The Characteristics And Mechanism Of Sediment Gravity Flow In Slope Area In Qiongdongnan Basin

In the recent years, deepwater depositional systems gradually become a hotspot in international geological research with the offshore oil and gas exploration advancing to the deep water area. Meanwhile, the gravity flow as an important way of delivering sediment from shallow to deep water is of significant importance in deep water depositional study. This thesis closely tracks the international frontier and focuses on shelf-slope system, and studied the sediment gravity flow in shelf-slope system. By characterizing the sediment gravity flow in shelf-slope of Qiongdongnan Basin, the evolutionary history of shelf-slope system in this basin could be better understood. Moreover, the internal architecture and paleoceanographical information within sediment gravity flow has some theoretical significance. The main achievements made by this thesis are listed below:1. Based on the slope morphology and internal architecture of Qiongdongnan Basin, four kinds of slope have been indentified from west to east:progradational slope, slumping slope, channelized slope, wide and gental slope. The progradational slope is characterized by rapid development of progradation and "S" type slope, and the stacking pattern of internal sequence shows a strong progradation with weak aggradation, the shelf break migrates in a smoothly ascending trend. The slumping slope is marked by steep slope morphology along with large-scale MTDs on slope, the stacking pattern of internal sequence shows a weak progradation and aggradation, the shelf break migrates in a up-dip pattern; when influenced by MTDs, the shelf-break will show a backward migrating trend. The channelized slope has the steepest slope morphology, characterized by a mass of canyon and channels on shelf and slope, the internal sequence stacks as aggradation along with weak retrogradation, the shelf-slope migrates in a slightly up-dip pattern and shows a backward trend. The wide and gental slope displays gentle slope morphology and the stacking pattern of the slope is characterized by aggradation, with no obvious shelf break.2. There are large-scale MTDs developed in slope and deep water areas of Qiongdongnan Basin, and the upper slope and lower slope have different MTDs elements. The upper slope is dominated by large-scale slides, escarpments and transported blocks, while the lower slope is mainly comprised of remnant blocks, stratum deformation and synsedimentary thrust faults. Moreover, MTDs on the lower slope shows more intensive erosion. In this study, the erosion caused by MTDs was proved to destroy the primary features of underlying submarine fan, leading to the fluid escaping. In addition, the irregular top surface of MTDs controls the succedent deposition in some extent, and the small-scale depression is an ideal place for turbidity currents to deposit. At the same time, the head of MTDs can also develops on gentle lower slope areas.3. MTDs developed in the east and west have different depositional features. In the Songnan3D survey, western part of the basin, the area of the MTDs becomes larger from the early stage to the later one in the Yinggehai Formation. The MTDs always have different lobes with clear outline, with larger thickness (up to200m). Meanwhile, thrusts are more developed in this area, which indicates that the compression here is more intense. However, in the wide and gental slope, eastern part of the basin, not only the dimision of the MTDs is much smaller, but also the MTDs have less compressional texture and erosional features. The different morphology between western and eastern part of the basin could be the reason for the different features developed in the MTDs.4. The mass transport deposits in the central basin have been divided into nine stages. Three kinds of features about the distribution of the MTDs have been figured out. Firstly, MTDs are well developed and nearly all of the central bansin has been covered by the MTDs except for the early stages. For instance, area of the MTDs in the last stage could occupy20%of the total area of the basin. Secondly, depocenters of MTDs of each stage keep staying at the Songnan and Lingshui depression, in which the slumping slopes are developed. Although there are MTDs developed in the region conresponding to channelized slope and progradational slope, the area and thickness is much smaller. In the wide and gental slope, there is only a small amount of MTDs. Thirdly, the area of the MTDs is becoming larger from the older stage to the younger one, and the area of the last stage could be up to9100km2.5. The slope canyon system developed in Qiongdongnan Basin can be divided into three sections based on its distribution features, whicht are Baodao slope canyon, Baodao-Shenhu transition zone slope canyon and Shenhu slope canyon. Although the Baodao slope canyon and Shenhu slope canyon were developed on different slope systems, which are channelized slope and wide and gental slope, respectively, the head part of the canyons are located on the upper slope and do not erode into the shelf, which are named blind canyons. Meanwhile, in the planview, the canyons developed on the Shenhu margin exhibit mushroom-shaped outline. However, the canyons developed on the Baodao-Shenhu transition zone erode into the shelf, just as the canyons developed elsewhere in the world, which are named general canyons. The ration of depth to width in these two kinds of canyons have similar shifting trend, which is lower in the proximal and distal part, and higher in the central part, but the value of the ratio is higher in the general canyons than blind ones.6. By using high-resolution3D seismic data, architecture and evolution of the7submarine canyons developed on the shelf margin in the Late Miocene has been studied. Genenrally, the length of the canyons are10-30kilometers, with a width of500-5000meters and a maximum relief of300meters. For the original architecture of the canyons on the slope has been destroyed by the well developed MTDs, they only could be distinguished on the shelf. There are four kinds of architecture elements developed in these canyons, which are erosional surfaces, basal lags, MTDs and lateral accretion packages (LAPs), and the lithology are dominated by mudstone and silty stone. Meanwhile, the morphology of them is characterized by U-shape at the proximal part and V-shape at the distal part.7. In the late Miocene, the slope canyon show significant unidirectional migration features. On seismic profiles, lateral accretion packages only developed on the west flank, while the canyons show erosional characteristics on the east. The maximum lateral migration distance can be up to10km. Along the canyon, the stacking pattern varies in different profiles. At the canyon head part, lateral aggradation is intensest and the canyons have maximum lateral migration. In the central part, the lateral migration becomes weaker and shows vertical aggradation, with the thalweg trend shows up-dip trend. At the distal part of the canyons, the distance of lateral migration becomes smaller or even disappears and canyons subsequently shows vertical aggradation. The thalweg trends are vertically upwards or large angle upwards with weak eastward migration. Although the morphology gradually varies during the entire evolution history of the canyon, there are also some canyons that evolved with abrupt changes in strike during a certain stage.8. Analyze the relations between the evolutions of late-Miocene canyons and the fault activities. Through the spatial configuration relationships between the fault developments in the studied areas and the shelf margin canyons, it was illustrated that the head of the canyon was gradually deflecting towards a series of small fault systems developed in the center of the studied area. Meanwhile, the plane scales of the faults in the shelf margin first grow large followed by a subsequent tendency of getting small, which is in accordance with the evolutions of the canyon scales. Furthermore, the deflection of the canyons is also well consistent with the spatial developments of the faults, which indicating that the formations and evolutions of the canyons are controlled by the fault activities in the study area.9. The bottom current generated by the Paleo-South China Sea Warm Current has a important controlling effect on the development of the lateral accretion packages (LAPs), for only the bottom current could result in the constantly asymmetric sediment supply. Interplay of the bottom current and the turbidity current results in the different stacking pattern. In the proximal part of the canyons, comparing to the bottom current, the intensity of the turbidity current is relatively weak, which leads to the well development of the LAPs on the west side of the canyons and subsequent migration of them. In the distal part of the canyons, the intensity of the turbidity current becomes stronger and could result in more erosion, so the stacking pattern of canyons is characterized by aggradation and the canyons show much less migration.10. The trigger mechanism of MTDs in the Qiongdongnan Basin has been summarized. The evolution of slope system in Qiongdongnan Basin is dominated by the provenances from Hainan Island and the Red River. Due to spatially closer to the sources, sedimentary rate is higher in the west part of the basin than the east, which subsequently results in the development of four different slope systems. According to the spatial relationship between the MTDs and the slope, we conclude that the architecture of the slope, surposed to be long term trigger, has an important control on the development of MTDs. Meanwhile, for the Qiongdongnan Basin has entered into accelerating subsidence stage since late Miocene, main faults of the basin have been experienced reactivation. In addition, in the east of slumping and channelized slope in the central section pf the Qiongdongnan basin, since the head part of the MTDs is generally around the faults developed on the shelf margin, it is reasonable to conclude that the reactivation of the faults since the Late Miocece and subsequent earthquicks due to the tectonics could be another trigger for the MTDs, which is suppose to be short-term trigger.