The Late Cretaceous–early Paleogene interval is globally associated with transient to long-term changes in the stable carbon isotopic composition of marine carbonates (δ13Ccarb). Based on biostratigraphic reconstruction, this critical period of Earth's history is thought to coincide with the deposition of world heritage Paleocene phosphate deposits (phosphorites) in northwestern Morocco. However, the detailed stratigraphy of the Gantour basin, one of the most important Moroccan phosphate deposits, has not yet been constrained. For instance, the former “Montian” Stage has been used to tentatively approximate the Danian, whereas the succeeding Selandian Stage remains to be identified. Here, we develop a detailed organic carbon isotopic (δ13Corg) curve from phosphorus-rich horizons of the western Gantour sedimentary sequence in an attempt to constrain their stratigraphic placement and depositional age model. Upsection, these strata host long-term negative and positive δ13Corg trends that tend to correlate with global δ13Ccarb records of the Cretaceous–Paleogene and mid-Thanetian transitional boundaries. The data support the presence of Danian and Selandian rocks in the Gantour basin, which are succeeded by strata containing characteristic signatures of the well-known Cenozoic δ13C maximum at 58–57.5 Ma (the Paleocene Carbon Isotope Maximum). Our results shift the previously proposed Cretaceous–Paleogene transition in the Gantour basin further down into the older sediment C2M layer without interfering with recorded massive biological turnover in faunal diversity and abundance. Moreover, the refined stratigraphy suggests that the deposition of the Gantour phosphorites spanned ~8.5 Myr. Our results confirm the utility of δ13Corg chemostratigraphy for dating and correlating phosphate-bearing deposits of the Tethyan province. They have important implications for deciphering Paleocene phosphogenesis, the co-evolution of associated vertebrate groups, and for prospecting phosphorus-rich mineral deposits.