## 4 Results  
### Time Series Analysis  
The natural gamma-ray (GR) series from Well EP17 underwent LOESS smoothing to eliminate a long-term trend spanning 251.906 m (Figure 2c). Subsequent spectral analysis using the 2\(\pi\) Multi-Taper Method (MTM) revealed significant cyclicities at depths of 94.3 m, 27.4 m, 10.4 m, and 4.6 m (Figure 2a), with amplitude peaks surpassing the 95% confidence level. The thickness ratios of these cycles (20.5:5.9:2.2:1) closely correspond to the theoretical orbital period ratios (405 kyr:126 kyr:40 kyr:20 kyr) derived from the astronomical solution by [PERSON] et al. (2010d) [33] for the 15-20 Ma interval (theoretical ratio: 20.15:6.3:2.1:1). These cycles are interpreted as follows: 405 kyr long eccentricity (94.3 m), 100 kyr short eccentricity (27.4 m), 40 kyr obliquity (10.4 m), and 20 kyr precession (4.6 m). The Evolutionary Fast Fourier Transform (eFFT) analysis further confirmed the high stability of the 94.3 m cycle thickness (Figure 2b), providing robust evidence for the preservation of Milankovitch signals within the GR series.  
Figure 2: Depth–domain analysis of the GR data series in the EP17 borehole. **(a)** Lithostriagraphy and lithology; **(b)** GR data series; **(c)** GR data series detrended using the LOESS filter to remove the 251.906 m trend; **(d)**\(\sim\)94.3 m Gaussian bandpass filter (passband: 0.0010344 \(\pm\) 0.0001 cycles/m); **(e)** 2\(\pi\)MTM spectral analysis of the detrended GR series, where E represents long eccentricity, e represents short eccentricity, O represents obliquity, and P represents precession.; **(f)** eFFT evolutionary spectral analysis of the detrended GR series, where E represents long eccentricity, e represents short eccentricity, O represents obliquity, and P represents precession.  
The COCO analysis identified an optimal sedimentation rate of 23.4 cm/kyr for Well EP17 (Figure 3a). The corresponding cyclic thicknesses and periods are as follows: 94.3 m (402 kyr), 27.4 m (118 kyr), 10.4 m (44 kyr), and 4.6 m (19.6 kyr). These measurements align closely with theoretical astronomical cycles, thereby confirming the reliability of the astronomical signals identified in the Zhujiang Formation of Well EP17.  
Based on the interpretation of astronomical cycles and the tracking of sedimentation rates, the detrended gamma-ray (GR) data were astronomically tuned using the [PERSON] 2010d scheme [6]. The results indicate that the duration of the upper section of the Zhujiang Formation in Well EP17 is 3.11 million years (Myr). The Multi-Taper Method (MTM) spectrum of the calibrated GR sequence reveals significant peaks at 405 kyr, 100 kyr, 40 kyr, and 20 kyr, which correspond to the theoretical eccentricity timescale (ETP) astronomical solution proposed by [PERSON] (2010d) [33] (Figure 4a,b). Gaussian bandpass filtering was employed to extract 18 cycles of the 405 kyr long eccentricity and 72 cycles of the 100 kyr short eccentricity (Figure 4c).  
Figure 3: Correlation coefficient method (COCO) analyses of geophysical logging data from the EP17 borehole. (**a**) COCO analysis of Well EP17, testing sedimentation rates within the range of 10–30 cm/kyr with a step size of 0.1 cm/kyr and 5000 Monte Carlo simulations,\(\rho\) Indicates the size of the correlation coefficient; (**b**) Null hypothesis of Well EP17; (**c**) Astronomical orbital matching numbers of Well EP17.