Dynamic evolution of the Eocene basin-source system in the Baiyun Sag， Pearl River Mouth Basin

The Pearl River Mouth Basin （PRMB）， located in the deep-water super-extensional region along the northern continental margin of the South China Sea， exhibits complex and diverse tectonic evolution in the rifting stage. Focusing on the Eocene Wenchang and Enping formations in the Baiyun Sag within the PRMB， northern South China Sea， we reconstruct the prototype basin of the entire Baiyun Sag on the scale of third-order sequences. This approach overcomes the limitations of previous analyses on the sag scale and under a second-order sequence stratigraphic framework while also leading to new insights into the tectono-sedimentary evolutionary mechanisms of the Baiyun Sag during rifting. Based on seismic stratigraphic trend analysis， we reconstruct the denuded thickness of the Baiyun Sag during the deposition of the Wenchang and Enping formations. Furthermore， based on an analysis of the regional tectonic setting and previous research results on sedimentary systems， we characterize the dynamic evolutionary process of the prototype basin of the Baiyun Sag. The results indicate that during the deposition of the Wenchang and Enping formations， the prototype basin evolved from small-scale narrow lake basin under strong extension to a wide， shallow-water lacustrine basin under weak extension， ultimately developing into a shallow-water， extensive basin under a stable subsidence setting. The denudation center migrated gradually from the initial axial intra-basin provenance area during the deposition of the lower member of the Wenchang Formation to the northern gentle slope during the deposition of the upper member of the Wenchang Formation and then to the northwestern area outside the basin during the deposition of the Enping Formation. The study area experienced a dynamic evolution of basin-source sedimentary and filling， shifting from proximal to distal provenance. Correspondingly， the basin-source system of the Baiyun Sag transitioned from a proximal provenance-steep slope type to a proximal provenance-gentle slope type and then to a distal provenance-gentle slope type. Specifically， the intra-basin axial supply of the proximal provenance-steep slope type dominant from the 6th to 4th sub-members of the Wenchang Formation during intense rifting changed to the supply of the proximal provenance-gentle slope type in the northern gentle slope zone predominating from the 3rd to 1st sub-members of the Wenchang Formation during weak rifting. Finally， it transitioned to the long-axis supply of the distal provenance-gentle slope type spanning from the 4th sub-member of the Enping Formation to the 1st and 2nd sub-members of the Enping Formation during the rifting-depression transition period. Both the migration of the denudation center and the transformation of the basin-source system are governed by a clockwise variation in the stress field in the northern South China Sea， shifting from NW to NNW and then to nearly NS.