Carbonate platform evolution across the Cenomanian/Turonian boundary (Akrabou Formation, Southeastern Morocco)

The Cenomanian-Turonian transition represents one of the major paleoenvironmental perturbations of the Cretaceous, characterized by global sea-level highstands, widespread marine transgression, and Oceanic Anoxic Event 2 (OAE2). Across North Africa, the transgression promoted the establishment of an epicontinental carbonate platform over previously continental domains, represented in southeastern Morocco by the up to 100 m thick Akrabou Formation. The Akrabou Formation evolves from shallow-water bioclastic wackestones and packstones with abundant micrite into deeper water laminated lime mudstones with calcareous plankton, fish debris, and ammonite-bearing nodules, and then again to shallow benthic carbonate production. In the upper part, high-energy conditions are reflected by coarser cross-bedded levels, while peritidal facies at the top mark platform emersion and the reestablishment of continental siliciclastic sedimentation. Biostratigraphic data place the Cenomanian-Turonian boundary shortly after the maximum deepening phase. Stable isotope analyses (δ13C and δ18O) performed on bulk carbonates reveal a broad positive δ13C excursion reaching +5‰ VPDB within the lower shallow-water interval, consistent with the global isotopic signature of OAE2 (Jarvis et al., 2006). This trend is followed by a rapid decrease to values around −2‰ VPDB within the laminated deeper-water interval, and by a subsequent return toward positive values up to +2‰ VPDB in the upper shallow-water deposits. Isotope data from the primary fibrous calcite of oyster shells display comparable trends, with slightly heavier values, supporting preservation of the δ13C primary depositional signal. We suggest that the laminated interval recorded a temporary retreat of shallow-water carbonate production under environmental stress likely related to rapid transgression and enhanced nutrient supply. This platform retreat postdates the peak positive carbon-isotope excursion (see also Wang et al., 2021), suggesting an environmental response not directly linked to OAE2, but rather controlled by basin-scale processes operating after the event. However, the crisis was not irreversible, as benthic carbonate communities recovered and platform sedimentation resumed.