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Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 56 – 65 www.elsevier.com/locate/palaeo

Conodont biostratigraphic control on transitional marine to non-marine Permian–Triassic boundary sequences in Yunnan–Guizhou, China I. Metcalfe a,⁎, R.S. Nicoll b b

a Asia Centre, University of New England, Armidale NSW 2351, Australia Earth and Marine Sciences, Australian National University, Canberra ACT 0200, Australia

Accepted 30 November 2006

Abstract The recovery of conodonts associated with ash beds and magnetostratigraphy in the key Zhongzhai Section, near Langdai, Liuzhi, Guizhou Province, provides precise and definitive control on the position of the Permian–Triassic boundary in the transition from marine to non-marine facies of western Guizhou and eastern Yunnan Provinces of southwestern China. In the Zhongzhai Section the boundary interval consists of a lower limestone, 20 cm thick, that contains fragments of Hindeodus sp. and Clarkina sp. This is overlain by a 50 cm thick black shale bed containing an abundant brachiopod fauna, but only a single conodont fragment. This bed is in turn overlain by a 23 cm thick limestone that contains Clarkina meishanensis, Merrillina ultima, Hindeodus changxingensis, H. praeparvus and H. eurypyge. Directly over this limestone is a 5 cm thick ash bed followed by a 10 cm thick black shale, which is overlain by a second, upper, ash bed that is 3 cm thick. On top of the upper ash bed is a 20 cm thick silty limestone containing an abundant dwarf conodont fauna, dominated by Hindeodus, and containing H. parvus but also including Clarkina tulongensis. The Permian–Triassic boundary is placed at the level of the black shale located between the two ash beds. © 2007 Elsevier B.V. All rights reserved. Keywords: Permian–Triassic boundary; Conodonts; Marine to non-marine transition; Yunnan–Guizhou, China

1. Introduction A major ongoing issue in resolving current debates on and identifying the causative mechanism(s) for the Permian–Triassic mass extinction, and subsequent recovery of global ecosystems, is precise correlation of Permian–Triassic transitional sequences and associated events within and between marine and non-marine sequences. The formally defined Permian–Triassic boundary (Yin et al., 2001) is located in a marine sequence at ⁎ Corresponding author. E-mail address: [email protected] (I. Metcalfe). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.11.034

Meishan, Zhejiang Province, South China, where there is an excellent conodont biostratigraphic record (Zhang et al., 1995; Mei et al., 1998; Nicoll et al., 2002; Jiang et al., 2007) which provides the formal definition of the base of the Triassic at the first appearance of Hindeodus parvus (Kozur & Pjatakova). In western Guizhou and eastern Yunnan Provinces, South China, there are wellexposed, complete Permian–Triassic sedimentary rock sequences that represent a transition from fully marine in the east, through paralic to non-marine environments in the west (Peng et al., 2005). We here present new unequivocal biostratigraphic constraints on the placement of the P–T boundary in these important marine to

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non-marine transitional sequences which is linked to regionally persistent volcanic ash bed markers and magnetostratigraphy (Mundil et al., 2004). Our work in Guizhou and Yunnan forms part of a much larger umbrella international research program led by the senior author which has been conducting multidisciplinary studies of P–T sequences at key marine and non-marine localities throughout China (Fig. 1). The P–T sequences of South China, including those in Guizhou and Yunnan, contain multiple volcanic ash/clay beds. The volcanic clay beds in Zhejiang and Sichuan Provinces have now been dated by our research group and have provided a robust definitive age of 252.6 +/− 0.2 Ma for the P–T boundary main mass extinction through U–Pb zircon dating at the Meishan and Shangsi marine sections (Mundil et al., 2001; Mundil et al., 2004). In addition to undertaking biostratigraphic, chemostratigraphic and isotopic dating, our group has also undertaken magnetostratigraphic studies of P–T transitional sequences in China, including the Zhongzhai section. The placement of the biostratigraphic P–T

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boundary here presented, and its relationship to magnetostratigraphy and regionally persistent volcanic ash “event” marker beds provide an effective means to correlate the P–T boundary and mass extinction levels in these marine to non-marine sequences in South China. 2. General Permian–Triassic stratigraphy and palaeogeography of eastern Yunnan and western Guizhou provinces Late Permian–Early Triassic strata in eastern Yunnan and western Guizhou, SW China, represent a transition from terrestrial non-marine deposition (lacustrine-swamp facies) in the west on the margin of the “Sichuan–Yunnan old land” through coastal marsh-littoral facies further east to littoral and fully marine neritic facies in the east (Fig. 2). These sequences overly the end Guadalupian Emeishan basalts. Lithologies in the non-marine western sequences are dominated by sandstones and siltstones with coal. Transitional coastal–marginal marine sequences include

Fig. 1. Map showing the location of the Zhongzhai section and other Permian–Triassic boundary sections studied in China.

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Fig. 2. Above: Late Permian (Changhsingian) palaeogeography of eastern Yunnan and western Guizhou and location of the Zhongzhai section. Below: generalised lithostratigraphy along the cross-sectional line A–B from west to east. After Peng et al. (2005).

sandstones, mudstones/shales and thin intercalated sandy limestones and limestones, whilst fully marine sequences in the east comprise dominantly limestones with minor mudstones, and chert (Nanjing Institute of Geology and Palaeontology, 1980; Peng et al., 2005). The Zhongzhai section, reported on here, is exposed in a road cutting located approximately 150 km WSW of Guiyang and 25 km SW of Liuzhi in western Guizhou Province. GPS coordinates for the base of the measured section at Zhongzhai are 26°09.110N 105°17.113E. Lithologies are predominantly siltstones, sandstones and mudstones with minor silty or sandy limestones, shale and thin volcanic ash/clay beds. 3. Lithostratigraphy of the Zhongzhai section A lithostratigraphic log, measured by the authors in 2001 is presented in Fig. 3. A total of 76.5 m of section

were measured in detail (Fig. 3). Samples were collected for conodont (Plate 1) extraction and for palynological investigations. Oriented drillcore samples were collected for palaeomagnetic investigations to determine the magnetostratigraphy of the section. The positions of all samples are indicated on the log of Fig. 3. The measured sequence spans the upper part of the Longtan Formation and the lower part of the Yelang Formation. The boundary between these two formations was placed by the Nanjing Institute of Geology and Palaeontology (1980) at the base of the 5 cm thick volcanic clay at 6.23 m in our section. The lower part of the Longtan Formation (not exposed and hence not measured by us) comprises 6+ m thick bedded limestone with fusulinids and brachiopods, overlain by 9.13m thick-bedded grey-green sandstone (Nanjing Institute of Geology and Palaeontology, 1980). The upper part of the Longtan Formation comprises grey-

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Fig. 3. Lithostratigraphical measured section and magnetostratigraphy of the upper Longtan and lower Yelang Formations of the Zhongzhai section showing conodont and palynology sample locations, palaeomagnetic drill core sample locations, and occurrence of important fossils. Four digit conodont and palynology sample numbers are prefixed by 640.

green carbonaceous siltstones and mudstones with thin sandy limestones and calcareous nodules containing brachiopods, bivalves, gastropods, ostracods and cepha-

lopods. The lower Yelang Formation comprises approximately 15.5 m mudstones with minor siltstones and thin silty limestones, overlain by, in ascending order, 35.5 m

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Plate 1. Conodonts from the Permian-Triassic boundary beds at Zhongzhai. All figures are Pa elements. 1. 2, 3, and 4. 5, 6. 7, 8. 9. 10, 11. 12.

Hindeodus parvus Kozur & Pjatakova, lateral view of specimen 1673, sample 6405549. Hindeodus eurypyge Nicoll, Metcalfe & Wang, oral, outer lateral and posterior views of specimen 1670, sample 6405548. Hindeodus eurypyge Nicoll, Metcalfe & Wang, oral and outer lateral views of specimen 1671, sample 6405548. Hindeodus changxingensis Wang, lateral views of specimens 1668 and 1669, sample 6405548. Clarkina tulongensis Tian, oral view of specimen 1672, sample 6405549. Clarkina meishanensis Zhang, Lai and Ding, inner lateral and oral views of specimen 1660, sample 6405548. Merrillina ultima Kozur, lateral view of specimen 1665, sample 6405548.

siltstones with silty limestone near the base, 4.5 m mudstones and 14+ m green sandstone with minor mudstone (Fig. 3). The bivalve genus Claraia (including

Claraia wangi) and the brachiopod genus Lingula are particularly abundant in the lower Yelang Formation. Both Claraia and Lingula appear in the section at 6 m in

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our section, immediately above the mass extinction and magnetic reversal levels. The P–T boundary interval, which also straddles the Longtan–Yelang Formations boundary, consists of a lower 20 cm thick limestone overlain by a 50 cm thick black shale bed containing an abundant brachiopod fauna. This bed is in turn overlain by a 23 cm thick limestone a 5 cm thick ash bed followed by a 10 cm thick black shale, a second, upper, ash bed that is 3 cm thick, then followed by a 20 cm thick silty limestone (Figs. 3 and 4). 4. The Permian–Triassic biostratigraphic boundary and its placement in the Zhongzhai section The Permian–Triassic boundary is defined by the first appearance of the conodont species Hindeodus parvus (Kozur & Pjatakova) (Yin et al., 2001). At the Global Stratotype Section and Point (GSSP) at Meishan, China, this occurs at the base of Bed 27c which is 17 cm above the main mass extinction level at the base of volcanic ash Bed 25 (Jin et al., 2000; Yin et al., 2001; Nicoll et al., 2002; Fig. 2). Lithological correlation between the Meishan GSSP and the Zhongzhai section, suggested that the P–T boundary should be positioned between the two volcanic ash beds in the sequence at 6.23 and 6.38 m in the section (Fig. 3). Macrofaunal and palynofloral data support this position (Nanjing Institute of Geology and Palaeontology, 1980; Peng et al., 2005) but to date important boundary defining conodonts were not reported from the section. We here report conodont faunas from the Zhongzhai section which unequivocally

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places the Permian–Triassic boundary between our samples 5548 and 5549 and almost certainly between the two volcanic ash beds at 6.23 and 6.38 m in the section, allowing these ash beds to be used for proxy correlation of the boundary into non-marine facies to the west. 4.1. Conodont faunas Two, and possibly three, distinctive age diagnostic faunas were recovered in this study (Table 1). Samples 6405548, and probably 6405546, contain a fauna typical of the latest Permian to earliest Triassic transition (Nicoll et al., 2002). All of the Hindeodus species are restricted to the immediate boundary interval, being found both just below and just above the boundary. According to Kozur (2005) Merrillina ultima, here recorded from sample 6405548, is restricted to the latest Permian, and represents the Late Permian (Changhsingian Stage) Merrillina ultima–Stepanovites? mostleri Zone. Sample 6405549 contains a fauna diagnostic of the earliest Triassic with Hindeodus parvus. The other Hindeodus species from this sample are found in both the latest Permian and earliest Triassic. Clarkina tulongensis co-occurs with Clarkina carinata and ranges from the latest Permian into the earliest Triassic. This fauna represents the Hindeodus parvus Zone. Samples 6405552 and 6405553 contain only broken fragments of conodonts. None of the elements are well enough preserved to attempt specific identification, but the nature of the denticles in the P-type elements suggests

Fig. 4. Photograph of the Permian–Triassic boundary beds of the Zhongzhai section and stratigraphically important conodonts.

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Table 1 Conodonts recovered from samples in the Zhongzhai section, see Fig. 3 for position Sample

Species/abundance

Sample wt.

CAI

6405553

2 conodont element fragments ? Neospathodus sp. indet. 2 fragments 7 conodont element fragments Indet. fragments (7) 0 conodonts Associated fauna/flora: wood fragments, 1 ostracod 0 conodonts 277 conodont elements and fragments Clarkina tulongensis Tian 1 Pa element Clarkina sp. indet. 1 Pa element fragment Hindeodus latidentatus Kozur, Mostler & Rahimi-Yazd 7 Pa elements Hindeodus parvus Kozur & Pjatakova 40 Pa elements Hindeodus typicalis (Sweet) 10 Pa elements Hindeodus spp. indet. 59 Pa elements Ramiform elements, all species 161 elements Associated fauna: ostracods, phosphatic fragments, gastropod, bivalve 500+ conodont elements and fragments Clarkina meishanensis Zhang, Lai and Ding 35 Pa elements Hindeodus changxingensis Wang 27 Pa elements Hindeodus eurypyge Nicoll, Metcalfe & Wang 41 Pa elements Hindeodus praeparvus Kozur 72 Pa elements Hindeodus n. sp. A Nicoll, Metcalfe & Wang 7 Pa elements Hindeodus spp. Pa fragments 125 elements Merrillina ultima Kozur 13 Pa elements Ramiform elements of all species uncounted Associated fauna: ostracods, forams, fish teeth, phosphatic fragments 1 conodont element fragment Genus and species indet. 1 element fragment Associated fauna: fish teeth (2), phosphatic fragments 27 conodont elements and fragments Clarkina sp. 1 element fragment Clarkina sp. indet. 2 element fragment Hindeodus sp. A 1 element fragment Hindeodus sp. B 10 element fragments Ramiform elements 13 fragments Associated fauna: forams (3 species), bivalves, crinoids, phosphatic fragments

5 kg

1

5 kg

1

5 kg

N/A

5 kg 5 kg

N/A 1

5 kg

1

5 kg

1

5 kg

1

6405552 6405551 6405550 6405549

6405548

6405547

6405546

Sample weight is that of processed sample. CAI = conodont color alteration index (Epstein et al., 1977).

they should be assigned to Neospathodus rather than Hindeodus. This would place this part of the section in the early, but not earliest Triassic (Induan). 5. Magnetostratigraphy and its relationship to the P–T boundary Global palaeomagnetic data show that there is a reversed to normal magnetic polarity transition just below the biostratigraphic P–T boundary and approximately coincident with the main end-Permian mass extinction level (Steiner et al., 1989; Zhu and Liu, 1999; Scholger et al., 2000; Szurlies et al., 2003; De Kock and Kirschvink, 2004; Glen et al., in preparation). Our group's magnetostratigraphic work at Shangsi and other P–T sections in China (Glen et al., in preparation)

confirms this (Fig. 4) and our results from the Zhongzhai section (Fig. 5) demonstrate a reversed to normal transition at 6 m in our section, just 33 cm below the P–T boundary here defined by conodonts (Nomade et al., 2002; Glen et al., in preparation). This magnetic reversal, coupled with the volcanic ash beds at the boundary serve as excellent proxies for identifying the boundary and mass extinction levels in non-marine sequences to the west of the Zhongzhai section. 6. Stable carbon isotope records It is now well established that a significant negative stable carbon isotope shift occurs globally at or immediately following the P–T mass extinction, and this is recorded by both carbonate and organic carbon isotope

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Fig. 5. Correlation of Permian–Triassic magnetostratigraphies demonstrating the consistent reversed to normal transition just below the Permian– Triassic boundary.

records in both marine and non-marine environments (e.g. Holser et al., 1989; Wang et al., 1994; Morante, 1996; Wignall et al., 1998; Krull and Retallack, 2000). The negative shift appears more consistently as a single large shift in the marine environment and for carbonate data (Holser and Magaritz, 1987; Baud et al., 1989; Krystyn et al., 2003; Krull et al., 2004) whereas observed negative shifts in non-marine and terrestrial environments are less clear, more facies and lithology dependent, and often marked by multiple shifts related to the major climatic perturbations during the extinction interval (Morante, 1996; Krull and Retallack, 2000; Twitchett et al., 2001; De Wit et al., 2002; Foster and Afonin, 2005). It is also more difficult to deconvolute shifts in organic carbon isotope compositions due to kerogen type from global climate and extinction related driving mechanisms (Poole et al., 2004). Stable carbon isotope data is only available for the nonmarine P–T Chahe section in eastern Yunnan and western Guizhou (see Fig. 1 for location). In this section, a negative isotope excursion, interpreted as the P–T boundary excursion by Peng et al. (2005) is recorded at a level corresponding to two clay beds (Fig. 6) that may be correlatives of the volcanic clay beds at the boundary at

Zhongzhai. A magnetostratigraphic study at Chahe would provide a vital new proxy for correlation and if the magnetic reversal identified at Zhongzhai and elsewhere can be located then correlation of the Chahe non-marine section with the standard marine GSSP could be robustly made. Stable isotope data for Zhongzhai would also aid this correlation. 7. Conclusions The Permian–Triassic boundary is unequivocally placed between our samples 6405548 and 6405549 in the Zhongzhai section. This level corresponds to two volcanic ash beds which appear to be regionally persistent allowing lithological correlation of the P–T boundary interval between marine and non-marine sections in SW China. A magnetostratigraphic reversed to normal transition occurs just below the boundary level at Zhongzhai, consistent with global records and this, together with stable isotope data may provide further proxy correlations of the marine to non-marine P–T sequences in the region. Further work is required to effect such proxy correlation. There are no published isotopic dates for the volcanic ash beds that occur at the

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Fig. 6. Organic carbon isotope curve and lithostratigraphy for the Chahe non-marine section, eastern Yunnan (from Peng et al., 2005).

boundary level in the Yunnan–Guizhou region and confirmation of the isochronous nature of these beds would be confirmed by precise dating of these volcanic layers.

authors wish to thank Xulong Lai, Kexin Zhang and Paul Wignall for their constructive suggestions that have improved the manuscript. References

Acknowledgements This work formed part of a larger research program on the Permian–Triassic boundary supported by an Australian Research Council grant to I. Metcalfe. The

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