Permian-Triassic boundary sections in the regions of Julfa (NW Iran) and Abadeh (Central Iran) display three characteristic rock units: (1) the Paratirolites Limestone (4-5 m thick) with the mass extinction horizon (P-Tr extinction event) at the top surface (2) the ‘Boundary Clay’, and (3) the Elikah Formation with the conodont P-Tr boundary near the base. The Paratirolites Limestone is the uppermost of the Permian carbonate formations. It is a pelagic red nodular marly limestone and represents a skeletal-dominated benthic carbonate factory. Microfacies studes show a decline in carbonate accumulation towards the top of this unit, where increasing numbers of reworked hardground clasts, bored and encrusted bioclasts and lithoclasts, ferruginous crusts, and dissolved cephalopod shells occur. The reduction in the carbonate accumulation culminates at the transition from the Paratirolites Limestone into the ‘Boundary Clay’, where red claystone and shales with only few marly nodules replace the carbonate factory. At this transition occurs also a significant reduction of biogenic components. In Central Iran, in the transition from the ‘Boundary Clay’ to the overlying grey platy limestone beds of the Elikah Formation occur one or more enigmatic ‘calcite fan’ layers of probably abiotic origin. The skeletal carbonate factory of the Late Permian was restorted with the deposition of micritic limestone with stromatolitic and thrombolitic fabric at the base of the Elikah Formation. Densely laminated bindstone, aggregate grain grapestone, wackestone with calcite spar-filled voids, and oncoid floatstone represent a microbial-dominated carbonate factory. Microbial carbonates deposited in the Early Triassic Elikah Formation are represented by different types of microbialite and stromatolite. In the sections near Julfa the organosedimentary deposits are controlled by benthic microbial communities; they can be classified based only on their microstructure. They comprise fine-grained agglutinated stromatolite, poorly structured thrombolite, and cryptic microbialite. In the sections near Abadeh, in contrast to the sections near Julfa, the occurrence of the microbialite and stromatolite is more diverse and large-scale features with conspicuous morphology and internal structure occur. They comprise heart-shaped, flower-shaped, isolated and amalgamated forms. Thrombolite mounds with clotted mesofabric, planar and domal stromatolites with laminated mesofabric predominate. The P-Tr boundary sections in Julfa area and Central Iran were deposited on a carbonate shelf (platform) in equatorial latitudes. Skeletal, biotically controlled benthic carbonate factory of the Late Permian has been replaced by the biotically induced carbonate factory of the Early Triassic, with periods of abiotic precipitates within the ‘Boundary Clay’ and in the Elikah Formation. Microbialites deposited in the aftermath of the P-Tr mass extinction are considered to be anomalous are mentioned as biotic responses to periods of unusual ocean chemistry.

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