Facies analysis and sequence stratigraphy of the Lower Cretaceous strata (Fahliyan Formation) in Izeh Zone, Zagros Basin, SW Iran

Petrographic studies were carried out to explain the main variations in sedimentary facies and environment characteristics. Based on these studies, 16 microfacies were recognized for the Fahliyan strata, which were deposited in various segments of the ramp platform, including tidal-flat, lagoon, shoal, and open marine settings. Moreover, interpreted depositional environment and fauna components and sedimentary textures of microfacies were applied to identify three-order depositional sequences (sequences A to C). Sequence A, the lowest part of the Fahliyan strata, comprises sedimentary facies related to shoal and open marine settings. Sequence B constitutes the middle part of the formation, and the open marine segment was considered the primary depositional environment for most of the recognized microfacies in this sequence. Microfacies in the upper part of the Fahliyan Formation were mainly deposited in shallow (lagoon) to deeper water (open marine), and they form the last depositional sequence of the formation (sequence C). The results of this study reveal that the lower successions mainly were deposited coincided with a sea-level fall in the basin, while the sea level shows a slight rise during deposition of the middle units of the formation. However, the prevailing microfacies related to the shallow water environment, such as lagoon setting through the uppermost parts of the Fahliyan Formation, represent a fall in sea level at that time. This study shows that the Mongasht section (center of the study area) with microfacies associated with deep-water environments was the deepest part of the basin (depocenter). Hopefully, it is attempted to make a sequence stratigraphy framework to analyze the variations of microfacies and sedimentary environment and the reasons behind these changes for this formation.