Abstract: The Qikou sag in the Bohai Bay basin, as a significant Cenozoic depression basin in the eastern North China craton, holds key implications for understanding regional tectonic-sedimentary responses and hydrocarbon accumulation mechanisms during the Paleogene. This study presents a systematic analysis of detrital zircon U-Pb geochronology and trace elements from clastic rock samples of the first (Es1) and second (Es2) members of the Shahejie Formation, obtained from three coring wells (X, Y, Z) in the southern Qikou sag. Integrated with provenance contribution inversion and tectonic setting discrimination, we investigate the controlling effects of the Paleogene tectonic background on the provenance system. Results reveal distinct detrital zircon age spectra between Es1 and Es2 samples: Es1 samples are dominated by age peaks at 120 Ma, 284 Ma, 1 866 Ma, and 2 515 Ma, whereas Es2 samples exhibit clusters at 252-246 Ma, 320-291 Ma, 1 882-1 877 Ma, and 2 542-2 532 Ma. The primary source of the Es1 sediments, according to provenance analysis, was the northern domain of the plate-margin orogenic belt bordering the North China block. Additional provenances included the Jiaodong and Liaodong Peninsulas. In contrast, Es2 sediments were primarily sourced from the Central Asian orogenic belt and the Sulu orogenic belt. Provenance contribution modeling further reveals that Es1 was mainly supplied by the Jiaobei (76.48%), Jiaodong (12.14%), and Liaodong (6.65%) regions, whereas Es2 was significantly influenced by the Central Asian orogenic belt (9.46%) and the Sulu orogenic belt (5.65%). Based on cumulative curves of the time difference between zircon crystallization and deposition ages (CADA) and the evolution of regional fault systems, this study proposes that the Qikou sag was generally under an extensional tectonic setting during the Oligocene. However, a significant provenance shift occurred from Es2 to Es1, reflecting a tectonic transition from a NE-trending to an approximately EW-trending fault system. This shift led to a change in sediment routing from NS to EW, further evidencing the deep-seated control of regional stress field transformation on tectonic-sedimentary processes in the basin under the influence of Pacific Plate subduction.

Key words: Bohai Bay basin, detrital zircon, U-Pb geochronology, Paleogene, trace elements, Pacific Plate, tectonic setting, oil and gas