Restudy of conodont biostratigraphy of the Permian–Triassic boundary section in Zhongzhai, southwestern Guizhou Province, South China

Highlights

• New conodont material discovered and documented from the only known shallow-water clastic-shelf marine PTB in South China.

• Discovery of the FLO of Hindeodus parvus 18 cm lower than the PTB currently accepted for the Zhonghai section.

• FADs of Hindeodus parvus in South China are diachronous.

Abstract

New conodont samples have been systematically collected at high stratigraphic resolution from the upper part of the Longtan Formation through to the lower part of the Yelang Formation at the Zhongzhai section, southwestern Guizhou Province, South China, in an effort to verify the first local occurrence of Hindeodus parvus in relation to the Permian–Triassic boundary at this section. The resampled conodont fauna from the Permian–Triassic boundary interval comprises five identified species and two undetermined species in Hindeodus and Clarkina. Most importantly, the first local occurrence of Hindeodus parvus is found for the first time from the bottom of Bed 28a, 18 cm lower than the previously reported first local occurrence of this species at this section. Considering the previously accepted PTB at the Zhongzhai section, well calibrated by conodont biostratigraphy, geochronology and carbon isotope chemostratigraphy, this lower (earlier) occurrence of H. parvus suggests that this critical species could occur below the Permian–Triassic boundary. As such, this paper provides evidence that (1) the first local occurrences of H. parvus are diachronous in different sections with respect to the PTB defined by the First Appearance Datum (FAD) of this species at its GSSP section in Meishan, China and that (2) the lower stratigraphic range of H. parvus should now be extended to latest Permian.

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.