Available online at www.sciencedirect.com 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,*, Jinnan Tong a, Zhiming Sun a,b, Michael J. Orchard c a 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 dis- tinguished. They are, in ascending order, Hindeodus typicalis zone, Neogondolella krystyni zone, Neospathodus kummeli zone, Neospath- odus 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 boundary (PTB) at the base of Bed 27c of the Meishan section D, Changxing, Zhejiang Province [1]. The second Establishment of the global stratotype section and Lower Triassic stage – the Induan and Olenekian bound- point (GSSP) is one of the main purposes of the Interna- ary (IOB) has been extensively studied in recent years. tional Commission of Stratigraphy (ICS) in the past By comparing with the H. himalayica in the Gondwanan twenty years. So far about 42 GSSPs have been set up Tethys region and the H. hedenstroemi in the boreal for the expected 96 Phanerozoic stages, and the rest of region, Zakharov et al. [2,3] proposed that the Tree the GSSPs are expected to be established by 2008. The Kamnya Cape Ravine section and the Abrek Bay section Triassic had its first GSSP and its base defined in from the Russian South Primorye, which are believed to 2001, that is the GSSP of the Permian and Triassic yield a mixed Boreal and Tethyan fauna, are candidates of the Induan–Olenekian boundary, and ammonoid * Corresponding author. Tel.: +86 27 62024366; fax: +86 27 67885096. Hedenstroemia bosphorensis was suggested as the index E-mail address: [email protected] (L. Zhao). 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 80 L. Zhao et al. / Progress in Natural Science 18 (2008) 79–90 But Dagys [4] pointed out that in the Induan and Olene- community sequences and 15 subbiocenozones based on kian boundary H. bosphorensis also occurred in the strata paleoecological studies of the rich fossils in the Majiashan where Gyronites dominates, so its age is questionable. section [32–35]. However, Hedenstroemia is a genus which prevailed in Most of the work in Chaohu focused on the biostratig- the high-latitude regions and scarcely occurred in the raphy, especially the conodont and ammonoid sequences, low-latitude regions, while Flemingites, Euflemingites, as well as the carbon and oxygen isotopic stratigraphy, Meekoceras and others have been commonly considered magnetic stratigraphy, lithostratigraphy and sedimentol- as the index fossils to identify this boundary in the low-lat- ogy. Special attention has been paid to the strata from itude regions. Though the conodonts in the high-latitude the Permian–Triassic boundary to the lower part of the regions are not so common as in the low-latitude regions, Olenekian [21–24,36–44]. So we consider the Chaohu sec- most of the key forms are still recognizable one. Therefore, tions fit for the GSSPs of the Lower Triassic boundaries, the conodonts should be considered in the definition of the maybe even the base of the Middle Triassic [20]. Induan–Olenekian boundary. Lower Triassic conodont biostratigraphy has been 2.1. North and West Pingdingshan sections of Chaohu, established in some areas of the world, such as the sections Anhui province in Lukac, Western Slovenia [5]; Tesero, Southern Alps [6]; Wadi Wasit, Central Oman [7]; Tahokamigumi and Kam- The North and West Pingdingshan sections are situ- ura, Southwest Japan [8]; Abadeh, Central Iran [9]; Muth, ated on the western limb of the Majiashan–Pingdingshan Himalaya [10,11]; Chaotian, Meishan, Bian Yang, Qing Syncline, about 3 km from the center of Chaohu and Yan and Wangmo, China [12–18]. The published data indi- north to the Chaohu Lake. At the West Pingdingshan cated that Lower Triassic conodont zonations used to section, the Lower Triassic is even better exposed except define the Induan–Olenekian boundary are very different that about 10 m strata at the base of the Lower Triassic from one area to the other in tropical Tethyan realm, even are covered by a road leading to a quarry on the hillside. for the same section. Therefore, detailed conodont study However, the Permian–Triassic boundary can still be on Lower Triassic strata at Chaohu (in centimeter scale observed. The sequence exposure around the Induan– around the Induan–Olenekian boundary) was conducted Olenekian boundary, up to the lower part of Spathian, in order to establish a complete Lower Triassic conodont is extremely good for study. The section is quite contin- sequence and its intercalibration with the ammonoid uous and all the boundaries between the formations are sequence. very distinct. Our attention was paid to the intervals around the Induan–Olenekian boundary, where continu- 2. Geological setting ous sampling was undertaken. Among the 186 samples from the 74.63 m-thick North Pingdingshan section The Lower Triassic in Chaohu is one of the well-devel- (Fig. 1), 80 yielded identifiable conodonts and some oped Lower Triassic sequences in South China. The Lower poorly preserved fragments. A total of 990 identifiable Triassic sequence of Chaohu not only yields abundant fos- elements were assigned to 40 species in 6 genera. Seven sils, but also provides a complete biostratigraphical conodont stratigraphic zones have been distinguished. sequence marked by conodonts and ammonoids [19–23], They are, in ascending order, Hindeodus typicalis zone, allowing for a well-defined chronostratigraphy. Neogondolella krystyni zone, Neospathodus kummeli zone, Early investigations on the Triassic geology of Chaohu N. dieneri zone, N. waageni zone, Neospathodus pingding- began in the 1970s. The best-studied is Lower Triassic shanensis zone, and N. homeri zone. sequence in Chaohu in the Majiashan section, where the Of the 286 horizons were sampled in the 112.8 m thick 1–200,000 regional geological survey was undertaken by West Pingdingshan section (Fig. 2), 172 productive sam- the Anhui Geological Surveying Team in the 1970s. Fossils ples contained identifiable conodonts. The extracted 2871 were very abundant and diverse throughout the Lower Tri- conodont elements included 54 stratigraphically signifi- assic, including three bivalve assemblages [24–26]. Six cant species in 11 genera: Seven conodont zones were Lower Triassic ammonoid zones were recognized in recognized in ascending order as follows: H. typicalis ascending order: Lytophiceras–Ophiceras zone, Prionolobus zone, Ng. krystyni zone, N. kummeli zone, N. dieneri zone, Flemingites zone, Anasibirites zone, Tirolites–Colum- zone, N. waageni zone, N. pingdingshanensis zone, and bites zone, and Subcolumbites zone by Guo [27] and Tong N. homeri zone (Fig. 2). et al. [28], and six conodont zones in ascending order: Clarkina carinata zone, Neospathodus dieneri zone, Neos- 2.2. South Majiashan section pathodus cristagalli zone, Neospathodus waageni zone, Icri- ospathodus collinsoni zone, and Neospathodus anhuinensis– The Lower Triassic of the Majiashan section is one of Neospathodus homeri zone by Ding [29]. Many other stud- the best-studied sequences in China. Thirty-three samples ies also found Lower Triassic ichthyosaur such as Chaohu- from 100 horizons in the 206 m South Majiashan section saurus geishanensis, Chensaurus (=Anhuihusaurus) [22], collected from the Helongshan and Nanlinghu for- chaoxianensis, C. faciles [30,31] and five Lower Triassic mations, yielded about 1000 conodonts assigned to 14 L. Zhao et al. / Progress in Natural Science 18 (2008) 79–90 81 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 recog- 3. Conodont zonations nized in ascending order as follows: N. waageni zone, N. pingdingshanensis zone, N. homeri zone and N. anhuin- A lot of new forms of conodonts have been extensively ensis zone. studied, and a comprehensive Lower Triassic conodont 82 L.