Mode and timing of the Early Cretaceous transgression in Iran: Insights from the stratigraphic evidence of sea-level changes and geodynamic events

The study of selected logs from the Central-Eastern Iran Microplate (CEIM; area of Arusan) and Sanandaj–Sirjan Zone (SSZ; areas of Esfahan east and Tiran) constrains timing and evolution of the Cretaceous successions, resting unconformably on older sediments or non-conformably on the metamorphic basement. The Cretaceous marine transgression occurred almost synchronously on a levelled topography, documenting a previous deep erosion. Above a basal siliciclastic unit, rudist- and orbitolinid-rich lower Aptian limestone (“Orbitolina Limestone”, Shak Kuh Formation, Taft Formation) were deposited. Later, the stratigraphic evolution of the three studied areas differentiates, documenting changes in the depositional environments, reflecting different subsidence rates. In the Tiran area, marls and cherty limestones covering shallow-water limestones record a deepening trend. A differentiation of depositional environment is observed at Arusan, where conglomerates to the west pass to basinal sandstone and marls to the east. Here, a Coniacian-Campanian reprise of the carbonate production (with skeletal limestones rich in rudists, ostreids, and abundant orbitoidids) is covered by cherty marls. In the Esfahan north area, glauconitic hardground marls covered by chert record important deepening during the early Cenomanian, followed by the reprise of shallow-water carbonate production (middle Cenomanian). The synchronous deposition of Aptian carbonate successions on large parts of the CEIM, SSZ and North-Iranian domains suggests a rapid regional transgression controlled by a sea-level rise. Stratigraphic differentiation occurring in late Aptian–Cenomanian times, recording the onset of tectonic events differently affecting the aforementioned domains, likely related to the opening of oceanic seaways in the CEIM and the SSZ.