Siliciclastics Sequence Models in Wide and Low-gradient Continental Margin of Northern South China Sea

Zhang, Shangfeng; Zhang, Changmin; Yin, Yanshu; Shi, Hesheng; Wang, Ruiliang; Du, Jiayuan

The northern shelf of the South China Sea , more than 250 km wide and less than 1 degree of terrain gradient averagely, is a typical wide and low-gradient continental shelf. Based on sedimentary facies association and stratigraphy stack patterns, 20 third-order siliciclastic sequences in Paleogene-Neogene in northern continental margin of the South China Sea is recognized and categorized to 3 types of sequence models.

Model 1: Tripartite sequence model in shelf margin. The sequences SQ13.8, SQ21 and SQ23.8 belong to this model. Sequences development is major controlled by relative sea-level change in this model,and characterized by stream rejuvenation and fluvial incision. Shelf-margin delta formed during rapid falling relative sea-level or low relative sea-level, is associated with basin floor turbidite fans. When the deltas are already at shelf margin, the subsequent fall, even a very small magnitude, may result in the shelf-edge dissection and the formation of basin-floor fans. In transgressive stage, the outer-shelf wave-dominated delta transforms to the mid-shelf delta. In high relative sea level stage the inner shelf tide-dominated delta and the bayhead delta develops.

Model 2: Tripartite sequence model in continental shelf. The sequence SQ14.8, SQ12.7, SQ11.7 and SQ8.5, are typical examples of this model. Sediment supply is dominated in the formation of this sequence model. Although the falling relative sea level cann't expose the entire shelf, rapid supply of sediment result in delta accumulation in inner shelf to form local depositional break. The high sediment supply drives the delta beyond the former shelf depositional break ,but incised valley doesn't occur in depositional break edge during lower relative sea level. Below the depositional slope break exists shelf-perched lowstand tidal sand ridges resulting from the reforming of preexisting delta, transgressive and highstand deposits are the inner shelf wave-dominated and especial tide-dominated deltaic sediments.

Model 3: The dimidiate sequence model in shelf margin. This model is the specific type of sequence SQ14.8, SQ12.7, SQ11.7 and SQ8.5. Relative sea-level and sedimentary supply control development of sequence. Lowstand systems tract doesn't occur. When the continental shelf is submerged and the sediment supply flux decreased, mid-shelf to inner shelf delta develop in transgressive stage, and the bay-head delta in high sea level stage.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013