1. Shenzhen Branch of CNOOC (China) Co., Ltd., Shenzhen 518064, Guangdong, China; 

 To systematically understand the sequence boundary identification and sedimentary environment evolution in the ultra?deep water area of the Baiyun Sag, Pearl River Mouth Basin, the authors conducted a compre? hensive analysis using trace and major element data from the carbonate rock section of the Late Oligocene Zhuhai Formation to Early Miocene Zhujiang Formation of the drilled Well A1 in Baiyun Sag. The results show: ①Three third?order sequence boundaries of SB24. 80, SB23. 03 and SB21. 00 were identified through the variation charac? teristics of parameters such as concentrations of Pb, Zn, Rb, Al 2O3 , Fe2O3 , and Th / U ratio. ②Based on the var? iability of Sr/ Ba, V/ Ni, Sr/ Cu, Al 2O3 / MgO, and MgO/ CaO ratios, it is confirmed that the sample section deposi? ted mainly in a sedimentary environment characterized by arid climate and relative sea?level rise. ③Ratios of V/ Cr, Ni / Co, V/ (V + Ni) suggest that the carbonate rocks in Well A1 are mainly developed in ancient marine wa? ter bodies of oxidizing environments, with shallow water bodies. In the later stage, the water level tended to rise to?wards hypoxic conditions, and the development of carbonate rocks was terminated by “drowning”.