Research On Conodont Fauna From Upper Permian Dalong Formation And The Permian And Triassic Boundary In Western Hubei Area

The end-Permian saw the largest biological mass extinction in earth life's history with 75% of terrestrial species and 95% of marine species disappearing.Therefore,as the hotspot and frontier of today's international geological research,it is most importance to exhaustively research the co-evolution of biology and environment,the mechanism of the mass extinction and the main factors of the Early Triassic biological recovery.Conodont,due to the rapid evolution and wide distribution,plays a key role in the study of the coevolution of the biological and the environment during the end Permian.There are many complete marine sequences in the Permian and Triassic boubdary(PTB)in the Yangtze region,South China,and numerous studies of PTB conodonts have been reported.However,these PTB sections are mostly in shallow water environment,and materials from deep-water areas are rare,which may because of the difficulty of extracting conodonts from siliceous strata.However,basinal sections have the advantage of recording more complete information across the PTB.Contemporaneously,the western Hubei Basin was a large rift basin that formed an embayment in the northeastern margin of the Middle of the Yangtze Block.The Dalong Formation occurs extensively in western Hubei,and is considered a record of deep-water basin and deeper-water slope environments during the latest Permian,which is a good place for studying the coevolution of the biological and the environment in deep-water facies during PermianTriassic period.Here we present a detailed study on the latest Wuchiapingian to earliest Triassic conodont biostratigraphy and carbon isotope profile from the deep-water basin facies of the Zhuqiao,Shiligou and Qiyangba sections in the Middle Yangtze region of western Hubei,South China.The main conclusions are as follows:(1)According to the main lithological characteristics of study sections,the lower siliceous rocks part of Dalong formation in the Zhuqiao section,Shiligou section and Qiyangba sections are considered to be the deep-water basin with anoxic condition,the upper limestone part of Dalong formation is deposited in the basin margin facies.The bioclastic sparite of Daye formation is belong to the carbonate platform slope facies;(2)Ten species belonging to three genera(Clarkina,Hindeodus and Isarcicella)of P1 conodont were identified from Zhuqiao setion,includeing C.orientalis,C.guangyuanensis,C.transcaucasica,C.liangshanensis,C.wangi,C.meishanensis,C.orchard,H.parvus,H.pisai and I.isarcica.And five conodont zones have been identified,in ascending order,they are C.orientalis Zone,C.wangi Zone,C.meishanensis Zone,H.parvus Zone and I.isarcica Zone;(3)Seven species belonging to three genera of P1 conodont were identified from Shiligou section,includeing C.orientalis,C.wangi,C.subcarinata,C.changxingensis,C.yini,C.deflecta and H.aff.parvus.And five conodont zones have been identified,in ascending order,they are C.orientalis Zone,C.wangi Zone,C.subcarinata Zone,C.changxingensis Zone and C.yini Zone;(4)Four species belonging to one genera of P1 conodont were identified from Qiyangba section,includeing C.wangi,C.subcarinata,C.changxingensis and C.yini.And three conodont zones have been identified,in ascending order,they are C.wangi Zone,C.subcarinata Zone and C.yini Zone;(5)On Basis of conodont zones,the Wuchiapingian-Changhsingian boundary(WCB)of the Zhuqiao section is placed at 10.85 m in the base of bed 25 a and the WCB of the Shiligou section should be laid at 7.24 m in the lower of bed 6,meanwhile,the PTB of the Zhuqiao section is drawn at 11.74 m in bed 27 and the PTB of the Shiligou section is placed at 35.39 m in the bottom of bed 17.(6)According to the C.orientalis,C.wangi and C.meishanensis Zones identified from Dalong Formation at the Zhuqiao section,the C.orientalis,C.wangi,C.subcarinata and C.yini Zones identified from Dalong Formation at the Shiligou section and the C.wangi,C.subcarinata and C.yini Zones found from Dalong Formation at the Qiyangba section.We confirm the age range of Dalong Formation in western Hubei area is the late Wuchiapingian to Changhsingian.(7)Three conodont zones(C.subcarinata Zone,C.changxingensis Zone and C.yini Zone)are absent from the Dalong Formation at the Zhuqiao section between C.wangi zone and C.meishanensis.This may partly reflect the difficulty of extracting conodonts from siliceous strata that dominate this section.Alternatively,the absence could be due to a hiatus,possibly be between bed 24 and 25 a at the Zhuqiao section.(8)There are both carbon isotope negative excursions occur near the WCB and PTB at the Zhuqiao and Shiligou sections.The WCB ?13Ccarb negative excursion is in the C. orientalis zone at the Zhuqiao section with the average-1.08‰,follow by the brief positive peak between +0.51‰ and +0.84‰ around the WCB.At the Shiligou section,the WCB ?13Ccarb negative excursion is in the C.orientalis Zone and C.wangi Zone,then there is a rapid recovery to stable values with the average value 0.64‰ from the C.subcarinata Zone to C.yini Zone.The PTB ?13Ccarb negative excursions at the Zhuqiao section ranges from the C.meishanensis Zone to I.isarcica Zone.At the Shiligou section,the PTB ?13Ccarb values begins to gradual decline at the base of C.yini,with a sharp negative shift 3.62‰ in the upper part of C.yini Zone,followed by stable negative values with the average-0.80‰ during the Early Triassic.The negative carbon isotope excursions in the study area are the results of biological mass extinction and environment changes.(9)A Systematic understanding of the conodont biostratigraphy and the inorganic carbon isotope(?13Ccarb)of the Dalong Formation and the Permian-Triassic boundary in the western Hubei area are documented in this study.Which could be contrasted well with other classical section around the world(e.g.Meishan,Shangsi).All in all,this study provides a strong conodont biostratigraphy evidences for researching the temporal and spatial distribution of Dalong Formation,the evolution of western Hubei Basin,the Wuchiapingian-Changhsingian boundary and Permian-Triassic boundary in western Hubei.Meanwhile,provides new carbon isotope profile for studying the process of the Late Permian mass extinction.Finally,provides new materials for studying the co-evolution of the biological and the environment in deep-water facies during Permian-Triassic period.