Sequence Architecture, Depositional Evolution and Controlling Processes of Continental Slope in Pearl River Mouth Basin, Northern South China Sea

The South China Sea (SCS) Basin is the largest marginal sea basin with abundant petroleum resources on the continental shelf of Southeast Asia. However, there is a lack of research on the relationship between the development and sedimentary evolution of South China Sea continental slope belt and the tectonics and dynamic processes of the South China Sea Basin. Based on integrated analyses of seismic, well-logging and core data, the sequence architecture, depositional-geomorphological evolution and controlling processes of the continental slope in the Pearl River Mouth Basin of the northern SCS have been systematically documented. The sedimentary infill of the marginal sea basin can be divided into seven composite sequences (CS1-CS7) that are bounded by regional unconformities. Each of the composite sequences CS3 to CS7 (Upper Oligocene to Quaternary) comprises of generally a regional transgressive-regressive cycle. The CS3 to CS7 can be further divided into 20 sequences that are defined by local unconformities and correlative conformities. Depositional systems recognized in the continental slope deposits mainly include outer shelf to shelf-edge deltas, prodelta-slope fans, shelf-margin slope clinoforms, unidirectionally-migrating bottom current-gravity flow composite slope channels, large-scale incised slope valleys, muddy slope fans, slope slump-debris-flow complexes and large-scale soft-sediment deformed beds. They were arranged with distinguishing patterns in different sequences, forming various styles of depositional architectures. The study shows that the short-term sea level changes (sequences) are generally comparable with those of the Haq global sea level curve, whereas long-term sea level changes (composite sequences) were apparently controlled or enhanced by tectonic uplift and subsidence. The depositional evolution of the continental shelf margin can be divided into (1) the early post-rift seafloor spreading (breakup sequence), (2) the late post-rift seafloor spreading, and (3) the post-seafloor spreading tectonic-depositional stages. The thermal uplift, tectonic differential subsidence, post-rift thermal subsidence and the eastern collision after Pliocene are all regarded to have played an important role in the formation of major unconformities and regional transgressive-regressive cycles. The composite sequences (CS3 and CS4) developed during the Late Oligocene to Middle Miocene recorded the depositional successions of the continental slope formed from the beginning to the stopping of the seafloor spreading. The composite sequence (CS3) deposited during early post-rift stage is regarded as a breakup sequences. It is characterized by the development of large-scale continental shelf edge delta and prodelta slope fan systems. Climate change and monsoon strengthening may have enhanced the sediment supply during the Late Oligocene to Early Miocene and the Pleistocene, which may be responsible for the sufficient sediment supply for the development of large-scale continental shelf marginal delta systems during these periods. The shelf margin delta-shoreline clastic systems and the associated slope fan systems on the continental margin have proven to be the most important targets for oil and gas exploration.