Along-strike variability in shelf-margin morphology and accretion pattern: An example from the northern margin of the South China Sea

Interplays among diachronous tectonism, uneven sediment supply, and local marine hydraulic processes make the northern margin of the South China Sea (SCS) an ideal location to investigate the complexity of along-strike variability in shelf margins. This study examines shelf-margin morphology, stratigraphy, and sedimentation from the northern SCS using multichannel seismic reflection profiles complemented with the data from commercial and ocean drilling sites. Analysis of seismic reflection profiles reveals three categories of shelf-margin cross-sectional profiles, the concave-up, linear, and sigmoidal, according to which five margin sectors were recognized. Results show that these margin segments differ in relief, shelf-edge trajectory, submarine canyon development, and long-term accretion pattern. The westernmost margin sector, or the Yinggehai (YGH)-western Qiongdongnan (QDN) margin, has appeared to be supply dominated since its commencement at ca. 10.5 Ma, which is characterized by well-developed prograding clinoforms, low-angle shelf-edge trajectories, and an absence of canyons. Presence of concave-up profiles is also suggestive of high sediment influx. In contrast, the eastern QDN margin was primarily regulated by local subsidence and faulting, leading to a stationary shelf-edge migrating pattern and linear upper-slope morphology. Densely distributed slope-confined gullies indicate the margin's disequilibrium and erosive nature. Further east, the Pearl River Mouth (PRM) margin formed much earlier (ca. 30 Ma) and experienced a more complicated accretion history, including three phases which were dominated by sequential marginal faulting (before ca. 30 Ma), basement structure (ca. 30-23 Ma), and sediment supply (ca. 23 Ma to the present). The overall sigmoidal morphology and truncated stratigraphy of this margin probably resulted from the sculpting of local marine processes, especially ocean currents and internal waves. The exception of the central PRM margin where concave-up profiles develop is mainly related to canyon erosion. Overall, this study highlights the vital role of local forcing factors in controlling along-margin variations and determining the final fates of different margin segments. A comparison between the northern SCS and other well-established examples reveals that concave-upward shelf-margin shapes, which are usually associated with high sediment supply, little influence from hydraulic regimes, or sometimes, high degree of canyon development, may be an indicator of good reservoir potential beyond the shelf edge.