Integrated analyses of the Ediacaran-Cambrian boundary sequence in northern Gondwana (Anti-Atlas platform, Morocco)

Abstract

The Ediacaran to Cambrian transition (ECT) represents a critical period in Earth’s history, marked by the rapid emergence and diversification of metazoan life. Understanding the precise timing and dynamics of this bio-event as well as their links to paleoenvironmental changes requires a well-defined stratigraphic framework. The Anti-Atlas platform in Morocco offers a key region for unravelling the tectono-sedimentary setting, chemostratigraphic and geochemical features of ECT in northern Gondwana margin. Stratigraphic record of this transitional interval lying in the Lower Adoudou Formation (Tabia-Tifnount Members transition) is well-constrained by δ¹³C_carb chemostratigraphy and geochronology. However, the basin evolution analysis based on sequence stratigraphy has received little attention, as well the exact location of the Ediacaran-Cambrian boundary in this region remains uncertain, partly due to the absence of typical small shelly fossils and trace fossil assemblages. In this study, we apply an integrated approach combining sedimentary, stratigraphic, paleontological, and geochemical analyses to investigate the Tabia section, one of the most continuous ECT sequences in northern Gondwana. The stratigraphic record reveals four key stages in the tectono-sedimentary evolution. The synrift stage comprises three tectonic pulses, each corresponding to distinct depositional sequences within the Tabia Member. The progressive extension starts with rift initiation forming a narrow rift branch and culminates during rift propagation with wide rift branch during the following two tectonic pulses. The postrift stage coincides with a shift to the stable carbonate platform of Tifnout Member. This geodynamic and paleogeographic transition is well documented by geochemical evidence. In contrast to the postrift strata, synrift sequences preserve increased syn-sedimentary fault-driven hydrothermal activity and related dolomitization process (e.g., higher ⁸⁷Sr/⁸⁶Sr ratios, Eu/Eu* anomalies, and elevated concentrations of metals). In addition, our results offer new fossils and δ¹³C_carb chemostratigraphy constraints on the precise placement of the Ediacaran-Cambrian boundary across the platform, suggesting that the synrift Tabia Member and postrift lower part of Tifnout member including Tamjout dolomite are late Ediacaran in age. The first negative excursion in the Lower Tifnout Member is interpreted as the basal Cambrian carbon isotope excursion (BACE), coinciding with the transition from synrift to post-rift settings in the western Anti-Atlas platform.