Foraminiferal Paleoecology and Sequence Stratigraphy of the upper Dhruma Formation, Saudi Arabia

This study analyzes foraminiferal assemblages in the Middle Jurassic upper Dhruma Formation in Central Saudi Arabia to support the refinement of the Jurassic sequence stratigraphic interpretation in the region. The disaggregation-acetic acid leaching technique was applied to 62 lithified limestone and marl samples to recover foraminifera, facilitating morphogroup analysis, species composition assessment, and the vertical distribution of species for paleoenvironmental interpretation, depositional sequence analysis, and maximum flooding horizon identification. The studied section covers the Atash and lower part of the Hisyan members of the D7 unit of the Dhruma Formation. The succession is dominated at the base of the Atash Member by calcareous epifaunal benthic species, transitioning to a diverse assemblage of shallow and deep infaunal agglutinated and calcareous benthic foraminifera, along with planktonic taxa, in the middle part of the Atash Member. This part of the section marks peak transgression and coincides with the J40 maximum flooding surface (MFS). A return to epifaunal foraminiferal dominance in the lower part of the overlying Hisyan member probably reflects a shallowing. Morphogroup and species composition analyses reveal sig nificant environmental shifts, correlating with a transgressive-regressive cycle. Biodiversity indices and clus ter analysis further delineate depositional settings, supporting a refined stratigraphic framework. A concep tual paleobathymetric model indicates progressive deepening in an epicontinental ramp setting, emphasizing the role of sea-level fluctuations and suggesting that the clay-rich unit (Hisyan Member) may represent a shallowing rather than a deepening trend. These findings enhance our understanding of Jurassic shallow ma rine carbonate platform dynamics and their broader implications for regional sequence stratigraphy.