Cenozoic tectonic subsidence in deepwater sags in the Pearl River Mouth Basin, northern South China Sea

Abstract

The Cenozoic tectonic subsidence of the deepwater area in the Pearl River Mouth Basin, northern South China Sea is studied by subsidence analysis via backstripping calculations based on data of newly interpreted sequence boundaries. In the subsidence analysis local porosity–depth parameters were estimated based on well data, the paleo-water depth parameters and lithology were estimated for all grid nodes based on well data and sedimentary system maps, and ages of sequence boundaries were adjusted according to the International Chronostratigraphic Chart v2013/01. Sensitivity analysis shows that these are essential to ensure the quality of the subsidence analysis in the slope areas. Maps of tectonic subsidence rates of 18 Cenozoic sequences were constructed and spatial–temporal variations of the tectonic subsidence were discussed. The subsidence was restricted in the Baiyun and Liwan Sags before 30 Ma and extended to uplifts after 30 Ma. This indicates that the 30 Ma unconformity is the breakup unconformity that separated the syn-rift and post-rift sequences. Four substages of post-rift subsidence were identified by cyclic changes in the rate and pattern of tectonic subsidence at boundaries of 23.3 Ma, 17.2 Ma and 11.9 Ma, respectively. Anomalous post-rifting tectonic subsidence was significant and totaled to ~ 1200 m at the center of the Baiyun Sag, and punctuated by rapid subsidence and uplift events. The Baiyun Event was manifested in the study area by a short-lived uplift event in 23.9–23.3 Ma followed by a short-lived rapid subsidence in 23.3–19.8 Ma in the central study area. This localized rapid subsidence caused northward jump and anti-clockwise rotation of the shelf break. Two-order teeterboard (seesaw)-like subsidence was observed in the study area in the post-rifting stage, suggesting elastic deformations of the crust and lithosphere. • Tectonic subsidence of deepwater sags estimated via backstripping on newest data • Maps and histograms of subsidence rate for 18 Cenozoic sequences are constructed. • Results indicate that the 30 Ma unconformity is the breakup unconformity. • Cyclic anomalous post-rift subsidence of ~ 1200 m confirmed for central Baiyun Sag • Baiyun event near Oligoc./Mioc. boundary is evidenced by rapid local subsidence.