The Ilam Formation in the Zagros basin, southwest Iran, is a rich petroleum reservoir rock that possesses considerable exploration potential. This study provides valuable insights into the paleoenvironmental and paleogeographic conditions of the Ilam Formation using sedimentological and geochemical analysis as well as sea-level changes during the Late Cretaceous time (Late Santonian-Late Campanian) in the Lorestan area. Based on petrographic analysis, four microfacies were identified in the studied area (Mehdi Abad section) in the Ilam Formation that were deposited in an outer ramp setting, from middle neritic to bathyal, in a homoclinal ramp. Sequence stratigraphic analysis led to detect two transgressive system tracts and one regressive system tract. In the current study, the paleogeochemical signatures, using redox-sensitive elements, show strong evidence of declining oxygen level during the Late Santonian and Late Campanian; however, increase of oxygen levels only observed during the Early Campanian time. The changes in oxic-anoxic conditions in the depositional basin during deposition of the Ilam Formation can be correlated with the final phase of the oceanic anoxic event in the Cretaceous. However, the closure event of the Neo-Tethys Ocean and subsequent changes in fault mechanisms and basin structure were causal mechanisms of oxic-anoxic conditions during this period.

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