Sedimentary Characteristics And Paleoclimate Evolution Of The Upper Triassic To Middle-Lower Jurassic In The Southwest Of The North Qiangtang Basin,Tibet

The middle of the Triassic?ca.225 Ma?is the key period for the evolution of the Qiangtang Basin.The basin is gradually depauperated due to the closure of the paleo-Tethys ocean and then formed the widely distributed paleo-weathering crusts on the top of the Upper Triassic Tumengela Formation,which mark the end of the evolution of the Qiangtang Basin.On the other hand,the Late Triassic?Norian-Rhaetian?volcanism and volcanic sedimentations are widely distributed on the Qiangtang Basin due to the rapid expansion of the Meso-Tethys Ocean?Bangong Lake-Nujiang Ocean?,which represent the onset of the newly evolution of the Qiangtang Basin.The sedimentary environment,basin nature,and paleoclimate have changed abruptly during the Late Triassic to Middle Jurassic.The Upper Triassic black/dark grey mudstones,formed on the basin depauperated period,are widely distributed on the lower of the Tumengela Formation,which have been considered as one of the most significant source rocks in the Qiangtang Basin and received more attention.In this paper,we selected the Upper Triassic Tumengela Formation?front of the sedimentary-tectonic transition boundary?and Upper Triassic Nadi Kangri Formation and Lower-Middle Jurassic Quemo Co Formation?rear of the sedimentary-tectonic transition boundary?in the southwest of the North Qiangtang Depression as the research object.The systematic sedimentology,sedimentary geochemistry,petrology,and mineralogy concentrating on these sediments are conducted to discuss the evolution of the sedimentary environment and paleoclimate during the Late Triassic to Middle Jurassic?front and rear of the sedimentary-tectonic transition boundary??Meanwhile,the zircon U-Pb geochronology and sedimentology?sedimentary sequences?have been constrain the nature and onset timing of the newly Qiangtang Basin,and then combined with previous regional tectonic evolution research to discuss its geodynamic mechanism.Finally,the paleoclimate,paleoproductivity,paleo-redox condition,provenance,and sedimentation rate during the Upper Triassic Tumengela Formation black/dark grey mudstones deposition are conducted,which are used to analyze the factors controlled the organic material accumulated and establish their formation model.Firstly,the sedimentary facies of the Tumengela,Nadikangri,and Quemo Co formations are recognized based on the lithofacies,sedimentary structure,grain size,palaeontology and geochemistry marks,and then reconstructed the evolution of the sedimentary environment in the front and rear of the sedimentary-tectonic transition boundary?during the Late Triassic to Middle Jurassic?.The Tumengela Formation is formed in the deltaic environment and can further recognize two sedimentary sub-facies including delta front and plain and five sedimentary micro-facies including subsea interdistributary bay,subsea interdistributary channel,interdistributary bay,interdistributary channel,and natural levee.The Nadi Kangri Formation is formed in the continental volcanic explosive,effusive and alluvial–diluvial environment.The Quemo Co Formation is formed in the braided fluvial,restricted platform and tidal flat environment and can further recognize eight sedimentary sub-facies including braided fluvial channel,braid bar,flood plain,lagoon,intra-platform shoal,low tidal flat,middle tidal flat,and high tidal flat.The Tumengela Formation is characteristic by a shallowing-upward regressive sequence and finally uplift to terrisian.After that,large-scale NadiKangri Formation volcanism and volcanic sedimentations occurred during the Late Triassic and then formed a deepening-upward transgressive sequence during the Quemo Co Formation deposition.Secondly,the sedimentary sequences and nature of the newly Qiangtang Basin have been constrained based on the sedimentary facies and sedimentary environment.The sedimentary sequences of the newly Qiangtang Basin are composed of continental volcanic sedimentary succession consisting of explosive and effusive facies associations?Nadi Kangri Fm.?,continental siliciclastic sedimentary succession consisting of alluvial–diluvial and braided fluvial facies associations?Nadi Kangri Fm.and first member of the Quemo Co Fm.?,and marine carbonate and siliciclastic sedimentary successions composed of restricted platform and tidal flat facies associations?second and third member of the Quemo Co Fm.?.The sedimentary sequences of the newly Qiangtang Basin have undergone a significant sedimentary overlap from continental volcanic and continental siliciclastic successions to marine carbonate and siliciclastic successions,which are similar to those of the representative rift basin.Volcanic-volcanoclastic rocks of the Nadi Kangri Formation are formed in the syn-rifting stage.The newly Qiangtang Basin is dominated by the rapid expansion of the Meso-Tethys Ocean.Thirdly,the onset timing of the newly Qiangtang Basin has been constrained based on the zircon U-Pb age of tuff and stratigraphic contact relationship.The basal conglomerate or volcanic/volcaniclastic rocks on the bottom of the Nadi Kangri Formation are the initial filling of the newly Qiangtang Basin,their ages represent the onset timing of the basin.The tuff on the bottom of the Nadi Kangri Formation is directly overlapped on the Upper Triassic Tumengela Formation in the Fanghu area,which have three volcanic-eruptive sedimentary cycle and their ages are 221.7±1.3 Ma,217.0±1.5 Ma,and 207.1±1.3 Ma,respectively.Combined with the previous zircon ages of the Nadi Kangri Formation,we consider that 221.7–220.4 Ma is the initial age of the onset of the newly Qiangtang Basin,and the rifting lasted for at least 19.3?Ma?221.7-202.4 Ma?.Fourthly,the paleoclimatic conditions in the southwest of the North Qiangtang Depression during the Tumengela,Nadikangri,and Quemo Co formations deposition are analyzed based on the methods of the geochemistry,petrology,and mineralogy,and then reconstructed the evolution of the paleoclimate in the front and rear of the sedimentary-tectonic transition boundary?during the Late Triassic to Middle Jurassic?.The paleoclimate is warm and humid,but show aridification-upward during the Tumengela Formation deposition,which have moderate chemical weathering index values?CIAcorr,PIAcorr,CIW and CPA?and decrease-upward,coal line,plant fossils,low feldspar contents,and some siderite.However,the paleoclimate is hot and arid during the Nadi Kangri and Quemo Co formations,which have low chemical weathering index values,red beds contain abundant caliche nodules,evaporative rocks such as dolomite,micrite,oolitic limestone and gypsum,high feldspar and hematite contents.The abrupt climate change is consistent with the timing of the sedimentary-tectonic transition?Late Triassic Norian-Rhaetian?,which may be controlled by the evolution of the Qiangtang Basin.The warm and humid climate during the early of the Late Triassic may be related to the closure of the paleo-Tethys Ocean,and the aridification may be related to the uplift of the Qiangtang Basin after collision.However,the hot and arid during the middle of the Late Triassic to early of the Middle Jurassic may be related to the rapid expansion of the Meso-Tethys Ocean.Abundant CO₂ have been released during the rapid expansion of the Meso-Tethys Ocean,which will lead to the temperature increased and trigger the hot and arid climate.However,this mechanism should be further certified in the next study.Finally,the main factors controlled the organic material accumulated and their formation model of the black/dark grey mudstones on the lower of the Upper Triassic Tumengela Formation?Woruo Mountain-Fanghu section?have been established based on the comprehensive analysis of their paleoclimate,paleo-redox,paleo-productivity,provenance,and sedimentation rate.The TOC contents in the lower black mudstones?0.71-3.29%,avg.1.67%?are distinctly higher than those of the upper gray mudstones?0.54-0.88%,avg.0.64%?.The paleoclimate condition was warm and humid during thse mudstones deposition,as indicated by moderate chemical index of alteration(CIA_corr=77-82).The water columns have relatively high primary productivity,which are supported by the relatively high P concentrations.The oxidizing water conditions were widely distributed on the Woruo Mountain-Fanghu area,as suggested by the paleoredox indicators(Mn,U/Th,and C_org/P_tot)and size of framboidal pyrites.However,the upper gray mudstones were formed in the more oxidizing conditions than the lower black mudstones.The TiO₂-Zr,Co/Th-La/Sc,La/Th-Hf,and La/Yb-?REE bivariate diagrams and??La?_N/??Yb?_N ratio values indicate that the provenance of these mudstones was primarily from felsic igneous rocks and with fast sedimentation rate.The relatively high organic carbon contents of mudstones are mainly related to high primary productivity and fast sedimentation rate,which will lead to preservation of some organic matter even when bottom waters are completely oxidizing.The felsic detrital matter input during the mudstones deposition would result in dilution of organic matter.The TOC contents of gray mudstones are significant lower than that of black mudstones may be related to more oxidizing conditions and relatively low paleoproductivity during them deposition.