Callovian (Middle Jurassic) holostratigraphy in the Arabian Plate: A Reference Section from Saudi Arabia

The Callovian stage of the Middle Jurassic represents a critical yet often incomplete interval in the global stratigraphic record, due to widespread hiatuses and condensed successions. In contrast, the shallow-marine carbonate succession along the Tuwaiq Escarpment in Saudi Arabia provides an exceptionally continuous and high-resolution archive of Callovian environmental and stratigraphic evolution. A key reference section at Khashm al Qaddiyah near Riyadh (327.5 m thick), integrated with 28 additional sections spanning approximately 1000 km, documents a nearly complete transition from the upper Dhruma Formation (Bajocian–Bathonian) through the Tuwaiq Mountain Formation (Callovian) to the basal Hanifa Formation (Oxfordian). A robust multi-proxy biostratigraphic framework underpins this record, incorporating ammonites, calcareous nannofossils, benthic foraminifera, brachiopods, nautiloids, ostracods, corals, and stromatoporoids. Ammonite zonation—from the gracilis to athleta–lamberti zones—allows precise subdivision of the Callovian. The Lower–Middle Callovian boundary is defined at the base of the Reineckeia anceps Zone, while the Middle–Upper boundary corresponds to the base of the Peltoceras athleta Zone. Calcareous nannofossil bioevents, including the first occurrences of Watznaueria ovata, Stephanolithion bigotii bigotii, and Watznaueria moshkovitzii, provide additional chronological control. Benthic foraminiferal assemblages define five biozones (Dm-6 to Tm-5), recording a shift from restricted shallow settings to open-platform and reefal environments, with peak biodiversity at the Middle–Upper Callovian transition. Sequence stratigraphically, the succession reflects a stepwise deepening trend beginning with a latest Bathonian–early Callovian transgression and culminating in widespread reef development during the Late Callovian. Bayesian age–depth modeling indicates a marked increase in sedimentation rates, from approximately 1–3.3 cm/kyr in the Early Callovian to 15–23.6 cm/kyr in the Middle–Late Callovian. This acceleration is attributed to enhanced accommodation driven by subsidence near Riyadh and increased sediment supply, likely linked to the Wadi ad-Dawasir delta system. The succession ends with progressive shallowing and a major sea-level fall across the Callovian–Oxfordian boundary. The carbon-isotope record further highlights the completeness and paleoenvironmental sensitivity of this archive. An early Callovian positive excursion (~+2.3‰) precedes the first occurrence of Stephanolithion bigotii bigotii, followed by another (~+2.5‰) at the Lower–Middle Callovian transition, coinciding with increased sedimentation rates. During the Middle Callovian, δ¹³C values rise steadily through the coronatum Zone (up to ~+3.7‰), culminating in a major excursion (~+3.2‰ to +4‰) across the coronatum–athleta transition. This interval reflects peak greenhouse conditions, heightened productivity, and significant organic carbon burial associated with the development of an Arabian intrashelf basin. The Late Callovian is characterized by continued isotopic variability, including a ~+3.2‰ excursion within coral–stromatoporoid bioherms, followed by a pronounced negative shift (~+0.2‰) and a final positive excursion (~+3‰) across the Callovian–Oxfordian boundary. This phase also records a major ecological turnover, marked by the disappearance of coral–stromatoporoid reefs and the transition to algal mudstone facies, preceding a regional unconformity. Overall, the Saudi Arabian Callovian succession represents one of the most complete and detailed carbonate platform records globally, providing critical insights into the interactions among sea-level change, climate dynamics, carbon cycling, and ecosystem evolution during the Middle Jurassic.