SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Successio
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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 Ediacaran-Cambrian 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 Dengying Formation (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 Yanjiahe Formation (Figs. 1b, 1c). The Dengying Formation, Block as a unit during the Early Neoproterozoic (Charvet, 2013). overlying the Doushantuo Formation, 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 limestone 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).