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Volume 31 Issue 1
Jan.  2020
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Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou, Gongjing Zhang. SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China. Journal of Earth Science, 2020, 31(1): 69-78. doi: 10.1007/s12583-019-1233-x
Citation: Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou, Gongjing Zhang. SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China. Journal of Earth Science, 2020, 31(1): 69-78. doi: 10.1007/s12583-019-1233-x

SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China

doi: 10.1007/s12583-019-1233-x
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  • The Ediacaran-Cambrian succession in South China records dramatic biological,oceanic and geochemical changes,but it is not well constrained geochronologically. This study reports a new SIMS U-Pb date of 543.4±3.5 Ma (MSWD=1.2) from a tuffaceous layer in the Zhoujiaao Section,and carbonate C-O isotopes in both Zhoujiaao and Sixi sections,Three Gorges area. This tuffaceous layer is present in the Upper Dengying Formation (i.e.,the Baimatuo Member) which is characterized by a stable δ13Ccarb plateau and the beginning of a negative δ13Ccarb shift near its upper boundary. In accordance with the existing biostratigraphic and chemostratigraphic data,this new date corroborates that the upper boundary of the Dengying Formation in South China is approximately equivalent to the Ediacaran-Cambrian boundary (ca. 541 Ma). This age also provides the minimum age of the last appearance of the Shibantan biota in the Three Gorges area,indicating that the terminal Ediacaran index fossils (e.g.,Cloudina,Sinotubulites) are not reliable stratigraphic markers for further subdivision of the uppermost Ediacaran.
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SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China

doi: 10.1007/s12583-019-1233-x
    Corresponding author: Daizhao Chen

Abstract: The Ediacaran-Cambrian succession in South China records dramatic biological,oceanic and geochemical changes,but it is not well constrained geochronologically. This study reports a new SIMS U-Pb date of 543.4±3.5 Ma (MSWD=1.2) from a tuffaceous layer in the Zhoujiaao Section,and carbonate C-O isotopes in both Zhoujiaao and Sixi sections,Three Gorges area. This tuffaceous layer is present in the Upper Dengying Formation (i.e.,the Baimatuo Member) which is characterized by a stable δ13Ccarb plateau and the beginning of a negative δ13Ccarb shift near its upper boundary. In accordance with the existing biostratigraphic and chemostratigraphic data,this new date corroborates that the upper boundary of the Dengying Formation in South China is approximately equivalent to the Ediacaran-Cambrian boundary (ca. 541 Ma). This age also provides the minimum age of the last appearance of the Shibantan biota in the Three Gorges area,indicating that the terminal Ediacaran index fossils (e.g.,Cloudina,Sinotubulites) are not reliable stratigraphic markers for further subdivision of the uppermost Ediacaran.

Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou, Gongjing Zhang. SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China. Journal of Earth Science, 2020, 31(1): 69-78. doi: 10.1007/s12583-019-1233-x
Citation: Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou, Gongjing Zhang. SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China. Journal of Earth Science, 2020, 31(1): 69-78. doi: 10.1007/s12583-019-1233-x
  • The Ediacaran-Cambrian (E-C) transitional period was a crucial time interval in the Earth history with remarkable biological evolution, characterized by the decline of the Ediacaran fauna, the subsequent remarkable metazoans radiation well known as the "Cambrian Explosion" (Maloof et al., 2010; Marshall, 2006), and profound oceanic changes, including increasing oxygen levels, large carbonate carbon isotope excursions and sulfur isotope variations (Lyons et al., 2014; Jiang et al., 2012; Wang et al., 2012a, b ; Goldberg et al., 2007). In South China, the well-developed E-C succession opens a window to explore the coevolution of the biosphere and geosphere. In the last decade, numerous paleontological (Chen et al., 2014, 2013; Duda et al., 2014; Guo et al., 2014; Steiner et al., 2007) and geochemical studies (Ding et al., 2018; Wang et al., 2014; Ishikawa et al., 2013, 2008; Li D et al., 2013, 2009; Jiang et al., 2012, 2007) have focused on the South China E-C transitional strata. However, the E-C strata are not well constrained by radiometric ages in South China, although previous biostratigraphic and chemostratigraphic studies have proposed feasible stratigraphic correlation schemes (Jiang et al., 2012; Wang D et al., 2012; Ishikawa et al., 2008; Zhou and Xiao, 2007; Zhu et al., 2007). Among these ages, a series of radiometric ages from ca. 538 to 522 Ma have been reported in the Zhujiaqing Formation (and its coeval strata), generally constraining the Terreneuvian Series and the minimum age for the E-C boundary in South China (Lan et al., 2017; Chen et al., 2015; Okada et al., 2014; Wang X Q et al., 2012; Zhu et al., 2009; Compston et al., 2008; Jenkins et al., 2002). In the shelf margin (Ganziping) and basin settings (Bahuang), two U-Pb zircon ages of 542.6±3.7 and 542.1±5.0 Ma from the Liuchapo Formation provide direct ages for the E-C boundary in South China (Chen et al., 2015). In comparison, the shallow-water Dengying Formation, generally considered as the terminal Ediacaran strata, has a widely accepted lower boundary age of approximately 551 Ma (Xiao et al., 2016; Condon, 2005) but no direct age constraint for its upper boundary. On the other hand, the Upper Dengying Formation generally underwent severe karstification and is unconformably overlain by the lowermost Cambrian successions (Wang et al., 2012a). In this case, uncertainties remain in the stratigraphic correlation of the uppermost Ediacaran to the lowermost Cambrian succession of South China.

    To provide further geological constraints on the E-C succession, a new second ion mass spectrometry (SIMS) U-Pb age is reported from a tuffaceous layer in the Baimatuo Member of the Upper Dengying Formation at the Zhoujiaao Section and carbonate C-O isotopes at both the Zhoujiaao and Sixi sections, Three Gorges area, western Hubei Province. The Three Gorges area possesses a relatively complete E-C boundary succession with credible biostratigraphic and chemostratigraphic markers (Guo et al., 2014; Jiang et al., 2012). In combination with previous radiometric ages and biological and geochemical data, this paper provides the first U-Pb age for the Upper Dengying Formation to constrain the E-C transitional strata in South China.

  • The Yangtze Block was amalgamated with the Cathaysian Block as a unit during the Early Neoproterozoic (Charvet, 2013). Afterwards, coincident with the breakup of Rodinia, the coupled South China Block was separated from other blocks, leading to the development of rift basins (Wang and Li, 2003). After the Cryogenian, the Yangtze Block had evolved into a drift stage based on its long time sequence stratigraphy (Jiang et al., 2011; Wang and Li, 2003), although recent provenance studies indicate the formation of a foreland basin in the southeast Yangtze Block (Yao et al., 2015). During the E-C transitional period, the Yangtze Block experienced block-tilting and intense extension (Liu et al., 2013; Chen et al., 2009). As a result, carbonate deposition developed on paleo-uplands mostly in the Mid-Upper Yangtze Block and was surrounded by chert deposits in the deep slope and basin environments (Fig. 1a) (Chen et al., 2009).

    Figure 1.  (a) Paleogeography of the Yangtze Block during the E-C transition; (b) simplified geological map of the Three Gorges area, with location of studied section. 1. Zhoujiaao; 2. Sixi; (c) generalized stratigraphic column of the Ediacaran-Cambrian succession with carbon isotope curve in the Three Gorges area (modified from Wang et al., 2014 and Jiang et al., 2012). HMJ. Hamajing Member; SJT. Shuijingtuo Formation.

    In the Three Gorges area, the E-C succession around the Huangling anticline includes the Dengying Formation and the Yanjiahe Formation (Figs. 1b, 1c). The Dengying Formation, overlying the Doushantuo Formation, is subdivided into the Hamajing Member, the Shibantan Member and the Baimatuo Member in ascending order (Fig. 1b) (Zhu et al., 2003). The Hamajing Member comprises mainly massive and medium to thick intraclastic/ooidal dolograinstone (Chen et al., 2014; Duda et al., 2014). The Shibantan Member is characterized by dark gray, banded limestone with horizontal lamination and hummocky cross-bedding (Chen et al., 2014; Duda et al., 2014). The Baimatuo Member consists of light gray massive dolostone (Duda et al., 2016; Zhou and Xiao, 2007; Zhu et al., 2007). Of these units, the Shibantan Member and the Lower Baimatuo Member are rich in fossils, including macro-algal fossils Vendotaenia (Weber et al., 2007; Sun, 1986), the Ediacaran-type fossils Yangtziramulus and Paracharnia (Shen et al., 2009; Xiao et al., 2005; Sun, 1986), trace fossils (Meyer et al., 2014; Chen et al., 2013; Weber et al., 2007) and tubular fossils Sinotubulites (Xiao et al., 2005; Ding et al., 1993). This fossil association is named the Shibantan (Xilingxia) biota (Zhu, 2010; Ding et al., 1992a). The Yanjiahe Formation, which underlies the Shuijingtuo Formation, is characterized by cherty and/or phosphatic dolomudstone in the lower part followed by dark gray shaly limestone intercalated with thin black shales in the middle and upper part (Jiang et al., 2012). Micrhystridium-like acritarchs and three small shelly fossil (SSF) assemblages are recognized in the Yanjiahe Formation (Fig. 1c) (Guo et al., 2014; Ding et al., 1992b). The Zhoujiaao Section, located at a quarry in Northwest Yichang, western Hubei Province (Fig. 1b), consists of the Upper Shibantan Member and Lower-Middle Baimatuo Member (Fig. 2). The upper part of the Shibantan Member is composed of dark gray limestone with abundant banded clay laminae. The overlying Lower Baimatuo Member is dominated by a 45 m medium to thick-bedded peloidal dolograinstone with meter-scale intercalations of thin bedded dolomudstone (Fig. 2a). The tuff layer was found at the top of this unit and collected for SIMS U-Pb analysis (Figs. 2c, 2d). The overlying succession is a 5 m-thick dolomudstone. The upper part of the Zhoujiaao Section is dominated by coarse dololaminite manifested by dolomudstone/dolopackstone couplets (Fig. 2b), which is indicative of a peritidal environment.

    Figure 2.  Representative photographs from Zhoujiaao Section. (a) Peloidal dolograinstone with meter-scale intercalation of thin bedded dolomudstone in the Lower-Middle Baimatuo Member; (b) coarse dololaminite marked by dolomudstone/dolopackstone couplets in the upper part of the Baimatuo Member; (c) location of the tuff layer in the Baimatuo Member of the Dengying Formation, standing person for scale: 172 cm; (d) close-up photograph of the tuffaceous bed, hammer for scale (35 cm).

    The Sixi Section, located in West Yichang, western Hubei Province (Fig. 1b), comprises the Upper Baimatuo Member (Dengying Formation) and the Yanjiahe Formation (Fig. 3). The Upper Baimatuo Member, similar to that at the Zhoujiaao Section, is composed of the characteristic coarse dololaminite showing dolomicrite/peloid-concentrated couplets deposited in a peritidal environment (Figs. 4a, 4e). The Yanjiahe Formation conformably overlies the Dengying Formation without subaerial exposure features (Fig. 4b). The Lower Yanjiahe Formation is composed of dolomudstone with a basal thin phosphatic dolostone layer (Figs. 4c, 4f). This unit is followed by a thin siliceous phosphorite layer and a 14 m-thick dark gray shaly limestone upward. The uppermost Yanjiahe Formation is 0.25 m of siliceous phosphorite, which is overlain by gray calcareous shales of the lowermost Shuijingtuo Formation (Fig. 4d).

    Figure 3.  Stratigraphic columns and carbon-oxygen isotope profiles of the Zhoujiaao and Sixi sections. SBT. Shibantan Member.

    Figure 4.  Representative photographs and photomicrographs from Sixi Section. (a) Coarse dololaminite marked by dolomudstone/dolopackstone couplets in the Baimatuo Member, marker pen for scale: 14 cm; (b) the Dengying-Yanjiahe boundary, characterized by abrupt transition from coarse dololaminite to phophatic dolostone, standing person for scale: 180 cm; (c) close-up photograph of the Dengying-Yanjiahe boundary, hammer for scale: 35 cm; (d) Yanjiahe-Shuijingtuo boundary, characterized by gradual transition from siliceous phosphorite to calcareous shale, hammer for scale: 35 cm; (e) photomicrograph of (a), characterized by mm-scale laminae of alternating peloid/micrite-rich couplets; (f) photomicrograph of the phosphatic dolostone in the basal Yanjiahe Formation under plane polarized light. Red arrows represent phosphorus contents.

  • Samples (BMT) were taken from tuff layers in the Baimatuo Member at the Zhoujiaao Section. Zircon crystals were separated from tuffaceous samples by standard magnetic and gravitational separation methods. Zircon grains, together with zircon standards Plešovice, Qinghu and 91500, were mounted in an epoxy resin which was then polished to expose the crystals for measurement. Prior to SIMS analysis, all zircon grains were imaged and examined with reflected and transmitted light photomicrographs and cathodoluminescence (CL) images for external and internal structures (Fig. 5). Euhedral to subhedral zircons which have clear oscillatory rims and are free of cracks and inclusions were selected for age determination.

    Figure 5.  Photomicrographs of representative zircon crystals analyzed in this study, the ellipses (30 μm×20 μm) show the spots of SIMS U-Pb analyses.

    The absolute abundances and isotope compositions of U, Pb and Th were analyzed using SIMS with a CAMECA 1 280 instrument in the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The size of the primary oxygen ion (O2-) beam spot was approximately 20-30 μm, and the extraction potential of the secondary ion was 10 kV. Pb+ peaks were separated from isobaric interferences using an energy window of 60 eV and a mass resolution of approximately 5 400 (at 10% peak height) in the secondary ion beam optics. The intensities of the secondary ion beam were measured by the peak jumping method in ion-counting mode using a single electron multiplier. The error of the U/Pb calibration curve was fitted relative to the Plešovice standard zircons and then applied to the unknown grains. The absolute abundances of U, Pb and Th were calibrated to the 91500 standard zircon (Wiedenbeck et al., 1995), and their ratios were determined relative to the Plešovice standard zircons (Sláma et al., 2008). Based on the assumption that the samples are mainly contaminated by common Pb during sample preparation, analyzed Pb isotope values were calibrated for common Pb by the 204Pb method using a current mean crustal composition. The corrections were small and did not affect the choice of common Pb compositions. The analysis of data was performed using the Isoplot 4.15 program. Uncertainty in the individual analyses of isotope ratios is displayed at the 1σ level. The weighted average age is displayed at the 2σ level. For the purpose of monitoring the U-Pb measurement accuracy, Qinghu standard zircons were measured as unknowns together with the BMT zircons. Twelve measurements of Qinghu zircons gave a 238U-206Pb weighted average of 159.6±1.4 Ma, which is coincident with the suggested result of 159.5±0.2 Ma within errors, suggesting that measurements of samples are reliable (Li X H et al., 2013).

    Twenty-four samples from the Zhoujiaao Section and 74 samples from the Sixi Section were collected at fresh exposures in the field work for the carbon isotope and oxygen isotope analysis. Sample powders were obtained using a dental drill to remove clay-rich laminae, post-depositional veins and diagenetic fabrics. Aliquots (~250 μg) were reacted with phosphoric acid for 200 s at 70 ℃. The extracted CO2 was then introduced into a mass spectrometer of Finnigan MAT-253 mass spectrometer at the IGGCAS. The accuracy and precision were routinely checked by running a carbonate standard IVA-CO-1 (δ13C=2.21‰, δ18O= -1.90‰), which was repeatedly calibrated using the international standard NBS-19 after every six sample measurements (Cui and Wang, 2014). Carbon and oxygen isotope data are displayed relative to the standard Vienna Peedee Belemnite (VPDB) with analytical precisions of 0.2‰ and 0.15‰, respectively.

  • Most zircons extracted from tuff sample BMT are euhedral and subeuhedral in morphology and 100-200 μm in length. Some of them have clear oscillatory zoning in CL images (Fig. 5). U contents range from 84 ppm to 3 876 ppm (mostly within 84 ppm-913 ppm). Th contents range from 83 ppm to 1 617 ppm (mostly within 84 ppm-856 ppm). Th/U values are between 0.208 and 2.524 (mostly within 0.208-1.750). A total of 29 analyses were performed on 29 zircons and their isotope results are exhibited in Table S1. Nine sets of data are rejected because of discordance or high common lead, and the remaining 20 analyses of zircon provide a 238U-206Pb weighted average of 543.4±3.5 Ma with MSWD=1.2 (Fig. 6).

    Figure 6.  (a) U-Pb concordia diagram and (b) weighted average analysis for the tuffaceous layer from the Baimatuo Member of the Dengying Formation at the Zhoujiaao Section.

    The results of δ13Ccarb18Ocarb are shown in Fig. 3 and Table S2. In the Zhoujiaao Section, the δ13Ccarb values are relatively stable between 2.48‰ and 3.55‰, with corresponding δ18O values ranging from -6.97‰ to -4.99‰. In the Sixi Section, the δ13Ccarb values range from -0.79‰ to 4.05‰ and the δ18Ocarb values vary between -8.84‰ and -5.39‰. A negative δ13Ccarb excursion with δ13Ccarb values changing from stable values of approximately 2‰ to a nadir of -0.79‰ occurs in the Dengying-Yanjiahe transition. Farther upward, a positive δ13Ccarb shift is present in the Middle and Upper Yanjiahe Formation, characterized by an upward δ13Ccarb increase to 4.05‰ (Fig. 3).

  • The primary δ13Ccarb values of carbonate are susceptible to post-depositional alteration. Because oxygen isotopes are more sensitive to diagenesis and diagenetic fluids generally have low oxygen isotope values, extremely low δ18Ocarb (< -10‰) values have been interpreted to reflect significant diagenetic alteration (Kaufman and Knoll, 1995). Furthermore, post-depositional alteration commonly produces decreases in both carbon and oxygen isotopes, leading to a positive covariation between δ13Ccarb and δ18Ocarb values (Li C et al., 2017; Li D et al., 2009). In our study, the samples from the Sixi Section exhibit weak negative correlation; all the δ18Ocarb values from the Sixi Section are greater than -10‰ (Fig. 7b). This relationship thus does not support significant diagenetic alteration. At the Zhoujiaao Section, samples show a weak positive correlations between δ13Ccarb and δ18Ocarb values (R2=0.15) (Fig. 7a). This weak positive correlation may possibly reflect some degree of diagenetic alteration. However, the δ18Ocarb values are no less than -7‰ and the δ13Ccarb profile of the Baimatuo Member is consistent with those from other sections across the Three Gorges area (see later discussion). On the other hand, a previous study showed a low Mn/Sr ratio (< 10) for the Baimatuo Member at the Sandouping-Yanjiahe Section (Wang D et al., 2012), indicating that δ13Ccarb was not significantly affected by diagenesis. Thus, the δ13Ccarb profile of the Zhoujiaao Section is considered to reflect the secular δ13C variations.

    Figure 7.  Crossplots of the C-O isotope from (a) Zhoujiaao Section and (b) Sixi Section.

  • Carbon isotope chemostratigraphy is a powerful method for subdivision and correlation of the E-C strata (Zhu et al., 2019, 2013, 2007, 2006; Xiao et al., 2016; Zhou and Xiao, 2007). Based on numerous carbon isotope patterns across the Yangtze Block (Zhu et al., 2019, 2013, 2007, 2006; Jiang et al., 2012, 2007; Zhou and Xiao, 2007), a composite δ13Ccarb curve for the E-C succession was constructed (Zhou and Xiao, 2007), consisting of (1) a stable δ13Ccarb pattern of 2‰-3‰ (Ediacaran intermediate values, EI) in the Upper Dengying Formation, (2) a dramatic negative δ13Ccarb shift (BACE or EN4) across the Dengying-Yanjiahe (or its correlatives) boundary, and (3) a large positive excursion (ZHUCE) in the Upper Yanjiahe (or its correlatives). Notably, the negative shift from the EI in the uppermost Dengying Formation to EN4 in the lowermost Yanjiahe Formation (or its correlatives) is associated with the extinction of the Ediacaran biota and the appearance of SSFs (Zhou et al., 2019). Thus, the EN4 is widely accepted as chemostratigraphic marker for E-C boundary in South China (Zhou et al., 2019; Li D et al., 2013; Jiang et al., 2012), which is also the case in other blocks (e.g., Oman) (Amthor et al., 2003).

    In our study, the Zhoujiaao Section and the lower part of the Sixi Section (0-8 m) exhibiting stable δ13Ccarb values of approximately 2‰-3‰ can be correlated with the EI (Fig. 8). Above the stable δ13Ccarb plateau at the Sixi Section, the negative excursion of δ13Ccarb from approximately 2‰ in the uppermost Dengying Formation to negative values in the lower part of the Yanjiahe Formation should be correlated to EN4 (BACE). In the Mid-Upper Yanjiahe Formation, the large positive carbon isotope excursion with a magnitude of 4‰ should be correlated to ZHUCE (Fig. 8). In summary, the δ13Ccarb profiles of the E-C boundary strata at the Zhoujiaao and Sixi sections correspond well with those in other sections around South China (Fig. 8). On the other hand, the occurrence of SSFs slightly above the negative excursion further confirms this correlation scheme and supports the interpretation that this negative excursion is approximately equivalent to the E-C boundary at Three Gorges (Guo et al., 2014).

    Figure 8.  Integrated chronostratigraphic correlations of the Zhoujiaao Section, Sixi Section with other representative Upper Ediacaran successions worldwide. Data sources, Three Gorges: δ13Ccarb from Wang et al. (2014) and Jiang et al. (2012), biostratigraphic data from Chen et al. (2014) and Jiang et al. (2007). Eastern Yunnan: δ13Ccarb from Li D et al. (2013) and Zhu et al. (2007), biostratigraphic data from Steiner et al. (2007), radiometric ages from Zhu et al. (2009) and Jenkins et al. (2002). Oman: δ13Ccarb and biostratigraphic data from Amthor et al. (2003), radiometric age from Bowring et al. (2007). The thicknesses of lithostratigraphic units in Oman are not to scale. DLT. Donglongtan Member; JC. Jiucheng Member; SYT. Shiyantou Formation.

    In this study, the tuffaceous layer from the Baimatuo Member at the Zhoujiaao Section is dated at 543.4±3.5 Ma. This new chronostratigraphic anchor point is present in the EI and reconciles with a SIMS U-Pb age of 546.3±2.7 Ma in the lower part of the EI at the Yinchangpo Section, eastern Yunnan (Yang et al., 2017). Although the Upper Baimatuo Member is missing at the Zhoujiaao Section, the tuffaceous layer is only approximately 5 m lower than the peritidal deposition, which is the characteristic of the Upper Baimatuo Member as suggested by the Sixi Section. Therefore the tuffaceous layer is not far below the upper boundary of the Dengying Formation and consequently suggests that the onset of EN4 above EI should slightly postdate 543.4±3.5 Ma, which is in accord with the tuffaceous layer BB-5 of 541.00±0.13 Ma in the zero crossing between a stable δ13Ccarb plateau (corresponding to EI) and a large negative δ13Ccarb excursion (corresponding to EN4) from Oman (Bowring et al., 2007; Amthor et al., 2003). Recent ages from tuffaceous beds in the Spitskopf Member and the Nomtsas Formation in Namibia suggest that the E-C boundary is close to 538.8 Ma (Linnemann et al., 2019). However, in Namibia, the globally distributed BACE is absent, and the E-C boundary is determined by the first occurrence of Cambrian-type trace fossils (Streptichnus narbonnei and Treptichnus cf. pedum) (Linnemann et al., 2019), which is considered to be diachronous between paleocontinents (Zhu et al., 2019). Thus whether this revised age of 538.8 Ma is accepted as the age of E-C boundary remains uncertain, and we still adopt the age of 541.00±0.13 Ma suggested by the International Commission on Stratigraphy. Therefore, the E-C boundary in South China should also be placed at the zero crossing of the EN4 (BACE), which is located approximately at the Dengying-Yanjiahe (or its correlatives) boundary (Fig. 8). Farther upwards, the E-C succession in eastern Yunnan is constrained by an age of 535.2±1.7 Ma obtained from a tuffaceous layer at the Meishucun Section (Zhu et al., 2009). This tuffaceous layer, occurring in the Bed 5 of the Middle Zhongyicun Member (corresponding to the Lower-Middle Yanjiahe Formation) and approximately 10 m above the first appearance datum of SSFs (near the nadir of EN4), is also consistent with our placement of the E-C boundary of the Yangtze Platform at the zero crossing of the EN4 (Dengying-Yanjiahe boundary).

  • The Ediacara biota can be grouped into three stages: the Avalon assemblage (ca. 575-560 Ma), the White Sea assemblage (ca. 560-550 Ma) and the Nama assemblage (ca. 550-541 Ma) (Narbonne, 2005; Grotzinger et al., 1995). The Nama assemblage, known in the Nama Group of Namibia, records the last evolutionary stage of the Ediacara biota (Xiao and Laflamme, 2009). Among fossils of the Nama assemblage, biomineralizing fossils including Cloudina, Sinotubulites, and Namacalathus have been reported from many terminal Ediacaran strata around the world, indicating that they can be used as index fossils for global correlations (Zhou et al., 2019; Cai et al., 2015; Hua et al., 2005). Moreover, the last appearance of the Ediacaran index fossils (Cloudina, Sinotubulites, Namacalathus) was traditionally used as a marker for the E-C boundary (ca. 541 Ma) (Amthor et al., 2003). In South China, the Deng-ying Formation contains the Shibantan biota in the Three Gorges area and the Gaojiashan biota in Southern Shaanxi (Cai et al., 2014; Hua et al., 2007). Fossil assemblages, affinity and geochrological data indicate that these two biotas can be correlated with the Nama assemblage (Yang et al., 2017; Chen et al., 2014). However, the new age in this study provides the minimum age of the last appearance of the Shibantan biota, indicating that the last appearance of the Ediacaran fauna in the Three Gorges area was earlier than 543 Ma. On the other hand, recent paleontological data from Southern Shaanxi (China), Maly Karatau (Kazakhstan), and the eastern Siberian Platform (Russia) show that the Ediacaran index fossils overlap with the SSFs in the E-C boundary successions (Cai et al., 2019; Zhu et al., 2017; Yang et al., 2016). For these reasons, the terminal Ediacaran index fossils (e.g., Cloudina, Sinotubulites) are less reliable than carbon isotope stratigraphy for further subdivision and correlation of the uppermost Ediacaran.

  • Detailed analyses of U-Pb geochronology and carbon isotope chemostratigraphy at Zhoujiaao and Sixi sections in the Three Gorges area lead us to the following conclusions. (1) Zircon crystals from a tuffaceous bed in the Baimatuo Member of the Dengying Formation in northwestern Yichang, western Hubei Province, is dated at 543.4±3.5 Ma, which is the first U-Pb age for the Dengying Formation of the Three Gorges area. (2) Carbon isotope profiles at the Zhoujiaao and Sixi sections can be correlated with those from other sections across the Yangtze Block and provide a framework for discussion of the new age. (3) This new U-Pb age provides a robust geochronological constraint on the uppermost Ediacaran strata in the Three Gorges area. Further integrating this new age with the carbon isotopes at Zhoujiaao and Sixi sections and previous biostratigraphic and chemostratigraphic data indicates that the E-C boundary is placed at the Dengying-Yanjiahe boundary. (4) The disappearance of the Shibantan biota occurred earlier than 543 Ma, suggesting that the extinction of Ediacaran index fossils was diachronous between continents and consequently not a reliable marker for the uppermost Ediacaran.

  • We thank Guoqiang Tang, Xin Liao and Liyu Zhang for their guide in SIMS U-Pb geochronological analysis, and Linlin Cui for help in carbon isotope analysis. This work is funded by the National Natural Science Foundation of China (Nos. 41472089, 91755210). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1233-x.

    Electronic Supplementary Materials: Supplementary materials (Tables S1, S2) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1233-x.

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