Depositional Evolution Of Upper Permian To Middle Triassic Sequence In South Tibet

The Permian-Triassic mass extinction?PTME?,which was the most severe biotic crisis in the Phanerozoic history,caused the extinction of over 90%of marine fauna among which highly calcified biota were most suffered.The Paleozoic carbonate factory in tropical seas collapsed due to the loss of calcified marine biota such as corals,calcareous algae and large benthic foraminifera.The carbonate factory in the aftermath of the PTME was characterized by widespread occurrence of anachronistic facies including microbialite,seafloor carbonate cement,wrinkle structures,suggesting peculiar oceans that were somewhat similar to Precambrian ones.Most research regarding carbonate factory focuses on low latitude regions with very few on middle latitude regions that were indispensable parts of global carbonate factory.The recovery of the carbonate factory in middle latitude regions and the knowledge on the pattern how carbonate factory in such regions reacts to the PTME can help deepen our understanding on the mechanism of the PTME in high latitude regions.South Tibet was located on the northern margin of peri-Gondwana with paleolatitude of?40°,which provides an opportunity to investigate the change of carbonate factory in middle latitude regions across the PTME.The carbonate factory spanning from the Late Permian to the Middle Triassic in South Tibet is reconstructed via the method of sedimentology,geochemistry,and petrology.Unique physio-chemo and biological conditions are needed for the development of anachronistic facies or peculiar sediments in carbonate factory.Therefore they are keys to recover paleo-environment of oceans.Here three types of peculiar sediments are present in this paper to recover the paleoenvironment of the Early Triassic South Tibet.Main conclusions and highlights are as following:1.The evolution of Late Permian to Middle Triassic carbonate factory in South Tibet was revealed.A Late Permian cool-water carbonate factory rich in bryozoans,thick-shelled brachiopods and echinoderms developed in South Tibet.The cool-water carbonate factory collapsed during the PTME,and subsequently,an auto-micrite factory developed in South Tibet in the immediate aftermath of the PTME.Dolomitic muds with the absence of metazoans comprise the auto-micrite carbonate factory in which abundant fossilized microbes were found.A change in carbonate factory composition occurred in the late Early Triassic during which the auto-micrite carbonate factory is changed to deposition of bioclastic packstone/wackestone in shallow-water regions and sea-floor carbonate precipitate in deep-water regions.The Middle Triassic carbonate factory was similar to the Early Triassic one due to the long-suppressed biotic recovery.However,the Middle Triassic carbonate factory is suppressed by the enhanced input of silicates due to high subsidence rates and consequently mixed carbonate-silicate deposited.2.A new type of anachronistic facies was reported from the turbidite deposit of Lower Triassic Zhongbei Group at Xiukang section.Eleven beds of sea-floor carbonate precipitates?SCP?consisted of pseudo-aragonite crystals were found with an average thickness of 0.5?1 cm.These crystals show a homogeneous pattern under cathodoluminescence light with the absence of diagenetic bands.This is further supported by SEM scanning showing homogeneous element distributions and the fact that?¹³C values?-5‰?in SCP are averagely close to overlying limestones.A new model as to the formation of SCPs was proposed that the mixing driven by turbidite flows of shallow seawater with deep sulfidic seawater resulted in the oversaturation of calcium carbonate.The occurrence of SCPs suggests euxinic and sulfidic conditions in Early Triassic deep seas.3.The deposition of Early Triassic dolomitic muds in South Tibet was promoted by the bloom of microbes.Dolostones were found in four Permian-Triassic boundary sections with thin dolostone?0.5 m in shallow-water section?Selong?and thick dolostone 1.5?4 m in relative deep-water sections?Tulong,Qubu,Gongpu?.The PTB dolostones consist of fine dolomite crystals that are tens of micrometers in length.Dolomite crystals showing homogenous cathodoluminescence patterns are commonly embedded by dark grey organic matters.Abundant fossilized microbes and extracellular polymeric substances?EPS?were found in dolomite crystals.Additionally,the occurrence of PTB dolomite was coeval with a significant sulfate drawdown of the Early Triassic seawater evidenced by data of?³⁴S.It is thus proposed that the bloom of microbes in the aftermath of PTME promoted the widespread precipitation of dolomites in South Tibet.Microbial induced dolostones were the main production of the carbonate factory in the Early Triassic South Tibet,and it also suggests a metazoan suppressed and microbe flourished marine ecosystem.4.Early Triassic carbonate red beds were reported at Tulong and Xialong sections in South Tibet.Early Triassic marine beds that consisted of nodular bioclastic limestone were only found in relative deep-water sections with thickness ranging from1 to 6 m.Abundant burrows were found in red beds.Roman spectrum evidence suggests that the red colour was pigmented by hematites.Stratigraphic correlations reveal that the occurrence of red beds was associated with the changes in redox conditions of deep waters evidenced by framboid pyrite diameters.We proposed that the Early Triassic marine red beds formed during the period when deep water redox conditions transformed from being anoxic to dysoxic.Therefore,Early Triassic red beds in South Tibet can be indicators of marine benthic redox condition.