Geochemistry and regional stratigraphy of the Upper Cretaceous succession of central Saudi Arabia: A record of foreland basin inception on the Arabian Plate

The inception of a foreland basin on the Arabian Plate and its subsidence history during the Late Cretaceous are recorded in its forebulge and backbulge sedimentary succession. Exposures of this succession in central Saudi Arabia were examined through a multi-disciplinary approach involving sedimentology, stratigraphy, biostratigraphy, and geochemistry to document the stratigraphic evolution of the depositional system. The succession is marked at its base by deltaic, estuarine, and open-coast tidal flat siliciclastic deposits of the Majma, Qibah, and Malihah formations. These units transition upward into the Aruma Formation which represents carbonate deposition on a carbonate ramp. A Campanian–Maastrichtian age for the Aruma Formation is consistent with the appearance of a negative carbon isotope excursion marking the Campanian-Maastrichtian Boundary Event. The lithological shift from siliciclastics to shallow-water carbonates marks an overall transgression, over a regional unconformity, which is contrary to the documented long-term eustatic sea-level fall from the Turonian through Maastrichtian. This was driven by progressive increase in flexural subsidence due to ophiolite obduction over the plate's eastern and northeastern margins, forcing the Aruma Formation carbonates to onlap onto the unconformity and backstep over the forebulge of the foreland basin. The maximum flexural subsidence is manifested by the deposition of a phosphoritic condensed section in the earliest Maastrichtian, which also signals to the timing of the farthest advancement of the ophiolites over the plate's margins. This study showcases the capacity of shallow-water carbonate systems as recorders of subsidence history in foreland basins.