Journal of Earth Science, Vol. 31, No. 1, p. 69–78, February 2020 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-019-1233-x

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

Taiyu Huang 1, 2, 3, Daizhao Chen *1, 2, 3, Yi Ding1, 2, 3, 4, Xiqiang Zhou1, 2, 3, Gongjing Zhang1, 2, 3

1. Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

2. University of Chinese Academy of Sciences, Beijing 100049, China

3. Institution of Earth Science, Chinese Academy of Sciences, Beijing 100029, China

4. Post-Doctoral Research Station of Geological Resource and Geological Engineering, Chengdu University of Technology, Chengdu 610059, China Taiyu Huang: https://orcid.org/0000-0002-1235-6667; Daizhao Chen: https://orcid.org/0000-0002-1131-3630

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 13 (i.e., the Baimatuo Member) which is characterized by a stable δ Ccarb plateau and the 13 beginning of a negative δ Ccarb 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. KEY WORDS: U-Pb dating, carbon isotope chemostratigraphy, Dengying Formation, Ediacaran-Cambrian boundary, Three Gorges area, Shibantan biota, geochemistry.

0 INTRODUCTION However, the E–C strata are not well constrained by radiometric The Ediacaran–Cambrian (E–C) transitional period was a ages in South China, although previous biostratigraphic and crucial time interval in the Earth history with remarkable bio- chemostratigraphic studies have proposed feasible stratigraphic logical evolution, characterized by the decline of the Ediacaran correlation schemes (Jiang et al., 2012; Wang D et al., 2012; fauna, the subsequent remarkable metazoans radiation well Ishikawa et al., 2008; Zhou and Xiao, 2007; Zhu et al., 2007). known as the “Cambrian Explosion” (Maloof et al., 2010; Mar- Among these ages, a series of radiometric ages from ca. 538 to shall, 2006), and profound oceanic changes, including increasing 522 Ma have been reported in the Zhujiaqing Formation (and oxygen levels, large carbonate carbon isotope excursions and its coeval strata), generally constraining the Terreneuvian Se- sulfur isotope variations (Lyons et al., 2014; Jiang et al., 2012; ries and the minimum age for the E-C boundary in South China Wang et al., 2012a, b; Goldberg et al., 2007). In South China, (Lan et al., 2017; Chen et al., 2015; Okada et al., 2014; Wang the well-developed E–C succession opens a window to explore X Q et al., 2012; Zhu et al., 2009; Compston et al., 2008; Jen- the coevolution of the biosphere and geosphere. In the last dec- kins et al., 2002). In the shelf margin (Ganziping) and basin ade, numerous paleontological (Chen et al., 2014, 2013; Duda et settings (Bahuang), two U-Pb zircon ages of 542.6±3.7 and al., 2014; Guo et al., 2014; Steiner et al., 2007) and geochemical 542.1±5.0 Ma from the Liuchapo Formation provide direct ages studies (Ding et al., 2018; Wang et al., 2014; Ishikawa et al., for the E-C boundary in South China (Chen et al., 2015). In 2013, 2008; Li D et al., 2013, 2009; Jiang et al., 2012, 2007) comparison, the shallow-water Dengying Formation, generally have focused on the South China E–C transitional strata. considered as the terminal Ediacaran strata, has a widely ac- cepted lower boundary age of approximately 551 Ma (Xiao et *Corresponding author: [email protected] al., 2016; Condon, 2005) but no direct age constraint for its © China University of Geosciences (Wuhan) and Springer-Verlag upper boundary. On the other hand, the Upper Dengying For- GmbH Germany, Part of Springer Nature 2020 mation generally underwent severe karstification and is uncon- formably overlain by the lowermost Cambrian successions Manuscript received February 11, 2019. (Wang et al., 2012a). In this case, uncertainties remain in the Manuscript accepted July 1, 2019. stratigraphic correlation of the uppermost Ediacaran to the

Huang, T. Y., Chen, D. Z., Ding, Y., et al., 2020. 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, 31(1): 69–78. https://doi.org/10.1007/s12583-019-1233-x. http://en.earth-science.net 70 Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou and Gongjing Zhang lowermost Cambrian succession of South China. Afterwards, coincident with the breakup of Rodinia, the coupled To provide further geological constraints on the E–C suc- South China Block was separated from other blocks, leading to cession, a new second ion mass spectrometry (SIMS) U-Pb age the development of rift basins (Wang and Li, 2003). After the is reported from a tuffaceous layer in the Baimatuo Member of Cryogenian, the Yangtze Block had evolved into a drift stage the Upper Dengying Formation at the Zhoujiaao Section and car- based on its long time sequence stratigraphy (Jiang et al., 2011; bonate C-O isotopes at both the Zhoujiaao and Sixi sections, Wang and Li, 2003), although recent provenance studies indicate Three Gorges area, western Hubei Province. The Three Gorges the formation of a foreland basin in the southeast Yangtze Block area possesses a relatively complete E-C boundary succession (Yao et al., 2015). During the E–C transitional period, the Yang- with credible biostratigraphic and chemostratigraphic markers tze Block experienced block-tilting and intense extension (Liu et (Guo et al., 2014; Jiang et al., 2012). In combination with previ- al., 2013; Chen et al., 2009). As a result, carbonate deposition ous radiometric ages and biological and geochemical data, this developed on paleo-uplands mostly in the Mid–Upper Yangtze paper provides the first U-Pb age for the Upper Dengying For- Block and was surrounded by chert deposits in the deep slope mation to constrain the E–C transitional strata in South China. and basin environments (Fig. 1a) (Chen et al., 2009). In the Three Gorges area, the E–C succession around the 1 GEOLOGICAL SETTINGS AND LITHOSTRATIGRAPHY Huangling anticline includes the Dengying Formation and the The Yangtze Block was amalgamated with the Cathaysian (Figs. 1b, 1c). The Dengying Formation, Block as a unit during the Early Neoproterozoic (Charvet, 2013). overlying the , is subdivided into the

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.

SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran–Cambrian Boundary 71

Hamajing Member, the Shibantan Member and the Baimatuo Upper Shibantan Member and Lower–Middle Baimatuo Member Member in ascending order (Fig. 1b) (Zhu et al., 2003). The (Fig. 2). The upper part of the Shibantan Member is composed of Hamajing Member comprises mainly massive and medium to dark gray with abundant banded clay laminae. The over- thick intraclastic/ooidal dolograinstone (Chen et al., 2014; Duda lying Lower Baimatuo Member is dominated by a 45 m medium et al., 2014). The Shibantan Member is characterized by dark to thick-bedded peloidal dolograinstone with meter-scale interca- gray, banded limestone with horizontal lamination and hum- lations of thin bedded dolomudstone (Fig. 2a). The tuff layer was mocky cross-bedding (Chen et al., 2014; Duda et al., 2014). The found at the top of this unit and collected for SIMS U-Pb analysis Baimatuo Member consists of light gray massive dolostone (Figs. 2c, 2d). The overlying succession is a 5 m-thick dolomud- (Duda et al., 2016; Zhou and Xiao, 2007; Zhu et al., 2007). Of stone. The upper part of the Zhoujiaao Section is dominated by these units, the Shibantan Member and the Lower Baimatuo coarse dololaminite manifested by dolomudstone/dolopackstone Member are rich in fossils, including macro-algal fossils Vendo- couplets (Fig. 2b), which is indicative of a peritidal environment. taenia (Weber et al., 2007; Sun, 1986), the Ediacaran-type fos- The Sixi Section, located in West Yichang, western Hubei sils Yangtziramulus and Paracharnia (Shen et al., 2009; Xiao et Province (Fig. 1b), comprises the Upper Baimatuo Member al., 2005; Sun, 1986), trace fossils (Meyer et al., 2014; Chen et (Dengying Formation) and the Yanjiahe Formation (Fig. 3). The al., 2013; Weber et al., 2007) and tubular fossils Sinotubulites Upper Baimatuo Member, similar to that at the Zhoujiaao Sec- (Xiao et al., 2005; Ding et al., 1993). This fossil association is tion, is composed of the characteristic coarse dololaminite show- named the Shibantan (Xilingxia) biota (Zhu, 2010; Ding et al., ing dolomicrite/peloid-concentrated couplets deposited in a peri- 1992a). The Yanjiahe Formation, which underlies the Shuijing- tidal environment (Figs. 4a, 4e). The Yanjiahe Formation con- tuo Formation, is characterized by cherty and/or phosphatic formably overlies the Dengying Formation without subaerial ex- dolomudstone in the lower part followed by dark gray shaly posure features (Fig. 4b). The Lower Yanjiahe Formation is limestone intercalated with thin black shales in the middle and composed of dolomudstone with a basal thin phosphatic dolo- upper part (Jiang et al., 2012). Micrhystridium-like acritarchs stone layer (Figs. 4c, 4f). This unit is followed by a thin siliceous and three small shelly fossil (SSF) assemblages are recognized layer and a 14 m-thick dark gray shaly limestone in the Yanjiahe Formation (Fig. 1c) (Guo et al., 2014; Ding et upward. The uppermost Yanjiahe Formation is 0.25 m of sili- al., 1992b). The Zhoujiaao Section, located at a quarry in North- ceous phosphorite, which is overlain by gray calcareous shales west Yichang, western Hubei Province (Fig. 1b), consists of the of the lowermost Shuijingtuo Formation (Fig. 4d).

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).

72 Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou and Gongjing Zhang

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 per- formed using the Isoplot 4.15 program. Uncertainty in the indi- vidual 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 zir- cons. 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 anal- ysis. Sample powders were obtained using a dental drill to re- move clay-rich laminae, post-depositional veins and diagenetic fabrics. Aliquots (~250 μg) were reacted with phosphoric acid for 200 s at 70 °C. 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 Figure 3. Stratigraphic columns and carbon-oxygen isotope profiles of the standard NBS-19 after every six sample measurements (Cui and Zhoujiaao and Sixi sections. SBT. Shibantan Member. Wang, 2014). Carbon and oxygen isotope data are displayed rel- ative to the standard Vienna Peedee Belemnite (VPDB) 2 METHOD with analytical precisions of 0.2‰ and 0.15‰, respectively. Samples (BMT) were taken from tuff layers in the Baimatuo Member at the Zhoujiaao Section. Zircon crystals 3 RESULTS were separated from tuffaceous samples by standard magnetic Most zircons extracted from tuff sample BMT are euhedral and gravitational separation methods. Zircon grains, together and subeuhedral in morphology and 100–200 μm in length. with zircon standards Plešovice, Qinghu and 91500, were Some of them have clear oscillatory zoning in CL images (Fig. mounted in an epoxy resin which was then polished to expose 5). U contents range from 84 ppm to 3 876 ppm (mostly within the crystals for measurement. Prior to SIMS analysis, all zircon 84 ppm–913 ppm). Th contents range from 83 ppm to 1 617 ppm grains were imaged and examined with reflected and transmitted (mostly within 84 ppm–856 ppm). Th/U values are between light photomicrographs and cathodoluminescence (CL) images 0.208 and 2.524 (mostly within 0.208–1.750). A total of 29 anal- for external and internal structures (Fig. 5). Euhedral to subhe- yses were performed on 29 zircons and their isotope results are dral zircons which have clear oscillatory rims and are free of exhibited in Table S1. Nine sets of data are rejected because of cracks and inclusions were selected for age determination. discordance or high common lead, and the remaining 20 analyses The absolute abundances and isotope compositions of U, of zircon provide a 238U-206Pb weighted average of 543.4±3.5 Pb and Th were analyzed using SIMS with a CAMECA 1 280 Ma with MSWD=1.2 (Fig. 6). 13 18 instrument in the Institute of Geology and Geophysics, Chinese The results of δ Ccarb-δ Ocarb are shown in Fig. 3 and Table 13 Academy of Sciences (IGGCAS). The size of the primary oxy- S2. In the Zhoujiaao Section, the δ Ccarb values are relatively 2- gen ion (O ) beam spot was approximately 20–30 μm, and the stable between 2.48‰ and 3.55‰, with corresponding δ18O val- + extraction potential of the secondary ion was 10 kV. Pb peaks ues ranging from -6.97‰ to -4.99‰. In the Sixi Section, the 13 18 were separated from isobaric interferences using an energy win- δ Ccarb values range from -0.79‰ to 4.05‰ and the δ Ocarb val- 13 dow of 60 eV and a mass resolution of approximately 5 400 (at ues vary between -8.84‰ and -5.39‰. A negative δ Ccarb ex- 13 10% peak height) in the secondary ion beam optics. The intensi- cursion with δ Ccarb values changing from stable values of ap- ties of the secondary ion beam were measured by the peak jump- proximately 2‰ to a nadir of -0.79‰ occurs in the Dengying- 13 ing method in ion-counting mode using a single electron multi- Yanjiahe transition. Farther upward, a positive δ Ccarb shift is plier. The error of the U/Pb calibration curve was fitted relative present in the Middle and Upper Yanjiahe Formation, character- 13 to the Plešovice standard zircons and then applied to the ized by an upward δ Ccarb increase to 4.05‰ (Fig. 3).

SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran–Cambrian Boundary 73

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.

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.

74 Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou and Gongjing Zhang

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.

13 4 DISCUSSION (2) a dramatic negative δ Ccarb shift (BACE or EN4) across the 4.1 Evaluation of Carbon Isotope Data Dengying-Yanjiahe (or its correlatives) boundary, and (3) a large 13 The primary δ Ccarb values of carbonate are susceptible to positive excursion (ZHUCE) in the Upper Yanjiahe (or its cor- post-depositional alteration. Because oxygen isotopes are more relatives). Notably, the negative shift from the EI in the upper- sensitive to diagenesis and diagenetic fluids generally have low most Dengying Formation to EN4 in the lowermost Yanjiahe 18 oxygen isotope values, extremely low δ Ocarb (< -10‰) values Formation (or its correlatives) is associated with the extinction have been interpreted to reflect significant diagenetic alteration of the and the appearance of SSFs (Zhou et al., (Kaufman and Knoll, 1995). Furthermore, post-depositional al- 2019). Thus, the EN4 is widely accepted as chemostratigraphic teration commonly produces decreases in both carbon and oxy- marker for E-C boundary in South China (Zhou et al., 2019; Li 13 gen isotopes, leading to a positive covariation between δ Ccarb D et al., 2013; Jiang et al., 2012), which is also the case in other 18 and δ Ocarb values (Li C et al., 2017; Li D et al., 2009). In our blocks (e.g., Oman) (Amthor et al., 2003). study, the samples from the Sixi Section exhibit weak negative 18 correlation; all the δ Ocarb values from the Sixi Section are greater than -10‰ (Fig. 7b). This relationship thus does not sup- port significant diagenetic alteration. At the Zhoujiaao Section, 13 samples show a weak positive correlations between δ Ccarb and 18 2 δ Ocarb values (R =0.15) (Fig. 7a). This weak positive correla- tion may possibly reflect some degree of diagenetic alteration. 18 However, the δ Ocarb values are no less than -7‰ and the 13 δ Ccarb profile of the Baimatuo Member is consistent with those from other sections across the Three Gorges area (see later dis- cussion). On the other hand, a previous study showed a low Mn/Sr ratio (<10) for the Baimatuo Member at the Sandouping- 13 Yanjiahe Section (Wang D et al., 2012), indicating that δ Ccarb 13 was not significantly affected by diagenesis. Thus, the δ Ccarb profile of the Zhoujiaao Section is considered to reflect the sec- ular δ13C variations.

4.2 Constraints on the E-C Boundary 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, 13 2007; Zhou and Xiao, 2007), a composite δ Ccarb curve for the E–C succession was constructed (Zhou and Xiao, 2007), con- 13 sisting of (1) a stable δ Ccarb pattern of 2‰–3‰ (Ediacaran Figure 7. Crossplots of the C-O isotope from (a) Zhoujiaao Section and (b) intermediate values, EI) in the Upper Dengying Formation, Sixi Section.

SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran–Cambrian Boundary 75

In our study, the Zhoujiaao Section and the lower part of the 543.4±3.5 Ma, which is in accord with the tuffaceous layer BB- 13 Sixi Section (0–8 m) exhibiting stable δ Ccarb values of approxi- 5 of 541.00±0.13 Ma in the zero crossing between a stable 13 mately 2‰–3‰ can be correlated with the EI (Fig. 8). Above the δ Ccarb plateau (corresponding to EI) and a large negative 13 13 stable δ Ccarb plateau at the Sixi Section, the negative excursion δ Ccarb excursion (corresponding to EN4) from Oman (Bowring 13 of δ Ccarb from approximately 2‰ in the uppermost Dengying et al., 2007; Amthor et al., 2003). Recent ages from tuffaceous Formation to negative values in the lower part of the Yanjiahe beds in the Spitskopf Member and the Nomtsas Formation in Formation should be correlated to EN4 (BACE). In the Mid– Namibia suggest that the E-C boundary is close to 538.8 Ma Upper Yanjiahe Formation, the large positive carbon isotope ex- (Linnemann et al., 2019). However, in Namibia, the globally dis- cursion with a magnitude of 4‰ should be correlated to ZHUCE tributed BACE is absent, and the E-C boundary is determined by 13 (Fig. 8). In summary, the δ Ccarb profiles of the E-C boundary the first occurrence of Cambrian-type trace fossils (Streptichnus strata at the Zhoujiaao and Sixi sections correspond well with narbonnei and Treptichnus cf. pedum) (Linnemann et al., 2019), those in other sections around South China (Fig. 8). On the other which is considered to be diachronous between paleocontinents hand, the occurrence of SSFs slightly above the negative excur- (Zhu et al., 2019). Thus whether this revised age of 538.8 Ma is sion further confirms this correlation scheme and supports the in- accepted as the age of E-C boundary remains uncertain, and we terpretation that this negative excursion is approximately equiva- still adopt the age of 541.00±0.13 Ma suggested by the Interna- lent to the E-C boundary at Three Gorges (Guo et al., 2014). tional Commission on Stratigraphy. Therefore, the E-C bound- In this study, the tuffaceous layer from the Baimatuo Mem- ary in South China should also be placed at the zero crossing of ber at the Zhoujiaao Section is dated at 543.4±3.5 Ma. This new the EN4 (BACE), which is located approximately at the chronostratigraphic anchor point is present in the EI and recon- Dengying-Yanjiahe (or its correlatives) boundary (Fig. 8). Far- ciles with a SIMS U-Pb age of 546.3±2.7 Ma in the lower part ther upwards, the E-C succession in eastern Yunnan is con- of the EI at the Yinchangpo Section, eastern Yunnan (Yang et strained by an age of 535.2±1.7 Ma obtained from a tuffaceous al., 2017). Although the Upper Baimatuo Member is missing at layer at the Meishucun Section (Zhu et al., 2009). This tuffa- the Zhoujiaao Section, the tuffaceous layer is only approxi- ceous layer, occurring in the Bed 5 of the Middle Zhongyicun mately 5 m lower than the peritidal deposition, which is the char- Member (corresponding to the Lower–Middle Yanjiahe For- acteristic of the Upper Baimatuo Member as suggested by the mation) and approximately 10 m above the first appearance da- Sixi Section. Therefore the tuffaceous layer is not far below the tum of SSFs (near the nadir of EN4), is also consistent with our upper boundary of the Dengying Formation and consequently placement of the E-C boundary of the Yangtze Platform at the suggests that the onset of EN4 above EI should slightly postdate zero crossing of the EN4 (Dengying-Yanjiahe boundary).

Figure 8. Integrated chronostratigraphic correlations of the Zhoujiaao Section, Sixi Section with other representative Upper Ediacaran successions worldwide. 13 Data sources, Three Gorges: δ Ccarb from Wang et al. (2014) and Jiang et al. (2012), biostratigraphic data from Chen et al. (2014) and Jiang et al. (2007). Eastern 13 Yunnan: δ Ccarb 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 13 et al. (2002). Oman: δ Ccarb 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.

76 Taiyu Huang, Daizhao Chen, Yi Ding, Xiqiang Zhou and Gongjing Zhang

4.3 Constraints on the Biostratigraphic Correlation their guide in SIMS U-Pb geochronological analysis, and Linlin The Ediacara biota can be grouped into three stages: the Cui for help in carbon isotope analysis. This work is funded by Avalon assemblage (ca. 575–560 Ma), the White Sea assem- the National Natural Science Foundation of China (Nos. blage (ca. 560–550 Ma) and the Nama assemblage (ca. 550–541 41472089, 91755210). The final publication is available at Ma) (Narbonne, 2005; Grotzinger et al., 1995). The Nama as- Springer via https://doi.org/10.1007/s12583-019-1233-x. semblage, known in the Nama Group of Namibia, records the last evolutionary stage of the Ediacara biota (Xiao and Electronic Supplementary Materials: Supplementary materi- Laflamme, 2009). Among fossils of the Nama assemblage, bio- als (Tables S1, S2) are available in the online version of this ar- mineralizing fossils including Cloudina, Sinotubulites, and ticle at https://doi.org/10.1007/s12583-019-1233-x. 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Geochronologic ying Formation contains the Shibantan biota in the Three Gorges Constraints on the Chronostratigraphic Framework of the Neoprotero- area and the Gaojiashan biota in Southern Shaanxi (Cai et al., zoic Huqf Supergroup, Sultanate of Oman. American Journal of Science, 2014; Hua et al., 2007). Fossil assemblages, affinity and geo- 307(10): 1097–1145. https://doi.org/10.2475/10.2007.01 chrological data indicate that these two biotas can be correlated Cai, Y. P., Hua, H., Schiffbauer, J. D., et al., 2014. Tube Growth Patterns and with the Nama assemblage (Yang et al., 2017; Chen et al., 2014). Microbial Mat-Related Lifestyles in the Ediacaran Fossil Cloudina, However, the new age in this study provides the minimum age Gaojiashan Lagerstätte, South China. Gondwana Research, 25(3): of the last appearance of the Shibantan biota, indicating that the 1008–1018. https://doi.org/10.1016/j.gr.2012.12.027 last appearance of the Ediacaran fauna in the Three Gorges area Cai, Y. P., Xiao, S. H., Hua, H., et al., 2015. New Material of the Biominer- was earlier than 543 Ma. On the other hand, recent paleontolog- alizing Tubular Fossil Sinotubulites from the Late Ediacaran Dengying ical data from Southern Shaanxi (China), Maly Karatau (Ka- Formation, South China. Precambrian Research, 261: 12–24. zakhstan), and the eastern Siberian Platform (Russia) show that http://doi.org/10.1016/j.precamres.2015.02.002 the Ediacaran index fossils overlap with the SSFs in the E-C Cai, Y. P., Xiao, S. H., Li, G. X., et al., 2019. Diverse Biomineralizing Ani- boundary successions (Cai et al., 2019; Zhu et al., 2017; Yang et mals in the Terminal Ediacaran Period Herald the Cambrian Explosion. al., 2016). For these reasons, the terminal Ediacaran index fossils Geology, 47(4): 380–384. https://doi.org/10.1130/g45949.1 (e.g., Cloudina, Sinotubulites) are less reliable than carbon iso- Charvet, J., 2013. The Neoproterozoic–Early Paleozoic Tectonic Evolution tope stratigraphy for further subdivision and correlation of the of the South China Block: An Overview. Journal of Asian Earth Sci- uppermost Ediacaran. ences, 74: 198–209. https://doi.org/10.1016/j.jseaes.2013.02.015 Chen, D. Z., Wang, J. G., Qing, H. R., et al., 2009. Hydrothermal Venting 5 CONCLUSIONS Activities in the Early Cambrian, South China: Petrological, Geochron- Detailed analyses of U-Pb geochronology and carbon iso- ological and Stable Isotopic Constraints. Chemical Geology, 258(3/4): tope chemostratigraphy at Zhoujiaao and Sixi sections in the 168–181. https://doi.org/10.1016/j.chemgeo.2008.10.016 Three Gorges area lead us to the following conclusions. (1) Zir- Chen, D. Z., Zhou, X. Q., Fu, Y., et al., 2015. New U-Pb Zircon Ages of the con crystals from a tuffaceous bed in the Baimatuo Member of Ediacaran-Cambrian Boundary Strata in South China. Terra Nova, the Dengying Formation in northwestern Yichang, western Hu- 27(1): 62–68. http://doi.org/10.1111/ter.12134 bei Province, is dated at 543.4±3.5 Ma, which is the first U-Pb Chen, Z., Zhou, C. M., Meyer, M., et al., 2013. Reply to Comment on “Trace age for the Dengying Formation of the Three Gorges area. (2) Fossil Evidence for Ediacaran Bilaterian Animals with Complex Behav- Carbon isotope profiles at the Zhoujiaao and Sixi sections can be iors” [Precambrian Research. 224 (2013) 690–701]. Precambrian Re- correlated with those from other sections across the Yangtze search, 231: 386–387. https://doi.org/10.1016/j.precamres.2013.04.002 Block and provide a framework for discussion of the new age. Chen, Z., Zhou, C. M., Xiao, S. H., et al., 2014. New Ediacara Fossils Pre- (3) This new U-Pb age provides a robust geochronological con- served in Marine Limestone and Their Ecological Implications. Scien- straint on the uppermost Ediacaran strata in the Three Gorges tific Reports, 4(1): 4180–4190. https://doi.org/10.1038/srep04180 area. Further integrating this new age with the carbon isotopes at Compston, W., Zhang, Z., Cooper, J. A., et al., 2008. Further SHRIMP Geo- Zhoujiaao and Sixi sections and previous biostratigraphic and chronology on the Early Cambrian of South China. American Journal of chemostratigraphic data indicates that the E-C boundary is Science, 308(4): 399–420. https://doi.org/10.2475/04.2008.01 placed at the Dengying-Yanjiahe boundary. (4) The disappear- Condon, D., 2005. U-Pb Ages from the Neoproterozoic Doushantuo For- ance of the Shibantan biota occurred earlier than 543 Ma, sug- mation, China. Science, 308(5718): 95–98. https://doi.org/10.1126/sci- gesting that the extinction of Ediacaran index fossils was dia- ence.1107765 chronous between continents and consequently not a reliable Cui, L. L., Wang, X., 2014. Determination of Carbon and Oxygen Isotopes marker for the uppermost Ediacaran. of Geological Samples with a Complicated Matrix: Comparison of Dif- ferent Analytical Methods. Anal Methods, 6(22): 9173–9178. ACKNOWLEDGMENTS https://doi.org/10.1039/c4ay01717j We thank Guoqiang Tang, Xin Liao and Liyu Zhang for Ding, L. F., Li, Y., Chen, H. X., 1992a. Discovery of Micrhystridium

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