The middle Eocene represents a shift from the Early Eocene greenhouse climate to a subsequent cooling trend, during which orbital-scale forcing exerted a strong influence on continental depositional systems. The third Member of the Hetaoyuan Formation (Eh3) in the Biyang Depression of the Nanxiang Basin preserves a thick lacustrine succession, but its third-order sequence subdivision has remained controversial because of insufficient temporal control. In this study, natural gamma-ray (GR) logs from wells A1 and A2 were analyzed using an integrated workflow that combines cyclostratigraphy, INPEFA (Integrated Prediction Error Filter Analysis), wavelet transform, and sedimentary noise modeling. The GR records reveal clear astronomical signals corresponding to ~405 kyr long eccentricity, ~100 kyr short eccentricity, ~40 kyr obliquity, and ~20 kyr precession. Astronomical tuning to the 405 kyr cycle yields a depositional duration of ~10.3 Myr and a basal age of ~49.6 Ma. Integrated stratigraphic analysis identifies nine third-order sequence boundaries and eight third-order sequences. In addition, sedimentary noise modeling detects a prominent ~1.2 Myr long-period obliquity modulation signal, which is interpreted to govern long-term lake-level fluctuations and third-order sequence development. These results provide a time-constrained framework for sequence subdivision and demonstrate the importance of orbital forcing in shaping lacustrine stratigraphic architecture.
Ye et al. (Wed,) studied this question.
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