Carbonate factory dynamics during a biocrisis (Barzakh condition), a case study of late Permian–Early Triassic successions in the Persian Gulf, Iran

Abstract Marine environmental conditions govern carbonate sediment types through their influence on carbonate factories. These factories, the sites of carbonate production, adjust to environmental shifts by altering precipitation forms. This study examines Permian-Triassic strata in the Persian Gulf to understand how environmental changes, particularly a major biocrisis, impacted carbonate sedimentation. Thin section analysis, core data, and gamma-ray logs were utilised to reconstruct this history. During the late Permian, a thriving tropical carbonate factory was established along the Neo-Tethys margin. Favourable environmental conditions and ample accommodation space facilitated its expansion and fostered the proliferation of diverse marine life. The Dalan Formation, characterised by grain-dominated facies, resulted, with peloids, ooids, and skeletal fragments making up the bulk of the sediment. Carbonate production was driven by both chemical precipitation and metazoan activity. The Permian-Triassic boundary extinction event profoundly impacted sedimentation. Metazoans nearly disappeared, creating a ‘Barzakh condition’ (microbial dominated system) in the Early Triassic, in which microbial populations were no longer suppressed by metazoan predators. Increased calcium carbonate saturation and nutrient levels, likely due to upwelling and consumer removal, favoured microbial carbonate factories. Consequently, the Lower Triassic succession in the Persian Gulf exhibits a clear shift towards microbial-mediated carbonate sediments. This study is significant due to its focus on shallow marine settings along the Arabian Plate, demonstrating how Earth system events, particularly the end-Permian mass extinction, fundamentally altered carbonate platform production and dynamics. These environmental changes also impacted reservoir properties, which are relevant for understanding the region's hydrocarbon resources.