## 6 Conclusions  
1. Based on the principles of cycloststratigraphy, the Milankovitch cycle signals preserved in the 15.98-19.09 Ma stratigraphic sequence of the Upper Zhujiang Formation in the Enping Depression have been extracted systematically by using spectral analysis, wavelet transform, and astronomical tuning techniques of natural gamma-ray loggings. Through sliding window spectral analysis and dynamic correction of sedimentation rates, an astronomical age scale containing 405 kyr long eccentricity, 100 kyr short eccentricity, 40 kyr obliquity, and 20 kyr precession cycles was successfully constructed. Its resolution is more accurate than that of traditional biostratigraphy, and it provides a key calibration standard for high-precision isochronous stratigraphic correlation of the Neogene in the northern South China Sea. 2. The inversion results based on the DYNOT orbital period decomposition algorithm and \(\rho\)1 sediment noise model show that the study interval has a significant 405 kyr long eccentricity period dominant characteristic relative to sea-level variation. The quantitative correlation analysis between cycle thickness and sedimentary flux shows that the high value period of long eccentricity (corresponding to a high insolation phase) is coupled with the warm and wet period of global climate, which promotes the extensive deposition of fine mudstone under the background of transgression, while the low value period (low insolation phase) triggers the regional dry and cold climate, enhances the supply of terrigenous debris, and forms the dominant sedimentary sequence of coarse delta-coastal sandstone. This law establishes a dynamic response model for the prediction of reserve-cap combination under the interaction of land and sea. Figure 9: Sedimentary pattern diagram of the upper section of the Zhujiang Formation in the Enping Sag. (**a**) Mode I—low-eccentricity progradational delta; (**b**) Mode II—high-eccentricity retrogradational delta. 3. The correspondence between the constant elements indicating environmental changes and astronomical cycle curves reveals that the 405 kyr long eccentricity period regulates the \"periodic modulation\" effect of precession amplitude, and both of them affect environmental changes such as climate change, weathering intensity, and rainfall variation in the study area, further affecting the sedimentary process. During the period of high eccentricity, the intensity of monsoon precipitation and chemical weathering increased sharply due to the increase in precession amplitude, and the base level rose due to the retreat of the ice sheet, which significantly altered the spatial distribution of the denudation flux and the sedimentary system in the source area. 4. The methodology and high-resolution timescale presented here lay a foundation for future comparative studies of orbital-scale sedimentary responses in monsoon-dominated systems, both within SE Asia and globally. This study is constrained by its regional focus on a single borehole in the Enping Depression, limiting direct comparisons with contemporaneous SE Asian basins due to scarce orbital-scale datasets. The temporal resolution of the astronomical timescale may overlook sub-Milankovitch events, while model assumptions (e.g., DYNOT attributing noise solely to sea level) might oversimplify tectonic or diagenetic influences. The reliance on GR and geochemical proxies (CIA, Al\({}_{2}\)O\({}_{3}\)) could underrepresent complex climate feedbacks, and restricted data accessibility hinders full reproducibility. Future work should integrate multi-proxy analyses and expand to adjacent basins to enhance regional and global applicability. Conceptualization, S.Z. and C.Z. ; methodology, E.X. ; software, C.Z. ; validation, S.Z., C.Z. and E.X. ; formal analysis, Y.W. ; investigation, R.Z. ; resources, R.H.; data curation, G.G. ; writing--original draft preparation, S.Z. ; writing--review and editing, C.Z. ; visualization, C.Z. ; supervision, E.X. ; project administration, Y.W. ; funding acquisition, S.Z.