Sediment Charateristics And Its Weathering And Provenance Implication Of The Pearl River And Rivers In South China

As the medium of land and ocean,rivers transport terrestrial debris into the ocean,which plays a vital role in the global geochemical cycle.The modern Pearl River originated in the southeast of the Tibet Plateau and eventually flows into the South China Sea.The evolution of the Pearl River is closely related to the tectonic evolution of the continental margin of southern China and the topographical changes caused by the uplift of the Tibet Plateau caused by the collision of the Indian and Eurasian plates.Meanwhile,as the largest river in south China,the Pearl River records the evolution history of the Asian monsoon system.As a link between the Asian monsoon climate system and the uplift of the Tibet Plateau,the sediments of Pearl River record the evolution information of structural units and geomorphic systems within the basin.In addition,as an important sediment source in the northern basin of the South China Sea,the evolution of the Pearl River basin is also of great significance for the prediction of high-quality reservoirs in the northern basin of the South China Sea.The evolution of the Pearl River has always been the focus of geomorphologists and geologists.The mineralogical and geochemical composition and provenance tracer studies of the Pearl River sediments are of great significance for the analysis of the mechanism of erosion and transport in the river sediments,restoration of the pearl River geological and historical evolution and other related issues,clarifying the evolution of the entire modern Asian geomorphic pattern,revealing the response of river evolution to monsoon climate evolution and structural geomorphologic change.Although some geologists have analyzed the provenance of the Pearl River sediments and some important knowledge has been gained.However,the previous provenance studies are relatively simple,including several traditional heavy mineral analysis and zircon chronology analysis of the main trunk stream,they do not combine the comprehensive petrology and heavy mineral analysis of sediments.Most of the previous sampling points were concentrated in the main stream,and the provenance information of the whole Pearl River,including the main stream and tributary,was lacking.So far,no studies of mineralogical and geochemical composition of fluvial sediments have done on the weathering of south China.Given the complexity of sediment composition and source-sink process in the Pearl River Basin,this study uses multi-methods,including the petrology,heavy mineral,clay mineral,detrital zircon U-Pb dating and geochemical analysis,combined with the mineral composition,detrital zircon U-Pb dating and geochemical analysis of the small coastal rivers in southeast China,southern tributaries of the Yangtze River and Taiwan rivers to have a comprehensive and systematic provenance study on the sediments from Pearl River,revealing the provenance and weathering controlling factors of the Pearl River sediments.The core samples from the northern South China Sea were added to further analyze the evolution of Pearl River.Then the provenance tracer system of the Pearl River sediments was established to clarify the controlling effect of provenance and weathering on sediment composition,and to provide a reference basis for studying the evolution of other rivers and reshaping global climate change.The results show that the evolution of Pearl River has gone through six stages with the movement of the South China Sea and the uplift of the Tibet Plateau:(a)In the early Cretaceous,when the Ancient Pacific plate was subducting towards the South China plate.The Andean magmatic belt was formed in the coastal area of South China under subduction,at which time the topography of the east of South China is higher than that of the west.During this period,the granites related to the island arc in South China transported sediments to the Pearl River Mouth Basin mainly through a small coastal river,which is considered to be the paleo-Dong river.(b)By the end of the Cretaceous,the back-arc extension in the South China Sea area began,and the rift basin in the north of the South China Sea formed.At this time,the topography in the east of South China continued to increase.The peleo-Bei river began to develop and connected with the paleo-Dong river.(c)During the Paleocene to Eocene period,the topography of the eastern South China slowed down due to the evolution of the South China Sea rift,the topography in the east was still higher than in the west.The uplift area in the east of South China provided sediments to Pearl River Mouth Basin.The catchment area of Pearl River was still limited to the paleo-Bei and paleo-Dong river,but larger than that in the end Cretaceous.(d)In the Oligocene,the sea floor of the South China Sea began to expand.The southeast Tibet Plateau uplifted rapidly and the topography of the eastern South China gradually decreased.The scale of Pearl River did not change much compared to Paleocene to Eocene,and the provenance of Pearl River Mouth Basin still came from the paleo-Bei and paleo-Dong River in the eastern South China.(e)In the late Oligocene,the northern South China Sea entered the late rifting stage,and the uplift of southeastern Tibet Plateau plate accelerated.At this time,the topography in the east and west South China plate has reversed,and the topography in the west is higher than that of the east.The adjustment of topography contributed to the rapid expansion of paleo-Pearl River to the west and connecting with the main tributaries upstream.(f)During the early and middle Miocene,the Pearl River had grown to its present configuration,providing a steady sources for the Pearl River Mouth Basin.Weathering and parent rock lithology have an obvious control on the mineralogy and geochemistry of the Pearl River sediments.The depletion in mobile alkali and alkaline-earth metals increases steadily from the drier western part of the Pearl River catchment to the wetter eastern part.Na and Ca are the most strongly depleted elements in Pearl River sediments,whereas the low concentration of Ca and Mg in river sand of coastal SE China also reflects the felsic character of parent rocks in the Cathaysia block.Eastward-increasing weathering conditions are most clearly highlighted by the progressive eastward increase in kaolinite across the Pearl River catchment,whereas the weathering effect in sand is blurred by quartz addition owing to widespread recycling of quartz-rich siliciclastic source rocks.This study underscores the care that should be taken while inferring climatic conditions from the geochemical composition of sediments and sedimentary rocks,which is controlled by multiple geological factors including the felsic versus mafic character of parent rocks and the presence of extensive carbonate or siliciclastic covers in source areas.Most reliable indications are provided by the kaolinite/(illite+chlorite)ratio and by ?Al indices for Na,Ca,Mg and Sr,which consistently increase eastward across South China thus paralleling the marked eastward increase in monsoonal precipitation towards the coast.In the Pearl River and southern Yangtze catchments,dominantly sedimentary and basaltic rocks of the Yangtze block shed quartzo-lithic sedimentaclastic sand,whereas mostly granitic and sedimentary rocks of the Cathaysia block generate feldspatho-quartzose sand.Rivers of the Fujian and Zhejiang Provinces draining the SE Coast Magmatic Belt carry feldspatho-litho-quartzose volcaniclastic sand.Within the Pearl River catchment,35-40% of the sand is derived from the Yangtze block,and60-65% from the Cathaysia block.South China hosts one of the largest and most spectacular karst areas on Earth,divided into Fengcong and Fenglin karst.In fengcong(cone)karst,carbonate detritus attacked by carbonic and sulfuric acids is partly preserved,whereas carbonate grains are completely dissolved in fenglin(tower)karst developed in wetter regions to the east.The strong climatic gradient from the dry Tibetan plateau in the west to monsoon-drenched coastal areas is faithfully reflected in illite-rich vs.kaolinite-rich clay-mineral assemblages,but in sand the weathering effect is much harder to isolate from the dominant effects of source-rock lithology and recycling.The different durability of tectosilicates(quartz > microcline > orthoclase >plagioclase)and the degree of corrosion displayed by detrital minerals offer essential qualitative information but cannot represent robust proxies to quantify weathering intensity.The sand generation index SGI reveals whether lithologies are under-or over-represented in detrital assemblages and proved to be most useful to trace different weathering regimes across South China.The relative proportions of durable minerals,complemented by zircon-geochronology data,indicates that Neogene sandstones exposed in western Taiwan were principally fed by a paleo-Yangtze River.Tectonics,weathering and sedimentation are interrelated and interactive.Tectonics and weathering control the formation and composition of sediments,and analysis on sediments can help trace the tectonic history of the source area and the degree of weathering experienced during transport.Based on the principle of "comparing the present with the past",the study of modern sediments can not only provide a strong basis for the evolution of the sedimentary environment in the past,it can also predict the trend of sediment evolution in the future.