the sedimentary system in the region ranges from tidal-flat to shallow-shelf facies. The Qiongzhusi Formation as a whole belongs to an anaerobic environment, with a reducing environment its base and the degree of anoxia decreasing upwards. The regional develop-ment was a shallow- to deepwater shelf sedimentary system.

CHEN WeiZhen, TIAN JingChun, LIN XiaoBing, LIANG QingShao, YANG YanRu, WANG Xing. Geochemical Characteristics and Paleoenvironmental Significance of Lower Cambrian Maidiping and Qiongzhusi Formations in Southwestern Sichuan Basin: A case study of well JS1[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1784-1798. doi: 10.14027/j.issn.1000-0550.2022.101
Citation: CHEN WeiZhen, TIAN JingChun, LIN XiaoBing, LIANG QingShao, YANG YanRu, WANG Xing. Geochemical Characteristics and Paleoenvironmental Significance of Lower Cambrian Maidiping and Qiongzhusi Formations in Southwestern Sichuan Basin: A case study of well JS1[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1784-1798. doi: 10.14027/j.issn.1000-0550.2022.101
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0.   引言

  • 埃迪卡拉纪—寒武纪作为地质历史上重大环境变化和生物演化的关键时期之一,冰期事件、全球气候、海洋环境、生物演化与构造演化等发生了剧烈变化。这一时期中国南方扬子板块早寒武世广泛沉积一套富有机质黑色岩系,为解译该时期的海洋环境和生命演化提供了关键信息。四川盆地连续沉积埃迪卡拉纪—寒武纪过渡时期海相沉积岩,为探索该时期古海洋沉积环境演化提供了一个可视化窗口。目前的研究工作主要集中在下寒武统烃源岩发育规模、有机质富集机理、储层特征、油气勘探与预测等研究,并取得了较好进展[1⁃5]。然而,对于氧化还原指标与沉积环境演化的认识存在着较大分歧,古环境与地球化学分析结果尚不统一[1⁃3,6⁃7]。众多学者认为下寒武统海洋环境存在浅水氧化和深水缺氧（甚至富氧）的氧化分层,且深层海水可能已经开始大规模氧化,发生在寒武纪早期著名的生物大爆发事件进一步暗示了深层海水的氧化[8⁃9]。在此背景下四川盆地西南缘分别发育了以潮坪相沉积环境为主的麦地坪组和以陆棚相沉积体系为主的筇竹寺组。然而,部分学者认为四川盆地西南缘麦地坪组—筇竹寺组沉积期始终处于浅水—深水陆棚相沉积环境之中[3⁃4,10]。

    本文以四川盆地西南缘地区JS1井下寒武统麦地坪组—筇竹寺组为研究对象,重点开展了稀土元素地球化学特征分析,结合主、微量元素示踪,对研究区物源属性、沉积—构造背景及区域氧化还原特征进行研究,为进一步明确川西南地区麦地坪组—筇竹寺组沉积特征和古环境演化提供依据。

1.   地质背景

  • 晚震旦世—早寒武世上扬子地区整体位于冈瓦纳大陆北缘的被动大陆边缘盆地[11],在Rodinia（罗迪尼亚）超大陆裂解和Gondwana（冈瓦纳）大陆聚合的共同影响下,经历了多期次构造运动,其中以挤压性质为主的桐湾运动和以裂陷性质为主的兴凯地裂运动,对四川盆地的影响最为广泛,使得在构造反转过程中,台地内进一步发生裂陷,形成“绵阳—长宁”裂陷槽,导致该时期的古地理格局和沉积—成岩环境发生了显著变化[12⁃14],形成了康滇古陆物源区—裂陷槽沉积区（西缓东陡）的沉积格局[5]。因此,下寒武统麦地坪—筇竹寺组沉积期,扬子板块经历由陆内裂谷—被动大陆边缘—稳定地台的演化阶段[15⁃16]。

    早寒武世,四川盆地整体表现为西高东低的古地理格局,自西向东依次发育滨岸相、潮坪相、浅水陆棚相、深水陆棚相与盆地相。川西南地区位于裂陷槽西侧缓坡带（图1）,寒武系出露较为完整,下寒武统可划分为麦地坪组、筇竹寺组、沧浪铺组和龙王庙组[4⁃5,17⁃19]。麦地坪组与下伏灯影组呈不整合接触,与上伏筇竹寺组呈平行不整合接触,筇竹寺组与上覆沧浪铺组呈整合接触[20]（图1）。

    Figure 1.  Early Cambrian sedimentary facies distribution (modified from reference [10]) and comprehensive stratigraphic histogram for well JS1, southwestern Sichuan Basin

    通过对JS1井岩屑与显微薄片观察,麦地坪组以含磷—磷质白云岩、砂屑磷块岩（图2a,b）、硅质磷块岩、含粉砂质页岩及黑色硅质岩（图2c）为主,硅质、碳酸盐与磷灰石为其主要矿物组分,含少量黏土质、磷质、硅质结核。垂向上,碳酸盐类矿物含量逐渐增加,硅质含量逐渐降低,白云岩与磷块岩互层发育,并以富磷质组分为其主要识别特征。筇竹寺组以灰色泥岩、粉砂质泥岩、深灰色页岩、灰质粉砂岩、灰—浅灰色粉砂岩,泥质粉砂岩不等厚互层为主。底部黑色页岩层段（图2d,f）,颜色较深,粒度较细,水平层理发育,石英颗粒多呈棱角—次棱角状,分选较好,随着粉砂质含量的增加,向上逐渐过渡为深灰色、灰黑色粉砂质页岩,有机质含量较高,富含生物化石（图2e）。

    Figure 2.  Photomicrographs of samples from the Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin

2.   材料与方法

  • 在进行实验测试分析之前,首先对采集样品进行了宏观筛选与微观薄片鉴定,以剔除明显受风化作用和成岩作用改造的样品。同时,选取矿物组分均一、岩性变化较小的样品,以确保测试分析数据的可靠性与真实性。在此基础上,我们在JS1井麦地坪组—筇竹寺组有代表性地筛选出16个样品进行分析,7个样品来自筇竹寺组,9个来自于麦地坪组。全岩主、微量元素与稀土元素分析在广州澳实矿物实验室进行测试,所有分析测试,严格按照实验方法与步骤进行。使用CS-Mat 5500型红外光谱仪测定有机碳丰度。主量元素采用P61-XRF26sX射线荧光光谱仪分析,实验流程根据GB/T 14506.28—2010、GB/T 14506.14—2010。微量、稀土元素使用M61-MS81电感耦合等离子体质谱测定,测试流程根据DZ/T 0223—2001、GB/T 14506.30—2010。全岩主微量、稀土元素的分析精度优于5%。

3.   测试结果

    3.1.   主量元素、总有机碳

  • 主量元素和总有机碳分析结果如表1所示,麦地坪组SiO2含量介于37.89%~52.33%,平均值为45.17%,CaO含量介于18.47%~24.26%,平均值为20.69%,MgO含量介于3.07%~5.69%,平均值为4.19%,MnO与TiO2含量较低,均小于0.5%。值得注意的是该组P2O5含量较高,介于7.36%~12.09%,平均值为9.61%,明显高于上伏筇竹寺组P2O5含量（P2O5=0.21%）,与前人认为的麦地坪组最为典型的特征即富集P2O5相一致,这种富集机制得益于生物代谢产物和磷质生物组分在沉积物中的大量保存[4⁃5]。

    样品编号井深/m岩性TOC/%质量分数/%
    SiO2K2ONa2OCaOMgOAl2O3TFe2O3MnOTiO2P2O5烧失量
    Q-33133 313灰色页岩0.1537.153.130.1115.9210.245.192.700.160.240.1224.89
    Q-33403 340灰色页岩0.1637.173.600.3614.498.758.423.640.110.470.1622.62
    Q-35033 503粉砂质页岩0.4657.933.312.755.243.0812.764.600.090.590.349.03
    Q-35733 573粉砂质页岩0.3654.412.852.419.392.8711.604.640.100.610.2110.59
    Q-36263 626粉砂质页岩0.3854.063.172.338.233.0812.395.420.160.620.229.92
    Q-36493 649粉砂质页岩0.5660.352.932.795.392.9212.114.220.090.610.238.12
    Q-36513 651粉砂质页岩0.4156.152.942.436.953.8111.534.160.110.570.2010.85
    Q-36973 697粉砂质页岩0.4045.351.180.3919.825.173.751.670.070.187.3714.90
    M-37023 702黑色页岩0.3637.891.560.9222.214.975.722.370.070.2910.0413.73
    M-37103 710黑色页岩0.4038.051.110.4921.995.693.592.140.070.188.4117.47
    M-37173 717黑色页岩0.4738.621.170.7224.264.204.232.160.070.2111.6212.48
    M-37223 722粉砂质页岩0.3149.481.330.7418.473.494.702.130.080.237.4711.67
    M-37283 728粉砂质页岩0.4252.340.960.5018.613.073.321.500.060.169.519.82
    M-37343 734粉砂质页岩0.5647.850.660.3921.473.392.221.390.070.1112.1010.13
    M-37403 740黑色页岩0.8051.550.530.3219.433.251.921.560.050.1010.7710.27
    M-37443 744黑色页岩0.6145.380.880.4119.924.492.921.590.070.149.2314.70

    Table 1.  Analysis results of major elements and total organic carbon in Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin

    筇竹寺组泥页岩矿物成分以SiO2、Al2O3和CaO为主,SiO2质量分数介于37.15%~60.35%,平均值为50.32%,Al2O3质量分数介于3.75%~12.75%,平均值为9.72%,CaO质量分数介于5.24%~19.82%,平均值为10.68%。MgO、Fe2O3、K2O、Na2O和P2O5平均质量分数逐渐降低。MnO和TiO2含量最低,均小于0.5%,与北美页岩平均值相比,表现为明显的CaO、MgO、Al2O3和Na2O富集,MnO相对亏损。麦地坪组TOC含量介于0.31%~0.80%,平均值为0.48%,筇竹寺组TOC含量介于0.15%~0.56%,平均值为0.35%。

    • 3.2.   微量元素

  • 微量元素测试分析了33种元素,包括：Li、Be、Sc、V、Cr、Co、Ni、Cu、Zn、Ga、Rb、Sr、Ba、Th、U、Pb、Ce、Nb、La、Eu、Sn、Cs、Pr、Nd、Sm、Tb、Gd、Dy、Ho、Er、Tm、Yb、Lu,结果见表2。为更好地明确研究区地球化学组成及元素的富集、亏损与迁移规律,分析其成岩环境和条件,对微量元素富集程度进行了表征,富集系数EF计算公式如下：

    XEF=X/Al样/X/Alucc (1)

    式中：UCC为上地壳平均值[21⁃22]。当XEF>1,表示元素X相对上地壳富集。若XEF<1,表示元素X相对上地壳亏损。测试样品的微量元素富集系数变化曲线如图3所示。

    样品编号微量元素质量分数/×10-6
    LiBeScVCrCoNiCuZnGaRbSrNbSnCsBaPbThULaCePrNdSmEuTbGdDyHoErTmYbLu
    Q-331313.80.74.345.744.06.8212.733.456.95.640.872.84.60.91.31 050.610.93.32.412.523.82.812.02.60.80.42.82.80.51.50.21.50.2
    Q-334029.21.38.895.852.710.125.723.050.210.466.8117.09.11.63.21 222.912.06.33.121.137.04.318.44.01.10.64.04.10.82.10.32.10.3
    Q-350333.11.910.5129.7112.011.933.125.2219.415.790.7136.610.82.23.31 555.633.59.94.032.050.46.024.14.61.30.64.54.00.82.20.32.10.3
    Q-357333.61.811.481.978.412.830.227.584.114.885.2236.711.12.13.52 049.332.99.32.629.947.65.723.64.71.50.74.44.10.82.20.32.30.3
    Q-362633.11.911.8102.8147.813.535.327.674.715.995.9222.212.22.54.42 788.632.79.23.730.650.16.124.54.81.60.74.74.30.82.50.42.50.4
    Q-364927.01.310.585.680.410.327.526.580.414.171.7153.910.92.02.51 628.027.18.22.225.040.24.819.94.31.30.64.34.00.82.20.32.20.3
    Q-365127.71.410.385.288.110.228.023.962.913.672.6147.510.51.92.92 199.322.18.02.224.941.34.920.74.41.60.64.44.00.72.20.32.20.3
    Q-369715.41.83.7129.066.44.417.717.045.35.131.0268.24.51.01.6637.516.23.119.634.425.84.820.84.01.00.85.66.11.44.10.63.40.5
    M-370219.81.75.5161.193.56.020.320.793.27.942.6368.96.41.31.91 131.224.24.620.541.132.56.126.45.01.41.06.97.51.75.10.74.10.6
    M-371015.41.73.7182.0118.684.857.725.2214.25.128.0398.94.91.11.45 943.847.33.115.938.126.05.424.24.72.81.06.67.41.74.90.74.00.6
    M-371716.21.54.2111.296.811.380.322.243.66.231.7406.46.21.51.61 200.231.03.317.450.928.87.032.26.31.71.49.210.42.57.11.05.50.8
    M-372216.71.45.0103.971.26.122.317.952.96.536.2291.84.51.01.81 162.726.63.68.942.128.65.926.65.21.41.17.27.91.85.10.74.10.6
    M-372832.51.43.498.979.84.219.917.752.24.9526.0324.83.40.81.4548.322.12.611.142.723.56.127.45.51.31.28.29.42.36.61.05.40.8
    M-373412.91.52.391.2197.317.131.720.2145.53.516.1401.34.30.81.2756.830.21.911.746.923.76.228.35.41.41.38.49.62.46.91.05.30.8
    M-374012.91.22.060.8178.85.826.217.4124.03.012.1365.43.50.71.11 160.127.21.99.043.424.06.026.95.21.41.27.78.92.26.50.95.00.7
    M-374417.71.32.975.1105.816.627.122.684.74.219.4375.72.70.81.41 503.239.92.48.643.526.06.529.35.91.71.38.610.42.47.11.16.00.9

    Table 2.  Analysis results of trace elements in Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin

    Figure 3.  Comparison between the trace element enrichment coefficients in the Maidiping Formation⁃Qiongzhusi Formation and upper crust, southwestern Sichuan Basin

    麦地坪组相对于筇竹寺组部分元素呈明显富集,如：U、Ba、Tm、Er、Ho、Dy、Lu和Yb元素,富集系数在2~4左右,Sm、Eu、Tb、La、Zn、Cr等元素,弱富集。而Sc、Rb、Th、Sn和Cs元素,亏损明显,特别是Sn和Rb元素,富集系数分别为0.19和0.24。筇竹寺组Ba和U元素呈明显富集,Eu、Pb、Dy、Li、Zn及Lu等六种元素相对于上地壳弱富集,其他元素则呈不同程度的亏损,特别是Sn和Sr元素最为亏损。Ba、U富集系数差异明显,可作为判别海洋环境是否存在热水沉积的标志[23⁃24],麦地坪组Ba含量介于548.3×10-6~5 943.83×10-6,平均值为1 560.46×10-6,为上地壳丰度的2.84倍。筇竹寺组Ba含量介于1 050.65×10-6~2 788.64×10-6,平均值为1 784.9×10-6,是上地壳丰度的3.25倍,U含量介于2.21×10-6~3.73×10-6,平均值为2.93×10-6,为地壳平均值的1.05倍,麦地坪组U含量介于8.67×10-6~20.54×10-6,平均值为13.68×10-6,是地壳值的4.89倍（BaUCC=550×10-6,UUCC=2.8×10-6）,明显高于前者。

    • 3.3.   稀土元素

  • 稀土元素分析结果及相关参数见表3,麦地坪组稀土总量ΣREE（113.95×10-6~165.54×10-6,平均值为141.17×10-6）,接近大陆上地壳平均稀土元素总量（146.4×10-6）,轻、重稀土元素分异明显（ΣLREE/ΣHREE比值介于2.96~4.03,平均值为3.46）,（La/Yb）N平均值为6.05,球粒陨石标准化（图4）曲线呈现轻微右倾,表明轻稀土相对富集,重稀土相对亏损。δEu（平均值为0.79）与δCe（平均值为0.36）的值均小于1（除单一样品δEu值正异常外）,δCe呈明显负异常,δEu表现为负异常。

    样品编号稀土元素质量分数/×10-6(La/Yb)N(La/Sm)N(Gd/Yb)N(Sm/Nd)NδEuδCe
    LaCePrNdSmEuGdTbDyHoErTmYbLuΣREEΣLREEΣHREELREE/HREE
    Q-331312.5823.812.8512.052.670.882.890.452.880.571.570.251.500.2465.2054.8410.365.295.652.961.550.680.970.92
    Q-334021.1437.014.3218.414.061.184.000.644.120.812.170.342.140.37100.7186.1314.595.916.653.271.510.680.880.88
    Q-350332.0850.466.0224.104.621.324.570.694.040.802.240.352.160.37133.83118.6015.237.7910.004.371.710.590.870.82
    Q-357329.9247.635.7123.634.741.504.460.724.180.822.260.362.390.38128.69113.1415.557.278.453.971.510.620.980.82
    Q-362630.6050.156.1224.514.851.694.700.734.390.882.520.402.570.41134.52117.9116.607.108.033.971.480.611.070.83
    Q-364925.0340.224.8919.904.341.344.350.674.090.832.230.372.230.39110.8895.7215.166.317.583.631.580.670.930.82
    Q-365124.9241.354.9820.714.461.624.470.674.060.782.240.352.250.36113.2398.0415.196.457.473.511.600.661.100.84
    Q-369734.4425.844.8120.854.051.075.600.896.111.474.190.623.470.56113.9591.0522.903.986.705.351.300.600.690.42
    M-370241.1832.596.1026.445.061.436.941.087.511.765.100.754.190.64140.75112.7927.964.036.625.121.340.590.740.44
    M-371038.1626.095.4924.234.782.826.671.077.471.744.950.744.090.66128.97101.5827.393.716.295.021.320.611.530.38
    M-371750.9928.827.0832.296.331.769.261.4710.452.517.111.065.550.87165.54127.2738.273.336.205.071.350.600.700.32
    M-372242.1528.605.9826.665.241.477.281.127.951.885.190.774.160.64139.10110.1129.003.806.835.061.410.610.730.38
    M-372842.7223.586.1627.445.511.398.221.289.412.326.681.015.490.87142.08106.8035.283.035.244.881.210.620.630.31
    M-373446.9623.776.2928.395.451.458.441.309.632.446.921.035.370.84148.28112.3135.973.125.895.421.270.590.650.29
    M-374043.4224.056.0526.915.231.477.701.208.932.256.520.965.040.78140.50107.1233.373.215.815.221.230.600.710.31
    M-374443.5026.046.5729.365.911.778.651.3810.412.497.161.116.060.93151.34113.1538.192.964.844.631.150.620.760.33
    球粒陨石[25]0.300.800.120.600.190.070.260.050.320.070.210.030.210.03
    UCC[22]30647.10264.500.883.800.643.500.802.300.332.200.32

    Table 3.  Analysis results for REE from Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin

    Figure 4.  Standardized REE distribution model of chondrite from Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin

    筇竹寺组稀土元素含量变化较大,ΣREE（65.19×10-6~134.52×10-6,平均值为112.44×10-6）,低于大陆上地壳平均含量（146.4×10-6）,相对亏损。轻、重稀土元素分异明显（ΣLREE/ΣHREE比值介于5.29~7.79,平均值为6.59）,稀土元素配分曲线显著右倾,（La/Yb）N值介于5.65~9.99,平均值为7.69,分异明显,强于麦地坪组。δEu（平均值为0.97）与δCe（平均值为0.85）的值均小于1,具弱的δCe和δEu负异常（图4）。

    在成岩演化过程中,稀土元素配分模式易受成岩作用的影响,造成Ce富集、Eu亏损,削弱了ΣREE对原始沉积环境的指示意义[26⁃27],导致δCe与δEu、ΣREE呈现出良好的相关关系。对研究区样品δEu、ΣREE与δCe的相关性判别显示（图5）,δCe与ΣREE、δEu均表现出较弱的相关性,相关系数分别为0.47、0.12,反映成岩作用对其影响较弱。因此,可使用稀土元素参数来识别沉积环境特征和物质来源。

    Figure 5.  Correlation diagram between δCe with ΣREE and δEu in Maidiping Formation⁃Qiongzhusi Formation, well JS1, southwestern Sichuan Basin

4.   讨论

    4.1.   物源与构造背景

  • 主、微量元素的含量和相关比值,可作为识别沉积物来源、判别物源区构造背景的重要指标[28⁃29]。基于测试分析结果（图6）,对研究区麦地坪组—筇竹寺组的沉积物来源与构造背景进行了分析。

    Figure 6.  Vertical evolution characteristics of geochemical parameters of Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin

    麦地坪组—筇竹寺组Al2O3和TiO2整体含量较低,两者之间具有正相关性（R2=0.989）（图7a）,但明显低于澳大利亚后太古代平均页岩（PAAS）[21]的Al2O3（18.90）、TiO2（1.00）含量,而且与SiO2之间没有明显相关性,相关系数分别为R2=0.32、R2=0.28（图7b,c）,表明沉积期受陆源物质影响不大。

    Figure 7.  Plots of (a) Al2O3 vs. TiO2; (b) Al2O3 vs. SiO2; and (c) TiO2 vs. SiO2 from Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin

    MnO/TiO2小于0.5时为大陆坡或边缘海环境,当MnO/TiO2值介于0.5~3.5时,指示开阔海洋环境[30]。麦地坪组与筇竹寺组MnO/TiO2比值分别为0.43、0.28,均小于0.5,表明两者均形成于大陆坡或边缘海环境（图6）。Fe2O3/TiO2-(Al2O3)/(Al2O3+TFe2O3)图解进一步揭示了,研究区在麦地坪组—筇竹寺组沉积期主要处于大陆边缘环境和远洋沉积环境（图7,8）。La/Yb与ΣREE交会图显示[21],数据点主要集中在沉积岩与花岗岩的交汇区（图9）。因此,麦地坪组—筇竹寺组沉积物物源以沉积岩和花岗岩为主。

    Figure 8.  Discrimination diagram for Fe2O3/TiO2 and (Al2O3)/(Al2O3+TFe2O3) in Maidiping Formation⁃Qiongzhusi Formation, well JS1, southwestern Sichuan Basin (modified from reference [30])

    Figure 9.  La/Yb and ΣREE diagrams of Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin (modified from reference [27])

    • 4.2.   热液作用

  • 正常海相沉积环境中Y/Ho值范围为44~74,岩浆岩中Y/Ho与PAAS值（Y/Ho值约为28）相近[31]。因此,可借助Y/Ho值来区分原始沉积环境类型属于正常海水沉积还是属于受热液作用影响的沉积环境。JS1井麦地坪组—筇竹寺组Y/Ho平均值为44.9,为典型海水沉积环境。由于海底热液活动往往富集Zn、Ni、Cu等元素,而Co元素则主要来自水成的沉积环境[32]。Ni-Co-Zn三元图[33]分析表明（图10）,测试数据均靠近Ni-Zn边界,部分落入热液沉积区。此外,海底热液活动通常导致δEu的正异常[34],研究区除麦地坪组1个数据和筇竹寺组2个数据呈δEu值正异常外,其他数据δEu值均小于1。因此,研究区麦地坪组与筇竹寺组沉积期间可能存在短暂或间歇性热液流体扰动,整体以海相沉积环境为主。

    Figure 10.  Ternary diagram of Ni⁃Co⁃Zn in Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin (modified from reference [33])

    • 4.3.   古水深与古盐度

  • Sr/Ba作为指示水体深度和氧化还原条件变化的重要指标之一[3],在正常海水中,Sr元素的溶解度与迁移能力明显高于Ba元素,Ba2+优先与SO42-离子结合形成沉淀,而留在水体中的Sr2+趋于富集,随着水体变浅和水动力条件的不断增强,Sr以SrSO4的形式形成沉淀,导致沉积物中的Sr浓度、Sr/Ba比值和古水深呈明显负相关关系[35]。麦地坪组Sr/Ba比值平均为0.34,筇竹寺组为0.087,明显低于前者,表明筇竹寺组沉积水体明显深于麦地坪组（图6）。垂向上,麦地坪组Sr/Ba值自下而上呈先增加后降低的趋势,而筇竹寺组整体变化不大。

    古盐度作为古环境研究中的一个重要指标,对恢复古环境演化机制具有重要作用[36]。沉积环境中Na+和K+离子作为海洋水体含量最高、活性较强的无机盐类,其含量可作为指示古盐度波动的最直接标志[37]。由于K+相比Na+更容易被黏土矿物吸附进入伊利石晶格,且随着盐度的增加,在海水中迁移更远,导致对Na+的吸附量就会越高。因此,K/Na比值在一定程度上反映了沉积水体的深度。选取对古盐度变化灵敏的Al/Cr、Fe/Ca与K/Na值进行投点发现（图6,11）,麦地坪组—筇竹寺组沉积期水体深度与古盐度变化基本一致,只有筇竹寺组上部两个数据点在K/Na值上变化明显,可能与筇竹寺组晚期沉积环境变化有关[38]。因此,麦地坪组沉积期,水体盐度相对较高,为相对封闭的浅海滞留环境。而筇竹寺组沉积期海侵作用显著,导致川西南地区原有的局限环境得以改善,水体加深,盐度降低[38],并逐渐形成以浅水—深水陆棚为主的沉积环境。

    Figure 11.  Intersection discriminant diagram of paleo⁃salinity parameters of Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin (modified from reference [38])

    • 4.4.   氧化还原特征

  • 前人研究认为Ce的富集、亏损程度与氧化还原环境有直接关系[39⁃41],广泛用于解释岩石成因和古环境信息,不受时间的影响[42]。值得注意的是,Ce异常的亏损程度与氧化还原环境、海平面变化、pH值等因素密切相关[43]。因此,在使用Ce异常判别水体氧化还原条件时需考虑其他因素对Ce异常的干扰。Ce异常主要与游离态Ce3+和稳定态Ce4+相对浓度有关,在成岩作用过程中（成岩后期）极易与其他稀土元素发生交换,导致Ce异常信号发生改变[44⁃45]。研究区Ce的异常系数δCe与稀土总量ΣREE、铕异常系数(δEu)相关性较差（R2(ΣREE/δCe)=0.39,R2(δEu/δCe)=0.18）,表明受成岩作用影响较弱（图5）。同时,La的异常富集也会造成海水Ce的异常,导致对Ce异常的过度计算（图12a）。基于Pr、Nd在地球化学行为上无明显相关性的前提下,对Pr异常值计算（δPr=2[Pr]N/（[Ce]N+[Nd]N））表明,真实的Ce负异常化学行为必然导致δPr>1,而Ce正异常则会引起δPr<1[44,46⁃48]。研究区麦地坪组样品δPr集中在1.16~1.36,平均值为1.27,Pr/Pr*-Ce/Ce*交会图显示（图12b）,均落入IIIB区域内,表明后期成岩作用对Ce异常影响不大,Ce异常值可真实反映海水的原始沉积特征。此外,由于不活动元素Th主要赋存于陆源碎屑,陆源碎屑物质的混入使得Th不断富集和稀土元素浓度不断升高[49]。然而,研究区ΣREE与Th相关性较差（R2=0.046）,两者之间无显著相关性。因此,Ce异常值能够真实反映原始海洋的沉积记录。

    Figure 12.  (La/Sm)N⁃δCe scatter plot and Pr/Pr*⁃Ce/Ce* diagram of Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin (modified from references [44,46])

    通常认为,中等—强烈的负Ce异常指示富氧、浅水沉积环境[42,44,46]。麦地坪组δCe值介于0.29~0.45,平均值为0.36,小于1,δCe负异常明显,指示研究区麦地坪组形成于氧化环境[27,50⁃51]。筇竹寺组δCe值介于0.81~0.92,平均值为0.85,负异常特征较弱,属于典型的贫氧—缺氧沉积环境。

5.   古环境特征

  • δCe异常指标分析显示,研究区麦地坪组沉积期底层水体整体以氧化环境为主,随着沉积过程的不断进行,δCe值逐渐增大,证实了沉积环境向缺氧还原条件的过渡。然而,对于麦地坪组沉积环境的研究目前存在多种观点：一种观点认为下寒武统麦地坪组—筇竹寺组沉积时期位于“绵阳—长宁”裂陷槽的边缘过渡带,以深水陆棚相沉积为主,受构造运动控制强烈,且存在区域性、沉积水体较深以及能量较低等特征[4,10,52⁃53]。也有观点认为早寒武世川西南地区整体处于伸展拉张的构造背景下,麦地坪组继承了晚震旦世开阔陆表海环境,受周期性海平面升降的影响,在裂陷槽西侧主要发育潮坪相沉积,并记录该时期独特的海洋化学信号[3,5,7,19,54⁃56]。

    针对麦地坪组沉积环境特征的研究,近年来相关学者使用不同的手段和方法开展了大量研究工作,包括沉积学[52,57]、古生物学[7,58]和有机地球化学[3,37,59]等,在不同程度上响应了研究区麦地坪组潮坪—浅水陆棚相的沉积特征。基于前人研究成果,将JS1井相关地球化学指标与周缘地区进行对比分析（图13）,认为川西南地区在早寒武世沉积期受康滇古陆与“绵阳—长宁”裂陷槽等因素的共同影响,麦地坪组整体继承了前寒武世凹隆相间的古地理格局,并在桐湾运动II幕的作用下,遭受了不同程度的构造抬升,水体逐渐变浅,尤其在古构造的较高部位。此外,全球性大规模海侵作用、间歇性热液活动和洋流活动携带的富营养化水体（P、Ba）的涌入,使得研究区逐渐演化成适宜生物生存与繁殖的理想场所,在一定程度上刺激了后生真核生物的辐射,如典型的小壳生物[7]；而富含生物和磷质有机化合物的形成,可能记录了该时期浅层氧化水体的δCe负异常信号。伴随着后生生物的大规模快速辐射,海水氧含量逐渐降低,缺氧程度加深。与此同时,康滇古陆的不断抬升,使得水体盐度逐渐增加,破坏了生物生存的宜居环境,生物数量大量减少与死亡,表现在U、V等氧化还原敏感元素逐渐富集。在横向上,向盆地内部,水体具有氧含量逐渐降低（Ce、Th/U）、水深加深,滞留程度不断加剧的趋势（Mo/U、Mo/TOC）,特别是裂陷槽发育区域（图13）,反映了川西南地区逐渐由潮坪相到深水陆棚相的沉积演化过程。此外,TOC丰度明显低于裂陷槽周缘地区,进一步揭示了潮坪环境,不利于有机质的大规模保存（图13）。

    Figure 13.  Comparison of geochemical indices of Maidiping Formation⁃Qiongzhusi Formation in well JS1, southwestern Sichuan Basin (data from references [19,38,60⁃62])

    筇竹寺组沉积初期受兴凯地裂运动和海平面变化的共同影响,导致盆地西缘整体抬升,与下伏地层之间发育沉积间断,形成广泛不整合面[3]。随着全球海侵作用的加剧,海平面上升,盆地西缘地区与广海沟通更为频繁,逐渐形成统一整体,连片发育,伴随着藻类和生物的大规模死亡,在沉降与分解过程中进一步增加了氧气的消耗,促进了缺氧环境的形成。与此同时,在地裂运动的影响下,深部热液间歇性扰动携带的富营养物质（图10）,可能造成生物的繁盛和初级生产力的迅速提高,有利于优质烃源岩的形成[60]。筇竹寺组晚期在海退作用的影响下,沉积环境逐渐向贫氧环境过渡,有机质含量降低,在纵向上TOC曲线呈先增加后降低的趋势（图13）。横向上,烃源岩厚度受古地理格局控制明显,裂陷槽发育区明显厚于周缘地区,水体盐度下降（Sr/Ba）,滞留程度降低（Mo/U、Mo/TOC）。因此,优质烃源岩的形成与分布受沉积相带控制明显[61⁃62],特别是斜坡相和盆地相（图13）。综上所述,筇竹寺组沉积期地层整体沉积厚度与分布受裂陷槽控制明显,经历由浅水—深水陆棚环境的演化过程,而相对缺氧—贫氧的水体环境更利于有机质的规模富集。

6.   结论

  • （1） 四川盆地西南缘地区JS1井麦地坪—筇竹寺组物源区主要为沉积岩和花岗岩。源区构造环境主要与大陆边缘环境相关。微量元素与稀土元素特征表明麦地坪组形成于浅水沉积环境,受海底热液活动影响较弱。筇竹寺组沉积期水深远大于麦地坪组,古盐度较低,热液活动频繁,有利于富有机质页岩的形成和保存。

    （2） 川西南地区麦地坪组沉积期以氧化环境为主,发育潮坪—浅水陆棚相沉积体系。筇竹寺组在全球海平面升降旋回和兴凯地裂运动的共同影响下,沉积分异显著,发育浅水—深水陆棚相沉积,经历缺氧—贫氧的海洋环境演化。

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