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A detailed Lower Triassic conodont biostratigraphy and its implications for the GSSP candidate of the Induan–Olenekian boundary in Chaohu, Anhui Province
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Progress in Natural Science 18 (2008) 79–90
A detailed Lower Triassic conodont biostratigraphy and its implications for the GSSP candidate of the Induan–Olenekian boundary in Chaohu, Anhui Province Laishi Zhao a
a,*
, Jinnan Tong a, Zhiming Sun
a,b
, Michael J. Orchard
c
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China b Institute of Geomechanics, Chinese Academy of Geological Science, Beijing 100081, China c Geological Survey of Canada, 101-605 Robson Street, Vancouver, Canada V6B5J3 Received 26 February 2007; received in revised form 24 April 2007; accepted 4 July 2007
Abstract Chaohu is located in a deep part of carbonate ramp on the Lower Yangtze Block, which belonged to the low-latitude eastern Tethyan archipelago during the Early Triassic. Fossils were very rich in the Lower Triassic of Chaohu. Bivalves, ammonoids, conodonts were very common throughout the Lower Triassic, while fish fossils were generally rich in some beds of the upper part. It is one of the most typical sections for the Early Triassic chronostratigraphy in the world. Although various fossils had been studied in the 1980s and 1990s, recent studies based upon new and more detailed collections from the Lower Triassic of Chaohu showed that the conodont zonation needs revision. We collected Lower Triassic conodont fossils from continuous sections of the West Pingdingshan, North Pingdingshan and South Majiashan, Chaohu, Anhui Province, and updated zonations were made for each section. Eight conodont zones have been distinguished. They are, in ascending order, Hindeodus typicalis zone, Neogondolella krystyni zone, Neospathodus kummeli zone, Neospathodus dieneri zone, Neospathodus waageni zone, Neospathodus pingdingshanensis zone, Neospathodus homeri zone, and Neospathodus anhuinensis zone. The first occurrence of Neospathodus waageni eowaageni of the N. w. eowaageni subzone (i.e. the base of the N. waageni zone) is suggested as the marker to define the Induan–Olenekian boundary. Ó 2007 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved. Keywords: Conodont biostratigraphy; Conodont zonation; Lower Triassic; Induan–Olenekian; Chaohu in Anhui Province
1. Introduction Establishment of the global stratotype section and point (GSSP) is one of the main purposes of the International Commission of Stratigraphy (ICS) in the past twenty years. So far about 42 GSSPs have been set up for the expected 96 Phanerozoic stages, and the rest of the GSSPs are expected to be established by 2008. The Triassic had its first GSSP and its base defined in 2001, that is the GSSP of the Permian and Triassic *
Corresponding author. Tel.: +86 27 62024366; fax: +86 27 67885096. E-mail address: [email protected] (L. Zhao).
boundary (PTB) at the base of Bed 27c of the Meishan section D, Changxing, Zhejiang Province [1]. The second Lower Triassic stage – the Induan and Olenekian boundary (IOB) has been extensively studied in recent years. By comparing with the H. himalayica in the Gondwanan Tethys region and the H. hedenstroemi in the boreal region, Zakharov et al. [2,3] proposed that the Tree Kamnya Cape Ravine section and the Abrek Bay section from the Russian South Primorye, which are believed to yield a mixed Boreal and Tethyan fauna, are candidates of the Induan–Olenekian boundary, and ammonoid Hedenstroemia bosphorensis was suggested as the index fossil to define the boundary.
1002-0071/$ - see front matter Ó 2007 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved. doi:10.1016/j.pnsc.2007.07.001
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But Dagys [4] pointed out that in the Induan and Olenekian boundary H. bosphorensis also occurred in the strata where Gyronites dominates, so its age is questionable. However, Hedenstroemia is a genus which prevailed in the high-latitude regions and scarcely occurred in the low-latitude regions, while Flemingites, Euflemingites, Meekoceras and others have been commonly considered as the index fossils to identify this boundary in the low-latitude regions. Though the conodonts in the high-latitude regions are not so common as in the low-latitude regions, most of the key forms are still recognizable one. Therefore, the conodonts should be considered in the definition of the Induan–Olenekian boundary. Lower Triassic conodont biostratigraphy has been established in some areas of the world, such as the sections in Lukac, Western Slovenia [5]; Tesero, Southern Alps [6]; Wadi Wasit, Central Oman [7]; Tahokamigumi and Kamura, Southwest Japan [8]; Abadeh, Central Iran [9]; Muth, Himalaya [10,11]; Chaotian, Meishan, Bian Yang, Qing Yan and Wangmo, China [12–18]. The published data indicated that Lower Triassic conodont zonations used to define the Induan–Olenekian boundary are very different from one area to the other in tropical Tethyan realm, even for the same section. Therefore, detailed conodont study on Lower Triassic strata at Chaohu (in centimeter scale around the Induan–Olenekian boundary) was conducted in order to establish a complete Lower Triassic conodont sequence and its intercalibration with the ammonoid sequence. 2. Geological setting The Lower Triassic in Chaohu is one of the well-developed Lower Triassic sequences in South China. The Lower Triassic sequence of Chaohu not only yields abundant fossils, but also provides a complete biostratigraphical sequence marked by conodonts and ammonoids [19–23], allowing for a well-defined chronostratigraphy. Early investigations on the Triassic geology of Chaohu began in the 1970s. The best-studied is Lower Triassic sequence in Chaohu in the Majiashan section, where the 1–200,000 regional geological survey was undertaken by the Anhui Geological Surveying Team in the 1970s. Fossils were very abundant and diverse throughout the Lower Triassic, including three bivalve assemblages [24–26]. Six Lower Triassic ammonoid zones were recognized in ascending order: Lytophiceras–Ophiceras zone, Prionolobus zone, Flemingites zone, Anasibirites zone, Tirolites–Columbites zone, and Subcolumbites zone by Guo [27] and Tong et al. [28], and six conodont zones in ascending order: Clarkina carinata zone, Neospathodus dieneri zone, Neospathodus cristagalli zone, Neospathodus waageni zone, Icriospathodus collinsoni zone, and Neospathodus anhuinensis– Neospathodus homeri zone by Ding [29]. Many other studies also found Lower Triassic ichthyosaur such as Chaohusaurus geishanensis, Chensaurus (=Anhuihusaurus) chaoxianensis, C. faciles [30,31] and five Lower Triassic
community sequences and 15 subbiocenozones based on paleoecological studies of the rich fossils in the Majiashan section [32–35]. Most of the work in Chaohu focused on the biostratigraphy, especially the conodont and ammonoid sequences, as well as the carbon and oxygen isotopic stratigraphy, magnetic stratigraphy, lithostratigraphy and sedimentology. Special attention has been paid to the strata from the Permian–Triassic boundary to the lower part of the Olenekian [21–24,36–44]. So we consider the Chaohu sections fit for the GSSPs of the Lower Triassic boundaries, maybe even the base of the Middle Triassic [20]. 2.1. North and West Pingdingshan sections of Chaohu, Anhui province The North and West Pingdingshan sections are situated on the western limb of the Majiashan–Pingdingshan Syncline, about 3 km from the center of Chaohu and north to the Chaohu Lake. At the West Pingdingshan section, the Lower Triassic is even better exposed except that about 10 m strata at the base of the Lower Triassic are covered by a road leading to a quarry on the hillside. However, the Permian–Triassic boundary can still be observed. The sequence exposure around the Induan– Olenekian boundary, up to the lower part of Spathian, is extremely good for study. The section is quite continuous and all the boundaries between the formations are very distinct. Our attention was paid to the intervals around the Induan–Olenekian boundary, where continuous sampling was undertaken. Among the 186 samples from the 74.63 m-thick North Pingdingshan section (Fig. 1), 80 yielded identifiable conodonts and some poorly preserved fragments. A total of 990 identifiable elements were assigned to 40 species in 6 genera. Seven conodont stratigraphic zones have been distinguished. They are, in ascending order, Hindeodus typicalis zone, Neogondolella krystyni zone, Neospathodus kummeli zone, N. dieneri zone, N. waageni zone, Neospathodus pingdingshanensis zone, and N. homeri zone. Of the 286 horizons were sampled in the 112.8 m thick West Pingdingshan section (Fig. 2), 172 productive samples contained identifiable conodonts. The extracted 2871 conodont elements included 54 stratigraphically significant species in 11 genera: Seven conodont zones were recognized in ascending order as follows: H. typicalis zone, Ng. krystyni zone, N. kummeli zone, N. dieneri zone, N. waageni zone, N. pingdingshanensis zone, and N. homeri zone (Fig. 2). 2.2. South Majiashan section The Lower Triassic of the Majiashan section is one of the best-studied sequences in China. Thirty-three samples from 100 horizons in the 206 m South Majiashan section [22], collected from the Helongshan and Nanlinghu formations, yielded about 1000 conodonts assigned to 14
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Fig. 1. Vertical range and zonation of conodonts, ammonoids and bivalves assemblage at North Pingdingshan section, Chaohu, Anhui Province. Modified after Zhao et al. [19] Key to sub/zones: 1 = N. dieneri M2; 2 = N. dieneri M3; 3 = N. w. eowaageni; 4 = N. pingdingshanensis.
species in 4 genera. Four conodont zones were recognized in ascending order as follows: N. waageni zone, N. pingdingshanensis zone, N. homeri zone and N. anhuinensis zone.
3. Conodont zonations A lot of new forms of conodonts have been extensively studied, and a comprehensive Lower Triassic conodont
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Fig. 2. Vertical range and zonation of conodonts, ammonoids and bivalves assemblage at West Pingdingshan section, Chaohu, Anhui Province. Modified after Zhao et al. [19].
zonation has been well defined, including eight conodont zones, most of which are regionally or globally correlated. The basic conodont data have been given in Ref. [21]. An
updated datum of conodonts from North and West Pingdingshan sections is listed in Figs. 1 and 2. Some key conodont species are introduced in Figs. 3 and 4.
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Fig. 3. Specimens from the West Pingdingshan section. 1–6, Neospathodus waageni eowaageni Zhao & Orchard. 1A, 1B, 1C are Sample (Bed): CP24-6-2 (Bed No. 24–16); 2, Sample (Bed): CP24-7 (Bed No. 24–20); 3A, 3B, 3C, Sample C24-8 (Bed No. 24–21); 4A, 4B, Sample (Bed): CP24-9 (Bed No. 24– 22); 5A, 5B, 5C, Holotype, Sample (Bed): CP27-1 (Bed No. 27); 6A, 6B, 6C, Sample (Bed): CP26-4 (Bed No. 26). 7–10, Neospathodus posterolongatus Zhao & Orchard. 7A, 7B, 7C, Sample (Bed): CP24-7 (Bed No. 24–20); 8A, 8B, 8C, Sample (Bed): CP24-9 (Bed No. 24–22); 9A, 9B, 9C, Sample (Bed): CP25-7 (Bed No. 25–25); 10A, 10B, 10C, Holotype, Sample (Bed): CP25-7 (Bed No. 25–25). 11, Neospathodus pakistanensis Sweet. 11A, 11B, Sample (Bed): CP25-7 (Bed No. 25–25). 12–13, Neospathodus spitiensis Geol. 12A, 12B, Sample (Bed): CP25-13 (Bed No. 25–31); 13A, 13B, Sample (Bed): CP25-7 (Bed No. 25–25).
3.1. Hindeodus typicalis zone The H. typicalis zone is based on occurrence of H. typicalis below the first occurrence of N. krystyni. Cooccurring conodonts are Neogondolella carinata, Neogon-
dolella planata, and Isarcicella? sp., in association with the ammonoids Ophiceras and Lytophiceras. The zone might range into lower strata but its base is undetermined because no conodonts have been recovered from the underlying Dalong Formation, which is composed
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Fig. 4. Specimens from the West Pingdingshan section. 1 and 2, Neospathodus waageni Sweet. Morphotype 1. 1A, Sample (Bed): CP25-8 (Bed No. 25–26); 2A, 2B, Sample (Bed): CP25-11 (Bed No. 25–29); 3A, 3B, 3C; 4A, 4B, 4C, N. waageni Sweet. Morphotype 2. Sample (Bed): CP25-11 (Bed No. 25–29); 5-7. N. waageni Sweet. Morphotype 4. 5A, 6A, 6B, Sample (Bed): CP25-11 (Bed No. 25–29); 7A, 7B, Sample (Bed): CP25-13 (Bed No. 25–31); 8–9. N. waageni Sweet. Morphotype 5. 8A, 8B, 8C, Sample (Bed): CP25-7 (Bed No. 25–25); 9A, 9B, Sample (Bed): CP25-10 (Bed No. 25–28); 10-11. N. waageni Sweet. Morphotype 6. Sample (Bed): CP25-7 (Bed No. 25–25); 12, 14–15. Neospathodus cf. waageni Sweet. Sample (Bed): CP25-6 (Bed No. 25–23); 13. Neospathodus cf. waageni Sweet. Sample (Bed): CP25-7 (Bed No. 25–25).
of chert beds. Hindeodus parvus, the index fossil of the Permian–Triassic Boundary (PTB), has not been found in Chaohu, so the PTB is placed at the middle of Bed 5 based on the correlation of the lithostratigraphic ‘‘PTB set’’ [37,38].
3.2. Neogondolella krystyni zone The Ng. krystyni zone is defined by the first occurrence of Ng. krystyni at its base and by the appearance of N. kummeli at its top. The index conodont co-occurs with
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Fig. 5. Induan–Olenekian boundary stratigraphical sequence at the West Pingdingshan section.
Ng. carinata and Ng. planata at the West Pingdingshan and North Pingdingshan sections, and with Ng. orchardi at West Pingdingshan. This is the first reported occurrence of Ng. krystyni in China. Both the H. typicalis and Ng. krystyni zones lie within the Ophiceras–Lytophiceras zone in Chaohu. The Ng. krystyni zone of Chaohu is believed to correspond to, or approximately, both the Ng. krystyni and Ng. discreta zones in Spiti sections [45], and can be also correlated with Isarcicella staeschei zone upper part, I. Isarcica zone and Neogondolella tulongensis–Ng. planata zone at Meishan D section [16].
shan and North Pingdingshan sections is N. dieneri Morphotype 1. At Chaohu, N. dieneri Morphotype 1 appears concurrently with N. kummeli, but we nevertheless choose to use the latter species to define a range zone. On the basis of its occurrences, the N. kummeli zone is largely Dienerian in age although Orchard and Tozer [46] reported it from the latest Griesbachian Bukkenites strigatus zone of the Arctic, and it also corresponds to the N. kummeli zone at the Meishan D section reported by Zhang et al. [16].
3.3. Neospathodus kummeli zone
The N. dieneri zone is subdivided into three subzones at the West Pingdingshan and North Pingdingshan sections in Chaohu based on the occurrence of the morphotypes that are differentiated on the basis of differences in the configu-
The N. kummeli zone is defined by the occurrence of N. kummeli. The associated conodont at the West Pingding-
3.4. Neospathodus dieneri zone
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ration of the cusp and penultimate denticle of the P1 elements. These subzones in ascending order are: Subzone N. dieneri Morphotype 1 defined at its base by the disappearance of N. kummeli, and its upper limit by the first occurrence of N. dieneri Morphotype 2; Subzone N. dieneri Morphotype 2 marked for the base by the first appearance of the name-giver and the upper limit by the first occurrence of N. dieneri Morphotype 3; and subzone of N. dieneri Morphotype 3 marked for the base by the first appearance of N. dieneri Morphotype 3 and the top by the first occurrence of N. waageni. The three morphotypes of N. dieneri appeared successively, but all range upwards into the higher subzones. The N. dieneri zone corresponds to N. cristagalli–N. dieneri zone of Zhang et al. [16] in the Meishan D section, and also to the N. dieneri zone and lower part of Neospathodus
pakistanensis zone of Wang et al. [17] in the Guandao section, southern Guizhou of China. 3.5. Neospathodus waageni zone The N. waageni zone is subdivided into two subzones based on the successive appearances of N. w. eowaageni (=N. waageni n. subsp. A, [21] and N. w. waageni. These two subspecies clearly occur in succession at the Pingdingshan sections. The N. w. eowaageni subzone is defined by the first occurrence of N. w. eowaageni, and the upper limit is marked by the first appearance of N. w. waageni. It comprises the previous N. waageni n.subsp. A (=N. w. eowaageni) Zhao and Orchard, Figs. 3, 1–6) subzone and N. waageni n.subsp. B (=Neospathodus posterolongatus Zhao and Orchard) subzone [20,21]. This subzone contains
Fig. 6. Conodont lineages across Induan–Olenekian boundary.
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all three morphotypes of N. dieneri, N. aff. cristagalli, N. aff. chaohuensis, N. cancavus, N. chii, N. posterolongatus (Figs. 3, 7–10), Neospathodus novaehollandiae, N. pakistanensis (Figs. 3, 11), N. n. sp. F, N. n. sp. K, and abundant ramiform elements at the West Pingdingshan section. The base of N. w. waageni subzone is determined by the first occurrence of N. w. waageni, and the top is marked by the first occurrence of N. pingdingshanensis. In this study, 5 morphotypes of N. waageni are distinguished (Fig. 4, 1– 11). Associated species include 5 morphotypes of N. waageni, N. cf. waageni (Fig. 4, 12–15), N. w. eowaageni, N. posterolongatus, N. aff. cristagalli, N. dieneri (all morphotypes), Neospathodus spitiensis (Fig. 3, 12–13), Neospathodus discretus, N. aff. chaohuensis, N. n. sp. K, Neospathodus concavus, N. novaehollandiae, N. pakistanensis, Neospathodus peculiaris, Neospathodus tongi, ‘Neospathodus alberti’, N. sp. G, N. sp. I, N. n. sp. H, Platyvillosus costatus, P. hamadai, Aduncodina unicosta, and numerous ramiform elements. Two subzones can be recognized at North Pingdingshan section. The base of the Flemingites–Eufemingites zone is 1.01 m above the base of the N. w. eowaageni. The first appearance datum (FAD) of N. w. waageni coincides with the base of Flemingites–Eufemingites beds. On the basis of its occurrence, the N. waageni zone corresponds to the N. waageni zone [47] in the western United States, to the N. waageni zone and Gondolella milleri zone of Krystyn [10] and to the N. waageni–Parachirognathus fauna in the Guandao section, southern Guizhou of China [17]. 3.6. Neospathodus pingdingshanensis zone The N. pingdingshanensis zone first found in China is defined by the appearance of N. pingdingshanensis, and its top is defined by the first occurrence of N. homeri. Associated conodonts in the zone are N. w. eowaageni, N. w. waageni, N. cf. abruptus, N. n. sp. R and N. n. sp. N. This zone has a narrow range compared with the overlying N. homeri zone and underlying N. waageni zone but it is distinctive with high abundance of the nominal species and a constant distribution at the sections in Chaohu. The associated conodonts at the North Pingdingshan section include Neogondolella aff. sweeti, Neogondolella elongata, Neospathodus aff. w. eowaageni, N. w. eowaageni, N. w. waageni, N. n. sp. R, Neospathodus eotriangularis, Neospathodus spathi, N. aff. crassatus, N. n. sp. O, N. n. sp. P, and N. n. sp. Q. The N. pingdingshanensis zone of Chaohu is believed to broadly correspond to the Neospathodus crassatus zone of Orchard [48] in California, also to the N. crassatus zone [17] in the Guandao section, southern Guizhou of China and lower part of the N. homeri–Neospathodus triangularis zone [18], Ganheqiao section of Wangmo, Guizhou. 3.7. Neospathodus homeri zone The N. homeri zone begins with the first occurrence of N. homeri, and the upper boundary is marked by the
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appearance of N. anhuinensis. Only the lower part of this zone exists at the Mt. Pingdingshan due to erosion of the Majiashan–Pingdingshan syncline. At the West Pingdingshan section, the associated conodonts are N. crassatus, N. spathi, N. eotriangularis, Cornudina breviramulis, N. n. sp. R, and abundant ramiform elements. The zonal fossil is also found at the North Pingdingshan section, in association with the conodonts Neospathodus abruptus, N. spathi, N. n. sp. R, and Cratognathus. The N. homeri zone was found in 18 samples at the South Majiashan section where, besides N. homeri and N. abruptus, existed the following species: N. spathi, N. brevissimus, C. breviramulis, N. eotriangularis, A. unicosta, and I. collinsoni. In Chaohu, we also include the specimen of I. collinsoni in the N. homeri zone. The N. homeri zone can broadly be correlated with the I. collinsoni zone and lower part of the N. triangularis zone of Orchard [48] in California, and also the Neospathodus symmetricus–N. homeri zone [17] in the Guandao section, southern Guizhou of China. 3.8. Neospathodus anhuinensis zone The N. anhuinensis zone, only known in the South Majiashan section, is defined by the first appearance of N. anhuinensis. The upper boundary is uncertain due to the rarity of conodonts from the strata in the topmost part of the section, which has been regarded as Anisian on the basis of regional correlation by Li [25]. The zone yields the species including N. anhuinensis, N. homeri, N. abruptus, N. spathi, C. breviramulis, and A. unicosta at the South Majiashan section. The species N. anhuinensis is an endemic form but it co-existed with the widespread species N. homeri. The zone is undoubtedly one of great values in the Lower Yangtze region, and has been recognized in several sections [49]. The N. anhuinensis zone in Chaohu is correlated with the upper part of N. symmetricus–N. homeri zone [17] in the Guandao section, southern Guizhou of China, and broadly corresponds to the N. symmetricus– Neogondolella regalis zones in North America [46]. 4. Induan–Olenekian boundary 4.1. Boundary strata The Induan–Olenekian boundary strata are composed of the mudstone–limestone alternations with three intercalated bentonite beds in the West Pingdingshan section, which provide good samples for various studies. In order to precisely construct the boundary sequence, lithologic sequence around the Induan–Olenekian boundary has been subdivided into about 60 subbeds mainly based on the natural beddings except for shale. The Induan–Olenekian boundary strata at the West Pingdingshan section have been examined in a great detail. The beds are subdivided into subbeds in centimeters and all data are based on the subbeds (Fig. 5). All studies for the boundary sequence
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have been based on this subdivision, including fossil collections and various samples. 4.2. Boundary index fossils In the boundary strata, the common fossils include conodonts, ammonoids and bivalves. Conodonts are very rich in the limestone beds and lens intercalated in mudstone while they are also observed in the mudstone beds. Conodonts, ammonoid and bivalves have been recovered from most of the subbeds in the section (Figs. 1, 2, 5). The conodont N. w. eowaageni (the first occurred subspecies of N. waageni zone) was suggested as the index fossil to define the boundary and the ammonoids Flemingites and Euflemingites are the reference markers. In the Pingdingshan sections (both North Pingdingshan and West Pingdingshan sections), N. dieneri is clearly grouped into three morphotypes, which occurred in an ascending order to define three subzones in the N. dieneri zone based on the differences in the configuration of the cusp and penultimate denticle of the P1 elements in Chaohu. Among the N. waageni Zhao and Orchard ever defined three subspecies, namely N. waageni n.subsp.A, N. waageni n.subsp.B and N. w. waageni [20,21]. And the N. waageni n.subsp. A is named N. w. eowaageni [19], and the N. waageni n.subsp. B is considered as a new species and named N. posterolongatus [19]. In Chaohu, the N. waageni group includes at least two characteristic forms: N. w. eowaageni (upright denticles) (=N. waageni n. subsp. A; N. w. eowaageni) and N. w. waageni Sweet, and they occur in succession at the section (Figs. 1, 2, 5). The fairly common and well-preserved specimens of N. w. eowaageni (upright denticles) were found in ten samples throughout the waageni zone. The repeated sampling and acidic analysis showed that the first appearance of N. w. eowaageni (upright denticles, Figs. 3, 1–6) is in subbed 24–16, that is 40.49 m above the base of the Yinkeng Formation. The first appearance of Flemingites lies in subbed 24–21. The base of the Flemingites–Euflemingites zone is 0.26 m above the base of the N. w. eowaageni subzone at the West Pingdingshan section. The exact point for the Induan–Olenekian boundary is the first appearance datum (FAD) of the conodont N. waageni. In Chaohu, this point is at the base of Subbed 24–16 in the West Pingdingshan section (Fig. 5). Consequently, the base of the N. w. eowaageni subzone (i.e., the base of the N. waageni zone) at the West Pingdingshan section has been proposed as the GSSP of the Induan–Olenekian boundary. The candidate section is also well-defined by magnetostratigraphy and carbon isotope stratigraphy [40,41,44]. According to the study of the conodont and ammonoid biostratigraphic sequences and various accumulated data, a new scheme of the global Induan and Olenekian Boundary is proposed and the FAD of conodont N. w. eowaageni is defined as the bottom of Olenekian. At North Pingdingshan section, N. w. eowaageni (upright denticles =N. waageni n. subsp. A; N. w. eowaa-
geni) appears in Sample A49-2, 0.36 m above the base of Bed 49, and 40.84 m above the base of the Yinkeng Formation. The Induan–Olenekian boundary defined by the FAD of N. w. eowaageni (upright denticles) at North Pingdingshan section is similar to that at West Pingdingshan section. 4.3. Regional occurrence The N. w. eowaageni (upright denticles) can also be recognized in Malaysia [50], Kashmir [51], Spiti [10] and Canada [52]. Fairly common and well-preserved specimens of N. posterolongatus were found in the six samples we collected. This species ranges through much of the N. waageni zone, appearing first in subbed 24–20 at 0.03 m below the FAD of the ammonoids Flemingites and Euflemingites at the West Pingdingshan section. N. posterolongatus is 40.77 m above the base of the Yinkeng Formation. It also has considerable potential as an index for the Induan– Olenekian boundary. 4.4. Conodont lineages across the Induan–Olenekian boundary In studying the conodonts from Spiti, Orchard [11] proposed a conodont lineage, N. pakistanensis–N. posterolongatus–N. spitiensis, across the Induan–Olenekian boundary. Based upon the data from Chaohu, the evolutionary lineages across the Induan–Olenekian boundary have been proposed including the boundary index fossils, N. dieneri Morphotype 3 – N. w. eowaageni–N. n. sp. R lineage and N. dieneri Morphotype 3 – N. w. waageni–N. pingdingshanensis lineage (Fig. 6). Therefore, The N. w. eowaageni (upright denticles) is an indicator of the Induan–Olenekian boundary. 5. Conclusions With the Lower Triassic conodont fossils in Chaohu, Anhui Province, detailed zonations are made. It implies that at least eight conodont stratigraphic zones can be distinguished. Thus, the representative conodont biostratigraphic sequences in the southern China are erected and precisely correlated with contemporaneous ones in the world. In the West Pingdingshan section, the first occurrence of N. w. eowaageni was recognized as the Induan– Olenekian boundary, and the distance between it and the base of the Flemingites–Euflemingites is 26 cm. These data are reliable evidence for erecting the global stratotype of the Induan–Olenekian boundary. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant Nos. 40325004, 40574028, 40621002), and the Geological Survey of Canada Project GK4700, IGCP 467 and GPMR2007. The authors are in-
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debted to Dr. Y. Zakharov for providing us with important data on ammonoid biostratigraphy, and to Hansen H. J. and Yin Hongfu for their comments on the manuscript. All of the SEM photographs were undertaken at the SEM Lab of Geological Survey of Canada. References [1] Yin H, Zhang K, Tong J, et al. The global stratotype section and point (GSSP) of the Permian–Triassic boundary. Episodes 2001;24(2):102–14. [2] Zakharov YD. Proposals on revision of the Siberian standard for the Lower Triassic and candidate stratotype section and point for the Induan–Olenekian boundary. Albertiana 1994;14:44–51. [3] Zakharov YD, Ukhaneva NG, Ignatijev AV, et al. Dorashamian, Induan, Olenekian, Anisian, Ladinian, Carnian, Norian and Rhaetian carbonates of Russia: stable isotopes, Ca/Mg ratio, and correlation. Albertiana 1999;22:27–30. [4] Dagys AS. Zonation of eastern boreal Lower Triassic and Induan– Olenekian boundary. Albertiana 1995;15:19–23. [5] Tea Kolar J, Bogdan J. First record of Hindeodus–Isarcicella population in Lower Triassic of Slovenia in the Ziri area of western Slovenia. Palaeogeogr Palaeoclimatol Palaeoecol 2007;252:72–81. [6] Perri MC, Farabegoli E. Conodonts across the Permian–Triassic boundary in the southern Alps. Cour Forschungs Senkenb 2003;245:281–313. [7] Krystyn L, Sylvain R, Aymon B, et al. A unique Permian–Triassic boundary section from the Neotethyan Hawasina Basin, Central Oman Mountains. Palaeogeogr Palaeoclimatol Palaeoecol 2003;191:329–44. [8] Koike T. Early Triassic Neospathodus (Conodonta) apparatuses from the Taho Formation, southwest Japan. Paleontol Res 2004;8(2):129–40. [9] Kozur H. Pelagic uppermost Permian and the Permian–Triassic boundary conodonts of Iran. Part I: Taxonomy. Hallesches Jahrbuch Geowiss 2004;18:39–68. [10] Krystyn L, Om NB, Devendra KB. Muth (Spiti, Indian Himalaya) – a candidate global stratigraphic section and point (GSSP) for the base of the Olenekian Stage. Albertiana 2005;33:51–3. [11] Orchard MJ, Krystyn L. Conodonts from the Induan–Olenekian boundary interval at Mud, Spiti. Albertiana 2007;35:30–5. [12] Ji Z, Yao J, Isozaki YK, et al. Conodont biostratigraphy across the Permian–Triassic boundary at Chaotian, in northern Sichuan, China. Palaeogeogr Palaeoclimatol Palaeoecol 2007;252:39–55. [13] Nicoll RS, Metcalfe I, Wang C. New species of the conodont genus Hindeodus and the conodont biostratigraphy of the Permian–Triassic boundary interval. J Asian Earth Sci 2002;20:609–31. [14] Luo G, Lai X, Jiang H, et al. Size variation of the end Permian conodont Neogondolella at Meishan section, Changxing, Zhejiang and its significance. Sci China (Ser D) 2006;49(4):337–47. [15] Wu Y. Conodonts, Reef evolution and mass extinction across Permian–Triassic boundary. Beijing: Geological Publishing House; 2005, 1–90. [16] Zhang K, Tong J, Shi G. Early Triassic conodont – palynological biostratigraphy of the Meishan D section in Changxing, Zhejiang Province, South China. Palaeogeogr Palaeoclimatol Palaeoecol 2007;252:4–23. [17] Wang H, Wang X, Li R, et al. Triassic conodont succession and stage subdivision of the Guangda section, Bianyang, Luodian, Guizhou. Acta Palaeontol Sin (in Chin) 2005;44(4):611–26. [18] Yao J, Ji Z, Wang Y, et al. Research on conodont biostratigraphy and age determination of the Lower–Middle Triassic boundary in southern part of Guizhou Province, China. Albertiana 2005;33:96–7. [19] Zhao L, Orchard MJ, Tong J, et al. Lower Triassic conodont sequence in Chaohu, Anhui Province, China and its global correlation. Palaeogeogr Palaeoclimatol Palaeoecol 2007;252:24–38.
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Progress in Natural Science 18 (2008) 79–90
A detailed Lower Triassic conodont biostratigraphy and its implications for the GSSP candidate of the Induan–Olenekian boundary in Chaohu, Anhui Province Laishi Zhao a
a,*
, Jinnan Tong a, Zhiming Sun
a,b
, Michael J. Orchard
c
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China b Institute of Geomechanics, Chinese Academy of Geological Science, Beijing 100081, China c Geological Survey of Canada, 101-605 Robson Street, Vancouver, Canada V6B5J3 Received 26 February 2007; received in revised form 24 April 2007; accepted 4 July 2007
Abstract Chaohu is located in a deep part of carbonate ramp on the Lower Yangtze Block, which belonged to the low-latitude eastern Tethyan archipelago during the Early Triassic. Fossils were very rich in the Lower Triassic of Chaohu. Bivalves, ammonoids, conodonts were very common throughout the Lower Triassic, while fish fossils were generally rich in some beds of the upper part. It is one of the most typical sections for the Early Triassic chronostratigraphy in the world. Although various fossils had been studied in the 1980s and 1990s, recent studies based upon new and more detailed collections from the Lower Triassic of Chaohu showed that the conodont zonation needs revision. We collected Lower Triassic conodont fossils from continuous sections of the West Pingdingshan, North Pingdingshan and South Majiashan, Chaohu, Anhui Province, and updated zonations were made for each section. Eight conodont zones have been distinguished. They are, in ascending order, Hindeodus typicalis zone, Neogondolella krystyni zone, Neospathodus kummeli zone, Neospathodus dieneri zone, Neospathodus waageni zone, Neospathodus pingdingshanensis zone, Neospathodus homeri zone, and Neospathodus anhuinensis zone. The first occurrence of Neospathodus waageni eowaageni of the N. w. eowaageni subzone (i.e. the base of the N. waageni zone) is suggested as the marker to define the Induan–Olenekian boundary. Ó 2007 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved. Keywords: Conodont biostratigraphy; Conodont zonation; Lower Triassic; Induan–Olenekian; Chaohu in Anhui Province
1. Introduction Establishment of the global stratotype section and point (GSSP) is one of the main purposes of the International Commission of Stratigraphy (ICS) in the past twenty years. So far about 42 GSSPs have been set up for the expected 96 Phanerozoic stages, and the rest of the GSSPs are expected to be established by 2008. The Triassic had its first GSSP and its base defined in 2001, that is the GSSP of the Permian and Triassic *
Corresponding author. Tel.: +86 27 62024366; fax: +86 27 67885096. E-mail address: [email protected] (L. Zhao).
boundary (PTB) at the base of Bed 27c of the Meishan section D, Changxing, Zhejiang Province [1]. The second Lower Triassic stage – the Induan and Olenekian boundary (IOB) has been extensively studied in recent years. By comparing with the H. himalayica in the Gondwanan Tethys region and the H. hedenstroemi in the boreal region, Zakharov et al. [2,3] proposed that the Tree Kamnya Cape Ravine section and the Abrek Bay section from the Russian South Primorye, which are believed to yield a mixed Boreal and Tethyan fauna, are candidates of the Induan–Olenekian boundary, and ammonoid Hedenstroemia bosphorensis was suggested as the index fossil to define the boundary.
1002-0071/$ - see front matter Ó 2007 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved. doi:10.1016/j.pnsc.2007.07.001
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But Dagys [4] pointed out that in the Induan and Olenekian boundary H. bosphorensis also occurred in the strata where Gyronites dominates, so its age is questionable. However, Hedenstroemia is a genus which prevailed in the high-latitude regions and scarcely occurred in the low-latitude regions, while Flemingites, Euflemingites, Meekoceras and others have been commonly considered as the index fossils to identify this boundary in the low-latitude regions. Though the conodonts in the high-latitude regions are not so common as in the low-latitude regions, most of the key forms are still recognizable one. Therefore, the conodonts should be considered in the definition of the Induan–Olenekian boundary. Lower Triassic conodont biostratigraphy has been established in some areas of the world, such as the sections in Lukac, Western Slovenia [5]; Tesero, Southern Alps [6]; Wadi Wasit, Central Oman [7]; Tahokamigumi and Kamura, Southwest Japan [8]; Abadeh, Central Iran [9]; Muth, Himalaya [10,11]; Chaotian, Meishan, Bian Yang, Qing Yan and Wangmo, China [12–18]. The published data indicated that Lower Triassic conodont zonations used to define the Induan–Olenekian boundary are very different from one area to the other in tropical Tethyan realm, even for the same section. Therefore, detailed conodont study on Lower Triassic strata at Chaohu (in centimeter scale around the Induan–Olenekian boundary) was conducted in order to establish a complete Lower Triassic conodont sequence and its intercalibration with the ammonoid sequence. 2. Geological setting The Lower Triassic in Chaohu is one of the well-developed Lower Triassic sequences in South China. The Lower Triassic sequence of Chaohu not only yields abundant fossils, but also provides a complete biostratigraphical sequence marked by conodonts and ammonoids [19–23], allowing for a well-defined chronostratigraphy. Early investigations on the Triassic geology of Chaohu began in the 1970s. The best-studied is Lower Triassic sequence in Chaohu in the Majiashan section, where the 1–200,000 regional geological survey was undertaken by the Anhui Geological Surveying Team in the 1970s. Fossils were very abundant and diverse throughout the Lower Triassic, including three bivalve assemblages [24–26]. Six Lower Triassic ammonoid zones were recognized in ascending order: Lytophiceras–Ophiceras zone, Prionolobus zone, Flemingites zone, Anasibirites zone, Tirolites–Columbites zone, and Subcolumbites zone by Guo [27] and Tong et al. [28], and six conodont zones in ascending order: Clarkina carinata zone, Neospathodus dieneri zone, Neospathodus cristagalli zone, Neospathodus waageni zone, Icriospathodus collinsoni zone, and Neospathodus anhuinensis– Neospathodus homeri zone by Ding [29]. Many other studies also found Lower Triassic ichthyosaur such as Chaohusaurus geishanensis, Chensaurus (=Anhuihusaurus) chaoxianensis, C. faciles [30,31] and five Lower Triassic
community sequences and 15 subbiocenozones based on paleoecological studies of the rich fossils in the Majiashan section [32–35]. Most of the work in Chaohu focused on the biostratigraphy, especially the conodont and ammonoid sequences, as well as the carbon and oxygen isotopic stratigraphy, magnetic stratigraphy, lithostratigraphy and sedimentology. Special attention has been paid to the strata from the Permian–Triassic boundary to the lower part of the Olenekian [21–24,36–44]. So we consider the Chaohu sections fit for the GSSPs of the Lower Triassic boundaries, maybe even the base of the Middle Triassic [20]. 2.1. North and West Pingdingshan sections of Chaohu, Anhui province The North and West Pingdingshan sections are situated on the western limb of the Majiashan–Pingdingshan Syncline, about 3 km from the center of Chaohu and north to the Chaohu Lake. At the West Pingdingshan section, the Lower Triassic is even better exposed except that about 10 m strata at the base of the Lower Triassic are covered by a road leading to a quarry on the hillside. However, the Permian–Triassic boundary can still be observed. The sequence exposure around the Induan– Olenekian boundary, up to the lower part of Spathian, is extremely good for study. The section is quite continuous and all the boundaries between the formations are very distinct. Our attention was paid to the intervals around the Induan–Olenekian boundary, where continuous sampling was undertaken. Among the 186 samples from the 74.63 m-thick North Pingdingshan section (Fig. 1), 80 yielded identifiable conodonts and some poorly preserved fragments. A total of 990 identifiable elements were assigned to 40 species in 6 genera. Seven conodont stratigraphic zones have been distinguished. They are, in ascending order, Hindeodus typicalis zone, Neogondolella krystyni zone, Neospathodus kummeli zone, N. dieneri zone, N. waageni zone, Neospathodus pingdingshanensis zone, and N. homeri zone. Of the 286 horizons were sampled in the 112.8 m thick West Pingdingshan section (Fig. 2), 172 productive samples contained identifiable conodonts. The extracted 2871 conodont elements included 54 stratigraphically significant species in 11 genera: Seven conodont zones were recognized in ascending order as follows: H. typicalis zone, Ng. krystyni zone, N. kummeli zone, N. dieneri zone, N. waageni zone, N. pingdingshanensis zone, and N. homeri zone (Fig. 2). 2.2. South Majiashan section The Lower Triassic of the Majiashan section is one of the best-studied sequences in China. Thirty-three samples from 100 horizons in the 206 m South Majiashan section [22], collected from the Helongshan and Nanlinghu formations, yielded about 1000 conodonts assigned to 14
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Fig. 1. Vertical range and zonation of conodonts, ammonoids and bivalves assemblage at North Pingdingshan section, Chaohu, Anhui Province. Modified after Zhao et al. [19] Key to sub/zones: 1 = N. dieneri M2; 2 = N. dieneri M3; 3 = N. w. eowaageni; 4 = N. pingdingshanensis.
species in 4 genera. Four conodont zones were recognized in ascending order as follows: N. waageni zone, N. pingdingshanensis zone, N. homeri zone and N. anhuinensis zone.
3. Conodont zonations A lot of new forms of conodonts have been extensively studied, and a comprehensive Lower Triassic conodont
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Fig. 2. Vertical range and zonation of conodonts, ammonoids and bivalves assemblage at West Pingdingshan section, Chaohu, Anhui Province. Modified after Zhao et al. [19].
zonation has been well defined, including eight conodont zones, most of which are regionally or globally correlated. The basic conodont data have been given in Ref. [21]. An
updated datum of conodonts from North and West Pingdingshan sections is listed in Figs. 1 and 2. Some key conodont species are introduced in Figs. 3 and 4.
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Fig. 3. Specimens from the West Pingdingshan section. 1–6, Neospathodus waageni eowaageni Zhao & Orchard. 1A, 1B, 1C are Sample (Bed): CP24-6-2 (Bed No. 24–16); 2, Sample (Bed): CP24-7 (Bed No. 24–20); 3A, 3B, 3C, Sample C24-8 (Bed No. 24–21); 4A, 4B, Sample (Bed): CP24-9 (Bed No. 24– 22); 5A, 5B, 5C, Holotype, Sample (Bed): CP27-1 (Bed No. 27); 6A, 6B, 6C, Sample (Bed): CP26-4 (Bed No. 26). 7–10, Neospathodus posterolongatus Zhao & Orchard. 7A, 7B, 7C, Sample (Bed): CP24-7 (Bed No. 24–20); 8A, 8B, 8C, Sample (Bed): CP24-9 (Bed No. 24–22); 9A, 9B, 9C, Sample (Bed): CP25-7 (Bed No. 25–25); 10A, 10B, 10C, Holotype, Sample (Bed): CP25-7 (Bed No. 25–25). 11, Neospathodus pakistanensis Sweet. 11A, 11B, Sample (Bed): CP25-7 (Bed No. 25–25). 12–13, Neospathodus spitiensis Geol. 12A, 12B, Sample (Bed): CP25-13 (Bed No. 25–31); 13A, 13B, Sample (Bed): CP25-7 (Bed No. 25–25).
3.1. Hindeodus typicalis zone The H. typicalis zone is based on occurrence of H. typicalis below the first occurrence of N. krystyni. Cooccurring conodonts are Neogondolella carinata, Neogon-
dolella planata, and Isarcicella? sp., in association with the ammonoids Ophiceras and Lytophiceras. The zone might range into lower strata but its base is undetermined because no conodonts have been recovered from the underlying Dalong Formation, which is composed
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Fig. 4. Specimens from the West Pingdingshan section. 1 and 2, Neospathodus waageni Sweet. Morphotype 1. 1A, Sample (Bed): CP25-8 (Bed No. 25–26); 2A, 2B, Sample (Bed): CP25-11 (Bed No. 25–29); 3A, 3B, 3C; 4A, 4B, 4C, N. waageni Sweet. Morphotype 2. Sample (Bed): CP25-11 (Bed No. 25–29); 5-7. N. waageni Sweet. Morphotype 4. 5A, 6A, 6B, Sample (Bed): CP25-11 (Bed No. 25–29); 7A, 7B, Sample (Bed): CP25-13 (Bed No. 25–31); 8–9. N. waageni Sweet. Morphotype 5. 8A, 8B, 8C, Sample (Bed): CP25-7 (Bed No. 25–25); 9A, 9B, Sample (Bed): CP25-10 (Bed No. 25–28); 10-11. N. waageni Sweet. Morphotype 6. Sample (Bed): CP25-7 (Bed No. 25–25); 12, 14–15. Neospathodus cf. waageni Sweet. Sample (Bed): CP25-6 (Bed No. 25–23); 13. Neospathodus cf. waageni Sweet. Sample (Bed): CP25-7 (Bed No. 25–25).
of chert beds. Hindeodus parvus, the index fossil of the Permian–Triassic Boundary (PTB), has not been found in Chaohu, so the PTB is placed at the middle of Bed 5 based on the correlation of the lithostratigraphic ‘‘PTB set’’ [37,38].
3.2. Neogondolella krystyni zone The Ng. krystyni zone is defined by the first occurrence of Ng. krystyni at its base and by the appearance of N. kummeli at its top. The index conodont co-occurs with
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Fig. 5. Induan–Olenekian boundary stratigraphical sequence at the West Pingdingshan section.
Ng. carinata and Ng. planata at the West Pingdingshan and North Pingdingshan sections, and with Ng. orchardi at West Pingdingshan. This is the first reported occurrence of Ng. krystyni in China. Both the H. typicalis and Ng. krystyni zones lie within the Ophiceras–Lytophiceras zone in Chaohu. The Ng. krystyni zone of Chaohu is believed to correspond to, or approximately, both the Ng. krystyni and Ng. discreta zones in Spiti sections [45], and can be also correlated with Isarcicella staeschei zone upper part, I. Isarcica zone and Neogondolella tulongensis–Ng. planata zone at Meishan D section [16].
shan and North Pingdingshan sections is N. dieneri Morphotype 1. At Chaohu, N. dieneri Morphotype 1 appears concurrently with N. kummeli, but we nevertheless choose to use the latter species to define a range zone. On the basis of its occurrences, the N. kummeli zone is largely Dienerian in age although Orchard and Tozer [46] reported it from the latest Griesbachian Bukkenites strigatus zone of the Arctic, and it also corresponds to the N. kummeli zone at the Meishan D section reported by Zhang et al. [16].
3.3. Neospathodus kummeli zone
The N. dieneri zone is subdivided into three subzones at the West Pingdingshan and North Pingdingshan sections in Chaohu based on the occurrence of the morphotypes that are differentiated on the basis of differences in the configu-
The N. kummeli zone is defined by the occurrence of N. kummeli. The associated conodont at the West Pingding-
3.4. Neospathodus dieneri zone
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ration of the cusp and penultimate denticle of the P1 elements. These subzones in ascending order are: Subzone N. dieneri Morphotype 1 defined at its base by the disappearance of N. kummeli, and its upper limit by the first occurrence of N. dieneri Morphotype 2; Subzone N. dieneri Morphotype 2 marked for the base by the first appearance of the name-giver and the upper limit by the first occurrence of N. dieneri Morphotype 3; and subzone of N. dieneri Morphotype 3 marked for the base by the first appearance of N. dieneri Morphotype 3 and the top by the first occurrence of N. waageni. The three morphotypes of N. dieneri appeared successively, but all range upwards into the higher subzones. The N. dieneri zone corresponds to N. cristagalli–N. dieneri zone of Zhang et al. [16] in the Meishan D section, and also to the N. dieneri zone and lower part of Neospathodus
pakistanensis zone of Wang et al. [17] in the Guandao section, southern Guizhou of China. 3.5. Neospathodus waageni zone The N. waageni zone is subdivided into two subzones based on the successive appearances of N. w. eowaageni (=N. waageni n. subsp. A, [21] and N. w. waageni. These two subspecies clearly occur in succession at the Pingdingshan sections. The N. w. eowaageni subzone is defined by the first occurrence of N. w. eowaageni, and the upper limit is marked by the first appearance of N. w. waageni. It comprises the previous N. waageni n.subsp. A (=N. w. eowaageni) Zhao and Orchard, Figs. 3, 1–6) subzone and N. waageni n.subsp. B (=Neospathodus posterolongatus Zhao and Orchard) subzone [20,21]. This subzone contains
Fig. 6. Conodont lineages across Induan–Olenekian boundary.
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all three morphotypes of N. dieneri, N. aff. cristagalli, N. aff. chaohuensis, N. cancavus, N. chii, N. posterolongatus (Figs. 3, 7–10), Neospathodus novaehollandiae, N. pakistanensis (Figs. 3, 11), N. n. sp. F, N. n. sp. K, and abundant ramiform elements at the West Pingdingshan section. The base of N. w. waageni subzone is determined by the first occurrence of N. w. waageni, and the top is marked by the first occurrence of N. pingdingshanensis. In this study, 5 morphotypes of N. waageni are distinguished (Fig. 4, 1– 11). Associated species include 5 morphotypes of N. waageni, N. cf. waageni (Fig. 4, 12–15), N. w. eowaageni, N. posterolongatus, N. aff. cristagalli, N. dieneri (all morphotypes), Neospathodus spitiensis (Fig. 3, 12–13), Neospathodus discretus, N. aff. chaohuensis, N. n. sp. K, Neospathodus concavus, N. novaehollandiae, N. pakistanensis, Neospathodus peculiaris, Neospathodus tongi, ‘Neospathodus alberti’, N. sp. G, N. sp. I, N. n. sp. H, Platyvillosus costatus, P. hamadai, Aduncodina unicosta, and numerous ramiform elements. Two subzones can be recognized at North Pingdingshan section. The base of the Flemingites–Eufemingites zone is 1.01 m above the base of the N. w. eowaageni. The first appearance datum (FAD) of N. w. waageni coincides with the base of Flemingites–Eufemingites beds. On the basis of its occurrence, the N. waageni zone corresponds to the N. waageni zone [47] in the western United States, to the N. waageni zone and Gondolella milleri zone of Krystyn [10] and to the N. waageni–Parachirognathus fauna in the Guandao section, southern Guizhou of China [17]. 3.6. Neospathodus pingdingshanensis zone The N. pingdingshanensis zone first found in China is defined by the appearance of N. pingdingshanensis, and its top is defined by the first occurrence of N. homeri. Associated conodonts in the zone are N. w. eowaageni, N. w. waageni, N. cf. abruptus, N. n. sp. R and N. n. sp. N. This zone has a narrow range compared with the overlying N. homeri zone and underlying N. waageni zone but it is distinctive with high abundance of the nominal species and a constant distribution at the sections in Chaohu. The associated conodonts at the North Pingdingshan section include Neogondolella aff. sweeti, Neogondolella elongata, Neospathodus aff. w. eowaageni, N. w. eowaageni, N. w. waageni, N. n. sp. R, Neospathodus eotriangularis, Neospathodus spathi, N. aff. crassatus, N. n. sp. O, N. n. sp. P, and N. n. sp. Q. The N. pingdingshanensis zone of Chaohu is believed to broadly correspond to the Neospathodus crassatus zone of Orchard [48] in California, also to the N. crassatus zone [17] in the Guandao section, southern Guizhou of China and lower part of the N. homeri–Neospathodus triangularis zone [18], Ganheqiao section of Wangmo, Guizhou. 3.7. Neospathodus homeri zone The N. homeri zone begins with the first occurrence of N. homeri, and the upper boundary is marked by the
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appearance of N. anhuinensis. Only the lower part of this zone exists at the Mt. Pingdingshan due to erosion of the Majiashan–Pingdingshan syncline. At the West Pingdingshan section, the associated conodonts are N. crassatus, N. spathi, N. eotriangularis, Cornudina breviramulis, N. n. sp. R, and abundant ramiform elements. The zonal fossil is also found at the North Pingdingshan section, in association with the conodonts Neospathodus abruptus, N. spathi, N. n. sp. R, and Cratognathus. The N. homeri zone was found in 18 samples at the South Majiashan section where, besides N. homeri and N. abruptus, existed the following species: N. spathi, N. brevissimus, C. breviramulis, N. eotriangularis, A. unicosta, and I. collinsoni. In Chaohu, we also include the specimen of I. collinsoni in the N. homeri zone. The N. homeri zone can broadly be correlated with the I. collinsoni zone and lower part of the N. triangularis zone of Orchard [48] in California, and also the Neospathodus symmetricus–N. homeri zone [17] in the Guandao section, southern Guizhou of China. 3.8. Neospathodus anhuinensis zone The N. anhuinensis zone, only known in the South Majiashan section, is defined by the first appearance of N. anhuinensis. The upper boundary is uncertain due to the rarity of conodonts from the strata in the topmost part of the section, which has been regarded as Anisian on the basis of regional correlation by Li [25]. The zone yields the species including N. anhuinensis, N. homeri, N. abruptus, N. spathi, C. breviramulis, and A. unicosta at the South Majiashan section. The species N. anhuinensis is an endemic form but it co-existed with the widespread species N. homeri. The zone is undoubtedly one of great values in the Lower Yangtze region, and has been recognized in several sections [49]. The N. anhuinensis zone in Chaohu is correlated with the upper part of N. symmetricus–N. homeri zone [17] in the Guandao section, southern Guizhou of China, and broadly corresponds to the N. symmetricus– Neogondolella regalis zones in North America [46]. 4. Induan–Olenekian boundary 4.1. Boundary strata The Induan–Olenekian boundary strata are composed of the mudstone–limestone alternations with three intercalated bentonite beds in the West Pingdingshan section, which provide good samples for various studies. In order to precisely construct the boundary sequence, lithologic sequence around the Induan–Olenekian boundary has been subdivided into about 60 subbeds mainly based on the natural beddings except for shale. The Induan–Olenekian boundary strata at the West Pingdingshan section have been examined in a great detail. The beds are subdivided into subbeds in centimeters and all data are based on the subbeds (Fig. 5). All studies for the boundary sequence
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have been based on this subdivision, including fossil collections and various samples. 4.2. Boundary index fossils In the boundary strata, the common fossils include conodonts, ammonoids and bivalves. Conodonts are very rich in the limestone beds and lens intercalated in mudstone while they are also observed in the mudstone beds. Conodonts, ammonoid and bivalves have been recovered from most of the subbeds in the section (Figs. 1, 2, 5). The conodont N. w. eowaageni (the first occurred subspecies of N. waageni zone) was suggested as the index fossil to define the boundary and the ammonoids Flemingites and Euflemingites are the reference markers. In the Pingdingshan sections (both North Pingdingshan and West Pingdingshan sections), N. dieneri is clearly grouped into three morphotypes, which occurred in an ascending order to define three subzones in the N. dieneri zone based on the differences in the configuration of the cusp and penultimate denticle of the P1 elements in Chaohu. Among the N. waageni Zhao and Orchard ever defined three subspecies, namely N. waageni n.subsp.A, N. waageni n.subsp.B and N. w. waageni [20,21]. And the N. waageni n.subsp. A is named N. w. eowaageni [19], and the N. waageni n.subsp. B is considered as a new species and named N. posterolongatus [19]. In Chaohu, the N. waageni group includes at least two characteristic forms: N. w. eowaageni (upright denticles) (=N. waageni n. subsp. A; N. w. eowaageni) and N. w. waageni Sweet, and they occur in succession at the section (Figs. 1, 2, 5). The fairly common and well-preserved specimens of N. w. eowaageni (upright denticles) were found in ten samples throughout the waageni zone. The repeated sampling and acidic analysis showed that the first appearance of N. w. eowaageni (upright denticles, Figs. 3, 1–6) is in subbed 24–16, that is 40.49 m above the base of the Yinkeng Formation. The first appearance of Flemingites lies in subbed 24–21. The base of the Flemingites–Euflemingites zone is 0.26 m above the base of the N. w. eowaageni subzone at the West Pingdingshan section. The exact point for the Induan–Olenekian boundary is the first appearance datum (FAD) of the conodont N. waageni. In Chaohu, this point is at the base of Subbed 24–16 in the West Pingdingshan section (Fig. 5). Consequently, the base of the N. w. eowaageni subzone (i.e., the base of the N. waageni zone) at the West Pingdingshan section has been proposed as the GSSP of the Induan–Olenekian boundary. The candidate section is also well-defined by magnetostratigraphy and carbon isotope stratigraphy [40,41,44]. According to the study of the conodont and ammonoid biostratigraphic sequences and various accumulated data, a new scheme of the global Induan and Olenekian Boundary is proposed and the FAD of conodont N. w. eowaageni is defined as the bottom of Olenekian. At North Pingdingshan section, N. w. eowaageni (upright denticles =N. waageni n. subsp. A; N. w. eowaa-
geni) appears in Sample A49-2, 0.36 m above the base of Bed 49, and 40.84 m above the base of the Yinkeng Formation. The Induan–Olenekian boundary defined by the FAD of N. w. eowaageni (upright denticles) at North Pingdingshan section is similar to that at West Pingdingshan section. 4.3. Regional occurrence The N. w. eowaageni (upright denticles) can also be recognized in Malaysia [50], Kashmir [51], Spiti [10] and Canada [52]. Fairly common and well-preserved specimens of N. posterolongatus were found in the six samples we collected. This species ranges through much of the N. waageni zone, appearing first in subbed 24–20 at 0.03 m below the FAD of the ammonoids Flemingites and Euflemingites at the West Pingdingshan section. N. posterolongatus is 40.77 m above the base of the Yinkeng Formation. It also has considerable potential as an index for the Induan– Olenekian boundary. 4.4. Conodont lineages across the Induan–Olenekian boundary In studying the conodonts from Spiti, Orchard [11] proposed a conodont lineage, N. pakistanensis–N. posterolongatus–N. spitiensis, across the Induan–Olenekian boundary. Based upon the data from Chaohu, the evolutionary lineages across the Induan–Olenekian boundary have been proposed including the boundary index fossils, N. dieneri Morphotype 3 – N. w. eowaageni–N. n. sp. R lineage and N. dieneri Morphotype 3 – N. w. waageni–N. pingdingshanensis lineage (Fig. 6). Therefore, The N. w. eowaageni (upright denticles) is an indicator of the Induan–Olenekian boundary. 5. Conclusions With the Lower Triassic conodont fossils in Chaohu, Anhui Province, detailed zonations are made. It implies that at least eight conodont stratigraphic zones can be distinguished. Thus, the representative conodont biostratigraphic sequences in the southern China are erected and precisely correlated with contemporaneous ones in the world. In the West Pingdingshan section, the first occurrence of N. w. eowaageni was recognized as the Induan– Olenekian boundary, and the distance between it and the base of the Flemingites–Euflemingites is 26 cm. These data are reliable evidence for erecting the global stratotype of the Induan–Olenekian boundary. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant Nos. 40325004, 40574028, 40621002), and the Geological Survey of Canada Project GK4700, IGCP 467 and GPMR2007. The authors are in-
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