Astronomically forced lake-level fluctuations with impact on sand-body distribution of the oligocene Huagang Formation in the Xihu Depression, east China sea shelf basin

The Late Oligocene Huagang Formation in the Xihu Depression, East China Sea Shelf Basin, records typical braided river delta deposits. However, the influence of astronomical forcing on sand-body distribution remains unclear. Based on core, logging, and seismic data, subaqueous distributary channels of braided river deltas were identified and multi-scale lake-level fluctuations were reconstructed. The main findings findings are: (1) The upper Huagang Formation records stable 1.2 Myr long obliquity cycles and 405 kyr long-eccentricity cycles. A 5.6 Myr floating astronomical time scale (ATS) was established using the stable 405 kyr eccentricity signal. (2) DYNOT analysis shows that both million-year and 100,000-year lake-level variations are astronomically driven. The 1.2 Myr cycle controls 3rd-order lake-level changes, influencing deltaic facies belts, while the 405 kyr cycle governs 4th-order fluctuations, affecting sand-body stacking patterns. (3) Obliquity and eccentricity cycles also jointly modulate climate. A high-obliquity, low-eccentricity mode reflects a warmer, wetter climate with lower lake levels, where sand bodies occur in tangential or superimposed patterns. A high obliquity, high eccentricity mode reflects a colder, drier climate with higher lake levels, where sand bodies are isolated. This study provides new theoretical insights for high-resolution reservoir characterization and development of braided river delta reservoirs.