Middle And Late Triassic Palynoflora And Paleoclimate In Ordos Basin

The Triassic was a relatively warm period on Earth,during which time the Antarctic was covered by dense warm-temperate forests,and the high latitude areas in the Arctic were green in color with warm-temperate vegetation too.However,in the Early Triassic,the arid climates prevailed due to the rapid global warming since the end-Permian when the largest biological extinction and ecosystem collapse in the Phanerozoic happened.On the supercontinent of Pangea,the arid climate ravaged the low and middle latitude regions of both hemispheres,except for the high latitudes near poles.The only surviving vegetation was distributed along seasonal rivers in the form of"gallery forests"during the Early Triassic time.This climate and vegetation situation was changed in the early Late Triassic（Carnian）,and the extremely arid climate in the Early and Middle Triassic shifted into a humid climate in the Late Triassic-Middle Jurassic,and this great change was named as the Carnian Pluvial Event（CPE）.In recent years,the research on this climate event has become one of the most popular hot-spots in the field of paleoclimatology,more and more works focus on the marine-continental facies and terrestrial deposits both in low latitude and high latitude areas.Macrofossil plants and microfossil spores and pollen would witness the vegetation changes during this climate shift period,especially in some large continental basins.Generally,fossil spores and pollen can reflect the vegetative landscape in the basin（including the drainage and periphery mountainous areas）and might record this big event（i.e.CPE）.However,due to the difficulties in the comparison of marine and continental strata,the lack of effective constraints on the boundary between the Middle and Upper Triassic in terrestrial deposits severely obstructs the study of the Carnian Pluvial Event.In northwestern China,large terrestrial basins developed in the Triassic period,such as the Tarim Basin,the Junggar Basin,and the Ordos Basin,and in these basin continuous deposits were well developed.Among them,the Ordos Basin is a large inland basin developed on the basis of Paleozoic epi-continental shelf.Triassic fillings in the basin are huge,and a large number of animal and plant fossils and spore-pollen fossils preserved in the lacustrine sediments,which pull out an important window for studying the Carnian Pluvial Event.Based on the spore-pollen fossils of the Middle to Late Triassic in the Ordos Basin,the vegetation succession and paleoclimate changes were observed in the present thesis.Meanwhile,geochemical analysis such as isotope dating and organic carbon isotopes,etc.were used to calibrate the geological age and characterize the environmental changes,especially the pluvial episodes.All the material were collected from the Middle to Late Triassic in the Yanchang Formation（sensu lato）in the Tongchuan and Zhidan areas of Shaanxi Province.They were mainly focused on the YC6,YC7,and YC8 members of the formation,in order to find out the Middle and Upper Triassic boundary and to see the feedback of the CPE in the lacustrine facies in the inland Ordos Basin,and indirectly serve for oil and gas exploration.Through field excursion mapping and sampling of the Yanchang Formation outcropped in Yunmeng Township,Yijun County,Tongchuan City,Shaanxi Province,and core description and sampling of the Y1011 borehole in Yongning Town,Zhidan County,Yan’an City,Shaanxi Province,and indoor spore-pollen identification and geochemical analysis,the Middle-Late Triassic vegetation and climate were reconstructed.1.a total of 110 species from 50 genera of sporo-pollen fossils were identified from 87 samples collected from YC8-1 to YC7-3 sub-members of the Yunmeng profile.Based on the vertical distribution of each taxon and cluster analysis（Tilia software）of specimens on genus and species level,two palynological assemblages were established from bottom to top,namely,Lundbladispora watangensis–Taeniaesporites combinatus（WC）assemblage（YC8-1）and Lundbladispora communis–Discisporites granulus（CG）assemblage（YC7-3）.Meanwhile,a total of166 species from 66 genera were identified from 68 core samples of Y1011 borehole,ranging from YC8-1 to YC6-4 sub-members,and three palynological assemblages were established in ascending order,namely,1)Lundbladispora communis–Protopinus latebrosa（CL）assemblage（YC7-3+lower part of YC7-2）,2)Baculatisporites elegans–Pinuspollenites divulgatus（ED）assemblage（upper part of YC7-2+YC7-1）,and 3)Apiculatisporis globosus–Taeniaesporites divisus（GD）assemblage（YC6-4）.Moreover,the diversity of genus and species was used in cluster analysis and consistent palynological assemblages were resulted on the above profile and borehole samples.By comparing and correlation of the palynological assemblages from Y1011 borehole and Yunmeng profile,four assemblages were organized in the Ordos Basin from bottom to top,namely,Lundbladispora watangensis–Taeniaesporites combinatus（WC）assemblage（YC8-1）,Lundbladispora communis–Protopinus latebrosa（CL）assemblage（YC7-3+lower part of YC7-2）,Baculatisporites elegans–Pinuspollenites divulgatus（ED）assemblage（upper part of YC7-2+YC7-1）,and Apiculatisporis globosus–Taeniaesporites divisus（GD）assemblage（YC6-4）.2.Based on the global geological age distribution of the known genera and species in the above-mentioned assemblages,especially the comparison of the palynological fossils with the Carnian GSSP（Global boundary Stratotype Section and Point）palynological markers,and other palynological records which age is calibrated by marine fossils,such as conodonts and ammonites in Europe and South China,the geological age of the Lundbladispora watangensis–Taeniaesporites combinatus assemblage（YC8-1）was assigned to latest Ladinian（Middle Triassic）,and the Lundbladispora communis–Protopinus latebrosa assemblage（YC7-3+the lower part of YC7-2）and the Baculatisporites elegans–Pinuspollenites divulgatus assemblage（upper part of the YC7-2+YC7-1）were aged Julian substage of Carnian（Late Triassic）,corresponding to Julian-1 and Julian-2,respectively.The age of the Apiculatisporis globosus–Taeniaesporites divisus assemblage（YC6-4）was assigned to Tuvalian substage of Carnian.Furthermore,the co-occurrence of Dictyophyllidites harrisii,Apiculatisporis bulliensis,Aratrisporites xiangxiensis,Piceaepollenites omoriciformis,Podocarpidites ornatus,Discisporites,and Classopollis in the palynoflora in northern China could be used as an indicator for the beginning of Carnian in the Triassic boreal palynological realm.3.Zircon U-Pb laser ablation inductively coupled plasma mass spectrometer（LA-ICP-MS）isotopic dating was engaged for three tuff layers in the YC7-3sub-member on Yunmeng profile.The weighted mean ages in ascending order were236.0±1.7 Ma,234.8±2.1 Ma,and 234.1±2.4 Ma,which constrained the geological age of the YC7-3 to the Late Triassic Carnian stage,and highly consistent with the palynological age.4.By comparing to the Middle and Late Triassic palynological works in the Ordos Basin done before,the assemblages from the Yunmeng profile and the Y1011borehole posses the characters of those in Shaanxi,Gansu,Shanxi,Henan and other areas,i.e.the characters of the North China palynoflora.The high diversity and abundance of Lundbladispora at the end of the Ladinian and the appearance of Ovalipollis in the Carnian indicate that the vegetation in southern China and northern China had already undergone intermingling during the latest Middle to earliest Late Triassic.This also provides practical linkage for the palynofloras between southern and northern China,and might imply that the North China Plate and Yangzi Plate begone to collide in the earliest Carnian.5.In order to study the detail succession of Triassic vegetation,phytoecological analysis of the palynofloras on the Yunmeng profile and Y1011 borehole was conducted including cluster analysis,and four sub-assemblages,CG-1 and CG-2 of the Lundbladispora communis-Discisporites granulus（CG）assemblage,as well as CL-1 and CL-2 of Lundbladispora communis-Protopinus latebrosa（CL）assemblage were recognized.They are well consistent both on the profile and in the borehole.In addition,two sub-assemblages,ED-1 and ED-2,were identified in the Baculatisporites elegans-Pinuspollenites divulgatus（ED）assemblage.Overall,the latest Ladinian to Carnian vegetation in the Ordos Basin was diverse,they cover lakes,swamps,dry or wet lowlands,highlands,and mountainous areas.However,big changes of the vegetation happen in the end-Ladinian time when 70%of sporophytes and 37%of gymnosperms were extinct.Only a few floating plants survived in the eutrophic lakes,while some xerophytic plants represented by the Cheirolepidiaceae were prevailed.The overall Carnian vegetation indicate that the early Carnian climate is with strong seasonal arid characteristics.By counting the abundance and content of each ecological type,four major humid events were identified from the latest Ladinian to Carnian,located at the top of the WC palynological zone,the top of the CL-1（CG-1）and CL-2 palynological sub-zones,and the bottom of the GD palynological zone.Therefore,three pluvial events might be revealed in Carnian,and the main Carnian Pluvial Event（CPE）in the Ordos Basin might be aged earliest Tuvalian substage,which can be compared to the CPE in Europe or the western coast of Paleotethys.6.In order to further determine the climate and terrestrial ecosystems from the latest Ladinian to Early Carnian,total organic carbon（TOC）content,organic carbon isotopes(δ¹³C_org),and major,trace,and rare earth element geochemistry were analyzed for 87 samples on the Yunmeng profile.The trace elements indicate that the depositional environment was in a deep-water anoxic conditions during the latest Ladinian to Julian-1（early Carnian）.Negative shifts ofδ¹³C_org,as well as decreases in the Chemical Index of Alteration（CIA）,Chemical Index of Weathering（CIW）,Plagioclase Index of Alteration（PIA）,Loss on Ignition（LOI）,Lanthanide Rare Earth Elements（LREE）/Heavy Rare Earth Elements（HREE）ratio,Sr/Ba,and Sr/Co ratios,indicate the existence of moist events during the latest Ladinian,which brought massive sediment and C₃plants into the basin due to rapid flood transport.The pluvial event indicated by CL-1（CG-1）palynological zone is characterized by a positive shift inδ¹³C_org,but no significant changes in weathering indices or element geochemistry.This may show another case that in a large and deep inland basin the pluvial climate cause algae flourished and the chemical weathering delayed.Unfortunately,due to sampling limitations,comparisons with drilling cores were not possible,and geochemical analysis of the flood events during the early Tuvalian was not conducted.It needs to be addressed in future work.