Early-Middle Silurian Sedimentary Successions and Their Response to Tectonism and Eustatic Fluctuations: A Case Study from the Outcrops in Keping Area, Tarim Basin
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摘要: 志留纪是晚加里东运动对塔里木盆地古地理和古构造格局产生重要影响的构造变格阶段, 以全盆地规模的不整合面为界, 前志留纪以海相碳酸盐岩沉积为主的古环境转变为志留纪以海相碎屑岩沉积为主的古环境.开展志留纪沉积充填序列及岩矿特征研究, 有助于进一步认识塔里木盆地该变格阶段构造运动特征及其沉积充填响应.为此, 精选柯坪露头区志留系出露连续、完整的大湾沟及四石场剖面, 进行了露头精细沉积学观察采样及室内分析测试工作, 描述和重建了早、中志留世沉积环境、沉积序列及其所反映的海平面变化, 在此基础上划分了层序地层单元.下、中志留统可识别出陆棚、三角洲和潮汐影响的海岸等沉积体系类型, 大型三角洲的存在则是柯坪露头区的特色.通过砂岩碎屑组分构成研究, 并与Dickinson的砂岩组构大地构造背景模式图解对比, 揭示该区早、中志留世砂岩沉积物主要来自碰撞造山带和前陆隆起区以及大陆块的物源区, 该结果反映了志留纪早期, 受晚加里东运动影响, 塔里木周缘发生汇聚碰撞, 并由此造成塔里木地块南部和北部构造抬升并形成剥蚀区的背景.综合分析揭示早、中志留世沉积序列对构造和海平面变化响应特征.受晚加里东运动影响, 早期(柯坪塔格组沉积期)构造活动较强, 海平面波动明显, 在陆棚基础上发育有大型三角洲沉积.其砂岩岩屑含量高、成分成熟度低, 总体反映了构造活动期快速抬升导致大量物源注入的特点; 后期(柯坪塔格组上部—依木干他乌组沉积期)构造活动逐渐减弱并趋于稳定, 海平面低幅度波动, 主要以潮坪、潮道、小型三角洲和泻湖沉积为主, 砂岩成分成熟度高, 细粒沉积物比例加大.总体上, 早、中志留世沉积序列反映了构造活动由强逐渐减弱并对凹陷区逐渐填平补齐的演化特征.Abstract: The Silurian, influenced by the Late Caledonian movement, is a significant change stage of palaeoenvironments and palaeotectonics in Tarim basin. The transformation from Pre-Silurian marine carbonate dominant deposits into Silurian marine clastic dominant deposits is bounded by the basin-scale unconformity surface at the bottom of Silurian. It is important to recognize the character of the tectonic movement and its depositional response by studying the sedimentary successions and mineral and lithological feature of sandstone in this period. Therefore, the outcrops in Keping area, where the Silurian depositional sequence is continuous and of perfect exposure, were selected for this study. Based on the detailed outcrop observation, sampling and laboratory tests, the sedimentary environments and successions of the Early-Middle Silurian, and its responses to sea-level changes are reconstructed, and then sequence stratigraphy units are divided. Continental shelf, deltas and tidal influenced shoreline etc. are recognized from the Lower-Middle Silurian in Keping area. Large scale deltas deposits have been founded in the Lower Silurian. The study results of sandstone framework mineralogy composition, compared with Dickinson's framework modes, reveal that the Early-Middle Silurian sandstone were mainly derived from the provenance of collision orogenes, forebulge, and continental block. It directly reflects that the orogene around the Tarim plate's impact was due to the Late Caledonian compression during the Early Silurian. As a result, the north and the south of the Tarim plate were uplifted and a large denudation area was formed. Sedimentary successions analysis reveals the tectonics and eustatic fluctuation in the period of the Early-Middle Silurian. During the early stage (Kepingtage stage) of the Early-Middle Silurian, tectonic activity was intensive and eustatic fluctuation was obvious under the influence of the Late Caledonian movement. As a result, large scale delta systems with lower maturity and higher lithic content sandstone on the background of continental shelf environments developed, which reflects that as a result of rapid uplifting, there was an abundance of source input from the north of Tarim block. During the later stage of the Early-Middle Silurian, tectonic activities decreased and got stabilized, and low-amplitude eustatic fluctuation was dominant. Tidal action was obvious, and tidal flat, tidal channel, small deltas, and lagoon were developed, which sandstone was charactered by higher maturity and lower lithic content. Sedimentary successions of the Early-Middle Silurian indicate that tectonic activities decreased, and the depression area was filled with sediments.
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Key words:
- Tarim basin /
- Silurian sedimentary successions /
- sandstone fabric /
- tectonics
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图 1 塔里木盆地早志留世柯坪塔格组沉积期构造古地理背景及研究区露头剖面位置(构造沉积环境图参考何登发(2004，内部资料)，经允许修改发表)
Fig. 1. Tectonic palaeogeographic setting during the Early Silurian Kepingtage stage and the location of the study outcrops, Tarim basin
图 2 阿克苏四石场剖面下志留统地层宏观沉积序列照片
Fig. 2. Outcrop macrographs and interpretation of sedimentary successions of the Lower Silurian on the Sishichang outcrop in Keping area
图 3 阿克苏四石场剖面下、中志留统地层沉积序列综合柱状图
沉积环境及岩性横向有变化，仅代表测量剖面岩性及沉积环境；Qm.单晶石英；F.单晶长石颗粒，Lt.岩屑总量
Fig. 3. Composite columnar section for sedimentary successions of Lower-Middle Silurian on Sishichang outcrop in Keping area
图 4 柯坪地区大湾沟剖面下、中志留统地层沉积序列综合柱状图
沉积环境及岩性横向有变化，仅代表测量剖面岩性及沉积环境；Qm.单晶石英；F.单晶长石颗粒，Lt.岩屑总量
Fig. 4. Composite columnar section for sedimentary successions of the Lower-Middle Silurian on Dawangou outcrop in Keping area
图 5 柯坪地区志留系典型界面野外照片
a.柯坪塔格组砂岩与下伏印干组深灰色泥岩突变接触；b.柯坪塔格组与下伏印干组分界面处的黄色粘土层和砂砾岩；c.柯坪塔格组上砂岩段大型水下分流河道砂岩与下伏中泥岩段深灰色泥岩突变接触；d.塔塔埃尔塔格组紫红色泥岩与下伏柯坪塔格组灰绿色泥岩突变接触；e.依木干他乌组紫红色泥岩与下伏塔塔埃尔塔格组砂岩接触关系；f.康克林组砂砾岩与下伏依木干他乌组紫红色泥岩角度不整合接触；a、b、c.大湾沟剖面照片；d、e、f.四石场剖面照片
Fig. 5. Typical outcrop photographs showing interfacies in the Silurian in Keping area
图 6 柯坪露头区四石场剖面下志留统砂岩岩石薄片镜下典型照片
a.S1k-1，岩屑砂岩，颗粒分选中等，次棱角-次圆状，孔隙泥质和沥青充填，见石英次生加大，正交偏光；b.S1k-15，岩屑砂岩，颗粒分选中等，次棱角-次圆状，泥质和铁质胶结，石英次生加大边发育，正交偏光；c.S1k-6岩屑砂岩，颗粒分选中等，次棱角状，见石英次生加大和铁质胶结，单偏光；d.S1tt-5岩屑砂岩，颗粒分选中等，次棱角-次圆状，粒间主要为方解石胶结，见石英次生加大边，正交偏光；e.S1k-8含岩屑砂岩，颗粒分选好，次棱角状，具石英次生加大，见铁质胶结，孔隙内见沥青充填，正交偏光；f.S1tt-6钙质砂岩，颗粒分选中等-好，磨圆次棱角-次圆状.粒间主要为方解石胶结，可见铁质胶结，单偏光；g.S1k-10岩屑砂岩，颗粒分选中等，次棱角-次圆状，粒间为为泥质和沥青充填，见石英次生加大，单偏光；h.S1tt-10含岩屑砂岩，颗粒分选中等，次圆状，粒间主要为沥青及泥质充填，见方解石胶结，石英次生加大发育，正交偏光
Fig. 6. Typical thin-section photomicrographs showing rock textures of the Lower Silurian sandstones on Sishichang outcrop in Keping area
图 7 柯坪地区下、中志留统砂岩大地构造背景分析三角图(据Dickinson and Suczek, 1979；Dickinson, 1985)
B.隆起基底；C.稳定克拉通；P.深成岩；V.火山岩
Fig. 7. Ternary diagrams showing the tectonic settings of the Lower-Middle Silurian sandstones in Keping area
表 1 柯坪地区下、中志留统砂岩碎屑组分含量
Table 1. Detrital mineral contents of sandstones in the Lower-Middle Silurian of Keping area
样品编号 层位 剖面 岩石定名 砂岩碎屑组分含量(%) Qt Qm Qp F K P Lv Lm Ls L Lt DW1 依木干他乌组 大湾沟 细-粉砂岩 64 64 0 5.0 2.0 3.0 0 0.0 19.0 19.0 19.0 DW2 依木干他乌组 大湾沟 细-粉砂岩 60 60 0 1.0 0.0 1.0 0 0.0 20.0 20.0 20.0 DW3 塔塔埃尔塔格组 大湾沟 细砂岩 80 79 1 5.0 2.0 3.0 10 0.0 0.0 10.0 11.0 DW5 塔塔埃尔塔格组 大湾沟 细砂岩 82 80 2 3.0 1.0 2.0 8 2.0 1.0 11.0 13.0 DW6 塔塔埃尔塔格组 大湾沟 细砂岩 76 74 2 4.0 2.0 2.0 8 2.0 0.0 10.0 12.0 DW4 塔塔埃尔塔格组 大湾沟 细-中砂岩 82 78 4 2.0 0.0 2.0 6 0.0 0.0 6.0 10.0 DW9 塔塔埃尔塔格组 大湾沟 细-中砂岩 70 67 3 2.0 0.0 2.0 8 0.0 0.0 8.0 11.0 DW8 塔塔埃尔塔格组 大湾沟 细砂岩 82 80 2 2.0 0.0 2.0 0 1.0 9.0 10.0 12.0 DW7 塔塔埃尔塔格组 大湾沟 粗-中砂岩 79 77 2 4.0 2.0 2.0 8 0.0 0.0 8.0 10.0 DW10 塔塔埃尔塔格组 大湾沟 细砂岩 80 77 3 2.0 1.0 1.0 5 1.0 0.0 6.0 9.0 DW19 柯坪塔格组 大湾沟 细砂岩 62 60 2 2.0 0.0 2.0 0 3.0 27.0 30.0 32.0 DW13 柯坪塔格组 大湾沟 细砂岩 72 72 0 1.0 0.0 1.0 10 1.0 0.0 11.0 11.0 DW14 柯坪塔格组 大湾沟 细砂岩 66 64 2 3.0 1.0 2.0 0 0.0 28.0 28.0 30.0 DW16 柯坪塔格组 大湾沟 细砂岩 69 66 3 3.0 1.0 2.0 0 3.0 20.0 23.0 26.0 DW18 柯坪塔格组 大湾沟 细砂岩 67 65 2 2.0 1.0 1.0 0 3.0 21.0 24.0 26.0 DW15 柯坪塔格组 大湾沟 细砂岩 70 70 0 2.0 0.0 2.0 0 0.0 23.0 23.0 23.0 S1tt-12 塔塔埃尔塔格组 四石场 细-中砂岩 81 78 3 1.0 0.5 0.5 0 1.0 11.0 12.0 15.0 S1tt-11 塔塔埃尔塔格组 四石场 细-中砂岩 84 80 4 0.5 0.0 0.5 0 0.5 6.0 6.5 10.5 S1tt-10 塔塔埃尔塔格组 四石场 细-中砂岩 84 80 4 0.5 0.0 0.5 2 1.0 4.5 7.5 11.5 S1tt-9 塔塔埃尔塔格组 四石场 细-中砂岩 79 75 4 0.5 0.0 0.5 0 0.0 5.5 5.5 9.5 S1tt-8 塔塔埃尔塔格组 四石场 不等粒砂岩 66 56 10 0.0 0.0 0.0 5 0.0 10.0 15.0 25.0 S1tt-7 塔塔埃尔塔格组 四石场 细砂岩 83 83 0 2.0 0.0 2.0 0 0.0 7.0 7.0 7.0 S1tt-6 塔塔埃尔塔格组 四石场 不等粒砂岩 78 74 4 2.0 0.0 2.0 0 0.0 5.0 5.0 9.0 S1tt-5 塔塔埃尔塔格组 四石场 细-中砂岩 78 75 3 2.0 0.5 1.5 0 0.0 4.0 4.0 7.0 S1tt-4 塔塔埃尔塔格组 四石场 细-中砂岩 87 80 7 1.0 0.5 0.5 0 0.0 2.0 2.0 9.0 S1tt-3 塔塔埃尔塔格组 四石场 细-中砂岩 92 85 7 1.0 0.5 0.5 0 1.0 2.0 3.0 10.0 S1tt-2 塔塔埃尔塔格组 四石场 细-中砂岩 88 78 10 0.0 0.0 0.0 0 0.0 5.0 5.0 15.0 S1tt-1 塔塔埃尔塔格组 四石场 细-中砂岩 84 80 4 1.0 0.0 1.0 0 2.0 7.0 9.0 13.0 S1k-15 柯坪塔格组 四石场 细-中砂岩 78 75 3 1.0 0.0 1.0 0 2.0 13.0 15.0 18.0 S1k-14 柯坪塔格组 四石场 细-中砂岩 79 75 4 1.0 0.0 1.0 0 3.0 12.0 15.0 19.0 S1k-13 柯坪塔格组 四石场 细-中砂岩 72 70 2 2.0 0.0 2.0 0 3.0 17.0 20.0 22.0 S1k-12 柯坪塔格组 四石场 细-中砂岩 75 74 1 1.0 0.0 1.0 0 0.0 18.0 18.0 19.0 S1k-11 柯坪塔格组 四石场 细-中砂岩 72 70 2 1.0 0.0 1.0 1 2.0 19.0 22.0 24.0 S1k-10 柯坪塔格组 四石场 细-中砂岩 67 65 2 1.0 0.5 0.5 6 4.0 17.0 27.0 29.0 S1k-9 柯坪塔格组 四石场 细-中砂岩 78 75 3 1.0 0.5 0.5 0 2.0 15.0 17.0 20.0 S1k-8 柯坪塔格组 四石场 细-中砂岩 69 65 4 1.0 0.5 0.5 0 11.0 15.0 26.0 30.0 S1k-7 柯坪塔格组 四石场 细-中砂岩 73 70 3 1.0 0.5 0.5 2 5.0 17.0 24.0 27.0 S1k-6 柯坪塔格组 四石场 细砂岩 58 55 3 1.0 0.0 1.0 0 0.0 36.0 36.0 39.0 S1k-5 柯坪塔格组 四石场 细砂岩 78 75 3 2.0 1.0 1.0 0 3.0 15.0 18.0 21.0 S1k-4 柯坪塔格组 四石场 细砂岩 65 65 0 1.5 0.5 1.0 0 2.0 26.5 28.5 28.5 S1k-3 柯坪塔格组 四石场 细-中砂岩 72 65 7 2.0 1.0 1.0 0 0.0 23.0 23.0 30.0 S1k-2 柯坪塔格组 四石场 细-中砂岩 83 80 3 1.0 0.0 1.0 0 2.0 9.0 11.0 14.0 S1k-1 柯坪塔格组 四石场 细砂岩 55 55 0 2.0 0.0 2.0 0 0.0 37.0 37.0 37.0 注：薄片由中国地质大学(武汉)黄思骥老师鉴定.Qt.稳定石英颗粒，其中包括Qm(单晶石英)和Qp(多晶石英，主要为燧石)；F.单晶长石颗粒，包括P(斜长石)和K(钾长石)；L.不稳定复晶岩屑，包括Lv(火山岩屑)、Ls(沉积岩屑)和Lm(变质岩屑)，Lt.岩屑总含量，包括Qp和L. 
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表 2 柯坪地区下志留统砂岩碎屑组分含量统计
Table 2. Statistical list of detrital mineral contents of sandstones in the Lower Silurian of Keping area
层位 剖面名称 单晶石英含量(Qm)(%) 长石含量(F)(%) 岩屑总含量(Lt)(%) 范围 平均值 综合平均值 范围 平均值 综合平均值 范围 平均值 综合平均值 柯坪塔格组 大湾沟 60~72 66.2 68.1 1~3 2.2 1.5 9~32 24.7 25.0 四石场 55~80 68.9 1~2 1.3 14~37 25.2 塔塔埃尔塔格组 大湾沟 67~80 76.5 76.8 2~5 3.0 1.8 9~13 11.0 11.5 四石场 74~85 77.0 0~2 1.0 7~25 11.8
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