Depositional environment, sequence stratigraphy and geochemistry of Lower Cretaceous carbonates (Fahliyan Formation), south-west Iran

The Fahliyan Formation is a carbonate sequence of Lower Cretaceous (Berriasian–Hauterivian) age and was deposited in the Zagros sedimentary basin, Iran. In this investigation, the Fahliyan Formation at the type section and in the subsurface has been studied. Facies analysis and petrographic studies led to the recognition of 10 microfacies that were deposited in three facies belts: lagoon, shoal and open marine. The observed facies patterns indicate a carbonate rimmed-shelf depositional environment. Based on field observations, microfacies analysis and sequence stratigraphic concepts, two-third-order sequences in the type section and three-third-order sequences in the subsurface section were identified. The transgressive deposits display a predominance of deep subtidal facies, while highstand deposits show shallow subtidal facies. Some petrographic evidence such as an abundance of aragonite skeletal and non-skeletal components shows that this formation was deposited in a sub-tropical environment with original aragonite mineralogy. Geochemical evidence such as high Sr/Na ratios also support original aragonite mineralogy. δ 18O and δ 13C values suggests that alteration occurred during burial diagenesis, in a closed system, with low water/rock interaction. Palaeotemperature calculation, based on the heaviest oxygen isotope value in micritic samples of the Fahliyan limestone, shows that ambient water temperature was around 24 °C during the deposition of this formation.