Lower Triassic Conodont Successions From The Enshi Area,Hubei Province,South China And Kashmir

The end-Permian mass extinction was the most severe in Earth history.The subsequent recovery of the marine ecosystems occurred gradually 1 Myr after the P-T mass extinction,which was a critical period that witnessed major changes in the form of Earth's biosphere from Paleozoic to Mesozoic.To better understand the time and rate of this recovery,a high-resolution timescale supported by an integrated biochronologic framework is crucial.Representatives of the conodont families survived up to,and crossed the PTB without major change,which make them significant in the study of the PTB.Hence,high-resolution conodont biostratigraphy offers one of the most useful and practical tools for developing a high-resolution timescale for the recovery of marine ecosystems,and an important role in the stratigraphiccorrelation between different marine deposition environment.Lower Triassic marine carbonate strata are well exposed in South China,which has a clear geologic record and abundant fossil fauna,making it an ideal region for biostratigraphic and chronostratigraphic research.When compared with those exposed in the Lower Yangtze and Upper Yangtze regions,the Lower Triassic strata of the Enshi area in Middle Yangtze region has received much less attention,and little biostratigraphic study has been undertaken.Paleogeographically,both Ganxi and Jianshi sections were situated on a carbonate ramp,while Ruiping section situated on a carbonate platform,which were located at the north of the Yangtze platform during the P-T transition.These sections have complete and continuous marine sedimentary strata,and are ideal for the study of the uppermost Permian to Lower Triassic conodont biostratigraphic and depositional environment.A fourth section,Guryul Ravine,was located on the northern margin of Gondwana,on the southern side of the Neotethys inner sea during the P-T transition,and was different from South China in paleogeographic deposition environment.It was once the candidate GSSP section of the PTB and regarded as an ideal section for PTB study.Lower Triassic conodont biostratigraphy was studied continually during 1970s-1980 s in this section,but the unstable political conditions in Kashmir have precluded access to and sampling of the section for the past 30 years.The emphasis of this thesis is to study the Permian-Triassic boundary and the Induan-Olenekian boundary in the Enshi area and Kashmir,to enhance the precision of stratigraphiccorrelation of the Lower Triassic,and to offer a high-resolution timescale.Lower Triassic conodont biostratigraphy in these sections contribute to the establishment of a high-resolution Lower Triassic conodont sequence,and enable correlations of the uppermost Permian to Lower Triassic conodont zones from the Meishan,Chaohu and Spiti sectons.The result of thestudy are given below:1.A high resolution Lower Triassic conodont successions has been established from theEnshi area,western Hubei Province,where the key conodont zones including: 1)Hindeodus parvus Zone,2)Isarcicella staeschei Zone,3)Hindeodus postparvus Zone,4)Hindeodus sosioensis Zone,5)Clarkina planata Zone,6)Neoclarkina aff.discreta Zone,7)Neospathodus dieneri Zone,and 8)Novispathodus waageni Zone.There are three conodont Zones: Clarkina yini Zone,Clarkina meishanensis Zone,and Hindeodus changxingensis Zone were established from uppermost Permian in Jianshi section;and four conodont zones had beed recognised in the P-T transtion in Ruiping section,which fill the gap of conodont biostratigraphy research during this period in the study section.In addition,three morphotypes of Ns.dieneri have been differentiated in both Jianshi and Ganxi setion,and the Ns.dieneri zone was subdivided into three subzones based on the successive appearance of the three morphotypes,which improve the stratigraphic subdivision and correlation effectively.2.First accurately placed the Permian–Triassic boundary at Bed 136-2 based on the appearance of Hindeodus parvus in Jianshi section,and recognised the typical P-T transition conodont elements both Hindeodus parvus and Isarcicella staeschei in Ganxi setion for the first time,which is of great significance for discussionof the PTB in the study section.Furthermore,the microbialite in P-T transition bearing both Hindeodus parvus and Isarcicella lobata in Ruiping section,which is important to study the age of the microbialite.3.Both the Nv.waageni eowaageni and Nv.waageni waageni subspecies had been recognised from Jianshi section for the first time.And the Induan–Olenekian boundary(IOB)is drawn precisely at Bed 225 + 40 cm based on the first appearance of Nv.waageni eowaageni.So far,no elements corresponding to Nv.waageni eowaageni were yet found in Ganxi,so the IOB is tentatively placed at Bed 64,152.34 m above the base of the Daye Formation based on the FO of Nv.waageni waageni in Ganxi section.4.A total of 198 samples were collected from the Guryul Ravine section in Kashmir.Abundant conodonts elements were recovered.A total of 6452 elements included 1621 P1 elements.Seven conodont zones were recognized,in ascending order: Hindeodus parvusZone,Isarcicella staeschei Zone,Neoclarkina krystyni Zone,Neospathodus dieneri Zone,Novispathodus waageni Zone,Novispathodus pingdingshanensis Zone,and Triassospathodus homeri Zone.The PTB is placed within Bed 52 in Unit E2,80 cm above the base of Unit E2 based on the FO of Hindeodus parvus,which corresponds to the minimum level for the FO of Hindeodus parvus reported by previous workers.Inaddition,the index conodont fossil Novispathodus pingdingshanensis,used to mark the Smithian-Spathian Boundary,was also identified from this section,which is of value for the stratigraphiccorrelation between highlatitudearea and South China.5.Based upon the conodont data from Jianshi section,an ontogenetic and evolutionary model for both Novispathodus waageni eowaageni and Eurygnathodus costatus have been proposed in this study.The collections of Nv.w.eowaageni from Jianshi section have enriched the inventory of the reported Nv.w.eowaageni materials.They supplement the study of conodont evolution around the IOB,and demonstrate the wide distribution of the taxon beyond the candidate GSSP Chaohu section.Moreover,based on the abundant conodont collections of Eurygnathodus costatus in the IOB interval at Jianshi section,four probable stages(A,B,C,D)of ontogenetic and evolutionary change for Eurygnathodus costatus have been proposed.This species can play a significant role as an auxiliary index to mark the IOB,as advanced in this study as a new research outcome for the conodont evolution during this period.6.A clearer understanding of conodont assemblages from Enshi and Kashmir area results from this study.The conodont sequence erected from the study areas compare well with their counterparts elsewhere in South China(e.g.Meishan,Chaohu and Spitisections).Overall,the established conodont zones from the study sections provide a higher precision time frame for the study of coevolution of biology and the environment during the P-T transition,and provide additional data for conodont differentiation and their evolution in this crucial period of Earth history.