Facies-dependent response of reef–shoal development to orbital pacing in the Changxing Formation, Sichuan Basin

Abstract Orbital forcing can produce contrasting stratigraphic expressions across carbonate-platform facies. We integrated lithology, thin-section microfacies, and high-resolution gamma-ray records from wells PS10 and PS2 in the Changxing Formation, Sichuan Basin, and applied spectral, time–frequency, COCO/eCOCO, band-pass filtering, and amplitude-modulation analyses to construct floating astrochronologies. PS10 records an intraplatform reef–shoal system dominated by MF1 grainstone and FA2 grain-shoal deposits, including three major shoal units and a reef-base–reef-core–reef-cap/grain-shoal complex. PS2 records three shallowing-upward alternations between FA2 grain shoals and FA4 intershoal–tidal-flat fine-grained deposits. Whole-interval COCO indicates mean sedimentation rates of 7.7 cm kyr − 1 for PS10 and 11.2 cm kyr − 1 for PS2. Both wells preserve 405-kyr long eccentricity, 125/95-kyr short eccentricity, 41-kyr obliquity, and 23.7-kyr precession rhythms. Precession-amplitude envelopes contain dominant 109–110-kyr modulation and show strong coherence with short eccentricity, indicating eccentricity-paced bundling of high-frequency depositional rhythms. Stepwise eCOCO Best SR changes coincide with facies transitions. Long-term warming, carbon-cycle change, and progressive deoxygenation defined the environmental context, whereas orbital pacing regulated buildup windows and growth pulses. Facies position filtered this forcing: the intraplatform high at PS10 favored sustained vertical aggradation, whereas the platform-margin setting at PS2 favored repeated shoal expansion, retreat, and fine-grained deposition. Thus, identical orbital rhythms generated different stratigraphic architectures across facies belts.