Establishment of an Astronomical Time Scale and Discussion on the Sedimentary Origin of the Member 4 Glutenite, Paleogene Wenchang Formation, Enping 21 Sag, Zhu III Depression, Pearl River Mouth Basin, South China Sea

Coarse-grained clastic systems in rift basins are often considered unfavorable for preserving continuous astronomical records. This study investigates the thick conglomeratic succession in the upper Member 4 (43.90–44.73 Ma) of the Wenchang Formation in the Enping 21 sub-sag, Yangjiang East Sag, Zhu III Depression, Pearl River Mouth Basin. Integrating cyclostratigraphy, petrography, and seismic-logging analyses, we established a high-resolution astronomical timescale (43.90–44.73 Ma) covering approximately 828 kyr. Cyclostratigraphic results indicate a highly continuous depositional record with an optimal sedimentation rate of 19.55 cm/kyr. Petrographic and geochemical evidence—including low structural maturity, dominant igneous lithic fragments, and high detrital zircon Th/U ratios—reveals a proximal volcaniclastic source, characterizing the studied interval as tephroid successions. Anomalously high GR responses and chaotic seismic facies further distinguish this succession from conventional terrigenous deposits, linked to a nearby identified magmatic body. Our results suggest that rapid accumulation of volcaniclastic flows and ash fallout, driven by proximal magmatic activity, reduced hydrodynamic reworking and facilitated the preservation of astronomical signals. This study highlights that in active lacustrine rift basins, magmatic activity significantly dictates sedimentary infill patterns by providing rapid volcaniclastic input, thereby enabling the establishment of precise chronostratigraphic frameworks even within complex, coarse-grained depositional systems.