Study On The Sedimentary Sequences And Basin Transformation Of The End-triassic To Early-middle Jurassic Period In The Northern Qiangtang Basin

The Qiangtang Basin,located in the northern Tibet,is the largest Mesozoic marine petroliferous sedimentary basin in China,while is still under-explored.During the end-Triassic period,the Qiangtang basin suddenly transformed from carbonate(or clastic sediment)into volcanic-volcanic clastic sediment,and then overlapped by Early-Middle Jurassic alluvial-fluvial facies deposition.The Qiangtang Basin transformed from foreland basin to rift basin,so,what the variation characteristics of the sedimentary sequences,paleoclimate,paleoenvironment as well as provenance during the depositional transformation,and when does the depositional transformation begin?At the same time,at the end of the Late Triassic,carbon isotope excursion and biological extinction events occurred in the marine strata of southern Tibetan and southern Qiangtang Basin,what's the relationship with the end-Triassic to Early-Middle Jurassic depositional transformation and Late Triassic magmatic activity?The solution of these problems is of great significance not only to deepen the understanding of sedimentary tectonic evolution of the Qiangtang Basin,but also to study the relationship of biological extinction,volcanic eruption and transgression at the end-Triassic in the eastern Tethys region.This paper chooses the well-outcrop and continuous sedimentary strata(including drilling cores and cuttings)of the end-Triassic to Early-Middle Jurassic period in the northern Qiangtang Basin as the research object.Through thin section,grain-size analysis,mineralogy,micropaleontology,element geochemistry and isotope chronology methods,the variation characteristics of the sedimentary sequences,paleoclimate,paleoenvironment as well as provenance during the sedimentary transformation period from the end-Triassic to Early-Middle Jurassic have been revealed,the Late Triassic volcanic eruption episode event and basin transformation time have been determined.The coupling relationship between the sedimentary transformation of end-Triassic to Early-Middle Jurassic and the contemporaneous volcanic eruption episodes and biological extinction event is also revealed.The main results are as follows:At the end of the Late Triassic(Carnian),the Qiangtang foreland basin is gradually shrinking.The Upper Triassic clastic strata,including the Bagong and the upper member of the Xiaochaka formations,is generally displayed a shallowing upward marine-continental transitional siliceous clastic sedimentary sequence.The upper member of the Xiaochaka Formation shows a complete regressive sequence,from prodelta facies to delta-front facies and then to delta plain facies,and subsequently overlapped by the alluvial-proluvial basal conglomerate deposits of the Upper Triassic Nadi Kanggri Formation.The Bagong Formation also displayed a transition from a restricted sea to a transitional environment,and finally transformed into continental volcanic clastic deposition,which was conformable contact with the overlying pyroclastic rocks.In the margin of the central Qiangtang uplift,the deposition are mainly delta-front and delta plain facies,and developed underwater distributary channels,estuarine bar and coal-bearing line swamp environment.At the end of the Late Triassic(Norian to Rhaetian),a volcano-sedimentary event occurred in the Qiangtang Basin,including the Nadikangri and E'erlongba formations,in general,it displayed continental volcanic sedimentary sequence,and in part it shows the subaqueous volcanic sedimentary sequence.According to the lithologic association characteristics of typical sections,three types of sedimentary processes can be divided:Type 1:The Upper Triassic Nadikangri Formation displays an angular unconformity sedimentary overlying the Paleozoic fold strata,which occurs in the Qiangtang central uplift and the northern margin of the basin;Type 2:The Late Triassic volcanic-sedimentary unconformable deposit on the Upper Triassic strata,and the unconformity interface is often characterized by paleokarst and alluvial-diluvial conglomerate deposition,which are found in the southwest margin and north of the basin.Type 3:The Late Triassic volcanic-sedimentary is conformable contact with the lower strata,mainly distributed in the eastern and central part of the northern Qiangtang Basin.The Lower-Middle Jurassic Quemo Co Formation mainly developed two sedimentary sequence types:type 1:The deposition started from alluvial-fluvial facies and gradually evolved into delta,tidal flat,lagoon and restricted platform sedimentary environment,indicating a progradational sequence from continental to marine facies,mainly distributed in the middle of northern Qiangtang Basin.Type 2:The Quemo Co Formation was less affected by marine transgression during this period,and deposited in offshore lacustrine siliceous clastic sedimentary sequence,and further divided into fluvial,evaporative salt lake and shallow lake sedimentary environment,mainly occurred in the eastern and northern areas in the northern Qiangtang Basin.The Late Triassic volcanic activity in Qiangtang Basin was episodic.A complete Late Triassic volcanic-sedimentary sequence is developed in the Quemo Co south section,which records four volcanic-sedimentary cycles,from bottom to top,the deposition ages of tuff samples are 220.0±1.1 Ma,210.9±0.9 Ma,208.3±0.8 Ma and 202.9±1.1 Ma,respectively.The youngest age is located near the Triassic/Jurassic boundary,which is basically consistent with the stratigraphic boundary of E'erlongba and Quemo Co formations.The volcanic eruption time of the Qiangtang Basin ranged from 202 Ma to 227 Ma,i.e.the Late Triassic Norian to Rhaetian period.The volcanic eruption duration was about 25 Ma,and showed five age peaks of 203 Ma,208 Ma,211 Ma,217 Ma and 221 Ma,and mainly distributed along the rift troughs.The northern Qiangtang Basin was transformed from a foreland basin to a rift basin during?227Ma.About?202 Ma,the early filling process of the Qiangtang Jurassic rift basin began.The paleoenvironment and palaeoclimate of northern Qiangtang Basin underwent significant changes during the depositional transformation from end-Triassic to Early-middle Jurassic.Before the depositional transformation,the average CIA,CIW and PIA values of the clastic rocks in northern Qiangtang Basin are 75.06,75.50 and 71.55,respectively,indicating moderate to high degree of chemical weathering.The results of geochemical indexes and microfossils show that the sedimentary environment was semi-arid to semi-humid,and minor semi-humid environment.The water condition was normal marine to brackish water environment,and evolved to be brackish water environment upward.During the sedimentary transformation period,the chemical weathering indexes decreased significantly,with the average values of 47.24,47.44 and 45.03,respectively.In this period,the paleo-weathering intensity maintained a low degree of chemical weathering,the overall paleoclimate was semi-arid,and some of the paleoclimate was extremely arid,and the water condition was mainly fresh to brackish water environment.After the depositional transformation,the degree of chemical weathering appeared a substantial rebound,the average values increase to 76.12,76.55 and 72.48,respectively,and the chemical weathering intensity maintained in moderate and high degree.The paleoclimate is mainly sub-humid,and minor in humid paleoclimate.After the depositional transformation,the water salinity is relatively low with mainly fresh to brackish water condition.By comparing with the previous studies,it reflected that the sedimentary paleoenvironment transformation was synchronous with carbon isotope excursion,contemporaneous volcanic eruption episodes and biological extinction events.Episodic volcanic eruptions in the end of Triassic and subsequent massive and rapid input of greenhouse gases(e.g.CO₂ and CH₄)into the atomophere-hydrosphere system caused extreme dry climate and ocean acidification,which may have contributed to the biotic crises in the southern Qiangtang area.In addition,the provenance also changed greatly before and after the end-Triassic to Early-Middle Jurassic depositional transformation.Before the depositional transformation,most of the detrital zircon ages of the clastic rocks are between 220 Ma and 250 Ma,and the provenance is mainly from the Qiangtang central uplift.While,after the depositional transformation,the zircon ages of the clastic rocks are mainly in the range of 1700?1900 Ma and 2100?2700 Ma,followed by the peak ages of 200?220 Ma,the Qiangtang central uplift and the Songpan-Ganzi complex are the main provenance areas of this period.Finally,the end-Triassic to Early-Middle Jurassic depositional transformation process is divided into three stages:the shrinkage of foreland basin,the onset of the rift basin and the early filling process of the rift basin,and sedimentary evolution model of this period is established,and further summarize the various stages of sedimentary evolution history.