Astronomical forcing and coal accumulation model of the Middle Jurassic Xishanyao Formation in the Turpan–Hami Basin, Northwestern China

The Xishanyao Formation in the Taibei Depression of the Turpan–Hami Basin consists predominantly of coal-bearing fluvial and lacustrine deposits characterized by frequent interbedding and systematic vertical facies transitions. These strata exhibit repetitive stacking patterns and thickness variations, suggesting the presence of sedimentary cycles. The cyclic architecture corresponds closely to astronomical forcing signals. In this study, natural gamma ray (GR) logs were employed as paleoclimate proxy indicators, and time-series analysis was conducted using Acycle. Astronomical periodicities were identified, and a floating timescale calibrated to the 405 ka long-eccentricity cycle was established. Comprehensive analysis of the sequence stratigraphic framework and timescale suggests that the top boundary of the Xishanyao Formation dates to 165.3 ± 0.8 Ma, corresponding to the top of the Bathonian Stage in the Middle Jurassic (165.3 ± 1.1 Ma). Sequence J 2 × 1 lasted 621.8 ka, with a thickness of 69 m and a sedimentation rate of ~ 11.1 cm/ka, during which braided-river delta and floodplain facies developed. Sequence J 2 × 2 lasted 4031.9 ka, with a thickness of 444 m and a sedimentation rate of 11.0 cm/ka, characterized by braided-river delta fronts, swamps, and shallow-lacustrine facies. Sequence J 2 × 3 lasted 3461.8 ka, with a thickness of 328 m and a sedimentation rate of 9.5 cm/ka, dominated by delta-front, swamp, and lacustrine facies. Sequence J 2 × 4 lasted 1869 ka, with a thickness of 191 m and a sedimentation rate of 10.2 cm/ka. Overall, the astronomical timescale established in this study provides a high-resolution chronostratigraphic framework for reconstructing the paleoenvironmental evolution of the Xishanyao Formation in the Taibei Depression. More importantly, this study provides a multi-well astronomical framework that quantitatively links orbital forcing, sediment accumulation, and peat-swamp development in a Middle Jurassic continental coal-bearing basin.