Marine anoxia, eustasy, and tectonic controls on carbonate factory dynamics in the Aptian Intrashelf Basin, Interior Fars Zone

The early Aptian was marked by Oceanic Anoxic Event 1a (OAE 1a), global sea-level oscillations, and tectonic reorganization. This study examines how the Dariyan Formation (Interior Fars Zone, Zagros Basin) recorded the interplay of global environmental changes and local tectonics within a shallow carbonate platform-intrashelf basin system. Through integrated analysis of calcareous nannofossil paleoecology, microfacies, sequence stratigraphy, and stable isotopes from a 241.5 m-thick outcrop section, we link ecological restructuring with stratigraphic development across a ramp-to-basin transect. Nannofossil assemblages reveal a transition from oligotrophic to mesotrophic-eutrophic conditions during OAE 1a. Microfacies document environments from restricted lagoons to deep basinal settings, organized into five third-order sequences within the lower and upper Aptian second-order cycles. OAE 1a exerted a primary control on carbonate production, driving nutrient enrichment, ecological stress, and episodic platform recovery. The Radiolarian Flood Zone, developed during DS2, represents the intrashelf basin expression of the Selli Level, with organic-rich condensation in basin centers and coeval progradation along margins. Stratigraphic patterns and thickness trends are consistent with reactivation of the Kazerun Fault System, whose differential subsidence influenced facies partitioning and accumulation rates. The carbon-isotope record broadly captures Aptian carbon-cycle perturbations, though some departures from global reference curves may reflect local overprints. Overall, the Dariyan Formation provides a sensitive record of OAE 1a in a tectonically segmented intrashelf basin, highlighting how global environmental perturbations were modulated by eustatic and local tectonic controls.