SEQUENCE ARCHITECTURE AND DEPOSITIONAL EVOLUTION OF PALEOGENE SYNRIFT TO EARLY POST-RIFT STRATA IN THE BAIYUN SAG OF THE PEARL RIVER MOUTH BASIN, SOUTH CHINA SEA

The South China Sea is located at the intersection of three plates and exhibits a complex evolution from synrift to post-rift to, locally, compressional stages. This study focuses on the Baiyun Sag and adjacent Panyu uplift of the Pearl River Mouth Basin with the goal of constructing the sequence architecture and depositional evolution of the Eocene – Early Oligocene synrift to the Late Oligocene early post-rift. The Baiyun Sag represents a half-graben and the Paiyun uplift to the north defines a hangingwall ramp. This study utilizes 3-D seismic data complemented by well logs and cores and reveals that the package of strata are comprised of three composite sequences made up of five third-order sequences for the synrift Wenchang Formation, three third-order sequences for the transitional rift-to-drift Enping Formation, and six third-order sequences for the early post-rift Zhuhai Formation. Composite sequence boundaries are defined by truncation unconformities and could be identified across the whole study area. In contrast, third-order sequence boundaries are recognized on the basis of toplap and offlap characteristics and are best developed in center of the Baiyun Sag. Based on different internal reflection configurations, external shapes and contact relationships, thirteen seismic facies are identified. Lithofacies are arranged into eight gamma ray well-log shapes. Based on the aerial distribution of seismic and lithofacies, the Wenchang and Enping formations consist of alluvial fan and fan-delta deposits adjacent to the southern fault scarp and prograding braid-delta facies on the adjacent ramp. Central rift basin deposits are dominated by lacustrine mudstones that record an upward shallowing of the lake. The overlying Zhuhai Formation is dominated by southward-prograding deltas. This study is the first documentation of a synrift to early post-rift depositional evolutionary history of the Pearl River Mouth Basin.