Lower-Middle Triassic Conodont Biostratigraphy In The Slope-basin Facies, South Guizhou

The conodonts are very common in the Triassic rocks, and have a wide range of distribution in sedimentary strata of different types. They evolved rapidly so they are usually used as a tool for the global stratigraphic correlation, and powerful in dating the successions from the Paleozoic to Triassic. During the Paleozoic-Mesozoic transition, which was the greatest transition in the Earth history, the geological and biological events caused the entire collapse of the Paleozoic ecosystems and the reconstruction of the Mesozoic-Cenozoic ecosystems.The Triassic strata are well developed in South China with various marine paleogeographic facies from platform to basin facies, being one of the best regions for the study of the great geological events from the latest Permian to the earliest Middle Triassic. A great achievement of bio-chronostratigraphy and sedimentary paleogeography have been well studied. The Qingyan Section is situated in the slope region, providing the complete sequence of extinction, survival, recovery and radiation during the Permian-Triassic interval. And this region yields the Qingyan Biota, which represented the great radiation of benthic organisms in the early Middle Triassic. The Sidazhai Section of Ziyun is deposited in deep-water basin and characterized by a major change from carbonate facies to siliciclastic facies through the Early/Middle Triassic transition. And such, Sidazhai is a good section to study the recovery and radiation process in the basin facies.This study aims to establish conodont biostratigraphy framework through the Lower and Middle Triassic sequence at two sections (Qingyan Section located at slope facies region, the joint zone betweend platform and basin; Sidazhai Section located at the basin facies region), for providing a detailed time-scale in the studing of evolution of ecological system during the Permian-Triassic interval.A total of30species in8genera were indentified at Qingyan Section, i.e. Neospathodus, Neogondolella, Neohindeodella, Cratognathodus, Xaniognathus, Chiosella, Cypridodella, Icriospathodus. And six conodont zones are recognized throught the Early and Middle Triassic strata, including Neospathodus dieneri Zone, Ns. cristagalli Zone, Ns. waageni Zone, Ns. homeri Zone, Chiosella timorensis Zone, and Neogondolella cf. bulgarica Zone. These conodont zones are precisely correlated with other section in South China. The Induan-Olenekian boundary at Qingyan Section is located in the lower-middle parts of Luolou Formation (the middle part of Bed22), wheras the Olenekian-Anisian boundary is situated at the bottom of Qingyan Formation (the base of Bed44).Ns. dieneri Zone includes the upper part of Shabaowan Formation and the base of Luolou Formation (Beds9-18), and belongs to the early Dienerian. The base of Ns. cristagalli Zone is defined by the First Occurrence (FO) of Ns. cristagalli, covering the lower-middle parts of Luolou Formation (Bed19-22), and belongs to late Dienerian. The Ns. waageni Zone covers the middle parts of Luolou Formation (Bed22-28) with the FO of Ns. waageni, and belongs to Smithian in age. The Ns. homeri Zone covers the upper part of Luolou and Anshun Formation, containing Ns. homeri and Ns. pingdingshanensis, and the base of this zone is defined by the FO of Ns. homeri. The occurrence of Ns. pingdingshanensis has extended the distribution range, which is first reported in Chaohu, Anhui Province. The Ch. timorensis Zone is the first zone of the Anisian, covering the lower part of Qingyan Formation, and the base part of this zone is defined by the FO of Ch. timorensis. The Neogondolella cf. bulgarica Zone occurred in the upper part of Mafengpo Member, suggesting an early Anisian age. The Leidapo Member containing Qingyan Biota belongs to early late Anisian in age based on the ammonoid zones and neighbouring conodont-bearing sections.A total of18species belonging to6conodont genera were uncovered at Sidazhai Section, i.e. Neospathodus, Neogondolella, Cratognathodus, Chiosella, Ctenognathus, and Icriospathodus. Four conodont zones are recognized in the Early-Middle Triassic, including Neospathodus waageni Zone, Ns. pingdingshanensis Zone, Ns. triangularis-Ns. homeri Zone, and Chiosella timorensis Zone.The Induan-Olenekian boundary at Sidazhai Section is located in Luolou Formation (the upper part of Bed22), wheras the Olenekian-Anisian boundary is situated at the base of Xinyuan Formation (Bed33).The Ns. waageni Zone began in the upper part of Bed14with the FO of Ns. waageni. This zone belongs to Smithian in age. The Ns. pingdingshanensis Zone appears in the upper part of Luolou Formation (Beds19-21), being marked by the FO of Ns. pingdingshanensis. The Ns. triangularis-Ns. homeri Zone ranges from the upper part of Luolou to Ziyun Formation (Bed22-the base of Bed33), being marked by the abundant occurrences of Ns. triangularis and Ns. homeri. This zone is assigned to the Spathian in age. The Chiosella timorensis Zone is marked by the incoming of eponymous species, which is also the sigin of the Olenekian-Anisian boundary worldwidely.Based on the study of conodont biostratigraphy, a sequence chronostrigraphic framework has been proposed:the Ns. dieneri Zone and Ns. cristagalli Zone belong to Dienerian; the Ns. waageni Zone is Smithian in age wheras Ns. pingdingshanensis Zone and Ns. triangularis-Ns. homeri Zone are suggested to Spathian age; the Chiosella timorensis Zone is the first zone of the Anisian, and the Ng. cf. bulgarica Zone assigned to early Anisian. The Lower-Middle Triassic conodont sequence in the two differenct facies is basically the same. The Induan-Olenekian boundary is defined by the FO of Ns. waageni, and the Olenekian-Anisian boundary is marked by the FO of Chiosella timorensis at these two sections. The conodont biostratigraphy at the slope and basin regions can be well correlated based on our conodont studies. The results of this research provides a time framework for the better understanding of the evolutionary process of the ecosystem structure in the aftermath of the end-Permian mass extinction and the coevolution between life and environment during the great transition from the Paleozoic to Mesozoic.