Cenozoic sedimentary evolution of deepwater sags in the Pearl River Mouth Basin, northern South China Sea

摘要

Recent exploration revealed the high potential for hydrocarbon in the deepwater sags, Pearl River Mouth Basin, northern South China Sea. This paper reports its Cenozoic sedimentary evolution through backstripping of high precision depth data of interpreted sequence boundaries. Local backstripping parameters were mapped based on well and geophysical data. Sensitivity analysis indicates that the reliability of decompaction results were largely improved by using the local porosity parameters and the lithological parameters that vary with grid nodes. Maps of sedimentation rates of 17 sequences from 65 Ma to the present were constructed, showing the spatial–temporal variation of the sedimentation rate. Three rapid depositional stages, 65–32, 29–23.3, 18.5–10.5 Ma, and three slow depositional stages, 32–29, 23.3–18.5, 10.5–0 Ma, were identified with abrupt changes of sedimentary patterns. The three rapid depositional stages were in accord with syn-rifting stage, the first post-rifting depositional stage, and the second post-rifting depositional stage, respectively. And the three slow depositional stages were in keeping with three tectonic events respectively. Several significant sedimentary discontinuities at 32, 23.3 and 10.5 Ma were observed and discussed. The comparison between the study area and the ODP Site 1148 at 32–23.3 Ma indicates that before ~29 Ma the ODP Site 1148 was at similar sedimentation regime as that in the Baiyun and Liwan sags, but significant diversity appeared after ~29 Ma, when a large quantity of terrigenous sediments was trapped by strong post-rifting subsidence in the Baiyun and Liwan sags and could not reach the lower slope areas. Study revealed that the most rapid accumulation from 18.5 to 17.5 Ma might be mainly owing to the large sediment supply during this strong monsoon period.