Biostratigraphy and microfacies of the Gadvan Formation: Coastal Fars zone (Zagros Basin, SW Iran)

In order to study the Lower Cretaceous sequence biostratigraphy, microfacies, and sedimentary environment of the Gadvan Formation, two stratigraphic sections of Dehnu and Kool-e-Kochek in the Coastal Fars region SW of Iran were selected. The lithological characteristic of both studied sections of the Gadvan Formation is comprised mainly of marl and marly limestone with some shale alternation. The lower and upper contacts of the Gadvan Formation are conformable with surrounding strata Fahliyan (below) and Dariyan (above). Foraminiferal biostratigraphy leads to the identification of Hedbergella sp. Zone in Dehnu section and Paleorbitolina lenticularis–Choffatella decipiens zone in Kool-e-Kochek section. According to described biozones, the age determination of the Gadvan Formation is assigned to Upper Barremian–Lower Aptian. The association of plankton and Calcispahaerulids represents beginning of open marine paleoenvironment in Dehnu section, whereas the orbitolinids are expanded in the Kool-e-Kochek as shallow water environment. The microfacies analysis of the both studied sections represent Bioclast wackestone, planktonic foraminifera bioclast mudstone, Calcisphaerulid wackestone/packstone, peloid grainstone, agglutinated foraminifera wackestone, Bioclast orbitolinid wackestone, orbitolinid packstone, and peloid bioclast wackestone microfacies that were identified and attributed to shallow lagoon, shoal and open marine paleoenvironment in both sections. The well development of both shallow water and open marine foraminifers indicates the change in deepening which is caused by Kazerun basement fault in the studied area during late Barremian to early Aptian time.