Tectono-eustatic controls on depositional setting and spatial facies distribution of Coniacian–Santonian sequences of the Zagros Basin in Fars area, S. Iran

Abstract

New high-resolution biostratigraphy and facies analyses indicate that a Coniacian-Santonian homoclinal carbonate ramp (Ilam Fm.) developed on the subaerially exposed mid-Turonian surface at the top of Sarvak Fm. in Fars zone of the Zagros Basin (northeastern margin of the Arabian Plate). Spatial facies distribution, sequence stratigraphy, and lateral variation in the thickness of Ilam Fm., as well as extend of erosion of the underlying Sarvak Fm. suggest that a combination of regional tectonic activities and global sea-level changes controlled the development of this succession. A major tectonic configuration in the Zagros belt including several basement faults subdivide the belt into different blocks with distinctive sedimentary records. Reactivation of these preexisting basement faults during Late Cretaceous provided a horst and graben geometry due to the closure of Neo-Tethys. Three of our studied sections are located on an upthrown block, bounded between Kazerun and Karebas basement faults, include inner and mid-ramp deposits. These sections represent an eastward deepening trend which correspond to eastward tilting of this fault block. The easternmost section is located in the eastern neighboring downthrown block , bounded between Karebas and Bastak faults, and includes outer ramp and basin deposits confirming a significant paleo-bathymetrical contrast between the two blocks. Furthermore, eastward thickening of the Ilam Fm. independently confirms the increase of accommodation space, as a result of deeper depositional condition, toward center of the Fars zone. Absence of subaerial exposure features at top of the Sarvak Fm. in the downthrown block shows that the global sea-level fall and regional tectonic regime also controlled the extend of unconformity underlying the Coniacian-Santonian platform deposits (Ilam Fm.).