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长江三峡东段（简称“峡东”）地区成冰系冰碛岩最早报道于20世纪初（Willis et al.，1907）。随后，Lee and Chao（1924）在峡东地区黄陵背斜创立“南沱砂岩”和“南沱冰碛岩”。20世纪60年代，刘鸿允和沙庆安（1963）将“南沱冰碛岩”修订为南沱组，将“南沱砂岩”更名为莲沱群，后莲沱群修改为莲沱组（马国干等，1980）。曾有学者认为，莲沱组可能包含了下冰期（Sturtian冰期）和间冰期的沉积地层（彭学军等，2004；王自强等，2006；尹崇玉和高林志，2013），但多组凝灰岩、凝灰质砂岩的U-Pb同位素年代学测试结果表明，莲沱组顶部的沉积时限在748±12~724±12 Ma之间，为成冰纪之前的沉积（马国干等，1984；高维和张传恒，2009；Lan Zhongwu et al.，2015；Pi Daohui and Jiang Shaoyong，2016）。20世纪80年代，有学者根据黄陵背斜南侧的鄂西长阳古城剖面，原南沱组包含上、下两套冰碛岩及二者之间含碳酸锰碳质砂页岩的岩石组合特征及年代学数据，将古城剖面南沱组自下而上解体为古城组、大塘坡组和南沱组（马国干等，1980；赵自强等，1985）。三个组依次分别对应Sturtian冰期、间冰期和Marinoan冰期沉积（赵彦彦和郑永飞，2011；周传明，2016；胡军等，2021）。
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目前，学界普遍认为，峡东地区普遍缺失Sturtian冰期古城组冰碛岩和间冰期大塘坡组泥页岩，Marinoan冰期南沱组冰碛岩直接不整合覆盖在拉伸系莲沱组或黄陵花岗岩之上（沙庆安等，1963；唐天福等，1978；赵小明等，2011；官开萍等，2016；胡蓉等，2016；周传明，2016；Hu Jun et al.，2020；岳勇等，2020）。峡东青林口剖面作为南沱组不整合覆盖在莲沱组之上的典型代表，以剖面露头好，接触关系清楚而闻名（安志辉等，2014）。然而，近年有学者对峡东青林口剖面南沱组冰碛岩中所夹的一套灰绿色页岩和下部的冰碛岩划归南沱组提出了质疑，认为青林口南沱组剖面的地层结构、含锰灰岩的岩石学及地球化学特征、CIA化学风化强度指标等均与鄂西长阳古城成冰系剖面相似，二者可进行横向对比，即认为青林口南沱组剖面同样发育完整的成冰纪地层序列，具有Sturtian冰期、Marinoan冰期夹间冰期的完整沉积记录，据此将青林口剖面南沱组自下而上划分为古城组、大塘坡组和南沱组（季泽龙和刘晓峰，2023）。青林口剖面仅发育南沱组或发育完整成冰系两种截然不同的认识，对峡东地区乃至扬子克拉通北部成冰纪地层发育及其对比划分、构造演化等将产生重大影响。本文通过对青林口成冰系剖面页岩夹层开展细致的野外调研和室内薄片观察，结合CIA数据等进行对比分析，以期确定该段页岩的沉积背景，据此厘清峡东地区成冰纪的岩石地层序列。
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1 研究区成冰系及青林口剖面简介
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峡东地区与鄂西长阳古城一带地处扬子克拉通北缘中段，是我国成冰系的典型发育区。峡东黄陵背斜南缘以庙河—青林口—秋千坪—莲沱一带发育的南沱组厚度最大，约70~110 m（沙庆安等，1963；安志辉等，2014）。传统观点认为峡东地区仅发育相当于Marinoan冰期的南沱组，缺失Sturtian冰期和间冰期沉积，南沱组直接覆盖在莲沱组砂岩或黄陵花岗岩之上（安志辉等，2014；胡军等，2021）。鄂西长阳古城成冰系则以发育两次冰期及间冰期碎屑沉积的完整沉积记录为特征，自下而上划分为古城组、大塘坡组和南沱组。
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峡东青林口南沱组剖面位于黄陵背斜西南翼（图1），地处湖北省宜昌市秭归县境内的芝茅公路旁，地理坐标为30°47′57.68″N；110°58′8.28″E。该剖面出露好，南沱组以微角度不整合覆盖在拉伸系莲沱组之上（图2b），整合伏于陡山沱组盖帽的白云岩之下，以发育典型的冰碛杂砾岩为特征，下部夹有泥页岩（图2a~c）。剖面详细介绍如下：
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上覆地层：陡山沱组浅灰色厚层白云岩
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————整合————
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南沱组：
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3.，灰绿色厚层—块状冰碛杂砾岩，不显层理，砾石成分复杂，有花岗岩、砂岩、片岩、灰岩等；砾石形态多样，大小混杂，排列无序，含量30%~40%；砾径多在3~15 cm之间，大者可达50 cm以上；砾石磨圆度以棱角状—次圆状为主，个别呈浑圆状；砂泥质及钙质胶结 55 m
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2.，灰绿色含杂砾粉砂质页岩，风化面多呈紫红色，顶部夹2~5 cm厚的棕褐色纹层状灰岩透镜体 6 m
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1.，灰绿色厚层—块状冰碛杂砾岩，不显层理，砾石成分复杂，有花岗岩、砂岩、片岩、灰岩等；砾石形态多样，大小混杂，排列无序，含量40%~60%；砾径多在1~10 cm之间，小者仅0.2~0.3 cm，大者可达40~50 cm；砾石磨圆度以棱角状—次圆状为主，个别浑圆状；砂泥质及钙质胶结 4 m
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~~~~~~微角度不整合~~~~~~
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下伏地层：莲沱组紫红色砂岩夹紫红色粉砂质页岩
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2 含杂砾粉砂质页岩的岩性特征
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青林口剖面南沱组下部含杂砾粉砂质页岩夹于两套厚层-块状冰碛杂砾岩之间（图1b，图2a、b），其下伏厚约4 m的杂砾岩微角度不整合于莲陀组砂岩之上（图2b）。该套含杂砾粉砂质页岩呈灰绿色，页理发育，局部可见水平纹层，厚2~6 m，风化后呈片状（图2c、d）。砾石在粉砂质页岩中分布杂乱无章，时而稠密，时而稀疏。在含量较多处，1 m2范围内可见十余个厘米级砾石，在含量稀疏处则可能几平方米范围内都看不到1个厘米级砾石。砾石成分复杂，以石英岩、砂岩、碳酸盐岩等砾石为主；砾石形态多样，整体磨圆较差，以棱角状—次棱角状为主，个别浑圆（图2e~i）；砾径大小混杂，小者仅0.2 cm，大者在2.0~3.0 cm之间，野外见到最大的一颗砾石直径约为20 cm；常见砾石导致泥质岩纹层塑性变形弯曲的现象（图2g），显示砾石具有坠石沉积的特点。
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通过对含杂砾粉砂质页岩进行切片观察，发现岩石呈泥质结构，水平纹层发育（图3a）；岩石中粗碎屑颗粒较多，主要为细砾石和粗砂，这些粗碎屑分选差，多在1~3 mm之间；碎屑的磨圆也较差，多呈棱角—次棱角状，个别为次圆状；以碳酸盐岩、石英和砂岩等碎屑为主，含量在5%~20%之间（图3a~f）。一些磨圆差的砾石明显切断泥质岩纹层，显示坠石的特征（图3f）。此外，岩石中还有大量细砂—粉砂级碎屑（图3a~f）。
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总体而言，该套含杂砾粉砂质页岩中砾石成分复杂，砾石大小混杂、磨圆度差，以棱角—次棱角状为主，特别是局部明显呈现砾石底部泥质岩纹层塑性变形或砾石直接切断泥质岩纹层的特点，指示其坠石的沉积特征。另外，整合于其上、下的两套杂砾岩均为典型的冰碛杂砾岩，表明这套含杂砾粉砂质页岩为冰碛沉积。此外，值得注意的是，这种含坠石构造的粉砂质页岩在峡东地区南沱组中较为常见，除青林口剖面外，峡东地区的其他剖面，如廖家沟、芝麻坪和五指山剖面等，南沱组中均发育含少量砾石的泥质岩，且砾石明显有坠石沉积的特点（图4a~e）。
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图1 研究区和邻区地质略图以及青林口剖面南沱组地层柱状图
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Fig.1 Geological sketch map of the study and adjacent areas and stratigraphic column of the Nantuo Formation of the Qinglinkou section
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（a）—峡东及邻区地质简图（据Tang Feng et al.，2006修改）；（b）—青林口剖面南沱组地层柱状图
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(a) —simplified geological map of the East Yangtze Gorges and adjacent areas showing location of Qinglinkou section (modified from Tang Feng et al., 2006) ; (b) —stratigraphic column of Nantuo Formation at Qinglinkou
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3 含杂砾粉砂质页岩的MnO含量及CIA指数讨论
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针对峡东青林口剖面南沱组下部含杂砾粉砂质页岩段和鄂西长阳古城大唐坡组的全岩MnO含量和CIA指数，前人曾开展过系统分析和研究工作（季泽龙和刘晓峰，2023）。诸多学者亦曾对贵州松桃县道坨、湖北古城、湖南、黔东及邻区大塘坡组的CIA值和含锰岩系（菱锰矿）MnO含量进行过报道（冯连君等，2003；周琦，2008；张飞飞等，2013a，2013b；朱祥坤等，2013；何志威等，2014；Wang Ping et al.，2020），本文基于上述研究数据和工作成果进行重新讨论。
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图2 三峡地区青林口剖面南沱组冰碛岩和含冰碛砾石页岩夹层
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Fig.2 Moraine-bearing shale interbed within the glacial diamictites of the Nantuo Formation in the Qinglinkou section, the Yangtze Gorges
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（a）—南沱组冰碛杂砾岩夹含冰碛砾石页岩；（b）—南沱组不整合于莲沱组之上；（c）—灰绿色含冰碛砾石页岩与下伏冰碛杂砾岩接触界面；（d）—纸片状页岩；（e~i）—南沱组页岩中的冰坠石
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(a) —moraine-bearing shale interbed within the glacial diamictites of the Nantuo Formation; (b) —the Nantuo Formation directly and unconformably sits on the Liantuo Formation; (c) —grey green moraine-bearing shale and underlying glacial diamictites of the Nantuo Formation; (d) —laminated shale; (e~i) —ice-rafted dropstones within the shale interbed of the Nantuo Formation
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图3 三峡地区青林口剖面南沱组含冰碛砾石页岩薄片图影
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Fig.3 Thin section photographs of moraine-bearing shale of the Nantuo Formation in the Qinglinkou section, the Yangtze Gorges
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（a）—纹层状含砾粉砂质页岩，粗砂和细砾石总含量约5%；（b~f）—含砾（粗碎屑）粉砂质页岩，粗碎屑磨圆差，以棱角状—次棱角状为主，砾石含量在10%~20%之间，其中（f）可见砾石切穿泥质岩纹层现象，显示坠石特征
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(a) —laminated gravel-bearing silty shale, with approximately 5% coarse sand and fine gravel content; (b~f) —gravel-bearing (coarse-clast) silty shale, predominantly angular to sub-angular coarse clasts, with gravel content ranging from 10% to 20%; among them, (f) exhibits gravel cutting through mudstone bedding, indicating characteristics of ice-rafted dropstones
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3.1 MnO含量对比分析
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青林口南沱组剖面含杂砾粉砂质页岩段除上部纹层状含锰灰岩透镜体样品（SQLK-26S）的全岩MnO含量为2.10%之外，其余样品的MnO含量普遍在0.12%~0.21%之间（平均为0.16%）；鄂西长阳古城剖面大塘坡组菱锰矿和含锰灰岩的MnO含量分别为24.40%和6.33%，其余样品的MnO含量在0.35%~0.63%之间（平均为0.50%）（季泽龙和刘晓峰，2023）。贵州松桃县道坨大塘坡组菱锰矿的MnO含量在18.15%~32.20%之间，页岩的MnO含量在0.29%~11.75%之间（平均为3.17%）（何志威等，2014）。可见，青林口剖面含杂砾粉砂质页岩与大塘坡组之间MnO含量差距显著。此外，青林口剖面纹层状含锰灰岩MnO含量2.10%，与湖北古城、湖南民乐、黔东及邻区的西溪堡、杨立掌和大坪盖地区的大塘坡组菱锰矿MnO含量（分别为29.0%、37.7%、31.6%、31.1%和32.7%）相比（周琦，2008；张飞飞等，2013a，2013b；朱祥坤等，2013）存在很大差距。此外，还有研究表明MnO含量的高低并不宜作为大塘坡组沉积的标志，如一些地区大塘坡组底部沉积的MnO含量可以低于2%（Yu Wenchao et al.，2022）。因此，将青林口剖面页岩中MnO含量作为与大塘坡组对比的依据明显不够充分。
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图4 三峡地区廖家沟剖面、芝麻坪剖面和五指山剖面南沱组含冰碛砾石页岩/泥岩
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Fig.4 Moraine-bearing shale/mudstone of the Nantuo Formation from Liaojiagou, Zhimaping and Wuzhishan sections in the Yangtze Gorges
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（a）—廖家沟剖面南沱组含冰碛砾石页岩整合于陡山沱组盖帽白云岩之下;（b）—廖家沟剖面南沱组页岩中的冰坠石;（c）—芝麻坪剖面南沱组含冰碛砾石页岩整合于陡山沱组盖帽白云岩之下，页岩中的冰坠石;（d）—芝麻坪剖面南沱组页岩中的冰坠石;（e）—五指山剖面南沱组泥岩中的冰坠石
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(a) —moraine-bearing shale of the Nantuo Formation conformably overlain by the cap dolomite of the Doushantuo Formation in the Liaojiagou section; (b) —ice-rafted dropstones are present within the shale of the Nantuo Formation in the Liaojiagou section; (c) —moraine-bearing shale of the Nantuo Formation conformably overlain by the cap dolomite of the Doushantuo Formation in the Zhimaping section; (d) —ice-rafted dropstones are present within the shale of the Nantuo Formation in the Zhimaping section; (e) —ice-rafted dropstones are present within the mudstone of the Nantuo Formation in the Wuzhishan section
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3.2 CIA指标对比分析
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化学蚀变指数（CIA）作为指示气候类型及风化程度的重要指标，在探索古气候、开展化学地层分析对比等方面发挥了重要作用（冯连君等，2003；王自强等，2009；邓乾忠等，2015；李明龙等，2019；Wang Ping et al.，2020）。冯连君等（2003）利用CIA分析湖南石门的新元古代地层，发现CIA值为60~70和70~75分别反映冰期和间冰期化学风化的特征，结合渫水河组CIA值为55~70，判断湖南石门的新元古代地层可能是冰期的沉积产物。峡东青林口成冰系剖面含杂砾粉砂质页岩段的CIA值在60.67~68.36之间（平均为 65.54），而鄂西长阳古城成冰系大塘坡组的CIA值则介于72.91~74.90之间（平均为74.13）（季泽龙和刘晓峰，2023）。贵州松桃、重庆秀山小茶园、笔架山的大塘坡组黑色页岩的CIA值分别为64~70（平均为67.6）、63.29~74.49（平均为69.28）和69.84~71.28（平均为70.58）（Wang Ping et al.，2020；田欢欢，2022）。上述结果表明，青林口剖面含杂砾粉砂质页岩段的CIA值明显低于其他各地大塘坡组页岩的CIA值，指示其可能是冰水沉积的产物，反映气候相对寒冷干燥、化学风化条件较低。其他各地大塘坡组的CIA值则指示其为温湿气候、中等化学风化条件下的产物。因此，CIA值的对比指示青林口剖面含杂砾粉砂质页岩段与其他各地大塘坡组之间的古气候背景及化学风化方式明显不同。
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综上，青林口南沱组剖面含杂砾粉砂质页岩段的MnO含量与鄂西长阳古城大塘坡组差异显著，其纹层状含锰灰岩的MnO含量更是无法与其他地区大塘坡组菱锰矿或含锰灰岩进行对比。同时，青林口南沱组剖面含杂砾粉砂质页岩段的CIA值反映寒冷干燥的古气候背景、低等化学风化条件，与典型的大塘坡组的CIA值以及其反映的古气候背景和化学风化方式差别巨大。故此套含杂砾粉砂质页岩不宜与大塘坡组对比，即不宜将青林口剖面南沱组解体为古城组、大塘坡组和南沱组。
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4 结论
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峡东青林口剖面南沱组灰绿色页岩中明显含砾石，这些砾石显示典型冰坠石的特征，表明此套含杂砾粉砂质页岩为冰碛沉积。同时，已有的MnO含量及CIA指数分析结果显示，此套岩石的MnO含量以及CIA值均与鄂西长阳古城和贵州松桃等地大塘坡组差异显著，特别是CIA值更支持此套泥质岩为寒冷气候下沉积的。综上所述，青林口剖面不发育间冰期沉积——大塘坡组，剖面所有成冰纪沉积均为南沱组。峡东地区成冰系的发育情况和地层结构与青林口剖面基本一致，即仅发育Marinoan冰期南沱组沉积，缺失Sturtian冰期和间冰期沉积。
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参考文献
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An Zhihui, Tong Jinnan, Ye Qin, Tian Li, Song Huyue, Zhao Xiaoming. 2014. Neoproterozoic stratigraphic sequence and sedimentary evolution at Qinglinkou section, East Yangtze Gorges area. Earth Science-Journal of China University of Geosciences, 39(7): 795~806 (in Chinese with English abstract).
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摘要
以往研究认为，三峡东部地区仅发育可与Marinoan冰期相对应的南沱组冰碛岩，南沱组直接不整合覆于拉伸系莲沱组或黄陵花岗岩之上。然而，近年有学者通过对峡东黄陵背斜西南翼青林口南沱组剖面开展研究提出新认识，认为青林口剖面南沱组包含了上、下两套冰碛岩与二者之间的页岩夹纹层状含锰灰岩。该剖面的地层结构、含锰灰岩的岩石学及地球化学特征、化学蚀变指数(CIA)等与鄂西长阳古城成冰系剖面相似，二者之间可进行横向对比，并据此将峡东青林口剖面南沱组自下而上解体为古城组、大塘坡组和南沱组，分别对应Sturtian冰期、间冰期和Marinoan冰期沉积。针对这一新认识，笔者对青林口南沱组剖面开展详细的野外地质调查工作，在被认为可与间冰期沉积大塘坡组相对比的灰绿色页岩中发现了许多砾度不等的冰坠砾石，指示这套灰绿色页岩实为含冰坠石的冰碛页岩，而非间冰期沉积。此外，这套灰绿色冰碛页岩无论是岩石学特征，还是CIA值或MnO值，均与鄂西长阳古城、贵州松桃大塘坡等剖面的大塘坡组差异显著，将二者作为同一时期地层进行横向对比的证据不可信。因此，本文认为不宜将三峡东部南沱组解体为古城组、大塘坡组和南沱组。
Abstract
Previous studies suggested that glacial deposits of the Nantuo Formation, correlated with the Marinoan glaciation, directly unconformably overlie the Tonian Liantuo Formation or Huangling Granite in the East Yangtze Gorges. However, in recent years, some scholars have raised doubts about the exclusive presence of the Nantuo Formation in the Qinglinkou section of the southwestern limb of the Huangling Anticline in the East Yangtze Gorges. They argue that the Qinglinkou section of the Nantuo Formation comprises both upper and lower sets of glacial diamictites, as well as interbedded shale and laminated manganese-bearing limestones. They point out that the stratigraphic architecture, lithological signatures, geochemical characteristics, and Chemical Index of Alteration (CIA) values of this section resemble those of the Cryogenian succession in Gucheng, Changyang, western Hubei Province. In their opinion, a lateral correlation can be made between the two sections, suggesting a disintegration of the Nantuo Formation in the Qinglinkou section from bottom to top into the Gucheng, Datangpo, and Nantuo formations. These formations would correspond to the Sturtian glaciation, interglacial period, and Marinoan glaciation deposits, respectively. Based on the aforementioned observations, the authors recently conducted a detailed field geological survey of the Nantuo Formation in the Qinglinkou section. During this survey, many ice-rafted dropstones of varying sizes were found within the gray-green shale beds. These beds were previously considered comparable to the Datangpo Formation, which is interpreted as interglacial deposits. The presence of dropstones indicates that these gray-green shale beds containing glacial dropstones are actually glacial diamictites (moraine-bearing shale) rather than interglacial deposits. Additionally, the gray-green moraine-bearing shale exhibits significant differences in lithological characteristics, CIA values, and MnO content compared to the Datangpo Formation observed in sections such as Gucheng in western Hubei Province and Datangpo in Songtao, Guizhou Province. These differences suggest that lateral correlation between the two should not be pursued. Therefore, it is not advisable to disintegrate the Nantuo Formation into the Gucheng, Datangpo, and Nantuo formations in the East Yangtze Gorges.
Keywords
Yangtze craton ； Neoproterozoic ； Cryogenian ； Nantuo Formation ； glacial diamictites
