1. Introduction
The Ediacaran–Cambrian transition records several dramatic changes in the history of life. Along with small chemical and physical changes built up during the Proterozoic Eon, metazoan life began to flourish and went through several pulses of radiations and extinctions, and the interactions between the biological groups and their physicochemical environments fundamentally altered the ecosystem of the Phanerozoic (Bottjer, Hagadorn & Dornbos, Reference Bottjer, Hagadorn and Dornbos2000; Zhu, Strauss & Shields, Reference Zhu, Strauss and Shields2007; Maloof et al. Reference Maloof, Porter, Moore, Dudás, Bowring, Higgins, Fike and Eddy2010a ; Erwin & Tweedt, Reference Erwin and Tweedt2012; Shu et al. Reference Shu, Isozaki, Zhang, Han and Maruyama2014; Erwin, Reference Erwin2015). However, the precise regional and/or global correlation of this interval across different palaeocontinents is still challenging due to many factors, including the lack of globally available diagnostic fossils in the lowermost Cambrian system and the lack of suitable age constraints that are applicable to all palaeocontinents (Zhu et al. Reference Zhu, Li, Zhang, Steiner, Qian and Jiang2001; Zhu, Babcock & Peng, Reference Zhu, Babcock and Peng2006; Zhu, Strauss & Shields, Reference Zhu, Strauss and Shields2007; Babcock et al. Reference Babcock, Peng, Zhu, Xiao and Ahlberg2014). Although ichnofossils have limited biostratigraphic applications, the trace fossils of the Ediacaran–Cambrian transition have significant implications because the patterns, complexity, and degree of bioturbations during this interval are clearly distinguished from Ediacaran ichnofossils, so they can provide a lot of information on the radiation of nonbiomineralizing metazoans before the radiation of small shelly fossils (SSFs) in the early Cambrian (Crimes, Reference Crimes1987; Narbonne, Kaufman & Knoll, Reference Narbonne, Kaufman and Knoll1994; Jensen Reference Jensen1997, Reference Jensen2003; Droser, Gehling & Jensen, Reference Droser, Gehling and Jensen1999; Bottjer, Hagadorn & Dornbos, Reference Bottjer, Hagadorn and Dornbos2000; Droser, Jensen & Gehling, Reference Droser, Jensen and Gehling2002; Carbone & Narbonne, Reference Carbone and Narbonne2014; Mángano & Buatois, Reference Mángano and Buatois2014). At present, the lowest occurrence of ichnofossil Treptichnus pedum (Trichophycus pedum) or the base of the T. pedum Ichnozone has been used to mark the base of the Cambrian since the ratification of the Cambrian GSSP (Global Boundary Stratotype Section and Point) in 1994 (Brasier, Cowie & Taylor, Reference Brasier, Cowie and Taylor1994; Landing, Reference Landing1994; Babcock et al. Reference Babcock, Peng, Geyer and Shergold2005, Reference Babcock, Peng, Zhu, Xiao and Ahlberg2014; Peng, Babcock & Cooper, Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmidtz and Ogg2012; Landing et al. Reference Landing, Geyer, Brasier and Bowring2013). However, due to many problems associated with the facies restriction, preservation and taxonomy, T. pedum at this interval is not readily available in many depositional settings other than Avalonia and Baltica (Zhu, Reference Zhu1997; Steiner et al. Reference Steiner, Li, Qian, Zhu and Erdtmann2007; Babcock et al. Reference Babcock, Peng, Zhu, Xiao and Ahlberg2014), and sometimes (e.g. South China) the lowest occurrence of T. pedum is stratigraphically higher than the lowest occurrence of SSFs (Li, Zhang & Zhu, Reference Li, Zhang and Zhu2001; Zhu et al. Reference Zhu, Li, Zhang, Steiner, Qian and Jiang2001; Steiner et al. Reference Steiner, Li, Qian, Zhu and Erdtmann2003, Reference Steiner, Li, Qian, Zhu and Erdtmann2007; Li et al. Reference Li, Steiner, Zhu, Yang, Wang and Erdtmann2007, Reference Li, Zhao, Gubanov, Zhu and Na2011). Therefore, it is difficult to use T. pedum and the associated ichnofossil assemblage as a sole primary correlation tool for the Ediacaran–Cambrian transition, so the regional correlations of this interval are largely dependent on SSFs or acritarchs, along with a distinctive negative carbon isotope excursion that is found globally, when the T. pedum assemblage is not locally available (Moczydłowska, Reference Moczydłowska1991; Li, Zhang & Zhu, Reference Li, Zhang and Zhu2001; Zhu et al. Reference Zhu, Li, Zhang, Steiner, Qian and Jiang2001; Steiner et al. Reference Steiner, Li, Qian, Zhu and Erdtmann2003, Reference Steiner, Li, Qian, Zhu and Erdtmann2007; Zhu, Babcock & Peng, Reference Zhu, Babcock and Peng2006; Li et al. Reference Li, Steiner, Zhu, Yang, Wang and Erdtmann2007, Reference Li, Zhao, Gubanov, Zhu and Na2011; Zhu, Strauss & Shields, Reference Zhu, Strauss and Shields2007; Li et al. Reference Li, Ling, Jiang, Pan, Chen, Cai and Feng2009, Reference Li, Ling, Shields-Zhou and Thirlwall2013; Maloof et al. Reference Maloof, Ramezani, Bowring, Fike, Porter and Mazouad2010b ; Kouchinsky et al. Reference Kouchinsky, Bengtson, Runnegar, Skovsted, Steiner and Vendrasco2012; Peng, Babcock & Cooper, Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmidtz and Ogg2012; Babcock et al. Reference Babcock, Peng, Zhu, Xiao and Ahlberg2014; Yang et al. Reference Yang, Steiner, Li and Keupp2014, Reference Yang, Steiner, Zhu, Li, Liu and Liu2016).
The utility of organic microfossils as a biostratigraphic correlation tool is usually limited relative to other skeletal metazoan fossils, due to their nonbiomineralizing nature that can only be preserved through narrow and specific taphonomic windows (Moczydłowska, Reference Moczydłowska1991, Reference Moczydłowska1998; Moczydłowska & Zang, Reference Moczydłowska and Zang2006). However, early Cambrian acanthomorphic acritarchs are morphologically distinctive, and some acanthomorphic acritarch assemblages are temporally limited to, and correlated with, other metazoan fossils indicative of the lower Cambrian; thus they are age-diagnostic taxa that can be used as a chronological reference for regional and global stratigraphic correlations (Moczydłowska, Reference Moczydłowska1991, Reference Moczydłowska1998, Reference Moczydłowska, Peng, Babcock and Zhu2001, Reference Moczydłowska2002; Vidal & Moczydłowska-Vidal, Reference Vidal and Moczydłowska-Vidal1997; Moczydłowska & Zang, Reference Moczydłowska and Zang2006). For example, Moczydłowska (Reference Moczydłowska1991) showed that the basal Cambrian acanthomorphic acritarchs are useful in biostratigraphic correlations between the East European Platform (EEP) and other major Cambrian successions by developing four acritarch assemblage zones (Moczydłowska, Reference Moczydłowska1991, Reference Moczydłowska1998, Reference Moczydłowska, Peng, Babcock and Zhu2001). The acritarch assemblage zones described by Moczydłowska (Reference Moczydłowska1991) are not universally recognized; however, the Asteridium tornatum – Comasphaeridium velvetum Zone, the first acritarch assemblage zone, is recognized in South China (Yin et al. Reference Yin1987a ,b; Ding, Li & Chen, Reference Ding, Li and Chen1992; Zang, Reference Zang1992; Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009), Tarim (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009), Lesser Himalaya (Tiwari, Reference Tiwari1999), South Australia (Moczydłowska & Zang, 2006), Spain (Palacios & Vidal, Reference Palacios and Vidal1992) and Avalonia (Palacios et al. Reference Palacios, Jensen, Barr, White and Miller2011), supporting the validity of acritarchs as a biostratigraphic correlation tool for the lowermost Cambrian System. The fossil record of phytoplankton and skeletal metazoans around this time indicates that Cambrian acanthomorphic acritarchs went through a major radiation that slightly preceded the radiation of skeletal metazoans (Moczydłowska, Reference Moczydłowska1991, Reference Moczydłowska1999, Reference Moczydłowska2002; Palacios & Moczydłowska, 1998; Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009). This implies that the temporal relationship between the first appearances of Cambrian acanthomorphic acritarchs and SSFs can provide insights into biogeochemical conditions associated with the co-radiation of phytoplankton and skeletal metazoans. In addition, where the first appearance data (FADs) of acanthomorphic acritarchs and SSFs are almost contemporaneous, especially in the Lower Cambrian successions globally, the presence of either fossil group in a rock profile can be used to indicate the lowermost Cambrian age in various types of sedimentary environments.
In South China, abundant and diverse SSFs from the lowermost Cambrian successions across the Yangtze Platform facilitated the establishment of SSF-based Meishucunian biostratigraphy, which is widely used in regional and global biostratigraphic correlations. Currently, the base of the Meishucunian Stage of South China is marked by the first appearance of SSFs of the Anabarites trisulcatus – Protohertzina anabarica Zone (Qian, Reference Qian1977, Reference Qian1978; Qian & Bengtson, Reference Qian and Bengtson1989; Qian et al. Reference Qian, Zhu, He and Jiang1996; Li, Zhang & Zhu, Reference Li, Zhang and Zhu2001; Qian, Li & Zhu, Reference Qian, Li and Zhu2001; Zhu et al. Reference Zhu, Li, Zhang, Steiner, Qian and Jiang2001, Reference Zhu, Zhang, Steiner, Yang, Li and Erdtmann2003; Steiner et al. Reference Steiner, Li, Qian, Zhu and Erdtmann2003; Li et al. Reference Li, Steiner, Zhu, Yang, Wang and Erdtmann2007, Reference Li, Zhao, Gubanov, Zhu and Na2011; Yang et al. Reference Yang, Steiner, Li and Keupp2014, Reference Yang, Steiner, Zhu, Li, Liu and Liu2016). Furthermore, acanthomorphic acritarchs and microfossils indicative of the basal Cambrian were also reported from many successions containing SSFs in the same stratigraphic profiles, especially from eastern Yunnan and the Yangtze Gorges area (Chen, Reference Chen1984; Ding & Qian Reference Ding and Qian1988; L. Yin, Reference Yin1995, Reference Yin, Gao and Xing1997; Qian, Li & Zhu, Reference Qian, Li and Zhu2001; Yin et al. Reference Yin, Wang, Zhao and Ou2016). The co-radiation of these microfossils and SSFs has much significance in defining the Ediacaran–Cambrian transition in South China, although not much is known about the exact stratigraphic range of the microfossils. These Cambrian acritarchs and associated microfossils are recognized as the Micrhystridium–Paracymatiosphaera–Megathrix assemblage (Yin, Reference Yin1995), which corresponds to the Asteridium tornatum – Comasphaeridium velvetum Zone of the EEP acritarch assemblage (Yin, Reference Yin1995; Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009). Since the taxonomic revisions by Yao et al. (Reference Yao, Xiao, Yin, Li and Yuan2005), the Micrhystridium–Paracymatiosphaera–Megathrix assemblage has been recognized as the Asteridium–Heliosphaeridium–Comasphaeridium (AHC) acritarch assemblage in the Yangtze Platform. According to the modified definition by Yao et al. (Reference Yao, Xiao, Yin, Li and Yuan2005), the lower limit of the assemblage is now defined by the abundant occurrences of Asteridium tornatum, Heliosphaeridium cf. lubomlense or H. ampliatum, and Comasphaeridium annulare, which corresponds to the Anabarites trisulcatus – Protohertzina anabarica Zone, and the upper limit is defined by the first appearance of acritarch genus Skiagia, which corresponds to the trilobite-bearing formations in the Yangtze Platform (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). This implies that the stratigraphic range of the AHC assemblage is either limited to the lower Meishucunian Stage, or spans the entire Meishucunian Stage (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). However, due to the lack of diagnostic microfossils in middle to upper Meishucunian rocks, the temporal and spatial relationships between the AHC acritarchs and other skeletal metazoans are not yet fully understood.
Aside from the palaeontological evidence for biological radiations at the lowermost Cambrian, various geochemical studies provide useful information about the interactions between biological and geologic events before and during the Ediacaran–Cambrian transition, including events that created suitable biogeochemical conditions for the radiation of metazoans (e.g., Brasier et al. Reference Brasier, Magaritz, Corfield, Luo, Wu, Ouyang, Jiang, Hamadi, He and Frazier1990, Reference Brasier, Shields, Kuleshov and Zhegallo1996; Knoll & Carroll, Reference Knoll and Carroll1999; Zhu, Babcock & Peng, Reference Zhu, Babcock and Peng2006; Zhu, Strauss & Shields, Reference Zhu, Strauss and Shields2007; Zhu, Zhang & Yang, Reference Zhu, Strauss and Shields2007; Maloof et al. Reference Maloof, Porter, Moore, Dudás, Bowring, Higgins, Fike and Eddy2010a ; Shields-Zhou & Zhu, Reference Shields-Zhou and Zhu2013; Sperling et al. Reference Sperling, Frieder, Raman, Girguis, Levin and Knoll2013; Lenton et al. Reference Lenton, Boyle, Poulton, Shields-Zhou and Butterfield2014; Chen et al. Reference Chen, Ling, Vance, Shields-Zhou, Zhu, Poulton, Och, Jiang, Li, Cremonese and Archer2015; Och et al. Reference Och, Cremonese, Shields-Zhou, Poulton, Struck, Ling, Li, Chen, Manning, Thirlwall, Strauss and Zhu2015). For example, carbon isotope excursions are now widely used as an important chronostratigraphic guide to constrain and correlate major geological and biological events across different palaeocontinents, and they are particularly practical in the lowermost Cambrian, because a prominent negative δ13C excursion at the base of the Cambrian (BACE; Basal Cambrian Carbon isotope Excursion) is readily identified globally, whereas the biostratigraphic guides of the corresponding time period show high variability in presence/absence, morphology and stratigraphic range by region (Babcock et al. Reference Babcock, Peng, Zhu, Xiao and Ahlberg2014). In South China, a well-established δ13C profile and biostratigraphic markers of Meishucunian successions have been used to constrain and correlate the Cambrian strata regionally and globally (e.g. Brasier et al. Reference Brasier, Magaritz, Corfield, Luo, Wu, Ouyang, Jiang, Hamadi, He and Frazier1990; Grotzinger et al. Reference Grotzinger, Bowring, Saylor and Kaufman1995; Lindsay et al. Reference Lindsay, Brasier, Dorjnamjaa, Goldring, Kruse and Wood1996; Zhang et al. Reference Zhang, Li, Zhou and Zhu1997; Zhou et al. Reference Zhou, Zhang, Li and Yu1997; Shen & Schidlowski, Reference Shen and Schidlowski2000; Zhu, Babcock & Peng, Reference Zhu, Babcock and Peng2006; Kouchinsky et al. Reference Kouchinsky, Bengtson, Pavlov, Runnegar, Torssander, Young and Ziegler2007; Zhu, Strauss & Shields, Reference Zhu, Strauss and Shields2007; Zhu, Zhang & Yang, Reference Zhu, Strauss and Shields2007; Ishikawa et al. Reference Ishikawa, Ueno, Komiya, Sawaki, Han, Shu, Li, Maruyama and Yoshida2008, Reference Ishikawa, Ueno, Shu, Li, Han, Guo, Yoshida and Komiya2013; Li et al. Reference Li, Ling, Jiang, Pan, Chen, Cai and Feng2009, Reference Li, Ling, Shields-Zhou and Thirlwall2013; Jiang et al. Reference Jiang, Wang, Shi, Xiao, Zhang and Dong2012). However, although the general pattern of the δ13C is recognized in many sections in South China, it is difficult to set the concrete temporal and spatial link between SSF-based biostratigraphy and carbon isotope profile in the Yangtze Platform, partly due to the high faunal provincialism of SSFs.
SSFs, acritarchs and carbon isotope data have been reported from the Yanjiahe Formation in the Yangtze Gorges area, representing one of the best stratigraphic sequences across the Ediacaran–Cambrian boundary interval in South China for resolving the stratigraphic problems addressed above. The purposes of the present study are: (1) to document the stratigraphic range of the AHC acritarch assemblage in the Yanjiahe Formation; (2) to correlate the AHC assemblage to the SSF assemblage in the same stratigraphic profile; and (3) to reconcile the temporal and spatial relations between the two biostratigraphies and the carbon isotope chemostratigraphy. This work improves the precise stratigraphic correlation of the basal Cambrian in South China, the chronostratigraphic frame for global correlation, and our understanding of the co-radiation of primary producers and metazoans in the early Cambrian.
2. Geological background
The Ediacaran to lower Cambrian successions of the Yangtze Gorges area are composed, in ascending order, of the Nantuo, Doushantuo, Dengying, Yanjiahe and Shuijingtuo formations (Chen, Reference Chen and Wang1987; Zhao et al. Reference Zhao, Xing, Ding, Liu, Zhao, Zhang, Meng, Yin, Ning and Han1988; Ding et al. Reference Ding, Li, Hu, Xiao, Su and Huang1996; Guo et al. Reference Guo, Li, Han, Zhang, Zhang, Ou, Liu, Shu, Maruyama and Komiya2008). The Yanjiahe Formation crops out on the south and west part of the Huangling anticline and represents one of the most well-developed and fossiliferous sequences across the Ediacaran–Cambrian boundary interval of South China. The materials for the present bio- and chemostratigraphic studies were collected from the Yanjiahe Formation near Jijiapo village (Fig. 1). Here the formation is c. 50 m thick, including the covered intervals between outcrops. The Yanjiahe Formation contains two cycles: the lower cycle is c. 30 m thick, and composed of grey to dark-grey dolostone, silty and/or chertified dolostone with intercalated thin-bedded fossiliferous black chert and minor phosphatic clasts embedded in the light-grey dolostone. The upper cycle is c. 20 m thick, and composed of light to dark grey dolostone, black limestone with intercalated thin black shale, with carbonate and chert nodules in the lower part and carbonate concretions in the upper part. Previously reported fossil associations of the formation are as follows. Small shelly fossils belonging to the Anabarites trisulcatus – Protohertzina anabarica (SSF Zone I) are reported from the phosphatic clasts of the lower cycle (Chen, Reference Chen1984; Guo et al. Reference Guo, Li, Han, Zhang, Zhang, Ou, Liu, Shu, Maruyama and Komiya2008, Guo, Li & Li, Reference Guo, Li and Li.2014); Purella antiqua and other SSFs indicative of SSF Zone II of South China are reported from the phosphatic chert nodules of the base of the upper cycle (Guo, Li & Li, Reference Guo, Li and Li.2014); SSFs associated with SSF Zone III, including Aldanella yanjiaheensis, are reported from the top of the upper cycle (Guo et al. Reference Guo, Li, Han, Zhang, Zhang, Ou, Liu, Shu, Maruyama and Komiya2008; Guo, Li & Li, Reference Guo, Li and Li.2014). Also, an acritarch taxon indicative of lower Cambrian age, Micrystridium regulare, was reported at the base of the formation (Ding, Li & Chen, Reference Ding, Li and Chen1992). The U–Pb age from the base of the Shuijingtuo Formation in the Yangtze Gorges area gives an age of 526.4±5.4 Ma (Okada et al. Reference Okada, Sawaki, Komiya, Hirata, Takahata, Sano, Han and Maruyama2014); thus the Yanjiahe Formation records the biogeochemical history of approximately the first 15 Ma of the Cambrian Period.
Locality map and simplified geological map of the Jijiapo section in the Yangtze Gorges area, Hubei, China (30°45′14″N, 41°1′57″E).
3. Materials and methods
To confirm the stratigraphic range of the acritarchs of the Yanjiahe Formation, dark, thin layers of chert samples were collected from the base to the top of the formation. A total of 576 petrographic thin-sections were made to examine acritarchs and other microfossils. All the prepared thin-sections were examined with a Nikon Eclipse Ni optical microscope, and photographed with a Nikon DS-Fi1c charge-coupled device (CCD). All the examined fossil specimens are held by the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS), Nanjing, China.
To pinpoint the isotopic values to the horizon of the fossil assemblages, 144 carbonate samples were collected for carbon isotope analyses from the same outcrop. Carbonate samples were collected at c. 0.5 m intervals throughout the section. Each sample was carefully examined and the most homogeneous and freshest part of each sample was powdered with a 2 mm micro-drill. Carbon and oxygen isotopes were then analysed with a Finnigan MAT 253 mass spectrometer at NIGPAS using the Chinese national standard, an Ordovician carbonate from a site near Beijing (reference number GBW 04405: δ13C=0.57±0.03‰ VPDB; δ18O=−8.49±0.13‰ VPDB).
4. Descriptions of the acritarch and other microfossils
Black chert from the Yanjiahe Formation displays a variety of microfossils including algae, cyanobacteria, a distinctive assemblage of acritarchs, as well as various organic remains of unknown affinities. The taxonomic study of the microfossils presented here is mainly focused on the biostratigraphically significant taxa associated with the AHC assemblage.
The acritarchs of the lowermost Cambrian are distinguished from older Ediacaran acritarchs, or younger Phanerozoic acritarchs, by having a relatively small vesicle diameter with simple processes. However, because of this simple general morphology, many Cambrian acanthomorphic acritarchs in early literatures are incorrectly described under the genus Michrystridium, a taxon characterized by small vesicles (<20 µm) with processes that may or may not communicate with the vesicle interior (Moczydłowska, Reference Moczydłowska1991). The genus thus became a ‘taxonomic wastebasket’ (Loeblich, Reference Loeblich1970; Moczydłowska, Reference Moczydłowska1991), referring to simple, small acanthomorphic acritarchs ranging from the Cambrian to recent (Moczydłowska, Reference Moczydłowska1991). To distinguish Cambrian micrhystrids from younger ones, Moczydłowska (Reference Moczydłowska1991) transferred some of the micrhystrids to new genera Asteridium and Heliophaeridium, where Asteridium refers to species with short processes separated from the vesicle interior, while Heliosphaeridium refers to species with longer, hollow processes communicating with the vesicle interior. For the taxonomic identification of the Yanjiahe materials, we followed the original and emended diagnosis of Moczydłowska (Reference Moczydłowska1991) and Yao et al. (Reference Yao, Xiao, Yin, Li and Yuan2005).
4.1. Acanthomorphic acritarchs
Group ACRITARCHA Evitt, Reference Evitt1963
Genus Asteridium Moczydłowska, Reference Moczydłowska1991
Type species. Asteridium lanatum (Volkova, Reference Volkova, Volkova, Zhuravleva, Zabrodin and Klinger1969) Moczydłowska, Reference Moczydłowska1991
Asteridium tornatum (Volkova, Reference Volkova, Volkova, Zhuravleva, Zabrodin and Klinger1968) Moczydłowska, Reference Moczydłowska1991
Figure 2a–d, h
Asteridium tornatum (Volkova, Reference Volkova, Rozanov, Missarzhevskii, Volkova, Voronova, Krylov, Keller, Korolyuk, Lendzion, Michniak, Pykhova and Sidarov1968) Moczydłowska, Reference Moczydłowska1991, p. 48, pl. 1, fig. A–C, and synonyms therein; Moczydłowska, 1998, p. 52, fig. 20C, and synonyms therein; Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005, p. 691, pl. 1, figs 1–4.
Thin-section photomicrographs of Cambrian acanthomorphic acritarchs from the Yanjiahe Formation. (a–c) Asteridium tornatum; (d) A. tornatum (upper and lower right) and H. ampliatum (lower right); (e–g) Heliosphaeridium ampliatum; (h) A. tornatum (left) and H. ampliatum (right); (i) Heliosphaeridium dissimilare; (j–l) Comasphaeridium annulare. Scale bars represent 10µ for a-l. (a) YB 0.6_1a_4_2; (b) SY2 2.0_4a_4_4; (3) SY2 2.0_4a_4_4; (d) SY2 2.0_1f_2_6; (e) YB 0.6_2a_1; (f) YB 0.4_2a_3_1; (g) YB 0.6_2a_1; (h) SY2 1.4_1d_11_3; (i) SY2_-0.3_2f_1_1; (j) YB -3.7_2a_9; (k) SY2 0.7_4b_1_4; (l) SY2 -3.1_1e_1_2.
Distribution. Occurrences in South China include the basal Niutitang Formation (Xie et al. Reference Xie, Tenger, Qian, Zhang, Bian and Yin2015). Occurrences in Tarim include the Xishanblaq Formation (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). Occurrences in Avalonia include the Ratcliffe Brook Formation (Palacios et al. Reference Palacios, Jensen, Barr, White and Miller2011). For occurrences in the East European Platform and Siberia, see Moczydłowska (Reference Moczydłowska1991, Reference Moczydłowska1998).
Description. Single-walled, oval to spherical vesicles covered by evenly distributed processes. Processes solid, short and conical with pointed tips. Processes separated from the vesicle interior.
Dimensions. Vesicle diameter 5.8–13.7 μm (mean 9.12 μm, n=145), process length 0.6–3.6 μm (mean 1.56 µm, n=145). Vesicle size is positively correlated with process length (Fig. 3). No excystment observed. A total of 145 fossils were measured.
Cross-plot and frequency diagram of vesicle diameter (VD) and process length (PL) of Asteridium tornatum from the Yanjiahe Formation.
Discussion. Genus Asteridium includes spherical to oval, single-layered, thin and organic-walled microfossils with many solid processes that are short, tapering, pointed, blunt or swollen (Moczydłowska, Reference Moczydłowska1991). A. tornatum is distinguished from other species of Asteridium by having very short conical processes around the vesicle (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). The Yanjiahe specimens clearly show the generic characteristics of A. tornatum, although the measured process is slightly longer than A. tornatum reported from the EEP and Tarim. In many specimens, processes are darkened due to taphonomic alterations. Clusters of several specimens are often observed, often including acritarchs of other taxa, most commonly with Heliosphaeridium. The lowest occurrence of A. tormatum from the Jijiapo section is c. 21.8 m above the base of the Yanjiahe Formation, and the last occurrence is 29.8 m above the base of the Yanjiahe Formation. The fossil shows maximum abundance at 27.7–29.0 m, which is 0.7 m above the A. trisulcatus – P. anabarica Assemblage Zone in the Jijiapo section. A. tornatum is the most common acritarchs taxon in the section, along with Heliosphaeridium ampliatum.
Genus Comasphaeridium Staplin, Jansonius & Pocock, 1965
Type species. Comasphaeridium cometes (Valensi, 1948) Staplin, Jansonius & Pocock, 1965
Comasphaeridium annulare (Wang, Reference Wang1985) Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005
Figure 2j–l
Comasphaeridium annulare (Wang, Reference Wang1985) Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005, p. 692, pl. 1, figs 5–7, and synonyms therein; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009, p. 35, figs 3.1 (lower specimen), 3.2.
Distribution. Basal Cambrian successions in South China and Tarim. Occurrences in South China include the Yangjiaping Formation (Wang, Reference Wang1985), the Taozichong Formation (Wang, Reference Wang1985), the Yanjiahe Formation (Yin et al. Reference Yin, Yue, Gao and Ding1992; Yin, Reference Yin1995; Yin, Reference Yin1997; Yin, Gao & Xing, Reference Yin, Gao and Xing2003; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009) and the Zhujiaqing Formation (Yin, Reference Yin1990). Occurrences in Tarim include the Yurtus Formation (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009).
Description. Oval to spherical vesicles with short, numerous, densely distributed radial processes. Processes stiff, simple, thin and of uniform length.
Dimensions. Vesicle diameter 5.1–17.0 μm (mean 10.87 μm, n=12). Processes 1.0–5.77 μm (mean 2.75 μm, n=12) in length, and <0.5 μm in thickness. No positive correlation between vesicle diameter and process length. No excystment observed.
Discussion. C. annulare is commonly found from the phosphorite–chert layers of the basal Cambrian successions from South China (Wang, Reference Wang1985; Yin, Reference Yin1990, Reference Yin1995; Yin, Reference Yin1997; Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009). Yao et al. (Reference Yao, Xiao, Yin, Li and Yuan2005) transferred four species of the genus Paracymatiosphaera Wang, Reference Wang1985 to the genus Comasphaeridium based on the process characters, assuming taphonomic alterations may have led to taxonomic confusion on specimens showing variations in the regularity of process length or distribution (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). Comasphaeridium specimens of the Yanjiahe Formation are relatively rare compared to other AHC acritarchs. The stratigraphic range of the C. annulare is 23.3–28.15 m above the base of the formation. The fossil shows maximum abundance at 23.3 m, which is 3.7 m below the A. trisulcatus – P. anabarica Assemblage Zone in the Jijiapo section.
Genus Heliospaheridium Moczydłowska, Reference Moczydłowska1991
Type species.Heliosphaeridium dissimilare (Volkova, Reference Volkova, Rozanov, Missarzhevskii, Volkova, Voronova, Krylov, Keller, Korolyuk, Lendzion, Michniak, Pykhova and Sidarov1969) Moczydłowska, Reference Moczydłowska1991
Heliosphaeridium ampliatum (Wang, Reference Wang1985) Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005
Figure 2d–g, h
Heliosphaeridium ampliatum (Wang, Reference Wang1985) Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005, p. 693, pl. 1, figs 8, 9, and synonyms therein; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009, figs 3.1 (upper specimen), 3.3, 3.4.
Distribution. Basal Cambrian successions in South China and Tarim. Occurrences in South China include the Zhujiaqing Formation (Wang, Zhang & Guo, Reference Wang, Zhang and Guo1983; Yin, Reference Yin1990), the Yangjiaping Formation (Wang, Reference Wang1985), the Taozichong Formation (Wang, Reference Wang1985; Braun & Chen Reference Braun and Chen2003), the Niutitang Formation (Wang, Reference Wang1985; Braun & Chen Reference Braun and Chen2003; Xie et al. Reference Xie, Tenger, Qian, Zhang, Bian and Yin2015), the Yanjiahe Formation (Yin, 1985; Reference YinYin, 1987a ; Ding, Li & Chen, Reference Ding, Li and Chen1992; Yin et al. Reference Yin, Yue, Gao and Ding1992; Yin, Gao & Xing, Reference Yin, Gao and Xing2003; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009) and the Kuanchuanpu Formation (Reference YinYin, 1987b ). Occurrences in Tarim include the Yurtus and Xishanblaq formations (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009).
Description. Single-walled spherical vesicles with long, evenly distributed processes. Processes are hollow, stiff, and tapering toward the distal end.
Dimensions. Vesicle diameter 6.3–13.3 μm (mean 9.2 μm, n=81); process length 3.5–14.8 µm (mean 7.2 µm, n=81). Vesicle size is positively correlated with process length (Fig. 4). No excystment observed. A total of 81 specimens were measured.
Cross-plot and frequency diagram of vesicle diameter (VD) and process length (PL) of Heliosphaeridium ampliatum from the Yanjiahe Formation.
Discussion. H. ampliatum differs from other species of Heliosphaeridium in having distinctively fewer but longer processes around the vesicle (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). Other species of Heliosphaeridium (e.g. Heliosphaeridium dissimilare, Fig. 2i) were also found from the Jijiapo section; however, the number of specimens is not enough to be of any statistical importance, so they were excluded from the focus of this study. In the measured specimens, the approximate ratio between vesicle diameter and process length is 3:4, however, processes longer than the vesicle diameter are commonly observed (Fig. 4). The stratigraphic range of H. ampliatum in the Yanjiahe Formation is 26.4–29.5 m from the base of the formation, and the taxon shows maximum abundance at 27.4–27.7 m, which is 0.4–0.7 m above the A. trisulcatus – P. anabarica Assemblage Zone in the Jijiapo section.
4.2. Tubular microfossils with cross walls
Genus Megathrix Yin, Reference Yin1987a, emend. Shang et al. Reference Shang, Liu, Yang and Chen2015
Type species. Megathrix longus Yin, Reference Yin1987a, emend. Shang et al. Reference Shang, Liu, Yang and Chen2015
Megathrix longus Yin, Reference Yin1987a, emend. Shang et al. Reference Shang, Liu, Yang and Chen2015
Figure 5a–g
Megathrix longus Yin, Reference Yin1987a, n. emend. Shang et al. Reference Shang, Liu, Yang and Chen2015, p. 5, figs 2A–2D, figs 3A–3L, figs 4A–4E, figs 5A–5N. figs. 6A–6F, 6H–6M, figs 11A–11J, and synonyms therein.
(a–g) Thin-section photomicrographs of Megathrix longus from the Yanjiahe Formation. Grey arrows indicate complete cross-walls; white arrows indicate incomplete cross-walls. Scale bars represent 50µ for (a–f). (a) SY2 0.2_1e; (b) SY2 0.7_3b_6; (c) SY2 1.4_1d_8; (d) SY2 0.7_5_6; (e) SY2 0.7_4a_5_1; (f) SY2_0.2_1e.
Distribution. Basal Cambrian successions in South China and Tarim. Occurrences in South China include the Yanjiahe Formation (Yin, Reference Yin1987a; Yin et al. Reference Yin, Yue, Gao and Ding1992; Yin, Gao & Xing, Reference Yin, Gao and Xing2003; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009; Shang et al. Reference Shang, Liu, Yang and Chen2015). Occurrences in Tarim include the Yurtus and Xishanblaq formations (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009).
Description. Long tubular microfossil with evenly spaced corrugated or curved cross walls within each tube. The tubes are 44.5–96.3 μm wide (mean 68.7 μm, n=89). The average length of the tubes cannot be measured because most of the filaments are partly preserved. However, each preserved segment of the tubes easily reaches several hundred μm, and the longest segment measured in thin-sections reaches 1972 μm, indicating that the intact tubes may reach up to several mm scale in length. Transverse walls commonly show two types: complete cross walls (grey arrows in Fig. 5c, e–g) and incomplete cross walls (white arrows in Fig. 5c, e–g). In many specimens, incomplete walls are inserted between cross walls, and both types of wall are observed even in the poorly preserved specimens (Fig. 5c), so incomplete walls do not seem to be a product of taphonomic artefacts. The spacing between the walls varies by specimen; however, in the measured specimens, wall spacing is c. 10 μm. Tubes are mostly solitary, and rarely branch.
Discussion. The taxon typically co-occurs with the AHC acritarchs assemblage (Fig. 5b), thus serving as a significant component of the AHC assemblage (Yin et al. Reference Yin1987 a,b; Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005; Dong et al. Reference Dong, Xiao, Shen, Zhou, Li and Yao2009). In the Jijiapo section, the stratigraphic range of M. longus is 26.15–28.4 m above the base of the formation, with the maximum abundance at the 27.7 m horizon. Although M. longus is a common component in the lowermost Cambrian microfossil assemblages, not much was known about its taxonomic affinity until recently, except that the general morphology of the taxon resembles Oscillatoriopsis-like cyanobacteria (Yin et al. Reference Yin1987 a). Shang et al. (Reference Shang, Liu, Yang and Chen2015) recently reviewed M. longus specimens from the Yanjiahe Formation, and concluded that M. longus might be closely associated with Oscillatoriaceae, based on general morphology, ecology and the growth pattern of the tube. Furthermore, the corrugated transverse wall of M. longus, the most distinctive character of the taxon, is a postmortem feature rather than the original biological feature. Our specimens show most of the key characters of the taxon presented by Shang et al. (Reference Shang, Liu, Yang and Chen2015); thus we agree that M. longus was a planktic Oscillatoriopsis-like organism in the Cambrian ocean, which radiated along with the AHC assemblage.
5. Discussion
5.1. Acritarch biostratigraphy
The known stratigraphic range of the Asteridium–Heliosphaeridium–Comasphaeridium acritarch assemblage is in the 21.8–29.8m interval above the base of the Yanjiahe Formation. Each component of the assemblage shows maximum abundance at 27–29 m, which is slightly above the A. trisulcatus – P. anabarica Assemblage Zone in the Jijiapo section (Fig. 6). The acritarchs belonging to the AHC assemblage have relatively simple morphology, so the shape, length and density of processes are the essential characteristics in the taxonomic identification. However, the petrographic observations on the thin sections indicate that the rocks underwent various early diagenetic processes, such as silicification, carbonization and phosphatization. Early silicification clearly helped the preservation of organic-walled microfossils; however, carbonization and phosphatization may have led to the loss of the key features of the acritarchs, leaving poorly preserved simple, darkened globules behind. The lowest occurrence of these globules is 7–8 m below the A. trisulcatus – P. anabarica Assemblage Zone.
Lithostratigraphy, acritarchs and SSF biostratigraphy, and composite profiles of δ13Ccarb (‰, VPDB), δ18O (‰, VPDB) for the Yanjiahe Formation at the Jijiapo section. Yellow shade indicates the stratigraphic range of the AHC acritarch assemblage.
In South China and Tarim, the abundant occurrence of AHC acritarchs marks the lower limit of the AHC assemblage, and the first appearance of acritarch genus Skiagia marks the upper limit of the assemblage (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). According to this definition, the lower limit of the AHC assemblage is placed at the 27 m horizon in the section, in accordance with that of the A. trisulcatus – P. anabarica Assemblage Zone, even though each key taxon of the AHC assemblage appears earlier (Fig. 6). The upper limit of the assemblage should be placed in the Shuijingtuo Formation, which corresponds to the Yu'anshan Formation in Yunnan where Skiagia was reported (Zang, Reference Zang1992; Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005). However, the black micritic limestone from the middle to upper part of the Yanjiahe Formation (30–43 m) does not yield any possible materials for acritarch biostratigraphy, and the black chert near the Yanjiahe–Shuijingtuo boundary does not yield any acritarchs associated with the AHC assemblage. Therefore in the Yanjiahe Formation, the AHC acritarchs are only found near the lower boundary of the AHC assemblage. Furthermore, the preliminary result on acritarch biostratigraphy in eastern Yunnan indicates that the FADs of AHC acritarchs are placed near the top of the Daibu Member, which is slightly below the FADs of SSFs in Yunnan. Furthermore, the upper limit of the AHC acritarchs cannot easily be confirmed in the Zhujiaqing Formation due to the lack of materials that can preserve the acritarchs. Thus, even though the stratigraphic range of the AHC assemblage in South China is either limited to the lower Meishucunian Stage, or spans the entire Meishucunian Stage (Yao et al. Reference Yao, Xiao, Yin, Li and Yuan2005), the lower limit of the AHC assemblage consistently corresponds to the A. trisulcatus – P. anabarica Assemblage Zone, and also with the BACE curve, in the Yangtze Gorges area and eastern Yunnan (Fig. 7).
Chemo- and biostratigraphic correlation of the Yanjiahe Formation to two reference sections in northeast Yunnan. Blue shade indicates the correlation of δ13Ccarb between the Yanjiahe and Zhujiaqing formations. Yellow shade indicates the stratigraphic range of the AHC acritarch assemblage at the Jijiapo section, and projected correlations in the Zhujiaqing Formation based on the FAD of AHC acritarchs (unpublished data), SSF biostratigraphy, and lithostratigraphy. SSF biostratigraphy data for the Jijiapo section is from Guo Li & Li (Reference Guo, Li and Li.2014); Carbon isotope data of the Laolin and Xiaotan sections are from Li et al. (Reference Li, Ling, Jiang, Pan, Chen, Cai and Feng2009), and Li et al. (Reference Li, Ling, Shields-Zhou and Thirlwall2013) respectively.
Recently, Chang et al. (Reference Chang, Feng, Clausen and Zhang2016) reported the earliest sponge spicules from the Yanjiahe Formation, slightly before the A-P Zone from the Luojiacun section. Although the section is c. 10 km from the Jijiapo section, the stratigraphic information of this section is not provided in detail. The A-P Zone is located c. 6 m above the Dengying–Yanjiahe boundary according to their simplified stratigraphic profile, so the majority of chert layers in the lower to middle part of the formation are missing in the Luojiacun section. Therefore, the temporal and spatial relationship between the reported sponge fossils and acritarchs in the Yanjiahe Formation remains unclear.
5.2. Carbon isotope stratigraphy
The carbon isotope analysis of the section shows the following features (Fig. 6; Table 1): (1) positive δ13C values around 0.5‰ in the uppermost part of the Dengying Formation; (2) δ13C values start to drop from c. 3 m above the base of the Yanjiahe Formation, and reach a nadir of −4.91‰ in the middle part of the formation, at c. 6–7 m below the A. trisulcatus – P. anabarica Zone (N1); (3) δ13C values become positive again in the middle–upper part of the formation which is dominated by black limestone (P1); (4) δ13C values drop to −2‰ at the topmost part of the Yanjiahe Formation. The N1 excursion in this study is comparable to N1 (Ishikawa et al. Reference Ishikawa, Ueno, Komiya, Sawaki, Han, Shu, Li, Maruyama and Yoshida2008) and CN1 (Jiang et al. Reference Jiang, Wang, Shi, Xiao, Zhang and Dong2012), and the general trend and magnitude of this negative excursion shows a typical large negative δ13C anomaly of the basal Cambrian System (Zhu, Babcock & Peng, Reference Zhu, Babcock and Peng2006), so is interpreted as the BACE curve. In the Yanjiahe Formation, the A. trisulcatus – P. anabarica Zone is placed in the upper part of the BACE curve, and the AHC assemblage embraces the nadir to the top of the excursion (Fig. 6). In other parts of the Yangtze Platform, the base of the Meishucunian Stage is defined by the first appearance of SSFs that are equivalent to the A. trisulcatus – P. anabarica Zone (Qian & Bengtson, Reference Qian and Bengtson1989; Li, Zhang & Zhu, Reference Li, Zhang and Zhu2001; Qian, Li & Zhu, Reference Qian, Li and Zhu2001; Zhu et al. Reference Zhu, Li, Zhang, Steiner, Qian and Jiang2001; Zhu, Babcock & Peng, Reference Zhu, Babcock and Peng2006; Li et al. Reference Li, Zhao, Gubanov, Zhu and Na2011). Therefore, the BACE and the A. trisulcatus – P. anabarica Zone in the lower to middle Yanjiahe Formation indicate that this part is equivalent to the Daibu and lower Zhongyicun members of the Zhujiaqing Formation in eastern Yunnan (Fig. 7). The P1 excursion is interpreted as ZHUCE, which corresponds to the Watsonella crosbyi Assemblage Zone in eastern Yunnan (Zhu, Babcock & Peng, Reference Zhu, Babcock and Peng2006; Li et al. Reference Li, Zhao, Gubanov, Zhu and Na2011). W. crosbyi is absent in the Yanjiahe Formation; however, Aldanella yanjiaheensis, a key SSF taxon in the W. crosbyi Zone, is reported from the top of the formation (Guo, Li & Li, Reference Guo, Li and Li.2014). Therefore, the coexistence of P1 and A. yanjiaheensis indicates that the upper part of the formation corresponds to the Dahai Member of the Zhujiaqing Formation in eastern Yunnan. However, unlike in eastern Yunnan, P1 starts from the middle part of the Yanjiahe Formation, while A. yanjiaheensis is reported from phosphatic clasts right below the Yanjiahe–Shuijingtuo boundary (Fig. 6). This is probably due to the lack of phosphatized rocks that can preserve SSFs in the middle to upper part of the formation, and A. yanjiaheensis reported from the topmost Yanjiahe Formation does not seem to represent the first appearance of the third SSF assemblage in the Yangtze Gorges area.
δ13CPDB and δ18OPDB of samples from the Jijiapo section, Yangtze Gorges area, South China
7. Conclusion
The presence of microfossils indicative of the basal Cambrian was confirmed in the Yanjiahe Formation. The upper limit of the AHC acritarch assemblage cannot be confirmed in the Yangtze Gorges area, but the lower limit of the AHC assemblage corresponds to the A. trisulcatus – P. anabarica Assemblage Zone, and also with the BACE curve. This indicates that the radiation of primary producers occurred slightly before, or almost synchronous with, the radiation of biomineralizing metazoans during the early Cambrian, and this radiation event pairs with the BACE curve. This also confirms the biostratigraphic utility of the AHC acritarch assemblage, indicating that the FAD of AHC assemblage can be used to pinpoint the specific horizon where the SSFs started to radiate when the SSFs are not available, and can also be used to indicate the upper part of the BACE excursion when carbon isotope data are not available. However, to apply this correlation to broader sedimentary environments with more confidence, similar bio- and chemostratigraphic investigations need to be carried out in more complete successions in the Yangtze Platform, as well as in Siberia, Avalonia and other type areas where the SSF biostratigraphy and chemostratigraphy are well understood.
Acknowledgements
We thank Aihua Yang, Lanyun Miao, Cui Luo and Jan-Peter Duda for assisting with the fieldwork, and Xiaoming Chen for isotope analyses. We thank Guoxiang Li, Junming Zhang and many other colleagues for helpful discussions. Pengju Liu and an anonymous reviewer provided helpful comments that greatly improved the manuscript. This work was supported by the CAS President's International Fellowship Initiative (2015PB001), the Strategic Priority Research Program (B) CAS (XDB18030304), the National Natural Science Foundation of China (40725005, 40930211, J0930006, 41372021, J2120006) and the Ministry of Science and Technology of China (2013CB835000).
