Lower and Upper Boundaries of Fortunian from Yichang, Hubei, China
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摘要: 对湖北宜昌地区滚子坳剖面寒武系纽芬兰统幸运阶顶底界线的位置进行较为细致的研究.通过切片法及酸蚀法对61个样品进行系统处理，获得了大量的小壳化石、疑源类化石及其他分类位置未定化石.系统鉴定和描述了23属44种.根据具刺疑源类化石Asteridium和小壳化石Protohertzina sp.的首现位置以及无机碳同位素BACE曲线最大负值位置可以推断出埃迪卡拉系-寒武系界线应该位于第1层底部往上约10.5 m处；根据Zhijinites longistriatus的首现位置以及无机碳同位素ZHUCE曲线最大值位置可以推断出幸运阶与第二阶的界线位置大致为第4层底部往上约2.78 m.此外，还将峡东地区的小壳化石Ⅱ带和Ⅲ带重新定义，即将Purella antiqua组合（Zone Ⅱ）修订为Purella antiqua-Siphogonuchites triangularis组合；将Aldanella yanjiahensis组合（Zone Ⅲ）修订为Aldanella yanjiahensis-Zhijiniteslongistriatus组合.Abstract: This paper is focused on the base of the Fortunian and Cambrian Stage 2 in the Gunziao section of Yichang, Hubei province. Thousands of SSFs has been found by dealing with 61 samples, 23 genera and 44 species to be identified and described. Additionally, some conclusions are drawn as follows. According to the first appearance datum of Asteridium and Protohertzina sp., and the large negative carbon isotope anomaly (BACE), the base of the Fortunian should be located at 10.5 meters above the Beds 1. According to the first appearance datum of Zhijinites longistriatus and the large positive carbon isotope anomaly (ZHUCE), it can be inferred that the base of Stage 2 is approximately 2.78 meters above the Beds 4. Purella antiqua Assemblage Zone (ZoneⅡ) and Aldanella yanjiahensis Assemblage Zone (ZoneⅢ) in Three Gorge area are redefined based on the new data, which is to be Purella antiqua-Siphogonuchites triangularis Assemblage Zone and Aldanella yanjiahensis-Zhijinites longistriatus Assemblage Zone.
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Key words:
- Cambrian /
- Fortunian Stage 2 /
- small shelly fossils /
- AHC assemblage zone /
- Gunziao section /
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图 3 湖北宜昌滚子坳剖面纽芬兰统岩家河组疑源类化石及其共生化石
a~c. Asteridium tornatum，内视图；d~e. Comasphaeridium annulare，内视图；f. Heliosphaeridium ampliatum，内视图；g~h. Megathrix longus，纵切面；图f引自Chang et al.(2019); 比例尺a~f为10 μm，g~h为50 μm
Fig. 3. The acritarchs and paragenetic fossils from Yanjiahe Formation of Gunziao section in Yichang, Hubei Province
图 4 湖北宜昌滚子坳剖面纽芬兰统岩家河组小壳化石ZoneⅡ
比例尺=100 μm; a~i为ZoneⅡ，j~w为ZoneⅢ；a~b. Siphogonuchitestriangularis，a.凹面视，b.侧视；c~d. Microcornus parvulus，侧视；e. Protohertzina unguliformis，侧视；f.Purella antiqua，顶视；g~i. Conotheca longiconica，侧视；j~k. Zhijinites longistriatus，侧视；m~n. Archaeooides sp.，外视；o~p. Turcutheca lubrica，侧视；q. Zhijinites cordiformis，侧视；r. Zhijinacus guizhouensis，侧视；s. Zhijintes pandurifomis，侧视；l, t~u. Zhijinites tumourifomis，侧视；v~w. Lapworthella hubeiensis，侧视；其中j~l, q~u潘时妹等(2018)
Fig. 4. The small selly fossils from Yanjiahe Formation of the Gunziao section in Yichang, Hubei Province
图 5 湖北宜昌滚子坳剖面与其他地区寒武系底部地层划分与对比
Fig. 5. The subdivision and correlation of the lower Cambrian between Gunziao section in Yichang and other area
图 6 湖北宜昌滚子坳剖面埃迪卡拉系-寒武系界线附近主要化石分布及无机碳同位素曲线
1.无机碳同位素图修改自(Ishikawa et al., 2008); 2.寒武系底界置于BACE负异常什么位置目前国际上无定论，本文有大量化石，根据化石的指示将BACE负值的峰值作为寒武系底界，第二阶底界的定义同理
Fig. 6. Distribution of fossils and inorganic carbon isotope curves near the Ediacaran-Cambrian boundary line in the Gunziao section of Yichang, Hubei Province
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