Characteristics Of Migrating Canyon Systems In Baiyun Sag Of Northern South China Sea And Their Inlfuences On Gas Hydrate Accumulation

A large-scale tectonic movement happened during Late Oligocene to earlyMiocene epoch in the South China Sea, which is called the Baiyun Movement.The outer shelf slope break of Pearl River Mouth Basin transited to a muchnorthern place after the movement, with the depocenter subsequently changed aswell. Baiyun Sag evolved to a deep-water slope environment. Large-scaleregression happened at about13.8Ma in Northern South China Sea. After thatcanyon system began to develop and four periods of canyons have developed sincethen.In this paper, I interpret the detailed characteristics of migrating canyons,slides and sediment waves using high-resolution3D seismic data of the study area.I also depict the reflecting features and distributing characteristics of the BottomSimulating Reflector (BSR) in this study, in order to research on the accumulationof gas hydrate. Then through analyzing the relationship between the sedimentarycharacteristics of canyon system and the features of BSR, some characteristics ofgas hydrate accumulation are summarized here.Four sub-periods of canyon can be recognized in the fourth period of thecanyon in Baiyun Sag, which corresponding to four relative sea-level changessince Quaternary period. The interpretation of the distribution range of the foursub-periods of canyons using3D seismic data shows obvious headward erosionand northeastern migration. Each sub-period of the canyons generally comprisesof four sedimentary facies which are erosion base, basal lag, mass transportdeposits and lateral inclined packages. In addition, analyzing the distribution ofthe third canyon period through seismic profiles and coherence slice shows that thefourth period of the canyons are with good inheritance to the earlier canyons on thelower slope, while with no inheritance on the upper slope. In fact, the heads ofthe fourth period canyons are developed above the normal bathyal deposits in theform of head slides and the erosion of the turbidity currents. The canyon migration due to the South China Sea Warm Current causes higher erosional rateand lower depositional rate on the eastern wall, while the western wall is just theopposite. BSR shows different characteristics on either side of the canyon ridgethat BSR is simply reflected, reversed polarity and approximate parallel to theseafloor on the eastern flank (western wall of canyon), while BSR is multiplyreflected, with strong amplitude and not parallel to the seafloor on the westernflank (eastern wall of canyon). We suggest that the difference of depositional rateon each walls of the canyon results in the difference of BSR behaviors. It is alsodeduced that BSR should be deeper on the depositional flank because of highersedimentary rate and lower thermal flow and BSR should be shallower on theerosional flank because of higher erosional rate and higher thermal flow. With thecontinuing migration of the canyon, BSR on the erosional flank will move down,and BSR on the depositional flank will move up.Large amounts of slides exist in the canyon system of Baiyun Sag, most ofwhich are of small scales. These slides play a significant role in the processes ofcanyon evolution. For one hand, slides at the heads of canyons promote theheadward erosion to upper slope. For the other hand, the slides on the deepereastern walls of canyons accelerate the migration to the northeast. The slides arecharacterized by chaotic-blank seismic reflection and sharp scars above the heads.A recently happened slide is found in this study. This slide locates east to thedistrict of BSR. It is suggested that this slide caused the eastern part of BSRbecame shallower or even disappeared. So there may be gas hydrate dissociatedlocally or loss of free gas to the seafloor.In addition, sediment waves can be found in the study area. I interpret thesediment waves to be caused by slides through analyzing seismic data. Thoughthese sediment waves show an upper slope migrating-like phenomenon, it is notresulted from the different thickness of each sides of the wave crest. Secondly,each wave group shares little similarity on waveform, wave length, wave heightand extension directions of wave crest. Besides, the boundaries of wave groupsare concave downward which look more like listric faults. The sediment waves may be result from slides and creepy deformation and some sliding faults cutacross the base of gas hydrate stability zone, causing parts of BSR disappeared. Itcan be seen that BSR is distributed as strips in a plane view where the sedimentwaves locate. However, we need more3D seismic data to research on the wavesbecause sedimentation on wavelike seabed may also result in such wavelikesediments.In conclusion, sedimentary processes like canyon migration, slides andsediment waves in the canyon system of Baiyun Sag do have some influences onthe accumulation of gas hydrate. The BSR is of dynamic change and nonuniformdistribution in this area.