Ediacaran-Cambrian Boundary in the Anti-Atlas belt (Morocco): A review of biostratigraphy, chemostratigraphy and geochronology

Abstract

The Ediacaran-Cambrian transition represents a pivotal geological marker, denoting the decline of the Ediacaran biota and the emergence of most modern phyla in an interval marked by perturbations to the carbon cycle (as evidence by carbonate carbon isotopes, δ¹³C_carb), biotic turnover, dynamic paleoredox regimes, and magnetic field instability. Following initial international expeditions to the Anti-Atlas belt in Morocco, numerous attempts have sought to pinpoint the exact stratigraphic level of the Ediacaran-Cambrian boundary (E-C) in this important succession, where ca. 3 km of continuous carbonate rocks provide one of the most complete successions for the establishment of a global δ¹³C_carb chemostratigraphic reference curve. Subsequently, a growing number of publications have explored the stratigraphy, paleontology, and geochronology of the Ediacaran-Cambrian transition in the Anti-Atlas. Despite extensive efforts, the precise position of the E-C boundary in the Anti-Atlas remains ambiguous, with multiple proposed positions ranging from within the Ouarzazate Group to the lower part of the Tifnout Member (Adoudou Formation, Taroudant Group). Here, we conduct a comprehensive review of the available chemostratigraphic, paleontological, and geochronological data associated with the late Ediacaran-Cambrian Ouarzazate Group and Adoudou Formation within the Anti-Atlas belt. The objective is to refine our understanding of the regional expression of the E-C boundary and offer clarity on the inconsistencies observed among biostratigraphy, chemostratigraphy, and geochronology. This review underscores that the currently proposed boundary relies primarily on the δ¹³C_carb excursion, lacking corroboration from other independent markers such as geochronology and globally significant ichnofossil assemblage constituents. Additionally, through the integration of newly available data and enhanced global chemostratigraphy, biostratigraphy, and geochronology, our findings suggest that the E-C boundary within the Western Anti-Atlas may be positioned as low as within the upper unit of the Tabia Member. However, this interpretation relies heavily on a Fortunian age for the ichnotaxa Monomorphichnus, in the absence of co-occurring specimens of the boundary-defining ichnospecies Treptichnus pedum. Moreover, a revised assessment of chemostratigraphic correlation and geochronological markers indicates that the Tabia and Tifnout members in the Central and Eastern Anti-Atlas do not correlate with the same named members in the Western Anti-Atlas. Both the Tabia and Tifnout members of the Central-Eastern Anti-Atlas may instead correlate with the middle part of Tifnout Member in the Western Anti-Atlas. This implies a late Ediacaran to early Cambrian ca. 10 m.y stratigraphic gap in the Central-Eastern Anti-Atlas and hence the E-C boundary in the Central-Eastern Anti-Atlas is likely situated within the unconformity between the Ouarzazate and Taroudant Groups. Lastly, we present unresolved questions regarding the biogenicity and affinities of some described discoidal structures and calcified microfossils from the Tabia Member, with the aim of further stimulating active engagement of the global scientific community in the establishment of robust paleontological and chronostratigraphic constraints for the E-C boundary within the Anti-Atlas belt.