Late Permian To Early Triassic Sedimentology,Sequence Stratigraphy And Palaeoclimatology Of North China

During the Permian-Triassic mass extinction(PTME)interval,both sea and land experienced major turnovers in biota and environments.This coincided with rapid warming and was followed by an extraordinary hyperthermal event followed this during the Early Triassic when the hottest temperatures of the Phanerozoic occurred.Sea-surface temperatures approached 40°C,with profound consequences for the ocean’s sedimentology and faunal distributions.However,the influences of the major climatic and environmental changes on land during the Late Permian to Early Triassic are still debated.Therefore,studies on Permian—Triassic transition terrestrial records are urgently needed.The well-developed Permian and Triassic continuous terrestrial outcrops in North China are prerequisites for relevant geological studies.During the Late Permian to Early Triassic,North China was located on the north side of the eastern part of the Palaeo-Tethys Ocean at low and middle latitudes.Its unique palaeogeographic location helps fill the gaps in the study of ecosystems and environments developed in a range of lake and river settings at low palaeolatitudes.Currently,studies of the region have focused on local areas or just a few attributes of the environments.It is therefore imperative to conduct a sedimentological study across a broad region of North China.This study includes three main themes: basin evolution and accompanying climate changes of the Late Permian to Early Triassic,unusual sedimentary structures in the Early Triassic,and the establishment of a sequence stratigraphy-based chronostratigraphic framework for the Late Permian to Early Triassic of North China.Other than tectonic controls,sedimentological characteristics are closely bound with palaeo-vegetation distributions and palaeoclimate fluctuations.The Permian-Triassic mass extinction in terrestrial settings is commonly linked with major sedimentological and climatic changes.The traditional viewpoint suggests there was a switch from meandering to braided fluvial systems and increased aridity;an idea first proposed in Karoo Basin,Africa and latter supported by evidence from the Kuznetsk Basin of Russia and Bowen Basin of Australia.We have examined the predominantly terrestrial strata(11 locations)of North China to reveal that,on the contrary,there was little substantial sedimentological change during the Permian-Triassic mass extinction.Instead,a significant sedimentological change is seen to occur slightly higher within the lowest Triassic.The transition saw a switch fluvial and floodplain conditions of the Sunjiagou Formation to dominantly braided fluvial and shallow lacustrine/fluviodeltaic conditions of the Liujiagou Formation.This study argues that the difference in global basins’ sedimentology is possibly caused by different tectonics and climates.Another possibility is that a poorly resolved stratigraphic framework of previous studies,linked to the uncertainty of the location of the Permian-Triassic boundary has led to earliest Triassic sedimentological changes being attributed to events at the boundary.In the North China basins,sedimentological changes are consistent with the lithostratigraphic transformations.(1)three depositional packages develop in Sunjiagou Formation: the lower part is dominated by grey-green sandstones interbedded with grey-green or dark-red mudstone,which is interpreted to form part of a distally-terminating alluvial fans.The middle part is dark-red,thick-bedded mud-siltstone with calcareous nodules development and thick-bedded sandstone,deposited in alluvial-fluvial plains with low-sinuosity rivers.The upper part,developed in the south-west of the region,consists of thin-bedded marls or silty calcarenite—mud-siltstone alternations with marine fossils,for example,bivalves and gastropods.These strata are interpreted to have formed in shallow lacustrine conditions with marine influence i.e.a coastal plain setting.In summary,the Sunjiagou Formation was deposited in low-sinuosity rivers—medial terminal fan—coastal plain settings.(2)The Liujiagou Formation is dominated by multi-storey sandstones with minor,fine-grained floodplain deposits interbedded in distal areas.Braided fluvial to coastal lacustrine environments are recorded but previously reported aeolian facies were not encountered in this study and were unlikely to have been a major constituent given the prevalence of lacustrine and fluvial conditions.(3)Later in the Early Triassic,fine-grained pedogenic deposits,playa lake,and alluvial plain facies became widespread(Heshanggou Formation).This study also provides a detailed examination of shallow lacustrine sediments from the Lower Triassic succession of North China,where abundant,unusual sedimentary structures are well-developed.These consist of diverse fluvial-to-shallow lacustrine sandstones with spectacular,coarse conglomerates composed of diverse,intraformational clasts reworked from the interbedded sediments.The conglomerate beds can show inverse grading,high angle,flat-pebble imbrication in their lower part,and vertically orientated flat pebble fans in their upper part.The cobbles include cemented-and-reworked conglomerate intraclasts and concentrically-laminated sandstone concretions that record multi-step growth and reworking.Calcite cementation of the sandy lake bed was pervasive and likely facilitated by high temperatures.The conglomerate record frequent,high-energy events capable of brecciation of a lithified lake bed and transporting cobbles in wave-influenced sediment-gravity flows.A depositional model is invoked in which powerful oscillatory flows brecciated and deflated the lake bed and subsequently helped sustain turbulence during short-distance lateral flow.It is possible that hurricanes originating from the adjacent hyperthermal Palaeo-Tethyan Ocean travelled into the major lakes of the North China continent during the Early Triassic.For palaeoclimate analysis,this study used the development of various palaeosol types and inferred annual,or interannual mean river discharges as markers for determining climate humidity changes.Palaeosols are present throughout the basin fill and record a transition from Aridisols and Vertisols in the Late Permian(Sunjiagou Formation)to Inceptisols in the Early Triassic(Liujiagou Formation),followed by gleyed Aridisols,Vertisols and Inceptisols later in the Early Triassic(Heshanggou Formation).Meanwhile,in the Liujiagou Formation,the lack of deep incision at channel bases indicates that there was only slight variance in peak discharge(major flood events would be expected to cause substantial erosion and incision which is not observed),which indicates relatively persistent humidity.Overall,the North China Basin experienced an increase in seasonal rainfall in the latest Permian and a base-level rise that introduced marine conditions into the southwest of the basin.The Early Triassic saw frequent seasonal or interannual rainfall and intense evaporation in a hot climate.The simultaneity of end-Permian marine and terrestrial mass extinctions is an important scientific question and a hot debate.However,the lack of key fossils and datable ash beds in terrestrial settings makes it much more difficult to correlate the marine and terrestrial stratigraphic sequences in North China.This study attempts to establish a chronostratigraphic framework based on the sequence stratigraphy,supplemented with age-significant palaeontological and geochemical data and newly-available palaeomagnetism results.Three sequences have been identified from the uppermost Permian to the Lower Triassic,divided by the sequence boundaries beneath the base of Liujiagou Formation,in the uppermost Liujiagou Formation or at the base of Heshanggou Formation,and at the top of Heshanggou Formation,which correspond with three hiatuses.The first hiatus likely spanned the Dienerian,whilst the second hiatus spanned the late Smithian to the early Spathian.The youngest hiatus started at the beginning of Anisian(Middle Triassic).Overall,three stages have been identified in the development of the North China Basin in this study.The basin was initially occupied by a low-sinuosity fluvial—coastal plain in the Late Permian and around the Permian-Triassic transition(Sunjiagou Formation),and then by braided fluvial-shallow lacustrine conditions in the early Early Triassic(Liujiagou Formation),and finally by shallow lacustrine—playa lake environments in the late Early Triassic(Heshanggou Formation).Substantial sedimentological changes occurred in the terrestrial Permo-Triassic environments of North China,but there was no abrupt transition in fluvial styles around the Permian—Triassic mass extinction.Meanwhile,the wet-dry climate gradually changed from the aridity of the Late Permian to increased precipitation in the Latest Permian,humidity increased further in the early Early Triassic,and finally to seasonal,semi-humidity was established in the late Early Triassic.Storm-generated sedimentary structures and concentrate-laminated concretions are thought to be the product of global warming in the Early Triassic.Additionally,an initial sequence stratigraphy-based chronostratigraphic framework is established.