Sedimentary environment evolution of marine-continental transitional facies in Xujiahe Formation, northeastern Sichuan Basin: Evidence from element geochemistry

  • Shangshang BO , 1, 2 ,
  • Jixian TIAN 3 ,
  • Yetong WANG 1, 2 ,
  • Shuang FU 1 ,
  • Hengchi SHEN 4 ,
  • Guoqiang SUN , 1
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  • 1. Northwest Institute of Eco⁃Environment and Resources,Chinese Academy of Sciences,Lanzhou 730000,China
  • 2. University of Chinese Academy of Sciences,Beijing 100049,China
  • 3. Research Institute of Petroleum Exploration and Development,Langfang 065007,China
  • 4. Northeastern Sichuan Gas Mine of PetroChina Southwest Oil & Gasfield Company,Dazhou 635000,China

Received date: 2023-08-12

  Revised date: 2023-09-18

  Online published: 2023-10-10

Supported by

The Prospective and Fundamental Major Technology Projects of CNPC(2021DJ0605)

Abstract

The marine and continental transitional sediments of Xujiahe Formation effectively record the information of the Late Triassic paleoclimate-environment changes in the Sichuan Basin. The analysis of sedimentary environment is of great significance for exploring the mechanism of organic matter enrichment and hydrocarbon accumulation. Based on field observation and description of the geological profiles of the Upper Triassic Xujiahe Formation in different locations in the northeast Sichuan Basin, petrology and element geochemical analysis of the samples in each section, combined with the regional tectonic setting, the depositional environment of the clastic rocks in the Xujiahe Formation in the northeastern Sichuan Basin is discussed. The results show that the clastic components of Xujiahe Formation in northeastern Sichuan Basin are mainly cuttings and quartz, and the types of sandstone are mainly medium-grained lithic sandstone and medium-grained arkose lithic sandstone. The parameters of CIA, CIW, PIA, and the correlation discrimination maps of ICV-CIA and WIP-CIA indicate that the Xujiahe Formation has suffered a moderate degree of chemical weathering, and the chemical weathering of the fourth (T3 x 4) and sixth (T3 x 6) members of Xujiahe Formation is the strongest and has undergone a slight sedimentary recycle. The ratios of Mo/Al, Mo/Mo*, V/Cr and the cross plot of V/Al-U/Al and V/ Al-Mo /Al indicate that the sedimentary period of Xujiahe Formation was an oxidizing environment, and the oxidation degree of the fourth and sixth members of Xujiahe Formation was higher. Affected by the combination of terrestrial source input, lake intrusion/sea intrusion events and climate change, the climate of the Xujiahe Formation sedimentary period has a trend of changing from warm and humid to hot and humid, the water body is salty-light alternate, the salinity changes greatly, the horizontal fluctuation is more frequent, the water depth of the fourth and sixth members of Xujiahe Formation is the shallower, the paleoproductivity level of the lake basin is moderately low, showing a trend of first rising, then decreasing, and then rising, and the terrestrial source input of water body is the main controlling factor of paleoproductivity. There is a significant linkage between the environmental evolution, stratigraphic rotation and two contractions (T3 x 4 and T3 x 6 water retreat) and one expansion (T3 x 5 water intake) experienced by the lake basin during the sedimentary period of the Xujiahe Formation.

Cite this article

Shangshang BO , Jixian TIAN , Yetong WANG , Shuang FU , Hengchi SHEN , Guoqiang SUN . Sedimentary environment evolution of marine-continental transitional facies in Xujiahe Formation, northeastern Sichuan Basin: Evidence from element geochemistry[J]. Natural Gas Geoscience, 2024 , 35(4) : 645 -660 . DOI: 10.11764/j.issn.1672-1926.2023.09.020

0 引言

中—晚三叠世,古特提斯洋闭合,扬子板块与华北板块拼接,秦岭造山带发生由东向西的穿时性碰撞隆升1,四川盆地进入海陆过渡相演化阶段,盆地西缘龙门山前陆构造带活动强烈,北缘米仓山—大巴山造山带开始活动并发育推覆构造,向盆内供源,形成了上三叠统须家河组海陆过渡相沉积2-4。遭受过海侵作用的须家河组5-6,有效地记录了四川盆地逐步由被动大陆边缘向前陆盆地转换过程中的古气候—环境变迁的信息37-8。已有众多学者579-13从不同角度对其进行了研究,认为须家河组主要接受西部龙门山、东北部大巴山—米仓山、东南部雪峰山以及盆地南部通过三角洲沉积体系提供的物源5,须家河组各段沉积时期的古气候对形成该组地层泥砂间互的“三明治”式沉积结构具有主控作用10。须家河组总体上处于热带—亚热带的临海、滨湖三角洲或河湖沼泽相沉积环境13,诺利末期以滨海相亚热带温湿型气候为主,在晚三叠世诺利—瑞替期之交气候波动频繁79
近期,四川盆地东北部(川东北)地区须家河组油气显示频繁,多口井已获得高产工业气流,但是勘探程度低,地质特征和成藏机理尚不明确14。针对川东北地区须家河组沉积环境的研究,前人101315-17多通过孢粉等微观组合对比和古植物群落重建,挖掘其中的信息,仅少数涉及元素地球化学特征研究3,这一现状极大地限制了对川东北地区须家河组沉积成岩机制的探索,制约了油气勘探的进程。因此,本文选取川东北地区不同剖面、不同层位的上三叠统须家河组样品为研究对象,从碎屑组分、主微量元素特征入手,利用各种判别图解、比值关系及相关性等解读保存在碎屑岩中的沉积环境信息,揭示川东北地区须家河组的风化和沉积再旋回程度、原生古环境垂向变化规律,进而为认识须家河组的沉积演化模式、古环境恢复以及有机质富集条件提供地球化学依据。

1 区域地质概况

四川盆地是中国重要的含油气盆地,也是晚三叠世以来中国最大的陆相盆地之一,位于上扬子板块西北部,秦岭造山带南侧18-19,形成于印支期,后经燕山运动改造20,在喜马拉雅期综合褶皱作用影响下形成了现今的构造面貌21。现今的四川盆地被周缘造山带围绕22图1(a)]。盆地地层发育较为齐全,除古近系和新近系缺失以外,其余地层均有发育23。须家河组几乎遍布全盆,自西北向东南逐渐减薄[图1(b)]。
图1 四川盆地及邻区构造背景(a)与四川盆地须家河组残余厚度等值线图(b)及东北部构造简图(c)[(a)图据文献[30-31]修改,(b)图据文献[32]修改,(c)图据文献[33]修改,上三叠统古水流数据据文献[34-36]]

Fig.1 The structural background map(a) of the Sichuan Basin and its adjacent areas, the isocontour map(b) of the residual thickness of Xujiahe Formation in the Sichuan Basin, and the structural sketch (c) in the northeast of the Sichuan Basin ((a) is modified from Refs.[30-31], (b) is modified from Ref.[32], (c) is modified from Ref.[33], Upper Triassic palaeocurrent data is from Refs.[34-36])

研究区位于四川盆地东北部,处于川北坳陷带和川东高陡构造带的交会区,受到米仓山—大巴山前缘冲断带与川东弧形褶皱带构造叠加作用影响1924图1(c)]。川东北地区须家河组是在印支中晚期龙门山冲断带远程效应、南秦岭上地壳载荷作用25以及雪峰山陆内挤压造山作用26-27的共同控制下产生缓慢沉降,覆于中三叠统碳酸盐岩之上的海陆过渡相地层28。区内须家河组发育广泛的辫状河三角洲沉积体系29,岩性以深灰色泥岩、页岩夹浅灰色砂岩为主14,自下而上发育须一段—须六段(须一段发育局限),与上覆下侏罗统白田坝组和下伏中三叠统雷口坡组呈整合接触。

2 样品与分析

2.1 样品采集

在川东北地区达州—梁平—奉节—万源一带7个须家河组野外剖面进行样品采集,在须二段—须六段(T3 x 2—T3 x 6)5个层段采集新鲜的砂泥岩样品(详细信息见表1),选取川东北五宝场地区五宝浅15井岩心样品进行碎屑组分分析。
表1 采集的野外露头样品信息

Table 1 Information on field outcrop samples collected

采集地 样品编号 层位 采样点坐标 岩性
宣汉县七里乡 Qlx-3 T3 x 3 N31°11.795′,E107°44.546′ 深灰色泥岩
Qlx-5 T3 x 5 N31°11.849′,E107°44.258′ 泥质粉砂岩
Qlx-6 T3 x 6 N31°11.837′,E107°44.175′ 灰色泥岩
复平镇黑天池 Htc-5 T3 x 5 N30°48.578′,E107°52.244′ 泥质粉砂岩
天城镇铁峰山 Tfs-2 T3 x 2 N30°55.383′,E108°20.588′ 灰色砂岩
朱衣镇黄井水库 Hjsk-2 T3 x 2 N31°03.368′,E109°15.476′ 灰色泥岩
Hjsk-5 T3 x 5 N31°03.908′,E109°15.552′ 黑色泥岩
大巴山地质公园 Dbs-3 T3 x 3 N31°39.256′,E108°15.488′ 粉砂岩
万源市固军水库 Gjsk-2 T3 x 2 N31°45.535′,E108°06.681′ 灰色泥岩
长石镇黄家沟 Hjg-4 T3 x 4 N32°04.8894′,E107°54.746′ 灰色泥岩

2.2 分析测试

显微镜下岩石薄片描述与鉴定是依据行业标准《岩石薄片鉴定》(SY/T5368—2000),利用ZEISS imager.A2偏光显微镜进行的。在进行主量、微量元素测试时,首先用锉刀处理样品,去除表面灰尘和风化部分,然后用去离子水冲洗,在105 ℃烘箱中烘干,用碎样机碎至200目,在显微镜下观察样品的组成和结构以确保用于测试的样品没有经历蚀变、矿化;另外,将砂岩和粉砂岩样品中的大颗粒剔除,只测试其中的泥级基质。用于主量元素分析的样品放在Bruker S2PUMA波长色散X射线荧光光谱仪中进行测试。微量元素分析时,将样品用HF+HNO3密封溶解,并用电感耦合等离子体质谱仪ICP-MS进行分析测试。

3 结果

3.1 岩石学特征

根据野外典型剖面自下而上对须二段—须六段进行了描述。须二段(T3 x 2):厚度较大,主要发育砾岩及砂岩夹砾岩层,大巴山地质公园剖面发育球枕构造[图2(a)],反映近物源、水动力强的特征;天城镇铁峰山剖面以灰白色中—细砂岩和泥岩互层为主,未见砾石层[图2(b)];朱衣镇黄井水库的须二段与下伏巴东组呈平行不整合接触,下部以黑色泥页岩为主,部分风化呈土黄色,上部发育砂岩夹薄层砾岩[图2(c)];宣汉县七里乡剖面为块状的灰色砂岩夹泥砾和炭屑[图2(d)],底部出现暗色泥岩夹煤线,反映稳定的水介质条件。须三段(T3 x 3):主要产出暗色砂岩、粉砂岩、粉砂质泥岩,出露情况差,被植物覆盖;宣汉县七里乡剖面须三段为暗色粉砂质泥页岩夹薄层砂岩[图2(e)];大巴山地质公园剖面须三段为泥岩夹薄层砾岩。须四段(T3 x 4):岩性包括浅灰色、灰白色细砂岩、深灰色泥质粉砂岩、灰白色中砂岩,顶部出现灰黑色页岩和煤层。须五段(T3 x 5):灰色砂岩与灰黑色泥页岩互层,粒度较细,厚度较小[图2(f)]。须六段(T3 x 6):大巴山地质公园剖面为中厚层砂岩,夹少量砾岩[图2(g)],复平镇黑天池剖面须六段为灰色粉砂岩,和上覆侏罗系接触面可见侏罗系珍珠冲段介壳灰岩[图2(h)]。
图2 野外露头典型地层剖面照片及岩性地层柱状图

(a)大巴山地质公园剖面须二段球枕构造;(b)天城镇铁峰山剖面须二段灰白色中细砂岩和泥岩互层;(c)朱衣镇黄井水库须二段与下伏巴东组呈平行不整合;(d)宣汉县七里乡剖面须二段块状灰色砂岩夹泥砾和炭屑;(e)宣汉县七里乡剖面须三段暗色粉砂质泥页岩夹薄层砂岩;(f)宣汉县七里乡剖面须五段暗色炭质泥岩夹泥质粉砂岩;(g)大巴山地质公园剖面须六段中厚层砂岩;(h)复平镇黑天池剖面三叠系和侏罗系接触面侏罗系珍珠冲段介壳灰岩;(i)五宝浅15井单井沉积相柱状图

Fig.2 Typical outcrop profile photos and lithostratigraphic column charts

总体来看,川东北须家河组发育辫状河三角洲平原和辫状河三角洲前缘沉积28-2937,由山前向川东北腹部地区,砾岩含量减少、泥质含量增加。
须家河组砂岩样品的碎屑组分中石英含量为25%∼70%,平均含量为61%,以单晶石英为主,含有少量复石英和燧石[图3(d)];长石含量为3%∼15%,平均含量为4%,以正长石为主,其次为斜长石[图3(e)];岩屑含量为20%∼70%,平均含量为35%,以变质岩屑为主[图3(f)],其次为沉积岩屑;胶结物以方解石和硅质为主,少量黏土胶结,黏土矿物主要为绿泥石和伊利石,无高岭石[图3(g)—图3(i)]。从石英—长石—岩屑(Q—F—L)分类图解中可以看出川东北须家河组砂岩主要为岩屑砂岩与长石岩屑砂岩[图3(a)],整体以中粒灰白色岩屑砂岩最发育[图3(b),图3(c)],说明沉积环境偏氧化。碎屑岩成分成熟度低、结构成熟度中等、碎屑颗粒多呈次棱角状—次圆状、分选较好、有颗粒支撑,说明水动力作用较强、近物源。
图3 须家河组的成分分类和显微照片

(a)须家河组Q—F—R三元图解;(b)五宝浅15井,2 663.19~2 663.32 m,T3 x 4,水下分流河道灰白色岩屑石英中砂岩;(c)五宝浅15井,T3 x 5,中粒岩屑砂岩(单偏光);(d)五宝浅15井,2 925.97 m,石英颗粒紧密接触(正交光);(e)万源市石冠寺野外露头,T3 x 2,石英—长石颗粒接触(正交光);(f)五宝浅15井,2 666.85 m,T3 x 4,千枚岩岩屑挤压变形(正交光);(g)大巴山地质公园野外露头,T3 x 2,方解石胶结(正交光);(h)五宝浅15井,2 922.55 m,T3 x 2,绿泥石薄膜和硅质胶结(正交光);(i)五宝浅15井,2 661.65 m,T3 x 4,伊利石胶结(正交光)。

注:Qz:石英;F:长石;Lv:火山岩屑;Ph:千枚岩岩屑;Cal:方解石胶结;Chl:绿泥石膜;Ill:伊利石胶结

Fig.3 Composition classification and micrograph of Xujiahe Formation

3.2 元素地球化学特征

须家河组样品的主量元素中SiO2含量最高,平均为56.41%,其次为Al2O3(平均值为15.97%)、Fe2O3(平均值为8.01%)、CaO和MgO(平均值分别为4.47%和2.21%),K2O和Na2O(平均值分别为3.15%和0.49%);微量元素中含量最高的是Ba(平均452.78×10-6),其次为Zr(平均值为186.63×10-6),Rb和Sr(平均值分别为111.13×10-6和102.92×10-6)、Zn和Cr(平均值分别为93.14×10-6和88.25×10-6);各样品数据见表2
表2 须家河组样品主量、微量元素含量

Table 2 Content of main elements and trace elements in samples from Xujiahe Formation

Qlx-3 Qlx-5 Qlx-6 Htc-5 Tfs-2 Hjsk-2 Hjsk-5 Dbs-3 Gjsk-2 Hjg-4

主量

元素

/%

SiO2 48.07 55.50 60.33 53.02 69.65 55.02 51.06 59.23 55.75 56.50
Al2O3 11.35 17.51 17.75 13.32 14.02 17.11 15.68 17.55 16.58 18.86
Fe2O3 6.49 9.15 6.99 7.92 4.06 10.02 7.56 9.16 10.36 8.40
CaO 9.87 3.24 3.19 6.36 3.16 3.48 5.31 3.25 3.65 3.17
MgO 2.67 2.15 1.96 2.36 1.02 2.34 3.41 1.90 2.48 1.83
K2O 2.90 3.38 2.65 2.99 3.49 3.35 3.79 2.41 3.87 2.77
Na2O 0.48 0.05 0.16 0.28 0.95 1.19 0.80 0.38 0.14 0.49
MnO 0.06 0.09 0.03 0.21 0.01 0.08 0.04 0.10 0.08 0.05
TiO2 0.48 0.94 0.86 0.66 0.34 0.83 0.72 0.82 0.74 0.91
P2O5 0.14 0.08 0.11 0.13 0.05 0.15 0.15 0.13 0.10 0.09
Total 82.52 92.07 94.02 87.23 96.74 93.56 88.52 94.92 93.76 93.07

微量

元素

/10-6

Ba 238.31 562.77 557.68 390.50 316.17 503.35 363.53 522.87 447.38 625.22
Zr 126.58 245.09 217.23 193.52 124.66 184.63 163.62 220.55 146.35 244.07
Rb 84.17 110.26 126.99 97.51 103.09 121.08 110.71 95.65 146.25 115.61
Sr 153.91 40.41 100.01 139.12 88.65 124.32 85.45 110.10 76.77 110.41
Zn 61.50 113.69 140.40 94.79 32.51 98.36 65.02 132.84 82.74 109.51
Cr 67.76 82.80 113.98 76.43 29.17 108.22 86.21 93.98 98.95 124.99
V 57.61 100.72 92.17 76.86 41.82 89.66 82.07 90.73 88.05 98.66
Ni 29.87 33.92 53.64 39.89 33.34 42.75 34.98 46.97 37.58 52.77
Co 21.42 30.59 23.84 27.09 24.60 33.07 25.31 31.65 33.56 28.94
Cu 24.36 26.43 47.40 21.90 5.85 30.49 19.16 36.00 36.63 5.63
Pb 15.37 12.94 22.25 20.00 5.04 19.19 10.93 23.64 20.52 101.90
Ga 13.75 19.23 23.85 14.51 17.42 24.98 20.55 20.01 22.00 27.65
Y 13.94 21.66 25.27 17.88 13.80 21.36 20.36 21.75 21.75 21.74
Nb 9.86 13.11 19.58 17.41 10.52 13.19 19.09 15.87 13.81 18.72
Th 6.35 7.64 18.57 7.77 5.68 10.76 11.67 12.45 9.48 16.85
Ge 1.13 5.36 4.77 8.19 57.78 1.49 3.85 6.99 1.08 6.55
Hf 3.24 4.05 11.43 5.69 2.06 6.79 6.94 11.87 11.48 1.39
Sc 23.97 1.06 0.48 12.32 0.23 1.46 9.29 0.97 2.57 0.23
U 2.59 6.09 4.29 2.40 2.11 4.74 2.27 2.67 2.82 1.75
Ta 0.61 1.37 1.87 0.74 1.44 1.37 0.95 1.27 1.42 1.46
Mo 0.61 1.08 0.88 0.75 0.37 1.02 0.74 0.78 0.91 0.84
Cd 0.22 0.16 0.96 0.18 1.05 0.74 0.08 0.50 0.36 0.20
Bi 0.01 0.02 0.04 0.25 0.24 0.02 0.12 0.09 0.02 0.16
根据UCC平均含量将样品主、微量元素标准化,从标准化配分模式图(图4)中可以发现须家河组Na2O含量相对UCC表现为强烈亏损,其余主量元素含量则与UCC相当;微量元素中Ge和Cd明显富集,Sr、Sc和Bi强烈亏损,其余微量元素含量则与UCC相当。
图4 主量元素(a)和微量元素(b)与大陆上地壳(UCC)标准化配分模式(UCC数据来自文献[47])

Fig.4 Major elements (a) and trace elements (b) are standardized partitioning patterns with continental upper crust (UCC)(UCC data is quoted from Ref.[47])

4 讨论

4.1 化学风化与沉积再旋回

沉积岩的地球化学成分会受到化学风化作用和沉积再旋回作用的影响。目前已经提出了化学蚀变指数(CIA)38-39、成分变异指数(ICV)40、化学风化指数(CIW)41等参数作为定量分析风化作用、沉积分选和再旋回的指标,计算公式见表3
表3 化学风化及沉积再旋回判别参数

Table 3 Chemical weathering and distinguishing parameters of sedimentary recycle

参数 公式

风化

相关性

来源 须家河组 须二段 须三段 须四段 须五段 须六段
CIA Al2O3/(Al2O3+Na2O +K2O+CaO*)×100 文献[38 66.97~83.92 71.63 76.27 80.35 75.86 83.92
ICV (Fe2O3+K2O+Na2O+CaO*+MgO+MnO+TiO2)/Al2O3 文献[40 0.79~1.44 1.13 1.17 0.84 1.13 0.79
CIW Al2O3/(Al2O3+Na2O+CaO*)×100 文献[41 81.43~99.88 88.19 90.53 92.16 91.30 97.14
PIA (Al2O3-K2O)/(Al2O3 +Na2O +CaO*-K2O)×100 文献[42 76.58~98.78 84.78 88.03 90.80 89.28 96.60
CIX Al2O3/(Al2O3+Na2O+K2O)×100 文献[48 72.38~84.97 75.18 79.34 83.19 78.65 84.97
WIP (2Na2O/0.35+MgO/0.9 +2K2O/0.25 +CaO*/0.7)×100 文献[49 29.84~50.93 45.30 33.89 34.29 39.74 29.84
MWPI [(Na2O+K2O+CaO*+MgO)/(Na2O+K2O+CaO*+MgO +SiO2+Al2O3+Fe2O3)]×100 文献[50 6.29~12.58 9.23 8.58 7.16 8.75 6.30
CPA (Al2O3)/(Al2O3+Na2O)×100 文献[51 89.76~99.51 93.60 95.01 95.92 95.30 98.55

注:式中氧化物均为摩尔含量,CaO*为硅酸盐中CaO的摩尔含量,计算方法来自文献[39]:①使用P2O5数据对磷灰石中的CaO进行校正(CaO′=CaO-10/3×P2O5);②CaO*=Min(CaO′,Na2O)

随着化学风化程度的增加、气候变得温暖湿润,自由阳离子的反应减少会产生更高的CIA值38。未经蚀变的岩石的CIA值一般低于50;CIA值介于50~65之间,反映寒冷干燥条件下的弱化学风化;CIA值介于65~85之间,则反映温暖湿润条件下的中等化学风化;CIA值介于85~100之间,指示炎热潮湿条件下的强化学风化,完全被蚀变的岩石的CIA值为1003842。须家河组样品的CIA值为66.97~83.92,平均值为75.93,表明遭受了中等的化学风化影响(表3图5),其中须六段风化作用最强,须四段次之。
图5 A—CN—K(a)和A—CNK—FM(b)三角图、ICV—CIA(c)和WIP—CIA(d)交会图[(a)底图据文献[42]修改,(b)底图据文献[57]修改,(c)底图据文献[58]修改,(d)底图据文献[43]修改]

((a) base map is modified from Ref.[42], (b) base map is modified from Ref.[57],

(c) base map is modified from Ref.[58], (d) base map is modified from Ref.[43])

Fig.5 Triangular plot of A-CN-K (a) and A-CNK-FM (b), Intersection diagram of ICV-CIA (c) and WIP-CIA (d)

在Al2O3—(CaO*+Na2O)—K2O(A—CN—K)三角图中[图5(a)],预测化学风化趋势线平行于A—CN边,近似平行于样点分布线,说明钾交代作用对样品的影响不明显,样点位于PAAS(澳大利亚后太古代平均页岩)和伊利石之间,说明风化作用进入以伊利石为标志的中级阶段。同样,在Al2O3—(CaO*+ Na2O+K2O)—(Fe2O3+MgO)(A—CNK—FM)三角图中[图5(b)],样点全部位于斜长石—FM连线与绿泥石—伊利石连线之间,也表明须家河组受到中等程度的化学风化影响。
成分变异指数(ICV)可以用来判断碎屑岩的成分成熟度、风化与再旋回作用强度40。当ICV<1,成分成熟度高,含有较多黏土矿物成分,可能经历了再旋回作用或是在首次沉积条件下经历了强烈的风化作用;当ICV>1,成分成熟度低,含有较多的未风化矿物成分,则趋向于首次沉积。须家河组样品的ICV值在1.0上下波动(0.79~1.44),平均值为1.07,须二段、须三段、须五段都大于1,为首次沉积;须四段和须六段都略小于1,可能经历了弱的沉积再旋回作用[表3图5(c)]。另外,CIW、PIA、CIX等参数均指示须四段和须六段化学风化和沉积再旋回程度较须二段、须三段、须五段强(表3)。
GARZANTI等43认为更高的CIA值和更低的WIP值指示更强的风化,石英稀释作用对WIP的影响很大,但对CIA没有影响,故提出用化学指标区分首次沉积和再旋回沉积,CIA—WIP图很容易显示沉积物中石英的富集,对于首次沉积的样品,CIA与WIP之间呈线性关系,图5(d)显示须家河组处于中等化学风化环境,基本属于首次沉积。综上所述,须家河组经历了中等的化学风化,沉积再旋回程度低,须二段、须三段、须五段为首次沉积,须四段和须六段经历了弱的沉积再旋回作用,风化作用相对更强。

4.2 氧化—还原环境

沉积岩中微量元素的分布、分配和迁移与沉积岩形成时的氧化—还原环境密切相关44。Mo、V和U是变价元素,其价态与活动性随氧化—还原环境的不同而改变45-46,可作为判断氧化—还原环境的灵敏指示剂,可采用Mo/Al、V/Al、U/Al值来判别环境的氧化—还原状态,当Mo/Al<0.4×10-4、V/Al<23×10-4、U/Al<1×10-4时指示氧化环境45。须家河组的Mo/Al、V/Al、U/Al值均小于上述指标,表明在氧化环境下须家河组亏损Mo、V和U元素[图6(a),图6(b)]。由于Mo与Ce—Pr在岩浆过程具有相似的分配系数,而Mo比Ce—Pr受氧化—还原环境的影响更大,因此,Mo/Mo*[Mo/(Ce×Pr)1/2]可以消除母岩成分的影响而常被用来判别氧化—还原环境52。须家河组的Mo/Mo*值均小于1,反映了Mo受到氧化作用的影响。
图6 须家河组V/Al-U/Al(a)、V/Al-Mo/Al(b)、V/Cr—U/Th(c)以及Ni/Co—U/Th(d)交会图[(a)和(b)底图据文献[45]修改]

Fig.6 Intersection diagram of V/Al-U/Al (a), V/Al-Mo /Al (b), V/Cr-U/Th (c) and Ni/Co-U/Th (d) of the Xujiahe Formation((a) and (b) base map is modified from Ref.[45])

在V/Cr—U/Th和Ni/Co—U/Th交会图中53图6(c),图6(d)],须家河组样点均处于氧化环境附近。此外,反映水体氧化—还原环境的δU、δEu指标总体上均反映须家河组沉积期为氧化环境,与前人观点一致54,且须四段和须六段的氧化程度更高。

4.3 古气候

石英抗风化能力较强,SiO2在温暖湿润的强风化作用中能够得到较多的保留,而长石抗风化能力较弱,Al2O3、K2O、Na2O等容易因风化作用而流失,在强风化条件下含量较低55。须家河组石英平均含量为61%,长石平均含量仅为4%,说明在温暖湿润的环境中遭受了较强的风化作用影响。样品在SiO2—(Al2O3+K2O+Na2O)图解中总体显示为潮湿环境[图7(a)]。
图7 SiO2—(Al2O3+K2O+Na2O)(a)和CIA—C值(b)交会图[(a)底图据文献[55]修改,(b)底图据文献[64]修改]

Fig.7 Intersection diagram of SiO2 -(Al2O3+K2O+Na2O)(a)and CIA-C value(b)((a) base map is modified from Ref.[55],(b) base map is modified from Ref.[64])

干旱环境中,蒸发强、水体碱度大,易促使盐类矿物沉淀,Ca、Mg、Sr、Ba、K、Na相对富集,而潮湿环境中Fe、Mn、Cr、Ni、V、Co相对富集,因此,C值[∑(Fe+Mn+Cr+Ni+V+Co)/∑(Ca+Mg+Sr+Ba+K+Na)]可作为反映古气候条件的指标56,须家河组C值介于0.40~1.12之间,反映了半湿润—湿润的气候条件[图7(b)]。另外,须家河组Na2O含量相对UCC表现为强烈亏损[图4(a)],较高的CIA值和较低的Mg/Ca值也同样反映了温暖湿润的古气候条件,且须四段和须六段有向炎热潮湿转变的趋势(表3),与前人101317结论较为一致。

4.4 沉积水体条件

干旱气候环境中CaO的含量高,Sr置换Ca2+的程度高,会使得岩石中Sr含量增大,导致其不适宜作为盐度判别指标59,而须家河组沉积时期温暖湿润,样品中CaO的含量不高,并不会对Sr含量造成影响,故本文应用Sr含量和Sr/Ba比值法划分了须家河组沉积水体盐度,来反映当时沉积介质古盐度的变化。须家河组低Sr含量以及Sr/Ba值小,反映了淡水环境,不同层位指标有浮动,判断须家河组沉积期间可能出现了咸—淡交替的水体环境,水体盐度的不稳定可能与该时期频繁波动的古气候条件17、湖侵/海侵事件660以及陆源输入有关。
沉积岩中的Ti和Al一般存在于黏土、长石和其他硅酸盐矿物中,通常被认为是碎屑通量的指示物61-62。须家河组TiO2和Al2O3的正相关性较强,表明这2种元素与陆源输入有一定的关系[图8(a)]。Zr亲陆,含量越高,说明离陆源区越近,沉积岩中的Zr受Al支配,因此,Zr/Al值能反映陆源组分的搬运距离及水深的变化,其值越大,离岸越近,水体越浅61。须家河组Ti—Zr/Al具有一定的相关性,同样说明有一定的陆源输入[图8(b)]。
图8 TiO2—Al2O3(a)和Ti—Zr/Al(b)交会图

Fig.8 Intersection diagram of TiO2 -Al2O3(a)and Ti-Zr/Al(b)

大型湖泊水深一般与离岸距离存在正相关关系,因此,反映离岸距离的(Al+Fe)/(Ca+Mg)值可以指示湖泊古水深的变化趋势,比值大反映近岸、浅水沉积环境63。须二段、须三段、须四段、须五段、须六段的(Al+Fe)/(Ca+Mg)值分别为3.27、2.88、4.72、3.54、4.14。须家河组沉积时期湖平面波动较为频繁,须四段、须六段沉积期水深最浅,这与须四段、须六段经历风化再旋回、氧化条件最强、向炎热潮湿转变相呼应(图9)。
图9 须家河组沉积环境判别参数纵向对比图

Fig.9 Longitudinal comparison map of sedimentary environment discrimination parameters of Xujiahe Formation

4.5 古生产力控制因素

沉积岩中Ni、Cu、Zn元素作为微量营养元素,主要通过与有机质络合后沉降到水体底部,进而被保留在沉积物中,因此,Ni+Cu+Zn值可评价水体古生产力65。沉积物中的Ba元素主要为生物成因富集,过剩钡(Ba过剩)可以用来指示沉积时期的生产力,其计算公式为Ba过剩=Ba样品-Al样品(Ba/Al)PAAS 66。2个参数相关性较好(图10),本文综合二者衡量水体古生产力。Ba过剩值(须二段=365.87、须三段=371.38、须四段=613.21、须五段=475.36、须六段=546.37)和Ni+Cu+Zn值(须二段=115.94、须三段=165.77、须四段=167.90、须五段=167.41、须六段=241.43)指示须家河组沉积时期湖盆的古生产力水平中等偏低,且呈现须二段—须四段升高,须四段—须五段降低,须六段再升高的规律。
图10 须家河组古生产力主控因素相关性图解

Fig.10 Correlation diagram of main controlling factors of paleoproductivity in Xujiahe Formation

从Ba过剩与沉积环境各指标的相关性图解(图10),可以看出Ba过剩值与Al2O3值、TiO2值、C值、(Al+Fe)/(Ca+Mg)值的相关性好(R 2分别为0.88、0.80、0.77、0.75),说明水体的陆源输入程度、气候条件和水深是须家河组古生产力的控制因素,其中陆源碎屑供应是主控因素,须家河组发育的大量植物碎屑也证实了这一观点317

5 沉积环境演化模式

晚三叠世初,四川盆地在印支运动的影响下,发生了广泛的构造抬升,海水退出,逐渐过渡到陆相沉积演化阶段31。须家河组沉积时期为一宽缓的湖盆,发生过多次水进和水退。综合上述基于元素地球化学的沉积环境分析,认为须家河组沉积时期风化作用、再旋回程度、氧化—还原条件、古气候、古盐度、陆源输入、古生产力与水平面的升降之间存在较明显的联动性31图9图10)。须家河组总体处于氧化、湿热、咸—淡交替、水平面波动的沉积环境中,沉积时期湖盆经历了2次收缩(T3 x 4和T3 x 6)和1次扩张(T3 x 5),对应着沉积环境演化和地层旋回的周期性变化。
第1阶段(T3 x 4收缩期)[图11(a)]:气候更加湿润,CIA值增大,充沛的地表流水和大气降水持续注入到湖泊中,将陆源生物碎屑等有机质加入湖盆中;在这一背景下,尽管湖水的含氧量增加,V/Cr、U/Th、Mo/Mo*等参数都表现出湖水有向更氧化环境变化的趋势,不利于有机质富集,但湖盆水体的营养物质基数大幅增加,Ba过剩值和Ni+Cu+Zn值指示古生产力水平提高;同时,随着气候变得更加炎热,蒸发量大于补给量,Mg/Ca值略有升高,湖平面快速下降,湖盆水深减小,(Al+Fe)/(Ca+Mg)值对应出现高值;ICV、CIW、WIP等指标一致指示化学风化程度增加,须四段沉积物粒度更粗(图9)。
图11 须家河组沉积环境演化模式

(a)第1阶段(T3 x 4收缩期);(b)第2阶段(T3 x 5扩张期)

Fig.11 Sedimentary environment evolution model of Xujiahe Formation

第2阶段(T3 x 5扩张期)[图11(b)]:淡水注入相对减少,陆源输入(Al、Ti、Zr等)随之减少,但温度由炎热向温暖转变,蒸发量减少,导致水深增加,(Al+Fe)/(Ca+Mg)值出现低值;在湖侵/海侵的背景下,即使化学风化减弱和氧化程度降低,有利于有机质富集,但营养元素减少,故出现了Ni+Cu+Zn值指示古生产力水平基本保持不变,而在相对还原条件下沉积物中的Ba元素发生溶解析出67-68,Ba过剩值指示古生产力水平降低的现象,须五段沉积了较细的泥岩、粉砂岩(图9)。
第3阶段(T3 x 6收缩期):与第1阶段相同,再次经历收缩,湖退明显,三角洲平原范围向湖内扩张,在须五段细粒沉积物上沉积了须六段中厚层砂岩夹少量砾岩。

6 结论

(1)四川盆地东北部须家河组碎屑组分以石英和岩屑为主,岩石类型主要为中粒岩屑砂岩与中粒长石岩屑砂岩;CIA、CIW、PIA等参数和ICV—CIA、WIP—CIA等相关性判别图解指示须家河组经历了中等程度的化学风化和弱的沉积再旋回作用,须四段和须六段化学风化程度和沉积再旋回作用最强;Mo/Al、Mo/Mo*、V/Cr等值和V/Al—U/Al、V/Al—Mo/Al等交会图均反映川东北须家河组沉积期为氧化环境,且须四段和须六段的氧化程度更高。
(2)受陆源输入、海侵事件和气候变化的共同影响,气候具有从温暖潮湿向炎热潮湿转变的趋势,水体呈咸—淡交替,盐度范围变化较大;须家河组沉积时期水平面波动较为频繁,须四段和须六段沉积期水深最浅,湖盆的古生产力水平中等偏低,且呈现出先升高、再降低、再升高的趋势,水体的陆源输入是古生产力的主控因素。
(3)须家河组沉积环境与湖盆演化和地层旋回之间存在明显的联动性。T3 x 4和T3 x 6沉积时期湖盆收缩,氧化、风化、沉积再旋回作用加强,水体变浅,古生产力水平升高,气候相对炎热潮湿,沉积物粒度较粗,T3 x 5沉积时期湖盆扩张,氧化、风化、沉积再旋回作用减弱,水体变深,古生产力水平降低,气候相对温暖潮湿,沉积物粒度较细。
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