The variability of shelf-margin morphology and controlling factors : a case study of a northwestern South China Sea shelf-margin

A shelf-margin from Qiongdongnan Basin, northwestern South China Sea was studied. Geometric parameters (clinoform heights, lengths, forest inclinations) and accretion rates (progradation rates and aggradation rates) were measured, shelf-edge trajectories were depicted, and ancient sediment flux across the shelf-edge area were calculated using Petter et al 2013 methodology. The shelf-margin was divided into three depositional packages in terms of different shelf-margin growth patterns. The lower most depositional package (10.5 - 5.5 My) exhibits strong progradation tendency but minimum to even negative aggradation, which is likely due to the anomalous post-rift subsidence that occurred along the northern Qiongdongnan Basin during Middle Miocene to Late Miocene. In contrast, the upper most package (1.9 My - present) mainly grew vertically and managed to maintain a relatively high-rising trajectory because the shelf had become very wide (200 – 280 Km, Xie et al 2008) since Late Miocene and the sediment supply was highly insufficient and fine-grained. It is also likely that this very high supply rate of mud results at least partly from along-strike mud delivery, possibly as littoral fluid mud plumes, from the mouth of the Red River Delta. The middle package exhibits somewhat a mixed progradational and aggradational character with moderate clinoform dimensions and moderate accretion rates, which is likely to be a result of a reasonable balance between supply and accommodation.