Spatial-Temporal Variability in Stratigraphic Architectures of the Pearl River Mouth Continental Shelf, Northern South China Sea: An Interactive Response to Regional and Local Controls
Stratigraphic organizations of continental shelves are products of complex interactions between a series of regional and local controls, e.g. sediment input, sea level, climate, tectonic regime and local hydraulic regime. Using newly-acquired high-resolution single-channel seismic grids (8000 km long) and a few shallow boreholes, this study systematically investigated the Quaternary stratigraphic evolution of the Pearl River Mouth continental shelves in northern South China Sea. Two major types of shelf-crossing unconformities were recognized, including the erosional sequence boundaries (SB) and the flat-lying flooding surfaces (FS). They subdivided the upper Quaternary section into six high-order sequences (SQ6 to SQ1 from bottom up) and corresponding systems tracts. Driven by the asymmetry of Pleistocene eustatic cycles (slow fall, rapid rise), regressive units dominated most of the stratigraphic sections, with transgressive and highstand units only locally distributed. The two types of unconformities occasionally merged where transgressive units were absent. Our results revealed a significant along-strike variability in the stratigraphic pattern in front and lateral of the Pearl River Mouth, including the incision relief along SBs (shallow or deep), geometry of prograding clinoforms (high- or low-angle) and preservation of transgressive units (high or low), etc. The position of sediment source was a very important control. However, local tectonic activity and hydraulic conditions also contributed a lot to the lateral stratigraphic variations. In the shelf around Dongsha region, evidence indicated that continuous local tectonic uplifting coupled with intense activity of internal waves exerted significant influence in forming the erosive stratal pattern. In temporal perspective, the stratigraphic architecture displayed contrasting geometry across a key horizon SB4, the age of which was dated ~ 600 ka after a calibration with shallow boreholes. Within sequences below SB4, fluvial incisions were barely present and clinoforms were small in height and confined within the middle shelf, compared with the wide-spread, large-scale incised valley systems and high-angle shelf-margin clinoforms above it. We suppose that this change might be closely linked with the mid-Pleistocene climate transition, which brought an obvious increase in the periodicity of glacial-eustatic cycles (i.e. 40 ky to 100 ky), finally driving the sea level to fall beyond the shelf-edge.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017