Sedimentary Evolution Of The Suonahu Formation In North Qiangtang Basin And Its Response To The Uplift Of Tibetan Plateau

The Tibetan Plateau is a largest and highest continental plateau on the earth's surface.The time and extent of the uplift of the Tibetan Plateau have always attracted researchers' attention.Studying the formation background and filling process of the Cenozoic basins in the Tibetan Plateau have an important significance for understanding the uplift history which had been recorded in the sediments.Therefore,it is necessary to analyze the formation background,filling and evolution mechanisms of these basins.The Qiangtang Basin is located in the central part of the Tibetan Plateau,providing an opportunity to study the history of the uplift of the Tibetan Plateau.The Cenozoic sedimentary strata recorded a large amount of information on the uplift of plateau and climate change,and it has always played an important role on Tibetan Plateau research.The sedimentation of the strata and the uplift of plateau were two results of the same dynamic process.The latter was the driver of the former.The sedimentation of the strata is an archive about the mechanism and the history of the plateau uplift,completely recording the entire history of plateau uplift.In this paper,a comprehensive study of the geochemical characteristics,carbon and oxygen isotopes,sulfur isotopes,detrital zircon U-Pb age,and pollen of the Suonahu Formation is carried out to address the sedimentary age,depositional environment and provenance of the Suonahu Formation.The Cenozoic stratigraphic evolution framework of the North Qiangtang Basin was reconstructed,and the uplift status of the Tibetan Plateau during that period was discussed.The paper mainly obtained the following results and understandings:(1)The sedimentary facies of Suonahu Formation are divided by petrographic,sedimentary structure and other indicators.The Suonahu Formation in the North Qiangtang Basin is a set of lacustrine deposits dominated by fine clastic rocks,with braided river facies at the bottom.The lithology is sandstone,gravel-bearing sandstone,and conglomerate,with normal grading,scouring surface and cross bedding in the lower part.The middle part is composed of lacustrine shoreline subfacies.The lithology is sandstone and silty mudstone.The upper part is shallow lake subfacies consisting mainly of purple-red mudstone,with horizontal bedding,and thin layered gypsum.The uppermost part is an evaporative salt lake environment with gypsum,anhydrite,and thin mudstone.In some areas,due to the precipitation of salt and the input of fresh water,algal limestone containing gypsum can also be seen.The overall depositional environment variations show transitions from braided river subfacies? lakeside subfacies? shallow lake subfacies,and finally into arid salt lake sedimentary environment.(2)Used the minimum detrital zircon U-Pb age(59.57�9.21Ma)and pollen assemblage characteristics(Distachya)to determine the sedimentary age of the Suonahu Formation.The Suonahu Formation in North Qiangtang was deposited in the Eocene-early Oligocene(51?28Ma),which is same age to the Yaxicuo Formation in the Hoh Xil Basin.(3)Through the comparative study of mineralogy and geochemical indicators(CIA,A�CN�K,etc.),the palaeoclimate characteristics,tectonic setting,material sources and provenance characteristics of the Suonahu Formation have been determined.The provenance of the Suonahu Formation is mainly from felsic igneous rocks,with minor input from intermediate rocks,and the source areas were weakly weathered and experienced a low to moderate sorting and recycling.Its tectonic background is a related-rift basin,and it is semi-arid to arid paleoclimatic conditions.(4)Through carbon and oxygen isotopes studies and sulfur isotopes studies,the paleoelevation of the Suonahu Formation was calculated and the paleo-lake environment was studied.During the deposition of the Suonahu Formation,the paleoelevation of the North Qiangtang Basin was about 2830+715/-862 m,and it was deposited in a semi-open-semi-closed shallow-water oxidation lake environment with high salinity.(5)U-Pb chronology of detrital zircons is used to explain the tectonic thermal events experienced by detrital zircons.The detrital zircons of the Suonahu Formation have experienced multiple tectonic thermal events including NeoarcheanPaleoproterozoic tectonic thermal events(2224?2668Ma),Mesoproterozoic Columbia supercontinent thermal event(1581?1929Ma),Neoproterozoic Rodinia supercontinent thermal event(622?1198Ma),Pan-African movement tectonic thermal event(422?578Ma),Paleo-Tethys-like closed thermal event(204?269Ma)and Middle Tethys ocean subduction thermal event(103?179Ma)),combined with the inland lake depositional enviroment in the Qiangtang Basin during the Suonahu Formation deposition period.These indicate that these zircons are recyclic zircons.The similarity of the U-Pb age distribution histograms of the three samples of detrital zircons indicates that the provenance of Suonahu Formation is relatively stable without major changes.(6)Analyzed the uplift status of the Tibetan Plateau during this period through a comprehensive study of the Suonahu Formation,using sedimentological methods,and divides the process of uplift into three stages.Affected by the collision of the India-Eurasian plates,the North Qiangtang Basin was already in the terrestrial environment during the Paleogene: 1)Paleocene to Eocene�Compression Orogenic Stage(the Kangtuo Formation deposition period >51Ma),sedimentary fluvial red molasses lithology combination,the overall performance is differential uplift;2)Eocene � Relatively Stable Uplift Stage/Overall Uplift(the Suonahu Formation deposition period 51?28Ma),sedimentary lacustrine facies fine clastic rocks,gypsum rocks and gypsum-bearing algae limestone,the internal topography of the basin has a small difference in elevation,and the Qinghai-Tibet Plateau is uplifted steadily;3)Late Eocene and early Oligocene� Rapid Uplift(Yulinshan volcanic rocks <28Ma): the detachment of the lithospheric mantle and the upwelling of deep materials caused rapid uplift of the crust.