Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period

The Cambrian is the first period of the Phanerozoic during which numerous major geological and biological events occurred. Understanding these events requires a high-resolution time scale. The Luoyixi section in Guzhang (Hunan Province, China) is the global stratotype section and point (GSSP) for the Guzhangian Stage. Here, magnetic susceptibility and δ13C data were used to conduct cyclostratigraphic analyses covering the upper Drumian through lower Guzhangian stages. The results of the power spectral analyses and average spectral misfit analyses indicate that the 25–25.53 m, 6.25–6.91 m, 1.90–2.03 m, and 1.08–1.19 m sedimentary cycles may represent long-eccentricity, short-eccentricity, obliquity and precession cycles, respectively. A ~1400 kyr floating astronomical time scale was constructed using the stable 405-kyr eccentricity cycles to calibrate the agnostoid trilobite zones. The obtained fundamental obliquity period (30.7 ± 0.7 kyr) implies 370,180 ± 1220 km (vs. 384,000 km for present day) for the Earth–Moon distance and 21.58 ± 0.18 h (vs. 23.93 h for present day) for the length of day during the middle Cambrian (500 Ma). •Reliable Milankovitch cycles were identified in the Middle Cambrian marine strata.•An ~1400 kyr ATS was constructed for the Drumian–Guzhangian transition.•The durations of the two complete agnostoid trilobite biozones were estimated.•Deep-time evolution of the Earth–Moon system was reconstructed.