Integrated Stratigraphy and Bayesian Age Modelling of the Tiout Section (an-Ti-Atlas, Morocco): Implications for Early Cambrian Geologic Time Scale Calibration

The calibration of the early Cambrian Geologic Time Scale (GTS) remains difficult because major carbon isotope excursions, biotic events, and geochronologic constraints are rarely occurring together within a single stratigraphic framework. The Tiout section (Anti-Atlas, Morocco) offers a rare opportunity to address this problem because it preserves a continuous 1205 m thick succession containing multiple volcanic ash beds, a refined trilobite biostratigraphy, and one of the most complete early Cambrian (Cambrian Age 2–Age 3) successions of West Gondwana. We integrate high-resolution δ¹³Ccarb stratigraphy, elemental geochemistry, U–Pb geochronology, and biostratigraphy within a Bayesian astrochronological age-depth model to produce a quantitative chronostratigraphic framework for the Cambrian Age 2–Age 3 interval. This integrated model provides numerical age constraints for both carbon isotope excursions and key evolutionary events, allowing direct comparison between regional stratigraphic archives and the global Cambrian time scale. The new δ¹³Ccarb record from Tiout display several globally significant excursions, including the termination of the SHIyantou Carbon isotope Excursion (SHICE), excursions II and III, and the Early Atdabanian/Repinaella Zone Excursion (EAREZE; excursion IV). The EAREZE peak occurs below the lowest local occurrence of mineralized trilobites, correlates into the EAREZE peak in the Repinaella Zone (trilobites) at the Zhurinsky Mys section in east Siberia (IV) and is dated at 520.046 ± 0.097 Ma, providing the first direct numerical calibration of this excursion within a single stratigraphic record bracketed by two ash beds. The age of the EAREZE peak is significantly older (ca. 2 Ma) than the U-Pb age of the CARE excursion recently determined in the Parabadiella Zone in South Australia and older than unrevised ages (ca. 518 Ma) on the oldest trilobite fragments from the Avalonia paleocontinent (South Wales). These results establish Tiout as a robust multi-proxy reference framework for synchronizing carbon isotope stratigraphy, fossil first appearances, and radioisotopic ages in the early Cambrian. More broadly, this study illustrates how integrated Bayesian age modelling can support transparent and reproducible refinement of Cambrian GTS calibration and reinforces the potential of Tiout as a candidate reference section for the base of Cambrian Series/Epoch 2.