Stratigraphic Position of Cryogenian Songluo Biota in Shennongjia Area
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摘要: 新元古代成冰纪末期“雪球地球”中的宏体化石是认识早期生命演化与极端环境变化的关键.神农架的宋洛生物群是华南成冰纪地层中唯一有详细报道的宏体藻类化石记录.然而，对这套宏体藻类化石赋存层位的认识存在归属南沱组还是大塘坡组两种不同的观点，因此其地层序列和宏体化石赋存层位亟需进一步厘定.笔者通过大量野外地质工作，对神农架宋洛地区宋洛生物群的赋存层位及空间分布开展调查研究.宋洛剖面沿公路出露时发育3个正断层，造成部分地层(如第9~11层)多次重复出现，易引起混淆，产生不同的认识.厘定后的宋洛剖面具有24层，其中第1~19层归属南沱组，而第20~24层为陡山沱组.基于南沱组中发育多个黑色页岩层、黑色页岩夹层上下岩性特征相似、不同黑色页岩夹层内化石面貌不同、神农架地区大塘坡组只分布在神农架西侧、以及黑色页岩具有极低的锰含量等5个原因，认为宋洛生物群的赋存层位为南沱组，而非大塘坡组.Abstract: Macrofossils in Late Neoproterozoic "Snowball Earth" are significant to the coevolution of early life and environment. Songluo biota of Nantuo Formation in Shennongjia area is the unique macroalgae record during the Cryogenian "Snowball Earth". However, because of lacking precise dating data, the stratigraphic position of fossil-bearing shale is controversial. One viewpoint correlates to Nantuo Formation, and the other correlates to Datangpo Formation. Stratigraphic correlation of Songluo biota is in need of restudy. The stratigraphic position and spatial distribution of Songluo biota were investigated under numerous field work. Three normal faults recorded in the Songluo Section along the roadside, leading to recurring problems for some layers(e.g. layers 9-11). Totally, 24 layers have been divided in the Songluo Section, of which the Nantuo Formation contains layers 1-19 and the Doushantuo Formation covers layers 20-24. Based on multiple black shales documented in Nantuo Formation, black shales sandwiched in similar calcareous diamictite, the fossil assemblages changed in different shales, the Datangpo Formation only recorded in western Shennongjia area, and extremely low concentration of Mn in black shales, the Songluo biota and hence the fossiliferous black shale at Songluo Section is part of the Nantuo Formation rather than the Datangpo Formation.
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Key words:
- Cryogenian /
- Nantuo Formation /
- Songluo biota /
- macroalgae /
- Shennongjia area /
- biostratigraphy /
- environmental geology
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图 1 研究区地质背景图
a. 新元古代晚期中国构造刚要图，中国地图底图审图号为GS(2019)1673号，研究区位于扬子板块北缘方框所示；b. 神农架地区地质简图和宋洛剖面位置，方框位置开展了详细的野外地质填图，即图c所展示地质图；c. 宋洛地区1∶5万地质图和实测剖面位置，方框位置为图 2所展示的实测剖面位置和范围
Fig. 1. Geological setting of study area
图 2 宋洛剖面沿X003县道走向平面图，断层及野外照片
a. 宋洛剖面沿公路走向平面图、地层分层和断层；b. 断层F1野外照片；c. 断层F2野外照片，黄色圆圈中野簿的长度为19 cm
Fig. 2. Stratigraphic division and faults in the Songluo Section
图 3 宋洛剖面新元古代地层综合柱状图
A1-A2，B1-B2，C1-C2和D1-D2分别是沿003县道的分段图切剖面，具体对应位置见图 2；红色虚线为地层对比线；右侧宋洛剖面为综合柱状图
Fig. 3. Column of the Songluo Section
图 4 宋洛剖面白云质胶结杂砾岩野外照片
a. 南沱组与神农架群角度不整合接触，黄色虚线之下为神农架群，之上为南沱组白云质胶结杂砾岩；b. 第3层砂岩砾石上的冰川擦痕；c. 第7层砂岩砾石上的冰川擦痕；d. 第3层砾岩砾石；e. 第9层白云岩砾石；f. 第3层棱角状白云岩砾石；红色箭头指示砾石，图片中笔杆的直径为0.8 cm，地质锤的长度为33 cm
Fig. 4. Field photos of calcareous diamictite in Songluo Section
图 5 宋洛剖面南沱组黑色页岩野外照片
a. 宋洛剖面第6层黑色页岩夹在白云质胶结杂砾岩之间；b. 宋洛剖面第6层富含有机质的黑色页岩；c. 宋洛剖面第6层中的宏体碳质压膜化石；d. 宋洛剖面第6层顶界面呈波状起伏，黄色虚线之下为黑色页岩，之上为白云质胶结杂砾岩；e. 黄色虚线之上为宋洛剖面第8层黑色粉砂质页岩-页岩，之下为白云质胶结杂砾岩；f. 凤头坪地区白云质胶结杂砾岩内夹黑色页岩层，两条黄色虚线之间为黑色页岩；图片中地质锤的长度为33 cm
Fig. 5. Field photos of black shale around Songluo Section
图 6 宋洛剖面含砾白云质泥岩野外照片
a. 第4层含砾白云质泥岩，发育沉积纹层；b. C1-C2分段剖面中第9层与第10层分界，黄色虚线之下为第9层灰黑色块状白云质胶结杂砾岩，黄色虚线之上为第10层灰色块状含砾白云质泥岩，第10层下部含砾石；c. 第10层下部发育沉积纹层；d. 第10层下部落石构造；e. C1-C2分段剖面中第10层顶部发育约40 cm的白云岩层；f. D1-D2分段剖面中第10层顶部出露两层厚约20 cm的白云岩层；图片中硬币的直径为2 cm，地质锤的长度为33 cm
Fig. 6. Field photos of calcareous siltstone with lonestones in Songluo Section
图 7 宋洛剖面第11层和陡山沱组地层野外照片
a. 第11层灰绿色粉砂质页岩-页岩；b. 第11层灰绿色粉砂质页岩-页岩，发育水平纹层；c. 第11层底部发育砾石；d. 第20层陡山沱组盖帽白云岩及其上下地层，黄色虚线之间为盖帽白云岩；e. 陡山沱组第2段下部第21层灰绿色粉砂质页岩-页岩夹白云岩；f. 陡山沱组第2段下部第22层灰色砂岩夹粉砂质页岩；红色箭头指向砾石，图片中笔杆的直径为0.8 cm，地质锤的长度为33 cm
Fig. 7. Field photos of layer 11 in Nantuo Formation and Doushantuo Formation
表 1 宋洛剖面南沱组黑色页岩夹层锰含量
Table 1. Concentration of Mn in black shales of the Songluo section
样品号 层号 位置(m) Mn(%) 样品号 层号 位置(m) Mn(%) SL+0 m 第6层 0 0.007 SL+2.7 m 第6层 2.7 0.005 SL+0.3 m 第6层 0.3 0.006 SL+3.0 m 第6层 3.0 0.005 SL+0.6 m 第6层 0.6 0.006 SL-ht+0.1 m 第8层 0.1 0.012 SL+0.9 m 第6层 0.9 0.005 SL-ht+0.4 m 第8层 0.4 0.006 SL+1.2 m 第6层 1.2 0.005 SL-ht+0.6 m 第8层 0.6 0.005 SL+1.5 m 第6层 1.5 0.006 SL-ht+0.8 m 第8层 0.8 0.007 SL+1.8 m 第6层 1.8 0.006 SL-ht+1.0 m 第8层 1.0 0.008 SL+2.1 m 第6层 2.1 0.005 SL-ht+1.2 m 第8层 1.2 0.006 SL+2.4 m 第6层 2.4 0.005 SL-ht+1.7 m 第8层 1.7 0.006 注：Mn含量为本研究实测数据；位置为距对应层底部的距离. 
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