Restudy of conodont biostratigraphy of the Permian–Triassic boundary section in Zhongzhai, southwestern Guizhou Province, South China
Graphical abstract
Introduction
The end-Permian mass extinction, the most severe of its kind in Earth history (Sepkoski, 1981, Erwin, 1994), has been studied for the last several decades. However, causes of this mass extinction remain unclear. In part, the challenge relates to the uncertainty surrounding the precise determination of the Permian−Triassic boundary at a specific location or section.
Globally, the Permian–Triassic boundary (PTB) has been determined by the First Appearance Datum (FAD) of the conodont species Hindeodus parvus at the Global Stratotype Section and Point (GSSP) of the Meishan section in Zhejiang Province of South China (Yin et al., 2001), where recent geochronology studies of interdispersed ash beds have also provided several firm radiometric age anchor points (Mundil et al., 2004, Shen et al., 2011). Actually, in South China, there have been numerous excellent marine Permian–Triassic boundary sections reported in the past three decades (Zhang, 1987, Dai and Zhang, 1989, Tian, 1993, Zhu et al., 1994, Shen et al., 1995, Zhang et al., 1995, Zhang et al., 2009, Yang et al., 1999, Wang and Xia, 2004, Ji et al., 2007, Jiang et al., 2007, Jiang et al., 2011, Metcalfe and Nicoll, 2007, Chen et al., 2008, Chen et al., 2009), but most of these previous studies have been focused on shallow-water carbonate platform sections, with few concerning clastic-rock facies from continental shelf settings. This is despite the significance of clastic-shelf PTB sections in serving as an important bridge in correlating marine and non-marine PTB sequences (Peng et al., 2005). In this regard, the Zhongzhai section is perhaps unique as it is the only known PTB section in South China to date representing the clastic-shelf rock facies. As such, this section has attracted much attention in a number of recent publications (e.g., Metcalfe and Nicoll, 2007, Shen et al., 2011, Lei et al., 2012).
After a brief note published as an abstract by Nicoll and Metcalfe, 2005, Metcalfe and Nicoll, 2007 formally reported, for the first time, Hindeodus parvus from Bed 30 of the Zhongzhai section, which enabled them to place the PTB at the base of this bed. However, our recent systematic bed-by-bed resampling of the entire section has discovered that Hindeodus parvus in fact first appears at Bed 28a, 18 cm lower than the first local occurrence (FLO) of this species previously reported by Metcalfe and Nicoll. This discovery is important as it would suggest, at first glance, that either the PTB needs to be lowered to the base of Bed 28a for the Zhongzhai section, or the first local occurrence of Hindeodus parvus is actually lower than the PTB at this section if the previously accepted PTB is kept unchanged for the Zhongzhai section. As discussed further below, the second scenario is strongly favored here as the PTB of the Zhongzhai section has been well constrained and calibrated by additional evidences from both geochronology and δ13C isotope chemostratigraphy. Consequently, a significant implication would become apparent from the present study: if a broad global synchronicity can be assumed for the PTB, the first local occurrences of Hindeodus parvus would have to be diachronous with respect to the PTB at its GSSP section in Meishan and to the first local occurrence of this species at this section. Further, it also implies that the lower stratigraphic range of H. parvus should now be extended to latest Permian.
Section snippets
Location and geological setting
Geographically, the Zhongzhai PTB section (26.1529°N and 105.2865°E) is situated about 1 km northeast of Zhongzhai (a local township), Liuzhi County, southwestern Guizhou Province (Fig. 1, Fig. 2). Paleogeographically, the Zhongzhai section was located in the western part of the Yangtze Basin in South China during the Permian. From Late Permian to Early Triassic, the basin contained a variety of depositional settings, consisting of shallow-water clastic-shelf, shallow-water carbonate platforms,
Material and methods
Eighty-two samples (3–5 kg per sample) were systematically collected from the claystone, calcareous mudstone, siltstone and limestone beds of the upper part of the Longtan Formation and the part of the lower Yelang Formation in the Zhongzhai section. All samples were corroded by 10% solution of acetic acid and then examined under a binocular microscope. Importantly, the samples of Beds 26, 28a, 28b, 29 and 30, which are the key layers of the Permian–Triassic boundary interval, were repeatedly
The conodont fauna
This Zhongzhai conodont fauna was first reported by Nicoll and Metcalfe (2005) in a very short note, in which they briefly described the lithology of Permian–Triassic interval strata from Beds 26–31, and placed the PTB at the bottom of Bed 30 based on the occurrence of Hindeodus parvus. Subsequently, Metcalfe and Nicoll (2007) formally reported and figured this fauna from the Zhongzhai section, which included Hindeodus parvus, H. eurypyge, H. changxingensis, Clarkina tulongensis, C. meishanensis
Discussion
At first glance, the newly discovered first local occurrence (FLO) of H. parvus may suggest that the PTB at the Zhongzhai section could be placed at the base of Bed 28a, instead of at the base of Bed 30 as was previously suggested by Metcalfe and Nicoll (2007). However, this suggestion is not tenable if multiple other lines of chronostratigraphic and biostratigraphic evidence are taken into account. First, when Nicoll and Metcalfe (2005; see also Metcalfe and Nicoll, 2007) reported H. parvus
Conclusions
The discovery of H. parvus from Bed 28a of the Zhongzhai section has yielded three significant stratigraphic implications. First, the first local occurrence (FLO) of H. parvus at this section is 18 cm lower than the PTB currently accepted. Second, this study has highlighted the inadequateness in using the first occurrence of H. parvus as an exclusive chronostratigraphic marker for defining the base of the Triassic in marine settings. Instead, other auxiliary biostratigraphic indicators as well
Acknowledgments
We are very grateful to Prof. Shu-zhong Shen, Dr. Hai-shui Jiang and Dr. Robert S. Nicoll for discussion on conodont taxonomy. We thank Chengchen Du, Qiang Zhang and Wei Zhang for their help in field sampling, Fei Teng for photography. This study was supported by Natural Science Foundation of China (Grant Nos. 41372030, 40872008, 41002012/D0202), the Ministry of Education of China (B08030 of 111 Project, NCET-10-0712), the Foundation of Geological Survey of China (12120113012500) and in part a
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2020, Earth-Science ReviewsCitation Excerpt :Thus, the local FO of H. parvus in any other section should be later or coeval with the Meishan D section (Fig. 1). However, several studies from South China indicated that the FOs of H. parvus in Wuzhuan, Zhongzhai and Daijiagou sections are all earlier than the FAD of H. parvus in Meishan (Yuan et al., 2014; Zhang et al., 2014; Brosse et al., 2016) while it has been accepted that the FO of H. parvus in the Shangsi section is later (Jiang et al., 2011; Yuan et al., 2018). Relative to deep water sections, the record from shallow water sections is less complete, which can also be shown by the result of this study (Figs. 12 and 13).