Early Triassic Conodont Biostratigraphy And Environmental Changes In Qinling Area And Its Correlation With South China

The Permian–Triassic（P–T）mass extinction was the most severe extinction event in geological history,and was succeeded in the Early Triassic by an interval characterized by frequent,large global carbon isotope excursions（CIEs）,multiple and widespread anoxic phases,hyperthermal climate and rapid cooling events.Marine organisms experienced several smaller-scale extinction events,especially around the Smithian–Spathian（S–S）boundary when the nektonic conodonts and ammonoids almost became extinct,and environmental conditions changed dramatically.The Qinling area of China is an ideal area to study the conodont biostratigraphy and environmental changes in the Early Triassic because of the huge thickness of the Early Triassic marine strata.It also provides a higher palaeolatitude record than South China in a setting with better connectivity to the Paleo-Tethys Ocean.After comprehensive study of the Yiwagou and Dengdengqiao sections in the west Qinling area,and the Erlongkou and Xiejiacao sections in northwestern South China,about 13,100 conodont specimens and 419 inorganic carbon isotope values were obtained,along with～560 thin sections and 68 polished rocks.The main conclusions are as follows:1.The high precision conodont biostratigraphic study at Yiwagou,Gansu Province enabled the establishment of 10 Early Triassic conodont zones at this section.In ascending order they are:Hindeodus parvus zone,H.postparvus zone,Neospathodus dieneri zone,Eurygnathodus costatus zone,E.hamadai zone,Novispathodus waageni-Scythogondolella mosheri assemblage zone,Pachycladina-Parachirognathus assemblage zone,Triassospathodus hungaricus zone,Ns.robustispinus zone and T.clinatus zone.Three late Permian conodont zones have been established at Erlongkou,in Fuling,Chongqing.In ascending order they are:Clarkina changxingensis zone,C.yini zone and C.meishanensis zone.Three Early Triassic conodont zones have been established at the Xiejiacao section in Guang’an,Sichuan.In ascending order they are:Parachirognthus-Pachycladina assemblage zone,T.hungaricus zone and T.symmetricus zone.Interestingly,a small number of conodonts spanning a long time range appeared in beds 5 to 10 at the Dengdengqiao section（Dangchang,Gansu Province）,they are T.triangularis of the Spathian age in Early Triassic,Chiosella timorensis of the early Anisian age in Middle Triassic,and Budurovignathus mungoensis of the late Ladinian age in Middle Triassic or the early Carnian age in Late Triassic.The Mosherella?fauna ranging from the Middle–Late Triassic transition to early Late Triassic was recovered from the upper part of bed 2 in a previous study,but has not been found in this study.According to the presence of the conodont B.mungoensis and the underlying strata,it can be inferred that the Dengdengqiao Formation is of Ladinian,Middle Triassic or younger age.The early–middle Triassic conodonts and a large number of algae fragments in the Dengdengqiao section may be the products of redeposition.2.Based on the conodont biostratigraphy andδ¹³C_carb records,the P–T,Induan–Olenekian（I–O）and S–S boundaries at Yiwagou and Xiejiacao have been defined.It is concluded that E.costatus cannot completely replace Nv.waageni as the marker for the global I–O boundary,because the latter is more widely distributed.A weathering crust in the Erlongkou section is placed at the bottom of C.meishanensis zone in Changhsingian.3.In total,in the carbonate-dominated west Qinling sections,8 microfacies were identified and grouped into 4 facies associations that correspond to 4 different environments:oolitic shoal,open-marine carbonate platform,storm-dominated inner platform in the platform and restricted carbonate platform.The open-marine carbonate platform and mixed-deposited platform facies were identified at Erlongkou and the open-marine carbonate platform and restricted carbonate platform facies were identified at Xiejiacao in the Smithian–Spathian strata.4.This study shows that the late Smithian crisis event occurs in both low-latitude South China and higher latitude Qinling area,and began around the middle–late Smithian boundary.This event not only caused severe extinctions of nektonic conodonts and ammonoids,but also affected other organisms such as bivalves,gastropods,foraminifera,implying a widespread and extensive extinction event.5.Several carbonate horizons in west Qinling have red pigmentations（marine red beds）due to the presence of mainly nano-to micron-sized goethite or hematite particles,indicating that they are marine authigenic iron oxides rather than terrigenous clastic inputs.In the Early Triassic,Smithian marine red beds were only distributed in the areas surrounding the Paleo-Tethys Ocean.Advection of shallow,anoxic ferruginous(Fe²⁺)waters from Paleo-Tethys Ocean onto the Qinling carbonate platform,followed by precipitation or iron oxides is likely the source of these marine red beds.Spathian marine red beds are also present and these show global distribution.Again,oxygenation of the global oceans may have seen waters from middle or deep settings of Paleo-Tethys Ocean supply ferruginous waters into the low productivity carbonate Qinling Sea where they oxidized and formed marine red beds.6.Early Triassic carbon isotope records show several rapid excursions,although their amplitudes vary considerably from region to region and also with water depth.For example,theδ¹³C_carbnegative excursion from early to middle–late Smithian（P2–N3）ranges in magnitude from～-9.7‰to～-2.9‰,the positive excursion from middle–late Smithian to early Spathian（N3–P3）ranges from～+2.6‰to～+11.9‰.The carbon isotope vertical gradientΔδ¹³C_vert values show large variations in the Nanpanjiang Basin,the Lower Yangtze Basin and the Arabian region,and there is no global signal.Up to now,the origins of the large excursions of carbon isotope values during Smithian and Spathian remain unclear and are not easily reconciled with global redox changes,and need further research.