Orbital Forcing and Formation Mechanism of Eocene Lacustrine Halite-Mudstone Rhythmites in the Jianghan Basin, China

Abstract

Astronomical orbital periods are widely documented in deep-time stratigraphy; however, the operational mechanisms of these periods and their sedimentary responses in lacustrine basins remain poorly constrained under varying paleogeographic and paleoclimatic settings. Focusing on the salt rhythms of the Eocene Qianjiang Formation in the Jianghan Basin, this study identifies two critical transitions in the recorded orbital signals at approximately 42.92 and 41.06 Ma through high-resolution astronomical tuning, coinciding with paleoclimate evolution. Stage I (43.7–42.92 Ma): Under a global warming background characterized by a warm-humid climate, the basin was dominated by thick mudstone interbedded with minor halite. The strata primarily record eccentricity, obliquity and precession periods, consistent with the classical Milankovitch forcing pattern observed in low-latitude regions. Stage II (42.92–41.06 Ma): Against a global cooling background, the study area was governed by periodic warm-humid monsoon precipitation, leading to the deposition of isopachous halite-mudstone interbeds. Concurrently, high-latitude sea-ice expansion amplified the global obliquity signal, resulting in a strong obliquity-driven signature recorded in this mid-low latitude basin. Stage III (41.06–40.2 Ma): With subsequent global warming and diminishing sea-ice extent, the meridional thermal gradient weakened, reducing the influence of obliquity on mid-low latitudes. The cyclicity transitioned from sole obliquity dominance to a regime jointly governed by obliquity, eccentricity and precession periods. Consequently, the regularity of halite-mudstone interbeds significantly weakened under the interference of multiple orbital forcings. Our results demonstrate that the astronomical signals preserved in the Eocene salt rhythms of the Jianghan Basin were controlled by high-to-low latitude interactions modulated by paleoclimate evolution. This study provides a reference for understanding the operational mechanisms of deep-time orbital periods and their controls on lacustrine sedimentation.