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Volume 30 Issue 5
Oct.  2019
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Latest Wuchiapingian to Earliest Triassic Conodont Zones and δ13Ccarb Isotope Excursions from Deep-Water Sections in Western Hubei Province, South China

  • Deep-water facies sections have advantages of recording complete information across the Permian-Triassic boundary (PTB). Here we present a detailed study on the conodont biostratigraphy and carbon isotope profile ranges from the Wuchiapingian-Changhsingian boundary (WCB) to the PTB of two deep-water facies sections at Zhuqiao and Shiligou in the Middle Yangtze region, western Hubei, South China. Fifteen species and three genera are identified. Eight conodont zones are recog-nized which in ascending order are the Clarkina orientalis, C. wangi, C. subcarinata, C. changxingensis, C. yini, C. meishanensis, Hindeodus parvus and Isarcicella isarcica zones. The onset of deposition of the deep-water siliceous strata of Dalong Formation in western Hubei began in the Late Wuchiapingian and persisted to the Late Changhsingian. Carbon isotope negative excursions occur near both the WCB and PTB in both sections. The WCB δ13Ccarb negative excursion is in the C. orientalis and C. wangi zones. The PTB δ13Ccarb negative excursion began in the C. yini Zone and extended to the I. isarcica Zone. The absence of several Changhsingian zones may indicate the difficulty of extracting conodonts from siliceous strata or the presence of an intra-Changhsingian hiatus.
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Latest Wuchiapingian to Earliest Triassic Conodont Zones and δ13Ccarb Isotope Excursions from Deep-Water Sections in Western Hubei Province, South China

    Corresponding author: Xulong Lai, xllai@cug.edu.cn
  • 1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
  • 2. Wuhan Center of Geological Survey, Wuhan 430205, China
  • 3. School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK
  • 4. State Laboratory of Geobiology and Environmental Geology, China University of Geosciences, Wuhan 430074, China
  • 5. Non-ferrous Metals Geological Exploration Bureau of Zhejiang Province, Shaoxing 312000, China
  • 6. State Laboratory of Paleontology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Nanjing 210008, China

Abstract: Deep-water facies sections have advantages of recording complete information across the Permian-Triassic boundary (PTB). Here we present a detailed study on the conodont biostratigraphy and carbon isotope profile ranges from the Wuchiapingian-Changhsingian boundary (WCB) to the PTB of two deep-water facies sections at Zhuqiao and Shiligou in the Middle Yangtze region, western Hubei, South China. Fifteen species and three genera are identified. Eight conodont zones are recog-nized which in ascending order are the Clarkina orientalis, C. wangi, C. subcarinata, C. changxingensis, C. yini, C. meishanensis, Hindeodus parvus and Isarcicella isarcica zones. The onset of deposition of the deep-water siliceous strata of Dalong Formation in western Hubei began in the Late Wuchiapingian and persisted to the Late Changhsingian. Carbon isotope negative excursions occur near both the WCB and PTB in both sections. The WCB δ13Ccarb negative excursion is in the C. orientalis and C. wangi zones. The PTB δ13Ccarb negative excursion began in the C. yini Zone and extended to the I. isarcica Zone. The absence of several Changhsingian zones may indicate the difficulty of extracting conodonts from siliceous strata or the presence of an intra-Changhsingian hiatus.

0.   INTRODUCTION
1.   GEOLOGICAL SETTING
  • In South China, during the Changhsingian (Late Permian), the Middle Yangtze Block was mostly carbonate platform (Fig. 1c), whilst siliceous strata of Dalong Formation accumulated in deep-water on the northern and southwest margin of the Block (Yin et al., 2014; Feng and Gu, 2002). Contemporaneously, the western Hubei Basin was a large rift basin that formed an embayment in the northwestern margin of the Middle Yangtze Block (Liu et al., 2019; Zhuo et al., 2009) (Figs. 1a, 1b). The Zhuqiao and Shiligou sections were located in the western Hubei Basin during the PTB (Fig. 1a). Dalong Formation occurs extensively in western Hubei, and is considered a record of deep-water basin and deeper-water slope environments during the Latest Permian (He et al., 2013). There are three sedimentary types of Dalong Formation in this area from west to east, and they are siliceous rock to limestone and mudstone type, mudstone to siliceous rock type and siliceous limestone to mudstone type (Niu et al., 2000). The first two types are of deep-water, basin origin, whereas the last type is a transitional type between the basin and platform facies.

    Figure 1.  (a) Paleogeography during the Changhsingian Stage in the Middle and Upper Yangtze Region, modified from He et al. (2013) and Feng et al. (1997); (b) paleogeography showing the position of South China during the End-Permian extinction, modified after Scotese, 2001; (c) distribution of deep-water environments during the Changhsingian in South China, modified from Yin et al. (2014) and Sun et al. (1989). ★Location of the Shiligou Section, ▲location of the Zhuqiao Section. NMBY. North Marginal Basin of Yangtze Platform; HGGB. Hunan-Guizhou-Guangxi Basin.

    The Zhuqiao Section is located next to the Zhuqiao hydropower station, 2 km north of Zhuqiao village, Wufeng County, Yichang City, western Hubei Province, South China (Fig. 2). The exposed strata at the Zhuqiao Section include the Late Permian Wuchiaping and Dalong formations, and the Early Triassic Daye Formation (Fig. 3). The top of Wuchiaping Formation is composed of grey, dolomitic limestone with bed thicknesses of more than 1 m. Dalong Formation consists of grey to dark grey thin-bedded siliceous mudstone and carbonaceous mudstone with a few interbeds of dark grey medium- to thin-bedded limestone, forming rhythmic stratification, and with well-developed thin laminations. Planktonic or nektonic fossils including ammonoids and radiolarians are common in the thin-bedded siliceous mudstone. Thus, predominately siliceous facies of Dalong Formation at Zhuqiao are considered deep-water basin facies and belong to the mudstone of siliceous rock type (Niu et al., 2000). Daye Formation consists of mudstone and muddy limestone with thin-bedded claystone.

    Figure 2.  Locations of the Zhuqiao and Shiligou sections, western Hubei Province, South China. ★Location of the Shiligou Section, ▲location of the Zhuqiao Section.

    Figure 3.  Photos of field outcrops and fossils at the Zhuqiao and Shiligou sections in western Hubei Province, South China. (a) Panorama of the Zhuqiao Section; (b) lithology of bed 25 to bed 29 at Zhuqiao, inserted is a dolomitic mudstone of bed 26; (c) Ophiceras in bed 27 of Daye Fm. at Zhuqiao; (d) rugged limestone surface at the base of bed 25a of Dalong Fm. at Zhuqiao; (e) overview of the WCB at Shiligou; (f) the PTB at Shiligou.

    The Shiligou Section is located in Xinglong Town, Fengjie County of Chongqing City, along the roadside of the "Tourism Circle Way" (Fig. 2). The strata at Shiligou belong to the western Hubei Basin and consist of the well exposed Dalong and Daye formations (Fig. 3). The lower part of Dalong Formation is grey to black, thin-bedded siliceous mudstone and calcareous mudstone, similar to the Zhuqiao Section, whereas the middle and upper parts consist of grey, medium- to thin-bedded limestone with calcareous mudstone interbedded. Dalong Formation of the Shiligou Section is also deep-water, basinal facies and belongs to the siliceous rock to limestone and mudstone type (Niu et al., 2000). The overlying Daye Formation is composed of grey, thin-bedded limestone with interbedded mudstone.

2.   MATERIALS AND METHODS
  • A total of 42 and 28 conodont samples (about 3–5 kg per sample) were collected from Dalong and Daye formations at Zhuqiao and Shiligou, respectively. Limestone samples were processed by dilute acetic acid (10%) (Jiang et al., 2007), mudstone samples were cracked by dithionite solution and hydrogen peroxide, and siliceous mudstone samples were dissolved by dilute hydrofluoric acid (5%). LST-an inorganic heavy liquid was used in conodont separation as described by Yuan et al. (2015). Fifteen species belonging to three genera (Clarkina, Hindeodus and Isarcicella) of conodont P1 elements were identified (1 553 from the Zhuqiao Section and 91 from the Shiligou Section), and some key species are illustrated in Plates 1 to 4.

    Figure Plate 1.  SEM photos of conodonts from the Zhuqiao section. All illustrations are P1 elements, scale bar=100 μm. 1. Clarkina guangyuanensis (Dai and Zhang, 1989), from sample ZQC-01; 2. Clarkina transcaucasica Gullo and Kozur, 1992, from sample ZQC-01; 3. Clarkina liangshanensis (Wang, 1978), sample ZQC-01; 4-25. Clarkina orientalis (Barskov and Koroleva, 1970); 4-7. sample ZQC-01; 8-25. sample ZQC-08-01.

    Figure Plate 2.  SEM photos of conodonts from the Zhuqiao section. All illustrations are P1 elements, scale bar=100 μm. 1-28. Clarkina orientalis (Barskov and Koroleva, 1970); 1-2. sample ZQC-19; 3-8. sample ZQC-20; 9-14. sample ZQC-21; 15-17. sample ZQC-22; 18-23. sample ZQC-23; 24-26. sample ZQC-24-01; 27-28. sample ZQC-25-01.

    Figure Plate 3.  SEM photos of conodonts from the Zhuqiao and Shiligou sections. All illustrations are P1 elements, scale bar=100 μm. 1-8. Clarkina orientalis (Barskov and Koroleva, 1970), Shiligou section, 1. sample SLG-01; 2-5. sample SLG-05; 6-7. sample SLG-08-02; 8. sample SLG-11-02; 9-23. Clarkina wangi (Zhang, 1987); 9-16. Zhuqiao section, sample ZQC-25-01; 17-23. Shiligou section; 17-21. sample SLG-06-01; 22-23. sample SLG-06-02; 24-27. Clarkina subcarinata (Sweet, 1973), Shiligou section, 24-26. sample SLG-08-02; 27. sample SLG-09; 28-33. Clarkina changxingensis (Wang and Wang, 1981), Shiligou section; 28-29. sample SLG-12; 30-31. sample SLG-13-01; 32-33. sample SLG-13-02.

    Figure Plate 4.  SEM photos of conodonts from the Zhuqiao and Shiligou sections. All illustrations are P1 elements, scale bar=100 μm. 1-5. Clarkina changxingensis (Wang and Wang, 1981), Shiligou section, 1. sample SLG-15-01; 2. sample SLG-15-02; 3-4. sample SLG-16-02; 5. sample SLG-16-03; 6. Clarkina deflecta (Wang and Wang, 1981), Shiligou section, sample SLG-15-01; 7-9. Clarkina yini Mei, 1998, Shiligou section; 7. sample SLG-15-01; 8. sample SLG-15-03; 9. sample SLG-16-02; 10-12. Clarkina meishanensis (Zhang et al., 1995), Zhuqiao section, sample ZQC-25-02; 14, 19-20. Hindeodus parvus (Kozur and Pjatakova, 1976), Zhuqiao section; 14. sample ZQC-28; 19. sample ZQC-34-01; 20. sample ZQC-34-02; 13, 15. Hindeodus sp., 13. Shiligou section, sample SLG15-01, 15. Zhuqiao section, sample ZQC-28; 16. Clarkina orchardi (Mei, 1996), Zhuqiao section, sample ZQC-28; 17. Hindeodus pisai Perri and Farabegoli, 2003, Zhuqiao section, sample ZQC-30-02; 18. Isarcicella isarcica (Huckriede, 1958), Zhuqiao section, sample ZQC-30-02; 21. Hindeodus aff. parvus, Shiligou section, sample SLG-17-03.

    For δ13Ccarb and δ18Ocarb measurement, 106 and 60 fresh, whole rock samples were taken from the Zhuqiao and Shiligou sections, respectively. Weathered surfaces and large calcite veins were avoided, and all samples were crushed to less than 200 meshes with a dentist drill. Analysis was conducted at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan) with a MAT-253 mass spectrometer with standard methodology (see Supplemental data).

    A positive correlation between δ13Ccarb and δ18Ocarb is usually explained as a sign of the influence of meteoric diagenesis (Meyers and Lohmann, 1985). Most of the δ18Ocarb values obtained from Dalong Formation in our sections range from -4.0‰ to -8.0‰, indicating a weak influence of meteoric diagenesis. This conclusion is supported by the finding of the weak correlation (R2=10-4 and 10-2) between δ13Ccarb and δ18Ocarb (see Supplemental data).

3.   CONODONT BIOSTRATIGRAPHY
  • A total of 8 conodont zones have been identified indicating ages ranging from the Latest Wuchiapingian to Earliest Triassic from the two sections in western Hubei area. In ascending order, they are the C. orientalis in the Latest Wuchiapingian, C. wangi, C. subcarinata, C. changxingensis, C. yini and C. meishanensis zones in the Changhsingian, H. parvus and I. isarcica zones in the Griesbachian. The Zhuqiao Section has the C. orientalis, C. wangi, C. meishanensis, H. parvus and I. isarcica zones, whilst the Shiligou Section has the C. orientalis, C. wangi, C. subcarinata, C. changxingensis and C. yini zones.

  • This zone is discovered at both Zhuqiao (0–10.85 m, beds 1–24) and Shiligou (0–7.24 m, beds 1–6) (Figs. 4, 5). The lower limit of the zone is undefined because there is no sampling below this level. The upper limit is defined by the first occurrence (FO) of C. wangi. C. orientalis is the dominant species in this zone, especially abundant at the Zhuqiao Section, where it occurs in most of the lower part of Dalong Formation. Besides the zonal conodont, only a few C. guangyuanensis, C. transcaucasica and C. liangshanensis were found in Sample ZQC-1 at Zhuqiao.

    Figure 4.  Distribution of conodont fossils at Zhuqiao, Wufeng County, Yichang City, western Hubei Province, South China.

    Figure 5.  Distribution of conodont fossils at the Shiligou Section in Xinglong Town, Fengjie County, Chongqing City, South China.

    The C. orientalis Zone was first established by Kozur (1975) in Archura, Transcaucasia and is widely known in South China (Shen and Mei, 2010; Shen et al., 2007; Jin et al., 2006; Nafi et al., 2006; Mei et al., 1994) and Iran (Shen and Mei, 2010; Yazdi and Shirani, 2002). Though the range of C. orientalis could extend into the C. wangi Zone of basal Changhsingian, Shen (2007) particularly researched the spatial and temporal distribution of C. orientalis and showed that population of the species was a distinct Late Wuchiapingian maker. The base of C. orientalis Zone at the Zhuqiao and Shiligou Sections are undefined, but the zone can be correlated with the Neogondolella orientalis-N. longicuspidata Zone and C. longicuspidata Zone at the Meishan Section (Yuan et al., 2014; Zhang et al., 2009), Ganxi Section (Nafi et al., 2006) and Shangsi Section (Shen et al., 2013) in South China and the C. orientalis Zone established by Shen and Mei (2010) in Iran. The age of the zone is Late Wuchiapingian.

  • This zone is identified at both Zhuqiao (10.85–10.96 m, bed 25a) and Shiligou (7.24–11.39 m, beds 6–8) (Figs. 4, 5). Lower limit: the first occurrence of C. wangi. The upper limit of the zone at the Shiligou Section is defined by the first occurrence of C. subcarinata. However, at the Zhuqiao Section, the upper limit is defined by the first occurrence of C. meishanensis in bed 25b. Associated conodonts are C. orientalis and C. sp.. C. wangi has its maximal abundance in the C. wangi Zone, especially in Zhuqiao, and we found hundreds of C. wangi specimens.

    Mei and Henderson (2001) first established the C. wangi-C. subcarinata Zone at the Meishan Section and C. wangi was not discovered outside South China at that time. Because the lower part of the zone is dominated by C. wangi, Mei et al. (2004) subsequently distinguished a C. wangi Zone from the C. wangi-C. subcarinata Zone. The Global Stratotype Section and Point (GSSP) for the basial boundary of the Changhsingian Stage was then defined at the first appearance datum of the conodont C. wangi within the C. longicuspidata to C.wangi lineage by Jin et al. (2006).

  • The Early Changhsingian zone is only recognized in the Shiligou Section (11.39–28.14 m, beds 8–12) (Fig. 5). Lower limit: the first occurrence of C. subcarinata. Upper limit: the first occurrence of C. changxingensis. Associated conodonts are C. orientalis, C. wangi, and C. sp.. This zone has been widely studied around the world (Yuan et al., 2014; Shen and Mei, 2010; Ji et al., 2007; Kozur, 2005, 2004). Nafi et al. (2006) failed to differentiate the zone from the C. wangi Zone at their Ganxi Section. The C. wangi Zone of Ganxi actually consists of both the C. wangi and C. subcarinata zones. The zone at Shiligou is equivalent to the C. subcarinata Zone of Meishan (Yuan et al., 2014) and Shangsi (Yuan et al., 2019; Shen et al., 2013) and sections in Iran (Shen and Mei, 2010).

  • This Middle Changhsingian zone is only identified in the Shiligou Section (28.14–31.64 m, beds 12–15) (Fig. 5). Lower limit: the first occurrence of C. changxingensis. Upper limit: the first occurrence of C. yini. Associated conodonts are C. deflecta and C. sp.. Wang and Wang (1981) first established the C. deflecta-C. changxingensis assemblage Zone at the Meishan Section in South China, and the C. changxingensis Zone has been widely reported around Paleotethys since then (Lyu et al., 2019; Bai et al., 2017; Yuan et al., 2014; Zhang et al., 2014; Shen and Mei, 2010; Zhang et al., 2009; Nafi et al., 2006; Mei et al., 1998; Orchard and Krystyn, 1998). The C. changxingensis Zone of the Shiligou Section is equivalent to the middle and upper parts of the Neogondolella changxingensis-N. deflecta Zone of Zhang et al. (2009), and the C. changxingensis Zone of Yuan et al. (2014) at the Meishan Section.

  • The Late Changhsingian zone is only found at Shiligou (31.64–35.39 m, beds 15–16) (Fig. 5). Lower limit: the first occurrence of C. changxingensis. We only found a few C. changxingensis and C. yini individuals in beds 15 and 16 at Shiligou. And abundant Early Triassic bivalve Claraia and ammonoid Ophiceras are found in the bottom of bed 17. We put the upper limit of the zone in the uppermost part of bed 16. It is largely equivalent to the Neogondolella yini Zone at the Meishan (Jiang et al., 2007) and Shangsi sections (Jiang et al., 2011), and the C. yini Zone of the Ganxi Section (Nafi et al., 2006).

  • The Late Changhsingian zone is only identified at Zhuqiao (10.96–11.01 m, bed 25b) (Fig. 5). Lower limit: the first occurrence of C. meishanensis. Upper limit is placed at the top of bed 25b. No conodont species were found from bed 25c to bed 27. We could not identify any zone in this interval. Just a few C. meishanensis were found in the zone and badly preserved, because a dithionite solution and hydrogen peroxide were used to crack the black mudstone of samples for conodont extraction. The zone was first established by Mei et al. (1998) at the Meishan Section. The C. meishanensis of the Meishan Section is in bed 25b of dark gray clay. The C. meishanensis at the Zhuqiao Section may be equivalent to the Neogondolella meishanensis zone at Meishan (Jiang et al., 2007).

  • This zone is only discovered at Zhuqiao (12.15–12.8 m, beds 28–30) (Fig. 4). Lower limit: the first occurrence of H. parvus. Upper limit: the first occurrence of Isarcicella isarcica. Associated taxa: C. orchard, H. sp., and C. sp.. The First Appearance Datum of H. parvus was used to define the GSSP for the basal boundary of the Triassic System by Yin et al. (2001), and it has been widely used ever since (Lyu et al., 2019; Bai et al., 2017; Yuan et al., 2014; Zhang et al., 2014; Zhao et al., 2013; Jiang et al., 2007; Perri and Farabegoli, 2003). However, the first occurrence of H. parvus in our study is in bed 28, 0.36 m higher than the first occurrence of Ophiceras in bed 27 which we use to define the beginning of the Triassic in our section.

    We have not found H. parvus in the Shiligou Section, but abundant Early Triassic Claraia and Ophiceras were found at the bottom of bed 17. Furthermore, we found H. aff. parvus (Pl. 4, Fig. 21) from Sample SLG-17C3 in bed 17. So, we consider the age of bed 17 at the Shiligou Section to be the Earliest Triassic.

  • The Mid Griesbachian zone is only found in the Zhuqiao Section (12.8–13.96 m, beds 30–34) (Fig. 4). Lower limit: the first occurrence of Isarcicella isarcica. Upper limit is uncertain, because no samples were taken from above bed 34. Associated taxa: H. parvus and H. sp.. I. isarcica was first obtained by Huckriede (1958) in northern Italy. The I. isarcica Zone usually overlies the I. staeschei Zone (Zhang et al., 2014; Zhao et al., 2013; Jiang et al., 2007). However, we failed to establish the zone at Zhuqiao, where the I. isarcica Zone of the Zhuqiao Section is believed to correspond to I. isarcica Zone at Meishan (Jiang et al., 2007) and Daxiakou (Zhao et al., 2013).

4.   CARBON ISOTOPE STRATIGRAPHY
  • Carbon isotope variations in the PTB interval show a rapid and large negative excursions in both δ13Ccarb and δ13Corg records and provide an age control independent of conodont stratigraphy (Shen et al., 2013; Song et al., 2012; Korte and Kozur, 2010; Xie et al., 2007). The carbon isotope excursion around the WCB is also well documented in South China (Wei et al., 2015; Shen et al., 2013; Jin et al., 2006; Shao et al., 2000). However, whether the δ13C excursion across the WCB is a global event or a special phenomenon influenced by regional factors is still unknown. Shao et al. (2000) documented a δ13C anomaly at the WCB in Guizhou and Guangxi of South China, and considered this WCB excursion to be a global negative carbon isotopic excursion because more 12C-rich CO2 was released into the atmosphere. Shen and Mei (2010) documented that the negative excursion around the WCB showed different magnitudes and patterns, and proposed that whether this excursion was due to local or global controls still remained unresolved. Wei et al. (2015) considered the significant negative excursion at the WCB was probably a global signal and mainly caused by the low primary productivity. Liao et al. (2016) suggested that the change in sedimentary environment had an important influence on the δ13C values across the WCB. Here, we document carbon isotope data from WCB to PTB at the Zhuqiao and Shiligou sections (Fig. 6).

    Figure 6.  Changhsingian δ13Ccarb chemostratigraphy at the Zhuqiao and Shiligou sections compared with the Meishan and Shangsi records from Shen et al. (2013).

    At Zhuqiao, the δ13Ccarb profile shows a gradual negative decline in the C. orientalis Zone with the values beginning to decline from 0.41‰ at 3.77 m in bed 7 to the minimum -2.85‰ at 9.96 m in bed 22, followed by the brief positive peak between +0.51‰ and +0.84‰ around the WCB. The negative δ13Ccarb excursion near the PTB is also showed. The excursion shows a negative excursion trend from the C. meishanensis Zone to I. isarcica Zone with the average -0.75‰. The δ13Ccarb values are variable around the PTB, and can not be compared with the Meishan Section (Shen et al., 2013; Xie et al., 2007) and other sections in South China (Yuan et al., 2015; Shen et al., 2013), which is difficult to explain. It seems there are some noises of the carbon isotope excursion at the PTB of Zhuqiao. The carbonate dolomitization is obvious near the PTB of Zhuqiao Section (Fig. 3b). The carbonate dolomitization may be one possible explanation.

    The δ13Ccarb data should show a clear negative excursion across the WCB at Shiligou. The δ13Ccarb curve indicates a negative excursion trend in the C. orientalis Zone and C. wangi Zone with the average -0.85‰, then the rapid recovery to stable values with the average value 0.64‰ from the C. subcarinata to the C. yini zones. The negative excursion around the PTB is more clearly observed at Shiligou. The δ13Ccarb values are stable during the most part of Changhsingian with the average +0.64‰ from the C. subcarinata Zone to the C. yini Zone which are consistent with the δ13Ccarb values in the Changhsingian of many other sections (Shen et al., 2013). The δ13Ccarb values begin to gradually decline at the base of C. yini Zone from 0.92‰ to -0.01‰, followed by a sharp negative shift 3.62‰ from 0.81‰ to -2.81‰ in the upper part of C. yini Zone, which may be compared with the sharp 5‰ decline within the C. meishanensis Zone at the Meishan Section (Xie et al., 2007). Above the PTB, the δ13Ccarb values are stable on the average -0.80‰ in bed 17.

5.   DISCUSSION
  • The Global Stratotype Section and Point (GSSP) for the base of the Changhsingian Stage was defined at the First Appearance Datum of the conodont C. wangi within the lineage from C. longicuspidata to C. wangi (Jin et al., 2006). C. wangi occurs in both the Zhuqiao and Shiligou sections, with the C. orientalis zone below. We suggest that the WCB of the Zhuqiao Section should be placed at 10.85 m in the base of bed 25a and the WCB of Shiligou Section should be laid at 7.24 m in the lower of bed 6.

  • At the Zhuqiao Section, the first occurrence of H. parvus is in bed 28 which is 0.36 m higher than the first occurrence of Ophiceras (Figs. 3b, 3c) in bed 27. We define the PTB of the Zhuqiao Section by the first occurrence of Ophiceras in bed 27 (11.74 m), which is 0.36 m below the first occurrence of H. parvus.

    H. parvus is not found at Shiligou, but abundant Early Triassic Claraia and Ophiceras are found in bed 17 instead, where H. aff. parvus also occurs. So, we consider the age of bed 17 at Shiligou Section should be the Earliest Triassic. Taking the negative excursion around the PTB from the Shiligou Section into account, we suggest the PTB of the Shiligou Section is at 35.39 m, which is the bottom of bed 17.

  • In South China, chert deposition in Dalong Formation has been found to persist into the Earliest Triassic in the Gaimao (Yang et al., 2012) and Xinmin sections (Zhang et al., 2014) of Guizhou Province, although generally this unit is considered to record latest Permian siliceous sedimentation in deep-water basin and slope environments (He et al., 2013; Chen et al., 2010).

    Since the definition of Dalong Formation was confirmed by Zhao et al. (1978), Dalong Formation in western Hubei area is usually considered to be of a Changhsingian age (He et al., 2013), although the ammonoids recorded by Niu et al. (2000) indicate a Late Wuchiapingian to Changhsingian age. Our conodont studies at Zhuqiao and Shiligou confirm the age range.

    Three conodont zones (C. subcarinata, C. changxingensis and C. yini Zones) are absent from Dalong Formation at the Zhuqiao Section between the C. wangi Zone and C. meishanensis Zone. This may partly reflect the difficulty of extracting conodonts from siliceous strata that dominate this section. Previously reported conodont studies at siliceous-dominated sections at Dongpan (Luo et al., 2008) and Xinmin (Zhang et al., 2014) also show that the conodont zones in both sections are not complete. If this is true, the three missing zones are supposed to be within the C. orientalis Zone, and further research is needed to differentiate them from the C. orientalis Zone. Alternatively, the absence could be due to a hiatus, possibly between bed 24 and 25a at the Zhuqiao Section. Evidences for this hiatus is: (1) a sudden change of lithology between bed 24 and bed 25a from siliceous mudrock to limestone; (2) the surface of limestone in bed 25a is irregular (Fig. 3d); (3) δ13Ccarb values show an increase at 24/25a boundary. If this reason is correct, the hiatus in Zhuqiao Section may be caused by a regional regression, which is potentially attributed to the regional crustal uplift which should begin from the south to north in the Western Hubei Basin, and as a result the Shiligou Section is almost unaffected and with no conodont zone absent. During the Late Permian, the western Hubei Basin was a longitudinal tensile depression rift basin with many faults, especially in the east, which may cause a regional crustal uplift (Zhuo et al., 2009).

6.   CONCLUSIONS
  • Fifteen species belonging to three genera (Clarkina, Hindeodus and Isarcicella) of conodont P1 elements are identified and eight conodont zones are recognized from Dalong to lower Daye Formations at the Zhuqiao and Shiligou Sections near western Hubei Province, South China. In ascending order, they are C. orientalis, C. wangi, C. subcarnata, C. changxingensis, C. yini, C. meishanensis, H. parvus and I. isarcica zones. Several zones are missing from the Changhsingian (C. subcarinata, C. changxingensis and C. yini Zones) either due to collection failure or the presence of a hiatus.

    There are both carbon isotope negative excursions near the WCB and PTB at the Zhuqiao Section and Shiligou Section. The WCB δ13Ccarb negative excursion is in the C. orientalis zone at the Zhuqiao Section with the average -1.08‰, followed by the brief positive peak between +0.51‰ and +0.84‰ around the WCB. At the Shiligou Section, the WCB δ13Ccarb negative excursion occurs in the C. orientalis and the C. wangi zones, and then there is a rapid recovery to stable values with the average value 0.64‰ from the C. subcarinata to the C. yini zones. The PTB δ13Ccarb negative excursions are also showed. At Zhuqiao, the negative excursion ranges from the C. meishanensis Zone to the I. isarcica Zone. At Shiligou, the PTB δ13Ccarb values begin with gradual decline at the base of C.yini Zone, with a sharp negative shift 3.62‰ in the upper part of C. yini Zone, followed by a stable negative value on the average -0.80‰ during the Early Triassic.

    On Basis of conodont zones and carbon isotope data, the WCB of the Zhuqiao Section is placed at 10.85 m in the base of bed 25a and the WCB of the Shiligou Section should be laid at 7.24 m in the lower of bed 6, and meanwhile, the PTB of the Zhuqiao Section is drawn at 11.74 m in bed 27 and the PTB of the Shiligou Section is placed at 35.39 m at the bottom of bed 17.

ACKNOWLEDGMENTS
  • This work is supported by the Natural Science Foundation of China (Nos. 41572002, 41830320, 41272044, 41472087, 4183000426, 41802016). All thanks are to Zaitian Zhang, Chunbo Yan, Rong Chen and Lina Wang for their help in field sampling. All SEM pictures are undertaken at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan). Finally, we thank the two anonymous reviewers for their constructive suggestions. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1018-2.

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