Subglacial Sedimentary Characteristics of the Bottom of Nantuo Formation in Three Gorges Area and Its Implications
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摘要: 峡东地区是我国原震旦系标准剖面所在地，该地区南沱组直接覆盖莲沱组之上，中间缺失下冰期和间冰期地层，究其原因尚不清楚.通过聚焦南沱组与莲沱组接触层位，利用现代冰川沉积物研究方法开展系统沉积学研究，结果表明：九龙湾周缘南沱组和莲沱组之间存在一层紫红色混合杂砾岩层，其内部砾石定向性、形态、磨圆、岩性以及显微构造与之上南沱组典型灰绿色杂砾岩区别明显，青林口剖面南沱组底部发育微观尺度上的变形沉积构造.表明南沱组底部与莲沱组接触层位为冰川底碛成因，而南沱组主体为冰海沉积成因.证明峡东地区南沱组与莲沱组之间地层缺失是由盛冰期的冰川剥蚀所致，南沱组可能仅代表了盛冰期之后冰消期的沉积记录.Abstract: Nantuo Formation directly overlies the Liantuo Formation in the Three Gorges area where the standard section for defining Sinian System is located. However, there is the absence of deposits of Sturtian glaciation and interglaciation below the Nantuo Formation. The interpretation for the depositional absence remains unclear. Our works focused on the boundary strata between the Nantuo Formation and the Liantuo Formation using sedimentological method in studying the contemporaryglacial deposits. The results show that there is a layer of purplish red diamictites between the Nantuo Formation and the Liantuo Formation in study area. The orientation, shape, roundness, lithology of the clasts and microscopic structure of diamictites in the boundary strata are obviously different from the typical Nantuo diamictites. In addition, subglacial deformational structure on microscale can be recognized at Qinlinkou section. We conclude that the inconformity between the Nantuo Formation and the Liantuo Formation is caused by subglacial erosion, and the Nantuo Formation may just record the retreat deposition of ice sheet from its glacial maximum.
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Key words:
- Nantuo Formation /
- Nanhua System /
- glacial deposition /
- glacial erosion /
- Three Gorges area /
- sedimentology
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图 1 研究区及剖面位置
a，b. 研究区所在大地构造背景；c. 黄陵南缘剖面所在位置
Fig. 1. Geological setting of study area and locations of sections
图 2 砾石形状统计三角图及磨圆分类、判断依据和统计直方图
Fig. 2. The diagram of clast-shape and roundness data and descriptive criteria for clast roundness categories
图 3 九龙湾剖面及周缘辅助剖面南沱组与莲沱组界线层野外特征
Fig. 3. Photos of the contact layers between Nantuo Formation and Liantuo Formation from the Jiulongwan sections
图 4 研究区莲沱组-南沱组界线层位地层柱状对比图
Fig. 4. Stratigraphic logs of the contact layers between Liantuo Formation and Nantuo Formation
图 5 青林口剖面南沱组底部野外特征
Fig. 5. Macroscopic characteristics of the lower of Nantuo Formation in Qinlinkou section
图 7 九龙湾南沱组底部砾石形态、磨圆和岩性统计图
VA. 极棱角状；A. 棱角状；SA. 次棱角状；SR. 次圆状；R. 圆状；WR. 极圆状；RA. 棱角状和极棱角状砾石占比；C40=c/a比值≤0.4的样本占比；Str. 发育擦痕砾石占比；n. 砾石统计数量
Fig. 7. Clast-shape, roundness and lithology data from the contact layer and the lower of Nantuo Formation in Jiulongwan section
图 8 九龙湾剖面紫红色杂砾岩显微构造
a，b. 紫红色杂砾岩底部角砾显微构造，蓝色方框位置表示碎屑颗粒相互挤压碰撞；c，d. 紫红色杂砾岩镜下特征，红色虚线代表颗粒定向堆积，黄色圆圈代表小颗粒围绕大颗粒旋转排列，白色实线代表基质受微剪切作用定向；e，f. 扫描电镜下黏土矿物层状定向排列
Fig. 8. Thin-section photographs showing microscopic features of the purplish diamictite in Jiulongwan section
图 9 青林口剖面莲沱组-南沱组接触层位显微构造特征
红色虚线代表颗粒定向堆积；黄色圆圈代表小颗粒围绕大颗粒旋转排列；白色实线代表基质受微剪切作用定向；蓝色方框代表碎屑颗粒接触挤压
Fig. 9. Thin-section photographs showing microscopic features of the contact layers
图 10 研究区南沱组冰期沉积模式
Fig. 10. Summary model of glacial depositional process of Nantuo Formation in the study area
表 2 不同冰川环境沉积物特征及识别标志总结
Table 2. Characteristics of glacial deposits in different glacial environmental setting
环境 冰下环境 冰河 冰湖 冰海近缘 冰海远缘 沉积物特征 砾石普遍具有擦痕、磨光面，具有特殊形态（子弹状、熨斗状），沉积厚度薄 砾石具有一定磨圆，搬运距离远近差别不同. 沉积厚度薄 具有季节性纹泥和漂砾. 沉积厚度较厚 块状和层状杂砾岩夹细碎屑岩层. 沉积厚度厚 细碎屑岩具层理，伴有漂砾. 沉积厚度较厚 沉积构造 砾石具有一定定向性 发育平行层理、交错层理和砾石层 韵律层理和落石构造 块状和层状，具粒序层理 发育层理、落石构造 物质来源 以基底和近源物质为主 以近源物质为主 多样 多样 多样 变形程度 脆性变形和塑性变形，常发育剪切面和叶理 几乎无 落石卸载塑性变形 重力流产生滑动等塑性变形 落石卸载塑性变形 注：据Benn and Evans（2000）; Arnaud and Etienne（2011）. 
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