| Junggar Basin,located in the north of Xinjiang,is one of the most important oil-bearing basins in northwest China.The Bogda Mountain in the southeast of the Junggar Basin is a branch of the East Tianshan orogenic belt,which separates the Turpan-Harmi Basin and the Junggar Basin.After the closure and welding of the ocean basin in early Carboniferous,the southeastern margin of the Junggar entered a critical period of ocean-land transition from late Paleozoic oceanic basin closure to Mesozoic intra-continental evolution.The tectonic evolution of the basin-mountain relationship in the southeastern margin,the initial uplift time of the Bogda Mountain,as well as the tectonic-climatic-sedimentary coupling mechanism have been international hot issues.Meanwhile,the Permian-Triassic sedimentary facies and provenance in the Fukang Sag and Jimsar Sag have troubled and restricted oil-gas exploration and development.We take the Permian-Triassic strata of the outcrop sections around the Bogda area and seismic and logging data from drilling cores in the Fukang Sag and the Jimsar Sag as a research case,use multidisciplinary methods such as sedimentology,geochemistry,detrital zircon U-Pb chronology and statistics etc.,analyze the middle Permian – early Triassic sedimentary facies,provenance,the paleoclimate change,determine the initial uplift time of the Bogda Mountain,rebuild the comprehensive evolution of tectono-climate-paleogeography from the middle Permian to the early Triassic in the southeast margin of the Junggar Basin.Based on comprehensive analysis of the outcrop sections,cores,seismic and logging data,the unconformity between the middle Permian and upper Permian is identified.In the Fukang sag and the Jimusar sag,the middle Permian strata shows the feature of thick in the south and thin in the north,while the upper Permian and lower Triassic strata is characterised for thick in the north and thin in the south,which indicates the subsidence center switched from the south to north after the middle Permian in the Bogda area.Sedimentary facies analysis shows a north to south direction braided river delta-lacustrine sedimentary system in the middle Permian Pingdiquan Formation.The upper Permian Wutonggou Formation deposited an alluvial fan system from south to north.The lower Triassic Jiucaiyuazi Formation shows a fan delta-lacustrine sedimentary features from south to north.The clastic composition of sandstone shows that the middle Permian Pingdiquan Formation is mainly lithic sandstone and feldspar lithic sandstone,with an average of32% lithic content.The lithic types are mainly sedimentary rocks,and the provenance is mainly from the island arc region.However,the upper Permian Wutonggou Formation and the Lower Triassic Jiucaiyuanzi formation are mainly lithic sandstone,with an average content of 80%.The lithic type is mainly volcanic rock lithic,and the provenance is mainly from the recurrent orogenic belt.The proportion of unstable heavy minerals from the upper Permian and lower Triassic sandstone is higher than the middle Permian samples,shows the change of provenance.The planar distribution of heavy minerals indicates that the provenance of the middle Permian Pingdiquan Formation is from north to south while the upper Permian Wutonggou Formation is oposite.A comparative provenance analysis based on detrital zircon U-Pb chronology shows that the northern and southern margins of the Bogda Mountain are in a unified source system in the Middle Permian,which may be derived from the magmatic basement of the Bogda Mountain and the late Carboniferous island arc of the Northern Tianshan Mountain.In the late Permian and the early Triassic,the provenance system of the northern and southern margins of the Bogda Mountain are significantly different.The sediments in the northern margin of the Bogda Mountain lacked material input from the Middle Tianshan Mountains compared to the southern margin,indicating the uplift of the Bogda Mountain which blocked sediments transportation.The petrochemistry and paleoclimate analysis of mudstone samples from the southeastern margin of the Junggar Basin show that the paleoclimate of the Middle Permian was semi-arid to arid,and the temperature of ancient water body is relatively stable around 29? in reduction-weak oxidation environment.In the late Permian,the paleoclimate changed from semi-arid to semi-humid dominated by reductive environment,and the mean ancient water temperature was 29.14?.The early Triassic paleoclimate was generally wet-semi-humid,with weak reduct-reductive environment and the average ancient water temperature was 29.52?.Overall,the climate in the south-eastern jungjungnean region from the Middle Permian to the early Triassic tended to be warm and humid,leading to a gradual increase in chemical weathering.Based on the analysis of sedimentary facies characteristics,provenance,and the paleoclimate change,the Southeast margin of the Junggar Basin tectonic-comprehensive climate-paleogeographic evolution process in the Middle Permian to Early Triassic is as follows: The Middle Permian inherited the rift background from the Early Permian,deposted a braided river delta-lacustrine sedimentary system under drought-semi-arid climate.The Late Permian is the tectonic inversion period.Due to the initial uplift of the Bogda Mountain and the re-uplift of Northern and Middle Tianshan Mountains,the early lake basin died out and alluvial fans formed in front of the north Slope of the Bogda Mountain.The provenance was also differentiated for the sediments in the south and the north of the Bogda Mountain.The Early Triassic is a period of denudation and deposition,when the tectonic environment in the study area was relatively stable,and the paleoclimate gradually tended to be warm and humid,which played a major role in controlling the regional weathering degree and sedimentary conditions.The Bogda Mountain suffered from intense weathering and denudation,and the sedimentary lake basin area on the north side gradually expanded,forming the fan delta-lacustrine sedimentary system. |
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