Carbonate platform dynamics across the Cenomanian/Turonian Boundary: the OAE2 record in the Akrabou Formation (Southeastern Morocco)

During Early Cretaceous, most of North Africa was characterized by continental sedimentation. It was at the Cenomanian/Turonian Boundary (early Late Cretaceous), that an important eustatic sea level rise led to the flooding of a huge portion of continental margin, from Egypt to Morocco, with the emplacement of an epicontinental carbonate platform known as North Saharian Platform. The Cenomanian/Turonian Boundary corresponds to one of the warmest periods in Earth history, highest sea level stands, and an important oceanic anoxic event (OAE2). Our research focuses on Southeastern Morocco, between the Central High Atlas and Anti-Atlas ranges, where the Akrabou Formation (Ferrandini et al., 1985) records this important event. The Akrabou Formation, up to 100 m thick, reveals a dynamic depositional history. Its lower portion is characterized by low-energy, shallow-water environments, testified by fine-grained wackestone/packstone with abundant micrite, very small oligotypic benthic foraminifera, ostracods, bivalves, and echinoderms; oysterrich beds alternate with fine limestones. Upwards, the succession changes to laminated lime-mudstones with planktonic foraminifera, chert lenses, and ammonite-bearing nodules, indicating a deepening. Subsequently, a shallowing trend is marked by fine-grained packstone/wackestone with an oligotypic assemblage of small benthic foraminifera, echinoderms, bivalves and macroforaminifera, with chert and ferruginous surfaces in interbeds. Abundant micrite suggests the return to low energy shallow-water environments. An increase in energy is observed upward with coarser, cross bedded levels. On top, peritidal facies mark the emersion of the platform and the re-establishment of continental clastic sedimentation. Stable isotope analysis (δ13C and δ18O) on bulk carbonates and oyster shells reveals a distinct δ13C positive broad peak of +3‰ VPDB, aligning with OAE2 values worldwide (Voigt et al., 2008). Highest δ13C values occur in the lower portion of the platform, and then decrease to around -2‰ VPDB, coinciding with a thinly stratified interval interpreted as the maximum flooding phase. OAE2 significantly stressed the epicontinental platform, as suggested by the temporary shift to deeper environments. However, this crisis was transient and the shallow benthic carbonate community was quickly reestablished. As observed by Wang et al. (2021), the maximum retreat of the platform, and hence the peak of its crisis, is not coincident with the stable isotopic record of OAE2, but rather occurs with a significant delay.