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

The Ediacaran-Cambrian transition represents a pivotal geological time, denoting the decline of the Ediacaran biota and the emergence of most modern animal 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. Thick and laterally extensive exposure of marine sedimentary rock along the Anti-Atlas (AA) belt of Morocco constitute an especially important succession for global Ediacaran-Cambrian (E-C) chronostratigraphy. Here, numerous attempts have sought to pinpoint the exact stratigraphic level of the E-C boundary. The AA belt comprises ca. 3 km of continuous carbonate rocks, providing one of the most complete successions for the establishment of a global δ¹³C_carb chemostratigraphic reference curve.

A growing number of publications in recent years have enhanced the stratigraphic, paleontological and geochronological record of the AA belt. However, despite extensive efforts, the precise position of the E-C boundary in the Anti-Atlas remains ambiguous. The δ¹³C_carb data from this region have been used to inform rates of change in global palaeomarine redox conditions, biotic innovation and turnover, but significant inconsistencies remain in global correlation.

Here, we conduct a comprehensive examination of the available chemostratigraphic, paleontological, and geochronological data associated with the late Ediacaran-Cambrian Ouarzazate Group and Adoudou Formation within the AA 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 biostratigraphic, chemostratigraphic and geochronological datasets. This review highlights that the stratigraphic level currently assumed to represent the E-C boundary in the AA belt relies primarily on δ¹³C_carb data and, in particular, a prominent negative δ¹³C_carb excursion. However, the precise level of the E-C boundary in this region lacks corroborating evidence from other independent markers such as geochronological data or, crucially, the presence of the boundary-defining ichnospecies Treptichnus pedum.

Through the integration of newly available data and interrogation of global chemostratigraphic, biostratigraphic, and geochronological information, 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 the presumed Fortunian age of the ichnotaxa Monomorphichnus, because no co-occurring specimens of T. pedum are yet known. Moreover, a revised litho- and chemostratigraphic correlation that employs a compilation of published 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.

Keywords: Anti-Atlas, Ediacaran-Cambrian boundary, Lower Cambrian ichnozone, Ouarzazate Group Adoudou Formation