Deep water sedimentary characteristics of expansion transformation margin in the Xisha area of the South China Sea

Xisha Area is located in the northern part of the South China Sea. The water depth is about 1000m to 3000m. Since Cenozoic , the Xisha area has experienced dual effects of extension and strike slip on the western margin, forming a special tectonic background of continental slope uplift. Overall, the Xisha area belongs to the continental slope system of the northern South China Sea. It is separated by the Qiongdongnan Basin to the north, on the west connected to the Yuedong Shelf , and  on the south and east ,adjacent to the deep-sea basin. It has developed three stages of sedimentation: fault depression, fault depression transformation, and depression. During the transition and depression periods, carbonate platform sediments developed due to rising sea levels and lacking of injection of terrestrial debris. After the Middle Miocene, the Xisha area was deep-water sedimentary environment on the continental slope, dominated by deep-sea to semi deep-sea sediments, with carbonate platform sediments developed on local uplifts. Under the special tectonic background, two types of deep-water sedimentary systems developed in the Xisha Sea during the Miocene period.

During Miocene ,the Xisha Sea area was marine sedimentary environment, with developping two different types of deep-water sedimentary bodies. One type is a deep-water channel supplied by terrestrial debris from the Yuedong River system, and the other is a deep-water channel supplied by carbonate debris from the Xisha Platform. Deep water channels supplied by terrestrial debris are mainly influenced by ancient topography and sea level rise and fall, with strong mobility and mutual cutting characteristics between channels; Deep water channels supplied by carbonate rock debris are mainly influenced by ancient topography, with vertical accretion as the main source and weak mobility of the channels.

The deep water channels filled with terrestrial debris developed in the research area are a process of low sea level and early marine invasion from the early to late stages of channel development. From the early stage to the late stage ,the limitation of deep water channels gradually becoming weaker, and the channels developping from a single channel with strong restriction in the early stage to a composite channel with multiple single channels cutting and overlapping each other in the middle stage, and then to late stage channel complexes and channelized lobes.

In the Late Miocene, deep-water channels filled with carbonate rock debris developed in the Xisha Sea area, mainly in the depressions between the Guangle Platform and the Xisha Platform  and on the east side of the Huaguang sag. This type of deep-water channel developed in the early Late Miocene, and on seismic profiles, the reflection characteristics in the upstream and downstream are relatively similar, mainly consisting of a single channel vertically stacked. However, the location of the channel is different, and the characteristics of the channel also changed. The filling material of this type of channel is carbonate rock debris from the Xisha Platform, which has strong paleotopographic limitations and is mainly vertically accreted.