Sequence architecture and depositional evolution of the Late Miocene to quaternary northeastern shelf margin of the South China Sea

Abstract

Based on an integrated analysis of seismic, well logging and paleontological data, the sequence architecture and depositional evolution of the northeastern shelf margin of the South China Sea since Late Miocene are documented. The slope deposits of the Late Miocene to Quaternary can be divided into two composite sequences (CS1 and CS2) bounded by regional unconformities with time spans of 3–7 Ma, and eight sequences defined by local unconformities or discontinuities with time spans of 0.8–2.3 Ma. Unconformities within CS1 feature shelf-edge channel erosion, while in CS2 they form truncations at the top of the shelf margin as prograding complexes and onlap contacts against the slope.Depositional systems recognized in the slope section include unidirectionally migrating slope channels, slope fans or aprons, shelf-edge deltas and large-scale slope clinoforms. CS1 (Late Miocene to Pliocene) is characterized by development of a series of shelf-margin channels and associated slope fan aprons. The shelf-margin channels, oriented mostly NW-SE, migrate unidirectionally northeastwards and intensively eroded almost the entire shelf-slope zone. Two types of channels have been identified: (1) broad, shallow and unconfined or partly confined outer-shelf to shelf-break channels; and (2) deeply incised and confined unidirectionally migrating slope channels. They might be formed by gravity flow erosion as bypassing channels and filled mostly with along-slope current deposits. Along the base of the shelf slope, a series of small-scale slope fans or fan aprons are identified, including three depositional paleo-geomorphological elements: (1) broad or U-shaped, unconfined erosional-depositional channels; (2) frontal splays-lobes; and (3) non-channelized sheets. CS2 (Quaternary) consists mainly of a set of high-angle clinoforms, shelf-margin deltas and lower slope unidirectionally migrating channels.The relative sea level changes reflected in the sequence architecture of the study area are basically consistent with Haq's global sea level curve, but the development of regional unconformities were apparently enhanced by tectonic uplift. The development of high-angle (thick) clinoforms in the Quaternary may be attributed to a high sediment supply rate and rapid tectonic subsidence. The formation of the unidirectionally migrating channels appears to have resulted from the combined effects of the northeastward South China Sea Warm Current (SCSWC) and downslope gravity flow. The formation of the slope channels in the outer-shelf to shelf-break zone may be predominately controlled by bottom current, whereas those developed along the middle to lower slope zone may be dominated by gravity flow.