Isotope stratigraphy (87Sr/86Sr, 13C) and depositional sequences of the Aruma Formation, Saudi Arabia: Implications to eustatic sea‐level changes

Lithofacies characteristics, depositional architecture (facies tracts and depositional sequences), stable isotopes (δ18O and δ13C), and 87Sr/86Sr ratios were analysed from the Aruma Formation at the Khashm Buwaibiyat section (north‐east Riyadh, Saudi Arabia). The Aruma Formation consists of three formal members (base to top), Khanasir, Hajajah, and Lina members. Based on 87Sr/86Sr and δ13C data, the lower age of the regionally distributed Aruma Formation (Khanasir Member) is now proposed as Late Campanian. This age assignment is supported by the identification of a distinct δ13C negative excursion (=Epsilon Event; Upper Campanian), and a globally conformable range of 87Sr/86Sr values (0.707604–0.707703). Additionally, a review of previous fauna (ammonites and nannofossils) also favours this age assignment. The 87Sr/86Sr values (0.707817–0.707848) from the upper part of the Hajajah Member reflects a Late Maastrichtian signature as also noted by the presence of a long‐term δ13C decrease. Additionally, based on lithofacies and depositional architecture for the Aruma Formation, nine facies types are identified. These are grouped into four facies associations (restricted peritidal, lagoon inner ramp, shoal transgressive lag, and coral reefs), reflecting four third‐order depositional sequences (transgression–regression cycles). These reflect strong influence of both eustatic sea‐level rise and regional tectonics (resulting in widespread unconformities and long‐time gaps). The shallowing and deepening of the sea level is inferred using sequence stratigraphy (system tracts) and lithofacies variations. This data suggests a gradual rise in sea level from the Upper Campanian to the Maastrichtian with an intervening fall around the Campanian‐Maastrichtian boundary.