The Quantitative Evolution Of Shelf-margin Clinoforms In The Qiongdongnan Basin

Shelf-edge trajectory analysis has received much attention, and has been the subject of increasingly intense study in recent years. The prediction of deep-water sands that can be tied to specific types of shelf-edge trajectories will help to assist greatly in locating deep-water reservoirs and in developing predictive stratigraphy. This dissertation uses two-dimensional (2-D) and three-dimensional (3-D) seismic, core, logging curves and acoustic impedance data in the Qiongdongnan Basin to propose quantitative measurement methods for shelf-edge trajectories, shelf-edge clinoforms and clinothems based on their types, shapes and geometric parameters; investigate the vertical migration evolution of shelf-margin clinoforms since late Miocene in the Qiongdongnan Basin; document the lateral difference between western and eastern shelf-margin clinoforms; summarize clinothem internal architecture and their stratal stacking patterns; discuss relationships between shelf-edge trajectories and regional stacking patterns with styles of deep-water sedimentation and volumes of coevally deposited deep-water sandstones; estimate semi-quantitatively the magnitude of sediment supply and shelf accommodation; address the controlling factors of the evolution of shelf-margin clinoforms; propose a quaternionic classification system for shelf-margin architecture based on the interplay of sediment supply, accommodation, climate and current-controlled processes; and predict the types and location of sand-prone deep-water depositional systems and coeval deep-water reservoirs. The main achievements made by this dissertation are listed as follows.1) Four main shelf-edge trajectories are recognized in seismic profiles:the first phase contains shelf edges that are strongly progradational with a flat to slightly rising shelf-edge trajectory (0°～0.5°); the second phase includes shelf edges that moderately progradational with low-angle rising shelf-edge trajectory (0.2°～3°); the third phase contains shelf edges that strongly aggregational with high-angle rising shelf-edge trajectory (3°～9°); the fourth phase includes shelf edges that vertically aggregational with ultra-high-angle rising shelf-edge trajectory (10°～20°).2) The nine types of the observed clinothems are identified in the research:Oblique, Tangential oblique, Tangential oblique chaotic, Sigmoidal symmetrical, Sigmoidal divergent, Sigmoidal chaotic, Asymmetrical top-heavy, Asymmetrical bottom-heavy and Complex. Oblique, Tangential oblique and Asymmetrical bottom-heavy are mainly developed during the S40-S30; Complex is mainly developed in the western studied area during the S30-S27, Asymmetrical top-heavy is mainly developed in the eastern studied area during the S30-S27; Sigmoidal symmetrical and Sigmoidal divergent are mainly developed during the S27-S20; Sigmoidal chaotic and Asymmetrical top-heavy are mainly developed during the S27-S20.3) Progradational or slightly downstepping stacking patterns are identified in S40-S30; Mixed progradational and aggradational stacking patterns are identified in western S30-S27 and S27-S20; Dominantly aggradational stacking patterns are identified in eastern S30-S27 and S20-S0.4) Flat to slightly falling shelf-edge trajectories and progradational or slightly downstepping stacking patterns are associated with sand-rich submarine fan, canyon channel sand complex (sand-bearing ratios>30%); slightly rising shelf-edge trajectories and progradational and aggradational stacking patterns are fronted by mixed sand/mud small-scale channel sand, levee-overbank sediments and deep-water lobes (15%<sand-bearing ratios<30%); steeply rising shelf-edge trajectories and dominantly aggradational stacking patterns are linked to areally extensive mass-transport systems (sand-bearing ratios<15%).5) The approximate magnitude of sediment supply can be evaluated by determining the aggradation rate (Ra), the progradation rate (Rp), and the cross-sectional net sediment flux (Fc). Low supply (low Qs) is measured as Ra of<500 m/My, Rp of<12.5 km/My, and Fc of<5 km2/My and high supply (high Qs) is measured as Ra of>500 m/My, Rp of>12.5 km/My, and Fc of>5 km2/My. Estimates of the magnitude of shelf accommodation can be determined from the magnitude of shelf-edge trajectory angle and contemporaneous overall slope angle. Flat to slightly falling and slightly rising shelf-edge trajectories naturally and low overall slope angle (represented by 2°<β<6°) reflect a low accommodation; moderately rising trajectories and low overall slope angle (represented by 2°<β<6°) are symptomatic of medium accommodation; steeply rising trajectories and high overall slope angle (represented by P>6°) are suggestive of high accommodation.6) The effects of variations in the interplay of sediment supply, accommodation, climate and Surface Water mass on architectural styles of shelf-margins have been semi-quantitatively investigated. Four different types of western shelf-margins are recognized during the scenario evolution:a. ow Qs+low 8(x) +weak Surface Water mass+arid climate; b. low Qs+medium 8(x)+weak Surface Water mass+variable monsoon; c. high Qs+medium 8(x)+strong Surface Water mass +variable monsoon; d. high Qs+high 8(x)+strong Surface Water mass+variable monsoon. Two different types of eastern shelf-margins are recognized during the scenario evolution:a. low Qs+medium 8(x)+weak Surface Water mass+arid climate; b. low Qs+high δ(x)+strong Surface Water mass+variable monsoon.7) Low δ(x) and arid climate are extremely important for forming sandy submarine fans or canyon channel; medium 8(x) and low overall slope angle with moderately rising shelf-edge trajectories tend to foster sand-rich deep-water lobes and highstand slope fans; high 8(x), narrow shelf width and river input from Hainan island yielded sand-prone lenticular submarine fans.The highlights of this research indicate that 1) a methodology was developed for the quantitative analysis of clinoform and clinothem architectures,2) accumulation mechanisms were built for sand-and sediment-budget partitioning into and across the shelf, and 3) estimation methods were approached the magnitude of sediment supply and shelf accommodation for shelf-margin architecture variability.