Characteristics And Evolution Model Of The Deepwater Slope Systems Under Different Sedimentary Settings In The Continental Margin Of Northern South China Sea

Deepwater slope sedimentology is one of the primary parts of the deepwater depositional theory.It has theoretical significance and practical application to carry out the research on deepwater slope sedimentology.This dissertation uses the seismic sedimentological methods to study and analyze the deep-water sedimentary systems.Firstly,according to the settings of sediment supply,the deep-water slopes are divided into shelf slope,peri-platform slope and volcanic slope.Secondly,based on the distribution relationship between deepwater slopes and other geomorphologic units,the deepwater slopes are divided into different steps.Then,we study systematically the sedimentary characteristics and evolution model of the deepwater slope under the different sedimentary settings.The results reveal that shelf slope has multi-steps and develop stepwise model of sediment transportation.The first-step shelf slope is characterized by canyon groups and creep-furrow groups,while the second-step slope is distinguished by large canyon complex.All the different-step slopes show strengthening at the upper part and weakening in the lower part in erosion,which results in sediment passing in the upper slope and sedimentation in the lower slope.Sediment accumulation in the first-step slope make it to be the sediment source of the second-step slope,showing a stepwise model of sediment transportation.This multi-steps of the shelf slope and its stepwise model of sediment transportation are controlled by both tectonic activities and sediment input,as well as by sea-level changes.This model first reveal the micro-distribution rules of the slope sedimentary processes and it is significant to guide deepwater hydrocarbon explorations and submarine engineering constructions.This study also reveals sedimentary processes and depositional model of the periplatform slope systems.There are at least four mixed depositional processes,namely,in-situ mixing,punctuated mixing,facies-edge mixing and volcanic-related mixing.A large channel complexes are recognized in the slope around the Liuhua carbonate platform and characterized by mixed channel-fill deposits.These channel-fill cyclic carbonate in the periplatform slope showed a good corresponding relationship to the highstand sea-level.Due to the slope sedimentation is sensitive to growing status of the carbonate platform,we speculated that Liuhua carbonate platform had high production and can export more carbonate debris to the channel during highstand.Around the modern carbonate platform,there develop sediment transporting network formed by “gullies-erosive-grooves-(contourite)channel-large canyon”,which links the neritic platform and the deepwater basins.According the observation and the analysis of the geomorphology,we recovered the sedimentary transporting styles,like sheet-like turbidity current,mass flow,bottom current and liner turbidity current.The periplatform sedimentary model reveals there develops much of seabed hazards in the periplatform slope and this provides a significant theoretical reference for decision-making to develop islands in the South China Sea.Furthermore,this study reveals that the sedimentation around the submarine volcano is different from the normal basin filling because volcanic activities changes the original source-sink system,accommodation and hydrodynamics.Based on the analysis of the seismic facies and lithological characteristics,we establish the evolution model of the volcano-controlled sedimentation.The evolution process of the perivolcano sedimentation is divided into pre-volcano stage,syn-eruptive stage,posteruptive stage and post-volcano stage.This has important sedimentological implications and practical applications.