Sedimentary Evolution Of Different Palaeogeographic Settings In The Northern Middle Yantze Platform During The Permian And Triassic Transion

The Permian–Triassic transition is an important turning point in history of earth.Not only occurred the most severe mass extinction event within the Phanerozoic during this critical time,but also sedimentary systems changed dramatically.This global sedimentolocial transformation is reflected in that lowerest Triasic strata display widespread anachronistic facies in shallow-water environments and black lithological units characterized by mudstone and marl limestone in deep-water environments,and chert and reef deposition in upper Permian strata end,documenting the evolution of multiple aspects of earth such as marine chemistry and tectonic setting.Because Permian–Triassic strata are well preserved in south China,here,we selected three sections which were distributed in a carbonate platform to basin transect from the northern of middle Yantze platform,and conducted analyses in sedimentary facies,carbonate microfacies,sedimentation rate,carbon isotope composition,and pyrite framboid of these three sections to explore sedimentary evolution of different water-depth environments from the northern middle Yantze platform during the Permia–Triassic transition and their environmental indications.Anachronistic facies refers mainly to carbonate sediments that reflect peculiar marine environments.The aftermath of end-Permian mass extinction is one of the periods when anachronistic facies ocurred diffusely.In South China,different types of anachronistic facies appeare orderly in time succession.Microbialites are often the first anachronistic sediments that formed immediately above the end-Permian mass extinction boundary in shallow carbonate platforms,and are subsequently followed by oolite and oncolite,which are sometimes intercalated by vermicular limestone.Because different types of anachronistic facies represent different depositional environments,this succession pattern of anachronistic facies preserved in the stratigraphy well records the evolution of marine environments following the end-Permian mass extinction.Based on the sedimentological analysis on the Yangjiawan section in South China,this study suggests that the transition from microbialites to oolitic limestone is associated with the rise of sea level and intensification of hydrodynamics,while the shift from oolite to oncolite reflects rapid rising of sea level in the beginning of the Early Triassic.However,the occurrence of vermicular limestones that are intercalated within the oolitic limestones is considered the products of temporarily-formed restricted platform environment.Anachronistic facies reach 75 m-thick at the Yangjiawan section.Stratratigraphic correlation reveals that this anachronistic facies interval approximately corresponds to the20 cm-thick sequence from beds 27 to 28 at the Global Stratotype Section and Point(GSSP)section of Meishan in Zhejiang Province,indicating much higher carbonate sedimentation rates in shallow-water platforms than in deeper environments in the aftermath of the end-Permian mass extinction.High rates of carbonate precipitation indicate supersaturation marine conditions with respect to calcium carbonate in shallow platforms,which may have resulted from the upwelling of alkaline anoxic deep waters accompanied with strong evaporation as a consequence of extremely high temperatures at that time.Nevertheless,it is the global sea-level rise in the early Triassic that provided accommodation space for the precipitation of great thickness of anachronistic carbonate sediments.The South China block was a major continental block,bordering Paleo-Tethys to the west and Panthalassa to the east.It was separated from the North China block by an ocean basin to the north,namely Qinling-Dabie Ocean,which was a northern part of Paleo-Tethys.In this oceanic basin,marine strata of Late Paleozoic and Early Triassic were well developed,clearly recording evolution of marine chemistry and tectonics during the Paleozoic–Mesozoic transition.Late Permian strata are composed of deep-water chert rich in ammonoids and radiolarians,but detrital breccia deposits is lacking,indicating a highly stable tectonic setting.The end-Triassic mass extinction event caused abruptly shifts in marine biology and sedimentary facies,but mainly carbonaceous shale and thin-bedded limestone of earliest Triassic indicate that it has inherited the Late Permian stable tectonic environment.However,our recent investigation at Chibi and Ruichang areas in Hubei of the northern margin of South China block revealed several beds of thick-bedded breccia limestone embedded in Early Triassic thin-bedded strata.The breccia is composed of lithified oolitic limestone formed in shallow-water environments.Storm origin proposed in previous studies cannot reasonably explain the formation of these breccia.Instead,earthquake is a preferred cause for breccia generation.Because these breccias were broken in shallow water but finally deposited in deep-water platform marginal environment,they must have been transported a long distance.Given the gentle slope in the northern margin of the South China block during Later Permian,breccias generated by earthquake were hardly fallen into basin along slope.Therefore,we suppose that earthquake generated a noticeably steep slope,giving rise to subsequent transportation of earthquake-triggered breccia to deep-water basin by gravity.The widespread distribution of near-identically earliest Triassic earthquake-trigged breccia limestone along the northern margin of the South China Block demonstrate that earthquake activated by plate motion was universal during Early Triassic.Although the Qinling-Dabie Ocean between the North and South China Blocks was finally terminated due to the Indosinian orogeny during Middle to Late Triassic,concentrated occurrences of earthquake-triggered breccia limestone along the northern margin of the South China Block reveal commencement of prelude of the Indosinian orogeny and shrinking of the Plaeo-Tethys at the beginning of Early Triassic.Despite high-resolution conodont biostratigraphy is unavailable and sedimentary facies is different,the there sections can be correlated through mass extinction boundary,event bed,sedimentary facies,and carbon isotope curve.Breccia limestone bed appears both in Chibi and Ruichang eariest Triassic strata,but number of the bed and characteristic of breccia are various.The breccias at Chibi are mainly composed of lithified oolitic limestone,while at Ruichang are micritic limestone,implying that their deposition are related to palaeogeographical background.These breccias were transported from shallow-water environment and deposited into deep-water environment.Compared with Ruichang area in deep basin,Chibi area was palaeogeographically located in ramp of middle water-depth and closer to breccia source.The sedimentary tranformation of entire Early Triassic strata of Chibi area that the first member of Daye formation is mainly mudstone and marl limestone,the second menber is thin-bedded limestone interbedded with two oolite breccia limestone beds,the third member is thin-bedded limestone interbedded with several ooid and oncolite limestone,the forth member is oolite,the Jialingjiang formation is micritic and breccia dolomite,and finally Middle Triassicc strata are teresstrial deposition,reflcets the gradual shallowing process from deep-water ramp to eventually continental environments,while tectonic activities have played a great role in causing the filling.The sedimentation rates in three sections all display abrupt changes across the end-Permian mass extinction boundary,increasing 13 times in Yangjiawan section of shallow-water platform and 43 times in Chibi section of deep-water ramp,respectively.This phenomenon has been widely proved in other global contemporaneous sections.Anslysis implied that the causes for this increase are divergent in different water-depth environment.Because deposition in shallow-water platforms are mainly anachronistic facies,the nature of which is carbonate,high sedimentation rates are linked to supersaturation with respect to carbonate,while earliest Triassic trangression provides the accommodation space.Sediments in deep-water settings are mudstone and marl limestone,so high sedimentation rates are related to intensification of terrigenous weathering and input.?¹³Ccarb background values are closely related to their palaeogeographical environment.?¹³Ccarb values in Chibi section of middle water-depth slolpe are lighter than those from Yangjiawan section of shallow-water carbonate platform and heavier than those from Ruichang section of deep-water basin.Strata erosion near the mass extinction boundary makes?¹³C_carb profile in Yangjiawan section slightly incomplete.The?¹³C_carbprofiles in three sections all show two negative excursions,but the exact timings are not identical.In addition,the largest negative carbon isotope excursion occures in Chibi section among the three sections,which may be ascribed to its ramp setting of middle water-depth and be easily affected by change of OMZ(oxygen minimum zone).Palaeoredox history of the three sections are various.In combination with a literature review,results show that in the northern deep-water environments,anoxia occurred before the mass extinction,and pulsed oxygenation event occurred after the mass extinction and returened to anoxia later;in middle and shallow water-depth environments,redox conditions before the mass extinction were oxygenic,and became anoxia immediately after the mass extinction but with several periods of alleviation of anoxia.