Study On Early-Middle Triassic Conodont Biostratigraphy In Nanpanjiang Area

The end-Permian mass extinction event and subsequent ecological environment recovery were one key period in the Earth history; meanwhile marine ecosystems had evolved from Paleozoic type to Mesozoic one. As a hot topic in scientific researching, the end-Permian mass extinction and its recovery in ecological environment had been under investigated for many years. The sections in the Nanpanjiang Basin could provide some important breakthroughs for a comprehensive recognition of the nature of the end-Permian mass extinction and the extraordinary nature of the Early Triassic world that followed. Marine microfossil, conodont, as a good index in biochronology, was used to subdivide the Early-Middle Triassic biostratigraphy, especially Olenekian Stage in this study. Meanwhile, event stratigraphy and Carbon stable isotope was combined to explain interrelationship between organism and environment. Five sections in the Early-middle Triassic were chosen in Nanpanjiang region, including platform facies Gaomao and Huaxi section (Guiyang, Guizhou province), basin margin facies Bianyang section (Luodian county, Guizhou province), basin facies Zuodeng section (Tiandong county, Guangxi province) and Wantou section (Fengshan county, Guangxi province).Bianyang section is located on the northwest flank of the Great Bank of Guizhou (GBG) in Nanpanjiang area. Strata from end-Permian and earlier Middle-Triassic were analysed through the high-resolution conodont biostratigraphy.10conodont zones was established in Bianyang section, in ascending order, they are:Clarkina yini Zone; Hindeodus changxingensis Zone; Hindeodus parvus Zone; Sweetospathodus kummeli Zone; Neospathodus dieneri Zone; Neospathodus cristagalli Zone; Discretella discreta Zone; Pachycladina-Parachirognathus assemblage zone; Icriospathodus collinsoni Zone and Triassospathodus homeri Zone, respectively. In Bianyang section II,5conodont zones were set up, in ascending order: Triassospathodus homeri Zone, Triassospathodus brochus Zone, Chiosella timorensis Zone, Nicoraella germanicus Zone and Nicoraella kockeli Zone. Based on the high-resolution conodont sequences, two conclusions were concluded:1) the Permian-Triassic boundary should be placed in the basal part of Bed6as the first appearance datum (FAD) of Hindeodus parvus. Whilst, we suggest the existence of H. changxingensis zone at this section represents the extent of sea level fall in the GBG and even the Nanpangjiang area.2) the Induan-Olenekian boundary was defined by the first occurrance of Discretella discreta and located at the94.4m higher than the basement of Bed1. Furthermore, the correlation of Discretella discrete and Novispathodus waageni was discussed. At the Bianyang section Ⅱ, Olenekian-Anisian boundary was defined at15.9m above the basal part of this section by the FAD of Chiosella timorensis.The previous study had demonstrated detailed conodont sequences at Zuodeng section Ⅰ. In this study, we just simply clarify conodont bioastratigrphy of the key strata through the controlled conodont samples collected. Six conodont zones are recognized, in ascending order:Hindeodus parvus Zone, Hindeodus sosioensis Zone, Neospathodus dieneri Zone, Discretella discreta Zone, Icriospathodus collinsoni Zone and Triassospathodus homeri Zone. Large samples were collected in the Zuodeng section Ⅱ,7conodont zones were recognized in the less than30m thickness strata, in ascending order, they are:Neospathodus dieneri Zone, Novispathodus waageni Zone, Pachycladina-Parachirognathus assemblage Zone, Novispathodus pingdingshanensis Zone, Icriospathodus collinsoni Zone, Triassospathodus homeri Zone and Triassospathodus brochus Zone. Some conodont elements were also identified in the Early-Middle Triassic boundary interval at Zuodeng section Ⅲ, such as Chiosella timorensis. According to the high resolution conodont sequences at Zuodeng section Ⅱ, Dienerian-Smithian boundary and Smithian-Spathian boundary can be defined at4.50m and11.08m above the base of Bed1by the FO of Novispathodus waageni and Novispathodus pingdingshanensis, respectively. The same situaiton is Olenekian-Anisian boundary can be placed at2m above the basal part by the FO of Chiosella timorensis at Zuodeng section Ⅲ. Comparing with previous study, the Permian-Triassic boundary is still defined at basal part of microbiolites. Additionally, conodont research reveals the existence of stratigraphic repetition between Zuodeng section Ⅰ and Zuodeng section Ⅱ, which is different from the earlier research.Large conodont samples were collected in the less than thickness13m strata in Wantou section. Three conodont zones were established as follows:Triassospathodus homeri Zone, Triassospathodus brochus Zone and Chiosella timorensis Zone. Moreover, the Olenekian-Anisian boundary was placed at8.22m higher than the base of Bed1according to the FAD of Chiosella timorensis. Meanwhile, conodont sequence was investigated in the Smithian-Spanthian boundary interval in the Laren section, Fengshan County. Then the Smithian-Spathian boundary was defined at the10m higher than the basal level by the FAD of Novispathodus pingdingshanensis.In platform facies Huaxi and Gaomao sections, Guiyang city, Guizhou province, Anshun Formation and Huaxi Formation mainly consist of dolomite, traditionally assigned to Middle Triassic strata. Whereas, conodont materials from the Gaomao section (comprising of Icriospathodus? crassatus, Icriospathodus collinsoni, Novispathodus abruptus, Triassospathodus symmetricus and Triassospathodus homeri) turned out that the lower part of Huaxi Formation in Gaomao section deposited in the mid-late Spathian period; conodont sequences from the Huaxi section (comprising of Novispathodus abruptus, Triassospathodus symmetricus and Triassospathodus homeri) revealed that the basal part of Huaxi Formation should correspond to the late-Spathian substage. Through the study of5sections from different sedimentary facies in the Nanpanjiang basin, it is in favor of better understanding of the Early Triassic conodont sequences in different facies. Two genera of Pachycladina and Parachirognathus can be found in the late-Spathian strata at Bianyang section (basin margin facies), Zuodeng section (basin facies) and Jiarong section (ramp facies). Therefore, we conclude that the sea level would be shallower in Nanpanjiang basin in this period. Based on the new and abundant conodont materials in the study, Triassospathodus brochus Zone was proposed to as one new conodont Zone to be established in the late-Spathian strata, the FAD of which is later than Triassospathodus homeri. This Zone can also be distinguished in Bianyang section II, Zuodeng section Ⅱ, Wantou section and other regions of the world.Different ecological condition could be reflected by different conodont genera. In this study the research focuses on the change of conodont genera and species in the episode of environment fluctuation in Early Triassic. Through analysis, Eurygnathodus and Icriospathodus appear usually in cooler climate period, corresponding to the positive fluctuation of δ13Ccarb. Therefore the two genera were suggested as the representative of cooler water elements and the biodiversity increasing was accompanied in this period. In this study, although Novispathodus pingdingshanensis had been confirmed to exist in ammoniod Xenocelites Zone in the latest-Smithian strata, it was still proposed as an index fossil to define Smithian-Spathian Boundary, because the species Novispathodus pingdingshanensis have the typical geological significance. The FAD of Novispathodus pingdingshanensis generally matched ralativly negative δ13Ccarb values, the highest values in the anoxic period and the hottest palaeo-ocean temperature value. Moreover, the range of Novispathodus pingdingshanensis was in the transitional phase. Afterwards, some new Spathian species had appeared."Green Bean Rock" beds were widely recongnized in Nanpangjian basin in Early-Middle Triassic boundary interval. For better understanding of this dispute on the correlation of "Green Bean Rock" and the FAD of Chiosella timorensis, conodont research was made in this interval. It convinces us that there is no exact temporal relationship between those two. Moreover, the numbers of "Green Bean Rock" beds were different in. the different regions and the range of the age was very longer than the former study. In the study, we tend to think the first appearance datum of Chiosella timorensis is contemporaneous in the different areas. Overall consideration, the FAD of Chiosella timorensis is more suitbale to define the early-middle Triassic Boundary, although it was identified that Chiosella timorensis could appear in the Haugi Zone in the latest-Spathian strata.