Refining the stratigraphy of Moroccan phosphate-bearing sequences to enhance preservation of phosphate resources

Séminaire de Jérémie Aubineau (post-doc à Géosciences Montpellier)

Abstract: The Moroccan phosphate series hosts abundant species of vertebrates, ranging over a period of nearly 25 Myr from the Maastrichtian to the Lutecian. However, the biostratigraphic background of some Paleocene sediments is absent or relatively unreliable. Therefore, I have explored several approaches to better constrain the stratigraphy of phosphate-rich sequences of Morocco in both the western High Atlas (WHA) and Gantour Plateau (~100 km northward of the WHA).
I first focused on the U-Pb dating of carbonate fluorapatite (CFA – Ca10-x-yNaxMgy(PO4)6-z(CO3)z(F)0.4zF2) grains, considering the high amount of uranium in phosphate minerals. Unfortunately, linking U-Pb phosphate ages to the age of deposition is not easy. Moreover, I have measured the carbon isotope ratios of CFA and carbonates in the WHA sediments to perform a chemostratigraphic study. The results yielded ∂13C values as low as -12‰, which is absolutely not consistent with the Paleogene ∂13Ccarb values. Finally, I explored the organic carbon isotopes (∂13Corg) of several Gantour phosphorites. A covariation was found between the global ∂13C trend of carbonates from deep-sea benthic foraminifera and the ∂13Corg trend in the Moroccan phosphorites, which allows stratigraphic correlations and provides new age constraints on the world’s largest phosphate accumulation.
The ∂13Corg chemostratigraphy seem to be a powerful approach to enhance interbasinal stratigraphic correlations that could optimize exploration and lead to a good knowledge of phosphate resources.